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 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 nitrogen 

and phosphorus 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 the 

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 

Figure 8-15. Nitrate concentrations increased in 7 of 24 groundwater study 

areas resampled after about a decade. (Modified from Rupert, 2008.) 

aquifer studies, the median concentration increased from 1.2 to 

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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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
Publishing support provided by the U.S. Geological Survey

Publishing Network, Sacramento and Tacoma Publishing Service Centers

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:

• Chester Zenone, Editorial Consultant

Graphic design and layout:

• Yvonne Roque, Bill Gibbs, and Sharon Wahlstrom, 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