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November 12, 1992 Working To Restore Nature i ARCO Station No 6100, Tracy, California Cuttings generated during drilling were placed on and covered with plastic and left at the site Soil samples were collected from the drill cuttings for analysis to evaluate disposal options Dillard Trucking of Byron, California, disposed of the drill cuttings at Redwood Landfill A copy of the memorandum from Dillard Trucking indicating the completion of soil disposal is attached Select soil samples collected from the borings and the sample from the drill cuttings were submitted to Sequoia Analytical Laboratory in Sacramento, California (Hazardous Waste ► Testing Certificate No 1624), for analysis of TPHg and benzene, toluene, ethylbenzene, and total xylene isomers (BTEX) using Environmental Protection Agency (EPA) Methods 5030 and 8020, respectively The sample from the drill cuttings was also analyzed for organic lead using the California LUFI' Method Analytical results of the soil samples are shown on Table 1. ► Analytical results indicated gasoline hydrocarbons were detected in soil samples collected from boring B-25 drilled in the shoulder of Patterson Pass Road, residual hydrocarbons were detected at concentrations ranging from 2,100 to 4,900 ppm from depths of 16 to 46 feet bgs Soil samples collected below the depth of 46 feet bgs in boring B-25 did not contain ? *� detectable hydrocarbons Gasoline hydrocarbons were not detected in the samples collected from boring B-23 Depth to Ground Water 'J The well log for the onsite water-supply well was obtained from the Department of Water ► Resources in Sacramento, California, and examined to evaluate the depth to ground water beneath the site and the well construction The well log indicates that a 8-5/8-inch-diameter steel well casing was installed in the boring drilled to a depth of 540 feet bgs in June 1969 The log also indicates that gravel pack was placed from 35 feet to 540 feet bgs, and the well casing was perforated from 396 to 416 feet bgs and 496 to 536 bgs The depth of the first 1 encountered ground water is not indicated on the log; however, the depth to water in the well after development was 100 feet bgs The log of sediments indicates gravel layers from 400 to 405 feet bgs, 507 to 515 feet bgs, and 525 to 531 feet bgs The United States Geodetic Survey Midway 7 5-minute Quadrangle shows an artesian well at similar ground surface elevation, approximately 1,800 feet southeast of the onsite water-supply well 1 RESNA's previous investigation indicated that high concentrations of gasoline hydrocarbons in soil are generally 20 to 50 feet bgs beneath and adjacent to the dispenser islands, and that below 50 feet the concentrations of gasoline hydrocarbons in soil attenuate with depth Relatively high concentrations of gasoline hydrocarbons were detected in soil at borings B- 14 and B-24, approximately 60 feet north of the water-supply well (RESNA Report No 30041-2, March 20, 1992) The concentrations of gasoline hydrocarbons (TPHg) detected 062kcar 300413 3