Maria Fernanda Mellano Antonio Emilio Ramirez

Transcription

GROUNDWATER ARSENIC IN THE AREA AROUND
MARIA ELENA, SANTIAGO DEL ESTERO PROVINCE,
NORTHWESTERN ARGENTINA
Hydrogeochemical characteristics, arsenic mobilization and experimental
studies on arsenic removal using natural clays
Maria Fernanda Mellano
Antonio Emilio Ramirez
September 2004
TRITA-LWR Master Thesis
ISSN 1651-064X
LWR-EX-04-40
Maria Fernanda Mellano & Antonio Emilio Ramirez
TRITA LWR Masters Thesis 04-40
ii
Groundwater arsenic in the area around Maria Elena, Santiago del Estero Province, Northwestern Argentina
GROUNDWATER ARSENIC IN THE AREA
AROUND MARIA ELENA IN SANTIAGO DEL
ESTERO PROVINCE, NORTHWESTERN
ARGENTINA
Hydrogeochemical characteristics, arsenic mobilization and
experimental studies on arsenic removal using natural clays
Linneaus-Palme Student Exchange Programme
KTH
Main Supervisor
Assoc. Prof. Dr. Prosun Bhattacharya
KTH- International Groundwater Arsenic Research Group (GARG)
Department of Land and Water Resources Engineering, Royal Institute of Technology (KTH), SE-100 44
STOCKHOLM, Sweden
Co-Supervisors
Prof. Dr. Jochen Bundschuh, Lic. Raúl Martín and Lic. Angel Storniolo
Departamento de Geología y Geociencias, Facultad de Ciencias Exactas y Tecnologias, Universidad Nacional de
Santiago del Estero, Santiago del Estero 4200, Argentina
Prof. Dra. Clara López Pasquali de Araya
Departamento de Química Analítica, Facultad de Agronomía y Agroindustrias, Universidad Nacional de
Santiago del Estero, Santiago del Estero 4200, Argentina
Reviewer
Md. Jakariya
Department of Land and Water Resources Engineering, Royal Institute of Technology (KTH) SE-100 44
STOCKHOLM, Sweden
Examiner
Prof. Em. Gunnar Jacks
Department of Land and Water Resources Engineering, Royal Institute of Technology (KTH) SE-100 44
STOCKHOLM, Sweden
Stockholm
September, 2004
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iv
Royal Institute of Technology
International Office
PREFACE
This study has been carried out within the framework of the Linnaeus Palme (LP)
Academic Exchange Programme for teachers and students, which is funded by the
Swedish International Development Cooperation Agency, Sida/Asdi and
administered by the International Programme Office (IPK), Sweden. The
participating Swedish university department has the operative responsibility of the
Linnaeus Palme Programme, while the International Office at the Swedish
unviersity, in this case KTH, the Royal Institute of Technology, Stockholm, has a
co-ordinating role.
The LP Student Scholarship Programme offers an opportunity for undergraudate
students registered at universities in Sweden (Linnaeus scholars) and at universites
in Africa, Asia or Latin America (Palme scholars) to undertake courses of one or
two semesters at universities in Africa, Asia or Latin America respectively in
Sweden. The LP exchange studies in regulars university courses, as in this case the
student's final degree project, should be an ordinary part of the student's university
degree and may, as in this case, result in an in-depth report, the student's Master of
Science thesis.
The main purpose of the LP Programme is to enhance the mobility of university
students and the possiblity of Swedish university students to study at universities in
countries outside the OECD region. The over all goals are to strengthen Swedish
university cooperation in Africa, Asia and Latin America and to enhance mutually
human resources competence capacity and the knowledge and understanding of
different cultures.
Sigrun Santesson
Coordinator
LP Programme
International Office
KTH, SE–100 44 Stockholm, Sweden, Phone: +46 8 790 7i83 , Fax: +46 8 790 8192, E-mail: [email protected]
Momsreg.nr/VAT: SE202100305401, www.kth.se
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TABLE OF CONTENTS
PREFACE ...................................................................................................................................................... V
TABLE OF CONTENTS ........................................................................................................................... VII
ABSTRACT...................................................................................................................................................IX
SAMMANFATTNING .................................................................................................................................XI
SUMMARIO .............................................................................................................................................. XIII
ACKNOWLEDGEMENTS .........................................................................................................................XV
1. INTRODUCTION...................................................................................................................................... 1
1 .1 A R S E N I C I N G R O U N D W A T E R O F T H E W O R L D ................................................................. 1
1 .2 H I G H - A R S E N I C G R O U N D W A T E R I N A R G E N T I N A A N D S A N T I A G O D E L E S T E R O P R O V I N C E 1
1 .3 E N V I R O N M E N T A L G E O C H E M I S T R Y O F A R S E N I C ............................................................. 2
1.3.1 Sources of arsenic .................................................................................................................................................................2
1.3.2 Speciation of arsenic.............................................................................................................................................................4
1.3.3 Adsorption-desorption..........................................................................................................................................................4
1 .4 T O X I C I T Y O F A R S E N I C ( H E A L T H I M P A C T S ) .................................................................... 5
1 .5 O B J E C T I V E S O F T H E P R E S E N T S T U D Y ............................................................................ 5
2. AREA OF STUDY....................................................................................................................................... 6
2 .1 G E O G R A P H I C L O C A T I O N .............................................................................................. 6
2 .2 D E M O G R A P H I C A N D S O C I O - E C O N O M I C S I T U A T I O N ........................................................ 6
2 .3 C L I M A T I C C H A R A C T E R I S T I C S ........................................................................................ 6
2 .4 G E O M O R P H O L O G I C A L S E T T I N G ................................................................................... 6
2.4.1 Loess plain ..........................................................................................................................................................................8
2.4.2 Río Dulce alluvial palaeoplain.............................................................................................................................................9
3. MATERIALS AND METHODS ..............................................................................................................10
3 .1 C O M P I L A T I O N O F B A C K G R O U N D D A T A ........................................................................ 10
3 .2 D E S K T O P S T U D I E S ...................................................................................................... 10
3 .3 . F I E L D I N V E S T I G A T I O N S ............................................................................................. 10
3.3.1 Inventory of wells and selection of sampling sites .................................................................................................................10
3.3.2 Groundwater sampling .....................................................................................................................................................10
3.3.3 Determinations of field parameters .....................................................................................................................................10
3.3.4 Drilling and sediment sampling..........................................................................................................................................10
3.3.5 Sampling of clays for adsorption experiments.....................................................................................................................12
3 .4 . L A B O R A T O R Y I N V E S T I G A T I O N S .................................................................................. 12
3.4.1 Groundwater analyses........................................................................................................................................................12
3.4.2 Textural analyses and clay mineralogy ...............................................................................................................................12
3.4.3 Geochemical analyses ........................................................................................................................................................13
3.4.4 Adsorption batch experiments............................................................................................................................................14
3 .5 D A T A P R O C E S S I N G ..................................................................................................... 15
3.5.1 Microsoft Excel software....................................................................................................................................................15
3.5.2 Aqua Chem ......................................................................................................................................................................15
3.5.3 AutoCAD........................................................................................................................................................................15
4. RESULTS AND DISCUSSION .................................................................................................................16
4 .1 C H A R A C T E R I Z A T I O N O F T H E U N S A T U R A T E D Z O N E ....................................................... 16
4 .2 S O U R C E S O F G R O U N D W A T E R A R S E N I C ......................................................................... 16
4 .3 S T R A T I G R A P H I C P R O F I L E S ........................................................................................... 16
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4.3.1 Profile 1 ............................................................................................................................................................................16
4.3.2 Profile 2 ............................................................................................................................................................................16
4 .3 H Y D R O G E O L O G Y ........................................................................................................ 16
4 .4 H Y D R O C H E M I C A L S T U D I E S .......................................................................................... 16
4.4.1 Hydrochemical characterization..........................................................................................................................................20
4.4.2 Geochemical classification...................................................................................................................................................22
4 .5 . R E L A T I O N S H I P S B E T W E E N H Y D R O C H E M I C A L P A R A M E T E R S .......................................... 24
4.5.1. Relationships between major ions ......................................................................................................................................24
4 .6 D I S T R I B U T I O N O F G R O U N D W A T E R A R S E N I C ................................................................. 26
4.6.1 Spatial variations ..............................................................................................................................................................26
4.6.2 Relationships between As and other hydrochemical parameters ...........................................................................................27
4 .7 S E D I M E N T C H A R A C T E R I S T I C S ...................................................................................... 28
4.7.1 Overview............................................................................................................................................................................28
4.7.2 Mineralogical characteristics of the sediments ......................................................................................................................29
4.7.3 Characteristics of the soil saturation extract .......................................................................................................................29
4.7.4 Cation exchange capacity ...................................................................................................................................................29
4.7.5 Iron content........................................................................................................................................................................30
4.7.6 Results of geochemical analyses ...........................................................................................................................................30
5. REMEDIATION OF ARSENIC CONTAMINATED GROUNDWATER ............................................ 33
5 .1 O V E R V I E W ................................................................................................................. 33
5 .2 S E L E C T I O N O F S E D I M E N T S .......................................................................................... 33
5 .3 E X P E R I M E N T A L R E S U L T S ............................................................................................ 34
5.3.1 Effects of reaction time and pH..........................................................................................................................................34
5.3.2 Role of pH for arsenic removal...........................................................................................................................................34
5 .4 D I S C U S S I O N ............................................................................................................... 35
6. CONCLUSIONS....................................................................................................................................... 36
7. RECOMENDATIONS ............................................................................................................................. 38
8. REFERENCES ......................................................................................................................................... 39
APPENDIX 1 ................................................................................................................................................ 42
APPENDIX 2................................................................................................................................................ 43
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ABSTRACT
Since the end of the 19th century, the presence of natural arsenic (As) in concentrations above the
limit of the World Health Organization (WHO; 10 µg L-1) has been observed in groundwaters
from different regions of Argentina. In the semiarid area around Maria Elena town (Banda
County), Santiago del Estero Province, located in the northern part of the Chaco-Pampean plain,
dissolved As, often exceeding the Argentine limit (50 µg L-1) has been found in the shallow
aquifer, which is used for water supply of the dispersed living rural population. The ingestion of
this compound by the rural population caused health implications in form of different chronical
diseases and resulted in some cases in fatalities due to cancer, degenerative effects or circulation
and neurotoxical problems. Hydrogeochemical investigations were performed to characterise
and to delimit zones, where As concentrations in the shallow aquifer are low enough hat they are
suitable for human consumption.
Water samples were collected from 19 wells in the shallow groundwater in June and August 2000
around the town of Maria Elena. The water samples were neutral or slightly alkaline, and were
predominantly of Na-HCO3 type (58%). The concentration of dissolved As ranged from 10 to
1900 µg L-1; 27% of the samples exceeded the Argentine limit (0.05 mg L-1). A moderate positive
correlation was found between the As and the Na+ concentration (R2=0.30) and HCO3 - (R2 =
0.121).
Natural clays from Santiago del Estero and Misiones Provinces were tested as adsorbents to
remove As from groundwater. Adsorption experiments were performed under different pH
values (4.5, 5.6, 8.63) and concentrations of dissolved As(V) (0.5, 1.0, 2 mg L-1). Adsorption was
studied during 48 hours of contact time between As solution and adsorbents.
Clays containing iron were found to be the most effective for As adsorption. In the clay (laterite;
29% beidelite, 71% caolinite) from Misiones province, As (V) was lowered by 99% (from 2 mg L1
to below <0.05 mg L-1), whereas the clays (include composition) from Santiago del Estero
Province (clay sample "Choya": 88% montmorillonite, clay sample "Lomas Coloradas": 60%
montmorillonite, 39% illite) adsorbed only between 40 and 53% of the dissolved As(V) in
solution. The results show that within 90 minutes, the maximum As adsorption took place
(independent from pH and original As concentration).
Keywords: Arsenic, groundwater, hydrogeochemistry, clay, adsorption, removal.
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SAMMANFATTNING
Förekomsten av naturlig arsenik med koncentrationer över gränsvärdet för
Världshälsoorganisationen (10 µg L-1) har rapporterats i grundvatten, från olika regioner av
Argentina. I det halvtorra området kring staden Maria Elena (Banda trakten), Santiago del Estero
Provinsen, i norra delen av Choco-Pampean slätten, har upplöst arsenik, ofta över det
Argentinska gränsvärdet (50 µg L-1), observerats i de grunda vattenlagren, vilket används som
dricksvatten av den spridda landsbygdsbefolkningen. Intagandet av arsenik hos
landsbygdsbefolkningen har orsakat allvarliga hälsoproblem i form av kroniska sjukdomar och i
vissa fall resulterat i dödsfall på grund av cancer, degenererade effekter eller cirkulations- och
neurotoxiska problem. Hydrogeokemiska undersökningar utfördes för att karaktärisera och
begränsa zoner, där arsenikkoncentrationerna i de grunda vattenlagren är tillräckligt låg för
mänsklig konsumtion.
I juni och augusti 2000 togs vattenprover från 19 brunnar av det grunda grundvattnet, av neutralt
eller lätt alkaliskt vatten och dominerande av Na-HCO3 typ (58 %). Koncentrationen av upplöst
arsenik sträckte sig från 10 till 1900 µg L-1 och 27 % av proven översteg det argentinska
gränsvärdet (0,05 mg L-1). En måttligt positiv korrelation upptäcktes mellan As och Na+
koncentrationen (R2 = 0,30) och HCO3- (R2 =0,121).
Naturliga leror från Santiago del Estero och Missiones Provinser testades som absorbenter för att
avlägsna arsenik från grundvatten. Absorptionsexperiment utfördes vid olika pH-värden (4.5, 5.6,
8.63) och koncentrationer på upplöst arsenik (V) (0.5, 1.0, 2.0 mg L-1). Absorption studerades
under 48 timmars kontakttid mellan arseniklösning och absorbenter.
Lera innehållande mest järn visade sig vara väldigt effektiv för arsenikabsorption. I leran (laterite;
29% beidelite, 71% calinite) från Missiones provinsen, As (V) sänktes med 99% (från 2 mg L-1 till
under <0.05 mg L-1), medan leran (inklusive sammansättning) från Santiago del Estero Provinsen
(lerprov ”Choya”: 88% montmorillonite, lerprov ”Lomas Coloradas”: 60% montmorillonite, 39%
illite) bara absorberade mellan 40 och 53% av den upplösta arseniken (V) i lösning. Resultaten
visar att inom 90 minuter skedde den maximala As adsorptionen (oberoende av pH och
ursprungskoncentrationen As).
Nyckelord: Arsenik, grundvatten, hydrogeokemi, lera, absorption, avlägsnande, effektivitet
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SUMMARIO
Desde finales del siglo 19, se ha observado la presencia de arsénico natural en las aguas
subterráneas de diferentes regiones de la Republica Argentina, en concentraciones que
sobrepasan el limite permitido por la Organización Mundial de la Salud (OMS), (10 µg L-1). En la
región semiárida, en el nordeste de la llanura Chaco-Pampeana, en la provincia de Santiago del
Estero, Departamento Banda, se encuentra la localidad de María Elena. En este lugar, el arsénico
disuelto, muchas veces excede el limite establecido por el Código Alimentario Argentino (50
µg L-1), encontrándose el contaminante en el acuífero superficial, el cual es usado como
abastecimiento de agua para la dispersa población rural de la zona. La ingestión de este
compuesto por la población rural, causa complicaciones en la salud, provocando enfermedades
crónicas, resultando fatales en algunos casos debido al cáncer, efectos degenerativos o problemas
circulatorios y neurotóxicos.
Fueron realizadas investigaciones hidrogeoquímicas para caracterizar y delimitar las zonas, donde
las concentraciones de arsénico en el acuífero superficial son bajas y el agua es apta para consumo
humano.
Se tomaron muestras de agua de 19 pozos en los acuíferos superficiales entre junio y agosto del
2000, y se encontró que las aguas fueron predominantemente del tipo Na-CO3 (58% de los casos)
y de pH neutro o levemente alcalino. La concentración de arsénico disuelto estuvo en el rango de
10 a 1900 µg L-1, pero el 27% de las muestras excedieron el limite argentino. Fue encontrada una
moderada correlación positiva entre las concentraciones de As y Na+ (R2 =0.30) y HCO3- (R2 =
0.121).
Fueron testeadas arcillas naturales de las provincias de Santiago del Estero y Misiones como
adsorbentes para remover arsénico desde el agua subterránea. Los experimentos de adsorción
fueron preparados bajo diferentes valores de pH (4.5, 5.6 y 8.63) y diferentes concentraciones de
arsénico (V) disuelto (0.5, 1.0 y 2.0 mg L-1). El fenómeno de adsorción fue estudiado durante 48
horas con diferentes tiempos de contacto entre soluciones de arsénico y el adsorbente.
Las arcillas que contienen principalmente hierro, fueron estudiadas por su eficiencia para la
adsorción de arsénico. En las arcillas de Misiones (laterita; 29% de beidelita y 71% de caolinita),
As (V) fue reducido un 99% (desde 2 mg L-1 hasta <0.05 mg L-1 ), en tanto que las arcillas de
Santiago del Estero ( Muestra “Choya”: 88% de montmorillonita, Muestra “Lomas Coloradas”:
(60% montmorilonita y 39% de illita) adsorben solamente 40 y 53% respectivamente del arsénico
disuelto en solución.
Los resultados muestran que sobre 90 minutos de tiempo de contacto, tiene lugar la máxima
adsorción de arsénico (independientemente del pH y la concentración de arsénico original).
Palabras claves: Arsénico, agua subterránea, hidrogeoquímica, arcillas, adsorción, remoción.
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ACKNOWLEDGEMENTS
This thesis concludes our Bachelor degree in Groundwater and Chemistry at the National
University of Santiago del Estero, Santiago del Estero, Argentina. The study was carried out in
the Royal Institute of Technology, KTH, Stockholm, Sweden, within the framework of
Linneaeus-Palme Academic Exchange Programme between KTH and National University of
Santiago del Estero (UNSE). The work is a result of the co-operation between Associate
Professor Dr. Prosun Bhattacharya at the Department of Land and Water Resources
Engineering, and Professor Dr. Jochen Bundschuh, Lic. Raul Martin and Lic. Angel Storniolo
Department of Geology and Dra. Clara L. P. De Araya, Department of Analytical Chemistry,
National University of Santiago del Estero. They were our supervisors during the project.
We would like to express our sincere thanks to the following persons who helped us along the
way:
Our supervisor in Sweden, Associate Professor Prosun Bhattacharya, Department of Land and
Water Resources Engineering for support throughout the project and for giving us the
opportunity to know about an incredible country, Sweden.
Our supervisor in Stockholm University, Magnus Mörth, for his patience in the laboratory.
Our supervisors in Argentina, Raúl Martín, Angel del Rosario Storniolo, Geology and
Geosciences Department, and Clara L. P. De Araya, Analytical Chemistry Department, for
helping us along the way.
Sigrun Santesson, Linnaeus-Palme Programmes coordinator, thanks for all little things that were
big things for us.
Ann Fylkner and Monica Löwen, at the chemical Laboratories of the Department of Land and
Water Resources Engineering, KTH for assisting us with chemical analysis and with great
patience.
Åsa Carlsson, our study coordinator in Stockholm, who helped us always when we had problems.
Additionally, we would like, to express our thanks to our families, who always helped us and
trusted in our capacity.
Lily and Ariel for their help to complete this dream.
Jochen Bundschuh, International Technical Cooperation Programme, for advice on the outline
of this study and his great patience. Thanks!
Finally, we would like to thank all our friends in Sweden who have supported our work and
contributed that these months became for ever unforgettable, specially to Gabriela, Fredrik,
Therese, Alberto, Mikael, Momoko, Tullia, Erik, Kenji and Anna.
Stockholm, September 2004.
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1. INTRODUCTION
Water plays an important role in human development. Supply of drinking water with good quality is
an issue of major concern, especially if the accelerated growth of the population and the quality of
life is taken into consideration. Under this social pressure, the modern civilization requires access to
large volume of water to satisfy its basic needs. This situation gets worse due to natural or
anthropogenic contamination of drinking water resources, which is in many cases massive, complex
and harmful (Rodier et al., 1990). Contamination of groundwater resources by As of geogenic origin
is an example.
northern boarder of Santiago del Estero province
and extends into the provinces of Salta and Chaco
(Figure 1.1).
The second area of Santiago del Estero Province
with groundwater As is the Río Dulce Alluvial
Cone located to the East of Santiago del Estero
city in the counties of Banda and Robles (Vargas
Gil et al., 1981, Bundschuh et al. 2004, Figure 1.1).
The shallow aquifer (depth mostly <15 m) and the
unsaturated zone of Río Dulce cone characterized
by a discontinuous accumulation of loessic
sediments (0 to few meters thick), which are
deposited over fluvial and aeolian sediments of
Quaternary age which are predominantly of fine
sand to silt fraction.
Some hypotheses intend to explain the origin of
As in groundwater. These include volcanic ash
layers, and volcanic glass dispersed in sediments
(Bundschuh et al., 2004).
The Maria Elena area, situated in the Banda
County (Figure 1.2), is characterized by a dispersed
rural population. The small portions of land,
which belong to the individual families, count in
most cases with a shallow handpump-driven well,
which extracts water from the shallow aquifer. In
most cases, this aquifer is the only available fresh
water source, which however in many cases is
contaminated by As (Herrera et al., 2000).
At present, the Simbolar-Añatuya aqueduct
provides suitable drinking water to one third of the
population of Maria Elena area. The drinking
water is stored in a communal central tank
(capacity 30 m3), located about 1.5 km South of
the Provincial Road 21 (Figure 1.2). The other two
thirds of the population, (approximately 280
people), obtain their water predominantly from
small household wells.
1.1 Arsenic in groundwater of the world
In many parts of the world, serious health
problems are directly related to the presence of
arsenic (As) in drinking water. This is a worldwide
catastrophe that has affected millions of people,
particularly in the developing countries. Chronic
arsenicosis is prevalent in countries as Bangladesh,
India, China, Argentina, Mexico, Chile, etc., where
groundwater has been used primarily for drinking
(Bhattacharya et al., 2002a). The World Health
Organization, (WHO), has recommended a
drinking water quality guideline of 10 µg L-1
(WHO, 2001)
1.2 High-arsenic groundwater in Argentina
and Santiago del Estero Province
The presence of natural As is reported in
groundwaters from several areas of Argentina.
Most of these groundwaters belong to shallow ,
and only few to deep aquifers, making them
unsuitable for human consumption.
In extensive areas of northwestern Argentina,
especially in the provinces of Santa Fe, Córdoba,
Tucumán, Salta, Chaco, and Santiago del Estero,
the presence of As in groundwater is also
accompanied by other geogenic contaminants such
as As and fluoride (e.g. Trelles et al., 1950).
Approximately a population of 1.2 million
particularly those living in the rural and peri-urban
settlements of Chaco-Pampean Plain (Figure 1.1)
are affected by groundwater As (Bundschuh et al.,
2004; El Liberal, 2004).
In the province of Santiago del Estero, which
belongs to the semi-arid Chaco plain (Figure 1.1),
two large areas with groundwater As can be
delimited. The first area is situated along the
1
P
68º
Pampa and
Chaco plain
A
60º
R
G
U
A
11,443,596 inhab.
978,634 km 2
Y
P
Distribution of
arsenic and
fluorine in the
groundwater
a
24º
60º
R
G
U
A
b
Y
24º
Salta
CH
ILE
Salta
A
H
C
Santiago
del Estero
28º
C
A
O
arsenic
Resistencia
Santiago
del Estero
fluorine
Resistencia
28º
28º
La Rioja
La Rioja
San Juan
San Juan
Cordoba
Santa
Fe
32º
Santa
Fe
Cordoba
32º
32º
32º
P
Mendoza
Mendoza
A
UR
UG
UA
Y
M
Buenos
Aires
Buenos
Aires
36º
P
36º
UR
UG
UA
Y
Santa
Rosa
36º
36º
A
Santa
Rosa
Bahia Blanca
40º
0
200 km
68º
64º
Sou
th
ntic
Atla
Bahia Blanca
an
Oce
60º
40º
0
200 km
40º
58º
68º
64º
Sou
th
ntic
Atla
60º
an
Oce
40º
58º
Figure 1.1: Location map showing the Chaco-Pampean Plains in Argentina and areas with prominent higharsenic and fluoride occurrences nation in groundwater.
(FeAsS), cobaltite (CoAsS), enargite (Cu3AsS4),
gerdsorfite (NiAsS), glaucodot ((Co,Fe)AsS), and
elemental As are other naturally occurring Asbearing minerals (Bhattacharya et al., 2002a).
1.3 Environmental geochemistry of arsenic
1.3.1 Sources of arsenic
Arsenic is naturally occurring element earth's crust,
which ranks 20th in abundance in relation to the
other elements. The average As concentration in
continental crust varies between 1-2 mg As kg-1
(Taylor and McLennan, 1985). Arsenic is widely
distributed in a variety of minerals, but commonly
occurs as arsenides of iron, copper, lead, silver and
gold, or as sulfides (Bhattacharya et al., 2002a).
Realgar (As4S4) and orpiment (As2S3) are the two
common As sulfides
During the processes of crystallization of magma,
As does not crystallize in the primary state.
Instead, it increases its concentration in residual
magma, specially in hydrothermal solutions,
forming minerals such as arsenical pyrite (FeAsS),
cobaltite (CoAsS) or ferrous arsenide (FeAs2)
(Rankama and Sahama, 1962). The weathering of
arsenic minerals forms soluble As-oxyanions and
may therefore result in As-rich waters in the
proximities of As deposits.
where As occurs in reduced forms while As occurs
in
oxidized form in the mineral arsenolite
(As2O3).
From its origin in the earth’s crust, As can enter
the
environment
through
natural
and
anthropogenic processes. Natural processes as
weathering, and volcanic eruptions, can release
Loellingite (FeAs2), safforlite (CoAs), niccolite
(NiAs), rammelsbergite (NiAs2), arsenopyrite
2
Groundwater arsenic in the area around Maria Elena in Santiago del Estero Province, Northwestern Argentina
___________________________________________________________________________________
San Isidro
Cerrillos
64º00'
Lag.
Colorada
San Felipe
Abra Grande
San Nicolas
Palo Quemado
Huyamampa
Ahi Veremos
Lag.
Cruzanita
La Loma
San
Antonio
Media
Flor
La Soledad
Qita Punco
San
Roque
Negra Muerta
La Aurora
Bajo Hondo
27º30'
Chaup Pozo
Pto. Viejo
a
lw
ampa fa
ult
rai
yF
.
.C
B.
M.
Huyam
G.
Los Soria
27º30'
San Ignacio
Jumi Pozo
San Isidro
Las Chacras
study area
CLODOMIRA
Bajo Granda
El Aibe
San Javier
Antaje
San Ramon
Tacoyoj
El Puestito
Los Naranjos
El Dean
Cnia.Rasquin
La Capilla
Las Palmitas
Libano Gran Porvenir
CANADA
ESCOBAR
El Simbolar
Pampa Muyoj
Isla Corral
Santa
Elena
Cuyoj
Nuevo
LA BANDA
Los Mojones
Mal Paso
Rodeana
Palmitas
Cara
Pujio
Simbolar
Senora
Pujio
El Alto Pampa Mayo
María Elena
V.Hipolita
lce
Du
Riv
SANTIAGO DEL
ESTERO
er
El Regugio
BELTRAN
Las Bahegas
LAS
FLORES
Morcillo
La Florida
EL
ZANJON
MACO
Pampa Muyoj
Los Arias
railway
settlements
Yanda
Santa
Maria
salt lake, salt flat
Santa
Rosa
department limit
Cara
Pujio
Tusca Pozo
0
10 km
Mili
Cardozos
San Juan
Manogasta
Las Lomitas
Fernandez
Mistol
Pozo Suni
San Ramon
swampy area
Alluvial cone
FORRES
El
La Rivera
Quebrachal
Villa Robles
Maquito
principal road
Chilca
Pto. Nuevo
28º00'
La Higuera
Campo Nuevo
64º00'
Figure 1.2: Map of the Santiago del Estero province showing the geological and geomorphological
characteristics and the location of the area of present study around Department of Robles
3
and becomes soluble in water according to the
reaction:
As into the environment. Arsenic may be
transported over long distances either as dust
through air or as suspended particulates or as
dissolved in water. However, industrial activities
are form the principal source of As emisssion,
which accounts for widespread As contamination
(Bhattacharya et al., 2002a).
As2O3 + 3 H2O ↔ 2 H3AsO30
This dissolution has the following acid reaction:
1.3.2 Speciation of arsenic
Aqueous speciation of As is important in
determining the extent of its interaction with the
aquifer solid phases and thereby its mobility in
groundwater. Arsenic is generally present in two
oxidation states: arsenate [As(V)] or arsenite
[As(III)] for Eh conditions prevalent in most
groundwaters (Figure 1.3).
Arsenic rarely occurs as native elemental form, and
the –3 oxidation state is found only in very
reducing environments. Arsenite [As(III)] has been
considered to be more toxic (U.S Environmental
Protection Agency, 1976), however, more recent
studies have show that most ingested As(V) can be
reduced to As(III). Thus, exposure to both forms
of As may result in similar toxicological effects
(National Research Council, 1999).
H 3 AsO 4
HAsO 4
H 3 AsO 3
AsO 4
HAsS 2
(AsS )
AsS 2
AsH 3
3H2O + As3+
(5)
↔
3H3O+ + AsO33-
(6)
The oxidation state +5 is generated by oxidation
of arsenite to arsenate (AsO43-) whose alkaline salts
are easily soluble in water.
In soft waters with abundant sodium bicarbonate,
Na3AsO4 and Na3AsO3 are the predominant As
species. It can then be predicted that As
concentration increses with increasing alkalinity
(Trelles et al., 1950). On the contrary, waters with
high concentrations of calcium and magnesium
salts (either as carbonates or sulphates) let expect
low concentrations of As.
3-
2-
1.3.3 Adsorption-desorption
Arsenic concentrations observed in most
groundwaters are orders of magnitude lower than
those of most As-bearing minerals.
Thermodynamic predictions show that some As
minerals
could
potentially
control
As
concentrations in groundwater. However, under
common geochemical conditions, these minerals
are rarely formed due to their low natural levels of
concentrations. Therefore, adsorption reactions
between As at mineral surfaces generally play the
most important control on the dissolved
concentration of As in groundwater environments.
Adsorption of As is a complex function of the
(As )
As H 3 =1
6.67
(4)
The alkaline salts of these three acids are soluble in
water and their dissolution has a basic character.
(aq)
P
↔ H3O+ + AsO33-
H3AsO3 + 3H2O
-
HAsO 3
(3)
In slightly acid, neutral or alkaline dissolutions
with a pH >7, anions like the arsenite AsO3-3 are
formed according to the reaction:
As 2 S 3
H 2 AsO 3
H2AsO3- + H2O ↔ H3O+ + HAsO32-
H3AsO3 + 3 H+ →
2-
0.177
(2)
The three acid forms are in equilibrium and
transform one to another according to the
prevailing geochemical environment. At pH <4
the arsenious acid (H3AsO3), preferably forms
As3+ cations according to the equation:
M ost gro und
waters
0.362
H3AsO3 + H2O ↔ H3O+ + H2AsO3-
HAsO32- + H2O
M ost s urfac e
waters
H 2 AsO 4 -
(1)
8.95
Figure 1.3: pH-Eh diagram showing the
thermodynamic stability fields of different forms
of arsenic.
In arsenious oxide (As2O3), the oxidation state is
+3. It is formed during the oxidation of zinc
compounds that contain As. It sublimates easily
4
___________________________________________________________________________________
1.5 Objectives of the present study
interrelationship between the properties of the
solid surface, the pH, the concentration of
dissolved As and other ions comppeting for the
same adsorption sites , and the speciation of
dissolved As.
The general objectives of the present investigation
are:
•
To study the hydrogeochemical characteristics
of the groundwater of the shallow aquifers
situated in Maria Elena area of Banda County
(Santiago del Estero Province, Argentina).
•
To relate the water type, the concentration of
the individual ions, and other chemical
characteristics such as pH, electrical
conductivity,
and
alkalinity
to
the
concentration of As dissolved in the
groundwater.
•
To identify zones with high concentration of
As in the groundwaters and delimit the zones
suitable for exploiting groundwater from the
shallow aquifers that comply with the drinking
water regulations. To identify the different
geomorphological units that promotes the
recharge and mobilization of the groundwater.
•
To evaluate the geochemical characteristics of
the clays and volcanic ashes from different
parts of Santiago del Estero Province, which
may be suitable for the removal of
groundwater As.
1.4 Toxicity of arsenic (health impacts)
The endemical chronical hydroarsenicism is a
social problem that affects the most economically
suppressed rural zones with no access to safe
drinking water. The bibliography explaining
damages caused by As to humans is quite
extensive. These damages can be classified as
acute, subacute or chronical.
It was believed that most of the toxic effects
caused by inorganic As are in its oxidation state
+3, because the oxidation state +5 interacts very
little with the tissues. At the time of intake,
inorganic As is methylated into organic forms like
monomethylarsonic and/or dimethilarsenic acid.
These two acids appear in the body about 6 to 8
hours after the intake and can remain in the urine
for 10 or even up to 70 days. After this time, the
organoarsines are eliminated through the urine. If
the intake of As is constant, the organism can not
eliminate all the As, resulting in its accumulation
within the organism. A dose of 0.1 g of As(III)
causes human mortality.
•
To develop possible low-cost methods of
removal of groundwater As, using natural
clays, which are available in Santiago del
Estero Province, and that can be applied on
household-scale or in community-scale in
small rural settlements.
The work was carried out within the framework of
the agreement as a part of a Linneaus- Palme
Academic Exhange Programme between Royal
Institute of Technology (KTH) Sweden and
Universidad Nacional de Santiago del Estero
(UNSE) Argentina.
Some of the toxic effects in organisms due to
prolonged exposure to inorganic As are:
•
Electrolytic unbalance associated to the loss of
liquids from the blood to the tissues.
•
Inflammation of the eyes and of the
respiratory tract.
•
Loss of appetite and weight.
•
Liver damages.
•
Dermatitis.
Several studies performed on patients living in a
typical area with hydroarsenicism in Cordoba
Province (Argentina) have shown that the minimal
required quantity to produce cancer varies
considerably in every organism. As a general rule,
after 20 years of intake of water containing high
levels of As, cancer can be developed (Arguello et
al., 1939; Yeh et al., 1968).
5
2. AREA OF STUDY
The climate of Santiago del Estero Province is in
general warm and uniform, occasionally reaching
extreme temperatures. The maximum temperature
can exceed the 45 °C during the Summer (JanuaryFebruary) (Prohaska, 1959) and the minimum
temperature can reach values below zero in winter
(July-August). An average of 300 days in the north
and 270 days in the south are free of frost (Boleta
et al., 1989).
The annual precipitation varies from 750 mm to
the east to 500 mm to the west of the province.
The maximum precipitation is measured during
the months of January and February and the
minimum during April and September. From April
to September, the evapotranspiration is highest
ranging from 1100 to 1200 mm. This data is close
to the results obtained through calculations based
on the methodology used by Thornwaite and
Penman, resulting in an evapotranspiration
between 1300 and 1400 mm observing that the
humidity loss is close to the average annual
precipitation, 532 mm (Herrera et al., 1999;
Bundschuh et al., 2004). In most areas of Santiago
del Estero Province (including the study area), the
hydrological balance results in loss of water during
all the months of the year
Precipitation during the years 1912-1999, contains
precipitation data from 87 years taken by the
Instituto Nacional de Tecnología Agropecuaria,
INTA in Santiago del Estero Province (Figure
2.2). The data has been presented in Table 2.1. In
the same the wet and dry seasons can be identified.
The values range from a maximum of 1036 mm
(1986-1987) to a minimum of 205 mm (19941995) with an average of 535 mm.
The precipitation during the last 30 years,
presented in Figure 2.2, indicates a variation
between a maximum of 1036 mm to a minimum
of 350 mm (median of 614 mm). Alteration of
humid and dry seasons, also affects the level of the
water table in the region.
2.1 Geographic location
The study area Maria Elena is located in Banda
county of Santiago del Estero Province
(Argentina), and extends between the latitudes
27.5 and 27.7° W and the longitudes 63.9 and
64.0° S. Maria Elena area lies approximately 30
km to the North of the provincial capital Santiago
del Estero City (Figure 1.2)
2.2 Demographic and socio-economic
situation
The rural population of Maria Elena area is
represented by approximately 400 inhabitants, all
spread out through the locality (INDEC, 1994).
The level of education of the inhabitants is in
general, low. Only a few have basic education.
The economy of the families is based on small
scale agriculture, which produces crops as corn,
lucerne and cucurbita and on breeding of animals
as goats, pigs and some birds for domestic
consumption. The agricultural and breeding
activities were increased after the installation of an
irrigation (and drainage) system, which obtains its
water from river Río Dulce. Despite the attempt to
promote the economy of the region by the
introduction of agricultural activities, the climate
however did not favour its development. Since
1998, the region has been affected by periodical
inundations that have sometimes led to total losses
of the annual harvests.
2.3 Climatic characteristics
The study area can be categorized as semi-arid, the
climate type which prevails in 87.6% of Santiago
del Estero Province. The remaining part of the
province is classified to represent dry sub-humid
climate (Torre Buchmann, 1981).
An interesting and important characteristic of the
zone is that its flat topography, that allows the free
circulation of winds coming from the north as well
as from the south. The northern winds are warm
and dry and have a strong evaporative influence
during the beginning of the spring (Boleta et al.,
1989).
2.4 Geomorphological Setting
The study area is situated at the northwestern
boundary of the Río Dulce alluvial cone. This cone
is delimited to the west by the Huyamampa fault
6
_____________________________________________________________________________________
1200
Precipitation Maria Elena Period (1912-1999)
Maximum1036mm
P(mm)
1000
800
Minimum350mm
Medium611 mm
Minimum
Last 30 years
600
400
200
911-1912
913-1914
915-1916
917-1918
919-1920
921-1922
923-1924
925-1926
927-1928
929-1930
931-1932
933-1934
935-1936
937-1938
939-1940
941-1942
943-1944
945-1946
947-1948
949-1950
951-1952
953-1954
955-1956
957-1958
959-1960
961-1962
963-1964
965-1966
967-1968
969-1970
971-1972
972-1973
974-1975
976-1977
978-1979
980-1981
982-1983
984-1985
986-1987
988-1989
990-1991
992-1993
994-1995
996-1997
998-1999
0
Figure 2.1: Annual precipitation in Maria Elena area, Santiago del Estero Province.
Legend
LOESSIC PLAIN
URBAN AREAS
FLOW DIRECTION
City of Clodomira
FAULT
PALAEOCHANNEL
OF RIO DULCE
PAVED ROAD
ABANDONED
MEANDER
SEASONAL CHANNELS
Figure 2.2: Satellite imagery showing the geomorphic characteristics of the region around Clodomira town.
7
Table 2.1 : Annual precipitation in Maria Elena, during the period between 1912-1999.
Year
1911-1912
1912 -1913
1913-1914
1914-1915
1915-1916
1916-1917
1917-1918
1918-1919
1919-1920
1920-1921
1921-1922
1922-1923
1923-1924
1924-1925
1925-1926
1926-1927
1927-1928
1928-1929
1929-1930
1930-1931
1931-1932
1932-1933
Precipitation
364.4
507
532
337
311
289
313
398
645
714
869
404
426
378
395
691
720
462
843
402
586
447
Year
Precipitation
1933-1934
388
1934-1935
519
1935-1936
391
1936-1937
205
1937-1938
481
1938-1939
615
1939-1940
609
1940-1941
497
1941-1942
501
1942-1943
461
1943-1944
446
1944-1945
294
1945-1946
726
1946-1947
316
1947-1948
604
1948-1949
596
1949-1950
367
1950-1951
291
1951-1952
471
1952-1953
525
1953-1954
386
1954-1955
483
Year
Precipitation
1955-1956
768
1956-1957
473
1957-1958
782
1958-1959
558
1959-1960
522
1960-1961
527
1961-1962
337
1962-1963
640
1963-1964
394
1964-1965
539
1965-1966
488
1966-1967
300
1967-1968
657
1968-1969
471
1969-1970
533
1970-1971
475
1971-1972
367
1972-1973
681
1973-1974
875
1974-1975
521
1975-1976
699
1976-1977
594
Year
1977-1978
1978-1979
1979-1980
1980-1981
1981-1982
1982-1983
1983-1984
1984-1985
1985-1986
1986-1987
1987-1988
1988-1989
1989-1990
1990-1991
1991-1992
1992-1993
1993-1994
1994-1995
1995-1996
1996-1997
1997-1998
1998-1999
Precipitation
624
579
539
711
703
358
586
1036
493
668
507
581
521
627
972
585
350
371
510
929
565
870
Pliocene and Miocene age are covered by
approximately 30 m thick Pampean loess. The
sediments within the Río Dulce alluvial cone are
generally stratified and comprise gravel, sands, silt
and clays together with the volcanic ash present as
discrete beds or as mixed with the fine grained
sediments form the different alluvial aquifers
(Claesson and Fagerberg, 2003; Bundschuh et al.,
2004).
(Figure 1.2). The Huyamampa fault, which runs
from North to South, was caused by a level
difference between two tectonic blocks. The Dulce
River crosses this fault depositing alluvial
sediments, which formed the Río Dulce alluvial
cone. The city of Santiago del Estero is located at
the western margin of the alluvial cone that
extends for about 30 km E-W, and 50 km N-S,
covering an area of 1500 km2 (Farias and Cortes,
1997). Towards the west of the fault the clays of
Four main geomorphological features have been
distinguished in the central part of the Santiago del
Estero Province (Bejarano Sifuentes & Nordberg,
2003). These are: i) the saline depressions
(represented by the Huyamampa lagoons); ii) the
active plain of the Rio Salado river; iii) the Rio
Salado river palaeo-plain and iv) the Rio Dulce
alluvial cone. In the area around Maria Elena, two
major geomorphologic units are distinguished on
the satellite imagery (Figure 2.2):
•
Loess plain
•
Río Dulce alluvial paleoplain
2.4.1 Loess plain
The Loess plain unit is formed by fine sediments
of aeolian origin. Topographically the loess plain is
more elevated towards the eastern part of the
study area, and the elevation declines gradually
towards the west.
8
_____________________________________________________________________________________
Due to the smooth slope of the loess plain, a welldefined water-draining pattern cannot be
determined. The slight inclination of the loess
plain to the west, in the wet season natural parallel
channels (surface runoff) run in the SW-NE
direction (Figure 2.2).
2.4.2 Río Dulce alluvial palaeoplain
It is a part from the old alluvial plain, distinguished
by the abandoned meander segments of the Río
Dulce channel. These abandoned meander
channels have left riverborne deposits, mostly
levees and meanders that are developed along both
the sides of the margins of the river, during the the
flood periods. In the aerial photographs, the
geomorphological characteristics of the palaeoriver channels could bee seen very clearly (Figure
2.2).
9
3. MATERIALS AND METHODS
protocol for sampling (Cardona, 1990; Cardona et
al. 1993). Polyethylene bottles (2 L) washed with
HCl and thoroughly rinsed with deionised water
were used for sampling. Prior to sampling, wells
were purged for a period of more than 15 minutes.
This was considered to be sufficient to obtain a
representative groundwater sample. Bottles were
rinsed at least three times with water to be
sampled, then completely filled to avoid any
contact between sample water and air, which may
cause chemical modifications of the sample. The
groundwater samples were labelled with
coordinates of the sampling points, date, owner’s
name and personal in charge of the sampling.
before their transportation to the laboratory of
UNSE, stored at approximately 4 °C and analysed
within the following 72 hours.
3.1 Compilation of background data
The compilation of existing information on the
study area comprises a review of literature, well
profiles, chemical groundwater analyses data,
satellite images, population census data etc.,
pursued by national, provincial and private
organizations as the Instituto Nacional de
Tecnologia Agropecuaria (National Institute of
Agriculture
and
Farming
INTA),
the
Administración Provincial de Recursos Hídricos
(Provincial
Administration
of
Hydraulic
Resources, A.P.R.H), the Dirección de Estadística
y Censo de la Provincia (Dirección General de
Planificación, 1991), and at the University of
Santiago del Estero UNSE (Forestry and Forests
Management Institute, Department Geology and
Geosciences, Surveying Department).
3.3.3 Determinations of field parameters
The groundwater levels were measured from the
studied wells, and a groundwater level elevation
contour map was drawn. pH, water temperature
and electrical conductivity (EC) were measured on
all the sampled wells. The pH meter was calibrated
at least once a day using pH 4 and 7 buffers.
Measurements of pH, EC, alkalinity, total
dissolved solids, and chloride concentration were
determined directly in the field.
3.2 Desktop studies
Desktop studies included mainly the analysis and
delimitation of the geomorphological units.
Therefore, the LANDSAT image (1: 250,000) was
used and enlarged to 1: 30,000 working scale.
To adjust the geomorphologic units on the map,
panchromatic vertical and black and white aerial
photographs were used (Military Geography
Institute I.G.M) 1: 63,000 scale. The mapped
geomorphological units are outlined in Figure 2.2.
3.3.4 Drilling and sediment sampling
To investigate the sediments of unsaturated zone,
9 drillings (Figure 2.2) were carried out using a
drilling shovel of 100 mm diameter. Sediment
samples were taken every meter or when visible
lithological changes occurred. In general,
sediments were collected from the bore holes at
intervals between of 0-3.0 m except for one
borehole Ibarra Juan where the samples were
collected upto a depth of 10m. From each drilling
site, 8 to 24 sediments samples of 1 kg were taken
(in total 120 samples). Samples were filled into
polyethylene bags and labelled for further
laboratory analyses. The samples were grouped
according to depth and changes in texture, colour,
degree of induration, etc were documented.
Textural classification of the sediments obtained
from 8 profiles in Maria Elena area, Banda
3.3. Field investigations
3.3.1 Inventory of wells and selection of
sampling sites
The census of wells consisted of gathering existing
data on static groundwater level, previous
groundwater sampling and analysis, and the
geographic location of wells using a personal
Satellite navigator Garmin II (Thir, 1999). A total
of 27 groundwater-sampling sites were selected
and referred by numbers 1 to 27 (Figure 3.1).
3.3.2 Groundwater sampling
The groundwater sampling was carried out during
the hydrologic period 1999-2000 according to
specifications by Rodier et al. (1990) and the
10
_____________________________________________________________________________________
Figure 3.1: Map of the study area around María Elena showing the locations of groundwater sampling.
N
SANTIAGO DEL ESTERO PROVINCE
ARGENTINA
Misiones
Province
M2-M10
M1
FIGUEROA
STORAGE LAKE
M6
M3
SIERRA DE GUASAYAN
RÍO HONDO
STORAGE LAKE
M9
M7-M10
M8
LA BANDA
SANTIAGO
M5
DEL ESTERO
M4
RÍ
O
M2
DU
LC
E
RÍ
O
0
50
SA
LA
DO
100 km
Figure 3.2: Map of Santiago del Estero with the location of the clay samples. Inset map of Argenina with the
location of Misiones.
11
examination of water, sewage and wastewater
(APHA, 1975).
Laboratory analyses was carried out only with
groundwater samples which presented no
alterations of their chemical-physical properties as
colour, suspended solids, turbidity, etc. The field
analyses data were additionally used to control
respective laboratory analysis data.
Electrical conductivity, dried residue (105 °C), and
total hardness were determined and, the samples
were analysed for calcium, magnesium, sodium,
chloride, bicarbonate and sulphate.
Alkalinity was determined using the standard
titration technique with sulphuric acid (0.02 N),
using 25 mL of sample, and phenolphthalein and
green bromine as indicators (Rodier et al., 1990).
The chloride content was analysed by titration
with silver nitrate (0.02 N) and potassium
chromate as indicator (Rodier et al., 1990).
The total As concentration was determined using
the silver dithiodicarbamate (SDDC) standardized
method (Rodier et al., 1990).
County, Santiago del Estero Province information
are documented in Table 4.1.
3.3.5 Sampling of clays for adsorption
experiments
Samples of nine clays were collected from Santiago
del Estero Province, Capital, Choya, Banda and
Figueroa localities, were taken. Additionally laterite
sample from Misiones Province was included for
comparison (Figure 3.2).
3.4. Laboratory investigations
3.4.1 Groundwater analyses
The
samples
were
analysed
in
the
hydrogeochemical laboratory of the Geology
Department at the National University. Santiago
del Estero (UNSE). The methodology for the
determination of the various groundwater
chemical parameters are presented in Table 3.1.
Table 3.1: Metodology and equipment used for
groundwater analysis.
Analyte
Method
Electrical Conductivity
Conductivity Method 120.1
pH
Potentiometric Method 150.1
Dried residue at 105ºC
Method 160.3
Total Hardness
EDTA titration. Method 130.2
Calcium
(EDTA 0.02 N) Method 215.2
Magnesium
Complexometric (Rodier et al.,
1990)
Sodium
Flame photometry. (Rodier et al.,
1990)
Chloride
Method 325.3.
Sulphates
Gravimetric Method 375.3
Carbonates
Volumetric Method 310.1
Arsenic
Method 206.4
3.4.2 Textural analyses and clay mineralogy
3.4.2.1 Textural analyses
In the laboratory the samples were dried in a stove
at 105 °C, then mechanically separated and
homogenised. For the textural analyses, ASTM
standard skimmers were used and the technique
suggested by Ingra (1983) was applied. The sieve
size was selected according to PHI grades to
separate in one hand, fraction of thick sand and
loam and on the other hand, fractions of
sediments with clayish nature. This study was
completed by the analysis of the distribution of
sediment
population,
the
mineralogical
components and their alterations that could be
contributing to the existence of groundwater As at
Maria Elena area.
3.4.2.2 Clay mineralogy
The clay mineralogy of the 9 clay samples from
Santiago del Estero and the sample from
Missiones Province were characterized by X-ray
diffractiometry. X-ray diffraction studies were
carried out at Tucumán University (Estratigraphy
and Sedimentology Miguel Lillo Institute) using
Philips PW2510 equipment. The pH value and the
electrical conductivity of their saturation extract
were determined. Cation analysis of the clays were
All analyses were performed under prevailing
norms of the Environmental Protection Agency
(USEPA, 1983) and the standard methods of the
12
_____________________________________________________________________________________
NaHCO3 extraction
performed at Department of Biogeochemistry,
Stockholm University (see Annexure 1).
The residues contents of the conical flasks from
the previous extractions step were kept after
decantation and liquid fraction removal, and used
for bicarbonate extraction by adding 100 mL of
0.01 M NaHCO. After shaking for 2 hours, and
reposing for 12 hours to allow sedimentation of
the solids particles, the procedure from previous
extraction step was repeated. Solutions were
filtered through OOK-filters, acidified and then
again filtered using 0.2 μm filters and stored in 22
mL vials. The bicarbonate extraction was only
applied on two samples M1 and M2 since all other
sediment samples had high pH values (7-10).
3.4.3 Geochemical analyses
Total 10 clay samples from Santiago del Estero
and Missiones (see section 3.3.5) were extracted by
7M HNO3, and the aliquots were analysed for
trace elements and for major ions follwing the
procedure outlined by Claesson and Fagerberg
(2003). Sequential leaching was also performed on
these clay samples.
3.4.3.1 7M HNO3 extraction
Sediment samples were dried at 30 °C. 2 grams of
sample were mixed in a conical flask with 15 mL 7
M HNO3 and thereafter boiled for 2.5 hours on a
sand bed with small glass balls assuring a good
circulation. The samples were digested at 70 °C for
the first 30 minutes, and then at 100 °C for the
next 2 hours. Then after cooling, the solutions
were filtrated through OOK filters, and diluted to
a total of 50 mL with distilled water (DIW).
NaoAc extraction
This step quantifies the elements bound to
carbonate phases. The remaining contents of the
conical flask from the previous step were left for
decantation and liquid fraction removal. 200 mL
of 1.0 M acetate (C2H3NaO2) was added to each
flask. The flasks were left for 2 hours on the
horizontal shaker and the solutions were filtered
through OOK filters, then acidified and stored in
50 mL bottles. In this case no precipitation
occurred so it was not necessary to filter again.
3.4.3.2 Sequenial extractions
Sequential leaching was employed to investigate
sediment composition, which is potentially
important to understand the possible adsorption
sites and hence for their application for
remediation of high As groundwaters of Santiago
del Estero.
Sequential extraction was performed on the 10
selected clay samples using in turn de-ionized
water (DIW), sodium bicarbonate (NaHCO3),
sodium acetate (NaoAc), oxalate (NH4C2O4) and
finally the residual using 7M HNO3. Each step
quantifies the amount of different components
that can be mobilized from the samples.
NH4C2O4 extraction
Selective extractions of the samples where made
using buffered ammonium oxalate (pH 3.5) in the
dark, to quantify the amount of Fe, Al and Mn
bound to the oxide and hydroxides in the
sediments and to quantify the fraction of As in
these phases. The remaining contents of the
conical flask from the previous extraction step
were left for decantation and then liquid fraction
was removed. Later 100 mL of 0.2 M oxalate
(NH4C2O4) was added to the conical flasks in
darkness and left for 4 hours on the horizontal
shaker. The contents of the conical flask from
these phases were left for decantation and the
majority of the liquid fraction was removed to
centrifuge cups. Samples were centrifuged for 15
minutes at 4000 g. The supernatants were filtered
with 0.45 µm Sartoruis filters and diluted 5 times
prior to the analysis.
De-ionized water (DIW) extraction
The sediment samples were leached at pH 6.95
with DIW to quantify the water-soluble fraction of
trace elements. In 250 mL conical flasks, 4 grams
of dry sample were mixed with 100 mL distilled
water. After shaking on a horizontal shaker for 2
hours, the solutions were filtered using OOKfilters, then acidified with 0.5 mL concentrated
HNO3 and stored in 50 mL plastic bottles. In case
that precipitation occurred in the recipients, the
solutions were filtered again using a 0.2 μm filter
and stored in 22 mL vials.
Residual fraction (7M HNO3.) extraction
The remaining sediment in the conical flasks was
dried overnight at 30 °C. Samples were thereafter
extracted in HNO3 following the same procedure
described before for the sediments. This quantifies
the contents in the residue, which were not
13
These clay sediments were chosen based on their
high concentrations of Fe, Al and Mn, present
mainly as oxides and hydroxides, which have
potential sites for As adsorption.
The experiments were carried out using 10 g of
sorbent (clay samples) and arsenate solutions
(K2H2AsO4) with concentrations of 0.5, 1 and 2
mg/L of As+5 and variable pH values (4.5, 5.9 and
8.6). Ionic strength was maintained constant by
addition of electrolyte (0.01 M NaCl). The flasks
were closed and put for 10 minutes on a
horizontal shaker. This system assumes no
competing anions in solution. Since the clays shall
be used on household scale in small filters, the
experiments were conducted without adjusting the
pH of the solutions. Equilibration time was
determined for arsenate. After a contact time of 30
minutes, 90 minutes, 4 hours, 1 day and 2 days,
solutions were filtered through OOK filter.
This experiment was carried out to determining
the amount of As adsorbed as function of contact
time ranging from 30 minutes to 48 h. Initially,
solution containing the adsorbates in single-species
system were prepared with 0.01M of NaCl as
background electrolyte. The soils and the solutions
were then added into each PP bottle to make up a
soil:solution ratio of 1:10. Soil-less blanks (with
addition of the adsorbate only) were also prepared
for determination of initial concentration of the
adsorbates, The suspensions in the bottles and
their pH values were periodically measured, in
order to measure the effect of changes in pH the
adsorption process througout. At the end of the
desired contact time, the bottles were left for
decantation and the supernatants were filtered
through the 0.45 μm Whatman membrane filters.
The filtrates were subsequently kept in the
refrigerator before the analysis.
Arsenic concentration was measured in an ICPOES, manufacturer: Varian, model: Vista Pro Ax.
At the same time, arsenate elimination by each clay
as a function of pH was determined.
The dependency of reaction time and the
speciation of As in adsorption processes has been
studied in recent years (Gulledge and O’Connor,
1973; Pierce and Moore, 1982; Soner et al., 2002;
Singh and Pant, 2004). The promising experiments
in Bangladesh (Larsson and Leiss, 1997) show that
for laterite most of the adsorption takes place
within 5-10 minutes. Since the dependency of time
on the adsorption characteristics is governed by
leached from the sediments by the previous
procedure.
All sediment extractions were analyzed at
Stockholm
University
(Department
of
Biogeochemistry using ICP-OES: Varian: Vista
Pro Ax).
3.4.4 Adsorption batch experiments
During recent years, considerable research has
been carried out to develop methods for efficient
As remediation suitable for applications at
household and/or community levels. The
principles of these methods for As removal are
based on chemical processes (Johnston et al.,
2000) such as:
•
Oxidation and transformation of As into
species that are more easily adsorbed.
•
Precipitation and co-precipitation of As
species allowing separation of solid phase As
from water by filtration.
•
Adsorption of As species to natural materials,
generally with high contents of metal- oxides
and hydroxides.
•
Physical separation of precipitated or
adsorbed As by sedimentation or filtration.
Different degrees of advanced filters can be
used, from denim textile to highly
sophisticated industrial filters.
In most cases a combination of these processes is
applied.
The present study focuses on As removal by
adsorption on natural materials followed by
sedimentation to separate the water from the Asadsorbing media. Natural materials containing
metal-oxides and -hydroxides could have Asadsorption features. Major As-binding minerals are
oxides and hydroxides of Fe, Al and Mn, of which
Fe has been most studied (Smedley and
Kinniburgh, 2002). In many soil types these oxides
and hydroxides occur as coatings on mineral
grains. Arsenate is generally more easily removed
since it occurs as negatively charged anionic
species though adsorption on the surface of the
positively charged media. Clays can under certain
conditions adsorb As because of the oxide-like
structures at their edges.
Based on the results of the sediment geochemistry
(see section 3.4.3), two clays from Santiago del
Estero and one from Misiones were selected.
14
_____________________________________________________________________________________
the geochemical characteristics of the experimental
conditions, this study aimed at measuring the As
concentration in the water first after a time interval
of 30 minutes after mixing the soil into the water.
Approximately 90 minutes, 4 hours, 24 hours and
48 hours later tests were made to assure others
reactions. After 30 minutes the most samples were
clear and transparent, but in some cases
sedimentation was slower and 30 minutes was not
enough for gravity settling. In such cases the
flaskswas left for further sedimentation until was
possible or filtering trough 0.45-μm Whatman
membrane filters. (The results are shown in
Annexe, Table 6).
3.5 Data processing
3.5.1 Microsoft Excel software
Microsoft Excel software was used for data
handling and to plot and correlate the various
hydrogeochemical and sediment geochemical data.
3.5.2 Aqua Chem
Graphical presentation of hydrogeochemical
analysis data was performed using AquaChem
software. Piper and Schoeller-Berkaloff plots were
prepared to classify the groundwater samples.
3.5.3 AutoCAD
The spatial variability of the physical-chemical and
chemical groundwater data within the study area
was processed by AutoCAD to prepare the maps
for the distribution and spatial variability of
different hydrogeochemical parameters such as
electrical conductivity, pH, alkalinity, sulphate,
chloride and others. The same was done to obtain
the spatial distribution of groundwater level
elevation contours.
The method of Stiff was selected for the graphical
representation of the spatial distribution of
analyses data. The obtained spatial distributions of
isolines and the distribution of the Stiff diagrams
were overlaid and plotted using AutoCAD
software on the map of the project area.
15
4. RESULTS AND DISCUSSION
sequence is predominantly of silt texture with
subordinate fine sand and clayey silt in PG8.
This homogeneity extends up to the PG1, where
the predominance of fine sand textured sediments
indicate a paleo-channel, and hence a zone with
higher hydraulic permeability.
4.1 Characterization of the unsaturated
zone
The unsaturated zone ranges from the ground
surface to a depth varying between 1.2 and 2.5 m.
The upper part is composed by fine loess-type
sediments of continental origin, which are
underlain by fluvial and aeolian sediments of
Quaternary age. The textural classification of the
sediments (Table 4.1) obtained from the 8 drilling
profiles (Herrera et. al., 2000) allowed to
characterise the majority of the sampled sediments
as unimodal sediments, of fine to very fine sand
textural composition. This sediment type
represents 64.3% of the sampled sediments. The
second class of sediments (35.7% of the samples)
have a texture of very fine sandy silt to fine silt,
which corresponds to a texture of loess-type
sediments. Minerals as sodium, calcium and
potassium aluminosilicates comprise the fine to
very-fine sand fraction.
The depth to the groundwater table is about 1.40 m in
the NW and slightly deeper in the southeast (1.6 m). In
the zone of the paleo-channel, a silt layer was observed
between a depth of 1.10 and 1.75 m.
4.3.2 Profile 2
This W-E cross-section (Figure 4.2), constructed
from the profiles PG3, PG7, and PG10, shows a
very uniform textural composition. The
predominance of medium, fine and very fine
sands, indicate higher depositional energy
environment compared to the NW-SE crosssection. As exceptions, there exist a single 2.7 m
thick layer of silt fraction (PG7), and a sandy silt
lentil between 2.4 and 4.0 m (PG17).
4.2 Sources of groundwater arsenic
4.3 Hydrogeology
The sediment samples of the unsaturated zone
contain in dispersed form 12% of volcanic ash,
mostly volcanic glass, which shows a high
deglassing level. Unlikely the neighbour county
Robles, in the study area distinct layers of volcanic
ash were not found. This volcanic ash is
considered a potential primary source of the As
dissolved in the groundwater.
The average groundwater level contours of the
shallow aquifer for the hydrological period 19992000 is shown in Figure 4.3. The elevation of the
phreatic groundwater level varies from 1.2 to 2 m
below ground surface. The first value corresponds
to the topographically lowest central zone of the
study area and the later one to the slightly higher
elevated zone towards the west and southeast. The
general groundwater flow direction is from NW to
SE. However, there is a local tendency towards the
center of María Elena town where the
groundwater flow is predominantly fron north to
south.
4.3 Stratigraphic profiles
Textural classification of the sediments were carried out
in the framework of a joint research project by Herrera
et al. (2000) with the aim to establish the lateral
interrelation of the sediment package on a regional
scale. A series of stratigraphic profiles along different
geographical orientations were drawn, from which only
the profiles located within the study area around Maria
Elena is discussed in the following sections.
4.4 Hydrochemical studies
Chemical composition of natural groundwaters are
controlled by the infiltration of surface water,
rock-water
interactions,
and
related
physicochemical, and biological processes, which
are partly climate dependent. The salient
physiochemical parameters of the groundwater
samples are presented in Table 4.2. The major ion
characteristics are presented in Table 4.3.
4.3.1 Profile 1
This cross-section, which was constructed from
the Profiles PG1, PG8, and PG20 crosses the
region in direction of the regional groundwater
flow direction (Figure 4.1).
The sediment
16
_____________________________________________________________________________________
Table 4.1: Textural classification of the sediments obtained from 8 profiles in Maria Elena area, Banda County,
Santiago del Estero Province (modified from Herrera et. al., 2000).
Area
% Retention
Sampling
interval
(m)
10
2
Las Palmitas
Pozo Nº 4
Acuña Juan
Pozo Nº 7
Alvarez Benito
Pozo Nº 3
Alberto Díaz
Pozo Nº 3
Gutiérrez Fernando
Pozo Nº 1
Saad Felipe
Pozo Nº 20
María ElenaAcuña
Rosalía, Pozo Nº 17
Ibarra Juan
Pozo Nº 10
0.0 – 1.2
1.0 – 1.75
1.75 – 2.9
0.0 – 0.6
0.6 – 1.3
1.3 – 1.7
1.7 – 2.0
2.0 – 2.4
2.4 – 2.8
0.0 – 0.6
0.6 – 0.8
0.8 – 1.6
1.6 – 2.0
2.0 – 2.2
2.2 – 3.0
0.0 – 1.0
1.0 – 1.3
1.3 – 1.7
1.7 – 1.9
1.9 – 2.0
2.0 – 2.5
2.5 – 3.0
0.0 – 0.2
0.2 – 0.5
0.5 – 1.1
1.1 – 1.75
1.75 – 2.2
2.2 – 2.6
2.6 – 2.9
0.0 – 0.15
0.15 – 0.45
0.45 – 1.5
1.5 – 1.75
1.75 – 2.4
2.4 – 3.0
3.0 – 3.5
0.0 – 1.2
1.2 – 2.4
0.0 – 1.2
1.2 – 1.8
1.8 – 2.7
2.7 – 4.1
4.1 – 5.6
5.6 – 6.2
6.2 – 10.0
18
1
1.1
1.5
1.5
1.3
4.0
0.8
1.1
0.9
1.2
0.9
1.7
2.1
2.6
2.5
1.6
3.5
5.4
3.6
Sieve size
35
60
120
Particle Diameter (mm)
0.5
2.3
3.4
3.2
2.4
2.6
2.3
3.4
2.9
2.8
5.3
6.9
7.7
5.7
6.9
11.5
1.5
1.7
1.6
1.2
0.9
1.1
5.8
7.8
9.5
5.0
9.7
5.9
9.6
3.3
3.6
4.1
5.2
3.1
2.8
3.9
2.1
1.7
6.1
6.8
3.3
12.4
20.1
26.2
22.1
0.25
5.9
7.1
4.9
6.3
5.7
4.6
4.9
3.2
4.2
11.6
13.4
13.5
12.7
21.4
28.9
3.5
4.1
4.1
2.8
2.5
2.1
2.6
11.8
16.7
12.9
7.9
22.8
22.2
28.2
6.1
6.9
7.76
7.9
5.4
5.7
8.9
4.2
4.2
13.5
19.8
15.5
24.2
32.4
34.0
33.6
17
0.125
9.3
8.2
13.0
14.6
14.3
7.9
6.8
11.5
14.7
14.9
18.3
18.6
15.4
28.7
27.2
10.0
13.5
12.4
7.2
6.5
6.3
8.1
17.8
21.8
13.8
10.8
28.6
34.7
29.9
14.8
16.2
12.6
11.9
14.4
11.6
12.1
14.6
16.5
21.2
25.1
24.0
31.2
25.2
16.7
28.3
Textural Classification
Sand
230
0.06
20.3
24.4
14.6
13.8
13.3
12.4
8.5
12.4
14.4
20.0
16.6
16.1
16.9
18.9
16.9
13.1
12.3
11.4
8.5
8.4
9.3
10.9
18.5
17.2
13.0
9.6
14.9
20.5
13.2
13.8
11.0
11.1
10.8
12.5
17.8
9.7
25.8
20.2
17.4
18.5
21.3
16.1
11.1
7.8
5.5
Bottom
< 0.06
62.2
56.9
64.3
62.9
64.1
72.7
76.4
69.1
64.0
48.3
44.8
42.7
47.9
22.8
11.6
72.0
68.5
70.5
80.2
82.6
81.4
77.3
46.1
36.5
51.0
66.7
24.0
15.9
18.0
61.9
62.4
63.5
63.0
64.6
62.1
65.5
53.4
57.41
40.9
28.9
36.0
12.8
5.6
7.5
4.4
37.8
43.1
35.7
37.1
35.9
27.3
23.6
30.9
36.1
51.7
55.2
57.3
52.1
77.2
88.4
28.0
31.5
29.6
19.8
17.4
18.6
22.7
53.9
63.5
49.1
33.3
76.1
84.2
82.0
38.1
37.6
36.5
37.0
35.4
37.9
34.5
46.6
42.6
59.1
71.1
64.0
87.2
94.4
92.5
95.6
Silt
+
Clay
62.2
56.9
64.3
62.9
64.1
72.7
76.4
69.1
64.0
48.3
44.8
42.7
47.9
22.8
11.6
72.0
68.5
70.5
80.2
82.6
81.4
77.3
46.1
36.5
51.0
66.7
23.9
15.9
18.0
61.9
62.4
63.5
63.0
64.6
62.1
65.5
53.4
57.4
40.9
28.9
36.0
12.8
5.6
7.5
4.4
Silt
49.8
45.6
51.5
49.8
51.3
57.3
61.1
55.3
51.2
29.0
35.9
34.1
28.7
16.0
9.3
57.6
54.8
56.4
64.2
66.1
65.1
54.1
25.5
29.2
40.8
53.4
19.2
12.7
14.4
49.6
49.9
50.8
50.3
51.7
49.7
52.4
32.0
42.4
31.9
20.2
21.6
6.9
3.4
5.3
2.6
Clay
12.4
11.4
12.8
13.1
12.8
15.4
15.3
13.8
12.8
19.3
9.0
8.5
19.1
6.9
2.3
14.4
13.7
14.1
16.1
16.5
16.3
23.2
20.6
7.3
10.2
13.3
4.8
3.2
3.6
12.4
12.5
12.7
12.7
12.9
12.4
13.1
21.4
15.0
9.0
8.7
14.4
5.9
2.2
2.2
1.8
GEOLOGICAL
PROFILE
GEOLOGICAL PROFAILE
N° 1 1
PG-8
0
PG-20
PG-1
1
NF
2
Profundidad en metros
3
4
REFERENCES
5
Sand fine
6
Sand Slime
Clay Slime
7
8
NF
Freatic Level
PG-1
Observation point
Drilling Wells
9
Geological Profaile
CONSEJO DE INVESTIGACIONES CIENTIFICAS Y TECNOLOGICAS
UNIVERSIDAD NACIONAL DE SANTIAGO DEL ESTERO
CENTRO DE INVESTIGACIONES HIDROGEOLOGICAS
FACULTAD DE CIENCIAS EXACTAS Y TECNOLOGIAS
10
Director
Titulo
Ggo. BONIFACIO M. FARIAS
Integrantes
11
Elaboro
Lic. JULIA CORTES
Lic. ANGEL del R. STORNIOLO
Lic. RAUL A. MARTIN
Proyecto
Lic. RAUL A. MARTIN
Lic. MARTIN THIR
Dibujo en CAD
T.H.S. ANTONIO E. RAMIREZ
E.S. GRACIELA FERREIRA SORAIRE
Figure 4.1: NW- SE geological cross section through Maria Elena study area (Santiago del Estero Province)
GEOLOGICAL
GEOLOGICAL
PROFAILE N°PROFILE
2
0
PG-7
PG-17
2
PG-10
PG-3
1
NF
2
P rofund id ad en m etros
3
4
REFERENCES
5
Sand fine
Sand Slime
6
7
NF
Freatic Level
PG-1
Observation point
Drilling Wells
8
Geological Profaile
9
CONSEJO DE INVESTIGACIONES CIENTIFICAS Y TECNOLOGICAS
UNIVERSIDAD NACIONAL DE SANTIAGO DEL ESTERO
CENTRO DE INVESTIGACIONES HIDROGEOLOGICAS
FACULTAD DE CIENCIAS EXACTAS Y TECNOLOGIAS
10
Director
Titulo
Ggo. BONIFACIO M. FARIAS
Integrantes
Elaboro
Lic. JULIA CORTES
Lic. RAUL A. MARTIN
Proyecto
Lic. RAUL A. MARTIN
Lic. MARTIN THIR
Dibujo en CAD
T.H.S. ANTONIO E. RAMIREZ
E.S. GRACIELA FERREIRA SORAIRE
11
Figure 4.2: E-W geological cross-section through Maria Elena study area (Santiago del Estero Province).
18
_____________________________________________________________________________________
Table 4.2: Physico-chemical parameters for the groundwater from María Elena area, Santiago del Estero
Province.
Sample
Coordinates
Level (m)
1
2
X
4394915
4394304
Y
6945824
6943541
Elevation
178
206
3
4
5
6
9
10
12
14
17
19
21
22
23
24
25
26
27
4394443
4388750
4394193
4394173
4394411
4393361
4392203
4392908
4391269
4394577
4389879
4394399
4394763
4389600
4394047
4395155
4386506
6943227
6944289
6943601
6945910
6946990
6944703
6945462
6943282
6945608
6945166
6944641
6945204
6947516
6945192
6944678
6946565
6941685
171
174
185
197
216
186
178
205
205
210
182
191
191
-
TDS
Phreatic
1.80
2.00
mg L-1
87
838
1.80
1.30
1.50
1.12
1.77
1.4
1.42
1.20
1.20
1.25
1.05
1.23
1.20
1.40
1.45
1.30
2.00
840
857
590
930
1248
1990
1166
1355
1582
550
1157
650
1498
940
1380
1940
1810
EC
pH
Hardness
Alkalinity
mg L-1
400
569
290
270
260
240
400
480
550
335
490
275
31
140
470
280
260
380
440
μS cm-1
1210
1309
7.2
7.5
mg L-1
275
569
1145
1208
843
1324
1760
850
1620
1930
2220
780
1651
950
2120
1360
2000
2778
2600
7.2
7.3
7
7.2
7.2
7.2
7.4
7.2
7.3
7.3
7.3
7.2
7.3
7.3
7.3
7.3
7.4
325
45
230
330
580
305
145
500
130
235
280
240
130
350
560
810
690
Figure 4.3: The average groundwater level contours of the shallow aquifer for the hydrological period 19992000 and the direction of groundwater flow in Maria Elena area, Santiago del Estero province.
19
Table 4.3: Results of groundwater composition in selected wells from Maria Elena.
Owner
Well
pH
EC
µS cm-1
TDS
mg L-1
HCO3mg L-1
Clmg L-1
Ca2+
mg L-1
Mg2+
mg L-1
Gutiérrez, Fernando
1
7.2
1300
900
518
85
68
24
Álvarez, Benito
3
7.2
1200
820
366
160
80
36
Martínez, José
5
7.1
860
602
323
85
50
28
González, Juan. C
9
7.3
1780
1240
605
163
196
26
Ibarra, Juan
10
7.1
2920
2040
603
376
70
36
Barrionuevo, Leonor
14
7.2
1970
1370
561
248
140
60
Ávila, Héctor
21
7.3
1660
1168
439
213
50
36
hardness established by WHO is 400 mg L-1 (CaCO3
equivalent) is exceeded, making them unsuitable for
human consumption.
4.4.1 Hydrochemical characterization
The characterization of groundwater is based on
the chemical composition and the physicalchemical parameters
4.4.1.4 pH
The spatial variation of the pH values is low.
Values range between 7 and 7.5 (average 7.27)
(Figure 4.4c). This is due to the presence of weakly
ionized acids and bases, which keep almost
constant pH in presence of dissolved CO2 and
HCO3-.
4.4.1.1 Electrical conductivity
The electrical conductivity (EC) of water is directly
related to concentration of dissolved ions
expressed as meq L-1. Most of the water samples
(74%) have electrical conductivities between 780
and 1930 µS cm-1. Only 26% have higher values
between 1930 and 2778 µS cm-1. The average value
is 1560 µS cm-1. The spatial distribution of the EC
isoclines (Figure 4.3a) indicates that the
groundwaters with the lowest EC values, and
hence the best quality are found in the eastern part
of the study area. This can be explained by the
presence of a paleo-river in this zone, whose better
permeable sediments result in higher groundwater
recharge and higher groundwater flow velocities.
4.4.1.5 Chloride
The chloride concentration ranges from 78 to 390
mg L-1, the average value is 200 mg L-1 (Figure
4.4d). Low chloride concentrations of less than
150 mg L-1, may result from increased infiltration
of low mineralized water from the paleo-river or
by infiltration of low mineralized irrigation water.
Chloride concentrations of all 10 samples are
below the chloride drinking water limit established
at 700 mg L-1 by the WHO.
4.4.1.2 Total dissolved solids (TDS)
The 19 analyzed groundwater samples have TDS values
between 550 and 1990 mg L-1 (average 1167 mg L-1),
which are below the WHO limit (2000 mg L-1). The
spatial distribution of the TDS isolines (Figure 4.4b)
corresponds to those of the electrical conductivity.
4.4.1.6 Alkalinity
The alkalinity of most of the samples ranges
between 300 and 500 mg L-1 (expressed as CaCO3)
(Figure 4.3c). Only the samples from two wells
have values above 500 mg L-1 and the remaining
have values bellow 300 mg L-1 of CaCO3. The
maximum allowed total alkalinity level expressed
as CaCO3 is 800 mg L-1.
4.4.1.3 Water hardness
The average value of the water hardness is 354 mg L-1
expressed as calcium carbonate equivalent (CaCO3).
Most values range between 130 and 350 mg L-1 and
only few exceed 500 mg L-1. In 5 of the 19 water
samples, the maximum allowed value for water
20
_____________________________________________________________________________________
EC
TDS
a
b
Cl-
pH
c
d
HCO3-
SO42-
e
f
Figure 4.4: Spatial variability of: a) electrical conductivity (EC); b) total dissolved solids (TDS); c) pH; d)
chloride (Cl-); e) bicarbonate (HCO3-); and f) sulfate (SO42-) in the shallow groundwater of Maria Elena. See
Figure 4.3 for legend.
21
4.4.1.7. Sulphate
of the Na-HCO3-Cl type, followed by 26% water
samples of Ca-Na-HCO3-Cl type and the rest 11%
characterized as Na-SO4-type (Figure 4.6). The
major anions show the following order of
predominance:
r HCO3- > r Cl- > r SO42The predominance of bicarbonate ions is probably
due to dissolution of evaporites found in the
sediments or due to redissolution of carbonates
from sediments of the unsaturated zone by
leaching through the infiltrating rain and irrigation
water containing organic acids.
The second abundant anion is chloride. Its
presence may be due to dissolution of evaporites
and the prevailing cation is Na+ indicating the
occurrence of ion exchange processes between
groundwater and sediment whereby the ions of the
earth alkaline elements in the groundwater are
replaced by Na+ resulting in sodium-bicarbonate
waters. The order of predominance of the major
cations are indicated by the relation:
The SO42--ion is less soluble and less mobile than
the chloride ion, and hence may precipitate under
excess of dissolved Ca2+ and Mg2+ ions. Sulphate
ions may be reduced by the Sporovibrium
Desulfuricans bacteria. It comes mainly from the
oxidation of metallic sulfides and the dissolution
of gypsum or anhydrate. Most of the water
samples have sulphate concentrations below 300
mg L-1 and only 5 samples exceed the 400 mg L-1
limit established by the WHO (Figure 4.4f).
4.4.1.8. Arsenic
Dissolved As could be detected in all water
samples, however the spatial variability of its
concentration is high (Figure 4.5). About 73% of
the samples have As concentrations above the
Argentine limit (0.05 mg L-1 Argentine Food
Codex), making them unsuitable for human
consumption.
4.4.2 Geochemical classification
Groundwater samples were classified (Table 4.4)
according to their percental main anion and cation
composition (expressed as meq%) and plotted in a
Piper diagram (Figure 4.6). When a major anion or
cation in water exceeds 50% of total anions or
cations respectively, the water is classified after the
respective anion or cation. In the case that the
50% limit is not reached, the two most abundant
ions are used instead.
Among the total 20 analyzed samples, 63% were
r Na+ > r Ca2+ > r Mg2+
The chemical groundwater analysis data were
plotted in Schoeller diagram (Figure 4.7).
Figure 4.5: Spatial distribution of arsenic concentration (mg L-1) in the shallow groundwaters of Maria Elena
area. See Figure 4.3 for legend.
22
_____________________________________________________________________________________
Table 4.4: Schoeller’s classification of the shallow groundwaters of Maria Elena area and their suitability for
drinking purposes.
Sample Anions
Cations
Classification
EC/ Type I Human
consumption
1
2
3
4
5
6
9
10
12
14
17
18
19
21
22
23
24
25
26
27
Na+ > Ca+2 >Mg+2
Na+ > Mg+2 > Ca+2
Na+ > Ca+2 >Mg+2
Na+ > Ca+2 >Mg+2
Na+ > Ca+2 >Mg+2
Na+ > Ca+2 >Mg+2
Ca+2 >Na+ > Mg+2
Na+ > Ca+2 >Mg+2
Na+ > Mg+2 > Ca+2
Na+ > Ca+2 >Mg+2
Na+ > Ca+2 >Mg+2
Na+ > Ca+2 >Mg+2
Na+ > Ca+2 >Mg+2
Na+ > Ca+2 >Mg+2
Na+ > Ca+2 >Mg+2
Na+ > Ca+2 >Mg+2
Na+ > Ca+2 >Mg+2
Na+ > Ca+2 >Mg+2
Na+ > Ca+2 >Mg+2
Na+ > Ca+2 >Mg+2
Na-HCO3- type
Na-HCO3- type
Na-HCO3- type
Na-HCO3- type
Na-HCO3- type
Na-HCO3- type
Ca-HCO3- type
Na-Cl- type
Na-HCO3- type
Na-Cl- type
Na-HCO3- type
Na-SO4- type
Na-HCO3- type
Na-Cl- type
Na-SO4- type
Na-HCO3- type
Na-HCO3- type
Na-SO4- type
Na-SO4- type
Na-SO4- type
5a/C-3
5a/C-3
5b/C-3
5b/C-3
5b/C-3
6b/C-3
6e/C-3
2b/C-3
5a/C-3
2b/C-3
6b/C-3
4b/C-4
6b/C-3
2b/C-3
3b/C-3
5b/C-3
5b/C-3
3b/C-3
1b/C-3
2b/C-3
HCO3- > Cl- > SO42HCO3- > Cl- > SO42HCO3- > Cl- > SO42HCO3- > Cl- > SO42HCO3- > Cl- > SO42HCO3-> SO42- > ClHCO3-> SO42- > ClCl- > HCO3-> SO42HCO-3 > Cl- > SO42Cl- > HCO3- > SO42HCO3-> SO42- > ClSO42-> HCO3- > ClHCO3-> Cl- > SO42Cl- > HCO3- > SO42SO42-> Cl- > HCO3HCO3- > Cl- > SO42HCO3- > Cl- > SO42SO42-> Cl- > HCO3Cl- > SO42- > HCO3Cl- > HCO3-> SO42-
suitable
suitable
suitable
suitable
suitable
suitable
suitable
suitable
suitable
suitable
suitable
not suitable
suitable
suitable
suitable
suitable
suitable
suitable
suitable
suitable
Figure 4.7: Major ion composition of the groundwater.
samples plotted on a Schoeller-diagram.
Figure 4.6: Major ion analyses plotted on a Piper
diagram illustrating the predominance of Na-HCO3,
Ca-Na-HCO3-Cl, and Na-SO4-types of groundwater
composition in Maria Elena.
23
Figure 4.8: Spatial variation of groundwater types presented as Stiff diagrams.
4.5.1.2. Cl- and HCO3-
The Stiff diagrams show the spatial variation of
the groundwater composition and groundwater
type within the study area (Figure 4.8). High
concentrations of Na+ in groundwater clearly
indicate cation exchange between groundwater
and aquifers sediments.
The Cl-/HCO3- ratio allows to trace an increasing
salt concentration along the groundwater
flowpath. The samples classified as Na-HCO3 type
waters have Cl/HCO3 ratios (in 58% of the
samples) ranging between 0.3 and 1.4. These
values indicate a continental effect (sea water has
values in the range of 20-50). Further, an increase
in the chloride concentration is observed in these
samples although the bicarbonate concentrations
were constant.
The plot between the major ions Na+ and Ca2+
against HCO3- indicates two distinct groups
(Figure 4.9a,b). High positive correlation (R2 =
0.93) was observed for the waters, which were
mostly Na-HCO3-type. However, the sample
population with lower Na+ levels also indicated a
moderate positive correlation (R2 = 0.46). The
Ca2+ concentrations in these water samples
indicate a positive correlation with HCO3- (R2 =
0.60) that indicate that the groundwater
composition is partially controlled by the
dissolution of carbonates. However, the samples
with low concentrations of Ca2+ do not show any
specific trend with HCO3- (Figure 4.9b).
4.5. Relationships between hydrochemical
parameters
4.5.1. Relationships between major ions
4.5.1.1 Mg2+and Ca2+
Along the groundwater flowpath and hence with
increasing time, Ca2+ and Mg2+ may be exchanged
with Na+ from the solid phase. This cation ion
exchange process reduces the hardness of the
groundwater, but it also increases its sodium
concentration, which favors the dissolution of
sulfates.
The Mg2+/Ca2+ ratio varies between 0.21 (sample
M9) and 1.63 (sample M2). These relative low
values indicate a continental geochemical effect,
with dominance of the calcium over magnesium
ions.
24
_____________________________________________________________________________________
800
400
y = 61.166e
600
0.0036x
2
R = 0.935
500
400
300
200
y = 0.4124x - 6.1513
a
100
Ca2+ (mg L-1)
Na+ (mg L-1)
700
350
y = 0.7925x - 186.84
300
R = 0.59
250
200
150
100
R = 0.456
0
0
200
400
HCO3-
250
600
800
(mg L )
200
400
Mg2+ (mg L-1)
c
(mg L )
2
R = 0.422
50
40
30
20
y = 0.0427x + 7.658
d
10
0
800
y = 11.978ln(x) - 25.457
60
100
600
-1
HCO3-
70
150
50
0
-1
y = 90.679ln(x) - 349.5
2
R = 0.822
200
Ca (mg L-1)
b
50
2
0
2
R = 0.29
0
0
100
200
SO42-
300
400
500
0
100
200
-1
SO42-
(mg L )
300
400
500
-1
(mg L )
600
600
y = 273.54ln(x) - 1077.1
y = 188.52ln(x) - 645.84
500
500
2
R = 0.797
400
300
200
100
y = 0.4184x + 60.099
2
R = 0.552
e
0
Na+ (mg L-1)
Na+ (mg L-1)
2
2
R = 0.928
400
300
200
y = 145.73ln(x) - 577.31
100
2
R = 0,870
f
0
0
100
200
SO42-
300
400
500
-1
0
100
200
300
-
(mg L )
400
500
-1
Cl (mg L )
Figure 4.9: Bivariate plots between HCO3- vs a) Na+ and b) Ca2+; SO42- vs c) Ca2+, d) Mg2+ and e) Na+; and Cl- vs Na+
showing two distinct groupsof groundwater compositions in the study area around Maria Elena, Santiago del Estero
Province.
4.5.1.3. SO42-and Cl-
concentrations of SO42- in these samples also
exhibit a moderate positive correlation with Mg2+
(R2 = 0.42). The group of samples with high Na+
concentrations shows a strong positive correlation
with SO42- (R2 = 0.79; Figure 4.9e) as well as with
Cl- (R2 = 0.92; Figure 4.9f). Similar trends of
positive correlations of Na+ were also found with
SO42- (R2 = 0.55) and Cl- (R2 = 0.87) for the other
group of samples characterized by low Na+
concentrations (Figure 4.9e,f).
The SO4 concentration in groundwater is
generally high and results in an increased salt
concentration. The relation between SO42-and Clshows similar behaviour similar to the previous
one. Six of the 19 samples have values higher than
1, while the others indicate values prevailing in the
range between 0.25 to 0.99.
Bivariate plots between SO42- concentrations in
these groundwaters show two distinct groups
(Figure 4.9c-e). A strong positive correlation is
observed between the distribution of Ca2+ and
SO42- (R2 = 0.82) in the samples with high Ca2+
concentrations indicating a probable dissolution of
gypsum or anhydrite from the sediments. The
2-
4.5.1.4 Ionic exchange capacity
The ionic exchange capacity (IC) is defined by the
proportionality of the relation between Cl- and
ions of alkaline elements such as Na+ and K+.
Negative IC values show the predominance of
25
the central area of Maria Elena area, where
groundwater flow is stagnant (Figure 4.10). Zones
with low groundwater As concentrations (<50 µg
L-1) indicate areas with groundwater most suitable
for drinking purposes (Fig. 4.10). A comparison of
the zones high and low in groundwater As (Figure
4.10) with the corresponding spatial distribution of
other parameters (Figures 4.4a-f) can be carried
out at least for the high As zone around Las
Abritas and the zone with low As concentrations
close to Las Palmitas, where most of the sampled
wells are located., whereas in the high As zone
around Bayo Muerto and the low As zone of
Rosquín, the sampling points are too few to do so.
Thereby it can be recognized that the zones with
highest As concentrations are coincident with
zones of stagnant groundwater (Figure 4.3), high
electrical conductivity (Figure 4.4a), high TDS
levels (Figure 4.4b) and high concentrations of Cl(Figure 4.4d) and SO42- (Figure 4.4f).
sodium and potassium ions over chloride.
Increasing negative values of IC indicate an
increase of dissolved Na+ (or ions of other alkaline
elements), as it may occur (1) by dissolution from
the sediments solid phase or (2) along
groundwater flowpaths or in stagnant groundwater
where increasing residence time of the
groundwater results in cation exchange (exchange
of Ca2+ and Mg2+ for Na+). This ion exchange and
the related reduction of water hardness explain
why the most negative IC values coincide with the
values of lower water hardness. In the study zone,
the minimum IC value is –2.40 (M12) and the
maximum is –0.06 (M26).
4.6 Distribution of groundwater arsenic
4.6.1 Spatial variations
The spatial variation in the distribution of As in
groundwater reveals two distinct zones with high
As concentrations (>50 µg L-1) (Figure 4.10). Both
correspond to geomorphological depressions in
Figure 4.10: Spatial distribution of the zones whit high and low concentration of arsenic in the area around Maria Elena.
26
_____________________________________________________________________________________
negative correlation was found between Mg2+ and
As (R2 = 0.56) (Figure 4.11b). The correlation
between As with the major ions Ca2+ and Cl- as
well as pH indicates two distinct groups (Figure
4.11c,d,f). Strong positive correlation of As and
Ca2+ characterizes the samples with low Ca2+
concentration (R2 = 0.81) while a trend of
increasing As concentration is found for waters
with increasing Cl- concentrations (R2=89). A
distinct positive correlation is noted amongst pH
and As (Figure 4.11f) with two distinct data
groups.
4.6.2 Relationships between As and other
hydrochemical parameters
The spatial variability of As concentrations in
groundwaters of Maria Elena area is clearly
reflected through the relationships between the
distributions of the major ions. The relationship of
arsenic has been plotted against the major cations
Na+, Mg2+, and Ca2+, the major anions HCO3- and
Cl-, and pH (Figure 4.11a-f).
A well defined positive correlation is observed
between As and Na+ (R2 = 0.63), As and HCO3(R2 = 0.69) (Figure 4.11a,e), while a distinct
500
500
a
300
200
y = 7.28e
100
b
400
As (µg L-1)
As (µg L-1)
400
y = -183.8ln(x) + 694.9
300
2
R = 0.561
200
0.0075x
100
2
R = 0.63
0
0
0
100
200
300
+
400
500
600
0
10
20
-1
2
70
d
y = 7.266x + 50.763
2
400
R = 0.807
As (µg L-1)
As (µg L-1)
60
500
c
y = 14.915x - 188.02
300
200
0.365
y = 4.527x
R = 0.36
y = 40.381ln(x) - 18.082
300
2
R = 0.894
200
100
2
R = 0.44
0
0
0
50
100
Ca
2+
150
200
200
300
400
500
-1
Cl (mg L )
(mg L )
500
f
61.689
y = 2E-51x
2
R = 0.972
400
0.008x
As (µg L-1)
y = 2.022e
2
R = 0.686
300
100
-
e
400
0
250
-1
500
As (µg L-1)
50
-1
Mg (mg L )
500
100
40
2+
Na (mg L )
400
30
200
100
300
200
y = 114.41x - 806.08
2
R = 0.55
100
0
0
0
200
400
HCO3-
600
800
-1
6,9
7,0
7,1
7,2
7,3
7,4
7,5
7,6
pH
(mg L )
Figure 4.11: Bivariate plots between As vs a) Na+; b) Mg2+; c) Ca2+; d) Cl-; e) HCO3- and f) pH in the shallow
groundwaters around Maria Elena, Santiago del Estero Province.
27
Natural materials containing metal-oxides and hydroxides may have As-adsorption features.
Major As-binding minerals are oxides and
hydroxides of Fe, Al and Mn, of which Fe has
been most studied (Smedley and Kinniburgh,
2002). In many soil types, these oxides and
hydroxides occur as coatings on mineral grains.
Arsenate oxy-anion, is adsorbed strongly on
positively charged surfaces of the oxides of Fe, Al
and Mn at specific pH conditions (Table 4.6).
4.7 Sediment characteristics
4.7.1 Overview
In this section, the salient geochemical
characteristics of the sediments from Santiago del
Estero Province considered to be important for
the removal of As from of water is discussed. The
overall geochemical characteristics of the
sediments will form the base for selecting the
specific local sediments, which may be used as
adsorbents for removal of As from groundwater.
Table 4.5: General mesoscopic characteristics and mineralogy of the clayey sediments from Misiones Province (M1) and
different parts of Santiago del Estero (SdE) Province (M2-M10). L= Low, H= High.
Sample Location
Color
Crystallinity
Principal clay minerals
M1
Misiones
Poor
M2
Guaptayan Choya (SdE)
Reddish
brown
Brick red
Poor
L-beidellite 28%, kaolinite 70%, quartz <1.5%, possibly
smectite (<0.5%)
L-beidellite 61%, illite 39%
M3
Rio Salado (SdE)
Brick red
Good
H-montmorillonite 38%, illite 58%, chlorite 6%
M4
25 de Mayo Choya (SdE)
Off-white
Moderate to poor
H-montmorillonite 99%, quartz 1%
M5
Dpto Capital (SdE)
Brick red
Moderate
M6
Dpto Figueroa (SdE)
Brick red
Moderate
M7
Cantera Becaria (SdE)
Brick red
Poor
L-montmorillonite 36%, illite 24%, muscovite 19%,
heulandite 19%, gypsum 2%
H-montmorillonite 25%, illite associated with Mmuscovite 69%, chlorite 6%
Kaolinite 10%, beidellite 19%, illite 71%
M8
Lomas Coloradas (SdE)
Brick red
Poor
L-montmorillonite 60%, illite 39%, chlorite 1%
M9
Volcanic ash (Banda County)
Off-white
Poor
M10
Brick red
Low
H-montmorillonite 17%, illite 71%, chlorite 6%,
heulandite 6%
H-montmorillonite 12%, illite 86%, chlorite 2%
Table 4.6: Geochemical characterization of the clay sediments from Misiones Province (M1) and clays and the volcanic
ash sediments from different parts of Santiago del Estero Province (M2-M10).
Sample Location
pH
CEC
meq 100g-1
EC
μS cm-1
Fe
mg g-1 solid
M1
M2
M3
M4
M5
M6
M7
M8
M9
M10
4.46
6.49
7.45
7.14
7.95
7.98
7.7
7.36
9.88
7.68
18.3
39
36.7
40
23
25
18
40
34.4
25
72.2
2910
2945
360
9240
5700
190
1250
5100
6210
40.3
27.7
7.6
17.8
3.5
5.7
2.3
7.8
3.6
4.2
Misiones
Guaptayan Choya,(SdE)
Rio Salado (SdE)
25 de Mayo Choya,(SdE)
Dpto Capital (SdE)
Dpto Figueroa (SdE)
28
_____________________________________________________________________________________
about neutral to slightly alkaline (6.8 to 8.0) (Table
4.6). The saturation extract of the volcanic ash
sample (M9) has a significantly higher pH of 9.9.
In contrast, the clay sediment sample from
Misiones Province (M1) is moderate acid
(pH=4.5), because of its low buffer capacity. This
is reflected by the low electrical conductivity of 72
μS/cm, compared the much higher EC values of
the saturation extracts of the clay sediments from
Santiago del Estero Province, with ranges from
1250 to 9240 μS cm-1, with exceptions of M4 with
360 μS cm-1 and M7 with 190 μS cm-1 which may
be due to the high degree of weathering of the
samples.
4.7.2 Mineralogical characteristics of the
sediments
X-ray diffraction studies on the 9 clay samples
from Santiago del Estero Province and on one
sample from Misiones Province revealed distinct
clay mineralogy. The clay samples from Misiones
revealed a dominance of kaolinite (70%) followed
by beidilite (28%). The clay samples from different
parts of Santiago del Estero Province revealed
presence of beidillite (19-61%), illite (24-71%),
montmorrillonite (25-99%), chlorite (1-6%),
heulandite (6-19%), quartz (~1%) and gypsum
(~2%). The detailed clay mineralogical
constitution is presented in Table 4.5.
4.7.4 Cation exchange capacity
The cation exchange capacity (CEC) of the
sediment samples is more or less uniform, ranging
from 18.3 to 40 meq 100g-1 of sample (Table 4.6).
4.7.3 Characteristics of the soil saturation
extract
The pH of the sediment saturation extracts from
the clay sediment samples from Santiago del
Estero Province (M2-M8, M10) show values from
Table 4.7: CEC values determined for some selected clay minerals in the samples of Misiones and Santiago del Estero.
S. No.
Clay Minerals
CEC meq 100g-1
1
2
3
4
5
6
7
Kaolinite
Halloisite
Illite
Chlorite
Vermiculite
Montmorillonite
Sepiolite-palygorskite
3-5
10-40
10-50
10-50
100-200
80-200
20-35
Table 4.8: Distribution of 7M HNO3 extractable Al, Fe and Mn and As in the sediment samples from Misiones and
Santiago del Estero.
Sample
Location
AlNO3
FeNO3
MnNO3
AsNO3
(g kg-1)
(g kg-1)
(g kg-1)
(mg kg-1)
M1
Misiones
50.08
103.36
0.63
2.90
M2
21.65
63.55
0.14
161.53
M3
Rio Salado (SdE)
26.65
14.54
0.33
5.10
M4
21.10
11.04
0.08
1.41
M6
Depto. Figueroa (SdE)
30.64
12.92
0.30
3.80
M8
25.54
30.03
0.44
9.57
M9.1
3.81
4.72
0.11
2.83
M9.2
3.98
4.99
0.11
3.15
MCV:1
Volcanic ash (Cuatro Horcones)
3.95
4.90
0.11
3.24
MCV:2
Volcanic ash (Cuatro Horcones)
4.52
5.61
0.12
3.28
29
4.7.6 Results of geochemical analyses
Choya and Departamento Figueroa. In general,
AlNO3 and FeNO3 concentrations in the volcanic
ash sediments vary in the range between 3.8-4.5 g
kg-1 and 4.7-5.6 g kg-1. MnNO3 concentrations were
lower (0.08-0.44 g kg-1) as compared to the AlNO3
and FeNO3 (see Figure 4.12). The distribution of
AsNO3 in the analyzed sediments indicates a
considerable variability (1.41-161.5 mg kg-1), while,
the volcanic ash samples had similar levels of
AsNO3 (2.8-3.3 mg kg-1), which are similar to those
reported by Claesson and Fagerberg (2003). The
most important aim was to find the best samples
for As adsorption using the amount of Al and Fe
in the soil samples.
4.7.6.1 7M HNO3 extraction
4.7.6.2 Sequential extraction
In general these values agree with the CEC values
from the literature (Ladeira and Ciminelli, 2004).
The CEC values for some of the selected clay
minerals are presented in Table 4.7.
4.7.5 Iron content
The iron content of the sediment, as an index for
potential As adsorption capacity, is highest in the
laterite from Misiones Province (M1, 40.3 g kg-1
Fe), while the iron contents of the samples from
Santiago del Estero Province show considerable
variability, ranging from 2 to 27 g kg-1.
The results of the sequential extractions of the
samples of the laterite from Missiones Province,
five clay sediment samples and four samples of
volcanic ash from Santiago del Estero province are
presented in Table 4.9 and Figure 4.13.
De-ionized water (DIW) extraction
Sequential extraction of the clay sediments with
de-ionized water (DIW) revealed considerable
variations in the amounts of Al ranging in
concentrations varying between 27.5 mg kg-1 and
173.2 mg kg-1. Maximum concentrations
1000,00
100,00
Al
10,00
Fe
Mn
1,00
As
0,10
Samples
Figure 4.12: Distribution of Al, Fe, Mn and As in the samples extracted by 7M HNO3 .
30
2
M
C
V:
M
C
V:
1
9.
2
M
9.
1
M
8
M
6
M
4
M
3
M
M
M
2
0,01
1
Al, Fe, Mn (g/kg), As (mg/kg)
Data on salient geochemical characteristics of the
laterite from Missiones Province, five clay
sediment samples and four samples of volcanic ash
from Santiago del Estero province are presented in
Table 4.8. 7M HNO3 extraction indicates high
concentrations of AlNO3, (50.1 g kg-1) and FeNO3,
(103.4 g kg-1) and low MnNO3 (0.63 g kg-1). The
clay samples from different areas of Santiago del
Estero Province revealed moderate high AlNO3,
(21.1-30.6 g kg-1) and high FeNO3 (30.0-63.6 g kg-1)
for the samples from Guaptayan-Choya and
Lomas Coloradas and a low FeNO3 (11-14.5 g kg-1)
in the clay samples from Rio Salado, 25 de Mayo-
_____________________________________________________________________________________
Table 4.9: Result of sequential extraction of the sediment samples from Misiones and Santiago del Estero.
Sample Location
AlDIW
AlHCO3 AloAc
Alox
Alres
FeDIW
FeHCO3 FeoAc
Feox
Feres
MnDIW MnHCO3 MnoAc Mnox
Mnres
AsDIW
AsHCO3 AsoAc
Asox
--------------(mg kg-1)------------- (g kg-1) --------------(mg kg-1)------------- (g kg-1) --------------(mg kg-1)------------- (g kg-1) -------------(mg kg-1)------------
M1
M2
Misiones
M3
M4
Rio Salado (SdE)
M6
M8
Depto. Figueroa (SdE)
M9.1
M9.2
Asres
(mg kg-1)
38.2
173.2
0.62
0.31
0.15
0.30
50.9
41.9
13.37
2.76
5.9
27.8
0.43
0.02
0.02
0.02
86.2
32.5
52.6
2.69
47.6
8.59
0.18
0.53
0.09
0.03
48.8
10.7
0.17
0.044
0.12
5.74
0.12
0.82
0.27
0.82
0.12
1.64
0.55
20.7
137.4
27.5
na
na
0.44
17.4
80.7
102.2
1.57
2.96
19.1
2.3
na
na
0.02
0.02
39.1
4.4
6.36
0.93
34.7
2.91
na
na
1.58
0.84
20.1
1.23
0.079
0.013
1.29
0.47
na
na
0.41
0.30
0.21
0.19
1.08
0.33
100.9
144.4
na
na
0.21
1.93
70.6
72.3
2.68
4.85
13.5
7.4
na
na
0.02
0.02
34.6
37.0
5.49
7.15
20.0
10.7
na
na
0.84
4.17
21.8
18.9
0.074
0.106
0.79
3.85
na
na
0.41
0.54
0.13
0.27
0.84
2.78
24.8
87.6
na
na
0.03
0.14
24.1
90.4
1.77
1.66
15.1
9.9
na
na
0.02
0.02
17.0
9.8
2.49
2.23
1.38
6.12
na
na
0.48
0.94
1.86
6.09
0.041
0.041
0.12
2.51
na
na
0.38
0.24
0.12
2.80
0.33
0.32
MCV:1 Volcanic ash (Banda County) 75.9
MCV:2 Volcanic ash (Banda County) 107.0
na
na
0.03
0.04
27.1
19.4
1.65
1.84
8.2
22.6
na
na
0.02
0.02
17.5
11.7
2.35
2.54
5.31
7.00
na
na
0.56
0.43
1.33
1.32
0.038
0.041
2.48
2.55
na
na
0.39
0.28
0.11
0.11
0.29
0.44
of AlDIW was found in the clays from Guaptayan
Choya, Lomas Coloradas and Rio Salado regions
with concentrations of 173.2 mg kg-1 144.4 mg kg-1
and 137.4 mg kg-1 respectively. Concentrations of
Fe and Mn in the DIW extracts ranged between
5.9-27.8 mg kg-1 and 1.38-47.6 mg kg-1 respectively
(Table 4.9), while As concentrations in the DIW
extracts were low (0.12-0.47 mg kg-1. However the
clay samples from Choya and Lomas Coloradas
indicated considerably high AsDIW concentrations
(1.29-5.74 mg kg-1).
The volcanic ash samples from Banda County also
indicated higher range of concentrations of AlDIW
(24.1-107 mg kg-1), FeDIW (8.2-22.6 mg kg-1),
MnsDIW (1.38-7.0 mg kg-1) as well as high
concentrations of AsDIW (2.48-2.55 mg kg1).However, one sample from (M9.1) revealed
lower concentration of As in the DIW extract (see
Table 4.9 and Figure 4.13).
between 0.03-0.84 mg kg-1, while the clay samples
from Rio Salado and Lomas Coloradas revealed
high concentrations of MnoAc in the range of 1.58
mg kg-1 and 4.17 mg kg-1 respectively. The
extractions of the volcanic ash samples indicate
low AloAc concentrations (0.03-0.14 mg kg-1), while
the concentrations of MnoAc are appreciably high
(0.43-0.94 mg kg-1) in the NaoAc extracts. As
concentrations in these extracts were quite similar
for of the extracted clay samples as well as the
volcanic ash and ranged between 0.24-0.41 mg kg1, however two samples of the clays from
Guaptayam Choya and Lomas Coloradas revealed
relatively higher concentrations 0.82 mg kg-1 and
0.54 mg kg-1 respectively.
NH4C2O4 extraction
Oxalate (NH4C2O4) extraction in the dark is one
of the important step in the sequential extraction
process. Oxalate extractable fractions of Al, Fe
and Mn quantify the amount of these elements
bound to the amorphous and/or poorly crystalline
oxides of Al, Fe and Mn and the amounts of As
released from these oxides during the process of
chelation. Oxalate extraction of laterite sample
from Misiones, reveal significantly high quantity of
Feox (52.6 mg kg-1) and Mnox (48.8 mg kg-1) and
comparably lower amount of Alox (13.4 mg kg-1).
Oxalate extraction of the Misiones laterite sample
also revealed leaching of As in minor quantity
(0.12 mg kg-1). The clay sediments from Santiago
del Estero province revealed high concentration of
Alox, (41.9-80.7 mg kg-1), Feox (32.5-39.1 mg kg-1),
and Mnox (10.7-21.8 mg kg-1).
It is interesting to note that the clay sample from
25 de Mayo, Choya leached appreciably high
quantity of Alox (102.2 mg kg-1) while the
concentrations of Feox (4.4 mg kg-1), and Mnox
NaHCO3 extraction
As mentioned in section 3.4.3, the bicarbonate
extraction was only applied on the samples from
Misiones (M1) and Guaptayan Choya, (M2), since
all other sediment samples had high pH values (710). The amount of AlHCO3 (0.31-0.62 mg kg-1),
FeHCO3 (0.02-0.43 mg kg-1), MnHCO3 (0.18-0.53 mg
kg-1), and AsHCO3 (0.12-0.82 mg kg-1) in the
extracts were very low in the samples (Table 4.9,
Figure 4.13).
NaoAc extraction
Sequential extraction of the clay sediments with
sodium acetate at pH 5.4 (NaoAc) revealed low
concentrations of AloAc (0.15-1.93 mg kg-1), except
for the clay sample from 25 de Mayo, Choya
where nearly 27.4 mg kg-1 Al was leached during
bicarbonate extraction. FeoAc concentration in the
NaoAc extracts were very low (Table 4.9), while
the MnoAc concentrations
generally ranged
31
100000,00
a
10000,00
1000,00
100,00
10,00
1,00
1000,00
100,00
10,00
1,00
0,10
0,10
0,01
0,01
1000,00
100,00
c
DIW
Extractable As (mg kg-1)
100,00
10,00
1,00
0,10
b
10000,00
Extractable Fe (mg kg-1)
0,01
HCO3
d
oAc
ox
10,00
Res
1,00
Samples
V:
1
V:
2
M
C
M
9.
2
M
C
M
8
M
9.
1
M
6
M
4
M
3
M
1
M
C
V:
V:
2
1
9.
2
C
M
M
9.
1
M
8
M
M
6
M
4
M
3
M
2
M
1
0,10
M
2
-1)
Extractable Al
Al (mg
(mg kg -1
Extractable
100000,00
Extractable Mn (mg kg-1)
Samples
Figure 4.13: Distribution of Al, Fe, Mn and As released during the sequential extraction of the sediment samples from
Misiones and Santiago del Estero.
residue, which were immobile from the sediments
during the preceding steps of sequebtial extraction.
The concentrations of Alres (13.37 g kg-1). and Feres
(52.6 g kg-1) were very high in the Misiones laterite.
In comparison, the clay samples revealed of Alres
and Feres concentrations varying in the range of
1.57-4.85 g kg-1 and 0.93-7.15 g kg-1 respectively
(Table 4.9). The volcanic ash samples from
Santiago del Estero revealed low concentrations
of residual fractions of Alres and Feres with
concentrations ranging between 1.66-1.84 g kg-1
and 2.23-2.54 g kg-1 respectively.
Concentration of Mnres (0.041-0.17 g kg-1) in the
residual fraction in all the samples, while Asres
fractions indicated a wider range of variability
between 0.29-2.78 mg kg-1. In the Guaptayan
Choya sample Asres concentration was however
high (20.7 mg kg-1, see Table 4.9 and Figure 4.13).
kg-1).were
(1.23 mg
lower as compared to the rest
of the clay samples (Table 4.9).Oxalate extractable
As concentration in most of the clays were low
(0.13-0.27 mg kg-1), except for the Guaptayan
Choya sample with Asox content of 1.64 mg kg-1.
In the volcanish ash samples from Banda County,
the fractions of Alox, Feox and Mnox ranged
between 19.4-27.1 mg kg-1, 11.7-17.5 mg kg-1 and
1.32-1.86 mg kg-1 respectively. The concentrations
of Asox in these samples of volcanic ash were low
(0.11-0.12 mg kg-1, Table 4.9). One of the sample
of volcanic ash (M9.2) from Baanda County
indicated appreciably high quantities of Alox (90.4
mg kg-1), Mnox (6.09 mg kg-1) and Asox (2.8 mg kg1). However the content of Fe (9.8 mg kg-1) was
ox
low compared to the rest of the samples of
volcanic ash in the region.
Residual fraction (7M HNO3.) extraction
Samples extracted in 7M HNO3 during the last
step of the sequential extraction procedure
quantify the contents of Al, Fe, Mn and As in the
32
_____________________________________________________________________________________
5. REMEDIATION OF ARSENIC CONTAMINATED GROUNDWATER
Hering, 2003; Genç-Fuhrman et al., 2004). The
highest As adsorption was correlated with the
highest content of Al- and Fe-oxihydroxides that
are characterized by large specific surface area
(Pierce and Moore, 1992; Sracek et al. 2004).
Goethite (α-FeOOH), which is second most
common naturally occurring iron oxide mineral
after hematite (Fe2O3), has also been reported to
be effective for As adsorption (Garcia-Sanchez et
al., 1999; Lenoble et al., 2002). The removal of As
strongly depends only on the solution pH. Several
studies on the influence pH on As adsorption on
oxides and hydroxides suggested that As (III) had
adsorption maximum at around pH 7 whereas As
(V) sorption reached a maximum sorption around
pH 4-7 and then decreased with more alkaline pH.
Maximum adsorption of As(V) occurred in the pH
range 3-6 (Manning and Goldberg, 1996; Matis et
al., 1997; Goldberg, 2002). Iron oxides are
adsorbents for other anions also form similar
surface complexes as the As(V) and As(III)
species.
Adsorption of As in soils and water may also be
significantly affected by the presence of anions
such as PO4-3, SO4-2, CO3-2 and Cl-, which compete
with As for the sorption sites (Manning and
Goldberg, 1996; Fendorf et al., 1997). The
competitive effect of equimolar As(V) on As(III)
adsorption was limited and apparent only on
kaolinite and illite in the pH range 6.5-9.
Proper hydrochemical characterization of
groundwaters with As contamination is thus a
basic pre-requisite for successful remediation using
the natural adsorbents. Disposal of the used
adsorbents (sludge) is an important aspect during
such water treatments, as unsafe disposal of these
sludges may consequently release As and
contaminate the environment.
5.1 Overview
Removal of As from water is an important
worldwide issue. Incidences of elevated As
concentrations in groundwater in the countries
with poor infrastructure demands techniques that
are cost-effective and affordable for safe drinking
water supply to the affected population.
Conversely, many people in the developed world
are also drinking water with unsafe levels of As.
Large number of treatment technologies are
available to remove As from water ranging from
sophisticated technology such as ion exchange and
reverse osmosis to the much simpler, and often
highly
effective
coagulation-flocculation
techniques (Bhattacharya et al., 2002a).
Several studies have been undertaken to examine
the removal of As from natural water both in
macro and in micro scale (Anderson et al., 1976;
Manning and Goldberg, 1996; Fendorf et al., 1997;
Jain et al., 1997; Goldberg 2002; Thirunavukkarasu
et al., 2002; Dixit and Hering, 2003; GençFuhrman et al., 2004; Zhang et al., 2004) using
metal oxides and clays. In general, As correlates
well with Fe in sediments (Bose and Sharma, 2002;
Bhattacharya et al., 2001, 2002b; Smedley and
Kinniburgh, 2002; Ahmed et al., 2004). Natural
attenuation of As involves processes such as
adsorption, desorption, reduction, oxidation, coprecipitation and dissolution. The processes that
control the fate, the transformation and the
transport of As in the subsurface are complex
since
oxidation-reduction
conditions
are
temporally variable. In addition, the fixation of As
is also dependent on the physical properties and
mineralogy of the clays in the soils and sediments.
In highly weathered soils, fixation of As-anions is
related to the presence of oxides of Fe and Al. as
the adsorbents. Fe- and Al-oxides are effective
adsorbents for anionic contaminants because of its
surface charge, large surface area, and favorable
geometry of terminal OH groups for specific
adsorption. (Ladeira and Ciminelli, 2003).
Therefore soils rich in Fe- and Al-oxides were
used to study the As removal by various researches
(Manning and Goldberg, 1996; Fendorf et al.,
1997; Thirunavukkarasu et al., 2002; Dixit and
5.2 Selection of sediments
Based on the geochemical characteristics of the
sediment samples from Santiago del Estero
Province, samples M4 and M8 were selected. The
sample M2 with highest Fe content, was not
included because of its chemical characteristics
33
similar to M4. Additionally, the sample M1 from
Misiones Province was selected for comparison.
most of the arsenate adsorption on clays took
place within the first 4 h of contact time.
5.3 Experimental results
5.3.2 Role of pH for arsenic removal
Under acidic conditions (pH 5.9) complete
removal of arsenate was found for the clays from
Misiones Province as an adsorbents. The removal
efficiency was very high at all the pH ranges from
4.5 to 8.6 (see Figure 5.2). The removal efficiency
ws very high for the Missiones clay for the batches
with different As concentration.
The removal was much lower with clays from
Santiago del Estero (Figure 5.2). At alkaline pH
ranges (pH >8.6), the clays were probably affected
by precipitation of Fe- and/Al-hydroxides (as
indicated by changes in the colour of the solution).
This observation was consistent with the removal
of As5+ from the suspensions. At elevated pH
(>8.6) the adsorption of As5+ was found to be
limited and hence As removal from the aqueous
phase decreased with an increase in pH. The
increase of pH in the solutions was not
proportional to As concentrations and for As
concentrations higher than 2 mg L-1, pH reached a
plateau and remained constant.
Many reasons may explaine the increased pH but
the most important (in the present context) is the
uptake of protons by mineral weathering and ionexchange reactions, combined with the effect of
evaporation in arid and semiarid regions. This pH
increase is commonly associated with the
development of salinity and the salinization of
soils. Under oxidising conditions
5.3.1 Effects of reaction time and pH
The purpose of the experiment was to identify the
reaction time required for adsorption to reach
equilibrium. The experiments were carried out
using 10 g of sorbent (clay samples) and arsenate
[As(V)] solutions (K2H2AsO4) with concentrations
of 0.5, 1 and 2 mg L-1 of As+5 at pH values
between 4.5-8.6. The effect of pH on adsorption
was studied by determining the amount of
adsorbed As at pH 4.5, 5.9 and 8.63.
Results are shown in Figure 5.1, for a solidsolution contact time between 90 minutes and 4
hours; the absorbed quantity was stable for all the
solids, and for arsenate. The addition of the
Missiones clays, resulted in a sudden drop from
pH 8.5 to 6.0 within the first 0.5 hours and finally
reaching to a pH of 5.0 after 48 hours of reaction
time. In the case of pH 4.5 and 6.9, the change in
the pH is within a narrow range. However, it is
observed that the batch experiments at the three
pH reaches at final pH of ca 5.0 after 48 hours of
equilibration (Figure 5.1a).
In the case of the adsorbent media with the Choya
clays and Lomas Coloradas from Santiago del
Estero province show an initial decrease in the pH
from 8.63 to 7.0 after the first 0.5 hours but
indicates a final equilibration pH of ca 8.0 (Figure
5.1 b-c). The change in the equilibrium pH
significantly affects the removal of As from the
aqueous media. Further, it could be observed that
10
8
8
8
6
6
6
pH
pH
10
pH
10
4
2
4
a
2
0
4
b
2
0
0
0.5
1.5
4
24
48
pH=4.5
pH=5.9
c
pH=8.63
0
0
0.5
Time (h)
1.5
4
Time (h)
24
48
0
0.5
1.5
4
24
48
Time (h)
Figure 5.1: Effect of the variation of pH as a function of time on the adsorption of As(V) in the different studied media. a) Misiones;
b) Choya, Santiago del Estero; and c) Lomas Coloradas, Santiago del Estero.
34
_____________________________________________________________________________________
5.4 Discussion
strongly depends only on the solution pH.
Maximum adsorption of As(V) occurred in the pH
range 3-6 (Matis et al., 1997). Iron oxides are
adsorbents for other anions also form similar
surface complexes as the As(V) and As(III)
species.because of surface charge, surface area, and
favorable geometry of terminal OH groups for
specific adsorption. (Ladeira and Ciminelli, 2003).
The highest As adsorption was correlated with the
(Pierce and Moore, 1992; Sracek et al., 2004).
Goethite (α-FeOOH), which is second most
common naturally occurring iron oxide minerals.
400
300
200
a
0
400
300
200
100
b
0
0
0.5
1.5
4
24
48
0
0.5
Time (h)
As adsorbed ( g L-1)
-1
As adsorbed
adsorbed ((µg
As
g LL-1))
400
300
200
100
d
0
0.5
1.5
4
24
1000
g
0
4
Time (h)
c
0.5
600
400
200
e
24
48
4
24
48
24
48
24
48
pH 8.6
800
600
400
200
f
0
0
0.5
1.5
4
24
48
0
2000
pH 5.9
1500
1000
500
1.5
1000
h
0.5
1.5
4
Time (h)
pH 8.6
1500
1000
500
i
0
0
1.5
200
Time (h)
2000
1500
0.5
400
0
Time (h)
pH 4.5
0
Lom.Col.
600
48
800
48
Asadsorbed
adsorbed( (µg
-1-1)
As
g LL
)
24
pH 5.9
Time (h)
500
4
0
0
2000
1.5
1000
pH 4.5
Choya
800
Time (h)
500
Misiones
pH 8.6
0
100
1000
pH 5.9
500
pH 4.5
500
Asadsorbed
adsorbed
As
( (µg
g L-1L)-1)
Fe and Al oxides are effective anion adsorbents
because of surface charge, surface area, and
favorable geometry of terminal OH groups for
specific adsorption. (Ladeira and Ciminelli, 2003).
The highest As adsorption was correlated with the
(Pierce and Moore, 1992; Sracek et al. 2004).
Goethite (γ-FeOOH), which is second most
common naturally occurring iron oxide mineral
after hematite (Fe2O3), has also been reported to
be effective for As adsorption (Garcia-Sanchez et
al., 1999; Lenoble et al., 2002). The removal of As
0
0.5
1.5
4
Time (h)
24
48
0
0.5
1.5
4
Time (h)
Figure 5.2: Sorption of arsenate on clays from Misiones Province, Choya and Lomas Coloradas, Santiago del Estero
Province, with varying time for equilibrium, at three different pH and arsenic concentrations. a-c) 0.5 mg L-1 As5+; d-f)
1.0 mg L-1 As5+; and g-i) 2.0 mg L-1 As5+.
35
6. CONCLUSIONS
Natural As is ubiquitously reported in groundwater
from several parts of Argentna where most
groundwater for drinking water supplies for the
rural population are abstracted from the shallow
aquifers generally at a few meters to tens of meters
in depth.
In northwestern Argentina, especially in the
provinces of Santa Fe, Córdoba, Tucumán, Salta,
Chaco, and Santiago del Estero, groundwater are
enriched with As accompanied by other geogenic
contaminants such as fluoride. In the province of
Santiago del Estero, which belongs to the semiarid Chaco plain, two large areas with groundwater
As contamination are found. The first area is
situated along the northern boarder of Santiago del
Estero province and extends into the provinces of
Salta and Chaco. The second area of Santiago del
Estero Province with groundwater As is the Río
Dulce Alluvial Cone located to the East of
Santiago del Estero city in the counties of Banda
and Robles. The shallow aquifer (depth mostly
<15 m) and the unsaturated zone of Río Dulce
cone
characterized
by
a
discontinuous
accumulation of loessic sediments (0 to few meters
thick), which are deposited over fluvial and aeolian
fine sand to silty sediments of Quaternary age. The
source of As in groundwater in the region include
volcanic ash layers, and volcanic glass dispersed in
aquifers.
The present study was carried out in Maria Elena,
situated in the Banda County with dispersed rural
population. Drinking water supplies to the
individual families are served in most cases with a
shallow hand pumped wells placed in the shallow
aquifers, as the only available fresh water source.
In Maria Elena the drinking water is supplied to
the one third of the population from the SimbolarAñatuya aqueduct. The drinking water is stored in
a communal central tank (capacity 30 m3), located
about 1.5 km South of the Provincial Road 21.
The other two thirds of the population,
(approximately 280 people), obtain their water
predominantly from small household wells.
Two main geomorphologic units are distinguished
within the study area: Loessic Plain and Rio Dulce
Alluvial Cone. The hydrogeochemical studies
indicate the following salient characteristics:
•
58% of the analyzed groundwater samples
are of Na-HCO3 type, 26% are of Na-Cl
type, and 16% are of Na- SO4 type
•
The most frequent SO42- concentration is
<300 mg L-1 and only 5 groundwater
samples exceeded 400 mg L-1 established
by the WHO as drinking water limit.
•
The predominant cation is Na+ followed
by Ca2+. The increased Na+ concentration
indicates cation exchange, where Ca2+,
Mg2+ of the groundwater is replaced by
Na+ from the solid matrix resulting in
sodic-bicarbonate waters with the
following evolution Na+ > Ca2+ > Mg2+.
Arsenic is present in all the analysed groundwater
samples. The As concentration of most of the
samples (75%) is less than the Argentine limit
(0.05 mg L-1) and 25% of the samples overpass
this limit. It can be observed that zones close to
Las Palmitas, Bayo Muerto and Rasquín present
As concentrations below the drinking water
guideline value of the WHO. The highest As
concentrations were detected in María Elena and
Las Abritas areas, where values range from 0.05 to
1.9 mg L-1.
The spatial distribution pattern reveals two distinct
zones with high As concentrations (>50 µg L-1)
and correspond to the geomorphological
depressions in the central part of Maria Elena area,
around Bayo Muerto where groundwater flow is
stagnant. Zones with low As concentrations in
groundwater (<50 µg L-1) with groundwater are
suitable for drinking purposes close to Las
Palmitas, where most of the sampled wells were
located.. Zones with highest As concentrations are
coincident with zones of stagnant groundwater
and characterised by high electrical conductivity,
TDS levels and high concentrations of Cl- and
SO42-. The spatial variability of As in groundwaters
of Maria Elena area are well correlated with the
distribution of the major ions. A well defined
positive correlation is observed between As and
Na+, As and HCO3-, while Mg2+ indicated a
distinct negative correlation with As. The
correlation between As with the major ions Ca2+
and Cl- as well as pH indicates two distinct groups.
36
_____________________________________________________________________________________
Strong positive correlation of As and Ca2+
characterizes the samples with low Ca2+. A trend
of increasing As concentration is found for waters
with increasing Cl- concentrations. A distinct
positive correlation is noted amongst pH and As.
In general, the mobility of As in groundwaters are
associated with aeolian Pampean or Postpampean
sediments, particularly those with high content of
volcanic glass. Bundschuh et al. (2004) explained
mobilization of As from the sediments and the
ash-layer (MCV) and suggest that it is controlled
by
•
Leaching and mobilisation of As along
down gradient with infiltrating water.
•
Upward mobility of As through capillary
pore water.
Arsenic adsorption was carried out on locally
available materials such as natural clays with high
content of amorphous iron and aluminum
hydroxide that present high adsorption capacity
for arsenate.
Using the information about physical and chemical
characteristics those clay samples could be
selected, which are most suitable for As
adsorption. Samples from Choya (M4) and Lomas
Coloradas (M8) were selected as most suitable
from Santiago del Estero Province; a clay sample
from Missiones Province (M1) was additionally
selected for comparison. Among the selected
samples for the adsorption experiments, Fe-, Aland Mn-oxides and hydroxides were more
abundant in M1 than in M4 and M8.
Laterite clay (M1) from the province Misiones had
the highest As retention (99% dissolved As was
adsorbed), whereas the adsorption by the clays
from Lomas Coloradas and Choya indicated As
adsorption efficiency of 53% and 40%
respectively.
• By fluctuations of the groundwater table.
Salient geochemical characteristics of the
sediments from Santiago del Estero Province were
evaluated for their suitability as adsorbents for As
removal from groundwater. The main constituents
of the samples were identified as quartz aluminium
silicates (predominantly kaolinite), and iron oxides
by the XRD studies.
Sequential extraction procedure was employed for
characterising the adsorbents. Concentrations of
Fe in the DIW extracts were higher than acetate
for the samples M1, M4 and M8, but Al
concentrations were also relatively high.
Comparison of the concentrations Fe, Al and Mn
extracted by the oxalate clearly indicates the
presence of Al in higher amounts as compared to
Fe and Mn. Amorphous Fe-, Al- and Mn-oxides
and hydroxides are readily dissolved in oxalate,
and therefore useful to quantify As associated with
these secondary phases. Oxalate extractions
indicated that Al and Fe are preferentially present
as oxides and hydroxides and in significantly
higher amounts as compared to Mn oxides and
oxyhydroxides in the analysed sediments. This
indicates that the oxides and hydroxides of Al and
Fe are more important that can be well suited for
removalk of As from the contaminated
groundwater. Arsenic concentrations in the
volcanic ash-layer (M10) are low, and this sample
also yields the lowest concentration of leached As
during the acetate and oxalate extractions steps.
Concentrations of leached iron, aluminium and
manganese are also lower for this sample
throughout all four extractions steps.
37
7. RECOMENDATIONS
Groundwater is the main source of potable water
for the population in a major part of the globe. In
South- and Southeast Asia, As in groundwater
occurs widely over the Bengal Delta Plain (BDP)
aquifers in Bangladesh and West Bengal, India.
Over the last few years, groundwater with elevated
As in the Central Gangetic Plains of Uttar Pradesh
and Bihar, and the Brahmaputra valley of Assam in
India, and in extended part of the alluvial plains of
the Indus river in Pakistan as well as in the Terai
belt of Nepal have emerged as potential areas of
environmental health disaster. Nearly 100 million
are estimated to be at the potential risk of As
exposure. Similarly, As has been discovered over
extended areas of Latin American countries
including Mexico, Argentina, Chile. Peru, Bolivia,
Nicaragua, Uruguay, Venezuela, Ecuador where
there are potential risks for human health through
drinking water sources.
The rising demand of water for human use, makes
evident that conflictive and more complex
situations will increase. The existing information
about the presence of As in groundwater in
Argentina, and more specifically in the province of
Santiago del Estero is far from being adequate. It
is important to launch information campaigns by
the
competent
authorities
to
increase
consciousness about the health risks and the other
problems associated with the utilization of
groundwater with high As concentrations.
As a recommendation for the future, it is advisable
to study the characteristics of the sediments of Rio
Dulce Alluvial Cone and their relation with
processes of As mobilization. It is important to
carry out detailed hydrogeological investigations
focused on identification of deeper aquifers with
low As for future exploitation.
Studies on As-removal techniques using natural
clays from Santiago del Estero Province, should be
further expanded. Adsorption experiments for the
evaluation of the locally available clays for As
removal which may provide a cost effective
solution in the region needs to be carried out in
detail for effectiveness. The lateritic clay from
Missiones Province which is found to be
promising for As remediation, further demands
cost-benefit analysis.
Studies using respective clays with pre-treatment
to optimize adsorption are required in order to
find the best adsorption capacity clays in Santiago
del Estero Province. Since As reduction from
groundwater by clays from Santiago del Estero
Province was found to range between 40 and 53%,
the As-rich water needs to pass several times the
clay filter. Since most adsorption occurs during the
first 90 minutes of contact time, several loops of
that period are recommended. However further
studies are recommended to optimize timing for
maximum adsorption.
38
_____________________________________________________________________________________
8. REFERENCES
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Sracek, O., 2004. Arsenic contamination in groundwater of alluvial aquifers in Bangladesh: An overview. Appl.
Geochem., 19(2): 181-200
Anderson, M.A., Ferguson, J.F., Gavis,, J. 1976. Arsenate adsorption on amorphous aluminium hydroxide. J. Colloid
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APHA, 1975. Standard Method for the Examination of Waters and Wastewaters. American Public Health Association, 14th Ed.,
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Arguello, R.A., Cenget, D.D., Tello, E.E., 1939. Cancer and regional endemic chronic arsenicism. Brit. J. Dermat. 51:548.
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Argentina. Master Thesis, Dept. of Land and Wat. Res. Eng., KTH, Stockholm, Sweden, TRITA-LWR-EX-0306, 2003, 40 pp.
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Perspective. In: Sarkar, B. (Ed.) Handbook of Heavy Metals in the Environment. Marcell Dekker, New York, pp. 145–
215.
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Bose, P., Sharma, A., 2002. Role of iron in controlling specieation and mobilization of arsenic in subsurface environment.
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2004. Groundwater arsenic in Chaco-Pampean Plain, Argentina: A Case study from Robles Department,
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San Luis Potosí. Geofísica Internacional 32(2): 277-286.
Claesson, M, Fagerberg, J., 2003. Arsenic in groundwater of Santiago del Estero – Sources, mobility patterns and remediation with
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Genç-Fuhrman, H., Tjell, J.C., McConchie, D., 2004. Adsorption of arsenic from water using activated neutralized red
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41
HNO3
Misiones
Sgo del Estero
Sgo del Estero
DIW
Misiones
Sgo del Estero
Sgo del Estero
Bicarbonate
Misiones
Sgo del Estero
Acetate
Misiones
Sgo del Estero
Sgo del Estero
Oxalate
Misiones
Sgo del Estero
Sgo del Estero
Residual Fraction
Misiones
Sgo del Estero
Sgo del Estero
Extractant
50083
21100
25540
38.22
27.51
144.4
0.62
0.31
0.29
34.79
3.86
50.95
102.16
72.29
13364.9
2959.1
4853.6
M1
M4
M8
M1
M2
M1
M4
M8
M1
M4
M8
M1
M4
M8
52618.66
926.08
7153.44
86.24
4.37
36.97
<0.80
<0.80
<0.80
0.43
<0.80
5.89
2.29
7.39
13000
66325
30026
Fe
mg kg-1
Al
mg kg-1
M1
M4
M8
Sample
Sediment analysis data (in mg kg-1)
APPENDIX 1
0.55
0.33
2.78
<5.20
0.19
0.27
0.54
0.59
1.07
<5.20
0.82
<5.20
0.47
3.85
2.90
1.41
9.57
mg kg-1
As
8.96
3.38
4.19
4.43
1.17
2.06
4.48
2.88
1.99
0.21
1.37
40.63
4.85
1.66
75.12
23.85
17.08
mg kg-1
Ba
22.66
110.4
122.71
4.22
4.29
3.68
43.3
820.1
742.17
2.00
251.5
704.3
2276.7
2062.3
733.3
3985
3214
mg kg-1
Ca
Cd
42
1.04
0.03
0.18
<0.89
0.05
<0.89
<0.89
<0.89
<0.89
<0.89
<0.89
0.02
<0.89
<0.89
2.49
0.32
0.75
mg kg-1
Co
3.97
0.31
2.73
2.21
0.04
0.07
<0.95
<0.95
<0.95
<0.95
0.03
0.73
0.04
0.05
21.90
2.49
7.53
mg kg-1
Cr
21.9
0.74
4.22
0.07
0.04
0.07
0.07
0.07
0.07
0.02
<0.48
<0.48
<0.48
0.03
53.2
6.57
14.7
mg kg-1
25.48
1.04
4.63
3.72
1.17
0.87
0.34
0.25
0.39
0.12
0.10
10.16
0.69
1.07
127.82
4.00
17.08
mg kg-1
Cu
82.01
419.68
964.82
100.18
168.29
129.59
218.5
972.1
908.2
20.1
194.4
64.5
263.6
331.4
464.0
1463.7
2564.7
mg kg-1
K
74.3
442
2732
0.69
5.80
8.97
4.1
226
284
0.15
104
73.6
441
439
397.1
4373
8915
mg kg-1
Mg
Time
0h
0.5h
1.5h
4h
24h
48h
Time
0h
0.5h
1.5h
4h
24h
48h
Time
0h
0.5h
1.5h
4h
24h
48h
pH=5.9
5.90
7.43
7.60
7.08
7.25
7.64
pH=5.9
5.90
8.07
7.66
7.01
7.23
7.49
pH=5.9
5.90
5.46
5.60
4.53
4.41
4.53
43
pH=8.63
8.63
7.35
7.60
7.04
7.10
7.70
pH=8.63
8.63
7.95
7.76
7.14
7.30
7.34
pH=8.63
8.63
5.72
5.84
5.70
5.20
4.56
As+5 conc. = 1.0 mg L-1
pH=4.5
pH=5.9
pH=8.63
time pH=4.5
4.50
5.90
8.63
0h
4.50
5.49
5.55
5.89
0.5h
5.65
5.48
5.6
6.00
1.5h
5.76
4.84
5.65
6.00
4h
3.99
4.50
5.82
5.90
24h
5.70
4.84
4.38
5.03
48h
4.30
SAMPLE FROM CHOYA (M4)
pH=4.5
pH=5.9
pH=8.63
time pH=4.5
4.50
5.90
8.63
0h
4.50
8.07
7.90
7.97
0.5h
8.02
7.93
8.15
8.12
1.5h
7.61
7.08
7.08
6.8
4h
6.98
7.12
7.16
7.25
24h
7.46
7.61
7.15
7.60
48h
7.63
SAMPLE FROM LOMAS COLORADAS (M8)
pH=4.5
pH=5.9
pH=8.63
time pH=4.5
4.5
5.90
8.63
0h
4.50
7.38
7.35
7.24
0.5h
7.56
7.50
7.25
7.38
1.5h
7.68
6.88
7.05
6.73
4h
7.20
7.35
7.30
7.50
24h
7.32
7.30
7.68
7.55
48h
7.70
As+5 conc. = 0.5 mg L-1
SAMPLE FROM MISIONES (M1)
Results of the Adsorption Experiments. pH dependence of time.
APPENDIX 2
time
0h
0.5h
1.5h
4h
24h
48h
time
0h
0.5h
1.5h
4h
24h
48h
time
0h
0.5h
1.5h
4h
24h
48h
pH=4.5
4.50
7.51
7.59
6.93
7.28
7.75
pH=4.5
4.50
7.54
8.20
6.86
7.12
7.50
pH=4.5
4.50
5.60
5.68
4.52
4.43
4.32
pH=5.9
5.90
7.52
7.56
6.96
7.30
7.74
pH=5.9
5.90
8.17
8.27
7.07
7.30
7.50
pH=5.9
5.90
6.00
6.05
6.33
4.82
4.50
pH=8.63
8.63
7.52
7.59
7.15
7.30
7.75
pH=8.63
8.63
7.30
8.07
7.00
7.35
7.55
pH=8.63
8.63
5.36
5.80
5.61
5.51
4.50
As+5 conc. = 2.0 mg L-1
_____________________________________________________________________________________

Maria Fernanda Mellano Antonio Emilio Ramirez

Transcription

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