Assessment of Intrinsic Vulnerability Using DRASTIC vs. Actual Nitrate Pollution: The Case of a Detrital Aquifer Impacted by Intensive Agriculture in Cádiz (Southern Spain)

Abstract

The degradation of groundwater quality due to nitrate is a widespread issue in heavily agricultural areas and a major concern for public health. Improving knowledge of the intrinsic vulnerability of aquifers with respect to the actual contamination is crucial for adequate water management and for complying with the European directives aimed at protecting this valuable resource. In this study, we applied the well-established DRASTIC method to assess the intrinsic vulnerability of the Benalup aquifer, a detrital aquifer located in the southern Iberian Peninsula that supports important agricultural activity. The model was compared with in situ measurements of this ion, evidencing a lack of agreement between the most vulnerable zones and those that display higher nitrate concentrations. This fact should not be interpreted as an inadequacy in the vulnerability model, but as a result of several factors such as (i) the marked heterogeneity in land uses and the spatial variability in contaminant sources, (ii) the construction and exploitation characteristics of the water boreholes, (iii) the sampling procedure and depth to the water table, and (iv) transport and degradation processes within the porous medium. All these aspects can lead to discrepancies between the actual distribution of contamination and vulnerability models such as DRASTIC. All these factors should be carefully considered in the design of a sampling network in order to achieve a representative picture indicating the extent of contamination and the overall chemical quality of the system.