Hydrogeochemical evolution and salinity risk assessment in hard rock aquifers of Southern India: A chemometric perspective

Peninsular India presents with its hard rock terrain and seasonal rainfall, faces increasing challenges in managing groundwater quality. This study evaluates the hydrogeochemical characteristics and seasonal variations in groundwater quality within the agriculturally dominated region of southern Tamil Nadu, based on 48 samples collected during pre- and post-monsoon periods. An integrative approach combining major ion chemistry (pH, EC, TDS, Ca²⁺, Mg²⁺, Na⁺, K⁺, Cl⁻, SO₄²⁻, HCO₃⁻), geochemical plots (Piper, Wilcox, Gibbs), and chemometric techniques was employed to evaluate the controlling processes. Irrigation water quality was examined using Sodium Adsorption Ratio (SAR), Sodium Percentage (Na%), Kelly’s Ratio (KR), Magnesium Adsorption Ratio (MAR), and Permeability Index (PI). The Groundwater Quality Index (GWQI)values indicated that during the pre-monsoon period, 54.16 % of samples were of poor quality (GWQI ≤ 60), while post-monsoon dilution improved quality, with no poor-quality samples and 54.17 % in the excellent category. Total dissolved solids exceeded WHO (2017) limits in 40 % of samples, with persistent salinity hotspots in the northwest. Hydrochemical facies were dominated by Ca²⁺-Mg²⁺-Cl⁻-SO₄²⁻, indicative of permanent hardness and evaporite dissolution. In the pre-monsoon period, 54.16 % of samples had poor quality, while post-monsoon conditions showed significant improvement. Statistical analyses identified TDS, Na⁺, Cl⁻, and Mg²⁺ as the primary determinants of groundwater quality, with elevated salinity, attributable to both geogenic processes such as evaporite dissolution and anthropogenic inputs from agriculture, emerging as a key controlling factor. After the monsoon, water quality improved due to dilution. These findings underscore the importance of season-based groundwater management in hard rock aquifers to balance drinking water safety and sustainable irrigation use.