USGS

Summary Document for the
USGS Workshop on
Mercury Cycling in the Environment
Golden, Colorado,
July 7-9, 1996

Sponsored by the
U.S. Geological Survey, Water Resources Division,
Office of Water Quality and Toxics Substances Hydrology Program
Reston, Virginia
October, 1996

by
David P. Krabbenhoft
U.S. Geological Survey
Madison, WI 53719
(608) 276-3843,

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PREFACE

On July 7-9, 1996 the U.S. Geological Survey's Office of Water Quality and Toxic Substances Hydrolgy Program sponsored a workshop on mercury cycling in the environment in recognition that public concern for fish and wildlife and human health from mercury toxicity has increased substantially over the past 5 to 10 years. These concerns are manifested primarily from the issuance of fish consumption advisories in the majority of U.S. states, Canada, and several European countries due to high levels of mercury in game fish. Although the precise causes for this contamination are poorly understood, it appears to result from both source and ecosystem-specific factors. Until recently, attempts to unravel this environmental contamination problem have been frustrated by both sampling and analytical barriers. For most aquatic ecosystems, atmospheric deposition is the primary source of mercury (although there are numerous instances of geologic and anthropogenic point-source contamination cases) and the resulting aqueous concentrations of mercury are generally less than 10 nanograms per liter. The challenge to scientists is to explain the series of processes that lead to toxic or near-toxic levels of mercury in organisms near the top of the food chain (the bioaccumulation process), when aqueous concentrations and source delivery rates are so low. To adequately understand this phenomenon an interdisciplinary approach is requisite. Due to recent great strides in sampling and analytical techniques, scientists can now routinely collect representative air, water, tissue, and sediment samples, and analyze for specific mercury species. The resultant data have provided new insights into the processes controlling the transport, cycling, and fate of mercury in aquatic ecosystems. In addition, new techniques that employ isotopic tracers have provided new insights about the specific processes at the root of this contamination problem: mercury methylation and demethylation.

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Scope and Objectives of the Workshop

Although the U.S. Geolgical Survey (USGS) plays a national-leadership role in water resources science, the current state of knowledge concerning mercury (sources, fate, controlling geochemical process, analytical and sampling methods) is --at the present --poorly distributed and implemented within USGS water programs. With this observation in mind, the workshop was organized to 1) transfer information and technology, 2) identify data and information gaps within the mercury knowledge base, 3) identify specific data and information gaps where the USGS might play a major role, and 4) apply information gained from the first three objectives to plan for a national mercury project funded by the Toxic Substances Hydrology Program.

Information and technology transfer were accomplished through 1.5 days of contributed and invited presentations by scientists from the USGS (Water Resources Division (WRD), Biological Resources Division (BRD), and Geologic Division (GD)), other federal and state agencies, universities, and private research entities. Many of the presentations were given by world leaders in the scientific mercury community. Their presentations highlighted many of the breakthrough studies over the past 10 years that have redefined our understanding of mercury sources, transport, cycling and transformation processes, biotic uptake and food-web transfer, and analytical methodologies.

On the final day of the Workshop objectives 2 and 3 were addressed. Workshop attendees were divided into three technical Work Groups to meet and discuss 1) what we know, 2) what we do not know, and 3) what can or should the USGS do to help fill information gaps. All the Work Groups reconvened at the end of the day so that each group could present their conclusions.

The following day, a committee of seven Workshop attendees met to discuss how the information gained from the Workshop could be used to guide the formulation of a work plan for a new project on mercury funded by the Toxic Substances Hydrology Program. The group sought to emphasize efforts that matched the strengths of the USGS with perceived information needs from the Workshop.

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Summary of the Technical Presentations

Concerns for Human Health:

  • The impact is real and potentially great, as was demonstrated in case studies where severe mercury poisoning has occurred.
  • The impact from low-level exposure (commonly observed today) is unclear, but is potentially great for unborn children.
  • Current standards for the issuance of fish-consumption advisories are intentionally conservative, and these standards should be kept in place as a protection barrier for the human health, or at least until we have improved information.

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Mercury in Fish and Aquatic Food Webs:

  • A tremendous amount of new information has been provided by scientific research over the past 10 years. These studies have shown that several key environmental parameters are linked with high levels of mercury in fish.
  • However, this is a very complex area of research that is controlled by ecosystem parameters (e.g., water chemistry, wetlands presence/absence), aqueous mercury speciation, food-web structure, size, age, and growth rate of organisms, population size, etc.
  • The effect of source strength and point-source impacts are unclear, as was illustrated by examples from Oak Ridge, TN; Carson River, NV; and the Everglades.

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Exposure and Health Risks for Piscivorous (fish eating) Wildlife:

  • This is maybe the area where most of the concern for health risk should be placed.
  • Continent-wide studies on common organisms (e.g. Loons) are beginning to show strikingly similar results that suggest mercury impacts piscivorous wildlife, particularly reproduction rates.
  • These studies are difficult to conduct and more controlled, experimental research needs to be performed before definitive conclusions can be reached. Recent studies that employ innovative methods, such "clean egg/dirty egg swapping will be key for unraveling controlling influences.

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Atmospheric Sources and Transport:

  • Studies in this area of research are very scale dependent; the scale at which research questions are asked can dictate the information that is needed or will be attained, and the consequent interpretations.
  • Although mercury contamination is truly a global pollution problem, regional, sub-regional, and local effects are clearly evident from recent studies.
  • Coal and oil combustion and municipal and medical waste incineration are the major anthropogenic sources to the atmosphere.
  • Abandoned mines and industrial effluents are unquantified point sources to aquatic ecosystems.
  • Natural emissions are important too (e.g. volatilization from the oceans and soils), but the natural:man-contributed ratio is still unresolved.
  • Recent evidence suggests that Asian and South American countries are major contributors to the global atmospheric load.

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Sampling and Analytical Methods for Mercury:

  • Sampling and analytical methods have rapidly developed over the past decade to include reliable sub part per trillion quantification of several mercury species in a variety of environmental samples (e.g., water, sediments, air, aerosols).
  • These developments were a key reason for the interpretive power of many recent mercury studies.
  • Sampling and analytical methods are continuing to evolve at a rapid rate.

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Mercury Methylation and Demethylation:

  • The biogeochemical processes of mercury methylation and demethylation are probably the most import bioaccumulative-controlling steps in the environmental mercury cycle.
  • Methylation is largely the result of intracellular processes of sulfate reducing bacteria, although other microorganisms can methylate mercury as can some abiotic processes.
  • Demethylation of mercury is also microbiallly mediated. There appear to be two pathways: the mer Operon (a lyase/reductase process), and an oxidative process.
  • Current research seeks to identify the organisms which mediate the demethylation processes, to quantify where and under what conditions each process dominates, and rates reactions.

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Historical Perspectives as Recorded by Lake Sediments, and the Global Mercury Cycle:

  • Dated sediment cores are an effective way to infer historical trends in mercury accumulation rates, and potential point-sources releases, in deep water lakes and reservoirs containing organic-rich sediments.
  • Cores taken over an area can be used to differentiate watershed versus atmospheric contributions to lakes and reservoirs, as well as regional trends in deposition.
  • Fine-scale sampling in well preserved cores show that atmospheric deposition rates of mercury may have already peaked, and in some local to regional areas are declining. On the global scale, however, Hg emissions from developing areas (e.g. South America, Asia) are rising, which may reverse this trend.
  • The global mercury cycle is important to consider for mercury researchers, and one of the most elusive aspects of this cycle is the relative contributions of natural to anthropogenic sources.
  • Modeling efforts suggest that past uses (as long ago as the 1800
  • s) of mercury by man may still be affecting the global mercury cycle.

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Processes at the Sediment/Water Interface:

  • Many biogeochemical processes operate under optimal conditions at the sediment/water interface, including mercury methylation and demethylation.
  • Recent studies -- involving detailed investigations of the sediment/water interface -- show that in many cases the interface a relatively unimportant source of inorganic mercury, but a dominant site for methylmercury production and flux.
  • These studies need to place an equal emphasis on quantifying groundwater fluxes, which is the dominant transport vector is most littoral zones.

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Mercury-DOC Interactions:

  • Dissolved organic carbon (DOC) is an effective complexing ligand for many trace metals including mercury. Recent studies have shown strong correlative relations between DOC and total and methyl mercury content in a variety of aquatic ecosystems.
  • The precise mechanisms for this relation are still poorly understood. Researchers need to place more emphasis on the quality of the DOC (elemental makeup, functional and sulfhydyl group concentrations, humic/fulvic fractions, etc.to clarify the role of DOC in the environmental mercury cycle.

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Summary of Work Group Presentations

I. Aquatic Biota, Wildlife, and Human Health Work Group:

What we know:

  • Methylmercury bioaccumulates in fish and many other aquatic organisms and biomagnifies in food chains.
  • The fraction of total mercury existing as methylmercury typically increases up aquatic food chains from primary producers to fish. Nearly all (95-100%) of the mercury present in fish is methylmercury, obtained mostly from the diet.
  • The structure of aquatic food webs can greatly influence mercury concentrations in fish.
  • Methylation and demethylation are key processes affecting concentrations of methylmercury in aquatic organisms in both grossly and lightly contaminated ecosystems.
  • Total concentrations of mercury in sediment, water, and biota in lower trophic levels (below fish) are not reliable predictors of methylmercury concentrations in fish.
  • Mercury concentrations in fish are low in some freshwater ecosystems having large inventories of inorganic mercury in sediments.
  • Certain fresh waters with fish-consumption advisories (i.e., high concentrations of mercury in sport fish) are lightly contaminated ecosystems in which inorganic Hg(II) is readily converted to methylmercury. These fresh waters include low-alkalinity lakes, newly flooded reservoirs, and certain wetland ecosystems.
  • The construction and flooding of new reservoirs increase mercury levels in fish by creating environmental conditions that greatly increase the microbial production of methylmercury from existing inorganic Hg(II).
  • Methylmercury is highly neurotoxic, damaging the central nervous system. The developing young (early life stages) of vertebrate organisms (including humans) are much more sensitive than adults to methylmercury.
  • Human exposure to methylmercury is almost entirely due to consumption of fish.
  • Fish-eating birds, mammals, and reptiles in ecosystems with mercury-contaminated fish have high dietary exposure to methylmercury, vastly exceeding the exposure of human populations (as indicated by mercury concentrations in blood).

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What we suspect:

  • Methylmercury adversely affects the reproductive success and developing young of fish-eating wildlife in ecosystems having fish with elevated mercury concentrations.
  • Most of the methylmercury (inventory) within an aquatic ecosystem at a given point in time resides in the fish.
  • Temperature may be a significant environmental variable affecting methylmercury production and uptake in fish and other biota in aquatic ecosystems.

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Critical information gaps:

  • The environmental variables and processes that most strongly influence the bioavailability of mercury and bioaccumulation of methylmercury in aquatic food chains.
  • The toxicological significance of dietary methylmercury exposure in fish-eating wildlife (birds, mammals, and reptiles), with emphasis on reproductive effects.
  • The relative contributions of external mercury inputs (e.g., atmospheric deposition) and watershed sources (sediments, soils, and geologic materials) of mercury to the quantities accumulated in fish.
  • The forms of methylmercury (e.g. CH3HgCl, CH3HgOH, (CH3)2Hg) that most readily cross biological membranes.
  • The influence of organic complexation on the biological uptake of methylmercury.

 

What is the most appropriate role for the USGS regarding the topic of methylmercury and human health?

Work Group Recommendation:

USGS studies should focus on understanding factors and processes influencing mercury levels in fish, the primary source of human exposure. Health effects of methylmercury exposure in humans is being addressed in large studies by other teams of investigators. In other words, USGS investigations should focus on processes affecting exposure to methylmercury, rather than on health effects of methylmercury on humans.

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II. Biogeochemistry and Cycling Work Group:

What we know:

  • Mercury is generally found at very low concentrations and is extremely reactive in the environment. It readily undergoes phase, species, and redox changes.
  • A good overall understanding of the factors controlling the formation, destruction, transport, and uptake of methylmercury is the most important aspect of aquatic mercury research.
  • Sulfate reducing bacteria are important mediators of methylmercury production medium.
  • Microbes are largely responsible for mercury demethylation in the environment, and they accomplish this through the mer operon and oxidative processes.
  • Sedimentation and evasion (water-air exchange of reduced gaseous mercury) are the primary sinks for mercury from an aquatic ecosystem.
  • Mercury concentrations and speciation varies spatially and temporally (daily to seasonal).
  • Mercury strongly associates with particulate matter, especially organic particulates.
  • The quality and quantity of DOC in an aquatic ecosystem can have a strong influence on the fate and transformation of mercury in the environment.
  • Low pH systems generally promote higher concentrations, mobility, and methylation of mercury.
  • Generally the vast majority of mercury in an aquatic ecosystem is in the inorganic form (about 95 to 99%).
  • Generally the vast majority of mercury in an aquatic ecosystem is found in the sediments.
  • Aqueous mercury is affected by photochemical processes (e.g. photo reduction).
  • The sediment water interface (or other interfaces where oxic/anoxic boundaries are present) is a dominant site for methylmercury production.

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Critical information gaps:

  • Absolute, in situ measurements of methylation and demethylation.
  • Quantification of mercury sources to ecosystems and pools across a variety of ecosystems.
  • The available species of mercury for methylation and how interactions with sulfide affect mercury availability in the environment.
  • Identification of novel types of bacteria and biochemical mechanism(s) for mercury methylation and demethylation.
  • The important methyl donors in the environment? Microorganisms, humics, chemicals?
  • What drives photochemical reactions of mercury reduction and subsequent evasion?
  • Are there other important organo-mercurials in the environment? (e.g., dimethylmercury, phenylmercury)
  • Analytical methods: good QA/QC, inter-lab comparisons, reference materials (especially aqueous standards), microbial assays for methylation and demethylation, interdisciplinary "team studies,

 

What is the most appropriate role for the USGS regarding the topic of biogeochemical cycling?

Work Group Recommendation:

USGS studies should focus on understanding what controls methylmercury production, destruction, and uptake by organisms in a wide variety of environments across the US. Emphasis should be placed on trying to understand why mercury becomes of toxicological concern to wildlife and humans in some aquatic ecosystems and not in others, when the reasons are not obvious. Sites should be chosen to span source types (e.g. atmospheric versus point source dominated), climate, geology, hydrology, and trophic structure. The goal of these studies should be to understand why contamination problems occur in some areas, to predict where mercury problems might occur where there is no information, and to provide useful information of resource managers and policy makers concerning whether mitigative measures are possible.

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III. Source-Transport Work Group:

What we know:

  • There are both natural and anthropogenic sources of mercury to the environment.
  • Coal combustion and municipal and medical waste incineration are the major anthropogenic sources to the atmosphere.
  • There is growing concern from abandoned mines where metallic mercury from the extraction process, and oxidizing tailings or remnant ore present a potentially large contamination source.
  • The atmosphere is the dominant transport vector of mercury to most ecosystems that are not affected by point sources (which is the general case).
  • Natural emissions are important too, including volatilization from the oceans and soils.
  • Forests accumulate dry deposition in equivalence to wet deposition. Vegetation is a source to the atmosphere (evasion from leaf surfaces) and to watersheds (leaf litter and throughfall).
  • Regional, sub-regional, and local source effects are evident.
  • Most depositing mercury is in the form of inorganic mercury, and the majority of that falls with precipitation.

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Critical information gaps:

  • The relative magnitude of natural to anthropogenic sources.
  • Knowledge of whether different Hg sources (e.g. atmospheric versus mines versus industrial release) result in differing contamination levels. Or, does the source strength scale linearly to food chain contamination.
  • Man's activities influence on the overall global mercury cycle
  • A national survey of historical records of mercury accumulation across the US. This could include inorganic compartments (soils), biologic compartments (end of food web), and possibly some trend analysis.
  • A nationally complete (good spatial coverage) wet and dry mercury deposition network across the coterminous United States.
  • A national inventory of sources and pools of Hg, including natural and human related.

 

What is the most appropriate role for the USGS regarding the topic of sources and transport?

Work Group Recommendation:

Due to less well developed expertise in the atmospheric sciences, USGS involvement in this area would need to be strongly partnered with other agencies, universities, and private labs. Joint efforts should focus on 1) providing a complete national scale framework for atmospheric deposition and sediment accumulation rates, and 2) a national, quantitative inventory of man-related and natural sources.

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