Assessment of groundwater vulnerability to nonpoint source pollution in a Mediterranean coastal zone (Mersin, Turkey) under conflicting land use practices

Abstract

Groundwater resources of the Tarsus coastal plain (Mersin, Turkey) are being exploited heavily for a variety of purposes and they are under a serious threat from nonpoint source pollution from the conflicting land use practices and saltwater intrusion due to overpumping. In this study, vulnerability of groundwater to nonpoint source pollution was assessed using GIS techniques and employing both Generic and Pesticide DRASTIC models. Calculated vulnerability indices ranged between 68–206 and 69–236 for Generic and Pesticide DRASTIC, respectively. Areas lying between “moderately high” to “high” vulnerability categories extend along the coastline in areas covered by the recent Eolian sand dunes where saltwater intrusion is prevalent. The “low” vulnerability category is restricted to a small portion of the area in the NW, and it is represented by poorly drained soils with topographic gradients above 18%. A sizeable portion of the area was classified into “moderately low” to “moderate” vulnerability category, which is typified by low topographic gradients (<2%) and fairly uniform distributions of hydraulic conductivity, net recharge and depth to water table. The validation of the DRASTIC models was accomplished through pairwise comparison of DRASTIC vulnerability maps (using Pearson's r correlation coefficient) with a total of 18 raster layers representing original DRASTIC input data, land use/land cover (LULC) features, and groundwater Cl− and NO3− data. Results from the correlation analysis indicate a significant association between high groundwater NO3− concentrations and distances from certain LULC types, including open-field farms, citrus orchards, industrial complexes, and residential buildings.