Assessment of Alluvial Aquifer Intrinsic Vulnerability by a Generic Drastic Model; A Discussion on Data Adequacy and Pragmatic Results

Abstract

DRASTIC is a model that is commonly used to assess vulnerability to groundwater contamination at the landscape scale. When sparse data are available to populate the layers of the model, it can be difficult to ascertain the true usefulness of the model produced map. In this research an alluvial aquifer, the Sahneh aquifer in Kermanshah province of western Iran, was mapped using the generic DRASTIC model. The data available for populating the model layers were generally sparse. The model was validated using a nitrate concentration map constructed from well water measurements within the DRASTIC map area. A Receiver Operating Curve (ROC) analysis was conducted by placing 500 random points in the DRASTIC generated map compared to the nitrate concentration map. The area under the curve was compared and yielded a value of 0.72 or 72% concordance, which means it has good validity. This investigation demonstrates that a generic DRASTIC model can yield acceptable results without modification or increasing its complexity. If the ROC analysis had yielded a value <50%, then the DRASTIC would have been considered to not be useful. A common mistake in the use of DRASTIC is to modify the method to greatly increase its complexity, which may actually decrease, not increase the resultant model usefulness.