Sediment and Water Quality Evaluation Framework for Disposal of Dredged Material in an Upland Rehandling Facility, Portland, Oregon

Because the regional open-water disposal site for the Portland area was recently closed, the Port of Portland has had to change from in-water to upland disposal to manage sediments from its maintenance dredging operations. The Port of Portland designed and built a dredged material rehandling facility to receive dredge slurry that is hydraulically offloaded from a barge. The rehandling facility contains a primary basin for the initial hydraulic discharge and a secondary basin for quiescent settling and enhanced clarification of the dredging elutriate. The facility provides temporary storage and dewatering of the dredged material until a suitable beneficial use can be identified. Typically, the dredged material is suitable for use as fill at Port development sites. Existing regulatory guidance for in-water disposal was not applicable to upland disposition, so the Port worked with the dredged material management agencies to develop a tiered evaluation framework that was protective of both surface water and groundwater. Four pathways were evaluated by which contaminants could potentially be released to the environment: (1) releases to the water column at the point of dredging; (2) releases to the Columbia River through the effluent return flow at the rehandling facility; (3) releases to the river by way of seepage under the rehandling facility berm; and (4) releases to groundwater by infiltration beneath the facility. The proposed evaluation framework includes initial bulk sediment screening criteria, as well as more advanced laboratory analysis of dredging elutriate or leachate using Corps of Engineers (i.e., modified elutriate test, column settling test) or EPA (i.e., synthetic precipitation leaching procedure) protocols, as necessary. The results of this project support the use of a tiered evaluation framework for characterizing potential environmental impacts at an upland rehandling or disposal site. Field verification sampling of water column and dredge elutriate chemistry was also conducted; in particular, field sampling of effluent return flow from the rehandling facility validated the results of the modified elutriate test.