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1. ATTACHMENT A SUMMARY OF PREVIOUS INVESTIGATIONS k Chevron 91452, 334 East Main Street, Ripon, California On September 27, 1995 four underground storage tanks (USTs) were removed, including two 10,000-gallon gasoline tanks, one 5,000-gallon gasoline tank, and one 1,000-gallon waste-oil + tank. Three 12,000-gallon gasoline USTs were installed northwest of the former UST complex. i Four groundwater monitoring wells have been installed previously at the site. Monitoring r and sampling at the site was discontinued in October 1997. At that time, laboratory analytical data reported no detectable concentrations of total petroleum hydrocarbons k quantified as diesel (TPHd), total petroleum hydrocarbons quantified as gasoline (TPHg), j benzene, toluene, ethyl benzene, and xylenes (BTEX), or methyl tertiary butyl ether (MtBE). Groundwater flow was reported consistently to the southwest at gradients ranging from F 0.0002 to 0.003 feet/feet from February 1996 to October 1997. i Quarterly monitoring and sampling of the four previously installed wells was resumed in November 2001. Groundwater samples from MW-1 and MW-2 were non-detect for all analyzed constituents. The groundwater sample from MW-4 contained TPHg and MtBE at concentrations of 85 micrograms per liter (µg/l) and 36 µg/l, respectively. MW-3 was inaccessible due to a car parked on top of it. Groundwater flow was to the northeast at a gradient of 0.0005 foot per foot (ft/ft). A closure request was submitted in June 1997. The request for closure was denied due to the monitoring wells being located upgradient of the current and former USTs. j 1 On July 26, 2001, three Geoprobe soil borings were advanced to collect soil and groundwater samples to approximately 40 feet below ground surface (bgs) southwest of the current and f former USTs. Two soil samples from each of the borings were submitted for chemical 1 analysis. A groundwater sample was collected between 36 to 38 feet bgs in each boring and submitted for chemical analysis. MtBE was detected in one soil sample at 0.19 milligrams per kilogram (mg/kg) in GP-21 at 32 feet bgs. No other target analytes were detected above the method reporting limits in the six soil samples analyzed by the laboratory. Geoprobe 1 groundwater sample results are summarized in Table A-1. 1 I I � I } 1 i a i i f 4