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n CNITERRA RRA VAC <br /> As stated previously, the site is now a vacant lot. The extent of future commercial <br /> development at the site is unknown but is expected to be limited to light commercial or <br /> retail enterprises. Any building is expected to have a slab on grade foundation. Any <br /> j structure constructed at this site is not expected to have a subgrade basement. As a <br /> result, default GSI/RBCA `enclosed space' values were used to evaluate the potential <br /> I inhalation pathway. The default values are a conservative representation of the potential <br /> health risks associated with the inhalation pathway. <br /> The site is presently is located in a mixed residential and light commercial area. The <br /> extent of future development on the adjacent properties is expected to continue as mixed <br /> residential and light commercial or retail enterprises. <br /> 6.1 Tier One Model Evaluation Results <br /> GSI/RBCA Tier One model results show that benzene has a representative concentration in <br /> soil of 0.11 mg/kg (calculated at a 95%Q upper confidence limit) and a calculated RBSL of <br /> 0.02 mg/kg. The benzene representative concentration is 6 times greater than the RBSL. <br /> Representative soil contaminant concentrations for ethylbenzene, toluene, and total xylene <br /> are all less than their RBSLs. Terra Vac did not use MtBE data in the Tier One model as <br /> MtBE was not detected in the vadose zone soils. This lack of data is shown on the model <br /> data sheets as "#DIV/O!". <br /> GSI/RBCA Tier One model results also show that benzene in groundwater has a <br /> representative concentration of 6.7 mg/L (calculated at a 95% upper confidence limit) and <br /> a calculated RBSL of 0.054 mg/L. The benzene representative concentration is 120 times <br /> greater than the RBSL. Representative groundwater contaminant concentrations for <br /> ethylbenzene, toluene, total xylene, and MtBE are all less than their respective RBSLs. <br /> Per the GSI/RBCA model, onsite commercial inhalation of volatilized benzene from soil <br /> and groundwater to indoor air from sources beneath the site represent the highest potential <br /> risk to human health. <br /> To summarize, ethylbenzene, toluene, and total xylene in soil and groundwater passed <br /> the Tier One evaluation; benzene did not. MtBE in soil was not considered as none was <br /> detected in vadose zone soils. MtBE in groundwater passed the Tier Two evaluation. <br /> Appendix F presents the GSIIRBCA Tier One model results. <br /> 7. GSI/RBCA TIER TWO MODEL EVALUATION <br /> Terra Vac applied the GSIIRBCA Tier Two evaluation using site parameters and <br /> hydrocarbon contaminant concentration data. Terra Vac developed a site specific Tier Two <br /> risk model based upon future onsite commercial usage and continued off site residential <br /> usage. Where site specific data were not available, Terra Vac used default GSIIRBCA <br /> values. Table 6 presents site specific data used in the Tier Two evaluation. <br /> The preprogrammed settings in the GSI/RBCA model were modified to include <br /> evaluations of the future building to be located above the hydrocarbon plume in addition <br /> to site specific plume and subsurface characteristics'. <br /> Project 3 0-02 12 9 c:lalton130-02121caplrev5lcap-rev5.doc <br /> July 15, 1998 <br />