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This study considered the temporal variations in rainfall and water level patterns as governing factors, which influence the geochemical process of coastal aquifer around Pondicherry, South India. Rainfall and water level data were collected from 2006 to 2016, which showed that the amount of rainfall from 2006 to 2011 was higher than that of 2011 to 2016. To understand the geochemical process governing groundwater, samples were collected during 2006 (n = 54), followed by 2011 (n = 93), and during 2016 (n = 63) as part of continuous observation. The major ions and stable isotopes (δ18O and δD) were analyzed in the samples to determine the geochemical variations. The predominant types were noted as Na-HCO3 and Na-Cl; Ca-HCO3 and Ca-Mg-Cl; and Na-Cl and Ca-Mg-Cl in 2006, 2011, and 2016, respectively. Saturation states of sulfate and carbonate minerals were compared for the study periods and it indicates that the saturation index (SI) values were increased from 2006 to 2011, but decreased from 2011 to 2016. PHREEQC inverse modeling revealed the predominance for the dissolution and leaching of carbonate minerals during increased rainy periods, and the increase of halite saturation during lesser rainfall period. AQUACHEM mixing studies suggested that geochemical signatures of 2006 and 2011 were preserved in samples of 2016 in different proportions. Considering the major factors, the main processes prevailing in the study area were inferred to be dissolution and leaching during 2006~2011 years and seawater intrusion along with ion exchange during 2011~2016 years. In all these periods of study, anthropogenic impact was also identified in the groundwater samples. Hence, this study revealed that the rainfall and water level gave a significant variation in the geochemical process of groundwater in the coastal aquifer system.

The irregular monsoonal rainfall and overexploitation of groundwater have resulted in the sharp decline of water resources in different parts across India. Various factors like geology, terrain morphology, hydrology, and hydrometeorology influence the recharge potentiality of a region. Application of remote sensing and geographical information system (GIS) tools might be effective in the delineation of groundwater recharge potential zones and help towards the sustainability of groundwater resources. In this study, it was attempted to identify the groundwater recharge potential zones in the Courtallam region located at Tamil Nadu state of South India by integrating multiple thematic layers involving lithology, geomorphology, land use/land cover, slope, lineament density, drainage density, and the spatial distribution of groundwater level and rainfall through weighted overlay analysis in Arc GIS. The recharge potential was compared with the electrical conductivity of groundwater for evaluating the water quality. It was characterized the groundwater recharge potential of the study area as very poor (8.7%), poor (32.5%), moderate (37%), good (16.3%), and very good (5.5%). one-third of this area has moderate groundwater recharge potentiality and the southeastern part of the study area with good quality of water is feasible for new bore-well construction.