Comprehensive Evaluation Of River Water Quality And Study Of Pollutant Inflow Based On Composite Model

With the rapid development of social economy and the acceleration of urbanization,The problem of water environment is particularly prominent.Section A of river F in north China is located in the upper reaches of the river,bearing the filling of drinking water,agricultural irrigation,urban industrial and domestic water.It is responsible to function for urban drinking water supply,farmland irrigation,urban industrial water,aquaculture water and reserve water.There is great significance for the protection and treatment of river water environment to accurately evaluate the water quality of the reach and reasonably assess the pollutant inflow.Taking section A of river F as the research object,this paper determined the main pollutants in this section by using principal component analysis,curve fitting equation and ROC analysis method,and comprehensively analyzed and evaluated the change law of the substance along the river and water quality change of the whole sction.The BOD-DO（Streeter-Phelps）coupling model was established to analyze the BOD₅attenuation law and DO recovery law of the river which based on an improved optimal search program of fastest descent method to solve the model parameters.Using simplified mechanical water quality models of NH₃-N and TP,the attenuation and migtation patterns of the two indexes in the section were analyzed.At the same time,the non-mechanical model of pollutant index and flow data of―along distance‖and―along time‖was established by using spline curve fitting equation expressed polynomial approximation.Combining with the above mechanical water quality model,the estimation method of pollutant import volume based on composite model is explored to assess the pollutant load of the river.The main conclusions are drawn:（1）The principal component analysis（pca）scores of each section were linearly correlated with the current water quality evaluation standard.The results of qualitative analysis of score and the non-mechanical model of score reflect the distribution of water quality along the path of river reach:Within the range of 0-99km,the water quality is good or medium（class I,II,III,IV and V）,and within the downstream range of 99km,the quality is poor（class inferior V）.（2）NH₃-N is the main pollutant index of river with prominent contribution rate.The monitoring value of NH₃-N index can be used as a marker pollutant index of the sample river section to reflect the comprehensive water quality.The warning range is as follows:When the NH₃-N concentration value is 0-0.45mg/L,the water quality is judged as good（class I,II and III）;when 0.45-0.84mg/L,the water quality evolves to medium（class IV and V）;when 0.84-1.59mg/L,the water quality is judged to be medium;when more than 1.59 mg/L,the water quality is judged to evolves to be inferior to class V.（3）In the process of establishing the BOD-DO（Streeter-Phelps）coupling model,an improved optimal search program for the joint calculation of unknown parameters of the model with the fastest descent method is proposed.The computer program overcomes the local minimum（optimal solution）problem existing in the traditional iterative calculation method,improves the calculation accuracy of parameters,and is helpful for the comprehensive evaluation of BOD₅ attenuation law and DO recovery law of rivers.（4）The pollutant import amount was estimated and compared with the compared with the comprehensive evaluation results of river water quality.Based on the unit distance import volume of river reach with good water quality,the 0-93 km section was determined to be a river reach with good water quality.The estimated annual unit distance import volume of NH₃-N was 0-6.0 t/km,TP was 0-1.0 t/km,and BOD₅ was 25-50 t/km.The pollutant load is within the self-purfication capacity range of the acceptability capacity of natural water bodies.The 99-145 km reach is the water quality deterioration section.The estimated annual unit distance import volume of NH₃-N was 146-336 t/km,TP was 8-21 t/km,and BOD₅ was 272-648 t/km.（5）In the deterioration section of water quality,the overall pollution load showed the rule that the unit distance pollution load of first and last stage was heavier and the unit distance pollution load of the middle stage was lighten.The variation trend of NH₃-N,TP and BOD₅ loads are different along the river.The trend of TP inflow load is consistent with that of the total pollutant inflow load along the river.On the basis of comforming to the general law,the pollution load of NH₃-N in the last stage is higher than that in the first stage.The pollution load of BOD₅ in the first stage is obviously higher than that in the last stage.The results suggested there were different pollution sources of NH₃-N and BOD₅,while TP pollution sources were related to them.In this paper,water quality assessment and river water quality modeling were used to estimate the water ecological pressure of natutal water,to master the overall―health‖status and―disease source‖of water,and to study the pollutant load of sample reach,so as to contribute to the proteciong and treatment of rivers.