Publication Summary

Sediment cores from Lake Ozette, Lake Sammamish, and Lake St. Clair were age-dated and analyzed for mercury as part of a long-term study evaluating mercury trends through sediment cores across the state. Mercury concentrations have significantly declined at Lake Sammamish since the 1940s, while Lake Ozette has experienced a leveling off of mercury loading with possible slight declines over the last decade. Mercury levels in sediments at Lake St. Clair have increased during the past 15-20 years.

See the link below to the focus sheet.

In 2005, the Washington State Legislature provided funding to the Washington State Department of Ecology (Ecology) to develop a long-term monitoring program for mercury in freshwater systems. Mercury is the first pollutant to be studied under the Persistent, Bioaccumulative, and Toxics (PBT) Reduction Strategy.

This report presents results of the first year of evaluating mercury deposition through the use of age-dated sediment cores.

During the fall of 2006, Ecology collected sediment cores and surface sediments from the following lakes: (1) Lake Ozette, located in the northwest corner of the Olympic Peninsula, (2) Lake Sammamish in east King County, and (3) Lake St. Clair in Thurston County. The selected lakes reflect potential impacts from different sources: trans-Pacific, large urban area, and the only coal-fired power plant in Washington, respectively.

Sediment cores were dated, using the constant rate of supply model, by examining stable lead, 210lead, and percent solids. Sediments were also analyzed for total mercury, total organic carbon, selenium, and grain size.

Lake Ozette sediments did not show mercury above background (natural) levels until the mid 1900s, and peaked in the mid 1990s. Sediment cores for Lakes Sammamish and St. Clair displayed increased mercury levels beginning in the early 1900s, and peaked during World War II industrialization in the regions.

Mercury concentrations in recently deposited sediments at Lake Ozette have remained steady with possible slight declines. Mercury levels in sediments at Lake Sammamish have steadily declined since reaching maximum levels in the 1940s. Recent trends at Lake St. Clair reveal steadily increasing levels of mercury over approximately the last 20 years.