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technology for the impacted soil and groundwater at this site The results of the Enhanced • Bioremediation pilot test study indicated that both oxygen and nutrient augmentation are feasible treatment technologies for the impacted soil and groundwater at this site Based on the pilot test results, air sparge/soil vapor extraction was selected as the most effective treatment technology for this site A conceptual design of the treatment system, technology implementation workplan, and proposed monitoring and reporting guidelines are included in this CAP report 2.0 Assessment of the Impacts ' Results of the soil and groundwater sampling conducted during the assessment portion of the project indicate that the horizontal extent of the hydrocarbon impact from the UST release has been delineated The vertical extent of the hydrocarbon impact is the underlying groundwater aquifer 2.1 Contaminant Characteristics The soil stratigraphy across the site is depicted in Cross Section (Figure 3) Cross Section A-A' (Figure 4) extends from monitoring well MW-6 (north) to MW-9 (south) and includes wells screened in the shallow and deep aquifers Cross Section B-B' (Figure 5) extends from monitoring well MW-7 (southwest) to MW-1 (northeast) and includes wells screened in the shallow and deep aquifers In general, shallow subsurface soil at the site consists of interbedded units of fine-grained (silt) deposits and coarse-grained (sand) deposits Based on the soil boring logs, relatively fine-grained soil consisting of silt and clay was encountered from surface to approximately 60 feet bgs These fine-grained soils appeared to be underlain primarily by deposits consisting of clayey to silty sand and sand to a depth of approximately 116 feet bgs A stiff silt layer was encountered from about 116 to 118 feet bgs The silt layer was underlain by ' silty sand and sand Soil boring and well construction details are presented in Appendix A A Soil Boring Location Map presented as Figure 6 shows the location of soil borings installed at the site and summarizes the analytical results Concentrations of adsorbed-phase hydrocarbons detected in the soil were compared to the EPA Region 9 DRGs The soil samples collected from the south and southeast walls of the UST excavation at depths between 14 and 19 feet below I ground surface (bgs) exceeded the Industrial PRGs for benzene (1 5 mg/kg), toluene (520 mg/kg), and xylenes (210 mg/kg) The benzene PRG was also exceeded in borings SB-2 (46 feet bgs) and SB-13D (46 5 feet bgs) TPH-G concentrations detected in these soil samples ranged from 10 3 to 17,600 mg/kg Soil analytical data are summarized in Table 1 2.2 Hydrogeologic Characteristics IInterpretation of the monitoring well gauging data indicates that there is a south to southwesterly hydraulic gradient across the site (Figure 7) A review of the groundwater level data at the two cluster well points (MW-3, MW-3A, MW-3D and MW-13, MW-13A, N4W-13B) indicates that the shallow aquifer appears to be at least partially isolated from the deeper aquifer Water level data from monitoring well MW-13B, screened in the deep aquifer, shows a consistently lower IEGUL,q�o 'T ® U ACES+