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S ECOR - .. -- - - 4.0 SITE CONCEPTUAL MODEL The goal of a SCM is to identify known and hypothetical exposure pathways relevant to an assessment of human health and environmental risks at a site (USEPA, 1992). This section consists of the following components(USEPA, 1989): • Current and future land uses; • Potentially exposed populations; • Water well surveys; and • Complete and potentially complete exposure pathways. 4.1 CURRENT AND FUTURE LAND USES The Site has been occupied by an active gasoline service station, and all immediate surrounding properties are utilized for commercial/industrial purposes. Based on the current land use, and the location of the Site (on a narrow strip of land surrounded by Interstate 5 on the west, E. Louise Avenue on the south, and S. Harlan Road on the west), it is expected that land use of the Site will remain commercial/industrial in the future. However, since residential homes are also found in the area (east of Harlan Road), hypothetical and conservative residential land use was also assumed in the Tier 1 RSCA, per the RWQCB's recommendations(RWQCB, 2005). 4.2 POTENTIALLY EXPOSED POPULATIONS Based on the current and likely future use of the Site and immediate surrounding land as commercial/industrial property, the on-site potentially exposed populations include commercial/industrial workers, customers, and utility/construction workers. Of these, the commerciallindustrial workers would spend the greatest time on-site and would therefore incur the greatest potential exposure. As such, on- site commercial/industrial workers were selected as potentially exposed populations of concern for the Site. In addition, utility/construction workers were evaluated in this RBCA due to the presence of shallow impacted soil at the Site, these utility/construction workers may be exposed to COPCs in soil via direct contact pathways. Off-site residents and commercial/industrial workers are not expected to be potentially exposed to the impacted water because groundwater monitoring data show that chemical contaminants are not migrating off-site and on-site contamination represents decreasing source strength due to biodegradation. As discussed above, within the recent four monitoring events from 6115106 to 3108107, DIPE was detected twice at MW-3 (13 NgIL on 12/07/06 and 10 NgIL on 3108107); MTBE was detected twice at MW-2 (0.69 Ng1L on 12107/06 and 0.77 NgIL on 3108107) and at MW-1 (0.5 NgIL on 12/07/06 and 0.76 NgIL on 3108107); and benzene was detected three times at MW-1 (1.6 NgIL on 9105106, 3.8 NgIL on 12107106, and 1.5 NgIL on 3108107). These levels were below(MTBE) or slightly above(benzene)their respective MCLs established by the State of California(13 NgIL for MTBE and 1 NgIL for benzene) (RWQCB,2003). 8