BIOGEOCHEMISTRY OF ARSENIC IN GROUNDWATER AND ITS IMPLICATION FOR EVALUATING THE SUSTAINABILITY OF AQUIFERS AS A LONG-TERM SAFE WATER SOURCES

Arsenic is a highly toxic and ubiquitous metalloid. Most arsenic problems related to drinking water exposure, however, are the result of arsenic mobilization under natural conditions. A number of large aquifers throughout the world have been identified with arsenic contamination, made it as a crucial water quality problem in many parts of the world, particularly in Bangladesh, it has turned to be a serious life threatening issue. The spatial extent of the problem and the fact that the most severely contaminated areas are in the geologically distinct low-lying flood plain, support the idea that the sediments themselves are the most likely arsenic source. However, the mobility of arsenic in the subsurface is influenced by a combination of the dissolved species present, minerals in aquifer solids, microbial activity, and by ambient geochemical parameters such as redox conditions.

The results from a detailed biogeochemical investigation of arsenic in an area of Bangladesh provide a unique opportunity to better understand the hydrological, geochemical and microbial interactions. A series of anaerobic incubations were conducted in replicate over 90 days using natural orange sands from deeper orange-colored Pleistocene sands initially containing 0.14 mg kg^-1 phosphate-extractable As, >99% as As(V), and 0.8 g kg^-1 of 1 hr HCl-leachable Fe, mostly as Fe(III). The sediment was resuspended in artificial groundwater, with or without lactate as a labile carbon source, and inoculated with metal-reducing Shewanella sp. ANA-3. Within 23 days, dissolved As concentrations increased to 17 mg L^-1 with lactate, mostly as As(III), and 2 mg L^-1 without lactate. The controls showed < 1 mg L^-1 solute As, even though significant Fe and Mn were released with lactate. Sedimentary phosphate-extractable As increased to 4-fold to ~ 0.6 mg kg^-1 by Shewanella (with or without lactate) in 23 days, with significant proportion as As(III) by 92 days. These observations indicate that metal-reducers such as Shewanella can trigger As release to groundwater by converting sedimentary As to a more mobilizable form without the addition of high levels of labile carbon. Such interactions need to be better understood to determine the vulnerability of these low-As containing sustainable aquifers from which drinking water is increasingly drawn in Bangladesh.