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i 2.3 Previous Investigations V &W Drilling, Lodi, California, mobilized a truck-mountcd hollow-stem auger drilling rig to the site on August 24, 1999 and drilled four soil borings under the supervision of Upgradient. The first boring(B1 in Figure 6) was drilled near the southwestern corner of the former UST excavation, and was sampled at 5400t intervals starting at a depth of 15 feet(close to the base of the former UST excavation). Upgradient reported moist soil at a depth of 40 feet bgs, suggesting that the capillary fringe had been reached. However, no groundwater reportedly entered the soil boring and the soil boring was terminated at this depth. V &W drilled four more borings (Figure 6.). Soil samples were collected at 5-foot intervals, 4 beginning at either 5 or 10 feet. These borings were terminated at 30 feet bgs and were plugged L with cement-bentonite grout. A geologist from Upgradient examined the samples and prepared boring logs. These logs are presented in Appendix 4. Subsurface Investigative Results The site is underlain at,shallow depths by beds of silt and fine- grained sand that vary in thickness and horizontal extent. The uppermost unit is a silt bed whose color and thickness are variable. This bed is missing in the former UST pit where it was likely removed during the over excavation (Upgradient, 1999). East of the excavation, the silt bed overlies a relatively thin sand bed that is fine grained to silty. Below the sand is a thick layer of relatively consolidated silt that contains leaf impressions, root holes, and other evidence of ongoing soil profile formation. Below the silt bed is another sand bed that coarsens downward but its base was not reached during the subsurface investigation. The lithologic contact between the silt bed and this lower sand 'bed varies from a depth of 24 feet bgs in soil boring B 1 to j l approximately 28 feet bgs in soil borings B2 and B5. This lithologic contact was reportedly not encountered in soil boring B4 (Upgradient, 1999). Strong to moderate petroleum hydrocarbon odors were detected in the upper silt bed-in soil boring B I. A slight odor was noted in the uppermost sand sample (25 feet bgs) from.this same Env ironmentaRSTETARSCRO 9102008.wpd 7 r