Evaluation of the comprehensive pollution control effectiveness in the Zouping section of the Xiaoqing River Basin based on water quality targets

Comprehensive pollution control is one of effective means to improve water environment and water quality, and the assessment of its effects is a key to test the effectiveness of these control measures. Xiaoqing River is located in the lower reaches of the Yellow River. It is an important drainage channel of Shandong Province. The river is a source of irrigation of farmland on both sides, and is used for river transport and access to the sea. However, the Zouping section of the Xiaoqing River basin has high concentrations of NH₃-N, TN, and TP, which do not meet water quality objectives. Focusing on the Zouping section of the Xiaoqing River Basin, three major pollutants affecting the water quality of the basin (NH₃-N, TN, TP) were treated. According to the source of the pollutant load, comprehensive control measures combining internal and external sources are considered from the aspects of upgrading the sewage treatment plant, centralised collection and treatment of rural sewage, reduction of fertiliser application, regional afforestation and reduction of upstream incoming water load. Five combinations were created. The SWAT model was established based on the basic data of the basin and the parameter rates were determined to simulate the five treatment scenarios under different precipitation conditions, and to analyse whether the concentrations of NH₃-N, TN, and TP could reach the target of Class III water quality of surface water after the implementation of the control measures. A combination of the R-R-V (Reliability-Resilience-Vulnerability)index method and the entropy method were used to assess the effectiveness of the combined measures in terms of reliability, reparability and vulnerability. On this basis the best measure were selected. The results showed that different combinations of control measures were able to improve the water environment condition. The concentrations of NH₃-N, TN and TP after the five treatment scenarios were significantly lower than those before the treatment under abundant, flat and dry precipitation, and the annual average concentrations of pollutants in the control scenarios of S2, S3, S4, and S5 were able to reach the target of Class III water quality. Under S5 (the scenario including urban sewage treatment rate increased to 100%, rural sewage treatment rate increased to 80%, bare land reduced by 30 km² , upstream load reduced by 40%, fertiliser application reduced by 40%), the monthly concentrations of pollutants in the Tangkouqiao section reached the target of Class III water quality during wet, normal, and dry years. The S5 scenario had the highest composite score under different rainfall scenarios, with 0.96 and 0.96 in the year of abundance and flat water, respectively, and 0.88 in the year of dry water, which demonstrated the best effect of treatment measures on water quality improvement. This study proves that the R-R-V index method and entropy method are good for assessing the effectiveness of combined control measures. This study also provides a way to implement water quality management targets in the Zouping section of Xiaoqing River Basin.