Grantee Research Project Results
2002 Progress Report: Watershed Influences on Transport, Fate, and Bioavailability of Mercury in Lake Superior
EPA Grant Number: R827629Title: Watershed Influences on Transport, Fate, and Bioavailability of Mercury in Lake Superior
Investigators: Hurley, James P. , Back, Richard C. , Armstrong, D. E. , Shafer, Martin M.
Current Investigators: Hurley, James P. , Back, Richard C. , Armstrong, D. E. , Shafer, Martin M. , Manolopoulos, Helen
Institution: University of Wisconsin - Madison , Wisconsin Department of Natural Resources , Lake Superior State University
Current Institution: University of Wisconsin - Madison , Lake Superior State University , Wisconsin Department of Natural Resources
EPA Project Officer: Packard, Benjamin H
Project Period: October 1, 1999 through September 30, 2002 (Extended to September 30, 2003)
Project Period Covered by this Report: October 1, 2001 through September 30, 2002
Project Amount: $829,384
RFA: Mercury: Transport and Fate through a Watershed (1999) RFA Text | Recipients Lists
Research Category: Watersheds , Heavy Metal Contamination of Soil/Water , Water , Safer Chemicals
Objective:
The overall objective of this research project is to assess the importance of watersheds in controlling sources, transport, fate, and bioavailability of mercury (Hg) in a northern temperate lake system. The specific objectives of the research project are to: (1) determine the speciation and bioavailability of Hg transported to Lake Superior by representative tributaries/watersheds; (2) determine the importance of watershed-specific characteristics (soil type, land use, surficial deposits) that control physical/chemical forms of Hg transported downstream; (3) identify key mechanisms controlling Hg bioavailability and speciation in near-shore zones relative to open lake regions; and (4) provide process-level information to compliment concurrent development of Hg fate and transport models of the Lake Superior ecosystem.
Our approach combines field and laboratory studies with modeling to assess the importance of watershed processes in controlling Hg fate and transport in Lake Superior. Each phase (field studies, laboratory studies, modeling efforts) is strongly linked to provide feedback for the remaining phases. Techniques that were developed and adapted by our group during previous projects (i.e., "clean" ultrafiltration, resin techniques, biota processing) are being supplemented with new techniques (i.e., stable isotope Hg analysis by inductively coupled plasma-mass spectrometry (ICP-MS); phytoplankton and zooplankton uptake experiments). Modeling efforts combine the use of ongoing geographical information system (GIS)-based watershed yield modeling with the Dynamic Mercury Cycling Model (D-MCM) for the development of a model for Lake Superior at Tetra Tech, Incorporated.
Progress Summary:
Field sampling was concluded in Year 3, with a focus on nearshore tributary mixing zones. Previous field studies conducted in the nearshore waters of the Duluth-Superior Harbor, Chequamegon Bay, and the Whitefish Bay of Lake Superior consistently showed the methyl mercury (MeHg) enrichment of particles in the offshore area, relative to the river plume and the open lake surface waters. To further investigate processes and mechanisms controlling the bioaccumulation of methyl mercury in Lake Superior, three studies were conducted during the later phase of our project on the nearshore waters of Whitefish Bay where the organic-rich waters of the Tahquamenon River mix.
Late Fall 2001. High dissolved organic carbon (DOC) river water and pristine oligotrophic lake water were collected from the mouth of the Tahquamenon River and the Iroquois Point, respectively. A gradient of mixed waters was produced by diluting river water with lake water. All water types were sampled for total, methyl, and reactive mercury to examine the effects of water composition, with respect to DOC concentration and conductivity on Hg partitioning, and to better understand Hg transport through the river-lake mixing zone.
Early Spring 2002. Water and biota samples were collected along a transect moving from the mouth of the Tahquamenon River, through the river-lake mixing zone, and toward the outer edge of the river plume in the offshore waters, to examine changes in the distribution and partitioning of mercury species in nearshore waters. Furthermore, size-specific seston was collected from the river mouth and the offshore waters to study the bioaccumulation of methyl mercury in the lower food chain.
Fall 2002. Previously developed ultrafiltration methods were used to isolate and concentrate fractions (< 10 kD, 10 kD - 0.45 µm) of filtered surface waters (< 0.45 µm) collected from the Iroquois Point and the mouth of the Tahquamenon River, as well as a 1:1 mixed sample meant to simulate water from the nearshore mixing zone. Each fraction was analyzed for total and methyl mercury to examine the distribution of mercury species between colloidal and truly dissolved water fractions. Furthermore, bioassay studies assessing methyl mercury bioavailability to phytoplankton were conducted with each fraction of each water type. The information collected from this study will help us understand transport mechanisms of Hg through nearshore mixing zones and processes controlling the assimilation and bioaccumulation of Hg species by lower food chain organisms.
Laboratory analyses for the three aforementioned studies will be concluded at the end of 2002, and findings currently are being summarized in a research paper that will be submitted by June 2003.
A considerable portion of our efforts in Year 3 were dedicated to the advancement of our laboratory studies directed at predicting the bioavailability of methyl mercury. The first main accomplishment in these studies was the successful development of the method under "clean" conditions for mercury experiments. We were able to clean our freshwater media (Fraquil) with a Chelex resin column to remove all methyl mercury (below detection limit) and most total mercury (about 1.0 ng/L). Selenastrum cultures achieved log growth in the clean Fraquil, while methyl mercury concentrations remained below the detection limit. We have monitored cultures during a 72-hour period for changes in cell density, cell mass, dissolved organic carbon, and concentrations of methyl and total mercury in both media and algae. Experiments involving the addition of methyl mercury (2.0 ng/L) to Selenastrum cultures have shown that the partitioning of methyl mercury from the dissolved phase to the cells is not instantaneous, but increases over a matter of days, reaching a maximum partitioning at 24 hours (Kd ~ 103); after which it remains constant. The partitioning of methyl mercury was found to be 80-90 percent intracellular and only 10-20 percent extracellular. In these experiments, algal growth remained similar between controls, and those spiked with methyl mercury and no photo-degradation of methyl mercury were observed under lighted conditions. Finally, similar experiments recently were conducted on Lake Superior nearshore waters to study differences in methyl mercury bioavailability to algae throughout the waters of the mixing zone and between colloidal (10 kD - 0.45 µm) and truly dissolved (< 10 kD) fractions of the filtered phase (< 0.45 µm).
In summarizing our results for a preliminary mass balance for Lake Superior (Rolfhus, et al., in review), we found that atmospheric deposition dominated the supply of HgT (primarily inorganic Hg) to the lake, followed by tributary inputs and particulate remineralization; perhaps not surprising in a large lake with a relatively small watershed area/lake area ratio. Evasion of Hg(0) was the principal removal term, on the same order as atmospheric deposition and sedimentation. Approximately half of the HgT in particulate matter reaching the sediments was estimated to be remineralized, and export and groundwater terms generally were much smaller. Estimates are lake HgT storage results in a residence time (pool size/removal rate) for HgT of 4.6 years in the aqueous phase, and 0.4 years in the particulate phase, only if sedimentation alone is considered.
Sources of MeHg to the lake are more evenly divided between the atmosphere, tributaries, groundwater, remineralization, and in situ methylation, whereas sedimentation and photo-demethylation were the dominant removal processes. We suggest that in-lake methylation occurred in the warmer, shallow sediments, whereas tributary methyl mercury was primarily supplied during the spring. Recent studies have shown that planktonic methyl mercury content peaks during the spring period in Lake Superior. Such observations imply that tributary MeHg is more available to the planktonic food web than MeHg is from in situ sources, though the benthic food web has yet to be thoroughly studied with regards to bioaccumulation. Estimates of methyl mercury residence time are 4.5 years for aqueous MeHg and 2.4 years for particulate MeHg, based only upon the sedimentation term. We also are evaluating the potential for methylation and demethylation in both nearshore and offshore zones of Lake Superior. Our results from eight sites in the open waters of the lake during 2000, reveal extremely low methylation rates (<0.1 percent per day).
Finally, our work examining the effects of groundwater and surface water interactions on methyl mercury production and transport within the Lake Superior watershed has been concluded and summarized in the form of a Master's Thesis (Stoor, 2002). Our geographic information systems (GIS) work on mapping the Lake Superior Basin for land use/land cover, surficial geology, and bedrock type necessary for the translation of total and methyl mercury export rates for unmonitored watersheds in the Mercury Cycling Model also is approaching completion. Results are being summarized in a research paper that will be submitted to the Journal of Great Lakes Research in early 2003.
Future Activities:
Future activities will focus on detailed bioassay studies and summarizing our results for both a final report and manuscripts to be submitted for publication.
Our remaining laboratory efforts have been greatly enhanced by the ability to analyze stable Hg isotopes, through our cooperative work with Dr. David Krabbenhoft of the U.S. Geological Survey's Mercury Research Laboratory in Middleton, WI. We will continue to use stable isotopic techniques in phytoplankton and zooplankton laboratory uptake and partitioning studies for Lake Superior.
Finally, we will continue to work with Tetra Tech, Inc., as they develop the Electric Power Research Institute (EPRI)-sponsored Lake Superior Mercury Cycling Model. The model will be calibrated with our results, and simulations will be run to determine long-term trends and various management scenarios for Lake Superior.
Journal Articles on this Report : 3 Displayed | Download in RIS Format
Other project views: | All 36 publications | 9 publications in selected types | All 9 journal articles |
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Back RC, Gorski PR, Cleckner LB, Hurley JP. Mercury content and speciation in the plankton and benthos of Lake Superior. Science of the Total Environment 2003;304(1-3):349-354. |
R827629 (2001) R827629 (2002) R827629 (Final) |
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Cleckner LB, Back R, Gorski PR, Hurley JP, Byler SM. Seasonal and size-specific distribution of methylmercury in seston and zooplankton of two contrasting Great Lakes embayments. Journal of Great Lakes Research 2003;29(1):134-144. |
R827629 (2002) R827629 (Final) |
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Rolfhus KR, Sakamoto HE, Cleckner LB, Stoor RW, Babiarz CL, Back RC, Manolopoulos H, Hurley JP. Distribution and fluxes of total methylmercury in Lake Superior. Environmental Science & Technology 2003;37(5):865-872. |
R827629 (2002) R827629 (Final) |
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Supplemental Keywords:
Great Lakes, EPA Region 5, heavy metals, biology, hydrology, environmental chemistry, ecosystem protection, environmental exposure, environmental risk, geographic area, waste, water, air deposition, bioavailability, ecology, ecosystems, environmental chemistry, fate and transport, mercury, state, Lake Superior, Michigan, Minnesota, Wisconsin, MI, MN, WI, aquatic, atmospheric deposition, colloidal particles, fish consumption, geochemistry, lake ecosystems, mercury cycling, metals, soils, watershed influences., Scientific Discipline, Water, Waste, Geographic Area, Ecosystem Protection/Environmental Exposure & Risk, Hydrology, Bioavailability, Environmental Chemistry, State, Fate & Transport, Air Deposition, Ecology and Ecosystems, Mercury, EPA Region, Great Lakes, fate and transport, aquatic, Minnesota, MN, colloidal particles, mercury cycling, soils, fish consumption, geochemistry, watershed influences, water quality, Lake Superior, Wisconsin (WI), wetland, Region 5, atmospheric deposition, lake ecosystems, Michigan (MI)Relevant Websites:
http://www.engr.wisc.edu/groups/mercury/ Exit
http://www.wri.wisc.edu/ Exit
http://www.engr.wisc.edu/interd/wcp/ Exit
Synthesis Report of Research from EPA’s Science to Achieve Results (STAR) Grant Program: Mercury Transport and Fate Through a Watershed (PDF) (42 pp, 760 K)
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.