Grantee Research Project Results
Final 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 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 goal of this research project was to assess the importance of watersheds in controlling sources, transport, fate, and bioavailability of mercury (Hg) in the Lake Superior basin, a northern temperate lake system. The specific objectives of the study were to: (1) determine the speciation and bioavailability of Hg transported to Lake Superior by rivers that drain representative and contrasting watersheds; (2) determine the importance of watershed-specific characteristics (e.g., land cover, land use, and surficial deposits) that control physical and 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 complement concurrent development of Hg fate and transport models of the Lake Superior ecosystem.
Summary/Accomplishments (Outputs/Outcomes):
An indepth study was conducted on the Tahquamenon River watershed to investigate characteristics and conditions that affect the production of methylmercury (MeHg) within the Lake Superior basin and its subsequent transport into the lake via tributaries. Groundwater sampled at two sites of contrasting land cover type (coniferous and deciduous forest) contained relatively high concentrations of MeHg (up to ~0.6 ng L-1 in August). During periods of low flow in the summer, anoxic and low redox conditions are promoted in the subsurface, which enhance microbial activity and MeHg production. During periods of high flow, in response to precipitation events or spring snowmelt, MeHg is flushed and transported to down gradient surface waters, depleting concentrations in the wells. A strong link between MeHg in the subsurface and the hydrologic cycle was observed at both sites. These results indicate that: (1) subsurface transport and/or production of MeHg can be an important source of MeHg to surface waters, and (2) MeHg can be exported from forested systems, not only wetlands as was thought previously. The importance of the hyporheic zone, the subsurface interface between groundwater and surface water, as a source of MeHg to streams in both wetland and forested areas also was revealed through this study. Concentrations of MeHg in both bottom pore waters and stream water increased during the summer when methylation was promoted in the hyporheic zone, and decreased in September when conditions were less favorable for methylation. Previous studies have calculated MeHg efflux to a stream by comparing MeHg concentrations in wellside streams and using differences in well level to calculate water flow. Our studies have shown the importance of also measuring hyporheic processes and potential efflux. The concentrations of MeHg in hyporheic porewaters are consistently higher than background well concentrations.
High loads of Hg entering Lake Superior through tributaries generally are retained in nearshore waters. Elevated Hg concentrations were observed in all lake compartments in these riverine mixing zones, emphasizing the significance of this source to nearshore waters and the biogeochemical cycling of Hg. To study the influence of river plumes on Hg dynamics in Lake Superior nearshore waters, we conducted a series of studies in the Tahquamenon River/Whitefish Bay mixing zone. Despite expected decreases in analyte concentrations caused by dilution of the river plume with lake water, an enrichment in the MeHg content of particles always occurred at the outer edge of the plume. An increase in the MeHg partitioning coefficient (log Kd) of more than an order of magnitude also was noted between the inner and outer edge of the plume. The biological enrichment of the suspended particle pool, indicated by the increase in chlorophyll a (chl-a) coupled with the concomitant increase in particulate MeHg relative to other parts of the lake, suggests an enhancement of MeHg bioaccumulation in these regions. This trend also was observed in other Lake Superior mixing zones (e.g., St. Louis River and Fish Creek).
Two studies were conducted to investigate the mechanisms of MeHg transport through the mixing zone by: (1) collecting size-fractioned seston (< 35, 35 - 153, 153 - 243, > 243 µm) from the river mouth and offshore to examine the changing distribution of chemical species within the particulate phase, and (2) collecting colloidal (110 kDa – 0.45 µm) and truly dissolved (< 10 KDa) fractions from the mouth, offshore, and a representative mixing zone sample to examine the changing distribution of chemical species within the aqueous phase (< 0.45 µm). The less than 35 µm fraction represented the majority of mass, MeHg, and chl-a at both sites, and overall concentrations decreased between the mouth and offshore with the exception of chl-a, which showed the opposite trend. When examined on a mass basis however, MeHg was more concentrated in the larger fractions with the 153–243 µm fraction being more important in the river mouth and the greater than 243 µm fraction in the offshore. As larger particles settled out between the mouth and offshore, this would suggest that MeHg is transported to the offshore through smaller particles where it is assimilated by larger, more biologically active particles. Our ultrafiltration study revealed that MeHg enters the lake from the river associated predominantly with colloidal material, but in the offshore, where the amount of dissolved organic carbon (DOC) and proportion of colloids decrease, MeHg is associated mainly with the “truly dissolved” phase.
A large portion of our study was devoted to developing appropriate tools and methods to examine the mechanisms of MeHg uptake by the lower food web in fresh waters. The greater challenge was to develop a method appropriate for Hg analysis incorporating clean trace metal techniques. A batch culture bioassay using Selenastrum capricornutum was developed successfully and applied to various water types. MeHg uptake experiments combined with modeling efforts revealed that uptake into cells was not a steady state process over the 48-hour study and was affected mainly by uptake rate (k1) and cell growth (kG), whereas the depuration rate (k2) was not noticeable. Uptake was rapid within the first 15 minutes and became comparatively slower between 1 and 6 hours. The partitioning of MeHg between the media and algae (bioconcentration factor) was approximately 105 L kg-1 from 0.25 hour through 24 hours, but decreased to 104 when the algae growth increased by a factor of 10. When the bioassay was conducted in high DOC water from the Tahquamenon River, uptake was inhibited relative to low DOC lake water, presumably because of complexation of Hg by DOC.
Using data collected through our field studies on the open waters and GIS-based studies estimating watershed loading to the lake, along with supplementary data from the literature, we constructed a preliminary mass balance for total mercury (HgT) and MeHg in Lake Superior. The mass balance indicates that the supply of HgT is dominated by atmospheric deposition (58%), followed by tributary inputs (21%) and particulate remineralization (19%), whereas sources of MeHg are divided more evenly between the atmosphere, tributaries, groundwater, remineralization, and in situ methylation. Evasion of elemental Hg (Hg0) is the principal removal process of HgT followed by sedimentation, whereas MeHg is removed mainly by sedimentation and photo-demethylation.
Three cruises conducted on Lake Superior aboard the EPA research vessel R/V Lake Guardian provided us with significant information on the temporal and spatial distribution of Hg species in the water, sediments, and biota. Total Hg in surface waters averaged 0.57 ± 0.07 and 0.47 ± 0.03 ng L-1 in April and August, respectively, whereas MeHg concentrations in August averaged 5.1 ± 0.9 pg L-1, accounting for approximately 1 percent of HgT. The observed sub-ng L-1 concentrations are more comparable to those measured in open ocean waters than inland lakes. Total Hg and MeHg in surface sediments (0–2 cm) averaged 83 ± 12 ng g-1 dw and 0.21 ± 0.03 ng g-1 dw, respectively. Distribution coefficients for HgT and MeHg (log Kd = 3.9 ± 0.2 and 2.4 ± 0.2, respectively) in Lake Superior’s clay-rich, organic-poor sediments generally are low relative to other aquatic systems, suggesting that Hg is more mobile in Lake Superior than in other aquatic systems. In situ methylation rates in deep sediments were found to be extremely small, less than 1 percent per day, but might be higher in shallower, warmer, nearshore sediments. Concentration gradients of aqueous HgT and MeHg were observed in surface waters and sediments, with the highest values occurring in April, particularly along the northwestern shoreline of the lake. This most likely is a result of mixing with tributary inputs, whereas consistently lower values were observed in offshore regions of Lake Superior.
Mercury concentrations in offshore bulk zooplankton collected in April ranged from 35 to 50 ng MeHg/gdw and from 80 to 130 ng HgT/gdw. In August, concentrations ranged from 15 to 25 ng MeHg/gdw and from 20 to 70 ng HgT/gdw. Concentrations of both HgT and MeHg measured in April were significantly higher than those measured in August, most probably the result of a dilution effect caused by an increase in zooplankton mass. MeHg in zooplankton generally ranged from 30 to 56 percent of HgT. Higher concentrations of MeHg were found in zooplankton from the epilimnion, which is consistent with the enhanced bioaccumulation of MeHg observed immediately below the thermocline at 37 m during our depth profile study. These observations suggest that Hg in surface waters deposited from the atmosphere is more bioavailable than Hg in deeper waters. Concentrations in Mysis relicta ranged from 33 to 54 ng MeHg/gdw throughout the lake, while concentrations in chironomid larvae were 8 ng MeHg/gdw and 32 ng MeHg/gdw in amphipods.
Journal Articles on this Report : 9 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, Hurley JP, Rolfhus KR. Watershed influences on the transport, fate and bioavailability of mercury in Lake Superior: field measurements and modelling approaches. Lakes & Reservoirs: Research and Management 2002;7(3):201-206. |
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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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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Hoffmann SR, Shafer MM, Babiarz CL, Armstrong DE. A critical evaluation of tangential-flow ultrafiltration for trace metal studies in freshwater systems. 1. Organic carbon. Environmental Science & Technology 2000;34(16):3420-3427. |
R827629 (Final) |
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Hurley JP, Manolopoulos H, Babiarz CL, Sakamoto H, Rolfhus KR, Back RC, Shafer MM, Armstrong DE, Harris R. Methylmercury in Lake Superior: offshore processes and bioaccumulation. Journal de Physique IV 2003;107(1):641-644. |
R827629 (Final) |
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Manolopoulos H, Hurley JP, Babiarz CL, Back RC, Rolfhus KR. Riverine mixing zones as regions of enhanced methylmercury bioaccumulation in Lake Superior. Journal de Physique IV 2003;107(1):805-808. |
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:
water, watersheds, groundwater, sediments, precipitation, chemical transport, bioavailability, particulates, heavy metals, biology, hydrology, environmental chemistry, limnology, Great Lakes, EPA Region 5, Wisconsin, WI, Michigan, MI, Minnesota, MN,, 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.