Predictive geospatial model for arsenic accumulation in Holocene aquifers based on interactions of oxbow-lake biogeochemistry and alluvial geomorphology

Machine-Learning Approach for Identifying Arsenic-Contamination Hot Spots: The Search for the Needle in the Haystack

“…Recent research advances indicate that detached, abandoned meandering-river bends (or oxbow lakes), their fine-grained sediment-filled counterparts (or clay plugs), and associated sand-prone point bars are potential sites with high levels of As contamination in the alluvial-basin landscape on a global scale (Figure ). , Porous and permeable sandy point bars stand out, induced by differential compaction, as topographical high grounds in the alluvial landscape, whereas fine-grained alluvial plain and clay-plug sediment is compacted, thereby reducing its porosity and permeability. Population nuclei on elevated point bars provide protection from yearly monsoonal river inundation.…”

“…As-concentration data from shallow (≤40 m deep) tube wells. Highest As concentrations in an enclosed point-bar aquifer . Jamuna River Basin, West Bengal, India.…”

“…Dissolved As(III) then migrates to sandy point bars by diffusion and advection along the porosity–permeability gradient, driven by gravity and clay compaction. , The compacted alluvial plain and clay plug are the low-permeable envelope that forms a four-way closure around the point-bar reservoir, initially at the surface in the alluvial plain and, upon burial by continued fluvial sedimentation in the subsiding Holocene alluvial basin, also overlying the point-bar sand in the subsurface. The resultant anisotropic sedimentary architecture constrains the groundwater flow paths and strongly reduces the recharge efficiency in the aquifer domain of the enclosed pockets of porous point-bar sand, leading to the accumulation of As with concentrations on the order of 500 μg/L (Figure ), i.e., far beyond the WHO-recommended maximum level of 10 μg/L. The point-bar/oxbow-lake/clay-plug geomorphological units are ubiquitous, with scattered locations in all major river channel belts in Holocene alluvial basins around the world, with a total areal extent of many millions of square kilometers.…”