Hydrogeochemical characterization of groundwater in the shallow aquifer system of Middle Ganga Basin, India

Middle Ganga Basin (MGB) is lifeline for millions of inhabitants relying heavily on the groundwater. This has resulted in depletion of water quality and quantity at a very rapid scale. The present study has emphasized on hydrogeochemical evolution of groundwater in the Middle Ganga Basin, covering an area of 99,058 sq. km. Around 400 water samples were evaluated to determine the geochemical evolution of the shallow groundwater in MGB. The weighted average water quality index (WAWQI) shows 20.2% of the groundwater are unsuitable for drinking purposes. The Ca-HCO3 water facies dominates in northern region in the interfluves of Ghaghara and West Rapti rivers whereas more evolved water types such as Mg-HCO3, Na-HCO3 are found in the interfluves area of Ghaghara, Ganga, Yamuna, and Gomati Rivers. The occurrence of more mineralized water with increasing residence time in the flow direction suggests geogenic control and evolution follow the Chebotarev sequence. The saline water type is observed in and around the settlement reflecting the in-situ enrichment due to stagnation and anthropogenic activities. Hierarchical cluster analysis classified the regional groundwater data into three distinct major groups G1, G2, and G3. Factor 1 is attributed to anthropogenic inputs and associated with subgroup G2B and group G3. Factor 2 is attributable to the geogenic factors and is associated with sub-groups G1A, G1B, and G2A. The various bivariate plots confirm the dominance of silicate weathering over carbonate weathering in the study area. Geochemical mass balance modeling suggests calcite and dolomite are in saturation to oversaturation conditions, restricting their further dissolution and primary silicate minerals con-trolling the groundwater chemistry. Our work reveals hydrogeochemical evolution on a regional scale in the shallow groundwater which will help to develop sustainable groundwater management strategies.