Redox control of arsenic mobilization in Bangladesh groundwater

Detailed hydrochemical measurements, delta(34)S(SO4) and H-3 analyses were performed on 37 groundwater samples collected 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-wide As surveys to span the entire spectrum of As concentrations in Bangladesh groundwater, and to represent 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), presumably of Holocene age. It remained mobile under SO4-reducing conditions, suggesting that authigenic sulfide precipitation does not constitute a significant sink for As in these groundwaters. The redox state of the water was characterized by a variety of parameters including dissolved O-2, NO3- Mn2+, Fe2+ concentrations, and SO42-/Cl- ratios. High dissolved [As] (>50 mug/l; or >0.7 muM) were always accompanied by high dissolved [HCO3-] (>4 mM), and were close to saturation with respect to calcite. Groundwater enriched in As (200-800 mug/l; or 2.7-10.7 muM) and phosphate (30-100 muM) but relatively low in dissolved Fe (5-40 muM) probably resulted from re-oxidation of reducing, As and Fe enriched water. This history was deduced from isotopic signatures of delta(34)S(SO4) and (H2O)-H-3 (H-3) to delineate the nature of redox changes for some of the reducing groundwaters. In contrast, As is not mobilized in presumed Pleistocene aquifers, both shallow (30-60 m) and deep (150-270 m), because conditions were not reducing enough due to lack of sufficient 02 demand. (C) 2003 Elsevier Ltd. All rights reserved.