Hydrogeochemistry and its relationship with land use pattern and monsoon in hard rock aquifer

Influences of land use and monsoon recharge on groundwater geochemistry in the hard rock aquifer in south India are studied using geochemical modelling, geochemical and geospatial tools, and Pearson correlation analysis (PCA). Groundwater samples were collected from 2017 to 2019 (n = 267) and analysed for EC, pH, major ions, and minor ions. Water samples were classified into urban (n = 89) and agricultural (n = 178) regions based on land use patterns. Groundwater is very hard (Total Hardness > 180 mg/l; 74%), and the predominant water types are Ca–Mg–Cl followed by Na-Cl. In the agricultural region, groundwater is less mineralized (TDS < 500 mg/l; 62%) compared to urban regions (38%). In the urban wells, ionic strength and Log pCO₂ justified the wastewater infiltration. The groundwater is undersaturated with halite, gypsum, anhydrite and fluorite and saturated/supersaturated with carbonate minerals. PCA and geochemical tools indicate that water chemistry is predominantly governed by silicate weathering and ion exchange reactions. The interconnection between Cl⁻, SO₄²⁻ and NO₃⁻ ensures the impact of nitrification/nitrate sources along with anthropogenic input. In 2018, Cl⁻ and NO₃⁻ contents were reduced by dilution due to high rainfall (2061 mm), whereas HCO₃⁻ was enriched owing to recharge-induced mineral dissolution. Geospatial tools explain that the groundwater quality was improved in 2018 and justified by NO₃⁻ (< 45 mg/l; 3291 to 4138 km²; > 100 mg/l; 176 to 9 km²), Cl (< 200 mg/l, 1975 to 2747km²) and HCO₃⁻ distributions. The outcome of this study highlights that urban wells are highly polluted and need proper management practices to protect this hard rock aquifer.