An important consideration when evaluating a remedy is whether
the compound is halogenated or nonhalogenated. A halogenated
compound is one onto which a halogen (e.g., fluorine, chlorine,
bromine, or iodine) has been attached. Typical halogenated VOCs
have been listed at the beginning of Subsection
2.4. The nature of the halogen bond and the halogen itself
can significantly affect performance of a technology or require
more extensive treatment than for nonhalogenated compounds.
As an example, consider bioremediation. Generally, halogenated
compounds are less amenable to bioremediate than nonhalogenated
compounds. In addition, the more halogenated the compound (i.e.,
the more halogens attached to it), the more resistant it is to
biodegradation. As another example, incineration of halogenated
compounds requires specific off-gas and scrubber water treatment
for the halogen, in addition to the normal controls that are
implemented for nonhalogenated compounds.
Therefore, the vendor of the technology being evaluated must
be informed whether the compounds to be treated are halogenated
or nonhalogenated. 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 halogenated compounds.
Subsurface contamination by VOCs potentially exists in four
- 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 1 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.