2026 Volume 49 Pages 89-98
Distinguishing between basin-derived loads (originating from, for example, domestic and industrial wastewater, and agricultural land) and atmospheric deposition–derived loads is crucial for riverine eutrophication control. However, completely separating the legacy components of atmospheric nitrogen accumulated in the basin over long periods remains challenging. In this study, rather than attempting a complete separation of all components, we propose a simplified top-down statistical model to estimate the mitigation potential—the riverine nitrogen load that can be rapidly reduced in response to air quality improvements—using only publicly available long-term monitoring data. Focusing on the strong correlation between total nitrogen (TN) and total phosphorus (TP) under stable water quality conditions, the model extracts the nitrogen component not associated with TP that tracks short-term time lags (0-2 months) relative to atmospheric NOx concentrations. In doing so, we incorporate a nonnegative constraint to ensure physical validity. Applying the model to the Katsura River in the Sagami River system, the maximum mitigation potential for riverine nitrogen load under stable, non-rainfall conditions as of 2021 was estimated to be approximately 9%, assuming the complete removal of atmospheric nitrogen oxides. These results suggest that, as of 2021, basin-based measures, such as wastewater treatment, continue to have a dominant effect in the target basin. Our method enables the estimation of the mitigation potential of air quality management using only existing data without the need for additional field surveys, facilitating rapid diagnosis for prioritizing air and water quality policies.
