by William M. Alley

 Thomas E. Reilly

 O. Lehn Franke

 

U.S. Geological Survey Circular 1186

Sustainability of

Ground-Water

Resources

Denver, Colorado

1999
U.S. DEPARTMENT OF THE INTERIOR

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY

Charles G. Groat, Director

The use of firm, trade, and brand names in this report is for identification purposes only and

does not constitute endorsement by the U.S. Government

Library of Congress Cataloging-in-Publications Data

Alley, William M.

Sustainability of ground-water resources / by William M. Alley, Thomas E. Reilly, and

O. Lehn Franke.

p. cm -- (U.S. Geological Survey circular : 1186)

Includes bibliographical references.

1. Groundwater--United States. 2. Water resources development--United States. I.

Reilly, Thomas E. II. Franke, O. Lehn. III. Title. IV. Series.

GB1015 .A66 1999

333.91'040973--dc21 99–040088

ISBN 0–607–93040–3

U.S. GOVERNMENT PRINTING OFFICE : 1999

Free on application to the

U.S. Geological Survey

Branch of Information Services

Box 25286

Denver, CO 80225-0286
III

FOREWORD

 oday, many concerns about the Nation’s ground-water resources involve questions about their 

future sustainability. The sustainability of ground-water resources is a function of many factors, including 

depletion of ground-water storage, reductions in streamflow, potential loss of wetland and riparian 

ecosystems, land subsidence, saltwater intrusion, and changes in ground-water quality. Each groundwater system and development situation is unique and requires an analysis adjusted to the nature of the 

existing water issues. The purpose of this Circular is to illustrate the hydrologic, geologic, and ecological 

concepts that must be considered to assure the wise and sustainable use of our precious ground-water 

resources. The report is written for a wide audience of persons interested or involved in the protection 

and sustainable use of the Nation’s water resources.

T

Charles G. Groat, Director

U.S. Geological Survey
IV

CONTENTS

Foreword III

Introduction 1

General Facts and Concepts about Ground Water 6

Ground-Water Development, Sustainability, and Water Budgets 15

Ground-Water Budgets 17

Hypothetical Examples of How Ground-Water Systems Change in Response to Pumping 22

Field Examples of How Ground-Water Systems Change in Response to Pumping 24

Long Island, New York 24

High Plains Aquifer 26

Effects of Ground-Water Development on Ground-Water Flow to and from Surface-Water Bodies 30

Streams 30

Lakes 40

Wetlands 42

Springs 43

Coastal Environments 44

Effects of Ground-Water Development on Ground-Water Storage 45

Storage Changes 45

Subsidence 55

Water-Quality Factors Affecting Ground-Water Sustainability 59

Land-Surface/Water-Table Connection 60

Ground-Water/Surface-Water Connection 62

Saltwater Intrusion 64

Meeting the Challenges of Ground-Water Sustainability 67

The Importance of Ground-Water Data 68

Use of Ground-Water Computer Models 70

Strategies for Sustainability 72

Concluding Remarks 76

Acknowledgments 77

References 78
V

BOXES

Box A -- Confined and unconfined aquifers respond differently to pumping 12

Box B -- Droughts, climate change, and ground-water sustainability 20

Box C -- Ground-water/surface-water interactions and water-resources sustainability:

Examples from the Northwestern United States 36

Box D -- High Plains aquifer: Egg carton or bathtub 48

Box E -- The connection between surface-water quality and ground-water quality 

in a karst aquifer 63

Box F -- Refinement of ground-water system understanding through time: 

Lessons from post audits 71

Box G--Examples of innovative approaches that contribute to ground-water sustainability 74
Under natural conditions, water levels in wells completed in many 

confined aquifers rise above the land surface, resulting in artesian 

flow. The well shown in the photograph was drilled near Franklin, 

Virginia, in 1941 to a depth of about 600 feet in confined aquifers. 

The initial water level in the well was about 7 feet above land surface. 

The above-ground structure shown in the photograph (Cederstrom, 

1945) was built in the 1940’s as a creative solution to measure the 

water level in this well. Measurements at the well were discontinued 

in 1960. Today, measurements at other nearby wells indicate that 

water levels are now 150 to more than 200 feet below land surface.
1

Ground water is one of the Nation’s most 

important natural resources. It provides about 

40 percent of the Nation’s public water supply. In 

addition, more than 40 million people, including 

most of the rural population, supply their own 

drinking water from domestic wells. As a result, 

ground water is an important source of drinking 

water in every State (Figure 1). Ground water is also 

the source of much of the water used for irrigation. 

It is the Nation’s principal reserve of freshwater and 

represents much of the potential future water 

supply. Ground water is a major contributor to flow 

in many streams and rivers and has a strong influence on river and wetland habitats for plants and 

animals.

The pumpage of fresh ground water 

in the United States in 1995 was estimated 

to be approximately 77 billion gallons per 

day (Solley and others, 1998), 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, the ground-water 

resource appears ample. Locally, however, the 

availability of ground water varies widely. 

Moreover, only a part of the ground water 

stored in the subsurface can be recovered by 

wells in an economic manner and without 

adverse consequences.

Sustainability of 

Ground-Water Resources

by William M. Alley

Thomas E. Reilly

O. Lehn Franke

INTRODUCTION

Ground water is one of the Nation’s most 

important natural resources.
2

The construction of surface reservoirs has 

slowed considerably in recent years (Figure 2). As 

surface-water resources become fully developed 

and appropriated, ground water commonly offers 

the only available source for new development. 

In many areas of the United States, however, 

pumping of ground water has resulted in significant depletion of ground-water storage. Furthermore, ground water and surface water are closely 

related and in many areas comprise a single 

resource (Winter and others, 1998). Ground-water 

pumping can result in reduced river flows, lower 

lake levels, and reduced discharges to wetlands 

and springs, causing concerns about drinkingwater supplies, riparian areas, and critical aquatic 

habitats. Increasingly, attention is being placed 

on how to manage ground water (and surface 

water) in a sustainable manner (Downing, 1998; 

Sophocleous, 1998; Gelt and others, 1999).

Resource sustainability has proved to be an 

elusive concept to define in a precise manner and 

with universal applicability. In this report, we 

define ground-water sustainability as 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. Some key goals related 

to ground-water sustainability in the United 

Kingdom are listed in Figure 3. These goals 

apply equally well in the United States.

WA

61

OR

40

NV

31

CA

45

AZ

60

ID

96

UT

56 CO

22

WY

57

MT

52

ND

57

SD

70

NE

87

KS

51

OK

34

TX

45 LA

61

AR

53

MO

54

IA

78

MN

80 WI

70

IL

33

IN

64

OH

46

PA

41

WV

43 VA

34

NC

50

SC

45 GA

41 AL

52

FL

93

KY

24

TN

47

MS

92

MI

46

NY

35

ME

60

VT

64

NH

61

MA

46

RI

26

CT

NJ 54

53

DE

MD 66

31

AK

62 HI

94

NM

90

EXPLANATION

60 Estimated percentage of

 population in a State

 using ground water as

 drinking water in 1995

Figure 1. Ground water 

is an important source of 

drinking water for every 

State. (U.S. Geological 

Survey, 1998.)

0

50

100

150

200

250

300

350

400

450

500

1880 1900 1920

YEAR

STORAGE CAPACITY, IN MILLION ACRE-FEET

1940 1960 1980 2000

Figure 2. Total surface-water reservoir capacity in 

the conterminous United States from 1880 to 1990. 

(Modified from Solley, 1995.)
3

Perhaps the most important attribute of the 

concept of ground-water sustainability is that it 

fosters a long-term perspective to management of 

ground-water resources. Several factors reinforce 

the need for a long-term perspective. First, ground 

water is not a nonrenewable resource, such as a 

mineral or petroleum deposit, nor is it completely 

renewable in the same manner and timeframe 

as solar energy. Recharge of ground water from 

precipitation continually replenishes the groundwater resource but may do so at much smaller 

rates than the rates of ground-water withdrawals. 

Second, ground-water development may take 

place over many years; thus, the effects of both 

current and future development must be considered in any water-management strategy. Third, 

the effects of ground-water pumping tend to 

manifest themselves slowly over time. For 

example, the full effects of pumping on surfacewater resources may not be evident for many 

years after pumping begins. Finally, losses from 

ground-water storage must be placed in the 

context of the period over which sustainability 

needs to be achieved. Ground-water withdrawals 

and replenishment by recharge usually are variable both seasonally and from year to year. 

Viewing the ground-water system through time, 

a long-term approach to sustainability may 

involve frequent temporary withdrawals from 

ground-water storage that are balanced by intervening additions to ground-water storage.

THE PRIORITIES FOR GROUND-WATER MANAGEMENT

TO PROTECT A PRICELESS NATIONAL ASSET

SUSTAINABLE LONG-TERM YIELDS FROM

AQUIFERS

EFFECTIVE USE OF THE LARGE VOLUME

OF WATER STORED IN AQUIFERS

PRESERVATION OF GROUND-WATER

QUALITY

PRESERVATION OF THE AQUATIC

ENVIRONMENT BY PRUDENT

ABSTRACTION OF GROUND WATER

INTEGRATION OF GROUND WATER AND

SURFACE WATER INTO A COMPREHENSIVE WATER AND ENVIRONMENTAL

MANAGEMENT SYSTEM

Figure 3. Vision statement of priorities for ground-water management in the United Kingdom. (Modified 

from Downing, 1998.)

Ground water is not a nonrenewable resource, 

such as a mineral or petroleum deposit, 

nor is it completely renewable in the same 

manner and timeframe as solar energy.
4

Three terms that have long been associated 

with ground-water sustainability need special 

mention; namely, safe yield, ground-water mining, 

and overdraft. The term “safe yield” commonly 

is used in efforts to quantify sustainable groundwater 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. The consequences 

of pumping should be assessed for each level 

of development, and safe yield taken as the 

maximum pumpage for which the consequences 

are considered acceptable. The term “groundwater 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 groundwater withdrawals.

In some situations, the focus of attention 

may be on extending the useful life of an aquifer 

as opposed to achieving long-term sustainability. 

This situation—for which the term ground-water 

mining is perhaps most fitting—is not addressed 

specifically in this report; however, many of the 

same hydrologic principles that we discuss herein 

still apply.

This introductory discussion indicates that 

the concept of ground-water sustainability and 

its application to real situations is multifaceted 

and complex. The effects of many human activities 

on ground-water resources and on the broader 

environment need to be clearly understood.

We begin by reviewing some pertinent 

facts and concepts about ground water and some 

common misconceptions about water budgets and 

ground-water sustainability. Individual chapters 

then focus on the interactions between ground 

water and surface water, on ground-water storage, 

and on ground-water quality as each aspect relates 

to the sustainability of ground-water resources. We 

conclude by discussing the importance of groundwater data, uses of ground-water models, and strategies to meet the challenges posed in assuring 

sustainable use of ground-water resources.

Throughout the report, we emphasize 

that development of ground-water resources 

has consequences to hydrologic and related environmental systems. We discuss relevant concepts 

and field examples in the body of the text, and 

provide more technical discussion of special topics 

and additional field examples in “boxes.” An exception is the next special section, “General Facts and 

Concepts about Ground Water.” Many readers 

familiar with ground-water concepts will want 

to go directly to the chapter on “Ground-Water 

Development, Sustainability, and Water Budgets.”
5

“If sustainable development is to mean anything, 

such development must be based on an appropriate 

understanding of the environment—an environment 

where knowledge of water resources is basic 

to virtually all endeavors.”

Report on Water Resources Assessment, WMO/UNESCO, 1991
GENERAL FACTS AND CONCEPTS 

ABOUT GROUND WATER

6

• Ground water occurs almost everywhere 

beneath the land surface. The widespread 

occurrence of potable ground water is the 

reason that it is used as a source of water 

supply by about one-half the population of 

the United States, including almost all of the 

population that is served by domestic watersupply systems.

• Natural sources of freshwater that become 

ground water are (1) areal recharge from 

precipitation that percolates through 

the unsaturated zone to the water table 

(Figure 4) and (2) losses of water from 

streams and other bodies of surface water 

such as lakes and wetlands. Areal recharge 

ranges from a tiny fraction to about one-half 

of average annual precipitation. Because 

areal recharge occurs over broad areas, 

even small average rates of recharge (for 

example, a few inches per year) represent 

significant volumes of inflow to ground 

water. Streams and other surface-water 

bodies may either gain water from ground 

water or lose (recharge) water to ground 

water. Streams commonly are a significant 

source of recharge to ground water downstream from mountain fronts and steep 

hillslopes in arid and semiarid areas and 

in karst terrains (areas underlain by limestone and other soluble rocks).

• The top of the subsurface ground-water body, 

the water table, is a surface, generally below 

the land surface, that fluctuates seasonally 

and from year to year in response to changes 

in recharge from precipitation and surfacewater bodies. On a regional scale, the configuration of the water table commonly is a 

subdued replica of the land-surface topography. The depth to the water table varies. 

In some settings, it can be at or near the land 

surface; for example, near bodies of surface 

water in humid climates. In other settings, the 

depth to the water table can be hundreds of 

feet below land surface. 

The following review of some basic facts and concepts about ground water serves as background 

for the discussion of ground-water sustainability.

Precipitation 

Evapotranspiration

Recharge to

water table

Unsaturated zone

Soil zone

Saturated zone

below the water table

(Ground water)

Capillary fringe

Water table
7

• Ground water commonly is an important 

source of surface water. The contribution 

of ground water to total streamflow varies 

widely among streams, but hydrologists 

estimate the average contribution is somewhere between 40 and 50 percent in small 

and medium-sized streams. Extrapolation of 

these numbers to large rivers is not straightforward; however, the ground-water contribution to all streamflow in the United States 

may be as large as 40 percent. Ground water 

also is a major source of water to lakes and 

wetlands.

• Ground water serves as a large subsurface 

water reservoir. Of all the freshwater that 

exists, about 75 percent is estimated to be 

stored in polar ice and glaciers and about 

25 percent is estimated to be stored as 

ground water. Freshwater stored in rivers, 

lakes, and as soil moisture amounts to less 

than 1 percent of the world’s freshwater. The 

reservoir aspect of some large ground-water 

systems can be a key factor in the development of these systems. A large ratio of total 

ground-water storage either to groundwater withdrawals by pumping or to natural 

discharge is one of the potentially useful 

characteristics of a ground-water system 

and enables water supplies to be maintained 

through long periods of drought. On the 

other hand, high ground-water use in areas 

of little recharge sometimes causes widespread declines in ground-water levels 

and a significant decrease in storage in 

the ground-water reservoir. 

Figure 4. The unsaturated zone, capillary fringe, water table, and saturated zone.

Water beneath the land surface occurs in two principal zones, the unsaturated zone and the saturated zone. 

In the unsaturated zone, the spaces between particle grains and the cracks in rocks contain both air and water. 

Although a considerable amount of water can be present in the unsaturated zone, this water cannot be pumped by 

wells because capillary forces hold it too tightly.

In contrast to the unsaturated zone, the voids in the saturated zone are completely filled with water. The approximate upper surface of the saturated zone is referred to as the water table. Water in the saturated zone below the water 

table is referred to as ground water. Below the water table, the water pressure is high enough to allow water to enter a 

well as the water level in the well is lowered by pumping, thus permitting ground water to be withdrawn for use.

Between the unsaturated zone and the water table is a transition zone, the capillary fringe. In this zone, the 

voids are saturated or almost saturated with water that is held in place by capillary forces.
8

• Velocities of ground-water flow generally are 

low and are orders of magnitude less than 

velocities of streamflow. The movement 

of ground water normally occurs as slow 

seepage through the pore spaces between 

particles of unconsolidated earth materials 

or through networks of fractures and solution openings in consolidated rocks. A 

velocity of 1 foot per day or greater is a high 

rate of movement for ground water, and 

ground-water velocities can be as low as 

1 foot per year or 1 foot per decade. In 

contrast, velocities of streamflow generally 

are measured in feet per second. A velocity 

of 1 foot per second equals about 16 miles 

per day. The low velocities of ground-water 

flow can have important implications, 

particularly in relation to the movement 

of contaminants.

• Under natural conditions, ground water 

moves along flow paths from areas of 

recharge to areas of discharge at springs 

or along streams, lakes, and wetlands. 

Discharge also occurs as seepage to bays 

or the ocean in coastal areas, and as transpiration by plants whose roots extend to near 

the water table. The three-dimensional body 

of earth material saturated with moving 

ground water that extends from areas of 

recharge to areas of discharge is referred to 

as a ground-water-flow system (Figure 5).

Water table

Water table

Unsaturated zone

Confined aquifer

Unconfined aquifer

Stream

Transpiration

by vegetation

High hydraulic-conductivity aquifer

Low hydraulic-conductivity confining unit

Very low hydraulic-conductivity bedrock

Direction of ground-water flow

EXPLANATION

Figure 5. A local scale ground-water-flow system.

In this local scale ground-water-flow system, 

inflow of water from areal recharge occurs at the water 

table. Outflow of water occurs as (1) discharge to the 

atmosphere as ground-water evapotranspiration (transpiration by vegetation rooted at or near the water table 

or direct evaporation from the water table when it is at 

or close to the land surface) and (2) discharge of ground 

water directly through the streambed. Short, shallow 

flow paths originate at the water table near the stream. 

As distance from the stream increases, flow paths to the 

stream are longer and deeper. For long-term average 

conditions, inflow to this natural ground-water system 

must equal outflow.
9

• The areal extent of ground-water-flow systems 

varies from a few square miles or less to 

tens of thousands of square miles. The 

length of ground-water-flow paths ranges 

from a few feet to tens, and sometimes 

hundreds, of miles. A deep ground-waterflow system with long flow paths between 

areas of recharge and discharge may be overlain by, and in hydraulic connection with, 

several shallow, more local, flow systems 

(Figure 6). Thus, the definition of a groundwater-flow system is to some extent subjective and depends in part on the scale of a 

study.

1

2

3

Local ground-water subsystem

Subregional ground-water subsystem

Regional ground-water subsystem

Water table

High hydraulic-conductivity aquifer

Low hydraulic-conductivity confining unit

Very low hydraulic-conductivity bedrock

Direction of ground-water flow

EXPLANATION

Unsaturated zone

Surface-water body

1

1

2

1

2

1

3

3

3

Figure 6. A regional ground-water-flow system that comprises subsystems at different scales and 

a complex hydrogeologic framework. (Modified from Sun, 1986.)

Significant features of this depiction of part of a regional ground-water-flow system include (1) local 

ground-water subsystems in the upper water-table aquifer that discharge to the nearest surface-water 

bodies (lakes or streams) and are separated by ground-water divides beneath topographically high areas; 

(2) a subregional ground-water subsystem in the water-table aquifer in which flow paths originating at 

the water table do not discharge into the nearest surface-water body but into a more distant one; and 

(3) a deep, regional ground-water-flow subsystem that lies beneath the water-table subsystems and is 

hydraulically connected to them. The hydrogeologic framework of the flow system exhibits a complicated 

spatial arrangement of high hydraulic-conductivity aquifer units and low hydraulic-conductivity 

confining units. The horizontal scale of the figure could range from tens to hundreds of miles.
10

• The age (time since recharge) of ground water 

varies in different parts of ground-waterflow systems. The age of ground water 

increases steadily along a particular flow path 

through the ground-water-flow system from 

an area of recharge to an area of discharge. In 

shallow, local-scale flow systems, ages of 

ground water at areas of discharge can vary 

from less than a day to a few hundred years. 

In deep, regional flow systems with long flow 

paths (tens of miles), ages of ground water 

may reach thousands or tens of thousands of 

years.

• Surface and subsurface earth materials are 

highly variable in their degree of particle 

consolidation, the size of particles, the size 

and shape of pore or open spaces between 

particles and between cracks in consolidated 

rocks, and in the mineral and chemical 

composition of the particles. Ground water 

occurs both in loosely aggregated and 

unconsolidated materials, such as sand and 

gravel, and in consolidated rocks, such as 

sandstone, limestone, granite, and basalt.

• Earth materials vary widely in their ability to 

transmit and store ground water. The ability 

of earth materials to transmit ground water 

(quantified as hydraulic conductivity) varies 

by orders of magnitude and is determined 

by the size, shape, interconnectedness, and 

volume of spaces between solids in the 

different types of materials. For example, 

the interconnected pore spaces in sand 

and gravel are larger than those in finer 

grained sediments, and the hydraulic 

conductivity of sand and gravel is larger 

than the hydraulic conductivity of the 

finer grained materials. The ability of earth 

materials to store ground water also varies 

among different types of materials. For 

example, the volume of water stored in 

cracks and fractures per unit volume of 

granite is much smaller than the volume 

stored per unit volume in the intergranular 

spaces between particles of sand and gravel. 

• Wells are the principal direct window to 

study the subsurface environment. Not 

only are wells used to pump ground water 

for many purposes, they also provide essential information about conditions in the 

subsurface. For example, wells (1) allow 

direct measurement of water levels in the 

well, (2) allow sampling of ground water 

for chemical analysis, (3) provide access for 

a large array of physical measurements in 

the borehole (borehole geophysical logging) 

that give indirect information on the properties of the fluids and earth materials in the 

neighborhood of the well, and (4) allow 

hydraulic testing (aquifer tests) of the earth 

materials in the neighborhood of the well to 

determine local values of their transmitting 

and storage properties. In addition, earth 

materials can be sampled directly at any 

depth during the drilling of the well.
11

• Pumping ground water from a well always 

causes (1) a decline in ground-water levels 

(heads; see Figure 7) at and near the well, 

and (2) a diversion to the pumping well of 

ground water that was moving slowly to its 

natural, possibly distant, area of discharge.

Pumping of a single well typically has a 

local effect on the ground-water-flow 

system. Pumping of many wells (sometimes 

hundreds or thousands of wells) in large 

areas can have regionally significant effects 

on ground-water systems. 

• Ground-water heads respond to pumping to 

markedly different degrees in unconfined 

and confined aquifers. Pumping the same 

quantity of water from wells in confined and 

in unconfined aquifers initially results in 

much larger declines in heads over much 

larger areas for the confined aquifers (see 

Box A). This is because less water is available from storage in confined aquifers 

compared to unconfined aquifers. At a 

later time, as the amount of water derived 

from storage decreases and the system 

approaches equilibrium, the response of 

the system no longer depends upon being 

confined or unconfined. The amount of head 

decline at equilibrium is a function of the 

transmitting properties of the aquifers and 

confining units, discharge rate of the well, 

and distance to ground-water-system 

boundaries. Many aquifers, such as the 

upper part of the deep flow subsystem 

shown in Figure 6, exhibit a response 

to pumping that is intermediate between 

a completely confined and a completely 

unconfined aquifer system.

Figure 7. The concept of “hydraulic head” or “head” 

at a point in an aquifer.

Consider the elevations above sea level at points A 

and B in an unconfined aquifer and C in a confined 

aquifer. Now consider the addition of wells with short 

screened intervals at these three points. The vertical 

distance from the water level in each well to sea level 

is a measure of hydraulic head or head, referenced to a 

common datum at each point A, B, and C, respectively. 

Thus, head at a point in an aquifer is the sum of (a) the 

elevation of the point above a common datum, usually 

sea level, and (b) the height above the point of a column 

of static water in a well that is screened at the point. 

When we discuss declines or rises in ground-water 

levels in a particular aquifer in this report, we are referring to changes in head or water levels in wells that are 

screened or have an open interval in that aquifer.

Land surface

Unsaturated

zone

Unconfined

aquifer

Confining

unit

Confined

aquifer

Sea level

A

B

C

Screened

interval

of well

Elevation

and head

at point A,

in feet

Head at

point B,

in feet

Elevation

of point C,

in feet

Undefined

interval

Water table

Head at

point C,

in feet

Elevation

of point B,

in feet
12

A

Confined and Unconfined Aquifers

Respond Differently to Pumping

The markedly different response of confined and 

unconfined aquifers to pumping (before the ground-water 

system returns to a new equilibrium) is demonstrated by 

calculations of drawdown resulting from a single pumping well 

in an idealized example of each type of aquifer (Figures A–1 

and A–2). The numerical values used in the calculations are 

listed in Table A–1. Inspection of these values indicates that 

they are the same except for the storage coefficient S. Herein 

lies the key, which we discuss further in this section. To a 

hydrogeologist, the values in Table A–1 indicate a moderately 

permeable (K) and transmissive (T) aquifer, typical values of 

the storage coefficient S for confined and unconfined aquifers, 

and a high rate of continuous pumping (Q) for one year (t).

A mathematical solution was developed by Theis 

(1940) to calculate drawdowns caused by a single well in 

an aquifer of infinite extent where the only source of water is 

from storage. This solution was used to calculate drawdowns 

at the end of one year of pumping for the confined and unconfined aquifers defined by the values in Table A–1. These drawdowns are plotted on Figure A–3. Inspection of Figure A–3 

shows that drawdowns in the confined aquifer are always 

larger than drawdowns in the unconfined aquifer, and that 

significant, or at least measurable, drawdowns occur at much 

larger distances from the pumping well in the confined aquifer. 

For example, at a distance of 10,000 feet (about 2 miles) 

from the pumping well, the drawdown in the unconfined 

Mineral

grains

Pore

water

Well discharge 

Original groundwater level (head) 

before pumping

Confining unit

(low hydraulic

conductivity)

Confined

aquifer

Confining

unit

Drawdown

Figure A–1. Pumping a single well in an idealized confined 

aquifer. Confined aquifers remain completely saturated during 

pumping by wells (saturated thickness of aquifer remains 

unchanged).
13

aquifer is too small to plot in Figure A–3, and the drawdown in the confined aquifer is about 10 feet. Furthermore, 

a measurable drawdown still occurs in the confined aquifer 

at a distance of 500,000 feet (about 95 miles) from the 

pumping well. Considering this information in a spatial sense, 

the cone of depression (Figure A–4) associated with the 

pumping well in the confined aquifer is deeper and much 

more areally extensive compared to the cone of depression in 

the unconfined aquifer. In fact, the total volume of the cone of 

depression in the confined aquifer is about 2,000 times larger 

than the total volume of the cone of depression in the unconfined aquifer for this example of a hypothetical infinite aquifer.

Table A–1. Numerical values of parameters used to calculate drawdowns in ground-water levels in response to pumping in 

two idealized aquifers, one confined and one unconfined

Parameter Confined aquifer Unconfined aquifer

Hydraulic conductivity, K 100 feet per day 100 feet per day

Aquifer thickness, b 100 feet 100 feet

Transmissivity, T 10,000 feet squared per day 10,000 feet squared per day

Storage coefficient, S 0.0001 0.2

Duration of pumping, t 365 days 365 days

Rate of pumping, Q 192,500 cubic feet per day 

(1,000 gallons per minute)

192,500 cubic feet per day 

(1,000 gallons per minute)

Well discharge 

Air

Water

around

grains

Water table and original

ground-water level (head)

before pumping 

Saturated zone

Unsaturated zone

Land

surface

Confining unit

Drawdown

Figure A–2. Pumping a single well in an idealized unconfined aquifer. Dewatering occurs in cone of depression 

of unconfined aquifers during pumping by wells (saturated 

thickness of aquifer decreases).
14

Center of

pumping well

The large differences in drawdowns and related 

volumes of the cone of depression in the two types of aquifers 

relate directly to how the two types of aquifers respond to 

pumping. In unconfined aquifers (Figure A–2) dewatering of 

the formerly saturated space between grains or in cracks or 

solution holes takes place. This dewatering results in significant volumes of water being released from storage per unit 

volume of earth material in the cone of depression. On 

the other hand, in confined aquifers (Figure A–1) the entire 

thickness of the aquifer remains saturated during pumping. 

However, pumping causes a decrease in head and an accompanying decrease in water pressure in the aquifer within 

the cone of depression. This decrease in water pressure 

allows the water to expand slightly and causes a slight 

compression of the solid skeleton of earth material in the 

aquifer. The volume of water released from storage per 

unit volume of earth material in the cone of depression in 

a confined aquifer is small compared to the volume of water 

released by dewatering of the earth materials in an unconfined aquifer. The difference in how the two types of aquifers 

respond to pumping is reflected in the large numerical difference for values of the storage coefficient S in Table A–1. 

The idealized aquifers and associated calculations of 

aquifer response to pumping discussed here represent ideal 

end members of a continuum; that is, the response of many 

real aquifers lies somewhere between the responses in these 

idealized examples.

Figure A–4. The cone of depression associated 

with a pumping well in a homogeneous aquifer.

Figure A–3. Comparison of drawdowns 

after 1 year at selected distances from single 

wells that are pumped at the same rate in an 

idealized confined aquifer and an idealized 

unconfined aquifer. Note that the distances 

on the x-axis are not constant or to scale.

0

10

20

30

Drawdown in

 confined aquifer

Drawdown in

 unconfined aquifer

40

DISTANCE FROM PUMPING WELL

Next to well

DRAWDOWN, IN FEET

10 feet

100 feet

1,000 feet

10,000 feet

(almost 2 miles)

100,000 feet

(almost 19 miles)

500,000 feet

(almost 95 miles)
15

A ground-water system consists of a mass of 

water flowing through the pores or cracks below 

the Earth’s surface. This mass of water is in motion. 

Water is constantly added to the system by recharge 

from precipitation, and water is constantly leaving 

the system as discharge to surface water and as 

evapotranspiration. Each ground-water system 

is unique in that the source and amount of water 

flowing through the system is dependent upon 

external factors such as rate of precipitation, location of streams and other surface-water bodies, 

and rate of evapotranspiration. The one common 

factor for all ground-water systems, however, is 

that the total amount of water entering, leaving, 

and being stored in the system must be conserved. 

An accounting of all the inflows, outflows, and 

changes in storage is called a water budget. 

Human activities, such as ground-water withdrawals and irrigation, change the natural flow 

patterns, and these changes must be accounted for 

in the calculation of the water budget. Because any 

water that is used must come from somewhere, 

human activities affect the amount and rate of 

movement of water in the system, entering the 

system, and leaving the system.

Some hydrologists believe that a predevelopment water budget for a ground-water 

system (that is, a water budget for the natural 

conditions before humans used the water) can 

be used to calculate the amount of water available 

for consumption (or the safe yield). In this case, 

the development of a ground-water system is 

considered to be “safe” if the rate of ground-water 

withdrawal does not exceed the rate of natural 

recharge. This concept has been referred to as 

the “Water-Budget Myth” (Bredehoeft and others, 

1982). It is a myth because it is an oversimplification of the information that is needed to understand the effects of developing a ground-water 

system. As human activities change the system, 

the components of the water budget (inflows, 

outflows, and changes in storage) also will change 

and must be accounted for in any management 

decision. Understanding water budgets and how 

they change in response to human activities is 

an important aspect of ground-water hydrology; 

however, as we shall see, a predevelopment water 

budget by itself is of limited value in determining 

the amount of ground water that can be withdrawn on a sustained basis.

GROUND-WATER DEVELOPMENT, 

SUSTAINABILITY, AND WATER BUDGETS 
16

Some hydrologists believe that a predevelopment 

water budget for a ground-water system (that is, 

a water budget for the natural conditions before 

humans used the water) can be used to calculate the 

amount of water available for consumption (or the 

safe yield). This concept has been referred to as the 

“Water-Budget Myth.”
17

Under predevelopment conditions, the 

ground-water system is in long-term equilibrium. 

That is, averaged over some period of time, the 

amount of water entering or recharging the system 

is approximately equal to the amount of water 

leaving or discharging from the system. Because 

the system is in equilibrium, the quantity of water 

stored in the system is constant or varies about 

some average condition in response to annual or 

longer-term climatic variations. This predevelopment water budget is shown schematically in 

Figure 8A.

We also can write an equation that describes 

the water budget of the predevelopment system as:

Recharge (water entering) =

Discharge (water leaving)

The water leaving often is discharged to 

streams and rivers and is called base flow. The 

possible inflows (recharge) and outflows (discharge) 

of a ground-water system under natural (equilibrium) conditions are listed in Table 1.

Ground-Water Budgets

Ground-water system

Recharge Discharge

Removal of water

stored in the system

Increase in

recharge

Decrease in

discharge

Pumpage

A

B

Figure 8. Diagrams illustrating water budgets for a ground-water system for predevelopment and development 

conditions.

(A) Predevelopment water-budget diagram illustrating that inflow equals outflow. (B) Water-budget diagram 

showing changes in flow for a ground-water system being pumped. The sources of water for the pumpage are 

changes in recharge, discharge, and the amount of water stored. The initial predevelopment values do not directly 

enter the budget calculation.

Table 1. Possible sources of water entering and leaving a 

ground-water system under natural conditions

Inflow

(recharge)

Outflow

(discharge)

1. Areal recharge 

from precipitation that percolates through 

the unsaturated 

zone to the 

water table.

1. Discharge to streams, lakes, 

wetlands, saltwater bodies 

(bays, estuaries, or oceans), 

and springs.

2. Recharge from 

losing streams, 

lakes, and 

wetlands.

2. Ground-water evapotranspiration.
18

Humans change the natural or predevelopment flow system by withdrawing (pumping) 

water for use, changing recharge patterns by irrigation and urban development, changing the type 

of vegetation, and other activities. Focusing our 

attention on the effects of withdrawing ground 

water, we can conclude that the source of water 

for pumpage must be supplied by (1) more water 

entering the ground-water system (increased 

recharge), (2) less water leaving the system 

(decreased discharge), (3) removal of water that 

was stored in the system, or some combination 

of these three.

This statement, illustrated in Figure 8B, can 

be written in terms of rates (or volumes over a 

specified period of time) as:

Pumpage = Increased recharge + Water removed 

from storage + Decreased discharge.

It is the changes in the system that allow 

water to be withdrawn. That is, the water pumped 

must come from some change of flows and from 

removal of water stored in the predevelopment 

system (Theis, 1940; Lohman, 1972). The predevelopment water budget does not provide information on where the water will come from to supply 

the amount withdrawn. Furthermore, the predevelopment water budget only indirectly provides 

information on the amount of water perennially 

available, in that it can only indicate the magnitude of the original discharge that can be 

decreased (captured) under possible, usually 

extreme, development alternatives at possible 

significant expense to the environment. 

The source of water for pumpage is supplied by 

(1) more water entering the ground-water system 

(increased recharge), (2) less water leaving the 

system (decreased discharge), (3) removal of 

water that was stored in the system, or some 

combination of these three.
19

Regardless of the amount of water withdrawn, the system will undergo some drawdown 

in water levels in pumping wells to induce the 

flow of water to these wells, which means that 

some water initially is removed from storage. 

Thus, the ground-water system serves as both a 

water reservoir and a water-distribution system. 

For most ground-water systems, the change in 

storage in response to pumping is a transient 

phenomenon that occurs as the system readjusts 

to the pumping stress. The relative contributions 

of changes in storage, changes in recharge, and 

changes in discharge evolve with time. The initial 

response to withdrawal of water is changes in 

storage. If the system can come to a new equilibrium, the changes in storage will stop and inflows 

will again balance outflows:

Pumpage = Increased recharge 

+ Decreased discharge

Thus, the long-term source of water to 

discharging wells is typically a change in the 

amount of water entering or leaving the system. 

How much ground water is available for use 

depends upon how these changes in inflow and 

outflow affect the surrounding environment and 

what the public defines as undesirable effects on 

the environment.

In determining the effects of pumping and 

the amount of water available for use, it is critical 

to recognize that not all the water pumped is 

necessarily consumed. For example, not all the 

water pumped for irrigation is consumed by 

evapotranspiration. Some of the water returns 

to the ground-water system as infiltration (irrigation return flow). Most other uses of ground 

water are similar in that some of the water 

pumped is not consumed but is returned to 

the system. Thus, it is important to differentiate 

between the amount of water pumped and the 

amount of water consumed when estimating 

water availability and developing sustainable 

management strategies. 

The possibilities of severe, long-term 

droughts and climate change also should be 

considered (see Box B). Long-term droughts, 

which virtually always result in reduced 

ground-water recharge, may be viewed as a 

natural stress on a ground-water system that in 

many ways has effects similar to ground-water 

withdrawals—namely, reductions in ground-water 

storage and accompanying reductions in groundwater discharge to streams and other surfacewater bodies. Because a climate stress on the 

hydrologic system is added to the existing or 

projected human-derived stress, droughts represent extreme hydrologic conditions that should 

be evaluated in any long-term management plan.

Consideration of climate can be a key, but 

underemphasized, factor in ensuring the 

sustainability and proper management of 

ground-water resources.
20

B

Droughts, Climate Change, and

Ground-Water Sustainability

The term “drought” has different meanings to different 

people, depending on how a water deficiency affects them. 

Droughts have been classified into different types such as 

meteorological drought (lack of precipitation), agricultural 

drought (lack of soil moisture), or hydrologic drought (reduced 

streamflow or ground-water levels). It is not unusual for a 

given period of water deficiency to represent a more severe 

drought of one type than another type. For example, a 

prolonged dry period during the summer may substantially 

lower the yield of crops due to a shortage of soil moisture 

in the plant root zone but have little effect on ground-water 

storage replenished the previous spring. On the other hand, 

a prolonged dry period when maximum recharge normally 

occurs can lower ground-water levels to the point at which 

shallow wells go dry.

Ground-water systems are a possible backup 

source of water during periods of drought. If ground-water 

storage is large and the effects of existing ground-water development are minimal, droughts may have limited, if any, effect 

on the long-term sustainability of aquifer systems from a 

storage perspective. In contrast, where ground-water storage 

and heads have been substantially reduced by withdrawals of 

ground water before a drought occurs, ground water may be 

less useful as a source of water to help communities and 

others cope with droughts. Furthermore, previous groundwater withdrawals can cause water levels and flows in lakes, 

streams, and other water bodies during droughts to be below 

limits that would have occurred in the absence of groundwater development. Likewise, reduced freshwater discharges 

to coastal areas during droughts may cause seawater to move 

beyond previous landward limits, or reduced heads in aquifers 

may cause renewed land subsidence (Figure B–1). 

400

300

200

500

600

12

6

–6

–12

0

1960 1965 1970 1975 1980 1985 1990 1995

DEPTH TO WATER BELOW

LAND SURFACE, IN FEET

COMPACTION,

IN INCHES

No data

Drought Drought

No data

Figure B–1. Effects of drought on ground-water levels and associated subsidence in the San Joaquin Valley of California. 

(Modified from Galloway and Riley, in press; and Swanson, 1998.)

The San Joaquin Valley is a major agricultural area that produces a large fraction of the fruits, nuts, and vegetables in the 

United States. Ground-water withdrawals during the 1930’s to early 1960’s caused water-level declines of tens to hundreds of 

feet in much of the valley. The water-level declines resulted in compaction of the alluvial deposits and extensive land subsidence. 

Subsidence in excess of 1 foot has affected more than 5,200 square miles in the San Joaquin Valley, representing perhaps the 

largest anthropogenic change in land-surface elevation in the world. Importation of surface water, beginning in the 1960’s, led to 

a decrease in ground-water withdrawals, which in turn led to rising ground-water levels and at least a temporary end to further

subsidence. During severe droughts in 1976–77 and 1987–93, deliveries of imported water were decreased. More ground water 

was pumped to meet water demands, resulting in a decline in the water table and a renewal of compaction and land subsidence.
21

A common response to droughts is to drill more wells. 

Increased use of ground water may continue after a drought 

because installation of wells and the infrastructure for delivery 

of ground water can be a considerable investment. Thus, a 

drought may lead to a permanent, unanticipated change in 

the level of ground-water development. Use of ground-water 

resources for mitigating the effects of droughts is likely to be 

most effective with advance planning for that purpose.

Ground-water systems tend to respond much more 

slowly to short-term variability in climate conditions than do 

surface-water systems. As a result, assessments of groundwater resources and related model simulations commonly 

are based on average conditions, such as average annual 

recharge or average annual discharge to streams. This use 

of average conditions may underestimate the importance 

of droughts. 

The effect of potential long-term changes in climate, 

including changes in average conditions and in climate 

variability, also merits consideration. Climate change could 

affect ground-water sustainability in several ways, including 

(1) changes in ground-water recharge resulting from changes 

in average precipitation and temperature or in the seasonal 

distribution of precipitation, (2) more severe and longer lasting 

droughts, (3) changes in evapotranspiration resulting from 

changes in vegetation, and (4) possible increased demands 

for ground water as a backup source of water supply. Surficial 

aquifers, which supply much of the flow to streams, lakes, 

wetlands, and springs, are likely to be the part of the groundwater system most sensitive to climate change; yet, limited 

attention has been directed at determining the possible effects 

of climate change on shallow aquifers and their interaction 

with surface water.

In summary, consideration of climate can be a key, but 

underemphasized, factor in ensuring the sustainability and 

proper management of ground-water resources. As increasing 

attention is placed on the interactions of ground water with 

land and surface-water resources, concerns about the effects 

of droughts, other aspects of climate variability, and the potential effects of climate change are likely to increase.

During the final preparation of this report in the summer of 1999, much of the Eastern United States was experiencing 

a severe drought, causing shallow wells to go dry in many areas. A few inches of rainfall in late August returned lawns to 

a healthy green color in this part of rural Virginia (note wellhead near home). However, these storms had little effect on 

ground-water levels because of the large cumulative moisture deficit in the unsaturated zone. Up to 6 inches or more of 

sustained precipitation from Tropical Storm Dennis over Labor Day weekend had a more substantial effect on groundwater levels, but parts of the Eastern United States unaffected by the tropical storm remained dry. In mid-September, 

Hurricane Floyd brought additional rain to the region. The drought intensified concerns about development of groundwater resources and the effects of possible interference between pumping wells, particularly in rapidly developing parts 

of the Piedmont where some of the fastest growing counties in the Nation are located.
22

Consider a ground-water system in which 

the only natural source of inflow is areal recharge 

from precipitation. The amount of inflow is thus 

relatively fixed. Further consider that the primary 

sources of any water pumped from this groundwater system are removal from storage, decreased 

discharge to streams, and decreased transpiration 

by plants rooted near the water table. 

If the above-described ground-water system 

can come to a new equilibrium after a period of 

removing water from storage, the amount of water 

consumed is balanced by less water flowing to 

surface-water bodies, and perhaps, less water available for transpiration by vegetation as the water 

table declines. If the consumptive use is so large 

that a new equilibrium cannot be achieved, water 

would continue to be removed from storage. In 

either case, less water will be available to surfacewater users and the ecological resources dependent 

on streamflow. Depending upon the location of the 

water withdrawals, the headwaters of streams may 

begin to go dry. If the vegetation receives less water, 

the vegetative character of the area also might 

change. These various effects illustrate how the 

societal issue of what constitutes an undesired 

result enters into the determination of groundwater sustainability. The tradeoff between water for 

consumption and the effects of withdrawals on the 

environment often become the driving force in 

determining a good management scheme.

In most situations, withdrawals from 

ground-water systems are derived primarily from 

decreased ground-water discharge and decreased 

ground-water storage. These sources of water 

were thus emphasized in the previous example. 

Two special situations in which increased recharge 

can occur in response to ground-water withdrawals are noted here.

Pumping ground water can increase 

recharge by inducing flow from a stream into 

the ground-water system. When streams flowing 

across ground-water systems originate in areas 

outside these systems, the source of water being 

discharged by pumpage can be supplied in part 

by streamflow that originates upstream from the 

ground-water basin. In this case, the predevelopment water budget of the ground-water system 

does not account for a source of water outside the 

ground-water system that is potentially available 

as recharge from the stream.

Another potential source of increased 

recharge is the capture of recharge that was 

originally rejected because water levels were at 

or near land surface. As the water table declines 

in response to pumping, a storage capacity for 

infiltration of water becomes available in the 

unsaturated zone. As a result, some water that 

previously was rejected as surface runoff can 

recharge the aquifer and cause a net increase in 

recharge. This source of water to pumping wells 

is usually negligible, however, compared to other 

sources.

In summary, estimation of the amount of 

ground water that is available for use requires 

consideration of two key elements. First, the use 

of ground water and surface water must be evaluated together on a systemwide basis. This evaluation includes the amount of water available from 

changes in ground-water recharge, from changes 

in ground-water discharge, and from changes in 

storage for different levels of water consumption. 

Second, because any use of ground water changes 

the subsurface and surface environment (that is, 

the water must come from somewhere), the public 

should determine the tradeoff between groundwater use and changes to the environment and 

set a threshold at which the level of change 

becomes undesirable. This threshold can then be 

used in conjunction with a systemwide analysis of 

the ground-water and surface-water resources to 

determine appropriate limits for consumptive use.

Systemwide hydrologic analyses typically 

use simulations (that is, computer models) to aid 

in estimating water availability and the effects of 

Hypothetical Examples of How Ground-Water 

Systems Change in Response to Pumping
23

extracting water on the ground-water and surfacewater system. Computer models attempt to reproduce the most important features of an actual 

system with a mathematical representation. If 

constructed correctly, the model represents the 

complex relations among the inflows, outflows, 

changes in storage, movement of water in the 

system, and possibly other important features. 

As a mathematical representation of the system, 

the model can be used to estimate the response 

of the system to various development options 

and provide insight into appropriate management 

strategies. However, a computer model is a simplified representation of the actual system, and the 

judgment of water-management professionals is 

required to evaluate model simulation results and 

plan appropriate actions. We return to the use of 

models in the final chapter of this report, “Meeting 

the Challenges of Ground-Water Sustainability.”

Because any use of ground water changes 

the subsurface and surface environment (that is, the 

water must come from somewhere), the public should 

determine the tradeoff between ground-water use and 

changes to the environment and set a threshold for 

what level of change becomes undesirable.
24

Long Island is bounded on the north by 

Long Island Sound, on the east and south by the 

Atlantic Ocean, and on the west by New York Bay 

and the East River. Long Island is divided into four 

counties—Kings, Queens, Nassau, and Suffolk. 

The two western counties, Kings and Queens, are 

part of New York City. 

Precipitation that infiltrates and percolates to 

the water table is Long Island's only natural source 

of freshwater because the ground-water system 

is bounded on the bottom by relatively impermeable bedrock and on the sides by saline ground 

water or saline bays and the ocean (Figure 9). 

About one-half the precipitation becomes recharge 

to the ground-water system; the rest flows as 

surface runoff to streams or is lost through evapotranspiration (Cohen and others, 1968). Much 

of the precipitation that reaches the uppermost 

unconfined aquifer moves laterally and discharges 

to streams and surrounding saltwater bodies; 

the remainder seeps downward to recharge 

the deeper aquifers. Water enters these deeper 

aquifers very slowly in areas where confining 

units are present but enters freely in other 

areas where confining units are absent. Water 

in the deeper aquifers also moves seaward and 

eventually seeps into overlying aquifers. Predevelopment water budgets for most of Nassau and 

Suffolk Counties on Long Island are shown in 

Figure 9.

Over the past three centuries, the island's 

ground water has been developed through three 

distinct phases. In the first, which began with 

the arrival of European settlers in the mid-17th 

century, virtually every house had its own shallow 

well, which tapped the uppermost unconsolidated 

geologic deposits, and also had its own cesspool, 

which returned wastewater to these same 

deposits. Because population was sparse, this 

mode of operation had little effect on the quantity 

and quality of shallow ground water. During 

the next two centuries, the population increased 

steadily, and, by the end of the 19th century, 

the individual wells in some areas had been 

abandoned in favor of shallow public-supply 

wells.

The second phase began with the rapid 

population growth and urban development that 

occurred during the first half of the 20th century. 

The high permeability of Long Island's deposits 

encouraged the widespread use of domestic 

wastewater-disposal systems, and the contamination resulting from increased wastewater 

discharge led to the eventual abandonment of 

many domestic wells and shallow public-supply 

wells in favor of deeper, high-capacity wells. In 

general, pumping these deep wells had only a 

small effect on the quantity of shallow ground 

water and related surface-water systems because 

most of the water was returned to the groundwater reservoir through domestic wastewaterdisposal systems. 

Field Examples of How Ground-Water Systems 

Change in Response to Pumping

LONG ISLAND, NEW YORK
25

Water-budget

area

Long

Atlantic Ocean

Island

Sound

Water

table

Water-budget area

QUEENS

COUNTY

KINGS

COUNTY

NASSAU

COUNTY

SUFFOLK

COUNTY

Long Island Sound

ATLANTIC OCEAN

74°

73°

72°

41° 41°

72°

73°

74°

LONG ISLAND

OVERALL PREDEVELOPMENT

WATER-BUDGET ANALYSIS

GROUND-WATER PREDEVELOPMENT

WATER-BUDGET ANALYSIS

INFLOW TO LONG ISLAND

HYDROLOGIC SYSTEM

OUTFLOW FROM LONG ISLAND

HYDROLOGIC SYSTEM

1. Precipitation 2,475

2. Evapotranspiration of precipitation

3. Ground-water discharge to sea

4. Streamflow discharge to sea

5. Evapotranspiration of ground water

6. Spring flow

 Total outflow

1,175

725

525

25

25

2,475

CUBIC

FEET PER

SECOND

INFLOW TO LONG ISLAND

GROUND-WATER SYSTEM

OUTFLOW FROM LONG ISLAND

GROUND-WATER SYSTEM

 7. Ground-water recharge 1,275

 8. Ground-water discharge to streams

 9. Ground-water discharge to sea

10. Evapotranspiration of ground water

11. Spring flow

 Total outflow

500

725

25

25

1,275

CUBIC

FEET PER

SECOND

2

4 1

3

Fresh 

ground water

Confining

unit

Very low

hydraulic-conductivity

bedrock

Saline

ground water

Figure 9. Ground-water budget for part of Nassau and Suffolk Counties, Long Island, New York. (Modified 

from Cohen and others, 1968.)

Block diagram of Long Island, New York, and tables listing the overall water budget and ground-water 

budget under predevelopment conditions. Both water budgets assume equilibrium conditions with little or 

no change in storage.
26

The third and present phase of groundwater development on Long Island began in the 

early 1950’s with the introduction of large-scale 

sewer systems in the more heavily populated 

areas. The purpose of the sewers was to prevent 

domestic wastewater from entering the aquifer 

system because contaminants from this source 

were being detected in deep public-supply wells. 

Even though the sewers protect the aquifers from 

further contamination, they also prevent the 

replenishment (recharge) that the wastewater had 

provided to the ground-water reservoir through 

the domestic wastewater-disposal systems. The 

wastewater is now diverted to sewage-treatment 

plants, whose effluent is discharged to the bays 

and oceans. The decrease in recharge has caused 

the water table in the sewered areas to be substantially lowered, the base flow of streams to be 

reduced or eliminated, and the length of perennial 

streams to be decreased.

In Nassau and Suffolk Counties, about 

200 cubic feet per second of wastewater (ground 

water that has been pumped and used) was 

discharged in 1985 by the three major sewer 

districts to the surrounding saltwater bodies 

(Spinello and Simmons, 1992). As previously 

noted, the only source of freshwater to the system 

is precipitation. Therefore, the water required to 

balance the loss from the ground-water system 

must come primarily from decreases in groundwater discharge to streams and to surrounding 

saltwater bodies. Capture of ground-water evapotranspiration, spring flow, and some surface runoff 

are also possible, but each of these sources is 

limited to a maximum of approximately 25 cubic 

feet per second (Figure 9). As the flow to the 

streams decreases, the headwaters of the streams 

dry up and the streams become shorter. As the 

discharge of ground water to surrounding saltwater bodies decreases, saline ground water 

moves landward as saltwater intrusion. Thus, this 

case is an example in which the determination of 

sustainable yields cannot be based solely on predevelopment water budgets. The specific response of 

the ground-water system to development must be 

taken into account in determining the appropriate 

limits to set on ground-water use.

The High Plains is a 174,000-square-mile area 

of flat to gently rolling terrain that includes parts 

of Colorado, Kansas, Nebraska, New Mexico, 

Oklahoma, South Dakota, Texas, and Wyoming. 

The area is characterized by moderate precipitation but generally has a low natural recharge rate 

to the ground-water system. Unconsolidated alluvial deposits that form a water-table aquifer called 

the High Plains aquifer (consisting largely of the 

Ogallala aquifer) underlie the region. Irrigation 

water pumped from the aquifer has made the 

High Plains one of the Nation’s most important 

agricultural areas.

During the late 1800’s, settlers and speculators moved to the plains, and farming became 

the major activity in the area. The drought of 

the 1930’s gave rise to the use of irrigation and 

improved farming practices in the High Plains 

(Gutentag and others, 1984). Around 1940, a rapid 

expansion in the use of ground water for irrigation 

began. In 1949, about 480 million cubic feet per 

day of ground water was used for irrigation. By 

1980, the use had more than quadrupled to about 

2,150 million cubic feet per day (U.S. Geological 

Survey, 1984). Subsequently, it declined to about 

1,870 million cubic feet per day in 1990 (McGuire 

and Sharpe, 1997). Not all of the water pumped 

for irrigation is consumed as evapotranspiration 

by crops; some seeps back into the ground and 

recharges the aquifer. Nevertheless, this intense 

use of ground water has caused major water-level 

HIGH PLAINS AQUIFER
27

declines (Figure 10A) and decreased the saturated 

thickness of the aquifer significantly in some 

areas (Figure 10B). These changes are particularly 

evident in the central and southern parts of the 

High Plains.

The southern part of the High Plains 

aquifer in Texas and New Mexico slopes gently 

from west to east, cut off from external sources of 

water upstream and downstream by river-carved 

escarpments, as shown in Figure 11A. Thus, 

ground-water recharge is due almost exclusively 

to areal recharge from precipitation. Although 

precipitation in the area is 15 to 20 inches per year, 

only a fraction of an inch recharges the aquifer due 

to high evapotranspiration from the soil zone. 

During predevelopment conditions, discharge as 

seeps and springs along the eastern escarpment 

equaled recharge. Today, the magnitude of natural 

recharge and discharge is small compared to withdrawals for irrigation.

NEW MEXICO 

COLORADO

TEXAS

NEBRASKA

KANSAS

WYOMING SOUTH DAKOTA

OKLAHOMA

Water-level change,

 in feet

EXPLANATION

More than 150

100 to 150

50 to 100

10 to 50

Boundary of the

 High Plains aquifer

Declines

More than 10

Rises

Less than 10-foot

 change

0

0 150 KILOMETERS

150 MILES

105°

40°

35°

100°

RIVER

RIVER

South

River

North 

Platte

River

PLATTE

RIVER

RIVER

C ANADIAN

ARKANSAS

REPUBLICAN

Platte A

Figure 10. Changes in (A) ground-water levels and (B) saturated thickness in the High Plains aquifer from 

predevelopment to 1997. (V.L. McGuire, U.S. Geological Survey, written commun., 1998.)

Extensive pumping of ground water for irrigation has led to ground-water-level declines in excess of 100 feet 

in parts of the High Plains aquifer in Kansas, New Mexico, Oklahoma, and Texas. These large water-level declines 

have led to reductions in saturated thickness of the aquifer exceeding 50 percent of the predevelopment saturated 

thickness in some areas. Lower ground-water levels cause increases in pumping lifts. Decreases in saturated thickness result in declining well yields. Surface-water irrigation has resulted in water-level rises in some parts of the 

aquifer system, such as along the Platte River in Nebraska.

NEW MEXICO 

COLORADO

TEXAS

NEBRASKA

KANSAS

WYOMING SOUTH DAKOTA

OKLAHOMA

Saturated-thickness

 change, in percent

EXPLANATION

More than 50

25 to 50

10 to 25

Boundary of the

 High Plains aquifer

Decreases

More than 10

Increases

Less than 10-percent

 change

0

0 150 KILOMETERS

150 MILES

105°

40°

35°

100°

RIVER

RIVER

South

River

North 

Platte

River

PLATTE

RIVER

RIVER

C ANADIAN

ARKANSAS

REPUBLICAN

Platte B
28

The predevelopment water budget and a 

water budget for average developed conditions in 

the southern High Plains aquifer during 1960–80 

are shown in Figure 11B. Comparison of these 

water budgets shows that, due to irrigation 

return flow, recharge to the High Plains aquifer 

increased more than twentyfold from an estimated 

24 million cubic feet per day during predevelopment to about 510 million cubic feet per day 

during 1960–80. This increase in recharge (about 

486 million cubic feet per day) together with 

the decrease in storage (about 330 million cubic 

feet per day) accounts for over 98 percent of the 

total pumpage (about 830 million cubic feet per 

day). Less than 2 percent of the pumpage is 

accounted for by decreases in natural discharge 

(about 14 million cubic feet per day). 

A long delay between pumping and its 

effects on natural discharge from the High Plains 

aquifer is caused by the large distance between 

many of the pumping wells and the location of the 

springs and seeps that discharge from the groundwater system. The southern High Plains is perhaps 

the best known example of significant, long-term 

nonequilibrium for a regional ground-water 

system in the United States. That is, water levels 

continue to decline without reaching a new 

balance (equilibrium) between recharge to and 

discharge from the ground-water system. 

Water table

High Plains

aquifer

Bedrock

Vertical scale greatly exaggerated

Springs

and

seeps

West East

System before development System during development

Natural

recharge

24

Pumpage

830

Recharge

and irrigation

return flow

510

Natural

discharge

10

Natural

discharge

24

Decrease

in storage

330

B

A

Figure 11. The effects of ground-water 

withdrawals on the southern High Plains 

aquifer.

Schematic cross section (A) of the 

southern High Plains aquifer illustrating 

that ground-water withdrawal in the middle 

of the southern High Plains aquifer has a 

negligible short-term effect on the discharge 

at the boundaries of the aquifer. (Modified 

from Lohman, 1972.) (B) Water budgets of 

the southern High Plains aquifer (all flows 

in million cubic feet per day) before development and during development. (Modified 

from Johnston, 1989; data from Luckey 

and others, 1986).
29

The preceding two field examples illustrate 

some of the complexities associated with the use 

of water budgets to determine the development 

potential of a ground-water system. Knowledge 

of the sources and discharges of water to and 

from the system and how they change with 

continuing development is needed to understand 

the response of ground-water systems to development, as well as to aid in determining appropriate 

management strategies and future use of the 

resource. 

The examples discussed here and those 

in the following chapters illustrate several of 

the principles summarized by Bredehoeft and 

others (1982) in their article on the “water-budget 

myth” and earlier by Theis (1940): 

• Some ground water must be removed from 

storage before the system can be brought 

into equilibrium. 

• The time that is required to bring a hydrologic 

system into equilibrium depends on the rate 

at which the discharge can be captured.

• The rate at which discharge can be captured 

is a function of the characteristics of the 

aquifer system and the placement of pumping 

wells. 

• Equilibrium is reached only when pumping is 

balanced by capture. In many circumstances, 

the dynamics of the ground-water system are 

such that long periods of time are necessary 

before even an approximate equilibrium 

condition can be reached.

In the next three chapters we discuss in 

more detail the effects of ground-water development on ground-water discharge to and recharge 

from surface-water bodies, the effects of groundwater development on ground-water storage, and 

water-quality factors affecting ground-water 

sustainability.
30

EFFECTS OF GROUND-WATER 

DEVELOPMENT ON GROUND-WATER FLOW 

TO AND FROM SURFACE-WATER BODIES

As development of land and water resources 

intensifies, it is increasingly apparent that 

development of either ground water or 

surface water affects the other.

Streams

Streams either gain water from inflow of 

ground water (gaining stream; Figure 12A) or lose 

water by outflow to ground water (losing stream; 

Figure 12B). Many streams do both, gaining in 

some reaches and losing in other reaches. Furthermore, the flow directions between ground water 

and surface water can change seasonally as the 

altitude of the ground-water table changes with 

respect to the stream-surface altitude or can 

change over shorter timeframes when rises in 

stream surfaces during storms cause recharge to 

the streambank. Under natural conditions, ground 

water makes some contribution to streamflow in 

most physiographic and climatic settings. Thus, 

even in settings where streams are primarily losing 

water to ground water, certain reaches may receive 

ground-water inflow during some seasons.

Losing streams can be connected to the 

ground-water system by a continuous saturated 

zone (Figure 12B) or can be disconnected from 

the ground-water system by an unsaturated zone 

(Figure 12C). An important feature of streams 

that are disconnected from ground water is that 

pumping of ground water near the stream does 

not affect the flow of the stream near the pumped 

well.

As development of land and water resources 

intensifies, it is increasingly apparent that development of either ground water or surface water affects 

the other (Winter and others, 1998). Some particular 

aspects of the interaction of ground water and 

surface water that affect the sustainable development of ground-water systems are discussed below 

for various types of surface-water features.
31

A pumping well can change the quantity and 

direction of flow between an aquifer and stream in 

response to different rates of pumping. Figure 13 

illustrates a simple case in which equilibrium is 

attained for a hypothetical stream-aquifer system 

and a single pumping well. The adjustments 

to pumping of an actual hydrologic system may 

take place over many years, depending upon the 

physical characteristics of the aquifer, degree of 

hydraulic connection between the stream and 

aquifer, and locations and pumping history of 

wells. Reductions of streamflow as a result of 

ground-water pumping are likely to be of greatest 

concern during periods of low flow, particularly 

when the reliability of surface-water supplies is 

threatened during droughts.

At the start of pumping, 100 percent of the 

water supplied to a well comes from ground-water 

storage. Over time, the dominant source of water 

to a well, particularly wells that are completed in 

an unconfined aquifer, commonly 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 surfacewater body such as a lake or wetland. The source 

of water to a well from a stream can be either 

decreased discharge to the stream or increased 

recharge from the stream to the ground-water 

system. The streamflow reduction in either case 

is referred to as streamflow capture.

In the long term, the cumulative streamflow capture for many ground-water systems 

can approach the quantity of water pumped 

from the ground-water system. This is illustrated 

in Figure 14, which shows the time-varying 

percentage of ground-water pumpage derived 

from ground-water storage and the percentage 

derived from streamflow capture for the hypothetical stream-aquifer system shown in Figure 13. The 

time for the change from the dominance of withdrawal from ground-water storage to the dominance of streamflow capture can range from weeks 

to years to decades or longer.

GAINING STREAM

Flow direction

Unsaturated zone

Water table

Saturated zone

A

LOSING STREAM

Flow direction

Water table Unsaturated

zone

B

C LOSING STREAM THAT IS DISCONNECTED

FROM THE WATER TABLE

Flow direction

Water table

Unsaturated

zone

Figure 12. Interaction of streams and ground water. 

(Modified from Winter and others, 1998.)

Gaining streams (A) receive water from the 

ground-water system, whereas losing streams (B) lose 

water to the ground-water system. For ground water 

to discharge to a stream channel, the altitude of the 

water table in the vicinity of the stream must be 

higher than the altitude of the stream-water surface. 

Conversely, for surface water to seep to ground water, 

the altitude of the water table in the vicinity of the 

stream must be lower than the altitude of the stream 

surface. Some losing streams (C) are separated from 

the saturated ground-water system by an unsaturated zone.
32

Stream

Land surface

Water table

Unconfined aquifer

Confining unit

Q1

Q2

Stream

Land surface

Water table

Unconfined aquifer

Confining unit

Stream

Land surface

Water table

Unconfined aquifer

Confining unit

Recharge area

A

B

C

Ground-water

divide

Figure 13. Effects of pumping from a hypothetical ground-water system that discharges to a stream. (Modified 

from Heath, 1983.)

Under natural conditions (A), recharge at the water table is equal to ground-water discharge to the stream. 

Assume a well is installed and is pumped continuously at a rate, Q1, as in (B). After a new state of dynamic equilibrium is achieved, inflow to the ground-water system from recharge will equal outflow to the stream plus the withdrawal from the well. In this new equilibrium, some of the ground water that would have discharged to the stream is 

intercepted by the well, and a ground-water divide, which is a line separating directions of flow, is established locally 

between the well and the stream. If the well is pumped at a higher rate, Q2, a different equilibrium is reached, as 

shown in (C). Under this condition, the ground-water divide between the well and the stream is no longer present, 

and withdrawals from the well induce movement of water from the stream into the aquifer. Thus, pumping reverses 

the hydrologic condition of the stream in this reach from ground-water discharge to ground-water recharge. Note 

that in the hydrologic system depicted in (A) and (B), the quality of the stream water generally will have little effect 

on the quality of ground water. In the case of the well pumping at the higher rate in (C), however, the quality of the 

stream water can affect the quality of ground water between the well and the stream, as well as the quality of the 

water withdrawn from the well. Although a stream is used in this example, the general concepts apply to all surfacewater bodies, including lakes, reservoirs, wetlands, and estuaries.
33

Most ground-water development is much 

more complex than implied in Figure 13; for 

example, it may comprise many wells pumping 

from an aquifer at varying pumping rates and at 

different locations within the ground-water-flow 

system. Computer models commonly are needed 

to evaluate the time scale and time-varying 

response of surface-water bodies to such complex 

patterns of ground-water development. From a 

sustainability perspective, the key point is that 

pumping decisions today will affect surface-water 

availability; however, these effects may not be 

fully realized for many years.

The eventual reduction in surface-water 

supply as a result of ground-water development 

complicates the administration of water rights. 

Traditionally, water laws did not recognize the 

physical connection of ground water and surface 

water. Today, in parts of the Western United States, 

ground-water development and use are restricted 

because of their effects on surface-water rights. 

Accounting for the effects of ground-water development on surface-water rights can be difficult. 

For example, in the case of water withdrawn 

to irrigate a field, some of the water will be lost 

from the local hydrologic system due to evaporation and use by crops, while some may percolate to 

the ground-water system and ultimately be 

returned to the stream. Related questions that arise 

include: how much surface water will be captured, 

which surface-water bodies will be affected, and 

over what period will the effects occur? Some of 

these issues are illustrated further in Box C.

From a sustainability perspective, the key 

point is that pumping decisions today will affect 

surface-water availability; however, these effects 

may not be fully realized for many years.

Water from storage

Water from streamflow capture

0

50

100

PERCENTAGE OF GROUND-WATER PUMPAGE

PUMPING TIME

Figure 14. The principal source of water to a well 

can change with time from ground-water storage 

to capture of streamflow.

The percentage of ground-water pumpage derived 

from ground-water storage and capture of streamflow 

(decrease in ground-water discharge to the stream or 

increase in ground-water recharge from the stream) 

is shown as a function of time for the hypothetical 

stream-aquifer system shown in Figure 13. A constant 

pumping rate of the well is assumed. For this simple 

system, water derived from storage plus streamflow 

capture must equal 100 percent. The time scale of the 

curves shown depends on the hydraulic characteristics 

of the aquifer and the distance of the well from the 

stream.
34

Ground-water pumping can affect not only 

water supply for human consumption but also the 

maintenance of instream-flow requirements for 

fish habitat and other environmental needs. Longterm reductions in streamflow can affect vegetation along streams (riparian zones) that serve 

critical roles in maintaining wildlife habitat and in 

enhancing the quality of surface water. Pumpinginduced changes in the flow direction to and from 

streams may affect temperature, oxygen levels, 

and nutrient concentrations in the stream, which 

may in turn affect aquatic life in the stream.

Perennial streams, springs, and wetlands in the Southwestern United States are highly valued as 

a source of water for humans and for the plant and animal species they support. Development of 

ground-water resources since the late 1800’s has resulted in the elimination or alteration of many 

perennial stream reaches, wetlands, and associated riparian ecosystems. As an example, a 1942 photograph of a reach of the Santa Cruz River south of Tucson, Ariz., at Martinez Hill shows stands of 

mesquite and cottonwood trees along the river (left photograph). A replicate photograph of the same 

site in 1989 shows that the riparian trees have largely disappeared (right photograph). Data from two 

nearby wells indicate that the water table has declined more than 100 feet due to pumping, and this 

pumping appears to be the principal reason for the decrease in vegetation. (Photographs provided by 

Robert H. Webb, U.S. Geological Survey.)
35

In gaining and in losing streams, water 

and dissolved chemicals can move repeatedly 

over short distances between the stream and the 

shallow subsurface below the streambed. The 

resulting subsurface environments, which contain 

variable proportions of water from ground water 

and surface water, are referred to as hyporheic 

zones (see Figure 15). Hyporheic zones can be 

active sites for aquatic life. For example, the 

spawning success of fish may be greater where 

flow from the stream brings oxygen into contact 

with eggs that were deposited within the coarse 

bottom sediment or where stream temperatures 

are modulated by ground-water inflow. The effects 

of ground-water pumping on hyporheic zones and 

the resulting effects on aquatic life are not well 

known.

Figure 15. The dynamic interface between ground 

water and streams. (Modified from Winter and others, 

1998.)

Streambeds are unique environments where ground 

water that drains much of the subsurface of landscapes 

interacts with surface water that drains much of the 

surface of landscapes. Mixing of surface water and 

ground water takes place in the hyporheic zone where 

microbial activity and chemical transformations 

commonly are enhanced.

Stream

Stream

Interface of ground-waterflow system, hyporheic zone,

and stream

Direction of

ground-water

flow

Direction of

ground-water

flow

Water table 

H y p o r h e i c z o n

e
36

C

Ground-Water/Surface-Water Interactions 

and Water-Resources Sustainability: 

Examples from the Northwestern United States

PUGET SOUND LOWLAND, WASHINGTON

A numerical model of a hypothetical basin in the Puget 

Sound Lowland of western Washington was used by Morgan 

and Jones (1999) to illustrate the effects of ground-water 

withdrawals on discharge to streams and springs in small 

basins typical of the region. The hypothetical basin shown in 

Figure C–1 consists of glacial deposits and alluvial sediments 

along streams that overlie low hydraulic-conductivity bedrock 

(see Figure C–2).

The results of three simulations are used here to 

illustrate the effects of pumping on streamflow for each 

of three different locations of water withdrawal from a well: 

(1) the unconfined aquifer near stream segment A, (2) the 

unconfined aquifer about 6,000 feet from stream segment A, 

and (3) a confined aquifer near stream segment A that is 

separated from the unconfined aquifer by about 25 feet of lowpermeability till. The pumping well in simulation 1 and that in 

simulation 3 have the same land-surface location. The three 

simulations (simulations 1–3) are for steady-state conditions; 

that is, the ground-water system has reached dynamic equilibrium with the pumping from the well. For each simulation, the 

percentage of the ground water withdrawn that comes from 

capture of discharge to streams and their associated springs 

is shown in Table C–1 for five stream segments labeled A to E 

in Figure C–1.

Figure C–1. Three-dimensional perspective view of a hypothetical basin typical of the Puget 

Sound Lowland showing topography, streams, and well locations for pumping simulations. 

(Modified from Morgan and Jones, 1999.)

The effects of ground-water withdrawals on streamflow and spring discharge have become a major concern in parts of 

the Northwestern United States as continuing population growth increases the demand for water and pressures mount on the 

water resources to meet minimum instream-flow requirements for recreation and for fish and wildlife habitat. Examples from 

Washington and Idaho illustrate some of the complexities in how ground-water pumping affects surface-water resources.

DRIFT PLAIN

BLUFFS

MAJOR

STREAM VALLEY

EXPLANATION

Stream-segment letter

SIMULATION 2 WELL

SIMULATION 1 AND 3 WELL

Stream

20 MILES

NORTH

14 M

ILES

500

FEET

D

C

C

C

B

E

D

A

B

B
37

At steady state, all of the ground-water withdrawal comes from capture of surface water for each of 

the three simulations. In looking at the distribution among 

stream reaches, virtually all of the captured water is 

from stream segment A when the water is pumped from 

a well in the unconfined aquifer near that stream segment 

(simulation 1). For a well located farther from stream 

segment A (simulation 2), most of the capture is still from 

stream segment A, but almost 25 percent of the capture is 

from the more distant stream segments B and E. Finally, 

if the water is withdrawn from the confined aquifer near 

stream segment A (simulation 3), about 50 percent of the 

capture comes from stream segment A, and the remainder 

comes from more distant stream segments. 

The results illustrate that, because the effects of 

pumping propagate in all directions, several surface-water 

bodies can be affected by the water-level drawdowns caused 

by a pumping well. A well pumping from an unconfined aquifer 

will tend to capture most of its discharge from the nearest 

stream reaches. The presence of a confining layer between 

the well and the streams causes the cone of depression of 

the well to extend greater distances to capture the natural 

discharge required to offset pumping. Morgan and Jones 

(1999) demonstrated through additional simulations that, 

as the depth of the well and the number of confining layers 

increase, capture of discharge to streams and springs is 

distributed over increasingly larger areas.

Bedrock

Glacial deposits

Major

stream

valley

Springs

East West

Alluvium

Upland springs Upland streams

Figure C–2. Simplified hydrogeologic section for basin shown 

in Figure C-1 showing generalized ground-water-flow paths 

under natural conditions. Confining units and their effect on the 

flow system are not shown. (Modified from Morgan and Jones, 

1999.)

Table C–1. Streamflow capture along five stream segments for three pumping simulations: (1) pumping from well in unconfined 

aquifer near stream segment A, (2) pumping from well in unconfined aquifer about 6,000 feet from stream segment A, and 

(3) pumping from well in a confined aquifer near stream segment A

Streamflow capture along stream segment

as percentage of pumpage

Stream segment Simulation 1 Simulation 2* Simulation 3

A 97 70 51

B <1 12 13

C <1 <1 5

D <1 3 5

E 1 12 26

*Sum of percentages for simulation 2 is less than 100 due to rounding.
38

EASTERN SNAKE RIVER PLAIN, IDAHO

The previous example illustrates how capture of 

surface water might be distributed over a basin after equilibrium of the ground-water system has occurred. Computer 

simulations of the Eastern Snake River Plain aquifer by 

Hubbell and others (1997) illustrate how the effects of 

pumping on streamflow might be distributed through time 

before the ground-water system reaches equilibrium.

Highly permeable basaltic rocks of the Eastern Snake 

River Plain in Idaho provide conduits for rapid recharge of 

precipitation and water from extensive irrigation. Groundwater flow in this basaltic aquifer is primarily from northeast to 

southwest, as shown in Figure C–3. The aquifer is connected 

to the Snake River and discharges to the river largely through 

major springs, such as Thousand Springs at the downstream 

end of the flow system.

The timing of the effects of pumping on spring 

discharge to the Thousand Springs reach and to the 

entire eastern Snake River were simulated for each of four 

potential well sites (sites A–D in Figure C–3). For simulations 

at sites A to C, pumping was simulated at a constant rate for 

100 years. Figure C–4 shows the depletion of flow to the river 

as a percentage of pumpage during the 100-year simulation. 

The river losses from pumping at site A are 50 percent 

of the pumpage after 23 years and about 90 percent after 

100 years. Slightly more than half of the river depletion 

caused by pumping at site A occurs along the Thousand 

Springs reach.

Pumping at site B, located much closer to the 

Thousand Springs reach, has almost 90 percent of the 

pumpage obtained from capture of spring flow after 10 years 

of pumping. Most of the losses are from the Thousand Springs 

reach; river flow in the other reaches is only slightly affected by 

pumping at site B.

Site C is more distant from the Snake River. Pumping 

at this location has little effect on flows to the river for more 

than 10 years; however, depletion continuously increases 

during the long period of continuous pumping. The river depletion after 100 years of pumping at site C is about 70 percent 

of the pumpage.

IDAHO

Thousand Springs

reach

A

A

B

C

D

EXPLANATION

Boundary of Eastern

 Snake River Plain

Generalized ground-

 water-flow path

Site location for

 hypothetical

 pumping well

0

0 50 KILOMETERS

50 MILES

Snake R.

Snake River

Figure C–3. Eastern Snake River Plain aquifer 

system. (Modified from Hubbell and others, 1997; 

reprinted with permission of the National Ground 

Water Association. Copyright 1997.)

Thousand Springs
39

The results of pumping at site D illustrate how surfacewater depletion can continue long after pumping is discontinued at a well. These residual effects are demonstrated at 

site D by simulating continuous pumping for 30 years followed 

by a 70-year nonpumping period. Depletion of flow to the river 

increases for approximately 15 years after pumping at site D 

is discontinued. Depletion is still occurring 70 years after 

pumping ceases at a rate equivalent to 15 percent of the 

average pumping rate during the 30 years of pumping at 

site D.

The simulated results for sites A to D indicate that the 

location of a well relative to the ground-water-flow system has 

a significant effect on where changes in flow in the system 

take place and how long the system continues to adjust before 

equilibrium is reached. These results highlight the importance 

of taking transient response times of ground-water systems 

into account in long-term water-resources planning.

Site A

Site C

Site B

Site D

0

10

20

30

40

50

60

70

80

90

100

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90 100 0 10 20 30 40 50 60 70 80 90 100

TIME, IN YEARS

RIVER LOSSES (PERCENTAGE OF PUMPAGE)

TIME, IN YEARS

EXPLANATION 

Eastern Snake River

Thousand Springs reach

Pumping Nonpumping

Figure C–4. Simulated losses to the Snake River resulting from pumping at well sites A to D. 

(Modified from Hubbell and others, 1997; reprinted with permission of the National Ground Water 

Association. Copyright 1997.)
40

Lakes, both natural and human made, 

are present in many different parts of the landscape and can have complex ground-water-flow 

systems associated with them. Lakes interact with 

ground water in one of three basic ways: some 

receive ground-water inflow throughout their 

entire bed; some have seepage loss to ground 

water throughout their entire bed; and others, 

perhaps most lakes, receive ground-water inflow 

through part of their bed and have seepage loss to 

ground water through other parts. Lowering of 

lake levels as a result of ground-water pumping 

can affect the ecosystems supported by the lake 

(Figure 16), diminish lakefront esthetics, and have 

negative effects on shoreline structures such as 

docks.

Lakes

Littoral Zone

Infrequent high

Frequent high

Minimum average

Frequent low

Infrequent low

Marsh

Hydric forests

Pine/oak uplands

High

Low ELEVATION ELEVATION High

Low 0 20 40 60

PERCENTAGE OF TIME LAKE LEVELS ARE EXCEEDED

80 100

Infrequent high

Historical level

Defined minimum level

Frequent high

Minimum average Frequent low

Infrequent low

A

B

Figure 16. Setting minimum water levels in Florida lakes. (Modified from McGrail and others, 1998.)

As part of efforts to prevent significant undesirable environmental consequences from water-resources development, water-management agencies in Florida are defining minimum flows and water levels for priority surface 

waters and aquifers in the State. For lakes, the minimum flows and water levels describe a hydrologic regime that is 

less than the historical or optimal one but allows for prudent water use while protecting critical lake functions. As 

an example, five possible minimum water levels defined for a lake are shown in A. An elevation and a percentage of 

time the level is exceeded characterize each of these levels. The upper curve in B shows the percentage of the time that 

the lake is historically above each corresponding level. The goal is to ensure that water withdrawals and other waterresource management actions continue to allow the lake water levels to be at or above the minimum levels shown by 

the lower curve in B for the percentage of time shown.
41

The chemistry of ground water and the

direction and magnitude of exchange with surface

water significantly affect the input of dissolved

chemicals to lakes. In fact, ground water can be the

principal source of dissolved chemicals to a lake,

even in cases where ground-water discharge is a

small component of a lake’s water budget.

Changes in flow patterns to lakes as a result of

pumping may alter the natural fluxes to lakes of

key constituents such as nutrients and dissolved

oxygen, in turn altering lake biota, their environment, and the interaction of both.

As a result of very low topographic relief, high rainfall, and a karst terrain, the Florida landscape

is characterized by numerous lakes and wetland areas. The underlying Floridan aquifer is one of

the most extensive and productive aquifers in the world. Over the past two decades, lake levels

declined and wetlands dried out in highly developed west-central Florida as a result of both extensive

pumping and low precipitation during these years. Differentiating between the effects of the drought

and pumping has been difficult. (Photographs courtesy of Florida Water Resources Journal, August,

1990 issue.)

Dock on Crooked Lake in central Florida in the

1970’s.

The same dock in 1990.
42

Wetlands are present wherever topography 

and climate favor the accumulation or retention 

of water on the landscape. Wetlands occur in 

widely diverse settings from coastal margins 

to flood plains to mountain valleys. Similar 

to streams and lakes, wetlands can receive 

ground-water inflow, recharge ground water, 

or do both. Wetlands are in many respects groundwater features.

Public and scientific views of wetlands 

have changed greatly over time. Only a few 

decades ago, wetlands generally were considered 

to be of little or no value. It is now recognized that 

wetlands have beneficial functions such as wildlife 

habitat, floodwater retention, protection of the 

land from erosion, shoreline protection in coastal 

areas, and water-quality improvement by filtering 

of contaminants.

The persistence, size, and function of 

wetlands are controlled by hydrologic processes 

(Carter, 1996). For example, the persistence of 

wetness for many wetlands is dependent on a relatively stable influx of ground water throughout 

changing seasonal and annual climatic cycles. Characterizing ground-water discharge to wetlands and 

its relation to environmental factors such as moisture content and chemistry in the root zone of 

wetland plants is a critical but difficult to characterize aspect of wetlands hydrology (Hunt and 

others, 1999).

Wetlands can be quite sensitive to the 

effects of ground-water pumping. Ground-water 

pumping can affect wetlands not only as a result 

of progressive lowering of the water table, but 

also by increased seasonal changes in the altitude 

of the water table. The amplitude and frequency 

of water-level fluctuations through changing 

seasons, commonly termed the hydroperiod, affect 

wetland characteristics such as the type of vegetation, nutrient cycling, and the type of invertebrates, fish, and bird species present. The effects 

on the wetland environment from changes to the 

hydroperiod may depend greatly on the time of 

year at which the effects occur. For example, lower 

than usual water levels during the nongrowing 

season might be expected to have less effect on the 

vegetation than similar water-level changes 

during the growing season. The effects of 

pumping on seasonal fluctuations in groundwater levels near wetlands add a new dimension 

to the usual concerns about sustainable development that typically focus on annual withdrawals 

(Bacchus, 1998).

Wetlands
43

Springs typically are present where the 

water table intersects the land surface. Springs 

serve as important sources of water to streams 

and other surface-water features, as well as being 

important cultural and esthetic features in themselves. The constant source of water at springs 

leads to the abundant growth of plants and, 

many times, to unique habitats. Ground-water 

development can lead to reductions in springflow, 

changes of springs from perennial to ephemeral, 

or elimination of springs altogether. Springs 

typically represent points on the landscape 

where ground-water-flow paths from different 

sources converge. Ground-water development 

may affect the amount of flow from these different 

sources to varying extents, thus affecting the 

resultant chemical composition of the spring 

water.

Springs

The highly productive Edwards aquifer, the first aquifer to be designated as a sole 

source aquifer under the Safe Drinking Water Act, is the source of water for more 

than 1 million people in San Antonio, Texas, some military bases and small towns, 

and for south-central Texas farmers and ranchers. The aquifer also supplies water 

to sustain threatened and endangered species habitat associated with natural springs 

in the region and supplies surface water to users downstream from the major springs. 

These various uses are in direct competition with ground-water development and have 

created challenging issues of ground-water management in the region. (Photograph by 

Robert Morris, U.S. Geological Survey.)

Comal Springs
44

Coastal areas are a highly dynamic interface between the continents and the ocean. The 

physical and chemical processes in these areas 

are quite complex and commonly are poorly 

understood. Historically, concern about ground 

water in coastal regions has focused on seawater 

intrusion into coastal aquifers, as discussed in a 

later chapter of this report. More recently, ground 

water has been recognized as an important 

contributor of nutrients and contaminants 

to coastal waters. Likewise, plant and 

wildlife communities adapted to particular 

environmental conditions in coastal areas can 

be affected by changes in the flow and quality 

of ground-water discharges to the marine 

environment.

Coastal Environments

In summary, we have seen that changes to surface-water bodies in response to ground-water 

pumping commonly are subtle and may occur over long periods of time. The cumulative effects of 

pumping can cause significant and unanticipated consequences when not properly considered in 

water-management plans. The types of water bodies that can be affected are highly varied, as are 

the potential effects.
45

EFFECTS OF GROUND-WATER 

DEVELOPMENT ON GROUND-WATER 

STORAGE

Previous chapters have discussed the 

“ground-water-flow system,” including recharge 

and subsequent flow of ground water through the 

system to areas of discharge, primarily bodies of 

surface water. In this context, the ground-waterflow system functions as a conduit that transports 

water, sometimes over considerable distances 

(miles, tens of miles), from areas of recharge 

to areas of discharge. In this chapter, the focus 

changes from the dynamic aspect of the groundwater-flow system to another aspect—the fact 

that the flowing ground water in the system 

represents a large, sometimes huge, volume of 

water in storage. In this context, it is appropriate 

to change terminology from “ground-water-flow 

system” to “ground-water reservoir,” which 

emphasizes the storage aspect of ground-water 

systems.

A key feature of some aquifers and ground-water 

systems is the large volume of ground water in 

storage, which allows the possibility of using 

aquifers for temporary storage, that is, managing 

inflow and outflow of ground water in storage 

in a manner similar to surface-water reservoirs.

Storage Changes

A change in the water level of any well 

(change in head) is a measure of a change in 

storage in the ground-water reservoir in the neighborhood of the open interval of the well. Thus, a 

rising water level in a well represents an increase 

in storage and a declining water level represents a 

decrease in storage in the ground-water reservoir. 

This situation is analogous to changes in water 

level in surface-water reservoirs. However, the 

relation between changes in water levels in 

wells and changes in the volume of water in 

storage is considerably more complex in groundwater reservoirs than in surface-water reservoirs 

(see Box A).

Even in aquifers and parts of aquifers that 

are not stressed by pumping wells, water levels 

in wells change continuously in response to 

changes in natural rates of recharge and discharge 

in the ground-water-flow system. Water levels 

in many wells exhibit an approximate annual 

cycle—water levels are highest during months 

of highest recharge, commonly the spring of 

the year, and lowest during months of lowest 

recharge, commonly the summer and early fall. In 

addition, large changes in recharge and discharge 

occur from year to year, which results in a potentially significant rise and decline in water levels 

during wet and dry years, respectively (Figure 17).
46

MAR

JUNE

SEPT

DEC

MAR

JUNE

SEPT

DEC

MAR

JUNE

SEPT

DEC

MAR

JUNE

SEPT

DEC

MAR

JUNE

SEPT

DEC

MAR

JUNE

SEPT

DEC

MAR

JUNE

SEPT

DEC

MAR

JUNE

SEPT

DEC

MAR

JUNE

SEPT

DEC

MAR

JUNE

SEPT

DEC

MAR

JUNE

SEPT

DEC

1940

5

10

15

20

25

41

42

43

44

45

46

47

1941 1942 1943 1944 1945 1946 1947 1948 1949 1950

MAR

JUNE

SEPT

DEC

MAR

JUNE

SEPT

DEC

MAR

JUNE

SEPT

DEC

MAR

JUNE

SEPT

DEC

MAR

JUNE

SEPT

DEC

MAR

JUNE

SEPT

DEC

MAR

JUNE

SEPT

DEC

MAR

JUNE

SEPT

DEC

MAR

JUNE

SEPT

DEC

MAR

JUNE

SEPT

DEC

MAR

JUNE

SEPT

DEC

1940 1941 1942 1943 1944 1945 1946 1947 1948 1949 1950

STREAM DISCHARGE, IN CUBIC FEET PER SECOND GROUND-WATER LEVELS, IN FEET ABOVE SEA LEVEL

QUEENS

COUNTY

KINGS

COUNTY

NASSAU

COUNTY

SUFFOLK COUNTY

74°

73°

72°

41° 41°

72°

73°

74° 0

0 10 20 30 KILOMETERS

10 20 30 MILES

EXPLANATION

Well

Streamflow-gaging station

Composite of three wells

 Composite of three streams

LONG ISLAND

A

B

Figure 17. Composite average monthly (A) ground-water levels in selected wells and (B) discharge of 

selected streams in Nassau County, Long Island, New York for the period 1940-50. (Modified from Franke 

and McClymonds, 1972.)

The highly transmissive surficial deposits of sand and gravel, low relief, and humid climate of Long 

Island create ideal conditions for good hydraulic connection between the unconfined aquifer and numerous 

small streams. Before development more than 90 percent of total streamflow was derived from groundwater inflow; thus, these streams have been described as “ground-water drains.” The good correspondence 

between ground-water levels in the unconfined aquifer and flow in nearby streams reflects the fact that in 

this ground-water system most of the streamflow is derived from ground water and there is good connection between the two systems.
47

Declines in heads and associated reductions 

in storage in response to pumping can be large 

compared to changes in unstressed ground-water 

systems. For example, declines in heads as a result 

of intense pumping can reach several hundred 

feet in some hydrogeologic settings. Widespread 

pumping that is sufficient to cause regional 

declines in heads can result in several unwanted 

effects. For example: (1) regional declines in heads 

may represent large decreases in aquifer storage, 

particularly in unconfined aquifers; (2) some 

wells may become dry because the lower heads 

are below the screened or open intervals of these 

wells; (3) pumping costs will increase because 

the vertical distance that ground water must be 

lifted to the land surface increases; (4) locally, the 

rate of movement of contaminated ground water 

and the likelihood that the contaminated ground 

water will be intercepted by a pumping well are 

increased; and (5) pumping of ground water may 

result in land subsidence or intrusion of saline 

ground water in some hydrogeologic settings. 

Because large and widespread changes in heads 

in aquifers are of interest to water managers and 

users of the ground-water resource, maps of heads 

(water levels) often are prepared periodically for 

individual, heavily pumped aquifers by water 

agencies. Comparisons of these synoptic-head 

maps permit changes in ground-water levels in 

wells to be documented through time for individual aquifers. Such histories of head change 

sometimes serve as the basis and catalyst for initiatives to manage the affected ground-water reservoir. The following examples illustrate aquifer 

response to pumping and associated changes in 

storage in different environmental settings.

High Plains aquifer—Let’s first return to 

a previous example, the High Plains aquifer 

(see section on “Ground-Water Development, 

Sustainability, and Water Budgets”). Groundwater pumping from this unconfined aquifer has 

resulted in the largest decrease in storage of any 

major aquifer in the Nation. In parts of the central 

and southern High Plains, more than 50 percent of 

the predevelopment saturated thickness has been 

dewatered (see Figure 10B). The water table 

continues to decline under much of the High 

Plains. During the past two decades, however, 

monitoring of water levels in wells indicates an 

overall reduced rate of decline of the water table 

(McGuire and Sharpe, 1997). This change is attributed to improved irrigation and cultivation practices, decreases in irrigated acreage, and above 

normal precipitation during this period. In parts 

of the High Plains, water-level rises have occurred 

because of seepage losses from surface-water irrigation or the recovery of local cones of depression 

as a result of decreased pumpage.

49

The actual situation lies somewhere between these 

two analogies. Water pumped by a well is withdrawn from the 

aquifer within the vicinity of the well; however, the cumulative 

effects of pumping many neighboring wells over many years 

result in regional water-level declines. These regional declines 

in water levels limit the influence that an individual irrigator 

has over his or her own future pumping lifts. The local effects 

of changes in the magnitude and direction of gradients in the 

water table also must be considered. That is, the net effects 

of the reduced pumping from a well will be distributed over an 

area that increases with time and that may not be limited to 

the well owner’s property. The end result is that the decreases 

in water withdrawals do not result in an equivalent future 

increase in ground-water storage directly under the pumped 

area.

A hypothetical pumping scenario illustrates these 

effects (Alley and Schefter, 1987). In this scenario, pumping 

is reduced from a base level by 25 percent for 10 years within 

management areas of various sizes in the High Plains. The 

effect of the reduced pumping was simulated for 10 years and 

20 years from the start of the water-conservation program; 

that is, at the end of 10 years of conservation and at 10 years 

after conservation ended. The results, shown in Figure D–1 

for both time periods, demonstrate that individual irrigators 

operating at the local scale have limited ability to “bank” water 

to decrease their future pumping lifts, but that opportunity 

exists at larger management scales to effectively reduce 

future pumping lifts. For example, after 20 years much less 

than 10 percent of the conserved water remains under areas 

of a few square miles or less (the size of many farms); but this 

increases to about 60 percent for areas of 250 square miles 

and to more than 85 percent for management areas in excess 

of 1,000 square miles. The results illustrate the potential 

usefulness of ground-water management areas and provide 

some insight into the effectiveness of different size areas.

100

0 250 500 750 1,000 1,250 1,500 1,750 2,000 2,250 2,500

80

60

40

20

0

MANAGEMENT AREA, IN SQUARE MILES

PERCENTAGE OF CONSERVED WATER

REMAINING UNDER MANAGEMENT AREA

10 Years

20 Years

Figure D–1. Percentage of water conserved by reduced 

pumping that provides increased ground-water storage under 

hypothetical management areas of different size overlying the 

High Plains aquifer. (Modified from Alley and Schefter, 1987.) 
50

Nassau County, New York—The response of 

an unconfined aquifer to stress in a humid climate 

that results from urban land-use practices is exemplified by the upper glacial aquifer on Long Island. 

It was noted previously that prior to installation 

of an extensive sewer system, a large proportion 

of the water pumped on Long Island for public 

supply and commercial use was returned to this 

unconfined aquifer by septic systems. After installation, water that formerly recharged the upper 

glacial aquifer from septic systems now was 

discharged directly into the ocean. This loss of 

recharge represented a significant change to the 

water budget of the ground-water system and 

resulted in a loss in storage of water in the upper 

glacial aquifer.

The effects of installation of the sewer 

system on aquifer storage in Long Island are 

reflected in the water-level record shown in 

Figure 18 for a well completed in the upper glacial 

aquifer in west-central Nassau County, an area 

where an extensive sewer system began operation 

in the early 1950’s. The upper horizontal line in 

Figure 18 (water level equals 68 feet above sea 

QUEENS

COUNTY

KINGS

COUNTY

NASSAU

COUNTY

SUFFOLK COUNTY

74°

73°

72°

41° 41°

72°

73°

74°

Estimated average ground-water level

before construction of the sewer system

Estimated average groundwater level after hydrologic

system has adjusted to the

effects of the sewer system

46

1940 1945 1950 1955 1960 1965 1970

WATER LEVEL, IN FEET ABOVE SEA LEVEL

1975 1980 1985 1990 1995 2000

48

50

52

54

56

58

60

62

64

66

68

70

72

74

Sewered area

Well

YEAR

LONG ISLAND

0

0 10 20 30 KILOMETERS

10 20 30 MILES

Figure 18. Water-level record for a well completed in the upper glacial aquifer in 

west-central Nassau County, Long Island, New York.
51

level) represents an average water-table altitude 

at the well before installation of the sewer system. 

The fluctuations in water level around the average 

value represent a response to the annual cycle of 

recharge and evapotranspiration and the differences in this cycle from year to year. The sewer 

system achieved close to its maximum discharge 

by the mid-1960’s for the existing population in 

the sewered area. The lower horizontal line (water 

level equals 54 feet above sea level) represents the 

average water level after the hydrologic system 

had adjusted to the effects of installation of the 

sewer system. The water-level fluctuations around 

the lower horizontal line again reflect annual 

recharge and evapotranspiration cycles.

Installation of the sewer system has resulted 

in an areally extensive (several tens of square 

miles) loss of storage in the unconfined aquifer. 

The most obvious undesirable effect of the 

lowered water-table elevations has been marked 

decreases in the flow and length of small, groundwater-fed streams in the area. The positive effect 

of installing the sewer system has been to reduce 

recharge of contaminated water from septic 

systems and thereby help maintain the quality 

of shallow ground water and the deeper ground 

water that is hydraulically connected to the 

shallow ground water.

Chicago/Milwaukee area—The long history 

of ground-water withdrawals from the CambrianOrdovician aquifer in the Chicago and Milwaukee 

areas is a well-documented example of the effects 

of heavy pumpage on heads in a confined aquifer. 

The first documented deep well in the Chicago 

area was drilled in 1864 to a depth of 711 feet and 

flowed at the land surface at a rate of 400 gallons 

per minute. During the next decades and into the 

20th century as the Chicago metropolitan area 

grew, the number of wells, the areal extent of 

pumping, and the total withdrawals from this 

aquifer increased substantially. Maximum withdrawals, about 180 million gallons per day, and 

maximum declines in heads of about 800 feet for 

the Cambrian-Ordovician aquifer, occurred in 

the eight-county Chicago area in about 1980 

(Figure 19). Since 1980, many public water 

suppliers in the Chicago area have shifted their 

source of water from ground water to additional 

withdrawals from Lake Michigan. This shift has 

resulted in a significant decrease in total withdrawals from the aquifer and a general recovery 

(increase) of heads in the areas of decreased withdrawal (Figure 20). Pumping continues in all parts 

of the Chicago and Milwaukee area, however, and 

may be increasing in some parts, so that heads in 

some localities may still be decreasing.

Milwaukee

OZAUKEE

FOND DU LAC SHEBOYGAN GREEN

LAKE

MARQUETTE

COLUMBIA WASHINGTON

MILWAUKEE

DODGE

KENOSHA

RACINE

WAUKESHA

WALWORTH

DANE

ROCK

BOONE

WINNEBAGO

OGLE

LEE

BUREAU

McHENRY

KANE

DE KALB

DU PAGE

LAKE

COOK

LAKE

PORTER

JASPER

KANKAKEE

WILL

KENDALL

PUTNAM

LA SALLE GRUNDY

IROQUOIS

NEWTON

WISCONSIN

ILLINOIS

INDIANA

ILLINOIS

GREEN STEPHENSON

JEFFERSON

Chicago

Rock

River

Des

Fox

River

River

River

Plaines Kank ake e

0

Base from U.S. Geological Survey 1:2,000,000 Digital Data

Albers Equal-area Conic projection

Standard parallels 33° and 45°, central meridian –89°

25 50 KILOMETERS

25

41°

42°

43°

89° 88° 87°

0 50 MILES

EXPLANATION

Line of equal water-level decline, 1864–1980—Dashed

 where approximate. Interval, in feet, is variable

Major ground-water divide

LAKE MICHIGAN

700

800

600

700

800

500

300

100

200

200

400

350

375

300

100

50

50

50

100

500

600

300

400

500

200

50

100

100

100

100

Figure 19. Decline in heads (water levels) in the 

Cambrian-Ordovician confined aquifer, Chicago and 

Milwaukee areas, 1864-1980. (Modified from Avery, 

1995.)
52

The volume of the cone of depression in the 

Chicago and Milwaukee area is large, even with 

the present decrease in withdrawal rates. A principal concern has been the possibility of beginning 

to dewater the confined aquifer and effectively 

convert it to an unconfined aquifer. This possibility 

was imminent at the center of the cone of depression in 1980 and was avoided by the subsequent 

decrease in withdrawal rates in this critical area.

The sustainability of confined aquifer 

systems like the Cambrian-Ordovician aquifer is 

typically controlled by the proximity of pumping 

centers to recharge and discharge areas or by the 

hydraulic connection with other aquifer systems. 

Walton (1964) defined the “practical sustained 

yield” of the Cambrian-Ordovician aquifer as 

the maximum amount of water that can be continuously withdrawn from existing pumping centers 

without eventually dewatering the most productive water-yielding formation. Using this definition, Walton estimated the practical sustained 

yield to be about 46 million gallons per day. 

He noted that with the existing distribution of 

pumping centers, the practical sustained yield 

was limited not by the rate of replenishment 

in recharge areas but by the rate at which water 

can move eastward through the aquifer from 

recharge areas. Walton estimated that the practical 

sustained yield could be increased more than 

40 percent to about 65 million gallons per day 

by (1) increasing the number of pumping centers, 

(2) shifting centers of pumping toward the 

recharge area, and (3) spacing wells at greater 

distances.

In all but the deeply buried parts of the 

Cambrian-Ordovician aquifer in the Chicago and 

Milwaukee area, the water is chemically suited for 

all uses. Thus, water quality has not been a major 

factor affecting the use of this aquifer. Because of 

their greater depth, however, confined aquifers 

often contain saline water or are hydraulically 

connected to other aquifers and confining units 

that contain water with high dissolved-solids 

concentration. Declines in head in the confined 

aquifer can cause the movement of poor quality 

water from surrounding aquifers (or confining 

units), which may limit development of the 

aquifer more than declines in heads and aquifer 

storage.

1940 –50

0

50

100

150

200

250

1950 1960 1970

YEAR

WATER LEVEL, IN FEET ABOVE SEA LEVEL

1980 1990 2000

Figure 20. Representative trend of water 

levels for a deep well in Cook County, Chicago 

area, since 1940. (From Visocky, 1997.)
53

Kings County, New York—The history 

of ground-water development in Kings County 

(Brooklyn), Long Island, New York since the 

early 1900’s is a well-documented example of a 

complete cycle of intensive development with 

significant decreases in heads and reduction in 

storage in the unconfined aquifer accompanied 

by intrusion of saline ground water, followed by a 

decrease in total pumpage and a gradual recovery 

of heads. In 1903, total ground-water withdrawals 

in Kings County were about 30 million gallons per 

day. Available information on the altitude of the 

water table indicates no obvious cones of depression at this time (Figure 21). Total pumpage in 

Kings County peaked in the 1920’s to early 1940’s 

(maximum annual pumpage about 75 million 

gallons per day). As shown in Figure 21, water 

levels in 1936 were near or below sea level 

throughout Kings County, and the cone of depression extended into southwestern Queens County. 

In 1947, public-supply pumpage ceased 

in Kings County. The source of water for public 

supply changed to the upstate surface-water 

system that supplies New York City through 

water tunnels. Furthermore, legislation was 

implemented during this period that required 

“wastewater” (including air-conditioning water) 

from some industrial/commercial uses be 

recharged to the aquifer system through wells. 

Concurrently, and partly as a result of these 

changes, industrial pumpage declined to a longterm stable rate of slightly less than 10 million 

gallons per day. These changes are reflected in 

the water-table map of 1965 shown in Figure 21 

in which heads have risen throughout Kings 

County and are at or below sea level only in 

northern parts of the county. Subsequent maps 

show a small but continuing recovery of the 

water table. 

KINGS COUNTY

QUEENS COUNTY

MANHATTAN

KINGS COUNTY

QUEENS COUNTY

MANHATTAN

KINGS COUNTY

QUEENS COUNTY

MANHATTAN

ATLANTIC OCEAN

ATLANTIC OCEAN

ATLANTIC OCEAN

0 1 2 3 4 KILOMETERS

0 2 3 4 MILES 1

1903 1936 1965

EXPLANATION

5

–25

–35

–15

–5

–5

0

0

5

15

5

5

35

25

15

15

0

0

5

5

10

15

20

–5

Water-level contour—

 Shows altitude of water

 level. Contour intervals

 5 and 10 feet. Datum is

 sea level

5 N

QUEENS

COUNTY

KINGS

COUNTY

NASSAU

COUNTY

SUFFOLK COUNTY

74°

73°

72°

41° 41°

72°

73°

74°

LONG ISLAND

Figure 21. Water-table altitudes in Kings and part of Queens Counties, Long Island, New York in 1903, 

1936, and 1965. (Modified from Franke and McClymonds, 1972.)
54

The history of ground-water development 

in Kings County has been influenced considerably 

by the strong hydraulic connection between 

the unconfined ground-water system and the 

surrounding bodies of saline surface water. The 

decision to stop pumping for public supply and 

to recharge high-quality wastewater back to the 

aquifer system was driven in large measure by 

concerns about ongoing and continuing intrusion 

of saline ground water into the naturally fresh part 

of the aquifer system. On the other hand, an unforeseen and undesirable effect of decreased pumpage 

and the accompanying rise in the altitude of the 

water table in Kings County is that basements of 

major buildings constructed in the 1920’s and 

1930’s now lie below the water table and require 

continuous pumping of dewatering systems to 

keep them dry.

Commonalities in the preceding four examples are noteworthy and include (1) the changes 

in storage resulted in observable changes in the 

ground-water system; (2) the changes in the 

ground-water system generally were viewed by 

local stakeholders as undesirable, at least to some 

extent; and (3) in at least three of the four examples, some response to mitigate the perceived 

undesirable effects of the change in storage was 

initiated. In examples such as the southern High 

Plains aquifer in Texas and New Mexico, and the 

unconfined aquifer in Brooklyn, New York, the 

long-term sustainability of the ground-water 

resource was perceived to be in jeopardy.
55

Land subsidence, which is a decline in 

land-surface elevation caused by removal of 

subsurface support, can result from a variety of 

human activities (Galloway and others, in press). 

Subsidence can severely damage structures such 

as wells, buildings, and highways, and creates 

problems in the design and operation of facilities 

for drainage, flood protection, and water conveyance. Human activities related to ground water 

cause land subsidence by three basic mechanisms: 

compaction of aquifer systems, dissolution 

and collapse of rocks that are relatively soluble 

in water (for example, limestone, dolomite, 

and evaporites such as salt and gypsum), and 

dewatering of organic soils.

Compaction of aquifer systems as a result 

of ground-water withdrawals and accompanying 

land subsidence is most common in heavily 

pumped alluvial aquifer systems that include 

clay and silt layers. As heads in the aquifer system 

decline due to pumping, some of the support 

for the overlying material previously provided 

by the pressurized water filling the sediment 

pore space shifts to the granular skeleton of the 

aquifer system, increasing the intergranular 

pressure (load). Because sand and gravel deposits 

are relatively incompressible, the increased intergranular load has a negligible effect on these 

aquifer materials. However, clay and silt layers 

comprising confining units and interbeds can be 

very compressible as water is squeezed from these 

layers in response to the hydraulic gradient caused 

by pumping.

So long as the intergranular load remains 

less than any previous maximum load, the 

deformation of the aquifer system is reversible. 

However, when long-term declines in head 

increase the intergranular load beyond the 

previous maximum load, the structure of clay and 

silt layers may undergo significant rearrangement, 

resulting in irreversible aquifer system compaction 

and land subsidence. The amount of compaction is 

a function of the thickness and vertical hydraulic 

conductivity of the clay and silt layers, and the 

type and structure of the clays and silts. Because 

of the low hydraulic conductivity of clay and silt 

layers, the compaction of these layers can continue 

for months or years after water levels stabilize in 

the aquifer. In confined aquifer systems that 

contain significant clay and silt layers and are 

subject to large-scale ground-water withdrawals, 

the volume of water derived from irreversible 

compaction commonly can range from 10 to 

30 percent of the total volume of water pumped 

(Galloway and others, in press). This represents 

a one-time mining of stored ground water and 

a permanent reduction in the storage capacity 

of the aquifer system.

The first recognized land subsidence in 

the United States from aquifer compaction as a 

response to ground-water withdrawals was in the 

area now known as “Silicon Valley” in California. 

Other areas experiencing significant land subsidence from ground-water withdrawals include 

the San Joaquin Valley of California (see Box B), 

the alluvial basins of south-central Arizona 

(Figure 22), Las Vegas Valley in Nevada, the 

Houston-Galveston area of Texas, and the 

Lancaster area near Los Angeles, California.

Subsidence
56

LAND SUBSIDENCE,

1952–1992, IN FEET

Phoenix

Tucson

Eloy

Stanfield

Queen Creek

Mesa Apache Junction

Casa Grande

Luke Air

Force Base

Areas affected by 

 subsidence

Water-level decline 

 greater than 100 feet

ARIZONA

0

10

20

Harquahala

 Plain West Salt 

River Basin

San Simon 

Valley Basin

Willcox

Basin

Avra

Valley

Picacho Basin

Tucson

Basin

Stanfield

Basin

East Salt River Basin

0

0 50 KILOMETERS

50 MILES

MAP AREA

EXPLANATION

Figure 22. Land subsidence in south-central Arizona. (Modified from Carpenter, in press.)

Ground-water development for agriculture in the basin-fill aquifers of south-central Arizona began in the late 

1800’s, and by the 1940’s many of the basins had undergone intensive ground-water development. Ground-water 

depletion has been widespread over these basins, and locally, water-level declines have exceeded 300 feet. These 

water-level declines have resulted in regional subsidence, exceeding 10 feet in some areas. A profile near Luke Air 

Force Base illustrates that subsidence is greater near the center of basins, where the aggregate thickness of the finegrained sediments is generally greater. In conjunction with widespread subsidence, numerous earth fissures have 

formed at and near the margins of subsiding basins or near exposed or shallow buried bedrock.

In many areas of the arid Southwest, earth 

fissures are associated with land subsidence. Earth 

fissures are caused by horizontal movement of 

sediment that occurs during compaction. These 

features start out as narrow cracks, an inch or less 

in width. They intercept surface drainage and can 

erode to widths of tens of feet at the surface and 

may extend more than 100 feet below the land 

surface. Fissures may be a few hundred feet to 

miles in length. One extraordinary fissure in the 

Picacho Basin, northwest of Tucson, Arizona, is 

10 miles long.
57

Sign warning motorists of subsidence hazard was erected after an earth fissure damaged a road 

in Pima County, Arizona (left photograph). Earth fissure near Picacho, Arizona (right photograph). 

(Photographs by S.R. Anderson, U.S. Geological Survey.)
58

Subsidence also occurs from local sinkhole 

collapse in areas underlain by limestone, dolomite, 

and other soluble rocks. Areas susceptible to sinkhole collapse are particularly common in the 

humid Eastern United States. Sinkhole development occurs naturally but may be enhanced by 

human activities, such as diversion and impoundment of surface water and pumping of ground 

water. Ground-water pumping can induce sinkholes by reducing the buoyant support of cavity 

walls and ceilings or by reducing the cohesion 

of loose, unconsolidated materials overlying 

preexisting sinkholes. The effects of ground-water 

pumping on sinkhole development can result 

from long-term declines in water levels or in 

response to rapid fluctuations of water levels 

caused by pumping wells. Some notable examples 

of rapid sinkhole development have occurred 

in the Southeastern United States. Though the 

collapse features tend to be highly localized, their 

effects can extend well beyond the collapse zone as 

a result of the introduction of contaminants from 

the land surface to the ground-water systems.

Finally, land subsidence can occur when 

organic soils are drained for agriculture or other 

purposes. Causes include compaction, desiccation, 

wind erosion, and oxidation of drained organic 

soil layers. These effects commonly are associated 

with the purposeful draining of the land surface 

but also may occur as a result of ground-water 

pumping near wetlands and other poorly drained 

areas. Subsidence at rates of an inch or more per 

year as a result of drainage has been observed over 

large areas such as the Sacramento-San Joaquin 

Delta in California and the Florida Everglades 

(Galloway and others, in press).

Development of a new irrigation well in west-central Florida 

triggered hundreds of sinkholes over a 20-acre area. The sinkholes ranged in size from less than 1 foot to more than 150 feet 

in diameter. (Photograph by Ann B. Tihansky, U.S. Geological 

Survey; see person in center for scale.)
59

Previous chapters have discussed quantities of water recharging, flowing through, and 

discharging from the ground-water system and 

quantities of water stored in the system. This brief 

discussion of ground-water quality adds a further 

dimension to ground-water resource sustainability; namely, the question of the suitability of 

ground water for different uses. Various measures 

of water quality such as taste and odor, microbial 

content, and dissolved concentrations of naturally 

occurring and manufactured chemical constituents 

define the suitability of water for different uses. 

The availability of ground water and the 

suitability of its quality for different uses are 

inextricably intertwined. To take an extreme 

example, salt brines having very high dissolvedsolids concentrations occur adjacent to fresh 

ground water almost everywhere. Although 

brines represent huge volumes of ground water 

in storage, these brines are not included in most 

inventories of available ground water because 

of their inherent unsuitability for almost all 

uses. Ground waters having somewhat lower 

dissolved-solids concentrations may be suitable 

for some uses but not for others. For example, 

some cattle can tolerate a higher dissolved-solids 

concentration in their drinking water than 

humans.

A key consideration in managing a groundwater resource is its vulnerability to sources of 

contamination that are located primarily at and 

near the land surface. Because of generally low 

ground-water velocities, once contaminants have 

reached the water table, their movement to nearby 

surface-water discharge areas or to deeper parts of 

the ground-water-flow system is slow. For the same 

reason, once parts of an aquifer are contaminated, 

the time required for a return to better waterquality conditions as a result of natural processes is 

long, even after the original sources of contamination are no longer active. Ground-water-quality 

remediation projects generally are very expensive 

and commonly are only partly successful. In some 

settings, steep gradients caused by ground-water 

pumping can greatly increase the rate at which 

contaminants move to deeper ground water. For 

these reasons, State and Federal environmental 

WATER-QUALITY FACTORS AFFECTING 

GROUND-WATER SUSTAINABILITY

The availability of ground water and the 

suitability of its quality for different uses 

are inextricably intertwined.
60

agencies seek to protect the ground-water resource 

by stressing regulatory efforts to prevent groundwater contamination.

Contamination of ground water is not 

always a result of the introduction of contaminants 

by human activities. Possible natural contaminants include trace elements such as arsenic and 

selenium, radionuclides such as radon, and high 

concentrations of commonly occurring dissolved 

constituents.

The first two subsections below involve two 

of the most significant linkages in hydrology—the 

land-surface/water-table connection and the 

ground-water/surface-water connection. The third 

subsection, saltwater intrusion, involves movement of naturally occurring, highly saline ground 

water into parts of adjacent aquifers that contain 

less saline water. Pumping of the less saline 

(commonly potable) ground water generally 

causes this movement.

In principle, virtually any human activity 

at and near the land surface can be a source of 

contaminants to ground water as long as water 

and possibly other fluids move from the land 

surface to the water table. Sources of chemicals 

introduced to ground water in this way include 

fertilizers, manure, and pesticides applied to 

agricultural lands; landfills; industrial-discharge 

lagoons; leaking gasoline storage tanks; cesspools 

and septic tanks; and domestically used chemicals. 

These sources commonly are classified as “point”

or “nonpoint” sources. For example, industrial 

lagoons, leaking storage tanks, and landfills are 

considered to be point sources. A considerable 

number of these sources and associated contaminant plumes have undergone intensive studies 

followed by a remediation program. Many of the 

chemicals associated with point sources—for 

example, gasoline and other manufactured organic 

chemicals—even at very low concentrations, 

render the contaminated ground water highly 

undesirable or useless as a source of domestic 

or public supply.

Croplands are a primary nonpoint source 

of contamination because of their large areal 

extent and significant applications per unit area 

of possible contaminants (fertilizers and pesticides) 

to ground water. Irrigated agriculture also has noteworthy effects on ground-water (and surfacewater) quality. Increased areal recharge from excess 

irrigation-water applications results in the potential 

for increased transport of contaminants from 

the land surface to ground water. Also, a marked 

increase in dissolved-solids concentrations in soil 

water and shallow ground water may result from 

evaporation of irrigation water during delivery of 

the water to the crops and from transpiration of the 

applied water by the crops. In addition to cropland, 

agricultural activities include numerous point 

sources such as animal feedlots, waste lagoons, 

and storage sheds for agricultural chemicals.

Land-Surface /Water-Table Connection
61

Although the area occupied by urban land 

is small compared to the total area of the Nation, 

the diverse activities in urban areas provide innumerable point sources of contamination that can 

affect the quality of shallow ground water. From a 

regional perspective, urban land can be considered 

as a nonpoint source that exhibits a wide range in 

water quality. These effects on ground-water 

quality are particularly important from a watermanagement viewpoint if the water-table aquifer 

beneath urban land is used or could be used as a 

source of water supply.

A noteworthy effort to protect ground-water 

quality and the sustainability of the local groundwater resource, specifically to protect the quality 

of ground water that is pumped from publicsupply wells, is the wellhead protection programs 

undertaken by the U.S. Environmental Protection 

Agency and the States. The approach of these 

programs is to estimate areas at the water table 

that contribute recharge to public-supply wells 

(Figure 23) and then to implement ground-water 

protection practices on the overlying land surface. 

Because many uncertainties exist in estimating 

areas contributing recharge to pumping wells 

(particularly for well-screen placements at some 

distance below the water table), and because areas 

contributing recharge may be located a considerable distance from the pumped wells, implementing ground-water protection practices at the 

land surface often poses considerable challenges.

Figure 23. Area contributing recharge to a single 

discharging well in a simplified hypothetical groundwater system: (A) cross-sectional view, and (B) map 

view. (Modified from Reilly and Pollock, 1993.)

The area contributing recharge to a pumping well 

can be defined as the surface area at the water table 

where water entering the ground-water system eventually flows to the well. If the system is at equilibrium, 

this area must provide an amount of recharge that 

balances the amount of water being discharged from the 

well. Thus, lower areal recharge rates result in larger 

contributing areas of wells. If a nearby surface-water 

body also contributes water to the discharging well, 

the area contributing recharge is reduced and is a function not only of the areal recharge rate but also of 

the amount of water obtained from the surface-water 

body. Depending on factors that describe the threedimensional flow system and the placement of the well, 

the area contributing recharge to a well does not necessarily have to include the location of the well itself.

Areal recharge

Discharging 

well

Discharging 

well

Area

contributing

recharge

to well

Area

contributing

recharge

Screened

interval

Water table

Land surface

Bounding

 flow paths

A

B
62

Water table

Unconfined aquifer

Stream

Waste

disposal

basin

Seepage

to stream

Ground-water

recharge

Evapotranspiration

from land surface

and soil zone

Precipitation

EXPLANATION

Approximate direction

 of ground-water

 movement

Not to scale

Contaminant

plume

Water table

The movement of water in both directions 

between ground-water systems and surface-water 

bodies has been discussed previously in this report. 

Chemical constituents are transported along with 

the moving water. Thus, contaminants in surface 

water can be transported into adjacent groundwater systems, and contaminants in ground water 

can be transported into adjacent surface-water 

bodies.

Because ground water commonly is a 

major component of streamflow, the quality of 

discharging ground water potentially can affect the 

quality of the receiving stream in many hydrologic 

settings (Figure 24). Because the proportion of 

streamflow contributed by ground water can vary 

greatly throughout the year, seasonal variations in 

the effects of ground-water quality on stream-water 

quality can occur.

Reductions in the quantity of ground water 

discharged to a stream as a result of pumping may 

have significant consequences where this discharge 

significantly dilutes the concentration of contaminants introduced to streams from point sources and 

surface runoff. In such situations, streamflow 

capture by pumping wells may reduce the contaminant-dilution capacity of the stream during periods 

of low flow below the dilution capacity assumed in 

setting discharge permits for the stream.

Contributing areas to wells often include 

surface-water bodies, and increasing attention is 

being placed on surface water as a potential source 

of contamination to wells. Possible contamination 

by induced infiltration of surface water adds 

several dimensions to the protection of ground 

water. These include consideration of the upstream 

drainage basin as part of the “contributing area” to 

the well and greater consideration of microbial 

contamination. Contaminated surface water may 

have a significant effect on the sustainable development of ground water near streams or on the need 

for treatment of ground water prior to use. Among 

the settings of greatest concern for contamination of 

ground water by streams are karst terrains where 

aquifers are hydraulically connected by sinkholes 

or other conduits that can channel river water 

directly into an aquifer with little or no filtration 

(see Box E).

In many aquifers, large changes in chemical 

oxidation conditions, organic-matter content, and 

microbial activity occur within a relatively thin (a 

few feet or even inches) zone or interface between 

ground water and surface water. Thus, conditions 

near the interface between ground water and 

surface water can significantly affect the transport 

and fate of nutrients, metals, organic compounds, 

and other contaminants between the two resources. 

Reactions at this interface commonly decrease the 

concentrations that might be transported between 

surface water and ground water (Winter and 

others, 1998).

Ground-Water/Surface-Water Connection

Figure 24. Simplified representation of a 

contaminant plume in ground water.

In this hypothetical example, sufficient 

time has elapsed for part of the plume of 

contaminated ground water to reach and 

discharge into a nearby stream. As shown, the 

stream intercepts the plume as it reaches the 

stream. In some situations, depending upon 

the geometry of the ground-water-flow system 

and the location of the plume in the flow 

system, part or all of the plume may flow 

under the stream and contaminate ground 

water on the other side of the stream.
63

E

The Connection Between Surface-Water Quality

and Ground-Water Quality in a Karst Aquifer

The Upper Floridan aquifer, which is the sole source 

of water supply for Valdosta, Georgia, and much of the 

surrounding area, receives large volumes of direct discharge 

from the Withlacoochee River through sinkholes in the 

streambed or off-channel. A highly interconnected conduit 

system has developed in the Upper Floridan aquifer in this 

area, which extends at least 15 miles from the sinkhole area. 

Chloride and isotopic data were used by Plummer and others 

(1998) to map the percentage of Withlacoochee River water 

in ground water in the Upper Floridan aquifer (Figure E–1). 

These data indicate that ground water in parts of the Upper 

Floridan aquifer contains high percentages of recently 

recharged Withlacoochee River water. Plummer and others 

(1998) note that, although the patterns shown in Figure E–1 

are generally true over the area, extreme variations can occur 

at a given location, as would be expected because of the large 

variations and discontinuities in hydraulic properties in the 

karst environment and time-varying inflows of river water into 

the aquifer.

The strong connection between the Withlacoochee 

River and ground water in the Valdosta area has created 

concerns about the potential for contamination of groundwater supplies by contaminants in the river. There also are 

concerns about the effects of natural organic matter in the 

river water. For example, in the early 1980’s, it was recognized that chlorination of aquifer water produced disinfection 

by-products in excess of drinking-water standards. This 

occurred as a result of reaction of chlorine with the high 

amounts of natural organic matter in the river water recharged 

to the aquifer.

The original wells for Valdosta were near the city, in the 

areas where the aquifer contains a high percentage of river 

water. The city completed a new set of water-supply wells 

in the well field indicated in Figure E–1, in an area where 

the aquifer contains a relatively low percentage of river water. 

Even with this added level of assurance, it is still necessary to 

protect the surface waters that supply the aquifer. The source 

area of concern for ground water is the entire Withlacoochee 

River Basin upstream from Valdosta. 

Sinkhole near the Withlacoochee River. 

(Photograph by Richard E. Krause, 

U.S. Geological Survey.)

Withlacoochee R.

Withlacoochee

R.

10

20

10

10

20

20

20

20

20

50

10

10

10

50

Valdosta

Valdosta watersupply well field

Sinkhole

area

Line of equal percentage of

 river water in ground water

EXPLANATION

0

0 5 KILOMETERS

5 MILES

Figure E–1. Estimated percentage of 

Withlacoochee River water in ground water 

in the Upper Floridan aquifer, June 1991. 

(Modified from Plummer 

and others, 1998.)
64

The fresh ground-water resource of the 

United States is surrounded laterally and below 

by saline water. This is most evident along coastal 

areas where the fresh ground-water system comes 

into contact with the oceans, but it is also true in 

much of the interior of the country where deep 

saline water underlies the freshwater. The fresh 

ground-water resource being surrounded by saltwater is significant because, under some circumstances, the saltwater can move (or intrude) into 

the fresh ground-water system, making the water 

unpotable.

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, as shown 

in Figure 25A. The boundary typically is not sharp 

and distinct as shown in Figure 25A, but rather is a 

gradation from fresh to saline water known as the 

zone of diffusion, zone of dispersion, or the transition zone. 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. That is, any pumpage 

will cause some movement in the boundary 

between the freshwater and the surrounding 

saltwater. 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 determines how 

quickly and by how much 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 (Figure 25B).

Saltwater Intrusion

Land surface

Water table

Water table

Fresh ground water

Fresh ground water

Ocean

Pumped well

Well discharge 

Land surface

Saline ground water

Saline ground water

A 

B 

Figure 25. Relation of fresh and saline ground water. 

(Modified from U.S. Geological Survey, 1984.)

(A) In coastal areas, fresh ground water discharges to 

the surrounding saline surface-water bodies by flowing 

over the denser saline ground water. (B) In both coastal 

and inland areas, large drawdowns in an individual well 

can cause underlying saline water to migrate upward 

into the well and cause contamination of the water being 

discharged. 
65

In 1969, the Task Committee on Saltwater 

Intrusion of the American Society of Civil 

Engineers (1969) indicated that saltwater intrusion of some type is an existing problem in 

nearly every State. Examples of saltwater intrusion are especially numerous along the coasts 

(U.S. Geological Survey, 1984). Some prominent 

examples follow.

Los Angeles and Orange Counties in 

California operate artificial-recharge programs 

to control saltwater intrusion caused by groundwater withdrawals. In Hawaii, several aquifers 

susceptible to saltwater intrusion underlie the 

island of Oahu. In Florida, saltwater intrusion 

occurs in the Jacksonville, Tampa, and Miami 

areas. Farther north on the Atlantic Coast, 

saltwater intrusion is occurring near Brunswick 

and Savannah, Georgia, and on Hilton Head 

Island, South Carolina. In New Jersey, aquifers 

underlying parts of Atlantic, Gloucester, 

Monmouth, Cape May, Ocean, and Salem 

Counties are being affected by saltwater intrusion. The threat of saltwater intrusion is always 

present on Long Island, New York, and Cape Cod, 

Massachusetts, because saltwater bodies surround 

both localities. A specific example of saltwater 

intrusion into the Old Bridge aquifer of New 

Jersey (Schaefer and Walker, 1981) is shown in 

Figure 26.

35

35

36

Raritan Bay

Keyport

Harbor

0

0

1 MILE

1 KILOMETER

1

10

100

19501955

1960

1965

197019751977

1,000

CHLORIDE CONCENTRATION,

IN MILLIGRAMS PER LITER

A

B

– 20

250

10

– 30

– 40

EXPLANATION

Line of equal head, 1977—Contour interval

 10 feet. Datum is sea level

Line of equal chloride concentration, in

 milligrams per liter, 1977—Contour interval is variable

Generalized direction of ground-water flow

10

– 40

N

Union Beach

well field

Figure 26. Saltwater intrusion into the Old Bridge 

aquifer, New Jersey. (Modified from Schaefer and 

Walker, 1981.)

(A) A composite graph of chloride concentration 

in water samples from wells screened at about the same 

depth in the Union Beach Borough well field. Chloride 

concentration in water samples from the Union Beach 

well field increased significantly above background 

levels beginning in about 1970 and increased steadily 

after that time. (B) As pumping in the area caused 

water levels to decline below sea level, saline ground 

water moved landward and caused the increase in 

chloride (and dissolved solids) in wells near the shore. 

Because of the increasing chloride and dissolved solids, 

pumpage was curtailed in the 1980’s, and the well field 

was abandoned in the early 1990’s and replaced by 

wells farther inland.
66

An inland area where saltwater intrusion is 

an important issue is the Mississippi River alluvial 

plain in Arkansas. For example, ground-water 

withdrawals from the alluvial aquifer for irrigation near Brinkley, Arkansas, have caused upward 

movement of saline water from the underlying 

Sparta aquifer into the alluvial aquifer (Morris and 

Bush, 1986). A confining unit separating the aquifers is discontinuous, and the intrusion appears to 

occur mainly where the confining unit is absent.

Many of the deeper aquifers in the central 

part of the United States contain saline water. 

Withdrawals from the overlying aquifers in these 

areas increase the potential for saltwater intrusion 

from below.

In summary, the intrusion of saltwater or 

mixing of fresh ground water with the surrounding 

saltwater, caused by withdrawals of freshwater 

from the ground-water system, can make the 

resource unsuitable for use. Thus, ground-water 

development plans should take into account potential changes in water quality that might occur 

because of saltwater intrusion.
67

As we have seen, the sustainability of 

ground-water resources is a function of many 

factors, including decreases in ground-water 

storage, reductions in streamflow and lake levels, 

loss of wetland and riparian ecosystems, land 

subsidence, saltwater intrusion, and changes in 

ground-water quality. Each ground-water system 

and development situation is unique and requires 

an analysis adjusted to the nature of the water 

issues faced, including the social, economic, and 

legal constraints that must be taken into account. 

A key challenge for achieving ground-water 

sustainability is to frame the hydrologic implications of various alternative management strategies 

in such a way that they can be properly evaluated. 

Ground-water scientists have developed an 

expanding capability to address issues associated 

with the development and sustainability of 

ground-water resources. Early efforts focused on 

methods of evaluating the effects of ground-water 

pumping on an aquifer’s long-term capacity to 

yield water to wells. Subsequently, methods 

were applied to evaluate various effects of groundwater development on surface-water bodies, land 

subsidence, and saltwater intrusion. Starting in the 

late 1970’s, increasing concerns about contamination of ground water by human activities led to an 

awareness of the great difficulty and expense of 

cleaning up contaminated aquifers and drew 

attention to the importance of prevention of 

ground-water contamination. With time, it has 

become clear that the chemical, biological, and 

physical aspects of ground-water systems are 

interrelated and require an integrated analysis, 

and that many issues involving the quantity, 

quality, and ecological aspects of surface water 

are interrelated with ground water. Thus, groundwater hydrologists are challenged continually by 

the need to provide greater refinement to their 

analyses and to address new problems and issues 

as they arise.

MEETING THE CHALLENGES OF 

GROUND-WATER SUSTAINABILITY

A key challenge for achieving ground-water 

sustainability is to frame the hydrologic 

implications of various alternative management 

strategies in such a way that they can be 

properly evaluated.
68

The foundation of any good ground-water 

analysis, including those analyses whose objective 

is to propose and evaluate alternative management strategies, is the availability of high-quality 

data. Principal types of data commonly required 

are listed in Table 2. Some, such as precipitation 

data, are generally available and relatively easy to 

obtain at the time of a hydrologic analysis. Other 

data and information, such as geologic and hydrogeologic maps, can require years to develop. Still 

other data, such as a history of water levels in 

different parts of ground-water systems, require 

foresight in order to obtain measurements over 

time, if they are to be available at all. Thus, a key 

starting point for assuring a sustainable future 

for any ground-water system is development of a 

comprehensive hydrogeologic data base over time. 

As examples, these data would include depths and 

thicknesses of hydrogeologic units from lithologic 

and geophysical well logs, water-level measurements to allow construction of predevelopment 

water-level maps for major aquifers as well as 

water-level maps at various times during development, ground-water sampling to document preand post-development water quality, and simultaneous measurements of streamflow and stream 

quality during low flows to indicate possible 

contributions of discharging ground water to 

surface-water quality. Many of the types of data 

and data compilations listed in Table 2 need to 

be viewed on maps. Thus, Geographic Information Systems (GIS) typically are an integral part 

of the data-base system to assist in organizing, 

storing, and displaying the substantial array of 

needed information.

The Importance of Ground-Water Data

The foundation of any good ground-water analysis, 

including those analyses whose objective is to 

propose and evaluate alternative management 

strategies, is the availability of high-quality data.
69

Table 2.—Principal types of data and data compilations required for analysis of ground-water systems

Physical Framework

Topographic maps showing the stream drainage network, surface-water bodies, landforms, cultural features, and locations of 

structures and activities related to water

Geologic maps of surficial deposits and bedrock

Hydrogeologic maps showing extent and boundaries of aquifers and confining units

Maps of tops and bottoms of aquifers and confining units

Saturated-thickness maps of unconfined (water-table) and confined aquifers

Average hydraulic conductivity maps for aquifers and confining units and transmissivity maps for aquifers

Maps showing variations in storage coefficient for aquifers

Estimates of age of ground water at selected locations in aquifers

Hydrologic Budgets and Stresses

Precipitation data

Evaporation data

Streamflow data, including measurements of gain and loss of streamflow between gaging stations

Maps of the stream drainage network showing extent of normally perennial flow, normally dry channels, and normally 

seasonal flow

Estimates of total ground-water discharge to streams

Measurements of spring discharge

Measurements of surface-water diversions and return flows

Quantities and locations of interbasin diversions

History and spatial distribution of pumping rates in aquifers

Amount of ground water consumed for each type of use and spatial distribution of return flows

Well hydrographs and historical head (water-level) maps for aquifers

Location of recharge areas (areal recharge from precipitation, losing streams, irrigated areas, recharge basins, and recharge 

wells), and estimates of recharge

Chemical Framework

Geochemical characteristics of earth materials and naturally occurring ground water in aquifers and confining units

Spatial distribution of water quality in aquifers, both areally and with depth

Temporal changes in water quality, particularly for contaminated or potentially vulnerable unconfined aquifers

Sources and types of potential contaminants

Chemical characteristics of artificially introduced waters or waste liquids

Maps of land cover/land use at different scales, depending on study needs

Streamflow quality (water-quality sampling in space and time), particularly during periods of low flow
70

During the past several decades, computer 

simulation models for analyzing flow and solute 

transport in ground-water and surface-water 

systems have played an increasing role in the evaluation of alternative approaches to ground-water 

development and management. The use of these 

models has somewhat paralleled advances in 

computing systems. Ground-water models are an 

attempt to represent the essential features of the 

actual ground-water system by means of a mathematical counterpart. The underlying philosophy is 

that an understanding of the basic laws of physics, 

chemistry, and biology that describe ground-water 

flow and transport and an accurate description 

of the specific system under study will enable a 

quantitative representation of the cause and effect 

relationships for that system. Quantitative understanding of cause and effect relationships enables 

forecasts to be made for any defined set of conditions. However, such forecasts, which usually are 

outside the range of observed conditions, typically 

are limited by uncertainties due to sparse and 

inaccurate data, poor definition of stresses acting 

on the system, and errors in system conceptualization (Konikow and Bredehoeft, 1992). Although 

forecasts of future events that are based on model 

simulations are imprecise, they nevertheless may 

represent the best available decision-making information at a given time. Because of the usefulness 

of computer simulation for decision making, the 

basic construction of computer simulation models, 

as well as model forecasts, need to be updated 

periodically as the actual ground-water system 

continues to respond to the physical and chemical 

stresses imposed upon it and as new information 

on the ground-water system becomes available.

Use of Ground-Water Computer Models

Although forecasts of future events that are 

based on model simulations are imprecise, they 

nevertheless may represent the best available 

decision-making information at a given time.
71

F

Refinement of Ground-Water System 

Understanding Through Time: 

Lessons from Post Audits

Computer simulation models of flow and transport 

are a principal means for evaluating the response of aquifer 

systems to ground-water withdrawals and other human activities. There is a tendency to view development of such models 

as a one-time activity. However, if a model is used to address 

questions about the future responses of a ground-water 

system that are of continuing significance to society, then field 

monitoring of the ground-water system should continue and 

the model should be reevaluated periodically to incorporate 

new information or new insights (Konikow and Reilly, 1999). 

For example, it might be desirable to add new capabilities to 

an existing model, such as interactions between ground water 

and surface-water bodies.

Ground-water models commonly are used to make 

forecasts for a decade or more in the future. Confidence in the 

reliability of a ground-water model is dependent in large part 

upon the quality and extent of historical data used to calibrate 

and test the model. In recent years, studies have been made 

of the accuracy of selected model forecasts several years 

after the date for which the forecasts had been made. Such 

studies, commonly referred to as post audits, offer a means 

to evaluate overall performance of a model and the nature and 

magnitude of model forecasting errors. Post audits also 

provide insights into possible future model enhancements.

As an example, a post audit was made for a groundwater model of the Blue River Basin, a heavily irrigated area in 

southeastern Nebraska. Forecasts of water-level declines for 

1982 made by the model in 1965 were compared to measured 

1982 water-level declines as shown in Figure F–1 (Alley and 

Emery, 1986). Overall, the forecasted and measured waterlevel declines were somewhat similar in magnitude, although 

clearly more complexity is shown by the measured water-level 

declines. Further examination during the post audit revealed 

that irrigation demand had been greatly underestimated for 

the forecast period between 1965 and 1982. If the actual 

ground-water withdrawals had been incorporated in the 1965 

model, forecasted water-level drawdowns for 1982 would 

have been much greater than the measured drawdowns in 

1982. Apparently, the aquifer storage coefficient used in the 

1965 model was too low, and the model underestimated 

contributions to pumpage from sources other than depletion of 

aquifer storage. For example, streamflow depletion appears to 

have been underestimated.

One of the limitations of the Blue River Basin model 

was that ground-water development was relatively limited 

at the time of original model calibration. For example, the 

only area of significant water-level decline in 1965 was in the 

northern part of the basin. A common finding of post audits 

of ground-water model forecasts is that the time period for 

matching historical conditions in the original model was too 

short to capture important elements of the ground-water 

system in the model. Processes or boundary conditions that 

are insignificant under the initial, lower stress regime may 

become important under a different and generally larger set 

of imposed stresses. Thus, a conceptual model founded on 

observed behavior of a ground-water system may provide 

inaccurate forecasts if existing stresses are increased or 

new stresses are added. In addition, as illustrated by the Blue 

River Basin modeling study, future projections of water withdrawals typically are highly uncertain and need to be refined 

with time. The possibility of periodic refinement and reuse of 

ground-water models highlights the importance of thorough 

documentation and careful archiving of these models and 

continued monitoring of the ground-water system.

30

25

20

20

20

15

10

5

15

25

5

5

25

20

20 15

5

5

10

15

10

20

15

30

15

10 5

5

98°

98°

97°

97°

98°

97°

97°

41 41° 41° 41° °

40° 40° 98°

40° 40° NEBRASKA

NEBRASKA

KANSAS

NEBRASKA

KANSAS

0

0 20 KILOMETERS

20 MILES

20

EXPLANATION

Line of predicted equal

 water-level decline, in feet

20 Line of measured equal

 water-level decline, in feet

Little Blue River

Big Blue River

Little Blue River

Big Blue River

A B

MAP AREA

Figure F–1. (A) Predicted and (B) measured ground-water-level declines in the Blue River Basin from predevelopment to 1982. 

(Emery, 1965; Johnson and Pederson, 1983.)
72

When broadly considered, alternative 

management strategies are composed of a small 

number of general approaches, as outlined below.

Use sources of water other than local 

ground water. The main possibilities are (1) shift 

the source of water, either completely or in part, 

from ground water to surface water, or (2) import 

water (usually, but not necessarily, surface water) 

from outside river-basin or ground-water system 

boundaries. In two previous examples given in the 

“Storage Changes” section—the Chicago metropolitan area and Kings County, Long Island—the 

ground-water systems were stressed sufficiently 

to cause undesirable effects, and surface-water 

sources were substituted for ground-water sources 

as a result. On the other hand, ground water 

currently is used or is being considered for use in 

many localities as a supplement for surface-water 

sources that are no longer adequate.

Change rates or spatial patterns of 

ground-water pumpage. Possibilities include 

(1) an increase in pumpage that results in a 

new equilibrium of the ground-water system, 

(2) a decrease in pumpage that results in a new 

equilibrium of the ground-water system, or 

(3) a change in the spatial distribution of pumpage 

to minimize its existing or potential unwanted 

effects. Management strategies might include 

varying combinations of these approaches.

Computer simulation models have value 

beyond their use as purely predictive tools. They 

commonly are used as learning tools to identify 

additional data that are required to better define 

and understand ground-water systems. Furthermore, computer simulation models have the capability to test and quantify the consequences of 

various errors and uncertainties in the information 

necessary to determine cause and effect relationships and related model-based forecasts. This 

capability, particularly as it relates to forecasts, 

may be the most important aspect of computer 

models in that information about the uncertainty 

of model forecasts can be defined, which in turn 

enables water managers to evaluate the significance, and possibly unexpected consequences, 

of their decisions.

If a model is used to address questions about 

the future responses of a ground-water system 

that are of continuing significance to society, 

then field monitoring of the ground-water 

system should continue and the model should 

be reevaluated periodically to incorporate new 

information or new insights.

Strategies for Sustainability
73

Increase recharge to the ground-water 

system. Usual options include (1) pumpage 

designed to induce inflow from surface-water 

bodies, or (2) recharge of surface water or reused 

water (ground water or surface water) of good 

quality by surface spreading or injection through 

wells. Examples of several of these options are 

presented in Box G.

Decrease discharge from the ground-water 

system. Possibilities include pumpage that is 

designed to decrease discharge (1) to streams, 

lakes, or springs, or (2) from ground-water evapotranspiration. Both of these possibilities can have 

undesirable effects on surface-water bodies or on 

existing biological resources.

Change the volume of ground water in 

storage at different time scales. Possibilities 

include (1) managed short-term (time scale of 

months and years) increases and decreases in 

storage in the ground-water reservoir, which 

suggests that the ground-water reservoir might 

be managed at a time scale that is comparable to 

the management of surface-water reservoirs, or 

(2) a continuing long-term (possible time scales of 

decades and centuries) decrease in ground-water 

storage. Of course, complete or almost complete 

depletion of aquifer storage is not a strategy for 

sustainability, but an extreme approach that may 

be considered in some situations.

Consideration of these general approaches 

indicates that they are not mutually exclusive; that 

is, the various approaches overlap, or the implementation of one approach will inevitably involve 

or cause the implementation of another. For 

example, changing rates or patterns of groundwater pumpage will lead to changes in the spatial 

patterns of recharge to or discharge from groundwater systems.

The short list of general approaches may 

suggest that proposing and evaluating alternative 

management strategies is deceptively simple. On 

the contrary, ground water is withdrawn from 

complex, three-dimensional systems, and many 

possible combinations of these approaches typically should be considered in developing management strategies for a particular ground-water 

system.

Innovative approaches that have been undertaken 

to enhance the sustainability of ground-water 

resources typically involve some combination of use 

of aquifers as storage reservoirs, conjunctive use of 

surface water and ground water, artificial recharge 

of water through wells or surface spreading, and 

the use of recycled or reclaimed water.
74

G

Examples of Innovative Approaches that 

Contribute to Ground-Water Sustainability

DAYTON, OHIO

Dayton, Ohio, is heavily dependent upon ground water 

to meet municipal and industrial water-supply needs. Nearly 

one-fourth of all ground water used in Ohio is withdrawn from 

wells completed in a sole-source sand and gravel aquifer that 

underlies the Dayton metropolitan area. Much of the water is 

pumped from a 30- to 75-foot thick shallow aquifer that underlies the Mad River Valley. To ensure that ground-water levels 

are maintained high enough to allow for large drawdowns by 

high-capacity wells, an artificial recharge system has been in 

place since the 1930’s. The source of recharge is streamflow 

diverted from the Mad River into a series of interconnected 

infiltration ditches and lagoons that occupy about 20 acres 

on Rohrers Island. To meet increasing demand, a new municipal well field was developed in the 1960’s in a section of the 

aquifer north of Dayton. Here, water is pumped from the 

Great Miami River into a series of ponds and lagoons, some 

of which also serve as water hazards on a city-owned golf 

course. Recharge lagoons at both well fields are periodically 

drained, and accumulated muck and silt are excavated to 

maintain a high rate of infiltration into the underlying aquifer. 

ORLANDO, FLORIDA

Large volumes of reclaimed water, which has undergone advanced secondary treatment, are reused through 

land-based applications in a 40-square-mile area near 

Orlando, Florida. These applications include citrus crop irrigation and artificial recharge to the surficial aquifer through rapid 

infiltration basins. 

High-capacity turbine pump installed on a municipal 

well at Rohrers Island. Recharge lagoon in background. 

(Photograph by Brent Means, U.S. Geological Survey.)

Rapid infiltration basins. (Photograph courtesy of 

Water Conserv II facility, Orlando, Florida.)

ORANGE COUNTY, CALIFORNIA

Orange County, near Los Angeles, California, receives 

an average of only 13 to 15 inches of rainfall annually, yet 

sustains a population of approximately 2.5 million people. A 

ground-water basin that underlies the northwestern half of the 

county supplies about 75 percent of the total water demand. 

As the area developed from a thriving agricultural center into a 

highly urbanized area, increased demands for ground water 

resulted in a gradual lowering of the water table below sea 

level and encroachment of saltwater from the Pacific Ocean. 

To prevent further saltwater intrusion and to replenish the 

ground-water supply, the Orange County Water District 

operates a hydraulic barrier system composed of a series 

of 23 multipoint wells that inject freshwater into the aquifer, 

A variety of innovative approaches have been undertaken to enhance the sustainability of ground-water resources. 

These approaches typically involve some combination of the use of aquifers as storage reservoirs, conjunctive use of surface 

water and ground water, artificial recharge of water through wells or surface spreading, and use of recycled or reclaimed water. 

These approaches commonly lead to scientific questions about the extent and nature of ground-water and surface-water interactions, geochemical effects of mixing water from different sources with aquifer water, production and degradation of by-products

from injection of treated water, and other issues.

Selected examples of innovative approaches that have been implemented are described below.
75

blocking further passage of seawater. The source of injection 

water is a blended combination of deep well water and recycled secondary effluent. The recycled product meets drinkingwater standards through advanced treatment processes of 

reverse osmosis and activated carbon adsorption.

LONG ISLAND, NEW YORK

Ground water is the sole source of freshwater for 

the more than 3 million people who live on Long Island outside 

the metropolitan New York City boundary. (The Long Island 

ground-water system was discussed earlier in the sections 

on “Ground-Water Development, Sustainability, and Water 

Budgets” and “Effects of Ground-Water Development on 

Ground-Water Storage.”) To help replenish the aquifer, as 

well as reduce urban flooding and control saltwater intrusion, 

more than 3,000 recharge basins dispose of storm runoff at an 

average rate of about 150 million gallons per day. Initially, 

many of these basins were abandoned gravel pits, but since 

1936 urban developers are required to provide recharge 

basins with new developments. Practically all basins are 

unlined excavations in the upper glacial deposits and have 

areas from less than 0.1 to more than 30 acres.

WILDWOOD, NEW JERSEY

Wildwood, New Jersey, is a resort town on a barrier 

island along the Atlantic Coast. As a resort community, it has a 

large influx of tourists in the summertime. The population can 

increase from about 5,000 during the winter to 30,000 at the 

height of the summer tourist season. The Wildwood Water 

Utility withdraws water from wells located about 5 miles inland 

from the barrier island. To supply water from these wells to 

meet the island’s needs during the summer would require a 

large pumping facility and transmission lines that would be 

little used the rest of the year. To avoid these excessive costs, 

the utility injects ground water into a shallow aquifer on the 

island during periods of low demand and withdraws the water 

in the summer by using dual injection and recovery wells. 

This system, operated since 1967, represents perhaps the 

oldest operational aquifer storage recovery (ASR) project in 

the United States. In ASR, water is injected underground, 

commonly into nonpotable or saline aquifers, where it forms 

a lens of good quality water for later recovery from the same 

well(s). Operation of a typical ASR installation is designed to 

smooth out annual variability in water demand by recharging 

aquifers during periods of low demand and recovering the 

water during periods of high demand. Advantages of ASR 

over other artificial recharge schemes are that it uses very 

little land (especially compared to surface spreading) and 

reduces the cost and maintenance of separate injection and 

recovery wells.

Water Factory 21 treatment facility. (Photograph 

courtesy of Orange County Water District.)

Wildwood beach in the summer. (Photograph courtesy of Cape May County Division of Tourism.)

Aerial photograph of development on Long Island 

showing recharge basin. (Photograph courtesy 

of Nassau County Department of Public Works.)
76

• The most important and most extensively 

discussed concept in this report is that 

volumes of water pumped from a groundwater system must come from somewhere 

and must cause a change in the groundwater system. Possible sources of water for 

pumpage are (1) more water entering the 

ground-water system (increased recharge), 

(2) less water leaving the system (decreased 

discharge), and (3) removal of water that was 

stored in the system. 

• One of the critical linkages in both unstressed 

and stressed ground-water systems is between 

ground water and surface water. Pumping 

water from aquifers that are hydraulically 

connected with surface-water bodies can have 

a significant effect on those bodies by reducing 

ground-water discharges to surface water and 

possibly causing outflow from those bodies 

into the ground-water system. Thus, an evaluation of ground-water management strategies 

needs to involve consideration of surfacewater resources, including closely related 

biological resources.

• A key feature of some aquifers and groundwater systems is the large volume of ground 

water in storage, which allows the possibility 

of using aquifers for temporary storage, that is, 

managing inflow and outflow of ground water 

in storage in a manner similar to surface-water 

reservoirs.

• From the standpoint of water use and water 

management, all ground water is not 

equal—the suitability of water, as measured 

by its quality, is a key consideration in developing water-management strategies. Furthermore, determining water suitability (or 

unsuitability) requires detailed information 

on the three-dimensional distribution and 

concentrations of potential contaminants, 

both naturally occurring contaminants and 

those resulting from human activities.

• Continuing large withdrawals of water from 

an aquifer often result in undesirable consequences. The most common of these consequences have been discussed throughout 

this report. From a management standpoint, 

water managers, stakeholders, and the public 

must decide the specific conditions under 

which the undesirable consequences can no 

longer be tolerated.

• The effects of ground-water development may 

require many years to become evident. Thus, 

there is an unfortunate tendency to forego the 

data collection and analysis that is needed to 

support informed decision making until well 

after problems materialize.

• Evaluation of possible ground-water management approaches (a) depends on the 

continuing collection, archiving, and analysis 

of a broad range of different types of information, and (b) can be assisted by well-designed 

computer simulation models.

Concluding Remarks

In conclusion, we would like to emphasize the following interrelated facts and concepts: 
77

ACKNOWLEDGMENTS

Technical review of this Circular was 

provided by John Vecchioli, J.W. LaBaugh, W.W. 

Lapham, S.A. Leake, and T.C. Winter. G.J. Beserra, 

M.J. Focazio, Cindy Gehman, P.D. Hays, R.E. 

Krause, P.J. Lacombe, V.L. McGuire, Jack Monti, 

D.S. Morgan, L.C. Murray, A.S. Navoy, D.L. Nelms, 

G.B. Ozuna, G.L. Rowe, M.P. Scorca, A.G. Spinello, 

and D.K. Yobbi provided information for some 

of the examples. J.V. Flager, M.A. Kidd, and 

Chet Zenone provided editorial and technical 

reviews. The final manuscript was prepared 

by M.J. VanAlstine, J.K. Monson, J.M. Evans, 

R.J. Olmstead, E.J. Swibas, and C.L. Anderson. 
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