Distinguishing sources of nitrate in groundwater

Anthropogenic inputs have significantly impacted the nitrogen cycle, with NO3 pollution being a major concern affecting water resources worldwide [Shukla and Saxena, 2019]. The EU Nitrates Directive [EC, 1991] aimed to protect water quality by preventing and reversing degradation of groundwater due to NO3 from agricultural, urban and industrial wastes; NO3 concentrations should not exceed the limit of 50 mg/L in water bodies. Therefore, assessing NO3 sources is a fundamental prerequisite for planning sustainable groundwater management. In Sardinia, according to the EC Nitrates Directive, the intensively farmed area of Arborea was designated as Nitrate Vulnerable Zone [R.A.S., 2005]. In 2011, a program aimed at groundwater quality monitoring started. The NO3 concentration in groundwater varied from <0.8 to 515 mg/L and median 19 mg/L; 23% of values were higher than 50 mg/L. Flat and hilly areas in eastern Sardinia were the zones mostly affected by NO3 contamination. To estimate potential NO3 sources, groundwater samples were collected at 174 sites from 2013 to 2019. Hydrogeochemistry, stable isotopes (D, O, N, S and B), statistics (Gaussian mixture and Bayesian isotope mixing models) and land uses were combined to assess NO3 sources. The use of manure was a primary contributor to NO3 pollution, whereas urban sewage and synthetic fertilizers were subordinated. Results of this study have been used by the Sardinian Regional Authorities to review critical areas, giving a relevant contribution in identifying the new vulnerable zones according to the Nitrate Directive.