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<br /> Five additional off-site wells (MW-8 through MW-12) were installed, and one off-site well
<br /> (MW-7) was destroyed in September 1998. Soil samples collected during the j
<br /> investigation at depths of 4 to 6 feet bgs contained petroleum hydrocarbon concentrations
<br /> of up to 1.5 ppm of TPHg and 0.0034 ppm of benzene. Fuel oxygenates including methyl
<br /> tertiary butyl ether(MtBE)were not detected (Alisto, 1999).
<br /> One 10,000-gallon UST, one 8,000-gallon UST, and one 6,000-gallon single-wall
<br /> fiberglass UST, two dispenser islands, related product lines, and one canopy were
<br /> replaced with two 12,000-gallon double-wall gasoline USTs, two dispenser islands, related
<br /> product lines, and one canopy in December 1998. The new USTs were installed in an
<br /> excavation located immediately adjacent to the former UST complex. These excavations
<br /> were hydraulically connected with pea gravel backfill material encountered in the northeast
<br /> corner of the former UST pit. Soil samples collected from the capillary fringe of the former
<br /> and new UST excavations at depths of 11 to 12 feet bgs contained petroleum
<br /> hydrocarbons at concentrations of up to 130 ppm of TPHg, 50 ppm of MtBE, 290 ppm of
<br /> ethanol, and 51 ppm of tertiary butyl alcohol (TBA). Benzene and fuel oxygenates were
<br /> not detected in soil samples from the UST excavations. TPHg, benzene, and fuel
<br /> oxygenates were not detected in soil samples collected from beneath the former product
<br /> lines (approximately 3 to 4 feet bgs). Groundwater was encountered in the excavations at
<br /> a depth of approximately 11 to 12 feet bgs. Groundwater was encountered within the
<br /> excavation at depths of 11 and 12 feet bgs. Grab groundwater samples SW and CW,
<br /> collected from beneath the former south and center USTs, contained 8,400 and 60,000
<br /> ppb of TPHg, 310 and 1,500 ppb of benzene, 100,000 and 130,000 ppb of MtBE, and 460
<br /> and 870 ppb of tertiary amyl methyl ether (TAME), respectively. Approximately 61,000
<br /> gallons of groundwater were removed from the former and present UST pits during UST
<br /> removal and replacement activities (Gettler-Ryan Inc.[GRI], UST and Product Line
<br /> Replacement Report, April 23, 1999).
<br /> During UST excavation and replacement activities, a set of first generation product lines
<br /> were discovered, and were subsequently removed. Concentrations of TPHg (50 ppm),
<br /> MtBE (0.17 ppm), and ethanol (64 ppm) were detected in soil samples collected from
<br /> beneath the product lines at depths between 2.5 feet and 4 feet bgs. The area
<br /> surrounding these samples was overexcavated. Four additional soil samples were
<br /> collected from the overexcavation sidewalls, and one additional soil sample was collected
<br /> from the overexcavation bottom (approximately 6 feet to 11.5 feet bgs). With the
<br /> exception of TPHg concentrations detected up to 950 ppm, soil samples did not contain
<br /> concentrations of petroleum hydrocarbons or fuel oxygenates.
<br /> Prior to backfilling and compacting of the former gasoline UST complex excavation, one
<br /> conductor casing (CC-WS) was placed near the northwest corner of the former UST
<br /> excavation. The casing was installed to facilitate the removal of groundwater from within
<br /> the former UST cavity. The conductor casing consisted of 11.5 feet of 6-inch diameter
<br /> polyvinyl chloride (PVC), and 10 horizontal feet of 6-inch PVC in an "L" configuration
<br /> extending eastward from the vertical section. Between January 1999 and March 26, 1999,
<br /> a total of approximately 102,760 gallons of groundwater were removed by WMIS.
<br /> Groundwater was extracted on a weekly basis from wells MW-3, MW-5, MW-10, and from
<br /> the conductor casing installed within the UST excavation (CC-WS) by Onyx Industrial f
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