The Quality of Our Nation’s Waters

Nutrients in the Nation’s Streams and Groundwater, 1992–2004

U.S. Department of the Interior

U.S. Geological Survey

Circular 1350

National Water-Quality Assessment Program
Cover: Aerial view of the Sacramento River in the Sacramento Valley, California. 

Photograph taken by Rand Schaal, Ph.D., pilot and photographer.
The Quality of Our Nation’s Water—

Nutrients in the Nation’s Streams and 

Groundwater, 1992–2004

By Neil M. Dubrovsky, Karen R. Burow, Gregory M. Clark, 

Jo Ann M. Gronberg, Pixie A. Hamilton, Kerie J. Hitt, David K. Mueller, 

Mark D. Munn, Bernard T. Nolan, Larry J. Puckett, Michael G. Rupert, 

Terry M. Short, Norman E. Spahr, Lori A. Sprague, and William G. Wilber 

National Water-Quality Assessment Program

Circular 1350

U.S. Department of the Interior

U.S. Geological Survey
ii

U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2010

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to reproduce any copyrighted materials contained within this report.

Suggested reference:

Dubrovsky, N.M., Burow, K.R., Clark, G.M., Gronberg, J.M., Hamilton P.A., Hitt, K.J., Mueller, D.K., Munn, M.D., Nolan, B.T., 

Puckett, L.J., Rupert, M.G., Short, T.M., Spahr, N.E., Sprague, L.A., and Wilber, W.G., 2010, The quality of our Nation’s 

waters—Nutrients in the Nation’s streams and groundwater, 1992–2004: U.S. Geological Survey Circular 1350, 174 p.

Additional information about this study is available at http://water.usgs.gov/nawqa/nutrients/pubs/circ1350

Library of Congress Cataloging-in-Publication Data

The quality of our Nation’s waters : nutrients in the Nation’s streams and groundwater, 1992–2004 / by N.M. Dubrovsky, K.R. 

Burow, G.M. Clark, J.M. Gronberg, P.A. Hamilton, K.J. Hitt, D.K. Mueller, M.D. Munn, B.T. Nolan, L.J. Puckett, M.G. Rupert, T.M. 

Short, N.E. Spahr, L.A. Sprague, and W.G. Wilber. 

 p. cm. -- (Circular ; 1350)

 ISBN 978-1-4113-2904-1

 1. Nutrient pollution of water--United States. 2. Water--Nitrogen content--United States. 

3. Water--Phosphorus content--United States. I. Dubrovsky, N. M. II. Geological Survey (U.S.) 

III. Title: The quality of our Nation’s waters: nutrients in the Nation’s streams and groundwater, 1992–2004. IV. Series: U.S. 

Geological Survey circular ; 1350.

 TD427.N87N875 2010

 363.739'0904--dc22 

 2010027122 
Foreword iii

Foreword

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 the Nation’s water resources 

is critical to ensuring long-term availability of water that is safe for drinking and recreation and is suitable 

for industry, irrigation, and fish and wildlife. Population growth and increasing demands for water make the 

availability of that water, now measured in terms of quantity and quality, even more essential to the long-term 

sustainability of our communities and ecosystems.

The USGS implemented the National Water-Quality Assessment (NAWQA) Program in 1991 to support national, 

regional, state, and local information needs and decisions related to water-quality management and policy 

(http://water.usgs.gov/nawqa). The NAWQA Program is designed to answer: What is the quality of our Nation’s 

streams and groundwater? How are conditions changing over time? How do natural features and human activities 

affect the quality of streams and groundwater, and where are those effects most pronounced? By combining 

information on water chemistry, physical characteristics, stream habitat, and aquatic life, the NAWQA Program 

aims to provide science-based insights for current and emerging water issues and priorities. From 1991to 2001, 

the NAWQA Program completed interdisciplinary assessments and established a baseline understanding of waterquality conditions in 51 of the Nation’s river basins and aquifers, referred to as Study Units 

(http://water.usgs.gov/nawqa/studyu.html).

National and regional assessments are ongoing in the second decade (2001–2012) of the NAWQA Program as 

42 of the 51 Study Units are selectively reassessed. These assessments extend the findings in the Study Units by 

determining status and trends at sites that have been consistently monitored for more than a decade, and filling 

critical gaps in characterizing the quality of surface water and groundwater. For example, increased emphasis has 

been placed on assessing the quality of source water and finished water associated with the Nation’s community 

water systems. During the second decade, NAWQA is addressing five national priority topics that build an 

understanding of how natural features and human activities affect water quality and establish links between 

sources of contaminants, the transport of those contaminants through the hydrologic system, and the potential 

effects of contaminants on humans and aquatic ecosystems. Included are studies of the fate of agricultural 

chemicals, effects of urbanization on stream ecosystems, bioaccumulation of mercury in stream ecosystems, 

effects of nutrient enrichment on aquatic ecosystems, and transport of contaminants to public-supply wells. In 

addition, national syntheses of information on pesticides, volatile organic compounds (VOCs), nutrients, trace 

elements, and aquatic ecology are continuing.

The USGS aims to disseminate credible, timely, and relevant science information to address practical and effective 

water-resource management and strategies that protect and restore water quality. We hope this NAWQA 

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.

The USGS recognizes that a national assessment by a single program cannot address all water-resource issues of 

interest. External coordination at all levels is critical for cost-effective management, regulation, and conservation 

of our Nation’s water resources. The NAWQA Program, therefore, depends on advice and information from other 

agencies—federal, state, regional, interstate, tribal, and local—as well as nongovernmental organizations, 

industry, academia, and other stakeholder groups. Your assistance and suggestions are greatly appreciated.

Matthew C. Larsen

Associate Director for Water
iv

This report is one of a series of publications, The Quality of Our Nation’s Waters, which describes major 

findings of the NAWQA Program on water-quality issues of regional and national concern. This report 

presents an assessment of the occurrence and distribution of nutrients in the Nation’s streams and 

groundwater based on water-quality data from about 500 streams and over 5,000 wells collected from 

1992 through 2001. Other topics addressed include the relation of nutrient conditions to aquatic life in 

streams, changes in nutrient conditions from 1988 through 2004, and implications for future changes. 

Other reports in this series focus on additional water-quality constituents of concern such as pesticides 

and volatile organic compounds in streams and groundwater, the effects of contaminants and habitat 

on the condition of aquatic communities in streams, and on 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. 

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 is relevant 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.

Donna N. Myers

Chief, Office of Water Quality

U.S. Geological Survey

Introduction to this report and the NAWQA series

The Quality of Our Nation’s Waters
Contents v

Chapter 1. National Findings and Their Implications.............................................................1

Chapter 2. Nutrient Primer .......................................................................................................21

Chapter 3. NAWQA’s Approach to Nutrient Assessment ...................................................41

Chapter 4. Occurrence and Distribution of Nutrients in Streams and Groundwater.......51

Chapter 5. Exchange of Nutrients between Surface Water and Groundwater ..............85

Chapter 6. Potential for Effects on Human Health .............................................................101

Chapter 7. Potential for Effects on Aquatic Life..................................................................113

Chapter 8. Changes in Nutrient Concentrations: Past and Predicted.............................131

References Cited ....................................................................................................................157

Glossary.....................................................................................................................................169

Abbreviations, Acronyms, and Units of Measure..............................................................174

Contents
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Chapter 1

1

Chapter

Do NAWQA findings substantiate national concerns for aquatic and 

human health?

National Water-Quality Assessment (NAWQA) findings indicate that 

nutrient concentrations in streams and groundwater in basins with significant 

agricultural or urban development are substantially greater than naturally 

occurring or “background” levels. For example, median concentrations 

of total nitrogen and phosphorus in agricultural streams are about 6 times 

greater than background levels. Findings also indicate that concentrations in 

streams routinely were 2 to 10 times greater than regional nutrient criteria 

recommended by the U.S. Environmental Protection Agency (USEPA) 

to protect aquatic life. Such large differences in magnitude suggest that 

significant reductions in sources of nutrients, as well as greater use of 

land management strategies to reduce the transport of nutrients to streams, 

are needed to meet recommended criteria for streams draining areas with 

significant agricultural and urban development. 

Nitrate concentrations above the Federal drinking-water standard—or 

Maximum Contaminant Level (MCL)—of 10 milligrams per liter (mg/L, 

as nitrogen) are relatively uncommon in samples from streams used for 

drinking water or from relatively deep aquifers; the MCL is exceeded, 

however, in more than 20 percent of shallow (less than 100 feet below the 

water table) domestic wells in agricultural areas. This finding raises concerns 

for human health in rural agricultural areas where shallow groundwater is 

used for domestic supply and may warn of future contamination of deeper 

groundwater pumped from public‐supply wells.

Are levels of nutrients in water increasing or decreasing?

A decadal assessment of trends in concentrations of nitrogen and 

phosphorus from about 1993 to 2003 shows minimal changes in those 

concentrations in the majority of studied streams across the Nation, and more 

upward than downward trends in concentrations at sites with changes. These 

findings underscore the need for reductions in nutrient inputs or management 

strategies that would reduce transport of nutrients to streams. Upward trends 

were evident among all land uses, including those only minimally affected 

by agricultural and (or) urban development, which suggests that additional 

protection of some of our Nation’s most pristine streams 

warrants consideration. 

The median of nitrate concentrations in groundwater from 495 wells 

also increased significantly from 3.2 to 3.4 mg/L (6 percent) during about the 

same period, and the proportion of wells with concentrations of nitrate greater 

than the MCL increased from 16 to 21 percent. Nitrate concentrations in 

water in deep aquifers are likely to increase during the next decade as shallow 

groundwater with elevated concentrations moves downward. The potential 

for future contamination of the deep aquifers requires attention because these 

aquifers commonly are used for public water supply, and because restoration 

of groundwater is costly and difficult. 

Long-term and consistent monitoring of nutrients, improved accounting 

of nutrient sources, and improved tracking and modeling of climatic and 

landscape changes will be essential for distinguishing trends in nutrient 

concentrations, understanding the causes of those trends, and accurately 

tracking the effectiveness of strategies implemented to manage nutrients.

National Findings and 

Their Implications

Although the use of artificial fertilizer 

has supported increasing food 

production to meet the needs of a 

growing population, increases in 

nutrient loadings from agricultural 

and, to a lesser extent, urban sources 

have resulted in nutrient concentrations 

in many streams and parts of aquifers 

that exceed standards for protection 

of human health and (or) aquatic life, 

often by large margins. 
2 Nutrients in the Nation’s Streams and Groundwater, 1992–2004

Introduction

New results—through monitoring and 

modeling—confirm and expand findings from 

earlier NAWQA studies.

This report is based on water-quality 

assessments conducted from 1992 through 

2001 in 51 major hydrologic systems across 

the Nation (referred to as Study Units) by the 

U.S. Geological Survey’s (USGS) National 

Water‐Quality Assessment (NAWQA) Program. 

The assessments were conducted using a 

nationally consistent approach in 20 Study 

Units during 1992–1995; in 16 Study Units 

during 1996–1998; and in 15 Study Units during 

1998–2001. A regional study of groundwater in 

the High Plains Aquifer continued into 2004, and 

additional special studies and trends assessments 

took place during 2002–2004.

Five measures of nitrogen- and 

phosphorus‐containing nutrients are discussed 

in this report, including nitrate, ammonia, total 

nitrogen, orthophosphate, and total phosphorus. 

Samples for analysis of nutrients in water were 

collected at 499 stream sites throughout the year, 

including high-flow and low-flow conditions. 

Most analyses in this report are based on 2 years 

of monthly data for each site. Sampling at a 

subset of sites was more intensive during the 

time of highest runoff and use of agricultural 

chemicals—generally weekly or twice monthly 

for a 4- to 9-month period.

Dissolved nutrients were analyzed in water 

samples from 5,101 wells, including monitoring, 

domestic, and public-supply wells. Unlike stream 

monitoring sites, which were sampled multiple 

times, wells were sampled only once because of 

the comparatively slow rate of change in most 

groundwater systems, relative to streams. Data 

analyses were based on one sample per well.

Sampling sites for streams and groundwater 

were selected among geographic areas that 

represent a wide range of physiographic and 

climatic settings, as well as different land 

uses associated with a variety of contaminant 

sources, including agricultural, urban, and 

natural sources. These broad land-use categories 

include a multitude of landscape modifications 

and land‐management strategies. Shallow 

groundwater (generally less than 20 feet below 

the water table) was sampled in agricultural, 

urban, and undeveloped areas, whereas deeper 

groundwater was sampled from wells that tap 

major aquifers, most of which are affected by 

a mixture of land uses and are important as 

potential sources of drinking water.

This analysis of NAWQA results builds 

upon an initial national assessment of nutrients 

in streams and groundwater that was based on 

results from investigations in NAWQA’s first 

20 Study Units (summarized in the first report of 

this series, U.S. Geological Survey, 1999). The 

more extensive data and expanded geographic 

coverage available for this report reinforce many 

of the previously reported findings, allow more 

detailed analyses of each topic, and support new 

analyses, including: (1) development of statistical 

models that extend the results from targeted 

NAWQA studies to areas of the Nation that have 

not been sampled; (2) evaluation of decadal 

changes in concentrations of nutrients in streams 

and groundwater along with modeled estimates 

for future trends; and (3) comparison of observed 

and modeled nutrient concentrations in streams 

to USEPA recommended nutrient criteria. 

Findings are summarized for major aspects 

of the occurrence of nutrients, groundwater 

and surface water interactions, implications 

for human and aquatic health, and changes in 

nutrients over time. Each topic is identified 

with the chapter in this report in which more 

detailed results, explanations, and references are 

provided. Key implications and applications to 

water-quality management are also summarized 

for each topic. The NAWQA approach and 

design are summarized in Chapter 3, and details 

on data-analysis methods, as well as all data used 

in this report, are available at http://water.usgs.

gov/nawqa/nutrients/pubs/circ1350.
Chapter 1

National Findings and Their Implications 3

17-0191_Fig_1-01_sw_boxplot_landuse.ai

Concentration, in milligrams per liter

0

2

4

6

8

10

12

Agricultural Urban Mixed Undeveloped

Total nitrogen

0.58

Agricultural Urban Mixed Undeveloped

Concentration, in milligrams per liter

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Total phosphorus

0.034

90th percentile

10th percentile

Median

75th percentile

25th percentile

Background 

concentration

EXPLANATION

0.58

Occurrence and Distribution 

of Nutrients in Streams and 

Groundwater

Significantly elevated concentrations 

of nutrients are widespread in streams and 

groundwater in areas of substantial agricultural 

and urban development. Concentrations vary 

in streams and groundwater across the Nation 

because of natural features and human activities 

that affect the sources and transport among 

different landscapes and during different seasons 

(Chapter 4).

Concentrations of all five nutrients—nitrate, 

ammonia, total nitrogen, orthophosphate, and 

total phosphorus—exceeded background levels 

at more than 90 percent of 190 streams draining 

agricultural and urban watersheds. Nitrate 

concentrations exceeded background levels in 

64 percent of 86 shallow aquifer studies sampled 

in agricultural and urban areas. Concentrations of 

other nutrients in groundwater were not greater 

than background levels in most areas; phosphorus 

tends to be relatively insoluble in water and thus 

sorbs onto soils and aquifer materials, and the 

other forms of nitrogen are readily transformed 

by microbes to nitrate or nitrogen gas. 

Concentrations of total nitrogen were higher 

in agricultural streams than in streams draining 

urban, mixed land use, or undeveloped areas, 

with a median concentration of about 4 mg/L, 

about 6 times greater than background levels 

(fig. 1-1). The highest concentrations of nitrogen 

in agricultural streams generally occurred in the 

Northeast, Midwest, and the Northwest (fig. 1-2), 

where some of the largest amounts of fertilizer 

and manure in the Nation are applied to soils. 

Agricultural practices influence the transport 

of nutrients in streams draining large areas of 

the United States. Such practices include (1) the 

prevalent use of tile drains and ditches in the 

Midwest, which help to drain clay-rich soils, 

facilitating the transport of nutrients to streams; 

and (2) intensive irrigation and large amounts of 

irrigation-return flow, such as in the Northwest, 

which can carry substantial amounts of nutrients 

to streams. Streams in agricultural areas in the 

Southeast had relatively lower concentrations 

of nitrogen than streams in other parts of the 

Figure 1-1. Concentrations of total nitrogen were highest in 

agricultural streams, with a median concentration of about 

4 mg/L (about 6 times greater than background levels), whereas 

concentrations of total phosphorus were similar in agricultural 

and urban streams, with a median concentration of about 

0.25 mg/L (also about 6 times greater than background levels).

country, partly because soil and hydrologic characteristics 

support greater losses of nitrogen through biological 

uptake and denitrification before overland flow or shallow 

groundwater reaches streams.
4 Nutrients in the Nation’s Streams and Groundwater, 1992–2004

17-0191_Fig_1-02_SWGW_NPconcbyLUqrt.ai

Agricultural Urban

Low (0.66)

Medium (0.66–3.17)

High (3.17)

Total nitrogen 

concentration in streams,

in milligrams per liter

2,900 2,900–15,000 15,000

Stream sites or 

groundwater studies, 

by land use

Data on nutrient inputs not available for Alaska and Hawaii.

340 340–2,730 2,730

Total phosphorus 

concentration in streams,

in milligrams per liter

Low (0.08)

Medium (0.08–2.6)

High (2.6)

Median concentration of 

nitrate in groundwater,

in milligrams per liter as N

Low (0.05)

Medium (0.05–0.28)

High (0.28)

Estimated 1997 nitrogen inputs from fertilizer, manure, and 

atmosphere, in pounds per square mile

Estimated 1997 phosphorus inputs from fertilizer and manure,

in pounds per square mile

Total nitrogen concentration in streams

Nitrate concentration in groundwater

Total phosphorus concentration in streams

Agricultural Urban

Figure 1-2. Elevated concentrations of nitrogen and phosphorus are associated with some of the highest fertilizer and 

manure inputs across the Nation. Because tile drains promote transport of nitrogen to streams but inhibit transport to 

groundwater, high concentrations in groundwater are not as prevalent as in surface water in the upper Midwest.
Chapter 1

National Findings and Their Implications 5

100 1,000 10,000

Nonpoint nitrogen input, in pounds per square mile

1

10

100

1,000

10,000

Total nitrogen outflow yield, in pounds per square mile

Percent nutrients

lost to streams

5%

50%

Increasing

nutrients lost

(or exported)

to streams

Agricultural 

 with 5 percent tile drains

 with ≥5 percent tile drains

Urban

Undeveloped

Land use

Concentrations of total nitrogen in urban streams 

were lower than those in agricultural streams, with a 

median concentration less than 2 mg/L (about 3 times 

greater than background levels) (fig. 1-1). The highest 

concentrations in urban streams were scattered across the 

Nation (fig. 1-2); high concentrations were sometimes 

associated with upstream wastewater-treatment facilities. 

In some parts of the country, such as the semiarid West, 

a large portion of the flow in some streams may be 

wastewater effluent, particularly during times when 

natural runoff is low. Atmospheric deposition can account 

for a significant portion of the nitrogen in streams in some 

relatively undeveloped watersheds, such as are found in 

the Northeast.

Concentrations of total phosphorus were highest 

in streams in both agricultural and urban areas, with 

a median concentration of about 0.25 mg/L (about 

6 times greater than background levels) (fig. 1-1). High 

concentrations of phosphorus in agricultural settings are 

associated with relatively high applications of fertilizer 

and manure (fig. 1-2). Other urban sources may include 

treated wastewater effluent and septic-system drainage (in 

less urbanized settings), as well as runoff from residential 

lawns, golf courses, and construction sites.

Geographic patterns in the occurrence and 

distribution of nutrient concentrations are complicated 

by naturally occurring seasonal fluctuations in climate 

(such as those that influence streamflow and uptake by 

aquatic and riparian vegetation) and human factors (such 

as those related to the application of fertilizer and manure 

and irrigation). At many sites in the eastern half of the 

United States, total nitrogen concentrations were highest 

in the spring when streamflow is highest and fertilizer is 

applied, whereas total phosphorus concentrations were 

highest in the summer and autumn when streamflow is 

lowest and less water is available to dilute effluent from 

point sources. At other sites, particularly in the upper 

Midwest, both nitrogen and phosphorus concentrations 

were highest during high streamflow in the spring. In the 

western half of the United States, seasonal patterns were 

less distinct due to the highly variable topography and 

climate and the ubiquitous presence and effects of dams 

and canals. The highest concentrations of phosphorus in 

western streams, particularly in rangeland areas of the 

interior West, often occurred during the summer (May 

through July), when snowmelt produces high streamflows 

and sediment-bound phosphorus is mobilized by erosion. 

High concentrations of nitrogen often occurred during the 

winter when streamflow generally is low and less water is 

available to dilute irrigation return flow and effluent from 

point sources. 

The amounts of nitrogen and phosphorus lost from 

watersheds to streams—referred to as yields (expressed as 

mass per unit area)—increased with increasing nutrient inputs 

regardless of land use. For example, between 5 and 50 percent 

of the nitrogen input from nonpoint sources was exported out 

of most (69 percent) watersheds (see dashed red lines, fig. 1-3). 

Nutrient inputs that are not taken up by plants, immobilized 

in soil, or volatilized to the atmosphere can be transported to 

a stream and exported from the watershed. Variability in the 

amounts of nutrients exported can be explained in part by 

differences in agricultural practices, and in soils, geology, and 

hydrology. For example, agricultural lands with tile drains in 

5 percent or more of the watershed were 3 times more likely 

to export more than 25 percent of nitrogen to streams than 

agricultural lands with fewer tile drains. Less nitrogen was 

exported from watersheds in the Southeast because of greater 

amounts of denitrification in the soils and in the shallow 

groundwater that ultimately discharges to streams. Similarly, 

less nitrogen was exported from western watersheds because of 

generally low amounts of precipitation and runoff, as well as the 

modification of flow systems by irrigation and impoundments. 

Phosphorus is less soluble than nitrogen and thus, the amount of 

phosphorus exported was lower than nitrogen at many sites. 

Figure 1-3. The amount of nitrogen exported from watersheds to 

streams (referred to as yields) increased with increasing nutrient 

inputs, regardless of land use. Between 5 and 50 percent of the 

nitrogen input from nonpoint sources was exported out of most 

(69 percent) watersheds regardless of land use. The proportion of 

nitrogen input that is exported by a stream as yield is much greater 

for agricultural streams with tile drains in 5 percent or more of the 

watershed than for agricultural streams with fewer tile drains.
6 Nutrients in the Nation’s Streams and Groundwater, 1992–2004

Figure 1-4. Nitrate concentrations were highest in oxic groundwater (i.e., 

water with greater than 0.5 mg/L dissolved oxygen) than in groundwater without 

dissolved oxygen regardless of land use or nitrogen input.

Oxic

Mixed

Reduced

Oxic

Mixed

Reduced

0 2 4 5 0 5,000 10,000 15,000 31

Median nitrate concentration,

in milligrams per liter as N

Median nitrogen input within 

500-meter radius of well,

 in pounds per year

Agricultural

Urban

Redox condition Redox condition

Farm fertilizer

Nonfarm fertilizer

Manure

Atmospheric

Nitrogen input

EXPLANATION

The location of the sources of nutrients 

within a watershed and the characteristics 

of the stream channel affect the export of 

nutrients to downstream water bodies, such 

as the Gulf of Mexico and Chesapeake Bay. 

Simulations made with a USGS nutrient 

model that relates instream nutrient loads to 

the locations of upstream nutrient sources 

and watershed characteristics that affect 

nutrient transport provide information on 

the delivery of nitrogen and phosphorus 

from 62,000 stream reaches to the Nation’s 

major rivers and estuaries (see Chapter 4, 

Using the SPARROW Model to Assess 

Nutrient Concentrations in Streams). For 

example, because the amount of biological 

processing of nitrogen in streams depends on 

the surface area of the stream bottom (where 

the organisms live) in relation to the stream’s 

volume, a greater percentage of the nitrogen 

delivered to large deep rivers is typically 

exported to downstream water bodies than 

nitrogen delivered to small streams. Other 

findings indicate that corn and soybean 

cultivation are the largest sources of nitrogen 

annually to the Gulf of Mexico, whereas 

animal manure on pasture and rangelands 

and corn and soybean cultivation are the 

largest contributors of phosphorus.

In groundwater, median concentrations 

of nitrate were highest (3.1 mg/L) in 

shallow wells in agricultural areas, which 

are associated with relatively large amounts 

of fertilizer and (or) manure applications. 

Median concentrations of nitrate were lower 

in shallow wells in urban areas (1.4 mg/L). 

The lowest concentrations occurred in deep 

wells beneath a mix of land uses.

Nitrate concentrations in groundwater 

varied across the Nation. Some of the 

highest concentrations occurred in the 

Northeast, Midwest, California, and the 

Northwest (fig. 1-2). Elevated nitrate 

concentrations are likely the result of high 

nitrogen inputs and natural conditions that 

favor transport. Concentrations of nitrate 

generally decrease with well depth because 

(1) recharge of deep, old groundwater most 

likely occurred when anthropogenic nitrogen 

inputs were relatively low; (2) the amount 

of time for microbial processes such as denitrification to take place increases 

as water moves downward through the aquifer; and (3) deep groundwater is 

commonly a mixture of water that has moved along flow paths that originate in 

areas of differing land uses and nitrogen inputs.

The median concentration of nitrate was significantly higher in oxic 

groundwater (i.e., water with greater than 0.5 mg/L dissolved oxygen) than 

in groundwater without dissolved oxygen within each land-use setting, and 

is strongly dependent on the redox condition (the presence or absence of 

oxygen) regardless of nitrogen input at the land surface (fig. 1-4). For example, 

nitrogen inputs in agricultural areas were about the same across the three 

redox classes, and the median concentration of nitrate was about 5.6 mg/L in 

oxic water, but near detection levels in water under reduced conditions. These 

results show that redox status of an aquifer is a critical factor to consider when 

assessing vulnerability to nitrate contamination.

Nitrate concentrations are closely associated with groundwater age and 

geochemical conditions. Groundwater recharged within the last 10 years is 

typically oxic and has higher nitrate concentrations (median of 1.6 mg/L) than 

old groundwater (median of 0.26 mg/L), which generally is deeper and more 

commonly in a reduced state than young groundwater. Nitrate concentrations 

greater than 10 mg/L were rarely detected in groundwater recharged prior to 

about 1950.
Chapter 1

National Findings and Their Implications 7

EXPLANATION

Percentage of total 

nitrate load contributed 

by base flow by tercile

≤37

37–54

54

(49 sites)

(50 sites)

(49 sites)

• Nutrient concentrations in streams 

and groundwater can be anticipated 

from information about land use and 

associated fertilizer applications, along 

with natural features and management 

practices that influence the amount and 

timing of nutrient transport over the 

land and to the groundwater system.

• The major role of groundwater age and 

geochemical conditions in governing 

concentrations of nitrate needs to 

be accounted for in groundwater 

assessments because these factors 

can obscure relations between 

elevated concentrations of nitrate in 

groundwater and nitrate sources.

• Hydrologic settings in which 

groundwater is least vulnerable to 

contamination are often those in which 

streams may be most vulnerable and 

vice versa. For example, tile drains 

may help to protect groundwater from 

contamination while at the same time 

increasing nutrient transport 

to streams.

• The productivity of poorly drained 

agricultural land has been greatly 

enhanced by engineering the landscape 

to promote rapid drainage of the land 

surface by artificial drains and ditches. 

The resulting large yields of nitrogen in 

streams in watersheds with tile drains 

suggest a need to balance consideration 

of stream water quality and 

crop production.

• Because there is less natural 

attenuation of nutrient yields in large 

deep rivers than in small streams, 

nutrient management strategies for 

the protection of estuaries and other 

downstream water bodies should 

pay particular attention to point and 

nonpoint sources of nutrients that 

discharge to main stem channels. 

Exchange of Nutrients Between Surface Water 

and Groundwater

Groundwater can contribute a substantial portion of the total annual 

load of dissolved nutrients—particularly nitrate—to streams. The relative 

contribution from groundwater varies in streams across the Nation 

because of differences in natural features and human activities that affect 

the interactions across different landscapes and during different seasons 

(Chapter 5).

Base flow—defined as streamflow conditions following extended 

periods of minimal precipitation when the source of streamflow primarily 

is groundwater discharge—contributed more than a third of the total annual 

load of nitrate in two-thirds of 148 streams studied. These findings are 

based on data from streams that drain relatively small watersheds (less than 

500 square miles), where contributions of nitrate from other sources during 

base flow are believed to be small or insignificant. 

The relative contribution of nitrate from base flow is not uniform 

across the Nation, but varies geographically in response to base flow 

sources, as well as to natural features and human activities that can affect 

transport of groundwater to streams. For example, base flow contributed 

more than 50 percent of the nitrate load to many streams in northern 

Alabama through Tennessee, Virginia, Maryland, and Pennsylvania, owing 

primarily to the relatively permeable and well-drained geologic settings 

in the Valley and Ridge physiographic province that promote groundwater 

transport to streams (fig. 1-5). In contrast, base flow contributions generally 

Figure 1-5. Base flow, consisting mostly of groundwater discharge, accounts 

for more than a third of the total annual load of nitrate in about two-thirds of 148 

streams draining watersheds smaller than 500 square miles across the Nation. 

The magnitude of contribution depends on natural features and human activities 

that can affect groundwater transport to streams, as well as the potential 

contribution of other nitrate sources.

Implications
8 Nutrients in the Nation’s Streams and Groundwater, 1992–2004

Implications

were low (less than or equal to 37 percent) 

in the Midwest. This is due, in large part, to 

the use of tile drains and ditches to remove 

water from poorly drained agricultural 

lands, which reduces recharge to the 

groundwater system and enhances runoff 

to streams. 

Findings from a national landscape 

analysis show an association between 

base flow which provides substantial 

contributions of nitrate to streams and 

the permeability of soils and underlying 

bedrock. Nitrate loads from base flow 

were significantly lower (contributing 

about 27 percent of total stream nitrate 

load) in streams draining landscapes with 

impermeable soils and bedrock than in 

those draining landscapes with permeable 

soils and (or) bedrock (44 to 47 percent). 

Permeability affects recharge to the 

aquifer system and the degree to which 

groundwater can move through the aquifer 

to streams.

Natural processes—including 

physical, chemical, and biological—can 

affect exchanges of water and its dissolved 

constituents between aquifers and streams. 

For example, in the shallow subsurface of 

riparian zones that contain organic-rich 

sediments with low dissolved-oxygen 

concentrations, bacteria convert dissolved 

nitrate in groundwater to innocuous, 

gaseous forms of nitrogen through the 

process of denitrification. Nutrients also 

can be removed by plants in riparian 

or buffer zones. The efficacy of these 

processes depends on the geometry of 

the local groundwater flow system. For 

example, in some settings, groundwater 

can flow along relatively deep flow paths 

beneath riparian zones such that nitrate 

in the groundwater is unaffected by the 

riparian zone and can discharge directly to 

streams. Findings show that riparian zones 

are most effective in settings with thin 

surficial aquifers underlain by a shallow 

confining layer, with organic-rich soils 

that extend down to the confining layer. 

Groundwater in these types of settings 

tends to flow through biologically reactive 

parts of the aquifer, which promotes the 

removal of nitrate.

Nitrate is not the only nutrient that can be 

contributed to streams through groundwater discharge. 

Groundwater can contribute significant amounts of 

phosphorus to streams, particularly where natural 

sources of phosphorus are present in the aquifer 

and reduced chemical conditions favor phosphorus 

transport. Such conditions have been documented in 

areas as different as the Coastal Plain of North Carolina, 

and the Tualatin River Basin of Oregon.

• Groundwater contributions of nutrients, 

particularly nitrate, to streams can be important 

and may merit special attention in settings in 

which natural features and human activities 

enhance the connections between groundwater 

and surface water. 

• For streams in which groundwater contributions 

of nutrients are substantial:

◦ Crop-management and irrigation practices, 

which commonly are designed to reduce 

or slow the movement of overland flow to 

streams, may have only a limited effect on 

nutrient loads to streams.

◦ Improvements in water quality as a result 

of reductions in nutrient inputs on the land 

may not be apparent in streams for decades 

because of the slow rate of groundwater 

movement from the land surface through the 

subsurface to streams.

• Full accounting and assessment of groundwater 

contributions of nutrients to surface water 

is a critical step in developing management 

strategies to meet water-quality goals for 

protection of drinking-water supplies and 

aquatic life. Specifically,

◦ Omission of groundwater contributions of 

nutrients from Total Maximum Daily Loads 

(TMDLs) calculations can result in errors 

in the allocation of loads to other sources 

that need to be controlled, thus limiting the 

effectiveness of TMDLs for restoring and 

protecting streams.

◦ Management practices such as riparian buffer 

zones can remove nitrate from groundwater 

discharging to a stream, but the efficacy of the 

buffer will depend, in part, on the geometry of 

the local groundwater flow system.
Chapter 1

National Findings and Their Implications 9

_Fig_1-06_swgw_pies.ai

20

0–20 

No exceedance

Percentage of samples at each stream site or groundwater

study exceeding the nitrate MCL

EXPLANATION

11.1 percent

17

percent

7.3 percent

Agriculture

Percentage of Streams

2 percent 3.3 percent

Urban Mixed

Percentage of Groundwater Studies

3 percent 5.3 percent

Agriculture Urban Major aquifers

39.6

percent

43.4

percent

48.5

percent

38.3

percent

Potential Significance to 

Human Health

Nutrients in streams seldom occurred 

at concentrations greater than water-quality 

benchmarks for human health. Concentrations of 

nitrate greater than the Maximum Contaminant 

Level (MCL) of 10 mg/L were more prevalent and 

widespread in groundwater used for drinking 

water, including that from domestic and—less 

frequently—public-supply wells (Chapter 6).

Nitrate was the most frequently detected 

nutrient in streams at concentrations greater 

than its human-health benchmark—the 

USEPA Maximum Contaminant Level (MCL) 

of 10 mg/L (the maximum permissible 

concentration of a contaminant in water that 

may be delivered to any user of a public water 

system). Concentrations of nitrate exceeded the 

MCL in 2 percent of all stream samples, and 

in at least one sample collected at 10 percent 

(50 of 499) of streams. Samples from 5 streams 

had concentrations of nitrite greater than its 

MCL of 1 mg/L. All of the samples with nitrite 

exceedances were collected downstream from 

wastewater-treatment facilities, and none of the 

streams were used as a source of drinking-water 

supply. Concentrations of ammonia in streams 

never exceeded its human-health benchmark—

the USEPA lifetime health advisory of 30 mg/L. 

Elevated concentrations of nitrate mostly 

occurred in streams that drain agricultural 

watersheds where the use of fertilizers and (or) 

manure is relatively high. At least one sample 

from nearly 30 percent of the agricultural streams 

had a nitrate concentration greater than the MCL, 

and concentrations greater than the MCL were 

detected in more than 20 percent of samples 

from about 11 percent of these streams (fig. 1-6). 

Concentrations exceeded the MCL at fewer 

streams draining urban land (about 7 percent), 

which likely reflects lower use of fertilizers on 

residential lands. Nitrate concentrations exceeded 

the MCL in samples from 5 percent of streams 

draining mixed land-use settings; none of the 

samples from streams draining undeveloped land 

had a concentration exceeding the MCL. 

Most surface-water samples with 

concentrations of nitrate exceeding the MCL 

were collected from small streams in the Corn 

Belt region, which encompasses Illinois, Indiana, 

Iowa, Nebraska, and Ohio, as well as parts of 

adjoining States. Fertilizer application rates 

in the Corn Belt are among the highest in the 

Nation, and as a result, elevated concentrations 

of nutrients are widespread in the region’s 

streams and rivers. These elevated concentrations 

also reflect landscape characteristics and landmanagement practices that promote rapid 

transport of runoff from fields to streams, 

including relatively impermeable soils and 

artificial drainage, such as tile drains. 

Figure 1-6. The Maximum Contaminant Level 

(MCL) for nitrate in drinking water of 10 milligrams 

per liter as nitrogen is exceeded far more frequently 

in streams in agricultural areas than in the other 

land-use categories. Much larger proportions of 

groundwater studies have MCL exceedances than do 

stream sites in all land-use categories.
10 Nutrients in the Nation’s Streams and Groundwater, 1992–2004

For perspective on the relevance of 

NAWQA findings to surface water used for 

drinking-water supplies, 12 percent of the 

Nation’s 1,679 public-water supply intakes 

withdraw water from streams that drain 

watersheds with predominantly agricultural land. 

Most (55 percent) of the Nation’s water-supply 

intakes are in watersheds draining undeveloped 

land, with the remaining intakes in areas of 

mixed and urban land use (32 and 

1 percent, respectively). 

Elevated concentrations of nitrate are 

more prevalent and widespread in groundwater 

used for drinking water than in streams; 

concentrations exceeded the MCL in samples 

from 7 percent of 2,388 domestic wells and 

in samples from 3 percent of 384 publicsupply wells. In addition, the water from one 

or more wells sampled in 57 percent of all 

agricultural, urban and major aquifer studies had 

concentrations of nitrate exceeding the MCL, 

compared to 10 percent of all stream sites. The 

broad distribution of concentrations of nitrate 

above the MCL is reflected in the large number 

of groundwater studies in which one or more 

MCL exceedances were detected: 83 percent 

of the agricultural studies, 52 percent of urban 

studies, and 44 percent of major aquifer studies 

(fig. 1-6). Elevated concentrations of nitrate were 

detected in groundwater in agricultural and urban 

areas throughout the United States, but least 

commonly in the Southeast because of aquifer 

and soil conditions that promote denitrification 

of nitrate to other forms, such as nitrogen gas. 

More than 20 percent of 406 shallow domestic 

wells located in agricultural areas exceeded the 

MCL. The proximity to land surface and the 

level of human activity, including the relatively 

heavy applications of fertilizers, increase the 

vulnerability of this resource to 

nitrate contamination. 

The lower frequency of exceedance of the 

MCL for nitrate in samples from public‐supply 

wells than in domestic wells likely reflects a 

combination of factors, including (1) greater 

depths to older and less contaminated 

groundwater; (2) longer travel times from the 

surface to the screened interval of the well, 

allowing degradation and (or) attenuation of 

contaminants during transport; and (3) the locations 

of public-supply wells near urban areas, where 

sources of nitrate are less prevalent than in rural, 

agricultural areas.

A national statistical model was developed 

to assess the vulnerability of relatively deep 

groundwater (more than 164 feet, or 50 meters, 

below land surface) to nitrate contamination 

(fig. 1-7). Model simulations predict moderate 

to severe nitrate contamination in groundwater 

underlying the High Plains, northern Midwest, and 

areas of intensive agriculture in the East (such as in 

eastern Pennsylvania and the Delmarva Peninsula) 

and the West (such as in the Columbia Plateau in 

Washington, the San Joaquin Valley in California, 

and the Snake River Plain in Idaho). Predicted 

elevated concentrations are associated with nitrogen 

inputs, such as those from fertilizer and manure; 

natural soil, landscape, and geologic features 

that promote rapid transport of groundwater; 

anthropogenic factors or practices such as irrigation; 

and the absence of denitrification processes.

The model helps to estimate the number of 

people who may use wells for drinking water that 

tap aquifers vulnerable to nitrate contamination. 

About 15 million people live in areas where nitrate 

concentrations are predicted to be greater than 

1 milligram per liter in wells more than 164 feet 

below land surface. (Nitrate can occur naturally, but 

nitrate concentrations greater than 1 milligram per 

liter are likely to indicate human activities.) Among 

the potential users, more than 1.2 million people 

live in areas predicted to have moderate nitrate 

contamination of groundwater (greater than 5 mg/L 

but less than or equal to the MCL of 10 mg/L). 

Additional well owners may be affected in 

the future as a result of shallow contaminated 

groundwater moving downward into the aquifer 

system. Statistical models suggest, for example, 

that the number of people exposed to nitrate 

concentrations greater than the MCL would be 

14 percent greater if they obtain their water from 

shallow wells (33 feet or less) than if they obtain 

their water from deep wells (164 feet or greater). 

Because well depth is a proxy for travel time in an 

aquifer, water-quality conditions at shallow depths 

serve as an early warning system for possible 

future conditions. 
Chapter 1

National Findings and Their Implications 11

rediction.ai

Predicted nitrate concentration, in milligrams per liter as N

≤1 1–5 5–10 10

EXPLANATION

Missing data

Model for deep groundwater used as drinking water (50-m simulation depth)

Groundwater

• Concentrations of nutrients in streams seldom exceeded humanhealth benchmarks, particularly in undeveloped watersheds. Thus, 

water utilities that withdraw water from streams in undeveloped 

or mixed watersheds, which account for 87 percent of the Nation’s 

public-water supply intakes, are unlikely to withdraw water with 

nitrate concentrations greater than the MCL of 10 mg/L. The 

likelihood of nitrate concentrations in streams being greater than 

10 mg/L is largest in streams draining agricultural areas, which 

account for about 12 percent of public-water supply intakes 

on streams.

• Individuals who obtain their drinking water from shallow 

domestic wells near existing or former agricultural settings have 

the highest probability of consuming water with elevated nitrate 

concentrations. More than 43 million people—about 15 percent of 

the U.S. population—rely on domestic 

(privately owned) wells as their source 

of drinking water. The quality and safety 

of water from domestic wells are not 

regulated by the Federal Safe Drinking 

Water Act nor, in most cases, by State 

laws. Rather, individual homeowners 

are responsible for maintaining their 

domestic well systems and for monitoring 

water quality. Nitrate, which can persist in 

groundwater for years and even decades, 

may be present because of previous land 

uses and practices, which emphasizes 

the importance of public education and 

water-quality testing in areas where land 

use has changed.

• Even in relatively protected settings, 

deep aquifers require some level of 

consideration to support long-term 

prevention of nitrate contamination. 

Groundwater at all depths is part of an 

integrated system and can be vulnerable 

to future contamination as water moves 

downward from shallower, contaminated 

groundwater systems. Future 

contamination in deep aquifers requires 

consideration because these aquifers 

commonly are used as sources of public 

supply and because restoration of the 

purity of this relatively inaccessible and 

slow-moving water is costly and difficult. 

• Strategies for managing nutrients require 

far more information than we can afford 

to measure directly for all the places 

and times of interest. Many strategic 

decisions—such as setting monitoring 

priorities and implementing management 

and wellhead protection strategies—

inherently depend on predicting the 

potential effects of nutrients for locations 

that have never been directly assessed. 

The continued development of nationalscale predictive models of water quality 

conditions with quantified reliability is 

increasingly possible and is a critical 

step for cost-effective management and 

protection of water resources.

Figure 1-7. Results of model simulations suggest moderate (shown in 

yellow and orange) to severe (shown in red) nitrate contamination in 

relatively deep groundwater (greater than 164 feet or 50 meters below land 

surface) underlying parts of California, the Northwest, the High Plains, and 

the East. These areas typically are associated with large nitrogen input; 

natural soil, landscape, and geologic features that promote rapid transport 

of groundwater; and a lack of biogeochemical processes that convert 

nitrate to other forms of nitrogen. 

Implications
12 Nutrients in the Nation’s Streams and Groundwater, 1992–2004

Potential Significance to 

Aquatic Life

Concentrations of ammonia seldom 

exceeded levels set to protect aquatic life 

in streams. Concentrations of nitrogen and 

phosphorus commonly exceeded recommended 

regional criteria in selected areas of the United 

States, particularly in agricultural and urban 

areas (Chapter 7).

Concentrations of ammonia in streams 

seldom exceeded recommended USEPA 

numeric criteria established to protect 

aquatic life from ammonia toxicity (fig. 1-8). 

Specifically, concentrations exceeded acute 

criteria in 33 samples (0.14 percent) at 

7 streams (1.4 percent), from among about 

24,000 samples collected from nearly 

500 streams. Concentrations exceeded the 

chronic criteria in 139 samples from 22 sites. 

The acute and chronic criteria generally were 

exceeded in streams in the semiarid West 

draining watersheds with urban and mixed land 

uses. For many of these streams, effluent from 

wastewater-treatment facilities comprises a 

significant portion of the streamflow during much 

of the year. Chronic criteria were exceeded less 

frequently at agricultural sites (fig. 1-8), despite 

relatively large fertilizer and manure sources. 

This finding indicates that ammonia from 

nonpoint agricultural sources is not reaching or 

persisting in these streams, but rather is sorbed 

onto soils, volatilized, converted from the 

un-ionized form (NH3) to nitrate, or taken up 

by plants. 

The USEPA has developed technical 

guidance and provided recommendations 

for aquatic life criteria based on estimated 

reference conditions for nitrogen, phosphorus, 

chlorophyll a, and turbidity in 14 nutrient 

ecoregions across the Nation. These ecoregions 

represent areas of similar climate and geology 

(see Chapter 7 sidebar, The U.S. Environmental 

Protection Agency’s National Nutrient Criteria 

Program). For comparison and perspective, 

average background concentrations of nitrogen 

and phosphorus were estimated for each of 

the 14 nutrient ecoregions using the USGS 

SPARROW model. The modeled background 

concentrations and recommended USEPA criteria 

generally follow similar patterns within the 

ecoregions; however, the recommended criteria 

are more stringent than modeled concentrations 

in several regions, particularly for phosphorus, 

suggesting that the recommended criteria may 

be difficult to attain in some regions (fig. 1-9). 

Estimation of concentrations indicative of 

reference conditions is an area of continuing 

research (Herlihy and Sifneos, 2008).

Median concentrations of nitrogen and 

phosphorus, measured at 135 agricultural 

streams, were about 2 to more than 10 times 

higher than recommended nutrient criteria 

(fig. 1-9). The relatively large differences suggest 

that significant reductions in sources of nutrients, 

as well as reductions in transport through the 

implementation of land and water management 

strategies, are needed to meet recommended 

criteria in these areas. These results span 

ecoregions throughout the United States, 

including the Willamette and Central Valleys, 

the Xeric West, the Mostly Glaciated Dairy 

Region in the upper Midwest, the Southeastern 

Temperate Forested Plains and Hills, and the 

Central and Eastern Forested Uplands.

Acute

criteria

Chronic

criteria

Percentage of sites where ammonia 

criteria were exceeded

0 5 10 15

Agricultural

Urban

Mixed

Watershed land use

Figure 1-8. Concentrations of ammonia in 

streams seldom exceeded acute or chronic criteria 

set to protect aquatic life. Sites at which the 

criteria were exceeded generally are on streams 

in the semiarid West that drain areas of urban 

or mixed land use and are potentially influenced 

by treated effluent from wastewater-treatment 

facilities located upstream.
Chapter 1

National Findings and Their Implications 13

A regression model was developed to estimate 

the probability of exceeding a specific total nitrogen 

concentration for any given stream. Simulation results 

predict widespread exceedance of recommended 

criteria, suggesting, for example, that there is a 

greater than 75 percent probability that total nitrogen 

concentrations would exceed recommended nutrient 

criteria in 83 percent of stream miles throughout the 

Nation. This proportion is similar to what would be 

expected considering how the criteria were designed 

(U.S. Environmental Protection Agency, 2001, 

fig. 5). Simulation results also predict that a 25-percent 

reduction in fertilizer in the Corn Belt and Northern 

Great Plains nutrient ecoregion would result in a small 

(about 2.5 percent) decrease in the number of stream 

miles expected to exceed the respective recommended 

nutrient criteria. These findings underscore the challenge 

of reducing nutrients to levels protective of aquatic 

life in streams in agricultural areas, and suggest that 

greater use of land management strategies to reduce the 

transport of nutrients to streams may be needed to meet 

recommended criteria. 

17-0191_Fig_1-09

7 16 48

12

14 9

12

7

16

49

12 16 9

13

90th percentile

Boxplot shown for ecoregions

with more than 6 sampling sites

Modeled background

USEPA recommended

nutrient criteria 75th percentile

Median

25th percentile

10th percentile

EXPLANATION

Total nitrogen

Modeled background

USEPA recommended

nutrient criteria

Total phosphorus

Site value

12 Number of sampling sites

U.S. Environmental Protection Agency nutrient ecoregions

Willamette and Central Valleys

Western Forested Mountains

Xeric West

Great Plains Grass and Shrublands

Central Cultivated Great Plains

Corn Belt and Northern Great Plains

Mostly Glaciated Dairy Region

Nutrient Poor Largely Glaciated Upper Midwest and Northeast

Southeastern Temperate Forested Plains and Hills

Texas-Louisiana Coastal and Mississippi Alluvial Plains

Central and Eastern Forested Uplands

Southern Coastal Plain

Southern Florida Coastal Plain

Eastern Coastal Plain

1

2

3

4

5

6

7

8

9

10

11

12

13

14

Criteria for USEPA nutrient ecoregion 13 are under development.

0.001

0.01

0.1

1

10

USEPA nutrient ecoregion

Total phosphorus concentration, in milligrams per liter

Agricultural sites

Total phosphorus

1 2 3 4 5 6 7 8 9 10 11 12 13 14

0.1

1

10

USEPA nutrient ecoregion

Total nitrogen concentration, in milligrams per liter

Agricultural sites

1 2 3 4 5 6 7 8 9 10 11 12 13 14

Total nitrogen

Figure 1-9. U.S. Environmental Protection Agency (USEPA) recommended nutrient criteria and U.S. Geological Survey 

modeled estimates for background nutrient concentrations follow similar patterns of variation within the 14 nutrient 

ecoregions; however, recommended criteria are more stringent than estimates of background concentrations in several 

regions, particularly for phosphorus, suggesting that the recommended criteria may be difficult to attain in some areas. 

Median concentrations of nitrogen and phosphorus measured in agricultural streams were from 2 to more than 10 times 

greater than recommended regional nutrient criteria. 
14 Nutrients in the Nation’s Streams and Groundwater, 1992–2004

Mean value

95 percent confidence interval

Total phosphorus, in milligrams per liter

40

50

60

70

80

90

100

Macroinvertebrates

Fish

Algae

Total nitrogen, in milligrams per liter

Biological condition, in percent

40

50

60

70

80

90

100

Macroinvertebrates

Fish

Algae

0.7 0.7–1.5 1.5 0.025 0.025–0.075 0.075

up to 46 percent of the variability in algal biomass—when 

stream characteristics, such as water temperature and canopy 

cover, were included in the analysis. 

• More stringent ambient water-quality criteria for 

ammonia have recently been proposed that would 

be more protective of sensitive species (U.S. 

Environmental Protection Agency, 2009), which may 

result in more streams being out of compliance with 

respect to ammonia.

• The large differences in magnitude between nitrogen 

and phosphorus concentrations measured in streams 

draining areas with significant agricultural and urban 

development and USEPA recommended nutrient 

criteria suggest that significant reductions in sources 

of nutrients, as well as greater use of land and water 

management strategies to reduce the transport 

of nutrients to streams, are still needed to meet 

recommended criteria for these streams. 

• The increased sensitivity of algal communities to 

nutrient enrichment compared to macroinvertebrate 

and fish communities suggests that algal communities 

should be considered for stream bioassessments to 

avoid underestimating the extent of water quality 

impairment. 

• The wide range in biological response to nutrient 

concentrations supports the need for a regional 

approach to nutrient criteria and for consideration 

of factors related to stream habitat and flow 

characteristics in the development of these criteria.

Implications

Stream ecosystem health can be assessed by determining 

the biological condition of a stream on the basis of the 

numbers and types of individuals comprising algal, 

macroinvertebrate, and fish communities. NAWQA findings 

suggest that the biological condition of all three communities 

are increasingly altered with increasing concentrations of 

nitrogen and phosphorus (fig. 1-10). Changes were most 

pronounced for algal communities, in which the average 

biological condition in streams with elevated nutrients (greater 

than 1.5 mg/L of nitrogen and 0.075 mg/L of phosphorus) was 

about 30 percentage points lower than the algal biological 

condition in streams with the lowest nutrient concentrations. 

This finding is consistent with the direct link between nutrient 

availability and algal growth, whereas the response of higher 

organisms, such as fish and macroinvertebrates, is indirect, 

mediated by transfer of energy from aquatic plants and algae 

via food chains of varying complexity. 

NAWQA findings show that the effects of nutrient 

enrichment on algal biomass (as indicated by chlorophyll a) 

are not uniform in streams across the Nation. Regional 

variations suggest that other factors can influence algal 

communities, such as streamflow, canopy cover, water 

temperature, and clarity. A regression model, based on data 

for 143 sites in 5 different agricultural areas across the 

United States, showed a weak relation between phosphorus 

concentrations and algal biomass. Specifically, only 

12 percent of the variability in algal growth was explained by 

concentrations of phosphorus. The relation was even weaker 

for nitrogen concentrations. Weak relations in some regions 

also can be explained by elevated nutrient concentrations 

that are greater than what plants require, so that additional 

increases in nutrients have little effect on plant biomass. The 

strength of the relation between nutrient concentrations and 

algal biomass for individual study areas increased—explaining 

Figure 1-10. Biological condition 

for algal, macroinvertebrate, 

and fish communities declined 

with increasing concentrations 

of nitrogen and phosphorus 

in streams. Changes were 

most pronounced for algal 

communities, in which the 

average biological condition in 

streams with the highest nutrients 

was about 30 percentage points 

lower than the algal biological 

condition in streams with the 

lowest nutrient concentrations. 
Chapter 1

National Findings and Their Implications 15

Nitrate concentration, in milligrams per liter as N

U.S. population, in millions of people

2.0

1.5

1.0

0.5

0.0

Nitrogen fertilizer input, in millions of tons

0

2

4

6

8

10

12

14

0

50

100

150

200

250

300

1900 1920 1940 1960 1980 2000

U.S. population

Nitrogen fertilizer input

1993–2003 period of record 

used in trend analysis of streams

Blackstone River at Millville, Massachusetts

San Joaquin River near Vernalis, California

Changes in Nutrient Concentrations: 

Past and Predicted

Although concentrations of nitrogen and phosphorus 

in most streams were relatively stable from 1993 to 2003, 

the number of streams with increasing concentrations 

outnumbered those with decreasing concentrations. Nitrate 

concentrations in groundwater also increased over about the 

same time span (Chapter 8).

NAWQA findings provide a framework for assessing 

whether nutrient concentrations in streams and groundwater 

are changing over time and for predicting concentrations in 

the future. The assessment considers many of the complexities 

in nutrient occurrence and transport, both in streams, which 

can respond quickly, even within hours, and in groundwater 

systems, which can take from years to decades for changes 

in quality to occur. In addition, nutrient trend assessments 

in streams are complex because nutrients are influenced by 

natural variations in precipitation and streamflow, as well 

as by human activities that affect nutrient sources (such as 

fertilizer applications) and transport to streams (including 

implementation of management practices such as conservation 

tillage or tile drainage). The natural and human factors 

often occur simultaneously and thus may either counteract 

or supplement each other. Some of these complexities 

are removed in the NAWQA analysis by “flow adjusting” 

measured nutrient concentrations in streams. Flow adjusting 

concentrations helps to remove variability in concentrations 

that may be caused by natural changes in precipitation and 

streamflow, thereby enabling changes caused by human or 

land-use changes to be better assessed. 

The use of nitrogen and phosphorus fertilizers in 

the United States has increased over the last century and, 

more specifically, increased 10-fold and 4-fold, respectively, 

between about 1950 and the early 1980s. There also has been 

a more than three-fold increase in population in the Nation 

since 1900. Increases in nitrogen inputs to the environment 

since 1900 are reflected in the large increases in concentrations 

of nitrate in streams in areas as disparate as the Blackstone 

River in urbanized Massachusetts and the San Joaquin River 

in the agricultural Central Valley of California (fig. 1-11). 

Figure 1-11. The rapid increase in nitrogen inputs to the environment since 1900 is 

reflected in the large increases in concentrations of nitrate in streams as disparate as the 

Blackstone River in urbanized Massachusetts and the San Joaquin River in the agricultural 

Central Valley of California. Since about 1980, application of nitrogen fertilizer has remained 

relatively stable, as have inputs from manure and nitrogen from atmospheric deposition. 

The NAWQA assessment reflects changes that may have occurred from 1993 to 2003 

(yellow vertical shaded area).
16 Nutrients in the Nation’s Streams and Groundwater, 1992–2004

NAWQA findings reflect periodic measurements of 

concentrations of phosphorus at 171 stream sites and of 

concentrations of nitrogen at 137 stream sites from 1993 

to 2003 (fig. 1-11, yellow vertical shaded area). This was a 

period of relatively small changes in nutrient inputs from 

nonpoint sources that followed larger, sustained increases 

during much of the 20th century. 

No significant flow-adjusted trends were noted at 

the majority of stream sites for nitrogen (63 percent) or 

phosphorus (51 percent), suggesting that human and land-use 

activities contributed minimally to nutrient changes over the 

decade at these sites. At the remaining sites, there were more 

upward than downward trends; flow-adjusted concentrations 

increased at 33 and 21 percent of sites for phosphorus and 

nitrogen, respectively, and decreased at 16 percent of sites 

for both nutrients. In some geographic areas and land uses, 

the flow-adjusted trends were related to changes in sources 

of nutrients, including fertilizer, manure, and urban sources. 

For example, at stream sites with greater than 40 percent 

agricultural land in the basin, increases in manure and 

fertilizer applications were associated with increasing trends in 

concentrations of phosphorus.

Geographic patterns in flow-adjusted trends were more 

obvious for phosphorus than for nitrogen, with upward 

trends most prevalent in the central and southwestern parts 

of the United States for phosphorus (fig. 1-12). In addition, 

patterns differed for the two nutrients among different land 

uses. Specifically, streams draining agricultural and mixed 

watersheds showed more upward trends in phosphorus 

(about 33 percent of sites) than in nitrogen (about 20 percent 

of sites). Increased fertilizer inputs throughout much of the 

central United States corresponded to widespread increases 

in phosphorus but not in nitrogen, indicating that the spatial 

differences between trends in the concentrations of phosphorus 

and nitrogen may be due to other factors, such as differences 

in their chemical characteristics. Specifically, some forms of 

nitrogen are more soluble in water and can move to streams 

in both surface runoff and groundwater, whereas phosphorus 

is less soluble in water, tends to attach to soil particles, and 

thereby preferentially moves to streams in surface runoff. 

In areas where groundwater contributions to streams are 

significant, changes in stream nitrogen concentrations can 

lag changes on the land surface by years or even decades as 

groundwater moves through the aquifer to streams.

Findings from the flow-adjusted analysis show that 

human activities contributed to more frequent increases in 

nutrient concentrations in streams with initial concentrations 

below the USEPA recommended nutrient criteria than in 

streams with initial concentrations above the criteria. Nearly 

40 and 30 percent of the sites at which concentrations were 

below the criteria showed upward trends in phosphorus and 

nitrogen, respectively (fig. 1-13A). Although concentrations 

at about half of these sites showed no trend, these findings 

suggest increased risk from human activities on some of our 

Nation’s most pristine streams. Downward trends in nutrient 

concentrations were more pronounced in streams in which the 

recommended nutrient criteria were exceeded; the trends in 

concentrations of phosphorus and nitrogen at nearly 20 percent 

of these sites were downward (fig. 1-13B).

Overall trends in nutrient concentrations in streams 

(trends for which concentrations have not been adjusted to 

remove effects of flow) were upward at fewer sites (24 percent 

for phosphorus and 11 percent for nitrogen) than flow-adjusted 

trends noted during 1993 to 2003 (fig. 1-12). This suggests 

that human-related activities such as those related to increased 

fertilizer applications did not always result in increased 

concentrations in streams because they were offset by changes 

in precipitation and streamflow. The largest difference in flowadjusted versus overall trends in phosphorus concentrations 

occurred in the Central and Southwestern United States, 

where drought conditions in the latter part of the study 

period resulted in reduced surface runoff and streamflow. 

Concentrations of phosphorus in the streams most likely 

would have been higher under more normal precipitation 

and streamflow conditions. The same is true for nitrogen; 

specifically, differences between upward flow‐adjusted trends 

and overall trends in nitrogen also can be accounted for by 

downward trends in streamflow at those sites, although these 

sites are more scattered across the Nation.

The number of downward overall trends was similar 

to the number of downward flow-adjusted trends in nutrient 

concentrations from 1993 to 2003 (about 16 percent each 

for both phosphorus and nitrogen). Streamflow at most of 

these sites did not change significantly from 1993 to 2003, 

which suggests that the decreases in nitrogen and phosphorus 

were often a result of either reduced sources (fertilizers and 

manure applications and point sources) or improved nutrient 

management. At sites where initial concentrations were above 

USEPA’s recommended nutrient criteria, concentrations 

decreased to below the criteria by 2003 in only 2 of 143 

streams for nitrogen and 5 of 103 streams for phosphorus. 
Chapter 1

National Findings and Their Implications 17

17-0191_Fig_1-12_SW_NPtrends_map.ai

EXPLANATION

Upward

No significant trend

Downward

Direction of trend (1993–2003)

Total phosphorus concentration

Total nitrogen concentration

Flow-adjusted trends in streams Overall trends in streams

Figure 1-12. The analysis of flow-adjusted trends indicates that human activities contributed to more increases in nutrient 

concentrations from 1993 to 2003 (33 and 21 percent for phosphorus and nitrogen, respectively) than decreases (about 

16 percent). In the Central and Southwestern United States and parts of the southeast, streamflow decreased at many sites. 

This resulted in fewer overall trends—which reflect the net effect of all influence on nutrient concentrations in streams— 

than flow-adjusted trends. Potential human-related increases in nutrient concentrations at these sites likely were offset by 

decreases in precipitation and runoff. 

The median concentration of nitrate measured in water 

samples from 495 wells increased from 3.2 to 3.4 mg/L 

(6 percent) between the first sampling period of 1988–1995 

and the second sampling period of 2001–2004. In shallow 

groundwater beneath agricultural areas, the median nitrate 

concentration increased from 4.8 to 5.7 mg/L, whereas in 

deep groundwater in major aquifers, the median nitrate 

concentration increased from 1.2 to 1.5 mg/L. The proportion 

of wells with concentrations of nitrate greater than the 

USEPA MCL of 10 mg/L increased from 16 to 21 percent a 

decade later. 
18 Nutrients in the Nation’s Streams and Groundwater, 1992–2004

0191_Fig_1-13

Total nitrogen (34 sites)

Total phosphorus (28 sites)

Total nitrogen (103 sites)

Total phosphorus (143 sites)

A. B. Streams with concentrations in 1993 above 

USEPA’s recommended ecoregional nutrient criteria 

Upward flowadjusted trends Downward flowadjusted trends No significant

change 

Change between 1993 and 2003

0

10

20

30

40

50

60

70

Upward flowadjusted trends Downward flowadjusted trends No significant

change 

Percentage of sites

Streams with concentrations in 1993 below 

USEPA’s recommended ecoregional nutrient criteria 

1930 1940 1950 1960 1970 1980 1990 2000 2010

0

20

40

60

80

Estimated date of recharge

Nitrogen fertilizer input, in millions of tons

Nitrate concentration, in milligrams per liter as N

0

4

8

12

14

10

6

2

Shallow land use wells

Nitrogen fertilizer input

Deep major aquifer wells

NAWQA sampling period

Figure 1-13. Findings from the flow-adjusted analysis show that increasing nutrient concentrations 

related to human and land-use activities from 1993 to 2003 are more common in streams having nutrient 

concentrations in 1993 below U.S. Environmental Protection Agency’s recommended nutrient criteria 

than in streams in which those criteria were exceeded. Although concentrations at about one-half of 

the sites showed no trend, these findings point to increased risk from human activities on some of our 

Nation’s most pristine streams. 

Estimates of recharge dates for individual 

groundwater samples analyzed for nitrate show that 

concentrations have generally increased since about 

1975 (fig. 1-14). Although these samples were collected 

within a recent and narrow range in time, estimated 

recharge dates indicate that the time required for the 

sampled water to travel from the water table to the well 

intake was in the range of years to decades. Elevated 

nitrate concentrations appear in shallow groundwater 

wells as early as the 1950s and 1960s, whereas elevated 

concentrations in the deep aquifers do not appear until 

the 1970s. Recharge dates of the sampled groundwater 

indicate that increases in nitrate concentrations over 

time are consistent with trends in historical fertilizer use 

in the United States.

Unlike in many streams, changes in nutrient 

concentrations in groundwater are probably not 

associated with current human activities but rather 

reflect changes in fertilizer applications and land-use 

practices over time. The lag between what occurs on 

the land surface and chemical changes in water at a 

well means that any improvements in water quality that 

might result from nutrient management practices on 

the surface may not be apparent for years or decades. 

Figure 1-14. Estimates of groundwater recharge dates 

for individual samples show that nitrate concentrations in 

groundwater have generally increased since about 1975. Although 

the samples were collected during a narrow range in time, the 

range in recharge dates represented by these data spans 

many decades.
Chapter 1

National Findings and Their Implications 19

As a result, in some aquifers, only small changes 

in nitrate concentrations may be observed from 

monitoring of relatively deep public-supply 

wells, whereas larger, more rapid changes in 

concentration may be observed during the same 

time period in water from shallow wells. 

Redox conditions (the presence or absence 

of oxygen) in an aquifer influence changes in 

nitrate in groundwater over time. In aquifers 

• Long-term and consistent monitoring of 

nutrients in streams and groundwater 

is essential for distinguishing actual 

trends from short-term fluctuations and 

for accurately tracking changes. This is 

particularly important because nutrient 

concentrations in streams and groundwater 

will continue to respond to climatic and 

landscape changes.

• Upward trends in concentrations of 

phosphorus and nitrogen in streams were 

evident among all land uses, including 

those only minimally affected (as indicated 

by nutrient concentrations lower than 

USEPA’s recommended nutrient criteria). 

These findings point to increased risk of 

impact from human activities on some of 

our Nation’s most pristine streams.

• Understanding the causes of trends in 

concentrations of nutrients in streams 

requires an understanding of streamflow 

and natural and landscape factors, as well 

as information on changes in land-use and 

nutrient management activities over time.

• Understanding the causes of trends in 

concentrations of nitrate in groundwater 

usually requires estimates of groundwater 

age, redox conditions, and an 

understanding of the groundwater flow 

system because of local and regional 

complexities in groundwater flow paths.

• Nitrate concentrations in deep oxic 

aquifers used as a source of water supply 

are likely to increase during the next 

decade as shallow groundwater with 

high concentrations moves deeper into 

aquifers. These increases will continue 

to occur for some period of time even if 

nitrogen inputs on the land 

surface decrease.

• Improvements in nutrient management 

practices on the land surface may take 

years to decades to produce improved 

groundwater quality because of the 

slow rate of groundwater flow. Similar 

time lags also are expected for streams 

that receive considerable groundwater 

discharge.

with chemically reduced conditions, changes in 

nitrate concentrations in the aquifer were very 

small because of the conversion of nitrate to 

inert nitrogen gas by bacteria (denitrification). 

In contrast, changes in nitrate concentration over 

time in groundwater under oxic conditions can 

be large because nitrate is stable and persistent 

under these conditions. 

Implications
20 Nutrients in the Nation’s Streams and Groundwater, 1992–2004

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Chapter 2

Chapter

2

Introduction

Nutrients—nitrogen and phosphorus—

are chemical compounds that are 

essential to life and important for 

natural plant and animal growth. 

Efforts to increase food production to 

meet the demands of a growing global 

population have greatly increased 

the use of fertilizers, which have, in 

turn, increased the occurrence, loads, 

and concentrations of nutrients in 

streams and groundwater. Elevated 

concentrations of nutrients can cause 

a variety of problems, including 

degradation of ecosystems, recreational 

activities, and human health. Many 

factors affect the transport and 

concentrations of nutrients in streams 

and groundwater, including the 

intensity and distribution of fertilizer 

use; land management practices and 

other anthropogenic activities; natural 

factors, such as soil and aquifer 

characteristics, and hydrology; and 

the chemical properties of the nutrient 

compounds themselves.

Photograph from Franklin D. Roosevelt Presidential Library Museum.

Nutrient Primer