Typical nonhalogenated SVOCs are listed at the beginning of Subsection 2.5. Subsurface contamination by
nonhalogenated 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 nonhalogenated SVOCs is much less than for
nonhalogenated 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 nonhalogenated SVOC
contaminants and contaminant groups are discussed below:
- PAHs: PAHs are generally biodegradable in soil
systems. Lower molecular weight PAHs are transformed much
more quickly than higher molecular weight PAHs. The less
degradable, higher molecular weight compounds have been
classified as carcinogenic PAHs (cPAHs). Therefore, the
least degradable fraction of PAH contaminants in soils is
generally subject to the most stringent cleanup
standards. This presents some difficulty in achieving
cleanup goals with bioremediation systems.
molecular weight PAH components are more water soluble
than higher molecular weight PAHs. Readily mobilized
compounds, such as naphthalene, phenanthrene, and
anthracene, are slightly water-soluble. Persistent PAHs,
such as chrysene and benzo(a)pyrene, present even lower
water solubilities. Pyrene and fluoranthene are
exceptions because these compounds are more soluble than
anthracene, but are not appreciably metabolized by soil
microorganisms. Other factors affect PAH persistence such
as insufficient bacterial membrane permeability, lack of
enzyme specificity, and insufficient aerobic conditions.
PAHs may undergo significant interactions with soil
Intermediate PAH degradation products (metabolites) in
soil treatment systems may also display toxicity.
Complete mineralization of PAHs is slow; intermediates
may remain for substantial periods of time.
- 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 their categorization on the applicability
of a substance or product to the type of pest control
desired. 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.