Hydrochemical facies distribution, controlling mechanisms and Natural background concentrations of major pollutants in Ganga-Yamuna Interfluve Aquifer, India

Evolution of groundwater genesis in Central Ganga Plain (CGP) is scrutinized with due consideration of hydrochemical and hydrodynamic environment within Quaternary alluviums. Wide variation in hydrochemical facies in CGP indicates a dynamic hydro-geochemical environment influenced from the seasonal rainfall, return flows, canal seepages, and anthropogenic activities. The Ca-HCO₃ facies retaining meteoric nature is characterized by shallow water levels, high recharge rate, high hydraulic conductivity, low salinity and trace elemental load. A noticeable increase in salinity and trace elemental load is reported as groundwater evolved from Ca-HCO₃ to Ca-Mg-HCO₃, Ca-Mg-Cl-SO₄, Ca-Na-HCO₃ type and Na-Cl-SO₄ types. The hydrochemical facies and bivariate plots infer Silicate and Carbonate weathering in a redox-ion exchange environment. The varying concentrations of radio-nuclides like U and Th in different hydrochemical facies depicts the migration attributed to oxidizing-reducing environment and acid-alkaline conditions. Aquifer heterogeneity and spatial variability of groundwater recharge sources including rainfall, return flows, canal seepages imparts changes that make it difficult to appoint sources. The shallow groundwater recharge zones are depleted in heavier isotopes (δ¹⁸O: -12‰ to -7‰) advocating recharge primarily from precipitation and surface water. The concomitant occurrences of HCO₃, Sr, and SiO₂ ascribe genesis from geogenic source more precisely the silicate weathering. Reverse geochemical modelling shows that groundwater is saturated with chalcedony, quartz, barite, talc and under-saturated with anhydrite, gypsum, halite, and sylvite. Analyses including contamination evolution and PPI, point that NO₃, Cl, SO₄, Sr, As, Mn are common pollutants stemming particularly from agricultural activities and industrial effluents pose a serious threat to groundwater sustainability.