Study On Cd Polluting Property And Migratory Behavior In The Soil Of Water Level Fluctuating Zone Of The Three Gorges Reservoir

The anti-seasonal adjustment model of“winter flooding and summer land”in the three Gorges Reservoir of the Yangtze River give rise to a long time and high intensity dry-wetting alternation in the reservoir area every year,which results in a special transitional ecosystem of water and land.The heavy metal cadmium（Cd）has become the primary pollution element in the soil of the three Gorges Reservoir zone,and the related studies have been concentrating on the Yangtze River main stream and lack of reports about tributary or backwater regions which influenced by main stream flow backward.In this paper,the soil of the Yangtze River tributaries in Pingshan,the interior of the three Gorges Reservoir area,is taken as the study area.Put the soil of the reservoir bank（elevation 177m）as the control-experiment,The physical and chemical properties of soil with different flooded heights of151m and 160m were obtained.Besides,based on the analysis of Cd concentration,spatial distribution and pollution assessment,the Cd pollution characteristics of Pingshan land were determined.Using the undisturbed soil column to simulate the effects on the soil endogenous colloid release and Cd migration in different moisture conditions（saturation,field moisture capacity,drought）.This paper aims to explore the possible physical and chemical properties change of soil,the distribution of Cd and Cd migration assisted by potential soil colloids during alternation of dry-wetting and different duration of flooding time.The main conclusions are as follows:1.Results showed Flooding has significant effects on some physical and chemical properties such as pH,CEC,bulk density,and mechanical composition in thewater level fluctuation zone.A weak acid to weak alkaline（6.29⁸.15）of soil in Pingshan zone.Long-term flooding could decrease soil pH.CEC in 151m was significantly higher than the other two altitudes（P<0.05）.Organic matter easily accumulated in the surface of the 151m because of long-term flooding,it also effected soil mechanical composition.Soil water retention curves in different altitudes had no significant difference in the low pressure section（h>-100cm）.Soil moisture in 160m rapidly decreased in high pressure section（h<-100cm）because of high sand content.Compared with 177m,curve of 151m changed more gently because it’s difficult to remove water under high pressure with abundance of small pores in soil.2.The total Cd content in soil at the height of 177m,160m,151m was0.85,0.80,0.72mg/kg respectively（There was no significant difference,P>0.05）,which was not up to the second grade of Chinese standard for soil environment（GB15618-19955）.The Cd concentration in the soil profile of the reservoir bank increased with depth,while the Cd content in both top soil（0¹0cm）and deep soil（30⁴0cm）was higher than that in the deep soil in water level fluctuation zone.The Cd content of the soil on the bank of the reservoir is higher than that of in water level fluctuation zone.“Tessier”extraction showed that the carbonate-bounded Cd in this area accounted for the largest proportion（29.81%⁴3.33%）.The proportion of Cd fractions at the height of 151m and 177m were carbonate bound>exchangeable form>iron-manganese oxide bound>residual form>organic and sulfide-bound.For 160m was carbonate bound>iron-manganese oxide bound state>exchangeable form>organic and sulfide-bound>residual form.The evaluation of the geoaccumulation index showed that the soil Cd pollution levels at the altitudes of 177m,160m,and 151m were medium-strong,medium-strong,and medium-grade,respectively.The average RAC value exceeded 50%,whichimplied strong bioavailability.Soil Cd at 151m has the strongest mobility and release ability.The potential risk assessment presented that the soil Cd pollution at three elevations reached a strong ecological risk degree,and the order was177m>160m>151m.In addition,exogenous addition of Cd caused the increase of carbonate bound fraction and residual form which can increase its ecological risk.3.In the undisturbed soil column experiment,the Br-penetration velocity slowed down with the decrease of soil moisture,and the breakthrough curve also showed asymmetry and tailing characteristics.Dispersion of soil water could promote the generation of preferential flow.The presence of preferential flow in the original soil was confirmed by a miniature water potentiometer.Soil pH decreased with the prolonged drought time,and increased in rainfall would cause the soil pH to neutral eventually.The trend of changes in Ca²⁺,Mg²⁺,and EC also has tailing characteristics.concentrationCa²⁺>Mg²⁺.The concentration in different stages was saturation stage>drought stage>field moisture capacity stage.In the stage of saturation and field moisture capacity,the release amount of endogenous colloids first decreased and then increased,while the colloid release trend in drought stage showed a“multi-peak”shape.The longer the interval between dry and wet,the more favorable the formation of colloidal particles.But if it exceeded its“critical drought time”,the initial colloid release significantly reduced.DOC outflows during the drought period increased significantly and also showed a“multi-peak”shape.Colloidal bounding Cd was the main formation of Cd in the transport processes.Due to the presence of preferential flow and colloidal flocculation,the release of colloidal bounding Cd did not reach a steady state.However,alternating wet and dry could enhance soil Cd adsorption,thus reduced the release of colloidal bound Cd and ecological risk of Cd.The Pearson correlation coefficient showed that the factors affected the release of endogenous colloids were different at different stages.Saturation stage:pH,EC,Ca²⁺,Mg²⁺,field moisture capacity stage:pH,drought stage:EC,Ca²⁺,Mg²⁺.The factors affected the release of colloidal binding Cd:saturation stage:colloidal concentration,field moisture capacity stage:colloidal concentration,drought stage:DOC.Colloids are more likely to assist cadmium migration in saturation and field moisture capacity stages.