Grantee Research Project Results
2005 Progress Report: Natural and Anthropogenic Sources of Mercury to the Atmosphere: Global and Regional Contributions
EPA Grant Number: R829796Title: 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 Period Covered by this Report: January 1, 2005 through December 31, 2006
Project Amount: $897,219
RFA: Mercury: Transport, Transportation, and Fate in the Atmosphere (2001) RFA Text | Recipients Lists
Research Category: Heavy Metal Contamination of Soil/Water , Air Quality and Air Toxics , Safer Chemicals , Air
Objective:
The importance of the atmosphere in transporting Hg intra- and interhemispherically has been established, and knowledge of the behavior and fate of Hg in the atmosphere is increasing. The assessment of natural and anthropogenic sources is uncertain, however, and the mechanisms by which Hg is removed from the atmosphere are not well constrained. Additionally, the linkages between inputs of anthropogenic Hg, especially from the atmosphere, and the bioaccumulation of MMHg in sensitive aquatic ecosystems have not been established firmly. Recently, however, we reported a strong correlation/connection between the MMHg content of mosquitoes and atmospheric Hg deposition and contamination. The objective of this research project is to address 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 research is focused on current measurement, reconstruction, quantification, and interpretation of the modern and historical variation in atmospheric Hg fluxes associated with the mid- and subtropical latitudes of North America.
Progress Summary:
Experimental Design
We are combining 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 that display a range in Hg deposition as determined from the Mercury Deposition Network (MDN). The regions include the west coast of North America, the midcontinental United States, the east coast of North America, and the southeastern 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 (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 especially are valuable because they occur over broad geographic regions. These natural archives particularly are 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 lake sediment. Recently, we used lake-sediment archives to reconstruct atmospheric Hg deposition to Arctic Alaska during the last several centuries. 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 (Fitzgerald, et al., 2005). The observed modern to pre-industrial enrichment of atmospheric Hg deposition is 3.2 ± 1.0 (n = 5), which is similar to other remote nonpolar regions and represents, principally, anthropogenic contributions from the global pool.
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 Hg and 210Pb precipitation collectors at Bartlett Cove, Glacier Bay National Park headquarters. Based on topographic maps and field observations, the lakes appear to have relatively small watersheds. Local vegetation is comprised of undisturbed (pristine) coastal rainforest (Sitka spruce, western hemlock) and open muskeg (peatlands) on more gently sloping surfaces. Geographic information systems (GIS) analyses were used to determine precise surface areas, bathymetry, and catchment characteristics. This lake work was conducted from a small Zodiac inflatable boat powered by an electric trolling motor. 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 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 laboratories in Minnesota 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 Hg 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 detection (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 agrees quite well with the observed ratio of 3.2 ± 1.0 for Arctic Alaska and is consistent with historical records from lake sediments and peat that indicate that atmospheric deposition of Hg has increased two to four times in the northern hemisphere since industrialization (Swain, et al., 1992; Fitzgerald, et al., 1998; Benoit, et al., 1998).
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. The lakes have relatively small watersheds, and the vegetation is comprised of a patchwork of shrub tundra, rocky barrens, and spruce/fir forests on protected slopes. 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.
210Pb dating of all 24 cores has been completed, and 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 Alaska 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. We selected remote regions of North America over which different Hg emission sources were expected to dominate. Lake sediment records indicate, however, a remarkable uniformity of change between southeastern Alaska and Newfoundland, with a two- to three-fold increase in Hg deposition since pre-industrial times. This is significant because it indicates that anthropogenic emissions have altered Hg fluxes to about the same degree throughout remote North America, even though contemporary Hg deposition exhibits large temporal and spatial variability.
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. 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 semiremote locations (Lamborg, et al., 2000). The correlation between Hg and 210Pb in rainwater suggests analogous removal mechanisms for these two species, namely, gas-phase conversion from a more volatile form (Hgº; 222Rn) to a particle-reactive form (Hg2+; 210Pb2+). The correlation appears to be nearly universal with agreement in observations from midcontinent, coastal, and midocean locations. We are exploring the utility of using 210Pb:Hg relationships as 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 have hypothesized that at less remote sites, such as urban regions, enhanced atmospheric Hg deposition that is derived locally/regionally 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 should remove the issue of site-to-site variation in climatology (e.g., rain depth, frequency, temperature, etc.) and permit direct comparison of sites from widely different locations.
Atmospheric Hg Deposition Collections in Southeastern Alaska. 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. Rainwater samples were collected weekly in Glacier Bay National Park from July 2003 to July 2005 (24 months). Precipitation Hg concentrations from southeastern Alaska show a “washout curve” similar to that reported for Arctic Alaska. Hg concentrations decrease with precipitation event size, suggesting that Hg is being removed from the atmosphere as a particle. The Hg content of precipitation ranges 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 yr-1, the estimated average atmospheric deposition rate is 4.6 µg m-2 yr-1. As noted, the average Hg accumulation in the lake sediments is 16 ± 4 µg m-2 yr-1. 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 addressing this through the use of GIS (i.e., watershed area) and watershed delivery factors to determine watershed Hg contributions. We also can compare the excess Pb inventories in sediments to the rain Pb flux to identify focusing/winnowing factors.
The uniformity of the empirically established 210Pb flux among Chichagof Island lake sediment core sites is an important and useful result. Given the grand mean of 0.34 ± 0.06 pCi cm-2 yr-1 for all cores from southeastern Alaska, the empirical relationship between atmospheric Hg and 210Pb deposition in remote regions (Lamborg, et al., 2000) and a recent average rainfall depth of 1.6 m yr-1 in the Tongass National Forest region, the 210Pb flux would predict a contemporary Hg deposition of 10 ± 3 µg m-2 yr-1. As stated above, the average atmospheric Hg deposition rate estimated from precipitation sampling is 4.6 µg m-2 yr-1, roughly one-half of that predicated from the 210Pb flux in the sediment cores. Because 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 (10 vs. 4.6 µg m-2 yr-1) indicates a focusing enhancement of about two times. This means that the actual lake-wide Hg accumulation rate is closer to about 8 µg m-2 yr-1, with roughly one-half (4.6 µg m-2 yr-1) from direct deposition to the lake surface and the remainder from watershed runoff (of atmospherically-derived Hg). This partitioning of Hg inputs is consistent with a mean watershed:lake-area ratio of 5 (a first approximation for these lakes) and an export of 20 percent of the atmospheric Hg deposited to the watersheds (similar to that observed elsewhere). The GIS mapping of lake and watershed areas will allow us to estimate more accurately the watershed Hg contribution, whereas analysis of the precipitation 210Pb collections will provide a more direct measure of the enhancement factor for sediment focusing.
Atmospheric 210Pb Deposition Collections. Bulk deposition collectors were used at selected MDN sites and Glacier Bay National Park, and 210Pb samples were retrieved by the operators of the sites on the same schedule as the Hg rain samples. The MDN sites include the west coast of North America, the midcontinental United States, the east coast of North America, and the southeastern United States. Precipitation is the principal means for removal of particle-bound species (i.e., 210Pb) from the atmosphere, and contamination virtually is 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 December 2004 at the Florida collection sites; July 2005 at the Alaska, Washington, Maine, and Newfoundland collection sites (18-22 months); and are ongoing at the Minnesota sites. Following collection, 210Pb samples were held for 1 year to allow establishment of secular equilibrium of 210Pb with its radioactive progeny, 210Po. 210Pb determinations are being conducted using alpha-spectrometry of the daughter, 210Po, spontaneously deposited on silver foil. Sample processing and alpha counting have been completed for 12-15 months at each collection site.
Mercury deposition closer to emission sources would be 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 that receive little local/regional Hg should show Hg:210Pb ratios similar to those observed at remote locations, whereas sites that receive Hg deposition of a more localized nature should show greater ratio values. Preliminary results indicate that this is the case, with the Glacier Bay sites and MDN sites in Minnesota, Newfoundland, and Maine receiving Hg deposition similar to the global-only deposition previously measured at remote sites. In contrast, sites in south Florida and Seattle receive Hg in excess of the remote Hg:210Pb relationship.
Future Activities:
210Pb analysis of rain samples collected at the MDN sites and Glacier Bay is being conducted at the University of Connecticut (12-15 months completed for each site). Alpha counting of rain samples will continue until about August 2006 (longer for the ongoing rain collections at the Minnesota sites).
References:
Benoit JM, Fitzgerald WF, Damman AWH. The biogeochemistry of an ombrotrophic bog: evaluation of use as an archive of atmospheric mercury deposition. Environmental Research 1998;78(2):118-133.
Engstrom DR, Swain ED, Henning TA, Brigham ME, Brezonik PL. Atmospheric mercury deposition to lakes and watersheds: a quantitative reconstruction from multiple sediment cores. In: Baker LA, ed. Environmental Chemistry of Lakes and Reservoirs. Washington, DC: American Chemical Society, 1994, pp. 33-66.
Engstrom DR, Swain ED. Recent declines in atmospheric mercury deposition in the upper Midwest. Environmental Science & Technology 1997;31(4):960-967.
EPRI (Electrical Power Research Institute). Protocol for estimating historic atmospheric mercury deposition, EPRI/TR-106768, Palo Alto, CA, 1996.
Fitzgerald WF, Engstrom DR, Mason RP, Nater EA. The case for atmospheric mercury contamination in remote areas. Environmental Science & Technology 1998;32(1):1-7.
Lamborg CH, Fitzgerald WF, Graustein WC, Turekian KK. An examination of the atmospheric chemistry of mercury using 210Pb and 7Be. Journal of Atmospheric Chemistry 2000;36(3):325-338.
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(4):51.1-51.11.
Swain EB, Engstrom DR, Brigham ME, Henning TA, Brezonik PL. Increasing rates of atmospheric mercury deposition in midcontinental North America. Science 1992;257(5071):784-787.
Journal Articles on this Report : 2 Displayed | Download in RIS Format
Other project views: | All 12 publications | 5 publications in selected types | All 5 journal articles |
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Fitzgerald WF, Engstrom DR, Lamborg CH, Tseng C-M, Balcom PH, Hammerschmidt CR. Modern and historic atmospheric mercury fluxes in northern Alaska: global sources and arctic depletion. Environmental Science & Technology 2005;39(2):557-568. |
R829796 (2005) |
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Hammerschmidt CR, Fitzgerald WF. Methylmercury in mosquitoes related to atmospheric mercury deposition and contamination. Environmental Science & Technology 2005;39(9):3034-3039. |
R829796 (2005) |
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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, Chemistry and Materials Science, Atmospheric Sciences, 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 vaporRelevant Websites:
http://www.teamHg.uconn.edu Exit
http://www.smm.org/SCWRS Exit
Progress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.