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Following the ASTM RBCA approach, the RWQCB also recommended a three-tiered approach for an environmental risk assessment (ERA) (the RWQCB uses the term ERA instead of RBCA, although this report uses RBCA throughout). Under Tier 1, sample data are directly compared to ESLs selected for the Site; and decisions are made regarding the need for additional site investigation, remediation, or a more detailed, site-specific risk assessment. Tier 1 ESLs are not required regulatory cleanup standards (RWQCB, 2003). Under Tier 2, selected model components used to develop the Tier 1 ESLs are modified to reflect site-specific conditions. Under Tier 3, alternative models and modeling assumptions can be used to estimate site-specific potential risks to human health and the environment. In general, risks to human health and the environment can be considered insignificant at sites where concentrations of chemicals of potential concern (COPCs) do not exceed their respective ESLs. The presence of chemicals at concentrations above the ESLs does not necessarily indicate that significant risks exist at a site. It generally indicates that additional investigation or a site-specific evaluation is warranted (RWQCB, 2003). To select the appropriate Tier 1 ESL lookup tables, the designated beneficial use of impacted or threatened groundwater beneath the Site and the depth below ground surface to the top of impacted soil need to be determined. Unconsolidated geologic units that are �-• comprised of less than 20% sand-size (or larger) materials or more than 30% clay-size materials are typically not considered to be viable aquifers or potential sources of useable drinking water (RWQCB, 2003). The RWQCB also states that groundwater is not considered to be a potential source of drinking water if: 1) total dissolved solids (TDS) level in groundwater is greater than or equal to 3,000 milligrams per liter (mg/1); OR 2) the water bearing unit is not sufficiently permeable to produce an average sustained yield of 200 gallons of water per day (RWQCB, 2003). V For evaluation of commercial/industrial properties, the RWQCB highly recommended that site data be compared to ESLs for both unrestricted/residential and commercial/industrial land uses. For sites where sample data are limited, the RWQCB recommended that maximum detected concentrations be compared to ESLs. For sites where an adequate number of data points are available, statistical methods can be used to estimate the 95% upper confidence limit of the arithmetic mean as the exposure point concentration (EPC). It was also recommended that sample data collected outside the impacted areas should generally not be included in the estimation of EPCs. Complete and Potentially Complete Exposure Pathways According to Cal-EPA and USEPA, an exposure pathway is considered complete or potentially complete if it meets four basic requirements: (1) presence of chemical sources, (2) release and transport within an environmental medium, (3) an exposure point for contact, and (4) an exposure route. Based on the location of the impact soil and groundwater beneath the site and the �• information provided by the City regarding the PWS wells in the vicinity of the site, the following exposure pathways are complete and potentially complete for the onsite commercial/industrial receptors (and construction workers): V • Inhalation of indoor or outdoor vapors from the impacted soil. The soil vapors could migrate vertically and laterally through permeable zones in the soil and along man- Workplan to Conduct Subsurface Soil Vapor Intrusion Study \\b// ATC Project 54.26874.0001 V ` Page 9