The Research Of Water Environmental Capacity Between Minjiang Bridge Section And Minjiang Dongqing Junction Section

The Minjiang River is an important tributary of the upper reaches of the Yangtze River and the most important water resource in the Chengdu Plain.Its water environment capacity is an important criterion for measuring the quality of the water environment in the region,and it also directly affects the management and control strategies of surrounding pollution sources.Meishan City,Sichuan Province is located in the middle reaches of the Minjiang River.The hydropower development,industrial enterprise production,livestock and poultry,and aquaculture industries in the city are highly dependent on the Minjiang River.The sources of small and medium-sized industrial enterprises,living sources and agricultural livestock sources are staggered,resulting in cross-sectional water quality.The guarantee rate is low,the seasonal variability of water environment capacity is strong,and the environmental management difficulty is high.In view of this,this study takes the central urban area of Meishan as the research area,and the Minjiang Bridge section and the Minjiang Dongqing junction section as the research section.Based on the water quality monitoring data of the section from2014 to 2019,the industrial enterprises within the catchment area of the research section were investigated and visited.There are 119 sewage treatment stations and 18 domestic sewage treatment stations.At the same time,considering the current pollution source production of about 500,000 mu of cultivated land in the agricultural area,the discharge coefficient method is used to evaluate the inflow of different river sections;the single-factor evaluation method and the Nemerow comprehensive pollution index method are used.Evaluate the characteristics of water quality changes in the research river reaches from 2014 to 2019,identify key water quality indicators,and verify the contributions of different pollution sources;based on the generalization of hydrological units to study the river reach,based on a two-dimensional water quality model,the Minjiang Bridge-Minjiang East was established.The water quality model of the green junction section takes into account the complete specular reflection mode of the shore,and introduces the non-uniformity coefficient to optimize and correct the traditional water environment capacity;with the help of Pearson type III frequency curve and Fischer formula,inversion under the most unfavorable conditions The hydrological parameters and pollution degradation coefficients of the research rivers were determined according to the water quality standard of the water quality standard of“Surface Water Environmental Quality Standard（GB3838-2002）”,and the method based on the water quality of the control section was adopted to evaluate the water quality of the research section under different flow guarantee rates and hydrological conditions.It also proposes a strategy for reducing pollution sources along the way,providing theoretical technology and basic data for water environment management in the central urban area of Meishan,and providing strong support for water ecological environmental protection in the upper reaches of the Yangtze River.The main research conclusions are as follows:（1）Through the calculation and analysis of the amount of pollution sources entering the river and the water environment quality in the study area,it is obtained that the amount of COD,NH₃-N and TP entering the river in the study area is 1.81×10⁷ kg/a,9.55×10⁵ kg/a and 1.55×10⁵kg/a,the largest contribution to the COD and TP pollution load is the large-scale livestock and poultry breeding pollution source,and the largest contribution to the NH₃-N load is the urban domestic sewage;the single-factor evaluation results show that each section exceeds the relevant regulations.Class III water quality standard,the evaluation result of Nemerow pollution index method shows that the section with the worst water quality is the section of Tiquan River Mouth.（2）Establish a two-dimensional steady-state water quality model for the study area,and optimize and obtain hydrological and water quality parameters with different guarantee rates and different hydrological periods by using the shore-side complete mirror reflection model,and use the IDL programming program and related statistical methods.The reliability and rationality of the required water quality parameters are verified,and the main water quality parameters are k _COD=0.2 d^-1、k _NH3-N=0.1 d^-1、k _TP=0.125 d^-1、D_y1=0.41 m²/s、D_y2=0.86 m²/s.Under different flow guarantee rates,the water environment capacity decreases with the increase of the guarantee rate.When the guarantee rate is 90%,the water environment capacity is the smallest.In different hydrological periods,the annual comprehensive water environmental capacities of COD,NH₃-N and TP were 3.08×10⁷ kg/a,1.71×10⁶ kg/a and 4.68×10⁴ kg/a,respectively.The proportion of the water environment capacity in the whole year is larger than that in the dry season.（3）Combined with the remaining water environment capacity of each research factor under different guarantee rates and different hydrological periods,it can be obtained that when the flow guarantee rate is greater than 50%,in the downstream Minjiang Pengdong-Minjiang Dongqing junction,there is no COD.Remaining water environmental capacity;when different hydrological periods and flow guarantee rate<90%,NH₃-N has residual water environmental capacity in the upper reaches of Minjiang Bridge-Pengdong junction of Minjiang River,corresponding to dry season and flow guarantee rate>25%,There is no remaining water environment capacity of NH3-N in the lower reaches of the Pengdong junction of the Minjiang River and the Dongqing junction of the Minjiang River,and the reduction amount increases with the increase of the guarantee rate.When the guarantee rate is 90%,the reduction amount of NH₃-N The maximum is 6.60×10⁵ kg/a,and the reduction rate is 76.63%;TP has no remaining water environmental capacity in different guarantee rates and different hydrological periods,and the corresponding reduction amount increases with the arrival of the wet season,when the guarantee rate is 90%,the reduction of TP in the reach L1and L2 reached the maximum,and the reduction rate reached 90.13%and 78.8%of the river entry;during the wet season,the maximum reduction of TP in the reach L2 is1.03×10⁴ kg/a.