Formatted Title:

Porewater Migration Assessment for Protective and Resilient Remedial Design at a Complex Site

Background/Objectives:

Implementation of a resilient remedy is vital to successful long-term remedy performance. However, implementing a resilient remedy presents a challenge on large and complex sites where decisions need to be made for an area with various site-specific design constraints and considerations. In 2000, the Portland Harbor Superfund Site (PHSS) was added to the National Priorities List due to sediment contamination concerns and the potential risks to human health and the environment. The most widespread contaminants found include polychlorinated biphenyls, polycyclic aromatic hydrocarbons, polychlorinated dibenzo-p-dioxins, and polychlorinated dibenzofurans. The Swan Island Basin (SIB) Project Area is located within the larger PHSS between approximately River Miles 8.1 and 9.2 on the northeast side of the tidally influenced Willamette River. Porewater migration analysis was implemented as part of a Pre-Design Investigation in 2022 and 2023 to provide additional environmental data needed to address data gaps that were utilized in the Basis of Design Report and will be used in Remedial Design. Porewater migration upward through bedded sediments directly affects the remedial design, performance of an engineered cap, and recontamination potential analysis. Porewater migration may be complicated at contaminated sediment sites with a history of dredging and filling to form both the landscape and the waterway. Mapping the location and magnitude of porewater discharge is essential for effective and resilient cap design. For these mapping efforts, a strategic, two-phased approach was used to optimize the value of a comprehensive site-specific study appropriate for the complexity and scale of the project area.

Approach/Activities:

The field porewater study was performed in the 115-acre project area to identify and characterize zones where porewater migration might influence remedial capping design and to help quantify the groundwater discharge at areas where porewater migration was observed. Porewater upwelling zones were mapped using a Trident Probe, which utilizes temperature and conductivity sensors to measure temperature and conductivity for both the subsurface sediment porewater and the overlaying surface water. Phase 2 of the study utilized ultrasonic flow meters (UltraSeep meters) and differential pressure piezometers. The UltraSeep meter incorporates the continuous, direct measurement of groundwater seepage rates using the time-transient ultrasonic technique. UltraSeep meters were co-located with differential pressure piezometers to measure the vertical hydraulic gradient of sediment porewater using the wet/wet differential method with a pressure sensor and mini piezometer. Both phases of the study were completed with diver-supported instrument deployment and retrieval.

Results/Lessons Learned:

The porewater study consisted of two phases: Phase 1 mapped porewater migration and Phase 2 quantified in situ porewater discharge rates in selected areas that were identified by the Phase 1 mapping. Porewater upwelling locations were identified and mapped using spatial variations in the temperature and conductivity contrast between porewater and near-bed surface water. Porewater upwelling zones identified during Phase 1 were targeted in the second phase of the study. Utilization of UltraSeep meters in Phase 2 allowed for site-specific measurement of maximum porewater upwelling that was used in cap evaluation and recontamination potential evaluation. Challenges encountered included the effects of uncertainty in variable weather conditions and the seasonal presence of stagnant, low-mixing zones in near-bed surface water observed during cold weather months. The results of this study provided valuable information needed for recontamination potential analysis, identification of optimum location for treatability study utilizing passive samplers, and for the robust remedial design of resilient engineered sediment caps.

Chris Smith

Affiliation

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Porewater Migration Assessment for Protective and Resilient Remedial Design at a Complex Site

(Group 2, Poster Board #58) Porewater Migration Assessment for Protective and Resilient Remedial Design at a Complex Site