Dissolution and entrapment of arsenic in fluvial point bars-case study of the Ganges River, Bihar, India

摘要

Spatially variable arsenic concentrations in shallow aquifers in Holocene Ganges River deposits in Bihar (India) suggest geological conditioning of the propagation of arsenic-contaminated water in terms of stratigraphy and sediment type. High arsenic concentrations are confined to sandy point bars in abandoned river bends. The arsenic is geogenic and occurs in solid state in clay minerals and hydrated iron-arsenic-oxide quartz grain coatings. Microbial respiration in a redox-controlled environment triggers the reductive dissolution of arsenic. Analysis of two 50-m-deep cored boreholes shows that lithofacies heterogeneity and inherent permeability contrast in stacked point-bar deposits and associated clay plugs is the key to the dissolution and stratigraphic entrapment of arsenic. An electromagnetic survey visualized the shape, size and spatial distribution of the stratigraphic entrapment in the shallow sub surface.