Formatted Title
Application of ERH and SEE to Address Complex Geology and High Groundwater Flux Zones to Treat a PCE DNAPL Source Zone
Background/Objectives
Thermal remediation at sites with complex surface and subsurface features present challenges that require using multiple technologies and innovative engineering approaches to achieve the remedial objectives safely and effectively. The Hamilton Labree Rds. Superfund site consisted of a complicated PCE DNAPL source zone with part of the treatment area beneath a road and a creek running through the middle. The treatment zone encompassed an area of 28,920 ft2 and ranged between depths of 30 and 50 ft bgs, for a total treatment volume of 11,900 cy. The hydrogeologic conditions included a surficial fine-grained alluvial “silt cap” overlaying coarse-grained upper glaciofluvial outwash deposits (sands and gravels), a low permeable till unit, lower glaciofluvial sands and gravels, and a confining clay unit at the bottom. artesian conditions over portions of the TTZ adjacent to the creek. High groundwater flux rates in the sand and gravel unites required special consideration of appropriate heating approaches, groundwater extraction, and monitoring designs to maintain hydraulic control during heating. Drying out the alluvial silt cap during ISTR could result in preferential pathways for groundwater flow and contaminants of concern (COCs) to migrate into the creek. Performance requirements included achieving soil concentrations of less than 10 mg/kg of PCE in each confirmation soil sample throughout the treatment area.
Approach/Activities
A total of 49 steam injection wells (SIWs) and 82 ERH electrodes, including stacked configurations in the same borehole within the treatment area and across the 50-ft treatment interval depth. The ISTR system was also equipped with 20 horizontal vapor extraction wells (HVEWs) to aid in mass recovery and pneumatic control near the surface and 15 multi-phase extraction wells (MPEs) to assist with deep mass removal and maintain pneumatic and hydraulic control. Subsurface temperatures and pressures within the TTZ were monitored at 16 temperature and pressure monitoring points. A specially engineered channel, incorporating insulation, was constructed to route the creek through the TTZ and protect it from COC flux and temperature impacts. Temperature sensors were also installed to monitor creek temperatures continuously and ensure that the insulated channel was functioning as intended and that the creek was protected.
Results/Lessons Learned
After ~150 days of heating and vapor and liquid extraction, the ERH and SEE systems delivered 1.7M and 3.4M kWh of energy to the subsurface, resulting in heating the TTZ to 100°C after ~105 days and maintaining steam temperatures for 45 days. A total of 100 samples, were collected at 15 boring locations throughout the TTZ and submitted for EPA 8260D analysis. All of the samples were below the 10 mg/kg PCE remedial goal with 80 of them non-detect for PCE. The average (mean) of all detected concentrations of PCE (including samples collected from the clay unit below the TTZ) was 4.1 mg/kg with a standard deviation of 8.1 mg/kg. The median of those same concentrations was 1.3 mg/kg. The total contaminant mass removed from the site was 7,800 lbs, which was more than three times the original mass estimate. Pneumatic and hydraulic control was maintained throughout operations with not evidence of temperature impacts to the creek. This presentation will focus on lessons learned during design, construction and operation of a thermal remediation system combining ERH and SEE to address a complex geologic system with high groundwater flux zones, while preventing COC migration and temperature impacts to a sensitive creek receptor.