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
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
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
- Solid phase: contaminants adsorbed or partitioned onto
the soil or aquifer material in both saturated and
- Aqueous phase: contaminants dissolved into pore water
according to their solubility in both saturated and
- 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
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
- 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.