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

ASPECT CONSULTING 94 FINAL PROJECT NO. AS190583A-08  MAY 21, 2025 metals data from well REC5-MW-01 (distinct and somewhat anomalous data as described by Aspect (2013a)), followed by the metals data for the rest of Unit B. 6.5.2.2.1 Groundwater Metals at Well REC5-MW-1/-1R The highest concentrations of dissolved arsenic, copper, and lead in Upland Area groundwater were historically detected at well REC5-MW-01, located on the downgradient edge of the former steam plant. Well REC5-MW-01 was damaged during mill demolition and replaced during the RI by REC5-MW-1R, located a few feet from the original well. The arsenic concentrations detected in groundwater samples collected from replacement well REC5-MW-1R exceeded the PCL in both groundwater samples collected, but the concentrations were substantially lower (less than 30 µg/L) than those measured in 2012 in original well REC5-MW-1. Although no elevated soil metals concentrations were detected in the area around wells REC5-MW-1 and -1R, it was inferred that a highly localized metals source existed in their vicinity. Therefore, during the second IA, a total of 368 tons of soil was excavated from the REC5-MW-1 area, and reported concentrations of arsenic, copper, and lead were below the soil PCLs in all of the excavation verification samples (Aspect, 2021). Apart from well REC5-MW-1/-1R, groundwater concentrations of arsenic, copper, lead, mercury, and nickel exceeding their PCLs are relatively widespread within Unit B, with somewhat different distributions as depicted on Figures 6-B5 through 6-B9, respectively. Zinc exceedances in groundwater are limited to one well in Unit B (Figure 6-B10). The Unit B metals data other than those from the REC5-MW-1/-1R area are described below. 6.5.2.2.2 Arsenic Arsenic is present above the soil PCL of 20 mg/kg in two soil samples collected from within Unit B (Figure 6-B5). A statistical evaluation of all arsenic soil data from Unit B indicates a 95 percent UCL of 5.1 mg/kg, which is below the soil PCL of 20 mg/kg, with less than 1 percent of the samples (2/229) exceeding the PCL. None of the reported concentrations of arsenic in soil in Unit B exceed two times the PCL (40 mg/kg). Therefore, Unit B arsenic soil concentrations are in compliance with the PCL. Arsenic has been detected in groundwater above the 9 µg/L PCL, which is based on natural background, in five Unit B well locations (Figure 6-B5). There is no apparent spatial correlation between arsenic exceedances in soil versus those in groundwater within Unit B (Figure 6-B5). With one exception, the concentrations of arsenic reported exceeding the 9 µg/L PCL range from 14.7 µg/L to 17.5 µg/L (Table 6-10). The March 2017 sample collected from well PM-MW-04 contained dissolved arsenic at 86.1 µg/L, which is significantly higher than concentrations of total arsenic previously reported in well PM-MW-04, due to the groundwater pH increase from 7.7 or less to 11.6 created by the CM placement (Table 6-10). There are no groundwater arsenic exceedances at any of the six shoreline wells in Unit B (Table 6-22). Dissolved arsenic concentrations were also less than the PCL in samples of intertidal porewater collected immediately west of Unit B (locations PW-03 and PW-04 on Figure 6-B5).