Soil Vapor Extraction (SVE) with Six-Phase Soil Heating (SPSH)
- Electrical power was delivered to the soil by steel electrodes inserted vertically in a circular array. Each electrode served as an SVE vent.
- Electric current passed through the soil creating steam and contaminant vapors.
- A blower pulled soil vapors from the SVE vents and through a knockout tank to a condenser.
- The condenser cooled and condensed hot vapors and separated the gas and liquid phases.
- The gas phase passed through a knockout tank and was discharged to the atmosphere.
- The liquid stream was treated by air stripping and was discharged on site.
CERCLA and State Record of Decision (ROD) date - August 8, 1997
|Regulatory Points of Contact:|
of Environmental Conservation
Anchorage, Alaska 99501
US EPA Region 10
1200 6th Street
Seattle, Washington 98101
USACE, Alaska District
P.O. Box 898
Anchorage, Alaska 99506-0898
US Army, Dept. of Public Works
Fort Richardson, Alaska
Organic Compounds - Volatiles (halogenated)
- 1,1,2,2-Tetrachloroethane (TCA)
- Tetrachloroethene (PCE)
- Trichloroethene (TCE)
- Maximum concentrations:
- 2,030 mg/kg TCA
- 159 mg/kg PCE
- 384 mg/kg TCE
Chlorinated solvents were used as a carrier for neutralization chemicals after burning of materials in disposal trenches.
Type/Quantity of Media Treated:
- 3,910 cubic yards (CY) or 7,150 tons of soil in situ
- Soil Moisture Content: 7.3 - 13.9%
- Air Permeability (within the soil volume): 1.6 x 10 -7 cm2
- Soil Porosity: 21 - 27%
Purpose/Significance of Application:
Successful demonstration of SVE with SPSH applied to contamination in saturated soils. Use of SPSH with SVE systems allows more rapid treatment, especially in colder environments.
Regulatory Requirements/Cleanup Goals:
- System performance was evaluated against three primary criteria:
1. The ability of each of the three six-phase heating arrays to heat soil in-situ;
2. Demonstrated removal of contaminants, as measured in the condenser off-gas and condensate; and
3. Demonstrated reduction of soil contamination, as measured in the pre- and post-treatment soil samples.
- The air stripper effluent was compared to the Alaska maximum contaminant levels (MCLs) for drinking water.
- The treatability study successfully achieved all of the criteria established for system performance.
- The air stripper effluent demonstrated compliance with the Alaska MCLs.
- The total cost for this project was $967,822.
- The total cost for treatment ranged from $189 to $288 per CY ($103 to $158 per ton) of soil. The soil treatment costs ranged from $726 to $2,552 per pound of contaminant removed.
- The large power requirement of the treatment equipment was a significant operating cost because the site was in a remote location and power was provided by diesel generators.
The PRDA was active from approximately 1950 to 1972. Chlorinated solvents were used as a carrier for neutralization chemicals that were applied after burning of materials in disposal trenches. These materials included chemical warfare agents, smoke bombs, and Japanese cluster bombs (detonated prior to burial). Four disposal areas have been identified in an area encompassing approximately 1.5 acres.
Two solvents, TCA and TCE, were found in higher concentrations and over a larger area than any other chemicals detected. PCE was also detected above action levels. A 1996 treatability study at the PRDA concluded that SVE was capable of removing solvent vapors from the subsurface, but at a rate that would require more than 10 years of treatment. Based on these results, it was recommended that SVE treatment enhanced with in-situ soil heating could be used at the site as a means for completing treatment more rapidly.
A treatability study was conducted between July and December 1997 to evaluate SVE enhanced by SPSH. Three arrays were constructed and operated at PRDA. Two arrays were 27 feet in diameter and one array was 40 feet in diameter. Each array was operated for six weeks after a shakedown period. The smaller arrays demonstrated over 90% removal of soil contaminants; the larger array demonstrated over 80% removal of contaminants. These results indicated that there may be limitations to the size of the array that can effectively treat soil at a particular site. The size of the array is limited by the resistivity of the soil and power requirements.