Formatted Title:

Quantifying Specific Discharge and PAH Flux Using Sediment Bed Passive Flux Meters to Optimize Sediment Cap Design

Background/Objectives:

Migration of non-aqueous phase liquid (NAPL) is impacting intertidal areas known as East Beach and North Shoal at the Wyckoff/Eagle Harbor Superfund Site in Bainbridge Island, Washington. The remedy selected to address these discharges includes dredging and capping. To determine if dissolved-phase polycyclic aromatic hydrocarbons (PAH) discharge also needs to be treated in the sediment cap, dissolved-phased PAH concentrations and rates of discharge within the groundwater-surface water transition zone at the base of the proposed cap were quantified using sediment bed passive flux meters (SBPFMs). The SBPFMs are a direct-push probe which contains a permeable internal sorbent located between two screened intervals. In the presence of a vertical hydraulic gradient, the screens allow water flow through the internal sorbent matrix that is impregnated with resident tracers. The tracers are displaced from the sorbent at rates proportional to the water flux through the sorbent. At the same time, dissolved contaminants present in transition zone water are retained on the sorbent at rates proportional to the ambient contaminant mass flux in the transition zone.

Approach/Activities:

SBPFMs were deployed during a sediment cap predesign investigation to measure time-averaged upward/downward discharge rates and associated mass discharges for dissolved-phase PAHs within the groundwater/surface water transition zone for 2 weeks. SBPFMs were installed at 11 East Beach and 15 North Shoal locations within the 2 to 4 foot interval below sediment surface for a period of 2 weeks. Using a PushPoint sampler, collocated transition zone water grab samples were also collected at four East Beach and six North Shoal SBPFM locations to measure dissolved-phased PAHs during a single ebb-tide event.

Results/Lessons Learned:

The SBPFM data allowed for estimates of net upward specific (upward minus downward) discharges and associated mass discharges across the study areas. The average net upward discharges for East Beach and North Shoal were 0.55 and 0.15 centimeters per day (cm/d). Consistent with past ebb tide event observations, the highest measured specific discharges were observed between the 0 and +2.5 foot beach elevation contours. Observations of NAPL staining on some SBPFMs correlated to elevated PAH mass discharges which when converted to average concentrations indicted that individual constituents were measured at concentrations in excess of effective solubilities. When excluding the NAPL-affected results, total dissolved PAH concentrations from SBPFMs ranged from not detected to 2,294 micrograms per liter (µg/L) at the East Beach and 1.2 to 3,128 µg/L at the North Shoal. The transition zone water grab sample results generally agreed with SBPFM concentrations, providing additional confidence in the SBPFM data set when the differences between the methods were considered. The SBPFM results confirmed the conceptual site model by verifying the primary areas of discharge within the cap target treatment zones. Excluding the NAPL-affected results, SBPFM results provided a range of time-weighted average transition zone concentrations that will be used to inform dissolved contaminant transport modeling for the purposes of cap design in the East Beach and North Shoal areas.

Bernadette Wright

Affiliation

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(Group 2, Poster Board #70) Quantifying Specific Discharge and PAH Flux Using Sediment Bed Passive Flux Meters to Optimize Sediment Cap Design