Abstract: A technique for the analysis of 1-D pollutant migration through a day layer of finite depth is presented. This formulation includes dispersive and advective transport in the clay as well as geotechnical reactions and permits consideration of the depletion of contaminant in the landfill with time as well as the effect of groundwater flow in a permeable stratum beneath the clay layer.  A limited parametric study is presented to illustrate the effect of considering these factors in the analysis.  It is shown that for the most practical situations the concentration of contaminant within the ground water beneath the landfill will reach a peak value at a specific time and will then decrease with subsequent time.  It is shown that the magnitude of this peak concentration and the time required for it to occur are highly dependent upon the mass of contaminant within the landfill and the sorption capacity of the clay.  Other important factors which re examined include the thickness of the clay layer, the advection velocity (relative to the dispersivity), and the ground-water flow velocity in any permeable strata beneath the clay layer.  The implications of these results for optimizing the design of clay liners is then discussed.

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 finite layer method for general three-dimensional diffusion.  Particular attention is focused on the effects of horizontal advective velocity and coefficient 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.

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 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.

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 the contaminant available for transport.  The implications of uncertainty with respect to fracture opening size and groundwater 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 groundwater velocity does not have a significant effect on predicted contaminant migration for the class of problems being considered.

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.

Abstract: The modeling 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 finite but uncertain service lives of different 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 geosyntheitc clay liners.

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, infiltration, and attenuation in the hydrolgeologic system on the contaminating lifespan of a landfill.  The potential impact of fracturing of the soil separating the landfill from n 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.

Abstract: This paper examines the potential impact on groundwater quality of contaminant migration from a landfill site, through a fractured till, and into an underlying aquifer.  The paper describes a simple, semi-analytic technique for modeling 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 finite 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 coefficient, dispersivity, effective porosity of the matrix, radioactive decay, Darcy velocity, thickness of the aquifer, distribution coefficient, and 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 coefficient, dispersivity, effective porosity of the matrix, radioactive decay, Darcy velocity, thickness of the aquifer, distribution coefficient, 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.

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 aquitards with appropriate modifications described in the paper.  A comparison is made between the results obtained fro 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.

Abstract: Some of the factors to be considered in performing impact assessments associated with proposals to locate landfill sites in fractured till are discussed.  These factors include the effect o the mass of contaminant, 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.  The influence of both a man-made (compacted clay liner) and natural intact clayey layer in contact with the fractured till is 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 primary engineering (i.e. the underdrain system) in a landfill.

Abstract: This paper reviews the applications for Geosynthetics in waste containment.  Consideration is given to geotexiles 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 fully “geosyntheitc design” which incorporates geotexiles, geosyntheitc drainage mats, geomembranes and geogrids (for reinforcement).

Abstract: An experimental investigation of diffusive transport of a non-reactive solute (chloride) in saturated, intact Queenston Shale is described.  Laboratory tests were preformed 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 of 22 ± 1ºC 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.

Abstract: An experimental determination of chloride diffusion coefficient in saturated, intact Bison mudstone is described.  Laboratory tests simulating one-dimensional diffusive transport were preformed 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 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 10º c ranged from 1.5 to 2.0 x 10^-6 cm² /sec, from which a corresponding tortuosity factor (r) ranging from .15 - .20 can be calculated. Based on the pore size measurement, 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.

Abstract: Leachate characteristics from five 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.

Abstract: Some technical advantages and disadvantages associated with different uses for a granular layer constructed beneath a landfill liner are examined.  The importance of diffusion is discussed and it is shown that there is potential for significant contaminant impact on an underlying aquifer even if all leachate escaping through the primary liner is collected by the secondary leachate collection system.

Abstract: A new 2D finite layer formulation which allows consideration of both vertical and horizontal migration in systems which may consist of both fracture 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.

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 applications 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.

Abstract: The short term 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 acidic 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.

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 through barriers and into adjacent aquifers, and finally, to methods of obtaining solutions of specific observations and recommendations are made.

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.

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 covers a full range of layer thickness between very thin and infinitely thick. It demonstrates that in general, the flow is greater than that predicted 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.

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 engineered systems re often incorporated, and the service life of these systems must be considered when assessing their potential impact.  The role of modeling in predicting the potential impacts due to the interaction between the hydrogeology and the proposed engineering is discussed.  The potential impact of different landfill designs on groundwater quality is examine3d for a hypothetical.