Pollution Load Estimation And Source Analysis In Cao’e River Basin

With the rapid development of economy and industry in global coastal cities,people attach great importance to the coastal ecological environment.Hangzhou Bay-Zhejiang offshore is the sea area with the most serious eutrophication and the most prominent ecological and environmental problems in China’s offshore.Cao’e River finally flows into Hangzhou Bay.The discharge of regional nitrogen and phosphorus nutrients causes deterioration of water quality,and the water environment pollution problem in the basin is still not optimistic.In this study,Cao’e River basin is taken as the research object,and samples are taken in January 2022（dry season）and June 2022（wet season）to measure the concentration and temporal and spatial distribution characteristics of different forms of nitrogen and phosphorus were analyzed.The historical water quality was evaluated by single factor evaluation method to determine the main pollutants in the area.Statistical method and output coefficient method are used to estimate the pollution load of point source and non-point source and analyze the source of pollutants.Calculate the water environment capacity of the basin,allocate the water environment capacity by using the optimization allocation method,the comprehensive allocation method and the contribution rate allocation method,refine the environmental capacity of each control unit,and propose the pollutant reduction amount and reduction rate in combination with the pollutant inflow of each control unit,so as to provide a basis for the water environment management in the Cao’e River basin.The main conclusions are as follows:（1）TN concentration in Cao’e River basin is relatively high,and NO₃^--N is the main form of TN.In space,the concentrations of TN and NO₃^--N show obvious differences,and the spatial distribution characteristics of NH₃-N and NO₂^--N are basically consistent.The concentrations of TP,DTP,PO₄^3-,PP in the dry season and DTP,PO₄^3-in the wet season showed a spatial downward trend from upstream to downstream,while the concentrations of TP and PP in the wet season were opposite to those in the dry season.It’s obvious that the single factor assessment results the COD,NH₃-N and TP of the four monitoring sections can meet the water quality objectives and meet the national Class III surface water quality standard.（2）Based on the analysis of the pollution sources of the basin,non-point source pollution is the main pollution,of which COD and NH₃-N are mainly from rural domestic pollution sources,and TN and TP are mainly from pollution sources of different land use types.In terms of spatial distribution,the amount of four pollutants entering the river has similar spatial distribution characteristics,and the contribution rates of COD,NH₃-N,TN and TP pollutants entering the river of control units 4,8,9,11 and 21 to Cao’e River are45.12%,41.53%,38.83%and 35.18%respectively.Therefore,these five control units should be taken as key units for control.（3）Based on the constraints of meeting the water quality standards of the monitoring section and the section into the sea,the water environmental capacity of COD,NH₃-N and TP of the basin water is 47945.17 t,2435.79 t and 474.98 t respectively.The optimal allocation method,comprehensive allocation method and contribution rate allocation method are used to allocate the water environmental capacity,and the environmental capacity of each control unit is refined.（4）According to the calculation,there is still residual environmental capacity for COD in the basin.Only NH₃-H and TP need to be reduced.NH₃-N needs to be reduced by 6.91 t,TP needs to be reduced by 128.36 t.TP reduction is relatively high,so it is necessary to strengthen the management of phosphorus pollutant discharge.