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11-_� S ECOR � - - - -- - NLs are health-based advisory levels for chemicals without available MCLs (Cal-EPA, 2006b). Derivation of NLs is similar to that of PHGs(Cal-EPA, 2006a). Recent groundwater monitoring results at the Site (last three quarters of 2006 and first quarter of 2007) have shown that only benzene was recently detected at a level slightly above its primary MCL of 1 pg/L at MW-1 (see Table C-3 of Appendix C). It should be noted that TPHg has not been detected in any onsite wells for nearly 3 years. Detections of low levels of benzene, MTBE, and DIPE in groundwater represent only random occurrences and should not be of concern. According to the RWQCB (RWQCB, 2004a), for groundwater that exceeds water quality objectives, as in the case of benzene at this Site, a NFAR can be obtained based on the following information: • Demonstration that the plume is stable with either an overall annual decrease in size or an annual decrease in contaminant concentration trend. • Calculations or modeling results, including monitoring verification of model conclusions, which show when water quality objectives are predicted to be achieved. • Verification that there are no current or anticipated uses of the impaired water within 'the timeframe projected to meet water quality objectives. (Institutional controls may be needed to prevent such use if this period is not sufficiently short.) As discussed above, there was a significant decrease in chemical concentration trend in groundwater, and possibly in soil, at the Site that can address the reporting requirement stated in bullet item No. 1 above. For example, TPHg and MTBE concentrations in onsite wells have reached their highest concentrations in 2001-2002, but have steadily decreased to ND levels since 2002. The rest of this RBCA will present all the relevant information regarding the reporting requirements stated in bullet items No. 2 and 3 above. 3.2 RBCA METHODOLOGY RBCAs are typically performed to provide a consistent decision-making process for the assessment and response to small petroleum-contaminated sites, based on protection of human health and the environment. The RBCA process is implemented in a tiered approach where upon completion of each tier, the results are evaluated and, if warranted, conservative default assumptions of the earlier tier are replaced with site-specific data and the analysis proceeds to the next tier(ASTM, 2002). In the Tier 1 analysis, concentrations of the detected analytes are compared to the applicable federal, state, or local water quality objectives and SLs for the impacted media (e.g., soil, groundwater, soil gas, air)to determine if site conditions satisfy the criteria for a quick regulatory closure or warrant a more site- specific evaluation. While MCLS and PHGs described above are evaluated for potentially potable groundwater, other SLs used in the Tier 1 RBCA may include other health-based levels such as ASTM's risk-based screening levels (RBSLs) (ASTM, 2002), Cal-EPA OEHHA California human health screening levels (CHHSLs) (Cal-EPA, 2005b), RWQCB San Francisco Region's environmental screening levels 6