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02 May 2013 AGE-NC Project No. 04-1168 Page 8 of 10 The estimated extent of dissolved TPH-g is depicted in Figure 6, and is representative of current groundwater conditions at the site. Based on the data, the dissolved extent can be separated into two contoured zones of 1,000 pg/I and 100 pg/I (i.e near non detect). The volume of each contour zones can be determined by utilizing the formula for an elliptical cylinder. Based on the estimated volumes of the above two contoured zone,the estimated volume of TPH-g impacted groundwater is approximately 0.35 pounds,equivalent to 0.056 gallons. Maximum dissolved mass-volume calculations are included in Appendix E. /6.0. RISK ASSESSMENT - SOIL VAPOR INTRUSION EVALUATION A soil vapor intrusion investigation has not been performed at the site. AGE believes that residual soil contamination left in p ace will not pose a significant threat to human health or to the environment based on the following: • Petroleum hydr=(Bl), soi wasencountered on ow the former USTs at depth 2 (B2,MW-1) and 45 feet bsg (B2). nzene was not detected in acted during the 2004 and 2005 ' estigations. • Although soil samples have not been laboratory analyzed from five and ten feet bsg, field samples at borings B1 and MW-1 had no observable hydrocarbon odor or positive photoionization detector (PID) readings (Table 1, Appendix C). • Based on the location and depths of residual impact hydrocarbon impact and the lateral distance from the site building (18 feet), it is not likely that vapor emitted from the plume would impact the on-site building (Figure 5). 7.0. DISCUSSION AND CONCLUSIONS Based on data collected during site assessment and groundwater monitoring events at the Arroyo's Smog Shop site, AGE concludes: • An unauthorized release of petroleum hydrocarbons has impacted soil beneath the formerly installed underground storage tanks. Once released to the subsurface,the petroleum hydrocarbon contaminants migrated vertically into the underlying silt and clay units to depths between 15 feet (bottom of former USTs) and 45 feet bsg. • The site is generally composed of alternating layers of silt and clay with intermittent small lenses of silty sand ( Appendix C and Figure 3). • The greatest adsorbed hydrocarbon concentrations were encountered within the former tank pit area near borings B1 and MW-1. Utilizing B1 as a mid-point, the Advanced GeoEnvironmental,Inc.