Cleaning up the Lower Duwamish

Sediment remediation/early action

Industrial and stormwater discharges from King County’s combined sewer overflows (CSOs) to the Lower Duwamish Waterway were reduced substantially during the past 20 years. Despite that achievement, however, historical, persistent chemical contaminants in sediment continue to pose a risk to aquatic life, wildlife and human health.

King County's Wastewater Treatment Division is carrying out a Sediment Management Plan to remediate sediment contamination near the County’s combined sewer overflow outfalls (CSOs). Most of that contamination is historical--from the first half of the 20th century. Since the 1960s, the county's Industrial Waste Program and CSO Control Program have dramatically reduced pollutants from CSOs.

For more information about the Sediment Management Program and its accomplishments visit:

• Sediment Management Program history
• Sediment Management Plan Studies (pre-Superfund)
• Duwamish/Diagonal Project  – removed 66,000 cubic yards of contaminated sediment from the Lower Duwamish
• Norfolk Project – removed more than 5,000 cubic yards of contaminated sediment from the Lower Duwamish
• Frequently Asked Questions