10 STAR

REPORT

Vol. 4 Issue 1 October 2000 A product of the National Center for Environmental Research

MERCURY TRANSPORT AND FATE IN WATERSHEDS

U.S. EPA Office of Research

and Development’s Science

To Achieve Results (STAR)

Research in Progress

In December 1997, EPA’s Mercury Study

Report to Congress identified mercury as a critical

human health and environmental problem needing

additional scientific and technical research. Mercury

poses risks to humans and wildlife, particularly

because it concentrates in the tissues of animals as

it moves up through a food chain. Observed adverse effects in mammals, fish and birds have

included behavioral and neurological abnormalities,

impaired growth and development, reproductive

abnormalities such as fetal deformities, and in some

cases complete reproductive failure. Fish consumption is the dominant exposure pathway for humans and

wildlife. In some cases, mostly in the past, wildlife

were killed by extreme environmental concentrations

of mercury, for example from seed grains treated with

mercury, or from unusally severe instances of waste

release. But the most pervasive wildlife effects involve

reduced breeding success, which now poses severe

consequences for water birds throughout North

America and some endangered species, including

panthers, in the Florida Everglades.

Sources & Paths of Mercury in the Environment

Hg0

Hg+

Hg++

 Industial Sources

Plastics manufacture

Power generation

Chlorine production

Mercury Deposition

Once on land

Mercury is bound

to organic and

inorganic matter

in soil as, e.g.,

sulfides (HgS)

Bioaccumulation

of Mercury higher

up the food chain

via consumption

of fish

Chemical reaction in sediments

Hg+

 > Hg++ > (CH3)

 2Hg

 Anerobic

 Mircoorganisms

Methyl Mercury

in Fish (CH3

)

 2Hg

Rain

Runoff

Bioaccumulation

of Mercury higher

up the food chain

via consumption

of fish
2

More U.S. waters are closed

to fishing because of mercury

contamination than any other toxic

contamination problem. Because

fish consumption can be a significant route of human exposure to

toxic chemicals, the U.S. Food and

Drug Administration (FDA) issues

guidelines to states on fish contamination levels that warrant

closures of fishing waters to

commercial or recreational fishing.

The principal sources of fish

contamination, perhaps surprisingly, are air emissions of mercury

from coal burning

power plants, municipal waste incinerators

and other industrial

sources. The emitted

mercury compounds

readily settle, either

directly on water, or on

land from which they

runoff to water bodies.

Additional significant

amounts reach surface

and ground water due to

urban runoff and leaching from

mines and waste disposal sites.

Mercury can also be directly

discharged in inadequately treated

municipal or industrial wastewaters. A number of biological and

chemical processes that are

partially understood occur in soils,

water bodies and sediments that

cause mercury to react with organic matter to form methyl mercury (MeHg), the most toxic form of

Hg, which is readily taken up by

plants and animals. A principal

focus of the research studies

described in this report is to

achieve a better understanding of

the rates at which formation or

transformation of such compounds

occurs in watersheds and

waterbodies and how these are

affected by releases of various

forms of mercury from natural

sources, from residues left by

historic human activities, and from

current human activities.

Many human sources of

mercury have been eliminated or

much reduced by laws and regulations. Its use in paints and pesticides has been banned, and

industrial and municipal emissions

have been greatly curtailed through

process changes and recovery

technologies. However, some

major human uses continue,

leading to on-going emissions.

Mercury compounds left

by past industrial

activities and mining

also continue to

cycle through

land and water

ecosystems. In

addition, there

are significant

natural sources.

Mercury’s various chemical forms

differ in toxicity. It is readily

changed from one form to another

in passing from air to land to

surface waters, wetlands and

sediments, and then released back

again to the atmosphere. It is a

highly challenging problem for

environmental managers and

scientists to understand the potential risks posed by mercury to

wildlife and humans, and to determine what regulatory and nonregulatory approaches will most

effectively eliminate those risks.

STAR MERCURY

TRANSPORT AND

FATE RESEARCH

PROJECTS

In 1999, EPA’s STAR program

funded a set of studies to develop

a better understanding of

mercury’s terrestrial and aquatic

fate and transformation processes

that influence ecological and

human exposures. In several

projects, there is a particular focus

on microbial processes, which are

at present poorly understood.

Another area of focus is model

development. Improved models

are needed to predict ecosystem

responses to changes in anthropogenic inputs of mercury that could

result from various emission

reduction options and scenarios.

Whole-Ecosystem

Studies

A research team from Syracuse University, Cornell University and Smith College is studying

chemical and biological control of

mercury cycling in upland, wetland

and lake ecosystems in the

Adirondack region of New York.

Many Adirondack lakes have

elevated mercury concentrations in

water and fish tissue, and health

advisories warning against freshwater fish consumption have been

mandated throughout the region.

The concentration of mercury

in fish tissue has been found to be

higher in more acidic water bodies.

High acidity can occur naturally,

but is exacerbated by acid deposition from coal-burning power
3

plants, mine drainage and other

pollution sources. Conversely,

high levels of dissolved organic

carbon, resulting from the decay of

organic matter such as plants, can

decrease mercury uptake by fish.

Consequently, lakes that receive

substantial drainage from wetlands

are often characterized by relatively low levels of mercury in fish.

Recent lake sediment studies in

the Northeast have shown more

than threefold increases in mercury

deposition since 1850, suggesting

the degree to which industrial

pollution has contributed to regional mercury contamination.

Moreover, for reasons that are not

yet understood, these studies

suggest that watershed retention of

atmospheric mercury has decreased markedly over the last 60

years, from 95% retention in the

1930s to 75% retention today.

The Syracuse-led team is gathering data on transport and transformation patterns of mercury compounds in an Adirondack forest,

wetlands and surface water. They

will focus on mechanisms controlling MeHg levels and transport in

wetland pore waters and open

waters. Based on these findings

and other current data, and on

historic patterns of mercury transport and fate, they will develop a

lake/watershed mercury cycling

model. This will be used to predict

impacts on ecosystem mercury

levels that might result from emission reductions in the Adirondack

region.

The opportunity to fully track

and quantify MeHg production,

transport and fate in two large

ecosystems is offered by a study in

the Experimental Lakes Area of

northwestern Ontario, and by

extensive work in the Florida

Everglades. An international

research team from the University

of Maryland’s Chesapeake

Biological Laboratory, the Academy of Natural Sciences Estuarine Research Center, and the

Freshwater Institute of the

Canada Department of Fisheries

and Oceans is conducting comparative studies on these large

systems. They are assessing

mercury methylation in a catchment (large watershed) that includes uplands, wetlands and a

lake. This STAR-funded work is

one portion of a larger scale,

whole-ecosystem experiment in

which all mercury fate, transport,

transformation and uptake, from

atmospheric loading through fish

mercury concentrations, is being

traced. The STAR study component focuses on how MeHg

changes in response to changes in

mercury loading. Stable isotope

tracing methods will be used to

measure accumulation, production

and degradation rates of MeHg in

upland and wetland soils and lake

sediments, together with key

biogeochemical parameters that

affect mercury methylation and

bioavailability. The tracer method

will allow them to track the fate of

"new" mercury deposits vs. "old"

mercury stored in sediments and

soils, and to track the bioavailability Great Lakes Watershed
4

of new mercury over time across

the components of the watershed.

The Experimental Lakes Area

(ELA) is representative of north

central and northeastern US

ecosystems affected by mercury.

To provide a comparison in a very

different ecosystem and region,

this team will conduct parallel

mercury tracing studies in the

Florida Everglades. Here, mercury

addition experiments will be

conducted within wetland enclosures. Additional field work,

together with transport and fate

data from other Everglades studies, will support estimates of MeHg

fate in this warm, highly organically

enriched system. Taken together,

these two studies will provide data

that bracket the most likely conditions in most U.S. watersheds

impaired by mercury pollution and

provide information on regional and

landscape variability in methylation

and subsequent mercury accumulation in fish. Results should

provide the ability to predict

changes in levels of various forms

of mercury in the environment

caused by reduced mercury

emissions.

Fish consumption advisories

have been issued for Lake Superior due to mercury contamination.

Transport, fate and bioavailability

of mercury in the whole Lake

Superior Basin ecosystem are

being studied by a team from the

University of Wisconsin at

Madison, Lake Superior State

University, and the Wisconsin

Department of Natural Resources. Preliminary studies of

several rivers in the area have

found that wetland- and forestdominated watersheds attenuate

total mercury transport in the

Basin. And, very importantly, these

watershed types enhance the

production of the MeHg, the most

bioaccumulative, toxic form of

mercury. Preliminary estimates

suggest that while air deposition

provides about 75% of the total

mercury input, inputs from watersheds provide more than 80% of

the MeHg. Furthermore, while

atmospheric inputs are distributed

across the vast surface area of the

Lake, watershed influences are

concentrated in certain nearshore

zones. The specific physical and

chemical factors regulating

bioavailability are not known. For

instance, perhaps the MeHg

produced in some components of

watersheds is largely unavailable

for bioconcentration due to its

association with certain high

molecular weight organic materials,

or specific soil types. This study

will investigate sites where it is

hypothesized that enhanced

bioaccumulation may occur,

including sub-watersheds, riverlake mixing zones and open

waters. Field work will be supplemented with lab studies of processes influencing fate, transport

and bioavailability of MeHg and

total mercury under similar conditions. Results will be incorporated

into models predicting the influence

of each part of the ecosystem on

the ultimate delivery of the more

toxic and bioaccumulative forms of

mercury to the Lake’s waters and

aquatic life.
5

The Re-emission

Problem

Recent national mercury

emissions inventories estimate that

re-emission of previously deposited

mercury from historic human

sources is similar in magnitude to

industrial emissions. U.S. industrial emission is approximately 240

tons per year, while estimates of

re-emission from soils, sediments

and terrestrial and aquatic vegetation range from 150 to 600 tons per

year. The role of soils, water

bodies and vegetation as mercury

sources will affect the success of

any program aimed at reducing

mercury loading in and from

watersheds. Processes that

convert mercury to the elemental

form, which is highly volatile, also

diminish its availability for methylation and uptake by living organisms. The University of Michigan

is examining an important hypothesis: that the processes through

which monovalent and divalent

mercury are converted to the

elemental form are enhanced by

sunlight. If true, it indicates that

when there is strong light penetration into water bodies, the monovalent and divalent forms of mercury

associated with the organic matter

abundant in wetlands and some

lakes may be preferentially converted to readily released elemental mercury. It would thus be far

less likely to form the bioavailable

MeHg that accumulates in food

chains. Lab and shipboard experiments will assess the following and

other factors in the Saginaw Bay/

Lake Huron system: 1) what

degradation products, containing

what forms of mercury, are created

when locally typical organic matter

decays in different amounts of

light?; 2) what intracellular or

extracellular pathways in organisms can result in methylation of

these compounds?; 3) whether

conversion of monovalent and

divalent mercury to its elemental

form is activated in solid phases in

soils and wetlands; and 4) what

effect does oxidation of elemental

mercury have on its tendency to be

released? Results will help distinguish which components of the

watershed are the most likely

souces of mercury re emission.

For example, perhaps clear lakes

could be identified as less serious

sources of mercury re-emission

than wetlands with murky waters,

and thus with less light-induced

conversion of mercury into the less

bioavailable forms. This will help in

modeling likely successes of

decreasing mercury contamination

in fish and other organisms for

given watershed if new mercury

emissions are decreased.

Emission from Water

to Air

The natural chemical processes of reduction and oxidation,

called the “redox” cycle, control the

proportions of mercury present in

various forms. This in turn affects

rates of mercury exchange between water bodies and the atmosphere. Princeton University is

performing a comprehensive

analysis of all components of this

cycle. Some water bodies release

mercury in large amounts to air,

while others do not, resulting in

major differences in the amounts of

mercury in the water, sediments

and organisms. A series of lab and

field experiments will study the

chemical and biological redox

mechanisms that transform mercury between its divalent form,

Hg(II), and the volatile elemental

form, Hg(0). The forms differ in the

electron charges of the mercury

atoms. The experiments test three

complementary hypotheses: 1)

biological reduction of Hg(II) to

Hg(0) normally occurs through a

two-electron transfer reaction

mediated by certain enzymes in

two groups of photosynthetic

microorganisms, phytoplankton

and cyanobacteria; 2) chemical

reduction of Hg(II) occurs in two

distinct one-electron transfer

reactions, first the reduction of

Hg(II) to Hg(I) which requires a

high energy reducing agent (typically formed in light) such as the

superoxide anion or an organic

radical (probably a semiquinone),

followed by the reduction of Hg(I)

to Hg(0) by organic matter typically

present in natural waters; and 3)

oxidation of elemental mercury

requires first oxidation of Hg(0) to

Hg(I), likely caused by the same

radicals, superoxide or

semiquinones, and then oxidation

of Hg(I) to Hg(II) by oxygen, which

is facilitated by chloride complexation of the ionic mercury species.

Starting with simple modeled

systems and building up to complex lab situations more like those

of natural waters, the lab experiments are designed to establish the
6

mechanisms and rates of these

processes and to provide techniques and probes for field work.

Field experiments will include a

range of values for the key parameters, and are designed to establish

the actual occurrence of the mechanisms in nature and to provide data

on process rates. Results should

let us understand how factors

including solar irradiation, dissolved

organic matter concentration, pH

and biological productivity control

rates of mercury loss from water

bodies to the atmosphere, and

hence why water bodies with similar

mercury inputs end up with different

mercury levels in water, sediments

and fish.

Sulfur-Mediated

Microbial Effects

Sulfur, in all its chemical

forms, plays important roles in the

methylation of mercury by naturally

occuring sediment and soil bacteria.

Researchers from the University of

Maryland’s Chesapeake Biological Laboratory and the Academy

of Natural Sciences Esturaine

Research Center are using lab and

field studies to investigate the

hypothesis that high levels of sulfide

(a reduced form of sulfur) diminish

methylation by lessening the degree

to which mercury is taken up by

methylating bacteria. The scientists

also hypothesize that when there is

little sulfur present as sulfate (an

oxidized form), little methylation

occurs because many methylating

bacteria need sulfate as a nutrient.

Also, they will investigate the role of

some sulfate-using bacteria in the

converse process of demethylation,

together with assessing rates under

various conditions of abiotic

demethylation, a purely chemical

process with no bacteria or other

organisms involved. Results will

provide a comprehensive picture of

the soil and sediment processes

leading to greater or lesser production of the more toxic and

bioaccumulative forms of mercury.

Mercury from Mine

Wastes

Mercury mining has left a

legacy of contaminated wastes

throughout the mining belts of the

United States. The problem is

compounded by weathering, which

transforms the mercury into additional chemical forms, redistributes

it in watersheds and through the air,

and may increase bioavailability to

organisms. Mine wastes can have

significant regional environmental

impacts, since airborne mercury

sometimes reaches uncontaminated ecosystems far from point

sources. Stanford University and

the University of Nevada at Reno

are studying the physical and

chemical processes that control

chemical transformation, release

and distribution of mercury from

mine wastes, with the overall

objective of assessing risks to local

and regional ecosystems.

The researchers will determine

the mix of mercury compounds in

environmental samples, and use lab

experiments to examine sorption

processes of mercury on mineral

particles typical of sediments

downstream from mine sites,

transport on colloidal particles, and

the effects of sulfate and chloride

on sorption processes. They will

also monitor atmospheric emissions

from mine waste sites representing

different weathering and climatic

regimes, and compare emission

levels of each type of mercury with

the distribution of types (the “speciation”) in the mine wastes. This

will test the hypothesis that emissions of mercury to the atmosphere

are closely linked to mercury

speciation in mine wastes, and are

influenced by the amount of available elemental mercury, the other

mineral components of the wastes,

and weather. Based on these

results, they will develop an area

emission model by incorporating the

speciation data into a Geographic

Information System. With this they

will “scale up,” to estimate mercury

emissions over entire contaminated

mine sites. If this research increases our understanding of the

contributions of various mercury

sources to local and regional

mercury budgets, results will help

assess whether particular control

options for various point sources

will be effective in reducing local

and regional risks. Results will also

provide information useful in global

mercury modeling efforts.
7

Mercury in a Coastal

Environment

When polluted freshwater

mixes with saline coastal water

chemical processes typically lead to

precipitation (settling out) of many

toxic pollutants, including mercury

compounds. Estuarine water

bodies are thus major repositories

for riverborne/watershed derived

mercury. The University of Connecticut is studying microbiological,

chemical and physical aspects of

mercury cycling in Long Island

Sound. For decades the Sound and

its river tributaries served as the

receiving water for discharges

associated with older industrial

facilities. Many of these facilities

no longer exist, but in some cases a

legacy of contamination remains,

particularly in sediments associated

with freshwater and saltwater

interfaces. Preliminary work has

found large emissions of elemental

mercury from the waters of the

Sound to the atmosphere. At

present it is not known how rates of

historic and new mercury inputs and

deposition compare to rates at

which it is being released to the air.

This research team will test the

following hypotheses: 1) that the

principal influences on mercury

distributions are the the amount that

is present from place to place in

bioavailable forms and the levels of

chemical reducing agents, bacterial

activity and solar radiation from

place to place; (2) that organic

matter production, reflected in the

difference between net primary

production and bacterial consumption, can serve as a surrogate for

the amount of active organic

substances that influence the

biogeochemical fate of mercury; (3)

that estuarine reactions (i.e., mixing

of riverborne mercury with seawater

high in chlorine and positively

charged ions) increase the proportion of bioavailable mercury available for reduction; 4) that direct

sewage discharge into contained

brackish waters leads to enhanced

localized production of elemental

mercury; (5) given the availability of

reactive mercury species, and the

limited light penetration in coastal

waters, elemental mercury is the

predominant mercury cycling

product of bacterial activity in the

zone of well oxygenated waters;

and 6) net synthesis of MeHg is

most significant in redox “transition

zones”, primarily the shallower

sediments and waters in basins that

experience seasonal low oxygen

conditions (i.e., the western and

central Sound). Results of these

studies will be incorporated into a

biogeochemical mercury cycling

and mixing model, aspects of which

will be relevant to other coastal

regions. Such a model can be used

to predict water quality and fish

mercury levels that may result from

alternate pollution abatement

strategies.

Historic Analysis of

Sources and

Environmental

Conditions

Efforts to reduce mercury

emissions tend to have been based

on the assumption that in most

water bodies, particularly lakes,

mercury accumulation in fish would

be more or less proportional to

atmospheric deposition. Given the

strong evidence that mercury

deposition is now about 3 to 4 times

greater than natural rates, a first

estimate would be that fish should

be 3 to 4 times more contaminated

than natural levels. But a team of

Minnesota researchers has determined, based on comparing modern

fish to fish preserved in the 1930s,

that mercury concentrations in fish

may have increased by a factor of

10 in the low alkalinity lakes that are

common in their region. It is

possible that enhanced

bioaccumulation could be caused

by changes in the biological community. For example, there could be
8

STAR Principal Investigators, Institutions and Project Titles

higher levels in food chains, and

thus more biomagnification, if there

have been changing prey abundances. However, recent sediment

cores from sixteen Minnesota lakes

demonstrate that, beginning about

1940, MeHg was an increasingly

large percentage of the total. This

suggests a fundamental change in

methylation efficiency, weakening

support for the competing hypothesis of longer food chains.

Researchers from the University of Minnesota, the Minnesota

Pollution Control Agency and the

Science Museum of Minnesota

are investigating the anthropogenic

ecosystem changes that have

become widespread since World

War II to assess which are the most

likely contributors to enhanced

methylation. The changes include

increased sulfate deposition,

increased nutrient loads and

increased loading of mercury to

wetlands because of soil disturbance. The study will establish the

relative importance of atmospheric,

in-lake, and various wetland

sources of MeHg to a lake in a

forested watershed. They will

determine mercury retention and

release in different wetland types

and conduct experiments to determine which methylation-enhancing

processes are occuring. Finally,

they will add a hydrologically-based

wetland geographic information

system module to a widely used

Mercury Cycling Model, so the

findings can be applied to other

lakes. The model will be, among

other applications, relevant to

states’ needs to develop the total

maximum daily load estimates

required to support water quality

protection actions.

G. E. Brown, Jr., T. R. Ireland, D.

Grolimund, C. S. Kim (Stanford University,

CA); M. S. Gustin (University of NevadaReno (NV)); J. J. Rytuba (collaborator,

U.S. Geological Survey, Menlo Park (CA).

Processes Controlling the Chemical/

Isotopic Speciation and Distribution of

Mercury from Contaminated Mine Sites.

C. T. Driscoll (Syracuse University, NY); J.

Yavitt (Cornell University, NY); R. Newton

(Smith College, MA); R. Munson

(TetraTech Inc.,UT). Chemical and

Biological Control of Mercury Cycling in

Upland, Wetland, and Lake Ecosystems in

the Northeastern U.S.

W. F. Fitzgerald, P.T. Visscher (University

of Connecticut, CT). Microbiological and

Physicochemical Aspects of Mercury

Cycling in the Coastal/Estuarine Waters of

Long Island Sound and Its River-Seawater

Mixing Zones

C. C. Gilmour, A. Heyes (The Academy of

Natural Sciences Estuarine Research

Center, MD); R. P. Mason (University of

Maryland, Chesapeake Biological

Laboratory); J. M. Rudd (collaborator,

Canada Department of Fisheries and

Oceans, Freshwater Institute). Response of Methylmercury Production and

Accumulation to Changes in Hg Loading:

A Whole-ecosystem Mercury Loading

Study

J. P. Hurley (University of Wisconsin at

Madison, and Wisconsin Department of

Natural Resources, WI); D.E. Armstrong,

M.M. Shafer (University of Wisconsin at

Madison, WI); Richard C. Back (Lake

Superior State University, MI). Watershed Influences on Transport, Fate, and

Bioavailability of Mercury in Lake

Superior

R. P. Mason (University of Maryland,

Chesapeake Biological Laboratory) and

C. C. Gilmour (TheAcademy of Natural

Sciences Estuarine Research Center,

MD). Understanding the Role of Sulfur in

the Production and Fate of Methylmercury in Watersheds

F. M. M. Morel (Princeton University, NJ).

The Redox Cycle of Mercury in Natural

Waters

J. O. Nriagu, G. Keeler, J. Lehrnan

(University of Michigan, MI); S. Lindberg,

Hong Zhang (collaborators, Oak Ridge

National Laboratory). Photo Induced

Reduction of Mercury in Lakes, Wetlands, and Soils

E. B. Swain, J. Jeremiason (Minnesota

Pollution Control Agency, MN); P. L.

Brezonik, E.Nater, J.Cotner (University

of Minnesota, MN); D. Engstrom, J.

Almendinger (Science Museum of

Minnesota, MN), Reed Harris (TetraTech

Inc., CA). Methylmercury Sources to

Lakes in Forested Watersheds: Has

Enhanced Methylation Increased

Mercury in Fish Relative to Atmospheric

Deposition?