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 waterquality 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

Total nitrogen outflow yield, in pounds per square mile

Nonpoint nitrogen input, in pounds per square mile

1

10

100

1,000

10,000

1,000 10,000 100,000

Percent nutrients

lost to streams

5%

50%

100,000

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 (72 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 

(72 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

2.0 percent 3.3 percent

Urban Mixed

Percentage of Groundwater Studies

3.0 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). 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 
22 Nutrients in the Nation’s Streams and Groundwater, 1992–2004

Concerns about Elevated Nutrients in Water

The Clean Water Act of 1972 requires that States establish water-quality 

standards for water bodies, such as lakes and streams, “taking into consideration 

their use and value for public water supplies, propagation of fish and wildlife, 

recreational purposes, and agriculture, industrial, and other purposes, and also 

taking into consideration their use and value for navigation.” [Section 303(c)(2)(A)]. 

Although nitrogen and phosphorus are essential nutrients, elevated concentrations 

can lead to a variety of direct and indirect impairment of water bodies for many 

uses, including drinking water, recreation, and aquatic life. 

Drinking-Water Supplies and Human Health

The USEPA has set a standard of 10 mg/L as N of nitrate for drinking water to 

protect against methemoglobinemia, or “blue-baby syndrome,” a disorder in which 

the oxygen-carrying capacity of the blood is compromised. Intake of nitrate and 

nitrite in drinking water also has been implicated in other human health problems, 

including specific cancers and reproductive problems, although more research 

is needed to verify these associations (see Chapter 6 sidebar, Nitrate in Drinking 

Water: Potential Health Effects; Ward and others, 2005). Indirect effects of nutrient 

enrichment in surface waters on human health are numerous and complex. For 

example, algal blooms caused by nutrient enrichment may lead to algal toxins 

in water supplies (Lopez and others, 2008), an increase in the production of 

carcinogenic disinfection byproducts during chlorination of drinking water, and 

the potential for increased disease transmission due to an increase in mosquito 

populations (Townsend and others, 2003). Objectionable taste and odor are 

secondary issues related to nutrient-induced algal blooms that, while not a health 

hazard, are costly for drinking-water managers. 

Recreational Uses

Elevated nutrients can cause algal blooms and over-growth of other aquatic 

plants, which can cause aesthetic impairment, and interfere with recreational 

swimming and boating. In Nebraska, for example, the Department of Environmental 

Quality posts health alerts to warn residents when algal toxins reach levels unfit 

for recreational activities in specific lakes (Nebraska Department of Environmental 

Quality, 2009). 

Nutrients and Their 

Sources

Nutrients are essential for healthy 

plant and animal populations and provide 

a range of benefits, including increased 

food production for a growing global 

population. Too many nutrients, however, 

are not necessarily a good thing, and can 

have adverse effects on water quality, 

drinking‐water sources, recreation, and 

aquatic life (see sidebar, Concerns about 

Elevated Nutrients in Water). For example, 

excessive nitrate in drinking water can 

lead to “blue-baby syndrome,” in which 

oxygen levels in the blood become too 

low, sometimes with fatal results. Elevated 

concentrations of nitrogen and phosphorus 

in surface water can cause excessive growth 

of algae and other nuisance aquatic plants 

(a condition known as eutrophication). 

These plants can clog pipes and interfere 

with recreational activities such as fishing, 

swimming, and boating. Subsequent decay 

of algae can result in foul odors and a 

decrease in the amount of dissolved oxygen 

in water, also known as hypoxia. Hypoxic 

conditions, such as those found in the Gulf 

of Mexico, can harm fish and shellfish 

that are economically and ecologically 

important to the Nation. Societal concerns 

for the quality of our surface waters 

continue to increase, as many of the 

Nation’s streams and coastal waters do not 

meet water-quality goals. Data submitted 

by States in 2004, the most recent reporting 

date to the U.S. Environmental Protection 

Agency (USEPA) (2009), indicate that 

51 percent of the waters they surveyed are 

too contaminated for basic uses, such as 

fishing and swimming, because of their 

nutrient content.

In Nebraska, the Department of 

Environmental Quality posts health 

alerts to warn residents when algal 

toxins in lakes reach levels that make 

them unfit for recreational activities. 

Photograph courtesy of Tadd Barrow, 

University of Nebraska, Lincoln.
Chapter 2

Nutrient Primer 23

B.

Texas Louisiana

95° 90°

31°

30°

29°

28°

27°

26°

0 200 MILES

0 200 KILOMETERS

Bottom oxygen

values, in

milligrams per liter

0–2

2–4

4

Estuaries with low dissolved oxygen A.

Biological Condition of Freshwaters

Excessive nutrients and the resulting excessive 

plant biomass can have negative impacts on aquatic 

life. Increased plant biomass due to nutrient 

enrichment can cause a wide range of problems 

including a reduction in levels of dissolved oxygen 

(critical for aquatic life), decrease in quality of 

habitat, decrease in water clarity, enhancement of 

the growth of invasive species and toxic algae, and 

overall alteration of biological health (Lopez and 

others, 2008). Less often, ammonia concentrations 

reach levels toxic to fish, and national standards 

have been established for both acute and chronic 

exposure. In some settings and during different times 

of year, excessive nutrients can lead to “Harmful Algal 

Blooms,” or HABs, in which the population of algae 

grows to abnormal levels and either produces toxic 

compounds and (or) causes severe oxygen depletion 

associated with the die-off of the algal bloom 

(Bushaw-Newton and Sellner, 1999; Lopez and others, 

2008). HABs are most commonly found in eastern 

coastal waters and are due primarily to tiny marine 

organisms called dinoflagellates. Although HABs are 

less well documented in freshwater, most cases in 

northern States are associated with blue-green algae 

(Lopez and others, 2008); however, the golden algae 

Prymnesium parvum is documented as a producer of 

a potent toxin to fish in freshwaters in southern States 

(Lopez and others, 2008). 

Figure 2-1. Excess nutrients in runoff have resulted in hypoxia—low 

dissolved oxygen concentrations in water—in estuaries around the 

country (A), including the Gulf of Mexico (B). (A, modified from National 

Science and Technology Council Committee on Environmental and Natural 

Resources, 2003; and B, data from National Oceanic and Atmospheric 

Administration, 2004.)

Fish kill associated with a bloom of the golden algae, 

Prymnesium parvum, on Lake Whitney in Texas forms a 

silver band of dead fish along the shoreline. Photograph 

courtesy of Joan Glass, Texas Parks and Wildlife 

Department, from Lopez and others (2008).

Hypoxia in Coastal Waters and the Great Lakes

Most recently, HABs have been identified as a major cause of ecological 

disturbance in large water bodies. The decay of excessive algal biomass can 

deplete the dissolved oxygen in water, creating a condition called hypoxia 

(dissolved oxygen less than 2 mg/L). This can result in stress or death of 

near-bottom or bottom-dwelling organisms. Hypoxia in coastal regions that 

receive excess nutrients from rivers is a serious global problem, and hypoxic 

conditions plague critical waters in the United States including the Gulf of 

Mexico near the Mississippi River, the Chesapeake Bay, Puget Sound, and 

Lake Erie. Development of hypoxic conditions can have consequences for 

economic activities including fisheries and tourism (fig. 2-1).
24 Nutrients in the Nation’s Streams and Groundwater, 1992–2004

Nutrient Forms

Five measures of nitrogen- and phosphorus-containing 

nutrients are discussed in this report. All forms of nutrients 

discussed in this report are expressed as concentrations 

of either nitrogen or phosphorus. For example, a nitrate 

concentration expressed as 10 milligrams per liter (mg/L) 

refers to a nitrate concentration of 10 mg/L as nitrogen (as N).

•	 Total nitrogen includes nitrogen in all its forms—

nitrate, nitrite, ammonia, and organic nitrogen. Nitrite 

generally is unstable in surface water and contributes 

little to the total concentration of nitrogen in water. 

Organic nitrogen (mostly from plant material or organic 

contaminants) can exist in considerable proportions 

and contribute substantially to concentrations of total 

nitrogen in streams. 

•	 Nitrate, a compound of nitrogen and oxygen, is the 

primary form of nitrogen dissolved in streams and 

groundwater. In this report, nitrate refers to the sum 

of nitrate plus nitrite, because nitrite concentrations 

commonly were negligible. 

•	 Ammonia, a compound of nitrogen and hydrogen, 

is a dissolved form of nitrogen that is less common 

than nitrate. As measured by the USGS laboratory, 

total ammonia includes ammonium ion and un-ionized 

ammonia. Un-ionized ammonia typically is a minor 

component of ammonia at acidity levels (or pH levels) 

commonly observed in streams and groundwater. 

•	 Total phosphorus is usually bound to sediment, 

and generally includes dissolved phosphates and 

particulate organic phosphorus (mostly from 

plant material).

•	 Phosphates, compounds of phosphorus, oxygen, 

and hydrogen, typically constitute the majority of 

the dissolved phosphorus in natural water, and are 

referred to as orthophosphate. Orthophosphate can be 

readily assimilated by aquatic plants.

to a 2.5-fold increase in nitrogen- and phosphorus‐driven 

eutrophication (Tilman and others, 2001). Control of 

the delivery of these nutrients into the environment is 

complicated by cycling, in that nitrogen and phosphorus can 

be reintroduced into a system because of remobilization and 

biological activity long after nutrient inputs into the system 

have decreased. 

Nutrients Occur in Dissolved and 

Particulate Forms

Nutrients occur in several forms, including total 

nitrogen, nitrate, ammonia, total phosphorus and phosphate 

(see “Nutrient Forms”). Nitrogen forms available for plant 

uptake include dissolved inorganic nitrogen (ammonia, 

nitrate, and nitrite) and organic nitrogen. Nitrate is 

the primary form of nitrogen dissolved in streams and 

groundwater. Organic nitrogen, mostly from plant material 

or organic contaminants, can be dissolved or particulate 

(bound to sediment or in plant and animal tissue) and can 

contribute substantially to total nitrogen levels in streams. 

The most readily available form of phosphorus for 

plants is dissolved phosphate, which typically constitutes 

the majority of the dissolved phosphorus in natural water, 

and is generally reported as orthophosphate. In addition to 

dissolved phosphate, total phosphorus includes other forms, 

such as particulate organic phosphorus, which is mostly 

from plant material and usually bound to sediment.

Nutrients Occur Naturally but Most are 

Derived from Human Sources

Although nitrogen gas constitutes 78 percent of the 

Earth’s atmosphere, most plants and animals cannot utilize 

it directly, and therefore, they rely on processes that convert 

nitrogen gas into useable inorganic forms. For example, 

lightning and nitrogen “fixing” organisms, including 

legumes, bacteria, and algae, can convert nitrogen gas 

into biologically useable forms, such as ammonium and 

nitrate. Phosphorus also occurs naturally, in large part, from 

phosphate minerals in sediment and rocks. 

The natural global nitrogen cycle has been extensively 

altered through production and application of nitrogen 

fertilizers, cultivation of nitrogen-fixing crops, animal 

waste disposal, wastewater and industrial discharges, and 

combustion of fossil fuels (Galloway and others, 1995; 

Vitousek and others, 1997). These human alterations have 

approximately doubled the rate of nitrogen inputs into 

the terrestrial nitrogen cycle and have greatly increased 

the transfer of nitrogen from rivers to estuaries and other 

sensitive receiving waters. Human activities also have 

profoundly influenced the cycling of phosphorus through 

the environment, doubling the rate of phosphorus delivery 

from land to the oceans (Howarth and others, 2000).

The next 50 years will likely see rapid agricultural 

expansion in response to demand for food and a projected 

50-percent increase in global population, which could lead 
Chapter 2

Nutrient Primer 25

Human-Related Sources Include Point 

and Nonpoint Sources of Contamination

Human-related sources of nutrients can be 

classified as either point or nonpoint contamination. 

Point sources, such as those from municipal and 

industrial discharges and concentrated animal feeding 

operations, are regulated by laws that place limits 

on the types and amounts of contaminants that can 

be released to water. Legislation, starting in 1972 

with the Clean Water Act, has resulted in upgrades to 

wastewater-treatment plants and reductions in industrial 

sources of nutrients. Although violations still occur, 

legislation has had a positive influence on limiting the 

nutrients that enter water systems.

Nonpoint sources of nutrients include applications 

of commercial fertilizers on agricultural and residential 

lands, and nutrients from livestock and pet wastes 

and from septic systems. Atmospheric deposition 

also is a nonpoint source of nitrogen, which is 

derived either naturally from chemical reactions 

(such as through lightning) or from the combustion 

of fossil fuels, such as in coal-fired power plants, or 

volatilization of ammonia from fertilizer and manure 

(U.S. Environmental Protection Agency, 2002). Under 

some circumstances, nitrogen fixed by leguminous 

crops or stored in the soil can become a nonpoint 

source. Nutrients derived from nonpoint sources can be 

transported to streams in runoff from precipitation or 

irrigated fields, with inflowing groundwater, or through 

drainage ditches and subsurface tile-drain systems 

(hereinafter referred to as tile drains). 

Limiting nutrients from nonpoint sources is 

difficult because these sources are widespread and 

thus more difficult to identify and quantify than point 

sources. In fact, nonpoint-source contamination is 

the leading and most widespread cause of nutrient 

degradation of water; NAWQA studies estimate that 

more than 90 percent of nitrogen and phosphorus 

released to the environment originates from nonpoint 

sources (Puckett, 1995); the remaining percentages are 

from point sources.

Coal-fired power plants are sources of nitrogen in atmospheric 

deposition. Photograph by U.S. Geological Survey.
26 Nutrients in the Nation’s Streams and Groundwater, 1992–2004

Fertilizer is used extensively in corn and soybean growing areas of Iowa. 

Photograph by Lynn Betts, U.S. Department of Agriculture, Natural Resources 

Conservation Service.

17-0191_Fig_2-02_Nut_input.ai

0

2

4

6

8

10

12

14

1940 1950 1960 1970 1980 1990 2000 2010

Nutrient input, in millions of tons

Phosphorus in manure

(1982, 1987, 1992, 1997)

Phosphorus fertilizer

(1945–2006)

Atmospheric nitrogen

(1985–2001)

Nitrogen in manure

(1982, 1987, 1992, 1997)

Nitrogen fertilizer

(1945–2006)

EXPLANATION

Nutrient Sources Have Increased Over the 

Last Century but Have Remained Stable 

Since about 1980

The use of nitrogen and phosphorus fertilizers has 

increased over the last century, by 10-fold and 4-fold, 

respectively, between about 1950 and the early 1980s 

(fig. 2-2). The increase was due, in large part, to increases in 

areal application rates (the amount of fertilizer applied to each 

acre for some major crops) and an increase in the acreage 

of treated land. For example, the areal application rate of 

nitrogen fertilizer to corn grown in the Midwest increased 

from 49 to 129 pounds per acre from 1965 to 1980, after 

which it remained stable (Cassman and others, 2002). 

Since about 1980, applications of nitrogen and 

phosphorus fertilizers have leveled off and have since 

remained relatively stable (fig. 2-2). This is due in large part 

to increasing fertilizer costs, growing environmental concerns, 

and changing agricultural and waste management practices, 

which have reversed or slowed the pace of increasing use. 

Nitrogen and phosphorus from manure and nitrogen from 

the atmosphere also have remained relatively stable since the 

1980s (fig. 2-2).

Nationally, commercial fertilizer is the largest single 

nonpoint source of nutrients. More than 10 million tons of 

nitrogen and nearly 2 million tons of phosphorus are applied 

each year as commercial fertilizer (fig. 2-2). Most of the 

applications are for agricultural purposes, although a small 

portion, about 2 to 4 percent, of the total nitrogen fertilizer 

is used in nonagricultural settings such as city parks, golf 

courses, and residential lawns. 

About 6 million tons of nitrogen and nearly 2 million tons 

of phosphorus are excreted as manure each year. The mass of 

nitrogen in manure produced by livestock is about one-half 

as much as that applied in fertilizer and has varied little since 

data became available in 1982. The mass of phosphorus in 

manure is about the same as phosphorus applied in fertilizer, 

and also has varied little since 1982 (fig. 2-2). 

A significant proportion of the commercial fertilizer and 

manure that is applied to crops is taken up by the plants and 

removed from the fields, and hence does not contribute to 

environmental problems. Nitrogen uptake efficiencies have 

increased over the past two decades, and although uptake 

efficiencies as high as 80 percent in major cereal crops have 

been attained in controlled research experiments, uptake 

efficiencies on production farms usually range from 18 to 

49 percent (Cassman and others, 2002). 

Atmospheric deposition contributes about one-half as 

much nitrogen as manure does, and less than one-quarter as 

much nitrogen as does fertilizer application. Specifically, 

about 2.5 million tons of nitrogen are deposited in the United 

States each year from the atmosphere (fig. 2-2). Phosphorus 

generally does not enter the environment with 

atmospheric deposition. 

Figure 2-2. The use of nitrogen and phosphorus fertilizers has 

increased 10-fold and 4-fold, respectively, between about 1950 

and the early 1980s. Since about 1980, however, applications of 

nitrogen and phosphorus fertilizers have leveled off and have 

remained relatively stable. Nitrogen and phosphorus from manure 

and nitrogen from the atmosphere also have remained relatively 

stable since the 1980s. (Modified from Ruddy and others, 2006.)
Chapter 2

Nutrient Primer 27

Figure 2-3. Different sources of nitrogen and phosphorus predominate in streams that represent different land-use 

categories. (See table 3-1 for definition of land-use categories.)

Figure 2-4. Both the areal rate of fertilizer application (A) and the total use 

(B) vary widely among major crops (U.S. Department of Agriculture, 2006).

0 20 40 60 80

Nutrient input, in percent

Mixed

0 20 40 60 80

Nutrient input, in percent

Undeveloped

0 20 40 60 80

Nutrient input, in percent

Urban

0 20 40 60 80

Atmospheric

Nonfarm

fertilizer

Manure

Farm

fertilizer

Phosphorus

Nitrogen

Agricultural

Nutrient input, in percent

Data on atmospheric phosphorus inputs not available.

Phosphorus

Nitrogen

0 75 150

Soybeans

Cotton

Wheat

Corn

Average nutrient application rate,

by selected crop, 1997,

in pounds per fertilized acre

A. B.

0 2 4 6

Soybeans

Cotton

Wheat

Corn

Estimated nutrient use,

by selected crop, 1997,

in millions of tons

Sources of Nutrients by Land Use and Crop Type

The NAWQA approach to categorizing stream basins results in different sources of nitrogen and phosphorus predominating 

in streams that represent different land-use categories. Commercial fertilizers constitute more than 60 percent of nitrogen and 

phosphorus inputs to agricultural basins, and more than 50 percent to urban basins (fig. 2-3). Manure also can be a significant 

source of nutrients in agricultural watersheds, accounting for about 25 percent and nearly 40 percent, respectively, of nitrogen 

and phosphorus inputs. Animal manure on pasture and rangelands, and atmospheric deposition, are the largest contributors of 

nutrients in undeveloped lands.

In general, the use of commercial 

fertilizers on agricultural land varies, 

depending on the type of crop, with 

the highest areal rates of application 

on corn (about 130 pounds of nitrogen 

per acre; fig. 2-4A). Corn also received 

the largest proportion of nitrogen from 

commercial fertilizer, representing 

about 40 percent of the total use in 1997 

(fig. 2-4B). In contrast, the areal rate of 

nitrogen fertilizer application on soybean 

plants is relatively low (about 30 pounds 

per acre; fig. 2-4A) because bacteria in 

the roots of soybean plants are capable 

of extracting, or “fixing,” nitrogen gas 

from the atmosphere. As a consequence, 

less than 2 percent of the nitrogen 

from commercial fertilizer was used on 

soybean cultivation in 1997 (fig. 2-4B). 
28 Nutrients in the Nation’s Streams and Groundwater, 1992–2004

From about 1940 to 1970, laundry 

detergent was a major source of phosphorus 

to the environment. Contributions decreased 

to almost negligible amounts, however, after 

the enactment of State bans on the use of 

phosphate detergent beginning in the 1970s 

and the voluntary cessation of phosphate use 

by detergent manufacturers. As a result, the use 

of phosphorus in detergents decreased from a 

peak of 220,000 metric tons in 1967 to less than 

10,000 metric tons in 1995 (Litke, 1999).

Sources of ammonia also have decreased 

since the early 1970s when legislation was 

enacted to improve the treatment of wastewater. 

The decreased ammonia concentrations achieved 

through treatment nitrification (or the oxidation 

of ammonia to nitrate) of wastewater prior to 

discharge also has increased levels of dissolved 

oxygen in streams, which can help to improve 

the health of fish communities. However, 

because ammonia is converted to nitrate during 

nitrification, nitrate has increased in some 

streams downstream from wastewater-treatment 

plants, such as in the Trinity River, Texas (see 

Chapter 8 sidebar, Past Changes Resulting from 

Nutrient Control Measures).

Sources of Nutrients Vary 

Geographically and by Land Use

The relative contributions of the different 

nonpoint nutrient sources vary in different 

geographic regions of the country (fig. 2-5) and 

can be explained, in large part, by the distribution 

of agricultural, urban, and undeveloped land 

use (see sidebar, Sources of Nutrients by 

Land Use and Crop Type). The highest rates 

of farm fertilizer nitrogen application are in 

the upper Midwest, where an application rate 

of 40,000 pounds per square mile (lb/mi2) is 

exceeded over an extensive area from Indiana to 

Nebraska. Farm fertilizer nitrogen application 

rates are also high in parts of the irrigated West, 

such as the Snake River Plain in southern Idaho, 

California’s San Joaquin Valley, parts of Florida, 

and parts of the east coast. 

Some parts of the Southeast and Southern 

Plains and the extensive western rangelands 

receive high loads of nitrogen (locally exceeding 

40,000 lb/mi2) in the form of manure, but 

such areas of high manure applications are not 

as extensive as those of high applications of 

commercial fertilizers. Manure from animals that 

are restricted to confined feeding operations, such 

as at dairies or feedlots, is usually stored onsite 

for later use on adjacent agricultural land; manure 

from free-ranging livestock is not recovered, 

but rather is distributed across the grasslands, 

shrublands, and forests used for grazing. 

Nitrogen applications of nonfarm fertilizer, 

such as that used on lawns in residential areas, 

generally are restricted to population centers, and 

rarely exceed 10,000 lb/mi2 outside of densely 

populated areas (fig. 2-5). 

Atmospheric sources of nitrogen are highest 

in the eastern part of the country, where the 

deposition rates are highest (fig. 2-5). In general, 

the rate of atmospheric deposition of nitrogen 

increases progressively from the west to east. The 

highest deposition rates of nitrogen occur in a 

broad band from the upper Midwest through 

the Northeast. 
Chapter 2

Nutrient Primer 29

Figure 2-5. The highest rates of commercial fertilizer applications of nitrogen are in agricultural areas of the upper Midwest, and 

parts of the Great Plains, the Northwest, California, and the southeast Atlantic Coastal Plain. Commercial fertilizers also are a leading 

source of nutrients in urban areas around the Nation. Manure sources dominate in parts of the South and Southeast and in extensive 

western rangelands. Atmospheric sources are highest in the eastern part of the country, where the deposition rates are highest, and 

increase from west to east.

17-0191_Fig_2-05_NInput_Maps_newcat_addlmap.ai

Nitrogen from sewage

Nitrogen from nonfarm fertilizer Nitrogen from atmospheric deposition

Nitrogen from farm fertilizer Nitrogen from livestock manure

Estimated 1997 nitrogen input rate from farm and nonfarm fertilizer, 

manure, atmospheric deposition, and sewage, in pounds 

per square mile

>0–≤1,000

>1,000–≤2,000

>2,000–≤3,000

>3,000–≤4,000

>4,000–≤5,000

>5,000–≤6,000

>6,000–≤8,000

>8,000–≤10,000

>10,000–≤20,000

>20,000–≤30,000

>30,000–≤40,000

>40,000

No nutrient input

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

Connecticut River, Connecticut

Susquehanna River, Pennsylvania White River, Indiana

Palouse River, Washington

San Joaquin River, California 

South Platte River, Colorado 

White River, Arkansas

Proportions of 

nitrogen sources for

selected watersheds

Atmosphere

Fertilizer Manure

Sewage

Fertilizer

Manure

Atmosphere

Sewage

EXPLANATION

Dominant source 

of nitrogen, by county

Different Sources of Nitrogen Predominate 

in Different Regions of the Country

Fertilizer is the predominant source of nitrogen in 

most agricultural areas (fig. 2-6), including in the Midwest 

and Great Plains (White River Basin, Indiana), the 

irrigated West (Palouse River Basin, Washington, and the 

San Joaquin River Basin, California), and in parts of the 

Southeast. Nutrients from manure, however, can dominate 

those from commercial fertilizers in some agricultural 

watersheds with large populations of confined livestock, 

such as in parts of the East, South, and Southeast (see areas 

exceeding 40,000 lb/mi2, fig. 2-5), including in the White 

River Basin in Arkansas, the Susquehanna River Basin in 

Pennsylvania, and in the extensive western 

rangelands (fig. 2-6).

Some urban watersheds receive significant nutrient inputs 

from wastewater-treatment plants, such as in the South Platte 

River Basin, Colorado (fig. 2-6). Occasionally, streams are 

dominated by wastewater effluent from treatment plants; this 

circumstance is more common in urban areas of the semiarid 

West, including parts of Arizona, Nevada, and southern 

California, where wastewater-treatment plant effluent may 

be the predominant source of water in a stream during

 dry seasons.

Atmospheric deposition is the largest nonpoint source 

of nitrogen in undeveloped watersheds in the eastern part of 

the country where the deposition rates are highest, such as in 

the Connecticut River Basin, Connecticut (fig. 2-6), and in 

areas near the Great Lakes and the arid and mountainous West 

where human development is very sparse.

Figure 2-6. Commercial fertilizers are the dominant source of nitrogen in the White River Basin in Indiana, 

which is typical of their intensive use in the upper Midwestern part of the Nation, and in parts of the Northwest 

and West, such as in the Palouse River Basin in Washington and the San Joaquin River Basin in California. 

Commercial nitrogen fertilizers also are the major source of nitrogen in urban basins, and some urban watersheds 

also have significant nutrient contributions from wastewater-treatment plants, such as in the South Platte River 

Basin, Colorado. Nutrients from manure can even dominate those from commercial fertilizers in some agricultural 

watersheds with large populations of confined livestock, such as in the White River Basin in Arkansas and in the 

Susquehanna River Basin, Pennsylvania, as well as in the extensive western rangelands. Atmospheric contributions 

of nitrogen are important in the eastern parts of the country, such as in the Connecticut River Basin.
Chapter 2

Nutrient Primer 31

Streams are particularly

vulnerable to nutrient

contamination because runoff

from agricultural and urban

areas flows directly into streams

along with both dissolved and

particle-associated nutrients.

Nonpoint sources are diffuse 

and widely dispersed, such as 

agricultural runoff and 

atmospheric deposition.

Nitrogen is transported to the 

atmosphere by burning of fossil 

fuels and may also volatilize 

from fertilizer and manure.

Nutrients are transported to 

groundwater mostly by recharge 

resulting from rainfall or irrigation 

within agricultural and urban 

areas. Other locally important 

sources include transport down 

leaky well casings and contaminated streams that lose water to 

the groundwater system.

Point-source contamination can

be traced to specific points of

discharge from wastewatertreatment plants and factories

or from combined sewers.

REGIONAL TRANSPORT

The atmosphere is a primary source of

nitrogen, which returns to earth with precipitation 

and dry deposition. In the natural nitrogen cycle, 

nitrogen gas in the atmosphere is converted to 

biologically-available forms by bacteria in soils or 

associated with plants.

Groundwater is an important source of 

nutrients to some streams during 

low-flow periods, when groundwater 

constitutes a major portion of streamflow.

DRY

DEPOSITION

GROUNDWATER RECHARGE

DISCHARGE

TO STREAMS

RUNOFF

RECHARGE

FROM

STREAMS

PRECIPITATION

ENTRY

THROUGH

WELLS

WASTEWATER

EVAPORATION

SPRAY DRIFT

WIND EROSION

RUNOFF

RECHARGE

Figure 2-7. Nutrients from nonpoint and point sources are cycled throughout the hydrologic system, but may be affected by different 

chemical, physical, and biological processes in different parts of the system. (Modified from Majewski and Capel, 1995.)

Nutrient Transport to Streams and 

Groundwater

Nutrients released into the environment as diffuse 

nonpoint sources or as point sources enter streams along with 

runoff from precipitation, irrigation, or through drainage 

ditches and tile drains; are transported to groundwater by 

infiltrating rainfall or irrigation; and are transported to the 

atmosphere by volatilization either directly from a source or 

from a contaminated surface-water body (fig. 2-7). Within 

each of the hydrologic compartments, nutrient concentrations 

are affected by physical features, such as soils and slope of 

the land, as well as by biological and geochemical processes 

that can change the chemical form of the nutrient and (or) 

transfer it from the water to a solid phase or to the atmosphere. 

The chemical, physical, and biological processes that 

influence nutrient transport vary in intensity among different 

environmental settings. 
32 Nutrients in the Nation’s Streams and Groundwater, 1992–2004

Agricultural Best Management Practices

Agricultural best management practices (BMPs) are designed to minimize the environmental impacts of 

agriculture while sustaining crop productivity. BMPs reduce nutrient losses to streams through management 

of nutrient inputs on the land surface and through curtailment of erosion and runoff of nutrients from the 

land surface to streams. Three common BMPs are conservation tillage, nutrient management plans, and 

conservation buffers (U.S. Environmental Protection Agency, 2008). 

•	 Conservation tillage—Conventional tillage exposes the soil surface by plowing crop residues into 

the soil. In contrast, conservation tillage limits tilling while retaining crop residue on the soil surface, 

thereby increasing infiltration and reducing erosion and runoff to streams.

•	 Nutrient management plans—Comprehensive nutrient management plans help guide decisions on 

the placement, rate, timing, form, and method of nutrient application to avoid inputs in excess of crop 

requirements and to minimize loss to streams, groundwater, or the atmosphere. Nutrient management 

plans can incorporate a variety of agronomic tests to balance the amount of nutrients currently 

available in the soil against the amount required for crop production, and to identify the ideal timing 

for crop growth and irrigation to minimize runoff and leaching.

•	 Conservation buffers—Conservation buffers are areas of permanent vegetation often planted 

adjacent to streams, lakes, ponds, and wetlands or along the edges of agricultural fields to help 

reduce runoff or leaching of nutrients by filtering out nutrients and sediments, enhancing infiltration, 

and increasing plant uptake. The vegetation in buffers also can provide wind breaks, wildlife habitat, 

cooler water temperatures, stream-bank stabilization, and improved aesthetics.

BMPs often are included as part of cooperative conservation plans developed through the U.S. 

Department of Agriculture’s (USDA) Farm Service Agency and Natural Resources Conservation Service, 

other Federal, State, and local agencies, private companies, and individual landowners. Four of the largest 

national USDA conservation programs are the Conservation Reserve Program (CRP), the Conservation 

Reserve Enhancement Program (CREP), the Wetlands Reserve Program (WRP), and the Grassland Reserve 

Program (GRP). These programs are all voluntary. Farmers, ranchers, and other producers enrolled in the 

CRP plant long-term vegetative covers, such as introduced or native grasses or hardwood trees, to improve 

water quality, control soil erosion, and enhance wildlife habitat. As of 2008, 34.72 million acres were enrolled 

in the CRP (U.S. Department of Agriculture, 2008). The CREP, which is part of the CRP, helps producers 

protect environmentally sensitive land, decrease erosion and runoff, restore wildlife habitat, and safeguard 

groundwater and surface water through BMPs such as conservation buffers. As of 2008, 1.13 million 

acres were enrolled in the CREP (U.S. Department of Agriculture, 2008). The WRP provides landowners 

financial incentives and technical assistance to restore wetland functions while maximizing wildlife habitat 

benefits. Wetlands reduce downstream flooding, protect drinking water, filter runoff, and provide vital 

habitat for many wildlife species. As of 2008, 2.0 million acres were enrolled in the WRP (U.S. Department 

of Agriculture, 2008). The GRP helps landowners restore and enhance grasslands on their property and 

protects vulnerable grasslands from conversion to crop land, urban land, or other uses. The GRP has 

enrollments in all 50 States and Puerto Rico (U.S. Department of Agriculture, 2008).

Measuring the cumulative effect of BMPs on water quality in streams across the Nation is a challenging 

task. In the large watersheds studied by the NAWQA Program, which range from less than 8 square 

miles to more than 80,000 square miles, BMPs are one of many influences on water quality in streams. 

Distinguishing the effects of BMPs from the effects of often interrelated changes in land use, source 

inputs, climate, or other factors can be a difficult task—a task that is further complicated by the lack of 

comprehensive national-scale data on when and where various BMPs are implemented across the Nation. 

More comprehensive and detailed data on BMPs could help improve our understanding of the factors 

affecting nutrient conditions in the Nation’s streams and groundwater. Our understanding of the effects of 

BMPs on agricultural landscapes will be advanced by a multi-scale assessment of the environmental effects 

of conservation practices and programs being conducted by the USDA’s Conservation Effects Assessment 

Program (U.S. Department of Agriculture, 2010).
Chapter 2

Nutrient Primer 33

Assessment of the occurrence and transport 

of nutrients in streams and groundwater, 

therefore, requires recognition of complicated 

interconnections among surface water and 

groundwater systems, atmospheric contributions, 

and the natural and human factors that can 

affect transport. Key natural factors, including 

soil type, geology, and slope of the land, can 

govern the amount and timing of transport of 

nutrients to streams and groundwater. Human 

factors and actions that can affect transport 

include irrigation, groundwater pumping, the 

presence of impervious surfaces, artificial 

subsurface drainage, and best management 

practices (see sidebar, Agricultural Best 

Management Practices). The result can be 

varying concentrations of nutrients in streams 

and groundwater, even in watersheds that may 

have similar land-use settings and rates of 

fertilizer use. 

Nutrient transport in water depends on the 

chemical makeup of the nutrients themselves, 

which can affect mobility and persistence. 

Some compounds, such as nitrate, readily 

dissolve and move with water in both streams 

and groundwater. Most forms of phosphorus, 

however, attach to soil particles rather than 

dissolve; these compounds are transported 

to streams with eroded soil, particularly 

during times of high runoff from precipitation 

or irrigation. Groundwater typically is not 

vulnerable to contamination by nutrients, such as 

phosphorus, that attach to soils. 

Nutrient transport in water also depends on 

geochemical conditions of the water, which can 

affect the persistence of a nutrient. For example, 

as long as dissolved oxygen is present in 

groundwater (referred to as oxic conditions) and 

organic carbon content is low, nitrate generally 

is stable and hence persistent in groundwater, 

sometimes for decades (see sidebar, Redox 

Conditions in Aquifers Affect the Stability of 

Nitrate in Groundwater). Groundwater inflow 

to streams, therefore, can be a large source of 

nitrate in some areas (Bachman and others, 

1998; Sprague and others, 2006). If the dissolved 

oxygen in groundwater is depleted, nitrate 

becomes unstable and is converted to nontoxic 

nitrogen gas, a process referred to 

as “denitrification.”

Conservation tillage allows for double-cropping 

of soybeans in the residue left by winter wheat, 

yielding two crops during a single growing season 

and protecting the land from erosion. Photograph 

by Joseph Larson, U.S. Department of Agriculture, 

Natural Resources Conservation Service.

Multiple rows of trees and shrubs, as well 

as a native grass strip, combine in a riparian 

conservation buffer to protect Bear Creek in Story 

County, Iowa. The buffer is a nationally designated 

demonstration area for riparian buffers. Photograph 

by Lynn Betts, U.S. Department of Agriculture,

Natural Resources Conservation Service.
34 Nutrients in the Nation’s Streams and Groundwater, 1992–2004

Streams

The transport of nutrients to streams is driven by 

precipitation or irrigation. In large part, it is driven by the 

timing of the precipitation and associated runoff and drainage 

relative to fertilizer and manure applications, and controlled 

by a multitude of natural and human factors that affect runoff 

and groundwater discharge to streams. Specifically, high 

rainfall, snowmelt, and (or) excessive irrigation, especially 

following recent fertilizer applications, can accelerate 

transport of nutrients to streams. Streams with steep slopes 

and sparse streamside vegetation to slow runoff and sediment 

movement generally are vulnerable to nutrients entering 

their channels. Clayey and compacted soils underlain by 

poorly drained sediment and (or) nonporous bedrock can 

create relatively impermeable surfaces for quick runoff, as 

can extensive urban pavement. Flat areas with tile drains and 

ditches also can provide quick pathways for runoff to streams. 

In contrast, some best management practices help to reduce 

transport of nutrients, particularly those nutrients that bind 

to soil and sediment, such as phosphorus. Examples of such 

practices include (1) the use of polyacrylamide, a chemical 

added to irrigation water to reduce erosion and enhance water 

infiltration, and (2) conversion from rill irrigation to sprinkler 

and drip irrigation systems, which use less water and thereby 

reduce erosion and surface runoff. 

The biology and physical habitat of a stream also can 

affect the transport and occurrence of nutrients in streams. 

Once nitrogen enters a stream, it cycles in and out of the 

water as a result of the activities of plants and animals. 

Production of plant biomass cycles nutrients from the water, 

while decay of plants and animals cycles nutrients back into 

the water. As nutrients cycle between the water and biota, 

they can be eventually lost to the stream either by exchange 

with the atmosphere (in the case of nitrogen) or by transport 

downstream. Nitrogen and phosphorus can both limit 

biological production, with streams in the eastern United 

States more likely to be phosphorus-limited and streams in the 

Western United States more likely to be nitrogen-limited. 

Three physical habitat factors—light, water temperature, 

and streamflow—affect the development and growth of algae 

and aquatic plants, which can thereby affect nutrient uptake 

and hence the subsequent transport of nutrients in a stream. 

Overhanging riparian vegetation can block light from reaching 

the water surface, and suspended sediments can increase 

turbidity and prevent light from penetrating the water. Growth 

of algae and aquatic plants is limited by low-light conditions, 

regardless of nutrient concentrations. Water temperature can 

regulate the rate at which cells can function and grow, with 

warmer water temperatures stimulating more rapid plant 

growth. Streamflow can limit the growth of algae and other 

aquatic plants by limiting nutrient and habitat availability 

during periods of low flow, or by scouring the bottom of the 

stream and reducing the abundance of attached vegetation 

during periods of high flow, such as after a heavy rainfall.

Groundwater

As with streams, transport of nutrients to groundwater 

is driven by precipitation or irrigation, which provide sources 

of water for recharge. Groundwater transport of nutrients is 

different from that in streams because only dissolved forms 

of nutrients can move substantial distances in groundwater. 

Nitrate readily dissolves and moves with water, and is 

therefore often the dominant nutrient species. Particle bound 

constituents, like phosphorus, are largely retained by the soil 

and aquifer materials. In addition, transport of nutrients to 

and within groundwater systems is much less predictable than 

transport in streams because flow is considerably slower and 

more complex than the flow of stream water. 

Natural features that accelerate nutrient transport to 

groundwater generally contrast with those discussed above for 

streams. Nitrate is readily transported to shallow groundwater 

in well-drained areas with rapid infiltration and highly 

permeable subsurface materials. Well-drained and permeable 

soils that are underlain by sand and gravel or carbonate rock 

(also referred to as “karst”) can enable rapid downward 

movement of water. Crop-management practices, which 

commonly are designed to reduce or slow the movement of 

sediment and nutrients to streams, also can increase infiltration 

of water and nutrients into the ground. Pumping groundwater 

can accelerate the flow of water and transport contaminants 

toward the pumped wells. 

Physical characteristics of streams, such as the amount of shading, influence 

nutrient transport. Photograph by U.S. Geological Survey.
Chapter 2

Nutrient Primer 35

390

396

402

408

414

420

426

432

Elevation, in meters

Water table

Otter Tail River

EXPLANATION Datum is sea level

Vertical scale greatly exaggerated

0 500 1,000 METERS Age of groundwater, in years

0–10

10–30

30–40

40

Redox Process

Oxygen reduction

Nitrate reduction

Iron reduction

Carbon dioxide reduction

Sand and gravel

surficial aquifer

Well cluster Land surface

Boundary (dashed where inferred)

Glacial

till

Well screen 

location

Otter Tail River, Minnesota

Redox Conditions in Aquifers Affect the Stability of Nitrate in Groundwater

Under the right conditions, nitrate may be removed from groundwater before it reaches the intake 

screen of a drinking-water well by a biogeochemical process called denitrification. In aquifers, bacteria 

are constantly breaking down natural organic carbon compounds to acquire the energy they need to 

live in much the same way that humans break down the organic carbon compounds in the food we 

eat. Because carbon is oxidized (by losing an electron) and other reactants are reduced (by gaining 

an electron), these are known as oxidation–reduction, or “redox” reactions. Less often, an inorganic 

constituent, such as the reduced form of iron in the minerals pyrite and glauconite, is oxidized. In 

the first reaction in the sequence, bacteria utilize the oxygen dissolved in groundwater to oxidize the 

available organic carbon. As long as dissolved oxygen is present in groundwater, nitrate is generally 

stable and hence persistent. Groundwater containing dissolved oxygen concentrations greater than 0.5 

mg/L is referred to as “oxic.” As oxygen is consumed, groundwater becomes reduced (concentrations 

of dissolved oxygen less than 0.5 mg/L). If dissolved oxygen in groundwater becomes depleted, the 

bacteria will use nitrate to oxidize the organic carbon in a redox reaction called denitrification. Through 

denitrification, nitrate (NO3

-) is converted to nitrogen gas (N2

), which is nontoxic. After nitrate is mostly 

gone, then bacteria can use organic carbon for other redox reactions such as manganese reduction, iron 

reduction, sulfate reduction, and reduction of carbon dioxide to methane. As a result of these reactions, 

nitrate may be removed from the groundwater, only to be replaced by manganese, iron, sulfide, or 

methane (considered markers of reduced waters). Often these various processes occur in succession as 

water moves down and through an aquifer, creating a layered or stratified sequence of water chemistry 

(fig. 2-8).

Figure 2-8. Biogeochemical processes can occur in succession as water moves down and through an aquifer, 

creating a layered or stratified sequence of water chemistry. In the aquifer adjacent to the Otter Tail River, 

Minnesota, nitrate is removed from the groundwater by denitrification in a zone near the water table. (Modified 

from Puckett and Cowdery, 2002.) 
36 Nutrients in the Nation’s Streams and Groundwater, 1992–2004

Natural processes can significantly reduce nutrient 

concentrations in groundwater through chemical and 

biological transformations under the right geochemical, 

or “redox,” conditions (see sidebar, Redox Conditions in 

Aquifers Affect the Stability of Nitrate in Groundwater). 

Through denitrification, bacteria can convert nitrate to 

innocuous, gaseous forms of nitrogen when groundwater 

passes through organic-rich sediment that is low in 

dissolved oxygen. 

Deep aquifers usually contain water with lower 

concentrations of nutrients than shallow aquifers because (1) it 

takes a long time—decades or more, in most cases—for water 

to move from the land surface to deep aquifers (resulting in 

long residence times for groundwater and any solutes, like 

nitrate, it may contain); (2) long travel distances increase 

the likelihood that nutrients will transform, such as through 

denitrification, or attach to aquifer materials; (3) protective 

low-permeability deposits (which inhibit flow and transport) 

may be present between the land surface and deep aquifers; 

and (4) mixing of water from complex flow paths over long 

distances and time periods tends to result in a mixture of landuse influences on the chemical character of deep groundwater, 

including contributions of nutrients from areas of undeveloped 

lands where concentrations are generally lower than those 

from developed lands.

It is important to note that groundwater at all depths is 

part of an integrated and interconnected hydrologic system and 

is subject to contamination as water moves downward from 

the land surface. Future contamination in deep major aquifers 

could pose serious concerns because these aquifers commonly 

are sources of private household and public water supplies, 

and restoration of the purity of this relatively inaccessible and 

slow-moving water would be costly and difficult.

Transport of Nutrients Varies Seasonally and 

with Hydrologic Events

Transport of nutrients to streams and groundwater 

varies seasonally, in large part following seasonal patterns in 

human activities. For example, in Granger Drain, Washington, 

concentrations of nitrate are highest during the non-irrigation 

season when groundwater is the primary source of water to the 

stream (fig. 2-9). Concentrations of total phosphorus, however, 

are highest during the irrigation season, when surface runoff 

delivers phosphorus-rich sediment to the drain. 

The transport of nutrients to streams also can vary 

in response to changes in precipitation and runoff and 

streamflow (see sidebar, Effects of Precipitation and 

Climate on Streamflow and Temporal Variability of Nutrient 

Concentrations). Increases in precipitation and streamflow 

in basins in which point-source discharges or nitrate-rich 

groundwater inflows predominate commonly are accompanied 

by decreases in nutrient concentrations as a result of greater 

instream dilution by the relatively clean runoff. The same 

increase in precipitation and streamflow in basins in which 

nonpoint sources predominate, however, can increase nutrient 

concentrations as a result of increased transport by surface 

runoff. For example, in the Hockanum River, which drains 

an urban watershed in Connecticut, greater dilution of the 

relatively constant nutrient input from point sources led to 

lower total phosphorus concentrations at higher streamflows 

(fig. 2‐10). The opposite pattern occurred in Sugar Creek, 

which drains an agricultural watershed in Indiana—increased 

nutrient transport in surface runoff from extensive corn 

and soybean fields upstream led to higher total phosphorus 

concentrations at higher streamflows.

The transport of nutrients to streams also can vary 

seasonally in response to rates at which nutrients are taken out 

of the water during growth by aquatic organisms (“uptake”). 

For example, total nitrogen concentrations were lowest in late 

summer in some agricultural streams due to biological uptake 

and relatively low rates of fertilizer applications. 
Chapter 2

Nutrient Primer 37

May June July Aug Sept Oct Nov Dec Jan

Total phosphorus concentration,

in milligrams per liter

0

0.2

0.4

0.6

0.8

1.0

1.2

Nitrate concentration,

in milligrams per liter as N

0

1

2

3

4

5

6

7

Irrigation season Non-irrigation season

1999 2000

Granger Drain at Granger, Washington

Total phosphorus concentration

Nitrate concentration

EXPLANATION

Streamflow, in cubic feet per second

Sugar Creek at New Palestine, Indiana

0 500 1,000 1,500 2,000

0.0

0.1

0.2

0.3

0.4

0.5

Hockanum River near East Hartford, Connecticut

Total phosphorus concentration,

in milligrams per liter

0 100 200 300 400 500 600 700

0.0

0.2

0.4

0.6

0.8

1.0

Part of the flow of the Yakima River is diverted to provide irrigation 

water for crops in Central Washington. Photograph by Henry Ngam, U.S. 

Geological Survey.

Figure 2-9. Concentrations of nitrate and total phosphorus 

in Granger Drain, Washington, vary in response to their major 

transport pathways. Nitrate concentrations are highest during 

the non-irrigation season, when groundwater is the primary 

source of water to the stream, whereas total phosphorus 

concentrations are highest during the irrigation season, when 

surface runoff transports phosphorus-rich sediment to the 

stream (Fuhrer and others, 2004). 

Figure 2-10. Total phosphorus concentrations in the Hockanum River, Connecticut, an urban site with 

numerous upstream point sources, decreased as streamflow increased because of greater instream 

dilution. In contrast, total phosphorus concentrations in Sugar Creek, Indiana, increased as streamflow 

increased as a result of greater surface runoff from extensive fields of corn and soybeans.
38 Nutrients in the Nation’s Streams and Groundwater, 1992–2004

Figure 2-11. Areas of the conterminous United States where long-term droughts 

(several years to decades) are positively correlated (red) or negatively correlated (blue) 

with changes in ocean temperature in the North Pacific (A) and the North Atlantic (B). 

(Modified from McCabe and others, 2004.)

A.

B.

Positive values are shaded red, and negative values are shaded 

blue. Darker shades indicate stronger correlation (>0.4 or <-0.4).

Effects of Precipitation and Climate on Streamflow and 

Temporal Variability of Nutrient Concentrations

Michael Dettinger, U.S. Geological Survey

Daily, seasonal, and long-term changes in climate can affect streamflow, which in turn affects nutrient 

variability in streams. Specifically, daily variability in surface temperature, light availability, and precipitation 

can contribute to short-term (minutes to days) changes in local hydrologic processes such as runoff, 

snowmelt, and evapotranspiration, that in turn affect streamflow. Seasonal changes (weeks to months) in 

large-scale weather patterns can bring seasonal dry or wet periods and the start and end of growing seasons. 

Longer changes (years) are tied to “El Nino” events in the tropical Pacific, which yield a recurring pattern 

of winter precipitation that favors a wetter-than-normal Southwest and Florida and a drier-than-normal 

Northwest. The opposite of an El Nino in the tropical Pacific is a “La Nina,” which reliably yields wetterthan-normal winters in the Northwest and drier-than-normal winters in the Southwest. Changes that last for 

decades at a time, such as prolonged droughts, are related to climatic fluctuations over the North Pacific and 

North Atlantic (fig. 2-11).
Chapter 2

Nutrient Primer 39

Onset of spring snowmelt

>20 days earlier

15–20

10–15

5–10

between 5 days earlier and 5 days later

5–10 days later

10–15

15–20

>20

Large circles indicate sites with trends that differ significantly from 

zero at 90-percent confidence level; small circles are not confidently identified.

140° 130° 120° 50°

45°

40°

35°

110°

Additional multi-decadal streamflow variability may be related to long-term climate change in 

response to increasing greenhouse-gas concentrations in the global atmosphere, or to naturally 

occurring variability in winter and spring temperatures and in the form and timing of precipitation. 

Warming has occurred across the West, upper Midwest, and Northeastern United States, while the 

Southeast has actually cooled overall. Warmer winters and springs in the Western United States 

have contributed to widespread hydrologic changes, including trends towards more precipitation as 

rain rather than as snow, less snowpack overall, earlier greening of vegetation, and earlier snowmelt 

(fig. 2-12).

Figure 2-12. Spring snowmelt onset—the date on which snowmelt runoff adds a large 

initial surge to rivers—occurs a week to almost three weeks earlier now than in the middle 

of the 20th century in the West (Dettinger, 2005). Changes in the timing of streamflow have 

implications for agricultural water management and hence nutrient transport. 

Nutrient concentrations respond to these short-term and longer-term temporal changes in 

streamflow. Specifically, nutrient concentrations can change throughout the year and over a 24-hour 

period in response to changes in light availability, temperature, and in turn, biological productivity 

(Kent and others, 2005; Peterson and others, 2005). Concentrations in some areas are highest during 

spring and summer wet periods following fertilizer application. Concentrations in other areas are 

highest during winter low-flow periods because of groundwater inflow and reduced biological uptake. 

Finally, annual and decadal changes in precipitation can lead to long-term changes in surface runoff or 

instream dilution, contributing to long-term changes in nutrient concentrations in streams.
40 Nutrients in the Nation’s Streams and Groundwater, 1992–2004

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Chapter 3

Chapter

3 NAWQA’s Approach 

to Nutrient 

Assessment 

Introduction

The NAWQA Program’s assessment of nutrients during 1992–2004 provides one of the most comprehensive 

analyses to date of nutrient occurrence in streams and groundwater of the United States. The assessment 

followed a national study design in which some of the most important hydrologic systems were studied on 

a rotational schedule using nationally consistent sampling and analytical methods. This approach yields 

an understanding of water‐quality conditions in a national context, while also supporting comparisons and 

assessments within and among individual watersheds, aquifers, and geographic regions. By systematically 

relating nutrient occurrence and transport to key factors that control nutrient contamination—such as 

nutrient use and properties, land use, hydrology, and other environmental features—NAWQA provides 

information needed for managing nutrients within the Nation’s diverse land-use and environmental settings.

EXPLANATION

National Water-Quality Assessment Study Units

Study Units sampled from 1992 to 2001

Study initiated in 1991

Study initiated in 1994

Study initiated in 1997

High Plains Regional 

Groundwater Study, 

initiated in 1999
42 Nutrients in the Nation’s Streams and Groundwater, 1992–2004

Sampling by NAWQA targeted specific land uses including 

agricultural (top), urban (middle), and undeveloped (bottom) lands.

Targeted Sampling Across the Nation’s Diverse 

Land Uses and Environmental Settings

This report is based primarily on results from NAWQA’s first decade 

of water-quality assessments, which were completed on a rotational 

schedule from 1992 to 2001 in 51 major hydrologic systems across 

the country—referred to as Study Units—as well as in the High Plains 

Regional Groundwater Study, using a nationally consistent study design. 

Assessments were conducted in 20 Study Units during 1992–1995; in 

16 Study Units during 1996–1998; and in 15 Study Units during 1998–

2001. Collectively, the 51 NAWQA Study Units and the High Plains 

Study cover a substantial portion of the Nation’s land area; account for 

more than 70 percent of total water use and more than 50 percent of the 

population served by public water supplies and domestic wells; and are 

representative of the Nation’s diverse landscapes, hydrologic systems, 

ecological resources, and land uses. Water samples were collected from 

selected streams and aquifers through 2004 to assess the effects of 

nutrient enrichment on stream ecosystems and to assess trends in nutrient 

concentrations (Chapters 7 and 8, respectively).

The primary objectives of the NAWQA nutrient assessment were to 

determine: (1) the occurrence and concentrations of nutrients in streams 

(ranging from small streams to large rivers) and groundwater; (2) where 

and when nutrients occur in relation to factors that govern their sources 

and transport in the hydrologic system; (3) whether any nutrients may be 

present at concentrations that could affect human health or aquatic life; 

and (4) how concentrations are changing over time. To address these goals, 

NAWQA employed a targeted assessment focusing on studies of:

• Streams and shallow groundwater in specific, relatively 

homogeneous land‐use and environmental settings to relate nutrient 

occurrence to individual types of nonpoint sources; and

• Streams and major aquifers (regionally extensive aquifers that 

are important groundwater resources for water supply) in areas 

of mixed land uses to evaluate the integrated effects of multiple 

sources of nutrients on their occurrence and concentrations.

Details on the sampling design and analytical methods, as well as 

all data used in this report, are available at: http://water.usgs.gov/nawqa/

nutrients/pubs/circ1350.

For the targeted assessment by land use, streams and shallow 

groundwater were sampled in agricultural and urban areas, and in 

undeveloped areas dominated by forest or rangeland. As described in more 

Photograph by Rand Schaal, Ph.D., pilot and photographer.

Photograph by Lynn Betts, U.S. Department of 

Agriculture, Natural Resources Conservation Service.

Photograph by R.G. Johnson, U.S. National Park Service.
Chapter 3

NAWQA’s Approach to Nutrient Assessment 43

detail below and in the accompanying sidebar, 

Unique Features of the NAWQA Approach, 

streams and groundwater were sampled most 

intensively in agricultural and urban areas 

because of the importance of assessing nutrient 

occurrence in areas with the largest manmade nutrient sources. The agricultural areas 

are diverse in climate, geography, and crop 

types, and span coastal, desert, and temperate 

environmental settings. They include, for 

example, areas dominated by concentrated 

animal feeding operations in the Midwest; fields 

of wheat and other grains in the Great Plains; 

mixed row crops and poultry in the East; and 

rangeland in the Southwest. The areas sampled 

in urban settings primarily were residential, 

typically with low-to-medium population 

densities (300 to 5,600 people per square mile). 

Some commercial or industrial areas also were 

included in the assessments, but point sources 

and extensive industrial and downtown urban 

areas generally were not assessed.

Unique Features of the NAWQA Approach

Water-quality assessments by NAWQA, which is a single program among many local, State, 

and Federal programs, were not designed to address all of the Nation’s water-resource information 

needs and issues. Listed below are several characteristics and limitations of the NAWQA approach 

that are important to consider when interpreting the findings on nutrients presented in this report.

•	 NAWQA assessments collected extensive data to characterize the hydrologic conditions 

at all stream and groundwater sites in the Study Units. These data are critical for 

understanding the physical and chemical factors that control transport of nutrients 

in the environment.

•	 NAWQA assessments characterized the quality of the available, untreated water resources, 

and not the quality of drinking water (as would be done by monitoring water from 

water-treatment plants or from household taps). By focusing on the quality of streams and 

groundwater in their present condition (ambient water quality), NAWQA studies complement 

many Federal, State, and local drinking-water monitoring programs.

•	 NAWQA assessments of nutrients focused primarily on nonpoint sources originating from 

fertilizer application, manure, and atmospheric deposition in agricultural, urban, and other 

land-use settings. Some sites—particularly those downstream from major metropolitan 

areas—also may have been influenced by point sources, such as discharges from 

wastewater-treatment plants.

•	 NAWQA assessments targeted specific land-use settings that are most extensive or 

important to water quality in a wide range of hydrologic and environmental settings across 

the Nation. This targeted approach gives priority to understanding the most critical factors 

influencing water quality. Extrapolation of local and regional results to national analyses, 

however, requires careful definition of each type of water resource and environmental 

setting for which conclusions are drawn and the use of statistical models to extrapolate 

results to resources that have not been measured. 

•	 NAWQA assessments did not focus on specific sites with known water-quality problems or 

narrowly defined issues, but rather on the condition of the total resource, including streams 

and groundwater in a wide range of hydrologic and land-use settings across the country.
44 Nutrients in the Nation’s Streams and Groundwater, 1992–2004

Sampling Design

The national sampling design for the NAWQA 

Program focuses on a characterization of water-quality 

conditions in streams and groundwater that includes 

consideration of a broad suite of physical and chemical 

characteristics of water, streambed sediments, and 

tissues from aquatic organisms, as well as measures of 

the condition of the algal, macroinvertebrate, and fish 

communities. The national assessment of nutrients is 

based on results from the analysis of 28,466 samples of 

water from streams, and 5,101 samples of groundwater 

from the 51 NAWQA Study Units. 

Stream Water

Water samples were collected at 499 stream 

sites for analysis of nutrients in water (fig. 3-1). The 

samples were collected from streams throughout the 

year, including during both high-flow and low-flow 

periods. 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. 

Each NAWQA stream site was classified to 

evaluate the potential land-use influences on the 

quality of water sampled. The sites were characterized 

by determining the proportions of each major land 

use within each stream’s contributing watershed. 

Table 3-1 lists the criteria used to classify stream 

sites, and figure 3-1 shows the national distribution 

of these categories for the conterminous United 

States. Streams classified as “mixed land use” drain 

mixtures of two or more land-use settings and do not 

meet the criteria described in table 3-1 for individual 

agricultural, urban, or undeveloped settings. Land‐use 

classifications were adjusted for a small number 

of streams that have watersheds with substantial 

areas that did not contribute streamflow during the 

study period. Most streams that were classified as 

agricultural, urban, or undeveloped also commonly 

have small amounts of other land uses in their 

watersheds. For example, and of particular importance 

to findings for nutrients, many streams classified as 

undeveloped have some agricultural or urban activity 

in their watersheds.

Table 3-1. Each stream sampled by NAWQA was 

classified according to the dominant land uses in its 

watershed.

[Modified from Gilliom and others (2006). The land-use data 

set used for these classifications was an enhanced version of 

the USGS 1992 National Land Cover Data (NLCD) (Nakagaki 

and others, 2007)] 

Land-use 

classification Watershed land-use criteria

Agricultural > 50 percent agricultural land and 

 ≤ 5 percent urban land

Urban > 25 percent urban land and 

≤ 25 percent agricultural land

Undeveloped ≤ 5 percent urban land and 

≤ 25 percent agricultural land

Mixed All other combinations of urban,

agricultural, and undeveloped land

Stream discharge and water-quality samples were collected 

throughout the year. Photograph by U.S. Geological Survey.
Chapter 3

NAWQA’s Approach to Nutrient Assessment 45

17-0191_Fig_3-01_SamplingDesignandNLCDe.ai

Agricultural

Urban

Undeveloped

Mixed

Land-use classification

EXPLANATION

Nationally consistent data not available

for Alaska and Hawaii

Groundwater studies

Mixed land use (94)

Land-use studies of shallow groundwater

Major aquifer studies

NAWQA Study Unit

High Plains Regional 

Groundwater Study

Agricultural (135 sites)

Urban (55)

Undeveloped (159)

Stream-water sampling sites

Mixed land use (150)

Watershed land use

NAWQA Study Unit

Agricultural (53 studies)

Urban (33)

Undeveloped (9)

Figure 3-1. Stream sampling sites and groundwater studies in agricultural, urban, and undeveloped areas were 

distributed across the Nation’s diverse environmental settings to evaluate the occurrence of nutrients within areas of 

specific land uses and in different environmental settings.
46 Nutrients in the Nation’s Streams and Groundwater, 1992–2004

Most of the wells sampled as part of land-use studies were new or existing 

observation wells. Photograph by U.S. Geological Survey.

Statistical models that relate nutrient concentrations to 

streamflow and time were used to compute flow-weighted, 

mean annual concentrations of ammonia, nitrate, total 

nitrogen, orthophosphate, and total phosphorus. Use of 

flow-weighted mean annual values facilitates comparisons 

among sites and avoids the potential bias to assessment results 

caused by differences in the number of samples collected at a 

site, sampling frequency, and varying flow conditions during 

the period of sampling. Models were developed to estimate 

nutrient loads and concentrations for each day of the year. 

The models were calibrated using nutrient concentrations 

from stream samples and mean-daily streamflow for the date 

of sampling. These daily values were then used to calculate 

unbiased summary statistics—including mean annual loads, 

yields, and flow-weighted concentrations—which were then 

used to make comparisons among sites.

Groundwater

NAWQA assessed nutrients in groundwater within 

specific land-use settings and in major aquifers influenced by 

a mixture of land uses. Dissolved nutrients were analyzed in 

water samples from 5,101 wells that were part of 189 land-use 

and major aquifer study networks (fig. 3-1). Unlike stream 

monitoring sites, which were sampled multiple times, wells 

were sampled only once because of the comparatively slow 

rates of change of water quality in most groundwater systems 

relative to the rates of change typical of streams. Data analyses 

were based on one sample per well.

Land-use studies focused on shallow groundwater 

primarily within agricultural and urban land-use settings, 

and to a lesser extent in undeveloped areas. Most of the 

wells sampled that were part of land-use studies were new 

or existing observation wells or domestic supply wells. Each 

study involved sampling about 20 to 30 randomly located 

wells within the targeted land-use area. In most of the wells, 

samples were collected from depths less than 20 feet below 

the water table, thus indicating, as directly as possible, the 

influence of each land use on shallow groundwater quality.

Major aquifer studies involved sampling about 20 to 

30 domestic or public-supply wells that withdraw water 

from aquifers or aquifer systems that are important sources 

of water supply. Wells that were sampled for these studies 

were randomly selected throughout the areas underlain by 

each major aquifer, without regard to land use. Thus, the 

groundwater sampled for the major aquifer studies reflects the 

effects of a mixture of different land uses and groundwater 

ages on water quality, often including water that was 

recharged long distances from the sampled wells and in a 

variety of different land-use settings. The nature and extent of 

each major aquifer sampled for these studies is described in 

the summary report for the NAWQA Study Unit in which it 

occurs (see http://water.usgs.gov/nawqa/).

NAWQA also conducted studies that describe the change 

in water quality along a groundwater flow path from the point 

of recharge at the land surface to the point of discharge—

for example a stream or lake in a natural flow system, or a 

pumped well. Design of these “flow‐system studies” was 

tailored to the topic being addressed as well as the specific 

environmental framework at each locale. Examples of topics 

addressed include transport of constituents to drinking-water 

wells, and transport of agricultural chemicals in groundwater 

to streams. 

Groundwater sampled during the NAWQA studies was 

classified into one of three categories of redox condition 

based on the concentrations of several diagnostic constituents 

(Jurgens and others, 2009):

• Oxic—groundwater with concentrations of dissolved 

oxygen greater than 0.5 mg/L. 

• Reduced—groundwater with concentrations of 

dissolved oxygen less than or equal to 0.5 mg/L, and 

containing chemical markers for iron and manganese 

reduction. 

• Mixed—groundwater with concentrations of dissolved 

oxygen greater than 0.5 mg/L and containing chemical 

markers for iron and manganese reduction.
Chapter 3

NAWQA’s Approach to Nutrient Assessment 47

Agricultural

Urban

Undeveloped

Mixed

0

10

20

30

40

50

60

70

NAWQA stream 

sampling sites

U.S. streams Streams with 

drinking-water intakes

Proportion of watersheds,

in percent

NAWQA Stream-Water Assessment in 

a National Context

Consistent with the NAWQA sampling design, which 

was targeted by land use, the nutrient findings discussed 

in this report generally are presented by land-use category. 

Aggregation of NAWQA findings for streams across all 

land-use categories would not accurately represent all 

streams in the conterminous United States (fig. 3-2), which 

were characterized by classifying the watersheds of all 

stream segments in the USEPA River Reach File (Nolan 

and others, 2002) using the land-use criteria in table 3-1. 

For example, nearly 30 percent of the streams sampled by 

NAWQA were agricultural streams, whereas agricultural 

streams represent only about 15 percent of all streams in 

the conterminous United States. Furthermore, as shown 

in figure 3-2, agricultural streams represent only about 

12 percent of all streams with public water-supply intakes. 

There are 1,679 public water-supply intakes on streams across 

the Nation for which land use could be characterized. The 

NAWQA design also over-represented urban streams and 

under-represented undeveloped streams compared with the 

national occurrence of streams in these land-use 

settings (fig. 3-2).

Nutrient concentrations in streams can fluctuate in 

response to changes in precipitation and streamflow (see 

Chapter 2, Nutrient Primer), so information on streamflow 

conditions at the time of sample collection is critical for 

assessing nutrient conditions. Because of the rotational 

sampling design used in the NAWQA assessment and because 

of regional variations in climate, hydrologic conditions during 

the NAWQA sampling periods were not uniform across all 

streams. Instead, regional patterns in climate resulted in many 

eastern streams having normal to above normal flow during 

Figure 3-2. The NAWQA design for stream assessments placed 

greater emphasis on sampling streams that drain agricultural and 

urban watersheds (as defined in table 3-1), relative to those in 

undeveloped watersheds. Streams sampled by NAWQA included 

higher proportions of those classified as agricultural and urban—

and lower proportions of those classified as undeveloped—

compared with all streams in the conterminous United States 

and those with drinking-water intakes. (Modified from Gilliom and 

others, 2006.)

their respective sampling period, whereas flows in western 

streams were variable, with more having below normal flow 

during their sampling period (fig. 3-3). Streamflow also varied 

within the sampling period at many sites. For example, the 

3-year sampling period for the Flint River in Alabama, the 

Great Miami River in Ohio, and the Spokane River in Idaho 

included both wet and dry periods (fig. 3-3). This hydrologic 

variability adds complexity to a comparison of sites by 

geographic region or land use.
48 Nutrients in the Nation’s Streams and Groundwater, 1992–2004

17-0191_Fig_3-03.ai

Flint River near Brownsboro, Alabama

Streamflow, in cubic

feet per second

100,000

10,000

1,000

100

10

1999 2000 2001

Oct

Jan

Apr

July

Oct

Jan

Apr

July

Oct

Jan

Apr

July

75th percentile

25th percentile

based on water

years 1934–1994

Streamflow

wet

dry

Water year

Great Miami River at Hamilton, Ohio

1999 2000 2001

Streamflow, in cubic

feet per second

100,000

10,000

1,000

100

10

Oct

Jan

Apr

July

Oct

Jan

Apr

July

Oct

Jan

Apr

July

75th percentile

25th percentile

based on water

years 1971–2000

Streamflow wet

dry

Water year

Spokane River near Post Falls, Idaho

Streamflow, in cubic

feet per second

50,000

10,000

5,000

500

1,000

100

1999 2000 2001

Oct

Jan

Apr

July

Oct

Jan

Apr

July

Oct

Jan

Apr

July

75th percentile

25th percentile

based on water

years 1970–2001

Streamflow wet

dry

Water year

Departure of streamflow during the 

NAWQA sampling period from the 

long-term mean streamflow (1971 

to 2000), in percent.

Much below normal (≤25)

Below normal (>25–75)

Normal (>75–125)

Above normal (>125–175)

Much above normal (>175)

EXPLANATION

Figure 3-3. Streamflow at many eastern stream sites was normal to above normal during their respective sampling 

periods, whereas flows at western stream sites were variable, with more being below normal during their sampling 

period. These contrasts reflect both the rotational sampling design used in the NAWQA assessment and regional 

variations in climate. Streamflow also varied within the sampling period at many sites; samples from the Flint River in 

Alabama (Woodside and others, 2004), the Great Miami River in Ohio (Rowe and others, 2004), and the Spokane River in 

Idaho (Clark and others, 2004) were collected during both wet and dry periods.
Chapter 3

NAWQA’s Approach to Nutrient Assessment 49

Area within 500-meter 

radius of NAWQA 

sampled wells

All U.S. area over

groundwater resource

0

10

20

30

40

50

60

70

Proportion of land area, 

in percent

Agricultural

Urban

Undeveloped

Major aquifers

NAWQA Groundwater Assessment in a 

National Context

NAWQA findings for groundwater, as for streams, are grouped 

by land use in this report. NAWQA’s targeted sampling design for 

groundwater over-represented areas with agricultural, urban, and 

mixed land use, and under-represented undeveloped areas when 

compared with the national distribution of these land-use settings 

overlying known groundwater resources (fig. 3-4). Comparisons 

of land-use distributions for the NAWQA-sampled wells with 

distributions for the entire Nation were based on NAWQA land-use 

classifications (as defined in table 3-1) for every square-kilometer 

area in the conterminous United States. Under‐representation of 

undeveloped areas results from the targeting of agricultural and urban 

areas by the studies of shallow groundwater, and the scarcity of 

water‐supply wells in areas that have little or no human activity. 

Extrapolation to Unsampled Areas

NAWQA assessments use an integrated approach that 

includes both monitoring and modeling. Monitoring provides 

direct observations, often over time, of water-quality properties 

and characteristics, whereas models are tools that help interpret 

these observations. Analysis of model simulations can advance 

understanding of the relation of water quality to human activities and 

natural processes that affect spatial variations in quality. 

Nutrient data collected by NAWQA during its first decade 

support the development and testing of a wide range of models, 

particularly statistical models, that are being used to predict nutrient 

concentrations in streams and groundwater for locations where 

nutrient concentrations have not been measured. This expansion 

of water-quality assessments from individual monitoring sites to 

unstudied locations by use of models for prediction, or spatial 

extrapolation, is fundamental to extending targeted local and regional 

studies to a comprehensive national assessment. 

NAWQA statistical models establish quantitative linkages 

between nutrient concentrations measured at sampling sites with the 

associated explanatory variables that represent sources of nutrients, 

hydrology, and characteristics of soils, geology, and other factors. The 

models are built using the explanatory variables that best correlate 

with, or explain, concentrations of selected nutrients observed in 

streams and groundwater. Although explanatory variables included in 

the models are significantly correlated with nutrient concentrations, 

the specific cause-and-effect relations responsible for the observed 

correlations are not always clear, and inferences regarding causes 

should be considered as hypotheses. Models complement the 

monitoring by helping to estimate the extent of concentrations of 

concern, or by indicating vulnerable areas that can benefit from 

enhanced monitoring and protection. The statistical models are 

constrained to representing areas within the conterminous United 

States because data on nutrient sources and transport factors are 

unavailable for Alaska and Hawaii.

Figure 3-4. NAWQA’s targeted sampling design for 

groundwater emphasized areas with urban, mixed, 

and agricultural land use—and under-represented 

undeveloped areas—when compared with the national 

distribution of land uses. (Modified from Gilliom and 

others, 2006.)

The Spokane River near Spokane, Washington. Photograph by Lan 

Tornes, U.S. Geological Survey.
50 Nutrients in the Nation’s Streams and Groundwater, 1992–2004

Development of Models for Total 

Nitrogen Concentrations in Streams

Models developed to extrapolate total 

nitrogen concentrations in streams used linear 

regression statistical techniques to relate 

flow-weighted, mean annual total nitrogen 

concentrations at 462 sites to characteristics 

(explanatory factors) associated with watersheds 

draining streams in the USEPA River Reach File 

(Spahr and others, 2010). The models relate total 

nitrogen concentrations to nitrogen inputs, basin 

characteristics, and hydrologic factors using data 

collected from 1992 to 2001. Optimal results 

were achieved by dividing the sites into two 

groups based on the percentage of agricultural 

land use in the upstream watershed: 220 sites 

referred to as “agricultural” drain watersheds 

with more than 25 percent of agricultural land 

use, and 242 sites referred to as “nonagricultural” 

drain watersheds with less than 25 percent 

agricultural land use. 

The total nitrogen concentration at each 

site was represented by a flow-weighted, 

mean annual concentration to represent the 

movement of nitrogen mass throughout the year. 

After testing numerous potential explanatory 

factors, eight were selected for the model of 

agriculturally influenced sites and four were 

selected for the model of nonagricultural sites. 

The regression models were used to extrapolate 

total nitrogen concentrations to the more than 

600,000 miles of streams depicted in the USEPA 

River Reach File (Nolan and others, 2002). The 

results of simulations made with both models 

were merged to produce a national map that 

shows the predicted mean estimates of the total 

nitrogen concentration. 

Development of the Nonlinear 

Nitrate Models for Groundwater

Two models based on nonlinear regression 

were developed by NAWQA to predict 

nitrate concentrations in groundwater of the 

conterminous United States (Nolan and Hitt, 

2006). The first model uses data primarily from 

monitoring wells to predict nitrate contamination 

of shallow (10 meters, or 33 feet), recently 

recharged groundwater. The second model uses 

data from domestic and public-supply wells to 

predict nitrate contamination in drinking water. 

The median depth of the drinking-water supply 

wells was 50 meters (164 feet). The shallow 

model is intended to maximize correlations with 

overlying land use, whereas the drinking-water 

model represents the used resource.

The models for predicting nitrate 

concentration in groundwater are based on 

averages of nitrate concentration and other 

characteristics of NAWQA groundwater studies. 

The average nitrate concentration of water in 

the wells sampled in each study was used to 

smooth local variability, so that the predominant 

factors that influence nitrate contamination at the 

national scale would be more readily apparent. 

The shallow groundwater model was based on 

data from 97 studies in which most samples 

were collected from monitoring wells, whereas 

the drinking-water model was based on data 

from 111 studies in which all samples were 

collected from domestic or public-supply wells. 

Data used to develop the models were collected 

during 1991–2003 and represent the first full 

decade of sampling by NAWQA. More than 

100 explanatory factors were tested in the models 

to identify those that most reliably predict nitrate 

concentrations in groundwater: 16 factors were 

selected for the shallow groundwater model, and 

14 factors were selected for the drinking-water 

model. Model predictions were made for each 

square-kilometer area in the conterminous United 

States for which a complete suite of input data 

was available, resulting in more than 8 million 

predictions. Uncertainty analysis was performed 

with the final models by varying model equations 

hundreds of times to produce a mean and 

variance of predictions at each groundwater 

study location. 
Chapter 4

Occurrence and 

Distribution of 

Nutrients in 

Streams and 

Groundwater

Introduction

NAWQA’s assessment of nutrients in 

streams and groundwater of the United 

States includes hydrologic systems 

representative of the Nation’s diverse 

landscapes, ecological resources, and 

climatic conditions. The assessment is 

targeted to specific land-use settings that 

are important to water quality across the 

Nation, allowing for characterization 

of the occurrence and distribution 

of nutrients in relation to land use 

and geographic patterns. Results of 

the assessment also can be used to 

identify critical factors influencing the 

occurrence and distribution of nutrients 

in streams and groundwater, such as 

anthropogenic (human) nutrient inputs, 

seasonal variability, and physical and 

chemical aquifer characteristics.

Photograph by Norman Spahr, U.S. Geological Survey.

Chapter

4
52 Nutrients in the Nation’s Streams and Groundwater, 1992–2004

Sites with minimal human impact, such as the Colorado River in Rocky Mountain National Park, 

were chosen to represent background nutrient conditions. Photograph by J.R. Deacon, U.S. 

Geological Survey.

Background Conditions

What would nutrient concentrations be in streams and 

groundwater if there were no effects of human development? 

An answer is not readily available for many parts of the Nation 

because human development is so extensive, and remaining 

pristine sites, even where present, are often not monitored. 

To help answer this question, the NAWQA Program collected 

samples from streams and wells, and gathered complementary 

data from other programs, in areas with minimal or no 

development (see sidebar, Estimation of Background 

Concentrations). These samples reflect nutrients that are 

present in streams and groundwater as a result of purely 

natural processes, as well as low-level inputs of nutrients from 

anthropogenic sources such as atmospheric deposition. Data 

from these samples were used to estimate what concentrations 

might be expected under minimally impacted or “background” 

conditions across the United States.

The nutrient concentrations estimated to represent 

national background concentrations are listed in table 4-1. 

Table 4-1. Estimates for national background concentrations of nutrients in streams and groundwater.

[ mg/L, milligrams per liter; –, not applicable]

Data set

Ammonia Nitrate Total Nitrogen Orthophosphate Total Phosphorus

Sites or 

wells

Concentration

(mg/L) 

Sites or 

wells

Concentration

(mg/L) 

Sites or 

wells

Concentration

(mg/L) 

Sites or 

wells

Concentration

(mg/L) 

Sites or 

wells

Concentration

(mg/L) 

Streams 89 0.025 108 0.24 88 0.58 89 0.010 84 0.034

Groundwater 177 0.10 419 1.0 – – 166 0.030 – –

These estimates can be compared with measurements made 

at other locations to determine whether development has 

resulted in elevated concentrations. Nutrient concentrations 

in undeveloped areas may vary considerably among regions 

of the country, but the number of sites sampled is insufficient 

to define small-scale variations. The regional variability in 

background nutrient concentrations is discussed in the context 

of the development of nutrient criteria protective of aquatic 

life in Chapter 7.

In areas of substantial agricultural or urban 

development, nutrient concentrations are likely to be elevated 

above background levels. Flow-weighted mean annual 

concentrations for all five nutrient constituents exceeded the 

background concentrations at 90 percent of 190 NAWQA 

agricultural and urban stream sites. Median concentrations 

of nitrate exceeded the groundwater background in 64 

percent of 86 shallow aquifer studies sampled by NAWQA 

in areas underlying agricultural or urban land use; however, 

concentrations of ammonia and orthophosphate were not 

greater than background levels in most areas.
Chapter 4

Occurrence and Distribution of Nutrients in Streams and Groundwater 53

EXPLANATION

Sites used in estimating background

concentrations of nutrients

Streams

Groundwater

Estimation of Background Concentrations

Estimates of background concentrations of nutrients in streams were based on data from 110 sites selected on the 

basis of criteria used by Clark and others (2000) (fig. 4-1).

1. The upstream basin had less than 5 percent urban land and less than 25 percent agricultural land,

2. The site was classified as a reference, forest indicator, or rangeland indicator site by local experts,

3. The basin area was less than 3,000 km2 (1,158 mi2), 

4. Data were sufficient for estimation of flow-weighted mean annual nutrient concentrations, and 

5. If two sites were on the same stream, only the upstream site was included.

Flow-weighted mean annual concentrations (see Chapter 3, NAWQA’s Approach to Nutrient Assessment) of the five 

nutrient constituents at these 110 sites were computed on the basis of several years of samples (see Mueller and Spahr, 

2005, for details). Background concentrations were estimated as the 75th percentile of the flow-weighted means for all 

sites that had adequate data for each constituent. Selection of the 75th percentile to represent background implies that 

these concentrations are exceeded at no more than 25 percent of minimally impacted streams.

For groundwater, background concentrations were estimated using data from 419 wells selected on the basis of criteria 

specified by Nolan and Hitt (2003): 320 “retrospective” wells from the USGS data base, and an additional 99 wells sampled 

by NAWQA (fig. 4-1). Background concentrations were estimated as the 75th percentile of a single representative sample 

from each of the 419 wells that had data for the nutrient in question. Only dissolved constituents were included because 

particulate nitrogen and phosphorus are negligible in groundwater.

Figure 4-1. Nutrient data from 110 stream sites and 419 wells in relatively undeveloped areas were used to 

estimate minimally impacted (background) concentrations for the United States.
54 Nutrients in the Nation’s Streams and Groundwater, 1992–2004

Concentrated animal feeding operations, also referred to as “CAFOs,” are potential sources of nutrients to streams and 

groundwater. Photograph by Jeff Vanuga, U.S. Department of Agriculture, Natural Resources Conservation Service.

Nutrient Concentrations in Streams 

Relations to Land Use

Variations in the occurrence and distribution of 

nutrients in streams reflect, in part, differences in land use 

and associated nutrient sources. Nonpoint inputs, such as 

those from fertilizers and manure from livestock, are the 

major sources of nutrients in agricultural areas, whereas 

both point and nonpoint sources—including wastewater 

effluent from municipal or industrial facilities; fertilizers 

applied to lawns, golf courses, and parks; septic systems; and 

atmospheric deposition—are the major sources in urban areas. 

In undeveloped areas, nonpoint inputs, such as atmospheric 

deposition and natural sources released by weathering of rocks 

and soil, typically are the largest sources. NAWQA sampling 

sites in watersheds were classified by land use: agricultural, 

urban, mixed, and undeveloped (see Chapter 3, NAWQA’s 

Approach to Nutrient Assessment, table 3-1). Urban sites were 

selected to avoid the effects of major wastewater-treatment 

plants and other point sources, but in some locations this was 

not feasible. 

Median flow-weighted nutrient concentrations for all 

constituents were higher in streams in developed areas—that 

is, agricultural, urban, or mixed land-use streams—than in 

streams in undeveloped areas (fig. 4-2). Concentrations of 

total nitrogen were higher in agricultural streams than in 

streams with urban, mixed, or undeveloped land use, with a 

median concentration of 3.8 mg/L—about 6 times greater than 

background levels. Concentrations of total nitrogen in urban 

streams were lower than those in agricultural settings, with a 

median concentration of 1.5 mg/L (about 3 times greater than 

background levels). Concentrations of total phosphorus were 

similarly elevated in streams in both agricultural and urban 

areas, with median concentrations of 0.26 and 0.25 mg/L, 

respectively—again about 6 times greater than background 

levels. Concentrations of ammonia and orthophosphate were 

similar in agricultural, urban, and mixed land-use streams. 

In urban and mixed land-use streams, the relatively higher 

concentrations of ammonia, orthophosphate, and total 

phosphorus compared to concentrations of nitrogen likely 

were due to the influence of point sources; many of the highest 

concentrations occurred at sites downstream from wastewatertreatment plants. Although samples from undeveloped streams 

had lower concentrations of total phosphorus than samples 

from streams in other land uses, the contrast was not as great 

as for other nutrients. The likely cause is naturally occurring 

phosphorus associated with the high suspended-sediment 

concentration in some undeveloped streams.
Chapter 4

Occurrence and Distribution of Nutrients in Streams and Groundwater 55

17-0191_Fig_4-02_sw_boxplot_landuse.ai

90th percentile

10th percentile

Median

75th percentile

25th percentile

EXPLANATION

Background concentration

0.025

Concentration, in milligrams per liter

0

2

4

6

8

10

12

Total nitrogen

Agricultural Urban Mixed Undeveloped

Nitrate

0.58

Concentration, in milligrams per liter as P Concentration, in milligrams per liter

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0.24

0.034

Agricultural Urban Mixed Undeveloped

Orthophosphate

Agricultural Urban Mixed Undeveloped

0.010

Concentration, in milligrams per liter as N

0

2

4

6

8

10

12

Agricultural Urban Mixed Undeveloped

Ammonia

Concentration, in milligrams per liter as N 

Agricultural Urban Mixed Undeveloped

0.025

0.0

0.1

0.2

0.3

0.4

0.5

Total phosphorus

Figure 4-2. In streams, concentrations of total nitrogen and nitrate were higher in agricultural streams than in urban 

or mixed land-use streams, whereas concentrations of ammonia, orthophosphate, and total phosphorus were similar in 

agricultural, urban, and mixed land-use streams.
56 Nutrients in the Nation’s Streams and Groundwater, 1992–2004

1

2

3

4

5

6

7

8

Geographic framework used in 

discussion of nutrients in streams

and groundwater

1—Northeast 

2—Southeast 

3—Midwest 

4—Northern Plains 

5—Southern Plains 

6—Southwest 

7—Northwest 

8—California 

EXPLANATION

(New England and Mid-Atlantic Water-Resources Regions)

(South Atlantic-Gulf and Tennessee Water-Resources Regions)

(Great Lakes, Ohio, Upper Mississippi, and Souris-Red-Rainy Water-Resources Regions)

(Missouri Water-Resources Region)

(Lower Mississippi, Arkansas-White-Red, and Texas-Gulf Water-Resources Regions)

(Rio Grande, Upper Colorado, Lower Colorado, and Great Basin Water-Resources Regions)

(Pacific Northwest Water-Resources Region)

(California Water-Resources Region)

Geographic Distribution

The geographic distribution of nutrients in streams 

reflects regional differences in nutrient sources, hydrologic 

characteristics of watersheds, and chemical and biological 

processes that affect specific nutrients. In this section, 

interactions among these factors are illustrated by comparing 

the geographic distributions of total nitrogen and total 

phosphorus in streams. The comparisons are illustrated using 

a series of national maps showing the relative range (high, 

medium, or low) of concentrations for sites downstream 

Geographic Framework 

Describing the geographic distribution of nutrients requires a spatial framework with a defined terminology to delineate 

different areas. For the purposes of this report, the continental United States was divided into eight contiguous areas 

on the basis of water-resources regions (fig. 4-3). Seaber and others (1987) used the boundaries of major river basins to 

define 21 water-resources regions. The areas designated on the map below are either single water-resources regions or 

combinations of regions. These area designations are used in the discussion of geographic distribution of nutrients in streams 

and groundwater. Alaska and Hawaii are individual regions.

Figure 4-3. For the purposes of this 

report, the continental United States 

was divided into eight contiguous areas 

that are based on water-resources 

regions. These area designations are 

used in this report in the discussion of 

geographic distribution of nutrients in 

streams and groundwater.

from specified land uses (fig. 4-4) (see sidebar, Geographic 

Framework). Sites at which concentrations were within the 

lowest 25 percent of the data are shown in the “Low” range, 

sites where concentrations were within the upper 25 percent of 

the data are shown in the “High” range, and all other sites are 

shown in the “Medium” range. The background shading on the 

maps identifies nonpoint-source input rates of nitrogen 

and phosphorus.
Chapter 4

Occurrence and Distribution of Nutrients in Streams and Groundwater 57

17-0191_Fig_4-04_SW_Nconc_byLUquart

Agricultural Urban

Low (0.66)

Medium (0.66–3.17)

High (3.17)

Total nitrogen concentration,

in milligrams per liter

Estimated 1997 nitrogen inputs from

fertilizer, manure, and atmosphere,

in pounds per square mile Undeveloped

2,900

2,900–15,000

15,000

Stream sites, by watershed land use

Data on nitrogen inputs not available 

for Alaska and Hawaii.

EXPLANATION

Mixed

Agricultural Urban

Undeveloped Mixed

Streams

Figure 4-4. Concentrations of total nitrogen in agricultural streams were high at sites in areas that receive large annual 

inputs of nitrogen in the Northeast, Midwest, and Northwest. Total nitrogen concentrations in urban and mixed streams 

generally were in the medium range, whereas concentrations in undeveloped streams generally were low.
58 Nutrients in the Nation’s Streams and Groundwater, 1992–2004

0

3,000

1,000

2,000

Streamflow, in cubic feet per second

Streamflow

Iowa River near Rowan, Iowa

Tile drain flow No tile drain flow

Jan Feb Mar Apr May June July Aug Sept Oct Nov

1997

0

20

10

Nitrate concentration,

in milligrams per liter as N

Dec

EXPLANATION

Tile drain nitrate concentration

Iowa River nitrate concentration

Figure 4-5. Tile drains can contribute substantial amounts of nitrate to 

streams and rivers from agricultural fields. Nitrate concentrations in the 

Iowa River near Rowan, Iowa, decreased when flow from tile drains ceased 

(Kalkhoff and others, 2000).

Concentrations of total nitrogen in agricultural streams 

were high at most sites in the Northeast, Midwest, and 

Northwest (fig. 4-4). These sites are in areas that receive large 

annual inputs of nitrogen in the form of fertilizer, manure, and 

(in some cases) atmospheric deposition. High concentrations 

in the Midwest are influenced by the prevalence of tile drains, 

which facilitate water movement from fields to streams. For 

example, nitrate concentrations in the Iowa River near Rowan, 

Iowa, were at their highest while tile drains were flowing, and 

decreased when flow from tile drains ceased (fig. 4-5). High 

concentrations at many sites in the Northwest, the Northern 

Plains, the Southwest, and California might be influenced by 

large applications of irrigation water. Irrigation return flow 

can carry substantial amounts of nitrate to streams (fig. 4-6). 

Most of the medium and low concentrations of total nitrogen 

in agricultural areas were at sites in the Southeast and Southern 

Plains, where inputs are more variable. These settings can 

contain substantial amounts of organic matter, which result in 

loss of nitrate due to denitrification before it reaches a stream; 

therefore, concentrations can be relatively low in comparison to 

inputs (see Chapter 5, Exchange of Nutrients Between Surface 

Water and Groundwater).

Total nitrogen concentrations downstream from urban 

areas generally were in the medium range, but sites with high 

concentrations were scattered from the Northeast through 

the Southwest and California (fig. 4-4). Many of the sites 

with high concentrations were downstream from wastewatertreatment plants, which provide an additional nitrogen 

input. The potential effect of wastewater-treatment plants is 

especially large in the arid West, where wastewater-treatment 

plant effluent can be the primary source of the flow in urban 

streams during parts of the year when precipitation is low.

At sites downstream from relatively undeveloped 

watersheds, total nitrogen concentrations generally were low 

(fig. 4-4). Medium-range concentrations in the Northeast 

and in eastern parts of the Midwest were in areas of 

greater atmospheric nitrogen deposition. Medium and high 

concentrations of total nitrogen in the Southwest and western 

parts of the Northern Plains (fig. 4-4) were detected at sites 

with greater loads of suspended sediment, which can be a 

source of organic matter containing particulate nitrogen. 

This was seen, for example, in the Yellowstone River Basin, 

where total nitrogen increases along with the concentration of 

suspended sediment (Peterson and others, 2004) (see sidebar, 

Contrasts among Undeveloped Watersheds: Forest and 

Rangeland; fig. 4-7).
Chapter 4

Occurrence and Distribution of Nutrients in Streams and Groundwater 59

Tile drains buried at shallow depths reduce the water content 

of poorly drained soils and divert shallow groundwater to 

nearby streams. Photograph by Jeffrey Martin and Nancy 

Baker, U.S. Geological Survey.

The Bighorn River, Wyoming, displays the high concentration of 

suspended sediment characteristic of streams that drain rangelands 

with highly erosive soils. Photograph by Nate Majerus, U.S. 

Geological Survey.

0

2

4

6

8

10

12

14

Nitrate concentration, in milligrams per liter as N

0

Streamflow, in cubic feet per second

Jan MarFeb Apr May June July Aug Sept Oct Nov Dec

1993

Orestimba Creek near Crows Landing, California

300

600

900

1,200

1,500

Nitrate

concentration

Non-irrigation

season

Non-irrigation

season

Irrigation season

Streamflow

10

1

0.1

Flow-weighted average concentration of 

total nitrogen, in milligrams per liter

Flow-weighted average concentration of 

suspended sediment, in milligrams per liter

10 100 1,000 10,000

Yellowstone River and tributaries, Montana and Wyoming

Figure 4-7. Because suspended sediment can be a source of organic 

matter containing particulate nitrogen, concentrations of total nitrogen 

were correlated with concentrations of suspended sediment in the 

Yellowstone River and its tributaries, Montana and Wyoming.

Figure 4-6. In Orestimba Creek, California, irrigation return flow can 

transport substantial amounts of nitrate to the stream during the spring 

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

0.001

0.01

0.1

1

10

Concentration, in milligrams per liter

Total

nitrogen

Total

phosphorus

Nitrate as N

Streams Groundwater

0.2

0.01

0.05

Rangeland

Forest

0.01

75th percentile

Median

25th percentile

90th percentile

Detection limit

10th percentile

EXPLANATION

Contrasts among Undeveloped Watersheds: 

Forest and Rangeland 

Water-quality conditions in undeveloped watersheds are related to the same factors that influence natural vegetation types 

in watersheds and hence were evaluated by sorting the water-quality data according to predominant land cover: forest or 

rangeland. Comparison of nutrient concentrations in streams shows that total phosphorus is significantly greater in rangeland 

areas than in forest areas (fig. 4-8). Most of the rangeland sites are in the semiarid and arid western parts of the United States, 

where phosphorus often occurs naturally in igneous and marine sedimentary rocks (Peterson and others, 2004). Also, because 

phosphorus tends to attach to soil sediment particles, erosion can transport sediment and attached phosphorus to streams, 

causing concentrations of total phosphorus in the streams to increase with increasing concentrations of suspended sediment. 

Rangeland sites are generally in areas where erosion potential is greater, because of slope, soil type, and climate.

Although the concentrations of total nitrogen in streams are not significantly different between forest and rangeland sites 

throughout the United States (fig. 4-8), total nitrogen in some areas, such as the Yellowstone River Basin, increases along with 

the amount of rangeland in the upstream watershed because suspended sediment can also be a source of organic matter 

containing particulate nitrogen. In groundwater, nitrate concentrations are much higher in rangeland areas than in forest areas 

(fig. 4-8). Studies have shown that naturally high concentrations of nitrate can accumulate in soils and groundwater in semiarid 

and arid regions (Walvoord and others, 2003).

The geographic distribution of total phosphorus 

concentrations in streams is similar to that of total nitrogen, 

with high concentrations found in agricultural streams 

throughout the country (fig. 4-9). In contrast to the distribution 

of total nitrogen concentrations, distributions of total 

phosphorus concentrations were sometimes relatively lower 

in agricultural streams (more sites in the medium and low 

ranges, fig. 4-9) and higher in urban streams (more sites 

in the high and medium ranges, fig. 4-9). In addition, total 

phosphorus concentrations were relatively higher in streams in 

undeveloped areas than were total nitrogen concentrations.

Figure 4-8. Rangeland streams have 

higher total phosphorus concentrations 

than streams in forested areas. Rangelands 

are primarily in the semiarid and arid 

Western United States, where phosphorus 

occurs naturally in geologic materials 

and the erosion potential is high. In 

groundwater, nitrate concentrations 

are higher in rangeland areas than in 

forested areas, most likely because high 

concentrations of nitrate can accumulate 

in soils and groundwater in semiarid and 

arid regions. (Modified from Mueller and 

Spahr, 2006.) 
Chapter 4

Occurrence and Distribution of Nutrients in Streams and Groundwater 61

17-0191_Fig_4-09_SW_Pconc_byLUquart.ai

Low (0.05)

Medium (0.05–0.28)

High (0.28)

Total phosphorus concentration,

in milligrams per liter

Estimated 1997 phosphorus 

inputs from fertilizer and manure, 

in pounds per square mile

340

340–2,730

2,730

Stream sites, by watershed land use

Data on phosphorus inputs not available 

for Alaska and Hawaii.

EXPLANATION

Agricultural Urban Undeveloped Mixed

Streams

Agricultural Urban

Undeveloped Mixed

Figure 4-9. High concentrations of total phosphorus were found in agricultural and urban streams; however, concentrations 

of total phosphorus were relatively higher than were total nitrogen concentrations in urban and undeveloped streams.
62 Nutrients in the Nation’s Streams and Groundwater, 1992–2004

Agricultural

0 50 100 0 50 100

Dissolved

orthophosphate

Dissolved

phosphorus

Total

Phosphorus

100

Rangeland

and forest Urban

Trinity River below

Dallas, Texas

Percentage of samples

0 50 0 50 100

Concentration, in

milligrams per liter

EXPLANATION

<0.01

0.01–0.1

>0.1

Figure 4-10. Samples from the Trinity River below Dallas, where the streamflow is dominated by effluent 

from regional wastewater-treatment plants, had higher concentrations of phosphorus than samples from 

other streams in the Trinity River Basin, Texas, which were affected only by nonpoint sources (Land and 

others, 1998).

≤0.024

0.024−0.039

0.039−0.055

0.055−0.071

0.071

EXPLANATION

Phosphorus, in percent by weight

No data

Figure 4-11. Phosphorus is naturally high in some 

geologic materials in western rangelands, where a 

high erosion potential may result in the transport of 

phosphorus-rich sediment to streams. (Data from 

U.S. Geological Survey, 2009a.)

At undeveloped sites, high and medium concentrations of 

total phosphorus were more common than high and medium 

concentrations of total nitrogen in the Northern Plains, 

Southwest, and California (figs. 4-4 and 4-9). Rangeland is the 

predominant land cover in these areas, whereas forest is the 

predominant land cover in most other undeveloped areas of 

the United States. In rangeland areas, phosphorus commonly 

occurs naturally in geologic materials (fig. 4-11) and erosion 

potential is high, which contributes to higher total phosphorus 

concentrations in streams here compared to those in streams 

in other undeveloped, forested areas (see sidebar, Contrasts 

among Undeveloped Watersheds: Forest and Rangeland).

Differences between the distributions of total nitrogen 

and total phosphorus concentrations in streams likely are a 

result of differences in sources—especially natural sources 

of phosphorus as well as its relatively high concentrations 

in wastewater-treatment plant effluent—and differences 

in the processes that transport nitrogen and phosphorus to 

streams. For example, water samples from the Trinity River 

below Dallas, where the streamflow is dominated by effluent 

from regional wastewater-treatment plants, had higher 

concentrations of phosphorus than water samples from other 

streams in the Trinity River Basin, which were affected only 

by nonpoint sources (Land and others, 1998) (fig. 4-10). 

Nonpoint sources of phosphorus in urban areas include runoff 

from lawns and construction sites.
Chapter 4

Occurrence and Distribution of Nutrients in Streams and Groundwater 63

17-0191_Fig_4-12_SW_model_Nconc.ai

≤0.5 0.5–1 1–5 5–10 10

Predicted total nitrogen concentration, in milligrams per liter

EXPLANATION

Streams Extrapolation of Total Nitrogen 

Concentrations in Streams to 

Unsampled Areas

The results of simulations made with 

two statistical models—one for agriculturally 

influenced streams and one for nonagricultural 

streams—were aggregated to predict the total 

nitrogen concentrations in streams across 

the conterminous United States (Spahr and 

others, 2010) (see Chapter 3, Extrapolation to 

Unsampled Areas, Development of Models 

for Total Nitrogen Concentrations in Streams). 

Model results for streams predict that the highest 

total nitrogen concentrations should be in areas 

with the highest nonpoint source nitrogen 

inputs, such as agricultural areas in the upper 

Midwest and selected coastal areas of California 

(fig. 4-12). The lowest concentrations are 

predicted in the mountainous regions of the West, 

the Appalachians, and the northern parts of New 

England. Simulated concentrations for much of 

the country are less than 5 mg/L and for major 

sections are less than 1 mg/L. 

The model for agriculturally influenced 

streams explains more than 70 percent of the 

variability of total nitrogen concentration. The 

explanatory factors that represent nitrogen 

sources in the model include atmospheric 

nitrogen input, fertilizer nitrogen input, manure 

nitrogen input, and percentage of undeveloped 

land in the watershed above the sampling site. 

Explanatory factors that represent transport 

to and within streams in the model include 

percentage of sand in soil, presence of subsurface 

drains, long-term average percentage Horton 

overland flow, and mean annual precipitation. 

Areas with greater nitrogen inputs and 

subsurface drainage tend to have greater 

nitrogen concentrations in streams, whereas 

areas with sandy soils, higher precipitation, and 

greater undeveloped areas, have lower nitrogen 

concentrations in streams.

The model for the nonagricultural streams explained 62 percent 

of the total nitrogen variability. The explanatory factors that represent 

nitrogen sources in the model for the nonagricultural sites include a 

nonpoint source term (representing fertilizer, manure, and atmospheric 

inputs) and population density; transport factors in the model are 

mean runoff and base flow index (the ratio of base flow to total flow 

volume for a given year). In the model, population density acts as a 

surrogate for nitrogen inputs associated with urban settings that are 

otherwise unaccounted for, such as point sources. In nonagricultural 

basins, streams with greater nitrogen inputs and greater population 

in their watersheds tend to have higher total nitrogen concentrations, 

whereas streams with greater streamflow and a higher percentage 

of streamflow derived from groundwater tend to have lower total 

nitrogen concentrations.

Figure 4-12. Model predictions of total nitrogen in streams show the 

highest concentrations (orange and red) in the upper Midwest. The linear 

regression models relate the concentrations of total nitrogen at 462 stream 

sites to several explanatory factors (Spahr and others, 2010).
64 Nutrients in the Nation’s Streams and Groundwater, 1992–2004

Nutrient Export in Streams

When nutrients are input to the land surface 

from fertilizer application, manure generation, 

atmospheric deposition, or other sources, they 

can be taken up by crops and natural vegetation, 

immobilized in the soil, taken up by aquatic 

organisms, or, in the case of nitrogen, lost to 

the atmosphere during denitrification. Any 

remaining nitrogen or phosphorus is potentially 

available for export out of the watershed by 

streams and rivers or by leaching through the 

soil to groundwater. Nutrients lost (exported or 

leached) from the watershed can contribute to 

eutrophication of downstream water bodies and 

economic losses through reduction in crop yields 

or expense for additional fertilizer.

The amount of nitrogen and phosphorus lost 

per unit area (expressed as “per square mile”) 

from watersheds to streams—referred to as the 

yield—increases with increasing nutrient inputs, 

regardless of land use (fig. 4-13A). Nitrogen 

input and yield generally differed among 

agricultural (median yield 6,518 pounds per 

square mile), urban (median yield 2,898 pounds 

per square mile), and undeveloped streams 

(median yield 904 pounds per square mile). 

Agricultural streams had the largest yields due to 

their relatively high 

nutrient inputs. 

Calculating the ratio of the yield to the input 

allows an assessment of the proportion of input 

that is “exported” from a watershed by its stream. 

Between 5 and 50 percent of the nitrogen input 

from nonpoint sources was exported out of most 

(72 percent) watersheds (see dashed red lines, 

fig. 4-13A). Variability in the amount of nutrient 

loss can be explained in part by differences 

in agricultural practices, soils, geology, and 

hydrology. In the Southeast, a relatively low 

proportion of nitrogen—usually less than 

25 percent of input—is exported at agricultural 

sites, possibly because of greater amounts 

of denitrification in shallow groundwater 

(fig. 4-13B). In the West, a relatively low 

proportion of nitrogen is exported because of 

generally low amounts of precipitation and 

runoff, and possibly highly modified hydrology. 

In contrast, agricultural sites with tile drains 

in 5 percent or more of the watershed are 

about 3 times more likely to export more than 

25 percent of the nitrogen input than those in 

watersheds with fewer tile drains (fig. 4-13B). 

The median yield of nitrogen from agricultural 

streams with tile drains also was slightly more 

than 3 times larger than the median yield for 

other agricultural streams (fig. 4-13C). 

Between 5 and 50 percent of the phosphorus 

input from nonpoint sources was exported out of 

51 percent of watersheds. Apparent phosphorus 

export is lower than nitrogen export for many 

sites, possibly reflecting the fact that phosphorus 

is not as mobile in the aquatic system as nitrogen 

(fig. 4-13A). Because phosphorus is relatively 

insoluble, tile drains do not increase phosphorus 

yields as markedly as they do nitrogen yields 

(fig. 4-13C). 

This comparison assumes that 

anthropogenic nonpoint-source inputs and stream 

yield account for all nitrogen and phosphorus 

movement into and out of these watersheds. 

These assumptions do not always hold true. 

For many sites, primarily those in undeveloped 

and urban areas, apparent export is greater than 

50 percent; in some cases, yield even exceeds 

input. The most likely explanation is that some 

natural inputs, such as geologic phosphorus or 

nitrogen fixation, or anthropogenic inputs such 

as effluent from wastewater-treatment plants, 

are being underestimated. For other sites, the 

apparent nutrient export from the watershed is 

less than 5 percent. At these sites, the proportion 

of nutrients lost to other mechanisms, such 

as crop harvesting, incorporation in natural 

vegetation, and immobilization in the soil, 

may be greater; or nutrient inputs may 

be overestimated. 
Chapter 4

Occurrence and Distribution of Nutrients in Streams and Groundwater 65

Figure 4-13. Nutrient yield generally increases with increasing input regardless of land use (A). Both the proportion of nitrogen 

input that is exported by a stream as yield (B), as well as the amount of yield (C) 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. Because phosphorus is relatively 

insoluble, tile drains do not increase phosphorus yields as markedly as nitrogen yields. (Modified from Mueller and Spahr, 2006.) Fig_4-13_yield_plot_corrected_May_2011 Total nitrogen outflow yield, in pounds per square mile

A.

Nonpoint nitrogen input, in pounds per square mile

1 10 100 1,000

Nonpoint phosphorus input, in pounds per square mile

0.1

1

10

100

1,000

Total phosphorus outflow yield, in pounds per square mile

5%

50%

B.

Median total nitrogen outflow yield, 

in pounds per square mile

C.

Agricultural

with <5

percent

tile drains

Agricultural

 with ≥5

percent

 tile drains 

Urban Undeveloped

Land use

Median total phosphorus outflow yield,

in pounds per square mile

Land use

Agricultural

with <5

percent

tile drains

Agricultural

 with ≥5 

percent

 tile drains 

Urban Undeveloped

1

10

100

1,000

10,000

1,000 10,000 100,000

Percent nutrients

lost to streams

5%

50%

100,000

10,000

0

50

100

150

200

250

300

350

400

450

0

2,000

4,000

6,000

8,000

10,000

12,000

14,000

10,000

Nitrogen export from

agricultural sites, in percent

<25

≥25

County with ≥5 percent tile drains

EXPLANATION

Increasing

nutrients lost

(or exported)

to streams

Agricultural 

 with <5 percent tile drains

 with ≥5 percent tile drains

Urban

Undeveloped

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

37

percent

18

percent

8 percent

12

percent

25

percent

Phosphorus

16

percent

14

percent

4 percent

9 percent

5 percent

52

percent

Nitrogen

Sources

Urban and populationrelated sources

Non-recoverable animal manure 

on pasture and rangeland

Corn/soybean crops

Other crops

Natural land

Atmospheric deposition

EXPLANATION

17-0191_Fig_4-14.ai

10

1

0.1

0.01

0.001

0.01 0.1 1 10

Water depth, in meters

Instream removal rate coefficient, in day-1

Published in the literature

Estimated in the SPARROW model

Nitrogen loss rates

Using the SPARROW Model to Assess 

Nutrient Conditions in Streams

To extend our assessment to areas that have not been sampled, 

USGS scientists have developed the statistical model, SPARROW 

(SPAtially Referenced Regression On Watershed attributes), to relate 

nutrient concentrations from a large network of monitoring stations to 

(1) upstream sources, such as fertilizer, manure, wastewater discharges, 

and the atmosphere; and (2) watershed characteristics affecting 

transport, including soil permeability, stream size, and streamflow 

(Smith and others, 1997). These sources and watershed characteristics 

are spatially referenced to a detailed network of stream reaches that 

represents pathways of water movement through the modeled region. 

Because of the spatial referencing, nutrient loss during both surface 

runoff and instream transport can be evaluated, and loads for each 

stream reach can be allocated to individual upstream sources and 

geographic regions. In addition, the statistical basis of the model 

provides measures of uncertainty for the model and its predictions. 

The SPARROW model can be used to address a variety of 

management issues (Smith and others, 1997). Nutrient concentrations 

and loads in unmonitored stream reaches can be predicted, thus 

allowing for identification of specific locations where regulatory 

standards or criteria are not likely to be met. Nutrients in a given 

stream reach, estuary, or other receiving water body can be traced to 

individual sources and locations in the upstream watershed, allowing 

for targeted management of important nutrient sources and contributing 

areas. Model inputs can be altered, allowing for simulation of 

hypothetical conditions, such as changes in nutrient sources associated 

with proposed nutrient control strategies. 

The SPARROW modeling approach has been applied at the 

national scale to provide estimates of nutrient loads and concentrations 

in 62,000 stream reaches in the United States and to improve our 

understanding of the sources, transport, and delivery of nutrients in 

streams (Smith and others, 1997; Alexander and others, 2008). For 

example, the rate of nitrogen loss in streams has been found to rapidly 

decline as water depth and stream size increase, 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 (fig. 4-14). Consequently, large deep rivers contribute 

a larger percentage of their nitrogen to downstream receiving water 

bodies than small streams (Alexander and others, 2000, 2008). This 

finding indicates that reductions in nitrogen sources near small 

streams will have a smaller benefit to receiving water bodies than 

reductions in nitrogen sources near large rivers. Recently developed 

national-scale models were used to document key nutrient sources, 

geographic regions, and transport processes that affected contributions 

of nitrogen and phosphorus to the northern Gulf of Mexico during 

1992 (Alexander and others, 2008). Results from the model revealed 

the dominance of agricultural nonpoint sources over point sources in 

the Mississippi River Basin (fig. 4-15). Corn and soybean cultivation 

Figure 4-14. The rate of nitrogen loss in streams 

rapidly declines as water depth and stream size 

increase; consequently, large rivers contribute a 

larger percentage of their nitrogen to downstream 

receiving water bodies than small streams. As a 

result, reductions in nitrogen sources near small 

streams will have a smaller benefit to receiving 

water bodies than reductions in nitrogen sources 

near large rivers (Alexander and others, 2008).

Figure 4-15. Corn and soybean cultivation was the 

largest contributor of nitrogen to the Gulf, resulting 

in large part from commercial fertilizer use and 

manure applications to crops as fertilizer. Corn and 

soybean cultivation and animal manure on pasture 

and rangelands were the largest contributors of 

phosphorus (Alexander and others, 2008).
Chapter 4

Occurrence and Distribution of Nutrients in Streams and Groundwater 67

NY

PA

MD

DE

VA

WV

Chesapeake Bay

NY

PA

MD

DE

VA

WV

Chesapeake Bay

76°

78°

80°

42°

40°

38°

76°

78°

80°

42°

40°

38°

EXPLANATION

Amount of nitrogen delivered 

to streams, in kilograms 

per hectare per year

0.0−1.7

1.7−3.4

3.4−5.1

5.1−6.8

6.8−8.5

8.5−10.2

10.2−11.9

11.9−13.6

13.6−15.3

>15.3

A.

EXPLANATION

Amount of nitrogen delivered

to Chesapeake Bay, in kilograms 

per hectare per year

0.0−1.7

1.7−3.4

3.4−5.1

5.1−6.8

6.8−8.5

8.5−10.2

10.2−11.9

11.9−13.6

13.6−15.3

>15.3

B.

was the largest contributor of nitrogen to the Gulf, resulting 

in large part from commercial fertilizer use and manure 

applications to crops as fertilizer. Corn and soybean cultivation 

and animal manure on pasture and rangelands were the largest 

contributors of phosphorus. Illinois, Iowa, Indiana, Missouri, 

Arkansas, Kentucky, Tennessee, Ohio, and Mississippi, which 

together make up only one-third of the 31-State Mississippi 

River drainage area, were found to contribute more than 

75 percent of the nitrogen and phosphorus to the Gulf. 

SPARROW models also have been developed for smaller 

regions to provide a more detailed focus on factors that 

influence water quality locally but that may not be important 

everywhere in the country (Preston and others, 2009). 

Previous regional models for 1992 conditions include those for 

the Chesapeake Bay watershed (Brakebill and others, 2001), 

Figure 4-16. Only a fraction of the total nitrogen delivered to streams locally in the Chesapeake Bay watershed (A) is delivered to 

Chesapeake Bay (B) because of losses in the stream during travel to the Bay. The proportion of nitrogen lost during instream travel 

was greatest for areas farthest from the Bay. (Data from Brakebill and others, 2001.)

selected North Carolina coastal drainages (McMahon and 

others, 2003), and the New England region (Moore and others, 

2004). In the Chesapeake Bay model, for example, simulation 

results indicate that only a fraction of the total nitrogen 

delivered to streams locally in the watershed (fig. 4-16A) 

was ultimately delivered to the Chesapeake Bay (fig. 4-16B) 

because of losses in streams during travel to the Bay. The 

proportion of nitrogen lost during instream travel was greatest 

for areas farthest from the Bay (fig. 4-16B). New regional 

models currently are being developed for 2002 conditions in 

the Northeast, Southeast, Midwest, Northern Plains, Southern 

Plains, and the Pacific Northwest regions (fig. 4-3). These new 

SPARROW models are based on updated geospatial data and 

stream-monitoring records from local, State, and other 

Federal agencies.
68 Nutrients in the Nation’s Streams and Groundwater, 1992–2004

Seasonal Variation of Nutrients in Streams

Geographic patterns in the occurrence and distribution 

of nutrient concentrations are complicated by seasonal 

variations that affect the timing of the highest and lowest 

nutrient concentrations in streams. Nutrient concentrations 

vary seasonally in response to natural factors (streamflow 

and uptake by aquatic organisms) and anthropogenic factors 

(the type and timing of nutrient sources, such as fertilizer 

applications, and streamflow alterations, such as diversions 

and flow augmentation from irrigation return flows). 

Evaluation of these seasonal patterns in nutrient concentrations 

in streams furthers our understanding of factors that contribute 

to high nutrient concentrations and can, thereby, facilitate 

design of effective monitoring and management strategies.

• Streamflow—In streams in which point-source 

discharge or groundwater inflows predominate, 

increases in flow driven by increases in precipitation 

and runoff often lead to a decrease in nutrient 

concentrations as a result of instream dilution. The 

same increases in precipitation and consequent flow 

in streams in which nonpoint sources predominate, 

however, can cause an increase in nutrient 

concentrations as a result of the increased surface 

runoff. Similarly, decreases in streamflow can result 

in increases in nutrient concentrations where point 

sources predominate, but decreases in concentrations 

where nonpoint sources predominate (see Chapter 2, 

Nutrient Primer). The seasonal pattern of streamflow 

varies across the United States. In the eastern United 

States (east of the 100th meridian), streamflows often 

are highest in the spring and lowest in the autumn, 

following seasonal precipitation patterns (fig. 4-17A). 

Seasonal patterns in streamflow in western streams are 

less distinct because of the highly variable geography 

and climate, as well as the ubiquitous use of dams 

and canals to modify streamflow and optimize the use 

of scarce water resources in this region. Streamflows 

in some areas of the West are highest in the spring 

when snowmelt from the mountains makes its way 

downstream; in other areas, streamflows are highest in 

the summer during periods of irrigation and when local 

rainstorms occur (fig. 4-17A). Streamflows often are 

lowest in the West during the autumn and winter, when 

little precipitation or snowmelt occurs.

• Timing of nutrient usage—When fertilizers and 

manure are applied in agricultural and urban areas, 

nitrogen and phosphorus become available for 

potential transport to streams. Runoff resulting from 

precipitation shortly after such applications can 

transport this nitrogen and phosphorus to streams, 

thereby increasing instream concentrations. As a result, 

some of the highest concentrations of nutrients in 

streams occur immediately after fertilizer and manure 

application. The timing of fertilizer and manure 

application is thus a critical factor, and varies across 

the United States depending on crop type, regional 

climatic conditions, and fertilizer type.

• Biological activity—Warm water temperatures and 

abundant sunlight in the summer months create ideal 

conditions for the growth of algae and aquatic plants. 

These organisms require nutrients for growth, and 

nutrient concentrations in streams across the country 

often decrease in the summer months because of 

uptake by aquatic organisms. During cooler months,

less nutrient uptake occurs because some organisms 

die, whereas others experience a reduction in metabolism.

Manure from concentrated animal feeding operations that is used to fertilize crops can be a source of nutrients to 

nearby streams. Photograph by Tim McCabe, U.S. Department of Agriculture, Natural Resources Conservation Service.
Chapter 4

Occurrence and Distribution of Nutrients in Streams and Groundwater 69

17-0191_Fig_4-17_seasonmap.ai

J F M MA AJ J S O N D

0 J F M MA AJ J S O N D

0 J F M A M J J A S O N D

J F M MA AJ J S O N D

E

E

A.

B.

C. Seasons with high total phosphorus

Seasons with high total nitrogen

Seasons with high streamflow

D G E

F

0 

10 

20 

30 

40 D. Mean monthly discharge, in cubic feet per second

10

20

30

Mean monthly discharge,

in cubic feet per second

E.

50 

100 

150 

200 F. Mean monthly discharge, in cubic feet per second

0 

500 

1,000 

1,500 

2,000 

2,500 G. Mean monthly discharge, in cubic feet per second

0.0 

2.0 

0.02 

0.07 

Mean monthly concentration,

in milligrams per liter 

Total phosphorus

Total nitrogen

1.0

4.0

0.05 

0.13 

Mean monthly concentration,

in milligrams per liter 

Total phosphorus

Total nitrogen

Mean monthly concentration,

in milligrams per liter 

Total phosphorus

Total nitrogen

5.0

0.0

0.15

0.00

0.5 

1.0 

0.01 

0.06 

Mean monthly concentration,

in milligrams per liter 

Total phosphorus

Total nitrogen

Merced River near Newman, California

Rock Creek at Twin Falls, Idaho

Sugar Creek at New Palestine, Indiana 

Lisha Kill northwest of

Niskayuna, New York

100th

meridian Winter (November–January)

Summer (May–July)

Autumn (August–October)

Spring (February–April)

EXPLANATION

Figure 4-17. The distribution of nutrient concentrations during the year is related to broad regional patterns in streamflow (A). In the 

eastern United States, many sites have higher nitrogen concentrations in the spring as compared with other seasons of the year (B), 

whereas phosphorus concentrations generally are higher in the summer and autumn during low streamflow (C). The seasonal pattern in 

nutrient concentrations in western streams tends to be opposite that in eastern streams: high phosphorus in western streams generally 

coincides with high summer streamflows (C), whereas high concentrations of nitrogen occur during the winter when streamflow is 

generally low. Examples of regional patterns are shown in D through G.
70 Nutrients in the Nation’s Streams and Groundwater, 1992–2004

Excess nutrients are the main cause of water-quality impairment in the Chesapeake Bay. Photograph from Chesapeake Bay Program.

The specific seasonal interactions among streamflow, 

nutrient sources, and biological activity vary regionally and 

locally (Mueller and Spahr, 2006). Examples of regional 

patterns are shown in figure 4-17 and are explained as follows:

• In the eastern United States, 63 percent of sites had 

significantly higher streamflows and 39 percent had 

significantly higher nitrogen concentrations in the spring 

as compared with other seasons of the year (figs. 4-17A

and 4-17B). Seasonal low streamflows and low nitrogen 

concentrations occurred during the autumn at 66 and 

42 percent of eastern sites, respectively. In contrast 

to nitrogen, phosphorus concentrations generally 

were higher in the summer and autumn during low 

streamflow and lower in the winter and spring when 

streamflows were high (fig. 4-17C). For example, 

nitrogen concentrations in Lisha Kill, New York, were 

high and phosphorus concentrations were low in March 

and April when streamflow was high (fig. 4-17D). The 

maximum phosphorus concentrations occurred during 

periods of low summer streamflow. The association 

of high nitrogen concentrations with high streamflow 

suggests transport of nonpoint source nitrogen, such as 

fertilizer by rainfall runoff, whereas the high phosphorus 

concentrations during low streamflow indicate point 

sources, such as wastewater-treatment plants. At an 

agricultural site in the upper Midwest (Sugar Creek, 

Indiana), both nitrogen and phosphorus concentrations 

paralleled the changes in streamflow, suggesting that 

both nutrients are mobilized from nonpoint sources 

(fig. 4-17E). Low summer nitrogen concentrations in 

eutrophic agricultural streams in the Midwest may result 

in part from uptake of the nitrogen by algae.

• As with streamflow, seasonal patterns in nutrient 

concentrations in western streams are less distinct than 

in eastern streams because of hydrologic modifications 

and the highly variable geography and climate. 

The seasonal pattern in nutrient concentrations in 

western streams tends to be opposite that in eastern 

streams. The highest concentrations of phosphorus 

in western streams were most common during the 

summer when snowmelt produced high streamflows 

(fig. 4-17C), whereas high concentrations of nitrogen 

in western streams occurred during the winter 

when streamflow was generally low (fig. 4-17B). 

The data for Rock Creek, Idaho (fig. 4-17F), are 

characteristic of this seasonal pattern, with high 

concentrations of phosphorus and low concentrations 

of nitrogen coincident with high spring and summer 

streamflows. Especially in the semiarid areas of the 

interior West, high phosphorus concentrations during 

high streamflow are consistent with mobilization 

of sediment-bound phosphorus by erosion. High 

nitrogen concentrations during low streamflow may 

be derived from irrigation-return flow, point sources, 

or groundwater discharge to streams—sources that are 

diluted by spring and summer snowmelt runoff. For 

example, in the Merced River, California (fig. 4-17G), 

dam releases of runoff with low nutrient concentrations 

from the undeveloped Sierra Nevada Mountains dilute 

the effects of runoff from agricultural land in the 

San Joaquin Valley during the irrigation season from 

March to July. Concentrations of both nitrogen and 

phosphorus increased in the late summer and early 

winter when reservoir releases were minimal.
Chapter 4

Occurrence and Distribution of Nutrients in Streams and Groundwater 71

Figure 4-18. Concentrations of nitrate in groundwater were higher in agricultural land-use areas than in urban areas, and were 

higher in urban areas than in major aquifers. Concentrations of phosphorus and ammonia varied little among land-use categories.

Most wells sampled for agricultural and urban land-use studies were 

monitoring wells installed by NAWQA. Photograph by U.S. Geological Survey.

90th percentile

10th percentile

Median

75th percentile

25th percentile

EXPLANATION

0.02 Detection limit

Concentration, in milligrams per liter as N

Concentration, in milligrams per liter as P

Agricultural Urban Major aquifers

0

2

4

6

8

10

12

14

16

18

0.00

0.05

0.10

0.15

0.20

Agricultural Urban Major aquifers

Nitrate Orthophosphate

Ammonia

Concentration, in milligrams per liter as N

0.0

0.1

0.2

0.3

0.4

0.5

Agricultural Urban Major aquifers

0.02

0.01 0.05

Nutrient Concentrations in 

Groundwater 

Relations to Land Use

Nitrate concentrations were significantly different among 

groundwater samples collected in agricultural, urban, and 

major aquifer studies (Burow and others, 2010) (fig. 4-18). 

Concentrations of nitrate were highest in groundwater beneath 

the agricultural land-use setting, with a median of 3.1 mg/L—

about 3 times the national background concentration of 

1.0 mg/L (table 4-1). Nitrate concentrations were lower 

in urban areas (median of 1.4 mg/L) than in agricultural 

areas, but were higher in urban areas than in major aquifers. 

Some of the differences in concentrations between the study 

types can be explained by differences in the depths of the 

wells from which samples were collected. Wells sampled in 

agricultural and urban land-use studies typically are shallow 

(median well depth, 33 feet), and thus represent mostly 
72 Nutrients in the Nation’s Streams and Groundwater, 1992–2004

recently recharged groundwater, whereas wells completed in 

the major aquifers typically are much deeper (median well 

depth, 152 feet), and represent older groundwater. Although 

the highest concentrations of nitrate in groundwater are higher 

than the highest nitrate concentrations in streams, median 

concentrations are similar, and the relative differences in 

concentrations among the different land-use categories 

are similar. 

Concentrations of orthophosphate (hereinafter referred 

to as dissolved phosphorus) and ammonia were much lower 

in groundwater than in streams, varied little among land-use 

categories, and generally were not greater than background 

levels (0.03 and 0.10 mg/L, respectively, table 4-1). Dissolved 

phosphorus and ammonia are not persistent or mobile enough 

to affect groundwater concentrations significantly under 

most conditions.

Geographic Distribution

Variations in the distribution of nitrate concentrations 

in groundwater are due to differences in types of sources, 

physical factors that either favor or inhibit transport of nitrate 

to groundwater, and chemical factors that result in persistence 

or removal of nitrate in groundwater over time. Nitrate 

concentrations are highest in shallow, oxic groundwater that 

receives high inputs of nitrogen from fertilizer, manure, and 

atmospheric deposition. Median concentrations of nitrate in 

shallow groundwater for agricultural land-use studies were 

higher in many studies in the Northeast, the Midwest, and 

the Northwest, and in a few studies in the Southeast and 

California (fig. 4-19). These concentrations were likely the 

result of high nitrogen inputs and conditions favorable to 

nitrate transport in groundwater. Median concentrations of 

nitrate in urban studies were mostly in the medium range; the 

relatively few studies with concentrations in the low category 

were distributed across the southern half of the country. Most 

nitrate concentrations in major aquifers were low or medium, 

with somewhat higher concentrations in parts of the Northeast, 

the Northern and Southern Plains, and the Southwest. High 

concentrations in the Northern and Southern Plains and 

Southwest could be influenced by irrigation practices, which 

may accelerate the downward movement of nitrate into 

groundwater. Sampled wells in the major aquifers of the 

Northeast are shallower than wells in most of the other major 

aquifer studies, and thus the samples may represent young 

groundwater that has moved rapidly through the system. 

Concentrations generally were low in the major aquifers of the 

Midwest and south through the Mississippi Valley to the coast, 

which can be attributed to a combination of physical and 

chemical properties that inhibit rapid nitrate transport to these 

deep aquifers. 

The distribution of nitrate concentrations in groundwater 

(fig. 4-19) contrasts with that of total nitrogen in streams 

(fig. 4-4). Whereas both agricultural and mixed land-use 

streams that have total nitrogen concentrations in the highest 

category are clustered in the upper Midwest, the nitrate 

concentrations in groundwater in agricultural areas and major 

aquifer studies do not show this pattern of distribution. The 

contrast between the spatial distribution of total nitrogen 

in streams and nitrate in groundwater is a consequence of 

hydrologic factors that promote rapid and efficient movement 

of nitrogen to streams yet tend to inhibit the movement of 

nitrate into groundwater. In the upper Midwest, for example, 

artificial subsurface drainage and ditches installed to drain 

dense, clay-rich glacial soils, route runoff and groundwater 

recharge to streams, while limiting the opportunity for 

infiltration to the water table.

In contrast to concentrations of phosphorus in streams, 

dissolved phosphorus concentrations in groundwater generally 

were low. Eighty-eight percent of the measured concentrations 

were less than 0.1 mg/L dissolved phosphorus, although 

locally some concentrations were as high as 4.3 mg/L. 

Phosphorus in groundwater originates from 

anthropogenic sources, such as fertilizer, manure, and 

waste effluent, as well as natural geologic sources. Elevated 

dissolved phosphorus concentrations in groundwater occur 

throughout the United States, with no definitive spatial pattern. 

Median concentrations were greater than the background 

concentration of 0.03 mg/L in 21 percent of major aquifer 

studies and 21 percent of agricultural and urban land-use 

studies. Additionally, dissolved phosphorus concentrations 

in groundwater showed no correlation to fertilizer and 

manure use in agricultural areas. This lack of correlation 

and the similarity of concentrations in both shallow and 

deep groundwater suggests that natural geologic sources of 

phosphorus may have a greater influence on concentrations in 

groundwater than do human-derived sources from activities on 

the land surface. 

The transport of phosphorus in groundwater typically 

is limited because most phosphorus compounds are not very 

soluble and tend to precipitate or adsorb to soil particles. 

Phosphorus can be mobilized under favorable geochemical 

conditions, however, such as high pH, high organic content, 

and geochemically reduced conditions. Dissolved phosphorus 

concentrations were significantly higher in reduced 
Chapter 4

Occurrence and Distribution of Nutrients in Streams and Groundwater 73

17-0191_Fig_4-19_GW_Nconc.ai

2,900

2,900–15,000

15,000

Shallow groundwater 

studies, by land use

Data on nitrogen inputs not available for Alaska and Hawaii.

EXPLANATION

Agricultural Urban

Major

aquifers

Low (0.08)

Medium (0.08–2.6)

High (2.6)

Median concentration of nitrate,

in milligrams per liter as N

Estimated 1997 nitrogen inputs from fertilizer, manure, and 

atmosphere, in pounds per square mile

Agricultural Urban

Major aquifers

Groundwater

Figure 4-19. High concentrations of nitrate in groundwater in agricultural areas and major aquifer studies are broadly distributed 

rather than clustered in the upper Midwest, where the nitrogen inputs are high.

groundwater than in oxic groundwater, and 

concentrations also were higher in groundwater 

with pH greater than 7.5. In selected areas 

with median phosphorus concentrations above 

background levels, dissolved phosphorus was 

significantly and positively correlated to pH 

and to the concentrations of ammonia, calcium, 

magnesium, and iron. 

Phosphorus is not regulated in drinking 

water, however, its concentration in groundwater 

can affect surface-water quality at locations 

where groundwater inflow contributes 

significantly to streamflow (see Chapter 5, 

Exchange of Nutrients between Surface Water 

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

Salt Lake City was among the locations where the quality of shallow groundwater beneath residential urban land was studied. 

Photograph by Steven Gerner, U.S. Geological Survey.

Nitrate Leaching to Groundwater

Leaching of nonpoint source nitrate to groundwater 

can pose a threat to drinking water supplies or streams. The 

amount of nitrate leached to groundwater, represented as the 

percentage of nonpoint source nitrogen inputs at the land 

surface that moves beyond the root zone and reaches the 

aquifer, varies as a function of crop uptake and soil conditions. 

The percentage of nonpoint source nitrogen leached to 

groundwater has been estimated in various USGS field studies 

(fig. 4-20). Although these studies used different methods to 

estimate the percentage of nitrate leached to groundwater, the 

estimates generally range from 10 to 50 percent. Leaching 

fractions are relatively low (less than 20 percent) in areas 

where tile drains divert percolating groundwater to streams, or 

where fine-grained sediments impede the vertical movement 

of water and thus provide more opportunity for denitrification 

to remove nitrate before the water reaches the aquifer. 

Aquifers that underlie areas with high nitrogen inputs to the 

land surface, combined with coarse-grained soils and rapid 

rates of vertical movement through the unsaturated zone, are 

the most vulnerable to nitrate contamination. 

Nitrate concentrations measured in oxic shallow 

groundwater were significantly correlated to estimated 

nonpoint source nitrogen input within a 500-meter 

(0.31 mile) radius around each well (fig. 4-21). Although 

the correlation between median nitrate input and median 

nitrate concentration is statistically significant, the correlation 

is weak, indicating that other factors have an important 

influence on concentrations. For example, concentrations of 

nitrate in groundwater in the agricultural land-use studies 

in the Upper Snake River Basin are relatively low despite 

high nitrogen inputs. In this area, the low background nitrate 

concentrations (less than 1 mg/L), high lateral groundwater 

flow rates, and dilution by irrigation water from the Snake 

River result in network medians from about 1 to 4 mg/L. In 

contrast, the median concentration of nitrate in groundwater 

in the agricultural land-use study in the Puget Sound Basin 

is high (about 13 mg/L) in spite of low nitrogen input. This 

study area has one of the highest recharge rates; the sandy 

aquifer sediments facilitate rapid transport of nitrate to these 

wells. The nonpoint source nitrogen input in urban landuse study areas is lower than that in agricultural areas, and 

nitrate concentrations are generally lower in response to 

this difference in nitrogen inputs. Concentrations of nitrate 

in groundwater in the Santa Ana Basin, the Great Salt Lake 

Basins, and South Platte River Basin studies are high relative 

to the concentrations in other urban land-use studies. The 

population density in these three study areas is greater than 

1,000 people per square kilometer. Population density is 

significantly correlated with nitrate concentrations within 

the urban land-use studies, suggesting that the intensity of 

development of urban land may better reflect nitrogen inputs 

in urban areas than inputs estimated on the basis of fertilizer 

and manure applications and atmospheric sources.
Chapter 4

Occurrence and Distribution of Nutrients in Streams and Groundwater 75

20_GW_N_leaching.ai

GroundwaterUnsaturated

zone

≤20

20–40

40

Fraction of nitrate 

leached, in percent

Estimated 1997 nitrogen inputs from

fertilizer, manure, and atmosphere,

in pounds per square mile

2,900

2,900–15,000

15,000

Media

EXPLANATION

Groundwater

17-0191_Fig_4_21_No3_totn_LUSoxic.ai

0

5

10

15

20

Nitrate concentration, in milligrams per liter as N

0 10,000 20,000 30,000

Nonpoint source nitrogen input, in pounds per year

USNK

GRSL

SPLT

PUGT

SANA

Agricultural 

Urban

Shallow groundwater study

EXPLANATION

USNK—Upper Snake River Basin

PUGT—Puget Sound Basin

SANA—Santa Ana Basin

GRSL—Great Salt Lake Basins

SPLT—South Platte River Basin

NAWQA Study Unit

Figure 4-20. It is estimated that 10 to 50 percent of the nonpoint source 

nitrogen inputs at the land surface reach the groundwater at the sites where 

detailed studies were conducted.

Figure 4-21. Although there is a statistically 

significant correlation between nonpoint source 

nitrogen inputs and median nitrate concentrations in 

oxic shallow groundwater, the correlation is weak, 

indicating that other factors have an important 

influence on concentrations. 
76 Nutrients in the Nation’s Streams and Groundwater, 1992–2004

Water-quality characteristics are measured at the site as samples are 

collected for laboratory analysis. Photograph by U.S. Geological Survey.

17-0191_Fig_4-22_GWredox_wtdep.ai

EXPLANATION

Depth to seasonally 

high water table in

the soil horizon, in feet

5

3–5

0–3

Redox condition

of groundwater 

Reduced

Mixed

Oxic

Geochemical Controls on Nitrate Groundwater

Occurrence in Groundwater

Redox conditions exert a strong control on 

nitrate occurrence in groundwater (Burow and 

others, 2010) (see Chapter 2, Nutrient Primer). 

When the geochemical environment is oxic, as 

indicated by the presence of dissolved oxygen 

greater than 0.5 mg/L in groundwater, nitrate may 

persist. When the geochemical environment is 

reducing, nitrate in groundwater may be removed 

by denitrification, which yields nitrogen gas. 

Reducing conditions predominate in areas where 

large amounts of organic carbon are present 

in the aquifer and where near-surface water 

saturation limits the penetration of oxygen gas. 

Reducing conditions also can evolve because of 

the progressive depletion of dissolved oxygen by 

reactions during flow, and hence, groundwater 

tends to become more “reduced” the longer it 

resides in the ground. Conditions favorable to 

reducing conditions are more common in humid, 

poorly drained areas of the eastern U.S.; oxic 

conditions favorable for persistence of nitrate are 

prevalent in the semiarid West (fig. 4-22).

Median nitrate concentrations in groundwater 

are significantly higher under oxic conditions than 

under other redox conditions, regardless of the 

land use (fig. 4-23). In addition, within each land 

use, nitrate concentrations are strongly dependent 

on redox condition regardless of nitrogen load. 

For example, nitrogen input in agricultural areas 

is similar across the three redox categories, but the 

median concentration of nitrate is about 5.6 mg/L 

in oxic groundwater, and is less than the detection 

level (0.05 mg/L) in reduced groundwater. 

Another striking illustration of the impact of 

redox condition is that nitrate concentrations are 

higher in oxic water in shallow urban wells with 

low nitrogen input, than in agricultural wells with 

much higher input but reduced water. Similar 

relations are seen in concentrations of nitrate in 

wells completed in major aquifers. In addition to 

potential denitrification in reduced groundwater, in 

some areas of poorly drained soils tile drains divert 

infiltrating water to streams, resulting in still less 

nitrogen load to groundwater. 

Figure 4-22. Groundwater with reducing conditions (indicated by red 

dots) is common in the Midwest and eastern United States, where a humid 

climate and poor drainage lead to saturated soils (indicated by shallow 

depth to a seasonally high water table). Oxic (oxygen-rich) conditions 

favorable for persistence of nitrate are prevalent in the semiarid West.
Chapter 4

Occurrence and Distribution of Nutrients in Streams and Groundwater 77

Oxic

Mixed

Reduced

Oxic

Mixed

Reduced

Oxic

Mixed

Reduced

Agricultural

Urban

Major aquifers

Redox condition Redox condition Redox condition

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

Farm fertilizer

Nonfarm fertilizer

Manure

Atmospheric

Nitrogen input

EXPLANATION

Figure 4-23. Nitrate concentrations are significantly higher in oxic 

groundwater regardless of the land use and source of nitrogen. Within each 

land-use category, nitrate concentration is strongly dependent on redox 

condition regardless of nitrogen input. (Modified from Burow and others, 2010.)

These results show that the redox 

condition of an aquifer is a critical factor to 

consider when assessing vulnerability to nitrate 

contamination. Resource managers could 

glean critical information on redox condition 

from data routinely collected on dissolved 

iron and manganese (which indicate reduced 

conditions when concentrations are elevated) 

for assessment of secondary drinking-water 

standards, or with additional monitoring of 

dissolved oxygen concentrations.
78 Nutrients in the Nation’s Streams and Groundwater, 1992–2004

Groundwater samples are analyzed for environmental tracers, such as 

tritium and chlorofluorocarbons, to determine the time elapsed since 

recharge. Tritium is analyzed by (clockwise from upper left) distillation, 

electrolytic enrichment, secondary distillation, and scintillation counting. 

Photographs by A. Abedini, U.S. Geological Survey.

AAXXXX_fig 01

Nitrate concentration, in milligrams per liter as N

0.05

0.1

1

10

100

Young groundwater

(post-1952)

Old groundwater

(pre-1952)

Age of groundwater, as determined by tritium concentration

Background

concentration

90th percentile

75th percentile

Median

25th percentile

10th percentile

EXPLANATION

0.05

Detection limit

Figure 4-24. Nitrate concentrations are significantly higher in younger 

groundwater than in older groundwater.

Physical Factors: Aquifer and 

Well Characteristics

Groundwater age reflects the aggregate effect 

of all physical factors that control the rate of 

movement of groundwater in an aquifer—those 

physical attributes that control the “intrinsic 

susceptibility” of an aquifer. Water samples 

collected from a subset of the NAWQA wells 

were analyzed for tritium, an environmental tracer 

that indicates whether the bulk of the water was 

recharged prior to or after 1952 (see sidebar, 

Groundwater Tracers as Indicators of Groundwater 

Age). Groundwater with tritium concentrations 

greater than 2.5 picocuries per liter is assumed to 

have been recharged after 1952.

Sorting groundwater samples into two 

groups—pre- and post-1952 by tritium “age”—

shows that nitrate concentrations are significantly 

higher in younger (post-1952) groundwater than 

in older groundwater (fig. 4-24). Specifically, most 

of the old groundwater has nitrate concentrations 

less than the background concentration of 1 mg/L, 

whereas 59 percent of young groundwater has 

concentrations greater than 1 mg/L. Groundwater Groundwater Tracers as Indicators of 

Groundwater Age 

Concentrations of certain anthropogenic 

chemicals in groundwater can be used to trace 

the flow of young water (water recharged within 

the past 50 years) and to determine the time 

elapsed since recharge (Plummer and Friedman, 

1999). Recent human activities have released 

an array of chemical and isotopic substances to 

the atmosphere. These atmospheric substances 

(such as tritium) have been incorporated into the 

hydrologic cycle and can be found in groundwater 

recharged within the last 50 years. The age of a 

groundwater sample is estimated by comparing 

the concentration of a tracer in the sample with 

the known history of the change in concentration 

of the tracer in the atmosphere. Information about 

the age of groundwater can be used to provide 

further understanding of nitrate concentrations in 

wells. For example, low concentrations of nitrate 

may be found in oxic groundwater with high 

nitrogen inputs. If the corresponding age of water 

in the well is greater than about 50 years, then the 

low nitrate concentration may reflect historically 

low input of nitrogen (at the time of recharge). 

Concentrations may increase in the future in this 

well as groundwater that reflects the currently 

high inputs reaches the well.
Chapter 4

Occurrence and Distribution of Nutrients in Streams and Groundwater 79

17-0191_Fig_4-25_no3_satthick.ai

0 10 20 30 40 50 60 70 80

200

400

600

800

1,000

0

Nitrate concentration, in milligrams per liter as N

Well depth below water table, in feet

Aquifer type

0.01

0.1

1

10

100

Nitrate concentration, in milligrams per liter as N

Confined Unconfined

0.05

90th percentile

75th percentile

Median

25th percentile

10th percentile

Data outside 90th

or 10th percentile

EXPLANATION

0.05

 Detection limit

age is not a definitive control on nitrate 

concentration, however. For example, areas 

with young groundwater may have low nitrate 

concentrations because no nitrate sources are 

present in their recharge areas, or reducing 

conditions may have removed any nitrate from the 

water. Areas with old groundwater also may have 

high nitrate concentrations, particularly in areas 

where most of the groundwater moves slowly but 

fractures or other high-permeability features allow 

rapid transport of small volumes of high nitrate 

water to wells (McMahon and others, 2008).

Nitrate concentrations decrease as the depth 

of a well below the water table increases (fig. 4-25) 

because of the following contributing factors:

• In general, it takes longer for groundwater 

to travel to greater depths, and hence, the 

recharge may have occurred at a time when 

nitrogen input at the surface was lower.

• Greater depth suggests a longer residence 

time, which increases the opportunity for 

denitrification to remove nitrate from water.

• The groundwater flow paths to wells 

become more complex and intermixed 

as the well gets deeper. This increase in 

depth increases the likelihood that the 

water sampled in a deep well is a complex 

mixture of water recharged in many 

different—and possibly distant—land uses 

with different nitrogen sources (or none at all).

Water from wells completed in confined 

aquifers had significantly lower concentrations 

of nitrate than water from unconfined wells 

(fig. 4-26). Confined aquifers are separated from an 

overlying unconfined aquifer by fine-grained layers 

of material that impede downward movement of 

younger water that may be carrying contaminants 

from the land surface. Many groundwater 

protection programs assume that a confined aquifer 

is not vulnerable to nitrate contamination. But finegrained confining layers may not be completely 

protective: 25 percent of wells classified as being 

completed in confined aquifers had concentrations 

of nitrate above national background levels 

(1 mg/L), including 2 percent with concentrations 

above the drinking-water limit of 10 mg/L. In 

some areas, confining layers are not uniformly 

extensive, or have preferential flow paths through 

which water can quickly cross the layers (see 

sidebar, Protection of Confined Aquifer from 

Contamination in the High Plains Aquifer). 

Figure 4-25. A number of factors contribute to a general 

decrease in nitrate concentrations with increasing depth 

of a well, including greater mixing and more opportunity for 

denitrification at greater depths, and greater nitrogen input 

in recent decades. 

Figure 4-26. Although wells in confined aquifers had lower 

concentrations of nitrate than did wells in unconfined aquifers, 

high concentrations—including some that exceeded the 

Maximum Contaminant Level of 10 milligrams per liter—indicate 

that confining layers are not always completely protective 

(Burow and others, 2010). 
80 Nutrients in the Nation’s Streams and Groundwater, 1992–2004

High Plains aquifer, Nebraska

Water table

Lower confined lenses

 (sand and silt)

Unconfined aquifer

Confining unit 

Upper confined aquifer 

Lower confining unit

Loess

Not to scale

Inactive well

Irrigation well

Public-supply well

(sand and gravel)

(sand)

Shale

(clayey silt till)

Recharge

Discharge

Direction of

groundwater flow

EXPLANATION

Well

Screened Interval

Protection of Confined Aquifer from 

Contamination in the High Plains Aquifer 

Confined aquifers are commonly assumed to be protected from contamination from activities on the land surface because 

relatively impermeable fine-grained layers impede downward movement of younger groundwater containing contaminants. 

However, highly contaminated water in unconfined aquifers can move downward or laterally to confined aquifers, particularly 

if those confined aquifers are heavily used (pumped) or if features exist that allow contaminants to short-circuit through or 

around the fine-grained layers.

In the northern High Plains aquifer in east-central Nebraska, young contaminated groundwater from the unconfined aquifer 

is reaching the deeper confined aquifer, where most of the public-supply wells are screened (fig. 4-27) (Landon and others, 

2008). The shallow unconfined aquifer beneath both urban and agricultural land has high nitrate concentrations (median 

concentration of 16 mg/L). Water in the confined aquifer generally is old and in a reduced state, so nitrate concentrations 

generally are low (median concentration of less than 0.06 mg/L). However, water from 2 of 11 of the public-supply wells 

screened exclusively in the confined aquifer had concentrations of nitrate greater than the MCL of 10 mg/L in at least one 

sample during the last decade.

On the basis of analysis of chemical tracers, sediment-chemistry data, and simulation modeling, it has been shown that 

nitrate and other contaminants in the unconfined aquifer have bypassed the highly reducing confining unit by moving through 

abandoned or inactive well bores to reach the screened intervals of wells completed in the confined aquifer (fig. 4-27) 

(McMahon and others, 2008; Landon and others, 2008). Movement of nitrate through confining units or along other rapid travel 

paths has been documented in other areas of the country as well, such as the Floridan aquifer in Florida (Katz and others, 

2007). These examples demonstrate that aquifer confinement does not ensure that an aquifer is safe from contamination from 

activities on the land surface.

Figure 4-27. Despite the presence of an extensive overlying confining unit, 

overlying unconfined water containing high nitrate concentrations is reaching 

drinking water wells in the confined aquifer in parts of the High Plains aquifer. 

(Modified from Jagucki and others, 2008.)
Chapter 4

Occurrence and Distribution of Nutrients in Streams and Groundwater 81

 Another factor that can influence the vulnerability of 

groundwater to nitrate contamination is the aquifer lithology—

the primary rock type (Miller, 2000; U.S. Geological Survey, 

2003; and U.S. Geological Survey, 2009b). Aquifer lithology 

can influence the speed at which contaminants move through 

the aquifer. Groundwater studies were grouped into categories 

on the basis of lithology of the principal aquifer. 

The relative vulnerabilities of the principal aquifer 

settings, as indicated by median nitrate concentration, can be 

summarized as follows (see fig. 4-28):

• Waters in unconsolidated sand and gravel (non‐glacial 

origin) and in basaltic and other volcanic-rock 

aquifer lithology groups have relatively high nitrate 

concentrations regardless of land use. This is consistent 

with the high permeability of these geologic materials.

• Waters in sandstone and semi-consolidated sand and 

gravel aquifers consistently have among the lowest 

nitrate concentrations within each land-use category. 

• The relative rank of the nitrate concentrations in water 

in aquifers in the other four lithology groups varies 

among the land-use categories. For example, the 

carbonate rock aquifer lithology group has the highest 

concentrations of nitrate among the lithology groups in 

agricultural areas, but among the lowest concentrations 

in urban and major aquifer networks.

Sandstone

Carbonate rock

Unconsolidated sand and gravel (non-glacial origin)

Crystalline rock

Glacial sand and gravel

Semi-consolidated sand and gravel

Sandstone and carbonate rock

Basaltic and other volcanic rock

EXPLANATION

Aquifer lithology group

Major aquifers Agricultural Urban

0 2 4 6 8

Glacial sand and gravel

Unconsolidated sand and gravel (non-glacial origin)

Basaltic and other volcanic rock

Sandstone and carbonate rock

Crystalline rock

Sandstone

Carbonate rock

Semi-consolidated sand and gravel

0 2 4 6 8 0 2 4 6 8

Median nitrate concentration, 

in milligrams per liter as N

Median nitrate concentration,

in milligrams per liter as N

Median nitrate concentration,

in milligrams per liter as N

Aquifer lithology group

no data

no data

Figure 4-28. Consistent with the high permeability of these lithologies, unconsolidated sand and gravel (non-glacial origin) and 

basaltic and volcanic-rock aquifer lithology groups contain groundwater with relatively high nitrate concentrations regardless 

of land use. In contrast, water in sandstone and semi-consolidated sand and gravel aquifers consistently have groundwater 

with among the lowest nitrate concentrations within each land-use category.
82 Nutrients in the Nation’s Streams and Groundwater, 1992–2004

17-0191_Fig4-29_tritium_redx_no3.ai

Young groundwater (post-1952)

Percentage of samples with tritium data for age dating (1885 total)

0 20 40 60

Oxic

Mixed

Reduced

0 20 40 60

Redox condition

≤1

EXPLANATION

Nitrate concentration, in milligrams per liter as N

1–10 10

Old groundwater (pre-1952)

0

Agricultural land-use studies

5,000 10,000 15,000 20,000 25,000 30,000

0

20

40

60

80

100

Percentage of oxic wells

Total nitrogen input within 500-meter radius of well, in pounds

Carbonate rock

Unconsolidated sand and gravel (non-glacial origin)

Sandstone and carbonate rock Glacial sand

and gravel Semi-consolidated sand and gravel

Sandstone

Basaltic and other volcanic rock

1 1–2 2

Median nitrate concentration, in milligrams per liter as N

EXPLANATION

Figure 4-29. The highest nitrate concentrations in groundwater occur where 

aquifer properties and redox conditions are favorable to nitrate transport and 

persistence. Nitrate concentrations were highest in young oxic groundwater; in 

contrast, nitrate concentrations greater than 10 milligrams per liter were rare in 

old groundwater in general and non-existent in old reduced groundwater (Burow 

and others, 2010). 

Figure 4-30. The aquifers most vulnerable to high nitrate 

concentrations are those with the highest nitrogen inputs, oxic 

conditions, and permeable lithologies, such as basaltic and 

volcanic-rock aquifers.

Integration of Physical and 

Chemical Factors that Control 

Nitrate in Groundwater

The highest nitrate concentrations 

occurred where both aquifer properties and 

redox conditions favor nitrate transport and 

persistence (Burow and others, 2010). Wells 

were grouped into two age groups (pre- and 

post-1952, on the basis of tritium activity 

data) and then split into redox categories to 

illustrate the relation between groundwater 

age and redox condition (fig. 4-29). Young 

water tends to be more oxic, and accounts 

for most samples with nitrate above 

background concentrations. In particular, 

very high nitrate concentrations—greater 

than 10 mg/L—are found in only 1.4 percent 

of old groundwater samples in general, and 

are nonexistent in old reduced groundwater. 

These results underscore the vulnerability of 

groundwater to contamination in areas where 

both physical and geochemical conditions 

favor the rapid movement and persistence of 

nitrate in aquifers.

The highest groundwater nitrate 

concentrations occur where relatively large 

nitrogen inputs coincide with physical 

and chemical factors that favor nitrate 

transport and persistence. The aquifers most 

vulnerable to high nitrate concentrations 

are those with the highest nitrogen inputs, 

a highly permeable lithology, and oxic 

conditions (fig. 4-30). The principal aquifer 

lithologies in which nitrogen inputs were 

generally high and redox conditions were 

predominantly oxic, such as the basaltic and 

volcanic-rock and the unconsolidated sand 

and gravel (non-glacial origin) aquifers, had 

the highest nitrate concentrations. In contrast, 

the sandstone principal aquifer lithology, 

which generally had low nitrogen inputs and 

predominantly reduced conditions, had the 

lowest nitrate concentrations. Because the 

factors that influence aquifer vulnerability 

to nitrate contamination are complex and 

interrelated, statistical models have been 

used to integrate aquifer vulnerability 

characteristics over large areas.
Chapter 4

Occurrence and Distribution of Nutrients in Streams and Groundwater 83

17-0191_Fig_4-31_GWAVA-S_model_prediction.ai

Predicted nitrate concentration, in milligrams per liter

EXPLANATION

≤1 1–5 5–10 10 Missing data

Model for shallow groundwater

Groundwater

Figure 4-31. The model for shallow groundwater predicts moderate (yellow 

and orange areas) to severe (red areas) nitrate contamination in areas with 

large nitrogen sources, factors that promote rapid transport of nitrogen in 

groundwater, and a lack of attenuation processes (Nolan and Hitt, 2006). 

Extrapolation of Nitrate 

Concentrations in Shallow 

Groundwater to Unsampled Areas

A statistical model was developed to predict 

nitrate concentrations in shallow groundwater 

(median depth of 10 meters, or 33 feet) based 

on concentrations measured in samples from 

wells in 97 studies across the conterminous 

United States along with selected explanatory 

factors (Nolan and Hitt, 2006) (see Chapter 3 

section, Extrapolation to Unsampled Areas, 

Development of the Nonlinear Nitrate Models for 

Groundwater). The model predicts moderate to 

severe nitrate contamination in the High Plains, 

northern Midwest, and other areas of intensive 

agriculture in both the East (eastern Pennsylvania 

and the Delmarva Peninsula) and the West 

(the Columbia Plateau in Washington, the San 

Joaquin Valley in California, and the Snake River 

Plain in Idaho) (fig. 4-31). The highest nitrate 

concentrations are predicted in areas with large 

nitrogen sources, factors that promote rapid 

transport of nitrogen in groundwater, and a lack 

of attenuation processes. 

The model for shallow groundwater 

explained about 80 percent of the variation in 

nitrate concentrations. Factors that represent 

nitrogen sources include farm fertilizer, manure 

from confined livestock, and population density, 

as well as the amount of agricultural land. 

Factors in the model that represent the rate at 

which nitrate is transported in groundwater 

include water input, rock type, the presence of 

drainage ditches, and percentage of clay. For 

example, rocks such as limestone in eastern 

Pennsylvania and basalt in the Columbia Plateau 

of Washington have features, such as fractures 

and solution cavities, that promote the transport 

of nitrate in water moving through the aquifer. In contrast, ditches in 

poorly drained agricultural areas, such as parts of the upper Midwest, 

divert water to nearby streams, which short-circuits the leaching process. 

This “diversion” results in low concentrations of nitrate in groundwater 

despite very large sources of nitrogen on the land surface. Factors that 

represent nitrate attenuation processes in the model include the presence 

of wetlands and the extent of soils rich in organic matter. Together, these 

variables represent the relatively high potential for denitrification in 

saturated soils with high organic carbon content. 
84 Nutrients in the Nation’s Streams and Groundwater, 1992–2004

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Chapter 5

5

Chapter

Photograph by Lynn Betts, U.S. Department of Agriculture, Natural Resources Conservation Service.

Exchange of 

Nutrients between 

Surface Water 

and Groundwater 

Introduction

Knowledge of the relative magnitudes of the multiple 

sources of streamflow—and of the contribution of 

groundwater in particular—is important because the 

concentrations of nutrients are commonly different 

among the different sources. In a natural setting, the 

sources of streamflow can include precipitation, surface 

runoff, groundwater discharge, interflow (that is, shallow 

subsurface flow), and the release of water temporarily 

stored within the watershed (bank storage, wetlands, 

lakes). In streams modified by development, streamflow 

also may include reservoir release, discharges from point 

sources in urban areas, and irrigation-return flows in 

agricultural areas. Changes in streamflow and nutrient 

concentrations over time will reflect the cumulative effects 

of all these factors. 

Streamflow can be separated into two general classes: 

quick flow, which consists of surface runoff and rapid 

interflow; and base flow, which consists of groundwater 

discharge, release from other watershed storages, and 

longer term interflow. Base flow has traditionally been 

attributed primarily to groundwater discharge, but it 

is important to note that other sources of water can 

contribute to base flow (Halford and Mayer, 2000).

Unlike nutrient sources on the land surface, groundwater 

contributions of nutrients to streams are less apparent and 

difficult to measure, and hence, often go unrecognized. 

Inputs of nitrate to a stream from groundwater can be 

substantial, and in this chapter the amounts and sources 

of nitrate in streams during base flow conditions will be 

evaluated, along with factors that affect the transport 

of nitrate from groundwater to streams. Examples of 

conditions under which groundwater may contribute 

phosphorus to streams also are described.
86 Nutrients in the Nation’s Streams and Groundwater, 1992–2004

EXPLANATION

Percentage of total nitrate

load contributed by base flow

by tercile

≤37

37–54

54

(49 sites)

(50 sites)

(49 sites)

Base Flow Contributions of Nitrate 

Load to Streams

The proportion of the nitrate load of a stream contributed 

by base flow was estimated for 148 relatively small watersheds 

(each less than 500 square miles) across the Nation (Spahr and 

others, 2010) (see sidebar, Calculation of the Contribution of 

Base Flow to the Nitrate Load in Streams). Many streams have 

a large percentage of their nitrate load contributed by base 

flow: 66 percent of streams had more than 37 percent of the 

total nitrate load contributed by base flow (fig. 5-1). Although 

broad regional patterns are not well defined, small clusters of 

sites at which similar processes result in similar proportions of 

nitrate contributions are distinct.

Low to moderate proportions of stream nitrate loads—

less than 54 percent of the total load—are contributed by base 

flow in most streams in the central portion of the country, from 

the Great Lakes to Texas (fig. 5-1):

• Sites in the Trinity River Basin of eastern Texas 

typically have less than 37 percent of their nitrate load 

contributed by base flow. The low base flow proportion 

at these sites results from periods of very low flow with 

low nitrate concentrations followed by short duration 

runoff events with greater nitrate concentrations (Land 

and others, 1998). 

• Sites in the eastern part of the Corn Belt (Indiana 

and Ohio) have base flow contributions to nitrate in 

streamflow that commonly are less than 37 percent of 

total instream nitrate. Relatively low contributions of 

groundwater to instream nitrate loads result from the 

use of tile drains in much of the Corn Belt (see sidebar, 

Hydrologic Modifications Create Seasonal Patterns in 

Nutrient Concentrations).

Streams with greater than 54 percent of their nitrate load 

from base flow drain a variety of hydrologic settings and 

include multiple sources of nitrate (fig. 5-1):

• The Sandhills region of central Nebraska represented 

by the Dismal River (the red symbol in central 

Nebraska) is an area with highly permeable soils and 

bedrock. Very little runoff occurs in this area because 

precipitation infiltrates directly to groundwater 

(Frenzel and others, 1998). Streamflow is almost 

entirely generated by groundwater discharge, resulting 

in a base flow nitrate load contribution of 97 percent.

• Seven sites in the Valley and Ridge physiographic 

province, which extends from northern Alabama 

through Tennessee, Virginia, Maryland, and 

Pennsylvania, have more than 54 percent of their 

nitrate load contributed by base flow. Base flow nitrate 

contribution is more than 70 percent of total instream 

nitrate for four sites that are underlain by carbonate 

aquifers. Although hydrologic characteristics vary 

within the Valley and Ridge carbonate aquifers, they

generally are considered unconfined, with karst 

features including sinkholes, springs, and caverns 

(Lindsey and others, 2009). The karst and solution 

features result in aquifers that are highly susceptible to 

contamination (Hampson and others, 2000) as well as 

extensive interaction between groundwater and 

surface water. 

• Many sites with snowmelt-dominated hydrology 

(such as sites in the mountainous areas of Colorado) 

have high base flow nitrate contributions. Nitrate 

concentrations at these sites are typically low, but 

concentrations during base flow periods were routinely 

greater than those during snowmelt runoff.

Figure 5-1. Sixty-six percent 

of the streams evaluated had 

more than 37 percent of their 

total nitrate load contributed 

by base flow. The percentage 

of total nitrate load contributed 

to streams by base flow 

varies greatly depending on 

the sources of the nitrate and 

factors that affect its transport 

in both groundwater and 

streams.
Chapter 5

Exchange of Nutrients between Surface Water and Groundwater 87

Clear Creek near Lancing, Tennessee

5/4/02 5/24/02 6/13/02

0

1,000

2,000

3,000

4/20/02

Streamflow, in cubic feet per second

Quick flow

Base flow

Calculation of the Contribution of Base Flow to the 

Nitrate Load in Streams

Estimation of the portion of a stream’s nitrate load contributed by base flow requires several steps: (1) estimation 

of the streamflow contributed by base flow, (2) identification of stream samples whose chemistry reflects base 

flow, and (3) use of the flow and chemical data to calculate annual nitrate loads that are due to base flow. The 

base flow proportion of streamflow at a subset of NAWQA surface-water sites was estimated using a hydrograph 

separation method presented by Wahl and Wahl (1988), which is in turn based on methodology developed by the 

Institute of Hydrology (1980). For example, the blue area in the hydrograph for Clear Creek, Tennessee, is the portion 

of streamflow attributed to base flow by this method (fig. 5-2). In this case, the base-flow component represents 

groundwater discharge, because the Clear Creek watershed is small and predominately forested, and is not affected 

by point sources, return flows, or flow regulation.

Average annual loads (mass per year) of nitrate were determined for base flow and total flow conditions for 

selected sites sampled during 1990–2006 (Spahr and others, 2010). The period of record for load computations ranged 

from 1 to 16 years, with an average of 6 years. Briefly, the annual hydrographs were separated into base flow and 

total flow proportions, resulting in daily total and base flows. Water samples collected at the sites were classified 

as either base flow or non-base flow samples. A base flow sample was one that was collected on any day when the 

base flow was 77 percent or more of the total flow. Using the base flow samples and the daily base flow values, base 

flow nitrate loads were determined using the statistical model LOADEST (Runkel and others, 2004). Total flow loads 

were determined on the basis of all nitrate concentration values and the total daily streamflow. The ratio of the base 

flow load to the total flow load represents the portion of nitrate load transported to the stream by base flow. The base 

flow load was calculated for 148 surface-water sites with watersheds less than 500 square miles in area. (See Spahr 

and others, 2010, for more details.) Sites suspected to be influenced by wastewater discharges or impoundments 

were excluded from the analysis.

Figure 5-2. This streamflow hydrograph example shows 

estimated partitioning of total flow into base flow and 

quick flow components for Clear Creek near Lancing, 

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

400

0

200

1994

10

0

5

Nitrate 

concentration

Streamflow

Nitrate concentration, in milligrams per liter as N

Streamflow, in cubic feet per second

Oct Nov Dec Jan Feb Mar Apr May June July Aug Sept

1993

EL68D Wasteway, Washington

0

0

Nitrate in tile drain

Nitrate in Sugar Creek

25

10

15

20

Nitrate concentration,

in milligrams per liter as N

1,600

400

800

1,200

Streamflow, in cubic

feet per second

Sugar Creek at New Palestine, Indiana

Dec Jan Feb Mar Apr May June July Aug Sept Oct Nov Dec

1993 1994 1995 1996

0

5

Jan Feb Mar Apr May June July Aug Sept Oct Nov Dec Jan Feb Mar Apr May June July Aug

Period when tile

drains generally

were not flowing

Period when tile

drains generally

were not flowing

Hydrologic Modifications Create Seasonal 

Patterns in Nutrient Concentrations

The seasonal exchange of water between streams and 

groundwater, and resulting changes in nutrient concentrations, 

can be radically affected by local man-made modifications of the 

hydrologic system. In the Central Columbia Plateau of eastern 

Washington, irrigation wasteways and other surface drains convey 

low-nitrate irrigation water diverted from the Columbia River during 

the growing season (April through October, fig. 5-3). When irrigation 

ceases and discharge of high-nitrate shallow groundwater becomes 

the predominant source of streamflow, nitrate concentrations in 

the stream increase by almost three-fold. The counter-intuitive 

consequence of this pattern of water management is minimum nitrate 

concentrations in the stream during the growing—and fertilizer 

application—season.

In the White River Basin, Indiana, nitrate concentrations of many 

streams are influenced by the effects of agricultural tile drains. 

Nitrate-rich shallow groundwater and water percolating through 

poorly drained soils are intercepted by drains and routed rapidly 

to streams. Elevated nitrate concentrations are observed in Sugar 

Creek when tile drains are flowing (fig. 5-4). When drainage ceases, 

nitrate concentrations decrease to background levels typical of those 

found in deeper aquifers that contribute water to the streams during 

these periods (Fenelon, 1998).

Figure 5-3. Nitrate concentrations in the EL68D 

Wasteway are low during the growing season, and highest 

during the winter and early spring, when the main source 

of streamflow is groundwater discharge with elevated 

concentrations of nitrate (Williamson and others, 1998).

Figure 5-4. In the White River Basin, Indiana, nitrate concentrations of many streams are influenced by agricultural tile drains. Nitraterich shallow groundwater and water percolating through poorly drained soils are intercepted by the drains and routed rapidly to streams. 

Elevated nitrate concentrations are observed in Sugar Creek when tile drains are flowing. When drainage ceases, nitrate concentrations 

return to background levels typical of those found in deeper aquifers that contribute water to the streams during these periods (Fenelon, 1998).
Chapter 5

Exchange of Nutrients between Surface Water and Groundwater 89

EXPLANATION

Landscape categories

Impermeable soils and impermeable bedrock

Impermeable soils and permeable bedrock

Permeable soils and impermeable bedrock

Permeable soils and permeable bedrock

Stream sampling site

90th percentile

75th percentile

Median

25th percentile

10th percentile

21 Number of samples

Data outside 90th 

or 10th percentile

Impermeable soils and

impermeable bedrock 

Impermeable soils and

permeable bedrock

Permeable soils and

impermeable bedrock

Permeable soils and

permeable bedrock

0

20

40

60

80

100

Percentage of total nitrate

load contributed by base flow

21 47 50 30

Regional Processes That Affect Base Flow Loads

Climate, as well as physical characteristics including soils and bedrock permeability and land-surface slope, are among the 

many factors that affect the mode of transport of nitrate from sources in a landscape to a stream. The influence of contrasting 

hydrologic landscapes on the sources of nitrate loads in streams was shown by classifying stream sites into one of four landscape 

categories (Wolock and others, 2004):

1. Landscapes with impermeable soils and impermeable bedrock. 

2. Landscapes with impermeable soils and permeable bedrock. 

3. Landscapes with permeable soils and impermeable bedrock. 

4. Landscapes with permeable soils and permeable bedrock. 

This classification of relative bedrock permeability is based on lithologic groups of principal aquifers and bedrock 

permeability classes. Areas with no principal bedrock aquifer were assigned the lowest permeability class (Wolock and others, 

2004). The classification did not consider the shallow aquifers of glacial origin in the Midwest.

Figure 5-5. The proportion of 

the total nitrate load in streams 

attributed to nitrate in base 

flow was lowest in landscapes 

where both soils and bedrock are 

impermeable, and significantly 

higher in areas with more 

permeable soils or bedrock. 

These impermeable areas could 

be most responsive to nutrient 

management practices designed 

to reduce nutrient transport to 

streams by runoff. 
90 Nutrients in the Nation’s Streams and Groundwater, 1992–2004

Overland flow is the dominant source of streamflow in landscapes in which both soils and bedrock are impermeable. 

Photograph by John T. Wilson, U.S. Geological Survey.

The Influence of Groundwater on 

Nitrogen Delivery to Chesapeake Bay

Groundwater discharge to stream channels contributes a significant amount of flow and 

nitrogen to streams in the Chesapeake Bay watershed, and is therefore an important pathway for 

nitrogen to reach the Bay. Groundwater contributes more than one-half (54 percent) of the total 

annual flow of streams in the Chesapeake Bay watershed, with a range in different streams of 16 

to 92 percent. The groundwater nitrate load contributes about one-half (48 percent) of the total 

annual nitrogen load of streams entering the Bay, with a range of 17 to 80 percent in different 

streams (Bachman and others, 1998).

The underlying rock type and physiographic province, described by “hydrogeomorphic 

regions” or HGMRs, influence groundwater contributions to total streamflow (fig. 5-6A). 

Groundwater contributions to total streamflow are highest in the carbonate rocks of the Valley 

and Ridge HGMR, where permeable soils and generally low relief promote infiltration, and a 

high degree of fracturing and solution-enlarged fractures transmit large quantities of water; in 

the Coastal Plain HGMRs, where well-drained soils and flat topography promote infiltration into 

shallow aquifers; and in the Piedmont Crystalline and Blue Ridge Crystalline HGMRs, where 

a permeable mantle of colluvium and regolith generally covers the slopes of hills and acts as 

a reservoir for groundwater (Bachman and others, 1998). Groundwater contributions to total 

streamflow are lowest in the Mesozoic Lowland Siliciclastic HGMR, where poorly drained soils 

and (or) steeper topography favor surface runoff over infiltration, and the bedrock has low 

fracture connectivity. In the four HGMRs with sufficient numbers of sites and data for statistical 

analysis, groundwater contributions to the total nitrate load are high in the Piedmont Crystalline 

HGMR and low in the Appalachian Plateau Siliciclastic HGMR (fig. 5-6B). In general, the highest 

groundwater nitrate yields are in HGMRs with higher amounts of agricultural land use, while the 

lowest groundwater nitrate yields are in HGMRs with highest percentages of forested land use 

(Bachman and others, 1998).

The age of groundwater in shallow aquifers in the Chesapeake Bay watershed ranges from 

modern (less than 1 year) to more than 50 years, with a median age of 10 years (Lindsey and 

others, 2003). If nitrate is assumed to move with groundwater, the age of groundwater can be 

used to estimate the rates of nitrate transport—in this case, nitrate in groundwater may have 

a transport time of years to decades, with a median transport time of about 10 years. The slow 

movement of groundwater relative to that of surface water has a significant impact on the total 

time it takes for nitrogen to move through the watershed and reach the Chesapeake Bay, and 

thereby creates a lag time between the implementation of management actions and the reduction 

of nitrogen loads in streams in the Chesapeake Bay watershed (Phillips and Lindsey, 2003).
Chapter 5

Exchange of Nutrients between Surface Water and Groundwater 91

Fig506chesbaybaseflowsidebarai

78°

42°

40°

76°

38°

80°

VA

WV

MD

PA

NY

DC

DE

Chesapeake Bay

Fall Line

Percentage of groundwater contribution

to total streamflow

Dominant hydrogeomorphic region

100

90

80 (56)

(50)

(15)

(14)

(9)

(58) (12)

(8)

70

60

50

40

30

20

10

0

APS BR PCR

BC A ABC D AB

100

90

80

70

60

50

40

30

20

10

0 B A AB AB

(14)

(6) (11)

APS PCR VRC VRS

B.

A.

Dominant hydrogeomorphic region

VRS VRC ML CPU CPD

C ABC ABC

Percentage of groundwater

nitrate load to total nitrate load

Number of observations

75th percentile

Median

25th percentile

Non-parametric Tukey test (Medians with

the same letter are not significantly different.) 

Data value less than or equal to 1.5 times

the interquartile range outside the quartile 

Outlier data value less than or equal to

3 times and greater than 1.5 times the

interquartile range outside the quartile 

ABC

APC

PCA

CPL

(12)

EXPLANATION

Hydrogeomorphic regions

Appalachian Plateau Carbonate 

Appalachian Plateau Siliciclastic 

Blue Ridge Crystalline

Coastal Plain Dissected Upland 

Coastal Plain Lowland 

Coastal Plain Undissected Upland 

Mesozoic Lowland Siliciclastic

Piedmont Carbonate

Piedmont Crystalline

Valley and Ridge Carbonate 

Valley and Ridge Siliciclastic 

APS

VRC

VRS

BR

ML

PCR

CPD

CPU

nutrient management practices designed to reduce nutrient 

transport to streams by runoff. Conversely, in areas where a 

large proportion of the stream nitrate load is derived from base 

flow, and the base flow is derived primarily from groundwater, 

changes in nutrient management will have less impact on 

stream loads until nitrate concentrations in the groundwater 

reaching the stream decrease. In these areas, slow rates of 

groundwater flow means that reductions in stream nitrate load 

could lag behind implementation of management measures 

for years to decades (see Chapter 8, Changes in Nutrient 

Concentrations: Past and Predicted).

Figure 5-6. Groundwater supplies a significant amount of water and nitrogen to streams in the Chesapeake Bay watershed and is 

therefore an important pathway for nitrogen to reach the Bay. Groundwater contributions to total streamflow are highest in the carbonate 

rocks of the Valley and Ridge hydrogeomorphic region (VRC), where permeable soils and generally low relief promote infiltration and 

solution-enlarged fractures transmit large quantities of water, and are lowest in the Mesozoic Lowland Siliciclastic hydrogeomorphic 

region (ML), where poorly drained soils and (or) steeper topography favors surface runoff over infiltration, and the bedrock has low 

fracture connectivity (A). In the four hydrogeomorphic regions with enough sites for statistical analysis, groundwater contributions to 

the total nitrate load are high in the Piedmont Crystalline hydrogeomorphic region (PCR) and low in the Appalachian Plateau Siliciclastic 

hydrogeomorphic region (APS) (B). (Modified from Bachman and others, 1998.)

A significantly lower proportion of the nitrate load is 

derived from base flow at sites in areas with impermeable 

soils and impermeable bedrock than in the other landscapes, 

although there is a large amount of variability within each 

group (fig. 5-5). Landscapes in which both soils and bedrock 

are impermeable are those in which overland flow is the 

dominant source of flow in streams. Landscapes with these 

characteristics are common in the productive agricultural 

land in the Corn Belt and Great Plains. These areas, as well 

as similar areas where most of the nitrate load in streams 

is not delivered by base flow, could be most responsive to 
92 Nutrients in the Nation’s Streams and Groundwater, 1992–2004

These sites are in agricultural or mixed landuse areas in the Midwest with large nitrogen 

input from fertilizer, and with tile drainage 

systems. In these areas, nitrate concentrations 

are highly variable in groundwater near the 

water table (ranging from less than 0.05 to 

17 mg/L), but consistently less than 0.05 mg/L 

in reduced groundwater from greater than 

25 feet below the water table. The high nitrate 

concentrations in the base flow samples at 

these sites likely represent a blend of shallow 

high-nitrate groundwater and deeper lownitrate groundwater, whereas low median 

concentrations for groundwater reflect the 

greater frequency of samples with low nitrate 

concentrations. Despite the contribution of 

some nitrate from groundwater during base 

flow, at these sites overland flow and tile 

drainage are the predominant sources of 

streamflow and nitrate load. 

Nitrate concentrations were elevated 

in both base flow and shallow groundwater 

in the vicinity of Tulpehocken Creek 

(fig. 5-7). This site is in an agricultural 

area of eastern Pennsylvania with highly 

permeable underlying bedrock (Fischer 

and others, 2004). Manure from livestock 

operations is commonly applied to farm 

fields in this area. Lindsey and others (1998) 

describe the rapid infiltration of nitrate from 

fertilizer and manure to groundwater in other 

agricultural areas of Pennsylvania with similar 

shallow and highly permeable bedrock. The 

correspondence between groundwater and 

base flow nitrate concentrations suggests a 

fairly rapid—and unattenuated—transport of 

nitrate in groundwater to this stream.

Although analysis of national-scale 

data and potential governing processes 

demonstrate some of the relations between 

nitrate concentrations in base flow and 

shallow groundwater, there is a large amount 

of variability among sites. Site-specific study 

is often required to completely understand the 

source of nitrate to a stream during base flow, 

and in particular, the role of groundwater. 

Nitrate Concentration in 

Base Flow and Shallow 

Groundwater

The mean annual nitrate concentrations in 

base flow were compared with the median nitrate 

concentrations in shallow groundwater samples 

in the same setting to illustrate the potential 

effect of the movement of shallow groundwater 

to streams (Spahr and others, 2010). Nitrate 

concentrations in groundwater typically were 

similar to or greater than nitrate concentrations 

in base flow (fig. 5-7). Areas where the contrast 

between elevated concentrations of nitrate in 

groundwater and low concentrations in base 

flow are greatest are commonly underlain by 

both permeable soils and permeable bedrock—

conditions that enhance infiltration of water. 

Most of these areas also have groundwater that is 

predominantly oxic—the chemical environment 

in which nitrate is stable. These characteristics—

high infiltration and oxic conditions—favor 

nitrate transport and persistence in groundwater. 

The lack of correspondingly high concentrations 

in the base flow of the paired surface-water sites 

may have multiple causes. In some settings, 

there has not been sufficient time for highnitrate shallow groundwater to migrate to the 

nearby stream. In these cases, stream nitrate 

concentrations lag behind those in the shallow 

groundwater, and concentrations may increase 

in the future as more high-nitrate groundwater 

reaches the stream. Alternatively, at some sites, 

chemical processes may be acting to rapidly 

remove nitrate as water moves from the aquifer 

into the stream channel. For example, high 

denitrification rates in riparian sediments along 

Fishtrap Creek, Washington, remove nitrate 

from groundwater before it enters the stream 

(Tesoriero and others, 2000) (fig. 5-7). 

Three sites where surface-water 

concentrations of nitrate were elevated, yet 

median groundwater concentrations were low, 

occur in areas with low infiltration due to 

impermeable soils and impermeable bedrock 

(see oval on left side of graph in fig. 5-7). 
Chapter 5

Exchange of Nutrients between Surface Water and Groundwater 93

Impermeable soils and impermeable bedrock

Impermeable soils and permeable bedrock

Permeable soils and impermeable bedrock

Permeable soils and permeable bedrock

EXPLANATION

Landscape categories

Mean annual base flow nitrate concentration, in milligrams per liter as N

Median nitrate concentration for land-use study wells,

in milligrams per liter as N

Fishtrap Creek,

Washington

Tulpehocken Creek,

Pennsylvania

0 3 6 9 12

0

3

6

9

12

15

15

Low infiltration and

reduced groundwater 

conditions

Greater infiltration and 

oxic groundwater conditions

1 to 1 line

Figure 5-7. Nitrate concentrations in groundwater are often greater than 

nitrate concentrations in base flow. Sites with particularly high nitrate 

concentrations in groundwater, but low nitrate concentrations in streams, 

have characteristics—high infiltration and oxic conditions—that favor 

nitrate transport and persistence in groundwater. Shallow groundwater with 

high nitrate concentrations may not have had time to reach these streams 

and (or) chemical processes may be removing nitrate as water moves 

from the aquifer into the stream channel. Conversely, sites where nitrate 

concentrations in groundwater are very low, yet have elevated nitrate 

concentrations in surface water, occur where overland flow is an important 

source of streamflow (areas with low infiltration due to impermeable soils), and 

where there are reduced conditions in groundwater (low dissolved oxygen).
94 Nutrients in the Nation’s Streams and Groundwater, 1992–2004

Denitrification and plant 

uptake in riparian zone Subsurface tile 

drains and ditches

Organic

rich soil 

Water table

Low nitrate Deep groundwater discharge

Low nitrate

Underflow

High nitrate

High nitrate

Nitrate Removal across the 

Groundwater–Surface-Water Interface

The capability of streamside riparian zones and 

streambeds to reduce nitrate concentrations in streams is of 

considerable interest to agricultural land managers. Nitrate can 

be removed from nitrate-rich groundwater as it moves through 

the riparian zone to the stream (fig. 5-8), and nitrate can be 

removed from stream water that flows through sediments in 

the streambed. Sediments in both of these environments can 

contain appreciable amounts of organic carbon and other 

reactants that support bacterial denitrification. In addition, 

the vegetation in riparian buffer zones can take up nitrate, an 

important plant nutrient. These processes have been studied 

in a variety of land-use and hydrologic settings by intensive 

instrumentation of this highly dynamic environment (Puckett, 

2004; Duff and others, 2008; Puckett and others, 2008).

Denitrification and plant uptake in riparian zones can 

remove 100 percent of nitrate in discharging groundwater 

in some environmental settings, but they may be relatively 

ineffective in others (Puckett, 2004). The removal of 

nitrate from groundwater is promoted by a combination of 

Figure 5-8. Nitrate may be removed from groundwater by various means before it discharges to 

streams. It may be lost through denitrification in the aquifer if sufficient organic carbon is present, 

or it may be taken up by plants in the riparian zone. However, nitrate may be delivered directly to 

streams in water intercepted by tile drains and ditches. In aquifers in which denitrification does 

not occur, nitrate-rich water may flow beneath the organic-rich, reducing soils of the riparian zone 

and discharge to streams. This nitrate-rich water may be diluted by nitrate-poor groundwater 

that recharged through the riparian zone, or by deeper, older groundwater that recharged prior to 

widespread use of fertilizers.

hydrogeologic, biological, and biogeochemical processes 

(table 5-1). Fine-grained sediments and low gradients result 

in slow flow rates that allow more time for denitrification 

to take place. If the surficial sediments are primarily silt 

and clay, however, the fine-grained sediments can form a 

confining layer that forces groundwater to flow below the 

biologically active zone and may result in less nitrate removal. 

Coarse‐grained sediments and high groundwater gradients 

may force groundwater to flow through the riparian zone faster 

than the biological processes can remove nitrate. In addition, 

if surface runoff occurs or if shallow groundwater is routed 

through tile drains and ditches, riparian zones can be bypassed 

and nitrate-rich water is discharged directly to streams. 

Riparian zones appear to be most effective in settings 

with thin surficial aquifers, underlain by a shallow confining 

layer, and with organic-rich soils that extend down to 

the confining layer. This combination of factors forces 

groundwater to flow through the biologically reactive portions 

of the aquifer and promotes nitrate removal. For example, 

in the Albemarle-Pamlico Drainage Basin (fig. 5-9), high 

concentrations of nitrate are completely removed from 

groundwater discharging from a surficial aquifer through a 

narrow zone adjacent to and beneath Middle Swamp, North 
Chapter 5

Exchange of Nutrients between Surface Water and Groundwater 95

Carolina (Tesoriero and others, 2005). Where these conditions 

do not exist, nitrate-laden groundwater can pass beneath 

the reactive soil layers of the riparian zone and discharge 

directly into streams; this outcome was seen in some streams 

in the Atlantic Coastal Plain including streams in New Jersey 

(Kauffman and others, 2001), Maryland (Böhlke and Denver, 

1995), and South Carolina (Puckett and Hughes, 2005). Sites 

where high-nitrate water in tile drains and ditches bypasses 

riparian zones include Cow Castle Creek in South Carolina 

(Puckett and Hughes, 2005), Plum Tree Branch in North 

Carolina (Tesoriero and others, 2005), and Leary Weber Ditch 

in Iowa (Baker and others, 2006). 

Table 5-1. Factors that promote transport or removal of nitrate in riparian zones and streambeds. 

Factors and Processes that 

Promote Nitrate Transport

Factors and Processes that 

Promote Nitrate Removal

Factors Hydrogeologic and 

biogeochemical processes Factors Hydrogeologic and biogeochemical processes

Well drained soils

Karst terrain

Rapid recharge to the

water table limits nitrate

removal by plant uptake or

denitrification.

Poorly drained soils

• Hydric soils

• Shallow water table

Hydric soils are saturated

(typically, by a shallow water

table) allowing oxygen to become

depleted and denitrifying bacteria

to convert nitrate to nitrogen gas.

Oxic groundwater

• Low carbon soils

• Absence of reduced

 minerals that support

 denitrification

Denitrification does not

occur and nitrate is

transported with minimal

loss.

Low oxygen groundwater

• Carbon-rich soils, aquifer

 materials, and streambeds

• Presence of reduced

 minerals that support

 denitrification

Carbon and reduced minerals

provide the conditions necessary

for bacteria to deplete dissolved

oxygen. Denitrifying bacteria can

then convert nitrate to nitrogen

gas.

Short groundwater residence times

• Coarse-grained sediment

• High groundwater gradient

• Tile drains and ditches

Groundwater moves

rapidly through aquifers

and streambeds, limiting

contact time with aquifer

materials that support

denitrification, thus

allowing nitrate to persist

and discharge to surface

waters. Subsurface tile

drains and ditches intercept

shallow groundwater

and shunt it to streams

before it can enter the

groundwater system.

Long residence times

• Fine-grained sediment

• Low groundwater gradient

Groundwater moves slowly

through aquifers and streambeds,

increasing contact time with

aquifer materials that support

denitrification. Even with slow

denitrification rates, long residence

times may allow most nitrate to be

removed.

Limited uptake by riparian

vegetation

• Overland flow

• Underflow 

• Tile drains and ditches 

Nitrate-rich surface

water flows directly to

streams, bypassing root

zones and soil horizons

where plant uptake and

denitrification can occur.

Overland flow moves

rapidly over the land

surface. Underflow is

shallow groundwater that

moves beneath the root

zone.

Rapid uptake by riparian

vegetation 

Plant roots take up nitrate before it

enters the groundwater system or

before discharging groundwater

enters a stream.
96 Nutrients in the Nation’s Streams and Groundwater, 1992–2004

Fig_5-09.ai

22

24

20

18

16

14

12

10

8

6

4

2

-2

4.1

EXPLANATION

Sampling point

Nitrate concentration, in milligrams per liter as N

0 500 1,000 METERS

Alluvial

aquifer

Sea level

Peedee Formation

Yorktown aquifer

Surficial aquifer

Confining unit

Water table

<0.1

<0.1

<0.1

Oxygen reducing zone

2.6

0.9

4.1

<0.1

<0.1

<0.1

<0.1 <0.1 <0.1

1.4

12

0.5

12

6.4

5.2

7.7

<0.1

<0.1 <0.1

<0.1

Elevation, in meters above or below sea level

Middle Swamp

3.9 Oxygen

reducing

zone

Nitrate reducing zone

Nitrate reducing

zone

Iron reducing zone

Iron reducing

zone

Carbon

dioxide

reducing

zone

Middle Swamp, North Carolina

Shallow sampling points were installed in streambeds to assess the 

chemistry of groundwater discharging to streams. Photograph by 

U.S. Geological Survey.

Figure 5-9. Nitrate is removed by natural processes from shallow groundwater in the narrow riparian zone and streambed before 

it discharges into Middle Swamp in North Carolina. Zones of oxygen reduction, nitrate reduction, iron reduction, and carbon dioxide 

reduction are shown along with nitrate concentrations at groundwater sampling locations. (Modified from Tesoriero and others, 2005.) 

Nitrate removal also can take place in an area of 

active exchange between groundwater and surface water 

in the streambed known as the hyporheic zone. Numerous 

hydrogeologic and biogeochemical processes can remove 

nitrate from stream water that enters the hyporheic zone, as 

well as from groundwater discharging into the streambed 

(table 5-1). In the Merced River in California (fig. 5-10), 

deep groundwater with nitrate concentrations of about 5 mg/L 

entered the middle of the streambed. This nitrate was then 

reduced by denitrification in the hyporheic zone as well as by 

mixing with lower-nitrate groundwater that flowed in laterally 

(fig. 5-10) and surface water that entered the hyporheic zone 

from the stream (Puckett and others, 2008).

Movement of surface water and (or) groundwater into the 

hyporheic zone is governed by coarse-grained bed materials 

and the difference in hydraulic head (the sum of elevation head 

plus pressure head) between surface water and groundwater. 

For example, if the hydraulic head of groundwater flowing 

into the hyporheic zone is greater than that of surface water, 

then the groundwater will dominate in the zone. The hydraulic 

head differential and streambed material coarseness also 

control the residence time of water in the hyporheic zone, 

which can range from a few hours to several weeks, with 

longer residence times leading to more complete nitrate 

removal. In addition, the capability of hyporheic zones to 

remove nitrate from surface water and groundwater may be 

limited in areas of fine-grained sediments, which limit water 

movement into the streambed (Puckett and others, 2008). 
Chapter 5

Exchange of Nutrients between Surface Water and Groundwater 97

The nitrate load in groundwater discharging to a stream (being measured at left) can make 

a substantial contribution to the total nitrate load in a stream (being measured at right). 

Photograph by Joseph Domagalski, U.S. Geological Survey.

Fig_5-10_Merced.ai

4.9 5.7

EXPLANATION

2.1

Sampling point

Nitrate concentration, 

in milligrams per liter as N

Direction of flow

Merced River

Hyporheic zone

High nitrate groundwater 

Low nitrate groundwater 

18

19

20

21

22

23

24

0 10 20 30 40

Elevation, in meters

0.1 3.8

Horizontal distance, in meters

Merced River, California 

0.3

0.1 0.4

0.2

0.5

0.4

2.1

1.3

1.2 0.2

0.4 1.4

South 

Bank

0.2

0.8

1.4

North 

Bank

<0.06

<0.06

<0.06

<0.06 <0.06 <0.06

<0.06

Figure 5-10. In the Merced River, California, deep groundwater with nitrate concentrations of about 

5 milligrams per liter enters the central portion of the streambed. Nitrate concentrations are then reduced 

as a result of denitrification and dilution by mixing with other groundwater having little or no nitrate, or 

mixing with stream water that has entered the hyporheic zone.

Under the right conditions, therefore, the 

hyporheic zone may serve as a “backup” for 

riparian zones that are inefficient at removing 

nitrate from groundwater. However, in areas 

where the rate of groundwater flow exceeds the 

denitrification rate, nitrate may pass through the 

hyporheic zone and contribute to stream-water 

loads. In a study of five sites across the country, it 

was found that in most low-gradient agricultural 

streams, little surface water entered the hyporheic 

zone except during high-flow periods, and 

thus groundwater discharge dominated in the 

hyporheic zone (Duff and others, 2008; Essaid 

and others, 2008; Puckett and others, 2008). 

These studies also showed that hyporheic 

processes removed from 45 to 75 percent of 

groundwater nitrate before it discharged to 

surface water. Once water was in the streams, 

however, biological processes removed less 

than 5 percent of nitrate, minimally reducing 

downstream loads.
98 Nutrients in the Nation’s Streams and Groundwater, 1992–2004

Groundwater Contribution of 

Phosphorus to Streams

The generally low concentrations of phosphorus in groundwater suggest 

that input of phosphorus to streams from groundwater is modest. A detailed 

analysis of phosphorus transport to five streams in predominantly agricultural 

watersheds showed that groundwater contributions to the instream phosphorus 

concentrations usually were minor in these areas (Tesoriero and others, 

2009). However, geologic sources of phosphorus in an aquifer, coupled with 

chemical conditions favorable to phosphorus transport, can result in high 

groundwater inputs of phosphorus to streams. In the Albemarle‐Pamlico 

Drainage Basin in North Carolina, high phosphorus concentrations in many 

streams result from discharge of groundwater with naturally high phosphorus 

concentrations (Spruill and others, 1998). Documenting this natural source 

of phosphorus has increased the accuracy of Total Maximum Daily Loads 

(TMDLs) calculated for some North Carolina streams. Analyses of samples 

collected from 25 sites during low‐flow conditions in August and September 

1995 indicate that dissolved phosphorus concentrations in groundwater 

were higher than those in streams at most sites throughout the Coastal Plain 

(fig. 5-11). The median dissolved phosphorus concentration in groundwater 

was 0.2 mg/L. Instream concentrations of dissolved phosphorus at 25 sites 

were significantly correlated to the concentrations in groundwater discharging 

to the streams, and stream concentrations decreased with increasing 

streamflow, indicating that groundwater discharge and point sources are 

the primary sources of dissolved phosphorus in the stream. In some areas, 

high phosphorus concentrations in groundwater are associated with swampy 

areas with decomposing organic matter, where groundwater or water in the 

streambed is geochemically reduced. In other areas, the streams have cut 

through deeper geologic formations that contain relatively soluble phosphate 

minerals (Tesoriero and others, 2005). 

Similarly, groundwater contributes phosphorus to the Tualatin River, 

Oregon, during summer low-flow conditions (Kelly and others, 1999). 

In spite of improvements in wastewater treatment and land management 

practices in this watershed, the high concentrations of phosphorus contributed 

from groundwater make it difficult to meet the TMDL. Dissolved phosphorus 

concentrations ranged from 0.1 mg/L in shallow groundwater to a maximum 

of more than 2 mg/L in deep groundwater. Groundwater discharge accounted 

for nearly one-fourth of the total phosphorus input to the river. The primary 

source of high phosphorus in groundwater is the decomposition of organic 

material at the interface between older valley-fill sediments and overlying 

catastrophic flood deposits (fig. 5-12). High phosphorus concentrations are 

contributed to the stream at locations where deep groundwater discharges 

along long flow paths, or where the river or its major tributaries cut through 

this interface between the two major geologic strata. 
Chapter 5

Exchange of Nutrients between Surface Water and Groundwater 99

 Groundwater

 Streams

0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

Phosphorus concentration,

in milligrams per liter

Albemarle-Pamlico Drainage Basins, North Carolina

Site number

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

Median total phosphorus concentrations, in milligrams per liter (mg/L), are in bold text.

Streambed piezometers receiving

local groundwater discharge

0.75 mg/L

Water table

Shallow aquifer sediments

 and flood deposits

Deep aquifer sediments

with woody debris and peat

Streambed piezometers receiving

regional groundwater discharge

1.23 mg/L

Deep

domestic wells

0.34 mg/L

Shallow

domestic wells

0.15 mg/L

Tualatin River, Oregon

Figure 5-11. Dissolved phosphorus concentrations in groundwater were 

higher than those in streams at most sites throughout the Coastal Plain in 

the Albemarle-Pamlico Drainage Basin, North Carolina. Concentrations in 

streams and groundwater discharging to the streams were significantly 

correlated, indicating that the groundwater was a significant source of 

phosphorus to the streams at low flow (Spruill and others, 1998).

Figure 5-12. Groundwater contributes phosphorus to the Tualatin River, Oregon, during summer low-flow conditions (Kelly and 

others, 1999). The primary source of high phosphorus in groundwater is the decomposition of organic material. 
100 Nutrients in the Nation’s Streams and Groundwater, 1992–2004

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Chapter 6

Potential for Effects 

on Human Health 

Introduction

Elevated concentrations of nutrients, 

particularly nitrate, in drinking water 

may have both direct and indirect 

effects on human health. One direct 

effect of ingestion of high levels of 

nitrate is methemoglobinemia, a 

disorder in which the oxygen-carrying 

capacity of the blood is compromised; 

the USEPA Maximum Contaminant 

Level (MCL) of 10 mg/L of nitrate for 

drinking water was adopted to protect 

people, mainly infants, against this 

problem. High nitrate concentrations 

in drinking water also have been 

implicated in other human health 

problems, including specific cancers 

and reproductive problems, but more 

research is needed to corroborate 

these associations (see sidebar, 

Nitrate in Drinking Water: Potential 

Health Effects). The indirect effects 

of nutrient enrichment of surface 

waters on human health are many and 

complex, including algal blooms that 

release toxins and the enhancement 

of populations of disease-transmitting 

insects, such as mosquitoes (Townsend 

and others, 2003).

Photograph by U.S. Geological Survey

6

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

Nitrate in Drinking Water: Potential Health Effects

Mary H. Ward, Division of Cancer Epidemiology and Genetics, National Cancer Institute, National Institutes of Health, 

Department of Health and Human Services, Bethesda, MD

Jean D. Brender, Department of Epidemiology and Biostatistics, Texas A&M Health Science Center, School of Rural 

Public Health, College Station, TX

Adverse health effects from nitrate in drinking water most likely result from complex interactions among 

(1) the amount of nitrate ingested, (2) the dietary intake of other constituents that may enhance or inhibit the 

formation of harmful compounds from ingested nitrate, and (3) medical and (or) genetic conditions that may 

increase susceptibility (Ward and others, 2005). To date, there have been few well-designed epidemiologic 

studies on the role of nitrate ingestion in specific chronic diseases. Thus, the results of epidemiologic 

studies have been varied: although positive associations between nitrate intake and adverse effects have 

sometimes been found, with the exception of methemoglobinemia, it is difficult for scientists to draw 

conclusions about human health risk.

Human biomonitoring studies show that ingestion of nitrate via drinking water contributes to the 

formation of N-nitroso compounds, a class of chemicals that may have ties to cancer, diabetes, and adverse 

reproductive outcomes. Additional studies that accurately assess drinking-water nitrate exposure are 

needed to further our understanding of the relation between drinking-water nitrate and health outcomes. 

This research is particularly relevant for rural users of privately owned wells because these wells are 

unregulated, and NAWQA studies show elevated nitrate concentrations in shallow groundwater in 

agricultural areas where rural users reside.

Methemoglobinemia 

The USEPA MCL for nitrate of 10 mg/L as N for drinking water was promulgated to protect infants from 

developing methemoglobinemia, also known as blue baby syndrome. Ingested nitrate is reduced to nitrite, 

which binds to hemoglobin to form methemoglobin. Too much methemoglobin interferes with the oxygencarrying capacity of the blood, and infants are particularly susceptible to this condition. 

A recent assessment of cases of methemoglobinemia associated with well water in the United States 

found cases occurring at nitrate levels between 20 and 30 mg/L. The concentration of nitrate in drinking 

water that produces elevated methemoglobin can be affected by several factors including other dietary 

sources of nitrate, certain medications, and concurrent exposure to enteric bacteria that cause diarrhea. 

Because fecal bacteria can co-occur with nitrate in domestic wells, a better understanding of the 

comparative roles of these contaminants as risk factors for methemoglobinemia is needed.

Cancer

Ingestion of nitrate via drinking water contributes to the formation of N-nitroso compounds in the 

body—or endogenously—by a process called “endogenous nitrosation.” Most N-nitroso compounds 

are potent animal carcinogens, and several N-nitroso compounds formed endogenously in humans from 

dietary precursors are considered probable human carcinogens. On the basis of human and animal studies, 

the International Agency for Research on Cancer (IARC) concluded that “ingested nitrate or nitrite under 

conditions that result in endogenous formation of N-nitroso compounds is probably carcinogenic 

to humans.” 

Most epidemiologic studies of nitrate in drinking water in relation to cancer have been ecologic in design, 

linking incidence or mortality rates to levels of nitrate in drinking water for large groups of people at the town 
Chapter 6

Potential for Effects on Human Health 103

or county level (Cantor and others, 2006). This study design is mostly useful for generating hypotheses 

about disease risk factors. Studies based on individual exposures are needed to establish cause and 

effect because of the complex and multi-step process by which ingested nitrate forms potentially 

carcinogenic N-nitroso compounds. Study designs that assess individual exposure include case-control 

studies, which compare a group of individuals with a disease or condition with a group who do not have 

the disease or condition, and cohort studies, which track a select group of people for a period of time.

In the past decade, several individual-based studies have evaluated historical nitrate levels in 

public-water supplies (largely below 10 mg/L) and risk of several cancers. A cohort study of older women 

in Iowa found 2.8-fold and 1.8-fold increases in the risk of bladder and ovarian cancers, respectively, 

associated with the highest quartile of nitrate exposure (> 2.46 mg/L). Significant inverse associations 

were observed for uterine and rectal cancer and no significant associations were observed for nonHodgkin lymphoma; melanoma; leukemia; and colon, rectal, pancreatic, kidney, and lung cancer. Dietary 

nitrate intake was not associated with increased risk of any cancer type. A case-control study of nonHodgkin lymphoma in Nebraska showed a significant positive association between the average nitrate 

level in public-water supplies over about 40 years and risk among men and women; however, subsequent 

studies of non-Hodgkin lymphoma have found no association. Other recent studies found no association 

between elevated levels of nitrate in public-drinking-water supplies and stomach cancer or childhood 

brain cancer; however, the children of one group of women who used private wells during pregnancy 

had a significantly increased risk of brain cancer. 

Some studies evaluated factors that enhance nitrosation, such as low vitamin C intake, to identify a 

potentially high risk subgroup. For example, although case-control studies of several types of cancer in 

Iowa found no overall association between cancer risk and average nitrate levels in drinking water over 

almost 30 years, there was a significant increase in the risk of colon cancer among a subgroup with both 

10 or more years of exposure above 5 mg/L of nitrate and factors that increase endogenous nitrosation 

(low vitamin C or high meat intake). Conversely, a case-control study in Nebraska of adult brain cancer 

showed no evidence of increased risk among subgroups with lower vitamin C intake and higher ingestion 

of nitrate in drinking water.

Adverse Reproductive Outcomes

Several studies have examined the association between drinking water that contains elevated 

levels of nitrate and adverse reproductive outcomes, such as spontaneous abortions, stillbirths, 

premature birth, or intrauterine growth retardation. Results of these studies are inconsistent, possibly 

indicating that there is no true effect of nitrate in water on reproductive outcomes at the nitrate levels 

evaluated. Alternatively, the inconsistencies may be due to the different periods over which exposure 

was assessed, different levels of nitrate in water across studies, or differences in exposure to other 

co-factors. 

Results of studies that evaluated nitrate in drinking water and congenital malformations in offspring 

also are mixed. Some studies found positive associations between nitrate in drinking water and 

outcomes, such as neural tube defects. In each of these studies, levels of nitrate in water associated 

with increased risk of these defects were below the MCL. Two such studies also examined dietary 

intake of nitrate and nitrite and neural tube defects, and found minimal or no effect on risk. In another 

study, higher levels of nitrates in food or drinking water during the periconceptional period significantly 

increased the risk of neural tube defects if women were exposed to nitrosatable drugs. A cohort study 

in Sweden noted a weakly positive association between maternal prenatal consumption of water with 

nitrate concentration of 2.0 mg/L (as nitrate) or greater and congenital heart defects in offspring.
104 Nutrients in the Nation’s Streams and Groundwater, 1992–2004

Photograph ©iStockphoto.com/Genesis Photography.

Fig_6-01_swgs_pies.ai

>20

>0–20 

No exceedance

Percentage of samples at each stream site or groundwater

study exceeding the nitrate MCL

EXPLANATION

Agriculture Urban Major aquifers

Agriculture Urban Mixed

Percentage of Streams

Percentage of Groundwater Studies

2.0 percent 3.3 percent

3.0 percent 5.3 percent

11.1 percent 7.3 percent

39.6

percent

43.4

percent

48.5

percent

38.3

percent

17

percent

Streams

Concentrations of nutrients in the 499 streams routinely 

sampled by NAWQA during the period 1992–2001 were seldom 

greater than human-health benchmarks. The USEPA lifetime 

health advisory for ammonia of 30 mg/L was not exceeded in any 

samples, and concentrations of nitrite greater than the Maximum 

Contaminant Level (MCL) of 1 mg/L were rare and limited to 

five sites. The waters at all five sites were heavily impacted by 

wastewater discharges and were not used as drinking-water sources. 

Concentrations of nitrate in streams exceeded the MCL 

much more frequently than did nitrite, but usually not in streams 

that would be used as a drinking-water supply. The nitrate MCL 

was exceeded in about 2 percent of all samples (566 of 27,555) 

distributed among 50 stream sites. Most of the streams with one or 

more samples with concentrations of nitrate above the MCL are in 

agricultural areas (fig. 6-1). Twenty-eight percent of the agricultural 

sites had one or more samples with concentrations above the MCL, 

and 11 percent of the agricultural sites had concentrations above 

the MCL in more than 20 percent of the samples. Only 7 percent 

of urban sites and 5 percent of mixed land use sites had samples 

with nitrate concentrations above the MCL. The nitrate MCL 

was not exceeded in any of the 6,728 samples collected at the 

159 undeveloped stream sites. 

For perspective on the relevance of NAWQA findings 

to surface water used as a source 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 

intakes are in watersheds draining undeveloped land, with the 

remaining (32 and 1 percent) in mixed and urban land 

use, respectively. 

Most streams with nitrate concentrations above the MCL are 

in the upper Midwest Corn Belt (fig. 6-2). Most of these are small, 

agricultural streams (median watershed size about 300 square 

miles), but some larger streams in this region that are used as 

drinking-water supplies are similarly affected (see sidebar, Nitrate 

Concentrations Vary Seasonally and are Moderated by Reservoirs 

in the Midwest). Application rates of commercial fertilizer and (or) 

manure are high in this region. Sites in the Corn Belt also are often 

characterized by dense, clay-rich soils and are artificially drained by 

surface ditches or buried tile drains. The combination of dense soils 

and artificial drainage promotes rapid transport of runoff and nitrate 

from farm fields to streams. Most of the concentrations above the 

nitrate MCL occur during the growing season (May, June, and July) 

at these sites, consistent with the agricultural origin of the nitrate. 

Land-use categories for the 15 sites with one or more exceedances 

that are scattered across the Western United States are more 

diverse: these sites are almost evenly divided between irrigated 

agricultural sites and urban or mixed land-use sites with large inputs 

of wastewater. Only four sites are in the Northeast (all in eastern 

Pennsylvania) and none are in the Southeast. 

Figure 6-1. 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. 
Chapter 6

Potential for Effects on Human Health 105

17-0191_fig 6-05

No exceedance

0–20

20

Percentage of samples at

each stream site 

exceeding the nitrate MCL 

(10 milligrams per liter as N)

Estimated 1997 nitrogen inputs from

fertilizer, manure, and atmosphere,

in pounds per square mile

Agricultural Urban

Major

aquifers

No exceedance

0–20

20

Percentage of samples for

each groundwater study 

exceeding the nitrate MCL

(10 milligrams per liter as N)

Shallow groundwater

studies, by land use

EXPLANATION

2,900

2,900–15,000

15,000

Stream sites, by watershed land use

Data on nitrogen inputs not available 

for Alaska and Hawaii.

Agricultural Undeveloped Mixed Urban Groundwater

Streams

Figure 6-2. Although stream sites with nitrate concentrations above the Maximum Contaminant Level (MCL) for nitrate in drinking 

water of 10 milligrams per liter are broadly distributed, this standard is most often exceeded in agricultural streams draining basins 

with dense clay soils and artificial drainage in the Corn Belt States of the upper Midwest. In contrast, the distribution of nitrate 

concentrations above the MCL in groundwater studies shows no strong spatial pattern. 
106 Nutrients in the Nation’s Streams and Groundwater, 1992–2004

39°

40°

41°

91° 90° 89°

0 10 20 30 40 MILES 50

0 10 20 30 40 KILOMETERS 50

88°

MAP LOCATION

Lower

Illinois

River

Basin

ILLINOIS

Decatur

Lake

Sangchris

Illinois River

Sangamon R.

Springfield Lake

Taylorville

Lake

Lake

Sangamon River 

at Monticello

Sangamon River

near Oakford

Oakford

Decatur

Springfield

EXPLANATION

Stream sampling site

City

Sangamon River at Monticello, Illinois

Sangamon River near Oakford, Illinois

Nitrate concentration, in milligrams per liter as N

Maximum Contaminant Level

20

18

16

14

12

10

8

6

4

2

0

Sept

Oct

Nov

Dec

Jan

Feb

Mar

Apr

May

June

July

Aug

Sept

Oct

Nov

Dec

Jan

Feb

Mar

Apr

May

June

July

Aug

Sept

1996 1997 1998

B.

Nitrate Concentrations Vary A.

Seasonally and Are Moderated 

by Reservoirs in the Midwest

Nitrate concentrations in the Sangamon 

River in Illinois were more variable upstream 

of water-supply reservoirs (at Monticello, 

Illinois) than downstream of the reservoirs 

(near Oakford, Illinois) (fig. 6-3A). During 

April through June of 1997 and 1998, 

concentrations were frequently above the 

MCL of 10 mg/L in the Sangamon River 

at Monticello, 29 miles upstream of Lake 

Decatur—the drinking-water source 

for the city of Decatur (fig. 6-3B). When 

nitrate concentrations exceed the MCL, 

the city of Decatur either treats the stream 

water to remove nitrate or mixes the highnitrate surface water with groundwater to 

dilute the nitrate concentrations. Nitrate 

concentrations also were highest in the 

spring in the Sangamon River near Oakford, 

which is downstream of Lake Decatur and 

other water-supply reservoirs used by the 

city of Springfield. Concentrations in samples 

from the Sangamon River near Oakford, 

however, seldom reached the nitrate MCL. 

From August through November, 

the above pattern was reversed: 

concentrations were low at both sites, but 

the concentrations at the site downstream of 

the reservoirs were greater than those at the 

upstream site. Similar concentrations and 

seasonal patterns occurred in other streams 

in the corn- and soybean-producing areas of 

the Midwest (Scribner and others, 1996).

Figure 6-3. Nitrate concentrations in the 

Sangamon River, Illinois, are affected by reservoirs 

and other factors. Monticello lies upstream of 

several multi-purpose reservoirs and the cities of 

Springfield and Decatur. Oakford lies downstream of 

the reservoirs (A). The reservoirs are used for water 

supply, and during spring, they are replenished by 

river water that exceeds the Maximum Contaminant 

Level for nitrate. Water leaving the reservoirs 

usually does not exceed the nitrate standard and 

has less variability in nitrate concentrations (B).
Chapter 6

Potential for Effects on Human Health 107

Groundwater

Concentrations of nitrate greater than the USEPA MCL 

of 10 mg/L occur more frequently and are more widespread 

in groundwater than in streams. About 9 percent of all wells 

sampled had nitrate concentrations greater than the MCL, 

compared to 2 percent of all stream samples. Similarly, 

57 percent of all agricultural, urban and major aquifer studies 

had one or more samples with a concentration exceeding the 

MCL, compared to 10 percent of all stream sites. In addition, 

a greater proportion of groundwater studies than stream sites 

had concentrations above the MCL in at least 20 percent 

of their samples (fig. 6-1). Concentrations exceeding the 

MCL were much more prevalent in samples from wells in 

agricultural areas (20 percent) than in samples from either 

wells in urban areas or in major aquifer wells (table 6-1). 

The overall rates of exceedance of the MCL in samples from 

wells in urban areas and major aquifers were similar—3 and 

4 percent, respectively.

Samples from about 7 percent of all domestic wells 

sampled had nitrate concentrations above the MCL (table 6-1). 

About 43 million people in the United States obtain water for 

household use from privately owned (domestic) wells (Hutson 

and others, 2004). The relatively high rate of exceedance of 

the MCL for nitrate in shallow domestic wells in agricultural 

areas is a potential human health concern because the water 

from these wells generally is used as household supply 

without treatment. The overall rate of exceedance of the 

MCL in public-supply wells was only 3 percent. The low 

rate of exceedance in the public wells reflects the relatively 

great depth of these wells, and hence, the long travel time 

of the water from the land surface to the well intakes, as 

well as measures taken to avoid groundwater with high 

nitrate concentrations. In contrast to water from domestic 

wells, source water for public-supply wells that exceeds the 

MCL is either treated or blended with water having a lower 

concentration of nitrate to attain the standard prior to being 

distributed to users. Because of the large number of people 

potentially affected, the complexity of the problem, and the 

high cost of treatment, the NAWQA Program is conducting 

detailed studies of public-supply well vulnerability (see 

sidebar, NAWQA Study of Public-Supply Well Vulnerability). 

The broad distribution of elevated concentrations of 

nitrate in groundwater is reflected in the high proportion 

of studies with one or more MCL exceedances: 83 percent 

of the agricultural studies, 52 percent of urban studies, and 

44 percent of major aquifer studies (fig. 6-1). Two subtle 

spatial patterns in the rate of exceedance of the nitrate MCL 

in groundwater studies are noteworthy. First, unlike streams, 

groundwater studies in which concentrations of nitrate exceed 

the MCL are broadly distributed across the United States 

(fig. 6-2); in particular, despite the large nitrogen inputs in the 

upper Midwest Corn Belt, the rates of MCL exceedance in this 

area are no different from the rates in other agricultural areas 

of the country. Second, the generally low rate of exceedance 

of the nitrate MCL in studies across the southeastern 

portion of the country may be attributed to the prevalence 

of aquifers with chemically reduced conditions that promote 

denitrification, and limestone aquifers where rapid flushing 

results in low nitrate concentrations.

The proportions of public-supply and domestic wells 

that had nitrate concentrations above the MCL of 10 mg/L are 

similar to, but slightly larger than, the results in a previous 

national study. Results from a USEPA survey of 1,300 wells 

sampled during 1985–1990 indicate that 1.2 percent of 

community water system (CWS) wells and 2.4 percent of rural 

domestic wells produced water that had nitrate concentrations 

greater than the MCL (U.S. Environmental Protection Agency, 

1990). CWS wells are those in piped drinking-water systems 

that have at least 15 connections or serve at least 25 permanent 

residents. Rural households were defined as those outside of 

incorporated or unincorporated areas with a population of 

2,500 or more and outside of urban fringe areas as defined by 

the U.S. Census Bureau.

Table 6-1. Samples from about 7 percent of domestic wells and 3 percent of public-supply wells had nitrate concentrations above the 

Maximum Contaminant Level (MCL) of 10 milligrams per liter as nitrogen. The rate of exceedance was highest in shallow groundwater 

under agricultural lands.

Type of groundwater study

Public-supply wells Domestic wells Monitoring wells

(unused) Total

Number 

sampled

Percentage 

exceeding 

MCL

Number 

sampled

Percentage 

exceeding 

MCL

Number 

sampled

Percentage 

exceeding 

MCL

Number 

sampled

Percentage 

exceeding

MCL

Agricultural 1 100 406 22 938 20 1,345 20

Urban 9 0 17 6 781 3 807 3

Major aquifer 374 2 1,965 4 183 8 2,522 4

Total number, or percent

for lumped studies 384 3 2,388 7 1,902 12 4,674 9
108 Nutrients in the Nation’s Streams and Groundwater, 1992–2004

Not to Scale

Water table

Aquifer

Domestic

well Days to years

Centuries to millennia

Public-supply

well

Years to decades

NAWQA Study of Public-Supply Well Vulnerability 

The NAWQA Program has implemented an intensive study focused on understanding the factors that affect the 

vulnerability of public-supply wells to contamination (Eberts and others, 2005). The study findings are intended to help water 

managers, policy makers, drinking-water suppliers, and scientists to:

•	 Better understand how and why contamination of public-supply wells occurs and whether water quality may get 

better or worse over time;

•	 Assess and predict the vulnerability of groundwater and public-supply wells to contamination, even in 

unmonitored areas; 

•	 Choose new areas for water-supply wells and develop and prioritize monitoring programs; and

•	 Evaluate various groundwater pumping, development, and land-management alternatives. 

NAWQA findings through 2009 show that important factors that affect public-supply well vulnerability include mixing 

of water of different ages in wells, “short circuiting,” and geochemical conditions in the aquifer. Because subsurface 

conditions and management practices differ among aquifers and water distribution systems, public-supply wells in different 

areas of the country are not equally vulnerable to contamination, even where contaminant sources are similar.

Water from public-supply wells is a mixture of water of various ages associated with different land uses and potential 

sources of contamination from the land surface (fig. 6-4). Wells that draw a notable amount of older water (greater than 50 

years) may not fully respond to land-surface activities and sources of contamination for decades. In these types of wells, a 

decrease in contaminant concentrations can lag years, decades, and even centuries behind clean up and protection efforts 

due to the mixing of a smaller proportion of young (more contaminated water) with a larger proportion of older water during 

pumping. In contrast, water quality in public-supply wells that produce predominantly young water (less than 50 years old)

can respond relatively quickly (from days to years) to land-use changes near the surface, including source protection efforts.

Under natural conditions, vertical and horizontal movement of groundwater is often slow, and water can take decades 

to millennia to move through an aquifer system. NAWQA findings, however, show that pumping at public-supply wells in 

the vicinity of “short circuits” commonly allows water and associated contaminants to move more quickly to the wells than 

expected. Short circuits can include natural features, such as fractures in glacial till, solution channels in carbonate rock, 

and sinkholes, as well as human-related factors, such as abandoned or poorly constructed wells.

Geochemical processes within the aquifer can affect contaminant transport and fate. Geochemical processes and 

conditions can result in contaminants being degraded, adsorbed, volatilized, dispersed, and (or) mobilized as they move in 

the aquifer. This study has developed a tool to help understand the geochemical conditions in the aquifer and contributing 

areas to wells, which is critical to determining the transport of contaminants to a public-supply well.

Figure 6-4. Even in urban areas, 

groundwater reaching public-supply 

wells is a mixture of water recharged 

under a variety of land uses, and is a 

mixture of ages (blue lines on diagram).
Chapter 6

Potential for Effects on Human Health 109

Samples of groundwater were collected from privately owned (domestic) wells; 

about 43 million people in the United States obtain water for household use from 

domestic wells. Photograph by U.S. Geological Survey.

0 2 4 6 8 10

≥300

200 to <300

100 to <200

<100

Percentage of wells exceeding nitrate MCL (10 milligrams per liter as N)

Depth below water table, in feet

The processes that influence the frequency of 

exceedance of the nitrate MCL in groundwater studies 

have already been discussed with respect to nitrate 

concentrations in general. Nitrate concentrations 

greater than the MCL are favored by oxic geochemical 

conditions, and exceedances are rare in chemically 

reduced groundwater: only 2 percent of samples from 

wells with reduced groundwater had concentrations 

above the MCL, whereas 12 percent of samples from 

wells with oxic groundwater had concentrations 

above the MCL. Exceedances are similarly infrequent 

in old groundwater (recharged prior to 1952): only 

about 1 percent of samples from wells producing old 

groundwater had concentrations of nitrate above the 

MCL, whereas about 12 percent of samples from wells 

with young groundwater had concentrations above 

the MCL. The much higher rate of exceedance in 

agricultural areas compared to that in urban areas is 

due in part to the higher input of nitrogen to the 

land surface.

The high rate of exceedance of the nitrate 

MCL in shallow wells in agricultural areas is cause 

for concern even though the aquifer represented by 

samples from these wells is not often directly used as a 

drinking‐water source. By design, agricultural land‐use 

study wells sample the shallowest, most recently 

recharged groundwater at each locale. As such, it is 

the first water affected by downward movement of 

contaminants from the land surface. In drinking‐water 

wells (domestic and public-supply wells), the rate 

of exceedance of the nitrate MCL decreases with 

increasing depth of the well below the water table 

(fig. 6-5). This distribution reflects the protective 

effect of the relatively long time it takes for surface 

contamination to reach the intakes of drinking‐water 

wells. The long travel time also represents an increased 

opportunity for removal of nitrate by denitrification 

or dilution by mixing. In many areas, the shallow 

groundwater and the deep aquifer zones pumped 

for domestic and public supply are part of a single, 

continuous hydrologic flow system. In these areas, 

downward movement of groundwater containing 

nitrate at concentrations exceeding the MCL could 

result in deterioration of drinking-water supplies in the 

future. Reliable prediction of this potential requires 

specific hydrologic and geochemical information 

unique to each locale. Although this level of detail for 

the entire country is beyond the scope of the NAWQA 

Program, some case studies examining the migration of 

nitrate-contaminated groundwater to supply wells have 

been done, and are described in Chapter 8, Changes in 

Nutrient Concentrations: Past and Predicted.

Figure 6-5. In drinking-water wells, the rate of exceedance of the 

U.S. Environmental Protection Agency Maximum Contaminant Level 

(MCL) for nitrate of 10 milligrams per liter decreases with increasing 

depth of the well below the water table. Downward movement of 

shallow groundwater with nitrate concentrations exceeding the MCL 

could potentially result in deterioration of drinking-water supplies in 

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

Fig_6-06_GWAVA-DW_model_prediction.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 Extrapolation of Nitrate 

Concentrations in Drinking 

Water to Unsampled Areas

As part of NAWQA’s integrated approach 

to monitoring and prediction, a statistical model 

was developed to assess the vulnerability of 

groundwater to nitrate contamination at the 

national scale (Nolan and Hitt, 2006). The model 

developed for deep groundwater (median depth 

of about 50 meters or 164 feet) that is used as 

drinking water predicts moderate to severe nitrate 

contamination in the High Plains, the northern 

Midwest, and other areas of intensive agriculture 

in both the East (eastern Pennsylvania and the 

Delmarva Peninsula) and the West (the Columbia 

Plateau in Washington, the San Joaquin Valley in 

California, and the Snake River Plain in Idaho) 

(fig. 6-6). The model predicts the highest nitrate 

concentrations in areas with large nitrogen 

sources, factors that promote rapid transport 

of nitrogen in groundwater, and a lack of 

attenuation processes. 

The model for nitrate in deep groundwater 

that is used as a drinking-water supply explained 

about 77 percent of the variation in the nitrate 

concentrations. Areas that are predicted to have 

nitrate concentrations greater than 10 mg/L (red 

in fig. 6-6) are less extensive than predicted 

by a similar model for shallow groundwater 

(see Chapter 4, Occurrence and Distribution of 

Nutrients in Streams and Groundwater). This 

is because the drinking-water wells are about 

130 feet deeper on average than the shallow 

wells, and nitrate concentration generally 

decreases with increasing well depth (fig. 6-5). 

Greater depth generally represents greater travel 

time between recharge point and well intake, 

and greater travel time favors nitrate attenuation. 

Greater depth also increases the likelihood of the 

existence of intervening, less permeable layers 

that restrict the downward migration of water and 

associated dissolved nitrate. In addition, deeper 

groundwater is older and may predate recent 

(1950s to present) increases in nitrogen sources, 

especially fertilizer, and thus in some cases may 

have low nitrate concentrations.

Figure 6-6. Results of model simulations suggest moderate (shown in yellow 

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

deep groundwater (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.

The model for deep groundwater was used as a basis for estimating 

the number of users of private wells in contaminated areas. The 

resulting population counts indicated that almost 15 million people 

live in areas with predicted nitrate concentrations greater than 1 mg/L

in water from wells that are 164 feet deep. Among these potential 

users, 1,240,000 people live in areas predicted to have moderate nitrate 

contamination of groundwater (>5 to ≤10 mg/L nitrate) (table 6-2). 

More users may be affected in the future, however, because shallow 

groundwater with high nitrate concentrations can migrate to greater 

depths within the aquifer. Because well depth is a proxy for travel time 

in the aquifer, conditions at shallow depths (33 feet) in essence constitute 

an early-warning system. Population counts corresponding to predicted 

nitrate concentrations greater than 1 mg/L are 3–14 percent greater at a 

well depth of 33 feet than at 164 feet (table 6-2). 
Chapter 6

Potential for Effects on Human Health 111

Areas where the model for deep groundwater predicts nitrate concentrations 10 milligrams per liter as N

Areas where the model for streams predicts total nitrogen concentrations 10 milligrams per liter as N

Areas where the model for deep groundwater predicts nitrate concentrations 10 milligrams per liter as N 

and the model for streams predicts total nitrogen concentrations 10 milligrams per liter as N

EXPLANATION

Streams and Groundwater

California Nebraska

Comparison of the maps of predicted total 

nitrogen concentrations in streams (see fig. 4-12) 

and nitrate in groundwater (fig. 6-6) shows a striking 

contrast in the distribution of areas predicted to 

have concentrations greater than 10 mg/L (fig. 6-7). 

Overlaying simulation results for the two models 

show that there are only small areas of overlap 

(red, in fig. 6-7) between areas predicted to have 

concentrations greater than 10 mg/L of total 

nitrogen in streams and areas predicted to have 

concentrations greater than 10 mg/L of nitrate in 

groundwater. The contrast in the maps reflects 

the contrast in physical and chemical aspects of 

the landscape that facilitate nitrogen transport to 

streams or to groundwater. For example, factors that 

promote rapid infiltration of rainfall into the ground, 

such as permeable soils, enhance the likelihood for 

contamination of groundwater; factors that inhibit 

infiltration and promote runoff, such as impermeable 

soils and artificial drainage, favor contamination 

of streams. 

Table 6-2. The model for deep groundwater was used to predict the 

number of people using private wells in areas with different nitrate 

concentrations for two well-depth scenarios. Conditions at shallow depths 

(33 feet) constitute an early-warning of changes that may occur as shallow 

groundwater with high nitrate concentrations migrates to greater depths in 

the aquifer. 

[mg/L, milligrams per liter]

Predicted nitrate 

concentration range

Population counts for 

simulation depth Percentage

change 164 feet 33 feet

 No data 1,710,000 1,710,000 Not applicable

Background:

 0 to ≤1 mg/L 20,000,000 19,400,000 –3.1

Elevated:

 >1 to ≤5 mg/L 13,000,000 13,300,000 +3.1

Moderate:

 >5 to ≤10 mg/L 1,240,000 1,400,000 +13.7

Severe:

 >10 mg/L 467,000 528,000 +13.0

Figure 6-7. The contrast in the areas predicted by the streams model (green) and by the groundwater model 

(brown) to have total nitrogen (in streams) or nitrate (in groundwater) concentrations greater than 10 milligrams 

per liter reflects the contrast in physical and chemical aspects of the landscape that facilitate nitrogen 

transport to streams or to groundwater. 
112 Nutrients in the Nation’s Streams and Groundwater, 1992–2004

0

2

4

6

8

10

12

≤1 1–10 10

Frequency of exceedance by an organic

contaminant, in percent 

Nitrate concentration range, in milligrams per liter as N

Percentage of wells at concentrations 

greater than one-tenth of HBSL or MCL 

Percentage of wells at concentrations

greater than HBSL or MCL 

Co-Occurrence of Organic Contaminants and Nitrate at 

Concentrations of Concern for Human Health 

Leslie DeSimone, U.S. Geological Survey

Data for 2,167 domestic wells sampled in major aquifers across the United States (DeSimone, 

2009) show that the frequency of encountering an organic contaminant—that is, a pesticide 

or volatile organic compound—at a concentration of greater than one-tenth of an MCL (U.S. 

Environmental Protection Agency, 2006) or USGS Health-Based Screening Level (HBSL) 

(Toccalino and Norman, 2006; Toccalino and others, 2006) increases as nitrate concentrations 

increase (fig. 6-8). The likelihood of encountering a concentration of an organic contaminant 

that exceeds one-tenth of an MCL or HBSL is about five times greater in samples with nitrate 

concentrations above the background level of 1 mg/L than in samples with nitrate concentrations 

below background levels (see Chapter 4, Background Conditions). Similarly, the frequency 

of occurrence of an organic contaminant at a concentration above an MCL or HBSL is about 

four times greater in groundwater with nitrate concentrations above background levels than 

in groundwater with nitrate concentrations below background levels (see red bars, fig. 6-8). 

Overall, 84 percent of samples with concentrations of organic compounds greater than MCLs or 

HBSLs also had nitrate concentrations above background levels. These findings indicate that 

when concentrations of nitrate are above background levels in a domestic drinking-water supply, 

additional analysis for organic contaminants should be considered. 

Figure 6-8. The likelihood of detecting an organic contaminant at a 

concentration of concern for human health is greater in samples with 

nitrate concentrations above the background level of 1 mg/L than in 

samples with nitrate concentrations below background levels. (HBSL, 

Health-Based Screening Level; MCL, Maximum Contaminant Level.)
Chapter 7

Potential for Effects 

on Aquatic Life

Introduction

Excessive nutrients and resulting 

instream plant biomass can have a 

wide range of impacts on aquatic 

ecosystems. There is increasing interest 

in establishing numeric nutrient criteria 

at scales that reflect the geographic 

variability in the natural factors 

affecting instream nutrient conditions. 

The responses of aquatic biota to 

nutrient enrichment, and the potential 

use of biological metrics as measures 

of water-quality status and biological 

condition, also are areas of active 

study. This chapter examines the status 

of streams with respect to ammonia 

criteria, geographic variability of 

background nutrient concentrations, 

and the response of aquatic biota to 

varying nutrient levels. 

Photograph by William S. Keller, U.S. National Park Service.

Chapter

7
114 Nutrients in the Nation’s Streams and Groundwater, 1992–2004

Potential Effects of Ammonia on 

Aquatic Life

Ammonia (NH3) and its ionized form, ammonium 

(NH4

+), occur naturally and are formed during decomposition 

of proteins, manure wastes, and other nitrogen-containing 

compounds. Ammonia is an excellent source of nitrogen 

for plants and is part of most natural and synthetic fertilizer 

formulations. Ammonia is soluble in water, and water 

contaminated with fertilizer runoff, animal wastes, or 

sewage may contain elevated concentrations of ammonia. 

In well‐oxygenated streams, microorganisms may convert 

ammonia to nitrite and nitrate through a process known 

as nitrification. 

The un-ionized form of ammonia (NH3) can be toxic 

to aquatic plants, invertebrates, and fish at concentrations 

less than 1 mg/L (U.S. Environmental Protection Agency, 

1999). Concentrations of ammonia in NAWQA samples were 

compared with aquatic health criteria for both acute and chronic 

effects using guidelines established by the USEPA for protection 

of aquatic life (U.S. Environmental Protection Agency, 1999). 

More stringent ambient water-quality criteria have recently been 

proposed which would be more protective of sensitive species 

(U.S. Environmental Protection Agency, 2009a) (see sidebar, 

Calculation of Potential Ammonia Toxicity). 

Calculation of Potential Ammonia Toxicity 

The toxicity of ammonia in water is dependent on the proportion of ammonia present in the un-ionized 

form. Because the distribution of ammonia between the ionized and un-ionized forms is both pH and 

temperature dependent, the pH and temperature of water samples are required to determine the toxicity 

of a specific stream environment to organisms. Toxicity increases as pH increases, and in general, 

as temperature decreases. Plants are more tolerant of ammonia than animals, and invertebrates are 

generally more tolerant than fish.

Concentrations in samples from streams were compared with criteria established to prevent both 

acute and chronic health effects caused by exposure of aquatic organisms to ammonia in water. Acute 

health effects are those caused by sudden and severe exposure to a toxicant, whereas chronic health 

effects are those that result from prolonged or repeated exposure over days, months, or years. The acute 

criteria for ammonia in water range from 1.3 to 49 mg/L of total ammonia for pH values of 6.5–9.0 and 

water temperatures of 0–30°C. The chronic criteria range from 0.18 to 6.7 mg/L of total ammonia for the 

same pH and water temperature ranges. Effects of chronic exposure generally are evaluated over a 4- to 

30-day averaging period (U.S. Environmental Protection Agency, 1999).

The USEPA has proposed new ambient criteria for ammonia that consider toxicological data 

published since development of the current criteria (beginning in 1985) (U.S. Environmental Protection 

Agency, 2009a). Although the influence of pH and temperature on ammonia in the proposed criteria is the 

same as in the past, the new criteria take into account the presence or absence of freshwater mussels. 

Freshwater mussels are particularly sensitive to ammonia, and many species are endangered or in 

decline. For example, laboratory tests show that ammonia concentrations as low as 0.03 mg/L can affect 

growth rates of freshwater mussels, and at 0.12 mg/L or greater can be lethal to mussels (Newton and 

Bartsch, 2007). Adoption of these more stringent criteria may result in more streams being listed as out of 

compliance with respect to ammonia.
Chapter 7

Potential for Effects on Aquatic Life 115

Fig_7-01.ai

Agricultural

Urban

Mixed

Watershed land use

Acute

criteria

Chronic

criteria

Percentage of sites where ammonia 

criteria were exceeded

0 5 10 15

Wastewater-treatment plant effluent provides much of the streamflow in the Santa Cruz River at Tubac, Arizona, during 

part of the year. Photograph by Gail E. Cordy, U.S. Geological Survey.

Figure 7-1. 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 

large areas of urban or mixed land and are 

potentially influenced by treated effluent from 

wastewater-treatment facilities located upstream.

The concentrations of ammonia at NAWQA stream sites seldom 

exceeded levels believed to adversely affect aquatic life. Ammonia 

concentrations in more than 24,000 samples from 499 sites were compared 

with acute and chronic criteria on the basis of pH and temperature of 

the samples. The acute criteria for ammonia were rarely exceeded; only 

33 samples (0.14 percent) at 7 sites (1.4 percent) had concentrations greater 

than the criteria. Samples that exceeded the acute criteria were split among 

urban (2 sites or 3.6 percent of urban sites), mixed (4 sites or 2.7 percent 

of mixed sites), and agricultural (1 site or 0.7 percent of agricultural sites) 

land-use settings (fig. 7-1). 

The chronic criteria for ammonia were exceeded in 139 samples 

(0.57 percent) at 22 sites (4.4 percent). Sites at which concentrations of 

ammonia exceeded chronic criteria also were primarily in urban (8 sites 

or 14 percent of urban sites) and mixed (9 sites or 6 percent of mixed 

sites) land use areas; only 10 samples at 5 agricultural sites (3.7 percent of 

agricultural sites) exceeded the criteria (fig. 7-1). Ninety-one exceedances 

(65 percent) of the chronic criteria occurred at only 4 sites (2 urban and 2 

mixed land use) in the semiarid West. 
116 Nutrients in the Nation’s Streams and Groundwater, 1992–2004

2,900

2,900–15,000

15,000

Stream sites, by watershed land use

Data on nitrogen inputs not available 

for Alaska and Hawaii.

EXPLANATION

Agricultural Urban Undeveloped Mixed

Streams

No exceedance

Chronic criteria exceeded

Acute criteria exceeded

Type of aquatic health

criteria exceeded for

ammonia

Estimated 1997 nitrogen inputs from

fertilizer, manure, and atmosphere,

in pounds per square mile

Figure 7-2. Ammonia at concentrations greater than the aquatic life criteria were infrequent, 

and most such sites were in the Western States. These sites generally are in areas of urban 

and mixed land use and are either known or suspected to be affected by the discharge of 

wastewater-treatment plants.

Although concentrations greater than ammonia criteria 

were infrequent, the sites where criteria were exceeded 

were predominantly in the Western States (shown by the 

red and yellow symbols in fig. 7-2). With few exceptions, 

sites with exceedances generally were in urban and mixed 

land-use areas. In many cases, treated effluent from 

wastewater‐treatment facilities was known or suspected to be 

the source of ammonia. NAWQA sampling sites were selected 

to represent regional-scale, ambient water‐quality conditions, 

and it must be reiterated that when feasible, sites were selected 

to avoid the effects of major wastewater-treatment plants and 

other point sources. Thus, these findings generally are not 

representative of overall stream water-quality conditions where 

treated effluent is a significant component of streamflow. 

Advances in the treatment of wastewater have resulted in 

decreases in concentrations of ammonia in some streams 

that receive treated effluent (see Chapter 8 sidebar, Past 

Changes Resulting from Nutrient Control Measures). 

Despite the large inputs of fertilizer and manure, 

sampling at 135 agricultural sites found only rare 

exceedances of ammonia criteria. This indicates that 

ammonia from nonpoint sources is not reaching or persisting 

in streams at high concentrations. Rather, ammonia in 

agricultural watersheds is either being sorbed onto soils, 

volatilized, converted to nitrate through the process of 

nitrification, and (or) rapidly removed from the water 

column by aquatic plants.
Chapter 7

Potential for Effects on Aquatic Life 117

Lowess smooth

South Platte River at 

Henderson, Colorado

Platte River at 

Louisville, Nebraska

Streamflow, in cubic feet per second

100 1,000 10,000 100,000

Concentration of total ammonia, in milligrams per liter as N10

1

0.1

0.01

110° 100°

42°

39°

0 50 100 MILES

0 40 80 KILOMETERS

South Platte 

River at 

Henderson

Platte River at 

Louisville

Denver

NEBRASKA

WYOMING

S. Platte R.

COLORADO

KANSAS

SOUTH DAKOTA

Platte River EXPLANATION

Major land use

Stream sampling site

Basin boundary

Agricultural

Urban

Forest

Rangeland

Other undeveloped

Water

MAP LOCATION

Changes in Ammonia Sources and 

Streamflow in Western Basins 

The relation between streamflow and ammonia concentration may reflect whether the ammonia is from a point 

source or a nonpoint source. Most exceedances of ammonia criteria occurred in western streams in semiarid settings 

where treated wastewater effluent is likely present. The South Platte River Basin in Colorado and Nebraska is typical of 

many semiarid western basins with large urban centers. In these basins, treated wastewater effluent is often a major 

component of streamflow. Concentrations of ammonia in the South Platte River at Henderson, just downstream of the 

Denver urban center, are an order of magnitude greater than those in the Platte River at Louisville, Nebraska, hundreds of 

miles downstream. This contrast primarily is a result of the source of ammonia and lack of dilution near Denver. Most of 

the watershed above the Henderson, Colorado, site is undeveloped, and because of the semiarid climate and upstream 

diversions, the treated wastewater from the Denver metropolitan area contributes about 69 percent of the streamflow in 

the South Platte River at this site (Dennehy and others, 1995). Consequently, when streamflow is low in the South Platte 

River at Henderson, the relative contribution from treated wastewater is high, so that concentrations of ammonia in the 

river are high (fig. 7-3). When snowmelt runoff from the undeveloped forests at higher elevation in the watershed reaches 

the streams, flow in the South Platte River at Henderson increases, the relative contributions from treated wastewater 

decrease, and concentrations of ammonia decrease. In contrast, the watershed of the Platte River at Louisville, Nebraska, 

is more than 10 times larger than the watershed of the South Platte River at Henderson, and about 83 percent of the Platte 

River watershed is agricultural land and rangeland. As flow in the Platte River at Louisville increases, concentrations 

of ammonia also increase (fig. 7-3). Nonpoint sources, primarily fertilizer and manure, are responsible for most of the 

nitrogen introduced to the watershed upstream of the Platte River at Louisville (Mueller and Spahr, 2005). The increase in 

ammonia concentrations with flow at the Platte River at Louisville is indicative of a watershed where nutrients primarily 

are from nonpoint sources and are transported to streams primarily as runoff after their application to the land surface.

Figure 7-3. The relation between streamflow and ammonia concentration at a site reflects whether the ammonia in a stream 

is from a point source or a nonpoint source. Discharges of treated wastewater effluent to the South Platte River downstream 

of Denver, Colorado, result in higher concentrations of ammonia when streamflow is low, and lower concentrations when 

streamflow is high. The relation between ammonia and streamflow at the Platte River at Louisville, Nebraska, is reversed: 

ammonia concentrations increase with increasing streamflow, indicating that the ammonia is primarily from nonpoint sources.
118 Nutrients in the Nation’s Streams and Groundwater, 1992–2004

1

The value of 0.125 mg/L for total phosphorus in region 10 is discounted as unreliable by the USEPA.

Background site data

 (75th percentile)

Modeled background

USEPA recommended 

nutrient criteria

National background

Background site data

 (75th percentile)

Modeled background

USEPA recommended 

nutrient criteria

National background

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.00

USEPA nutrient ecoregion

Total phosphorus concentration, in milligrams per liter

Total nitrogen Total phosphorus

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

Background site data shown for ecoregions with at least 10 sites. Criteria for USEPA nutrient ecoregion 13 are under development.

0.0

0.5

1.0

1.5

2.0

2.5

USEPA nutrient ecoregion

Total nitrogen concentration, in milligrams per liter

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

Both the modeled background concentrations and 

the USEPA recommended nutrient criteria have similar 

patterns of variation among the nutrient ecoregions (fig. 7-4). 

Modeled background concentrations represent a condition 

of no human development and generally are less than the 

recommended criteria, which represent minimal human impact. 

However, the recommended criteria are less than modeled 

background concentrations in several regions, particularly 

for total phosphorus. Most of the recommended criteria also 

are somewhat less than regional background concentrations 

estimated by the 75th percentiles of flow-weighted means 

for minimally impacted sampling sites. This comparison 

suggests that it may be difficult to meet the USEPA criteria for 

phosphorus in streams in developed basins in some nutrient 

ecoregions, in particular for total phosphorus in streams in 

basins that have even a modest degree of development, such as 

those in the Great Plains Grass and Shrublands ecoregion (4). 

Other approaches to estimating concentrations indicative 

of reference conditions are found in the literature, and research 

is continuing in this area. Herlihy and Sifneos (2008) used data 

from sites sampled as part of the USEPA’s Wadeable Streams 

Assessment (U.S. Environmental Protection Agency, 2006) 

to evaluate different approaches to setting regional nutrient 

criteria. Although the values for total nitrogen were relatively 

consistent with the USEPA recommended criteria, values for 

total phosphorus were always greater than the recommended 

criteria, and were greater by a factor of 3.8 for ecoregion 4.

Regional Nutrient Criteria and 

Background Estimates

Concentrations of nutrients in minimally impacted 

streams can be affected by geographic variability of natural 

conditions, such as geology and climate. Recognizing this 

variability, the USEPA has developed recommended criteria 

for total nitrogen and total phosphorus for the protection 

of aquatic life for 14 nutrient ecoregions (see sidebar, The 

U.S. Environmental Protection Agency’s National Nutrient 

Criteria Program). These criteria are based on the 25th 

percentile of data available at the time of criteria development 

in each ecoregion, and are meant to approximate reference 

conditions. Current criteria for the 14 ecoregions range 

from 0.12 to 2.18 mg/L for total nitrogen and from 0.010 to 

0.0761 mg/L for total phosphorus; the criteria also include 

recommendations for turbidity and chlorophyll a (U.S. 

Environmental Protection Agency, 2002b).

Smith and others (2003) used the data from Clark 

and others (2000) to calibrate a simulation model and 

estimated background concentrations of total nitrogen and 

total phosphorus for all stream basins in the conterminous 

United States. From these estimates, they computed the 

average background concentration for each of the 14 USEPA 

nutrient ecoregions. These modeled values are compared with 

the USEPA recommended criteria in figure 7-4. The 75th 

percentiles of flow-weighted mean concentrations also are 

shown for the four ecoregions that have at least 10 minimally 

impacted sites in the data set used for this report.

Figure 7-4. 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. (See fig. 7-7 for list of ecoregions.)
Chapter 7

Potential for Effects on Aquatic Life 119

0.001

0.01

0.1

1

10

0.001

0.01

0.1

1

10

USEPA nutrient ecoregion

USEPA nutrient ecoregion

Total phosphorus concentration, in milligrams per liter Total phosphorus concentration, in milligrams per liter

Agricultural sites

Urban sites

Total phosphorus

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

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

7 16 48

12

14 9

12

8

8

10

14

0.1

1

10

0.1

1

10

USEPA nutrient ecoregion

Total nitrogen concentration, in milligrams per liter

Agricultural sites

USEPA nutrient ecoregion

Total nitrogen concentration, in milligrams per liter

Urban sites

A.

B.

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

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

Total nitrogen

7

16

49

12 16 9

13

9

8

10

14

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

Criteria for USEPA nutrient ecoregion 13 are under development.

Concentrations of both total nitrogen and total 

phosphorus measured in agricultural and urban streams 

typically are much higher than the recommended ecoregional 

criteria, as well as the modeled estimates for background 

concentrations (fig. 7-5A and 7-5B). For example, the 

median concentration for sites within a specific nutrient 

ecoregion ranges from about 2 to more than 10 times the 

respective ecoregional criterion for both total nitrogen and 

total phosphorus. The magnitudes of the differences between 

concentrations measured in agricultural and urban landscapes 

and recommended criteria show that large reductions in 

concentration would be necessary if recommended criteria for 

these areas are to be met.

Figure 7-5. Concentrations of total nitrogen and total phosphorus measured at both agricultural (A) and urban (B) stream sites were 

from 2 to more than 10 times greater than modeled regional background concentrations and U.S. Environmental Protection Agency 

(USEPA) recommended nutrient criteria. These comparisons provide a perspective on the magnitude of the challenge necessary to 

meet the USEPA recommended nutrient criteria for streams in developed basins. (See fig. 7-7 for list of ecoregions.) 
120 Nutrients in the Nation’s Streams and Groundwater, 1992–2004

The U.S. Environmental Protection Agency’s 

National Nutrient Criteria Program 

“To restore and maintain the chemical, physical and biological integrity of the Nation’s waters” 

–Clean Water Act Section 101(a) 

Nutrients consistently rank as one of the top five causes of water body impairment for waters surveyed 

by States, Tribes, and Territories and reported to USEPA. USEPA’s current figures indicate that for those 

waters assessed, nutrients have a combined direct and indirect effect on the impairment of 51 percent of 

river and stream miles, 52 percent of lake acres, and 58 percent of bay and estuarine square miles (U.S. 

Environmental Protection Agency, 2009b). The influence of nutrients on streams was further documented 

in a recent USEPA study that reported that 30 percent of the streams in the United States contained 

impaired biological communities with nitrogen and phosphorus as the leading causes of impairment (U.S. 

Environmental Protection Agency, 2006).

To address these issues, the USEPA started the National Nutrient Program as an offshoot of the 

1998 Clean Water Action Plan to reduce eutrophication that results from human activity. The USEPA is 

partnering with States, authorized Tribes, and Territories to identify and adopt appropriate numeric nutrient 

criteria into State, Tribal, and Territorial water-quality standards and implement those standards in their 

water-quality programs (fig. 7-6).

USEPA developed recommended criteria for eutrophication causal and response variables for rivers/

streams and lakes/reservoirs. Recommended criteria for rivers/streams were developed for 13 nutrient 

ecoregions1

 (fig. 7-7). USEPA also provided technical guidance for developing nutrient criteria for wetlands 

and for estuarine and coastal marine waters. USEPA provided three options to States and Tribes for 

adopting nutrient criteria in rivers/streams and lakes/reservoirs:

•	 Adopt the ecoregional criteria recommendations that USEPA published under section 304(a) of 

the Clean Water Act; 

•	 Modify the section 304(a) criteria to reflect site-specific conditions; or 

•	 Adopt criteria based on other scientifically defensible methods.

Most States are utilizing the last approach, basing their criteria on the biological responses to nutrients 

in waters of their State. This approach requires extensive field data, and statistical analysis of those 

data. USEPA supports the States by funding data gathering efforts and making national experts available 

to analyze that data, and provides technical assistance as States consider and derive nutrient criteria 

endpoints. USEPA published a status report in 2008 (U.S. Environmental Protection Agency, 2008) on the 

progress States are making in adopting recommended nutrient criteria into Water-Quality Standards. This 

report can be found on the Nitrogen and Phosphorus Pollution Program webpage: http://www.epa.gov/

waterscience/criteria/nutrient/strategy/status.html. In addition to supporting States, Tribes, and Territories 

in developing numeric nutrient criteria, USEPA also is engaged in the following activities:

•	 Working with the State of Florida to develop nutrient and biological criteria for both inland and 

estuarine coastal waters in response to a lawsuit from the Florida Wildlife Federation; 

•	 Developing a document with recommendations for using empirical approaches derived field data to 

establish nutrient criteria; and 

•	 Working with State and national organizations to assess innovative approaches to reducing 

nitrogen and phosphorus pollution.

1

Criteria for USEPA nutrient ecoregion 13 are under development.
Chapter 7

Potential for Effects on Aquatic Life 121

National

criterion

Ecoregional

criterion

Starting point

(304(a) criteria)

Ecoregion

subcategory

criterion

GOAL 

Site-specific

criterion

Relatively costly

Range

of 

possible

values

Resources needed

to develop criteria

(Time, Funds)

Geographic scales of nutrient criteria development

1

2

3

4

5

6

7

8

9

11

13

14

11

10

4

8

2

12

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

EXPLANATION

Figure 7-6. In recognition of both the 

wide range in nutrient concentrations 

across the Nation, as well as the large 

amount of resources needed to develop 

nutrient criteria, the USEPA developed 

recommended criteria for ecoregions 

as a starting point for States, Tribes, and 

Territories to begin developing criteria 

specific to their conditions.

Figure 7-7. The U.S. Environmental Protection Agency has developed recommended 

nutrient criteria for rivers and streams for 14 ecoregions in the conterminous United States 

(U.S. Environmental Protection Agency, 2002b). 
122 Nutrients in the Nation’s Streams and Groundwater, 1992–2004

Comparison of Nutrient Criteria with 

Predicted Total Nitrogen in Streams 

Based on NAWQA data 

A statistical model developed on the basis 

of NAWQA data for total nitrogen in streams 

predicts widespread exceedance of the proposed 

nutrient criteria for total nitrogen concentrations, 

and only a modest decrease in concentration 

in response to reduced fertilizer inputs (Spahr 

and others, 2010) (see Chapter 3, Extrapolation 

to Unsampled Areas, Development of Models 

for Total Nitrogen Concentrations in Streams). 

The model was used to calculate the probability 

that the flow‐weighted average concentration of 

total nitrogen in a given stream reach is greater 

than the recommended criteria for that nutrient 

ecoregion (U.S. Environmental Protection 

Agency, 2000). The model simulations show that 

streams in which total nitrogen concentrations 

have a high probability (75 percent or greater) 

of exceeding the recommended criteria are 

broadly distributed across the Nation (fig. 7-8A), 

and constitute 83 percent of all stream miles 

(fig. 7-8B). This proportion is similar to what 

would be expected considering how the criteria 

were designed (U.S. Environmental Protection 

Agency, 2001, fig. 5). In the Corn Belt and 

Northern Great Plains (nutrient ecoregion 6), the 

recommended nitrogen criterion is likely to be 

exceeded in almost 70 percent of stream miles 

with a probability of at least 90 percent.

The models also can be used to provide 

broad regional estimates of how total nitrogen 

concentrations in streams may change with 

reductions in nonpoint sources of nitrogen. 

For example, when nitrogen inputs from 

fertilizer use were reduced by 25 percent in the 

Corn Belt and Northern Great Plains nutrient 

ecoregion, the number of stream miles with a 

high probability (greater than 90 percent) of 

exceeding the nutrient criterion decreased by 

only about 2.5 percent. Although the reduction 

in fertilizer use resulted in a decrease in the 

estimated nitrogen concentrations, concentrations 

in most streams still greatly exceeded the nutrient 

criterion. Fertilizer inputs are only one potential 

source of nitrogen to streams, and this exercise 

did not consider change in other nitrogen 

sources. Although this scenario does not capture 

the complex response to a change in nutrient 

input in a specific stream, it does illustrate the 

magnitude of the challenge of reducing nutrient 

impact on streams through source reduction 

alone. The results suggest that greater use of land 

management strategies to reduce the transport 

of nutrients to streams may be needed to meet 

recommended criteria for streams draining 

areas with appreciable agricultural and urban 

development. For example, installation of simple 

bioreactors to treat tile drainage has been shown 

to be an effective way to reduce transport of 

nitrogen to streams (Jaynes and others, 2008).
Chapter 7

Potential for Effects on Aquatic Life 123

Streams

Corn Belt and Northern

Great Plains nutrient

ecoregion

A.

50 60 70 80 90

0

20

40

60

80

100

100

Probability of total nitrogen concentration

exceeding the USEPA recommended regional criteria

Percentage of stream miles 

B.

83

75

Probability that predicted average nitrogen concentration exceeds recommended nutrient

criteria for that ecoregion 

EXPLANATION

≤25 25–50 50–75 75–90 90

Figure 7-8. Model simulations show that streams in which total nitrogen 

concentrations have a high probability (75 percent or greater) of exceeding 

the recommended criteria are broadly distributed across the Nation (A), and 

constitute 83 percent of all stream miles (B) (Spahr and others, 2010). 
124 Nutrients in the Nation’s Streams and Groundwater, 1992–2004

Photograph by Carmen Burton, U.S. Geological Survey.

Photograph by Steve Fend, U.S. Geological Survey.

Photograph by Jason May, U.S. Geological Survey.

Some State water-quality programs use assessments of algal, 

macroinvertebrate, or fish communities.

Influence of Elevated Nutrients on 

Aquatic Life

Although nutrients, primarily nitrogen and 

phosphorus, are critical for a healthy ecosystem, 

excessive levels of nutrients in streams can lead to 

eutrophic conditions characterized by increased plant 

growth followed by a decline in oxygen in the water 

when dead plant material (algae or macrophytes) 

decomposes. Excessive plant growth can further affect 

stream health by reducing stream velocity and thereby 

causing greater deposition of fine sediments along the 

stream bottom. Increased deposition of fine sediments 

reduces quality and availability of streambed habitat, 

often resulting in reduction or loss of bottom-dwelling 

organisms. Although nutrient enrichment from elevated 

concentrations of nitrogen and phosphorus is a major 

contributing factor to stream eutrophication, enrichment 

effects can vary from one stream to another as a result of 

differences in patterns of streamflow, amount of riparian 

shading, water temperature, water clarity, and the extent 

of groundwater and surface water exchange.

One of the most common approaches to assessing 

the effects of eutrophication is to use the presence or 

absence of certain plants and animals as indicators of 

stream health. These kinds of approaches are referred 

to as bioassessments, and commonly are used in 

many Federal and State water-quality programs. Most 

of these programs are based on assessments of the 

numbers and kinds of individuals comprising the algal, 

macroinvertebrate, or fish communities. Data on aquatic 

organisms add strength to stream-quality assessments 

because organisms integrate the effects of stream 

conditions over a period of time ranging from days to 

years, whereas a measurement of nutrient concentration 

represents a single point in time. Information about the 

composition of stream communities can be used to assess 

the overall health of a stream ecosystem, based on the 

degree to which biological communities differ from 

a natural state (see sidebar, Assessment of Biological 

Condition). Such differences can indicate whether a 

stream has been influenced by human disturbance or, 

conversely, how much it has recovered or improved 

from some previous condition. Calculation of biological 

condition provides a measure that can be used to compare 

individual sites or groups of stream sites across 

different landscapes.
Chapter 7

Potential for Effects on Aquatic Life 125

Mean value

95 percent confidence interval

Total phosphorus, in milligrams per liter

40

50

60

70

80

90

100

Total nitrogen, in milligrams per liter

Biological condition, in percent

40

50

60

70

80

90

100

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

Macroinvertebrates

Fish

Algae

Macroinvertebrates

Fish

Algae

Assessment of 

Biological Condition 

The NAWQA Program used a consistent 

approach to assess biological condition for 

algal, macroinvertebrate, and fish communities 

at about 1,400 stream sites across the 

Nation. Biological condition was assessed 

by comparing observed (O) community 

characteristics (such as the number of taxa) 

to those expected (E) if the community was 

minimally disturbed by human activities. 

The observed condition (O) is derived from 

a sample collected at a site, whereas the 

expected (E) condition is estimated based 

on data from a set of environmentally similar 

reference sites. Because natural variation in 

environmental settings is accounted for in this 

approach, departures of observed condition 

from expected condition are likely the result 

of human activities. The ratio of observed to 

expected conditions (O/E) can be expressed as 

a percentage of the reference condition scaled 

from 0 percent to 100 percent. The lower the 

percentage, the more altered the biological 

community, whereas a higher percentage 

means that the biological community is more 

similar to reference conditions.

Decreases in Biological Condition Were 

Associated with Nutrient Enrichment 

Biological condition for algal, macroinvertebrate, and fish 

communities declined with increasing concentrations of nitrogen 

and phosphorus in streams (fig. 7-9). Streams were grouped into 

three categories of nutrient enrichment according to concentration 

ranges of total nitrogen and total phosphorus based on published 

nutrient enrichment categories (Dodds and others, 1998). Changes 

in biological condition corresponding to increasing nutrient 

concentrations were more pronounced for algal communities than 

for macroinvertebrate or fish communities. In streams with the 

highest concentrations of total nitrogen or total phosphorus, average 

biological condition scores for algal communities were about 30 

percentage points lower than algal condition in streams with the 

lowest nutrient concentrations. In contrast, biological condition 

scores in streams with the highest nutrient concentrations were 

about 14 percentage points lower for fish communities and about 

10 percentage points lower for macroinvertebrate communities 

compared to 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 to nutrients is mostly indirect, mediated by transfer of 

energy (food resources) from primary producers (aquatic plants and 

algae) via food chains of various complexity. Algal communities 

are a useful biological indicator of eutrophication because of their 

sensitivity to nutrient enrichment (see sidebar, Algae as Indicators 

of Nutrient Enrichment). 

Figure 7-9. 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 nutrient 

concentrations was about 30 percentage points lower than the algal biological condition in streams 

with the lowest nutrient concentrations. 
126 Nutrients in the Nation’s Streams and Groundwater, 1992–2004

Fig_7-10.ai

75th percentile

90th percentile

Median

25th percentile

10th percentile

EXPLANATION

Algal biological condition, in percent

0

20

40

60

80

100

Undeveloped Agricultural Urban

Algal Community Condition Was Reduced 

for Many Streams in Agricultural and 

Urban Land-Use Settings

The relatively low median biological condition 

scores for algal communities in urban and agricultural 

streams indicated that stream health may be degraded 

in these land‐use settings compared to that in streams 

in undeveloped areas (fig. 7-10). The median biological 

condition scores of algal communities in agricultural and 

urban streams were 33 and 49 percentage points lower, 

respectively, than the reference condition score. Although 

the condition of algal communities was significantly 

reduced in agricultural and urban streams, the wide range 

of condition scores (indicated by the difference between 

the tenth and ninetieth percentiles) shows that algal 

condition was highly variable—that is, streams with high 

(expected in relatively undisturbed stream environments) 

as well as low (expected in highly disturbed stream 

environments) biological condition were found in each of 

these land-use settings. Algal condition scores generally 

were lower at urban sites than at agricultural sites, despite 

significantly higher concentrations of total nitrogen at 

agricultural sites. This lack of correspondence suggests 

that other factors, such as habitat, the specific forms of 

nitrogen or phosphorus present, or the presence of other 

chemical stressors, also are influencing the condition of 

algal communities at urban sites. 

Figure 7-10. The relatively low median biological condition 

scores for algal communities in urban and agricultural streams 

indicate that stream health may be degraded in these land-use 

settings compared to that in undeveloped streams. 

Fish are electroshocked, collected, surveyed, and released back to the 

stream to determine a census of fish population. Photograph by Terry 

Maret, U.S. Geological Survey.

Macroinvertebrates are collected with a Slack sampler. Photograph 

by Dennis Sun, U.S. Geological Survey.
Chapter 7

Potential for Effects on Aquatic Life 127

Streams

Estimated 1997 nitrogen inputs from

fertilizer, manure, and atmosphere,

in pounds per square mile

2,900

2,900–15,000

15,000

EXPLANATION

Percentage of

eutrophic diatoms

0.0–3.5

3.5–12.7

12.7–31.0

31.0–89.5

Algae as Indicators of Nutrient Enrichment 

The biological condition of algal communities is an especially effective indicator of human-influenced changes in water 

and habitat quality because many algal species have specific environmental requirements for growth and development. These 

requirements—such as food and habitat preferences, reproductive behavior, and lifespan—are all part of each species’ life 

history strategy for survival. Because all algae species have unique combinations of life history strategies and environmental 

preferences, their presence in a stream indicates a specific—and sometimes narrow—range of environmental conditions. 

Species occurring in streams in which water and habitat quality are degraded generally are limited to those organisms that are 

tolerant of existing physical and chemical properties of the stream environment.

As an example, the relative abundance of eutrophic diatoms in algal communities increased as concentrations of nitrogen 

and phosphorus increased (Porter and others, 2008). Eutrophic diatoms, which are algal species that prefer streams with 

elevated levels of nutrients, had a higher relative abundance in areas of the country with intensive agriculture, such as the 

upper Midwest, and in heavily urbanized areas (fig. 7-11). Eutrophic diatoms also were a strong indicator of nutrient enrichment 

in most—but not all—of the major river basins in the country (Porter and others, 2008). The relative abundance of other algal 

species may reflect other important environmental conditions, such as the concentration of dissolved oxygen or salinity. Algal 

indices are increasingly being considered in State and Tribal bioassessment programs (U.S. Environmental Protection Agency, 

2002a), as well as in development of nutrient criteria at the State level (Ponader and others, 2005; Belton and others, 2006).

Photographs printed with permission from the Academy of Natural Sciences, Patrick Center for Environmental Research Algae Image Database.

Figure 7-11. Eutrophic diatoms—

those that prefer streams with 

elevated levels of nutrients—have a 

high relative abundance in areas of 

the country with intensive agriculture, 

such as the upper Midwest, and in 

intensively urbanized areas. (Modified 

from Porter and others, 2008.)
128 Nutrients in the Nation’s Streams and Groundwater, 1992–2004

B.

Targeted

area

Study area

boundary

A.

Central Columbia

Plateau and

Yakima River Basin

Central

Nebraska

Basins

White River Basin

and Great and Little

Miami River Basins

Georgia

Coastal

Plain

Delmarva

Peninsula

Ozark

Plateaus

Upper Mississippi

River Basin

Upper Snake

River Basin

EXPLANATION

Study area started in 2001

Study area started in 2005

Agricultural watershed

Sampling site

Nutrient–Biota Interactions in 

Agricultural Streams

In 2001, NAWQA began a study of the effects of nutrient 

enrichment on stream ecosystems in five areas with extensive 

agricultural land. The objective of the study was to improve 

understanding of the interactions of nutrients, habitat, and 

biological community structure (algal and macroinvertebrate 

communities) and function (nutrient transformations and 

metabolism). Currently, the concentrations of nitrogen and 

phosphorus, along with algal biomass (chlorophyll a), are used 

by the USEPA and States for establishing nutrient criteria and 

Effects of Nutrient Enrichment on Stream Ecosystems:

Study Approach 

To address nutrient–biota interactions in agricultural streams, the NAWQA study on nutrient enrichment sampled 20 to 

30 independent, wadeable stream sites distributed along a gradient of nutrient concentrations in each of eight study areas 

(fig. 7-12). Sites were selected on the basis of geographic data, predicted nutrient loads and measured nutrient concentrations, 

habitat, and stream size. Data collected during the growing season included nitrogen and phosphorus in water, biological 

communities (algae and invertebrates), algal chlorophyll a, and stream habitat; stream metabolism (primary production and 

respiration) was collected at approximately one-third of the sites. Geographic Information Systems were used to evaluate 

riparian and land-use data. More information on study design and findings can be found at http://wa.water.usgs.gov/neet/.

for assessing nutrient enrichment in streams. One common 

approach to establishing nutrient criteria is to develop a 

statistical model that can predict plant biomass on the basis of 

specific nutrient concentrations, so that the model can be used 

to determine a target nutrient concentration for maintaining 

plant biomass below a specific level. Findings from this study 

should help State and Federal agencies with the development 

of nutrient criteria in various subregions of the United States. 

Although the study is currently (2010) in progress, preliminary 

findings are presented below. (Details on the study approach 

are presented in the sidebar, Effects of Nutrient Enrichment on 

Stream Ecosystems: Study Approach.)

Figure 7-12. The effects of nutrient enrichment on stream ecosystems in agricultural landscapes are being studied 

in eight geographic areas (A), by sampling 20 to 30 sites in each area along a gradient of nutrient conditions (B).
Chapter 7

Potential for Effects on Aquatic Life 129

Algal biomass limited 

by low nutrients

High nutrients, but algal biomass limited 

by physical conditions

High algal biomass reduces nutrients

Increasing nutrient concentration

Increasing chlorophyll 

a

High algal biomass reduces nutrients High algal biomass and nutrients not limiting High algal biomass and nutrients not limiting

eutrophic

oligotrophic

Algal biomass limited 

by low nutrients

High nutrients, but algal biomass limited 

by physical conditions

Nutrients and Algal Biomass

Although increases in the concentrations of nitrogen 

and phosphorus can cause an increase in algal biomass, 

the relation is often weak, varies geographically, and can 

be opposite of what one would expect (Munn and others, 

2010). Figure 7-13 is a Nutrient–Algal Biomass Conceptual 

Diagram, developed as part of the NAWQA studies on nutrient 

enrichment, that describes the hypothetical relation between 

nutrient concentrations and algal biomass, represented by 

chlorophyll a. There is a general assumption that sites fall 

along a line that runs from the lower left quadrant, where algal 

biomass increases as a function of nutrient availability (an 

oligotrophic stream), to the upper right quadrant, where the 

nutrient concentrations exceed what algae require (a eutrophic 

stream) and, therefore, algal biomass becomes limited by other 

factors, such as habitat. However, sites also can fall into the 

upper left and lower right quadrants. Sites in the upper left 

quadrant have algal biomass that accumulates to levels high 

enough to effectively remove nitrogen and phosphorus from 

the water column such that their concentrations decrease to 

lower levels, sometimes even below detection. The implication 

is that some sites may have concentrations below nitrogen 

or phosphorus criteria, but still be eutrophic, with high algal 

biomass and problems similar to those associated with low 

concentrations of dissolved oxygen. In contrast, sites in the 

lower right quadrant have elevated nutrient concentrations, 

but plant growth does not occur because of habitat limitations, 

for example, reduced light levels due to high concentrations 

of suspended sediment, or shading by canopy cover. These 

streams have insufficient solar energy to convert nutrients 

into algal biomass, and thus nutrients are transported to 

downstream receiving waters. It is, therefore, critical to 

consider a mixture of variables when determining whether 

streams meet nutrient criteria. 

Figure 7-13. This Nutrient–Algal Biomass Conceptual Diagram illustrates the relation between the concentration of nutrients (total 

nitrogen and total phosphorus) and the resulting algal biomass represented by chlorophyll a. The three photographs illustrate the type of 

streams in each quadrant: the top photograph is Clear Creek near Columbus, Nebraska (taken by M.D. Munn, U.S. Geological Survey), the 

lower left photograph is North Sylamore Creek near Fifty Six, Arkansas (taken by James Petersen, U.S. Geological Survey), and the lower 

right photograph is Lime Creek, near Cobb, Georgia (taken by Alan Cressler, U.S. Geological Survey).
130 Nutrients in the Nation’s Streams and Groundwater, 1992–2004

Total phosphorus, in milligrams per liter

0.001 0.01 0.1 1 10

Benthic chlorophyll a, in milligrams per square meter

1,000

100

10

1

0.1

Georgia Coastal Plain

Central Columbia Plateau and Yakima River Basin

Central Nebraska Basins

Delmarva Peninsula

White River Basin and Great and Little Miami River Basins

Figure 7-14. Although there is a positive relation between total phosphorus concentration and 

algal biomass in agricultural streams nationwide, the relation is partly the result of aggregating 

data for regions with very different relations. (Modified from Munn and others, 2010.)

Whereas a positive relation exists between total 

phosphorus and algal biomass in agricultural streams 

nationwide, figure 7-14 suggests that the relation is partly 

driven by aggregating regions with very different relations 

(Munn and others, 2010). Data from the 143 agricultural sites 

distributed throughout the five study areas were aggregated to 

examine the Nutrient–Algal Biomass concept. The regression 

model developed to determine if benthic algal biomass 

(chlorophyll a) on fine-grained sediment could be predicted 

on the basis of total phosphorus concentration explained only 

12 percent of the variability in chlorophyll a (fig. 7-14). The 

regression model for total nitrogen was not significant. 

The lack of a relation between algal biomass and 

nutrients in some regions indicates that most of the sites 

within those study areas have nutrient concentrations greater 

than what the algae require, so that additional increases in 

nutrients would have minimal effect on algal biomass (the 

upper right quadrant in fig. 7-13). For example, the lowest 

concentration of total phosphorus reported for streams in 

the Central Nebraska study was 0.14 mg/L, which is well 

above the concentration needed for algal growth. Regression 

models developed for each study area told divergent stories. 

Total phosphorus explained 32 percent of the variation 

in chlorophyll a in the Delmarva Peninsula study area, 

but the other study areas either did not show a significant 

relation, or displayed a negative relation (as for the Central 

Columbia Plateau and Yakima River study area) between total 

phosphorus and algal biomass that is due to factors other than 

nutrients. The relative strength of the relation in the Delmarva 

Peninsula study area is likely due in part to the relatively 

low—and large range in—phosphorus concentrations.

Regression models were improved by the addition of 

habitat variables, such as water temperature and canopy 

cover. For example, the amount of variation in chlorophyll a

concentrations explained increased from 12 percent, when 

nutrients alone were used as input variables, to 32 percent 

when both nutrients and habitat variables were included. 

This increase in the amount of variation explained by 

habitat variables was also found in the Delmarva Peninsula 

study area, where the amount of variation in chlorophyll a 

concentrations explained by the statistical model increased 

from 32 to 46 percent with the addition of a single habitat 

variable, canopy cover. The wide range in biological response 

to nutrient levels supports the need for a regional approach to 

establishing nutrient criteria, but also suggests that specific 

measures of habitat may need to be part of any criteria and (or) 

assessment to better understand expectations of changes in 

management practices meant to reduce nutrient loading 

to streams.
Chapter 8

Photograph by Lynn Betts, U.S. Department of Agriculture, Natural Resources Conservation Service.

Chapter

8 Changes in Nutrient 

Concentrations: 

Past and Predicted

Introduction 

Long-term changes in nutrient 

concentrations in streams and 

groundwater are determined by 

natural climatic changes and human 

changes in the use and management 

of agricultural, urban, and industrial 

sources of nutrients. Detecting changes 

in nutrient concentrations in streams 

and in groundwater presents different 

challenges. In streams, nutrient 

concentrations can vary widely over a 

few hours or days, making it difficult to 

detect smaller sustained changes over 

several years. In groundwater, the slow 

flow rates may result in very gradual 

changes in nutrient concentrations. 

Both circumstances necessitate longterm monitoring to discern trends in 

nutrient concentrations. Long-term, 

consistent monitoring can provide 

insight into how and why nutrient 

concentrations are changing over 

time—insight that is critical for 

evaluating progress towards waterquality goals, anticipating where action 

may be necessary to prevent degradation 

of water resources, and planning 

effective remediation strategies.
132 Nutrients in the Nation’s Streams and Groundwater, 1992–2004

Nutrient Trends in Streams

Nutrient inputs to streams from agricultural 

and urban sources have increased over the last 

century (fig. 8-1A). As these inputs increased, 

nutrient concentrations in streams increased in 

environmental settings as diverse as the humid 

urban Northeast and the semiarid agricultural 

Central Valley of California—as shown through 

long-term records of the Blackstone River in 

Massachusetts and the San Joaquin River in 

California (fig. 8-1B). Within the last two decades, 

increasing fertilizer costs, growing environmental 

concerns, and changing agricultural and waste 

management practices have reversed or slowed 

the pace of these increases (fig. 8-1A) (see 

sidebar, Past Changes Resulting from Nutrient 

Control Measures). NAWQA’s assessment 

of nutrient trends between 1993 and 2003 

at 171 stream sites for total phosphorus and 

137 stream sites for total nitrogen encompassed 

only recent changes and consequently does not 

reflect the changes that occurred earlier in the 

20th century.

Prior to widespread use of commercial fertilizer, livestock waste was the major source of nutrients for crops on 

family farms. Photograph by U.S. Department of Agriculture, Natural Resources Conservation Service.
Chapter 8

Changes in Nutrient Concentrations: Past and Predicted 133

A.Population, in millions of people

Nutrient input, in millions of tons

B.Nitrate concentration, in milligrams per liter as N

1900 1920 1940 1960 1980 2000

2.0

1.5

1.0

0.5

0.0

Blackstone River at Millville, Massachusetts

San Joaquin River near Vernalis, California

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

Phosphorus fertilizer input

1993–2003 period of record 

used in trend analysis of streams

Figure 8-1. Nutrient inputs to streams from agricultural and 

urban sources have increased over the last century (A). As these 

inputs increased, nutrient concentrations in streams increased in 

environmental settings as diverse as the humid urban Northeast 

and the semiarid agricultural Central Valley of California—as shown 

through long-term records of the Blackstone River in Massachusetts 

and the San Joaquin River in California (B). NAWQA’s assessment 

of nutrient trends in streams reflects only changes that may have 

occurred from 1993 to 2003 (yellow vertical shaded area). 
134 Nutrients in the Nation’s Streams and Groundwater, 1992–2004 1970 1972 1974 1976 1978

1980198219841986198819901992199419961998200020022004

0.00

0.45

0.90

1.35

1.80

Total phosphorus concentration, in milligrams per liter

2006

Phosphorus consumption for detergent, in million metric tons

0.30

0.25

0.20

0.15

0.10

0.05

0.00

Contentnea Creek at Hookerton, North Carolina

Total phosphorus 

concentration in

Phosphorus

consumption for

detergent in the

United States

Contentnea Creek

North Carolina’s ban on

phosphates in detergent

Past Changes Resulting from Nutrient Control Measures

Although efforts to control nutrient enrichment have been many and varied, it is often difficult 

to determine the efficacy of specific management measures because of the lack of suitable data 

on the degree or spatial extent of the measure, concurrent changes in other sources or factors, 

or a general lack of nutrient monitoring following the implementation of a measure. But there 

have been notable success stories in which changes in some aspect of nutrient management 

resulted in improved water quality.

Figure 8-2. North Carolina’s 1988 ban on phosphate detergents likely 

contributed to decreased total phosphorus concentrations in Contentnea 

Creek, North Carolina.

Phosphate detergent ban—The use of phosphorus in laundry detergents began around 1940 and 

increased rapidly through 1970 to become a major source of phosphorus to the environment. In 1970, 

the detergent industry voluntarily agreed to limit phosphate use in manufacturing, and individual 

States began instituting phosphate detergent bans. As a result, the use of phosphorus in detergents 

decreased from a peak of 220,000 metric tons in 1967 to less than 10,000 metric tons in 1995 (Litke, 

1999). These phosphorus control measures have resulted in decreased phosphorus concentrations 

in many streams affected by wastewater-treatment plant effluent and septic-system leakage. For 

example, concentrations of total phosphorus in Contentnea Creek, North Carolina, decreased after 

1988 when the State implemented a phosphate detergent ban (Harned, 2003) (fig. 8-2).
Chapter 8

Changes in Nutrient Concentrations: Past and Predicted 135

2.0

1.5

1.0

0.5

0.0

100

80

60

40

20

0

Concentration, in milligrams per liter

Streamflow, in cubic feet per second

1997 1998 1999 2000 2001

Granger Drain at Granger, Washington

Total

phosphorus

Streamflow

197019721974197619781980198219841986198819901992199419961998200020022004

0

5

10

15

20

Concentration, in milligrams per liter as N

B.

2006

Number of fish species

30

5

10

15

20

25

0

Concentration, in milligrams per liter

A.

0

5

10

15

20

Trinity River below Dallas, Texas

Ammonia concentrations

Nitrate concentrations

Fish species

Dissolved-oxygen concentrations

Ammonia concentrations, as N

Figure 8-4. Decreases in total phosphorus concentrations 

in Granger Drain, Washington between 1997 and 2001 may be 

related to the increased use of agricultural Best Management 

Practices (BMPs) in the basin (Fuhrer and others, 2004). Data 

provided by the Roza-Sunnyside Board of Joint Control.

Figure 8-3. Improvements in wastewater treatment have decreased 

ammonia concentrations, increased dissolved oxygen concentrations, 

and improved fish communities in the Trinity River below Dallas, Texas 

(A). Because the ammonia is converted to nitrate during treatment, 

however, nitrate concentrations in the river have increased (B). Data on 

fish species modified from Land and others (1998).

Implementation of agricultural Best Management 

Practices—Agricultural Best Management Practices 

(BMPs), such as conservation tillage and drip irrigation, 

also have improved water quality in some streams. Many 

BMPs are designed to reduce erosion, and as a result are 

effective at reducing instream concentrations of nutrients 

such as phosphorus that bind strongly to eroded sediment. 

For example, decreased concentrations of total phosphorus 

between 1997 and 2001 in Granger Drain, Washington, 

may be related to implementation of BMPs in the basin, 

including (1) the use of polyacrylamide, a chemical added 

to irrigation water to reduce erosion and enhance water 

infiltration, and (2) conversion from rill irrigation to sprinkler 

and drip irrigation, which use less water and reduce erosion 

and surface runoff (Fuhrer and others, 2004) (fig. 8-4). 

Concentrations of nitrate, which does not bind strongly to 

eroded sediment, did not show a similar decrease.

Improvements in the treatment of wastewater—During the 

same period, improvements in the treatment of wastewater 

resulted in decreased ammonia concentrations in some 

streams. Decreased ammonia concentrations achieved through 

nitrification—the oxidation of ammonia to nitrate—of wastewater 

prior to discharge can lead to increased concentrations of 

dissolved oxygen in streams, which in turn can improve the health 

of fish communities. For example, fish were almost nonexistent 

in the Trinity River below Dallas in the early 1970s (Texas Parks 

and Wildlife Department, 1974). When ammonia concentrations 

in the river decreased as a result of upgrades to wastewatertreatment plants after 1980, dissolved oxygen concentrations 

and the number of fish species in the river increased (Land and 

others, 1998) (fig. 8-3A). Because ammonia is converted to nitrate 

during nitrification, however, nitrate concentrations can increase 

downstream of wastewater-treatment plants, as occurred in the 

Trinity River (fig. 8-3B).
136 Nutrients in the Nation’s Streams and Groundwater, 1992–2004

Although the change in the availability of 

some nutrient sources slowed in the latter part of 

the 20th century, there were notable national and 

regional changes between the early 1990s and 

early 2000s (fig. 8-5). Among these changes:

• Livestock production shifted from a large 

number of small farms to fewer, larger 

concentrated animal feeding operations 

(CAFOs). CAFOs sometimes produce 

large volumes of manure with more 

nutrients than can be fully assimilated by 

crops and pasture on nearby agricultural 

land (U.S. Environmental Protection 

Agency, 2004). Shifts in the location and 

numbers of confined livestock contributed 

to changes in nutrient inputs from 

manure, with widespread increases in 

manure inputs occurring in the central and 

southeastern United States—particularly 

in North Carolina, the State with the 

second largest production of hogs and 

pigs, and where the number of these 

animals nearly doubled between 1992 and 

1997 and then leveled off between 1997 

and 2002 (National Agricultural Statistics 

Service, 2008) (fig. 8-5A).

• Corn received the largest proportion of 

nutrients from fertilizer, representing 

about 40 percent of the total fertilizer use 

in the United States (U.S. Department of 

Agriculture, 2008). Changes in nutrient 

inputs from fertilizer often corresponded 

to changes in corn acreage and yield, 

with relatively large increases in acreage 

occurring in Arkansas, Missouri, Kansas, 

South Dakota, and North Dakota, and 

relatively large decreases occurring in 

North Carolina and Georgia (National 

Agricultural Statistics Service, 2008) 

(fig. 8-5B). (Large percentage increases 

in fertilizer inputs in parts of California, 

Nevada, and Maine occurred in areas with 

small amounts of agricultural land and 

fertilizer inputs; see Chapter 2, fig. 2-2 for 

estimates of 1997 inputs from fertilizer, 

manure, and atmospheric deposition). 

Changes in the acreage of other crops, 

crop yields, climate, and economic factors 

such as the price of fertilizer and crops, 

likely also contributed to changes in 

fertilizer inputs during this period.

• Nitrogen oxide emissions from vehicles, factories, and other 

sources throughout the United States decreased 12 percent from 

1993 to 2002—with the largest decreases in the Northeastern and 

Western States and the smallest decreases in the Rocky Mountains 

and Great Plains States (U.S. Environmental Protection Agency, 

2003). These decreases in emissions largely coincided with 

widespread stable or decreasing nitrogen inputs from atmospheric 

deposition from 1993 to 2003 (fig. 8-5C).

• Population growth varied regionally between 1990 and 2000, 

with greater increases in the West (19.7 percent) and South 

(17.3 percent) and much lower increases in the Midwest 

(7.9 percent) and Northeast (5.5 percent) (U.S. Census Bureau, 

2001) (fig. 8-5D). For example, the population of Pennsylvania 

increased by 3.4 percent, from 11.9 million to 12.3 million 

people, while the population of Nevada increased by 66 percent, 

from 1.2 million to 2.0 million people. At the county level, the 

largest decreases in population primarily took place in the Great 

Plains States, while the largest increases took place in the South 

and West.

Throughout the United States, nutrient sources changed at different 

rates and in different directions, making it difficult to determine 

their effect on nutrient trends in streams. Further complicating this 

determination, data on many nutrient sources and management practices 

are of unknown reliability, are inconsistent over time and space, or are 

not collected at all.

Geographic shifts in the location and numbers of livestock changed the distribution of 

nutrient inputs in some areas of the country. In North Carolina, the number of hogs and 

pigs nearly doubled between 1992 and 1997. Photographs by Lynn Betts, U.S. Department 

of Agriculture, Natural Resources Conservation Service (top), and Steve Reid, North 

Carolina Division of Water Quality, Raleigh, North Carolina (bottom).
Chapter 8

Changes in Nutrient Concentrations: Past and Predicted 137

Fig_8-05_change

EXPLANATION

-100 – -50

 -50 – -10

 -10 – 10

 10 – 50

 50 – 200

 200

Percent change, by county

Phosphorus inputs

Phosphorus inputs

Nitrogen inputs 

A.

B. Change in fertilizer, 1993 to 2003

Change in population,

1990 to 2000 

D. Change in nitrogen inputs from 

atmospheric deposition, 1993 to 2003

C.

Change in manure, 1992 to 2002

Nitrogen inputs 

Figure 8-5. Nutrient sources changed at different rates and in different directions 

throughout the United States during NAWQA’s assessment period, making it difficult to 

determine their effect on nutrient trends in streams. (See Chapter 2, fig. 2-2 for estimates 

of 1997 inputs from fertilizer, manure, and atmospheric deposition.)
138 Nutrients in the Nation’s Streams and Groundwater, 1992–2004

EXPLANATION

Upward Downward No significant trend

Direction of trend (1993–2003)

Streams

Human activities are not the only influence on nutrient concentrations 

in streams. Natural changes in precipitation and streamflow also can 

influence concentrations by altering the amount of instream dilution of 

nutrients and surface runoff carrying nutrients to the stream. At most sites 

used in the analysis of trends in nutrient concentrations, streamflow did not 

change significantly from 1993 to 2003 (fig. 8-6). Where changes did occur, 

the trends were downward at all but two sites. The streamflow decreases 

occurred primarily at sites in the Central, Southwestern, and parts of the 

Southeastern United States, often as a result of drought conditions in the 

latter part of the study period.

Human and natural changes often occur simultaneously, and they 

can either counteract or supplement each other. For example, nutrient 

concentrations in a stream may change very little if a decrease in 

concentration resulting from improvements at a wastewater-treatment plant 

is offset by decreased instream dilution resulting from a natural decrease in 

streamflow. In the analysis in the following section on Flow-Adjusted Trends 

in Concentration, the large amount of variability in nutrient concentrations 

in streams caused by natural changes in streamflow is removed, allowing 

trends caused by human changes to be more directly assessed. In the analysis 

in the following section on Overall Trends in Concentration, the net effects 

of all simultaneous influences on nutrient concentration are evaluated, 

allowing for the assessment of concentrations in streams relative to waterquality standards and the condition of aquatic communities. More details 

on the methods used to evaluate trends in nutrient concentrations in streams 

are presented in the sidebar, Methods Used to Evaluate Trends in Nutrient 

Concentrations in Streams.

Figure 8-6. At most sites used in the analysis of trends in concentration, 

streamflow did not change significantly from 1993 to 2003. Where changes did 

occur, the trends were downward at all but two sites. The streamflow decreases 

primarily occurred at sites in the Central, Southwestern, and parts of the 

Southeastern United States, often as a result of drought conditions in the latter 

part of the study period.

Methods Used to Evaluate 

Trends in Nutrient Concentrations 

in Streams

The analysis of nutrient trends in streams 

from 1993 to 2003 included data from the 

USGS’s National Water Information System 

(NWIS) database (collected through the 

NAWQA Program and in cooperation with 

other Federal, State, and local agencies) 

and selected data from the USEPA’s Storage 

and Retrieval (STORET) database. A total of 

244 sites were included in the analysis on 

the basis of the following general criteria: (1) 

record of nutrient concentrations beginning 

in 1993 or earlier and ending in 2003 or later; 

(2) approximately quarterly sampling each 

year; (3) continuous streamflow record 

between 1993 and 2003 at that site or a 

nearby representative site; (4) data gaps 

no longer than 2 years and only during the 

middle 6 years of record; (5) representative 

coverage of samples over the range of the 

hydrograph to avoid bias toward low or high 

streamflows; and (6) representative coverage 

of all seasons to avoid bias towards certain 

times of year (Sprague and others, 2009).

The statistical procedures used here 

to evaluate trends assume that the trend 

is monotonic—that is, the values in the 

population of observations increase or 

decrease over time with no substantial 

reversals in direction. Trend results are 

reported as the overall change between 

the beginning and ending dates rather than 

the year-to-year variation from 1993 to 

2003. Shorter duration changes over a few 

hours, days, or months can cause nutrient 

concentrations in streams to vary by orders 

of magnitude. The “noise” caused by 

frequent short-duration changes can make 

it difficult to detect the “signal” of a small 

sustained trend over several years. As a 

result, some smaller trends in concentration, 

such as those resulting from the conversion 

of farmland to suburban areas, may not be 

detectable until many years have passed.
Chapter 8

Changes in Nutrient Concentrations: Past and Predicted 139

0

1

2

3

4

Total phosphorus concentration,

in milligrams per liter

0

10

20

30

40

A.

1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002

1993–2003 1993–early 2002

Salt River near Roosevelt, Arizona B. Salt River near Roosevelt, Arizona

Total phosphorus

Total phosphorus, below detection level

The noise, or variability, also can lead to uncertainty in estimating the magnitude of a trend. The amount of uncertainty can be 

represented as a confidence interval, a range of values that likely includes the true trend magnitude. Confidence intervals are wider 

for data sets having greater variability (Helsel and Hirsch, 1992). Estimated trend magnitudes are reported here with 95-percent 

confidence intervals—the interval contains the true trend magnitude with 95-percent confidence. If a 95-percent confidence 

interval contains both positive and negative values, the trend is considered to be non-significant. If a 95-percent confidence interval 

contains only positive or only negative values, however, the trend is considered to be significant (upward or downward, respectively), 

regardless of the interval width.

The magnitude and significance of each trend is sensitive to conditions at the beginning and end of the record. Thus a trend 

from 1993 to 2003 might be different from a trend during a different period, such as from 1985 to 2003 or from 1993 to 2002. The use of 

different periods of record for analysis can lead to different, yet equally valid, trend results for the same site (fig. 8-7). Because the 

outcome of the trend analysis is sensitive to the period of record, the same period of record (1993 to 2003) was used at all sites to 

facilitate comparison and summary of trends among sites. NAWQA assessments at some sites did not begin until 1996 or 1998; these 

sites were not included in the trend analysis unless earlier data were available from another source.

Flow-adjusted trends in concentration were compared with contemporaneous changes in nutrient sources, including atmospheric 

deposition of nitrogen, phosphorus and nitrogen loading from fertilizer and manure, and population (a surrogate for urban sources of 

nutrients) to help understand the factors that affect the trends over time. The goal of this analysis was not to explain all factors that 

affected each trend at individual sites—that was beyond the scope of the study. Rather, the goal was to identify on a broader scale 

those individual factors that potentially affected trends at groups of related sites. The groups included sites in the Eastern, Central, and 

Western United States and agricultural, urban, and undeveloped sites. Because the number of sites meeting the NAWQA criteria for 

agricultural and urban land-use classification (see Chapter 3, NAWQA’s Approach to Nutrient Assessment, table 3-1) was too small for 

statistical analysis, watershed land-use criteria were adjusted to greater than 40 percent agricultural land use (instead of greater than 

50 percent) and greater than 10 percent urban land use (instead of greater than 25 percent).

Figure 8-7. The record of total phosphorus concentrations at the Salt River near Roosevelt, Arizona, shows large 

increases in 2002 and 2003 (A). These increases likely were due in large part to runoff after the Rodeo-Chediski fire, 

which burned more than 452 square miles in the Salt River drainage (Arizona Game and Fish Department, 2006). The 

record of total phosphorus concentrations between 1993 and early 2002 (preburn) shows no evidence of an increase 

of similar magnitude (B). Flow-adjusted trends during both periods were upward, though the magnitude of the trend 

was much larger from 1993 to 2003 than from 1993 to 2002. (Note different y-axis scales in A and B.)
140 Nutrients in the Nation’s Streams and Groundwater, 1992–2004

EXPLANATION

Upward Downward No significant trend

Direction of trend (1993–2003)

Flow-adjusted trends in streams Overall trends in streams

Total phosphorus concentration

Total nitrogen concentration

Flow-Adjusted Trends in Concentration

Flow-adjusted trends in the concentration of phosphorus 

and nitrogen from 1993 to 2003 were not significant at the 

majority of sampling sites nationwide (51 and 63 percent of 

sites for total phosphorus and total nitrogen, respectively) 

(Sprague and others, 2009) (fig. 8-8). In the remaining 

streams, there were more upward than downward flowadjusted trends—trends were upward at 33 and 21 percent 

of sites for total phosphorus and total nitrogen, respectively, 

but downward at only 16 percent for both nutrients (fig. 8-8). 

These findings indicate that human activities such as changes 

in nutrient inputs or implementation of management practices 

did not contribute to net changes in nutrient concentrations 

in most sampled streams. Human activities did, however, 

contribute to more increases than decreases in the remaining 

streams, despite relatively stable fertilizer application rates 

during 1993–2003 (fig. 8-1).

In the Central and Southwestern United States, flowadjusted trends in total phosphorus concentrations generally 

were upward or nonsignificant (fig. 8-8). Flow-adjusted trends 

in total nitrogen concentrations generally were downward or 

nonsignificant in the Northwestern United States.

Figure 8-8. The patterns of flow-adjusted trends in nutrient concentrations indicate that human changes in the use and management 

of nutrient sources did not contribute to net changes in their concentrations in most sampled streams from 1993 to 2003, and contributed 

to more increases than decreases in the remaining streams. In Central, Southwestern, and parts of the Southeastern United States, 

where streamflow decreased at many sites, there were fewer upward overall trends (which reflect the net effect of all simultaneous 

influences on nutrient concentrations in streams) than upward flow-adjusted trends. Anthropogenic increases in nutrient sources at 

sites in these areas likely were offset by decreases in precipitation and surface runoff. Without the decrease in surface runoff carrying 

nutrients to streams, concentrations probably would have been higher than were actually observed.
Chapter 8

Changes in Nutrient Concentrations: Past and Predicted 141

Upward flowadjusted trends Downward flowadjusted trends No significant

change 

0

10

20

30

40

50

60

70

80

90

100

Percentage of sites

Upward flowadjusted trends Downward flowadjusted trends No significant

change 

0

10

20

30

40

50

60

70

80

90

100

Percentage of sites

Undeveloped (41) Mixed (62)

Agricultural (11 sites) Urban (16)

Undeveloped (59) Mixed (73)

Agricultural (13 sites) Urban (19)

EXPLANATION

Upward

No significant trend

Downward

Direction of trend (1993–2003) Watershed land use

Agricultural

Urban

Undeveloped

Mixed

Undetermined

Total phosphorus concentration

Total nitrogen concentration

Flow-adjusted trends in streams by land use

Although the trend sites meeting the NAWQA criteria 

for agricultural and urban land-use classification were small 

in number and geographically clustered (fig. 8-9), a limited 

comparison of flow-adjusted trends by land-use category can 

be made. There were more upward than downward trends 

in streams in all land-use categories for total phosphorus, 

particularly in agricultural and mixed land-use streams 

(fig. 8-9). By comparison, the percentage of upward and 

downward flow-adjusted trends was comparable for total 

nitrogen in streams of all land uses except agricultural streams, 

where no downward flow-adjusted trends were observed. The 

geographic coverage of these sites was sporadic, however, 

particularly for urban and agricultural sites. For example, 

coverage of agricultural areas was limited primarily to parts 

of the Corn Belt, the Central Valley of California, the Ozark 

Plateaus, and the Potomac and Columbia River Basins. Data 

from additional sites would be needed to more fully describe 

the pattern of trends among land uses.

Figure 8-9. There were more upward than downward flow-adjusted trends in streams in all land-use categories for total 

phosphorus, especially in agricultural and mixed land-use streams. In comparison, the percentages of upward and downward 

flow-adjusted trends were comparable for total nitrogen in streams of all land uses except agricultural streams, where no 

downward flow-adjusted trends were observed. 
142 Nutrients in the Nation’s Streams and Groundwater, 1992–2004

Figure 8-10. In streams with concentrations of nutrients below U.S. Environmental Protection Agency’s 

recommended ecoregional nutrient criteria in 1993, human activities contributed to more increases 

(upward flow-adjusted trends) than decreases (downward flow-adjusted trends) in concentration (A). 

Decreases in concentrations occurred more frequently in streams with nutrient concentrations above the 

criteria in 1993 (B).

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 

Total nitrogen (34 sites)

Total phosphorus (28 sites)

Streams with concentrations in 1993 above 

USEPA’s recommended ecoregional nutrient criteria 

Total nitrogen (103 sites)

Total phosphorus (143 sites)

Upward flowadjusted trends Downward flowadjusted trends No significant

change 

Change between 1993 and 2003

A. B.

Flow-adjusted trends were not significant at nearly 

one-half of the streams that were least affected by nutrient 

enrichment in 1993 (streams with nutrient concentrations 

in 1993 below USEPA’s recommended ecoregional nutrient 

criteria), but human activities contributed to more than three 

times as many increases as decreases in total phosphorus 

concentrations and more than ten times as many increases as 

decreases in total nitrogen concentrations at the remainder 

(fig. 8-10A). More decreases in nutrient concentrations 

occurred in streams with concentrations that were above 

USEPA’s recommended ecoregional nutrient criteria in 1993, 

particularly with respect to total nitrogen concentrations 

(fig. 8-10B)—downward flow-adjusted trends were observed 

in nearly 20 percent of these more highly affected streams. 

These findings indicate that while modest gains have been 

made in nutrient quality in some affected streams, human 

activities have led to greater nutrient enrichment in some of 

the Nation’s least-affected streams. 

In some land-use and specific geographic areas, 

flow‐adjusted trends in phosphorus concentrations were 

related to changes in major sources of nutrients—fertilizer, 

manure, and urban sources (represented by population here).

• Agricultural sources—Changes in manure (fig. 8-11A) 

and fertilizer (fig. 8-11B) inputs were associated with 

changes in total phosphorus concentrations at sites 

with greater than 40 percent agricultural land use 

in the basin. 

• Population—Changes in population were associated 

with changes in total phosphorus concentrations at 

undeveloped sites and at sites in the Western United 

States (figs. 8-11C and 8-11D).

At all sites, there likely were additional factors 

influencing nutrient trends. Some factors, such as changes in 

inputs from atmospheric deposition, were not associated with 

concentration trends in any particular geographic or land-use 

area, but were likely important at individual stream sites. 

Other factors, such as changes in loading from point sources 

and implementation of nutrient management strategies, could 

not be evaluated because national-scale, consistently derived 

data sets for these factors are not currently available. 

In general, factors influencing the trends in 

concentrations were more difficult to identify for total nitrogen 

than for total phosphorus. Nitrogen is more readily leached 

through the soil and generally is more mobile in groundwater 

than phosphorus. Because it may take many years for 

groundwater to travel from recharge areas to streams, there 

may be a substantial lag between changes in nitrogen use on 

the land surface and changes in concentrations in streams. 

As a result, trends in total nitrogen concentrations in streams 

that receive nitrogen inputs from groundwater may have been 

influenced more by changes on the land surface that took place 

prior to 1993. 
Chapter 8

Changes in Nutrient Concentrations: Past and Predicted 143

Figure 8-11. In some geographic and specific land-use areas, flow-adjusted trends in phosphorus 

concentrations from 1993 to 2003 were related to changes in inputs of major sources of nutrients, including 

manure at agricultural sites (A), fertilizer at agricultural sites (B), urban sources (represented by population) at 

undeveloped sites (C), and urban sources at sites in the Western United States (D). Factors associated with flowadjusted trends in nitrogen concentrations were more difficult to identify; because groundwater can take many 

years to travel to streams, there may be a substantial lag between changes in nitrogen use on the land surface 

and changes in nitrogen concentrations in streams.

EXPLANATION

Trend estimate

Lower 95-percent confidence interval**

Upper 95-percent confidence interval**

** There is a 95-percent probability that the true trend is between the 

upper and lower confidence interval (see sidebar, Methods Used to 

Evaluate Trends in Nutrient Concentrations in Streams).

Change line

*Sites with greater than 40 percent agricultural land use in the basin.

Change in population, in percent

Western United States

-200

0

200

400

600

800

1,000

1,200

1,400

0 20 40 60 80 100

-200

0

200

400

600

800

1,000

1,200

1,400

Change in population, in percent

Undeveloped

-80 -40 0 40 80 -100

0

100

200

300

400

500

Change in phosphorus inputs

from fertilizer, in percent

Agricultural*

-50 0 50 100

-60 -40 -20 0 20 40 -100

0

100

200

300

400

500

Change in phosphorus inputs 

from manure, in percent

Agricultural*

Flow-adjusted trends in total phosphorus

concentration, in percent

Flow-adjusted trends in total phosphorus

concentration, in percent

Flow-adjusted trends in total phosphorus

concentration, in percent

Flow-adjusted trends in total phosphorus

concentration, in percent

A. B.

C. D.

It is important to note that human activities may have had 

a positive effect even in streams where the flow-adjusted trend 

was not downward. Often, multiple and possibly counteracting 

factors are influencing nutrient concentrations in a stream. 

One or more factors contributing to decreases in concentration 

(such as implementation of BMPs) can be offset by additional 

factors contributing to increases in concentrations (such as 

increased application of manure or fertilizer), resulting in net 

increases in or relatively stable concentrations. Without those 

factors contributing to decreases in concentration, instream 

concentrations might have been higher. 
144 Nutrients in the Nation’s Streams and Groundwater, 1992–2004

Fig_8-12.ai

0

10

20

30

40

Flow-adjusted

trend

Overall

trend

Percentage of sites with upward trends

between 1993 and 2003

Streams with upward trends

Total phosphorus

Total nitrogen

seven streams with downward trends, the overall trends were 

similar to the flow-adjusted trends, suggesting that human 

activities played a large role in the concentration decreases. 

In the remaining stream, the overall trend was much larger 

than the flow-adjusted trend, suggesting that a natural change 

in streamflow was the primary influence in the concentration 

decrease. The successes were not limited to streams in which 

only a small decrease was needed to reach a criterion; they 

also occurred in more highly affected streams where larger 

decreases were needed to reach a criterion. Partial progress 

towards the criteria was made in more streams (sites with 

downward overall trends, criterion not reached, fig. 8-13B), 

including some of the most highly affected streams. For 

example, total nitrogen concentrations in the Neuse River, 

North Carolina, decreased an estimated 37 percent (95-percent 

confidence interval of -45 to -27 percent) to approach the 

recommended total nitrogen criterion of 0.69 mg/L by 

2003 (fig. 8-13C). In most streams, however, the change 

in concentration was insufficient to reach the criteria 

(fig. 8-13B)—and in some cases, concentrations increased 

and nutrient quality declined. For example, total nitrogen 

concentrations in Flint Creek, Oklahoma, increased an 

estimated 52 percent (95-percent confidence interval of 33 to 

73 percent), thus moving further from the recommended total 

nitrogen criterion of 0.31 mg/L by 2003 (fig. 8-13C).

Overall Trends in Concentration

There were fewer upward overall trends (reflecting all 

simultaneous influences on concentrations in streams) than 

upward flow-adjusted trends (reflecting the influence of human 

activities; fig. 8-12), indicating that anthropogenic increases in 

nutrient sources or transport did not always result in increasing 

concentrations in the stream (Sprague and others, 2009). These 

differences primarily were in the Central and Southwestern 

United States for phosphorus and more scattered throughout 

the Nation for nitrogen, generally at sites where streamflow 

decreased. At these sites, anthropogenic increases in nutrient 

sources likely were offset by natural decreases in surface 

runoff needed to transport the nutrients to the streams. Without 

the decreases in surface runoff, concentrations in these streams 

probably would have been higher than were actually observed. 

The number of downward overall trends during 

1993–2003 was similar to the number of downward 

flow‐adjusted trends in nutrient concentrations—about 

16 percent of sites each for both phosphorus and nitrogen. At 

many sites where both the overall and the flow-adjusted trend 

were downward, streamflow did not change significantly from 

1993 to 2003, which suggests that the decreases in nitrogen 

and phosphorus concentrations were a result of either reduced 

sources (fertilizer and manure applications and point sources) 

or improved nutrient management.

Broad regional patterns were less apparent in overall 

trends than in flow-adjusted trends (fig. 8-8). Overall trends 

in total nitrogen concentrations generally were downward or 

nonsignificant in the Northwestern United States (fig. 8-8). 

Upward overall trends were large enough that 

concentrations generally increased to above USEPA’s 

recommended ecoregional nutrient criteria in 6 of 28 streams 

(21 percent) with total phosphorus concentrations below the 

criteria in 1993 and in 6 of 34 streams (18 percent) with total 

nitrogen concentrations below the criteria in 1993 (fig. 8-13A). 

At sites at which concentrations increased to above the 

criteria, the overall trend was similar to the flow-adjusted 

trend, suggesting that human activities played a large role in 

the increases. In the majority of these least-impacted streams, 

however, no significant change occurred (fig. 8-13A) and 

nutrient quality was maintained.

In comparison, downward overall trends caused 

concentrations to decrease to below USEPA’s recommended 

ecoregional nutrient criteria in only 2 of 143 streams 

(1 percent) with total phosphorus concentrations above 

the recommended criteria in 1993, and in 5 of 103 streams 

(5 percent) with total nitrogen concentrations above the 

recommended criteria in 1993 (fig. 8-13B). In six of these 

Figure 8-12. There were fewer upward overall 

trends than upward flow-adjusted trends in nutrient 

concentrations during 1993–2003, indicating that 

anthropogenic increases in nutrient sources and (or) 

transport did not always result in increasing instream 

concentrations because of the counteracting effects of 

decreasing surface runoff and streamflow.
Chapter 8

Changes in Nutrient Concentrations: Past and Predicted 145

Fig_8-13.ai

U.S. Environmental

Protection Agency

recommended 

ecoregional nutrient

criterion

Trend line

EXPLANATION

Total nitrogen (103 sites)

Total phosphorus (143 sites)

B.

Downward

overall trend,

criterion not

reached

Downward 

overall trend,

criterion

reached

Upward

overall

trend

No significant

change

Streams with concentrations in 1993 above

USEPA’s recommended ecoregional nutrient criteria

A.

0

20

40

60

80

Upward overall

trend, criterion

not reached

Upward overall

trend, criterion

reached

Downward

overall

trend

No significant

change

Total nitrogen (34 sites)

Total phosphorus (28 sites)

Percentage of sites

Streams with concentrations in 1993 below

USEPA’s recommended ecoregional nutrient criteria

0

1

2

3

4 Neuse River at Kinston, North Carolina

1993

1994

1995

1996

1997

1998

1999

2000

2001

2002

2003

Total nitrogen concentration,

in milligrams per liter

0

1

2

3

4

5

6 Flint Creek near Kansas, Oklahoma

19931994199519961997199819992000200120022003

C. Downward overall trend,

criterion not reached

Upward overall trend

Figure 8-13. Upward overall trends were large enough that concentrations increased to above U.S. 

Environmental Protection Agency’s (USEPA) recommended ecoregional nutrient criteria by 2003 in 

21 percent of streams with total phosphorus concentrations below the criteria in 1993 and in 18 percent of 

streams with total nitrogen concentrations below the criteria in 1993 (A). By comparison, downward overall 

trends were large enough that concentrations decreased to below the criteria by 2003 in only 1 percent of 

streams with total phosphorus concentrations above the criteria in 1993, and in 5 percent of streams with 

total nitrogen concentrations above the criteria in 1993 (B). For example, from 1993 to 2003, total nitrogen 

concentrations in the Neuse River, North Carolina, decreased to approach the recommended total nitrogen 

criterion, whereas total nitrogen concentrations in Flint Creek, Oklahoma, increased, moving further from 

the recommended total nitrogen criterion during the same period (C).
146 Nutrients in the Nation’s Streams and Groundwater, 1992–2004

1993–2003 period of

record reported here

LOWESS (locally weighted scatterplot 

smoothing) trend line

EXPLANATION

0.0

0.1

0.2

0.3

0.4

0.5

0.6

Total phosphorus concentration, in milligrams per liter

Ohio River near Grand Chain, Illinois

197219741976197819801982198419861988

1990

1992199419961998

2000

200220042006

The trends in nutrient concentrations 

in U.S. streams between 1993 and 2003 are 

notably different than those between 1975 and 

1994, when more downward trends in nutrient 

concentrations occurred (Alexander and Smith, 

2006). In some U.S. streams, improvements 

in water quality realized after enactment of 

the Clean Water Act of 1972 have been offset 

somewhat by the effects of more recent changes 

in climate, population, land use, or agricultural 

and industrial activities. For example, an 

estimated 60 percent decrease (95-percent 

confidence interval of -76 to -44 percent) in 

Figure 8-14. In some parts of the United States, improvements in water 

quality realized after the enactment of the Clean Water Act in 1972 have been 

offset somewhat by the effects of more recent changes in climate, population, 

land use, or agricultural and industrial activities. For example, the increase in 

total phosphorus concentrations between 1993 and 2003 in the Ohio River near 

Grand Chain, Illinois, followed a sustained decrease that occurred between 

1972 and 1992.

total phosphorus concentrations between 1972 

and 1992 in the Ohio River near Grand Chain, 

Illinois, was followed by an estimated 61 percent 

increase (95-percent confidence interval of 17 

to 121 percent) in concentrations between 1993 

and 2003 (fig. 8-14). In the long term, human 

activities and climate will inevitably continue to 

change, affecting stream concentrations. Future 

nutrient conditions in the Nation’s streams 

will depend on both natural and anthropogenic 

factors, and consideration of the full range of 

possible climatic conditions can help in shaping 

future water-quality management decisions.
Chapter 8

Changes in Nutrient Concentrations: Past and Predicted 147

EXPLANATION

Type of groundwater study

Agricultural land use

Major aquifers

Significant increase

No significant change

Significant decrease

Change in nitrate concentrations

Groundwater

Samples for groundwater are sealed in glass ampoules 

in the field prior to shipping to the laboratory for 

determination of the recharge date. Photograph by U.S. 

Geological Survey.

Changes in Nitrate Concentrations in 

Groundwater

Nitrate concentrations increased significantly in 495 wells 

first sampled during 1988–1995, and then resampled during 

2001–2004. Median concentration of nitrate across all wells 

increased significantly from 3.2 to 3.4 mg/L; changes at 

individual wells were highly variable and ranged from 0 to 

33 mg/L. In shallow groundwater beneath agricultural areas, 

the median concentration of nitrate increased from 4.8 to 

5.7 mg/L, whereas in deep groundwater sampled in major 

aquifer studies, the median concentration increased from 1.2 to 

Figure 8-15. Nitrate concentrations increased in 7 of 24 groundwater study 

areas resampled after about a decade. (Modified from Rupert, 2008.) 

1.5 mg/L. The proportion of wells with nitrate concentrations 

greater than the USEPA Maximum Contaminant Level (MCL) 

was 16 percent at the start of the assessment period and 

increased to 21 percent of the wells sampled about a decade 

later. Nearly 90 percent of the wells in which concentrations 

of nitrate were greater than the MCL were completed in 

shallow aquifers beneath agricultural land-use study areas. 

The concentrations of nitrate increased significantly in 7 of 

the 24 groundwater study areas that were resampled (Rupert, 

2008). Nitrate concentrations decreased in only one study area, 

the Willamette Valley of Oregon (fig. 8-15). 
148 Nutrients in the Nation’s Streams and Groundwater, 1992–2004

9 0

0

0

0

0

Date of sample collection

1 1940 1950 1960 1970 1980 1990 2000 2010

0

20

40

60

80

Nitrate concentration, in milligrams per liter as N

1930 1940 1950 1960 1970 1980 1990 2000 201

0

2

4

6

8

Estimated date of recharge

Nitrogen fertilizer input, in millions of tons

A.

0

4

8

12

14

10

6

2

B.

3 0

Shallow land use wells

Nitrogen fertilizer input

Deep major aquifer wells

Figure 8-16. Estimates of groundwater recharge dates for individual samples show that nitrate concentrations have generally 

increased since about 1975. Although the samples were collected during a narrow range in time (A), the recharge dates represented by 

these data span many decades (B). Comparison between observed concentration of nitrate in groundwater and nitrate loading at the 

land surface would be difficult without an appraisal of the recharge dates of the sampled groundwater. (Modified from Rupert, 2008.)

Estimates of groundwater recharge dates 

for individual samples also show that nitrate 

concentrations generally have increased 

since about 1975. Data on concentrations of 

environmental tracers were used to estimate 

the date that sampled water entered the aquifer 

(Plummer and Friedman, 1999). Nitrate 

concentrations plotted at the date the sample was 

collected (fig. 8-16A) and nitrate concentrations 

plotted at the date the water recharged the 

aquifer (fig. 8-16B) provide completely different 

perspectives of how concentrations vary over 

time. Although the samples were collected within 

a very narrow range in time (1993–2003), the 

recharge dates for these samples span many 

decades. The recharge dates show that water in 

samples collected from 1993 to 2003 took from 

years to decades to travel from the water table 

to a well intake. The wide range in travel times 

from recharge areas to the wells reflects the wide 

range in hydrogeologic conditions across the 

Nation. The data also show that in addition to 

increasing over time, elevated concentrations of 

nitrate appear in the shallow wells (blue symbols) 

as early as the 1950s and 1960s, whereas 

elevated concentrations in the deep major aquifer 

wells (red symbols) do not appear until the 

1970s (fig. 8-16B). 

The appraisal of the recharge dates of the 

sampled groundwater shows that increases in 

nitrate concentrations over time are consistent 

with trends in historical fertilizer use in the 

United States. Fertilizer use in the United States 

increased dramatically starting about 1950, 

primarily because of the production of synthetic 

nitrogen fertilizer. Nitrate concentrations in 

groundwater increased in response to the increase 

in the use of nitrogen fertilizer (fig. 8-16B), 

although the fertilizer use appears to have 

increased more rapidly in the 1960s and 1970s 

than did nitrate concentrations in most of the 

samples collected during this period. Data for 

domestic and monitoring wells in a regional 

aquifer in Wisconsin show that an increase 

in nitrate concentrations in groundwater is 

strongly correlated to estimates of nitrate input 

from fertilizer use (Tesoriero and others, 2007) 

(fig. 8-17). The best fit between fertilizer use 

and groundwater concentrations assumes that 

20 percent of the nitrate in the fertilizer is 

leached to groundwater. 

Nitrate is stable and persistent in oxic 

water (water in which oxygen is present at 

concentrations greater than 0.5 mg/L), but 

nitrate concentrations are commonly very low in 

reduced water because denitrification by bacteria 
Chapter 8

Changes in Nutrient Concentrations: Past and Predicted 149

1950 1960 1970 1980 1990 2000

0

5

10

15

20

Estimated date of recharge

Nitrate concentration in recharge,

in milligrams per liter as N

Estimated nitrate concentration

in recharge based on fertilizer use

Monitoring wells

Median concentration in domestic wells

52

-40

-20

0

20

40

Oxic Mixture of oxic

and reduced

Reduced

Change in nitrate concentrations between

near-decadal sampling events,

in milligrams per liter as N

Redox condition

318

41

90th percentile

Median

Data outside 90th

or 10th percentile

41

75th percentile

25th percentile

10th percentile

Number of wells

EXPLANATION

Figure 8-19. Changes in nitrate concentrations 

were much greater in shallow groundwater than 

in deep groundwater because the shallow water 

is younger and reflects the most recent changes 

in activities and nutrient management practices at 

the land surface. 

-20

0

10

20

30

40

Change in nitrate concentrations between

near-decadal sampling events,

in milligrams per liter as N -10

-30

-40

Well depth, in feet

0 200 400 600 800 1,000

transforms the nitrate to nitrogen gas (see Chapter 2, 

Nutrient Primer). Changes in nitrate concentrations in 

groundwater over time can be very large in oxic waters, 

but changes are typically minimal to negligible in reduced 

waters because of denitrification (fig. 8-18). In most wells 

sampled, the redox conditions in the aquifers were stable 

throughout the sampling period; in a few wells, however, 

nitrate concentrations increased because of a transition from 

reduced to oxic conditions. Short-term changes in aquifer 

redox conditions can occur because of transient conditions, 

such as changes in the water-table elevation or contaminant 

loading; long-term changes from reduced to oxic conditions 

can occur because of depletion of the organic carbon or 

reduced iron minerals in the aquifer sediments.

Figure 8-17. An increase in nitrate concentrations in 

water from domestic and monitoring wells in a regional 

aquifer in Wisconsin parallels the increase in nitrate in 

recharge estimated to have resulted from the effects of 

fertilizer use. (Modified from Tesoriero and others, 2007.)

Figure 8-18. Changes in nitrate concentrations are much larger 

in oxic groundwater than in water under either reduced or mixed 

redox conditions. Changes in nitrate concentrations in reduced 

water are negligible because most of the nitrate has been 

transformed by denitrifying bacteria. (Modified from Rupert, 2008.)

Change Varies by Depth and Well Type

Changes in nitrate concentrations in wells are 

dependent on the position of the wells in the aquifer. Nitrate 

concentrations are typically highest and change most 

rapidly in the shallow part (that is, the upper part) of an 

aquifer. Water in the shallow part of the aquifer is young, 

and it reflects variations in nitrate inputs that may result 

from recent changes in activities and nutrient management 

practices at the land surface. Groundwater that lies deeper in 

the aquifer usually is older and is a composite that averages 

changing nitrate inputs over time. As a consequence, 

changes in nitrate concentrations were much greater in 

shallow groundwater than in deep groundwater (fig. 8-19). 
150 Nutrients in the Nation’s Streams and Groundwater, 1992–2004

It is important to monitor water quality in various well 

types in an aquifer because the apparent rate of change in 

water quality will depend on the type of well, and hence depth 

interval, sampled. Typically, each type of well—monitoring, 

domestic supply, or public supply—taps a different depth 

interval in the aquifer system, so that samples collected from 

each type of well reflect a different part of the history of 

nitrogen input in that region. The importance of considering 

the depth and type of well sampled is illustrated by comparing 

nitrate concentrations in two contrasting sand and gravel 

aquifers: the surficial aquifer on the Delmarva Peninsula of 

Delaware, Maryland, and Virginia (fig. 8-20A), and in the 

eastern San Joaquin Valley of California (fig. 8-20B). Both 

areas are intensively and extensively farmed, and nitrogen 

fertilizer inputs in both increased about 5-fold from 1950 to 

the 1980s. Water moves at similar rates in both aquifers, but 

the aquifer on the Delmarva Peninsula is much thinner than 

the eastern San Joaquin Valley aquifer, and thus the distance 

that recharge water must travel from the land surface to 

the well intakes is much shorter. Concentrations of nitrate 

are most variable and highest near the water table, and 

concentrations decrease with depth as nitrate is increasingly 

mixed with older, low nitrate water with increased distance 

from the water table. The following are important points 

when comparing nitrate concentrations in groundwater in the 

Delmarva Peninsula and San Joaquin Valley aquifers:

• In both the Delmarva Peninsula and eastern San 

Joaquin Valley study areas, concentrations of nitrate 

were high in water from shallow monitoring wells in 

agricultural areas (Burow and others, 2008a; Debrewer 

and others, 2008), with median concentrations above 

the MCL for nitrate of 10 mg/L during 2001–2003 

(fig. 8-20). During the study period, median nitrate 

concentrations in oxic groundwater near the water 

table increased from 8.3 to 12 mg/L on the Delmarva 

Peninsula and from 11 to 14 mg/L in the eastern San 

Joaquin Valley. The highest concentrations and greatest 

changes in concentration reflect the short travel time 

from the land surface to the well.

• The median concentration of nitrate in domestic 

wells on the Delmarva Peninsula, which are at an 

intermediate depth, increased significantly from 9 to 

12 mg/L from 1988 to 2001 (fig. 8-20C). Domestic wells 

in the eastern San Joaquin Valley are more than 100 feet 

deeper than those on the Delmarva Peninsula, and nitrate 

concentrations were lower and increased more slowly, 

from 6.1 to 7.3 mg/L from the period 1993–1995 to 

2001–2002. Water produced by most of the Delmarva 

Peninsula wells entered the aquifer within the last decade. 

In the eastern San Joaquin Valley, however, water tapped 

by domestic wells was older, likely entering the aquifer 

more than a decade ago. The concentrations in the San 

Joaquin wells therefore reflect the lower rate of fertilizer 

use prior to the 1980s (Spurlock and others, 2000; Burow 

and others, 2008a).

• The median concentration of nitrate of 5.5 mg/L in 

the deepest wells on the Delmarva Peninsula—the 

public supply wells—was lower than concentrations in 

the shallower part of the system. Nitrate concentrations 

in public-supply wells in the eastern San Joaquin Valley 

were lower than those in the Delmarva Peninsula 

and increased only slightly, from 3.4 to 3.7 mg/L, 

from the 1990s to the period 2001–2002 (fig. 8-20C). 

Concentrations in the public‐supply wells in the eastern 

San Joaquin Valley were also much lower than those 

in the shallower domestic wells. The average age of 

groundwater in the Delmarva Peninsula public-supply 

wells is generally less than 20 years old (Ferrari, 2002), 

whereas the public‐supply wells in the eastern San 

Joaquin Valley integrate water with ages ranging from 

several decades to several hundred years (Burow and 

others, 2008b). 

In general, concentrations of nitrate will change more 

slowly in public-supply wells than in other wells because of 

their relatively greater depth and because their generally longer 

intake screens integrate groundwater with a wide range of ages, 

and hence, nitrate input history. As a result, in some aquifers, 

concentrations of nitrate in public-supply wells may only 

change slightly during the same period in which concentrations 

at shallow depths are increasing rapidly. Nitrate concentrations 

are likely to increase in public-supply wells during the next 

decade in oxic parts of these aquifers as the water with high 

concentrations now being tapped by domestic wells moves 

deeper into the aquifer. 
Chapter 8

Changes in Nutrient Concentrations: Past and Predicted 151

Fig_8-20_XSECS.ai

Groundwater age, in years 

since recharged

5

10

15

Agricultural

Confining unit

Surficial aquifer

Urban

publicsupply

well

domestic

well monitoring

well

Forest

Water table

0

25

50

75

100

Depth below land surface, in feet

Surficial aquifer on the Delmarva Peninsula, 

Delaware, Maryland, and Virginia

A.

Eastern San Joaquin Valley,

California

B.

Urban

Agricultural

monitoring

well

10

20

60

200

Groundwater age, in years since recharged

domestic

well

public-supply

well

Water table

25

50

75

100

125

150

175

200

225

250

275

Depth below land surface, in feet

325

300

0

Nitrate concentration,

in milligrams per liter as N

<5

5–10

>10 

EXPLANATION

Unsaturated Zone

Direction of 

groundwater flow

Maximum Contaminant Level

Public-supply wells

Domestic wells

Shallow

 monitoring wells

15

10

5

0

15

10

5

0

15

10

5

0

15

10

5

0

15

10

5

0

15

10

5

0

Median nitrate concentration, in milligrams per liter as N

2001–

2002

1990s

no data

2000

2001–

2002

1993–

1995

2001–

2003

1988

Eastern San Joaquin Valley

2001–

2002

1993–

1995

Surficial aquifer on the

Delmarva Peninsula

1988 2001–

2003

C.

Figure 8-20. It is critical to monitor water 

quality in various types of wells in an aquifer 

because the apparent rate of change in 

chemical characteristics will depend on the 

type of well, and hence, the depth interval 

sampled. Median nitrate concentrations in 

domestic wells increased significantly in 

both the Delmarva Peninsula (A) and the 

eastern San Joaquin Valley (B) during the 

sampling period (C), whereas the change in 

concentrations from the 1990s to 2001–2002 

was insignificant in the relatively deep publicsupply wells in the eastern San Joaquin 

Valley (C). Concentrations of nitrate exceeded 

the Maximum Contaminant Level of 10 mg/L 

(represented by the red line) in most of the 

shallow monitoring wells in both areas, as well 

as in the slightly deeper domestic wells in the 

Delmarva Peninsula (C). 
152 Nutrients in the Nation’s Streams and Groundwater, 1992–2004

Datum is sea level

Vertical scale greatly exaggerated

0 500 1,000 METERS

EXPLANATION

Well screen location and

estimated groundwater 

recharge date based on

environmental tracer

1987 Groundwater recharge 

dates based on 

groundwater flow 

models

1975

390

396

402

408

414

420

426

432

Altitude, in meters

Otter Tail River

Water table

Well cluster Land surface

Otter Tail River, Minnesota

1987

1987

1948

1958

1972 1985

1945

1935

1955

1965

1975 1987

1970

1973

1948

<1945

Glacial

till

Boundary (dashed where inferred)

Nitrate Contamination in Shallow 

Groundwater—A Legacy Issue

Evaluations of NAWQA data for streams and wells 

during the first decade of the Program show that nutrient 

concentrations have increased rather than decreased in 

many study areas, especially in groundwater. In addition, 

concentrations of nitrate in young, shallow groundwater in 

agricultural areas are much higher than concentrations in 

the deep resource usually used for drinking water. These 

observations, along with the potential for this reservoir of 

shallow, high-nitrate groundwater to move toward valued 

water resources, yield two important implications: first, 

concentrations of nitrate in deep groundwater used as a 

source of drinking water, and in streams with appreciable 

groundwater inputs, are likely to continue to increase in 

many areas of the country under current nutrient management 

practices; and second, there may be a significant lag time 

between the implementation of improved nutrient management 

practices and resulting decreases in nutrient concentrations.

As precipitation and irrigation waters infiltrate the soil, 

they move vertically downward and usually reach the water 

table in a matter of weeks to months. Once at the water table, 

this recharged water enters the groundwater system and flow 

directions gradually change from near vertical to horizontal 

until the water nears streams and lakes, where the flow is 

generally upward. Because groundwater flow rates typically 

are slow, on the order of a few tens to a few hundreds of feet 

per year, it can take decades or even centuries for groundwater 

to move from the most distant recharge areas to the discharge 

areas at wells, streams, lakes, or the coast. Consequently, most 

surficial aquifers appear stratified, with younger water near 

the surface and older water at progressively greater depths, 

and the aquifer system may contain water that has entered the 

system at various times over the last 50–100 years or more 

(fig. 8-21). During this same period, particularly in the post 

World War II era, 1945–2004, nitrate use in the United States 

in the form of commercial fertilizer increased more than 

20-fold, from about 0.6 to about 13 million tons annually. 

More than 50 percent of the nitrate applied to the land surface 

may leach to groundwater (Tesoriero and others, 2007), and in 

oxic aquifers that nitrate can persist indefinitely. NAWQA has 

sampled groundwater at various depths and distances along 

flow paths to determine past nitrate inputs and the potential for 

nitrate to move deeper into aquifers to the intervals at which 

water-supply wells are screened (Böhlke and Denver, 1995; 

Puckett and Cowdery, 2002; Puckett and Hughes, 2005; Green 

and others, 2008; McMahon and others, 2008a; Steele and 

others, 2008). This approach has allowed NAWQA to examine 

the potential for groundwater nitrate to discharge to surface 

waters now and in the future. At selected sites, these data and 

related findings were used to construct numerical models that 

can be used to evaluate possible future scenarios.

Figure 8-21. Most surficial aquifers appear stratified, with young water near the surface and older water at progressively 

greater depths. In a glacial outwash aquifer in Minnesota, groundwater recharge dates ranged from 1987 near the water 

table to 1935 in the deepest and most distal portion of the aquifer. (Modified from Puckett and Cowdery, 2002.)
Chapter 8

Changes in Nutrient Concentrations: Past and Predicted 153

Fig_8-22_NJ_Glassboror_GW.ai

1940 1960 2000 2040 2080 2100

0

5

1

2

3

4

1980 2020 2060

Nitrate concentration, in milligrams per liter as N

Public-supply wells

Gradual reduction

Immediate ban

Historical input

Fixed input

Future Changes in Nitrate Transport 

to Drinking-Water Wells

Nitrate concentrations generally are higher 

in wells that are screened in the shallow part of 

an aquifer than in the deeper parts, especially 

in agricultural settings. In view of the fact that, 

on average, 40 percent of the land within a 

500 meter radius (1,640 feet) circle around major 

aquifer wells is agricultural, the fate of the high 

levels of nitrate in shallow groundwater is an 

important concern for domestic well owners and 

public well operators. A consequence of the high 

nitrate concentrations already present in shallow 

groundwater is that even if nitrate inputs are 

decreased, concentrations in deep groundwater 

are expected to increase as the contaminated 

young shallow water moves downward.

The impact of the lag time between 

changes in nitrate inputs at the land surface and 

consequent changes in nitrate concentrations 

in groundwater was evaluated for an aquifer 

near Glassboro, New Jersey, using numerical 

simulation techniques (Kauffman and others, 

2001). Given the concentrations of nitrate in the 

aquifer in 2000, even with an immediate ban 

on nitrate inputs, it would take about 30 years 

to reduce concentrations in public-supply wells 

below the background level (fig. 8-22). Under 

a more likely scenario of a gradual reduction in 

input concentrations to near zero over 50 years, 

nitrate concentrations in public-supply wells 

would continue to rise for about 10 years 

before declining gradually over a period of 

50 to 70 years. The lag time between changes 

in nitrate input at the surface and changes in 

nitrate concentrations at the well predicted by 

the model for the aquifer in southern New Jersey 

is the result of a suite of factors including the 

hydrogeologic and geochemical conditions of 

the aquifer.

The lag time between changes in nitrate 

inputs at the land surface and consequent changes 

in nitrate concentrations in groundwater from 

public-supply wells can range from several 

years to hundreds of years, depending on the 

hydrogeologic setting and hydrologic conditions 

within the aquifer. Detailed field studies were 

used to construct numerical models in four 

contrasting hydrogeologic settings: a carbonaterock aquifer (the Floridan aquifer system, 

Tampa, Florida); a thin unconsolidated sand 

Figure 8-22. At the rate of nitrate input to shallow groundwater in 2000 in 

this study area in southern New Jersey, concentrations of nitrate in publicsupply wells are predicted to continue to increase in the future. Even if nitrate 

use is reduced, it may take decades for groundwater concentrations to reach 

pre-commercial fertilizer levels. (Modified from Kauffman and others, 2001.)

Urban encroachment may result in public supply wells withdrawing 

groundwater that recharged through agricultural lands. Photograph by Lynn 

Betts, U.S. Department of Agriculture, Natural Resources Conservation Service.

and gravel aquifer (Glacial aquifer system, Woodbury, Connecticut); 

and two thick unconsolidated sand and gravel aquifers (the Central 

Valley aquifer system, Modesto, California, and the High Plains 

aquifer system, York, Nebraska) (McMahon and others, 2008b). 

Using the same nitrate input characteristic of historical changes in 

fertilizer use (Burow and Green, 2008), simulations showed that 

nitrate concentrations in a public-supply well completed in the thin 

unconsolidated sand and gravel aquifer responded most rapidly, 
154 Nutrients in the Nation’s Streams and Groundwater, 1992–2004

reaching the MCL of 10 mg/L in less than 10 years after 

nitrate in recharge reached that level (fig. 8-23A). It took 

more than a decade for nitrate concentrations in the well in 

the carbonate-rock aquifer to reach the MCL, whereas the 

wells in the thick unconsolidated sand and gravel aquifers 

took more than 60 years to reach concentrations above the 

MCL. These results illustrate the effects of the physical 

features of the aquifer systems alone, as denitrification was 

not accounted for in these simulations. 

The nitrate concentrations in the four different 

hydrogeologic settings also responded differently to 

simulated urbanization. Simulations were done in which 

the proportion of agricultural land in the recharge area 

of the well decreased at a rate of 2 percent annually 

beginning in the 1970s to represent loss to urban 

encroachment (McMahon and others, 2008b). Because 

most of the water reaching the wells in the carbonaterock and thin unconsolidated sand and gravel aquifers is 

less than 15 years old, simulated concentrations peak and 

begin to decline after a delay of 6 to 12 years (fig. 8-23B). 

The response to changes in nitrate inputs in the thick 

unconsolidated sand and gravel aquifers was much more 

gradual, and peak concentrations were much lower 

because a significant proportion of water reaching these 

wells is old.

Adding denitrification in selected aquifer zones 

to the models increased lag times slightly in two of the 

four systems. Removal of nitrate by denitrification may 

decrease the response of groundwater concentrations to 

nitrate input in the carbonate-rock aquifer (Crandall and 

others, 2009), and in the unconsolidated sand and gravel 

aquifer of the High Plains aquifer system (Clark and 

others, 2007). The lack of attenuation by denitrification 

in the unconsolidated sand and gravel aquifers of the 

Central Valley, California (Burow and others, 2008b), 

and Woodbury, Connecticut (Starn and Brown, 2007), is 

attributed to the predominantly oxic conditions and (or) 

low denitrification rates. In aquifers with chemically more 

reduced conditions, changes in nitrate input would be 

buffered by rapid denitrification, and nitrate concentrations 

in public-supply wells would likely not change. Even in 

aquifers in which the reactants required for denitrification 

reactions are in limited supply and the reactions are very 

slow, long travel times may allow appreciable amounts 

of nitrate to be removed before the water reaches a well 

intake. In some aquifers, however, the reactants required 

for denitrification will gradually be depleted, and nitrate 

will move deeper into the aquifer and persist longer, 

creating a greater problem in the future.

Fig_8-23_Future_TANCruns_double.ai

Nitrate in well in urban area—2% annual loss of agricultural land

0

5

10

15

Nitrate concentration,

in milligrams per liter as N

20

Nitrate in 

recharge

Time, in years

0 20 40 60 80 100

0

5

10

15

0 20 40 60 80 100

Nitrate concentration,

in milligrams per liter as N

20

Nitrate in well in agricultural area

Nitrate in recharge

Time, in years

Thin unconsolidated sand and gravel—Glacial 

Carbonate rock—Floridan 

Thick unconsolidated sand and gravel—Central Valley

Thick unconsolidated sand and gravel—High Plains 

Time-series nitrate concentration in

well in selected principal aquifers

6 years

12 years

26 years

30 years

A.

B.

Maximum

Contaminant

Level

Maximum

Contaminant

Level

Lag time

EXPLANATION

Figure 8-23. Model simulations of the effects of physical 

features of four aquifer systems on nitrate transport in 

groundwater show that nitrate concentrations in public-supply 

wells in carbonate-rock and thin unconsolidated sand and 

gravel aquifers respond quickly to increases in nitrate input (A) 

and show a delay of 6 to 12 years before concentrations begin 

to decline when nitrate input concentrations were decreased 

due to loss of agricultural land in the recharge area (B). The 

response to changes in nitrate inputs in deep alluvial aquifers 

was much more gradual, and peak concentrations were much 

lower. (Modified from Burow and Green, 2008; and McMahon 

and others, 2008b.) 
Chapter 8

Changes in Nutrient Concentrations: Past and Predicted 155

Fig_8-24_GW_age_alt.ai

Stream

Water table

Direction of flow

Groundwater age

Young Old

EXPLANATION

1940 1960 2000 2040 2080 2100

0

5

1

2

3

4

1980

Streams

Nitrate concentration, in milligrams per liter as N

2020 2060

Historical input

Fixed input

Gradual reduction

Immediate ban

Future Changes in Nitrate in 

Streams

The slow rate at which nitrate is transported 

in groundwater also can result in a considerable 

lag time between changes in nitrate inputs at the 

land surface and changes in nitrate concentrations 

in groundwater discharging to streams. As 

groundwater reaches streams, the stratified age 

distribution results in younger water with higher 

nitrate concentrations discharging along the 

stream margins and older water containing lower 

nitrate concentrations discharging towards the 

center of the stream (fig. 8-24). Consequently, 

the concentration of nitrate in a stream is a 

mixture of waters of varying age and nitrate 

concentration. However, as the older groundwater 

is replaced by younger water containing higher 

concentrations of nitrate, the concentration of 

nitrate in discharging groundwater is projected to 

become more uniform across the entire width of 

the stream, and concentrations in the stream 

will increase.

A numerical model of the aquifer near 

Glassboro, New Jersey, illustrates the impact 

of the lag time between changes in nitrate 

inputs at the land surface and changes in 

nitrate concentrations in streams (Kauffman 

and others, 2001). At the current rate of nitrate 

input, concentrations in streams in this area are 

predicted to continue to increase in the future 

(fig. 8-25). Under a scenario of gradual reduction 

in input nitrate concentrations over 50 years, 

it may take decades for concentrations in the 

streams to reach pre‐commercial fertilizer 

levels. Even with an immediate decrease in 

nitrate input to background levels at the water 

table, it would take about 30 years to reduce 

concentrations below background in the streams 

(fig. 8-25). The actual response of a specific 

stream to nitrate inputs from groundwater 

would depend on a large number of factors, 

including the presence of riparian buffer zones 

and streambed sediments capable of removing 

nitrate from the groundwater (see Chapter 5, 

Exchange of Nutrients between Surface Water 

and Groundwater). 

Figure 8-24. When groundwater discharges to a stream, younger waters 

discharge along the stream margins, and older waters are focused toward 

the center. Because older groundwater presently contains lower nitrate 

concentrations than younger groundwater, this creates a pattern of 

decreasing nitrate concentrations towards the center of the stream. In the 

future, as groundwater with higher nitrate concentrations replaces older, 

low-nitrate groundwater, nitrate concentrations will increase in the stream. 

(Modified from Modica and others, 1998.) 

Figure 8-25. Concentrations of nitrate in streams near Glassboro, New 

Jersey, are predicted to continue to increase in the future at the current 

rate of nitrate input. Even in areas where nitrate use is now being reduced, 

it may take decades for concentrations of nitrate in streams to decrease 

to levels present prior to the use of commercial fertilizers. (Modified from 

Kauffman and others, 2001.)
156 Nutrients in the Nation’s Streams and Groundwater, 1992–2004

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Livingston, July 1972 through April 1974: Austin, Tex., 

Texas Parks and Wildlife Department, Inland Fisheries and 

Environmental Branches, 47 p.

U.S. Census Bureau, 2001, Population change and 

distribution, 1990 to 2000: U.S. Census Bureau Report 

C2KBR/01-2, 7 p., accessed June 17, 2010, at http://www.

census.gov/prod/2001pubs/c2kbr01-2.pdf.

U.S. Department of Agriculture, 2008, Data Sets, Fertilizer 

Use and Price, accessed June 17, 2010, at http://www.ers.

usda.gov/Data/FertilizerUse/.

U.S. Environmental Protection Agency, 2003, National air 

quality and emissions trends report: U.S. Environmental 

Protection Agency Report EPA 454/R-03-005, 190 p., 

accessed June 17, 2010, at http://www.epa.gov/air/airtrends/

aqtrnd03/.

U.S. Environmental Protection Agency, 2004, Risk 

assessment evaluation for concentrated animal feeding 

operations: U.S. Environmental Protection Agency Report 

EPA/600/R-04/042, 138 p., accessed June 17, 2010, at 

http://www.epa.gov/nrmrl/pubs/600r04042/600r04042.pdf.
168 Nutrients in the Nation’s Streams and Groundwater, 1992–2004

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Glossary

Agricultural stream A stream draining a 

watershed with more than 50 percent agricultural 

land (cropland or pasture) and 5 percent or less of 

urban land.

Algae Chlorophyll-bearing nonvascular plants, 

primarily aquatic species that have no true roots, 

stems, or leaves; most algae are microscopic, but 

some species can be as large as vascular plants.

Algal bloom Sudden spurts of algal growth, 

which can affect water quality adversely and 

indicate potentially hazardous changes in local 

water chemistry. 

Ammonia A compound of nitrogen and 

hydrogen (NH3) that is a common by-product 

of human and animal waste. Ammonia readily 

converts to nitrate in soils and streams.

Anoxic Groundwater that has no dissolved 

oxygen or a very low concentration of dissolved 

oxygen (that is, less than 0.5 milligram per liter).

Aquatic health guidelines Specific levels of 

water quality which, if reached or exceeded, 

may adversely affect aquatic life. These 

are nonenforceable guidelines issued by a 

governmental agency or other institution.

Aquifer A water-bearing layer of soil, sand, 

gravel, or rock that will yield usable quantities of 

water to a well. 

Atmospheric deposition The transfer of 

substances from the air to the surface of the 

Earth, either in wet form (rain, fog, snow, dew, 

frost, hail) or in dry form (gases, aerosols, 

particles).

Background concentration A concentration 

of a substance in a particular environment that 

is indicative of minimal influence by human 

(anthropogenic) sources.

Base flow Hydrologic regime in streams, 

following extended periods of minimal 

precipitation, during which streamflow is derived 

primarily from groundwater discharge. 

Benthic Refers to plants or animals that live on 

the bottom of lakes, streams, or oceans.

Best management practice (BMP) An 

agricultural practice that has been determined to 

be an effective, practical means of preventing or 

reducing nonpoint source pollution.

Bioassessment Use of organisms to evaluate 

environmental quality.

Biomass The amount of living matter, in the 

form of organisms, present in a particular habitat, 

usually expressed as weight per unit area.

Blue-baby syndrome A condition that can 

be caused by ingestion of high amounts of 

nitrate resulting in the blood losing its ability to 

effectively carry oxygen. It is most common in 

young infants and certain elderly people. Also 

called methemoglobinemia.

Carbonate rocks Rocks (such as limestone 

or dolostone) that are composed primarily 

of minerals (such as calcite and dolomite) 

containing the carbonate ion (CO3

2-).

Chlorophyll A green photosynthetic pigment 

found in most plants, algae and cyanobacteria. 

Confined aquifer An aquifer that is completely 

filled with water under pressure and that is 

overlain by material that restricts the movement 

of water. 

Conservation tillage Planting and growing 

crops with reduced disturbance of the surface 

soil.

Cubic foot per second (ft3/s, or cfs) Rate 

of water discharge representing a volume of 

1 cubic foot passing a given point during 1 

second, equivalent to approximately 7.48 gallons 

per second or 448.8 gallons per minute or 

0.02832 cubic meter per second.

Denitrification A process by which oxidized 

forms of nitrogen such as nitrate (NO3

-

) are 

reduced to form nitrites, nitrogen oxides, 

ammonia, or free nitrogen, commonly brought 

about by the action of denitrifying bacteria and 

usually resulting in the escape of nitrogen to 

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

Discharge Rate of fluid flow passing a given 

point at a given moment in time, expressed as 

volume per unit of time.

Domestic well A privately-owned well that 

typically serves one home and supplies water for 

human consumption and other homeowner uses.

Drinking-water standard or guideline A 

threshold concentration in a public drinkingwater supply, designed to protect human 

health. As defined here, standards are U.S. 

Environmental Protection Agency regulations 

that specify the maximum contamination levels 

for public water systems required to protect the 

public welfare; guidelines have no regulatory 

status and are issued in an advisory capacity. See 

also MCL, water-quality guidelines, and waterquality standards.

Ecoregion An area of similar climate, landform, 

soil, potential natural vegetation, hydrology, or 

other ecologically relevant variables.

Effluent Outflow from a particular source, such 

as a stream that flows from a lake or liquid waste 

that flows from a factory or sewage-treatment 

plant.

Eutrophication The process by which water 

becomes enriched with plant nutrients, most 

commonly phosphorus and nitrogen.

Horton overland flow Occurs when the rate 

of rainfall on a surface exceeds the rate of 

infiltration and any depression storage has 

been filled. 

Hydraulic head Hydraulic head is an indicator 

of the total energy available to move groundwater 

through an aquifer. Hydraulic head is measured 

by the height to which a column of water will 

stand above a reference elevation (or “datum”), 

such as mean sea level.

Hydric soil Soil that formed under conditions 

of saturation, flooding, or ponding long enough 

during the growing season to develop anoxic 

conditions in the upper part of the soil profile.

Hydrograph Graph showing variation of water 

elevation, velocity, streamflow, or other property 

of water with respect to time. 

Hydrologic system The assemblage of 

pathways by which water travels as it circulates 

beneath, at, and above the Earth’s surface 

through various processes such as precipitation, 

runoff, evaporation, infiltration, transpiration, and 

groundwater flow.

Hyporheic zone A zone in streambeds in which 

active exchange of groundwater and surface 

water can occur and where many biogeochemical 

processes can remove nitrate and other 

contaminants.

Hypoxia/Hypoxic waters Waters with dissolved 

oxygen concentrations of less than 2 parts per 

million, the level generally accepted as the 

minimum required for most marine life to survive 

and reproduce. 

Impermeability The incapacity of a rock to 

transmit fluid. 

Intrinsic susceptibility A measure of the ease 

with which a contaminant in water can enter and 

move through an aquifer. It is a characteristic of 

the aquifer and overlying material and hydrologic 

conditions, and is independent of the chemical 

characteristics of the contaminant and its sources. 

See also vulnerability.

Irrigation return flow The part of irrigation 

water applied to the land surface that is not 

consumed by evapotranspiration or uptake by 

plants and that either infiltrates downward to an 

aquifer or finds its way to a surface-water body.

Karst A type of topography that results from 

dissolution and collapse of carbonate rocks such 

as limestone and dolomite, and characterized 

by closed depressions or sinkholes, caves, and 

underground drainage. 

Land-use study A network of existing or 

installed shallow wells in an area having a 

relatively uniform land use. These studies 

have the goal of relating the quality of shallow 

groundwater to land use. 

Leaching The removal of materials in solution 

from soil or rock to groundwater; refers to 

movement of pesticides or nutrients from land 

surface to groundwater.

Linear regression A statistical method for 

analyzing and estimating the magnitude of a 

response variable as a function of one or more 

explanatory variables.

Lithology The physical character of a rock 

based on such characteristics as color, structure, 

mineralogical composition, and grain size.

Macroinvertebrate An animal that is large 

enough to be seen without magnification and has 

no backbone or spinal column.
Glossary 171

Macrophyte An aquatic plant growing in or 

near water that is either emergent, submergent, 

or floating. 

Major aquifer A regionally extensive 

subsurface geologic formation or group of 

formations that is used, or has the potential to be 

used, as a significant groundwater resource. 

Major aquifer studies NAWQA investigations 

involving the sampling of 20 to 30 domestic and 

(or) public-supply wells that withdraw water 

from major aquifers. 

Maximum contaminant level (MCL) Maximum 

permissible level of a contaminant in water that 

is delivered to any user of a public water system. 

MCLs are enforceable standards established by 

the U.S. Environmental Protection Agency. See 

also drinking-water standard.

Mean The average of a set of observations, 

unless otherwise specified. 

Median The middle or central value in a 

distribution of data ranked in order of magnitude. 

The median is also known as the 50th percentile. 

Mixed-land-use streams Streams draining 

watersheds in which no single type of land 

use (agricultural, urban, or undeveloped) 

predominates. These include all streams not 

meeting the specific land-use criteria for 

agricultural, urban, or undeveloped streams.

Nitrate An ion consisting of nitrogen and 

oxygen (NO3

-

). Nitrate is a plant nutrient and is 

very mobile in soils. 

Nitrite An ion consisting of nitrogen and 

oxygen (NO2

-

). The nitrite and organic species 

are unstable in aerated water and generally 

constitute a small fraction of the dissolved 

nitrogen species.

Nitrification A microbial process by which 

reduced nitrogen compounds (primarily 

ammonia) are sequentially oxidized to nitrite 

and nitrate.

Nonpoint source A pollution source that cannot 

be defined as originating from discrete points 

such as pipe discharge. Areas of fertilizer and 

pesticide applications, atmospheric deposition, 

manure generation, and natural inputs from 

plants and trees are types of nonpoint 

source pollution. 

Nutrient Element or compound essential for 

animal and plant growth. Common nutrients in 

fertilizer include nitrogen, phosphorus, 

and potassium. 

Oligotrophic stream Nutrient poor stream with 

relatively low primary production

Overland flow The flow of rainwater or 

snowmelt over the land surface toward stream 

channels. After it enters a stream, it 

becomes runoff.

Oxic A term used to describe water in which 

the concentration of dissolved oxygen is greater 

than or equal to 0.5 milligrams per liter.

Permeability A measure of the relative ease 

with which a porous medium can transmit a fluid.

Point source A contaminant source at a discrete 

location such as a discharge pipe, drainage ditch, 

tunnel, well, concentrated livestock operation, or 

floating craft. 

Principal aquifer A regionally extensive aquifer 

or aquifer system that has the potential to be used 

as a source of potable water.

Public-supply well A privately or publicly 

owned well that provides water for public use to: 

(1) community water systems, (2) transient noncommunity water systems, such as campgrounds, 

or (3) non-transient, non-community systems, 

such as schools.

Recharge Water that infiltrates the ground and 

reaches the saturated zone. 

Redox condition As used in this report, redox 

condition refers to the geochemical status or 

position of a groundwater system on a scale 

between very oxidizing and very reducing.

Reduced Groundwater in which concentrations 

of dissolved oxygen are less than 0.5 mg/L, 

and that contains chemical markers for iron 

(concentrations of iron greater than 100 

milligrams per liter) and (or) manganese 

(concentrations of manganese greater than 50 

milligrams per liter) reduction.

Reference site A NAWQA sampling site 

selected for its relatively undisturbed conditions. 

Residence time The amount of time that a 

constituent, particle, organism, or other entity 

spends within a given environmental medium.
172 Nutrients in the Nation’s Streams and Groundwater, 1992–2004

Retrospective analysis Review and analysis 

of existing data in order to address NAWQA 

objectives, to the extent possible, and to aid in 

the design of NAWQA studies. 

Riparian Areas adjacent to rivers and streams 

with a high density, diversity, and productivity 

of plant and animal species relative to nearby 

uplands.

Saturated zone The region in the subsurface in 

which all the interstices or voids are filled with 

water under a pressure exceeding that of the 

atmosphere.

Simulation model A mathematical model 

used to predict the combined effects and (or) 

consequences of one or more processes of 

interest by reproducing these effects using 

mathematical relations and (or) numerical 

techniques, typically through the use of 

computer programs.

Statistical model A model used to represent 

the effects of one or more processes of interest 

by quantitative, probabilistic relations (such as 

regressions) between one or more explanatory 

variables and a particular response variable.

Statistical significance The likelihood 

(commonly expressed as a probability, p) that 

the result of a statistical test may have occurred 

solely by chance. Observations associated with 

p values of 0.05 or less (a 95 percent or greater 

confidence level) are typically deemed to be 

statistically significant, and thus, are unlikely to 

have occurred solely by chance.

Study Unit A major hydrologic system of the 

United States in which NAWQA studies are 

focused. Study Units are geographically defined 

by a combination of ground- and surface-water 

features and generally encompass more than 

4,000 square miles of land area.

Subsurface tile-drain systems Perforated pipes 

that are buried to shallow depths in the ground to 

reduce the water content of poorly drained soils 

and divert shallow groundwater to 

nearby streams.

Surface runoff That part of the runoff which 

travels over the soil surface to the nearest stream 

channel. It is also defined as that part of the 

runoff of a drainage basin that has not passed 

beneath the surface since precipitation.

Taxon (plural taxa) Any identifiable group of 

taxonomically related organisms.

Tile drain See Subsurface tile-drain systems 

Tolerant species Those species that are 

adaptable to (tolerant of) human alterations to the 

environment and that often increase in number 

when human alterations occur. 

Total concentration Refers to the concentration 

of a constituent regardless of its form (dissolved 

or bound) in a sample. 

Total Maximum Daily Load (TMDL) A 

calculation of the highest amount of a pollutant 

that a water body can receive and safely meet 

water quality standards set by the state, territory, 

or authorized tribe.

Total nitrogen The sum of inorganic nitrogen 

(nitrate, nitrite, ammonia) and organic nitrogen.

Total phosphorus The sum of inorganic and 

organic phosphorus.

Unconfined aquifer An aquifer whose upper 

surface is a water table; an aquifer containing 

unconfined groundwater. 

Undeveloped stream A stream draining a 

watershed with 25 percent or less of agricultural 

land and 5 percent or less of urban land.

Unsaturated zone The subsurface region of 

earth materials above the water table in which the 

pore spaces may contain a combination of air 

and water.

Urban stream A stream draining a watershed 

with more than 5 percent of residential, 

commercial, transportation, urban recreational 

areas, and (or) industrial land, and 25 percent or 

less of agricultural land.

U.S. Environmental Protection Agency 

recommended ecoregional nutrient 

criteria Numerical values for phosphorus 

and nitrogen concentrations associated with 

the prevention and assessment of eutrophic 

conditions. These recommended ecoregional 

nutrient criteria represent conditions of surface 

waters that have minimal impacts caused by 

human activities.

Vulnerability The tendency or likelihood for 

contaminants to reach a specified position in 

the groundwater system after introduction at 

some location above the uppermost aquifer. 

The vulnerability of a groundwater resource 
Glossary 173

to contamination depends on its intrinsic 

susceptibility as well as the locations and types of 

sources of naturally occurring and anthropogenic 

contamination, relative location of wells, and the 

fate and transport of the contaminant(s). 

Water-quality criteria Specific levels of water 

quality which, if reached, are expected to render 

a body of water unsuitable for its designated 

use. Commonly refers to water-quality criteria 

established by the U.S. Environmental Protection 

Agency. Water-quality criteria are based on 

specific levels of pollutants that would make the 

water harmful if used for drinking, swimming, 

farming, fish production, or industrial processes. 

Water-quality guidelines Specific levels of 

water quality which, if reached, may adversely 

affect human health or aquatic life. These 

are nonenforceable guidelines issued by a 

governmental agency or other institution.

Water-quality standards State-adopted and 

U.S. Environmental Protection Agency-approved 

ambient standards for water bodies. Standards 

include the use of the water body and the waterquality criteria that must be met to protect the 

designated use or uses. 

Watershed The portion of the surface of the 

Earth that contributes water to a stream through 

overland run-off, including tributaries and 

impoundments. 

Water table The point below the land surface 

where groundwater is first encountered and 

below which the earth is saturated. Depth to the 

water table varies widely across the country. 

Yield The mass of material or constituent 

transported by a river in a specified period of 

time divided by the drainage area of the 

river basin. 
174 Nutrients in the Nation’s Streams and Groundwater, 1992–2004

Abbreviations, Acronyms, and Units of Measure

BMPs Best Management Practices

CAFO Concentrated animal feeding operation

HBSL Health-Based Screening Level

km kilometer

L liter

lb pound

m meter

MCL Maximum Contaminant Level

mg milligram

mi mile

N nitrogen

NAWQA National Water-Quality Assessment Program (USGS)

SPARROW SPAtially Referenced Regression On Watershed attributes

TMDLs Total maximum daily loads

USDA U.S. Department of Agriculture

USEPA U.S. Environmental Protection Agency

USGS U.S. Geological Survey

U.S. United States
For more information concerning the research in this report, contact:

Chief, National Water-Quality Assessment Program

U.S. Geological Survey

413 National Center

12201 Sunrise Valley Drive

Reston, VA 20192

or visit our Web site at:

http://water.usgs.gov/nawqa/nutrients/pubs/circ1350

Acknowledgments

Special thanks to the following individuals and organizations for their contributions:

NAWQA personnel in study areas across the Nation for their contributions of data and 

research results.

NAWQA’s many partners in the governmental and non-governmental sectors who have 

helped guide scientific efforts and ensure that NAWQA information meets the needs of 

local, state, tribal, regional, and national stakeholders.

USGS reviewers:

• Brian G. Katz

• Keith W. Robinson

• Dennis A. Wentz

• Gary L. Rowe, Jr.

External reviewers:

• Susan Holdsworth, USEPA, Office of Wetlands, Oceans, and Watersheds

• Jane Houlihan, Environmental Working Group

• Denise Keehner, USEPA, Office of Wetlands, Oceans, and Watersheds

• Robert L. Kellogg, USDA, Natural Resources Conservation Service

• Jerrell L. Lemunyon, USDA, Natural Resources Conservation Service

• Anne S. Marsh, The Heinz Center

• Jan McGoldrick, PG Environmental, LLC 

• Olga Naidenko, Environmental Working Group

• Roberta Parry, Office of Water

• Marc Ribaudo, USDA, Economic Research Service

• Steven H. Wolfe, Florida Department of Environmental Protection

Technical editor:

• Linda Rogers, USGS; Chester Zenone, Editorial Consultant

Technical Illustration:

• Yvonne Roque, USGS

Layout Production:

• Sharon Wahlstrom, USGS

Graphic Design:

• Bill Gibbs, USGS

Publishing support provided by the U.S. Geological Survey

Publishing Network, Sacramento and Tacoma Publishing Service Centers
Dubrovsky and others— Nutrients in the Nation’s Streams and Groundwater, 1992–2004—Circular 1350