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

ASPECT CONSULTING 92 FINAL PROJECT NO. AS190583A-08  MAY 21, 2025 TPH concentrations in three samples (8,400 to 28,000 measured by NWTPH-Dx) exceed the proposed area-specific PCL based on residual saturation (7,700 mg/kg). 6.5.2.1.2 Groundwater TPH and PAH Data Outside of the USTs 71, 72, 73 IA area, TPH-D+O and naphthalene concentrations in Unit B groundwater samples were less than PCLs (Figures 6-B2 and 6-B4). Low-level total cPAH (TEQ) exceedances occurred sporadically in wells throughout Unit B (Figure 6-B3). Within the USTs 71, 72, 73 IA area, the results from two years of quarterly post-IA groundwater monitoring at wells UST71-MW-103 and -104, located directly downgradient of the residual inaccessible TPH-impacted soils on the south and north ends of the excavation, respectively, did not identify concentrations of TPH-D+O in groundwater above the PCL. However, concentrations of total cPAHs (TEQ) were detected above the PCL in the groundwater samples collected from well UST71-MW-103 during each post-IA groundwater sampling event and naphthalene was detected above the PCL in one of eight post-IA groundwater samples collected from well UST71-MW-103 (Table 6-14). At well UST71-MW-102, located downgradient of the central portion of the IA excavation, away from the residual inaccessible soil, motor oil-range TPH exceedances (up to 2,200 µg/L) and total cPAHs (TEQ) exceedances (up to 0.136 µg/L) were each detected in post-IA groundwater samples collected; the TPH and total cPAHs TEQ exceedances occurred in different sampling rounds. In addition, naphthalene exceeding its PCL (based on VI) was reported in one of eight groundwater samples collected from well UST-MW-103 (Table 6-14). In this area, the two wells with detected TPH and/or PAH exceedances in groundwater— UST71-MW-102 and -103—hit refusal on buried structure during drilling, so the well screens could not be placed to the depth planned and they thus intercepted a limited saturated thickness. The wells recovered slowly and could not be developed to the degree desired following installation, and consequently their groundwater samples were consistently more turbid than observed in the other wells in this area (Aspect, 2015d). For example, over the 2-year period of monitoring, wells UST71-MW-102 and -103 had the average turbidities of 47 and 95 NTU, respectively, compared to 8 and 7 NTU in wells UST71-MW-101 and -104, respectively, suggesting that turbidity bias contributed to the detected groundwater TPH/PAH exceedances in wells UST71-MW-102 and -103 (turbidity data in Table 6-11). Outside of the USTs 71,72, 73 IA area, total cPAHs (TEQ) were detected above the PCL in groundwater at five monitoring wells within Unit B (Figure 6-B3). For each of those five wells (BA-MW-03, BA-MW-05, PM-MW-02, PM-MW-03, and PM-MW-05), the total cPAHs (TEQ) exceedances were very low—below 0.04 µg/L (4 in 100 billion)—and were not reproduced in repeated sampling (Table 6-14). The cPAHs are hydrophobic compounds with low mobility in groundwater, particularly within the organic-rich dredge fill that largely makes up the Upland Area. Therefore, the concentrations of total cPAHs (TEQ) in Unit B groundwater samples may be influenced by small quantities of solids that are suspended in the groundwater samples. In accordance with the RI/FS Work Plan, the 2013 through 2015 groundwater samples were centrifuged