title: A comparison of data-driven groundwater vulnerability assessment methods.
abstract: Increasing availability of geo-environmental data has promoted the use of statistical methods to assess groundwater vulnerability. Nitrate is a widespread anthropogenic contaminant in groundwater and its occurrence can be used to identify aquifer settings vulnerable to contamination. In this study, multivariate Weights of Evidence (WofE) and Logistic Regression (LR) methods, where the response variable is binary, were used to evaluate the role and importance of a number of explanatory variables associated with nitrate sources and occurrence in groundwater in the Milan District (central part of the Po Plain, Italy). The results of these models have been used to map the spatial variation of groundwater vulnerability to nitrate in the region, and we compare the similarities and differences of their spatial patterns and associated explanatory variables. We modify the standard WofE method used in previous groundwater vulnerability studies to a form analogous to that used in LR; this provides a framework to compare the results of both models and reduces the effect of sampling bias on the results of the standard WofE model. In addition, a nonlinear Generalized Additive Model has been used to extend the LR analysis. Both approaches improved discrimination of the standard WofE and LR models, as measured by the c-statistic. Groundwater vulnerability probability outputs, based on rank-order classification of the respective model results, were similar in spatial patterns and identified similar strong explanatory variables associated with nitrate source (population density as a proxy for sewage systems and septic sources) and nitrate occurrence (groundwater depth).
authors.affiliation: European Commission, Joint Research Centre, Institute for Environment and Sustainability, Via E. Fermi 2749, I-21027 Ispra (VA), Italy.Dipartimento di Scienze della Terra "A. Desio," Università degli Studi di Milano, Via Mangiagalli 34, 20133 Milan, Italy.United States Geological Survey, 954 National Center, Reston, VA 20192.Istituto per la Dinamica dei Processi Ambientali, Consiglio Nazionale delle Ricerche (CNR-IDPA), Piazza della Scienza 1, 20126 Milan, Italy.
authors.forename: Alessandro
authors.initials: A
authors.lastname: Sorichetta
authors.forename: Cristiano
authors.initials: C
authors.lastname: Ballabio
authors.forename: Marco
authors.initials: M
authors.lastname: Masetti
authors.forename: Gilpin R
authors.initials: GR
authors.lastname: Robinson
authors.forename: Simone
authors.initials: S
authors.lastname: Sterlacchini
mesh_terms: D060587:Groundwater; D016015:Logistic Models; D009566:Nitrates; D018570:Risk Assessment; D014874:Water Pollutants, Chemical; D014876:Water Pollution
chemical_list: D009566:Nitrates; D014874:Water Pollutants, Chemical