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A field and reactive transport model study of arsenic in a basaltic rock aquifer

A field and reactive transport model study of arsenic in a basaltic rock aquifer
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  A field and reactive transport model study of arsenic in a basaltic rock aquifer Bergur Sigfusson a,b, ⇑ , Sigurdur R. Gislason b , Andrew A. Meharg a a School of Biological Sciences, University of Aberdeen, AB243UU Aberdeen, UK  b Institute of Earth Sciences, University of Iceland, 101 Reykjavik, Iceland a r t i c l e i n f o  Article history: Available online 7 January 2011 a b s t r a c t The use of geothermal energy as a source for electricity and district heating has increased over recentdecades.DissolvedAscanbeanimportantconstituentofthegeothermalfluidsbroughttotheEarth’ssur-face. Here the field application of laboratory measured adsorption coefficients of aqueous As species onbasaltic glass surfaces is discussed. The mobility of As species in the basaltic aquifer in the Nesjavellirgeothermal system, Iceland was modelled by the one-dimensional (1D) reactive transport model PHRE-EQC ver. 2, constrained by a long time series of field measurements with the chemical composition of geothermal effluent fluids, pH, Eh and, occasionally, Fe- and As-dissolved species measurements. Di-,tri-andtetrathioarsenicspecies ð As ð OH Þ S 2  2  ,AsS 3 H 2  , AsS 3  3  andAs ð SH Þ  4 )werethedominantformof dis-solvedAsingeothermalwatersexitingthepowerplant(2.556 l Mtotal As)butconvertedtosomeextenttoarsenite(H 3 AsO 3 )andarsenate HAsO 2  4   oxyanionscoincidingwithrapidoxidationofS  2  toS 2 O 2  3  andfinally to SO 2  4  during surface runoff before feeding into a basaltic lava field with a total As concentrationof 0.882 l M following dilution with other surface waters.Acontinuous25-adatasetmonitoringgroundwaterchemistryalongacrosssectionofwarmspringsonthe Lake Thingvallavatn shoreline allowed calibration of the 1D model. Furthermore, a series of groundwater wells located in the basaltic lava field, provided access along the line of flow of the geothermaleffluent waters towards the lake. The conservative ion Cl  moved through the basaltic lava field(4100m) in less than10a but As was retarded considerably due to surface reactions and has entered agroundwaterwell850mdowntheflowpathasarsenateinaccordancetothepredictionofthe1Dmodel.The1Dmodel predicted acomplete breakthrough of arsenate in theyear 2100. Inareduced systemarse-nite should be retained for about 1ka.   2011 Elsevier Ltd. All rights reserved. 1. Introduction Geothermal areas frequently produce spring and streamwaterswith elevated As concentrations (Webster and Nordstrom, 2003).Utilisation of geothermal areas can increase the discharge of geo-thermal waters towards the surface with associated As and metalcontamination (Olafsson, 1992; Baba and Armannsson, 2006; Gal-lup, 2007;Aksoyetal., 2009). Re-injectionofspent geothermalflu-ids to deep aquifers is rapidly increasing around the world and isrecommended as the standard procedure to avoid adverse impactsof geothermal utilisation (Baba and Armannsson, 2006).Arsenic is believed to primarily enter crustal fluids during crys-tallisingofdeepplutonsashotvolatile-richmagmaticfluidsescapefrom the plutons and segregate into vapour and brine resulting inthe formation of porphyry-style and epithermal ore deposits andfumarole activity in volcanic areas (Goldschmidt, 1954; Ballantyneand Moore, 1988). Arsenic concentrations in volcanic gases sam-pled between 400 and 900  C often range between 1 and 10ppmof the vapour condensates (Mambo et al., 1991) corresponding toAs enrichment in the gas phase between 100 and 1000 withrespect to the magma body suggesting an important transfer of As into the hydrosphere and atmosphere during magma degassingand volcanic eruptions (Pokrovski et al., 2002). Arsenic preferen-tially concentrates into the liquid aqueous phase in water domi-nated active hydrothermal systems below 350  C (Ballantyne andMoore,1988). ArsenicmainlyoccursasAs(III)hydroxide,primarilyas As(OH) 3  aqueous species and to lesser extent, the sulfideH 0–3 As 1–3 S 3–6 , species in natural hydrothermal solutions depend-ingontemperature, pHandH 2 Scontent(Akinfievetal., 1992;Helzet al., 1995;Pokrovskiet al., 1996) .  Accordingtodissolutionexper-iments, As occurs to a large extent in a soluble form in intermedi-ate and silicic volcanics in New Zealand, probably as salts onmineral grain surfaces (Ellis and Mahon, 1964). The ratio of As toCl insolutionindeepaquifersinIcelandis similartothat of tholei-itic basalts indicating the As may be leached congruently from therock surfaces as meteoric water reacts with those rocks in high-temperaturegeothermalsystems(Giroud,2009)oratlowerdepthsin colder environments (Arnorsson, 2003). 0883-2927/$ - see front matter    2011 Elsevier Ltd. All rights reserved.doi:10.1016/j.apgeochem.2011.01.013 ⇑ Corresponding author. Present address: Reykjavik Energy, Baejarhals 1, 110Reykjavik, Iceland. E-mail address: (B. Sigfusson).Applied Geochemistry 26 (2011) 553–564 Contents lists available at ScienceDirect Applied Geochemistry journal homepage:  Arsenic has been reported to be highly mobile in shallowground waters up to 90  C in basaltic terrain (Arnorsson, 2003),however it may not be considered as conservative as Cl  and B inhydrothermal solutions in Iceland (Giroud, 2009). It may be incor-porated into secondary sulfide minerals such as pyrite (FeS 2 ), real-gar (As 2 S 2 ) and orpiment (As 2 S 3 ) (Cleverley et al., 2003) depending on H 2 S (aq)  concentrations. As the hydrothermal solutions risetowards the surface and mix with cold ground waters at shallowlevels the As may be effectively removed from the solution bycoprecipitation with or adsorption onto ferric hydroxides (Giroud,2009;Arnorsson,2003)orsorbedonbasalticglasssurfaces(Sigfus-son et al., 2008).Hydrothermal fluids may rise towards the surface with limitedinteractionwithcoldgroundwaters, eithernaturallyintheformof springs and fumaroles or through discharging boreholes utilisedfor geothermal power production. Recent advances in analyticaltechniques have made possible direct determination of thioarsenicspecies in geothermal waters either in the field (this study) or inthe laboratory after preservation (Wallschlager and Roehl, 2001;Stauder et al., 2005; Planer-Friedrich et al., 2007). Planer-Friedrich TowardsMt. Hengill Fig. 1.  Location of sampling sites, the geological cross section and the 1D model path. Green line near the shore depicts the traverse in Fig. 3. Modified from Hafstað et al. (2007). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)554  B. Sigfusson et al./Applied Geochemistry 26 (2011) 553–564  etal.(2007)foundthatasthedischargedwatersfromasulfidichotspringflowedonthe surface, thioarsenates preferablytransformedinto arsenite with less thioarsenates transforming stepwise by li-gandexchangetoarsenateandfinallythearseniteoxidisedtoarse-nate as all thioarsenates and H 2 S had disappeared.From chromatographic methods, the As contained in alkalinesulfidic solutions can occur either mainly as thioarsenate or thi-oarsenite(Wilkinet al., 2003; Stauderetal., 2005; Planer-Friedrichet al., 2007), however, Beak et al. (2008) have pointed out that the chromatographic methods cannot independently determine theoxidation state of As and peaks generated by thioarsenates mayalso be explainedby thioarsenitespecies. Inany case, the thioarse-nic species, incorporating the As in either oxidation state, breakdown under oxidised conditions at the surface in a series of kinet-icallycontrolledreactions(Planer-Friedrichetal., 2007). Therefore,thechemicalcompositionof thewater andthereactiontimeattheEarth’s surface determine the As speciation in the geothermalwater once it runs on the surface or seeps back into the bedrock.An accurate quantification of the individual species may as a con-sequence, be detrimental for As transport prediction by geochem-ical modelling.The transport of As in shallow groundwater aquifers has beenstudied extensively due to its toxicity, e.g. Charlet et al. (2007).The reduced form of As, arsenite (As(III)) is generally consideredmore toxic than the oxidised form, arsenate (As(V)) (Fergusonand Gavis, 1972). The harmfulness of arsenite is based on its reac-tion with SH-groups of proteins (Squibb and Fowler, 1983) and,therefore, Stauder et al. (2005) suggested the toxicity of As couldbe decreased by formation of thioarsenates (addition of SH-groupstoAs)fromarseniteinsulfidicenvironments. Themobilityof theseAs species depends on the chemical characteristics of the speciesthemselves as well as the surface properties of the aquifer matrix.The surface of the aquifer in this contribution is primarily com-posed of basaltic glass. Sigfusson et al. (2008) measured sorptionprocesses of arsenite and arsenate on basaltic glass and reporteddecreasing mobility of arsenite as pH increased from 3 to 10whereas arsenate was immobile at pH3 and highly mobile atpH10. Stauder et al. (2005) predicted that thioarsenates shouldbe less mobile than arsenite and arsenate on soil materials suchas pyrite and goethite due to the shift from a neutral to weak an-ionic form to a strong anionic As complex as a result of thioarse-nates. This would be the case under acidic conditions whereasthe thioarsenates should be more mobile at alkaline conditionsas encountered in the high-temperature geothermal fluids in thecurrent contribution.The aim of this study was to predict the transport of As in abasaltic aquifer within the Nesjavellir geothermal system in SWIceland. To this end, laboratory measured adsorption coefficientsof aquatic As species on basaltic glass surfaces were incorporatedinto the one-dimensional (1D) reactive transport model PHREEQCver. 2 (Parkhurst and Appelo, 1999) constrained by 25-a long timeseries of field measurements of chemical composition of geother-mal effluent fluids and ground waters, pH, Eh and occasionallyFe- and As-dissolved species measurements. 2. Geological setting and power production  2.1. Geological setting  The Nesjavellir geothermal power plant is a so called co-gener-ation power plant, producing both 88  C hot water by heat ex-change, and electricity. It lies in the eastern rim of the Hengill NK-1NL-4NL-7NL-9NL-8Grámelur NK-2 m.a.s.l Legend Fault, displacement in meters; scoria; se sediments;groundwater well in cross section; groundwater well outsideof cross section BedrockHagavíkurhraun lavaNesjahraun lavaStangarhólshraun lava Lækjarhvarf  Fig. 2.  (a)Geological crosssectionof themodelledarea(modifiedfromHafstaðet al., 2007).Geographical locationof thecross sectionisdisplayedasyellowlineinFig. 1. (b) The 1Dmodel for water flowfromLækjarhvarf (Site 7) to Grámelur (Site 13) corresponding to the cross sectionin Fig. 2a. (For interpretation of the references to color in thisfigure legend, the reader is referred to the web version of this article.) B. Sigfusson et al./Applied Geochemistry 26 (2011) 553–564  555  central volcano complex in south western Iceland (Fig. 1). TheHengill central volcano complex is the northernmost complex of the Western volcanic rift zone of Iceland that extends from thetip of the Reykjanes peninsula which then submerges to form theMid Atlantic ridge (MAR). To the north of the Hengill central vol-cano, Lake Thingvallavatn fills a tectonic graben associated withWNW tectonic movement of the North American plate and theHreppar micro plate which separates the North American and Eur-asian plates in the area (Einarsson, 2008). The lake is primarilyspring fed by shallow aquifers to the north and supports a rela-tively high productivity ecosystem for its latitude ( Jonasson,1992) .  The Nesjavellir power plant lies in the Nesjavellir grabenextending from Hengill central volcano to Lake Thingvallavatn(Fig.1).Thebedrocktopographyischaracterisedbypredominantlybasaltic lavas formed at interglacial intervals or predominantlyhyaloclastite ridges which are mostly basaltic glass, formed subglacially during glacial intervals (Tomasson and Saemundsson,1967).The area is one of the most active volcanic areas in Iceland. Onthe surface, Holocene lavas of variable age have flowed fromeruption sites in the vicinity and extended far into the lake. Of the surface lavas, Stangarhólshraun is the oldest and erupted inearly Holocene (8ka) from a crater row extending from westerlyparts of Stangarháls toward the lake in a NE–SW direction (Figs. 1and 2a). The lava flows in this area are of AA type with thickglassy scoria at the base and top of the lava flows facilitating fastgroundwater flow at the base of the lavas. The lava is partiallycovered by the younger Hagavíkurhraun (5.5ka) which eruptedfrom a crater row to the east of the Nesjavellir graben and flowedover the graben (Figs. 1 and 2a). The lava is composed of oxidisedscoria at the top and base of the flow while the centre of the flowis more dense and crystalline. The youngest lava, Nesjahraun(2ka), erupted near the Hagavíkurhraun and covers a large por-tion of the older lavas as shown in Figs. 1 and 2a (Saemundsson, 1995). The lava primarily covers the eastern parts of the Nesjavel-lir graben between the power plant and Lake Thingvallavatn andextends 1km into the lake to 40m depth. A series of tectonicevents occurred between the eruptions of the two youngest lavasleaving a series of faults in the Nesjavellir graben (Fig. 2a). Thesubsided area formed in these tectonic events was later filledwith the Nesjahraun lava (Saemundsson, 1995). The Holocene la-vas overlie the hyaloclastite rock formation consisting of basalt of variable crystallinity such as tuff, breccias, pillow lavas and pillowfragments, and glacial till can also be found in the bedrock (Haf-stað et al., 2007) (Fig. 2a). Fig. 2a represents a lateral cross section extending along the main groundwater flow path. Fig. 2b depictsthe proposed mass balance scheme for the As transport as de-scribed later.  2.2. Power production at the Nesjavellir site The power plant was commissioned in September 1990.Initially, four wells were connected to the production line. Priorto electrical power production in 1998, the 198  C geothermal    D   i  s   t  a  n  c  e   N  o  r   t   h   t  o   S  o  u   t   h   (  m   )    C   l   (  m   M   )   D   i  s   t  a  n  c  e   N  o  r   t   h   t  o   S  o  u   t   h   (  m   )    S   O    4  -   2     (  m   M   )   D   i  s   t  a  n  c  e   N  o  r   t   h   t  o   S  o  u   t   h   (  m   )    S   i   O    2    (  m   M   )   D   i  s   t  a  n  c  e  w  e  s   t   t  o  e  a  s   t   (  m   ) 0123456789101112    1   9   8   3   1   9   8   5   1   9   8   7   1   9   8   9   1   9   9   1   1   9   9   3   1   9   9   5   1   9   9   7   1   9   9   9   2   0   0   1   2   0   0   3   2   0   0   5   2   0   0   7   2   0   0   9    A  s   (      µ    M   ) MarkagjáVarmagjáSigguvíkEldvíkMarkatangiStapavíkMarkagjáVarmagjáSigguvíkEldvíkMarkatangiStapavíkMarkagjáVarmagjáSigguvíkEldvíkMarkatangiStapavíkMarkagjáVarmagjáSigguvíkEldvíkMarkatangiStapavík abcd Fig. 3.  Measured Cl, SO 4 , SiO 2  and As concentrations from the springs along LakeThingvallavatn shoreline from 1983 to December 2008. Sampling locations aredisplayed in Fig. 1. Black line in 1990 marks commission of the power plant, blackline in 1998 marks the expansion of the power plant to electricity generation. AllsampleswerecollectedbytheresearchdepartmentofReykjavikEnergy, whichalsodetermined Cl  , SO  24  and SiO 2  concentrations. Arsenic concentrations weremeasured from archived samples. (See separate file for grey tones version for printcopy.) Fig. 4.  Sample chromatogram for As and anion speciation from water measured atSite 6<1min after sampling. B. Sigfusson et al./Applied Geochemistry 26 (2011) 553–564  557
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