Nitrate transformation and source tracking of rivers draining into the Bohai Sea using a multi-tracer approach combined with an optimized Bayesian stable isotope mixing model

Excessive levels of NO3 -can result in multiple eco-environmental issues due to potential toxicity, especially in coastal areas. Accurate source tracing is crucial for effective pollutant control and policy development. Bayesian models have been widely employed to trace NO3 -sources, while limited studies have utilized optimized Bayesian models for NO3 - tracing in the coastal rivers. The Bohai Rim is highly susceptible to ecological disturbances, particularly N pollution, and has emerged as a critical area. Therefore, identification the N fate and understanding their sources contribution is urgent for pollution mitigation efforts. In addition, understanding the influenced key driven factors to source dynamic in the past ten years is also implication to environmental management. In this study, water samples were collected from 36 major river estuaries that drain into the Bohai Sea of North China. The main transformation processes were analyzed and quantified the sources of NO3 -using a Bayesian stable isotope mixing model (MixSIAR) with isotopic approach (815N-NO3- and 818O-NO3-). The overall isotopic composition of 815N-NO3- and 818O-NO3- in estuary waters ranged from -0.8-19.3%o (9.3 +/- 4.6%o) and from -7.1-10.5%o (5.0 +/- 4.3%o), respectively. The main sources of nitrate in most river estuaries were manure & sewage, and chemical fertilizer, while weak denitrification and mixed processes were observed in Bohai Rim

Automated summary

Excessive levels of NO3- can cause environmental issues, particularly in coastal areas. Accurate source tracing is crucial for effective pollution control and policy development. This study collected water samples from major river estuaries in the Bohai Sea of North China and used stable isotope analysis to determine the sources of NO3-.