Geochemistry and mineralogy of arsenic in (natural) anaerobic groundwaters J.A. Saunders a, * , M.-K. Lee a , M. Shamsudduha a,1 , P. Dhakal a , A. Uddin a , M.T. Chowdury b , K.M. Ahmed b a Department of Geology and Geography, Auburn University, Auburn, AL, USA b Department of Geology, University of Dhaka, Dhaka, Bangladesh article info Article history: Available online 5 July 2008 abstract Here new data from field bioremediation experiments and geochemical modeling are reported to illustrate the principal geochemical behavior of As in anaerobic groundwaters. In the field bioremediation experiments, groundwater in Holocene alluvial aquifers in Ban- gladesh was amended with labile water-soluble organic C (molasses) and MgSO 4 to stim- ulate metabolism of indigenous SO 4 -reducing bacteria (SRB). In the USA, the groundwater was contaminated by Zn, Cd and SO 4 , and contained <10 lg/L As under oxidized conditions, and a mixture of sucrose and methanol were injected to stimulate SRB metabolism. In Ban- gladesh, groundwater was under moderately reducing conditions and contained 10 mg/L Fe and 100 lg/L As. In the USA experiment, groundwater rapidly became anaerobic, and dissolved Fe and As increased dramatically (As > 1000 lg/L) under geochemical conditions consistent with bacterial Fe-reducing conditions. With time, groundwater became more reducing and biogenic SO 4 reduction began, and Cd and Zn were virtually completely removed due to precipitation of sphalerite (ZnS) and other metal sulfide mineral(s). Following precipitation of chalcophile elements Zn and Cd, the concentrations of Fe and As both began to decrease in groundwater, presumably due to formation of As-bearing FeS/FeS 2 . By the end of the six-month experiment, dissolved As had returned to below background levels. In the initial Bangladesh experiment, As decreased to virtually zero once biogenic SO 4 reduction commenced but increased to pre-experiment level once SO 4 reduc- tion ended. In the ongoing experiment, both SO 4 and Fe(II) were amended to groundwater to evaluate if FeS/FeS 2 formation causes longer-lived As removal. Because As-bearing pyrite is the common product of SRB metabolism in Holocene alluvial aquifers in both the USA and Southeast Asia, it was endeavored to derive thermodynamic data for arsenian pyrite to better predict geochemical processes in naturally reducing groundwaters. Including the new data for arsenian pyrite into Geochemist’s Workbench, its stability field com- pletely dominates in reducing Eh–pH space and ‘‘displaces” other As-sulfides (orpiment, realgar) that have been implied to be important in previous modeling exercises and reported in rare field conditions. In summary, when anaerobic bacterial metabolism is optimized by providing both elec- tron donors and acceptors, As is mobile under Fe-reducing conditions, immobile under SO 4 -reducing conditions, and arsenian pyrite is the likely stable mineral phase formed under SO 4 -reducing conditions, instead of pure As–S phases such as realgar or orpiment. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction Natural As enrichment of groundwater occurs around the world by a variety of geochemical processes 0883-2927/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.apgeochem.2008.07.002 * Corresponding author. E-mail address: saundja@auburn.edu (J.A. Saunders). 1 Present address: Department of Geography, University College, London, UK. Applied Geochemistry 23 (2008) 3205–3214 Contents lists available at ScienceDirect Applied Geochemistry journal homepage: www.elsevier.com/locate/apgeochem