This report summarizes existing information on water quality and other indicators of the environmental conditions of Lake Union/Ship Canal.  King County and other agencies have collected data on water quality, sediment chemistry, and benthic communities in Lake Union/Ship Canal. These data were evaluated and summarized as a part of this assessment in order to describe current conditions, identify long-term trends, and review compliance with Washington State standards. Data include physical parameters in the water column (temperature, dissolved oxygen, salinity, pH, and turbidity/total suspended solids), chlorophyll-a, nutrients, fecal coliform bacteria, metals, and various groups of organic compounds. Data on metals and organic compounds in sediments, sediment toxicity, and benthic community structure, were also analyzed.

Results showed that bacteria concentrations exceeded water quality standards and are typically highest near the Ballard Locks and decrease moving upstream.  Despite occasionally not meeting water quality standards, concentrations in these areas have declined over the past several decades.  This decline is likely due, in part, to increased CSO control.  Water temperature and dissolved oxygen (DO) did not meet water quality standards.  The high temperatures and low DO in Lake Union/Ship Canal threaten salmonids and other aquatic life at certain times of the year.  Salinity in Lake Union/Ship Canal is substantially influenced by the intrusion of salt water via the Locks during the summer; the amount varies greatly year to year. Nutrient concentrations in the system have decreased over the years. Metals data indicate that concentrations in Lake Union/Ship Canal have not exceeded the Washington State water quality criteria for aquatic life or the EPA Human Health Criteria.  Polychlorinated biphenyls (PCBs) and bis(2-ethylhexyl)phthalate (BEHP) were sometimes detected in the limited number of samples collected at concentrations exceeding EPA Human Health Criteria. Although data indicate that concentrations of other organic chemicals have not exceeded water quality criteria. Sediments in many parts of Lake Union and the Ship Canal did not meet sediment quality standards.  Sediment toxicity samples indicated toxicity at many sites in Lake Union and the Ship Canal. The benthic community structure is mostly comprised of pollution-tolerant taxa and is less diverse than Lake Washington and Lake Sammamish.