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ethylbenzene, and total xylenes (BTEX). Although samples collected from the soil borings reported non-
<br /> detectable (ND) levels for all of the analytes, IT noted that "due to the nature of the site's coarse grained
<br /> soils any hydrocarbons may have migrated vertically instead of laterally" (IT, 1991).
<br /> A San Joaquin County Public Health Services (SJCPHS) letter received on August 28, 1992, documented
<br /> the completion of site investigation and remedial activities.
<br /> In December 1994, Emcon advanced soil borings TB-1 through TB-3 to 20 feet bgs and TB 4 to 71 feet
<br /> bgs. Concentrations of benzene (0.18 parts per million [ppm]), toluene (0.005 ppm), ethylbenzene
<br /> (0.031 ppm), and total xylenes (0.040 ppm) were detected in the soil sample collected from TB-4 at 65
<br /> feet bgs. Total xylenes were detected at 0.005 ppm in TB-2 at 20 feet bgs. Total petroleum
<br /> hydrocarbons as oil and grease (TOG)was detected in soil samples from TB-1 at 10 feet bgs,TB-3 at 10
<br /> and 20 feet bgs, and TB-4 at 65 feet bgs(EMCON, 1994).
<br /> Based on a March 19, 1996 letter from the California Regional Water Quality Control Board (CRWQCB)—
<br /> Central Valley Region,the site (then BP Station No.11190) was granted No Further Action status.
<br /> In preparation for sale of the site, in March 2005, SECOR International (SECOR [now Stantec]) advanced
<br /> six soil borings (B-1 through B-6) at the site. TPHg,TPHd, benzene, methyl tert-butyl ether(MTBE) and
<br /> tert-butyl alcohol (TBA) concentrations were reported as maximum concentrations of 1,600 milligrams
<br /> per kilograms(mg/kg), 1,300 mg/kg, 3 mg/kg, 6.3 mg/kg, and 0.47 mg/kg, respectively (SECOR, 2005).
<br /> In January 2007, SECOR observed the drilling of soil borings B-7 through B-11 to depths of 50 to 55 feet
<br /> bgs in the vicinity of the existing UST's and gasoline dispensers. Gasoline-range organics(GRO [aka
<br /> TPHg]), diesel-range organics (DRO [aka TPHd]), and benzene were only detected in soil boring B-7, at
<br /> concentrations of 8,000 mg/kg, 32 mg/kg, and 25 mg/kg, respectively. MTBE was reported in several
<br /> borings at a maximum concentration of 8.0 mg/kg (boring B-11 at 50 feet bgs). Groundwater was
<br /> encountered in two borings, B-7 and B-11, at depths of 45 to 47 feet bgs and groundwater samples were
<br /> collected. GRO and BTEX were detected at concentrations of up to 6,000 µg/L, 150 µg/L, 970 pg/L, 200
<br /> pg/L and 1,000 µg/L, respectively in the groundwater sample collected from B-7. The highest MTBE and
<br /> TBA concentrations (60,000 pg/L and 7,700 pg/L, respectively) were detected in the groundwater
<br /> sample collected from B-11 (SECOR, 2007).
<br /> In March 2008,SECOR supervised the installation of on-site groundwater monitoring wells MW-1
<br /> through MW-4 to a total depth of approximately 55 feet bgs (SECOR, 2008).
<br /> In February 2010, Stantec oversaw the installation of monitoring wells MW-5 through MW-9 to an
<br /> approximate depth of 65 feet bgs. MTBE was detected in soil samples collected from wells MW-6
<br /> through MW-9, with a maximum concentration of 6.8 mg/kg in soil sample MW-7 at a depth of 62 feet
<br /> bgs, while TBA was detected in soil samples collected from well MW-7 at a maximum concentration of
<br /> 1.2 mg/kg (Stantec, 2010).
<br /> On November 26, 2012,three soil borings (VP-1 through VP-3)were advanced on-site to depths of
<br /> approximately five feet below ground surface (bgs). Each soil boring was completed as a temporary
<br /> vapor sampling point. Due to rainy conditions through much of December and January,sampling was
<br /> postponed until a sufficient dry period was predicted, per the DTSC guidelines.
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