Summary Document for the
USGS Workshop on
Mercury Cycling in the Environment
July 7-9, 1996
Sponsored by the
U.S. Geological Survey, Water Resources Division,
Office of Water Quality and Toxics Substances Hydrology Program
David P. Krabbenhoft
U.S. Geological Survey
Madison, WI 53719
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.
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
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.
- 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.
- 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
- 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.
- 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
- 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.
- 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
- 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.
- 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.
- 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.
- 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
- 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
- Modeling efforts suggest that past uses (as long ago as
- s) of mercury by man may still be affecting the global mercury
- Many biogeochemical processes operate under optimal conditions
at the sediment/water interface, including mercury methylation
- 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.
- 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.
- 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
- 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).
- 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.
- 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.
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.
- 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
- 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
- Mercury concentrations and speciation varies spatially and
temporally (daily to seasonal).
- Mercury strongly associates with particulate matter, especially
- 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.
- The sediment water interface (or other interfaces where
oxic/anoxic boundaries are present) is a dominant site for
- 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,
- 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
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.
- 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.
- 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
- 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
- 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.
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
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