Impact of cyclic freezing on LNAPL movement in a single fracture

Olumide Iwakun, David Sego, Kevin Biggar

In the proceedings of: GEO2010 Calgary: 63rd Canadian Geotechnical Conference & 6th Canadian Permafrost Conference

Session: W2-A

ABSTRACT: Porous media including fractured bedrock in cold regions undergo cyclic freezing and thawing due to seasonal variation in temperature. Spilled fuel in such environment is difficult to cleanup because of inadequate understanding of its subsurface behavior. In this study, a freezing cell consisting of two parallel glass plates with emplaced glass beads was constructed in the laboratory to simulate a bedrock fracture and determine the impact of cyclic freezing on LNAPL movement. The test procedure involved introduction of LNAPL atop the freezing cell that contained water mixed with fluoroscein dye, in a top-down freezing test. LNAPL migration was observed and measured using a high-resolution digital camera with time-lapse photography. Both diesel and soluble oil were used for the experiment. Tests with soluble oil involved thorough mixing at 12.5% volume ratio with the fluoroscein-water in the freezing cell. The results showed upward mobility of free phase diesel under cyclic freezing, and downward progressive expulsion of the soluble oil ahead of freezing front during freezing. The results corroborated literature findings on organic solute expulsion ahead of freezing front, and provided insight into the behavior of trapped LNAPL below the water table when subjected to freezing conditions. Résumé Les médias poreux comprenant la roche en place rompue dans des régions froides subissent congélation cyclique et dégel dû à la variation saisonnière de la température. Le combustible renversé dans un tel environnement est difficile nettoyer en raison de arrangement insatisfaisant de son comportement à fleur de terre. Dans cette étude, une cellule de congélation se composant de deux plaques de verre parallèles avec les perles en place a été construite dans le laboratoire pour simuler une fracture de roche en place et pour déterminer l'impact de la congélation cyclique sur le mouvement de LNAPL. La procédure d'essais a comporté l'introduction de LNAPL placé sur la cellule de congélation qui a contenu de l'eau mélangée au colorant de fluoroscein, dans un essai de gélivité de haut en bas. La migration de LNAPL a été observée et mesurée utilisant un appareil photo numérique à haute résolution avec la photographie de temps-faute. De l'huile diesel et soluble ont été employées pour l'expérience. Les essais avec de l'huile soluble ont comporté le mélange complet au rapport de 12.5% volumes à la fluoroscein-eau dans la cellule de congélation. Les résultats ont montré la mobilité ascendante du diesel libre de phase sous la congélation cyclique, et l'expulsion progressive de haut en bas d'huile soluble en avant d'avant de congélation pendant la congélation. Les résultats ont corroboré des résultats de littérature sur l'expulsion organique de corps dissous en avant de l'avant de congélation, et si l'aperçu du comportement de LNAPL emprisonné au-dessous de la nappe phréatique une fois soumis à la congélation conditionne. 1 INTRODUCTION Successful remediation of spilled fuel in porous media including fractured bedrock requires adequate understanding of its subsurface behavior. Most spilled fuels (i.e., gasoline and diesel) are subsets of light nonaqueous phase liquid (LNAPL) because they are less dense than water and mostly immiscible with it. In temperate regions, the behavior of spilled fuel is well-documented in the literature. However, in cold regions overlapping permafrost environments, subsurface behavior of spilled fuel is an ongoing research area. Permafrost is often viewed as a barrier to contaminant migration in cold region, which has often contributed to improper environmental practices involving spilled fuel (Barnes and Chuvilin, 2009). Studies have shown that ice or completely ice-saturated frozen soil without defects have very low permeability in the order of 10-15 cm2, and are mostly impervious. This formed the basis for the development and use of frozen core barrier for contaminant mitigation. A study by Anderson et al. (1996) on such barrier's resistance to ice erosion by liquid contaminant showed that minimization of ice erosion requires full ice saturation and barrier temperature below the freezing point depression of the contaminant. However, in ice-saturated frozen soil, there is a natural propensity for microcracks development, especially at temperatures below freezing (Yershov et al., 1988) because frozen ground is a spatially inhomogeneous system (Frolov, 1982). Laboratory experiment by Biggar and Neufeld (1996) on vertical migration of diesel into columns of saturated silty-sand subjected to freeze-thaw cycles showed no contamination in the permanently frozen soil layer at depth after eight cycles of freeze-thaw. The diesel contamination was limited to the saturated soil up to the thaw depth. Thus, they postulated that the contaminant movement into the saturated soil was due to migration in fissures induced during freezing However, site investigations of fuel migration 156

RÉSUMÉ: of cyclic freezing on LNAPL movement in a single fracture

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