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 spatial patterns and contributions to surface waters. Annual wet mercury deposition averaged 7.4 mu g Hg/m(2).yr and showed significant variations between 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 found 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 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.