Characterization of recharge processes in shallow and deeper aquifers using isotopic signatures and geochemical behavior of groundwater in an arsenic-enriched part of the Ganga Plain

摘要

Arsenic concentrations in groundwater extracted from shallow aquifers in some areas of the Ganga Plain in the states of Bihar and Uttar Pradesh, exceed 50μgL~(-1) and locally reach levels in the 400μgL~(-1) range. The study covered 535km2 of active flood plain of the River Ganga, in Bihar where a two-tier aquifer system has been delineated in a multi-cyclic sequence of Quaternary sand, clay, sandy clay and silty clay all ≤～250m below ground surface. The research used isotopic signatures (δ ~(18)O, δ ~2, ~3H, ~(14)C) and major chemical constituents HCO3-,SO42-,NO3-,Cl-,Ca2+,Mg2+,Na+,K+,Astotal of groundwater to understand the recharge processes and groundwater circulation in the aquifers. Values of δ ~(18)O and δ ~2 combined with ~3H data indicate that the recharge to the As-enriched top 40m of the deposits is modern (<50a), predominantly meteoric, with some evaporation during infiltration, and partly from tanks and other surface water bodies. The lower part of the upper aquifer is vulnerable to mobilization of As with increasing groundwater extraction. The low As lower aquifer (max. 5μgL~(-1)) is hydrologically isolated from the upper aquifer and is characterized by lower ~(14)C concentration and lower (more negative) δ ~(18)O values. Groundwater in the lower aquifer is ～3ka old, occurs under semi-confined to confined conditions, with hydrostatic head at 1.10m above the head of the upper aquifer during the pre-monsoon. The recharge areas of the lower aquifer lies in Pleistocene deposits in basin margin areas with the exposed Vindhyan System, at about 55km south of the area.