144
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th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
18
(Thoma et al. 1993). Although several studies have been
undertaken towards the development of models that can
handle the combined advective and diffusive mass
transport of consolidating contaminated media (e.g., Smith
2000, Peters and Smith 2002, Alshawabkeh et al. 2005,
Alshawabkeh and Rahbar 2006, Fox 2007a,b, Fox and Lee
2008, Lee and Fox 2009), comparatively fewer
experimental studies for this scenario have been
undertaken (e.g., Wang et al. 1991, Tang et al. 2005, Lee et
al. 2009, Meric et al. 2010). Nonetheless, the issue of
contaminant
migration including diffusion from
consolidating contaminated porous media remains an
important area of research (e.g., Fox and Shackelford
2010).
Air
Water
Contaminated
Sediments
Capping Layer
Advection
+
Diffusion
Figure 23. Schematic scenario of subaqueous cap for isolating
contaminated sediments in situ.
6 CONCLUSIONS
The role of diffusion in environmental geotechnics was
reviewed. Diffusion has been shown to be a significant
contaminant transport process through low-permeability
barrier materials, including natural and engineered clay
barriers such as compacted clay liners (CCLs) and
geosynthetic clay liners (GCLs), with values of hydraulic
conductivity,
k
h
, lower than 10
-9
m/s, and a dominant
transport process for
k
h
values lower than about 2-5 x 10
-10
m/s. The increasing significance of diffusion with
decreasing
k
h
results in a situation whereby design of
engineered clay barriers solely based on achieving low
k
h
is not only incorrect but also unconservative with respect
to the duration of contaminant containment in such
situations. As a result, achieving low
k
h
is a necessary, but
not sufficient condition for assuring effective containment
of contaminants with low-
k
h
barriers.
The existence of semipermeable membrane behavior is
shown to affect the diffusion of simple salt solutions
through bentonite based GCLs via ion exclusion. The
greater the magnitude of the membrane behavior, the lower
the effective diffusion coefficient. However, membrane
behavior also is shown to diminish with increasing salt
concentration, such that membrane behavior likely will
play a minor, if any, role in affecting solute diffusion
through traditional sodium bentonite based GCLs in many
practical applications, such as landfills. Nonetheless,
membrane behavior is likely to play a more significant role
in terms of the diffusion of contaminants through other
types of bentonite based barriers. For example,
semipermeable membrane behavior is likely to be
important in assessing diffusion of radionuclides through
the highly compacted bentonite buffers being considered
for containment of high-level radioactive waste, especially
given the extremely long containment durations (e.g.,
10,000 yr) associated with this application.
Diffusion is known to be the dominant liquid-phase
transport process of VOCs through intact geomembrane
liners (GMLs), either alone or as a component of a
composite liner overlying and in intimate contact with an
underlying CCL or GCL. In this regard, relatively recent
data from clay-lined and composite-lined landfills in
Wisconsin, USA, indicate that the GML component of
composite liners offers virtually no added resistance to
VOC diffusion relative to the CCLs. Diffusion also may be
an important consideration for contaminant transport
through slurry based vertical cutoff walls, but the
significance of diffusion in this case likely is governed by
the magnitude of the hydraulic gradient,
i
h
, across the wall,
with the significance of diffusion increasing with
decreasing
i
h
. Finally, diffusion through subaqueous caps
used for in situ containment of contaminated dredged
sediments has been an area of significant study,
particularly in terms of long-term environmental impacts.
Matrix diffusion, whereby contaminants diffuse from
interconnected pores or fractures into the surrounding
intact clay or rock matrix, can be an important attenuation
mechanism in assessing the potential environmental impact
of migrating contaminants, both on a global scale such as
beneath a landfill located over fissured or fractured clay or
rock, and on a local scale such as through a GCL
comprised of granular bentonite. However, the resulting
contamination of the clay or rock matrix may result in
ineffective and/or prolonged remediation of the sites due to
the process of reverse matrix or back diffusion.
Finally, gas-phase diffusion also can play a significant
role in environmental geotechnics, particularly since
diffusion via the gas phase can be significantly faster than
that via the liquid phase. Two examples where gas-phase
diffusion is important include the diffusion of oxygen
through covers resulting in oxidation of sulphidic bearing
mine tailings and the subsequent generation of acid
drainage, and the release of radon from uranium bearing
tailings to the surrounding atmosphere.
7 ACKNOWLEDGMENTS
The author expresses his sincere gratitude to David Daniel,
R. Kerry Rowe, Robert Quigley (deceased), John Cherry,
Robert Gillham, and Donald Gray for their guidance and
assistance during his PhD graduate studies on the topic of
diffusion through clay barriers. The author also thanks
Mario Manassero, Chair of ISSMGE Technical Committee
TC215 on Environmental Geotechnics, for his support in
receipt of the first
R. Kerry Rowe Honorary Lecture
which
served as the basis for this paper. Finally, the author
appreciates the assistance of his Ph.D. graduate student,
Kristin Sample-Lord, in the preparation of this paper.
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