Geochemistry, stable isotopes and statistic tools to estimate threshold and source of nitrate in groundwater (Sardinia, Italy)

• Published in: Water research (Oxford) 2023-04, Vol.232, p.119663-119663, Article 119663
• Main Authors: Biddau, Riccardo, Dore, Elisabetta, Da Pelo, Stefania, Lorrai, Mario, Botti, Paolo, Testa, Maurizio, Cidu, Rosa
• Format: Article
• Language: English
• Subjects: China; Environmental Monitoring - methods; Groundwater; Groundwater - chemistry; Italy; Manure - analysis; Mixing models; Nitrate pollution; Nitrates - analysis; Nitrogen - analysis; Nitrogen Isotopes - analysis; Sardinia; Sewage; Stable isotopes; Sulfur; Water Pollutants, Chemical - analysis
• ISSN: 0043-1354; 1879-2448
• DOI: 10.1016/j.watres.2023.119663

•Groundwater samples were collected to assess NO3− potential sources contamination.•Statistical tools were applied to define local NO3− threshold.•Combined geochemistry and multiple stable isotopes assess potential NO3− sources.•Mixing model results showed the prevalent NO3− sources (sewage and/or manure).•11B isotopic signatures in groundwater indicated the manure as predominant source. 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 δ18OSO4 of SO42− in groundwater samples indicated different sources of SO42−. Sulfur isotopic features attributed to marine SO42− were consistent with groundwater circulation in marine-derived sediments. Other source of SO42− were recognize due to the oxidation of sulfide minerals, to fertilizers, manure, sewage fields, and SO42− derived from a mix of different sources. Values of δ15N and δ18ONO3 of NO3− in groundwater samples indicated different biogeochemical processes and NO3− sources. Nitrification and volatilization processes might have occurred at very few sites, and denitrific