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a a ATTACHMENT A SUMMARY OF PREVIOUS INVESTIGATIONS Chevron 9-6171, 6633 Pacific Avenue, Stockton, CA The subject site is located at the intersection of Pacific Avenue and Benjamin Holt Drive in Stockton, California Topography in the area is relatively flat Land use in the area is primarily commercial and residential, including three known groundwater contamination cases where sources of volatile organic compounds (VOCs), specifically tetrachloroethene (PCE), have been identified In December 1986, the station was remodeled, and the three underground storage tanks (USTs), fuel island, and product piping were replaced at the site The former UST pit was backfilled, and the new USTs were relocated to the southwestern portion of the site At the time of the remodel an additional fuel island was installed, and the former 550-gallon waste oil tank was replaced with a 1,000-gallon waste oil tank In January 1987, Groundwater Technology, Inc (GTI) performed a subsurface investigation, which included the installation of one monitoring well in the former UST pit Gasoline odors were noted to a depth of 39 feet below ground surface (bgs) Groundwater analytical results included 990 parts per billion (ppb) total petroleum hydrocarbons calculated as gasoline (TPHg), and benzene, toluene, and xylenes at concentrations of 32 0 . ppb, 47 ppb, and 43 ppb, respectively In late July to early August 1990, Sierra Environmental Services (Sierra) installed groundwater monitoring wells MW-2 through MW-5 Hydrocarbons were not detected in any of the soil samples from the borings Soil stratigraphy was described as interbedded silty sand and clayey silt to silty clay, with first water encountered in silty sand at an average depth of 54 feet Sierra reported the top of a potential confining layer in boring MW-5 at a depth of 68 feet, however, the thickness of this layer was not explred Groundwater analytical results for samples from MW-2 through MW-5 indicated nondetectable concentrations of TPHg, benzene, toluene, ethylbenzene, and xylenes (BTEX), ethylene dibromide (EDB), and lead MW-4 groundwater was also analyzed for total coliform due to its proximity to the sewer lines at the site, total coliform was not detected MW-5 groundwater was additionally analyzed for metals and total oil and grease due to its proximity to the waste oil UST Cadmium, chromium, and zinc were detected at concentrations of 30 ppb, 400 ppb, and 1,100 ppb, respectively On July 30, 1992, Sierra installed groundwater monitoring well MW-6 on-site Soil samples did not contain detectable TPHg or BTEX Groundwater samples were not collected from MW-6 until the third quarter 1992 On July 31, 1992, Sierra abandoned groundwater-monitoring well MW-1 MW-1 was no longer usable as groundwater elevations at the site declined below the screened interval, • since at least September 1990