Spatial and statistical assessment of groundwater vulnerability to nitrate pollution

Worldwide Non-Point Sources (NPS) pollutants are recognized as the major contributors to surface and groundwater contamination. Characteristically NPS pollutants present some common features: are difficult to trace to a specific source; enter the environment over an extensive area; have the potential for maintaining a relatively long active presence in groundwater. So assessment of NPS pollution in groundwater at the local or regional scale is a key component to a sustainable environmental policy. Nitrates are among the most common global NPS pollutants and groundwater nitrate contamination and the associated health concerns are among the most common problems adversely affecting groundwater quality worldwide. Considering its behaviour in groundwater, given the wide range of nitrate sources and the frequency with which it has been measured in groundwater, nitrate is a good indicator to study groundwater contamination, to NPS and factors affecting it. Logistic regression and Weights of Evidence Modeling Technique have been used to analyze both natural and anthropogenic factors influencing the occurrence of high nitrate concentrations in groundwater resources located in the central part of the Po Plain (Northern Italy). The proposed methodology was applied in the Lodi District (central part of the Po Plain, Northern Italy), a wide area identified as a Nitrate Vulnerable Zone by the European Community, as a result of the implementation of EU Nitrate Directive (91/976/EC). It combines measurements of nitrate concentrations (response variable) with spatial data representing both categorical and numerical variables (explanatory variables or predictors). The measurements were carried out by means of a monitoring net consisting of 69 wells, irregularly distributed in the study area (Fig. 1), while the geo-environmental factors describes the potential sources of nitrate and the relative ease with which it may migrate towards groundwater. They include population density, nitrogen fertilizer loading, groundwater recharge, soil protective capacity, vadose zone permeability, groundwater depth, and saturated zone permeability.