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Journal ArticleDOI

Influence of Sorption Intensity on Solute Mobility in a Fractured Formation

01 Jan 2009-Journal of Environmental Engineering (American Society of Civil Engineers)-Vol. 135, Iss: 1, pp 1-7
TL;DR: In this paper, a numerical model is developed that couples matrix diffusion and nonlinear sorption at the scale of a single fracture using the dual-porosity concept, and the influence of both favorable and unfavorable sorption intensities on solute mobility is investigated using the method of spatial moments.
Abstract: Diffusive mass transfer between fracture and matrix accompanied with sorption significantly influences the efficiency of natural attenuation in hard rocks. While these processes have extensively been studied in a fractured formation, limited information exists on the sorption nonlinearity. For this purpose, a numerical model is developed that couples matrix diffusion and nonlinear sorption at the scale of a single fracture using the dual-porosity concept. The study is limited to a constant continuous solute source boundary condition. The influence of both favorable and unfavorable sorption intensities on solute mobility is investigated using the method of spatial moments. The differing capacities of available sorption sites between fracture surfaces at the fracture-matrix interface and the solid grain surfaces within the rock matrix result in a slower migration of solutes along the fracture, and a larger amount of diffusive mass transfer away from the high permeability fracture.
Citations
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Journal ArticleDOI
TL;DR: A two-dimensional mathematical model is presented to predict the response of municipal solid waste (MSW) of conventional as well as bioreactor landfills undergoing coupled hydro-bio-mechanical processes and underscores the importance of considering the effect of coupled processes while examining the stability and integrity of the liner and cover systems.

42 citations

Journal ArticleDOI
TL;DR: A critical review of the previous research efforts on coupled processes is provided to help landfill engineers understand, design and operate municipal solid waste (MSW) landfills safely and efficiently as discussed by the authors.
Abstract: Disposal of municipal solid waste (MSW) in engineered landfills is one of the most widely used waste management practices in the USA and worldwide. During its design life, landfilled MSW undergoes various complex mechanisms controlled by physical (hydraulic and mechanical), chemical, thermal, and biological processes and their interrelated behaviors. A thorough understanding of these coupled MSW interactions is critical in designing a stable, effective and well-operational landfill. However, to date, the current practices associated with mathematical modeling as well as long-term monitoring of landfill performance(s) are mostly empirical and limited to site-specific conditions. Moreover, they fail to substantially quantify the changes in the geotechnical properties of MSW that result from coupled processes, especially the highly uncertain biological processes that result in MSW degradation in landfills. Furthermore, the spatially and temporally varied waste composition, heterogeneous and anisotropic nature of field MSW together with leachate and landfill gas production due to biodegradation results in atypical differential MSW settlement and, therefore, can adversely impact the long-term performance of landfills. Over the years, numerous experimental studies, field studies and mathematical modeling studies that focus on these landfill processes have been performed to optimize MSW landfill performance. In this study, a critical review of the previous research efforts on coupled processes is provided to help landfill engineers understand, design and operate MSW landfills safely and efficiently. Moreover, key research issues and challenges related to numerical modeling of landfilled MSW undergoing coupled processes are presented.

39 citations

Journal ArticleDOI
TL;DR: In this paper, an improved mathematical model is suggested that better describes fluid flow through a coupled fracture-matrix system using a dual-porosity approach, which differs from a conventional model as the fracture flow equation contains a hyperbolic term in addition to the conventional dispersive term.
Abstract: The present paper addresses critical issues that describe the transient transfer of stored rock-matrix flow into high-permeable fractures and rate-limited diffusive solute flux into low-permeable rock matrix using a typical dual-porosity approach. An improved mathematical model is suggested that better describes fluid flow through a coupled fracture-matrix system using a dual-porosity approach. The suggested model differs from a conventional model as the fracture flow equation contains a hyperbolic term in addition to the conventional dispersive term. The matrix flow equation contains the coupling term that controls the transient nature of fluid exchange from the stored rock matrix into the hydraulic conductors. The Langmuir sorption isotherm is suggested to describe the limited sorption sites available on fracture walls, while the Freundlich sorption isotherm is recommended to describe the unlimited sorption sites available within the rock matrix. The dispersion mechanism in a coupled fracture-ma...

34 citations

Journal ArticleDOI
TL;DR: Results indicate that the variation in the different parameters assumed has a significant influence on the successful distribution of the moisture in a typical bioreactor landfill cell using a DB as the LRS.
Abstract: A drainage blanket (DB) is a recently introduced leachate recirculation system (LRS) in bioreactor landfills, which involves the use of a blanket of high permeable material that is spread over a large area of the municipal solid waste (MSW). Based on the laboratory and field observations documented in the literature, the results of the performance and efficiency of bioreactor landfills vary greatly due to the empirical method followed to design the LRS. Therefore, a rational LRS design methodology that achieves an efficient bioreactor landfill and creates an optimal and safe environment is necessary. Two-phase flow modeling was performed in this study by representing the relative permeabilities of leachate and landfill gas with the van Genuchten function and fluid flow with Darcy’s law. The effects of heterogeneous-anisotropic MSW, the leachate injection rate, and the saturated and unsaturated hydraulic conductivities of the MSW on the moisture distribution in a typical bioreactor landfill cell using a DB as the LRS were modeled. Those results included saturation levels, maximum pore water and gas pressures, maximum influenced lateral spread (wetted width), maximum influenced wetted area, and outflow collected at leachate collection and removal system at the bottom of the landfill. The results indicate that the variation in the different parameters assumed has a significant influence on the successful distribution of the moisture. Unsaturated hydraulic properties considerably affect moisture flow and distribution in landfilled MSW. And, the intermittent mode of leachate recirculation has the potential for the development of gas pressures that must be considered to evaluate the stability of the landfill slopes.

20 citations

Journal ArticleDOI
TL;DR: In this article, a numerical two-phase flow model was used to examine the effects of unsaturated hydraulic properties on the moisture distribution, pore fluid pressures, and the stability of a bioreactor landfill slope with horizontal trench as leachate recirculation system.
Abstract: In bioreactor landfill, the leachate flow and moisture distribution depend upon saturated and unsaturated hydraulic properties of municipal solid waste (MSW). The effects of unsaturated parameters have not been studied because of scarcity of the data and variation in unsaturated parameters due to MSW heterogeneity, degree of decomposition, and pore structure. In this study, a numerical two-phase flow model was used to examine the effects of unsaturated hydraulic properties on the moisture distribution, pore fluid pressures, and the stability of a bioreactor landfill slope with horizontal trench as leachate recirculation system. Unsaturated hydraulic parameters were based on the van Genuchten model and obtained from previously published laboratory studies. The unsaturated hydraulic properties of MSW are found to significantly influence the leachate distribution, pore water and capillary pressures, and landfill slope stability during the operations of leachate injection and subsequent gravity drainage. Further research is needed for better understanding and accurate measurement of hydraulic properties and shear strength parameters of unsaturated MSW.

18 citations

References
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Journal ArticleDOI
TL;DR: In this paper, the authors analyzed the accessibility of the rock matrix to radio-nuclides and showed that the diffusion of the nuclides into the rock matrices and their sorption onto the surfaces of the microfissures are the main mechanisms retarding migration from a repository.
Abstract: This paper discusses migration of radionuclides in the bedrock surrounding a repository. Currently available models use either a surface reaction or a bulk reaction concept to describe the retardation of migrating nuclides. The first model assumes that the nuclide reacts only with the surface of the fissures. This implies that the rock matrix is not utilized as a sink. The other model implies that the whole bulk of the rock is accessible to the nuclides. The paper analyzes the accessibility of the rock matrix to the radio-nuclides. The transport mechanisms are shown to be flow of water and nuclides in the fissures and transport of nuclides from the water in the fissures into water in the microfissures of the rock by pore diffusion. The diffusion of the nuclides into the rock matrix and their sorption onto the surfaces of the microfissures are the main mechanisms retarding migration from a repository. The diffusivity of the nuclide may be as important as its sorption equilibrium constant. Diffusivities in the pores and microfissures in such dense rocks as granite under confining pressure of hundreds of bars can be expected to be 6–20% of the diffusivity in water. These data are obtained from electrical resistivity measurements of saltwater-filled granites. Porosity of such granites varies from 0.4 to 0.9%. The apparent diffusivities in the granites will then vary between 0.25 · 10−12/Kdρp and 10 · 10−12/Kdρp m2/s, where Kdρp is the volume equilibrium constant. This varies from the porosity of the rock for nonsorbing species to up to and over 104. For a 100-year contact time a nonsorbing nuclide can be expected to penetrate tens of meters of the rock matrix and a strongly sorbing nuclide with Kdρp larger than 104 will penetrate a few millimeters. The diffusion into the rock matrix can enhance the retardation by many orders of magnitude as compared to retardation by surface reaction in fissures only. The retardation may, on the other hand, be many orders of magnitude smaller than the maximum value that could be obtained if all the rock matrix were accessible. This depends very much on the fissure widths and spacings.

810 citations

Journal ArticleDOI
TL;DR: In this paper, a general analytical solution is developed for the problem of contaminant transport along a discrete fracture in a porous rock matrix, which takes into account advective transport in the fracture, longitudinal mechanical dispersion in a fracture, molecular diffusion along the fracture axis, adsorption into the face of the matrix, adhesion within the matrix and radioactive decay.
Abstract: A general analytical solution is developed for the problem of contaminant transport along a discrete fracture in a porous rock matrix. The solution takes into account advective transport in the fracture, longitudinal mechanical dispersion in the fracture, molecular diffusion in the fracture fluid along the fracture axis, molecular diffusion from the fracture into the matrix, adsorption into the face of the matrix, adsorption within the matrix, and radioactive decay. Certain assumptions are made which allow the problem to be formulated as two coupled, one-dimensional partial differential equations: one for the fracture and one for the porous matrix in a direction perpendicular to the fracture. The solution takes the form of an integral which is evaluated by Gaussian quadrature for each point in space and time. The general solution is compared to a simpler solution which assumes negligible longitudinal dispersion in the fracture. The comparison shows that in the lower ranges of groundwater velocities this assumption may lead to considerable error. Another comparison between the general solution and a numerical solution show excellent agreement under conditions of large diffusive loss. Since these are also the conditions under which the formulation of the general solution in two orthogonal directions is most subjectmore » to question, the results are strongly supportive of the validity of the formulation.« less

772 citations

Journal ArticleDOI
TL;DR: In this paper, the authors present data supporting a previously sug gested alternative dual-mode model of sorption in which dissolution and hole-filling mechanisms take place concurrently, as in glassy organic polymers.
Abstract: The widely accepted dissolution (partition) model of sorption to soil organic matter (SOM) has been challenged by evidence that SOM has a non-uniform sorption potential. This study presents data supporting a previously sug gested alternative dual-mode model of sorption in which dissolution and hole-filling mechanisms take place concurrently, as in glassy organic polymers. The holes are postulated to be nanometer-size voids within the organic matrix that provide complexation sites. The main focus was on sorption of chlorobenzene, 1,2-dichlorobenzene, and 1,3-dichlorobenzene, but some experiments were carried out also on 2,4-dichlorophenol and the herbicide metolachlor. Sorption from water to high-organic soils, humic acid particles, and poly(vinyl chloride) is nonlinear, competitive, and predictably responsive to conditions that affect hole populations such as temperature and co-solvent addition. Sorption to a peat soil and its components became progressively nonlinear and competitive in the order humic ac...

761 citations

Journal ArticleDOI
TL;DR: In this article, current levels of understanding of the reactions and processes comprising sorption phenomena are discussed, as well as the forms and utilities of different models used to describe them, and the translation of these concepts into functional models for characterizing sorption rates and equilibria.

602 citations

Book
04 May 1993
TL;DR: In this paper, a compilation of nine articles dealing with various aspect of flow in fractured media is presented, ranging from radionuclide waste to multiphase flow in petroleum reservoirs to practical field test methods.
Abstract: This book is a compilation of nine articles dealing with various aspect of flow in fractured media. Articles range from radionuclide waste to multiphase flow in petroleum reservoirs to practical field test methods. Each chapter contains copious figures to aid the reader, but is also a detailed in-depth analysis of some major flow problem. The subjects covered are as follows: an introduction to flow and transport models; solute transport in fractured rock with application to radioactive waste repositories; solute transport models through fractured networks; theoretical view of stochastic models of fracture systems; numerical models of tracers; multiphase flow models in fractured systems and petroleum reservoirs; unsaturated flow modeling; comparative analysis of various flow modeling techniques in fractured media; and, a summary of field methods for measuring transfers of mass, heat, contaminant, momentum, and electrical charge in fractured media.

586 citations