From Ground Water Currents, April 1995, Issue No. 11

DNAPL Technologies Evaluated

By William H. Engelmann, EPA Environmental Monitoring Systems Laboratory, Las Vegas

The EPA's Robert S. Kerr Environmental Research Laboratory has published a report of a project that reviewed and evaluated in situ technologies for remediation of dense nonaqueous phase liquids (DNAPLs) contamination occurring below the ground watertable. The report reviews various in situ technologies and evaluates them on the basis of their theoretical background; field implementation; level of demonstration and performance; waste, technical and site applicability/limitations; and cost and availability. The processes discussed are: biological; electrolytic; containment and ground modification; soil washing; air stripping; and thermal. A summary of the project's conclusions follows.

The report concludes that the remediation of DNAPLs faces challenges posed by the site stratigraphy and heterogeneity, the distribution of the contamination and the physical and chemical properties of the DNAPL. A successful technology has to be able to overcome the problems posed by the site complexity and be able to modify the properties of the DNAPL to facilitate recovery, immobilization or degradation. In addition, methodology must be adaptable to different site conditions and must be able to meet the regulatory goals.

Thermally based technologies are regarded as among the most promising, with steam enhanced extraction (SEE) as probably the most promising candidate. The CROW process relies on similar mechanisms; however, it was not clear whether the injection of hot water and low quality steam offers an advantage over SEE. Radio frequency heating, which relies on in-situ steam generation to be effective, has only been tested in the vadosezone.

The report concluded that the next group of promising technologies are the soil washing technologies because they can manipulate chemical equilibria and reduce capillary forces. A blend of akalis, cosolvents and surfactants is probably the best combination for a soil washing application, each important for its own reasons. Alkalis can saponify certain DNAPLs and affect wetability and sorption; cosolvents provide viscous stability and enhance solubility and mass transfer between the aqueous phase and the DNAPL; surfactants have the largest impacts on solubilty and interfacial tension reduction. Water flooding is best applied in highly contaminated areas as a precursor to these methods.

The thermal and soil washing technologies are considered as best suited for areas that are highly contaminated with DNAPLs. However, these techniques by themselves still may not be able to achieve the currently mandated regulated cleanup standards. Thus, consideration should be given to using these technologies in combination with the technologies suitable for long-term plume management. The bio-remediation techniques and permeable treatment walls hold the best promise.

A special problem is posed by mixed wastes, heavy metals and radionuclides mixed with DNAPLs since recovery at the ground surface may not be desirable in many instances. In such instances, solidification/stabilization (S/S) and vitrificationare among the most viable in situ technologies. Excluding radionuclides, in situ S/S is the most promising candidate because of its broadly demonstrated effectiveness, cost andapplicability to the saturated zone.

A copy of the report, EVALUATION OF TECHNOLOGIES FOR IN-SITU CLEANUP OF DNAPL CONTAMINATED SITES" (Order No. PB94-195039), can be obtained for $27.00 (subject to change) from the National Technical Information Service, 5285 Port Royal Road, Springfield,VA 22161 (Telephone: 703-487-4650).