Mechanisms of surface water nitrogen pollution during flood events in a mountainous agricultural watershed: Implications for non-point source management

Agricultural non-point source pollution poses a persistent threat to drinking water security in mountainous watersheds, yet the seasonal and event-scale dynamics of nitrate sources remain poorly constrained. This study investigated nitrate sources and transformation mechanisms in the upper Hutuo River Basin—the primary water source for Shijiazhuang City (population >10 million)—by integrating hydrochemical analysis, nitrate dual isotopes (δ¹⁵N–NO₃⁻ and δ¹⁸O–NO₃⁻), water stable isotopes (δ²H–H₂O and δ¹⁸O–H₂O), and the MixSIAR model. Surface water sampling was conducted during the dry season (May), wet season (August), and normal season (October) of 2023, coupled with high-frequency monitoring during a typhoon-induced flood event. Nitrate concentrations followed a distinct seasonal pattern: highest in the dry season (15.1 mg/L), lowest in the wet season (6.7 mg/L), and intermediate in the normal season (9.7 mg/L). Soil nitrogen dominated in the dry season (82% contribution), whereas manure & sewage (39.1%) and chemical fertilizers (22.2%) increased markedly during the wet season due to surface runoff flushing. During the flood event, NO₃⁻ concentration peaked at 20.38 mg/L at 5.3 h after the flood peak (778 m³/s). At the NH₄⁺ peak (July 30, 10:00), manure & sewage dominated (75.0%, 95% CI: 52.4–93.8%), reflecting first-flush mobilization of surface-accumulated sewage and manure. In contrast, at the NO₃⁻ peak (July 31, 14:00), chemical fertilizers dominated (80.1%, 95% CI: 53.6–96.9%), reflecting subsequent flushing of fertilizer-derived nitrate. Nitrification was the dominant nitrogen transformation process across most seasons, as indicated by δ¹ ⁸O-NO₃⁻ values generally between + 1.0‰ and + 4.8‰ at most sites. These findings reveal asynchronous NO₃⁻ and NH₄⁺ responses during flood events: NH₄⁺ peaked before the flood peak due to first-flush mobilization of surface-accumulated manure and sewage, whereas NO₃⁻ peaked 5.3 h after the flood peak, reflecting rapid mobilization of fertilizer-derived nitrate through shallow subsurface and riparian pathways. These findings highlight the critical need for seasonally adaptive agricultural management: reducing fertilizer application before monsoon rainfall, optimizing timing and placement of nitrogen inputs, and strengthening rural sewage interception to protect drinking water quality in mountainous agricultural watersheds.