Identification of groundwater nitrate sources in an urban aquifer (Alborz Province, Iran) using a multi-parameter...

High concentrations of NO₃̄ in water resources are detrimental to both human health and aquatic ecosystems. Identification of NO₃̄ sources and biogeochemical processes is a crucial step in managing and controlling NO₃̄ pollution. In this study, land use, hydrochemical data, dual stable isotopic ratios and Bayesian Stable Isotope Mixing Models (BSIMM) were integrated to identify NO₃̄ sources and estimate their proportional contributions to the contamination of the Karaj Urban Aquifer (Iran). Elevated NO₃̄ concentrations indicated a severe NO₃̄ pollution, with 39 and 52% of groundwater (GW) samples displaying the concentrations of NO₃̄ in exceedance of the World Health Organization (WHO) standard of 50 mg NO₃̄ L^-1 in the rainy and dry seasons, respectively. Dual stable isotopes inferred that urban sewage is the main NO₃̄ source in the Karaj Plain. The diagram of NO₃̄/Cl‾ versus Cl‾ confirmed that municipal sewage is the major source of NO₃̄. Results also showed that biogeochemical nitrogen dynamics are mainly influenced by nitrification, while denitrification is minimal. The BSIMM model suggested that NO₃̄ originated predominantly from urban sewage (78.2%), followed by soil organic nitrogen (12.2%), and chemical fertilizer (9.5%) in the dry season. In the wet season, the relative contributions of urban sewage, soil nitrogen and chemical fertilizer were 87.5, 6.7, and 5.5%, respectively. The sensitivity analysis for the BSIMM modeling indicates that the isotopic signatures of sewage had the major impact on the overall GW NO₃̄ source apportionment. The findings provide important insights for local authorities to support effective and sustainable GW resources management in the Karaj Urban Aquifer. It also demonstrates that employing Bayesian models combined with multi-parameters can improve the accuracy of NO₃̄ source identification.