Six-Year Trend (1990−1995) of Wet Mercury Deposition in the Upper Midwest, U.S.A.

Total wet mercury deposition was monitored weekly at six Upper Midwest, USA, sites for a period of six years, 1990--1995, to assess temporal and spatial patterns and contributions to surface waters. Annual wet mercury deposition averaged 7.4 {micro}g Hg/m{sup 2}{center{underscore}dot}yr and showed significant variations between sites and increased trends averaging 0.60 {micro}g Hg/m{sup 1}{center{underscore}dot}yr [8%/yr] over the period. Warm (rain) season wet mercury deposition was found to average 77% of total annual wet deposition. Warm (rain) and cold (snow) season trends showed increases of 0.31 [5.5%/yr] and 0.29 {micro}g Hg/m{sup 2}{center{underscore}dot}yr [17%/yr], respectively. Average annual precipitation depth showed a nonsignificant increase of 0.25 cm/yr [0.4%/yr] for the period. Differences in precipitation depth, mercury concentrations, and wet mercury deposition among sites were noted. Methylmercury wet deposition, measured in 36 weekly samples, averaged about 0.18 ng/L [1.5% of total mercury] in rain and strongly correlated with total mercury, major ions, and precipitation depth.