Significant Project Features
- Mechanical dredging of 5,000 CY of sediment from the Willamette River contaminated with PAHs, TPH, BTEX, free cyanide, lead, mercury, and zinc
- Water generated during dredging operations was decanted and collected for treatment offsite
- Stabilization of sediment with Portland cement and offsite T+D
- Placement of cap and cover system consisting of sand and GAC
History & Location Details
Sevenson performed mechanical dredging of contaminated sediments in a segment of the Willamette River adjacent to the former Portland Gas Manufacturing site, owned by the State of Oregon and managed by the Oregon Department of State Lands. The stretch of river which required remediation was approximately 800 feet between the Burnside and Steel bridges in downtown Portland. The contaminants of concern (COC) were PAHs, TPH, BTEX, free cyanide, and three target metals (lead, mercury, and zinc).
The site had a concrete pathway which was used by pedestrians. Prior to initiating remediation, this pathway was closed, and bicycles and pedestrians were temporarily re-routed along the existing multi-use lane on the NW Naito Parkway. Temporary fencing was installed at the north and south ends to safely divert bicyclists and pedestrians around the installation work area.
The site was bounded to the west by a near-vertical seawall. Exact locations of the base of the seawall were determined through hydrographic surveys and loaded into the dredging software to ensure no dredging or spudding occurred within 10-feet from the base of the seawall. Remedial action did not compromise seawall stability.
Debris removal preceded dredging operations. A large (several feet) mound of offshore debris composed of large pieces of brick wall, concrete, lumber, riprap and sheet metal was identified in three areas. Approximately 16,000 SF of debris was removed. A John Deere 850D excavator equipped with a grapple was staged on a spud barge was used to remove the debris. Debris was placed into material bares and transferred to the offloading facility. Debris removal occurred within the moon pool containment system.
Dredging operations were performed with an excavator with a 2 CY environmental bucket for dredging the inner 3 to 1 slope along the seawall, where precision was more important. In deeper water, a crane with a 7 CY environmental bucket was used. Barges were used for handling dredged material. Sevenson used the dredge positioning software DREDGEPACK to accurately determine the position and ensure target depths were met. The excavator or crane was staged on a spud barge.
The dredge will operate within the limits of a mobile moon pool containment system to create a physical engineered barrier around the dredge area. This will limit the spread of suspended sediment and reduce the dispersion of turbidity and contaminants outside the area. The system consists of double-walled, near full length turbidity curtains attached to a framework of interlocking floats, enclosed a 40-foot by 40-foot work area.
Dredging was sequenced to minimize the exposure of the interim dredge cut along the base of the seawall, which is the area with the highest potential for exposing buries contamination. Tar-like material was removed first and segregated for disposal. Dredging progressed from the offshore to nearshore areas, followed by the inner side slope along the seawall, which was dredged last. This way, the transition will be the last area dredged and the first area to receive a GAC-amended cover.
Dredging operations adjacent to the seawall did not utilize a moon pool system due to proximity to the seawall. Instead, a single-walled turbidity curtain was used. The double-walled moon pool curtains deployed from the surface were complemented with a bedload baffle installed on the riverbed. The bedload baffle was an anchored curtain with a flotation to keep it near vertical, extending 8 feet above the mudline around the perimeter of the dredging side slopes. The bedload baffle was installed in a fixed position for the duration of the dredging work, unlike the moon pool containment system which moved with the dredge inside the bedload baffle. Together, the moon pool and the bedload baffle provided overlapping coverage of the entire depth of the water column for controlling sediment resuspension.
Once the tar-like material (TLM) was removed, it was segregated into a separate barge while the remaining sediment from the remediation area was removed and staged on watertight barges. The material barges transferred the material to be dewatered and amended with Portland cement. Following amendment, sediment was transported by tugboat and barge approximately 85 nautical miles to the permitted offloading facility in The Dalles, Oregon.
Water from dredged sediment during dewatering operations was not returned to the river. Two water management barges were set up with one bulk 140,000-gallon containment cell and two 21,000-gallon Baker Tanks on each. One Baker Tank was dedicated to containing water generated during the area in which TLM. This water was tested and disposed of separately.
Capping and Cover Placement
An amendment blend of GAC and sand was placed in Sediment Decision Unit (SDU) D, while a blend of gravelly sand and GAC was placed in the other areas (SDUs A, C1 and E). Mixing occurred in batches at the material supply yard using variable speed hopper drives conveying material into a quad-screw feed hopper. Once blended, the material was loaded onto a deck barge using an excavator.
Capping operations were completed using an excavator with a 2.25 CY material handling bucket staged on a spud barge. Decontaminated scows transported the capping materials to the PGM site for placement.
Cap and cover materials were placed at or just below the water surface in 6-inch lifts to minimize dredging residuals and to maintain uniformity of cap thickness.
Like the dredging operations, capping operations were sequenced to minimize the exposure of the dredge cut along the base of the seawall, which is the area with the highest potential for exposing buried contamination. Dredging operations finished with the inner side of the slope along the seawall in SDU A, so this is where the capping commenced. Capping progressed from the nearshore to the offshore areas.
Installation of Long-Term Sampling Ports
Four monitoring ports were placed above the chemical isolation layer prior to the placement of the rock armor layer. The ports were constructed of reinforced concrete manholes, approximately 2-2.5-ft. by 3-ft. A Sea Hawk crane was used with diver assistance to ensure the installation was level. The sampling ports were placed directly over the GAC-amended layer, filled with sand and covered with a steel ring and a lid. These ports will be accessed during the long-term monitoring program to allow collection of samples until a sufficient thickness of new sediment has accumulated over the armor layer.
Protection of Local Aquatic Species
In response to guidance from the National Oceanic and Atmospheric Administration Fisheries, several measures were taken to protect aquatic species and minimize fish entrapment during moon pool operations. Best Management Practices developed included:
- Under normal operating conditions, the bas of the moon pool was to be hung between 2 and 8 feet above the mudline, depending on water depth, to allow room for fish to escape
- The moon pool curtains were left extended as much as practicable during equipment moves, depending on the water depth and strength of the current
- The curtains were lowered to their final depth on the three upstream sides of the moon pool first, and the downstream curtain panel was lowered last to help minimize entrapment
- If necessary, the speed at which the curtains were lowered was reduced to allow fish more time to escape
- A long-handled net was available on deck in case any stranded fish were observed in the moon pool
Following completion of the PGM Site Remediation, all temporary safety screen materials were removed from the park. No further site restoration actions were necessary.