Quantification of geogenic arsenic in clay-plug sediment of Holocene floodplains

Shallow aquifers in Holocene flood basins around the world are loci for the pollution of arsenic, which poses a severe health threat to millions of people. Groundwater is the prime source of water for consumption and irrigation in densely-populated areas of developing countries. Arsenic concentrations are conditioned by the meandering-river geomorphology of the Holocene flood basins, and show a high spatial variability. The combination of high organic carbon deposition rates and the presence of solid-state arsenic constitutes a potential locus for pollution in clay-filled clay plugs. After the reductive dissolution in anoxic conditions, the mobilized arsenic migrates to adjacent sandy point-bar aquifers, and accumulates in high concentrations by stratigraphic entrapment. In this paper the pollution threat is quantified through the calculation and analysis of solid-state arsenic in clay plug sediment. To assess the arsenic volume in clay-plug sediment, the bulk sediment volume of twenty clay plugs on the Middle Ganges Plain of Bihar (India) was calculated by combining surface area analysis of Sentinel-2 satellite data with side-scan sonar depth profiling of oxbow lakes, and with sedimentological data from four cored shallow wells. Arsenic concentrations in the clay-plug sediment, obtained elemental analysis (ICP-MS based) of 18 core sub-samples and complemented with published concentration data, yielded an average arsenic content of 28.75 mg/kg sediment in the 12 m thick clay plugs, and a total arsenic weight of 0.07 – 3.13 . 106 kg per clay plug. Arsenic and iron concentrations in the sediment decrease with depth, and this testifies to the increased dissolution.