Groundwater vulnerability assessment of an urban coastal phreatic aquifer in India using GIS-based DRASTIC model

The groundwater pollution in shallow aquifers is usually an outcome of urbanisation resulting from the discharge of industrial and domestic effluents. Maps of aquifer vulnerability to pollution are becoming more in demand because it addresses the degree of sensitivity of the groundwater system towards changes in intrinsic and extrinsic factors. With this aim, the DRASTIC method was adopted to address the vulnerable groundwater zones for shallow aquifers of Greater Cochin and the surrounding region. Seven hydrogeological parameters either measured in-situ for 98 dug wells or derived from secondary sources and satellite data were integrated into a GIS platform and modelled using DRASTIC. Rating for the DRASTIC factors was derived based on weights assigned to each parameter. The resultant map depicted vulnerability index values ranging 79 to 174 reclassified as very high (>144), high (144–131), medium (<131 to 115) and low (<115). About 68% of the total study area, characterised by porous and permeable aquifer media with shallow groundwater levels, displayed a very high to high vulnerability index value. The majority of the highly vulnerable area falls in the urban centre with high population density within the lowland region. Sensitivity analysis shows that aquifer media has the biggest effect on vulnerability assessment. Water quality analysis was carried out for groundwater samples collected from 41 dug wells. The degree of contamination was calculated by adding the concentration of metals. The susceptible zones based on the degree of contamination (Cd) cross verified with vulnerability index depicted a significant correlation between vulnerability zones. The study indicates that contamination and vulnerability to contamination is a main cause of concern for more than 3.82 million people living in the region. The study suggests that DRASTIC model can be used as an efficient tool to manage water-sensitive urban development and groundwater sustainability.