History of mercury inputs to Minnesota lakes: Influences of watershed disturbance and localized atmospheric deposition

The history of mercury (Hg) inputs to 55 Minnesota (U.S.A.) lakes was reconstructed from ²¹⁰Pb (lead-210)-dated sediment cores to determine if erosion of soils from agriculture and urbanization contributes a significant loading of Hg to lakes, and whether lakes near Hg-emitting facilities receive appreciable local atmospheric deposition. Modern (1994-1997) Hg accumulation and Hg flux ratios (modern : preindustrial) increase significantly with the percentage of watershed area under urban or agricultural land-use. Both past and modern Hg accumulation rates are strongly correlated with the flux of total aluminum (Al), a tracer for soil erosion. Modern Hg accumulation rates are substantially higher in the Minneapolis-St. Paul metropolitan area and in agriculturally dominated south-central Minnesota than in the forested northeastern part of the state, largely because of erosional inputs of soil-bound Hg from disturbed catchments. Modern Hg loading from direct atmospheric deposition is also greater in the metropolitan region than in the rural areas of south-central or northeastern Minnesota. However, some of the excess loading to urban lakes may also be a legacy of formerly high Hg deposition to urban watersheds. A decline in local Hg emissions from peak levels in the 1970s coupled with reduced erosional inputs has cut Hg loading to many metro-area lakes by more than half.