King County's Combined Sewer Overflow Water Quality Assessment
Assessing Water and Sediment Quality in the Duwamish River and Elliott Bay
About the WQA
The Duwamish River and Elliott Bay
The Duwamish River (157K)
Elliott Bay (123K)
Photos by Ned Ahrens
Poet Richard Hugo grew up fishing in the Duwamish Slough. In his autobiography, The Real West Marginal Way, he describes his experience:
Porgies, shiners we called them, were so numerous there that after the out tide drained the slough, we continued to catch them out of the small puddles left standing in the mud flats....The river was alive with salmon. They rolled and splashed everywhere. I could hear them in the fog, and some I could see, huge shadows that climbed the air and slipped back into the water so close to the boat I could have touched them.
The Duwamish Slough no longer exists, and the Duwamish River and Elliott Bay have changed significantly since the mid-1800s. Over the past 100 years, human activities have eliminated most of the original habitat in the river corridor and have affected fish runs, shellfish harvests, and wildlife populations, as well as recreation such as fishing, boating, clamming, and bird watching. Human activities include the diversion of the Black and White rivers for navigation and flood control; the dumping of ship ballast, dredged materials, and soils; the discharges of untreated sewage, stormwater, and combined sewer overflows (CSOs); and the discharges and runoff from shipbuilding and other industrial and manufacturing processes.
The following graphics illustrate changes in the Duwamish River due to human activities.
Graphics courtesy of the Port of Seattle
|Shallows and flats
|Developed floodplain and shoreline
|Under developed floodplain and shoreline
|Sewer overlows (CSO)
|Mean Low Water
In the 1980s, studies identified pollution problems in the river and the bay. To reduce pollution, King County and other agencies undertook the following:
- secondary treatment of wastewater
- rerouting treatment plant discharge from the river to Puget Sound
- sediment capping and cleanup of contaminated areas
- controlling toxicants from industries and stormwater runoff
- reducing CSOs to meet federal and state regulations
Due to these pollution-control efforts, temperature, dissolved oxygen, and fecal coliform bacteria readings improved. Lead and other heavy metals in the water from smokestacks, drains, and street runoff were drastically reduced; ammonia concentrations and garbage were reduced; and restoration of habitat was undertaken along the river.
Combined Sewer Overflows
Denny Way CSO (122K)
Photo by Ned Ahrens
As mentioned previously, two of the discharges to the Duwamish River and Elliott Bay include untreated sewage and stormwater. When combined and discharged into water bodies from a single pipe, these discharges are known as combined sewer overflows, or CSOs.
Diagram of combined sewer system (6K)
During dry weather and smaller rainstorms, CSOs do not occur because the wastewater system can usually channel the wastewater to a treatment plant. However, during some high-intensity or long-duration storms, the capacity of the wastewater system may be exceeded because of the additional stormwater. To protect the treatment plants as well as to avoid sewage backups into residences, businesses, and industrial areas, the combined sewers discharge the flows directly into a water body.
Although the sewage in CSOs is greatly diluted by the stormwater, CSOs may be harmful to public health and aquatic life because of bacteria and chemicals present in the sewage and stormwater runoff. Just as with other discharges, the chemicals and bacteria can remain suspended in the water or become attached to sediment. These chemicals and bacteria may harm the health of fish, as well as animals and humans who come in contact with the water or who eat the fish.
Map of CSO locations (92K)
As shown on the map, there are sixteen King County CSO outfalls which discharge about 1.4 billion gallons of CSO into the Duwamish River and Elliott Bay in a year of average rainfall. To meet the Washington State Department of Ecology's (Ecology) CSO requirements, King County has undertaken CSO projects in the past and is planning for others in the next five to thirty years. King County is currently working to meet Ecology's requirement of reducing CSOs to one discharge a year on average at each CSO outfall as a long-term goal. Meeting Ecology's requirements involves a significant financial investment of approximately $600 million through 2030.
CSO Water Quality Assessment
In order to have a better understanding of the dynamics of the Duwamish River/Elliott Bay estuary and the impacts of CSOs relative to other pollutant sources, King County undertook an extensive study: King County Combined Sewer Overflow Water Quality Assessment for the Duwamish River and Elliott Bay (hereafter referred to as "the study"). Some of the questions we looked at included:
- What is the existing quality of the water and sediment?
- What are the flow patterns in the water and how are chemicals dispersed and/or ingested by aquatic life, wildlife, and humans?
- What are the health risks to humans and other creatures that come in contact with the estuary with CSOs and without CSOs?
- Will the waters and sediment become cleaner and safer as a result of our CSO control efforts? If so, to what extent?
The study was undertaken from 1996 through 1998. A key objective of the study was to consider the interests of the variety of people and groups who use the estuary. To achieve this objective, we sought a diversity of perspectives for the study. Thus, the study team included a core project team of scientists, planners, engineers, and other professionals; a stakeholder committee (composed of representatives from local communities, businesses, environmental organizations, tribal governments, and agencies); and a national peer review panel.
What The Study Found
As mentioned previously, major objectives of the study were to determine existing conditions of the Duwamish River and Elliott Bay with CSOs and the conditions of these water bodies if CSOs were eliminated. It was beyond the scope of this study to look at the specific impacts of other pollutant sources such as stormwater run-off, agricultural sources, and leaky septic tanks. More research would need to be undertaken to specifically categorize the impacts of each of the other sources.
For most risks, removing CSOs will have little impact. However, removing CSOs will reduce risk of infection from human pathogens, most notably near the Denny Way CSO (which will be controlled by 2003). Removing CSOs will also reduce risks to sediment-dwelling organisms near the CSO discharges.
To find out more about the study and/or its results, please see the documents listed under WQA Reports at the beginning of this site or see the public information document, King County Combined Sewer Overflow Water Quality Assessment for the Duwamish River and Elliott Bay Summary.
For more information, please contact:
|King County DNRP
||Fax: (206) 296-0192|
|Water and Land Resources Division
||Phone: (206) 296-1986|
|King Street Center, 201 S. Jackson Street
|Seattle, WA 98104-3855