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When can point-source buffering be overridden? Diagnostic evidence for concentration-discharge regime transitions
Guoshuai Zhang1, Peng Zhang2, Shunxing Qin2
1Chinese Academy of Environmental Planning, Beijing, 100041, China; United Center for Eco-Environment in Yangtze River Economic Belt, Beijing, 100041, China.
Water Research
|June 27, 2026
Summary
Continuous point-source discharges can reduce nonpoint-source nutrient signals in rivers. Our study shows increasing point-source intensity generally leads to less nutrient enrichment, though exceptions highlight complex interactions needing pollutant-specific management.
Area of Science:
- Environmental Science
- Hydrology
- Water Quality Management
Background:
- Concentration-discharge (C-Q) relationships are key to understanding riverine nutrient dynamics.
- The impact of continuous point-source (PS) discharges on episodic nonpoint-source (NPS) nutrient signals in mixed-source rivers is not well understood.
- Nutrient pollution (TN, TP, NH₃-N) remains a significant environmental challenge.
Purpose of the Study:
- To investigate how point-source intensity influences nutrient C-Q regimes in a mixed-source river system.
- To identify conditions where the buffering effect of point sources on nonpoint sources may be weakened or masked.
- To provide insights for pollutant-specific nutrient management strategies.
Main Methods:
- Analysis of high-frequency water quality data, daily discharge, and point-source effluent records at seven monitoring stations.
- Application of a multi-scale diagnostic framework including GLS-corrected C-Q classification, seasonal decomposition, and hydrograph-limb analysis.
- Utilized XGBoost-SHAP for residual attribution to understand drivers of deviations from C-Q relationships.
Main Results:
- Increased point-source intensity correlated with lower TN C-Q slopes, shifting behavior from enrichment towards chemostasis.
- Observed deviations from expected C-Q trends at specific sites, suggesting factors like dissolved-particulate phosphorus superposition, headwater NPS flushing, and urban wet-weather inputs.
- Seasonal decomposition improved model explanatory power, revealing wet-dry regime reversals and highlighting pollutant-specific drivers (biogeochemistry, antecedent discharge, effluent variability).
Conclusions:
- Continuous point-source discharges can significantly alter nutrient C-Q dynamics in mixed-source rivers.
- Specific site and pollutant conditions can override the general buffering effect of point sources.
- Findings support the need for tailored, pollutant-specific nutrient management approaches in complex river systems.
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