ASSESSMENT OF GROUNDWATER VULNERABILITY IN OHL RESERVOIR WATERSHED USING A GIS-BASED MODIFIED DRASTIC MODEL

This study was designed to assess the potential for groundwater contamination in the watershed surrounding Ohl Reservoir, which is a source of public drinking water supply for the greater Lock Haven area in Clinton County, PA. This study area is vulnerable to groundwater contamination from various factors including agricultural practices and anthropogenic influences. The DRASTIC model has been in use since 1987 to determine potential vulnerability for groundwater contamination using seven hydrogeologic parameters that influence and control groundwater movement through soil and aquifer layers, namely the depth to the groundwater table (D), net recharge (R), aquifer media (A), soil media (S), topography (T), impact of the vadose zone (I), and hydraulic conductivity (C). All of these parameters were determined for the study area, and were converted into ArcGIS data layers that were compiled into two groundwater vulnerability index maps, one using standard procedure and the other using a modified net recharge values. All of these parameters were determined using standard weights, ranges, and ratings published in the literature. The standard vulnerability index map shows the agricultural land areas have the potential for higher groundwater contamination. The modified vulnerability index map showed relatively lower potential for groundwater contamination for 94.3% of the study area including the agricultural land; however, the reservoir and wetland areas show a higher susceptibility for groundwater contamination.