SITE INFORMATION AND CORRESPONDENCE

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25775 PATTERSON PASS ROAD - 2 - • 14 March 2000 oxygenates. In December 1999, ARCO requested Sequoia Analytical (Sequoia) to review available chromatograms to determine if MtBE was present in soil samples collected and analyzed by EPA Method 8020. Sequoia reviewed 28 chromatograms and estimated MtBE concentrations at <0.100 mg/kg to <2.50 mg/kg. Sequoia stated that their instrumentation was not calibrated to specifically look for MtBE in 1991, and the results of their review are qualitative at best. Board chemist, Mae Hoe also reviewed the chromatograms, and stated that an unidentified peak is present prior to benzene in some of the chromatograms, which may indicate the presence of MtBE in the sample. The vertical extent of soil contamination has been defined on-site to a depth of 115 feet below ground surface (bgs), or approximately 165 feet above mean sea level (MSL). The depth to TPHg and BTEX contamination in soil increases from the northwest to the southeast as shown on Figures 3 and 4 from Pinnacle's Hydrocarbon Mass Evaluation. Boring logs indicate cemented silts that may have restricted the downward movement of site contaminants, causing the distribution of soil contaminants as shown on the figures. The IT Group has estimated that 32,537 gallons of TPHg remain in soils both on-site and offsite. Groundwater was not encountered in soil borings at a total depth of 116 feet bgs, and monitoring wells have not been installed. On 8 March 2000, Ms. Lagorio and I visited the site to determine the distance to the nearest offsite water supply well and the California Aqueduct. During this site visit, we recognized the ground surface elevation at vapor wells VW-18, VW-19, and VW-20 is approximately 20 feet lower than the on-site vapor wells. Based on the completed depths of these"vapor"wells from 73 to 88 feet below ground surface (bgs), and the increase in water levels throughout San Joaquin County since 1996, groundwater may be present in these wells. The on-site water supply well was installed in 1969. This well is at the southwest comer of the property near Patterson Pass Road approximately 150 feet from the former USTs. The surface elevation at this well is approximately 280 feet above MSL. The well is constructed with 8-inch casing to 540 feet bgs, screened from 396 to 416 and from 496 to 536 feet bgs, and gravel packed from 35 to 540 feet bgs. According to the driller's log, a sanitary seal was not installed. One soil boring was completed to 45 feet bgs near this well in March 1992, and only low concentrations of toluene and xylenes were identified at 10 feet bgs. A water sample from this well was analyzed using EPA Method 8021 and 8260B in August 1999, and all constituents were nondetect. The depth to water measured when the well was sampled was 39.85 feet bgs. Since this well is gravel packed from 35 feet to 540 feet, contaminants intercepting the gravel pack may disperse vertically into deeper water bearing units. A sensitive receptor survey was not completed for this site; however, County staff identified a second water supply well approximately 2,500 feet northwest of the site at 26116 Patterson Pass Road. The well is owned by Eugene Sparks, and the surface elevation is approximately 300 feet above MSL. The Sparks well is constructed with 6-inch casing to 193 feet bgs, screened from 153 to 193 feet bgs, gravel packed from 50 to 193 feet bgs, with a sanitary seal to 50 feet bgs. The depth to water after well completion was listed at 63 feet bgs. The California Aqueduct is approximately 1,200 feet northeast of the site. In a 29 February 2000 Department of Water Resources (DWR) letter, the DWR described a slope failure along the California Aqueduct in Merced County resulting in petroleum-contaminated groundwater from an upslope UST site entering the aqueduct. I contacted Mr. David Duval of the DWR Byron Field Division to determine if the petroleum release at the subject site could impact the aqueduct. Mr. Duval stated that streams