Environmental Engineering Reference
In-Depth Information
The geometry of the interface between coarse- and i ne-grained aquifer strata plays an important
role in determining the degree to which matrix diffusion will occur. Compared to thick sand beds,
thin sand beds interbedded with clays and silts provide a larger sand/clay contact surface area across
which diffusion may occur. In strongly heterogeneous conditions, contaminant l ow in sand lenses
less than 10 cm thick will be retarded by aqueous-phase diffusion, causing a signii cant mass of
solute to reside in immobile dissolved-phase storage. Most aquifers are characterized as highly
heterogeneous and anisotropic and exhibit sharp contrasts of hydraulic conductivity, often on the
scale of a few centimeters (Payne et al., 2008).
The concentration gradient that drives diffusion across interfaces from high to low hydraulic
conductivities will rapidly dissipate as 1,4-dioxane or other solutes invade i ne-grained pores.
The l ux density will decrease in response to a decreased concentration gradient, according to
Fick's law:
d
C
(3.46)
J
=Q
D
,
total
eff
d
z
where
Θ total is total porosity (dimensionless), D eff is effective diffusivity (in m 2 /day), d C /d z is the solute
concentration gradient [i.e., the change in mass concentration over distance, in (kg/m 3 ) · m −1
=
kg/m 2 ],
and J is the l ux density [in
g/(cm 2 · s)]. The diffusive l ux density will rapidly decay as contaminant
concentration increases in the “immobile porosity,” that is, in the micropores in the clays and silts,
thereby decreasing the solute concentration gradient, d C /d z. In calculations performed by Payne et al.
(2008), the diffusive l ux for a trichloroethylene gradient with an initial source-side concentration of
200
μ
g/L will decline 10-fold in 100 days and 1000-fold in 1000 days.
There are two main consequences of appreciable diffusive mass l ux density (Payne et al.,
2008):
μ
The diffusive loss of solute from the mobile porosity slows the solute velocity relative to
groundwater velocity.
Immobile porosity in clays and silts serves as a repository for dissolved-phase solute that
enters via aqueous-phase diffusion
A further consequence of diffusion is contaminant transport to sites unavailable to microorgan-
isms, precluding biodegradation. For example, in a silt loam, half the measured pore volume con-
sists of pores with radii less than 1
μ
m, whereas the mean diameter of most soil bacteria is
0.5-0.8
m (Alexander, 1994).
Therefore, diffusion of chemical contaminants into pores smaller than those occupied by soil
microbes is an important factor controlling the rate of biodegradation of these compounds.
μ
m, and the mean diameter of soil pores occupied by bacteria is 2
μ
BIBLIOGRAPHY
Abe, A., 1999, Distribution of 1,4-dioxane in relation to possible sources in the water environment. The Science
of the Total Environment 227: 41-48.
Adamus, J.E., May, H.D., Paone, D.A., Evans, P.J., and Parales, R.E., 1995, United States Patent 5,474,934:
Biodegradation of ethers. Assignee: Celgene Corporation, Warren, NJ.
Ahlers, J., 1998, Screening information data set initial assessment report: 2-Methylbut-3-yn-ol; 8th SIDS2
Initial Assessment Meetings, Paris, France. Nairobi, Kenya: UNEP (United Nations Environment
Programme) Organization for Economic Cooperation and Development (OECD).
Alarie, Y., Nielsen, G.D., Andonianhaftvan, J., and Abraham, M.H., 1995, Physicochemical properties of non-
reactive volatile organic chemicals to estimate RD 50 : Alternatives to animal studies. Toxicology and
Applied Pharmacology 134(1): 92-99.
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