Grantee Research Project Results
2001 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. , 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, 2000 through September 30, 2001
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 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. Specific objectives of the study 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 by new techniques (i.e., stable isotope Hg analysis by ICP-MS; phytoplankton and zooplankton uptake experiments). Modeling efforts combine the use of ongoing geographic information systems (GIS)-based watershed yield modeling with the Dynamic Mercury Cycling Model (D-MCM) for the development of a model for Lake Superior.
Progress Summary:
Our research efforts in the first year of the project focused on: (1) investigating differences between processes that influence offshore and nearshore bioaccumulation of Hg in Lake Superior, particularly with regards to the spatial/temporal distribution of Hg in near-shore and offshore environments; (2) investigating watershed processes that enhance production and transport of methyl Hg to tributaries; (3) developing laboratory techniques for "trace-metal clean" plankton culturing/uptake studies; and (4) providing information for initial simulations of the Tetra Tech-Electric Power Research Institute (EPRI) Lake Superior Mercury Cycling Model.
We have continued our Hg work in the open water of Lake Superior. In August 2001, we participated in the Environmental Protection Agency (EPA) Great Lakes Monitoring Cruise aboard the R/V Guardian. We followed the cruise along the northern shore of Lake Superior from Sault Ste. Marie to Duluth, MN. Our main objectives were to: examine the spatial distribution and speciation of mercury throughout the water column during thermal stratification by sampling epilimnetic and hypolimnetic waters along a depth profile at the deepest point in the lake; zooplankton from the entire water column, epilimnion, and hypolimnion, were sampled to examine mercury bioaccumulation; benthic organisms, waters from the nepheloid layer, and sediments were sampled to assess sediments as a source of MeHg to benthic organisms and overlying waters; and to examine nearshore waters affected by urban activities (Duluth-Superior Harbor and Thunder Bay) as a source of Hg/MeHg to Lake Superior.
Given the dimictic nature of Lake Superior, it is considered to be well mixed,
and variations in Hg concentration with depth were not expected to be found.
However, during thermal stratification in August, elevated concentrations of
unfiltered HgT and MeHg on particles relative to the surface, were observed
within the thermocline layer where fluorescence also reached a maximum. The
depth of maximum MeHg concentration on particles coincided with the exact depth
of maximum fluorescence (37 m) and biological activity, suggesting a bioaccumulating
effect on MeHg. Similar peaks in MeHg P also were noted in the upper hypolimnion
and in the nepheloid layer where fluorescence was at a minimum (settling particles).
Concentrations of HgT in zooplankton were greater in the hypolimnion versus
the epilimnion, while MeHg in zooplankton remained constant with depth. We continue
to analyze samples from our transect in the more contaminated Thunder Bay. Our
ongoing work in estuarine mixing zones of the lake (St. Louis River, Fish Creek,
and Tahquamenon River) continues to show enhanced bioaccumulation of MeHg in
biota relative to offshore sites.
Formation of MeHg within the Lake Superior watershed, specifically the role
of groundwater as a source of MeHg to headwater streams, has been a major focus
of our research. The role of watershed composition on MeHg transport is being
examined in two focused studies: (1) homogeneous watershed subunits have been
identified for the south shore of Lake Superior (based upon GIS surficial geology
and land use data), whose tributaries have been sampled for Hg speciation in
surface waters; and (2) the role of groundwater in producing/transporting MeHg
within a forest-wetland dominated watershed. We have completed an extensive
exercise to fully map the Lake Superior Basin, including Canadian watersheds,
for land use/land cover, surficial geology, and bedrock type. This information
is critical to developing a mass-balance approach for Hg cycling in the basin.
We have used these data to interface a watershed component into the Mercury
Cycling Model. Watersheds have been grouped and clipped to fit specific cells
for the lake portion of the model. Our GIS information allows for translation
of Hg and MeHg export rates to unmonitored watersheds in the basin.
We have continued to study the interaction of groundwater and surface water, and the effects on MeHg production and transport. An extensive set of monitoring wells has been placed in the East Creek Watershed and near a temporary stream near the mouth of the Tahquamenon River. They are influenced by both wetland and forest land covers. Groundwater, stream water, and porewater samples also are being taken to detect areas of enhanced MeHg production and transport. Through graduate student research, we have observed two watershed processes important in delivering MeHg to the mixing zones of Lake Superior. First, in both sites, we observed a displacement of groundwater rich in MeHg during the spring melt period. Second, we have observed active methylation in the hyporheic zone of streams in both forested and wetland regions. These observations help explain our previous observations of elevated MeHg in streams that drain predominantly forested watersheds. We believe that forested zones of Lake Superior, in addition to wetland areas, are active zones of production and transport of MeHg in the basin.
In addition to watershed production and transport of MeHg, 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). We continue to compare additional onshore and offshore sites, while addressing seasonal dynamics through a graduate student research project.
Finally, we have continued our laboratory studies directed at predicting bioavailability of Hg and MeHg in algal and zooplankton cultures, through a cooperative effort with the Wisconsin State Hygiene Biomonitoring Laboratory. We currently have five freshwater species in culture, and the cultures have been shown to have consistent log growth in "Chelexed" Fraquil media, both with and without the presence of EDTA. The algae are monitored for the amount of dissolved organic carbon (DOC) produced during growth (high DOC concentrations could complex Hg species). We have conducted experiments to determine optimal flask cleaning procedures, optimal flask material, whether flasks need to be pre-conditioned for MeHg additions, and if low level MeHg concentrations (1.0 ng/L) will photo-degrade under lighted culture conditions.
Future Activities:
Our field-related activities will conclude during 2002. We have designed a complex sampling scheme to better understand transformations occurring in the estuarine mixing zone of the Tahquamenon River in Whitefish Bay. We will add additional sampling sites to our previous efforts in an attempt to better understand the fate of both Hg species, DOC, and the MeHg uptake processes for phytoplankton and zooplankton in the mixing zone gradient. We also will collect large-volume samples of "clean" water for both laboratory uptake studies and for DOC characterization. We will work cooperatively with the U.S. Geological Survey (USGS) laboratory in Boulder, CO, for our DOC work, with a goal of quantifying the importance of DOC-sulfur complexation.
Our laboratory efforts have been greatly enhanced by the ability to analyze for stable Hg isotopes, through our cooperative work with Dr. David Krabbenhoft of the USGS Mercury Research Laboratory in Middleton, WI. We will continue to use stable isotopic techniques in phytoplankton and zooplankton laboratory uptake and partitioning studies, and in evaluating sites of methylation and demethylation in nearshore and offshore Lake Superior.
Finally, we will continue to work with Tetra Tech, Inc., on the development of the 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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Babiarz CL, Hoffmann SR, Shafer MM, Hurley JP, Andren AW, Armstrong DE. A critical evaluation of tangential-flow ultrafiltration for trace metal studies in freshwater systems. 2. Total mercury and methylmercury. Environmental Science & Technology 2000;34(16):3428-3434. |
R827629 (2001) R827629 (Final) |
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Babiarz CL, Hurley JP, Hoffmann SR, Andren AW, Shafer MM, Armstrong DE. Partitioning of total mercury and methylmercury to the colloidal phase in freshwaters. Environmental Science & Technology 2001;35(24):4773-4782. |
R827629 (2001) R827629 (Final) |
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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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Supplemental Keywords:
groundwater, sediments, estuary, heavy metals, terrestrial, environmental chemistry, biology, hydrology, limnology, zoology, Great Lakes, Wisconsin, WI, Michigan, MI, Minnesota, MN., Ecosystem Protection/Environmental Exposure & Risk, Scientific Discipline, Geographic Area, Waste, Water, Air Deposition, EPA Region, Fate & Transport, Mercury, State, Environmental Chemistry, Bioavailability, Hydrology, Great Lakes, Ecology and Ecosystems, wetland, geochemistry, MN, water quality, aquatic, soils, watershed influences, atmospheric deposition, Region 5, fate and transport, lake ecosystems, Lake Superior, mercury cycling, Minnesota, colloidal particles, Michigan (MI), Wisconsin (WI), metalsRelevant Websites:
http://www.engr.wisc.edu/groups/mercury/ Exit
http://www.wri.wisc.edu/ Exit
http://www.engr.wisc.edu/interd/wcp/ Exit
http://www.lssu.edu/academics/science/default.html 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.