Distribution And Migration Of Lead And Zinc In Bijiang Watershed Under The Influence Of Lanping Lead-zinc Mine

Heavy metal pollution caused by metal mining activities and its environmental hazards have attracted widespread attention at home and abroad.However,the quantitative impact of mine development on heavy metal contents in river and the migration process of heavy metal pollutants along the river are still unclear.This study takes the largest Lanping lead-zinc mine in Asia and the Bijiang River(the first tributary of the Lancang River)as the research object.Soil samples were collected in the Bijiang watershed and dynamic monitoring for lead(Pb)and zinc(Zn)pollutants on 18 different sections of the Bijiang River in different media(water,bed sediment and suspended sediment)was carried out at different time scales(dry season,flood season and rainstorm-flood process).We clarified the temporal and spatial distribution characteristics of Pb and Zn contents in soil and river under the mining,revealed the migration characteristics of Pb and Zn contents around the mining area,and further quantitatively evaluated the contribution of Jinding lead-zinc mining to the Bijiang watershed by calculating the metal-fluxes and relative contribution rate.The main results are as follows:(1)Temporal and spatial differentiation characteristics of lead and zinc in the Bijiang watershed under the influence of mine:The average contents of Pb and Zn in mined soil were significantly higher than those in unmined ones by nearly 10 times.The Zn content in surface water,Pb and Zn contents in the bed sediment and also in suspended sediment around the mining area were all significantly higher than those in the upstream of the mining area,especially during the rainstorm-flood process.Seasonally,the average contents of Zn in the water and of Pb and Zn in the bed sediment in the dry season were higher than those in the flood season.Under the rainstorm condition,the Zn content in the water of the mining area was significantly higher than that of Pb,and the highest content of Pb and Zn in the suspended sediment was significantly higher than the upstream of the mining area by about 50 times.(2)Migration characteristics of lead and zinc pollutants around mining area during a rainstorm-flood event:the changes of Pb and Zn contents in water and suspended sediment were significantly different with the rainfall process,respectively.The variation trends of Pb and Zn contents in suspended sediment around mining area were almost consistent with the rainfall process,and influenced by surface runoff process,Pb and Zn contents reached two distinct peaks with the rainfall up to 56 mm and 76 mm,respectively.However,Zn content in the surface water around mining area was significantly higher than Pb content,and the highest value of Zn content occurred after22 hours from the end of rainfall.(3)Quantitative impact of mine development on lead and zinc contents in the Bijiang watershed:Under the influence of mining activities,Pb and Zn contents in mined soil were extremely high.Compared with the national standard for the risk screening values for soil contamination of agricultural land,both Pb and Zn contents in cultivated land around the mining area were nearly 10 times higher.Meanwhile,the relative contributions of mining on Pb and Zn contents in bed sediment around the downstream of the mining area,that is,C_(Pb)upand C_(Zn)upwere in a range of 12%?89%and 5%?77%,respectively.The relative contribution for Pb and Zn contents in suspended sediment was in a range of 13%?63%and 33%?85%,respectively.During the storm-flood process,the Zn flux in the water of the Bijiang River increased by99.98%,and the flux of Pb and Zn pollutants in suspended sediment increased by88.94%and 94.80%,respectively.This study showed that the Lanping lead-zinc mine exploration had a significant impact on the contents of Pb and Zn in soil and river sediment,and its effect on Zn was higher than that of Pb,especially in the rainstorm-flood process.The Pb and Zn pollutants from the Lanping Jinding mine mainly entered to the Bijiang River through surface runoff.Corresponding measures should be taken to limit the heavy metal pollutants generated by surface runoff,and cut down their migration.The study also provided a scientific basis for the prevention and control of heavy metal pollution in river under the influence of superlarge mine exploitation in mountainous areas.