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<br /> Subsurface lnvestigaLion Report-Tosco(Former BP)Service Station No. 11195 Lathrop,California
<br /> April 11,2002
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<br /> One 10,000-gallon, one 8,000-gallon,and one 6,000-gallon single-wall fiberglass gasoline USTs, two dispenser
<br /> islands,related product lines, and one canopy were replaced with two 12,000-gallon double-wall gasoline USTs,
<br /> two dispenser islands,related product lines,and one canopy in December 1998. The new USTs were installed
<br /> in an excavation located immediately adjacent to the former UST complex. These excavations are hydraulically
<br /> connected with pea gravel backfill material encountered in the northeast corner of the former UST pit.
<br /> Soil samples collected from the capillary fringe of the former and new UST excavations at depths of 11 to 12 feet
<br /> bgs contained petroleum hydrocarbons at concentrations of up to 130 ppm of TPHg,50 ppm of MtBE,290 ppm
<br /> of ethanol,and 51 ppm of tertiary butyl alcohol(TBA). All of the soil samples were reported as not detected for
<br /> benzene and the remaining oxygenate compound analytes. Soil samples collected from beneath the former
<br /> product lines at depths of 3 to 4 feet bgs were reported as not detected for TPHg, benzene, MtBE, and the six
<br /> oxygenate compounds. A set of first generation product lines were discovered during construction activities. Soil
<br /> samples were collected from beneath these product lines at depths of 2.5 to 4 feet bgs. Two of these samples were
<br /> reported as not detected for all analytes except that one sample contained 0.17 ppm of MtBE and the other sample
<br /> contained 64 ppm of ethanol. The third sample contained 50 ppm of TPHg and was not detected for all other
<br /> analytes. A 13 feet by 13 feet by 11.5 feet deep overexcavation was excavated around this sample. Four sidewall
<br /> soil samples and one bottom sample were collected at depths ranging from 6 to 11.5 feet bgs. These samples
<br /> contained TPHg concentrations ranging from not detected to 950 ppm. The remaining analytes were reported
<br /> as not detected.
<br /> Groundwater was encountered in the excavations at a depth of approximately 11 to 12 feet bgs. Two grab
<br /> groundwater samples were collected from the former UST excavation and analyzed for TPHg,BTEX,MtBE,and
<br /> six oxygenate compounds. Grab groundwater samples SW and CW,collected from beneaththe former south and
<br /> center USTs,contained 8,400 and 60,000 ppb of TPHg, 310 and I,500 ppb of benzene, 100,000 and 130,000
<br /> ppb of MtBE,and 460 and 870 ppb of tert-amyl methyl ether(TAME), respectively(Gettler-Ryan Inc., UST and
<br /> Product Line Replacement Report, dated April 23, 1999).
<br /> Approximately 61,000 gallons of groundwater were removed from the former and present UST pits during the
<br /> 1998 construction activities, stored on-site on portable tanks, then transported by Waste Management Industrial
<br /> Services(WMIS)of Benicia,California to the Tosco Refinery in Rodeo, California for treatment and disposal.
<br /> Groundwater sample W-STI was collected on February 1, 1999 and analyzed for TPHg,BTEX, and MtBE to
<br /> comply with Tosco Refinery disposal requirements. The sample contained 600 ppb of TPHg, 8.0 ppb of benzene,
<br /> and 12,000 ppb of MtBE(Gettler-Ryan Inc., 1999).
<br /> Prior to backfilling and compacting of the former gasoline UST complex excavation,one conductor casing was
<br /> placed near the northwest comer of the former UST excavation. The casing was installed to facilitate the removal
<br /> of groundwater from within the former UST cavity. The conductor casing consisted of 11.5 vertical feet of 6-inch
<br /> diameter polyvinyl chloride (PVC) and 10 horizontal feet of 6-inch PVC in an "L" configuration extending
<br /> eastward from the vertical section. The bottom 10 feet of the vertical casing and the horizontal casing consist of
<br /> 0.02-inch machine slotted well screen. The location of the conductor casing is shown on Figure 2.
<br /> Between February 26 and March 26, 1999, approximately 41,760 gallons of groundwater were removed from
<br /> the former UST backfill utilizing the conductor casing. Groundwater removal and transportation was performed
<br /> by WMIS. The water was transported to the Tosco Refinery for hutment and disposal. A total of approximately
<br /> 102,760 gallons of groundwater were removed from the former and present UST pits and transported to the Tosco
<br /> Refinery between January 1999 and March 26, 1999.
<br /> 140193.11-2
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