Research Reports

The following research reports on contamiant transport, landfill design, and the use of the POLLUTE and MIGRATE software and can be downloaded using the links below.


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1-D Pollutant Migration

ABSTRACT

A technique for the analysis of 1-D pollutant migration through aclay layer of finite depth is presented. This formulation includes dispersive andadvective transport in the clay as well as geochemical reactions and permitsconsideration of the depletion of contaminant in the landfill with time as wellas the effect of ground-water flow in a permeable stratum beneath the claylayer. A limited parametric study is presented to illustrate the effect of consid-ering these factors in the analysis. It is shown that for most practical situationsthe concentration of contaminant within the ground water beneath the landfillwill reach a peak value at a specific time and will then decrease with subse-quent time. It is shown that the magnitude of this peak concentration and thetime required for it to occur are highly dependent upon the mass of contami-nant within the landfill and the sorption capacity of the clay. Other importantfactors which are examined include the thickness of the clay layer, the advec-tion velocity (relative to the dispersivity), and the ground-water flow velocityin any permeable strata beneath the clay layer. The implications of these resultsfor optimizing the design of clay liners is then discussed.

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A Finite Layer Technique for Calculating Three-Dimensional Pollutant Migration in Soil

ABSTRACT

A technique for the analysis of two- and three-dimensional pollutant migration through a layered soil medium is described. An earlier solution for plane diffusion in a single homogeneous layer of soil is extended using the ?nite layer method for general three-dimensional di?usion. Particular attention is focused on the effects of horizontal advective velocity and coeffcient of hydrodynamic dispersion within the aquifer together with the thickness of the aquifer. A parametric study is presented to demonstrate some characteristics of contaminant migration in a layered soil system, taking into account the fact that the surface concentration does not remain constant because of contaminant transport into the deposit. The advantages of the approach are most pronounced when attempting to determine concentrations away from the landfill at modest to large times.

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A Semi-Analytic Model for Contaminant Migration in a Regular Two or Three Dimensional Fractured Network: Conservative Contaminants

ABSTRACT

A new semi-analytical solution for the transport of a conservative contaminant species in a fractured medium having a regular two or three dimensional fracture network is presented. The application of the technique and some of the practical implications arising from an examination of contaminant migration in fractured systems is discussed. Particular consideration is given to the effects of Darcy velocity, fracture spacing, matrix porosity, dispersivity and the mass of contaminant available for transport. The implications of uncertainty with respect to fracture opening size and ground-water velocity is also discussed and it is shown that provided one can obtain a reasonable estimate of the hydraulic gradient and hydraulic conductivity for the rock mass, uncertainty regarding the magnitude of the opening size and the groundwater velocity does not have a signi?cant effect on predicted contaminant migration for the class of problems being considered.

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An Efficient Analysis of Pollutant Migration through Soil

ABSTRACT

The potential for contamination of groundwater is now a major consideration in the design and construction of waste disposal sites in many countries. Often the movement of contaminant from the disposal pit is controlled either by siting the land?ll in a natural clayey soil or by constructing a compacted clay liner between the disposal pit and the surrounding soil. The selection of the type and thickness of the liner requires consideration of the expected concentrations at observation points beneath the liner, at the boundaries of the disposal site and possibly, at speci?c monitoring points outside the site. These contaminant migration analyses can be performed using time-marching finite element techniques, however, to obtain accurate results at both small and large times, this requires a relatively refined finite element mesh (to accommodate high concentration gradients at low times) and considerable computational effort.

Many soil deposits are horizontally layered and it is not really necessary to use the ?nite element method. In these cases, an alternative finite layer procedure proposed by Rowe and Booker can be adopted for directly calculating the concentration of contaminants of speci?ed locations and times. This approach, which will be described in this chapter, takes account of the fact that the concentration of contaminant within the disposal pit may decrease as mass is transported into the soil while also allowing for the possible presence of a more permeable underlying stratum (aquifer) beneath the clay liner. The technique to be adopted involves taking the Laplace and, where appropriate, Fourier transforms of the governing equations, finding an analytic solution in transform space and then numerically inverting the transforms to obtain the concentrations of contaminant at selected positions and times. The solution will be developed initially for the case of 1D (vertical) advective-dispersive transport in layered soil but allowing for horizontal transport in an underlying aquifer. The procedure will then be generalized to two and three dimensions. The appliction of these techniques will be discussed and will be illustrated by a number of examples.

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Analysis of Contaminant Transport Through Fractured Rock at an Ontario Landfill

ABSTRACT

The effects of fracture spacing. fracture opening size, Darcy velocity and dispersion upon the calculated contaminant plume in a fractured shale are examined. It is shown that the calculated contaminant plume, based on a reasonably hydrogeologic input, is consistent with the observed, very limited, extent of the contaminant plume at a 15 year old landfill in Burlington, Ontario. The results demonstrate that matrix diffusion can play a very significant role in the attenuation of contaminant migrating in fractured porous media.

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Background Documentation for POLLUTE and MIGRATE

ABSTRACT

This document provides supplementary information concerning computer programs POLLUTE and MIGRATE and should be read in conjunction with the Users Manuals for these programs.

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Calculating Leakage through Composite Liner Systems

ABSTRACT

A new semi-analytic solution for the leakage of fluid through a circular hole in an otherwise essentially impermeable geomembrane underlain by a clay liner is presented. This solution coveres the full range of layer thickness between very thin (one-dimensional flow) and infinitely thick (Forchheimer's solution). It demonstrates that, in general, the flow is greater than that produced by the limiting cases. The solution can be used for a wide range of practical problems where the radius of the hole may range from a pinhole to a large quasi-circular wrinkle in a perforated geomembrane (i.e. where the "hole" is considered to the the area where there is no contact between the geomembrane and clay). The solution assumes perfect contact between the geomembrane away from the hole but does allow consideration of hydraulic anisotropy of the clay layer. Usinng the proposed theory, a very simple, analytic, approximate expression is obtained. This solution can be used in hand calculations to establish the leakage rate in many practical design situations.

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Composite Liners as Barriers: Critical Considerations

ABSTRACT

The finite service life of engineered components of composite liner systems is a critical consideration in the design of such systems. Four different barriers incorporating composite liners are examined with respect to service life, leakage through the geomembrane, and the hydraulic conductivity of the geosynthetic clay liner.

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Consideration of Uncertainty regarding the Service lives of Engineered Systems in Assessing Potential Contaminant Impact

ABSTRACT

The modelling of contaminant transport through barrier systems will be discussed in the context of uncertainty regarding the service life of various components of the engineered barrier systems. A technique for performing a stochastic analysis that takes consideration of ?nite but uncertain service lives of di?‘erent components of the system is discussed and will be illustrated by a number of examples. The barrier systems to be considered will include conventional clay liner systems that include multiple leachate collection systems, systems involving composite (geomembrane and clay) liners, and systems involving geosynthetic clay liners.

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Contaminant Impact Assessment and the Contaminating Lifespan of Landfills

ABSTRACT

Some of the factors to be considered in performing impact assessments for proposed municipal and non-hazardous waste landfill sites are discussed. These factors include the effect of the mass of contaminant, in?ltration. and attenuation in the hydrogeologic system on the contaminating lifespan of a land?ll. The potential impact of fracturing of the soil separating the land?ll from an underlying aquifer is examined. The influences of a compacted clay liner and (or) a natural, intact clayey layer below the fractured soil are examined. The concept of developing “triggers" to initiate leachate control measures. and the associated potential impact on groundwater, is discussed in the context of the potential design life of the underdrain system in a landfill.

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Contaminant Impact Due to Clogging

ABSTRACT

Recent advances in the development of finite layer theory allow the modelling of changes in the operation of an engineered barrier system for landfills. Factors that can be considered include changes in the operation of the system as the primary leachate system clogs, changes in the operation of secondary leachate collection and hydraulic control layers, and changes in the diffusive and hydraulic characteristics of geomembranes. The application of the theory is illustrated with reference to a number of standard barrier system designs and it is shown that for a large landfill some of these may eventually give rise to unacceptable impact when consideration is given to the finits service life of the engineered systems for the conditions examined.

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Contaminant Migration through Fractured till into an Underlying Aquifer

ABSTRACT

This paper examines the potential impact on groundwater quality of contaminant migration from a land?ll site, through a fractured till, and into an underlying aquifer. The paper describes a simple, semi-analytic technique for modelling contaminant transport through the fractured till, including consideration of diffusion of contaminants from the fractures into the till matrix, sorption, and radioactive decay. The model also considers the ?nite mass of contaminant and dilution due to the flow of groundwater in the aquifer. The model can be readily implemented on a microcomputer. The model allows examination of variations in fracture spacing, fracture opening size, thickness of the fractured zone, diffusion coef?cient, dispersivity, effective porosity of the matrix, radioactive decay, Darcy velocity, thickness of the aquifer, distribution coef?cient, and mass of contaminant. The paper describes the results of a limited parametric study that, inter alia, examines the effects of uncertainty in fracture spacing, the thickness of the fractured till, and the effective porosity of the till matrix. Some of the practical implications are discussed.

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Effect of multiple contaminant migration on diffusion and adsorption of some domestic waste contaminants in a natural clayey soil

ABSTRACT

This paper describes laboratory model tests involving the placement of domestic landfill leachate on top of a layer of saturated undisturbed clayey soil and allowing chemical constituents to migrate into the soil by diffusion only. During the testing period (15 days), samples from the overlying leachate were regularly collected and analyzed for the chemical constituents of interest (i.e., Cl, Na, K, Mg, and Ca). At the end of the test, the soil layer was sectioned to determine the pore-water and adsorbed concentration variations with depth for each species.

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Evaluation of the Hydraulic Conductivity of Aquitards

ABSTRACT

The evaluation of the bulk vertical hydraulic conductivity of an aquitard based on its response to the pumping of an adjacent aquifer is examined using Biot's theory. Consideration is given to the errors in interpretation of the results of pumping tests which arise as a result of the time lag associated with different types of piezometers as well as the length of the piezometer. Factors to allow for correction of these errors are presented. Although these factors are originally developed for isotropic aquitards. they can be used for anisotropic aquttards with appropriate modifications described in the paper. A comparison is made between the results obtained from diffusion theory (as assumed in the development of techniques currently used in practice) and the more rigorous Biot's theory. The application of the technique is illustrated by two examples.

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Fractured till: Its Implications for Contaminant Impact Assessment

ABSTRACT

Some of the factors to be considered In performing impact assessments associated with proposals to locate landfill sites In fractured tlll are discussed. lhese factors include the effect of the mass of contaminant, Infiltration into the landfill and the contaminant transport pathway on the contaminating lifespan of a landfill. The effects of fracturing of the till on the potential impact of contaminants on underlying aquifers are examined with respect to a hypothetical case. lhe influence of both a man-made (compacted clay liner) and natural intact clayey layer in contact with the fractured till Is examined. lhe concept of developing "triggers" to initiate leachate control measures. and the associated potential impact on groundwater, Is discussed In the context of the potential design life of the primary engineering (i.e. the underdrain system) in a landfill.

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Geosynthetics: Environmental Applications in Waste Containment

ABSTRACT

This paper reviews the applications for geosynthetics in waste containment. Consideration is given to geotottiles as filters and separators in otherwise conventional landfill design. The use of drainage mats as a replacement for conventional granular drainage layers, the use of geomembrane liners as a supplement to natural or compacted clay barriers, and finally to a fully geosynthetic design which incorporates geotextiles, geosynthetic drainage mats, geomembranes and geogrids (for reinforcement).

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Laboratory Determination of Chloride Diffusion Coefficient in an Intact Mudstone

ABSTRACT

An experimental determination of chloride diffusion coefficient in saturated, intact Bison mudstone is described. Laboratory tests simulating one dimensional diffusive transport were performed by placing distilled water directly above a sample of Bison Mudstone having a high initial concentration of chloride in its pore water. Chloride and other species naturally occurring in the pore water were then permitted to diffuse out of the sample and into the distilled water reservoir for a period of up to 34 days. At the end of the test, the sample was sectioned, and the chloride pore water concentration profile measured. Fickian diffusion theory was then used to deduce the diffusion coefficient (D). The diffusion coefficient for chloride at a temperature of 10C ranged from 1.5 to 2.0 x 10-6 cm2/sec, from which a corresponding tortuosity factor (1) ranging from .15 to .20 can be calculated. Based on pore size measurements, double layer thickness and consideration of the hydrated ionic diameter of chloride, the effective porosity available for chloride diffusion is approximately equal to the total porosity calculated from the moisture content of the rock. For comparison, an attempt was made to obtain the diffusion coefficient for bromide diffusing into the sample, simultaneous with chloride diffusing out. It was found, however, that the concentration profile obtained for bromide could not be fitted by the Fickian diffusion theory, due to interactions between bromide and other species naturally occurring in the rock sample.

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Laboratory Determination of Chloride Diffusion Coefficient in an Intact Shale

ABSTRACT

An experimental investigation of diffusive transport of a nonreactive solute (chloride) in saturated, intact Queenston Shale is described. Laboratory tests were performed by placing distilled water in contact with samples of shale having a high initial concentration of chloride in their pore water. Chloride was then permitted to diffuse out of the shale and into the distilled water reservoir for a period of up to 65 days. At the end of each test, the shale sample was sectioned to determine the variation in chloride pore-water concentration with depth through the sample. Fickian diffusion theory was then used to deduce the diffusion coef?cient (D). The experimental diffusion coefficient for chloride at a temperature of 22C ranged from 1.4 x 10-6 to 1.6 x 10=6 cm2/s, which corresponds to a tortuosity (r) ranging from 0.095 to 0.108. Based on pore size measurements and consideration of the ionic diameter of hydrated chloride, the effective porosity available for chloride diffusion is estimated to be greater than 75% of the total porosity calculated from the moisture content of the shale.

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Leachate Characteristics for MSW Landfills

ABSTRACT

Leachate characteristics from Ontario landfills are compared with typical values for both European and U.S. landfills. The time history of key constituents are examined and the half-life for first order decay is estimated based on the available data. An examination of the mass of chloride in North America's MSW indicates that it likely represents about 0.18% of the total mass of MSW.

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Long-term performance of containinant barrier systems

ABSTRACT

This lecture describes the latest findings with respect to the long-term performance of modern municipal solid waste (MSW) landfill barrier systems. Field data relating to the clogging of leachate collection systems and the latest techniques for predicting their performance are examined. It is indicated that the primary leachate collection system may have service lives that range from less than a decade to more than a century, depending on the design details, waste characteristics and mode of operation. Recent data indicate that landfill liner temperatures can be expected to reach at least 30 to 40C for normal landfill operations. With recirculation of leachate the liner temperature increases faster than under normal operating conditions and may be expected to exceed 40C. Temperatures (up to 40 to 60C) may occur at the base of landfills where there is a significant leachate mound. Temperature is shown to have a significant impact on both contaminant migration and the service life of the liner system. Field measurements and theoretical calculations show that composite liners are substantially better than single liners in terms of controlling leakage from landfills. Also, the leakage rates with a composite liner are very small, and diffusion will dominate as a transport mechanism for contaminant that can readily diffuse through a geomembrane (GM). Composite liners involving a GM over a geosynthetic clay liner (GCL) gave rise to substantially less leakage than those involving a compacted clay liner (CCL).

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Modeling of 2D Contaminant Migration in a Layered and Fractured Zone Beneath Landfills

ABSTRACT

A new 2D finite layer formulation which allows consideration of both vertical and horizontal contaminant migration in systems which may consist of both fractured and unfractured layers is described. The practical application of the theory is illustrated with respect to a number of hypothetical cases. The results indicate that even relatively widely spaced small fractures can have a significant effect on potential impact. It is also shown that when dealing with relatively impermeable tills, significant impact on an underlying aquifer may not occur until after the landfill leachate is at a low strength; but the impact may be quite significant and may last for hundreds of years.

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Movement of Pollutants through Clayey Soil

ABSTRACT

This paper examines a number of factors which should be considered when attempting to predict the impact of landfill sites on groundwater contamination. The relative importance of transport mechanisms such as diffusion, dispersion and advectiOn are discussed as well as the significance of attenuation mechanisms. Techniques for determining relevant parameters are outlined and the applicability of laboratory techniques for determining diffusion and distribution coefficients is discussed with respect to the observed migration of contaminants beneath the Sarnia Landfill.

Simple but effective models for calculating the migration of contaminant from landfills are discussed and their application illustrated by a number of examples. Finally, factors such as the impact of the leachate collection system and the migration of contaminant from landfills designed to have an inward gradient is examined.

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Municipal Solid Waste Landfilling

ABSTRACT

The short and long term performance of clayey barriers (the cheapest way to encapsulate waste) is the subject of this paper. Municipal solid waste leachate varies from a moderately saline, slightly organic, slightly acid liquid when fresh to a non-threatening liquid once aged and diluted. Biological activity within the waste is responsible for extensive carbonate and sulphide dumping which tends to clog drainage systems. Concurrent advection and diffusion play major roles in salt and organic transfer through clay barriers. Typical salt fluxes are presented for barriers of differing thickness to illustrate the great importance of diffusion as a transfer process.

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Pollutant Transport Through Barriers

ABSTRACT

Methods of predicting contaminant transport through saturated and unsaturated clayey barriers are reviewed. Particular consideration is given to the relative importance of advection and dispersion as transport mechanisms, the soil properties controlling transport, transport through saturated and unsaturated soils, contaminant transport through barriers and into adjacent aquifers, and finally, to methods of obtaining solutions to the transport equations. Based on this review, a number of specific observations and recommendations are made.

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Two-dimensional pollutant migration in soils of finite depth

ABSTRACT

A technique for analyzing two-dimensional migration of contaminant from a land?ll into a homogeneous clayey layer is described. The analysis takes acount of the fact that the concentration of contaminant in the land?ll will decrease as contaminant is transported into the soil. The analysis also allows for advective—dispersive transport within a permeable stratum (aquifer), which underlies the homogeneous layer. Consideration is given to chemical retardation arising from sorption/ desorption of contaminant in the clayey layer. Some of the more important effects arising from the use of a 2-D analysis are illustrated by means of a limited parametric study. It is shown that the diffusion of contaminant from the aquifer into the clayey layer will substantially reduce the concentrations of contaminant in the groundwater away from the land?ll. It is also shown that there is a critical velocity in the aquifer at which the maximum concentration at a point occurs. At greater or lesser velocities, signi?cantly smaller concentrations may result. Thus design for limiting velocities is not necessarily conservative. Finally, the results of the 2-D analysis are compared with those from a 1—D analysis and the applicability of 1—D solutions is discussed.

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Waste Disposal Facility Site Selection and Design Considerations

ABSTRACT

Considerations associated with the selection and design of a suitable waste disposal facility are discussed. These considerations include the potential for protection of groundwater quality, predictability of groundwater movement, and potential for disruption of groundwater users. In the design of a waste disposal facility engineeered systems are often incorporated, and the service life of these systems must be considered when assessing their potential impact. The role of modelling in predicting the potential impacts due to the interaction between the hydrogeology and the proposed engineering is discussed. The potentual impact of different landfill designbs on groundwater quality is examined.

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