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• ATTACHMENT A •
<br />SUMMARY OF PREVIOUS INVESTIGATIONS
<br />Chevron Service Station 9-2033, 508 West Charter Way, Stockton, CA
<br />SITE BACKGROUND AND PREVIOUS INVESTIGATIONS
<br />The site is an operating Chevron Service Station located in a primarily commercial area
<br />at the southwest corner of the intersection of West Charter Way and South Lincoln
<br />Street in Stockton, California. The site currently contains three gasoline underground
<br />storage tanks (USTs) that share a common excavation near the northwest site boundary,
<br />four dispenser islands under a common canopy, and a station building.
<br />A Texaco Service Station operates directly east of the site (across Lincoln Street) and an
<br />operating ARCO Service Station is located northeast of the site across the intersection
<br />of West Charter Way and South Lincoln Street. The City of Stockton maintenance yard,
<br />located to the south, is reported to have, or have had, one or more USTs.
<br />Potential chemicals of concern at the site include total petroleum hydrocarbons
<br />calculated as TPHg; aromatic hydrocarbons including BTEX; and oxygenates, including
<br />MtBE, tertiary butanol, DIPE, EtBE, TAME, 1,2 -DCA, EDB, ethanol, and methanol.
<br />In March 1995, Groundwater Technology installed three monitoring wells (MW -1, 2,
<br />and 3) onsite to approximately 50 feet below ground surface (bgs), and one soil boring
<br />(SB -1) to 35 feet bgs. Soil analytical results indicated no detectable levels of TPHg in
<br />soil borings for wells MW -1 and MW -2. Concentrations of TPHg in MW -3 soil were
<br />reported at 3.8 and 250 milligrams per kilogram (mg/kg) at 25 and 35 feet bgs,
<br />respectively. Soil boring SB -1 test results indicated TPHg concentrations of 4.2 and 28
<br />mg/kg at 30 and 35 feet bgs, respectively. Generally, aromatic hydrocarbons including
<br />BTEX were reported for soil samples collected from the 20 to 25 -foot depth range near
<br />the former UST area (i.e. SB -1) and in the 30 to 35 foot depth range, (i.e. MW -3)
<br />consistent with the first occurrence of groundwater. Metals analyses results for soil
<br />samples from MW -1 indicated concentrations of chromium, lead, nickel and zinc at 23,
<br />7.8, 21, 50 mg/kg (10 feet bgs) and 31, 6.5, 42, and 59 mg/kg (35 feet bgs), respectively.
<br />Quarterly sampling of monitoring wells MW -1 through MW -3 has been performed
<br />since May 1995. Dissolved concentrations of TPHg in MW -1 increased from 340 to
<br />5,900 micrograms per Liter (µg/L) during this period. Dissolved benzene
<br />concentrations in MW -1 have also increased from 67 µg/L in May 1995 to 1,100 µg/L
<br />in August 1997. Review of MW -1 analytical results also suggests an increasing trend in
<br />1,2 dichloroethane (DCA). Analytical results for MW -2 have consistently indicated no
<br />detectable concentrations of TPHg, BTEX, and MtBE. Review of analytical results for
<br />MW -3 suggests a decreasing trend of hydrocarbon concentrations between May 1995
<br />and August 1997, from 44,000 µg/L to 7,100 µg/L TPHg, and 2,900 µg/L to 800 µg/L
<br />benzene, respectively. Copies of groundwater monitoring reports are included as
<br />Appendix D.
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