Geochemical Evolution of Groundwater in the Pleistocene Aquifers of South Ganga Plain, Bihar

The study area is spread over 1950 sq. km and covers apart of Pleistocene deposists in the Ganga Plain. A two tier aquifer system made up of sands of various grades occurs in the area and caters to the entire water demand to the tune of 0.12 MCM/sq. km/year. The top 30 m of the alluvial deposits are dominated by clay, sandy clay, silt with thin lenses of sands. The latter constitutes the Shallow Aquifer, occurring under unconfined condition. The Deep Aquifer is made up of interconnected sand layers below 30 m depth, forming a potential zone, where groundwater occurs under semiconfined condition. Sluggish hydraulic conductivity in Shallow Aquifer results in higher mineralization of groundwater than In Deep Aquifer. Principal Component Analysis with 10 chemical constituents, and plots in Expanded Durov Diagram indicate distinctly different geochemical processes in Shallow and Deep Aquifers. In Shallow Aquifer the processes shaping up the chemical character of groundwater are non-exchange, sediments dissolution return seepage from irrigation water and rain-water lnfiltration. But the major processes in Deep Aquifer are leakage from Shallow Aquifer, followed by ion-exchange and weathering of silicate minerals. In the process of ion-exchange, Na⁺ from the aquifer matrix dominated by clays and sandy clays replaces Ca⁺² in groundwater, resulting in formation of patches of calcium carbonate nodules.