Ecological Risk And Health Assessment Of Hengshishui River After Environmental Treatment

Dabaoshan mine is a large polymetallic mine located in the mountainous area of northern Guangdong.The illegal mining activities were becoming increasingly serious since1980s.The acid mine drainage(AMD)containing Cd,Mn,Pb and other heavy metals produced in the process of mining and ore washing was discharged directly into Hengshishui River without treatment,which caused serious damage to the water quality and ecological environment of the watershed.In recent years,Dabaoshan Mining Co.,LTD.has built a sewage treatment plant in the upstream Hengshishui River and carried out environmental treatment on the river,then the water quality of Hengshishui River has been improved.The main contents of this study include:(1)Distribution characteristics of heavy metal in the water and sediments of Hengshishui River and potential ecological risk assessment;(2)The Community structure of Macroinvertebrate and biological assessment of water quality in Hengshishui River;(3)Functional integrity of Hengshishui River ecosystem assessment based on litter decomposition rate.Purpose is by comparing with previous research results,to understand Hengshishui River water after environmental treatment,water and sediment content of heavy metals and chemical speciation of heavy metal in sediments,the change of Macroinvertebrate community structure and litter decomposition rate,In order to further studying the restoration and health assessment of Hengshishui River ecosystem to provide scientific basis.1.Distribution characteristics and potential ecological risk assessment of heavy metals in Hengshishui RiverEleven sampling sites were set from the tailing dam of Dabaoshan to Yaobu bridge at the estuary of Beijiang River(about 100 km from Dabaoshan mine).The contents of Cu,Zn,Mn,Pb and Cd in water and sediment from each sites were determined,and using the BCR continuous extraction procedure to anaylsed the chemical speciation of heavy metal in sediment.The results showed,After environmental treatment,the concentration of heavy metal ions in Hengshishui River water was significantly less than in the past,The contents of Cu,Zn,Pb and Cd are conform to theⅢgrade of the surface water standard,except Mn exceed standard in individual sites.But according to the trend of the chemical speciation and distribution characteristics of heavy metals in sediments,the proportion of bioavailable fraction of heavy metal(exchangeable fraction,oxides Fe-Mn fraction and organic matter sulfidic fraction)was high in most contaminated sites,except two reference sites and Yaobu bridge at the estuary of Beijiang river has high proportion of residual fraction(biological unavailable fraction)of heavy metal.It is indicated after environmental treatment,the high risk of heavy metal contamination in the sediments of contaminated sites still exist.Because of the high background value of heavy metals in Hengshishui River,Hakanson potential ecological risk index method was adopted in this study,In our study,Hakanson potential ecological risk index method was used to assess the potential ecological risk of heavy metals in the sediments of Hengshishui River by measuring the total content of heavy metals and the content of exchangeable fraction.The result showed,The hazard degree of 5 heavy metal elements is from high to low:Cd>Cu>Pb>Zn>Mn,and the ecological risk of Cd is much higher than the other four heavy metals.According to the measuring method of the total content of heavy metals,In Hengshishui River watershed,except the Yaobu bridge at lowest downstrem is medium risk,the heavy metals in the sediments of other samples all have strong and extremely strong level of potential ecological risk.But by the content of exchangeable fraction measuring method,the potential ecological risk of heavy metals in sediments of two reference sites,Liangqiao and Yaobu bridge are at slight level.other sites are still exist strong and extremely strong potential ecological risk.The measurement method of exchangeable fraction eliminates the residual fraction content that is not available for biology,therefore,compare with the assessment results are more close to actual situation compare with the general measurement method of the total content of heavy metals.2.Eco-system health assessment of Hengshishui River based on litter decomposition rateSeven sampling sites were set in Dabaoshan tailing dam and hengshishui river watershed (5 interference sites,2 reference sites),placed Dracontomelon duperreanum,Schima superba and Castanopsis fissa three kinds of leaf litter.The model of litter decomposition rate was used to evaluate the ecological functional integrity of Hengshishui River.According to the results of our previous study in 2007,the environmental scores of all sampling sites but reference sties were 0,which means the ecological function was seriously impacted by AMD.The results of this study showed that the environmental score of the three reachs at the tailing dam,Liangqiao and Tangxin is still 0,but the environmental score of the two downstream sites,Shangba and Hengshishui bridge is 1(indicating the ecological function is affected to a certain extent).All reference sites show that the score is 0 which means the ecological function was not affected.The results showed that the ecological function of the sites in the downstream of Hengshishui River was restored after environmental treatment,and the water quality and ecosystem health were improved to a certain extent.3.Macroinvertebrate community structure and bioassessment of water quality in Hengshishui RiverIn September,October and November,2018,the macroinvertebrate communities were investigated at the above 6 sampling sites in Hengshishui River watershed(since there is no biological presence in the tailing dam,this site were not included),A total of 74 species of macroinvertebrates belonging to 33 families of 5 classes were collected,67 species of aquatic insects,accounting for about 90%species of macroinvertebrates.Compared with the results of our previous study in 2010,the composition and species of macroinvertebrate communities in Hengshishui River changed after environmental treatment.The number of species in Tangxin and Shangba has increased from 6 and 19 to 13 and 35.Trichoptera insects began to appear in Tangxin,and some sensitive groups such as Hydropsychidae and Baetidae become the main dominant species of Shangba and Hengshishui bridge in downstream.The average density of macroinvertebrates at Liangqiao and Shangba also increased from 18 ind/m~2 and30 ind/m~2 to 74 ind/m~2 and 349 ind/m~2,respectively.The results showed that the macroinvertebrate community structure in Hengshishui River had been restored a certain extent at present,and the species and density of sensitive groups among the dominant species had been increased.The water quality of Hengshishui River watershed was assessed by biodiversity index and biological index.The results of water quality bioassessment showed that after environmental treatment of Hengshishui River,the water quality of Liangqiao and Tangxin at upstream was still in pollution,but the water quality of the reference sites,Shangba and Hengshishui bridge at downstream was good,which was consistent with the results of the assessment by using the decomposition rate of litters.