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

Abstract

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.