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⁺-SO₄²⁻-HCO₃⁻-Cl⁻ during pre-monsoon, shifting to Ca²⁺-Cl⁻-SO₄²⁻-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.

Submarine groundwater discharge (SGD) study is essential for groundwater in coastal terrace at Tiruchendur. The famous Murugan Temple is located in the area and around 25,000 people who visit this temple use the SGD well water at NaaliKinaru (a small open well) as holy water and drink it. The rock and soil type are sandy clay, silt, beach sand, calcarenite, kankar, gneissic rock and charnockite in base rock. Megascopic identification method was used to identify the porous and permeable rocks such as calcarenite, sandstone and kankar to support to increase SGD flux. Grain size study was used to identify the paleo-coastal estuarine environment with sediment deposits in the terrace. The square array electrical resistivity method was used to study the subsurface geology and aquifer depth. The 2d ERT technique was used to identify the subsurface shallow perched aquifer of freshwater. The magnetotelluric survey method was used to scan the entire subsurface geological and tectonic uplift, coastal ridges, rock folded subsurface structural features of continental and oceanic tectonism. Darcy’s law was used to calculate the SGD flux rate in the above study area.

Rapid urbanization across Southern Asia’s coastal regions has significantly increased electricity demand, driving India’s solar sector expansion under the National Solar Mission and positioning the country as the world’s fourth-largest solar market. Nonetheless, methodological limitations remain in applying GIS-based multi-criteria decision analysis (MCDA) frameworks to coastal urban microclimates, which involve intricate land-use dynamics and resilience constraints. To address this gap, this study proposes a multi-criteria GIS- based Analytical Hierarchy Process (AHP) framework, incorporating remote sensing and geospatial data, to assess Solar Farm Sites (SFSs) suitability, supplemented by sensitivity analysis in Thoothukudi coastal city, India. Ten parameters—covering photovoltaic, climatic, topographic, environmental, and accessibility factors—were used, with Global Horizontal Irradiance (18%), temperature (11%), and slope (11%) identified as key drivers. Results show that 9.99% (13.61 km²) of the area has excellent suitability, mainly in the southwest, while 28.15% (38.33 km²) exhibits very high potential along the southeast coast. Additional classifications include good (22.29%), moderate (32.41%), and low (7.16%) suitability zones. Sensitivity analysis confirmed photovoltaic variables as dominant, with GHI (0.25) and diffuse radiation (0.23) showing the highest impact. The largest excellent zone could support approximately 390 MW, with excellent and very high zones combined offering up to 2080 MW capacity. The findings also underscore opportunities for dual-use solar deployment, particularly on salt pans (17.1%), as well as elevated solar installations in flood-prone areas. Overall, the proposed framework provides robust, spatially explicit insights to support sustainable energy planning and climate-resilient infrastructure development in coastal urban settings.