Impact of seawater intrusion in the coastal aquifers and its effects on agriculture in the delta region of Tamil Nadu

Abstract The current study evaluates the influence of seawater intrusion on groundwater and its implications for agriculture in the coastal belt of Tamil Nadu, India. This region has suffered from extensive groundwater salinization and low agricultural yield for several years. The salinity intrusion functions as a catalyst to damage the agriculture practices. Therefore, it is crucial to address the depletion of groundwater quality in the study area. In this study, 177 groundwater samples were obtained during pre-monsoon (2023). Subsequently, an integrated approach involving hydrogeochemical evaluation, geochemical modelling, chemical indices, GIS and multivariate statistics is applied in this study. The GQISWI and Seawater Mixing Index (SMI) suggest that the Southern part of the study area is largely affected by seawater mixing. According to the Sodium Adsorption Ratio (SAR), Electric Conductivity (EC), Sodium Percentage (%Na) and Permeability Index (PI) distribution maps, it is identified that the groundwater in the southern part of the study area is not suitable for irrigation purposes. The hydrogeochemical data suggests that Ca-Cl type, Ca-Mg-Cl type − are predominant water types in the study area. The bivariate plots reveal that ion exchange, reverse ion exchange, silicate weathering, seawater mixing and anthropogenic activities are controlling factors influencing groundwater evolution. The Chloro-alkaline index suggests that 87.6% of the samples indicate reverse ion exchange controlling the groundwater system. Saturation Index (SI) indicates that the groundwater is saturated (>0) with dolomite and calcite, due to seawater intrusion. Pearson correlation, R-mode factor analysis, Principal Component Analysis (PCA) and Hierarchical Cluster Analysis (HCA) also indicate the influence on groundwater from seawater mixing. In addition, overexploitation of groundwater through deep borewells for agriculture and domestic purposes, as well as climate change, may result in the salinization of coastal aquifers in the study area. These findings will assist local communities and policymakers in devising effective management and mitigation measures to increase crop yields and prevent seawater mixing in the research area.