Theme: 01. Remediation Technology Innovations

Topic Area: 01q. Electrical Resistance Heating: Best Practices and Lessons Learned

Formatted Title:

In Situ Thermal Treatment of DNAPL Site Using Sheet Pile Electrodes

Background/Objectives:

A former wastewater treatment facility in Salem, New Hampshire, was impacted with chlorinated volatile organic compounds (CVOCs). The site lithology was comprised of an Upper Sand Unit (USU), extending from surface grade to approximately 20 to 25 feet below ground surface (bgs). An underlying Silty Clay Unit (SCU) extended to 30 to 35 feet bgs with an underlying Lower Sand Unit (LSU), extending to 45 to 50 feet bgs. The flow of groundwater is generally west. The maximum trichloroethene (TCE) concentration in the treatment zone was 31,200 mg/kg. The site remedial objectives were to thermally treat CVOC-impacted soil in the dense non-aqueous phase liquid (DNAPL) source area and the larger surrounding area with soil CVOC concentrations exceeding New Hampshire Department of Environmental Services Soil Remediation Standards (NH-SRS) for TCE, tetrachloroethene (PCE), cis-1,2-dichloroethene (cDCE), vinyl chloride (VC), and 1,1,1-trichloroethane (1,1,1-TCA). Groundwater monitoring of polychlorinated biphenyls (PCBs) and per- and polyfluoroalkyl substances (PFAS) was also required to evaluate concentrations before, during, and after thermal remediation.

Approach/Activities:

The property owner was motivated by a potential property transfer and implemented an expedited electrical resistance heating (ERH) remediation across a 34,735 square foot treatment area, using 161 sheet pile electrodes, extending to varying depths between 18 and 37 feet bgs. Sheet pile electrodes reduce costs significantly due to their high surface area, which allows for wider spacing. Additionally, sheet piles can be driven into the subsurface, rather than drilled, which reduces waste generation. Temperature was monitored in the subsurface using 16 temperature monitoring points (TMPs), with sensors distributed at depths across the treatment zone. During heating, vacuum was applied to the subsurface and vapors were extracted through vapor recovery screens co-located with electrodes. A regenerative thermal oxidizer (RTO) was used to treat the extracted vapors. The maintenance of pneumatic control was evaluated through 16 pressure monitoring points installed across the site. To evaluate whether hydraulic control was maintained, three perimeter groundwater monitoring wells co-located with temperature monitoring points were installed.

Results/Lessons Learned:

The site lithology was conducive for efficient sheet pile electrode installation. System operations were completed in October 2023. The sheet pile electrodes power output exceeded anticipated levels, which will be taken into account in future ERH designs in similar lithologic and hydrogeologic settings. The site achieved a maximum average temperature of 102.4ºC. Measurements collected at the PMP and perimeter monitoring wells indicated pneumatic and hydraulic control were maintained during operations. As anticipated, with the ERH remediation targeting steaming conditions, no marked changes in PCB and PFAS groundwater concentrations were observed. Emissions from the RTO were in compliance with the established air permit. The ERH system removed approximately 12,200 pounds of VOCs over 113 days of heating, achieving a 99.99 percent reduction in soil TCE concentrations. The DNAPL Cleanup and NH-SRS Cleanup Performance Goals were achieved post thermal remediation. The 95 percent UCL of the mean TCE, PCE, cDCE, VC, and 1,1,1-TCA concentrations in post thermal remediation soil samples were all less than the established NH-SRSs.

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Michael Dacey