Evaluation of water quality fluctuation in the tidal reach under the impact of on shore wastewater discharges based on MIKE 21 model in dongguan, China

The text discusses the study of water quality simulation in the lower reaches of the Dongjiang River basin—tidal river section. Utilizing MIKE 21 software, an integrated 2D hydrodynamic-water quality coupled mathematical model was constructed for the lower reaches and estuary area of the Dongjiang River. The model simulated the hydrodynamic characteristics near the wastewater outlet and the impact of pollutant discharge on the water quality in the study area. The simulation results indicate: (1) During wet and dry seasons, the overall flow pattern remains consistent, with the ebb current velocity slightly greater than the flood tide current velocity beside the outlet. The flow velocity is generally greater during wet season compared to dry season. Interestingly, the flow direction during flood and ebb tides is completely opposite and tides have a great impact on the maximum flow velocity in Xialu Connecting channel. (2) Under normal discharge conditions, the increment of COD, NH₃-N, and TP in most areas of Zhongtang Waterway, Daoyun Waterway, and Hengchong Waterway is less than 5 mg/L, 0.1 mg/L, and 0.02 mg/L, respectively, indicating a minor impact on water quality; While in the event of an accidental discharge, the concentration increment is in the range of 5–30 mg/L, 0.1–0.25 mg/L, and 0.02–0.05 mg/L, respectively. COD causes the most significant water quality changes and will severely impact the water environment of the Dongjiang River basin. Furthermore, pollutant dispersion is more extensive during dry seasons due to reduced river flow and weaker dilution capacity. (3) Overall, the results underscore the critical importance of accounting for both tidal and seasonal dynamics in simulation of estuarine water quality.