Comprehensive hydrogeochemical characterization and seasonal water quality index analysis for sustainable groundwater management in Valliyur region, Southern Tamil Nadu, India

Groundwater contamination and seasonal quality variations pose significant challenges in crystalline terrains where rock-water interactions control hydrogeochemical evolution. Despite increasing demographic and agricultural pressures, the Valliyur region in Southern Tamil Nadu lacks comprehensive seasonal hydrogeochemical characterization. This study aims to characterize seasonal hydrogeochemical evolution, assess drinking and irrigation water suitability using comprehensive indices, and identify controlling processes for sustainable groundwater management. The groundwater showed alkaline nature (pH 7.14-8.35) with electrical conductivity (206-11283 µS/cm) and total dissolved solids (96-7334 mg/L) exceeding WHO guidelines in 32% of samples. Total hardness exceeded standards in 48.33% (pre-monsoon) and 43.33% (post-monsoon) samples. ANOVA statistical analysis confirmed that only potassium showed statistically significant seasonal variations (p < 0.001), while geological processes dominated other parameters. The rock-water interactions and weathering process controlled the groundwater geochemistry. The Piper trilinear diagram reveals water types dominated by Ca²⁺-Mg²⁺-HCO₃^--Cl^- and Ca²⁺-SO₄^2--HCO₃^--Cl^- during pre-monsoon, shifting to Ca²⁺-Cl^--SO₄^2--HCO₃^- dominance post-monsoon. Saturation indices revealed calcite equilibrium (52%), normal ion exchange dominance (95%), and bivariate analysis confirmed silicate weathering as the primary control. The Water Quality Index showed seasonal improvement from 48.33% "Excellent" (pre-monsoon) to 70% "Excellent" (post-monsoon). Health risk assessment identified 25% of the study area posing moderate to high health risks during pre-monsoon periods requiring water treatment or alternative sources. Irrigation quality assessment confirmed suitability for agricultural use with enhanced quality during post-monsoon. These findings demonstrate groundwater suitability for irrigation but seasonal variability for drinking purposes, highlighting the need for comprehensive monitoring and management for long-term sustainability.