In the deltaic plain of the Ganges-Meghna-Brahmaputra Rivers, arsenic concentrations in groundwater commonly exceed regulatory limits (>50 &mgr;g L-1) because FeOOH is microbially reduced and releases its sorbed load of arsenic to groundwater. Neither pyrite oxidation nor competitive exchange with fertilizer phosphate contribute to arsenic pollution. The most intense reduction and so severest pollution is driven by microbial degradation of buried deposits of peat. Concentrations of ammonium up to 23 mg L-1 come from microbial fermentation of buried peat and organic waste in latrines. Concentrations of phosphorus of up to 5 mg L-1 come from the release of sorbed phosphorus when FeOOH is reductively dissolved and from degradation of peat and organic waste from latrines. Calcium and barium in groundwater come from dissolution of detrital (and possibly pedogenic) carbonate, while magnesium is supplied by both carbonate dissolution and weathering of mica. The 87Sr/86Sr values of dissolved strontium define a two-component mixing trend between monsoonal rainfall (0.711¿0.001) and detrital carbonate (<0.735). ¿ 2001 American Geophysical Union