Deconstruction of historic mercury accumulation in lake sediments, northeastern United States.

Total atmospheric contribution of mercury (Hg(T)) to lake sediment was estimated using 210Pb-dated sediment cores. Algorithms based on estimates of lake and watershed processes were applied to more accurately assess anthropogenic contributions of Hg to the environment and Hg(T). Factors addressed include: lake-specific background accumulation rates of Hg (Hg(B)), variability of sediment accumulation rates that caused variation in Hg accumulation during the last 100-150 years (Hgv), and variable flux of anthropogenic Hg from the atmosphere (Hg(A)). These fluxes were normalized for sediment focusing using a regional, unsupported 210Pb correction factor to yield Hg(A,F). Time series maps of Hg(A,F) allow for comparison across time and space, and are provided for 1900, 1950, 1975 and 1990 across eastern New York and New England, USA. Deconstruction algorithms reduce inter-/intra-lake variability in Hg accumulation rates and improve temporal coherence. Hg(A,F) started to increase near the end of the 19th century to a maximum between 1970 and 1990, depending on the lake. Maximum Hg(T) across the region ranges from 27.1 to 175 microg/m2 year. Maximum Hg(A,F) ranged from 10.4 to 66.3 microg/m2 year. The timing of Hg(A,F) declines in response to decreased atmospheric deposition may be controlled by in-lake and in-watershed storage and transport of Hg-bearing sediment.

Paleolimnological assessment of Grove and Plow Shop Ponds, Ayer, Massachusetts, USA--a superfund site.

Three sediment cores from each of severely polluted Grove and Plow Shop Ponds, Ayer, Massachusetts, USA, were dated using (210)Pb, characterized for plant macrofossil assemblages, and analyzed for H(2)O, loss-on-ignition, stable Pb isotopes, and concentrations of As, Ca, Cd, Cr, Cu, Fe, Hg, methyl-Hg, Ni, Pb, and Zn. A core from nearby kettle Spectacle Pond, Littleton, Massachusetts, was similarly characterized (except for plant macrofossil assemblages) to assess the regional air pollution signal in sediment for comparison with the six cores. Accumulation rates for metals (mass per area per year), the anthropogenic component (mass per area per year), and total accumulation of the anthropogenic component (mass per area) indicate that As, Cd, Cr, Cu, Hg, methyl-Hg, Ni, Pb, and Zn have accumulated in sediment as a consequence of point source pollution from within the drainage basins of Grove and Plow Shop Ponds. Three distinct sources of pollution are inferred. As is entering Plow Shop Pond via groundwater in the southwest. Cd, Ni, Pb, and Zn are entering the system predominantly at the eastern end of, or upstream from, Grove Pond. Pb also comes from the northwest corner of Grove Pond, the principal source of Cr, Cu, and Hg. These results are consistent with chemistry of modern surface sediments. The history of pollution extends back more than 100 years. Intra- and inter-core variability of concentrations and accumulation rates indicate that much of the pollution was likely in particulate form with little physical redistribution. Recently, concentrations and accumulation rates have generally decreased substantially for those elements present in excessive concentrations in the past. This is a consequence of accumulation of recent, less polluted sediment. In Spectacle Pond, the nearby reference lake, accumulation rates for As, Cd, Hg, and Pb, adjusted for background values and changes in sedimentation rate, increased above background starting in the late 19th century, peaked about 1980, and declined substantially to 2000. These decreases suggest that the anthropogenic (pollution) component of atmospheric deposition of these elements declined after 1980 by at least 50%(As), 80%(Cd), 80%(Hg), and 80%(Pb).