Final Report: Natural and Anthropogenic Sources of Mercury to the Atmosphere: Global and Regional Contributions

EPA Grant Number: R829796
Title: Natural and Anthropogenic Sources of Mercury to the Atmosphere: Global and Regional Contributions
Investigators: Fitzgerald, William F. , Engstrom, Daniel
Institution: University of Connecticut , Science Museum of Minnesota
EPA Project Officer: Chung, Serena
Project Period: January 1, 2003 through December 31, 2005
Project Amount: $897,219
RFA: Mercury: Transport, Transportation, and Fate in the Atmosphere (2001) RFA Text |  Recipients Lists
Research Category: Mercury , Air Quality and Air Toxics , Safer Chemicals , Air

Objective:

The atmosphere provides the primary pathway for the transport and deposition of mercury (Hg) at the earth’s surface. Both natural (e.g., volcanoes) and human-related sources (e.g., coal-fired power plants) contribute to the atmospheric burden, dispersion, deposition, and accumulation of Hg in soils, fresh and marine waters, sediments, and biota. Two of the principal issues relating to environmental Hg cycling and contamination are: (1) deciphering the connections between atmospherically derived Hg loadings and the biologically mediated production of methylmercury (MeHg) and its bioaccumulation in aqueous systems; and (2) identifying the local, regional, and global sources contributing to Hg deposition. The risks of human exposure to MeHg, especially prenatally, and the potential deleterious ecological consequences from localized to global-scale mercury pollution, have given much impetus to mercury studies and regulatory activities internationally. This project is tackling the challenge of determining Hg loadings, attributing sources to local, regional, or global pools, and assessing the anthropogenic impact in important regions of North America (i.e., West Coast [Glacier Bay, Alaska and Washington State], East Coast of North America [Cormack, Newfoundland and Maine], Mid-Continent [Minnesota], and Southeastern United States. [Florida]).Three research strategies have been used: (1) Hg deposition archives provided by dated and analyzed lake sediment cores; (2) estimates of contemporary Hg fluxes provided by Hg deposition collectors; and (3) 210Pb deposition collections to provide an index that would yield precise estimates of the local/regional contributions to Hg deposition. The findings to date show secular increases (associated with the Industrial Revolution) in Hg accumulation patterns in lake sediments from SE Alaska and western Newfoundland that are consistent with sources associated primarily with the global pool of Hg (modern/pre-industrial enrichment of 2.2–2.9), some evidence of recent Asian contributions to the SE Alaskan region, and local impact at the Florida sites.

The importance of the atmosphere in transporting mercury (Hg) intra- and interhemispherically (elemental Hg [Hg0] is the major species transported) has been established, and knowledge of the behavior and fate of Hg in the atmosphere is increasing. However, the relative contribution of natural and anthropogenic sources is uncertain, the mechanisms by which Hg is removed from the atmosphere are poorly understood, and the processes and reactions linking inputs of anthropogenic Hg, especially from the atmosphere, and the bioaccumulation of methylmercury (MeHg) in sensitive aquatic ecosystems have not been firmly established. Recently, we have been investigating the cycling and fate of MeHg in freshwater (Hammerschmidt and Fitzgerald, 2006a; Hammerschmidt, et al., 2006) and marine systems (Fitzgerald, et al., 2007), and have reported a strong correlation/connection between the MeHg content of freshwater fish and mosquitoes with atmospheric Hg deposition and contamination (Hammerschmidt and Fitzgerald, 2005; Hammerschmidt and Fitzgerald, 2006b). This project addresses questions relating to natural and anthropogenic contributions from global and localized sources, the identification of Hg deposition with a regional origin (e.g., United States), and the examination of spatial and temporal trends (e.g., increases, declines) in atmospheric Hg deposition for predictive/modeling purposes. The specific objectives were to address the following hypotheses:

(1) Atmospheric mercury deposition in southeastern Alaska can be viewed as an integrated sample of global Hg pollution in the Northern Hemisphere, and thus provides a refined estimate for the hemispheric component of Hg deposition and a baseline for sites closer to local and regional emission sources.

(2) Atmospheric mercury deposition in Canada’s maritime provinces is elevated above the northern hemispheric average by regional contributions from the industrialized northeast/midwest United States and southeast Canada, and can be separated into global and regional components by a comparison of sedimentary archives.

(3) The linear correlation between Hg and 210Pb found in rainwater from other remote and semi-remote locations is observed in southeastern Alaska, and this behavior can be used to constrain the global-scale wet atmospheric flux of Hg to lakes and watersheds of temperate North America.

(4) At less remote sites, enhanced atmospheric Hg deposition that is locally/regionally derived is indicated by deviations from the Hg and 210Pb relationship observed in southeastern Alaska.

(5) Sediment archives will show, when corrected for climatology using 210Pb, that the Pacific and Atlantic seaboards of North America received equivalent preindustrial atmospheric Hg fluxes, and provide a baseline for assessing the global component of anthropogenic Hg deposition at any given locality.

This study has yielded high quality biogeochemical data needed for quantitative assessment of the scale and historical record of potentially enhanced atmospheric Hg deposition related to increased human-related Hg emissions over the past 150 years (Fitzgerald and Lamborg, 2003). This information will be especially useful in improving models of the global and regional biogeochemical and atmospheric cycling of Hg, and assessing the impact associated with atmospherically transported pollutant-derived Hg in the environment.

Summary/Accomplishments (Outputs/Outcomes):

Experimental Design

We combined two research strategies: (1) archives provided by lake sediment cores; and (2) collocated Hg and 210Pb deposition collectors, to derive precise estimates of the modern Hg flux to eastern and western North America. Our lake studies were conducted in the lacustrine environs of the Tongass National Forest of southeastern Alaska (June 2003) and Deer Lake/Cornerbrook, Newfoundland (August 2004). This work is complemented by event-scale Hg and 210Pb depositional investigations at the lake study areas and other key geographic regions of the United States that display a range in Hg deposition as determined from the Mercury Deposition Network (MDN). The regions included the west coast, mid-continent, east coast, and the southeastern regions of the United States.

Lake Sediment Archives

Contemporary and historical information provided by careful measurements and scrupulous examination of natural archives has been critically important to the assessment and understanding of major biogeochemical cycles (Electric Power Research Institute [EPRI], 1996). Lake sediments, peat bogs, and ice cores have been used successfully for regional and global studies of modern and historical Hg atmospheric depositional patterns (Swain, et al., 1992; Engstrom, et al., 1994; Engstrom and Swain, 1997; Fitzgerald, et al., 1998; Benoit, et al., 1998; Lamborg, et al., 2002; Fitzgerald, et al., 2005). Lakes are especially valuable, because they occur over broad geographic regions. These natural archives are particularly well suited to examine the global/regional nature of atmospheric Hg dispersion and deposition, and to assess the impact of human-related Hg emissions on the natural Hg cycle as recorded in accumulating lakes sediment. Recently, we used lake-sediment archives to reconstruct atmospheric Hg deposition to Arctic Alaska over the last several centuries (Fitzgerald, et al., 2005). The lake-sediment results allowed us to evaluate polar Hg depletion events and constrain a contemporary lake/watershed mass balance with real-time measurement of Hg fluxes in rainfall, runoff, and evasion.

We evaluated recent changes in atmospheric mercury deposition for two regions of North America over which different mercury emission sources should dominate. Trends in atmospheric deposition were inferred from Hg accumulation rates in 210Pb-dated sediment cores. We used the archives provided by lake sediments to make an assessment of the extent to which atmospherically transported Hg has impacted southeastern Alaska, a region remote from localized emission sources (hypothesis 1). The magnitude of increase in Hg accumulation in these sedimentary records provides a global reference against which changes in Hg deposition from other regions may be compared. There are no significant anthropogenic Hg sources anywhere in coastal southeastern Alaska and limited human development within several hundred kilometers of lakes in the region. Historical records from lake sediments and peat indicate that atmospheric deposition of Hg has increased 2–4 times in the northern hemisphere since industrialization (i.e., Swain, et al., 1992; Benoit, et al., 1998; Fitzgerald, et al., 1998). However, our preliminary data from Glacier Bay National Park suggested that the increase in southeastern Alaska was at the lower end of this range (ca 1.8–2.1 times; Engstrom and Swain, 1997). We used lake-sediment cores collected in Newfoundland to assess the increase in atmospheric Hg deposition experienced by a region “down wind” of the relatively industrialized eastern North America (hypothesis 2). We expected the magnitude of change in these sedimentary archives should be greater than that measured in southeastern Alaska, with the difference attributable to regional emission sources affecting the Atlantic coast. Our previous efforts have indicated that in Nova Scotia, and by extension in Newfoundland, as much as 40% could be due to regional source and/or enhanced oxidation phenomena (Lamborg, et al., 2002). Cores from Nova Scotia lakes suggest that present-day Hg deposition in maritime Canada is elevated over preindustrial rates by a factor of 4.3–5.4 times (Lamborg, et al., 2002). We also expected that sediment archives would show, when corrected for climatology using 210Pb (hypothesis 5), that the Pacific and Atlantic seaboards of North America received equivalent preindustrial atmospheric Hg fluxes, and provide a baseline for assessing the global component of anthropogenic Hg deposition at any given locality.

Southeastern Alaska. In June 2003, sediment cores were collected from four carefully chosen study lakes near Pt. Adolphus on Chichagof Island in the Tongass National Forest, southeastern Alaska. The selected lakes occur in a relatively tight cluster and are located within 25 km of our atmospheric mercury and 210Pb precipitation collectors at Bartlett Cove, Glacier Bay National Park headquarters. GIS analyses were used to determine precise surface areas, bathymetry, and catchment characteristics. Six sediment cores were obtained from widely spaced locations in the deeper regions of each lake by means of an HTH-Teknik gravity corer equipped with 7 cm diameter polycarbonate core tubes. The cores were sectioned into 1–2 cm increments in the field, and later shipped to the St. Croix Watershed Research Station (SCWRS) labs (MN) for processing and 210Pb dating. All core intervals were analyzed for water content, bulk density, and loss-on-ignition (organic content), and then freeze-dried. Hg analyses were carried out at the University of Connecticut using an automated direct mercury analyzer (Milestone DMA 80) that requires no sample preparation prior to analysis. Additionally, comparison Hg analyses were done using a microwave/acid digestion procedure with cold vapor atomic fluorescence spectroscopy (CVAFS; Lamborg, et al., 2002).

There is striking uniformity in the empirically established 210Pb flux among core sites surrounding Chichagof Island. The grand-mean for all 24 cores from the Tongass National Forest is 0.34 ± 0.06 pCi cm-2 yr-1. The modern Hg flux (i.e., atmospheric and watershed contributions) estimated from Hg measurements made in these lake sediment cores is 16 ± 4 μg m-2 yr-1. The present day Hg accumulation rate normalized to pre-industrial values indicates that atmospheric deposition of Hg has increased by a factor of 2.9 ± 0.5. This increase is substantially greater than that measured in the three cores collected by Engstrom and Swain (1997) in Glacier Bay National Park in 1987 (2.0 ± 0.1). Moreover, the increase in the 2005 Chichagof cores for stratigraphic intervals corresponding to about 1987 is very close to that measured at the top of the 1987 Glacier Bay cores; that is to say, the two core-sets show almost identical changes in mercury accumulation from about 1800 to 1987, while the new Chichagof cores record a substantial up-tick in mercury flux within the last two decades. This is a surprising and critical finding, as it has implications for changes in emission sources affecting southeastern Alaska and the northwest coast of North America.

Newfoundland. Using the field methods described above, sediment cores were collected from four carefully chosen study lakes near Deer Lake/Cornerbrook, western Newfoundland in August 2004. Three of the selected lakes are close together, approximately 30 km west of the atmospheric precipitation collectors (MDN station) at Cormack; the fourth site is about 60 km to the east of Cormack. Six sediment cores were obtained from widely spaced locations in the deeper regions of each lake. As for the southeast Alaska cores, GIS analyses have been used to determine precise surface areas, bathymetry, and catchment characteristics. Newfoundland lake cores were sectioned into 1–2 cm increments for processing, 210Pb dating, and Hg analysis according to the same procedures described for the southeastern Alaska cores.

The empirically established 210Pb flux for western Newfoundland is 0.33 ± 0.21 pCi cm-2 yr-1. This indicates that there is more variability than was associated with the southeastern AK lakes, but cores within lakes are similar. The modern Hg flux (i.e., atmospheric and watershed contributions) estimated from Hg measurements made in these lake sediment cores is 18 ± 4 μg m-2 yr-1. The present day Hg accumulation rate normalized to pre-industrial values indicates that atmospheric deposition of Hg has increased by a factor of 2.2 ± 0.3 in Newfoundland. This is significantly lower than the increase observed in the lake cores from southeastern Alaska (2.9 ± 0.5), and furthermore, is very gradual over the last two decades and does not exhibit the sharp increase evident in the Alaskan records. We initially expected the opposite pattern—that Newfoundland would show a larger historical increase in mercury deposition as compared to Alaska, due to the influence of Hg emission sources in the industrial Northeast. However, these results suggest an entirely different scenario, one in which rising Hg emissions from Asia (and China in particular) have substantially increased Hg deposition in southeastern Alaska, while steady or declining mercury emissions from North American sources have caused Hg deposition to level off in Newfoundland. Recent emission inventories (Pacyna, et al., 2006) clearly show that China has become the dominant source region for global Hg emissions, and that these Asian emissions have likely offset declining emissions from North America and Europe (Lindberg, et al., 2007).

Current Hg and 210Pb Deposition

The accuracy of contemporary and historic reconstructions of atmospheric depositional patterns for Hg is aided greatly by incorporating the constraint of real-time precipitation deposition determinations into the experimental design. In the current research, we measured Hg deposition at Glacier Bay, and made use of data from relevant MDN stations (2007). The modern atmospheric Hg flux estimated using archival data from lake sediments has been compared to these “ground truthing” measurements. Biogeochemical investigations benefit from the availability of tracers for processes, reactions, and spatial/temporal scales. Indeed, significant environmental insight can be gained by examining substances of interest (e.g., Hg) relative to specific environmental indicators. Our past research has pointed to a linear correlation between Hg and 210Pb in rainwater from remote and semi-remote locations (Lamborg, et al., 2000; Fitzgerald, et al., 2005). The correlation between Hg and 210Pb in rainwater suggests analogous removal mechanisms for these two species, namely, gas-phase conversion from a more volatile (Hg0; 222Rn) form to a particle-reactive form (Hg2+; 210Pb2+). The correlation appears to be nearly universal with agreement in observations from mid-continent, coastal, and mid-ocean locations. The Hg:210Pb relationship from remote locations, like southeastern Alaska, provides a robust measure of global-source Hg deposition (hypothesis 3), one that is independent of local climatological conditions and thus broadly applicable across large geographic regions. Differences in precipitation depth-weighted Hg concentrations in precipitation at the MDN sites raise important questions relating to the identification of sources and reactions in our developing understanding and modeling of the atmospheric cycling of Hg. For example, do higher levels of Hg in precipitation reflect localized emissions of Hg (hypothesis 4), and are these localized sources natural (e.g., enhanced particle scavenging) or anthropogenic? We have used the Hg:210Pb relationship as a means of: (1) determining Hg deposition associated with precipitation; and (2) identifying and quantifying the contributions of global and localized emissions to atmospheric Hg deposition. We hypothesized that at less remote sites, such as urban regions, enhanced atmospheric Hg deposition that is locally/regionally derived will be indicated by deviations from the Hg:210Pb relationship observed in southeastern Alaska. In effect, the application of 210Pb as a normalizing tracer of particulate scavenging removes the issue of site-to-site variation in climatology (rain depth, frequency, temperature, etc.) and permits direct comparison of sites from widely different locations.

The MDN sites selected display a range in Hg deposition. The specific regions are West Coast North America, Mid-Continent United States, East Coast North America, and Southeastern United States. The west coast stations are at Glacier Bay (non-MDN) and King County, Washington, near Seattle (WA18). As outlined, Glacier Bay provided a measure of Hg and 210Pb associated with incoming open ocean-global circulation. The MDN site in Seattle is an urban site, and the Hg and 210Pb ratios in precipitation were expected to reflect the presence of significant, locally generated Hg contributions. The mid-continental stations are in Minnesota at the Marcell Experimental Forest (MEF; MN16) in a northern forested region, and Lamberton (MN27) in the agricultural region in the southwestern part of the state. Minnesota provides a location that is in the prevailing west to east air transport across the United States-, which will be influenced by localized and regional emissions and deposition. The east coast sites are at Acadia National Park, Maine (ME96) and at Cormack, Newfoundland (NF09). While the precipitation depth-weighted values for Hg are low in Maine and Newfoundland, local/regional emissions and deposition were expected to be pronounced. Such sources should be evident in the Hg and 210Pb distribution in precipitation. In the southeastern U.S., the stations selected are in Florida at Andytown (near Miami, FL04) and the more rural and less populated Everglades Nutrient Removal Project (ENR; FL34). The precipitation depth-weighted Hg levels in south Florida precipitation are high and the sources are unclear. This may be a locally derived phenomenon, or it may be associated with longer range transported Hg that may be scavenged efficiently as a consequence of upper level air oxidation processes.

Atmospheric Hg Deposition Collections in Southeastern Alaska and Newfoundland. Current wet deposition of Hg associated with precipitation has been assessed using an “ultra-clean” wet-only Hg collector in close proximity to the lake study region within the Tongass National Forest of southeastern Alaska. We have run a similar and successful site in the environs of the Toolik Lake Arctic LTER Station (Tseng, et al., 2004; Fitzgerald, et al., 2005). Rainwater samples were collected approximately weekly in Glacier Bay National Park, AK, from July 2003 to July 2005 (24 months). Total Hg was analyzed using methods designed for seawater (Bloom and Crecelius, 1983; Fitzgerald and Gill, 1979; Gill and Fitzgerald, 1985) that are similar to the U.S. Environmental Protection Agency (EPA) Method 1631. Precipitation Hg concentrations from southeastern Alaska show a “washout curve” similar to that reported for Arctic Alaska (Fitzgerald, et al., 2005). Mercury concentrations decrease with precipitation event size, suggesting that Hg is being removed from the atmosphere as a particle. The Hg content of precipitation ranged from 1.1 to 24.4 ng/L, with a volume-weighted average of 2.6 ng Hg/L. As recent average rain depths near Glacier Bay average 1.8 m y-1 (National Climatic Data Center [NCDC]; 2007), the estimated average atmospheric deposition rate is 4.7 ± 1.0 μg m-2 y-1 (Table 1). Similarly, rain water Hg concentrations ranged from 0.9 to 21.3 ng/L in weekly collections from July 2003 to July 2005 at the Cormack, NF (NF09) MDN site, with a volume-weighted average of 4.5 ng Hg/L, and an atmospheric deposition rate of 4.2 ± 0.4 μg m-2 y-1 (MDN, 2007). Hg deposition was highest at the south Florida (FL04 and FL34) and Lamberton (MN27) sites (9.5–22.5 μg m-2 y-1; Table 1), intermediate at the Seattle (WA18), MEF (MN16) and Acadia (ME98) sites (6.3-7.2 μg m-2 y-1), and lowest at Cormack and Glacier Bay.

Table 1. Atmospheric Deposition of Hg and Pb-210 at MDN Study Sites and Glacier Bay (AK)

Site

MDN Sampling
Period

Hg
Deposition
(μg m-2 y-1)

Rain
Depth
(m y-1)

Pb-210
Sampling Period

Pb-210 Deposition
(pCi cm-2 y-1)

WA18 (Seattle)

7/03–7/05

6.7 ± 0.9

0.8 ± 0.1

9/03–7/05

0.09 ± 0.01

NF09 (Cormack)

7/03–7/05

4.2 ± 0.4

1.1 ± 0.1

10/03–7/05

0.16 ± 0.02

FL04 (Andytown)

2/03–2/05

22.5 ± 3.2

1.5 ± 0.2

5/03–11/04

0.16 ± 0.03

FL34 (ENR)

2/03–2/05

11.8 ± 2.2

1.1 ± 0.2

6/03–11/04

0.13 ± 0.03

MN27 (Lamberton)

10/03–10/05

9.5 ± 1.4

0.7 ± 0.1

10/03–8/05

0.31 ± 0.05

MN16 (MEF)

10/03–10/05

6.3 ± 0.7

0.7 ± 0.1

10/03–8/05

0.18 ± 0.03

ME96 (Acadia)

7/03–7/05

7.2 ± 0.7

1.5 ± 0.2

10/03–3/05

0.51 ± 0.1

AK (Glacier Bay)

--

4.7 ± 1.01

1.8 ± 0.32

10/03–3/05

0.25 ± 0.05

1 Hg flux based on volume-weighted average Hg concentration for same collection dates as Pb-210 samples; 2 NCDC (2007)

As noted, the average Hg accumulation in the lake sediments is 16 ± 4 μg m-2 y-1 for southeastern Alaska and 18 ± 4 μg m-2 y-1 for Newfoundland. As compared to atmospheric deposition, the higher Hg flux in the cores results from: (1) Hg contributions from the lake watersheds; and (2) the preferential focusing of fine-grained sediments (and associated Hg) into the deeper parts of the lake basin. As discussed below, we have ways of dealing with this through the use of GIS (i.e., watershed area) and watershed delivery factors to get at watershed Hg contributions. We have also compared the excess Pb inventories in sediments to the rain Pb flux to get at focusing/winnowing factors.

The uniformity of the empirically established 210Pb flux among Chichagof Island lake sediment core sites is an important and useful result. The grand-mean is 0.34 ± 0.06 pCi cm-2 yr-1 for all cores from southeastern Alaska, and the 210Pb atmospheric deposition measured at Glacier Bay (AK) is 0.25 ± 0.05 pCi cm-2 yr-1. Since 210Pb in lake sediments is derived solely from direct atmospheric inputs to the lake surface (no watershed contributions), a comparison of these two Hg deposition estimates (0.34 vs. 0.25 pCi cm-2 y-1) indicates a factor of 1.4 focusing enhancement. This means that the actual lake-wide Hg accumulation rate is closer to about 12 μg m-2 y-1, with about a third (4.7 μg m-2 y-1) from direct deposition to the lake surface, and the remainder from watershed runoff (of atmospherically derived Hg). In comparison, given the grand-mean of 0.33 ± 0.21 pCi cm-2 yr-1 for sediment cores from Newfoundland, and a current 210Pb flux of 0.16 ± 0.02 pCi cm-2 yr-1 for Cormack (NF), a factor of 2 focusing enhancement is indicated, and the actual lake-wide Hg accumulation is about 9 μg m-2 y-1. The average atmospheric Hg deposition rate estimated from precipitation sampling (4.2 μg m-2 y-1) is approximately half the actual lake-wide Hg accumulation. GIS mapping of lake and watershed areas indicate an average watershed:lake-area ratio of 9.9 ± 4.6 for the Alaska lakes and 5.3 ± 2.7 for the Newfoundland lakes. It would thus take only a small portion of the wet Hg deposition to the watershed (16% for Alaska and 22% for Newfoundland) to balance the mercury accumulation in the lake sediments. This calculation does not take into account dry deposition of reactive gaseous mercury (RGM) to the watershed (which would further lower the percentage of total watershed deposition delivered to the lakes) or evasion of elemental mercury from the lake surface (which would require a larger watershed input to balance mercury sedimentation).

Atmospheric 210Pb Deposition Collections. Bulk deposition collectors were used at selected MDN sites and Glacier Bay National Park (AK), and 210Pb samples were retrieved by the operators of the sites on the same schedule as the Hg rain samples. Precipitation is the principal means for removal of particle-bound species (210Pb) from the atmosphere, and contamination is virtually impossible, so a simple bulk precipitation collector with an evaporation trap sufficed for collection of 210Pb samples. Rainwater collections for 210Pb analysis began in the summer/fall of 2003 at each collection site, and ended in November 2004 at the FL04 and FL34 collection sites, July 2005 at the AK, WA18, ME98, and NF09 collection sites (18–22 months), and are ongoing at the MN sites. Following collection, 210Pb samples were held for a year to allow establishment of secular equilibrium of 210Pb with its radioactive progeny, 210Po. Pb-210 determinations were conducted using alpha-spectrometry of the daughter, 210Po, spontaneously deposited on silver foil (Benoit and Hemond, 1988).

Atmospheric 210Pb deposition was highest at the Acadia (ME96) and Lamberton (MN27) sites (0.31–0.51 pCi cm-2 y-1; Table 1), intermediate at the Glacier Bay, MEF (MN16), Cormack (NF09), and Andytown (FL04) sites (0.16–0.25 pCi cm-2 y-1), and lowest at Seattle (WA18) and ENR (FL34; 0.09-0.13 pCi cm-2 y-1). Mercury deposition closer to emission sources was expected to deviate from that in remote regions, and this has been demonstrated by direct Hg and 210Pb depositional measurements at the lake study-areas and other geographic regions, both urban and rural. We hypothesized that sites which receive little local/regional Hg should show Hg:210Pb ratios similar to those observed at remote locations (Lamborg, et al., 2000), while sites that receive Hg deposition of a more localized nature should show greater ratio values. Glacier Bay (AK) and Cormack (NF09) receive Hg deposition similar to the global-only deposition previously measured at remote sites, with Hg:210Pb ratios less than those estimated previously. This is consistent with the low precipitation volume-weighted values for Hg at Glacier Bay and Cormack, with no evidence of local/regional emissions and deposition of Hg. Acadia (ME98) showed a Hg:210Pb ratio similar to remote regions, but both Hg and Pb-210 deposition are elevated (Table 1) as compared to Glacier Bay and Cormack. In contrast, sites in south Florida (FL04 and FL34) and Seattle (WA18) receive Hg in excess of the remote Hg:210Pb relationship. Results from the south Florida sites are consistent with the high precipitation volume-weighted Hg levels measured in that region and indicate the presence of significant local/regional sources of Hg. Similarly, despite the low 210Pb deposition at the urban Seattle site (Table 1), the Hg:210Pb ratio in precipitation was expected to reflect the presence of significant, locally generated Hg contributions.

Conclusions:

  1. There is striking uniformity in the empirically established 210Pb flux among sediment core sites in southeastern Alaska. The grand-mean for all 24 cores from the Tongass National Forest is 0.34 ± 0.06 pCi cm-2 yr-1. The modern Hg flux estimated from Hg measurements made in these lake sediment cores is 16 ± 4 μg m-2 yr-1.
  2. The empirically established 210Pb flux for western Newfoundland is 0.33 ± 0.21 pCi cm-2 yr-1. This indicates that there is more variability than was associated with the southeastern AK lakes, but sediment cores within lakes are similar. The modern Hg flux estimated from Hg measurements made in these lake sediment cores is 18 ± 4 μg m-2 yr-1.
  3. The present day Hg accumulation rate normalized to pre-industrial values indicates that atmospheric deposition of Hg has increased by a factor of 2.9 ± 0.5 in southeastern Alaska, and 2.2 ± 0.3 in Newfoundland. Cores collected in 1987 in southeastern Alaska show a much smaller increase in mercury accumulation (2.0 ± 0.1), indicating that there has been a substantial increase in mercury deposition to southeastern Alaska within the last two decades.
  4. We selected remote regions of North America over which different mercury emission sources were expected to dominate. Although we hypothesized that Newfoundland would show a larger historical increase in mercury deposition as compared to Alaska, the opposite was observed. These results suggest that rising Hg emissions from Asia have increased Hg deposition to southeastern Alaska in recent times. At the same time, declining emission from North America may have caused Hg deposition in the Canadian Maritimes to level off. Such results are consistent with observed trends in global mercury emission inventories.
  5. Mercury deposition changes closer to emission sources deviate from those in remote regions, and this has been demonstrated by direct Hg and 210Pb depositional measurements at the lake study-areas and other geographic regions. Sites which receive little local/regional Hg show Hg:210Pb ratios similar to those previously observed at remote locations, while sites that receive Hg deposition of a more localized nature show greater ratio values. Collection sites in Glacier Bay (Alaska), Newfoundland, and Maine show Hg:210Pb ratios similar to remote sites, while sites in south Florida and Seattle receive Hg in excess of the remote relationship.

Acknowledgments:

Laboratory assistance has been provided by Larissa Graham, Allan Hutchins, Jillian Weber, and Ming Chung. Our program has benefited from interested colleagues associated with current operations and facilities (National Parks and the MDN sites), and their cooperation was needed for efficient access to remote lakes and watersheds. We wish to thank Rob Tordon and Steve Beauchamp (Environment Canada, Nova Scotia) and Hazel Crocker (Newfoundland); David Manski, Bob Breen, and Emily Seger (Acadia National Park); Larry Fink, Nichole Niemeyer, and Joseph Jean-Jacques (S. Florida Water Management District); Ed Swain (Minnesota Pollution Control Agency), Deacon Kyllander, and Art Elling (U.S. Forest Service, MN); Lee Klossner and LaMoine Nickel (University of Minnesota); Bob Brunette, Megan Vogt, and Nicholas McMillan (Frontier Geosciences, WA); and Rusty Yerxa and Mary Kralovec (Glacier Bay National Park, AK).

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Lamborg CH, Fitzgerald WF, Damman AWH, Benoit JM, Balcom PH, Engstrom DR. Modern and historic atmospheric mercury fluxes in both hemispheres: global and regional mercury cycling implications. Global Biogeochemical Cycles 2002;16:1104.

Lindberg S, Bullock R, Ebinghaus R, Engstrom D, Feng X, Fitzgerald W, Pirrone N, Prestbo E, Seigneur C. A synthesis of progress and uncertainties in attributing the sources of mercury in deposition. Ambio 2007;36:19-32.

MDN (Mercury Deposition Network). National Atmospheric Deposition Program, 2007, http://nadp.sws.uiuc.edu Exit .

NCDC (National Climatic Data Center). NOAA Satellite and Information Service, U.S. Department of Commerce, 2007, http://www.ncdc.noaa.gov Exit .

Pacyna EG, Pacyna JM, Steenhuisen F, Wilson S. Global anthropogenic mercury emission inventory for 2000. Atmospheric Environment 2006;40:4048-4063.

Swain EB, Engstrom DR, Brigham ME, Henning TA, Brezonik PL. Increasing rates of atmospheric mercury deposition in midcontinental North America. Science 1992;257:784-787.

Tseng C-M, Lamborg CH, Fitzgerald WF, Engstrom DR. Cycling of dissolved elemental mercury in arctic Alaskan lakes. Geochimica et Cosmochimica Acta 2004;68(6):1173-1184.

Journal Articles:

No journal articles submitted with this report: View all 13 publications for this project

Supplemental Keywords:

chemical transport, heavy metals, scaling, environmental chemistry, northeast, southeast, Midwest, Pacific Northwest,, Scientific Discipline, Air, INTERNATIONAL COOPERATION, Waste, Ecosystem Protection/Environmental Exposure & Risk, POLLUTANTS/TOXICS, Air Quality, air toxics, Environmental Chemistry, Chemicals, Fate & Transport, Environmental Monitoring, Atmospheric Sciences, Chemistry and Materials Science, fate and transport, air pollutants, Hg, mercury, mercury emissions, modeling, mercury cycling, chemical kinetics, atmospheric mercury chemistry, mercury chemistry, atmospheric chemistry, atmospheric mercury cycling, atmospheric deposition, contaminant transport models, heavy metals, mercury vapor

Relevant Websites:

http://www.teamHg.uconn.edu Exit
http://www.smm.org/SCWRS Exit

Progress and Final Reports:

Original Abstract
  • 2003 Progress Report
  • 2004 Progress Report