Research On Migration And Accumulation Of Heavy Metals In Reservoir Sediments

The dam blocks the connectivity of the upstream and downstream of the river,causing a large amount of suspended matter and various pollutants carried by it to settle in the reservoir.Heavy metals are refractory,persistent,bioaccumulative and toxic,which seriously endanger ecosystems and human health.Reservoir deposits,while accumulating heavy metals,will release heavy metals to the overlying water in the event of disturbance or changes in surrounding environmental parameters,posing a hidden danger to safe water supply.In this study,the sediments of the A Reservoir were taken as the main research object.Combined with the hydrological characteristics of the reservoir,the distribution characteristics,sedimentation process and migration and enrichment behavior of heavy metals in different areas of the reservoir were analyzed,and the pollution and ecological risk of heavy metals were evaluated.For heavy metals in sediments close to the dam,comparison between the A Reservoir and other typical drinking water reservoirs in Liaoning and Jilin provinces were made.The results will provide reference point for an in-depth understanding of the hydrological cycle of heavy metals and pollution management of sediments in the reservoir.The main research contents and results are as follows:(1)The water level in the K area,the upstream rainfall,and the hydrological data such as the flow,sediment concentration and sediment transport rate of the upstream gauging station on the X River were collected and analyzed.Combined with the stratification structure of the sediment at the sampling site,the improved flood bed interpretation method was used to determine the deposition rate of the site and the deposition process of heavy metals was reconstructed.The results show that the K area has been flooded for more than 65%since the establishment of the reservoir dam.The sediment peaks are affected by flood peak flow and upstream rainfall intensity.The flood caused by short-term heavy rainfall shows higher sediment concentration.The average deposition rate of the sampling site is 0.97-1.10 cm/a,which is similar to the calculation of the terrain elevation difference before and after the reservoir dam construction.(2)In order to reveal the effects of sediment grain size,organic matter,bed topography and reservoir hydrodynamics(such as turbidity current)on migration and enrichment of heavy metals,distribution characteristics of total heavy metals and different fractionations in different areas and environmental medium of A Reservoir were studied.The results show that the total amount of heavy metals and the proportion of acid soluble and reducible phases increase obviously from the fluctuation area to the underwater area or dam.Manganese(Mn)is obvious with its greatest enrichment effect.Mn in sediments is mainly acid soluble and reducible phases,lead(Pb)is mainly reducible and residual phases,iron(Fe),copper(Cu),cadmium(Cd)and zinc(Zn)are mainly residual and reducible phases,and the proportion of residual chromium(Cr)is more than 90%.The content of heavy metals is positively correlated with fine sediments,organic matter content and water depth,and the bed topography and reservoir hydrodynamics have a greater impact on the former two.The porewater Mn content and the content and proportion of acid soluble Mn are positively correlated with water depth(p<0.05)and negatively correlated with the distance to the dam(p<0.05).This indicates that soluble Mn is enriched along the thalweg to the dam under the action of reservoir hydrodynamics(such as turbidity current).(3)Core sediments close to the dams of A Reservoir,B Reservoir,C Reservoir,D Reservoir and E Reservoir were collected to reveal the vertical distribution of heavy metals.Correlation analysis and principal component analysis were used to investigate the sources of heavy metal pollution.The results show that the contents of organic matter and heavy metals in the surface sediments of A Reservoir increase significantly with time,with the highest increase rate of Mn reaching 280%;the heavy metals in the surface sediments of B Reservoir and Huofang Reservoir increase significantly with time,indicating an increasing heavy metal input in recent years;the shape of D Reservoir is regular and simple,resulting in serious siltation in front of the dam and a similar increase or decrease trend of heavy metals due to flooding;the average content of Mn in the sediments of the E Reservoir is the highest,while the content of other heavy metals are relatively low.Statistical analysis shows that the sources of heavy metals pollution in the five reservoirs include mining metallurgy,agricultural planting,industrial and domestic sewage,etc.(4)Based on the total amount and fraction of heavy metals in the sediments collected,the pollution and ecological risks of heavy metals were evaluated.The results show that the accumulation and pollution level of heavy metals in the sediments of A Reservoir are gradually increased from H Island and K area to the underwater area or the dam.The accumulation and pollution of Cd is the most serious,and Cu,Mn and Cr are relatively moderate;the possibility of heavy metals poisoning benthic organisms gradually becomes stronger,and the order of contribution to toxic effects is Cr>As>Cd(:-Pb>Zn>Cu.Heavy metals in the core sediments of C Reservoir,B Reservoir and A Reservoir are toxic to benthic organisms,while heavy metals in the sediments of D and E Reservoirs are not toxic to benthic organisms.The biotoxicity evaluation of heavy metals relies heavily on the types of heavy metals involved in the evaluation and the evaluation requires comprehensive consideration of the total amount,form and bioavailability of heavy metals.In summary,the flood bed interpretation method considering hydrological factors such as sediment concentration and rainfall can well reconstruct the sedimentation rate in the reservoir riparian zone.The heavy metals in reservoir sediments(soils)obviously accumulate along the direction of surface runoff,especially Mn.The serious pollution problem of Cd in reservoir sediments is worthy of attention.The increasing trend of heavy metals in surface sediments near the dam indicates that there is a long way to go to prevent and control heavy metal pollution in reservoirs.