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

In the deltaic plain of the Ganges-Meghna-Brahmaputra rivers, arsenic concentrations in groundwater commonly exceed regulatory limits (50 μg l) 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 come from microbial fermentation of buried peat and organic waste in latrines. Concentrations of phosphorus of up to 5 mg l 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, whilst magnesium is supplied by both carbonate dissolution and weathering of mica. The Sr/Sr values of dissolved strontium define a two component mixing trend between monsoonal rainfall (0.711 ± 0.001) and detrital carbonate (≤ 0.735).