Study On The Water Quality And Pollutant Flux In The Middle And Lower Reachesof Sihe River

In recent years,the rapid development of China’s economy,the continuous promotion of urbanization,the change of underlying surface and the over development of water and soil resources have seriously affected the water cycle and hydrological process in China,resulting in a series of problems such as water shortage,water ecological damage,water environment pollution,etc.As one of the important tributaries of Nansi Lake,the quality of Si River directly affects the coordinated development of social,economic and ecological environment of Nansi Lake and the receiving area along the East Route of the South-to-North Water Diversion Project.Therefore,it is necessary to study the water quality and pollutant flux of Si river.In this paper,the water quality of the monitoring year of the section is evaluated by the matter element extension method and BP neural network method with the Xian River estuary of the middle and lower reaches of the Si River and the Xiaoyi River Estuary as the research reach and Shuyuan hydrological station as the typical section.Through comparison,the water quality of the section in 2008-2018 years is comprehensively evaluated by BP neural network algorithm,and its change rules is analyzed;Secondly,the pollutant fluxes of Permanganate Index(CODmn),Ammonia Nitrogen(NH3-N),Total Nitrogen(TN)and Total Phosphorus(TP)were calculated by the estimation method of period flux,and the main sources,variation rules of different pollutants and their relationship with water quality and flow were analyzed;Finally,the coupling model of hydrodynamic and water quality is built by using Mike21 software,and different flow schemes are made to simulate the change process of pollutants in the study river reach and the entrance of Xiaoyi River downstream.The conclusions of the study are as follows:(1)In the annual average concentration of each index,the total nitrogen(TN)exceeds the standard seriously;except that the chemical oxygen demand(COD)slightly exceeds the grade Ⅴ in 2009,the rest of the annual average is below the gradeⅣ,and the pollution degree of ammonia nitrogen(NH3-N)and total phosphorus(TP)is relatively low,reaching the grade Ⅲ;the chemical oxygen demand(COD)and total nitrogen(TN)are mainly from point source pollution,while ammonia nitrogen(NH3N)and total phosphorus(TP)are greatly affected by Non-point source pollution confluence in flood season.(2)BP neural network method is more applicable than matter element extension method in Si River water quality evaluation.The evaluation results show that the water quality evaluation grade in 2008-2013 is mainly class V,and the water quality situation has improved significantly since 2014,all of which can reach grade Ⅳ water quality standard of water function area.(3)The interannual variation of the flux of each index is great,except that the beginning year of TN flux calculation is 2008,the maximum values of other indexes appear around 2003,and after 2013,the flux of each index decreases significantly,and the trend of variation is stable,which basically corresponds to the water quality evaluation results.(4)According to the hydrological analysis and calculation of permanganate index(CODmn)and ammonia nitrogen(NH3-N),the flux in the extremely high flow year is significantly higher than low flow year,but the flux change between the high flow year and the normal flow year is less significant;the proportion of the flux in the main flood season in the annual flux is significantly higher than the others months,and the flux change trend in the main flood season is basically the same as that in the annual flux,which shows that the calculation of the flux in the Considering the two factors of flow and pollutant concentration,the pollutant concentration is greatly affected by human activities,but there is a certain linear correlation between flow and flux when flow is the dominant factor.(5)When the initial water quality of the simulated reach is grade Ⅳ and the inflow water quality of Xiaoyi River is grade Ⅴ,the water quality of the main stream will gradually improve with the dilution self purification capacity of the river,and the greater the river flow is,the stronger the dilution self purification capacity of pollutants will be;In the simulation of water quality at the entrance of Xiaoyi River,it is found that TP has the strongest self purification capacity.When the model runs stably,TP concentration can reach grade Ⅱ,followed by COD.When the model runs stably,COD concentration can reach grade Ⅲ,The self purification ability of NH3-N is lower.When the model is stable,the concentration of NH3-N reaches grade Ⅳ,and the total nitrogen(TN)self purification ability is the worst.When the model is stable,the TN concentration seriously exceeds grade Ⅴ water quality standard.