3815_PGSF WMVD Pkg 2_Vol 4-2_03.03.2026_BXWA_Certified

ASPECT CONSULTING 64 FINAL PROJECT NO. AS190583A-08  MAY 21, 2025 independent Phase 2 ESA. Two sample foc values were excluded from the calculation of the site-wide average foc: the 0.0564 foc measurement in sample GF-B-3-11-12.5 was determined to be a statistical outlier (high), and sample GF-B-9-7.5-9 (foc = 0.0048) contained an elevated diesel-range TPH concentration (6,400 mg/kg). In addition, two soil samples that were subsequently removed during interim action excavations—thus are not representative of current Site soil conditions—were excluded from the calculation. Although large quantities of aggregate were imported to the Site for use as excavation backfill during the interim actions, these materials are not contaminated and were, therefore, excluded from the determination of Site-specific soil foc in accordance with MTCA (WAC 173-340-747(5)(b)(i)). Table 5-5 presents the Site-specific soil foc data used in the calculation, and the excluded values. Because Ecology’s CLARC database does not define a soil organic carbon-water partition coefficient (Koc) or Henry’s Law constant for cPAH mixtures, the values for those parameters assigned for total cPAHs in the calculations was the average of CLARC’s listed values for the seven individual cPAH compounds. As agreed to with Ecology, the soil-to-groundwater pathway is not considered for constituents that have not been positively identified in groundwater at concentrations exceeding groundwater PCLs. Constituents for which the soil-to-groundwater pathway is considered in the derivation of soil PCLs are presented in Tables 5-3 and 5-4 (“Y” in the column labeled “Groundwater Exceedances Confirmed Empirically for Analyte?”). The soil concentrations predicted by this MTCA-default methodology are intentionally conservative for the purposes of data screening in the RI. Because this default methodology is a conservative predictive model rather than an empirical measurement, it is possible that soil concentrations greater than these PCLs are not actually leaching contaminants to groundwater at concentrations exceeding groundwater PCLs. MTCA provides a range of options to evaluate Site-specific soil concentrations protective of groundwater, including the use of soil leaching tests and empirical groundwater quality data, as outlined in WAC 173-340-747. Empirical groundwater data indicating that groundwater meets PCLs is the strongest and most reliable evidence that soil concentrations in that vicinity (upgradient) are protective of groundwater. The demonstration of whether soil concentrations are protective of groundwater is evaluated based on all RI data (Section 6.3), which factors into soil cleanup levels established for the Upland Area. 5.3.2.3 Risk-Based Soil PCLs for Petroleum Mixtures Site-specific, risk-based Method C soil PCLs can be calculated for TPH mixtures, addressing all exposure pathways (including soil leaching to groundwater) using volatile petroleum hydrocarbon (VPH) and/or extractable petroleum hydrocarbon (EPH) data to quantify concentrations of aromatic and aliphatic hydrocarbons in specific carbon ranges for the specific petroleum product, in accordance with MTCA (WAC 173-340- 745(5)(b)(iii)(B)(III)). This approach is implemented for specific areas of industrial land use in the Upland Area where residual petroleum remains in place following interim actions as presented in Section 6.5, and the MTCA calculations are detailed in Appendix F. Because of the multiple petroleum types beneath the warehouse building, MTCA