Deciphering geochemical controls and seasonal dynamics of groundwater quality in a semi-arid Deccan Basalt aquifer, central India

This study presents a comprehensive seasonal assessment of groundwater hydrogeochemistry and quality in Ashti tahsil, a predominantly agricultural region overlying Deccan basalt aquifers in central India. Physicochemical parameters, major ion chemistry, mineral saturation indices, and multi-parameter water quality indices were evaluated for pre-monsoon and post-monsoon periods across forty wells. Groundwater exhibited a neutral to slightly alkaline pH (7.1-8.0), controlled by carbonate buffering through the dissolution of calcite and dolomite. The dominant hydrochemical facies was Ca-Mg-HCO 3 , reflecting carbonate dissolution and water-rock interaction with subordinate agricultural inputs. Total dissolved solids exceeded the Bureau of Indian Standards (BIS) acceptable limit of 500 mg/L in 55% and 45% of pre- and post-monsoon samples, respectively, indicating moderate anthropogenic influence, mainly from agricultural return flow and the leaching of fertiliser-derived soluble salts (nitrate, chloride and sulphate), which enhance dissolved ion loads in the shallow aquifer. Nitrate contamination emerged as the primary public health concern, with 40-45% of wells exceeding the 45 mg/L BIS threshold (maximum: 176 mg/L), primarily due to excessive agrochemical application. PHREEQC thermodynamic modelling confirmed near-equilibrium carbonate mineral controls, while consistently undersaturated evaporite phases (halite SI: −8.24 to −5.89; gypsum SI: −4.02 to −1.42) confirmed freshwater character. The entropy-weighted water quality index (EWQI) classified 40.0% and 42.5% of samples as excellent during the pre- and post-monsoon seasons, respectively, with post-monsoon dilution improving the proportion of high-quality samples. Principal component analysis (PCA) resolved three components explaining 85.2% of total variance, identifying mineralisation, carbonate buffering, and agricultural contamination as the principal hydrogeochemical controls, while hierarchical cluster analysis resolved three hydrochemically distinct groups that remained stable across seasons. Seasonal hydrogeochemical processes were structurally stable, with monsoon recharge inducing transient dilution rather than long-term chemical evolution. These findings underscore the urgent need for integrated nutrient management strategies and targeted aquifer protection measures to prevent irreversible degradation of this critical groundwater resource.