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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 a S13-1) 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, 5 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 (Ikg/L) during this period Dissolved benzene
<br /> concentrations in MW-1 have also increased from 67 µg/L in May 1995 to 1,100 Itg/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/I'
<br /> benzene, respectively Copies of groundwater monitoring reports are included as
<br /> Appendix D _ `T�)Ur4. %$ Ivo Pt, (?6)R-74' i<' 1�
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