Description
Full Project Description Located at: View PDF
Project: EHC Pilot Study for Oregon DEQ's Dry Cleaner Program at a Former Dry Cleaner Site, OREGON USA
Project manager/regulatory contact: Don Hanson, Oregon Department of Environmental Quality
Summary
Groundwater at the former Serry’s dry cleaner site in Corvallis, Oregon, was impacted with chlorinated volatile organic compounds (CVOCs). The key CVOCs found at the site include PCE, TCE, DCE, and VC, as high as
22,000, 1,700, 3,100, and 7 ppb, respectively. Field scale pilot tests were performed for the Oregon DEQ’s Dry
Cleaner Program to evaluate the performance of EHC®, an in situ integrated biological and chemical reduction
(ISCR) technology. The method of injection was also evaluated to determine the effectiveness of the injection
method in distributing EHC, given the low permeability of soils at the site (sandy silt and silty clay). Subsequent
field monitoring has shown greater than 99.9% removal of total CVOC concentrations two years after the
injections, without the accumulation of catabolites or rebound of CVOCs.
The Challenge
The groundwater flow direction at the site changes with seasonal conditions, and is either toward the south-east or
the north-west. The groundwater velocity is unknown, but assumed to be low due to the type of soils at the site. No
confirmed source of PCE has been found, however groundwater concentrations suggest that there may be two hot
spots on site: the north-west corner and south-east corner of the building. The building is currently occupied and
access is difficult to obtain. The primary goal of the pilot-scale treatment is to reduce groundwater concentrations at
the southeastern hot-spot that may have historically contributed to indoor air vapor intrusion at nearby residences.
Test Injection
A test injection was conducted prior to full-scale implementation to evaluate the effectiveness of direct push
methods in distributing EHC in the low permeability soils. A total of 450 lbs of EHC was injected from 13 to 25 ft
bgs (4 discrete layers, spaced 4 ft apart) using GeoProbe's pressure activated injection tip. A high pressure
pneumatic grout mixing and pumping system was used to mix and pump the EHC to the injection tip. Six soil
cores were obtained around the injection point (from 0.5 to 5 ft from the injection point) to assess the radius of
influence of the injection (Figure 1). EHC fractures were found at the farthest sampling points indicating that the
radius of influence was up to at least 5 ft. Figure 2, 3, 4 and 5 show some of the EHC bands or fractures observed
in the soil cores.
The following lithology was observed from the soil sampling:
- Brown sandy, clayey silt from 8 to 20 ft bgs;
- Stiff brown silty clay from 20 to 22 ft bgs;
- Stiff gray clay below 22 ft bgs;
- Water bearing and loose sand layers were encountered at
approximately 26 ft bgs.
Field-Scale Pilot Study
In August 2006, a total of 10,250 lbs of
EHC was injected into an area measuring
approximately 825 ft2 x 20 ft deep (from
10 to 30 ft bgs), which resulted in an
average application rate of 0.6% to soil
mass. The EHC was supplied as a dry
powder in 50-lb bags and mixed with
water on site into slurry containing about
29% solids (Figure 6). Using conventional
direct push technology, the EHC slurry
was injected at 3 to 6 GPM at a pressure of
approximately 200-400 psi. The depth
interval targeted was from the groundwater
table (ca. 10 ft bgs) to approximately 30 ft
bgs, where a less permeable layer was
encountered. The EHC was injected in
discrete layers using the Geoprobe
pressure-activated injection tip and high
pressure grout mixing unit.
The EHC was added to a total of 32 injection points (Figure 7), including nine additional points south of the
building. The additional points were added due to issues with surfacing: it was not possible to achieve the originally
planned application rate of 1% to soil mass
in the tight soil formation at the targeted
flow rate of approximately 5 GPM due to
pressure build-up in the subsurface. The
treatment area was therefore increased
resulting in a lower application rate of 0.6%
to soil mass within the treatment area. In
addition, a relatively tight injection spacing
of 5 ft was used for the EHC injections
based on the results from the test injection. Approximately 18 lbs of EHC was added
per vertical foot on average.
Figure 7 also shows the locations of four
new monitoring wells (MW-6 to MW-9)
installed in January 2007. Previous
monitoring performed in February 2005 and
August and September 2006 was conducted
via direct push probe exploration locations
(HC-22 to HC-29).
The Result
Following injection of EHC into the suspected source area, PCE was decreased to below the detection limit of 1
ug/L and total CVOCs decreased from a maximum of 25,606 ug/L to below 100 ug/L at all locations within 12
months (Figure 8). The PCE degradation resulted in an initial build-up of TCE, DCE (primarily cis-DCE) and VC,
indicating that sequential dehalogenation was occurring (biological pathway). However, conversion rates were less
than Stoichiometric and concentrations of all constituents appeared to decrease simultaneously, suggesting that
abiotic treatment mechanisms also occurred at significant rates. Observed generation of VC was limited, indicating
that abiotic beta-elimination was a primary pathway for cis-DCE degradation. A smaller concentration of
chloroethane (CA) was also observed (maximum concentration of 44 ug/L measured in February 2008 - 18-month
data), suggesting that hydrogenation of VC might be occurring.
Note: The data from Feb 2005, Aug 2006, and Sep 2006 was obtained via direct push probe exploration locations. Subsequent monitoring was conducted via groundwater monitoring wells installed close to each of the previous direct push sampling clusters.
Historically, CVOC concentrations in groundwater fluctuate heavily with the seasons in the study area. Figure 9
shows the groundwater elevation and total CVOCs concentrations measured since the well installations. Transient
rebound was observed during the rainy season
(18-month data), presumably due to the
introduction of CVOCs from the smear zone as
a result of a 6 to 7 ft (1.8 to 2.1 m) increase in
the groundwater table. While the groundwater
table remained high through June 2008 (22-
month data), total CVOC concentrations
decreased, suggesting continued treatment via
resident, active EHC. By August 2008 (dry
season), PCE and TCE both decreased to below
the detection limit of 1 ppb at all locations. The
rebound observed during the following wet
season (March 2009) was much smaller than
the previous year, suggesting that treatment of
the smear zone is being achieved as desorption
is promoted.
Degradation end products. An increase in ethene and ethane levels confirms that complete degradation is
occurring (Figure 10); ethene levels of up to 760 ug/L were measured in July 2007 (11-month data) which
constitutes an increase of 96% compared with
the maximum concentrations measured in
August 2006. As total dissolved
concentrations of chlorinated ethenes
decreased within the treatment zone, the
concentration of dissolved gases has also
decreased. However, concentrations of ethane
remain above baseline levels, suggesting that
a smaller concentration of sorbed CVOCs
remain and that degradation is still ongoing. A
spike in ethane concentrations were observed
at the NE and SW monitoring wells during the
rainy season (18-month data), as CVOCs
sorbed to the smear zone entered the system
and became available for degradation.
EHC indicator parameters. To assess whether the wells are in fact under the EHC zone of influence, the
groundwater was sampled for total organic carbon (TOC) and ferrous iron (Fe(II)). A significant increase was
observed at all sampling locations, confirming that successful product placement and sufficient distribution had
been achieved. TOC and Fe(II) has been measured at a maximum concentration of 2,800 and 130 mg/L
respectively.
Redox indicator parameters: Following the EHC injections, the oxidation-reduction potential (ORP) decreased
within the injection zone. Sulfate concentrations decreased from a baseline concentration of 7 to 34 mg/L to below
the detection limit of 0.1 mg/L, suggesting that sulfate reducing conditions was established within the injection
zone. Furthermore, an increase in methane suggests methanogenic conditions and an excess amount of dissolved
carbon. As dissolved carbon concentrations declined over time, ORP has increased. However, methane levels
remain high, suggesting that reducing conditions are maintained within the treatment zone.
The Cost
The material cost of using EHC was $1.24/ft³
($44/m³). The injections were completed in 5 days. Results of this pilot
study have shown that ISCR using the EHC technology offers a safe,
effective and cost-efficient remedial solution for dry cleaning and
related sites.
For more information, see the Oregon DEQ Project Description at: http://www.deq.state.or.us/wdr/?p=20107
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