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4.0 RISK ASSESSMENT To identify known and hypothetical exposure pathways relevant to an assessment of human health and environmental risks at a site, a conceptual site model (CSM) has been developed. The original USTs, which were the primary source of the hydrocarbon impact and therefore the primary potential source of potential exposure to hazardous chemical, have been replaced. Secondary sources of potential exposure exist and include residual concentrations of TPHd, BTEX or MTBE in the soil and dissolved-phase MTBE in groundwater. The following sections identify site-specific potentially exposed populations as well as evaluate the presence of complete and/or potentially complete exposure pathways for these populations. Based on current land use, zoning, and the location of the Site near Interstate 5, it is expected that the Site itself and the adjacent Storage Pro facility (15550 South Harlan Road) will be used for commercial purposes in the future. Residential areas are located in close proximity to the Site. The on-site potentially exposed populations include commercial workers, customers, and construction workers at truck stop. Of these, commercial workers spend the greatest amount of time on-site and would therefore incur the greatest potential exposure to hydrocarbon vapors, if any. It appears that although soil impact is very minimal and limited in extent, MTBE- impacted groundwater impact has spread north and northeast beneath the adjacent commercial properties and formerly beneath residential properties along Patricia Place and Lottie Way. Potentially exposed off-site receptors would include residents possibly exposed to vapors. Typically, potentially exposed sensitive populations at a site may also include off-site school age children, medically compromised people, and the elderly. There does not appear to be any schools, daycares, hospitals, convalescent centers, senior centers, or skilled nursing facilities within 1,000 feet of the site. Based on the minimal nature and location of DRO, BTEX, and MTBE-impacted soil (see section 3.3.1) and the distribution and extent of MTBE-impacted groundwater beneath the Site and site vicinity (see section 3.3.2), the current and future planned commercial use of the property, and the lack of nearby water supply wells or sensitive human populations (see section 2.3), the only exposure pathway that appears complete or potentially complete is inhalation of vapors emanating from impacted soil and/of groundwater. Those receptors to this potential inhalation risk are on-site commercial workers or customers at the Joe's Truck property or the adjacent Storage-Pro Facility. In August 2009, a soil gas survey was conducted to evaluate this pathway. The objective of this soil gas investigation was to evaluate TPHg, TPHd, BTEX, and MTBE concentrations in shallow soil vapor, if any, and to compare these results with environmental screening levels (ESLs) to determine the need for a Tier 2 human health risk assessment (HHRH). To accomplish these objectives, Stratus advanced and sampled eight temporary soil gas sampling points (V-1 through V-8; Figure 213). The locations of the eight soil gas sampling points were selected to evaluate if potential inhalation risks to workers at the site property and to occupants of surrounding commercial and residential properties in areas overlying or near known impacted groundwater are present. The surrounding properties that were considered were the (commercial) Storage Pro ( 1 facility and residential properties adjoining the site property to the east along Lisa Lane/Patricia J Page 16 5TKATU,5