An important consideration when evaluating a remedy is whether the compound is halogenated or nonhalogenated. A halogenated compound is one onto which a halogen ion (e.g., fluorine, chlorine, bromine, or iodine) has been attached. Typical halogenated SVOCs are listed at the beginning of Subsection 2.6. 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 this form of treatment than nonhalogenated compounds. In addition, the more halogenated the compound (i.e., the more halogens attached to it), the more refractive it is toward 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 halogenated SVOCs potentially exists in four phases:

• Gaseous phase: contaminants present as vapors in saturated zone.

• Solid phase: contaminants adsorbed or partitioned onto the soil or aquifer material 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 NAPLs primarily in saturated zone.

One or more of the three fluid phases (gaseous, 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).

Contaminant flow may occur through a number of mechanisms. Volatilization from residual saturation or bulk liquid into the unsaturated pore spaces produces a vapor plume. While the degree of volatilization from halogenated SVOCs is much less than for halogenated VOCs, this process still occurs.

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 or low solubility organic contaminants may be present as NAPLs. DNAPLs will tend to sink to the bottom of surface waters and ground water aquifers. LNAPLs will float on top of surface water and ground water. In addition, 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.

Properties and behavior of specific halogenated SVOC contaminants and contaminant groups are discussed below:

• PCBs: PCBs encompass a class of chlorinated compounds that includes up to 209 variations or congeners with different physical and chemical characteristics. PCBs were commonly used as mixtures called aroclors. The most common aroclors are Aroclor-1254, Aroclor-1260, and Aroclor-1242. PCBs alone are not usually very mobile in subsurface soils or water; however, they are typically found in oils associated with electrical transformers or gas pipelines or sorbed to soil particles, which may transport the PCBs by wind or water erosion.

• Pentachlorophenol (PCP): PCP is a contaminant found at many wood-preserving sites. PCP does not decompose when heated to its boiling point for extended periods of time. Pure PCP is chemically rather inert. The chlorinated ring structure tends to increase stability, but the polar hydroxyl group facilitates biological degradation. All monovalent alkali metal salts of PCP are very soluble in water. The protonated (phenolic) form is less soluble, but this degree of solubility is still significant from an environmental standpoint. PCP can also volatilize from soils. It is denser than water, but the commonly used solution contains PCP and petroleum solvents in a mixture less dense than water. Therefore, technical grade PCP floats on the top of ground water as a LNAPL.

• Pesticides: The term pesticide is applied to literally thousands of different, specific chemical-end products. Pesticides include insecticides, fungicides, herbicides, acaricides, nematodicides, and rodenticides. There are several commonly used classification criteria that can be used to group pesticides for purposes of discussion. Conventional methods of classifying pesticides base categorization on the applicability of a substance or product to the type of pest control desired. (For example, DDT is used typically as an insecticide.) The RCRA hazardous waste classification system is based on waste characterization and sources. Neither of these classification formats is suitable for use in this document because they have no bearing on applicable pesticide treatment technologies.