CONTROL OF AQUIFER MATERIAL ON GROUNDWATER MINERALIZATION AND MAPPING CONTAMINANTS IN A MULTI-AQUIFER SYSTEM OF THE CENTRAL GANGA PLAIN, INDIA

The densely populated Central Ganga Plain of India comprises thick, highly permeable sand layers interbedded with less permeable silt, clay, and Kankar carbonate deposits. The occurrence of alternate sequences of sand, silt, and clay has resulted in a multitier aquifer system. The shallow, unconfined aquifer, typically less than 100 meters deep, is heavily exploited for domestic and agricultural purposes through hand-pumps, private motorized wells, and intensive cultivation. Unregulated extraction has led to rapid groundwater depletion and increased vulnerability to contamination. To address these issues, a study was conducted in Varanasi district to investigate factors controlling the vertical and spatial variability of groundwater chemistry in this region with prevailing aquifer heterogeneity. Groundwater (n=150) along rainwater (n=60) and surface water (n=19) samples were analyzed for a suite of physicochemical parameters. Chemometric, statistical, and isotopic analyses revealed groundwater chemistry is independent of inputs from rainwater and surface water and primarily dependent on existing aquifer materials. A strong association between Na⁺, Ca²⁺, Mg²⁺, K⁺, F^-, and HCO₃^- reflects the dominant role of silicate weathering in controlling groundwater chemistry. The unbalanced Na⁺ over Cl^- suggests additional sources of Na⁺ from sodic and saline soils rich in Na₂CO₃ and NaHCO₃, particularly in the northwestern part of the study area. Data further revealed that F^- enrichment is spatially located around shallow groundwater and is linked to the presence of minerals such as fluorite, mica, and amphibole in the soil. Higher NO₃^- values in areas adjacent to agricultural lands clearly depicts inputs from NPK fertilizers. The findings indicate that groundwater chemistry is primarily influenced by mineral dissolution processes within aquifer materials, which is main reason for spatial and vertical variability in aquifer hydrochemistry. Insights gained from this study can contribute to the development of sustainable groundwater management strategies in similar hydrogeological settings in order to mitigate the impacts of excessive groundwater extraction and promote long-term aquifer health.