Spatial variation patterns of water quality in the reclaimed-water-receiving reach of the Zhangcun River

To provide a preliminary assessment of spatial water-quality variation in reclaimed-water- receiving rivers, the reclaimed-water-receiving reach of the Zhangcun River was selected as the study area. In October 2023, eight sampling sites were established upstream and downstream of the wastewater treatment plant, and dissolved oxygen (DO), pH, electrical conductivity (EC), turbidity, total nitrogen (TN), ammonia nitrogen (NH₄⁺-N), nitrite nitrogen (NO₂⁻-N), nitrate nitrogen (NO₃⁻-N), and total phosphorus (TP) were measured to characterize the spatial distributions of basic physicochemical parameters and nutrients in the receiving reach. The results indicated pronounced spatial heterogeneity in water quality within the Zhangcun River receiving reach. For basic physicochemical parameters, DO in the downstream reach exhibited an overall increasing trend, pH remained relatively stable within a slightly alkaline range, EC remained high downstream, whereas turbidity was markedly higher upstream than downstream. For nutrients, NO₃⁻-N was the dominant inorganic nitrogen species, and the trend of TN was largely consistent with that of NO₃⁻-N. Overall, nitrogen showed a pattern of higher concentrations upstream, decreases near the outfall, followed by localized downstream fluctuations and a subsequent decline. TP increased significantly in the near-outfall segment and then decreased longitudinally. These observations suggest that the spatial water-quality pattern in the Zhangcun River reclaimed-water-receiving reach could not be explained by reclaimed-water input alone, but likely reflected the combined influence of upstream background inflow, effluent replenishment, and longitudinal in-stream adjustment.