Contaminant Fate and Transport - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 278

A
B

A΄

B΄

FIGURE 8.3
Effects of dispersion on subsurface migration in a
porous medium.

where
J is the flux density [M/L 2 × T]
D is the Fickian mass transport coefficient [L2/T] 2 /T]
C is chemical concentration [M/L 3 ]
x is the distance over which a concentration change is being considered [L]

Note : In simple calculations, the minus sign is often omitted if the direction of Fickian
transport is clear.
As a contaminated fluid flows through a porous medium, it will mix with non-con-
taminated water. The result will be a dilution of the contaminant by a process known as
dispersion (Fetter 1993). Longitudinal dispersion occurs along streamlines as the contami-
nated water moves downgradient. Lateral or transverse dispersion occurs at right angles
to longitudinal dispersion. Figure 8.3 shows the basic causes of longitudinal and lateral
dispersion. As a fluid moves through pores in an aquifer, it follows a tortuous path, mov-
ing faster through the center of the pore and slower along the edges of the soil particles. As
a result, some of the fluid will travel a longer path.
General factors that influence or control the rate of migration of a contaminant include
(Hornsby 1990):

• Physical properties of the contaminants themselves
• The geological environment where the release occurs
• Climatological factors
• Vegetation factors

Specific physical properties affecting migration of contaminants in soil include (USGS
2006a; Rogers et al. 2007; Payne et al. 2008):

• Solubility in water. The more soluble a contaminant is in water, the more mobile it
will be in the subsurface environment.
• Vapor pressure. As vapor pressure increases, affinity to volatilize increases and
the more likely a contaminant will be present in the gas phase.
• Molecular weight. The higher the molecular weight, the greater the energy
requirement to transport the contaminant in the horizontal direction. Increased
molecular weight may induce the contaminant to migrate downward in areas of
steep slopes.