Salt Blocking Effect Of The Clay Layer In Saline Water Overflow Process

In the groundwater flow system,the clay layer,as the weakly permeable layer,is an important part of the groundwater system in the plain area,and it is also a protective barrier for groundwater resources.A large number of previous studies have shown that the clay layer has properties similar to a semi-permeable membrane,which can intercept the salt in the groundwater and block the migration of salt in the saline water.However,the evaluation and research on the ability of clay layer to block salt transport is still relatively preliminary,and the research on the transport process of salt in clay layer and the form and influencing factors of the blocking effect of clay layer on salt is still not clear and in-depth,and lack of sufficient data support and quantitative evaluation.Therefore,in view of the actual problem of salt water leakage in Cangzhou area,this paper used the undisturbed clay sample to carry out the experimental simulation to study on the saline water overflow process under the in-situ environmental conditions,and used the PHREEQC software to carry out the numerical simulation of the saline water overflow process.Based on the investigation and research results of predecessors in Cangzhou area,the vertical distribution and structural characteristics of Quaternary aquifer groups and the vertical variation of lithology and mineral composition of Quaternary layers in the study area are clarified through drilling survey,test analysis and detection of Quaternary disturbance soil samples.Based on the understanding of the water-rock interaction phase balance in the long-term stable groundwater system,this paper creatively discussed the hydrochemical evolution of aquifers in the vertical structure.The depth ranges of the saline water distribution and downward movement in Cangzhou area is 0-115 m.Affected by the distribution and downward movement of saline water,the Hydrochemical environment of the Quaternary aquifer groups is gradually changed follows the order of SO4·Cl?HCO3·SO4·Cl?HCO3·SO4?HCO3·Cl?HCO3 from top to bottom.The downward movement of saline water increases the adsorption of Na+ and Cl-by the aqueous medium and the ion exchange capacity of Na+ with Ca2+ and Mg2+ between groundwater and aquifer medium,and the dissolution precipitation equilibrium of calcite,dolomite and gypsum in groundwater shifts to the precipitation direction.The transformation of water-rock interaction between aquifer medium and groundwater increases the loss of salt ions in the saline water overflow process,and blocks the downward movement of saline water.In this paper,the laboratory experiments were carried out to simulate the saline water overflow process in the undisturbed clay layer samples using the self-designed and manufactured clay high-pressure reverse osmosis test device.It was found that after the high-pressure infiltration of saline water,the average particle size of the test clay sample decreased,the specific surface area increased,and the content of Na+ and Cl-of the soluble salt component of the clay samples increased,while the content of Ca2+,Mg2+,HCO3-and SO42-decreased.In the experiments,as the time of saline water infiltration increases,the content of Na+ and Cl-in the leachate increased,the content of Ca2+ and Mg2+ decreased,and the hydrochemical type transformed from HCO3·Cl-Na·Mg and HCO3·SO4·Cl-Na·Mg to Cl-Na·Mg and Cl-Na,the chemical osmotic pressure caused by the differences in the concentration of solute at both ends of the clay layer gradually decreased,the adsorption of Na+ in the saline water slowed down,the cation exchange between Na+ in the saline water and Ca2+,Mg2+in the clay layer gradually weakened,and the dissolving and precipitation reactions of different minerals gradually shifted in the direction of dissolution.The change of various water-rock interactions between saline water and clay layer causes the resistance of clay soil layer to the salt transport of salt water to decrease,and the salt resistance coefficient ?of clay layer to decrease.But,due to the decrease of the average particle size of the clay layer,the surface area increases,the permeability decreases,the film efficiency coefficient of the clay layer increases,and the membrane performance is enhanced.The saline water overflow process in the high-pressure reverse osmosis experiment is simulated by PHREEQC software.The simulation results are basically the same as the experiment results,and the fit of the simulation results and the experiment results is 0.96 and 0.97,respectively.The simulation results of the influence of different parameters on the salt blocking effect of the clay layer in the saline water overflow process show that the permeability is the most critical parameter affecting the ability of clay layer salt resistance;and the influence of clay layer thickness on salt resistance capacity is only obvious in the initial stage of saline water infiltration;while the influence of salt water solute concentration becomes stronger with the increase of time.At the same time,the influence of adsorption and cation exchange on the salt resistance capacity will decrease with the increase of saline water seepage time.This paper mainly studies and discusses the blocking form and influence factors of the clay layer in the saline wateroverflow process,and the relevant research results can effectively fill the data gap in the research on the process and mechanism of the salt blocking effect of clay in China.It provides scientific basis for the development and utilization of water resources and the improvement and utilization of saline water,and provides a theoretical basis for the design and evaluation of clay barriers in scenes such as landfills and contaminated sites.