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

ASPECT CONSULTING PROJECT NO. AS190583A-08  MAY 21, 2025 FINAL 77 6.5 Nature and Extent of Contamination by Unit The nature and extent of contamination under current (post-IA) Upland Area conditions is presented for Units A through E in Sections 6.5.1 through 6.5.5, respectively. As presented in Table 6-1, following completion of the two IAs, there are no constituent concentrations in Upland Area soil exceeding criteria based on industrial direct contact. For those portions of the Upland Area outside of the Warehouse Subarea, the evaluation of contaminant nature and extent in soil focuses solely on the soil-to-groundwater pathway— i.e., whether current concentrations of a constituent in soil are leaching to the extent of causing exceedances of the respective PCL in groundwater. In accordance with MTCA (WAC 173-340-747(9)), if groundwater contaminant concentrations meet the applicable groundwater cleanup level, concentrations of that contaminant in adjacent soil are determined to be protective of groundwater, as long as all requirements of WAC 173-340-747(9)(b) are met. To that end, viewing the soil and groundwater data together on the exceedance maps is a primary basis for evaluation of the soil-to-groundwater pathway for each constituent in each unit. For metals in soil, the RI data is evaluated by unit using statistical methods to determine compliance with MTCA cleanup levels. To confirm compliance with MTCA cleanup levels, the following three criteria must be met (WAC 173-340-740(7) and -745(8)): 1. The 95 percent upper confidence limit (UCL) on the mean concentration is below the PCL. 2. No single concentration can be greater than two times the PCL, or if the PCL is based on natural background, a higher exceedance factor (also referred to as the magnitude of exceedance) can be calculated based on Site-specific data and used instead per Ecology, 1992. 3. Less than 10 percent of sample concentrations can exceed the PCL, or if the PCL is based on natural background, a higher exceedance frequency can be calculated based on Site-specific data (per Ecology, 1992) and used instead of the default 10 percent. Modifications to the magnitude and frequency of exceedance were made to account for background for copper, mercury, nickel, and zinc in accordance with Attachments 1 and 2 of Ecology’s 1992 statistical guidance (Ecology, 1992). The statistical evaluation for metals in soil was completed using EPA’s ProUCL 5.1 to evaluate data distribution and calculate statistics for each data set, including outliers and nondetect results. The ProUCL backup is provided in Appendix I. 6.5.1 Unit A The IAs completed within Unit A have consisted of:  Removal of approximately 9,700 tons of soil contaminated with petroleum hydrocarbons, primarily TPH-O (Bunker C fuel oil) with lesser quantities of TPH-G, from the Bunker C ASTs area. The south and east ends of the Bunker C ASTs area excavation were advanced as close as feasible to the edges of the warehouse structure.