| In recent decades,with the rapid development of China’s economy and society,the problem of groundwater pollution has become increasingly serious.The risk management of groundwater pollution has become one of the hot spots of environmental workers at home and abroad.The risk management of groundwater pollution includes institutional control,engineering control and active remediation,incorporating the removal and control of pollution sources,the control of pollution plume spread and the remediation of contaminated groundwater.The cut-off of polluted groundwater plays an important role in the remediation of groundwater pollution.It can effectively limit the diffusion of pollutants within a certain period of time,and provides an economic and effective method for groundwater pollution remediation.Materials commonly used for cutoff groundwater pollution include bentonite,cement,and clay.The key issues in the study of groundwater pollution cutoff materials at home and abroad are:(1)Can the barrier materials maintain the required low hydraulic conductivity in the extreme acid,alkali,high salt and non-aqueous phase liquid(NAPL)pollutant environments?(2)What kind of contamination(type of pollutants,degree of pollution)and environmental conditions(p H,etc.)are suitable for blocking materials?(3)How to judge the compatibility of the barrier materials and the pollutant,and whether they can be applied to the pollutant environments.As a result,this paper takes bentonite/cement/clay based in-situ barrier materials as the research objects,and is carried out by combining with different vertical engineering barriers.Firstly,the containment performance of three series of barrier materials is studied,and the concentration and volume of bentonite slurry injected into different lithology strata,the hydraulic conductivity of cement based material with different cement ratio,the influence of clay and bentonite or modifying agent dosage on the hydraulic conductivity of clay based barrier material are determined.Secondly,the compatibility of three series of barrier materials in serious environmental pollution(acid,alkaline,salt and organic pollution)is researched.Through monitoring the change law hydraulic conductivity of barrier materials under different solution permeation,the compatibility of barrier materials with pollutants can be evaluated intuitively.Simultaneously,the interaction mechanism between barrier materials and pollutants are clarified,and the modification effect of the modified barrier materials is analyzed.Finally,the applicability of three series of barrier materials in different environments is evaluated comprehensively.The main conclusions are incorporated as follows:(1)The bentonite slurry can effectively and quickly intercept the groundwater pollution,and the low permeability area can be formed immediately near the injection point after injecting bentonite slurry into the aquifer.For fine sand,medium sand and coarse sand aquifers,bentonite slurry with concentration of 6%,7% and 10% and corresponding volume of 0.6 PV,0.8 PV and 0.6 PV are required to be injected to meet the hydraulic conductivity requirement of vertical engineered barrier.The suitable p H of bentonite based barrier material ranges from 2.0 to 13.0,and it is appropriate for dissolving organic pollution environment,not suitable for Ca Cl2 saline environment with a concentration between 10 and 1000 m M and pure phase organic pollution environment.(2)The hydraulic conductivity of cement based material decreases linearly with hydration time(in logarithmic coordinate system),and the larger the cement content is,the faster the hydraulic conductivity decreases.In a certain time,the hydraulic conductivity of cement based barrier material can reduced to 1.0×10-7 cm/s.Cement based blocking material should not be injected near the strong acidic environment(p H = 1.0)where the corresponding calcium salt is soluble and the strong basic environment(p H = 13.0),but can be injected their upstream or downstream for a certain distance.It is suitable for sulfate environment,dissolved phase and pure phase organic pollution environment.(3)The hydraulic conductivities of clay-based barrier materials with bentonite addition of 5% and 6% are basically identical and can achieve the order of 10-8 cm/s.The suitable p H of clay based barrier material ranges from 2.0 to 13.0.It is appropriate for Ca Cl2 saline environment with a concentration no more than 100 m M and for dissolving organic pollution environment.However,calcium ions and some of organic pollutant can penetrate clay-based material.Meanwhile,clay based barrier material does not apply to pure phase organic contaminated environment.(4)The addition of sodium pyrophosphate reduces the hydraulic conductivity of the clay based barrier material and increases the adsorption capacity of the clay based barrier material to Ca2+,which is suitable for Ca Cl2 saline environment with a concentration no more than 100 m M.The addition of activated carbon increased the adsorption capacity of clay based materials for organic pollutants,and the clay based materials modified by activated carbon with the addition amount of 0% ~ 5% were suitable for phenol pollution.(5)In this paper,a method for determining the compatibility of vertical cut-off wall for groundwater pollution is proposed,and a method for evaluating the suitability of barrier materials and groundwater pollution environments is constructed.The main innovations of this thesis are as follows: the compatibility and action mechanism of bentonite/cement/clay based barrier materials with different pollutants are clarified,and the blocking effect and application scope of different barrier materials in heavily contamination environments(acidic,basic,saline and organic pollution)are determined.It has been proved that the interaction between polluted groundwater and porous media of aquifer,which leads to the change of permeability for porous media.This paper puts forward the judgment method of the compatibility performance of the groundwater pollution barrier materials,and constructs the evaluation system for groundwater pollution barrier materials.To sum up: This paper lays the theoretical and application foundation for the evaluation of the compatibility of pollutants and barrier wall materials in the application of groundwater pollution cutoff technology,and provides a basis for the long-term effectiveness evaluation of groundwater pollution vertical engineered walls. |
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