Integrated Water Quality Analysis And Partition Water Quality Model Study In Dongping Lake

The South-to-North Water Transfer Project is an indispensable strategic and important basic project in China.It plays an important role in the economic development,ecological environment and social production and life in northern China.Dongping Lake,as the last stage of the East Route Project of South-to-North Water Transfer,its water quality is directly related to the safety of water supply in the receiving area of the East Route Project of South-to-North Water Transfer.Based on the monitoring data of water quality spatial distribution in Dongping Lake from 2013 to2017,the Dongping Lake is divided into three districts,i.e.area I,area II and area III.Four analysis methods,namely,single factor evaluation method,comprehensive pollution index method,comprehensive water quality identification index method and grey relational analysis method,are used to analyze different districts and monitoring points of Dongping Lake.At the same time,the water quality model of Dongping Lake was established to analyze the main influencing factors and the main causes of water pollution.The main research results are as follows:（1）All the four analysis methods reflect that the water quality of Dongping Lake District has been improved significantly from 2013 to 2017.By 2017,the water quality of Dongping Lake District reached the overall standard.Except for 2013,the water quality of Dongping Lake District I was better than that of District II and District III,and the water quality in spring was better than that in autumn.The main water quality indexes affecting the water environment of Dongping Lake were T-P,T-N and COD_Cr.And the influence of T-N concentration is the main water quality index in District I of Dongping Lake,while the influence of T-P and COD_Crr concentration is the main water quality index in District II and III of Dongping Lake.（2）According to the single factor evaluation method,the water quality of Dongping Lake changed from V toⅢin 2013-2017,and the evaluation grade obtained by this method was the most stringent,and was not affected by the weight.According to the water quality comprehensive pollution index method,the water quality comprehensive pollution index P value of Dongping Lake decreased from 0.96 to 0.66in 2013-2017,and the decline rate was 31.3%.The water quality of Dongping Lake changed from basically reaching the standard to reaching the standard.This method has the best water quality condition,but can only determine the overall reaching the standard,but can not determine the type of water body.The result of this method is influenced by the weight greatly,and based on the grey relational analysis method,the result of the analysis is the same as that of other methods,and the influence of the weight is the least.（3）From the monitoring point of view,the pollution area of Dongping Lake gradually shifted from area I to area II and area III from 2013 to 2017,and the water pollution in the densely populated areas such as Laohu Town Wharf and Da’anshan Wharf was more serious;from the water quality in spring and autumn,the average concentration of COD_Mn,COD_Cr,NH₃-N,T-P and T-N in Dongping Lake in autumn was higher than that in other seasons.In spring,it was 1.03,1.01,1.26,1.25 and 1.04 times as much as in spring,indicating that the water quality of Dongping Lake in spring was better than that in autumn on the whole.Compared with May 2013,the average concentration of COD_Mn,COD_Cr,NH₃-N,T-P and T-N decreased by-55.5%,2.0%,67.5%,49.5%and 53.1%respectively in April 2017,P value decline rate was 28.6%,and in April 2015 P value decline rate was the largest,October 2015 P value decline rate was the smallest,indicating that the overall water quality of Dongping Lake in April 2015 is the best,October 2015 is the worst,in spring and autumn 2017 P value is0.65,water quality is up to the standard,Dongping Lake water quality improvement effect is significant.（4）Based on the monitoring data of topography,water level,discharge and water quality of Dongping Lake,the mixed water quality model of Dongping Lake was established by using the quality conservation principle of each water quality index.The degradation rates of COD_Cr,T-P and T-N were calculated in spring and autumn 2016and spring and autumn 2017 respectively.The concentration of T-N in Dongpinghu I area is generally on the rise,and the water environment in this area is mainly affected by nitrogen concentration;the concentration of T-P in Dongping Lake II and III areas is on the rise,and the water environment in these two areas is mainly affected by phosphorus concentration.It is concluded that the water quality of the upper water diversion,the water quality of Dawen River,the endogenous release of Dongping Lake and non-point source pollution may be the main reasons for the increase of T-N and T-P concentration in the districts.Finally,countermeasures for water pollution in Dongping Lake are put forward.Through the comprehensive analysis of water quality of Dongping Lake and the study of zoning water quality model,the changing trend of water quality in Dongping Lake is revealed,and the areas with poor water environment,the main water quality indexes and main causes of water pollution are found out.It can provide scientific basis for the prevention and control of Dongping Lake water quality,prediction and the implementation of related control projects,and provide guarantee for the sustainable development of Dongping Lake and its surrounding regional economy.