Evaluation of groundwater vulnerability to pesticides: A comparison between the pesticide drastic index and the PRZM leaching quantities

Abstract The current use of pesticides in agriculture is giving rise to an increasing number of cases of groundwater contamination. Facing this fact, people concerned with the use of either pesticides or groundwater are preoccupied with understanding and predicting such risks of contamination. Some of the current approaches are aimed at evaluating the degree of vulnerability of the resource to contamination, mainly through indexation methods. Other approaches consist of modeling the processes that control the migration of contaminants in soil; these are mostly mathematical models for transport simulation. While the latter approaches are based on processes, the former usually rely solely on certain parameters and characteristics of the medium. The DRASTIC rating system, one of several indexation methods, offers a version adapted for agricultural use. Among the currently available simulation models, the PRZM model has been specifically developed to simulate the fate of pesticides. Results from these two methods were compared, attempting to identify a correlation between the pesticide DRASTIC index and the leaching quantities provided by the PRZM model. No significant correlation was found between their results. The only factors taken into account in both methods that influence the correlation of the results, are the depth to groundwater and the vadose zone media type. The most significant conclusion resulting from this work is that chemical characteristics of the potential contaminants, which are not considered in the DRASTIC index, are important, if not critical, when evaluating the vulnerability or the potential for contamination of groundwater by agricultural chemicals. In addition, simulation models appear to be the best tools for evaluating the groundwater vulnerability, because they quantify this pollution potential in terms of physical magnitudes, like contaminant concentrations, which can be compared with water quality criteria.