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

Highlights: • NO₃⁻/Cl⁻ variations suggested that NO₃⁻ mainly from manure and sewage in the upper Mun. • Theoretical and measured δ¹⁸O suggested nitrification was the dominate processes. • Denitrification proved to dominate the nitrate transformation in middle and lower Mun. • Chemical fertilizer, soil N and Manure and sewage contributed more than 90% of nitrate inputs. 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 δ¹⁵N-NO₃⁻ and δ¹⁸O-NO₃⁻ ranged from −3.9‰ to +16.6‰ and from −5.2‰ to +40.0‰, respectively. The results of nitrate isotopes indicated that NO₃⁻ mainly originated 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 for further researches and approaches to assess the effects of tropical climate and agriculture on nitrate accumulation in watershed.