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Site Conceptual Model October 3, 2002 ARCO Facility No 6100 6 POTENTIAL EXPOSURE PATHWAYS AND SENSITIVE RECEPTORS An exposure pathway model for the site is presented in Figure 7 The primary potential source of exposure is due to spills and leaks related to current and historical gasoline station operations at the site Potential secondary sources for exposure include subsurface soils, free-phase liquid hydrocarbons, and dissolved petroleum hydrocarbons in perched groundwater Soils beneath the site are a potential source of exposure due to dermal contact and vapor inhalation for workers excavating in these areas 6.1 Onsite Receptors A water supply well is present in the southwestern portion of the site Water samples from this well have been collected and analyzed on a quarterly basis since the first quarter 2001 Petroleum hydrocarbons or fuel oxygenates have never been detected in any of the samples Given that the supply well is screened from 396 and 536, it appears unlikely that hydrocarbons from this site have impacted this well Further, if water from the gravel pack around this well is supplying water to Sand 1 or Sand 2, it is even less likely that this well could be impacted by . hydrocarbons However, since the full extent of impact at the site is not completely characterized, it seems prudent to continue sampling of the supply well Volatilization of petroleum hydrocarbons to outdoor air is unlikely to represent an exposure risk Construction workers involved in excavation in these areas could be exposed through dermal contact or inhalation of volatilized hydrocarbons Volatilization of hydrocarbons to indoor air is also unlikely unless the slab foundation beneath the store has been compromised There appears to be little risk of exposure to petroleumm hydrocarbon vapors beneath the site 6.2 Offsite Receptors 6 2 1 Water Supply Wells A well search conducted by Delta identified eight water supply wells located within a 0 5 mile radius of the site A table summarizing details associated with each well, a figure illustrating the locations of the wells, and DWR well completion reports for each well are presented in Appendix E Locations or usage of the wells identified in the DWR records search have not been verified in the field Two wells (Map ID #2 and #3) were installed northwest of the ARCO facility on property owned • by Mr Eugene Sparks Delta personnel searched the area northwest of the ARCO facility and were unable to locate these wells No signs of any water use (imgation, domestic, livestock) were observed on the property Given that no groundwater appears to be in use, and that the P 1Bp-Arco for URS161001Reports16100 SCM doc Page 12 Stratus