Contributions of floodplain stratigraphy and evolution to the spatial patterns of groundwater arsenic in Araihazar, Bangladesh

Extreme spatial heterogeneity has emerged as a salient characteristic of groundwater arsenic in many complex fluviodeltaic environments. Here we examine patterns of arsenic heterogeneity in the shallow (<23 m) groundwaters of a well-studied floodplain setting in Araihazar, Bangladesh. Ninety-five augers and 200 shallow wells sampled at spacings of 10(1)-10(3) m in the 25 km(2) area indicate that the concentration of arsenic in shallow groundwater largely varies with the grain size, thickness, and distribution of fine-grained (<63 mu m) sediments that overlie buried aquifer sands. The overall pattern shows that lower arsenic concentrations are typically found where aquifer sands outcrop at or near the surface, whereas higher arsenic levels typically underlie, or are adjacent to thicker, fine-grained deposits. Furthermore, chronostratigraphic reconstructions of aquifer sediments indicate that sediment distribution, and consequently the patterning of dissolved arsenic, is readily explained in the context of local river history and floodplain development within the past 1000 yr. An important implication is that complex patterns of groundwater arsenic in afflicted fluviodeltaic settings can be better understood through reconstructions of local aquifer history. This finding is especially relevant because the village and tube-well locations are closely linked with surface landforms such as former levees and bars. An additional and worrisome finding is that the artificial filling of villages to protect from flooding can mimic the natural fine-grained stratigraphy commonly associated with high concentrations of arsenic.