Arsenic in groundwater: Testing pollution mechanisms for sedimentary aquifers in Bangladesh

In the deltaic plain of the Ganges-Meghna-Brahmaputra Rivers, arsenic conce ntrations in groundwater commonly exceed regulatory limits (>50 mug L-1) be cause FeOOH is microbially reduced and releases its sorbed load of arsenic to groundwater. Neither pyrite oxidation nor competitive exchange with fert ilizer phosphate contribute to arsenic pollution. The most intense reductio n and so severest pollution is driven by microbial degradation of buried de posits of peat. Concentrations of ammonium up to 23 mg L-1 come from microb ial fermentation of buried peat and organic waste in latrines. Concentratio ns of phosphorus of up to 5 mg L(-)1 come from the release of sorbed phosph orus when FeOOH is reductively dissolved and from degradation of peat and o rganic waste from latrines. Calcium and barium in groundwater come from dis solution of detrital land possibly pedogenic) carbonate, while magnesium is supplied by both carbonate dissolution and weathering of mica. The Sr-87/S r-86 values of dissolved strontium define a two-component mixing trend betw een monsoonal rainfall (0.711 +/- 0.001) and detrital carbonate (<0.735).