Water Environment Quality Assessment And Eutrophication Risk In Fenghe Basin

The Fenghe Basin is located in the southwestern of Xi’an City,Shaanxi Province.It plays an important role in the water environment of Weihe Basin and the production and livelihood of the local residents.Based on the investigation of environment background and water pollution status of the Fenghe Basin,8 sections and 15 evaluation indexes were monitored.The chemical characteristics of the indexes,the trends and regulars of the wet season,dry season and normal season in this hydrological year were discussed by analyzing the data from September 2018 to August 2019.The methods such as Single-factor exponential method,Nemerow method,Fuzzy Comprehensive Evaluation（FCE）and Grey Correlation Analysis（GRA）were used to describe the degree of polluted water for each section.They were also performed to analyze main pollution factors and sources.In addition,eutrophication risk analysis was carried out on middle and downstream sections.According to the assessment results and the analysis of its pollution sources,measures to prevent and control water environment pollution in the Fenghe Basin are proposed.The main research results are:（1）The influences of PO₄-P and NO₂-N on the change of water quality were very slight.There was a strong correlation between N and P which have large influence on water quality.The pollution status of the sections during the same period was upstream<downstream<midstream,the section Fengyu in the upstream had the best water quality,while the section Yurufeng in the midstream was polluted seriously.Compared to different water periods,the wet season was least polluted than the normal season,with the dry season was polluted the most serious.（2）Generally speaking,the pollution status of sections from light to heavy were Fengyu,Gaoguanyu,Yanjiaqu,Sanliqiao,Qinduzhen,Taipingyu,Fenghekou,Yurufeng.The Single-factor index method evaluated that 12.5%sections were level IV and 87.5%were level V in Fenghe Basin.The traditional Nemerow method evaluated that 12.5%sections were level IV and V,and 87.5%were inferior level V.The proportion of level V water in the traditional FCE is as high as 91.8%.The evaluation results of these precious three methods are pessimistic,for the reasons of evaluation results depend largely on the most polluted factor-TN.The improved Nemerow method which reduces the effects of TN,evaluated that 12.5%were level II and III,54.2%were level IV,and 33.3%were level V.The GRA evaluated the water quality that 75%were in level I and 25%were in level V.The main difference of this method from others may be the evaluation factors were standardized in different ways.The improved FCE has both advantages of Analytic Hierarchy Process（AHP）and entropy weight method,weaken the influencing factors of TN,balance the proportion of COD_Mn and DO,and make the weight distribution more reasonable.The level I,II and III were accounted for 49.9%,while levels IV and V were accounted for 50.1%.Take the characteristics of these methods into account,the conclusion of this study chose the result of improved FCE.（3）According to the assessment of eutrophication risk,the midstream and downstream of the Fenghe Basin had the risk of mild or even moderate eutrophication.The growth of phytoplankton is likely to be restricted by N,P,DO and water temperature.There was a strong correlation between the N content in surface water and sediment,mainly for the reason of endogenous N has a great influence on the surface water.（4）The main pollution sources in the upstream were domestic sewage and agricultural pollution,while the midstream and downstream included domestic sewage,industrial sewage and agricultural pollution.The main pollution indexes are NH₃-N,TP and TN.The key direction of water quality protection is to control nutrient pollution.To create a more suitable living environment,while carrying out the publicity work about water environment,the targeted pollution prevention and control measures include:the development and use of green bait and excellent seedlings for ecological breeding in the aquaculture industry;applying fertilizer scientifically and rationally in agriculture,carrying out soil and water conservation at the same time;energy conservation and emission reduction are the main means in industry;on the premise of supporting the construction of sewage collection pipe network,combining with the construction of wetland project to prevent the large-scale scattered discharge of sewage.