Disentangling agricultural and non-agricultural drivers of groundwater nitrate contamination using an integrated footprint–statistical approach

: Nitrate (NO₃-) contamination of groundwater is a widespread environmental and public health concern, particularly in intensively managed agricultural regions. This study investigates the spatial and temporal dynamics of groundwater NO₃- pollution across the Po Valley (northern Italy), integrating the Grey Water Footprint (GWF) framework with multivariate statistical and geospatial analyses. Using data from approximately 2100 monitoring sites between 2013 and 2023, nitrogen loads to groundwater were estimated and partitioned into agricultural and non-agricultural contributions. Results indicate that while agriculture remains an important source of NO₃- pollution-especially in Emilia-Romagna and Piedmont-it does not fully explain observed contamination patterns. Non-agricultural sources, including urban and industrial activities, contribute significantly, particularly in Lombardy and, to a lesser extent, Veneto. The estimated non-agricultural nitrogen load (Lnon-agr) exceeded 10 kg N/ha/yr in over 50% of sites across the study area, highlighting the relevance of diffuse anthropogenic pressures beyond farming. Principal Component Analysis (PCA) reveals that both anthropogenic factors (population density, industrial activity, and agricultural intensity) and natural drivers (precipitation, soil texture, and drainage) jointly control NO₃- distribution. Natural factors, especially precipitation and soil properties, often play a dominant role in regulating nitrate transport and accumulation, explaining regional differences that cannot be attributed solely to nitrogen application rates. Temporal analysis further shows that the relative importance of these drivers varies over time, influenced by climatic variability. Uncertainty analysis underscores the influence of poorly constrained parameters, particularly groundwater recharge and natural background NO₃- concentrations, on model outputs. Overall, the study demonstrates that groundwater nitrate pollution in the Po Valley arises from a complex interplay of sources and processes. These findings highlight the need for integrated management strategies that address both agricultural and non-agricultural inputs while accounting for hydrogeological and climatic conditions.