Aquifer vulnerability assessment using Weights of Evidence modelling technique: application to the Province of Milano, Northern Italy

The occurrence of elevated nitrate concentration in groundwater in the Province of Milan (Northern Italy) was related to both natural and anthropogenic variables, using the weight of evidence model to perform groundwater vulnerability assessment. The study represents an evolution of a first application of the method due to the improvement of the available database in quality and accuracy and an improved statistical control onto the final results. According to weight values, the most important factors determining susceptibility are population density, urban areas, groundwater depth and total infiltration. Groundwater vulnerability has been expressed in terms of susceptibility classes: comparison between areal distribution of these classes and the frequency of occurrences of nitrate in wells shows high degree of correlation both for low and high nitrate concentration. Also correlation between post probability classes and mean nitrate concentrations in wells located in each of these classes gives a high R2 = 0.98 indicating the general good quality of results.