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f sE C 0R i 7.0 TIER 2 RBCA RESULTS . F i The purposes of the Tier 2 RSCA are to use site-specific data, whenever available, to address'SL exceedances in the Tier 1 RBCA and to quantify complete or potentially complete exposure pathways not addressed by the ESLs. This Tier 2 RBCA was conducted using simple fate and transport models and USEPA or Cal-EPA exposure factors and toxicity values. r � 7.1 SOIL Because COPC concentrations in soils can be reduced with time through biodegradation, dissolution in water, and volatilization (collectively known as weathering), the current EPCs must be much lower than the historical measurements in 1999. . The reduction in the COPC concentrations due to weathering is i faster in the shallow, unsaturated soils than in the saturated zone due to the short pathway to surface through which vapors migrate and aerobic conditions that are conducive to biodegradation (Howard et al., 1991). To provide an estimate of the current maximum EPCs for BTEX in the soils, the maximum detected concentrations reported in historical studies (1999) can be used in first-order decay equations (USEPA, 1996). While these equations will more closely reflect the current site conditions, the predicted i EPCs will still be conservatively high as-first-order decay equations do not take into account dissolution'in water and volatilization. Current soil EPC was not estimated for TPHg due to the complexity of this mixture and there are no published'physical and chemical data that can be used to predict the biodegradation TPH as a mixture. Using the most conservative published half-lives for COPCs in soil from Howard et al. (1991) and the following First-order decay equation below(USEPA, 1996), the current EPCs of COPCs were calculated: Cr C. xe-dr (Eq. 1) Where:. C, = Current COPC concentration at time t; Co - Original measured concentration in 1999 as time=D; e = Natural exponential; A - First-order decay constant(days'), equals 0.6931t,12, where tVZ is half-life; t - Time elapsed from the original sampling. It should be noted that the parameters A-and t should have consistent units, which can be any unit of time (hours, days, months,1or years). The literature reported values of half-lives for COPCs in soil and i comparison of their estimated current EPCs to respective ESLs are presented in Table 4. The Tier 2 RBCA results indicated that all COPCs, including xylenes, in soil have biodegraded to levels much below their corresponding residential and industrial ESLs. Although current soil EPC was not 15