This report summarizes the approach, findings, and recommendations of the Water Quality Assessment and Monitoring Study (“Study”). King County undertook the Study to inform the 2018 update to its combined sewer overflow (CSO) control plan and inform future activities undertaken by King County and other entities to improve water quality in the region. The Study assessed past and present water quality conditions in the waterbodies where the County’s remaining 14 CSOs that overflow more than an average of once per year discharge: Lake Union/Ship Canal, Elliott Bay, and the Duwamish Estuary. The assessment estimated annual contaminant loadings to these study areas from different pollutant pathways, reviewed planned projects to understand how contaminant loadings may change, and identified impairments that will remain in 2030. Nine technical reports describe different aspects of water quality in the study areas.
Actions in the past 50 years have improved water quality: nutrient and bacteria have decreased in surface waters and sediment cleanups have improved sediment quality. Despite improvements, some water quality is still impaired in the three study areas. King County CSOs contribute to identified impairments. Other pathways also contribute to identified impairments, such as stormwater discharges, leaching from boat bottom vessel paint, and leaching from creosote-treated wood pilings. Planned actions will reduce contaminant loadings by 2030. Reduction in frequency of untreated CSO discharges will reduce bacteria loadings. Stormwater management and treatment will reduce loadings from stormwater. Laws to limit copper content in antifouling vessel bottom paint, and automobile brake pads will reduce copper. Creosote-treated wood piling removal will reduce organic chemical loading, and contaminated sediment cleanup will reduce sediment contamination levels. However, water quality challenges will remain in 2030. Recommended next steps for the region include: reduce the frequency of untreated CSO discharges, implement planned water quality improvement projects and programs, and monitor changes and add to the body of knowledge over time.
