Cleaning Up the Columbia Slough’s Sediment

The contamination in the Columbia Slough's sediments is widespread and at relatively low concentrations. It reflects years of historic agricultural and industrial activities in the areas around the slough that contributed long-lasting pollutants to the sediments. With more than 200 stormwater outfalls that carry runoff from public streets and neighborhoods directly to the slough, the City also has a role in the cleanup of slough sediment.

The Columbia Slough Sediment Program is responsible for meeting the requirements of the state’s environmental cleanup law. The requirements are outlined in the 2005 Oregon Department of Environmental Quality Record of Decision for Slough Sediments. Since 1993, Environmental Services and the Oregon Department of Environmental Quality (DEQ) have been working together to investigate environmental conditions in the slough and implement actions to reduce contaminant discharges and improve watershed health.

Our Cleanup Approach

• Identify and work with businesses to prevent pollution from industrial and commercial facilities.
• Stop unauthorized and illegal discharges to storm sewers that flow directly to the slough.
• Implement citywide actions to meet the requirements of the City’s Municipal Separated Stormwater Sewer permit and Total Maximum Daily Load Implementation Plan.
• Install pollution reduction facilities, such as green streets and stormwater filters, to reduce contaminant levels in stormwater runoff from selected public roadways before discharge to the slough.
• Test sediment quality and fish tissue in the slough over time to assess progress in reducing pollutant levels.
• Stop soil erosion.
• Encourage property owners to landscape with native plants and reduce pesticide use.
• Build and restore wetlands to filter stormwater.
• Add native plants and trees along the banks of the slough to buffer the water from human activity.
• Engage residents of the Columbia Slough watershed to ensure they know how they can help keep rivers and streams clean.
• Eliminate sanitary waste overflows into the slough. In 2000, Environmental Services finished building the Columbia Slough combined sewer overflow projects, which eliminated combined sewer overflows to the slough.

Annual Report

The City and DEQ continued their ongoing collaboration to clean up the Columbia Slough in 2024-2025. Key accomplishments of the past year include:

• 21 properties are undergoing source control evaluations and/or clean-up actions to prevent sediment recontamination.
• Stormwater treatment design is underway in 13 priority outfall basins to improve water quality and reduce runoff from selected City rights-of-way that discharge to the slough.
• 145 stormwater and stormwater solids samples were collected from nine high-priority outfall basins to help identify possible sources of stormwater contamination to the Slough and refer sites to appropriate regulatory programs.
• A slough-wide fish tissue sampling event was initiated to collect 118 fish for 22 composite samples that will be assessed for concentrations of PCBs, metals, pesticides, and PFAS.

Columbia Slough Story Map

The Columbia Slough Story Map is an interactive resource for people interested in learning more about the Department of Environmental Quality's Columbia Slough Sediment Cleanup Plan. This detailed resource includes a description of the Columbia Slough sediment cleanup plan, an overview of individual sediment cleanups, and a timeline of key events.

More Information

• Learn more about the Columbia Slough watershed, its health, and efforts underway to improve conditions.
• Find the latest watershed report card for the slough.
• Learn more about current slough outfall projects such as Outfall 57.
• Learn more about the Oregon Department of Environmental Quality's Columbia Slough Sediment Project.
• Find information on Columbia Slough fish advisories.