U.S. Department of the Interior

U.S. Geological Survey

Circular 1323

Ground-Water Availability in the United States

Ground-Water Resources Program

Water Quality

Water Quantity

Aquifers
The cover shows three maps from the report that represent components of

ground-water availability—the water-quality map of ground-water arsenic 

samples is figure 21, the water-quantity map of water-level declines is figure 12, 

and the aquifers map showing principal aquifers of the United States is figure 2.
A contribution of the Ground-Water Resources Program

Ground-Water Availability in the United States

By Thomas E. Reilly, Kevin F. Dennehy, William M. Alley, and William L. Cunningham

Circular 1323

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

DIRK KEMPTHORNE, Secretary

U.S. Geological Survey

Mark D. Myers, Director

U.S. Geological Survey, Reston, Virginia: 2008

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reproduce any copyrighted materials contained within this report.

Suggested citation:

Reilly, T.E., Dennehy, K.F., Alley, W.M., and Cunningham, W.L., 2008, Ground-Water Availability in the United States: 

U.S. Geological Survey Circular 1323, 70 p., also available online at http://pubs.usgs.gov/circ/1323/

Library of Congress Cataloging-in-Publication Data

Ground-water availability in the United States / by Thomas E. Reilly ...

[et al.].

 p. cm. -- (U.S. Geological Survey circular ; 1323)

 Includes bibliographical references and index.

 ISBN 978-1-4113-2183-0 (alk. paper)

 1. Groundwater--United States. I. Reilly, Thomas E. II. Title:

Groundwater availability in the United States.

 GB1015.G758 2008

 553.7’90973--dc22

 2008026582
Foreword

Ground water is among the Nation’s most important natural resources. It provides half our drinking water and is essential 

to the vitality of agriculture and industry, as well as to the health of rivers, wetlands, and estuaries throughout the country. 

Large-scale development of ground-water resources with accompanying declines in ground-water levels and other effects 

of pumping has led to concerns about the future availability of ground water to meet domestic, agricultural, industrial, 

and environmental needs. The challenges in determining ground-water availability are many. This report examines what 

is known about the Nation’s ground-water availability and outlines a program of study by the U.S. Geological Survey 

Ground-Water Resources Program to improve our understanding of ground-water availability in major aquifers across the 

Nation. The approach is designed to provide useful regional information for State and local agencies who manage groundwater resources, while providing the building blocks for a national assessment. The report is written for a wide audience 

interested or involved in the management, protection, and sustainable use of the Nation’s water resources.

 Robert M. Hirsch, Associate Director for Water

 U.S. Geological Survey

Blue Springs, Blue Springs State Park, Barbour County, Alabama

Photograph by Alan M. Cressler, USGS

Contents

Introduction ....................................................................................................................................................1

Challenges in Determining Ground-Water Availability ............................................................................3

Ground Water—The Hidden Resource .............................................................................................5

Time Scales of Ground-Water Systems ............................................................................................6

What Do We Know About Ground-Water Availability in the United States? .......................................7

Location and Description of Major Aquifers ....................................................................................7

Water Use ............................................................................................................................................11

Changes in Ground-Water Levels and Ground-Water Storage ..................................................13

Recharge ..............................................................................................................................................20

Ground-Water Discharge ..................................................................................................................26

Ground-Water Quality ........................................................................................................................26

Regional-Scale Approach to National Assessment ..............................................................................29

Regional Ground-Water Budgets .....................................................................................................30

Selection of Regional Ground-Water Flow Systems .....................................................................30

Regional Studies .................................................................................................................................37

Examples of Regional Aquifer Assessments ...........................................................................................39

Middle Rio Grande Basin ...................................................................................................................40

California Central Valley Aquifer System ........................................................................................44

Coastal Plain Aquifer System ...........................................................................................................50

Great Lakes Basin ...............................................................................................................................54

High Plains Aquifer .............................................................................................................................57

Future Directions..........................................................................................................................................62

Acknowledgments .......................................................................................................................................62

Selected References ...................................................................................................................................63

Boxes

A—Terms Used in Describing Ground-Water Availability ......................................................................4

B—The U.S. Geological Survey National Water-Quality Assessment 

 (NAWQA) Program .......................................................................................................................29

C—The U.S. Geological Survey Regional Aquifer-System Analysis 

 (RASA) Program ...........................................................................................................................36

D—Relation of Ground-Water Modeling to Assessment and Monitoring .........................................38
Artesian well at Woonsocket, well throws a 3-inch-wide stream to height of 97 feet, 

Sanborn County, South Dakota, Circa 1900 (Darton, N.H., 1905, Plate 60-B).
O.E. Meinzer in 1934.

Ground-Water Availability in the United States

By Thomas E. Reilly, Kevin F. Dennehy, William M. Alley, and William L. Cunningham

Introduction 

Ground water is one of the Nation’s most valuable natural resources. It occurs almost everywhere 

beneath the Earth’s surface and is a major source of 

water supply worldwide. Ground water has a crucial 

role in sustaining streamflow between precipitation 

events and especially during 

protracted dry periods. 

In addition to human uses, 

many ecosystems are 

dependent on ground-water 

discharge to streams, lakes, 

and wetlands.

Although humans have 

been digging wells and 

tunnels for water supply for 

thousands of years, extensive 

use of ground water is 

relatively recent, with the 

advent of rural electrification 

and more effective drilling 

and pumping technologies 

during the past 75 years. 

A growing awareness of 

ground water as a critical 

natural resource leads to 

some basic questions. How 

much ground water do we have? Are we running 

out? Where are ground-water resources most stressed 

by human development? Where are the resources 

most available for future supplies? Although these 

questions seem simple, providing the answers is 

complex because a meaningful assessment of groundwater availability in the United States requires a 

multidisciplinary evaluation of the hydrologic system, 

as well as an understanding of the different water 

issues that exist across the Nation. Furthermore, the 

information available to support a broad assessment of 

the resource varies across the Nation (Alley, 2006).

During the past century, several ground-water 

assessments have been completed by the U.S. Geological Survey (USGS) on a national scale. The first 

of these assessments was completed by O.E. Meinzer 

(1923) who has been called 

the “father of ground-water 

hydrology” (Lohman, 1986, 

p. 51). Meinzer’s publication 

was followed several decades 

later with State-by-State 

summaries on ground-water 

resources (McGuinness, 

1951 and 1963); by summary 

appraisals for 21 regions 

of the Nation in the 1970s 

(U.S. Geological Survey Professional Papers 813A– U); 

a State-by-State summary 

(U.S. Geological Survey, 

1985); and by the Regional 

Aquifer-System Analysis 

(RASA) Program in which 

25 of the Nation’s most 

important regional groundwater systems were evaluated 

(Sun and Johnston, 1994). These national and regional 

evaluations have improved our knowledge about the 

Nation’s ground-water resources. Repeated evaluations of the resource through time are needed as new 

information on ground-water resources and connected 

surface-water systems becomes available; new 

methods and technologies for resource assessment are 

developed; and the places ground water is used, water 

demands, and the issues of concern change with time.
Report to Congress

Concepts for National Assessment

of Water Availability and Use

Circular 1223

U.S. Department of the Interior

U.S. Geological Survey

2 Ground-Water Availability in the United States

“Finding scientific and technical solutions 

to problems of water availability and 

quality will require extensive cooperation 

and collaboration among Federal, State, 

and local agencies, private sector water 

experts, stakeholders, and the public...”

From “A Strategy for Federal Science and Technology to 

Support Water Availability and Quality in the United States,” 

National Science and Technology Council (2007)

Environmental decision making has grown more

complex with society demanding ample water for human 

use along with environmental protection and preservation at the same time. When O.E. Meinzer (1923) 

published his first national ground-water assessment, 

indoor plumbing still was not commonly used, lowcapacity wells were sufficient for most purposes, and the 

population of the country was more dispersed. Today, 

lifestyles generally require large amounts of water and 

a complex infrastructure to deliver water to urban and 

suburban population centers. Even if water resources 

are abundant regionally, heavy water use in centralized areas can create local stresses. As water-related 

problems evolve in complex ways, an up-to-date and 

comprehensive evaluation of ground-water resources 

that builds on the foundation of previous studies is 

needed to meet society’s ever-changing water demands.

A goal of ground-water resource assessment is 

to provide information on the current status of the 

resource that provides insights about the future availability of ground water. Ground-water management 

decisions in the United States are made at a local level, 

such as the State, municipality, or a special district 

formed for water-resources management. Many aquifer 

systems cross these political boundaries. Thus, a key 

role of national and regional assessments is to provide 

consistent and integrated information across political 

boundaries that is useful to those who use and manage 

the resource. The State and local agencies manage the 

water-resources system and collect and analyze local 

data. Federal scientific agencies support this function 

by developing methods of analysis and analyzing the 

water-resources system across political boundaries. 

This partnership between State and local agencies 

and the USGS enables the resource to be understood 

on a multi-State, regional, and national basis.

With these considerations in mind, the purpose of 

this report is to identify the challenges in determining 

ground-water availability, summarize the current state 

of knowledge from a national perspective, and outline 

an approach for developing the needed understanding 

of future water availability. This report is an outgrowth 

of a pilot study, National Assessment of Water Availability and Use, that began in 2005 at the request of 

Congress (Barlow and others, 2002). The report also 

builds on regional ground-water availability studies 

recently undertaken as part of the USGS Ground-Water 

Resources Program (Dennehy, 2005). The approach 

to national ground-water assessment described in the 

section “Regional-Scale Approach to National Assessment” of this report, is a key element of the water 

census of the United States, which has been proposed 

as a strategic science direction of the USGS (U.S. Geological Survey, 2007), as well as part of the proposed 

Federal science strategy to meet nationwide water challenges by the National Science and Technology Council 

(2007) Subcommittee on Water Availability and Quality.
Challenges in Determining Ground-Water Availability 3

Challenges in Determining 

Ground-Water Availability

Although determining the amount of ground 

water available in the Nation may seem straightforward, it is actually quite complex. Some key 

difficulties are as follows:

1. In contrast to rivers and lakes, ground-water 

systems are hidden from direct observation 

and measurement, 

2. The sources of water to ground-water systems 

and the time required for the effects of withdrawals to propagate through the system and 

be observed are different for each system, 

3. The amount of detail (spatial scale) needed to 

describe the resource depends on the objectives 

and purpose of the desired information, 

4. The amount of change in ground-water levels 

that is important is different for different 

ground-water systems, 

5. Not all water pumped is consumed and much 

of the water pumped is redistributed and 

changes the ground-water flow system, and

6. The chemical quality of the water is important 

in determining its suitability (and thus its availability) for various uses.

These challenges are considered throughout this report.

As a foundation for the discussion in this report, it is helpful to consider the meaning of the 

terms water availability and ground-water availability. Although the quantities of water in 

a hydrologic system usually can be measured, computed, or estimated, water availability 

is a more elusive and multifaceted concept. Water availability is a function not only of the 

quantity and quality of water in a basin or aquifer system but also the physical structures, 

laws, regulations, and socioeconomic factors that control its demand and use. This report 

discusses physical and chemical characteristics that are important as indicators of groundwater availability. At the local level, these characteristics must be considered jointly with 

societal factors as determinants of actual ground-water availability and society’s tolerance 

of the consequences of its use. Societal perspectives and constraints change with time just 

as the ground-water resource does (Alley and Leake, 2004).

Determining ground-water availability means 

more than calculating the volume of ground water 

underlying a particular area or within an aquifer. One 

must not only consider that some of the water may not 

be economically recoverable or of poor quality but 

also that ground water is connected to the rest of the 

hydrologic system. Ground-water withdrawals can and 

usually do affect the amount (and quality) of surface 

water. For example, depletion of a small part of the 

total volume of ground water in storage (sometimes 

only a few percent) can have substantial and 

undesirable effects on the availability of surface water 

that becomes the limiting factor to development of the 

ground-water resource (Alley, 2007). Increasingly, 

contributions of ground water to surface water are 

considered an important part of ecosystem needs, and 

in some cases, plant and animal communities depend 

partly or completely on ground water to maintain their 

current composition and function. Thus, an assessment of ground-water availability requires consideration of the response of the entire hydrologic system 

to ground-water withdrawals. 

In discussions of ground-water availability, terms 

such as ground-water mining and safe yield frequently

are used to indicate the status of the resource. The 

use of different terms can sometimes exacerbate 

the difficulties in answering questions about water 

availability because these terms can have different 

meanings. Four of these terms are defined in Box A. 
4 Ground-Water Availability in the United States

Terms Used in Describing Ground-Water Availability

term ground-water mining typically refers to 

a prolonged and progressive decrease in the 

amount of water stored in a ground-water 

system, as may occur, for example, in heavily 

pumped aquifers in arid and semiarid regions. 

Ground-water mining is a hydrologic term 

without connotations about water-management 

practices (U.S. Water Resources Council, 

1980). The term overdraft refers to withdrawals 

of ground water from an aquifer at rates 

considered to be excessive and, therefore, 

carries the value judgment of overdevelopment. 

Thus, overdraft may refer to ground-water 

mining that is considered excessive as well as 

to other undesirable effects of ground-water 

withdrawals. The terms safe yield, ground-water 

mining, and overdraft are not used in this report.

When discussing water availability, the 

topic of well yields (that is, how much water a 

particular well can produce) frequently arises. 

Well yields, however, only address the efficiency 

with which a well will allow water to be removed 

from the ground-water system. The well must be 

placed in the context of the flow system around 

the well and the amounts of water being withdrawn by other wells in the area. Just because a 

well initially can pump a certain amount of water 

does not mean that the ground-water system 

can supply that amount of water indefinitely. An 

analysis of the ground-water system is required 

to determine the source of the water that is 

being withdrawn from the ground-water system 

and to determine if there is a sufficient amount 

of water available from these sources. Thus, 

well yields, in and of themselves, do not address 

regional ground-water availability.

A number of terms are used to describe 

ground-water availability. For example, groundwater resources often are discussed in terms 

of their sustainability. As defined in Alley and 

others (1999), ground-water sustainability is the 

“development and use of ground water in a manner that can be maintained for an indefinite time 

without causing unacceptable environmental, 

economic, or social consequences.” The definition of “unacceptable consequences” is largely 

subjective and may involve a large number of 

criteria. Furthermore, ground-water sustainability 

must be defined within the context of the complete hydrologic system of which ground water is 

a part. For example, what may be established as 

an acceptable rate of ground-water withdrawal 

with respect to changes in ground-water levels 

may reduce the availability of surface water to 

an unacceptable level. Determination of groundwater sustainability essentially is the end result of 

the public acceptance of the tradeoffs of development. A key role of hydrologists is to provide 

information, such as is described in this report, 

on the long-term consequences of pumping and 

other factors affecting ground-water resources 

that can aid societal decisions related to sustainability. We use the term ground-water sustainability in this report in this general context.

The term safe yield commonly is used in 

efforts to quantify sustainable ground-water 

development. The term should be used with 

respect to specific effects of pumping, such as 

water-level declines, reduced streamflow, and 

degradation of water quality. Alley and Leake 

(2004) describe the history of the term and its 

relation to the concept of sustainability. The 

A
Ground-water data collection. Photographs by USGS staff: 

Alan M. Cressler, William L. Cunningham, Kevin F. Dennehy, 

Matthew J. Gilbert, Edward H. Martin, Lester J. Williams, 

and Douglas D. Zettwoch.

Challenges in Determining Ground-Water Availability 5

Ground Water—The Hidden Resource

 Ground water is a hidden resource, in that we 

cannot visually observe its movement and status. 

We must, therefore, gain information by measurements 

obtained from wells, by measurements of flow and 

water levels at its boundaries, and through indirect 

methods of measurement (such as surface geophysics) 

to assess the resource. These data are used to infer the 

actual occurrence and movement of ground water and to 

develop a conceptual model of the ground-water system. 

This conceptual model can be used to explain the extent 

of the ground-water system, the sources of water to 

the system, and the movement (rate and direction) of 

water through the inferred hydrogeologic units. The 

conceptual model can never be exact and is subject 

to uncertainty and error because of the indirect nature 

of the measurement methods and the complexities of 

the subsurface and natural systems in general.

Ground-water systems store and transmit water. One 

of the advantages of ground water is that it exists almost 

everywhere across the Nation and, thus, is available away 

from surface sources of water. This advantage enables

communities, individual well owners, and irrigators the 

opportunity to obtain water without investing in pipelines and storage facilities. The water in a ground-water 

system is stored naturally in the pore space or fractures 

of the earth. As ground water is withdrawn at a well, the 

connected pore spaces or fractures serve as the pipeline 

to move the water from one part of the hydrogeologic 

system to where it is being withdrawn. Understanding the 

movement of water through the ground-water system 

and understanding the limits of the sources of water are 

key aspects of a ground-water availability assessment.

The amount of detail needed to describe and 

assess the resource depends on the objectives and 

purpose of the estimate. The pumpage of fresh ground 

water in the United States in 2000 was estimated to be 

approximately 83 billion gallons per day (Hutson and 

others, 2004), which is about 8 percent of the estimated 1 trillion gallons per day of natural recharge to 

the Nation’s ground-water systems (Nace, 1960). From 

an overall national perspective, therefore, the groundwater resource appears ample. Throughout the Nation, 

however, the availability of ground water varies widely. 
Water from storage

Water from streamflow capture

0

50

100

PUMPING TIME 

PERCENTAGE OF GROUND-WATER PUMPAGE

Figure 1. The principal source of water to a well can 

change with time from ground-water storage to capture 

of streamflow (modified from Alley and others, 1999).

6 Ground-Water Availability in the United States

Time Scales of Ground-Water Systems

There are two main processes to consider in determining time scales of ground-water systems. One is 

the time it takes water levels to respond to changes in 

stress (such as pumping) on the ground-water system; 

the other is the time it takes the water to travel through 

the ground-water system. The time frame of changes 

in water levels depends on how quickly the change in 

water levels propagates through the system after water 

is removed from storage. The time of travel of water 

flowing through the system depends on the velocity 

of the water and the distance between the recharge 

and discharge boundaries. These two times are very 

different for most ground-water systems. 

The amount of water coming from different sources 

to a discharging well changes through time until, if 

possible, a new steady-state or equilibrium condition is 

established. For example, figure 1 illustrates the sources 

of water in a simple idealized stream-aquifer system 

supplying one well. At the start of pumping, 100 percent 

of the water supplied to the well comes from groundwater storage. Over time, the dominant source of water 

to the well changes from ground-water storage to surface 

water. The surface-water source for purposes of discussion here is a stream, but it may be another surface-water 

body, such as a lake or wetland. The source of water to a 

well from a stream can be either decreased ground-water 

discharge to the stream or increased flow (recharge) from 

the stream into the ground-water system. The streamflow reduction in either case is referred to as streamflow 

capture. The adjustments to pumping of an actual hydrologic system may take place over many years, decades, 

or longer, depending on the physical characteristics of 

the aquifer, degree of connection between the stream 

and aquifer, and locations and pumping history of wells.

Most ground-water systems are much more 

complex than implied in figure 1; for example, the 

system may comprise many wells pumping from 

an aquifer at varying pumping rates and at different 

locations within the ground-water flow system. From an 

availability perspective, the key point is stated by Theis 

(1940) as, “All water discharged by wells is balanced by 

a loss of water somewhere.” The ground-water system 

is part of the encompassing hydrologic cycle, and 

water taken from the ground-water system has to come 

from storage or other parts of the hydrologic cycle 

that are connected to the ground-water system. These 

connected systems might be surface-water bodies, such 

as streams, or they could be plant communities that 

use the water for growth, such as riparian vegetation. 

Pumping decisions made today will affect water availability for the surrounding ecosystem; however, these 

effects may not be fully realized for many years.

The time of travel of the water flowing through a 

ground-water system is different from the time it takes 

the water levels to respond and is widely variable, 

ranging from less than a day to more than a million 

years (Bentley and others, 1986). As a result, water 

stored within the system can range in age (the time since 

recharge) from recent precipitation to water trapped in 

the sediments as they were deposited in geologic time. 

For the 48 contiguous States, Nace (1960) estimated that 

there are about 60,000 trillion gallons of ground water 

in storage. Assuming net recharge of 1 trillion gallons a 

day, about 160 years of recharge is stored in the ground. 

In the humid East, the average storage time of ground 

water is probably shorter, and in the arid and semiarid 

central and western States, it is probably longer. 

For large systems with long flow paths and large 

travel times, past climate variability may be important in 

the initial saturation of the aquifer and the distribution of 

water in storage. Future climate variability and change 

also may be important in determining water availability. 

For example, in the mountains of the western United 

States, precipitation in recent decades has come more 

frequently in the form of rain rather than snow (Knowles 

and others, 2006), and snowpacks have thinned (Mote 

and others, 2005), which likely changes the quantity 

and distribution of recharge in the mountains as well 

as recharge from mountain runoff. As climate changes, 

ground-water systems will respond, but the effects may 

take long periods of time to fully develop.

Figure 1. The principal source of water to a well can 

change with time from ground-water storage to capture

of streamflow (modified from Alley and others, 1999).
Perspective view of overlapping aquifer systems in the 

Alabama, Florida, Georgia, and South Carolina area as 

described in text and shown in figure 2 (from Miller, 2000).

Surficial aquifer system

Sand and gravel aquifer

Biscayne aquifer

Intermediate aquifer system

Floridan aquifer system

Southeastern Coastal Plain

 aquifer system

Piedmont and Blue Ridge aquifers

Valley and Ridge aquifers

Appalachian Plateaus aquifers

Interior Low Plateaus aquifers

What Do We Know About Ground-Water Availability in the United States? 7

What Do We Know About GroundWater Availability in the United States?

In order to determine the availability of water, 

we need information about the resource (the supply) 

and about its use (the demand). The amount of detail 

needed is dependent on the objectives and scale of 

the analysis. On a national scale, we know quite a 

bit about the Nation’s ground-water resource; however, much of the information is generalized and has 

limitations when attempts are made to plan for the 

future. Even though national-scale information has its 

limitations, it provides a framework for a systematic 

comparison of the resource across the Nation. This 

section of the report reviews the information available 

on a national scale and also indicates the limitations of 

that information when determining water availability.

Location and Description of Major Aquifers

An aquifer is a geologic formation, a group of 

formations, or a part of a formation that contains 

sufficient saturated permeable material to yield 

significant quantities of water to wells and springs. 

The areal and vertical location of the major aquifers 

is fundamental to the determination of ground-water 

availability for the Nation. 

The location, hydrologic characteristics, and 

geologic characteristics of the principal aquifers 

throughout the 50 States, Puerto Rico, and the 

U.S. Virgin Islands are described in the Ground 

Water Atlas of the United States (Miller, 2000; 

http://capp.water.usgs.gov/gwa/). The groundwater information summarized in the atlas has been 

collected over many years by the USGS and other 

partner agencies. The Atlas provides key descriptive 

information in a regional and national context. 

A two-dimensional map representation of the 

principal aquifers of the Nation (U.S. Geological 

Survey, 2003) is shown in figure 2. The map, which 

is derived from the Ground Water Atlas of the United 

States, indicates the areal extent of the uppermost 

principal aquifers on a national scale. Although the 

map is two dimensional, it provides a useful visual representation of the Nation’s complex three-dimensional 

ground-water resource. In some places, other productive aquifers underlie those shown on the map. For 

example, the highly productive limestone that forms 

the Floridan aquifer system of the southeastern United 

States underlies the entire Florida Peninsula and 

extends into Georgia, Alabama, and South Carolina. 

Only small areas of this aquifer system are shown 

on the map, because it is covered in many places by 

younger sand aquifers. Likewise, some aquifers in 

sedimentary rocks are overlain by confining units and 

extend into the subsurface beyond the areas shown on 

the map. Local aquifers, such as stream-valley aquifers that might overlie the aquifers mapped in figure 2, 

are not shown because of the scale of the figure. 
Figure 2. Principal aquifers of the United States (modified from Principal Aquifers U.S. Geological Survey, 2003).

C A N A D A

MEXICO

120°

45°

115° 110°

105° 100°

95°

40°

35°

30°

O

C

E

A

N

P

A

C

I

F

I

C

G U L F

Gulf of California

160° 159° 158° 157° 156° 155°

22°

21°

20°

19°

18°30'

17°30'

68° 67° 66° 65° 64°

19°30'

170° 160° 150° 140° 

130°

180°

170°E

70°

50°

60°

P A C I F I C

O C E A N

US

UK

A T L A N T I C O C E A N

P A C I F I C O C E A N

RUSSIA

CANADA

GULF OF ALASKA

Caribbean Sea

Principal Islands of

HAWAII

Albers equal area projection, standard parallels 8°N and 18°N,

central meridian 157°W

Puerto Rico and the Virgin Islands

Albers equal area projection, standard parallels

8° N and 18°N, central meridian 66° 30'W

ALASKA

0 100 200 300 400 MILES

0 100 200 300 400 500 600 KILOMETERS

Albers equal area projection, standard parallels

55°N and 65°N, central meridian 154°W

18

18

28

41

20

19

44

40

33

49

61

47

33

25

25

25

36

35

11

26

45

27

14

46

38

19

12

2 43

11

2

24

37

25

41

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36

4

35

60

9

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4

42

1

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1

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42

57

6 4

5

7

58

4

8

58

3

10

4

1

16

56 17

55

13

20

8 Ground-Water Availability in the United States

Figure 2. Principal aquifers of the United States (modified from Principal Aquifers, U.S. Geological Survey, 2003).
90°

85°

75°

70°

25°

0 100 200 300 400 MILES

0 100 200 300 400

Compiled by U.S. Geological Survey

Revised 2003

Albers equal area projection

standard parallels 29°30'N and 45°30'N

central meridian 96°W

500 600 KILOMETERS

A T L A N T I C

O C E A N

M E X I C O

O F

Ontario

Lake

Lake

Huron

Lake Superior

Michigan Lake

Georgian Bay

Lake

St Lawernce River

Erie 80°

33

52

51

54

22

15

62

21

31

39

62

22

29

53

30

52

50

50

53

39

48

40

47

31

32

47

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31

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48

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47

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31

18

20 21

13

44 29

15

29

52

40

40

Other—Rocks that are minimally permeable

 but may contain locally 

 productive aquifers

Southern Nevada volcanic-rock aquifers

Pacific Northwest basaltic-rock aquifers

Snake River Plain basaltic-rock aquifers

Columbia Plateau basaltic-rock aquifers

Hawaiian volcanic-rock aquifers—Locally

 overlain by sedimentary deposits

Piedmont and Blue Ridge crystalline-

 rock aquifers

Igneous and metamorphic-rock aquifers

Basin and Range carbonate-rock aquifers

Roswell Basin aquifer system

Ozark Plateaus aquifer system

Blaine aquifer

Arbuckle–Simpson aquifer

Silurian–Devonian aquifers

Ordovician aquifers

Upper carbonate aquifer

Floridan aquifer system

Biscayne aquifer

New York and New England

 carbonate-rock aquifers

Piedmont and Blue Ridge carbonate-

 rock aquifers

Castle Hayne aquifer

North Coast Limestone aquifer system

 (Puerto Rico)

Kingshill aquifer (Virgin Islands)

Carbonate-rock aquifers

Edwards–Trinity aquifer system

Valley and Ridge aquifers—

 Carbonate-rock aquifers

 are patterned

Mississippian aquifer

Paleozoic aquifers

Sandstone and carbonate-rock aquifers

Colorado Plateau aquifers

Denver Basin aquifer system

Lower Cretaceous aquifers

Rush Springs aquifer

Central Oklahoma aquifer

Ada–Vamoosa aquifer

Early Mesozoic basin aquifers

New York sandstone aquifers

Pennsylvanian aquifers

Marshall aquifer

Cambrian–Ordovician aquifer system

Jacobsville aquifer

Lower Tertiary aquifers

Upper Cretaceous aquifers

Upper Tertiary aquifers 

Sandstone aquifers

Sand and gravel aquifers north of the limit

 of Quaternary continental glaciation

 and east of the Rocky Mountains. The

 aquifers are mostly in glacial deposits—

 Gray is combined with color of

 underlying aquifer

Basin and Range basin-fill aquifers

Rio Grande aquifer system

California Coastal Basin aquifers

Pacific Northwest basin-fill aquifers

Columbia Plateau basin-fill aquifers

Snake River Plain basin-fill aquifers

Puget Sound aquifer system

Willamette Lowland basin-fill aquifers

Northern Rocky Mountains Intermontane

 Basins aquifer system

Central Valley aquifer system

High Plains aquifer

Pecos River Basin alluvial aquifer

Mississippi River Valley alluvial aquifer

Seymour aquifer

Surficial aquifer system

Unconsolidated-deposit aquifers (Alaska)

South Coast aquifer (Puerto Rico)

Coastal Plain aquifer systems in 

 semi-consolidated sand

Coastal lowlands aquifer system

Texas coastal uplands aquifer system

Mississippi embayment aquifer system

Southeastern Coastal Plain 

 aquifer system

Northern Atlantic Coastal Plain 

 aquifer system

Unconsolidated and semiconsolidated

sand and gravel aquifers

8

9

10

11

12

13

14

15

16

17

1

2

3

4

7

5

6

18

23

42

43

44

45

46

47

57

58

59

60

62

61

48

49

50

51

52

53

54

55

56

24

25

26

38

39

40

27

28

29

30

31

32

33

34

35

36

37

19

20

21

22

41

What Do We Know About Ground-Water Availability in the United States? 9
Aquifer

Confining unit

Contact—Dashed where

 approximately located

NOT TO SCALE

EXPLANATION Lower Potomac–Raritan–Magothy aquifer

Middle Potomac–Raritan–Magothy aquifer

Upper Potomac–Raritan–Magothy aquifer

Confining unit between Lower and Middle Potomac–Raritan–Magothy aquifers

Confining unit between Middle and Upper Potomac–Raritan–Magothy aquifers

Merchantville–Woodbury confining unit 

Englishtown

aquifer

Fall Line

Holly Beach

water-bearing

zone

Estuarine clay

confining unit Confining unit

overlying Rio Grande

water-bearing zone

Vincentown

aquifer

Navesink–Hornerstown

confining unit

Basal Kirkwood confining unit

Piney Point aquifer

Vincentown–Manasquan confining unit

Wenonah–Mount Laurel aquifer

Upper Kirkwood–Cohansey aquifer

Northwest Southeast

Atlantic

Ocean

Bedrock

Marshalltown Wenonah confining unit

Lower Kirkwood–Cohansey and confined Kirkwood aquifer

Figure 3. Figure 3. Generalized hydrogeologic section of the New Jersey Coastal Plain (modified from Voronin, 2004). Generalized hydrogeologic section of the New Jersey Coastal Plain (modified from Voronin, 2004).

10 Ground-Water Availability in the United States

Some of the principal aquifers shown in figure 2 

are systems of multiple aquifers. For example, the 

“Northern Atlantic Coastal Plain aquifer system” is 

identified on the national map as a principal aquifer 

composed of semiconsolidated sand that is present 

in several States, including southern New Jersey. 

At a regional level, however, the “Northern Atlantic 

Coastal Plain aquifer system” is actually a system of 

aquifers and confining units (Voronin, 2004) as shown 

in figure 3 for the New Jersey Coastal Plain. At a local 

level, the distribution of sands and clays are even more 

variable than those shown at the regional level.

Thus, we have a broad understanding of where 

the principal water-bearing formations in the United 

States are located. The level of detailed understanding 

of ground-water systems varies widely across the 

United States. The principal aquifers, shown in 

figure 2, provide a framework to classify and study 

ground-water systems regionally. These potential 

areas for regional investigations can be prioritized in 

conjunction with other information on sources and 

uses of water, as discussed later in this report.
0

10,000

20,000

30,000

40,000

50,000

60,000

1960 1965 1970 1975 1980 1985 1990 1995 2000

GROUND-WATER WITHDRAWALS FOR IRRIGATION,

IN MILLION GALLONS PER DAY 

17 western States

31 eastern States

What Do We Know About Ground-Water Availability in the United States? 11

Water Use

The USGS has partnered with State and local 

agencies to compile estimates of ground-water and 

surface-water withdrawals for the Nation at 5-year 

intervals since 1950. The data currently are compiled at 

the county, State, and national levels for eight categories of water use—public supply, domestic, irrigation, 

livestock, aquaculture, self-supplied industrial, mining, 

and thermoelectric power. The most recent compilation 

is for the year 2000 (Hutson and others, 2004).

“The United States should accurately 

assess the quantity and quality of its water 

resources, should accurately measure how 

water is used, and should know how water 

supply and use change over time.”

From “A Strategy for Federal Science and Technology 

to Support Water Availability and Quality in the United 

States,” National Science and Technology Council (2007)

The precision of water-use data over time is 

affected by the status of State water-use reporting 

programs and the development of techniques for 

estimating water use. Programs to collect wateruse data in each State are highly variable and are 

summarized in a review of the USGS water-use 

program (National Research Council, 2002).

Some water-use data, such as public supply for 

household uses and withdrawals by some industrial 

users, are obtained by direct measurement, and some 

are estimated as the amount reported or allowed by 

permit. Many uses, such as for self-supplied domestic 

use, irrigation, and some industries, commonly are 

estimated using coefficients that relate water use to 

another characteristic. For example, water use for a 

particular type of industry might be estimated by using 

information on employment or production in terms of 

gallons per day per employee or per unit of product.

Despite the inherent differences in data-collection 

methods from year to year and State to State, the USGS 

water-use data represent a unique record of withdrawals 

over time and throughout the country. The data 

provide a broad-based 50-year history of changes in 

water withdrawals. For example, the data indicate that

ground-water withdrawals more than doubled between 

1950 and 1975 but subsequently have remained fairly 

steady, that the percentage of ground water compared 

to surface water for public supply increased from 

about 26 percent in 1950 to about 40 percent in 1985 

and has remained just under 40 percent during the past 

two decades, and that the percentage of ground water 

compared to surface water for irrigation increased 

from about 23 percent in 1950 to about 42 percent in 

2000 (Hutson and others, 2004). In the most recent 

compilation (for year 2000), ground-water withdrawals 

for irrigation accounted for about two-thirds of total 

ground-water withdrawals (Hutson and others, 2004). 

The temporal trends of ground-water withdrawals for 

irrigation have been somewhat different between the 

western and eastern States (fig. 4). 

Figure 4. Groundwater withdrawals 

for irrigation in the 

western and eastern 

conterminous United 

States. Ground-water 

withdrawals for irrigation 

decreased in the western States 

in recent decades as a result of 

expanding urban areas, an increase in dryland farming, 

and increased efficiencies of application. In contrast, 

ground-water withdrawals for irrigation in the eastern half 

of the country increased steadily over the same period, in 

part, as a supplemental source of water to protect against 

dry periods. (Data compiled from U.S. Geological Survey 

Circulars titled “Estimated use of water in the United 

States,” published in 5-year intervals for the years 

1960 to 2000.)
WITHDRAWALS, IN MILLION GALLONS PER DAY

0 2,000 4,000 6,000 8,000 10,000 12,000 14,000 16,000 18,000 20,000

High Plains aquifer

Central Valley aquifer system

Basin and Range basin-fill aquifers

Mississippi River Valley alluvial aquifer

Floridan aquifer system

Glacial sand and gravel aquifers

California Coastal Basin aquifers

Snake River Plain basaltic-rock aquifers

Coastal lowlands aquifer system

Alluvial aquifers (Not shown in figure 2)

Other

Rio Grande aquifer system

Northern Atlantic Coastal Plain aquifer system

Mississippi embayment aquifer system

Columbia Plateau basaltic-rock aquifers

Cambrian–Ordovician aquifer system

Pacific Northwest basin-fill aquifers

Southeastern Coastal Plain aquifer system

Biscayne aquifer

Edwards–Trinity aquifer system

11

10

13

1

50

3

59

18

2

22

20

60

33

4

21

51

38

Figure 5. Principal aquifers that provided most of the total withdrawals for irrigation, public-supply, and 

self-supplied industrial water uses in the United States during 2000 (from Maupin and Barber, 2005).

Figure 5. Principal aquifers that provided most of the total withdrawals for irrigation, public-supply, and self-supplied 

industrial water uses in the United States during 2000 (from Maupin and Barber, 2005). See figure 2 for aquifer locations.

12 Ground-Water Availability in the United States

As part of the 2000 compilation of water-use 

data, estimates of ground-water withdrawals were 

made for 66 principal aquifers in the United States for 

three major categories of water use—public supply, 

irrigation, and self-supplied industrial (Maupin and 

Barber, 2005). The results indicate that 20 principal 

aquifers (including an “other” principal aquifer 

category) account for about 90 percent of the groundwater withdrawals in the United States for the three 

major categories combined (fig. 5).

In estimating ground-water use, it is important to 

recognize that not all the water pumped is consumed. 

When water is pumped from the ground and used, the 

water molecules are not destroyed; the water is simply 

moved to different places. For example, when water 

is used for self-supplied domestic use, some of it is 

consumed and some of it is redistributed back into the 

environment. Solley and others (1998, p. 24) estimated 

that only 26 percent of the water used for self-supplied 

domestic use is consumed. Consumed water is assumed 

to be evaporated, transpired, incorporated into products 

or crops, consumed by humans or livestock, or otherwise removed from the immediate water environment. 

The rest of the water gets redistributed back into the 

environment, such as sewage disposal into streams and 

additional recharge from excess irrigation. Even the 

water consumed, however, is not really lost; it goes 

into the atmosphere or into products or living tissue. 

When analyzing the amount of ground-water available, 

it is important to consider where the water pumped 

will end up. Thus, ideally, information on groundwater use should include estimates of consumptive use 

and return flow as well as withdrawals, but this type of 

information can be difficult to estimate for many uses.
Thickness

below

stream

bottom

Ground-water system

NOT TO SCALE

Height of water table

above stream bottom

Stream

Land surface

Water table

Figure 6. A cross section of a hypothetical stream-aquifer 

system, showing that a small change in the ground-water level 

can result in a large change in gradient and flow to the stream.

Water table

High Plains aquifer

Bedrock

Springs

and

seeps

West East

VERTICAL SCALE GREATLY EXAGGERATED

NOT TO SCALE

Figure 7. A diagrammatic cross section of the southern High 

Plains aquifer system, illustrating that in areally extensive 

systems, large changes in water levels and in the amount of 

water stored may occur before the effects of the water-level 

changes reach the boundaries of the system (modified from 

Lohman, 1972).

What Do We Know About Ground-Water Availability in the United States? 13

Changes in Ground-Water Levels and 

Ground-Water Storage

The response of a ground-water system to withdrawals was explained succinctly by Theis (1940). 

Two important points raised by Theis (1940, p. 10) 

are: “All water discharged by wells is balanced by a 

loss of water somewhere,” and “This loss is always to 

some extent and in many cases largely from storage 

in the aquifer. Some ground water is always mined.” 

Discharge from a well always requires some drawdown 

(change in water level) to create hydraulic gradients 

to move water to the well. Thus, all discharging wells 

have to remove some water from storage, and all 

systems must conserve mass. This conservation of 

the mass of water is referred to as a mass balance.

The altitude of the top of the water column in a 

well is called the water level. Water levels in carefully constructed wells with short screens indicate the 

energy or head of the water and can be used to define 

the direction and rate of movement of the water in the 

ground-water system (for more information see Taylor 

and Alley, 2001). Water levels are important because 

they define the state of the saturated ground-water 

system. Decreases in water levels caused by pumping 

not only indicate changes in the amount of water 

stored in the aquifer but also that water probably is 

being “captured” from a neighboring surface source. 

The relative significance of water-level changes with 

respect to these two aspects depends on the location 

of the water-level changes in relation to the boundaries 

of the ground-water system.

In systems that have streams that are nearby 

and not deeply incised in the aquifer (fig. 6), a small 

change in water level can change the ground-water 

flow to the stream by a large percentage. If the water 

level in the ground-water system drops below the 

stream stage (the water level of the stream), the stream 

can become a major source of flow to the groundwater system. The change in the amount of water 

stored in the ground-water system would not change 

that much in this example, but the effects on the 

nearby stream system could be substantial. In systems 

that are expansive and have boundaries that are far 

away, such as the southern High Plains aquifer system 

(fig. 7), there can be large changes in both water 

levels and the amount of water in storage before the 

distant bounding streams or springs are affected.

Ground-water systems are continuous saturated 

systems made up of different earth materials. As 

a simplified classification, these saturated earth 

materials can be classified as either aquifers or 

confining beds. As previously defined, an aquifer 

contains sufficient saturated permeable material to 

yield significant quantities of water to wells and 

springs. A confining bed is a rock unit of very low 

hydraulic conductivity that restricts the movement 

of ground water either into or out of adjacent aquifers 

(Heath, 1983). A ground-water system can be made 

up of many aquifers and confining beds. The top 

boundary of the saturated ground-water system is 

the water table. 

Figure 6. A cross section of a hypothetical stream-aquifer 

system, showing that a small change in the ground-water level 

can result in a large change in gradient and flow to the stream.

Figure 7. A diagrammatic cross section of the southern 

High Plains aquifer system, illustrating that in areally extensive 

systems, large changes in water levels and in the amount of 

water stored may occur before the effects of the water-level 

changes reach the boundaries of the system (modified from 

Lohman, 1972).
Figure 8. Approximate location of maximum subsidence 

in United States identified by research efforts of Joseph 

Poland (pictured). Signs on pole show approximate altitude 

of land surface in 1925, 1955, and 1977. The pole is near 

benchmark S661 in the San Joaquin Valley southwest of 

Mendota, California (from Galloway and Riley, 1999).

14 Ground-Water Availability in the United States

The response of an aquifer through time to 

pumping depends in part on whether the aquifer is 

“confined” or “unconfined.” Aquifers that contain 

the water table are referred to as unconfined aquifers. 

In unconfined aquifers, the drawdowns through 

time in response to pumpage usually are moderated 

because of the large source of water derived from 

storage as water is drained from the pore space of the 

aquifer material as the water level drops. In addition, 

in unconfined aquifers, surface-water sources of water 

can be nearby and can limit the extent of the waterlevel declines. 

Aquifers that are fully saturated and have 

confining units above them are referred to as confined

(or artesian) aquifers. Drawdowns through time in 

response to pumping confined aquifers usually are 

larger than those in unconfined aquifers for the same 

transmitting properties. The water table at the very 

top of the entire ground-water system is hydraulically 

distant from the direct influence of the pumping in a

confined aquifer and hence the water-table aquifer does 

not drain in response to the pumping. Rather, as water

levels drop in response to pumpage, the water pressure 

in the confined aquifer decreases, and water is released 

from storage as the water expands and the aquifer 

material compresses. In addition, in confined aquifers 

the boundaries of the aquifer are usually farther away 

physically (or hydraulically) and, thus, cannot readily 

serve as a source of water to be captured. 

If the ground-water system eventually comes to 

a new equilibrium in response to the pumping, the 

effect of storage on the magnitude of the drawdown is 

no longer a factor because water levels are no longer 

changing, regardless if the aquifer is confined or unconfined. The magnitude of the drawdown under equilibrium conditions depends on the transmitting properties 

and the location and magnitude of the sources of water.

In some hydrogeologic terrains, the removal of water 

can cause some fine-grained confining beds to compact 

substantially causing subsidence of the land surface. 

Land subsidence can be a gradual or sudden sinking 

of the Earth’s surface caused by subsurface movement 

of earth materials (Galloway and others, 1999). Some 

areas of the Nation, such as the San Joaquin Valley, 

California (fig. 8), are more prone to subsidence 

because of the presence of fine-grained compressible 

confining beds. Thus, water-level declines become 

very important in areas susceptible to subsidence.
Figure 9. Areas of water-table decline or artesian water-level decline in excess of 40 feet 

in at least one aquifer since predevelopment (modified from U.S. Geological Survey, 1984).

What Do We Know About Ground-Water Availability in the United States? 15

The volume of ground water in storage is 

decreasing in many areas of the United States in 

response to withdrawals. If these water-level declines 

are sustained over time, the effect is often described 

as ground-water depletion. Among the consequences 

of ground-water-level declines are increased pumping 

costs, deterioration of water quality, reduction of 

water in streams and lakes, and land subsidence. 

Such negative effects, while variable, happen to some 

degree with any ground-water use. As with other 

natural resources, society must weigh the benefits 

gained by the use of the natural resource against the 

consequences of such use. These effects should be 

observed over time in order to determine their impact.

The extent of ground-water-level declines across 

the United States has not been monitored or computed 

on a regular basis. Previous compilation efforts have 

included national maps of “Areas where significant 

cones of depression have been developed by pumping 

from wells” and “Ground-water reservoirs with perennial overdraft” (Thomas, 1951, Plates II and III), and 

a national map showing the “Area in which significant 

ground-water overdraft is occurring” (U.S. Water 

Resources Council, 1978, p. 59). The most recent 

national summary of ground-water-level declines was 

compiled by the USGS in 1983. The map from this 

compilation is shown in figure 9, which delineates 

areas of water-table decline or artesian water-level 

decline in excess of 40 feet (ft) in at least one aquifer 

since predevelopment (U.S. Geological Survey, 1984).

There are inherent difficulties with a national 

compilation of this kind. Aquifers may be confined or 

unconfined, the ground-water system may be directly 

connected or distant from a surface-water body, and 

ground-water development may be uneven in space 

and time. Ground-water-level declines occur at scales 

ranging from a single well to aquifer systems underlying several States. The extent of the resulting effects 

depends on several parameters, including pumping 

and natural discharge rates, physical properties of 

the aquifer, and natural and human-induced recharge 

rates. At the local scale, monitoring of water-level 

declines commonly focuses on local effects (such 

as local well interference), and at the regional scale, 

monitoring commonly focuses on more widespread 

effects (such as streamflow depletion). 

Figure 9. Areas of water-table decline or artesian water-level decline in excess of 40 feet 

in at least one aquifer since predevelopment (modified from U.S. Geological Survey, 1984).
Figure 10. Areas of water-level decline in excess of 40 feet in at least one confined aquifer since predevelopment, 

and areas of water-table decline in excess of 25 feet in the water-table aquifer since predevelopment.

16 Ground-Water Availability in the United States

To illustrate the extent of ground-water-level 

declines over a national and regional scale, information was gathered in 2007 from USGS State Water 

Science Centers throughout the Nation for areas of 

water-level decline in excess of 40 ft in at least one 

confined aquifer since predevelopment and areas of 

water-level decline in excess of 25 ft in an unconfined 

aquifer since predevelopment (fig. 10). In order to be 

included on the map, the areal extent of the waterlevel decline had to be approximately 500 square 

miles (mi2) or larger. In some cases, USGS State 

Water Science Centers suspected additional areas of 

ground-water decline, but sufficient supporting data 

were not available to define the extent of the areas; 

therefore, these areas were not included on the map.

Because ground-water-level declines are necessarily the result of ground-water use, these areas of 

extensive ground-water-level declines correspond well 

to the highly used principal aquifers. A description of 

selected areas of ground-water-level declines follows, 

with reference to the principal aquifers shown in 

figure 2. In comparison to figure 9, some areas shown 

in figure 10 have expanded since the 1983 compilation.

Figure 10. Areas of water-level decline in excess of 40 feet in at least one confined aquifer 

since predevelopment, and areas of water-table decline in excess of 25 feet in the water-table 

aquifer since predevelopment.

In other areas, the water-level changes took place during 

initial development, and water levels are relatively stable 

today (2007). For example, most of the area shown as 

declines in the upper Midwest (mainly in South Dakota 

and Iowa) is the result of drawdowns that occurred 

early in the 1900s (U.S. Geological Survey, 1984).

Although this approach to evaluate significant 

areas of ground-water-level decline is useful, it is not 

comprehensive. In some aquifers, such as the Basin 

and Range basin-fill aquifers, the geologic structure 

of the aquifer limits the areal extent of the drawdown 

(though it may exacerbate the rate of decline). Basin 

and Range basin-fill aquifers generally are composed 

of unconsolidated material in basins bounded by 

mountains. Individual basins may not be large enough 

to meet the criteria used in this analysis.

Because ground-water-level decline can have 

impacts at a variety of scales, a national map showing 

potential local effects also was compiled. This is not 

a comprehensive evaluation of water-level declines in 

all areas across the Nation, because a comprehensive 

database of all ground-water-level monitoring data 

in the United States does not exist. In fact, the 
Figure 11. Individual wells in the USGS National Water Information System database where the 

difference between the lowest and highest water-level measurement over time is at least 40 feet. 

Each well is one blue circle, and apparent miscellaneous shapes are due to overlapping circles.

Figure 11. Individual wells in the USGS National Water Information System database where the 

difference between the lowest and highest water-level measurement over time is at least 40 feet. 

Each well is one blue circle, and apparent miscellaneous shapes are due to overlapping circles.

What Do We Know About Ground-Water Availability in the United States? 17

availability of ground-water levels and rates of change 

is “not adequate for national reporting” according to 

the report, “The State of the Nation’s Ecosystems” (The 

H. John Heinz III Center for Science, Economics, and 

the Environment, 2002). A followup report from the 

Heinz Center (The H. John Heinz III Center for Science, 

Economics, and the Environment, 2006) identified 

ground-water levels as “one of the 10 highest priority 

data gaps that must be filled to improve the nation’s 

ability to report on ecosystem conditions and use, and 

to make sound policy and operational decisions.”

Even though it does not contain all the wells 

measured by other agencies, the USGS National 

Water Information System (NWIS) database contains 

the most complete ground-water-level dataset for 

the Nation. NWIS contains ground-water-level data 

from USGS Federal programs, programs with other 

Federal agencies, and cooperative programs with State 

and local governments. Water-level data in the NWIS 

database may or may not include predevelopment 

measurements, so the total water-level declines reported 

from an individual well may be underestimated. In 

addition, water-level changes for this analysis were 

determined by calculating the difference between 

the highest and lowest water level measured over 

time. This approach does not account for seasonal 

water-level recovery, or long-term recovery, in some 

cases. Figure 11 illustrates individual wells where the 

difference between the highest and lowest water level 

measured is at least 40 ft. It is clear from figure 11 

that few areas of the Nation have been spared from 

the effects of substantial water-level declines.

Summaries of some of the areas experiencing 

substantial water-level declines are provided below. 

Other areas, including the California Central Valley 

aquifer system, the High Plains aquifer system, and 

the aquifer systems surrounding the Great Lakes, 

are described later in this report as case studies and 

are not presented here. The summaries below and in 

the case studies suggest that water-level declines are 

widespread across the Nation and can result from 

different circumstances.
18 Ground-Water Availability in the United States

Pacific Northwest Region

Evaluating the effect of ground-water use on the 

history and extent of water-level declines in some 

parts of the country is complicated by the fact that 

excess surface-water irrigation can contribute to 

water-level increases or reduce declines that would 

have occurred otherwise. Two aquifer systems in the 

Pacific Northwest provide such examples.

Water use for irrigation, public supply, and 

industrial uses in the area of the Columbia Plateau 

basalt aquifer system of Washington and Oregon has 

resulted in significant water-level changes. Water 

levels have risen (more than 300 ft in some areas) to 

near land surface beneath about 1.2 million irrigated 

acres as a result of the application and infiltration of 

surface water. Water-level declines resulting from 

ground-water pumping for irrigation began in the 

1960s. The declines continue today (2007) and have 

exceeded 200 ft in some parts of the central plateau 

that are not irrigated by surface water (Vaccaro, 1999).

The Snake River Plain aquifer in Idaho provides 

extensive water for irrigation as well as much of the 

flow of the Snake River through springs. Prior to about 

1960, surface-water irrigation raised water levels in 

parts of the Snake River Plain aquifer by 60–70 ft 

and increased ground-water discharge to the river and 

springs (Idaho Department of Water Resources, 1999). 

Since the high water-level period of the 1950s, water 

levels and spring discharge have decreased because of 

intensive use of ground water for agriculture and more 

efficient irrigation practices (Lindholm, 1996). Areas 

of water-level decline in these two principal aquifers 

have increased since the 1983 compilation.

Mississippi River Valley and Gulf Coast Region

Ground-water use in the Mississippi River Valley 

alluvium, the Coastal lowlands, and the Mississippi 

embayment aquifer systems has been fundamental 

to the region’s agriculture, industry, and some 

municipalities. In the Houston, Texas area, extensive 

pumpage of ground water to support economic and 

population growth has caused water-level declines 

of approximately 400 ft, resulting in extensive 

land-surface subsidence of as much as 10 ft. The City 

of Houston is addressing this problem by shifting 

to surface-water sources as their primary supply. 

Ground-water pumpage for Baton Rouge, Louisiana, 

increased more than tenfold between the 1930s and 

1970, resulting in ground-water-level declines of 

approximately 200 ft. Baton Rouge is underlain by a 

series of aquifers, and as one aquifer experiences large 

water-level declines, pumping has shifted to others 

over time. Ground-water declines in the Sparta aquifer 

in Arkansas, Louisiana, Mississippi, and Tennessee 

have raised concerns about the sustainability of 

the resource. Some areas have shifted their source 

of supply to surface water. Regional water-level 

declines of as much as 70 ft have resulted in interstate 

concerns over continued and increased pumpage in 

the Memphis, Tennessee area.

Atlantic Coastal Region

Although the humid eastern seaboard has abundant renewable freshwater resources, ground-water 

resources in the highly productive principal aquifers 

are under increasing stress in some areas as coastal 

populations continue to increase. This increase in 

ground-water use from the Floridan, Northern Atlantic 

Coastal Plain, and Southeastern Coastal Plain aquifer 

systems has resulted in a significant expansion of 

the areas affected by ground-water declines since 

the 1983 compilation. The aquifers of the Atlantic 

Coastal Region generally are confined aquifers, with 

productive units commonly overlying one another. 

The area depicted for the Atlantic Coastal Region in 

figure 10 consists of 45 individual areas of drawdown 

calculated using water levels from predevelopment 

and the year 2000 that have been merged together for 

this map (dePaul and others, 2008).

Basin and Range Aquifers

Ground-water-level declines in Basin and Range 

aquifers in California, Arizona, Nevada, and Utah 

become observable when examining the declines 

from individual wells. Ground-water use from Basin 

and Range aquifers is the fourth highest among the 

Nation’s principal aquifers. Water is needed in the 

area to support a fast-growing population as well 

as agricultural production in the desert southwest. 

Drawdown patterns from ground-water use can be 

seen in southern Arizona, southern California, the 

Las Vegas, Nevada area, and central Utah.
Figure 12. Water-level declines. Red regions indicate areas in excess of 500 square miles that have water-level 

decline in excess of 40 feet in at least one confined aquifer since predevelopment, or in excess of 25 feet of decline 

in unconfined aquifers since predevelopment. Blue dots are wells in the USGS National Water Information System 

database where the measured water-level difference over time is equal to or greater than 40 feet.

Figure 12. Water-level declines. Red regions indicate areas in excess of 500 square miles that have waterlevel decline in excess of 40 feet in at least one confined aquifer since predevelopment, or in excess of 25 feet of 

decline in unconfined aquifers since predevelopment. Blue dots are wells in the USGS National Water Information 

System database where the measured water-level difference over time is equal to or greater than 40 feet.

What Do We Know About Ground-Water Availability in the United States? 19

When the necessary data do not exist, additional 

monitoring is needed to refine the knowledge of 

water-level declines on a national scale. Moreover, 

observation of ground-water-level changes is only 

one piece of the puzzle concerning ground-water 

availability. Measurements that indicate changes 

in base flow of streams are also good indicators of 

changes in ground-water levels. In addition, water-level 

declines occur in three dimensions, and differences in 

changes with depth are important although they cannot 

be shown on the two-dimensional national maps.

New methods and scientific advances can 

continue to improve our understanding and ability 

to document water-level and storage changes. For 

example, the use of microgravity measurements on 

the ground (Pool and Eychaner, 1995) and from space 

(Swenson and others, 2003) is being developed and 

refined to estimate storage changes. These methods 

for estimating changes in ground-water storage are 

promising at the local and regional scales. 

The information presented in figure 12 combines 

the regional water-level declines presented in figure 10 

with the more local water-level declines presented 

in figure 11. Figure 12 represents the current state of 

knowledge about ground-water declines on a national 

scale. Our knowledge is incomplete in some areas 

because there are not enough water-level data and 

perhaps in other areas because data have not been 

compiled. A national effort is needed to organize 

available information on changes in ground-water 

storage, similar to what was done for the High Plains 

aquifer (see High Plains aquifer case study). To 

address this need, the Advisory Committee on Water 

Information in 2007 created a Subcommittee on 

Ground Water (http://acwi.gov/sogw/index.html) to 

develop and encourage implementation of a nationwide, long-term ground-water quantity and quality 

monitoring framework. 
EXPLANATION

Mean annual precipitation, in inches

Less than 5

5–12

12–20

20–30

30–40

40–50

50–70

70–100

Greater than 100

Figure 13. Mean annual precipitation for the contiguous United States, 1890 to 2002 (from Anderson and Woosley, 2005).

20 Ground-Water Availability in the United States

Recharge

Estimates of recharge quantify the amount of 

water entering the saturated ground-water system. 

These estimates, however, are dependent on the scale 

of the estimate, the method used to make the estimate, 

and the time over which the estimate is made. The 

intended use of the estimated recharge value is important in determining the scale, time period, and accuracy 

needed. The following discussion is limited to recharge 

to the water table (as opposed to interaquifer recharge). 

Recharge can be diffuse or localized. Diffuse 

recharge refers to the widespread movement of 

water from land surface to the water table as a result 

of infiltration of precipitation over large areas and 

percolation through the unsaturated zone. Localized 

recharge, on the other hand, refers to the movement of 

water from surface-water bodies to the ground-water 

system and is less uniform in space than diffuse 

recharge. Most ground-water systems receive both 

diffuse and localized recharge. In general, the importance of diffuse recharge decreases as the aridity of a 

region increases. Information available on recharge 

at the national scale pertains to diffuse recharge. The 

information available does not account for the effects 

of irrigation and other human activities, which can 

have major effects on recharge in many areas. 

The amount of precipitation represents the maximum amount available (an upper limit) of possible 

natural diffuse recharge to the ground-water system. 

Precipitation frequently is the principal source of 

water to the ground-water system in the absence of 

irrigation. Examination of an average precipitation 

map for the United States (fig. 13) is useful in determining broad areas of potential high and low recharge.

Figure 13. Mean annual precipitation for the conterminous United States, 1890 to 2002 

(from Anderson and Woosley, 2005).
EXPLANATION

Precipitation minus PET, in inches per year

Greater than 0 to 12.5

12.5 to 34

34 to 72

72 to 262

Figure 14. Difference between annual precipitation and potential evapotranspiration (PET) 

rates across the conterminous United States (from Healy and others, 2007

–63 to 0

What Do We Know About Ground-Water Availability in the United States? 21

The amount of water available for natural 

recharge to the ground-water system and as surface 

runoff to streams is represented by the amount of 

precipitation minus the amount of evapotranspiration. 

Evapotranspiration is the water lost to the atmosphere 

by two processes: evaporation and transpiration by 

plants. In the total water budget for an area, evapotranspiration is a significant budget component. It is 

estimated that evapotranspiration for the conterminous 

United States accounts for 67 percent of the outflow 

of water from precipitation (Hanson, 1991). In arid 

areas, evapotranspiration often exceeds precipitation 

in the long term, and areal recharge occurs only 

sporadically following extreme rainfall events. 

Evapotranspiration amounts are difficult to measure 

and are usually estimated. 

Figure 14 is an indicator of potential natural 

recharge on a national scale. The map of the 

conterminous United States in figure 14 shows the 

distribution of the difference between precipitation and potential evapotranspiration (Healy and 

others, 2007). Potential evapotranspiration is “the 

water loss that would occur by evapotranspiration 

if there was never a deficiency of water in the soil 

for use by vegetation” (Wilson and Moore, 1998). 

Where precipitation substantially exceeds potential 

evapotranspiration, one can expect a water surplus, 

and where potential evapotranspiration substantially 

exceeds precipitation (the negative numbers), one 

can expect a water deficit. The map indicates that 

the eastern United States has more precipitation than 

potential evapotranspiration and the Great Plains 

and southwestern United States has more potential 

evapotranspiration than precipitation.

Figure 14. Difference between annual precipitation and potential evapotranspiration (PET) rates 

across the conterminous United States (modified from Healy and others, 2007).
Available precipitation—Average from 1934 to 2002, in

 inches per year. Number of counties in parentheses

0 to 5 (603)

5 to 10 (414)

10 to 15 (525)

15 to 20 (698)

20 to 25 (464)

Greater than or 

 equal to 25 (407)

EXPLANATION

Figure 15. Available precipitation (difference between monthly precipitation and potential evapotranspiration (PET), sum of months with nonzero values) for the conterminous United States (modified 

from Roy and others, 2005; reprinted with permission, American Water Resources Association).

22 Ground-Water Availability in the United States

Figure 15 (Roy and others, 2005) takes the 

analysis shown in figure 14 one step further and 

estimates the precipitation available for use in the 

United States; that is, how much water is available for 

recharge to the ground-water system or as runoff to 

streams. This estimate was calculated as the difference 

between precipitation and potential evapotranspiration 

summed for all months in the year in which 

precipitation exceeded potential evapotranspiration. 

The map in figure 15 shows values of precipitation 

available for use at the county level estimated from 

data from 1934 to 2002 at the 344 National Oceanic 

and Atmospheric Administration climate divisions 

covering the conterminous United States. This map 

Figure 15. Available precipitation (difference between monthly precipitation and potential evapotranspiration [PET], sum of months with nonzero values) for the conterminous United States (modified 

from Roy and others, 2005; reprinted with permission, American Water Resources Association).
r

r

r

r r

r

r

r

r

r

r

r

r

r

r

r

d

d

d

d

d

d

s

s

s

s

s

s

s

s

s

s

s

s

s

s

s2

s2

s2

s2

Little or no water deficiency in any season—Occurs only in moist climates

Moderate seasonal moisture variation with summer the driest season—

 Summer water deficiency in moist climates; winter water surplus in dry climates

Large seasonal moisture variation with summer the driest season—

 Summer water deficiency in moist climates; winter water surplus in dry climates

Little or no water surplus in any season—Occurs only in dry climates

s

r

s2

d

EXPLANATION

Figure 16. Seasonal variation of effective moisture for the conterminous United States (modified 

from Thornthwaite, 1948; reprinted with permission, The American Geographic Society).

What Do We Know About Ground-Water Availability in the United States? 23

clearly shows that most of the western United States, 

except for some coastal areas, has far less water 

available for ground-water recharge and use than the 

rest of the country. Thornthwaite (1948) presented 

a similar map (fig. 16) that showed qualitative areas 

of water deficiency across the conterminous United 

States. Figure 16 shows areas of little or no water 

deficiency, areas of seasonal moisture variation 

(summer deficiency and winter surplus), and areas 

with little or no water surplus. The maps by Healy 

and others (2007), Roy and others (2005), and 

Thornthwaite (1948) describe a similar assessment 

of the distribution of water available for recharge 

and runoff across the United States.

Figure 16. Seasonal variation of effective moisture for the conterminous United States 

(modified from Thornthwaite, 1948; reprinted with permission, The American Geographic Society).
EXPLANATION

Mean annual ground-water recharge, 

 in inches per year

0.0 to 0.5

0.5 to 1.0

1.0 to 5.0

5.0 to 10.0

10.0 to 15.0

15.0 to 20.0

Greater than 20.0

Figure 17. Estimated mean annual natural ground-water recharge in the conterminous United States (data from Wolock, 2003a).

24 Ground-Water Availability in the United States

Estimated values of natural ground-water recharge 

are available for the entire United States (Wolock, 

2003a) based on base-flow separation techniques. 

The component of streamflow that is contributed 

by ground-water discharge as opposed to surface 

runoff is called base flow. Wolock (2003a) produced 

a 1-kilometer resolution raster dataset (fig. 17) as 

an estimate of mean annual natural ground-water 

recharge. The dataset was created by multiplying a grid 

of base-flow index values (Wolock, 2003b) by a grid 

of mean annual runoff values derived from a 1951–80 

mean annual runoff contour map (Gebert and others, 

1987). The concept used to construct the estimate 

is based on two assumptions: (1) long-term average 

natural ground-water recharge is equal to long-term 

average natural ground-water discharge to streams, 

and (2) the base-flow index reasonably represents, 

over the long term, the percentage of natural groundwater discharge in streamflow. These estimates 

(fig. 17), based on very broad assumptions, give a 

“big picture” of natural ground-water recharge but 

cannot characterize the variability over space and 

time at more local scales. In addition, in areas where 

streams are regulated, irrigation return flows are large, 

or urbanization has occurred, the basic assumptions 

may not hold. For example, three of the case studies 

presented later in this report (Middle Rio Grande 

Basin, California Central Valley aquifer system, and 

the High Plains aquifer) illustrate several-fold increases 

in recharge from natural to developed conditions.

Figure 17. Estimated mean annual natural ground-water recharge in the conterminous United States 

(data from Wolock, 2003a).
EXPLANATION

Base-flow

measurement 

site

Recharge in inches

0.0 to 4.0

4.1 to 8.0

8.1 to 12.0

12.1 to 16.0

Greater than 16.0

No data

A. B.

0 10 20 KILOMETERS

0 10 20 MILES

Wisconsin

88°0’

45°0’

44°0’

87°30’ 88°0’

45°0’

44°0’

87°30’

Figure 18. Recharge rates in Hydrologic Unit Code (HUC) watershed 04030102, Door-Kewaunee, 

estimated for two different scales (A) regionally and (B) subregionally (from Gebert and others, 2006).

Figure 18. Recharge rates in Hydrologic Unit Code (HUC) watershed 04030102, Door–Kewaunee, 

estimated for two different scales (A) regionally and (B) subregionally (modified from Gebert and 

others, 2006).

What Do We Know About Ground-Water Availability in the United States? 25

There are estimates of recharge for large parts of 

the United States, such as the Great Lakes Basin 

(Neff and others, 2006), and there are estimates 

for smaller areas (Gebert and others, 2006). Gebert 

and others (2006) illustrate (fig. 18) that estimates 

of recharge at different scales and times can show 

marked differences in variability even though similar 

base-flow separation techniques were used in all 

cases. In figure 18, the regional estimate produces 

only one value for the entire Door–Kewaunee 

watershed in Wisconsin, whereas the more detailed 

subregional examination shows a wide range of 

values. Gebert and others (2006) also note that the 

range in recharge can be expected to increase as the 

spatial scale becomes smaller, or different techniques 

are used. The objectives of a study will determine 

which estimates are sufficient.
26 Ground-Water Availability in the United States

Ground-Water Discharge

Understanding the amounts of water and where 

the water enters and leaves the saturated ground-water 

system is a prerequisite to evaluating ground-water 

availability. Water can naturally discharge from 

ground-water systems to surface-water bodies or to 

plants and the atmosphere. In addition, withdrawals 

by people also account for discharge leaving the 

system, as discussed previously.

Evapotranspiration directly from the groundwater system (as opposed to evapotranspiration at 

land surface) is mostly due to transpiration because 

direct evaporation from the water table through the 

unsaturated zone is limited. Most of the ground-water 

evapotranspiration takes place where the water table 

is close to the land surface and plant roots have easy 

access to the ground water. This condition usually 

is found in riparian zones near streams. Groundwater evapotranspiration can be a major discharge 

in ground-water systems in the southwestern United 

States but usually is not the major component of 

discharge in the humid eastern United States (Healy 

and others, 2007, p. 28). There are no national 

compilations of ground-water evapotranspiration, 

because it is highly variable and locally dependent 

on the depth to water and the vegetation present. 

Ground-water discharge to surface-water bodies 

is a key component of most ground-water systems. 

Base flow is less variable over time than surface 

runoff and is responsible for keeping many streams 

flowing when there is no precipitation, even in times 

of drought. The percentage of streamflow that is 

accounted for by base flow for any particular stream 

is variable across the Nation. Wolock (2003b) 

estimated the ratio of base flow to total flow, 

expressed as a percentage (which is called the baseflow index) for the conterminous United States. 

The ground-water contribution to streamflow 

is an important aspect of the water budget of a 

ground-water system because it frequently is the 

primary source of water that can be captured for 

use. Understanding the changes in the discharge 

of a ground-water system to development through 

time is fundamental to understanding ground-water 

availability (Bredehoeft, 2007). 

Ground-Water Quality

The chemical quality of ground water affects 

its suitability for different uses. Natural chemical 

reactions between the water and the rock it flows 

through add dissolved substances and compounds 

to the water. Human-made contaminants at the land 

surface also can percolate into ground water and 

affect its suitability. The sources of ground-water 

contamination are numerous and diverse and include 

point and nonpoint (dispersed) sources. Contamination 

from point sources is particularly difficult to 

characterize from a national or regional perspective. 

A basic suitability criterion is the amount of 

total dissolved solids or salinity of the water. Feth 

and others (1965) identified the depth to saline water 

for much of the United States (fig. 19). This national 

information, however, provides only a rough idea as to 

where salinity is an issue; the depth to saline water at 

any particular well can be substantially different from 

what is shown on the national map.

Saline water in coastal as well as inland areas 

has the potential to move into a fresh ground-water 

system and affect its suitability. Freshwater is less 

dense than saline water and tends to flow on top of the 

surrounding or underlying saline ground water. Under 

natural conditions, the boundary between freshwater 

and saltwater maintains a stable equilibrium (fig. 20). 

The boundary typically is not sharp and distinct but 

is a gradation from fresh to saline water shown by the 

transition zone in figure 20. When water is pumped 

from an aquifer that contains or is near saline ground 

water, the saltwater/freshwater boundary will move in 

response to this pumping. If the boundary moves far 

enough, some wells become saline, thus contaminating 

the water supply. The location and magnitude of the 

ground-water withdrawals with respect to the location 

of the saltwater determine how quickly and how far the 

saltwater moves. Even if the lateral regional movement 

of saltwater is negligible, individual wells located near 

the saltwater/freshwater boundary can become saline 

as a result of significant local drawdowns that cause 

underlying saltwater to “upcone” into the well. 

The availability of ground water and the suitability of its quality for different uses are inextricably 

intertwined. As just described, saltwater occurs 
EXPLANATION

Depth to saline ground water, in feet

Less than 500

500 to 1,000

Greater than 1,000

Inadequate information

Figure 19. Depth to saline ground water in the conterminous United States (generalized from Feth and others, 1965).

Water table

Land surface

Freshwater

Sea

Trans i t i on z o ne

Impermeable bedrock

Saltwater

Figure 20. Ground-water flow patterns and the freshwatersaltwater transition zone in an idealized coastal aquifer. 

Circulation of saltwater from the sea to the transition zone and 

then back to the sea is induced by freshwater and saltwater 

mixing in the transition zone (from Barlow, 2003; modified from 

Cooper, 1964). 

NOT TO SCALE

What Do We Know About Ground-Water Availability in the United States? 27

in the subsurface nationwide. Although saltwater 

represents huge volumes of ground water in storage, 

these volumes are not included in most inventories of 

available ground water because of their inherent unsuitability for almost all uses without treatment. Ground 

water of lower dissolved-solids concentrations, however, may still be suitable for some uses. For example, 

some cattle can tolerate a higher dissolved-solids 

concentration in their drinking water than can humans. 

Economics is an additional factor to consider in 

determining the chemical suitability of a water supply. 

Water can be treated to remove most chemicals at a 

cost. For example, saltwater can be desalinated and 

used for drinking. Thus, when considering the chemical 

suitability of a source of water, one must determine the 

sources of all water types and the amount of money 

that people are willing to spend to treat it for various 

requirements. Likewise, there may be other issues, 

such as the cost of energy and environmental effects 

of waste disposal, when considering desalination. 

Figure 19. Depth to saline ground water in the conterminous United States (generalized from Feth and others, 1965).

Figure 20. Ground-water flow patterns and the freshwatersaltwater transition zone in an idealized coastal aquifer. 

Circulation of saltwater from the sea to the transition zone 

and then back to the sea is induced by freshwater and 

saltwater mixing in the transition zone (from Barlow, 2003; 

modified from Cooper, 1964). 
Figure 21. Ground-water arsenic samples collected in 1973–2001 (from Ryker, 2001, accessed 

July 17, 2006, at http://water.usgs.gov/nawqa/trace/pubs/geo_v46n11/fig1.html).

EXPLANATION

Concentration of arsenic, 

 in micrograms per liter

1 to 3

3 to 5

5 to 10

10 to 50

At least 50

28 Ground-Water Availability in the United States

The quality of ground water varies in three 

dimensions (areally and with depth). Some natural 

constituents, such as dissolved-solids concentrations, 

can be mapped broadly on a national scale, for 

example the depth to saline water as shown in 

figure 19. Other natural constituents, such as arsenic 

(fig. 21), are associated with geologic formations. 

Although difficult to map on a national scale because 

of local chemical variability, the distribution of a 

concentration of a particular constituent in water 

from wells does provide some ability to highlight 

areas of the Nation that may have potential problems.

Many of the chemical constituents that affect the 

suitability of water for use, however, come from point 

and nonpoint sources of anthropogenic contamination 

near or at the land surface. Contamination from point 

sources is extremely variable over a range of distances 

and depths and is difficult to portray at a national 

scale. Likewise, contamination from nonpoint sources 

at the land surface, such as pesticide use for crops or 

onsite wastewater-disposal systems for residential 

areas, usually is relatively local when considering 

the Nation and varies considerably with depth even 

at the local scale.

National- and watershed-scale information is 

available on some constituents through State and 

Federal programs such as the USGS National WaterQuality Assessment (NAWQA) Program (see Box B). 

Because of their variable spatial distribution, the 

information available at a national and watershed scale 

for many constituents consists largely of statistical 

information about their concentrations, frequency of 

occurrence, and associations with variables such as 

land use, chemical application rates, and other causative factors. Information from the NAWQA Program 

is available on a study-unit basis and as national 

overviews. National overviews include selected findings from 1991 to 2001 (Hamilton and others, 2004), 

pesticides (Gilliom and others, 2006), and volatile 

organic compounds (Zogorski and others, 2006).

Figure 21. Ground-water arsenic samples collected in 1973–2001 (from Ryker, 2001, accessed 

on July 17, 2006, at http://water.usgs.gov/nawqa/trace/pubs/geo_v46n11/fig1.html).
B

Regional-Scale Approach to National Assessment 29

Regional-Scale Approach to 

National Assessment

Thus far, the discussion has centered on the 

availability and limitations of national-scale information about the ground-water resources of the United 

States. As noted, the primary issues affecting groundwater availability vary from location to location and 

commonly require analysis in the context of groundwater flow systems to achieve a meaningful perspective. 

With this principle in mind, the remainder of this report 

focuses on how regional evaluations of the Nation’s 

principal aquifers could form the foundation for a 

national assessment of ground-water availability. 

The information obtained on ground-water systems 

achieved through these regional studies, complemented

“We do not have an adequate picture of 

water availability at national, regional, 

and local levels.”

From “Science and Technology to Support Fresh 

Water Availability in the United States,” National 

Science and Technology Council (2004)

by additional ground-water information available 

from other USGS programs, other Federal agencies, 

States, Tribes, and local governments could form the 

basis for developing a more complete picture of the 

Nation’s ground-water availability. Regional aquifer 

studies currently underway in the USGS GroundWater Resources Program (Dennehy, 2005) form the 

initial stages of this proposed effort.

The USGS implemented the National WaterQuality Assessment (NAWQA) Program in 1991 to 

develop long-term consistent and comparable information on streams, rivers, ground water, and aquatic 

systems in support of national, regional, State, and 

local information needs and decisions related to 

water-quality management and policy. The NAWQA 

Program is designed to determine the condition of 

our Nation’s streams, rivers, and ground water, and 

the changes in these conditions over time.

The program collects and interprets data about 

surface- and ground-water chemistry, hydrology, 

land use, stream habitat, and aquatic life in the 

United States by using a nationally consistent study 

design and uniform methods of sampling analysis. 

From 1991 to 2001, the NAWQA Program conducted 

interdisciplinary assessments and established a 

baseline understanding of water-quality conditions

in 51 of the Nation’s river basins and aquifers 

(Hamilton and others, 2004). 

The U.S. Geological Survey National Water-Quality 

Assessment (NAWQA) Program

NAWQA activities during the second decade 

(2001–2012) of the program are focusing in large 

part on national and regional assessments, all of 

which build on continued monitoring and assessments of the study areas. Selected major activities during the second decade include continuing 

national syntheses of information on pesticides, 

volatile organic compounds, nutrients, selected 

trace elements, and aquatic ecology; studies on 

five national priority topics, including the fate of 

agricultural chemicals, effects of urbanization on 

stream ecosystems, bioaccumulation of mercury 

in stream ecosystems, effects of nutrient enrichment on aquatic ecosystems, and transport of 

contaminants to public-supply wells; and regional 

assessments of water quality and trends in major 

river basins and principal aquifers. Additional 

information on the NAWQA Program is available 

online at http://water.usgs.gov/nawqa/ and in 

Gilliom and others (2001).
30 Ground-Water Availability in the United States

Regional Ground-Water Budgets

Regional ground-water budgets are key to 

understanding the sources of water to a groundwater system and how water withdrawals change the 

components of flows in the hydrologic cycle. A water 

budget quantitatively accounts for inflows, outflows, 

and changes in storage of a hydrologic system. A 

regional ground-water budget is based on the conservation of mass within a defined regional ground-water 

flow system. That is, the amount of water entering 

the ground-water system plus the amount being 

removed from storage must balance the amount of 

water leaving the system over the time scale of interest. For further discussion of water budgets, see Healy 

and others (2007). In the case studies described in this 

report, volumetric fluxes, estimated as the average 

rates of change in cubic feet per second, are used to 

quantify the flow of water for the specified systems.

Each ground-water system in the United States is 

unique in terms of climate, hydrogeologic framework, 

and boundary conditions (both type and location), and 

each system responds differently to stress. Thus, the

sources of water (that is, the location and magnitudes of 

changes in inflows, outflows, and storage) that supply 

withdrawals from major aquifer systems in the United 

States are highly variable. This variability is shown 

in figure 22 by the results from model simulations of 

the USGS Regional Aquifer-System Analysis (RASA) 

Program. The Floridan and Edwards–Trinity aquifer 

systems, which equilibrate rapidly after pumping, were 

simulated as steady state with no long-term change in 

storage. In contrast, the Southern High Plains, with 

most natural discharge occurring far from pumping 

wells, and the deeply buried Great Plains aquifer 

system have had significant changes in ground-water 

storage. The distinction between changes in recharge 

and changes in discharge shown in figure 22 is, in 

part, a function of how the system is defined (that is, 

a gain to one system may result in a loss from an 

adjoining system). For example, ground-water withdrawals from confined aquifers (such as the Northern 

Atlantic and Gulf Coastal Plains) can cause flow to 

be diverted from shallow aquifers into the deeper 

regional flow regime (more recharge from the perspective of the confined aquifer system) that otherwise 

would discharge to streams in the outcrop areas (less 

discharge from the perspective of the entire groundwater system). Ground-water recharge in a region 

also can increase as a result of human modifications, 

such as return flow of excess irrigation water, which 

has occurred in the California Central Valley. Further 

examples of the diversity of responses to ground-water 

development are given in the case studies that follow.

Selection of Regional Ground-Water 

Flow Systems

Many different regional ground-water flow systems

can be defined for the United States. One approach 

for selection of regional ground-water flow systems 

to be studied focuses attention on the primary areas of 

ground-water use nationwide. This approach identifies 

a set of principal aquifers primarily on the basis of 

water use. This set of regional ground-water flow 

systems could be supplemented by additional aquifers

in hydrogeologic terrains that may be underrepresented 

by the principal aquifers described and that are 

important current or future sources of ground water.

Figure 23 shows the set of 30 principal aquifers 

that collectively account for about 94 percent of the 

Nation’s total ground-water withdrawals for public 

supply, irrigation, and self-supplied industrial uses 

combined. The principal aquifers shown in 

figure 23 include both individual principal aquifers 

and combinations of principal aquifers (where these 

form connected hydrologic systems) identified in the 

Ground Water Atlas of the United States (Miller, 2000). 

Figure 23 was derived by ranking the principal aquifers 

using the water-use data from Maupin and Barber 

(2005). Table 1 lists the estimated water withdrawals 

for 2000 (the last year these estimates were available) 

for the 30 principal aquifers shown in figure 23. 

The top ranked aquifers by water use are dominated 

by irrigation withdrawals. In addition to covering a 

substantial part of the Nation’s ground-water use, the 

set of 30 principal aquifers covers a large spatial area 

of the Nation, includes a variety of hydrogeologic 

terrains, and includes aquifers being used to provide 

substantial amounts of public water supply.

Each of these 30 principal aquifers has been 

studied previously and to varying degrees of comprehensiveness. The Ground Water Atlas of the United 

States (Miller, 2000) describes all of the systems. 

Many were studied as part of the RASA Program from 

1978 to 1994 (see Box C). Table 2 lists the 30 principal 

aquifers, the years they were studied as part of the 

RASA Program, if applicable, and more recent studies.
Figure 22. Sources of water that supply withdrawals from selected major aquifer systems in the 

United States based on model simulations for various periods through the 1970s and 1980s. The 

results illustrate the variety of ways in which overall ground-water budgets can change in 

response to large-scale pumping. The simulations of the aquifer systems were for different stages 

in the development of each system, and in many cases, are not representative of current (2007) 

conditions given the dynamic nature of ground-water systems (from Alley and others, 2002).

California

 Central Valley

Eastern Snake

River Plain

Great Plains Northern Midwest

Northern Atlantic

 Coastal Plain

Gulf Coastal Plain

Edwards–Trinity

Southern

 High Plains

Southeastern

 Coastal Plain

Floridan

EXPLANATION

Increased recharge

Decreased discharge

Decreased storage

Induced recharge

Infiltration of 

 irrigation water

Figure 22. The sources of water that supply withdrawals from selected major aquifer systems in 

the United States based on model simulations for various periods through the 1970s and 1980s. The 

results illustrate the variety of ways in which overall ground-water budgets can change in response 

to large-scale pumping. The simulations of the aquifer systems were for different stages in the 

development of each system and, in many cases, are not representative of current (2007) conditions 

given the dynamic nature of ground-water systems (modified from Alley and others, 2002).

Regional-Scale Approach to National Assessment 31

The principal aquifers shown in figure 23 were 

defined on the basis of geologic and hydrologic 

characteristics. Some of the principal aquifers, such 

as the California Central Valley aquifer system, define 

a regional ground-water flow system. Other principal 

aquifers do not define a single distinct regional flow 

system but provide a good starting point for defining 

regional aquifer systems to be analyzed for national 

assessment. In these cases, the studies undertaken 

would include combinations of parts of multiple 

principal aquifers that define a regional flow system. 

For example, the alluvial aquifers, which make up a 

principal aquifer distributed throughout the United 

States, would be part of many of the regional aquifer 

systems evaluated. Examples of how regional aquifer 

systems can be defined from the principal aquifers are 

provided in several case studies described in the section on “Examples of Regional Aquifer Assessments.”
Columbia Plateau basaltic-rock

 and basin-fill aquifers

Basin and Range basin-fill and

 carbonate-rock aquifers

Biscayne aquifer

California Coastal Basin aquifers

Cambrian–Ordovician aquifer system

Central Valley aquifer system

Coastal lowlands aquifer system

Northern Rocky Mountains

 intermontane basins aquifer system

Lower Cretaceous aquifers 

 (IA, KS, MN, and NE)

Edwards–Trinity aquifer system

Floridan aquifer system

High Plains aquifer

Mississippi River Valley alluvial aquifer

Mississippi embayment aquifer system

Mississippian aquifers

Northern Atlantic Coastal Plain 

 aquifer system

Glacial sand and gravel aquifers

Volcanic-rock aquifers (Hawaii)

Snake River Plain basaltic-rock and 

 basin-fill aquifers

Pacific Northwest basaltic-rock

 and basin-fill aquifers

Puget Sound aquifer system

Rio Grande aquifer system

Roswell Basin aquifer system

Silurian–Devonian aquifers

Southeastern Coastal Plain aquifer system

Surficial aquifer system

Texas coastal uplands aquifer system

Valley and Ridge carbonate-rock 

 and other aquifers

Willamette Lowland basin-fill aquifers

Intermediate aquifer system (Florida)

EXPLANATION

Thirty of the principal aquifers

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

26

27

29

2

3 1

4

5

6

7

8 9

10

11

12

13

14

14

14

15

16

17

18

19 20

21

22

23 24 25

26

27

28

28

29

30

30

32 Ground-Water Availability in the United States

Figure 23. Thirty principal aquifers that collectively account for about 94 percent of the Nation’s total 

ground-water withdrawals for public supply, irrigation, and self-supplied industrial uses combined.
Regional-Scale Approach to National Assessment 33

Table 1. The 30 regional principal aquifers with the greatest amount of ground-water use. Water-use estimates from Maupin and 

Barber (2005). 

Regional principal aquifer

Principal aquifer 

number in 

figure 2

Total water use Irrigation Public supply Self-supplied 

industrial

Million gallons per day

High Plains aquifer 11 17,488 17,000 389 99

Central Valley aquifer system 10 9,808 8,910 839 59

Mississippi River Valley alluvial aquifer 13 9,290 9,150 70 70

Basin and Range basin-fill and 

carbonate-rock aquifers

1 and 42 5,695 4,550 1,080 65

Glacial sand and gravel aquifers shaded area 4,075 1,170 2,273 632

Floridan aquifer system 50 3,645 1,930 1,330 385

California Coastal Basin aquifers 3 3,446 1,760 1,580 106

Snake River Plain basaltic-rock and 

basin-fill aquifers

6 and 59 3,075 2,900 151 24

Coastal lowlands aquifer system 18 2,368 933 1,010 425

Pacific Northwest basaltic-rock and 

basin-fill aquifers

4 and 58 1,340 1,206 121 13

Rio Grande aquifer system 2 1,119 867 240 12

Columbia Plateau basaltic-rock and 

basin-fill aquifers

5 and 60 1,077 810 223 44

Northern Atlantic Coastal Plain 

aquifer system

22 1,035 70 793 172

Mississippi embayment aquifer system 20 946 195 576 175

Cambrian–Ordovician aquifer system 33 933 92 590 251

Southeastern Coastal Plain 

aquifer system

21 860 382 340 138

Biscayne aquifer 51 812 114 698 0

Edwards–Trinity aquifer system 38 740 282 411 47

Surficial aquifer system 

(southeastern United States)

15 650 364 263 23

Volcanic-rock aquifers (Hawaii) 61 429 171 243 15

Willamette Lowland basin-fill aquifers 8 420 245 99 76

Roswell Basin aquifer system 43 386 364 21 1

Texas coastal uplands aquifer system 19 381 188 148 45

Northern Rocky Mountains intermontane basins aquifer system9 377 264 78 35

Valley and Ridge carbonate-rock and 

other aquifers

39 363 7 226 130

Intermediate aquifer system (Florida) not shown 354 292 61 1

Lower Cretaceous aquifers 

(Iowa, Kansas, Minnesota, 

and Nebraska only)

25 317 259 53 5

Mississippian aquifers 40 285 6 211 68

Puget Sound aquifer system 7 260 45 192 23

Silurian–Devonian aquifers 47 246 27 164 55
34 Ground-Water Availability in the United States

Table 2. Previous investigations of the 30 regional principal aquifers with the greatest amount of ground-water use.—Continued 

[Acronyms used in the table are: RASA, Regional Aquifer-System Analysis Program; NAWQA, National Water-Quality Assessment Program; and

GWRP, Ground-Water Resources Program]

Regional principal 

aquifer

Dates of RASA study 

that included area Recent regional investigations Selected references for previous regional studies

High Plains aquifer 1978–86 Selected areas have had hydrologic investigations. A 

water-quality investigation under the NAWQA program 

took place from 1999 to 2006.

McMahon and others, 2007; 

Luckey and Becker, 1999; 

Weeks and others, 1988

Central Valley 

aquifer system

1978–82 A followup geochemical investigation (RASA Phase 2) 

was completed in 1990. A GWRP study started in 2004 

and has recently been completed.

Bertoldi and others, 1991; Claudia C.

Faunt, U.S. Geological Survey, 

written commun., 2007

Mississippi River 

Valley alluvial 

aquifer

1980–91 Followup quantitative studies have been conducted 

in Arkansas.

Ackerman, 1996; Czarnecki, 

Clark, and Reed, 2003; 

Czarnecki, Clark, and Stanton, 

2003; Grubb, 1998; Reed, 2003; 

Stanton and Clark, 2003

Basin and Range 

basin-fill and 

carbonate-rock 

aquifers

1978–1990

(Arizona);

1980–88

(Nevada and Utah)

Selected areas have been studied since the RASA Program 

including Death Valley Regional Flow System, 

Antelope Valley–Western Mojave, Mojave River 

Ground-Water Basin, Joshua Tree Ground-Water 

Subbasin–Mojave Desert, Northern Utah Valley, 

and Upper San Pedro Basin, Arizona.

Anderson, 1995; Belcher, 2004; 

D’Agnese and others, 2002; 

Harrill and Prudic, 1998; 

Leighton and Phillips, 2003; 

Nishikawa and others, 2005; 

Pool and Dickinson, 2007; Stamos 

and others, 2001; Thiros, 2006

Glacial sand and 

gravel aquifers

1981–89 The RASA study covered the Northeast Glacial aquifer system. 

The remainder of the glacial system has not had a systematic 

flow analysis but is the subject of a current water-quality 

investigation under the NAWQA program. Selected areas 

that were not covered by the RASA study have been studied, 

including Colville Basin, Washington; Spokane Valley–

Rathdrum Prairie, Washington–Idaho, and Methow Basin 

Washington. A current study of water availability in the 

Great Lakes basin will include the glacial aquifer system.

Ely and Kahle, 2004; Grannemann 

and Reeves, 2005; Hsieh and 

others, 2007; Konrad and 

others, 2005; Randall, 2001; 

Warner and Arnold, 2005

Floridan 

aquifer system

1978–86 Several recent regional investigations have been carried out 

in Georgia and Florida since the RASA study, however, 

they have not covered the entire aquifer.

Johnston and Bush, 1988; 

Payne and others, 2005, 2006; 

Provost and others, 2006; 

Sepulveda, 2002

California Coastal 

Basin aquifers

1989–94 A RASA study included intensive study of the Santa Clara–

Calleguas Basin. Since then, substantial work has been 

done on other coastal basins including Pajaro, Santa Barbara, Los Angeles, San Bernardino, and Santa Clara Valley.

Danskin and others, 2006; Freckleton and others, 1998; Hanson, 

2003; Hanson and others, 2003; 

Hanson and others, 2004; 

Reichard and others, 2003

Snake River Plain 

basaltic-rock and 

basin-fill aquifers

1979–90 Lindholm, 1996

Coastal lowlands 

aquifer system

1980–91 A followup study, including a subsidence model, was 

conducted for the northern part of the aquifer system 

in Texas.

Grubb, 1998; Kasmarek and 

Robinson, 2004; Martin and 

Whiteman, 1999

Pacific Northwest 

basaltic-rock and 

basin-fill aquifers

Studies of the Upper Deschutes Basin and the 

Portland Basin have been undertaken.

Gannett and Lite, 2004; 

Morgan and McFarland, 1996

Rio Grande 

aquifer system

1978–90 A comprehensive study of ground water in the Middle Rio 

Grande Basin, which is the basin that includes Albuquerque, New Mexico, was undertaken from 1995 to 2000.

Bartolino and Cole, 2002; 

Wilkins, 1998

Columbia Plateau 

basaltic-rock and 

basin-fill aquifers

1982–89 Vaccaro, 1999

Northern Atlantic 

Coastal Plain 

aquifer system

1979-87 Since the RASA study, selected areas have had hydrologic 

investigations. The GWRP study of the North Carolina–South 

Carolina area has recently been completed in 2007. In New 

Jersey, the RASA model has been updated and numerous 

smaller studies have been conducted. A study of the Coastal 

Plain Aquifer in the Mid-Atlantic area is in progress.

Ator and others, 2005; Bruce G. 

Campbell, U.S. Geological

Survey, written commun., 2007; 

Shedlock and Bolton, 2006; 

Trapp and Meisler, 1992; 

Voronin, 2004
Regional-Scale Approach to National Assessment 35

Table 2. Previous investigations of the 30 regional principal aquifers with the greatest amount of ground-water use.—Continued 

[Acronyms used in the table are: RASA, Regional Aquifer-System Analysis Program; NAWQA, National Water-Quality Assessment Program; and

GWRP, Ground-Water Resources Program]

Regional principal 

aquifer

Dates of RASA study 

that included area Recent regional investigations Selected references for previous regional studies

Mississippi 

embayment 

aquifer system

1980–91 Followup studies and models of the Sparta aquifer 

have been conducted in southeast Arkansas and 

north-central Louisiana.

Arthur and Taylor, 1998; Grubb, 

1998; McKee and Clark, 2003;

McKee and others, 2004

Cambrian–

Ordovician 

aquifer system

1978–84 Since the RASA study, selected areas have been studied. 

In Wisconsin, a 10-county regional ground-water 

assessment was conducted. The current study of water 

availability in the Great Lakes basin will include the 

Cambrian–Ordovician aquifers.

Feinstein and others, 2005; 

Grannemann and Reeves, 2005; 

Young, 1992

Southeastern 

Coastal Plain 

aquifer system

1979–88 Since the RASA study, selected areas have had hydrologic 

investigations. The GWRP study of the North Carolina–

South Carolina area was recently completed in 2007.

Bruce G. Campbell, 

U.S. Geological Survey, written 

commun., 2007; Miller, 1992

Biscayne aquifer A numerical model was completed in 2001. Langevin, 2001

Edwards–Trinity 

aquifer system

1986–94 Since the RASA study, significant work has been undertaken 

studying the Edwards aquifer and a three-dimensional 

ground-water model was completed in 2004. 

Lindgren and others, 2004

Surficial aquifer 

system (southeastern United States)

Volcanic-rock 

aquifers (Hawaii)

1982–88 The RASA study was of the island of Oahu. A simulation of 

the aquifer in the Pearl Harbor area was completed in 2005. 

Studies on other islands have been done since the RASA 

Program, including Molokai; Kona, Hawaii; Hawi aquifer, 

Hawaii; and Southern Lihue Basin, Kauai.

Izuka and Gingerich, 1998; 

Izuka and Oki, 2002; Nichols 

and others, 1997; Oki, 1997, 

1999, 2002, 2005, 2006; 

Underwood and others, 1995

Willamette Lowland 

basin-fill aquifers

1989–94 There has been a recent geologic and hydrogeologic 

framework study.

Conlon and others, 2005; 

Gannett and Caldwell, 1998

Roswell Basin 

aquifer system

Texas coastal uplands 

aquifer system

1980–91 Grubb, 1998

Northern Rocky 

Mountains intermontane basins 

aquifer system

A RASA study was planned and begun in 1992 but not 

completed because of budget cuts.

Clark and Kendy, 1992

Valley and Ridge 

carbonate-rock and 

other aquifers

1988–93 The RASA study of the Appalachian Piedmont, Valley and

Ridge, and Blue Ridge Provinces included simulation of the 

Cumberland Valley in Pennsylvania (PA). Several localscale studies have been conducted in PA, Maryland, Tennesssee, West Virginia (WV), and Virginia (VA). A study 

of the Shennandoah Valley in VA and WV is in progress.

Chichester, 1996; Haugh, 2002; 

Swain and others, 2004

Intermediate aquifer 

system (Florida)

A study was recently published in 2006. Knochenmus, 2006

Lower Cretaceous 

aquifers (IA, KS, 

MN and NE only)

State-wide studies have been done on different parts of 

the system (IA, Iowa; KS, Kansas; MN, Minnesota; 

NE, Nebraska).

Burkart 1984; Woodward 

and Anderson, 1986

Mississippian 

aquifers

Puget Sound 

aquifer system

1989–94 The hydrogeologic framework has been studied and there 

has been a model analysis of Thurston County.

Drost and others, 1999; 

Vaccaro and others, 1998

Silurian–Devonian 

aquifers

1988–91 The RASA study of the Midwestern Basins and arches glacial 

and carbonate regional aquifer system included part of the 

Silurian–Devonian aquifers. Two studies of the Silurian–

Devonian aquifer in a part of southeastern Michigan and 

in part of Iowa were recently completed. A current study 

of water availability in the Great Lakes basin will include 

the Silurian–Devonian aquifers.

Bugliosi, 1999; Grannemann 

and Reeves, 2005; 

Reeves and others, 2004; 

Tucci and McKay, 2006
C

36 Ground-Water Availability in the United States

The Regional Aquifer-System Analysis 

(RASA) Program began in response to the 

1977 drought and recommendations by the 

U.S. National Water Commission and the 

U.S. Comptroller General. From 1978 to 1995, 

25 of the Nation’s most important ground-water 

systems were evaluated as part of the RASA 

Program (fig. C1). Computer models were used 

to develop estimates of current and future water 

availability for many of these systems. In addition, the Ground-Water Atlas of the United States 

(Miller, 2000; http://capp.water.usgs.gov/gwa/) 

was compiled as a general source of information on ground-water resources.

The RASA Program provided a baseline of 

knowledge on the aquifer systems studied that 

will prove useful for many decades to come. 

However, as valuable as the RASA Program was, 

it leaves some important needs unfulfilled. The 

program did not examine many of the shallower 

or less productive aquifers that are very important to rural and small community water users 

and in sustaining flow in streams. The RASA 

Program also was static, describing the aquifers 

at a point in time. Virtually all of the information 

used in the RASA ground-water studies is now 

more than 15 to 20 years old. Ground water is 

dynamic, and aquifers need to be re-examined 

over time as conditions and issues change.

The U.S. Geological Survey Regional Aquifer-System 

Analysis (RASA) Program

16

6 8

11

22

7

13

19

21

4

10

2

20

5

10

18

14

3

9

25

17

23

1

15

24

12

Figure C1. The 25 regional aquifers studied under the Regional Aquifer-System 

Analysis (RASA) Program from 1978 to 1995 (from U.S. Geological Survey, 1998).

EXPLANATION

Northern Great Plains

High Plains

Central Valley, California

Northern Midwest

Southwest alluvial basins

Floridan

Northern Atlantic Coastal Plain

Southeastern Coastal Plain

Snake River Plain

Central Midwest

Gulf Coastal Plain

Great Basin

Northeast glacial aquifers

Upper Colorado River Basin

Oahu, Hawaii

Caribbean Islands

Columbia Plateau

San Juan Basin

Michigan Basin

Edwards–Trinity

Midwestern basins and arches

Appalachian valleys and Piedmont

Puget–Willamette Lowland

Southern California alluvial basins

Northern Rocky Mountain 

 Intermontane Basins

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

RASA study areas

Figure C1. The 25 regional aquifers studied under the Regional Aquifer-System 

Analysis (RASA) Program from 1978 to 1995 (from U.S. Geological Survey, 1998).
Regional-Scale Approach to National Assessment 37

Regional Studies

The ground-water assessment undertaken for 

each regional ground-water flow system emphasizes 

the integrated use of monitoring data, ground-water 

modeling, and other existing information to place 

the status of ground-water resources in the context 

of the complete water budget for that aquifer system 

(see Box D). This assessment is structured to provide 

a national perspective on the Nation’s ground-water 

resources, while simultaneously emphasizing the 

value of the regional assessments to those who 

manage and use the resources regionally and locally. 

The previous RASA assessments are mostly more 

than 20 years old and the new proposed assessments 

would build on the information developed under the 

RASA Program where appropriate but take advantage 

of new information and techniques to assess the 

ground-water resources of the Nation.

As a first step in the investigations, existing 

information would be compiled and synthesized 

on ground-water availability for the entire regional 

ground-water flow system. This information includes 

what is known about changes in ground-water levels, 

storage, recharge, and discharge and identification of 

major regional ground-water availability issues. The 

second step would involve regional ground-water 

modeling to estimate historic changes in water budgets 

and provide a tool for estimating system response 

to future stresses. Depending on the situation, the 

entire regional aquifer system or selected parts of 

the regional aquifer system would be modeled. For 

example, in some cases, a regional aquifer system may 

consist of many individual basins complicating meaningful simulation of the entire aquifer system. In other 

cases, information is most needed on particular parts 

of the aquifer system. If previous ground-water flow 

models exist for the area or part of the area, they could 

be used as a starting point to develop more up-to-date 

and accurate models. More focused studies would be 

used to understand processes and effects (for example, 

stream-aquifer interactions) that are important 

regionally but occur at finer scales. Regional modeling 

studies would be emphasized to the extent practicable, 

so that the results from all regional aquifer studies 

could be assembled to provide a national assessment. 

Development and testing of new approaches to 

regional assessment would be an important part of the 

studies to advance the approaches taken over time. 

As noted in Box D, the completion of model 

analysis of aquifer systems provides an opportune 

time to summarize insights on the value of the existing 

monitoring networks and possible gaps in coverage. 

Thus, the regional modeling studies would provide 

feedback to others on existing monitoring networks. 

Overall, products from the regional studies 

would include:

1. Water budgets of major aquifer systems;

2. Current estimates and historic trends in groundwater use, storage, recharge, and discharge;

3. Ground-water models that provide a regional 

context for more local studies and a tool for

others to make future projections of groundwater availability;

4. Regionwide estimates of key hydrologic 

variables (for example, aquifer properties 

and recharge) for major aquifers; 

5. An evaluation of the existing networks for 

monitoring ground-water availability; and 

6. Testing and evaluation of new approaches 

for analysis of regional aquifers. 

These products would provide the foundational 

information and modeling tools to help State and local 

agencies make water-availability decisions based on 

local water-management constraints and goals. 

The regional studies would build on a foundation of previous and ongoing ground-water studies. 

Additionally, whenever possible, future groundwater availability studies would be scheduled to take 

advantage of other coincident efforts for the purpose of 

leveraging resources to the maximum extent possible. 

Each regional study would require 3 to 6 years for 

completion, depending on complexity. Given the 

current funding level of the USGS Ground-Water 

Resources Program, a national assessment would 

require 2 to 3 decades, although interim information 

would likely be released periodically to summarize 

the state of knowledge at that time. 

Water availability involves both the quantity and 

quality of the resource. Interpretation of groundwater quality is best achieved with an understanding 

of ground-water flow systems. Thus, various ways 

would be explored to link studies of the NAWQA 

Program with the hydrologic studies of the regional 

aquifer systems. Likewise, improved understanding 
D

38 Ground-Water Availability in the United States

A ground-water model attempts to reproduce, or 

simulate, the operation of a ground-water system with a 

mathematical counterpart. Models are commonly used 

to evaluate changes to the water budget of an aquifer 

resulting from land-use changes, water withdrawals, 

and climate, and how these changes affect streamflow, 

lake levels, water quality, and other important variables. 

An important role of ground-water modeling is to 

place current conditions defined by monitoring data in 

the context of the slow changes that may be taking place 

in the hydrologic system. Many aquifer systems have 

undergone several decades of development and may be 

far from equilibrium. Data on current conditions may not 

indicate, for example, how future streamflow depletion 

will evolve from the pumping that has already occurred, 

but this can be estimated by the use of models. 

Monitoring and computer modeling are complementary activities, but too often are treated separately, 

ignoring important linkages and feedbacks. An idealized 

framework for integration of monitoring and modeling 

in the context of ground-water assessment is illustrated 

in figure D1. In this figure, the top row signifies a longterm network that is systematically monitored over 

time. The second row represents the development of 

models and their periodic updates to advance understanding of how the aquifer system responds to human 

development, integrate new information from scientific 

studies, and address new questions as they may arise. 

Monitoring data serve as primary information for calibration of computer models. Conversely the process of model 

calibration and use provides insights into the adequacy of 

and gaps in monitoring data. This is shown by the arrows 

representing long-term monitoring as input to modeling 

and a feedback loop to evaluate long-term monitoring networks on the basis of modeling. Unfortunately, the second 

step, evaluation of monitoring networks at the conclusion 

of a modeling study too rarely occurs. 

Figure D1 explicitly recognizes that every simulation 

model is built upon an underlying conceptual model of how 

the ground-water system works. More often than not, data 

will fit more than one conceptual model, and good calibration of a model does not ensure a correct conceptual model 

(Bredehoeft, 2003). The appropriateness of the conceptual 

model is tested as a ground-water model is built, and field 

observations are compared to the model simulations. The 

conceptual model of a system should be evaluated periodically and updated as an important part of updating models. 

The final row in figure D1 signifies that periodic studies in addition to long-term monitoring networks should 

be integrated into each stage of model development. For 

example, information about water sources and the age of 

the water (time since recharge) obtained from environmental tracers can be compared to ground-water ages 

and flow paths inferred from modeling. Likewise, geologic 

and geophysical studies may provide new insights into the 

hydrogeologic framework.

Relation of Ground-Water Modeling to Assessment and Monitoring

Simulation

Model

Conceptual

Model

Environmental Tracer Data

Geologic and Hydrologic Studies

Simulation

Model

Conceptual

Model

Environmental Tracer Data

Geologic and Hydrologic Studies

Time (not to scale)

Past Present Future

Long-Term Monitoring Networks (water levels, water quality, streamflow, and other components)

Simulation

Model

Conceptual

Model

Environmental Tracer Data

Geologic and Hydrologic Studies

Simulation

Model

Conceptual

Model

Environmental Tracer Data

Geologic and Hydrologic Studies

Figure D1. A framework for integration of monitoring and modeling in the 

context of ground-water assessment (modified from Alley, 2006).
, USGS Richard R. Luckey

Center-Pivot Irrigation on the High Plains

The Bernalillo Riverside Drain, 

New Mexico

James R. Bartolino, USGS

Examples of Regional Aquifer Assessments 39

of geologic frameworks provided by the USGS 

National Cooperative Geologic Mapping Program 

and other geologic mapping by the State Geological 

Surveys would be important input to the regional-scale

models developed as part of the national assessment. 

Complementary data collection and sharing of information among the USGS and State and local agencies 

would enhance the ability of the regional studies to be 

relevant and useful to those managing the resources. 

Examples of Regional Aquifer Assessments

Five case studies (fig. 24) are used to illustrate the 

diversity of water-availability issues, the approaches 

to regional assessment, and the resultant information 

produced by these studies. All but one of the case 

studies include overall water budgets for the aquifer system under predevelopment and development conditions. 

The first case study summarizes a comprehensive 

multiagency evaluation of the Middle Rio Grande Basin 

undertaken in the late 1990s. The next two case studies

(California Central Valley and Coastal Plain) are 

recently completed regional aquifer assessments by the 

USGS Ground-Water Resources Program. The fourth 

case study features an ongoing study in the Great Lakes 

Basin, with a focus on the Lake Michigan ground-water 

basin. Finally, the High Plains aquifer, although lacking 

a recent ground-water model for the complete system, 

is used to illustrate the value of a long-term waterlevel monitoring program and the relation of water 

quality to water availability at the regional scale. 

Figure 24. Five case 

studies that document the 

assessment of regional 

ground-water systems.

Great Lakes

Basin

Coastal

Plain

Regional aquifer

 assessments

California

Central

Valley

EXPLANATION

High Plains

Middle Rio

Grande Basin

Lake Michigan from Manistique, Michigan

An orchard 

 in California’s 

Central Valley

Figure 24. Five case studies that document the 

assessment of regional ground-water systems.

Rose M. McGowan, USGS

Thomas A. Abrahamsen, USGS

May River in Beaufort County, South Carolina

Claudia C. Faunt, USGS
Arroyo Chico

Rio Puerco

Rio Puerco

Jemez River

Arroyo de las

Calabacillas

Salado

Rio

Rio Salado

Jemez Canyon

Reservoir

Galisteo

Res

Tijeras Arroyo

San Jose 

Rio

Cochiti Lake

Arroyo

Tonque

River Fe

 Arroyo Abo

Creek

Galisteo

Creek

Hells Canyon Wash

RIO

GRANDE 

RIO 

GRANDE 

Las

tas er Hu

San

Ysidro Cochiti

Santo Domingo

San Felipe

Algodones

Bernalillo

Rio

Rancho

Albuquerque Isleta

Los Lunas

Belen

Bernardo

San Acacia

McKINLEY

CIBOLA

VALENCIA

SOCORRO

TORRANCE

SANTA FE

BERNALILLO

SANDOVAL

Santa

Rio Grande

 inner valley

Middle

 Rio Grande

 Basin

0 10 20 MILES

0 10 20 KILOMETERS

Figure 25. Major physiographic and hydrologic features of 

the Middle Rio Grande Basin, New Mexico (from Bartolino 

and others, 2002).

35°30'

35°15'

35°

34°45'

34°30'

107°15' 107° 106°45' 106°30' 106°15' Sierra Nacimiento

Jemez

Mountains

Cochiti 

Dam

Manzano Mountains

Manzanita

Mountains

Los Pinos

Mountains

Ladron

Peak

Sierra Lucero

Mesa

Lucero

San Juan Basin

Sandia Mountains

Espanola Basin ~

NEW MEXICO

Area of 

enlarged 

map

Figure 25. Major physiographic and hydrologic 

features of the Middle Rio Grande Basin, New Mexico 

(modified from Bartolino and others, 2002).

40 Ground-Water Availability in the United States

Middle Rio Grande Basin

Introduction

The Middle Rio Grande Basin is an area of about 

3,060 mi2 in central New Mexico within the Rio 

Grande valley extending from about Cochiti Lake 

downstream to about San Acacia (fig. 25). The Middle 

Rio Grande Basin is part of the principal aquifer 

referred to as the “Rio Grande aquifer system” in 

figure 23. The Middle Rio Grande Basin covers about 

10 percent of this principal aquifer and includes an 

area of considerable ground-water use by the City 

of Albuquerque and surrounding communities. The 

climate over most of the basin is semiarid, with mean 

annual precipitation ranging from 7.6 inches at Belen to 

12.7 inches at Cochiti Dam (Bartolino and Cole, 2002). 

Most water-bearing units of the Middle Rio 

Grande Basin are unconsolidated deposits of the 

Tertiary-age Santa Fe Group. The Tertiary-age Santa 

Fe Group deposits are basin-fill deposits representing 

the transition from a topographically closed basin to 

the current through-flowing Rio Grande. Because the 

Santa Fe Group and post-Santa Fe Group deposits are 

hydraulically connected, they are commonly grouped 

together as the Santa Fe Group aquifer system (fig. 26). 

The thickness of the Santa Fe Group in the 

Middle Rio Grande Basin is highly variable because of 

complex faulting during sedimentation. Total thickness 

ranges from about 1,400 ft at basin margins to approximately 14,000 ft in localized areas in the center of the 

basin (Bartolino and others, 2002). Ground water is 

withdrawn mostly from the sands and gravels of the 

upper and middle parts of the aquifer; only about the 

upper 2,000 ft of the aquifer is used for ground-water 

withdrawal. The depth to water in the aquifer system 

varies widely from about 0 ft near the Rio Grande to 

as much as 1,180 ft in an area west of Albuquerque.

Water Availability Issues 

In 2000, the population of the Middle Rio Grande 

Basin was about 690,000 or about 38 percent of the 

population of New Mexico. Water for municipal and 

domestic supply is currently (2007) almost exclusively from ground water. The New Mexico Office 

of the State Engineer administers the appropriation 

and use of the water resources of New Mexico and 

has declared the basin a “critical basin;” that is, a 

ground-water basin faced with rapid economic and 

population growth where there is less than adequate 

technical information as to the available water supply 

(Bartolino and others, 2002). In addition, surface-water 

flows of the Rio Grande are considered fully appropriated by the New Mexico Office of the State Engineer, 

and an equivalent surface-water right must be obtained 

to offset any ground-water withdrawals that deplete the 

river (Bartolino and Cole, 2002, p. 69). In this semiarid 

environment, understanding the amount of water available for use and the interaction between surface water 

and ground water is critical to planning for the future.
Sandia Mountains

Rio Grande

Ziana horst

San Francisco fault

Sand Hill fault

WEST EAST

Calabacillas subbasin

EXPLANATION

Sante Fe Group aquifer system

Axial gravel

Eolian sand and silt

Piedmont-slope deposits

Pre-Santa Fe Group units

Contact

NOT TO SCALE

Fault

Blanket gravel, sand

Medium-coarse sand

Silt and fine sand

Medium sand

Fine-medium sand

Figure 26. Generalized west-east cross section of hydrogeologic deposits 

in the Middle Rio Grande Basin (modified from Bartolino and Cole, 2002).

Examples of Regional Aquifer Assessments 41

Approach to Assessment

The Middle Rio Grande Basin study was a 6-year 

effort by the USGS and other agencies to improve 

the understanding of the hydrology, geology, and 

land-surface characteristics of the Middle Rio Grande 

Basin (Bartolino and Cole, 2002). The approach 

undertaken was to develop an up-to-date understanding of the hydrogeology, ground-water quality, water 

use, mountain recharge, and hydraulic connection 

between the Rio Grande and the ground-water system. 

The conceptual geologic framework of the 

Middle Rio Grande Basin was revised and updated 

by mapping the surficial and bedrock deposits of the 

Middle Rio Grande Basin and adjoining areas. Several 

different geophysical methods were used to aid in 

developing the revised hydrogeologic framework. 

In particular, high-resolution aeromagnetic surveys 

delineated faults that offset water-bearing units in the 

aquifer system, which may play an important role 

in the ground-water flow system (fig. 27). Groundwater samples were extensively analyzed for environmental tracers and other chemical constituents in 

order to date ground water, to define zones of differing 

water quality, and to locate areas of recent recharge 

(Plummer and others, 2004). The interaction between 

the Rio Grande and the ground-water system was 

estimated by electromagnetic surveys and measurement of streamflow losses. Research on mountain 

front recharge indicated that the amount of water 

being recharged to the ground-water system is less 

than was previously estimated (Anderholm, 2000).

All of these improvements in the understanding 

of the ground-water system were incorporated in a 

quantitative ground-water flow model of the basin 

(McAda and Barroll, 2002). The model enables estimates to be made of the water budget of the groundwater system under predevelopment conditions and 

through time as human influences changed the system. 

Figure 26. Generalized west-east cross section of hydrogeologic deposits in the 

Middle Rio Grande Basin (from Bartolino and Cole, 2002).
Aeromagnetic anomaly map

0

–500

–900

500

1,000

1,350

NANOTESLAS

0 1 2 3 KILOMETERS

0 1 2 3 MILES

Modified from U.S. Geological Survey and Sander Geophysics, Ltd., 1998

Rio Grande

Rio Grande

Buildings

Water

tank

Bridge Pipeline

Pipeline

Hydrogeologic features

EXPLANATION

Igneous rock

Figure 27. Aeromagnetic anomaly map of an area south of Albuquerque and simplified map of important hydrogeologic features. 

Many geologic features and human-made structures can be seen on the anomaly map, which is displayed in color and shaded 

as though it were a relief map illuminated from the east. The most evident hydrogeologic features on the aeromagnetic map are 

faults and igneous rocks, which are depicted on the simplified map (from Bartolino and others, 2002).

Exposed

Estimated at 49-foot depth

Estimated at 490-foot depth

Interpreted fault trace

Figure 27. Aeromagnetic anomaly map of an area south of Albuquerque and simplified map of important 

hydrogeologic features. Many geologic features and human-made structures can be seen on the anomaly map, 

which is displayed in color and shaded as though it were a relief map illuminated from the east. The most evident 

hydrogeologic features on the aeromagnetic map are faults and igneous rocks, which are depicted on the 

simplified map (from Bartolino and others, 2002).

42 Ground-Water Availability in the United States

Selected Findings 

Before major human influences (predevelopment), 

the ground-water system in the Middle Rio Grande 

Basin was in an approximate state of dynamic equilibrium. All the water recharging the system naturally 

discharged from the system. As estimated by the 

ground-water flow model, predevelopment recharge 

to the aquifer from the Rio Grande, Jemez River, 

other rivers, mountain front recharge, and underflow 

from neighboring basins amounted to approximately 

200 cubic feet per second (ft3/s) (McAda and Barroll, 

2002; Douglas P. McAda, U.S. Geological Survey, 

written commun., 2007), and most of the water discharged as evapotranspiration (fig. 28). Once humans 

began development of the basin, canals were built for 

irrigation, wells were drilled for supply, drains were 

built to prevent water logging of soils, and water was 

removed from storage. These changes drastically altered 

the hydrologic budget in the ground-water system. 

Ground-water levels have declined in many parts 

of the Middle Rio Grande Basin; the water table 

has declined more than 160 ft since 1945 in some 

areas (Bartolino and Cole, 2002). Ground-water 

flow model estimates (McAda and Barroll, 2002; 

Douglas P. McAda, U.S. Geological Survey, written 

commun., 2007) for 1999 indicate that total inflows 

(including water from storage) and total outflows 

increased to about 560 ft3/s, with withdrawals from 

wells accounting for more than 200 ft3/s of the outflows. The flow rates presented in McAda and Barroll 

(2002) for the Rio Grande and Jemez River were 

net flow rates that combined inflows and outflows; 

additional information (Douglas P. McAda, U.S. 

Geological Survey, written commun., 2007) allowed 

for the separation of inflows and outflows as shown 

in figure 28. Recharge to the aquifer system is from 

the runoff from the mountain fronts, intermittent 

tributaries, underflow from bounding ground-water 

systems, canal seepage, the Rio Grande, and the 
A. Predevelopment

Recharge from Recharge from other

Rio Grande and rivers, mountains, 

Jemez River and boundaries

125 ft3/s 75 ft3/s

Evapotranspiration

180 ft3/s

B. Postdevelopment (1999)

Recharge from

Rio Grande,

Jemez River, and

riverside drains

225 ft3/s

Recharge from other rivers, 

mountains, canals, and boundaries

195 ft3/s

Irrigation and 

septic return flow

55 ft3/s

Discharge to Rio Grande,

Jemez River, 

and all drains

230 ft3/s

Withdrawals 210 ft3/s

Evapotranspiration

120 ft3/s

Discharge to Rio Grande 

and Jemez River 

20 ft3/s

Change in storage 85 ft3/s

Figure 28. Ground-water budgets before development and 

postdevelopment (annual rates during 1999) for the Middle 

Rio Grande Basin aquifer system: (A) predevelopment, and 

(B) postdevelopment (1999) (data from McAda and Barroll, 

2002; Douglas P. McAda, U.S. Geological Survey, written 

commun., 2007). [ft3

/s, cubic feet per second]

Figure 28. Ground-water budgets before development and 

postdevelopment (annual rates during 1999) for the Middle 

Rio Grande Basin aquifer system: (A.) predevelopment, and 

(B.) postdevelopment (1999) (data from McAda and Barroll, 2002; 

D.P. McAda, U.S. Geological Survey, written commun., 2007). 

[ft3/s, cubic feet per second]

The Bernalillo Riverside Drain, 

New Mexico. Photograph by 

James R. Bartolino, USGS.

Examples of Regional Aquifer Assessments 43

Jemez River. As determined from the model, recharge 

to the ground-water system from the Rio Grande and 

Jemez River was about 125 ft3

/s during predevelopment conditions and recharge from the Rio Grande, 

Jemez River, and the constructed riverside drains to 

the ground-water system became 225 ft3

/s in 1999, 

and discharge as evapotranspiration decreased from 

180 ft3

/s to about 120 ft3/s. Because of the scale of the 

model, the Rio Grande and the riverside drains were 

both represented in the same model cells. Thus, they 

tend to act as one combined component in the model 

(Douglas P. McAda, U.S. Geological Survey, written 

commun., 2007). Interior drains installed to prevent 

water logging as a result of irrigation became a major 

discharge mechanism of the system, accounting for 

about 185 ft3

/s of the 230 ft3/s of outflow to the Rio 

Grande, Jemez River, and all drains. The results of the 

study demonstrate that the ground-water system has 

changed substantially since predevelopment, and the 

Rio Grande is a major source of water to the groundwater system in response to development. The model 

can be used to estimate and understand the consequences of changes in water use on the ground-water 

system as well as on the exchange of flow between 

the ground-water system and the Rio Grande.
44 Ground-Water Availability in the United States

California Central Valley Aquifer System

Introduction

For more than 50 years, California’s Central 

Valley has been one of the most productive agricultural 

regions of the world, which is due in large part to an

ample supply of water for irrigation. On less than 

1 percent of the total farmland in the United States, 

the Central Valley supplies 8 percent of the national 

agricultural output (by value). In an area of about 

20,000 mi2, the Central Valley produces 250 different 

crops (Great Valley Center, 1998) with an estimated 

value of 17 billion dollars per year in 2002 (Great 

Valley Center, 2005) (fig. 29). This irrigated agriculture relies heavily on a combination of water supply 

from surface-water diversions and ground-water 

pumping (Bertoldi and others, 1991). Approximately 

one-sixth of the Nation’s irrigated land is in the Central 

Valley (Bureau of Reclamation, 1994), and about oneeighth of the Nation’s ground-water pumpage is from 

its aquifers (Maupin and Barber, 2005).

The Central Valley is a large structural trough 

filled with continental sediments that contains most 

of the Valley’s freshwater resources (Berkstresser, 

1973; Page, 1973). The average thickness of the 

continental sediments is about 2,400 ft. Sediments 

vary in thickness but generally thicken from south 

to north. Aquifer sediments contain mostly fluvial 

and interbedded lacustrine deposits. These continental deposits consist predominantly of lenses of 

gravel, sand, silt, and clay, with more than half of the 

total thickness composed of fine-grained sediments 

(Page, 1986). Most of these fine-grained lenses are not 

areally extensive; however, there is one major unit, the 

Corcoran Clay member, in the San Joaquin Valley that 

is worthy of mention because it can locally function 

as a confining unit (fig. 29). In the past 50 years, the 

drilling of numerous long-screened irrigation wells 

through the unit has minimized the influence of the 

Corcoran Clay on hydraulic heads above and below 

the clay. Therefore, the Central Valley aquifer can 

be thought of as a single heterogeneous aquifer in 

which the vertical movement of water is dependent 

on the properties of fine-grained sediments and local 

influences of high-capacity ground-water wells.

Climate in the Central Valley is arid to semiarid 

with precipitation being greater in the northern part 

of the valley (13 to 26 inches) than in the southern 

part (5 to 16 inches). In contrast to low precipitation 

in the valley, mean annual precipitation in high 

altitude mountains that nearly surround the valley can 

be three times greater than what falls on the valley 

floor. Streamflow is almost entirely dependent on 

precipitation in the Sierra Nevada and part of the 

Klamath Mountains to the north (fig. 29). Depth to 

water is highly variable but typically can be within 

100 ft of land surface and sometimes even deeper, 

particularly in the south. Prior to ground-water 

development, the direction of flow was toward the 

center of the valley, and discharge was primarily as 

evapotranspiration from wetlands or a small amount 

as seepage to streams. Precipitation was the principal 

source of recharge to the aquifer. These historical 

patterns of ground-water discharge and recharge 

have been significantly altered by agricultural water 

development. For additional information on the 

geologic framework and hydrologic setting of the 

Central Valley, see Bertoldi and others (1991).

Water Availability Issues

In addition to demands on the water system from 

agriculture, population growth is also placing demands 

on the water resources. Between 1990 and 2002, about 

4 percent of the Central Valley’s irrigated farmland was 

converted to other uses, primarily for housing and other 

urban uses (Great Valley Center, 2005). In the past 

20 years, population in the Central Valley has nearly 

doubled, and future growth is projected to continue. 

These large increases in population have intensified 

the competition for water within the Central Valley 

and statewide. Other water issues, such as conservation of agricultural lands, conjunctive use, artificial 

recharge, hydrologic implications of land-use change, 

and climate change, have added to the complexity 

of how to evaluate the individual and joint effects of 

these different factors on overall water availability. 
S

acramento River

Sutter

Buttes

Tulare

Basin

King's River

Carquinez

Straits

San Francisco Bay

Tulare

Lake bed

0 20 40 60 80

100 KILOMETERS

100 MILES

0 20 40 60 80

EXPLANATION

Line of section (figure 30)

Extent of San Joaquin Basin

 fluvial fans

Major streams and 

 selected canals

Precipitation station and name

Historic lake

Geomorphic province

Basin

Delta

Dissected uplands

Sacramento River flood plain

Sutter Buttes

Fan deposits

S

i

e

r

r

a

N

e

v

a d a

S

a

c

r

a

m

e

n

t

o

V

a

l

l

e

y

S

a

n

J o a q u i n

V a l l e y

C

o

a

s

t

R

a

n

g e

s

Cascade Range

Klamath Mountains

124° 123° 122° 121° 120° 119°

40°

39'

38°

37°

36°

35°

Tehachapi Mts

Pacific

Ocean

Corcoran Clay thickness

High : 200 feet

Low : 0

Red Bluff

Davis

Davis

Fresno

Bakersfield

Figure 29. Central Valley aquifer and major geomorphic provinces (modified from Davis and others, 1959; Olmstead and 

Davis, 1961; and Jennings, 1977), fluvial fans of the San Joaquin Basin (Weissmann and others, 2005), and extent and 

thickness of Corcoran Clay, California (modified from Page, 1986; Burow and others, 2004).

Pacific Ocean

San

Francisco

Central

Valley

C

A

L

I

F

O R N I A

San

Joaquin

River San Joaquin Basin

Figure 29. Central Valley aquifer and major geomorphic provinces (modified from Davis and others, 1959; 

Olmstead and Davis, 1961; and Jennings, 1977), fluvial fans of the San Joaquin Basin (Weissmann and others, 

2005), and extent and thickness of Corcoran Clay, California (modified from Page, 1986; Burow and others, 2004).

Examples of Regional Aquifer Assessments 45
46 Ground-Water Availability in the United States

Approach to Assessment

The Central Valley ground-water system has 

been studied to varying degrees since about the late 

1880s. Starting about the 1940s and continuing for 

the next four decades, the use of ground water for 

irrigation increased steadily to a point where ground 

water was providing about 50 percent of water use 

for irrigation (Williamson and others, 1989; Bertoldi 

and others, 1991). This growing dependence on 

ground water and decrease in the availability of 

imported surface water because of drought resulted in 

recognition of the need to begin to investigate groundwater resources across large areas of the valley. 

The information acquired made it possible to better 

define geologic features and begin to delineate the 

hydraulic properties of aquifer sediments. Quantitative 

assessments using ground-water flow models for 

parts of the Central Valley aquifer began in the 1970s. 

However, the first regionally comprehensive model of 

ground-water flow for the entire Central Valley was 

conducted by Williamson and others (1989), as part 

of the USGS RASA Program. The RASA Program 

provided a wealth of information on the geology, 

hydrology, and water chemistry of the Central Valley. 

The ground-water flow model by Williamson and 

others (1989) simulated conditions from 1961 to 

1977, a period of large and variable stresses on the 

ground-water system. The resulting model provides 

an overall representation for this large region but 

is generally inadequate at scales less than about 

500 mi2; a scale at which water-management 

decisions typically are made.

A variety of investigators from local organizations, 

State agencies, and private groups have continued to 

collect data, perform studies, and develop ground-water 

flow models of various parts of the Central Valley. It has 

been some 25 years since the original RASA groundwater flow model provided valuable insights to the 

regional ground-water system. Because of the dynamic 

character of the ground-water flow system, increased 

geologic and hydrologic knowledge, and the intense 

competition for available ground-water resources, the 

USGS re-examined the aquifer beginning in 2004. 

Selected Findings

An important step in the USGS’s most recent 

effort to assess the ground-water resources of the 

Central Valley was to better characterize the hydraulic properties of aquifer sediments. The hydrologic 

system in the Central Valley is complex, in part 

because of the heterogeneous nature of the hydrogeologic setting. Fine-grained deposits, including one 

mappable clay body—the Corcoran Clay—are spread 

throughout the valley fill. These valley deposits can be 

conceptualized as one large heterogeneous aquifer. 

A database of approximately 8,500 drillers’ logs 

was compiled to organize information on subsurface 

lithology in the Central Valley. Texture was used as 

a basis for constructing a three-dimensional spatial 

correlation model of the percentage of coarse-grained 

deposits in the Central Valley (fig. 30). This spatial 

correlation model correlated reasonably well to 

the geomorphic provinces shown in figure 29. The 

textural distribution likely is caused by differences 

in alluvial and fluvial depositional environments in 

concert with the distribution of the sediment source 

material. It was further assumed that the textural 

distribution correlates to hydraulic conductivity, 

enabling the development of a heterogeneous 

hydraulic conductivity distribution to represent the 

framework of the Central Valley ground-water system.

Water development for irrigation has had a 

pronounced effect on the hydrologic budget of the 

Central Valley. The development of surface-water 

and ground-water resources in support of agriculture 

has fundamentally altered the recharge and discharge 

components of the Valley’s water budget. The USGS 

Central Valley ground-water availability study 

developed a water budget for the period 1962–2003, 

which indicated that irrigation water supply was 

about evenly split between surface water and ground 

water with slightly more coming from surfacewater delivery (Claudia C. Faunt, U.S. Geological 

Survey, written commun., 2007). The relative annual 

contribution from either surface water or ground 

water, however, can vary substantially in response to 

climate conditions (wet or dry year). The large amount 
Percent coarse-grain material

0 to 10

10 to 20

20 to 30

30 to 40

40 to 50

50 to 60

60 to 70

70 to 80

80 to 90

90 to 100

Inactive in model

Approximate extent

of Tulare Lake bed

EXPLANATION

Model 

boundary

Corcoran

Clay

Line of section

shown in figure 29

Figure 30. Block diagram of texture model for the Central Valley aquifer 

(from Claudia Faunt, U.S. Geological Survey, written commun., 2007).

Figure 30. Block diagram of texture model for the Central Valley aquifer 

(from Claudia C. Faunt, U.S. Geological Survey, written commun., 2007).

Examples of Regional Aquifer Assessments 47
An orchard in California’s 

Central Valley. Photograph 

by Claudia C. Faunt, USGS.

48 Ground-Water Availability in the United States

of irrigation has altered the amount and distribution of 

recharge to the system and can change the water-table 

configuration and volume of surface-water discharge. 

The predevelopment water budget (fig. 31) indicates 

that prior to irrigation development, the amount of 

inflow to the system was equal to outflow (estimated 

at about 2,800 ft3

/s). For the postdevelopment 

period (1962–2003), average discharge increased 

to 15,500 ft3

/s and average recharge increased to 

13,500 ft3

/s. Overall, the postdevelopment average 

discharge and recharge for the aquifer system during 

this period were about five times greater than the 

predevelopment values. Postdevelopment average 

recharge was predominantly from irrigation return 

flow, and average discharge was overwhelmingly from 

ground-water pumpage. The average rate of decrease 

in aquifer storage including subsidence (compaction 

of fine-grained beds), for the period 1962 through 

2003 was estimated to be about 1,900 ft3

/s.

Agricultural development of the Central Valley 

water resources has resulted in large diversions and 

redistribution of surface water together with intensive 

pumpage from wells to meet various crop demands, 

especially during dry periods. Ground-water pumpage 

greatly exceeded natural recharge and resulted in 

large water-level declines. More pronounced in the 

San Joaquin Valley, but evident elsewhere in the 

Central Valley, these water-level declines resulted 

in subsidence. Land subsidence was significant and 

widespread, causing a variety of practical and economic 

problems such as damage to canals and drainage 

systems and loss of irrigation wells. The effects of 

unchecked water development and the possibility of 

prolonged drought caused water managers to increase 

surface-water imports and decrease ground-water 

pumpage (since the late 1970s). These actions have 

halted land subsidence in seriously affected areas, but 

the possibility of subsidence occurring again remains.

 The updated ground-water flow model was 

improved by an increase in its spatial (both laterally 

and vertically) and temporal resolution, thus enabling 

enhanced water-budget detail in the upper part of the 

system. This detail also allows for more accuracy 

in the estimates of the volume and distribution of 

subsidence. Perhaps most strikingly, the model now 

includes the routing of the surface water through 

the vast series of interconnected rivers and canals, 

allowing for better simulation of the interaction of 

ground water and surface water and more detailed 

water-budget accounting. This is accomplished by 

using the Farm Process developed for the USGS 

modular three-dimensional finite-difference groundwater flow model, MODFLOW-2000 (Schmid and 

others, 2006) linked with the Streamflow-Routing 

Package (Prudic and others, 2004). As an example, 

the overall water budget for the final year (2003) 

of the postdevelopment period comes at the end of 

5 years of below-average rainfall. Surface-water 

agricultural deliveries (13,000 ft3

/s) were more than 

double the ground-water agricultural deliveries 

(5,900 ft3

/s) and the resulting combined recharge and 

surface-water inflow (10,000 ft3

/s) was slightly greater 

than the combined total agricultural and municipal 

pumpage (8,000 ft3

/s) for the year 2003 (fig. 31).
Recharge from

precipitation

2,100 ft3/s

Surface-water 

inflow

700 ft3/s

Evapotranspiration

2,400 ft3/s

B. Postdevelopment (average from 1962 to 2003)

Recharge from

precipitation and

irrigation-return flow

9,900 ft3/s

Surface-water

inflow

3,600 ft3/s

Surface-water 

outflow

2,500 ft3/s

Withdrawals

13,000 ft3/s

Change in storage (including

subsidence) 1,900 ft3/s

Surface-water 

outflow

400 ft3/s

C. Postdevelopment (2003)

Recharge from

precipitation and 

irrigation-return flow

6,500 ft3/s

Surface-water

inflow

3,500 ft3/s

Surface-water 

outflow

2,400 ft3/s

Withdrawals

8,000 ft3/s

Farm process

Ground-water

agricultural deliveries

5,900 ft3/s

Precipitation

22,100 ft3/s

Evapotranspiration

32,000 ft3/s

Surface-water 

agricultural deliveries 

13,000 ft3/s

Runoff 2,500 ft3/s

A. Predevelopment

 

Change in storage (including

subsidence) 300 ft3/s

Figure 31. Change in Central Valley aquifer water budget due to development: (A.) predevelopment (data from Bertoldi and others, 1991), (B.)

postdevelopment (average from 1962 to 2003) (data from Claudia Faunt, U.S. 

Geological Survey, written commun., 2007), and (C.) postdevelopment (2003) 

(data from Claudia Faunt, U.S. Geological Survey, written commun., 2007). 

[ft3/s, cubic feet per second]

Figure 31. Change in Central Valley aquifer water budget due to development: 

(A) predevelopment (data from Bertoldi and others, 1991), (B) postdevelopment 

(average from 1962–2003) (data from Claudia C. Faunt, U.S. Geological Survey, 

written commun., 2007), and (C) postdevelopment (2003) (data from Claudia C. Faunt, 

U.S. Geological Survey, written commun., 2007). [ft3/s, cubic feet per second]

Examples of Regional Aquifer Assessments 49
0 100 200 MILES

0 100 200 KILOMETERS

Atlan tic Ocea n

WEST

VIRGINIA

VIRGINIA

OHIO

KENTUCKY

NORTH

CAROLINA

SOUTH

CAROLINA

MARYLAND

TENNESSEE

GEORGIA

FLORIDA

PENNSYLVANIA NEW

JERSEY

A

A‘ Fall Line

N COASTAL

P L A I N

Study area

Northern extent Coastal Plain 

 physiographic province

Line of section—See figure 33

EXPLANATION

A A‘

Figure 32. The areal extent of the Coastal Plain aquifer system 

study area within the Coastal Plain physiographic province 

and location of cross section (modified from B.G. Campbell, 

U.S. Geological Survey, written commun., 2007).

Figure 32. The areal extent of the Coastal Plain aquifer 

system study area within the Coastal Plain physiographic 

province and location of cross section (from Bruce G. 

Campbell, U.S. Geological Survey, written commun., 2007).

50 Ground-Water Availability in the United States

Coastal Plain Aquifer System

Introduction

The study area encompasses the entire Coastal 

Plain region of North Carolina and South Carolina and 

extends into parts of Virginia and Georgia (fig. 32). 

This region is part of the Atlantic and Gulf Coastal 

Plain physiographic province, which stretches from 

Long Island, New York, southward and westward 

into Texas. The portion of the aquifer system in North 

and South Carolina covers approximately 42,500 mi2 

and is a significant source 

of water used to meet the 

needs of a growing population. Most of this area is 

underlain by at least one, 

and in many cases several, 

highly productive aquifers. 

Water from these aquifers 

is used for diverse needs, 

including municipal water 

supply, as well as industrial 

and agricultural uses. Many 

places within this part of the 

aquifer system have intensively developed the groundwater resource, while in other 

areas the resource remains 

largely undeveloped. Some 

municipalities, where development of the aquifers in 

previous years created large 

areas of water-level decline 

in the aquifer potentiometric 

surfaces, have switched 

to surface-water sources, 

allowing the ground-water 

levels to partially recover. 

Saltwater encroachment is 

occurring in several places within the aquifer system 

and is directly related to the lowering of the potentiometric surfaces. Most of the North and South Carolina 

aquifers in the Coastal Plain contain freshwater (total 

dissolved solids less than 1,000 milligrams per liter), 

however, brackish water and saltwater exist in the 

aquifer system, especially near the coast.

The sediments that compose the Coastal Plain 

range in age from Cretaceous to Holocene (Miller, 

1992; Trapp and Horn, 1997). These sediments typically were deposited in shallow marine environments 

during a series of transgressions and regressions of 

the Atlantic Ocean. The aquifer system consists of 

a variety of sediment types from sand, silt, and clay 

to various types of consolidated carbonate rocks 

(fig. 33). The boundary to the east of the Coastal Plain 

is the Atlantic Ocean and the boundary to the west 

of the Coastal Plain is the Piedmont Province. This 

boundary separating the 

Coastal Plain deposits from the Piedmont 

soils is also called the 

Fall Line. Coastal Plain 

sediments thicken from 

the Fall Line toward 

the Atlantic Ocean and 

in many cases extend 

many miles offshore 

beneath the sea floor.

The climate of the 

study area is temperate 

and characterized by 

hot, humid summers 

and moderate winters. 

Annual precipitation in 

the area can range from 

approximately 35 to 

65 inches but averages 

about 50 inches across 

the two-State area. The 

water table commonly 

is near land surface, 

but a thick (as much 

as 200 ft) unsaturated 

zone exists in parts 

of the upper aquifers. 

The overall direction of ground-water flow is downdip toward the Atlantic Ocean. Locally, the surfacewater features can influence flow directions. Aquifers 

primarily are recharged by precipitation in the outcrop 

areas along the Fall Line. Discharge is primarily to 

nearby streams through the shallow flow system, to 

other aquifers by leakage, to the ocean, and to wells.
A A‘

0 50 100 MILES

0 50 100 KILOMETERS

VERTICAL SCALE GREATLY EXAGGERATED

Georgia South Carolina North Carolina

L1 Surficial

EXPLANATION

L2 Yorktown—Upper Floridan

L3 Yorktown

 Upper Floridan

L4 Castle Hayne–Pungo River

 Middle Floridan

L5 Castle Hayne–Pungo River

 Middle Floridan

L6 Beaufort–Gordon

L7 Beaufort–Gordon

L8 Peedee–Crouch Branch

L9 Peedee–Crouch Branch

L10 Black Creek–McQueen Branch

L11 Black Creek–

 McQueen Branch

L12 Upper Cape Fear–Charleston

L13 Upper Cape Fear–

 Charleston

L14 Lower Cape Fear–Grambling

L15 Lower Cape Fear–

 Grambling

L16 Lower Cretaceous

Figure 33. Generalized cross-section of hydrogeologic units through the central part of 

the Coastal Plain aquifer system study area (from B.G. Campbell, U.S. Geological Survey, 

written commun., 2007). Line of section shown in figure 32. 

Aquifer Confining unit

Model layer

L2

L4

L6

L8

L10

L12

L14

L2

L4

L6

L14

L12

L1–L16

Figure 33. Generalized cross section of hydrogeologic units through the central part 

of the Coastal Plain aquifer system study area (from Bruce G. Campbell, U.S. Geological 

Survey, written commun., 2007). Line of section shown in figure 32. 

Examples of Regional Aquifer Assessments 51

Water Availability Issues

Ground-water withdrawals from the aquifers 

along the coast in North Carolina and South Carolina have increased substantially over the past couple 

of decades in response to demands for water for a 

rapidly growing population. Both States have sought 

to increase their development of surface-water 

supplies to meet the needs of the coastal populations 

but also recognize that additional information 

is needed about the availability of ground-water 

resources. For example, the effects of ground-water 

withdrawals on the quantity of freshwater discharge to 

streams, estuaries, and wetlands are largely unknown. 

A complicating factor in some areas is the concern 

about saltwater intrusion. Adequate ground-water 

supplies and declining water levels in the study 

area aquifers of North Carolina and South Carolina 

became a problem in the late 20th century. Water-level 

declines have brought attention to the need to better 

manage withdrawals to maintain ground-water availability for the future. Large, regional areas of waterlevel decline currently (2007) are present in several 

areas and are as much as 100–200 ft deep. When these 

large water-level declines occur near the coast, saltwater intrusion into coastal aquifers is a possibility. 

Drought compounds the problem of declining water 

levels. In response to the recent drought of 1998–2002 

across the southeastern United States, near-surface 

ground-water levels in the aquifer system declined to 

some of the lowest levels on record.
52 Ground-Water Availability in the United States

Approach to Assessment

Separate independent numerical models were 

developed to simulate the ground-water flow in the 

North Carolina (Giese and others, 1997) and South 

Carolina (Aucott 1988; 1996) Coastal Plain aquifer 

system as part of the USGS RASA Program. The North 

Carolina RASA model simulated conditions from 1900 

to 1980, based on a hydrogeologic framework composed of 10 aquifers and associated confining units. 

The South Carolina RASA model simulated conditions 

from 1935 to 1982, based on a hydrogeologic framework composed of five aquifers and associated confining units. These inconsistent conceptualizations of the 

aquifer system and resultant ground-water flow models 

from adjoining States are now more than 25 years old. 

The need to address the study area’s ground-watersupply issues and identified differences in the conceptual models of the hydrogeologic framework led to the 

most recent assessment of ground-water availability 

that began in 2004. The current regional ground-water 

flow model took advantage of the large amount of 

new hydraulic, geologic, water-level, and water-use 

data available in conducting the hydrologic analysis. 

USGS scientists constructed a flow model using the 

updated hydrogeologic framework that was developed 

in conjunction with State agency partners. This regional 

aquifer assessment benefited from a variety of investigations conducted by local organizations, State 

agencies, and private groups. The updated regional 

model of the aquifer system was calibrated to groundwater levels and stream discharges for both steady-state 

(predevelopment) and transient (postdevelopment) 

conditions. An essential part of the modeling effort is 

collaboration with cooperators and stakeholders and 

the formation of several project liaison committees. An 

advantage of this close working relationship with local 

partners is the development of a regional management 

tool that can be used to address interstate water issues 

that involve several of the southeastern States.

Selected Findings

Water development for an expanding population 

has had a decided effect on the hydrologic budget of 

the Coastal Plain aquifer system. The simultaneous 

development of surface-water and ground-water 

resources to satisfy the needs of the growing 

population has altered the recharge and discharge 

components of the ground-water budget of the 

area. This alteration can be observed by examining 

the ground-water budget results from the recent 

May River in Beaufort County, South Carolina. Photograph by Thomas A. Abrahamsen, USGS.
Recharge from

precipitation

1,900 ft3/s

Leakage

from rivers

20 ft3/s

Net leakage and

lateral flow

300 ft3/s

B. Postdevelopment (annual average from 1999 to 2004)

Recharge from

precipitation

1,520 ft3/s

Net leakage 

and

lateral flow 

310 ft3/s

Leakage from rivers

30 ft3/s

Discharge 

to rivers

1,430 ft3/s

Withdrawals

830 ft3/s

Change in storage 400 ft3/s

Discharge 

to rivers

1,620 ft3 /s 

Figure 34. Change in Atlantic Coastal Plain aquifer system 

water budget due to development: (A.) predevelopment, and 

(B.) postdevelopment (annual average from 1999 to 2004)

(data from B.G. Campbell, U.S. Geological Survey, written 

commun., 2007). [ft3/s, cubic feet per second]

A. Predevelopment

Figure 34. Change in Atlantic Coastal Plain aquifer system 

water budget due to development: (A) predevelopment, and 

(B) postdevelopment (Annual average from 1999 to 2004). 

(data from Bruce G. Campbell, U.S. Geological Survey, 

written commun., 2007). [ft3/s, cubic feet per second] 

Examples of Regional Aquifer Assessments 53

modeling study. The water-budget analysis indicates 

that there is an increase in the capture of surface water 

in the upper aquifers with both an increase in leakage 

into the aquifer system and a decrease in discharge 

out of the aquifer system (Bruce G. Campbell, U.S. 

Geological Survey, written commun., 2007). 

The water budget for the aquifer system in 

the study area was estimated for predevelopment 

and postdevelopment conditions using the regional 

ground-water flow model. The results are shown 

in figure 34. Prior to ground-water development, 

the amount of inflow to the system (estimated at 

about 1,920 ft3/s) was equal to outflow from the 

system. For the simulated postdevelopment period 

of 1999 to 2004, average discharge (including 

pumpage) increased to 2,260 ft3/s, with recharge 

from precipitation (1,520 ft3/s) averaging less than 

predevelopment amounts because of drought conditions during the period. This combination has resulted 

in measured declines in water levels where pumping 

is most active and where additional surface water has 

not significantly augmented ground-water withdrawals 

in meeting the overall water demand. These changes 

in water levels are the result of an average rate of 

400 ft3/s of water being removed from storage over the 

1999 to 2004 postdevelopment period. Over the past 

couple of decades, the system has recovered in some 

areas because of the augmentation of ground-water 

pumpage with surface-water supplies. 

It is also important to recognize that the net 

leakage and lateral flow of the system changed 

direction between predevelopment and postdevelopment. In predevelopment, a net rate of 300 ft3

/s 

discharged to the overlying surficial aquifer, the 

ocean, and the lateral boundaries. For the 1999–2004 

period, however, an average net rate of 310 ft3/s 

recharged the aquifer system from the overlying 

surficial aquifer and lateral boundaries. The flows 

from “net leakage and lateral flow” represent the sum 

of leakage into the aquifer system from the surficial 

aquifer being represented as a source of water, leakage 

out of the system into the surficial aquifer, discharge 

into the surrounding saltwater bodies, and flow into 

and out of the modeled area from lateral boundaries. 

As the system underwent development and water 

levels declined, the flow from the surficial aquifer into 

the underlying aquifer system increased, and discharge 

from the aquifer system into the surficial aquifer 

and the surrounding saltwater bodies decreased; it is 

merely a coincidence that this net leakage is about the 

same amount but in different directions for the predevelopment and the postdevelopment conditions. 

Development of the water resources has resulted in 

localized declines in water levels caused by intensive 

pumpage from wells primarily to meet public supply 

demands, which is exacerbated during extended dry 

periods. When these localized areas of water-level 

decline occur near the coast, there can be an increased 

potential for the occurrence of saltwater encroachment. 

The effects of intense localized ground-water development and climate variability in the form of drought have 

caused water-resource managers to secure increased 

surface-water resources and decrease or make plans to 

decrease ground-water pumpage in selected areas.
Figure 35. The Great Lakes Basin in the United States and Canada.

LAKE

HURON

LAKE ERIE

LAKE ONTARIO

LAKE MICHIGAN

LAKE SUPERIOR

WISCONSIN

ILLINOIS

INDIANA OHIO

PENNSYLVANIA

NEW

YORK

ONTARIO

QUEBEC

MINNESOTA

IOWA

M C H

G

A

N

I I

90° 80°

42°

48°

Base from ESRI, 2001; U.S. Army Corps of Engineers, 1998; and Environment Canada, 1995

CANADA

U.S.A

A

A'

EXPLANATION

Boundary of Lake Michigan Basin 

ground-water flow model

A A' Line of section—See figure 36

Great Lakes Basin

KILOMETERS

0 100 200 MILES

0 100 200

Figure 35. The Great Lakes Basin in the United States and Canada.

54 Ground-Water Availability in the United States

Great Lakes Basin

Introduction

The Great Lakes Basin straddles the United 

States–Canadian border (fig. 35) and covers an area of 

approximately 296,000 mi2, including the surface area 

of the Great Lakes, which is about 30 percent of the 

total basin area. The Great Lakes watershed includes 

parts of eight States (Minnesota, Wisconsin, Illinois, 

Indiana, Michigan, Ohio, Pennsylvania, and New 

York) in the United States and part of the Canadian 

province of Ontario. About one-third of the basin land 

area lies within the United States.

The climate of the Great Lakes Basin is prone to 

extremes. In winter, cold arctic air moves across the 

open water of the lakes and absorbs moisture, which 

is released as snowfall on the leeward side of the lakes 

as the air mass cools when approaching land. In the 

summer, the area is dominated by warm, humid air 

from the Gulf of Mexico (Hodgkins and others, 2007). 

Mean annual precipitation ranges from less than 

27 inches in the west to more than 47 inches in the 

east. Areal recharge from precipitation is the primary

source of water to the ground-water system; with 

local recharge rates ranging from less than 1 inch to 

more than 20 inches per year (Neff and others, 2006).

Ground water discharges directly to the Great Lakes as 

seepage or indirectly as base flow in streams and rivers.

The amount of water flowing through the subsurface varies, depending on the characteristics of the 

water-bearing rocks and sediments. Unconsolidated 

materials that were deposited at or near land surface 

as a result of large-scale glacial advances and retreats 

make up the most productive aquifers. Most glacial 

deposits are composed of mixtures of sand and 

gravel, and silt and clay. Glacial deposits can be as 

much as 1,200 ft thick in parts of Michigan and are 

several hundred feet thick in buried bedrock valleys in 

Illinois, Wisconsin, and New York (Grannemann and 

others, 2000). Deposits can be thin or nonexistent in 

areas where bedrock is exposed at land surface due to 

removal of material by glaciers. 

Bedrock aquifers are generally widespread throughout the region and are more continuous than the aquifers 

in the glacial deposits (fig. 36). Some bedrock aquifers 

extend beyond the watershed boundaries. The extent and 
 VERTICAL SCALE GREATLY EXAGGERATED

 Datum is sea level

0 50 100 KILOMETERS

0 50 100 MILES

–10,000

–8,000

–6,000

–4,000

–2,000

0

2,000 Wisconsin Ohio

Lake Michigan

BEND INSECTION

EXPLANATION

Mississippian aquifers

Silurian–Devonian

Cambrian–Ordovician 

 aquifer system

Precambrian

Surficial aquifer system Confining unit

Pennsylvanian aquifers

A A‘

Figure 36. Generalized hydrostratigraphic section through the central part of the Great Lakes Basin 

(modified from Sheets and Simonson, 2006). Line of section shown in figure 35.

Figure 36. Generalized hydrostratigraphic section through the central part of the Great Lakes 

Basin, United States (from Sheets and Simonson, 2006). Line of section shown in figure 35.

Examples of Regional Aquifer Assessments 55

boundaries of the ground-water basin may be different 

than the surface expression of watersheds (ground-water 

divides and watershed boundaries may not coincide). 

Additionally, as the depth to these aquifers increases so 

does the likelihood that water quality will degrade and 

the less likely the aquifers will be used for water supply.

Water Availability Issues

Most large public water supplies in the basin are 

taken directly from the Great Lakes. Ground water has 

always been an important source of drinking water in 

rural areas; however, it is becoming an increasingly 

important source of drinking water (supplying about 

8.2 million people within the watershed) as the population spreads beyond the large municipalities into 

the suburbs (Grannemann and others, 2000). In the 

past, agriculture alone dominated rural water use, but 

competing demands have resulted in the conversion of 

some areas of agricultural water use to drinking water 

supplies. With continued population growth come 

additional demands for ground water to satisfy other 

uses such as manufacturing, power generation, and 

transportation. Ground-water availability is directly 

linked to the suitability of the ground-water quality 

for its intended use. From a ground-water perspective, 

major resource issues related to water availability are 

focused around finding adequate supplies, understanding the relation between surface water and ground 

water, the effect of changing water quality on water 

use, and ecosystem health in relation to quantity and 

quality of the water resource. The issue of diverting 

water from the Great Lakes remains a keen interest. 

Approach to Assessment

In 2005, the USGS began a pilot effort in the Great 

Lakes Basin to develop a potential nationwide program 

that would describe the status and trends in the availability and use of the Nation’s freshwater resources 

(Barlow and others, 2002; Grannemann and Reeves, 

2005). Prior to this most recent endeavor, the groundwater resources of the basin were comprehensively 

assessed in the early 1970s (Allen and Waller, 1975). 

The USGS studies of regional aquifer systems by the 

RASA Program during 1978–95 contributed greatly to 

the geologic and hydrologic knowledge of the groundwater resources of the area (Young, 1992; Westjohn 

and Weaver, 1998; Bugliosi, 1999; Randall, 2001). 

The Great Lakes Basin pilot study is building on this 

information together with other available information 

on surface-water flows and storage of water.
Recharge from

precipitation

22,400 ft3/s

Net lateral flow

300 ft3/s

Discharge to rivers,

lakes, and ponds

26,700 ft3/s

Discharge to

Lake Michigan

500 ft3/s

River leakage

4,500 ft3/s

Predevelopment

Figure 37. Predevelopment Lake Michigan Basin 

ground-water budget (data from H.W. Reeves, 

U.S. Geological Survey, written commun., 2007). 

[ft3/s, cubic feet per second]

Figure 37. Predevelopment Lake Michigan Basin 

ground-water budget (data from Howard W. Reeves, 

U.S. Geological Survey, written commun., 2007). 

[ft3/s, cubic feet per second]

56 Ground-Water Availability in the United States

Selected Findings

The Great Lakes Basin pilot study is at its 

halfway point (2007); therefore, a basinwide assessment of water availability is incomplete. However, 

several interim findings are available that improve 

understanding of the most important factors affecting 

the Great Lakes Basin hydrologic system now and 

for the future. Initially, to improve understanding of 

water resources in the Great Lakes, it is important to 

identify the role ground water plays in the hydrologic 

budget (Grannemann and others, 2000). In estimates 

of sources of water to the Great Lakes, direct groundwater discharge generally is assumed to be small. 

This perspective, however, does not account for the 

amount of ground water that flows indirectly into 

the Great Lakes as a component of streamflow. A 

recent study by Neff and others (2005) analyzed 

streamflow-monitoring stations across the basin 

and determined that the ground-water component of 

streamflow averages 66 percent of total streamflow 

for the basin and is a significant source of water to 

the Great Lakes.

The location of ground-water divides determines 

whether ground water flows toward or away from 

one of the Great Lakes (Sheets and Simonson, 2006). 

In the shallow ground-water system, ground-water 

divides typically coincide approximately with watershed divides. In deeper aquifers, however, this is 

not necessarily the case. Additionally, the location 

of ground-water divides can change if the system 

is stressed, whether naturally (drought) or through 

intense development of the ground-water system.

Consistent and accurate estimates are needed to 

understand how recharge might affect ground-water 

availability and use. Some recent progress has been 

made in estimating recharge on a basinwide scale 

for the Great Lakes Basin. Neff and others (2006) 

provide the first integrated study of long-term average 

ground-water recharge to the shallow aquifers in the 

United States and Canada within the Great Lakes 

Basin. Results are limited to long-term averages, 

broad spatial scales, and fluxes to the shallow (less 

than 100 ft) system. Additional studies of deep aquifer 

recharge and the temporal variability of recharge are 

needed to gain a better understanding of ground-water 

recharge in the Great Lakes Basin.

In the United States part of the Great Lakes 

Basin, four regional aquifer-system studies—Northern 

Midwest, Midwestern Basins and Arches, Northeast 

Glacial Aquifers, and the Michigan Basin—were 

conducted by the USGS as part of the RASA Program.

The Great Lakes Basin ground-water system (in the 

United States) is composed of three major aquifer 

systems: Cambrian–Ordovician aquifer system, 

the Silurian–Devonian aquifers, and the surficial 

aquifer system; and three minor aquifer systems: 

Pennsylvanian sandstone aquifer, the Pennsylvanian 

sandstone and carbonate-rock aquifer, and the 

Mississippian sandstone aquifer (U.S. Geological 

Survey, 2003). Recent examination of regional studies 

indicates that the regional ground-water divides for the 

Cambrian–Ordovician aquifer system and the Silurian–

Devonian aquifers have changed over time and differ 

from surface-water divides in some areas (Sheets and 

Simonson, 2006). These differences are a result of 

either pumping or structural geologic features and make 

the assessment of individual water-budget components 

more difficult and highly dependent on which areas are 

being evaluated. Currently (2007), the USGS is developing a ground-water flow model of the Lake Michigan 

ground-water basin (fig. 35). Preliminary results from 

the steady-state simulation for the water budget indicate 

that recharge from precipitation accounts for about 

90 percent of the inflow to the Lake Michigan Basin 

(Howard W. Reeves, U.S. Geological Survey, written 

commun., 2007) (fig. 37). The water budget depicted 

in figure 37 is for the predevelopment Lake Michigan 

Basin watershed; the net-lateral inflow component 

represents the flow in the ground-water system that 

crosses the topographic watershed boundary. 
Examples of Regional Aquifer Assessments 57

High Plains Aquifer

Introduction

The High Plains aquifer is one of the largest and 

most productive aquifers in the Nation. It underlies 

an area of about 111 million acres (174,000 mi2) in 

parts of eight western States (fig. 38). The area is 

a remnant of a vast plain formed by sediments that 

were deposited by streams flowing eastward from the 

ancestral Rocky Mountains. Mean annual precipitation ranges from 16 inches in the western part of the 

High Plains to about 28 inches in the east. Evaporation 

rates measured from free-water surfaces in the High 

Plains range from 60 inches in the north to 105 inches 

in the south (Gutentag and others, 1984). These rates 

are among the highest in the Nation and are attributed 

to high summer temperatures and persistent winds. 

Because evaporation rates are high relative to precipitation, there is little water available to recharge the 

aquifer, and in some areas, the time between recharge 

events may be years, decades (Luckey and Becker, 

1999), or even longer (McMahon and others, 2006).

Depending on location, the High Plains aquifer 

consists of one or more hydraulically connected 

geologic units. In most of the area, the Ogallala 

Formation of Miocene age and overlying hydraulically 

connected Quaternary deposits, if present, are the 

principal geologic units of the aquifer (fig. 39). The 

geologic units that form the High Plains aquifer generally were deposited by streams or wind and contain 

varying amounts of clay, silt, sand, and gravel. Bedrock units that underlie the High Plains aquifer are for 

the most part consolidated and are not as transmissive 

as the sediments that compose the High Plains aquifer. 

Gutentag and others (1984) provide an overview of the 

geologic history and setting of the High Plains.

The saturated thickness of the High Plains aquifer

ranges from about zero to more than 1,000 ft and 

averages about 200 ft. Depth to water ranges from 

near land surface to 500 ft and has an average depth 

of about 100 ft. The ground-water flow direction 

generally is from west to east and locally toward 

streams. Pumping from numerous irrigation wells across 

the High Plains is the primary mechanism for groundwater discharge. Precipitation is the principal natural 

source of recharge to the aquifer. Regional variability 

in water-level changes in the High Plains aquifer results 

from regional differences in climate, soils, land use, 

and ground-water withdrawals for irrigation. 

Water Availability Issues

The High Plains aquifer is the source of irrigation water that has transformed a part of the Great 

Plains into one of the major agricultural regions of the 

world. About 97 percent of the water pumped from 

the aquifer is used for irrigation, which accounts for 

about 30 percent of the ground water withdrawn for 

irrigation in the United States (Maupin and Barber, 

2005). Of the total crop production in the United 

States, the High Plains aquifer area accounts for about 

19 percent of the wheat, 19 percent of the cotton, 

15 percent of the corn, and 3 percent of the sorghum 

(U.S. Department of Agriculture, 1999). Irrigation 

first began in the southern High Plains in the late 

1880s, but it was not until after the drought in the 

1930s that it became clear that increased development 

of water for irrigation was necessary. The postwar 

economic boom of the 1950s, the development of 

irrigation technologies, and federally funded irrigation projects in the 1960s allowed farmers to work the 

land and become independent of the natural dry and 

wet weather cycles that had previously frustrated their 

efforts to cultivate the plains (Dennehy and others, 

2002). By 1992, irrigated crop land (13 million acres) 

represented about 12 percent of the High Plains land 

use (Qi and others, 2002). In addition to crops, the 

region accounts for nearly 18 percent of the total 

cattle production in the United States and is rapidly 

becoming a center for swine production. The aquifer 

also provides drinking water to most of the people 

who live within its boundaries. 

Successful development of the ground-water 

resources does not come without a cost. Substantial 

pumping of the High Plains aquifer for irrigation has 

resulted in water-level declines in some parts of the 

aquifer of more than 150 ft (McGuire, 2007). In 1984, 

concern about these declines led Congress to mandate 

a water-level monitoring program for the aquifer. 

Approach to Assessment 

To date, there have been three regionally extensive ground-water assessment studies of the High 

Plains aquifer (Johnson, 1901, 1902; Lohman, 

1953; Weeks and others, 1988). The latest and most 

comprehensive investigation to quantify the groundwater resources was conducted by Weeks and others (1988) as part of the RASA Program to evaluate 
Midland

Odessa

Lubbock

Amarillo

Guymon

Wichita

Garden

City

Colby

North

Platte

Wayne Scottsbluff

Cheyenne

TEXAS

COLORADO

KANSAS

NEW MEXICO

NEBRASKA

OKLAHOMA

WYOMING

SOUTH DAKOTA

Base information from U.S. Geological Survey digital data, 1:100,000

Albers Equal-Area projection

Standard Parallels 29°30' and 45°30', central meridian -96°

0 50 100 MILES

0 50 100 KILOMETERS

Loup River

Platte River

Republican River

Elkhorn River

N Platte River

S Platte River

Arkansas River

Cimarron River

Canadian River

108° 106° 104° 102° 100° 98°

32°

34°

36°

40°

42°

Northern

High Plains

Southern

High Plains

Central

High Plains

EXPLANATION

Elevation, in feet 

above sea level

8,000

1,000

Figure 38. The High Plains aquifer in the west-central United States. Figure 38. The High Plains aquifer in the west-central United States.

58 Ground-Water Availability in the United States
Alluvial systems

Quaternary deposits

Sand Hills eolian deposits

Plio-Pleistocene deposits

Ogallala Formation

Arikaree/Brule Formations

Aquifer not present

Hydrogeologic units

0 50 100 MILES

0 50 100 KILOMETERS

EXPLANATION

Figure 39. Hydrogeologic units of the High Plains aquifer 

(modified from Gutentag and others, 1984). 

TEXAS

COLORADO KANSAS

NEW MEXICO

NEBRASKA

OKLAHOMA

WYOMING

SOUTH DAKOTA

IOWA

N

Figure 39. Hydrogeologic units of the High Plains aquifer 

(modified from Gutentag and others, 1984). 

Examples of Regional Aquifer Assessments 59

the historical and future effects of ground-water 

development on the High Plains aquifer. The High 

Plains RASA assembled extensive amounts of information used to define the aquifer’s hydrogeologic 

framework, various water-quality characteristics, 

and hydrologic budget and stresses. These data were 

used to perform a quantitative analysis using the first 

computer model of the entire aquifer (Luckey and 

others 1986; Luckey and others, 1988). The computer 

model enabled investigators to examine the system 

response to a variety of ground-water development 

strategies. Intensive ground-water withdrawals 

continue to support the agricultural lifestyle that has 

existed in the area since the early 1900s. There have 

been no system-wide updates to the computer models 

that were constructed more than 20 years ago.

To better predict the future availability of ground 

water, it is necessary to improve our understanding 

of a variety of topics, including ground-water quality, 

because ground-water quality has a direct effect on 

how water can be used. The first regionally comprehensive ground-water-quality assessment of the High 

Plains aquifer began in 1998 as part of the USGS 

NAWQA Program (Dennehy, 2000; McMahon and 

others, 2007). The purpose of this study was to determine the current ground-water quality conditions of 

the High Plains aquifer and to increase the scientific 

understanding of the factors that affect ground-water 

quality. The quantity and quality of ground water in 

the High Plains aquifer need to be considered together 

when assessing ground-water availability.

Selected Findings

Ground-water withdrawals, especially during 

the past 50 years, have resulted in significant waterlevel declines that have adversely affected the 

long-term sustainability of the High Plains aquifer. 

Water-level changes in the High Plains aquifer from 

predevelopment to 2005 are shown in figure 40. 

About 24 percent of the area or about 27 million 

acres had more than 10 ft of water-level decline 

from predevelopment to 2002 (McGuire, 2007). 

Large areas with more than 50 ft of decline predominate in the southern High Plains and extend 

north into southwest Kansas. Additional information on historical water levels is available at the 

High Plains Water-Level Monitoring Study web site 

(http://ne.water.usgs.gov/ogw/hpwlms/). During the 

past half century or so, water in storage in the High 

Plains aquifer declined about 200 million acre-feet 

(equivalent to 65 trillion gallons) with 62 percent of 

the total ground-water volume loss occurring in Texas. 

Although the rate at which water levels are declining 

has slowed, the downward trend continues in many 

areas across the High Plains (McGuire, 2007).

The water used for irrigation is not coincident 

with the largest reservoir of water in storage. Groundwater withdrawals initially began in the southern 

High Plains and moved northward. The largest drawdowns in the High Plains aquifer are contained within 

the southern High Plains. Therefore, it is misleading to 

look at a water budget for the entire High Plains when 

the southern half of the aquifer has undergone greater 

withdrawals from a smaller reservoir of available 
TEXAS

COLORADO

KANSAS

NEW MEXICO

NEBRASKA

OKLAHOMA

WYOMING

SOUTH DAKOTA

IOWA

0 50 100 MILES

0 50 100 KILOMETERS

N

Water-level change, in feet

EXPLANATION

DECLINES

More than 150

NO SUBSTANTIAL CHANGE

Area of little or no

saturated thickness

RISES

More than 50

25 to 50

+10 to –10

10 to 25

10 to 25

25 to 50

50 to 100

100 to 150

Figure 40. Water-level changes in the High Plains aquifer, predevelopment to 2005 (modified from McGuire, 2007). Figure 40. Water-level changes in the High Plains aquifer, predevelopment to 2005 

(modified from McGuire, 2007).

60 Ground-Water Availability in the United States

ground water. Reduction in storage primarily due to 

ground-water development for irrigation is responsible 

for about a 7-percent reduction of the original total 

water volume when considering the entire High Plains 

aquifer. If we compare water budgets developed 

for just the south-central High Plains aquifer for 

predevelopment and for 1997, however, an 18-percent 

decline of water in storage is computed for that area 
Recharge

310 ft3/s

Discharge

to drains

68 ft3/s

B. Postdevelopment (end of 1997)

Recharge from 

precipitation and

irrigation-return flow

820 ft3/s

Discharge to drains

60 ft3/s

Withdrawals

3,690 ft3/s

Change in storage 3,080 ft3/s

Discharge

to streams

242 ft3/s

Discharge to streams

150 ft3/s

A. Predevelopment

Figure 41. Ground-water budget in the south-central 

High Plains aquifer study area: (A.) predevelopment and 

(B.) at the end of 1997 (from Luckey and Becker, 1999). 

[ft3/s, cubic feet per second]

600

500

400

300

200

100

0

0 3,000 6,000 9,000 12,000 15,000

 Cimarron (Central)

 Lincoln County #4 (Northern)

 Exponential age distribution

 EXPLANATION

 Base of aquifer 

 Well cluster (region of High Plains aquifer)

 SCREEN DEPTH BELOW WATER TABLE, IN FEET

Recharge rate = 

0.02 feet per year

Recharge rate = 

0.05 feet per year

A. Fewer than 3 high-capacity 

 wells in 3-mile radius

B. More than 10 high-capacity 

 wells in 3-mile radius

0 3,000 6,000 9,000 12,000 15,000

 Castro (Southern)

 Hugoton (Central)

APPARENT GROUND-WATER AGE, IN RADIOCARBON YEARS

Figure 42. Apparent ground-water age and depth below the 

water table in the High Plains aquifer: (A) flow system relatively 

undisturbed by pumping and (B) flow system highly perturbed 

by pumping (McMahon and others, 2007).

Examples of Regional Aquifer Assessments 61

(Luckey and Becker, 1999). As shown in figure 41, 

more than 80 percent of the ground-water withdrawals 

at the end of 1997 in the south-central High Plains 

aquifer comes from ground-water storage. 

Water in the High Plains aquifer generally is 

of high quality and is suitable for most agricultural, 

domestic, and industrial uses without treatment. The 

intensive agricultural land use and large-scale changes 

in the hydrologic regime, however, potentially affect 

the availability of the ground-water resource through 

changes in water quality. Transport of human and 

agricultural chemicals (nitrogen and pesticides) in 

ground-water recharge from land surface to the water 

table is perhaps the most obvious way in which 

ground-water quality has been degraded, but other 

factors should be considered. For example, infiltration 

of water from multiple irrigation applications can 

increase the concentration of dissolved solids through 

evaporation. Likewise, conversion of natural rangeland to agricultural lands can adversely affect shallow 

ground-water quality by changing local recharge and 

flushing salts that have accumulated at the surface 

Figure 41. Ground-water budget in the south-central 

High Plains aquifer study area: (A) predevelopment 

and (B) at the end of 1997 (data from Luckey and 

Becker, 1999). [ft3/s, cubic feet per second]

Figure 42. Apparent ground-water age and depth 

below the water table in the High Plains aquifer: 

(A) flow system relatively undisturbed by pumping 

and (B) flow system highly perturbed by pumping 

(McMahon and others, 2007).

over long periods of time to underlying aquifers 

(Scanlon and others, 2005; McMahon and others, 

2006). Intensive pumping and resulting declines in 

water levels can enhance the upward movement of 

mineralized water from below by increasing upward 

hydraulic gradients into the aquifer. The combination 

of degraded water from above and mineralized ground 

water from below can limit the unaffected water to 

a layer in the center of the aquifer that has not been 

affected by human activities. A confounding situation 

can exist, however, if the density of high-capacity 

pumping wells is sufficient enough to mix the water 

in the aquifer and thus begin producing water the 

quality of which is a blend of the conditions across 

the saturated thickness. This situation is illustrated 

using ground-water ages in figure 42. Ground-water 

ages typically increase exponentially with depth, 

which can result in a relatively thin layer of modern 

recharge overlying a much thicker zone of old 

recharge; whereas, in the second graph of figure 42 

ground-water ages do not change with depth because 

of vertical mixing caused by pumping.
62 Ground-Water Availability in the United States

Future Directions

 Ground-water systems change in response 

to development and changes in climate, and need 

to be monitored and evaluated on a regular basis 

to quantify the amount of water available for use 

and the ramifications of using the resource. Each 

regional ground-water system is unique in terms of 

climate, hydrogeologic framework, and boundary 

conditions (both type and location), and each system 

responds differently to stresses from human development and climate. For example, the five case studies 

presented here illustrate how water budgets and individual budget components differ geographically and 

demonstrate how human intervention has influenced 

water movement in several major aquifer systems. 

“Continue to conduct research and provide information—at a scale that is useful 

to states and local entities—about such 

matters as the safe, or sustainable, yield 

of aquifers (and methods for determining 

that yield); water-use data; and delineating 

boundaries and water budgets of threedimensional watersheds, including scientifically based and cost-effective methods 

of quantifying interactions between ground 

water and surface water.”

Recommended actions to USGS and State Geological 

Surveys from “Ground Water Report to the Nation,” 

Ground Water Protection Council (2007)

An approach for broad-scale assessment of the 

Nation’s ground-water resources is proposed that is 

adaptable over time and provides quantitative regional 

analyses of major areas of ground-water use. The 

proposed program builds on the results of a long history

of partnerships among the USGS, other Federal agencies, 

States, Tribes, and local governments to collect groundwater data and undertake investigative studies of groundwater systems. The proposed program would provide 

current estimates and historic trends in ground-water use, 

storage, recharge, and discharge (water-budget analysis); 

computer models of regional ground-water systems;

regionwide estimates of hydraulic properties for major 

aquifers; evaluation of existing networks for monitoring 

ground-water availability; and testing and evaluating 

new approaches for analysis of regional aquifers.

The proposed program is designed to allow both 

“scaling up” to a national synthesis and “scaling down” 

to provide information relevant to issues of more local 

concern. With respect to scaling up, the regional studies 

undertaken by the program together with additional 

information from the USGS and other agencies could be 

synthesized periodically to provide to the extent possible 

a comprehensive up-to-date picture of the Nation’s 

ground-water resources. Such information on aquifer 

conditions and long-term factors affecting groundwater resources would help address broad-based 

societal issues related to present day availability and 

long-term sustainability of the ground-water resources 

that are essential to our Nation’s water supply, agricultural and industrial production, and ecosystem health. 

With respect to scaling down, it is recognized that 

ground-water management decisions in the United 

States are made by States, municipalities, and special 

districts formed for ground-water management. Thus, 

information and models provided at the regional scale 

would be designed to provide a regional framework 

for more detailed studies and models by those who 

make management decisions at the local level. In 

addition, the regional studies would be partnered, 

where possible, with interested agencies and organizations to enhance their relevance to local concerns.

Acknowledgments

The assistance of Eve L. Kuniansky, Devin L. 

Galloway, Dennis W. Risser, and Patrick Tucci of the 

USGS in assembling table 2 is gratefully acknowledged. The ground-water-level decline data from 

the USGS National Water Information System were 

compiled with the assistance of George A. Karavitis. 

The reviews by Mark T. Anderson, James R. Bartolino, 

James W. LaBaugh, and Keith R. Prince improved the 

technical content of the entire report. Review of individual case studies and provision of additional unpublished information by Bruce G. Campbell, Claudia C. 

Faunt, Douglas P. McAda, Peter B. McMahon, and 

Howard W. Reeves of the USGS improved the content and accuracy of the summaries. The editorial 

assistance of Kay E. Hedrick and Sandra C. Cooper 

is also gratefully acknowledged. Sharon L. Qi provided the Geographical Information System support. 

And lastly, the final report preparation and layout by 

Caryl J. Wipperfurth is especially appreciated. 
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