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 phases:

• Gaseous phase: Contaminants present as vapors in unsaturated zone.

• 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 unsaturated zones.

• 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 aqueous solubilities.

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.