Arsenic concentrations in shallow groundwater of Araihazar, Bangladesh. Part I. Geological control through floodplain evolution

This study examines the lateral heterogeneity of shallow ( < 23 m) groundwater arsenic concentrations at lateral scales of 101-103 m in the context of floodplain morphology and evolution in Araihazar, Bangladesh. A total of 95 auger cores and 200 shallow wells sampled over a 25 km2 area indicate that the concentration of arsenic in shallow groundwater varies with the thickness and distribution of fine-grained ( < 63μm) sediments that cap aquifer sands. Lower arsenic concentrations are typically found where sands outcrop at or near the surface; higher arsenic levels are observed underneath thicker fine-grained deposits. Optical luminescence, 210Pb, and 137Cs measurements indicate that the distribution of these deposits is not random and reflects local river history and floodplain development. A worrisome finding is that artificial raising of villages to protect them from flooding mimics the natural stratigraphy that is associated with high concentrations of arsenic in shallow groundwater