Arsenic-contaminated groundwater causes a wide-spread, serious health risk affecting millions of people worldwide. Focus of the research is the floodplain of the Ganges River in the State of Bihar (India) where groundwater is the principal source of drinking water and irrigation, and where the levels of arsenic exceed the World Health Organization (WHO) guidelines of 10 μg L-1 for safe drinking water by more than 40 times. To date, no reliable tools exist to measure the hydro-geochemical variability in space and time related to transport of dissolved arsenic in the subsurface. Shallow aquifers in the Ganges River channel belt in our study area in Bihar have high and spatially variable concentrations of arsenic contamination. The arsenic is of geogenic origin and is dissolved from fluvial sediments to groundwater by reductive release under anaerobic conditions in oxbow lakes, where the high amount of organic material acts as reagent for Fe-As-oxide dissolution. Fluvial point bars adjacent to the oxbow lakes act as stratigraphic traps for the arsenic-enriched groundwater. Core and well log analyses from two shallow boreholes in a fluvial point bar, in combination with time-domain electro-magnetic surveys, show the control of geological heterogeneity on the spatial distribution of arsenic contamination.


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