Description
Project Update January 2011
Application: EHC PRB for Treatment of CT, installed April 2005
Site: Grain Silo Facility, Kansas (Confidential Client)
Lead Consultant - Malcolm Pirnie, Inc.
Installation Contractor: Redox Tech, LLC
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Summary
EHC®has been shown to effectively treat carbon tetrachloride (CT) and its catabolites. A large-scale field effort was undertaken to: i) validate EHC performance under field conditions, and ii) assess the effectiveness of the construction methodology (i.e., direct injection of EHC slurry). 24 tons of EHC were injected into a 270 ft (83 m) long permeable reactive barrier (PRB) intersecting the groundwater plume downgradient of the suspected source area. Subsequent field monitoring showed a 76% and 88% decrease in CT concentrations after just 4 months, without the accumulation of recognized catabolites, such as chloroform (CF). Continued monitoring has shown that the PRB has remained active for over 5 years and that the decreasing trend in CT concentrations has migrated a significant distance downgradient from the PRB with a 99.5% decline in CT observed 70 ft (21 m) from the PRB and a 91% decline observed 600 ft (183 m) from the PRB.
The Challenge
Site groundwater is impacted with CT at concentrations of up to 2,700 ppb. The CT plume extends approximately 2,500 ft (760 m) from a grain elevator where it discharges into a small creek (Figure 1 - Right Column). The remedial goal is to treat CT to <5 ppb, CF to <100 ppb, Chloromethane (CM) to < 20 ppb and methylene chloride (MC) to <5 ppb. The CT source area is elusive, but impacts are likely the result of using CT as a fumigant in the grain silos on the site. There is a complex geologic history in the study area which resulted in a mix of interbedded sand, gravel, clay and silt. Two primary saturated sand units have been identified, named as the upper and lower saturated sand units. The upper sand unit is approximately 3 to 4 ft (0.9 to 1.2 m) thick. The thickness of the lower sand unit varies considerably across the site and ranges from 0 to 13 ft (0 to 4 m) thick. The groundwater table is encountered at approximately 23 ft (7 m) bgs at the PRB area; CT impacts extend down to a maximum of approximately 45 ft (14 m) bgs. The bedrock rises to an elevation approximately 10 feet (3 meters) above the present day water table at the presumed source area. The CT is believed to have transported along the topography of the bedrock surface to the downgradient aquifer. Access restrictions due to residential properties further complicates source area clean-up.
Lab-Scale Validation of the EHC® Technology
Preliminary reactant screening was accomplished by performing batch tests and measuring changes in CF, CT,
DCM and CM concentrations with time. Complete removal of CT and all its daughter products was achieved after 65 days of batch treatment using EHC (Table 1 — Right Column).
Field-Scale Pilot Study
In March 2005, a
total of 48,000 lbs (21,818 kg) of EHC® were injected into a 270 ft (83m)
long PRB across the plume. The EHC was applied at a rate of approximately
1% to soil mass into the upper and lower saturated sand units. The EHC®
was supplied as a dry powder in 50-lb (23 kg) bags and mixed with water
(Figures 2 and 3 - Right Column) on site into slurry containing about
40% solids. The EHC® slurry was injected into twenty-seven injection
groups composed of 126 injection borings using a direct injection
technique. Injection borings targeted a specific depth in either the upper
or lower saturated sand unit. Each injection group was spaced 10 ft (3 m)
apart. This injection approach was employed to avoid injecting the
majority of EHC® slurry into the bottom intervals. Soil samples were collected to verify proper placement of EHC. Horizontal (Figure 4 - Right Column) as well as vertically rising
fractures (Figure 5 - Right Column) were observed.
The Result
Figure 6 (Below) shows the concentration of CT and daughter products in groundwater measured upgradient and downgradient of the EHC PRB. The downgradient monitoring wells are located 70 ft (21 m), 140 ft (43 m) and 600 ft (183 m) downgradient of the PRB, which equals approximately 39, 78 and 333 days in terms of groundwater travel time. Groundwater sampling results as early as four months after installation of the PRB showed a 76% and 88% decline in CT concentration at monitoring wells located 70 and 140 ft (21 and 43 m) downgradient of the PRB, respectively. CT removal rates peaked 16 months after installation with >99 percent removal observed 70 ft (21 m) downgradient of the PRB (from a baseline of 1,000 ppb to <5 ppb). Two years after installation these rates decreased slightly to approximately 95 percent removal, and have stabilized there. CF concentrations initially increased as a result of the CT degradation. However, by February 2007 CF had decreased to 8.4 ppb, which is below background and initial concentrations. DCM and CM have remained below the detection limits in both the upgradient and downgradient wells since the May 2006 monitoring event. A well at the edge of the plume remains non-detect (100% removal) for all analytes since August 2005. In the seven most recent sampling events (22 to 61 months after the EHC injections), effects of the PRB were observed 600 ft (183 m) downgradient from the PRB, with a 91% decline in CT concentration measured in April 2008 (from an initial concentration of 140 to 13 ppb) and again, no accumulation of catabolites. Meanwhile, inflowing concentrations have fluctuated with a high of 2,700 ppb measured in February 2007.
Figure 6: Influence of EHC PRB on concentrations of CT and related degradation products in groundwater.
Figure 7 (Below) shows the estimated extent of CT plume prior to and 22 to 61 months after installation of the EHC PRB. Groundwater levels measured over time did not indicate a change in groundwater direction following the installation of the PRB.
Figure 7: Impact of EHC PRB on extent of CT plume.
Degradation Rates
After an initial acclimatization period of approximately 5 months, a half life of between 1 and 5 days has been maintained for CT within the reactive zone (Figure 8 - Below). Degradation rates did initially increase with time as acclimatization occurred, but has slowly decreased since reaching its peak value of 1.3 days after 17 months, which is likely due to consumption of the EHC amendment. CF concentrations initially were detected as a result of the CT degradation (net negative half lives not shown on graph), but had decreased to below background and initial concentrations by May 2006, maintaining an estimated net half life of 3 to 5 days. DCM and CM have remained below the detection limits (<2 ppb) in both the inflowing and downgradient wells since the May 2006 monitoring event.
Half lives were calculated using a 1D equation assuming first order decay:
C = C0e-k(x/v)
Where:
C = concentration [M/L3]
C0 = initial concentration [M/L3]
k = decay constant [1/T]
x = PRB width [L]
v = flow velocity through PRB [L/T]
Figure 8: CT and CF half lives within PRB.
The Cost
The EHC material cost for the PRB measuring 270 ft long x 9.7 ft deep on average was less than $100,000, resulting $37/ft2 ($395/m2) of PRB cross-section. The installation was completed in 12 days (between March 30 and April 10, 2005). Using an estimated linear groundwater velocity of 1.8 ft/day (1.6 to 2.2 ft/day estimated) and a porosity of 30%, the PRB is treating an estimated total of 516,000 ft3 (14,600 m3) of groundwater per year (270 ft long x 9.7 ft average depth x 365 days x 1.8 ft/day x 30%). With a confirmed life of at least 5 years, the PRB has treated an estimated total of 2,580,000 ft3 (73,000 m3) of groundwater during its life-time at a product cost of $0.04/ft3 ($1.32/m3). It can therefore be concluded that ERD/ISCR using the EHC technology offers a safe, effective and cost-efficient remedial solution for similarly impacted environments. The reactive zone constitutes a green solution (no energy requirements) and maintenance costs are limited to groundwater monitoring over the life of the PRB.
The Conclusion
Removal Efficiency: Groundwater sampling results have shown up to 99.5% decline in CT concentration at the core of the plume 70 ft downgradient of the PRB (from an initial concentration of 1,000 ppb to 5 ppb measured in August 2006), without accumulation of catabolites.
Longevity: A single application of EHC has remained active for a period of >5 years, continuously supporting >94% removal of CT, without the accumulation of catabolites.
Plume Impacts: The reactive zone has had significant impacts on the extent of the CT plume. In the last seven sampling events conducted 22 to 61 months after the EHC injections, effects of the PRB has been observed 600 ft (183 m) downgradient from the PRB, with a 91% decline in CT concentrations measured in April 2008.
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