Mercury contamination is widespread globally, originating from natural and human-related sources, including 

air transport from coal combustion, waste incineration, 

and mining. (Photograph by Phillip J. Redman, U.S. 

Geological Survey).

U.S. Department of the Interior

U.S. Geological Survey

Mercury in Stream Ecosystems—

New Studies Initiated by the U.S. Geological Survey

By Mark E. Brigham, David P. Krabbenhoft, and Pixie A. Hamilton

Why study mercury? 

Mercury can adversely affect humans 

and wildlife through consumption of contaminated fish, particularly by sensitive 

individuals, such as children and women 

of childbearing age. Mercury is currently 

the leading cause of impairment in the 

Nation’s estuaries and lakes and was cited 

in nearly 80 percent of fish-consumption 

advisories (2,242 of 2,838) reported by 

states in 2000. The geographic extent 

of mercury advisories covers more than 

10 million acres of lakes and more than 

400,000 stream miles—increases of about 

7 and 48 percent, respectively, over advisories reported in 1998 (U.S. Environmental Protection Agency, 2002a).

Understanding the mercury cycle

Our understanding of mercury contamination has evolved considerably 

since the 1950’s, when the problem was 

perceived to be associated primarily 

with a limited number of point 

sources of severe pollution, such 

as industrial or mining wastes. We 

now know that mercury contamination is global and affects 

many waters that have no obvious 

mercury source. This is because 

mercury emissions generally 

disperse widely in the atmosphere 

before being deposited to the 

earth’s surface. Mercury is emitted by natural sources, such as 

volcanoes, geothermal springs, 

geologic deposits, and the ocean. 

Human-related sources primarily 

include coal combustion, waste 

incineration, industrial uses, and 

mining. During the last 150 years, 

human activities have more than 

doubled natural amounts of mercury in the atmosphere (Schuster 

and others, 2002).

Inorganic mercury (the form 

emitted to the environment) 

is generally not a health concern—it is poorly absorbed by the 

digestive tract. The real issue is 

methylmercury—an organic form 

that is highly toxic to the nervous 

system. Methylmercury is produced from inorganic mercury by 

methylation, a microbial process 

that is controlled by certain bacteria and 

enhanced by chemical and environmental 

variables, such as the presence of organic 

matter and oxygen. More than 95 percent 

of all mercury in fish is methylmercury, 

and this form of mercury biomagnifies 

to high concentrations at the top of food 

chains (Wiener and others, 2002). 

During the last 15 years, scientists 

have increased their understanding of 

mercury in standing water, such as lakes, 

reservoirs, and wetlands. Less understood 

are details of the mercury cycle—from 

source to fish—in stream ecosystems. 

Data are sparse for mercury in all components of stream ecosystems, including 

water, sediment, and aquatic biota. Data 

also are lacking on the transport, biological and chemical transformation, and 

bioaccumulation of mercury in streams 

in diverse geographical locations and 

ecological settings. USGS studies are 

designed to enhance scientific understanding of mercury in stream ecosystems.

Printed on recycled paper

Facts about mercury:

• Highly toxic to the nervous system

• Persistent in the environment

• Bioaccumulates (higher concentrations in tissues of aquatic plants 

and animals than in water)

• Biomagnifies (higher concentrations at increasingly higher levels 

in the food chain)

• Numerous chemical forms in air, 

water, sediment, and biota

• Responsible for nearly 80 percent 

of U.S. fish-consumption advisories

Fact Sheet 016–03

Month 2003
USGS mercury studies

USGS mercury studies address two 

primary questions: (1) What are the 

concentrations of mercury in water, sediment, and fish? and (2) How do biological, chemical, and other environmental 

characteristics govern the methylation, 

transport, and bioaccumulation of mercury in streams? These studies include 

stream ecosystems in relatively undeveloped forested areas and in highly urbanized areas (such as Portland, Oregon; 

Milwaukee, Wisconsin; and, Orlando, 

Florida) to allow comparisons among 

these contrasting land uses. The studies 

also cover a wide range of landscapes 

with varying geology, climate, hydrology, 

and wetland density to better understand 

environmental factors controlling methylation, mercury transport, and bioaccumulation. Finally, the stream ecosystems 

vary in food-web complexity, which can 

influence mercury bioaccumulation and 

biomagnification. Specifically, the studies 

assess:

• Atmospheric deposition of inorganic 

mercury to ecosystems.

• Processes affecting the formation 

and degradation of methylmercury, 

including (1) methylmercury dissolved in water and bound to sediment (which can differ in uptake by 

aquatic biota); (2) methylation rates 

in relation to microbial processes, 

chemical characteristics in water 

and sediment (including acidity 

(pH), temperature, dissolved organic 

carbon, and sulfate), environmental settings, and seasons; and (3) 

demethylation processes, which 

convert methylmercury to less toxic 

forms of mercury and, thereby, 

decrease the amount of methylmercury available to aquatic biota.

• Other geochemical forms of mercury in the atmosphere, water, and 

sediment.

• Food-web complexity (the totality 

of feeding interactions—from algae 

to aquatic insects to forage and 

predatory fish) and its influence on 

bioaccumulation of mercury. 

Mercury is studied in stream ecosystems across the Nation

USGS is assessing the extent and 

magnitude of mercury contamination 

in stream ecosystems in 14 major river 

basins across the continental United 

States (referred to as “Study Units” by 

the National Water-Quality Assessment 

(NAWQA) Program) (see map). Samples 

of water, sediment, and predatory fish 

were collected at 120 stream sites in 2002 

to assess the occurrence of total mercury, methylmercury, and other chemical 

constituents, such as sulfate and dissolved 

organic carbon. This one-time, “snapshot” assessment covers a multitude of 

diverse environmental settings.

In addition, USGS is assessing processes affecting transport, biological and 

chemical transformations, and bioaccumulation of mercury in 3 of the 14 

Study Units. These more intensive studies 

began in 2001 in the Willamette Basin, 

Georgia-Florida Coastal Plain Drainages, 

and Western Lake Michigan Drainages. 

Similar studies in six additional Study 

Units will begin in 2004 or 2007. 

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Samples for analysis of mercury were collected at 120 stream sites in 14 major river basins 

(NAWQA “Study Units”) in 2002. More detailed studies on mercury methylation, transport, and bioaccumulation are ongoing in 3 of the 14 Study Units—the Willamette Basin, Georgia-Florida Coastal 

Plain Drainages, and Western Lake Michigan Drainages

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Mercury levels in fish are determined by 

(1)—mercury sources, such as atmospheric 

emissions from burning coal; (2) methylation 

efficiency, which is controlled by certain 

biological, chemical, and environmental characteristics; and (3) food-web complexity, the 

totality of feeding interactions—from algae to 

predatory fish—in an ecological community.

Fish consumption warnings are posted to 

warn anglers. The number of states that have 

issued mercury advisories has risen steadily 

from 27 in 1993 to 44 in 2002 (U.S. Environmental Protection Agency, 2002b)
How USGS information can be 

used

Identifying high-priority regions

USGS mercury data will help to fill 

information gaps for stream ecosystems 

in many regions of the United States for 

which little or no information on mercury 

contamination exists. These assessments 

will help to better define and anticipate 

regional variations in aquatic mercury 

levels and to identify contaminated and 

high-priority regions for protection and 

restoration.

Improved criteria and standards for protecting aquatic and human health

USGS findings will contribute to 

improved mercury criteria and standards. The current U.S. Environmental 

Protection Agency criterion is based on 

mercury concentrations in fish because 

there is not enough information about 

mercury in water or about bioaccumulation of mercury in fish to establish a 

water standard. Because USGS studies 

relate concentrations of mercury in water 

to mercury concentrations in fish, the 

validity for establishing a water standard 

can be evaluated, which will ultimately 

allow a more timely and cost-effective 

approach for regulating mercury. In addition, USGS data and research may aid in 

the development of more rigorous models 

that relate water quality to mercury bioaccumulation, thereby enhancing capabilities for predicting mercury contamination 

in fish.

Improved strategies for controlling emissions and managing lands

USGS findings will provide a better understanding of the methylation 

of mercury in various environmental 

settings. This understanding will help to 

clarify the most appropriate strategies 

for defining and achieving safe mercury 

emission levels. For example, if sulfur is 

linked conclusively to the mercury cycle, 

there may be implications for the control 

of both sulfur and mercury emissions 

from power plants. In addition, research 

on environmental characteristics that 

enhance mercury methylation may help 

to avoid mercury contamination in the 

creation of reservoirs and flooding of 

lands in vulnerable areas.

While most research in the last 15 years has focused on mercury contamination in lakes, reservoirs, and wetlands and in portions of rivers receiving point-source discharges of industrial and 

mining wastes, USGS studies will help to fill large information gaps in terms of basic data and 

detailed studies of stream ecosystems across the Nation, such as in the Willamette River Basin 

shown above. (Photograph by Dennis A. Wentz, U.S. Geological Survey).

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Combustion from coal-fired utilities and industrial boilers accounts for more than 85 percent of 

the transmission of inorganic mercury to the atmosphere (U.S. Environmental Protection Agency, 

1997). USGS findings will provide a better understanding of what drives methylation of inorganic 

mercury in certain environmental settings, and thereby help to clarify appropriate strategies 

regarding mercury emissions.
Multiple USGS programs collaborate on mercury studies

USGS mercury assessments are 

planned and implemented in large part 

through the National Water-Quality 

Assessment (NAWQA) Program ( a map 

can be accessed at the NAWQA Web site 

at http://water.usgs.gov/nawqa). In 1991, 

NAWQA began evaluating the quality 

of streams, ground water, and aquatic 

ecosystems in 51 major river basins 

and aquifer systems (known as “Study 

Units”) across the Nation. The assessments characterize the ambient water 

resource—the source of about 60 percent 

of the Nation’s drinking water and water 

for industrial, irrigation, and recreational 

uses. During its first decade, NAWQA 

made baseline assessments of pesticides, 

nutrients, volatile organic compounds, 

trace elements, dissolved solids, and 

radon in streams and ground water, as 

well as the condition of aquatic habitats 

and fish, insect, and algal communities. 

These findings are described in hundreds 

of reports (access NAWQA Web site 

above). 

In the next decade of studies, 42 of 

the Study Units will be intensively reassessed to determine trends at many of the 

streams and ground-water monitoring 

sites; fill critical gaps in characterizing 

water-quality conditions; and build upon 

earlier NAWQA findings that show how 

natural features and human activities 

affect water quality and aquatic ecosystems. Mercury is one of five national 

priority topics that will be addressed in 

the second decade of NAWQA studies. 

Four remaining topics include (1) effects 

of urbanization on stream ecosystems; 

(2) ecological effects of nutrient enrichment; (3) transport of contaminants to 

public-supply wells; and, (4) sources, 

transport, and fate of agricultural chemicals.

The NAWQA work is enhanced 

through research conducted by the USGS 

Toxic Substances Hydrology (TOXICs) 

Program, National Research Program 

(NRP), and various programs within 

the USGS Geology and Biology disciplines. This collaborative research largely 

focuses on chemical and biological 

processes controlling mercury transport, 

cycling, and biomagnification in stream 

ecosystems. Specifically, the different 

programs provide the following expertise:

Cited literature:

Schuster, P.F., Krabbenhoft, D.P., 

Naftz, D.L., Cecil, L.D., Olson, M.L., 

DeWild, J.F., and Green, J.R., 2002, A 

270-year ice core record of atmospheric 

mercury deposition to western North 

America, Environmental Science and 

Technology, v. 36, pp. 2303-2310.

U.S. Environmental Protection 

Agency, 2002a, National water quality 

inventory—2000 Report, U.S. Environmental Protection Agency Report EPA841-R-02-001, Washington D.C.

U.S. Environmental Protection 

Agency, 2002b, Update: National listing of fish and wildlife advisories, EPA 

Fact Sheet EPA-823-F-02-007, Office of 

Water, Washington, D.C.

U.S. Environmental Protection 

Agency, 1997, Mercury study report to 

Congress, EPA 452R-97-004, Washington D.C.

Wiener, J.G., Krabbenhoft, D.P., 

Heinz, G.H., and Scheuhammer, A.M., 

2002, Ecotoxicology of mercury, chap. 

16, in D.J. Hoffman, B.A.Rattner, G.A. 

Burton, Jr., and J. Cairns, Jr., eds., Handbook of ecotoxicology, 2nd ed.: Boca 

Raton, Fla., CRC Press, p. 407-461.

For suggested further reading on 

mercury in stream ecosystems, 

refer to:

http://infotrek.er.usgs.gov/mercury

Toxic Substances Hydrology 

Program (TOXICs)—Mercury 

cycling; chemical transformations and 

physical partitioning of mercury; stateof-the-art analytical methods (http:

//toxics.usgs.gov).

National Research Program—

Microbial and chemical reaction rates 

(including sulfate reduction and mercury 

methylation and demethylation); mercury 

bioaccumulation (http://water.usgs.gov/

nrp).

Programs in the USGS Geology 

Discipline—Mercury methylation as it 

relates to sulfur geochemical cycling and 

sources (http://geology.usgs.gov).

Programs in the USGS Biology 

Discipline—Effects of mercury on fish 

health, including reproductive success, 

hormone levels, and genetic damage; 

occurrence of mercury in fish-eating 

birds (http://biology.usgs.gov).

The USGS promotes public access 

to water-quality information. Visit 

USGS Web sites listed above to 

access reports, water-quality data, 

and maps. For additional information, please contact:

Mark E. Brigham, Mercury 

Team Leader, (763) 783-3274, 

mbrigham@usgs.gov

David P. Krabbenhoft, Mercury 

Team Lead Scientist, (608) 821-3843, 

dpkrabbe@usgs.gov

Lynda S. Chasar, Georgia-Florida 

Coastal Plain Drainages, (850) 942-9500, 

ext. 3010, lchasar@usgs.gov

Barbara C. Scudder, Western Lake 

Michigan Drainages, (608) 821-3832, 

bscudder@usgs.gov

Dennis A. Wentz, Williamette Basin, 

(503) 251-3296, dawentz@usgs.gov

Timothy L. Miller, Chief of Water 

Quality Programs, USGS, (703) 648-

5716, tlmiller@usgs.gov