NONPOINT AND POINT SOURCES OF NITROGEN

IN MAJOR WATERSHEDS OF THE UNITED STATES

By Larry J. Puckett

U.S. GEOLOGICAL SURVEY

Water-Resources Investigations Report 94–4001

Reston, Virginia

1994
U.S. DEPARTMENT OF THE INTERIOR

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY

ROBERT M. HIRSCH, Acting Director

First printing, February 9, 1994

Second printing (with revisions), March 3, 1994

For additional information write to: Copies of this report can be purchased from:

Chief, National Water Quality U.S. Geological Survey

Assessment Program Earth Science Information

U.S. Geological Survey Open-File Reports Section

12201 Sunrise Valley Drive Box 25286, Mail Stop 517

Mail Stop 413 Denver Federal Center

Reston, VA 22092 Denver, CO 80225
CONTENTS

Abstract .................................................................................................................................................................... 1

Introduction.............................................................................................................................................................. 1

Why Be Concerned About Nitrogen? ...................................................................................................................... 2

What are the Major Sources of Nitrogen? ............................................................................................................... 2

Fertilizer ......................................................................................................................................................... 2

Animal Manure............................................................................................................................................... 2

Atmospheric Depositon.................................................................................................................................. 3

Point Sources.................................................................................................................................................. 3

Proportions of Nonpoint and Point Sources of Nitrogen ......................................................................................... 4

What Are the Implications for National Water-Quality Programs?......................................................................... 5

The National Water-Quality Assessment Program.................................................................................................. 8

References Cited ...................................................................................................................................................... 9

FIGURES

 1.–2. Map showing estimated nonpoint-source inputs of nitrogen:

1. Applied in commercial fertilizer, 1987.................................................................................................... 2

2. In animal manure, 1987 ........................................................................................................................... 3

3. Pie diagram showing sources of nitrogen-oxide emissions in the United States, 1988 .............................. 3

4. Map showing estimated atmospheric deposition of nitrogen in the United States ..................................... 4

5. Map showing locations of and pie diagrams showing proportions of nonpoint and point sources

of nitrogen in selected National Water-Quality Assessment Program watersheds ..................................... 5

6. Map showing locations of and bar graphs showing annual amounts of nitrogen transported in streams

and stream discharges in selected National Water-Quality Assessment Program watersheds.................... 6

7. Map showing locations of and pie diagrams showing proportions of in-stream nitrogen accounted for

by point sources in selected National Water-Quality Assessment Program watersheds............................. 7

8. Map showing locations of National Water-Quality Assessment Program study units and their

proposed implementation dates ................................................................................................................... 8

Contents
Introduction 1

Abstract

Estimates of nonpoint and point sources of

nitrogen were made for 107 watersheds located

in the U.S. Geological Survey’s National WaterQuality Assessment Program study units throughout the conterminous United States. The proportions of nitrogen originating from fertilizer,

manure, atmospheric deposition, sewage, and

industrial sources were found to vary with climate, hydrologic conditions, land use, population, and physiography. Fertilizer sources of

nitrogen are proportionally greater in agricultural

areas of the West and the Midwest than in other

parts of the Nation. Animal manure contributes

large proportions of nitrogen in the South and

parts of the Northeast. Atmospheric deposition

of nitrogen is generally greatest in areas of greatest precipitation, such as the Northeast. Point

sources (sewage and industrial) generally are predominant in watersheds near cities, where they

may account for large proportions of the nitrogen

in streams. The transport of nitrogen in streams

increases as amounts of precipitation and runoff

increase and is greatest in the Northeastern

United States. Because no single nonpoint nitrogen source is dominant everywhere, approaches

to control nitrogen must vary throughout the

Nation. Watershed-based approaches to understanding nonpoint and point sources of contamination, as used by the National Water-Quality

Assessment Program, will aid water-quality and

environmental managers to devise methods to

reduce nitrogen pollution.

INTRODUCTION

Congress has charged the U.S. Geological

Survey’s National Water-Quality Assessment

(NAWQA) Program with conducting an assessment

of the quality of the Nation’s water resources. The

goals of the NAWQA Program are to describe the status and trends in the quality of a large representative

part of the Nation’s streams and ground-water

resources and to provide a sound, scientific understanding of the primary natural and human factors

that affect the quality of these resources (Leahy and

Wilber, 1991). Since its passage in 1972, the Federal

Water Pollution Control Act (commonly referred to

as the Clean Water Act) has focused on efforts to

reduce discharges of pollutants from sewage-treatment plants and other point sources. The ultimate

goal of the Clean Water Act is to restore and maintain the chemical, physical, and biological integrity of

the Nation’s waters. However, in 1990, approximately 37 percent of the U.S. river miles that were

tested still did not fully support the uses designated to

them by the States (U.S. Environmental Protection

Agency, 1992).

Until recently, little was known about the magnitudes of the various nonpoint and point sources of

nitrogen. This knowledge gap has made it difficult to

develop a national strategy for nonpoint-source pollution prevention and control.

Many of the water-pollution-control measures

instituted since 1972 have focused on reducing discharges from point sources. However, one recognized

limitation of the Clean Water Act is its lack of controls on nonpoint-source pollution (Knopman and

Smith, 1993). These sources are called “nonpoint”

because they contribute pollutants to the receiving rivers and streams at numerous and widespread locations, rather than at a single discharge point. Commercial fertilizer and animal manure are two

Nonpoint and Point Sources of Nitrogen In Major Watersheds of

the United States

By Larry J. Puckett
Less than 1

Greater than or equal to 1 and less than 3

Greater than or equal to 3 and less than 7

Greater than or equal to 7

Nitrogen, in tons per square mile, by county

EXPLANATION

Figure 1. Estimated nonpoint-source inputs of nitrogen applied in commercial fertilizer, 1987.

important nonpoint sources of nitrogen. Both are

applied to agricultural land throughout the United

States. Atmospheric deposition is another important

nonpoint source of nitrogen but receives much less

recognition than other sources.

WHY BE CONCERNED ABOUT

NITROGEN?

Eutrophication of surface waters is the major

environmental effect of excessive nutrient inputs.

The abundant growth of algae and aquatic plants that

may develop in nutrient-rich waters is often unsightly

and can restrict recreational uses of lakes and slowmoving rivers and streams. When this vegetation

decomposes, it consumes large quantities of oxygen,

which leads to fish kills, foul odors and tastes, and

increased water-treatment costs.

WHAT ARE THE MAJOR SOURCES OF

NITROGEN?

Fertilizer

On an annual basis, about 11.5 million tons of

nitrogen are applied as commercial fertilizer for agricultural purposes throughout the United States

(fig. 1). Between 1945 and 1985, commercial nitrogen fertilizer use increased twentyfold in the United

States, from about 594,000 to almost 11.5 million

tons per year (Alexander and Smith, 1990). A portion of the fertilizer that is applied to fields returns to

the atmosphere as ammonia gas, and most of the rest

is either taken up by plants or converted to nitrate in

the soil. Consequently, most of the dissolved nitrogen that enters streams from runoff of agricultural fertilizer occurs as nitrate. Nitrate is a very mobile form

of nitrogen—it is not readily retained by the soil and

is highly soluble in water. Because of the mobility of

nitrate, farmers may apply it in greater quantities than

crops require. Also, given its high solubility, nitrate

may be washed into adjacent streams by rain, or it

may leach into the ground-water system.

Animal Manure

Each year the 7 billion farm animals in this country produce millions of tons of manure that contains an

estimated 6.5 million tons of nitrogen (fig. 2). If not

properly handled and disposed of, this manure can add

to the nitrogen in streams. Where farm animals are

allowed to roam freely, large amounts of nitrogen are

distributed over the landscape and represent a true nonpoint source of nutrients. However, where animals are

confined to feedlots, barns, or sheds, they become more

of a point-source nitrogen problem. In these situations,

large quantities of manure commonly are concentrated

in one location, and the nutrients that leach to ground

2 Nonpoint and Point Sources of Nitrogen in Major Watersheds of the United States
Less than 1

Greater than or equal to 1 and less than 2

Greater than or equal to 2 and less than 4

Greater than or equal to 4

Nitrogen, in tons per square mile, by county

EXPLANATION

and surface waters from storage areas may pose a waterquality problem.

Atmospheric Deposition

The nitrogen in atmospheric deposition originates primarily from the combustion of fossil fuels,

such as coal and oil (fig. 3). Atmospheric deposition

may be in a wet form as rain, snow, hail, fog, and

freezing rain or in a dry form as particulates, gases,

and droplets. The largest sources are point sources—

coal- and oil-burning electric utilities and large industries that together account for about 53 percent of

nitrogen emissions. However, automobiles, trucks,

buses, and other forms of transportation account for

approximately 38 percent of nitrogen emissions.

 Atmospheric inputs, in particular, have been

largely ignored within the context of nonpoint-source

pollution because they do not fit the traditional definition of a nonpoint source. For example, releases of nitrogen into the air from point sources, such as the combustion processes of powerplants and industries, are

called nonpoint sources of water pollution when that

nitrogen reaches water bodies through precipitation.

More than 3.2 million tons of nitrogen are deposited in

the United States each year from the atmosphere (fig.

4). This means that about 54 percent of the nitrogen

emitted from fossil-fuel-burning plants, vehicles, and

other sources in the United States (Sisterson, 1990) is

deposited on U.S. watersheds.

Point Sources

Point sources of nitrogen consist primarily of

a variety of large and small industries and publicly

and privately owned wastewater-treatment plants.

They are distinguished from nonpoint sources in that

they discharge directly into streams at a discrete

point. The most recent nationwide estimates of

What Are the Major Sources of Nitrogen 3

Figure 2. Estimated nonpoint-source inputs of nitrogen in animal manure, 1987.

Figure 3. Sources of nitrogen oxide emissions in the United

States in 1988.

Miscellaneous

1.1 percent

Transportation

38.4 percent

Electric utilities

35.4 percent

Industrial fuel

combustion

17.2 percent

Commercial/residential

combustion

3.8 percent

Industrial processes

4.2 percent

Source: Kohout, E.J, D.J. Miller, L. Nieves, D.S. Rothman, C.L. Saricks, 

and R. Stodolsky, 1990. Month and state current emission trends for 

NO x, SO x, and VOC: methodology and results. Argonne National 

Laboratory, Argonne, IL.
Less than or equal to 0.9

Greater than 0.9 and less than or equal to 1.3

Greater than 1.3 and less than or equal to 1.8

Greater than 1.8

Nitrogen, in tons per square mile

EXPLANATION

nonpoint- and point-sources of pollution were compiled in 1984 (Gianessi and Peskin, 1984). These

estimates indicated that during the period 1978

through 1981, point sources discharged approximately 1.3 million tons of nitrogen per year compared to 21.4 million tons from nonpoint sources.

Industrial sources and municipal sewage-treatment

plants accounted for about 26 and 74 percent, respectively, of the point-source total. However, point

sources represented only 5.7 percent of the total nitrogen added to the environment, whereas agricultural

nonpoint sources accounted for 93.5 percent.

PROPORTIONS OF NONPOINT AND

POINT SOURCES OF NITROGEN

The contribution of nonpoint sources to the total

nitrogen added to major watersheds of the United States

varies nationally from nearly zero in some predominantly urban watersheds to as much as 100 percent in

agricultural and other rural watersheds (fig. 5). In spite

of this variability, some broad generalizations can be

made. In the Western United States, where agriculture

is intensive, commercial fertilizers are the dominant

source of nitrogen. Atmospheric deposition is the second most important source, particularly in those western

watersheds devoted to forestry or in remote headwater

areas. In watersheds of the Central and the Southeastern

United States, commercial fertilizers are the dominant

nitrogen source, again due to intensive agricultural. Animal manure is an important secondary source in the Central and the Southeastern watersheds as a result of cattle,

hog, poultry, and other livestock production. In the

Northeastern United States, where agriculture is less

intensive, atmospheric deposition is the dominant source

of nitrogen in most watersheds, and animal manure is

the second most important source.

Areas of the country with the greatest rainfall

and atmospheric pollution also have the greatest

amounts of nitrogen deposited from the atmosphere

(fig. 4). For example, in the Northeastern United

States, atmospheric deposition of nitrogen in rain,

snow, and other forms accounts for about one-third of

the total nitrogen inputs to watersheds. The amount

of nitrogen carried by streams also is generally greatest where runoff is greatest. Runoff is greatest in the

Northeastern United States as are the amounts of

nitrogen transported in streams (fig. 6).

The proportion of in-stream nitrogen

accounted for by point sources is also variable in the

NAWQA Program watersheds (fig. 7). Streams near

large cities such as Denver in the South Platte River

Basin, Colorado, Dallas-Fort Worth in the Trinity

River Basin, Texas, and Atlanta in the AppalachicolaChattahoochee-Flint River Basin, Georgia (fig. 7)

receive relatively large proportions (up to 77 percent)

of their nitrogen from point sources, such as sewagetreatment plants. However, point-source inputs of

4 Nonpoint and Point Sources of Nitrogen in Major Watersheds of the United States

Figure 4. Estimated atmospheric deposition of nitrogen in the United States (R. Alexander, U.S. Geological Survey, written

commun., 1993).
What are the Implications for National Water-Quality Programs? 5

Figure 5. Proportions of nonpoint and point sources of nitrogen in selected National Water-Quality Assessment Program

watersheds.

nitrogen account for less than 10 percent of the

in-stream nitrogen in more than 50 percent of the

watersheds studied, and less than one-half of the

nitrogen in 90 percent of the watersheds studied.

This means that in more than 90 percent of those

watersheds, nonpoint sources of nitrogen account for

more than one-half of the nitrogen in streams.

In watersheds that are largely agricultural,

such as the Red River of the North in North Dakota

and Minnesota and the Palouse River in Washington,

nitrogen from commercial fertilizers accounts for 84

and 87 percent, respectively, of the total nitrogen

added to the watersheds. Similarly, in watersheds

where animals are raised in large quantities, such as

the Susquehanna River in Pennsylvania and the

White River in Arkansas, nitrogen from animal

manure accounts for 54 and 56 percent, respectively,

of the total nitrogen added to the watersheds.

Because dominant nitrogen sources vary among

watersheds, it would be difficult to implement a single

management strategy for nitrogen reduction that would

be effective throughout the Nation. The proportion of

nutrients deposited in the watershed that eventually find

their way into streams also is variable. In most of the

watersheds examined, only a small percentage of the

nitrogen deposited in the watershed immediately

reaches and is measured in the stream—the rest is

removed with harvested crops, incorporated into woody

vegetation, returned to the atmosphere by biological and

chemical processes, or transported to the ground-water

system. If water enters the ground-water system, then it

may take a long time before the water is discharged to a

stream. Therefore, the effects of current land-use practices may continue to show up in streams years or even

decades from now.

WHAT ARE THE IMPLICATIONS

FOR NATIONAL WATER-QUALITY

PROGRAMS?

Proportions of nonpoint and point sources

of nitrogen vary from watershed to watershed

throughout the United States, not necessarily

Trinity River, White River Ark. Apalachicola River, Fla.

Tex.

South Platte River,

Colo.

San Joaquin 

River, Calif.

Snake River, 

Idaho

Willamette River, 

Oreg.

Palouse River, 

Wash.

Platte River, 

Nebr.

Red River of the North,

Minn., N. Dak.

White River, 

Ind.

Connecticut River, 

Conn.

Susquehanna River, 

Penn.

Potomac River, 

D.C.

Tar River, N.C.

Altamaha River, Ga.

EXPLANATION

Atmosphere

Fertilizer

Manure

Point sources

Nitrogen source
following political boundaries. These proportions

vary as a function of land use, population, hydrologic

conditions, climate, and physiography. This variability suggests that in addition to efforts to reduce nitrogen releases from all sources to the extent feasible,

pollution-prevention plans need to be developed on

an individual watershed basis. In this way, the importance of sources, as well as differences in climate,

soils, and water-management practices, can be taken

into consideration. Developing a more complete

understanding of which sources have the greatest

effects on water chemistry in watersheds, and the timing of those effects, is essential to providing effective

prevention and control programs.

Point sources are commonly a major source

of nitrogen to streams near large urban areas. In

many areas of the United States waste-treatment

plants and other urban point sources still release large

amounts of nitrogen to downstream river reaches.

Localized effects of these point sources can exceed

those of nonpoint sources scattered throughout the

watershed. Therefore, concentrating pollution-prevention efforts entirely on nonpoint sources may not

remedy water-quality problems near major cities.

Atmospheric deposition of nitrogen can be

a major source of nitrogen that is not addressed

by water-quality legislation. Because most of the

sources of atmospheric deposition are point sources,

this form of pollution is currently controlled by reducing nitrogen oxide emissions. Commonly these point

sources are located outside of the political boundaries

of watersheds that receive this atmospherically deposited nitrogen and, therefore, may not be controlled

through State and local government regulations.

Recent amendments to the Clean Air Act (1990) have

mandated a 2-million-ton reduction in nitrogen oxide

emissions (approximately 10 percent) below 1980 levels by 2000. However, after 2000, nitrogen oxide

emissions are projected to increase again, which

means that atmospheric deposition of nitrogen will

6 Nonpoint and Point Sources of Nitrogen in Major Watersheds of the United States

Figure 6. Annual amounts of nitrogen transported in streams and stream discharges in selected National WaterQuality Assesessment Program watersheds.

EXPLANATION

Apalachicola River, 

Fla.

South Platte River, Colo. Trinity River, Tex. White River Ark.

San Joaquin 

River, Calif.

Snake River, 

Idaho

Willamette River, 

Oreg.

Palouse River, Wash. Platte River, Nebr.

Red River of the North,

Minn., N. Dak. White River, Ind.

Connecticut River, 

Conn.

Susquehanna River,

Pa.

Potomac River, 

D.C.

Tar River, N.C.

Altamaha River, Ga.

Nitrogen

Discharge

4.5

3.0

1.5

0

4.5

3.0

1.5

0

NITROGEN, IN TONS PER

SQUARE MILE

DISCHARGE, IN CUBICFEET PER SECOND PERDAY PER SQUARE MILE

White River Ark.
remain a factor that needs to be considered in nitrogen-management plans.

Eutrophication of large rivers, lakes, reservoirs, estuaries, and shallow marine environments

is the most immediate environmental consequence

of nitrogen pollution in surface waters. Estuarine

and shallow marine environments, such as the Gulf of

Mexico, Puget Sound, Chesapeake Bay, Albemarle

and Pamlico Sounds, Long Island Sound, and others,

are considered to be sensitive to large inputs of nitrogen. Environmental planners for the Chesapeake Bay

have found it necessary to control the cumulative

amount of nitrogen from all watersheds that contribute to the bay as part of their management plan

(Fisher and Oppenheimer, 1991). Water-quality managers in other areas of the Nation will probably find

that a similar approach is required in their watersheds. For example, nitrogen discharged to the Gulf

of Mexico from the Mississippi River may originate

as fertilizer, animal manure, or atmospheric deposition in Minnesota, Arkansas, or Ohio.

Quantitative information is meager or

unavailable for several potentially important

sources of nitrogen. Leachate from septic systems,

urban runoff, combined sewage overflow, and contaminated ground water are several potentially important nitrogen sources that are difficult to quantify,

either because they are poorly understood or because

little data on them exists. It is important to determine

the amount of nitrogen contributed by these undocumented sources in watersheds to target those that contribute most to nitrogen-pollution problems. For

example, it might be of little value to mandate reductions in commercial fertilizer application rates if combined sewage overflows contribute a proportionally

greater amount of the total nitrogen to a stream.

Data needed to develop pollution-prevention plans are either often not being collected or

not readily available. A significant limitation in

determining the importance of nonpoint and point

sources of nitrogen or other contaminants is the general lack of coordination between point-sourceWhat are the Implications for National Water-Quality Programs? 7

Figure 7. Proportion of in-stream nitrogen accounted for by point sources in selected National Water-Quality Assessment

Program watersheds.

Point

Nonpoint

Trinity River, White River Ark. Apalachicola River, Fla.

Tex.

South Platte River,

Colo.

San Joaquin 

River, Calif.

Snake River, 

Idaho

Willamette River, 

Oreg.

Palouse River, 

Wash.

Platte River, 

Nebr.

Red River of the North,

Minn., N. Dak.

White River, 

Ind.

Connecticut River, 

Conn.

Susquehanna River, 

Penn.

Potomac River, 

D.C.

Tar River, N.C.

Altamaha River, Ga.

EXPLANATION

Nitrogen source
EXPLANATION

Studies started in

 fiscal year 1991

Studies started in

 fiscal year 1994

Studies proposed for

 fiscal year 1997

Boundary of

 study unit

effluent monitoring and water-quality monitoring programs (Zogorski and others, 1990). For example,

most wastewater-treatment plants are required to

monitor their effluent for ammonia nitrogen, but not

for nitrate or organic nitrogen. Therefore, the total

amount of nitrogen that enters critical estuaries and

water bodies must be estimated. Source estimates

could be improved by mandating that chemical-constituent data required by effluent-monitoring programs be consistent with those required by streamwater-quality monitoring programs.

THE NATIONAL WATER-QUALITY

ASSESSMENT PROGRAM

The U.S. Geological Survey implemented the

NAWQA Program in 1991 to provide water-quality

information that will be useful to policymakers and

managers at the national, State, and local levels. To

meet this goal, the NAWQA Program is currently

assessing the status and trends in the quality of a

large representative part of the Nation’s streams and

ground waters. Liaison committees of interested

national, State, and local managers and scientists

meet regularly with the NAWQA Program staff to

ensure that program findings are relevant to the needs

of water-resources managers, provide review of products, provide assistance in field activities, and identify management implications of findings. The

NAWQA Program also is designed to provide a

sound, scientific understanding of the natural and

human factors that affect water quality (Leahy and

Wilber, 1991). In addition to determining the sources

and assessing the effects of nitrogen and other nutrients on water quality, the NAWQA Program investigators are evaluating the effects of pesticides and

volatile organic compounds in water and collecting

aquatic biological data as indicators of overall waterquality conditions.

The NAWQA Program consists of studies of 60

major river and aquifer systems, which are referred to as

“study units” (fig. 8). As a group, the 60 study units

account for about one-half of the land area of the conterminous United States and 60 to 70 percent of its water

use and population served by public water supply. The

NAWQA Program is using a watershed-based approach

to assess the quality of the Nation’s ground and surface

waters. The NAWQA Program results have already

influenced State legislation and management plans for

pesticides and organic compounds in Kansas, Washington, and elsewhere.

The water-quality and much of the nutrient

source data in this report were retrieved from electronic data bases, verified, and analyzed by the following U.S. Geological Survey personnel: Scott K.

8 Nonpoint and Point Sources of Nitrogen in Major Watersheds of the United States

Figure 8. Locations of National Water Quality Assessment Program study units and their proposed implementation dates.
Anderholm, Clyde E. Asbury, Richard W. Bell, Hugh

E. Bevans, Joel D. Blomquist, Bernadine A. Bonn,

Gary R. Buell, Gregory M. Clark, James C. Ebbert,

Suzanne R. Femmer, Gary T. Fisher, Douglas A.

Freehafer, Elizabeth A. Frick, Karen E. Greene, Robert A. Hainly, Charles R. Kratzer, David W. Litke,

Jeffrey D. Martin, Gerard McMahon, Patrick J. Phillips, David C. Reutter, Dale M. Robertson, John K.

Stamer, Lan H. Tornes, Peter C. Van Metre, Michael

D. Woodside, and Marc J. Zimmerman. Four of the

illustrations were compiled and formatted from electronic data bases by Kerie J. Hitt.

REFERENCES CITED

 Alexander, R.B., and Smith, R.A., 1990, County-level estimates of nitrogen and phosphorus fertilizer use in the

United States, 1945 to 1985: U.S. Geological Survey

Open-File Report 90-130, 12 p.

Fisher, D.C., and Oppenheimer, M., 1991, Atmospheric

nitrogen deposition and the Chesapeake Bay Estuary: Ambio, v. 20, p. 102-108.

Gianessi, L.P., and Peskin, H.M., 1984, An overview of

the RFF Environmental Data Inventory—Methods

and preliminary results: Resources for the Future,

Washington, D.C., 111 p.

Kohout, E.J., Miller, D.J., Nieves, L., Rothman, D.S., Saricks, C.L., and Stodolsky, R., 1990, Month and state

current emission trends for NOx, SOx, and VOC—

Methodology and results: Argonne National Laboratory.

Knopman, D.S., and Smith, R.A., 1993, Twenty years of

the Clean Water Act—Has U.S. water quality

improved?: Environment, v. 35, no. 1, p. 16-41.

Leahy, P.P., and Wilber, W.G., 1991, National Water-Quality Assessment Program: U.S. Geological Survey

Open-File Report 91-54, 2 p.

Sisterson, D.L., 1990, Detailed SOx-S and NOx-N mass

budgets for the United States and Canada, in Acidic

deposition—State of Science and Technology,

Appendix 8A: National Acid Precipitation Assessment Program, Washington, D.C., p. 8A-1–8A-10.

U.S. Environmental Protection Agency, 1992, National

Water Quality Inventory—1990 report to Congress:

U.S. Environmental Protection Agency, p. 5.

Zogorski, J.S., Blanchard, S.F., Romack, R.D., and Fitzpatrick, F.A., 1990, Availability and suitability of

municipal wastewater information for use in a National

Water-Quality Assessment—A case study of the Upper

Illinois River Basin in Illinois, Indiana, and Wisconsin:

U.S. Geological Survey Open-File Report 90-375, 68 p.

References Cited 9

For further information on this and related studies, contact:

Chief, NAWQA Program

U.S. Geological Survey

Water Resources Division

12201 Sunrise Valley Drive

National Center, MS 413

Reston, VA 22092