Hydrogeochemical and isotopic study for evaluation of seawater intrusion into shallow coastal aquifers of Udupi District, Karnataka, India

Groundwater is the major source of drinking in coastal areas and the gradual decline in its quality is a major concern. The present study was aimed to evaluate the groundwater quality and its deterioration in coastal aquifer in the Udupi district of Karnataka state (southwest India) by studying the hydrogeochemical characteristics and salinization processes. We collected 49 groundwater samples from open well to shallow bore wells apart from one surface water sample and one seawater sample during the pre-monsoon season. Samples were analyzed for major ions and oxygen isotopes (delta O-18) and the results reveal that the water is generally acidic to mild alkaline in nature, and the total dissolved solids (TDS) vary from 40 to 12,100 mg/l. delta O-18 values (-2.73% to -0.6%) indicate highly saline nature of the groundwater. Widely accepted plots such as Piper plot and Gibbs plot and ratios such as sodium/chloride, chloride/bicarbonate, and calcium/magnesium were used to identify the intrusion of saltwater into the coastal aquifers. The total hardness, Piper plot, and Gibbs plot suggest that the groundwater samples are brackish, mixed, and saline in nature. Based on the seawater percentage (SW%) to quantify the salinization of groundwater, we conclude that the saline water intrusion into the shallow aquifers from seawater is a threat in this region.