Skip to main content
King County logo
Sediment is material deposited on the bottom of streams, rivers, lakes, and bays. Sediments can come from surface water runoff, a pipe, or from upstream locations where loose bottom material mixes with flowing water.

Contamination or pollutants in sediment can come from a variety of sources. Heavy metals (such as copper and zinc) and oils (polycyclic aromatic hydrocarbons) can come from automobiles. Other chemicals (such as PCBs) can come from industrial discharges that took place years ago or from yards and streets where people use chemicals (pesticides and fertilizers). Some pollutants found in sediment can date back to times when there wasn't wastewater treatment or containment of polluted discharges. Others are in the air and dust that get deposited on the ground and washed into our waterways.

In general, the pollutants come from all of us. Each person can contribute to the pollution found in sediments by many common things we do in living our daily lives.

Regulation of contaminated sediments in Washington State falls under the authority of the state Department of Ecology .
The seven sites identified to date are located on the Seattle Waterfront in Elliott Bay or in the Duwamish Waterway.
King County has taken the responsibility to implement the cleanups identified in the Sediment Management Plan. King County continually explores ways to partner with other agencies and projects that have interest in the cleanup areas.
The current estimate to clean up sites King County has identified is $35 million dollars.
King County will pay for the cleanups under the Model Toxics Control Act (MTCA) program. Because a lot of the cleanup entails historical contamination, there is a state fund to cover ½ of the costs of cleanup for municipalities.
Sites that are managed by King County may generally take 2-3 years to clean up. Sites that fall under a federal or state clean up program may take longer.
Yes. King County monitors sites for at least five years to determine if any recontamination has occurred. Each site presents different circumstances, so monitoring periods may vary. Monitoring reports are available in the each of the projects' libraries
Due to King County's industrial waste program, industrial wastewater is controlled in the combined sewer system and sent to West Point for treatment. Based on studies by King County and other groups, sites that have been cleaned up have a very low likelihood of recontamination from CSOs. Since the CSO control program will take longer to implement, it is important to remove the historical contaminants like PCBs and mercury sooner rather than later.