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�../ tis 02 May 2013 AGE-NC Project No. 04-1168 Page 9 of 10 adsorbed hydrocarbon plume extends toward the north approximately 13 feet, toward the east approximately 8 feet, toward the south approximately 3 feet and toward the west approximately 7 feet (Figure 5). • Utilizing Figures 4 and 5 to model the adsorbed TPH-g impact, the mass of TPH-g in soil was determined by separating the soil plume into two contoured zones (i.e. 100 mg/kg and 1 mg/kg). Based on mass-volume estimates, AGE calculated an approximate 26.9 pounds (lbs), or 4.3 gallons of TPH-g impact soil at the site (Appendix E). • Based on the most recent groundwater monitoring data collected at the site (December 2012), the dissolved TPH-g plume is located directly beneath the former underground storage area, in the location of well MW-1. The dissolved plume appears to radiate surrounding the wells location, but is currently defined to the northwest and west by wells MWA and MW-2. The plume is undefined to the north to the east and the south of the location of well MWA. However, low concentrations reported in all three wells suggests that the plume is laterally limited to the areas surrounding the well network.Vertically,groundwater samples have not been collected at depths deeper than the installed well network at 70 feet bsg (Tables 3; Figure 6). • The estimated volume of TPH-g impacted groundwater at the site is approximately 0.35 pounds, equivalent to 0.056 gallons. Maximum dissolved mass-volume calculations are included in Appendix E. • Based on all historical data and remediation performed at the site AGE concludes that petroleum hydrocarbons remaining beneath the site appear to pose limited risk to human health and beneficial use of groundwater down-gradient from the site. • A sensitive receptor survey previously performed at the site did not identified any drinking water wells in the immediate vicinity of the site. Domestic water is currently supplied to areas surrounding the site by the California Water Service Company and the residual groundwater plume related to site is not likely to threaten this water supply. 8.0. RECOMMENDATIONS Based upon data collected from the site, AGE recommends the following: • No further subsurface investigation at this time. The Arroyo's Smog Shop site has adequately assessed soil and groundwater associated with the UAR of gasoline. Advanced GeoEnvironmentat,Inc. v