Controle abiótico dos íons principais, pH, fósforo e nitrogênio, e

UNIVERSIDADE FEDERAL DE MINAS GERAIS
INSTITUTO DE CIÊNCIAS BIOLÓGICAS
Controle abiótico dos íons principais, pH, fósforo e
nitrogênio, e suas implicações nos processos de
acidificação e eutrofização dos ecossistemas fluviais
de Minas Gerais – Brasil
Maria Elisa Castellanos Solá
Belo Horizonte, fevereiro de 2008
UNIVERSIDADE FEDERAL DE MINAS GERAIS
INSTITUTO DE CIÊNCIAS BIOLÓGICAS
Controle abiótico dos íons principais, pH, fósforo e
nitrogênio, e suas implicações nos processos de
acidificação e eutrofização dos ecossistemas fluviais de
Minas Gerais – Brasil
Maria Elisa Castellanos Solá
Tese apresentada ao Programa de
Pós-graduação
em
Ecologia,
Conservação e Manejo de Vida
Silvestre da Universidade Federal
de Minas Gerais como parte dos
requisitos para obtenção do título de
Doutor em Ecologia
Orientador:
Prof. Dr. José Pires de Lemos Júnior
Co-Orientadora:
Profa. Dra. Claudia Maria Jacobi
Belo Horizonte, 29 de fevereiro de 2008
Dedico a
minha mãe, Maria Elisa Solá de Castellanos
meu pai, Manuel Castellanos Solá
AGRADECIMENTOS
Aos meus orientadores José Pires de Lemos Filho e Claudia Jacobi, pela amizade, pelo apóio nos
momentos mais difíceis e orientação nos momentos incertos. Sempre presentes.
Aos meus colegas da turma de 2002 que fizeram com que os cursos teóricos e de campo fossem
momentos inesquecíveis pelo convívio, pela disponibilidade de compartilhar o conhecimento de
cada um e pela alegria das tradicionais festas na casa da Maíra.
A meu colega Marco Aurélio, companheiro constante em toda a jornada.
Ao professor Rogério Parentoni Martins pelo incentivo à leitura dos temas mais variados dentro
disciplina de Ecologia e disponibilidade para emprestar seus muitos e interessantes livros.
Aos professores do curso ECMVS que me apoiaram e incentivaram quando necessário.
Aos funcionários da secretaria que, mesmo com dificuldades, sempre acabaram resolvendo os
problemas.
Às bibliotecárias da Biblioteca do ICB que sempre buscaram atender prontamente minhas
solicitações.
Aos funcionários do IGAM por me fornecer as bases cartográficas do monitoramento das águas e
todas as informações sobre o banco de dados utilizado nesta tese.
Ao Prof. Dr. Heinz Charles Kohler – PUC/MG pelo constante incentivo e apóio desde a minha
decisão em iniciar o doutoramento.
Aos Dres. Sidneide Manfredini, Leandro Gonçalves Oliveira, Arnola Rietzler, Heinz Charles
Kohler e Francisco Barbosa por ter aceitado fazer parte da banca.
À Brandt Meio Ambiente por me liberar do trabalho nas últimas semanas da redação da tese.
Ao CNPq, pela bolsa de doutorado.
Ao Programa de Pós-Graduação em Ecologia, Conservação e Manejo da Vida Silvestre e à
UFMG.
A todos os que de alguma forma colaboraram,
meus agradecimentos.
Sumário
Página
Apresentação do tema e estrutura da tese................................................................
Resumo e Abstracts....................................................................................................
Capítulo 1. Controle geológico do pH nas águas de rios tropicais.........................
Introdução..................................................................................................................
Material e Métodos....................................................................................................
Resultados e Discussão..............................................................................................
1. Íons principais....................................................................................................
2.Classificação química dos rios............................................................................
3. Principais compartimentos geológicos..............................................................
4. Rochas cristalinas..............................................................................................
5. Rochas carbonáticas...........................................................................................
6. Bacias hidrográficas...........................................................................................
Conclusões.............................................................................................................
Referências............................................................................................................
Capítulo 2. Controle abiótico sazonal do pH em rios tropicais..............................
Introdução..................................................................................................................
Material e Métodos....................................................................................................
Resultados e Discussão .............................................................................................
1. pH......................................................................................................................
2. Íons principais....................................................................................................
3. Descarga, Sólidos totais em suspensão e sólidos totais dissolvidos..................
4. STS, STD, íons principais, fosfatos e metais pesados.......................................
5. Carga instantânea...............................................................................................
6. Alcalinidade, pH e sensibilidade à acidificação................................................
7. ANC calculada, alcalinidade e pH.....................................................................
8. Depressão de ANC dilution effect and major ions............................................
9. Solo, vegetação, trajetos das águas e efeito de diluição....................................
Conclusões................................................................................................................
Referências...............................................................................................................
Capítulo 3. Influência do clima úmido-seco nos padrões de P e N, nutrientes
limitantes e estado trófico em rios tropicais.............................................................
Introdução..................................................................................................................
Material e Métodos....................................................................................................
Resultados e Discussão..............................................................................................
1.Fósforo e nitrogênio............................................................................................
2. Estado trófico.....................................................................................................
3. Nutrientes limitantes..........................................................................................
4. Concordância entre os estados tróficos de TN e TP e potencial de limitação
da produtividade primária ......................................................................................
5. Risco de eutrofização
Conclusões.................................................................................................................
Referências................................................................................................................
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APRESENTAÇÃO DO TEMA E ESTRUTURA DA TESE
1. Apresentação do Tema
Desde a segunda metade do século XX, a população humana vem apresentando um
crescimento acentuado, tendo como conseqüência tanto o aumento do uso dos recursos
naturais como a geração de resíduos poluentes. Dentre os efeitos adversos das atividades
humanas, destaca-se a alteração dos ciclos biogeoquímicos nos diversos compartimentos dos
ecossistemas, seja pelo aumento das taxas de entrada de substâncias, pelo consumo de
substâncias em taxas maiores do que a reposição natural, ou pela alteração do tempo de
residência nos compartimentos. De acordo com Vitousek et al. (1997), os principais ciclos
biogeoquímicos que estão sendo alterados são os ciclos da água, carbono, nitrogênio, fósforo,
enxofre e metais. E, além da alteração dos ciclos biogeoquímicos, assistimos a mudanças nos
sistemas climáticos (Vörösmarty et al. 2000) e biodiversidade (Pimm et al. 1995,
McMichael et al. 1999, Sala 2000, Rojstaczer et al.2001).
Um dos compartimentos mais afetados pela alteração dos ciclos biogeoquímicos são
as águas superficiais (Postel et al. 1996, Vörösmarty e Sahagian 2000, Meybeck 2004),
onde a poluição das águas se processa num ritmo muito mais intenso e com um número muito
maior de compostos nocivos do que a poluição da atmosfera (Fellemberg 1980). Dentre as
atividades humanas que ameaçam os ecossistemas aquáticos, podem ser citados como
exemplos: alterações na estrutura física dos rios [construção de barragens, canalização,
desvios, alteração do canal natural dos rios, drenagem de áreas alagadas], interferências na
bacia hidrográfica [desmatamento, agricultura (Socolow 1999, Tilman 1999, Tilman et al.
1999) urbanização], interferência na biota [pesca excessiva, introdução de espécies exóticas].
O transporte fluvial é um elo importante no fluxo de material que permeia todo o
planeta, onde as águas correntes movimentam enormes quantidades de materiais rapidamente,
via fluxos superficiais e subterrâneos, na escala de centímetros até milhares de quilômetros
em resposta à gravidade. (Reiners e Driese 2003). Conforme Allan (1996), talvez nenhum
outro ecossistema tenha sido tão significativamente modificado pelas atividades humanas
como os rios. A degradação da saúde dos ecossistemas aquáticos é comum em todos os
lugares onde ocorreram assentamentos humanos significativos.
i
As águas superficiais geralmente estão contaminadas por diversas substâncias e ainda
é um desafio entender como o conjunto destas substâncias afeta a saúde dos ecossistemas e a
saúde humana. Estudos ecotoxicológicos têm buscado entender a liberação das substâncias no
meio ambiente, o transporte das substâncias na paisagem geográfica, a exposição da biota às
substâncias e a resposta da biota às substâncias (Miller 1978). Por outro lado, ainda é pouco
conhecido como as misturas dos poluentes afetam a relação dose-resposta (Chasin and
Azevedo 2003). Com relação à saúde humana, pesquisas sobre a “sensibilidade a químicos
múltiplos” ou “eco-síndrome” começam a ser realizadas, tendo sido proposto que exposições
a baixos teores de numerosas substâncias químicas presentes no ambiente podem provocar
doenças (Triendl et al. 1999, Kipen and Fiedler 2002, Terr 2003, Robinson and McDonell
2004). A maioria das pesquisas sobre a ação dos poluentes na biota tem sido centrada na ação
de compostos individuais. Estes resultados têm sido empregados como base dos critérios de
qualidade da água e padrões para proteção da vida aquática, ignorando o fato de que as
interações entre os poluentes podem ocorrer levando a sinergismo ou antagonismo da
toxicidade (Enserink et al. 1991, Otitoloju 2003). Além disto, em muitos paises, como por
exemplo Brasil, os valores de background natural não são conhecidos e os padrões ainda não
consideram as diferenças intra-regionais.
Com relação às alterações do ciclo do fósforo e suas conseqüências nos cursos de
água, Bennet et al. (2001) mencionam que as ações humanas como mineração de fósforo e
seu transporte como fertilizantes, alimentos animais e produtos agrícolas entre outros, estão
alterando o ciclo global do fósforo provocando seu acúmulo nos solos. O aumento dos níveis
de P no solo eleva o potencial de transporte de P para os ecossistemas aquáticos. Isto, devido
a que o fósforo geralmente entra nestes ecossistemas adsorvido às partículas do solo que são
erodidas para os lagos, córregos e rios. Grande parte deste transporte ocorre durante as
tormentas. Desta forma, o potencial de poluição por P está fortemente influenciado pelo uso
da bacia hidrográfica e pela concentração de fósforo no solo, sendo que qualquer fator que
aumente a erosão ou a quantidade de fósforo no solo aumenta o potencial de transporte para
os rios e lagos. O enriquecimento dos ecossistemas aquáticos com nutrientes pode causar a
produção excessiva nos lagos levando à eutrofização, o que pode impedir o uso das águas
devido a florescimento de algas e eventos anóxicos. Carpenter (2005) comenta que a
eutrofização tem se tornado um problema global que será intensificado nas próximas décadas
devido ao aumento da população humana, demanda de alimento, a conversão da terra, o uso
de fertilizantes e a deposição de nitrogênio. Através do uso de modelos de simulação o autor
ii
concluiu que são necessárias centenas de anos para que um lago se recupere da eutrofização
provocada pelas atividades agrícolas. Em princípio, a eutrofização é reversível; entretanto, na
escala de tempo humana a eutrofização dos lagos pode parecer permanente, senão ocorrerem
mudanças substanciais no manejo da terra e novas tecnologias para a redução do conteúdo de
fósforo nos solos ou para reduzir os processos de erosão.
Uma outra conseqüência da alteração dos ciclos biogeoquímicos é a presença da
“chuva ácida” e consequente acidificação dos ecossistemas (Binkley and Richter 1987).
Galloway (2001) menciona que as principais causas de acidificação são a produção de
energia com formação de NO e SO2, e a produção de alimentos com a formação de NH3 por
processos de nitrificação. A ocorrência de chuvas ácidas já é um sério problema que atinge os
países industrializados e em especial o dos países de clima temperado do Hemisfério Norte
(Kallend et al. 1983, Rodhe and Grant 1984). Solos, vegetação e água se encontram
contaminados com sérios efeitos na biota como a redução da biodiversidade, modificação das
estruturas tróficas e diminuição do estoque de peixes (Moissenko 2005), havendo grandes
esforços para a recuperação dos ecossistemas afetados (Evans et al. 2001). Laudon et al.
(1999) mencionam que a diminuição dos valores de pH durante as enchentes de primavera é
um fenômeno ubíquo em várias regiões do mundo, sendo que naquelas regiões acidificadas
este fenômeno tem sido atribuído à deposição de enxofre e nitrogênio em combinação à
diluição da capacidade tampão. Uma das ações de recuperação dos ecossistemas aquáticos
tem sido adicionar calcário aos rios durante o período de acidificação, sendo que, por
exemplo, o governo da Suécia tem subsidiado esta atividade com mais de 125 milhões US$ na
última década. Atualmente os pesquisadores realizam esforços para identificar os fatores
naturais que acidificam os cursos de água buscando evitar que águas naturalmente ácidas
sejam tratadas com calcário, elevando artificialmente seu pH (Bishop et al. 2000, Laudon et
al. 2004).
Nos trópicos em geral, assim como no Brasil, a questão da acidificação ainda não faz
parte das agendas governamentais e acadêmicas a não ser em casos pontuais de alguns
núcleos urbanos ou distritos industriais do país. Entretanto, a acidificação representa um
ameaça crescente nos países em desenvolvimento em terras tropicais e subtropicais
(Kuylenstierna et al. 2001). Sendo que no Brasil ainda podem existir grandes extensões de
territórios com pouca ou nenhuma influência de chuvas ácidas, há uma oportunidade ímpar de
estudar os processos que controlam a acidez dos ecossistemas em estado natural.
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Com relação à liberação de poluentes, as fontes de poluição geralmente são divididas
em pontuais e difusas. As fontes pontuais são locais pontuais identificáveis que apresentam a
qualidade e quantidade da emissão de poluentes aproximadamente constantes, não sendo
influenciados pelos fenômenos meteorológicos. Freqüentemente podem ser monitorados
mensurando a descarga e a concentração química periodicamente num local, sendo
relativamente simples de controlar. As principais fontes de poluição pontual são os efluentes
urbanos e industriais, que podem ser eliminadas por estações de tratamento de efluentes. As
fontes difusas não apresentam locais pontuais identificáveis, sendo disseminadas e
extremamente dinâmicas, compreendendo uma gama de atividades distribuídas na paisagem.
A extensão e a magnitude da poluição difusa estão estreitamente relacionadas com fatores
meteorológicos (precipitação pluvial, vento) assim como com as condições geográficas e
geológicas, podendo diferir muito de um lugar para o outro. As principais fontes de poluição
difusa incluem áreas agrícolas, urbanas e de mineração. Conseqüentemente, as fontes difusas
são difíceis de identificar no seu ponto de origem, de mensurar e de controlar. O controle das
fontes difusas está centrado nas práticas de manejo do solo e regulamentação das emissões de
poluentes na atmosfera. Pelas suas características, as fontes de poluição difusa constituem um
desafio para a gestão ambiental, mas têm recebido pouca atenção devido à dificuldade que
apresentam na sua identificação e mensuração (Carpenter et al. 1998, Vink et al. 1999,
Novotny 2003, Ferrier et al. 2004).
Considerando as características das fontes de poluição, pode-se esperar que as regiões
de clima tropical, devido à alta pluviosidade, sejam particularmente sensíveis à poluição por
fontes difusas. Conforme Bailey (1996), o Domínio do Trópico Úmido pode ser diferenciado,
com base na distribuição sazonal da precipitação, em a Divisão das Savanas e a Divisão das
Florestas Pluviais. Na Divisão das Savanas, situada entre as latitudes 10º e 30º, ocorre
alternância de períodos secos e úmidos, correspondendo ao clima Aw (quente e úmido com
inverno seco) segundo a classificação de Köppen. Já a Divisão das Florestas Pluviais, situada
entre o equador e as latitudes 10º, apresenta alta pluviosidade durante o ano inteiro.
Desta forma, pode-se depreender que dentre as regiões tropicais, as que se encontram
na Divisão das Savanas são duplamente sensíveis à poluição: no inverno seco, devido à baixa
vazão dos rios, acentuam-se os efeitos da poluição pontual; e no verão chuvoso, há grande
aporte de poluição difusa. Portanto, é prioritário estudar a dinâmica sazonal dos ciclos
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biogeoquímicos buscando compreender os processos de poluição nos cursos de água dessas
regiões.
Os esforços governamentais para o controle da poluição em Minas Gerais têm sido
tradicionalmente voltados para as fontes pontuais de poluição, sendo que não existe ainda um
conjunto de políticas públicas endereçadas ao controle da poluição por fontes difusas. Como
visto acima, o controle das fontes de poluição difusas constitui um desafio que vários paises
vem enfrentando, e este desafio se torna ainda maior nos países de climas tropicais.
Para reverter ou controlar as alterações provocadas nos ciclos biogeoquímicos é
necessário estudar os aspectos estruturais e dinâmicos dos mesmos, de modo a entender como
os ecossistemas acumulam, transformam e degradam as substâncias poluentes. Como
Chappin III (2002) menciona, o estudo de ecossistemas busca entender os fatores que
regulam as quantidades e fluxos de matérias e energias através dos sistemas ecológicos. A
este respeito, Allan (1996) cita que os fluxos de energia (expressos através da matéria
orgânica ou carbono) e o fluxo de material (elementos, minerais ou nutrientes) são as áreas
principais das investigações no nível dos ecossistemas buscando compreender os processos
que governam o movimento e a transformação de energia e materiais de um estado para outro,
sejam estes classificados como orgânicos ou inorgânicos.
Considerando o anteriormente exposto, os trabalhos apresentados nesta tese abordam
aspectos relacionados com os ciclos biogeoquímicos do fósforo e do nitrogênio, suas
alterações, e possíveis conseqüências nos processos de eutrofização e acidificação das águas
dos rios de Minas Gerais.
Os trabalhos aqui apresentados encontram-se em escala regional, abarcando todo o
Estado de Minas Gerais, tendo utilizado para tal a base de dados do ano 2000 do Instituto
Mineiro de Gestão das Águas – IGAM, pertencente ao Sistema Estadual de Meio Ambiente.
O monitoramento das águas superficiais vem sendo realizado desde o ano 1997, sendo
realizadas as análises químicas no Centro Tecnológico de Minas Gerais – CETEC pertencente
ao Sistema Estadual de Ciência e Tecnologia.
Quanto à escala regional de trabalho, cabe mencionar que os temas aqui abordados
carecem de estudos nos trópicos sul-americanos e que os trabalhos de síntese mundiais
geralmente constam com apenas poucos dados para a região da bacia do Amazonas (Stallard
v
1980) e muito raramente do Paraná (Meybeck 1979). Portanto, os estudos aqui realizados
poderão contribuir com uma perspectiva do trópico seco-úmido complementando a visão que
atualmente se tem, restrita aos trópicos úmidos. Sendo assim, sempre que possível buscou-se
discutir e inserir os resultados em escalas regionais e mundiais e nas diferenças com os climas
temperados do Hemisfério Norte.
Quanto à escolha do ano 2000 na base de dados do IGAM para a elaboração dos
presentes estudos, é necessário esclarecer que estudos preliminares mostraram a grande
influência que a variação inter-anual da precipitação pluvial exerce nos níveis de poluição e,
além disto, mostraram que o período 2000-2004 sofreu fortes alterações por influência dos
fenômenos El Niño e La Niña. Neste contexto, utilizou-se os dados de 2000 que mostrou ser
um ano de pluviosidade ‘normal’ sem secas acentuadas, permitindo detectar adequadamente
os efeitos decorrentes dos vários aspectos do ciclo hidrológico tais como escoamentos
superficiais e os decorrentes processos de erosão.
Finalmente, deve-se mencionar a base de dados do IGAM, que mostrou ser uma base
de dados consistente e criteriosamente gerenciada tanto quanto à coleta dos dados de campo
pelos técnicos do IGAM quanto às análises químicas e físico-químicas realizadas no CETEC.
Poucos Estados no Brasil realizam o monitoramento sistemático da qualidade das águas
superficiais dos seus territórios e, além disto, quando os dados estão a disposição pública,
estes não parecem atrair suficientemente a comunidade científica, sendo este o primeiro
estudo de que temos notícia que explora amplamente a potencialidade que esta base de dados
oferece. Ao longo da realização dos nossos estudos, foram detectadas algumas
complementações à bases de dados que poderiam melhorar a possibilidade e análise dos
mesmos; estas sugestões serão devidamente encaminhadas ao IGAM em momento oportuno.
2. Objetivo Geral
Identificar padrões sazonais ou geográficos com relação ao pH, nitrogênio e fósforo e
suas implicações na acidificação e eutrofização de córregos e rios em escala regional
(estadual), contribuindo para :
vi
(i) - Compreensão dos processos naturais que controlam as características físicas e químicas
de ecossistemas fluviais, ou seja, as condições e os recursos disponíveis para a biota;
(ii) - Compreensão dos processos de poluição decorrentes de distúrbios antrópicos nos
ecossistemas fluviais;
(iii) – Compreensão da sensibilidade dos ecossistemas aos distúrbios antrópicos.
3. Justificativa
1 - Atualmente os ciclos biogeoquímicos mais alterados pelas atividades humanas são os da
água, fósforo, nitrogênio e metais. Os ecossistemas fluviais são os mais afetados por estas
alterações.
2 - No Brasil, os ecossistemas fluviais ainda são pouco estudados, tendo sido realizados
principalmente estudos na escala de micro-bacias.
3 - Além disto, pouco se conhece sobre os ecossistemas fluviais dos trópicos úmido-secos sulamericanos.
4. A Estrutura da Tese
Os trabalhos apresentados nesta Tese constam de três capítulos, sendo dois referentes
a pH e alcalinidade, e um referente aos nutrientes fósforo e nitrogênio. Cada capítulo foi
redigido já no formato aproximado de trabalhos independentes para facilitar a publicação
posterior. Porém, como no presente documento não há limitação de espaço como ocorre nas
revistas científicas, houve a oportunidade de ampliar o conteúdo dos mesmos ilustrando
melhor os resultados obtidos assim como as discussões.
O Capítulo I, intitulado Geologic control of pH in tropical river waters, teve como
objetivo avaliar as variações de pH frente às principais características geológicas e
geográficas do Estado.
O Capítulo II, Seasonal abiotic control of pH in tropical river waters, também se
refere a pH e alcalinidade, mas desta vez explorando as variações sazonais e identificando um
padrão de comportamento.
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O Capítulo III, Influence of dry-wet climate on P and N patterns, limiting nutrients
and throphic status in tropical river waters, teve como objetivos identificar padrões sazonais
das concentrações de fósforo e nitrogênio e de limitação de nutrientes e testar um método de
classificação trófica dos rios.
Dada a qualidade do banco de dados de que dispomos em Minas Gerais, o apresentado
nesse trabalho é somente uma amostra do conhecimento que é possível advir a partir de outros
estudos que ajudarão a compor o cenários das características das águas superficiais de Minas
Gerais.
Espera-se que este primeiro trabalho de caráter regional sirva de estímulo a outros
pesquisadores para continuar o estudo dos processos que controlam os ciclos biogeoquímicos
em escalas mais detalhadas.
5. Referências
Allan, J.D. 1996. Stream Ecology. Structure and function of running waters. Chapman &
Hall, London. 388 pp.
Bailey, R.G. 1996. Ecosystem geography. Springer-Verlag, New York, 204 pp.
Bennet, E.M., Carpenter, S.R., Caraco, N.F. 2001. Human impact on erodable phosphorus and
eutrophication: a global perspective. BioScience 51(3):227-234.
Binkley, D. and Richter, D. 1987. Nutrient cycles and H+ budgets of forest ecosystems.
Advances in Ecological Research 16:1-51.
Bishop, K.H. and Laudon, H. 2000. Separating the natural and anthropogenic components of
spring flood pH decline: a method for areas that are not chronically acidified. Water
Resources Research 36(7):1873-1884.
Carpenter, S.R., Caraco, N.F., Correll, D.L., Howarth, R.W., Sharpley, A.N., Smith, V.H.
1998. Nonpoint Pollution of Surface Waters with Phosphorus and Nitrogen. Ecological
Applications 8(3):559-568.
Carpenter, S.R. 2005. Eutrophication of aquatic ecosystems: biostability and soil phosphorus.
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xi
RESUMO
Nas últimas décadas os ciclos biogeoquímicos vêm sendo intensamente alterados, em
especial os ciclos da água, carbono, nitrogênio, fósforo, enxofre e metais, afetando os
ecossistemas e a saúde das populações humanas. Nos ecossistemas, um dos compartimentos
mais afetados são as águas superficiais. Em Minas Gerais os estudos das águas superficiais
têm privilegiado lagos e reservatórios, havendo, portanto, carência de informação sobre os
processos que governam os ecossistemas fluviais. O presente trabalho teve como objetivo
fazer um primeiro estudo exploratório em escala regional (estadual) de parâmetros
limnológicos fluviais básicos e cujos ciclos biogeoquímicos vêm sendo mais afetados, sendo
estes: hidrogênio (pH), fósforo e nitrogênio. Buscou-se identificar padrões sazonais ou
geográficos, de forma a contribuir para a compreensão dos processos que alteram estes ciclos
biogeoquímicos, assim como subsidiar ações de gestão e manejo dos rios. Para tal, foi
utilizada a base de dados do monitoramento das águas superficiais de Minas Gerais sob
responsabilidade do Instituto de Gestão das Águas – IGAM.
O trabalho Geologic control of pH in tropical river waters teve como objetivo avaliar
as variações sazonais do pH frente a características geológicas ou geográficas. O trabalho
inicia-se investigando os principais íons presentes nos rios e analisando o perfil químico dos
mesmos quanto à sua ocorrência em ambientes de rochas cristalinas e carbonáticas.
Posteriormente é investigada a relação das variações de pH frente aos compartimentos de
rochas cristalinas e carbonáticas bem como nas principais bacias hidrográficas do Estado. Os
resultados mostram que a região é sensível à acidificação dos cursos de água e que existem
grandes diferenças regionais e sazonais nos valores de pH. Os resultados também mostram
que mudanças geológicas abruptas se refletem na fragmentação da paisagem química dos rios.
Desta forma, o trabalho alerta sobre a necessidade de (i) rever os critérios de pH permitidos
pela legislação tendo em vista as variações espaciais e sazonais, de modo a não comprometer
os ecossistemas fluviais permitindo que ambientes naturalmente ácidos sejam alcalinizados e
vice-versa; (ii) elaborar um programa de gestão e monitoramento dos processos de
acidificação das águas superficiais, bem como do solo e do ar.
O trabalho Seasonal abiotic control of pH in tropical river waters identifica um
padrão sazonal de acidificação das águas durante o período das chuvas e investiga suas causas
xii
através da análise da capacidade de neutralização de ácidos (ANC) aplicada a um modelo
simples de mistura de águas. São discutidas as características dos solos assim como os
processos do ciclo hidrológico que possivelmente comandariam o padrão encontrado.
Verificou-se que esta variação sazonal é um processo natural de diluição das águas dos rios,
intensificado por ações antrópicas que aumentam o escoamento superficial, não se tratando de
um processo decorrente da poluição por agentes ácidos no sistema fluvial. Este resultado
corrobora a sensibilidade da região para os processos de acidificação. É reforçada a
necessidade da gestão e monitoramento dos processos de acidificação bem como das fontes de
poluição ácida. Além disto, é mostrada a necessidade da elaboração de um programa de
gestão ambiental do uso do solo.
O trabalho Influence of dry-wet climate on P and N patterns, limiting nutrients and
throphic status in tropical river waters, teve como objetivo identificar padrões sazonais das
concentrações de fósforo e nitrogênio e da potencial limitação da produtividade primária por
nutrientes, bem como testar um método de classificação trófica de rios. Os resultados
mostraram que (i) durante o período das chuvas houve aumento nas concentrações e cargas de
nutrientes nos rios; (ii) fontes difusas de poluição, devido à erosão acelerada e ao escoamento
superficial, seriam responsáveis pelo aumento das concentrações e cargas de nutrientes; (iii)
durante o período das chuvas aumentou o estado trófico devido ao incremento de nutrientes
nos rios; (iv) os rios tendem a estar limitados por fósforo durante o período da seca e a estar
limitados por nitrogënio durante o período das chuvas, devido à maior carga de fósforo em
relação ao nitrogênio alterando a razão N:P; (v) o risco de ocorrência de bloom de
cianobactérias é maior durante o período da seca (P-limitante) quanto às cianobactérias nãofixadoras de nitrogênio devido à maior disponibilidade do íon amônio, enquanto é maior no
período das chuvas (N-limitante) para as bactérias fixadoras de nitrogênio devido à baixa
disponibilidade relativa de nitrato e do íon amônio; (vi) o método de classificação do estado
trófico de nutrientes mostrou-se adequado para os rios de Minas Gerais apresentando
concordância com os nutrientes limitantes.
O clima tropical úmido-seco de Minas Gerais reflete-se nas mudanças sazonais da
composição química da água dos rios. Durante o período das chuvas ocorre: diminuição dos
valores de pH; diminuição da concentração de oxigênio dissolvido, aumento das
concentrações de sólidos totais em suspensão, nutrientes e poluentes (metais pesados e
coliformes); aumento do estado trófico de nutrientes; mudança da limitação por fósforo para
xiii
nitrogênio, e aumento do risco de ocorrência de blooms por cianobactérias fixadoras de
nitrogênio. Verificou-se que os padrões sazonais estão sendo invertidos ou acelerados pelas
atividades antrópicas quanto ao uso do solo, ocorrendo mudanças nos fluxos hidrológicos.
Estas mudanças provocam o incremento dos fluxos do fósforo e nitrogênio, alterando assim as
condições e recursos disponíveis para a biota dos ecossistemas fluviais.
O estudo mostra a necessidade de que a gestão do uso do solo seja efetivamente
incorporada às políticas setoriais de meio ambiente, principalmente no que respeita às águas
superficiais e subterrâneas. Além disto, o estudo mostra que é necessário revisar os padrões
legais de qualidade da água, de forma a atender às diferenças regionais e sazonais dos
ecossistemas incorporando critérios ecológicos. Finalmente, considera-se que é necessário
fomentar os estudos de ecologia fluvial para embasar as ações de gestão.
xiv
ABSTRACTS
In the last decades, biogeochemical cycles have been intensely changed, especially the
hydrological cycle, as well as the carbon, nitrogen, phosphorus, sulfur and metal cycles,
affecting ecosystems and human health. In ecosystems, one of the more affected
compartments are the surface waters. In Minas Gerais, the studies of the surface waters have
been privileging lakes and reservoirs, existing therefore, lack of information on the processes
that govern the fluvial ecosystems. The present work was a first exploratory study in regional
scale (State) of limnological basic fluvial parameters, whose biogeochemical cycles have been
more affected, being these: Hydrogen (pH), phosphorus and nitrogen. The study sought to
identify seasonal or geographical patterns, in order to contribute for understanding the
processes govern the biogeochemical cycles and changes of these cycles, as well as to
subsidize rivers administration and management actions. For such, it was used the surface
waters monitoring database of Minas Gerais under the Instituto de Gestão das Águas de
Minas Gerais - IGAM responsibility.
The study Geologic control of pH in tropical river waters evaluated the seasonal
variations of pH with main geological or geographical characteristics. The work initiated
investigating the major ions and analyzing the chemical profile, regarding its occurrence in
environments of crystalline and carbonate rocks. Afterwards it was investigated the pH
variations with the principal geologic compartments as well as in the main hydrological
basins. The results showed that the region is sensitive to water courses acidification and that
there are great regional and seasonal differences in pH values. The results also show that
abrupt geological changes reflect in the rivers chemical landscape fragmentation. Thus, the
work alert on the need to (i) review pH criteria allowed by legislation, having in mind the
geographical and seasonal variations, so as to do not allow fluvial naturally acid ecosystems
became alkaline and vice-versa; (ii) prevent and monitoring the acidification of surface
waters, as well as of soil and air.
The study Seasonal abiotic control of pH in tropical river waters identified a
seasonal acidification pattern in rivers waters during the rainfalls period and investigated it
causes through the analysis of the acids neutralization capacity (ANC) applied to a waters
mixture simple model. Are discussed the characteristics of soils as well as hydrological
xv
processes that would be commanding the found pattern. Was verified that this seasonal
variation is a waters dilution natural process of the rivers, intensified by anthropogenic actions
that increase the overland flow, not treating itself a process due to the pollution for acid
agents in the fluvial system. This result corroborates the sensibility of the region for the
acidification processes. It is reinforced the need to manage and monitor of anthropogenic
acidification processes as well as of the sources of acid pollution. Beyond of this, it is shown
the need of an environmental soil management program.
The study Influence of dry-wet climate on P and N patterns, limiting nutrients
and throphic status in tropical river waters, aimed to identify phosphorus and nitrogen
concentrations seasonal patterns and the potential limitation of primary productivity by
nutrients, as well as test a river trophic classification method. The results showed that (i)
during the rainfalls period there was increased nutrients concentrations and loads on the
rivers; (ii) diffuse sources of pollution, due to the accelerated erosion and to the superficial
flow, would be responsible for the nutrients concentrations and loads increase; (iii) during the
rainfalls period increased the trophic state due to the nutrients increment on the rivers; (iv) the
rivers tend to be limited by phosphorus during the drought period and to be limited by
nitrogen during the rainfalls period, due to the largest phosphorus relative the nitrogen loads,
changing the N:P ratio; (v) the occurrence risk of cyanobacteria bloom is larger during the
drought period (P-limiting) regarding nitrogen non-fixing cyanobacteria due to the largest
ammonium ion availability, while it is larger in the rainfalls period (N-limiting) for the
nitrogen fixing bacteria due to the low relative availability of nitrate and ammonium; (vi) the
trophic state classification method of nutrients showed to be adequate for Minas Gerais' rivers
presenting concordance with the limiting nutrients.
Minas Gerais' dry-wet tropical climate reflects in the seasonal changes of water
chemical composition of rivers. During the rainfalls period occurs: pH values decrease;
dissolved oxygen concentration decrease; suspended total solid concentrations increase;
nutrients and pollutant (heavy metals and coliforms) increase; nutrients trophic state increase;
limitation change from phosphorus to nitrogen; and the risk of nitrogen-fixing cyanobacteria
blooms increase. Was verified that the seasonal patterns are being inverted or accelerated by
human activities and soil use, occurring changes in the hydrological flowspaths. These
changes provoke the phosphorus and nitrogen loads increment, changing thus the conditions
and available resources for the biota in fluvial ecosystems.
xvi
The study showed the need to incorporate the soil use management to the
environmental policies, mostly with respect to the superficial and subterranean waters.
Beyond of this, the study show that is necessary to revise the water quality legal standards to
attend to the regional and seasonal differences of the ecosystems and incorporating ecological
criterias. Finally, was considered that it is necessary to foment the studies of fluvial ecology
to subsidize management actions.
xvii
CAPÍTULO 1
Controle geológico do pH nas águas de rios tropicais
GEOLOGIC CONTROL OF pH IN TROPICAL RIVER WATERS
Trabalho a ser submetido à revista Biogeochemistr y
1
CAPÍTULO 1
GEOLOGIC CONTROL OF pH IN TROPICAL RIVER WATERS
Introduction
Natural surface waters are influenced by the interaction of acids and bases, and the
resulting pH is of great significance in the formation, alteration and dissolution of minerals
and thus in the composition and quality of water. The pH values of the majority of natural
waters generally lie within a thin range from 6 to 9, and are relatively constant (Stumm and
Morgam 1996). Values below 5 and above 9 are considered extreme and are harmful to most
organisms (Allan 1996). The composition of natural waters may be affected by the quantity
and nature of rainfall and evaporation, the geology and landscape of the watershed, rock
weathering, temperature and biota (Berner and Berner 1996). Weathering is a fundamental
process for ecosystems since many essential elements are derived from the underlying rocks
and transported to the oceans by rivers and streams (Bricker and Rice 1989, White and
Blum 1995). Chemical weathering is also important due to its ability to neutralize natural or
anthropogenic acids.
Human activities have altered the composition of natural waters, as revealed by
several studies on the acidification of rivers and lakes in Europe, Canada, and the USA
(Freedman 1995). Water acidification has been widely recognized as a structuring factor for
aquatic communities (Gunn and Sandoy 2003). In tropical countries such as Brazil, the study
of soil and surface waters acidification is still incipient, and most research has focused
primarily on acid rain in industrial and metropolitan areas (Moreira-Nordermann el al.
1998). However, to understand the effect of pollution on aquatic ecosystems and to be able to
propose environmental management actions, it is necessary to identify the main influences of
the natural factors mentioned above.
The aim of the present study was to evaluate pH variations of surface waters related to
geologic and geographic characteristics on a regional scale. For this we used a data base from
Minas Gerais State, southeastern Brazil, which covers a huge territory (equivalent to France),
2
a climate with two well-defined seasons (rainy summers and dry winters), and contrasting
geological characteristics (acid and alkaline rocks).
Materials and Methods
1. Study Area
The state of Minas Gerais occupies approximately 7% of Brazil, and is located in the
southeastern region, between the parallels 14º13’58”S and 22º54’00”S, and the meridians
39º51’32”W and 51º02’35”W. According to the Brazilian Institute of Geography and
Statistics (IBGE), it encompasses 586,528.3 km2. In 2000, the total population of 17,891,494
was strongly concentrated (82%) in urban areas, of which the most important is its state
capital, Belo Horizonte, and suburbs, known as the Belo Horizonte Metropolitan Region
(BHMR) with more than 3.5 million inhabitants.
The climate is characterized by two well-defined seasons: a dry period (April to
September) whose maximum coincides with the lowest winter temperatures in July, and a
rainy period (about 300 mm/month from October to March) in which the heaviest rains
coincide with the highest temperatures, around December-January. The headwaters of 15
Brazilian watersheds are located in Minas Gerais. Among these, the São Francisco River is
outstanding, because of its length (1135 km) and the fact that its watershed drains 40% of
Minas Gerais, and also the Paranaíba River and the Grande River, which form the Paraná
River.
Regarding the presence of acid or alkaline rocks, schematically, the State has three
major compartments: (i) relief modeled in Precambrian acid igneous and metamorphic rocks
(granite-gneiss and quartzite dominant rocks); (ii) Paraná Sedimentary Basin with igneous
rocks (basalt) and sedimentary basic rocks (calcareous and calciferous sandstones); (iii)
Bambuí Sedimentary Basin with basic rocks (calcareous and dolomite) (Fig.1a). Minas Gerais
is currently the largest cement producer in Brazil, which in turn is the tenth world’s largest
producer.
3
Figure 1a. Minas Gerais major compartments regarding the presence or dominance of acid
or alkaline rocks. In red: granite-gneiss and quartzite; in green: basalts with calcareous,
calciferous sandstones, and conglomerates; in yellow: calcareous and dolomites. The black
lines represent the watershed limits and the blue dots indicate the sample sites.
Sources: samples sites: IGAM 2000; geology: COMIG 2002.
2. Sample Sites
The monitoring of surface water quality is the responsibility of the Minas Gerais
Institute of Water Management (IGAM). The monitoring network includes the eight main
federal watersheds, and has 244 long-term sampling sites in which 40 water quality
parameters are measured. IGAM performs four annual sampling campaigns, corresponding to
summer (rainy season), autumn, winter (dry season) and spring. However, in this report we
will only consider the summer (January, February and March) and winter (July, August and
September) data. Samples are taken preferentially from the rivers´ main channels, since most
of the sampling points are located on bridges, and the pH is measured “in loco”. This report
used the available database of the year 2000 (IGAM 2002), except for 8 sample sites which
did not have the pH values for the two seasons, totalizing 236 sample sites. Eight major
4
federal watersheds were studied. Because of its large area, six sub-basins from the São
Francisco River watershed were analyzed separately, resulting in 14 watersheds studied (Fig.
1b). Spatial analyses were performed with the MapInfo Professional 6.0 software, using a
databank that integrated the maps locating the sample sites, seasonal pH values, the Minas
Gerais Geological Map (COMIG 2002), as well as digital topographic and hydrological maps
at the 1:100 000 scale (GeoMinas Project), based on IBGE maps.
Figure 1b. Sample sites distribution through the 14 major watersheds in Minas Gerais
State. The major basins that contribute to the São Francisco River watershed were
highlighted:Urucúia, Paracatú, Verde Grande, Pará, Paraopeba, Velhas.
Sources: sample sites: IGAM 2000; watersheds: delimited from Geominas / Prodemge
Project – MG digital database.
Results and Discussion
1. Major ions
Major cations and anions average from Minas Gerais and World rivers compared with
drainage dominant rock types are shown in Fig.2. Minas Gerais shows a cation profile similar
to the South American and World rivers. Relative to rock types, the cations concentrations
5
and ratios resemble more crystalline (igneous and metamorphic) than carbonate rocks.
Regarding the anion composition, Minas Gerais present low HCO3- values, again more similar
to crystalline rocks than to carbonate rocks; and lower than South American and World rivers.
So, this result shows the dominant influence of crystalline rocks over carbonate rocks in the
chemical composition of rivers, although the influence of carbonate rocks can be seen through
the higher Ca and HCO3- concentrations. These results indicate a general low acid
neutralizing capacity and hence, rivers sensitivity to acidification.
Figure 2. Mean composition of river water of Minas Gerais, South America and the World
compared with watersheds dominat rock types. Data from Wetzel 2001.
Figure 2 shows that Na concentrations are higher in Minas Gerais, South America and
World rivers when compared with average rocks. Also, Cl appears in these rivers while in
rocks is not a relevant anion. Beyond this, Figure 2 also shows an excess of Na relative to Cl.
The main sources of chloride are sea salts from rain and dry fallout (cyclic salts), dissolution
of halite in evaporites or dispersed in shales, thermal and mineral springs in volcanic areas,
and pollution such as domestic and industrial sewage. According to Berner and Berner
(1996) studies, in world average river waters cyclic salts contribute with 13% and pollution
contribute with 30%; the remaining Cl in river waters (57% of total Cl) comes predominantly
from the weathering of NaCl. Only about 8% of river water Sodium is from cyclic salts, 28%
are from pollution, 42% from evaporites, shales and thermal springs; and the remaining 22%
from silicates weathering. Another possible source of Na is from cation exchange of dissolved
Ca2+ with Na+ on detrital clay minerals, where cation exchange will produce excess of Na
6
relative to Cl in river water similar to silicate weathering. The authors considered that at a
global scale there are not enough clay minerals with exchangeable Na. As mineral sources of
Na, Hounslow (1995) mentions (i) ferromagnesian silicates: amphiboles -variety homblende
NaCa2(Mg,Fe,Al)5Si8O22(OH)2 ; (ii) feldespars: plagioclase - variety albita NaAlSi3O8 ; (iii)
cation exchange: Na-montemorillonita reacts with Ca and Mg releasing Na.
2. Chemical classification of rivers
According to Gibbs 1970, the major controls on rivers water chemisty are (1)
atmospheric precipitation; (2) rock weathering; (3) evaporation and crystallization. When the
ratio of the major cations Na/(Na + Ca) or major anions Cl/(Cl + HCO3-) in surface waters are
plotted against total dissolved solids (TDS), rivers fall in the areas dominated by the three
mecanisms, in the three corners of a “boomerang”. Figure 3 shows the world river
classification and Minas Gerais rivers.
Figure 3. Chemical classification of rivers.
To the left: Gibbs 1970 classfication. The upper corner is related to evaporation and
chemical precipitation (crystallization). The lower corner is related to atmospheric
precipitation. The center corner is related to rock weathering processes.
To the right: Minas Gerais results. Different dot colors refer to the major Minas
Gerais watersheads. Yellow and cyan outliers correspond to Mucurí and
Jequitinhonha watersheds respectively.
7
The results show that most Minas Gerais rivers were in the central corner of the
“boomerang” corresponding to rock-dominated rivers. The rock-dominated corner presents
intermediate TDS values and low Cl/(Cl + HCO3-) values. For these rivers, rock weathering
supplies most of the dissolved ions. According to Gibbs studies, most of the world rivers are
included in this category.
Carbonate minerals are not as abundant as silicate minerals, but they are dominant in
river chemistry. Most of the dissolved Ca and HCO3- in world rivers come from carbonate
dissolution. Calcium carbonate dissolves more rapidly than silicate minerals, and small
amounts present in a rock can dominate the groundwater compostion. So, it is expected that
rivers dominated by rock weathering consist mainly of Ca and HCO3- and result in low values
for Cl/(Cl + HCO3- ) ratios (Berner and Berner 1996).
According to Gibbs, rivers controlled mainly by atmospheric precipitation show a
chemical composition similar to rainfall, with low TDS and high Na or Cl, relative to Ca or
HCO3- respectively. Rivers controlled by evaporation-crystallization have high concentrations
of TDS and also high Na or Cl relative to Ca or HCO3- respectively, due to CaCO3 removal
by crystallization. Evaporation increases TDS concentrations and hence the concentrations of
Na and Cl; however, Ca and HCO3- are removed by precipitation of CaCO3. Atmospheric
precipitation-controlled rivers are in areas of high rainfall and evaporation-crystallization
rivers are in arid areas, while rock-dominated rivers are in areas of intermediate rainfall.
Stallard and Edmond (1983), as Berner and Berner (1996) mentioned, believe that the
chemical composition of some dilute Amazon tributaries results from their geology and
erosional regime and not from atmospheric precipitation. The high Na/Ca ratios is because the
rocks are mainly more siliceous, and the low TDS is because the sediments and soils have
already undergone intense weathering and are relatively unreactive.
Four sample sites from the Mucurí (MU009/11/13) and Jequitinhonha (JE025) rivers
presented the highest Cl/(Cl + HCO3-). They are located about 100 km far from the Atlantic
sea coast in granite-gneiss rocks and within one of the hottest areas of the State. Therefore, it
is possible that these sites were under evaporation due to high temperatures, along with the
sea NaCl contribution, a special case in Gibbs classification (see Chapter3 , Section 9.3).
8
3. The majors geologic compartments
Water chemistry reflects both bedrock and erosional regime. The most concentrated
waters are those draining evaporites, and waters draining limestones are more concentrated
than those draining silicate rocks (Drever 2002). As mentioned before, calcium carbonate
dissolves more rapidly than silicate minerals, and small amounts present in a rock can
dominate the groundwater composition.
As a rule, the acids present in rain are neutralized when they pass through the soil.
However, in watersheds over crystalline acid rocks (igneous and metamorphic) with reduced
buffer capacity and with large contribution of rainfall relative to underground water, the river
water will be acid even in the absence of pollution (Allan 1996). According to Stumm and
Morgam (1996), weathering is a process that consumes hydrogen ions (H+), which are also
important in buffering. Rainfall causes soil and surface water acidification if neutralization by
weathering is low. Forests and intensively cultivated areas increase acidity because, as more
cations are incorporated by the plants, more H+ will be released through the roots. These
protons react with the rock minerals to produce new cations needed by the plants. The balance
of H+ in the soil, given by the production in roots and consumption by weathering, is very
fragile.
The dissolution rate in calcareous rock regions is much faster than in those where
crystalline rocks prevail. Thus, in non-calcareous soils the rate of release of H+ by litter may
exceed the consumption rate of H+ by weathering, causing soil acidification. In calcareous
soils, the consumption rate of H+ by weathering equals or exceeds the rate of release by
organisms, and the soil releases cations, maintaining its buffering capacity (Stumm and
Morgam 1996) as shown below by Baird (1998):
CaCO3 (s) + H+ (aq) → Ca2+ (aq) + HCO3- (aq)
HCO3- (aq) + H+ (aq) → H2CO3 (aq) → CO2 (g) + H2O (aq)
From what was stated above, we can expect that rivers of calcareous areas are at risk
of pollution by basic compounds, especially in the dry season, and that those in areas of acid
rocks (igneous and metamorphic) are at risk from pollution by acidic compounds, especially
during the wet season.
9
Figure 4 shows the average alkalinity concentrations values in the three major
geologic compartments (see Fig.1a).
Figure 4. Average alkalinity concentrations in the
major geologic compartments:
A) granite-gneiss and quartzite
B) basalts with calcareous, calciferous sandstones and
conglomerates
C) calcareous and dolomites, and karst regions.
Alkalinity behavior was as expected: calcareous and dolomites and karst regions >
basalts with calcareous, calciferous sandstones and conglomerates > granite-gneiss and
quartzite. However, calcareous, dolomites and karst regions during the rainy season presented
a relatively lower alkaline concentration. During the wet period, the calcareous regions’ lower
alkalinity concentrations may be due to the influence of crystalline rocks located upstream
present in the the catchment. Figure 5 shows some selected sites from karst regions and noncarbonate watersheds, and illustrates the variability among different karst regions.
10
Figure 5. Carbonate and non-carbonate (karst regions) selected
sample sites. Acronyms refer to the specific identification of
each sample site by watershed: MU = Mucurí; BG = Grande;
PB = Paranaíba; BV = Velhas; SF = São Francisco; PT =
Paracatú; VG = Verde Grande.
4. Crystalline Rocks
Figure 6a shows, for both the dry and wet seasons, the association of the more acidic
pH values (dry season = 6.1- 6.5 and wet seaons = 5.2-6.0) with granite-gneiss and quartzite
acid rocks regions. Figure 6b shows that these more acidic sites also are related with high
altitude relief (more than 800 meters). Berner and Berner (1996) mention that a greater
relief results in greater physical erosion of the rock and faster exposure to chemical
weathering. The rapid transport away of the weathered material causes an incomplete
chemical weathering in the soil. So, both lithology (carbonates versus silicates) and relief are
important. On the other hand, with low relief, chemical weathering is more complete and
thick soils develop over the bedrock, so variations of the geology become less important.
11
The IGAM’s surface water quality map (IGAM 2000), shows that low pH values sites
(Fig. 6) coincide with high level of contamination by toxic compounds, suggesting that these
low pH values may be consequence of anthropogenic pollution that cannot be
counterbalanced by the low buffering capacity of crystalline acid rocks. As an example, the
Verde River (Grande River headwaters) presents the lowest pH values in its entire course,
both during the dry and wet seasons.
The Verde River region contains the highest altitudes and pluviometric indices of the
State, and is an area where rainforest remnants can still be found in the mountain slopes.
Hence, it is a propitious region for low infiltration and high surface runoff, which, added to
the release and transport of humic acids by forest litter, may contribute to a low regional
buffering capacity.
Studies comparing the Minas Gerais cerrado (savanna) areas with Pinus plantations
showed that the latter causes significant alterations in soil acidity up to at least 2 m depth. In
the cerrado areas pH varied from 5.0 to 6.0, while in the pine tree plantations the range was
4.0 to 5.0. This acidity was attributed to the influence of litter biomass produced by the forest,
releasing acidic organic compounds during decomposition (Brandão and Lima 2002).
The southern region of Minas Gerais (located in the granite-gneiss domain) is under
increasing anthropogenic pressure, so this region is very vulnerable to acid rain and other acid
pollutions. It should also be noted that several sample sites with low pH are near or upstream
of heavy metal ore deposits, and that a low pH may increase their availability in the
environment by modifying their solubility.
12
Figure 6. Lower pH values in the dry and rainy seasons.
Sample sites in red: dry season, pH 6.1-6.5; sample sites in blue: rainy season, pH 5.2-6.0.
Up: (Fig.6a): acid rocks areas in yellow (granite-gneiss and quartzite).
Below: (Fig. 6b): highest altitudes areas in green (above 800 meters).
Source: sample sites:IGAM 2000; geology: COMIG 2002.
13
5. Carbonate Rocks
Regarding the more alkaline pH values, Fig. 7 shows, both for the dry and wet season,
(pH 7.5 – 8.5 and pH 7.0-7.5 respectively) the association of pH with the areas of occurrence
of exo karst or calcareous / dolomite mines. Baird (1998), and many others, mention that in
calcareous waters, where pH is determined by CO2 and CaCO3 saturation, the typical pH
values are alkaline and vary between 7 and 9.
Figure 7. Highest pH values in the dry and rainy seasons.
Red color: dry season, pH 7.5 – 8.5; Blue color: rainy season, pH 7.0-7.5.
Dark green color: exo karst areas; Light green triangles: calcareous and dolomite mines.
Source: sample sites: IGAM 2002; geology and mines: COMIG 2002.
14
Sample sites located at dolomite mines areas showed, as expected, higher Mg/Mg +
Ca ratios than calcareous sites. Dolomite sites showed a ratio >0.3 while the calcareous sites
ratio was <0.3 (Fig. 8), which indicates dolomite-limestone weathering (Hounslow1995).
Dissolution of dolomite can be written as:
CaMg(CO3)2 + 2CO2 + 2H2O = Ca2+ + Mg 2+ + 4 HCO3Groundwater in equilibrium with dolomite should have a Ca2+ / Mg2+ molar ratio close
to unit. Because usually limestone is present with the dolomitic rocks, the ratio of
groundwaters is generally grater than unit (Langmuir 1997). The data showed that the ratio
for dolomite sample sites was below 2, according to the stoichiometric.
Figure 8. Dolomite and calcareous rivers Mg/(Mg + Ca) ratios.
Stoichiometric: Dissolution of dolomite, ratio = 0.5 ; dissolution of dolomite in the
presence of limestone, ratio = 0.3
As seen in Fig. 7, there are many calcareous and dolomite mines inserted in the
granite-gneiss compartment (crystalline acid rocks). The need to study the impacts of
calcareous mining outside mining areas is evident, since wind and water act as dust dispersal
vectors beyond the limits of the mines (see Figueirêdo 1999 for an example of the MRBH).
15
Calcareous dust not only directly alters the water natural pH but also has effects on the
dynamics of terrestrial ecosystems adapted to more acidic conditions. As an example,
Mahonia aquifolium, a North-American native plant introduced in Europe in the XIX century,
invaded sites where the acid pH resulting from pine tree forests had been increased by the
calcareous dust of surrounding industries (Auge and Amarell 1997). Besides mines, another
source of calcareous dust are agricultural activities, that use carbonates to improve tropical
acid soils, and may pollute watersheds by soil dust dispersal during the dry season and water
runoff in the rainy season.
In turn, karst regions located near large urban centers, such as the Lagoa Santa Karst
Region near the Belo Horizonte Metropolitan Region north border, may suffer the influence
of acid precipitation, resulting in increasing rock weathering rates and deleterious effect in
caves ecosystems as well as archaeological and paleontological heritage sites.
6. Watersheds
Figure 9 shows the relationship between average pH and alkalinity values, considering
the sample sites located in the crystalline portion of each watershed. The more vulnerable
sites are those from the São Francisco, Jequitinhonha, Paranaiba and Grande rivers during the
rainy season, with the lowest pH and the lowest alkalinity.
Figure 9. Relationship between average pH and alkalinity values considering crystalline
sample sites in each watershed.
SF = São Francisco; Pb = Paranaíba; Je = Jequitinhonha; Gr = Grande; Pd = Pardo; Do =
Doce; Pa = Pará; PS = Paraíba do Sul; Paraopeba; Ve = Velhas; Mu = Mucuri. For spatial
locations
see10Fig.
1a,b.the chemical composition detail for the San Francisco headwaters.
Figure
shows
16
Figure 10 shows the chemical composition for the San Francisco headwaters. Sample
site SF001 catchment is in quartzite and San Francisco river water, showing very low major
ions concentrations. SF002 catchment is affluent of the San Francisco river and its catchment
is in a karst region, showing very high concentrations of alkalinity, Ca and Mg, while the
other ions remain low. SF003, located in the San Francisco river downstream SF002 mouth
showed intermediate values between SF001 and SF002. During the dry season all three sites
showed a similar pH value around 7, but during the rainy season the SF001 pH was 5.6, and
of SF002 and SF003 still around 7. So, areas of geologic contacts (ecotones) can create patchy
chemical enviroments within short river distances.
Figure 10. São Franciso river headwaters chemical composition. SF001 is the first headwater
sample site, SF003 is downstream and SF002 is affluent to SF003.
Distances straight line:SF001 ⇒ SF003 = 90 km; SF002 ⇒ SF003 = 9 km.
SF001 is in quartzite rock and SF003 catchments are in Sao Fransisco Supergroup - Bambui
Group.
SF002 cathment is within the Arcos-Pains Karst Region.
17
Conclusions
The results indicate that river water chemistry is strongly influenced by the nature of
rocks and relief characteristics as well as the the seasonality of the dry-wet climate.
Rivers in crystalline acid rocks and in high altitude regions showed the lowest
alkalinity and pH values, and, as a consequence, are vulnerable to pollution by acidic agents.
The southern region of Minas Gerais (that is, in the granite-gneiss domain) is under increasing
anthropogenic pressure, so this region is the most vulnerable to acid rain and other acid
pollutants, as happens for example with Canadian and Scandinavian Precambrian regions.
The results point out that not only soil characteristics (as in Cinderby 1998, Melfi et
al. 2004), but also geological characteristics should be considered when establishing acid
sensitive regions.
pH is considered a water quality master variable because of the alteration and
dissolution of minerals, and thus the composition and quality of water depend on it,
influencing the whole fluvial ecosystems. Therefore, the management of pH in rivers has to
also consider the whole chemical environment, especially potential toxic elements such as
metals.
This study also showed the need to modify the pH standard values allowed by law for
surface waters in order to protect aquatic ecosystems. It is necessary to consider pH
differences between seasons and the natural pH limits of each geographic region.
References
Allan, J.D. 1996. Stream Ecology. Structure and Function of Running Waters. Chapman &
Hall, London, 388 pp.
Auge, H. and Amarell, U. 1997. Does environmental pollution facilitate plant invasions? A
case study on seedling recruitment and spatial spread in Mahonia aquilifolium
(Berberidaceae). 4th International Conference on the Ecology of Invasive Alien Plants.
Technische Universität, Berlin.
Baird, C. 1998. Environmental Chemistry. W. H. Freeman and company, 528 pp.
18
Berner, E.K. and Berner, R.A. 1996. Global environment. water, air, and geochemical cycles.
Prentice Hall, New Jersey, 376 pp.
Brandão, S.L. and Lima, S.C. 2002. PH and electric conductivity in soil solution, in Pinus and
Cerrado areas in Uberlândia (MG). Caminhos da Geografia 3(6), 46-56. (In Portuguese).
Bricker, O.P. and Rice, K.C. 1989. Acidic depositions to streams: a geology-based methods
predicts their sensitivity. Environ. Sci. Technol. 23, 379-385.
Cinderby S., Cambridge, H.M., Herrera, R., Hicks, W.K., Kuylenstirna, J.C.I., Murray, F.,
Olbrich, K. 1998. Global assessment of ecosystem sensitivity to acidic deposition, Stockholm
Environment Institute, Stockholm, 20 p.
Companhia de Mineradora de Minas Gerais – COMIG 2002. Geological atlas of the State of
Minas Gerais. (In Portuguese).
Drever, J.I. 2002. The geochemistry of natural waters. Surface and groundwater
environments. Prentice Hall, New Jersey, 436pp.
Figuerêdo, D.V. 1999. Influence of calcareous soil particulates on acid rain: Belo Horizonte
Metropolitan Region, Brazil. Ambio 26 (6), 514-518.
Freedman, B. 1995. Environmental Ecology. The Ecological Effects of Pollution,
Disturbance, and Other Stresses. Academic Press, New York, 605 pp.
Gibbs, R.J. 1970. Mechanisms controlling world water chemistry. Science 170: 1088.
Gunn, J.M. and Sandøy, S. 2003. Introduction to the Ambio Special Issue on biological
recovery from acidification: Northern Lakes Recovery Study. Ambio 32(3),162-164.
Hounslow, A.W. 1995. Water quality data. Analysis and interpretation. Lewis Publishers,
Boca Raton, 397pp.
Instituto de Gestão das Águas de Minas Gerais - IGAM 2000. Surface water quality report
2000. (In Portuguese).
Langmuir, D. 1997. Aqueous environmental geochemistry. Prentice Hall, New Jersey, 602pp.
Melfi, A.J., Montes, C.R., Carvalho, A., Forti, M.C. 2004. Use of pedological maps in the
identification of sensitivity of soils to acidic deposition: application to Brazilian soils. An.
Acad. Bras. Cienc. 76(1), 139-145.
Moreira-Nodermann, L.M., Forti, M.C., Di Lacio, V.L., Espírito Santo, C.M. et al. 1998.
Acidification in Southeastern Brazil. In: Acidification in Tropical Countries. Rodhe, H., and
Herrera, R., (eds). SCOPE Report No. 36, John Wiley and Sons, Chichester, 405 pp.
Stumm, W. and Morgan, J.J. 1996. Aquatic Chemistry. Chemical Equilibria and Rates in
Natural Waters. John Wiley and Sons, New York, 1022 pp.
19
Wetzel, R.G. 2001. Limnology. Lake and River Ecosystems. Academic Press, San Diego,
1006 pp.
White, A.F. and Blum, A.E. 1995. Effects of climate on chemical weathering in watersheds.
Geochim. Cosmochim. Acta 59(9), 1729-1747.
20
CAPÍTULO 2
Controle abiótico sazonal do pH em rios tropicais
SEASONAL ABIOTIC CONTROL OF pH IN TROPICAL RIVER WATERS
Trabalho aceito pela revista Ambio
21
CAPÍTULO 2
SEASONAL ABIOTIC CONTROL OF pH IN TROPICAL RIVER WATERS
Introduction
Natural surface waters are influenced by the interaction of acids and bases, and the
resulting pH is of great significance in the formation, alteration and dissolution of minerals,
and thus in the composition and quality of water. PH values below 5 and above 9 are
considered harmful to most organisms (Cowling and Linthurst 1981, Allan 1996, Stumm
and Morgam 1996) and the acidification of rivers and lakes can increase the leaching of
pollutants like trace metals and nutrients from contaminated sediments into inland waters
(Gundersen and Steinnes 2001).
The composition of natural waters may be affected by rainfall, evaporation, geology
and landscape characteristics, weathering, temperature, and biota (Berner and Berner 1996).
Human activities have altered the composition of natural waters, and acidification of surface
water is regarded as a global problem that occurs when regions potentially vulnerable to
acidification are impacted by acidic deposition or other acidic pollutants such as acid-mine
drainage. In North America and Europe, attention to acid depositions began in the seventies
and recently other countries, particularly from Japan, China and India, have studied this issue
(Galloway 2001, Ohte and Tokuchi 1995, Granat et al. 2001, Larsen et al. 2006). Rodhe
et al. (1995) advocate the need for extensive and long-term emission inventories and an
integrated monitoring of terrestrial and aquatic ecosystems. Brazil is only now beginning to
build a database that allows the study of soil and water acidification in regional scales, but the
chemistry of rainwater is only known for the major capitals (Moreira-Nordenmann et al.
1998), and few states have water quality, soil or land cover monitoring programs.
Studies in acid-sensitive areas have shown that stream acidification is controlled by
soil characteristics (thickness, base-cation content and SO4= adsorption capacity) and the
bedrock geology underlying the watersheds (Galloway et al. 1983, Bricker and Rice 1989,
Cronan and Schofield, 1990 and many others). Previous studies have shown that the
Brazilian territory is a region sensitive to acid depositions (Kuylenstierna et. al. 2001). Melfi
22
et al. (2004) constructed maps of vulnerability to acid deposition based on soil buffering
capacity, following the method of Cinderby et al. (1998) and using the FAO World Soil
Map (1992). The study indicated that Brazilian soils have the highest sensitivity to acid
depositions (classes 1 and 2) and are distributed all over the country, covering about 80% of
its surface.
To be able to propose environmental management actions, and to obtain a better
understanding of the effect of pollution on aquatic ecosystems, it is necessary to identify the
main influences of natural factors. The biogeochemical influence on the water chemistry and
the factors governing pH dynamics have been studied mostly in small-scale catchments and in
forest temperate regions already affected by acid rain, while few studies have been conducted
in tropical South America and in areas with little or no influence of acid rain.
The aim of this report is to evaluate the seasonal variations in pH and chemical
composition of Minas Gerais State surface waters. Minas Gerais was chosen because (i) the
majority of its territory is covered by the highest sensitive areas to acid depositions; (ii) it is
one of the world areas with potential acid deposition problems; (iii) it encompasses a huge
territory, roughly equivalent to France; (iv) it is subjected to a climate with two well-defined
seasons (rainy summers and dry winters) that allow the identification of seasonal pH patterns.
Materials and Methods
1. Study Area
The state of Minas Gerais occupies approximately 7% of Brazil, and is located in the
southeastern region, between parallels 14º13’58”S and 22º54’00”S, and meridians
39º51’32”W and 51º02’35”W. According to the Brazilian Institute of Geography and
Statistics (IBGE), it encompasses about 587,000 km2 with a population of 19 millions
strongly concentrated (82%) in urban areas (in 2000), of which the most important is its
capital and suburbs, known as the Belo Horizonte Metropolitan Region (BHMR) with more
than 3.5 million inhabitants.
The climate is characterized by two well-defined seasons: a dry period (April to
September) in which the lowest precipitation amount coincides with the lowest winter
23
temperatures in July, and a rainy period in which the heaviest rains coincide with the highest
temperatures, around December-January. Most part of its territory has a high relief and the
mean annual rainfall varies regionally between 700 and 2500 mm. Regarding the presence of
acid or alkaline rocks, the State has three major compartments: (i) igneous and metamorphic
rocks (granite-gneiss and quartzite dominant rocks); (ii) igneous (basalt) with sedimentary
rocks (calcareous and calciferous sandstones); (iii) calcareous and dolomite. The main soils
(88% of Minas Gerais area) are latisols (42%), podzolic soils (21%), cambisoils (17%), and
litosoils (8%). Latisols are highly weathered and virtually without the primary minerals or the
secondary minerals less resistant to weathering and they have low cation exchange capacity.
Generally, they are very deep and strongly acid. Podzolic soils are less deep than latisols and
deeper than cambisoils. The presence of clay in the Bt horizon decrease permeability,
promoting overland flow and erosion. Cambisoils and litosoils are young soils, shallow and
poorly developed, with primary minerals less resistant to weathering. Usually they are highly
acidic soils with very low permeability and with erosion risk. Litosoils are very shallow soils
over rocky substrate and usually are associated with cambisoils and rocky scarps (Embrapa
1999, Guerra and Cunha 1998). Rainforests predominate in the granite-gneiss areas, and
savannah vegetation cover (cerrado) in the others, although only 34% is left of the original
vegetation.
2. Sample Sites and Water Chemistry Determinations
The Minas Gerais Institute of Water Management (IGAM) monitors since 1997 a
network of 244 long-term sampling sites in which 40 water quality parameters are measured
in four annual sampling campaigns. In this report we considered only the summer wet season
(January, February and March) and the winter dry season (July, August and September) data.
PH is measured “in loco”; alkalinity, major ions, conductivity, turbidity, total phosphorus and
heavy metals are measured in accordance with the Standard Methods for the Examination of
Water and Wastewater (APHA 1998) by the government agency Technological Center of
Minas Gerais - CETEC in partnership with IGAM. This report used the available database of
the year 2000 (IGAM 2000), except for 9 sample sites which did not have the pH values for
the two seasons, thus totalizing 235 sample sites in Minas Gerais major watersheds (Fig.1).
Discharge data was available for 116 sample sites.
24
Figure 1. Sample sites distribution and major watersheds in Minas Gerais State, SE
Brazil.
Sources: sample sites: IGAM 2000; watersheds: delimited from Geominas / Prodemge
Project – MG digital database.
Results and Discussion
1. pH
pH values of the majority of natural waters generally lie within the range of 6 to 9
(Stumm and Morgan 1996). In most rivers in Minas Gerais, pH values were within these
limits, both in the dry and wet seasons (Fig. 2a,b). Only a few sites (15 among 236) showed
values below the lower limit of 6.0 (always during the wet season), and none exhibited values
above the upper limit of 9.
25
Figure 2a. pH values for the 236 sample sites,
in the dry (winter) and rainy (summer) seasons.
Figure 2b. Frequency of sample sites
according to pH ranges in the dry
(winter) and rainy (summer) seasons.
Sites showed a seasonal acidification pattern. In 92% of the sample sites the pH values
were slightly more acidic in the wet season than in the dry season. During the wet season pH
ranged from 5.2 to 7.5 (mean = 6.7), and during the dry season pH values ranges from 6.1 to
8.6 (mean = 7.4). Figure 2b shows that in the majority of sites the pH varied from 6.5 to 7.0 in
the wet period and from 7.0 to 7.5 in the dry period. In the Global Environment Monitoring
System (Gems/Water 2004) data bank, the global median pH value is 7.7, slightly more
alkaline than 6.9 in our results. Some lakes from the Rio Doce Valley (Tundisi and Saijo,
1997), eastern Minas Gerais, showed a similar pattern, indicating that this could be a general
pattern for lakes also, but it is necessary to analyze a larger data base, including reservoirs.
Among the sites that showed a seasonal acidification pattern, some (fifteen) became
acidic (below pH 6) during the wet season. The majority of these acidic sites are located in
geologically sensitive areas to acidification (granite-gneiss / quartzite), and the acidic
character can be due to an extreme case of seasonal natural acidification or due to pollution.
Among these sites, only one appears to be polluted, with high levels of phosphate (3.91 mg.l1
) and sulphate (15.1 mg.l-1).
On the other hand, during the wet period 19 sites exhibited alkalization (more alkaline
than during the dry season), a behavior opposite to the expected pattern, although pH values
lie within the acceptable limits (6 to 9). Among the 25 sites with pH ≥ 7 during the wet season
26
(Fig. 2b), 10 were the alkalized sites located in the Velhas River watershed within the BHMR,
which suggests that during the rainy season urban / industrial runoff is bringing more alkaline
compounds. Figuerêdo’s research (Figuerêdo 1999) demonstrated that the BHMR is affected
by prevailing winds carrying calcareous dust from the Lagoa Santa Karst Region. During the
dry period, this dust is deposited in the BHMR and would help to buffer the acidity of the first
rains. In this way, the calcareous dust would decrease the acidifying effects of air pollution
caused by sulfate and nitrogen oxides released by industries and vehicles.
Considering the seasonal pH variations in each sample site, in the majority of the sites
the difference lies below 1.0, but some sites (in the Velhas and Jequitinhonha rivers) varied
almost 2.0. Studies focusing on seasonal pH variations and changes in watershed land use
have demonstrated that in rural-urban watersheds there is an increase in the magnitude of pH
depressions as a result of increased urbanization (Cannon and Whitfield 2001). If the
behavior found in our data is due to pollution, it is a matter of great concern since such pH
seasonal depressions may be lethal to several organisms (Wetzel 2001).
There was a strong positive correlation (r = 0.98) between ΔH+rainy-dry and H+rainy
(Fig.3a and 3b). The acidic sites were related to the highest H+ seasonal depressions and the
alkalized sites were related to lower seasonal H+ variations.
Figure 3a,b. Relationship between the [H+] depressions and [H+ ] during the rainy season
(Fig.3a). [H+] depression was calculated as the difference between the rainy and the dry
season. In red: acid sites (below pH 6); in lilac alkalized sites (higher values during the
rainy season).
27
Seasonal or episodic acidification of surface waters have been reported by many
authors in several countries in temperate regions, especially related to snowmelt water. These
studies showed that precipitation, in the form of rain or snow, decrease pH and that
hydrologic pathways are important factors to explain the observed patterns.
In temperate regions, seasonal variations in pH have been noticed in places where
snowpack accumulates during the wintertime. Spring meltwater flows are relatively more
acidic due to soil saturation at the time of snowmelt so that there is relatively little
neutralization of the precipitation acidity by the soils (Gundersen and Steinnes 2001).
Comparative studies in Canada, UK and Norway showed similar features, in which pH and
ANC (acid neutralizing capacity) decline with increasing discharge. The authors noted that
the reason for this pattern was not that rainfall passed directly through the catchment to
provide higher stream water volumes, but that the large variations in stream chemistry was
related to the hydrologic flowpath through the terrestrial catchment prior to emergence at the
surface (Christophersen et al. 1994). Studies in a Japanese snow covered region also showed
that streams decrease from ~6.5 in the summer to 6.0 during the snowmelt (Ohte et al. 2001).
Also in northern Sweden, with relatively low levels of acid depositions, studies showed that
pH declined (1 to 3 units) in spring melt events (Laudon et al. 2000). In the Andirondack
temperate forest region of New York, subjected to acid depositions, the relations between pH
and flow were similar to those of ANC and flow: decreased pH with increased flow
(Lawrenceet al. 2004). White and Blum (1995) noted that increased precipitation and
moisture may decrease soil solution concentrations and decrease soil pH. Driscoll et al.
(2001) mention that surface waters are often more acidic after rain events, when the discharge
is higher. Precipitation can raise the water table from the subsoil into the upper soil horizons,
where acid neutralizing processes are generally less effective than in the subsoil. Water
draining into surface waters during high flows is therefore more likely to be acidic than water
that has discharged from the subsoils, which predominates during low flows.
2. Major ions
Considering the total average concentration values, among anions the order of
dominance is HCO3- > SO4= ≥ Cl-, and among cations it is Ca2+ > Na+ > Mg2+ ≥ K+. This order
of cations proportion obeys the typical sequence of cation mobility in silicate rocks
undergoing weathering (Berner and Berner 1996) and suggests a dominance of silicate rocks
28
in the solute chemical composition of the sample sites. The relative proportion of the
individual ions is maintained during the wet and the dry seasons (Fig.4).
Figure 4. Cations and anions mean concentrations in
the dry and wet seasons.
HCO3- was the dominant (>50%) anion in 85% of the sample sites during the wet
season and in 92% during the dry season. In most sites (84%), HCO3- dominated in both
seasons. Few sites were dominated by SO4=or Cl-. Among the cations, Ca2+ was the main
cation in 45% of the sample sites during the rainy season and in 42% of the sites during the
dry season. Only in 28% of the sample sites did Ca2+ dominate in both seasons. Few sites
were dominated by Na+ and no sites were dominated by K+.
Figure 5 shows the ternary plot for cations and anions during both seasons. During the
dry season, anions were almost all restricted to the HCO3- dominance, while during the rainy
season more sites showed dominance by or co-dominance with SO4= or Cl-. However, cations
showed a pattern that only varied slightly between seasons. Cations were more restricted to
the Ca2+and Mg2+ dominance with little co-dominance or dominance by other cations. The
different behavior between anions and cations indicate that for some sample sites cations
other than major cations, or even strong acids, could be present.
29
Figure 5. Ternary plots for the major cations and anions concentrations during the
dry and the rainy seasons (relative values).
Bicarbonate can result from carbon of carbonate minerals (calcite and dolomite) or
from the reaction of CO2 dissolved in soil and groundwater with carbonate or silicate minerals
(Berner and Berner 1996). Carbon dioxide is derived almost entirely from bacterial
decomposition of soil organic matter. As alkalinity, Ca and Mg are originated almost entirely
from rock weathering, mainly from calcite and dolomite, and in minor proportions from Ca
and Mg silicates. Calcium carbonates dissolve more rapidly than silicates, and small amounts
in a rock can dominate the composition of groundwaters.
In addition, there were strong correlations between concentrations of alkalinity and the
sum of the cations during the dry (r = 0.95) and wet seasons (r = 0.91). Among the cations,
Ca2+ showed the best correlations with alkalinity both in the dry (r = 0.89) and the wet
30
seasons (r = 0.92). The other cations presented weak correlations with alkalinity, except for
Mg2+ during the wet period (r = 0.83). Thus, alkalinity may explain the variability in Ca2+
concentration in both seasons as well as the variability of Mg2+ during the wet period.
3. Discharge, Total Suspended Solids and Total Dissolved Solids
The available discharge (m3.s-1) data for 116 sample sites showed that during the base
flow (dry) season 89% of the sample sites had less than 50% of the high flow (rainy) season
discharge. Sample sites had an average discharge value 4.3 times greater during summer
(rainy season) than during winter (dry season).
Considering the contribution to Total Solids (TS) values, Total Dissolved Solids
(TDS) were dominant during the dry season, while during the rainy season Total Suspended
Solids (TSS) prevailed. During the dry and the rainy seasons TDS average concentrations
were 65 and 68 mg.l-1 respectively, and TSS average concentrations were 22 and 271 mg.l-1
respectively (Fig.6). This result indicates that overland flow may be a significant water
pathway carrying particulate matter into the rivers by erosion processes. The results also
indicate that the magnitude of this pathway is important due to the 10 times increase in TSS
concentrations.
Figure 6. Proportion of Total Dissolved Solids (TDS) and Total Suspended Solids (TSS) in
Total Solids (TS) during the dry and wet seasons.
31
4. TSS, TDS, major ion, phosphates and heavy metals concentrations
Turbidity, trace elements, and phosphate had higher concentration values during the
high flow season, while alkalinity, conductivity, sum of cations (Ca2+, Mg2+, K+, Na+) and
sum of anions (HCO3-, SO4=, Cl-) had the opposite behaviour, with lower values during the
high flow season according to the pH pattern (Tab.1). Alkalinity showed lower concentration
values in 92% of the sample sites during the wet season, and so did conductivity (89%), sum
of cations (86%), HCO3- (92%), Ca2+ (84%), Mg2+ (63%), K+ (72%), Na+ (80%), Cl- (80%)
and SO4= (81%). In essence, during the rainy season the parameters generally associated with
the particulate phase (turbidity, phosphate and heavy metals) increased, while those associated
with base flows (alkalinity, conductivity, sum of the cations, etc.) decreased, consistent with
TSS and TDS behaviour. The higher concentrations of phosphates and heavy metals during
the rainy season corroborate the influence of overland flow and erosion on water quality.
Our results also show that there were strong correlations, between the dry and wet
seasons, for alkalinity (r = 0.92), sum of cations (r = 0.89), and conductivity (r = 0.91), as well
as Ca2+ (r = 0.84), Mg2+ (r = 0.78) and Na (r = 0.78), and weak correlations for K+ and pH.
These results suggest that in both the wet and dry seasons these ions may have the same
origin, and so, a possible dilution effect during the rainy season.
Table 1. Range and average values of water quality parameters in the dry and the rainy seasons.
Parameter
Unit
Conductivity
PH
Total alkalinity
Total hardness
Magnesium hardness
Calcium hardness
Chlorides
Sulfates
Sodium
Potassium
Turbidity
Total phosphate
Nickel
Copper
Zinc
Manganese
µmho/cm
mg.l-1 HCO3mg.l-1 CaCO3
mg.l-1 CaCO3
mg.l-1 CaCO3
mg.l-1 Cl
mg.l-1 SO4
mg.l-1 Na
mg.l-1 K
NTU
mg.l-1 P
mg.l-1 Ni
mg.l-1 Cu
mg.l-1 Zn
mg.l-1 Mn
Range
Dry season
8.4 - 1082
6.1 – 8.5
1.65-190.69
5.4 - 285
1.1 – 82.0
3.0 - 264
0.3 - 84
1 - 51
0.3 - 138
0.07 – 12.7
1.5 - 153
0.010 – 6.1
0.004 – 0.047
0.007 – 0.040
0.01 – 0.83
0.05 – 4.17
Wet season
6.5 - 714
5.2 – 7.5
0.305-150.49
3.4 - 225
1.3 - 108
1.0 - 145
0.3 - 60
1 - 39
0.3 - 44
0.21 – 7.53
3.6 - 4790
0.010 – 3.9
0.004 – 0.079
0.007 – 0.112
0.01 – 0.58
0.03 – 3.55
Average
Dry season
100.9
7.1
20.37
32.3
10.1
22.3
4.3
4.3
7.0
2.2
20.2
0.15
0.008
0.008
0.055
0.135
Wet season
68.7
6.6
13.32
24.7
9.1
15.6
3.3
3.3
3.9
1.75
334
0.19
0.015
0.023
0.084
0.287
32
White and Blum (1995) mention that a decrease in solute concentration with runoff is
commonly attributed to increased dilution of base flow chemistry. Our findings concur with
theirs, and with Gregory and Walling (1973), who state that the relationship between
dissolved solids concentrations and discharge are the result of two processes: runoff
production and solute production, reflecting surface and subsurface dynamics of catchments.
The relationship between total dissolved solids concentrations and streamflow is usually
inverse (a straight line plot on logarithmic coordinates) primarily due to a dilution effect, with
an increased contribution from storm runoff which carries a lower solute content than
baseflow. Under dry conditions, when baseflow is undiluted, there is a tendency of maximum
concentrations at low discharges, and under wet conditions, when high discharges occur, the
dilution effect becomes progressively less significant, and concentrations are dominated by
the near-constant solute content of the storm runoff component.
Our results showed that during the rainy season the parameters generally associated
with the particulate phase (turbidity, phosphate, heavy metals) increased, while those
associated with base flows (conductivity, alkalinity, sum of the cations, etc.) decreased. This
behavior reflects different flow paths: particulate matter results from surface or shallow
subsurface flow during the rainy season, while the dissolved phase is predominantly
associated with baseflow since many of these solutes are products of rock weathering
(Winston and Criss 2002). Similar results were obtained in southeastern Australia rivers,
where high major ion concentrations were attributed mainly to the influence of groundwater
inflows (dry period), while increased concentrations of nutrients, organic carbon, and trace
elements were attributed to anthropogenic inputs like discharges from sewage treatment
plants, diffuse and agricultural runoff (wet period) (Markich and Brown 1998).
5. Instantaneous Loads
To investigate a possible dilution effect, we analysed the load (concentration x river
water discharge) variations during the dry and wet seasons. In approximately 90% of the
sample sites, alkalinity and major ions loads were higher during the rainy season than during
the base flow season. The results indicate that during the wet season loads increased, but in
spite of this, dilution effects prevailed due to high water discharges, lowering the
concentrations. Regarding the number of sample sites (percentage of the total) that have a
contribution to the load over 100% during the wet season referred to the dry season, alkalinity
33
showed 71%, Ca2+ (68%), Mg2+ (71%), K+ (56%), Na+ (47%) and Cl- (47%). The only ion
that did not show any site with a contribution to the load over 100% during the wet season
was SO4= indicating that acid sulphates may not have influence over pH depression.
As for concentrations, loads showed that there was strong correlations for the major
ions between the dry and the wet seasons (except SO4=): alkalinity (r = 0.93), Ca (r = 0.91),
Mg (r = 0.90), Cl (r = 0.82), K (r = 0.92), Na (r = 0.93). Also, the difference between rainydry loads (load increment during the rainy season) was strongly correlated between many
ions: alkalinity and Ca (r = 0.97), alkalinity and Mg (r = .91), alkalinity and K (r = 0.93);
chloride and Na (r = 0.95), chloride and K (r = 87); whereas sulphate showed only weak
correlations with the cations (r = ~0.63). As for concentrations (section 4) these results also
indicate a possible dilution effect during the rainy season.
6. Alkalinity, pH and sensitivity to acidification
Alkalinity varied from 0.31 mg.l-1 HCO3- (5.08 μeq.l-1) to 190.69 mg.l-1 (3,125 μeq.l-1)
HCO3- and showed an average value of 20.37 mg.l-1 (334 μeq.l-1) in the dry season and 13.32
mg.l-1 (218 μeq.l-1) in the wet season. These results are slightly lower than the average value
of 24.4 mg.l-1 mentioned by Berner and Berner (1996) for South America and more than a
half of the world average value of about 51 mg.l-1 cited by Berner and Berner (1996) and
Wetzel (2001).
Figure 7 shows that the distribution of alkalinity during the rainy season was displaced
toward lower concentration values in relation to the dry season. The highest number of sample
sites showed concentrations between 50 and 300 μeq.l-1 in the wet season, and between 100
and 200 μeq.l-1 during the dry season. Values above 1000 μeq.l-1 (50 mg.l-1) corresponded
mainly to karst or metropolitan areas.
Freshwaters with alkalinity ≤ 200 μeq.l-1 (12.20 mg.l-1) are considered acid-sensitive
and with alkalinity ≤ 100 μeq.l-1 are considered very sensitive (Adriano 2001). During the
rainy season there were 67 (28%) very acid-sensitive and 102 (43%) sensitive sample sites,
while during the dry season only 23 sites were very acid-sensitive and 103 acid-sensitive.
34
Figure 7. Histogram of alkalinity concentration
categories during the dry and rainy seasons.
Figure 8 shows the relationship between alkalinity and pH. In the wet season pH rised
with increasing alkalinity to a maximum pH around 7.5, and the relationship between pH and
log alkalinity was strong (r2 = 0.52). In the dry season this behavior was not so clear
indicating interferences in the carbonate buffer system.
The relationship between log alkalinity (as meq.l-1) and pH during the wet season (pH
= 0.7117 * log alkalinity + 6.9423) showed that 40 μeq.l-1 translates to a pH 5.95 and 50
μeq.l-1 to a pH 6.02. Very similar results were obtained for Canada. Dupont et. al. (2005)
mention that the pH–alkalinity relationship confirmed that a 40 μeq/L ANC [acid neutralizing
capacity] translates approximately to a pH 6 for Quebec lake waters. Brazilian water quality
laws do not established standards for alkalinity but our results show that a limit value of 50
μeq.l-1 HCO3- will be appropriated to attend the Brazilian lower legal limit of pH 6. Regarding
the tolerance of aquatic organisms, several studies specify the alkaline limit value of 20 μeq.l1
while others adopt 50 μeq.l-1 (Dupont et. al. 2005, and cited references). To establish an
appropriate legal limit in Minas Gerais ecosystems it is necessary, therefore, to study the
tolerance of its aquatic biota.
35
Figure 8. Relationship between alkalinity and pH during the dry and the rainy season.
Up: Alkalinity versus pH. Below: log alkalinity versus pH. In red: adidic sites during the
rainy season (pH <6). In lilac: alkalized sites during the rainy season (increased pH values
during the rainy season, inverse pattern than expected). In blue: regular sites during the
rainy season. I green: regular sites during the dry season.
As mentioned in section1, during the rainy season 15 sites were below pH 6 (acidic
sites) and 19 sites exhibited alkalization, in other words more alkaline than during the dry
season, (alkalized sites). Figure 8 shows in red the acidc sites and in lilac the alkalized sites
for the rainy season and also these sites were marked in the dry season to investigate their
transition from the dry to the rainy season. The acidc sites during the dry season showed the
lowest alkalinity values and during the rainy season they continued showing the lowest
alkalinity values, indicating that probably this low pH < 6 could not due to acid pollution. For
the alkalized sites, there was another behavior. During the dry season, the sites were
distributed along the curve showing large alkalinity and pH variability. During the rainy
36
season they moved to the upper side of the curve among the highest alkalinity and pH sites,
indicating the possibility of pollution. As noticed in section 1 these sites are located in the
Velhas River watershed within the BHMR, and this result confirms the possibility of
pollution.
Figure 9 shows the distribution of the acid and alkalized sites with the alkalinity
concentration categories. The acidic sites were restricted to the very sensitive (13 sites) and
sensitive (2 sites) categories. On the other hand, the alkalized sites were distributed along all
the categories excluding the very sensitive to acidification.
Figure 9. Histogram of alkalinity categories
during the rainy season. In red: acidic sites. In
lilac: alkalizes sites.
7. Calculated ANC, alkalinity and pH
The alkalinity and pH depressions during the rainy season (highflow) observed before
can be thought of as the result of (i) dilution of base flow (main flowpath expected to
contribute to river water discharge during the dry season); (ii) presence of mineral acids (MA)
37
from rainwater; (iii) release of MA and/or strong organic acids from the catchment. In order
to analise these factors, the acid neutralization capacity (ANC) was calculated.
Acid neutralization capacity (ANC) calculations were based on the charge balance
definition of ANC, where, according to the electroneutrality principle, the equivalent sum of
base cations (BC) must equal the sum of acid anions anions (MA) (Stumm and Morgan
1997). So, ANC can be calculated as:
ANC = sum of base cations (–) sum strong acid anions
Sum of strong base cations = [Ca] + [Mg] + [Na] + [K]
Sum of strong acid anions = [SO4] + [NO3] + [Cl}
This equation shows that:
-
for every base cation charge that is removed from the water corresponds to a
proton added to the water.
-
for every anionic charge that is removed from the water corresponds to a
proton removed from the water.
Dissolution of CO2 in river water affect pH but not ANC, and for this reason ANC is
more adequate and robust for modeling purposes (Eshleman and Hemond 1988). IGAM
database did not include organic carbon measurements that allow estimating strong organic
acids. ANC / alkalinity and pH are different concepts. Alkalinity/ANC is the net concentration
of base cations in excess of strong acids; is a capacity factor (buffer capacity) while pH is an
intensity factor (H+ concentration). Alkalinity/ANC indicates how resistant to change is the
pH, how effective is the buffering, the homeostactic processes (Morel and Hering 1993),
which reflects in the ecossystem resilience.
ANC was strongly correlated with total alkalinity in both seasons (Fig. 10). If the
sample site with the highest alkalinity value during the wet season is ignored, r2 rises from
0.95 to 0.98. So, during the rainy season, calculated ANC can be best estimated from
alkalinity through the equation: ANC = 1.0549*alkalinity + 0.0908. This result indicates that
alkalinity is well represented by ANC, and also indicates that the major ions are the main
38
responsible for alkalinity. Therefore, organic strong acids could have only restricted
influences. As for alkalinity, ANC was well correlated with pH during the wet season (r2 =
0.53) and poorly correlated during the dry season (r2 = 0.06) (Fig.11).
Figure 10. Relationship between alkalinity and ANC during the dry and the wet seasons.
Figure 11. Relationship between pH and ANC during the dry and the rainy season.
8. ANC depression, dilution effect and major ions
8.1. Dilution effect
Considering the water cycle, during the dry season the water of stream and rivers is at
its baseflow, which is derived from the permanent groundwater system. During the rainy
39
season, the baseflow is augmented by the contributions from interflow (infiltrated water that
migrates laterally), overland flow, and directly rain falling into the river channel (Drever
2002 and many others).
Hypothetically, the highest dilution effect on ANC (defined as ANC
dilution)
could be
obtained when solutes during the baseflow period (dry season) are diluted to the water river
discharge of the highflow period (rainy season) without considering any other solute
contribution from neither overland flow nor interflow, or other sources. As mentioned before,
results showed that alkalinity and major ions concentrations decreased while loads increased
during the rainy season, indicating a dilution effect. It is thus expected that ANC during the
rainy season was lower than during the dry season. In addition, it is expected that ANC during
highflow was higher than ANC
dilution.
Figure 12 shows that this was the pattern for the
majority of the sample sites.
Figure 12. Relationship between ANC highflow, ANC baseflow and ANC dilution.
N = 116 sample sites.
40
In order to investigate the dilution effect, a simple water mixing model was applied to
the data (Petts and Foster 1985 and many others) as follows:
C baseflow x V baseflow + C new water x V new water = C highflow (V baseflow + V new water)
C baseflow = solute concentrations of baseflow (dry season)
C highflow = solute concentrations of highflow (rainy season)
C new water = C highflow – C baseflow
V baseflow = volume of flow from baseflow (dry season)
V highflow = volume of flow from highflow (rainy season)
V new water = volume of flow from new water = V highflow – V baseflow
New water solute concentration = C new water / V new water
This model was applied to ANC and individual cations and anions. Figure 13 shows
the relationship between ANC
new water.
baseflow
and ANC
new water,
and between ANC
There was not a very good correlation between ANC
baseflow
highflow
and ANC
and ANC
new water
(r2 =
0.62), but ANC highflow and ANC new water were very strongly correlated (r2 = 0.91).
Figure 13. Relationship between ANC baseflow, ANC highflow, and ANC new water.
ANC baseflow x ANC new water: r2 = 0.62
ANC highflow x ANC new water: r2 = 0.91
ANC baseflow x ANC highflow : r2 = 0.82
N = 116 sample sites
41
This result demonstrates that ANC during the rainy season was highly controlled by
the chemical characteristics of the new water that enters into the system. But, the correlation
between ANC baseflow and highflow was also high: ANC highflow = 1.4327*ANC
baseflow - 31.131 (r2 = 0.82).
Figure 14 shows the major ions for baseflow (dry season), highflow (rainy season) and
new water. The chemical profile for the rainy period is very similar to the new water profile,
except by Na+ that is substantially lower for new water than for the dry and wet seasons. This
result reinforces the evidences that during the rainy season there is not a simple dilution
effect, but that rivers are highly controlled by the chemical characteristics of the new water
that enters to the system.
Figure 14. Major ions in the dry season (baseflow), rainy season (highflow) and ‘new water’.
Figures 15 and 16 show the relationships between baseflow versus new water and
highflow versus new water, while Figure 17 shows the profile of the coefficients of
determination (r2) obtained.
42
Figure 15. Relationships between river and anion loads verus new water loads during the
rainy season (highflow) and the dry season (baseflow).
43
Figure 16. Relationships between cations loads verus new water loads during the rainy season
(highflow) and the dry season (baseflow).
44
Figure 17.Coefficients of determination profile of the relationships
between cations loads verus new water loads during the rainy
(highflow) and the dry seasons (baseflow). (See Figs. 16 and 17).
All the major ions show a stronger correlation between highflow and new water than
between baseflow and new water. Anions Cl- and SO4= showed lower correlations, and no
correlations (r2 = 0.30 and 0.20) between baseflow and new water. These results show that,
although new water controlled highflow major ions, they are remaining an identity between
baseflow and highflow.
8.2. ANC depression, Base Cations (BC) and Acids Anions (MA)
ANC depression was considered as the difference between ANC during the dry season
(baseflow) and ANC during the rainy season (higflow). BC is the sum of base cations and
MA the sum of acid anions (see page 41).
ANC depression can be defined as ΔANC = ANC
baseflow
– ANC
highflow
and also by
the difference between ΔBC and ΔMA [ΔANC = ΔBC (-) ΔMA], where ΔBC = BCbaseflow –
BChighflow and ΔMA = MAbaseflow – MAhighflow. If BCbaseflow is higher than BChighflow, ΔBC is
positive, and if BCbaseflow is lower than BChighflow, ΔBC is negative. The same reasoning is used
for ΔMA and ΔANC. In this way, it is expected that in order to obtain an ANC depression
during the rainy season [ΔANC +] it is necessary a [ΔBC+] (decreased concentrations in the
highflow period) or/and a [ΔMA –] (increased concentrations in the highflow period):
45
[ΔANC+] = [ΔBC+] – [ΔMA-]
So, it is the relative proportion between ΔBC and ΔMA concentrations that will
determine the occurrence of an ANC depression. As mentioned before, ANChighflow was lower
than ANCbaseflow in most sites (80%), and only in 46 (20%) of the sample sites ANChighflow was
higher. Each sample site was analysed regarding all the possibilities for the relative behavior
of ΔBC and ΔMA behavior affecting ΔANC, and the results are in Table 2.
Table 2. ANC depression during the rainy season and relative behavior of BC and MA
Type
[BC[ and [MA]
Δ ANC+
3
4
5
Nºsítes
[MA] ↑
[ANC] ↓
28
[BC] ↓ > [MA] ↓
[ANC] ↓
156
[BC] ↑ < [MA] ↑
[ANC] ↓
5
[BC] ↓ < [MA] ↓
[ANC] ↑
18
[BC] ↑
[MA] ↓
[ANC] ↑
23
[BC] ↑ > [MA] ↑
[ANC] ↑
5
[BC] ↓
1
2
[ANC]
Δ ANC -
6
Total
189
(80%}
46
(20%)
N = 235 sample sites
A - ANC depression during the highflow season: Δ ANC (+)
Type1: ΔBC is positive, [BC] were lower during the rainy season; and ΔMA is negative,
[MA] were higher during the rainy sason (12% of the sample sites)
Type 2: both ΔBC and ΔMA are positive, but the dilution of [BC] is higher than that of
[MA], resulting in a relative increment of [MA] (66% of the sample sites)
Type 3: both ΔBC and ΔMA are negative, but the increment of [MA] is higher than the
increment of [BC] (2% of the sample sites).
B – ANC increment during the highflow season: Δ ANC (-)
Type 4: both ΔBC and ΔMA are positive, but the dilution of [MA] is higher than the dilution
of [BC], resulting in a relative increment of [BC] (8% of the sample sites).
Type 5: ΔBC is negative and MA is positive (10% of the sample sites)
Type 6: both ΔBC and ΔMA are negative, but the increment of [BC] is higher than the
increment of [MA] (2% of the sample sites).
These results show that among the sites with ANC depression during the rainy season,
82% was due to a greater dilution of [BC], in 14% is due to [BC] dilution and [MA] increases
and in 6% to a relative increase of [MA]. So, these results are consistent with a dilution
hypothesis for the observed pH depression during the rainy season.
46
Table 3 shows the relationship between the ANC and pH behaviour. As expected,
there is a good agreement between ANC depression and pH depression during the rainy
season. Also, acidic sites (pH<6) are related to a [BC] dilution (Types 1,2), although 4 of
them are related also to higher [MA] values during the rainy season (Type 1).
Among the 189 sample sites with ANC depression during the rainy season, only 12
sites showed increased pH values, a behavior opposite to the expected. This indicates that
other strong basic compounds, aside those of the major anions used to calculate ANC, are
present in this rivers.
Table 3. ANC depression and pH behaviour
N=235
pH < 6
pH ↑
Nº sítes
pH ↓ (25)
4
pH ↑ (3)
28
[ANC] ↓
pH ↓ (147)
11
pH ↑ (9)
156
3
[ANC] ↓
pH ↓ (5)
4
[ANC]↑
pH ↓ (17)
pH ↑ (1)
18
5
[ANC]↑
pH ↓ (18)
pH ↑ (5)
23
6
[ANC]↑
pH ↓ (4)
pH ↑ (1)
5
Type
ANC
1
[ANC] ↓
2
[ANC] ↓
(189)
[ANC]↑
(46)
pH ↓
5
In red: pH behaviour not in agreement with ANC.
Among the 46 sample sites with increased ANC during the rainy season (sites that do
not follow the general acidification pattern), there was a disagreement with the expected pH
behaviour in 39 samples sites. These sites showed an acidification processes while ANC
increases, which indicate that other strong acids were present. Also, alkalinized sites are
among these 46 sample sites.
These sample sites with discordant behavior need to be further investigated. It is
possible that natural processes or pollution were responsible for those behaviours. Some
47
natural and biologically-mediated processes that occur in waters may influence alkalinity and
pH, for example: organic matter decomposition with organic acids release, photosynthesis
(with NO3- increase pH, with NH4+ decrease pH), respiration (the opposite process of
photosynthesis), nitrification (decreases pH), denitrification (increases pH), and CaCO3
dissolution (increases alkalinity) (Stumm and Morgan 1997). Organic acids can strongly
influence pH variabilities in some ecosystems. Laudon et al. (2000), studying stream
episodic pH declines in northern Sweden, found that for the majority of the events, organic
acids contributed over 75% of the acidity at peak runoff.
9. Soils and vegetation, water pathways and the dilution effect
Our results raised the question if the pH acidification pattern during the rainy season,
by base cations dilution lowering ANC, can also be due to a pattern of hydrologic flowpath in
soils.
Two physical factors are particularly important to the chemical composition of natural
waters: residence time and the pathways (routes) along which water moves through the
system. The residence time is important because the longer the residence time in an
environment, the more opportunities there are for reactions between water and the materials
with which it is in contact. The pathways determine which materials water will contact during
its passage through the hydrologic system. In general, water that follows shallow pathways
contact more weathered and, consequently, less reactive materials than water that moves in
deeper pathways (Bricker and Jones 1995). Christophersen et al. (1994) mentioned that
rainfall does not pass directly through the catchment to provide merely higher stream water
volumes, but that the large variations in stream chemistry were related to the hydrologic
flowpath through the terrestrial catchment prior to emergence at the surface.
9.1. Soils and Vegetation
As mentioned by Kellman and Tacaberry (1997), tropical soil profiles and their
underlying weathered mantles are generally deep and contain large quantities of clay-sized
soil particles which allow large storage reservoirs of soil moistures. The soils sometimes may
contain horizons of high clay content, unweathered bedrock or ironstone cuirass (canga),
48
which are impediment for the vertical movement of soil water. According to this description,
as mentioned in the Introduction, the main soils in Minas Gerais are latisols, which are highly
weathered, virtually without primary minerals and with low cation exchange capacity,
generally deep and strongly acidic. Others are cambisoils, litosoils and podzolic soils, also
strongly acid. Litosoils are found in mountain regions in quartzite and granite-gneiss bedrocks
(crystalline acid rocks).
To understand soil acid and basic behavior, we follow Reuss and Johnson (1986)
with notes from other authors. An important characteristic of soils is the cation-exhange
complex that is negative charges on clay minerals or on soil organic matter. In alkaline or
neutral soils the cation-exchange complex is dominated by base cations, whereas in acid
mineral soils it is generally dominated by aluminum species formed by the dissolution of soil
minerals in acid systems. In acid organic soils, H+ may be the dominant exchangeable cation.
Processes that acidify the soil include those that increase the negative charges (organic matter
accumulation, clay formation) or those that remove base cations. Processes that make the soil
more basic add base cations (weathering of soil minerals) or reduce negative charge
(destruction of organic matter by fire).
Forest and intensive agriculture increase soil acidity because they uptake base cations
and release H+ by the roots. These protons react with minerals and release new cations
necessary for the trees. This proton–cation balance is very fragile. The dissolution rate of
carbonate rates is very fast, so in soils from carbonate rocks the rate of H+ consumption by
weathering may be equal to or higher than the rate of H+ release from the biota. In soils from
crystalline rocks the dissolution rate is slow, and the rate of H+ release from the biota may be
higher than the rate of consumption by weathering and turn the soil more acid (Stumm and
Morgam 1996). Blum et al (1998), studying the flow of mineral weathering to surface waters
in the crystalline Himalaya, pointed out that 82% of the HCO3- flux was derived from the
carbonate weathering and only 18% was derived from silicate weathering, even though the
predominant rock was quartzofeldspathic gneiss and granite with only about 1% of carbonate
in the watershed, showing the importance of trace amounts of bedrock carbonate in
controlling the water chemistry. As seen in Chapter 1, Minas Gerais karst regions may have a
great influence on alkalinity and pH, especially those karst regions near or within the
crystalline rocks domains.
49
Harvest removal of vegetation causes the export of base cations, and regrowth after
harvest may cause further acidification of soils. Without harvesting the acidifying effect of
cation accumulation will continue as long as biomass accretion continues. Fire (a common
practice in Minas Gerais agriculture management) will reverse this accumulation but may
cause surface alkalization without substantially reversing the subsoil acidification caused by
vegetation uptake.
The second major process by which base cations are removed from soils is leaching. It
requires that removal of base cations in solution take place only in association with mobile
anions (electroneutrality). Generally, the dominant anion in solution is HCO3- and its
concentration is a function of pH and partial pressure of CO2. In soils, biological activity such
as roots and microbial respiration increases CO2, with the result that that partial pressures in
soils are commonly in the range of 1 to 5%, whereas in surface waters they are close to the
atmospheric level (0.03%). Increments in CO2 increase the concentrations of H2CO3 and its
dissociation products are H+ and HCO3-. The H+ formed does not remain in solution because
it tends to exchange with other cations or it dissolves soil minerals releasing Aluminum,
which then replaces other cations on the exchange complex. The increased content in base
cations and HCO3- is reflected as alkalinity. Thus, in the CO2 enriched atmosphere of the soil,
even in acid soils, the solutions may have positive alkalinity. When the soil solutions are
released to the lower CO2 environments of the surface waters, pH increases as CO2 degassing
takes place. The result is that, even in regions where soils are acid, in surface waters without
organic anions the predominant anion is HCO3- and the pH is > 6. Natural acid waters can
exist in many regions and in the majority of the cases organic anions predominate. These
waters occur where drainage from organic soil horizons is discharged directly into surface
waters without passing through mineral horizons, where the organic acids precipitate as iron
or Aluminum complexes.
9.2. Water flowpaths
Stream flow represents the sum of contributions from four sources: overland flow,
throughflow, groundwater flow and direct precipitation (Petts and Foster 1985 and many
others). Dunne (1978) developed a model for the major water flowpath related to climate,
vegetation, land use, soil and topography (Fig.18).
50
Figure 18. Dunne (1978) major water flowpaths related to climate,
landcover, soil and topography.
According to this model it can be expected that, during the rainy season, subsurface
flowpath be a dominant process in Minas Gerais. But also overland flow is possibly
significant, due to poor soil conservation practices, as well as direct precipitation (as in the
San Francisco River floodplain pond systems). In the Minas Gerais dry-wet climate two main
water flowpaths can be considered: (i) groundwater flow during the dry season and (ii)
overland flow, throughflow, and groundwater flow to the surface waters during the rainy
season (Fig.19). Generally, water flowpaths are classified as rain fluxes (surface / shallow
subsurface soil water with fast river responses) and base fluxes (deep soil water with slow
river responses). Next these main concepts are presented in order to be able to discuss our
results.
51
Figure 19. Water flowpaths during the dry and the rainy seasons.
Left: dry season: groundwater flow
Right: rainy season: groundwater flow, overland flow; troughflow (subsurface).
(i) Groundwater flow during the dry season (baseflow): Soils experiencing a prolonged
dry season undergo a progressive drying of the profile as the season advances (Kellman and
Tacaberry 1997). As a result, the effect of lithology on riverflow is particularly apparent
because the entire discharge in a river system may come from groundwater storage (rocks and
deep soils). River baseflow may be sustained by discharge from springs or by diffuse seepage
and the contribution of groundwater is related to the water-table level and the flow
characteristics of the geological strata (Petts and Foster 1985). Baseflow represents historical
rainwater events stored in the soil and rocks with long residence time (Fig.20). Also, during
the dry season the pollution from point sources is at the highest concentrations. This is the
case in Minas Gerais with average 6 months of dry weather but with perennial rivers and
streams, which demonstrates the high water storage capacity of the region. So, it can be
expected that the river baseflow during the dry season shows a strong influence from the
watershed geologic characteristics.
52
Figure 20. Groundwater flowpaths to rivers showing the different water
residence time.
Source: Oram, M.B. Water cycle and watersheds. http://www.wilkes.edu
(ii) Overland flow during the rainy season: Once precipitation reaches the land surface,
some may be intercepted by growing vegetation or buildings and evaporate back to the
atmosphere, but the largest portion will go through the vegetation cover to reach the ground.
When rainwater is intercepted by vegetation, it may became more acidic as the result of
washoff of the accumulated dry acid materials deposition in polluted regions, or by leaching
organic acids anions from the foliage (Reuss and Johnson 1986). When rainwater reaches
the ground, if precipitation inputs exceed soil infiltration rates, an overland flow is expected.
In Minas Gerais, overland flow could be a significant waterflow contributor to rivers. Our
53
results showed that during the rainy season rivers discharges increased in an average value
about 4 times, and that there was a shift from TDS dominance during the dry season to a TSS
dominance during the rainy season, where TSS increased 10 times (see pg 34 – section 3). As
mentioned before (section Study Area), only 34% is left of the Minas Gerais original
vegetation, and poor agriculture and land use practices turn soil less permeable and increases
the sensitivity to overland flow and erosion. Our further calculations of rainfall erosivity
showed that in 2000 erosivity reached a high annual 771 t/ha.mm/h. This value is consistent
with others in Brazil: 600 - 950 t/ha.mm/year and 815 t/ha.mm/year in Mato Grosso in the
High Paraguay River (MMA 1997) and Manaus in the Amazonas River (Oliveira Jr. and
Medina, 1990). So, the watersheds sensitivity to overland flow is enhanced in Minas Gerais
by the dry-wet climate that concentrates the high quantities of the annual rainfall with high
erosivity in only few months.
iii) Water throughflow during the rainy season: Precipitation that infiltrates the land
surface will percolate the soil profile to reach the groundwater (recharge) or move laterally
(troughflow) down to the river. In tropical dry-wet climates, the early rains during the wet
season reach a soil that has been depleted of a large water storage. Sequential rains gradually
increase the water storage, and, as a result, soil water does not start to drain from the soil
profile until well after the rain has begun. Once the profile becomes fully wetted, the volume
of water moving through the soil is large, because the rainfall is received as high intensity
storms. Where a horizon with low permeability exists in the soil profile, vertical movement of
drainage water may be accompanied by periodic saturation, and considerable lateral
downsolope flow above the horizon. During high intensity rainstorms this may result in
overland flow (saturations) (Kellman and Tacaberry 1997).
The presence of macropores in tropical soils may transmit large volumes of water
rapidly, essentially bypassing the micro-pores of the soil aggregates and the soil solutions
contained by these. This process suggests that much of the soil solution remains isolated from
the percolating water during rainstorms. Phosphates applied to the soil surface will be leached
by macro-pore flow, while nutrients released gradually into the soil solution from organic
matter decomposition would be little affected (Kellman and Tacaberry 1997). Considering
this scenario, a short residence time for water throughflow in the soils can be expected. This
new water mixed with the old groundwater (baseflow) will result in a dilution effect of major
ions in river waters.
(iv) Recharge of groundwater: Percolating rainwater recharges the groundwater store, and
raise the water table causing water to drain out more quickly. This drainage, in turn, lowers
54
the elevation and gradient of the water table, reducing the contribution to streams. Thus,
recharge and drainage interacts to produce both short-term and long-term and seasonal
variations of base-flows (Petts and Foster 1985). As water leaves the soil, and passes
through the groundwater system, the concentration of major ions generaly increases as a result
of the reaction between the water and the enclosing rock. Stream waters generally reflect the
composition of near-surface groundwater (Drever 2002). Driscoll et al. (2001) mention that
in the elevation of the water table from the subsoil into the upper soil horizons, water is in
contact with soils where acid neutralizing processes are generally less effective than in the
subsoil. Water draining into surface waters during high flows is therefore more likely to be
acidic than water that has discharged from the subsoil, which predominates during low flows.
9.3. Dilution effect
From what was exposed, a dilution effect can result from two different events: (i) a
dilution in the river channel by mixture of the contribution of the concentrated water from the
groundwater and the contribution of diluted sources (overland flow, shallow throghflow); (ii)
a dilution in the watersoil/groundwater by new water infiltrating the soil before water arrives
into the river channel. It can be assumed that in the first case the dilution effect will be greater
than in the second case, because already diluted groundwater is again diluted by overland
flow or shallow throughflow. Also, in the first case, it can be expected that major ions load
will be relatively lower than in the second case, because overland flow / shallow throughflow
has less opportunity to aquire major ions due to the shorter residence time in contact with
soils and also because the soil is heavily leached. In agriculture soils with limestone addition
or vegetation burn practices it can be expected that overland flow increases major ions loads
or even concentrations.
From our results and all the above exposed, it is possible to hypothesize a general
pattern for water pathways in a dry-wet climate to explain the pH depression and the dilution
effect: during the dry season, when in the groundwater (water that saturates soils or rock
materials) the level is lower, only the saturated zone affects the stream water chemistry. River
water has higher pH, higher major ions concentrations, and low TSS. During the rainy season,
soil water is diluted and pH decreases, also the growing natural vegetation and agriculture
may increase soil acidity because they uptake base cations. Rainwater infiltrates into the soil
surface and translocates down the soil profile, raising the groundwater level and diluting the
55
saturated zone. This infiltrating water can carry with it solutes such as nutrients and pollutants
from the soil surface, but is still more diluted than groundwater. The new water that the
groundwater receives is more diluted and also more acidic. This new groundwater zone
permanently feeds the river waters during the rainy season and will feed the rivers during the
next dry season. Throughout the next dry season this new groundwater will be turning
increasingly basic due to the long residence time and the contribution of weathering
processes. To the acidity of this new groundwater also contributes the fact that rising the
water table the groundwater is in contact with the upper soil horizons, where acid neutralizing
processes are generally less effective than in the subsoil. So, during the rainy season river
water has lower pH, alkalinity and major ions, and also low TSS. However, river waters also
receive the contribution of overland flow. Overland flow will periodically (in rainfall events)
add water to the rivers, carring also sediments, nutrients (P and N), heavy metals, bacteria and
organic matter from the topsoils. Overland flow can be sometimes a dominant water pathway,
especially in disturbed regions or watershed sections where soils are saturated and the
groundwater is close to the soil surface. Organic matter decomposition with organic acids
release can lower pH. The intensity of all these processes will be dependent on rainfall
patterns. For example, if the rainfall is slight and the soil is dry, infiltration will be completed
and only the saturated zone near the stream channel will contribute to the river waters; if the
rainfall is concentrated and abundant, it will excede infiltration rates and generate overland
flow; finally if the rainfall is very high and continuous the discharges from subsurface water
will be higher.
Summarizing, during the rainy season there is a change in the hydrological pathway as
water becomes increasingly dominated by near surface / surface flow than by the deeper
subsurface flow. This hydrological shift implicates also in a river chemical shift where pH
depression takes place and sensitivity to acidification is increased due to low alkalinities
values.
At other scales other hydrologic patterns may appear and other factos may drive river
water quality. As an example, in the semi-arid region of Middle Jequitinhonha Valley
(northeast Minas Gerais) the groundwater is under a salinization process. As mentioned
previously (Chap.1), sample sites from the Mucurí and Jequitinhonha rivers have the highest
Cl/(Cl + HCO3-) of the State. They are located about 100 km from the Atlantic sea coast in
granite-gneiss rocks and within one of the hottest areas of the State. So, it is possible that
56
these sites undergo evaporation due to high temperatures, along with the sea NaCl vapour
contribution. In fact, studies (Menegasse et al. 2003) concluded that in this region the
following mechanisms prevail: precipitation → soil saturation → evaporation → solute
concentration → infiltration of the concentrated solution during the rainfall. So, this pattern is
opposite to our study general pattern, because water from the soils that arrives into the
groundwater is more concentrated (by evaporation) than the groundwater and not more
diluted than groudwater as in our general pattern.
In contrast with our study, in a disturbed watershed on highly weathered oxysoils in
the Brazilian Amazon (Markewitz et al. 2002) pH ranged from 4.5 in the dry season to 6.0 in
the wet season. Alkalinity, conductivity, Ca, Mg and K also showed higher values during the
wet season, following the pH pattern. The authors argue that the inputs of cations and
alkalinity to the stream water from soils are controled by biogeochemical cycling within the
plant/surface-soil component of the ecosystem. Here, a biologically-mediated process
replaced deep soil mineral weathering as the process that controls stream water chemistry.
Conclusions
This study suggests that in tropical regions with dry-wet climate, such as Minas Gerais
in southeastern Brazil, there is a strong climatic influence on variations in recharge, waterrock interactions, and residence time that reflects on the rivers and streams solute chemical
composition, affecting pH, alkalinity and major ions.
River waters show a natural pattern of ANC and pH depression mainly due to base
cation dilution. Acidic sites (pH <6) were found during the rainy season, hence the maximun
stress for the biota is produced during the rainy season when pH is at a minimun. Also, ANC
depression indicates an ecosystem with lower resilience, sensitive to acidification processes.
In contrast to this pattern, results also indicate that some sample sites are experiencing
alkalinization during the rainy season, and that this process may be due to antropogenic
influences like urban diffuse pollution sources, limestone mining activities, etc. This result
highlights that regions like Minas Gerais, which have a low alkalinity and a low pH pattern,
are not only sensitive to acidification but also to alkalinization of the surface waters. Brazilian
57
water quality legislation currently allows pH values ranging from 6 to 9. Such wide and
general standard makes legally possible to promote alkalinization or acidification of aquatic
ecosystems. Our study emphasizes the need to establish pH legal standards in accordance with
regional seasonal characteristics as well as to introduce standards for alkalinity.
The ANC dilution effect, and hence sensitivity to acification, is dependent on rain
precipitation waterflow paths. The increasing land use activities may eventually favour
overland flow and shallow throughflow/interflow. All these influences on the hydrologic
cycle could result in increasing major ions dilution, low buffer capacity and rivers
acidification with deleterious consequences to river ecosystems.
Models used to predict ecosystems sensitivity to acidification are based on soil
characteristics (Cinderby et al. 1998, Melfi et al. 2004). However, our results indicate that
bedrock may be an important parameter to consider, as stated by Bricker and Rice (1989), as
well as hydrologic pathways. In some ecosystems, it may be also important to consider the
plant/surface-soil interactions, as shown by Markewitz et al. (2002).
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61
Capítulo 3
Influência do clima úmido-seco nos padrões de P e N,
nutrientes limitantes e estado trófico em rios tropicais
INFLUENCE OF DRY-WET CLIMATE ON P AND N
PATTERNS, LIMITING NUTRIENTS, AND TROPHIC STATE IN
TROPICAL RIVERS
Trabalho a ser apresentado à revista The Science of the Total Environment
62
Capítulo 3
Influence of dry-wet climate on P and N patterns, limiting nutrients and
trophic state in tropical rivers
Introduction
Global nutrient cycles have been greatly altered by land-use changes over the last
century. Studies in the temperate zone suggest that river export of nitrogen has increased by
3- to 20-fold in developed areas since industrialization (Howarth et al. 1996, Tilman 2001,
Foley et al. 2005, Martinelli and Howarth 2006). Large-scale land-use changes are
occurring in the tropical Americas, and these have important implications for the future of
freshwater and marine ecosystems. Freshwaters ecosystem alteration in the tropics is
increasing even more rapidly than changes impacting temperate zones. Downing et al. (1999)
also mention that tropical lands are most frequently converted to agricultural or degraded
landscapes, which have dramatically altered regional hydrology, and are subject to large
losses of nutrient elements such as nitrogen or phosphorus. It is likely that tropical aquatic
ecosystems will frequently experience increased silt, water, and nutrient loads as land-use
perturbations proceed.
Phosphorous and nitrogen pollution have been pointed as the major agents in the
eutrophication processes in rivers and streams. Eutrophication is the process of an ecosystem
becoming more productive by nutrient enrichment. Although this term is most commonly
applied to freshwater lakes and reservoirs, it also have been applied to flowing waters (Hilton
et al 2006), estuaries, and coastal marine waters (Jong 2006, Orive et al. 2007). The
increasing eutrophication is a main concern topic because it can affect the ecological health of
rivers and also prevent human use of water resources (Caraco 1993, Carpenter et al. 1995,
Wetzel 2001 and may others).
In Minas Gerais, as in the rest of Brazil, freshwater ecosystems researchers have paid
much attention to lakes and reservoirs and there is little information about rivers and streams.
Hence, it is urgent to understand the eutrophication process in tropical rivers and streams as
63
well as to develop criteria for setting nutrient levels in fluvial ecosystems. In this way,
improved water quality regulations could help water policy makers and ecosystem managers.
In order to predict the effects of N and P inputs on receiving waters, it is necessary to
be able to predict how water body nutrient concentrations vary as the external N and P inputs
are changed. In this study we examine the seasonal variation of phosphorus, nitrogen and N:P
ratio across Minas Gerais State, as well as the implications for the trophic states of rivers.
Materials and Methods
Study Area
The state of Minas Gerais covers approximately 587,000 km2 (7 %) of Brazil, and is
located in the southeastern region, between the parallels 14º13’58”S and 22º54’00”S, and the
meridians 39º51’32”W and 51º02’35”W. In 2000, the total population of 17,891,494 was
strongly concentrated (82%) in urban areas. The climate is characterized by two well-defined
seasons: a dry period (April to September) whose maximum coincides with the lowest winter
temperatures in July, and a rainy period (about 200 mm/month from October to March) in
which the heaviest rains coincide with the highest temperatures, around December-January
(Fig.1). Mean annual precipitation is around 700 and 1000 mm, while in the highest altitudes
can reach 1200 –2500 mm.
The main soils (88% of Minas Gerais area) are latisols (42%), podzolic soils (21%),
cambisols (17%), and litosoils (8%). Latisols are highly weathered and virtually without the
primary minerals or the secondary minerals less resistant to weathering and they have low
cation exchange capacity. Generally, they are very deep and strongly acid. Podzolic soils are
less deep than latosols and deeper than cambisols. The presence of a clay horizon decrease
permeability promoting overland flow and erosion. Cambisols and litosols are young soils,
shallow and poor developed, with primary minerals less resistant to weathering. Almost
always they are highly acidic soils. The high content of silt and shallow characteristics of
cambisols make these soils with very low permeability and erosion risk. Litosols are very
shallow soils over rocks substrates and usually they are associated with cambisols and rocky
scarps (Embrapa 1999, Guerra and Cunha 1998). Rainforests predominate in the granite-
64
gneiss areas, and savannah vegetation cover (cerrado) in the others, although only 34% is left
of the original vegetation.
Figure 1. Climate diagram of Minas Gerais.
Data from PUC Minas (2002).
2. Sample Sites and Analytical Methods
The monitoring of surface water quality is the responsibility of the Minas Gerais
Institute of Water Management (IGAM). The monitoring has 244 long-term sampling sites in
which 40 water quality parameters are measured. IGAM performs four annual sampling
campaigns, corresponding to summer (rainy season), autumn, winter (dry season) and spring.
However, in this report we will only consider the summer (January, February and March) and
winter (July, August and September) data. Samples are taken preferentially from the rivers´
main channels, since most of the sampling points are located on bridges. This report used the
available database of the year 2000 (IGAM 2000), except for 8 sample sites which did not
have all the data for the two seasons, totalizing 236 sample sites (Fig.2). This study
considered total phosphorus data and organic nitrogen, ammonia, nitrite and nitrate. Total
nitrogen was considered as the sum of all the nitrogen fractions above. Analytical methods are
in Table.1.
65
Figure 2. Sample sites of Minas Gerais major rivers (IGAM 2002).
Table 1. Analytical methods for Total Phosphorus and Nitrogen
Parameter
Total phosphorus
Analytical Method
APHA 4500-P C
Unit
mg/l P
Ammonium
ABNT NBR 10560/1988
mg/l N
Nitrate
APHA 4500-NO3- E
mg/l N
Nitrite
ABNT NBR 12619/1992
mg/l N
Total organic nitrogen
APHA 4500-Norg B
mg/l N
Total suspended solids ABNT NBR 10664/1989
mg/l
66
3. Limiting Nutrients
Vollenweider (1982) proposed for temperate lake phytoplancton a TN:TP ratio = 9:1
(by weight), where lake systems with values above 9 are considered to be potentially limited
by phosphorus. This ratio was used by Salas and Martino (2001) to propose a methodology
to evaluate eutrophication in tropical lakes, and was adopted in this study.
4. Trophic Status
The classification of the trophic state is an approach for determining the eutrophication
by human activities (Dodds 2002, 2007). Recently, Dodds (1998) proposed a trophic
classification method for temperate rivers using chlorophyll, total nitrogen and phosphorus
for rivers and streams. This classification method was applied in our study. In this trophic
classification the boundary between oligotrophic and mesotrophic was considered as: TN =
700 and TP = 25 μg.l-1, and the boundary between mesotrophic and eutrophic as TN = 1500
and TP = 75 μg.l-1.
Results
1. Phosphorus and Nitrogen
Phosphorus and nitrogen concentrations are highly dependent on land use and human
population. Major sources of P and N to surface waters results primarily from agriculture
(fertilizers and manure), industries and urban activity (sewage and drainage systems)
(Carpenter et al 1998, Caraco and Cole 1999, Khan and Ansari 2005). Point sources, like
urban sewage, are a relatively constant supply of nutrients and high flows will dilute while
low flows will concentrate the material. On the other hand, if heavy rainfall flushes the
materials from the terrestrial soils, including agricultural fertilizers, concentrations could
increase at high flows (Allan 1996).
1.1. Total Phosphorus (TP) and Total Nitrogen (TN) concentrations
Total phosphorus (TP) and total nitrogen (TN) varied widely. TP ranged from 0.010 to
3.910 mg.l-1 (mean = 0.192 ± 0.304) during the wet season and from 0.010 to 6.130 mg.l-1
67
(mean = 0.152 ± 0.546) during the dry season, and TN ranged from 0.21 to 11.64 mg.l-1
(mean = 1.12 ± 1.25) in the wet season and ranged from 0.21 to 24.12 mg.l-1 (mean = 1.69 ±
3.66) during the dry season. Both TP and TN showed higher maximum values during the dry
season (Table 2). The total phosphorus concentrations varied by about 613-fold during the dry
season and only 391-fold during the rainy season. Total nitrogen varied less than phosphorus,
115-fold during the dry season and 55-fold during the wet season. There was no correlation
between TP and TN.
Table 2. Total phosphorus and total nitrogen mean values during the
dry and the rainy season (mg.l-1).
Total Phosphorus
dry
rainy
Min
Max
Mean
SD
0.010
6.130
0.152
0.546
0.010
3.910
0.192
0.304
Total Nitrogen
dry
rainy
0.21
24.12
1.69
3.66
0.21
11.64
1.12
1.25
Figure 3 shows TP and TN histograms during both seasons. For TP, during the dry
season the majority of the sites were in the first categories (0.010 – 0.050 mg.l-1) and during
the wet season the majority of the sites were in higher categories (0.100 – 0.500 mg.l-1). For
TN, during the dry season the majority of the sites were in the first categories (0.21 – 1.00
mg.l-1), but during the rainy season the major increase was only in the second category (0.50 –
1.00 mg.l-1).
Figure 3. Total phosphorus (TP) and total nitrogen (TN) histograms
68
Figure 4 shows the different seasonal behavior of TP and TN at each sample site.
During the rainy season the majority of the sample sites showed higher TP concentrations
than during the dry seasons. On the other hand, similar numbers of sample sites were found
for TN during both seasons. Considering at each site both, TP and TN concentrations, Figure
5 highlights that during the rainy season 45% of the sample sites showed a TP and TN
increase, 32% of the sites showed a TP increase and TN decrease, 17% decreased TP and TN
and 6% of the sites showed a TP decrease and a TN increase.
Figure 4. N and P concentration behaviour
1.2. Nitrogen
forms
during
the dry and
the rainy season.
Figure 5. N and P concentration behaviour
together at each sample site.
Figure 6a shows the average concentrations for the main TN chemical forms. During
the dry season, ammonia dominates TN and during the rainy season there is a shift to Norganic dominance. Figure 6b shows that N-org and ammonium had more sample sites with
higher concentrations during the rainy season than nitrate. Nitrate had an opposite behavior
with more sample sites with higher concentrations during the dry season. Figure 7 shows that
during the dry season the highest TN values are due to ammonia and N-organic while during
the rainy season are due to ammonia, but N-organic dominate the lower and intermediate
values, indicating clearly that during the rainy season there was a high input of N-organic into
the rivers.
69
Figure 6a. (left) Nitrogen forms average concentrations during the dry and the rainy seasons.
Figure 6b. (right) Number of sample sites where nitrogen forms shows higher concentrations
during the dry and the rainy seasons.
Figure 7. Nitrogen forms according to increasing Total Nitrogen during the dry and the
rainy season.
1.3. TN and TP concentrations and the legal criteria
1.3.1. Total Phosphorus legal criteria
In non-polluted natural waters, total phosphorus concentrations extend over a very
wide range from <0.001 mg.l-1 to more than 200 mg.l-1 in some closed saline lakes and the
concentrations of most uncontaminated surface waters are between 0.01 mg.l-1 to 0.05 mg. l-1.
70
Variation can be related to characteristics of regional geology. Phosphorus levels are
generally lowest in mountainous regions of crystalline bedrock and increase in lowland waters
derived from sedimentary rock deposits (Wetzel 2001). Berner and Berner (1996) mention
that natural dissolved total phosphorus levels are low in rivers (0.025 mg l-1). According to
Weiner (2000) P dissolved concentrations in the rivers are generally in the range of 0.01-0.1
mg l-1 and seldom exceed 0.2 mg l-1.
When considering TP mean values (dry = 0.152; rainy = 0.192), during the dry season
values were according to the range observed by Weiner (2000) and during the rainy season
values were higher. When considering the range 0.01 mg.l-1 to 0.05 mg. l-1 for non-polluted
rivers (Wetzel 2001), 30% fall in this range during the dry season and only 23% were during
the rainy season. When considering the value 0.025 mg l-1 (Berner and Berner 1996), during
the dry season only 30% of the sample sites were under this limit, and during the rainy season
this figure dropped to only 19%. Whichever the criteria, the results show that rivers and
streams in Minas Gerais are highly polluted by phosphorus during both seasons, and
particularly during the rainy season.
Minas Gerais legal standards establish 0.025 mg.l-1 as the highest allowed
concentration for total phosphorus, coinciding with Berner and Berner (1996) values for
unpolluted sites. Dodds (1998) also considered 0.025 mg.l-1 as the boundary value between
oligotrophic and mesotrophic river waters. According to this, Minas Gerais and Brazilian laws
use very strict pollution criteria with the objective of maintaining an oligotrophic TP status in
rivers and streams. So, our results indicate that rivers undergo a strong pollution process by
phosphorus, but also indicates that it is necessary to study the natural background values
across Minas Gerais, as there are many regions with phosphatic rocks and mining, like those
in the municipalities of João Pinheiro, Tapira, Coromandel, Lagamar, Patos de Minas,
Felixlândia, Cedro do Abaeté, and Araxá.
1.3.2. Nitrogen legal criteria
Wetzel (2001) presents the average values of nitrogen forms for unpolluted world
rivers. Calculating TN from Wetzel figures, a value of 0.4 mg.l-1 (Table 3) was found for this
71
study. Dodds (1998) considered a higher value (0.7 mg.l-1) for the boundary between
oligotrophic and mesotrophic TN status. Our results show TN average values about three
times higher (1.7 and 1.1 mg.l-1 for the dry and the rainy seasons) than the Wetzel value for
unpolluted sites.
Table 3. Average Nitrogen forms concentrations during the dry and the rainy seasons.
mg.l-1 (min – max)
Wetzel (2001)
This study
dry season
rainy season
NH4 –N
NO2 - N
NO3 - N
Dis. N-organic
* total N-organic.
0.018 (0.005 – 0.04)
0.0012
0.101 (0.05-0.2)
0.260 (0.05-1.0)
0.9 (0.1 - 22.5)
0.019 (0.001 – 0.310)
0.28 (0.01 – 1.53)
0.5 (0.1 – 11.1)*
0.3 (0.1 – 8.3)
0.015 (0.001 – 0.199)
0.16 (0.01 – 1.08)
0.6 (0.1 –3.2)*
According to Minas Gerais water quality standards in 2000, there was not mention to a
criterion for TN or organic nitrogen, but do have criteria for nitrate (10 mg.l-1), nitrite (1 mg.l1
), and ammonium (1 mg.l-1). The nitrate criterion seems to be established with relation to the
human health: concentrations above 10mg.l-1 in drinking water are undesirable because they
can cause methemoglobinemia (Stumm and Morgam 1996, Baird 1999).
Our results show that the TN average values are ten times lower than the legal limit. In
our nitrate data, the highest values were 1.08 mg.l-1 for the dry season and 1.53 mg.l-1 for
rainy season. These are very low values (an order of magnitude) when compared with the
legal threshold, and indicate that a revision of this legal criterion is necessary from the aquatic
ecosystem point of view. Considering that Dodds (1998) adopted the eutrophic TN boundary
as 1.5 mg.l-1, it can be concluded that while law maintain TP in an oligotrophic status, the law
allows TN to reach a eutrophic status, or even a higher status, because there are no criteria for
organic nitrogen.
1.4. Discharge versus TSS and TDS
The available discharge (m3.s-1) data for 116 sample sites was analysed. Discharges
varied from 1 m3.s-1 to 1319 m3.s-1. Sample sites had an average discharge value 4.3 times
greater during summer (rainy season) than during winter (dry season). During the dry season
72
Total Dissolved Solids (TDS) were dominant while during the rainy season Total Suspended
Solids (TSS) prevailed. Average TDS concentrations remained with similar values in both
seasons (dry = 65 mg.l-1 and wet = 68 mg.l-1), while TSS increased from 22 mg.l-1 during the
dry season to 271 mg.l-1 during the rainy season. TDS concentrations are in agreement with
the values for South America (69 mg.l-1) according to Livingstone 1963 as cited by Gregory
and Walling 1973 and Wetzel 2001. These are low values when compared with extratropical rivers. TSS also showed low concentration values when compared to the estimated
average world value (535 mg/l) (Berner and Berner 1996).
Figure 8 shows TSS and TDS from our results and from the Gems/Water
Programme Report (2002) world rivers data base. Minas Gerais exhibits high TSS values
and very low TDS values when compared with North America and Europe, and this result
may reflect a higher erosion process in Minas Gerais, as discussed in Chapter 2.
Figure 8. Minas Gerais and World Total Suspended Solids (TSS) and Total Dissolved
Solids (TDS).
World data calculated from Gems/Water Programme Report (2002) data base.
TDS loads increased from 2183 g.s-1 during the dry season to 9117 g.s-1 during the
rainy season; annual average was 5650 g.s-1 (178,178 ton/year). TSS loads increased from 658
g.s-1 during the dry season to 43,250 g.s-1 during the rainy season; annual average was 21,954
g.s-1 (692,341 ton/year). TDS loads increased 4.1 times, according with the river discharge
increase of 4.3 times, while TSS increased 66 times. This is consistent with the world river
pattern where dissolved loads do not vary as greatly as suspended sediments (Gregory and
Walling 1973).
73
These values indicate a great influence of TSS in river ecosystems during the rainy
season. During the rainy season TSS and TDS showed a high linear correlation (r2 =0.80), but
during the dry season they were not correlated and also showed a low correlation after log-log
transformation (r2 = 0.36).
Latrubesse and Sinha (2005), studying the available data for tropical rivers, found
that rivers draining platforms or cratonic areas in savanna and wet tropical climates are
characterized by low sediment yield; and that rivers draining platforms, cratonic areas or a
combination of different geological domains in savannas or mixed savanna/forest
environments as Paraná, show lower sediment yield when compared to tropical mountain
rivers, but higher values when compared with rainforest cratonic plateau rivers. The authors
found a logarithmic relation between mean annual discharge (m3/s) and sediment yield
(t/km2/yr) and mentioned that this logarithmic trend shows that sediment yield is quite
variable for medium size basin but decreases sharply for large rivers. Considering that water
discharges are related to drainage area, the logarithmic relationship is an indicator of the
influence of a natural threshold of sediment yield for fluvial systems.
Our data also showed a logarithmic trend, but it was between discharge (m3.s-1) and
loads g.s-1, as we do not have the contribution area for each sample site to calculate the yields
(Fig.9). During the rainy season there was a clearer trend than during the dry season, and there
was a river discharge threshold about 200 m3.s-1. Below this threshold, there was a high
variability in TSS values but above it, TSS values remained more or less constant, at about
100 kg.s-1 - 300 kg.s-1 range.
Figure 9. Relationship between TSS and river discharges during the dry and the rainy seasons.
74
Regarding TDS, a strong linear relationship was found between river discharge and
TDS during both the dry and the rainy season (Fig.10).
Figure 10. River discharges and Total Dissolved Solids (TDS) during the dry and the rainy
season.
1.5. TP, TN and N-forms loads
TN average loads were 50.63 g.s-1 and 168.43 g.s-1 during the dry and the rainy
seasons respectively, and TP average loads were 3.41 g.s-1 and 29.43 g.s-1. TP increased 8.6
times while TN increased 3.3 times. TN loads increased 3715 ton/year and N-org increased
2489 ton/year, which correspond to 67% of the increment for TN, demonstrating that during
the rainy season there was a disproportional high input of N-organic into the rivers.
Figure 11. Total Phosphorus, Total Nitrogen and N-forms average loads during the dry
and the rainy seasons.
75
1.6. Loads correlations of TSS, TDS, TP, TN and N-forms
Table 4 shows the correlations between TSS, TDS, TP, TN and N-forms. During the
rainy season there were strong linear correlations between TSS and TP, TN and N-organic,
and to a lesser degree with ammonium and nitrate. TDS showed a different behaviour because
it showed correlations with nutrients only after a log-log transformation. During the dry
season both TSS and TDS needed to be transformed. TSS showed low correlations and TDS
showed better correlations. So, as happened with water discharge, TSS and TDS showed
different types of correlations, which may indicate different processes on course. TN showed
strong linear correlations with N-org and nitrate during the dry season and also during the
rainy season.
Table 4. TSS, TDS, TP, TN and N-forms loads correlations.
TSS x TP
TSS x TN
TSS x N-org
TSS x NH4+
TSS x Nitrate
TDS x TP
TDS x TN
TDS x N-org
TDS x NH4+
TDS x Nitrate
TN x N-org
TN x NH4+
TN x Nitrate
•
Phosphorus
Dry
0.46 log -log
0.59 log-log
Rainy
0.88
Nitrogen
Dry
Rainy
0.39 log-log
0.41 log-log
0.33 log-log
-
0.77
0,85
0.56*
0.47
0.65 log-log
0.68 log-log
0.41 log-log
0.69 log-log
0.80 log-log
0.78 log-log
0.63 log-log
0.76 log-log
0.68 log-log
0.86
0.62
0.87**
0.91
0.41
0.76
without the five sample sites with highest concentrations, ** without the sample sites
with highest TN loads (above 200 g.s-1)
Figure 12 shows the relationship between TN, TP and total suspended solids (TSS)
loads for the sites where the TN and TP loads increased during the rainy season (102 and 110
sample sites respectively). The TN and TP loads increments were considered as being the
differences between rainy and dry seasons. Figure 10 shows that there was a high correlation
between TP and TSS increments (r2 = 0.88) and between TN and TSS increments (r2 = 0.81),
suggesting the presence of P and N particulate matter during the rainy season.
76
Figure 12. Relationship between TSS load increment versus TP and TN load increment
Increment = (loads rainy – loads dry season).
1.7. Major phosphorus sources during the rainy season
Mainstone and Parr (2002) mention that during the dry season phosphorus is
dominated by sewage point sources with highly bioavailable and readily degradable organic
matter. During the rainy season total phosphorus is generally delivered in surface run-off
attached to soil particles. However, soluble phosphorus occurs when livestock excreta or
soluble inorganic fertilisers are washed off the land soon after application. Sub-surface
drainage and leaching may be important pathways particularly if the soil is overloaded with
phosphorus. Sandy soils and underlying sandstone geology are particularly vulnerable since
they have a very low adsorption capacity for phosphorus. Other soils and geologies are less
vulnerable, but may be more at risk due to macropore and fissure flow within the soil-rock
structure.
From the author above, three factors mentioned are particularly important in the Minas
Gerais case: (i) surface run-off attached to soil particles (ii) livestock excreta and (iii) soluble
inorganic fertilisers washed off the land soon after application. Minas Gerais has 62% of its
territory covered by pastures for cattle (beef cattle herd = 21 million) in extensive
management. Land use is dominated by pastures representing 62% of Minas Gerais area.
Also, only 5% of about 3 million m3 of sewage effluents are treated before released to rivers,
77
and is possible that river sediments acts as a sink. In Brazil, there was an increase rate of 6.5%
of nutrient agriculture use from 1970 to 2000, equivalent to 27 kg/ha in 1970 to 129 kg/ha in
2000 (Isherwood 2000). The data for Minas Gerais indicated that in 2000 were sold 280,000
tons of N, and 300,000 tons of phosphate as P2O5 with an average use of 43 kg/ha of
nitrogen and 52 kg/ha of phosphate (IBGE 2003).
1.7.1. Total Suspended Solids and Total Phosphorus
Phosphorus, besides participating in the biochemical structure of several molecules, is
the main element in ATP formation, responsible for the energy storage and transportation in
the cellular metabolism, particularly of the photosynthesis. But the availability for the biota
depends on its biogeochemical cycle characteristics. Unlike nitrogen, phosphorus does not
have a gaseous phase, and its cycle is linked to the lithosphere, where it tends to form
chemical compounds relatively insoluble in water, not being easily weathered nor released to
the hydrologic phase. Generally it occurs in the phosphate form combined with calcium,
potassium, magnesium or iron. These minerals are poorly soluble in water, and phosphorus
becomes available slowly through soil particles and rock weathering (Botkin and Keller
2000).
Even when soluble phosphate is released by weathering, phosphorus is usually quickly
tied up in the soil as iron, aluminum and calcium phosphates or by clay minerals to produce
insoluble forms (Berner and Berner 1996). The typical total P concentrations in soils range
from 300 to 1200 mg/kg and is composed of solid and solution phases, with the majority
(>99%) in solid phase. The solid phase is composed by organic P, Fe, Al and Ca phosphates,
and also P sorbed to surfaces of Fe and Al oxides. The solution phase is mostly
orthophosphate (H2PO4-) and small amounts of dissolved organic P or P bound to colloidal
organic matter and Fe oxide. A fraction of the solid phase (<25%) is in dynamic equilibrium
with the solution phase (sorption-desorption, precipitation-dissolution, immobilizationmineralization reactions) and is termed labile P, phytoavailable or bioavailable P; where the
total bioavailable P in a water sample is the sum of the dissolved inorganic P and the
bioavailable sediment P (Ward and Elliot 1995).
Chemical reactions with soil minerals play a key role in controlling phosphorus
availability in soils. Phosphate forms are controlled by pH, where H2PO4- dominates in acid
78
soil solutions. This is the less charged chemical form of phosphate and is the more mobile in
soil, and therefore the more available to biota. At low pH, also Al, Fe and Mn are soluble and
react with H2PO4- to form insoluble compounds. At low pH, phosphorus also can be sorbed
onto the surfaces of clays and oxides of iron and aluminum, initially by anion exchange and
afterwards by covalent bonds with metals on the mineral surface. This is why highly
weathered soils as Oxisols and Utilsols have extremely low phosphorus availability and why
forests on those soils are typically P-limited. On the other hand, at high pH in soils with high
concentrations of exchangeable calcium and calcium carbonate, phosphate combines with Ca
to form insoluble precipitates. Due to this phosphate characteristic at low and high pH,
phosphate is only available in a narrow range about pH 6.5. Organic matter can compete with
phosphate ions for binding sites on the surface of oxides and also can chelate metals and
prevent their reaction with phosphate, decreasing phosphorus fixation (Chapin III et al.
2002).
The strong binding of phosphate to organic matter or minerals in most soils results in
that 90% of the phosphorus loss occurs through surface runoff and erosion of particulate
phosphorus rather than through leaching of soluble phosphate to groundwater (Tiessen 1995).
Phosphorus generally enters aquatic ecosystems sorbed to soil particles that are eroded
into lakes, streams, and rivers. Much of this runoff occurs during major erosion-causing
storms. Potential P pollution of aquatic ecosystems is thus strongly influenced by watershed
land use and the concentration of P in watershed soils: any factor that increases erosion or the
amount of P in the soil increases the potential P runoff to downhill aquatic ecosystems.
Dissolved losses can be significant in some soils, especially if the iron (Fe), aluminum (Al),
and calcium (Ca) absorption capacity of the soil is saturated, allowing P to move more readily
through the soil toward aquatic ecosystems. Of particular concern is that large amounts of soil
P can be mobilized by exceptional precipitation and erosion events or by changes in land
management practices (Bennett et al. 2001).
Our results evidenced that during the rainy season TP had higher concentrations on
average, as well as the majority of the sample sites also showed higher concentrations. Also,
the load increased during the rainy season in the order of 8.6 times (an increase of 829
ton/year). TSS were moderately correlated with TDS during the rainy season. Loads showed a
strong linear correlation with TSS and a log-log relationship with TDS. These results suggest
79
that during the rainy season there was a transport of phosphorus in a soluble phase and also in
a particulate phase, binded to organic matter or minerals. The majority of the soils in Minas
Gerais are highly weathered acid soils with a strong presence of Fe and Al and in many
regions there are calcareous rocks, so the biogeochemical environment favours P soil fixing,
as Chapin III and colleagues mentioned. Minas Gerais dry-wet climate with high precipitation
concentrated in few months have a high erosivity that results in overland flow and erosion in
agriculture areas and bare soils, as well as in urban runoff. During the rainy season, as the
level of the water in the river channel rises, rivers can also receive inputs of organic matter
from the margins in the flooded area, as well as resuspend sediments with P storage from the
river bed. As the load during the rainy season increased by the order of 8.6 times, it is
probable that this high increment be the result of fertilizers carried to the rivers by overland
flow / shallow interflow from agriculture areas.
The particular behaviour of the Brazilian soils with high P binding capacity due to Fe
and Al oxides could diminish the adverse impact intensity in aquatic systems because P
remains binded to the soil particles in the water body and not in solution (Isherwood 2000).
However, chemicals such as P that appear to be sequestered in the soil may later be mobilized
by a geologic, hydrologic, chemical, or climatic event such acid precipitation or heavy
summer thunderstorms. P can also be mobilized by changes in land-management practices or
some unpredictable mechanism. In this sense, P accumulation in the watershed can be viewed
as what Stigliani (1991) and others term a ‘chemical time bomb’ in the soil (Bennett et al.
1999).
Bennett et al. (2001) using a global budget approach estimated the increase in net P
storage in terrestrial and freshwater ecosystems to be at least 75% greater than pre-industrial
levels of storage. The agricultural mass balance indicated that a large portion of this P
accumulation occurs in agricultural soils and that the rate of P accumulation is decreasing in
developed nations but increasing in developing nations. Developing countries are responsible
for about 60% of the world consumption.
1.7.2. Livestock excreta
Considering that each bovine produces 24 kg of manure by day with 0,55% of N and
0,25% of P2O5 (Isherwood 2000), could be estimated that 21 million of beef cattle herd
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produces 651,420 tons of nitrogen and 128,310 tons of phosphorus available by year in
pastures for overland flow transport to rivers in Minas Gerais. In fact, these numbers indicate
that livestock excreta are a relevant potential source of diffuse pollution for rivers.
Coliforms are indicators of animal pollution. If livestock excreta were washed off by
overland flow during the rainy season, it is expected that total coliforms and fecal coliforms
concentrations increas during the rainy season. Further analysis of our data base showed that
in 73% of the sample sites there was an increase in both total and fecal coliforms
concentrations.
Jamieson et al. 2004 mention that the primary sources of microbial pollution in
agricultural watersheds is fecal matter from livestock production from point sources such as
storage facilities and feedlots, and from non-point sources such as grazed pastures and
rangelands. Fecal contamination of watercourses from organic waste in agricultural runoff is
an important, but often overlooked, aspect of freshwater pollution (Rodgers et al. 2003). The
transport of pathogens from non-point sources areas to rivers and streams is linked to variable
factors which affect runoff, like timing of manure application, areas where manure is applied
in the soil surface or incorporated to the soil, areas where manure is directly deposited by
livestock, non-manured areas, access of livestock to streams, livestock confinement and
grazing schedules, location of feedlots, etc. (Walker et al. 1990, Edwards et al. 1997).
As Jamieson et al. (2004) also mention, the literature indicate that the majority of
enteric bacteria in soils and aquatic systems are associated with sediments and that these
associations influence their survival and transport characteristics. Extended survival patterns
have been noted for bacteria that have attached to sediment particles and settled to the bottom
of streams and lakes, and several studies have shown that concentrations of indicators
organisms are typically higher in sediment as opposed to the overlying water column in both
marine and freshwater systems. It has been proposed that enteric bacteria can survive longer,
and possibly grow within stream bottom sediment. The survival of fecal bacteria in sediments
is primarily attributed to the availability of soluble organics and nutrients as well as to the
increased protection from predatory protozoans. The disturbance of the stream bottom
sediment (storms, cattle entering the streams, etc.) can play a significant role in elevated water
column fecal coliforms concentrations. Thus, our results indicate that livestock excreta were
carried by overland flow and may be contributing with TN and TP increasing loads during the
81
rainy season. In Minas Gerais not only the excreta could be carried from pastures to rivers by
overland flow but also is common that cattle drink water in rivers and streams and rest at the
margins of the rivers, then also resuspended bottom sediment can contribute to TN and TP
increasing loads.
1.7.3. Fertilizers
The P sources used as fertilizers can be classified as: (i) ground rock phosphate (RP),
(ii) totally acidulated (TA), (iii) thermophosphate (TP). The TA fertilizers are highly soluble
in water and the most used worldwide. The RP and TP are generally lower in cost per unit of
P, less water-soluble and effective under specific conditions of soil management (Prochnow
2006). TA fertilizers are simple superphosphate, triple superphosphate, monoammonium
phosphate (MAP), diammonium phosphate (DAP).
Brazil produces about 53% of nitrogen, 68%of phosphorus and 15% of potassium used
in fertilizers. Phosphate fertilizers used in Brazil are the highly soluble TA fertilizers, mainly
in the form of MAP (NH4H2PO4) (43%), single superphosphate (Ca(H2PO4)2.2H20+CaSO4)
(30%) and triple superphosphate Ca(H2PO4)2.2H20 (15%) (Isherwood 2000).
When good phosphatic rocks are used, the TA fertilizers obtained present high water
solubility and readily for plant use. In contrast, when are used rocks with low quality, with
significant amounts of cation impurities as Fe and Al, other compounds can be produced and
present low water solubility. This is particularly true for Brazilian phosphatic rocks which
present low P and high cationic impurities concentrations. Considering this, the Brazilian
fertilizers law established a criterion of high soluble P contents (Prochnow 2006). So,
phosphate fertilizers can contribute to river waters with insoluble and soluble P forms as well
as ammonium ions.
Figure 13 shows the phosphorus cycle in soils, considering the inputs from fertilizers
and biota residues. As seen before, during the rainy season TP is strongly related to TSS. TP
related to TSS can be in the form of inorganic or organic origin and not readily available to
the biota. But TP is also related to TDS, and both fertilizers and soil organic matter could
contribute to the soluble P-form pool in soils and thus be readily available to the aquatic biota.
82
Figure 13. Phosphorus cycle in soils. Modified from Havlin et al. 1999
A data base with a detailed P forms, differentiating organic, inorganic, soluble and
insoluble phosphorus would permit identify better the sources and pathways of P pollution
and their relative contributions to the aquatic systems.
1.7.4. Sewage and urban runoff
Urban runoff has been pointed as one of the major sources of pollution of rivers and
lakes in Europe and USA. However, there is little understanding of the way that urbanization
contributes to this problem and in developing countries much attention has been paid to urban
point source pollution control but not to urban runoff control (Ellis 1986, Tsihrintzis and
Hamid 1997, Taebi and Droste 2004, Carle et al. 2005).
Jarvie et al. (2005) examined the interplay between dissolved and particulate P in
southern England. They examined riverbed sediments for a wide range of agricultural
subcatchments and main river sites to see whether or not bed sediments act as sources or sinks
of soluble reactive phosphorus - SRP under low flows and times of greatest eutrophication
risk. Rivers had elevated SRP linked to sewage effluent discharges and at these sites bed
83
sediments acted as SRP net sinks. In contrast, bed sediments acted as net sources of SRP
when sewage inputs were subject to large hydrological dilution by water of low SRP
concentration. The critical importance of sewage sources of P was emphasised for agricultural
regions.
Thus, it is possible that some rivers that receive sewage P loads during the year also
receive inputs of soluble P loads from bed sediments during the rainy season.
The probable main sources of nitrogen and phosphorus are shown in Fig. 14. During
the dry season sewage organic matter contributes with easy solubilized N and P. During the
rainy season overland flow contributes with P and N suspended solids from soil erosion and
flushed organic matter. Overland flow also carries soluble nutrients from livestock excreta
and fertilizers. Sewage effluents also contribute with P and N during the dry season, but are
subject to a dilution by increased river discharge.
Figure. 14. Sources of phosphorus and nitrogen pollution.
84
1.8. Nitrate and N-organic in large rivers
The origin of nitrogen in river water is complex because nitrogen exists as a major
constituent of atmosphere and is also an essential component of living tissues. N cycle is not
linked to the lithosphere because nitrogen minerals are rarely found in nature and also are
readily soluble. Nitrogen as an atmospheric gas must be fixed by microorganisms in order to
be used by plants and organisms.
In the terrestrial nitrogen cycle there are three major land inputs: biological fixation,
precipitation and dry deposition of the previously fixed nitrogen, and application of fertilizers.
Nitrogen is commercially recovered from the air as ammonia, which is produced by
combining nitrogen in the atmosphere with hydrogen from natural gas. Ammonia is then
converted to other nitrogen compounds, the most important of which are urea (NH2CONH2),
nitric acid (HNO3), ammonium nitrate (NH4NO3), and ammonium sulfate (NH4)2SO4. With
the exception of nitric acid, these compounds are widely used as fertilizer. When plants or
animal die, the organic matter releases ammonia, part of which is dissolved in soil water as
NH4+ and part of which escape from the soil as NH3 (g). In the soil, NH4+ is also retained by
ion exchange or clays, and excess of NH4+ can be washed into the rivers. In the presence of
O2, NH4+ is oxidized to NO2- and them to NO3-, which is not retained in the soil and is readily
eluted to groundwater. Plants convert dissolved NO3- and NH4+ (from fertilizer, rain, or
recycling of organic matter) into plant organic matter (Berner and Berner 1996, Stumm and
Morgam 1996).
Most of the organic matter is close to the soil surface. About of 95% of the soil
nitrogen and 25% of the phosphorus are contained in the organic matter (Pimentel and
Kounang 1998), or in the case of ammonium ions, it can be sorbed by clays and organic
matter. In these forms they are immobile and not available to plants. Nitrogen is lost form the
soil primarily by erosion, crop harvesting and nitrate leaching. The immobile forms (organic
and part of ammonium) can be converted to mobile nitrate. Also, a part of ammonium may
exist in soil water as free ammonium ions which are also mobile. In high pH calcareous soils,
the ammonium ion is converted to gaseous ammonia that can volatilize from soils or be
dissolved in soil water. The mobile forms are available to plants and can be transported by
soils water and infiltrated into groundwater (Novotny 2003).
85
Nitrogen retention within ecosystems is dominated by biological processes (plant
uptake and microbial immobilization). When these are disrupted by intense or prolonged
disturbance such as cultivation, the initial response is nitrogen mobilization leading to a large
increase in nitrogen availability that can support a burst of “pioneer” agriculture. With
continued cultivation, however, a large proportion of total soil nitrogen eventually is lost to
harvest, erosion, leaching to streamwater and groundwater, and volatilization and
denitrification. The residual nitrogen is in organic forms that are highly refractory to
decomposition (Vitousek and Howarth 1991).
As in soils, when ammonium is in the water in the presence of O2, it is oxidized to
nitrite and nitrate. This nitrification processes only occur in oxic waters, and nitrifying
bacteria compete for ammonium with primary producers. Only ammonium is used by the
cellular metabolism, so many primary producers prefer ammonium to nitrite or nitrate because
they do not have to expend energy in transformations. Anoxic processes include
denitrification of nitrate releasing N2, and ammonification of nitrate releasing ammonium.
Nitrification lowers the pH values while ammonification of nitrate raises the pH values. High
ammonium concentrations in rivers are frequently associated to anoxic water inputs
(groundwater) or pollution (Dodds 2002).
Large rivers are likely to be dominated by decomposition processes due to high
turbidity and depth that limits light availability for photosynthesis. Autothrophs are only
dominant in certain circumstances, such as large slow rivers or streams in deserts and
grasslands (Allan 1996).
The Gems/Water Programme Report (2002) on 82 major watersheds of the world
over the period 1976-1990, mentions that in most South American rivers nitrate levels are
very low, less than 0.88 N-NO3- mg.l-1. Similar levels were found in most African rivers. In
such rivers, nitrate is always a very minor component of the ionic balance. These low
concentration levels are more than 50 times lower than the WHO standard for drinking water
(50 mg.l-1 N-NO3-). Figure 15 shows that our results agree with Gems/Water Program
Report (2002). The nitrate mean annual values for our data ranged from 0.10 to 1.03 mg.l-1
N-NO3- (mean = 0.23) (see Table 3).
86
Figure 15. World and Minas Gerais rivers
average nitrate concentrations.
Red: Minas Gerais; Lilac: South
America; Green: World
Minas Gerais highest nitrate values are
from Velhas and Doce rivers.
World data from Gems/Water Program
Report (2002).
Turner et al. (2003) in a study of the world’s large rivers found that nitrate-N, not
ammonium, particulate or dissolved organic nitrogen, was the major contributor to the total
nitrogen pool above a minimal threshold TN concentration of about 0.075 mg-at.l-1.
Considering the annual average values from our data, there were strong relationships
between TN and N-organic (r2 = 0.62) but not between TN and nitrate. So, on an annual basis,
Minas Gerais rivers showed a different result from world rivers, where the N-organic forms
dominate. This may be a result of erosion and overland flow in higher levels as we comented
previously about (Fig.8). To confirm this result only for large rivers, we analysed the data for
the São Francisco River (considered as one of the world’s large rivers) and Doce River (also a
large river) on both annual and seasonal basis.
On an annual basis (Fig.16), N-organic was clearly dominant in S. Francisco River. A
sharp decrease in N-organic value was at SF0015 sample site which is located at the effluent
waters from Três Marias reservoir, showing that the reservoir retained N-organic. On the
other hand, Doce River showed an alternate dominance between nitrate and N-organic.
Comparing both rivers, San Francisco River showed lower nitrate than Doce River and higher
N-organic than Doce River. In general, for both rivers N-organic and nitrate increased to
downriver.
87
Figure 16. Annual average nitrogen forms along the São Francisco River and Rio Doce River.
SF015: effluent waters from Tres Marias reservoir; SF023: after Rio das Velhas tributary;
RD013: after Nova Ponte city; RD035: after Piracicaba river tributary.
Allan (1996) mentions that in large rivers, almost invariably the higher nutrient
concentrations are in the lower course of the rivers due to the suitable lowlands for agriculture
and settlement and thus a large variety of human activities. Our results are consistent with the
author’s observations, at least with respect to the agriculture, but also may result from the low
capacity of rivers to processes the high inputs of organic matter and thus the organic matter
accumulated progressively downriver.
Downing et al. (1999) mention that tropical rivers and streams are often relatively rich
in dissolved organic nitrogen (~35% of total N) as is also true for undisturbed temperate
systems. Our results showed that N-organic contributed with 29% and 55% of total N during
the dry and the rainy season with an average value of 42%, consistent with Downing et.al.
Higher values during the rainy season could be due to particulated N-organic. Downing et al.
(1999) also mention that in temperate regions, increased inputs of nitrogen from human
activity result in a proportionally greater riverine export of nitrogen in the form of nitrate.
This is true whether the major inputs of nitrogen are from agriculture or atmospheric
deposition, and whether atmospheric depositions are dominated by NOy or NHx forms.
Nitrate fluxes from the largest rivers in the world suggest that this is also true for disturbed
tropical watersheds, as nitrate fluxes are correlated with watershed population densities in
both temperate and tropical regions. On an annual basis, our results indicated that this pattern
mentioned by Downing et al. (op.cit.) could also be applied as a trend for Minas Gerais rivers,
88
as the Doce River watershed is more disturbed and with higher population densities than the
San Francisco River watershed.
Figure 17. N – organic, and N – nitrate concentrations in São Francisco River and Doce River
during the dry and the wet seasons.
Thus, on an annual basis, our results showed a different general pattern (N-organic
dominance) from that mentioned by Turner and colleagues (nitrate dominance) for world
rivers and on the other hand, our results agree with Downing et al. that nitrate river fluxes
increase in disturbed watersheds. Despite the annual difference between the two large rivers,
on a seasonal basis this difference was not evident. During the dry season both rivers showed
nitrate dominance while during the rainy season they shifted to N-organic dominance due to
an increase in N-organic concentration along with a nitrate decrease (Fig.17). San Francisco
River only showed a correlation, a weak correlation (r2 = 0.43), between N-organic and nitrate
89
during the dry season, and Doce River only showed a correlation, a strong correlation (r2 =
0.85), during the rainy season. This opposite behaviour between the two rivers indicates that
different driving processes were responsible for nitrogen fluxes into the rivers.
Brigante et al. (2003) studied the Mogi-Guaçú river watershed located southeast of
Minas Gerais State and northest of São Paulo State. This watershed has intensive agriculture
and several industries. In 2000-2001, the authors found that the highest sediment contents of
organic matter occurred during the rainy season. They also mentioned that these increased
organic concentrations were due to the vegetation decomposition processes in the river
margins. Also, springs in well-preserved native forest areas showed the highest N-organic
content in the sediment. In the São Paulo watershed area, the water column showed higher
suspended organic matter than during the dry season. During the rainy season, inorganic
suspended matter dominated over organic suspended matter. According to the data presented
by the authors we calculate, without the headwater outliers, that organic suspended solids in
average contributed to TSS about 17% during the dry season and 14% during the rainy
season. Only for reference, applying these values to our data, results in 3.74 mg/l of organic
suspended solids during the dry season and 37mg/l during the rainy season, showing the
influence of overland flow and erosion.
To investigate a putative sewage effect of pollution on the seasonal pattern showed by
large rivers we analysed lower scale systems; first we looked at a highly polluted urban
stream (Arrudas) and then we analysed a highly polluted medium-sized river (Velhas River).
1.9. Nitrogen forms in a highly polluted urban stream (Arrudas stream)
The Arrudas stream collects Belo Horizonte Metropolitan Region waters (BV155 –
Arrudas). This urban stream is tributary of Velhas River, and a sample site in this river was
also selected (BV 083), which is located about 4 km downriver from where Arrudas meets
with Velhas River. Results are in Fig.18.
90
A
B
C
Figure 18. N-organic, N-Ammonium and NNitrate concentrations in the Arrudas stream
(BV155) and a Velhas River (BV083) sample
site downriver its confluence with Arrudas.
Figure 18-A shows that during the dry season the sewage of Arrudas stream (BV155)
was dominated by ammonium and secondly by N-organic, and a high dilution effect can be
noted in the Velhas River (BV083) receiving this sewage water. Figure 18-C shows BV083
exhibited high ammonium values as BV155 does, but also higher nitrate values than BV155.
This behaviour is not in accordance with the general pattern we found for the dry season and
represented in Fig. 17. During the dry season the general pattern was nitrate dominance and
BV083 showed a N-organic and ammonium dominance under the influence of BV155.
Instead, during the rainy BV083 behaviour was as expected by the general pattern. Figure 18B shows that during the rainy season both stream and river exhibit a similar profile where Norganic dominated, where ammonium is at the lowest value and where nitrate also exhibit low
values following the pattern shown in Fig.17. So, this example shows that the influence of
sewage and urban runoff can be detected during the dry season by the presence of both high
N-organic and ammonium.
1.10. N-forms in a highly polluted river (Velhas River)
Figure 19 shows that Velhas River exhibits during the dry and the rainy seasons high
N-organic concentrations. During the dry season the upper portions of the river showed a
trend of dominance by N-organic and only the lower portions or the river (BV 142-149)
91
showed the nitrate-dominance general expected pattern for the dry season. During the rainy
season, the expected N-organic dominance was observed along the river. However, the main
difference between Velhas River and São Francisco/Doce rivers was related to the presence of
high ammonium values. Figure 19 shows that while the large rivers presented low ammonium
values, Velhas River shows very high concentrations values. In Velhas River ammonium
values are coupled with N-organic, with high correlations during the dry (r2 = 0.72) and the
rainy seasons (r2 = 0.68), showing the influence of sewages.
Figure 19. N- Nitrate and N-organic concentrations in the Velhas River during the dry
and the rainy seasons.
Figure 20. Ammonium concentrations
in the São Francisco, Doce and Velhas
rivers during the dry and the rainy
seasons.
92
1.11. N-forms and dissolved oxygen (DO)
The main DO sources for surface waters are atmosphere and photosynthesis and the
main losses are oxygen consumption by oxidation of organic matter, losses to the atmosphere,
organism respiration, and metal oxidations. Thus, it is expected that during the rainy season
high inputs of organic matter and suspended solids may decrease DO levels by organic matter
decomposition and lower photosynthesis due to lower transparency. DO solubility also
depends on temperature an atmospheric pressure, so with higher temperatures during the rainy
season a natural DO depression is expected compared with the cooler winter. It is also
expected that hotter regions exhibit lower DO concentrations than colder regions of the State.
When considering all the sites, DO showed a seasonal pattern in which during the
rainy season there are lower concentrations than during the dry season (Fig. 21). Low DO
sample sites during the dry season (below 6 mg.l-1) exhibit higher values during the rainy
season, an opposite behaviour than the majority of the sites, indicating that they could be
polluted sites.
Figure 21. Dissolved oxygen during the dry and the rainy seasons.
Ammonium was well correlated with dissolved oxygen during the dry season, but not
so well with N-organic. The relationship with nitrate is no clear but seems to have an opposite
behaviour than N-organic and ammonium. Ammonium and DO were related through an
exponential decay curve (r2 = 0.58) (Fig.22). During the rainy season no correlations were
found. These results suggest the anoxic process of ammonification from nitrate.
93
DO (mg/l)
y = 7.5331e -0.1248x
14
12
10
8
6
4
2
0
R2 = 0.5781
0
5
10
15
20
25
+
NH4 (m g/l)
2
R = 0.2605
14
DO (mg/l)
12
10
8
6
4
2
0
0.00
2.00
4.00
6.00
8.00
10.00
12.00
N-organic (mg/l)
14
DO (mg/l)
12
10
8
6
4
2
0
0
0.5
1
1.5
2
nitrate (mg/l)
Figure 22. Relationship between dissolved oxygen (DO) and nitrogen forms during the dry
season.
94
Ammonification of nitrate raises pH values and oxigen levels:
NO3- + H2O + 2H+ → NH4+ + 2O2
This reaction is an anoxic process that happens into low O2 environment. So, we analysed the
relationship between pH and NO3- at the sites with the lowest O2 concentrations, below 2.5
mg/.l-1 There was found a high relationship (r2 = 0.60), demostrating the ammonification
process in these sample sites (Fig. 23).
Fig. 23. Relationship between ammonium and pH at the lowest oxigen
concentrations sample sites.
Analysing the large rivers, São Francisco and Doce, the general pattern was
maintained, with DO concentrations higher during the dry season and lower during the rainy
season. The majority of the sites showed DO values above 6 mg.l-1. For the Velhas River, the
pattern was only maintained for the sections of the river with lower pollution (Fig. 24).
Wetzel (2001) mention that in rivers DO decline during the hot summers and that among
large rivers there are marked variations in oxygen concentrations, often coupled directly to
discharge and loading of organic mater. Dissolved organic matter leached from soils
following precipitation events or loaded from organic sewage sources can also increase
microbial-mediated consumption. During the dry season Velhas River showed a clear DO
depression in the sections with higher ammonium concentrations, showing a high negative
correlation between ammonium and DO (r2 = 0.87) (Fig. 25). So, the high ammonium values
along with a DO depression that Velhas exhibit, can be explained by the ammonification from
nitrate in an anoxic environment due to microbial respiration of organic matter. This pattern
was also kept during the rainy season however with less intensity (Fig.25).
95
Figure 24. Dissolved oxygen (DO) in
São Francisco, Doce and Velhas rivers
during the dry and the rainy seasons.
Note that the more polluted Velhas
River sections present values below 6
mg.l-1 especially low during the dry
season.
Figure 25. Velhas River dissolved oxygen (DO) and ammonium concentrations.
96
2. Trophic state categories
Following Dodds et al. (1998) trophic classification, the boundary between
oligothrophic and mesotrophic states was considered as: TN = 700 and TP = 25 μg.l-1, and the
boundary between mesotrophic and eutrophic states as TN = 1500 and TP = 75 μg.l-1. Figure
25a,b shows the TN and TP trophic state categories for Minas Gerais rivers.
Figure 25a. Trophic state categories comparison of TN during the dry and the rainy
seasons, and TP during the dry and the rainy seasons.
Figure 25b. TN and TP trophic state categories during the dry and the rainy seasons.
During the dry season, TN showed similar numbers of sample sites in each trophic
status and TP showed a slightly higher number of sample sites in the mesotrophic status.
During the rainy season, TN showed a decrease in the oligotrophic category and an increase
in the mesotrophic status, while eutrophic sites remainded the same. TP showed a clear shift
to eutrophic status, decreasing the number of oligo and mesotrophic status. Comparing TN
and TP behaviour, during the dry season they showed a somewhat similar behaviour, whereas
97
during the rainy season TN increased to the mesotrophic status and TP increased to the
eutrophic status (Fig.25b).
To detail this behaviour, the transition of the trophic state from the dry to the rainy
season was analysed for each sample site. The transition possibilities were: the trophic status
could remain in the same category, transit to a lower trophic state or transit to a higher trophic
state.
When TN and TP remained in the same trophic category (Fig.26a) there was a clear
difference between both, the number of TN sample sites was higher in the oligotrophic status
while the number of TP sites was higher in the eutrophic status. When TN and TP moved to a
lower trophic category (Fig.26b) also TN and TP showed a different behaviour: the number of
TN sample sites that transit to oligotrophic or mesotrophic state was higher when compared
with TP. In contrast, when TN and TP moved to higher throphic levels (Fig.26c), the number
of TP sites that transited to eutrophic status was higher than the number of TN sites.
A
C
B
Figure 26. Transition TN and TP trophic
state from the dry to the rainy season.
Acronyms: The first word refers to the site
trophic status during the dry season and the
second word referes to its transition to the
rainy season. Ex. oli-oli = oligotrophic in
the dry season and oligotrophic in the rainy
season.
98
So, TN and TP showed a clear different seasonal dynamic behaviour. When transiting
from the dry to the rainy season, the TN tendency is to remain in the same low category or to
transit to a lower category, while the TP tendency is to remain in high categories or transit to
higher categories.
3. Limiting nutrients
The TN:TP ratios (by weight) showed similar ranges during the dry and the rainy
sesons (dry from 0.08 to 232 and rainy from 0.03 to 206). However, mean values were higher
during the rainy season than during the dry season (rainy season average = 12.7 and dry
season average = 22.7).
Figure 28 shows the TN:TP ratios for each sample site. There was a clear difference
between both seasons, where in the rainy season the TN:TP ratio was lower than during the
dry season. During the rainy season the majority of the sites (64%) showed a TN:TP ratio
lower than 10, while during the dry season the majority of sample sites presented a TN:TP
ratio distributed around a maximum of 15 – 25 (Figs.27, 28). This TN:TP ratio distribution,
where during the rainy season the threshold of the highest frequencies was around 10, is close
to the threshold proposed by Vollenweider (1982) of 9.1.
Figure 27. TN:TP ratio during the dry and
the rainy season.
Figure 28. TN:TP ratios frequency
distribution
99
Figure 29 shows the relatioship between TN and TP and the threshold TN:TP = 9:1
(by weight), proposed by Vollenweider (1982). In the dry season most sites were above the
threshold, indicating potential limitation by phosphorus. In the rainy season, TN:TP ratios
appear below the threshold, indicating potential limitation by nitrogen.
Figure 29. Total Phosphorus versus Total Nitrogen and TN:TP ratios during the dry and
rainy seasons.
TN:TP < 9 shows potential limitation by nitrogen and TN:TP > 9 potential limitation by
phosphorus.
During the dry season 84% of the sample sites showed TN:TP > 9.1 and during the
wet season 60% of the sample sites showed TN:TP < 9.1 (Fig.30). Thus, during the rainy
season not only TP and TN concentrations changed, but also the quality of the relation
between them. During the rainy season there was a strong shift from a potential limitation by
phosphorus to a potential limitation by nitrogen.
Figure 30. TN:TP ratios during the dry
and the rainy seasons, illustrating the
shift from phosphorus limiting in the dry
season to nitrogen limiting during the
rainy season.
100
Figure 31 shows the the relatioship between N-forms versus TP and the threshold
TN:TP = 9:1 All nitrogen forms followed the same behaviour as TN: during the dry season
sample sites are above the TN:TP threshold (potential limitation for phosphorus) and during
the rainy season sample sites were below the TN:TP threshold (potential limitation for
nitrogen).
Figure 31. Total Phosphorus versus N-forms and TN:TP threshold.
Solid line: TN:TP threshold
TN:TP < 9 shows potential limitation by nitrogen (lilac) and TN:TP > 9 potential
limitation by phosphorus (red = dry seaosn, blue = rainy season).
101
Figure 32 shows the TN:TP ratio along the TP distribution. During the dry season the
majority of the sample sites were potentially limited by phosphorus, and the few sites that
were potentially limited by nitrogen were those with the highest phosphorus concentrations.
Figure 32. TN:TP ratios according TP distribution during the dry and the rainy seasons
During the rainy season, only sites with low phosphorus concentrations (< 0.050 mg.l1
) showed potential phosphorus limitation, while the sites with higher phosphorus
concentrations (> 0.100 mg.l-1) showed potential nitrogen limitation (Figure 33). So, low TP
sites were potentially limited by phosphorus while high TP sites tended to be potentially
limited by nitrogen, particularly during the rainy season.
Figure 33. TN:TP ratios according the TP number of samples sites in each concentration
category during the dry and the rainy seasons
102
3.1. TN:TP ratios, N-forms and phosphorus
Figure 34 shows N-forms and TP average values for the potential N-limited and Plimited sample sites during both seasons. Both seasons presented the same pattern: in Plimited sites there are high N-organic and ammonium concentrations along with low total
phosphorus concentrations; in the N-limited sites there are high N-organic concentrations and
high total phosphorus concentrations. Regarding nitrogen, the difference between P-limiting
and N-limiting sites is the absence of high ammonium concentrations in the N-limiting sites.
Figure 34. N-forms and TP average values for the potential N-limited and P limited
sample sites during both seasons.
103
3.2. N-limitation and P-limitation: terrestrial versus aquatic ecosystems
Vitousek and Howarth (1991) comment that the widespread occurrence of nitrogen
limitation to net primary production in terrestrial and marine ecosystems is something of a
puzzle; it would seem that nitrogen fixers should have a substantial competitive advantage
wherever nitrogen is limiting, and that their activity in turn should reverse limitation.
Nevertheless, there is substantial evidence that nitrogen limits net primary production much of
the time in most terrestrial biomes and many marine ecosystems. The biogeochemical
mechanisms that favor nitrogen limitation include: (i) the substantial mobility of nitrogen
across ecosystem boundaries, which favors nitrogen limitation in the
source
ecosystem,
especially where denitrification is important in sediments and soils, or in terrestrial
ecosystems where fire is frequent; (ii) differences in the biochemistry of nitrogen as opposed
to phosphorus (with detrital N mostly carbon-bonded and detrital P mostly ester-bonded),
which favor the development of nitrogen limitation where decomposition is slow. Smith et al.
(1999) mention that several studies have pointed that N is the key limiting element that
determines the productivity, diversity, dynamics and species composition of terrestrial
ecosystems (plants have high requirements for N; the parent materials from which soils have
formed did not contain N because the most stable form of N is gas, N2 ; thus, the N content of
soils is mainly of biological origin, formed via microbial N fixation). Although both N and P
can be limiting nutrients in aquatic and terrestrial ecosystems, the primary productivity of
freshwater ecosystems more often is limited by P than by N, whereas the opposite occurs in
terrestrial ecosystems. This difference is explained by a physical process: nitrate is readily
leached from soils, whereas phosphate has a movement rate through soil that is orders of
magnitude slower. Tanner et al. (1998) mention that tropical montane soils in Costa Rica
usually have more soil organic matter per unit ground area; N mineralization levels are lower
at higher altitudes, and extractable and total soil P are lower in sites with lower litterfall P
concentrations. The authors speculate that many lowland forests are limited by P and many
montane forests by N. Martinelli et al. (1999) cited that evidences suggest that N in most
tropical forests is relatively more available than in most temperate forests. More N circulates
annually through lowland tropical forests, and does so at higher concentrations than through
temperate forests. The major exceptions to this generalization in the tropics are forests on
white-sand soils and montane tropical forests. Downing et al. (1999) mention that pristine
tropical landscapes may have relatively high N loss rates which result from high rates of N
fixation and the general lack of N limitation in tropical terrestrial ecosystems. Therefore, not
104
disturbed tropical watersheds conserve less N, so that N concentrations exported from
disturbed tropical watersheds may be even greater than those seen in temperate systems. Our
results are consistents with this N-limiting terrestrial ecosystems theory, as they indicate that
rivers were potentially limited by phosphorus and that, probably due to diffuse sources of P
pollution from the watershed, there was a shift to a potentially nitrogen limitation during the
rainy season.
Huszar et al. (2006), studying the available data base for 192 tropical lakes and
reservoirs, verified that the TN:TP ratio was high, indicating potential P-limitation (mean =
26 and median = 19.8). On an annual average basis, our river data showed also P-limitation,
with lower TN:TP ratios (mean = 17.7 and median = 11.9) than the tropical lakes cited above
and slightly higher values than those calculated from Gems database for world rivers (Tab.5).
On a seasonal basis, the dry season showed values more similar to tropical lakes with
P-limitation, and the rainy season values were closer to world large rivers, wich tend to an Nlimitation. This seasonal behavior may be due to lower residence time for rivers during the
dry season, that can be thought of as a river lenthic phase and thus, more similar to lakes
regarding hydraulic conditions.
Table 5 Summary statistics of TN:TP ratios (by wight) from world tropical lakes (136 lakes,
56 reservoirs), world rivers (44 rivers) and this study (236 sites).
WorldTropical
Lakes*
Minas Gerais Rivers
(this study)
World
Rivers**
N-NO3:P-PO4
Dry season
Rainy season
Median
19.8
16.3
7.4
Mean
26.0
22.7
12.7
SD
30.9
20.9
21.2
Minimum
0.7
0.08
0.03
Maximum
221.5
232
206
*Data from Huszar et al (2006): world tropical lakes samples: annual values
mixed waters.
** Data from Gems/Water Programme Report (2002)
8.6
14.8
24.5
0.3
150
of surface
For lakes, Huszar et al. (2006) discussed that nutrient addition experiments both in
tropical and temperate systems (Elser et al. 1990 as cited) do not show uniform N limitation
but rather suggest that systems can vary between N-limitation, P limitation and co-limitation
105
of N and P. The authors mentioned that Arcifa et al. (1995), reviewing experimental nutrient
enrichment studies in 10 Brazilian lakes and reservoirs did not find clear-cut N-limitation;
rather, limitation varied both between systems, and seasonally within single systems.
Similarly, nutrient limitation in other tropical areas, as inferred from nutrient additions,
physiological indicators, or dissolved N:P ratios do not show uniform N or P limitation but
rather show seasonal and between-system variance (Fisher et al. 1995). From these reports, it
may be supposed that as many lakes and all reservoirs are directly connected with the
watershed rivers nutrient inputs, the seasonal behaviour referred above may be explained, at
least in part, by rivers seasonal TN:TP shifts like the general pattern we found in this study.
4. TN and TP trophic state concordance with limiting nutrients
Figure 35 shows the relationship between TN and TP trophic states versus TN:TP
ratios during the dry and the rainy seasons. During the dry season, the few N-limited sites
were mainly those where TP is eutrophic and TN is not eutrophic and also some sites where
both are eutrophic. This trend was kept during the rainy season when sites shifted to a Nlimited condition. N-limited sites were mainly those where only TP is eutrophic along some
sites where both TN and TP are eutrophic.
Figure 35. Relationship between trophic states and TN:TP ratios during the dry and the
rainy seasons.
TN:TP < 9 shows potential limitation by nitrogen and TN:TP > 9 potential limitation by
phosphorus
Red (dry season) and Blue (rainy season): TN and TP are not eutrophic
Ciano: TN and TP both are eutrophic
Yellow: TP is eutrophic and TN is not
Green: TN is eutrophic and TP is not
Black line: TN:TP ratio = 9.1
106
The detailed analysis of the relationship between TN and TP trophic status and the
implication for nutrient limitation are shown in Fig.36. The first group of sites (A) is when
TN and TP had an equal trophic status: both were oligotrophic, mesotrophic or eutrophic.
Both for the dry and the rainy seasons, when TN and TP were oligotrophic or mesotropic, the
ratio TN:TP was > 9 (P-limited) and when both were eutrophic the ratio was <9 (N-limited).
The second group of sites (B) is when TN had a higher trophic status. Both for the dry and the
wet seasons, TN:TP was > 9 (P-limited). The third group (C) is when TN trophic status is
lower than TP trophic status. Here there is a mixed behaviour with few N-limited sites during
the dry season and with more N-limited sites during the rainy season. The exception is when
TN is oligotrophic and TP is eutrophic, where all the sites were N-limited.
These results showed a good agreement between trophic status categories and the
limiting nutrient status. Phosphours limitation occurred in almost all the trophic categories
and nitrogen limitation was concentrated in the highest P trophic status during the rainy
season.
Figure 36. TN and TP trophic status versus TN:TP ratios during the dry and the wet seasons.
TN:TP < 9 shows potential limitation by nitrogen and TN:TP > 9 potential limitation by
phosphorus.
107
5. Eutrophication risk
In temperate Northern Hemisphere regions, the growing season is during the summer
while the rainy season is during the fall and winter, i. e. the hot period is uncoupled from the
rainy period. Unlike the Northern Hemisphere, tropical dry-wet regions as Minas Gerais
exhibit these two periods, hot and rainy, concentrated during the summer. In this way, it is
expected that in tropical dry-wet climates the mobilization, transport and avalability of
nutrients to the biota show different consequences for terrestrial and aquatic ecosystems.
In rivers, point sources of nutrient pollution are subject to a dilution effect during the
rainy season, while there is an increment of nutrients concentrations from the income of
diffuse pollution by influence of the rain (overland flow and erosion). Point sources dominate
during the dry season whereas during the rainy season diffuse sources dominate the river
chemistry. Thus, during the rainy and hot growing season it is expected that the biota is more
exposed to the increasing effects of diffuse sources of pollution than to the decreasing effects
of point sources. So, in the tropical dry-wet climate there is the coincidence of the biota high
nutrient demand (growing season) with the higher nutrient loads from watershed diffuse
sources. Then, diffuse sources disturbances can affect the biota in sensitive life stages.
Our results showed that the number of sites in TN eutrophic state did not vary
seasonally (33%), but the number of sites in TP eutrophic state increased form 26% during the
dry season to 68% during the rainy season. Although many streams and rivers worldwide
exhibit high nutrient concentrations, a prevailing view for many years held that rivers are
insensitive to nutrient inputs. This argument was based upon the assumption that other
physical, chemical, and biotic factors potentially restrict the effects of nutrient enrichment on
algal growth in rivers and streams. For example: (i) restriction of light penetration into the
water column by high concentrations of inorganic suspended solids can potentially limit the
growth of both benthic and suspended algae in rivers; (ii) the hydraulic flow regime can alter
periphyton standing crops in flowing waters; (iii) herbivore grazing often is noted as an
additional biological constraint on periphyton growth and productivity. For many years,
flowing waters were frequently perceived as nutrient saturated, because factors such as light
limitation and short hydraulic residence times should restrict or prevent any potential algal
responses to nutrient enrichment. However, evidence from studies in a wide variety of
108
geographical locations now suggests that flowing waters are sensitive to anthropogenic inputs
of N and P (Smith et al. 1999).
The production of high concentrations of biomass is closely linked with eutrophication
of surface waters, and nitrogen limitation of phytoplankton may encourage blooms of N2
fixers such as cyanobacteria. Cyanobacteria present several negative effects, notably reduced
water transparency, decreased biodiversity, elevated primary production and the potential
occurrence of oxygen depletion, which may result in massive fish kills, odor and taste
compounds, as well as production of toxins. Cyanobacterial toxins pose severe potential
health hazard, such as skin irritation, sublethal intoxication and liver damage (Reynolds 1999,
Chorus 1993). In 1996, there was a fatal poisoning of sixty dialysis patients in Brazil after
receiving water supplied by a bloom-affected reservoir (Pouria et al. 1998). In 2000 was
reported a cyanobacteria bloom in São Simão reservoir at the Paranaíba River (CastellanosSolá
and
Pinto-Coelho
2003,
Pinto-Coelho
and
Castellanos-Solá
2003).
In
septmber/october 2007 there was a huge bloom in the Velhas River that extended downstream
to the São Francisco River. The water state agency COPASA has a cyanobacteria monitoring
programme for the Middle São Francisco River along the cities of Ibiaí, São Romão, Retiro,
São Francisco, Januária, Matias Cardoso and Manga, and results showed that during the rainy
season the presence of cyanobacteria was less intensive than during the dry season in all the
studied areas. So, the presence of cyanobacteria blooms is a current reality in Minas Gerais
State, corroborating that rivers are sensitive to nutrient pollution (Ladeia et al. 2007).
The factors affecting the abundance of cyanobacteria in freshwaters include light,
nutrients, temperature, ionic composition and pH of the surrounding water and river flow.
According to a Dokulil and Teubner (2000) review, hypotheses to explain the success of
cyanobacteria are several and include the following: (i) elevated temperatures as the cause of
increased abundance of cyanobacteria especially during summer because of their usual higher
temperature optima compared to other algal groups; (ii) low light-energy requirements of
cyanobacteria as the driving factor for bloom formation; (iii) superior uptake kinetics for
inorganic carbon (low CO2/high pH-hypothesis) was postulated to be responsible for
cyanobacterial dominance. In lakes of low alkalinity, carbon dioxide availability did not
initiate blue-green maxima but was largely responsible for their maintenance; (iv) low
TN/TP-ratios are beneficial for both nitrogen and non nitrogen-fixing species of
cyanobacteria. In some cases, it is the timing when the critical ratio is reached, rather than the
109
ratio itself, which is important for the dominance of one species or another. (v) the inorganic
nitrogen hypothesis suggests that the forms and amounts of inorganic nitrogen favor different
algal groups. Non-N-fixing cyanobacteria are favored by ammonium-nitrogen, while
eukaryotic phytoplankton develops when nitrate-nitrogen is the main N-component present.
Scarcity of nitrogen induces nitrogen-fixation and hence favors the development of species
capable of fixing molecular nitrogen; (vi) cyanobacteria migrating from the sediment to the
water column gain competitive advantage by storage of internal phosphorus reserves; (vii)
cyanobacteria have higher requirements for trace elements compared with eukaryotic
phytoplankton; (viii) the buoyancy hypothesis is related to forms, which bear gas-vesicles,
such as Microcystis and Planktothrix, and are therefore able to use the water column stability
as a resource. They can either accumulate at some intermediate depth where conditions favour
them or rise to the water surface where light and carbon dioxide are available. Other
cyanobacteria, such as Limnothrix or Aphanizomenon, are more dependent on higher
turbulence; (ix) the minimization of mortality through immunity to grazing by zooplankton;
(x) suppression of the growth of other algae through the excretion of organic compounds; (xi)
toxin production by toxigenic strains of cyanobacteria affecting natural grazers and other
aquatic biota. Species of the genera Oscillatoria and Anabaena are among the most
distributed toxin producers in eutrophicated freshwaters.
Next we analyse the factors mentioned above that are available in our results:
Retention time: Retention time is the main characteristic that distinguishes rivers from lakes.
If the retention time of a lake is shorter than the doubling time of planktonic algae in the lake,
the algae will be flushed out at a rate faster than they can utilize nutrients and can grow; then,
irrespective of the nutrient conditions, the development of a biomass of planktonic algae,
large enough to cause nuisance conditions, will not occur (Vollenweider 1982). Water flows
faster in the upper reaches of rivers, with lower residence time than the downriver reaches. In
the upper reaches probably the doubling time of planktonic algae will be lower than the
residence time, so it is expected that macrophytes and attached algae will dominate. On the
other hand, in the lower reaches, where the river is large and deep, there are long retention
times which may be much longer than algal doubling times, so that phytoplankton can
develop a large biomass. During the dry season retention time will be higher than during the
rainy season. The increasing sediment deposition in the river channels from the erosion
processes during the rainy season also contributes to increase the retention time.
110
Light limitation: The River Continuum Concept (Vannote 1980) makes references to light
limitations. In a forest environment, the upper streams are more influenced by riparian
vegetation where shadow limits photosynthesis and there is a large input of organic matter;
here the river metabolism is heterotrophic. With the river enlargement the relative influence
of the riparian vegetation diminishes and light that reaches the river waters increase. This
condition is favorable to periphyton and macrophytes, increasing also the phytoplankton
biomass. In large rivers, turbidity, depth and substratum instability limit photosynthesis and
the rivers turns to be heterotrophic. From our results it can be expected that during the rainy
season, in high TSS environment rivers will be light-limited, but not during the dry season
due to the low TSS. Also, many streams and rivers upper reaches do not have riparian
vegetation due to the ecosystem characteristics (grasslands) or due to deforestation, and then
do not present ligh limitations.
pH / CO2 : Each alga has an optimal pH range for its growth, which is related to its carbon
uptake ability. When the pH level rises as a result of algal photosynthesis (18 H+ are
consumed with 106 C atoms), the phytoplankton community structure changes (Yamamoto
and Nakara 2005). Cyanobacteria have better CO2 kinetics than do the green algae. Initiation
of blue-green algae maxima does not depend on low CO2 or high pH, but once the blue-green
algae become abundant they ensure their dominance by reducing CO2 concentrations to levels
available only for them (Shapiro 1997). In our previous study (Chapters 1 and 2) there was a
pH seasonal pattern where the dry season showed higher pH than the rainy season. So, the
more alkaline environment during the dry season can favour cyanobacteria competition.
NH4+ / NO3- : As mentioned above, non-N-fixing cyanobacteria are favoured by ammoniumnitrogen, while eukaryotic phytoplankton develops when nitrate-nitrogen is the main Ncomponent present. Scarcity of nitrogen induces nitrogen-fixation and hence favors the
development of species capable to fix molecular nitrogen. Our results show that low
ammonium and nitrate concentrations are found in N-limiting sites, which coincides with the
advantage to fix nitrogen.
From these considerations, we can speculate that the dry season is the more propitious
time for a cyanobacteria nuisance. Comparing the dry season with the rainy season, the water
environment is more alkaline, without turbidity, nutrients from organic compounds are more
readily available, and retention time is higher. However, temperature during the dry season is
111
cold in almost all the state, but the north region of the State has high temperatures all the year
round, and is therefore sensitive for cyanobacteria blooms (in this region is located the section
of the São Francisco River that was affected by the cyanobacteria bloom mentioned above).
Also, as Fig.1 shows, at the begining and at the end of the dry season (March and September /
October) there are high temperatures and low rain precipitations, which ensures high water
temperatures and transparency for photosynthesis (São Francisco River bloom was during
September and October). But during the dry season P-limiting conditions predominate, which
does not favour N-fixing cyanobacteria, however 16% of the sample sites were N-limiting
during the dry season, and thus sensitive to cyanobacteria. Regarding N-fixing and non-fixing
cyanobacterias, Figure 37 shows the circumstances that would favour each type of
cyanobacteria during the dry season. The trend is that non-fixing cyanobacteria will benefit in
P-limiting environments due to average high ammonium concentrations found in these sites,
and fixing cyanobacteria will benefit in N-limiting sites due to low ammonium and nitrate
concentrations that these sites exhibit. As the number of sample sites that are P limiting
during the dry season are more (84%) than those that are N-limiting (16%) it is expected that
non-fixing cyanobacteria have more chances in this season.
In Fig. 37, we considered the number of sample sites that showed high ammonium
concentrations as being those above 0.1 mg.l-1, and low inorganic nitrogen was considered
when the sum of ammonium and nitrate was lower than 0.5 mg/l. It can be seen that using this
criteria, 28% of the sample sites are suitable to non-fixing cyanobacteria and 12% to fixing
cianobacteria, so 30% of the sample sites will be at risk to cyanobacteria nuisance. This
cenario is a matter of concern not only for riverine ecosystems but also for reservoirs that
receive these river waters. Residence time in reservoirs is higher than in rivers, so there is
more time for nutrients to be processed by the biota, and also during the rainy season
incoming sediments have time to be deposited and transparency be higher, allowing
photosynthesis. Thus, reservoirs could be also at risk during the rainy season.
112
Figure 37. Conceptual model showing the chemical charactristics that would
favour nitrogen fixing and non-fixing cyanobacteria. High ammonium
concentrations were considered as above 0.1 mg/l. Low inorganic nitrogen
were considered when the sum of ammonium and nitrate was lower than 0.5
mg/l.
Concluding Remarks
Diffuse sources of pollution due to accelerated erosion and overland flow during the
rainy season had as a consequence the increased nutrient eutrophication, due to nitrogen and
especially phosphorus loads. Also, these processes caused the shift from a prevalent Plimiting condition during the dry season to a N-limiting condition during the rainy season,
which will favour N-fixing cyanobacteria. However, best conditions for cyanobacteria
maxima are during the dry season.
113
This study highlights the importance of managing use of soil from an ecological point
of view; but, moreover, the importance of integrating terrestrial and aquatic ecosystems
management. Also this study indicates that diffuse sources of pollution are actually a matter
of concern for both nitrogen and phosphorus pollution, in contrast to the prevalent view that
nitrogen pollution will be solved by sewage control and that phosphorus is not a problem
because is imobilized in soils and sediments.
Current criteria for phosphorus and nitrogen need to be revised having in mind
ecological criteria, considering regional and seasonal characteristics. For this, it is necessary
to foment the study of fluvial ecoystems. After this study was done, legal limits for
phosphorus and nitrogen were changed (Conama resolution Nº 357, March 17, 2005) and the
criteria for nutrients maxima concentrations were increased, especially for nitrogen forms.
The consequences for Minas Gerais aquatic ecosystems are still to be appraised.
114
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Photos:
Gruta do Janelão: Grupo Bambuí de Pesquisas Espeleológicas
Others: Margi Moss www.brasildasaguas.com.br
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