Historical and present fluxes of mercury to Vermont and New Hampshire lakes inferred from 210Pb dated sediment cores
Introduction
Environmental mercury (Hg) contamination of aquatic ecosystems is a pervasive environmental problem, with potentially severe toxicological consequences for humans and piscivorous wildlife (USEPA, 1997; Evers et al., 1998; National Academy of Sciences, 2000). The majority of Hg contaminating aquatic ecosystems is understood to be anthropogenically derived and atmospherically deposited (Fitzgerald et al., 1998). In poorly buffered, undisturbed lakes, Hg is transported through watersheds by high molecular weight dissolved organic matter, and the proportion of this Hg which is neither methylated nor re-evaded as Hg0 is deposited to the sediments (Lee and Iverfeldt, 1991; Mierle and Ingram, 1991; Driscoll et al., 1994a; Hurley et al., 2000). Several studies have underscored the importance of watershed size in controlling Hg fluxes to sediments (Engstrom et al., 1994; Mielli, 1995; Lorey and Driscoll, 1999). Wetland area (Driscoll et al., 1994a; St. Louis et al., 1994), land use (Hurley et al., 2000), and pH (Rada et al., 1993) have also been shown to influence delivery of Hg to sediments.
Paleolimnological studies have been used to estimate whole-lake surficial sediment Hg burdens (Gilmour et al., 1992; Rada et al., 1993), and, when coupled with fine-resolution 210Pb dating, to estimate fluxes of Hg to lake sediments for both modern and historical time frames (Ouellet and Jones, 1983; Engstrom et al., 1994; Von Gunten et al., 1997; Hermanson, 1998; Lockhart et al., 1998; Lorey and Driscoll, 1999). Numerous multiple lake-sediment studies show anthropogenic Hg contamination to be a recent phenomenon (∼1850 to present), coincident with industrialization, and fossil fuel and waste combustion (Landers et al., 1998; Pirrone et al., 1998). Engstrom and Swain (1997) have shown that Hg deposition to lakes down-gradient of Midwestern urban centers is declining in response to recent reductions in Hg emissions. While there exists significant uncertainty in the estimation of Hg fluxes to individual lakes (Mielli, 1995; Gottgens et al., 1999), the pattern evident in so many paleolimnological Hg studies is clear: anthropogenically derived Hg has increased by a factor of 2–8× in the sediments of lakes throughout the Northern Hemisphere (Landers et al., 1998).
In the present study, we analyzed a series of single, short-cores taken from undisturbed lakes in Northern New England, to evaluate four specific hypotheses: (1) that Hg fluxes have increased proportionally to increases observed in other studies; (2) that Hg fluxes have decreased in recent years; (3) that Hg fluxes increase with increasing watershed area to lake area ratio; and, (4) that paleolimnologically inferred atmospheric total Hg deposition estimates compare well with measured total wet+dry Hg deposition.
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Site characteristics
The lakes selected for this study lie within the borders of Vermont and New Hampshire, and are characteristic of undisturbed lakes within the Northeastern Highlands Ecoregion (Omernik, 1987; USEPA, 2000). All are small, 8.1–38.9 hectare drainage lakes occupying undisturbed forested catchments, which are a mix of deciduous or coniferous vegetation overlying soils ranging from stony to silty loams. Bedrock geology is largely schistose or granitic, and most watersheds are poorly buffered. Some
210Pb dating and sedimentation rates
For all ten lakes, supported 210Pb concentrations ranged from 0.28 to 2.5 pCi g−1, and the number of deeper core intervals from which supported 210Pb was estimated ranged from one (McConnell Pond) to six (Intervale Pond). Inventories of unsupported 210Pb in the ten cores ranged from 6.77 to 22.02 pCi cm−2, which is equivalent to 210Pb fluxes of 0.22–0.71 pCi cm−2 yr−1 (Fig. 2 and Table 2). These 210Pb fluxes are similar to regional estimates of atmospheric 210Pb deposition (0.5 pCi cm−2 yr−1), which
Summary
Estimated Hg fluxes across the 10 lakes sampled in this study provide three distinct signals. First, there exists a synchronous increase in Hg fluxes across all lakes corresponding to the period 1850–1875, and Hg fluxes peak between 1955 and the present. Peak Hg fluxes are on average 3.9 times greater than average pre-1850 values, which is attributable to increased atmospheric deposition of Hg over the core record. Second, the relationship between the watershed:lake area ratio and Hg flux has
Acknowledgements
We thank Steve Couture, Bob Estabrook, and Steve Landry of the NH Department of Environmental Services for their project support; Ed Glassford, Kate Peyerl, and Kellie Merrell of the VT Department of Environmental Conservation, for analytical chemistry, coordinating both field sampling and lab processing, and for figure preparation; Kelly Thommes of the St. Croix Watershed Research Station for assistance in the 210Pb dating; and, Dr. Ruth Mickey of the University of Vermont for assistance with
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