DNAPL REMEDIATION
SILRESIM Superfund Site
Current Environmental Solutions is a wholly owned subsidiary of Terra Vac Corporation
offering the Six-Phase Heating™ / Electrical Resistance Heating technology.
Abstract
TerraVacs subsidiary, Current Environmental Solutions (CES™)
was subcontracted by Foster Wheeler Corporation (FW) to implement the
Electrical Resistance Heating (ERH) technology using Six Phases (SPH™) as a
pilot study system at the Silresim Superfund site in Lowell,
MA. CES was
formed by TerraVac Corporation in 1997 as a
technology pipeline for the new Six Phase Heating™ technology. The
subcontract was executed in July 2002, and actual site work took place between
August 2002 and January 2003.
The subsurface soil and
groundwater have been impacted by a variety of volatile and semi-volatile
compounds. VOCs are comprised of both
dense and light non aqueous phase liquids (DNAPL and LNAPL). The DNAPL compounds consist of a wide variety
of volatile and semivolatle contaminants including
chlorinated solvents and PAHs. The LNAPL compounds consist of aromatic
compounds including BTEX and styrene.
The contamination is a result of heavy industrial use since the 1900’s,
but the most significant cause of the contamination was due to operations of
the Silresim Company solvent waste recovery facility
that operated from 1971 through 1977.
The Silresim Company declared bankruptcy in
1977 and abandoned one million gallons of hazardous materials on site.
The pilot test area was
located in an area known to have high levels of VOC contamination in soil and
groundwater. The area of the one array
pilot study was approximately 850 ft².
The depth of treatment extended to 40 ft below grade surface (ft bgs). The subsurface consists of silty sand from
grade to ten ft bgs followed by a varved silty clay
unit that extends to 40 ft bgs.
Groundwater is typically found at 6 ft bgs. The main objectives of the study were to
prove the efficacy of the ERH technology as a remedial alternative at the site
by significantly reducing soil and groundwater concentrations.
The ERH system was designed
to heat and decontaminate soil and groundwater within the 850 ft2
area from grade to 40 ft bgs, corresponding to a total treatment volume of soil
and groundwater of approximately 1,250 yd³.
The subsurface installation consisted of 7 deep and 7 shallow vented
electrodes, 4 horizontal vacuum extraction (HVE) wells, 4 thermowells,
and three clusters of groundwater monitoring wells. Above surface equipment included a 500 kW
Six-Phase Heating™ Power Supply Unit (PSU), an electrolyte addition system, a
steam condenser system, granular activated carbon (GAC), and a water collection
and transfer system for managing secondary wastewater on site.
Installation
of the ERH system commenced in August 2002. System
start-up began in early October 2002.
Heating operations were completed over a three-month period ending early
January 2003. Boiling temperatures were
achieved after 5 weeks of operations.
Based on field screening techniques, approximately 1,500 pounds of vapor
phase volatile organic compounds (VOCs) were removed during the test. Based on soil analytical results and mass removal
estimates performed by FW, the mass in the target treatment area was estimated
to be 660 lbs and the post treatment mass in the target area was estimated to
be 13 lbs. Additional contamination was
degraded as indicated by high levels of chloride ion in the groundwater. As a result, shallow groundwater
contamination located in the target treatment zone was reduced by greater than
99% (to 24 feet bgs) and greater than 77% in deeper groundwater contamination. Total electrical energy input was approximately
286,200 kWh or 93% of the predicted value.
The ERH pilot test functioned
extremely well. The core component for
the application of the technology, the electrodes, was very efficient at
coupling power to the soil. There were
never any signs of power degradation.
The spacing of the electrodes as well as the diameter of the electrode
bore was adequate as well. These sizes
were based on conventional design parameters CES™ routinely uses. Site heat-up also progressed well. During uninterrupted heating phases,
temperature heat-up rates were typically above 5ºC per day. Based on the performance of the electrodes,
the spacing of the electrodes could be increased to 17.5 ft spacing.
Contaminant Levels at Beginning and End of treatment.
The subsurface soil and
groundwater at the site is impacted with volatile organic compounds (VOCs) such
as Trichloroethene (TCE), 1,1,1-
Trichloroethane (TCA), and other chlorinated
aliphatic and aromatic compounds. Dense
non-aqueous phase liquid (DNAPL) was detected in the pilot area and in other
areas surrounding the pilot site. The
SPH pilot system was used to remove VOCs through direct vaporization, in situ
steam stripping and thermally enhanced degradation.
The reported initial mass in
place was 800 lbs, however, the SPH system removed
1600 lbs during operations. Based on
chloride increases, as much as 1600 lbs may have been degraded in place. As a result of the ERH treatment, the shallow
groundwater concentrations at MW717, 4 to 24 ft bgs, showed a 100% removal of
TCE, 100% removal of TCA, with an overall total VOC removal rate of 99.7%. One month after the end of operations, the
groundwater concentrations drifted up slightly resulting in a total VOC
reduction of 93%.
In the lower portions of the
treatment zone (i.e. 27 to 37 ft bgs and 40 to 50 ft bgs), there were
significant reductions in CVOCs at 92% and 79%
respectively. Sharp increases in acetone
reduced the total VOC percentage removal to 84% and 39%, respectively. These results may have been impacted by the
potential influx of contamination from outside of the pilot which may have
contributed to the rebound in groundwater.
Groundwater
Treatment Standards and Levels Achieved
The main goal of the SPH
pilot study is to lower the VOCs levels by greater than 99% from pretest levels
during a set treatment duration. Another important goal is that of maintaining
effective vapor control of the SPH process.
The system removed more than double the reported mass in place, while
nearing or surpassing the 99% concentration reduction in all monitoring points.
Geology and Soil Types Encountered
The site soils consisted of fill and fine sand from
grade to 10 ft below grade (bg), clay clay and silt
layers from 10 to 30 ft bg, then silt and sand layers from 40 to 50 ft bg. Groundwater was encountered between five and
ten ft.
Treatment
Times Required to Meet Defined Treatment Standards
The set duration for this
pilot test was 90 days of operations.
During this time, the system removed more than double of the reported
mass in place.
