Study On Water Pollution Control Of Mountain River Based On EFDC Model

With the development of modern society,China is in a period of rapid development of urbanization.At the same time as urban expansion,it brings pollution to rivers,lakes and other water bodies in the region with different limits.Therefore,improving the urban water environment has become an urgent task.Mountain rivers have typical characteristics such as large terrain height difference,fast water flow,short storm current peaks,and severe initial scouring of surface pollutants.They bear a greater pollution load and pose a huge challenge to water pollution control research.Water environment research usually refers to a large space-time span and complex pollutant components and environmental change processes,but laboratory analysis or on-site water quality monitoring is not sufficient to restore the water environment change process.Therefore,numerical simulation of water environment is realized by building a mathematical model and exploring the evolution law of river water environment has become a hot topic of water environment protection.In this paper,the Changsheng River in Chayuan New District of Chongqing City is taken as the research object,and the Changsheng River hydrodynamic water quality model is constructed based on the water environment mathematical model EFDC software platform.Based on the simulation analysis and statistical analysis,the change of the Changsheng River water environment capacity is studied.It provides a scientific basis for water pollution control and water environment treatment of Changsheng River.The main contents of this study are as follows:（1）The pollution sources of the Changsheng River were investigated in detail,and the inflow pollution load during the monitoring period was analyzed.The outcomes indicated that the primary pollution load comes from the effluent of the upstream Leijiaqiao reservoir,point source pollution in the watershed,and non-point source pollution from rainwater and the effluent of the two tributaries.In the course of the water quality monitoring period,the inflow pollution load of COD,TN,TP,and NH₃-N were602.099 t,54.108 t,4.028 t and 23.933 t,respectively.（2）On account of the measured water quality data,the water quality of the Changsheng River was evaluated.The water quality of the Changsheng River was basically unevenly distributed between ClassⅡand inferior ClassⅤof water quality.The upstream water quality was basically between ClassⅡto ClassⅤof water quality,the middle and lower reaches are between ClassⅣand inferior ClassⅤof water quality.The control factors that determine the water environmental capacity of the Changsheng River are determined as COD,TN,TP,and NH₃-N,of which TN and NH₃-N exceeding the standard is obvious and need to be controlled.（3）Based on Delft3D＿3.28 software,a model orthogonal curve grid suitable for the Changsheng River was drawn.A total of 985 horizontal grids were set and three layers were vertically layered.The automatic survey technology of unmanned ship and motor ship equipped with depth sounder was used to obtain the Changsheng River underwater topographic survey data,and a 3D underwater topographic map of the Changsheng River model was constructed.（4）Based on the EFDC＿Explorer7.1 software,a hydrodynamic model of the Changsheng River was constructed.By comparing the three simulation indicators of flow velocity,water level,and temperature with the measured data,the outcomes indicated that the average relative errors of the three indicators were 29.640%,3.834%,and 3.916%,The average root-mean-square errors are 0.006,0.233,and 1.227,and the average root-mean-square relative errors are 22.568%,16.467%,and 9.962%.The analysis of the flow field characteristics of the model shows that it takes 30 days for the downstream of the stain tracer to reach the stable concentration.Water age analysis shows that the water age gradually increases from upstream to downstream,the upstream water age is about 6 days,the middle water age is about 9 days,and the downstream water Age is about 15 days.（5）In the light of the hydrodynamic model of the Changsheng River,a water quality model was constructed.COD,TN,TP,NH₃-N and Chlorophyll a（Chl-a）are selected for the simulated water quality indicators.The parameters of the model were calibrated using the first 6 water quality monitoring data,and the model was verified by the last 6monitoring data.The calibration results show that the average relative errors of the five simulated water quality indicators are 13.693%,9.370%,11.979%,11.400%,and21.145%,and the average root mean square errors are 2.078,0.178,0.014,0.110,3.926,and average root mean square.The relative errors are 28.739%,22.998%,26.878%,26.318%,26.203%;the verification results show that the average relative errors of the simulated five indicators are 16.711%,10.314%,14.225%,14.727%,23.045%,the average root mean square errors are 1.948,0.183,0.013,0.139,3.300,and the average root mean square relative errors are 44.770%,31.912%,38.646%,38.047%,and 30.790%.（6）The water quality-pollution source response relationship was established based on the Changsheng River EFDC hydrodynamic water quality model.The linear environmental model of the Changsheng River was used to calculate the water environmental capacity of the three sections of the Changsheng River.The outcomes indicated that the upstream COD 582.726 t,TN-21.174 t,TP 4.938 t,NH₃-N 0.821 t;the midstream COD 322.789 t,TN-19.590 t,TP 2.211 t,NH₃-N 0.109 t;downstream COD702.746 t,TN-8.615 t,TP2.522 t,NH₃-N 0.115 t.Taking the surface water environment Class III water quality as the control target,the annual pollution load of Changsheng River and the water environment capacity will be compared and analyzed.The water environment capacities of COD and TP are both surplus.Respectively for 82.176 t and17.374 t,the reduction ratios are 250.6%and 94.3%.（7）In the light of the Changsheng River hydrodynamic water quality model,three water environment improvement programs are proposed for different pollution sources and simulated water quality changes.The results show that the average reduction of water quality after eliminating point source pollution is COD 11.049%,TN 7.863%,TP12.333%,NH₃-N 8.845%;the average reduction rate of water quality after mitigation of non-point source pollution is COD 39.261%,TN 9.722%,TP 22.747%,NH₃-N 12.399%;the average reduction rate of water quality after comprehensive treatment is COD48.962%,TN 16.296%,TP 33.325%,NH₃-N 19.532%.The simulation results prove that it is feasible to comprehensively control the water environment of Changsheng River,and can achieve positive results.（8）A comprehensive set of water quality assurance technologies suitable for the Changsheng River is proposed through comprehensive consideration of the five aspects of sewage interception,channel dredging,initial rainwater control,water ecological restoration and water reoxygenation.