Contaminant Fate and Transport - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 285

• Distribution of plant matter
• Biological interaction between the contaminant and indigenous microorganisms
All these factors must be well understood before an accurate assessment of the fate and
transport of a contaminant in soil can be made.
Contaminants migrate through the soil by two basic processes: diffusion and mass flow.
The rates of diffusion and mass flow greatly depend upon the local geology and the physi-
cal chemistry of the contaminant. Diffusion of substances through soil and aquifer materi-
als occurs in response to differences in energy from one point to another. These energy
gradients may be caused by differences in temperature or chemical concentrations within
the contaminated area. In most cases, however, the principal process moving a contami-
nant through soil is mass flow or advection because contaminants generally want to move
downward through the soil under the force of gravity (USGS 2006a).
Contaminant-specific physical and chemical attributes affecting the migration of con-
taminants in soil include (USEPA 1999, 1996b,c; Wiedemeier et al. 1999)
• Solubility
• Vapor pressure
• Density
• Chemical stability
• Persistence
• Adsorption potential
In soil, solid phase contaminants migrate much more slowly than liquid phase contami-
nants and tend to remain relatively close to their point of release or deposition (USEPA
1999). Before they can migrate a significant distance, solid phase contaminants must change
phase or undergo a transformation process. For example, heavy metals—a solid phase
contaminant—typically remain at their point of release or deposition. If they undergo oxi-
dation, however, their solubility and other properties enabling migration may increase
(Lindsay 1979). And, once a contaminant begins to dissolve in water, it may also be subject
to further transformation reactions induced by indigenous bacteria present in the surface
soil (Sutherson and Payne 2005).
If a source continues to emit contaminant in solid or liquid form that dissolves in water,
the underlying soil will eventually become saturated. The leading edge of contamination
will migrate either horizontally or vertically or both as long as the retention capacity of
the soil is exceeded (USEPA 1999). When the contaminant release stops, the migration of
the liquid contaminant will significantly decrease as the soil regains its retention capacity
(Kaufman et al. 2009).
Since soil is also composed of gas, contaminant migration through the vapor phase is
often observed with contaminants having higher relative vapor pressures. VOCs are fre-
quent participants in this type of migration. Capillary forces can also induce the migration
of liquid phase contaminants.
8.3.4 Fate and Transport of Contaminants in Surface Water
The transport of contaminants in surface water is dominated by turbulent advective flow.
Because the rate of flow in a river or stream varies significantly by location and over time,