Experiment Study On The Membrane Effect Of Cohesive Soil In The Saltwater Leakage Process

The deep water occurs under the salt water in North China Plain, which forms thedouble layer structure of the water quality of “salt on the light”. Because of the excessiveexploitation of the deep water, the upper salty water recharges the deep water through theflow. The important issue in the water resources security is whether or not the clay layercan have the effect of filtering salt. The purpose of the experiment is to study the influencefactor which has the membrane effect except convection, dispersion and adsorption in theprocess of the salt migration.To test and verify the membrane effect and the filtering salt effect of the clay,Tangshan coastal clay from salt-water area, Tangshan city clay from freshwater area andNa-montmorillonite are selected as the experimental material in the reverse osmosisexperiment. The penetration curves of Cl-and Na+concentration are regularly monitored inoutlet, the breakthrough cures of each ion are gotten at the end of the experiment, and thedispersion coefficient D and retardation coefficient R are respectively calculated by usingCXTFIT software. At the end of experiment, the ion concentration on the membranesurface is tested and the whole rock is analyzed by layering the soil column.Experimental results show that the infiltration solution concentration monitored at theend of the experiment presents concentration phenomena and the solute migration rateshows a trend of slow, while the seepage solution concentration is greater than the initialvalue of the infiltration solution, which verifies the existence of membrane effect. In theurban soil of unsaturated adsorption, the molar concentration of Na+penetration curve issignificantly lower than the Cl-; in coastal soil of saturated adsorption, Na+and Cl-penetration curve have almost synchronous growth, and it indicates that cationic is blockedfor ion exchange adsorption by clay.In Na-montmorillonite, the molar concentration of Na+penetration curve is significantly higher than the Cl-. According to the theory of the reverseosmosis, the membrane surface concentration C0and the membrane efficiency σ arecalculated. The influence factors of the membrane efficiency σ are determined. Themaximum adsorption amout can be calculated through the clay adsorption experiment.Theactual interception amout of sodium ions is calculated through the whole rock analysis.Theinterception amout is greater than the adsorption amout.It is comprehensively analyzed that clay has membrane properties, the filtering saltmechanism is similar to the charged membrane, which is commonly lead by sieving andDonnan effect. The membrane effect of the clay makes the seepage solution enriched andseparated and makes the solute migration blocked, while the solute migration rate is veryslower that migration under the condition of convection-diffusion. The clay membranemakes the infiltration solution concentration enrich gradually. When the clay membraneeffect reaches equilibrium with the dispersion effect, the solute migration velocity canachieve stability, the seepage solution concentration will tend to be stable. Clay membraneperformance can slow the migration rate of the salt, but under the condition of long-termclosed infiltration, clay interlayer does not prevent salt ions through completely.There is a huge water head difference between the deep aquifer and shallow aquifer inthe eastern funnel area in North China Plain. Experiment show that if there is certainhydraulic gradient, upper salty water will recharge deep fresh water. The membrane effectof the clay layer as the weak permeable layer makes the down rate of the salty waterbottom slow.