Investigating the environmental response mechanisms of urban basin to reclaimed water system: A case study in Northeast China

Reclaimed water (RW) replenishment is an effective approach to addressing regional water scarcity in river basins; however, such artificial interventions significantly influence the basin environment. This study investigates the environmental response mechanisms in the Hejiagou (RHJ) River Basin in Harbin, a typical water-scarce city in Northeast China, under RW replenishment. Through in-situ monitoring from 2021 to 2024, MIKE21 modeling simulations, and statistical analysis, this study explores the basin's environmental response mechanisms under RW replenishment. The results indicate that RW recharge increased channel storage capacity by 34.62%. During non-frozen periods and when water shutdown conditions were not present, flow rates stabilized at 2 ~ 3m³/s, with velocities exceeding 2.5 m/s. Fluctuations in discharge water quality emerged as the primary factor influencing downstream river segments, with seasonal and spatial variations determining differential quality levels across different reaches. The physical, chemical, and biological properties of sediments and microorganisms downstream of the discharge point were highly correlated with water quality. Long-term discharge and scouring elevated the downstream riverbed by 0.5 ~ 1.0m, with an average scour depth of 0.9m/a. Hydrodynamic simulations revealed substantial impacts of the discharge point on all river segments. Recommendations for strategies, such as wastewater treatment plant upgrades and dredging, provide scientifically grounded guidance for mitigating similar environmental issues in catchment areas.