Photographs of field instruments and monitoring activities. 

Photograph captions and credits given where photographs 

are displayed in body of report.
Ground-Water-Level 

Monitoring and the 

Importance of Long-Term 

Water-Level Data

U.S. Geological Survey Circular 1217

by Charles J. Taylor

William M. Alley

Denver, Colorado

2001
U.S. DEPARTMENT OF THE INTERIOR

GALE A. NORTON, 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

Taylor, Charles J.

Ground-water-level monitoring and the importance of long-term water-level data / by

Charles J. Taylor, William M. Alley.

p. cm. -- (U.S. Geological Survey circular ; 1217)

Includes bibliographical references (p. ).

1. Groundwater--United States--Measurement. 2. Water levels--United States. I. Alley,

William M. II. Geological Survey (U.S.) III. Title. IV. Series.

GB1015 .T28 2001

551.49'2'0973--dc21

2001040226

Reston, Virginia 2001

Free on application to the

U.S. Geological Survey

Branch of Information Services

Box 25286

Denver, CO 80225-0286
III

FOREWORD

round water is among the Nation’s most precious 

natural resources. Measurements of water levels in wells 

provide the most fundamental indicator of the status of this 

resource and are critical to meaningful evaluations of the quantity and quality of ground water and its interaction with surface 

water. Water-level measurements are made by many Federal, 

State, and local agencies. It is the intent of this report to highlight the importance of measurements of ground-water levels 

and to foster a more comprehensive and systematic approach 

to the long-term collection of these essential data. Through 

such mutual efforts, the Nation will be better positioned in 

coming decades to make wise use of its extensive groundwater resources.

G

Robert M. Hirsch

Associate Director for Water

(Signed)
IV

CONTENTS

Foreword III

Introduction 1

Essential Components of Water-Level Monitoring Programs 7

Selection of Observation Wells 7

Frequency of Water-Level Measurements 8

Quality Assurance 9

Data Reporting 10

Uses and Importance of Long-Term Water-Level Data 15

Ground-Water Development in the High Plains and Gulf Coastal Plain 17

The High Plains Aquifer 18

The Gulf Coastal Plain Aquifer System 19

Memphis, Tennessee 21

Houston, Texas 24

Baton Rouge, Louisiana 26

Changes in Regional Ground-Water Flow 29

Drought Monitoring in Pennsylvania 31

Estimation of High Ground-Water Levels in Massachusetts and Rhode Island 34

Ground-Water and Surface-Water Interaction in Oregon 36

Wetland Hydrology in Michigan 40

Relevance of Water-Level Data to Ground-Water Quality Issues 42

Saline Water Intrusion in New Jersey 43

Upwelling of Saline Water in Utah 44

Effects of Residential Development in Montana 47

Innovative and Emerging Applications 50

Status of Water-Level Data-Collection Programs 57

Challenges and Future Opportunities 63

Acknowledgments 66

References 67
V

BOXES

Box A -- Hydraulic Head and Factors Causing Changes in Ground-Water Levels 3

Box B -- Ground-Water-Level Measurements: Why the Choice of Frequency Matters 12

Box C -- Statistical Design of Water-Level Monitoring Networks 52

Box D -- Ground-Water-Level Monitoring in the 1930’s, 1950’s, and Today 61

1

Ground-Water-Level Monitoring and the 

Importance of Long-Term Water-Level Data

by Charles J. Taylor

William M. Alley

INTRODUCTION

Ground water is one of the Nation’s most 

important natural resources. It is the principal 

source of drinking water for about 50 percent of the 

United States population, providing approximately 

96 percent of the water used for rural domestic 

supplies and 40 percent of the water used for public 

supplies (Solley and others, 1998). In addition, 

more than 30 percent of the water used for agricultural purposes is withdrawn from wells. Ground 

water also is a significant, but often unrecognized, 

component of the Nation’s surface-water resources. 

Much of the flow in streams and the water in lakes 

and wetlands is sustained by the discharge of ground 

water, particularly during periods of dry weather. 

Ground-water systems are dynamic and adjust 

continually to short-term and long-term changes in 

climate, ground-water withdrawal, and land use 

(Box A). Water-level measurements from observation wells are the principal source of information 

about the hydrologic stresses acting on aquifers and 

how these stresses affect ground-water recharge, 

storage, and discharge. Long-term, systematic 

measurements of water levels provide essential data 

needed to evaluate changes in the resource over 

time, to develop ground-water models and forecast 

trends, and to design, implement, and monitor the 

effectiveness of ground-water management and 

protection programs. 

“Water-level measurements from 

observation wells are the principal 

source of information about the 

hydrologic stresses acting on aquifers 

and how these stresses affect groundwater recharge, storage, and discharge.”
2

The U.S. Geological Survey (USGS) has 

collected water-level data for more than a hundred 

years, and many State and other agencies have a 

long history of water-level monitoring. However, 

water-level monitoring in the United States is 

fragmented and largely subject to the vagaries of 

existing local projects. A stable, base network of 

water-level monitoring wells exists only in some 

locations. Moreover, agency planning and coordination vary greatly throughout the United States with 

regard to construction and operation of water-level 

observation networks and the sharing of collected 

data.

For many decades, periodic calls have been 

made for a nationwide program to obtain more 

systematic and comprehensive records of water 

levels in observation wells as a joint effort among 

USGS and State and local agencies. O.E. Meinzer 

described the characteristics of such a program over 

65 years ago:

The program should cover the water-bearing formations 

in all sections of the country; it should include beds with watertable conditions, deep artesian aquifers, and intermediate 

sources; moreover, it should include areas of heavy withdrawal 

by pumping or artesian flow, areas which are not affected by 

heavy withdrawal but in which the natural conditions of intake 

and discharge have been affected by deforestation or breaking 

up of prairie land, and, so far as possible, areas that still have 

primeval conditions. This nation-wide program should furnish 

a reliable basis for periodic inventories of the ground-water 

resources, in order that adequate provision may be made 

for our future water supplies.

—O.E. Meinzer, 1935, Introduction to “Report 

of the Committee on Observation Wells, 

United States Geological Survey” (Leggette 

and others, 1935)

More recently, the National Research Council 

(2000) reiterated, “An unmet need is a national 

effort to track water levels over time in order to 

monitor water-level declines.” 

This report reviews the uses and importance 

of data from long-term ground-water-level monitoring in the United States. Case studies are 

presented to highlight the broad applicability of 

long-term ground-water-level data to water-resource 

issues commonly faced by hydrologists, engineers, 

regulators, and resource managers. It is hoped that 

this report will provide a catalyst toward the 

establishment of a more rigorous and systematic 

nationwide approach to ground-water-level 

monitoring—clearly an elusive goal thus far. The 

time is right for progress toward this goal. Improved 

access to water data over the Internet offers the 

opportunity for significant improvements in the 

coordination of water-level monitoring and the 

sharing of information by different agencies, as well 

as the potential means for evaluation of water-level 

monitoring networks throughout the United States.
3

A

Hydraulic Head and Factors Causing

Changes in Ground-Water Levels

This section describes some basic ground-water terms 

and provides a general description of natural and human 

factors that affect ground-water levels (heads). It is intended 

as background information for the reader who may have 

limited knowledge of ground-water hydrology.

Hydraulic head (often simply referred to as “head”) 

is an indicator of the total energy available to move ground 

water through an aquifer. Hydraulic head is measured by the 

height to which a column of water will stand above a reference 

elevation (or “datum”), such as mean sea level. A water-level 

measurement made under static (nonpumping) conditions is a 

measurement of the hydraulic head in the aquifer at the depth 

of the screened or open interval of a well (Figure A–1). 

Because hydraulic head represents the energy of water, 

ground water flows from locations of higher hydraulic head 

to locations of lower hydraulic head. The change in hydraulic 

head over a specified distance in a given direction is called 

the “hydraulic gradient.”

Land surface

Well 1 Well 2

Unsaturated

zone

Unconfined

aquifer

Confining

unit

Confined

aquifer

Sea level

A

B

Head at

point A,

in feet

Elevation

of point B,

in feet

Undefined

interval

Water table

Head at

point B,

in feet

Elevation

of point A,

in feet

Figure A–1. Sketch showing the relation between hydraulic 

heads and water levels in two observation wells—Well 1 

screened in an unconfined aquifer and Well 2 screened in a 

confined aquifer. Hydraulic heads in each of these two aquifers are determined by the elevation of the water level in the 

well relative to a vertical datum—in this case, sea level. 
4

Two general types of aquifers—unconfined and 

confined—are recognized (Figure A–2). In unconfined 

aquifers, hydraulic heads fluctuate freely in response to 

changes in recharge and discharge. Water levels measured 

in wells completed in the upper part of an unconfined aquifer 

help define the elevation of the water table, which is the top 

of the saturated zone. In confined aquifers, sometimes known 

as “artesian” aquifers, water in the aquifer is “confined” under 

pressure by a geological body that is much less permeable 

than the aquifer itself. Water levels in tightly cased wells 

completed in confined aquifers often rise above the elevation 

of the top of the aquifer (Figure A–2). These water levels 

define an imaginary surface, referred to as the potentiometric 

surface, which represents the potential height to which water 

will rise in wells completed in the confined aquifer. Many aquifers are intermediate between being completely unconfined or 

confined.

Ground-water levels are controlled by the balance 

among recharge to, storage in, and discharge from an aquifer. 

Physical properties such as the porosity, permeability, and 

thickness of the rocks or sediments that compose the aquifer 

affect this balance. So, too, do climatic and hydrologic factors, 

such as the timing and amount of recharge provided by 

precipitation, discharge from the subsurface to surface-water 

bodies, and evapotranspiration. When the rate of recharge 

to an aquifer exceeds the rate of discharge, water levels 

or hydraulic heads will rise. Conversely, when the rate 

of ground-water withdrawal or discharge is greater than 

Precipitation

Confining unit

Unconfined aquifer

Water level (head)

in well

Confined aquifer

Observation well

Recharge

Water table

Evaporation

Stream

Ground-water

flow direction

Figure A–2. Cross-section sketch of a typical ground-water-flow system showing the relation between an unconfined 

and confined aquifer, a water table, and other hydrologic elements.
5

the rate of ground-water recharge, the water stored in the 

aquifer becomes depleted and water levels or hydraulic 

heads will decline.

Water levels in many aquifers in the United States 

follow a natural cyclic pattern of seasonal fluctuation, 

typically rising during the winter and spring due to greater 

precipitation and recharge, then declining during the summer 

and fall owing to less recharge and greater evapotranspiration. 

The magnitude of fluctuations in water levels can vary greatly 

from season to season and from year to year in response 

to varying climatic conditions. Changes in ground-water 

recharge and storage caused by climatic variability commonly 

occur over decades, and water levels in aquifers generally 

have a delayed response to the cumulative effects of drought.

The range and timing of seasonal water-level fluctuations may vary in different aquifers in the same geographic 

area, depending on the sources of recharge to the aquifers 

and the physical and hydraulic properties of each. This is 

illustrated by hydrographs for two wells (GW–11 and MW–1) 

completed in a layered alluvial aquifer system near the Ohio 

River in northern Kentucky (Figure A–3). The two wells are 

approximately 250 feet apart; however, well GW–11 is 

completed in a shallow aquifer zone consisting of a mixture 

of silty clay and sand approximately 40 feet thick, while 

well MW–1 is completed in a deeper aquifer zone consisting 

of a mixture of sand and gravel approximately 20 feet thick.

Because the silty clay does not easily transmit water, 

the shallow aquifer zone exhibits a relatively muted response 

to a seasonal increase in recharge that typically occurs at this 

420

418

416

414

412

410

410

400

390

380

November December January February March April May June

1995

November December January February March April May June

1995

WATER LEVEL, IN FEET ABOVE SEA LEVEL WATER LEVEL, IN FEET ABOVE SEA LEVEL

1996

1996

GW-11

MW-1

Missing

data

Missing

data

Missing

data

Figure A–3. Example hydrographs showing the difference in timing and range of waterlevel fluctuations in two observation wells (GW–11 and MW–1) in an alluvial aquifer near 

the Ohio River, northern Kentucky.
6

location during the late winter and spring. As seen on the 

hydrograph, water levels in well GW–11 fluctuate slightly 

from November to June in response to individual precipitation 

events, but exhibit an overall seasonal increase of less than 

2 feet. In contrast, the more permeable sand and gravel in 

the deeper aquifer zone transmits water very easily, and 

the deeper aquifer zone exhibits a much greater response 

to the seasonal increase in recharge. On the hydrograph 

for well MW–1, water levels increase gradually at first from 

November through January, then more sharply from February 

to June, and exhibit an overall seasonal increase of more 

than 12 feet.

Superimposed on natural, climate-related fluctuations 

in ground-water levels are the effects of human activities that 

alter the natural rates of ground-water recharge or discharge. 

For example, urban development, deforestation, and draining 

of wetlands can expedite surface runoff and thus reduce 

ground-water recharge. Agricultural tillage, the impoundment 

of streams, and creation of artificial wetlands can increase 

ground-water recharge. Long-term water-level monitoring 

during periods of significant land-use change is important to 

the protection of aquifers. The effects of such human-induced 

changes on ground-water recharge and storage are often 

incremental, and the cumulative effects may not become 

evident for many years.

The withdrawal of ground water by pumping is the most 

significant human activity that alters the amount of ground 

water in storage and the rate of discharge from an aquifer. The 

removal of water stored in geologic materials near the well 

sets up hydraulic gradients that induce flow from more distant 

parts of the aquifer. As ground-water storage is depleted 

within the radius of influence of pumping, water levels in the 

aquifer decline. The area of water-level decline is called the 

cone of depression, and its size is controlled by the rate and 

duration of pumping, the storage characteristics of the aquifer, 

and the ease with which water is transmitted through the 

geologic materials to the well. The development of a cone of 

depression can result in an overall decline in water levels over 

a large geographic area, change the direction of ground-water 

flow within an aquifer, reduce the amount of base flow to 

streams, and capture water from a stream or from adjacent 

aquifers. Within areas having a high density of pumped wells, 

multiple cones of depression can develop within an aquifer. 

As the reader examines the case studies discussed in 

this report, it is instructive to identify the natural and humaninduced stresses on the aquifers described and the relative 

and combined effects of each on ground-water levels. This will 

illustrate the primary point of emphasis—that ground-waterlevel data must be collected accurately and over periods of 

sufficient time to enable the proper development, management, and protection of the Nation’s ground-water resources.

Measuring water level in dewatering well near Yuma, Arizona. Photograph by Sandra J. Owen-Joyce, U.S. Geological Survey.
7

All water-level monitoring programs depend 

on the operation of a network of observation 

wells—wells selected expressly for the collection of 

water-level data in one or more specified aquifers. 

Decisions made about the number and locations of 

observation wells are crucial to any water-level data 

collection program. Ideally, the wells chosen for an 

observation well network will provide data representative of various topographic, geologic, climatic, and 

land-use environments. Decisions about the areal 

distribution and depth of completion of observation 

wells also should consider the physical boundaries 

and geologic complexity of aquifers under study. 

Water-level monitoring programs for complex, 

multilayer aquifer systems may require measurements in wells completed at multiple depths in 

different geologic units. Large, regional aquifers that 

extend beyond State boundaries require a network 

of observation wells distributed among one or more 

States. If one of the purposes of a network is to 

monitor ambient ground-water conditions, or the 

effects of natural, climatic-induced hydrologic 

stresses, the observation network will require wells 

that are unaffected by pumping, irrigation, and land 

uses that affect ground-water recharge. These and 

many other technical considerations pertinent to the 

design of a water-level observation network are 

discussed in more detail in technical papers by 

Peters (1972), Winter (1972), and Heath (1976).

Commonly overlooked is the need to collect 

other types of hydrologic information as part of a 

water-level monitoring program. Meteorological 

data, such as precipitation data, aid in the interpretation of water-level changes in observation wells. 

Where observation wells are located in alluvial aquifers or other aquifers that have a strong hydraulic 

connection to a stream or lake, hydrologic data, 

such as stream discharge or stage, are useful in 

examining the interaction between ground water 

and surface water. Meteorological and streamflow 

data commonly are available from other sources; 

but if not, some monitoring of variables such as 

streamflow and precipitation may be needed to 

supplement the water-level data, particularly in 

remote areas or in small watersheds. In addition, 

water-use data, such as pumping rates and volumes 

of pumped water, can greatly enhance the interpretation of trends observed in water levels and explain 

changes in the storage and availability of ground 

water that result from water withdrawals over time.

ESSENTIAL COMPONENTS OF 

WATER-LEVEL MONITORING PROGRAMS

Before discussing the uses and importance 

of long-term water-level data, it is useful to review 

essential components of a water-level monitoring 

program. These include: (1) selection of observation 

Selection of Observation Wells

wells, (2) determination of the frequency of waterlevel measurements, (3) implementation of quality 

assurance, and (4) establishment of effective practices for data reporting. 

Well with tipping-bucket rain 

gage mounted on top.
8

The frequency of water-level measurements is 

among the most important components of a waterlevel monitoring program. Although often influenced by economic considerations, the frequency of 

measurements should be determined to the extent 

possible with regard to the anticipated variability of 

water-level fluctuations in the observation wells and 

the data resolution or amount of detail needed to 

fully characterize the hydrologic behavior of the 

aquifer. These aspects are discussed more fully in 

Box B. 

Typically, collection of water-level data 

over one or more decades is required to compile 

a hydrologic record that encompasses the potential 

range of water-level fluctuations in an observation 

well and to track trends with time. Systematic, 

long-term collection of water-level data offers the 

greatest likelihood that water-level fluctuations 

caused by variations in climatic conditions and 

water-level trends caused by changes in land-use or 

water-management practices will be “sampled.” The 

availability of long-term water-level records greatly 

enhances the ability to forecast future water levels. 

Therefore, observation wells should be selected with 

an emphasis on wells for which measurements can 

be made for an indefinite time. 

Frequency of Water-Level Measurements

Typically, collection of water-level data 

over one or more decades is required 

to compile a hydrologic record that 

encompasses the potential range of 

water-level fluctuations in an observation 

well and to track trends with time.

Downloading water-level data from an observation well in Oregon instrumented with a downhole transducer/logger. Photograph by David S. 

Morgan, U.S. Geological Survey.
9

Good quality-assurance practices help to 

maintain the accuracy and precision of water-level 

measurements, ensure that observation wells reflect 

conditions in the aquifer being monitored, and 

provide data that can be relied upon for many 

intended uses. Therefore, field and office practices 

that will provide the needed levels of quality assurance for water-level data should be carefully thought 

out and consistently employed. 

Some important field practices that will ensure 

the quality of ground-water-level data include the 

establishment of permanent datums (reference 

points for water-level measurements) for observation wells, periodic inspection of the well structure, 

and periodic hydraulic testing of the well to ensure 

its communication with the aquifer. The locations 

and the altitudes of all observation wells should 

be accurately surveyed to establish horizontal 

and vertical datums for long-term data collection. 

Inaccurate datums are a major source of error 

for water-level measurements used as control 

points for contoured water-level or potentiometricsurface maps and in the calibration and sensitivity 

analysis of numerical ground-water models. Recent 

advances in the portability and operation of 

traditional surveying equipment, and in Global 

Positioning System (GPS) technology, have simplified the process of obtaining a fast, accurate survey 

of well location coordinates and datums.

Existing wells selected and used for long-term 

water-level monitoring should be carefully examined 

to ensure that no construction defects are present 

that might affect the accuracy of water-level 

measurements. This may entail the use of a downhole video camera to inspect the well screen and 

casing construction. Over time, silting, corrosion, 

or bacterial growth may adversely affect the way the 

well responds to changes in the aquifer. Any well 

selected for inclusion in an observation network 

should be hydraulically tested to ensure it is in 

good communication with the aquifer of interest. 

Hydraulic tests should be repeated periodically to 

ensure that hydraulic communication between the 

well and the aquifer remains optimal and that the 

hydraulic response of the well reflects water-level 

(head) fluctuations in the aquifer as accurately as 

possible.

To help maintain quality assurance, a 

permanent file that contains a physical description of well construction, location coordinates, 

the datum used for water-level measurements, 

and results of hydraulic tests should be established 

for each observation well. Recent water-level 

measurements should be compared with previous 

measurements made under similar hydrologic conditions to identify potential anomalies in water-level 

fluctuations that may indicate a malfunction of 

measuring equipment or a defect in observationwell construction.

Quality Assurance

Hydraulic tests should be repeated 

periodically to ensure that the hydraulic 

response of the well reflects water-level 

(head) fluctuations in the aquifer 

as accurately as possible.
10

Water-level data reporting techniques vary 

greatly depending on the intended use of the data, 

but too often water-level measurements are simply 

tabulated and recorded in a paper file or electronic 

database. Simple tabulation is useful for the determination of average, maximum, and minimum 

water levels but does not easily reveal changes or 

trends caused by seasonal and annual differences 

in precipitation, water use, or other hydrologic 

stresses.

Water-level hydrographs—graphical plots 

showing changes in water levels over time—are 

a particularly useful form of data reporting. Such 

hydrographs provide a visual depiction of the range 

in water-level fluctuations, seasonal water-level variations, and the cumulative effects of short-term and 

long-term hydrologic stresses. In general, the value 

and reliability of the information presented by a 

water-level hydrograph improves with increasing 

frequency of measurement and period of record. 

Hydrographs that are constructed from infrequent 

water-level measurements, or that have significant 

gaps in time between the measurements, generally 

are difficult to interpret and may lead to biased or 

mistaken interpretations about the frequency and 

magnitude of water-level fluctuations and their 

causes. Depending on the frequency of water-level 

measurement and period of hydrologic record, 

water-level hydrographs can be constructed to illustrate historical water levels, compare recent and 

historical water-level data, and present descriptive 

statistics for water-level measurements (Figure 1).

The accessibility of water-level data is 

greatly enhanced by the use of electronic databases, especially those that incorporate Geographic 

Information System (GIS) technology to visually 

depict the locations of observation wells relative 

to pertinent geographic, geologic, or hydrologic 

features. The availability of electronic information 

transfer on the Internet greatly enhances the capability for rapid retrieval and transmittal of water-level 

data to potential users. Water-level hydrographs, 

maps of observation-well networks, tabulated waterlevel measurements, and other pertinent information all can be configured for access on the Internet. 

A significant advantage of this method of data 

reporting is the ease and speed with which groundwater-level data can be updated and made available 

to users. 

Data Reporting

Measuring well “stickup” to establish water-level 

measurement datum. Photograph by Heather Handran, 

U.S. Geological Survey.
11

A

B

C

18

20

22

24

26

18

20

22

24

26

October

1986

Oct

Oct Nov Dec Jan Feb Mar

MONTH

Apr May June July Aug Sept

Nov Dec Jan Feb Mar Apr May June July Aug Sept

October

1988

October

1990

October

1992

October

1994

October

1996

WATER LEVEL, IN FEET BELOW LAND SURFACE WATER LEVEL, IN FEET BELOW LAND SURFACE

19

20

21

22

23

24

25

26

WATER LEVEL, IN FEET BELOW LAND SURFACE

Maximum

75th percentile

Median

25th percentile

Minimum

EXPLANATION

EXPLANATION

Range of monthly 

 water levels for

 period of record

Monthly water levels 

 for current water

 year

Average monthly

 water levels for

 period of record

Figure 1. Hydrographs for a well in Vanderburgh County, Indiana, showing (A) continuous record of daily water-level measurements made over about a decade, (B) comparison between water-level measurements made in a single year to historical high and low 

water-level measurements, and (C) statistical distribution (boxplots) of water levels for 

each month. 
12

B

Ground-Water-Level Measurements: 

Why the Choice of Frequency Matters

Figure B–1. Common environmental factors that influence the choice of frequency of water-level 

measurements in observation wells.

The frequency of measurement is one of the most 

important considerations in the design of a water-level monitoring program. The development of a plan for water-level 

monitoring that will be used for each well in the observation 

network is dependent on the objectives of the program and the 

intended use and level of analysis required of the data. The 

frequency of measurement should be adequate to detect 

short-term and seasonal ground-water-level fluctuations of 

interest and to discriminate between the effects of short- and 

long-term hydrologic stresses (Figure B–1).

Water-level monitoring may involve “continuous” or 

periodic measurements. Continuous monitoring involves the 

installation of automatic water-level sensing and recording 

instruments that are programmed to make measurements in 

observation wells at a specified frequency. Continuous monitoring provides the highest level of resolution of water-level 

fluctuations. Hydrographs constructed from frequent waterlevel measurements collected with continuous monitoring 

equipment can be used to accurately identify the effects of 

various stresses on the aquifer system and to provide the 

most accurate estimates of maximum and minimum waterlevel fluctuations in aquifers. For these reasons, it is often 

advisable that new observation wells initially be equipped 

with continuous monitoring equipment to identify stresses on 

the aquifer and the magnitude and frequency of water-level 

fluctuations. Continuous monitoring may not be required 

where the hydraulic response of an aquifer to stresses is 

slow and the frequency and magnitude of water-level changes 

in an observation well are not great. However, it is often the 

best technique to use for monitoring fluctuations in groundwater levels during droughts and other critical periods when 

hydraulic stresses may change at relatively rapid rates. Near 

real-time data collection also can be accomplished using a 

continuous recording device and telecommunication or radio 

transmitter equipment. 

Periodic ground-water-level measurements are those 

made at scheduled intervals (weeks, months, or years) and 

are generally used for water-table or potentiometric surface 

mapping and to reduce the costs of long-term monitoring. 

Periodic water-level measurements are made by manually 

using electronic-sensor tapes, chalked metal tapes, or 

acoustic sounding devices. Potential drawbacks to periodic 

monitoring are that hydraulic responses to short-term stresses 

may occur between measurements and may be missed, 

extreme water-level fluctuations cannot be determined with 

certainty, and apparent trends in water levels potentially are 

biased by the choice of measurement frequency. 

More

frequent

Shallow, 

unconfined Rapid

Greater

withdrawal

More

variable

Less

frequent

Deep,

confined

Slow Less

withdrawal

Less

variable

Frequency of

water-level

measurements

Aquifer

type and

position

Groundwater flow

and

recharge

rate

Aquifer

development

Climatic

conditions
13

Synoptic water-level measurements are a special 

type of periodic measurement in which water levels in wells 

are measured within a relatively short period and under 

specific hydrologic conditions. Synoptic water-level measurements provide a “snapshot” of heads in an aquifer. Synoptic 

measurements commonly are taken when data are needed 

for mapping the altitude of the water table or potentiometric 

surface, determining hydraulic gradients, or defining the 

physical boundaries of an aquifer. Regional synoptic measurements made on an annual or multiyear basis can be used as 

part of long-term monitoring to complement more frequent 

measurements made from a smaller number of wells.

An example of the effects of different measurement 

frequencies is provided by water-level hydrographs for an 

observation well in Massachusetts. The well is completed in 

bedrock to a depth of 740 feet, and the characteristics of this 

well fall in the middle range of the temporal response categories shown in Figure B–1.

A daily water-level hydrograph for the Massachusetts 

well and hydrographs that would have been obtained for the 

same well if measurements had been made only monthly or 

quarterly are shown in Figure B–2. Comparing the effects of 

different measurement frequencies on the hydrographs illustrates several features. First, monthly water-level measurements for this well generally are adequate to discern overall 

seasonal patterns in water levels and long-term trends but 

miss some short-term effects from pumping or recharge. 

Second, unless quarterly measurements correspond with 

regular patterns of seasonal variability of water levels, it can 

be difficult or impossible to discern anything beyond simple 

long-term water-level trends. Figure B–3, which overlays the 

daily and quarterly hydrographs from Figure B–2, illustrates 

how less frequent water-level measurements lead to lower 

estimates of the range of fluctuations in water levels in an 

observation well. 

Figure B–2. Water-level hydrographs for well PDW 23 in western Massachusetts, based on daily, monthly, and quarterly 

measurements, plotted to same scale but vertically offset. 

10-01-91 09-30-92 09-30-93 09-30-94 09-30-95 09-30-96 09-30-97

Daily

Monthly

Quarterly

GROUND-WATER LEVEL
14

9

12

15

18

10-01-91 09-30-92 09-30-93 09-30-94 09-30-95 09-30-96 09-30-97

WATER LEVEL, IN FEET BELOW LAND SURFACE

Daily measurements

EXPLANATION

Quarterly measurements

Figure B–3. Overlaid daily and quarterly hydrographs for well PDW 23 in western Massachusetts.
15

To determine the hydraulic properties 

of aquifers (aquifer tests)

Mapping the altitude of the water 

table or potentiometric surface

Monitoring short-term changes in 

ground-water recharge and storage

Monitoring long-term changes in 

ground-water recharge and storage

Monitoring the effects of climatic 

variability

Monitoring regional effects of groundwater development

Statistical analysis of water-level 

trends

Monitoring changes in ground-water 

flow directions

Monitoring ground-water and 

surface-water interaction

Numerical (computer) modeling of 

ground-water flow or contaminant 

transport

Intended use of water-level data

Days/weeks Months

Typical length of data-collection effort or hydrologic record required

Years Decades

Most applicable for

 intended use

Sometimes applicable

 for intended use

EXPLANATION

Water-level data are collected over various 

lengths of time, dependent on their intended use(s). 

Short-term water-level data are collected over 

periods of days, weeks, or months during many 

types of ground-water investigations (Table 1). For 

example, tests done to determine the hydraulic 

properties of wells or aquifers typically involve the 

collection of short-term data. Water-level measurements needed to map the altitude of the water table 

or potentiometric surface of an aquifer are generally 

collected within the shortest possible period of time 

so that hydraulic heads in the aquifer are measured 

under the same hydrologic conditions. Usually, 

water-level data intended for this use are collected 

over a period of days or weeks, depending on the 

logistics of making measurements at different 

observation-well locations. 

USES AND IMPORTANCE OF 

LONG-TERM WATER-LEVEL DATA

Table 1. Typical length of water-level-data collection as a function of the

intended use of the data
16

In this report, the systematic collection of 

long-term water-level data is emphasized. Longterm data are fundamental to the resolution of 

many of the most complex problems dealing with 

ground-water availability and sustainability (Alley 

and others, 1999). As stated previously, significant 

periods of time—years to decades—typically are 

required to collect water-level data needed to assess 

the effects of climate variability, to monitor the 

effects of regional aquifer development, or to obtain 

data sufficient for analysis of water-level trends 

(Table 1).

Many of the applications of long-term waterlevel data involve the use of analytical and numerical 

(computer) ground-water models. Water-level 

measurements serve as primary data required for 

calibration and testing of ground-water models, and 

it is often not until development of these models 

that the limitations of existing water-level data are 

fully recognized. Furthermore, enhanced understanding of the ground-water-flow system and data 

limitations identified by calibrating ground-water 

models provide insights into the most critical needs 

for collection of future water-level data. Unfortunately, this second step of using ground-water 

models to help improve future water-level monitoring is rarely taken. 

The uses and importance of long-term waterlevel data are more fully realized by examining 

actual case studies. Several are presented here to 

demonstrate the applicability of water-level data to a 

wide range of water-resource issues. These include 

the effects of ground-water withdrawals and other 

hydrologic stresses on ground-water availability, 

land subsidence, changes in ground-water quality, 

and surface-water and ground-water interaction.

Enhanced understanding of the 

ground-water-flow system and data 

limitations identified by calibrating 

ground-water models provide insights 

into the most critical needs for collection 

of future water-level data. Unfortunately, 

this second step of using ground-water 

models to help improve future water-level 

monitoring is rarely taken.
17

Ground-Water Development in the 

High Plains and Gulf Coastal Plain

Gulf Coastal Plain

aquifer system

High Plains

aquifer

COLORADO KANSAS

OKLAHOMA

NEBRASKA

ARIZONA

NEW MEXICO

TEXAS

MINNESOTA

IOWA

WISCONSIN

ILLINOIS

MISSOURI

ARKANSAS

TENNESSEE

KENTUCKY

INDIANA OHIO

MISSISSIPPI

ALABAMA GEORGIA

LOUISIANA

FLORIDA

MICHIGAN NEW YORK

WEST

VIRGINIA

VIRGINIA

NORTH CAROLINA

SOUTH

CAROLINA

PENNSYLVANIA

MD DEL.

R.I.

N.J.

CONN.

MASS.

MAINE

VT.

N.H.

0 500 MILES

0 500 KILOMETERS

EDGE OF CONTINENTAL SHELF

29°

29°

37° 37°

95°

95°

85°

85°

WASHINGTON

OREGON

CALIFORNIA

NEVADA

IDAHO WYOMING

MONTANA NORTH DAKOTA

SOUTH DAKOTA

UTAH

Houston Baton Rouge

Memphis

Figure 2. Location of the High Plains aquifer and the Gulf Coastal Plain aquifer system.

In areas where aquifers are undergoing 

development, a long-term record of water-level 

measurements may encompass the transitional 

period between the natural and the developed 

state of the aquifer. Such records are invaluable 

in understanding and addressing problems that 

have developed in response to local and regional 

patterns of withdrawal, land use, and other human 

activities. This is demonstrated by the history of 

ground-water development of the High Plains 

aquifer and the Gulf Coastal Plain aquifer system 

(Figure 2).
18

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

THE HIGH PLAINS AQUIFER

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

area of flat to gently rolling terrain that includes 

parts of eight States from South Dakota to Texas. 

The area is characterized by moderate precipitation 

but in general 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 underlie the region. During the 

late 1800’s, settlers and speculators moved to the 

plains, and farming became the major land-use 

activity in the area. Since that time, irrigation water 

pumped from the aquifer has made the High Plains 

one of the Nation’s most important agricultural 

areas.

Changes in ground-water levels in the 

High Plains aquifer are tracked annually through 

the cooperative effort of the USGS and State and 

local agencies in the High Plains region. Typically, 

water-level measurements are collected from about 

7,000 wells distributed throughout the aquifer. 

Water-level measurements are made in the spring 

prior to the start of the irrigation season to provide 

consistency across the region. Information gathered 

in this multi-State cooperative effort reveals how 

changes in water stored in the aquifer vary from 

place to place depending on soil type, irrigation 

practices, recharge from precipitation, and the 

areal extent and magnitude of water withdrawals.

Over the years, the intense use of ground 

water for irrigation in the High Plains has caused 

major water-level declines (Figure 3) and decreased 

the saturated thickness of the aquifer significantly 

in some areas. For example, in parts of Kansas, 

New Mexico, Oklahoma, and Texas, ground-water 

levels have declined more than 100 feet. Decreases 

in saturated thickness of the aquifer exceeding 

50 percent of the predevelopment saturated 

thickness have occurred in some areas. In other 

parts of the aquifer, such as along the Platte River 

in Nebraska, the recharge provided by the infiltration of excess irrigation water has caused groundwater levels to rise. The multi-State ground-waterlevel monitoring program has allowed all of these 

changes to be tracked over time for the entire High 

Plains region and has provided data critical to evaluating different options for ground-water management. This level of coordinated ground-water-level 

monitoring is unique among major multi-State 

regional aquifers.

Figure 3. Changes in ground-water levels in 

the High Plains aquifer from before groundwater development to 1997. (V.L. McGuire, 

U.S. Geological Survey, written commun., 

1998.)
19

The Gulf Coastal Plain aquifer 

system represents a good example 

of the need to measure water levels 

in wells completed at different depths 

and in the context of a three-dimensional 

ground-water-flow system.

THE GULF COASTAL PLAIN AQUIFER 

SYSTEM

The Gulf Coastal Plain aquifer system 

consists of a large and complex system of 

aquifers and confining units that underlie about 

290,000 square miles extending from Texas to 

westernmost Florida, including offshore areas 

to the edge of the Continental Shelf. The Gulf 

Coastal Plain aquifer system represents a composite 

example of many of the issues for which long-term 

water-level data are collected and used. Water withdrawals from the aquifer system have caused 

lowering of hydraulic heads at and near pumping 

centers; reduced discharges to streams, lakes, and 

wetlands; induced movement of saltwater into parts 

of aquifers that previously contained freshwater; 

and caused land subsidence in some areas as a 

result of the compaction of interbedded clays within 

aquifers. 

The Gulf Coastal Plain aquifer system represents a good example of the need to measure water 

levels in wells completed at different depths and in 

the context of a three-dimensional ground-waterflow system. For example, in order to simulate 

ground-water flow for the entire aquifer system, 

Williamson and Grubb (in press) subdivided the 

aquifer system into 17 regional aquifers and 

confining units, most of which are shown in the 

vertical section in Figure 4. Even this level of 

subdivision represents a very coarse subdivision 

of the aquifer system given its complexity and 

variability. Numerous more refined subdivisions of 

parts of the aquifer system for smaller scale studies 

have been made during the long history of groundwater studies in the region. 

The value of long-term water-level data for 

the Gulf Coastal Plain aquifer system is illustrated 

by briefly examining the history of ground-water 

development near three large cities (Memphis, 

Tennessee; Houston, Texas; and Baton Rouge, 

Louisiana) and by examining some fundamental 

changes in the regional ground-water-flow system.
20

MISSISSIPPI EMBAYMENT

AQUIFER SYSTEM

COASTAL LOWLANDS

AQUIFER SYSTEM

11 6

5

4

3

2

7 8

9

10

11

16

17

15

13

14

Coast

Gulf of Mexico

Seawater

N S

0

0

50 100 KILOMETERS

50 100 MILES

Aquifer and model layer number—

 Patterned area contains saline water

Confining unit and model layer

 number

 

Unit truncated at edge of Continental

 Shelf

Illinois Louisiana

5,000

FEET

SEA

LEVEL

5,000

10,000

15,000 SCALE IS APPROXIMATE

7

15

12

EXPLANATION

Geopressured sediments

Figure 4. Aquifers and confining units and designation of layers in a regional model of the Gulf 

Coastal Plain aquifer system. (Modified from Williamson and Grubb, in press.)
21

Memphis, Tennessee

The Memphis aquifer (Memphis Sand) is the 

principal source of water for municipal, commercial, and industrial uses in the Memphis area of 

Tennessee. Pumpage increased from completion of 

the first well in 1886 until about 1974, when rates 

stabilized. Prior to development, the potentiometric 

surface of the Memphis aquifer is presumed to have 

been a smooth surface with a gentle slope to the 

west-northwest (Figure 5). Water-level data indicate 

that over the years a regional cone of depression 

has developed in the potentiometric surface of 

the aquifer, centered near downtown Memphis 

(Figure 6). As a result of ground-water withdrawals, 

the general direction of ground-water flow is toward 

the center of the regional cone of depression. 

Smaller cones of depression are superimposed 

upon this regional cone in areas heavily pumped 

by municipal and industrial wells.

230

220

230

240

250

260

280

280

270

270

270

Base from U.S. Geological Survey

1:24,000 and Mississippi River

Commission 1:62,500 quadrangles

90 ̊15' 90 ̊00' 89 ̊45'

35 ̊15'

35 ̊00'

0 5 MILES

0 5 KILOMETERS

TN AR

MS Study area

EXPLANATION

Potentiometric contour—

 Shows altitude at which

 water level would have

 stood in tightly cased

 wells. Contour interval

 10 feet. Datum is sea

 level

Well for which water-level

 measurements were

 used as control

Inferred limit of

confining bed

overlying

Memphis aquifer

ARKANSAS

TENNESSEE

MISSISSIPPI

Loosahatchie River

Wolf

River

MISSISSIPPI RIVER

Nonconnah Creek

Memphis

230

Figure 5. Inferred potentiometric surface of the Memphis aquifer prior to ground-water development. The observation wells shown were selected for their early records away from initial pumping 

centers. (Modified from Criner and Parks, 1976.)
22

Water-level hydrographs for two selected 

wells through 1995 show the effects of long-term 

pumpage (Figure 7). One well (Figure 7A) is near 

the center of the regional cone of depression and 

has one of the longest nearly continuous records 

of water-level measurements in the United States. 

Between 1928 and 1975, the water level in this 

well declined about 70 feet and then stabilized 

as the pumping rates stabilized. A second well 

(Figure 7B) is east of the center of the regional 

cone of depression. Water levels in this well have 

declined steadily since records began in 1940, 

Base from U.S. Geological Survey

1:24,000 and Mississippi River

Commission 1:62,500 quadrangles

90 ̊15' 90 ̊00' 89 ̊45'

35 ̊15'

35 ̊00'

0 5 MILES

0 5 KILOMETERS

TN AR

MS Study area

EXPLANATION

230

Memphis Light, Gas and

 Water Division well field

Potentiometric contour—

 Shows altitude at which

 water level would have

 stood in tightly cased

 wells. Dashed where

 approximately located.

 Hachures indicate

 depression. Contour

 interval 10 feet. Datum

 is sea level

Well for which water-level

 measurements were

 used as control

Inferred limit of

confining bed

overlying

Memphis aquifer

Nonconnah Creek

Loosahatchie River

ARKANSAS

180

190

200

210

220

230

240

250

170

160

l l l l l l l

140

l l l l l l l l l l l l l l l l l l l l l l l l

l l l l l l l l l

l l l l l l l l l

l l l l l

l l l l

l l l l

l lll ll ll l

l ll

lll

l l l l l l l l

150

140

130

120

210

260

270

150

160

170

180

190

200

210

220

230

240

260

170

250

240

170

l l

l l

lll

l

190l l l l l l l l l l l l l l l l l l l l l l l

l l l l l l l l l l l 180

l l l l l l l l l l l l l l l l150

l l l l l l l l l l l l l l l l l

140

130 ll lll ll lll

Sh:P-76

Sh:Q-1

120

Wolf

River

MISSISSIPPI RIVER

TENNESSEE

MISSISSIPPI

Memphis

Figure 6. Potentiometric surface of the Memphis aquifer in 1995 showing cones of depression and 

location of observation wells Sh:P–76 and Sh:Q–1. (Modified from Kingsbury, 1996.) 
23

suggesting that the cone of depression continued 

to expand eastward for at least 20 years past the 

overall stabilization in pumping rates. Note that the 

seasonal fluctuation in water levels recorded in these 

observation wells is primarily a result of seasonal 

differences in water demand and pumping (as 

opposed to changes in aquifer recharge) and is 

much greater near the center of the cone of depression (Figure 7A) than in outlying areas (Figure 7B). 

Long-term monitoring of water levels in the 

Memphis aquifer continues to provide essential 

information for management of this critical aquifer. 

As noted, monitoring is important not only near the 

major pumping centers but also in outlying areas.

50

120

110

100

90

80

70

60

70

80

90

100

110

120

130

140

150

1925 1930 1935 1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995

1925 1930 1935 1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995

WATER LEVEL, IN FEET BELOW LAND SURFACE WATER LEVEL, IN FEET BELOW LAND SURFACE

Missing

record

Well Sh:P-76 at Memphis

Land-surface altitude: 287 feet

Well depth: 488 feet

Well Sh:Q-1 near Cordova

Land-surface altitude: 330 feet

Well depth: 384 feet

B

A

Figure 7. Declining water-level trends in two long-term observation wells in the Memphis area. 

Locations of wells are shown in Figure 6.
24

Houston, Texas

Trends in ground-water withdrawals in the 

Houston, Texas, area are related to population and 

industrial growth, replacement of ground water by 

surface water as a source of supply in some parts of 

the area, and a shift from withdrawal for irrigation 

to public supply as a result of urban expansion. 

Ground-water withdrawals more than doubled every 

20 years in the area during 1900–60 (Wood and 

Gabrysch, 1965). Ground water was the sole source 

of public-water supply for Houston until 1954, 

when surface water was introduced from the 

San Jacinto River. As a result of the increased 

use of surface water and reduced ground-water 

withdrawals, ground-water levels stabilized in the 

industrial district of Houston in the mid-1970’s and 

began to recover in 1977 (Figure 8A). However, 

ground-water withdrawals continued to increase 

to the north and west of Houston because of urban 

development. As a result, water levels in these 

areas continued to decline (Figure 8B).

Wells: LJ-65-23-138 and LJ-65-23-139

Screen: 624–805 feet

Well: LJ-65-23-219

Screen: 698–1,235 feet

Well: LJ-65-21-805

Screen: 706–1,007 feet

Well: LJ-65-21-302

Screen: 710–1,650 feet

Well: LJ-65-23-220

Screen: 329–465 feet

1931

440

400

360

320

280

240

200

160

120

80

40

440

400

360

320

280

240

200

160

120

80

40

1936 1941 1946 1951 1956 1961 1966 1971 1976 1981 1986 1991

1931 1936 1941 1946 1951 1956 1961 1966 1971 1976 1981 1986 1991

WATER LEVEL, IN FEET BELOW LAND SURFACE WATER LEVEL, IN FEET BELOW LAND SURFACE

A

B

Figure 8. Water-level trends in selected wells in the Houston area showing 

(A) stabilization and recovery of water levels in the industrial district and 

(B) declining water levels north and west of Houston. (Modified from Grubb, 

1998.)
25

Extensive land subsidence has occurred in 

the greater Houston area as a consequence of the 

decline in ground-water levels. Long-term waterlevel measurements in the Houston area are invaluable indicators of the potential for subsidence. So 

long as hydraulic heads (indicated by water-level 

measurements) remain above previous minimum 

heads, the deformation of the aquifer is reversible. 

When hydraulic heads decline below previous lows, 

the structure of interbedded clay and silt layers may 

undergo significant rearrangement, resulting in 

irreversible aquifer-system compaction and land 

subsidence. In this low-lying coastal environment, 

as much as 10 feet of subsidence has increased 

the vulnerability of much of the area to flooding, 

caused permanent inundation of some areas, and 

activated faults causing damage to buildings, highways, and other structures. Subsidence to the east 

of Houston has been arrested as imported surfacewater supplies have been substituted for groundwater pumpage, but the fast-growing areas to 

the north and west, which still depend largely on 

ground water, are subsiding in response to declining 

ground-water levels (Figure 9).

1975 1995 1985

Change in ground-water levels in wells in the

Evangeline aquifer, 1977–1997

Some arrested subsidence

1974–1997

Continuing subsidence

1974–1997

0

0

40

–40

–40

80

–80

–80

120

–120

160

–160

Houston

Texas City

Baytown

1975 1995 1985

2.4

3.0

1.2

1.8

0.6

0

0.8

0.4

0

SUBSIDENCE,

IN FEET

SUBSIDENCE, IN FEET

Extensometer sites for

 measuring subsidence

Addicks

Lake Houston

Baytown

Clear Lake

Texas City

Areas with rising 

water levels

Areas with declining water levels

Line of equal

ground-water

level change

(interval 40 feet)

D

E

B

A

A

B

C

D

E

C

EXPLANATION

B

A

D

E

C

Figure 9. Relation between water-level trends and land subsidence 

in the Houston area. (Modified from Kasmarek and others, 1997; Coplin 

and Galloway, 1999.)
26

Baton Rouge, Louisiana

Trends in ground-water levels in the Baton 

Rouge area reflect growth in population and 

industry. Withdrawals increased more than tenfold 

from the 1930’s to 1970 and have since leveled off 

to some extent. In 1995, about 140 million gallons 

per day (Mgal/d) of ground water were pumped in 

the Baton Rouge area.

Sand layers at depths between about 400 and 

2,800 feet are major aquifers in the Baton Rouge 

area. Locally, the aquifers are referred to by the 

general depth of the top of the aquifer in the area, 

for example, the “2,000-foot” sand. The effects of 

overall increases of withdrawals on ground-water 

levels, as well as of a shift in pumpage from shallower to deeper sands, are shown for wells in the 

industrial area of Baton Rouge in Figure 10.

The hydrograph in Figure 10 for the 

shallower ground water is a composite of water 

levels in three wells monitored over the years in 

the “400-foot” and “600-foot” sands. During the 

early 1940’s to mid-1950’s, the “400-foot” and 

“600-foot” sands were the most heavily pumped 

aquifers in the Baton Rouge area, and pumpage 

reached a peak of 35 Mgal/d about 1942 

(Kuniansky, 1989). The hydrograph indicates that, 

after reaching record water-level lows in the mid1950’s, water levels (heads) in these aquifers rose 

steadily from the late 1950’s to 1990. During that 

period, deeper aquifers were developed, pumpage 

from the “400–600 foot” sands declined (to about 

12 Mgal/d in 1990), and pumping centers became 

distributed over wide areas. Water levels again 

declined in the 1990’s as withdrawals from the 

–250

–200

–150

–100

–50

0

50

100

1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000

WATER LEVEL, IN FEET ABOVE OR BELOW SEA LEVEL

Well EB-90 "2,000-foot" sand

Screen: 2,025–2,120 feet

Well EB-15 "600-foot" sand

Screen: 320–679 feet

Well EB-78 "400-foot" sand

Screen: 332–423 feet

Well EB-870 "600-foot" sand

Screen: 687–692 feet

Missing record

Missing record

Figure 10. Water-level trends in the Baton Rouge area, Louisiana, 1940–99. (Modified from 

Grubb, 1998.)
27

30°28'

91°12' 91°10' 91°08'

30°26'

30°24'

0 1 2 MILES

0 1 2 KILOMETERS

Pumping center

for public supply

Monitor well

Saltwater

encroachment

Saltwater front for

specific year

Area of

map

SALTWATER AREA

BATON

ROUGE

FAULT

LOUISIANA

Mississippi River

1992

1977

1965

BATON ROUGE

shallow aquifers increased (pumpage was about 

20 Mgal/d in 1995). Water-level declines in the 

well shown (well EB–870) were limited, however, 

because the pumping was less concentrated near 

that well location. 

Prior to about 1920, pumpage from the 

“2,000-foot” sand was small (less than 0.5 Mgal/d) 

and had little effect on heads in the aquifer (Torak 

and Whiteman, 1982). Pumping increased sharply 

to more than 10 Mgal/d after 1940 and has 

become redistributed in the Baton Rouge area as 

the locations of the major pumping centers have 

changed. A long-term hydrograph for well EB–90 

(Figure 10) completed in the “2,000-foot” sand 

shows that, as water use from this deeper aquifer 

increased, heads in the aquifer continued to decline 

from 1940 to the 1970’s. After reaching a 

maximum rate of 44 Mgal/d in 1976, pumpage 

from the “2,000-foot” sand began to decline to 

about 32 Mgal/d by 1985, resulting in a slight 

recovery in heads. From 1985 to 1995, pumpage 

increased, and water levels in well EB–90 declined 

again in the 1990’s, albeit at a slower rate than 

before. 

The large water-level (head) declines in the 

Baton Rouge area caused saltwater encroachment 

from the south in several of the sand aquifers 

(Figure 11). Long-term water-level monitoring is 

essential to continued understanding and forecasting 

movement of saltwater in the Baton Rouge area (as 

well as in other areas of the country, as discussed in 

later examples).

Figure 11. Saltwater encroachment in 

the “1,500-foot” sand aquifer moving 

toward pumping centers in the Baton 

Rouge area, Louisiana. A low-hydraulicconductivity fault zone retards saltwater 

movement in the area. Nevertheless, saltwater has leaked through the fault zone 

in some areas in response to pumping. 

(Modified from Tomaszewski, 1996.) 
28

The preceding examples for Memphis, 

Houston, and Baton Rouge illustrate how the 

history of ground-water development is reflected in 

long-term water-level records and how these records 

are essential to monitoring the effects of development and providing data needed for quantitative 

assessments of future management and development options. For all three metropolitan areas, 

individual long-term monitoring wells have provided 

valuable information about water-level trends at 

specific locations, but multiple wells are needed to 

track conditions in different aquifers and changes 

in cones of depression as pumping centers evolve. 

Furthermore, the examples show how information 

about ground-water withdrawals can be critical to 

the interpretation of water-level data.

Individual long-term monitoring wells 

have provided valuable information 

about water-level trends at specific 

locations, but multiple wells are needed 

to track conditions in different aquifers 

and changes in cones of depression 

as pumping centers evolve.

Information about ground-water 

withdrawals can be critical to the 

interpretation of water-level data.
29

Changes in Regional Ground-Water Flow 

As illustrated in the previous examples, more 

than 100 years of ground-water withdrawals have 

greatly altered ground-water conditions in the Gulf 

Coastal Plain aquifer system. As a result, there have 

been large-scale, regional changes in directions of 

horizontal flow, changes in vertical direction of flow 

between aquifers, increases in regional recharge to 

aquifers, and decreases in regional discharge from 

aquifers. 

Ground-water withdrawals from deeper aquifers in the Gulf Coastal Plain aquifer system have 

caused a reversal of vertical-flow directions from 

upward to downward throughout thousands of 

square miles. This was evident locally for the Baton 

Rouge area by the crossing of the water-level hydrographs in Figure 10. That is, heads in the upper 

sands were lower than heads in the underlying 

“2,000-foot” sand prior to the early 1960’s, 

resulting in upward flow. As withdrawals shifted 

to the deeper aquifers, heads in the “2,000-foot” 

sand declined below those in the shallower sands, 

reversing the vertical direction of flow from upward 

to downward. 

The relative changes in heads with depth 

and the magnitude and direction of vertical flow 

between aquifers are significant factors affecting 

future pumping lifts, base flow to streams, saltwater 

intrusion, and land subsidence. Such changes 

in an aquifer system typically are evaluated using 

computer model simulations. For example, the 

simulated widespread reversal of vertical-flow 

directions from predevelopment to 1987 for 

the upper part of the Gulf Coastal Plain aquifer 

system is shown in Figure 12. Model calibration 

and estimation of model accuracy required water 

levels measured at different depths before and 

after development and relied heavily on a compilation of water-level data collected by many prior 

studies throughout the region (Williamson and 

Grubb, in press).

Measuring water level in observation 

well in Colorado. Photograph by 

Heather S. Eppler, U.S. Geological 

Survey.
30

KENTUCKY

TENNESSEE

MISSISSIPPI

TEXAS LOUISIANA

ALABAMA

FLA.

ARKANSAS

MISSOURI

ILLINOIS

GULF OF MEXICO

Study area boundary

Edge of Continental Shelf

30°

35°

100°

100°

95°

90°

30°

35°

95°

90°

0

0 50 100 150 200 KILOMETERS

50 100 150 200 MILES

Vicksburg

Natchez

Baton Rouge

New

Orleans

Mathis

Galveston

Houston

Carrizo

Springs

Marianna

Wynne Memphis

Little Rock

Frio

San

Brazos

Navasota R

Angelina

Nueces

River

Calcasieu

Sunflower R

R

R

Black R

Big

Black

R

Yalobusha R

Tallahatchie R

River

Colorado

Trinity

Sabine

River

River

River

River

River

River Galveston

Bay

River

Red

Saline R

White R

Arkansas R

Ouachita

Pearl

River

Leaf

Alabama R

River

R

Neches

R

R

MISSISSIPPI

St. Francis

River Cache

RIVER

Anto nio

Guadalupe R

Lake Pontchartrain

Ya zoo

Chickas awhay R

Pascagaoula R

Tombigbee R

RIO

GRANDE

Hickman

Ohio R.

Base from U.S. Geological Survey

Scale 1:2,500,000

EXPLANATION

Area where simulated vertical flow was reversed

 from upward under predevelopment conditions

 to downward by 1987

Updip boundary of uppermost aquifer layer

 (Downdip boundary is edge of Continental Shelf)

Figure 12. Areas where vertical flow between uppermost aquifer layers reversed from upward under 

predevelopment conditions to downward by 1987, as simulated in regional model of Gulf Coastal 

Plain aquifer system. (Modified from Grubb, 1998.) 
31

More than 40 million people in the 

United States supply their own drinking water 

from domestic wells. Many of these wells are 

shallow and vulnerable to extended droughts. Yet, 

relatively few observation wells are measured regularly to provide an indication of the response of 

ground water to climatic conditions. Wells for such 

purposes are needed in relatively undeveloped 

recharge areas where water-level fluctuations primarily reflect climatic variation rather than groundwater withdrawals or human-induced recharge. 

The timeliness of water-level data also is a critical 

factor. Most wells are measured monthly or less 

frequently. Even if wells are equipped with a digital 

water-level recorder, the data must be retrieved and 

processed before they are available. As a result, 

available water-level data commonly lag behind 

current conditions from one to several months. 

Continuous collection, processing, and 

transmittal of water-level data by satellite and other 

telecommunication methods are increasingly being 

used to display real-time ground-water conditions 

on the Internet. The need for this type of information became evident during the summer of 1999, 

when drought in the Eastern United States resulted 

in drought declarations or water restrictions in 

15 States. Following a relatively dry spring and 

summer, rainfall from several large storms, including 

Tropical Storm Dennis and Hurricane Floyd, 

occurred in many of these States during the months 

of August and September 1999. After each storm, 

questions arose about whether water restrictions 

should be lifted. Each time, information on groundwater conditions was sought to help provide a 

complete picture of the drought conditions. The 

information typically was limited and not current. 

The State of Pennsylvania was an exception.

Drought Monitoring in Pennsylvania

Continuous collection, processing, 

and transmittal of water-level data by 

satellite and other telecommunication 

methods are increasingly being used 

to display real-time ground-water 

conditions on the Internet.
32

In 1931, in response to concerns about 

ground-water-level declines caused by the drought 

of 1930, a statewide well network was established 

in Pennsylvania to monitor water-level fluctuations. 

Today, this network consists of about 50 wells 

(Figure 13) operated by the USGS in cooperation 

with the Pennsylvania Department of Environmental Protection. The primary purpose of the 

observation-well network is to monitor groundwater conditions for indications of drought. The 

Pennsylvania Emergency Management Council 

uses data from the wells when categorizing counties 

for a drought declaration. Currently (2001), water 

levels for about 80 percent of the network wells are 

transmitted by satellite telemetry and displayed on 

the USGS Web pages for Pennsylvania. 

An observation-well network of 23 wells 

established in 1973 provides additional spatial 

resolution for ground-water conditions in Chester 

County, Pennsylvania (Schreffler, 1997). The 

Chester County network was established through a 

cooperative agreement between the Chester County 

Above normal

EXPLANATION

July 1999 average water levels

 in observation wells

Normal

Below normal

No data

Record high

Record low

Normal is defined as a water level 

between the 25th percentile and the 

75th percentile of monthly mean 

water levels for all previous years 

of record for July.

Prepared by the U.S. Geological Survey, in cooperation with the 

Bureau of Watershed Conservation, Pennsylvania Department of 

Environmental Protection

Figure 13. Location of drought-index wells and ground-water-level conditions in Pennsylvania 

in July 1999. 
33

Water Resources Authority and the USGS. The 

wells are distributed throughout the county in 

different geographic and geologic settings. A waterlevel hydrograph for a well that is in both the statewide network and the Chester County network is 

shown in Figure 14 for water years 1998 and 

1999.

Data from the Pennsylvania network were 

used by the State to help respond to the 1999 

drought. When drought emergency was declared 

in 55 Pennsylvania Counties in July 1999, one-third 

of the State’s network wells had record-low 

seasonal levels. The Governor was able to note 

that “(ground-) water levels we’re seeing today—in 

the middle of summer—are on par with levels we 

would see in September or October...Groundwater 

levels typically won’t begin to recharge until the 

leaves are off the trees and we get sustained 

rains in the fall” (Pennsylvania Department 

of Environmental Protection, news release, 

July 20, 1999).

6

7

8

9

10

11

12

13

14

15

16

17

18

DEPTH TO WATER, IN FEET BELOW LAND SURFACE

Oct Nov Dec Jan Feb Mar Apr May June

1997 1998 1999

July Aug Sept Jan Feb Mar Apr May June July Aug Sept Oct Nov Dec

Measured

EXPLANATION

Warning

Emergency

Figure 14. Hydrograph for drought-index well in Chester County, Pennsylvania, showing 

water levels from October 1997 to September 1999, compared to established monthly 

drought-warning and drought-emergency water levels. (D.W. Risser, U.S. Geological 

Survey, written commun., 2000.)
34

Statistical evaluations of water-level data 

collected for one or more decades can be used 

to estimate future high, low, and medium or 

“normal” water levels. The accuracy of these waterlevel estimates improves as the length of record 

increases.

In populous areas of coastal Massachusetts 

and Rhode Island, water levels normally change by 

several feet annually but can change by as much as 

20–30 feet (Socolow and others, 1994). This 

potentially wide range of ground-water fluctuation 

can result in adverse effects to home and building 

construction. Estimates of the maximum (highest) 

probable ground-water levels are needed to assess 

the likely chances for basement flooding, damage 

to building foundations due to increased hydrostatic 

pressure, and the potential failure of septic tanks 

and leach fields in unsewered areas (Figure 15). 

Estimation of High Ground-Water Levels

in Massachusetts and Rhode Island

Figure 15. Sketch showing effects of unanticipated high groundwater-level fluctuations on housing structures. (Modified from 

Socolow and others, 1994.) 
35

To address this problem, USGS hydrologists 

in Massachusetts developed a technique to estimate 

the potential maximum ground-water level at a site 

where only a single measurement of water level may 

be available (Frimpter, 1980; Frimpter and Fisher, 

1983). The technique uses a water-level measurement taken at the site of interest in combination 

with information on the concurrent water level and 

statistical distribution of water levels in a long-term 

observation well chosen as an “index” well and 

information on the range of water-level fluctuations 

at observation wells in similar geologic and hydrologic settings. The index well should be unaffected 

by pumping and other human-induced hydraulic 

stresses, completed in the same geologic material as 

that underlying the site of interest, and located in a 

similar topographic setting. Moreover, the index 

well must have a hydrologic record sufficiently long 

to provide for a statistically based determination of 

the range in water-level fluctuations. 

In Massachusetts, water-level measurements 

from nine index wells having 16–28 years of hydrologic record and about 160 wells having shorter 

periods of record were used to map five zones of 

different ranges in annual water-level fluctuations 

in glacial sand, gravel, and till deposits that underlie 

Cape Cod (Frimpter and Fisher, 1983). Subsequent 

application of the technique in Rhode Island was 

limited by the distribution of suitable index wells 

(Socolow and others, 1994). Approximately 

15 wells completed in glacial sand and gravel 

deposits and having hydrologic records that span 

the period between 1946 and 1989 were determined to be of suitable length for use as index 

wells. Because of relatively short (generally less 

than 5 years) or discontinuous hydrologic records, 

however, no suitable index wells were identified 

among the observation wells available in the till 

deposits of Rhode Island. 

Water well instrumented for satellite transmission and real-time 

reporting on the Internet. Photograph by William L. Cunningham, 

U.S. Geological Survey.
36

The effect of ground-water development on 

surface-water resources is increasingly a focus of 

ground-water studies (Winter and others, 1998). 

Yet, stream-gaging and water-level monitoring 

networks are rarely jointly designed with this use of 

data in mind. The upper Deschutes Basin, an area 

of rapid population growth in central Oregon, 

provides an example of the importance of combined 

ground-water and surface-water data. 

Quantitative assessments of the groundwater system and its interaction with surface-water 

resources of the upper Deschutes Basin have been 

crucial in supporting resource-management decisions in the basin. Surface-water resources in the 

area have been closed by the State to additional 

appropriation for many years. Thus, virtually all new 

development in the basin must rely on ground water 

as a source of water supply.

Locations of long-term observation wells and 

field-located wells for measurements during a recent 

multiyear study of the basin (Caldwell and Truini, 

1997; Gannett and others, 2001) are shown in 

Figure 16. The temporal distribution of water-level 

measurements from these wells during and prior 

to the study is shown as a three-dimensional plot 

in Figure 17 for the period 1977–98. The sporadic 

nature of the available water-level data is evident 

from Figure 17 and, as in many areas, complicates 

analysis of the data. Only a few wells—primarily 

those measured as part of a statewide network by 

the Oregon Department of Water Resources—have 

periods of record of 10 or more years.

Recharge resulting from leakage from streams 

and irrigation canals and from on-farm irrigation 

losses greatly exceeds recharge from precipitation 

in the dry plains of the eastern and central part of 

the basin. Examples of combined use of water-level 

and stream-gaging data to provide information on 

the streams and canals as a source of recharge to 

the basin are shown in Figures 18 and 19. Understanding these relations is critical to quantitative 

modeling of the basin hydrologic system. 

Figure 18 shows hydrographs of the 

stage of the Deschutes River at Benham Falls 

and water levels in wells 500 and 5,000 feet from 

the river. This is a reach in which the river loses 

Ground-Water and Surface-Water 

Interaction in Oregon

The effect of ground-water development 

on surface-water resources is increasingly 

a focus of ground-water studies. 

Yet, stream-gaging and water-level 

monitoring networks are rarely jointly 

designed with this use of data in mind.
37

OREGON

Study area

Located well during well inventory

EXPLANATION

Long-term observation well

0

0 10 20 KILOMETERS

10 20 MILES

44°00'

44°00'

43°30' 43°30'

44°30' 44°30'

121°00'

121°30'

121°00'

121°00'

122°00'

121°30'

122°00'

Figure 16. Location of field-located wells in upper Deschutes Basin study 

area, Oregon. (Modified from Caldwell and Truini, 1997; Gannett and 

others, 2001.)
38

about 100 cubic feet per second into permeable 

lava. Stream-gaging data show that the rate of 

loss is proportional to the river stage. The well 

closer to the river is near the gaging station. The 

well farther from the river is about 4 miles downstream from the station. Water levels in both 

wells respond to changes in river stage, and the 

effect is attenuated and delayed with distance 

from the river. 

Figure 19 shows the relation between the 

static water level in a 690-foot well and the stage in 

an irrigation canal about one-half mile away. Canal 

leakage is a significant source of local recharge 

in the more arid areas of the basin. The canals 

commonly operate during the irrigation season from 

April through the beginning of October and also 

are used periodically at other times to fill stock 

ponds and other storage facilities. Isotopic evidence 

(Caldwell, 1998) substantiates that the canal (and 

possibly the Deschutes River from which this and 

other canals originate) is likely the predominant 

source of water to this well and other wells in 

areas traversed by irrigation canals. 

Figure 17. Plot showing location of measured wells in 

upper Deschutes Basin, Oregon, and times of water-level 

measurement, 1977–98. Measurements from long-term 

observation wells are shown in red. (M.W. Gannett, 

U.S. Geological Survey, written commun., 2000.)
39

4,150

4,145

4,140

4,135

4,130

4,125

3,775

3,770

3,765

3,760

3,755

3,750

RIVER STAGE AND WATER LEVEL IN WELL 500 FEET

FROM RIVER, IN FEET ABOVE SEA LEVEL

WATER LEVEL IN WELL 5,000 FEET FROM RIVER,

IN FEET ABOVE SEA LEVEL

JAN

1994

JAN

1995

JAN

1996

JAN

1997

River stage

Well 500 feet from river

Well 5,000 feet from river 

EXPLANATION

Figure 18. Hydrographs of the stage of the Deschutes River at Benham 

Falls and water levels in wells 500 and 5,000 feet from the river. (Gannett 

and others, 2001.) 

140

JAN

1994

JAN

1995

JAN

1996

JAN

1997

120

100

80

60

40

20

0

CANAL DISCHARGE, IN CUBIC FEET PER SECOND

DEPTH TO WATER, IN FEET BELOW LAND SURFACE

639

640

641

642

643

644

645

646

Discharge, Arnold Canal

Depth to water, well

EXPLANATION

Figure 19. Relation between the static water level in a well in upper 

Deschutes Basin, Oregon, and the stage in an irrigation canal about 

one-half mile away. Although over 600 feet below land surface, the water 

level in the well starts to rise shortly after the canals start flowing and 

starts to drop soon after they are shut off for the season. The water level 

also responds to periodic short-term operation of the canal. (Gannett and 

others, 2001.)
40

Wetlands provide many beneficial functions 

such as flood control, water-quality modification, 

and habitat for wildlife. Increasingly, artificially 

constructed wetlands are used in flood mitigation 

and for treatment of acid-mine and wastewater 

discharges. While they are often thought of 

only in the context of surface water, most wetlands 

are ground-water-discharge areas. The storage of 

water is crucial to wetland ecology and hydrologic 

functions (Carter, 1996). In many wetlands, the 

depth to ground water and ground-water-level 

fluctuations largely control the capacity for water 

storage. Moreover, ground-water levels are often 

important in maintaining the physical and chemical 

conditions in the root zone that promote healthy 

and stable growth of wetland plants (Hunt and 

others, 1999).

Because of the complex interaction between 

surface and ground water in wetlands, ground-water 

discharge and storage commonly are difficult 

components of the wetland hydrologic system to 

characterize. Restoration of former wetlands or 

construction of functional artificial wetlands requires 

knowledge of ground-water-flow gradients and the 

natural range in seasonal fluctuations in the water 

table. One example of the need for water-level data 

to assess the efforts required to restore a wetland is 

Wetland Hydrology in Michigan

 N

EXPLANATION

Data-collection sites

Ground water

Surface water

Ground water and

 surface water

Wetland pool

River, creek, or ditch

0 1 2 KILOMETERS

0 2 MILES 1

SENEY NATIONAL 

WILDLIFE REFUGE

MICHIGAN

Marsh Creek

Walsh Ditch

Walsh Ditch

Ducey Creek

Walsh Cr

Sweeney Cr

Marsh Creek

Driggs River

Driggs River

Diversion Ditch

Figure 20. Ground-water and surface-water observation stations in the watershed management 

area of the Seney National Wildlife Refuge wetlands, in the Upper Peninsula region, Michigan. 

Photograph shows hydrologists making flow measurements in a perennially flooded pool in the 

wetland area at the refuge. (Courtesy of “People, Land, and Water,” October 1998, published by 

U.S. Department of the Interior, Washington, D.C.)
41

highlighted by a project underway in the Seney 

National Wildlife Refuge, in the Upper Peninsula 

region of Michigan (Figure 20). Wetlands in the 

wildlife refuge were drained in 1912 in a failed 

attempt to convert the land to agricultural use. 

Research began in 1998 to evaluate the potential 

for restoration of the wetland ecosystem in approximately 33,500 acres of the refuge (Sweat, 2001). 

Engineering controls will be used to rehydrate 

wetland soils and increase the altitude of the water 

table. However, the natural range of ground-waterlevel fluctuation within the wetland area is not 

known. If ground-water levels decline significantly or 

are subject to severe seasonal fluctuations, wetland 

ecosystems can be disrupted and the function and 

sustainability of the wetland can be impaired. 

Because available water-level data were 

not sufficient to determine seasonal trends and the 

range of ground-water-level fluctuations, investigators have installed 11 long-term ground-water 

observation wells and 7 combined ground-water 

and surface-water gaging stations (Figure 20). Data 

collected at these sites will be used to assess the 

average range of water-level fluctuations under the 

existing conditions, to determine how much groundwater levels need to be raised to support wetland 

ecologic functions, and to manage wildlife habitat 

and flood control in a perennially flooded pool in 

the wetland.

Downloading data from automatic water-level recorder. 

Photograph by Michael D. Unthank, U.S. Geological 

Survey.
42

The role of water-level data in the investigation of ground-water quality or contamination problems is sometimes underappreciated. To a large 

degree, predictions about the speed and direction of 

movement of ground-water contaminants are based 

on determination of the gradient (slope) of the water 

table or potentiometric surface in the affected 

aquifer. While the data needed for these determinations typically are obtained by synoptic water-level 

surveys, longer term water-level measurements are 

often needed to develop an understanding of how 

ground-water contaminants migrate from their 

sources through the ground-water system. For 

example, an examination of water-level hydrographs and graphs of contaminant concentrations 

over time may reveal a relation between the occurrence of event-related or seasonal changes in 

ground-water recharge and fluctuations in the 

contaminant concentrations.

Increasingly, computer-based solute-transport 

models are used to simulate subsurface migration 

and behavior of ground-water contaminants. Waterlevel data of sufficient duration and frequency of 

measurement are needed to calibrate and evaluate 

the reliability of the flow component of these 

models before realistic simulations of contaminant 

transport can be made. 

Many ground-water-quality problems develop 

over long periods due to human-induced changes 

in hydraulic heads and resultant changes in the 

dynamics of a ground-water-flow system. Degradation of freshwater aquifers by the intrusion of saline 

water is a particularly common ground-water-quality 

problem of this type. 

The use of long-term water-level data to 

address saline-water intrusion is presented in two 

examples here. These are followed with an example 

related to concerns about ground-water degradation 

from residential development.

Relevance of Water-Level Data

to Ground-Water Quality Issues

The role of water-level data in the 

investigation of ground-water quality 

or contamination problems is 

sometimes underappreciated.
43

1

10

100

1950195519601965197019751977

1,000

CHLORIDE CONCENTRATION,

IN MILLIGRAMS PER LITER

35

35

36

Raritan Bay

Keyport

Harbor

0 1 MILE

–20

250

10

–30

–40

2.0

2.6

7.3

98

3.1

1.8

2.8

2.0

660

Union Beach

Well Field

232

EXPLANATION

Line of equal head—

 Contour interval 10 feet.

 Datum is sea level

Line of equal chloride con-

 centration, in milligrams

 per liter—Contour interval 

 is variable

Location of chloride

 data— Chloride concen-

 tration, in milligrams per

 liter

Generalized direction of

 ground-water flow

10

–40

2.0

N

NEW

JERSEY

Raritan

Bay

SALINE WATER INTRUSION IN 

NEW JERSEY

The relation between the intrusion of 

saline water and declining hydraulic heads due to 

extensive aquifer development is well illustrated by 

the aquifers in the Coastal Plain of New Jersey 

(Lacombe and Rosman, 1997). Since the 1800’s, 

the principal source of public-water supply in the 

Coastal Plain of New Jersey has been ground water 

obtained from wells in 10 major confined aquifers. 

The aquifers are arranged in a dipping, layered 

ground-water system similar to that of the Gulf 

Coastal Plain aquifer system. Because of large 

ground-water withdrawals, regional cones of depression have developed in each of the aquifers. By 

1978, the potentiometric surfaces of most 

of the aquifers had been lowered below sea level, 

and natural flow directions in some areas were 

reversed. Consequently, saline water that is naturally present in the deeper parts of the aquifers was 

induced to migrate toward pumping centers, and 

chloride and dissolved-solids concentrations 

increased significantly in parts of these aquifers. 

As an example, pumping by public-supply 

wells completed in the Upper Potomac-RaritanMagothy aquifer near the New Jersey coastline 

resulted in a decline in hydraulic heads to more 

than 40 feet below sea level (Schaefer and Walker, 

1981). The development of this large cone of 

depression in the potentiometric surface in the 

aquifer also resulted in the landward reversal of 

ground-water flow and migration of saline water. 

Throughout the 1970’s, ground water in parts 

of the aquifer became progressively degraded by 

sharply rising chloride concentrations, as shown for 

the Union Beach well field in Figure 21. Although 

pumping was curtailed in the 1980’s, degradation 

of the aquifer by saline water was sufficiently extensive that the well field was later abandoned and 

replaced by wells farther inland.

Because of the continued potential 

threat of degradation of the freshwater parts 

of the aquifers, ground-water withdrawals are carefully monitored and regulated by the New Jersey 

Department of Environmental Protection (NJDEP). 

In addition, the NJDEP and USGS have developed 

a cooperative program to monitor changes in water 

levels and chloride concentrations at 5-year intervals 

in each of the confined aquifers. As part of this 

Figure 21. Relation between reductions in 

heads from pumping and chloride concentrations in the Upper Potomac-Raritan-Magothy 

aquifer, New Jersey, 1977. Chloride concentrations shown in the graph are a composite of 

concentrations in water samples from wells 

screened at about the same depth in the Union 

Beach well field. (Modified from Schaefer and 

Walker, 1981.)
44

monitoring program, water-level hydrographs are 

prepared from continuous measurements collected 

in 99 long-term observation wells and used to assess 

seasonal trends in ground-water recharge and 

storage. Water-level measurements are made in 

approximately 1,000 additional observation wells 

and used to construct potentiometric maps showing 

any significant changes in the size of the cones 

of depression developed in the aquifers. Water 

samples are collected from selected observation 

wells for analysis of chloride and dissolved-solids 

concentrations, and these data are compiled to 

monitor changes in the relation between hydraulic 

heads, ground-water-flow directions, and groundwater quality. Using this combined water-level and 

water-quality monitoring program, the NJDEP can 

evaluate the effects of water-management decisions 

on the aquifers and carefully monitor the improvement or further degradation of water quality in the 

aquifers.

UPWELLING OF SALINE WATER 

IN UTAH

Chloride contamination also can occur in 

noncoastal areas where the freshwater aquifer is 

invaded by saline water or brines upwelling from 

deeply buried sedimentary rocks. Spangler and 

others (1996) documented an example of this 

problem in a study of the quality of water in the 

Navajo aquifer in southeastern Utah.

The Navajo aquifer, composed of the 

Entrada and Navajo Sandstone formations, is 

one of several aquifers separated by confining 

units within a large sedimentary basin that underlies San Juan County, Utah (Figure 22). Within 

the basin, the top of the Navajo aquifer averages 

about 550 feet below land surface, and the thickness of the aquifer generally ranges from 750 to 

1,000 feet. The Navajo aquifer is recharged mainly 

by infiltration where the sandstones crop out at the 

surface along several mountain ranges that surround 

the basin. The Navajo aquifer is confined above by 

the Wanakah Formation and below by the ChinleMoenkopi Formation. Artesian pressures are so 

great in parts of the aquifer that ground water 

discharges naturally at the land surface from 

open, flowing wells. 

Much of the sedimentary basin that contains 

the Navajo aquifer has been explored and developed 

for oil and gas. Several oil fields were developed 

in the basin in the 1950’s, and exploration and 

production generally have increased since then. 

The main oil-producing zones in the basin are 

in carbonate rocks of the Paradox Formation, at 

depths ranging from 5,000 to 6,000 feet below 

land surface. Over time, as oil was extracted and 

oil-field pressures declined, the technique of water 

flooding—the injection of freshwater from alluvial 

aquifers along the San Juan River to flush residual 

oil—was used to boost production in the Paradox 

Formation oil wells. This practice began in the late 

1950’s and continues to the present. Brine water, 

obtained from the Paradox Formation as a byproduct of the water flooding and oil recovery 

process, was reinjected into deep wells completed 

in the oil-producing zones for disposal. 

Water-quality problems associated with 

increased chloride concentrations in wells drilled 

into the Navajo aquifer began to be reported in the 

1950’s. A review of historical water-level records 

indicated that hydraulic heads in the Navajo aquifer 

had declined by as much as 178 feet since the early 

1950’s because of increased development. The 

decline in hydraulic heads in the Navajo aquifer had 

resulted in an increased upward hydraulic gradient 

between the upper Paleozoic aquifer and Navajo 

aquifer (Figure 23). This indicated that ground 

water from the upper Paleozoic aquifer could 

provide recharge to the Navajo aquifer in locations 

where the Chinle Formation confining unit was 

breached by fractures or by improperly sealed 

wells. 

The information from historical water-level 

measurements was used to guide water-quality 

sampling needed to identify the source of the chloride contamination in the Navajo aquifer. Water 

samples were collected from wells completed 

in the Navajo aquifer, the upper Paleozoic aquifer, 

the Paradox Formation, and other deep aquifers 

(Spangler and others, 1996). Samples of the brine 
45

Figure 22. Geologic section showing the stratigraphic relations and movement of ground 

water between the Navajo aquifer, upper Paleozoic aquifer, and other major aquifers and 

confining units, San Juan County, southeastern Utah. (Modified from Spangler and others, 

1996.)

San Juan River

South North

Redwall-Leadville Limestone

Paradox Formation

Navajo aquifer (Entrada-Navajo Sandstones)

Confining unit

Aquifer

Potential direction of ground-

 water flow determined by

 differences in hydraulic head

 between aquifers

EXPLANATION

Chinle-Moenkopi

 Formation

Honaker Trail Formation

Pinkerton Trail Formation

Halgaito Member

Upper Paleozoic aquifer

(Cutler Formation)

Morrison Formation

UTAH San Juan

County

water being reinjected at producing oil wells also 

were collected. Detailed chemical analyses of these 

water samples indicated that the degradation of 

water quality in wells completed in the Navajo 

aquifer was caused primarily by the upwelling and 

mixing of saline water from the upper Paleozoic 

aquifer. The brine water reinjected into the Paradox 

Formation was determined to be an unlikely source 

of the chlorides in the Navajo aquifer. The oil 

and gas production activities may have contributed 

indirectly to the water-quality problem, however, 

as a review of well-construction logs identified 

over 200 active and abandoned oil wells that 

may be inadequately cased or sealed. These wells 

could provide conduits by which ground water 

migrates upward from the upper Paleozoic aquifer 

and intermingles with ground water in the Navajo 

aquifer.
46

McElmo

Creek

Gothic

Creek

SAN

JUAN

RIVER

Tsitah

Wash

Creek

Desert

Montezuma

Creek

Recapture Creek

Cottonwood

Wash

262

41

262

163

262

262

191

191

191

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

1983 Universal Transverse Mercator projection, Zone 12

EXPLANATION

37 15'

37 °00'

37 30'

109 30' 109 15' 109 00'

5,060

5,020

4,980

4,860 4,900

4,940

4,820

4,780

4,740

4,700

4,660

4,620

4,580

4,540

4,500

4,580

4,700

4,660

4,620

4,740

4,540

4,820

4,860

4,900

4,940

4,980

5,060

5,100

5,020

5,140

4,580

4,540

4,500

4,460

4,420

4,500

4,780

5,100

0 5 10 KILOMETERS

0 5 10 MILES

° °

°

°

°

Well completed in the Navajo aquifer

Well completed in upper Paleozoic aquifer

UTAH

UTAH

COLORADO MONTEZUMA COUNTY

SAN JUAN COUNTY

ARIZONA

Area where the relative difference between hydraulic head

 in the upper Paleozoic aquifer and the Navajo aquifer

 indicates upward direction of ground-water flow (Modified

 from Freethey and Cordy, 1991)

Boundary of study area

Potentiometric contour—Shows altitude at which water level would

 have stood in tightly cased wells completed in the Navajo aquifer; 

 contour interval 40 feet, dashed where approximate. Datum is sea

 level

Figure 23. Potentiometric map of the Navajo aquifer showing locations of wells used for 

water-level measurements and the inferred area of upward ground-water flow from the upper 

Paleozoic aquifer, San Juan County, southeastern Utah. (Modified from Spangler and others, 

1996.)
47

EFFECTS OF RESIDENTIAL 

DEVELOPMENT IN MONTANA

The early stages of land or aquifer development is an opportune time to establish a combined 

water-level and water-quality monitoring network 

that can define baseline conditions and track important changes with time in the quantity and quality of 

the resource. Examples are provided for the Gallatin 

and Helena areas in southwestern Montana, which 

are among many parts of the Western United States 

where rapid changes in land development have the 

potential to affect ground-water resources. 

The Gallatin Valley is an intermontane 

basin that consists of an alluvial plain flanked by 

higher elevation benches (Figure 24). The alluvial 

plain is used primarily for irrigated agriculture and 

the benches for dryland farming. In recent years, 

residential and commercial development has 

replaced considerable areas of farmland on both the 

alluvial plain and the benches. Much of the population increase has been outside of established cities 

and towns, in areas where each home has its own 

well and septic system. The residential development 

has raised concerns regarding the potential effects 

of infiltrating septic wastewater on the quality of 

ground water. In response, the Gallatin Local Water 

Quality District (GLWQD) was established in 1995, 

and efforts were undertaken to monitor the quality 

of ground water and surface water. 

Long-term water-level measurements are 

needed to provide information on trends or variations in annual recharge that may affect either 

the amount of dilution or the additional loads of 

contaminants that may be introduced to the groundwater system from the septic wastewater. Since the 

late 1940’s, periodic surveys have been made of 

water levels in the valley, but only two wells have 

been measured consistently from year to year. 

Both wells are near or within the flood plain of 

the Gallatin River. Water levels in the two wells 

primarily represent recharge from the river or from 

local diversions of river water for irrigation. Little 

water-level monitoring has been done for the aquifers underlying the benches (Kendy, 2001). To help 

address these issues, in 1997 the USGS designed a 

long-term water-level monitoring network in cooperation with the GLWQD that consists of 101 wells. 

An attempt was made to include as many previously monitored wells as possible and to expand 

the network to represent all developed aquifers 

in the GLWQD.

Like Gallatin Valley, the Helena, Montana, 

area has experienced marked growth in recent 

years. Public concerns about depletion or contamination of ground water in the bedrock areas 

surrounding the Helena Valley led to a hydrologic 

The early stages of land or aquifer 

development is an opportune time 

to establish a combined water-level 

and water-quality monitoring network 

that can define baseline ground-water 

conditions and track important 

changes with time in the quantity 

and quality of the resource.
48

study by the USGS in cooperation with the Lewis 

and Clark County Water Quality Protection District 

(Thamke, 2000) that is similar to the study 

described previously for the Gallatin area.

Monthly measurements of water levels in 

112 wells from 1993 to 1998 were an important 

part of the Helena bedrock area study, and waterlevel measurements currently (2001) continue 

to be made in 25 wells. Again, few long-term waterlevel monitoring wells existed prior to the study. 

Water-level data available for two wells from 1976 

to 1998 are shown in Figure 25 and illustrate 

the value of longer term measurements. The 

hydrograph for well 60 shows that though the 

period 1992–98 was one of generally rising water 

levels for this well, water levels in the well generally 

declined during the full period (1976–98). For 

well 174, the long-term trend is more difficult to 

determine because of relatively large gaps for some 

parts of the record. Water-level trends in the Helena 

bedrock are likely to vary across the area as a result 

of differences in precipitation, human influences, 

and the heterogeneous character of the bedrock 

aquifer. Thus, a network of long-term monitoring 

wells is needed to develop an overall perspective 

on the ground-water resources.

Figure 24. Perspective block diagram of the Gallatin Local Water Quality District, Montana, 

showing areas of urban and residential development. (Modified from Kendy, 2001.)
49

Figure 25. Long-term hydrographs for two wells in the Helena bedrock area and corresponding monthly precipitation at Helena, Montana. Trend lines are based on simple linear 

regression between water level and time. (Modified from Thamke, 2000.)
50

Several innovative uses of long-term waterlevel monitoring have been proposed in addition 

to the more conventional uses described thus far. 

For example, van der Kamp and Schmidt (1997) 

demonstrated a method in which the soil-moisture 

balance for a relatively large area was determined 

on the basis of water levels in wells completed 

in a thick clay layer. After removing the effects of 

barometric loading and Earth tides, the remaining 

changes in water pressure (water levels) represent 

changes in loading on a relatively large scale 

resulting from the balance between infiltration 

of precipitation and losses by evapotranspiration. 

Separately, Narasimhan (1998) emphasized 

that valuable insights about the dynamic attributes 

of ground-water systems can be gained by longterm passive monitoring of responses of groundwater systems to barometric changes and earth 

tides.

The use of geophysical techniques in combination with water-level data can enhance delineation and interpretation of water-level changes over a 

region. For example, microgravity methods can be 

used to measure extremely small gravitational 

changes resulting from changes in ground-water 

storage over an area. An example of the combined 

use of water-level measurements and microgravity 

measurements is shown in Figure 26 for the 

Tucson Basin in Arizona. The patterns of changes 

in ground-water storage based on microgravity 

measurements (Figure 26A) and patterns of 

changes in water levels (Figure 26B) are similar. 

Differences between the two maps result from 

the different locations of measurement, spatial 

variations in specific yield, and water stored in 

the unsaturated zone that is measured by microgravity measurements but not by the water-level 

measurements.

A second geophysical technique, Interferomic 

Synthetic Aperture Radar (InSAR), is proving to be 

a powerful new tool that uses repeat radar signals 

from space to measure land subsidence at high 

degrees of measurement resolution and spatial 

detail (Galloway and others, 1999). The combination of InSAR information with long-term waterlevel data from different locations and depths 

provides a means to map land subsidence as 

well as evaluate its causative factors.

Innovative and Emerging Applications

Scientist making microgravity measurement as part of study 

to determine aquifer storage changes near Tucson, Arizona. 

Photograph by Alice Konieczki, U.S. Geological Survey.
51

EXPLANATION

Ground-water storage

 change, in feet of water

–6 to –8

–4 to –6

–2 to –4

0 to –2

0 to 3

Microgravity station

0 5 KILOMETERS

0 5 MILES

PINAL CO. 111° 110°45'

PIMA CO.

111°15' 32°30'

32°

15'

EXPLANATION

–40 to –60

–30 to –40

–20 to –30

–10 to –20

0 to –10

0 to 40

0 5 KILOMETERS

0 5 MILES

Monitoring well

Water-level change,

 in feet

PINAL CO. 111° 110°45'

PIMA CO.

111°15' 32°30'

32°

15'

A

B

Figure 26. Estimated changes from 1989 to 1998 in the 

Tucson Basin in (A) ground-water storage based on microgravity 

survey data, and (B) ground-water levels based on measurements 

in monitoring wells. (Modified from Pool and others, 2000.)
52

C

Statistical Design of Water-Level

Monitoring Networks

Statistical techniques have found limited application 

to the design of water-level monitoring networks for several 

reasons. First, sufficient data are needed to reliably estimate 

the parameters required by the techniques. Second, waterlevel monitoring networks typically have multiple objectives, 

some of which are difficult to express quantitatively. Despite 

these limitations, statistical analysis of data from existing 

networks can provide useful guidance in evaluating these 

networks and a firmer basis for network modifications. 

Examples of the use of two well-known statistical techniques, 

geostatistical analysis and principal-components analysis, 

are described here.

GEOSTATISTICAL ANALYSIS

Geostatistics encompasses a set of probabilistic techniques aimed at determining estimates of spatial data (in this 

case, water levels) at unmeasured locations as combinations 

of nearby measured values. The method provides estimates 

of uncertainty that can be used to aid network design.

A typical application of geostatistics is to evaluate the 

relation between the number or density of monitoring wells 

and the uncertainty of a potentiometric map. Olea (1984) 

presented an example of this type of application for the 

Equus Beds aquifer, an intensively used aquifer in central 

Kansas. A map of the water-table elevation in the Equus Beds 

aquifer, based on data from the existing network of 244 observation wells, is shown in Figure C–1. Note that the density of 

monitoring wells in Figure C–1 is not homogeneous—about 

80 percent of the wells are located in the southern half of the 

area. From this network, Olea (1984) identified a reduced 

network of 47 wells by laying a regular hexagonal pattern 

(Figure C–2) over the area and randomly selecting from 

among the existing monitoring wells in each hexagon. 

A map of water-table elevation based on the revised network 

of 47 wells is shown in Figure C–3 and is similar to the map 

shown in Figure C–1. About 95 percent of the values in the 

two contour map grids differ by less than 5 percent. From the 

geostatistical analysis, the estimated average standard error 

of the water-table elevations increased about 20 percent from 

10 feet for the map of Figure C–1 to 12 feet for the map shown 

in Figure C–3.

Figure C–1. Water-table elevation in the 

Equus Beds aquifer, based on data from 

network of 244 observation wells. Circles show 

locations of observation wells. (Modified from 

Olea, 1984.)

1,480

1,440

1,400

1,360

0 5 MILES

0 5 KILOMETERS
53

Information provided by the previously described type 

of analysis may lead to reductions in the number of monitoring 

wells in some areas. The savings can be used to establish 

additional monitoring wells in areas with less adequate 

coverage, to increase the frequency of measurement, or to 

otherwise upgrade the network. The limitations of this type 

of analysis should be kept fully in mind, however, in that the 

analysis focuses on the overall ability to accurately represent 

a regional potentiometric surface. Other objectives of the 

network might need to be factored into any decisions about 

network design, such as objectives to quantify drawdowns 

in particular areas, to identify possible flow paths for waterquality analysis, or to evaluate the interactions of ground water 

and surface water. Likewise, geostatistical analysis assumes 

that further ground-water development will not greatly alter the 

estimated spatial correlations. 

Figure C–3. Water-table elevation in the Equus 

Beds aquifer, based on data from network of 

47 wells selected using 16-square-mile hexagons. Circles show locations of observation 

wells. (Modified from Olea, 1984.)

0 5 MILES

0 5 KILOMETERS

1,480

1,440

1,400

1,360

1,440

Figure C–2. Example of hexagonal 

sampling. Olea (1984) found the 

hexagonal pattern to be more efficient 

than a square pattern for selecting 

wells.
54

PRINCIPAL-COMPONENTS ANALYSIS

Principal-components analysis (PCA) is a data transformation technique used to search for structure in multivariate data sets. The goal of PCA is to determine a few linear 

combinations (principal components) of the original variables 

that can be used to summarize the data without losing much 

information. An example of PCA applied to water-level 

measurements near Williams Lake in Minnesota is discussed 

here (Winter and others, 2000).

Williams Lake is located in the glacial terrain of 

northern Minnesota. More than 300 measurements of water 

levels were made at each of 50 wells surrounding the lake 

(Figure C–4). In applying PCA to these data, the first two principal components (PC–1 and PC–2) were found to mimic 

basic patterns of water-level fluctuations in the wells and 

together accounted for 93 percent of the variance (variability) 

in the water-level data. For example, in Figure C–5, compare 

Figure C–4. Location of observation wells near Williams Lake in Minnesota. Well groups are based on the delineations shown 

in Figure C–6 and discussed in the text. (Modified from Winter and others, 2000.)

Group A

Group B

Group C

Ungrouped

Water-table contour—

 Shows altitude of the

 water table, in meters

 above sea level

EXPLANATION

15

24

um24

22

423.7

Water-table

 observation well

0 100 200 300 METERS

0 500 1,000 FEET

Doe Lake

Mary Lake

Williams Lake

Crystal Lake

420

417.6

419.1

420.6

422.1

423.7

um5

um20

um15 um1

um24

21

30

um2, 3, 4

4

6

um6, 7, 8

um9

19 um25

20

5

8

9 11

19

24

26, 28, 29

10

18

16

17

um

18

22

um26, 27, 28

um29

25

um32

2

3

7

13

um30

15

14

16

um31

12

Williams

Lake

Duluth

Min

Minneapolis nesota River

MINNESOTA

ST. PAUL
55

the hydrograph of water levels for well 15 with the graph 

of component scores for PC–1. Likewise, compare the 

hydrograph of water levels for well 22 with the graph of component scores for PC–2. A third hydrograph, for well 20, appears 

to be a mixture of PC–1 and PC–2.

The relative weighting of the water-level patterns 

represented by PC–1 and PC–2 for a well are reflected in 

the principal-component loadings. The component loadings 

are the correlation coefficients between the water-level 

measurements for the well and each principal component. A 

plot of the component loadings for each well with respect to 

PC–1 and PC–2 (Figure C–6) indicates that most wells fall into 

three groups. A large number of wells have high loadings on 

PC–1 and low loadings on PC–2 (Group A). At the other 

extreme, a few wells have high loadings on PC–2 and low 

loadings on PC–1 (Group B). Many wells have relatively high 

WATER LEVEL, IN METERS ABOVE SEA LEVELWATER LEVEL, IN METERS ABOVE SEA LEVEL

COMPONENT SCORESCOMPONENT SCORES 418.5

419.0

419.5

420.0

420.5

–3

–2

–1

0

1

2

3

–3

–2

–1

0

1

2

3

424.5

425.0

425.5

426.0

424.0

81 82 83 84 85 86

YEAR YEAR

YEAR YEAR

87 88 89 90 81 82 83 84 85 86 87 88 89 90

Principal Component 1 Principal Component 2

A B

C D

421.0

421.5

422.0

422.5

420.5

81 82 83 84 85 86 87 88 89 90

Williams Lake stage

F

418.5

419.0

419.5

420.0

418.0

81 82 83 84 85 86 87 88 89 90

E

Hydrograph for well 20

Hydrograph for well 15 Hydrograph for well 22

Figure C–5. Selected graphs for the Williams Lake area of Minnesota, including (A) component scores for principal 

component 1, (B) component scores for principal component 2, (C) water level in well 15, (D) water level in well 22, 

(E) water level in well 20, and (F) stage of Williams Lake. The variable spacing for each year on the x-axis reflects the 

number of measurements made for the year at each site. Principal-components analysis requires that measurements 

be made for all wells for each date used in the analysis, but the number of measurements per year can vary. (Modified 

from Winter and others, 2000.)
56

loadings on both PC–1 and PC–2 (Group C). Wells 15, 22, 

and 20, whose hydrographs are plotted in Figure C–5, are 

examples of wells from Groups A, B, and C, respectively.

The three patterns of water-table fluctuations reflect 

variations in recharge as related to the depth to the water 

table and whether the wells are upgradient or downgradient 

from the lake. For example, all Group A wells are upgradient 

from Williams Lake, and the water table is relatively deep 

at these wells. In contrast, the water table is very shallow 

at the three Group B wells. All but one of the Group C wells 

are downgradient from Williams Lake, and the pattern of 

water-table fluctuations shows some similarity to the stage 

of Williams Lake (Figure C–5).

The results of the PCA thus provide some basic 

insights into the similarities and dissimilarities in patterns of 

water-level fluctuations among the wells and might be useful 

in selecting wells for long-term monitoring. For example, a first 

consideration might be to select wells from each of the three 

groups. In addition, wells that fall outside the three groups 

might be individually reviewed to consider whether they represent critical hydrologic settings for long-term monitoring not 

represented by wells in the three groups.

PRINCIPAL COMPONENT 1

PRINCIPAL COMPONENT 2

1.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0

–0.2 –0.1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

C

B

A

um1

um2

um3

um4

um5

um6

um7

um8

um9 um25

um15

um16

um18 um17

um19

um20

um24

um26 um27,28

um29 um30,32

um31

2

3

4 6

5 8

7

9

10 11

12

13

14

15

16

19

21

22

24

26 28

20,18

29

30

25

Figure C–6. Plot of component loadings for principal component 1 versus principal component 2 for wells in the Williams 

Lake area. (Modified from Winter and others, 2000.)
57

To aid in preparation of this report, State and 

local water-resources agencies and USGS District 

offices were asked to provide information about the 

design, operation, and history of long-term groundwater observation wells in their respective State. 

“Long term,” as defined here, refers to any well 

being used to collect water-level measurements for 

5 years or more, or having at least 5 years of hydrologic record. It is worth repeating that water-level 

measurements typically must be collected from an 

observation well without interruption over one or 

more decades in order to compile a hydrologic 

record that represents the potential range of natural 

water-level fluctuations and tracks trends over time. 

Five years is therefore a relatively short period for 

water-level data collection, but it is at least sufficient 

to provide a record of several seasons of groundwater-level fluctuations. 

Sixty-two State and local water-management 

or regulatory agencies provided information, as did 

USGS offices in all 50 States and Puerto Rico. A 

surprising revelation from the results was how difficult it is to obtain information about the actual 

number of observation wells monitored, the 

frequency of water-level measurements, the average 

period of hydrologic record, and changes in the 

monitoring program over time. The reasons for this 

varied, but often the ability of the respondents to 

provide information was hindered by a lack of 

formal documentation about the design of the 

observation-well networks, limited “institutional 

memory,” and the lack of an accessible database. 

Another common problem encountered was that 

responsibilities for collecting water-level data are not 

always clearly defined. 

The level of effort in collecting long-term 

water-level data varies greatly throughout the 

United States. Although difficult to define precisely, 

the information collected indicated that there 

are on the order of 42,000 long-term (5 or more 

years of record) observation wells distributed 

throughout the United States. Approximately 

11,000 (less than one-third) of the reported 

number of long-term observation wells are presently 

monitored through the USGS Cooperative Water 

Program. This number is significantly less than the 

18,300 long-term observation wells reported in a 

1997 inventory of hydrologic monitoring stations 

operated under the Cooperative Water Program 

(Lew, 1998). The difference between the two 

numbers, in part, reflects a difference in the definition of “long-term” observation wells. However, a 

continuing decrease in the number of long-term 

observation wells monitored under the USGS 

Cooperative Water Program is consistent with the 

national trends noted in the 1997 inventory and in 

tracking USGS data-collection activities. 

In many States, a lack of sufficient financial 

resources impedes the construction of new observation wells in areas of need. To eliminate costs 

incurred by drilling and well construction, most 

agencies use private water wells or existing monitoring wells for the collection of water-level data. 

These “wells of opportunity” are often useful as 

long-term observation wells, but a problem reported 

by many States is the difficulty in locating suitable 

existing wells in specific aquifers or geographic locations. Limitations in funding and staffing also impair 

observation-well maintenance, upgrades to waterlevel-monitoring equipment, and consistency in 

water-level monitoring activities conducted from 

year to year. 

A proper evaluation of the suitability 

of existing observation-well networks is best done 

at the State and regional level, where the diversity 

in topographic, climatic, and geologic settings, 

ground-water use, and other factors can be 

properly considered. Two indicators of the status of 

observation-well networks are presented here that 

may be useful in comparing the approximate magnitude of long-term observation-well networks by 

State or region. The first indicator, observation-well 

density, is the ratio of the reported number of longterm observation wells in each State to the area (in 

1,000 square miles) enclosed within State boundaries (Figure 27). The second indicator, which 

relates water-level data collection to ground-water 

use, is the ratio of the reported number of long-term 

observation wells to the total amount of ground 

water withdrawn (in 100 million gallons per day) 

from each State (Figure 28). 

STATUS OF WATER-LEVEL 

DATA-COLLECTION PROGRAMS
58

Figure 27. Number of long-term water-level observation wells per 1,000-squaremile area in each State and in Puerto Rico.

Figure 28. Number of long-term water-level observation wells per hundred million 

gallons of ground water withdrawn per day in each State and in Puerto Rico.
59

The information presented by the maps in 

Figures 27 and 28 provides some indication of the 

relative magnitude of long-term ground-water-level 

data collection in various parts of the Nation. The 

data do not indicate the degree to which observation wells are distributed geographically and among 

aquifers in any particular State. Large observationwell networks in States having comparatively high 

values of one or both indicators may be good candidates for network evaluation designed to determine 

if monitoring sites may be reduced or redistributed 

to enhance data collection or reduce operational 

costs (see Box C). Conversely, comparatively low 

values of one or both indicators generally reflect a 

sparse number of wells relative to geographic area 

or to ground-water use in the indicated State. In 

these cases, in particular, a larger number of observation wells may be needed to ensure that sufficient 

water-level data are being collected, at a minimum, 

where ground-water withdrawals are concentrated 

or where sensitive environmental areas are located. 

As with streamflow and precipitation 

data, ground-water-level data become increasingly 

valuable with length and continuity of the records. 

Yet, unlike streamflow and meteorological records, 

ground-water-level records in most parts of 

the Nation are less than 40 years in length. 

Forty-four percent of agencies reported having 

observation-well networks in which the typical 

hydrologic record was 25–40 years, 31 percent 

reported having observation-well networks in which 

the typical hydrologic record was 10–25 years, and 

2 percent reported having networks in which the 

typical hydrologic record was less than 10 years. 

Twenty-two percent of the agencies reported that 

observation wells in their networks had periods of 

hydrologic record too varied to characterize. 

In recent years, the USGS and many State 

and local agencies have experienced difficulties 

in maintaining long-term water-level-monitoring 

programs because of limitations in funding and 

human resources. Where fiscal or personnel 

constraints have forced agencies to revise priorities 

for environmental data collection, preference typically has been given to water-quality monitoring, 

often at the expense of basic ground-water-level 

monitoring. Although water-level and ground-waterquality monitoring are complementary activities, 

these two types of data commonly are treated 

as mutually exclusive, and separate agencies 

commonly are responsible for each. Greater 

attention is needed to the long-term value of waterlevel data collected as part of water-quality monitoring and to the potential synergies between waterquality and water-level-monitoring networks.

In many States, observation wells tend 

to be concentrated in areas where aquifers are 

heavily developed. Few long-term observation 

wells are intentionally located away from the influence of pumping, irrigation, and other human 

activities to allow for monitoring of the natural 

effects of climate variability and to provide baseline 

data against which ground-water levels monitored 

during short-term investigations can be better evaluated in a longer term climatic perspective. The 

U.S. Geological Survey presently operates a sparse 

Greater attention is needed to the 

long-term value of water-level data 

collected as part of water-quality 

monitoring and to the potential 

synergies between water-quality and 

water-level-monitoring networks.
60

national network of about 140 climate-response 

wells (Figure 29), and a few States have droughtmonitoring networks that include climate-response 

observation wells, such as previously noted for 

Pennsylvania. Increased numbers of climateresponse observation wells and long-term monitoring of naturally occurring fluctuations in groundwater levels are needed to develop more complete 

ongoing assessments of droughts and the cumulative effects of other climatic phenomena (Alley, 

2001). During drought conditions, the effective 

management of ground-water resources, and monitoring of ground-water availability and ground-water 

and surface-water interaction, require the ability to 

rapidly collect water-level measurements and track 

trends. Therefore, more efforts should be made to 

construct climate-response and other observation 

wells capable of collecting “real-time” water-level 

measurements, and to make all collected water-level 

data more rapidly and readily accessible through 

electronic transmittal.

Figure 29. Location of observation wells in the USGS national 

climate-response ground-water-level network.

Increased numbers of climate-response 

observation wells and long-term monitoring 

of naturally occurring fluctuations in 

ground-water levels are needed to develop 

more complete ongoing assessments of 

droughts and the cumulative effects of 

other climatic phenomena.

WA

OR

CA

NV

UT

AZ

WY

CO

ND MN

WI

IL IN

KY

TN

MS

PR

HI

AK

AL

GA

FL

NC

WV VA

OH PA

NY

VT ME

NH

MA

RI CT

NJ

DE

MD

SC

MI

IA

MO

AR

LA

SD

NE

KS

OK

TX

NM

MT

ID
61

D

Ground-Water-Level Monitoring 

in the 1930’s, 1950’s, and Today

The severe drought of the 1930’s in much of the United 

States created widespread concern that declining water levels 

in wells and diminished flow of springs may be warnings of the 

eventual exhaustion of the Nation’s ground-water supplies. 

During the drought years of the 1930’s, considerable interest 

arose in the establishment of systematic programs for monitoring water levels in observation wells. It is instructive to 

compare the status of water-level monitoring during the 

1930’s, during the 1950’s (a second severe drought period), 

and today at the beginning of the 21st century.

1930’s—In 1933, about 3,000 observation wells were 

being measured periodically by the USGS and by State agencies, and about 115 of these wells were equipped with automatic (continuous) water-level recorders. Records of water 

levels covering many years were available for only a few 

areas, notably southern California, Honolulu, the Roswell 

Basin in New Mexico, and Long Island, New York. Other 

areas of heavy withdrawals had more sporadic water-level 

records. In 1936, the USGS released the first annual report on 

the fluctuations of ground-water levels and artesian pressures 

in the United States (Meinzer and Wenzel, 1936). This report 

was envisioned “as a step in the realization of a nationwide 

program of water-level records.” At the time, it was noted that 

the availability of water-level records was dependent upon 

ongoing investigations and that some of the most valuable 

records were in danger of being discontinued because of lack 

of funds for the projects that supported the monitoring. The 

need also was expressed for more observation wells outside 

of areas of major ground-water withdrawals to provide information on the effects of climatic variations on water levels. In 

addition, increased automatic monitoring of water levels was 

recommended.

1950’s—Ground-water levels at the end of 1954 were 

at or near record lows throughout most of the southern twothirds of the United States, creating renewed concern about 

the possible exhaustion of the Nation’s ground-water supplies 

(Fishel, 1956). Federal, State, and local agencies measured 

water levels in about 20,000 long-term observation wells 

across the country with records for many of the observation 

wells dating back to the 1930’s. Fishel (1956) used water-level 

records from nine States to illustrate how in most areas the 

low water levels were largely a function of the dry climate 

conditions and would recover after the drought ended. Fishel 

also noted that significant water-level declines in some areas, 

including “some of the best and most important aquifers,” 

were caused by large ground-water withdrawals, and that 

water-level declines in these areas would likely persist or 

worsen after the drought ended.

Today (2001)—There are on the order of 42,000 longterm observation wells in the United States with 5 or more 

years of water-level record. These wells are distributed 

throughout all States, and the level of effort varies greatly 

among States. No nationwide, systematic water-level monitoring program exists. Observation wells are still largely 

selected from existing wells that are part of specific studies, 

and the continuity of records is difficult when studies draw 

to a close. The ease of making data available on the Internet 

enhances the value of automatic water-level monitoring 

beyond that of the previous decades, but automatic measurement of water levels in long-term observation wells remains 

limited (for example, less than 10 percent of USGS long-term 

monitoring wells have continuous monitoring). Relatively little 

long-term monitoring takes place outside of major withdrawal 

areas. Concerns about the exhaustion of ground-water 

supplies exist for parts of the United States, but no longer for 

the Nation as a whole. Concerns about the effects of pumping 

on surface-water bodies, about water quality, and about the 

effects of possible climate change on ground-water and 

surface-water resources are much greater than in the 1930’s 

and 1950’s.
62

Chilgrove House

Southern

England

40

60

J FMAMJ J A SONDJ FMAM J J A SONDJ FMAM J J A SOND

1933 1934 1935

WATER LEVEL, IN METERS ABOVE DATUM

Maximum recorded

monthly value

Observed waterlevel trace

Mean monthly value for

period of record

Minimum recorded

monthly value

Ground-water levels have been measured from 1836 to the present on an almost continual basis 

at the Chilgrove House well in the south of England (Monkhouse and others, 1990). The well is 

completed in a chalk aquifer, and the hydrologic record for the well represents the longest period of 

measurement for any well in the United Kingdom. Snapshots of the water-level record for this well 

show the intensity of drought conditions from 1933 to 1935 in the context of the more than 160 years 

of record at the site. (Photograph by Terry J. Marsh, Centre for Ecology and Hydrology, Wallingford, 

England.)
63

The focus of this report has been to illustrate 

the importance of systematic, long-term collection 

of water-level data. Such data are crucial to the 

investigation and resolution of many complex waterresources issues commonly faced by hydrologists, 

engineers, water-supply managers, regulatory agencies, and the public. To ensure that adequate waterlevel data are being collected for present and anticipated future uses, observation-well networks and 

water-level monitoring programs at the local, State, 

and Federal level need to be evaluated periodically. 

In the course of these evaluations, several questions 

might be asked. Are data being collected from areas 

that represent the full range in variation in topographic, hydrogeologic, climatic, and land-use environments? Are plans to ensure long-term viability of 

observation-well networks and data-collection 

programs being made? How are the data stored, 

accessed, and disseminated? Who are the principal 

users of water-level data, and are the needs of these 

users being met? 

CHALLENGES AND 

FUTURE OPPORTUNITIES

To ensure that adequate water-level data 

are being collected for present and 

anticipated future uses, observation-well

networks and water-level monitoring

programs at the local, State, and Federal 

level need to be evaluated periodically.
64

Careful planning and design are required to 

ensure the collection of high-quality water-level data 

over the period of time needed to compile a useful 

hydrologic record of water-level changes. A further 

challenge is to supplement the long-term monitoring wells as hydrologic conditions in aquifers 

evolve. A comprehensive monitoring program 

should consider aquifers substantially affected by 

ground-water pumping, areas of future groundwater development, surficial aquifers that serve as 

major areas of ground-water recharge, and links 

with water-quality and surface-water monitoring. 

A commitment to long-term monitoring 

is needed to avoid data gaps resulting from an 

inadequate distribution of observation wells or 

periods of no measurements in a hydrologic 

record. Many agencies lack formalized written plans 

for the design and operation of ground-water-level 

networks, and many agencies have difficulty maintaining funding and program continuity necessary 

to ensure long-term collection of water-level data. 

Disruptions in the hydrologic record provided by 

water-level data collection and the gaps in data 

coverage can hinder the ability of water-resources 

managers to make sound resource-management 

decisions. Where water-level data are not available, 

hydrologic information needed to address critical 

ground-water problems may be impossible to 

obtain. Much recent effort has been made in the 

application of computer modeling techniques to 

forecast future ground-water levels. However, the 

successful application of even these advanced 

methods requires that sufficient water-level data 

are available.

More effort is needed to increase the amount 

of ground-water-level data stored in electronic databases, to increase the compatibility between databases, and to improve access to ground-water-level 

data on the Internet. Although some water-level 

databases can be accessed in this way, detailed 

and complete records of historical water-level data 

usually are limited or unavailable. In many agencies, 

large backlogs of historical ground-water-level data 

have not been entered into electronic databases, let 

alone made available on the Internet. Consequently, 

potentially useful data are residing in paper files 

where accessibility and utility are very limited.

Finally, to increase the collection and accessibility of water-level data, agencies need to examine 

ways to increase interagency coordination in 

constructing and maintaining observation-well 

networks, collecting water-level measurements, and 

sharing and disseminating data. Greater interagency 

cooperation will help ensure that data-collection 

efforts are sufficient to address issues relevant to the 

greatest variety of local, State, regional, and 

national water-resources issues.

In many agencies, large backlogs of 

historical water-level data have not been 

entered into electronic databases, let 

alone made available on the Internet. 

Consequently, potentially useful data are 

residing in paper files where accessibility 

and utility are very limited.
65

To increase the collection and 

accessibility of water-level data, agencies 

need to examine ways to increase 

interagency coordination in constructing 

and maintaining observation-well 

networks, collecting water-level 

measurements, and sharing and 

disseminating data.

Members of State and Federal agencies and local citizens group 

discuss results of ground-water-level monitoring at a landfill 

research site in Connecticut. Photograph by Susan Soloyanis.
66

ACKNOWLEDGMENTS

We are grateful to our colleagues at the U.S. Geological 

Survey and at the many State agencies who provided information 

about water-level data collection in their States. O. Lehn Franke, 

Virginia de Lima, and Geoffrey Freethey provided insightful 

technical reviews of this manuscript. Appreciation is expressed to 

the following USGS personnel for providing information presented 

as case studies: Marshall Gannett, John Kilpatrick, Dennis Risser, 

Larry Spangler, Michael Sweat, and Dan Tomaszewski. Joy Monson 

and Margo VanAlstine prepared the final manuscript and 

illustrations. 
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