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ARSENIC IN GEOTHERMALWATERS OF COSTA RICAA Minor Field StudyLotta HammarlundJuan PiñonesDecember 2009TRITA-LWR Master ThesisISSN 1651-064XLWR-EX-09-02

Lotta Hammarlund & Juan Piñones TRITA LWR Masters Thesis 09-02ii

Arsenic in Geothermal Waters of Costa RicaARSENIC IN GEOTHERMAL WATERS OFCOSTA RICAA Minor Field StudyLotta HammarlundJuan PiñonesM.Sc. ProgramEnvironmental Engineering and Sustainable InfrastructureKTHMain SupervisorProf. Prosun BhattacharyaKTH-International Groundwater Arsenic Research Group (GARG)Department of Land and Water Resources Engineering, Royal Institute of Technology (KTH)SE-100 44 STOCKHOLM, SwedenCo-SupervisorProf. Jochen BundschuhProf. Guillermo E. AlvaradoInstituto Costarricense de Electricidad (ICE), San Jóse, Costa RicaExaminerProf. Jon-Petter GustafssonDepartment of Land and Water Resources Engineering, Royal Institute of Technology (KTH) SE-100 44STOCKHOLM, SwedenStockholmDecember, 2009iii

Lotta Hammarlund & Juan Piñones TRITA LWR Masters Thesis 09-02iv

Arsenic in Geothermal Waters of Costa RicaRoyal Institute of TechnologyInternational OfficePREFACEThis study has been carried out within the framework of the Minor Field StudiesScholarship Programme, MFS, which is funded by the Swedish InternationalDevelopment Cooperation Agency, Sida.The MFS Scholarship Programme offers Swedish university students anopportunity to carry out two months’ field work, usually the student’s final degreeproject, in a Third World Country. The results of the work are presented in an MFSreport which is also the student’s Master of Science Thesis. Minor Field Studies areprimarily conducted within subject areas of importance from a developmentperspective and in a country where Swedish international cooperation is ongoing.The main purpose of the MFS Programme is to enhance Swedish universitystudents’ knowledge and understanding of these countries and their problems andopportunities. MFS should provide the student with initial experience of conditionsin such a country. The overall goals are to widen the Swedish human resourcescadre for engagement in international development cooperation and to promotescientific exchange between universities, research institutes and similar authoritiesin 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 naturalsciences in Sweden.Sigrun SantessonProgramme OfficerMFS Programme/LP ProgrammeInternational Office, MFSKTH, SE–100 44 Stockholm, Sweden, Phone: +46 8 790 7i83 , Fax: +46 8 790 8192, E-mail: sigrun@kth.sewww.kth.se/student/utlandsstudier/examensarbete/mfsv

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Arsenic in Geothermal Waters of Costa RicaTABLE OF CONTENTSTABLE OF CONTENTS........................................................................................................................... VIIACKNOWLEDGEMENTS ..........................................................................................................................IXABSTRACT...................................................................................................................................................XISAMMANFATTNING..............................................................................................................................XIIISINTESIS ....................................................................................................................................................XV1. BACKGROUND.......................................................................................................................................... 11.1 GEOTHERMAL SYSTEMS................................................................................................ 11.1.1 Classification of geothermal systems......................................................................................................................................11.1.2 Classification of geothermal systems......................................................................................................................................11.2 ENVIRONMENTAL EFFECTS OF GEOTHERMAL ACTIVITY................................................. 31.3 ARSENIC IN GEOTHERMAL SYSTEMS.............................................................................. 31.3.1 Dispersion of arsenic and environmental contamination........................................................................................................31.3.2 Health effects of arsenic and risks ........................................................................................................................................31.4 HISTORY OF EXPLOITATION OF GEOTHERMAL ENERGY IN C OSTA R ICA ......................... 41.5 RATIONALE OF THE PRESENT STUDY ............................................................................ 41.6 T HE STUDY AREAS....................................................................................................... 51.6.1 Miravalles geothermal field...................................................................................................................................................51.6.2 Rincon de la Vieja geothermal field......................................................................................................................................91.7 AIMS AND OBJECTIVES................................................................................................. 91.8 LIMITATIONS............................................................................................................... 92. ARSENIC IN GEOTHERMAL WATER.................................................................................................102.1 T HE SOURCE OF ARSENIC IN GEOTHERMAL SYSTEMS ....................................................102.1.1 The source and nature of geothermal fluids .........................................................................................................................102.1.2 The source of As in geothermal fluids.................................................................................................................................112.1.3 Arsenic from host rock leaching..........................................................................................................................................112.2 SPECIATION OF ARSENIC IN GEOTHERMAL FLUIDS .......................................................112.2.1 Dissolved arsenic................................................................................................................................................................112.2.2 Low sulphide, reduced fluids ..............................................................................................................................................112.2.3 High sulfide, reduced fluids ................................................................................................................................................112.2.4 Redox state........................................................................................................................................................................122.3 ARSENIC DEPOSITION FROM GEOTHERMAL FLUIDS.......................................................122.4 ARSENIC IN HOT SPRING DEPOSITS AND SCALES ..........................................................122.5 T HE FATE OF ARSENIC FROM GEOTHERMAL SOURCES ...................................................132.6 SURFACE WATERS .......................................................................................................132.7 GROUNDWATER..........................................................................................................143. MATERIALS AND METHODS ...............................................................................................................153.1 FIELD INVESTIGATIONS ..............................................................................................153.1.1 Field location and sampling ...............................................................................................................................................153.1.2 Collection of field data........................................................................................................................................................153.2 LABORATORY INVESTIGATIONS ...................................................................................213.2.1 Alkalinity .........................................................................................................................................................................213.2.2 Major ions.........................................................................................................................................................................223.2.3 Trace elements....................................................................................................................................................................223.2.4 Arsenic (III)......................................................................................................................................................................223.2.5 DOC ................................................................................................................................................................................22vii

Lotta Hammarlund & Juan Piñones TRITA LWR Masters Thesis 09-023.2 T REATMENT OF ANALYTICAL DATA .............................................................................224. RESULTS.................................................................................................................................................. 234.1 GEOTHERMAL WELLS ..................................................................................................234.1.1 Field measured parameters.................................................................................................................................................234.1.2 Major ions.........................................................................................................................................................................234.1.3 Trace elements....................................................................................................................................................................254.1.4 Correlation between various chemical parameters ................................................................................................................254.2 T HERMAL SPRINGS -NEUTRAL AND A CID .....................................................................264.2.1 Field measured parameters.................................................................................................................................................264.2.2 Major ions.........................................................................................................................................................................264.2.3 Trace elements....................................................................................................................................................................284.2.4 Correlation between various chemical parameters ................................................................................................................304.3 C OLD SURFACE WATERS ..............................................................................................314.3.1 Field measured parameters.................................................................................................................................................314.3.2 Major ions.........................................................................................................................................................................314.3.3 Trace elements....................................................................................................................................................................314.4 ANOMALOUS BORON CONCENTRATIONS.......................................................................325. DISCUSSION........................................................................................................................................... 335.1 ARSENIC CONCENTRATIONS IN MIRAVALLES AND RINCON DE LA VIEJA........................335.1.1 Geothermal wells................................................................................................................................................................335.1.2 Thermal springs.................................................................................................................................................................335.1.3 Cold surface waters ............................................................................................................................................................335.2 C OMPARISON OF THE DATA WITH OTHER GEOTHERMAL FIELDS IN N EW Z EALAND, USAAND PHILIPPINES.............................................................................................................335.2.1 Concentrations of arsenic and variability ............................................................................................................................345.2.2 Arsenic concentrations in Miravalles and Rincon de la Vieja ............................................................................................345. CONCLUSIONS....................................................................................................................................... 367. REFERENCES......................................................................................................................................... 37viii

Arsenic in Geothermal Waters of Costa RicaACKNOWLEDGEMENTSThis Master of Science thesis comprises the final part of our degree in EnvironmentalEngineering at KTH. This study could never have been realized without the assistance andsupport 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 theDepartment of Land and Water Resources Engineering (LWR) at KTH for advice and guidancethroughout this study.Our local supervisors Jochen Bundschuh and Guillermo E. Alvarado from InstitutoCostarricense de Electricidad (ICE), who provided invaluable guidance and support during ourfieldwork.Antonio Yock, Paul Moya and other staff at ICE, for support and help during our work and formaking our stay in Costa Rica a pleasant and memorable experience.Sida/SAREC for partly financing this project and Sigrun Santesson at the MFS Programmeoffice.Ann Fylkner and Monika Löwén, LWR-KTH, for laboratory assistance and supervision at thelaboratories of LWR-KTH.Carl-Magnus Mörth, Universitetslektor på Institutionen för Geologi och geokemi, for laboratoryanalysis.…and all the sympathetic people in Costa Rica that made this project an unforgettableexperience.Lotta HammarlundJuan PiñonesStockholm, December 2009ix

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Arsenic in Geothermal Waters of Costa RicaABSTRACTArsenic (As) contaminated land and groundwater is a serious health problem in a globalperspective. Long-term intake of water with high As concentration is directly linked to variouslife threatening diseases such as cancer and skin lesions. In geothermal areas As contaminationoccurs in two main processes: i) natural contamination where the gothermal waters reach thesurface 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 ofgeothermal waters as an energy resource causes mobilisation of As and other heavy metalscontained in the geothermal waters to reach the surface and then contaminate surface andshallow groundwater bodies. Both of these processes may affect the environment and the healthof humans and animals. Typical symptoms that arise due to the use of contaminated watersinclude 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 inCosta Rica. This area is rich in geothermal resources and the geothermal waters and thermal mudcontain high levels of As. Concentrations up to 30 mg As/l have been recorded and thus presenta problem for a country which is utilising these geothermal resources. It also indicates potentialhealth 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 overallcharacterisation of the geothermal waters in the areas of Miravalles and Pailas-Borinquiengeothermal fields. The specific objectives are to characterize geothermal waters extracted fromgeothermal wells used for power generation, natural thermal springs and cold springs in thesurroundings of Miravalles and Pailas-Borinquien geothermal fields. The results from this studyhave also been compared with the geochemical characteristics of the geothermal waters fromgeothermal 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 andthe 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 placesAs concentration exceed the WHO limit for safe drinking water. All the geothermal well fluidsgreatly exceed the WHO limit while all the cold springs fall below that limit. Sampled geothermalwell fluids are generally of Na-Cl-B type with an almost neutral pH. They contain an extremelyhigh As concentration. Sampled thermal springs can be divided into neutral thermal waters andacidic thermal waters. The neutral thermal springs (pH almost neutral) are generally of HCO 3 -Cltype while the acidic thermal springs (pH 1.97-3.25) are generally of SO 4 -S-Cl-Al type. Theycontain low to high As concentrations. Sampled cold surface waters are generally of Si-SO 4 -HCO 3 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 forgeothermal 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

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Arsenic in Geothermal Waters of Costa RicaSAMMANFATTNINGArsenikförorenad mark och grundvatten är ett allvarligt, globalt hälsoproblem. Konsumtion avvatten med hög arsenikhalt under lång tid är direkt förknippat med flera livshotande sjukdomar, t.ex. cancer och störningar på centrala nervsystem. I geotermiska områden förekommerarsenikföroreningar i huvudsakligen i två processer: i) naturliga föroreningar där geotermisktvatten når markytan genom naturliga källor och som sedan blandar sig med ytvatten ellergrundvatten som används både för bevattning och som dricksvattenkälla, och ii) exploatering avgeotermiska naturresurser för energiutvinning leder till att arsenik och andra tungmetaller somfinns i det geotermiska vattnet når markytan och förorenar yt- och grundvattenkällor. Båda dessaprocesser påverkar miljön och hälsotillståndet hos människor och djur. Vanliga symptom vidkonsumtion av arsenikförorenat vatten är olika typer av cancer.De aktuella studieområdena är belägna i vulkanområdena Miravalles och Rincón de la Vieja iCosta Rica. Här finns rikligt av geotermiska naturresurser med höga halter av arsenik i bådegeotermisktvatten och lera. Arsenik koncentrationer har uppmäts till 30 mg/L och är ett problemför ett land som utvinner energi från dessa naturresurser. Dessa halter indikerar en hälsorisk dåhalterna 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 iområdet runt Miravalles och Pailas-Borinquien geotermiska kraftverk. Målet är också attkarakterisera vattnet från: de geotermiska naturtillgångarna som används för att utvinna energi, denaturliga termiska källorna samt ytvattnen i omgivningarna runt Miravalles och Pailas-Borinquiens geotermiska kraftverk. Resultatet från den här studien har jämförts medkaraktäriseringen 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 vulkanområdenaMiravalles och Rincón de la Vieja. Vattenprover togs från 3 olika typer av reservoarer;geotermiska kraftverk, termiska källor och kalla ytvatten. Arsenikhalten översteg WHO:sgränsvärde för säkert dricksvatten i 35 av de 50 proverna. Alla prover från de geotermiskakraftverken översteg kraftigt WHO:s gränsvärden medan alla prover från de kalla ytvattnen föllunder gränsvärdet. De geotermiska vattnen från kraftverken är av vattentypen Na-Cl-B med ettneutralt pH.. De innehöll extremt höga As värden.Proverna från de termiska källorna kan delas in i neutrala eller sura vatten. De neutrala termiskakällorna (pH nära 7) är generellt av vattentypen HCO 3 -Cl medan de sura termiska källorna (pH1.97-3.25) generellt har vattentypen SO 4 -S-Cl-Al. De innehåller både låga och höga arsenikhalter.De kalla ytvattnen var av vattentypen Si-SO 4 -HCO 3 med ett neutralt till basiskt pH och innehölllåga arsenikhalter.Jämförelser med arsenikhalterna i Nya Zeland, USA och Phillipinerna visar att vattnen somanvänds 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

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Arsenic in Geothermal Waters of Costa RicaSINTESISLa contaminación de agua subterránea causada por arsénico es un problema serio para la salud yde importancia a nivel global. El consumo prolongado de agua con altas concentraciones dearsénico esta relacionado con diferentes tipos de enfermedades mortales, tales como cáncer ycon efectos degenerativos del sistema nervioso y de la piel. En areas geotérmicas la concentraciónde arsénico ocurre en dos procesos principales: i) contaminación natural causada cuando lasaguas geotermales alcanzan la superficie en forma de ballestas naturales y consecuentemente semezclan con corrientes de aguas superficiales o sistemas llanos de aguas subterráneas usadas porlos habitantes para la irrigación o como provisiones de agua potable y ii) la explotación de aguasgeotermales como recursos de energía causa una mobilisación de arsénico y otros metalespesados que se transportan con las aguas geotermales y asi alcanzan la superficie y sistemas llanosde aguas subterráneas. Ambos síntomas que surgen por uso de aguas contaminadas incluyenlesiones 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 altasconcentraciones de arsénico. Las concentraciones de arsénico registradas han alcanzado hasta 30mg/L en agua subterránea y por lo tal presenta un potencial problema de salud a un pais quehace uso de sus recursos geotermales. Adicionalmente señala un riesgo para la salud al exceder lasindicaciones de 10µg/L en agua potable determinada por OMS.El objetivo de este estudio es de investigar las concentraciones de As y hacer una caracterizacióngeneral de las aguas geotermales en los alrededores de los campos geotérmicos en Miravalles yPailas-Borinquien. Los objetivos específicos son de hacer caracterizaciones de las aguasgeotermales extraídas de pozos geotermales y utilizadas para la generación de energía, fuentestermales naturales y fuentes frías en los alrededores de los campos de aguas geotermales enMiravalles y Pailas-Borinquien. Los resultados tambien han sido estudiados en relación con lascaracterísticas de las aguas geotermales de pozos geotermales y fuentes de aguas termales enNueva Zelándia, Las Filipinas y Estados Unidos.Un total de 50 muestras fueron recogidas en diferentes lugares a los alrededores de las áreasvolcánicas de Miravalles y el Rincón de la Vieja. Las aguas fueron recogidas en tres diferentestipos de reservas, corrientes de pozos geotermales, arroyos termales y aguas frías llanas. En 35 delas 50 muestras analizadas la concentración de arsénico sobrepasó el límite de recomendación dela OMS en agua potable. Todas las corrientes de aguas geotermales de los pozos sobrepasaronnotablemente el límite postulado por la OMS para agua potable. Mientras que las ballestas deaguas frías estaban bien por debajo del mismo límite. Por lo general los iones dominantes enmuestras de corrientes de pozo geotermales son Na-Cl-B con pH neutral. Sin embargo, estascontenían altas concentraciones de arsénico. Las ballestas de aguas termales pueden ser divididasen agua termal neutral y agua termal acída. Los iones dominantes fueron generalmente delHCO 3 -Cl en las aguas neutrales (pH casí 7) y SO 4 -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 llanassobrepasaron el límite de la OMS en agua potable. Los iones dominantes fueron Si-SO 4 -HCO 3con pH neutral y contenían baja concentración de arsénico.Una comparación con los resultados de Nueva Zelandia, Estados Unidos y Las Filipinas muestraque las aguas de pozo usadas para la energía geotermal en Costa Rica contienen concentracionesextremedamente elevadas de Arsénico.Palabras claves: Energía geotérmica, Arsénico; boro; sistemas geotérmicos, aguas termales,Costa Rica; Miravalles; Rincón de la Viejaxv

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Arsenic in Geothermal Waters of Costa Rica1. BACKGROUNDGeothermal waters are related to active or inactivevolcanism and change their specific chemicalcomposition during their pathway from deepseated sources to the surface of the earth. Thepresence of arsenic (As) is high in these waters.Several gaps exist in our present day knowledge ofthe genesis and mobility of As in geothermalsystems and its geochemical processes and kineticsin the near-surface environment. This lack ofknowledge is of particular concern where countriesutilise geothermal waters for energy generation andactively facilitate the movement of potentiallycontaminated water at depth to the surface, whichis the case in Costa Rica. This study forms a partof a recently initiated collaboration betweenInstituto Costarricense de Electricidad (ICE),Costa Rica and the Department of Land andWater Resource Engineering at the Royal Instituteof Technology (KTH), Stockholm.1.1 Geothermal systems1.1.1 Classification of geothermal systemsGeothermal systems can be classified into threedifferent categories:• Low temperature (< 90° C)• Moderate-temperature (90° - 150° C)• High temperature (> 150° C)The temperature determines the method used toextract the energy.High temperature resources are generally usedonly for electric power generation. Wells ofvarying depth tap steam and hot water to driveturbines that drive electric generators. The threetypes of geothermal power plants most commontoday are:• Dry steam plants directly use geothermalsteam to turn turbines.• Flash steam plants pull deep, highpressurehot water into lower-pressuretanks and use the resulting flashed steamto drive turbines.• Binary-cycle plants transfer heat frommoderately hot geothermal water to asecondary fluid with a much lower boilingpoint causing the secondary fluid to boil.The resulting vapour is then used to driveturbines.Low- and moderate-temperature resources can bedivided into two categories:• Direct use, which uses the heat in water(over short distances) without a heatpump or power plant to heat buildings,industrial processes, greenhouses andresorts. Geothermal district heatingsystems supply heat by pumpinggeothermal water through a heatexchanger, which transfers the heat towater in separate pipes that is pumpedinto buildings. After passing through theheat exchanger, the geothermal water isinjected back into the reservoir where itcan be recharged and used again. Thismethod involves the extraction of waterfrom the ground and can thereforeimpact geothermal features.• Geothermal heat pump system, whichconsists of pipes buried in shallow groundnear a building or inserted in a verticalwell, a heat exchanger and ductwork inthe recipient building. In winter, heatfrom warmer ground is passed through aheat exchanger and used to warm anycooler environment (e.g. a building or abody of water). The process can work inthe opposite way in warm environments;drawing cool air from below ground into abuilding and using the ground as a heatdump (National Park Service, NPSWestern Energy Summit, January 21-23,2003 ).1.1.2 Classification of geothermal systemsHigh temperature geothermal systems occur allover the world; in the Philippines, New Zealand,USA, Hawaii, Japan, Russia, Italy, Iceland, Chile1

Lotta Hammarlund & Juan Piñones TRITA LWR Masters Thesis 09-02Figure 1: Map of Central America showing principal sites of geothermal resources (Birkle & Bundschuh 2007).and in all of the central American countries(except for Belize).All Central American countries (with the exceptionof Belize) are endowed with significant geothermalpotential due to their location within the PacificRim volcanic zone. The geothermal systems ofCentral America are so-called ‘convectionsystems’, related to an active volcanic belt, andderive their heat from magmatic bodies at shallowto intermediate levels. Such geothermal systemsprovide a natural energy source for manydevelopmental activities. The countries in CentralAmerica utilizing these sources for powergeneration are Guatemala, El Salvador, Nicaraguaand Costa Rica. An initial assessment study ofgeothermal resources in Central America wasperformed by the relevant national authoritiesduring the 1960’s and 70’s (Birkle & Bundschuh2007). The most important sites are showed inFigure 1.High temperature geothermal systems are generallyassociated with three distinct plate tectonicenvironments:i) within proximity of the plate boundaries,ii) regions around ‘hot spots’, where local magmachambers rise from near the mantle to shallowdepths in the earths crust, andiii) in rift zones where the tectonic plates diverge.Geothermal active areas occur where deeplycirculating groundwaters are conductively heatedin the crust and an unusually high geothermalgradient allows hot water or steam to appear at theearth’s surface. The temperature of geothermalfluids may be elevated by only a few degrees,above ambient levels (Ellis & Mahon 1967). Heatmay be derived from volcanic or magmaticactivity, metamorphism, faulting and radioactivity.The hot fluids are of lower density thansurrounding waters and rise through the host rock.As geothermal fluids rise through the crust, adecrease in pressure allows high temperaturesingle-phase fluids to separate at shallow depthinto two phases: steam and water. This processcan also occur wherever there is a sudden decreasein pressure due to the presence of a rock fractureor fissure (Webster & Nordstrom 2003). Manygeothermal fields have been developed, or aretargeted for development, to generate energy fromthe steam and hot water reservoirs beneath theearth’s surface (Bundchuh et al. 2002). As part of2

Arsenic in Geothermal Waters of Costa Rica___________________________________________________________________________________the exploitation process, reservoir fluid is drilledand brought to the surface under pressure. Whenit reaches the surface the water is “flashed” to adesirable temperature to generate the steam to runsteam turbines. The extent of steam and waterseparation (boiling) can be artificially manipulatedto maximise plant efficiency (Webster 1999).1.2 Environmental effects of geothermalactivityDevelopment of geothermal fields for powergenerations tends to increase the rate and volumeof geothermal fluids reaching the surface. Thewater formed during the process of separationbecomes a waste product. This wastewater oftenhas higher contaminant concentrations thannatural hot spring water because the processes thatremove or immobilise contaminants in naturalgeothermal features, such as the precipitation ofmineral-rich sinters, have been bypassed. Disposalof these waste waters can be problematic. In mostmodern geothermal power stations, waste watersare re-injected back into the field. However, atsome older fields such as the Wairakei GeothermalField in New Zealand these waters are stilldischarged into surface drainage systems (Webster& Nordstrom 2003).One of the most important environmental effectsresulting from geothermal activity is thecontamination of natural drainage systems by (As)and other harmful elements. Arsenic is a typicalcomponent of active geothermal systems. Itcommonly occurs together with otherenvironmental contaminants such as boron (B),mercury (Hg), antimony (Sb), selenium (Se),thallium (Tl), lithium (Li), fluoride (F) andhydrogen sulphide (H 2 S). These elements are alsorecognized as typical contaminants of geothermalsystems (Webster & Nordstrom 2003). Theconcentrations of As frequently exceed 10 μg/Land have been measured as high as 2000 μg/Lfrom some areas of geothermal activity. Waterflow through the surface and subsurfacecatchments therefore has the potential to transportAs and other harmful elements beyond theboundary of geothermal fields. In the areasurrounding geothermal systems, contamination ofsurface- and groundwaters is common (Webster &Nordstrom 2003). The exploitation of geothermalresources can also change the landscape with thedrilling of wells, lying of pipelines andconstruction of power plants. Exploitation canalso lead to ground subsidence if the fluidwithdrawal exceeds the natural inflow (i.e if reinjectionis not effective). It also causes emissionsof incondensable greenhouse gases, H 2 S and otherpollutants (Bargagli et al. 1997).1.3 Arsenic in geothermal systemsWorldwide, little is known about geothermal As,its genesis, speciation and mobility. Little is alsoknown about the geochemical and biochemicalprocesses which occur with the fluxes of As fromthe geothermal sources at the surface, wherepressure, temperature and oxidation conditionschange. It is also an issue of primaryenvironmental concern, to investigate thecharacteristics and behaviour of geothermal Asand its impact on the natural ecosystems. It is alsoimportant for developing strategies to improve thesocio-economic conditions of affected areas.1.3.1 Dispersion of arsenic and environmentalcontaminationArsenic contamination occurs due to two mainprocesses:1) Natural processes: Geothermal waters reach thesurface as natural springs and then mix withsurface water flows or shallow groundwaterbodies. These fresh water sources may be used byhumans for both irrigation and drinking watersupply.2) Geothermal exploitation: Human exploitationof geothermal waters as an energy source, causesmobilisation of As and other heavy metalscontained in the geothermal waters to the surface.This then increases the likelihood of surface andshallow groundwater contamination.Both of these processes may affect theenvironment and the health of humans andanimals. Typical symptoms that arise due to theuse of contaminated waters include skin lesions,hyperkeratosis, melanosis and different forms ofcarcinoma and lung cancer (WHO 1993, 2008;Webster & Nordstrom 2003).1.3.2 Health effects of arsenic and risksThe acute and chronic toxic effects of As are welldocumented. Arsenic has been declared a humancarcinogen, contributing to a high incidence ofskin and other cancers in populations exposed tohigh levels of As in drinking water (WHO 1993,3

Lotta Hammarlund & Juan Piñones TRITA LWR Masters Thesis 09-022008). The WHO drinking water guideline for Aswas lowered from 50 μg/L to 10 μg/L in 1993,and the new value has since been adopted by manycountries as a drinking water standard (Table 1).Table 1: Guideline values for arsenic recommended by WorldHealth Organisation (WHO 1993) for different purposes.Use of waterµg/lDrinking water limit 10Protection of aquatic life 19Stockwatering 20Irrigation 10Arsenic concentrations in natural surface drainagesystems frequently exceed 10μg/L in areas withgeothermal activity. Symptoms of chronic Aspoisoning such as skin lesions and high Asconcentrations in hair and nails have beenreported from geothermal areas but this may notalways be as a direct consequence of drinkingwater contamination (Webster & Nordstrom2003). At the Mt. Apo geothermal field in thePhilippines, for example, the two rivers drainingthe field carry elevated As concentrations due tohot springs activity in the river beds. Duringdevelopments of the field, alternative cleandrinking water supplies were provided and used bylocal residents, but the symptoms of high Asexposure appeared to persist (Webster 1999). Theaccumulation of As in edible aquatic plants is likelyto have been to blame, as high levels of As havebeen reported in aquatic weeds in other riversystems receiving geothermal fluids (Webster &Nordstrom 2003).1.4 History of exploitation of geothermalenergy in Costa RicaDuring the 1970’s, Costa Rica satisfied itselectricity needs using hydro (70%) and thermal(30%) energy sources. The continuous rise in oilprices, especially during the 1973 crisis, motivatedthe national electricity authority (InstitutoCostarricense de Electricidad [ICE]), to study thepossibility of using other energy sources for thegeneration of electricity, including geothermalenergy (Moya 2005).The first three deep wells were constructed 1979-80 close to the foot of the Miravalles volcano. ICEstarted to generate electricity in the beginning of1994. Since then more wells have been constructedand the installed capacity today is about 163MWe.Even though only about 8.6% of the geothermalcapacity was installed in 1994, the energy producedrepresented about 15% of the total energyproduced in Costa Rica, according to thecomparison made during 2004 (Paul Moya, ICE,pers. comm.).The most studied areas for geothermalexploitation are the volcanoes of Miravalles,Rincón de la Vieja and Tenorio (Figure 2B) (Moya2005). The largest thermal manifestations in thearea are in the Las Hornillas zone on theMiravalles volcano and in the Las Pailas zone onthe Rincon de la Vieja-Santa Maria volcaniccomplex. They contain lakes and acid-sulphatesprings, steaming ground, fumaroles emissions,small craters and mud volcanoes. The geothermalreservoir in this area are high-temperature andliquid-dominated (Gherardi et al. 2002). As aconsequence of increasing exploitation ofgeothermal resources, there has been emergingconcern about associated environmental impactsin the surrounding region. The geothermal watersand mud in Costa Rica are known to contain highlevels of As; concentrations up to 30 mg/L havebeen recorded in the geothermal waters. Arsenicpresents a serious potential environmentalcontamination and a risk for human health. Asconcentrations in many places are above the WHOdrinking water guidelines limit of 10 µg/l, (WHO2008). However, the geothermal waters havehighly varied characteristics that indicatedifferential patterns of hydro geochemicalevolution. It is therefore difficult to predictenvironmental risks on any practical level withoutdetailed study.1.5 Rationale of the present studySeveral gaps exist in our present day knowledge ofthe genesis and mobility of As in geothermalsystems and the geochemical processes and theirkinetics in near-surface environments. Since thereare an increasing number of areas where outflowof geothermal waters come in contact with humanbeings and grazing animals understanding of Asbehaviour is of great importance for the futuredevelopment and management of geothermalresources. It is a trend of increasing exploitation ofgeothermal resources for energy and tourismtherefore the understanding of geochemicalprocesses that occur along the pathway ofgeothermal waters is essential. Hydro-geochemicaland geochemical investigations of surface and4

Arsenic in Geothermal Waters of Costa Rica___________________________________________________________________________________deep geothermal waters and the sediments foundin the areas surrounding thermal springs andfumaroles are essential for developing effectivestrategies to safeguard the human health and theenvironment.The geological history of southern CentralAmerica is dominated by the growth of aCenozoic magmatic arc that is superimposed on aJurassic-Eocene ophiolite basement that consistsof basalt and volcanic – sedimentary units. CostaRica is tectonically complex because of theinterplay of four plates: Cocos, Caribbean, Nazca,and the South American block (Figure 2A). Thefigure shows the subduction of the Cocos Platebeneath the Caribbean Plate, which has caused thedevelopment of an inter-oceanic volcanic arc allalong the Middle American Trench.The Costa Rican Volcanic Front is associated withthe north eastward subduction of the Cocos platebeneath the Caribbean plate with a well-definedBenioff zone with a maximum depth of seismicityat 200 km and a subduction velocity of 9-10cm/year at an angle of 60 to 70˚ (Alvarado et al.1999; Montero 1999). The four mountain rangesin Costa Rica, known as Guanacaste, Tilarán,Central and Talamanca ranges. The Guanacasterange is a NW-SW trending chain of Quaternarystratovolcanos; predominantly andesitic incomposition. The range comprises mainly ofpyroclastic (fallout, surge, and flow) rocks, lavaflows, and fluvio-lacustrine deposits that haveformed gently sloping plateaus on both sides ofthe range. The area is under constant regionalstress due to the subduction of the Cocos Plateunder the Caribbean Plate and due to the regionaluplift of the volcanic arc (Vega et al. 2005).Surface indications of geothermal activity includenumerous springs, fumaroles, vents and boilingmud pots. These geothermal expressions formbeautiful landscapes in combination withvolcanoes and tropical rain forest. The uniquevisual nature of this landscape combined with itshigh biodiversity has created a region of high valuetourism. This industry contributes significantly tothe economic and social development of thecountry (Vega et al. 2005).1.6 The Study areasThe present study focuses on investigate theoccurrences of As as well as an overallcharacterisation of the geothermal waters in thevicinity of the volcanoes of Miravalles and Rincónde la Vieja based on the sampling of thegeothermal wells, natural thermal springs and coldsprings. The following section gives a briefoverview of the study areas.1.6.1 Miravalles geothermal fieldMiravalles is the most studied geothermal area inCosta Rica. Here 53 wells are drilled; from which32 of them are used for production while 14 areused for gravity injection of residual waters(Maniere 2005).Figure 2: A) Tectonic sketch map of Central America. Thesubduction of the Cocos Plate beneath the Caribbean Plate isshowed, which caused the development of an inter-oceanic volcanicarc all along the Middle American Trench. B) Geographical mapof Costa Rica showing location of main volcanoes and Miravallesand Rincon de la Vieja geothermal fields. The Guanacaste arcconsist of the Orosi-Cacao, Rincon de la Vieja-Santa Maria,Miravalles-Paleo Miravalles and Tenorion-Montezuma volcaniccomplexes (Gherardi et al. 2002).5

Lotta Hammarlund & Juan Piñones TRITA LWR Masters Thesis 09-02Cross section ofFigure 4Figure 3: Geological-structural map of Miravalles geothermal reservoir and well field located in Guayabo caldera. The Map section of LaFortuna Graben is enclosed by a black frame (Birkle and Bundchuh 2007).6

Arsenic in Geothermal Waters of Costa Rica___________________________________________________________________________________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-sectionsare shown in Figure 3 (Birkle and Bundchuh 2007).Miravalles geothermal area is located in the northwesternpart of Costa Rica in the Guanacastemountain range of Guanacaste Province. TheMiravalles volcano is a Quaternary strato-volcanorising to 2028 m a.s.l. and is currently inactive. Thegeothermal reservoir is high-temperature andliquid-dominated. The Miravalles field area ischaracterised by the Guayabo caldera (Figure 3)which is an active hydrothermal system of acaldera-type collapse structure of about 15 kmdiameter (Bundschuh et al. 2002; Vega et al. 2005).The highest reservoir temperature measured was255 ˚C with typical measured temperatures rangingbetween 230-240˚C. The north-eastern part of theGuanacaste Cordillera seems to be the mainrecharge zone. Regional groundwaters andreservoir fluids have been identified to haveseveral mixing trends (Gherardi et al. 2002). Inorder to study the hydrologic relations between thereservoir and the surface water bodies’ sampleswere taken from zones surrounding the Miravallesgeothermal field and from geothermal fluids.1.6.1.1 Geo-volcanological frameworkThe Miravalles geothermal field is located insidethe Guayabo caldera (Figure 3), which formedduring the Pleistocene after three separate phasesof volcanic activity. Within the Guayabo calderathe volcanic activity gradually shifted in anorthwest direction and then south-westwardsduring these phases. Revealing the existence ofstrong tectonic activity, four main fault systemshave been identified in the area (Gherardi et al.2002) all of which contribute to the hydraulicpermeability of the geothermal reservoir (Vega etal. 2005).In order of decreasing age, they are oriented:1. NW–SE direction sub-parallel to that ofthe main Central-American mountainchain, and, in particular, to the axis of theGuanacaste volcanic belt.2. N–S, formed during the Holocene, andhas produced a graben-like structureabout 3 km in width known as La FortunaGraben (Figure 3). The La FortunaGraben has displaced part of the Guayabocaldera and forms the eastern and westernmargin of the geothermal field.3. NE–SW, sub-parallel to the migratingeruptive centres of the Miravalles volcano.7

Lotta Hammarlund & Juan Piñones TRITA LWR Masters Thesis 09-02Figure 5: Conceptual model of Miravalles geothermal reservoir, with a heat source related to Miravalles volcano. Alsoshown are main aquifer systems of the geothermal reservoir with flow directions, which are influenced by major faults (e.g. LasHornillas fault) (Birkle and Bundchuh 2007).4. E–W, intersects the graben surface, andhas been encountered in severalproductive wells. This is the youngestfault system and is expressed at thesurface 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 geologicalmapping combined with lithological descriptionsfrom the 53 geothermal wells reveal differentstratigraphic units (Figure 4). Three phases ofvolcanic activity can be distinguished: pre-calderavolcanism, caldera volcanism and post calderavolcanism (Birkle & Bundchuh 2007; Vega et al.2005).To get to know the temperature distribution in thegeothermal field, information on water-rockinteractions and resulting hydrothermal alterationwas obtained from the 53 geothermal wells. Threetemperature zones, partly overlapping, could bedistinguished:1. a smectit zone (

Arsenic in Geothermal Waters of Costa Rica___________________________________________________________________________________1.6.2 Rincon de la Vieja geothermal fieldRincon de la Vieja is the only active volcano of thevolcanoes in the Guanacaste Volcanic Range. It isan andesitic stratovolcano located in the northwestof 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 complexvolcano, with a maximum elevation of 1916 m(Santa Maria cone), consisting of seven craters,forming a 8 km long NW trending ridge within anolder, wider caldera. The majority of thermalemissions of Rincon de la Vieja volcano arelocated in the western outer flank of the ActiveCrater. The origin of the thermal discharges ismainly due to the boiling of a shallow aquiferheated by inputs of magmatic-related hot fluidsand the expressions of geothermal activity in thearea mainly consist of hot mud pools and hotsprings (Tassi et al. 2005).The largest thermal manifestations on the LasPailas zone in Rincon de la Vieja are the SantaMaria volcanic complex which contains lakes andacid-sulphate springs, steaming ground, fumarolesemissions, small craters and mud volcanoes(Gherardi et al. 2002).1.8 LimitationsThe chemical behaviour of As in sulphide-richfluids has been the subject of considerable andongoing debate as there are several major obstaclesto the accurate prediction of As speciation.In part, this reflects the lack of a completethermodynamic database for As species. Stabilityconstants for ion pairs and complexes between theoxyanions and polyvalent cations are few andlimited in applicability. Also, polymerisation ofarsenite, arsenate and thioarsenite complexes inhigh As or high sulphur solutions appears likely,but is largely unconfirmed. These reactions couldsignificantly complicate As speciation. However,reliable thermodynamic data are available for someAs species, and these can be used to interpret Asbehaviour in many geothermal systems and surfacewater environments, as long as the limitations arerecognized (Webster & Nordstrom 2003).1.7 Aims and objectivesThe purpose of the study is to investigate theoccurrences of As as well as an overallcharacterisation of the geothermal waters in theareas of Miravalles and Pailas-Borinquiengeothermal fields.The specific objectives are to characterize watersextracted from:• Geothermal wells used for powergeneration.• Natural thermal springs.• Cold springs.• Suggest protection measures to avoid Ascontamination.This study is expected to give an opportunity tocompare similarities and dissimilarities of Asconcentration with results from geothermal wellsand thermal springs in New Zealand, Philippinesand USA where these issues have beeninvestigated.9

Lotta Hammarlund & Juan Piñones TRITA LWR Masters Thesis 09-022. ARSENIC IN GEOTHERMAL WATERInorganic As is known to be potentially toxic and ahazardous to humans. Arsenic is a strongly redoxsensitiveelement and in natural aquifer systems ispresent in two dominant redox states; As(ΙΙΙ) andAs(V). As(V) is generally the stable species inoxidizing environments while As(ΙΙΙ) speciespredominates under reducing conditions. Thetoxicity (and mobility) of As(ΙΙΙ) is greater thanthat of As(V). As(ΙΙΙ) is toxic since it coagulatesproteins, forms complexes with coenzymes, andinhibits ATP production (necessary for metabolicprocesses) (Bhattacharaya et al. 2002).The uprising geothermal waters are normallyreducing (suggesting the presence ofpredominantly As(ΙΙΙ) species) at depth. While itcomes in contact with shallow aquifers or mixeswith surface waters, the redox conditions becomeoxidising. In such oxidized systems, the mobilityof As is a function of the redox transformation ofthe As(ΙΙΙ) to the oxidised As(V) species. TheAs(V) species is then sorbed on oxide minerals,i.e., amorphous Al-, Mn- and Fe oxides andhydroxides. However, if residence time of waterwithin the geothermal system is short, redoxtransformations are rather incomplete and thusAs(ΙΙΙ) species may be present at the surfaceoutlets of thermal springs. The mobility of As inreduced systems is also governed by thedissolution of Fe- and Mn-oxyhydroxides in theaquifer sediments due to microbial mediatedbiogeochemical interactions (Bhattacharaya et al.2002).Geothermal waters, which are mineralised withover 1g/L of total dissolved solids change theirspecific chemical composition during theirpathway from deep seated sources to the surfaceof the earth. Since geothermal waters are related toactive or inactive volcanism, the presence of As iseverywhere in these waters. The concentration andspeciation of As depends on the thermal sources atdepth and subsequent hydro-chemical evolution ofwater chemistry along its pathway up to thesurface. Due to interactions with the sedimentsand other unstable constituents generated duringthe volcanic activities (fumaroles, gas vents etc.)the concentration and speciation of As change andleave behind hydrothermal mud (Webster &Nordstrom 2003).Geothermal fluids may contain as much as 50mg/L As, although concentrations between 1 and10 mg/L are more typical (Ballantyne & Moore1988).2.1 The source of arsenic in geothermalsystems2.1.1 The source and nature of geothermalfluidsActive geothermal areas occur where an unusuallyhigh geothermal gradient allows hot water orsteam to reach the earth’s surface. Heat sourcesmay be related to magmatic or volcanic activity,faulting, radioactivity or metamorphism.Regardless of the heat source, deeply circulatinggroundwaters are heated in the crust.The hot fluids (that may be heated by only a fewdegrees or by hundreds of degrees above thesurroundings) are of lower density thansurrounding waters, and rise through the hostrock, to complete the circulation system. Thepressure decreases when the geothermal fluids risethrough the crust and the fluid separates into asteam phase and a water phase. This usually occursat a shallow depth, but can occur wherever there isa sudden decrease in pressure due to rock fracture.Different kind of surface features can be seen inareas of geothermal energy. They can be groupedinto two main types, on the basis of theirrelationship to the rising geothermal fluids. Hotwater springs is a direct discharge of the hot waterphase. They are rich in chloride and silica and havean almost neutral pH.Steam vents and acidic, sulphate-rich springs areformed by the interaction of the steam phase withshallow aquifer waters. This mixture of water andsteam lead to precipitation and sublimation ofelemental sulphur which follows by microbialoxidation to sulphuric acid.Intense alteration of the host rock caused by thefluid can lead to an unstable ground surface andthe formation of bubbling “mudpools”. Geysersand carbonate-rich springs are also examples ofhot water discharges and include steam-heatingand steam-phase mixing (Webster & Nordstrom2003).10

Arsenic in Geothermal Waters of Costa Rica___________________________________________________________________________________2.1.2 The source of As in geothermal fluidsThe presence of As in geothermal fluids has beenknown since the mid 19 th century. Many studieshave been performed in the Yellowstone NationalPark which contains one of the largest geothermalsystems in the world. Here As concentrations inthe thermal features generally range from

Lotta Hammarlund & Juan Piñones TRITA LWR Masters Thesis 09-02Reactions between arsenate ions and dissolvedsulphide can result in the successive replacementof oxygen with sulphur. The exact nature of suchthio-As complexes still remains much debated. Byassuming that a trimmer structure occurs in asaturated geothermal fluid the following reactiongives the solubility of orpiment:1.5As 2 S 3 + 1.5HS + 0.5 H + = H 2 As 3 S 6-The solubility of orpiment increases with pH andsulphide concentration but is not greatly affectedby temperature changes below 200°C (Webster &Nordstrom 2003).2.2.4 Redox stateWhen the redox conditions in low sulphide fluidsbecome sufficiently oxidizing, oxidation ofarsenious acid to arsenate (H 2 As ν O 4- or HAs ν O 42-)is likely to occur. Oxidation occurs when a risinggeothermal fluids is exposed to atmosphericoxygen or mixed with another oxidizing fluid, suchas a shallow groundwater. Arsenate ions appear tobe formed in hot springs and their surfacedrainage systems. Hot springs form from reservoirfluids contain mainly As(III) , whereas acid-sulfateand bicarbonate features are more enriched inAs(V) . Important to notice is that reported As(III)concentrations may be underestimated if thesample has not been appropriately preserved priorto analysis. To preserve the sample it is importantto perform on site filtration through a 0.45μm (orfiner) membrane to remove bacteria. The sampleshould be stored cold and without air contact. It isalso often recommended to acidify the samplewith HNO 3 or HCl to pH 1.5-2.0 because thisinhibits oxidation of As(III) by chemical oxidantssuch as Fe(III). Although, in some waters,acidification has been observed to change Asspeciation slightly, reducing As(V) to As(III).Geothermal water collected immediately aftersteam separation, and before entering the drain,does not show a similar rate of As oxidation.Instead there is no noticeable As(III) oxidation inthe waste water (collected directly into a sterileglass bottle), even if bubbled with air for severalhours.Rapid As(III)/As(V) oxidation kinetics isapparently catalysed by microbial activity, althoughthe specific mechanisms by which bacteria couldfacilitate arsenite oxidation at temperatures > 50˚Cremain not completely understood. Bacteriainfluence the As(III) oxidation in geothermalsystems and therefore studies is being made in theidentification of the thermophilic bacteria that canoxidise As(III).The ability of H 2 S and thiosulphate (S 2 O 3 ) toreduce As(V) to As(III) is well known becausegeothermal waters containing sulphide orthiosulphate will preserve As as As(III) until thereduced sulphur is oxidised or volatilised (Webster& Nordstrom 2003).2.3 Arsenic deposition from geothermalfluidsDeposits of As, Sb, Au and Hg occurpredominantly near the surface in geothermalsystems while base metals such as Ag, Cu, Pb, Znwill be deposited at greater depth. The metalzonation is due to a three-step process of fluidboiling, gas phase transport and acid reactionswithin metal-bearing waters. Base metalprecipitation occurs due to boiling and the pHincrease which occurs when CO 2 gas is driven off.During boiling, As remains soluble as an oxyanionunder the higher pH and lower sulphideconcentrations present in the fluid. Precipitationof orpiment then occurs in response to theacidification of hot spring waters with acidsulphatewaters, subsurface cooling of the fluid orincreased H 2 S concentrations.At many geothermal fields it has been noted thatAs was mainly concentrated in pyrite at depth.Arsenic minerals such as arsenopyrite (FeAsS)appear to be uncommon in the rocks ofgeothermal reservoirs themselves but a range ofAs minerals are precipitated from geothermalsurface features such as hot springs (Webster &Nordstrom 2003).2.4 Arsenic in hot spring deposits andscalesThe coloured precipitate forming at the peripheryof hot springs, hot pool and geysers can have ahigh concentration of As ranging from two to acouple of hundreds mg/kg. It can be difficult todetermine the mineralogy of these colouredprecipitates though As:S ratios may be affected byother sulphide-bearing minerals and the colour canvary. For example, amorphous realgar has a brightred colour but so have stibnite and cinnabar too.12

Arsenic in Geothermal Waters of Costa Rica___________________________________________________________________________________Orpiment has a yellow-green colour that remind ofnative sulphur. However, in hot springs with lowlevels of antimony and mercury, the yellowprecipitate is commonly orpiment. Realgar doesnot seem to deposit in hot springs but does occuras coating and veins in the altered rocks of thegeothermal field. Other types of deposits have alsobeen reported from around the world but so farlittle is known.The scales which form in pipes and drains of adeveloped field can also be rich in As. The Asdeposition may occur due to adsorption on Feoxide(Webster & Nordstrom 2003).2.5 The fate of arsenic from geothermalsourcesOne of the most significant negativeenvironmental effects of geothermal activity is thecontamination of natural drainage systems by As.While direct soil contamination will occur to somedegree near a geothermal field, this is typically onlya local effect generally considered acceptable. Themain risk is presented by the transportation of Asbeyond the boundary of the geothermal field viawater flow through the surface and subsurfacecatchments. It is chemical contamination ofsurface water, rather than groundwaters, which ismost commonly detected in the vicinity ofgeothermal systems. In surface waters, As enters acycle of chemical and biochemical reactions whichaffect its potential impact (Webster & Nordstrom2003).2.6 Surface watersHot springs, geysers and steam features allproduce an excess of fluid at the surface thatusually drains directly into the nearest catchmentssystem. Even after orpiment precipitation, manyhot spring fluids will still contain >1mg/kg Asbecause orpiment is a relatively soluble salt. Forthat reason, water used for drinking, stockwatering and irrigation or supporting aquatic lifemay have unacceptably high As concentrationsdownstream of a geothermal system.Arsenic contamination of drinking water maycause serious health problems and has thereforereceived lots of attention.Arsenic speciation is important in surface watersbecause acute toxicity of As(III) is greater thanthat of As(V) or the organic forms of As. Forhuman chronic toxicity, the redox form of As maynot matter because As is largely reduced to As(III)and methylated. Another issue is the processes bywhich As interacts with sediments and organic andbiotic substrates in freshwaters, as this affects bothAs concentration and speciation.Studies of As in the rivers draining two geothermalsystems: Yellowstone National Park and theTaupo Volcanic Zone, have shown that As isprincipally transported in dissolved form.“Dissolved” in this case is defined as passingthrough a 0.45μm filter membrane. On a macroscaleAs also appears to behave conservativelydownstream of these geothermal systems, withlittle change in As mass flux down the river or inlakes and estuaries. Studies of geothermal-derivedAs in other freshwater systems support thisobservation (Webster & Nordstrom 2003).2.6.1 Dissolved arsenic speciationThe presence of bacteria in the oxidation processmay be very important both in river waters andaround the outlets of the geothermal springs.Wilkie & Hering (1998) noted that As oxidation inHot Creek did not occur if the river water wasfiltered under sterile conditions or after anantibiotic had been added. Their conclusion fromthis was that bacteria attached to submerged plantswere mediating As(III) oxidation. Some of themost common bacteria capable of As(III)oxidation are Pseudomonas sp, Xanthomonas andAcromobacter. Significantly, while As(V) may bethe predominant form of As found in anenvironment, the As(III) form can still occur dueto seasonal changes in water and microbialconditions(Webster & Nordstrom 2003).2.6.2 Transport and removal from the water columnPart of the As released into a river from ageothermal system will be incorporated into thebiochemical cycle, interacting with plants, biota,suspended material and bed sediments.A strong association between As and Fe-oxides inriver and lake sediments has been reported. Thisassociation is attributed to the adsorption ofarsenate onto the Fe-oxide coating on sedimentparticles.The life-cycle of aquatic plants may also affect theconcentration of As in the water column. Theuptake of As by aquatic macrophytes (plants) hasbeen reported in rivers contaminated bygeothermal waters. There is often a significant13

Lotta Hammarlund & Juan Piñones TRITA LWR Masters Thesis 09-02degree of As enrichment relative to the watercolumn. High phosphate concentrations caninhibit As uptake by plants, as well as Asadsorption on Fe-oxide surfaces.Finally, As interacts with macrobiota either directlyor via the food chain. As well as the issue oftoxicity, there is the potential problem of Asaccumulation in animal flesh (Webster &Nordstrom 2003).2.7 GroundwaterEven though As-rich groundwaters are common,there are relatively few examples where the As isclearly related to geothermal activity. Geothermalfluids are themselves effectively deeply circulatinggroundwaters. The thermal attractive force to thesurface limits the spreading of these As-rich fluidsat depth. However, at or near the surfacecontamination of shallow aquifer systems can anddoes occur.Groundwater contamination is a concern forgeothermal developers because there are severalpotential pathways for aquifer contamination(Webster & Nordstrom 2003). These include:• Unintentional re-injection of spentgeothermal waste water into an aquifer.This will normally occure only if theintegrity of the casing around a well isbreached.• Seepage from poorly-lined or unlined,holding ponds for the retention ofgeothermal fluids and from pipelines.• Burial of As-rich sludge from wastetreatment process or from drains andpipes.14

Arsenic in Geothermal Waters of Costa Rica___________________________________________________________________________________3. MATERIALS AND METHODS3.1 Field investigationsFieldwork was carried out during September 2005.It consisted of field measurements and watersampling. Totally 50 sample were collected at 49different places in the surroundings of theMiravalles and the Rincon de la Vieja volcanoareas. Water from three types of reservoirs wascollected e.g. geothermal well fluids (GTW),thermal springs (TS) and cold surface waters (CS)(Table 2).3.1.1 Field location and sampling3.1.1.1 MiravallesTwenty eight 28 samples were collected aroundthe 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 arepumping up the water from the geothermal well ata depth varying between 960-1998 meters of thegeothermal field of Miravalles (Figure 6).11 samples (Termal Guayabal, Sitio 13, Corralito,Termal union, U1, Hornillas Miravalles, Colegiofortuna, San bernardo 1, San bernardo 2, Salitralbagaces and Panteon) were collected from thermalsprings and 4 samples (Toma guayabo, Toma casamaquinas, Toma fortuna, and Toma colonia) werecollected from cold surface springs (Figure 7).3.1.1.2 Rincon de la ViejaIn the Rincon de la Vieja volcano area, 21 sampleswere collected. Among these, 4 samples (PGP-01,PGP-03, PGP-04 and PGB-01) were extracted bypump equipment directly from the geothermal wellat a depth varying between 1414-2595 meters ofthe geothermal field of Pailas-Borinquen (Figure8). 16 samples were collected from thermal springs(Figure 9) among which 5 were extracted atHornillas hotel Borinquen, Hornillas Parque,Pailas agua, Santa Maria Caliente and Azufrales.The remaining samples were collected from 11 hotwater springs in this area from Rio Salitral, RioTizate1, Rio Tizate 2, Salitral las lilas 1, Salitral laslilas 2, Salitral las lilas 1+2, Pedernal, NacientesNieve Arraya1 and Nacientes Nieve Arraya 2. 1sample was collected from the cold spring ofHotel Guachipelin. Figure 10 show an overviewover Rincon de la Vieja area and Figure 11 showhow the sampling at Rincon de la Vieja was carriedout.3.1.2 Collection of field dataThe following data was collected/measured in thefield:• GPS coordinates, information given fromICE• Well depth, information given from ICE• Temperature, using a portable EcoScan pH6 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 6meter (-1000-1000mV, +/- 2 mV))• Dissolved oxygen, sing a CyberScan DO100• -Conductivity, using a portable EcoScan pH6 meter• Field estimation of total As concentration,using a Hach Field test kit (range: 10-500µg/l).The equipments were checked and calibratedbefore the field measurements. The pH-meter wascalibrated once a day using buffer solutions andthe redox electrode was checked with Zobell’ssolution several times during the investigationperiod. Field measurements of redox potential(Eh) are known to be problematic withquestionable reliability (Appelo & Postma 1993).Water samples were taken from each location andbrought back to the laboratory of the Departmentof Land and Water Resources Engineering, RoyalInstitute of Technology Stockholm for furtheranalyses. A set of four bottles was taken, oneplastic bottle of 50 ml volume, one 50 ml bottlewith dark glass and two plastic bottles of 22 ml(see below).15

Lotta Hammarlund & Juan Piñones TRITA LWR Masters Thesis 09-02Figure 6: Map pf Miravalles geothermal field with sampled geothermal wells.16

Arsenic in Geothermal Waters of Costa Rica___________________________________________________________________________________Figure 7: Map of spring locations around Miravalles17

Lotta Hammarlund & Juan Piñones TRITA LWR Masters Thesis 09-02Figure 7:Figure 8: Map of Las Pailas-Borinquien geothermal fields and the sampled geothermal wells.18

Arsenic in Geothermal Waters of Costa Rica___________________________________________________________________________________Figure 9: Map of spring locations around Rincon de la Vieja.Figure 10: Ooverview map Rincon de la Vieja area.19

Lotta Hammarlund & Juan Piñones TRITA LWR Masters Thesis 09-02Figure 11: Sampling at Las-Pailas- Borinquien (1-3) and Miravalles geothermal fields (4). 1) Construction of pump equipment 2) Winchused to lower and extract sample equipment into boreholes 3) Cooling ponds 4) Releasing the gas before taking sample at Miravallesgeothermal field.20

Arsenic in Geothermal Waters of Costa Rica___________________________________________________________________________________Table 2: Overview of the samples collected from Miravalles and Rincon de la Vieja geothermal sitesSample nr Sample ID Location Sample source WL84 WL84 X Y Elevation Well_Depth Max Reservoir TempW N Lambert North Lambert East m(asl) m °C1 PGM-11 Miravalles Geothermal well 85 10´55.774" 10 43´6.000" 299,780 407,149 719 1450 2422 PGM-14 Miravalles Geothermal well x x x x 702 1396 See graph3 PGM-62 Miravalles Geothermal well 85 11´15.855" 10 43´7.997" 300,577 406,791 692 1758 2384 PGM-31 Miravalles Geothermal well 85 11´25.249" 10 42´37.539 299,843 406,539 643 1724 2345 PGM-08 Miravalles Geothermal well 85 11´44.568" 10 42´25.736" 298,908 406,251 809 1200 2326 PGM-45 Miravalles Geothermal well 85 11´54.829" 10 41´59.767" 298,547 405,663 593 960 2357 PGM-43 Miravalles Geothermal well 85 12´4.878" 10 42´3.971" 297,750 405,350 578 953 2298 PGM-12 Miravalles Geothermal well 85 11´43.921" 10 41´18.524" 297,880 405,044 516 1596 2279 PGM-20 Miravalles Geothermal well 85 11´50.390" 10 41´25.179" 296,482 405,677 517 1700 23110 PGM-46 Miravalles Geothermal well 85 11´43.145" 10 41´47.299" 296,687 405,481 584 1198 23311 PGM-29 Miravalles Geothermal well 85 10´30.171" 10 40´40.117" 297,366 405,703 473 1380 23012 PGM-07 Miravalles Geothermal well 85 10´45.090" 10 42´58.249" 295,296 407,915 738 1998 23913 PGM-19 (öppen) Miravalles Geothermal well 85 11´23.373" 10 41´51.316" 299,541 407,473 608 1260 23214 Termal Guayabal Miravalles Thermal spring 85 11´37.173" 10 45´16.444" 303,791 405,902 610 x x15 Sitio 13 Miravalles Thermal spring 85 11´52.648" 10 44´18.953" 302,026 405,427 575 x x16 Corralito Miravalles Thermal spring 85 11´38.53" 10 44´20.46" 590 x x17 Termal union Miravalles Thermal spring 85 11´46.396" 10 43´8.110" 299,849 405,611 550 x x18 U1 Miravalles Thermal spring 85 11´37.876 10 42´33.761" 298,793 405,867 625 x x19 Hornillas (Miravalles) Miravalles Thermal spring 85 10´45.83" 10 42´51.05" 754 x x20 Colegio Fortuna Miravalles Thermal spring 85 12´35.667" 10 40´25.262" 294,850 404,100 380 x x21 San Bernardo 1 Miravalles Thermal spring 85 12´10.239" 10 35´56.798" 286,600 404,850 335 x x22 San Bernardo 2 Miravalles Thermal spring x x x x 335 x x23 Salitral Bagaces Miravalles Thermal spring 85 14´24.114" 10 35´49.765" 286,396 400,780 165 x x24 Panteon Miravalles Thermal spring 85 12´57.37" 10 42´16.52" 510 x x25 Toma Guayabo Miravalles Cold spring 85 11´27.084" 10 43´34.234" 300,650 406,200 610 x x26 Toma casa maquinas Miravalles Cold spring 85 10´50.520" 10 43´46.701 301,030 407,312 780 x x27 Toma Fortuna Miravalles Cold spring 85 11´40.640" 10 40´46.309" 295,492 405,774 480 x x28 Toma Colonia Miravalles Cold spring 85 11´14.620" 10 40´37.395" 295,216 406,564 455 x x29 PGP-01 Rincón de la Vieja Geothermal well 85 21´42.980 10 45´51.235" x x 659 1418 24030 PGP-03 Rincón de la Vieja Geothermal well 85 21´6.635" 10 46´19.767 305,788 388,607 757 1772 24431 PGP-04 Rincón de la Vieja Geothermal well 85 21´32.571 10 45´29.037" 304,232 387,814 631 1418 22932 PGB-01 Rincón de la Vieja Geothermal well 85 24´30.555 10 48´24.866 309,652 382,425 699 2595 27633 Hornillas hotel Borinquen Rincón de la Vieja Thermal spring 85 24´54.507" 10 48´49.132" 310,400 381,700 535 x x34 Hornillas parque Rincón de la Vieja Thermal spring 85 22´24.0" 10 47´3.8" 307,150 386,260 760 x x35 Pailas agua Rincón de la Vieja Thermal spring 85 20´43.8" 10 46´21.9" 305,850 389,300 763 x x36 Santa Maria Caliente Rincón de la Vieja Thermal spring 85 19´30.0" 10 45´38.6" 304,460 391,540 715 x x37 Azufrales Rincón de la Vieja Thermal spring 85 19´39.5" 10 45´33.2" 304,350 391,250 725 x x38 Rio salitral (Hotel buena vista) Rincón de la Vieja Thermal spring 85 24´24.9" 10 48´41.1" 310,150 382,600 630 x x39 Rio tizate 1 Rincón de la Vieja Thermal spring 85 26´14.7" 10 46´28.9" 306,100 379,250 330 x x40 Rio tizate 2 Rincón de la Vieja Thermal spring 85 26´26.175" 10 46´25.602" 306,000 378,900 300 x x41 Salitral las lilas 1 Rincón de la Vieja Thermal spring 85 28´11.9" 10 48´19.2" 309,500 375,700 245 x x42 Salitral las lilas 2 Rincón de la Vieja Thermal spring 85 28´18.5" 10 48´12.6" 309,300 375,500 250 x x43 Salitral las lilas 1+2 Rincón de la Vieja Thermal river 85 28´18.5" 10 48´12.6" 309,300 375,500 250 x x44 Pedernal (NE de C Fortuna) Rincón de la Vieja Thermal spring 85 27´26.0" 10 49´1.6" 310,800 377,100 315 x x45 Naciente Nieves Araya 2 Rincón de la Vieja Thermal spring 85 19´39.4" 10 53´52.9" 319,700 391,300 435 x x46 Naciente Nieves Araya 1 Rincón de la Vieja Thermal spring 85 19´39.4" 10 53´52.9" 319,700 391,300 435 x x47 El volcancito Rincón de la Vieja Thermal spring 85 19´29.7" 10 54´35.2" 321,000 391,600 410 x x48 Las Avestruces Rincón de la Vieja Thermal spring x x x x 400 x x49 El albergue agroecologico Rincón de la Vieja Thermal spring x x x x 400 x x50 Hotel Guachipelin Rincón de la Vieja Cold spring 85 21´46.2" 10 45´42.6" 304,650 387,400 595 x xWater samples collected from each well involved:• 50 ml filtered samples for alkalinity andmajor anion analysis.• 22 ml filtered acidified samples (14MHNO 3 ), for major cation and trace elementsanalysis.• 22 ml acidified samples (14M HNO 3 ), keptfrozen when possible, for DOC analysis.• 50 ml filtered through a DisposableCartridge TM filter that removes species ofAs(V) and then acidified in a dark glassbottle, for As(III) analysis.The dark glass bottle was used for storing thewater meant for As analyses. Before bottling thewater was filtered through a cartridge filter withthe pore size 0.45 µm. to remove most of thecolloidal material and micro organisms that caneffect the dissolved As(ΙΙΙ/ V) ratio.Some of the samples were also acidified with 5 mlHNO 3per liter sample. Bottles were closedavoiding air bubbles. The samples were put in acooled box during transportation and storedrefrigerated until the analysis was completed inStockholm during October, 2006.3.2 Laboratory investigationsThe water was analyzed for major ions, total Asand other trace elements and further for As(III),DOC and alkalinity. Below follows a descriptionof the methods used for each of the analysis.3.2.1 AlkalinityThe alkalinity was measured following thestandard SS-EN ISO 9963-2 (SIS 1996).21

Lotta Hammarlund & Juan Piñones TRITA LWR Masters Thesis 09-023.2.2 Major ionsAnions were analyzed at the laboratory of theDepartment of Land and Water ResourceEngineering at KTH on a Dionex DX-120 IonChromatograph. In order to save the sensitiveequipment samples were diluted between 1 and100 times prior to analysis yielding lowerconcentrations of ions. Dilution was made withguidance from the field-measured conductivity.For anion analysis untreated samples and anIonPac AS9-SC coloumn was used. Thechromatograph was run several times until almostall results were within the calibrated interval set bythe standard solutions used for verifying theresults.Cations were analysed with ICP-OES (OpticalEmission Spectroscopy) from Varian Vista Ax..These analyses were performed at the StockholmUniversity.3.2.3 Trace elementsAcidified samples were analyzed at the StockholmUniversity with an ICP-OES (Optical EmissionSpectroscopy) from Varian Vista Ax.3.2.4 Arsenic (III)Samples used were directly filtered in the fieldthrough Disposable Cartridge filters that removesAs(V) and then acidified. Analyzes was performedat the Stockholm University using the sameinstrument as for trace elements, see above.3.2.5 DOCDissolved organic carbon was analyzed at thelaboratory of the Department of Land and WaterResource Engineering at KTH on a TOC-5000SHIMADZU Total Organic Carbon Analyzer asNPOC (Non-Purgeable Organic Carbon).3.2 Treatment of analytical dataAquaChem 4.0 (Waterloo Hydrogeologic, 1997)was used to determine water types and create piperdiagrams of major ions in the sampled waters.Scatter and Box plots was also carried out.22

Arsenic in Geothermal Waters of Costa Rica___________________________________________________________________________________4. RESULTSResults of field parameters and the concentrationsof major ions, selected minor ions, trace elementsand DOC in geothermal wells, thermal springs andcold springs from the study areas of Rincon de laVieja and Miravalles are summarized in Tables 3-9.4.1 Geothermal wells4.1.1 Field measured parametersA summary of the field measured parameters forwater samples are presented in Table 3. Thetemperature of the geothermal well fluids had tobe cold down before measuring could be carriedout. The measurements were taken at atemperature of 9-90 °C at Miravalles and 24-29 °Cat Rincon de la Vieja. The reservoir temperature atMiravalles and Rincon de la Vieja varies between230-255°C and 229-276°C respectively.The pH in the geothermal wells is neutral andvalues ranges between 7-7.8 in samples collected atthe wellhead at Miravalles with three exceptions ofPGM-29,PGM-07 and PGM-19 which are weaklyalkaline (pH 4.8-5.7). The samples collected at thebottom of the wells in Rincon de la vieja (PGP-01at 1414 m, PGP-03 at 1773 m, PGP-04 at 1419 mand PGB-01 at 2595m depths) were neutral toweakly alkaline (pH 5.72-7.77). Field measuredredox potential (Eh) in the geothermal well fluidsvaries between 99-431 mV at Miravalles and 160-366 mV at Rincon de la Vieja.Electrical conductivity (EC) is high, rangesbetween 8.940-15.150 mS/cm (with one exception130.310 mS/cm) at Miravalles and 8.00-12.85mS/cm at Rincon de la Vieja.4.1.2 Major ionsVariation of major ion concentration in samples atMiravalles (MV) and Rincon de la Vieja (RV) ispresented Table 4 and also plotted in Box andWhisker diagram in Figure 12.Cl - and Na + are the dominant ions in bothgeothermal waters. Na + is by far the mostpredominant 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), Ca 2+ (MV:23–109 mg/L, RV: 23–109 mg/L) andMg 2+( MV:0.04-7 mg/L, RV: 0.26-0.78 mg/L).Table 3: Summary of the field measured parameters forwater samples at the study areasS.No. Sample ID Date T pH Eh EC As (Field)°C mV µS/cm µg/lMiravalles1 PGM-11 sep-05 90,7 7,72 99 1300 >5002 PGM-14 sep-05 15,9 7,74 212 1290 >5003 PGM-62 sep-05 17,1 7,2 200 1236 >5004 PGM-31 sep-05 31,8 7,81 236 1238 >5005 PGM-08 sep-05 34,9 7,53 292 1271 >5006 PGM-45 sep-05 10,6 7,16 395 1370 >5007 PGM-43 sep-05 29,6 7,57 262 13031 >5008 PGM-12 sep-05 9,8 7,3 333 1515 >5009 PGM-20 sep-05 11,6 7,71 308 1472 >50010 PGM-46 sep-05 9,4 7,41 289 1492 >50011 PGM-29 sep-05 25,2 5,74 339 894 >50012 PGM-07 sep-05 12,3 4,91 392 1225 >50013 PGM-19 (Open) sep-05 13,3 4,8 431 960 >50014 Termal Guayabal sep-05 62,9 2,28 683 484 >50015 Sitio 13 sep-05 33,7 3,25 192 255 25-5016 Corralito sep-05 35,6 2,72 635 257 50-10017 Termal union sep-05 38,1 5,9 456 40

Lotta Hammarlund & Juan Piñones TRITA LWR Masters Thesis 09-02Figure 12: Major ion chemistry of the geothermal waters plotted in Box and Whisker diagram from the productions wells at Miravalles (tothe 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 Ca 2+ K + Mg 2+ Na + Cl - HCO3 - SO4 2- NO3 - Ion Balance Water typemg/l mg/l mg/l mg/l mg/l mg/l mg/ mg/l %Miravalles1 PGM-11 54,8 307 0,11 1855 3843 93,8 61,6 1,6 -9,9 Na-Cl-B2 PGM-14 69,1 305 0,09 1734 3437 48,6 46,7 3,6 -6,5 Na-Cl-B3 PGM-62 60,9 298 0,07 1760 3828 42,8 140,5 6,5 -12,3 Na-Cl-B4 PGM-31 74,8 299 0,13 1807 4117 58,7 144,5 6,0 -14,4 Na-Cl-B5 PGM-08 71,8 313 0,06 1825 4333 30,5 149 15,5 -16,2 Na-Cl-B6 PGM-45 82,4 363 0,09 2002 4822 26,9 155,5 24 -16,5 Na-Cl-B7 PGM-43 85,0 331 0,05 1988 4543 28,3 142,5 10 -14,1 Na-Cl-B8 PGM-12 109,2 343 0,19 2179 4947 49,1 254 23 -14,5 Na-Cl-B9 PGM-20 103,6 338 0,11 2107 8935 57,6 297 34 -42,2 Na-Cl-B10 PGM-46 94,7 313 0,12 1983 4316 65,9 199 58,5 -12,8 Na-Cl-B11 PGM-29 66,7 193 1,31 1290 2865 273,5 143 9,0 -15,7 Na-Cl-B12 PGM-07 29,7 309 3,63 1769 3421 0,0 237 32,5 -8,1 Na-Cl-B13 PGM-19 (open) 23,1 302 7,32 1656 2974 0,0 329,5 8,5 -5,4 Na-Cl-BRincon de la Vieja29 PGP-01 60,1 245 0,35 1125 2456 120,7 92,75 3,8 -11,4 Na-Cl-B30 PGP-03 99,3 457 0,68 1810 4444 41,8 144,5 14,5 -15,1 Na-Cl-B31 PGP-04 101,0 331 0,27 1566 3635 58,7 169 18,5 -13,6 Na-Cl-B32 PGB-01 94,8 274 0,78 1438 3148 121,0 135,5 10,0 -11,5 Na-Cl-BTable 5: Selected trace element chemistry of the geothermal waters at Miravalles and Rincon de la Vieja sites.Sample nr Sample ID As (tot) As(III) B Al Fe Mn Ba Li Si DOCmg/l mg/l mg/l mg/l mg/l mg/l mg/l mg/l mg/l mg/lMiravalles1 PGM-11 25,58 6,79 49,3 0,31 0,003 0,05 0,44 8,77 305 1,662 PGM-14 24,46 6,84 47,5 0,32 0,008 0,02 0,26 bdl 305 1,543 PGM-62 25,13 6,90 50,3 0,27 0,018 0,02 0,29 bdl 296 1,094 PGM-31 25,70 6,82 51,8 0,26 0,043 0,05 0,60 8,17 275 1,575 PGM-08 25,58 7,13 51,4 0,30 0,015 0,02 0,30 bdl 280 1,836 PGM-45 27,42 7,69 57,1 0,28 0,213 0,03 0,31 bdl 275 2,917 PGM-43 27,46 7,31 57,5 0,26 0,039 0,03 0,26 bdl 285 0,868 PGM-12 29,13 7,60 60,1 0,24 0,009 0,04 0,77 7,55 241 1,169 PGM-20 29,07 7,58 59,7 0,26 0,069 0,04 0,69 bdl 258 1,2810 PGM-46 26,92 7,55 56,7 0,26 0,044 0,05 0,58 7,50 251 1,1511 PGM-29 11,86 3,63 33,9 0,04 6,416 1,25 0,16 6,75 199 3,2412 PGM-07 26,34 7,58 54,2 0,19 0,795 0,58 0,23 8,24 314 0,4113 PGM-19 (open) 25,89 7,31 49,2 0,07 1,479 1,73 0,18 7,06 271 0,76Rincon de la Vieja29 PGP-01 7,81 2,34 20,6 0,04 0,560 0,23 0,04 5,90 176 4,2230 PGP-03 13,00 3,45 33,1 0,03 0,891 0,45 0,06 bdl 203 3,7331 PGP-04 12,80 3,57 30,1 0,05 0,891 0,33 0,06 bdl 206 4,0832 PGB-01 5,99 1,63 27,1 0,04 0,054 0,03 0,03 bdl 170 4,3624

Arsenic in Geothermal Waters of Costa Rica___________________________________________________________________________________4.1.3 Trace elementsA summary of trace elements for the geothermalwells are presented in Table 5. Extremely highconcentration 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 thesegeothermal waters.The concentrations of Fe and Mn were low in allgeothermal wells at both Miravalles and Rinconde la Vieja sites with concentration ranges between0.003-0.89 mg/L and 0.02-0.58 mg/L respectively.However, in the wells PGM 29 and PGM-19, theconcentrations of Fe was high with respectivevalues of 6.4 mg/L and 1.5 mg/L while thecorresponding Mn levels of 1.25 mg/L and 1.73mg/L.Some of the geothermal well fluids have high Liconcentrations. Seven sampled wells at Miravalleshave Li concentrations ranging between 6.75-8.77mg/L and one of the sampled geothermal fluids inRincon de la Vieja revealed Li concentration of 5.9mg/L.All of the sampled geothermal wells have high Siconcentrations (MV: 198-313 mg/L and RV: 169-205 mg/L).In general, the concentrations of DOC and HCO 3-is quite low in the geothermal well fluids ofMiravalles area with one exception PGM-29 whichhas high DOC and HCO 3- values. At Rincon de laVieja the DOC is high while the HCO 3- is low.4.1.4 Correlation between various chemicalparametersFigure 13-15 show Bivariate plots of thegeothermal wells (GTW) showing relationshipbetween the concentrations of:13. Total As with a HCO 3 , b SO4, c Fe, d B14. DOC and a As, b HCO 315. SO 4 and a Ca+Mg, b CaArsenic has high positiv correlation with HCO 3-(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 B25

Lotta Hammarlund & Juan Piñones TRITA LWR Masters Thesis 09-02Figure 14: Bivariate plots of the geothermal waters showing relationship between the DOC concentration and a As, b HCO3Figure 15: Bivariate plots of the geothermal waters showing relationship between the SO4 concentration and a Ca+Mg, b CaHigh correlation are also seen between As andDOC (MV: 0.68 and RDLV: 0.74) and betweenHCO 3- 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 doessulphate with Ca 2+ (MV: 0.74 and RDLV: 0.88).4.2 Thermal springs -Neutral and Acid4.2.1 Field measured parametersThe temperature ranged between 33.7-88.9°C inMiravalles and between 26-91˚C at Rincon de lavieja. A summary of the field measured parametersfor the thermal springs are presented in Table 3.The thermal springs have pH interval rangingbetween 1.97-7.84. Four of the sampled thermalwater in Miravalles and four of the sampledthermal waters in Rincon de la vieja were acidic(pH 1.97-3.25 resp 2.17-3.98) while the rest had aneutral or weakly alkaline pH (5.63-7.25 resp 5.00-7.63). The two most acidic springs at Miravalleshave the highest measured temperatures (HornillasMiravalles: pH 1.97 and temperature 88.9˚C andThermal Guayabal: pH 2.28 and temperature62.9˚C) and the same states for Rincon de la vieja(Pailas aguas: pH 2.17 and temperature 87.3˚C andHornillas parque: pH 3.75 and temperature91.2˚C).The redox potential (Eh) were between 192-683mV at Miravalles and between 9-564 mV atRincon de la vieja. The electric conductivity (EC)at Miravalles was between 0.26-4.84 mS/cm and atRincon de la vieja 0.143-9.58 mS/cm.4.2.2 Major ionsThe thermal springs can be divided into two maintypes:Acid thermal springs (TSA) andNeutral thermal springs (TSN).The major ion characteristics of the thremalsprings at the two study sites are presented inTable 6. Variation of major ion concentration insamples collected at the thermal springs atMiravalles and Rincon de la Vieja are presented inBox and Whisker plots in Figure 16 for Acidthermal springs and in Figure 17 for Neutralthermal springs. Figure 18 show Piper diagramsfor Neutral thermal springs (TSN) at Miravallesand Rincon de la Vieja.26

Arsenic in Geothermal Waters of Costa Rica___________________________________________________________________________________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 atMiravalles (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

Lotta Hammarlund & Juan Piñones TRITA LWR Masters Thesis 09-02The acidic thermal springs (TSA) had highSO 42- content (MV: 493-2699 mg/L and RV: 35-2275 mg/L). The Cl - content is low (MV: 21-642mg/L and RV: 2-434 mg/) as well as the NO 3-(MV: 1.4 -16 mg/L and RV: 1-5 mg/L) and theHCO 3- (MV: 0 mg/L and RV: 0 mg/L) content.Ca 2+ (MV: 19-145 mg/L and RV: 15-164 mg/L)predominates over Mg 2+ (MV: 8-91 mg/L and RV:6-26 mg/L), Na + (MV: 15-73 mg/L and RV: 6-79mg/L) and K + ( MV: 6-13 mg/L and RV: 2-14mg/L ) in the acid springs reflecting SO 4 -watertype:(MV: 62-146 mg/L and RV: 64-188 mg/L) andMn + (MV: 0.32-6.1 mg/L and RV: 0.23-4.23mg/L). The DOC content is low (MV: 0.4-1.23and RV: 0.9-1.39 mg/L with one exception ofPailas aguas 9.62 mg/L).The neutral thermal springs (TSN) are rich inHCO 3- (MV: 0-608 mg/L and RV: 0-535 mg/L),Cl - (6-2914 mg/L) and SO 42- (MV: 6-237 mg/Land RV: 23-812 mg/L). Mg 2+ concentrations arelow (MV: 2-19 mg/L and RV: 5-301 mg/L) as wellas K + (MV: 8-22 mg/L and RV: 2-186mg/L). Thewater type can be divided into HCO 3 -Si-SO 4 , Cl-Na and Cl-SO4.Two of the samples (Salitral las lilas 1 and 2) inRincon de la Vieja differs from the rest with areally high Cl - (2667 resp 2914 mg/L) and Na +(1242 resp 1250 mg/L) concentration.Concentrations of Ca 2+ (145 resp 150 mg/L), K +(186 resp 180 mg/L), HCO 3- (250 resp 310 mg/L),SO 42- (210 resp 212 mg/L) are quite low.One hundred meters downstream the river fromSalitral las lilas 1 and 2 is Salitral las lilas 1+2sampled. The concentration of Cl - (76 mg/L) andNa + (46 mg/L) are much lower in this sample incomparison with Salitra las lilas 1 and 2. Ca 2+ (15mg/L), K + (6 mg/L), HCO 3- (47 mg/L), SO 42- (25mg/L) and Mg 2+ (5 mg/L) are also much lower.The water is designated as Cl-HCO 3 -Na-Si type.4.2.3 Trace elementsA summary of the trace elements for the thermalsprings are presented in Table 7.Figure 19: Box and Whisker plots for Acidic thermal watersat Miravalles for As, B and Fe.The neutral springs (TSN) have a low DOC. Feand Mn are both low while the As concentration(0.005-10.9 mg/L) exceeds the WHO limit in 13of the 20 samples. High boron concentrationsoccur in all the samples (0.65-25.7 mg/L).Two of the samples (Salitral las lilas 1 and 2) inRincon de la Vieja differs from the rest with reallyhigh As (10.9 resp 10.6 mg/L) and boron (25.7resp 25.6 mg/L) concentrations. The Mgconcentration (13 resp 14 mg/L) is also muchhigher than in the other samples.One hundred meters downstream the river fromSalitral las lilas 1 and 2 is Salitral las lilas 1+2sampled. The concentrations of B (3.09 mg/L)and As (0.13 mg/L) is still high here but incomparison with Salitral las lilas 1 and 2 theconcentration has to be considered low.The acidic thermal springs (TSA) have a highcontent of A, B and Fe as showed in the Box andWhisker plots in Figure 19.Other trace elements in these waters include S 2-(MV: 202-814 mg/L and RV: 57-598 mg/L), Al 3+(MV: 20-453 mg/L and RV: 0-141 mg/L), Si 4+28

Arsenic in Geothermal Waters of Costa Rica___________________________________________________________________________________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 Ca 2+ K + Mg 2+ Na + Cl - HCO3 - SO4 2- NO3 - Ion Balance Water typemg/l mg/l mg/l mg/l mg/l mg/l mg/ mg/l %Miravalles14 Termal Guayabal 104 7 25 65 642 bdl 2700 16 12.7 Al-SO4-Cl15 Sitio 13 312 14 92 74 148 bdl 1613 1 10.8 Ca-Mg-SO416 Corralito 146 9 22 41 177 bdl 1303 2 14.3 Al-Ca-SO417 Termal union 36 9 13 26 13 190 40 1 0.4 Na-Ca-Mg-HCO318 U1 60 19 16 55 19 318 66 1 6.2 Na-Ca-HCO319 Hornillas (Miravalles) 20 8 8 16 22 bdl 494 5 43.8 Al-SO420 Colegio Fortuna 46 14 20 45 16 386 58 1 12.7 Ca-Na-Mg-HCO321 San Bernardo 1 43 20 17 91 100 395 27 1 19.7 Na-Ca-HCO3-Cl-B22 San Bernardo 2 42 21 17 94 62 396 25 18 15.3 Na-Ca-HCO3-B23 Salitral Bagaces 9 9 2 39 17 140 6 bdl 23.2 Na-HCO3-B24 Panteon 100 22 76 74 51 608 238 4 4.8 Mg-Ca-Na-HCO3-SO4Rincon de la Vieja33 Hornillas hotel Borinquen 14 6 6 10 10 33 42 bdl 21.5 Ca-Mg-B-HCO3-SO434 Hornillas parque 15 5 7 14 3 bdl 36 2 -8.5 Na-Ca-Mg-B-SO435 Pailas agua 15 3 9 7 2 bdl 2275 1 46.2 Al-SO436 Santa Maria Caliente 22 4 9 17 7 35 96 0 4.5 Ca-Na-Mg-SO4-HCO337 Azufrales 29 9 11 30 23 bdl 294 3 Na-Ca-SO438 Rio salitral (buena vista) 26 5 8 26 11 141 24 2 Na-Ca-HCO3-SO439 Rio tizate 1 23 7 13 34 13 174 52 1 7.1 Na-Ca-Mg-HCO3-SO440 Rio tizate 2 23 6 13 34 13 182 53 2 9.3 Na-Ca-Mg-HCO3-SO441 Salitral las lilas 1 145 187 14 1243 2667 251 210 13 22.9 Na-Cl-B42 Salitral las lilas 2 150 180 15 1251 2914 310 213 9 25.8 Na-Cl-B43 Salitral las lilas 1+2 15 6 5 46 77 48 26 3 32.3 Na-B-Cl44 Pedernal (NE de C Fortuna) 11 3 6 8 6 69 8 2 31.5 Ca-Mg-B-HCO345 Naciente Nieves Araya 2 325 130 280 307 1200 535 813 9 4.9 Mg-Ca-Na-Cl-SO446 Naciente Nieves Araya 1 340 147 294 323 1326 510 894 21 6.2 Mg-Ca-Na-Cl-SO447 El volcancito 354 144 301 334 973 531 667 19 -7.4 Mg-Ca-Na-Cl-SO448 Las Avestruces 63 14 14 58 92 37 178 5 28.6 Ca-Na-B-SO4-Cl49 El albergue agroecologico 164 14 26 79 435 bdl 353 5 15.1 Ca-Cl-SO4Table7: Selected trace element chemistry of the thermal waters at Miravalles and Rincon de la Vieja sitesSample nr Sample ID As (tot) As(III) B Al Fe Mn Ba Li Si DOCmg/l mg/l mg/l mg/l mg/l mg/l mg/l mg/l mg/l mg/lMiravalles14 Termal Guayabal 4.564 1.531 2.730 453.646 76.738 1.775 0.017 0.172 133.6 0.7915 Sitio 13 0.224 0.193 3.029 56.809 16.817 6.104 0.015 0.060 62.1 0.6316 Corralito 0.099 0.158 2.098 133.971 28.176 2.696 0.008 0.048 85.8 0.4017 Termal union 0.008 0.012 0.994 0.007 0.001 0.004 0.063 0.011 77.6 0.5918 U1 0.012 0.013 1.557 0.013 0.006 0.002 0.013 0.094 104.0 0.8019 Hornillas (Miravalles) 0.017 0.018 1.833 20.449 16.802 0.325 0.051 0.008 146.6 1.2320 Colegio Fortuna 0.010 0.011 0.925 0.010 0.003 0.802 0.056 0.016 74.0 0.9021 San Bernardo 1 0.271 0.017 2.355 0.010 0.002 0.000 0.087 0.379 80.7 0.9422 San Bernardo 2 0.281 0.024 2.432 0.010 0.003 0.000 0.085 0.392 82.2 0.8723 Salitral Bagaces 0.188 0.012 1.145 0.039 0.017 0.019 0.084 0.110 64.4 1.4924 Panteon 0.005 0.012 0.923 0.015 2.850 0.334 0.056 0.036 59.2 1.74Rincon de la Vieja33 Hornillas hotel Borinquen 0.008 0.014 1.709 0.062 0.681 0.084 0.050 0.005 44.5 4.9634 Hornillas parque 0.005 0.009 1.534 2.682 5.859 0.269 0.075 0.015 84.0 1.3935 Pailas agua 0.024 0.026 0.729 141.387 194.899 0.424 0.055 0.005 188.9 9.6236 Santa Maria Caliente 0.015 0.008 0.657 0.181 0.039 0.119 0.036 0.022 30.9 1.4037 Azufrales 0.007 0.010 0.595 0.860 0.120 0.226 0.032 0.011 64.1 0.9138 Rio salitral (buena vista) 0.006 0.007 1.670 0.011 bdl 0.000 0.030 0.015 51.1 2.5039 Rio tizate 1 0.007 0.008 0.755 0.015 0.002 0.000 0.041 0.031 70.2 1.1640 Rio tizate 2 0.007 0.015 0.722 0.012 0.004 0.001 0.041 0.031 70.9 0.3741 Salitral las lilas 1 10.853 1.629 25.724 0.011 0.030 0.747 0.357 6.440 54.7 0.6142 Salitral las lilas 2 10.633 3.256 25.634 0.011 0.008 0.838 0.338 6.356 52.5 0.7443 Salitral las lilas 1+2 0.132 3.044 3.092 0.065 0.050 0.025 0.047 0.245 39.4 1.4644 Pedernal (NE de C Fortuna) 0.005 0.008 1.907 0.006 0.002 0.000 0.034 bdl 59.2 1.3545 Naciente Nieves Araya 2 0.053 0.033 2.514 0.011 4.396 1.485 0.047 0.519 93.8 2.5346 Naciente Nieves Araya 1 0.053 0.034 2.758 0.012 0.513 1.312 0.049 0.561 97.8 1.2447 El volcancito 0.050 0.033 2.798 0.009 3.496 1.125 0.052 0.598 98.8 0.7148 Las Avestruces 0.017 0.016 6.764 0.073 0.710 1.095 0.036 0.296 63.9 2.6449 El albergue agroecologico 0.009 0.008 1.724 11.238 0.010 4.235 0.040 0.092 82.9 1.2329

Lotta Hammarlund & Juan Piñones TRITA LWR Masters Thesis 09-024.2.4 Correlation between various chemicalparametersBivariate plots the relationship between theconcentrations of total As with B and Fe in theacid thermal springs (TSA) waters are presented inFigure 20. Arsenic has high correlation with B(MV: r=0.96) and low correlation with Fe (MV:r=0.01) at Miravalles while there is no correlationat Rincon de la Vieja.The relationship between SO 42- concentrations ofthe thermal waters with Cl - , Mg 2+ and Ca ispresented in Figure 21. SO 42- has high correlationwith Cl - (MV: r=0.89) while its low with Mg 2+(MV: r=0.08) and Ca 2+ (MV: r=0.05) while there isno correlation at Rincon de la Vieja.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 SO 4 concentration and a) Cl, b) Mg and c) Ca30

Arsenic in Geothermal Waters of Costa Rica___________________________________________________________________________________4.3 Cold surface waters4.3.1 Field measured parametersA summary of the field measured parameters forthe cold springs are presented in Table 3.The cold springs had a temperature range of 14.2-23.6°C at Miravalles while the cold spring atRincon de la vieja had a temperature of 27.1degrees.The pH is almost neutral for the cold springs atMiravalles (6.57-7.18) while it is weakly alkaline atRincon de la Vieja (6.01).Field measured redox potential (Eh) rangebetween 419-439 mV and the Electric conductivity(EC) ranges from 0.141-0.304 mS/cm.4.3.2 Major ionsVariation of major ion concentration in samplescollected at the cold springs at Miravalles (MV)and Rincon de la Vieja (RV) is presented Table 8and also plotted in Box and Whisker diagram inFigure 22.The dominated ions in the cold springs atMiravalles are HCO 3- (38-97 mg/L), SO 42- 22-49mg/L), Si 4+ (38-44 mg/L) followed by Ca 2+ (15-26 mg/L), Na + (7-11 mg/L) and S (6-14 mg/L) inless concentrations.The dominated ions at the cold spring at Rinconde la vieja are Si 4+ (49.8 mg/L), SO 42- (49.1 mg/L)and HCO 3- (41.1 mg/L) followed by Cl-, Ca 2+ ,Na + , Mg 2+ and K + . The sampled cold surfacewaters are generally of HCO 3 -SO 4 - Si or HCO 3 -Sitype.4.3.3 Trace elementsA summary of trace elements for the cold springsare presented in Table 9.Elevated concentrations of As and B occur in thecold springs (0.0052-0.007 mg/L respective 0.7-1.42 mg/L). The DOC concentration of the areasis quite low as well as the Fe and the Mn.Figure 22: Major ion chemistry of the cold surface waters plotted in Box and Whisker diagram from the productions wells at Miravalles (tothe 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 Ca 2+ K + Mg 2+ Na + Cl - HCO3 - SO4 2- NO3 - Ion Balance Water typemg/l mg/l mg/l mg/l mg/l mg/l mg/ mg/l %Miravalles25 Toma Guayabo 21.7 2.6 7.1 8.5 6.5 97.8 22.4 0.9 22.8 Ca-Mg-B-HCO326 Toma casa maquinas 15.9 2.3 4.0 7.3 4.1 38.6 29.0 0.2 -4.2 Na-Ca-Mg-HCO3-SO427 Toma Fortuna 26.9 3.2 7.1 11.4 6.4 86.8 49.3 0.7 19.4 Ca-B-HCO3-SO428 Toma Colonia 25.1 3.0 7.0 10.1 5.8 85.5 45.0 0.9 21.3 Ca-B-HCO3-SO4Rincon de la Vieja50 Hotel Guachipelin 19.9 2.4 7.6 7.6 10.4 41.1 49.1 0.1 22.6 Ca-Mg-B-SO4-HCO331

Lotta Hammarlund & Juan Piñones TRITA LWR Masters Thesis 09-02Table9: Selected trace element chemistry of the cold surface waters at Miravalles and Rincon de la Vieja sitesSample nr Sample ID As (tot) As(III) B Al Fe Mn Ba Li Si DOCmg/l mg/l mg/l mg/l mg/l mg/l mg/l mg/l mg/l mg/lMiravalles25 Toma Guayabo 0.0052 0.0075 1.42 0.0101 0.0075 0.0003 0.0336 bdl 44.0 1.1626 Toma casa maquinas 0.0064 0.0000 0.716 0.0092 0.0035 0.0002 0.0132 bdl 40.3 1.2627 Toma Fortuna 0.0052 0.0113 1.2497 0.04 0.012 0.0011 0.0181 0.0031 39.2 1.0228 Toma Colonia 0.0065 0.0102 1.1014 0.0178 0.0036 0.0003 0.0149 bdl 38.3 0.74Rincon de la Vieja50 Hotel Guachipelin 0.0052 0.0086 1.0709 0.064 0.0019 0.0067 0.0558 bdl 49.8 0.864.4 Anomalous boron concentrationsExtremly high boron (B) concentrations werefound in all the samples. Especially high was theamount of boron in the geothermal well fluids,between 47-60 mg/L in the Miravalles area andbetween 20-33 mg/L in the Rincón de la Viejaarea.High concentrations of boron (ranging between0.66-6.76 mg/L) were found in the hot springswith two exceptions which differed a lot from therest. These two were the springs called Salitral lasLilas 1 and 2 with extremely high boronconcentrations (25.7 mg/L resp. 25.6 mg/L). Thethird sample were collected at the site where thesetwo springs entered a stream about 100 metersdown from where these springs entering the river(called Salitral las Lilas 1+2) which indicated a Bconcentration of 3.1 mg/L. The concentration ofB in the cold springs ranged between 0.72-1.25mg/L similar to a previous study by Nable et al(1997). All the samples exceed the WHO drinkingwater guideline for boron which is 0.5 mg/L.High concentrations of boron may occur naturallyin the soil or in groundwater. It can also be addedto the soil from mining, fertilisers, or irrigationwater. B-rich soils are also of importance thoughthey might cause B toxicity in the field anddecrease crop yields. One typical visible symptomof B toxicity is leaf burn. It can also lead to fruitand bark disorders. Although there are variousmethods of determine the levels of B in soils,these analyses can only provide little more than ageneral risk assessment for B toxicity. There areseveral methods to ameliorating high boron soilsbut to get rid of boron in soil and water isextremely difficult. At present neither soil norplant analysis can be recommended to preciselypredict the growth of plants on high B soils.If B contaminated water sources in Costa Rica areused by humans for irrigation and drinking watersupply are still not known. Neither are theenvironment effects on animals and plants fromboron contaminated water (Nable et al 1997).32

Arsenic in Geothermal Waters of Costa Rica___________________________________________________________________________________5. DISCUSSION5.1 Arsenic concentrations in Miravallesand Rincon de la viejaFindings indicate that high As concentrationsoccur through out the entire study area. 35 of thetotal 50 samples exceed the WHO drinking waterguideline for As, which is 10 μg/L5.1.1 Geothermal wellsThe results show that the highest Asconcentrations occur in the geothermal wells. Thegeothermal wells at Miravalles (mean 25.9 mg/L),show higher concentrations than those at Rinconde la Vieja, (mean 10.3 mg/L). Of particularimportance is that the As concentrations in thegeothermal waters used for power generation areextremely high at both Miravalles and Rincon de laVieja.Power generation requires substantial amounts ofwater to be extracted. During the separation ofsteam and bore water in geothermal powergeneration, Ass is retained in the waste bore water.High As content is one of the main problems inthe disposal of this water (Webster & Nordstrom2003). To prevent contamination of cold watersused for irrigation, re-injection of thermal watersto the reservoir is necessary. This also acts torecharge the aquifers of the thermal waters(Gemici & Tarcan 2002).We have stated that high As concentrations occurin the water used for geothermal energy in CostaRica. It is unknown if any leakage occurs in thegeothermal power plants in Costa Rica. Furtherinvestigations are needed to answer this question.5.1.2 Thermal springsHigh As concentrations occur in the thermalsprings. The As concentration in the Miravallesarea ranging from 0.005- 0.28 mg/L with theexception of one sample, collected at ThermalGuayabal, with a measured concentration of 4.6mg/L. The concentrations of As in the Rincón dela Vieja area ranged from 0.005-0.13 mg/L withtwo exceptions, Salitral las lilas 1 and Salitral laslilas 2 with As concentrations of 10.9 and 10.6mg/L respectively.This high concentration of As affects the areasurrounding these springs.Salitral las lilas 1 and Salitral las lilas 2 have theiroutlet into a river. To find out how diluted the Asconcentration becomes when entering the river asample Salitral las Lilas 1+2 was collected 100meters downstream in the river. The results showan As concentration of 0.13 mg/L, which is higherthan the WHO limit for drinking water. Thisshows that thermal springs with highconcentrations of As are mixing with cold surfacewater of low As concentration.5.1.3 Cold surface watersThe concentration of As in the cold springs do notexceed WHOs guidelines for drinking water.Although the As concentrations in the coldsprings are relatively low (Miravalles 0.0052 -0.0065 mg/L respective 0.0052 mg/L in Rincónde la Vieja) the risk of thermal water mixing withcold spring waters can increase the As content inthe cold waters. This might occur due to i.eflooding during the rain season or due to leakagefrom the geothermal field for power generation.It is unknown whether contaminated watersources in Costa Rica are used by humans forirrigation and drinking water supply. Neither arethe environmental effects on animals and plantsfrom As contaminated water in the area ofMiravalles and Rincon de la Vieja.We do know that thermal springs with highconcentrations of As are mixing with cold surfacewater of low As concentration which might lead toproblems if this is used by humans and animals.Therefore further investigations are needed tomitigate where the highest risk of mixing betweenthermal waters of high As concentration and coldsurface waters occur and investigate how thismixing process affects humans and animals in thearea in the long term.5.2 Comparison of the data with othergeothermal fields in New Zealand, USAand PhilippinesThe results show extremely high Asconcentrations in the geothermal wells andthermal springs of both Miravalles and Rincon dela vieja area. So, how high are the As levels33

Lotta Hammarlund & Juan Piñones TRITA LWR Masters Thesis 09-02Table 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 theWaikato River, and Mt Apo in the Phillippines.Field As (mg/kg) ReferenceGeothermal wellsWairakei, NZ 1.0 - 5.2 Ritchie (1961)Waiotapu, NZ 2.1 - 3.9 Ellis and Mahon (1977)Ohaaki/Broadlands, NZ 5.7 - 9.0 Ewers and Keays (1977)Tongonan, Philippines 28 (mean) Darby (1980)Miravalles, Costa Rica 11.86 - 29.13 mean value 25.8919Rincon de la Vieja, Costa Rica 5.99 - 13.00 mean value 10.302Thermal springsYellowstone Nat Park, US 0.16 - 10 Stauffer and Thomson (1984); Unpublished data, USGSWairakei, NZ 0.23 - 3.0 Ritchie (1961)Waiotapu, NZ 0.71 - 6.5 Webster (1990)Ohaaki/Broadlands, NZ 1.0 Ellis and Mahon (1977)Mt Apo, Philippines 3.1 - 6.2 Webster (1999)Miravalles, Costa Rica 0.005 - 4.56 mean value 0.0991Rincon de la Vieja, Costa Rica 0.005 - 10.85 mean value 0.015levels in these areas in comparison withgeothermal fields in other countries?Let us take a look at table 10 where Asconcentrations from New Zealand, USA and thePhilippines are presented.5.2.1 Concentrations of arsenic and variabilityArsenic concentrations reported in geothermalwells from geothermal fields in New Zealand(Wairakei 1.0 - 5.2 mg/kg, Waiotapu 2.1 - 3.9mg/kg and Ohaaki/Broadlands 5.7 - 9.0 mg/kg)are substantially lower than the ones from thisstudy in Costa Rica.The mean As concentration presented from thePhilippines (28 mg/kg) are higher than reportedfrom this study in Costa Rica (25.9 and 10.30mg/L).After this comparison it is possible to state thatthe As concentrations in the geothermal wells ofCosta Rica are extremely high.If we then compare the result from the thermalwells we can see that Rincon de la vieja has thelowest as well as the highest reported Asconcentration of all (0.005 - 10.85 mg/L), whileMiravalles 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(6.5 mg/L) in New Zealand. It also has got a lowerconcentration than reported from USA (10 mg/L)and the Philippines (6.2 mg/L).5.2.2 Arsenic concentrations in Miravalles andRincon de la ViejaThe thermal springs can be divided into to maintypes: Acid thermal springs (TSA) and Neutralthermal springs (TSN).The acid thermal springs consist of hot water andsteam having a low pH and are rich in sulphate.They are formed by the interaction of the steamphase with shallow aquifer waters. This mixture ofwater and steam lead to precipitation andsublimation of elemental sulphur which follows bymicrobial oxidation to sulphuric acid. Intensealteration of the host rock caused by the fluid(Webster & Nordstrom 2003) has lead to anunstable ground surface and the formation ofbubbling “mudpools” at several samplinglocations.The neutral thermal springs having an almostneutral pH, are rich in carbonate, chloride andsilica. They are examples of hot water dischargesand include steam-heating and steam-phase mixing(Webster & Nordstrom 2003).Two of the neutral thermal springs (Salitral las lilas1 and 2) stand out from the others with As34

Arsenic in Geothermal Waters of Costa Rica___________________________________________________________________________________concentrations reaching 10.8 mg/L and 10.6 mg/Lrespectively and the water of Cl-Na type differsfrom the other thermal springs.Measurements from these two springs have morein common with recordings from the geothermalwells used for power generation. Similarities notedare a neutral pH, high conductivity, similar Cl-Nawater type and high concentrations of both As andB.Possible reasons for this similarity might be:1. Water feeding Salitral las lilas 1 and 2come from the same deep naturalgeothermal source as the water extractedfor power generation, while the otherthermal springs in the area get fed byshallower geothermal sources. The highconcentration of As and boron recordedat the surface of the springs suggests thatthis possibility may be unlikely however.This is because water feeding natural hotsprings goes through several chemicalprocesses (such as precipitation) on theway to the surface which substantiallyreduces the levels of dissolvedcontaminants that reach the surface. Incontrast to this, the rapid extraction ofgeothermal water for power generationprevents these chemical changes.Contaminant levels are therefore relativelyunchanged when the water reaches thesurface.2. The development of the geothermal fieldsincluding the drilling and extraction ofwater has disturbed the naturalgroundwater systems and increased therate and volume of geothermal fluidsreaching the surface. Two things whichmay have artificially increased the level ofcontaminants in the springs are leakage offluid from the extraction pipes or anincrease in the natural spring flow rate.Increasing the flow rate might lead to areduction in the intensity of the naturalchemical processes that occur in the wateron the way to the surface.3. Reinjected waste water is feeding into theaquifer of Salitral las lilas 1 and 2.If the thermal springs of Salitral las lilas 1 and 2are influenced by the activity at the geothermalfield, it means that the reservoir fluids from thegeothermal field are mixing with the groundwaterin the area. More investigations of groundwaterand sediment samples are needed to drawconclusions on whether geothermal extraction isaffecting nearby groundwaters.35

Lotta Hammarlund & Juan Piñones TRITA LWR Masters Thesis 09-025. CONCLUSIONSThe present study indicates that high Asconcentrations occur throughout the entire studyarea. 35 of the total 50 samples exceed the WHOdrinking water guideline for As which is 10 μg/L.Extremely high As values were measured from thegeothermal wells (11.86-29.13 mg/L) whileconcentrations in thermal springs (0.0052- 4.56mg/L) were low to high and cold surface water(0.0052- 0.065 mg/L) was low. High boronconcentrations (33.9-60.1 mg/L) also occurthrough out the entire area.• Arsenic concentrations exceed the WHO limitfor safe drinking water (10μg/L) in 35 of the 50samples. Geothermal well fluids greatly exceedthe WHO limit while all the cold springs fallbelow that limit.• Boron concentrations exceed the WHO limit forsafe drinking water (0.5 mg/L is the B limitrecommended for drinking water by the WHO(1998) in all of the 50 samples.• Sampled geothermal well fluids are generally ofNa-Cl-B type with an almost neutral pH andwith oxidizing conditions. They containextremely high As and boron concentrations.• Sampled thermal springs can be divided intoneutral thermal waters and acidic thermal waters.The neutral thermal springs (pH almost neutral)are generally of HCO 3 -Cl type with reducingconditions. The acidic thermal springs (pH 1.97-3.25) are generally of SO 4 -S-Cl-Al type and withreducing conditions. They contain low to highAs concentrations.• Sampled cold springs are generally of Si-SO 4 -HCO 3 type with a neutral to alkaline pH withreducing conditions. They contain low Asconcentrations.• As comparison with New Zealand, USA andPhilippines show that the well fluids used forgeothermal energy in Costa Rica have extremelyhigh As concentrations.• Reinjection of geothermal fluids in Miravallesand Rincon de la Vieja is needed although thewaste fluid contains extremely highconcentrations of both As and B.36

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Lotta Hammarlund & Juan Piñones TRITA LWR Masters Thesis 09-02Webster, 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 EasternSierra Nevada. Environ. Sci. Technol..32: 657-66238