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

The implementation of integrated management measures is one of the effective means to improve the water environment of the basin and to enhance water quality, and the assessment of the effects is the key to testing the effectiveness of the management 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 management 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 treatment 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 treatment 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 treatment 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 0.956 and 0.956 in the year of abundance and flat water, respectively, and 0.881 in the year of dry water, which demonstrated the best effect of treatment measures on water quality improvement. Therefore, the R-R-V index and the entropy method are useful for assessing the effectiveness of integrated measures and selecting the best measures. This study provides a foundation for implementing water quality management objectives in the Zouping section of the Xiaoqing River Basin.