Identifying the sources of nitrate contamination using a combined dual isotope, chemical and Bayesian model approach in a tropical agricultural river: Case study in the Mun River, Thailand

Environmental issues triggered by increasing nitrate in agricultural river has become global concern. Identifying nitrate sources and transformation are crucial for water sources protection and eliminating nitrate contamination in an agricultural watershed. In this study, chemical and dual isotopic compositions of nitrate were employed to trace the nitrate sources and transformation processes, and proportional contribution of NO3- source were estimated by SIAR based on Bayesian model. NH4+ concentrations in middle Mun and lower Mun in wet season were significantly higher than NO3-, suggesting enhanced runoff processes by flood promote agricultural fertilized NH4+ leaching into the river. Higher Cl- concentration and NO3-/ Cl- indicated that manure and sewage was the dominate nitrate source in the Lam Takhong River and the upper Mun. The overall values of delta N-15- NO3- and delta O-18-NO(3)(-)ranged from -3.9 parts per thousand to +16.6 parts per thousand and from -5.2 parts per thousand from soil N nitrogen, chemical fertilizer, and manure and sewage wastes. Spatially, soil N and chemical fertilizer contributed the most nitrate in the mainstream of lower Mun, middle Mun, and the Lam Takhong River; whereas over 60% of nitrate was derived from manure and sewage in the upper Mun. The spatial variation of water discharge and rainfall, together with the nitrate concentration and isotopes inferred that the nitrate sources and transformations in rain- fed river in tropical zone were distinguished from other rivers. High water discharge driven by rainfall events accelerated the nitrate export and the contribution of atmospheric deposition in wet season, and enlarged the contribution of manure and sewage in dry season. This study provided an example

Automated summary

This study highlighted the importance of identifying nitrate sources and transformations in agricultural watersheds to protect water sources. The study employed chemical and dual isotopic compositions of nitrate to trace sources and processes, estimating the proportional contribution of different nitrate sources. Results showed that NH4+ concentrations were significantly higher than NO3- in certain river sections during the wet season, indicating enhanced runoff processes and fertilized NH4+ leaching. Additionally, manure and sewage were identified as dominant nitrate sources in certain river sections.