Historical Trends of Mercury Deposition in Adirondack Lakes

There is interest in historical patterns of total mercury (HgT) deposition because of concern over contamination to remote lakes by atmospheric Hg deposition. Sediment cores were collected from eight remote lakes in the Adirondack region of New York. Concentration and flux profiles were reconstructed from preindustrial to modern values based on 210Pb dating. All cores exhibited an increase in HgT since 1850. The ratio of modern HgT flux to preindustrial HgT flux in sediments ranged from 1.6 to 5.7, with an average value of 3.5. The lake cores showed an increase in sediment HgT flux with increasing ratio of watershed area to lake surface area. Our analysis suggests that in 1850 Adirondack watersheds retained 95% of atmospheric deposition of Hg. This fraction has decreased since about 1930 to 78% currently, a value that agrees with watershed mass balance studies. Extrapolation of the relationship between sediment HgT flux and watershed area to lake surface area to a value of 1 suggests that the flux of atmospheric HgT deposition was 5.0 μg m-2 yr-1 in preindustrial times and is 8.9 μg m-2 yr-1 currently. This modern estimate of Hg deposition to lake sediments is somewhat lower (∼36%) than direct estimates of atmospheric HgT deposition.