Story

Project

Grain Silo Facility, Kansas
Lead Consultant - Malcolm Pirnie, Inc.

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 remains active over 28 months and that the decreasing trend in CT concentrations has migrated a significant distance downgradient from the PRB with a >97% decline in CT observed 70 ft (21 m) from the PRB and a 90% 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). 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).

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) 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.

The Result

Figure 6 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.

Groundwater sampling results 13 to 28 months after installation of the PRB have shown a >97% decline in CT concentration at the core of the plume 70 ft (21 m) downgradient of the PRB. CF concentrations initially increased as a result of the CT degradation. However, by August 2007 CF had decreased to 10.2 ppb, which is below background and initial concentrations. DCM and CM were below the detection limits in both the inflowing and downgradient wells at the August 2007 monitoring event. The edge of the plume remains non-detect (100% removal) for all analytes since August 2005. At the two most recent sampling events conducted in February and August 2007, affects of the PRB were observed 600 ft (183 m) downgradient from the PRB, with a 90% decline in CT concentration measured in August 2007 (from an initial concentration of 140 to 14.5 ppb) and again, no accumulation of catabolites. Meanwhile, inflowing concentrations have increased from 770 to a high of 2,700 ppb measured in February 2007.

Figure 7 shows the estimated extent of CT plume prior to and 28 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.

The Cost

The material cost of using EHC® was $37/ft2 ($395/m2) of PRB cross-section (270 ftlong x 9.7 ft deep on average). 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 an estimated life of at least 5 years, the PRB is expected to treat a 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). Furthermore, continued monitoring is expected to show that ERD/ISCR using the EHC® technology offers a safe, effective and cost-efficient remedial solution for similarly impacted environments.