Chapter 5

Exchange of Nutrients between Surface Water and Groundwater 99

 Groundwater

 Streams

0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

Phosphorus concentration,

in milligrams per liter

Albemarle-Pamlico Drainage Basins, North Carolina

Site number

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

Median total phosphorus concentrations, in milligrams per liter (mg/L), are in bold text.

Streambed piezometers receiving

local groundwater discharge

0.75 mg/L

Water table

Shallow aquifer sediments

 and flood deposits

Deep aquifer sediments

with woody debris and peat

Streambed piezometers receiving

regional groundwater discharge

1.23 mg/L

Deep

domestic wells

0.34 mg/L

Shallow

domestic wells

0.15 mg/L

Tualatin River, Oregon

Figure 5-11. Dissolved phosphorus concentrations in groundwater were 

higher than those in streams at most sites throughout the Coastal Plain in 

the Albemarle-Pamlico Drainage Basin, North Carolina. Concentrations in 

streams and groundwater discharging to the streams were significantly 

correlated, indicating that the groundwater was a significant source of 

phosphorus to the streams at low flow (Spruill and others, 1998).

Figure 5-12. Groundwater contributes phosphorus to the Tualatin River, Oregon, during summer low-flow conditions (Kelly and 

others, 1999). The primary source of high phosphorus in groundwater is the decomposition of organic material. 
100 Nutrients in the Nation’s Streams and Groundwater, 1992–2004

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Chapter 6

Potential for Effects 

on Human Health 

Introduction

Elevated concentrations of nutrients, 

particularly nitrate, in drinking water 

may have both direct and indirect 

effects on human health. One direct 

effect of ingestion of high levels of 

nitrate is methemoglobinemia, a 

disorder in which the oxygen-carrying 

capacity of the blood is compromised; 

the USEPA Maximum Contaminant 

Level (MCL) of 10 mg/L of nitrate for 

drinking water was adopted to protect 

people, mainly infants, against this 

problem. High nitrate concentrations 

in drinking water also have been 

implicated in other human health 

problems, including specific cancers 

and reproductive problems, but more 

research is needed to corroborate 

these associations (see sidebar, 

Nitrate in Drinking Water: Potential 

Health Effects). The indirect effects 

of nutrient enrichment of surface 

waters on human health are many and 

complex, including algal blooms that 

release toxins and the enhancement 

of populations of disease-transmitting 

insects, such as mosquitoes (Townsend 

and others, 2003).

Photograph by U.S. Geological Survey

6

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

Nitrate in Drinking Water: Potential Health Effects

Mary H. Ward, Division of Cancer Epidemiology and Genetics, National Cancer Institute, National Institutes of Health, 

Department of Health and Human Services, Bethesda, MD

Jean D. Brender, Department of Epidemiology and Biostatistics, Texas A&M Health Science Center, School of Rural 

Public Health, College Station, TX

Adverse health effects from nitrate in drinking water most likely result from complex interactions among 

(1) the amount of nitrate ingested, (2) the dietary intake of other constituents that may enhance or inhibit the 

formation of harmful compounds from ingested nitrate, and (3) medical and (or) genetic conditions that may 

increase susceptibility (Ward and others, 2005). To date, there have been few well-designed epidemiologic 

studies on the role of nitrate ingestion in specific chronic diseases. Thus, the results of epidemiologic 

studies have been varied: although positive associations between nitrate intake and adverse effects have 

sometimes been found, with the exception of methemoglobinemia, it is difficult for scientists to draw 

conclusions about human health risk.

Human biomonitoring studies show that ingestion of nitrate via drinking water contributes to the 

formation of N-nitroso compounds, a class of chemicals that may have ties to cancer, diabetes, and adverse 

reproductive outcomes. Additional studies that accurately assess drinking-water nitrate exposure are 

needed to further our understanding of the relation between drinking-water nitrate and health outcomes. 

This research is particularly relevant for rural users of privately owned wells because these wells are 

unregulated, and NAWQA studies show elevated nitrate concentrations in shallow groundwater in 

agricultural areas where rural users reside.

Methemoglobinemia 

The USEPA MCL for nitrate of 10 mg/L as N for drinking water was promulgated to protect infants from 

developing methemoglobinemia, also known as blue baby syndrome. Ingested nitrate is reduced to nitrite, 

which binds to hemoglobin to form methemoglobin. Too much methemoglobin interferes with the oxygencarrying capacity of the blood, and infants are particularly susceptible to this condition. 

A recent assessment of cases of methemoglobinemia associated with well water in the United States 

found cases occurring at nitrate levels between 20 and 30 mg/L. The concentration of nitrate in drinking 

water that produces elevated methemoglobin can be affected by several factors including other dietary 

sources of nitrate, certain medications, and concurrent exposure to enteric bacteria that cause diarrhea. 

Because fecal bacteria can co-occur with nitrate in domestic wells, a better understanding of the 

comparative roles of these contaminants as risk factors for methemoglobinemia is needed.

Cancer

Ingestion of nitrate via drinking water contributes to the formation of N-nitroso compounds in the 

body—or endogenously—by a process called “endogenous nitrosation.” Most N-nitroso compounds 

are potent animal carcinogens, and several N-nitroso compounds formed endogenously in humans from 

dietary precursors are considered probable human carcinogens. On the basis of human and animal studies, 

the International Agency for Research on Cancer (IARC) concluded that “ingested nitrate or nitrite under 

conditions that result in endogenous formation of N-nitroso compounds is probably carcinogenic 

to humans.” 

Most epidemiologic studies of nitrate in drinking water in relation to cancer have been ecologic in design, 

linking incidence or mortality rates to levels of nitrate in drinking water for large groups of people at the town 
Chapter 6

Potential for Effects on Human Health 103

or county level (Cantor and others, 2006). This study design is mostly useful for generating hypotheses 

about disease risk factors. Studies based on individual exposures are needed to establish cause and 

effect because of the complex and multi-step process by which ingested nitrate forms potentially 

carcinogenic N-nitroso compounds. Study designs that assess individual exposure include case-control 

studies, which compare a group of individuals with a disease or condition with a group who do not have 

the disease or condition, and cohort studies, which track a select group of people for a period of time.

In the past decade, several individual-based studies have evaluated historical nitrate levels in 

public-water supplies (largely below 10 mg/L) and risk of several cancers. A cohort study of older women 

in Iowa found 2.8-fold and 1.8-fold increases in the risk of bladder and ovarian cancers, respectively, 

associated with the highest quartile of nitrate exposure (> 2.46 mg/L). Significant inverse associations 

were observed for uterine and rectal cancer and no significant associations were observed for nonHodgkin lymphoma; melanoma; leukemia; and colon, rectal, pancreatic, kidney, and lung cancer. Dietary 

nitrate intake was not associated with increased risk of any cancer type. A case-control study of nonHodgkin lymphoma in Nebraska showed a significant positive association between the average nitrate 

level in public-water supplies over about 40 years and risk among men and women; however, subsequent 

studies of non-Hodgkin lymphoma have found no association. Other recent studies found no association 

between elevated levels of nitrate in public-drinking-water supplies and stomach cancer or childhood 

brain cancer; however, the children of one group of women who used private wells during pregnancy 

had a significantly increased risk of brain cancer. 

Some studies evaluated factors that enhance nitrosation, such as low vitamin C intake, to identify a 

potentially high risk subgroup. For example, although case-control studies of several types of cancer in 

Iowa found no overall association between cancer risk and average nitrate levels in drinking water over 

almost 30 years, there was a significant increase in the risk of colon cancer among a subgroup with both 

10 or more years of exposure above 5 mg/L of nitrate and factors that increase endogenous nitrosation 

(low vitamin C or high meat intake). Conversely, a case-control study in Nebraska of adult brain cancer 

showed no evidence of increased risk among subgroups with lower vitamin C intake and higher ingestion 

of nitrate in drinking water.

Adverse Reproductive Outcomes

Several studies have examined the association between drinking water that contains elevated 

levels of nitrate and adverse reproductive outcomes, such as spontaneous abortions, stillbirths, 

premature birth, or intrauterine growth retardation. Results of these studies are inconsistent, possibly 

indicating that there is no true effect of nitrate in water on reproductive outcomes at the nitrate levels 

evaluated. Alternatively, the inconsistencies may be due to the different periods over which exposure 

was assessed, different levels of nitrate in water across studies, or differences in exposure to other 

co-factors. 

Results of studies that evaluated nitrate in drinking water and congenital malformations in offspring 

also are mixed. Some studies found positive associations between nitrate in drinking water and 

outcomes, such as neural tube defects. In each of these studies, levels of nitrate in water associated 

with increased risk of these defects were below the MCL. Two such studies also examined dietary 

intake of nitrate and nitrite and neural tube defects, and found minimal or no effect on risk. In another 

study, higher levels of nitrates in food or drinking water during the periconceptional period significantly 

increased the risk of neural tube defects if women were exposed to nitrosatable drugs. A cohort study 

in Sweden noted a weakly positive association between maternal prenatal consumption of water with 

nitrate concentration of 2.0 mg/L (as nitrate) or greater and congenital heart defects in offspring.
104 Nutrients in the Nation’s Streams and Groundwater, 1992–2004

Photograph ©iStockphoto.com/Genesis Photography.

Fig_6-01_swgs_pies.ai

>20

>0–20 

No exceedance

Percentage of samples at each stream site or groundwater

study exceeding the nitrate MCL

EXPLANATION

Agriculture Urban Major aquifers

Agriculture Urban Mixed

Percentage of Streams

Percentage of Groundwater Studies

2 percent 3.3 percent

3 percent 5.3 percent

11.1 percent 7.3 percent

39.6

percent

43.4

percent

48.5

percent

38.3

percent

17

percent

Streams

Concentrations of nutrients in the 499 streams routinely 

sampled by NAWQA during the period 1992–2001 were seldom 

greater than human-health benchmarks. The USEPA lifetime 

health advisory for ammonia of 30 mg/L was not exceeded in any 

samples, and concentrations of nitrite greater than the Maximum 

Contaminant Level (MCL) of 1 mg/L were rare and limited to 

five sites. The waters at all five sites were heavily impacted by 

wastewater discharges and were not used as drinking-water sources. 

Concentrations of nitrate in streams exceeded the MCL 

much more frequently than did nitrite, but usually not in streams 

that would be used as a drinking-water supply. The nitrate MCL 

was exceeded in about 2 percent of all samples (566 of 27,555) 

distributed among 50 stream sites. Most of the streams with one or 

more samples with concentrations of nitrate above the MCL are in 

agricultural areas (fig. 6-1). Twenty-eight percent of the agricultural 

sites had one or more samples with concentrations above the MCL, 

and 11 percent of the agricultural sites had concentrations above 

the MCL in more than 20 percent of the samples. Only 7 percent 

of urban sites and 5 percent of mixed land use sites had samples 

with nitrate concentrations above the MCL. The nitrate MCL 

was not exceeded in any of the 6,728 samples collected at the 

159 undeveloped stream sites. 

For perspective on the relevance of NAWQA findings 

to surface water used as a source for drinking-water supplies, 

12 percent of the Nation’s 1,679 public-water supply intakes 

withdraw water from streams that drain watersheds with 

predominantly agricultural land. Most (55 percent) of the Nation’s 

intakes are in watersheds draining undeveloped land, with the 

remaining (32 and 1 percent) in mixed and urban land 

use, respectively. 

Most streams with nitrate concentrations above the MCL are 

in the upper Midwest Corn Belt (fig. 6-2). Most of these are small, 

agricultural streams (median watershed size about 300 square 

miles), but some larger streams in this region that are used as 

drinking-water supplies are similarly affected (see sidebar, Nitrate 

Concentrations Vary Seasonally and are Moderated by Reservoirs 

in the Midwest). Application rates of commercial fertilizer and (or) 

manure are high in this region. Sites in the Corn Belt also are often 

characterized by dense, clay-rich soils and are artificially drained by 

surface ditches or buried tile drains. The combination of dense soils 

and artificial drainage promotes rapid transport of runoff and nitrate 

from farm fields to streams. Most of the concentrations above the 

nitrate MCL occur during the growing season (May, June, and July) 

at these sites, consistent with the agricultural origin of the nitrate. 

Land-use categories for the 15 sites with one or more exceedances 

that are scattered across the Western United States are more 

diverse: these sites are almost evenly divided between irrigated 

agricultural sites and urban or mixed land-use sites with large inputs 

of wastewater. Only four sites are in the Northeast (all in eastern 

Pennsylvania) and none are in the Southeast. 

Figure 6-1. The Maximum Contaminant Level (MCL) 

for nitrate in drinking water of 10 milligrams per 

liter as nitrogen is exceeded far more frequently in 

streams in agricultural areas than in the other land-use 

categories. Much larger proportions of groundwater 

studies have MCL exceedances than do stream sites in 

all land-use categories. 
Chapter 6

Potential for Effects on Human Health 105

17-0191_fig 6-05

No exceedance

0–20

20

Percentage of samples at

each stream site 

exceeding the nitrate MCL 

(10 milligrams per liter as N)

Estimated 1997 nitrogen inputs from

fertilizer, manure, and atmosphere,

in pounds per square mile

Agricultural Urban

Major

aquifers

No exceedance

0–20

20

Percentage of samples for

each groundwater study 

exceeding the nitrate MCL

(10 milligrams per liter as N)

Shallow groundwater

studies, by land use

EXPLANATION

2,900

2,900–15,000

15,000

Stream sites, by watershed land use

Data on nitrogen inputs not available 

for Alaska and Hawaii.

Agricultural Undeveloped Mixed Urban Groundwater

Streams

Figure 6-2. Although stream sites with nitrate concentrations above the Maximum Contaminant Level (MCL) for nitrate in drinking 

water of 10 milligrams per liter are broadly distributed, this standard is most often exceeded in agricultural streams draining basins 

with dense clay soils and artificial drainage in the Corn Belt States of the upper Midwest. In contrast, the distribution of nitrate 

concentrations above the MCL in groundwater studies shows no strong spatial pattern. 
106 Nutrients in the Nation’s Streams and Groundwater, 1992–2004

39°

40°

41°

91° 90° 89°

0 10 20 30 40 MILES 50

0 10 20 30 40 KILOMETERS 50

88°

MAP LOCATION

Lower

Illinois

River

Basin

ILLINOIS

Decatur

Lake

Sangchris

Illinois River

Sangamon R.

Springfield Lake

Taylorville

Lake

Lake

Sangamon River 

at Monticello

Sangamon River

near Oakford

Oakford

Decatur

Springfield

EXPLANATION

Stream sampling site

City

Sangamon River at Monticello, Illinois

Sangamon River near Oakford, Illinois

Nitrate concentration, in milligrams per liter as N

Maximum Contaminant Level

20

18

16

14

12

10

8

6

4

2

0

Sept

Oct

Nov

Dec

Jan

Feb

Mar

Apr

May

June

July

Aug

Sept

Oct

Nov

Dec

Jan

Feb

Mar

Apr

May

June

July

Aug

Sept

1996 1997 1998

B.

Nitrate Concentrations Vary A.

Seasonally and Are Moderated 

by Reservoirs in the Midwest

Nitrate concentrations in the Sangamon 

River in Illinois were more variable upstream 

of water-supply reservoirs (at Monticello, 

Illinois) than downstream of the reservoirs 

(near Oakford, Illinois) (fig. 6-3A). During 

April through June of 1997 and 1998, 

concentrations were frequently above the 

MCL of 10 mg/L in the Sangamon River 

at Monticello, 29 miles upstream of Lake 

Decatur—the drinking-water source 

for the city of Decatur (fig. 6-3B). When 

nitrate concentrations exceed the MCL, 

the city of Decatur either treats the stream 

water to remove nitrate or mixes the highnitrate surface water with groundwater to 

dilute the nitrate concentrations. Nitrate 

concentrations also were highest in the 

spring in the Sangamon River near Oakford, 

which is downstream of Lake Decatur and 

other water-supply reservoirs used by the 

city of Springfield. Concentrations in samples 

from the Sangamon River near Oakford, 

however, seldom reached the nitrate MCL. 

From August through November, 

the above pattern was reversed: 

concentrations were low at both sites, but 

the concentrations at the site downstream of 

the reservoirs were greater than those at the 

upstream site. Similar concentrations and 

seasonal patterns occurred in other streams 

in the corn- and soybean-producing areas of 

the Midwest (Scribner and others, 1996).

Figure 6-3. Nitrate concentrations in the 

Sangamon River, Illinois, are affected by reservoirs 

and other factors. Monticello lies upstream of 

several multi-purpose reservoirs and the cities of 

Springfield and Decatur. Oakford lies downstream of 

the reservoirs (A). The reservoirs are used for water 

supply, and during spring, they are replenished by 

river water that exceeds the Maximum Contaminant 

Level for nitrate. Water leaving the reservoirs 

usually does not exceed the nitrate standard and 

has less variability in nitrate concentrations (B).
Chapter 6

Potential for Effects on Human Health 107

Groundwater

Concentrations of nitrate greater than the USEPA MCL 

of 10 mg/L occur more frequently and are more widespread 

in groundwater than in streams. About 9 percent of all wells 

sampled had nitrate concentrations greater than the MCL, 

compared to 2 percent of all stream samples. Similarly, 

57 percent of all agricultural, urban and major aquifer studies 

had one or more samples with a concentration exceeding the 

MCL, compared to 10 percent of all stream sites. In addition, 

a greater proportion of groundwater studies than stream sites 

had concentrations above the MCL in at least 20 percent 

of their samples (fig. 6-1). Concentrations exceeding the 

MCL were much more prevalent in samples from wells in 

agricultural areas (20 percent) than in samples from either 

wells in urban areas or in major aquifer wells (table 6-1). 

The overall rates of exceedance of the MCL in samples from 

wells in urban areas and major aquifers were similar—3 and 

4 percent, respectively.

Samples from about 7 percent of all domestic wells 

sampled had nitrate concentrations above the MCL (table 6-1). 

About 43 million people in the United States obtain water for 

household use from privately owned (domestic) wells (Hutson 

and others, 2004). The relatively high rate of exceedance of 

the MCL for nitrate in shallow domestic wells in agricultural 

areas is a potential human health concern because the water 

from these wells generally is used as household supply 

without treatment. The overall rate of exceedance of the 

MCL in public-supply wells was only 3 percent. The low 

rate of exceedance in the public wells reflects the relatively 

great depth of these wells, and hence, the long travel time 

of the water from the land surface to the well intakes, as 

well as measures taken to avoid groundwater with high 

nitrate concentrations. In contrast to water from domestic 

wells, source water for public-supply wells that exceeds the 

MCL is either treated or blended with water having a lower 

concentration of nitrate to attain the standard prior to being 

distributed to users. Because of the large number of people 

potentially affected, the complexity of the problem, and the 

high cost of treatment, the NAWQA Program is conducting 

detailed studies of public-supply well vulnerability (see 

sidebar, NAWQA Study of Public-Supply Well Vulnerability). 

The broad distribution of elevated concentrations of 

nitrate in groundwater is reflected in the high proportion 

of studies with one or more MCL exceedances: 83 percent 

of the agricultural studies, 52 percent of urban studies, and 

44 percent of major aquifer studies (fig. 6-1). Two subtle 

spatial patterns in the rate of exceedance of the nitrate MCL 

in groundwater studies are noteworthy. First, unlike streams, 

groundwater studies in which concentrations of nitrate exceed 

the MCL are broadly distributed across the United States 

(fig. 6-2); in particular, despite the large nitrogen inputs in the 

upper Midwest Corn Belt, the rates of MCL exceedance in this 

area are no different from the rates in other agricultural areas 

of the country. Second, the generally low rate of exceedance 

of the nitrate MCL in studies across the southeastern 

portion of the country may be attributed to the prevalence 

of aquifers with chemically reduced conditions that promote 

denitrification, and limestone aquifers where rapid flushing 

results in low nitrate concentrations.

The proportions of public-supply and domestic wells 

that had nitrate concentrations above the MCL of 10 mg/L are 

similar to, but slightly larger than, the results in a previous 

national study. Results from a USEPA survey of 1,300 wells 

sampled during 1985–1990 indicate that 1.2 percent of 

community water system (CWS) wells and 2.4 percent of rural 

domestic wells produced water that had nitrate concentrations 

greater than the MCL (U.S. Environmental Protection Agency, 

1990). CWS wells are those in piped drinking-water systems 

that have at least 15 connections or serve at least 25 permanent 

residents. Rural households were defined as those outside of 

incorporated or unincorporated areas with a population of 

2,500 or more and outside of urban fringe areas as defined by 

the U.S. Census Bureau.

Table 6-1. Samples from about 7 percent of domestic wells and 3 percent of public-supply wells had nitrate concentrations above the 

Maximum Contaminant Level (MCL) of 10 milligrams per liter as nitrogen. The rate of exceedance was highest in shallow groundwater 

under agricultural lands.

Type of groundwater study

Public-supply wells Domestic wells Monitoring wells

(unused) Total

Number 

sampled

Percentage 

exceeding 

MCL

Number 

sampled

Percentage 

exceeding 

MCL

Number 

sampled

Percentage 

exceeding 

MCL

Number 

sampled

Percentage 

exceeding

MCL

Agricultural 1 100 406 22 938 20 1,345 20

Urban 9 0 17 6 781 3 807 3

Major aquifer 374 2 1,965 4 183 8 2,522 4

Total number, or percent

for lumped studies 384 3 2,388 7 1,902 12 4,674 9
108 Nutrients in the Nation’s Streams and Groundwater, 1992–2004

Not to Scale

Water table

Aquifer

Domestic

well Days to years

Centuries to millennia

Public-supply

well

Years to decades

NAWQA Study of Public-Supply Well Vulnerability 

The NAWQA Program has implemented an intensive study focused on understanding the factors that affect the 

vulnerability of public-supply wells to contamination (Eberts and others, 2005). The study findings are intended to help water 

managers, policy makers, drinking-water suppliers, and scientists to:

• Better understand how and why contamination of public-supply wells occurs and whether water quality may get 

better or worse over time;

• Assess and predict the vulnerability of groundwater and public-supply wells to contamination, even in 

unmonitored areas; 

• Choose new areas for water-supply wells and develop and prioritize monitoring programs; and

• Evaluate various groundwater pumping, development, and land-management alternatives. 

NAWQA findings through 2009 show that important factors that affect public-supply well vulnerability include mixing 

of water of different ages in wells, “short circuiting,” and geochemical conditions in the aquifer. Because subsurface 

conditions and management practices differ among aquifers and water distribution systems, public-supply wells in different 

areas of the country are not equally vulnerable to contamination, even where contaminant sources are similar.

Water from public-supply wells is a mixture of water of various ages associated with different land uses and potential 

sources of contamination from the land surface (fig. 6-4). Wells that draw a notable amount of older water (greater than 50 

years) may not fully respond to land-surface activities and sources of contamination for decades. In these types of wells, a 

decrease in contaminant concentrations can lag years, decades, and even centuries behind clean up and protection efforts 

due to the mixing of a smaller proportion of young (more contaminated water) with a larger proportion of older water during 

pumping. In contrast, water quality in public-supply wells that produce predominantly young water (less than 50 years old)

can respond relatively quickly (from days to years) to land-use changes near the surface, including source protection efforts.

Under natural conditions, vertical and horizontal movement of groundwater is often slow, and water can take decades 

to millennia to move through an aquifer system. NAWQA findings, however, show that pumping at public-supply wells in 

the vicinity of “short circuits” commonly allows water and associated contaminants to move more quickly to the wells than 

expected. Short circuits can include natural features, such as fractures in glacial till, solution channels in carbonate rock, 

and sinkholes, as well as human-related factors, such as abandoned or poorly constructed wells.

Geochemical processes within the aquifer can affect contaminant transport and fate. Geochemical processes and 

conditions can result in contaminants being degraded, adsorbed, volatilized, dispersed, and (or) mobilized as they move in 

the aquifer. This study has developed a tool to help understand the geochemical conditions in the aquifer and contributing 

areas to wells, which is critical to determining the transport of contaminants to a public-supply well.

Figure 6-4. Even in urban areas, 

groundwater reaching public-supply 

wells is a mixture of water recharged 

under a variety of land uses, and is a 

mixture of ages (blue lines on diagram).
Chapter 6

Potential for Effects on Human Health 109

Samples of groundwater were collected from privately owned (domestic) wells; 

about 43 million people in the United States obtain water for household use from 

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

0 2 4 6 8 10

≥300

200 to <300

100 to <200

<100

Percentage of wells exceeding nitrate MCL (10 milligrams per liter as N)

Depth below water table, in feet

The processes that influence the frequency of 

exceedance of the nitrate MCL in groundwater studies 

have already been discussed with respect to nitrate 

concentrations in general. Nitrate concentrations 

greater than the MCL are favored by oxic geochemical 

conditions, and exceedances are rare in chemically 

reduced groundwater: only 2 percent of samples from 

wells with reduced groundwater had concentrations 

above the MCL, whereas 12 percent of samples from 

wells with oxic groundwater had concentrations 

above the MCL. Exceedances are similarly infrequent 

in old groundwater (recharged prior to 1952): only 

about 1 percent of samples from wells producing old 

groundwater had concentrations of nitrate above the 

MCL, whereas about 12 percent of samples from wells 

with young groundwater had concentrations above 

the MCL. The much higher rate of exceedance in 

agricultural areas compared to that in urban areas is 

due in part to the higher input of nitrogen to the 

land surface.

The high rate of exceedance of the nitrate 

MCL in shallow wells in agricultural areas is cause 

for concern even though the aquifer represented by 

samples from these wells is not often directly used as a 

drinking‐water source. By design, agricultural land‐use 

study wells sample the shallowest, most recently 

recharged groundwater at each locale. As such, it is 

the first water affected by downward movement of 

contaminants from the land surface. In drinking‐water 

wells (domestic and public-supply wells), the rate 

of exceedance of the nitrate MCL decreases with 

increasing depth of the well below the water table 

(fig. 6-5). This distribution reflects the protective 

effect of the relatively long time it takes for surface 

contamination to reach the intakes of drinking‐water 

wells. The long travel time also represents an increased 

opportunity for removal of nitrate by denitrification 

or dilution by mixing. In many areas, the shallow 

groundwater and the deep aquifer zones pumped 

for domestic and public supply are part of a single, 

continuous hydrologic flow system. In these areas, 

downward movement of groundwater containing 

nitrate at concentrations exceeding the MCL could 

result in deterioration of drinking-water supplies in the 

future. Reliable prediction of this potential requires 

specific hydrologic and geochemical information 

unique to each locale. Although this level of detail for 

the entire country is beyond the scope of the NAWQA 

Program, some case studies examining the migration of 

nitrate-contaminated groundwater to supply wells have 

been done, and are described in Chapter 8, Changes in 

Nutrient Concentrations: Past and Predicted.

Figure 6-5. In drinking-water wells, the rate of exceedance of the 

U.S. Environmental Protection Agency Maximum Contaminant Level 

(MCL) for nitrate of 10 milligrams per liter decreases with increasing 

depth of the well below the water table. Downward movement of 

shallow groundwater with nitrate concentrations exceeding the MCL 

could potentially result in deterioration of drinking-water supplies in 

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

Fig_6-06_GWAVA-DW_model_prediction.ai

Predicted nitrate concentration, in milligrams per liter as N

≤1 1–5 5–10 10

EXPLANATION

Missing data

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

Extrapolation of Nitrate Groundwater

Concentrations in Drinking 

Water to Unsampled Areas

As part of NAWQA’s integrated approach 

to monitoring and prediction, a statistical model 

was developed to assess the vulnerability of 

groundwater to nitrate contamination at the 

national scale (Nolan and Hitt, 2006). The model 

developed for deep groundwater (median depth 

of about 50 meters or 164 feet) that is used as 

drinking water predicts moderate to severe nitrate 

contamination in the High Plains, the northern 

Midwest, and other areas of intensive agriculture 

in both the East (eastern Pennsylvania and the 

Delmarva Peninsula) and the West (the Columbia 

Plateau in Washington, the San Joaquin Valley in 

California, and the Snake River Plain in Idaho) 

(fig. 6-6). The model predicts the highest nitrate 

concentrations in areas with large nitrogen 

sources, factors that promote rapid transport 

of nitrogen in groundwater, and a lack of 

attenuation processes. 

The model for nitrate in deep groundwater 

that is used as a drinking-water supply explained 

about 77 percent of the variation in the nitrate 

concentrations. Areas that are predicted to have 

nitrate concentrations greater than 10 mg/L (red 

in fig. 6-6) are less extensive than predicted 

by a similar model for shallow groundwater 

(see Chapter 4, Occurrence and Distribution of 

Nutrients in Streams and Groundwater). This 

is because the drinking-water wells are about 

130 feet deeper on average than the shallow 

wells, and nitrate concentration generally 

decreases with increasing well depth (fig. 6-5). 

Greater depth generally represents greater travel 

time between recharge point and well intake, 

and greater travel time favors nitrate attenuation. 

Greater depth also increases the likelihood of the 

existence of intervening, less permeable layers 

that restrict the downward migration of water and 

associated dissolved nitrate. In addition, deeper 

groundwater is older and may predate recent 

(1950s to present) increases in nitrogen sources, 

especially fertilizer, and thus in some cases may 

have low nitrate concentrations.

Figure 6-6. Results of model simulations suggest moderate (shown in yellow 

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

deep groundwater (164 feet or 50 meters below land surface) underlying 

parts of California, the Northwest, the High Plains, and the East. These areas 

typically are associated with large nitrogen input; natural soil, landscape, and 

geologic features that promote rapid transport of groundwater; and a lack of 

biogeochemical processes that convert nitrate to other forms of nitrogen.

The model for deep groundwater was used as a basis for estimating 

the number of users of private wells in contaminated areas. The 

resulting population counts indicated that almost 15 million people 

live in areas with predicted nitrate concentrations greater than 1 mg/L

in water from wells that are 164 feet deep. Among these potential 

users, 1,240,000 people live in areas predicted to have moderate nitrate 

contamination of groundwater (>5 to ≤10 mg/L nitrate) (table 6-2). 

More users may be affected in the future, however, because shallow 

groundwater with high nitrate concentrations can migrate to greater 

depths within the aquifer. Because well depth is a proxy for travel time 

in the aquifer, conditions at shallow depths (33 feet) in essence constitute 

an early-warning system. Population counts corresponding to predicted 

nitrate concentrations greater than 1 mg/L are 3–14 percent greater at a 

well depth of 33 feet than at 164 feet (table 6-2). 
Chapter 6

Potential for Effects on Human Health 111

Areas where the model for deep groundwater predicts nitrate concentrations 10 milligrams per liter as N

Areas where the model for streams predicts total nitrogen concentrations 10 milligrams per liter as N

Areas where the model for deep groundwater predicts nitrate concentrations 10 milligrams per liter as N 

and the model for streams predicts total nitrogen concentrations 10 milligrams per liter as N

EXPLANATION

Streams and Groundwater

California Nebraska

Comparison of the maps of predicted total 

nitrogen concentrations in streams (see fig. 4-12) 

and nitrate in groundwater (fig. 6-6) shows a striking 

contrast in the distribution of areas predicted to 

have concentrations greater than 10 mg/L (fig. 6-7). 

Overlaying simulation results for the two models 

show that there are only small areas of overlap 

(red, in fig. 6-7) between areas predicted to have 

concentrations greater than 10 mg/L of total 

nitrogen in streams and areas predicted to have 

concentrations greater than 10 mg/L of nitrate in 

groundwater. The contrast in the maps reflects 

the contrast in physical and chemical aspects of 

the landscape that facilitate nitrogen transport to 

streams or to groundwater. For example, factors that 

promote rapid infiltration of rainfall into the ground, 

such as permeable soils, enhance the likelihood for 

contamination of groundwater; factors that inhibit 

infiltration and promote runoff, such as impermeable 

soils and artificial drainage, favor contamination 

of streams. 

Table 6-2. The model for deep groundwater was used to predict the 

number of people using private wells in areas with different nitrate 

concentrations for two well-depth scenarios. Conditions at shallow depths 

(33 feet) constitute an early-warning of changes that may occur as shallow 

groundwater with high nitrate concentrations migrates to greater depths in 

the aquifer. 

[mg/L, milligrams per liter]

Predicted nitrate 

concentration range

Population counts for 

simulation depth Percentage

change

164 feet 33 feet

 No data 1,710,000 1,710,000 Not applicable

Background:

 0 to ≤1 mg/L 20,000,000 19,400,000 –3.1

Elevated:

 >1 to ≤5 mg/L 13,000,000 13,300,000 +3.1

Moderate:

 >5 to ≤10 mg/L 1,240,000 1,400,000 +13.7

Severe:

 >10 mg/L 467,000 528,000 +13.0

Figure 6-7. The contrast in the areas predicted by the streams model (green) and by the groundwater model 

(brown) to have total nitrogen (in streams) or nitrate (in groundwater) concentrations greater than 10 milligrams 

per liter reflects the contrast in physical and chemical aspects of the landscape that facilitate nitrogen 

transport to streams or to groundwater. 
112 Nutrients in the Nation’s Streams and Groundwater, 1992–2004

0

2

4

6

8

10

12

≤1 1–10 10

Frequency of exceedance by an organic

contaminant, in percent 

Nitrate concentration range, in milligrams per liter as N

Percentage of wells at concentrations 

greater than one-tenth of HBSL or MCL 

Percentage of wells at concentrations

greater than HBSL or MCL 

Co-Occurrence of Organic Contaminants and Nitrate at 

Concentrations of Concern for Human Health 

Leslie DeSimone, U.S. Geological Survey

Data for 2,167 domestic wells sampled in major aquifers across the United States (DeSimone, 

2009) show that the frequency of encountering an organic contaminant—that is, a pesticide 

or volatile organic compound—at a concentration of 10 percent or more of an MCL (U.S. 

Environmental Protection Agency, 2006) or USGS Health-Based Screening Level (HBSL) 

(Toccalino and Norman, 2006; Toccalino and others, 2006) increases as nitrate concentrations 

increase (fig. 6-8). The likelihood of encountering a concentration of an organic contaminant 

that exceeds 10 percent of an MCL or HBSL is about five times greater in samples with nitrate 

concentrations above the background level of 1 mg/L than in samples with nitrate concentrations 

below background levels (see Chapter 4, Background Conditions). Similarly, the frequency 

of occurrence of an organic contaminant at a concentration above an MCL or HBSL is about 

four times greater in groundwater with nitrate concentrations above background levels than 

in groundwater with nitrate concentrations below background levels (see red bars, fig. 6-8). 

Overall, 84 percent of samples with concentrations of organic compounds greater than MCLs or 

HBSLs also had nitrate concentrations above background levels. These findings indicate that 

when concentrations of nitrate are above background levels in a domestic drinking-water supply, 

additional analysis for organic contaminants should be considered. 

Figure 6-8. The likelihood of detecting an organic contaminant at a 

concentration of concern for human health is greater in samples with 

nitrate concentrations above the background level of 1 mg/L than in 

samples with nitrate concentrations below background levels. (HBSL, 

Health-Based Screening Level; MCL, Maximum Contaminant Level.)
Chapter 7

Potential for Effects 

on Aquatic Life

Introduction

Excessive nutrients and resulting 

instream plant biomass can have a 

wide range of impacts on aquatic 

ecosystems. There is increasing interest 

in establishing numeric nutrient criteria 

at scales that reflect the geographic 

variability in the natural factors 

affecting instream nutrient conditions. 

The responses of aquatic biota to 

nutrient enrichment, and the potential 

use of biological metrics as measures 

of water-quality status and biological 

condition, also are areas of active 

study. This chapter examines the status 

of streams with respect to ammonia 

criteria, geographic variability of 

background nutrient concentrations, 

and the response of aquatic biota to 

varying nutrient levels. 

Photograph by William S. Keller, U.S. National Park Service.

Chapter

7
114 Nutrients in the Nation’s Streams and Groundwater, 1992–2004

Potential Effects of Ammonia on 

Aquatic Life

Ammonia (NH3) and its ionized form, ammonium 

(NH4

+), occur naturally and are formed during decomposition 

of proteins, manure wastes, and other nitrogen-containing 

compounds. Ammonia is an excellent source of nitrogen 

for plants and is part of most natural and synthetic fertilizer 

formulations. Ammonia is soluble in water, and water 

contaminated with fertilizer runoff, animal wastes, or 

sewage may contain elevated concentrations of ammonia. 

In well‐oxygenated streams, microorganisms may convert 

ammonia to nitrite and nitrate through a process known 

as nitrification. 

The un-ionized form of ammonia (NH3) can be toxic 

to aquatic plants, invertebrates, and fish at concentrations 

less than 1 mg/L (U.S. Environmental Protection Agency, 

1999). Concentrations of ammonia in NAWQA samples were 

compared with aquatic health criteria for both acute and chronic 

effects using guidelines established by the USEPA for protection 

of aquatic life (U.S. Environmental Protection Agency, 1999). 

More stringent ambient water-quality criteria have recently been 

proposed which would be more protective of sensitive species 

(U.S. Environmental Protection Agency, 2009a) (see sidebar, 

Calculation of Potential Ammonia Toxicity). 

Calculation of Potential Ammonia Toxicity 

The toxicity of ammonia in water is dependent on the proportion of ammonia present in the un-ionized 

form. Because the distribution of ammonia between the ionized and un-ionized forms is both pH and 

temperature dependent, the pH and temperature of water samples are required to determine the toxicity 

of a specific stream environment to organisms. Toxicity increases as pH increases, and in general, 

as temperature decreases. Plants are more tolerant of ammonia than animals, and invertebrates are 

generally more tolerant than fish.

Concentrations in samples from streams were compared with criteria established to prevent both 

acute and chronic health effects caused by exposure of aquatic organisms to ammonia in water. Acute 

health effects are those caused by sudden and severe exposure to a toxicant, whereas chronic health 

effects are those that result from prolonged or repeated exposure over days, months, or years. The acute 

criteria for ammonia in water range from 1.3 to 49 mg/L of total ammonia for pH values of 6.5–9.0 and 

water temperatures of 0–30°C. The chronic criteria range from 0.18 to 6.7 mg/L of total ammonia for the 

same pH and water temperature ranges. Effects of chronic exposure generally are evaluated over a 4- to 

30-day averaging period (U.S. Environmental Protection Agency, 1999).

The USEPA has proposed new ambient criteria for ammonia that consider toxicological data 

published since development of the current criteria (beginning in 1985) (U.S. Environmental Protection 

Agency, 2009a). Although the influence of pH and temperature on ammonia in the proposed criteria is the 

same as in the past, the new criteria take into account the presence or absence of freshwater mussels. 

Freshwater mussels are particularly sensitive to ammonia, and many species are endangered or in 

decline. For example, laboratory tests show that ammonia concentrations as low as 0.03 mg/L can affect 

growth rates of freshwater mussels, and at 0.12 mg/L or greater can be lethal to mussels (Newton and 

Bartsch, 2007). Adoption of these more stringent criteria may result in more streams being listed as out of 

compliance with respect to ammonia.
Chapter 7

Potential for Effects on Aquatic Life 115

Fig_7-01.ai

Agricultural

Urban

Mixed

Watershed land use

Acute

criteria

Chronic

criteria

Percentage of sites where ammonia 

criteria were exceeded

0 5 10 15

Wastewater-treatment plant effluent provides much of the streamflow in the Santa Cruz River at Tubac, Arizona, during 

part of the year. Photograph by U.S. Geological Survey.

Figure 7-1. Concentrations of ammonia in 

streams seldom exceeded acute or chronic 

criteria set to protect aquatic life. Sites at 

which the criteria were exceeded generally 

are on streams in the semiarid West that drain 

large areas of urban or mixed land and are 

potentially influenced by treated effluent from 

wastewater-treatment facilities located upstream.

The concentrations of ammonia at NAWQA stream sites seldom 

exceeded levels believed to adversely affect aquatic life. Ammonia 

concentrations in more than 24,000 samples from 499 sites were compared 

with acute and chronic criteria on the basis of pH and temperature of 

the samples. The acute criteria for ammonia were rarely exceeded; only 

33 samples (0.14 percent) at 7 sites (1.4 percent) had concentrations greater 

than the criteria. Samples that exceeded the acute criteria were split among 

urban (2 sites or 3.6 percent of urban sites), mixed (4 sites or 2.7 percent 

of mixed sites), and agricultural (1 site or 0.7 percent of agricultural sites) 

land-use settings (fig. 7-1). 

The chronic criteria for ammonia were exceeded in 139 samples 

(0.57 percent) at 22 sites (4.4 percent). Sites at which concentrations of 

ammonia exceeded chronic criteria also were primarily in urban (8 sites 

or 14 percent of urban sites) and mixed (9 sites or 6 percent of mixed 

sites) land use areas; only 10 samples at 5 agricultural sites (3.7 percent of 

agricultural sites) exceeded the criteria (fig. 7-1). Ninety-one exceedances 

(65 percent) of the chronic criteria occurred at only 4 sites (2 urban and 2 

mixed land use) in the semiarid West. 
116 Nutrients in the Nation’s Streams and Groundwater, 1992–2004

2,900

2,900–15,000

15,000

Stream sites, by watershed land use

Data on nitrogen inputs not available 

for Alaska and Hawaii.

EXPLANATION

Agricultural Urban Undeveloped Mixed

Streams

No exceedance

Chronic criteria exceeded

Acute criteria exceeded

Type of aquatic health

criteria exceeded for

ammonia

Estimated 1997 nitrogen inputs from

fertilizer, manure, and atmosphere,

in pounds per square mile

Figure 7-2. Ammonia at concentrations greater than the aquatic life criteria were infrequent, 

and most such sites were in the Western States. These sites generally are in areas of urban 

and mixed land use and are either known or suspected to be affected by the discharge of 

wastewater-treatment plants.

Although concentrations greater than ammonia criteria 

were infrequent, the sites where criteria were exceeded 

were predominantly in the Western States (shown by the 

red and yellow symbols in fig. 7-2). With few exceptions, 

sites with exceedances generally were in urban and mixed 

land-use areas. In many cases, treated effluent from 

wastewater‐treatment facilities was known or suspected to be 

the source of ammonia. NAWQA sampling sites were selected 

to represent regional-scale, ambient water‐quality conditions, 

and it must be reiterated that when feasible, sites were selected 

to avoid the effects of major wastewater-treatment plants and 

other point sources. Thus, these findings generally are not 

representative of overall stream water-quality conditions where 

treated effluent is a significant component of streamflow. 

Advances in the treatment of wastewater have resulted in 

decreases in concentrations of ammonia in some streams 

that receive treated effluent (see Chapter 8 sidebar, Past 

Changes Resulting from Nutrient Control Measures). 

Despite the large inputs of fertilizer and manure, 

sampling at 135 agricultural sites found only rare 

exceedances of ammonia criteria. This indicates that 

ammonia from nonpoint sources is not reaching or persisting 

in streams at high concentrations. Rather, ammonia in 

agricultural watersheds is either being sorbed onto soils, 

volatilized, converted to nitrate through the process of 

nitrification, and (or) rapidly removed from the water 

column by aquatic plants.
Chapter 7

Potential for Effects on Aquatic Life 117

Lowess smooth

South Platte River at 

Henderson, Colorado

Platte River at 

Louisville, Nebraska

Streamflow, in cubic feet per second

100 1,000 10,000 100,000

Concentration of total ammonia, in milligrams per liter as N10

1

0.1

0.01

110° 100°

42°

39°

0 50 100 MILES

0 40 80 KILOMETERS

South Platte 

River at 

Henderson

Platte River at 

Louisville

Denver

NEBRASKA

WYOMING

S. Platte R.

COLORADO

KANSAS

SOUTH DAKOTA

Platte River EXPLANATION

Major land use

Stream sampling site

Basin boundary

Agricultural

Urban

Forest

Rangeland

Other undeveloped

Water

MAP LOCATION

Changes in Ammonia Sources and 

Streamflow in Western Basins 

The relation between streamflow and ammonia concentration may reflect whether the ammonia is from a point 

source or a nonpoint source. Most exceedances of ammonia criteria occurred in western streams in semiarid settings 

where treated wastewater effluent is likely present. The South Platte River Basin in Colorado and Nebraska is typical of 

many semiarid western basins with large urban centers. In these basins, treated wastewater effluent is often a major 

component of streamflow. Concentrations of ammonia in the South Platte River at Henderson, just downstream of the 

Denver urban center, are an order of magnitude greater than those in the Platte River at Louisville, Nebraska, hundreds of 

miles downstream. This contrast primarily is a result of the source of ammonia and lack of dilution near Denver. Most of 

the watershed above the Henderson, Colorado, site is undeveloped, and because of the semiarid climate and upstream 

diversions, the treated wastewater from the Denver metropolitan area contributes about 69 percent of the streamflow in 

the South Platte River at this site (Dennehy and others, 1995). Consequently, when streamflow is low in the South Platte 

River at Henderson, the relative contribution from treated wastewater is high, so that concentrations of ammonia in the 

river are high (fig. 7-3). When snowmelt runoff from the undeveloped forests at higher elevation in the watershed reaches 

the streams, flow in the South Platte River at Henderson increases, the relative contributions from treated wastewater 

decrease, and concentrations of ammonia decrease. In contrast, the watershed of the Platte River at Louisville, Nebraska, 

is more than 10 times larger than the watershed of the South Platte River at Henderson, and about 83 percent of the Platte 

River watershed is agricultural land and rangeland. As flow in the Platte River at Louisville increases, concentrations 

of ammonia also increase (fig. 7-3). Nonpoint sources, primarily fertilizer and manure, are responsible for most of the 

nitrogen introduced to the watershed upstream of the Platte River at Louisville (Mueller and Spahr, 2005). The increase in 

ammonia concentrations with flow at the Platte River at Louisville is indicative of a watershed where nutrients primarily 

are from nonpoint sources and are transported to streams primarily as runoff after their application to the land surface.

Figure 7-3. The relation between streamflow and ammonia concentration at a site reflects whether the ammonia in a stream 

is from a point source or a nonpoint source. Discharges of treated wastewater effluent to the South Platte River downstream 

of Denver, Colorado, result in higher concentrations of ammonia when streamflow is low, and lower concentrations when 

streamflow is high. The relation between ammonia and streamflow at the Platte River at Louisville, Nebraska, is reversed: 

ammonia concentrations increase with increasing streamflow, indicating that the ammonia is primarily from nonpoint sources.
118 Nutrients in the Nation’s Streams and Groundwater, 1992–2004

1

The value of 0.125 mg/L for total phosphorus in region 10 is discounted as unreliable by the USEPA.

Background site data

 (75th percentile)

Modeled background

USEPA recommended 

nutrient criteria

National background

Background site data

 (75th percentile)

Modeled background

USEPA recommended 

nutrient criteria

National background

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.00

USEPA nutrient ecoregion

Total phosphorus concentration, in milligrams per liter

Total nitrogen Total phosphorus

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

Background site data shown for ecoregions with at least 10 sites. Criteria for USEPA nutrient ecoregion 13 are under development.

0.0

0.5

1.0

1.5

2.0

2.5

USEPA nutrient ecoregion

Total nitrogen concentration, in milligrams per liter

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

Both the modeled background concentrations and 

the USEPA recommended nutrient criteria have similar 

patterns of variation among the nutrient ecoregions (fig. 7-4). 

Modeled background concentrations represent a condition 

of no human development and generally are less than the 

recommended criteria, which represent minimal human impact. 

However, the recommended criteria are less than modeled 

background concentrations in several regions, particularly 

for total phosphorus. Most of the recommended criteria also 

are somewhat less than regional background concentrations 

estimated by the 75th percentiles of flow-weighted means 

for minimally impacted sampling sites. This comparison 

suggests that it may be difficult to meet the USEPA criteria for 

phosphorus in streams in developed basins in some nutrient 

ecoregions, in particular for total phosphorus in streams in 

basins that have even a modest degree of development, such as 

those in the Great Plains Grass and Shrublands ecoregion (4). 

Other approaches to estimating concentrations indicative 

of reference conditions are found in the literature, and research 

is continuing in this area. Herlihy and Sifneos (2008) used data 

from sites sampled as part of the USEPA’s Wadeable Streams 

Assessment (U.S. Environmental Protection Agency, 2006) 

to evaluate different approaches to setting regional nutrient 

criteria. Although the values for total nitrogen were relatively 

consistent with the USEPA recommended criteria, values for 

total phosphorus were always greater than the recommended 

criteria, and were greater by a factor of 3.8 for ecoregion 4.

Regional Nutrient Criteria and 

Background Estimates

Concentrations of nutrients in minimally impacted 

streams can be affected by geographic variability of natural 

conditions, such as geology and climate. Recognizing this 

variability, the USEPA has developed recommended criteria 

for total nitrogen and total phosphorus for the protection 

of aquatic life for 14 nutrient ecoregions (see sidebar, The 

U.S. Environmental Protection Agency’s National Nutrient 

Criteria Program). These criteria are based on the 25th 

percentile of data available at the time of criteria development 

in each ecoregion, and are meant to approximate reference 

conditions. Current criteria for the 14 ecoregions range 

from 0.12 to 2.18 mg/L for total nitrogen and from 0.010 to 

0.0761 mg/L for total phosphorus; the criteria also include 

recommendations for turbidity and chlorophyll a (U.S. 

Environmental Protection Agency, 2002b).

Smith and others (2003) used the data from Clark 

and others (2000) to calibrate a simulation model and 

estimated background concentrations of total nitrogen and 

total phosphorus for all stream basins in the conterminous 

United States. From these estimates, they computed the 

average background concentration for each of the 14 USEPA 

nutrient ecoregions. These modeled values are compared with 

the USEPA recommended criteria in figure 7-4. The 75th 

percentiles of flow-weighted mean concentrations also are 

shown for the four ecoregions that have at least 10 minimally 

impacted sites in the data set used for this report.

Figure 7-4. U.S. Environmental Protection Agency (USEPA) recommended nutrient criteria and U.S. Geological Survey modeled 

estimates for background nutrient concentrations follow similar patterns of variation within the 14 nutrient ecoregions; however, 

recommended criteria are more stringent than estimates of background concentrations in several regions, particularly for 

phosphorus, suggesting that the recommended criteria may be difficult to attain in some areas. (See fig. 7-7 for list of ecoregions.)
Chapter 7

Potential for Effects on Aquatic Life 119

0.001

0.01

0.1

1

10

0.001

0.01

0.1

1

10

USEPA nutrient ecoregion

USEPA nutrient ecoregion

Total phosphorus concentration, in milligrams per liter Total phosphorus concentration, in milligrams per liter

Agricultural sites

Urban sites

Total phosphorus

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

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

7 16 48

12

14 9

12

8

8

10

14

0.1

1

10

0.1

1

10

USEPA nutrient ecoregion

Total nitrogen concentration, in milligrams per liter

Agricultural sites

USEPA nutrient ecoregion

Total nitrogen concentration, in milligrams per liter

Urban sites

A.

B.

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

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

Total nitrogen

7

16

49

12 16 9

13

9

8

10

14

90th percentile

Boxplot shown for ecoregions

with more than 6 sampling sites

Modeled background

USEPA recommended

nutrient criteria 75th percentile

Median

25th percentile

10th percentile

EXPLANATION

Total nitrogen

Modeled background

USEPA recommended

nutrient criteria

Total phosphorus

Site value

12 Number of sampling sites

Criteria for USEPA nutrient ecoregion 13 are under development.

Concentrations of both total nitrogen and total 

phosphorus measured in agricultural and urban streams 

typically are much higher than the recommended ecoregional 

criteria, as well as the modeled estimates for background 

concentrations (fig. 7-5A and 7-5B). For example, the 

median concentration for sites within a specific nutrient 

ecoregion ranges from about 2 to more than 10 times the 

respective ecoregional criterion for both total nitrogen and 

total phosphorus. The magnitudes of the differences between 

concentrations measured in agricultural and urban landscapes 

and recommended criteria show that large reductions in 

concentration would be necessary if recommended criteria for 

these areas are to be met.

Figure 7-5. Concentrations of total nitrogen and total phosphorus measured at both agricultural (A) and urban (B) stream sites were 

from 2 to more than 10 times greater than modeled regional background concentrations and U.S. Environmental Protection Agency 

(USEPA) recommended nutrient criteria. These comparisons provide a perspective on the magnitude of the challenge necessary to 

meet the USEPA recommended nutrient criteria for streams in developed basins. (See fig. 7-7 for list of ecoregions.) 
120 Nutrients in the Nation’s Streams and Groundwater, 1992–2004

The U.S. Environmental Protection Agency’s 

National Nutrient Criteria Program 

“To restore and maintain the chemical, physical and biological integrity of the Nation’s waters” 

–Clean Water Act Section 101(a) 

Nutrients consistently rank as one of the top five causes of water body impairment for waters surveyed 

by States, Tribes, and Territories and reported to USEPA. USEPA’s current figures indicate that for those 

waters assessed, nutrients have a combined direct and indirect effect on the impairment of 51 percent of 

river and stream miles, 52 percent of lake acres, and 58 percent of bay and estuarine square miles (U.S. 

Environmental Protection Agency, 2009b). The influence of nutrients on streams was further documented 

in a recent USEPA study that reported that 30 percent of the streams in the United States contained 

impaired biological communities with nitrogen and phosphorus as the leading causes of impairment (U.S. 

Environmental Protection Agency, 2006).

To address these issues, the USEPA started the National Nutrient Program as an offshoot of the 

1998 Clean Water Action Plan to reduce eutrophication that results from human activity. The USEPA is 

partnering with States, authorized Tribes, and Territories to identify and adopt appropriate numeric nutrient 

criteria into State, Tribal, and Territorial water-quality standards and implement those standards in their 

water-quality programs (fig. 7-6).

USEPA developed recommended criteria for eutrophication causal and response variables for rivers/

streams and lakes/reservoirs. Recommended criteria for rivers/streams were developed for 13 nutrient 

ecoregions1

 (fig. 7-7). USEPA also provided technical guidance for developing nutrient criteria for wetlands 

and for estuarine and coastal marine waters. USEPA provided three options to States and Tribes for 

adopting nutrient criteria in rivers/streams and lakes/reservoirs:

• Adopt the ecoregional criteria recommendations that USEPA published under section 304(a) of 

the Clean Water Act; 

• Modify the section 304(a) criteria to reflect site-specific conditions; or 

• Adopt criteria based on other scientifically defensible methods.

Most States are utilizing the last approach, basing their criteria on the biological responses to nutrients 

in waters of their State. This approach requires extensive field data, and statistical analysis of those 

data. USEPA supports the States by funding data gathering efforts and making national experts available 

to analyze that data, and provides technical assistance as States consider and derive nutrient criteria 

endpoints. USEPA published a status report in 2008 (U.S. Environmental Protection Agency, 2008) on the 

progress States are making in adopting recommended nutrient criteria into Water-Quality Standards. This 

report can be found on the Nitrogen and Phosphorus Pollution Program webpage: http://www.epa.gov/

waterscience/criteria/nutrient/strategy/status.html. In addition to supporting States, Tribes, and Territories 

in developing numeric nutrient criteria, USEPA also is engaged in the following activities:

• Working with the State of Florida to develop nutrient and biological criteria for both inland and 

estuarine coastal waters in response to a lawsuit from the Florida Wildlife Federation; 

• Developing a document with recommendations for using empirical approaches derived field data to 

establish nutrient criteria; and 

• Working with State and national organizations to assess innovative approaches to reducing 

nitrogen and phosphorus pollution.

1

Criteria for USEPA nutrient ecoregion 13 are under development.
Chapter 7

Potential for Effects on Aquatic Life 121

National

criterion

Ecoregional

criterion

Starting point

(304(a) criteria)

Ecoregion

subcategory

criterion

GOAL 

Site-specific

criterion

Relatively costly

Range

of 

possible

values

Resources needed

to develop criteria

(Time, Funds)

Geographic scales of nutrient criteria development

1

2

3

4

5

6

7

8

9

11

13

14

11

10

4

8

2

12

U.S. Environmental Protection Agency 

nutrient ecoregions

Willamette and Central Valleys

Western Forested Mountains

Xeric West

Great Plains Grass and Shrublands

Central Cultivated Great Plains

Corn Belt and Northern Great Plains

Mostly Glaciated Dairy Region

Nutrient Poor Largely Glaciated Upper Midwest and Northeast

Southeastern Temperate Forested Plains and Hills

Texas-Louisiana Coastal and Mississippi Alluvial Plains

Central and Eastern Forested Uplands

Southern Coastal Plain

Southern Florida Coastal Plain

Eastern Coastal Plain

1

2

3

4

5

6

7

8

9

10

11

12

13

14

EXPLANATION

Figure 7-6. In recognition of both the 

wide range in nutrient concentrations 

across the Nation, as well as the large 

amount of resources needed to develop 

nutrient criteria, the USEPA developed 

recommended criteria for ecoregions 

as a starting point for States, Tribes, and 

Territories to begin developing criteria 

specific to their conditions.

Figure 7-7. The U.S. Environmental Protection Agency has developed recommended 

nutrient criteria for rivers and streams for 14 ecoregions in the conterminous United States 

(U.S. Environmental Protection Agency, 2002b). 
122 Nutrients in the Nation’s Streams and Groundwater, 1992–2004

Comparison of Nutrient Criteria with 

Predicted Total Nitrogen in Streams 

Based on NAWQA data 

A statistical model developed on the basis 

of NAWQA data for total nitrogen in streams 

predicts widespread exceedance of the proposed 

nutrient criteria for total nitrogen concentrations, 

and only a modest decrease in concentration 

in response to reduced fertilizer inputs (Spahr 

and others, 2010) (see Chapter 3, Extrapolation 

to Unsampled Areas, Development of Models 

for Total Nitrogen Concentrations in Streams). 

The model was used to calculate the probability 

that the flow‐weighted average concentration of 

total nitrogen in a given stream reach is greater 

than the recommended criteria for that nutrient 

ecoregion (U.S. Environmental Protection 

Agency, 2000). The model simulations show that 

streams in which total nitrogen concentrations 

have a high probability (75 percent or greater) 

of exceeding the recommended criteria are 

broadly distributed across the Nation (fig. 7-8A), 

and constitute 83 percent of all stream miles 

(fig. 7-8B). This proportion is similar to what 

would be expected considering how the criteria 

were designed (U.S. Environmental Protection 

Agency, 2001, fig. 5). In the Corn Belt and 

Northern Great Plains (nutrient ecoregion 6), the 

recommended nitrogen criterion is likely to be 

exceeded in almost 70 percent of stream miles 

with a probability of at least 90 percent.

The models also can be used to provide 

broad regional estimates of how total nitrogen 

concentrations in streams may change with 

reductions in nonpoint sources of nitrogen. 

For example, when nitrogen inputs from 

fertilizer use were reduced by 25 percent in the 

Corn Belt and Northern Great Plains nutrient 

ecoregion, the number of stream miles with a 

high probability (greater than 90 percent) of 

exceeding the nutrient criterion decreased by 

only about 2.5 percent. Although the reduction 

in fertilizer use resulted in a decrease in the 

estimated nitrogen concentrations, concentrations 

in most streams still greatly exceeded the nutrient 

criterion. Fertilizer inputs are only one potential 

source of nitrogen to streams, and this exercise 

did not consider change in other nitrogen 

sources. Although this scenario does not capture 

the complex response to a change in nutrient 

input in a specific stream, it does illustrate the 

magnitude of the challenge of reducing nutrient 

impact on streams through source reduction 

alone. The results suggest that greater use of land 

management strategies to reduce the transport 

of nutrients to streams may be needed to meet 

recommended criteria for streams draining 

areas with appreciable agricultural and urban 

development. For example, installation of simple 

bioreactors to treat tile drainage has been shown 

to be an effective way to reduce transport of 

nitrogen to streams (Jaynes and others, 2008).
Chapter 7

Potential for Effects on Aquatic Life 123

Streams

Corn Belt and Northern

Great Plains nutrient

ecoregion

A.

50 60 70 80 90

0

20

40

60

80

100

100

Probability of total nitrogen concentration

exceeding the USEPA recommended regional criteria

Percentage of stream miles 

B.

83

75

Probability that predicted average nitrogen concentration exceeds recommended nutrient

criteria for that ecoregion 

EXPLANATION

≤25 25–50 50–75 75–90 90

Figure 7-8. Model simulations show that streams in which total nitrogen 

concentrations have a high probability (75 percent or greater) of exceeding 

the recommended criteria are broadly distributed across the Nation (A), and 

constitute 83 percent of all stream miles (B) (Spahr and others, 2010). 
124 Nutrients in the Nation’s Streams and Groundwater, 1992–2004

Photograph by Carmen Burton, U.S. Geological Survey.

Photograph by Steve Fend, U.S. Geological Survey.

Photograph by Jason May, U.S. Geological Survey.

Some State water-quality programs use assessments of algal, 

macroinvertebrate, or fish communities.

Influence of Elevated Nutrients on 

Aquatic Life

Although nutrients, primarily nitrogen and 

phosphorus, are critical for a healthy ecosystem, 

excessive levels of nutrients in streams can lead to 

eutrophic conditions characterized by increased plant 

growth followed by a decline in oxygen in the water 

when dead plant material (algae or macrophytes) 

decomposes. Excessive plant growth can further affect 

stream health by reducing stream velocity and thereby 

causing greater deposition of fine sediments along the 

stream bottom. Increased deposition of fine sediments 

reduces quality and availability of streambed habitat, 

often resulting in reduction or loss of bottom-dwelling 

organisms. Although nutrient enrichment from elevated 

concentrations of nitrogen and phosphorus is a major 

contributing factor to stream eutrophication, enrichment 

effects can vary from one stream to another as a result of 

differences in patterns of streamflow, amount of riparian 

shading, water temperature, water clarity, and the extent 

of groundwater and surface water exchange.

One of the most common approaches to assessing 

the effects of eutrophication is to use the presence or 

absence of certain plants and animals as indicators of 

stream health. These kinds of approaches are referred 

to as bioassessments, and commonly are used in 

many Federal and State water-quality programs. Most 

of these programs are based on assessments of the 

numbers and kinds of individuals comprising the algal, 

macroinvertebrate, or fish communities. Data on aquatic 

organisms add strength to stream-quality assessments 

because organisms integrate the effects of stream 

conditions over a period of time ranging from days to 

years, whereas a measurement of nutrient concentration 

represents a single point in time. Information about the 

composition of stream communities can be used to assess 

the overall health of a stream ecosystem, based on the 

degree to which biological communities differ from 

a natural state (see sidebar, Assessment of Biological 

Condition). Such differences can indicate whether a 

stream has been influenced by human disturbance or, 

conversely, how much it has recovered or improved 

from some previous condition. Calculation of biological 

condition provides a measure that can be used to compare 

individual sites or groups of stream sites across 

different landscapes.
Chapter 7

Potential for Effects on Aquatic Life 125

Mean value

95 percent confidence interval

Total phosphorus, in milligrams per liter

40

50

60

70

80

90

100

Total nitrogen, in milligrams per liter

Biological condition, in percent

40

50

60

70

80

90

100

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

Macroinvertebrates

Fish

Algae

Macroinvertebrates

Fish

Algae

Assessment of 

Biological Condition 

The NAWQA Program used a consistent 

approach to assess biological condition for 

algal, macroinvertebrate, and fish communities 

at about 1,400 stream sites across the 

Nation. Biological condition was assessed 

by comparing observed (O) community 

characteristics (such as the number of taxa) 

to those expected (E) if the community was 

minimally disturbed by human activities. 

The observed condition (O) is derived from 

a sample collected at a site, whereas the 

expected (E) condition is estimated based 

on data from a set of environmentally similar 

reference sites. Because natural variation in 

environmental settings is accounted for in this 

approach, departures of observed condition 

from expected condition are likely the result 

of human activities. The ratio of observed to 

expected conditions (O/E) can be expressed as 

a percentage of the reference condition scaled 

from 0 percent to 100 percent. The lower the 

percentage, the more altered the biological 

community, whereas a higher percentage 

means that the biological community is more 

similar to reference conditions.

Decreases in Biological Condition Were 

Associated with Nutrient Enrichment 

Biological condition for algal, macroinvertebrate, and fish 

communities declined with increasing concentrations of nitrogen 

and phosphorus in streams (fig. 7-9). Streams were grouped into 

three categories of nutrient enrichment according to concentration 

ranges of total nitrogen and total phosphorus based on published 

nutrient enrichment categories (Dodds and others, 1998). Changes 

in biological condition corresponding to increasing nutrient 

concentrations were more pronounced for algal communities than 

for macroinvertebrate or fish communities. In streams with the 

highest concentrations of total nitrogen or total phosphorus, average 

biological condition scores for algal communities were about 30 

percentage points lower than algal condition in streams with the 

lowest nutrient concentrations. In contrast, biological condition 

scores in streams with the highest nutrient concentrations were 

about 14 percentage points lower for fish communities and about 

10 percentage points lower for macroinvertebrate communities 

compared to streams with the lowest nutrient concentrations. 

This finding is consistent with the direct link between nutrient 

availability and algal growth, whereas the response of higher 

organisms to nutrients is mostly indirect, mediated by transfer of 

energy (food resources) from primary producers (aquatic plants and 

algae) via food chains of various complexity. Algal communities 

are a useful biological indicator of eutrophication because of their 

sensitivity to nutrient enrichment (see sidebar, Algae as Indicators 

of Nutrient Enrichment). 

Figure 7-9. Biological condition for algal, macroinvertebrate, and fish communities declined with 

increasing concentrations of nitrogen and phosphorus in streams. Changes were most pronounced 

for algal communities, in which the average biological condition in streams with the highest nutrient 

concentrations was about 30 percentage points lower than the algal biological condition in streams 

with the lowest nutrient concentrations. 
126 Nutrients in the Nation’s Streams and Groundwater, 1992–2004

Fig_7-10.ai

75th percentile

90th percentile

Median

25th percentile

10th percentile

EXPLANATION

Algal biological condition, in percent

0

20

40

60

80

100

Undeveloped Agricultural Urban

Algal Community Condition Was Reduced 

for Many Streams in Agricultural and 

Urban Land-Use Settings

The relatively low median biological condition 

scores for algal communities in urban and agricultural 

streams indicated that stream health may be degraded 

in these land‐use settings compared to that in streams 

in undeveloped areas (fig. 7-10). The median biological 

condition scores of algal communities in agricultural and 

urban streams were 33 and 49 percentage points lower, 

respectively, than the reference condition score. Although 

the condition of algal communities was significantly 

reduced in agricultural and urban streams, the wide range 

of condition scores (indicated by the difference between 

the tenth and ninetieth percentiles) shows that algal 

condition was highly variable—that is, streams with high 

(expected in relatively undisturbed stream environments) 

as well as low (expected in highly disturbed stream 

environments) biological condition were found in each of 

these land-use settings. Algal condition scores generally 

were lower at urban sites than at agricultural sites, despite 

significantly higher concentrations of total nitrogen at 

agricultural sites. This lack of correspondence suggests 

that other factors, such as habitat, the specific forms of 

nitrogen or phosphorus present, or the presence of other 

chemical stressors, also are influencing the condition of 

algal communities at urban sites. 

Figure 7-10. The relatively low median biological condition 

scores for algal communities in urban and agricultural streams 

indicates that stream health may be degraded in these land-use 

settings compared to that in undeveloped streams. 

Fish are electroshocked, collected, surveyed, and released back to the 

stream to determine a census of fish population. Photograph by Terry 

Maret, U.S. Geological Survey.

Macroinvertebrates are collected with a Slack sampler. Photograph 

by Dennis Sun, U.S. Geological Survey.