SITE INFORMATION AND CORRESPONDENCE

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GEOMATRIX human health under specific exposure scenarios. Region 9 PBGs for residential and industrial land use are based on simultaneous exposure via three routes: ingestion, dermal contact, and inhalation of chemicals in soil. Because the site is Iikely to be industrial in the future, the industrial PRGs represent the most appropriate criteria. The exposure assumptions and methodologies incorporated into the development of industrial PRGs are consistent with assumptions that would be used in a site-specific risk assessment. The Region 9 PRGs are based on toxicity criteria published by the U.S. EPA, The California Environmental Protection Agency (Cal-EPA) independently developed toxicity criteria for certain chemicals. Therefore, for this assessment, Region 9 PRGs were recalculated using the Region 9 methodology and Cal-EPA toxicity criteria to generate so-called"Cal-Modified PRGs." Gal-Modified PRGs were calculated for benzene, benzo(a)pyrene, and dibenz(a,h)anthracene because they all have Cal-EPA cancer slope factors that differ from the U.S. EPA values. Additionally, Cal-Modified PBGs were calculated for five other PAHs (benzo(a)anthracene,benzo(b)fluoranthene,benzo(k)fluoranthene, chyrsene, and indeno(1,2,3-cd)pyrene)because the benzo(a)pyrene slope factor is used to derive their PRGs. Table 6 presents a comparison of soil analytical results to industrial PBGs. The maximum detected concentration and maximum sample quantitation limit (SQL), or method detection limit, are presented for soil from 0 to 15 feet bgs; because 15 feet bgs represents the maximum Probable depth to which an industrial and/or construction worker might be exposed to soil. As indicated in Tables 3 and 4, only five petroleum constituents have been detected in soil: xylenes, chrysene, fluorene, phenanthrene, and pyrene. It should be noted that xylenes were the only constituents detected at depths less than 10 feet bgs. All five of these chemicals were detected at maximum concentrations 200 to 40,000 times lower than their respective PBGs. Therefore, simultaneous exposure to these chemicals in soil between the depths of 0 and 15 feet bgs is unlikely to pose a unacceptable risk to human health under an industrial scenario. For two chemicals,benzo(a)pyrene and dibenz(a,h)anthracene,the maximum SQLs exceed industrial PRGs. As shown in Table 7,the SQL exceeded the PRG for benzo(a)pyrene in 18 of 24 samples from 0 to 15 feet bgs. However, in 17 of those 18 samples the SQL exceeded the industrial PRG by a factor of only 1.5 (SQL of 0.33 mg/kg compared with PRG of 0.22 mg/kg), and in one by a factor of 3 (SQL of 0.66 mg/kg). The SQL was less than the industrial PRG for dibenz(a,h)anthracene in 23 of the 24 samples; the SQL only slightly exceeded the industrial PRG for dibenz(a,h)anthracene in sample SB-8 at 15 feet bgs(SQL of 4.66 mg/kg compared to \\st3Wgtaata\Mc_safe\50000O ns1 HRA Rptdac 11