Mercury Cycling in the Environment Workshop, July 7-9, 1996

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Mail Stop 412                                           October 31, 1996


Subject:  Mercury Cycling in the Environment Workshop, July 7-9, 1996

Over the past five years, the public has become increasingly concerned 
about the possibility of mercury toxicity problems in humans, fish, and 
wildlife. In response, on July 7-9, 1996, the U.S. Geological Survey 
(USGS) and the Toxic Substances Hydrology (Toxics) Program sponsored a 
workshop on Mercury Cycling in the Environment to identify: (1) what 
science presently knows about the sources, transport, bioaccumulation, 
and biological effects of mercury; (2) the gaps in our present knowledge; 
and (3) how the USGS can collaborate with other organizations to help 
fill the knowledge gaps. The workshop began with a day and a half of 
technical presentations by scientists from the USGS, other state and 
Federal agencies, universities, and private entities. The presentations 
were followed by a day of open forum and small group discussions to 
identify critical knowledge gaps about mercury and potential roles for 
the USGS. Attached is a summary of the technical presentations and the 
conclusions from the discussions. This report, along with the Workshop 
Program and Abstracts, will be available on the Toxics Program home page 
in December, 1996. The URL for the home page is


Following the workshop, a subgroup of the Workshop attendees met to 
discuss how the USGS could fill the identified information needs by 
initiating a new project within the Toxics Program. A proposed work plan 
for that project will be distributed some time during the first months of 

                                  Janice Ward signed for
                                David A. Rickert
                                Chief, Office of Water Quality


This memorandum does not supercede any existing memoranda.

Distribution:   Chief, Office of Hydrologic Research
                Distribution A, B, DC
                District Water-Quality Specialists
                OWQ Staff
                NWQL Senior Staff
                Regional Water-Quality Specialists
                Workshop Attendees

- ----------------------

                       Summary Document for the

                   USGS Workshop on Mercury Cycling 
                         in the Environment

                  Golden, Colorado, July 7-9, 1996

           U.S. Geological Survey, Water Resources Division
                      Office of Water Quality
                         Reston, Virginia
                          October, 1996


On July 7-9, 1996 the USGS-WRD, Office of Water Quality 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.

            Scope and Objectives of the Workshop

Although the 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 (WRD, 
BRD, and 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 

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.

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.

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.

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.

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 
        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 
        Recent evidence suggests that Asian and South American countries are 
          major contributors to the global atmospheric load.

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 

Mercury Methylation and Demethylation:
        The biogeochemical processes of mercury methylation and 
          demethylation are probably the most import 
          bioaccumulative-controlling steps in the environmental mercury 
        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.

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. 

Processes at the Sediment/Water Interface:
        Many biogeochemical processes operate under optimal conditions at       
          the sediment/water interface, including mercury methylation and 
        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.

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.

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).

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.

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 

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.

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 
        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 
        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 
        The sediment water interface (or other interfaces where oxic/anoxic 
          boundaries are present) is a dominant site for methylmercury 

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 
        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 United States.  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.

III. Source-Transport Work Group: 

What we know:
        There are both natural and anthropogenic sources of mercury to the 
        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.

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
        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
        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 

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