Hydrogeochemical processes and physicochemical parameters controlling the groundwater chemistry of a tropical river basin, Southern India

The hydrogeochemical characteristics of groundwater in the Achankovil River Basin (ARB), Southern Kerala, India, were investigated to elucidate the dominant geochemical processes influencing water quality. A total of 75 groundwater samples were collected from dug wells during pre-monsoon and post-monsoon seasons (2021–2022) and analysed. Most of the groundwater samples in ARB were within the permissible limits of potable water quality. To evaluate the spatio-temporal variability, a combination of statistical and graphical approaches was applied, including correlation matrix, hierarchical cluster analysis (HCA), Gibbs and Chadha plots, ion exchange indices and the Entropy Weighted Water Quality Index (EWQI). Groundwater exhibited an acidic to near-neutral pH, with ionic dominance patterns of Cl⁻ (64 mg/L) > HCO₃⁻ (45.3 mg/L) > SO₄²⁻ (32.8 mg/L) > Ca²⁺ (24.2 mg/L) > Na⁺ (13.9 mg/L)> Mg²⁺ (11.7 mg/L > K⁺ (8.8 mg/L). In the post-monsoon, the order shifted slightly to: Cl⁻ (47.4 mg/L) > HCO₃⁻ (31.6 mg/L) > SO₄²⁻ (24.3 mg/L) > Ca²⁺ (17 mg/L) > Na⁺ (12.7 mg/L) > Mg²⁺ (8.1 mg/L) > K⁺ (5.9 mg/L). Gibbs and Chadha plots, along with ionic ratios, suggest that rock-water interactions, dominated by silicate weathering, are the principal hydrogeochemical controls. The predominance of positive chloro-alkaline indices (CAI) indicates active cation exchange reactions. Based on EWQI, 81.3% of groundwater samples were classified as excellent, 17.3% as good, and 1.3% as moderate during the pre-monsoon season. Post-monsoon, excellent quality increased to 86.6%, with 12% rated as good and 1.3% as moderate. Most samples ranged from “excellent” to “good” for drinking, though moderate quality appeared in some coastal sub-regions. These findings highlight the need for seasonal monitoring and emphasize the importance of sustainable groundwater management in the region.