Sediment Management Program
Contaminated sediment sample from the Lower Duwamish River. December 8, 2004
King County's Wastewater Treatment Division is carrying out a Sediment Management Plan developed in the late 1990s to remediate sediment contamination near county outfalls for combined sewer overflows, or CSOs, in Seattle. Those areas are contaminated with variety heavy metals (lead, copper, zinc), phthalates, polychlorinated biphenyls (PCBs), and polycyclic aromatic hydrocarbons (PAHs). Cleaning up these areas is estimated to cost the county $26 million to $36 million.
During the past 20 years, industrial and stormwater discharges through CSOs to Elliott Bay and the Duwamish River were reduced substantially. Despite that achievement, however, historical, persistent chemical contaminants in sediment continue to pose a risk to aquatic life, wildlife and human health.
Areas with contaminated sediment exist near some CSOs now operated by King County. 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.
Part of the cleanup process for these sites is identifying sources of pollution both past and present. In previous cleanups, many of the sources of contamination have come from historic uses upstream of King County CSOs including sewer outfalls built early in Seattle's history that are now intercepted and treated at one of the regional treatment plants.