SITE INFORMATION AND CORRESPONDENCE

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. A Ms.Lori Duncan November 15,2011 RE: 2911 E.Fremont St,Stockton,CA Page 12 of 14 Two other exposure parameters,body weight of 70 kg and breathing rate of 20 cubic meters per day,are intrinsic to the Unit Risk Factor (URF) and Reference Concentration in Air (RfC) specific to each contaminant. The URF and RfC values were developed by Cal/EPA and USEPA. For calculating inhalation risk,the URF is equivalent to the inhalation unit risk(NR). The equations for calculating site specific cancer risk and hazard quotient, respectively are: Risk = C9ndoor.lair x ET x EF x ED x IUR AT x 365 days/year x 24 hours/day Hazard Quotient = Cindoor air x ET x EF x ED AT x 365 days/year x 24 hours/day x RfC Table 5 summarizes concentrations of detected contaminants in air and the calculated hazard quotient/risk for the same constituents. The cumulative hazard quotients of the air samples collected at the site ranged from 3.97E-02(ambient air AA2)to 1.84E-01 (indoor air IAl),indicating insignificant health risk for all samples. A cumulative hazard index of<1 is generally considered insignificant risk to health. The sum of the incremental cancer risk from inhalation of ambient air ranged from 1.11 E-06 to 1.75E-06,while the sum of the incremental cancer risk from inhalation of indoor air ranged from 4.01 E-06 to 4.58E-06,all of which are slightly above the conservative target risk of 1.0E-06. The acceptable range recommended by the US EPA is 1.0E-04 to 1.0E-06. Remediation is generally not warranted at sites where the estimated cancer risk is less than 1.0E-06;remediation is generally warranted at sites where the estimated cancer risk exceeds 1.0E-04; and the need for remediation is generally evaluated on a site specific basis for sites where the estimated cancer risk is between 1.0E-04 and 1.0E-06. In other words, risks within this range are potentially acceptable, depending upon site specific considerations. CONCLUSIONS AND RECOMMENDATIONS Based upon the concentrations of chemicals in soil samples collected within and below the area of excavation, an estimated mass of between approximately 280 and 380 pounds of gasoline (TPHg) was present in soil prior to excavation,and approximately 125 to 190 pounds of residual gasoline remains in the subsurface after excavation activities. Based upon these estimates, excavation activities resulted in the removal of between 49%and 55%of the soil contamination. The configuration of the site(inside the building)prevented further excavation at the site. Direct exposure to remaining soil contamination is not an exposure pathway at the site because residual contamination in soil exists at depths greater than 15 feet and are overlain by concrete. In addition,groundwater sampling at the site detected only very low levels of PCE, but no residual gasoline consitituents, which suggests that native clay and silt zones present between the residual contamination and the known water table at the site act as an effective barrier to migration of contaminants downward to the water table. 10759-EXC SUMMARY LTR