Using Dual Isotopes and a Bayesian Isotope Mixing Model to Evaluate Nitrate Sources of Surface Water in a Drinking Water Source Watershed, East China

A high concentration of nitrate (NO3-) in surface water threatens aquatic systems and human health. Revealing nitrate characteristics and identifying its sources are fundamental to making effective water management strategies. However, nitrate sources in multi-tributaries and mix land use watersheds remain unclear. In this study, based on 20 surface water sampling sites for more than two years' monitoring from April 2012 to December 2014, water chemical and dual isotopic approaches (delta N-15-NO3- and delta O-18-NO3-) were integrated for the first time to evaluate nitrate characteristics and sources in the Huashan watershed, Jianghuai hilly region, China. Nitrate-nitrogen concentrations (ranging from 0.02 to 8.57 mg/L) were spatially heterogeneous that were influenced by hydrogeological and land use conditions. Proportional contributions of five potential nitrate sources (i.e., precipitation; manure and sewage, M & S; soil nitrogen, NS; nitrate fertilizer; nitrate derived from ammonia fertilizer and rainfall) were estimated by using a Bayesian isotope mixing model. The results showed that nitrate sources contributions varied significantly among different rainfall conditions and land use types. As for the whole watershed, M & S (manure and sewage) and NS (soil nitrogen) were major nitrate sources in both wet and dry seasons (from 28% to 36% for manure and sewage and from 24% to 27% for soil nitrogen, respectively). Overall, combining a dual isotopes method with a Bayesian isotope mixing model offered a useful and practical way to qualitatively analyze nitrate sources and transformations as well as quantitatively estimate the contributions of potential nitrate sources in drinking water source watersheds, Jianghuai hilly region, eastern China.