Water Quality Simulation And Control Based On Critical Source Areas Identification

Goal-oriented management is the key in water environment refined management currently.Identifying critical source areas?CSAs?,and adopting targeted control measures can enhance investment efficiency and improve water environment quality comprehensively.Taihu Basin is one of the most important watersheds in China,and more attention has been paid to water environment management.As one of counties in the watershed,the water pollution characteristics of Changxing County was analyzed,the CSAs was identified based on SWAT model and the key control area of the water environment was identified too.In order to impove the water quality of the key control area,the water quality model was used to simulate various measures and the cost-benefit analysis is used to find optimal solution.The main results are as follows:?1?The discharge pollution load of various pollution sources in Changxing County was calculated based on the export coefficient method.It showed that the county discharged about 6640.17 tons of COD,558.70 tons of ammonia nitrogen?NH₃-N?,1677.14 tons of TN and 333.90 tons of TP in 2017.The surface water quality of Changxing County has been improved from 2014 to 2017,but water quality in some sections still exceeded the standard or threshold.?2?SWAT model was used to simulate the nutrient loss in Changxing County.In2017,the annual loss load of TN was about 1649.55t,and the average loss load per unit area was 1152.73kg/km².The upstream was in a relatively low loss,and the load around the central and eastern plains and the lake inlet was higher.The loss load of TP was up to385.39t,and the average loss load per unit area was 269.32kg/km².The spatial distribution of TP was as the same as that of TN.According to the spatial distribution of nitrogen and phosphorus loss,the area with highest pollution load was defined as the CSAs based on the load per unit area index?LPUAI?.The CSAs accounted for about 3%of the total area of the county,but contributed more than 8%of the TN loss load and 11.5%of the TP loss load.The loss intensity of the TN and TP reached 28.37kg/ha and 4.00 kg/ha,which were significantly higher than other areas.?3?The key control areas was identified based on the CSAs.HEC-RAS model was established to simulate water quality in the area.The results showed that the water quality of the main sections was up to standard,and the water quality of upstream and Hexixingang was relatively good.However,the poor water quality areas mainly concentrated in the middle and lower reaches of Changxinggang,especially in the the rivers of urban area.?4?Fifteen water environment pollution control measures were chosen and analyzed by their investment and water quality improvement.The cost-benefit analysis was used to analyze the proposed measures under different scenarios such as the water quality of the key sections supposed to reach the standard,the water quality of all regional sections supposed to reach the standard,the cumulative environmental benefit was highest,and the single water quality index was best.The results showed that the measures of agricultural planting,aquaculture,livestock and poultry breeding,pollution control of rural life and drainage blockage and investigation had the best benefits.On the other hand,the construction of sewage pipe network had better environmental benefits.Due to its large investment,we suggested to promote gradually.As to urban area,it is recommended to give priority to the project of creating zero discharge community.Integrated sewage treatment equipment or implement moat displacement treatment project are supposed to be added on condition focusing on the pollution sources in this area.