arsenic in geothermal waters of costa rica - Mark

Transcription

ARSENIC IN GEOTHERMAL
WATERS OF COSTA RICA
A Minor Field Study
Lotta Hammarlund
Juan Piñones
December 2009
TRITA-LWR Master Thesis
ISSN 1651-064X
LWR-EX-09-02
Lotta Hammarlund & Juan Piñones
TRITA LWR Masters Thesis 09-02
ii
Arsenic in Geothermal Waters of Costa Rica
ARSENIC IN GEOTHERMAL WATERS OF
COSTA RICA
A Minor Field Study
Lotta Hammarlund
Juan Piñones
M.Sc. Program
Environmental Engineering and Sustainable Infrastructure
KTH
Main Supervisor
Prof. 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-Supervisor
Prof. Jochen Bundschuh
Prof. Guillermo E. Alvarado
Instituto Costarricense de Electricidad (ICE), San Jóse, Costa Rica
Examiner
Prof. Jon-Petter Gustafsson
Department of Land and Water Resources Engineering, Royal Institute of Technology (KTH) SE-100 44
STOCKHOLM, Sweden
Stockholm
December, 2009
iii
iv
Royal Institute of Technology
International Office
PREFACE
This study has been carried out within the framework of the Minor Field Studies
Scholarship Programme, MFS, which is funded by the Swedish International
Development Cooperation Agency, Sida.
The MFS Scholarship Programme offers Swedish university students an
opportunity to carry out two months’ field work, usually the student’s final degree
project, in a Third World Country. The results of the work are presented in an MFS
report which is also the student’s Master of Science Thesis. Minor Field Studies are
primarily conducted within subject areas of importance from a development
perspective and in a country where Swedish international cooperation is ongoing.
The main purpose of the MFS Programme is to enhance Swedish university
students’ knowledge and understanding of these countries and their problems and
opportunities. MFS should provide the student with initial experience of conditions
in such a country. The overall goals are to widen the Swedish human resources
cadre for engagement in international development cooperation and to promote
scientific exchange between universities, research institutes and similar authorities
in developing countries and in Sweden.
The International Office at the Royal Institute of Technology, KTH, Stockholm,
administers the MFS Programme for the faculties of engineering and natural
sciences in Sweden.
Sigrun Santesson
Programme Officer
MFS Programme/LP Programme
International Office, MFS
KTH, SE–100 44 Stockholm, Sweden, Phone: +46 8 790 7i83 , Fax: +46 8 790 8192, E-mail: [email protected]
www.kth.se/student/utlandsstudier/examensarbete/mfs
v
vi
TABLE OF CONTENTS
TABLE OF CONTENTS ........................................................................................................................... VII
ACKNOWLEDGEMENTS ..........................................................................................................................IX
ABSTRACT...................................................................................................................................................XI
SAMMANFATTNING .............................................................................................................................. XIII
SINTESIS ....................................................................................................................................................XV
1. BACKGROUND.......................................................................................................................................... 1
1 .1 G E O T H E R M A L S Y S T E M S ................................................................................................ 1
1.1.1 Classification of geothermal systems......................................................................................................................................1
1.1.2 Classification of geothermal systems......................................................................................................................................1
1 .2 E N V I R O N M E N T A L E F F E C T S O F G E O T H E R M A L A C T I V I T Y ................................................. 3
1 .3 A R S E N I C I N G E O T H E R M A L S Y S T E M S .............................................................................. 3
1.3.1 Dispersion of arsenic and environmental contamination ........................................................................................................3
1.3.2 Health effects of arsenic and risks ........................................................................................................................................3
1 .4 H I S T O R Y O F E X P L O I T A T I O N O F G E O T H E R M A L E N E R G Y I N C O S T A R I C A ......................... 4
1 .5 R A T I O N A L E O F T H E P R E S E N T S T U D Y ............................................................................ 4
1 .6 T H E S T U D Y A R E A S ....................................................................................................... 5
1.6.1 Miravalles geothermal field...................................................................................................................................................5
1.6.2 Rincon de la Vieja geothermal field......................................................................................................................................9
1 .7 A I M S A N D O B J E C T I V E S ................................................................................................. 9
1 .8 L I M I T A T I O N S ............................................................................................................... 9
2. ARSENIC IN GEOTHERMAL WATER.................................................................................................10
2 .1 T H E S O U R C E O F A R S E N I C I N G E O T H E R M A L S Y S T E M S .................................................... 10
2.1.1 The source and nature of geothermal fluids .........................................................................................................................10
2.1.2 The source of As in geothermal fluids.................................................................................................................................11
2.1.3 Arsenic from host rock leaching..........................................................................................................................................11
2 .2 S P E C I A T I O N O F A R S E N I C I N G E O T H E R M A L F L U I D S ....................................................... 11
2.2.1 Dissolved arsenic................................................................................................................................................................11
2.2.2 Low sulphide, reduced fluids ..............................................................................................................................................11
2.2.3 High sulfide, reduced fluids ................................................................................................................................................11
2.2.4 Redox state........................................................................................................................................................................12
2 .3 A R S E N I C D E P O S I T I O N F R O M G E O T H E R M A L F L U I D S ....................................................... 12
2 .4 A R S E N I C I N H O T S P R I N G D E P O S I T S A N D S C A L E S .......................................................... 12
2 .5 T H E F A T E O F A R S E N I C F R O M G E O T H E R M A L S O U R C E S ................................................... 13
2 .6 S U R F A C E W A T E R S ....................................................................................................... 13
2 .7 G R O U N D W A T E R .......................................................................................................... 14
3. MATERIALS AND METHODS ...............................................................................................................15
3 .1 F I E L D I N V E S T I G A T I O N S .............................................................................................. 15
3.1.1 Field location and sampling ...............................................................................................................................................15
3.1.2 Collection of field data........................................................................................................................................................15
3 .2 L A B O R A T O R Y I N V E S T I G A T I O N S ................................................................................... 21
3.2.1 Alkalinity .........................................................................................................................................................................21
3.2.2 Major ions.........................................................................................................................................................................22
3.2.3 Trace elements....................................................................................................................................................................22
3.2.4 Arsenic (III)......................................................................................................................................................................22
3.2.5 DOC ................................................................................................................................................................................22
vii
3 .2 T R E A T M E N T
OF ANALYTICAL DATA
............................................................................. 22
4. RESULTS.................................................................................................................................................. 23
4 .1 G E O T H E R M A L W E L L S .................................................................................................. 23
4.1.1 Field measured parameters.................................................................................................................................................23
4.1.2 Major ions.........................................................................................................................................................................23
4.1.3 Trace elements....................................................................................................................................................................25
4.1.4 Correlation between various chemical parameters ................................................................................................................25
4 .2 T H E R M A L S P R I N G S -N E U T R A L A N D A C I D ..................................................................... 26
4.2.2 Major ions.........................................................................................................................................................................26
4.2.3 Trace elements....................................................................................................................................................................28
4.2.4 Correlation between various chemical parameters ................................................................................................................30
4 .3 C O L D S U R F A C E W A T E R S .............................................................................................. 31
4.3.2 Major ions.........................................................................................................................................................................31
4.3.3 Trace elements....................................................................................................................................................................31
4 .4 A N O M A L O U S B O R O N C O N C E N T R A T I O N S ....................................................................... 32
5. DISCUSSION........................................................................................................................................... 33
5 .1 A R S E N I C C O N C E N T R A T I O N S I N M I R A V A L L E S A N D R I N C O N D E L A V I E J A ........................ 33
5.1.1 Geothermal wells................................................................................................................................................................33
5.1.2 Thermal springs.................................................................................................................................................................33
5.1.3 Cold surface waters ............................................................................................................................................................33
5 .2 C O M P A R I S O N O F T H E D A T A W I T H O T H E R G E O T H E R M A L F I E L D S I N N E W Z E A L A N D , USA
A N D P H I L I P P I N E S ............................................................................................................. 33
5.2.1 Concentrations of arsenic and variability ............................................................................................................................34
5.2.2 Arsenic concentrations in Miravalles and Rincon de la Vieja ............................................................................................34
5. CONCLUSIONS....................................................................................................................................... 36
7. REFERENCES ......................................................................................................................................... 37
viii
ACKNOWLEDGEMENTS
This Master of Science thesis comprises the final part of our degree in Environmental
Engineering at KTH. This study could never have been realized without the assistance and
support from several persons to whom we would like to express our gratitude:
Prosun Bhattacharya, Jon Petter Gustafsson, Gunnar Jacks and all the other people at the
Department of Land and Water Resources Engineering (LWR) at KTH for advice and guidance
throughout this study.
Our local supervisors Jochen Bundschuh and Guillermo E. Alvarado from Instituto
Costarricense de Electricidad (ICE), who provided invaluable guidance and support during our
fieldwork.
Antonio Yock, Paul Moya and other staff at ICE, for support and help during our work and for
making our stay in Costa Rica a pleasant and memorable experience.
Sida/SAREC for partly financing this project and Sigrun Santesson at the MFS Programme
office.
Ann Fylkner and Monika Löwén, LWR-KTH, for laboratory assistance and supervision at the
laboratories of LWR-KTH.
Carl-Magnus Mörth, Universitetslektor på Institutionen för Geologi och geokemi, for laboratory
analysis.
…and all the sympathetic people in Costa Rica that made this project an unforgettable
experience.
Lotta Hammarlund
Juan Piñones
Stockholm, December 2009
ix
x
ABSTRACT
Arsenic (As) contaminated land and groundwater is a serious health problem in a global
perspective. Long-term intake of water with high As concentration is directly linked to various
life threatening diseases such as cancer and skin lesions. In geothermal areas As contamination
occurs in two main processes: i) natural contamination where the gothermal waters reach the
surface as natural springs and then mix with surface water flows or shallow groundwater bodies,
used by humans for both irrigation and drinking water supply; and ii) human exploitation of
geothermal waters as an energy resource causes mobilisation of As and other heavy metals
contained in the geothermal waters to reach the surface and then contaminate surface and
shallow groundwater bodies. Both of these processes may affect the environment and the health
of humans and animals. Typical symptoms that arise due to the use of contaminated waters
include skin lesions and different forms of cancer.
The study area is located at the areas close to the volcanoes Miravalles and Rincón de la Vieja in
Costa Rica. This area is rich in geothermal resources and the geothermal waters and thermal mud
contain high levels of As. Concentrations up to 30 mg As/l have been recorded and thus present
a problem for a country which is utilising these geothermal resources. It also indicates potential
health problem as it exceeds the WHO drinking water guideline (10 μg/L).
The purpose of the study is to investigate the occurrences of As as well as an overall
characterisation of the geothermal waters in the areas of Miravalles and Pailas-Borinquien
geothermal fields. The specific objectives are to characterize geothermal waters extracted from
geothermal wells used for power generation, natural thermal springs and cold springs in the
surroundings of Miravalles and Pailas-Borinquien geothermal fields. The results from this study
have also been compared with the geochemical characteristics of the geothermal waters from
geothermal wells and thermal springs in New Zealand, Philippines and USA.
Totally 50 sample were collected at 49 different places in the surroundings of the Miravalles and
the Rincon de la Vieja volcano areas. Water from three types of reservoirs was collected;
geothermal well fluids, thermal springs and cold surface waters. In 35 of the 50 sampled places
As concentration exceed the WHO limit for safe drinking water. All the geothermal well fluids
greatly exceed the WHO limit while all the cold springs fall below that limit. Sampled geothermal
well fluids are generally of Na-Cl-B type with an almost neutral pH. They contain an extremely
high As concentration. Sampled thermal springs can be divided into neutral thermal waters and
acidic thermal waters. The neutral thermal springs (pH almost neutral) are generally of HCO3-Cl
type while the acidic thermal springs (pH 1.97-3.25) are generally of SO4-S-Cl-Al type. They
contain low to high As concentrations. Sampled cold surface waters are generally of Si-SO4HCO3 type with a neutral to alkaline pH and contain low As concentrations.
As comparison with New Zealand, USA and Philippines show that the well fluids used for
geothermal energy in Costa Rica have extremely high As concentrations.
Keywords: Geothermal energy, arsenic; boron, geothermal systems; thermal springs, Costa Rica;
Miravalles; Rincón de la Vieja.
xi
xii
SAMMANFATTNING
Arsenikförorenad mark och grundvatten är ett allvarligt, globalt hälsoproblem. Konsumtion av
vatten med hög arsenikhalt under lång tid är direkt förknippat med flera livshotande sjukdomar, t.
ex. cancer och störningar på centrala nervsystem. I geotermiska områden förekommer
arsenikföroreningar i huvudsakligen i två processer: i) naturliga föroreningar där geotermiskt
vatten når markytan genom naturliga källor och som sedan blandar sig med ytvatten eller
grundvatten som används både för bevattning och som dricksvattenkälla, och ii) exploatering av
geotermiska naturresurser för energiutvinning leder till att arsenik och andra tungmetaller som
finns i det geotermiska vattnet når markytan och förorenar yt- och grundvattenkällor. Båda dessa
processer påverkar miljön och hälsotillståndet hos människor och djur. Vanliga symptom vid
konsumtion av arsenikförorenat vatten är olika typer av cancer.
De aktuella studieområdena är belägna i vulkanområdena Miravalles och Rincón de la Vieja i
Costa Rica. Här finns rikligt av geotermiska naturresurser med höga halter av arsenik i både
geotermisktvatten och lera. Arsenik koncentrationer har uppmäts till 30 mg/L och är ett problem
för ett land som utvinner energi från dessa naturresurser. Dessa halter indikerar en hälsorisk då
halterna överstiger WHO:s gränsvärde för säkert dricksvatten (10 μg/L).
Syftet med den här studien är att undersöka förekomsten av arsenik i de geotermiska vattnen i
området runt Miravalles och Pailas-Borinquien geotermiska kraftverk. Målet är också att
karakterisera vattnet från: de geotermiska naturtillgångarna som används för att utvinna energi, de
naturliga termiska källorna samt ytvattnen i omgivningarna runt Miravalles och PailasBorinquiens geotermiska kraftverk. Resultatet från den här studien har jämförts med
karaktäriseringen av vatten från geotermiska kraftverk och termiska källor i Nya Zeland,
Phillipinerna och USA.
50 vattenprover från 49 olika plaster samlades in i omgivningarna runt vulkanområdena
Miravalles och Rincón de la Vieja. Vattenprover togs från 3 olika typer av reservoarer;
geotermiska kraftverk, termiska källor och kalla ytvatten. Arsenikhalten översteg WHO:s
gränsvärde för säkert dricksvatten i 35 av de 50 proverna. Alla prover från de geotermiska
kraftverken översteg kraftigt WHO:s gränsvärden medan alla prover från de kalla ytvattnen föll
under gränsvärdet. De geotermiska vattnen från kraftverken är av vattentypen Na-Cl-B med ett
neutralt pH.. De innehöll extremt höga As värden.
Proverna från de termiska källorna kan delas in i neutrala eller sura vatten. De neutrala termiska
källorna (pH nära 7) är generellt av vattentypen HCO3-Cl medan de sura termiska källorna (pH
1.97-3.25) generellt har vattentypen SO4-S-Cl-Al. De innehåller både låga och höga arsenikhalter.
De kalla ytvattnen var av vattentypen Si-SO4-HCO3 med ett neutralt till basiskt pH och innehöll
låga arsenikhalter.
Jämförelser med arsenikhalterna i Nya Zeland, USA och Phillipinerna visar att vattnen som
används för utvinning av energi i Costa Rica innehåller extremt höga arsenik koncentrationer.
Nyckelord: Geotermisk energi, arsenik; bor, Geotermiska system; termiska källor, Costa Rica;
Miravalles; Rincón de la Vieja,
xiii
xiv
SINTESIS
La contaminación de agua subterránea causada por arsénico es un problema serio para la salud y
de importancia a nivel global. El consumo prolongado de agua con altas concentraciones de
arsénico esta relacionado con diferentes tipos de enfermedades mortales, tales como cáncer y
con efectos degenerativos del sistema nervioso y de la piel. En areas geotérmicas la concentración
de arsénico ocurre en dos procesos principales: i) contaminación natural causada cuando las
aguas geotermales alcanzan la superficie en forma de ballestas naturales y consecuentemente se
mezclan con corrientes de aguas superficiales o sistemas llanos de aguas subterráneas usadas por
los habitantes para la irrigación o como provisiones de agua potable y ii) la explotación de aguas
geotermales como recursos de energía causa una mobilisación de arsénico y otros metales
pesados que se transportan con las aguas geotermales y asi alcanzan la superficie y sistemas llanos
de aguas subterráneas. Ambos síntomas que surgen por uso de aguas contaminadas incluyen
lesiones de la piel, diferentes tipos de cáncer.
La área de estudio está localizada en Miravalles y Rincón de la Vieja en el norte de Costa Rica.
En esta área se encuentran grandes recursos geotermales y las aguas subterráneas contienen altas
concentraciones de arsénico. Las concentraciones de arsénico registradas han alcanzado hasta 30
mg/L en agua subterránea y por lo tal presenta un potencial problema de salud a un pais que
hace uso de sus recursos geotermales. Adicionalmente señala un riesgo para la salud al exceder las
indicaciones de 10µg/L en agua potable determinada por OMS.
El objetivo de este estudio es de investigar las concentraciones de As y hacer una caracterización
general de las aguas geotermales en los alrededores de los campos geotérmicos en Miravalles y
Pailas-Borinquien. Los objetivos específicos son de hacer caracterizaciones de las aguas
geotermales extraídas de pozos geotermales y utilizadas para la generación de energía, fuentes
termales naturales y fuentes frías en los alrededores de los campos de aguas geotermales en
Miravalles y Pailas-Borinquien. Los resultados tambien han sido estudiados en relación con las
características de las aguas geotermales de pozos geotermales y fuentes de aguas termales en
Nueva Zelándia, Las Filipinas y Estados Unidos.
Un total de 50 muestras fueron recogidas en diferentes lugares a los alrededores de las áreas
volcánicas de Miravalles y el Rincón de la Vieja. Las aguas fueron recogidas en tres diferentes
tipos de reservas, corrientes de pozos geotermales, arroyos termales y aguas frías llanas. En 35 de
las 50 muestras analizadas la concentración de arsénico sobrepasó el límite de recomendación de
la OMS en agua potable. Todas las corrientes de aguas geotermales de los pozos sobrepasaron
notablemente el límite postulado por la OMS para agua potable. Mientras que las ballestas de
aguas frías estaban bien por debajo del mismo límite. Por lo general los iones dominantes en
muestras de corrientes de pozo geotermales son Na-Cl-B con pH neutral. Sin embargo, estas
contenían altas concentraciones de arsénico. Las ballestas de aguas termales pueden ser divididas
en agua termal neutral y agua termal acída. Los iones dominantes fueron generalmente del
HCO3-Cl en las aguas neutrales (pH casí 7) y SO4-S-Cl-Al en las aguas acídas (pH 1.97-3.25).
Estas contenían de baja a altas concentraciones de arsénico. Las muestras de aguas frías llanas
sobrepasaron el límite de la OMS en agua potable. Los iones dominantes fueron Si-SO4-HCO3
con pH neutral y contenían baja concentración de arsénico.
Una comparación con los resultados de Nueva Zelandia, Estados Unidos y Las Filipinas muestra
que las aguas de pozo usadas para la energía geotermal en Costa Rica contienen concentraciones
extremedamente elevadas de Arsénico.
Palabras claves: Energía geotérmica, Arsénico; boro; sistemas geotérmicos, aguas termales,
Costa Rica; Miravalles; Rincón de la Vieja
xv
xvi
1. BACKGROUND
•
Geothermal waters are related to active or inactive
volcanism and change their specific chemical
composition during their pathway from deep
seated sources to the surface of the earth. The
presence of arsenic (As) is high in these waters.
Several gaps exist in our present day knowledge of
the genesis and mobility of As in geothermal
systems and its geochemical processes and kinetics
in the near-surface environment. This lack of
knowledge is of particular concern where countries
utilise geothermal waters for energy generation and
actively facilitate the movement of potentially
contaminated water at depth to the surface, which
is the case in Costa Rica. This study forms a part
of a recently initiated collaboration between
Instituto Costarricense de Electricidad (ICE),
Costa Rica and the Department of Land and
Water Resource Engineering at the Royal Institute
of Technology (KTH), Stockholm.
Binary-cycle plants transfer heat from
moderately hot geothermal water to a
secondary fluid with a much lower boiling
point causing the secondary fluid to boil.
The resulting vapour is then used to drive
turbines.
Low- and moderate-temperature resources can be
divided into two categories:
•
Direct use, which uses the heat in water
(over short distances) without a heat
pump or power plant to heat buildings,
industrial processes, greenhouses and
resorts. Geothermal district heating
systems supply heat by pumping
geothermal water through a heat
exchanger, which transfers the heat to
water in separate pipes that is pumped
into buildings. After passing through the
heat exchanger, the geothermal water is
injected back into the reservoir where it
can be recharged and used again. This
method involves the extraction of water
from the ground and can therefore
impact geothermal features.
•
1.1 Geothermal systems
1.1.1 Classification of geothermal systems
Geothermal systems can be classified into three
different categories:
•
Dry steam plants directly use geothermal
steam to turn turbines.
Geothermal heat pump system, which
consists of pipes buried in shallow ground
near a building or inserted in a vertical
well, a heat exchanger and ductwork in
the recipient building. In winter, heat
from warmer ground is passed through a
heat exchanger and used to warm any
cooler environment (e.g. a building or a
body of water). The process can work in
the opposite way in warm environments;
drawing cool air from below ground into a
building and using the ground as a heat
dump (National Park Service, NPS
Western Energy Summit, January 21-23,
2003 ).
•
Flash steam plants pull deep, highpressure hot water into lower-pressure
tanks and use the resulting flashed steam
to drive turbines.
1.1.2 Classification of geothermal systems
High temperature geothermal systems occur all
over the world; in the Philippines, New Zealand,
USA, Hawaii, Japan, Russia, Italy, Iceland, Chile
• Low temperature (< 90° C)
• Moderate-temperature (90° - 150° C)
• High temperature (> 150° C)
The temperature determines the method used to
extract the energy.
High temperature resources are generally used
only for electric power generation. Wells of
varying depth tap steam and hot water to drive
turbines that drive electric generators. The three
types of geothermal power plants most common
today are:
1
Figure 1: Map of Central America showing principal sites of geothermal resources (Birkle & Bundschuh 2007).
ii) regions around ‘hot spots’, where local magma
chambers rise from near the mantle to shallow
depths in the earths crust, and
iii) in rift zones where the tectonic plates diverge.
Geothermal active areas occur where deeply
circulating groundwaters are conductively heated
in the crust and an unusually high geothermal
gradient allows hot water or steam to appear at the
earth’s surface. The temperature of geothermal
fluids may be elevated by only a few degrees,
above ambient levels (Ellis & Mahon 1967). Heat
may be derived from volcanic or magmatic
activity, metamorphism, faulting and radioactivity.
The hot fluids are of lower density than
surrounding waters and rise through the host rock.
As geothermal fluids rise through the crust, a
decrease in pressure allows high temperature
single-phase fluids to separate at shallow depth
into two phases: steam and water. This process
can also occur wherever there is a sudden decrease
in pressure due to the presence of a rock fracture
or fissure (Webster & Nordstrom 2003). Many
geothermal fields have been developed, or are
targeted for development, to generate energy from
the steam and hot water reservoirs beneath the
earth’s surface (Bundchuh et al. 2002). As part of
and in all of the central American countries
(except for Belize).
All Central American countries (with the exception
of Belize) are endowed with significant geothermal
potential due to their location within the Pacific
Rim volcanic zone. The geothermal systems of
Central America are so-called ‘convection
systems’, related to an active volcanic belt, and
derive their heat from magmatic bodies at shallow
to intermediate levels. Such geothermal systems
provide a natural energy source for many
developmental activities. The countries in Central
America utilizing these sources for power
generation are Guatemala, El Salvador, Nicaragua
and Costa Rica. An initial assessment study of
geothermal resources in Central America was
performed by the relevant national authorities
during the 1960’s and 70’s (Birkle & Bundschuh
2007). The most important sites are showed in
Figure 1.
High temperature geothermal systems are generally
associated with three distinct plate tectonic
environments:
i) within proximity of the plate boundaries,
2
___________________________________________________________________________________
the exploitation process, reservoir fluid is drilled
and brought to the surface under pressure. When
it reaches the surface the water is “flashed” to a
desirable temperature to generate the steam to run
steam turbines. The extent of steam and water
separation (boiling) can be artificially manipulated
to maximise plant efficiency (Webster 1999).
construction of power plants. Exploitation can
also lead to ground subsidence if the fluid
withdrawal exceeds the natural inflow (i.e if reinjection is not effective). It also causes emissions
of incondensable greenhouse gases, H2S and other
pollutants (Bargagli et al. 1997).
1.2 Environmental effects of geothermal
activity
Worldwide, little is known about geothermal As,
its genesis, speciation and mobility. Little is also
known about the geochemical and biochemical
processes which occur with the fluxes of As from
the geothermal sources at the surface, where
pressure, temperature and oxidation conditions
change. It is also an issue of primary
environmental concern, to investigate the
characteristics and behaviour of geothermal As
and its impact on the natural ecosystems. It is also
important for developing strategies to improve the
socio-economic conditions of affected areas.
1.3 Arsenic in geothermal systems
Development of geothermal fields for power
generations tends to increase the rate and volume
of geothermal fluids reaching the surface. The
water formed during the process of separation
becomes a waste product. This wastewater often
has higher contaminant concentrations than
natural hot spring water because the processes that
remove or immobilise contaminants in natural
geothermal features, such as the precipitation of
mineral-rich sinters, have been bypassed. Disposal
of these waste waters can be problematic. In most
modern geothermal power stations, waste waters
are re-injected back into the field. However, at
some older fields such as the Wairakei Geothermal
Field in New Zealand these waters are still
discharged into surface drainage systems (Webster
& Nordstrom 2003).
One of the most important environmental effects
resulting from geothermal activity is the
contamination of natural drainage systems by (As)
and other harmful elements. Arsenic is a typical
component of active geothermal systems. It
commonly
occurs
together
with
other
environmental contaminants such as boron (B),
mercury (Hg), antimony (Sb), selenium (Se),
thallium (Tl), lithium (Li), fluoride (F) and
hydrogen sulphide (H2S). These elements are also
recognized as typical contaminants of geothermal
systems (Webster & Nordstrom 2003). The
concentrations of As frequently exceed 10 μg/L
and have been measured as high as 2000 μg/L
from some areas of geothermal activity. Water
flow through the surface and subsurface
catchments therefore has the potential to transport
As and other harmful elements beyond the
boundary of geothermal fields. In the area
surrounding geothermal systems, contamination of
surface- and groundwaters is common (Webster &
Nordstrom 2003). The exploitation of geothermal
resources can also change the landscape with the
drilling of wells, lying of pipelines and
1.3.1 Dispersion of arsenic and environmental
contamination
Arsenic contamination occurs due to two main
processes:
1) Natural processes: Geothermal waters reach the
surface as natural springs and then mix with
surface water flows or shallow groundwater
bodies. These fresh water sources may be used by
humans for both irrigation and drinking water
supply.
2) Geothermal exploitation: Human exploitation
of geothermal waters as an energy source, causes
mobilisation of As and other heavy metals
contained in the geothermal waters to the surface.
This then increases the likelihood of surface and
shallow groundwater contamination.
Both of these processes may affect the
environment and the health of humans and
animals. Typical symptoms that arise due to the
use of contaminated waters include skin lesions,
hyperkeratosis, melanosis and different forms of
carcinoma and lung cancer (WHO 1993, 2008;
Webster & Nordstrom 2003).
1.3.2 Health effects of arsenic and risks
The acute and chronic toxic effects of As are well
documented. Arsenic has been declared a human
carcinogen, contributing to a high incidence of
skin and other cancers in populations exposed to
high levels of As in drinking water (WHO 1993,
3
2008). The WHO drinking water guideline for As
was lowered from 50 μg/L to 10 μg/L in 1993,
and the new value has since been adopted by many
countries as a drinking water standard (Table 1).
Even though only about 8.6% of the geothermal
capacity was installed in 1994, the energy produced
represented about 15% of the total energy
produced in Costa Rica, according to the
comparison made during 2004 (Paul Moya, ICE,
pers. comm.).
The most studied areas for geothermal
exploitation are the volcanoes of Miravalles,
Rincón de la Vieja and Tenorio (Figure 2B) (Moya
2005). The largest thermal manifestations in the
area are in the Las Hornillas zone on the
Miravalles volcano and in the Las Pailas zone on
the Rincon de la Vieja-Santa Maria volcanic
complex. They contain lakes and acid-sulphate
springs, steaming ground, fumaroles emissions,
small craters and mud volcanoes. The geothermal
reservoir in this area are high-temperature and
liquid-dominated (Gherardi et al. 2002). As a
consequence of increasing exploitation of
geothermal resources, there has been emerging
concern about associated environmental impacts
in the surrounding region. The geothermal waters
and mud in Costa Rica are known to contain high
levels of As; concentrations up to 30 mg/L have
been recorded in the geothermal waters. Arsenic
presents a serious potential environmental
contamination and a risk for human health. As
concentrations in many places are above the WHO
drinking water guidelines limit of 10 µg/l, (WHO
2008). However, the geothermal waters have
highly varied characteristics that indicate
differential patterns of hydro geochemical
evolution. It is therefore difficult to predict
environmental risks on any practical level without
detailed study.
Table 1: Guideline values for arsenic recommended by World
Health Organisation (WHO 1993) for different purposes.
Use of water
Drinking water limit
Protection of aquatic life
Stockwatering
Irrigation
µg/l
10
19
20
10
Arsenic concentrations in natural surface drainage
systems frequently exceed 10μg/L in areas with
geothermal activity. Symptoms of chronic As
poisoning such as skin lesions and high As
concentrations in hair and nails have been
reported from geothermal areas but this may not
always be as a direct consequence of drinking
water contamination (Webster & Nordstrom
2003). At the Mt. Apo geothermal field in the
Philippines, for example, the two rivers draining
the field carry elevated As concentrations due to
hot springs activity in the river beds. During
developments of the field, alternative clean
drinking water supplies were provided and used by
local residents, but the symptoms of high As
exposure appeared to persist (Webster 1999). The
accumulation of As in edible aquatic plants is likely
to have been to blame, as high levels of As have
been reported in aquatic weeds in other river
systems receiving geothermal fluids (Webster &
Nordstrom 2003).
1.4 History of exploitation of geothermal
energy in Costa Rica
1.5 Rationale of the present study
Several gaps exist in our present day knowledge of
the genesis and mobility of As in geothermal
systems and the geochemical processes and their
kinetics in near-surface environments. Since there
are an increasing number of areas where outflow
of geothermal waters come in contact with human
beings and grazing animals understanding of As
behaviour is of great importance for the future
development and management of geothermal
resources. It is a trend of increasing exploitation of
geothermal resources for energy and tourism
therefore the understanding of geochemical
processes that occur along the pathway of
geothermal waters is essential. Hydro-geochemical
and geochemical investigations of surface and
During the 1970’s, Costa Rica satisfied its
electricity needs using hydro (70%) and thermal
(30%) energy sources. The continuous rise in oil
prices, especially during the 1973 crisis, motivated
the national electricity authority (Instituto
Costarricense de Electricidad [ICE]), to study the
possibility of using other energy sources for the
generation of electricity, including geothermal
energy (Moya 2005).
The first three deep wells were constructed 197980 close to the foot of the Miravalles volcano. ICE
started to generate electricity in the beginning of
1994. Since then more wells have been constructed
and the installed capacity today is about 163MWe.
4
___________________________________________________________________________________
deep geothermal waters and the sediments found
in the areas surrounding thermal springs and
fumaroles are essential for developing effective
strategies to safeguard the human health and the
environment.
The geological history of southern Central
America is dominated by the growth of a
Cenozoic magmatic arc that is superimposed on a
Jurassic-Eocene ophiolite basement that consists
of basalt and volcanic – sedimentary units. Costa
Rica is tectonically complex because of the
interplay of four plates: Cocos, Caribbean, Nazca,
and the South American block (Figure 2A). The
figure shows the subduction of the Cocos Plate
beneath the Caribbean Plate, which has caused the
development of an inter-oceanic volcanic arc all
along the Middle American Trench.
The Costa Rican Volcanic Front is associated with
the north eastward subduction of the Cocos plate
beneath the Caribbean plate with a well-defined
Benioff zone with a maximum depth of seismicity
at 200 km and a subduction velocity of 9-10
cm/year at an angle of 60 to 70˚ (Alvarado et al.
1999; Montero 1999). The four mountain ranges
in Costa Rica, known as Guanacaste, Tilarán,
Central and Talamanca ranges. The Guanacaste
range is a NW-SW trending chain of Quaternary
stratovolcanos; predominantly andesitic in
composition. The range comprises mainly of
pyroclastic (fallout, surge, and flow) rocks, lava
flows, and fluvio-lacustrine deposits that have
formed gently sloping plateaus on both sides of
the range. The area is under constant regional
stress due to the subduction of the Cocos Plate
under the Caribbean Plate and due to the regional
uplift of the volcanic arc (Vega et al. 2005).
Surface indications of geothermal activity include
numerous springs, fumaroles, vents and boiling
mud pots. These geothermal expressions form
beautiful landscapes in combination with
volcanoes and tropical rain forest. The unique
visual nature of this landscape combined with its
high biodiversity has created a region of high value
tourism. This industry contributes significantly to
the economic and social development of the
country (Vega et al. 2005).
vicinity of the volcanoes of Miravalles and Rincón
de la Vieja based on the sampling of the
geothermal wells, natural thermal springs and cold
springs. The following section gives a brief
overview of the study areas.
1.6.1 Miravalles geothermal field
Miravalles is the most studied geothermal area in
Costa Rica. Here 53 wells are drilled; from which
32 of them are used for production while 14 are
used for gravity injection of residual waters
(Maniere 2005).
Figure 2: A) Tectonic sketch map of Central America. The
subduction of the Cocos Plate beneath the Caribbean Plate is
showed, which caused the development of an inter-oceanic volcanic
arc all along the Middle American Trench. B) Geographical map
of Costa Rica showing location of main volcanoes and Miravalles
and Rincon de la Vieja geothermal fields. The Guanacaste arc
consist of the Orosi-Cacao, Rincon de la Vieja-Santa Maria,
Miravalles-Paleo Miravalles and Tenorion-Montezuma volcanic
complexes (Gherardi et al. 2002).
1.6 The Study areas
The present study focuses on investigate the
occurrences of As as well as an overall
characterisation of the geothermal waters in the
5
Cross section of
Figure 4
Figure 3: Geological-structural map of Miravalles geothermal reservoir and well field located in Guayabo caldera. The Map section of La
Fortuna Graben is enclosed by a black frame (Birkle and Bundchuh 2007).
6
___________________________________________________________________________________
Figure 4: Geological-structural cross section of parts of the Miravalles geothermal field: A) from well 51 in the south to well 58 in the north,
and b) from well 58 in the northwest to well 64 in the southeast. The estimated 220˚C isotherm is also plotted. Locations of the cross-sections
are shown in Figure 3 (Birkle and Bundchuh 2007).
during the Pleistocene after three separate phases
of volcanic activity. Within the Guayabo caldera
the volcanic activity gradually shifted in a
northwest direction and then south-westwards
during these phases. Revealing the existence of
strong tectonic activity, four main fault systems
have been identified in the area (Gherardi et al.
2002) all of which contribute to the hydraulic
permeability of the geothermal reservoir (Vega et
al. 2005).
In order of decreasing age, they are oriented:
1. NW–SE direction sub-parallel to that of
the main Central-American mountain
chain, and, in particular, to the axis of the
Guanacaste volcanic belt.
2. N–S, formed during the Holocene, and
has produced a graben-like structure
about 3 km in width known as La Fortuna
Graben (Figure 3). The La Fortuna
Graben has displaced part of the Guayabo
caldera and forms the eastern and western
margin of the geothermal field.
3. NE–SW, sub-parallel to the migrating
eruptive centres of the Miravalles volcano.
Miravalles geothermal area is located in the northwestern part of Costa Rica in the Guanacaste
mountain range of Guanacaste Province. The
Miravalles volcano is a Quaternary strato-volcano
rising to 2028 m a.s.l. and is currently inactive. The
geothermal reservoir is high-temperature and
liquid-dominated. The Miravalles field area is
characterised by the Guayabo caldera (Figure 3)
which is an active hydrothermal system of a
caldera-type collapse structure of about 15 km
diameter (Bundschuh et al. 2002; Vega et al. 2005).
The highest reservoir temperature measured was
255 ˚C with typical measured temperatures ranging
between 230-240˚C. The north-eastern part of the
Guanacaste Cordillera seems to be the main
recharge zone. Regional groundwaters and
reservoir fluids have been identified to have
several mixing trends (Gherardi et al. 2002). In
order to study the hydrologic relations between the
reservoir and the surface water bodies’ samples
were taken from zones surrounding the Miravalles
geothermal field and from geothermal fluids.
1.6.1.1 Geo-volcanological framework
The Miravalles geothermal field is located inside
the Guayabo caldera (Figure 3), which formed
7
Figure 5: Conceptual model of Miravalles geothermal reservoir, with a heat source related to Miravalles volcano. Also
shown are main aquifer systems of the geothermal reservoir with flow directions, which are influenced by major faults (e.g. Las
Hornillas fault) (Birkle and Bundchuh 2007).
4. E–W, intersects the graben surface, and
has been encountered in several
productive wells. This is the youngest
fault system and is expressed at the
surface as hydrothermal alterations,
sulfataras, mud volcanoes and hot springs
(e.g., Las Hornillas at Hornillas fault)
(Gherardi et al. 2002; Vega et al. 2005).
In the Guayabo caldera, surface geological
mapping combined with lithological descriptions
from the 53 geothermal wells reveal different
stratigraphic units (Figure 4). Three phases of
volcanic activity can be distinguished: pre-caldera
volcanism, caldera volcanism and post caldera
volcanism (Birkle & Bundchuh 2007; Vega et al.
2005).
To get to know the temperature distribution in the
geothermal field, information on water-rock
interactions and resulting hydrothermal alteration
was obtained from the 53 geothermal wells. Three
temperature zones, partly overlapping, could be
distinguished:
1. a smectit zone (<165˚C)
2. transition zone (140-220˚C)
3. an illite zone (<220˚C)
The highest temperatures are found in the north
of the production zone indicating that, (1) the heat
source is related to the Paleo-Miravalles and
Miravalles volcanoes, and (2) that fluid movement
is generally north to south. This provides the
conceptual model of Miravalles geothermal
reservoir shown in Figure 5. From this figure can
be observed that the temperature of the
production zone decreases to the south, east and
west (Vega et al. 2005).
1.6.1.2 Characteristics of the manifestations
There are three types of water associated with
natural manifestations in the Miravalles area, other
than the water of the geothermal reservoir:
•
Cold water from streams, shallow aquifers
and rivers,
•
Neutral pH thermal waters,
• Sulphate-rich acid thermal waters.
The neutral pH waters emerge from the inside of
the caldera, following a N–S trending belt that
coincides with La Fortuna graben, while the
manifestations of sulphate-rich acid waters are
found along the slopes of the volcano at relatively
high elevation (> 500 m a.s.l.).
The surface waters are present over the whole area
and form a dense hydro graphic network of
streams of relatively low flow-rate. The largest
thermal manifestations in the Miravalles volcano
are in Las Hornillas zone. The thermal
manifestations contain lakes and acid-sulphate
springs, steaming ground, fumaroles emissions,
small craters and mud volcanoes (Gherardi et al
2002).
8
___________________________________________________________________________________
1.8 Limitations
1.6.2 Rincon de la Vieja geothermal field
Rincon de la Vieja is the only active volcano of the
volcanoes in the Guanacaste Volcanic Range. It is
an andesitic stratovolcano located in the northwest of Costa Rica near the border of Nicaragua,
at about 25 km NE from the city of Liberia
(Figure 2B). Rincon de la Vieja is a complex
volcano, with a maximum elevation of 1916 m
(Santa Maria cone), consisting of seven craters,
forming a 8 km long NW trending ridge within an
older, wider caldera. The majority of thermal
emissions of Rincon de la Vieja volcano are
located in the western outer flank of the Active
Crater. The origin of the thermal discharges is
mainly due to the boiling of a shallow aquifer
heated by inputs of magmatic-related hot fluids
and the expressions of geothermal activity in the
area mainly consist of hot mud pools and hot
springs (Tassi et al. 2005).
The largest thermal manifestations on the Las
Pailas zone in Rincon de la Vieja are the Santa
Maria volcanic complex which contains lakes and
acid-sulphate springs, steaming ground, fumaroles
emissions, small craters and mud volcanoes
(Gherardi et al. 2002).
The chemical behaviour of As in sulphide-rich
fluids has been the subject of considerable and
ongoing debate as there are several major obstacles
to the accurate prediction of As speciation.
In part, this reflects the lack of a complete
thermodynamic database for As species. Stability
constants for ion pairs and complexes between the
oxyanions and polyvalent cations are few and
limited in applicability. Also, polymerisation of
arsenite, arsenate and thioarsenite complexes in
high As or high sulphur solutions appears likely,
but is largely unconfirmed. These reactions could
significantly complicate As speciation. However,
reliable thermodynamic data are available for some
As species, and these can be used to interpret As
behaviour in many geothermal systems and surface
water environments, as long as the limitations are
recognized (Webster & Nordstrom 2003).
1.7 Aims and objectives
The purpose of the study is to investigate the
occurrences of As as well as an overall
characterisation of the geothermal waters in the
areas of Miravalles and Pailas-Borinquien
geothermal fields.
The specific objectives are to characterize waters
extracted from:
•
Geothermal
generation.
•
Natural thermal springs.
•
Cold springs.
wells
used
for
power
•
Suggest protection measures to avoid As
contamination.
This study is expected to give an opportunity to
compare similarities and dissimilarities of As
concentration with results from geothermal wells
and thermal springs in New Zealand, Philippines
and USA where these issues have been
investigated.
9
2. ARSENIC IN GEOTHERMAL WATER
Inorganic As is known to be potentially toxic and a
hazardous to humans. Arsenic is a strongly redoxsensitive element and in natural aquifer systems is
present in two dominant redox states; As(ΙΙΙ) and
As(V). As(V) is generally the stable species in
oxidizing environments while As(ΙΙΙ) species
predominates under reducing conditions. The
toxicity (and mobility) of As(ΙΙΙ) is greater than
that of As(V). As(ΙΙΙ) is toxic since it coagulates
proteins, forms complexes with coenzymes, and
inhibits ATP production (necessary for metabolic
processes) (Bhattacharaya et al. 2002).
The uprising geothermal waters are normally
reducing
(suggesting
the
presence
of
predominantly As(ΙΙΙ) species) at depth. While it
comes in contact with shallow aquifers or mixes
with surface waters, the redox conditions become
oxidising. In such oxidized systems, the mobility
of As is a function of the redox transformation of
the As(ΙΙΙ) to the oxidised As(V) species. The
As(V) species is then sorbed on oxide minerals,
i.e., amorphous Al-, Mn- and Fe oxides and
hydroxides. However, if residence time of water
within the geothermal system is short, redox
transformations are rather incomplete and thus
As(ΙΙΙ) species may be present at the surface
outlets of thermal springs. The mobility of As in
reduced systems is also governed by the
dissolution of Fe- and Mn-oxyhydroxides in the
aquifer sediments due to microbial mediated
biogeochemical interactions (Bhattacharaya et al.
2002).
Geothermal waters, which are mineralised with
over 1g/L of total dissolved solids change their
specific chemical composition during their
pathway from deep seated sources to the surface
of the earth. Since geothermal waters are related to
active or inactive volcanism, the presence of As is
everywhere in these waters. The concentration and
speciation of As depends on the thermal sources at
depth and subsequent hydro-chemical evolution of
water chemistry along its pathway up to the
surface. Due to interactions with the sediments
and other unstable constituents generated during
the volcanic activities (fumaroles, gas vents etc.)
the concentration and speciation of As change and
leave behind hydrothermal mud (Webster &
Nordstrom 2003).
Geothermal fluids may contain as much as 50
mg/L As, although concentrations between 1 and
10 mg/L are more typical (Ballantyne & Moore
1988).
2.1 The source of arsenic in geothermal
systems
2.1.1 The source and nature of geothermal
fluids
Active geothermal areas occur where an unusually
high geothermal gradient allows hot water or
steam to reach the earth’s surface. Heat sources
may be related to magmatic or volcanic activity,
faulting,
radioactivity
or
metamorphism.
Regardless of the heat source, deeply circulating
groundwaters are heated in the crust.
The hot fluids (that may be heated by only a few
degrees or by hundreds of degrees above the
surroundings) are of lower density than
surrounding waters, and rise through the host
rock, to complete the circulation system. The
pressure decreases when the geothermal fluids rise
through the crust and the fluid separates into a
steam phase and a water phase. This usually occurs
at a shallow depth, but can occur wherever there is
a sudden decrease in pressure due to rock fracture.
Different kind of surface features can be seen in
areas of geothermal energy. They can be grouped
into two main types, on the basis of their
relationship to the rising geothermal fluids. Hot
water springs is a direct discharge of the hot water
phase. They are rich in chloride and silica and have
an almost neutral pH.
Steam vents and acidic, sulphate-rich springs are
formed by the interaction of the steam phase with
shallow aquifer waters. This mixture of water and
steam lead to precipitation and sublimation of
elemental sulphur which follows by microbial
oxidation to sulphuric acid.
Intense alteration of the host rock caused by the
fluid can lead to an unstable ground surface and
the formation of bubbling “mudpools”. Geysers
and carbonate-rich springs are also examples of
hot water discharges and include steam-heating
and steam-phase mixing (Webster & Nordstrom
2003).
10
___________________________________________________________________________________
2.1.2 The source of As in geothermal fluids
The presence of As in geothermal fluids has been
known since the mid 19th century. Many studies
have been performed in the Yellowstone National
Park which contains one of the largest geothermal
systems in the world. Here As concentrations in
the thermal features generally range from <0.1
mg/kg to 10 mg/kg. This range is observed in
most active geothermal fields except for the
geothermal fields in Iceland and Hawaii where As
concentrations of <0.1 mg/kg are consistently
reported. The reason for low levels of As in those
areas is the occurrence of fresh basaltic host rock,
low in As, in the reservoir of the respective
geothermal fields. High As concentrations of >20
mg/kg are not uncommon in geothermal well
fluids. Therefore the As concentration from
geothermal fluid are usually three orders of
magnitude greater than those in uncontaminated
surface and groundwaters (Webster & Nordstrom
2003).
As concentrations are higher in neutral pHchloride hot springs compared to acid-sulfate
features. As is retained in the wastewater, during
the separation of steam and bore water in
geothermal power generation. The high content of
As in the wastewater from geothermal fields is one
of the main problems in the disposal of this water
(Webster & Nordstrom 2003).
2.2 Speciation of arsenic in geothermal
fluids
2.2.1 Dissolved arsenic
The chemical behaviour of As in sulphide-rich
fluids has been the subject of considerable and
ongoing debate as there are several major obstacles
to the accurate prediction of As speciation. Part of
this problem is the lack of a complete
thermodynamic database for As species. However,
reliable thermodynamic data are available for some
As species, and these can be used to interpret As
behaviour in many geothermal systems and surface
water environments, as long as limitations are
recognized (Webster & Nordstrom 2003).
2.1.3 Arsenic from host rock leaching
Significant quantities of As were leached from
non-mineralised andesite during hot water-rock
leaching experiments performed by Ellis and
Mahon (1964; 1967) with concentrations up to 1.3
mg/kg in the leachate. Experimental leaching of
greywacke with hot water also indicates significant
leaching of As, together with Sb, Se and S. This
leaching occurred despite the experiment being
performed at temperature less than those in many
geothermal fields. It can therefore be assumed that
neither magmatic fluid input, nor As
mineralization at depth, is a requirement for high
As concentrations in geothermal fluids. Most
reservoir fluids are under-saturated with respect to
arsenopyrite and other As minerals and for this
reason As leaching, rather than As precipitation, is
predicted to occur in a reservoir.
Most geothermal waters show a positive
correlation between As and Cl but the
interpretation of trends in the As/Cl ratio requires
caution; the association is more a function of
common behaviour, than of a common source or
direct chemical association. Chloride ions may be
sourced from host rock leaching, seawater or may
be gaseous HCl associated with magma intrusion,
while As is derived mainly by host rock leaching.
As and Cl remain in the fluid phase during subsurface boiling and phase separation.
2.2.2 Low sulphide, reduced fluids
As is likely to be transported as arsenius acid
(H3AsΙΙΙO3) in most hydrothermal solutions,
according to Ballantyne and Moore (1988).
H3AsO3 is considered to be the product of both
As oxide (As4O6) and orpiment (As2S3) dissolution
in reduced fluids. This occurs over a wide
temperature range and at an acid to neutral pH.
Orpiment solubility increases with temperature, to
at least 300 degrees. The orpiment solubility
reaction:
0.5As2S3 + 3H2O Ù H3AsO3 + 1.5H2S
The solubility of Orpiment is independent of pH
under acidic, or acid-neutral pH conditions, but
increases with further increase in pH (Webster &
Nordstrom 2003).
2.2.3 High sulfide, reduced fluids
In sulphide-rich hydrothermal solutions, orpiment
solubility as H3AsO3 is inhibited but solubility as
thioarsenite complexes increases. The thioarsenites
are a group of covalently bonded As-sulphide
complexes that may or may not include oxygen.
11
facilitate arsenite oxidation at temperatures > 50˚C
remain not completely understood. Bacteria
influence the As(III) oxidation in geothermal
systems and therefore studies is being made in the
identification of the thermophilic bacteria that can
oxidise As(III).
The ability of H2S and thiosulphate (S2O3) to
reduce As(V) to As(III) is well known because
geothermal waters containing sulphide or
thiosulphate will preserve As as As(III) until the
reduced sulphur is oxidised or volatilised (Webster
& Nordstrom 2003).
Reactions between arsenate ions and dissolved
sulphide can result in the successive replacement
of oxygen with sulphur. The exact nature of such
thio-As complexes still remains much debated. By
assuming that a trimmer structure occurs in a
saturated geothermal fluid the following reaction
gives the solubility of orpiment:
1.5As2S3 + 1.5HS + 0.5 H+ = H2As3S6The solubility of orpiment increases with pH and
sulphide concentration but is not greatly affected
by temperature changes below 200°C (Webster &
Nordstrom 2003).
2.3 Arsenic deposition from geothermal
fluids
Deposits of As, Sb, Au and Hg occur
predominantly near the surface in geothermal
systems while base metals such as Ag, Cu, Pb, Zn
will be deposited at greater depth. The metal
zonation is due to a three-step process of fluid
boiling, gas phase transport and acid reactions
within metal-bearing waters. Base metal
precipitation occurs due to boiling and the pH
increase which occurs when CO2 gas is driven off.
During boiling, As remains soluble as an oxyanion
under the higher pH and lower sulphide
concentrations present in the fluid. Precipitation
of orpiment then occurs in response to the
acidification of hot spring waters with acidsulphate waters, subsurface cooling of the fluid or
increased H2S concentrations.
At many geothermal fields it has been noted that
As was mainly concentrated in pyrite at depth.
Arsenic minerals such as arsenopyrite (FeAsS)
appear to be uncommon in the rocks of
geothermal reservoirs themselves but a range of
As minerals are precipitated from geothermal
surface features such as hot springs (Webster &
Nordstrom 2003).
2.2.4 Redox state
When the redox conditions in low sulphide fluids
become sufficiently oxidizing, oxidation of
arsenious acid to arsenate (H2AsνO4- or HAsνO42-)
is likely to occur. Oxidation occurs when a rising
geothermal fluids is exposed to atmospheric
oxygen or mixed with another oxidizing fluid, such
as a shallow groundwater. Arsenate ions appear to
be formed in hot springs and their surface
drainage systems. Hot springs form from reservoir
fluids contain mainly As(III) , whereas acid-sulfate
and bicarbonate features are more enriched in
As(V) . Important to notice is that reported As(III)
concentrations may be underestimated if the
sample has not been appropriately preserved prior
to analysis. To preserve the sample it is important
to perform on site filtration through a 0.45μm (or
finer) membrane to remove bacteria. The sample
should be stored cold and without air contact. It is
also often recommended to acidify the sample
with HNO3 or HCl to pH 1.5-2.0 because this
inhibits oxidation of As(III) by chemical oxidants
such as Fe(III). Although, in some waters,
acidification has been observed to change As
speciation slightly, reducing As(V) to As(III).
Geothermal water collected immediately after
steam separation, and before entering the drain,
does not show a similar rate of As oxidation.
Instead there is no noticeable As(III) oxidation in
the waste water (collected directly into a sterile
glass bottle), even if bubbled with air for several
hours.
Rapid As(III)/As(V) oxidation kinetics is
apparently catalysed by microbial activity, although
the specific mechanisms by which bacteria could
2.4 Arsenic in hot spring deposits and
scales
The coloured precipitate forming at the periphery
of hot springs, hot pool and geysers can have a
high concentration of As ranging from two to a
couple of hundreds mg/kg. It can be difficult to
determine the mineralogy of these coloured
precipitates though As:S ratios may be affected by
other sulphide-bearing minerals and the colour can
vary. For example, amorphous realgar has a bright
red colour but so have stibnite and cinnabar too.
12
___________________________________________________________________________________
Orpiment has a yellow-green colour that remind of
native sulphur. However, in hot springs with low
levels of antimony and mercury, the yellow
precipitate is commonly orpiment. Realgar does
not seem to deposit in hot springs but does occur
as coating and veins in the altered rocks of the
geothermal field. Other types of deposits have also
been reported from around the world but so far
little is known.
The scales which form in pipes and drains of a
developed field can also be rich in As. The As
deposition may occur due to adsorption on Feoxide (Webster & Nordstrom 2003).
not matter because As is largely reduced to As(III)
and methylated. Another issue is the processes by
which As interacts with sediments and organic and
biotic substrates in freshwaters, as this affects both
As concentration and speciation.
Studies of As in the rivers draining two geothermal
systems: Yellowstone National Park and the
Taupo Volcanic Zone, have shown that As is
principally transported in dissolved form.
“Dissolved” in this case is defined as passing
through a 0.45μm filter membrane. On a macroscale As also appears to behave conservatively
downstream of these geothermal systems, with
little change in As mass flux down the river or in
lakes and estuaries. Studies of geothermal-derived
As in other freshwater systems support this
observation (Webster & Nordstrom 2003).
2.5 The fate of arsenic from geothermal
sources
One of the most significant negative
environmental effects of geothermal activity is the
contamination of natural drainage systems by As.
While direct soil contamination will occur to some
degree near a geothermal field, this is typically only
a local effect generally considered acceptable. The
main risk is presented by the transportation of As
beyond the boundary of the geothermal field via
water flow through the surface and subsurface
catchments. It is chemical contamination of
surface water, rather than groundwaters, which is
most commonly detected in the vicinity of
geothermal systems. In surface waters, As enters a
cycle of chemical and biochemical reactions which
affect its potential impact (Webster & Nordstrom
2003).
2.6.1 Dissolved arsenic speciation
The presence of bacteria in the oxidation process
may be very important both in river waters and
around the outlets of the geothermal springs.
Wilkie & Hering (1998) noted that As oxidation in
Hot Creek did not occur if the river water was
filtered under sterile conditions or after an
antibiotic had been added. Their conclusion from
this was that bacteria attached to submerged plants
were mediating As(III) oxidation. Some of the
most common bacteria capable of As(III)
oxidation are Pseudomonas sp, Xanthomonas and
Acromobacter. Significantly, while As(V) may be
the predominant form of As found in an
environment, the As(III) form can still occur due
to seasonal changes in water and microbial
conditions(Webster & Nordstrom 2003).
2.6 Surface waters
Hot springs, geysers and steam features all
produce an excess of fluid at the surface that
usually drains directly into the nearest catchments
system. Even after orpiment precipitation, many
hot spring fluids will still contain >1mg/kg As
because orpiment is a relatively soluble salt. For
that reason, water used for drinking, stock
watering and irrigation or supporting aquatic life
may have unacceptably high As concentrations
downstream of a geothermal system.
Arsenic contamination of drinking water may
cause serious health problems and has therefore
received lots of attention.
Arsenic speciation is important in surface waters
because acute toxicity of As(III) is greater than
that of As(V) or the organic forms of As. For
human chronic toxicity, the redox form of As may
2.6.2 Transport and removal from the water column
Part of the As released into a river from a
geothermal system will be incorporated into the
biochemical cycle, interacting with plants, biota,
suspended material and bed sediments.
A strong association between As and Fe-oxides in
river and lake sediments has been reported. This
association is attributed to the adsorption of
arsenate onto the Fe-oxide coating on sediment
particles.
The life-cycle of aquatic plants may also affect the
concentration of As in the water column. The
uptake of As by aquatic macrophytes (plants) has
been reported in rivers contaminated by
geothermal waters. There is often a significant
13
degree of As enrichment relative to the water
column. High phosphate concentrations can
inhibit As uptake by plants, as well as As
adsorption on Fe-oxide surfaces.
Finally, As interacts with macrobiota either directly
or via the food chain. As well as the issue of
toxicity, there is the potential problem of As
accumulation in animal flesh (Webster &
Nordstrom 2003).
2.7 Groundwater
Even though As-rich groundwaters are common,
there are relatively few examples where the As is
clearly related to geothermal activity. Geothermal
fluids are themselves effectively deeply circulating
groundwaters. The thermal attractive force to the
surface limits the spreading of these As-rich fluids
at depth. However, at or near the surface
contamination of shallow aquifer systems can and
does occur.
Groundwater contamination is a concern for
geothermal developers because there are several
potential pathways for aquifer contamination
(Webster & Nordstrom 2003). These include:
•
Unintentional re-injection of spent
geothermal waste water into an aquifer.
This will normally occure only if the
integrity of the casing around a well is
breached.
•
Seepage from poorly-lined or unlined,
holding ponds for the retention of
geothermal fluids and from pipelines.
•
Burial of As-rich sludge from waste
treatment process or from drains and
pipes.
14
___________________________________________________________________________________
3. MATERIALS AND METHODS
sample was collected from the cold spring of
Hotel Guachipelin. Figure 10 show an overview
over Rincon de la Vieja area and Figure 11 show
how the sampling at Rincon de la Vieja was carried
out.
3.1 Field investigations
Fieldwork was carried out during September 2005.
It consisted of field measurements and water
sampling. Totally 50 sample were collected at 49
different places in the surroundings of the
Miravalles and the Rincon de la Vieja volcano
areas. Water from three types of reservoirs was
collected e.g. geothermal well fluids (GTW),
thermal springs (TS) and cold surface waters (CS)
(Table 2).
3.1.2 Collection of field data
The following data was collected/measured in the
field:
•
GPS coordinates, information given from
ICE
•
Well depth, information given from ICE
•
Temperature, using a portable EcoScan pH
6
meter
(temp
range
0-100˚C).
Temperatures for the geothermal wells:
information given from ICE
•
pH, using a portable EcoScan pH 6 meter
(pH range 0-14, +/- 0.01)
•
Eh (redox), using a portable EcoScan pH 6
meter (-1000-1000mV, +/- 2 mV))
•
Dissolved oxygen, sing a CyberScan DO100
•
-Conductivity, using a portable EcoScan pH
6 meter
•
Field estimation of total As concentration,
using a Hach Field test kit (range: 10-500
µg/l).
3.1.1 Field location and sampling
3.1.1.1 Miravalles
Twenty eight 28 samples were collected around
the Miravalles volcano area.
13 of these samples (PGM-11, PGM-14, PGM-62,
PGM-31, PGM-08, PGM-45, PGM-43, PGM-12,
PGM-20, PGM-46, PGM-29, PGM-07, PGM-19)
were extracted directly from the pipes which are
pumping up the water from the geothermal well at
a depth varying between 960-1998 meters of the
geothermal field of Miravalles (Figure 6).
11 samples (Termal Guayabal, Sitio 13, Corralito,
Termal union, U1, Hornillas Miravalles, Colegio
fortuna, San bernardo 1, San bernardo 2, Salitral
bagaces and Panteon) were collected from thermal
springs and 4 samples (Toma guayabo, Toma casa
maquinas, Toma fortuna, and Toma colonia) were
collected from cold surface springs (Figure 7).
The equipments were checked and calibrated
before the field measurements. The pH-meter was
calibrated once a day using buffer solutions and
the redox electrode was checked with Zobell’s
solution several times during the investigation
period. Field measurements of redox potential
(Eh) are known to be problematic with
questionable reliability (Appelo & Postma 1993).
Water samples were taken from each location and
brought back to the laboratory of the Department
of Land and Water Resources Engineering, Royal
Institute of Technology Stockholm for further
analyses. A set of four bottles was taken, one
plastic bottle of 50 ml volume, one 50 ml bottle
with dark glass and two plastic bottles of 22 ml
(see below).
3.1.1.2 Rincon de la Vieja
In the Rincon de la Vieja volcano area, 21 samples
were collected. Among these, 4 samples (PGP-01,
PGP-03, PGP-04 and PGB-01) were extracted by
pump equipment directly from the geothermal well
at a depth varying between 1414-2595 meters of
the geothermal field of Pailas-Borinquen (Figure
8). 16 samples were collected from thermal springs
(Figure 9) among which 5 were extracted at
Hornillas hotel Borinquen, Hornillas Parque,
Pailas agua, Santa Maria Caliente and Azufrales.
The remaining samples were collected from 11 hot
water springs in this area from Rio Salitral, Rio
Tizate1, Rio Tizate 2, Salitral las lilas 1, Salitral las
lilas 2, Salitral las lilas 1+2, Pedernal, Nacientes
Nieve Arraya1 and Nacientes Nieve Arraya 2. 1
15
Figure 6: Map pf Miravalles geothermal field with sampled geothermal wells.
16
___________________________________________________________________________________
Figure 7: Map of spring locations around Miravalles
17
Figure 7:
Figure 8: Map of Las Pailas-Borinquien geothermal fields and the sampled geothermal wells.
18
___________________________________________________________________________________
Figure 9: Map of spring locations around Rincon de la Vieja.
Figure 10: Ooverview map Rincon de la Vieja area.
19
Figure 11: Sampling at Las-Pailas- Borinquien (1-3) and Miravalles geothermal fields (4). 1) Construction of pump equipment 2) Winch
used to lower and extract sample equipment into boreholes 3) Cooling ponds 4) Releasing the gas before taking sample at Miravalles
geothermal field.
20
___________________________________________________________________________________
Table 2: Overview of the samples collected from Miravalles and Rincon de la Vieja geothermal sites
Sample nr Sample ID
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
PGM-11
PGM-14
PGM-62
PGM-31
PGM-08
PGM-45
PGM-43
PGM-12
PGM-20
PGM-46
PGM-29
PGM-07
PGM-19 (öppen)
Termal Guayabal
Sitio 13
Corralito
Termal union
U1
Hornillas (Miravalles)
Colegio Fortuna
San Bernardo 1
San Bernardo 2
Salitral Bagaces
Panteon
Toma Guayabo
Toma casa maquinas
Toma Fortuna
Toma Colonia
PGP-01
PGP-03
PGP-04
PGB-01
Hornillas hotel Borinquen
Hornillas parque
Pailas agua
Santa Maria Caliente
Azufrales
Rio salitral (Hotel buena vista)
Rio tizate 1
Rio tizate 2
Salitral las lilas 1
Salitral las lilas 1+2
Pedernal (NE de C Fortuna)
Naciente Nieves Araya 2
El volcancito
Las Avestruces
El albergue agroecologico
Hotel Guachipelin
Location
Sample source
Miravalles
Miravalles
Miravalles
Miravalles
Miravalles
Miravalles
Miravalles
Miravalles
Miravalles
Miravalles
Miravalles
Miravalles
Miravalles
Miravalles
Miravalles
Miravalles
Miravalles
Miravalles
Miravalles
Miravalles
Miravalles
Miravalles
Miravalles
Miravalles
Miravalles
Miravalles
Miravalles
Miravalles
Rincón de la Vieja
Rincón de la Vieja
Rincón de la Vieja
Rincón de la Vieja
Rincón de la Vieja
Rincón de la Vieja
Rincón de la Vieja
Rincón de la Vieja
Rincón de la Vieja
Rincón de la Vieja
Rincón de la Vieja
Rincón de la Vieja
Rincón de la Vieja
Rincón de la Vieja
Rincón de la Vieja
Rincón de la Vieja
Rincón de la Vieja
Rincón de la Vieja
Rincón de la Vieja
Rincón de la Vieja
Rincón de la Vieja
Rincón de la Vieja
Geothermal well
Geothermal well
Geothermal well
Geothermal well
Geothermal well
Geothermal well
Geothermal well
Geothermal well
Geothermal well
Geothermal well
Geothermal well
Geothermal well
Geothermal well
Thermal spring
Thermal spring
Thermal spring
Thermal spring
Thermal spring
Thermal spring
Thermal spring
Thermal spring
Thermal spring
Thermal spring
Thermal spring
Cold spring
Cold spring
Cold spring
Cold spring
Geothermal well
Geothermal well
Geothermal well
Geothermal well
Thermal spring
Thermal spring
Thermal spring
Thermal spring
Thermal spring
Thermal spring
Thermal spring
Thermal spring
Thermal spring
Thermal spring
Thermal river
Thermal spring
Thermal spring
Thermal spring
Thermal spring
Thermal spring
Thermal spring
Cold spring
WL84
W
85 10´55.774"
x
85 11´15.855"
85 11´25.249"
85 11´44.568"
85 11´54.829"
85 12´4.878"
85 11´43.921"
85 11´50.390"
85 11´43.145"
85 10´30.171"
85 10´45.090"
85 11´23.373"
85 11´37.173"
85 11´52.648"
85 11´38.53"
85 11´46.396"
85 11´37.876
85 10´45.83"
85 12´35.667"
85 12´10.239"
x
85 14´24.114"
85 12´57.37"
85 11´27.084"
85 10´50.520"
85 11´40.640"
85 11´14.620"
85 21´42.980
85 21´6.635"
85 21´32.571
85 24´30.555
85 24´54.507"
85 22´24.0"
85 20´43.8"
85 19´30.0"
85 19´39.5"
85 24´24.9"
85 26´14.7"
85 26´26.175"
85 28´11.9"
85 28´18.5"
85 28´18.5"
85 27´26.0"
85 19´39.4"
85 19´39.4"
85 19´29.7"
x
x
85 21´46.2"
Water samples collected from each well involved:
•
50 ml filtered samples for alkalinity and
major anion analysis.
•
22 ml filtered acidified samples (14M
HNO3), for major cation and trace elements
analysis.
•
22 ml acidified samples (14M HNO3), kept
frozen when possible, for DOC analysis.
WL84
N
10 43´6.000"
x
10 43´7.997"
10 42´37.539
10 42´25.736"
10 41´59.767"
10 42´3.971"
10 41´18.524"
10 41´25.179"
10 41´47.299"
10 40´40.117"
10 42´58.249"
10 41´51.316"
10 45´16.444"
10 44´18.953"
10 44´20.46"
10 43´8.110"
10 42´33.761"
10 42´51.05"
10 40´25.262"
10 35´56.798"
x
10 35´49.765"
10 42´16.52"
10 43´34.234"
10 43´46.701
10 40´46.309"
10 40´37.395"
10 45´51.235"
10 46´19.767
10 45´29.037"
10 48´24.866
10 48´49.132"
10 47´3.8"
10 46´21.9"
10 45´38.6"
10 45´33.2"
10 48´41.1"
10 46´28.9"
10 46´25.602"
10 48´19.2"
10 48´12.6"
10 48´12.6"
10 49´1.6"
10 53´52.9"
10 53´52.9"
10 54´35.2"
x
x
10 45´42.6"
X
Lambert North
299,780
x
300,577
299,843
298,908
298,547
297,750
297,880
296,482
296,687
297,366
295,296
299,541
303,791
302,026
Y
Lambert East
407,149
x
406,791
406,539
406,251
405,663
405,350
405,044
405,677
405,481
405,703
407,915
407,473
405,902
405,427
299,849
298,793
405,611
405,867
294,850
286,600
x
286,396
404,100
404,850
x
400,780
300,650
301,030
295,492
295,216
x
305,788
304,232
309,652
310,400
307,150
305,850
304,460
304,350
310,150
306,100
306,000
309,500
309,300
309,300
310,800
319,700
319,700
321,000
x
x
304,650
406,200
407,312
405,774
406,564
x
388,607
387,814
382,425
381,700
386,260
389,300
391,540
391,250
382,600
379,250
378,900
375,700
375,500
375,500
377,100
391,300
391,300
391,600
x
x
387,400
Elevation
m(asl)
719
702
692
643
809
593
578
516
517
584
473
738
608
610
575
590
550
625
754
380
335
335
165
510
610
780
480
455
659
757
631
699
535
760
763
715
725
630
330
300
245
250
250
315
435
435
410
400
400
595
Well_Depth Max Reservoir Temp
m
°C
1450
242
1396
See graph
1758
238
1724
234
1200
232
960
235
953
229
1596
227
1700
231
1198
233
1380
230
1998
239
1260
232
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
1418
240
1772
244
1418
229
2595
276
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
colloidal material and micro organisms that can
effect the dissolved As(ΙΙΙ/ V) ratio.
Some of the samples were also acidified with 5 ml
HNO3 per liter sample. Bottles were closed
avoiding air bubbles. The samples were put in a
cooled box during transportation and stored
refrigerated until the analysis was completed in
Stockholm during October, 2006.
•
3.2 Laboratory investigations
50 ml filtered through a Disposable
CartridgeTM filter that removes species of
As(V) and then acidified in a dark glass
bottle, for As(III) analysis.
The dark glass bottle was used for storing the
water meant for As analyses. Before bottling the
water was filtered through a cartridge filter with
the pore size 0.45 µm. to remove most of the
The water was analyzed for major ions, total As
and other trace elements and further for As(III),
DOC and alkalinity. Below follows a description
of the methods used for each of the analysis.
3.2.1 Alkalinity
The alkalinity was measured following the
standard SS-EN ISO 9963-2 (SIS 1996).
21
3.2.2 Major ions
Anions were analyzed at the laboratory of the
Department of Land and Water Resource
Engineering at KTH on a Dionex DX-120 Ion
Chromatograph. In order to save the sensitive
equipment samples were diluted between 1 and
100 times prior to analysis yielding lower
concentrations of ions. Dilution was made with
guidance from the field-measured conductivity.
For anion analysis untreated samples and an
IonPac AS9-SC coloumn was used. The
chromatograph was run several times until almost
all results were within the calibrated interval set by
the standard solutions used for verifying the
results.
Cations were analysed with ICP-OES (Optical
Emission Spectroscopy) from Varian Vista Ax..
These analyses were performed at the Stockholm
University.
3.2.3 Trace elements
Acidified samples were analyzed at the Stockholm
University with an ICP-OES (Optical Emission
Spectroscopy) from Varian Vista Ax.
3.2.4 Arsenic (III)
Samples used were directly filtered in the field
through Disposable Cartridge filters that removes
As(V) and then acidified. Analyzes was performed
at the Stockholm University using the same
instrument as for trace elements, see above.
3.2.5 DOC
Dissolved organic carbon was analyzed at the
laboratory of the Department of Land and Water
Resource Engineering at KTH on a TOC-5000
SHIMADZU Total Organic Carbon Analyzer as
NPOC (Non-Purgeable Organic Carbon).
3.2 Treatment of analytical data
AquaChem 4.0 (Waterloo Hydrogeologic, 1997)
was used to determine water types and create piper
diagrams of major ions in the sampled waters.
Scatter and Box plots was also carried out.
22
___________________________________________________________________________________
4. RESULTS
Table 3: Summary of the field measured parameters for
water samples at the study areas
Results of field parameters and the concentrations
of major ions, selected minor ions, trace elements
and DOC in geothermal wells, thermal springs and
cold springs from the study areas of Rincon de la
Vieja and Miravalles are summarized in Tables 3-9.
S.No. Sample ID
Miravalles
1
PGM-11
2
PGM-14
3
PGM-62
4
PGM-31
5
PGM-08
6
PGM-45
7
PGM-43
8
PGM-12
9
PGM-20
10 PGM-46
11 PGM-29
12 PGM-07
13 PGM-19 (Open)
14 Termal Guayabal
15 Sitio 13
16 Corralito
17 Termal union
18 U1
19 Hornillas (Miravalles)
20 Colegio Fortuna
21 San Bernardo 1
22 San Bernardo 2
23 Salitral Bagaces
24 Panteon
25 Toma Guayabo
26 Toma casa maquinas
27 Toma Fortuna
28 Toma Colonia
Rincon de la Vieja
29 PGP-01
30 PGP-03
31 PGP-04
32 PGB-01
33 Hornillas hotel Borinquen
34 Hornillas parque
35 Pailas agua
36 Santa Maria Caliente
37 Azufrales
38 Rio salitral (buena vista)
39 Rio tizate 1
40 Rio tizate 2
41 Salitral las lilas 1
42 Salitral las lilas 2
43 Salitral las lilas 1+2
44 Pedernal (NE de C Fortuna)
45 Naciente Nieves Araya 2
46 Naciente Nieves Araya 1
47 El volcancito
48 Las Avestruces
49 El albergue agroecologico
50 Hotel Guachipelin
4.1 Geothermal wells
4.1.1 Field measured parameters
A summary of the field measured parameters for
water samples are presented in Table 3. The
temperature of the geothermal well fluids had to
be cold down before measuring could be carried
out. The measurements were taken at a
temperature of 9-90 °C at Miravalles and 24-29 °C
at Rincon de la Vieja. The reservoir temperature at
Miravalles and Rincon de la Vieja varies between
230-255°C and 229-276°C respectively.
The pH in the geothermal wells is neutral and
values ranges between 7-7.8 in samples collected at
the wellhead at Miravalles with three exceptions of
PGM-29,PGM-07 and PGM-19 which are weakly
alkaline (pH 4.8-5.7). The samples collected at the
bottom of the wells in Rincon de la vieja (PGP-01
at 1414 m, PGP-03 at 1773 m, PGP-04 at 1419 m
and PGB-01 at 2595m depths) were neutral to
weakly alkaline (pH 5.72-7.77). Field measured
redox potential (Eh) in the geothermal well fluids
varies between 99-431 mV at Miravalles and 160366 mV at Rincon de la Vieja.
Electrical conductivity (EC) is high, ranges
between 8.940-15.150 mS/cm (with one exception
130.310 mS/cm) at Miravalles and 8.00-12.85
mS/cm at Rincon de la Vieja.
Date
T
°C
pH
Eh
mV
EC
µS/cm
As (Field)
µg/l
sep-05
sep-05
sep-05
sep-05
sep-05
sep-05
sep-05
sep-05
sep-05
sep-05
sep-05
sep-05
sep-05
sep-05
sep-05
sep-05
sep-05
sep-05
sep-05
sep-05
sep-05
sep-05
sep-05
sep-05
sep-05
sep-05
sep-05
sep-05
90,7
15,9
17,1
31,8
34,9
10,6
29,6
9,8
11,6
9,4
25,2
12,3
13,3
62,9
33,7
35,6
38,1
51,2
88,9
42,8
44,9
45,5
41,1
35,5
23,3
14,2
24,1
23,6
7,72
7,74
7,2
7,81
7,53
7,16
7,57
7,3
7,71
7,41
5,74
4,91
4,8
2,28
3,25
2,72
5,9
7,25
1,97
6,44
6,25
6,3
5,63
6,32
6,63
7,18
6,57
6,78
99
212
200
236
292
395
262
333
308
289
339
392
431
683
192
635
456
349
240
376
396
388
428
272
439
437
419
426
1300
1290
1236
1238
1271
1370
13031
1515
1472
1492
894
1225
960
484
255
257
40
71
180
64
94
97
26
122
22
14
30
25
>500
>500
>500
>500
>500
>500
>500
>500
>500
>500
>500
>500
>500
>500
25-50
50-100
<10
10
25
<10
50-100
50-100
50-100
<10
<10
<10
<10
<10
sep-05
sep-05
sep-05
sep-05
sep-05
sep-05
sep-05
sep-05
sep-05
sep-05
sep-05
sep-05
sep-05
sep-05
sep-05
sep-05
sep-05
sep-05
sep-05
sep-05
sep-05
sep-05
26
28
24,8
29
26,4
91,2
87,3
36,4
43,7
44,4
38,4
37,9
71,2
70,7
26,6
39
55
28,4
61,3
36,9
36,4
27,1
6,37
5,72
6,1
7,77
7,63
3,75
2,17
5,27
3,32
7,6
6,6
6,58
6,09
6,1
7,84
7,15
5,96
6,68
6,62
5
3,98
6,01
294
366
160
304
309
181
404
9
120
293
392
393
239
233
334
385
263
358
237
69
564
430
800
1285
1043
968
19
49
353
28
76
32
39
40
946
958
38
14
525
585
577
73
190
21
>500
>500
>500
>500
25-50
50
50
25
25-okt
<10
<10
<10
>500
>500
50
<10
25-50
25-50
25-50
25
<10
<10
The water from geothermal wells has high Cl
content range between 2865-4947 mg/L at
Miravalles (MV) with one exception of very high
concentration as high as 8935 mg/L. The Cl
concentrations varied between 2455-4444 mg/L at
Rincon de la Vieja (RV). The SO42- and HCO3concentrations are relatively low (MV: 46-329 and
0-273 mg/ L, respectively and RV: 93-169 and 41121 mg/ L). All the geothermal water is of Cl-Na
type.
4.1.2 Major ions
Variation of major ion concentration in samples at
Miravalles (MV) and Rincon de la Vieja (RV) is
presented Table 4 and also plotted in Box and
Whisker diagram in Figure 12.
Cl- and Na+ are the dominant ions in both
geothermal waters. Na+ is by far the most
predominant major cation (MV: 1289–2178 mg/L,
RV: 1125–1809 mg/L ), followed by K+ (MV:
192–362 mg/L, RV: 245–457 mg/L), Ca2+ (MV:
23–109 mg/L, RV: 23–109 mg/L) and
Mg2+(MV:0.04-7 mg/L, RV: 0.26-0.78 mg/L).
23
Figure 12: Major ion chemistry of the geothermal waters plotted in Box and Whisker diagram from the productions wells at Miravalles (to
the left) and Rincon de la Vieja (to the right) sites.
Table 4: Major ion chemistry of the geothermal waters from the productions wells at Miravalles and Rincon de la Vieja sites.
Sample nr Sample ID
Miravalles
1
PGM-11
2
PGM-14
3
PGM-62
4
PGM-31
5
PGM-08
6
PGM-45
7
PGM-43
8
PGM-12
9
PGM-20
10
PGM-46
11
PGM-29
12
PGM-07
13
PGM-19 (open)
Rincon de la Vieja
29
PGP-01
30
PGP-03
31
PGP-04
32
PGB-01
Ca2+
mg/l
K+
mg/l
Mg2+
mg/l
Na+
mg/l
Clmg/l
HCO3mg/l
SO42mg/
NO3mg/l
Ion Balance
%
Water type
54,8
69,1
60,9
74,8
71,8
82,4
85,0
109,2
103,6
94,7
66,7
29,7
23,1
307
305
298
299
313
363
331
343
338
313
193
309
302
0,11
0,09
0,07
0,13
0,06
0,09
0,05
0,19
0,11
0,12
1,31
3,63
7,32
1855
1734
1760
1807
1825
2002
1988
2179
2107
1983
1290
1769
1656
3843
3437
3828
4117
4333
4822
4543
4947
8935
4316
2865
3421
2974
93,8
48,6
42,8
58,7
30,5
26,9
28,3
49,1
57,6
65,9
273,5
0,0
0,0
61,6
46,7
140,5
144,5
149
155,5
142,5
254
297
199
143
237
329,5
1,6
3,6
6,5
6,0
15,5
24
10
23
34
58,5
9,0
32,5
8,5
-9,9
-6,5
-12,3
-14,4
-16,2
-16,5
-14,1
-14,5
-42,2
-12,8
-15,7
-8,1
-5,4
Na-Cl-B
Na-Cl-B
Na-Cl-B
Na-Cl-B
Na-Cl-B
Na-Cl-B
Na-Cl-B
Na-Cl-B
Na-Cl-B
Na-Cl-B
Na-Cl-B
Na-Cl-B
Na-Cl-B
60,1
99,3
101,0
94,8
245
457
331
274
0,35
0,68
0,27
0,78
1125
1810
1566
1438
2456
4444
3635
3148
120,7
41,8
58,7
121,0
92,75
144,5
169
135,5
3,8
14,5
18,5
10,0
-11,4
-15,1
-13,6
-11,5
Na-Cl-B
Na-Cl-B
Na-Cl-B
Na-Cl-B
Table 5: Selected trace element chemistry of the geothermal waters at Miravalles and Rincon de la Vieja sites.
Sample nr Sample ID
Miravalles
1
PGM-11
2
PGM-14
3
PGM-62
4
PGM-31
5
PGM-08
6
PGM-45
7
PGM-43
8
PGM-12
9
PGM-20
10
PGM-46
11
PGM-29
12
PGM-07
13
PGM-19 (open)
Rincon de la Vieja
29
PGP-01
30
PGP-03
31
PGP-04
32
PGB-01
As (tot)
mg/l
As(III)
mg/l
B
mg/l
Al
mg/l
Fe
mg/l
Mn
mg/l
Ba
mg/l
Li
mg/l
Si
mg/l
DOC
mg/l
25,58
24,46
25,13
25,70
25,58
27,42
27,46
29,13
29,07
26,92
11,86
26,34
25,89
6,79
6,84
6,90
6,82
7,13
7,69
7,31
7,60
7,58
7,55
3,63
7,58
7,31
49,3
47,5
50,3
51,8
51,4
57,1
57,5
60,1
59,7
56,7
33,9
54,2
49,2
0,31
0,32
0,27
0,26
0,30
0,28
0,26
0,24
0,26
0,26
0,04
0,19
0,07
0,003
0,008
0,018
0,043
0,015
0,213
0,039
0,009
0,069
0,044
6,416
0,795
1,479
0,05
0,02
0,02
0,05
0,02
0,03
0,03
0,04
0,04
0,05
1,25
0,58
1,73
0,44
0,26
0,29
0,60
0,30
0,31
0,26
0,77
0,69
0,58
0,16
0,23
0,18
8,77
bdl
bdl
8,17
bdl
bdl
bdl
7,55
bdl
7,50
6,75
8,24
7,06
305
305
296
275
280
275
285
241
258
251
199
314
271
1,66
1,54
1,09
1,57
1,83
2,91
0,86
1,16
1,28
1,15
3,24
0,41
0,76
7,81
13,00
12,80
5,99
2,34
3,45
3,57
1,63
20,6
33,1
30,1
27,1
0,04
0,03
0,05
0,04
0,560
0,891
0,891
0,054
0,23
0,45
0,33
0,03
0,04
0,06
0,06
0,03
5,90
bdl
bdl
bdl
176
203
206
170
4,22
3,73
4,08
4,36
24
___________________________________________________________________________________
A summary of trace elements for the geothermal
wells are presented in Table 5. Extremely high
concentration of As (MV: 11.9-29.1 mg/L, RV:
5.99-13.0 mg/L) and B (MV: 33-60 mg/L, RV:
20.6-33.1 mg/L) are the typical characterise these
geothermal waters.
The concentrations of Fe and Mn were low in all
geothermal wells at both Miravalles and Rincon
de la Vieja sites with concentration ranges between
0.003-0.89 mg/L and 0.02-0.58 mg/L respectively.
However, in the wells PGM 29 and PGM-19, the
concentrations of Fe was high with respective
values of 6.4 mg/L and 1.5 mg/L while the
corresponding Mn levels of 1.25 mg/L and 1.73
mg/L.
Some of the geothermal well fluids have high Li
concentrations. Seven sampled wells at Miravalles
have Li concentrations ranging between 6.75-8.77
mg/L and one of the sampled geothermal fluids in
Rincon de la Vieja revealed Li concentration of 5.9
mg/L.
All of the sampled geothermal wells have high Si
concentrations (MV: 198-313 mg/L and RV: 169205 mg/L).
In general, the concentrations of DOC and HCO3is quite low in the geothermal well fluids of
Miravalles area with one exception PGM-29 which
has high DOC and HCO3- values. At Rincon de la
Vieja the DOC is high while the HCO3- is low.
4.1.4 Correlation between various chemical
parameters
Figure 13-15 show Bivariate plots of the
geothermal wells (GTW) showing relationship
between the concentrations of:
13. Total As with a HCO3, b SO4, c Fe, d B
14. DOC and a As, b HCO3
15. SO4 and a Ca+Mg, b Ca
Arsenic has high positiv correlation with HCO3(MV: r=0.76 and RDLV: r=0.94),
Figure 13: Bivariate plots of the geothermal waters showing relationship between As concentration and a HCO3, b SO4, c Fe and d B
25
Figure 14: Bivariate plots of the geothermal waters showing relationship between the DOC concentration and a As, b HCO3
Figure 15: Bivariate plots of the geothermal waters showing relationship between the SO4 concentration and a Ca+Mg, b Ca
High correlation are also seen between As and
DOC (MV: 0.68 and RDLV: 0.74) and between
HCO3- and DOC (r=0.81 for both areas)
Sulfate with Ca+Mg show a positive correlation
(MV:r=0.75 and RDLV: r=0.87) and so does
sulphate with Ca2+ (MV: 0.74 and RDLV: 0.88).
Hornillas parque: pH 3.75 and temperature
91.2˚C).
The redox potential (Eh) were between 192-683
mV at Miravalles and between 9-564 mV at
Rincon de la vieja. The electric conductivity (EC)
at Miravalles was between 0.26-4.84 mS/cm and at
Rincon de la vieja 0.143-9.58 mS/cm.
4.2 Thermal springs -Neutral and Acid
4.2.2 Major ions
The thermal springs can be divided into two main
types:
Acid thermal springs (TSA) and
Neutral thermal springs (TSN).
The major ion characteristics of the thremal
springs at the two study sites are presented in
Table 6. Variation of major ion concentration in
samples collected at the thermal springs at
Miravalles and Rincon de la Vieja are presented in
Box and Whisker plots in Figure 16 for Acid
thermal springs and in Figure 17 for Neutral
thermal springs. Figure 18 show Piper diagrams
for Neutral thermal springs (TSN) at Miravalles
and Rincon de la Vieja.
The temperature ranged between 33.7-88.9°C in
Miravalles and between 26-91˚C at Rincon de la
vieja. A summary of the field measured parameters
for the thermal springs are presented in Table 3.
The thermal springs have pH interval ranging
between 1.97-7.84. Four of the sampled thermal
water in Miravalles and four of the sampled
thermal waters in Rincon de la vieja were acidic
(pH 1.97-3.25 resp 2.17-3.98) while the rest had a
neutral or weakly alkaline pH (5.63-7.25 resp 5.007.63). The two most acidic springs at Miravalles
have the highest measured temperatures (Hornillas
Miravalles: pH 1.97 and temperature 88.9˚C and
Thermal Guayabal: pH 2.28 and temperature
62.9˚C) and the same states for Rincon de la vieja
(Pailas aguas: pH 2.17 and temperature 87.3˚C and
26
___________________________________________________________________________________
Figure 16: Major ion chemistry of the Acid thermal waters plotted in Box and Whisker diagram from the productions wells at Miravalles
(to the left) and Rincon de la Vieja (to the right) sites.
Figure 17: Major ion chemistry of the Neutral thermal waters plotted in Box and Whisker diagram from the productions wells at
Miravalles (to the left) and Rincon de la Vieja (to the right) sites.
Figure 18: Piper diagram for Neutral thermal waters at Miravalles (to the left) and Rincon de la Vieja (to the right) sites.
27
The acidic thermal springs (TSA) had high
SO42- content (MV: 493-2699 mg/L and RV: 352275 mg/L). The Cl- content is low (MV: 21-642
mg/L and RV: 2-434 mg/) as well as the NO3(MV: 1.4 -16 mg/L and RV: 1-5 mg/L) and the
HCO3- (MV: 0 mg/L and RV: 0 mg/L) content.
Ca2+ (MV: 19-145 mg/L and RV: 15-164 mg/L)
predominates over Mg2+ (MV: 8-91 mg/L and RV:
6-26 mg/L), Na+ (MV: 15-73 mg/L and RV: 6-79
mg/L) and K+ ( MV: 6-13 mg/L and RV: 2-14
mg/L ) in the acid springs reflecting SO4-water
type:
(MV: 62-146 mg/L and RV: 64-188 mg/L) and
Mn+ (MV: 0.32-6.1 mg/L and RV: 0.23-4.23
mg/L). The DOC content is low (MV: 0.4-1.23
and RV: 0.9-1.39 mg/L with one exception of
Pailas aguas 9.62 mg/L).
The neutral thermal springs (TSN) are rich in
HCO3- (MV: 0-608 mg/L and RV: 0-535 mg/L),
Cl- (6-2914 mg/L) and SO42- (MV: 6-237 mg/L
and RV: 23-812 mg/L). Mg2+ concentrations are
low (MV: 2-19 mg/L and RV: 5-301 mg/L) as well
as K+ (MV: 8-22 mg/L and RV: 2-186mg/L). The
water type can be divided into HCO3-Si-SO4, ClNa and Cl-SO4.
Two of the samples (Salitral las lilas 1 and 2) in
Rincon de la Vieja differs from the rest with a
really high Cl- (2667 resp 2914 mg/L) and Na+
(1242 resp 1250 mg/L) concentration.
Concentrations of Ca2+ (145 resp 150 mg/L), K+
(186 resp 180 mg/L), HCO3- (250 resp 310 mg/L),
SO42- (210 resp 212 mg/L) are quite low.
One hundred meters downstream the river from
Salitral las lilas 1 and 2 is Salitral las lilas 1+2
sampled. The concentration of Cl- (76 mg/L) and
Na+ (46 mg/L) are much lower in this sample in
comparison with Salitra las lilas 1 and 2. Ca2+ (15
mg/L), K+ (6 mg/L), HCO3- (47 mg/L), SO42- (25
mg/L) and Mg2+ (5 mg/L) are also much lower.
The water is designated as Cl-HCO3-Na-Si type.
Figure 19: Box and Whisker plots for Acidic thermal waters
at Miravalles for As, B and Fe.
The neutral springs (TSN) have a low DOC. Fe
and Mn are both low while the As concentration
(0.005-10.9 mg/L) exceeds the WHO limit in 13
of the 20 samples. High boron concentrations
occur in all the samples (0.65-25.7 mg/L).
Two of the samples (Salitral las lilas 1 and 2) in
Rincon de la Vieja differs from the rest with really
high As (10.9 resp 10.6 mg/L) and boron (25.7
resp 25.6 mg/L) concentrations. The Mg
concentration (13 resp 14 mg/L) is also much
higher than in the other samples.
One hundred meters downstream the river from
Salitral las lilas 1 and 2 is Salitral las lilas 1+2
sampled. The concentrations of B (3.09 mg/L)
and As (0.13 mg/L) is still high here but in
comparison with Salitral las lilas 1 and 2 the
concentration has to be considered low.
A summary of the trace elements for the thermal
springs are presented in Table 7.
The acidic thermal springs (TSA) have a high
content of A, B and Fe as showed in the Box and
Whisker plots in Figure 19.
Other trace elements in these waters include S2(MV: 202-814 mg/L and RV: 57-598 mg/L), Al3+
(MV: 20-453 mg/L and RV: 0-141 mg/L), Si4+
28
___________________________________________________________________________________
Table 6: Major ion chemistry of the thermal waters from the productions wells at Miravalles and Rincon de la Vieja sites.
Sample nr Sample ID
Miravalles
14
Termal Guayabal
15
Sitio 13
16
Corralito
17
Termal union
18
U1
19
20
Colegio Fortuna
21
San Bernardo 1
22
San Bernardo 2
23
Salitral Bagaces
24
Panteon
Rincon de la Vieja
33
34
Hornillas parque
35
Pailas agua
36
37
Azufrales
38
Rio salitral (buena vista)
39
Rio tizate 1
40
Rio tizate 2
41
42
43
44
45
46
47
El volcancito
48
Las Avestruces
49
Ca2+
mg/l
K+
mg/l
Mg2+
mg/l
Na+
mg/l
Clmg/l
HCO3mg/l
SO42mg/
NO3mg/l
Ion Balance
%
Water type
104
312
146
36
60
20
46
43
42
9
100
7
14
9
9
19
8
14
20
21
9
22
25
92
22
13
16
8
20
17
17
2
76
65
74
41
26
55
16
45
91
94
39
74
642
148
177
13
19
22
16
100
62
17
51
bdl
bdl
bdl
190
318
bdl
386
395
396
140
608
2700
1613
1303
40
66
494
58
27
25
6
238
16
1
2
1
1
5
1
1
18
bdl
4
12.7
10.8
14.3
0.4
6.2
43.8
12.7
19.7
15.3
23.2
4.8
Al-SO4-Cl
Ca-Mg-SO4
Al-Ca-SO4
Na-Ca-Mg-HCO3
Na-Ca-HCO3
Al-SO4
Ca-Na-Mg-HCO3
Na-Ca-HCO3-Cl-B
Na-Ca-HCO3-B
Na-HCO3-B
Mg-Ca-Na-HCO3-SO4
14
15
15
22
29
26
23
23
145
150
15
11
325
340
354
63
164
6
5
3
4
9
5
7
6
187
180
6
3
130
147
144
14
14
6
7
9
9
11
8
13
13
14
15
5
6
280
294
301
14
26
10
14
7
17
30
26
34
34
1243
1251
46
8
307
323
334
58
79
10
3
2
7
23
11
13
13
2667
2914
77
6
1200
1326
973
92
435
33
bdl
bdl
35
bdl
141
174
182
251
310
48
69
535
510
531
37
bdl
42
36
2275
96
294
24
52
53
210
213
26
8
813
894
667
178
353
bdl
2
1
0
3
2
1
2
13
9
3
2
9
21
19
5
5
21.5
-8.5
46.2
4.5
Ca-Mg-B-HCO3-SO4
Na-Ca-Mg-B-SO4
Al-SO4
Ca-Na-Mg-SO4-HCO3
Na-Ca-SO4
Na-Ca-HCO3-SO4
Na-Ca-Mg-HCO3-SO4
Na-Ca-Mg-HCO3-SO4
Na-Cl-B
Na-Cl-B
Na-B-Cl
Ca-Mg-B-HCO3
Mg-Ca-Na-Cl-SO4
Mg-Ca-Na-Cl-SO4
Mg-Ca-Na-Cl-SO4
Ca-Na-B-SO4-Cl
Ca-Cl-SO4
7.1
9.3
22.9
25.8
32.3
31.5
4.9
6.2
-7.4
28.6
15.1
Table7: Selected trace element chemistry of the thermal waters at Miravalles and Rincon de la Vieja sites
Sample nr Sample ID
Miravalles
14
Termal Guayabal
15
Sitio 13
16
Corralito
17
Termal union
18
U1
19
20
Colegio Fortuna
21
San Bernardo 1
22
San Bernardo 2
23
Salitral Bagaces
24
Panteon
Rincon de la Vieja
33
34
Hornillas parque
35
Pailas agua
36
37
Azufrales
38
Rio salitral (buena vista)
39
Rio tizate 1
40
Rio tizate 2
41
42
43
44
45
46
47
El volcancito
48
Las Avestruces
49
As (tot)
mg/l
As(III)
mg/l
B
mg/l
Al
mg/l
Fe
mg/l
Mn
mg/l
Ba
mg/l
Li
mg/l
Si
mg/l
DOC
mg/l
4.564
0.224
0.099
0.008
0.012
0.017
0.010
0.271
0.281
0.188
0.005
1.531
0.193
0.158
0.012
0.013
0.018
0.011
0.017
0.024
0.012
0.012
2.730
3.029
2.098
0.994
1.557
1.833
0.925
2.355
2.432
1.145
0.923
453.646
56.809
133.971
0.007
0.013
20.449
0.010
0.010
0.010
0.039
0.015
76.738
16.817
28.176
0.001
0.006
16.802
0.003
0.002
0.003
0.017
2.850
1.775
6.104
2.696
0.004
0.002
0.325
0.802
0.000
0.000
0.019
0.334
0.017
0.015
0.008
0.063
0.013
0.051
0.056
0.087
0.085
0.084
0.056
0.172
0.060
0.048
0.011
0.094
0.008
0.016
0.379
0.392
0.110
0.036
133.6
62.1
85.8
77.6
104.0
146.6
74.0
80.7
82.2
64.4
59.2
0.79
0.63
0.40
0.59
0.80
1.23
0.90
0.94
0.87
1.49
1.74
0.008
0.005
0.024
0.015
0.007
0.006
0.007
0.007
10.853
10.633
0.132
0.005
0.053
0.053
0.050
0.017
0.009
0.014
0.009
0.026
0.008
0.010
0.007
0.008
0.015
1.629
3.256
3.044
0.008
0.033
0.034
0.033
0.016
0.008
1.709
1.534
0.729
0.657
0.595
1.670
0.755
0.722
25.724
25.634
3.092
1.907
2.514
2.758
2.798
6.764
1.724
0.062
2.682
141.387
0.181
0.860
0.011
0.015
0.012
0.011
0.011
0.065
0.006
0.011
0.012
0.009
0.073
11.238
0.681
5.859
194.899
0.039
0.120
bdl
0.002
0.004
0.030
0.008
0.050
0.002
4.396
0.513
3.496
0.710
0.010
0.084
0.269
0.424
0.119
0.226
0.000
0.000
0.001
0.747
0.838
0.025
0.000
1.485
1.312
1.125
1.095
4.235
0.050
0.075
0.055
0.036
0.032
0.030
0.041
0.041
0.357
0.338
0.047
0.034
0.047
0.049
0.052
0.036
0.040
0.005
0.015
0.005
0.022
0.011
0.015
0.031
0.031
6.440
6.356
0.245
bdl
0.519
0.561
0.598
0.296
0.092
44.5
84.0
188.9
30.9
64.1
51.1
70.2
70.9
54.7
52.5
39.4
59.2
93.8
97.8
98.8
63.9
82.9
4.96
1.39
9.62
1.40
0.91
2.50
1.16
0.37
0.61
0.74
1.46
1.35
2.53
1.24
0.71
2.64
1.23
29
r=0.01) at Miravalles while there is no correlation
at Rincon de la Vieja.
The relationship between SO42- concentrations of
the thermal waters with Cl-, Mg2+ and Ca is
presented in Figure 21. SO42- has high correlation
with Cl- (MV: r=0.89) while its low with Mg2+
(MV: r=0.08) and Ca2+(MV: r=0.05) while there is
no correlation at Rincon de la Vieja.
4.2.4 Correlation between various chemical
parameters
Bivariate plots the relationship between the
concentrations of total As with B and Fe in the
acid thermal springs (TSA) waters are presented in
Figure 20. Arsenic has high correlation with B
(MV: r=0.96) and low correlation with Fe (MV:
Figure 20: Bivariate plots of the thermal waters showing relationship between total As with a) B,and b) Fe
´
Figure 21: Bivariate plots of the thermal waters showing relationship between the SO4 concentration and a) Cl, b) Mg and c) Ca
30
___________________________________________________________________________________
The dominated ions in the cold springs at
Miravalles are HCO3- (38-97 mg/L), SO42- 22-49
mg/L), Si4+ (38-44 mg/L) followed by Ca2+ (1526 mg/L), Na+ (7-11 mg/L) and S (6-14 mg/L) in
less concentrations.
The dominated ions at the cold spring at Rincon
de la vieja are Si4+ (49.8 mg/L), SO42- (49.1 mg/L)
and HCO3- (41.1 mg/L) followed by Cl-, Ca2+,
Na+, Mg2+ and K+. The sampled cold surface
waters are generally of HCO3-SO4- Si or HCO3-Si
type.
4.3 Cold surface waters
A summary of the field measured parameters for
the cold springs are presented in Table 3.
The cold springs had a temperature range of 14.223.6°C at Miravalles while the cold spring at
Rincon de la vieja had a temperature of 27.1
degrees.
The pH is almost neutral for the cold springs at
Miravalles (6.57-7.18) while it is weakly alkaline at
Rincon de la Vieja (6.01).
Field measured redox potential (Eh) range
between 419-439 mV and the Electric conductivity
(EC) ranges from 0.141-0.304 mS/cm.
A summary of trace elements for the cold springs
are presented in Table 9.
Elevated concentrations of As and B occur in the
cold springs (0.0052-0.007 mg/L respective 0.71.42 mg/L). The DOC concentration of the areas
is quite low as well as the Fe and the Mn.
4.3.2 Major ions
Variation of major ion concentration in samples
collected at the cold springs at Miravalles (MV)
and Rincon de la Vieja (RV) is presented Table 8
and also plotted in Box and Whisker diagram in
Figure 22.
Figure 22: Major ion chemistry of the cold surface waters plotted in Box and Whisker diagram from the productions wells at Miravalles (to
the left) and Rincon de la Vieja (to the right) sites.
Table 8: Major ion chemistry of the cold surface waters from the productions wells at Miravalles and Rincon de la Vieja sites.
Sample nr Sample ID
Miravalles
25
Toma Guayabo
26
Toma casa maquinas
27
Toma Fortuna
28
Toma Colonia
Rincon de la Vieja
50
Hotel Guachipelin
Ca2+
mg/l
K+
mg/l
Mg2+
mg/l
Na+
mg/l
Cl mg/l
HCO3mg/l
SO42mg/
NO3mg/l
Ion Balance
%
Water type
21.7
15.9
26.9
25.1
2.6
2.3
3.2
3.0
7.1
4.0
7.1
7.0
8.5
7.3
11.4
10.1
6.5
4.1
6.4
5.8
97.8
38.6
86.8
85.5
22.4
29.0
49.3
45.0
0.9
0.2
0.7
0.9
22.8
-4.2
19.4
21.3
Ca-Mg-B-HCO3
Na-Ca-Mg-HCO3-SO4
Ca-B-HCO3-SO4
Ca-B-HCO3-SO4
19.9
2.4
7.6
7.6
10.4
41.1
49.1
0.1
22.6
Ca-Mg-B-SO4-HCO3
31
Table9: Selected trace element chemistry of the cold surface waters at Miravalles and Rincon de la Vieja sites
Sample nr Sample ID
Miravalles
25
Toma Guayabo
26
Toma casa maquinas
27
Toma Fortuna
28
Toma Colonia
Rincon de la Vieja
50
Hotel Guachipelin
As (tot)
mg/l
As(III)
mg/l
B
mg/l
Al
mg/l
Fe
mg/l
Mn
mg/l
Ba
mg/l
Li
mg/l
Si
mg/l
DOC
mg/l
0.0052
0.0064
0.0052
0.0065
0.0075
0.0000
0.0113
0.0102
1.42
0.716
1.2497
1.1014
0.0101
0.0092
0.04
0.0178
0.0075
0.0035
0.012
0.0036
0.0003
0.0002
0.0011
0.0003
0.0336
0.0132
0.0181
0.0149
bdl
bdl
0.0031
bdl
44.0
40.3
39.2
38.3
1.16
1.26
1.02
0.74
0.0052
0.0086
1.0709
0.064
0.0019
0.0067
0.0558
bdl
49.8
0.86
plant analysis can be recommended to precisely
predict the growth of plants on high B soils.
If B contaminated water sources in Costa Rica are
used by humans for irrigation and drinking water
supply are still not known. Neither are the
environment effects on animals and plants from
boron contaminated water (Nable et al 1997).
4.4 Anomalous boron concentrations
Extremly high boron (B) concentrations were
found in all the samples. Especially high was the
amount of boron in the geothermal well fluids,
between 47-60 mg/L in the Miravalles area and
between 20-33 mg/L in the Rincón de la Vieja
area.
High concentrations of boron (ranging between
0.66-6.76 mg/L) were found in the hot springs
with two exceptions which differed a lot from the
rest. These two were the springs called Salitral las
Lilas 1 and 2 with extremely high boron
concentrations (25.7 mg/L resp. 25.6 mg/L). The
third sample were collected at the site where these
two springs entered a stream about 100 meters
down from where these springs entering the river
(called Salitral las Lilas 1+2) which indicated a B
concentration of 3.1 mg/L. The concentration of
B in the cold springs ranged between 0.72-1.25
mg/L similar to a previous study by Nable et al
(1997). All the samples exceed the WHO drinking
water guideline for boron which is 0.5 mg/L.
High concentrations of boron may occur naturally
in the soil or in groundwater. It can also be added
to the soil from mining, fertilisers, or irrigation
water. B-rich soils are also of importance though
they might cause B toxicity in the field and
decrease crop yields. One typical visible symptom
of B toxicity is leaf burn. It can also lead to fruit
and bark disorders. Although there are various
methods of determine the levels of B in soils,
these analyses can only provide little more than a
general risk assessment for B toxicity. There are
several methods to ameliorating high boron soils
but to get rid of boron in soil and water is
extremely difficult. At present neither soil nor
32
___________________________________________________________________________________
5. DISCUSSION
Salitral las lilas 1 and Salitral las lilas 2 have their
outlet into a river. To find out how diluted the As
concentration becomes when entering the river a
sample Salitral las Lilas 1+2 was collected 100
meters downstream in the river. The results show
an As concentration of 0.13 mg/L, which is higher
than the WHO limit for drinking water. This
shows that thermal springs with high
concentrations of As are mixing with cold surface
water of low As concentration.
5.1 Arsenic concentrations in Miravalles
and Rincon de la vieja
Findings indicate that high As concentrations
occur through out the entire study area. 35 of the
total 50 samples exceed the WHO drinking water
guideline for As, which is 10 μg/L
5.1.1 Geothermal wells
The results show that the highest As
concentrations occur in the geothermal wells. The
geothermal wells at Miravalles (mean 25.9 mg/L),
show higher concentrations than those at Rincon
de la Vieja, (mean 10.3 mg/L). Of particular
importance is that the As concentrations in the
geothermal waters used for power generation are
extremely high at both Miravalles and Rincon de la
Vieja.
Power generation requires substantial amounts of
water to be extracted. During the separation of
steam and bore water in geothermal power
generation, Ass is retained in the waste bore water.
High As content is one of the main problems in
the disposal of this water (Webster & Nordstrom
2003). To prevent contamination of cold waters
used for irrigation, re-injection of thermal waters
to the reservoir is necessary. This also acts to
recharge the aquifers of the thermal waters
(Gemici & Tarcan 2002).
We have stated that high As concentrations occur
in the water used for geothermal energy in Costa
Rica. It is unknown if any leakage occurs in the
geothermal power plants in Costa Rica. Further
investigations are needed to answer this question.
5.1.3 Cold surface waters
The concentration of As in the cold springs do not
exceed WHOs guidelines for drinking water.
Although the As concentrations in the cold
springs are relatively low (Miravalles 0.0052 0.0065 mg/L respective 0.0052 mg/L in Rincón
de la Vieja) the risk of thermal water mixing with
cold spring waters can increase the As content in
the cold waters. This might occur due to i.e
flooding during the rain season or due to leakage
from the geothermal field for power generation.
It is unknown whether contaminated water
sources in Costa Rica are used by humans for
irrigation and drinking water supply. Neither are
the environmental effects on animals and plants
from As contaminated water in the area of
Miravalles and Rincon de la Vieja.
We do know that thermal springs with high
concentrations of As are mixing with cold surface
water of low As concentration which might lead to
problems if this is used by humans and animals.
Therefore further investigations are needed to
mitigate where the highest risk of mixing between
thermal waters of high As concentration and cold
surface waters occur and investigate how this
mixing process affects humans and animals in the
area in the long term.
5.1.2 Thermal springs
High As concentrations occur in the thermal
springs. The As concentration in the Miravalles
area ranging from 0.005- 0.28 mg/L with the
exception of one sample, collected at Thermal
Guayabal, with a measured concentration of 4.6
mg/L. The concentrations of As in the Rincón de
la Vieja area ranged from 0.005-0.13 mg/L with
two exceptions, Salitral las lilas 1 and Salitral las
lilas 2 with As concentrations of 10.9 and 10.6
mg/L respectively.
This high concentration of As affects the area
surrounding these springs.
5.2 Comparison of the data with other
geothermal fields in New Zealand, USA
and Philippines
The results show extremely high As
concentrations in the geothermal wells and
thermal springs of both Miravalles and Rincon de
la vieja area. So, how high are the As levels
33
Table 10: Arsenic concentrations in hot springs and in production or exploration well brines at Yellowstone National Park in the USA,
which is mainly drained by the Madison and Yellowstone Rivers, the Taupo Volcanic Zone in New Zealand, which is drained by the
Waikato River, and Mt Apo in the Phillippines.
Field
As (mg/kg)
Reference
Wairakei, NZ
Waiotapu, NZ
Ohaaki/Broadlands, NZ
Tongonan, Philippines
1.0 - 5.2
2.1 - 3.9
5.7 - 9.0
28 (mean)
Ritchie (1961)
Ellis and Mahon (1977)
Ewers and Keays (1977)
Darby (1980)
Miravalles, Costa Rica
Rincon de la Vieja, Costa Rica
11.86 - 29.13 mean value 25.8919
5.99 - 13.00 mean value 10.302
Geothermal wells
Thermal springs
Yellowstone Nat Park, US
Wairakei, NZ
Waiotapu, NZ
Ohaaki/Broadlands, NZ
Mt Apo, Philippines
0.16 - 10
0.23 - 3.0
0.71 - 6.5
1.0
3.1 - 6.2
Stauffer and Thomson (1984); Unpublished data, USGS
Ritchie (1961)
Webster (1990)
Ellis and Mahon (1977)
Webster (1999)
Miravalles, Costa Rica
Rincon de la Vieja, Costa Rica
0.005 - 4.56 mean value 0.0991
0.005 - 10.85 mean value 0.015
(6.5 mg/L) in New Zealand. It also has got a lower
concentration than reported from USA (10 mg/L)
and the Philippines (6.2 mg/L).
levels in these areas in comparison with
geothermal fields in other countries?
Let us take a look at table 10 where As
concentrations from New Zealand, USA and the
Philippines are presented.
5.2.2 Arsenic concentrations in Miravalles and
Rincon de la Vieja
The thermal springs can be divided into to main
types: Acid thermal springs (TSA) and Neutral
thermal springs (TSN).
The acid thermal springs consist of hot water and
steam having a low pH and are rich in sulphate.
They are formed by the interaction of the steam
phase with shallow aquifer waters. This mixture of
water and steam lead to precipitation and
sublimation of elemental sulphur which follows by
microbial oxidation to sulphuric acid. Intense
alteration of the host rock caused by the fluid
(Webster & Nordstrom 2003) has lead to an
unstable ground surface and the formation of
bubbling “mudpools” at several sampling
locations.
The neutral thermal springs having an almost
neutral pH, are rich in carbonate, chloride and
silica. They are examples of hot water discharges
and include steam-heating and steam-phase mixing
(Webster & Nordstrom 2003).
Two of the neutral thermal springs (Salitral las lilas
1 and 2) stand out from the others with As
5.2.1 Concentrations of arsenic and variability
Arsenic concentrations reported in geothermal
wells from geothermal fields in New Zealand
(Wairakei 1.0 - 5.2 mg/kg, Waiotapu 2.1 - 3.9
mg/kg and Ohaaki/Broadlands 5.7 - 9.0 mg/kg)
are substantially lower than the ones from this
study in Costa Rica.
The mean As concentration presented from the
Philippines (28 mg/kg) are higher than reported
from this study in Costa Rica (25.9 and 10.30
mg/L).
After this comparison it is possible to state that
the As concentrations in the geothermal wells of
Costa Rica are extremely high.
If we then compare the result from the thermal
wells we can see that Rincon de la vieja has the
lowest as well as the highest reported As
concentration of all (0.005 - 10.85 mg/L), while
Miravalles have a higher maximum concentration
(4.56 mg/L) than Ohaaki/Broadlands (1.0 mg/L)
and Wairakei 3.0 mg/L) but lower than Waiotapu
34
___________________________________________________________________________________
concentrations reaching 10.8 mg/L and 10.6 mg/L
respectively and the water of Cl-Na type differs
from the other thermal springs.
Measurements from these two springs have more
in common with recordings from the geothermal
wells used for power generation. Similarities noted
are a neutral pH, high conductivity, similar Cl-Na
water type and high concentrations of both As and
B.
Possible reasons for this similarity might be:
1. Water feeding Salitral las lilas 1 and 2
come from the same deep natural
geothermal source as the water extracted
for power generation, while the other
thermal springs in the area get fed by
shallower geothermal sources. The high
concentration of As and boron recorded
at the surface of the springs suggests that
this possibility may be unlikely however.
This is because water feeding natural hot
springs goes through several chemical
processes (such as precipitation) on the
way to the surface which substantially
reduces the levels of dissolved
contaminants that reach the surface. In
contrast to this, the rapid extraction of
geothermal water for power generation
prevents these chemical changes.
Contaminant levels are therefore relatively
unchanged when the water reaches the
surface.
2. The development of the geothermal fields
including the drilling and extraction of
water has disturbed the natural
groundwater systems and increased the
rate and volume of geothermal fluids
reaching the surface. Two things which
may have artificially increased the level of
contaminants in the springs are leakage of
fluid from the extraction pipes or an
increase in the natural spring flow rate.
Increasing the flow rate might lead to a
reduction in the intensity of the natural
chemical processes that occur in the water
on the way to the surface.
3. Reinjected waste water is feeding into the
aquifer of Salitral las lilas 1 and 2.
If the thermal springs of Salitral las lilas 1 and 2
are influenced by the activity at the geothermal
field, it means that the reservoir fluids from the
geothermal field are mixing with the groundwater
in the area. More investigations of groundwater
and sediment samples are needed to draw
conclusions on whether geothermal extraction is
affecting nearby groundwaters.
35
5. CONCLUSIONS
The present study indicates that high As
concentrations occur throughout the entire study
area. 35 of the total 50 samples exceed the WHO
drinking water guideline for As which is 10 μg/L.
Extremely high As values were measured from the
geothermal wells (11.86-29.13 mg/L) while
concentrations in thermal springs (0.0052- 4.56
mg/L) were low to high and cold surface water
(0.0052- 0.065 mg/L) was low. High boron
concentrations (33.9-60.1 mg/L) also occur
through out the entire area.
• Arsenic concentrations exceed the WHO limit
for safe drinking water (10μg/L) in 35 of the 50
samples. Geothermal well fluids greatly exceed
the WHO limit while all the cold springs fall
below that limit.
• Boron concentrations exceed the WHO limit for
safe drinking water (0.5 mg/L is the B limit
recommended for drinking water by the WHO
(1998) in all of the 50 samples.
• Sampled geothermal well fluids are generally of
Na-Cl-B type with an almost neutral pH and
with oxidizing conditions. They contain
extremely high As and boron concentrations.
• Sampled thermal springs can be divided into
neutral thermal waters and acidic thermal waters.
The neutral thermal springs (pH almost neutral)
are generally of HCO3-Cl type with reducing
conditions. The acidic thermal springs (pH 1.973.25) are generally of SO4-S-Cl-Al type and with
reducing conditions. They contain low to high
As concentrations.
• Sampled cold springs are generally of Si-SO4HCO3 type with a neutral to alkaline pH with
reducing conditions. They contain low As
concentrations.
• As comparison with New Zealand, USA and
Philippines show that the well fluids used for
geothermal energy in Costa Rica have extremely
high As concentrations.
• Reinjection of geothermal fluids in Miravalles
and Rincon de la Vieja is needed although the
waste
fluid
contains
extremely
high
concentrations of both As and B.
36
___________________________________________________________________________________
7. REFERENCES
Alvarado, G., Acevedo, A.P., Monsalve, M.L., Espindola, J.M., Gómez, D., may, M., Naranjo, J.A., Pulgarin,
B., Raigosa, J., Sigarán, C., & Van der Laat, R. (1999) El desarrollo de la Vulcanología en Latinoamérica en
el último Cuarto de Siglo XX. Rev. Geofísica 51: 785-241
Appelo, C.A.J., Postma, D. (1993). Geochemistry, Groundwater and Pollution. A.A.Balkema, Rotterdam.
Bhattacharya, P., Jacks, G., Ahmed, K.M., Routh, J., Khan, A.A. (2002) Arsenic in groundwater of the Bengal delta
plain aquifers in Bangladesh. Bull. Environ. Cont. Toxicol. 69: 538-545.
Ballantyne, J.M., Moore, J.N. (1988) Arsenic geochemistry in geothermal systems. Geochimica et Cosmochimica
Acta 52: 475-483.
Bargagli, R., Cateni, D., Nelli, L., Olmastroni, S., Zagarese, B. (1997) Environmental impact of trace element
emissions from geothermal power plants. Arch. Environ.Contam.Toxicol. 33: 172-181
Birkle, P., Bundschuh, J. (2007) High and low enthalpy geothermal resources and potentials. In: Bundschuh, J.
& G.E. Alvarado (eds.) (2007): Central America: Geology, Resources and Hazards. Taylor and Francis/CRC
Press, Publisher, The Netherlands. pp.705-776 (ISBN: 9780415416474)
Bundschuh, J., Alvarado, G., Reyes, E., A.R.R. Rodriguez, Palma, M.J., Zuñíga, A., Castillo, G. (2002)
Geothermal energy in the Central-American region. In: Chandrasekharam, D. & Bundschuh, J. (eds.)
Geothermal Energy for Developing Countries. Balkerna Publisher, The Netherlands; pp. 313-364.
Ellis, A.J., Mahon, W.A.J. (1964) Natural hydrothermal systems and experimental hot-water/rock interactions.
Geochimica et Cosmochimica Acta 28: 1329-1367.
Ellis, A.J., Mahon, W.A.J. (1967) Natural hydrothermal systems and experimental hot-water/rock interactions
(Part II). Geochimica et Cosmochimica Acta 31: 519-538.
Gemici, Ü., Tarcan, G., (2002) Distribution of boron in thermal waters of western Anatola, Turkey and
examples of their environmental impacts. Environmental Geology 43: 87-98.
Gherardi, F., Panichi C., Yock, A., Gerardo-Abaya, J. (2002) Geochemistry of the surface and deep fluids of
the Miravalles volcano geothermal system (Costa Rica). Geothermics 31: 91–128.
Kloppman, W., Bianchini, G., Charalambides, A., Dotsika, E., Guerrot, C., Klose, P., Marei, A., Pennisi, M..,
Vengosh, A., Voutsa, D. (2005) Boron contamination of Mediterranean groundwater resources: extent,
sources and pathways elucidated by environmental isotopes. Geophysics Research Abstracts. 7: 10162.
Manieri, A. (2005) Costa Rica country update report. Proc. World Geothermal Congress, Antalya, Turkey, 24-29 5pp
Montero, W. (1999): Sismícidad, Sismotectónica y Amenaza sismica en América Central. Rev. Geofísica 51: 243259
Moya, P. (2005) La Energía geotérmica en Costa Rica p. 1-14.
Nable, R.O., Bañuelos, G.S., Paull, J.G. (1997) Boron toxicity. Plant and Soil 193: 181-198.
National Park Service (2003) U.S. Department of the Interior, Geothermal Energy Development Overview, Geologic
Resources Division, NPS Western Energy Summit January 21–23, 2003.
Tassi, F., Vaselli, O., Capaccioni, B., Giolito, C., Duarte, E., Fernandez, E., Minissale, A., Magro, G. (2005)
The hydrothermal-volcanic system of Rincon de la Vieja volcano (Costa Rica): A combined (inorganic and
organic) geochemical approach to understand the origin of the fluid discharges and its possible application
to volcanic surveillance. J. Volcanology and Geothermal Research 148: 315-333.
Vega, E., Chavarría, L., Barrantes, M.., Molina, F., Hakanson, C.E., Mora, O. (2005) Geologic model of the
Miravalles geothermal field, Costa Rica. Proc. World Geothermal Congress, Antalya, Turkey, April 24-29 pp. 15.
Webster, J.G.(1999) The source of arsenic (and other elements) in the Marbel – Matingo river catchment,
Mindanao, Philippines. Geothermics 28: 95–111.
37
Webster, J.G.& Nordstrom, D. K. (2003) Geothermal Arsenic. In: Welch, A. & Stollenwerk, K.G. (eds.) 2003:
Arsenic in groundwater, Elsevier, pp 101-125.
WHO (1993) Guidelines for drinking water quality, vol 2. Health criteria and other supporting information.
World Health Organization,, Geneva.
WHO (2008) Guidelines for Drinking-Water Quality, Incorporating the First and Second Addenda, Volume 1
(Third Edition), World Health Organization, Geneva. 515p.
Wilkie, J. A., Hering, J. G. (1998) Rapid Oxidation of Geothermal Arsenic (III) in Streamwaters of the Eastern
Sierra Nevada. Environ. Sci. Technol..32: 657-662
38

arsenic in geothermal waters of costa rica - Mark

Transcription

Similar documents

8 Natalie Kruse Geothermal

CENTRAL EUROPE Project Transenergy - Transboundary

Presentazione standard di PowerPoint

Winkelman Presentation

Growers passport

United Country Land Pros, LLC RR 1 Box 73F Memphis, MO 63555

Remote Binary Power Development at Hveravellir, Central Iceland

Divya Bhaskar e-Paper Gujarat, Ahmedabad, Vadodra

Geothermal Energy in the Netherlands:

Forage géothermique profond ECOGI