Variation Characteristics Of Nutrients And Typical Heavy Metals At Sediment-Water Interface In The Outlet Of Nansi Lake

Nansi Lake is the largest lake in Shandong Province and one of the large freshwater lakes in China.The geochemical behavior of nutrients and heavy metals at the sediment-water interface has been widely explored.In this study,the diffusion gradient in thin films（DGT）technique,high-resolution dialysis（HR-Peeper）technique and traditional chemical component analysis are used.The occurrence characteristics,bioavailability,migration characteristics,release risks and diffusion fluxes of nutrients（P,NH₄⁺-N and NO₃^--N）and typical heavy metals（As,Co,Cr,Mo,Pb and Zn）in the sediment-water interface of the outlets of Nansi Lake are studied,and the interaction between phosphorus and heavy metals migration is also studied.In order to provide scientific basis for nutrients and heavy metals pollution control and ecological environment restoration management in outlets of Nansi Lake in the future.The main research results are as follows:（1）The pH of surface water at the outlets of Nansi Lake is 7.62,showing a weak alkalinity.The contents of total nitrogen（TN）and total phosphorus（TP）in surface water are1.97 mg/L and 0.15 mg/L respectively,which exceeded the standard content of Class IV water,higher than the contents of Weishan Lake of the subordinate lake of Nansi Lake,and have exceeded the occurrence concentration of nitrogen and phosphorus eutrophication.This region is in a"hypereutrophication"state.（2）The contents of DGT-labile P and Fe and soluble P and Fe in HR-Peeper are similar and significantly positively correlated in vertical distribution,indicating that the migration of phosphorus in sediments may be related to the reduction release of Fe,and the reduction dissolution of iron oxides can induce the release and diffusion of Fe²⁺and P in anaerobic environment.Calcium-bound phosphorus（Ca-P）is the main form of phosphorus in solid phase of sediments,and the unstable phosphorus content of shallow sediments is higher than that of bottom sediments,and it is easier to release phosphorus.The contents of NH₄⁺-N and NO₃^--N fluctuated greatly in sediment-water interface,and they may intertransform with each other under specific physical and chemical conditions.（3）The contents of soluble As,Cr,Pb and Zn in the overlying water of the outlet of Nansi Lake all meet the standards of Class Ⅲ water,and the contents of soluble Co and soluble Mo also meet the limits of the specific project standards of surface water source for centralized drinking water.The content of DGT labile and HR-Peeper soluble heavy metals at the sediment-water interface is irregularly distributed,and is mainly affected by manganese oxide,organic matter,phosphate and other factors.The content of heavy metals in solid phase of surface sediments is higher than the background value,and mainly in residual state（F4）.（4）According to the risk assessment parameters calculated by BCR,there is no release risk of Cr and Pb in the surface sediments at the outlets of Nansi Lake,As and Mo belong to low risk level,and Co and Zn belong to medium risk level.Based on R value,nutrients P,NO₃^--N and heavy metals As,Co,Mo and Zn in surface sediments were partial supplement types.NO₃^--N,Cr and Pb belong to the non-replenishment type,and the resupply capacity of interstitial water from sediment fixation is low.（5）According to the net diffusion flux calculated by the DGT-labile concentration,the sediment is the"source"of P,NH₄⁺-N,As,Co,Cr and Pb,and its release direction at the sediment-water interface is from the sediment to the overlying water.The negative diffusion fluxes of available NO₃^--N,Mo and Zn indicate that the sediment is a"sink"of NO₃^--N,Mo and Zn,and the labile NO₃^--N,Mo and Zn accumulate to the sediment.