Kouli_etal_2008_Groundwater modelling_BOOK.pdf - Pantelis ...

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Z. Yu, Y. Huang, A. Baron et al.
where J is mass flux, C is contaminant concentration, and q is the volumetric darcian flux of
water.
Because of the natural Brownian motion of dissolved ions and molecules, mass transport
in fluids can also occur via diffusion. A form of Fick's law of diffusion can be written as
follows:
J
d
∂C
= -θ Dd
(11)
∂z
where D d is the diffusion coefficient of a porous medium.
Mixing during mass transport is controlled by the combination of diffusion and
mechanical dispersion. The mechanical dispersion is caused by the variation in the field of
local velocity and can be described by an equation similar to Equation 11, but with a
mechanical dispersion coefficient (D m ). In some cases, the mechanical dispersion is scaledependent
and more important than diffusion. Because D d and D m are similar, the coefficient
of hydrodynamic dispersion (D=D d +D m ) is used to account for the combined effect of
diffusion and mechanical dispersion (Domenico and Schwartz, 1990). A general form of mass
transport in one dimension can then be described as:
∂(
θC)
=
∂t
∂C
[( θ D
z ∂z
∂
∂
) - q C]
(12)
This equation can be further generalized to incorporate terms for chemical reactions and
sorption effects. Various numerical schemes (e.g., finite difference and finite element
methods) are available for solving for the temporal and spatial distribution of contamination
in one, two, and three dimensions.
5. Detecting and Monitoring Contaminant Movement
After potentially contaminated sites are identified, preliminary monitoring-related activities
are required in order to detect and monitor contaminant movement. An understanding of
surface soil and subsurface geology and their effect on storage and fluid transmission helps
determine the type of transport mechanisms (Kramer and Keller, 1995) at work. Geologic
material and the interaction between its variants vary, of course, from site to site, can be
determined through site visits, geologic maps, and subsurface investigation (e.g., drilling).
Various hydraulic parameters can be estimated through field experiments and laboratory
analysis. In addition to defining flow conditions, site characterization also typically includes
an estimation of the mass balance and phase transfer of contamination. Durant and Myers
(1995) described various approaches used by the United States Environmental Protection
Agency (EPA) to monitor the vadose zone at RCRA facilities, and the EPA's National
Exposure Research Laboratory has produced a useful CD-ROM entitled "Site
Characterization Library" (EPA/600/C-98/001, April 1998). A general, phased strategy for
site investigations begins with noninvasive techniques, methods that do not appreciably
disrupt the subsurface environment or cause a change in contaminant transport properties.