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ASPECT CONSULTING <br />46 FINAL PROJECT NO. AS190583A-08  MAY 21, 2025 <br />analyzed for helium, which was used as a tracer gas during leak testing to evaluate <br />potential dilution of the sub-slab samples from indoor air intrusion at the sample point <br />locations. <br />4.2.2 Intertidal Porewater and Seep Sampling (February and August <br />2014) <br />Groundwater in the Upland Area discharges to the marine environment of the East <br />Waterway. The results of groundwater sampling at upland monitoring wells located along <br />the Upland Area shoreline indicate concentrations of cPAHs, select metals, un-ionized <br />ammonia, and sulfide that exceed the screening levels based on protection of marine <br />surface water. However, attenuation of constituent concentrations can occur within the <br />tidally influenced nearshore groundwater/surface water mixing zone (transitional zone) <br />prior to discharge to the sediment bioactive zone and then the marine water column. <br />Therefore, intertidal sediment porewater sampling was conducted to evaluate the quality <br />of groundwater entering the East Waterway marine environment in accordance with an <br />Ecology-approved Addendum to the RI/FS Work Plan (Aspect, 2014c). Porewater <br />sampling locations were positioned downgradient of upland shoreline wells where the <br />highest concentrations of metals and/or un-ionized ammonia had been detected in prior <br />groundwater sampling. <br />Porewater sampling was completed in late February 2014 (wet season) and mid-August <br />2014 (dry season). Seven sampling locations (PW-1 through PW-7) were proposed; <br />however, PW-1, PW-2, and PW-6 could not be sampled because shoreline armoring <br />(riprap) extended well below the tide level, and there was no accessible intertidal substrate <br />from which to sample porewater during the lower low tidal stages. A substantial surface <br />seep (Seep-1) was observed emanating from the riprap up the shore slope from proposed <br />location PW-1 during both sampling events. Samples of sediment porewater were, <br />therefore, collected from Seep-1, PW-3, PW-4, PW-5, and PW-7 on exposed intertidal <br />beach during lower low tide conditions. The samples were collected during the last <br />3 hours of the ebb tide and up to 1 hour past the lower low water slack tide. No sample <br />locations were inundated at time of sampling. (Although surface flow was observed at the <br />Seep-1 sample location, the water sample was collected from below the sediment mudline <br />to avoid contact with the atmosphere, using the procedure described below.) <br />The porewater wellpoint assembly proposed in the Work Plan Addendum (Aspect, 2014b) <br />had insufficient open area to produce measurable water over a period of nearly <br />20 minutes, despite attempts in multiple locations. Therefore, porewater samples were <br />collected by inserting the proposed sample tubing directly into the substrate to a depth of <br />4 to 10 inches below mudline. This collection method avoided sample contact with the <br />atmosphere and produced a steady low flow sufficient for sample collection. Porewater <br />from the intertidal sediment was pumped at a rate of less than 0.25 liter per minute <br />through a flow-through cell with continuous reading of field parameters (temperature, pH, <br />specific conductance, oxidation-reduction potential (ORP), and dissolved oxygen). Once <br />the field parameters had stabilized, the flow-through cell was disconnected, and the <br />sample was collected directly from the tubing. In-line filtering (0.45-micrometer [µm] <br />filter) was used for samples to be analyzed for dissolved metals and dissolved sulfide.