| Basin is an integral and comprehensive ecological region,which is a necessary place for the growth and reproduction of plants and animals and the living and production of human beings.Wei River,the fir-st major tributary of the Yellow River,flows through Gansu,Ningxia and Shaanxi.It is a very important river basin in the northwest China.With the rapid development of social economy and the expansion of human activities,the ecological risk in Wei River basin has become increasingly prominent,which seriously restricts the development of social economy and ecological civilization in the Wei River basin.Taking the Wei River basin as the research area,a comprehensive ecological risk assessment of the Wei River basin was conducted in this paper.By using the ecological risk assessment system,the comprehensive assessment of the Wei River basin was conducted from three aspects,namely,physics,chemistry and biology.As for physical component,Human activity index(HAI),Vegetation buffer width(VBW),Bank erosion(BE),Water flow condition(WFC),Water quality(WQ),and Substrate composition(SC),Habitat complexity(HC),River meandering degree(RMD)were established.As for chemical component,the heavy metal Cr,Pb,Hg,As and Zn in the river sediments were measured.As for biotic component,a comprehensive ecological risk assessment was conducted on the Wei River basin based on the Shannon-Weiner diversity index,Margalef richness index,Pielou evenness index and Simpson diversity index of zooplankton,which provided theoretical basis and technical support for ecological governance and planning of Wei River basin in the future.The main research results are as follows:(1)Based on the ecological environment conditions of Wei River basin and the evaluation indexes of river habitat quality in semi-arid area,the weight of each index was calculated by using the analytic hierarchy process(AHP),river morphology and structure of(0.5396)is the key to determine the physical habitat quality of Wei River basin,and the sediment composition(0.2912)is to determine the key indicators of river morphology and structure.In terms of hydraulic geometry,b/f is 1.079,and greater than 1 indicates that the channel shape belongs to the wide-shallow type,f<b indicates that with the increase of flow,the increase of water depth in Wei River basin is less than that of river width,m/f is 0.553,indicating that with the increase of flow,the increase of water depth in Wei River basin is greater than that of flow velocity.For the whole Wei River basin,the restoration of physical habitats in Wei River,Jing River and Beiluo River should,on the basis of comprehensive water resource management and water pollution control,improve the width and composition diversity of vegetation buffer zone in the riparian zone,reduce the intensity of agricultural development and improve the ratio of environmental flow demand.(2)Based on the collection and determination of heavy metals in sediments of Wei River basin,the total heavy metal Cr,Pb,Zn,As,Hg,average were 59.5,43.625,68.3125,5.3717,0.0261 mg/kg respectively,Pb is greater than the average of Shaanxi soil background value,mean value of Cr,Zn,Hg and Shaanxi soil background values was flat,the As below the average of soil background values in Shaanxi province.In addition,the coefficient of variation of Zn and Hg is higher than that of other elements,indicating that the level difference is relatively large,showing more drastic fluctuations and greatly affected by human activities.Pearson correlation analysis and cluster analysis were carried out on the 5 heavy metals,and the results showed that there was a significant correlation between Cr,Pb,Zn,As and Hg in the sediments of Wei River basin(R was 0.681-0.897.P<0.01),indicating that the above 5 elements had certain homology.The results of element clustering are divided into two categories:one is Hg,and the other includes Pb,As,Zn and Cr.Hg is different from other sources of heavy metals,in which Pb,As and Zn have strong similar change rules,and Cr also has certain similarity.For the whole Wei River Basin,it is necessary to strengthen the waste water discharge standards and treatment and recycling in factory production.(3)A total of 19 species of zooplankton were found,including 10 species of rotifers,4 species of cladocerans and 5 species of copepods through the collection of zooplankton in Wei River basin.The dominant zooplankton species in Wei River basin are Asplachnapriodonta,Brachionuscalyciflorus,Brachionusurceus and Nauplii.The correlation analysis of zooplankton community structure and water environment factors in Wei River basin shows that water temperature,dissolved oxygen,altitude and electrical conductivity are important factors affecting the zooplankton community structure.Cluster analysis was used to analyze zooplankton at each point in Wei River basin.With 15 as the fixed scale,zooplankton can be divided into two categories.The first category includes ZJS,LJH,HX,YJP,YLP,LT,JKH,ZT,XY,BD and LJC.The second category is WJB.For the Wei River basin as a whole,the Wei River,Jing River and Beiluo River biological structures should be in so as to ensure normal water supply,on the basis of improving aggregate of soil and water conservation work,as well as the management and control of the area of industrial wastewater and urban sewage discharge and reduce the intensity of agricultural development,so as to improve the quality of river basin ecological environment.(4)Comprehensive ecological risk assessment is made by physical factors,chemical factors and biological factors of space vector in combination with the result of the interaction,comprehensive ecological risk assessment of Wei river basin CERI is 0.35,is medium grade,but the ecological risk of BD,WJB,JKH and ZT are poor,CERI assessment were 0.44,0.42,0.47 and 0.41,and the CERI of LJC,XY,LT,HX,YJP,YLP,ZJS,LJH were 0.23,0.26,0.35.0.31.,0.33,0.24,0.40 and 0.36,ecological risk assessment is medium.Based on the results of the comprehensive ecological risk assessment of the three major river systems of Wei river,Jing river and Beiluo river,it can be seen that the ecological restoration of the Wei river system should improve the vegetation area of the riparian zone,increase the complexity of the physical habitat of the Wei river system,reduce the discharge of industrial sewage and improve the treatment degree,and improve the aquatic biodiversity.The ecological restoration of Jing river system should protect the physical habitat in the middle and upper reaches,strengthen the habitat management in the lower reaches,reduce the interference intensity of human activities along the riverbank,control agricultural non-point source emissions,improve the habitat quality of aquatic organisms,and improve the biodiversity.The ecological restoration of the Beiluo river system should strengthen the management of physical habitats in the middle and lower reaches,improve habitat complexity and vegetation coverage,strictly supervise the discharge of waste water from oil refineries,reduce the interference intensity of human activities along the riverbank,control agricultural non-point source emissions,improve the habitat quality of aquatic organisms and improve biodiversity. |
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