1. Erosion Control
    2. Vegetation Management

    Cleaning Up the Duwamish River

    July 1, 2004
    A public-private partnership is working to clean up a marine waterway in an industrial area of Seattle, WA, to give an important salmon run a new lease on life.The late-2003 and early-2004 sediment cleanup on the Duwamish River is expected to be the first step in a larger project that now has won Superfund designation. The cleanup, conducted after salmon returned to their home streams for spawning last fall, is part of a process started 10 years before the area became a Superfund site. Cleaning up this 7 ac area partially fulfills the terms of a 1991 federal consent decree settling a Natural Resource Damages lawsuit. This cleanup is one of nine projects funded by the consent decree.After the Superfund designation in 2001, the work that began in 1991 and any future EPA work became intertwined, explains Jeff Stern, a senior water quality planner for King County, WA, and the lead for the county on these projects. Although the current work and planned future research are not too complicated to explain, the places where one project ends and the next begins can get a little hazy.Dredging Contaminated Sediments From the WaterwayThe bottom line is this: Contaminated sediment is being dredged from the waterway and transported by train in lined containers to an eastern Washington landfill owned by Rabanco of Seattle. A 3 ft or deeper layer of contaminated sediment is being dredged from the site to provide the space necessary for installation of an engineered sediment cap to prevent future contamination of the waterway without increasing the existing bottom elevations. In the consent decree, $24 million was provided to conduct sediment cleanup, habitat restoration, and source control in Puget Sound, an inlet of the Pacific Ocean that includes Elliott Bay, and the Duwamish River, which flows into the bay.King County’s regional wastewater treatment utility is managing the current dredging and capping in the Duwamish. The state and county project is being coordinated with EPA, however, because EPA research likely will uncover the need for more cleanup in adjacent areas, according to the final report from the Washington State Department of Ecology on what is called the “Sediment Management Standards Cleanup Action Decision: Duwamish/Diagonal CSO/SD.” The Superfund project is still in the research stage.The mechanical dredging in this phase is being done with a clamshell bucket. According to Stern, the project managers did extensive research on the possibility that the contaminated sediment would scatter beyond the site, and through modeling determined they shouldn’t expect effects outside the dredge site. Therefore isolation methods, such as turbidity curtains, were not required. “At these sites, we minimize the scattering through the use of dredging best management practices that minimize the release of particulates into the water column,” he says.Stern notes most isolation methods would not have been effective here because in large tidal fluctuation sites the flow rates and the change in direction cannot be handled without failure in the system. Isolation by more-solid structures might have been possible but would have been difficult for the same reasons turbidity curtains would fail.Many of the same partners and scientists are working on both projects, reports Stern, which makes their work more seamless but makes the differences more difficult to explain to others. The partners involved include the City of Seattle, King County, the State of Washington, the Port of Seattle, EPA, the Boeing Company, and the Muckleshoot and Suquamish tribes. Boeing has facilities near the Duwamish, and the two tribes have a treaty to fish in the area. Fishing is not allowed in the cleanup area, however; the dredging is taking place in an industrial waterway through which salmon pass on their way from Elliott Bay to the Green River. The Duwamish, an artificial waterway at the mouth of the Green River, was dredged out years ago to allow deeper-water vessels to pass through. “The site is much different from being what you would consider a cleanup on a river,” Stern explains. “It’s much more like cleanups in Commencement Bay, even though technically you could argue [the Duwamish] is a river.” Stern says in most rivers, sediment buildup from river flow is the concern. “Here, the significant drivers for the sediments are the tidal currents. The river flow is a thin layer on top, and it doesn’t have any direct effect.”Complex Site HistoryThe area has a complicated history of contamination. Four sewer or stormwater discharge pipes are located inshore of the cleanup site. According to the state report on this project, the historic outfall for a sewage treatment plant that closed in 1969 is located upstream of the cleanup site. This pipe was an emergency bypass for the Duwamish Pump Station that receives combined sewage and stormwater from an area upstream. Discharges from this facility have been controlled to less than one outflow per year, and none is known to have occurred since 1989. A sewage outflow could be triggered at this facility in flood conditions.

    The dredging bucket preparing to deposit sediment on the dredging barge.Separated stormwater continues to flow into the Duwamish through a different pipe from a 2,585 ac drainage area and is estimated to have an annual average volume of 1,230 million gal/yr, which varies depending on rainfall. The two other pipes contribute much smaller amounts of stormwater drainage. The discharge often includes an oil sheen of unknown origin, and phthalate compounds are commonly present. According to the state report, the site also contains high levels of polychlorinated biphenyls (PCB) in sediments offshore from the outfalls. Recent sampling for the site assessment report found high PCBs in river sediments as well.Since the early 1900s, industrial waste has been discharged into the waterway, either by way of wastewater or stormwater. Past and present discharges to the waterway come from boat manufacturing and repair, marina operations, airplane parts manufacturing, and metals fabrication. This industrial pollution has contributed to the contamination traced to the stormwater drains and wastewater overflows. Yet no list of polluters is available because a lengthy, potentially responsible-party search only recently has started at this site as part of the Superfund project. Pollution at the site has been reduced through several different programs, but the storm drains and combined sewers still discharging at the site continue to get polluted, according to Stern.The Duwamish River area has a complex history. The waterway initially was dug out almost 100 years ago. “It’s a straightened channel made for navigational purposes,” Stern explains. The channel is surrounded by industry. There are few natural places in the area, making it more of a port than a river. It is also much wider than most rivers in this area. Part of the project area last was dredged when the shoreline was modified in 1977.The Duwamish flows northwest from the Green River near Tukwila, WA, splitting around Harbor Island and emptying into Elliott Bay south of the Seattle waterfront. The lower waterway has been heavily industrialized for more than 80 years. It supports one of the largest salmon runs in the region. Both the Suquamish and the Muckleshoot tribes have a treaty right to fish commercially for Chinook salmon and harvest up to 26,000 a year. Recreational anglers also find salmon, steelhead trout, smelt, and bottom fish in the waterway. The salmon spend little time in the river and probably are not tainted by the contaminated sediment, but resident fish—especially such bottom fish as sole—probably are affected, according to a recent EPA study that cautions against eating sole caught in the Duwamish.A view of the dredging barge, crane, and bucket as the bucket is lowered into the Duwamish River.Combined federal and state research data identified a number of pollutants in the Duwamish River that exceeded state sediment-quality standards, including six metals, three chlorobenzenes, two phthalates, total low-molecular-weight polycyclic aromatic hydrocarbons, total high-molecular-weight polycyclic aromatic hydrocarbons, and PCBs. The four chemicals of concern chosen for defining the cleanup area are mercury, PCBs, bis (2-ethylhexyl) phthalate, and butyl benzyl phthalate. Capping Contaminants The primary objective of the project is to restore natural resources in the contaminated area. The cap that will be installed after the dredging will isolate the remaining contamination. Superfund most likely will continue the cleanup and widen the area of work. Several possibilities for treatment of the site were explored before the state decided to cap the sediment in place after removing enough to make room for the cap. Although two kinds of caps are used in this kind of project—either a 1 ft cap or a 3 ft cap—only the thicker cap was considered for this site because it is a busy industrial shipping waterway, which requires more durability. “A thick cap was selected because this approach is considered to provide environmental protection [similar to that provided by] removing most of the contaminated sediments but at a significantly lower cost,” says the final report on the project. About 62,000 yd3 of contaminated sediments containing PCBs and other chemicals are being evacuated to provide space for the cap. Approximately the same amount of clean material will be used to install the cap and return the site to its predredge bottom elevations. The minimum cap thickness will be about 3 ft, but in many places the cap will be thicker. After the cap is installed, the existing bank will be given a dressing layer of armor stone and fish mix, a layer of sand and gravel (about 1,700 yd3), to ensure long-term stability of the slope and create a more fish-friendly area. Just as the project managers avoided dredging during salmon spawning times, the cap installation is set to be completed in time to avoid affecting the juvenile salmon as they pass through the area on their way to the Pacific Ocean.Although the possibility was explored, none of the contaminated dredged materials will be treated and reused for a beneficial purpose. “Treatment was evaluated as an option for the project but was eliminated due to cost effectiveness,” Stern says. “Treatment is usually only cost-effective for very large projects or for very contaminated material—neither of which relates to this project.”Don Theiler, director of the King County Wastewater Treatment Division, calls the cleanup project an important achievement on behalf of the environment: “This is a major milestone in eventually returning the Duwamish to a healthy, protective waterway. It is one of our early action sites, and the partners have worked hard to ensure we are able to move quickly with this work.”Seattle Mayor Greg Nickels echoes that statement on the city’s Web site. “We are committed to working with EPA, [the State Department of] Ecology, and community and stakeholder groups to improve the health of the Duwamish as soon as possible,” Nickels says. “We can achieve real environmental and public health improvements now while supporting the basic requirements and intent of Superfund.”