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ASPECT CONSULTING <br />102 FINAL PROJECT NO. AS190583A-08  MAY 21, 2025 <br />metals for purposes of this RI/FS, but the interim action excavation was not expanded <br />based on the record sample results (Aspect, 2021). One saturated, excavation base sample <br />(HBX-B-02) located within a few feet of the HB-MW-1R location contained TPH-D+O at <br />a concentration of 19,100 mg/kg (without silica gel cleanup), which is well above the <br />2,000 mg/kg PCL that is based on accumulation of free-phase petroleum product on <br />groundwater. A slight petroleum sheen was observed on water in the Hydraulic Barker <br />area excavation while it was open, but no sheen was observed at the sample location and <br />no free product accumulation was observed anywhere in the open excavation. <br />6.5.3.1.2.1 SITE-SPECIFIC SOIL TPH PCL FOR HYDRAULIC BARKER AREA <br />The 12-foot soil sample from HB-MW-1R was submitted for laboratory analysis of <br />hydrocarbon fractions using the EPH Method to allow the calculation of risk-based soil <br />cleanup levels in accordance with MTCA (WAC 173-340-745(5)(b)(iii)(B)(III)), using the <br />same methodology as described in Section 6.4.1.1.1 above. The TPH concentration <br />detected by that method was 4,210 mg/kg, 54 percent of the 7,710 mg/kg TPH <br />concentration measured by the NWTPH-Dx analysis. The results of the MTCAtph <br />calculations for this area are detailed in Appendix F. <br />As discussed in Appendix F, the elevated residual TPH concentration at the base of the <br />IA 2 excavation is protective of industrial direct contact and leaching to groundwater, but <br />exceeds the proposed area-specific PCL based on residual saturation (7,700 mg/kg). <br />6.5.3.2 Metals <br />Within Unit C, the SHB-MW-1 IA successfully removed the high soil copper <br />concentration (565 mg/kg) targeted for removal, but soil concentrations of copper, <br />mercury, and zinc greater than default soil PCLs based on groundwater protection remain <br />in the excavation sidewalls (Aspect, 2015a). Following the IA, arsenic, copper, and <br />mercury were detected in Unit C soil and groundwater samples at concentrations <br />exceeding respective PCLs. <br />Lead and zinc were also detected in one or more Unit C soil samples at concentrations <br />exceeding default soil PCLs based on groundwater protection (Figure 6-C7 and -C10, <br />respectively; Table 6-9). However, lead and zinc exceedances are not detected in Unit C <br />groundwater (outside of the Former Log Pond), indicating that lead and zinc <br />concentrations in Unit C soil are protective of groundwater in accordance with MTCA <br />(Table 6-10). <br />The data for metals in Unit C media are depicted on Figures 6-C5 through 6-C10, and the <br />data for the three metals that exceed the PCL in groundwater are discussed in the <br />following subsections. <br />6.5.3.2.1 Arsenic <br />A statistical evaluation of all arsenic soil data from Unit C (including the Log Pond) <br />indicates a 95 percent UCL of 7.2 mg/kg, which is below the soil PCL of 20 mg/kg, with <br />3 percent of samples (3/107) exceeding the PCL, and no samples with concentrations <br />greater than two times the PCL. Arsenic soil concentrations within Unit C comply with the <br />PCL.