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

ASPECT CONSULTING PROJECT NO. AS190583A-08  MAY 21, 2025 FINAL 49 In addition, the accumulated sulfide mass in the DGT gel samplers was measured using purge-and-trap followed by the colorimetric method (methylene blue) in accordance with Aspect and Anchor QEA (2015a). The accumulated sulfide measured in the DGT and length of sampler deployment were used to calculate porewater concentrations based on diffusive flux relationships presented in Anchor QEA (2015a). 4.2.6 Supplemental Site-Wide Groundwater Sampling (March 2017) Based on elevated pH and associated metal concentrations in groundwater due to placement of the Crushed Material (see Section 6.3), an additional round of Site-wide wet- season groundwater sampling and analysis was conducted in early March 2017, near the end of the wettest wet season on record for the Puget Sound region. During this event, 56 wells were sampled for field parameter measurements and chemical analysis. Of the 56 groundwater samples collected, all were analyzed for dissolved metals, 6 were analyzed for PAHs, and 8 were analyzed for PCB congeners. The wells selected for PAH and PCB congener analyses were generally positioned to be at or immediately downgradient of areas where those constituents were detected at higher concentrations in the Crushed Material and/or soil. Aspect submitted to Ecology a memorandum summarizing the March 2017 groundwater quality data, as well as groundwater pH data collected subsequently in July and September 2017 (Aspect, 2017e). This memorandum can be viewed on Ecology’s website using the weblink provided in Section 1. 4.2.7 Supplemental Investigation of Log Pond (March-July 2017) Additional investigation of the Log Pond Area was conducted in response to information provided to Ecology in March 2016 by a former Scott Paper employee stating that they had witnessed the placement of demolition debris originating outside the mill property, and debris and barrels (drums) from the mill, during filling of the Log Pond in the late 1970s. A phased investigation was, therefore, conducted between March and July 2017 to supplement the previously collected data from the Log Pond (soil sampling at 18 locations and groundwater sampling from five wells) in accordance with an Ecology-approved Addendum to the RI/FS Work Plan (Aspect, 2017c). The first step of the investigation was a surface geophysical survey, employing both electromagnetic and magnetometer methods, conducted on north-south transects spaced at 5-foot intervals across the entire footprint of the Log Pond to assess presence of subsurface anomalies potentially representing buried drums or debris (Figure 2-4 shows the extent of the geophysical survey). Based on the geophysical survey results, eight soil borings and two test trenches were advanced at locations of identified geophysical anomalies, with locations agreed to by Ecology. The borings were advanced to depths ranging from 40 to 70 feet, penetrating the entire thickness of Log Pond fill into underlying native soil, using rotosonic methods. The rotosonic drilling method provided continuous, 6-inch-diameter soil cores for field screening and visual observation of non-soil material, and based on that information, soil sampling for chemical analysis. In addition, two test trenches (approximately 25 to 30 feet long) were excavated to expose the former conveyor foundation identified by the geophysical survey and sample soil adjacent to it. In total, 25 soil samples were collected from the borings and trenches for analysis a broad suite of chemicals (gasoline-range total petroleum hydrocarbons