U.S. EPA Contaminated Site Cleanup Information (CLU-IN)


U.S. Environmental Protection Agency
U.S. EPA Technology Innovation and Field Services Division

Dense Nonaqueous Phase Liquids (DNAPLs)

Treatment Technologies

Bioremediation

Multi-Component Waste

In situ bioremediation of mixture classes is not likely to be effective for the NAPL part of the contamination. Contamination from coal tars and creosotes is generally the result of disposal practices (coal tars) and industrial practices that concentrated them in large quantities. Their viscosity does not lend them to forming small ganglia and globules like those associated with chlorinated solvents. Heavy oils are even more viscous than coal tars and creosotes.

While coal tars and creosotes can move as a mass in the subsurface, they will begin a partitioning process that separates the lighter, more soluble components from the main mass of the contamination. These soluble components (often BTEX, naphthalene, and alkylated naphthalenes) generally are degradable under aerobic and anaerobic conditions. The compounds remaining in the mass as aging occurs are much less soluble and very difficult to biodegrade; hence, unlike the chlorinated solvents, accelerating the dissolution process will not eventually lead to the removal of the NAPL source. However, accelerating the dissolution process may lead to a more rapid depletion of the soluble components.

If bioremediation technologies are used at mixture sites, they are most likely to be used as part of a treatment train. For example, at creosote and coal tar sites the NAPL-containing soil is often excavated and disposed of off site, disposed of on site in RCRA-style aboveground containment units, or treated by aboveground low-temperature thermal treatment. If the groundwater is contaminated, a variety of technologies, of which bioremediation is one, can be used to clean up the dissolved fraction of the mixture.

The soluble components from the NAPL source zone will usually deplete the dissolved oxygen in the zone immediately downgradient from the NAPL source. This often results in an anaerobic halo projecting from the source zone. The anaerobic conditions will usually dissipate as one moves further downgradient from the source area, and other electron acceptors may be present. Biodegradation of soluble components may occur in the anaerobic zone (e.g., under SO4-reducing conditions), and may also occur further downgradient, under more oxidizing conditions (e.g., under aerobic, nitrate-, or iron-reducing conditions).

At some creosote sites, the contaminated soil will be excavated and separated into NAPL-saturated soil for off-site or other treatment, with the less contaminated soil treated on site by ex situ land treatment (also known as land farming). Bioremediation by land treatment requires a treatability study to show regulators that concentrations in the soil are not overly toxic to the microbes.