Assessment of alluvial aquifer intrinsic vulnerability by a generic DRASTIC model: a discussion on data adequacy and pragmatic results

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 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. Based on the data collected and analyzed, a range of values from 107 to 177 was found. Therefore, the final vulnerability map was divided into only two classifications, moderate and high, instead of the normal five classifications typically used. The results showed that there was no significant relationship with nitrate concentration by statistical methods. However, the field conditions and the method used have good accuracy. Field evaluations showed that karst water recharge acts as a factor in diluting nitrate concentrations. Evaluation of all final map classification methods (generic DRASTIC, natural breaks (Jenks), equal intervals, quantile, geometrical interval, and fuzzy classifiers) showed that based on cross-validation, the most consistent classifier is the generic DRASTIC based one with 62% and the quantile method is in second place with 36%. Changing the weights and rates of layers to justify the model within the measured nitrate concentrations does not show the inherent nature of the aquifer to contamination. The results of this model emphasize the use of generic DRASTIC with correct data and field study before any optimization. In addition, if optimization is to be performed, it is necessary to add a layer or layers that relate to those factors having the greatest impact on aquifer pollution or dilution of the ionic content of groundwater in the study area. Based on the results obtained from this research, it is concluded that the DRASTIC model can be used to produce a reliable result, useful for planning to reduce groundwater contamination potential, particularly in vulnerable areas.