In Situ Thermal Treatment (ISTT)
ISTT and enhanced soil vapor extraction (SVE) were used to remediate contaminated subsurface soil and overburden groundwater located within the source area. Details regarding the systems operation are provided below:
- The treatment system at the Site included a subsurface network of 64 electrode wells; 143 electrodes; 29 shallow SVE wells; 15 multi-phase extraction (MPE) wells; 16 temperature sensor wells; 12 temperature, pressure and vacuum sensor wells; a concrete vapor cover; extraction conveyance lines and manifolds; SVE, MPE and sensor well heads; and a SVE and liquid extraction system for control and treatment of vapors and liquids.
- For each electrode well, 1 to 3 electrodes were installed. The number of electrodes was dependent on the depth of the treatment zone. There were four treatment zones (A, B, C, and D) with treatment depths ranging from 10 to 45 feet below ground surface.
- The forty electrode well locations outside the Site building were comprised of standard, 8-inch diameter electrode wells. The electrode wells were spaced in an array approximately 19.7 feet on center.
- Twenty-four electrode wells with 6-inch diameters were installed within the former Valley Manufacturing Products Company (VMPC) building. These electrode wells were spaced in an array approximately 16.4 feet on center.
- Temperature sensor wells were installed between electrode pairs or in the center of electrode well triangles in 16 locations. Each temperature sensor well was equipped with a digiTAM™ digital temperature sensor that included a series of individual temperature sensors vertically spaced at 3 foot intervals from treatment depth to approximately ground surface.
- Pressure/vacuum sensor wells were installed at 12 locations in ISTT Areas A, C, and D and on the perimeter of the entire treatment area. These sensors measured pressure changes due to water level changes and were also equipped with a temperature sensor well.
- As part of the systems extraction and treatment components, a vapor recovery network system and a liquid extraction and treatment system were installed. This system was comprised of heat exchangers, a cooling tower, moisture knockout tanks, two rotary lobe displacement 40 horsepower vacuum blower, process equipment skids and granular activated carbon units. The liquid extraction system equipment included a pair of bag filers, a heat exchanger, an oil/water separator and pneumatic groundwater extraction pumps.
- Several upgrades to the system were conducted after a nonaqueous phase liquid (NAPL) breakthrough of the liquid treatment system was identified.
U.S. Environmental Protection Agency (EPA)
EPA Task Order Project Officer
Environmental Protection Agency
5 Post Office Square
Boston, Massachusetts 02119-3912
MassDEP Project Manager
Massachusetts Department of Environmental Protection
One Winter Street
Boston, Massachusetts 02108
Nobis Project Manager
Nobis Engineering, Inc.
585 Middlesex Street
Lowell, Massachusetts 01851
Volatile Organic Compounds (VOCs), Trichloroethene (TCE), cis-1,2-dichloroethene (cis-1,2-DCE)
Manufacturing processes at the Groveland Wells Nos. 1 & 2 Superfund Site used cutting oils, lubricants and degreasers. Historical records indicate that wastes associated with the manufacturing process were disposed in subsurface disposal systems and a leach field. USTs containing cutting oil, solvents and mineral spirits were also present onsite. When the facility was in operation, an estimated 3,000 gallons of contaminants were discharged to the environment from several surface and subsurface sources and from routine manufacturing processes. A spill of 500 gallons of TCE was also reportedly released to the environment in 1973.
Type/Quantity of Media Treated:
Purpose/Significance of Application:
The use of ISTT at the Site demonstrated in situ thermal treatment and enhanced soil vapor extraction was an effective alternative in remediating VOCs in groundwater and soil at the Site.
Regulatory Requirements/Cleanup Goals:
The overall ISTT system performance objectives for the Site were to reduce contaminant concentrations in the source area soils to meet site cleanup levels as specified in the 2007 Explanations of Significant Differences (ESD) for operable unit 2 (OU2) and to reduce the amount of time that the groundwater treatment system would need to operate. A performance objectives metrics system was developed to evaluate system performance and determine when a point of diminishing returns was reached. A summary of the performance objectives is provided below:
Metric goals developed for target temperatures were defined by the consultant as:
- The target temperature for the vadose zone was 90 degrees Celsius, where the vadose zone is assumed to be 0 to 25 feet below ground surface (bgs).
- The target temperature for the saturated zone was the boiling point of water where the saturated zone is assumed to be 25 to 45 feet bgs.
- 85 percent of the temperature sensors within the vadose zone reach 90 degrees Celsius
- 85 percent of the temperature sensors within the saturated zone reach the boiling point of water
- 100 percent of the temperature sensors within both the vadose and saturated zones reach a minimum of 60 degrees Celsius
Temperature sensors were located in ISTT Areas A, C and D. The sensor locations were spatially distributed to adequately monitor the entire treatment zone. Since Area B was the smallest of the treatment zones and was surrounded by the other zones, temperature sensors were not located in Area B.
In Area A, 80 percent of the temperature sensors in the vadose zone achieved the metrics goal of reaching 90 degrees Celsius and 50 percent of temperature sensors in the saturated zone achieved the metrics goal of reaching the boiling point of water. For Areas C and D, 80 percent of the sensors achieved the target temperature in the vadose zone and 100 percent achieved the target temperature in the saturated zone. All temperature sensors exceeded the minimum temperature metric of 60 degrees Celsius.
As part of performance monitoring, baseline and confirmatory soil and groundwater samples were collected. TCE mass reduction in groundwater was observed in most wells after ISTT remediation. Significant TCE mass reduction, ranging from 86 to 100 percent was observed in 11 of the 16 wells sampled as part of the remedial action. Groundwater from two of the 16 wells monitored was non-detect for TCE throughout the remedial action. However, at three of the 16 wells, TCE concentrations increased to concentrations as high as 38 micrograms per liter (µg/L). For cis-1,2-DCE, mass reductions of 75 to 100 percent were observed in nine monitoring wells. Six of the 16 monitoring locations sampled exhibited cis-1,2-DCE concentrations below the laboratory detection limit during both baseline and confirmation sampling. One monitoring location was observed to have a cis-1,2-DCE concentration increase from 6.6 µg/L to 17 µg/L. The wells where TCE and cis-1,2-DCE concentrations increased were along the edge or were outside the treatment zone. These increases were attributed to changes in contaminant migration patterns when the source area extraction wells were shut off in preparation for ISTT and following treatment.
Primary cost elements are described below:
- ISTT Subcontract Cost (Includes ISTT design, construction, operation and maintenance, demobilization, and reporting): $3,617,610
- Other Subcontractor Costs (Includes clearing & grubbing, utility survey, structural engineering assessment, mold assessment, monitoring well abandonment, well replacement, waste disposal, fence removal and installation, land surveying, soil borings, laboratory analysis, and electrical/automated controls services): $284,452
- ISTT Electricity Cost (Direct cost of electricity to operate the ISTT heating and extraction systems): $603,963
- Other Costs (Includes contractor labor costs, preparation of project plans and specifications, subcontractor procurement, field oversight, ISTT operation and maintenance, planning and execution of baseline and confirmatory soil and groundwater investigations, ambient air sample collection, report preparation, data management and evaluation, and costs associated with transportation, consumables, sampling equipment and supplies): $1,757,975
- Overall Project Cost: $6,264,000
Groveland Wells Nos. 1 & 2 Superfund Site is located in Groveland, Massachusetts. Historical releases of TCE have impacted the property. Cis-1,2-DCE, a breakdown product, is also present in groundwater at the site. The VMPC manufactured metal and plastic parts from 1963 through 2001. Cutting oil, lubricants, solvents, and degreasers were used in the manufacturing process. Underground storage tanks (USTs) containing cutting oil, solvents, and mineral spirits were removed from the southern portion of the Site. When the facility was in operation, an estimated 3,000 gallons of contaminants were reportedly discharged to the environment from accidental releases and via underground injection systems.
Based on environmental investigations conducted in 2006 and 2009 at the Site, the total area of the treatment zone was calculated to be approximately 14,830 square feet with a total volume of 17,450 cubic yards. Four treatment zones were delineated, each with a different treatment depth ranging from zero to 45 feet bgs.
The ISTT and SVE system began operation in August 2010 and applied electrical resistance heating Electro-Thermal Dynamic Stripping Process (ET-DSP™) in the vadose and saturated zones. The treatment system included ET-DSP™ electrodes installed in the subsurface, vapor extraction components, liquid extraction components, and a vapor cover that prevented surface water infiltration and provided vapor containment. Contaminated groundwater was treated at the existing groundwater pump and treatment system.
The ISTT system operated from August 2010 to February 2011 with one 9-day system shut-down of the liquid extraction and ET-DSP™ components in October 2010 due to a nonaqueous phase liquid (NAPL) breakthrough. Upgrades were made to the ISTT liquid extraction system and the system was restarted. A steam enhanced heating system was incorporated in December 2010 to supplement the ISTTs heating capability.
Confirmatory sampling results indicated a 97% reduction of TCE in groundwater at the Site. Despite these significant reductions, concentrations of TCE and cis-1,2-DCE remained elevated above cleanup goals in two small areas below a paved portion of the Site. The decision to shut down the ISTT system in February 2011 was made based on an evaluation of diminishing returns for contaminant removal compared to cost of operating the ISTT system.