2004 Progress Report: Natural and Anthropogenic Sources of Mercury to the Atmosphere: Global and Regional ContributionsEPA 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 Period Covered by this Report: January 1, 2004 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
Knowledge of the behavior and fate of mercury (Hg) in the atmosphere is increasing. The assessment of natural and anthropogenic sources, however, is uncertain, 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 monomethylmercury (MMHg) in sensitive aquatic ecosystems have not been firmly established. Recently, however, we reported a strong correlation/connection between the MMHg content of mosquitoes and atmospheric Hg deposition and contamination (Hammerschmidt and Fitzgerald, 2005). The objective of his 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 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.
We are combining two research strategies:
- archives provided by lake sediment cores;
- and collocated Hg and 210Pb deposition collectors, to derive precise estimates of the modern Hg flux to eastern and western North America.
Our lake projects 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, mid-continent of the United States, east coast of North America, and the southeastern United States. Our program benefits from the cooperation and assistance of interested colleagues associated with current operations and facilities (National Parks and the MDN sites).
Lake Sediment Archives
Contemporary and historical information provided by careful measurements and scrupulous examination of natural archives (EPRI, 1996) has been critically important to enhancing knowledge, understanding, environmental perspective, assessment of anthropogenic interferences, and modeling of major biogeochemical cycles. Lake sediments, peat bogs, and ice cores have been used successfully for regional and global studies of modern and historical Hg atmospheric depositional patterns, and we have much experience with this scientific approach (Swain, et al., 1992; Fitzgerald, et al., 1998; Engstrom and Swain, 1997; Engstrom, et al., 1994; Benoit, et al., 1998; Lamborg, et al., 2002). Lakes are especially valuable, because they occur over broad geographic regions. These natural archives are particularly wellsuited 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 over the last several centuries, to evaluate polar Hg depletion events, and to constrain a contemporary lake/watershed mass balance with real-time measurement of Hg fluxes in rainfall, runoff, and evasion ( Fitzgerald, et al., 2005).
We are evaluating recent changes in atmospheric mercury deposition for two regions of North America over which different mercury emission sources should dominate. Trends in atmospheric deposition will be inferred from Hg accumulation rates in 210Pb-dated sediment cores. We have hypothesized that atmospheric Hg deposition in southeastern Alaska can be viewed as an integrated sample of global Hg pollution in the Northern Hemisphere, and, therefore, represents a baseline in Hg deposition for comparison to sites closer to local and regional emission sources. There are no significant anthropogenic Hg sources anywhere in coastal southeastern Alaska, and human development is limited within several hundred kilometers of lakes in the region. We also have hypothesized that atmospheric Hg deposition in Canada’s Maritime Provinces (east coast of North America) 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. We are using lake-sediment cores collected in Newfoundland to assess the increase in atmospheric Hg deposition experienced by a region “downwind” of the relatively industrialized eastern North America. 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 indicate that in Nova Scotia, and by extension Newfoundland, as much as 40 percent of Hg deposition could be caused by regional source and/or enhanced oxidation phenomena (Lamborg, et al., 2002). We also hypothesize that s ediment 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.
Southeastern Alaska and Newfoundland
In June 2003, sediment cores were collected from four carefully chosen study lakes near Point 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. Similarly, sediment cores were collected from four carefully chosen study lakes near Deer Lake/Cornerbrook, Newfoundland in August 2004. These lakes are 30-60 km from the atmospheric precipitation collectors located in Cormack, Newfoundland.
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, southeastern Alaska is 0.34 ± 0.06 pCi cm-2 yr-1. Mercury analysis has been completed on all cores from southeast Alaska. The modern Hg flux estimated from Hg measurements made in the same lake sediment cores is 16 ± 4 µg m-2 yr-1. The present day Hg accumulation rate normalized to preindustrial values indicates that atmospheric deposition of Hg has increased by a factor of 2.9 ± 0.5. Historical records from lake sediments and peat indicate that atmospheric deposition of Hg has increased two- to four-fold in the northern hemisphere since industrialization (Swain, et al., 1992; Benoit, et al., 1998; Fitzgerald, et al., 1998). In contrast, 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 (Lamborg, et al., 2002).
Sediment cores were collected from four carefully chosen project lakes in the Dear Lake region of western Newfoundland. Six cores were collected from widely spaced locations in the deeper regions of each lake by means of a HTH-Teknik gravity corer equipped with 7-cm polycarbonate core tubes. Collected core lengths range from 24 to 44 cm. Lake work was conducted from a small Zodiac inflatable boat powered by an electric trolling motor. Three of the selected lakes occur in a relatively tight cluster and are located approximately 30 km west of the MDN station at Cormack; the fourth site is located about 60 km east of Cormack. Based on topographic maps and field observations, the lakes appear to have relatively small watersheds. Local vegetation is comprised of a patchwork of shrub tundra, rocky barrens, and spruce/fir forests on protected slopes. GIS analyses, currently in progress, will determine precise surface areas, bathymetry, and catchment characteristics.
Cores were sectioned into 1-2 cm increments in Cornerbrook, and later shipped to the St. Croix Watershed Research Station (SCWRS) laboratories (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. Lead-210 dating of all 24 cores has been in progress since the fall of 2004 and now is nearly complete (20 of 24 cores are done). T he empirically established 210Pb flux for the Newfoundland cores dated thus far is 0.37 ± 0.21 pCi cm-2 yr-1. This indicates that there is more variability than that associated with the southeastern Alaska lakes, but cores within lakes are similar. Selected samples (based on the 210Pb results) from roughly one-half of the cores have been sent to the University of Connecticut for mercury analysis.
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 are measuring Hg deposition in precipitation at Glacier Bay, Alaska, and making use of data from seven MDN stations. Our past research has pointed to a linear correlation between Hg and 210Pb in rainwater from remote and semi-remote locations (Lamborg, et al., 1999, 2000; Tseng, et al., 2004). Although preliminary, the precipitation correlation of Hg and 210Pb offers a hypothetical tool for discerning regional and global influences on local Hg depositional fluxes. We have hypothesized that the linear correlation between Hg and 210Pb found in rainwater from other remote and semi-remote locations will be observed in southeastern Alaska and that this behavior can be used to constrain the global-scale wet atmospheric flux of Hg to lakes and watersheds of temperate North America. We also have hypothesized that at less remote sites, enhanced atmospheric Hg deposition that is locally/regionally derived will be indicated by deviations from the Hg and 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 (rain depth, frequency, temperature, etc.) and permit direct comparison of sites from widely different locations.
The uniformity of the empirically established 210Pb flux among Chichagof Island lake sediment core sites is an important and useful result. The grand-mean for all cores from southeastern Alaska is 0.34 ± 0.06 pCi cm-2 yr-1. Based on the empirical relationship between atmospheric Hg and 210Pb deposition in remote regions (Lamborg, et al., 2000), and recent rainfall depths of 1.4-1.8 m y-1, this 210Pb flux would predict a contemporary Hg deposition of 11 ± 3 µg m-2 yr-1. As stated above, the modern Hg flux estimated from Hg measurements made in these lake sediment cores is 16 ± 4 µg m-2 yr-1. The Hg in the cores includes watershed contributions for Hg, but there is very little 210Pb from the watershed in the lake sediments. When corrected for the size of the watershed, the lake core results will predict a lower Hg deposition rate, likely closer to the estimate calculated using 210Pb.
Atmospheric Hg and 210Pb Deposition Collections
Weekly rainwater sample collections for Hg analysis began in Glacier Bay, Alaska, in July 2003 and will be continued until July 2005 (24 months). There have been several weeks of low rainfall, resulting in 2- to 3-week collection periods. Samples are analyzed within several weeks following collection, and all samples collected through December 2004 have been analyzed. Precipitation Hg concentrations show a “washout curve” similar to that reported by Fitzgerald, et al. (2005) 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 and shows a volume-weighted average of 2.6 ng Hg/L. As recent rain depths near Glacier Bay average 1.8 m y-1, the estimated average atmospheric deposition rate is 4.6 µg m-2 yr-1. The watershed effect likely contributes to the difference between this precipitation Hg flux estimate and the estimates based on lake sediment cores. Additionally, the 210Pb flux in the cores is not necessarily a lake-wide average (no shallow-water cores) and is probably biased high by sediment focusing. Therefore, we would expect Hg flux estimates based on the cores to be higher than the atmospheric flux estimates.
Weekly rainwater collections for 210Pb analysis were started in May 2003 at the Florida MDN sites included in our study (ENRP FL34 and Andytown FL04), and began in July 2003 at the Seattle, Washington, (WA18) and Glacier Bay, Alaska, sites. Collections at the Minnesota (Marcell MN16 and Lamberton MN27), Acadia National Park (ME96), and Cormack, Newfoundland (NF09) sites began in October 2003. For the Florida and Washington sites, there is consistent coverage for nearly every sampling week, with one sample from each site (samples from WA18 pooled because of low rainfall). There also is weekly coverage at both Minnesota sampling sites, and we typically receive only one sample (pooled—low rain volume) per collection week from each site. Duplicate samples are received approximately monthly in our laboratory at the University of Connecticut from the Arkansas, Maine, and Newfoundland collection sites. As stated above for rain Hg collections, there have been many weeks of low rainfall, resulting in a number of 2-or 3-week collection periods at the Glacier Bay, AK site.There has been roughly weekly coverage through the sampling period at Acadia National Park (Maine), but rain volume has often been low. Although weekly sample collections began in October 2003 in Cormack, Newfoundland, we have experienced problems with sample freezing, and no samples were collected from January until early April 2004. The 210Pb collectors in Cormack now are insulated in the same manner as the collectors at the Minnesota sites, and we have experienced more consistent collections through the current winter season.
Following collection, 210Pb samples must be held for 1 year to allow for the establishment of secular equilibrium of 210Pb with its radioactive progeny, 210Po. Therefore, following method development during the summer and fall of 2004, sample processing began during the winter of 2004. The initial project objectives were for 18 months of sample collection at each site. This goal was met in December 2004 at the Florida collection sites, and will be accomplished by April 2005 at the remaining sites. At the current rate of sample processing/alpha counting, 210Pb analysis should be completed for 9-12 months of collection at each site by the end of 2005. We plan to continue alpha counting of rain samples past the end of the regular project period (December 2005).
Our program benefits 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 and Nichole Niemeyer (South Florida Water Management District); Ed Swain (Minnesota Pollution Control Agency), Art Elling and Deacon Kyllander (U.S. Forest Service, MN), and Lee Klossner and LaMoine Nickel (University of Minnesota); Bob Brunette, Megan Vogt, and Nicholas McMillan (Frontier Geosciences, WA); and Rusty Yerxa (Glacier Bay National Park, AK).
We will begin Hg analyses of sediment cores collected from Newfoundland during the spring of 2005 at the University of Connecticut. Each lake coring campaign generates roughly 500-600 sediment samples for 210Pb and Hg analysis. 210Pb analysis of rain samples (following a suitable grow-in period) collected at the MDN sites and Glacier Bay are being conducted at the University of Connecticut and will be ongoing through the study. With the exception of the Florida sites (collection ended December 2004), we plan to continue collecting rain samples at each site until July 2005. A total of 600-700 210Pb precipitation samples will be received from the eight collection locations during the 18-22 month collection period. Alpha counting of rain samples will be continued past the end of the regular project period (December 2005). Mercury analysis of precipitation samples from Glacier Bay will be ongoing at the University of Connecticut. A total of approximately 60-70 Hg precipitation samples will be received for analysis by the end of the collection period (July 2005).
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 EB, Henning TA, Brigham ME, et al. 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 EB. 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, Lamborg CH, Tseng C-M, et al. Modern and historic atmospheric mercury fluxes in northern Alaska: global sources and arctic depletion. Environmental Science & Technology 2005;39(2):557-568.
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.
Hammerschmidt CR, Fitzgerald WF. Methylmercury in mosquitoes related to atmospheric mercury deposition and contamination. Environmental Science & Technology 2005;39(9):3034-3039.
Lamborg CH, Rolfhus KR, Fitzgerald WF, Kim G. The atmospheric cycling and air-sea exchange of mercury species in the South and equatorial Atlantic Ocean. Deep-Sea Research Part II-Topical Studies in Oceanography 1999;46(5):957-977.
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, et al. 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, et al. Increasing rates of atmospheric mercury deposition in midcontinental North America. Science 1992;257(5071):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.