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I
<br /> URS
<br /> . Ms. Victoria McCartney
<br /> Page 2
<br /> San Joaquin County Environmental Health Department
<br /> ARCO 5450
<br /> December 22,2005
<br /> the northwest at rates ranging from 0.0006 ft/ft to 0.03 ft/ft with fluctuations to the southwest and
<br /> southeast. Groundwater elevations, gradients, and flow directions are summarized in Tables I and 2.
<br /> Previous Investigations
<br /> 1 Initial Investigation October 195)
<br /> On October 5, 1995, soil sampling was conducted during facility product piping upgrades. Fourteen soil
<br /> samples were collected 18 inches beneath the product piping trenches. Laboratory analysis reported
<br /> maximum concentrations of gasoline-range total petroleum hydrocarbons (TPHg), ,benzene, toluene,
<br /> ethylbenzene, and total xylenes (BTEX) of 360,000 milligrams per kilogram (mg/kg) ,400 mg/kg,
<br /> 6,700 mg/kg, and 39,000 mg/kg, respectively, near the southwestern dispenser island. Approximately 35
<br /> cubic yards of soil were removed during over excavation activities of the impacted soil.Following over-
<br /> excavation, concentrations of BTEX were 0.087 mg/kg, 0.026 mg/kg, 0A064 mg/kg, and 0.044 mg/kg,
<br /> respectively, at approximately 11.5 feet bgs. TPHg was not detected at 11.5 feet bgs.
<br /> 1
<br /> Problem Assessment(April 1996
<br /> On July 30, 1996, one 2-inch-diameter groundwater monitoring well (MW-1, shown on Figure 2) was
<br /> drilled and installed to a depth of 25 feet bgs near the southwestern dispenser island. Laboratory
<br /> analysis reported total xylenes in all three soil samples analyzed with concentrations ranging from
<br /> 0.0 64 mg/kg to 0.0089 mg/kg. No other hydrocarbons were detected.
<br /> i
<br /> W-1 was sampled on July 30, 1996, November 15, 1996, and March 19, 1997. Laboratory analysis
<br /> reported concentrations of methyl tert-butyl ether(MtBE) of 35 micrograms per liter(gg/L), 8µg/L, and
<br /> 19 µg/L, respectively, per sampling event. TPH-g and BTEX were not detected in any of the three
<br /> sampling events. Depth to groundwater ranged from 8.89 feet to 12.37 feet bgs. X6,-fce':,d e�itz�ii2c 4 a
<br /> o,, 7/301$1 y,,.- �'Lror 4
<br /> On August 4, 1997, the California Regional Water Quality Control Board — Central Valley Region
<br /> (CRWQCB-CVR) issued a"No Further Action Required" status for the site.
<br /> ' Site Investigation November 1999
<br /> On November 24, 1999, soil sampling was performed during facility product piping and dispenser
<br /> upgrade activities. Ten soil samples were collected beneath the product piping lines and eight soil
<br /> samples were collected beneath the product dispensers. In soil sample PL9-5.5, laboratory analysis
<br /> reported concentrations of toluene, ethylbenzene, xylenes, and MtBE of 0.0436 mg/kg, 0.0124 mg/kg, 5
<br /> 0.0925 mg/kg, and 0.201 mg/kg, respectively. Xylene concentrations of 0.0448 and 0.0109 mg/kg were
<br /> also reported in soil samples PL-44.0 and DP84.5, respectively. PL9 and-Pb4 samples were collected
<br /> from the east of the dispensers. Neither hydrocarbons nor volatile organic compounds (VOCs) were v`t �"
<br /> detected in any other soil sample collected. Analytical data for the November 1999 investigation are
<br /> presented in Attachment C. ak 44
<br /> KAWprocess1[5 BP ARCO1054501.Soil Closure 051Draft of 5450 Closure Report.doc
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