- 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 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 zone.
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.
Lower 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 organic matter.
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.