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TERRA VAC The Uniform Building Code Section 1205 (c) states, "In lieu of required exterior openings for natural ventilation, a mechanical ventilation system may be provided. Such a system shall be capable of providing two air changes per hour...... Terra Vac modified the air exchange rate from the preprogrammed twenty air exchanges per day (0.00023 L/sec) to two air exchanges per hour (0.00056 L/sec). The California Environmental Protection Agency's (CAL-EPA) toxicity value for benzene is more conservative than the default value, used by GSI/ASTM. To reflect this, the oral and inhalation slope factors (SF) were changed from the Federal slope factor of 0.029 to California State slope factor of 0.1 in the GSI/RBCA chemical database. The Tier Two model applies acceptable risk levels (1X10-5) and site specific data and back calculates corresponding SSTLs. The representative concentrations are compared to the SSTLs for the completed exposure pathway predicated for commercial land use. The SSTL value represents an allowable concentration level of benzene at the source area that may remain and not present a health risk to a human population. 7.1 Tier Two Model Evaluation Results Terra Vac considered onsite commercial usage with continued off site residential usage adjacent to the site. 7.1.1 Volatilization from Vadose Zone Soils GSI/RBCA Tier Two model results show that the volatilization of benzene in soil to indoor air poses a threat to human health. Modeling results show that the benzene representative concentration in soil is 0.11 mg/kg compared to a SSTL of 0.02 mg/kg. The benzene representative concentration is 6 times greater than the SSTL. The representative concentration of all other hydrocarbon contaminant constituents are below the calculated SSTLs for on site commercial property usage. The pathway for exposure to volatilized benzene to indoor air, from vadose zone soil, does appear to pose a risk to human health at its current level. 7.1.2 Volatilization from Groundwater Tier Two modeling results also show that volatilization of benzene from groundwater to indoor air is a contaminant of concern with a calculated 95% upper confidence limit representative concentration in groundwater of 6.7 mg/L and a SSTL of 0.054 mg/L. The representative concentration is 120 times greater than the SSTL. Representative groundwater contaminant concentrations for ethylbenzene, toluene, and total xylene are all less than their SSTLs. The pathway for exposure to volatilized benzene to indoor air, from groundwater, does appear to pose a risk to human health at its current level. 8. DISCUSSION The purpose of this section is to discuss correction action alternatives to minimize health risks associated with hydrocarbon impacted soil and groundwater. Site specific cleanup levels for constituent hydrocarbons (in soil and groundwater) will be established to Project 30-0212 10 c:lalton]30-02121caplrev5lcap-rev5.doc July 15, 1998