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F <br /> ASSOCIATES I N C . <br /> 4.0 SUBSURFACE CONDITIONS <br /> ' Soils encountered beneath the site consist primarily of sand and clay to a depth of approximately <br /> 11 feet below grade, the maximum depth explored. From the surface to approximately 10 bgs, <br /> ' soil consists of poorly graded sand. From 10 to 11 feet bgs, the maximum depth explored, soil <br /> consists of lean clay. <br /> Groundwater beneath the site was encountered at approximately 10 feet below grade. Based on <br /> the San Joaquin County Flood Control and Water District, Spring 1995 report, groundwater <br /> ' flows to the north. Groundwater depth and flow direction have likely been influenced by on-site <br /> dewatering efforts related to construction of the road extension. <br /> ' 5.0 RESULTS <br /> ' 5_1 Soil Contamination <br /> Results of laboratory analyses performed on selected soil samples indicate that the majority of <br /> petroleum hydrocarbons present in the soil beneath the site have been removed. Soil samples <br /> from the excavation sidewalls at 14 feet bgs and from the excavation bottom at 17 feet bgs <br /> contained nondetectable to low concentrations of TPHd (4.0 ppm) and xylenes (0.058 ppm). <br /> TPHg was detected in one sidewall sample but was reported as gas and an unidentified <br /> hydrocarbon. MTBE was also detected in two of the sidewall samples but its presence was not <br /> confirmed by EPA method 8260. Samples from perimeter borings SBI through S134 contained <br /> i' only very low concentrations of TPHd (1.0 to 2.7 ppm). None of the soil samples contained <br /> detectable concentrations of BTEX or TPHg. In addition, none of the soil samples contained <br /> ' detectable concentrations of oxygenate fuel additives. The extent of impacted soil is shown on <br /> Figure 3. <br /> Analytical results of soil samples are summarized in Table 1. <br /> ' <br /> 5_2 Groundwater Contamination <br /> Results of laboratory analyses performed on groundwater samples indicate that petroleum <br /> hydrocarbons are present in the groundwater beneath the site. Groundwater samples collected <br /> ' from the excavation contained elevated concentrations of TPHd. Xylenes were detected but at <br /> concentrations below the maximum contaminant level. TPHg was detected but was reported as <br /> an unidentified hydrocarbon. MTBE was also detected in one sample but its presence was not <br /> confirmed by EPA method 8260. Perimeter borings SB 1 through SB4 contained nondetectable to <br /> very low concentrations of TPHd (58 parts per billion). None of the groundwater samples <br /> wA82254%sunupt.doc 5 <br />