This paper is the result of hydrogeological survey engineering.

Objective To evaluate the water environment quality of the water source area during the initial stage of the South-to-North Water Diversion Project's Middle Route and to identify the key pollution factors limiting water quality and their sources.

Methods This study utilizes annual water quality monitoring data from the upper reaches of the Danjiangkou reservoir area (2016-2019) as the research subject. Six indicators were selected for evaluation: Permanganate index, ammonia nitrogen, total phosphorus, total nitrogen, sulfide, and fecal coliforms. The comprehensive pollution index method and pollution contribution rate were employed to assess the spatiotemporal variations in water quality. Additionally, data from wastewater treatment plants and the export coefficient model (ECM) were used to estimate point-source and agricultural non-point-source total nitrogen (TN) loads, respectively. Based on hydrological data, the TN flux and contribution of the main inflow rivers were calculated.

Results From 2016 to 2019, concentrations of all evaluated indicators except total nitrogen (TN) declined markedly, and by 2019 most monitoring sections had met or were approaching the Grade Ⅱ criterion of the Environmental Quality Standards for Surface Water (GB 3838-2002). TN concentrations, by contrast, increased steadily. Spatially, enrichment was most pronounced at section A6 (Hanzhong Basin reach of the Hanjiang River, downstream of the headwater zone) and section A12 (Danjiang River inflow section); by 2019, TN at most sections exceeded the Grade Ⅱ limit (0.5 mg/L) by more than fivefold and accounted for 67%–85% of the total pollution load, making TN the dominant factor constraining water quality in the source area. At the section level, the reaches under Hanzhong and Shiyan jurisdictions (sections A1–A10, middle-upper Hanjiang, together with the Danjiang inlet A12 and its surrounding tributaries) showed steeper TN increases and more severe exceedance than those under Ankang and Shangluo. Along the Hanjiang mainstem, concentrations rose progressively from the headwaters to the Hanzhong Basin (A1–A6), with A6 marking the river-wide maximum. At the Danjiang inlet A12 and the adjacent tributaries, compounded urban and agricultural inputs drove TN in 2019 to 5–7 times the Grade Ⅱ limit, establishing this area as the primary nitrogen source for the reservoir bays and inlet zone. The spatial concentration gradient across these reaches is consistent with the ranking of inflow contributions (Hanjiang 64.37%, followed by the Danjiang), indicating that section-level differences are governed primarily by upstream load intensity rather than by riverine dilution. Source-apportionment results identify non-point sources as the dominant TN input to the reservoir. In 2016–2017, the contribution ranking was livestock and poultry farming (~38%) > cropland runoff (~34%) > rural domestic sewage (~27%), a compound-dominated regime involving all three sources. After 2018, declining livestock inventories reduced the intensity of the breeding-source input, while alternating drought and flood conditions intensified nitrogen loss from fertilized cropland; cropland runoff consequently rose to ~39% and overtook the other sources to become the leading contributor. Regarding point sources, effluent TN from urban wastewater treatment plants (5.40–7.96 mg/L) met the Class 1A standard (15 mg/L) of the Discharge Standard of Pollutants for Municipal Wastewater Treatment Plants (GB 18918-2002), but still exceeded the surface-water Grade Ⅱ criterion (0.5 mg/L) by over an order of magnitude, sustaining localized concentration hotspots in the immediate vicinity of the outfalls. The Hanjiang River is the core TN delivery pathway, contributing 64.37% of the total inflow and requiring a reduction of 44,449.9 t/a; the Duhe and Danjiang rivers rank next. TN total-load control is therefore required for all major inflow rivers.

Conclusions This study can provide a comparable baseline for establishing a long-term dynamic assessment and adaptive management strategy for total nitrogen (TN) along the entire chain—from "sources (key cities)" to "pathways (major rivers)" and finally to the "target (reservoir area)"—in the water source region of the South-to-North Water Diversion Middle Route.