Geoelectrical and Hydrochemical Insights in Tracing the Seawater Intrusion Along Coastal Aquifers of the East Coast of South Andaman Island, India

The December 2004 Indian ocean earthquake and its devastating tsunami resulted in exacerbated seawater intrusion into the coastal aquifers and inundation along the east coast of south Andaman Island, severely impacting freshwater resources. Following the 2004 earthquake recurring storm surges, tidal variation, and increasing groundwater abstraction caused by the region’s fast urbanization and population growth further stressing the groundwater resources. To improve the understanding of the current status of seawater intrusion, the long-term impact of tsunami-induced seawater intrusion on aquifers and subsequent recovery, this study aims to map freshwater and saline zones and assess the vulnerability of aquifers two decades after the subsidence and tsunami. The present study reports the results of an integrated approach combining 12 geoelectrical vertical resistivity soundings and profiling utilizing Schlumberger configuration along the east coast at four potential sites that were affected by inundation during the 2004 event. The geoelectrical resistivity data show low resistivity zones (0–10 Ωm) at shallow surface up to (50 m) as probable zones of sea water intrusion. The resistivity profiling illustrates maximum effect of the salinization up to ~ 1.5 km far away from the coast. Moreover, hydrochemical analysis of groundwater samples from dug and bore wells during pre-monsoon and post-monsoon periods at VES locations suggest Na-Cl water type and contamination. The total dissolved solids (TDS) content exceeded the allowed limits, pH and elevated electrical conductivity, mostly caused by Cl^-, Na⁺, K⁺, Ca²⁺ and Mg^2+, indicating high salinity and dissolved salt content. SO4^2- levels surpass permissible limits, whilst Na⁺ and K⁺ concentrations suggest contamination. The integrated findings indicate the complex interplay between aquifer geometry, coastal geomorphology, such as low-lying coastal zones and stress from recurring cyclones and anthropogenic activities significantly influences the sea water intrusion and the ocean flushing of contamination along the east coast of south Andaman. The study underlines the importance of determining aquifer susceptibility and understanding the mechanisms of tsunami-induced seawater intrusion in order to enhance the resilience of groundwater resources in tsunami-prone areas.