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i I In 1997, GHH Engineering removed three 12,000-gallon gasoline underground storage tanks (USTs), one 3,000-gallon diesel UST, and fuel pipelines The highest total petroleum hydrocarbons as diesel (TPHd) concentration detected was 5,000 milligrams per kilogram (mg/kg) in the soil sample collected beneath the diesel fuel dispenser The highest TPHg concentration detected was 4,800 mg/kg in the soil sample collected beneath the gasoline USTs Benzene was detected at concentrations up to 4 2 and 6 4 mg/kg in the soil samples collected beneath the USTs and dispensers, respectively Methyl tertiary butyl ether (MTBE) was detected at concentrations up to 28 and 18 mg/kg in the soil samples collected beneath the USTs and dispensers, respectively Approximately 400 cubic yards of soil excavated during UST and associated pipeline removal was disposed of off-site at a licensed landfill GHH Engineering estimated the mass of residual gasoline at the site to be approximately 163 gallons per a Request for No Further Action report dated February 2000 GHH Engineering performed quarterly groundwater monitoring and sampling from July 1997 to September 2002 The historic groundwater gradient direction has ranged from northeast to the southeast GROUNDWATER MONITORING • Water level measurements were collected from wells MW-1P through MW-6P, MW-1 and MW-2 on May 18, 2004 The locations of the monitoring wells are shown on Figures 2 and 3 Depth to water levels ranged from 62 67 to 63 78 feet below the tops of the well casings The groundwater elevation increased an average of 0 04 feet since the previous quarter The water level data were used to develop the groundwater elevation contour map (Figure 3) The average hydraulic gradient on May 18, 2004 was calculated to be 0 009 ft/ft or approximately 48 ft/mile, and flowed generally towards the northwest A summary of groundwater monitoring data is presented in Table 1 SAMPLING ACTIVITIES On May 18, 2004, ATC personnel collected groundwater samples from monitoring wells MW-1P through MW-6P, MW-1, and MW-2 Prior to collection of groundwater samples, the depth to water, pH, electrical conductivity, and temperature were measured in the groundwater purged from the monitoring wells and recorded Aquifer parameters were allowed to stabilize and a minimum of three well casing volumes were purged from each well prior to sampling The wells were allowed to recover and samples were collected from each well using dedicated disposable bailers Purged well water was contained on site in DOT approved 55-gallon drums and labeled for disposal pending receipt of laboratory results The groundwater samples collected from each well were submitted to Argon Laboratories, a state-certified analytical laboratory (FLAP cert no 2359) located in Ceres, California, for . analyses Laboratory analyses consisted of TPHg utilizing EPA Method 8015B, BTEX utilizing EPA Method 8021B, and oxygenate fuel additives MTBE, ethyl tertiary butyl ether (ETBE), di- slenvironmenta11259i210003 StocktonlQuanly12QR-2004doc 2 I