United States Marine Corps Site, North Carolina
Site groundwater was impacted with chlorinated volatile organic compounds (CVOCs) at concentrations as high as 80,000 ppb. A total of 24,100 lbs of EHC was injected into areas of elevated CVOC concentration to yield enhanced in situ chemical reduction (ISCR). Six months after the EHC injection, TCA levels had decreased by 94% and TCE levels had decreased by 84 to 90% at the targeted areas, without generation of problematic catabolites.
The surficial aquifer consists of unconsolidated, inter-fingering beds of fine sand, silt, clay, shell, and peat beds with scattered deposits of coarser grained material as part of relic beach ridges and alluvium. The aquifer averages about 40 feet thick and has an estimated average hydraulic conductivity of 10 ft/day. Site groundwater under a sensitive operational facility was impacted by TCE, cis 1,2-DCE, 1,1,1-TCA and 1,1-DCA with maximum detected concentrations of 73,000, 6,600, 24,000 and 4,800 ppb, respectively. These impacts extended from about 10 ft bgs to a depth of approximately 20 ft bgs. The goal was to reduce the CVOCs mass in a timely and effective manner.
There were two hot spot areas on site targeted for remediation; these were located under Buildings “A” and “B”. In January 2005, a total of 24,100 lbs of EHC were injected into the two hot spot areas. Approximately 13,400 lbs of EHC were injected at Building A into an area measuring approximately 110 ft long x 150 ft wide x 10 ft deep, giving an EHC application rate of approximately 0.07% to soil mass. Approximately 10,700 lbs of EHC were injected at Building B into an area measuring approximately 100 ft long x 150 ft wide x 10 ft deep, giving an EHC application rate of approximately 0.06% to soil mass.
The EHC was supplied as a dry powder in 50-lb bags and mixed with water on site into slurry. The EHC slurry was injected into a total of 47 injection locations from 10 to 20 ft bgs. The injections were conducted using a direct injection technique and the injection points were spaced approximately 25 ft apart. Figure 1 and 2 show the injection layout at Buildings A and B, respectively.
As summarized below, EHC treatment effectively removed CVOCs without the accumulation of catabolic intermediates.
TCA: Six months following the EHC injection, rigorous performance monitoring showed that TCA levels decreased by 94% (from 24,000 to 1,400 ppb) at the hottest area at Building B, without accumulation of problematic catabolites. TCA levels remained non-detect at Building A.
TCE: Six months following the EHC injection, TCE levels decreased by 91% (from 49,000 to 4,400 ppb) at Building A, with a slight increase in DCE levels (from 3,027 to 5,819 ppb). TCE levels decreased by 84% (from 7,400 to 1,200) and total DCE levels decreased by 72% (from 5,300 to 1,488 ppb) at Building B.
The material cost of using EHC was $3.90/yd3 ($5.00/m3) at Building A and $3.40/yd3 ($4.40/m3) at Building B. With an estimated life of >36 months, continued monitoring is expected to show that in situ chemical reduction (ISCR) using the EHC technology offers a safe, effective and cost-efficient remedial solution for similarly impacted environments.