Redoxcontrolofarsenicmobilizationin Bangladesh groundwater Y.Zheng a,b, *,M.Stute b,c ,A.vanGeen b ,I.Gavrieli b,d ,R.Dhar a , H.J. Simpson b,e ,P.Schlosser b,e,f ,K.M.Ahmed g a Queens College, City University of New York, Flushing, NY 11367, USA b Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY 10964, USA c Barnard College, New York, NY 10027, USA d Geological Survey of Israel, Jerusalem 95501, Israel e Department of Earth and Environmental Science, Columbia University, New York, NY 10027, USA f Department of Earth and Environmental Engineering, Columbia University, New York, NY 10027, USA g Department of Geology, University of Dhaka, Dhaka 1000, Bangladesh Abstract Detailed hydrochemical measurements,  34 S SO4 and 3 H analyses were performed on 37 groundwater samples col- lected during February 1999, January and March 2000 from 6 locations in eastern and southeastern Bangladesh to examine redox processes that lead to As mobilization in groundwater. The study sites were chosen based on available nation-wideAssurveystospantheentirespectrumofAsconcentrationsinBangladeshgroundwater,andtorepresent 3 of 5 major geological units of the Ganges-Brahmaputra Delta: uplifted Pleistocene terrace, fluvial flood plain and delta plain. Arsenic was found to be mobilized under Fe-reducing conditions in shallow aquifers ( < 35 m depth), pre- sumablyofHoloceneage.ItremainedmobileunderSO 4 -reducingconditions,suggestingthatauthigenicsulfideprecipita- tiondoesnotconstituteasignificantsinkforAsinthesegroundwaters.Theredoxstateofthewaterwascharacterizedbya varietyofparametersincludingdissolvedO 2 ,NO 3  ,Mn 2+ ,Fe 2+ concentrations,andSO 4 2 /Cl  ratios.Highdissolved[As] ( > 50 mg/l;or > 0.7 mM)werealwaysaccompaniedbyhighdissolved[HCO 3  ]( > 4mM),andwereclosetosaturation withrespecttocalcite.GroundwaterenrichedinAs(200–800 mg/l;or2.7–10.7 mM)andphosphate(30–100 mM)butrela- tivelylowindissolvedFe(5–40 mM)probablyresultedfromre-oxidationofreducing,AsandFeenrichedwater.Thishis- torywasdeducedfromisotopicsignaturesof  34 S SO4 and 3 H 2 O( 3 H)todelineatethenatureofredoxchangesforsomeof thereducinggroundwaters.Incontrast,AsisnotmobilizedinpresumedPleistoceneaquifers,bothshallow(30–60m)and deep(150–270m),becauseconditionswerenotreducingenoughduetolackofsufficientO 2 demand. # 2003ElsevierLtd.Allrightsreserved. 1. Introduction Arsenic is a highly toxic and ubiquitous metalloid (Cullen and Reimer, 1989), and realization is growing that water-borne As now poses a significant threat to humanandecosystemhealthworldwide(Nriagu, 1994). The recent decision by the US Environmental Protec- tion Agency that the Maximum Contamination Level (MCL)forAsindrinkingwaterwillbeloweredfrom50 mg/l (0.7 mM)to10 mg/l (0.1 mM) reflects re-evaluation of health risks associated with ingestion of this metal- loid (NRC, 1999). Because of this threat, it is critically important to understand the factors controlling As 0883-2927/$ - see front matter # 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.apgeochem.2003.09.007 Applied Geochemistry 19 (2004) 201–214 www.elsevier.com/locate/apgeochem * Corresponding author. Tel.: +1-718-997-3329; fax: +1- 718-997-3299. E-mail address: yan_zheng@qc.edu (Y. Zheng).