Atmospheric Mercury Deposition to Lakes and Watersheds

Abstract

Historic increases in atmospheric mercury loadings caused by anthropogenic emissions are documented from sediment cores from seven remote headwater lakes in Minnesota and northern Wisconsin. Whole-basin changes in Hg accumulation, determined from lakewide arrays of 210Pb-dated cores, show that regional atmospheric Hg deposition has increased by a factor of 3.7 since preindustrial times. The relative increase is consistent among lakes, although preindustrial Hg accumulation rates range from 4.5 to 9 µg/m2 per year, and modern rates range from 16 to 32 µg/m2 per year. The distribution of these rates is highly correlated with the ratio of catchment area to the lake surface area. Modern and preindustrial atmospheric deposition rates of 12.5 and 3.7 µg/m2 per year are calculated from this relationship, along with the relative contribution of Hg from the terrestrial catchment. Release of atmospheric Hg from catchment soils accounts for 20—40% of the Hg loading to the lakes, depending on catchment size. This export represents about 25% of the atmospheric Hg falling on the catchments under both modern and preindustrial conditions. The absence of geographic trends in either Hg deposition rates or their increase implies regional if not global sources for the Hg entering these lakes.