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5.0 POTENTIAL EXPOSURE PATHWAYS AND SENSITIVE RECEPTORS An exposure pathway model for the site is presented in Figure 10 Based on the analytical data collected to date, the chemicals of concern (COC's) at this site include TPHG, TPHD, benzene, and MTBE The primary source of exposure to COC's appears to be spills and leaks related to current and historical gasoline station operations at the site Potential secondary sources for exposure to COC's appear to include residual hydrocarbons in subsurface soils, and dissolved hydrocarbons in the groundwater Non-aqueous phase hydrocarbons, impacted surface soils, or impacted surface water are not potential sources of exposure at this site Potential exposure pathways associated with these secondary sources include volatilization to indoor and outdoor air, and the potential for dermal contact and ingestion of impacted soil or groundwater related to subsurface construction activities at the site 51 On-site Receptors • The primary source of human exposure to COC's at the site (leaks from the fuel storage and delivery system) appears to have been removed when the USTs were replaced Potential exposure routes due to the secondary sources of petroleum hydrocarbons at the site (impacted subsurface soil and groundwater beneath the site) appears limited to volatilization to indoor or outdoor air and exposure to construction workers The ground surface in the area of the identified residual hydrocarbons is paved with concrete and asphalt These paving materials are not impervious to soil vapors, but the volatilization of petroleum hydrocarbon vapors to outdoor air is unlikely to represent an exposure risk This is especially true when considering that there are not really any restrictions to airflow constructed at the site that might allow impacted soil vapors to concentrate Normal surface air movement (wind, etc ) is likely to dilute and remove impacted soil vapors from the site before concentrations reach risk levels The primary COC present in the dissolved phase beneath the study site is MTBE Other, more volatile COC's such as TPHG or benzene do not appear to be present in appreciable amounts in the groundwater Previous assessment activities have suggested that dissolved MTBE has traveled under the station building on-site, as well as under several buildings associated to the self storage facility north of the site Due to the hydrophilic nature of MTBE, however, it is unlikely that significant quantities of dissolved MTBE would volatilize Even if the building slab is cracked or compromised, it appears unlikely MTBE volatilization from groundwater poses a significant risk P Voe's Truck Plaza\Reports\SCM_09-20031Joes truck plaza SCM doe Page 12 SMAWS