U.S. Department of the Interior

U.S. Geological Survey

USGS Fact Sheet 116–99

May 1999

Lower

Susquehanna

River Basin

Western

Lake

Michigan

Drainage

Nevada Basin

and Range

Central

Columbia

 Plateau

San

JoaquinTulare

Basins

Central

Columbia

 Plateau

Willamette

Basin

Upper

Snake River

Basin

Red River

of the North Basin

Rio

Grande

Valley

Ozark

Plateaus

Central Nebraska

 Basins

South Platte

River Basin 

ApalachicolaChattahoocheeFlint River Basin

Western

Lake

Michigan

Drainages

White

River

Basin

Albemarle-Pamlico

Drainage

Potomac River Basin

Connecticut, Housatonic,

and Thames River Basins

Hudson River

Basin

Lower

Susquehanna

River Basin

San

JoaquinTulare

Basins

Trinity

River

Basin

Georgia-Florida 

Coastal Plain

Nevada Basin

and Range

0 400 MILES

0 400 KILOMETERS

Extent of nutrient contamination

and possible concerns

Nitrate, the form of nitrogen most related

to human health, generally does not pose a

health risk for residents whose drinking

water comes from streams or from aquifers

buried relatively deep beneath the land.

Health risks increase in aquifers located in

vulnerable geologic settings, such as in

sand, gravel, or karst (weathered carbonate

rock), that allow rapid movement of water.

In 4 of 33 major drinking-water aquifers

sampled, the U.S. Environmental

Protection Agency (USEPA) drinkingwater standard for nitrate was exceeded in

more than 15 percent of samples collected.

These aquifers, all of which underlie

intensive agricultural areas, are in

vulnerable geologic settings in the Central

Valley of California, the Great Plains, and

parts of the Mid-Atlantic region.

The most prevalent nitrate contamination was detected in shallow ground water

(less than 100 feet below land surface)

beneath agricultural and urban areas,

where about 15 percent of all samples

exceeded the USEPA drinking-water

standard. This finding raises potential

concerns for human health, particularly in

rural agricultural areas where shallow

ground water is used for domestic water

supply. Furthermore, high levels of nitrate

in shallow ground water may serve as an

early warning of possible future contamination of older underlying ground water,

which is a common source for public

water supply.

Concentrations of nitrogen and

phosphorus commonly exceed levels that

can contribute to excessive growth of

algae and other nuisance plants in streams.

For example, average annual concentraNational and regional insights on nutrients and pesticides in streams and ground water are based on

findings from studies completed in 1998 by the National Water-Quality Assessment (NAWQA) Program in 20

of the Nation’s most important river basins and aquifer systems.

STREAMS AND GROUND WATER in

basins with significant agricultural or

urban development, or with a mix of

these land uses, almost always contain

complex mixtures of nutrients (nitrogen

and phosphorus compounds) and

pesticides. These mixtures are

composed of chemicals in current use,

others that were used historically (such

as DDT, which was banned in the early

1970s), and chemical breakdown

products. The types and concentrations

of nutrients and pesticides found in

streams and ground water are closely

linked to land use and the chemicals

applied in each setting, such as

fertilizers and pesticides applied in

agricultural and urban areas, and

nutrients from animal and human

wastes. Thus, local and regional

management of fertilizer and pesticide

use can go a long way toward

improving water-quality conditions.

Land and chemical use are not,

however, the sole predictors of water

quality. Concentrations of nutrients and

pesticides vary considerably from

season to season, as well as among

watersheds with differing vulnerability

to contamination. Natural features,

such as geology and soils, and landmanagement practices, such as tile

drainage and irrigation, can affect the

movement of chemicals over land or to

aquifers and can thereby exert important

local and regional controls on water

quality. Understanding the national,

regional, and local importance of land

and chemical use, natural features, and

management practices on water quality

increases the effectiveness of policies

designed to protect water resources in

diverse settings.

The Quality of Our Nation’s Waters

Nutrients and Pesticides—A Summary
RELATIVE LEVEL OF CONTAMINATION

tions of total phosphorus in three-fourths

of streams in urban and agricultural areas

were greater than the USEPA desired goal

for preventing nuisance plant growth,

including algae, in streams. Such growth

can clog water intake pipes and filters and

can interfere with recreational activities,

such as fishing, swimming, and boating.

In addition, subsequent decay of algae can

result in foul odors, bad taste in drinking

water, and low dissolved oxygen in

aquatic habitats.

Extent of pesticide contamination

and possible concerns

The NAWQA Program measured 83

pesticides and breakdown products in

water and 32 pesticides in fish or bed

sediment. At least one pesticide was found

in almost every water and fish sample

collected from streams and in more than

one-half of shallow wells sampled in

agricultural and urban areas. Moreover,

individual pesticides seldom occurred

alone. Almost every sample from streams

and about one-half of samples from wells

with a detected pesticide contained two or

more pesticides. Although pesticides

frequently are found in water, their

potential effects on humans and aquatic

life are not fully understood. Potential

effects must be gauged from a combination of established water-quality standards

and guidelines, and by careful consideration of uncertainties and the potential for

unaccounted influences due to complexities related to pesticide occurrence.

The good news is that concentrations

of individual pesticides in samples from

wells and as annual averages in streams

were almost always lower than current

USEPA drinking-water standards and

guidelines. Standards and guidelines have

been established for 46 pesticides and

breakdown products. Effects of pesticides

on aquatic life, however, are a concern

based on U.S. and Canadian aquatic-life

guidelines, which have been established

for 28 of the pesticides measured. More

than one-half of agricultural and urban

streams sampled had concentrations of at

least one pesticide that exceeded a guideline for the protection of aquatic life.

 Potential risks to humans and aquatic

life implied by NAWQA pesticide findings

can be only partially addressed by

comparison to established standards and

guidelines. Many pesticides and their

breakdown products do not have standards

or guidelines, and current standards and

guidelines do not yet account for exposure

to mixtures and seasonal pulses of high

concentrations. In addition, potential

effects on reproductive, nervous, and

immune systems, as well as on chemically

sensitive individuals, are not yet well

understood. For example, some of the

most frequently detected pesticides are

suspected endocrine disrupters that have

potential to affect reproduction or

development of aquatic organisms or

wildlife by interfering with natural

hormones.

The widespread occurrence of

pesticides in water and the pervasive

uncertainty in assessing potential effects

on humans and aquatic life make pesticide

contamination a particularly difficult

water-quality problem to resolve. More

information is needed on potential effects

that are not well understood. In the meantime, our understanding of patterns of

pesticide contamination in relation to land

use, pesticide use, and the natural

characteristics of hydrologic systems can

help us to reduce the amounts of pesticides

that reach streams and ground water.

Nutrients and pesticides and their connection to land use

Relative levels of contamination are closely linked to land use and to the amounts and types of chemicals used

in each setting. Some of the highest concentrations of nitrogen and herbicides, including those most heavily

used (such as atrazine, metolachlor, alachlor, and cyanazine) were detected in samples collected from streams

and shallow ground water in agricultural areas. Some of the highest concentrations of phosphorus and

insecticides, including those currently used (such as diazinon, carbaryl, and malathion) and those historically

used (such as DDT, dieldrin, and chlordane) were detected in samples collected from urban streams.

Currently used

insecticides

Historically used

insecticides

Streams

Urban Agricultural Undeveloped

areas areas areas

Nitrogen Medium Medium–High Low

Phosphorus Medium–High Medium–High Low

Herbicides Medium Low–High No data

Medium-High Low–Medium No data

Medium-High Low–High Low

Currently used

insecticides

Historically used

insecticides

 Shallow Ground Water

Urban Agricultural

areas areas

Nitrogen Medium High

Phosphorus Low Low

Herbicides Medium Medium–High

Low–Medium Low–Medium

Low-High Low-High
Importance of seasonal and

geographic patterns in determining

protection strategies

Seasonal patterns in water quality of

streams emerged in most basins. The

patterns reflect many factors, but mainly

the timing and amount of chemical use,

the frequency and magnitude of runoff

from rainstorms or snowmelt, and specific

land-management practices, such as

irrigation and tile drainage. Concentrations

of nutrients and pesticides are highest

during runoff following chemical

applications. The seasonal nature of these

factors dictate the timing of elevated

concentrations in drinking-water sources

and aquatic habitats.

The geographic distribution of natural

features (including topography, geology,

soils, hydrology, and climate) and landmanagement practices (including tile

drainage, irrigation, and conservation

strategies) also affect the occurrence of

nutrients and pesticides in water. These

factors make some areas more vulnerable

to contamination than other areas, thus,

concentrations of nutrients and pesticides

can vary among seemingly similar land

uses and types of chemical applications.

Ground water is most vulnerable to

contamination in well-drained areas with

permeable soils that are underlain by sand

and gravel or karst. Examples are the

Platte River Valley in Colorado and

Nebraska, and karst regions within the

Susquehanna and Potomac River Basins in

Pennsylvania, Maryland, and Virginia. In

contrast, streams are most vulnerable in

basins with poorly drained clay soils, steep

slopes, or limited vegetation to slow

runoff. Tile drains and ditches also provide

Concentrations of nutrients and pesticides in streams and shallow ground

water generally increase with increasing amounts of agricultural and urban

land. This pattern is evident within small watersheds, as well as regionally,

where similar land-use settings and chemical applications extend over

broad areas. For example, intensive herbicide and fertilizer use in the Upper

Midwest has resulted in elevated concentrations of atrazine, nitrogen, and

phosphorus in streams throughout the region, including the Mississippi River.

Management strategies that are successful in reducing use and transport of

herbicides and fertilizers could lead to regional improvements in water quality.

Concentrations of nutrients and pesticides generally are higher and more prevalent in streams than in ground

water; however, indications of emerging ground-water contamination are important because ground-water

contamination is difficult to reverse. Ground-water flow rates are slow, and a contaminated aquifer can take

years or even decades to recover.

quick pathways for nutrient and pesticide

runoff to streams, such as in the White

River Basin in Indiana.

Patterns in regional vulnerability are

evident where similar natural features,

land use, and land-management practices

extend over broad areas. For example,

ground water underlying intensive

agriculture in parts of the Upper Midwest

is minimally contaminated where it is

protected by relatively impermeable soils

and glacial till that cover much of the

region. Local hotspots of nitrate and

pesticide contamination occur in the

region where ancient glacial streams

deposited sand and gravel, which enable

rapid infiltration and downward movement

of water and chemicals. Another example

is in the Southeast, where streams and

ground water contain relatively low

concentrations of nitrogen, partly because

soil and hydrologic characteristics in this

region favor conversion to nitrogen gas.

In contrast, relatively high nitrogen

concentrations occur in streams and

shallow ground water in the Central Valley

of California and parts of the Northwest,

Great Plains, and Mid-Atlantic regions,

because natural characteristics favor

transport of nitrogen.

Peter A. Steeves

Thomas L. Huntzinger
Water-quality changes

Water quality is constantly changing, from

season to season and from year to year.

Long-term trends, as captured by the

question “Are things getting better or

worse?,” are sometimes difficult to

distinguish from short-term fluctuations.

For many chemicals, it is too early to tell

whether conditions are better or worse,

because historical data are insufficient or

too inconsistent to measure trends. Despite

these challenges, some trends are evident

from monitoring of pesticides and

nutrients. These trends show that changes

in water quality over time frequently are

controlled by factors similar to those that

affect geographic variability, including

natural features, chemical use, and

management practices.

 One of the most striking trends is a

national reduction in concentrations of

organochlorine insecticides, such as DDT,

dieldrin, and chlordane, in whole fish.

Concentrations of DDT also have

decreased in sediment, as indicated in

sediment-core samples from urban and

agricultural

reservoirs and

lakes. Just as

notable as

these declines,

however, is

that these

persistent

insecticides

still are found at elevated levels in fish and

streambed sediment in many urban and

agricultural streams across the Nation.

Historical data also show that total

nitrogen concentrations have remained

stable over the past 20 years in rivers

downstream from wastewater treatment

plants, such as in the Trinity River in

Dallas, Texas. Improved treatment has

resulted in decreased

concentrations of

ammonia and

phosphorus despite

urban population

growth, but has also

resulted in changes in

the forms of nitrogen

in the river. Ammonia

is converted to nitrate,

which makes the

discharge less toxic to fish but may not

resolve problems with excessive plant

growth.

Changes in concentrations of modern,

short-lived pesticides follow changes in

use, often focused in specific regions and

land-use areas. For example, increases in

acetochlor and decreases in alachlor are

evident in some streams in the Upper

Midwest, where acetochlor partially replaced

alachlor for control of weeds in corn and

soybeans beginning in 1994. The changes in

use are reflected quickly in stream quality,

generally within 1 to 2 years.

In contrast, ground-water quality

responds more slowly to changes in

chemical use or land-management

practices, typically lagging by many years

and even decades. Local variations in

natural features, such as soil types and

amounts of recharge, can result in variable

rates of ground-water flow, which thereby

affect the long-term response to land

practices. For example, concentrations of

nitrate decreased significantly (from about

18 milligrams per liter in the mid-1980s to

less than 2 milligrams per liter in the mid1990s) in ground water underlying parts of

the Central Platte Natural Resources

District, Nebraska, after implementation of

fertilizer management strategies. Yet,

despite implementation of the strategies,

the response has been delayed in other

parts of the District because of differences

in local features controlling ground-water

flow. Specifically, concentrations of nitrate

remained greater than two times the

USEPA drinking-water standard in nearly

one-fourth of wells in one area sampled by

the District in the mid-1990s.

Science-based lessons for waterquality management and policy

Reductions of nutrient and (or) pesticide

concentrations in streams and ground water

clearly require management strategies that

focus on reducing chemical use and subsequent transport in the hydrologic system.

For these strategies to

be effective, they

should be developed

with careful consideration of the patterns

and complexities of

contaminant occurrence, behavior, and

influences on water

quality. NAWQA

results indicate four

basic considerations that are critical for

managing and protecting water resources

in diverse settings across the Nation.

First, local and regional management

strategies are needed to account for

This Fact Sheet accompanies a USGS

nontechnical publication titled

“The Quality of Our Nation’s Waters—

Nutrients and Pesticides” (USGS

Circular 1225). These publications are

available through the NAWQA Program.

For information, contact:

Chief, NAWQA Program

U.S. Geological Survey

413 National Center

Reston, VA 20192

phone: (703) 648-5716

fax: (703) 648-6693

email: nawqa_whq@usgs.gov

http://water.usgs.gov/lookup/get?nawqa

Terry R. Maret

Kevin F. Dennehy

geographic patterns in land use, chemical

use, and natural factors, which govern

hydrologic behavior and vulnerability to

contamination. Second, nutrients and

pesticides are readily transported among

surface water, ground water, and the

atmosphere and, therefore, environmental

policies that simultaneously address the

entire hydrologic system are needed to

protect water quality. Third, a top priority

should be to reduce the uncertainty in

estimates of the risks of pesticides and

other contaminants to humans and aquatic

life. This will require improved

information on the nature of exposure and

effects, and development of standards,

guidelines, and monitoring programs that

address the many complexities in

contaminant occurrence. For example,

neither current standards and guidelines

nor associated monitoring programs,

particularly with regard to pesticides,

account for contamination that occurs as

mixtures of various parent compounds and

degradation products, or that is

characterized by lengthy periods of low

concentrations punctuated by brief,

seasonal periods of higher concentrations.

Finally, continued development of reliable

predictive models is an essential element

of cost-effective strategies to anticipate

and manage nutrient and pesticide

concentrations over a wide range of

possible circumstances, over broad

regions, and for the long term. An understanding of these considerations will help

water managers and policy makers in their

implementation of environmental control

and protection strategies, in investments in

monitoring and science, and in the

development of future environmental

policies, standards, and guidelines.