Seawater intrusion vulnerability in the coastal aquifers of southern India-an appraisal of the GALDIT model, parameters' sensitivity, and hydrochemical indicators.

An appraisal of seawater intrusion into the coastal aquifers is one of the major issues for groundwater resource management. The GALDIT model applies to the analysis of multiple parameters using systematic GIS techniques for mapping and assessment of seawater intrusion vulnerability. It demarcates the mapping of potential vulnerability that shows a higher vulnerability to seawater intrusion in various parts of the coast and the estimated vulnerability index value of 7.50 and 9.64. An area of 33.0 km2 spread in the low-lying coastal area comprising estuaries, salt marshes, and saltpans shows the high vulnerability condition with an estimated vulnerability value of 6.42-7.50. An area of 73.20 km2 spread over coastal and alluvial plains experiences moderate vulnerability (temporal salinity in the groundwater sources) with an estimated vulnerability index value of 5.46-6.42. Aquifers underlying coastal uplands (hard rock formations) and some parts of accretionary beaches (2.05 km2) are relatively protected fresh groundwater sources, wherein the estimated vulnerability index is 4.55-5.46. The vulnerability mapping of the GALDIT model using hydrochemical analysis of primary groundwater parameters such as TDS, Cl-, HCO3, and Cl-/HCO3 ratio is validated. Higher concentration of TDS (2637-4162 mg/l) and Cl- (1268-2347 mg/l) is taken for the areas falling under higher vulnerability to seawater intrusion, especially in the placer mining sites and coastal areas facing erosion. Similarly, the groundwater sources of the low-lying areas including estuaries, salt marshes, saltpans, and backwater were noted to have higher values of Cl-/HCO3 with a rationality of 9.87-12.18. Hydrological facies shows the highest concentration of NaCl in the groundwater sources within the proximity of eroded beaches, saltwater bodies, and sand mining areas. A hydrochemical facies evolution (HFE) diagram represents the hydrochemical facies of groundwater elements that shows an intrusion of seawater into the coastal aquifers underlying the very high vulnerable zones. Higher bicarbonate concentration (233-318 mg/l) is noticed in the upland areas and some parts of dunes and accreted beaches, sandy coasts, and uplands. Vulnerability analysis reveals that those areas near saltwater bodies and eroding coasts are prone to lateral and vertical diffusion of saltwater. The geodatabase developed through such modeling studies can help in planning and developing activities for sustainable groundwater resource management in coastal areas.