Assessment of groundwater vulnerability for seawater intrusion using DRASTIC model in coastal area at Patuakhali District, Bangladesh

Seawater intrusion is a growing concern for coastal regions in Bangladesh. Overexploitation of groundwater, low rainfall, a high rate of evaporation, rising sea level, cyclones, storm surges, and shrimp cultivation are the causes of seawater intrusion that polluted the groundwater near the coastal area. High values of EC, TDS, Na+, and Cl- ions in groundwater indicate the seawater intrusion. Piper's Trilinear and Durov diagrams of groundwater indicated that most gathered groundwater show predominantly Na-Cl-type of water. Based on the USSL diagram, it is observed that the salinity of the study area is medium to high and the alkalinity is medium to very high. Also, in view of the Wilcox outline, most of the samples in the area of interest are permissible to doubtful, and doubtful to unsuitable. Chloride is moderately correlated with Na+ (R2 = 0.590) and poorly correlated with (HCO3- + CO32-) (R2 = 0.128), which indicates seawater intrusion. According to a comparison of the levels of Fe and Mn in the groundwater of the study area with those recommended by the World Health Organization (WHO) and Department of the Environment (DOE), twenty-two out of thirty-one samples exceed the acceptable limit for Fe and thirty out of thirty-one samples fall within the acceptable limit for Mn. The calculated water quality index reveals that most samples are of low drinking water quality. The research area's poor water type is found in the north-central and southern parts. The WQI geographic variation map reveals that the study's water has a WQI of more than 100, making it unsuitable for consumption. The final seawater intrusion vulnerability map clearly identifies the moderate to highly vulnerable zone of the study area using DRASTIC index parameters. DRASTIC index parameters revealed that some portions of the northwestern part of the sample area show a moderately vulnerable zone. And the rest of the sampling area indicates the highly vulnerable zone.