| Dabaoshan Mine located at northern Guangdong is a large multi-metal sulfide ore,which has been exploited for more than 40 years.Dabaoshan Mining activities has caused serious damage to its surrounding environment,especially the downstream farmland ecosystem.Previous studies on heavy metal pollution in the surrounding areas of Dabaoshan Mine have been carried out,but the researchs on the distribution,migration and transformation of heavy metal pollution at the source of Dabaoshan Mining area are still rare.In this paper,soil,tailings,water,sediment and plant samples were systematically collected from the source of Dabaoshan mining area and its adjacent downstream villages（Liangqiao Village）.The concentration and morphological characteristics of heavy metals（Cd,Pb,As,Zn,Cr,Ni,Cu）were analyzed,and the spatial distribution characteristics of heavy metal pollution were described by geostatistics software.At the same time,the pollution sources of mining area were analyzed by Pb stable isotope analysis,and the characteristics and influencing factors of heavy metal migration and transformation in mining area were expounded by combining X-ray diffraction analysis,transmission electron microscope-energy spectrum analysis（TEM-EDS）and Raman analysis of typical minerals in mining area,as well as laboratory simulated leaching experiments.On the one hand,the accumulation index(Igeo),enrichment factor（EF）,potential ecological risk assessment（Er）and comprehensive quality index method are used to evaluate the soil heavy metal pollution in the farmland-crop system of Liangqiao Village in the lower reaches of the mining area.On the other hand,the health risks caused by ingestion of agricultural products and mishandled soil are used to clarify the effects of heavy metal pollution in Dabaoshan mining area.The main results are as follows:（1）Heavy metal elements（Cd,Pb,As,Zn,Cr,Ni,Cu）were generally detected in soil,tailings,water and plant samples in Dabaoshan mining area.Cd,Pb and As are the main pollutants,and concentration exceeding rates（soil control standard limits）are 100%,75%and50%in soil and tailings samples,100%,85%and 100%in sediments,respectively.The concentrations of mining water samples are generally exceeded the standard limits of farmland irrigation and class V of surface water.Among the soil/tailings samples,Cd,Pb and As are mainly distributed in the mining region,cut-off dam,old dump（Xinshan region）and dam（Liwu mud dam）region,and the pollution of the old dump is the most serious.(Igeo)analysis of accumulation index showed that Cd,As,Pb,Cu and Zn were enriched and polluted in sediments of mining area,and all samples had high ecological risk.Morphological analysis showed that the forms of Cd,Pb and As in soil and sediment samples in mining area are mainly residual phase,accounting for 85%～95%of the total,followed by iron and manganese oxide bound form（1%～15%）.（2）The results of X-ray diffraction（XRD）,Raman spectroscopy and field emission transmission electron microscope（（TEM））equipped with energy spectrum analysis（EDS）show that the main mineral types in soil and tailings in Dabaoshan mining area are pyrite（Fe S2）,chalcopyrite（Cu Fe S2）and metal oxides,as well as a small amount of sphalerite（Zn S）and galena（Pb S）.The result of nitric acid leaching shows that acid condition（p H=3.0）is beneficial to the release and migration of Cd and As from soil,tailings and minerals（pyrite,chalcopyrite）,and from residual phase to non-residual phase.The morphological migration path of Cd under acidic condition may be as follows:residual phase/organically bound phase/iron-manganese oxide bound phase/weak acid binding phase and exchangeable phase.The migration path of Pb is as follows:residual iron and manganese oxide bound weak acid bound phase and transferable phase.Lead isotope analysis showed that the lead in soil,tailings,sediment and mining water samples mainly came from the release of metal minerals in mining area,and the contribution of diesel in mining area was less than 30%.Compared with the mining area,the lead in downstream river is mainly originated from diesel,accounting for more than 60%,while the lead in river from metal minerals is lower than 40%.Spearman correlation,principal component analysis and multiple regression analysis showed that Pyrite,Chalcopyrite,Sphalerite and Metal oxide are main sources of the heavy metals in soil and sediment in the mining area,in which Cd,As and Pb were mainly contributed by sphalerite and metal oxide,the contribution rate to the first two was 100%,and the contribution rate to Pb was 66%.The Pb of the rest of the sample comes from the contribution of pyrite,accounting for 34%.（3）The farmland soil and crops in Liangqiao Village were seriously polluted by the target heavy metals（Cd,Pb,As,Zn,Cr,Ni,Cu）.The main polluted heavy metal elements were Cd,As and Pb.100%,100%and 75%of the soil samples exceeded the screening value of farmland soil（GB15618-2018）,and 75%,90%and more than 90%of the crop samples exceeded the national standard of food pollutants（GB2762-2017）.The average migration factors of Cd,As and Pb from farmland soil to crops were 0.099,0.034 and 0.026,respectively.The total potential ecological risk index（RI）of soil samples in Liangqiao Village was generally higher than 600,which had a very high ecological risk.Among them,Cd,As and Pb contributed the most,the contribution rates were 77.8%,14.4%and 3.1%,respectively.The analysis of pollution sources shows that the soil Cu and Pb in Liangqiao Village mainly come from the pollution sources in the mining area,and the contribution rates are 100%and 87%for Cd,As and Zn respectively,while they come from the pollution sources and human production activities（pesticides,chemical fertilizers and pig farms）in the mining area,and the contribution rates of the two sources are the same.The health risk assessment showed that Liangqiao village residents had a higher carcinogenic risk(carcinogenic risk>10-6)and non-carcinogenic health risk（hazard quotient>1）through ingestion of agricultural products and miseating soil. |
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