GEOLOGY AS A PREDICTIVE TOOL IN DELINEATING GROUNDWATER ARSENIC CONTAMINATED AREAS- AN APPROACH FROM MIDDLE GANGA PLAIN, INDIA

Our study in the Middle Ganga Plain (MGP), in the upstream of Garo_Rajmahal Gap reveals a role of geology in the pattern and extent of groundwater arsenic concentration within ~50 m below ground confined within the post Upper Pleistocene deposits. The lithology is fine to very fine sand (micaceous), silt and mud of gray to dark gray/black in colour. The sand zones covered by variously thick mud/clay at surface, form the shallow aquifers, are major source for drinking and irrigation.

Research reveals that the geomorphic sub-environments and the sedimentary facies, controls the incidence of groundwater arsenic. The relation between arsenic content, aquifer-aquitard hydraulics, ground water flow regime and local-scale lithologic variations of can be effectively used for mapping the arsenic-safe areas in the otherwise declared affected administrative units. The elevated concentrations (>50 µg/L) are found to be spatially related with dark gray to black coloured organic rich clay sediments found both in the Active (T₀-surface) and Older Floodplain (T₁-surface) terraces.

The thickness of the dark gray to black coloured clay/mud cover overlying the shallow sand layer and the organic carbon content in the argillaceous sedimentary bodies are the controlling factor of arsenic release in groundwater by facilating reductive dissolution of hydrated iron oxide (HFO) coatings on sediment grains. The reductive dissolution of HFO is evident by high concentration of dissolved iron in groundwater. The spread of the reducing environment depends upon the volume of organic carbon release, hydraulic conductivity of the formation, groundwater flow direction, and the volume of fresh oxic water recharge that reaches the spreading anoxic front.

The investigation suggests that the geologic tool can be adopted in the preliminary stage to broadly delineate the suspected corridors of groundwater arsenic contamination.