Soil Properties And Element Migration Characteristcs Of Living Source Pollutants Under Dry-wet Cycle

Dry-wet cycling conditions of polluted fluids from domestic sources not only have a significant impact on soil properties,but also change the migration characteristics of elements in soil.This will not only threaten the safety of Engineering construction,but also pose a challenge to environmental protection.In order to study the soil properties and element migration characteristics under the condition of dry-wet cycling of source pollution,the silty clay in Xuzhou area was taken as the research object.Based on the analysis of inorganic ions and metabolites of organic matter in Domestic Landfill leachate,the mixed solution of Na₂CO₃ and Na₃PO₄ was selected as the contamination solution.The soil properties and element migration characteristics under wetting and drying cycles were tested and analyzed by a wetting and drying cycle test system consisting of wetting cycles and natural evaporation.The change mechanism of soil properties and element migration mechanism under the condition of dry-wet cycling of domestic pollution are revealed.Research shows:?1?For the process of cyclic humidification,the density,shear strength,parameters and resistivity of soil are negatively correlated with the times of cyclic humidification in the first 4-6 cycles(v_r?v_t?v_re<0).Thereafter,with the increase of cyclic humidification times,except for the slight increase of soil density?v_r>0?,the other two have little change(v_t?v_re?0).The three factors above are positively correlated with seepage depth.The permeability coefficient of soil,the number and average width of surface cracks are positively correlated with the number of cycles and negatively correlated with the depth of seepage under cyclic humidification.For the first to second and fifth to sixth cycles of humidification,the relationship between v_k%and seepage depth is negative and positive respectively.For the seventh to eighth and last two cycles of humidification,the influence of seepage depth on v_k%is small.?2?For the natural evaporation process,the density and resistivity of shallow and middle-layer soil are negatively and positively correlated with evaporation time,respectively.The density/resistivity of medium-deep soil increases first and then decreases/changes little with evaporation time.The density/resistivity of deep soil is not positively correlated with evaporation time.The density and resistivity of soil are positively and negatively correlated with depth,respectively.With the increase of evaporation time,the shear strength and its parameters of shallow soil increase first and then decrease,while the shear strength and its parameters of middle and lower soil increase.The shear strength of soil and its parameters first decrease and then increase with the increase of depth.With the increase of depth,v_rincreases and v_tdecreases in each evaporation stage.In the early stage of evaporation,v_tincreases first and then decreases with the increase of depth.During the middle and late evaporation period,v_tis positively correlated with depth.Under natural evaporation conditions,the number and average width of surface cracks are positively correlated with evaporation time.?3?Generally speaking,the wetting infiltration process of dry-wet cycle will increase the moisture content and permeability of soil,and decrease the density,shear strength,parameters and resistivity of soil.When the soil changes from wetting infiltration to evaporation,the density of the soil decreases,and the shear strength,parameters and resistivity increase.?4?Under the condition of humidification cycle,increasing the number of cycles can increase the growth rate of Q_a.Under the same cycle number,Q_a is negatively correlated with seepage depth.Natural evaporation can reduce the water content of shallow and middle layers of soil;in addition,increasing depth can inhibit the reduction of water content of soil by natural evaporation.?5?The density of Si,Al and Ca in soil decreases during the humidification cycle.At the same time,The density of P,Na and C increased.Increasing the depth of seepage can inhibit the reduction of element density of Si,Al and Ca in soil by humidification cycle.Evaporation can increase the density of elements in shallow-layer soil and decrease the density of elements of Na and C in middle-layer soil.Increasing depth can reduce the density of Si,Al and Ca elements in soil,but has little effect on the density of P elements.?6?In general,the wetting infiltration process of the dry-wet cycle of the polluted substances from domestic sources will reduce the density of Si,Al and Ca elements and increase the density of Na,P and C elements in soil.The density of Si,Al,Ca and P increases when the soil changes from wetting infiltration to evaporation.?7?Study of mechanism:?1?The seepage transport not only directly results in element migration,but also reduces the density and strength of soil,meanwhile,improves the permeability of soil.?2?On the one hand,the precipitation produced by chemical reaction between soil and water promotes the migration of elements from water to soil,on the other hand,it increases the density of soil and decreases its permeability,which has certain influence on the resistivity and strength characteristics of soil.?3?For one thing,evaporation force directly promotes the diffusion of water on the surface of soil to the atmosphere,resulting in the decrease of soil moisture content in a certain range below the surface,and then initiating the corresponding changes in soil density,strength,resistivity and water conductivity;at the same time,the volume shrinkage of soil not only lead to cracks,but also causes the overall migration of elements in the soil.?4?Gravity infiltration,ion adsorption and solute transport also change the soil properties and element migration characteristics under the condition of dry-wet cycling of source contaminants.?8?The basic differential equation of element migration in soil under the condition of dry-wet cycling of polluted matter from domestic sources is established.