Hydrochemical characterisation and identification of seawater intrusion in the Visakhapatnam coastal aquifer

Groundwater dependency has increased due to several factors like population increase, industrialisation, and climate change impact. Analysis of groundwater is very important to know the present status and also for better management of natural resources. Groundwater samples from 41 locations were collected along the Visakhapatnam coastal aquifer during the pre- and post-monsoon seasons, and samples were analysed for pH, EC, total dissolved solids (TDS), major cations, and anions like Ca2+, Mg2+, Na+, K+, Cl-, HCO3-, and SO42-. From the results of the chemical analysis, seawater intrusion is identified through qualitative approaches, i.e., Simpson ratio (Cl/(HCO3 + CO3) and Na/Cl ratio. The Simpson ratio ranges between 0.316 and 2.119 during pre-monsoon and 0.124 and 3.947 during post-monsoon, and from the Na/Cl ratio, 30 samples fell under seawater intrusion during pre-monsoon and 38 samples during post-monsoon. The Simpson ratio results also show that seawater intrusion is reducing during the post-monsoon due to increasing groundwater levels caused by rains. From the water quality index classification, 4.88% of the water samples fell under an excellent category in both seasons, and the rest of the samples were all in the remaining four classes. The spatial analysis was also done to understand the changes in groundwater quality and seawater intrusion over space.