Grantee Research Project Results

Final Report: Hg and Fe Biogeochemistry

EPA Grant Number: R825433C016
Subproject: this is subproject number 016 , established and managed by the Center Director under grant R825433
(EPA does not fund or establish subprojects; EPA awards and manages the overall grant for this center).
Center: UC Davis Center for Children's Environmental Health and Disease Prevention
Center Director: Van de Water, Judith
Title: Hg and Fe Biogeochemistry
Investigators: Suchanek, Thomas , Goldman, Charles R. , Richerson, Peter
Institution: University of California - Davis
EPA Project Officer: Packard, Benjamin H
Project Period: October 1, 1996 through September 30, 2000
RFA: Exploratory Environmental Research Centers (1992) RFA Text |  Recipients Lists
Research Category: Center for Ecological Health Research , Targeted Research

Objective:

The objective of this research project was to investigate the route of mercury (Hg) into and through Clear Lake, from the formation of Hg ore to its eventual bioaccumulation in plankton, benthic invertebrates, and higher trophic levels (see other research projects). We studied the movement of Hg from the Sulphur Bank Mercury Mine to various regions of the lake, its transformation from inorganic Hg to toxic methylmercury, and, ultimately, its bioaccumulation and effects on the biological/ecological system of the lake.

Summary/Accomplishments (Outputs/Outcomes):

Long-term research projects such as those sponsored by the Center for Ecological Health Research have enabled us to collect data over much longer periods of time than the typical grant period. The duration of this project allowed us to observe the effects of natural interannual variability on the dynamics of various system components such as Hg methylation. Specifically, we monitored interannual variation in Hg loading into Clear Lake, and we propose more realistic hypotheses about how the system functions. During this long-term study, we observed processes that have altered our perception of how ongoing Hg contamination enters the Clear Lake ecosystem, and how it interacts with other natural and anthropogenic stressors. We propose potential solutions to this problem. Our most recent data suggest that the flow of Hg-laden acid mine drainage (AMD) continues to enter Clear Lake from the Sulphur Bank Mercury Mine and may extend into Clear Lake at least 300 meters from the mine site. We continue to investigate this hypothesis with funding other than the Center's funding. This should provide useful information for the U.S. Environmental Protection Agency (EPA) Superfund in evaluating remediation options.

In coordination with Project R825433C015, we investigated the biogeochemical processes in sediments that supported Hg methylation, and we have found significantly elevated methylation rates at the Oaks Arm sites (where the Sulphur Bank Mercury Mine is located). These results also provide Hg loading data for developing total maximum daily load (TMDL) criteria at Clear Lake.

We also developed water, sulfur, and Hg budgets for Clear Lake, and worked with the U.S. Army Corps of Engineers to address the influence of proposed wetland restoration on ancillary methylmercury production. Our Hg budget included estimates of Hg loading to Clear Lake in 1992 and 2003. These data suggest that the U.S. EPA Superfund's mine remediation in 1992 did not lower significantly Hg loading to Clear Lake.

Once we identified the biogeochemical pathways and processes responsible for the source, creation, and transport of methylmercury, we followed the bioaccumulation of methylmercury through higher trophic pathways. We conducted a trophic analysis of Hg within the biotic compartments of Clear Lake. Using diet data and stable isotopes ¹³C and ¹⁵N, we followed the flow of Hg from input stream terrestrial detritus and autochthonous primary producers within Clear Lake to primary consumers and most major biomass-producing species within the system.

The following activities were accomplished:

• We developed a model of Hg uptake and bioaccumulation for the Clear Lake aquatic ecosystem. Utilizing data from all of the Clear Lake projects, we developed a model incorporating methylmercury production and trophic transfer between abiotic and biotic components that accurately describes Hg concentrations in higher trophic level biota. This model provides unique insights to Hg bioaccumulation in this system, and will aid Hg researchers in predicting Hg concentrations in comparable lake systems.

• Our report shows that Clear Lake is the first reported site at which Hg affects population- and community-level parameters of biological organization. Until now, nearly all of the known effects of Hg have been reported at the individual level. We showed that Hg in Clear Lake negatively impacts invertebrate population abundance and diversity metrics. In addition, fish populations for several species were shown to be negatively correlated with proximity to the mine. With these new data, researchers can better assess Hg effects on higher level population- and community-level processes.

• Our data show that Hg loading to Clear Lake has not diminished in the 11 years following the remediation of the Sulphur Bank Mercury Mine site. We compared surficial sediment Hg concentrations from 34 sites around Clear Lake in the fall of 1992 and 2003, and it appears that Hg loading has not decreased during this period, despite the U.S. EPA Superfund Program’s attempt to reduce Hg loading by the remediation of soils and waste rock piles on the mine site in 1992. This finding will help federal and state regulatory agencies drive the remediation process so that ongoing Hg loading can be significantly reduced or eliminated.

• We show that AMD from the Sulphur Bank Mercury Mine contributes on an ongoing basis to Hg loading to Clear Lake. By discovering, monitoring, and further investigating hot spot sources of AMD, increased acidity, increased sulfate, and elevated total and methylmercury at sites immediately adjacent to the mine, we have confirmed that the Sulphur Bank Mine continues to be a significant contributor to the production of methylmercury for the entire Clear Lake aquatic ecosystem. With these findings, additional studies should be conducted by the U.S. EPA Superfund Program to determine how best to reduce this source of Hg loading.

• We found that a major source of methylmercury production at Clear Lake is intimately tied to the production of a flocculent precipitate derived from AMD from the mine. Through a series of experimental laboratory microcosms, the addition of an AMD-derived flocculent precipitate (floc), found in abundance near the mine, was shown to stimulate the production of methylmercury by up to 20-fold above normal levels in Clear Lake sediments. This finding is consistent with the observations of ongoing Hg loading to Clear Lake from AMD, the lack of diminished Hg concentrations in surficial sediments, and the lack of a significant reduction in fish Hg concentrations since 1992. Therefore, understanding the linkage between AMD and Hg loading to Clear Lake will help in determining an appropriate set of remedial actions that can reduce Hg in higher trophic level species like the largemouth bass and catfish.

• We found that methylmercury and copper reacted additively as multiple stressors on zooplankton. A series of laboratory multiple stress toxicity and reproductive trials were conducted (on the standard laboratory benchmark cladoceran zooplankter Ceriodaphnia dubia and the indigenous Daphnia pulex from Clear Lake), exposing them to methylmercury and copper separately and in combination. The response to the copper-methylmercury mixture varied from additive to slightly less than additive. These data will be useful when formulating a risk assessment for the multiple contaminant stressors that affect Clear Lake biota by providing bounds on the level of effects from single stressors or combinations thereof.

• Our project represents one of the first ecosystem-wide, indepth studies of trophic transfer of Hg for any system investigated to date. We determined that Hg transport/bioaccumulation occurs primarily on the benthic pathway, with source material derived from the planktonic pathway. This information will be valuable to other investigators studying trophic transfer of Hg, and can assist federal and state regulatory agencies in determining the most appropriate strategies for remediating the Hg contamination problem at Clear Lake.

• We developed a water balance model for Clear Lake. This exercise enhanced our understanding of the communication between the mine site and Clear Lake, and thus improved our understanding of the mechanisms by which AMD impacts the aquatic ecosystem of Clear Lake.

• We developed a whole-lake sulfur budget for Clear Lake. Because of the importance of sulfur in the Hg methylation process, and because the Sulphur Bank Mercury Mine is a highly significant source of sulfur loading to Clear Lake, it was important to develop a budget that addressed the fluctuations and dynamics of sulfur cycling in Clear Lake. Clear Lake may be rather unique in the magnitude of sulfur that is entering the system, and this may provide unique opportunities (as we have documented) for atypical (nonsulfate-reducing bacteria) microorganisms to participate in Hg methylation.

• We developed a whole-lake, mass balance Hg budget for Clear Lake. Clear Lake is on the U.S. EPA's 303(d) list of California's impaired water bodies because of Hg contamination, and the Regional Water Quality Control Board is mandated to develop a TMDL remediation strategy for reducing Hg loading into Clear Lake. Using data from all of the years of the Center for Ecological Health research program, we were able to significantly contribute to the development of a mass balance Hg budget for Clear Lake. This also contributed to the development of the short- and long-term Hg loading estimates for the lake (see above).

• We found that methylmercury production in Clear Lake is decoupled from total inorganic Hg loading. Clear Lake is one of the most Hg-contaminated lakes in the world, based on sediment total Hg concentrations. We determined that there is a poor relationship between total Hg in sediments and water versus relative methylmercury bioaccumulation in the higher trophic levels compared with other contaminated systems. This finding may be because of a combination of factors, including: (1) the high pH (alkaline) nature of Clear Lake water; (2) the highly productive/eutrophic status of the lake's ecosystem; (3) the shallow depth regime of Clear Lake, which prevents stratification and widespread seasonal anoxic conditions; (4) the potential inhibitory influence of sulfur binding with Hg near the mine site; and (5) the lowered bioavailable nature of Clear Lake's inorganic Hg. This unique set of factors may provide special opportunities for more effective remediation than might otherwise be feasible for such an Hg-contaminated system.

• We found that profiles of methylmercury in sediment cores taken from Clear Lake were not the result of diagenetic upward mobilization of methylmercury, but represented vertical stability of that compound within the sediment column, a finding that will represent a significant contribution to the literature.

Supplemental Keywords:

ecosystem, ecosystem protection, environmental exposure and risk, geographic area, international cooperation, water, terrestrial ecosystems, aquatic ecosystem, aquatic ecosystem restoration, aquatic ecosystems and estuarine research, biochemistry, ecological effects, ecological indicators, ecological monitoring, ecology and ecosystems, environmental chemistry, restoration, state, water and watershed, watershed, watershed development, watershed land use, watershed management, watershed modeling, watershed restoration, watershed sustainability, agricultural watershed, exploratory research environmental biology, California, CA, Clear Lake, Lake Tahoe, anthropogenic effects, aquatic habitat, biogeochemical cycling, ecological assessment, ecology assessment models, ecosystem monitoring, ecosystem response, ecosystem stress, environmental stress, environmental stress indicators, fish habitat, hydrologic modeling, hydrology, integrated watershed model, lake ecosystems, lakes, land use, nutrient dynamics, nutrient flux, water management options, water quality, wetlands., RFA, Scientific Discipline, INTERNATIONAL COOPERATION, Water, ECOSYSTEMS, Waste, Ecosystem Protection/Environmental Exposure & Risk, Aquatic Ecosystems & Estuarine Research, Water & Watershed, Contaminated Sediments, mercury transport, Restoration, Aquatic Ecosystem, Environmental Microbiology, Terrestrial Ecosystems, Biochemistry, Environmental Monitoring, Ecology and Ecosystems, Aquatic Ecosystem Restoration, Watersheds, Mercury, anthropogenic stress, contaminant exposure, mercury uptake, watershed management, biodiversity, Clear Lake watershed, contaminated marine sediment, microbial degradation, nutrient loading, aqueous mercury, acid mine drainage, agricultural watershed, contaminated sediment, marine biogeochemistry, restoration strategies, Clear Lake, integrated watershed model, bioremediation of soils, iron, methylmercury, aquatic ecosystems, environmental stress, contaminated groundwater, mercury methylation, ecosystem stress, ecological impact, mercury chemistry, ecological research, watershed restoration, acid mine runoff

Relevant Websites:

http://ice.ucdavis.edu/cehr/ Exit

Progress and Final Reports:

Original Abstract

1997

1998

1999

Main Center Abstract and Reports:

R825433    UC Davis Center for Children's Environmental Health and Disease Prevention

Subprojects under this Center: (EPA does not fund or establish subprojects; EPA awards and manages the overall grant for this center).
R825433C001 Potential for Long-Term Degradation of Wetland Water Quality Due to Natural Discharge of Polluted Groundwater
R825433C002 Sacramento River Watershed
R825433C003 Endocrine Disruption in Fish and Birds
R825433C004 Biomarkers of Exposure and Deleterious Effect: A Laboratory and Field Investigation
R825433C005 Fish Developmental Toxicity/Recruitment
R825433C006 Resolving Multiple Stressors by Biochemical Indicator Patterns and their Linkages to Adverse Effects on Benthic Invertebrate Patterns
R825433C007 Environmental Chemistry of Bioavailability in Sediments and Water Column
R825433C008 Reproduction of Birds and mammals in a terrestrial-aquatic interface
R825433C009 Modeling Ecosystems Under Combined Stress
R825433C010 Mercury Uptake by Fish
R825433C011 Clear Lake Watershed
R825433C012 The Role of Fishes as Transporters of Mercury
R825433C013 Wetlands Restoration
R825433C014 Wildlife Bioaccumulation and Effects
R825433C015 Microbiology of Mercury Methylation in Sediments
R825433C016 Hg and Fe Biogeochemistry
R825433C017 Water Motions and Material Transport
R825433C018 Economic Impacts of Multiple Stresses
R825433C019 The History of Anthropogenic Effects
R825433C020 Wetland Restoration
R825433C021 Sierra Nevada Watershed Project
R825433C022 Regional Transport of Air Pollutants and Exposure of Sierra Nevada Forests to Ozone
R825433C023 Biomarkers of Ozone Damage to Sierra Nevada Vegetation
R825433C024 Effects of Air Pollution on Water Quality: Emission of MTBE and Other Pollutants From Motorized Watercraft
R825433C025 Regional Movement of Toxics
R825433C026 Effect of Photochemical Reactions in Fog Drops and Aerosol Particles on the Fate of Atmospheric Chemicals in the Central Valley
R825433C027 Source Load Modeling for Sediment in Mountainous Watersheds
R825433C028 Stress of Increased Sediment Loading on Lake and Stream Function
R825433C029 Watershed Response to Natural and Anthropogenic Stress: Lake Tahoe Nutrient Budget
R825433C030 Mercury Distribution and Cycling in Sierra Nevada Waterbodies
R825433C031 Pre-contact Forest Structure
R825433C032 Identification and distribution of pest complexes in relation to late seral/old growth forest structure in the Lake Tahoe watershed
R825433C033 Subalpine Marsh Plant Communities as Early Indicators of Ecosystem Stress
R825433C034 Regional Hydrogeology and Contaminant Transport in a Sierra Nevada Ecosystem
R825433C035 Border Rivers Watershed
R825433C036 Toxicity Studies
R825433C037 Watershed Assessment
R825433C038 Microbiological Processes in Sediments
R825433C039 Analytical and Biomarkers Core
R825433C040 Organic Analysis
R825433C041 Inorganic Analysis
R825433C042 Immunoassay and Serum Markers
R825433C043 Sensitive Biomarkers to Detect Biochemical Changes Indicating Multiple Stresses Including Chemically Induced Stresses
R825433C044 Molecular, Cellular and Animal Biomarkers of Exposure and Effect
R825433C045 Microbial Community Assays
R825433C046 Cumulative and Integrative Biochemical Indicators
R825433C047 Mercury and Iron Biogeochemistry
R825433C048 Transport and Fate Core
R825433C049 Role of Hydrogeologic Processes in Alpine Ecosystem Health
R825433C050 Regional Hydrologic Modeling With Emphasis on Watershed-Scale Environmental Stresses
R825433C051 Development of Pollutant Fate and Transport Models for Use in Terrestrial Ecosystem Exposure Assessment
R825433C052 Pesticide Transport in Subsurface and Surface Water Systems
R825433C053 Currents in Clear Lake
R825433C054 Data Integration and Decision Support Core
R825433C055 Spatial Patterns and Biodiversity
R825433C056 Modeling Transport in Aquatic Systems
R825433C057 Spatial and Temporal Trends in Water Quality
R825433C058 Time Series Analysis and Modeling Ecological Risk
R825433C059 WWW/Outreach
R825433C060 Economic Effects of Multiple Stresses
R825433C061 Effects of Nutrients on Algal Growth
R825433C062 Nutrient Loading
R825433C063 Subalpine Wetlands as Early Indicators of Ecosystem Stress
R825433C064 Chlorinated Hydrocarbons
R825433C065 Sierra Ozone Studies
R825433C066 Assessment of Multiple Stresses on Soil Microbial Communities
R825433C067 Terrestrial - Agriculture
R825433C069 Molecular Epidemiology Core
R825433C070 Serum Markers of Environmental Stress
R825433C071 Development of Sensitive Biomarkers Based on Chemically Induced Changes in Expressions of Oncogenes
R825433C072 Molecular Monitoring of Microbial Populations
R825433C073 Aquatic - Rivers and Estuaries
R825433C074 Border Rivers - Toxicity Studies