New approaches to integrate the time dimension in groundwater vulnerability assessments

European Union Directives (2000/60/EC, 2006/118/EC) require the identification of areas showing groundwater quality deterioration, as well as groundwater contamination by anthropogenic pollutants. Nitrate is one of the most common contaminants that affects groundwater in Northern Italy and all over the world. We present different approaches of groundwater vulnerability assessment to nitrate contamination, of the Po Plain area in Lombardy region, in order to meet the European Union requirements. Groundwater vulnerability is assessed through the spatial statistical method Weights of Evidence (WofE). WofE is a method based on the Bayesian conditional probability, which enables observations of the individual role and the combined effect of both anthropogenic and natural factors in relation to contamination in groundwater. Three scenarios have been evaluated: a) the status of nitrate contamination and anthropogenic sources in 2009; b) the change in nitrate contamination and the evolution of anthropogenic sources in the period 2000-2009; c) the combination of a) and b). Anthropogenic sources of nitrate contamination are fertilizer and manure from agricultural activities, and sewer leakages in urban areas. The first source is represented by the amount of nitrate loadings, while the second by the urbanization derived from satellite scatterometer data (QuikSCAT-DSM data). Natural factors representing geological and hydrogeological conditions (e.g., soil protective capacity and groundwater depth) are considered not time dependent in the analyses. The analyses show the differences and the similarities of the distribution of the vulnerable areas among the three scenarios. In these areas, the combination of natural and anthropogenic factors involves a given absolute level of contamination in the aquifer and a deterioration of groundwater quality. Moreover, these approaches and the available data sets enable the development of future scenarios that consider land use changes and their impact on groundwater quality