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Frequently asked questions

Frequently Asked Questions (FAQ) about how King County is addressing historical sediment contamination near County combined sewer overflow (CSO) outfalls, including the County’s Sediment Management Plan (SMP).

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. Rain that falls on streets, parking areas, sports fields, gravel lots, rooftops or other developed land mixes with what is on the ground and flows directly into the Duwamish. 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. Here are some things you can do to prevent harmful chemicals from getting into the Duwamish.

Regulation of contaminated sediments in Washington State falls under the authority of the state Department of Ecology.

The eight sites identified to date are located on the Seattle Waterfront in Elliott Bay, in the Duwamish Waterway, and in Portage Bay.

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 contributed to the contamination in the cleanup areas.

The estimate to clean up all these sites is tens of millions of 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 are grant opportunities to compete for a state fund to cover up to ½ of the costs of cleanup for municipalities.

Site cleanups that King County manages may generally take 3-4 years to clean up. Sites that fall under a federal or state cleanup program with multiple responsible parties take longer.

Yes. King County monitors sites they have cleaned up 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 each of the projects' libraries.

King County’s past investments in CSO control have paid off. Sediment quality is good at most CSOs and sediment quality overall has been improving in spite of growing population, increased development and thriving economy.

In addition, King County's Industrial Waste program regulates businesses to make sure that they pretreat their wastewater before they release it to our combined sewer system. This helps keep industrial pollution out of our wastewater. Based on responses at past cleanups and studies by King County contained in our SMP Update, there is a low likelihood that CSOs will re-contaminate cleaned up sites.

King County will continue to implement the CSO control program through 2030 but we are not waiting until then to remove the historical contaminants like PCBs and mercury. Instead, at sites were recontamination is not expected, we are actively working on cleanups. See the Sediment Management Plan for more information.