Adaptive Management of PAH and NAPL-Impacted Dredge Residuals – Former Manufactured Gas Plant Site Located in the Portland Harbor Superfund Site
Joe Smith, P.E., Environmental Engineer, Anchor QEA, LLC
Co-Authors: Ryan Barth, Principal Engineer, Anchor QEA, LLC and Bob Wyatt, NW Natural
Environmental dredging is a remedial technology commonly employed at contaminated sediment sites. Monitoring during these projects has documented that surface sediments remaining after dredging contained elevated concentrations, posing a residual risk to aquatic biota. The magnitude of these remaining generated residuals often dictates the success of an environmental dredging project. The U.S. Environmental Protection Agency (EPA), NW Natural, and EPA’s partners worked collaboratively to develop an adaptive post-dredge residuals management framework for the Gasco Sediments Site located in the Portland Harbor Superfund Site to manage elevated polycyclic aromatic hydrocarbon (PAH) and non-aqueous phase liquid (NAPL) residuals. Extensive pre- and post-dredge characterization findings at multiple environmental dredging projects were considered during development of the framework, including site-specific dredging equipment, controlled operations, and best management practices. The framework includes an approach to adaptively manage measured residuals using established design criteria during construction. Residual management includes placement of three 6 inch layers of clean sand cover sequenced as follows: immediately following confirmation that the dredge residuals are below concentrations of concern after design dredge elevations are achieved in a subarea, following completion of a dredge season, and following completion of all dredging activities after multiple seasons. The framework includes a decision tree that summarizes the collection of stepwise, multipoint, composited post-dredge and post-cover verification samples throughout dredging subareas and tiered subsampling to evaluate for missed inventory and generated residuals concentrations. If elevated residual concentrations or NAPL are identified in a subarea, the remedial design of the cover layer will be adaptively modified during construction in accordance with pre-established design alternatives. These alternatives include incorporation of active amendments in the clean cover or additional sand placement. The framework also provides clear decision points for EPA to confirm that dredging and cover activities are complete to minimize construction delays.
Joe Smith is a licensed Professional Engineer at Anchor QEA in North Carolina. He has 6 years of sediment remediation and navigation dredging experience at project sites across the United States and in Canada, including former MGP facilities in Oregon and New York. Mr. Smith has a background in site investigation and characterization, remedial alternatives evaluations, remedial and navigation dredging design, cost estimating, and construction management. He enjoys working to develop long-lasting, protective remedies for high profile environmental sites. Mr. Smith holds a BS in Environmental Systems Engineering from Pennsylvania State University and an MS in Environmental Engineering from Washington State University.