Source apportionment methods for upstream inflows of nitrate nitrogen in the reservoir using stable isotopes

WEI Jian; CHEN Li-li; CHEN Jun-jun; XIE Rong-rong; LI Jia-bing; MA Xiao-dan; WU Qiong-yao; SHI Cheng-chun

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China Environmental Science ›› 2026, Vol. 46 ›› Issue (3) : 1543-1553.

Environmental Ecology

Source apportionment methods for upstream inflows of nitrate nitrogen in the reservoir using stable isotopes

WEI Jian¹, CHEN Li-li², CHEN Jun-jun², XIE Rong-rong^1,3, LI Jia-bing^1,3, MA Xiao-dan^1,3, WU Qiong-yao¹, SHI Cheng-chun²

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Abstract

In this study, the ShuiKou Reservoir (SR) in the Min River Basin and its upstream inflows (T1~T3) were selected as the research area. The Bayesian Isotope Mixing Model (MixSIAR) was employed to conduct source apportionment of nitrate contributions during both non-flood and flood seasons. Two approaches were implemented: conventional nitrate nitrogen-oxygen isotopes and a newly developed "water hydrogen-oxygen stable isotope-nitrate concentration" method. The results from both methods were validated through a complete mixing model and the deviation information criterion (DIC) values from MixSIAR. The findings revealed that nitrate nitrogen concentrations in non-flood seasons exceeded those in flood seasons. Among the upstream inflows, the Shaxi River (T3), which is adjacent to an industrial area and has the highest proportion of construction land in its vicinity, demonstrated the most severe nitrate pollution. During flood seasons, a decline of δ¹⁸O-NO₃^- values in T3and SR were observed, alongside a reduction in both water δ¹⁸O-H₂O and δD-H₂O values within the entire research area. The results indicated that rainfall with lower δ¹⁸O-H₂O and δD-H₂O signatures brought pollutants with lower δ¹⁸O-NO₃^- to the research area. Secondly, both methods showed comparable source apportionment results during flood seasons (maximum error 13.04%), but exhibited significant discrepancies in non-flood seasons. Conventional isotope analysis under 0fractionation yielded contributions: T2 (49.4%) > T3 (26.1%) > T1 (24.5%), whereas the new method indicated: T3 (44.8%) > T1 (29.6%) > T2 (25.6%). Thirdly, the novel method demonstrated superior reliability, with lower DIC than those obtained using conventional methods under different fractionation. The nitrate concentration errors validated by the complete mixing model at merely 1.22% (flood season) and 0.89% (non-flood season). Those further confirmed its highest credibility. This methodology shows potential for extension to other pollutant source tracking studies, providing solution for watershed water quantity-quality integration and pollution control management.

Key words

stable isotopes / MixSIAR model / inflow contribution apportionment / nitrate nitrogen

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WEI Jian, CHEN Li-li, CHEN Jun-jun, XIE Rong-rong, LI Jia-bing, MA Xiao-dan, WU Qiong-yao, SHI Cheng-chun. Source apportionment methods for upstream inflows of nitrate nitrogen in the reservoir using stable isotopes[J]. China Environmental Science. 2026, 46(3): 1543-1553

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