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6A TIER 1 RBCA RESULTS Consistent with the methodology of a Tier 1 RBCA, EPCs of chemicals detected in onsite soil and groundwater were compared to their appropriate SLs (RWQCB, 2005; USEPA, 2004a). The comparison results are presented in Table 3. 6.1 SOIL The residual 0-10 feet bgs soil EPCs(95% UCL)were conservatively compared to Tier 1 soil ESLs based on direct contact, indoor air impact, soil leaching/groundwater protection, and ceiling values (odor, taste, etc.) for the current and future land use evaluation. Although the receptors of concern at the Site are commercial/industrial workers and utility/construction workers, residential ESLs were also presented in Table 3 for reference purposes (RWQCB, 2005). Tier 1 soil ESLs were exceeded for residential direct contact by xylenes (slight exceedance, such as 95% UCL of 289.9 mg/kg versus USEPA Region 9 PRG of 270 mg/kg), for residential and commercial/industrial direct contact by TPHg, for soil leaching/water protection by all COPCs, and for ceiling value by TPHg. It should be noted that the use of historical 1999 soil concentrations is very conservative. It is likely that COPC concentrations in soil have decreased significantly — even to ND levels at this moment in time. This is substantiated by the fact that TPHg was not detected in onsite groundwater since 2001. The current COPC concentrations in soil can also be estimated in the Tier 2 RBCA, using first-order decay equation. It should be noted that the USEPA soil vapor guidance indicates that soil VOC data are unreliable for assessing exposures via the indoor vapor intrusion pathway(USEPA, 2002). On Page 29 of the USEPA 2002 document "Draft Guidance for Evaluating the Vapor Intrusion to Indoor Air Pathway from Groundwater and Soils"for RCRA and CERCLA sites, the USEPA states: "Soil (as opposed to soil gas) sampling and analysis is not currently recommended for assessing whether or not the vapor intrusion pathway is complete. This is because of the large uncertainties associated with measuring concentrations of VOCs introduced during soil sampling, preservation, and chemical analysis, as well as the uncertainties associated with soil partitioning calculations." Since there are adequate groundwater data for the Site and groundwater at the Site is already impacted with COPCs, groundwater, not soil, data should be used for assessing all relevant exposure pathways (e.g., indoor air or groundwater ingestion). It is reasonable to assume that chemicals leached from surface and subsurface soils tend to reach an equilibrium state within the soil-water compartments if given sufficient residence or contact time (Bonazountas and Kailidromitou, 1993; Knox et al, 1993). As such, if SLs for groundwater are not exceeded for the groundwater ingestion pathway in this Tier 1 RBCA (even though SLs for soil based on soil leaching/groundwater protection are exceeded) it is concluded that this exposure pathway will not result in a significant risk or hazard to onsite receptors. 13