Low grade, meta-sedimentary, fractured, calcarous pelite beneath 1-3m of overburden. The attitude of the slaty cleavage is 50 degrees.
Targeted Environmental Media:
 Fractured Bedrock
The dissolved gasoline, BTEX and MTBE plumes stabilized at 200m long and 50m wide. Contaminated monitoring wells and residual water supplies both up-and down gradient from the site fall along the strike of the slaty cleavage, whereas monitoring wells directly down gradient but oblique to strike were never contaminated. The plume followed the strike of cleavage rather than the direction expected from head gradient.
Major Contaminants and Maximum Concentrations:
- Benzene-toluene-ethylbenzene-xylene (BTEX) (var)
- Coring
Comments:
Shallow bedrock fracture zone flow paterns differ from overburden and from deeper bedrock and can be recognized using test pits, direct-push sampling, and coring. Understanding the relationship of these zones of weather-enhancing, preferential permeability to water table elevation makes efficient free product recovery.
- Pump and Treat
- Other
Comments:
Remediation included overburden pump-and-treat to recover 35 gallons of free product in 1991 followed by excavation of 800 square meters of petroleum contaminated soil and weathered bedrock in 1994. In 1995 horizontal soil vapor extraction (SVE) extracted approximately 100 gallons of gasoline from unsaturated weathered bedrock. A bedrock ground water pump and treat system ran from 1992 to 1997.
None provided
Prior to the 1994 excavation, a key monitoring well showed up to 4000 ppb of gasoline range organics (GRO) but dropped to <100 ppb within six months of excavation. Annual monitoring from 1995-1999 showed that low levels of GRO persisted across the site but no more drinking water wells became contaminated. Natural attenuation will be a final remedy.
In January, 2004 the state was in the process of dismantling the remediation equipment and will initiate closeout procedures
The lesson learned at this site was to first place bedrock monitoring wells along strike of predominant fractures then to place sentinel wells across-strike to monitor seasonal variation in head gradient and contaminant migration.
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