Spatial and temporal patterns in mercury contamination in sediments of the Laurentian Great Lakes.

Data from recent sediment surveys have been collated and mapped in order to determine the spatial distribution of mercury in sediments across the entire Great Lakes basin. Information from historical surveys has also been collated in order to evaluate temporal trends. Lake Huron (2002) exhibited the lowest mercury concentrations (lakewide average concentration, 0.043 microg/g); Lakes Michigan (1994-1996) and Superior (2000) also exhibited relatively low levels (lakewide averages of 0.088 and 0.078 microg/g, respectively). The western basin of Lake Erie (1997-1998, 0.402 microg/g) and Lake Ontario (0.586 microg/g) exhibited the highest levels. Sources of mercury contamination in Lakes Erie and Ontario are primarily attributed to loadings from historical sources, including chlor-alkali production in the Detroit, St. Clair, and Niagara Rivers. The spatial distributions of mercury in sediments of Lakes Huron and Superior suggest that natural geochemical factors are an influence. Surficial sediment mercury contamination was found to have decreased markedly since the late 1960s and 1970s. Decreases in lakewide average sediment concentrations of mercury over this time period ranged from approximately 25% for Lake Ontario to 80% for Lake Huron.