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the truck-wash-rack filter unit), two locations near the southern boundary of the Site where the UST was located and where a diesel fuel spill occurred in 1995, and one location near a storm water drop inlet near the eastern boundary of the southern parcel, which appears to be one of the lowest areas of the Site. To evaluate vertical migration of VOCs, soil gas samples will also be collected at a depth of 15 feet bgs at locations SG-16 through SG-21. Field duplicate quality control samples will also be collected at three of these sample locations, as indicated on Table 1. Thus, a minimum of 30 soil gas samples will be collected and analyzed. An onsite mobile laboratory will perform the analysis for volatile organic compounds in accordance with the LARWQCB guidance,using modified EPA Method 8260B. Based on the initial results of the soil gas samples at 15 feet bgs, and in consultation with the DTSC, additional soil gas samples may be collected at 25 feet and/or at 35 feet bgs, or at additional locations at appropriate depths. Soil gas samples will be analyzed for target VOCs specified in Section 3.1 of the LARWQCB guidance using modified EPA method 8260B. The soil gas samples will also be analyzed for methyl tertiary butyl ether (MTBE), di- isopropyl ether (DIPE), ethyl tertiary butyl ether (ETBE), tert-amyl methyl ether (TAME), tertiary butyl alcohol (TBA), and ethanol. The detection limit for benzene and vinyl chloride will be no greater than 0.5 ug/L and the detection limit for the other target VOCs will be no greater than 1 ug/L. If no VOCs are detected in the initial soil gas samples, additional samples (at a rate of no less than 10 percent of the total number of previously collected samples)will be collected from the specific areas of concern and analyzed for benzene and vinyl chloride with a detection limit of 0.1 ug/L or less. All soil gas samples will also be tested for methane using an appropriate hand-held instrument (such as a Landtech GA-90) having a detection limit of no greater than 500 ppm,. In order to obtain lithologic information for the Site and to check that the planned intervals for the soil vapor probes will be set in soil structures with adequate transport mechanisms, a continuously-cored pilot soil boring will be completed to 15 feet bgs at the first soil gas location (SG-16). When completing the remaining soil vapor probes, the soil lithology will be similarly reviewed at each planned sample interval. Adjustments to sample intervals warranted by this review of lithology will be presented to the DTSC Project Manager during field operations. Although not currently anticipated, should encountered site soils consist primarily of tight formations such as clay or silty clay, URS will make a recommendation to DTSC regarding whether soil gas sampling should proceed or whether the field crew should instead collect soil matrix samples for VOC analysis using the USEPA-approved method 5035 preparation technique. URS would proceed as directed by DTSC. After collection and preservation by method 5035, analysis for VOCs by USEPA method 8260B would be performed. In addition to visual identification of soil type,one check that will be used to indicate the need for soil matrix sampling is when soil gas vacuum readings exceed approximately 5 inches of mercury. The soil gas survey will not be conducted during rain events or within seven days after significant rainfall. The soil gas survey protocol is presented in Section 4.2.4. Quality control samples including field blanks and duplicates will be analyzed by the onsite laboratory during the soil gas survey. The mobile laboratory will maintain records and perform required quality control and operational procedures in accordance with the LARWQCB guidance and applicable USEPA methods. Please refer to the next section regarding soil physical parameters to be analyzed that will support fate and transport modeling of volatile emissions at the Site. Page 14 PAPEA Lockeford\Work PlanVL keford WP.doc