- Cascade
TerraTherm’s work on the Solvents Recovery Service of New England (SRSNE) Superfund Site in Southington, CT is a great example of how thermal treatment can get to the bottom of your project needs. After a total of 304 days of In Situ Thermal Remediation (ISTR) treatment, we were able to successfully remove approximately 500,000 pounds of contaminant mass from the subsurface and meet all of the remediation objectives of the project. Overall, the SRSNE Superfund Site, and our work there, is indicative of the scale and efficiency with which Cascade and it’s team of partners can work. Continue reading for an in-depth explanation of the jobsite, it’s history, it’s challenges, and our work to surmount them.
The SRSNE Site was operational from 1955 through 1991. The facility received spent solvents from customers and distilled them to remove impurities. Disposal to lagoons, poor housekeeping, and other releases at the site resulted in the formation of a multi-component Non-Aqueous Phase liquid (NAPL) source zone in overburden soil and fractured bedrock, along with associated aqueous phase plumes.
In 1983, the site was listed on the National Priorities List (NPL), and in 1990, the USEPA conducted a formal Remedial Investigation (RI). In the late 90’s, various non-time critical removal actions were implemented, and RI/FS studies were completed between 1996 and 2004. In 2005, the EPA issued the Record of Decision (ROD) which established the multi-step clean-up plan for the site.
The EPA established specific goals for the ISTR remedy in their 2005 study to reduce Volatile Organic Compound (VOC) contamination to “levels that are not indicative of the presence of pooled or residual NAPL.”
Here are the specific soil cleanup levels determined by the EPA that we sought to achieve:
Due to the presence of VOC NAPLs present in the overburden soils, ISTR was selected to address the area. The ISTR source zone was approximately 74,000 square feet with treatment depths ranging from 15 to 32 feet below ground surface (bgs), totaling 57,000 cubic yards and estimated to contain between 500,000 and 2,000,000 pounds of contaminants.
The team utilized a total of 607 thermal conduction heater wells, 551 vapor extraction wells and over 300 linear feet of horizontal vapor extraction wells to heat and capture volatilized contaminants. Extracted vapors were treated through a thermal oxidizer and acid gas scrubber at a flow rate of approximately 1,500 standard cubic feet per minute.
To address the variance in bedrock depth, TerraTherm custom fabricated heater casings to the appropriate length for the given site. Heater lengths and heater circuiting were subsequently customized to ensure treatment across the varying bedrock surface. Redesign and fabrication of heaters, as well as reconfiguration of electrical circuits also occurred in real-time. No down-time occurred.
To address potential NAPL migration, a customized drilling and well installation procedure was developed to prevent further mobilization of NAPL into bedrock during or after well installation. Based on data from a significant number of downgradient monitoring wells, we observed zero evidence of downward migration of NAPL due to drilling.
To address the large contaminant mass, the wellfield was designed to operate in two phases. Each phase covered approximately half of the wellfield, with overlap in the middle to reduce the peak daily contaminant loading ~10,000 lbs VOCs/day). This allowed the aboveground treatment train to be of standard size.
As documented in USEPA’s September 2015 “Remedial Action Report Approval” and the associated ISTR Construction Completion Report, the ISTR system operated from May 2014 to February 2015, resulting in the removal of 496,400 lbs of VOC contamination, which equates to 99.7% VOC mass removed from soils. The soil cleanup levels were met at all 139 post-treatment sample locations.
Average soil concentrations dropped from 2,795 mg/kg to 5.62 mg/kg Total VOCs (TVOCs), approximately two orders of magnitude below requirements. Within the ISTR zone, a 97% reduction in average groundwater TVOC concentrations was achieved, with no “rebound” to date, approximately 18 months after completion of thermal treatment. A 99%+ destructive removal efficiency was achieved for treatment of extracted vapors.
As documented in USEPA’s Second Five-Year Review for the site (September 2015), in addition to the 496,400 pounds of VOC mass removed through ISTR, an estimated additional 660,000 pounds of VOCs have been removed through degradation (both abiotic and biotic) since completion of the RI in 1996.
Since completion of ISTR in February 2015, groundwater temperatures in the treated zone have declined to ~90- 95⁰F, and the resultant warm (55 – 75⁰F ) water plume has spread ~180 feet downgradient through a downgradient hydraulic containment area. Post-ISTR groundwater sampling and qPCR analysis confirmed the presence of chlorinated compound degrading bacteria such as DHC in the ISTR treatment zone and downgradient area. VOC concentrations have declined by an order of magnitude at the extraction system immediately downgradient of the ISTR area. The order of magnitude reduction in VOC concentration in the extracted groundwater was observed at a point in time reasonably consistent with the travel time from the ISTR treatment zone, and occurred simultaneously with an eight-fold increase in chloride concentration in the extracted water, suggesting the positive correlation between temperature and chlorinated VOC degradation rates. Since completion of the thermal remedy, the number of wells being pumped in the NTCRA1 area has been reduced and wells within the NTCRA1 area show an average 98% reduction in TVOCs since 2010.
This site was high on the EPA’s list of priorities, and it presented a few distinct challenges, namely uneven subsurface and the sheer volume of contaminants. Nevertheless, through careful design and steady execution, we were able to remove 99.7% of all contamination from the job site. This level of precise planning and execution is exemplary of the sort of work we are able to accomplish at Cascade.
