The hydraulic conductivity, k, of clayey soils is strongly influenced by the physicochemical properties of permeating liquid hydrocarbons. Tests on natural Sarnia soils mixed with pure liquids at a void ratio of 0.8 yielded k values that increased from 5 × 10−9 to 1 × 10−4 cm/s as the dielectric constant of the permeant decreased from 80 to 2.Sequential permeation of compacted, water-wet samples (k ≈ 10−8 cm/s) showed no changes in hydraulic conductivity when permeated with water-insoluble hydrocarbons of low dielectric constant (benzene, cyclohexane, xylene). These hydrophobic liquids were forced through microchannels or macropores and displaced less than 10% of the pore water from samples at a void ratio of unity.Permeation with water-soluble alcohols resulted in extensive removal of the pore water and up to 10-fold increases in k. Subsequent permeation with liquid aromatics of very low dielectric constant resulted in 1000-fold increases in k with only 30% of the pore space occupied by the aromatics. Association liquids such as alcohol that are mutually soluble in water and the aromatics seem to be required to initiate huge increases in k over testing periods of short duration. Key words: hydraulic conductivity, liquid hydrocarbons, clay barriers, dielectric constant.

Water-soluble organic liquids may, under some circumstances, dramatically increase the hydraulic conductivity, k, of water-saturated clayey soils, thus destroying their effectiveness as barriers for contaminants. Double-layer contraction at constant void ratio with resultant increases in the pore space available for flow has been identified as a primary mechanism for such increases in k. This paper shows, however, that the increased viscosity of solutions of some organics in water results in significant decreases in k.Pure alcohols (ethanol and methanol) and other water solubles (dioxane) when permeated through water-compacted clay samples produce an initial decrease in k, followed by a gradual increase to equilibrium values. The "hydrocarbon" front raises the viscosity of the pore fluid and causes an initial drop in k. Tests using aqueous solutions of ethanol and dioxane show decreases in k at concentrations up to ~70%. Only at high concentrations does the dielectric constant, double-layer effect overcome the viscosity effect and produce net increases in k.Effective stresses σ′vof 160 kPa during permeation with ethanol and dioxane prevent the increases in k. Physicochemically induced consolidation and increased K0-shearing effects that close shrinkage cracks are operative. Key words: hydraulic conductivity, liquid hydrocarbons, clay, viscosity, dielectric constant.

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The 99-ha Keele Valley Landfill is located in a former sand and gravel pit at Maple, Ontario. The base and sides of the pit are lined with a minimum of 1.2 m of excavated clayey silt till recompacted to achieve a design hydraulic conductivity of 1 × 10−8 cm/s or less. Extensive construction controls and monitoring programs have been implemented to determine the hydraulic conductivity and advective performance of the liner. A total of 267 postcompaction laboratory hydraulic conductivity (k) tests indicated that the first two stages of the liner had a geometric mean k of 7.7 × 10−9 cm/s. Calculations of in situ hydraulic conductivity based on lysimeter effluent collection rates show decreases in k to field values close to the laboratory values. In situ electrical conductivity sensors and lysimeter effluent chemistry measurements have monitored the advance of leachate-derived chemicals into the liner. Concurrent field verification by liner exhumation and chemical analysis has confirmed the importance of diffusion as the dominant migration mechanism through this low-k liner. Similar concentration trends for major ions have been observed in the field lysimeter effluents, effluents from laboratory liner–leachate compatibility tests, and pore water extracted from core samples of sections of exhumed liner exposed to leachate. The multicomponent field and laboratory testing and monitoring programs have shown good cross-agreement, and the actual performance of the liner has been close to preconstruction predictions. Key words : landfill, clayey liner, field hydraulic conductivity, field diffusion, municipal solid waste leachate, field lysimeter test, laboratory hydraulic conductivity, liner–leachate compatibility.

The magnitude of the effective stresses acting on a clay barrier appear to play a critical role in preserving low hydraulic conductivity, k, during exposure to soluble organic liquids. This paper summarizes the results of a research study on a natural clay from southwestern Ontario permeated with various mixtures of landfill leachate and water-soluble organic liquids at effective stresses ranging from 0 to ~300 kPa. Laboratory testing of "unconfined", water-compacted clays indicated that municipal solid waste leachates containing ethanol and dioxane at concentrations in excess of 70% can cause damaging increases in k of up to 1000-fold. Predamage application of vertical effective stresses can reduce or even eliminate these increases in k because of chemically induced consolidation and closure of macropores that develop as a result of double layer collapse. The levels of static effective stress required to prevent the increases in k were much higher for nonpolar dioxane (> 160 kPa) than for intermediately polar ethanol (20–40 kPa). Postdamage application of static effective stresses successfully eliminated the increases in k produced by ethanol permeation at near zero stress but could not heal the clay damaged by permeation of nonpolar dioxane. The practical significance of this work is discussed relative to clay barrier design. Key words: hydraulic conductivity, clay, organic liquids, leachate, liner, effective stress healing.