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

ASPECT CONSULTING 44 FINAL PROJECT NO. AS190583A-08  MAY 21, 2025 Throughout the IA, Aspect collected verification soil samples for chemical analysis from the sidewalls and bottoms of the excavations to assess compliance with IACLs. Chemical analyses for the verification soil samples specific to each interim cleanup area were as approved by Ecology. If the excavation verification sampling results indicated that IACLs were not achieved in an area of the excavation, that portion of the excavation was extended (overexcavated) to remove additional soil to meet IACLs to the extent practicable. Soil impacted by petroleum hydrocarbons was left in place beneath very large foundation elements within the USTs 71, 72, and 73 area, and beneath the distribution warehouse (Bunker C ASTs area). In addition, residual soil concentrations of selected metals (primarily copper, mercury, and zinc) within the excavation areas exceed IACLs based on leaching to groundwater for protection of the marine environment. The first IA included excavation dewatering as needed to facilitate excavation/handling of soil. Water produced during dewatering was treated on-Site using a temporary water treatment system prior to discharge of the treated water to the City’s sanitary sewer and publicly owned treatment works (POTW) for further treatment, in accordance with City Discharge Authorization (DA) number 254-13 granted to K-C in July 2013. More than 5.6 million gallons of groundwater were removed from the collective excavation areas for treatment and discharge to the POTW, and more than 6,200 gallons of water with free-phase petroleum product were collected and properly disposed of off-Site. Monitoring was conducted by qualified archeological personnel from SWCA during excavation within the Bunker C ASTs area, where penetration into underlying native soil was considered possible. The archeological monitoring methods and findings are summarized above in Section 2.3.1.1. Approximately 24,650 cubic yards of imported aggregate were chemically tested and then used for excavation backfill during the IA. In addition, approximately 1,700 cubic yards of geotechnically suitable overburden soil were excavated from the Bunker C ASTs, Heavy Duty Shop Sump, Naval Reserve Parcel UST, and Naval Reserve Parcel South areas and temporarily stockpiled for sampling and analysis. To satisfy compaction criteria, the overburden soil was used as backfill only above the water table. All IA data representing in-place soil (i.e., representing current conditions) are incorporated into this RI/FS. 4.1.2 Interim Action Confirmational Groundwater Monitoring (2014- 2016) Postconstruction confirmational groundwater monitoring was conducted for the IA areas, in accordance with the Ecology-approved Confirmational Groundwater Monitoring Work Plan (Aspect, 2014d). The goal of the groundwater monitoring is to assess whether the completed IA soil cleanup activities are protective of groundwater (i.e., whether sources of leachable contaminants have been eliminated). The monitoring program included the monitoring of 38 wells, 36 of which were newly installed, at 14 excavation areas.15 15 Groundwater monitoring was not conducted at the Heavy Duty Shop Sump area since soil contamination was not encountered (refer to IA Report; Aspect, 2015a).