Due to construction during the 1950s, most of the soil on the site was removed; therefore, mudstone bedrock is typically encountered at 5 feet below ground surface (bgs).
Targeted Environmental Media:
 Dense Non-aqueous Phase Liquids (DNAPLs) - Fractured Bedrock
A tetratrachloroethene (TCE) plume covers more than 5 acres and contamination extends up to 200 feet bgs.
Major Contaminants and Maximum Concentrations:
- Trichloroethene (0 µg/L)
- cis-1,2-Dichloroethene (0 µg/L)
- Vinyl chloride (0 µg/L)
No technologies selected.
- Thermal Treatment (In Situ)
- Other (Thermal Conductive Heating (TCH))
Comments:
Since the mid-1990s, a pump and treat system has treated groundwater contaminated with chlorinated volatile organic compounds (CVOCs) at concentrations of 600 milligrams/liter (mg/L) and higher. The TCH field demonstration consisted of 15 heater borings equipped with a vapor extraction screen. The borings ranged from 5 to 55 feet bgs and electricity was applied to the borings at a rate of 210 kilowatt (kW) in order to reach a temperature of 100 degrees Celsius (C). The system was operated for 98 days and a total of 493,000 kilowatt hours (kWh) were applied to the treatment area. Zones between 0 and 35 feet bgs reached temperatures between 99 to 110 degrees C (in situ groundwater boiling temperatures); however, zones between 40 to 50 feet bgs did not reach boiling, instead reaching temperatures between 70 to 80 degrees C.
From the bedrock, 15 stainless steel well screens connected to flexible hoses withdrew air, steam, and fluid, which were then fed through an off-gas treatment system. The system consisted of a 55 square foot heat exchanger, a 150 standard cubic foot per meter positive displacement blower, two 1,000-pound (lb) granular activated carbon units, and an 18-ton chiller. Water from the system was treated at an on-site treatment plant.
Not identified in the references cited.
From the 740 cubic yard pilot area, approximately 500 pounds (lbs) of total VOCs were removed by vapor extraction and an additional 30 lbs were removed from the water and condensed stream (based on photoionization detection and laboratory data, respectively). Total VOC concentrations after heating ranged from below 5 milligram/kilogram (mg/kg) to 275 mg/kg. The higher results were associated with distinct fracture zones which had an influx of cold groundwater during the treatment; this influx possibly led to incomplete heating of the rock matrix. Matrix blocks with no apparent fractures showed VOC concentrations less than 5 mg/kg. Overall mass reduction was calculated to range between 69 and 84 percent.
Anticipated groundwater extraction rates between 0.1 and 0.2 gallons per minute (gpm) were greatly exceeded by actual rates of 2 to 3 gpm and the increase was attributed to: (1) vapor recovery wells co-located within the heater well boreholes and (2) wells screens extending below the water table. About 270,000 gallons of water were extracted, greatly exceeding the anticipated range of 8,600 to 17,200 gallons. The high rate of groundwater extraction initiated groundwater flow through the bedrock fractures and limited subsurface heating and VOC removal. In the full-scale application, higher rates of VOC extraction are anticipated, which will allow the full-scale system design to limit groundwater extraction and inflow.
The actual system heat loss exceeded the anticipated heat by 15%; this was also attributed to the increased groundwater flow. While this is not expected to be a major issue for full-scale application, the full scale application may limit groundwater influx by using larger-diameter vapor extraction wells, installing a hydraulic barrier, or injecting steam into water-bearing fractures to remove the groundwater and heat the fractures.
A concurrent laboratory study evaluating VOC removal rates, temperatures needs, and the duration of TCH applications was ongoing at the time the reference was prepared. The results of this study would be included with the results of the pilot study in the Environmental Security Technology Certification Program (ESTCP) cost and performance report anticipated to be released in 2011; a recent fact sheet released by ESTCP for this pilot study is listed below.
References:
Lebron, Carmen; and Devon Phelan. 2010. U.S Navy Demonstrates Thermal Conducting heating for DNAPL Removal in Fractured Rock. Reported in Technology News and Trends, Issue 51, December. http:cluin.orgdownloadnewsltrstnandt1210.pdf
DNAPL Removal from Fractured Rock Using Thermal Conductive Heating (ER-200715). Accessed July 5, 2011. www.serdp.orgProgram-AreasEnvironmental-RestorationContaminated-GroundwaterPersistent-ContaminationER-200715ER-200715(language)eng-US#factsheet-5152-technology
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