Geologic framework and isotope tracing of the arsenious Quaternary Aquifer of the southwestern North Bengal Plain, West Bengal, India

The aim of the paper is to understand the geological control of groundwater, the recharge sources, the relative age of groundwater and the interaction between surface water and groundwater of the southwestern part of North Bengal Plain in the alluvium filled gap between the Rajmahal hills on the west and the Garo hills on the east. The area is covered by Quaternary alluvia of two different ages, viz. the Older Alluvium and the Newer Alluvium. The Older Alluvium of Pleistocene age is made up of argillaceous bed reddish brown in color and interspread with 'kankar' and laterite debris and the Newer Alluvium is dark, loosely compacted and has a high moisture content. A hydrostarigraphic model of the top 120 m geological column has been built by direct field observation, constructing Fence diagram, lithofacies and isotope analyses. The model indicates an oscillating environment of deposition of sediments from bottom to top with oxidizing at the bottom followed by reducing and then oxidizing environment at the top. The hydrogeochemistry of groundwater also suggests a predominantly reducing condition of the aquifer with high HCO3-, low SO42- and NO3- concentrations. The shallow groundwater at places contains heavy metals such as copper, cadmium, manganese, iron, chromium and arsenic. The delta O-18 and tritium values of groundwater are within the range of monsoon precipitation composition which indicates that groundwater is probably recharged primarily from precipitation. The plots of delta O-18 and delta D show slight deviation from the Local Mean Water Lines suggesting that some evaporation of rainfall occurs prior to or during infiltration. Therefore groundwater occurs under unconfined condition. Lithofacies analysis indicates that the sediments below the depth of 40 m are the potential water bearing formation. Depletion or enrichment of delta O-18 and delta D with depth was not observed and tritium content is also similar at various depths indicating possibility of mixing of groundwater from various depths due to pumping which may lead to contamination of the deeper aquifer by heavy metals and arsenic.