The spatial distribution of arsenic contamination in fluvial sediment of the Ganges River

Abstract

Shallow aquifers in the Ganges River channel belt (Bihar, India) have high and spatially variable concentrations of arsenic contamination. The arsenic is of geogenic origin. Hydrated iron-arsenic-oxide coatings on quartz and clay minerals occur in the Ganges River deposits. The arsenic is subsequently released to the groundwater in a redoxcontrolled environment whereby microbial respiration triggers the reductive dissolution of iron and arsenic. Recent data acquisition in Holocene fluvial deposits of the Ganges River suggests that high arsenic concentrations are associated with sandy point bars in the inner bends of abandoned meandering river bends. Analysis of two 50 m deep cored boreholes shows the lithofacies heterogeneity in the Holocene Ganges River deposits. Stacked point bar deposits and associated clay plugs occur in the first 28 m of the succession. From 28-50 m depth the gravel and coarse grained braided river deposits are the characteristic lithofacies type. In addition, a high resolution time domain electromagnetic survey (TemFast) was performed to visualise the spatial distribution of the fluvial deposits in the shallow subsurface. In both boreholes an anomalously high arsenic concentration (up to 800 ppb) occurs at the boundary between both lithofacies types. Further TemFast data acquisition and analysis should result in the construction of a static 3D geological model which subsequently will be used for flow calculations.