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M ERM PROBLEM ASSESSMENT REPORTM FRIM REMEDIAL ACTION PLAN -4- Farmer Beacon Station No. 494, 2185 East Fremont Street Stockton, CA--AMV Project No. 19005.07 Table 3 contains the analytical results from this s sampling event. TPHd was nondetectable in all samples. TPHg ranged from nondetectable in monitoring well MW-1 to 3,600 micrograms per liter(µg/1 or ppb) in monitoring well MW-3. Benzene concentrations ranged from nondetectable in monitoring well MW-1 to 180 ppb in monitoring well MW-3. Appendix E contains copies of the certified analytical reports. ' 3.2 December 1992 Drilling ' .2.1 Drilling and Soil C nditions On December 14, 1992, AMV advanced six 10-inch-diameter soil borings to depths ranging ' from approximately 83 to 87.5 feet below grade. Borings MW-4 and MW-5 were completed as monitoring wells with 4-inch-diameter casings. Borings VW-1 through VW-4 were completed ' as vapor extraction well nests, each nest containing a pair of 2-inch vapor extraction wells designated "a" and "b" (e.g., VW-la and VW-1b). In each nest, the screened interval for the "a" well is about 20 feet above the screened interval for the "b" well. Figure 2 illustrates the locations of monitoring wells MW-4 and MW-5, and vapor extraction wells VW-la and VW-lb through VW-4a and VW-4b. ' As borings were advanced, AMV collected soil samples at vertical intervals of 5 feet. Appendix A describes methods used to collect soil samples. Soils encountered in the six borings advanced in December 1992 are similar to those described from borings advanced in April 1992 (Section 3.1.1). Appendix B contains the soil boring logs. The boring logs indicate that contacts between soil strata vary between sharp and gradational, and that soil units are not laterally continuous across the site. 3.2.2 Soil Sample Collection and Anal tical Results ' Based on PID readings and consideration of sample location with respect to possible hydrocarbon source, AMV selected 43 of the soil samples collected for laboratory analysis. The soil samples collected in December 1992 were analyzed for BTEX and TPHg. Table 1 includes the analytical results for the 43 soil boring samples p submitted omitted for laboratory analysis from the December 1992 drilling event. Only samples from vapor extraction well ' borings VW-1, VW-2, and VW-4 contained TPHg at concentrations exceeding 1 mg/kg. The highest TPHg result in VW-1 was 5,400 mg/kg in the sample from 30 feet, and in VW-2, the highest result was 410 mg/kg in the sample from 40 feet. For VW-4, the highest result was ' 250 mg/kg in the sample from 40 feet. Appendix F contains copies of the certified analytical reports. 1 rPtWSW