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Activated Carbon-Amended Enhanced Natural Recovery (ENR): Biological Lines of Evidence from a Pilot Study in the Lower Duwamish Waterway
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The use of Activated Carbon (AC), as a stand-alone in situ treatment or to augment Enhanced Natural Recovery (ENR), is an increasingly recognized remedy to reduce the bioavailability of hydrophobic, bioaccumulative compounds in sediment. Concerns persist regarding the potential for adverse AC-associated effects to benthic invertebrates in AC‑amended sediments or ENR layers. As part of a pilot study to test the use of AC to augment an ENR layer composed of sand or gravelly sand material in the Lower Duwamish Waterway (Seattle, WA, USA), we evaluated the potential for AC-associated effects to benthic invertebrates using multiple biological lines of evidence.


ENR and ENR amended with 3-4% (by weight) granular AC (ENR+AC) plots were constructed in 2017 in three, separate one-acre plots in scour, subtidal, and intertidal areas. Multiple biological lines of evidence from the one-half acre ENR subplots were compared to the one-half acre ENR+AC subplots, respectively, to evaluate potential differences in biological responses resulting from the addition of AC. In 2018, 2019, and 2020, Sediment Profile Imaging/Plan View (SPI/PV) surveys were conducted to evaluate the presence of mature Stage 3 benthic communities, characterized by deep dwelling, head-down deposit feeding benthic invertebrates. In 2020, monitoring included a 28-day laboratory bioaccumulation study in which polychaete worms (Nephtys caecoides) and bivalve clams (Mya arenaria) were exposed to core samples obtained from the subtidal ENR and ENR+AC subplots; this study also compared the growth and survival of the test organisms between ENR and ENR+AC. In 2020, a benthic macroinvertebrate survey was conducted at all three plots to compare multiple metrics (abundance, Annelid abundance, richness, diversity, evenness, dominance, composition of major taxa) quantifying the health of the benthic communities established in the ENR and ENR+AC subplots.

Results/Lessons Learned.

The multiple lines of biological evidence evaluated in the pilot study indicated that AC did not adversely affect benthic invertebrates. The three annual SPI/PV surveys indicated no consistent differences in benthic community colonization between ENR and ENR+AC subplots. The percentages of Stage 3 benthic communities in 2020 in the ENR subplots were neither consistently higher nor lower than in the ENR+AC subplots for the intertidal (25% vs 25%), scour (100% vs 67%), or subtidal (25% vs 67%) plots; differences observed were attributed primarily to differences in natural sedimentation or scour in each respective subplot. These results were reinforced by the laboratory test results. Mean wet weight organism masses measured at the end of the exposures were not significantly different (p > 0.05) between ENR and ENR+AC for either clams (7.0 vs. 7.3 g/individual) or polychaetes (0.33 vs. 0.31 g/individual). Mean survival was not significantly different (p > 0.05) between ENR and ENR+AC for either clams (99% vs. 93%) or polychaetes (94% vs. 92%). Among 18 statistical comparisons of the benthic community metrics, only four indicated a significant difference (p < 0.05) between ENR and ENR+AC; these differences could be explained by disparate physical conditions and silt deposition between the ENR and ENR+AC subplots.

Publication Summary

  • Geosyntec Authors: Jason Conder
  • All Authors: Jason Conder (Geosyntec, Costa Mesa, CA, USA), Gene Revelas (Integral, Olympia, WA, USA), Victor Magar (Ramboll, Chicago, IL, USA), (Gretchen Heavner (Floyd Snyder, Seattle, WA, USA), Lis Nelis (Ramboll, Seattle, WA, USA), Cliff Whitmus (Wood, Seattle, WA, USA), Debra Williston and Jeff Stern (King County, Seattle, WA, USA), Lindsey Erickson and Joe Flaherty (The Boeing Company, Seattle, WA, USA), David Schuchardt, Peter Rude, and Allison Crowley (City of Seattle, Seattle, WA, USA), Joanna Florer (Port of Seattle, WA, USA)
  • 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