Site soils are primarily derived from erosion of the underlying Triassic red bed shales. Based on the RI/FS,
soil encountered was typically two to twenty feet thick consisting of gray silty clay/clayey silt with rock
fragments. The rock exposed in the vicinity of the Hunterstown Road Site consists of northwest dipping
sedimentary rock of the Gettysburg Formation. The igneous intrusives have altered the sedimentary rocks at the Hunterstown Road Site by increasing their hardness and changing their color and chemical composition. Such effects are a product of thermal metamorphism, where the changes are due to heat rather than heat and pressure or just pressure. In addition to the "baking" of the rock, hot water contained within the sedimentary rocks and/or provided by the magma also alters the mineralogy. The soft argillaceous red shale, typical of the Gettysburg Formation, is altered by this process to hard hornfels colored dark red to dark purple to black. All joints dipped steeply, between 73 degrees and vertical.
Targeted Environmental Media:
 Dense Non-aqueous Phase Liquids (DNAPLs) - Fractured Bedrock
Contaminated ground water moves through a complex network of bedding plane and
vertical fractures to the west-southwest. There is vertical contamination to at least 800 feet bgs.The USGS reviewed the Remedial Investigation Report and believes that DNAPLs may have migrated to over 2000 feet deep.
There are three separate plumes at the site.
Major Contaminants and Maximum Concentrations:
- Trichloroethene (48,000 µg/L)
- 1,1,1-Trichloroethane (13,000 µg/L)
- 1,1-Dichloroethane (Not given)
- 1,1-Dichloroethene (290 µg/L)
- Vinyl chloride (1,400 µg/L)
- 1,2-Dichloroethane (Not given)
- Borehole Geophysics
- Vertical Chemical Profiling
- Coring
- Pump and Treat
Comments:
The groundwater extraction system uses air stripping followed by vapor phase carbon. The system was made operational in 2004 and consists of six extraction wells ranging in approximate depth from 195 feet to 500 feet below ground surface (bgs). The treatment rate is 66 to 115 gpm for the total system.
A finding of technical impracticability was made for contamination below 800 feet bgs. For the groundwater above 800 feet bgs background levels of contamination, practically defined as no detection of VOCs using method EPA 524.2 Practical Quantitation Limits.
The system is functioning as designed. With the exception of Pumping Well PW-LA, five of the six pumping wells exhibit decreases in total VOC concentrations since the baseline sampling event was performed in September 2003. Influent samples from these recovery wells have decreased approximately 25.4 to 96.3 percent and have shown an overall decrease in total VOC concentrations of 70 percent since the start-up of the groundwater recovery/treatment system. It is believed that Pumping Well PW-LA continues to capture impacted groundwater from the Lagoon source area, which accounts for increasing VOC concentrations at this location.
Seven conventional monitoring wells show a general decreasing trend in total VOC concentrations, while six monitoring wells show an increasing trend in total VOC concentrations.
Total VOC concentrations in the multi-port monitoring wells show either a decreasing trend or have no obvious trend.
http://cfpub.epa.gov/supercpad/cursites/csitinfo.cfm?id=0301595
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