Geophysical investigation for seawater intrusion in the high-quality coastal aquifers of India: a review.

Around the globe, seawater intrusion in the coastal aquifer is a significant problem. Excessive groundwater extraction because of population growth, industrialization, tourism, and other anthropogenic activities and geogenic processes initiates and accelerates this problem. The contaminated groundwater impacts the health, economic activities, and social and cultural development of coastal regions. This work aims to explore the current status and a holistic comprehending review of geophysical studies applied to delineate the seawater intrusion in the high-quality coastal aquifers in India, as well as its origin and causes, mitigation strategies, and recent advancements in geophysical techniques to access the qualitative and quantitative properties of the complex aquifer system. In the future, it is recommended to do a detailed subsurface imaging of the entire coastal belt of India to decipher the lateral and vertical variation of the lithological conditions and seawater intrusion in space and time with improved/advanced geophysical techniques, which can lead toward sustainable development.

Integrating magnetic susceptibility, hydrogeochemical, and isotopic data to assess the seawater invasion in coastal aquifers of Digha, West Bengal, India.

Seawater intrusion in coastal aquifers is a major concern due to geogenic and anthropogenic activities leading to declining groundwater quality. The present study focuses on deciphering the sea water intruded zones and its extent in the Quaternary alluvial aquifer system in the coastal belt of Digha, West Bengal, India. In this study, 36 groundwater samples were collected during pre-monsoon (2020). Subsequently, an integrated approach of hydrogeological, hydrogeochemistry, bulk magnetic susceptibility, isotopic, multivariate statistical, and geochemical modeling is adopted. Spatial distribution maps of hydrological parameters (salinity, conductivity, TDS) and major ion concentration (Na+, K+, Ca2+, Mg2+, Cl-, SO42-, F-, and Br-) suggest that the northern, south-west, and eastern parts of the study area are largely affected by saltwater intrusion and are corroborated with seawater mixing index (SMI). Based on sodium adsorption ratio (SAR), sodium percentage (Na%), and Permeability index (PI) distribution maps, the same locations are identified under critical condition for the suitability of groundwater for irrigation. The order of concentration of cations and anions in the water samples are Na+ > Ca2+ > Mg2+ > K+ and HCO3- > SO4- > Cl- > Br- > F- respectively. Piper diagram shows three principal hydrochemical water types with water composition changes from fresh (86%) to saline water mix (14%). The hydrochemical facies evolution diagram depicts 81% of water samples are in the freshening phase, and 19% are in the intrusion phase. The various bivariate plots revealed that ion exchange, reverse ion exchange, silicate weathering, seawater mixing, and anthropogenic inputs are the governing factors that control groundwater evolution. R-mode factor analysis, principal component analysis (PCA), and agglomerative hierarchical cluster (AHC) also indicate the influence on groundwater from seawater mixing and/or seawater intrusion. The superlativeness of bulk magnetic susceptibility (χ) analysis of water samples in delineating seawater intruded zones is elaborated. Saturation index (SI) shows that groundwater is saturated (> 0) with calcite, dolomite, and aragonite, plausibly due to seawater ingression. Stable isotopic analysis of δ2H (- 53.979 to - 16.9578‰) and δ18O (- 7.00183 to - 1.37 ‰) suggests precipitation recharge/paleo-water at some locations and evaporation enrichment of groundwater. It is recommended to increase groundwater recharge, reduce groundwater extraction at critically affected locations, and have regular monitoring and management to control seawater intrusion.

Geophysical and hydrogeological investigation for the saline water invasion in the coastal aquifers of West Bengal, India: a critical insight in the coastal saline clay-sand sediment system.

The Digha-Shankarpur area in West Bengal suffers from the problem of saline water intrusion in the near-surface to shallow subsurface aquifers. In the present study, geophysical surveys were conducted and integrated with measured hydrogeological parameters to delineate the possible locations of saline water-invaded zones in the shallow aquifers. One hundred eighty-eight groundwater samples were collected, and parameters like salinity, EC, total dissolved solids (TDS), pH, temperature, and water level were measured. The geophysical survey, such as resistivity profiling, self-potential, and electrical resistivity imaging techniques, was applied. High TDS, salinity, and EC were observed in various places. Resistivity profiling survey indicates a low resistivity zone (< 10 Ωm), self-potential anomaly indicates a positive anomaly and resistivity imaging survey indicated very low resistivity zones (0-3 Ωm) in near-surface to shallow subsurface locations which are concurrent with the other coastal aquifers in the eastern part of India. These low resistivity zones are interpreted as saline water intrusion zone mixed with clay/sand layers up to a depth of 15 m possibly due to the ingression of seawater and also due to anthropogenic activities. Hence, protection from seawater intrusion from the canals into the coastal aquifers (shallow and deep) and human-made activities should be restricted to minimize the effect of saline water pollution.