| Being an important pollutant in lake systems,heavy metals pose a potential threat to ecological safety by being highly toxic and non-degradable.The highly dynamic hydrodynamic conditions at lake inlet,which acts as a sedimentation zone for heavy metals,may result in the sediment becoming a secondary source of pollution and releasing heavy metal elements into the overlying water column.In this study,the content,distribution characteristics,ecological risk and release risk of heavy metals(As,Cu,Pb,Zn and Cd)in the sediment-water interface of the Zhuzhaoxin River estuary were analyzed using the diffusion gradients in thin films(DGT)and high-resolution dialysis(HR-peeper)techniques,combined with a modified BCR sequential extraction method,to provide a basis for the future prevention and control of heavy metals at the inlet of the Newzhuzhao River.The main research are as follows:(1)The total content of heavy metals As,Cu,Pb,Zn and Cd in the sediments were all higher than the background values in the Nansi Lake Basin.The BCR sequential extraction results showed that the residual state occupied the major part of As,Cu,Pb and Zn,the nonresidual state occupied the major part of Cd.In addition,the content of non-residual Cd in the sediment surface is relatively high,reaching 0.34 mg/kg and mainly in exchangeable state,which indicates that Cd is more easily released from the sediment than other heavy metals.(2)The concentrations of dissolved heavy metals meet the national surface water environmental standard for Class III,indicating the water quality is relatively healthy;the statistical results of the DGT-labile concentrations indicated that the bioavailable content of Cd was higher at lake inlet.The distributions of dissolved and DGT-labile As,Cu,Pb,Zn and Cd were spatially heterogeneous,and its distribution characteristics were the result of the combination of many factors such as the chemical forms of heavy metals,Fe/Mn oxides and sulphur,etc.The results of the correlation analysis indicate that the dissolved,exchangeable and reducible states are all important sources of DGT-labile As and Cd.(3)The ecological risk of the heavy metals was evaluated by the species sensitivity distribution(SSD)showed that the risk quotient of DGT-labile Zn in the overlying water and sediment are 5.91 and 1.26 respectively,which are higher than other heavy metals and reach a medium ecological risk level.The DGT-labile concentrations,which represent the bioavailable fraction,reflect a more realistic environmental risk characteristic than the dissolved concentrations.The results of the multi-substance potential affected fractions based on DGTlabile concentration indicate that 14.92% and 9.64% of the species in the overlying water and sediment,respectively,will be exposed to heavy metals,and that the overall ecological risk of heavy metals in the inlet is at a mild level.Compared with the traditional ecological risk assessment methods,the coupling SSD and DGT-labile concentration can reflect the real environmental risk characteristics,and can be used as a powerful tool for ecological risk assessment in the lake estuary with complex environment.(4)The risk assessment criteria for Cd reached 33.07%,with the highest potential release risk,followed by As(7.13%),which was in the low release risk.The sediment buffering capacity showed that the five heavy metals had not reached the full buffer level and the release risk was small,among which the buffer capacity of As was the highest 0.42.In addition,the diffusion flux results show that the sediment is a ’sink’ for Cu,Zn and Cd,and a ’source’ for As and Pb,especially As has a clear tendency to diffuse into the overlying water bodies,and there is a potential release risk. |
