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t%-' S E C O R - 8.0 CONCLUSIONS This report demonstrates, using a standard RBCA Analysis, that the residual concentrations of petroleum hydrocarbons in the soils and groundwater at the ARCO 6080 Site do not pose a threat to human health or the environment. First, it was shown that the plume is stable with an annual decrease in contaminant concentration trend. Second, there are no current or anticipated uses of the impacted groundwater within the timeframe projected to meet water quality objectives. To be conservative, both residential and commercial/industrial RWQCB San Francisco Region's ESLs and USEPA Region 9 PRGs were used in the Tier 1 RBCA for comparison with soil and groundwater EPCs at the Site. Using the 95% UCL of 1999 historical soil concentrations, the results of the Tier 1 RBCA indicated that Tier 1 soil ESLs were exceeded for residential direct contact by xylenes (slight exceedance), 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 results of the Tier 2 RBCA took biodegradation into account and indicated that residual COPC concentrations in soil will not pose any unacceptable health risks to residential and industrial/commercial workers. California MCL, Tier 1 ESI. based on residential drinking water toxicity, and USEPA Region 9 residential drinking water PRG were exceeded (slight exceedance) by benzene. Tier 1 groundwater ESLs, based on indoor air impact levels, were not exceeded by any COPCs and there are no corresponding ESLs for DIPE. This Tier 1 RBCA comparison for groundwater is extremely conservative for the following reasons: • Groundwater is not a potential direct contact exposure route at the Site: o There are no current or anticipated uses of the impacted water within the timeframe projected to meet water quality objectives. o There are no existing water supply wells identified within a radius of 2,000 feet from the Site. o Chemical constituents are not migrating offsite (as such, estimation of the distance the residual plume will travel before water quality objectives are achieved is not needed). o On-site contamination represents a decreasing source strength due to biodegradation. Applying chemical-specific first order decay rate in the Tier 2 RSCA, benzene is expected to reach its drinking water MCL of 1 pg/L within 0.17 to 1.1 years. 18