Fate And Transport Of Heavy Metals In Ice-water-sediment Multi-medium

Migration and tendency of heavy metals in lakes are the research hotspots of the scholars at home and abroad.Along with the rapid development of industry & agriculture and urbanization in China,heavy metals enter into the lake system through surface runoff,atmospheric deposition,direct wastewater discharge,etc.The heavy metals cannot be easily degraded in the lake system,and only can migrate among different environment phases or enrich in a certain environment phase,and the capability of the migration among different environment phases and the final tendency are determined by the environment characteristics of the lake.The Ulansuhai Lake,located at the high latitude area in northern China,has the ice growth period of about 3-4 months and the unfrozen period of about 7-8 months.For the lake,the environment characteristics of the unfrozen period are significantly different from those of the ice growth period.During the ice growth period,the ice start to growth,and the volume of ice medium constantly increased,while the volume of water medium constantly decreased.When water freezes,the solute will be excluded to sub-glacial water to keep the ice crystal pure,which is a solute removal process,so the heavy metals have different environmental behaviors in the lake during the two periods.Therefore,the representative heavy metals----Cu,Zn,Pb,Cd and Cr were selected as the research object in this paper.Specifically,on the basis of the environment characteristics of the unfrozen period and the ice growth period of the lake in cold area and the influence of the environment change on heavy metal migration,the contents,the morphologies and the spatial distribution characteristics of the heavy metals----Cu,Zn,Pb,Cd and Cr in the environment media of the Ulansuhai Lake during the ice growth period and the unfrozen period were experimentally analyzed;then,the improved QWASI environment model was adopted to simulate the rule for the migration among different environment media and the tendency of the heavy metals during two periods;finally,the following conclusions were obtained:1.The QWASI model was improved through sediment re-suspension and suspended matter deposition experiment,and applied it to simulate the concentration change tendency of the heavy metals----Cu,Zn,Cr,Cd and Pb in the water body and in the sediments of the Ulansuhai Lake during the unfrozen period.The results show that the modeling results can simulate the changing trend and migration process of heavy metals accurately.The margins of error between the contents of the heavy metals in the water body and in the sediments calculated by the model and the measured values are respectively 2.8%~49.5% and 0.2%~19.3%.2.The QWASI model was improved further through freezing experiment test,the ice medium was added to the model to simulate the concentration change tendency of the heavy metals----Cu,Zn,Cr,Cd and Pb in the ice body,in the water body and in the sediments of the Wuliangsu Lake during the ice growth period.The results show that the improved model can simulate the migration process of heavy metals----Cu,Zn,Cr,Cd and Pb between ice,water and sediment medium.The margins of error between the contents of the heavy metals in the water body and in the sediments calculated by the model and the measured values are respectively 2.6%~61.3% and 0.9%~10.2%.3.The concentration change of the heavy metals in the water body and sediments during the unfrozen period is mainly influenced by sediment re-suspension and suspended matter deposition.According to the model parameter sensitivity analysis,various environment parameters significantly differently influence the output results of the model,orderly as follows: suspended matter deposition flux > sediment re-suspension flux > inflow water concentration > atmospheric deposition > mass transfer coefficient > sediment burial.The concentrations of the heavy metals in ice,water and sediment of t he Ulansuhai are comprehensively influenced by various migration processes.According to the model parameter sensitivity analysis,various environment parameters significantly differently influence the output results of the model,orderly as follows: suspended matter deposition flux > sediment re-suspension flux > mass transfer coefficient >inflow water concentration > sediment burial > surface sediment thickness.4.The concentrations of various heavy metals in the water body during the unfrozen period are not significantly changed,and the concentrations of the heavy metals except Pb in the water body during the ice growth period are greater than that during the unfrozen period.The sediments of various heavy metals during the unfrozen period tend to be increased,and the concentrations of the heavy metals----Cu and Cd in the sediments during the ice growth period are greater than those during the unfrozen period,while the concentrations of the heavy metals----Zn,Cr and Pb in the sediments during the ice growth period are less than those during the unfrozen period.5.The contents of the heavy metals----Cu,Zn,Cr,Cd and Pb in suspended matters are greater than those in the sediments.The migrating morphologies of the heavy metals----Cu,Zn,Cr,Cd and Pb in the sediments respectively occupy 29%,31.4%,20%,62.87% and 24.91%,while the migrating morphologies of the heavy metals----Cu,Zn,Cr,Cd and Pb in the suspended matters respectively occupy 41.1%,60%,27%,76.1% and 55.3%.Obviously,the suspended matters have high content of migrating heavy metals,while the sediments have relatively low content of migrating heavy metals.Additionally,the heavy metals with the highest migrating proportion in the sediments and in the suspended matters are both Cd,thus indicating that Cd has the strongest mobility.6.The average concentrations of the heavy metals----Cu,Zn,Cr,Cd and Pb in the ice bodies at various sampling points of the Ulansuhai during the ice growth period are respectively 1.13?g/L,9.98?g/L,2.26?g/L,0.118?g/L and 0.66?g/L,and the content of the heavy metals in the ice body is less than that in the water body below the ice.The survey result also shows that the content of the heavy metals in the upper layer of the ice body is greater than that in the middle and lower layers of the ice body.