非等温分布条件下三层复合衬垫中有机污染物一维瞬态运移规律研究

针对非等温分布条件下由土工膜(geomembrane,简称GMB)+土工复合膨润土衬垫(geosynthetic clay liner,简称GCL)+压实黏土衬垫(compacted clay liner,简称CCL)组成的3层复合衬垫中有机污染物一维瞬态运移问题,考虑了对流、扩散、机械弥散、吸附、降解和热扩散等因素,建立了相应的数学模型。该模型可考虑扩散系数和渗透系数等参数随温度变化。利用有限差分法,获得了该模型的数值解。通过将所建模型计算结果分别与试验结果、已有解析模型计算结果和COMSOL软件计算结果进行对比,验证了所建模型的正确性。基于所定义的击穿时间t_b,以甲苯作为代表性有机污染物分析和讨论了不同因素对运移行为的影响。结果表明：(1)非等温分布条件会使渗透系数、扩散系数和线性吸附系数等参数发生变化,其中渗透系数和扩散系数的变化会加快运移过程,尤其是扩散系数,而线性吸附系数的变化会减慢运移速率;(2)复合衬垫上部和底部温度差ΔT的增大会使击穿时间t_b减小,底部运移通量J_b增大;当ΔT为0、10、20、30、40 ...     

考虑热扩散的黏土衬垫中污染物运移简化计算

以单层压实黏土衬垫为研究对象,在传统对流弥散方程的基础上考虑温度梯度下污染物的热扩散运移,得到简化条件下半无限边界的解析解。研究表明,考虑热扩散效应所得的衬垫底部相对浓度和通量均大于传统对流弥散方程所得结果,S_T越大时相对浓度和通量也越大,当S_T=0.5 K-1时稳定通量达到S_T=0时稳定通量2.5倍。热扩散通量是对流通量的-AS_TD/v倍,两者均随时间的增加而增大;分子扩散通量则先增加达到峰值后减小趋于零。同时考虑温度对运移参数的影响和热扩散效应时,温度升高使衬垫底部相对浓度增大,导致击穿时间提前,污染物通量显著增大;当温度从30℃升高至70℃时,击穿时间从8.5 a提前至5.1 a,稳定通量增大约1.7倍。在进行衬垫设计时不考虑温度的影响将使污染风险大幅增加。     

非等温条件下有机污染物在粘土衬垫中的扩散分析

填埋场中垃圾降解作用产生的热量在黏土衬垫中的运移会形成温度场,温度场会对污染物的运移产生影响。文章在多孔介质热力学理论和污染物运移理论相结合的基础上,主要考虑温度场对污染物扩散的影响,建立了非等温条件下有机污染物在黏土衬垫中的一维扩散模型,并采用分离变量法得到了模型的解析解。基于文中模型,对比了非等温条件和等温条件下有机污染物在黏土衬垫中的运移。非等温条件的黏土衬垫底部浓度比等温条件下的浓度大,非等温模型比等温模型更偏于安全。解析解可为实验数据的拟合及分析、衬垫的设计提供参考。进一步的参数分析表明:增大阻滞因子Rd和黏土厚度L可以减小黏土衬垫底部由温度梯度引起的有机污染物浓度。     

非等温条件下有机污染物在黏土衬垫中的扩散分析

填埋场中垃圾降解作用产生的热量在黏土衬垫中的运移会形成温度场,温度场会对污染物的运移产生影响。文章在多孔介质热力学理论和污染物运移理论相结合的基础上,主要考虑温度场对污染物扩散的影响,建立了非等温条件下有机污染物在黏土衬垫中的一维扩散模型,并采用分离变量法得到了模型的解析解。基于文中模型,对比了非等温条件和等温条件下有机污染物在黏土衬垫中的运移。非等温条件的黏土衬垫底部浓度比等温条件下的浓度大,非等温模型比等温模型更偏于安全。解析解可为实验数据的拟合及分析、衬垫的设计提供参考。进一步的参数分析表明:增大阻滞因子Rd和黏土厚度L可以减小黏土衬垫底部由温度梯度引起的有机污染物浓度。     

基于有机污染物扩散的复合衬垫厚度的概率设计

有机污染物在复合衬垫中的运移以扩散方式为主。目前关于运移参数不确定性及其对衬垫性能影响的研究还不多,复合衬垫厚度的设计也没有考虑不确定性的影响。总结了运移参数已有的成果,采用蒙特卡罗法研究了半无限条件下运移参数的变异性对衬垫底部污染物质量浓度和通量的影响,并对衬垫厚度进行了设计。研究表明:土工膜中有机污染物的分配系数和扩散系数的变异性对质量浓度和通量的影响可以忽略,而土中有机污染物的扩散系数和阻滞系数的变异性对其影响则非常显著。确定性方法计算的结果偏于危险,有必要加强对土中有机污染物的扩散系数和阻滞系数概率特性的研究。以甲苯为例进行黏土衬垫厚度设计,结果表明:概率计算方法得到的黏土衬垫厚度约为确定性方法的1.71~1.81倍,采用概率方法进行衬垫设计更合理。     

渗透系数的变异性对压实黏土衬垫性能的影响

压实黏土衬垫是填埋场等环境岩土工程中常用的屏障材料,其厚度和渗透系数是主要的设计参数。考虑渗透系数的变异性,用衬垫底部污染物相对浓度和通量等指标分析了设计参数对衬垫有效性的影响。研究表明,渗透系数的变异性对衬垫性能有很大的影响：当衬垫厚度较小或渗透系数均值较大时,衬垫底部出现高浓度（接近于1）的概率很大,这个浓度区间可能比相对浓度的均值高很多,对周围环境污染有较大的威胁。从非反应性溶质的运移来看,我国规范中对衬垫厚度和渗透系数的规定是合理的。相对于污染物的通量,厚度和渗透系数对衬垫底部相对浓度的影响更为显著,相对浓度更适合作为评价衬垫性能的指标。     

复合衬层中变系数有机污染物迁移规律分析

土工膜和下伏压实黏土组成的复合衬层已被广泛用作填埋场的防渗屏障系统。有机污染物在复合衬层中迁移时,其主要运移机制是扩散作用。假设有机污染物在土工膜中稳态扩散,并假设在下伏衬层中扩散系数为迁移距离的线性函数,且考虑降解作用的影响,建立了有机污染物在复合衬层中的一维扩散模型,针对零浓度下边界条件,获得了模型解析解。基于该解析解,分析讨论了相关参数的敏感性。结果表明,相关参数对计算结果影响很大,降解半衰期对污染物运移同样存在较大影响。该解析模型仅适用于有机污染物在两层复合衬垫中的迁移问题,可为填埋场的初步设计提供参考     

煤矸石淋滤液中污染物在GCL中的扩散性能

为验证土工合成黏土衬垫（Geosynthetic Clay Liners,GCL）作为煤矸石处置场衬垫层建设材料的可行性,采用垂直式双筒扩散装置进行室内扩散试验,研究了煤矸石淋滤液中典型污染物Zn2+、Mn2+和SO₄2-在人工钠化膨润土GCL中的扩散性能,使用有限层法模拟软件POLLUTE V7对污染物随时间变化的曲线进行拟合,确定3种污染物在人工钠化膨润土GCL中的扩散系数。研究结果表明,污染物在GCL中的扩散系数低于它们在传统压实黏土衬垫层中的扩散系数,从而能更有效地控制煤矸石淋滤液中污染物在GCL中的扩散迁移行为。      

有机污染物在完好复合衬垫中的迁移分析

在复合衬垫中,有机污染物除了在土工膜缺陷处发生迁移外,在完好土工膜中的迁移也是主要方式。考虑有机污染物在土工膜边界处的浓度跳跃现象,建立了有机污染物在完好复合衬垫中迁移的一维模型,得到了解析解。通过与有限差分解的比较,验证了解答的合理性。应用该解答分析了GCL复合衬垫和CCL复合衬垫防止有机污染物渗漏的效果。结果表明,CCL复合衬垫的效果较好,可以通过黏土改性和增加黏土厚度的方法来提高其有效性。有机污染物类型的选择对GCL复合衬垫的分析结果影响较大,而阻滞系数的变化对分析结果影响较小。     

运移参数对GCL中污染物运移的影响研究

分析了运移参数的变化对GCL中污染物运移的影响。从衬垫底部浓度比、对流和扩散通量、污染物累积运移量等方面,对污染物在GCL中的运移进行了计算。结果表明,减小渗透系数和扩散系数可以有效减缓衬垫底部浓度比的增加速度。渗透系数对对流通量的大小、扩散通量的变化速度的影响较大;扩散系数主要影响最大扩散通量。当渗透系数较大时,可以忽略扩散累积运移量,用对流累积运移量代替总累积运移量引起的误差很小。当渗透系数较小时,忽略扩散运移量可能产生较大的误差,尤其是在扩散系数也较大的情况下误差更加明显。      

Landfill Base Liners: Assessment of Material Equivalency and Impact to Groundwater

This paper gathered available flow and transport solutions and used them for two composite liners, consisting of geomembrane (GM) overlying either a compacted clay liner (CCL) or a geosynthetic clay liner (GCL). Its aim is to provide a guiding framework for the possible choices of (a) approaches to bottom liner design, (b) respective analytical solutions to flow and transport equations, as well as (c) parameters required for each type of solution. On the basis of the obtained results, the following recommendations are made. When the goal of analysis is to determine material equivalency, leachate flow rate is an adequate key parameter for GM-CCL composite liners. For GM-GCL composite liners, it is necessary to compute contaminant concentration or mass flux, considering (a) transport through defects for inorganic contaminants and (b) diffusion and the contribution of any available attenuation layer for organic contaminants. When the goal of analysis is to assess impact to groundwater, it is advised to calculate both discharge rate and contaminant mass flux regardless of liner type. The critical parameter for the transport calculations is the retardation factor of the contaminant, for the case of CCLs, while the results for GCLs are much less sensitive to this parameter.     

The 14-year Performance of a Compacted Clay Liner used as Part of a Composite Liner System for a Leachate Lagoon

The migration of contaminants through a 2.9 m thick compacted clay liner (CCL) for a landfill leachate lagoon is examined 14 years after construction. The clay liner formed the lower portion of the composite liner system but the geomembrane (GM) was found to have defects that had allowed leachate to migrate between the GM and CCL. Chloride, sodium, potassium, calcium and magnesium pore water profiles through the CCL are examined. It is shown that chloride migrated approximately 1.7 m into the CCL during the 14 years of the lagoon operation, sodium approximately 1.2 m, and potassium 0.7 m. Diffusion and sorption data from laboratory diffusion testing are utilized in combination with a finite layer contaminant transport model to predict field contaminant migration profiles through the composite liner system and to establish the time of ‘failure’ of the geomembrane at sometime between 0 and 6 years after installation. Relatively high sorptive uptake of potassium by the CCL soil is observed from batch testing and diffusion testing with field data suggesting an even larger amount of sorption. It is hypothesized that organic sludge matter at the base of the lagoon is responsible for potassium uptake from the leachate. This field case highlights the importance of the compacted clay liner as part of the composite liner system in acting as a diffusion barrier during the lifetime of the lagoon as well as using relatively non-conservative contaminants such as chloride and sodium to estimate geomembrane ‘failure’ times     

An analytical solution for one-dimensional contaminant diffusion through multi-layered system and its applications

An analytical solution for one-dimensional contaminant diffusion through multi-layered media is derived regarding the change of the concentration of contaminants at the top boundary with time. The model accounts for the arbitrary initial conditions and the conditions of zero concentration and zero mass flux on the bottom boundary. The average degree of diffusion of the layered system is introduced on the basis of the solution. The results obtained by the presented analytical solutions agree well with those obtained by the numerical methods presented in the literature papers. The application of the analytical solution to the problem of landfill liner design is illustrated by considering a composite liner consisting of geomembrane and compacted clay liner. The results show that the 100-year mass flux of benzene at the bottom of the composite liner is 45 times higher than that of acetone for the same composite liner. The half-life of the contaminant has a great influence on the solute flux of benzene diffused into the underlying aquifer. Results also indicates that an additional 2.9–5.0 m of the conventional (untreated) compacted clay liner under the geomembrane is required to achieve the same level of protection as provided by 0.60 m of the Hexadecyltrimethylammonium (HDTMA)-treated compacted clay liners in conjunction with the geomembrane. Applications of the solution are also presented in the context of a contaminated two-layered media to demonstrate that different boundary and initial conditions can greatly affect the decontamination rate of the problem. The method is relatively simple to apply and can be used for performing equivalency analysis of landfill liners, preliminary design of groundwater remediation system, evaluating experimental results, and verifying more complex numerical models.     

Analytical solutions for organic contaminant diffusion in triple-layer composite liner system considering the effect of degradation

Acta Geotechnica - This paper presents analytical solutions for predicting one-dimensional diffusion of an organic contaminant through a triple-layer composite liner system comprising a geomembrane...     

CCL吸附特性及孔隙率降低对污染物运移的影响

假定孔隙均匀地分布于土体的物质空间内和土骨架对污染物的吸附特性服从平衡线性,对基本体积质量关系进行分析,提出了由于土体对污染物的吸附而引起的孔隙率降低的估算公式。在考虑土体孔隙率变化的条件下,建立了污染物一维运移的控制方程,并考虑垃圾生物降解效应、压实黏土衬里（CCL）防渗层、下覆有限厚度含水层等实际情况,确定了初始条件和边界条件。对所建立的初边值问题进行了数值求解,且对某假想填埋场情况进行了变动参数与对比计算,结果表明,由于土颗粒对污染物的吸附所引起的孔隙率降低,显著地降低了污染物对压实黏土衬里的穿透能力。与常孔隙率情况相比,CCL中污染物的峰值浓度降低近10 %,含水层中污染物浓度降低更显著。当考虑土体孔隙率变化时,弥散对污染物运移具有控制作用,分布系数对污染物的运移具有重要影响。     

Diffusion and sorption experiments using a DKS permeameter

The analysis of contaminant transport through clay liner is a relevant aspect in the design of industrial, urban and mining waste disposal systems, since these areas must be designed and operated to prevent contaminating substances from reaching underground water systems in unacceptable concentrations. The design requires an estimate of the potential contaminant transport rate. However, before any attempt at quantification can be made, values for transport mechanism control parameters must be established. Clayey materials are frequently used as contaminant barriers. In these materials, which have low hydraulic conductivity, the main contaminant transport mechanism is molecular diffusion. Parameters controlling transport for these conditions are the diffusion coefficient and sorption parameters. These parameters depend on soil constituents and characteristics as well as on the chemical constitution of the contaminant. The great complexity of the factors involved makes it necessary to determine the parameters of each type of soil. This paper discusses an equipment called DKS permeameter (diffusion, convection, sorption), for the study of soil-contaminant transport mechanisms, designed at the Institute for Soil Mechanics of the Ruhr-University Bochum, and some results obtained from its use at COPPE/Federal University of Rio de Janeiro (UFRJ), Brazil. This equipment determines the effective diffusion coefficient and sorption parameter with a better reflection of field conditions. The soil under study is a mix of sodium–bentonite that has low hydraulic conductivity (k=10⁻⁹ cm/s) with adequate liner characteristics. The result indicated the relevance of determining sorption parameters for structured soils, since the sorption perceived from batch test results using pulverised soil represents maximum soil capacity. Designs based on this parameter would overestimate the attenuation capacity of the liner.