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Multiple Lines of Evidence for Upstream Background Sediment Concentrations at the Portland Harbor Superfund Site
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Viable management solutions at contaminated sites often require the establishment of background concentrations that represent wide-ranging contaminant sources unrelated to the site. Background concentrations at sediment sites must also reflect the concentrations to which a site could recontaminate after remediation. Upstream background conditions were investigated as part of the 2018 pre-remedial design investigation and baseline sampling study (PDI) for the Portland Harbor Superfund Site (Site). The upstream area evaluated included the Downtown Reach (in downtown Portland), immediately upstream of the Site (river mile [RM 11.8 to RM 16.6]), and the Upriver Reach (RM 16.6 to 28.4). Both reaches (i.e., the D/U Reach) were included. This background study included an assessment of the current concentrations of chemicals of concern (COCs) in the mobile fraction of upstream surface sediments in three media: fine-grained bedded sediments that are potentially mobile,  suspended solids collected in sediment traps, and the particulate fraction of surface water samples. The goals of this investigation were to establish the current, representative upstream background conditions for the Site and better understand the potential for Site recontamination after remediation.



Following Site reconnaissance to locate areas of soft, fine-grained sediment in the 17 mile stretch of upstream Willamette River, spatially representative surface sediment samples (n=59; collected as 3-point composites, 0 to 30 cm depth) were collected from randomly placed locations within stratified areas (i.e., those representative of the grain size distributions of the Site's depositional areas). Two upstream composite surface water samples (each including particulate-bound and dissolved fractions) were collected along transects across three sampling events (i.e., low-flow, storm-flow, and high-flow events). Additionally, sediment traps were placed along two transects and sampled at three events to estimate the incoming sediment load to the Site. All media were analyzed for the six focused COCs specified in the Record of Decision (ROD) including PCBs, PAHs, DDx, and three dioxin/furan congeners (i.e., TCDD, PeCDF, and PeCDD). Upper statistical limits and other descriptive statistics were estimated forfocused COCs in each media and data were visualized by RM scatterplots to assess spatial distribution and variability, and boxplots for comparisons across media.

Results/Lessons Learned.

Multiple lines of evidence provide support for the upstream sediment background concentrations established using the 2018 PDI data. For example, the upstream sediment 95% UCL for total PCBs was 20.4 μg/kg. Similar concentrations were measured in the surface water particulate fraction within the D/U reach (mean = 20.2 μg/kg, normalized by mass of total suspended solids). Sediment trap data also showed consistent results (mean = 15.1 μg/kg; 95% UCL = 19.1 μg/kg). Further analysis of  PCBs in the depositional areas of the Site demonstrated a SWAC of 20.6 μg/kg, also supporting the 2018 PDI upstream sediment background value. These findings indicate that the upstream background value of 20.4 μg/kg for total PCBs reflects the quality of sediments depositing in the Site. Background concentration estimates for other Site COCs also will be presented; these recommendations have a strong foundation due to the robust sampling design used in this investigation and the extensive sampling implemented.

Publication Summary

  • Geosyntec Authors: Wendy R. Hovel, Anne Fitzpatrick, Jennifer Arblaster, Jason Conder, Luke Smith
  • All Authors: Wendy R. Hovel (Geosyntec, San Diego, CA), Anne Fitzpatrick(Geosyntec, Seattle, WA), Jennifer Arblaster (Geosyntec, Milton, VT), Jason Conder (Huntington Beach, CA), Luke Smith (Geosyntec, Seattle, WA), Betsy Ruffle (AECOM, Chelmsford, MA), Kristen Durocher (AECOM, Manchester, NH)
  • Title: 2023 Battelle Sediments Conference
  • Event or Publication: Event
  • Practice Areas: Sediment assessment and remediation, Contaminated site assessment and clean up
  • Citation: Battelle's International Conference on the Remediation and Management of Contaminated Sediments at the JW Marriott in Austin, Texas, on January 9 through 12, 2023
  • Date: January 9 through 12, 2023
  • Location: JW Marriott in Austin, Texas
  • Publication Type: Platform Presentation