The Quality of Our Nation’s Waters

Mercury in the Nation’s Streams—Levels, Trends, and Implications

U.S. Department of the Interior

U.S. Geological Survey

Circular 1395
Cover: Front cover photographs, clockwise from top left: (1) Brook trout (© iStock); 

(2) Sixmile Brook and adjacent wetland, New York (Dennis A. Wentz); (3) Coal-burning 

power plant (© iStock); (4) Lookout Creek, Oregon (Dennis A. Wentz).
The Quality of Our Nation’s Waters

Mercury in the Nation’s Streams—Levels, 

Trends, and Implications

By Dennis A. Wentz, Mark E. Brigham, Lia C. Chasar, Michelle A. Lutz, and 

David P. Krabbenhoft

Circular 1395

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2014

For more information on the USGS—the Federal source for science about the Earth, its natural and living 

resources, natural hazards, and the environment, visit http://www.usgs.gov or call 1–888–ASK–USGS.

For an overview of USGS information products, including maps, imagery, and publications, 

visit http://www.usgs.gov/pubprod

To order this and other USGS information products, visit http://store.usgs.gov

Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the 

U.S. Government.

Although this information product, for the most part, is in the public domain, it also may contain copyrighted materials 

as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner.

Suggested citation:

Wentz, D.A., Brigham, M.E., Chasar, L.C., Lutz, M.A., and Krabbenhoft, D.P., 2014, Mercury in the Nation’s streams—

Levels, trends, and implications: U.S. Geological Survey Circular 1395, 90 p., http://dx.doi.org/10.3133/cir1395.

Library of Congress Cataloging-in-Publication Data

Wentz, Dennis A.

The quality of our nation’s waters : mercury in the nation’s streams—levels, trends, and implications / by Dennis A. 

Wentz, Mark E. Brigham, Lia C. Chasar, Michelle A. Lutz, and David P. Krabbenhoft.

90 p. cm. — (Circular ; 1395)

Includes bibliographic references and index.

ISBN 978-1-4113-3786-2 (alk. paper)

1. Mercury–Environmental aspects—United States. 2. Stream ecology—United States. 3. Water quality—United 

States. I. Wentz, Dennis A. II. Geological Survey (U.S.) III. Series: U.S. Geological Survey circular ; 1395.

 TD427.M4W45 2014

 628.1'683–dc23

2014019008

ISSN 1067-084X (print)

ISSN 2330-5703 (online)
iii

Foreword

The United States has made major investments in assessing, managing, regulating, and conserving 

natural resources, such as water, minerals, soil, and timber. Sustaining the quality of the Nation’s water 

resources and the health of our ecosystems depends on the availability of sound water-resources data 

and information to develop effective, science-based policies. Effective management of water resources 

also brings more certainty and efficiency to important economic sectors. Taken together, these actions 

lead to immediate and long-term economic, social, and environmental benefits that make a difference 

to the lives of millions of people.

The U.S. Geological Survey (USGS) is committed to providing the Nation with reliable scientific 

information that helps to enhance and protect the overall quality of life and that facilitates effective 

management of water, biological, energy, and mineral resources (http://www.usgs.gov). Information 

on water resources is critical to ensuring long-term availability of water that is safe for drinking and 

recreation, and that is suitable for industry, irrigation, and fish and wildlife. Population growth and 

increasing demands for water make the availability of water, measured in terms of quantity and quality, 

essential to the long-term sustainability of our communities and ecosystems.

Mercury is a pervasive contaminant of streams and lakes, and has resulted in fish consumption 

advisories in all 50 States. The current report, “Mercury in the Nation’s Streams—Levels, Trends, and 

Implications,” presents a summary of results from USGS investigations conducted since the late 1990s 

on the sources, occurrence, trends, transport, and bioaccumulation of mercury in stream ecosystems. 

The report draws from studies conducted by several USGS Programs, including the National WaterQuality Assessment, Toxics Substances Hydrology, and National Research Programs. This report is 

one of a series of publications, The Quality of Our Nation’s Waters, which describe major findings of 

the USGS on water-quality issues of regional and national concern. Other reports in this series focus 

on the occurrence and distribution of nutrients, pesticides, and volatile organic compounds in streams 

and groundwater; the effects of contaminants and streamflow alteration on the condition of aquatic 

communities in streams; and the quality of untreated water from private domestic and public supply 

wells. Each report builds toward a more comprehensive understanding of the quality of regional and 

national water resources (http://water.usgs.gov/nawqa/nawqa_sumr.html). 

The information in this series is intended primarily for those interested or involved in resource 

management and protection, conservation, regulation, and policymaking at regional and national levels. 

In addition, the information should be of interest to those at a local level who wish to know more about 

the general quality of streams and groundwater in areas near where they live and how that quality 

compares with other areas across the Nation. We hope this publication will provide you with insights 

and information to meet your needs, and will foster increased citizen awareness and involvement in the 

protection and restoration of our Nation’s waters.

William H. Werkheiser

Associate Director for Water

U.S. Geological Survey
iv

Abbreviations, Acronyms, and Conventions 

Used in This Report

AMNet Atmospheric Mercury Network

CMAQ-Hg Community Multiscale Air Quality modeling system 

(mercury)

Hg Chemical symbol for mercury

MDN Mercury Deposition Network

METAALICUS Mercury Experiment to Assess Atmospheric 

Loadings in Canada and the United States

MPCA Minnesota Pollution Control Agency

NADP National Atmospheric Deposition Network

NCBP National Contaminant Biomonitoring Program

ppb Parts per billion

ppm Parts per million

TMDL Total Maximum Daily Load

USEPA United States Environmental Protection Agency

USGS United States Geological Survey

Mercury in fish muscle tissue is measured as total mercury; however, it 

has been shown that greater than 95 percent of such mercury is in the 

methylmercury form (Bloom, 1992). Thus, all fish tissue mercury discussed 

in this report is assumed to be methylmercury.

Mercury concentrations in fish and bird tissue are reported on a wet 

weight basis, unless otherwise noted. Mercury concentrations in soil and 

sediment are reported on a dry weight basis.

Water samples for mercury analysis were typically collected by the USGS 

during low streamflow, when differences between whole water (unfiltered) 

and dissolved (0.7-micrometer filtered) mercury concentrations are small 

and unimportant. We do not distinguish between these forms in this report.

The first use of each term listed in the Glossary is given in bold underline

font.
v

Contents

Chapter 1. Major Findings and Implications..............................................................................................1

Chapter 2. Why Is Mercury in Fish a Concern? ......................................................................................11

Chapter 3. How Does Mercury Cycle Through Aquatic Ecosystems?................................................21

Chapter 4. Where Are Environmental Mercury Concentrations Highest Across the Nation? .......43

Chapter 5. How Do Environmental Mercury Levels Vary over Time?..................................................53

References Cited .......................................................................................................................................73

Glossary.........................................................................................................................................................85

Appendix 1. Supporting Information for Selected Figures ..................................................................89
Photograph by Dennis A. Wentz.

Lookout Creek, Oregon
Major Findings 

and Implications

Introduction

Mercury is a potent neurotoxin that 

accumulates in fish to levels of concern 

for human health and the health of fisheating wildlife. Mercury contamination of 

fish is the primary reason for issuing fish

consumption advisories, which exist in every 

State in the Nation. Much of the mercury 

originates from combustion of coal and can 

travel long distances in the atmosphere before 

being deposited. This can result in mercurycontaminated fish in areas with no obvious 

source of mercury pollution.

A posted advisory communicates the risk of eating fish that contain high 

concentrations of mercury. (Photograph by Dennis A. Wentz.)

Three key factors determine the level of mercury 

contamination in fish—the amount of inorganic mercury

available to an ecosystem, the conversion of inorganic 

mercury to methylmercury, and the bioaccumulation of 

methylmercury through the food web. Inorganic mercury 

originates from both natural sources (such as volcanoes, 

geologic deposits of mercury, geothermal springs, and 

volatilization from the ocean) and anthropogenic sources (such 

as coal combustion, mining, and use of mercury in products 

and industrial processes; fig. 1-1). Humans have doubled 

the amount of inorganic mercury in the global atmosphere 

since pre-industrial times, with substantially greater increases 

occurring at locations closer to major urban areas. 

Chapter 1

Major Findings and Implications 1

Chapter

1
In aquatic ecosystems, some inorganic 

mercury is converted to methylmercury, the 

form that ultimately accumulates in fish. The 

rate of mercury methylation, thus the amount 

of methylmercury produced, varies greatly 

in time and space, and depends on numerous 

environmental factors, including temperature 

and the amounts of oxygen, organic matter, and 

sulfate that are present. 

Methylmercury enters aquatic food 

webs when it is taken up from water by algae 

and other microorganisms. Methylmercury 

concentrations increase with successively higher 

trophic levels in the food web—a process 

known as bioaccumulation. In general, fish at 

the top of the food web consume other fish and 

tend to accumulate the highest methylmercury 

concentrations. 

This report summarizes selected stream

studies conducted by the U.S. Geological Survey 

(USGS) since the late 1990s, while also drawing 

on scientific literature and datasets from other 

sources. Previous national mercury assessments 

by other agencies have focused largely on 

lakes. Although numerous studies of mercury 

in streams have been conducted at local and 

regional scales, recent USGS studies provide the 

most comprehensive, multimedia assessment 

of streams across the United States, and yield 

insights about the importance of watershed

characteristics relative to mercury inputs. 

Information from other environments (lakes, 

wetlands, soil, atmosphere, glacial ice) also is 

summarized to help understand how mercury 

varies in space and time.

Figure 1-1. Burning coal for energy production was the single largest 

component of anthropogenic mercury emissions in the United States during 

2005, accounting for slightly more than one-half of the total.

2 Mercury in the Nation’s Streams—Levels, Trends, and Implications

Combustion of coal and 

 other fossil fuels

Metal mining and production

Gold mining and production

Cement production

Chlor-alkali production

Waste incineration

Other 

EXPLANATION
Highlights of Major Findings and Implications 

• Methylmercury concentrations in fish exceeded 

the U.S. Environmental Protection Agency 

criterion for the protection of human health 

at about one in four streams across the United 

States. High methylmercury concentrations in 

fish are the primary cause of fish consumption 

advisories, which exist in every State in the 

Nation. The predominant source of mercury in fish 

is deposition of atmospheric inorganic mercury 

produced by coal combustion. In response to the 

widespread contamination of fish, mercury has been 

effectively removed from many products and waste 

streams, resulting in about a 60-percent decrease 

in emissions in the United States since 1990. 

However, to reduce mercury levels in fish to fully 

meet human health criteria, further reductions in 

mercury emissions are necessary.

• Wetlands increase the amount of inorganic 

mercury that is converted to methylmercury, the 

form that accumulates to harmful levels in fish.

Wetland characteristics, such as limited dissolved

oxygen concentrations and abundant organic matter, 

provide favorable environments for microorganisms 

to effect the conversion of inorganic mercury to 

methylmercury. Thus, wetland construction or 

restoration (for example, to improve habitat or to 

filter nutrients and sediment) should balance the 

potential for increased methylmercury production 

against the anticipated ecological and water-quality 

benefits of the wetlands. 

• In contrast to other environmental contaminants, 

mercury emission reduction strategies need to 

consider global mercury sources in addition to 

domestic sources. Reductions in domestic mercury 

emissions are likely to result in lower mercury 

levels in fish in the Eastern United States, where 

domestic emissions contribute a large portion of 

atmospherically deposited mercury. In contrast, 

emission controls will provide smaller benefits in 

the Western United States, where reduced domestic 

emissions may be offset by increased emissions 

from Asia. Implementation of the recently adopted 

U.S. Mercury and Air Toxics Standards and 

worldwide Minamata Convention goals should lead 

to reductions in both U.S. and global 

mercury emissions. 

• Existing mercury monitoring programs focus 

mostly on methylmercury concentrations in 

fish, and lack design elements and data to link 

these levels to mercury sources. Most programs 

do not track methylmercury concentrations in 

fish over time in ways that support rigorous, 

nationally consistent trend assessments. Given 

the complexities of mercury emissions, transport 

pathways, and ecological factors that influence 

the extent of methylmercury contamination in 

fish, a multimedia monitoring approach is critical 

to track the effectiveness of management actions 

intended to reduce mercury emissions and resulting 

environmental mercury levels. 

Chapter 1

Major Findings and Implications 3

(Photograph by Dennis A. Wentz.)
EXPLANATION

Non-mined Mined

≤ 0.10

> 0.10 – ≤ 0.20

> 0.20 – ≤ 0.30

> 0.30

Methylmercury, game fish fillet, 

in parts per million

Major Findings

(1) Methylmercury concentrations in fish 

exceeded the U.S. Environmental Protection 

Agency criterion for the protection of human 

health at about one in four streams across the 

United States. 

Fish methylmercury concentrations 

exceeded 0.3 ppm—the U.S. Environmental 

Protection Agency (USEPA) fish tissue mercury 

criterion for the protection of human health—in 

predator fish from about one-fourth of nearly 

300 streams sampled in a nationwide survey 

(fig. 1-2). Methylmercury concentrations in 

largemouth bass—a common predator fish—

exceeded the USEPA criterion at more than 

one-half of sites where they were collected. 

Similarly, recent national surveys by the 

USEPA found that methylmercury levels in fish 

exceeded the criterion from about one-fourth of 

stream kilometers assessed, and in predator fish 

from about one-half of lakes sampled.

Fish methylmercury concentrations in 

streams typically were highest in wetlanddominated landscapes, particularly in coastal 

plain streams of the Southeastern United States. 

Fish methylmercury levels also were high in the 

Western United States, but only in streams that 

historically had been mined for mercury or gold. 

Methylmercury levels were low in fish from 

streams in major urban and agricultural areas. 

Fish consumption advisories based on high 

methylmercury concentrations exist in every 

State. These advisories are based on the USEPA’s 

reference dose for mercury and States’ fish 

monitoring data, and are issued by individual 

States and Tribes. It is important for people 

consuming locally caught fish to be aware of the 

fish consumption advisories that apply in their area. 

Figure 1-2. Fish methylmercury concentrations exceeded the 

USEPA criterion for protection of human health (0.3 ppm) in predator 

fish from about one-fourth of streams sampled nationwide during 

1998–2005. Fish methylmercury concentrations generally were highest 

in wetland-dominated streams in the Southeastern United States or 

in streams draining basins that had been mined for mercury or gold in 

the West.

4 Mercury in the Nation’s Streams—Levels, Trends, and Implications
Wetlands increase the amount of inorganic mercury that is converted to methylmercury. 

(Photograph by Dennis A. Wentz.)

(2) The abundance and characteristics of 

wetlands are key factors that affect the ability 

of stream ecosystems to transform mercury into 

methylmercury.

Methylmercury concentrations in stream 

water, fish, and other aquatic organisms, correlate 

strongly with wetland abundance in stream 

basins. Wetland characteristics, such as limited 

dissolved oxygen concentrations and abundant 

organic matter, provide favorable environments 

for microorganisms to convert inorganic mercury 

to methylmercury. Methylmercury production in 

wetlands and other aquatic ecosystems generally 

increases with increasing sulfate, which can be 

contributed by anthropogenic sources, such as 

emissions from coal burning. Thus, decreasing 

sulfate emissions, in response to implementation 

of the Clean Air Act, are expected to cause 

decreasing methylmercury concentrations in 

some areas of the United States.

Water-level fluctuations, including drying 

and wetting of soil and aquatic sediment, also 

exacerbate mercury methylation. Fluctuating 

water levels can result from water-management 

actions, such as dam construction and operation. 

In addition, climate change is likely to increase 

the frequencies and intensities of droughts and 

storms, thus amplifying water-level fluctuations 

and increasing methylmercury concentrations.

Chapter 1

Major Findings and Implications 5
(3) Methylmercury concentrations in fish 

depend more on the amount of methylmercury 

in an ecosystem than on the amount of 

inorganic mercury released to the ecosystem.

Methylmercury concentrations in fish 

correlate strongly with methylmercury 

concentrations in stream water, indicating 

that the amount of methylmercury available 

to the base of the food web is an important 

control on fish methylmercury concentrations. 

Fish near the top of a food web have higher 

methylmercury concentrations than fish at lower 

trophic levels, because with each increase in 

trophic level, the methylmercury in the prey

organism is accumulated into the tissue of the 

consumer (fig. 1-3). 

Across the United States, methylmercury 

concentrations in fish and stream water 

generally were highest in undeveloped areas 

with abundant wetlands, which provide ideal 

conditions for methylmercury production. In 

contrast, methylmercury levels in largemouth 

bass from urban streams were the lowest 

of all land uses and land covers studied 

(fig. 1-4). This occurred even though inorganic 

mercury inputs were higher in urban settings 

than in agricultural, undeveloped, or mixed 

land use/land cover settings. Methylmercury 

concentrations were lower than expected in 

urban streams because factors conducive to 

methylmercury production, such as the amount 

of wetlands and dissolved organic carbon, also 

generally are low in these ecosystems. These 

findings contrast starkly with those for many 

other contaminants in rivers and streams, which 

tend to be high in urban and agricultural areas.

Although methylmercury concentrations 

in fish from some mined basins were as high as 

anywhere in the Nation, with values up to 50 

times the USEPA criterion for the protection of 

human health, most fish tissue mercury levels 

in mined basins were no higher than in rural 

undeveloped basins. Some streams draining 

mined basins in the West have concentrations 

of inorganic mercury in water and sediment that 

are hundreds-of-thousands of times greater than 

streams in unmined areas. However, a relatively 

small portion of the inorganic mercury typically 

is converted to methylmercury because wetlands 

and dissolved organic carbon generally are 

low in these ecosystems. The large amounts of 

mercury in mined ecosystems still contaminate 

fish decades after mining activity has ceased and, 

without costly remediation, will likely continue 

to contaminate fish into the future.

6 Mercury in the Nation’s Streams—Levels, Trends, and Implications
tac11-0651_fig01-03

Biomagnification Bioconcentration

Methylmercury concentration, in parts per million

USEPA fish tissue mercury criterion = 0.3 parts per million

Water

Algae and other

microorganisms

Invertebrates

Top

predator fish

Forage fish

0.00000001

0.0000001

0.000001

0.00001

0.0001

0.001

0.01

0.1

1

10

Increasing trophic level

Figure 1-3. Methylmercury concentrations in aquatic organisms increase with increasing 

methylmercury concentrations in water and with increasing trophic level. Fish at the top of the 

food web tend to have the highest concentrations of methylmercury.

0

0.5 

1.0 

1.5 

2.0 

2.5 

Undeveloped Mixed Mined Agricultural Urban 

Median methylmercury concentration in 

largemouth bass, in parts per million per meter

Figure 1-4. Mercury concentrations in largemouth bass were 

lowest in streams draining urban areas.

Chapter 1

Major Findings and Implications 7
(4) Mercury concentrations in lake sediment, 

fish tissue, and precipitation have decreased in 

some areas during recent decades, coincident 

with legislation regulating discharges of 

contaminants to air and water. 

Downward trends of mercury in lake 

sediment, fish, and precipitation coincide with 

implementation of the Clean Air Act (1970), the 

Clean Water Act (1972), and other legislation 

designed to limit pollutants to the environment. 

These measures address reductions in mercury 

use, controls on mercury emissions from waste 

incinerators, and incidental capture of mercury by 

controlling sulfur and particulate emissions from 

coal-fired power plants.

Lake Sediment.—From 1970 to 2000, downward 

trends of mercury in lake sediment cores, which 

record the history of mercury delivery to a lake, 

outnumbered upward trends by about a 2:1 ratio. 

Downward trends were most common in lakes 

in dense urban areas, and are consistent with 

controls on industrial discharges of mercury 

and a shift in coal combustion from residential 

and commercial heating to electrical power 

generation. The relative lack of decreasing 

mercury concentrations in reference lakes (less 

than 1.5 percent urban land) reflects stable or 

increasing global atmospheric mercury sources.

Fish Tissue.—During 1969–87, downward 

trends in fish methylmercury concentrations 

were measured at 20 of 22 sites outside the 

Southeastern United States. The numbers of 

upward and downward trends were about equal 

in the Southeast. Decreasing concentrations 

occurred primarily during the 1970s, followed by 

relatively stable concentrations during the 1980s. 

The rate of decrease ranged from 3 to 12 

percent annually. 

Precipitation.—Total mercury concentrations 

in precipitation at Mercury Deposition Network 

sites decreased in almost one-half of 49 sites 

monitored during 1996–2005, but showed 

no discernible change at the remaining sites 

(fig. 1-5). Decreases were particularly evident 

in the Northeast and are consistent with large 

reductions in mercury emissions—especially 

from medical and municipal incinerators—during 

this period.

Insufficient data for 

 trend determination

Trends in mercury concentration 

in precipitation

No significant trend

Significant decreasing trend

EXPLANATION

Figure 1-5. Total mercury 

concentrations in precipitation 

decreased at about one-half of 

monitoring sites during 1996–2005. 

8 Mercury in the Nation’s Streams—Levels, Trends, and Implications
This report is organized around the following questions: 

• What are mercury concentrations in 

streams across the Nation, and where 

are concentrations highest? (Chapter 4) 

• How do environmental mercury levels 

vary over time? What is the outlook 

for the future? (Chapter 5) 

• Why is mercury in fish a concern? 

(Chapter 2)

• Where does mercury in aquatic ecosystems 

come from? How does mercury move 

through stream ecosystems? Why are 

mercury concentrations in fish high when 

concentrations in stream water are typically 

low? Why do some fish species have higher 

mercury concentrations than others, and what 

factors control mercury bioaccumulation in 

fish? (Chapter 3)

Chapter 1

Major Findings and Implications 9
Little Wekiva River, Florida

Photograph ty Dennis A. Wentz.
Why Is Mercury in 

Fish a Concern?

This chapter addresses the question: Why 

is mercury in fish a concern? The toxicity 

of methylmercury to humans and wildlife 

is summarized, and fish methylmercury 

concentrations that may be health risks for 

consumers of fish are discussed. 

Mercury is a naturally occurring element and a potent 

neurotoxin with no known beneficial function related to the 

growth and development of plants or animals. The toxicity of 

mercury became widely recognized in the 1970s, when several 

mass mercury poisonings were reported in different parts of 

the world. One of the better-known examples documented 

the deaths of more than 1,000 people due to consumption of 

fish and shellfish that were contaminated by methylmercury 

discharged by a chemical plant to Minamata Bay, Japan. We 

now know that aquatic ecosystems produce methylmercury 

naturally, and that fish bioaccumulate this methylmercury to 

levels that can be toxic to people and wildlife.

By Elsie M. Sunderland1, James G. Wiener2, and Mark E. Brigham3

Methylmercury contamination of Minamata Bay, Japan, led to the discovery 

that fish can accumulate methylmercury at levels that are toxic to humans. 

(Photographs by William Eugene Smith and Aileen M. Smith, © Aileen M. 

Smith, used with permission.)

Chapter 2

Why Is Mercury in Fish a Concern? 11

Chapter

2

1

Department of Environmental Health, Harvard School of Public Health, 

Harvard University. 2

University of Wisconsin-La Crosse, River Studies Center.

3

U.S. Geological Survey.
1992 1994 1996 1998 2000 2002 2004 2006 2008 2010 1992 1994 1996 1998 2000 2002 2004 2006 2008 2010

A. Rivers B. Lakes

C.

0

2,000,000

4,000,000

6,000,000

8,000,000

10,000,000

12,000,000

14,000,000

16,000,000

18,000,000

Mercury

0

200,000

400,000

600,000

800,000

1,000,000

1,200,000

1,400,000

Mercury

All other

contaminants

All other

contaminants

Total lake area under fish consumption 

advisory, in acres

Total river length under fish consumption

 advisory, in miles

Advisories for specific waterbodies

Statewide freshwater advisory

Statewide freshwater advisory

and additional advisories on

specific waterbodies

Statewide coastal advisories

Statewide advisory for lakes only

EXPLANATION

Methylmercury Toxicity is a Global Concern

Methylmercury is an organic form of mercury that 

readily bioaccumulates in aquatic food webs, reaching its 

highest concentrations in predatory fish, fish-eating wildlife, 

and humans that consume these animals. The toxicity of 

mercury has been known for centuries; however, it gained 

widespread attention during the early 1970s, when a number 

of mass poisonings occurred around the world. In one of the 

most highly publicized incidents, it has been reported that 

more than 1,000 people have died since about 1960 from 

eating fish and shellfish contaminated with methylmercury 

discharged by a chemical manufacturing company to 

Minamata Bay, Japan (Harada, 1995).

Certain bacteria in water bodies and wetlands can 

naturally convert inorganic mercury to methylmercury. 

Between 1962 and 1970, for example, mercury discharged 

from a chlor-alkali plant to the English-Wabigoon River 

system in Ontario, Canada, was rapidly methylated and 

bioaccumulated, causing extremely high concentrations of 

methylmercury in fish and exposing many First Nations 

subsistence fishers to harmful amounts of methylmercury 

(Rudd and others, 1983; Wheatley and Paradis, 1995). We 

now understand that much lower levels of methylmercury 

exposure than those in Minamata Bay or the English-Wabigoon 

River are known to adversely affect human health, especially 

for children and pregnant women.

In many areas of North America, methylmercury levels 

in fish and in fish-eating wildlife are associated with harmful 

effects, such as diminished health and reproduction. These 

effects have been documented at tissue concentrations that 

are equivalent to or lower than the USEPA fish tissue mercury 

criterion for the protection of human health (0.3 ppm).

As environmental monitoring of mercury increased 

in the 1970s and 1980s, high mercury concentrations were 

discovered in fish inhabiting many water bodies that lacked 

direct anthropogenic sources of mercury in their watersheds. 

By the 1990s, a scientific consensus had emerged that mercury 

emitted to the atmosphere—and subsequently deposited 

onto the Earth’s surface—is the primary cause of mercury 

contamination in these ecosystems. Today, all of the Earth’s 

aquatic ecosystems are almost certainly contaminated by 

mercury from atmospheric emissions. In the United States, 

mercury is the primary cause of fish consumption advisories, 

accounting for 81 percent of all advisories in 50 States, 

1 Territory, and 3 Tribes during 2010 (fig. 2-1). 

Figure 2-1. Since 1993, fish consumption advisories related to mercury have increased substantially for both (A) rivers and (B) lakes. 

As of 2010, fish consumption advisories were issued for about 42 percent of the Nation’s lake area and 36 percent of river length, with 

mercury accounting for most. (C) Fish consumption advisories have been issued by all 50 States to inform the public about health risks 

associated with mercury in fish. (Data from U.S. Environmental Protection Agency, 2011a; see appendix 1 for additional information.)

12 Mercury in the Nation’s Streams—Levels, Trends, and Implications
Atlantic 

Coast

(1.55 ppb)

Inland Northeast

(0.77 ppb)

Great Lakes Coast

(0.80 ppb)

Inland Midwest

(0.63 ppb) Inland West

(0.73 ppb)

Inland South

(0.74 ppb)

Gulf Coast

(0.96 ppb)

Pacific 

Coast

(1.18 ppb)

Blood mercury concentration, 

in parts per billion (ppb)

Exposure to Methylmercury is 

Associated with Human Health Risks

Most mercury exposure in the U.S. 

population is from fish consumption. 

Methylmercury exposure from fish consumption 

has been associated with various adverse effects 

on human health, ranging from central nervous 

system toxicity in adults exposed at extremely 

high levels to diminished cardiovascular health 

and endocrine disruption at lower exposure 

levels (Clarkson and others, 2003; Choi and 

Grandjean, 2008; Tan and others, 2009). Longterm reductions or impairments in brain function 

in children associated with methylmercury 

exposure have been reported in many studies 

around the world. The most well established 

effects on humans at the relatively low levels 

of methylmercury exposure typical of fishconsuming populations are neurological impacts 

on children exposed in the womb, particularly 

during the third trimester (Mahaffey and others, 

2011). Data on children followed from before 

birth into childhood provide the scientific basis 

for the current reference dose for methylmercury 

used by the USEPA and established by a National 

Academy of Sciences panel of experts in 2000 

(National Research Council, 2000). The reference 

dose is the level of daily intake (of a chemical) 

that is not associated with an appreciable increase 

in risk of adverse health effects during a lifetime. 

For methylmercury, the USEPA reference dose 

is equivalent to 0.1 microgram per kilogram 

of body weight per day. This intake rate is 

equivalent to approximately 5.8 micrograms 

of mercury per liter (ppb) in blood. Data from 

the National Health and Nutrition Examination 

Survey suggest that from 4 to 8 percent of 

U.S. women of childbearing age exceed the 

USEPA reference dose for methylmercury. This 

corresponds to hundreds of thousands of children 

born in the United States each year with blood 

mercury concentrations exceeding the USEPA 

reference dose (Mahaffey and others, 2009).

Blood mercury levels in the U.S. population vary with fish 

consumption patterns and, in general, are higher in coastal areas with 

greater fish consumption rates than in inland communities (fig. 2-2). Most 

seafood consumed in the United States is sold in the commercial market. 

Methylmercury exposure is a function of how much seafood people eat and 

Figure 2-2. Average (geometric mean) blood mercury concentrations 

in humans are higher in coastal areas than in many inland areas, 

likely because fish consumption is higher in coastal areas. (Data from 

2004 National Health and Nutrition Examination Survey; modified from 

Mahaffey and others, 2009.)

Chapter 2

Why Is Mercury in Fish a Concern? 13
0 1 2 3 4

Tuna

Shrimp

Pollock

Salmon

Cod

Whiting

Flatfish

Clams

Crab

Scallops

Squid

Herring

Oysters

Sardine

Lobster

Mackerel

Halibut

Haddock

Swordfish

0 10 20 30 40

Tuna (canned, frozen, and fresh)

Swordfish

Pollock

Shrimp

Cod

Crab

Salmon (canned, frozen, and fresh)

Anchovies, herring, and shad

Orange Roughy

Halibut

Flounder, Plaice, and Sole

Haddock, hake, and monkfish

Grouper, rockfish, and sea bass

Snapper, porgy, and sheepshead

Mackerel

A. B.

Seafood consumption, in grams per day Population-wide methylmercury intake, 

in percent

concentrations of methylmercury in the seafood 

they choose. Tuna (both fresh and canned) 

are the largest contributor to methylmercury 

exposure in the United States because they are 

eaten in large quantities (fig. 2-3) and contain 

moderately high mercury concentrations (see 

“Balancing the Benefits and Risks of Fish 

Consumption,” p. 16). Species, such as pollock, 

are low in mercury but contribute substantially 

to population-wide exposure because people 

consume large quantities of these fish. 

Freshwater environments are important sources 

of methylmercury exposure for some of the most 

highly exposed human populations, such as 

recreational fishers and Native Americans, that 

harvest large quantities of fish from local lakes 

and streams (Mergler and others, 2007). 

Figure 2-3. Per capita seafood consumption (A) and methylmercury intake from commercial estuarine 

and marine fish and shellfish (B) in the United States indicate that several species with some of the highest 

mercury concentrations also are species that are most likely to be consumed. (Modified from Sunderland, 

2007; see appendix 1 for additional information.)

14 Mercury in the Nation’s Streams—Levels, Trends, and Implications
High levels of exposure to inorganic 

mercury (primarily through inhalation of 

elemental mercury) also can cause negative 

health effects, such as kidney failure and central 

nervous system toxicity. However, inorganic 

mercury concentrations in the atmosphere are 

much lower than levels known to elicit such 

effects, even adjacent to point sources, such 

as coal-fired power plants. Dental amalgams

contain elemental mercury and may result in lowlevel inorganic mercury exposures, but are not 

thought to be associated with substantial health 

risks for the general population (Clarkson and 

others, 2003; Bellinger and others, 2006). Unlike 

methylmercury, inorganic mercury consumed 

orally is not well absorbed by the human body 

and does not cross the blood-brain or 

placental barriers.

One of the challenges associated with 

understanding the impacts of methylmercury 

exposure on the developing brain is the 

co-occurrence of beneficial nutrients, such as 

polyunsaturated omega-3 fatty acids, with 

mercury in fish. Increasing dietary intake 

of omega-3 fatty acids during pregnancy 

is associated with improved childhood 

performance on neurocognitive tests (Choi and 

Grandjean, 2008; Strain and others, 2008). In 

addition, a variety of studies show benefits to 

cardiovascular health from omega-3 fatty acid 

intake (Mozaffarian and Rimm, 2006). These 

effects make it difficult to detect the full extent 

of methylmercury impacts on the developing 

brain and cardiovascular health because the main 

source of dietary exposure (seafood) contains 

both methylmercury and omega-3 fatty acids. 

For example, recent studies of neurocognitive 

performance in children in the Seychelles, 

an island republic in the Indian Ocean, show 

significant adverse effects from methylmercury 

exposure are only apparent after adjusting for the 

positive impacts of omega-3 fatty acids (Strain 

and others, 2008; Lynch and others, 2011).

A variety of studies also have suggested that 

selenium may be helpful in mitigating the effects 

of mercury toxicity (Choi and Grandjean, 2008). 

However, there have been few studies designed 

to assess the role of selenium in preventing 

methylmercury induced developmental 

neurotoxicity in humans. For example, selenium 

did not confer protection against methylmercury 

associated neurocognitive deficits in Faroe Island 

residents (Choi and others, 2008). Environmental 

interactions between mercury and selenium 

remain an area of active research.

Chapter 2

Why Is Mercury in Fish a Concern? 15
Balancing the Benefits and Risks of 

Fish Consumption

Fish are an important food resource despite 

the risk of methylmercury exposure. Rather than 

replacing fish in the diet with other foods, it is 

best to choose fish that are high in beneficial 

nutrients and low in methylmercury (Mahaffey 

and others, 2011). For example, average 

mercury levels for some commercial fish and 

seafood are given in figure 2-4.

Public health advice on safe fish 

consumption choices is provided by Federal, 

State, and local agencies. Guidance on safe fish 

consumption choices—such as eating smaller 

and younger fish, and harvesting fish from 

unpolluted ecosystems—can be found at: http://

fishadvisoryonline.epa.gov/Contacts.aspx. The 

USEPA and the Food and Drug Administration 

have issued the following joint advisory for 

women who may become pregnant, women 

who are pregnant, nursing mothers, and children 

(U.S. Environmental Protection Agency, 2004).

 “Fish and shellfish are an important 

part of a healthy diet. Fish and 

shellfish contain high quality protein 

and other essential nutrients, are low 

in saturated fat, and contain omega-3 

fatty acids. A well-balanced diet 

that includes a variety of fish and 

shellfish can contribute to heart health 

and children’s proper growth and 

development. So, women and young 

children in particular should include 

fish or shellfish in their diets due to 

the many nutritional benefits.

However, nearly all fish and shellfish 

contain traces of mercury. For most 

people, the risk from mercury by 

eating fish and shellfish is not a health 

concern. Yet, some fish and shellfish 

contain higher levels of mercury that 

may harm an unborn baby or young 

child’s developing nervous system. 

The risks from mercury in fish and 

shellfish depend on the amount of 

fish and shellfish eaten and the levels 

of mercury in the fish and shellfish. 

0.001 0.010 0.100 1.000

Clams

Shrimp

Salmon: fresh and frozen USEPA fish tissue

Salmon: canned mercury criterion

Mussels and oysters

Ocean Perch

Haddock, hake, and monkfish

Anchovies, herring, and shad

Flounder, Plaice, and Sole

Scallops

Pollock

Crab

Squid

Whitefish

Croaker

Cod

Tuna: canned lightmeat

Skate

Mackerel

Lobster

Snapper, porgy, and sheepshead

Sablefish

Sea trout

Halibut

Lingcod and Scorpionfish

Grouper, rockfish, and sea bass

Bluefish

Tuna: canned albacore/white

Tuna: fresh and frozen

Marlin

Orange Roughy

Swordfish

Shark

Tilefish

Average methylmercury concentration, 

in parts per million

≤ 0.1 

 > 0.1 – 0.3

> 0.3 – 0.5 

> 0.5 

EXPLANATION

Average methylmercury concentration, 

 in parts per million

Figure 2-4. Average mercury 

concentrations in most 

commercial fish and seafood 

fall within levels considered 

safe for people who consume 

average amounts of fish. 

(Data from the Food and Drug 

Administration, summarized 

by Sunderland, 2007; see 

appendix 1 for additional 

information.)

16 Mercury in the Nation’s Streams—Levels, Trends, and Implications
Therefore, the Food and Drug Administration 

(FDA) and the Environmental Protection Agency 

(EPA) are advising women who may become 

pregnant, pregnant women, nursing mothers, and 

young children to avoid some types of fish and eat 

fish and shellfish that are lower in mercury.

 By following these three recommendations [below] 

for selecting and eating fish or shellfish, women and 

young children will receive the benefits of eating 

fish and shellfish and be confident that they have 

reduced their exposure to the harmful effects 

of mercury.

1. Do not eat Shark, Swordfish, King Mackerel, 

or Tilefish because they contain high levels of 

mercury.

2. Eat up to 12 ounces (2 average meals) a week of 

a variety of fish and shellfish that are lower 

in mercury. 

Fish is the primary source of methylmercury exposure for 

osprey and other wildlife that consume large amounts of 

fish. (Photogrph by Mark E. Brigham.)

a. Five of the most commonly eaten fish that 

are low in mercury are shrimp, canned light 

tuna, salmon, pollock, and catfish.

b. Another commonly eaten fish, albacore 

(“white”) tuna has more mercury than 

canned light tuna. So, when choosing your 

two meals of fish and shellfish, you may 

eat up to 6 ounces (one average meal) of 

albacore tuna per week.

3. Check local advisories about the safety of fish 

caught by family and friends in your local lakes, 

rivers, and coastal areas. If no advice is available, 

eat up to 6 ounces (one average meal) per week 

of fish you catch from local waters, but don’t 

consume any other fish during that week.

Follow these same recommendations when 

feeding fish and shellfish to your young child, 

but serve smaller portions.”

Methylmercury Poses Toxicological Risks to Wildlife

Methylmercury poses a significant threat to wildlife—including 

fish, amphibians, reptiles, birds, and mammals—because of its high 

bioavailability, its substantial bioaccumulation in food webs, and its 

extreme toxicity (Wiener and others, 2003; Scheuhammer and others, 

2012). Aquatic food webs are important pathways for exposure of wildlife, 

particularly in mercury methylating landscapes, such as wetlands, or 

at polluted sites. Processes and factors that control the production of 

methylmercury in ecosystems or its concentration at the base of food 

webs strongly influence its concentration in fish and wildlife supported by 

those food webs. In some ecosystems, exposure levels are high enough to 

adversely affect the reproduction and health of wildlife (Scheuhammer and 

others, 2007; Evers and others, 2011; Sandheinrich and Wiener, 2011).

Methylmercury can diminish the health and fitness of wildlife 

through various harmful effects. The adverse effects of methylmercury 

on the central nervous system and on neurological function in wild birds 

and mammals have been well documented (Scheuhammer and others, 

2012). Exposure to methylmercury impairs the reproduction of wildlife 

at environmentally relevant concentrations in field and laboratory studies 

(Scheuhammer and others, 2007; Depew and others, 2012a, 2012b), partly 

because of its action as an endocrine disruptor (Crump and Trudeau, 2009; 

Chapter 2

Why Is Mercury in Fish a Concern? 17
Tan and others, 2009). Methylmercury also can affect 

the immune system, making wildlife more susceptible to 

disease. Moderately high exposure to methylmercury can 

damage cells and tissues. In freshwater fish, for example, 

liver tissue undergoes changes in color and biochemical 

properties in direct relation to methylmercury exposure 

(Sandheinrich and Wiener, 2011).

It is becoming increasingly evident that the scope and 

severity of the mercury problem for wildlife have been 

substantially underestimated. Recent findings show that 

methylmercury impairs the health and reproduction of fish 

and birds at much lower dietary or tissue concentrations than 

previously recognized (Evers and others, 2011; Sandheinrich 

and Wiener, 2011; Depew and others, 2012a). For example, 

concentrations of methylmercury in piscivorous fish from 

many North American fresh waters exceed estimated 

threshold levels (0.5 ppm in axial muscle tissue or 0.3 ppm 

in whole fish) that are associated with altered biochemical 

processes, damage to cells and tissues, and diminished 

reproduction (Dillon and others, 2010; Sandheinrich and 

Wiener, 2011). In birds, methylmercury in the diet of 

reproducing females is transferred to the developing egg—

the most sensitive life stage (Scheuhammer and others, 

2007; Heinz and others, 2009a). Reduced reproductive 

success has been associated with methylmercury exposure in 

field studies of several aquatic and marsh birds.

Criteria established to protect human health from 

the adverse effects of methylmercury exposure are not 

protective of fish-eating wildlife. The USEPA fish tissue 

criterion for methylmercury, established to protect the health 

of humans who eat noncommercial fish, is 0.3 ppm (Borum 

and others, 2001). For comparison, the production of young 

common loons (Gavia immer), a fish-eating aquatic bird, is 

substantially reduced if mercury concentrations in the prey fish 

eaten by adult loons on the breeding grounds exceed 0.18 ppm 

(Depew and others, 2012b).

Species of fish and wildlife can differ greatly in their 

sensitivity to methylmercury exposure, although comparative 

data on toxicity are limited largely to birds. In a landmark 

study, Heinz and others (2009b) examined dose-response 

curves for embryo survival in several species of birds exposed 

to methylmercury in egg injection experiments. The median

lethal dose for bird eggs varied 20-fold among the species 

tested, ranging from 0.12 ppm in the American kestrel (Falco 

sparverius) to 2.4 ppm in the double-crested cormorant 

(Phalacrocorax auritus). They classified 5 of the species tested 

as having high sensitivity to injected methylmercury (lethal 

dose less than 0.25 ppm), 13 as having medium sensitivity 

(lethal dose in the range of 0.25 to 0.99 ppm), and 6 as having 

low sensitivity (lethal dose greater than 1.0 ppm) (Heinz and 

others, 2009b). Avian species also differ in their ability to 

demethylate methylmercury, a factor that may contribute to 

variation among bird species in susceptibility to methylmercury 

toxicity (Scheuhammer and others, 2008; Eagles-Smith and 

others, 2009).

Aquatic food webs containing fish have for decades 

been considered the principal pathway for potentially 

harmful exposure of wildlife to methylmercury, and most 

ecotoxicological studies have therefore focused on piscivorous 

species. Unexpectedly high concentrations of methylmercury 

have been recently documented in songbirds and bats that feed 

on invertebrates (Evers and others, 2012). Most songbirds 

with elevated concentrations have been linked to food webs 

in mercury methylating environments—such as wetlands, 

streams, or lakes—and feed on spiders or emergent insects with 

aquatic larval stages (Edmonds and others, 2012). The recent 

discovery that some invertivores also are at risk to harmful 

methylmercury exposure foretells a substantial expansion in 

the perceived scope of the mercury problem for wildlife.

Acknowledgments

James G. Weiner was supported by the University of 

Wisconsin System Distinguished Professors Program and the 

University of Wisconsin-La Crosse Foundation during the 

preparation of this chapter.

Reproductive success of fish-eating birds, such as the common loon, 

can be significantly impaired by high methylmercury concentrations in 

fish. (Photograph by David Braithwaite, used with permission.)

18 Mercury in the Nation’s Streams—Levels, Trends, and Implications
This page left intentionally blank

Chapter 2

Why Is Mercury in Fish a Concern? 19
Fishing Brook, New York

Photograph by Dennis A. Wentz.
How Does Mercury 

Cycle Through Aquatic 

Ecosystems?

This chapter addresses the questions: Where does mercury in aquatic ecosystems come from? 

How does mercury move through stream ecosystems? Why are mercury concentrations in fish 

high when concentrations in stream water are typically low? Why do some fish species have 

higher mercury concentrations than others, and what factors control mercury bioaccumulation 

in fish? Major sources of inorganic mercury to aquatic ecosystems, transformation of 

inorganic mercury to methylmercury, and bioaccumulation of methylmercury in aquatic food 

webs are discussed.

In the past, industrial wastewater discharges were important sources of inorganic and organic mercury; however, today 

most mercury emissions in the United States are from coal combustion. This mostly inorganic mercury is carried tens to 

thousands of kilometers by the atmosphere, and a portion is eventually deposited back to Earth, either in precipitation or 

dry deposition. Because mercury can be transported long distances, emissions from other countries are an important source of 

mercury deposited in the United States. In aquatic ecosystems, inorganic mercury is converted to methylmercury (an organic 

form) by natural bacterial processes that are particularly active in wetlands. Some methylmercury enters aquatic food webs, 

where it is bioaccumulated, resulting in elevated concentrations in fish and other organisms at or near the top of the food web.

Mercury from power plants and other sources enters aquatic ecosystems and is bioaccumulated through food 

webs. (Graphic courtesy of Washington State Department of Health.)

Chapter 3

How Does Mercury Cycle Through Aquatic Ecosystems? 21

Chapter

3
Hg2+ MeHg

Oxidation

Atmospheric

depositon

Wet Dry

Re-emission

Methylation

Cropland and other settings with 

erodible soils contribute sedimentbound mercury to surface waters.

Emissions from coal-fired 

power plants are the largest 

anthropogenic source of 

mercury to the atmosphere

in the United States.

Gaseous elemental mercury in the 

atmosphere originates from natural 

and anthropogenic sources and 

can be transported long distances 

before it is deposited onto the 

earth’s surface. 

Some gaseous elemental 

mercury is oxidized to reactive 

mercury, which is more readily 

deposited. 

Gaseous elemental mercury, reactive gaseous 

mercury, and particulate mercury are mostly 

deposited by precipitation and by dry deposition. 

Mercury deposition rates are typically 

greater near large urban areas than 

in more remote areas. 

Mercury mines, gold mines, and 

some industries have historically 

contributed high concentrations of 

mercury to aquatic ecosystems. 

Some of this “legacy mercury” 

remains a problem today.

Natural bacteria convert reactive inorganic 

mercury to methylmercury—the form accumulated 

in fish, wildlife, and humans. Wetlands are 

especially important zones of mercury methylation.

Some gaseous elemental mercury 

emissions are deposited locally, 

but most are transported long 

distances through the atmosphere.

Most reactive gaseous and 

particulate forms of mercury tend 

to be deposited regionally.

Organic carbon from wetlands 

binds with mercury and facilitates 

its transport in natural waters. 

The Mercury Cycle Describes How Mercury Moves Through the Environment

The mercury cycle (fig. 3-1) describes where mercury comes from, how it enters an aquatic ecosystem, how it is transported 

through the system, what processes transform it along the way, where it accumulates, and how it leaves the system. In the United 

States, the predominant source of mercury to most aquatic ecosystems is emission of inorganic mercury to the atmosphere from 

burning coal for energy production. This mercury takes one of three forms: gaseous elemental mercury, reactive gaseous 

mercury, or particulate mercury. The atmospheric mercury enters aquatic ecosystems through wet and dry deposition. In 

Figure 3-1. The mercury cycle illustrates where mercury originates, how it enters an aquatic ecosystem, how it 

is transported and transformed within the ecosystem, and how it leaves the ecosystem. (Drawing © Frank Ippolito/

Production Post Studios/www.ProductionPost.com.)

22 Mercury in the Nation’s Streams—Levels, Trends, and Implications
addition, many ecosystems in the Western United States have 

been contaminated by past mercury and gold mining, which 

can provide large sources of inorganic mercury to streams, 

lakes, and wetlands. Some inorganic mercury is converted 

to methylmercury by bacteria in organic-rich areas, such as 

Methylmercury Concentrations in Fish Are 

Determined by Three Primary Factors

Three primary factors—represented by the 

axes of a 3-dimensional graph (fig. 3-2)—explain 

methylmercury concentrations in fish. The three factors 

are (1) mercury source (the amount of inorganic 

mercury available to an aquatic ecosystem), (2) 

mercury methylation (the rate at which inorganic 

mercury is converted to organic methylmercury within 

the ecosystem), and (3) mercury bioaccumulation 

(the increase of methylmercury concentration in biota 

at successively higher trophic levels of an aquatic 

food web). Ecosystems with low values for all three 

factors will contain fish with low methylmercury 

concentrations. Ecosystems with high values for 

all three factors will contain fish that are highly 

Source

Bioaccumulation

Methylation

Methylmercury

Methylmercury

Figure 3-2. Three basic factors—source, methylation, and 

bioaccumulation—explain methylmercury concentrations in fish. 

When all three factors are low, methylmercury levels in fish are low 

(represented by the smaller fish). When all three factors are high, 

methylmercury levels in fish are high (represented by the larger fish). 

When the three factors vary in magnitude, fish methylmercury levels vary 

depending on factor values and how the factors interact. (Modified from 

Mumley and Abu-Saba, 2002.)

wetlands, before entering streams and lakes in association 

with organic matter. Inorganic and methylmercury are mostly 

retained within the soil; however, some mercury leaves the 

system via streamflow, and some is re-emitted from the soil 

back to the atmosphere.

contaminated with methylmercury. Ecosystems with high 

values for one factor, and low to intermediate values for 

the other two factors, may or may not contain fish with 

high methylmercury concentrations, depending on the 

strength of each factor and how the three factors interact. 

For example, ecosystems that are contaminated with 

mercury (such as from industry or mining) can contain 

fish with high methylmercury concentrations, even 

when the potential for methylation and bioaccumulation 

are low, whereas ecosystems in remote areas with low 

mercury sources can contain fish with high methylmercury 

concentrations if the potential for mercury methylation 

and/or bioaccumulation is high.

Chapter 3

How Does Mercury Cycle Through Aquatic Ecosystems? 23
0 

100 

200 

300 

400 

500 

600 

700 

800 

900 

China India United

States

Russia Indonesia South

Africa

Brasil Australia South

Korea

Columbia 

Mercury emissions, in metric tons 

Combustion of coal and 

other fossil fuels

Metal mining and production

Gold mining and production

Cement production

Chlor-alkali production

Waste incineration

Other 

EXPLANATION

Mercury Sources to Aquatic 

Ecosystems Are Both Anthropogenic 

and Natural

Worldwide, mercury inputs to aquatic 

ecosystems are primarily from emissions—

anthropogenic and natural—transported by the 

atmosphere. Total global emissions of mercury 

to the atmosphere are estimated at 1,960 metric 

tons for 2010 (United Nations Environment 

Programme, 2013a). Of these, 30 percent were 

new emissions from human activities. Another 

60 percent were re-emissions of previously 

deposited mercury, most of which also were 

anthropogenic in origin. Only 10 percent of 2010 

mercury emissions were from natural sources—

including volcanoes, geothermal vents, wildfires, 

and the oceans.

Artisanal and small-scale gold mining 

contributed 37 percent of new anthropogenic 

worldwide mercury emissions and was the single 

largest component. Most of these activities were 

in East and Southeast Asia, Sub-Saharan Africa, 

and South America. Burning coal accounted for 

24 percent of total global anthropogenic mercury 

emissions and was the second largest component. 

The vast majority of mercury emissions from 

human activities were attributed to Asia (about 

931 metric tons); North America contributed an 

estimated 61 metric tons.

In the United States, the predominant 

source of anthropogenic mercury emissions 

is from burning coal. During 2005, the last 

year for which detailed source data are readily 

available, U.S. anthropogenic mercury emissions 

were the third largest worldwide, and coal 

combustion accounted for about 55 percent of the 

total (fig. 3-3) (Arctic Monitoring Assessment 

Programme / United Nations Environment 

Programme, 2008). Anthropogenic mercury 

emissions in the United States during 2005 also 

resulted from metallurgical processes, waste 

Figure 3-3. The United States contributed the third largest amount of mercury to worldwide emissions 

from human activities during 2005. Burning coal and other fossil fuels was the single largest component of 

anthropogenic mercury emissions in the United States, accounting for slightly more than one-half of the 

total. (Data from Arctic Monitoring Assessment Programme / United Nations Environment Programme, 2008; 

see appendix 1 for additional information.) 

24 Mercury in the Nation’s Streams—Levels, Trends, and Implications
Amount of total mercury emitted, 

Type of mercury source in kilograms per year

EXPLANATION

Coal combustion

Other fuel combustion

Incineration

Metallurgical and mining

Manufacturing and other

5 – 10

10 – 100

100 – 1,000

1,000 – 5,500

incineration, and numerous manufacturing and other sources, with most mercury emission sources 

located in the eastern half of the Nation (fig. 3-4). Most industrial wastewater discharges of mercury 

have been eliminated in the United States, thus mercury inputs to water are largely restricted to 

specific areas, such as historical mercury and gold mining areas of the West.

Figure 3-4. Most large point sources of mercury emissions to the atmosphere in the United States are coal-fired 

power plants located in the eastern half of the Nation. Canada has relatively few sources, and Mexico’s sources 

are mostly metallurgical processing, waste incineration, and other manufacturing. (Compiled by Mark Cohen, 

National Oceanic and Atmospheric Administration; see appendix 1 for additional information.)

Chapter 3

How Does Mercury Cycle Through Aquatic Ecosystems? 25
A recent global scale mercury deposition 

model suggests that about 70 percent of current 

mercury deposition to the United States is 

anthropogenic, of which 20 percent is from 

North American emissions and the remainder 

is from the rest of the world, including 

re-emission of older anthropogenic mercury that 

has accumulated in soil and the oceans since 

pre-industrial times (Selin and others, 2008). 

North American emissions represent a major 

portion of mercury deposition in the Eastern 

United States—up to about 60–80 percent in the 

industrialized Midwest—but a relatively minor 

portion in Western States (Seigneur and others, 

2004; Selin and others, 2007).

Mercury Reaching the Nation’s 

Aquatic Ecosystems Is Transported 

Primarily by the Atmosphere 

Atmospheric transport and deposition 

constitute the predominant pathway of 

anthropogenic mercury to most aquatic 

ecosystems in the United States, especially those 

in remote areas (Fitzgerald and others, 1998). 

Mercury is emitted either as gaseous elemental 

mercury, reactive (oxidized) gaseous mercury, or 

mercury adsorbed to solid particles and aerosols. 

Combustion of coal, which contains mercury as 

an impurity associated with pyrite (Tewalt and 

others, 2001), releases mercury to the atmosphere 

in all three forms.

Gaseous elemental mercury comprises 

more than 95 percent of global atmospheric 

mercury (Grigal, 2002) and becomes rapidly and 

efficiently mixed in the atmosphere. Average 

atmospheric transport distances for gaseous 

elemental mercury are tens of thousands of 

kilometers (Schroeder and Munthe, 1998; Lin 

and Pehkonen, 1999; Selin, 2009), thus giving 

rise to the designation of mercury as a global 

pollutant. Some gaseous elemental mercury is 

converted in the atmosphere to more reactive 

forms that fall out quickly and/or are absorbed 

by vegetation.

Less than about 5 percent of global 

atmospheric mercury is in the reactive 

form, either as reactive gaseous mercury or 

particulate mercury. These reactive mercury

forms have transport distances of only tens to 

hundreds of kilometers, and they are typically 

deposited closer to their sources than gaseous 

elemental mercury (Schroeder and Munthe, 

1998; Seigneur and others, 2006; Selin and 

others, 2007; Selin, 2009). The shorter transport 

distances for reactive mercury forms than for 

gaseous elemental mercury may lead to higher 

total mercury deposition rates near urban areas 

(where atmospheric sources are common) than in 

remote areas (Van Metre, 2012). 

Atmospheric Mercury Deposition Rates Are Higher Near Major Urban Centers Than in Remote Areas

By Peter C. Van Metre, U.S. Geological Survey

Analyses of sediment cores collected during 1999–2009 from 12 lakes distributed across the United States indicate 

that atmospheric mercury deposition rates are higher near major urban centers than in remote areas (Van Metre, 2012). 

The lakes are 10–310 kilometers downwind from the center of the nearest large city (fig. 3-5). On average, modern 

(post‐1990) atmospheric mercury deposition rates to lakes within 50 kilometers of urban areas were about 5 times higher 

than deposition rates to lakes that were farther than 150 kilometers from urban areas. In addition, the modern deposition 

rates were about 10 times historical background rates in lakes near urban areas, but only 3–4 times background rates for 

remote lakes.

26 Mercury in the Nation’s Streams—Levels, Trends, and Implications
Portland

Tampa

Orlando

Hartford

New Haven

Boston

Albany

Montréal

Chicago

Minneapolis

Denver

Salt Lake City

Sampling site

City

EXPLANATION

Mercury deposition rate, in micrograms per square meter per year

Distance to major city, in kilometers

80

60

0 100 200 300

HgWET

HgTOTAL

0

20

40

100

Figure 3-5. Sediment cores, used to 

assess historical rates of mercury 

deposition, were collected from 12 lakes that 

are 10–310 kilometers downwind from major U.S. 

cities. (Modified from Van Metre, 2012.)

Figure 3-6. Post-1990 total mercury deposition rates 

increased rapidly as the distance to a major urban 

area decreased; however, increases in wet mercury 

deposition rates were much smaller. (Modified from 

Van Metre, 2012.)

Modern atmospheric mercury deposition rates 

measured in the lakes correlate strongly with distance 

from the nearest city, and to total population and 

estimated mercury emissions near the lake. The 

single strongest correlation with modern atmospheric 

mercury deposition rates measured in the lakes is the 

distance to the nearest major city (HgTOTAL; fig. 3-6), 

which explained 87 percent of the variation in mercury 

deposition rate. Modern mercury deposition rates 

also correlate strongly with total population within 

a 100-kilometer radius of each lake (72 percent of 

variation explained) and to emissions of reactive 

gaseous plus particulate mercury within about 

150 kilometers of each lake (77 percent of variation 

explained). These results indicate that some urban 

mercury emissions are deposited locally, with an 

apparent distance of influence of the urban airshed on 

the order of 100 kilometers (fig. 3-6). Urban air quality 

might play a role in near-urban mercury deposition by 

altering local atmospheric mercury to reactive forms that 

are more likely to be deposited. 

The increase in total atmospheric deposition rate 

of mercury measured using lake sediment cores is 

considerably larger than the increase in wet deposition

rate (HgWET; fig. 3-6), as measured at nearby Mercury 

Deposition Network sites, thus suggesting that dry 

deposition is more important near urban areas. A number 

of sources and atmospheric processes in or near urban 

areas might contribute to wet and dry mercury deposition, 

including point and diffuse sources of mercury, emissions 

from contaminated soil, increased concentrations of 

oxidants (such as ozone) in urban air, and elevated 

concentrations of atmospheric particulate mercury. 

Reactive gaseous mercury and particulate mercury can 

be especially high in urban areas and tend to be deposited 

more rapidly than gaseous elemental mercury. 

Chapter 3

How Does Mercury Cycle Through Aquatic Ecosystems? 27
Re-emission Throughfall

Wet deposition Dry deposition

Litterfall

Figure 3-7. Atmospheric mercury is deposited in wet deposition (rain or snow) or as dry deposition 

(gaseous or particulate forms). Some mercury reaches the Earth surface as throughfall when precipitation 

trickles through vegetation washing mercury from surfaces, and some reaches the Earth surface as 

litterfall (leaves and other plant tissue). 

Mercury in the atmosphere can be deposited onto the 

Earth’s surface, either as wet deposition (rain or snow) or as 

dry deposition (gas phase or particulate deposition to surfaces 

during precipitation-free periods) (fig. 3-7). Wet deposition 

is dominated by reactive gaseous mercury and particulate 

mercury, which are subject to washout from the atmosphere. 

A small amount of mercury in precipitation—typically about 

1 percent—occurs as methylmercury (Grigal, 2002).

28 Mercury in the Nation’s Streams—Levels, Trends, and Implications
0 200400600 800 1,000 KILOMETERS

0 200 400 600 800 1,000 MILES

0 ≥ 18

Mercury wet deposition, 2005–2011, in micrograms per square meter

EXPLANATION

0.001 

0.01 

0.1 

1 

10 

0.1 1 10 100 1,000 

Orlando, Florida 

Wet mercury deposition, 

in micrograms per square meter

Precipitation, in millimeters

As one might expect from the distribution of mercury 

emission sources (fig. 3-4), average annual wet mercury 

deposition rates generally were higher in the Eastern than 

in the Western United States for the 7-year period from 

2005 to 2011 (fig. 3-8). This is due to the greater number 

of emissions sources (fig. 3-4) and the generally larger 

precipitation amounts in the Eastern United States. In 

addition, wet mercury deposition rates show a nearly 5-fold 

increase from the Northeastern United States to the Gulf 

of Mexico. This increase is driven largely by a similar 

increase in annual precipitation from the Northeast to the 

Gulf. The strong relationship between wet deposition of total 

mercury and precipitation amount has been demonstrated 

for numerous sites in and adjacent to Florida (Fulkerson 

and Nnadi, 2006) and is illustrated by data collected near 

Orlando, Florida (fig. 3-9). 

Although dry deposition of mercury is difficult to 

quantify, it is generally agreed to be of equal or greater 

importance than wet deposition, especially in forested 

basins, where leaves take up a substantial amount of 

mercury (Schroeder and Munthe, 1998; Grigal, 2002). This 

dry deposited mercury can be washed from vegetation by 

precipitation falling through the forest canopy (throughfall) 

or can be carried to the forest floor when leaves and other 

vegetation dies and falls to the ground (litterfall). Dry mercury 

deposition was estimated to be 2–3 times greater than wet 

mercury deposition in forested watersheds of northwestern 

Ontario (St. Louis and others, 2001; Evers, 2005), and dry 

deposition contributed more than one-half (55 percent) of total 

mercury deposition in the Northeastern United States (Risch 

and others, 2011). 

Figure 3-8. Average annual wet mercury deposition rates 

during 2005–2011 generally were higher in the Eastern than in the 

Western United States, and rates increased from north to south 

in the Eastern United States. (Map provided by Michael Tate, 

U.S. Geological Survey, written commun., January 15, 2013; see 

appendix 1 for additional information.) 

Figure 3-9. Wet mercury deposition rates correlate strongly 

with the amount of precipitation. (Note logarithmic scales of both 

axes. Data from National Atmospheric Deposition Program; see 

appendix 1 for additional information.) 

Chapter 3

How Does Mercury Cycle Through Aquatic Ecosystems? 29
Mercury Deposition 

Network (MDN) site

Atmospheric Mercury 

Network (AMNet) site

MDN and AMNet site

Sites

EXPLANATION

Figure 3-10. Mercury Deposition Network (wet mercury deposition) 

and Atmospheric Mercury Network (dry and total mercury deposition) 

sites are located across North America, although most are in the Eastern 

United States. (Data from National Atmospheric Deposition Program, 

http://nadp.sws.uiuc.edu.)

Monitoring and Modeling Help to Assess 

Mercury Deposition in the United States

Atmospheric mercury deposition is the linkage 

between mercury emissions and the amount of mercury 

entering aquatic ecosystems. Given the importance of 

wet and dry mercury deposition, multiple approaches are 

used to quantify these processes across the United States. 

Continued monitoring and modeling of mercury transport 

and depositional processes is critical to better understand 

the efficacy of emission controls. 

National Monitoring Networks.—The Mercury 

Deposition Network (MDN) is a national scale 

network that monitors total mercury concentrations 

in precipitation and total mercury deposition rates 

(fig. 3-10). It is part of the National Atmospheric 

Deposition Program (NADP) and is a collaborative effort 

among States, Tribal and local governments, educational 

institutions, private companies, nongovernmental 

organizations, and Federal agencies (including the 

USGS). The MDN comprises more than 100 monitoring 

sites across the United States and Canada. Most sites 

30 Mercury in the Nation’s Streams—Levels, Trends, and Implications
(Continued on page 32)

The Mercury Deposition Network site at the H.J. Andrews 

Experimental Forest, Oregon, is one of more than 100 locations 

across the United States and Canada where precipitation has been 

collected for analysis of mercury. (Photograph by Dennis A. Wentz.)

The USGS mobile atmospheric mercury laboratory can be deployed 

across the United States to measure key forms of atmospheric 

mercury—information that will improve our understanding of 

mercury transport and deposition from different emission sources. 

(Photograph by Mercury Research Laboratory staff.)

are in rural areas and are somewhat removed from large 

mercury sources, although a few MDN sites are in or 

near urban areas. Standardized procedures are used at 

all MDN monitoring sites to measure total mercury at 

concentrations less than 1 part per trillion. The MDN 

Program produces annual maps for the contiguous United 

States and southern Canada showing average mercury 

concentrations in precipitation and average mercury wet 

deposition rates.

The Atmospheric Mercury Network (AMNet) was 

established in 2009 by the NADP to measure gaseous 

elemental mercury, reactive gaseous mercury, particulate 

mercury, and various meteorological variables needed 

for estimating dry mercury deposition. As of February 

2013, 17 sites were included in AMNet (fig. 3-10). Most 

AMNet sites are co-located with existing MDN sites.

USGS Mobile Atmospheric Mercury 

Laboratory.—The USGS Mercury Research Laboratory 

in Middleton, Wisconsin, has a mobile atmospheric 

mercury laboratory that can be driven to various locations 

to collect data on mercury chemical forms in response 

to specific local and/or regional objectives (Kolker 

and others, 2007; U.S. Geological Survey, 2010). The 

laboratory is equipped with instrumentation that measures 

gaseous elemental mercury, reactive gaseous mercury, 

particulate mercury, wet mercury deposition, atmospheric 

gases (ozone, nitrogen oxides, sulfur dioxide, carbon 

monoxide), and a full suite of standard meteorological 

variables. Dry mercury deposition of reactive mercury 

(reactive gaseous mercury plus particulate mercury) can 

be calculated from the various measurements. 

Data from 13 locations across the United States as of 

June 2012 show that atmospheric mercury concentrations 

near urban sites were 2–6 times those at rural and coastal 

sites, depending on the form of the mercury (Engle and 

others, 2010). These values are consistent with impacts 

by local and regional mercury sources at urban sites. 

Urban sites also received considerably greater dry 

deposition of reactive mercury than did rural and 

coastal sites. 

Chapter 3

How Does Mercury Cycle Through Aquatic Ecosystems? 31
(Continued from page 31)

0 200 400 600 800 1,000 KILOMETERS

0 200 400 600 800 1,000 MILES

Mercury mine

Gold mine

EXPLANATION

Computer Models.—Computer models can help to 

understand how mercury deposition varies geographically by 

providing estimates of mercury deposition in areas with little 

or no data. Because atmospheric deposition models incorporate 

emissions, they also can be used to assess potential changes in 

mercury deposition that may result from reductions in mercury 

emissions. For example, the USEPA has used the Community 

Multiscale Air Quality modeling system (CMAQ-Hg) to make 

national assessments of mercury deposition in its Regulatory 

Impact Analysis of the Clean Air Mercury Rule (Bullock and 

Brehme, 2002; U.S. Environmental Protection Agency, 2005) 

(fig. 3-11). The model indicates that, on average, dry mercury 

deposition is approximately two-thirds of total mercury 

deposition across the continental United States. Total mercury 

deposition is estimated to be greatest in the heavily industrialized 

corridor of Ohio, West Virginia, Pennsylvania; parts of the 

arid Southwest; and near major cities. Brief descriptions of the 

CMAQ-Hg and other atmospheric mercury models are provided 

in U.S. Environmental Protection Agency (2008). 

Historical Mercury and 

Gold Mining Are Sources of 

Mercury to Some Streams

In addition to atmospheric 

mercury deposition, past mining 

of mercury and previous use of 

mercury to enhance the extraction 

of gold and silver can be locally 

or regionally important sources. 

Mining of mercury as a principal 

commodity ceased in the United 

States in 1992, and cyanide leaching 

has replaced mercury amalgamation 

as the preferred method for gold 

recovery (Booz Allen & Hamilton, 

Inc., 2002). However, historical 

gold and mercury mining have left a 

legacy of contamination, primarily 

in the Western United States, that 

will continue to release mercury 

directly to aquatic ecosystems or 

indirectly through emissions to the 

atmosphere (fig. 3-12). 

Figure 3-11. Estimates from the Community 

Multiscale Air Quality modeling system (CMAQ-Hg) 

indicate that total mercury deposition (sum of 

modeled wet deposition and dry deposition) is 

greatest in the heavily industrialized Northeast, 

parts of the arid Southwest, and near major cities. 

(Modified from Lin and others, 2007.)

Total Mercury Deposition Rate, in micrograms per 

square meter per year

48

0

EXPLANATION

Figure 3-12. Most mercury and gold production mines are in the Western United 

States. Mines in this region have left a legacy of mercury contamination that will 

continue into the future. (Data from Mineral Availability System–Mineral Industry 

Location System [MAS-MILS] of the U.S. Bureau of Mines and from Mineral 

Resources Data System [MRDS] of the U.S. Geological Survey, 2004.) 

32 Mercury in the Nation’s Streams—Levels, Trends, and Implications
Mercury in ore deposits occurs primarily 

as cinnabar—a distinctive red mercury sulfide 

mineral that typically forms near recent volcanic 

activity or geothermal springs (Berry and Mason, 

1959; Bailey and others, 1973; Gray, 2003a; 

Rytuba, 2003). In the past, mercury ore in the 

United States was roasted at high temperatures 

after extraction to vaporize the mercury and 

separate it from the sulfur (Rytuba, 2003). The 

mercury vapor was subsequently cooled, and 

liquid metallic mercury was recovered. The 

roasting process at most mines was inefficient, 

and much mercury was lost to the atmosphere. 

Soluble mercury compounds also were left 

behind in the rock waste tailings discarded at 

mined sites. 

Elemental mercury (front center)—a liquid at room 

temperature—is surrounded by granular massive cinnabar 

(upper right) and crystalline cinnabar. (© Paul Silverman, 

Fundamental Photographs, New York, used with permission.)

Figure 3-13. Historically, U.S. mercury production 

was cyclic and accounted for only about 12 percent 

of world production. (Data from U.S. Geological 

Survey, 2007.)

California, Nevada, Texas, and Oregon 

(in that order) were the top four producers 

of mercury in the United States through 

1971 (Bailey and others, 1973). Together the 

four States accounted for 97 percent of U.S. 

production, whereas California alone accounted 

for 84 percent. From 1900 to 1992 (when the last 

active U.S. mercury mine closed), U.S. mercury 

production was cyclic (fig. 3-13); peak production 

was 1,790 metric tons during World War II (U.S. 

Geological Survey, 2007). Historically, the United 

States contributed only about 12 percent of total 

world mercury production.

tac11-0651fig03-13

0 

2,000 

4,000 

6,000 

8,000 

10,000 

12,000 

1900 1920 1940 1960 1980 2000 

Annual mercury production, in metric tons

Year 

U.S. production

EXPLANATION

World production 

Gold occurs predominantly in the native 

(elemental) form. It is mined either from 

lode deposits (consolidated rock containing gold 

veins) or from placer deposits (sand and gravel 

deposits containing gold particles eroded from 

exposed lode deposits). Historically, elemental 

mercury was used to enhance the recovery of 

gold (and, to a lesser extent, silver) from both 

lode and placer deposits (Hunerlach and Alpers, 

2003; Lawrence, 2003; Alpers and others, 2005). 

Lode deposits were finely ground; elemental 

mercury was added to amalgamate the gold; 

and the mixture was heated to vaporize the 

mercury, which was condensed and recovered 

for reuse. Placer deposits were flushed through 

sluice boxes to separate the heavier gold 

particles from the lighter sand and gravel. 

Elemental mercury was added to the bottom 

of the sluice boxes to facilitate the separation, 

and the gold-mercury amalgam was collected 

and heated to isolate the gold and recover the 

mercury. However, mercury recovery from both 

types of gold mining operations was notoriously 

inefficient. It has been estimated that nearly 

6,000 metric tons of mercury were lost to the 

environment in California alone (Alpers and 

others, 2005), mostly from placer mining. 

Chapter 3

How Does Mercury Cycle Through Aquatic Ecosystems? 33
Subsurface flow

Inflow/runoff Soil and sediment erosion is 

a key transport process for 

particulate (sediment-bound) 

inorganic and methylmercury 

in agricultural and other settings 

where erosion is prevalent.

Wetlands contribute inorganic and 

methylmercury, mostly bound to 

dissolved organic matter, to streams 

through surface runoff and

subsurface flow.

Methylation (conversion of inorganic 

mercury to methylmercury) is an important 

process in organic-rich, low-oxygen 

environments, such as wetlands.

Most mercury in watersheds

is stored in surface soil.

Chemical and Hydrologic Factors Control Mercury Transport and Methylation of Inorganic Mercury 

As mercury moves through a watershed, it undergoes transformations that influence how it is stored or transported. On an 

annual basis, a substantial portion of total mercury deposition leaves the watershed by re-emission to the atmosphere (Grigal, 

2002; Hartman and others, 2009). The remaining mercury is retained in the soil and vegetation, or it is carried to streams by 

surface runoff or by water percolating through the soil (fig. 3-14). Some inorganic mercury is converted to methylmercury, 

which is readily accumulated by living organisms. Because virtually all mercury in fish is methylmercury (Bloom, 1992), 

understanding where and how methylmercury is formed is critical to understanding fish mercury contamination.

Figure 3-14. After mercury is deposited on a watershed, it undergoes various transport and transformation processes. In 

some settings, such as cultivated agriculture, suspended sediment dominates mercury transport, whereas in others, mercury 

bound to dissolved organic carbon is the primary transport mechanism. Various microbial and chemical reactions change the 

form of mercury within aquatic ecosystems. (Drawing © Frank Ippolito/Production Post Studios/www.ProductionPost.com.)

Surface Soil Typically Is the Largest Reservoir of 

Mercury in Watersheds

Within a watershed, soil generally is the largest reservoir 

of atmospherically deposited mercury (Nater and Grigal, 

1992; Mason and others, 1994; Krabbenhoft and others, 2005). 

Mercury binds strongly to natural organic material, thus 

organic-rich surface soil accumulates mercury over time.

Several loss mechanisms can partially counter mercury 

accumulation in soil. Mercury may be lost by soil erosion 

or by export with dissolved organic matter (fig. 3-14). In 

addition, volatile forms of mercury can be re-emitted from the 

land surface to the atmosphere (Kuiken and others, 2008). 

Soil mercury also can be volatilized during forest fires 

(Sigler and others, 2003; Woodruff and others, 2009). For 

example, northeastern Minnesota forests that had not burned 

during approximately the past 100 years contain a median 

of about 1 milligram mercury per square meter of surface 

soil. Intensely burned areas, on the other hand, contain only 

about 0.1 milligram per square meter (Woodruff and Cannon, 

2010). Atmospheric mercury deposition in the region is tens to 

hundreds of times smaller than mercury in soil storage (Swain 

and others, 1992; St. Louis and others, 2001), suggesting that 

soil storage represents many decades of accumulation. 

34 Mercury in the Nation’s Streams—Levels, Trends, and Implications
tac11-0651_fig03-15

1

10

100

1,000

10,000

100,000

1,000,000

Annual total mercury load exported from 

1 10 100 1,000 10,000 100,000 1,000,000

stream basin, in grams per year

Annual atmospheric deposition of total mercury to 

stream basin, in grams per year

1:1 line

0

1

2

3

4

5

Particulate mercury

Dissolved mercury

Annual total mercury export, 

in micrograms per square meter per year 

EXPLANATION

Lookout Creek, Oreg.

Beaverton Creek, Oreg.

Pike River, Wis.

Evergreen River, Wis.

Oak Creek, Wis.

St Marys River, Fla.

Santa Fe River, Fla.

Little Wekiva, Fla.

McTier Creek, S.C.

Edisto River, S.C.

Fishing Brook, N.Y.

Hudson River, N.Y.

Sleepers River, Vt.

Allequash Creek, Wis.

Mercury in Streams Is Controlled by 

Dissolved Organic Carbon, Particulate 

Matter, and Streamflow

Streamflow generally transports considerably less 

mercury from the watershed than is deposited by wet 

and dry depositional processes. For example, the annual 

mercury load—amount or mass of mercury carried 

by stream water from a stream basin in a year—was 

less than one-half the amount of mercury entering the 

stream basin by atmospheric deposition for 16 streams 

sampled across North America (Shanley and others, 

2008; Brigham and others, 2009; Journey and others, 

2012) (fig. 3-15). Soil in undisturbed settings tends 

to accumulate and slowly release mercury over time, 

which suggests that it could take a long time for 

some ecosystems to respond to reductions in mercury 

emissions to the atmosphere.

Hydrologic transport of mercury from watersheds 

is controlled primarily by dissolved organic carbon and 

suspended particulate matter—two important carriers 

of mercury in aquatic ecosystems (Hurley and others, 

1995; Grigal, 2002; Yin and Balogh, 2002; Brigham and 

others, 2009)—in concert with rates of streamflow. In 

watersheds with minimal surface erosion, most mercury 

is bound to dissolved organic carbon and is exported 

in the dissolved fraction (fig. 3-16). In watersheds with 

soil disturbances, mercury is bound mostly to suspended 

particles and is exported in the particulate fraction. 

Watersheds that export little dissolved organic carbon 

or suspended particulate matter typically transport 

little mercury. In some ecosystems, percolation 

through the soil to groundwater also can result in 

a loss of considerable mercury from the system 

(Krabbenhoft and Babiarz, 1992). 

Figure 3-15. Annual total mercury loads exported in 

streamflow from watersheds across the United States 

typically account for less than one-half of the mercury 

deposited from the atmosphere. (Data from Shanley and 

others, 2008; Brigham and others, 2009; Journey and 

others 2012.)

Figure 3-16. Annual total mercury export by U.S. streams can be dominated by dissolved or 

particulate mercury transport. (Data from Shanley and others. 2008; Brigham and others, 2009; 

Journey and others, 2012.)

Chapter 3

How Does Mercury Cycle Through Aquatic Ecosystems? 35
Total mercury

EXPLANATION

Methylmercury 

0.01

0.1

1

10

1 10 100 1 10 100

Mean dissolved organic carbon, in parts per million

Mean total mercury and methylmercury, 

in parts per trillion

A. Streams B. Lakes

Across a wide range of environmental conditions and 

spatial scales, lakes and streams with high concentrations of 

dissolved organic carbon generally have high concentrations 

of mercury (Wiener and others, 2006; Brigham and others, 

2009; Scudder and others, 2009). For example, total mercury 

and methylmercury concentrations correlate strongly with 

dissolved organic carbon concentrations for selected streams 

across the United States (Oregon, Wisconsin, New York, 

South Carolina, and Florida) (fig. 3-17A) and for lakes in 

northern Minnesota (fig. 3-17B). These findings are consistent 

with our understanding that organic carbon binds strongly with 

total and methylmercury (Grigal, 2002; Hall and others, 2008; 

Riscassi and Scanlon, 2011).

In some watersheds, mercury is transported primarily by 

suspended particulate matter, which includes both suspended 

sediment (predominantly silt and clay) and particulate organic 

matter (mainly decaying plant material) (Balogh and others, 

2005; Shanley and others, 2008). Increases in soil erosion due 

to land disturbance, such as agricultural tillage, increase export 

of particle-bound mercury from the land surface to aquatic 

ecosystems (Balogh and others, 1998).

The USGS has observed strong correlations between 

total mercury concentrations in water and streamflow at eight 

streams across the United States (Brigham and others, 2009). 

These correlations occur because rainfall and/or snowmelt 

move terrestrial mercury from watersheds to streams during 

precipitation events. Similar results have been widely 

observed by others (for example, Balogh and others, 1998).

In contrast to total mercury, the relationship between 

methylmercury and streamflow is more variable. In some 

streams, methylmercury concentrations increase with 

increasing flow (Bishop and Lee, 1997; Balogh and others, 

2008; Shanley and others, 2008; Brigham and others, 2009; 

Babiarz and others, 2012). During low flow, these streams 

may be fed by groundwater with low methylmercury 

concentrations; however, higher flows can inundate riparian

or floodplain wetlands and flush methylmercury to the stream. 

Conversely, methylmercury concentrations decrease with 

increasing flow in a number of streams (Bishop and Lee, 1997; 

Brigham and others, 2009; Tsui and others, 2010; Schelker 

and others, 2011; Babiarz and others, 2012). For example, 

during high flows, streams originating in wetlands with high 

methylmercury concentrations can have methylmercury 

levels diluted by upland runoff with low methylmercury 

concentrations. Lastly, some streams show no significant 

relationship between methylmercury and streamflow (Brigham 

and others, 2009).

Figure 3-17. Mercury concentrations correlate strongly with dissolved organic carbon 

concentrations in (A) streams from across the United States (data from Brigham and others, 2009; 

Journey and others, 2012) and (B) lakes in northern Minnesota (data from Goldstein and others, 2003; 

Wiener and others, 2006).

36 Mercury in the Nation’s Streams—Levels, Trends, and Implications
Seeps from wetlands contribute methylmercury to streams in the 

Adirondack region of New York. (Photograph by Dennis A. Wentz.)

0 

0.1 

0.2 

0.3 

0.4 

Mean methylmercury concentration in water, in parts per trillion

0 10 20 30 40 

Wetland abundance, in percent of basin area

Wetlands Are Important Areas 

for Methylmercury Production

USGS studies have found that 

wetland abundance is one of the 

most important factors explaining 

methylmercury concentrations in 

stream water for watersheds spanning 

large ranges of mercury deposition 

rates, and ecological, landscape, 

and hydroclimatic conditions.

Methylmercury concentrations in 

streams correlate strongly with wetland 

abundance, and increase about 0.1 part 

per trillion methylmercury for each 

10-percent increase in upstream wetland 

area (Balogh and others, 2008; Brigham 

and others, 2009; Journey and others, 

2012) (fig. 3-18). 

Figure 3-18. Mean methylmercury concentrations in water correlate strongly 

with wetland abundance for streams in Oregon, Minnesota, Wisconsin, New York, 

South Carolina, and Florida. (Data from Balogh and others, 2008; Brigham and 

others, 2009; Journey and others, 2012. See appendix 1 for additional information.)

Wetlands provide ideal conditions for converting inorganic mercury to 

methylmercury; however, they also confer many ecological benefits. 

(Photograph by Perry M. Jones.)

Chapter 3

How Does Mercury Cycle Through Aquatic Ecosystems? 37
Wetlands are efficient at converting inorganic mercury deposited 

by the atmosphere into methylmercury because they provide favorable 

conditions for mercury methylation, including abundant organic matter 

and low dissolved oxygen levels (St. Louis and others, 1994; Grigal, 

2002). Mercury methylation occurs naturally under these conditions 

as sulfate-reducing bacteria obtain energy by breaking down organic 

matter, using sulfate much like we use oxygen to respire (Compeau and 

Bartha, 1985; Gilmour and others, 1992; St. Louis and others, 1994). 

Methylmercury is formed as an incidental byproduct of this process and 

is transported from the wetlands bound to dissolved organic carbon. 

Moderate sulfate concentrations in wetlands have been shown to 

exacerbate methylmercury formation (see “Anticipated Future Trends and 

Possible Confounding Factors,” p. 65).

Wetlands also confer many ecological benefits, including habitat for 

plant and animal species, and trapping and retaining nutrients and sediment 

from runoff. More than one-half of all wetlands originally present in the 

contiguous 48 States have been lost largely due to drainage for agriculture, 

urban development, and other human uses (Dahl, 1990, 2011). In some 

areas, the loss has been substantially greater. California, for example, has 

lost more than 90 percent of its original wetlands (Dahl, 1990). Efforts 

to restore previously drained or degraded wetlands, or to construct new 

wetlands for ecological benefit, should seek to optimize these benefits 

while mitigating factors that promote methylmercury production.

Bioaccumulation Causes 

Methylmercury Concentrations 

in Aquatic Biota to Increase with 

Trophic Level

Bioaccumulation within an aquatic food 

web can cause methylmercury concentrations 

in fish to increase to levels that are potentially 

harmful to both people and fish-eating 

wildlife. Methylmercury is incorporated and 

bioconcentrated at the base of the food web 

by algae and other microorganisms attached to 

submerged surfaces in smaller streams or by 

algae and other microorganisms suspended in the 

water of larger rivers or lakes. Higher in the food 

web, the accumulation of methylmercury from 

one trophic level to the next (biomagnification) 

occurs almost exclusively through dietary 

consumption. Thus, algae are eaten by aquatic 

invertebrates; invertebrates are eaten by small 

fish; small fish are eaten by larger fish; and 

larger fish are eaten by wildlife, such as wading 

birds and mammals, or by humans. At each 

step, the methylmercury in the prey organism 

is accumulated in the tissue of the consumer. 

Methylmercury in fish occurs predominantly 

in muscle tissue in association with sulfur 

(Kuwabara and others, 2007), and because 

methylmercury in this form is eliminated slowly, 

it biomagnifies, or increases in concentration as it 

moves up the food web. 

Bioaccumulation caused methylmercury 

concentrations to increase by nearly 1 million 

times from water to top predator fish in USGS 

studies conducted nationwide during 2002–2009 

(fig. 3-19). Most of the increase (10,000 times) 

occurred during bioconcentration (from 

water to algae), whereas the increase due to 

biomagnification (from invertebrates to top 

predator fish) was only about 100 times. These 

results are typical of other freshwater food webs 

(Bodaly and others, 1997; Cleckner and others, 

1998; Wiener and others, 2003; Kuwabara and 

others, 2005). 

Sediment from a rice field in the Yolo Bypass Wildlife Area, California, is analyzed to 

determine how much methylmercury is produced in this type of agricultural setting. 

(Photograph by Mark Marvin-DiPasquale.)

38 Mercury in the Nation’s Streams—Levels, Trends, and Implications
Biomagnification Bioconcentration

Methylmercury concentration, in parts per million

USEPA fish tissue mercury criterion = 0.3 parts per million

Water

Algae and other

microorganisms

Invertebrates

Top

predator fish

Forage fish

0.00000001

0.0000001

0.000001

0.00001

0.0001

0.001

0.01

0.1

1

10

Increasing trophic level

Figure 3-19. Methylmercury concentrations were about 1 million times higher in predator fish than in 

stream water. [Note logarithmic scale of y-axis. The top and bottom of each circle represent the range of 

measured methylmercury concentrations. Data are for streams in Oregon, Wisconsin, and Florida sampled 

during 2002–2006 (Brigham and others, 2008; Chasar and others, 2008), and in New York and South Carolina 

sampled during 2007–2009 (Riva-Murray and others, 2011).]

As discussed previously, concentrations of methylmercury 

in fish depend on three basic factors (fig. 3-2)—the quantity 

of inorganic mercury available, the ability of the local 

environment to convert inorganic mercury to methylmercury, 

and the amount of methylmercury bioaccumulation through 

the food web. Thus, even in areas with similar mercury inputs, 

environmental factors that enhance mercury methylation 

and availability, such as wetland abundance and food-web 

structure, can cause mercury concentrations in fish to vary 

by a factor of about 10 (Wiener and others, 2006; Ward and 

others, 2010).

Chapter 3

How Does Mercury Cycle Through Aquatic Ecosystems? 39
Figure 3-21. Mean methylmercury concentrations 

in (A) invertebrates, (B) forage fish, and (C) predator 

fish from streams in Oregon, Wisconsin, and 

Florida correlate strongly with percentages of 

wetland in the upstream basins. (Modified from 

Chasar and others, 2009. Because methylmercury 

concentrations for invertebrates are typically 

reported on a dry weight basis, methylmercury 

concentrations for forage and predator fish also are 

shown on a dry weight basis in this figure for ease of 

comparison among the graphs.)

0.1 

1 

10 

0.01 0.1 1 10 

Methylmercury concentration in water, in parts per trillion 

Methylmercury concentration in largemouth bass

in parts per million per meter 

0 

0 

50 

100 

150 

200 

200 

400 

600 

800 

0 

2,000 

4,000 

6,000 

0 10 20 30 40 

Percent wetland 

Mean methylmercury concentration, in parts per billion dry weight 

A. Invertebrates

B. Forage fish

C. Predator fish 

Methylmercury Levels in Stream Biota Are 

Determined by the Supply of Methylmercury to 

the Base of the Food Web

A nationwide study during 1998 demonstrated that 

methylmercury concentrations in fish correlate strongly with 

methylmercury concentrations in stream water (Brumbaugh 

and others, 2001). Methylmercury in water accounted for 51 

percent of the variation of mercury levels in largemouth bass 

(fig. 3-20), and about 39 percent of the variation for all fish 

species combined. Other variables, such as dissolved organic 

carbon concentration, sulfate concentration, pH, wetland 

percentage in the basin, and methylmercury concentration 

of bed sediment, showed significant, though weaker, 

correlations with fish methylmercury levels. Methylmercury 

concentrations in invertebrates, forage fish, and top predator 

fish also correlate strongly with methylmercury concentrations 

in water from streams in Oregon, Wisconsin, and Florida 

(Chasar and others, 2009). The strong correlations of 

methylmercury concentrations in fish and aquatic invertebrates 

with methylmercury concentrations in stream water suggest 

that the supply of methylmercury to the base of the food web 

is the most important factor influencing methylmercury levels 

in biota across these diverse ecosystems.

Methylmercury concentrations in invertebrates, forage 

fish, and predator fish collected from streams in Oregon, 

Wisconsin, and Florida correlate strongly with the percentage 

of wetland in the upstream basin (fig. 3-21) (Chasar and 

others, 2009). This is expected, given (1) that methylmercury 

supply to the base of the food web is an important control 

on mercury bioaccumulation in these streams, and (2) there 

is a strong positive relationship between methylmercury 

concentration in water and wetland percentage for these 

streams (fig. 3-18). The relationships in figure 3-21 may 

provide useful and simple methods for estimating the 

potential for mercury contamination in streams with similar 

environmental characteristics to the streams studied. 

Figure 3-20. Methylmercury concentrations 

in water correlate strongly with methylmercury 

concentrations in largemouth bass collected across 

the United States during 1998. (Modified from 

Brumbaugh and others, 2001; see appendix 1 for 

additional information.)

40 Mercury in the Nation’s Streams—Levels, Trends, and Implications
Methylmercury Levels in Stream 

Biota also Are Determined by Trophic 

Position within the Food Web

Across the large ranges of environmental 

conditions encompassed by streams in Oregon, 

Wisconsin, and Florida, the increases of mercury 

through food webs were similar to those for 

previously studied lakes and other aquatic 

ecosystems (Wiener and others, 2003; Chasar 

and others, 2009). Methylmercury concentrations 

in attached algae and other microorganisms 

were 2,900–1,700,000 times the methylmercury 

concentrations in water (Bell and Scudder, 

2007) (fig. 3-19) and compare well to previously 

published values of 10,000–1,000,000 (Ward 

and others, 2010). In addition, methylmercury 

concentrations increased by 2.1–8.6 times from 

invertebrates to forage fish, and by 1.2–8.2 

times from forage fish to predator fish (Chasar 

and others, 2009)—values that are close to the 

increases of 2–5 times previously reported (Ward 

and others, 2010). 

Food webs for urban streams in Oregon, 

Wisconsin, and Florida were simpler (top 

predator fish generally were smaller and/or less 

piscivorous) than those for nonurban streams, 

and average trophic positions for top predator 

fish from urban streams were considerably 

lower than for nonurban streams (Chasar and 

others, 2009). Consequently, the methylmercury 

biomagnification potential for urban streams 

was considerably lower than for nonurban 

streams, and urban streams tended to have lower 

methylmercury concentrations in fish at the top 

of the food web.

Trophic position also can be an important 

control on mercury levels in biota when streams 

with similar environmental conditions are 

compared. For example, median trophic positions 

for forage and predator fish were higher for 

streams in South Carolina than in New York, and 

these differences apparently were sufficient to 

account for higher measured fish methylmercury 

levels for streams in South Carolina (RivaMurray and others, 2011). This was true despite 

methylmercury concentrations (potentially 

available to the base of the food webs) for 

streams in South Carolina that were, on average, 

only about one-half those for streams in 

New York.

Chapter 3

How Does Mercury Cycle Through Aquatic Ecosystems? 41
Evergreen River, Wisconsin

Photograph by Dennis A. Wentz.
This chapter addresses the questions: What 

are mercury concentrations in streams across 

the Nation, and where are concentrations 

highest? Concentrations of mercury in soil, 

streambed sediment, stream water, and fish 

across the United States are summarized in 

relation to mercury sources and the role of 

land use/land cover in modifying mercury 

concentrations. 

Mercury occurs naturally at trace concentrations in 

surficial geologic materials around the world. In the United 

States, economically mineable deposits have been found in 

Alaska and parts of the West. Human activities—including 

coal combustion, mercury and gold mining, urbanization, 

and manufacturing—have redistributed much of the mercury, 

thus increasing inorganic mercury levels in some areas and 

affecting mercury cycling in aquatic ecosystems.

Top: Burning coal for energy production contributes large amounts of mercury 

to the atmosphere. (Photograph by Phillip J. Redman.)

Bottom:The mill at Black Butte Mine in Oregon was a significant source of 

mercury in the Willamette River Basin during the early 1900s. (Photograph 

courtesy of the Cottage Grove Historical Society, Cottage Grove, Oregon.)

Where Are Environmental 

Mercury Concentrations 

Highest Across the Nation?

Chapter 4

Where Are Environmental Mercury Concentrations Highest Across the Nation? 43

Chapter

4
Natural Mercury Levels Are Low in Most Rocks, Soil, and Bed Sediment

Figure 4-1. Estimated background and baseline concentration ranges for total mercury in rocks, soil, 

streambed sediment, and lakebed sediment of the United States generally are mostly less than 0.1 ppm. 

Background and Baseline Mercury Concentrations Provide References for Comparison with 

Current Mercury Levels

Background mercury concentrations are natural levels that are unaffected by human activities (Gough and others, 

1993; Reimann and Garrett, 2005). In this report, background mercury levels are defined by concentrations in rocks, or 

in lake sediment or glacier ice deposited prior to industrialization. Baseline mercury levels are concentrations that have 

been documented for a specified area during a specified time period. Baseline concentrations typically are impacted 

to some extent by anthropogenic influences, such as global atmospheric deposition of mercury. Moreover, baseline 

concentrations are strictly valid only for the area where, and time period when, they are determined. 

Both background and baseline concentrations are most appropriately expressed as ranges. Estimates of background 

or baseline mercury concentration ranges can help to (1) ascertain whether concentrations in other areas are enriched, 

for example by geologic or anthropogenic sources; (2) determine if concentrations are changing over time; (3) assess 

the practicality of environmental guidelines; and (4) evaluate the effectiveness of proposed mitigation strategies.

Published background and baseline concentrations 

for total mercury in rocks, soil, streambed sediment, and 

lakebed sediment are mostly less than 0.1 ppm (fig. 4-1). 

The ranges of values are relatively similar, even though 

the various studies used different approaches to estimate 

background and baseline concentrations. The data provide a 

way to compare current mercury levels in a given location with 

estimated natural concentrations.

0 0.1 0.2 0.3 0.4

Total mercury concentration, in parts per million

Worldwide crustal abundance—Igneous and sedimentary rocks 

(except clays and shales) (Parker, 1967; Rudnick and Gao, 2003)

U.S. soil (Shacklette and Boerngen, 1984; R.C.Severson, U.S. Geological Survey, written commun.,1987)

U.S. streambed sediment (Horowitz and Stephens, 2008)

U.S. lake bed sediment (Mahler and others, 2006)

EXPLANATION

Background

Baseline

44 Mercury in the Nation’s Streams—Levels, Trends, and Implications
0.382

0.333

0.291

0.254

0.222

0.193

0.169

0.147

0.129

0.112

0.098

0.085

0.075

0.065

0.057

0.05

0.043

0.038

0.033

Soil mercury concentration, 

in parts per million

EXPLANATION

98 ̊W

Mercury Concentrations in Soil Generally Are Higher in the Eastern Than in the 

Western United States

Soil is a primary storage compartment for mercury and can be readily transported to other parts of 

the ecosystem (see “Surface Soil Typically Is the Largest Reservoir of Mercury in Watersheds,” p. 34). 

Baseline mercury concentrations in soil are low across most of the United States, with values ranging 

from 0.0091 to 0.37 ppm (fig. 4-1) (Shacklette and Boerngen, 1984; R.C. Severson, U.S. Geological 

Survey, written commun., 1987). In addition, USGS studies have shown that mercury concentrations 

in surface soil east of the 98th meridian (which runs approximately from eastern North Dakota through 

east-central Texas) typically are about twice those west of this meridian (Shacklette and Boerngen, 

1984; Gustavsson and others, 2001) (fig. 4-2). High mercury concentrations in the Eastern United States 

generally reflect greater rates of atmospheric mercury deposition in this region (fig. 3-8).

Figure 4-2. Mercury concentrations in soil generally are higher in the Eastern than in the Western United 

States. (Modified from Gustavsson and others, 2001.)

Chapter 4

Where Are Environmental Mercury Concentrations Highest Across the Nation? 45
0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14

Population

density

≥ 50th

percentile

Urban land

Rangeland

Forest

Pasture

Cropland

Total mercury concentration, in parts per million

Baseline

concentration

range

Land use/land cover 

Mercury Concentrations in Streambed Sediment Are Higher in Urbanized 

Areas Than in Areas Affected by Other Land Uses or Land Covers

USGS studies have shown that baseline total mercury concentrations in streambed sediment 

have a range of about 0.02–0.06 ppm (fig. 4-1; calculated from data in Horowitz and Stephens, 2008). 

Except for areas contaminated by mercury and gold mining, the largest enrichments of mercury in 

bed sediment are in urbanized areas—those basins with more than 25 percent urban land or with 

population densities greater than the 50th percentile (fig. 4-3) (Horowitz and Stephens, 2008; Scudder 

and others, 2009). In these areas, anthropogenic sources of mercury are numerous, and can include 

wastewater- treatment plants, waste incinerators, crematoriums, steel mills, cement kilns, and disposal 

sites for batteries, thermometers, fluorescent bulbs, and a variety of other mercury-containing wastes. 

Basins with more than 50 percent forested land showed a slight enrichment of mercury greater than 

baseline values (fig. 4-3), an effect attributed primarily to dry deposition on leaf surfaces, and possibly 

to elevated organic matter in these systems (Horowitz and Stephens, 2008).

Figure 4-3. Total mercury concentrations in streambed sediment are somewhat 

enhanced in streams draining forested land; however, urbanized areas, with their 

many anthropogenic mercury sources, exhibit concentrations that are about two 

to three times baseline levels and are higher than for any other land use/land 

cover. (Data from Horowitz and Stephens, 2008; see appendix 1 for additional 

information.) 

46 Mercury in the Nation’s Streams—Levels, Trends, and Implications
0 

2 

4 

6 

8 

10 

12 

1,000 10,000 

Mercury methylation efficiency, in percent 

Mercury source index 

Areas with mercury primarily from 

geologic sources, typically mercury or 

gold mining (mostly Western States)

Areas with mercury primarily from atmospheric 

sources and with high wetland abundance 

(mostly Eastern, Southeastern, and Midwestern States) 

Areas with mercury primarily from atmospheric 

sources and with low wetland abundance 

(States throughout the Nation) 

Mostly atmospheric Mostly geologic

10 100 

Streams draining watersheds with an abundance of wetlands tend to 

have high levels of methylmercury. (Photograph by Elizabeth A. Nystrom.)

Methylmercury Concentrations in Streams Vary with Wetland Abundance and the Amount of 

Available Inorganic Mercury

Methylmercury concentrations in stream water correlate strongly with areal percentages of wetland in the upstream basin and 

with dissolved organic carbon concentrations of the water for streams sampled across the Nation during 1998–2005 (Scudder 

and others, 2009). The wetlands produce dissolved organic carbon, which binds strongly to the methylmercury and facilitates its 

transport. Similarly, methylmercury concentrations in streambed sediment correlate strongly with percentages of wetland in the 

basin and with organic carbon concentrations in the sediment.

Mercury methylation efficiency and the amount of available 

inorganic mercury also affect methylmercury concentrations in streams 

(fig. 4-4). The highest mercury methylation efficiencies are associated 

with areas containing the greatest abundances of wetlands, including 

States in the East, Southeast, and Upper Midwest. Inorganic mercury 

concentrations in these areas are contributed mostly by atmospheric 

deposition, and methylmercury concentrations in streams can be high 

if mercury methylation efficiencies are high. Large geologic mercury 

sources typically are found in areas impacted by mercury and gold 

mining, such as those in the Western United States (South Dakota, Utah, 

Montana, Idaho, Nevada, California, and Colorado). These areas have 

low mercury methylation efficiencies because conditions favorable to 

methylation, such as wetlands and dissolved organic carbon, generally 

are less abundant in mined areas than in unmined areas. On the other 

hand, methylmercury concentrations can be elevated in streams draining 

mined areas if inorganic mercury levels are extremely high.

Chapter 4

Where Are Environmental Mercury Concentrations Highest Across the Nation? 47

Figure 4-4. Mercury 

methylation efficiency and 

mercury source affect 

mercury concentrations in 

U.S. streams. High mercury 

methylation efficiencies 

occur in watersheds with 

high wetland abundance. 

Watersheds with high 

geologic mercury sources 

relative to atmospheric 

sources are typical of areas 

mined for mercury or gold. 

Watersheds with high 

atmospheric mercury sources 

relative to geologic sources 

are typical of unmined areas. 

(Note logarithmic scale of 

x-axis; data from Bauch and 

others, 2009; see appendix 1

for additional information.) 
0 200 400 600 800 1,000 KILOMETERS

0 200 400 600 800 1,000 MILES

EXPLANATION

Non-mined Mined

< 0.10

0.10 – 0.19

0.20 – 0.29

≥ 0.30

Total methylmercury, game fish 

fillet, in parts per million

Fish Methylmercury Concentrations Are Highest 

in Predator Fish, Particularly for Streams 

Draining Forest or Wetland-Dominated Basins 

Fish methylmercury concentrations for 291 sites sampled 

nationwide by the USGS during 1998–2005 generally were 

highest in forest or wetland-dominated coastal plain streams in 

the Southeastern United States (fig. 4-5) (Scudder and others, 

2009). Although a small number of fish from western streams 

impacted by historical mercury or gold mining were extremely 

contaminated with mercury (fig. 4-5), on average, fish 

methylmercury concentrations were not significantly different 

between mined and unmined basins across the United States. 

This is consistent with the results for methylmercury in stream 

water and bed sediment, which also did not differ significantly 

between mined and unmined basins.

Median concentrations of methylmercury in all 

predator fish combined and in largemouth bass—the 

most commonly sampled predator fish—were 0.17 

and 0.33 ppm, respectively, for U.S. streams (Scudder 

and others, 2009). Fish methylmercury concentrations 

exceeded the USEPA criterion of 0.3 ppm for the 

protection of human health at about one-fourth of all 

stream sites and at more than one-half of sites where 

largemouth bass were collected. Similarly, for streams 

sampled across the country by the USEPA during 2008–

2009, methylmercury concentrations in all fish combined 

exceeded the criterion at about one-fourth of the sites (U.S. 

Environmental Protection Agency, 2013). In contrast, 

median methylmercury concentrations in predator fish 

from U.S. lakes sampled by the USEPA during 2000–2003, 

were slightly higher (0.28 ppm) than for streams sampled 

by the USGS, and a greater proportion (about one-half) 

exceeded the fish tissue mercury criterion (Stahl and 

others, 2009).

Figure 4-5. Fish methylmercury concentrations generally were highest in streams draining forest or wetland-dominated basins in 

the Southeastern United States or in streams draining basins that had been mined for mercury or gold in the Western United States. 

(Modified from Scudder and others, 2009.) 

48 Mercury in the Nation’s Streams—Levels, Trends, and Implications
0

0.5 

1.0 

1.5 

2.0 

2.5 

Undeveloped Mixed Mined Agricultural Urban 

Median methylmercury concentration in largemouth bass,

 in parts per million per meter

The highest median fish methylmercury 

concentration for streams was 0.42 ppm for 

spotted bass, a mostly piscivorous fish (Scudder 

and others, 2009). The median methylmercury 

concentration for smallmouth bass—a somewhat 

more omnivorous species than spotted or 

largemouth bass—was 0.20 ppm. Conversely, the 

lowest median fish methylmercury concentration 

was 0.07 ppm for rainbow and cutthroat trout. 

Low mercury concentrations in trout probably 

occur because their diets include significant 

portions of insects and other invertebrates, which 

occupy lower food-web trophic positions (thus, 

are lower in mercury), than do the diets of most 

predator fish, which feed at higher trophic levels 

(Scott and Crossman, 1973). 

Methylmercury concentrations in 

largemouth bass during 1998–2005 typically 

were highest for streams draining undeveloped 

basins and basins with mixed land use/land cover 

(fig. 4-6) (Scudder and others, 2009). These areas 

contain high percentages of forest and wetland, 

and high concentrations of methylmercury, 

all of which are strong predictors of mercury 

concentrations in fish (Krabbenhoft and others, 

1999; Brumbaugh and others, 2001; Scudder 

and others, 2009). In fact, methylmercury levels 

in largemouth bass correlated most strongly 

with percentages of the upstream basin in 

evergreen forest and woody wetland. 

Methylmercury concentrations in 

largemouth bass were lowest in urban streams 

(fig. 4-6) (Scudder and others, 2009), even 

though these streams had the highest total 

mercury concentrations in streambed sediment. 

The low mercury levels in fish from urban 

streams are attributed to a relative absence 

of factors conducive to methylmercury 

production (such as wetlands) compared 

to forested, undeveloped basins, and to the 

generally simpler food webs in urban streams, 

whereby top predator fish are smaller and 

less piscivorous than in nonurban streams. 

Methylmercury concentrations in largemouth 

bass from mined and agricultural areas were 

intermediate between those from undeveloped/

mixed areas and urban areas.

Mining of Mercury and Gold Can 

Cause High Mercury Concentrations 

in Stream Water, Streambed 

Sediment, and Fish

Mercury concentrations in stream water, 

streambed sediment, and fish from selected 

mercury mining areas of Alaska and California 

and gold mining areas of California and Nevada 

have been summarized by Gray (2003b, 

table 3). Maximum mercury concentrations 

downstream of these mining areas can be 

extremely high and can have adverse effects on 

aquatic ecosystems. Figure 4-6. Length-normalized methylmercury 

concentrations in largemouth bass were highest in 

streams draining undeveloped basins and basins 

with mixed land use/land cover. (Data from Bauch 

and others, 2009; see appendix 1 for additional 

information.) 

Chapter 4

Where Are Environmental Mercury Concentrations Highest Across the Nation? 49
0.0001

0.001

0.01

0.1

1

10

100

1,000

10,000

100,000

Total mercury Methylmercury Total mercury Methylmercury Methylmercury

Maximum mercury concentration, in parts per billion (water) and 

parts per million (bed sediment and fish) 

Stream Water Streambed Sediment Fish

Mercury Mines—Coast Ranges, Calif.

Mercury Mines—Southwest, Alaska

Gold Lode Mines—Carson River, Nev.

Gold Placer Mines—Sierra Nevada, Calif.

Baseline Streams—Southwest, Alaska

EXPLANATION

USEPA fish tissue

mercury criterion

(0.3 parts per million)

Maximum concentrations of total mercury and 

methylmercury in stream water were highest downstream 

of mercury mines in the California Coast Ranges (fig. 4-7). 

Maximum total mercury concentrations for mined areas 

were about 2,000–300,000 times greater than maximum 

baseline concentrations for total mercury in streams 

draining unmined areas of southwestern Alaska. Maximum 

methylmercury concentrations for mined areas were about 

6–350 times greater than corresponding maximum baseline 

concentrations.

Figure 4-7. Total mercury and methylmercury 

concentrations in stream water were highest 

downstream of mercury mines in the California 

Coast Ranges, as were the highest methylmercury 

concentrations in streambed sediment. Total 

mercury concentrations in streambed sediment 

were highest downstream of gold placer mines 

in the Sierra Nevada. Methylmercury levels 

in fish were highest downstream of gold lode 

mines in Nevada, but were likely controlled more 

by species and trophic position than by type of 

mining. (Note logarithmic scale of y-axis; data from 

Gray, 2003b, table 3.) 

Maximum concentrations of methylmercury in 

streambed sediment also were highest downstream of mercury 

mines in the California Coast Ranges (fig. 4-7). Maximum 

methylmercury concentrations were about 70–500 times 

greater than maximum baseline concentrations, except for 

downstream of gold placer mines in the Sierra Nevada, 

California, where they were similar to baseline concentrations. 

In contrast to methylmercury concentrations, the highest total 

mercury concentrations in bed sediment (up to 26,000 ppm, 

or 2.6 percent) were measured downstream of gold placer 

50 Mercury in the Nation’s Streams—Levels, Trends, and Implications
operations in the Sierra Nevada, California, and are consistent with the 

large losses of mercury attributed to these placer operations. Maximum total 

mercury concentrations in bed sediment were 100–30,000 times greater than 

maximum baseline concentrations. 

The maximum methylmercury concentration in bed sediment 

downstream of gold placer mines in the Sierra Nevada was similar to the 

maximum baseline methylmercury concentration, despite having total 

mercury concentrations that were more than 30,000 times greater than 

baseline concentrations (fig. 4-7). This observation indicates that basins 

mined for placer gold in the Sierra Nevada may have environmental 

characteristics that are less conducive to mercury methylation than baseline 

areas and is consistent with the degraded nature of stream channels that have 

undergone significant placer mining in this area (Alpers and others, 2005).

Maximum methylmercury concentrations in fish collected downstream 

of mined areas in Alaska, California, and Nevada were about 4–200 times 

greater than in fish from baseline streams (fig. 4-7); however, results from 

the different areas can not be directly compared because the fish were from 

varying, and sometimes unspecified, species. Maximum fish methylmercury 

levels were about 2–50 times greater than the USEPA criterion of 0.3 ppm for 

the protection of human health. 

The highest reported fish methylmercury concentration (16 ppm) was 

in a walleye (a piscivorous fish) captured in Lahontan Reservoir, which is 

downstream of the Comstock Lode in Nevada and part of the Carson River 

Mercury Superfund site (Lawrence, 2003) (fig. 4-7). Fish methylmercury 

levels in largemouth, smallmouth, and spotted bass from five California 

reservoirs affected by historical hydraulic gold mining were as high as 

1.5 ppm (May and others, 2000). The maximum fish methylmercury 

concentration in streams from southwestern Alaska (0.62 ppm) was in a Dolly 

Varden—an omnivorous resident salmonid (Gray and others, 2000). The 

maximum measured fish methylmercury levels are consistent with trophic 

positions typically occupied by the fish.

Chapter 4

Where Are Environmental Mercury Concentrations Highest Across the Nation? 51
Healing Springs, South Carolina

Photograph by Dennis A. Wentz.
Chapter

How Do Environmental 

Mercury Levels Vary 

Over Time?

This chapter addresses the questions: How do 

environmental mercury levels vary over time? 

What is the outlook for the future? Historical 

and recent U.S. trends of atmospheric mercury 

deposition and mercury concentrations in fish 

are presented, and expectations for the future 

are discussed. 

Important questions currently facing natural resource 

managers, and environmental scientists in general, include: 

“How much of the mercury falling on our land and water from 

atmospheric deposition is derived from human activities?,” 

“Are mercury concentrations in aquatic ecosystems and fish 

increasing or decreasing over time?,” “How are mercury 

concentrations likely to change in the future?.” The answers 

to these questions have important implications regarding the 

effectiveness of regulations designed to control emissions of 

mercury to air and discharges of mercury to water.

(Photograph by Mark E. Brigham.)

Chapter 5

How Do Environmental Mercury Levels Vary Over Time? 53
tac11-0651_fig05-01

Pre-industrial (background)

"Unknown"

Gold Rush (circa 1850–1884)

Krakatau (1883)

WWII manufacturing (circa 1940–1945)

Industrialization (circa 1880–present)

Mount St. Helens (1980)

2000

1950

1900

1850

1800

1750

1700

0 5 10 15 20 25 30 35 40

Total mercury concentration, in parts per trillion

Year

Tambora (1815)

Total mercury in precipitation 

(2005–2011)

Humans Have Increased Atmospheric 

Mercury Deposition Relative to 

Natural Levels 

Although mercury is emitted to the 

atmosphere by natural processes, anthropogenic 

mercury emissions from numerous sources have 

increased the amount of mercury that falls from 

the atmosphere worldwide. From pre-industrial 

(pre-1850) to recent times, ice and lake sediment 

core records have recorded large increases 

in atmospheric mercury deposition. In recent 

decades (since about 1970), mercury deposition 

has decreased at many U.S. locations relative to 

peak deposition rates. 

Potential mercury emissions from coal 

burning increased in the United States from 

about 1960 to the early 1990s; however, 

changing patterns of coal combustion over 

that period complicated the actual amount of 

mercury emitted (Engstrom and Swain, 1997). 

Historically, coal was burned largely for space 

heating; emissions entered the atmosphere 

through low smokestacks, resulting in localized 

mercury deposition. More recently, coal is burned 

primarily by power plants for energy production. 

The coal is lower in mercury than in the past, 

and some of the mercury is captured by controls 

implemented to reduce particulate and sulfur 

dioxide emissions. In addition, the much taller 

smokestacks at power plants result in longer 

range transport of the emitted mercury.

Ice Cores Reveal a Modern Industrial 

Footprint on Mercury Deposition in 

Remote Areas 

The most detailed long-term documentation 

of atmospheric mercury deposition is based 

on ice cores collected during the 1990s in the 

Upper Fremont Glacier in west-central Wyoming 

(Krabbenhoft and Shuster, 2002; Schuster and 

others, 2002) (fig. 5-1). The remoteness of this 

site limits influences from local mercury sources 

and provides an opportunity to evaluate the 

importance of regional and global variations in 

mercury deposition. 

Figure 5-1. Ice cores from the Upper Fremont 

Glacier, Wyoming, show that background total 

mercury concentrations prior to 1840 were 3–4 parts 

per trillion. Large, but relatively short-lived, mercury 

concentrations resulted from major volcanic 

eruptions, and a substantial increase occurred 

following the California Gold Rush. Since about 

1880, anthropogenic mercury contributions from the 

industrialization of America (especially during World 

War II), have generally increased, peaking in about 

1984. Mercury concentrations at the top of the ice 

core are similar to the range of annual average total 

mercury concentrations in precipitation at a nearby 

MDN collection site (WY08) during 2005–2011. 

(Modified from Krabbenhoft and Shuster, 2002; 

Schuster and others, 2002. Precipitation data for 

WY08 from http://nadp.sws.uiuc.edu/sites/siteinfo.

asp?net=MDN&id=WY08.)

54 Mercury in the Nation’s Streams—Levels, Trends, and Implications
Portable boats and coring equipment are used to collect 

sediment cores from remote lakes, where mercury is 

deposited predominantly by atmospheric deposition. 

(Photograph by Peter C. Van Metre.)

Sediment layers from lake cores are age dated and 

analyzed for mercury to determine how atmospheric 

mercury deposition has varied over time. (Photograph by 

Daniel R. Engstrom, Science Museum of Minnesota.)

Ice and Lake Sediment Cores Are Archives of 

Past Mercury Deposition

Ice cores from glaciers are used extensively to study the 

Earth’s climate history. They also can be used to examine 

concentration trends in chemicals, such as mercury. The weight 

of snow accumulating at the top of a glacier compresses buried 

snow into layers of ice, trapping chemicals that fell with the 

snow. Changes in mercury concentration within a core can be 

placed into the context of known historical events, such as the 

California Gold Rush and the industrialization of our economy. 

The ages of ice layers are accurately determined through 

use of annual dust layers, the radioactive isotopes tritium 

and chlorine-36 produced by atmospheric nuclear testing, 

carbon-14, and increased electrical conductivity of the ice 

caused by known volcanic eruptions that occurred in 

recorded history. 

Extensive studies of lake sediment cores from many 

researchers, including the USGS, provide temporal detail on 

anthropogenic contributions to mercury fallout across North 

America and elsewhere. Similar to ice cores, as new material 

is added, older material is buried, thus creating a temporal 

record. Determination of mercury concentrations and ages (or 

dates) in a sediment core profile allows scientists to reconstruct 

historical trends of mercury inputs to lakes. If a lake and its 

watershed are not impacted by direct discharges of mercurycontaining wastes, then mercury concentrations in the sediment 

core predominantly reflect atmospheric mercury deposition 

rates. Sediment deposited over the past 100–150 years can be 

aged, or dated, through analysis of lead-210 (a decay product 

of naturally occurring uranium-238) and cesium-137 (from 

atmospheric nuclear testing). Age dating not only provides 

historical context, but also allows determination of the rates 

at which sediment associated chemicals accumulate. Mercury 

has been determined in many more lake cores than glacial ice 

cores, providing a much broader geographic distribution of 

historical mercury deposition.

Chapter 5

How Do Environmental Mercury Levels Vary Over Time? 55
Three distinct volcanic events were recorded 

by the Upper Fremont Glacier ice-core profile. 

The eruptions of Mount Tambora and Mount 

Krakatau—the two largest in recorded history—

are easily distinguished despite their location in 

equatorial Indonesia. The eruption of Mount St. 

Helens, although orders of magnitude smaller 

than those of the two Indonesian volcanoes, also 

is easily seen because of its relative closeness 

(about 600 kilometers) to the Upper Fremont 

Glacier. These three events together contributed 

6 percent of the total mercury in the ice core.

Two mercury peaks are consistent with the 

timing of the California Gold Rush, during which 

mercury was used as an amalgamating agent to 

enhance the recovery of gold (see “Historical 

Mercury and Gold Mining Are Sources of 

Mercury to Some Streams,” p. 32). The high 

volatility of mercury, along with its high loss 

rate during gold mining, are expected to cause 

contamination considerable distances from mined 

areas. The first Gold Rush peak in the ice core 

approximates the period from the discovery of 

gold in 1848 until its peak production in 1852 

(Hill, 1926). The second (later) Gold Rush peak 

coincides approximately with a large increase in 

mercury production that may have been fueled 

by the widespread use of hydraulic mining (Hill, 

1926; Alpers and Hunerlach, 2000). The Sawyer 

Decision (issued in 1884) considerably limited 

this form of mining (Alpers and Hunerlach, 

2000), and subsequent mercury peaks from 

mining were not seen in the ice core. The two 

peaks that correlate with gold mining activities 

in California account for 13 percent of the total 

mercury in the ice core.

Beginning about 1880, total mercury 

concentrations in the Upper Fremont Glacier 

ice core were slightly greater than background 

concentrations until the period encompassed by 

World War II, then gradually increased to 20–25 

parts per trillion until about 1984. These elevated 

concentrations coincide with the industrialization 

of the United States and are likely due to 

emissions from coal-fired power plants, waste 

incinerators, chlor-alkali plants, and other 

industries. Decreasing mercury concentrations in 

the ice core since 1984 correspond with declines 

in U.S. and world mercury production (fig. 3-13), 

and with implementation of U.S. legislation 

designed to control industrial mercury emissions 

(see “Nationwide Trends of Mercury in Fish 

Generally Were Downward during the 1970s and 

1980s,” p. 60). Mercury concentrations at the top 

of the ice core are similar to annual average total 

mercury concentrations measured in precipitation 

at the nearest MDN collection site (WY08) 

during 2005–2011 (fig. 5-1). Anthropogenic 

mercury sources throughout the post-1900 period 

of industrialization have contributed most of 

mercury deposited during that time. 

Natural mercury contributions (background 

and volcanic eruptions) and anthropogenic 

contributions (Gold Rush, World War II, 

and industrialization) to the Upper Fremont 

Glacier ice core account for 48 and 52 percent, 

respectively, of total mercury in the core. 

It is important to realize, however, that all 

anthropogenic mercury contributions have 

occurred since about 1840 and make up a much 

larger percentage of the total contributions since 

that time.

Historical Rates of Mercury 

Deposition Reflected by Lake 

Sediment Cores Have Increased 

Since Pre-Industrial Times, and the 

Increases Are Greatest in Major 

Urban Areas 

Sediment cores from lakes in extremely 

remote regions, unaffected by regional 

anthropogenic emissions, typically show that 

mercury inputs during recent years are about 

double input rates in pre-industrial times 

(before about 1850). Cores from lakes in the 

remote Canadian Arctic, for example, show an 

approximate doubling of mercury accumulation 

rate relative to pre-industrial times (fig. 5-2). 

Similar findings for other remote areas, including 

northern Canada (Lockhart and others, 1995), 

southeastern Alaska (Engstrom and Swain, 1997), 

northern Alaska (Fitzgerald and others, 2005), 

and northern Scandinavia (Johansson, 1985; 

Verta and others, 1989; Bindler and others, 2001) 

typically show about a 1.5- to 3-fold increase in 

mercury concentration and accumulation rate, 

with a central value close to a 2-fold increase 

(Verta and others, 1989; Fitzgerald and 

others, 1998). 

56 Mercury in the Nation’s Streams—Levels, Trends, and Implications
tac11-0651

fig05-02

1800 

1850 

1900 

1950 

2000 

2050 

0 10 20 30 40 50 60 70 80 90 

Year 

Mercury accumulation rate, in micrograms per square meter per year 

 Remote Canadian Arctic Rural Continental United States 

 Major U.S. Urban Area 

Sediment cores collected from undisturbed, 

rural lakes across the continental United States 

typically show about a 3- to 5-fold increase 

in mercury accumulation rate relative to preindustrial times (fig. 5-2; Swain and others, 

1992; Engstrom and Swain, 1997; Lorey and 

Driscoll, 1999; Van Metre and Fuller, 2009; 

Drevnick and others, 2012; Van Metre, 2012). 

These are greater increases than for remote 

locations in Alaska and Canada, as discussed 

above. Similarly large increases in mercury 

concentrations and accumulation rates have 

been observed in southern Scandinavia—

close to continental Europe (Johansson, 1985; 

Bindler and others, 2001). The greater mercury 

deposition rates, relative to remote regions, 

reflect the many sources of mercury emissions 

within the continental United States (see 

“Mercury Sources to Aquatic Ecosystems Are 

Both Anthropogenic and Natural,” p. 24) and 

within continental Europe. 

Undisturbed lakes within or near major 

urban areas receive much greater mercury 

deposition than more rural lakes, and therefore 

show much larger increases relative to preindustrial times (fig. 5-2). A USGS study of lakes 

across the United States, in settings ranging 

from rural to urban, found that recent mercury 

accumulation rates in near-urban lakes averaged 

about 10 times the accumulation rates in preindustrial times, although this factor varied 

considerably among locations (Van Metre, 2012). 

Other studies have similarly found larger modern 

mercury accumulation rates in urban lakes 

compared to undisturbed rural settings (Engstrom 

and others, 2007).

Figure 5-2. Mercury accumulation rates in lake sediment cores reflect historical increases in 

anthropogenic mercury emissions. Typical sediment core profiles are shown for three general settings: 

a remote area (Little Coal Lake, Yukon Territories in the Canadian Arctic); rural United States (Hobbs 

Lake, Wyoming); and a major urban area (Carlton Lake, Florida). The core profiles reveal increases of 

approximately 2, 4.5, and 10 times, respectively, compared to background concentrations measured at the 

bottoms of the remote and rural cores. (Modified from Van Metre and Wilson, 2003; Van Metre. 2012.)

Chapter 5

How Do Environmental Mercury Levels Vary Over Time? 57
tac11-0651_fig05-03

Alaska Rural continental

United States Near major 

U.S. urban areas

Total annual mercury deposition, in micrograms per square meter

Deposition source

EXPLANATION

Natural (background)

Global

Regional

Urban

7.4 19 78

The spatial and temporal information 

derived through analysis of lake cores in 

remote areas of Alaska, rural continental United 

States, and near major U.S. urban areas, can 

be used to apportion mercury sources into 

four generalized types (fig. 5-3). Three scales 

of anthropogenic mercury are important: 

global—mercury emissions that are dispersed 

widely in the atmosphere; regional—mercury 

that falls out at a regional to national scale; and 

local—mercury that falls out within tens of 

kilometers of major urban areas. In addition, 

natural (or background) mercury is deposited 

from the atmosphere. Background mercury 

levels can be determined from deeper sedimentcore layers that reflect mercury deposition 

before industrialization, and therefore lack 

significant anthropogenic mercury. Lakes in 

or near urban areas receive mercury from all 

four of these general source types and typically 

have the highest mercury inputs. Knowledge 

of general source types helps State and Federal 

environmental regulatory agencies set realistic 

goals with respect to fish mercury levels and 

desired reductions in mercury emissions.

Lake Sediment Cores Show 

Decreasing Mercury Deposition in 

Recent Decades, Particularly in 

Urban Areas

Trends in mercury concentrations were 

determined for sediment cores collected from 31 

lakes and reservoirs nationwide (fig. 5-4) (Mahler 

and others, 2006). The trends were evaluated for 

the period after 1970 (generally 25–30 years), 

which corresponds to the period following 

enactment of the National Environmental Policy 

Act and the Clean Air Act (1969 and 1970, 

respectively)—legislation that affected emissions 

of mercury and other trace elements to the 

atmosphere (see “Nationwide Trends of Mercury 

in Fish Generally Were Downward during the 

1970s and 1980s,” p. 60).

Figure 5-3. Atmospheric mercury source types include anthropogenic (global, regional, and urban) and 

natural (background). Remote regions, such as Alaska, receive roughly equal portions of background and 

global anthropogenic mercury. Rural regions within the continental United States receive additional mercury 

from regional to national emission sources. Large U.S. urban areas receive a still greater mercury load. (See 

appendix 1 for additional information.)

58 Mercury in the Nation’s Streams—Levels, Trends, and Implications
tac11-0651_fig05-04

Atlanta

Denver

Rocky Mountain 

National Park

Glacier

National

Park

Upper 

Fremont 

Glacier

San Diego

Los Angeles

Las 

Vegas

Seattle

Houston

Fort Worth Dallas

Minneapolis

Chicago

Orlando

Boston

New York

Newark

Washington, D.C.

Sampling site, by trend and land cover type

Natural feature or park

Light

urban

Dense

urban Reference

Significant trend

No significant trend

EXPLANATION

About one-half of the lakes studied 

showed no trend in total mercury 

concentration; for the lakes that did show 

trends, decreasing trends outnumbered 

increasing trends by about 2:1 (Mahler 

and others, 2006). Dense urban areas 

(greater than 52 percent urban land) showed 

more decreasing trends than light urban 

(6–43 percent urban land) or reference 

areas (less than 1.5 percent urban land), 

but decreasing trends were noted for all 

three categories. For example, there were 

significant decreases in mercury concentration 

at Newbridge Pond, located in a dense urban 

area just east of New York City; at Lake 

Washington, in a light urban area east of 

downtown Seattle, Washington; and at Mills 

Lake, in a remote area of Rocky Mountain 

National Park, Colorado. The decreasing 

mercury concentrations at Mills Lake since 

1970 are in general agreement with data from 

the ice core at the Upper Fremont Glacier, 

which is about 450 kilometers northwest 

of Mills Lake. Moreover, the prevalence of 

decreasing mercury concentrations in dense 

urban areas is consistent with decreasing 

discharges of point and nonpoint sources 

of mercury to streams in urbanized areas, 

despite stable or increasing global atmospheric 

sources.

Other regional scale studies also have found 

a mix of decreasing and increasing mercury 

trends. Sediment cores from 91 natural, mostly 

forested, inland lakes throughout the Great Lakes 

region, from Minnesota to New Hampshire 

(Drevnick and others, 2012) showed mostly 

decreasing trends. These small inland lakes 

likely receive mercury inputs predominantly 

from atmospheric deposition. Eighty-four 

percent of these lakes showed decreases in 

mercury accumulation rates from peak values 

in the late-1980s to values that were about 20 

percent lower than peak in more recent decades. 

These lower values are similar to those measured 

during the 1970s.

Total mercury concentrations and 

accumulation rates in sediment cores collected 

during 2002–03 from nine lakes in Rocky 

Mountain and Glacier National Parks (located 

within 700 kilometers of the Upper Fremont 

Glacier; see fig. 5-4 for locations), also began 

to increase above background levels in about 

1900 and peaked sometime after 1980 (Mast 

and others, 2010). However, subsequent to 

1980, sediment cores from some lakes showed 

decreases in mercury concentrations and 

accumulation rates, whereas sediment cores from 

other lakes did not. 

Figure 5-4. Since 1970, mercury concentrations in sediment cores showed decreasing trends in more lakes in dense urban areas 

than in light urban or reference areas. (Modified from Mahler and others, 2006; see appendix 1 for additional information.)

Chapter 5

How Do Environmental Mercury Levels Vary Over Time? 59
tac11-0651_fig05-05

West

Southeast

Midwest

Northeast

West

Southeast

Midwest

Northeast

Trends in fish methylmercury concentration

Increasing trend Decreasing trend No trend

A. 1969 to 1987

B. 1988 to 2005

EXPLANATION

Mercury accumulation rates in lakes in 

rural Minnesota and urban Minneapolis began 

to increase during the mid-1800s and continued 

to increase into the 1950s (Engstrom and 

Swain, 1997). Accumulation rates in four rural 

eastern Minnesota lakes and four Minneapolisarea lakes peaked in the 1960s and 1970s, and 

decreased in the 1980s. The decreasing mercury 

accumulation rates in the 1980s are attributed to 

reductions in regional mercury sources, resulting 

from decreased industrial uses of mercury, 

improved technologies at coal-burning facilities, 

a shift from coal to natural gas for heating, and 

decreased waste incineration.

The more prevalent decreases in mercury 

concentrations in dense urban areas compared 

to other areas likely reflects both a decrease 

in mercury used in industrial processes and 

products, as well as a shift in coal combustion 

patterns. Since the 1940s, there has been a largescale shift away from burning coal for residential 

and commercial heating, whereby emissions 

enter the atmosphere through low smokestacks 

resulting in localized mercury deposition. The 

increase in coal-related mercury emissions was 

driven primarily by increased use of coal for 

energy production by power plants, where tall 

smokestacks result in longer range mercury 

transport. This trend has continued into the 1970s 

and beyond.

Nationwide Trends of Mercury in Fish 

Generally Were Downward during the 

1970s and 1980s 

The only national analysis of temporal 

trends for mercury in freshwater fish assessed 

U.S. rivers and lakes during two periods: 

1969–87 (50 sites) and 1988–2005 (40 sites) 

(Chalmers and others, 2011). Fish methylmercury 

levels decreased at 22 of 50 sites sampled from 

1969 to 1987, and levels increased at only 4 

sites (fig. 5-5A). These sites were sampled by 

the U.S. Fish and Wildlife Service as part of the 

Figure 5-5. (A) Fish methylmercury levels decreased at 22 of 50 sites 

sampled across the United States during 1969–87, and downward trends 

far outnumbered upward trends. (B) Only one-third of sampled sites 

showed significant trends during 1988–2005, and the number of sites with 

decreasing methylmercury concentrations was approximately equal to the 

number of sites with increasing concentrations. (Modified from Chalmers 

and others, 2011.)

60 Mercury in the Nation’s Streams—Levels, Trends, and Implications
National trends for methylmercury in fish are 

predominantly downward. Changes in mercury

emissions are unknown.

Mercury emissions decline in the Northeast, 

Midwest, and Southeast; however, data needed 

to assess trends for methylmercury in fish 

from these areas are inadequate. Limited 

data for some Midwestern and Southeastern 

States show that trends are about equally 

split between upward and downward, but 

results are not definitive.

1970

1975

1980

1985

1990

1995

2000

2005

2010

1965

Use of mercury in batteries phased out (1996)

Clean Air Act (1970) 

Clean Water Act (1972)

Mercury emissions from medical waste incineration limited (1997)

Resource Conservation and Recovery Act (1976)

Safe Drinking Water Act (1974)

Clean Air Mercury Rule vacated (annulled) by US Supreme Court (2008)

Mercury emissions from boilers, heaters, cement plants limited (2010)

USEPA issues Clean Air Mercury Rule (2005)

Last commercial mercury mining operation closes (1992) 

Clean Air Act Amendments require reductions in mercury emissions 

(1990)

USEPA establishes reference dose of 0.1 micrograms per kilogram of 

body weight per day for oral intake of methylmercury by humans (2001) 

and water- quality criterion of 0.3 ppm for methylmercury in fish (2001)

Mercury banned in interior latex paint (1990) / exterior (1991)

Food and Drug Administration establishes action level of 0.5 ppm for 

mercury in commercial seafood (1969)

Food and Drug Administration raises action level for mercury in 

commercial seafood to 1 ppm (1979)

Mercury emissions from municipal waste incineration limited (1995)

Mercury export from United States banned (2008)

USEPA issues Mercury and Air Toxics Standards for power plants (2012) 

National Environmental Policy Act (1969)

2015

At the Minamata Convention, more than 140 Nations agree to a treaty to 

reduce mercury emissions and releases to the atmosphere (2013)

EXPLANATION

Activities related primarily to water

Activities related primarily to air

General activities

tac11-0651_fig05-06 Relative fish methylmercury level

1965 1970 1975 1980 1985 1990

0

2

1

3

National Contaminant Biomonitoring Program 4

(NCBP) (Schmitt and others, 1999). For sites 

with downward trends, much of the decrease 

in fish methylmercury levels occurred during 

the 1970s, followed by relatively stable levels 

during the 1980s (fig. 5-6). The rate of decrease 

ranged from 3 to 12 percent annually. These 

decreasing fish methylmercury concentrations 

occurred in concert with implementation of 

legislation regulating contaminant discharges 

to air and water (fig. 5-7), and with elimination 

of many industrial uses of mercury. The 

pattern of predominantly decreasing trends for 

methylmercury in fish was similar to that for 

mercury in lake sediment cores for a similar time 

period (Mahler and others, 2006).

Figure 5-6. National 

Contaminant 

Biomonitoring 

Program (NCBP) 

sites indicate a rapid 

decrease in relative 

fish methylmercury 

levels during the 

1970s, followed by 

relatively stable 

levels in the 1980s. 

(Modified from 

Chalmers and others, 

2011; see appendix 1

for additional 

information.)

Figure 5-7. Decreasing methylmercury concentrations 

in fish across the United States during the 1970s and 1980s

coincide with major legislation regulating contaminant 

discharges to water and air, although changes in 

mercury emissions during that period are undocumented. 

Mercury emissions declined in Northeastern, 

Midwestern, and Southeastern States during the 1990s 

to early 2000s (Butler and others, 2008), when many 

mercury-specific regulations were implemented, but 

insufficient data are available to assess effects on 

methylmercury concentrations in fish.

Chapter 5

How Do Environmental Mercury Levels Vary Over Time? 61
For 1988–2005, consistent 

records for methylmercury in fish 

tissue were available for only 40 sites 

in Southeastern and Midwestern 

States; these sites were primarily 

sampled by various State resource 

agencies (Chalmers and others, 2011). 

Only 13 of 40 sites showed trends, 

and these were about equally split 

between downward (7) and upward 

(6) trends (fig. 5-5B). Elimination 

of nationally coordinated fish 

contaminant monitoring after 1987 

has resulted in insufficient data to 

analyze national trends since that 

time. The higher ratio of upward to 

downward trends for methylmercury 

in fish in the Southeast may be 

partly due to the greater importance 

of global mercury sources in the 

Southeast compared to the Northeast 

and Midwest (Seigneur and others, 

2004; Chalmers and others, 2011).

Changes in methylmercury 

concentrations in fish over the 

decadal time scales of the above 

studies may have affected human 

exposure through fish consumption 

in some areas. Of the 39 sites with 

detectable trends of methylmercury 

concentrations in fish (fig. 5-5A and 

5-5B), 6 sites had concentrations that 

started above the USEPA criterion of 

0.3 ppm for the protection of human 

health, but ended below the criterion; 

all were in the Northeast, Midwest, 

or West and were sampled during 

1969–87 (Chalmers and others, 2011). 

In the Southeast, on the other hand, 

three sites—all sampled in the 1988–

2005 period—had upward trends that 

started below the USEPA criterion but 

ended above the criterion. 

(Photograph by Mark E. Brigham.)

In contrast to the approximately equal numbers 

of upward and downward trends for other parts of the 

Southeast reported by Chalmers and others (2011), 

trends for mercury in largemouth bass for more than 60 

sites in the Florida Everglades were almost exclusively 

downward during approximately the same period 

(Atkeson and others, 2005; Axelrad and others, 2011). 

Moreover, a recent analysis of fish monitoring data from 

the Great Lakes region of the United States and Canada 

found that mercury concentrations in largemouth bass 

generally decreased during 1970–2009; however, walleye 

mercury concentrations decreased early in that period, 

then leveled off or began to increase during the 1990s 

(Monson and others, 2011). An explanation for this 

apparent inconsistency was not given, but it was noted 

that decreasing fish mercury levels would be expected 

given recent reductions in mercury emissions in North 

America and Europe. 

62 Mercury in the Nation’s Streams—Levels, Trends, and Implications
Insufficient data for 

 trend determination

Trends in mercury concentration 

in precipitation

No significant trend

Significant decreasing trend

EXPLANATION

Total mercury concentrations in 

precipitation decreased at about one-half of 

49 MDN sites across the United States and 

Canada during 1996–2005; no trends were found 

at the other one-half (fig. 5-8) (Prestbo and 

Gay, 2009). Decreasing trends were particularly 

evident in eastern North America.

Figure 5-8. Total mercury concentrations in precipitation decreased 

at about one-half of monitoring sites during 1996–2005. (Modified from 

Prestbo and Gay, 2009.)

A similar analysis of a slightly shorter time 

period (1998–2005) also found predominantly 

decreasing total mercury concentrations in 

precipitation at most MDN sites in the Eastern 

United States, especially in the Northeast 

and Midwest (Butler and others, 2008). 

Anthropogenic mercury emissions in the United 

States were decreasing during this same time 

period (Butler and others, 2008).

Mercury in Precipitation Has Typically Decreased during the 1990s and 2000s

Chapter 5

How Do Environmental Mercury Levels Vary Over Time? 63
Recent Trends in Environmental Mercury 

Concentrations Are Consistent with Legislation 

Designed to Limit Mercury Discharges

Table 5-1 summarizes how environmental mercury 

concentrations in the United States have changed over time 

based on the information presented in this chapter. Ice cores 

and lake sediment cores provide considerable evidence that 

environmental mercury levels began to rise above background 

levels during the mid- to late-1800s in concert with the 

industrialization of the Nation (fig. 5-1). Concentrations 

continued to increase until the 1970s or 1980s, when world 

Table 5-1. Trends in U.S. environmental mercury levels are consistent with trends in atmospheric source levels and with legislation 

designed to limit mercury discharged to air and water.

Preindustrial

(pre-1850s)

Industrial

(1850s–1970s)

Recent

(post-1970s)

Environmental Drivers

Natural background 

mercury concentrations 

resulted primarily from 

volcanic and geothermal 

emissions, and rock 

weathering.

Mercury concentrations increased 

due to increasing world 

production and to increasing 

anthropogenic emissions, 

including burning of coal for 

heat and electricity, waste 

incineration, and industrial uses.

U.S. and worldwide mercury 

production declined. U.S. and 

worldwide emissions of mercury 

to the atmosphere decreased. 

The United States implemented 

legislation to regulate discharges of 

mercury and other contaminants to 

air and water.

Environmental Mercury Responses

Ice Core Background concentrations 

averaged 3–4 parts per 

trillion.

Concentrations continued to 

rise above background levels.

Concentrations peaked at 20–25 parts 

per trillion in the mid-1980s, then 

decreased to about 10–15 parts per 

trillion during the late 1980s and 

early 1990s.

Lake Sediment Cores Background concentrations 

were low and stable.

Concentrations doubled in globally 

remote areas, increased by 3–5 

times in U.S. rural areas, and 

exhibited greater increases in 

large U.S. metropolitan areas. 

Concentrations generally 

leveled off or decreased.

Fish No data No data Concentrations decreased during 

the 1970s–80s, but showed no 

widespread trends during the 

1990s–2000s.

Precipitation No data No data Concentrations in the Eastern United 

States ranged from about 5 to 13 

parts per trillion and generally 

decreased during the 1990s–2000s.

production of mercury began to decrease, and when 

U.S. consumption and emissions of mercury began to 

decline in response to legislation intended to reduce 

uses of mercury and its discharge to air and water. Since 

then, the preponderance of monitoring data indicates that 

U.S. environmental mercury levels have generally been 

stable or decreasing. Based on the above associations 

of environmental mercury concentrations with mercury 

use and consumption statistics, it is likely that further 

reductions in mercury use would result in subsequent 

decreases in environmental mercury levels.

64 Mercury in the Nation’s Streams—Levels, Trends, and Implications
Coal-fired power plants are currently the largest source of mercury 

emissions in the United States. (Photograph by Mark E. Brigham.)

Anticipated Future Trends and Possible 

Confounding Factors

Many of the most acute sources of direct mercury 

discharges to our Nation’s waters have been identified 

and controlled since the late 1960s, leaving atmospheric 

emissions as the primary route of contamination of streams 

and lakes nationwide. Starting in the 1990s, mercury was 

effectively removed from many products and waste streams, 

resulting in substantial reductions in emissions of mercury 

to the atmosphere. As a result, mercury emissions in the 

United States decreased by nearly 60 percent (132 metric 

tons) between 1990 and 2005 (Schmeltz and others, 2011). 

Much of the decrease was due to reduced emissions from 

municipal and medical waste incineration, a result of 

legislation passed during 1995 and 1997 to limit mercury 

emissions from these sources (fig. 5-7).

Further reductions in mercury emissions to the 

atmosphere are anticipated as State, Federal, and 

international efforts to reduce mercury emissions are 

implemented. Mercury emissions in the State of Minnesota, 

for example, decreased by 71 percent from 1990 to 2005 

[see “The Minnesota Statewide Mercury Total Maximum 

Daily Load (TMDL),” p. 66]. However, in order to achieve 

desired reductions in fish mercury levels, anthropogenic 

emissions from outside Minnesota—both nationally 

and globally—must be curtailed by 93 percent. Recent 

Federal and international policies will help achieve 

desired reductions. Implementation of the new Mercury 

and Air Toxics Standards (U.S. Environmental Protection 

Agency, 2011b) will control mercury emissions from U.S. 

coal-fired power plants, currently the largest source of 

mercury emissions in the United States. Internationally, 

implementation of recent Minamata Convention goals will 

control mercury emissions from mining, coal combustion, 

cement production, and numerous other sources (Lubick and 

Malakoff, 2013; United Nations Environment Programme, 

2013b). As U.S. emissions are reduced, ecosystems within 

and close to major urban areas—which typically have large 

numbers of emission sources—will see greater reductions in 

mercury deposition than those in rural areas, where regional 

and global sources account for a greater proportion of total 

mercury deposition. 

Chapter 5

How Do Environmental Mercury Levels Vary Over Time? 65
EXPLANATION

Northeast (NE)

Southwest (SW)

By Edward B. Swain and Bruce A. Monson, Minnesota Pollution 

Control Agency

In response to widespread mercury contamination 

of fish, the Minnesota Pollution Control Agency 

(MPCA) began developing a statewide mercury 

TMDL in 2004—a TMDL that was officially approved 

by the USEPA in 2007. The MPCA proposed that 

the TMDL cover a broad geographic region because 

the mercury load to surface water, which was mostly 

delivered by the atmosphere, was not attributable 

to specific dischargers or even specific atmospheric 

emission sources. Instead, Minnesota’s TMDL 

presented evidence that atmospheric deposition of 

mercury was relatively uniform across the State, 

supplied more than 99.5 percent of the load to fishable 

waters, and that 90 percent of the atmospheric load 

originated outside Minnesota.

The statewide TMDL actually consists of two 

State separate TMDLs (fig. 5-9). Bioaccumulation of 

the atmospherically delivered mercury is more efficient 

in the northeast region of the State (NE, dominated by 

forests and wetlands), producing fish with significantly 

more mercury contamination, on average, than fish in 

the southwest region (SW, dominated by agriculture). 

Therefore, separate TMDLs were calculated for the 

two regions.

Minnesota’s TMDL goal is for mercury 

concentrations in fish fillets to not exceed 0.2 ppm. The 

needed reduction in mercury load is proportional to 

the necessary reduction in fish mercury concentration 

to achieve 0.2 ppm. Needed regional reductions from 

1990 baseline levels are 65 percent for the NE region 

and 51 percent for the SW region. Although reduction 

goals could have been set for each region, mercury 

source reductions are expected to result in relatively 

uniform deposition reductions across the State. The 

greater reduction needed (65 percent) was adopted 

as a statewide reduction goal, which simultaneously 

satisfied the 51-percent reduction goal for the 

SW region.

Of the mercury deposited in Minnesota, 

70 percent is a consequence of human activity and 

is, therefore, potentially controllable. To achieve 

a 65-percent reduction in deposition means that 

anthropogenic emissions must be reduced by much 

more than 65 percent. In fact, anthropogenic emissions 

that contribute to atmospheric loading in Minnesota 

must be reduced by 93 percent from the 1990 

baseline (65 percent divided by 70 percent). The 

waste load allocation in the TMDL requires total 

water point source loads to stay below 1 percent of 

the total mercury load to the State.

As of 2005, Minnesota had achieved a 

71-percent reduction in statewide mercury emissions 

since 1990. As of 2012, Minnesota’s emission 

reductions are on track to achieve the 93-percent 

reduction goal by 2025, as scheduled in Minnesota’s 

Implementation Plan. However, the goal of clean 

fish will not be met until all anthropogenic sources 

outside of Minnesota that contribute to deposition in 

the State also are reduced by 93 percent. Minnesota’s 

TMDL Implementation Plan is counting on Federal 

action to help achieve the needed reductions in 

national and international mercury emissions. The 

Federal government is pursuing emission reductions 

within the United States, and recently participated 

in the Minamata Convention on Mercury, an 

international treaty to reduce worldwide mercury 

emissions—reductions that are necessary to achieve 

the goals of Minnesota’s mercury TMDL.

More information on the Minnesota Statewide 

Mercury TMDL, including Minnesota’s plan to 

reduce mercury emissions by 93 percent, is available 

at http://www.pca.state.mn.us/wfhy9ef.

Figure 5-9. Separate mercury TMDLs have been 

developed for northeastern and southwestern 

Minnesota because the areas differ in their sensitivities 

to mercury bioaccumulation. (Modified from Minnesota 

Pollution Control Agency, 2007.)

The Minnesota Statewide Mercury Total Maximum Daily Load (TMDL)

66 Mercury in the Nation’s Streams—Levels, Trends, and Implications
Reductions in mercury emissions should 

result in decreased mercury levels in aquatic 

ecosystems, including fish. As described 

previously, concentrations of mercury in fish in 

streams generally have been declining in recent 

decades as a result of reductions in both mercury 

emissions to the atmosphere and direct discharge 

from industrial sources (Chalmers and others, 

2011). In addition, controlled experiments have 

shown a direct relationship between the amount 

of added inorganic mercury and the resulting 

methylmercury concentrations in lake water and 

aquatic biota (Orihel and others, 2006, 2007). 

A study in the Experimental Lakes Area 

of Canada—the Mercury Experiment to Assess 

Atmospheric Loadings in Canada and the United 

States (METAALICUS)—examined the fate of 

mercury added to the lake surface, an adjacent 

wetland, and the upland forested catchment. 

Traceable mercury, in the form of specific, stable 

(nonradioactive) isotopes was added each year 

from 2001 to 2007 by aerial application (Harris 

and others, 2007b; U.S. Geological Survey, 

2013a). Most of the mercury that was deposited 

onto the upland catchment accumulated in the 

plants and soil. Although runoff contained a 

substantial amount of mercury that had been 

deposited to the land surface prior to the study, 

less than 1 percent of the new mercury was 

transported to the lake in runoff (Harris and others, 

2007b). This finding showed that in upland, 

terrestrial environments, it takes longer than a 

decade for newly fallen mercury to be transported 

in runoff. In contrast, methylmercury levels in 

lake water and biota responded rapidly to mercury 

added directly to the lake surface during the first 

3 years of study (Harris and others, 2007b). 

The METAALICUS study has yielded 

several important lessons (Harris and others, 

2007b). Water bodies that receive most mercury 

inputs by direct deposition onto the water surface, 

such as lakes with relatively small watersheds, 

should respond within a few years to mercury 

emission reductions. In contrast, water bodies that 

receive most mercury inputs through runoff from 

the terrestrial environment, including streams 

and lakes with large watersheds relative to the 

lake surface area, could take as long as centuries 

to reach a new, lower mercury level in response 

to mercury emission reductions, due to the slow 

release of mercury from soil. Many lakes receive 

a mix of direct mercury deposition to the lake 

surface and mercury in runoff from the watershed. 

Fish methylmercury levels in these lakes should 

exhibit both a rapid decrease as a result of 

decreased deposition to the lake surface, and a 

slower response driven by slow equilibration of 

newly deposited mercury with mercury in runoff.

The United States Agrees to a Global Treaty to Reduce Mercury in 

the Environment

In January 2013, more than 140 United Nations member states, including the United 

States, agreed to a global, legally binding treaty to prevent emissions and releases of 

mercury to the atmosphere (Lubick and Malakoff, 2013; United Nations Environment 

Programme, 2013b). Known as the Minamata Convention on Mercury, the agreement 

provides controls and reductions across a wide range of products, processes, and industries 

where mercury is used, released, or emitted. 

The treaty will require actions to reduce mercury emissions to the air from power 

plants and other sources (industrial boilers, smelters, waste incinerators, and cement 

plants), reduce the use of mercury in a variety of industrial processes and products 

(batteries, switches, relays, fluorescent lamps, soaps, cosmetics, thermometers, and blood 

pressure devices), and address mercury supply and trade. The treaty also will require the 

development of strategies to reduce the amount of mercury used by artisanal and small-scale 

miners and to increase public awareness of the dangers inherent in this process.

Chapter 5

How Do Environmental Mercury Levels Vary Over Time? 67
The Mercury Experiment to Assess 

Atmospheric Loadings in Canada and the 

United States (METAALICUS) 

To directly test the response of fish mercury 

concentrations to changes in mercury deposition rate, 

an international team of scientists conducted a whole 

ecosystem experiment—known as METAALICUS—

at a remote lake in Canada starting in 1999 (Harris 

and others, 2007b). Different stable (nonradioactive) 

mercury isotopes were aerially applied to the 

lake surface, an adjacent wetland, and to the 

upland watershed. Isotopes allowed researchers to 

distinguish between experimentally applied mercury 

and mercury already present in the ecosystem. The 

METAALICUS study demonstrated several key 

findings that advanced our understanding of mercury 

cycling within watersheds. 

Fish methylmercury concentrations increased 

rapidly with increases in mercury deposition onto 

the lake surface over the first 3 years of study. In 

contrast, less than 1 percent of the mercury isotope 

deposited to the upland watershed was exported to 

the lake. Mercury entering the lake in runoff from 

the uplands was mostly “older” mercury—that 

is, mercury that had been deposited prior to the 

beginning of the experiment, aged in the upland 

ecosystem, and exported along with dissolved 

organic matter. 

The importance of mercury deposited 

to wetlands was strongly dependent on their 

hydrologic connectivity. In the METAALICUS 

study, the wetland surface was hydrologically 

disconnected from anoxic zones within the wetland. 

Thus, methylmercury formation and delivery to the 

lake from the wetland was minimal (Harris and others, 

2007b). Conversely, in wetlands with greater hydrologic 

connectivity, newly deposited mercury is more readily 

converted to methylmercury and more readily transported 

to adjacent lakes (Branfireun and others, 2005).

METAALICUS findings indicate that reductions 

in mercury emissions will yield rapid (on the order of 

years) reductions in fish methylmercury concentrations—

particularly in lakes that receive a significant portion 

of their mercury by deposition onto the lake surface 

(Harris and others, 2007b). However, a full response 

will be delayed by the gradual export of mercury stored 

in associated watersheds. In lakes with larger watershed 

areas relative to lake areas, responses may take longer. 

Traceable mercury isotopes were applied to the METAALICUS 

experimental watershed in Canada to distinguish added mercury 

from previously existing mercury. (Photograph by Vincent L. St. Louis, 

University of Alberta.)

Variability in ecosystem responses is expected, due 

to the numerous environmental and ecological factors that 

relate to mercury cycling and bioaccumulation; these factors 

vary both temporally and spatially. In addition to mercury 

inputs, disturbances, such as sulfate inputs, wetland 

restoration and water management, and climate change can 

have marked effects on methylmercury production within 

aquatic ecosystems. 

Sulfate Inputs.—Sulfate stimulates microbial 

methylmercury formation in low-sulfur environments, 

thus decreasing sulfate concentrations should result in 

lower fish methylmercury levels in many of the Nation’s 

waters. In the Florida Everglades, the USGS showed that 

agricultural drainage containing elevated sulfate—primarily 

from agricultural use—in combination with high regional 

atmospheric mercury deposition resulted in elevated 

methylmercury levels (Orem and others, 2011). Sulfur is widely 

used in agriculture, both as sulfate, and as elemental sulfur, 

which oxidizes to sulfate (Hagstrom, 1986). However, outside 

the Everglades, the role of agricultural sulfate in the mercury 

cycle has largely not been studied.

Atmospheric deposition also is an important source of 

sulfate—particularly for ecosystems that lack agricultural and 

other sources. In north-central Minnesota, sulfate deposition to a 

natural wetland was artificially increased for 4 years. The added 

sulfate markedly increased both methylmercury concentrations 

within the wetland and the export of methylmercury from the 

wetland to downstream waters (Jeremiason and others, 2006). 

68 Mercury in the Nation’s Streams—Levels, Trends, and Implications
2011). High methylmercury levels are produced by sulfatereducing bacteria, as flooded organic matter decays and 

sulfate is converted to sulfide. The sulfide can be re-oxidized 

to sulfate when the sediment dries, thus providing a renewed 

source of sulfate to fuel additional mercury methylation. 

Water management and hydrologic cycles associated with 

agricultural wetlands also have the potential to exacerbate 

mercury methylation (see “Wetland Restoration and Mercury 

in the San Francisco Bay and California’s Central Valley,” p. 71).

In the years following the experiment, after sulfate additions 

had ceased, methylmercury levels decreased (Coleman 

Wasik and others, 2012). 

Increased atmospheric sulfate deposition since preindustrial times likely has increased methylmercury 

concentrations in many locations more than would be 

expected based solely on anthropogenic mercury emissions. 

More recently, atmospheric sulfate deposition has been 

decreasing—by about 57 percent nationally between 1985 

and 2009—due to controls mandated as part of the Clean 

Air Act Amendment (Burns and others, 2011; Lehmann 

and Gay, 2011). Continued decreases in sulfate emissions 

and sulfate deposition are projected in coming decades, as 

further controls are implemented (Burns and others, 2011). 

These controls on sulfate should confer a benefit in the form 

of lower methylmercury production (Coleman Wasik and 

others, 2012) and, therefore, lower methylmercury levels 

in fish. For example, in a study of lakes within Isle Royale 

National Park, decreases in fish methylmercury levels were 

attributed to reductions in sulfate deposition in the absence 

of any decrease in mercury deposition (Drevnick and 

others, 2007).

Due to the complex interactions of mercury, sulfur, and 

natural organic matter, the side benefit of decreased sulfatedriven methylmercury formation may not be realized in all 

locations. Dissolved organic matter concentrations have been 

increasing in many acid-sensitive aquatic ecosystems, partly 

as a response to decreased sulfate deposition (Clark and 

others, 2010). In some locations, the increased mobilization 

of dissolved organic matter has concomitantly increased 

mercury concentrations in runoff (Hongve and others, 2012). 

However, other locations where dissolved organic matter has 

increased have not had increases in mercury in runoff (Eklöf 

and others, 2012). 

Wetland Restoration and Water Management.—The 

potential for mercury methylation should be considered 

among the many ecological and water-quality characteristics 

that may be altered by wetland restoration and watermanagement activities. Restoration of wetlands is a major 

thrust in many parts of the Nation due to the loss of about 

53 percent of the original wetlands in the lower 48 States 

(Dahl, 1990). 

Changes in water management, such as areal extent and 

frequency of inundation, also can exacerbate methylmercury 

formation. Recently flooded terrestrial soil (in newly 

constructed reservoirs and wetlands) and repeated cycles 

of wetting and drying of sediment (in reservoirs managed 

for flood control) are known to exacerbate mercury 

methylation (Bodaly and others, 1997, 2007; Snodgrass and 

others, 2000; Brigham and others, 2002; Hall and others, 

2004, 2005; St. Louis and others, 2004; Orem and others, 

Elevated methylmercury levels are commonly associated with 

newly constructed impoundments and with large waterlevel fluctuations in impoundments. (Photograph by Mark E. 

Brigham.)

Salt evaporation ponds (top) in South San Francisco Bay, 

California, are being restored to functional wetlands (bottom). 

(Photographs by Mark Marvin-DiPasquale.)

Chapter 5

How Do Environmental Mercury Levels Vary Over Time? 69
Climate Change.—Changes in hydrology due to 

climate change have the potential to change methylmercury 

production in aquatic ecosystems. For example, precipitation 

patterns in many regions are predicted to shift to a 

regime of greater extremes in response to climate change 

(Intergovernmental Panel on Climate Change, 2007). 

Increased precipitation intensity and frequency of storm 

events are likely to lead to increased mercury inputs to 

aquatic systems (Krabbenhoft and Sunderland, 2013). Also, 

extreme dry periods punctuated by large rainfall events will 

likely increase methylmercury production in many settings 

(Monson, 2009; Orem and others, 2011).

Are Current Monitoring and Assessment 

Programs Sufficient to Evaluate Future 

Mercury Trends?

As the amount and global distribution of mercury 

emissions to the atmosphere change, effective monitoring 

of mercury emissions, deposition, and environmental levels 

is needed to understand how aquatic ecosystems respond to 

the changes. Global emissions of mercury to the atmosphere 

seem to have peaked in the 1970s, and worldwide 

anthropogenic mercury emissions have been relatively 

stable since at least 1990 (United Nations Environment 

Programme, 2013a). Although emissions in Asia rose 

steadily from 1990 to 2005 due to increased coal burning, 

these increases were offset by decreases in North America 

and Europe. However, recent evidence suggests that mercury 

emissions from industrial sectors (coal combustion, and 

production of cement, pig iron and steel, and nonferrous 

metals) may be increasing.

Estimates of mercury emissions have improved over 

time, but some sources still remain poorly characterized. 

This results in considerable uncertainty in estimates of total 

anthropogenic mercury emissions to the atmosphere. For 

example, the 2010 estimate for worldwide anthropogenic 

mercury emissions is 1,960 metric tons, but the estimated 

range for this estimate is quite large (1,010–4,070 metric 

tons) (United Nations Environment Programme, 2013a). 

The Mercury Deposition Network (MDN) provides a 

useful framework for monitoring mercury in precipitation 

across the Nation. This program provides standardized 

equipment and procedures to participating agencies, 

based on a specific agency’s needs and available funding. 

However, because participation in the MDN is voluntary, 

some parts of the country are under-represented. 

Furthermore, the collaborator funding model used by the 

MDN has resulted in some long-term sites being discontinued 

in response to changing priorities or budget shortfalls. As 

sites are discontinued, the ability to assess spatial or temporal 

trends in mercury deposition is compromised.

Dry deposition of mercury accounts for a large portion of 

total atmospheric mercury deposition, yet our understanding 

of geographic and time trends for this component is hampered 

by a lack of standardized methods and limited data. In 2009, 

the MDN established the Atmospheric Mercury Network 

(AMNet) to help fill this data need (see “Monitoring and 

Modeling Help to Assess Mercury Deposition in the United 

States,” p. 30); however, this relatively new network is small 

with substantial gaps in geographic coverage. 

Data suitable for assessing long-term trends in fish 

mercury concentrations have not been collected across the 

United States since the mid-1980s, when the NCBP was 

discontinued. The USEPA currently analyzes mercury in fish 

tissue and bed sediment on a 5-year cycle as part of their 

National Aquatic Resource Surveys for lakes, streams, coastal 

waters, and wetlands. In addition, State agencies monitor 

fish mercury levels, mostly in support of fish consumption 

advisories. However, none of these efforts is both (1) 

national in scope and (2) comprised of consistently and 

routinely sampled locations—two characteristics essential for 

adequately determining long-term trends. 

Continued monitoring and assessment are important 

to determine how changing mercury emissions to the 

atmosphere ultimately impact mercury levels in aquatic 

ecosystems. Currently, there are no nationally consistent, 

continuing programs to assess mercury concentrations in 

U.S. aquatic ecosystems over time. A national monitoring 

and research plan to assess ecosystem responses to mercury 

emission reductions has been proposed (Mason and others, 

2005; Harris and others, 2007a; Negra and Lambert, 2009; 

Schmeltz and others, 2011), but has not been implemented. 

A consistent, stable, mercury monitoring program would 

provide important information on the efficacy of reductions in 

mercury emissions, whereas the lack of such a program makes 

addressing key policy questions at the national level difficult.

70 Mercury in the Nation’s Streams—Levels, Trends, and Implications
American Avocet (top) and White-faced Ibis (bottom) are 

wading birds commonly found in wetland areas, such as 

the Merced National Wildlife Refuge, California. (© Donald 

Quintana, used with permission.)

Wetland Restoration and Mercury in 

the San Francisco Bay and California’s 

Central Valley

By Mark Marvin-DiPasquale, U.S. Geological Survey

Draining approximately 40 percent of 

the State of California, the San Francisco Bay 

watershed—including California’s Central 

Valley—represents the largest estuarine 

ecosystem on the West Coast of the Americas. 

The Central Valley is part of the Pacific Flyway, 

a major route for migratory birds of western 

North America. From the 1850s through the 

1980s, approximately 90 percent of the wetlands 

in the region were lost (Garone, 2011). In the 

Central Valley, wetlands were drained and 

converted to cropland; along the San Francisco 

Bay, wetland habitats were lost largely to 

urbanization. However, in recent decades there 

has been a sustained effort to restore wetlands, 

with numerous projects ongoing or planned 

throughout both the saline and freshwater 

portions of the system. While this is a positive 

development for critical wildlife habitat, 

wetlands also are effective environments for 

converting inorganic mercury to methylmercury 

(Zillioux and others, 1993; Marvin-DiPasquale 

and others, 2003), which can bioaccumulate to 

toxic levels. As a consequence, concerns have 

been raised that increased wetland area may 

inadvertently exacerbate mercury contamination 

of local food webs. 

In addition to atmospheric mercury 

deposition from contemporary regional 

and global sources, the San Francisco Bay 

watershed contains a large amount of mercury 

contamination from historical gold and mercury 

mining. Nearly 6 million metric tons of mercury 

were released in the mining areas of California 

during the gold rush era (Alpers and others, 

2005). Much of the mercury lost during this 

period still remains in the original mining areas, 

and is transported to the Central Valley and San 

Francisco Bay Delta during high flow events 

(Domagalski, 1998; Domagalski and others, 

2004; Springborn and others, 2011). Such 

historical mercury inputs may be responsible 

for the generation of new methylmercury in 

downstream reservoirs and the Delta (Fleck 

and others, 2011; Marvin-DiPasquale and 

others, 2011). Major urban areas, including San 

Francisco, San Jose, and Sacramento, also release 

mercury from numerous industrial sources and 

products. To reduce mercury contamination in 

fish and wildlife in the San Francisco Bay and 

its watershed, the California State Water Board 

is developing TMDL targets for both the estuary 

(Central Valley Regional Water Quality Control 

Board, 2006) and the Central Valley, including 

the Sacramento-San Joaquin Delta (Central 

Valley Regional Water Quality Control Board, 

2011).

Chapter 5

How Do Environmental Mercury Levels Vary Over Time? 71
Rice fields in the Yolo Bypass Wildlife Area, California, are flooded 

each fall, and this may increase production of methylmercury. (Top

photograph by Mark Marvin-DiPasquale; bottom photograph by 

© Michael Layefsky, used with permission)

The South Bay Salt Pond Restoration Project, one 

of the largest in the Nation, is converting more than 

65 square kilometers of former salt production ponds 

to either managed ponds or tidal wetlands (South Bay 

Salt Pond Restoration Project, 2013). Because runoff 

from the largest historical mercury mining area in 

North America (New Almaden) drains right through 

the restoration area, mercury monitoring and research 

are a primary component of the restoration project 

(Ackerman and others, 2013). Results from these studies 

indicate that fish-eating bird species are at particular risk 

from mercury contamination in this region of the Bay 

(Ackerman and others, 2010), and that across the greater 

South San Francisco Bay there is a correlation between 

the percentage of total mercury as methylmercury in 

wetland sediment and the amount of mercury in the 

blood of marsh song sparrows (Grenier and others, 2010).

The salt pond restoration efforts also are changing 

the hydrology of one of the largest and most mercury 

contaminated slough channels in South San Francisco 

Bay. Of an estimated 1,650 kilograms of mercury 

currently buried in the top 2 meters of slough sediment, 

about 125 kilograms of mercury are expected to be 

mobilized by changes in hydrology associated with 

the restoration (Marvin-DiPasquale and Cox, 2007). 

Although, it is currently unclear exactly where this 

sediment associated mercury will be deposited, one 

recent study concluded that the degree of methylmercury 

production should actually decrease in the most 

contaminated salt pond once it is returned to full tidal 

flushing (Grenier and others, 2010), a transition currently 

scheduled for 2015.

Another wetland environment that has received 

increased attention with respect to mercury cycling 

and bioaccumulation are agricultural wetlands—

specifically rice growing areas of the Central Valley. A 

recent research effort in this environmental setting has 

resulted in a re-evaluation of the importance of these 

managed wetland areas with respect to methylmercury 

production, bioaccumulation, and export in the context 

of the San Francisco Bay Delta mercury TMDL process. 

One important finding from this work is that resident 

biota within these agricultural wetlands can readily 

bioaccumulate mercury, and that most of the export 

of methylmercury from these areas to the larger Delta 

region takes place during the high precipitation events 

that occur in winter. The extent of methylmercury 

production also appears to be linked to how rice straw 

is managed during the post-harvest period. For 

example, flooding the rice field (post-harvest) for the 

purpose of rotting the rice straw during the fall/winter 

period may increase methylmercury production 

(Marvin-DiPasquale and others, 2013; WindhamMyers and others, 2013). A project description and 

list of publications is provided in U.S. Geological 

Survey (2013b).

72 Mercury in the Nation’s Streams—Levels, Trends, and Implications
References Cited

Ackerman, J.T., Eagles-Smith, C.A., Herzog, M.P., 

Herring, G., Strong, C., and Mruz, E., 2010, Response 

of waterbird breeding effort, nest success, and mercury 

concentrations in eggs and fish to altered salt pond water 

management—Data summary: U.S. Geological Survey, 

Western Ecological Research Center, Davis, Calif., 28 p.

Ackerman, J.T., Marvin-DiPasquale, Mark, Slotton, Darrell, 

Eagles-Smith, C.A., Herzog, M.P., Hartman, C.A., Agee, 

J.L., and Ayers, Shaun, 2013, The South Bay Mercury 

Project—Using biosentinels to monitor effects of wetland 

restoration for the South Bay Salt Pond Restoration Project: 

U.S. Geological Survey, Report prepared for the South 

Bay Salt Pond Restoration Project and Resources Legacy 

Fund, 227 p., accessed September 18, 2013, at http://www.

southbayrestoration.org/documents/technical.

Acosta-Ruiz, Gildardo, and Powers, Bill, 2001, Preliminary 

atmospheric emissions inventory of mercury in Mexico: 

Sonora, Mexico, Acosta and Associates, Prepared for 

Commission for Environmental Cooperation, 14 p., 

accessed September 18, 2013, at http://www.epa.gov/

ttnchie1/conference/ei12/mexico/acosta-ruiz.pdf.

Alpers, C.N., and Hunerlach, M.P., 2000, Mercury 

contamination from historic gold mining in California: U.S. 

Geological Survey Fact Sheet FS-061-00, 6 p., http://pubs.

er.usgs.gov/publication/fs06100.

Alpers, C.N., Hunerlach, M.P., May, J.T., and Hothem, R.L., 

2005, Mercury contamination from historical gold mining in 

California: U.S. Geological Survey Fact Sheet 2005-3014, 

Version 1.1, 6 p., http://pubs.usgs.gov/fs/2005/3014.

Arctic Monitoring Assessment Programme/United Nations 

Environment Programme, 2008, Technical background 

report to the global atmospheric mercury assessment: 

Geneva, Switzerland, Arctic Monitoring and Assessment 

Programme, United Nations Environment Programme 

Chemicals Branch, 159 p.

Atkeson, T.D., Pollman, C.D., and Axelrad, D.M., 2005, 

Recent trends in mercury emissions, deposition, and biota 

in the Florida Everglades—A monitoring and modeling 

analysis, in Pirrone, N., and Mahaffey, K.R., eds., Dynamics 

of mercury pollution on regional and global scales—

Atmospheric processes and human exposures around the 

world: New York, Springer, p. 637–655.

Axelrad, D.M., Lange, T., Gabriel, M.C., Aiken, G.R., 

Brandon, A., Cunningham, M.W., DeBusk, T., Dierberg, F., 

Donner, B.A., Frederick, P., Gilmour, C., Harris, R., Jansen, 

D., Krabbenhoft, D.P., McCray, J.M., Orem, W.H., Oronato, 

D.P., Pollman, C.D., Rumbold, D.G., White, G., Wright, 

A.L., and Ye, R., 2011, Mercury and sulfur monitoring, 

research and environmental assessment in South Florida, 

2011 South Florida environmental report: West Palm Beach, 

Florida, South Florida Water Management District and 

Florida Department of Environmental Protection, p. 53.

Babiarz, C., Hoffmann, S., Wieben, A., Hurley, J., Andren, A., 

Shafer, M., and Armstrong, D., 2012, Watershed and 

discharge influences on the phase distribution and tributary 

loading of total mercury and methylmercury into Lake 

Superior: Environmental Pollution, v. 161, p. 299–310.

Bailey, E.H., Clark, A.L., and Smith, R.M., 1973, Mercury: 

U.S. Geological Survey Professional Paper 820, p. 401–414, 

http://pubs.er.usgs.gov/publication/pp820.

Balogh, S.J., Meyer, M.L., and Johnson, D.K., 1998, Transport 

of mercury in three contrasting river basins: Environmental 

Science and Technology, v. 32, no. 4, p. 456–462.

Balogh, S.J., Nollet, Y.H., and Offerman, H.J., 2005, A 

comparison of total mercury and methylmercury export 

from various Minnesota watersheds: Science of the Total 

Environment, v. 340, p. 261–270.

Balogh, S.J., Swain, E.B., and Nollet, Y.H., 2008, 

Characteristics of mercury speciation in Minnesota rivers 

and streams: Environmental Pollution, v. 154, no. 1, 

p. 3–11.

Bauch, N.J., Chasar, L.C., Scudder, B.C., Moran, P.W., Hitt, 

K.J., Brigham, M.E., Lutz, M.A., and Wentz, D.A., 2009, 

Data on mercury in water, streambed sediment, and fish 

from selected streams across the United States, 1998–2005: 

U.S. Geological Survey Data Series 307, 32 p., http://pubs.

usgs.gov/ds/307.

Bell, A.H., and Scudder, B.C., 2007, Mercury accumulation 

in periphyton of eight river ecosystems: Journal of the 

American Water Resources Association, v. 43, no. 4, 

p. 1–12.

Bellinger, D.C., Trachtenberg, F., Barregard, L., Tavares, 

M., Cernichiari, E., Daniel, D., and McKinlay, S., 2006, 

Neuropsychological and renal effects of dental amalgam 

in children—A randomized clinical trial: The Journal of 

the American Medical Association, v. 295, no. 15, 

p. 1,775–1,783.

References Cited 73
Berry, L.G., and Mason, B.H., 1959, Mineralogy—Concepts, 

descriptions, determinations: San Francisco, Calif., W.H. 

Freeman and Company, A Series of books in geology, 630 p.

Bindler, R., Olofsson, C., Renberg, I., and Frech, W., 2001, 

Temporal trends in mercury accumulation in lake sediments 

in Sweden: Water Air and Soil Pollution Focus, p. 343–355.

Bishop, K.H., and Lee, Y.-H., 1997, Catchments as a source of 

mercury/methylmercury in boreal surface waters, in Sigel, 

A., and Sigel, H., eds., Metal ions in biological systems: 

New York, Marcel Dekker, p. 113–130.

Bloom, N.S., 1992, On the chemical form of mercury in edible 

fish and marine invertebrate tissue: Canadian Journal of 

Fisheries and Aquatic Sciences, v. 49, no. 5, p. 1,010–1,017.

Bodaly, R.A., Jansen, W.A., Majewski, A.R., Fudge, R.J., 

Strange, N.E., Derksen, A.J., and Green, D.J., 2007, 

Postimpoundment time course of increased mercury 

concentrations in fish in hydroelectric reservoirs of 

northern Manitoba, Canada: Archives of Environmental 

Contamination and Toxicology, v. 53, no. 3, p. 379–389.

Bodaly, R.A., St. Louis, V.L., Paterson, M.J., Fudge, R.J.P., 

Hall, B.D., Rosenberg, D.M., and Rudd, J.W.M., 1997, 

Bioaccumulation of mercury in the aquatic food chain in 

newly flooded areas, in Sigel, A., and Sigel, H., eds., Metal 

ions in biological systems: New York, Marcel Decker, Inc., 

p. 259–287.

Booz Allen & Hamilton, Inc., 2002, Multi-state inventory of 

gold ore processing facilities: Tysons Corner, Va., Booz 

Allen & Hamilton, Inc., Document Control No. REPA2-

09920-004.

Borum, D., Manibusan, M.K., Schoeny, R., and Winchester, 

E.L., 2001, Water quality criterion for the protection of 

human health—Methylmercury: U.S. Environmental 

Protection Agency, EPA-823-R-01-001, 306 p.

Branfireun, B.A., Krabbenhoft, D.P., Hintelmann, H., Hunt, 

R.J., Hurley, J.P., and Rudd, J.W.M., 2005, Speciation and 

transport of newly deposited mercury in a boreal forest 

wetland—A stable mercury isotope approach: Water 

Resources Research, v. 41, no. W06016, p. 1–11.

Brigham, M.E., Duris, J.W., Wentz, D.A., Button, D.T., and 

Chasar, L.C., 2008, Total mercury, methylmercury, and 

ancillary water-quality and streamflow data for selected 

streams in Oregon, Wisconsin, and Florida, 2002–06: U.S. 

Geological Survey Data Series 341, 11 p., http://pubs.usgs.

gov/ds/341.

Brigham, M.E., Krabbenhoft, D.P., Olson, M.L., and DeWild, 

J.F., 2002, Methylmercury in flood-control impoundments 

and natural waters of Northwestern Minnesota, 1997–99: 

Water, Air, and Soil Pollution, v. 138, no. 1/4, p. 61–78.

Brigham, M.E., Wentz, D.A., Aiken, G.R., and Krabbenhoft, 

D.P., 2009, Mercury cycling in stream ecosystems. 1. Water 

column chemistry and transport: Environmental Science and 

Technology, v. 43, no. 8, p. 2,720–2,725.

Brumbaugh, W.G., Krabbenhoft, D.P., Helsel, D.R., 

Wiener, J.G., and Echols, K.R., 2001, A national pilot 

study of mercury contamination of aquatic ecosystems 

along multiple gradients—Bioaccumulation in fish: 

U.S. Geological Survey, Biological Resources Division, 

Biological Science Report USGS/BRD/BSR—2001-0009, 

25 p., http://pubs.er.usgs.gov/publication/bsr20010009.

Bullock, O.R., and Brehme, K.A., 2002, Atmospheric 

mercury simulation using the CMAQ model—Formulation 

description and analysis of wet deposition results: 

Atmospheric Environment, v. 36, no. 13, p. 2,135–2,146.

Burns, D.A., Lynch, J.A., Cosby, B.J., Fenn, M.E., Baron, 

J.S., and U.S. Environmental Protection Agency Clean 

Air Markets Division, 2011, National Acid Precipitation 

Assessment Program report to Congress 2011—An 

Integrated Assessment: National Science and Technology 

Council, 114 p.

Butler, T.J., Cohen, M.D., Vermeylen, F.M., Likens, G.E., 

Schmeltz, D., and Artz, R.S., 2008, Regional precipitation 

mercury trends in the eastern USA, 1998–2005—Declines 

in the Northeast and Midwest, no trend in the Southeast: 

Atmospheric Environment, v. 42, no. 7, p. 1,582–1,592.

Central Valley Regional Water Quality Control Board, 2006, 

Mercury in San Francisco Bay, Proposed basin plan 

amendment and staff report for revised total maximum 

daily load (TMDL) and proposed mercury water quality 

objectives: California Regional Water Quality Control 

Board, San Francisco Bay Region, 116 p., accessed 

September 18, 2013, at http://www.waterboards.ca.gov/

rwqcb2/water_issues/programs/TMDLs/sfbaymercurytmdl.

shtml.

Central Valley Regional Water Quality Control Board, 2011, 

The Water Quality Control Plan (Basin Plan) for the 

California Regional Water Quality Control Board, Central 

Valley Region, The Sacramento River Basin and the San 

Joaquin River Basin, fourth edition, revised October 2011: 

Central Valley Regional Water Quality Control Board, 

148 p., accessed September 18, 2013, at http://www.

waterboards.ca.gov/rwqcb5/water_issues/basin_plans/sacsjr.

pdf.

74 Mercury in the Nation’s Streams—Levels, Trends, and Implications
Chalmers, A.T., Argue, D.M., Gay, D.A., Brigham, M.E., 

Schmitt, C.J., and Lorenz, D.L., 2011, Mercury trends in 

fish from rivers and lakes in the United States, 1969–2005: 

Environmental Monitoring and Assessment, v. 175, no. 1, 

p. 175–191.

Chasar, L.C., Scudder, B.C., Bell, A.H., Wentz, D.A., and 

Brigham, M.E., 2008, Total mercury, methylmercury, 

and carbon and nitrogen stable isotope data for biota 

from selected streams in Oregon, Wisconsin, and Florida, 

2002–04: U.S. Geological Survey Data Series 349, 10 p., 

http://pubs.usgs.gov/ds/349.

Chasar, L.C., Scudder, B.C., Stewart, A.R., Bell, A.H., and 

Aiken, G.R., 2009, Mercury cycling in stream ecosystems. 

3. Trophic dynamics and methylmercury bioaccumulation: 

Environmental Science and Technology, v. 43, no. 8, p. 

2,733–2,739.

Choi, A.L., Budtz-Jorgensen, E., Jorgensen, P.J., Steuerwald, 

U., Debes, F., Weihe, P., and Grandjean, P., 2008, 

Selenium as a potential protective factor against mercury 

developmental neurotoxicity: Environmental Research, 

v. 107, p. 45–52.

Choi, A.L., and Grandjean, P., 2008, Methylmercury exposure 

and health effects in humans: Environmental Chemistry, 

v. 5, no. 2, p. 112–120.

Clark, J.M., Bottrell, S.H., Evans, C.D., Monteith, D.T., 

Bartlett, R., Rose, R., Newton, R.J., and Chapman, P.J., 

2010, The importance of the relationship between scale and 

process in understanding long-term DOC dynamics: Science 

of the Total Environment, v. 408, no. 13, p. 2,768–2,775.

Clarkson, T.W., Magos, L., and Myers, G.J., 2003, Human 

exposure to mercury–The three modern dilemmas: Journal 

of Trace Elements in Experimental Medicine, v. 16, no. 4, 

p. 321–344.

Cleckner, L.B., Garrison, P.J., Hurley, J.P., Olson, M.L., 

and Krabbenhoft, D.P., 1998, Trophic transfer of 

methyl mercury in the northern Florida Everglades: 

Biogeochemistry, v. 40, no. 2/3, p. 347–361.

Coleman Wasik, J.K., Mitchell, C.P.J., Engstrom, D.R., 

Swain, E.B., Monson, B.A., Balogh, S.J., Jeremiason, 

J.D., Branfireun, B.A., Eggert, S.L., Kolka, R.K., and 

Almendinger, J.E., 2012, Methylmercury declines in a 

boreal peatland when experimental sulfate deposition 

decreases: Environmental Science and Technology, v. 46, 

no. 12, p. 6,663–6,671.

Compeau, G.C., and Bartha, R., 1985, Sulfate-reducing 

bacteria—Principal methylators of mercury in anoxic 

estuarine sediment: Applied and Environmental 

Microbiology, v. 50, no. 2, p. 498–502.

Crump, K.L., and Trudeau, V.L., 2009, Mercury-induced 

reproductive impairment in fish: Environmental Toxicology 

and Chemistry, v. 28, no. 5, p. 895–907.

Dahl, T.E, 1990, Wetlands losses in the United States, 1780’s 

to 1980’s: Washington, D.C., U.S. Department of the 

Interior, Fish and Wildlife Service, 13 p.

Dahl, T.E., 2011, Status and trends of wetlands in the 

conterminous United States, 2004 to 2009: U.S. Fish and 

Wildlife Service Report to Congress, 107 p., accessed 

September 18, 2013, at http://www.fws.gov/wetlands/

Status-And-Trends-2009/. 

Daly, C., Halbleib, M., Smith, J.I., Gibson, W.P., Doggett, 

M.K., Taylor, G.H., Curtis, J., and Pasteris, P.P., 2008, 

Physiographically sensitive mapping of climatological 

temperature and precipitation across the conterminous 

United States: International Journal of Climatology, v. 28, 

no. 15, p. 2,031–2,064.

Depew, D.C., Basu, N., Burgess, N.M., Campbell, L.M., 

Devlin, E.W., Drevnick, P.E., Hammerschmidt, C.R., 

Murphy, C.A., Sandheinrich, M.B., and Wiener, J.G., 2012a, 

Toxicity of dietary methylmercury to fish—Derivation 

of ecologically meaningful threshold concentrations: 

Environmental Toxicology and Chemistry, v. 31, no. 7, 

p. 1,536–1,547.

Depew, D.C., Basu, N., Burgess, N.M., Campbell, L.M., 

Evers, D.C., Grasman, K.A., and Scheuhammer, A.M., 

2012b, Derivation of screening benchmarks for dietary 

methylmercury exposure for the common loon (Gavia 

immer)—Rationale for use in ecological risk assessment: 

Environmental Toxicology and Chemistry, v. 31, no. 10, 

p. 2,399–2,407.

Dillon, T., Beckvar, N., and Kern, J., 2010, Residue-based 

mercury dose-response in fish—An analysis using lethalityequivalent test endpoints: Environmental Toxicology and 

Chemistry, v. 29, no. 11, p. 2,559–2,565.

Domagalski, J., 1998, Occurrence and transport of total 

mercury and methyl mercury in the Sacramento River 

Basin, California: Journal of Geochemical Exploration, 

v. 64, p. 277–291.

References Cited 75
Domagalski, J.L., Alpers, C.N., Slotton, D.G., Suchanek, 

T.H., and Ayers, S.M., 2004, Mercury and methylmercury 

concentrations and loads in the Cache Creek watershed, 

California: Science of the Total Environment, v. 327, 

no. 1–3, p. 215–237.

Drevnick, P.E., Canfield, D.E., Gorski, P.R., Shinneman, 

A.L.C., Engstrom, D.E., Muir, D.C.G., Smith, G.R., 

Garrison, P.J., Cleckner, L.B., Hurley, J.P., Noble, R.B., 

Otter, R.R., and Oris, J.T., 2007, Deposition and cycling 

of sulfur controls mercury accumulation in Isle Royale 

fish: Environmental Science and Technology, v. 41, 

p. 7,266–7,272.

Drevnick, P.E., Engstrom, D.R., Driscoll, C.T., Swain, E.B., 

Balogh, S.J., Kamman, N.C., Long, D.T., Muir, D.G.C., 

Parsons, M.J., Rolfhus, K.R., and Rossmann, R., 2012, 

Spatial and temporal patterns of mercury accumulation in 

lacustrine sediments across the Laurentian Great Lakes 

region: Environmental Pollution, v. 161, p. 252–260.

Duffus, J.H., Nordberg, Monica, and Templeton, D. M., 2007, 

IUPAC glossary of terms used in toxicology (2nd ed.): Pure 

and Applied Chemistry, v. 79, no. 7, p. 1,153–1,344.

Dunne, Thomas, and Leopold, L.B., 1978, Water in 

environmental planning: San Francisco, W.H. Freeman and 

Company, 818 p.

Eagles-Smith, C.A., Ackerman, J.T., Yee, J., and Adelsbach, 

T.L., 2009, Mercury demethylation in waterbird livers—

Dose-response thresholds and differences among species: 

Environmental Toxicology and Chemistry, v. 28, 

p. 568–577.

Edmonds, S.T., O’Driscoll, N.J., Hillier, N.K., Atwood, J.L., 

and Evers, D.C., 2012, Factors regulating the bioavailability 

of methylmercury to breeding rusty blackbirds in 

northeastern wetlands: Environmental Pollution, v. 171, 

p. 148–154.

Eklöf, K., Fölster, J., Sonesten, L., and Bishop, K., 2012, 

Spatial and temporal variation of THg concentrations in 

run-off water from 19 boreal catchments, 2000–2010: 

Environmental Pollution, v. 164, p. 102–109.

Engle, M.A., Tate, M.T., Krabbenhoft, D.P., Schauer, J.J., 

Kolker, A., Shanley, J.B., and Bothner, M.H., 2010, 

Comparison of atmospheric mercury speciation and 

deposition at nine sites across central and eastern North 

America: Journal of Geophysical Research—Atmospheres, 

v. 115, no. D18, p. D18306.

Engstrom, D.R., Balogh, S.J., and Swain, E.B., 2007, 

History of mercury inputs to Minnesota lakes—Influences 

of watershed disturbance and localized atmospheric 

deposition: Limnology and Oceanography, v. 52, p. 2,467–

2,483.

Engstrom, D.R., and Swain, E.B., 1997, Recent declines in 

atmospheric mercury deposition in the Upper Midwest: 

Environmental Science and Technology, v. 31, no. 4, 

p. 960–967.

Environment Canada, 2005, National pollutant release 

inventory, 2005 data: Government of Canada, accessed June 

21, 2013, at http://www.ec.gc.ca/inrp-npri.

Evers, D.C., 2005, Mercury connections—The extent and 

effects of mercury pollution in northeastern North America: 

Ecotoxicology, v. 14, variously paged.

Evers, D.C., Jackson, A.K., Tear, T.H., and Osborne, C.E., 

2012, Hidden risk—Mercury in terrestrial ecosystems of the 

Northeast: Gorham, Maine, Biodiversity Research Institute, 

33 p.

Evers, D.C., Wiener, J.G., Driscoll, C.T., Gay, D.A., Basu, 

N., Monson, B.A., Lambert, K.F., Morrison, H.A., Morgan, 

J.T., Williams, K.A., and Soehl, A.G., 2011, Great Lakes 

mercury connections—The extent and effects of mercury 

pollution in the Great Lakes region: Biodiversity Research 

Institute BRI 2011-18, 44 p.

Fitzgerald, W.F., Engstrom, D.R., Lamborg, C.H., Tseng, 

C.-M., Balcom, P.H., and Hammerschmidt, C.R., 2005, 

Modern and historic atmospheric mercury fluxes in 

northern Alaska—Global sources and Arctic depletion: 

Environmental Science and Technology, v. 39, p. 557–568.

Fitzgerald, W.F., Engstrom, D.R., Mason, R.P., and 

Nater, E.A., 1998, The case for atmospheric mercury 

contamination in remote areas: Environmental Science and 

Technology, v. 32, no. 1, p. 1–7.

Fleck, J.A., Alpers, C.N., Marvin-DiPasquale, M., Hothem, 

R.L., Wright, S.A., Ellett, K., Beaulieu, E., Agee, J.L., 

Kakouros, E., Kieu, L.H., Eberl, D.D., Blum, A.E., and 

May, J.T., 2011, The effects of sediment and mercury 

mobilization in the South Yuba River and Humbug 

Creek Confluence Area, Nevada County, California—

Concentrations, speciation, and environmental fate, Part 

1—Field characterization: U.S. Geological Survey OpenFile Report 2010-1325A, 104 p., http://pubs.usgs.gov/

of/2010/1325A.

76 Mercury in the Nation’s Streams—Levels, Trends, and Implications
Fulkerson, M., and Nnadi, F.N., 2006, Predicting mercury wet 

deposition in Florida—A simple approach: Atmospheric 

Environment, v. 40, p. 3,962–3,968. 

Garone, P., 2011, The fall and rise of wetlands of California’s 

Great Central Valley: Berkeley, University of California 

Press, Stephen Bechtel Fund Book on Ecology and the 

Environment, 448 p.

Gary, Margaret, McAfee, Robert, Jr., and Wolf, C.L., eds., 

1972, Glossary of geology: Washington, D.C., American 

Geological Institute, 805 p.

Gilmour, C.C., Henry, E.A., and Mitchell, R., 1992, Sulfate 

stimulation of mercury methylation in freshwater sediments: 

Environmental Science and Technology, v. 26, no. 11, 

p. 2,281–2,287.

Goldstein, R.M., Brigham, M.E., Stuewe, L., and Menheer, 

M.A., 2003, Mercury data from small lakes in Voyageurs 

National Park, northern Minnesota, 2000–02: U.S. 

Geological Survey Open-File Report 03-480, 18 p., http://

pubs.er.usgs.gov/publication/ofr2003480.

Gough, L.P., Asher-Bolinder, S., Bales, C.A., Runberg, 

S.K., and Wells, R.K., 1993, Understanding our fragile 

environment: U.S. Geological Survey Circular 1105, 34 p., 

http://pubs.er.usgs.gov/publication/cir1105.

Gray, J.E., 2003a, Introduction, in Gray, J.E., ed., Geologic 

studies of mercury by the U.S. Geological Survey: U.S. 

Geological Survey Circular 1248, p. 1–7, http://pubs.usgs.

gov/circ/2003/c1248.

Gray, J.E., 2003b, Summary, in Gray, J.E., ed., Geologic 

studies of mercury by the U.S. Geological Survey: U.S. 

Geological Survey Circular 1248, p. 39–40, http://pubs.

usgs.gov/circ/2003/c1248.

Gray, J.E., Theodorakos, P.M., Bailey, E.A., and Turner, R.R., 

2000, Distribution, speciation, and transport of mercury 

in stream-sediment, stream-water, and fish collected 

near abandoned mercury mines in southwestern Alaska, 

USA: Science of the Total Environment, v. 260, nos. 1–3, 

p. 21–33.

Grenier, L., Marvin-DiPasquale, M., Drury, D., Hunt, J., 

Robinson, A., Bezalel, S., Melwani, A., Agee, J., Kakouros, 

E., Kieu, L., Windham-Myers, L., and Collins, J., 2010, 

South Baylands Mercury Project: San Francisco Estuary 

Institute, U.S. Geological Survey, and Santa Clara Valley 

Water District, 97 p. 

Grigal, D.F., 2002, Inputs and outputs of mercury from 

terrestrial watersheds—A review: Environmental Reviews, 

v. 10, no. 1, p. 1–39.

Gustavsson, N., Bølviken, B., Smith, D.B., and Severson, 

R.C., 2001, Geochemical landscapes of the conterminous 

United States—New map presentations for 22 elements: 

U.S. Geological Survey Professional Paper 1648, 38 p., 

http://pubs.er.usgs.gov/publication/pp1648. 

Hagstrom, G.R., 1986, Fertilizer sources of sulfur and their 

use, in Tabatabai, M.A., ed., Sulfur in Agriculture: Madison, 

Wis., American Society of Agronomy, Inc., Crop Science 

Society of America, Inc., and Soil Science Society of 

America, Inc., chap. 20, p. 567–581.

Hall, B.D., Aiken, G.R., Krabbenhoft, D.P., MarvinDiPasquale, M., and Swarzenski, C.M., 2008, Wetlands as 

principal zones of methylmercury production in southern 

Louisiana and the Gulf of Mexico region: Environmental 

Pollution, v. 154, no. 1, p. 124–134.

Hall, B.D., St. Louis, V.L., Rolfhus, K.R., Bodaly, R.A., Beaty, 

K.G., Paterson, M.J., and Cherewyk, K.A., 2005, Impacts 

of reservoir creation on the biogeochemical cycling of 

methyl mercury and total mercury in boreal upland forests: 

Ecosystems, v. 8, p. 248–266.

Hall, B.D., St. Louis, V.L., and Bodaly, R.A., 2004, The 

stimulation of methylmercury production by decomposition 

of flooded birch leaves and jack pine needles: 

Biogeochemistry, v. 68, no. 1, p. 107–129.

Harada, Masazumi, 1995, Minamata disease: Methylmercury 

poisoning in Japan caused by environmental pollution: 

Critical Reviews in Toxicology, v. 25, no. 1, p. 1-24.

Harris, R., Krabbenhoft, D.P., Mason, R., Murray, M.W., 

Reash, R., and Saltman, T., 2007a, Ecosystem responses to 

mercury contamination–Indicators of change: Boca Raton, 

Florida, CRC Press, 216 p.

Harris, R.C., Rudd, J.W.M., Amyot, M., Babiarz, C.L., Beaty, 

K.G., Blanchfield, P.J., Bodaly, R.A., Branfireun, B.A., 

Gilmour, C.C., Graydon, J.A., Heyes, A., Hintelmann, H., 

Hurley, J.P., Kelly, C.A., Krabbenhoft, D.P., Lindberg, S.E., 

Mason, R.P., Paterson, M.J., Podemski, C.L., Robinson, A., 

Sandilands, K.A., Southworth, G.R., St. Louis, V.L., and 

Tate, M.T., 2007b, Whole-ecosystem study shows rapid 

fish-mercury response to changes in mercury deposition: 

Proceedings of the National Academy of Sciences, v. 104, 

no. 42, p. 16,586–16,591.

Hartman, J.S., Weisberg, P.J., Pillai, R., Ericksen, J.A., 

Kuiken, T., Lindberg, S.E., Zhang, H., Rytuba, J.J., and 

Gustin, M.S., 2009, Application of a rule-based model to 

estimate mercury exchange for three background biomes in 

the continental United States: Environmental Science and 

Technology, v. 43, no. 13, p. 4,989–4,994.

References Cited 77
Heinz, G.H., Hoffman, D.J., Klimstra, J.D., and Stebbins, 

K.R., 2009a, Rapid increases in mercury concentrations in 

the eggs of mallards fed methylmercury: Environmental 

Toxicology and Chemistry, v. 28, no. 9, p. 1,979–1,981.

Heinz, G.H., Hoffman, D.J., Klimstra, J.D., Stebbins, K.R., 

Kondrad, S.L., and Erwin, C.A., 2009b, Species differences 

in the sensitivity of avian embryos to methylmercury: 

Archives of Environmental Contamination and Toxicology, 

v. 56, no. 1, p. 129–138.

Hill, J.M., 1926, California gold production, 1849–1923: 

Society of Economic Geologists, Inc., v. 21, p. 172–179.

Hongve, D., Haaland, S., Riise, G., Blakar, I., and Norton, S., 

2012, Decline of acid rain enhances mercury concentrations 

in fish: Environmental Science and Technology, v. 46, no. 5, 

p. 2,490–2,491.

Horowitz, A.J., and Stephens, V.C., 2008, The effects of land 

use on fluvial sediment chemistry for the conterminous 

U.S.—Results from the first cycle of the NAWQA 

Program—Trace and major elements, phosphorus, carbon, 

and sulfur: Science of the Total Environment, v. 400, 

nos. 1–3, p. 290–314.

Hunerlach, M.P., and Alpers, C.N., 2003, Studies of mercury 

contamination from gold mining–Mercury contamination 

from hydraulic gold mining in the Sierra Nevada, 

California, in Gray, J.E., ed., Geologic studies of mercury 

by the U.S. Geological Survey: U.S. Geological Survey 

Circular 1248, p. 23–27, http://pubs.usgs.gov/circ/2003/

c1248.

Hurley, J.P., Benoit, J.M., Shafer, M.M., Andren, A.W., 

Sullivan, J.R., Hammond, R., and Webb, D.A., 1995, 

Influences of watershed characteristics on mercury levels in 

Wisconsin rivers: Environmental Science and Technology, 

v. 29, no. 7, p. 1,867–1,875.

Intergovernmental Panel on Climate Change, 2007, Section 

10.3.6.1—Precipitation Extremes, in Contribution of 

Working Group I to the Fourth Assessment Report of the 

Intergovernmental Panel on Climate Change—The Physical 

Science Basis: Intergovernmental Panel on Climate Change, 

accessed September 18, 2013, at http://www.ipcc.ch/

publications_and_data/ar4/wg1/en/ch10s10-3-6-1.html.

Jeremiason, J.D., Engstrom, D.R., Swain, E.B., Nater, 

E.A., Johnson, B.M., Almendinger, J.E., Monson, 

B.A., and Kolka, R.K., 2006, Sulfate addition increases 

methylmercury production in an experimental wetland: 

Environmental Science and Technology, v. 40, 

p. 3,800–3,806.

Johansson, K., 1985, Mercury in sediment in Swedish forest 

lakes: Verhandlungen Internationale Vereinigung für 

Theoretische und Angewandte Limnologie, v. 22, 

p. 2,359–2,363.

Journey, C.A., Burns, D.A., Riva-Murray, K., Brigham, M.E., 

Button, D.T., Feaster, T.D., Petkewich, M.D., and Bradley, 

P.M., 2012, Fluvial transport of mercury, organic carbon, 

suspended sediment, and selected major ions in contrasting 

stream basins in South Carolina and New York, October 

2004–September 2009: U.S. Geological Survey Scientific 

Investigations Report 2012-5173, 125 p., http://pubs.er.usgs.

gov/publication/sir20125173.

Kolker, A., Engle, M.A., Krabbenhoft, D.P., and Olson, 

M.L., 2007, Investigating atmospheric mercury with the 

U.S. Geological Survey mobile mercury laboratory: U.S. 

Geological Survey Fact Sheet 2007-3071, http://pubs.usgs.

gov/fs/2007/3071.

Krabbenhoft, D., and Shuster, P., 2002, Glacial ice cores reveal 

a record of natural and anthropogenic atmospheric mercury 

deposition for the last 270 years: U.S. Geological Survey 

FS-051-02, 2 p., http://toxics.usgs.gov/pubs/FS-051-02.

Krabbenhoft, D.P., and Babiarz, C.L., 1992, The role of 

groundwater transport in aquatic mercury cycling: Water 

Resources Research, v. 28, no. 12, p. 3,119–3,128.

Krabbenhoft, D.P., Branfireun, B.A., and Heyes, A., 2005, 

Biogeochemical cycles affecting the speciation, fate, and 

transport of mercury in the environment, in Parsons, M.B., 

and Percival, J.B., eds., Mercury—Sources, Measurements, 

Cycles, and Effects: Quebec, Mineralogical Society of 

Canada, p. 297.

Krabbenhoft, D.P., and Sunderland, E.M., 2013, Global 

change and mercury: Science, v. 341, p. 1,457–1,458.

Krabbenhoft, D.P., Wiener, J.G., Brumbaugh, W.G., Olson, 

M.L., DeWild, J.F., and Sabin, T.J., 1999, A National pilot 

study of mercury contamination of aquatic ecosystems 

along multiple gradients, in Morganwalp, D.W., and 

Buxton, H.T., eds., U.S. Geological Survey Toxic 

Substances Hydrology Program—Proceedings of the 

Technical Meeting, Charleston, South Carolina, March 

8-12, 1999—Volume 2 of 3—Contamination of Hydrologic 

Systems and Related Ecosystems: U.S. Geological Survey 

Water-Resources Investigations Report 99-4018-B, 

p. 147–160, http://toxics.usgs.gov/pubs/wri99-4018/

Volume2/sectionB/2301_Krabbenhoft/index.html.

78 Mercury in the Nation’s Streams—Levels, Trends, and Implications
Kuiken, T., Gustin, M., Zhang, H., Lindberg, S., and Sedinger, 

B., 2008, Mercury emission from terrestrial background 

surfaces in the eastern USA, II—Air/surface exchange 

of mercury within forests from South Carolina to New 

England: Applied Geochemistry, v. 23, no. 3, p. 356–368.

Kuwabara, J.S., Arai, Yuji, Topping, B.R., Pickering, I.J., and 

George, G.N., 2007, Mercury speciation in piscivorous fish 

from mining-impacted reservoirs: Environmental Science 

and Technology, v. 41, no. 8, p. 2,745–2,749.

Kuwabara, J.S., Topping, B.R., Moon, G.E., Husby, Peter, 

Lincoff, Andrew, Carter, J.L., and Croteau, M-N., 

2005, Mercury accumulation by lower trophic-level 

organisms in lentic systems within the Guadalupe River 

watershed, California: U.S. Geological Survey Scientific 

Investigations Report 2005-5037, 52 p., http://pubs.usgs.

gov/sir/2005/5037.

Langbein, W.B., and Iseri, K.T., 1960, Manual of hydrology, 

Part I, General surface-water techniques—General 

introduction and hydrologic definitions: U.S. Geological 

Survey Water-Supply Paper 1541–A, 29 p., http://pubs.

er.usgs.gov/publication/wsp1541A.

Latysh, N.E., and Wetherbee, G.A., 2012, Improved mapping 

of National Atmospheric Deposition Program wetdeposition in complex terrain using PRISM-gridded data 

sets: Environmental Monitoring and Assessment, v. 184, 

p. 913–928.

Lawrence, S.J., 2003, Mercury in the Carson River Basin, 

Nevada, in Gray, J.E., ed., Geologic studies of mercury 

by the U.S. Geological Survey: U.S. Geological Survey 

Circular 1248, p. 29–33, http://pubs.usgs.gov/circ/2003/

c1248.

Lehmann, C.M.B., and Gay, D.A., 2011, Monitoring long-term 

trends of acidic wet deposition in US precipitation—Results 

from the National Atmospheric Deposition Program: Power 

Plant Chemistry, v. 13, no. 7, p. 378–385.

Lewis, M.E., and Brigham, M.E., 2004, Low-level mercury: 

U.S. Geological Survey Techniques of Water-Resource 

Investigation, book 9, chap. A5, section 5.6.4.B, 26 p., 

http://water.usgs.gov/owq/FieldManual/index.html.

Lin, C.-J., and Pehkonen, S.O., 1999, The chemistry 

of atmospheric mercury—A review: Atmospheric 

Environment, v. 33, no. 13, p. 2,067–2,079.

Lin, C.-J., Pongprueksa, P., Bullock, O.R., Jr., Lindberg, 

S.E., Pehkonen, S.O., Jang, C., Braverman, T., and Ho, 

T.C., 2007, Scientific uncertainties in atmospheric mercury 

models II—Sensitivity analysis in the CONUS domain: 

Atmospheric Environment, v. 41, no. 31, p. 6,544–6,560.

Lockhart, W.L., Wilkinson, P., Billeck, B.N., Hunt, R.V., 

Wagemann, R., and Brunskill, G.J., 1995, Current and 

historical inputs of mercury to high-latitude lakes in Canada 

and to Hudson Bay: Water, Air, and Soil Pollution, v. 80, 

nos. 1–4, p. 603–610.

Lorey, P., and Driscoll, C.T., 1999, Historical trends of 

mercury deposition in Adirondack lakes: Environmental 

Science and Technology, v. 33, no. 5, p. 718–722.

Lubick, Naomi, and Malakoff, David, 2013, With pact’s 

completion, the real work begins: Science, v. 341, 

p. 1,443–1,445.

Lynch, M.L., Huang, L.-S., Cox, C., Strain, J.J., Myers, G.J., 

Bonham, M.P., Shamlaye, C.F., Stokes-Riner, A., Wallace, 

J.M.W., Duffy, E.M., Clarkson, T.W., and Davidson, P.W., 

2011, Varying coefficient function models to explore 

interactions between maternal nutritional status and 

prenatal methylmercury toxicity in the Seychelles Child 

Development Nutrition Study: Environmental Research, 

v. 111, no. 1, p. 75–80.

Mahaffey, K.R., Clickner, R.P., and Jeffries, R.A., 2009, Adult 

women’s blood mercury concentrations vary regionally 

in the Unites States: association with patterns of fish 

consumption (NHANES 1999-2004): Environmental Health 

Perspectives, v. 117, no. 1, p. 47–53.

Mahaffey, K.R., Sunderland, E.M., Chan, H.M., Choi, 

A.L., Grandjean, P., Mariën, K., Oken, E., Sakamoto, 

M., Schoeny, R., Weihe, P., Yan, C.-H., and Yasutake, A., 

2011, Balancing the benefits of n-3 polyunsaturated fatty 

acids and the risks of methylmercury exposure from fish 

consumption: Nutrition Reviews, v. 69, no. 9, p. 493–508.

Mahler, B.J., Van Metre, P.C., and Callender, E., 2006, Trends 

in metals in urban and reference lake sediments across the 

United States, 1970 to 2001: Environmental Toxicology and 

Chemistry, v. 25, no. 7, p. 1,698–1,709.

Marvin-DiPasquale, M., Agee, J.L., Kakouros, E., Kieu, 

L.H., Fleck, J.A., and Alpers, C.N., 2011, The effects of 

sediment and mercury mobilization in the South Yuba 

River and Humbug Creek confluence area, Nevada County, 

California—Concentrations, speciation and environmental 

fate—Part 2— Laboratory Experiments: U.S. Geological 

Survey Open-File Report 2010−1325B, 53 p., http://pubs.

usgs.gov/of/2010/1325B.

Marvin-DiPasquale, M., and Cox, M.H., 2007, Legacy 

mercury in Alviso Slough, South San Francisco Bay, 

California—Concentration, speciation and mobility: U.S. 

Geological Survey Open-File Report 2007-1240, 99 p., 

http://pubs.usgs.gov/of/2007/1240/.

References Cited 79
Marvin-DiPasquale, M., Windham-Myers, L., Agee, J.L., 

Kakouros, E., Kieu, L.H., Fleck, J., Alpers, C.N., and 

Stricker, C., 2013, Methylmercury production in sediment 

from agricultural and non-agricultural wetlands in the Yolo 

Bypass, California: Science of the Total Environment, 

accessed November 1, 2013, at http://www.sciencedirect.

com/science/article/pii/S0048969713011364.

Marvin-DiPasquale, M., Agee, J.L., Bouse, R.M., and Jaffe, 

B.E., 2003, Microbial cycling of mercury in contaminated 

pelagic and wetland sediments of San Pablo Bay, California: 

Environmental Geology, v. 43, no. 3, p. 260–267.

Mason, R.P., Abbott, M.L., Bodaly, R.A., Bullock, O.R., Jr., 

Driscoll, C.T., Evers, D., E., L.S., Murray, M., and Swain, 

E.B., 2005, Monitoring the response to changing mercury 

deposition: Environmental Science and Technology, v. 39, 

no. 1, p. 14A-22A.

Mason, R.P., Fitzgerald, W.F., and Morel, F.M.M., 1994, 

The biogeochemical cycling of elemental mercury—

Anthropogenic influences: Geochimica et Cosmochimica 

Acta, v. 58, no. 15, p. 3,191–3,198.

Mast, M.A., Manthorne, D.J., and Roth, D.A., 2010, Historical 

deposition of mercury and selected trace elements to highelevation National Parks in the Western U.S. inferred from 

lake-sediment cores: Atmospheric Environment, v. 44, 

nos. 21–22, p. 2,577–2,586.

May, J.T., Hothem, R.L., Alpers, C.N., and Law, M.A., 2000, 

Mercury bioaccumulation in fish in a region affected 

by historic gold mining—The South Yuba River, Deer 

Creek, and Bear River watersheds, California, 1999: 

U.S. Geological Survey Open-File Report 00-367, 30 p., 

accessed September 20, 2013, at http://ca.water.usgs.gov/

archive/reports/ofr00367/.

Mergler, D., Anderson, H.A., Chan, L.H.M., Mahaffey, 

K.R., Murray, M., Sakamoto, M., and Stern, A.H., 2007, 

Methylmercury exposure and health effects in humans—A 

worldwide concern: Ambio, v. 36, no. 1, p. 3–11.

Minnesota Pollution Control Agency, 2007, Minnesota 

statewide mercury total maximum daily load: Minnesota 

Pollution Control Agency, March 2007, 56 p., accessed 

September 19, 2013, at http://www.pca.state.mn.us/index.

php/view-document.html?gid=8507. 

Monson, B., Staples, D., Bhavsar, S., Holsen, T., Schrank, 

C., Moses, S., McGoldrick, D., Backus, S., and Williams, 

K., 2011, Spatiotemporal trends of mercury in walleye and 

largemouth bass from the Laurentian Great Lakes Region: 

Ecotoxicology, v. 20, no. 7, p. 1,555–1,567.

Monson, B.A., 2009, Trend reversal of mercury concentrations 

in piscivorous fish from Minnesota lakes—1982–2006: 

Environmental Science and Technology, v. 43, no. 6, 

p. 1,750–1,755. 

Mozaffarian, D., and Rimm, E.B., 2006, Fish intake, 

contaminants, and human health—Evaluating the risks 

and the benefits: The Journal of the American Medical 

Association, v. 296, no. 15, p. 1,885–1,899.

Mumley, T.E., and Abu-Saba, K.E., 2002, The view from 

downstream—A TMDL strategy to attain water quality 

standards for mercury in San Francisco Bay: Proceedings of 

the Water Environment Federation, National TMDL Science 

and Policy 2002 Specialty Conference, p. 533–555.

Nater, E.A., and Grigal, D.F., 1992, Regional trends in 

mercury distribution across the Great Lakes States, north 

central USA: Nature, v. 358, p. 139–141.

National Research Council, 2000, Toxicological effects of 

methylmercury: Washington, D.C., National Academy 

Press, 344 p.

Negra, C., and Lambert, K.F., 2009, MercNet—Establishing a 

comprehensive national mercury monitoring network: U.S. 

Environmental Protection Agency, EPA 430-K-09-001, 

35 p.

Orem, W., Gilmour, C., Axelrad, D., Krabbenhoft, D., Scheidt, 

D., Kalla, P., McCormick, P., Gabriel, M., and Aiken, G., 

2011, Sulfur in the south Florida ecosystem—Distribution, 

sources, biogeochemistry, impacts, and management for 

restoration: Critical Reviews in Environmental Science and 

Technology, v. 41, no. 6, supp. 1, p. 249–288.

Orihel, D.M., Paterson, M.J., Blanchfield, P.J., Bodaly, R.A., 

and Hintelmann, H., 2007, Experimental evidence of a 

linear relationship between inorganic mercury loading 

and methylmercury accumulation by aquatic biota: 

Environmental Science and Technology, v. 41, no. 14, 

p. 4,952–4,958.

Orihel, D.M., Paterson, M.J., Gilmour, C.C., Bodaly, R.A., 

Blanchfield, P.J., Hintelmann, H., Harris, R.C., and 

Rudd, J.W.M., 2006, Effect of loading rate on the fate of 

mercury in littoral mesocosms: Environmental Science and 

Technology, v. 40, p. 5,992–6,000.

Parker, R.L., 1967, Composition of the Earth’s crust: U.S. 

Geological Survey Geological Survey Professional Paper 

440-D, 19 p., http://pubs.er.usgs.gov/publication/pp440D. 

Prestbo, E.M., and Gay, D.A., 2009, Wet deposition of 

mercury in the U.S. and Canada, 1996–2005—Results and 

analysis of the NADP mercury deposition network (MDN): 

Atmospheric Environment, v. 43, no. 27, p. 4,223–4,233.

80 Mercury in the Nation’s Streams—Levels, Trends, and Implications
Reid, G.K., and Wood, R.D., 1976, Ecology of inland waters 

and estuaries (2nd ed.): New York, D. Van Nostrand 

Company, 485 p.

Reimann, C., and Garrett, R.G., 2005, Geochemical 

background—Concept and reality: Science of the Total 

Environment, v. 350, nos. 1–3, p. 12–27.

Riscassi, A.L., and Scanlon, T.M., 2011, Controls on stream 

water dissolved mercury in three mid-Appalachian forested 

headwater catchments: Water Resources Research, v. 47, 

no. 12, p. W12512.

Risch, M.R., DeWild, J.F., Krabbenhoft, D.P., Kolka, R.K., 

and Zhang, L., 2011, Litterfall mercury dry deposition in the 

eastern USA: Environmental Pollution, v. 161, p. 284–290.

Riva-Murray, K., Chasar, L.C., Bradley, P.M., Burns, D.A., 

Brigham, M.E., Smith, M.J., and Abrahamsen, T.A., 2011, 

Spatial patterns of mercury in macroinvertebrates and fishes 

from streams of two contrasting forested landscapes in the 

eastern United States: Ecotoxicology, v. 20, p. 1,530–1,542.

Rudd, J.W.M., Turner, M.A., Furutani, I., Swick, A.L., 

and Townsend, B.E., 1983, The English-Wabigoon 

River system—I. A synthesis of recent research with a 

view towards mercury amelioration: Canadian Journal 

of Fisheries and Aquatic Sciences, v. 40, no. 12, 

p. 2,206–2,217.

Rudnick, R.L., and Gao, S., 2003, Composition of the 

continental crust, in Heinrich, D.H., and Karl, K.T., eds., 

Treatise on geochemistry: Oxford, Pergamon, p. 1–64.

Rytuba, J.J., 2003, Mercury mine studies—Environmental 

impact of mercury mines in the Coast Ranges, California, 

in Gray, J.E., ed., Geologic studies of mercury by the U.S. 

Geological Survey: U.S. Geological Survey Circular 1248, 

p. 13–17, http://pubs.usgs.gov/circ/2003/c1248.

Sandheinrich, M.B., and Wiener, J.G., 2011, Methylmercury 

in freshwater fish—Recent advances in assessing toxicity 

of environmentally relevant exposures, in Beyer, W.N., and 

Meador, J.P., eds., Environmental contaminants in biota—

Interpreting tissue concentrations (2nd ed.): Boca Raton, 

Florida, Taylor and Francis, p. 169–190.

Schelker, J., Burns, D.A., Weiler, M., and Laudon, H., 2011, 

Hydrological mobilization of mercury and dissolved 

organic carbon in a snow-dominated, forested watershed—

Conceptualization and modeling: Journal of Geophysical 

Research—Biogeosciences, v. 116, no. G1, p. G01002.

Scheuhammer, A.M., Basu, N., Burgess, N.M., Elliott, 

J.E., Campbell, G.D., Wayland, M., Champoux, L., and 

Rodrigue, J., 2008, Relationships among mercury, selenium, 

and neurochemical parameters in common loons (Gavia 

immer) and bald eagles (Haliaeetus leucocephalus): 

Ecotoxicology, v. 17, p. 93–101.

Scheuhammer, A.M., Basu, N., Evers, D.C., Heinz, G.H., 

Sandheinrich, M.B., and Bank, M.S., 2012, Ecotoxicology 

of mercury in fish and wildlife—Recent advances, in

Bank, M.S., ed., Mercury in the environment—Pattern 

and process: Berkeley, California, University of California 

Press, chap. 11, p. 223–238.

Scheuhammer, A.M., Meyer, M.W., Sandheinrich, M.B., 

and Murray, M.W., 2007, Effects of environmental 

methylmercury on the health of wild birds, mammals, and 

fish: Ambio, v. 36, no. 1, p. 12–18.

Schmeltz, D., Evers, D., Driscoll, C., Artz, R., Cohen, M., 

Gay, D., Haeuber, R., Krabbenhoft, D., Mason, R., Morris, 

K., and Wiener, J., 2011, MercNet—A national monitoring 

network to assess responses to changing mercury 

emissions in the United States: Ecotoxicology, v. 20, no. 7, 

p. 1,713–1,725.

Schmitt, C.J., Zajicek, J.L., May, T.W., and Cowman, D.F., 

1999, Organochlorine residues and elemental contaminants 

in U.S. freshwater fish, 1976–1986—National Contaminant 

Biomonitoring Program, in Ware, G.W., Nigg, H.N., 

and Doerge, D.R., eds., Reviews of environmental 

contamination and toxicology: New York, Springer-Verlag, 

p. 43–104.

Schroeder, W.H., and Munthe, J., 1998, Atmospheric 

mercury—An overview: Atmospheric Environment, v. 32, 

no. 5, p. 809–822.

Schuster, P.F., Krabbenhoft, D.P., Naftz, D.L., Cecil, L.D., 

Olson, M.L., DeWild, J.F., Susong, D.D., Green, J.R., 

and Abbott, M.L., 2002, Atmospheric mercury deposition 

during the last 270 years—A glacial ice core record of 

natural and anthropogenic sources: Environmental Science 

and Technology, v. 36, no. 11, p. 2,303–2,310.

Scott, W.B., and Crossman, E.J., 1973, Freshwater fishes of 

Canada—Bulletin 184: Ottawa, Fisheries Research Board of 

Canada Bulletin, v. 184, 966 p.

Scudder, B.C., Chasar, L.C., Wentz, D.A., Bauch, N.J., 

Brigham, M.E., Moran, P.W., and Krabbenhoft, D.P., 2009, 

Mercury in fish, bed sediment, and water from streams 

across the United States, 1998–2005: U.S. Geological 

Survey Scientific Investigations Report 2009-5109, 74 p., 

http://pubs.usgs.gov/sir/2009/5109.

References Cited 81
Seigneur, C., Lohman, K., Vijayaraghavan, K., Jansen, J., and 

Levin, L., 2006, Modeling atmospheric mercury deposition 

in the vicinity of power plants: Journal of the Air and Waste 

Management Association, v. 56, no. 6, p. 743–751.

Seigneur, C., Vijayaraghavan, K., Lohman, K., 

Karamchandani, P., and Scott, C., 2004, Global source 

attribution for mercury deposition in the United States: 

Environmental Science and Technology, v. 38, p. 555–569.

Selin, N.E., 2009, Global biogeochemical cycling of 

mercury—A review: Annual Review of Environment and 

Resources, v. 34, no. 1, p. 43–63.

Selin, N.E., Jacob, D.J., Park, R.J., Yantosca, R.M., Strode, 

S., Jaeglé, L., and Jaffe, D., 2007, Chemical cycling and 

deposition of atmospheric mercury—Global constraints 

from observations: Journal of Geophysical Research, v. 112, 

no. D2, p. D02308.

Selin, N.E., Jacob, D.J., Yantosca, R.M., Strode, S., Jaeglé, 

L., and Sunderland, E.M., 2008, Global 3-D land-oceanatmosphere model for mercury—Present-day versus 

preindustrial cycles and anthropogenic enrichment factors 

for deposition: Global Biogeochemical Cycles, v. 22, no. 2, 

p. GB2011. 

Shacklette, H.T., and Boerngen, J.G., 1984, Element 

concentrations in soils and other surficial materials of 

the conterminous United States: U.S. Geological Survey 

Survey Professional Paper 1270, 63 p., http://pubs.usgs.gov/

pp/1270.

Shanley, J.B., Mast, M.A., Campbell, D.H., Aiken, G.R., 

Krabbenhoft, D.P., Hunt, R.J., Walker, J.F., Schuster, 

P.F., Chalmers, A., Aulenbach, B.T., Peters, N.E., 

Marvin-DiPasquale, M., Clow, D.W., and Shafer, M.M., 

2008, Comparison of total mercury and methylmercury 

cycling at five sites using the small watershed approach: 

Environmental Pollution, v. 154, p. 143–154.

Sigler, J.M., Lee, X., and Munger, W., 2003, Emission and 

long-range transport of gaseous mercury from a large-scale 

Canadian boreal forest fire: Environmental Science and 

Technology, v. 37, no. 19, p. 4,343–4,347.

Snodgrass, J.W., Jagoe, C.H., Bryan, A.L., Jr., Brant, H.A., 

and Burger, J., 2000, Effects of trophic status and wetland 

morphology, hydroperiod, and water chemistry on mercury 

concentrations in fish: Canadian Journal of Fisheries and 

Aquatic Sciences, v. 57, no. 1, p. 171–180.

South Bay Salt Pond Restoration Project, 2013, The largest 

tidal wetland restoration project on the West Coast: 

Website, accessed November 30, 2013, at http://www.

southbayrestoration.org/. 

Springborn, M., Singer, M.B., and Dunne, T., 2011, Sedimentadsorbed total mercury flux through Yolo Bypass, the 

primary floodway and wetland in the Sacramento Valley, 

California: Science of the Total Environment, v. 412–413, 

p. 203–213.

St. Louis, V.L., Rudd, J.W.M., Kelly, C.A., Beaty, K.G., 

Bloom, N.S., and Flett, R.J., 1994, Importance of wetlands 

as sources of methyl mercury to boreal forest ecosystems: 

Canadian Journal of Fisheries and Aquatic Sciences, v. 51, 

p. 1,065–1,076.

St. Louis, V.L., Rudd, J.W.M., Kelly, C.A., Bodaly, R.A., 

Paterson, M.J., Beaty, K.G., Hesslein, R.H., Heyes, A., 

and Majewski, A.R., 2004, The rise and fall of mercury 

methylation in an experimental reservoir: Environmental 

Science and Technology, v. 38, p. 1,348–1,358.

St. Louis, V.L., Rudd, J.W.M., Kelly, C.A., Hall, B.D., 

Rolfhus, K.R., Scott, K.J., Lindberg, S.E., and Dong, 

W., 2001, Importance of the forest canopy to fluxes of 

methyl mercury and total mercury to boreal ecosystems: 

Environmental Science and Technology, v. 35, no. 15, 

p. 3,089–3,098.

Stahl, L.L., Snyder, B.D., Olsen, A.R., and Pitt, J.L., 

2009, Contaminants in fish tissue from US lakes and 

reservoirs—A national probabilistic study: Environmental 

Monitoring and Assessment, v. 150, p. 3–19.

Strain, J.J., Davidson, P.W., Bonham, M.P., Duffy, E.M., 

Stokes-Riner, A., Thurston, S.W., Wallace, J.M.W., Robson, 

P.J., Shamlaye, C.F., Georger, L.A., Sloane-Reeves, J., 

Cernichiari, E., Canfield, R.L., Cox, C., Huang, L.S., 

Janciuras, J., Myers, G.J., and Clarkson, T.W., 2008, 

Associations of maternal long-chain polyunsaturated 

fatty acids, methyl mercury, and infant development 

in the Seychelles Child Development Nutrition Study: 

Neurotoxicology, v. 29, no. 5, p. 776–782.

Sunderland, E.M., 2007, Mercury exposure from domestic 

and imported estuarine and marine fish in the U.S. seafood 

market: Environmental Health Perspectives, v. 115, no. 2, 

p. 235–242.

Swain, E.B., Engstrom, D.R., Brigham, M.E., Henning, T.A., 

and Brezonik, P.L., 1992, Increasing rates of atmospheric 

mercury deposition in midcontinental North America: 

Science, v. 257, p. 784–787.

Tan, S.W., Meiller, J.C., and Mahaffey, K.R., 2009, The 

endocrine effects of mercury in humans and wildlife: 

Critical Reviews in Toxicology, v. 39, no. 3, p. 228–269.

82 Mercury in the Nation’s Streams—Levels, Trends, and Implications
Tewalt, S.J., Bragg, L.J., and Finkelman, R.B., 2001, Mercury 

in U.S. coal—Abundance, distribution, and modes of 

occurrence: U.S. Geological Survey Fact Sheet 095-01, 4 p., 

http://pubs.usgs.gov/fs/fs095-01.

Tsui, M.T.K., Finlay, J.C., Balogh, S.J., and Nollet, Y.H., 

2010, In situ production of methylmercury within a stream 

channel in northern California: Environmental Science and 

Technology, v. 44, no. 18, p. 6,998–7,004.

U.S. Environmental Protection Agency, 2003, Methodology 

for deriving ambient water quality criteria for the protection 

of human health (2000)—Technical support document, 

Volume 2, Development of national bioaccumulation 

factors: U.S. Environmental Protection Agency, EPA–

822–R–03–030, variously paged.

U.S. Environmental Protection Agency, 2004, What you 

need to know about mercury in fish and shellfish: U.S. 

Environmental Protection Agency, 2 p., accessed September 

19, 2013, at http://water.epa.gov/scitech/swguidance/

fishshellfish/outreach/advice_index.cfm.

U.S. Environmental Protection Agency, 2005, Regulatory 

impact analysis of the final Clean Air Mercury Rule: U.S. 

Environmental Protection Agency, EPA-452/R-05-003, 

570 p.

U.S. Environmental Protection Agency, 2008, Model-based 

analysis and tracking of airborne mercury emissions 

to assist in watershed planning: U.S. Environmental 

Protection Agency’s Office of Wetlands, Oceans, and 

Watersheds under EPA Contracts EP-W-08-018 and 

68-W-03-028, accessed November 23, 2013, at http://

water.epa.gov/lawsregs/lawsguidance/cwa/tmdl/mercury/

upload/2008_10_28_tmdl_pdf_final300report_10072008.

pdf.

U.S. Environmental Protection Agency, 2011a, 2010 Biennial 

National Listing of Fish Advisories: U.S. Environmental 

Protection Agency, EPA-820-F-11-014, 7 p.

U.S. Environmental Protection Agency, 2011b, Mercury and 

air toxics standards for power plants: U.S. Environmental 

Protection Agency, 6 p., accessed September 19, 2013, at 

http://www.epa.gov/mats/pdfs/20111221MATSsummaryfs.

pdf.

U.S. Environmental Protection Agency, 2013, National rivers 

and streams assessment 2008–2009—A collaborative 

survey (draft): U.S. Environmental Protection Agency, EPA 

841-D-13-001, 110 p.

U.S. Geological Survey, 2004 Mineral Resources Data System 

(MRDS) [digital data]: U.S. Geological Survey, accessed 

September 19, 2013, at http://tin.er.usgs.gov/mrds.

U.S. Geological Survey, 2007, Mercury statistics, in Kelly, 

T.D., and Matos, G.R., comps. Historical statistics for 

mineral and material commodities in the United States: 

U.S. Geological Survey Data Series 140, 5 p. accessed 

September 20,2013, at http://pubs.usgs.gov/ds/2005/140/.

U.S. Geological Survey, 2010, USGS mobile atmospheric 

mercury lab: U.S. Geological Survey, accessed September 

19, 2013, at http://wi.water.usgs.gov/mercury-lab/mobileatmospheric-mercury-lab.html.

U.S. Geological Survey, 2013a, The METAALICUS project: 

Website, accessed November 30, 2013, at http://wi.water.

usgs.gov/mercury/metaalicus-project.html.

U.S. Geological Survey, 2013b, Mercury studies in the Yolo 

Bypass, California: Website, accessed November 30, 2013, 

at http://ca.water.usgs.gov/mercury/riceFields.html.

United Nations Environment Programme, 2013a, Global 

mercury assessment 2013—Sources, emissions, releases 

and environmental transport: Geneva, Switzerland, United 

Nations Environment Programme Chemicals Branch, 32 p.

United Nations Environment Programme, 2013b, Minamata 

convention agreed by nations: United Nations Environment 

Programme, accessed September 20, 2013, at http://www.

unep.org/newscentre/default.aspx?DocumentID=2702&Art

icleID=9373. 

Van Metre, P.C., 2012, Increased atmospheric deposition 

of mercury in reference lakes near major urban areas: 

Environmental Pollution, v. 162, p. 209–215.

Van Metre, P.C., and Fuller, C.C., 2009, Dual-core massbalance approach for evaluating mercury and 210Pb 

atmospheric fallout and focusing to lakes: Environmental 

Science and Technology, v. 43, no. 1, p. 26–32.

Van Metre, P.C., and Wilson, J.T., 2003, Sediment-core 

samples, in Schuster, P.F., Water and sediment quality in the 

Yukon River basin, Alaska, during Water Year 2001: U.S. 

Geological Survey Open-File Report 03-427, chap. 15, 

p. 67–114, http://pubs.usgs.gov/of/2003/ofr03427/. 

Verta, M., Tolonen, K., and Simola, H., 1989, History of heavy 

metal pollution in Finland as recorded by lake sediments: 

Science of the Total Environment, v. 87/88, p. 1–18.

Ward, D.M., Nislow, K.H., and Folt, C.L., 2010, 

Bioaccumulation syndrome—Identifying factors that 

make some stream food webs prone to elevated mercury 

bioaccumulation: Annals of the New York Academy of 

Sciences, v. 1195, p. 62–83.

Wetzel, R.G., 2001, Limnology (3rd ed.): San Diego, 

California, Academic Press, 1,006 p.

References Cited 83
Wheatley, B., and Paradis, S., 1995, Exposure of Canadian 

aboriginal peoples to methylmercury: Water, Air, and Soil 

Pollution, v. 80, nos. 1–4, p. 3–11.

Wiener, J.G., Knights, B.C., Sandheinrich, M.B., Jeremiason, 

J.D., Brigham, M.E., Engstrom, D.R., Woodruff, L.G., 

Cannon, W.F., and Balogh, S.J., 2006, Mercury in soils, 

lakes, and fish in Voyageurs National Park (Minnesota)—

Importance of atmospheric deposition and ecosystem 

factors: Environmental Science and Technology, v. 40, 

p. 6,261–6,268.

Wiener, J.G., Krabbenhoft, D.P., Heinz, G.H., and 

Scheuhammer, A.M., 2003, Ecotoxicology of mercury, in

Hoffman, D.J., B.A. Rattner, G.A. Burton, Jr., and J. Cairns, 

Jr., eds., Handbook of Ecotoxicology (2nd ed.): Boca Raton, 

Florida, CRC Press, chap. 16, p. 409–463.

Windham-Myers, L., Marvin-DiPasquale, M., Stricker, C., 

Agee, J.L., Kieu, L.H., and Kakouros, E., 2013, Mercury 

cycling in agricultural and managed wetlands of California, 

USA—Experimental evidence of vegetation-driven 

changes in sediment biogeochemistry and methylmercury 

production: Science of The Total Environment, accessed 

July 1, 2013, at http://www.sciencedirect.com/science/

article/pii/S0048969713005718.

Woodruff, L.G., and Cannon, W.F., 2010, Immediate and 

long-term fire effects on total mercury in forests soils 

of northeastern Minnesota: Environmental Science and 

Technology, v. 44, no. 14, p. 5,371–5,376.

Woodruff, L.G., Sandheinrich, M., Brigham, M.E., and 

Cannon, W.F., 2009, Impact of wildfire on levels of 

mercury in forested watershed systems–Voyageurs National 

Park, Minnesota: U.S. Geological Survey Scientific 

Investigations Report 2009-5151, 19 p., http://pubs.usgs.

gov/sir/2009/5151.

Yin, X., and Balogh, S.J., 2002, Mercury concentrations in 

stream and river water—An analytical framework applied 

to Minnesota and Wisconsin (USA): Water, Air, and Soil 

Pollution, v. 138, p. 79–100.

Zillioux, E.J., Porcella, D.B., and Benoit, J.M., 1993, Mercury 

cycling and effects in freshwater wetland ecosystems: 

Environmental Toxicology and Chemistry, v. 12, no. 12, 

p. 2,245–2,264.

84 Mercury in the Nation’s Streams—Levels, Trends, and Implications
Glossary

Aerosol Microscopic particles in the 

atmosphere.

Amalgam An alloy of mercury with another 

metal, often gold or silver.

Average A general term used to describe the 

central tendency of a group of observations.

Background Natural levels of an 

environmental chemical that are unaffected 

by human activities (Gough and others, 1993; 

Reimann and Garrett, 2005). 

Baseline Levels of an environmental chemical 

that are documented for a specified area during 

a specified time period; they are typically 

impacted to some extent by anthropogenic 

influences (Gough and others, 1993; Reimann 

and Garrett, 2005).

Basin (or Drainage Basin) See watershed.

Bioaccumulation An increase in the 

concentration of a chemical in the tissue of 

an organism that occurs when the rate of 

intake through ingestion, respiration, and 

other mechanisms from all environmental 

sources exceeds the rate of removal. Both 

bioconcentration and biomagnification are 

forms of bioaccumulation (U.S. Environmental 

Protection Agency, 2003; Duffus and others, 

2007; Chasar and others, 2008).

Bioconcentration An increase in the 

concentration of a chemical in the tissue of an 

organism relative to the environmental medium 

to which it is exposed; it occurs through direct 

(non-dietary) uptake from the medium (U.S. 

Environmental Protection Agency, 2003; Duffus 

and others, 2007).

Biomagnification An increase in the 

concentration of a chemical in the tissue of 

an organism that is associated with increasing 

trophic level of the organism in a food web, 

primarily through the mechanism of dietary 

accumulation. At its simplest, an increase in 

the concentration of a chemical in an organism 

relative to its food (U.S. Environmental 

Protection Agency, 2003; Duffus and 

others,2007).

Chlor-alkali plant An industrial facility that 

produces chlorine and sodium hydroxide by 

electrolysis of sodium chloride solutions. Older 

chlor-alkali plants used mercury cathodes, and 

losses of mercury often contaminated local water 

resources.

Cinnabar A red mercuric sulfide (HgS) that 

forms at relatively low temperatures and at 

shallow depths, typically near recent volcanic 

activity or geothermal springs. The most 

common mercury mineral (Bailey and others, 

1973; Rytuba, 2003).

Consumer An animal that eats plants or other 

animals.

Demethylate (demethylation) The process by 

which organic methylmercury is converted to 

inorganic reactive mercury. 

Dissolved Used to describe chemicals that pass 

through a filter of known pore size and that are 

analytically determined by a defined procedure.

Dry deposition Accumulation of particles 

and gases by impact on environmental surfaces 

during periods of no precipitation.

Ecosystem The aggregation of the biological 

community, the physical environment, and the 

interactions between them in a given area. 

Ecotoxicological Pertaining to the study of the 

nature, effects, and interactions of substances that 

are harmful to the environment.

Elemental mercury The basic chemical form of 

mercury; it cannot be chemically broken down 

into simpler substances. Mercury is the only 

metal that is a liquid at room temperature.

Endocrine disruption The process whereby a 

chemical interferes with the normal functioning 

of hormones in animals. Endocrine disruptors 

are known to affect brain function and sexual 

development, and to cause various cancers.

Glossary 85
Fish consumption advisory A recommendation 

to limit or avoid consumption of certain species 

of locally caught fish to help protect human 

health. Advisories may be directed toward 

the general public (including recreational and 

subsistence fishers) or sensitive populations 

(such as women of childbearing age, nursing 

mothers, and children). Federal, state and local 

governments issue fish consumption advisories.

Food web Feeding relationships among 

the various species of organisms in a natural 

community. A diagram of these relationships 

typically resembles a spider’s web (Reid and 

Wood, 1976).

Forage fish Smaller fish that are primary 

consumers (herbivores) and/or secondary 

consumers (omnivores or carnivores), and that 

serve as prey for larger predator fish. 

Game fish A fish caught by anglers for sport. 

They are often large predator fish at or near the 

top of the food web.

Gaseous elemental mercury Elemental 

mercury in the gaseous phase in the atmosphere.

Geometric mean The mean of the logarithms 

of a group of observations transformed back to 

the original measurement units of the data. The 

numeric value of the geometric mean is similar to 

the median.

Inorganic mercury Used to describe elemental 

mercury and/or reactive mercury not associated 

with carbon atoms. 

Invertebrate Aquatic invertebrates include 

organisms, such as insect larvae, snails, and 

crayfish. Invertebrates eat other small animals, 

plant material, or particulate organic matter, and 

comprise the lower levels of aquatic food webs. 

Invertebrates are eaten primarily by forage fish, 

which, in turn, are eaten by predator fish.

Invertivore An animal that feeds primarily on 

invertebrates.

Isotopes Atoms of the same element that 

contain the same number of protons but different 

numbers of neutrons. Stable isotopes, which 

can be used to trace the movement of certain 

chemicals through the environment, are not 

radioactive.

Land use/land cover Land use describes how 

people use the land for particular purposes. 

Land cover describes the vegetation and human 

construction, such as roads and buildings that 

cover the earth’s surface. 

Larva (larval) The immature form of an aquatic 

insect, such as a mayfly, that does not change 

greatly in appearance as it grows. Aquatic insect 

larvae often comprise much of the invertebrate 

population of a stream. 

Litterfall Freshly fallen leaves, twigs, and 

other plant tissues. Litterfall is an important 

mechanism for cycling nutrients and other 

constituents, including mercury, in forested 

ecosystems.

Lode deposit A mineral deposit consisting of 

veins in consolidated rock. 

Logarithm (logarithmic) The base 10 

logarithm (Y) of a number (X) is determined as 

Y=log10 (X); it is the power to which the base 

(10) must be raised to obtain the original number 

(10Y=X). For example, log10 (1,000,000) = 6, 

because 106 = 1,000,000. Base 10 (or common) 

logarithms are used in this report when a 

variable spans an extremely large range.

Mean The sum of all observations divided by 

the number of observations. A measure of central 

tendency.

Median The central value of a group of 

observations when the data are ranked in order 

by magnitude. The 50th percentile. A measure of 

central tendency.

86 Mercury in the Nation’s Streams—Levels, Trends, and Implications
Median absolute deviation (MAD) For a 

series of observations, the absolute values 

of the differences (deviations) between each 

observation and the median of the observations 

are determined. The median of these deviations is 

the MAD. 

Mercury A chemical element designated by the 

symbol Hg; it has an atomic number of 80. In this 

report, the term mercury also is used to describe 

the sum of all mercury forms (inorganic mercury 

plus methylmercury) in a given medium.

Methylate (methylation) The bacterial 

conversion of inorganic reactive mercury to 

organic methylmercury.

Methylmercury Used to describe the organic 

form of mercury with one methyl group attached 

to a mercury atom (monomethylmercury). 

Although dimethylmercury exists in the natural 

environment, the preponderance of environmental 

methylmercury is monomethylmercury, and the 

term methylmercury has become synonymous 

with that form.

Neurocognitive Of or related to the process of 

acquiring knowledge and understanding through 

thought, experience, and the senses.

Omega-3 fatty acid Polyunsaturated fatty acids 

that are essential for human growth and nutrition. 

Salmon and other fatty fish are important sources 

of omega-3 fatty acids.

Omnivore (omnivorous) An animal that 

consumes food from more than one trophic 

level. For example, crayfish eat both animals and 

plants. 

Order of magnitude Typically, 10 times greater 

or lesser than another value.

Particulate In air, used to describe chemicals 

attached to particles and aerosols. For mercury, 

this is typically reactive mercury. In water, used 

to describe chemicals attached to particles that 

are retained on a filter. For mercury, the filter 

pore size used in USGS studies has typically 

been 0.7 micrometers. 

Piscivore (piscivorous) An animal that feeds 

almost exclusively on fish.

Placer deposit A surficial mineral deposit 

formed when a heavy mineral or element, such as 

gold, is concentrated from weathered debris by 

the action of moving water, such as a stream.

Predator An animal that eats other animals, 

including fish and invertebrates.

Prey An animal that is pursued and eaten 

for food.

Reactive gaseous mercury Reactive inorganic 

mercury in the gaseous phase in the atmosphere. 

Reactive mercury Used to describe 

nonelemental forms of mercury that can 

chemically combine with other elements or 

molecules. Reactive mercury also is known as 

oxidized mercury.

Reference dose The level of daily intake 

(of a chemical) that is not associated with an 

appreciable increase in risk of adverse health 

effects during a lifetime.

Riparian Of or related to the narrow corridor 

of land adjacent to both sides of a stream, where 

terrestrial and aquatic ecosystems interact.

River Sometimes used to refer to large streams.

Salmonid Any fish species of the family 

Salmonidae, including salmon, trout, char, 

grayling, and whitefish.

Sluice box Long, narrow, wooden boxes used 

to separate fine placer gold particles from stream 

water and sediment by amalgamation.

Stream A natural, well-defined channel with 

flowing water on the earth’s surface, regardless of 

size (Langbein and Iseri, 1960; Gary and others, 

1972).

Glossary 87
Sulfate-reducing bacteria Bacteria that live in low-oxygen 

conditions and use sulfate (instead of oxygen) to break down 

organic matter and obtain energy. Some sulfate-reducing 

bacteria can methylate mercury.

Throughfall Water from precipitation that reaches the ground 

by falling unimpeded through plants or that drips onto the 

ground from branches and leaves. 

Top predator fish The species of fish occupying the highest 

trophic level in a food web. Top predator fish may be 

piscivorous, but are often opportunistic, feeding on any prey 

item that is readily available.

TMDL (Total Maximum Daily Load) A calculation of the 

maximum amount of a pollutant that a water body can receive 

and still safely meet established water-quality standards 

(http://water.epa.gov/lawsregs/lawsguidance/cwa/tmdl/index.

cfm; accessed February 22, 2013).

Total mercury Used to describe the sum of inorganic 

mercury plus methylmercury in a given medium.

Trophic level A given level of a food web. A trophic 

level consists of multiple, functionally equivalent species 

of organisms that compete with each other for available 

resources, particularly food. (Wetzel, 2001)

Trophic position Similar to trophic level, except that trophic 

position is a quantifiable number. Trophic position typically is 

calculated as the difference in the abundance of nitrogen-15 

(a stable isotope of nitrogen) in organisms at a given trophic 

level and those at the base of the food web (Chasar and others, 

2009).

Watershed Used to describe the entire surface of the earth 

that is defined by a drainage divide and that drains water 

falling as precipitation to a common outlet on a stream 

channel. The term is synonymous with drainage basin (Gary 

and others, 1972; Dunne and Leopold, 1978).

Wet deposition Rain, snow, or fog that deposits particles or 

dissolved chemicals onto environmental surfaces.

88 Mercury in the Nation’s Streams—Levels, Trends, and Implications
Appendix 1 89

Chapter 1. 

No supporting information.

Chapter 2. 

Figure 2-1. Data are from U.S. Environmental Protection 

Agency (2011a) (http://water.epa.gov/scitech/swguidance/

fishshellfish/fishadvisories/upload/technical_factsheet_2010.

pdf). The figures were simplified relative to their original form. 

The data for figure 2-1A are from figure 4 in the USEPA document, 

and the data for figure 2-1B are from figure 3 in the USEPA 

document. The category “all other contaminants” combines PCBs 

(polychlorinated biphenyls), chlordane, dioxins, DDT, and other 

contaminants in the original USEPA document. Figure 2-1C is a 

map of advisories as of 2010 and is from figure 2 in the original 

USEPA document. 

Figure 2-3. The graph is modified from Sunderland (2007). Data 

were supplied by Elsie Sunderland (Harvard University, written 

commun., June 26, 2012). 

Figure 2-4. Data are from the Food and Drug Administration; 

compiled by Elsie Sunderland (Harvard University, written 

commun., November 29, 2012).

Chapter 3.

Figure 3-3. Data are from Arctic Monitoring Assessment 

Programme / United Nations Environment Programme (2008; Table 

AppA.6, p. 132). The category “Metal Mining and Production” 

includes nonferrous metals plus pig iron and crude steel; the 

category “Chlor-alkali Production” is equivalent to caustic soda 

production. 

Figure 3-4. U.S. data are from U.S. Environmental Protection 

Agency (2008); Canadian data are from Environment Canada 

(2005); Mexican data are from Acosta-Ruiz and Powers (2001). 

Sources smaller than 5 kilograms per year are not shown. 

Data were compiled by Mark Cohen (National Oceanic and 

Atmospheric Administration, written commun., January 16, 2013).

Figure 3-8. The map was produced by Michael Tate (U.S. 

Geological Survey, written commun., January 15, 2013), using 

methods of Latysh and Wetherbee (2012). Annual mean 

precipitation-weighted total mercury concentrations were 

obtained from the Mercury Deposition Network (http://nadp.

sws.uiuc.edu/mdn). Concentration data for individual sites 

were spatially interpolated using inverse distance weighting. 

Interpolated concentrations were then multiplied by average 

annual modeled precipitation from the Parameter-elevation 

Regressions on Independent Slopes Model (PRISM) model (Daly 

and others, 2008), resulting in a map showing continuous mean 

wet deposition of total mercury for the continental United States. 

Figure 3-9. Weekly total mercury deposition and precipitation 

volume for September 2003–September 2007 at Orlando, Florida 

(site FL32), were downloaded from National Atmospheric 

Deposition Program at http://nadp.sws.uiuc.edu/sites/siteinfo.

asp?net=MDN&id=FL32.

Figure 3-18. Each dot represents the mean filtered (dissolved) 

methylmercury concentration from 20 to 59 samples collected 

at each stream over several years, encompassing a range of 

seasons and streamflow conditions. Data from Balogh and others 

(2008) were provided by Steven Balogh (Metropolitan Council 

Environmental Services, written commun., February 7, 2006).

Figure 3-20. Mercury concentrations, expressed on a wet 

weight basis, were divided by the length of fish (mean length 

was used for composite samples). This removes the effect of fish 

length on mercury concentration, so sites with a range of fish 

lengths can be compared. Note that these length-normalized units 

cannot be compared to the USEPA fish tissue mercury criterion 

for the protection of human health. 

Chapter 4.

Figure 4-3. Data are from Horowitz and Stephens (2008; 

table 3, p. 300). The baseline concentration range for mercury 

in streambed sediment was calculated as the median baseline 

mercury concentration plus or minus the median absolute 

deviation (MAD) for mercury (Horowitz and Stephens, 2008; 

table 2, p. 294). The calculated range is 0.04 ± 0.02, or 

0.02–0.06 ppm. This is the range presented in figure 4-1.

Appendix 1. Supporting Information for Selected Figures
90 Mercury in the Nation’s Streams—Levels, Trends, and Implications

Figure 4-4. Data are from Bauch and others (2009). Mercury 

methylation efficiency was calculated as the average ratio of 

methylmercury to total mercury concentration for water and bed 

sediment, and has been used as an indicator of an ecosystem’s 

propensity to methylate mercury (Krabbenhoft and others, 1999). 

Mercury source index was calculated as total mercury concentration 

in streambed sediment divided by organic content of the sediment and 

by local wet plus dry deposition rate. Wet mercury deposition rate was 

the average for 2000–2003, as measured by the Mercury Deposition 

Network. Dry deposition rate was modeled by Seigneur and others 

(2004); gridded data were supplied by the authors (Krish Vijayaraghavan, 

Atmospheric & Environmental Research, Inc., written commun., 

February 3, 2004). Wet and dry deposition rates for each stream location 

are reported in Bauch and others (2009; appendix 5). The units of the 

mercury source index are complex and are not included on the figure; 

however, the units are consistent and allow a comparison of the sites 

on a relative basis. The mercury source index is based on the AHA 

(Atmospheric Hg Accumulation) index in Krabbenhoft and others (1999). 

Both methylation efficiency and mercury source index are median 

values by study area. 

Figure 4-6. Land use/land cover categories are described in Scudder 

and others (2009; p. 3, p. 6). For a description of the y-axis label units, see 

supporting information for Figure 3-20 (above). 

Chapter 5. 

Figure 5-3. Background and global anthropogenic mercury deposition 

were determined from lake sediment cores in southeast Alaska and 

rural Midwestern United States (data from Engstrom and Swain, 

1997). Regional anthropogenic mercury deposition was determined by 

subtracting background plus global anthropogenic mercury deposition 

from total mercury deposition recorded in sediment cores from six rural 

lakes in the continental United States (data from Van Metre, 2012). Urban 

mercury deposition was determined by subtracting background plus 

global anthropogenic plus regional anthropogenic mercury deposition 

from total mercury deposition recorded in sediment cores from six urban 

lakes across the continental United States (data from Van Metre, 2012).

Figure 5-4. This map displays trend results from Mahler and others 

(2006); data for individual sites were supplied by Barbara Mahler (U.S. 

Geological Survey, written commun., August 18, 2013). 

Figure 5-6. Relative fish methylmercury levels were obtained 

by dividing the mercury concentration of each fish by the mean 

concentration for a given fish species and site (Chalmers and others, 

2011).
For more information concerning the research in this report, contact:

Chief, National Water-Quality Assessment Program

U.S. Geological Survey

413 National Center

12201 Sunrise Valley Drive

Reston, Virginia 20192

or visit our Web site at:

http://water.usgs.gov/nawqa/mercury/

Back cover photographs show locations and sampling activities 

associated with mercury research discussed in this report.

Acknowledgments

Special thanks to the following individuals and organizations for their contributions:

USGS personnel in study areas across the Nation for their contributions of data and 

research results.

USGS’s many partners in the governmental and non-governmental sectors who have 

helped guide scientific efforts and ensure that USGS information meets the needs of 

local, State, Tribal, regional, and national stakeholders.

Technical peer reviewers:

• James Kuwabara, U.S. Geological Survey

• Edward Swain, Minnesota Pollution Control Agency

Other reviewers:

• Jeff Bigler, U.S. Environmental Protection Agency

• Judy Campbell Bird, U.S. Geological Survey contractor

• Doug Burns, U.S. Geological Survey

• Ruth Chemerys, U.S. Environmental Protection Agency

• Mark Cohen, National Oceanic and Atmospheric Administration

• Ken Davis, U.S. Environmental Protection Agency

• Neil Dubrovsky, U.S. Geological Survey

• Stan Durkee, U.S. Environmental Protection Agency

• Barbara Eikenberry, U.S. Geological Survey

• Colleen Flannagan and Kristi Morris, National Park Service

• David Gay, National Atmospheric Deposition Program

• Ellen Kurlansky, U.S. Environmental Protection Agency

• Leonard Levin, Electric Power Research Institute

• Greg Masson, U.S. Fish and Wildlife Service

• David Schmeltz, U.S. Environmental Protection Agency

• Pete Van Metre, U.S. Geological Survey 

• John Wathen, U.S. Environmental Protection Agency

Technical editor:

• Linda Rogers, U.S. Geological Survey

Layout Production:

• Sharon Wahlstrom, U.S. Geological Survey

Graphic Design:

• Bill Gibbs, U.S. Geological Survey

Publishing support provided by the U.S. Geological Survey

Publishing Network, Tacoma Publishing Service Center
Wentz and others— Mercury in the Nation’s Streams—Levels, Trends, and Implications—Circular 1395

ISSN 1067-084X (print)

ISSN 2330-5703 (online)

http://dx.doi.org/10.3133/cir13954