Dilution parameter have been proposed for the superficial unconfined aquifer in Alessandria plain (Piedmont – northwestern Italy). The aquifer vulnerability map of the study area has been compared by GIS methodologies with the pollution load and the nitrate concentration map. Some incongruities have been explained by considering the proposed dilution parameters: the effective porosity and the equivalent horizontal hydraulic conductivity of the aquifer. The parameters have been evaluated by assigning a value of effective porosity and hydraulic conductivity to each lithologic layer of the subsoil and by calculating the mean values of ne and Kx in different zones of the aquifer.