Watershed Influences on Transport, Fate, and Bioavailability of Mercury in Lake Superior  

Period Covered by the Report: 1 October 1999 – 30 September 2000

Date of Report: 22 December 2000

EPA Agreement Number: R827629

Investigators: James P. Hurley 1, 2 (hurley@wri.wisc.edu), Principal Investigator; Richard C. Back 3, Co-Investigator; David E. Armstrong 2, Co-Investigator; Martin M. Shafer 2, Co-Investigator, Kristofer R. Rolfhus, Co-Investigator2.

Institution: (1) Bureau of Integrated Science Services, Wisconsin Department of Natural Resources, Monona, WI 53716; (2) University of Wisconsin-Madison, Madison, WI 53706; (3) Lake Superior State Univ., Sault Ste. Marie, MI 49783.

Research Category: Mercury: Transport and Fate through a Watershed

Project Period: October 1, 1999 - September 30, 2002.

 

OBJECTIVES:

The goal of this study 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 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 efforts of ongoing GIS-based watershed yield modeling with the Dynamic Mercury Cycling Model (D-MCM) model development at Tetra Tech, Inc.

Progress Summary/Accomplishments:

Our research efforts during the first year of the project were focused on: 1) Investigating differences between processes influencing offshore and near-shore 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; and, 3) Developing laboratory techniques for “trace-metal clean” plankton culturing/uptake studies.

Two cruises on Lake Superior aboard the EPA research vessel R/V Lake Guardian were conducted during 2000 (April, August). Nineteen open water stations on Lake Superior were visited in order to determine the spatial/temporal distribution and speciation of Hg.  Our results indicate that, as expected, Hg species concentrations are quite low in Lake Superior, similar to Lake Michigan and oceanic waters.  Total Hg averaged 0.49 ± 0.22 ng L-1 (mean, sd) for all stations, depths, and cruises, with little difference found between surface and deep samples; 74% was 0.7 mm filter passing.  Surface water total Hg samples were elevated along the Minnesota north shore in April, 2000, indicative of river inputs during this high-flow period.  During August 2000, methyl Hg (MeHg) averaged 6.4 ± 3.8 pg L-1 (with no discernable spatial trends), dissolved gaseous Hg was 20 ± 10 pg L-1, and reactive Hg averaged 45 ± 33 pg L-1.  For the August 2000 cruise, aqueous samples averaged 1.5% MeHg, 3.5% Hgo, 10% reactive Hg(II), and 85% unreactive organic Hg(II) complexes. Initial comparisons of phytoplankton revealed about a two to threefold enrichment of MeHg in riverine mixing zones versus offshore regions of the lake.  Analyses of the plankton and sediments (solid phase and pore waters) are ongoing.

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. An extensive set of monitoring wells has been placed in the East Creek watershed in the Tahquamenon River; groundwater, stream water and porewater samples are also being taken to detect areas of enhanced MeHg production and transport. Initial results from 2000 sampling indicate that groundwater and stream porewater are significant sources of MeHg, with wetland-dominated sites exhibiting the highest MeHg concentrations, at times exceeding 12 ng/L.

We have also initiated laboratory studies directed at predicting bioavailability of Hg and MeHg in algal and zooplankton cultures, through a cooperative effort with the Wisconsin State Lab of Hygiene Biomonitoring Laboratory, implementing trace-metal clean culturing techniques and constructing laboratory areas for Hg-clean research.  We have begun testing various algal growth media (Fraquil, Nutrient Enhanced Fraquil, and Bold’s Basal Media) to assess both algal growth and background Hg levels, and are evaluating Hg speciation using MINEQL.

Publications/Presentations:

Hurley, J.P., L.B. Cleckner, and M.M. Shafer.  2000. Watershed influences on mercury transport to Lake Superior.  Verh. Internat. Verein. Limnol. (In Press).

Cleckner, L.B., J.P. Hurley, K.R. Rolfhus, R.C. Back, C.L. Babiarz, and D.E. Armstrong.  2000. Watershed influences on transport, fate, and bioavailability of mercury in Lake Superior.  American Society of Limnology and Oceanography, Copenhagen, Denmark.  June.

Hurley, J.P., C.L. Babiarz, L.B. Cleckner, K.R. Rolfhus and M.M. Shafer and R.C. Back. 2000.  Partitioning and transport of total and methyl mercury in Lake Superior tributaries.  International Association for Great Lake Research, Cornwall, ONT, Canada.  May.

Hurley, J.P., K.R. Rolfhus, L.B. Cleckner, M.M. Shafer, C.L. Babiarz, D.E. Armstrong, R.C. Back and R. Harris.  2000.  Watershed Influences on Fate and Transport of Total and Methyl Mercury in the Lake Superior Basin. Air and Waste Management Association, Annual Meeting.  Banff, AB  Canada.  April.

Future Activities:

Our field-related activities during 2001 will target key periods of MeHg input and bioaccumulation in nearshore zones of Lake Superior.  Our work has shown that the majority of MeHg enters the lake during spring melt and we will target specific field and laboratory efforts  to better evaluate processes affecting MeHg bioaccumulation both at melt and during events in summer and fall.  We will compare melt periods with baseflow and event conditions during summer and fall.  We expect to conduct detailed transect sampling from nearshore mixing zones to offshore regions.  Our watershed work will expand to include a hydrologic component to determine yields of HgT and MeHg from contrasting land use/land cover characteristics.  

Our laboratory efforts will be greatly enhanced by the ability to include the use of 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 use stable isotopic techniques in laboratory uptake and partitioning studies and in evaluating sites of methylation and demethylation in Lake Superior.

 

Supplemental Keywords:

Groundwater, sediments, estuary, heavy metals, terrestrial, environmental chemistry, biology, hydrology, limnology, zoology, Great Lakes, Wisconsin (WI), Michigan (MI), Minnesota (MN)

 

Relevant Web Sites:

http://www.wri.wisc.edu/

http://www.engr.wisc.edu/interd/wcp/

http://www.lssu.edu/academics/science/default.html

 

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