Quantitative identification of nitrate sources in a coastal peri-urban watershed using hydrogeochemical indicators and dual isotopes together with the statistical approaches.

Surplus nutrient load and complex migration and transformation processes are the challenges for water quality management in the peri-urban coastal watershed, leading to increasing concerns worldwide. We investigated the spatio-temporal variation of hydrogeochemical parameters in surface water of Jimei Lake watershed, and distinguished the sources and transformation of nitrate-N (NO-N) using dual isotopes of nitrate (δN and δO in NO) with hydrogeochemical indicators. Principal component analysis (PCA) on hydrogeochemical parameters demonstrated that surface water was seriously polluted by nutrients, especially in the southeast of the downstream. There were signs of seawater intrusion and increased wastewater discharge in the mid-lower reaches with high ammonium concentrations. Nitrification occurred throughout the monitoring period with lower δN and δO values and NO derived from mixed pollution sources. Results of Bayesian model showed that dominant NO input originated from manure and sewage (M&S, 71% and 76% in the wet and dry season, respectively) and atmospheric deposition (22% and 16%, respectively). This result implied that the controls and treatment of M&S discharges are essential to alleviate of NO pollution. The proposed method is helpful to understand the origins of NO and may be suitable to develop measures for the reducing of nitrogen loadings in the peri-urban watershed.