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MERCURY DEPOSITION AND WATER QUALITY IN THE UPPER MIDWEST, USA
Citation:
Glass, G E., J. A. Sorensen, AND G. R. Rapp Jr. MERCURY DEPOSITION AND WATER QUALITY IN THE UPPER MIDWEST, USA. Presented at Save Lake Superior Association Annual Meeting, Split Rock State Park, MN, August 12, 2000.
Description:
Total wet mercury deposition was monitored weekly at six Upper-Midwest, USA sites for a period of six years, 1990-195, to assess temporal and spatial patterns, and contributions to surface waters. Annual wet mercury deposition averaged 7.4 g Hg/m2yr., showed significant variations between sites and increased trends averaging 0.60 g Hg/m2yr. (8% /yrs.) over the period (p<0.0001).
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 g Hg/m2yr.) (17 % /yr.), respectively (p>0.01). Average annual precipitation depth showed a non-significant increase of 0.25 cm/yr. (0.4% /yr.) for the period.
Significant differences in precipitation depth, mercury concentrations and wet mercury deposition among sites are attributed to differences in meteorology and proximity to emission source areas. Methylmercury wet depositon, 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. Total mercury and methylmercury concentrations in precipitation are compared with surface water concentrations after watrshed processing.
Water quality and fish sampling were conducted on eighty Minnesota lakes over the two year period, 1995-96. Samples of surface water and northern pike and/or walleye specimens (10, over a range of desired sized) were obtained from each lake. More than 1,400 mercury analyses were performed to make quantitative comparisons with fish data obtained previously from the same lakes up to 20 years earlier.
The results indicate that out of 80 study lakes, 75 had sufficient past residue data for statistical comparisns. Of those, 43 lakes (57%) show lower fish mercury levels for recent data compared to those reported previously, 19 lakes (25%) show greater mercury concentrations, and 13 lakes (17%) show no significant difference.
Water quality data statistical analyses, comparing past and present measurements, show significan increases in pH (+0.2 to +0.3) and alkalinity (+ 19 to + 39 ueq/L) accompanied by a decrease (- 1.2 to - 1.5 mg/L) in sulfate concentrations. These trends are in agreement with observed trends in wet sulfate deposition which have been steadily decreasing for over a decade since state regulatory control measures were enacted.
Watershed factors influence the differing trends in mercury residue levels. Fish mercury concentrations show positive correlations with water color and methylmercury concentrations, and plankton mercury, and negative correlations with pH and alkalinity. In general, reservoir lakes showed decreases in fish mercury, while lakes with low watershed to lake area ratios, and increased development, showed increases.