Clay-plug sediment as the locus of arsenic pollution in Holocene alluvial-plain aquifers

Shallow aquifers in many Holocene alluvial basins around the world have in the last three decades been identified as arsenic pollution hotspots, in which the spatial variation of natural (or: geogenic) arsenic concentration is conditioned by the meandering-river geomorphology and the fluvial lithofacies distribution. Despite the large amount of publications on the specifics of the pollution, still many uncertainties remain as to the provenance and processes that lead to arsenic enrichment in aquifers. In this paper, arsenic in abandoned and sediment-filled meandering-river bends (or: clay-plugs) is highlighted as a primary source of aquifer pollution. The combination of high organic-carbon deposition rates and the presence of chemically-bound natural arsenic in sediment of this specific geomorphological setting creates the potential for microbially-steered reductive dissolution of arsenic in an anoxic environment, and subsequent migration of the desorbed arsenic to, and stratigraphic entrapment in, adjacent sandy point-bar aquifers. To assess the magnitude of the arsenic source in clay-plug, bulk sediment volume calculations were made of twenty clay plugs on the Middle Ganges Plain of Bihar (India), by combining clay-plug surface area analysis of Sentinel-2 satellite data, side-scan sonar depth profiling of oxbow lakes and the Ganges River, and sedimentological data from five cored shallow wells. ICP-MS based elemental analysis of 36 core sub-samples, complemented with published concentration data in a similar geomorphological setting in West Bengal, India, yielded an average arsenic content of 28.75 mg/kg sediment in the 12-m-thick clay plugs, which amounts to a total arsenic volume of 0.07 – 3.13 . 106 kg per clay plug. A scenario is presented for the release of arsenic from the clay-plug sediment by microbial metabolism, followed by migration of the desorbed arsenic to the bordering point-bar sands.