Chapter 4

Occurrence and Distribution of Nutrients in Streams and Groundwater 57

17-0191_Fig_4-04_SW_Nconc_byLUquart

Agricultural Urban

Low (0.66)

Medium (0.66–3.17)

High (3.17)

Total nitrogen concentration,

in milligrams per liter

Estimated 1997 nitrogen inputs from

fertilizer, manure, and atmosphere,

in pounds per square mile Undeveloped

2,900

2,900–15,000

15,000

Stream sites, by watershed land use

Data on nitrogen inputs not available 

for Alaska and Hawaii.

EXPLANATION

Mixed

Agricultural Urban

Undeveloped Mixed

Streams

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

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

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

0

3,000

1,000

2,000

Streamflow, in cubic feet per second

Streamflow

Iowa River near Rowan, Iowa

Tile drain flow No tile drain flow

Jan Feb Mar Apr May June July Aug Sept Oct Nov

1997

0

20

10

Nitrate concentration,

in milligrams per liter as N

Dec

EXPLANATION

Tile drain nitrate concentration

Iowa River nitrate concentration

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

streams and rivers from agricultural fields. Nitrate concentrations in the 

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

(Kalkhoff and others, 2000).

Concentrations of total nitrogen in agricultural streams 

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

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

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

(in some cases) atmospheric deposition. High concentrations 

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

which facilitate water movement from fields to streams. For 

example, nitrate concentrations in the Iowa River near Rowan, 

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

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

concentrations at many sites in the Northwest, the Northern 

Plains, the Southwest, and California might be influenced by 

large applications of irrigation water. Irrigation return flow 

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

Most of the medium and low concentrations of total nitrogen 

in agricultural areas were at sites in the Southeast and Southern 

Plains, where inputs are more variable. These settings can 

contain substantial amounts of organic matter, which result in 

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

therefore, concentrations can be relatively low in comparison to 

inputs (see Chapter 5, Exchange of Nutrients Between Surface 

Water and Groundwater).

Total nitrogen concentrations downstream from urban 

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

concentrations were scattered from the Northeast through 

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

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

input. The potential effect of wastewater-treatment plants is 

especially large in the arid West, where wastewater-treatment 

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

streams during parts of the year when precipitation is low.

At sites downstream from relatively undeveloped 

watersheds, total nitrogen concentrations generally were low 

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

and in eastern parts of the Midwest were in areas of 

greater atmospheric nitrogen deposition. Medium and high 

concentrations of total nitrogen in the Southwest and western 

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

with greater loads of suspended sediment, which can be a 

source of organic matter containing particulate nitrogen. 

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

where total nitrogen increases along with the concentration of 

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

Contrasts among Undeveloped Watersheds: Forest and 

Rangeland; fig. 4-7).
Chapter 4

Occurrence and Distribution of Nutrients in Streams and Groundwater 59

Tile drains buried at shallow depths reduce the water content 

of poorly drained soils and divert shallow groundwater to 

nearby streams. Photograph by Jeffrey Martin and Nancy 

Baker, U.S. Geological Survey.

The Bighorn River, Wyoming, displays the high concentration of 

suspended sediment characteristic of streams that drain rangelands 

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

Geological Survey.

0

2

4

6

8

10

12

14

Nitrate concentration, in milligrams per liter as N

0

Streamflow, in cubic feet per second

Jan MarFeb Apr May June July Aug Sept Oct Nov Dec

1993

Orestimba Creek near Crows Landing, California

300

600

900

1,200

1,500

Nitrate

concentration

Non-irrigation

season

Non-irrigation

season

Irrigation season

Streamflow

10

1

0.1

Flow-weighted average concentration of 

total nitrogen, in milligrams per liter

Flow-weighted average concentration of 

suspended sediment, in milligrams per liter

10 100 1,000 10,000

Yellowstone River and tributaries, Montana and Wyoming

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

matter containing particulate nitrogen, concentrations of total nitrogen 

were correlated with concentrations of suspended sediment in the 

Yellowstone River and its tributaries, Montana and Wyoming.

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

transport substantial amounts of nitrate to the stream during the spring 

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

0.001

0.01

0.1

1

10

Concentration, in milligrams per liter

Total

nitrogen

Total

phosphorus

Nitrate as N

Streams Groundwater

0.2

0.01

0.05

Rangeland

Forest

0.01

75th percentile

Median

25th percentile

90th percentile

Detection limit

10th percentile

EXPLANATION

Contrasts among Undeveloped Watersheds: 

Forest and Rangeland 

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

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

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

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

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

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

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

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

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

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

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

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

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

and arid regions (Walvoord and others, 2003).

The geographic distribution of total phosphorus 

concentrations in streams is similar to that of total nitrogen, 

with high concentrations found in agricultural streams 

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

of total nitrogen concentrations, distributions of total 

phosphorus concentrations were sometimes relatively lower 

in agricultural streams (more sites in the medium and low 

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

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

phosphorus concentrations were relatively higher in streams in 

undeveloped areas than were total nitrogen concentrations.

Figure 4-8. Rangeland streams have 

higher total phosphorus concentrations 

than streams in forested areas. Rangelands 

are primarily in the semiarid and arid 

Western United States, where phosphorus 

occurs naturally in geologic materials 

and the erosion potential is high. In 

groundwater, nitrate concentrations 

are higher in rangeland areas than in 

forested areas, most likely because high 

concentrations of nitrate can accumulate 

in soils and groundwater in semiarid and 

arid regions. (Modified from Mueller and 

Spahr, 2006.) 
Chapter 4

Occurrence and Distribution of Nutrients in Streams and Groundwater 61

17-0191_Fig_4-09_SW_Pconc_byLUquart.ai

Low (0.05)

Medium (0.05–0.28)

High (0.28)

Total phosphorus concentration,

in milligrams per liter

Estimated 1997 phosphorus 

inputs from fertilizer and manure, 

in pounds per square mile

340

340–2,730

2,730

Stream sites, by watershed land use

Data on phosphorus inputs not available 

for Alaska and Hawaii.

EXPLANATION

Agricultural Urban Undeveloped Mixed

Streams

Agricultural Urban

Undeveloped Mixed

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

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

Agricultural

0 50 100 0 50 100

Dissolved

orthophosphate

Dissolved

phosphorus

Total

Phosphorus

100

Rangeland

and forest Urban

Trinity River below

Dallas, Texas

Percentage of samples

0 50 0 50 100

Concentration, in

milligrams per liter

EXPLANATION

<0.01

0.01–0.1

>0.1

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

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

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

others, 1998).

≤0.024

0.024−0.039

0.039−0.055

0.055−0.071

0.071

EXPLANATION

Phosphorus, in percent by weight

No data

Figure 4-11. Phosphorus is naturally high in some 

geologic materials in western rangelands, where a 

high erosion potential may result in the transport of 

phosphorus-rich sediment to streams. (Data from 

U.S. Geological Survey, 2009a.)

At undeveloped sites, high and medium concentrations of 

total phosphorus were more common than high and medium 

concentrations of total nitrogen in the Northern Plains, 

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

predominant land cover in these areas, whereas forest is the 

predominant land cover in most other undeveloped areas of 

the United States. In rangeland areas, phosphorus commonly 

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

potential is high, which contributes to higher total phosphorus 

concentrations in streams here compared to those in streams 

in other undeveloped, forested areas (see sidebar, Contrasts 

among Undeveloped Watersheds: Forest and Rangeland).

Differences between the distributions of total nitrogen 

and total phosphorus concentrations in streams likely are a 

result of differences in sources—especially natural sources 

of phosphorus as well as its relatively high concentrations 

in wastewater-treatment plant effluent—and differences 

in the processes that transport nitrogen and phosphorus to 

streams. For example, water samples from the Trinity River 

below Dallas, where the streamflow is dominated by effluent 

from regional wastewater-treatment plants, had higher 

concentrations of phosphorus than water samples from other 

streams in the Trinity River Basin, which were affected only 

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

Nonpoint sources of phosphorus in urban areas include runoff 

from lawns and construction sites.
Chapter 4

Occurrence and Distribution of Nutrients in Streams and Groundwater 63

17-0191_Fig_4-12_SW_model_Nconc.ai

≤0.5 0.5–1 1–5 5–10 10

Predicted total nitrogen concentration, in milligrams per liter

EXPLANATION

Extrapolation of Total Nitrogen Streams

Concentrations in Streams to 

Unsampled Areas

The results of simulations made with 

two statistical models—one for agriculturally 

influenced streams and one for nonagricultural 

streams—were aggregated to predict the total 

nitrogen concentrations in streams across 

the conterminous United States (Spahr and 

others, 2010) (see Chapter 3, Extrapolation to 

Unsampled Areas, Development of Models 

for Total Nitrogen Concentrations in Streams). 

Model results for streams predict that the highest 

total nitrogen concentrations should be in areas 

with the highest nonpoint source nitrogen 

inputs, such as agricultural areas in the upper 

Midwest and selected coastal areas of California 

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

predicted in the mountainous regions of the West, 

the Appalachians, and the northern parts of New 

England. Simulated concentrations for much of 

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

sections are less than 1 mg/L. 

The model for agriculturally influenced 

streams explains more than 70 percent of the 

variability of total nitrogen concentration. The 

explanatory factors that represent nitrogen 

sources in the model include atmospheric 

nitrogen input, fertilizer nitrogen input, manure 

nitrogen input, and percentage of undeveloped 

land in the watershed above the sampling site. 

Explanatory factors that represent transport 

to and within streams in the model include 

percentage of sand in soil, presence of subsurface 

drains, long-term average percentage Horton 

overland flow, and mean annual precipitation. 

Areas with greater nitrogen inputs and 

subsurface drainage tend to have greater 

nitrogen concentrations in streams, whereas 

areas with sandy soils, higher precipitation, and 

greater undeveloped areas, have lower nitrogen 

concentrations in streams.

The model for the nonagricultural streams explained 62 percent 

of the total nitrogen variability. The explanatory factors that represent 

nitrogen sources in the model for the nonagricultural sites include a 

nonpoint source term (representing fertilizer, manure, and atmospheric 

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

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

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

surrogate for nitrogen inputs associated with urban settings that are 

otherwise unaccounted for, such as point sources. In nonagricultural 

basins, streams with greater nitrogen inputs and greater population 

in their watersheds tend to have higher total nitrogen concentrations, 

whereas streams with greater streamflow and a higher percentage 

of streamflow derived from groundwater tend to have lower total 

nitrogen concentrations.

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

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

regression models relate the concentrations of total nitrogen at 462 stream 

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

Nutrient Export in Streams

When nutrients are input to the land surface 

from fertilizer application, manure generation, 

atmospheric deposition, or other sources, they 

can be taken up by crops and natural vegetation, 

immobilized in the soil, taken up by aquatic 

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

the atmosphere during denitrification. Any 

remaining nitrogen or phosphorus is potentially 

available for export out of the watershed by 

streams and rivers or by leaching through the 

soil to groundwater. Nutrients lost (exported or 

leached) from the watershed can contribute to 

eutrophication of downstream water bodies and 

economic losses through reduction in crop yields 

or expense for additional fertilizer.

The amount of nitrogen and phosphorus lost 

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

from watersheds to streams—referred to as the 

yield—increases with increasing nutrient inputs, 

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

and yield generally differed among agricultural 

(median yield 972 pounds per square mile), urban 

(median yield 432 pounds per square mile), and 

undeveloped streams (median yield 135 pounds 

per square mile). Agricultural streams had the 

largest yields due to their relatively high 

nutrient inputs. 

Calculating the ratio of the yield to the input 

allows an assessment of the proportion of input 

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

Between 5 and 50 percent of the nitrogen input 

from nonpoint sources was exported out of most 

(69 percent) watersheds (see dashed red lines, 

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

loss can be explained in part by differences 

in agricultural practices, soils, geology, and 

hydrology. In the Southeast, a relatively low 

proportion of nitrogen—usually less than 

25 percent of input—is exported at agricultural 

sites, possibly because of greater amounts 

of denitrification in shallow groundwater 

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

proportion of nitrogen is exported because of 

generally low amounts of precipitation and 

runoff, and possibly highly modified hydrology. 

In contrast, agricultural sites with tile drains in 5 

percent or more of the watershed are more likely 

to export more than 25 percent of the nitrogen 

input than those in watersheds with fewer tile 

drains (fig. 4-13B). The median yield of nitrogen 

from agricultural streams with tile drains was 

slightly more than 3 times larger than the median 

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

Between 5 and 50 percent of the phosphorus 

input from nonpoint sources was exported out of 

46 percent of watersheds. Apparent phosphorus 

export is lower than nitrogen export for many 

sites, possibly reflecting the fact that phosphorus 

is not as mobile in the aquatic system as nitrogen 

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

insoluble, tile drains do not increase phosphorus 

yields as markedly as they do nitrogen yields 

(fig. 4-13C). 

This comparison assumes that 

anthropogenic nonpoint-source inputs and stream 

yield account for all nitrogen and phosphorus 

movement into and out of these watersheds. 

These assumptions do not always hold true. 

For many sites, primarily those in undeveloped 

and urban areas, apparent export is greater than 

50 percent; in some cases, yield even exceeds 

input. The most likely explanation is that some 

natural inputs, such as geologic phosphorus or 

nitrogen fixation, or anthropogenic inputs such 

as effluent from wastewater-treatment plants, 

are being underestimated. For other sites, the 

apparent nutrient export from the watershed is 

less than 5 percent. At these sites, the proportion 

of nutrients lost to other mechanisms, such 

as crop harvesting, incorporation in natural 

vegetation, and immobilization in the soil, 

may be greater; or nutrient inputs may 

be overestimated. 
Chapter 4

Occurrence and Distribution of Nutrients in Streams and Groundwater 65

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

that is exported by a stream as yield (B), as well as the amount of yield (C) is much greater for agricultural streams with tile drains in 

5 percent or more of the watershed than for agricultural streams with fewer tile drains. Because phosphorus is relatively insoluble, tile 

drains do not increase phosphorus yields as markedly as nitrogen yields. (Modified from Mueller and Spahr, 2006.) 17-0191_Fig_4-13_yield_plot_post_review

1

10

100

1,000

10,000

Total nitrogen outflow yield, in pounds per square mile

A.

100 1,000 10,000

Nonpoint nitrogen input, in pounds per square mile

0.1 1 10 100 1,000

Nonpoint phosphorus input, in pounds per square mile

0.01

0.1

1

10

100

1,000

Total phosphorus outflow yield, in pounds per square mile

Increasing

nutrients lost

(or exported)

to streams

Percent nutrients

lost to streams

5%

50%

5%

50%

B.

0

250

500

750

1,000

1,250

1,500

1,750

2,000

Median total nitrogen outflow yield, 

in pounds per square mile

C.

Agricultural

with 5

percent

tile drains

Agricultural

 with ≥5

percent

 tile drains 

Urban Undeveloped

Land use

0

10

20

30

40

50

60

70

Median total phosphorus outflow yield,

in pounds per square mile

Land use

Agricultural

with 5

percent

tile drains

Agricultural

 with ≥5 

percent

 tile drains 

Urban Undeveloped

Agricultural 

 with 5 percent tile drains

 with ≥5 percent tile drains

Urban

Undeveloped

Land use

Nitrogen export from

agricultural sites, in percent

25

≥25

County with ≥5 percent tile drains

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

37

percent

18

percent

8 percent

12

percent

25

percent

Phosphorus

16

percent

14

percent

4 percent

9 percent

5 percent

52

percent

Nitrogen

Sources

Urban and populationrelated sources

Non-recoverable animal manure 

on pasture and rangeland

Corn/soybean crops

Other crops

Natural land

Atmospheric deposition

EXPLANATION

17-0191_Fig_4-14.ai

10

1

0.1

0.01

0.001

0.01 0.1 1 10

Water depth, in meters

Instream removal rate coefficient, in day-1

Published in the literature

Estimated in the SPARROW model

Nitrogen loss rates

Using the SPARROW Model to Assess 

Nutrient Conditions in Streams

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

USGS scientists have developed the statistical model, SPARROW 

(SPAtially Referenced Regression On Watershed attributes), to relate 

nutrient concentrations from a large network of monitoring stations to 

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

and the atmosphere; and (2) watershed characteristics affecting 

transport, including soil permeability, stream size, and streamflow 

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

are spatially referenced to a detailed network of stream reaches that 

represents pathways of water movement through the modeled region. 

Because of the spatial referencing, nutrient loss during both surface 

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

stream reach can be allocated to individual upstream sources and 

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

provides measures of uncertainty for the model and its predictions. 

The SPARROW model can be used to address a variety of 

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

and loads in unmonitored stream reaches can be predicted, thus 

allowing for identification of specific locations where regulatory 

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

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

individual sources and locations in the upstream watershed, allowing 

for targeted management of important nutrient sources and contributing 

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

hypothetical conditions, such as changes in nutrient sources associated 

with proposed nutrient control strategies. 

The SPARROW modeling approach has been applied at the 

national scale to provide estimates of nutrient loads and concentrations 

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

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

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

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

decline as water depth and stream size increase, because the amount 

of biological processing of nitrogen in streams depends on the surface 

area of the stream bottom (where the organisms live) in relation to the 

stream’s volume (fig. 4-14). Consequently, large deep rivers contribute 

a larger percentage of their nitrogen to downstream receiving water 

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

finding indicates that reductions in nitrogen sources near small 

streams will have a smaller benefit to receiving water bodies than 

reductions in nitrogen sources near large rivers. Recently developed 

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

geographic regions, and transport processes that affected contributions 

of nitrogen and phosphorus to the northern Gulf of Mexico during 

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

the dominance of agricultural nonpoint sources over point sources in 

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

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

rapidly declines as water depth and stream size 

increase; consequently, large rivers contribute a 

larger percentage of their nitrogen to downstream 

receiving water bodies than small streams. As a 

result, reductions in nitrogen sources near small 

streams will have a smaller benefit to receiving 

water bodies than reductions in nitrogen sources 

near large rivers (Alexander and others, 2008).

Figure 4-15. Corn and soybean cultivation was the 

largest contributor of nitrogen to the Gulf, resulting 

in large part from commercial fertilizer use and 

manure applications to crops as fertilizer. Corn and 

soybean cultivation and animal manure on pasture 

and rangelands were the largest contributors of 

phosphorus (Alexander and others, 2008).
Chapter 4

Occurrence and Distribution of Nutrients in Streams and Groundwater 67

NY

PA

MD

DE

VA

WV

Chesapeake Bay

NY

PA

MD

DE

VA

WV

Chesapeake Bay

76°

78°

80°

42°

40°

38°

76°

78°

80°

42°

40°

38°

EXPLANATION

Amount of nitrogen delivered 

to streams, in kilograms 

per hectare per year

0.0−1.7

1.7−3.4

3.4−5.1

5.1−6.8

6.8−8.5

8.5−10.2

10.2−11.9

11.9−13.6

13.6−15.3

>15.3

A.

EXPLANATION

Amount of nitrogen delivered

to Chesapeake Bay, in kilograms 

per hectare per year

0.0−1.7

1.7−3.4

3.4−5.1

5.1−6.8

6.8−8.5

8.5−10.2

10.2−11.9

11.9−13.6

13.6−15.3

>15.3

B.

was the largest contributor of nitrogen to the Gulf, resulting 

in large part from commercial fertilizer use and manure 

applications to crops as fertilizer. Corn and soybean cultivation 

and animal manure on pasture and rangelands were the largest 

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

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

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

River drainage area, were found to contribute more than 

75 percent of the nitrogen and phosphorus to the Gulf. 

SPARROW models also have been developed for smaller 

regions to provide a more detailed focus on factors that 

influence water quality locally but that may not be important 

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

Previous regional models for 1992 conditions include those for 

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

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

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

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

selected North Carolina coastal drainages (McMahon and 

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

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

results indicate that only a fraction of the total nitrogen 

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

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

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

proportion of nitrogen lost during instream travel was greatest 

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

models currently are being developed for 2002 conditions in 

the Northeast, Southeast, Midwest, Northern Plains, Southern 

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

SPARROW models are based on updated geospatial data and 

stream-monitoring records from local, State, and other 

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

Seasonal Variation of Nutrients in Streams

Geographic patterns in the occurrence and distribution 

of nutrient concentrations are complicated by seasonal 

variations that affect the timing of the highest and lowest 

nutrient concentrations in streams. Nutrient concentrations 

vary seasonally in response to natural factors (streamflow 

and uptake by aquatic organisms) and anthropogenic factors 

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

applications, and streamflow alterations, such as diversions 

and flow augmentation from irrigation return flows). 

Evaluation of these seasonal patterns in nutrient concentrations 

in streams furthers our understanding of factors that contribute 

to high nutrient concentrations and can, thereby, facilitate 

design of effective monitoring and management strategies.

• Streamflow—In streams in which point-source 

discharge or groundwater inflows predominate, 

increases in flow driven by increases in precipitation 

and runoff often lead to a decrease in nutrient 

concentrations as a result of instream dilution. The 

same increases in precipitation and consequent flow 

in streams in which nonpoint sources predominate, 

however, can cause an increase in nutrient 

concentrations as a result of the increased surface 

runoff. Similarly, decreases in streamflow can result 

in increases in nutrient concentrations where point 

sources predominate, but decreases in concentrations 

where nonpoint sources predominate (see Chapter 2, 

Nutrient Primer). The seasonal pattern of streamflow 

varies across the United States. In the eastern United 

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

are highest in the spring and lowest in the autumn, 

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

Seasonal patterns in streamflow in western streams are 

less distinct because of the highly variable geography 

and climate, as well as the ubiquitous use of dams 

and canals to modify streamflow and optimize the use 

of scarce water resources in this region. Streamflows 

in some areas of the West are highest in the spring 

when snowmelt from the mountains makes its way 

downstream; in other areas, streamflows are highest in 

the summer during periods of irrigation and when local 

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

lowest in the West during the autumn and winter, when 

little precipitation or snowmelt occurs.

• Timing of nutrient usage—When fertilizers and 

manure are applied in agricultural and urban areas, 

nitrogen and phosphorus become available for 

potential transport to streams. Runoff resulting from 

precipitation shortly after such applications can 

transport this nitrogen and phosphorus to streams, 

thereby increasing instream concentrations. As a result, 

some of the highest concentrations of nutrients in 

streams occur immediately after fertilizer and manure 

application. The timing of fertilizer and manure 

application is thus a critical factor, and varies across 

the United States depending on crop type, regional 

climatic conditions, and fertilizer type.

• Biological activity—Warm water temperatures and 

abundant sunlight in the summer months create ideal 

conditions for the growth of algae and aquatic plants. 

These organisms require nutrients for growth, and 

nutrient concentrations in streams across the country 

often decrease in the summer months because of 

uptake by aquatic organisms. During cooler months,

less nutrient uptake occurs because some organisms 

die, whereas others experience a reduction in metabolism.

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

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

Occurrence and Distribution of Nutrients in Streams and Groundwater 69

17-0191_Fig_4-17_seasonmap.ai

J F M MA AJ J S O N D

0 J F M MA AJ J S O N D

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

J F M MA AJ J S O N D

E

E

A.

B.

C. Seasons with high total phosphorus

Seasons with high total nitrogen

Seasons with high streamflow

D G E

F

0 

10 

20 

30 

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

10

20

30

Mean monthly discharge,

in cubic feet per second

E.

50 

100 

150 

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

0 

500 

1,000 

1,500 

2,000 

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

0.0 

2.0 

0.02 

0.07 

Mean monthly concentration,

in milligrams per liter 

Total phosphorus

Total nitrogen

1.0

4.0

0.05 

0.13 

Mean monthly concentration,

in milligrams per liter 

Total phosphorus

Total nitrogen

Mean monthly concentration,

in milligrams per liter 

Total phosphorus

Total nitrogen

5.0

0.0

0.15

0.00

0.5 

1.0 

0.01 

0.06 

Mean monthly concentration,

in milligrams per liter 

Total phosphorus

Total nitrogen

Merced River near Newman, California

Rock Creek at Twin Falls, Idaho

Sugar Creek at New Palestine, Indiana 

Lisha Kill northwest of

Niskayuna, New York

100th

meridian Winter (November–January)

Summer (May–July)

Autumn (August–October)

Spring (February–April)

EXPLANATION

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

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

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

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

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

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

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

The specific seasonal interactions among streamflow, 

nutrient sources, and biological activity vary regionally and 

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

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

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

significantly higher streamflows and 39 percent had 

significantly higher nitrogen concentrations in the spring 

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

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

concentrations occurred during the autumn at 66 and 

42 percent of eastern sites, respectively. In contrast 

to nitrogen, phosphorus concentrations generally 

were higher in the summer and autumn during low 

streamflow and lower in the winter and spring when 

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

nitrogen concentrations in Lisha Kill, New York, were 

high and phosphorus concentrations were low in March 

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

maximum phosphorus concentrations occurred during 

periods of low summer streamflow. The association 

of high nitrogen concentrations with high streamflow 

suggests transport of nonpoint source nitrogen, such as 

fertilizer by rainfall runoff, whereas the high phosphorus 

concentrations during low streamflow indicate point 

sources, such as wastewater-treatment plants. At an 

agricultural site in the upper Midwest (Sugar Creek, 

Indiana), both nitrogen and phosphorus concentrations 

paralleled the changes in streamflow, suggesting that 

both nutrients are mobilized from nonpoint sources 

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

eutrophic agricultural streams in the Midwest may result 

in part from uptake of the nitrogen by algae.

• As with streamflow, seasonal patterns in nutrient 

concentrations in western streams are less distinct than 

in eastern streams because of hydrologic modifications 

and the highly variable geography and climate. 

The seasonal pattern in nutrient concentrations in 

western streams tends to be opposite that in eastern 

streams. The highest concentrations of phosphorus 

in western streams were most common during the 

summer when snowmelt produced high streamflows 

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

in western streams occurred during the winter 

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

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

characteristic of this seasonal pattern, with high 

concentrations of phosphorus and low concentrations 

of nitrogen coincident with high spring and summer 

streamflows. Especially in the semiarid areas of the 

interior West, high phosphorus concentrations during 

high streamflow are consistent with mobilization 

of sediment-bound phosphorus by erosion. High 

nitrogen concentrations during low streamflow may 

be derived from irrigation-return flow, point sources, 

or groundwater discharge to streams—sources that are 

diluted by spring and summer snowmelt runoff. For 

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

dam releases of runoff with low nutrient concentrations 

from the undeveloped Sierra Nevada Mountains dilute 

the effects of runoff from agricultural land in the 

San Joaquin Valley during the irrigation season from 

March to July. Concentrations of both nitrogen and 

phosphorus increased in the late summer and early 

winter when reservoir releases were minimal.
Chapter 4

Occurrence and Distribution of Nutrients in Streams and Groundwater 71

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

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

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

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

90th percentile

10th percentile

Median

75th percentile

25th percentile

EXPLANATION

0.02 Detection limit

Concentration, in milligrams per liter as N

Concentration, in milligrams per liter as P

Agricultural Urban Major aquifers

0

2

4

6

8

10

12

14

16

18

0.00

0.05

0.10

0.15

0.20

Agricultural Urban Major aquifers

Nitrate Orthophosphate

Ammonia

Concentration, in milligrams per liter as N

0.0

0.1

0.2

0.3

0.4

0.5

Agricultural Urban Major aquifers

0.02

0.01 0.05

Nutrient Concentrations in 

Groundwater 

Relations to Land Use

Nitrate concentrations were significantly different among 

groundwater samples collected in agricultural, urban, and 

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

Concentrations of nitrate were highest in groundwater beneath 

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

about 3 times the national background concentration of 

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

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

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

Some of the differences in concentrations between the study 

types can be explained by differences in the depths of the 

wells from which samples were collected. Wells sampled in 

agricultural and urban land-use studies typically are shallow 

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

recently recharged groundwater, whereas wells completed in 

the major aquifers typically are much deeper (median well 

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

the highest concentrations of nitrate in groundwater are higher 

than the highest nitrate concentrations in streams, median 

concentrations are similar, and the relative differences in 

concentrations among the different land-use categories 

are similar. 

Concentrations of orthophosphate (hereinafter referred 

to as dissolved phosphorus) and ammonia were much lower 

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

categories, and generally were not greater than background 

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

phosphorus and ammonia are not persistent or mobile enough 

to affect groundwater concentrations significantly under 

most conditions.

Geographic Distribution

Variations in the distribution of nitrate concentrations 

in groundwater are due to differences in types of sources, 

physical factors that either favor or inhibit transport of nitrate 

to groundwater, and chemical factors that result in persistence 

or removal of nitrate in groundwater over time. Nitrate 

concentrations are highest in shallow, oxic groundwater that 

receives high inputs of nitrogen from fertilizer, manure, and 

atmospheric deposition. Median concentrations of nitrate in 

shallow groundwater for agricultural land-use studies were 

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

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

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

result of high nitrogen inputs and conditions favorable to 

nitrate transport in groundwater. Median concentrations of 

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

relatively few studies with concentrations in the low category 

were distributed across the southern half of the country. Most 

nitrate concentrations in major aquifers were low or medium, 

with somewhat higher concentrations in parts of the Northeast, 

the Northern and Southern Plains, and the Southwest. High 

concentrations in the Northern and Southern Plains and 

Southwest could be influenced by irrigation practices, which 

may accelerate the downward movement of nitrate into 

groundwater. Sampled wells in the major aquifers of the 

Northeast are shallower than wells in most of the other major 

aquifer studies, and thus the samples may represent young 

groundwater that has moved rapidly through the system. 

Concentrations generally were low in the major aquifers of the 

Midwest and south through the Mississippi Valley to the coast, 

which can be attributed to a combination of physical and 

chemical properties that inhibit rapid nitrate transport to these 

deep aquifers. 

The distribution of nitrate concentrations in groundwater 

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

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

streams that have total nitrogen concentrations in the highest 

category are clustered in the upper Midwest, the nitrate 

concentrations in groundwater in agricultural areas and major 

aquifer studies do not show this pattern of distribution. The 

contrast between the spatial distribution of total nitrogen 

in streams and nitrate in groundwater is a consequence of 

hydrologic factors that promote rapid and efficient movement 

of nitrogen to streams yet tend to inhibit the movement of 

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

artificial subsurface drainage and ditches installed to drain 

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

recharge to streams, while limiting the opportunity for 

infiltration to the water table.

In contrast to concentrations of phosphorus in streams, 

dissolved phosphorus concentrations in groundwater generally 

were low. Eighty-eight percent of the measured concentrations 

were less than 0.1 mg/L dissolved phosphorus, although 

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

Phosphorus in groundwater originates from 

anthropogenic sources, such as fertilizer, manure, and 

waste effluent, as well as natural geologic sources. Elevated 

dissolved phosphorus concentrations in groundwater occur 

throughout the United States, with no definitive spatial pattern. 

Median concentrations were greater than the background 

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

studies and 21 percent of agricultural and urban land-use 

studies. Additionally, dissolved phosphorus concentrations 

in groundwater showed no correlation to fertilizer and 

manure use in agricultural areas. This lack of correlation 

and the similarity of concentrations in both shallow and 

deep groundwater suggests that natural geologic sources of 

phosphorus may have a greater influence on concentrations in 

groundwater than do human-derived sources from activities on 

the land surface. 

The transport of phosphorus in groundwater typically 

is limited because most phosphorus compounds are not very 

soluble and tend to precipitate or adsorb to soil particles. 

Phosphorus can be mobilized under favorable geochemical 

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

and geochemically reduced conditions. Dissolved phosphorus 

concentrations were significantly higher in reduced 
Chapter 4

Occurrence and Distribution of Nutrients in Streams and Groundwater 73

17-0191_Fig_4-19_GW_Nconc.ai

2,900

2,900–15,000

15,000

Shallow groundwater 

studies, by land use

Data on nitrogen inputs not available for Alaska and Hawaii.

EXPLANATION

Agricultural Urban

Major

aquifers

Low (0.08)

Medium (0.08–2.6)

High (2.6)

Median concentration of nitrate,

in milligrams per liter as N

Estimated 1997 nitrogen inputs from fertilizer, manure, and 

atmosphere, in pounds per square mile

Agricultural Urban

Major aquifers

Groundwater

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

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

groundwater than in oxic groundwater, and 

concentrations also were higher in groundwater 

with pH greater than 7.5. In selected areas 

with median phosphorus concentrations above 

background levels, dissolved phosphorus was 

significantly and positively correlated to pH 

and to the concentrations of ammonia, calcium, 

magnesium, and iron. 

Phosphorus is not regulated in drinking 

water, however, its concentration in groundwater 

can affect surface-water quality at locations 

where groundwater inflow contributes 

significantly to streamflow (see Chapter 5, 

Exchange of Nutrients between Surface Water 

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

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

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

Nitrate Leaching to Groundwater

Leaching of nonpoint source nitrate to groundwater 

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

amount of nitrate leached to groundwater, represented as the 

percentage of nonpoint source nitrogen inputs at the land 

surface that moves beyond the root zone and reaches the 

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

The percentage of nonpoint source nitrogen leached to 

groundwater has been estimated in various USGS field studies 

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

estimate the percentage of nitrate leached to groundwater, the 

estimates generally range from 10 to 50 percent. Leaching 

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

where tile drains divert percolating groundwater to streams, or 

where fine-grained sediments impede the vertical movement 

of water and thus provide more opportunity for denitrification 

to remove nitrate before the water reaches the aquifer. 

Aquifers that underlie areas with high nitrogen inputs to the 

land surface, combined with coarse-grained soils and rapid 

rates of vertical movement through the unsaturated zone, are 

the most vulnerable to nitrate contamination. 

Nitrate concentrations measured in oxic shallow 

groundwater were significantly correlated to estimated 

nonpoint source nitrogen input within a 500-meter 

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

the correlation between median nitrate input and median 

nitrate concentration is statistically significant, the correlation 

is weak, indicating that other factors have an important 

influence on concentrations. For example, concentrations of 

nitrate in groundwater in the agricultural land-use studies 

in the Upper Snake River Basin are relatively low despite 

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

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

flow rates, and dilution by irrigation water from the Snake 

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

contrast, the median concentration of nitrate in groundwater 

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

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

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

aquifer sediments facilitate rapid transport of nitrate to these 

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

nitrate concentrations are generally lower in response to 

this difference in nitrogen inputs. Concentrations of nitrate 

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

Basins, and South Platte River Basin studies are high relative 

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

population density in these three study areas is greater than 

1,000 people per square kilometer. Population density is 

significantly correlated with nitrate concentrations within 

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

development of urban land may better reflect nitrogen inputs 

in urban areas than inputs estimated on the basis of fertilizer 

and manure applications and atmospheric sources.
Chapter 4

Occurrence and Distribution of Nutrients in Streams and Groundwater 75

20_GW_N_leaching.ai

GroundwaterUnsaturated

zone

≤20

20–40

40

Fraction of nitrate 

leached, in percent

Estimated 1997 nitrogen inputs from

fertilizer, manure, and atmosphere,

in pounds per square mile

2,900

2,900–15,000

15,000

Media

EXPLANATION

Groundwater

17-0191_Fig_4_21_No3_totn_LUSoxic.ai

0

5

10

15

20

Nitrate concentration, in milligrams per liter as N

0 10,000 20,000 30,000

Nonpoint source nitrogen input, in pounds per year

USNK

GRSL

SPLT

PUGT

SANA

Agricultural 

Urban

Shallow groundwater study

EXPLANATION

USNK—Upper Snake River Basin

PUGT—Puget Sound Basin

SANA—Santa Ana Basin

GRSL—Great Salt Lake Basins

SPLT—South Platte River Basin

NAWQA Study Unit

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

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

detailed studies were conducted.

Figure 4-21. Although there is a statistically 

significant correlation between nonpoint source 

nitrogen inputs and median nitrate concentrations in 

oxic shallow groundwater, the correlation is weak, 

indicating that other factors have an important 

influence on concentrations.