|
Site History and Previous Investigations
<br />Former Chevron Service Station No. 94054
<br />October 11, 2016
<br />borings were advanced in the former pump island area (SECOR International Incorporated,
<br />2004b). The greatest concentrations of petroleum hydrocarbon constituents detected in the soil
<br />samples collected during the associated direct push advancements were in the soil samples
<br />collected from DPS-5 at 20 feet bgs, DPS-5 at 25 feet bgs, DPS-10 at 12.5 feet bgs and DPS-12 at
<br />20 feet bgs. TPH-GRO was detected at a concentration of 8,500 mg/kg in the soil sample
<br />collected from DPS-12 at 20 feet bgs. BTEX was detected at concentrations of 26, 160, 130 and
<br />930 mg/kg, respectively, in the soil sample collected from DPS-5 at 20 feet bgs. MTBE was
<br />detected at a concentration of 0.0007 mg/kg in the soil sample collected from DPS-5 at 25 feet
<br />bgs. Methanol was detected at a concentration of 0.27 mg/kg in the soil sample collected from
<br />DPS-10 at 12.5 feet bgs. The greatest concentrations of petroleum hydrocarbon constituents
<br />detected in the groundwater samples collected during the associated direct push
<br />advancements were in the groundwater samples collected from DPW-1 at 37 feet bgs, DPW-8 at
<br />50 feet bgs, DPW-9 at 22.5 feet bgs and
<br />DPW-10 at 13 feet bgs. TPH-GRO, toluene, ethylbenzene, total xylenes and MTBE were detected
<br />at concentrations of 87,000, 1,900, 2,400, 15,000 and 23 pg/L, respectively, in the groundwater
<br />sample collected from DPW-9 at 22.5 feet bgs. Benzene was detected at a concentration of
<br />11,000 pg/L in the groundwater sample collected from DPW-1 at 37 feet bgs. TBA was detected
<br />at a concentration of 87 pg/L in the groundwater sample collected from DPW-8 at 50 feet bgs.
<br />Methanol was detected at a concentration of 210 pg/L in the groundwater sample collected
<br />from DPW-10 at 13 feet bgs.
<br />In March 2004, SECOR supervised the installation of two extraction wells (EW-1 and EW-2) and
<br />five temporary piezometers (PZ-1, PZ-2, PZ-3, PZ-4 and PZ-5). Extraction well EW-1 was installed
<br />slightly south of the former gasoline UST pit and EW-2 was installed slightly north of the former
<br />gasoline UST pit. PZ-1 was installed near the former pump island area. Piezometer PZ-2 was
<br />installed north of the former gasoline UST pit, PZ-3 was installed slightly further north of the former
<br />gasoline UST pit and PZ-4 was installed even further north of the former gasoline UST pit,
<br />approximately 35 feet north of the former gasoline UST pit (SECOR International Incorporated,
<br />2004b). Step drawdown and constant discharge pumping tests were performed separately in
<br />extraction wells EW-1 and EW-2 for the purpose of assessing the hydraulic properties of
<br />transmissivity, storativity and hydraulic conductivity at the site. Prior to the tests, pressure
<br />transducers with built in data loggers capable of record groundwater levels at one minute
<br />intervals were installed in the extraction wells, the piezometers and monitoring wells MW-3 and
<br />MW-8. For the tests performed in EW-1 the transmissivity geometric mean was 2,650 gallons per
<br />day per foot (gpd/ft), the storativity geometric mean was 0.009 and the hydraulic conductivity
<br />geometric mean was 0.015 centimeters per second (cm/sec). For the tests performed in EW-2
<br />the transmissivity geometric was 4,000 gpd/ft, the storativity geometric mean was 0.0000018 (1.8
<br />10-6 ) and the hydraulic conductivity geometric mean was
<br />0.030 cm/sec (SECOR International Incorporated, 2004b). Subsequently, a capture zone analysis
<br />was performed for EW-1 and EW-2. Capture zone parameters of 38 and 60 feet were calculated
<br />in the down gradient and cross gradient directions, respectively, for EW-1 and capture zone
<br />parameters of 31 and 48 feet were calculated in the down gradient and cross gradient
<br />directions, respectively, for EW-2 (SECOR International Incorporated, 2004b).
<br />In April 2004, SECOR supervised the destruction of temporary piezometers PZ-1 through PZ-5. The
<br />temporary piezometers were destroyed according to San Joaquin County Environmental Health
<br />Department (SJCEHD) and California Well Standards (SECOR International Incorporated, 2004b).
<br />7
|