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

ASPECT CONSULTING PROJECT NO. AS190583A-08  MAY 21, 2025 FINAL 87 6.5.1.2.2.3 NICKEL Dissolved nickel concentrations exceeding the 8.2 µg/L groundwater PCL were detected in four of five Unit A shoreline well locations but not in any of the Unit A wells located inland (Figure 6-A10). At each of the shoreline wells, the nickel exceedances were only detected in a single groundwater sample collected in 2012 and subsequent sampling results did not detect nickel in groundwater above the PCL (Table 6-10). Soil nickel concentrations throughout Unit A are less than the 48 mg/kg soil PCL (based on natural background concentrations) in all samples (Table 6-9; Figure 6-A10). For Unit A, there is no apparent correlation between nickel concentrations in groundwater versus soil, and thus no identifiable soil sources for the groundwater exceedances in shoreline wells (Figure 6-A12). Given the lack of identifiable sources of nickel in Unit A soil, and the fact that higher groundwater nickel concentrations are detected at shoreline wells than inland wells, a possible cause for elevated groundwater nickel in shoreline groundwater is release of nickel from oxidation of naturally occurring, nickel-containing iron sulfide minerals (pyrite or amorphous forms) in the dredge fill matrix. Sulfides are unstable in oxidized conditions, and release of nickel from pyrite oxidation is a phenomenon reported in the literature (Larsen and Postma, 1997; Kjoller, 2001). Both pyrite- and nickel-bearing ore minerals (e.g., limonite) are known to exist, and have been commercially mined in some localities33 of the very large Snohomish River watershed, which supplied the sediment that became Upland Area dredge fill. Relatively high concentrations of total sulfide (up to 3,800 mg/kg), albeit not mineral-specific, are documented in sediments of Port Gardner Bay and specifically the East Waterway (SAIC, 2009; Ecology, 2013). Groundwater in the Unit A shoreline wells has generally higher dissolved oxygen (D.O.) than in inland wells, which is indicative of seawater intrusion into the nearshore portion of the fill. An exception is shoreline well REC7-MW-4, located at the east end of the slip, which has relatively low D.O. (Table 6-11). The chart below shows the relationship of average nickel and average D.O. measured in Unit A groundwater wells, which shows the generally higher D.O. and generally higher nickel concentrations in the shoreline wells. 33 For example, mined in the Sultan and Index Mining Districts (Washington Department of Conservation and Development, 1942).