This paper describes a technique for determining the diffusion coefficient and the distribution coefficient for contaminants using saturated, intact (undisturbed) clayey soil samples. The technique is illustrated with reference to a number of laboratory tests involving advective-diffusive migration of potential contaminants through an intact clayey soil from Sarnia, Ontario. An important aspect of the proposed technique is that the mass of contaminant in the system is kept constant and so significant decrease in source leachate concentration occurs during each test. A simple theoretical model is used to analyze this case and it is shown that this phenomenon can be used to deduce both the diffusion coefficient and the distribution/partitioning coefficient from a single test. Parameters are deduced for a number of salt solutions passing through the clay from the Sarnia area. On the basis of these tests it is suggested that for the Sarnia soil and advective velocities up to the maximum examined (0.035 m/a), mechanical dispersion does not measurably affect the magnitude of the "coefficient of hydrodynamic dispersion" (i.e., there is no significant dispersion). It is also suggested that the effective porosity corresponds to that deduced from the water content of the soil. Key words: contaminant migration, soil, laboratory study, experimental, analysis, diffusion, advection, clays, groundwater.

The construction and evaluation of a compacted clayey till test pad constructed over a stone layer are described. The evaluation of the clayey liner involved (i) excavation of six test pits through the liner, followed by careful visual inspection for defects in the liner; (ii) sampling of the liner using standard 75 mm diameter Shelby tubes, a 150 mm diameter piston sampler, and block sampling; (iii) triaxial hydraulic conductivity tests on samples of liner material consolidated to a number of stress levels relevant to the proposed design; and (iv) diffusion tests on samples of liner material. Based on the results it is concluded that it was possible to construct a low-permeability liner (hydraulic conductivity less than 1.4 X cm/s under expected field stress conditions). Geotextiles from above and below the compacted liner were carefully exhumed and subjected to a series of laboratory tests to examine (i) the effect of construction damage on the geotextile's strength; (ii) the effectiveness of the geotextile to minimize intrusion of the clay liner through the geotextile and into the stone layer(s) under expected field stress conditions; (iii) the effectiveness of the geotextile as a filter for the compacted liner material under high upward gradient conditions; and (iv) the friction angle between the geotextile and clay, and geotextile and stone. The geotextiles exhumed from the test liner showed some evidence of construction damage; however, based on the field observations and subsequent laboratory tests, it is concluded that they performed adequately.La construction et 1'Cvaluation d'une planche d'essai en till argileux construite sur une couche de pierres sont dCcrites. L'Cvaluation de la membrane argileuse impliquait : (i) l'excavation de six puits d'essai B travers la membrane suivie d'une inspection visuelle attentive pour identifier des dCfauts dans la membrane; (ii) I'Cchantillonnage de la membrane au moyen de tubes Shelby standard de 75 mm de diambtre, d'un Cchantillonneur B piston de 150 mm, et de blocs; (iii) des essais de conductivitk thermique dans un appareil triaxial sur des Cchantillons du rnatkriau de la membrane consolidCs B un certain nombre de niveaux de contraintes correspondant B la conception proposCe;(iv) des essais de diffusion sur des Cchantillons du matCriau de la membrane. En partant de ces rCsultats, il est conclu qu'il Ctait possible de construire une membrane de faible permCabilit6 (conductivitC hydraulique infkrieure B 1,4 X mls sous des conditions de contraintes prCvues sur le terrain). Des gCotextiles placCs dessus et dessous la membrane compactie ont Ct C exhumCs avec grand soin et soumis B une sCrie d'essais de laboratoire pour examiner : (i) l'effet du dommage dO B la construction sur la rCsistance du gCot extile : (ii) I'efficacitC du gCotextile pour minimiser l'intrusion de la membrane argileuse B travers le gCotextile et dans la ou les couches de pierres dans des conditions de contraintes prCvues sur le terrain; (iii) I'efficacitC du gkotextile comme filtre pour le matCriau...

Considerations related to the design of the Halton Landfill as a "hydraulic trap" are summarized together with the research that was conducted to support the design concept. The interrelationship between hydrogeology and the engineered design is examined. Laboratory experiments demonstrated that there can be diffusion away from a source, even with significant inward velocity. Existing theory was found to provide a good prediction of the observed concentration profile in these experiments. It is also shown that a pressurized air pocket below the clay effectively acts as a zero-flux boundary and hence, with respect to migration of chloride, could be conservatively neglected in the impact assessment. The results of the impact calculations predict only a small increase in chloride concentration in the receptor aquifer while there is negligible predicted impact due to organic contaminants. The landfill was designed and constructed with a granular "sub-liner contingency layer" (SLCL) beneath the compacted liner. The operation of this layer is discussed. Finally, the construction of the compacted clayey liner with a hydraulic conductivity of 1 × 10-8 cm/s is documented.Key words: landfill, hydraulic containment, liner, field case, construction.

Considerations related to the design of the Halton Landfill as a "hydraulic trap" are summarized together with the research that was conducted to support the design concept. The interrelationship between hydrogeology and the engineered design is examined. Laboratory experiments demonstrated that there can be diffusion away from a source, even with significant inward velocity. Existing theory was found to provide a good prediction of the observed concentration profile in these experiments. It is also shown that a pressurized air pocket below the clay effectively acts as a zero-flux boundary and hence, with respect to migration of chloride, could be conservatively neglected in the impact assessment. The results of the impact calculations predict only a small increase in chloride concentration in the receptor aquifer while there is negligible predicted impact due to organic contaminants. The landfill was designed and constructed with a granular "sub-liner contingency layer" (SLCL) beneath the compacted liner. The operation of this layer is discussed. Finally, the construction of the compacted clayey liner with a hydraulic conductivity of 1 × 10 -8 cm/s is documented.Key words: landfill, hydraulic containment, liner, field case, construction.Résumé : On présente le résumé de considérations associées à la constructiion du site d'enfouissement de Halton conçu comme une « trappe hydraulique » de même que la recherche qui a été réalisée pour appuyer cette idée de conception. L'interrelation entre l'hydrogéologie et la conception de l'ouvrage est examinée. Des expériences en laboratoire ont montré qu'il peut y avoir de la diffusion s'éloignant de la source même avec une vélocité appréciable en direction de la source. On a trouvé que les théories existantes fournissent une bonne prédiction du profil de concentration observé dans ces expériences. On montre également qu'une poche d'air sous pression au-dessous de l'argile agit efficacement comme une barrière d'écoulement nul qui, en ce qui concerne la migration de chlorure, pourrait ainsi être négligé de façon sécuritaire dans l'évaluation environnementale. Les résultats des calculs d'impact prédisent seulement une faible augmentation de la concentration de chlorure dans l'aquifère récepteur et un impact négligeable des contaminants organiques. Le site d'enfouissement a été conçu et construit avec une couche granulaire de sécurité placée sous la membrane d'étanchéité (SLCL) d'argile compactée. On discute de l'opération de cette couche. Finalement, on présente des informations concernant la construction de la membrane d'étanchéité d'argile compactée ayant une conductivité hydraulique de 1 × 10 -8 cm/s.

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