Influence of nitrogen-based fertilization on nitrates occurrence in groundwater of hilly vineyards

Nitrogen losses from intensive agricultural production may end up as high nitrate (NO₃^-) concentrations in groundwater, with a long-term impact on groundwater quality. The main objective of this study was to evaluate the impact of fertilization practices used for grape cultivation on groundwater quality of Tidone Valley, northwest of Italy, following an integrated socio-hydrogeological approach that consists on (i) the involvement of 175 farmers in the description of agricultural and fertilization practices, using a survey of ad hoc questionnaires, (ii) the evaluation of NO₃^- occurrence in groundwater and (iii) the identification of NO₃^- sources through isotopic and hydrochemical analysis. In this area, as for certain particular Apennines shallow aquifers, groundwater is of reduced interest due to its limited storage capacity and there are insufficient wells currently monitored by the local Environmental Agency (ARPAE) to evaluate the impact of agricultural fertilization on existing local aquifers. Farmers' questionnaires results highlighted an extensive use of inorganic nitrogen fertilization and a tendency of farmers to follow their own experience for fertilization. Chemical analyses revealed high variability of major and trace elements concentrations isotope data. NO₃^- concentrations were significantly higher in deeper wells with respects to shallow wells. Isotopic results indicated that groundwater NO₃^- origin is inorganic, in agreement with the land use and the declared viticultural practices. Comparing groundwater NO₃^- occurrence from the studied area with values of entire Emilia-Romagna Region, only 7.7% of groundwater samples showed values above the EQS. (50 mg NO₃^-/L) between Nov 2017 and Sept 2018, while in the entire region 11.5% of groundwater samples showed values above the EQS in the same period. Considering that the vineyards surface in the studied area represents almost 75% of the entire regional vineyard surface, the obtained results suggest a low to moderate impact of viticulture on NO₃^-concentration of regional groundwater.