The subsurface at the pilot area consists of alluvial material to a depth of approximately 40 to 50 feet below ground surface (bgs) that overlies fractured and weathered sandstone of the Denver Formation (bedrock). Ground water generally flows north-northeast from Basin F. The velocity of ground water flow is approximately 0.2 feet/day. Ground water flow occurs in both the alluvium and the weathered bedrock. The zone where the demonstration was conducted (44 to 54 feet bgs) consists of an upper 1- to 3-foot-thick interval of well-graded sand and gravel and a lower 7- to 8-foot-thick interval of unconsolidated sand to poorly cemented sandstone.
Targeted Environmental Media:
 Fractured Bedrock
Major Contaminants and Maximum Concentrations:
- Dieldrin (0 µg/L)
- Dicyclopentadiene (0 µg/L)
- Tetrachloroethene (20 µg/L)
- Trichloroethene (100 µg/L)
- Methylene chloride (0 µg/L)
- Chloroform (0 µg/L)
- Flow
Comments:
A tracer study conducted in 2001 indicated that local ground water flow was predominantly to the northeast, with an easterly component.
- Bioremediation (In Situ)
- Reductive Dechlorination (In Situ Bioremediation)
Comments:
In February 2000, a laboratory treatability study was conducted on soil collected from below the ground water table at the pilot area to evaluate the potential for anaerobic remediation of diisopropylmethyl phosphonate (DIMP), dieldrin, and volatile organic compounds (VOC). The study indicated that the aquifer could support degradation of all the contaminants tested with the addition of Hydrogen Release Compound (HRC). DIMP decreased from 422 milligrams per liter (mg/L) to 81.7 mg/L after 35 days in the laboratory microcosm studies. A significant reduction in concentrations of chloroform, tetrachloroethene (PCE), trichloroethene (TCE), and daughter products was achieved within 28 days. Field results would be expected to occur more slowly than in the laboratory.
A pilot demonstration was conducted from May 2001 to October 2002 at the North of Basin F site. A total of 4,200 pounds of HRC was injected in the saturated zone at depths of approximately 44 to 54 feet bgs. HRC was injected in an L shape because of the northerly and easterly flow paths. The 500- by 300-foot L-shaped permeable HRC barrier consisted of three staggered rows of 14 injection points (42 total) on 6-foot centers.
The primary objective of the HRC pilot demonstration was to evaluate the ability of this in situ ground water treatment technology to significantly reduce the concentrations of each of the primary contaminants of concern (COC) in ground water.
The time-series plot for DIMP shows a decreasing trend for wells immediately downgradient of the HRC barrier. Overall decreases in DIMP concentrations ranged from 50 to 80 percent. DIMP concentrations in downgradient wells farther from the HRC barrier were variable.
Before injection p-chlorophenylmethylsulfide (CPMS) was not detected at concentrations above 2 micrograms per liter (µg/L) in the downgradient wells. Shortly after HRC was injected, concentrations of CPMS in the downgradient wells increased. The concentrations remained elevated for 6 to 8 months and then exhibited decreasing trends. The increase in CPMS concentrations in several wells appears to correspond to initial decreases in p-chlorophenylmethylsulfone (CMPSO2) concentrations, which suggests reduction of CPMSO2.
The concentration of PCE, which ranged from 10 to 20 µg/L, decreased during the demonstration in downgradient wells to less than 5 µg/L. The concentration of TCE, initially 20 to 100 µg/L, exhibited reductions of 20 to 30 percent. Concentrations of 1,2-dichloroethene (DCE) downgradient of the barrier increased from 5 µg/L to between 10 and 40 µg/L during the demonstration. For some wells, the magnitude of the increase in the DCE concentration correlated to the magnitude of TCE degradation.
Concentrations of dicyclopentadiene (DCPD) decreased in most of the downgradient wells: several wells showed a reduction in concentration of up to 90 percent.
Concentrations of dieldrin did not show any well-defined trends.
The concentrations of chloroform decreased rapidly after HRC was injected. Methylene chloride, the first daughter product of chloroform, was detected near the barrier, but concentrations decreased rapidly.
The variable concentrations of DIMP in the wells farther downgradient and the fact that concentrations of DIMP remained above 100 µgL at the end of the evaluation period reinforce the results of the laboratory treatability study, which indicated that DIMP is one of the more difficult compounds to treat in the ground water at the pilot area.
References:
Todd, Levi; Larry Kimmel; Ellen Kaastrup; Stephen Smith; Ed LaRock. 2004. Hydrogen Release Compound Field Demonstration at the Rocky Mountain Arsenal. The Fourth International Conference on Remediation of Chlorinated and Recalcitrant Compounds, Monterey, California, May 24-27.
|
