127
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
Rowe lecture
The role of diffusion in environmental geotechnics
Conférence Rowe
Le rôle de la diffusion en géotechnique environnementale
Shackelford C.
Colorado State University, Fort Collins, Colorado, USA
ABSTRACT: Diffusion of contaminants can play a significant if not dominant role in many applications encountered within
the field of environmental geotechnics. The objective of this paper is to provide an overview of the important role diffusion
plays in such applications. The presentation proceeds from a historical perspective, beginning with the recognition in the late
1970s to early 1980s that diffusion may be an important process in assessing contaminant migration through low
permeability barriers in waste containment applications. Data from the literature and simplified model simulations are used
to illustrate under what conditions diffusion is important, and the significance of diffusion with respect to different barrier
components and types of barriers in waste containment applications is illustrated. The barriers considered include natural
clays, compacted clay liners, geomembrane liners, geosynthetic clay liners, composite liners, vertical cutoff walls,
subaqueous caps for contaminated sediments, and highly compacted bentonite buffers for high level radioactive waste
containment. The significance of semipermeable membrane behavior on liquid-phase diffusion through bentonite-based
barriers also is highlighted. The potential importance of matrix diffusion as an attenuation mechanism for contaminant
transport also is illustrated, and the roles of both liquid-phase and gas-phase diffusion under unsaturated conditions are
discussed. Finally, the role of diffusion in terms of remediation applications is illustrated via an example analysis illustrating
the impact of reverse matrix or back diffusion on the effectiveness of pump-and-treat remediation.
RÉSUMÉ:
La diffusion de contaminants peut jouer un rôle significatif si ce n’est dominant dans le domaine de la
géotechnique environnementale.
L’objectif de cet article est de fournir une vue d’ensemble du rôle important
de la diffusion
dans de telles applications. La présentation suit une perspective historique : elle commence avec la reconnaissance vers la fin
des années 70 au début des années 80, du fait que l
a diffusion peut être un processus important dans l’évaluation de la
migration de contaminants à travers des barrières à perméabilité réduite dans des applications de confinement de déchets.
Des données tirées de la littérature et des simulations avec des modèles simplifiés sont utilisées pour mettre en lumière sous
quelles conditions la diffusion est importante. L’importance de la diffusion pour divers matériaux de barrières et types de
barrières dans les applications de confinement des déchets est illustrée ; les barrières considérées comprennent les argiles
naturelles, les liners d’argile compactée, les liners en géomembrane, les liners d’argile géosynthétique, les liners composites,
les murs de confinement verticaux, les couvertures subaquatiques pour sédiments contaminés, et des zones tampons en
bentonite
fortement compactée pour le confinement des déchets radioactifs. L’importance du comportement des membranes
semi-perméables sur la diffusion en phase liquide à travers des barrières à base de bentonite, telles que les liners en argile
géosynthétique, est aussi présentée. L’importance potentielle de la diffusion en matrice en tant que mécanisme d’atténuation
pour le transport de contaminants est aussi illustrée, et les rôles de la phase liquide comme de la phase gazeuse dans des
conditions non saturées sont examinés. Finalement, le rôle de la diffusion en terme d’applications de dépollution est illustré
via l’analyse d’un exemple qui décrit l’impact de la diffusion arrière sur l’efficacité de la dépollutio
n « pump-and-treat »
(pompage-écrémage-filtration).
KEYWORDS: Advection; Containment; Contaminant; Diffusion; Fick's laws; Membrane behavior; Remediation
1 INTRODUCTION
The advent of the formal sub-disciplinary field of
geotechnical engineering known as environmental
geotechnics can be traced to the early to mid 1970s, soon
after the formation of environmental regulatory agencies,
such as the United States Environmental Protection
Agency (US EPA) formed in 1970, whose purpose was to
enforce environmental regulations promulgated for the
protection of human health and the environment
(Shackelford 1999, 2000). One of the first orders of
business for these regulatory agencies was to provide
guidelines and regulations for the safe disposal of a variety
of liquid and solid wastes, including hazardous solid waste
(HSW) and municipal solid waste (MSW). For example,
the Resource Conservation and Recovery Act (RCRA)
promulgated in the US in 1976 provided detailed
guidelines for the use of low permeability barriers of
recompacted clay, since known as compacted clay liners
(CCLs), to minimize the migration of liquids and
contaminants emanating from HSW and MSW in the form
of RCRA Subtitles C and D, respectively. Until this period
of time, wastes had been disposed largely with relatively
little or no regard for any potential environmental
consequences, often in unlined pits and dumps or in
facilities that relied primarily upon the inherent low
permeability of any natural soil within the vicinity of the
disposal location.
Because of the lack of concern for environmental
consequences resulting from waste disposal prior to this
period, contamination at numerous disposal sites (hundreds
to thousands) had already occurred over the previous
decades, such as the infamous Love Canal site located in
Niagara Falls, New York, USA. Public awareness of the
potential environmental health concerns from such existing
contamination resulted in the realization of the need to
clean up or remediate the existing contamination from sites
that had already been polluted. An example of this
Lecture
