Hydrogeochemical, seawater intrusion and oxygen isotope studies on a coastal region in the Puri District of Odisha, India

Groundwater is the major source of freshwater in coastal areas, and gradual declining of water quality is a major cause of concern. The present study is focused on a coastal aquifer, to study the groundwater chemistry, hydrogeochemical characteristics, and salinization processes in a coastal area of the Puri District of Odisha, southeastern coast of India. Groundwater chemistry reveals, water compositions are generally near neutral to slightly alkaline nature in pH, and the total dissolved solids (TDS) concentrations varies from 150 mg/l in the inland area to 4006 mg/l towards the shorelines. Piper plot shows four principal hydrochemical water types prevailed in the groundwater zones with water composition changes from fresh water to the saline water mixing. The oxygen isotope (δ18O) values are found between −5.3‰ to and −2.96‰, which indicates groundwater compositions were influenced by the evaporation process. Based on Cl− concentrations (0.4-35 meq/l), the saline end-member is mixing of seawater with the groundwater. Calculations of ionic deltas in groundwater show deficiency of Na+, Ca2+, Mg2+, SO42− ions and significantly mixed with seawater and subsequent reactions governed by ion exchange processes in the aquifer. Saturation index shows groundwater were subsaturated to near equilibrium conditions with mineral phases such as dolomite, gypsum, halite and under-saturated with calcite, aragonite and anhydrite. Sulphate depletion observed in groundwater indicates the seawater mixing. Groundwater flow path shows, there is a gradual increasing of TDS concentrations from inland recharge areas to towards the discharge areas of shoreline and groundwater facies changes from Na-K-HCO3 to Na-Mg-Cl type. The coastal aquifers are subjected to the continuous influence of seawater mixing, dissolution of carbonate phase minerals, aided with rock-water interaction, and ion exchange processes are the significant governing factors, which controls the groundwater evolution.