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ASPECT CONSULTING <br />82 FINAL PROJECT NO. AS190583A-08  MAY 21, 2025 <br />Table 6-15d uses the highest detected indoor air concentration of each constituent of the <br />TPH mixture to calculate the total TPH concentration and site-specific TPH cleanup levels <br />for commercial and unrestricted use. <br />Because of the prevalence of hydrocarbon concentrations in urban air, Section 4.7 of <br />Ecology’s (2022) VI guidance recommends that measured indoor air concentrations be <br />corrected for contribution from area background sources, when site-specific concurrent <br />ambient air measurements are available as part of a Tier 2 VI assessment. Hydrocarbons <br />from vehicle exhaust and other sources are ubiquitous in urban air, and the concentrations <br />detected in the indoor air samples are comparable to those detected in samples of ambient <br />outdoor air collected away from the warehouse (outdoor air sample locations <br />BACK-AA-1 and BACK-AA2 shown on Figure 6-A1). Tables 6-15a, 6-15b, and 6-15c <br />present the average measured indoor air and ambient air concentrations31 for each <br />constituent, and their calculated differences (indoor minus ambient). The differences are <br />very small, and for some constituents are negative, indicating that the VI contribution to <br />indoor air concentrations within the warehouse is likewise very small. <br />For assessing risk from petroleum mixtures, Ecology guidance requires evaluation of the <br />additive effects of non-carcinogenic petroleum fractions and VOCs, individual <br />carcinogenic compounds, and cumulative cancer risk. Each of these requirements is <br />discussed below (Ecology, 2022). <br />Ecology’s guidance (Ecology, 2022) establishes a generic Method B (unrestricted land <br />use) air cleanup level for TPH based on noncarcinogenic effects, as well as an adjusted air <br />cleanup level for Method C (industrial land use), but also allows for the calculation of a <br />Site-specific total TPH cleanup level using Site-specific data. The indoor air data was used <br />to calculate a total TPH concentration (summation of all constituents) using the highest <br />concentration of each compound detected in any indoor air sample. The total TPH <br />concentration for the warehouse, calculated using the maximum detected concentration of <br />each compound from the existing indoor air data, is 106 µg/m3 (Table 6-15d). The same <br />data were used to calculate Site-specific cleanup levels for unrestricted and commercial <br />uses. The Site-specific TPH air cleanup levels are 110 µg/m3 to 938 µg/m3 for unrestricted <br />and commercial uses, respectively (Table 6-15d). The maximum detected indoor air total <br />TPH concentration is below the Site-specific TPH cleanup level for both potential use <br />scenarios, indicating that the additive effects of non-carcinogenic TPH in indoor air do not <br />exceed the MTCA hazard index threshold of 1, and do not pose an unacceptable risk to <br />human health (Table 6-15d). <br />Carcinogenic TPH compounds that are potentially present at the Site consist of benzene <br />and naphthalene. Tables 6-15b and 6-15c present the direct comparison of the reported <br />carcinogen concentrations in indoor air to their individual PCLs based on carcinogenic <br />risk for commercial, and unrestricted land uses, respectively. Naphthalene was not <br />detected in any of the indoor air samples at concentrations above the laboratory reporting <br />limits, which ranged from 0.67 to 0.78 µg/m3; however, the reporting limits are above the <br />indoor air PCLs for naphthalene under commercial (0.34 µg/m3) and unrestricted <br /> <br />31 Assuming ½ the reporting limit for nondetects.