Study On The Migration And Transformation Of Pollutants And Their Environmental Capacity In Tianjin Coastal Waters

In recent years,with the rapid development of the economy and society,a large amount of wastewater has been discharged into the sea,resulting in the deterioration of the water quality in the coastal waters and the decline of the marine carrying capacity.Environmental capacity,as a limited marine resource,should be rationally planned and utilized.As an important part of the Bohai Rim Economic Belt,Tianjin enjoys rich marine resources.Because it is located on the west coast of the Bohai Bay,its economic development depends on marine resources,while its rapid population growth and industrial development have a great impact on the marine environment.In order to prevent the deterioration of marine ecological environment and balance marine environmental capacity and the demand of industrial development,it is imperative to calculate the environmental capacity and control the emission of pollutants reasonably.Based on Delft3 D software,a three-dimension hydrodynamic and water quality model of Tianjin coastal waters are established.The hydrodynamic model is verified by the measured data of water level,current and the harmonic constants observed by tide stations around Bohai Sea.The results show that the established hydrodynamic model can better reflect the tidal current in the coastal waters of Tianjin.On this basis,a water quality model of Tianjin coastal waters is established to simulate the migration and transformation of chemical oxygen demand,inorganic nitrogen and active phosphate in Tianjin coastal waters in 2017.The water quality model is verified according to the monitoring data and the results show a good agreement.In order to explore the influence of atmospheric subsidence on the water quality of Tianjin coastal waters,the atmospheric subsidence of inorganic nitrogen is considered in the water quality model,and it is found that the deposition of inorganic nitrogen accounted for about 10-20% of the total amount of inorganic nitrogen at the monitoring point.The shallower the water depth is,the greater the impact of inorganic nitrogen deposition on the water quality at the monitoring station has.According to the simulation results of water quality,the response concentration of each pollutant in each estuary under the actual discharge flux is obtained.Control points and their standards are determined based on the division of functional areas in Tianjin's coastal waters.Two different control point schemes are set up to calculate environmental capacity in this paper.The calculation results show that,under the limitation of 81 control points,the COD has residual capacity in Tianjin coastal waters.The discharge of COD should be reduced in Yongdingxin River and Dagu Drainage River and the residual capacity can be mainly discharged through the Haihe River,Duoliu River,etc.The residual capacity of inorganic nitrogen is small,and the discharge flux in the first quarter should be controlled.Inorganic nitrogen can be discharged in the third quarter through Haihe River,Duliujian River,Ziyaxin River and Qingjinghuang Drainage Channel.The residual capacity of active phosphate is large and can be discharged through Haihe River,Duliujianhe River,Ziyaxin River and Beipaishui River.Under the limitation of 107 control points,compared with the calculated results at 81 control points,the environmental capacity of COD and active phosphate decreased,the COD environmental capacity decreased 119,490 tons,and the active phosphate decreased 1687 tons.The residual capacity of inorganic nitrogen has2781 tons at 81 control points,while at 107 control points the current capacity should be cut by 28.39% to meet the standard of control points.The number and position of control points have great influence on the calculation result of environmental capacity.