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

ASPECT CONSULTING 76 FINAL PROJECT NO. AS190583A-08  MAY 21, 2025 6.4 Ammonia and Sulfide in Groundwater As discussed in the following sections, un-ionized ammonia and hydrogen sulfide—the toxic forms of ammonia and sulfide—are present at select Upland Area groundwater and intertidal sediment porewater/seep locations at concentrations exceeding screening levels based on marine surface water protection. Both compounds persist in geochemically reducing conditions and are quickly transformed to less-toxic compounds in the presence of oxygen. Appendix H presents background information regarding the environmental geochemistry of ammonia and sulfide, and discusses the methods used to calculate, from analytical data, the concentrations of un-ionized ammonia and hydrogen sulfide. As described in Appendix H, the dredged marine sediments that were used historically to create most of the soil within the Upland Area commonly contain between 0.01 percent and 1 percent (100 to 10,000 mg/kg) bulk sulfide, largely in the form of insoluble iron and manganese sulfide minerals. Sulfides in mineral form are considered generally nontoxic. Conversely, hydrogen sulfide (H2S) is highly toxic to benthic and aquatic organisms and, therefore, federal/state water quality standards are based on H2S. The detected concentrations of dissolved sulfide 28 in field-filtered water samples collected at the Upland Area between 2012 and 2014 may include significant amounts of colloidal sulfide minerals that can pass through a 0.45-micron filter, and, therefore, greatly overstate the concentration of H2S that is the measure of sulfide toxicity in water. Therefore, starting in 2015, sampling of groundwater and intertidal seeps and porewater was conducted using the in situ diffusive-gradients-in-thin-films (DGT) passive sampling protocol that is designed specifically to quantify free H2S concentrations, as outlined in Appendix H. Consequently, for water sample locations with older data from the dissolved sulfide analysis (quantifying sulfide in all forms) and newer data from the DGT analysis (quantifying H2S), the latter data are used in this RI to assess nature and extent of sulfide in the form of H2S. A draft memorandum presenting the collective Upland Area ammonia and sulfide data and associated information was transmitted to Ecology in January 2018 (Aspect and Anchor QEA, 2018). Ecology provided comments on the draft document in March 2018 (Ecology, 2018). During a follow-up meeting on June 19, 2018, Ecology and K-C agreed that an in-water cap constructed of granular materials allowing oxygenation of groundwater discharge along affected portions of the shoreline would be included as a component of Ecology’s selected cleanup action for the East Waterway site. Ecology agreed that the in-water granular cap may be an effective remedy for treating un-ionized ammonia and hydrogen sulfide discharging from the Upland Area. Any requirements for engineering design analysis or modeling to demonstrate its treatment effectiveness will be included as a component of the selected cleanup action for the East Waterway site. The parties agreed that the Upland Area sulfide and ammonia data would be included and discussed in this Upland Area RI, but that remediation of those compounds would not be addressed in the Upland Area FS. 28 Analyzed using Standard Method 4500-S2-D.