U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 076–02

August , 2002

The U.S. Geological Survey (USGS) 

has monitored and assessed the quality of our Nation’s streams and ground 

water since its inception in 1879. Today, 

USGS provides information on issues 

such as the suitability of water for public 

supply and irrigation, aquatic ecosystem 

health, effects of agriculture and urbanization on water resources, acid rain, and 

disposal of radioactive waste. Through 

the integration of its six major waterquality programs (described on pages 5 

and 6), the USGS continues its mission 

to provide timely and relevant waterresources data and information that is 

freely available to all levels of government, non-governmental organizations, 

industry, academia, and the general 

public. The information provides a scientifi c basis for decision making related 

to resource management and restoration, 

and how we as individuals interact with 

our environment. 

Characteristics of USGS Water 

Quality Activities Provide 

Unique Perspective 

USGS programs provide a unique 

perspective on water-quality conditions, complementing much of the work 

conducted by local, state, and other 

Federal agencies, the private sector, and 

the university community. Six common 

characteristics of these programs are 

described.

First, USGS programs typically evaluate ambient water-quality conditions 

over long time scales with a regional 

and national perspective. This makes it 

possible to detect changes across time 

and space and to provide insights into 

natural and human factors that may be 

responsible for those changes. USGS 

programs are designed to address issues 

at several scales. Some address local 

issues in a particular stream or aquifer 

Monitoring and Assessing our Nation’s Water Quality

USGS Complements the Work of Others 

USGS water-quality assessments characterize the ambient water resource, 

which is the source of the Nation’s drinking water and of water for industrial, 

irrigation, and recreational uses. The USGS assessments thereby complement 

much of the compliance and regulatory monitoring conducted at the state level 

and by the U.S. Environmental Protection Agency (EPA) and other Federal 

agencies. Because of their regulatory responsibility, states and EPA typically 

focus on resources with the greatest levels of concern. This often makes it 

diffi cult for states and EPA to assess the 

total water resource. Also, state assessments are made against a backdrop of 

water-quality standards that differ from 

state to state. This makes regional and 

national assessments problematic. As 

water moves between and across state 

boundaries, the USGS has been able to 

provide information to multiple parties 

that are all interested in the same resource, 

but in different jurisdictional areas.

or in a particular county, while others 

look at broad regional systems that cross 

jurisdictional boundaries.

Second, USGS programs recognize 

the totality of the resource, including 

all components of the hydrologic cycle 

and the interconnections among these 

components. USGS addresses conditions 

in ground water, in the unsaturated zone, 

and in streams, lakes and reservoirs, and 

recognizes surface-water and groundwater interactions and atmospheric 

contributions. Inclusion of all hydrologic 

components allows a full accounting of 

all available sources, increases understanding of factors controlling waterquality degradation, and maximizes the 

effectiveness of water-resource utilization, protection, and restoration.

Third, USGS programs recognize the 

interconnections between water quality 

and biological systems. USGS addresses 

the susceptibilities of specifi c aquatic 

organisms, such as algae, macroinvertebrates, and fi sh to water-quality degradation, and determines how biological 

responses vary among the diverse environmental settings across the Nation. 

Such assessments lead to improved 

biological monitoring and consistent 

methods for assessing water-resource 

and environmental results. In addition, 

USGS assesses microbial processes and 

their effects on chemical degradation 

and water-quality conditions.

Fourth, USGS water-quality programs 

evaluate water quality in an overall 

hydrologic context. Chemical and biological data for streams are interpreted 

within the context of streamfl ow measurements, and ground-water chemistry 

is interpreted with an understanding 

of the ground-water fl ow system and 

aquifer characteristics. This is important 

because contaminants and their potential effects on drinking-water supplies 

and aquatic habitats vary over time and 

largely depend on the amount of water 

fl owing in streams and the directions of 

ground-water fl ow.

Photo from NAWQA Mississippi Embayment staff 
Fifth, USGS adheres to a national 

quality-assurance program with uniform methods of sampling and analysis. 

Monitoring data collected from representative sites across the Nation can, 

thereby, be combined into comprehensive regional and national assessments 

that identify and analyze trends in waterquality conditions.

Sixth, low levels of detection—often 

10 to 1,000 times lower than EPA drinking-water standards—are used to allow 

an early warning of contaminants before 

they reach levels of regulatory concern. 

In addition, the low-levels of detection 

allow an improved understanding of the 

connections among sources, transport, 

and fate of chemicals—generally not 

possible with data limited to measurements above regulatory levels.

Characteristics of USGS Water-Quality Programs:

• Ambient water-resource evaluation over long time scales with a regional 

and national perspective

• Interactions among surface water, ground water, and the atmosphere

• Interconnections between water quality and biological systems

• Water quality in a hydrologic context

• Uniform methods of sampling and analysis

• Low-levels of chemical detection

Long-Term Monitoring, Assessment, and Research Lead to 

Effective Water Management

USGS has organized its programs 

around three key components that are 

critical for successful water-resource 

management, including 1) long-term 

monitoring, 2) resource assessment, and 

3) research.

USGS long-term data collection

provides a quantitative means to answer 

the question “Are things getting better 

or worse?” Water quality is constantly 

changing, from season to season and 

from year to year. Consistent and systematic information over the long term 

is needed to (1) distinguish long-term 

trends from short-term fl uctuations 

and natural fl uctuations from effects 

of human activities; (2) evaluate how 

environmental controls and strategies 

are working; and, (3) choose the most 

cost-effective resource strategies for the 

future.

San Francisco Bay LongTerm Study

Long-term studies conducted over 

26 years in San Francisco Bay help 

resource managers and resource 

protection agencies understand the 

response of the Bay’s aquatic community to the combined effects 

of climate variability, changes in 

the hydrologic system, and the 

introduction of organic contaminants and trace elements (http:

//sfbay.wr.usgs.gov/). USGS resource assessment addresses 

the many complexities of contaminant 

occurrence and transport, which vary 

seasonally and among watersheds 

because of differences in land use and 

chemical applications, land-management 

practices, degree of watershed development, and natural factors, such as soils, 

geology, hydrology, and climate. Even 

among similar land uses and sources of 

contamination, differences in hydrology and other natural factors can result 

in different degrees of vulnerability to 

contamination and different ways that 

water-management strategies can lead to 

improved water quality.

and innovative study approaches for 

addressing contamination issues; and 

develops methodology for cost-effective 

hydrologic assessment. USGS continually improves techniques to understand 

and model sources and transport of contaminants, and processes affecting water 

quality in watersheds and aquifer systems. Continued development of reliable 

models helps to forecast the fate and 

transport of contaminants over different 

time frames, geographic areas, and environmental settings. Thus, improvements 

in modeling will help provide decisionsmaking tools needed by stakeholders at 

all levels to cost-effectively prioritize, 

manage, and protect their resource.

MTBE Study

USGS was fi rst to document 

the national extent of occurrence of methyl tert-butyl ether 

(MTBE), a fuel oxygenate, in the 

environment. MTBE was added 

to gasoline in some parts of the 

country to help meet the goals of 

the Clean Air Act. Because MTBE 

entered ground and surface waters 

from leaking gasoline storage 

tanks and by other means, the 

addition of MTBE to gasoline had 

the unintended consequence of 

introducing a potentially harmful chemical into water resources 

(http://wwwsd.cr.usgs.gov/

nawqa/vocns/). 

USGS research identifi es emerging 

contaminants (such as pesticide degradates, hormones, steroids, and pharmaceuticals); provides new information Landsat-7 image distributed by USGS EROS Data Center, 

Distributed Active Archive Center
Case Study: Nutrient Transport 

in the Mississippi River Basin

USGS studies of nutrient transport in the 

Mississippi River basin provide a good 

example of how insights obtained from 

the integration of long-term monitoring, 

resource assessment, and research can 

help to better understand an important 

regional and national water-quality 

issue. Excessive nitrogen in parts of the 

Mississippi River Basin threatens wildlife 

habitat, recreation, and drinking-water 

quality. The use of fertilizers is among 

the human factors that cause excess 

Microbiological Studies

Microbiological studies led to the 

identifi cation of processes that 

signifi cantly affect the distribution 

of toxic metals, such as arsenic, 

selenium, 

and mercury. 

Research studies of methyl 

mercury formation have provided important 

insights into the 

microbial action 

and chemical 

reactions that 

affect the forms 

of mercury and, 

thus its toxicity, in aquatic 

ecosystems. 

nitrogen in the Mississippi River Basin, 

along with wastewater discharges and 

atmospheric deposition. In addition, 

channelization and loss of wetlands have 

decreased the degree of natural transformation of nitrogen to innocuous gaseous 

forms (known as denitrifi cation), which 

naturally reduces the amount of nitrogen 

transported in water. USGS monitoring 

at more than 40 watersheds throughout 

the Mississippi River Basin during 1980 

to 1996 has helped to defi ne the relative 

nitrogen yields from different parts of 

the watershed (defi ned as the amount 

of nitrogen leaving a square kilometer 

of land, calculated from annual loads of 

nitrogen going past a stream gage and 

the area above the gage).

Each spring, excess nitrogen transported in the Mississippi River to the 

Gulf of Mexico causes the formation of a hypoxic zone, an area of low 

dissolved oxygen 

concentrations (http:

//wwwrcolka.cr.usgs.

gov/midconherb/

hypoxia.html). The 

resulting lack of oxygen can cause stress 

or death in bottomdwelling organisms 

that cannot escape 

to more oxygen-rich 

areas of the Gulf. Data 

collected by university 

scientists (Rabalais 

and others, 1999) 

have shown that the 

size of the hypoxic 

zone has more than 

doubled since it was 

Techniques for Age Dating Ground Water

USGS pioneered the use of several techniques for age dating ground 

waters to identify sources of water within an aquifer, and to assess its 

vulnerability to contamination. Tritium was fi rst used by the USGS 

for age dating in 1959. A similar technique was applied to chlorofl uorocarbons (CFCs) to date recent ground water. The age dating 

techniques have produced important insights on sources of water 

and on understanding human impacts on its quality. For example, the 

age-dating fi ndings demonstrated that improvements in ground-water 

quality, such as on the Delmarva Peninsula, may not be evident for 

years or even decades after farmers change their land-management 

practices because of slow ground-water movement 

(http://water.usgs.gov/lab/cfc/). 

fi rst systematically mapped in 1985. 

USGS long-term monitoring of water 

quality and streamfl ow demonstrate that 

the amount of nitrogen delivered to the 

Gulf of Mexico by the Mississippi River 

also has increased since the late 1960s 

(Goolsby and Battaglin, 2000). The 

amount of nitrogen delivered to the Gulf 

varies from year to year because of fl ow 

conditions. For example, the amount 

was low during the drought in the late 

1980s but extremely high during the 

fl ood of 1993, even though the amount 

of nitrogen applied to fi elds in the basin 

was not signifi cantly different.

In addition to monitoring, USGS 

conducts research and special assessments on processes that affect nitrogen 

transport and transformation in small 

watersheds and large river channels 

throughout the Mississippi River 

Basin. The research provides increased 

understanding of how subsurface fl ow, 

atmospheric cycling, and different channel sizes affect whether nitrogen is transformed to different chemical forms, is 

transported downstream, or dissipates to 

the atmosphere. USGS scientists developed the mass-balance regression model 

known as SPARROW (for SPAtiallyReferenced Regression On Watershed 

attributes) to better understand the links 

between sources of contaminants and 

factors that affect downstream transport 

(http://water.usgs.gov/nawqa/sparrow). The method correlates stream 

nitrogen fl ux (the mass of nitrogen 

transported downstream past a point 

on a river) with upstream nitrogen 

inputs (such as fertilizer use, manure, 

wastewater discharges, and atmospheric 

Photo by David L. Nelms, U.S. Geological Survey

Photo by Mark Marvin-DiPasquale, U.S. Geological Survey
deposition) and with factors controlling 

nitrogen transport in watersheds, including landscape and basin characteristics 

(such as soil permeability, channel size, 

and water velocity). Results show that 

nitrogen removal in streams declines 

rapidly as channel size increases (Alexander and others, 2000). The proximity 

of nitrogen sources to large streams and 

rivers, therefore, increases the ultimate 

transport of nutrients to the Gulf of 

Mexico. Despite the long travel times, 

many watersheds located along large 

rivers more than 2,500 kilometers from 

the Gulf of Mexico deliver signifi cantly 

larger fractions of nitrogen to coastal 

waters than watersheds located on 

smaller streams less than a few hundred 

kilometers from the Gulf. The delivery 

of nitrogen to coastal systems from point 

and nonpoint sources, therefore, is not a 

simple function of the distance of these 

sources from coastal waters. 

USGS monitoring, assessment, and 

research in the Mississippi River Basin 

continues to have important implications for nutrient management. The 

information is used by states, federal 

1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000

0

0.5

1.0

1.5

2.0

Nitrate flux, in millions of metric tons per year

5

10

15

20

25

30

Nitrate flux, in millions of metric tons per year

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The total amount of nitrate delivered by the Mississippi River depends 

on the concentration of nitrate in the water and the amount of water 

fl owing in the river (Goolsby and Battaglin, 2000; 

http://ks.water.usgs.gov/Kansas/pubs/fact-sheets/fs.135-00.html).

agencies, and other watershed stakeholders to develop and evaluate strategies 

for improved water-quality conditions. 

Specifi cally, the information is used to 

(1) help reduce nutrient losses from agricultural fi elds, urban runoff, and wastewater discharges; (2) guide wetlands 

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EXPLANATION

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More than 4,000 water-quality measurements and continuous streamgaging at 42 representative watersheds 

during 1980 to 1996 were used to defi ne the relative nitrogen yields from different areas within the Mississippi River Basin (Modifi ed from Goolsby and Battaglin, 2000; http://ks.water.usgs.gov/Kansas/pubs/factsheets/fs.135-00.html).

restoration efforts and management of 

navigation control structures to increase 

denitrifi cation; and, (3) identify watersheds with signifi cant contributions of 

nitrogen and where nutrient management 

strategies might have the most cost-effective opportunity for improvement. 
USGS Water-Quality Programs

USGS water-quality programs are 

designed to support the three components described above—monitoring, 

resource assessment, and research needs 

of the nation—although, generally each 

program tends to emphasize one component over the others. Each of the six programs are independently managed and 

funded; however, USGS organizes and 

integrates the different activities over 

time and across the Nation to capitalize 

upon existing resources. Resources are 

furthered leveraged through the collaboration and coordination with a multitude 

of federal, state, and local agencies, 

universities, public interest groups, and 

the private sector. Collaboration and 

coordination also help to guide and 

complement USGS scientifi c efforts and 

ensure that USGS information meets 

the needs of local, state, regional, and 

national stakeholders.

National Water-Quality Assessment 

Program (NAWQA), which began in 

1991, focuses on water quality in 42 

major river basins and aquifer systems, 

referred to as “study units.” A study 

unit boundary frequently crosses state 

boundaries and usually encompasses 

more than 10,000 square kilometers 

(about 3,900 square miles). Collectively 

the study units include water resources 

available to more than 60 percent of the 

population served by municipal supply 

and irrigated agriculture in watersheds 

that cover about half of the land areas 

of the conterminous United States. 

NAWQA was designed to answer the 

questions: “What is the condition of our 

streams and ground water? What are the 

factors affecting the condition? Is water 

quality changing over time?” One-third 

of all study areas are intensively investigated at any given time for 3 to 4 years, 

and trends are assessed about every 10 

years. For additional information, see 

http://water.usgs.gov/nawqa.

Toxic Substances Hydrology (TOXICS) Program, which began in 1983, 

conducts research and methods development related to contamination problems 

that are widespread and pose signifi cant 

risk to human health and the environment. The scale of studies is highly variable, including subsurface point-source 

studies, such as industrial spills, leaking 

fuel storage tanks, and solid waste landfi lls, and watershed- and regional-scale 

studies, such as agricultural chemicals in 

the Upper Midwest Corn Belt, hypoxia 

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Study Units— �������������������

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The National Water-Quality Assessment (NAWQA) Program assesses water-quality conditions 

in 42 major river basins and aquifer systems across the Nation (http://water.usgs.gov/nawqa).

in the Gulf of Mexico, and effects of 

mercury on aquatic ecosystems in the 

Everglades, Florida. Long-term research 

sites and regional- and national-scale 

monitoring networks provide a framework for interdisciplinary research teams 

that include non-USGS scientists. New 

methods are developed to make environmental measurements at low levels and 

to assess emerging water-quality issues, 

such as pharmaceuticals, hormones, 

and other wastewater contaminants. 

Models are developed to predict the 

transport and fate of contamination and 

to evaluate water-quality conditions 

under different management scenarios. 

For additional information, see http:

//toxics.usgs.gov.

The Cooperative Water program 

(COOP), which began in the early 

1900’s, includes studies in every state, 

protectorate and territory of the United 

States. These studies are conducted in 

cooperation with about 1,400 state and 

local agencies, which provide at least 

half of the funding. The studies generally focus on local (county or multiple 

counties) and State issues, related to, for 

example, development of water quality and biology monitoring networks, 

source-identifi cation of contaminants, 

effects of watershed development 

on quality and aquatic life, use of 

alluvial aquifers as public water 

supplies, pathogens and pathogen 

indicators, effects of animal feeding 

operations, and development of 

models and other analytical assessment tools. Because USGS waterquality studies incorporate streamfl ow measurements, many of the 

data collected at the local and State 

scales in the COOP projects are 

useful in the development of Total 

Maximum Daily Loads (TMDLs) 

for impaired stream reaches. For 

additional information see http:

//water.usgs.gov/coop.

Hydrologic Benchmark Network (HBN), which began in 1964, 

is comprised of stations across the 

Nation in areas with minimal infl uence from human activities. The 

HBN covers about 55 watersheds, 

ranging from a few to 50 square 

miles. Primary infl uences on water 

fl ow and quality at HBN sites are 

atmospheric inputs and the natural 

geologic conditions. HBN water-
Yukon. NASQAN data help to identify 

major source areas of contaminants and 

natural constituents, how they are transported and accumulated downstream, 

and areas of major deposition within the 

river basin. For additional information 

see http://water.usgs.gov/nasqan. 

National Trends Network (NTN), 

which began in 1983, is part of the 

National Atmospheric Deposition 

Program. The network is operated 

in collaboration with more than 100 

Federal, State, local, and private sector organizations. The NTN Program 

provides regional and national information on atmospheric deposition on the 

land surface through the collection of 

precipitation samples and analysis for 

major ions and nutrients at more than 

240 sites. A major benefi t of NTN has 

been identifi cation of the acidity of 

atmospheric deposition and changes in 

precipitation chemistry as a result of 

Clean Air Act reductions in the emission 

of oxides of nitrogen and sulfur. A subnetwork of NTN sites is used to monitor 

mercury deposition from precipitation. 

For additional information see http:

//bqs.usgs.gov/acidrain.

Printed on recycled paper

USGS Data Are Readily 

Available

The large USGS database on 

water-quality conditions and other 

information is publicly available 

and can be readily accessed via 

the Internet (http://water.usgs.gov/

nwis). It includes chemical data 

from rivers, streams, lakes, 

springs, and ground water from 

more than 335,000 sites, streamfl ow data from more than 20,000 

sites, and water levels from more 

than 1 million wells. Selected fi eld 

measurements, such as pH, temperature, and specifi c conductance, 

are available in real-time (updated 

at intervals of 4 hours or less) at 

more than 800 USGS sites.

Mississippi Basin

Rio Grande Basin

Colorado Basin

Columbia Basin

Yukon Basin

Active NASQAN Station

Inactive NASQAN Station

NAWQA Integrator Station

EXPLANATION

USGS measures water quality in fi ve of the Nation’s largest river basins through its National Stream Quality 

Accounting Network (NASQAN) (http://water.usgs.gov/nasqan).

Goolsby, D.A. and W.A. Battaglin, 

2000. Nitrogen in the Mississippi 

River Basin—Estimating Sources and 

Predicting Flux to the Gulf of Mexico. 

U.S. Geological Survey Fact Sheet 

135-00 (http://ks.water.usgs.gov/

Kansas/pubs/fact-sheets/fs.135-

00.html). 

Rabalais, N.N., R.E. Turner, J. 

Dubravko, J., Q. Dortsch, and W.J. 

Wisman, Jr., 1999, Characterization 

of Hypoxia—Topic 1 Report for the 

Integrated Assessment on Hypoxia 

in the Gulf of Mexico: Silver Spring, 

Md., NOAA Coastal Ocean Offi ce, 

NOAA Coastal Ocean Program Decision Analysis Series No. 17, 167 p.

By Gail E. Mallard and Pixie A. Hamilton

Layout and design by Phillip J. Redman

Visit our Web site at 

http://water.usgs.gov for direct access to 

reports and other interpretive information, monitoring data, technical reports, 

and maps. 

For additional information, 

please contact:

Chief, Offi ce of Water Quality 

Programs, USGS

(703) 648-5716 (voice)

(703) 648-6693 (fax)

nawqa_info@usgs.gov

sheds are used to assess infl uences from 

atmospheric emissions regulated through 

the Clean Air Act. For additional information see http://water.usgs.gov/hbn. 

National Stream Quality Accounting 

Network (NASQAN) was established 

in 1973. Currently the program measures the fl ux and transport of selected 

compounds in fi ve of the Nation’s largest river basins (ranging from 250,000 

to 1,200,000 square miles), including 

the Mississippi River, Columbia River, 

Colorado River, Rio Grande, and the 

References Cited

Alexander, Richard B., R.A. Smith 

and G.E. Schwarz, 2000. Effect of 

Stream Channel Size on the Delivery 

of Nitrogen to the Gulf of Mexico. 

Nature 403: 758-761.