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0 Field Sampling Plan for the Former UST Location at Stockton FMS #24 California Army National Guard than primary MCLs laterally in all directions from boreholes B30 and B32; except, have been defined to the south by boring B31, which had concentrations of xylenes less than MCLs. In addition, the groundwater contamination beneath the former diesel and gasoline UST location by not have entirely originated from the former USTs. Concentrations of TPHD, TPHG, and BTEX within soil beneath the former UST location above the groundwater plume are not indicative of the magnitude of contamination that has been detected in groundwater. Benzene was detected in soil at a maximum concentration of 0.12 mg/kg at 40 feet bgs in boring B32; however, benzene was detected in groundwater (approximately 45 feet bgs) at the same boring at a concentration of 24,000 µg/L. The magnitude of the difference between the two samples is 100 times within 5 feet of vertical distance. Several other contaminant concentrations also are similar; thus, additional contamination may also be attributing to the groundwater contaminant plume beneath the former UST location. To the north and adjacent to the former diesel and gasoline UST location is a wash pad and holding tank for the wash pad. No samples have been collected within the vicinity of the wash pad/holding tank. However, similar chemicals are also used and present at the wash pad and within the holding tank and could contribute to the groundwater contamination plume if a leak was present within the holding tank. 5.0 RECOMMENDATIONS Based on the information obtained for FMS #24, the following is recommended. • Drill five exploratory HSA borings and collect soil and groundwater samples to determine the extent of soil and groundwater contamination beneath the former diesel and gasoline UST location. Analyze soil and groundwater samples for TPHG and TPHD, VOCs (to include BTEX, and oxygenates), and metals. 6.0 FIELD SAMPLING PLAN 6.0.1 This FSP describes the strategy that will be used to characterize the levels of TPHD and TPHG, VOCs (including BTEX and oxygenates), and metal concentrations in soil and groundwater that may have originated from the former diesel and gasoline UST location. To characterize the extent of contamination beneath the former diesel and gasoline UST location, HSA drilling and soil and groundwater sampling is proposed within the vicinity. The proposed sampling locations for this investigation are shown on Figure 6-1. The sampling and analytical matrix is provided in Table 6-1. 6.0.2 General field, drilling, sampling and analysis, documentation, and data review procedures and strategies will follow guidelines presented in the CA ARNG Site Investigation Quality Assurance Project Plan (QAPP) (Radian International, 2000a). The QAPP also describes quality assurance activities to be performed, quality control samples to be collected, and procedures for equipment decontamination and disposal of contaminated materials. All health and safety issues are addressed in the CA ARNG Health and Safety Plan (Radian International, 2000b). This FSP follows the guidelines presented in the California Code of Regulations Title 23 (Sections 2649 and 2672). 6.0.3 Location of former diesel and gasoline USTs. Five HSA borings (FMS24SB01 through FMS24SB05) will be drilled and sampled to determine the extent of soil and groundwater contami- nation that is present beneath the former diesel and gasoline UST location. Borehole locations FMS24SBOI through FMS24SB05 will have soil and soil gas samples collected from 2 feet beneath the bottom of the fill material (if encountered) within native soil and be sampled down to ground- water at designated depths (15 to 25, 30, 35 to 40, first groundwater, and second groundwater 1 K \Wprocess\00251\CA ARNG\Stockton FSP\FINAUStockton FSP.doc 20 August 2006