Six-year trend (1990-1995) of wet mercury deposition in the Upper Midwest.USA

Total wet mercury deposition was monitored weekly at six Up per Midwest, U. S.A. sites for a period of six ye a rs, 1990-1995, to assess temporal and s patial patterns and contributions to surface waters. Annual wet mercury dep osition averaged 7.4 mu g Hg/m(2).yr and showed significant variations betw een sites and increased trends averaging 0.60 mu g Hg/m(2).yr [8%/yr] over the period (p < 0.0001). Warm (rain) season wet mercury deposition was foun d to average 77% of total annual wet deposition. Warm train) and cold (snow ) season trends showed increases of 0.31 [5.5%/yr] and 0.29 mu g Hg/m(2).yr [17%/yr], respectively (p < 0.01). Average annual precipitation depth show ed a nonsignificant increase of 0.25 cm/yr [0.4%/yr] for the period. Differ ences in precipitation depth, mercury concentrations, and wet mercury depos ition 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 an d strongly correlated with total mercury, major ions, and precipitation dep th.