Influence Of Soil Erosion On The Migration Process Of Heavy Metals In Cascade Large Deep Reservoirs

The Lancang River is a transboundary river in southwest China,the sixth largest river in the world,and extremely rich in biological resources.Understanding the heavy metal enrichment of sediments in the water system is one of the prominent problems in ecological environmental protection of water resources projects and an important task in water environment management in China.Heavy metals can be produced naturally in aquatic ecosystems through rock weathering,surface runoff and soil erosion,and it is necessary to explore the potential impact of sediment from soil erosion into river ecosystems on the mechanisms of heavy metal migration and transformation in reservoir sediments.To this end,this thesis collected data on Rainfall,Topography,Soil property,Vegetation cover and Land use from 2000-2019 in the Middle and Lower reaches of the Lancang River Basin(ML-LRB)as the study area,and calculated soil erosion in the MLLRB.Sediments were collected in December 2019 at the pre-reservoir,mid-reservoir and aft-reservoir of two deep large reservoirs in the ML-LRB,and the deposition and migration patterns of Cadmium(Cd),Iron(Fe),Manganese(Mn)and Sulfur(S)in the sediment-water profiles were analyzed.By analyzing the trend changes of soil erosion and the vertical distribution of heavy metals at the sediment-water interface in the subbasins where the sampling sites were located,the response patterns between the deposition and migration processes of heavy metals in the reservoir sediments and the intensity of soil erosion were studied.The results of the study are as follows:(1)Soil erosion in the ML-LRB from 2000 to 2019 showed a spatial temporal distribution characteristic of decreasing from north to south with annual,but from 2015 onwards the trend of increasing began to appear in the area above the central part of the watershed.The area occupied by the trend of decreasing soil erosion in the watershed was much larger than that of increasing,and soil erosion in Dali City,located in the north of the study area,and Xishuangbanna,located in the south,showed a concentrated trend of increasing.The mutation in soil erosion in S1 in 2002 and the ML-LRB at low risk of erosion in 2009 were mainly influenced by NDVI.Although the results of the study found significant improvement in soil erosion in the watershed,the increase in soil erosion in the northern part of the ML-LRB in recent years needs more attention in the future.(2)Cd,Fe,Mn and S in the two large deep reservoirs show a fluctuating decreasing trend from the sediment bottom to the sediment-water interface,with a larger trend in Nuozhadu(NZD)Reservoir and a larger trend in front of Xiaowan(XW)Reservoir.There is no obvious fluctuating increasing or decreasing trend of labile Cd in the pore water,and labile Fe,Mn and S show an obvious decreasing trend.There is a strong correlation between Cd and Mn and S deposition processes in NZD Reservoir sediments,which is due to the stable Cd,Fe and S from sediment input in the downstream area;part of labile Cd in pore water comes from the supplementation of Silt and Sand,and part is inherent in pore water,and the vertical distribution patterns of labile Cd and labile Mn are similar.However,only weak correlation between Cd and Mn deposition processes was found in the sediments of XW Reservoir,which indicates that Cd and Mn are more stable in the sediment intercepted by XW Reservoir and sediment input from the upstream area.The labile Cd in the pore water of XW Reservoir is inherent in the pore water for the most part,and the sediment replenishes very little.The cascade construction of the large deep reservoirs of the Lancang River has changed the source of sediment in the upstream and downstream reservoirs,making Mn in the sediment of NZD Reservoir the main mechanism controlling Cd migration.(3)Under soil erosion,which is mainly controlled by Rainfall,Vegetation cover,and Land use,the sediment produced by it sinks into the reservoir in a sub-regional manner under the action of stepwise hydropower development,bringing a comprehensive impact on the deposition and migration process of heavy metals in the reservoir sediment.