Ground water contamination at the site has been detected in the thin(<10 ft.) overburden (generally clay with fill materials) and uppermost bedrock (an interbedded limestone-shale sequence). Contaminant migration pathways can be traced to an individual bedding plane or discrete zone of bedding planes less than 5.0 ft. thick, over a distance of 50-100 ft.

Bulk hydraulic conductivities, estimated through slug tests range between 1x10-6 cm/sec and 1x10-4 cm/sec. During high vacuum pilot tests, immediate drawdowns of several feet were observed in wells located more than 100 ft. from the extraction wells. Storage capacity within the bedrock is extremely low indicating a generally low degree of fracturing. There is minimal contamination in deeper wells. Despite the generally low degree of fracturing of the bedrock, rapid response of fluid levels in both bedrock and overburden wells to rain indicates a relatively high degree of communication between the surface and uppermost bedrock.

• Dense Non-aqueous Phase Liquids (DNAPLs)
• Fractured Bedrock

In August 1998, a limited amount of DNAPL (0.3 ft.) was identified in one well placed at a suspected fracture confluence. DNAPL had not previously been identified and has been detected only intermittently since initial discovery. Reductive dechlorination (PCE to 1,2-DCE) has been detected near the source. This process is likely enhanced by the mixed nature of the plume, with the presence of hydrocarbons acting as a carbon source for microbial reductive dechlorination reactions.

The horizontal extent of ground water contamination in the uppermost bedrock ranges from 400-800 ft. downgradient of the suspected source area. The vertical extent of contamination is typically less than 30-40 ft. below ground surface. The absence of deeper contamination is documented by a series of clean down gradient wells, which extend up to 120 ft below the elevation of the suspected source.

• Tetrachloroethene (DNAPL)
• Trichloroethene (DNAPL)
• Benzene-toluene-ethylbenzene-xylene (BTEX) (LNAPL)

• Fluid Loggings
  • Temperature
• Vertical Chemical Profiling
  • Packer Isolation
  • Cluster Wells
• Coring

Comments:
While detailed characterization efforts have been sucessful, numerous field observations illustrate the complexity of a bedrock fracture system. Despite the generally low degree of fracturing of the bedrock, rapid response of fluid levels in both bedrock and overburden wells to rain events indicates a relatively high degree of communication between the surface and uppermost bedrock. Wells initially installed to monitor a single ground water regime, have been found to be screened across multiple distinct fracture systems; subsequently these wells have been recompleted to monitor distinct intervals. Some well interconnections initially identified during packer tests were not confirmed during later vacuum tests.

• Multi Phase Extraction
• Other

None provided

Approximately 40 gsallons of NAPL have been recovered. As a result, concentrations of NAPL related constituents (PCE, TCE, 1,1,1-TCE, toluene, and xylenes) have decreased by between 70 and 99% in the extraction wells with similar reductions have occurred in the source area monitoring wells.

In October, 2003, the system was still operating and providing analytical data every six months. Althoug concentrations have decreased significantly, they are still in the ppm range.

The overall decreases have been interrupted by spikes of increased concentrations, which may be associated with intervals of relatively high precipitation and infiltration. The rapid infiltration of precipitation into the bedrock is demonstrated in the observed increases (up to a factor of three) in system ground water production following significant rain events. The small concentration spikes in the NAPL-related constituents are accompanied by increases in relatively soluble (non-NAPL) constituents (e.g., acetone, methyl ethyl ketone and methylene chloride). This phenomenon illustrates the complexity of remediation of a multi-component plume in a fractured media.