An important consideration when evaluating a remedy is whether
the compound is nonhalogenated or halogenated. A nonhalogenated
compound is one which does not have a halogen (e.g., fluorine,
chlorine, bromine, or iodine) attached to it. Typical
nonhalogenated VOCs have been listed at the beginning of Subsection 2.3.
The vendor of the technology being evaluated must be informed
whether the compounds to be treated are nonhalogenated or
halogenated. In most instances, the vendor needs to know the
specific compounds involved so that modifications to technology
designs can be made, where appropriate, to make the technology
successful in treating nonhalogenated compounds.
Subsurface contamination by nonhalogenated VOCs potentially
exists in four phases:
- Gaseous phase: Contaminants present as vapors in
- Solid phase: Contaminants in liquid form adsorbed on soil
particles in both saturated and unsaturated zones.
- Aqueous phase: Contaminants dissolved into pore water
according to their solubility in both saturated and
- Immiscible phase: Contaminants present as non-aqueous
phase liquids (NAPLs) primarily in unsaturated zone.
One or more of the fluid phases (gaseous, liquid, aqueous, or
immiscible) may occupy the pore spaces in the unsaturated zone.
Residual bulk liquid may be retained by capillary attraction in
the porous media (i.e., NAPLs are no longer a continuous phase
but are present as isolated residual globules).
Residual saturation of bulk liquid may occur through a number
of mechanisms. Volatilization from residual saturation or bulk
liquid into the unsaturated pore spaces produces a vapor plume.
Lateral migration of this vapor plume is independent of ground
water movement and may occur as a result of both advection and
diffusion. Advection is the process by which the vapor plume
contaminants are transported by the movement of air and may
result from gas pressure or gas density gradients. Diffusion is
the movement of contaminants from areas of high vapor
concentrations to areas of lower vapor concentrations.
Volatilization from contaminated ground water also may produce a
vapor plume of compounds with high vapor pressures and high
Dissolution of contaminants from residual saturation or bulk
liquid into water may occur in either the unsaturated or
saturated portions of the subsurface with the contamination then
moving with the water. Even low-solubility organics may be
present at low concentrations dissolved in water.
Insoluble organic contaminants may be present as NAPLs. Dense
NAPLs (DNAPLs) have a specific gravity greater than unity and
will tend to sink to the bottom of surface waters and ground
water aquifers. Light NAPLs (LNAPLs) will float on top of surface
water and ground water. In addition, DNAPLs and LNAPLs may adhere
to the soil through the capillary fringe and may be found on top
of water in temporary or perched aquifers in the vadose zone.