Study On The Scheme Of Water Quality Stably Reaching Standards Of Seasonal Water-Shortage River Based On EFDC Model

In recent years,the problem of water environmental pollution in China has become increasingly serious,and the total amount control of pollutant discharge based on water environmental capacity has become the development trend of water environmental management.This paper takes the second tributary of the Han River—Gun River Basin as the object,and conducts research on water environment capacity accounting,allowable pollution load distribution and total amount control of pollutant discharge capacity,in order to propose the scheme of water quality stably reaching standard,and provide technical support for the comprehensive treatment of water environment pollution in the basin.Firstly,the Gunhe River Basin is divided into 3 water environment function areas and4 control units,and the 8 determined control sections are evaluated for the current water quality and change trend,and the main water quality pollution factors are identified as COD and NH₃-N,and the key control areas are the Juwan-Guanyinsi section and the tributary Shahe section.At the same time,6 sub-basin catchment areas were divided according to distributed hydrological model（Soil and Water Assessment Tool,SWAT）,and the statistical method and output coefficient method were used to calculate the pollution load of COD and NH₃-N into the river in the catchment area in 2019 was 1097.39 t/a and 87.15 t/a respectively.The contribution rates of point source COD and NH₃-N pollution into the river are 53.69%and 51.01%,and the contribution rates of non-point source COD and NH₃-N pollution into the river are 46.31%and 48.99%.Then,in order to determine the total emission control objectives,it is necessary to conduct research on water environment capacity verification and pollution load distribution.The Environmental Fluid Dynamics Code（EFDC）is establishes the source-sink dynamic response relationship between the pollution amount in the river and the water quality.The linear programming equation is solved by the water quality response coefficient in the wet,flat,and dry water seasons of the year,and the formula method was used for secondary verification.After error analysis and calibration,the water environment capacities of COD and NH₃-N are 698.10 t/a and 56.01 t/a respectively.Based on the water environment capacity,the safety margin is deducted and the total emission control target was determined,Under the concept of Total Maximum Daily Loads（TMDL）in the United States,the three-layer distribution is performed to obtain the allowable pollution amount of COD and NH3-N into the river is 665.19 t/a and 53.21 t/a respectively.The proportion of pollutant load and emission reduction targets for the six sub-basin are determined,which can realize hierarchical and zoning control management of the total amount of pollution.Finally,twelve scenarios of water quality standards were set up,including load reduction of sewage outlets,deep upgrading of tail water and ecological replenishment.Based on the simulation calculation of EFDC model,and according to the pollutant emission reduction targets of six sub-basins,specific engineering measures for water pollution prevention and control were put forward.From four aspects of point source pollution control,non-point source pollution prevention and control,water ecological restoration and long-term guarantee mechanism,a comprehensive pollution control system supporting water quality stably reaching standards standards in the Gunhe River Basin was constructed.