Geochemistry, stable isotopes and statistic tools to estimate threshold and source of nitrate in groundwater...

In the European Union, nitrate vulnerable zone (NVZ) should be designed for the mitigation of nitrate (NO₃^-) contamination caused by agricultural practices. Before establishing new NVZ, the sources of NO₃^- must be recognized. A geochemical and multiple stable isotopes approach (hydrogen, oxygen, nitrogen, sulfur and boron) and statistical tools were applied to define the geochemical characteristics of groundwater (60 samples), calculate the local NO₃^- threshold and assess potential sources of NO₃^- contamination in two study areas (hereafter Northern and Southern), located in a Mediterranean environment (Sardinia, Italy). Results of the integrated approach applied to two case study, permits to highlight the strengths of integrating geochemical and statistical methods to provide nitrate source identification as a reference by decision makers to remediate and mitigate nitrate contamination in groundwater. Hydrogeochemical features in the two study areas were similar: near neutral to slightly alkaline pH, electrical conductivity in the range of 0.3 to 3.9 mS/cm, and chemical composition ranging from Ca-HCO₃^- at low salinity to Na-Cl^- at high salinity. Concentrations of NO₃^- in groundwater were in the range of 1 to 165 mg/L, whereas the nitrogen reduced species were negligible, except few samples having NH₄⁺ up to 2 mg/L. Threshold values in the studied groundwater samples were between 4.3 and 6.6 mg/L NO₃^-, which was in agreement with previous estimates in Sardinian groundwater. Values of δ³⁴S and δ¹⁸O_SO4 of SO₄^2- in groundwater samples indicated different sources of SO₄^2-. Sulfur isotopic features attributed to marine SO₄^2- were consistent with groundwater circulation in marine-derived sediments. Other source of SO₄^2- were recognize due to the oxidation of sulfide minerals, to fertilizers, manure, sewage fields, and SO₄^2- derived from a mix of different sources. Values of δ¹⁵N and δ¹⁸O_NO3 of NO₃^- in groundwater samples indicated different biogeochemical processes and NO₃^- sources. Nitrification and volatilization processes might have occurred at very few sites, and denitrification was likely to occur at specific sites. Mixing among various NO₃^- sources in different proportions might account for the observed NO₃^- concentrations and the nitrogen isotopic compositions. The SIAR modeling results showed a prevalent NO₃^- source from sewage/manure. The δ¹¹B signatures in groundwater indicated the manure to be the predominant NO₃^- source, whereas NO₃^- from sewage was recognized at few sites. Geographic areas showing either a predominant process or a defined NO₃^- source where not recognize in the studied groundwater. Results indicate widespread contamination of NO₃^- in the cultivated plain of both areas. Point sources of contamination, due to agricultural practices and/or inadequate management of livestock and urban wastes, were likely to occur at specific sites.