Airborne Mercury in Precipitation in the Lake Superior Region

Mercury was measured in accumulated snow (March 1982) sampled from around Lake Superior and in rainfall from Duluth, Minnesota (June–September 1982 and March–November 1983), Forbes Township, and Dorset in northwestern and central Ontario, respectively (May–September 1983). Methods of melting snow and collecting rain samples were investigated to avoid loss of mercury during the melting process and sample shipment and storage. Low concentrations in snow and rain required greater analytical sensitivity. A detection limit of 0.008 ± 0.004 μg/L of mercury (N = 26) was attained using the cold vapor technique; and by utilizing a gold gauze amalgam accessory for preconcentration, a detection limit of 0.005 ± 0.003 μg/L (N = 13) was attained. Regional comparisons of mercury accumulation in the snow pack across the northern parts of Minnesota, Wisconsin, Michigan, and Ontario show highest values, 0.10 ± 0.17 μg/L Hg, in the Grand Rapids, Minnesota, area and lowest values in remote areas of Minnesota and Ontario. Rainfall concentrations of total mercury were substantially higher than snow accumulation and were mainly of inorganic forms (73%). The highest amounts of mercury were measured in rainfall at Duluth, Minnesota (3.4 μg/L) during June 1982 and at Forbes Township, Ontario, (1.2 μg/L) during July 1983. Substantially lower concentrations were observed in the other months of 1982 and 1983. There is no unique association of the observed mercury concentration with the air parcel history. However, the single highest mercury concentration in rainfall observed at Duluth, Minnesota, occurred when the 850-mb air parcel back trajectory indicated upwind corridors to the N-NW and the highest concentration at Forbes Township, Ontario, was associated with an 850-mb trajectory from W-NW. Significant positive correlation coefficients (0.05) were obtained with other measured components in rain at Duluth, Minnesota, including S, B, Ca, Mg, Na, Fe, Mn, and Zn in 1983. Mercury concentrations in precipitation and in lake water for precipitation-dominated lakes are similar and indicate that atmospheric deposition can be the major source of mercury for some lakes in the Lake Superior region.