Heavy Metal Pollution And Ecological Risk Assessment Of A Small Watershed Draining A Mining Area In Tongling

Mining not only leads to heavy metal pollution in the mining area,but also causes the heavy metal pollution of downstream areas through surface runoff.In this paper,a small watershed where an abandoned pyrite mine located in Tongling City was taken as a research object,in order to investigate the heavy metal pollution status 4 years after the suspension of this pyrite mine,a distributed sampling analysis in this watershed were conducted.In the downstream of the mine and the tributaries not affected by the mine,the samples of river water during the normal dry,flood rising and retreating period and sediment during the normal dry and flood period were collected.The soil samples from mining area,riparian zone and farmland,as well as the samples of control sites where not affected by runoff polluted by the mining area were collected mainly from the surface layer（0-50 cm）,and 6of them from middle（50-100 cm）and deep（100-150）layer.Then the elements of Cd,Pb,As,Cu and Zn were detected to analyze the content,spatial distribution and sources of heavy metals in the watershed.Finally,the single factor pollution index and potential ecological risk index（RI）were calculated to evaluate the river water,the Geo-accumulation index(I_geo)and RI for sediments and the soil.The findings are as follows:In the river water,the highest content of heavy metals was detected at the sampling site close to the mining area,and the content of downstream showed a decreasing trend,the lowest content at control sites.In the downstream of the mine,during the normal dry and flood rising period,the average content of Cd was 0.016 and 0.021 mg/L,respectively,which were 1.6 and 2.1 times than the relevant value of ClassⅤStandards for Environmental Quality of Surface Water（GB 3838-2002）.During the flood retreating period,the average content of Cd was 1.4 times than the Class III.As for other elements,the average content was lower than the Class III in all these three periods.Most heavy metals have relatively highest content during the flood rising period,then in the normal dry period and lowest in the receding period,which should be related to the accumulation and releasing by erosion of heavy metals in mining areas and riverbed during different periods.In the sediment,the content of heavy metals in midstream was the highest,and upstream and downstream much lower,and the control sites lowest.During the normal dry period,the content of Cd downstream of the mine was relatively highest,with the content between 3.66and 59.63 mg/kg.Its average content was 21.8 mg/kg,which was 10.9 times the relevant value of Control Value of Soil Pollution Risk for Agricultural Land（GB 15618-2018）,and the minimum content also exceeded the Control Value.The average contents of Pb and As were 841.33 and 280.63 mg/kg,respectively,which were 68%and 87%higher than the Control Value.The average content of Cu and Zn were 1178.33 and 2893.33 mg/kg,respectively,which were 23.6 and 24.5 times than the Control Value.The content of each heavy metal in the flood period was significantly lower than that in the normal dry period,which may be related to the erosion effect of flood on the riverbed.In the soil of mining area,the average content of Cd,Pb,As,Cu and Zn was 1.964,2513.00,402.00,144.47 and 667.40 mg/kg,respectively,which were 6.5,27.9,10.1,2.9 and3.3 times than the Screening Value of Soil Pollution Risk for Agricultural Land（GB 15618-2018）,and the average content of Pb and As were significantly higher than the Control Value.Comparing with the mining area,the content of heavy metals at the edge was much lower.In the soil of riparian zone,the average content of heavy metals was 1.067,636.28,242.40,126.88 and 443.54 mg/kg,respectively,which were 3.6,7.1,6.1,2.5 and 2.2 times than the Screening Value,and Pb and As were higher than the Control Value.The content in the middle stream and upstream of the riparian zone was highest.In the soil of farmland,the average content of heavy metals was 0.506,180.13,27.00,45.81 and 176.00 mg/kg,respectively,and Cd and Pb were 69%和100%higher than the Screening Value.The content of heavy metals in farmland was much higher in the sampling sites with low terrain in the middle and downstream,while the much lower content farther away from the river or with the higher terrain.In general,the content of each heavy metal in soil showed the order that the mining area>polluted riparian zone>polluted farmland.In the soil,Cu showed the lower content with the deeper depth,while Cd,Pb,As and Zn showed the highest content in the middle layer soil,the lowest in the deep layer.It indicated that these elements migrated downwards due to the long-term pollution.The spatial distribution characteristics of heavy metals indicated that the heavy metals in the watershed mainly originated from the mining area.The correlation analysis results showed that the correlation of heavy metals in river water was the weakest,followed by sediment,and the correlation of heavy metals in soil was the strongest,which generally indicated that the main sources of heavy metals in the watershed were consistent and might originate from the mining area.Principal component analysis showed that the mining area was the major source of pollution.The single factor pollution index and potential ecological risk index of river water showed the pollution and risk of Cd were the highest,then Zn,other elements were free of pollution and risk,and the heavy metal pollution and risk mainly concentrated in the middle stream and upstream.The geo-accumulation index and potential ecological risk index of sediment showed that the pollution and risk levels of Cd,Pb and As were much higher than Cu and Zn,and the pollution and risk in the middle stream were the highest.The geo-accumulation index and potential ecological risk index of soil showed that in the mining area the pollution and risk of Pb,As and Cd were very high,Cu and Zn were much lighter.The pollution and potential ecological risk at the mining area were the highest,and the pollution and risk at the edge were relatively lower.In the riparian zone downstream the mining area,Pb and As were at heavy-pollution grade.And the pollution and risk mainly occurred at middle stream and upstream.In the farmland soil,the elements with pollution and risk soil were Pb and Cd,and the others were at low-pollution and risk grade.In control sites,there was almost non-pollution and risk.