Mercury Cycling in the Environment Workshop, July 7-9, 1996
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cc: "Nana L Frye, Secretary (OA), Reston, VA "
Subject: OFFICE OF WATER QUALITY TECHNICAL MEMORANDUM 97.02
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From: "Nana L Frye, Secretary (OA), Reston, VA "
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Subject: OFFICE OF WATER QUALITY TECHNICAL MEMORANDUM 97.02
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Date: Fri, 01 Nov 1996 11:17:24 -0500
From: "David A Rickert, Chief, OWQ, Reston, VA"
In Reply Refer To:
Mail Stop 412 October 31, 1996
OFFICE OF WATER QUALITY TECHNICAL MEMORANDUM 97.02
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
http://toxics.usgs.gov/toxics
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
1997.
Janice Ward signed for
David A. Rickert
Chief, Office of Water Quality
Attachment
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
- ----------------------
ATTACHMENT
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
PREFACE
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
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.
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
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.
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.
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.
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
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.
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
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.
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.
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 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
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.
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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