Geochemistry, stable isotopes and statistic tools to estimate threshold and source of nitrate in groundwater (Sardinia, Italy). (1st April 2023)

• Record Type: Journal Article
• Authors: Biddau, Riccardo; Dore, Elisabetta; Da Pelo, Stefania; Lorrai, Mario; Botti, Paolo; Testa, Maurizio; Cidu, Rosa
• Is Part Of: Water research. Volume 232(2023)
• Publication Date: 2023-04-01
• Subjects: Groundwater -- Nitrate pollution -- Stable isotopes -- Mixing models -- Sardinia; Water -- Pollution -- Research -- Periodicals; 363.7394
• DOI: 10.1016/j.watres.2023.119663
• Languages: English
• ISSNs: 0043-1354

Abstract

Highlights: 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. Abstract: 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 δ 34 S and δ 18 OSO4 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 δ 15 N and δ 18 ONO3 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 δ 11 B 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. Graphic abstract: Image, graphical abstract