DECLINES IN HG FLUXES TO LAKES INFERRED FROM VT AND NH LAKE SEDIMENTS - ARE WATERSHEDS LEAKING HG?

Paleolimnological assessments of deepwater lake sediments provide an excellent tool for measuring historical Hg accumulation, and for estimating present-day Hg fluxes. In the present study, we measured total Hg in 210Pb-dated sediment cores from 13 lakes across VT and NH. These lakes had watersheds which were least-disturbed, and which ranged widely in their watershed-lake area ratios.

Across the study set, fluxes rates ranged from between 6 and 22 ug/m2/yr at the core bases, to between 20 and 82 ug/m2/yr at modern times. These values are in agreement with results from Midwestern and Adirondack lake sets. In the present lake set, there exists a synchronous increase in flux rates to all lakes, corresponding approximately to the 1850 core strata. In all but two lakes, Hg fluxes are presently declining significantly, and this reversal began in all cases by the 1980 core strata. We attribute these declines to reductions in atmospheric Hg contributions.

The historical direct atmospheric Hg contribution to the study lake set was estimated by the method of Swain and Engstrom at 9.3 ug/m2/yr, and the modern direct contribution at 19.7 ug/m2/yr. This latter value is within the range of atmospheric wet-Hg deposition reported from a site at Underhill, VT over the past several years. Watershed Hg retention for historic and present times was estimated by the method of Lorey and Driscoll and indicates an increasing importance of Hg release from watersheds. Isotopic d13C and d15N are presented in the context of Hg stratigraphy.

Our findings present evidence that: Hg fluxes to northeastern lake waters are presently in decline; these fluxes remain elevated in relation to baseline contributions, and; reductions in fluxes may be ‘damped’ by watershed influences. The present study-set provides a baseline against which projected declines in atmospheric Hg deposition can be evaluated.