Spatial distribution and trends of total mercury in waters of the Great Lakes and connecting channels using an improved sampling technique

Environment Canada recently developed a clean method suitable for sampling trace levels of metals in surface waters. The results of sampling for total mercury in the Laurentian Great Lakes between 2003 and 2009 give a unique basin-wide perspective of concentrations of this important contaminant and represent improved knowledge of mercury in the region. Results indicate that concentrations of total mercury in the offshore regions of the lakes were within a relatively narrow range from about 0.3 to 0.8 ng/L. The highest concentrations were observed in the western basin of Lake Erie and concentrations then declined towards the east. Compared to the offshore, higher levels were observed at some nearshore locations, particularly in lakes Erie and Ontario. The longer-term temporal record of mercury in Niagara River suspended sediments indicates an approximate 30% decrease in equivalent water concentrations since 1986.

► Basin-wide concentrations of total mercury in Great Lakes surface waters are provided for the first time.
► A clean sampling method is described, stressing isolation of the sample from extraneous sources of contamination.
► Sub-ng/L concentrations of total mercury are observed in most Great Lakes offshore areas.
► Concentrations in the western basin of Lake Erie are consistently the highest observed in the basin.
► The longer-term record of mercury in Niagara River suspended sediments indicates an approximate 30% decrease since 1986.