The Quality of Our Nation’s Waters

Factors Affecting Public-Supply-Well 

Vulnerability to Contamination: 

Understanding Observed Water Quality and 

Anticipating Future Water Quality

National Water-Quality Assessment Program

U.S. Department of the Interior

U.S. Geological Survey

Circular 1385

The Quality of Our Nation’s Waters

Factors Affecting Public-SupplyWell Vulnerability to Contamination: 

Understanding Observed Water 

Quality and Anticipating Future 

Water Quality

By Sandra M. Eberts, Mary Ann Thomas, and Martha L. Jagucki

National Water-Quality Assessment Program

Circular 1385

U.S. Department of the Interior

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

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2013

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Suggested citation:

Eberts, S.M., Thomas, M.A., and Jagucki, M.L., 2013, The quality of our Nation’s waters—Factors affecting publicsupply-well vulnerability to contamination—Understanding observed water quality and anticipating future water 

quality: U.S. Geological Survey Circular 1385, 120 p. 

Available online at http://pubs.usgs.gov/circ/1385/

Library of Congress Cataloging-in-Publication Data

Eberts, Sandra M.

The quality of our nation’s waters : factors affecting public-supply-well vulnerability to contamination : understanding 

observed water quality and anticipating future water quality / by Sandra M. Eberts, Mary Ann Thomas, and Martha 

L. Jagucki.

p. cm. -- (Circular ; 1385)

Factors affecting public supply well vulnerability to contamination : understanding observed water quality and anticipating future water quality

Includes bibliographical references.

1. Water quality--United States. 2. Water--Pollution potential--United States. 3. Well water--United States. 4. 

Drinking water--Contamination--United States. I. Thomas, Mary Ann, 1956- II. Jagucki, Martha L. III. Geological 

Survey (U.S). IV. Title. V. Title: Factors affecting public supply well vulnerability to contamination : understanding 

observed water quality and anticipating future water quality. VI. Series: U.S. Geological Survey circular ; 1385.

TD223.E24 2013

628.1’68--dc23

 2013005514

ISBN 978-1-4113-3556-1
iii

Foreword

The U.S. Geological Survey (USGS) is committed to providing the Nation with reliable scientific information 

that helps to enhance and protect the overall quality of life and that facilitates effective management of 

water, biological, energy, and mineral resources (http://www.usgs.gov/ ). Information on the Nation’s water 

resources is critical to ensuring long-term availability of water that is safe for drinking and recreation and is 

suitable for industry, irrigation, and fish and wildlife. Population growth and increasing demands for water 

make the availability of that water, measured in terms of quantity and quality, even more essential to the longterm sustainability of our communities and ecosystems.

The USGS implemented the National Water-Quality Assessment (NAWQA) Program in 1991 to support 

national, regional, State, and local information needs and decisions related to water-quality management 

and policy (http://water.usgs.gov/nawqa). The NAWQA Program is designed to answer: What is the quality 

of our Nation’s streams and groundwater? How are conditions changing over time? How do natural features 

and human activities affect the quality of streams and groundwater, and where are those effects most pronounced? By combining information on water chemistry, physical characteristics, stream habitat, and aquatic 

life, the NAWQA Program aims to provide science-based insights for current and emerging water issues and 

priorities. From 1991 to 2001, the NAWQA Program completed interdisciplinary assessments and established 

a baseline understanding of water-quality conditions in 51 of the Nation’s river basins and aquifers, referred to 

as Study Units (http://water.usgs.gov/nawqa/studies/study_units.html).

National and regional assessments are ongoing in the second decade (2001–2012) of the NAWQA Program as 

42 of the 51 Study Units are selectively reassessed. These assessments extend the findings in the Study Units 

by determining water-quality status and trends at sites that have been consistently monitored for more than a 

decade, and filling critical gaps in characterizing the quality of surface water and groundwater. For example, 

increased emphasis has been placed on assessing the quality of source water and finished water associated 

with many of the Nation’s largest community water systems. During the second decade, NAWQA is addressing five national priority topics that build an understanding of how natural features and human activities affect 

water quality, and establish links between sources of contaminants, the transport of those contaminants 

through the hydrologic system, and the potential effects of contaminants on humans and aquatic ecosystems. 

Included are studies on the fate of agricultural chemicals, effects of urbanization on stream ecosystems, 

bioaccumulation of mercury in stream ecosystems, effects of nutrient enrichment on aquatic ecosystems, and 

transport of contaminants to public-supply wells. In addition, national syntheses of information on pesticides, 

volatile organic compounds (VOCs), nutrients, trace elements, and aquatic ecology are continuing. 

The USGS aims to disseminate credible, timely, and relevant science information to address practical and effective water-resource management and strategies that protect and restore water quality. We hope this NAWQA 

publication will provide you with insights and information to meet your needs, and will foster increased citizen 

awareness and involvement in the protection and restoration of our Nation’s waters. 

The USGS recognizes that a national assessment by a single program cannot address all water-resource 

issues of interest. External coordination at all levels is critical for cost-effective management, regulation, 

and conservation of our Nation’s water resources. The NAWQA Program, therefore, depends on advice and 

information from other agencies—Federal, State, regional, interstate, Tribal, and local—as well as nongovernmental organizations, industry, academia, and other stakeholder groups. Your assistance and suggestions 

are greatly appreciated.

 William H. Werkheiser

USGS Associate Director for Water
iv

Acknowledgments

SPECIAL THANKS TO THE FOLLOWING INDIVIDUALS AND ORGANIZATIONS FOR THEIR CONTRIBUTIONS:

NAWQA personnel across the Nation for their contributions of data, research results, and 

review of findings

NAWQA’s many partners in the governmental and nongovernmental sectors 

who have helped guide scientific efforts and ensure that NAWQA information

meets the needs of local, State, Tribal, regional, and national stakeholders

REVIEWERS

Frederick Bloetscher, Florida Atlantic University; American Water Works Association

Darcy Campbell, U.S. Environmental Protection Agency

Kevin Dennehy, U.S. Geological Survey

Mike Ekberg, Miami (Ohio) Conservancy District

Richard Johnson, Oregon Health and Science University

Wayne Lapham, U.S. Geological Survey

Barbara Mahler, U.S. Geological Survey

Mike Muse, U.S. Environmental Protection Agency

Gary Rowe, U.S. Geological Survey

Chi Ho Sham, The Cadmus Group, Inc.; American Water Works Association

William Wilber, U.S. Geological Survey

Michael Wireman, U.S. Environmental Protection Agency

TECHNICAL EDITOR

C. Michael Eberle, U.S. Geological Survey

GRAPHIC DESIGN AND LAYOUT

David R. Jones, U.S. Geological Survey

All photos by USGS personnel unless otherwise noted
v

Contents

Chapter 1 Overview—Major Findings and Implications......................................................................1

Chapter 2 Introduction.............................................................................................................................. 15

 Understanding Water Quality in Public-Supply Wells .......................................................16

 Public-Supply-Well Vulnerability Primer..............................................................................22

Chapter 3 Measures of Vulnerability..................................................................................................... 29

 Sources of Recharge ..............................................................................................................31

 Geochemical Conditions.........................................................................................................37

 Groundwater-Age Mixtures ...................................................................................................48

Chapter 4 Preferential Flow Pathways ................................................................................................. 61

 Why Preferential Flow Pathways Matter.............................................................................64

 Implications for Monitoring and Protecting Public-Supply Wells ..................................66

 Recognizing the Influence of Preferential Flow Pathways ..............................................74

 Wellbores as Preferential Flow Pathways—Scenario Modeling....................................75

Chapter 5 Vulnerability Assessment Examples....................................................................................79

 Vulnerability of Water from Individual Wells.......................................................................80

 Relative Vulnerability Among Wells in Different Aquifers.................................................88

Chapter 6 Conclusions—Opportunities and Challenges....................................................................91

References Cited..........................................................................................................................................95

Glossary.......................................................................................................................................................101

Appendix TOOLBOX...................................................................................................................................107
vi
vii

ABBREVIATIONS, ACRONYMS, AND UNITS OF MEASURE

ACR Area contributing recharge

bls Below land surface

C/C0 Ratio of predicted concentration to initial

concentration

CA California

CFC	 Chlorofluorocarbon

CFC-11	 Trichlorofluoromethane

CFC-12	 Dichlorodifluoromethane

CT Connecticut

Delta Enrichment or depletion relative to a standard

of known composition

Fe Iron

FL Florida

ft Foot (feet)

gal/min Gallons per minute

gpm Gallons per minute

H2S	 Hydrogen sulfide

HBSL Health-Based Screening Level

in. Inch

km Kilometer

L Liter

MCL Maximum Contaminant Level

mg Milligram

mg/L Milligrams per liter

mi Mile

mi2	 Square mile

Mn Manganese

MODFLOW	 USGS modular groundwater-flow model

MTBE Methyl tert-butyl ether

N Nitrogen

NAWQA National Water-Quality Assessment Program (USGS)

NE Nebraska

NJ New Jersey

NLCDE Enhanced National Land Cover Data

NM New Mexico

NO3 Nitrate

NV Nevada

O2 Oxygen

OH Ohio

PCE Perchloroethene

per mil Parts per thousand

SF6	 Sulfur hexafluoride

SO4 Sulfate

TCE Trichloroethene

TX Texas

USEPA U.S. Environmental Protection Agency

USGS U.S. Geological Survey

U.S. United States

UT Utah

VOC Volatile organic compound

yr Year

ZOC Zone of contribution

ZOI	 Zone of influence

ZOT Zone of transport

2

H Deuterium

3-D Three dimensional

3

H Tritium

18O Oxygen-18

μg Microgram

μg/L Micrograms per liter

> Greater than

≥	 Greater than or equal to

< Less than

≤	 Less than or equal to

% Percent
About 35 percent of the population in the United States 

receives its drinking water from public-supply wells, such 

as the one here. Consequently, it is important to understand 

what factors affect public-supply-well vulnerability to 

contamination from chemicals or microorganisms in the 

groundwater. 
Overview—

Major Findings and Implications

An Issue of Human 

Health and Economics

A Water-Management 

Challenge

As part of the U.S. Geological Survey (USGS) National WaterQuality Assessment (NAWQA) Program, a study was conducted from 2001 to 2011 to shed light on factors that affect the vulnerability of water from public-supply wells*

 to contamination. The study was 

designed as a follow-up to earlier NAWQA studies that found mixtures of 

contaminants at low concentrations in groundwater near the water table in 

urban areas across the Nation and, less frequently, in deeper groundwater 

typically used for public supply (Hamilton and others, 2004). Although 

contaminants were less frequently detected in public-supply wells than 

in shallower monitoring wells, a separate study showed that contaminant 

concentrations were greater than drinking-water standards or other human 

health benchmarks in about 22 percent of public-supply-well samples 

(Toccalino and Hopple, 2010). 

These previous NAWQA findings imply that water from nearly one in 

five public-supply wells in the United States might need to be treated or 

blended with more dilute water sources to decrease concentrations of 

drinking-water contaminants before delivery to the public. Therefore, 

understanding factors that affect the vulnerability of water from publicsupply wells to contamination (also referred to herein as public-supply-well 

vulnerability) is important because removing contaminants from water 

intended for drinking is difficult, expensive, and becoming increasingly 

necessary (Job, 2011). Of particular concern is recent evidence that existing 

water-treatment systems do not effectively remove some contaminants now 

being found in groundwater (Hopple 

and others, 2009). Even when effective water-treatment technologies 

exist, it is especially difficult for 

small system operators to implement such technologies because 

their small customer base might not 

be able to cover the costs. 

Well-field operators, water-resource 

managers, drinking-water regulators, and scientists have long 

realized that water from publicsupply wells is vulnerable to 

contamination. However, it has been 

unclear why water from some wells 

becomes more contaminated than 

water from other wells, even when 

contaminant sources are similar. 

The role that water resource development (drilling and pumping of 

*

Italicized words are defined in the glossary.

1

A better understanding of public-supply-well 

vulnerability to contamination can lead to a 

reduced need for treatment of drinking water. 
2 Factors Affecting Public-Supply-Well Vulnerability to Contamination: Understanding Observed Water Quality and Anticipating Future Water Quality

new wells) and well operation play in public-supply-well water quality also 

has been unclear.

NAWQA’s study of public-supply-well vulnerability to contamination found 

that it is possible to understand—as well as anticipate—the quality of water 

from a public-supply well by knowing what factors to evaluate and how 

to go about evaluating them. Factors affecting public-supply-well vulnerability to contamination are the topic of this circular. Measures that can 

be used to determine which factor (or factors) plays a dominant role at an 

individual public-supply well also are described. Case-study examples are 

used throughout the circular to show how such information can be used to 

improve water quality. 

Study findings can be applied by drinking-water practitioners not only 

to devise improved programs for monitoring public-supply wells and the 

aquifers they tap but also to identify the most beneficial protection mechanisms for a particular well. Insights gained from the study will aid managers 

in making informed decisions in siting and constructing new public-supply 

wells and will help them anticipate the response of different wells to changes 

in management practices. Ultimately, the findings of this study can be used to 

update and enhance existing assessments of public-supply-well vulnerability 

across the Nation and to design strategies for preventing future contamination 

of such wells—thus helping to sustain the Nation’s water supply. 

Study findings demonstrate that groundwater vulnerability and publicsupply-well vulnerability are not the same. Groundwater vulnerability 

depends on three factors. First is the presence of manmade or natural 

contaminant sources; for example, leaky underground petroleum storage 

tanks and arsenic-rich aquifer sediments can be sources of contaminants 

entering groundwater. Second is the combination of chemical and physical 

processes in the subsurface that affect contaminant concentrations in an 

aquifer; for example, microorganisms can break down or degrade some 

chemical contaminants in groundwater, and unsorted sediments can cause 

Is There a Difference 

Between Groundwater 

Vulnerability and 

Public-Supply-Well 

Vulnerability?

The long well screens and high pumping 

rates that are typical of public-supply 

wells draw in water from multiple 

groundwater flow pathways within an 

aquifer. These flow pathways often 

originate over broad areas. In contrast, 

monitoring and domestic wells tend to 

capture water from more localized parts 

of an aquifer. Because groundwater 

contaminants generally are not uniformly 

distributed throughout an aquifer, 

differences in the location, design, 

construction, operation, and maintenance 

of different wells can lead to differences 

in the quality of water from different wells 

within the same aquifer.

Well

screen

Well

casing

Public-supply well

Recharge

Cone of depression Monitoring wells

Domestic well

Water table

Groundwater

flow pathways
Chapter 1—Major Findings and Implications 3

dispersion of contaminants as they move through an aquifer. Third is the 

ease with which water and contaminants can travel to and through an 

aquifer, also referred to as intrinsic susceptibility; for example, a thick layer 

of dense clay can reduce groundwater vulnerability by acting as a barrier to 

the movement of water and contaminants.

The vulnerability of a public-supply well depends on all of the above factors 

(contaminant input, contaminant mobility and persistence, and intrinsic 

susceptibility) but is further affected by the location, design, construction, 

operation, and maintenance of the well. For example, the location of a well 

determines whether a particular contaminant source is in the area that 

contributes water to the well. The placement of the well screen determines 

which chemical and physical processes in the aquifer have influenced the 

water before it is pumped from the well and, therefore, which contaminants 

might be present in the water as it enters the well, and at what concentrations. The depth of the well screen and the pumping rate of the well 

determine how quickly water and contaminants can travel from the water 

table to that particular well, and from what distance. Because well design, 

construction, and operation directly influence water quality, water produced 

by different types of wells (public-supply, domestic, and monitoring wells) 

might not contain the same concentrations of contaminants, even if the 

wells are completed at similar depths within the same aquifer. 

What Measures of 

Vulnerability are Useful 

for Individual PublicSupply Wells?

Study findings indicate that information on contaminant input, contaminant 

mobility and persistence, and intrinsic susceptibility within the area that 

con tributes water to a well can help answer the question, “Which contaminants in an aquifer might reach the well, and when, how, and at what 

concentration might they arrive?” 

Study-team scientists found that the following measures—each related to a 

different aspect of public-supply-well vulnerability—are particularly useful 

for understanding the quality of water pumped from individual publicsupply wells: 

(1) the sources of recharge that contribute water to a well, and the contaminants associated with the recharge—a measure of contaminant input; 

(2) the geochemical conditions encountered by water drawn into a well—a 

measure of contaminant mobility and persistence; and 

(3) the groundwater-age mixture of different waters that blend (or mix) in a 

well—a measure of intrinsic susceptibility. 

These measures of public-supply-well vulnerability and examples illustrating their utility for decisionmaking are discussed herein. Related implications for public-supply-well water quality are noted. 
4 Factors Affecting Public-Supply-Well Vulnerability to Contamination: Understanding Observed Water Quality and Anticipating Future Water Quality

Sources of Recharge

The quality of water pumped from a public-supply well depends on the 

proportions of (and contaminant concentrations in) waters from different 

recharge areas that enter and mix in the well. In this study, several 

methods were used to identify the sources of recharge for public-supply 

wells so that the quality of water from the wells could be better understood. For example, study-team scientists used chemical characteristics 

that were unique to different water sources to determine the origins of 

water produced by a public-supply well in Nebraska. Contrary to original 

expectations, not all of the water from the well was from the confined 

aquifer tapped by the well. Instead, at least 12 percent of the water was 

from an overlying aquifer that was previously thought to be hydraulically 

isolated from the confined aquifer. Understanding that another aquifer 

was contributing urban recharge to the well helped explain why anthropogenic contaminants were detected in water from the well. Knowing which 

parts of an aquifer system contribute water to a well can prompt scientists 

or water-resource managers to explore the mechanism that enables lowquality water to reach the well, potentially uncovering an important vulnerability for the water supply. 

Different groundwater flow pathways can bring different sources of water to a well. For 

example, different sources of water for the well in this figure include recharge through 

urban land (shown in orange), recharge through agricultural land (green), infiltrating 

stream water (blue green), and water that recharged centuries ago, before urban and 

agricultural development in the area (dark blue). The different sources of water would 

be associated with different types and amounts of contaminants, which would mix in the 

well and collectively contribute to the quality of water from the well.

Water affected by

urban recharge

Water affected

by agricultural

recharge

Predevelopment water

Agricultural

area 

Urban

area 

Public-supply well

Water affected by 

recharge from stream

Water table

• Identifying the sources of recharge 

that contribute water to different 

wells will help explain differences 

in contaminants and contaminant 

concentrations in wells that are close 

to each other but that draw in water 

from different recharge areas.

• Changes in the quality of water 

produced by a public-supply well over 

time—even in general characteristics 

such as temperature, pH, alkalinity, 

and dissolved-solids concentration—

warrant investigation so that any new 

source(s) of water (and associated 

contaminants) for the well can be 

identified and managed to minimize 

adverse effects.

SOURCES OF RECHARGE

Implications
Chapter 1—Major Findings and Implications 5

Geochemical Conditions

Geochemical conditions influence whether a contaminant that has been 

released to the groundwater will travel with the groundwater, react with the 

aquifer material, or degrade before reaching a public-supply well. USGS 

scientists used various methods to understand the effects of geochemical 

conditions on the quality of water from public-supply wells. Most notably, 

study-team scientists developed a simple and inexpensive method for characterizing whether the water from a well is derived from oxic (dissolved 

oxygen ≥ 0.5 milligram per liter), anoxic (dissolved oxygen < 0.5 milligram 

per liter), or mixed geochemical conditions in an aquifer, and the method 

was applied to wells in each study area. The utility of the method can be 

illustrated with the glacial aquifer system of the Northern United States. In 

the glacial aquifer system, geochemical conditions identified by using the 

method were a good indicator of the likelihood of detecting (or not detecting) 

arsenic concentrations greater than the drinking-water standard. 

An unexpected finding of this study was that human activities altered 

recharge or changed groundwater flow in ways that led to changes in aquifer 

geochemical conditions in most study areas. These changes resulted in 

chemical reactions between the groundwater and the solid aquifer material, 

releasing naturally occurring drinking-water contaminants (such as arsenic 

or uranium) into the groundwater. As a result, concentrations of these chemicals in water from study wells increased.

Geochemical conditions in an aquifer—especially dissolved oxygen content, pH, and 

alkalinity—strongly influence whether chemical contaminants are mobile and travel 

with the groundwater, whether they are bound to aquifer materials, or whether they 

degrade before reaching a well. For example, nitrate is stable under oxic conditions but 

is converted to harmless nitrogen gas in anoxic conditions through a process called 

denitrification. In this figure, nitrate in the urban recharge (shown in orange) would 

persist along most flow pathways between the urban area and the well because the 

water remains within the oxic zone of the aquifer. However, nitrate in the agricultural 

recharge (green) would degrade (“denitrify,”shown as change to blue) before reaching 

the well because of the anoxic conditions encountered. 

Water affected by

urban recharge

Water affected by 

agricultural recharge

Water table

Agricultural

area 

Urban

area 

Public-supply well

Water affected by 

recharge from stream

• Communities that determine whether 

the water from their public-supply 

well(s) is being drawn from oxic, anoxic, 

or multiple geochemical zones within 

an aquifer will have insight into which 

drinking-water contaminants in the 

groundwater are likely to reach their 

well(s) and which are likely to be 

assimilated by the aquifer before the 

water arrives at a well.

• Caution is needed when developing 

aquifers for increased water supply 

because modifying groundwater flow in 

some aquifers has altered geochemical 

conditions and caused contaminants 

such as arsenic and uranium (which 

occur naturally in aquifer rocks 

and sediments) to dissolve into the 

groundwater, where they were then 

drawn into public-supply wells.

GEOCHEMICAL CONDITIONS

Implications
6 Factors Affecting Public-Supply-Well Vulnerability to Contamination: Understanding Observed Water Quality and Anticipating Future Water Quality

Groundwater-Age Mixtures

The water that is pumped from a public-supply well did not recharge the 

surrounding aquifer at a single point in time. Rather, public-supply wells 

produce water with a mixture of groundwater ages. (Groundwater age 

refers to the number of years since water entered an aquifer system at the 

water table.) The groundwater-age mixture for a well reflects recharge and 

discharge rates and physical properties of the aquifer that together control 

the movement of water and contaminants to a well. An estimate for the 

age mixture of the water from a well provides insight into the likelihood 

of contamination from both anthropogenic and natural sources. Young 

(recently recharged) groundwater that enters a well is more vulnerable 

to contamination from human activities near the land surface than older, 

deeper groundwater. Old groundwater, however, is not necessarily free of 

contaminants. Old groundwater can contain naturally occurring chemical 

elements that can contaminate the water for the purpose of drinking. 

If the age mixture of a particular well’s water corresponds to a period of 

recharge that is longer than the duration of contaminant input, then some 

in-well dilution of contaminants entering the well is indicated. The dilution 

will occur because some of the water entering the well will have recharged 

the aquifer either before or after the time during which the contaminants 

were released to the groundwater. Consequently, wells that produce water 

with a wide range of groundwater ages have some degree of protection from 

high levels of contamination. 

To help explain water quality in public-supply wells, study-team scientists 

developed a tool (TracerLPM) for estimating the groundwater-age mixture 

for a well on the basis of measured concentrations of chemical tracers. Once 

the groundwater-age mixture for a well has been estimated by using the 

tool, the age mixture can then be used to forecast water-quality changes at 

the well in response to changes in contaminant input across the water table.

Because public-supply wells have long 

well screens and are pumped at relatively high rates, they draw in water that 

has been in the surrounding aquifer for 

different amounts of time. The resulting 

“age mixture” for the water from a well can 

be used to estimate how long it will take 

contaminants in the surrounding aquifer 

to arrive at the well and can affect the 

concentration of contaminants in the water 

produced by the well. For example, groundwater flow pathways that converge on the 

well in this figure simultaneously deliver 

water to the well that recharged the aquifer 

a few years ago and centuries ago. Only the 

younger water would contain contaminants 

associated with recent land use. The water 

that takes centuries to reach the well would 

not contain anthropogenic contaminants 

and would dilute the contaminants in the 

young water as the different waters mix in 

the well. However, the older water could 

bring naturally occurring drinking-water 

contaminants to the well.

Water affected by

urban recharge

Water affected by 

agricultural recharge

Water table

Predevelopment water

Agricultural

area 

Urban

area 

Public-supply well

Water affected by 

recharge from stream
Chapter 1—Major Findings and Implications 7

USGS scientists used information on the age mixture 

of the water from study wells in Connecticut and 

Nebraska to forecast how the quality of water from two 

very different wells might change in response to the 

same hypothetical 25-year contaminant release to the 

water table across the area contributing recharge to 

each well. The maximum contaminant concentration 

likely to be reached at each well—relative to the 

concentration in the recharge—is shown by the height 

of the curves in this figure. How rapidly contaminants 

would be flushed from the wells after input at the water 

table ceases is shown by the horizontal extent of the 

curves. The very different response in the two wells 

is entirely due to differences in the groundwater-age 

mixtures for the wells. More than 90 percent of water 

entering the Connecticut well is less than 10 years 

old. As a result, a contaminant could rapidly reach the 

well but also could be rapidly flushed from the well. In 

contrast, more than 95 percent of the water entering the 

Nebraska well is greater than 10 years old, and much of 

the water is hundreds to thousands of years old. As a 

result, old water entering the well would continuously 

dilute contaminants in younger water entering the well, 

but the contaminants would not be completely flushed 

from the well for a very long time.

Groundwater-age mixtures affect water-quality response

Contaminant

concentration,

in percent

 CONNECTICUT STUDY WELL

• Rapid response

• Small dilution

• Rapid flush

0

40

80

0

40

80

Time, in years

0 50 100

Recharge concentration

Concentration in well

 NEBRASKA STUDY WELL

• Slow response

• Large dilution

• Slow flush

• Knowledge of groundwater-age mixtures is important for prioritizing 

protection efforts. For example, managing contaminant sources within source 

water protection areas defined by groundwater traveltime is more beneficial 

for wells that produce predominantly young water than for wells that produce 

water that is generally older than the specified traveltime.

• There is a tradeoff when a well is deepened to decrease vulnerability to 

anthropogenic contaminants in young, shallow groundwater: deeper water, which 

has been in contact with the aquifer material for a longer time, often contains higher 

concentrations of naturally occurring drinking-water contaminants.

• Several years or even decades of monitoring will not be enough to characterize 

water-quality changes in public-supply wells that result from changes in land 

use or land management if most of the water from the wells is relatively old. 

A combined monitoring and modeling approach is needed for such wells to 

determine how long contaminant concentrations will continue to increase in 

the produced water after nonpoint-source-contaminant input at the water table 

is reduced.

GROUNDWATER-AGE MIXTURES

Implications
8 Factors Affecting Public-Supply-Well Vulnerability to Contamination: Understanding Observed Water Quality and Anticipating Future Water Quality

What Are Preferential 

Flow Pathways and 

Why Do They Matter?

Study findings show that vulnerability measures often reveal the presence of 

preferential flow pathways and their influence on the transport of groundwater 

and contaminants. Preferential flow pathways are pathways that provide little 

resistance to flow. They can be naturally occurring (for example, fractures 

in rocks or interconnected high-permeability sediments) or manmade (for 

example, a wellbore). Preferential flow pathways are widely present, because 

their effects were observed in all study areas. Although difficult to locate, 

they are important to understand because they can affect every other factor 

contributing to the vulnerability of water from a public-supply well.

Groundwater and contaminants traveling along preferential flow pathways 

will be accelerated more than groundwater and contaminants traveling 

through the aquifer matrix when pumping of a well begins. This acceleration can cause groundwater-flow rates within preferential flow pathways to 

be high compared with contaminant degradation rates, resulting in reduced 

effectiveness of natural processes that degrade groundwater contaminants 

(for example, denitrification). Rapid flow through preferential flow pathways 

has increased concentrations of anthropogenic and naturally occurring 

contaminants in groundwater, the latter by bringing together water and 

aquifer materials that are not in chemical equilibrium. Preferential flow 

pathways also create favorable conditions for the transport of pathogens: 

rapid traveltimes (which reduce the opportunity for microorganism die-off) 

and relatively large interconnected openings (which reduce the removal of 

microorganisms through filtration or sorption to sediments or rocks).

A USGS study of water quality in public-supply wells (Toccalino and 

Hopple, 2010) found that about one-third of the detections of anthropogenic 

contaminants were in groundwater from aquifers underlying low-permeability confining units. This surprising result suggests that preferential flow 

pathways across confining units might be common. As part of the study 

summarized in this circular, USGS scientists identified various ways to 

recognize instances in which preferential flow pathways are affecting the 

quality of water from a public-supply well.

Water affected by

urban recharge

Water affected by 

agricultural recharge

Water table

Predevelopment water

Agricultural

area 

Urban

area 

Public-supply well

Preferential flow pathways often deliver Inactive well

the youngest, most vulnerable water to 

public-supply wells, along with most of the 

anthropogenic contaminants that enter such 

wells. For example, in this figure, an inactive 

well near the public-supply well enables 

water with urban contaminants (shown in 

orange) to enter the public-supply well at 

a depth that would otherwise draw in only 

old, uncontaminated water (dark blue). As 

a result, the total amount of urban contaminants entering the well is higher than if the 

inactive well had not provided such an easy 

route for the contaminated water to travel.
Chapter 1—Major Findings and Implications 9

Local-scale groundwater-flow models

were constructed for several study 

areas and used to estimate the area 

at the land surface that contributes 

recharge to a typical public-supply well 

in each study area. The groundwater 

traveltimes from these recharge areas to 

the wells also were calculated. Results 

from the California and Florida models, 

which include enough detail to simulate 

preferential flow, demonstrate that some 

of the shortest traveltimes to a well 

(shown in shades of red in the figures) 

can be associated with recharge areas 

that are at great distances from the well. 

In these study areas, preferential flow 

occurs either through overlapping lenses 

of coarse-grained sediments or naturally 

occurring conduits and cavernous zones 

in limestone bedrock. Source water 

protection efforts that focus on the 

areas closest to a public-supply well will 

not protect the well from contaminated 

recharge entering the aquifer further 

away from the well but traveling rapidly to 

the well along preferential flow pathways.

Preferential-flow pathways make it difficult to know which areas to protect

CITY OF 

TEMPLE

TERRACE

MODESTO

Public-supply well

0 1 KILOMETER

0 1 MILE

<6.4

>6.4 to 16

>16 to 26

>26 to 37

>37 to 50

>50 to 66

>66 to 127

Estimated traveltime

from recharge area to

public-supply well,

in years

8 to 10

>10 to 20

>20 to 30

>30 to 40

>40 to 50

>50 to 60

>60 to 70

>70 to 80

>80 to 90

>90 to100

>100 

Estimated traveltime

from recharge area to

public-supply well,

in years

Public-supply well 0 1 KILOMETER

0 1 MILE

California Study Area

Florida Study Area

Groundwater travels preferentially through 

overlapping lenses of coarse-grained sediments.

Preferential flow occurs within naturally occurring sinkholes 

and conduits caused by dissolution of limestone bedrock.

• A general campaign to engage everyone in groundwater protection is worthwhile because 

preferential flow pathways make it difficult to know where the youngest groundwater that is drawn 

into a well actually originates. Protecting a delineated area around a public-supply well from 

contamination is necessary but may be insufficient if preferential flow pathways are present.

• Characterizing the groundwater chemistry in different parts of an aquifer system (in addition to 

characterizing contaminant occurrence) increases the likelihood of detecting the influence of 

preferential flow pathways on the quality of water from a public-supply well. Although widely present, 

preferential flow pathways occupy just a fraction of an aquifer system and are difficult to locate, 

whereas groundwater-chemistry data have been successfully used to identify situations where water 

from unexpected parts of an aquifer system is entering a well.

• Seasonal water-quality fluctuations in deep public-supply wells should alert water-resource 

managers to the possibility of preferential flow because recharge water is unlikely to travel from the 

water table through the aquifer matrix to a deep well screen within a single season.

• If manmade preferential flow pathways (such as wells screened through multiple aquifers or 

screened through different geochemical zones within the same aquifer) are affecting the quality of 

water being produced by a public-supply well, resource managers have an opportunity to devise 

effective means of preventing or minimizing flow through these features and reduce the vulnerability 

of the water from the well to contamination.

PREFERENTIAL FLOW PATHWAYS

Implications
How Can Information 

on the Factors Affecting 

Public-Supply-Well 

Vulnerability—Including 

Preferential Flow 

Pathways—Be Used 

to Guide Protection 

Decisions?

Study findings indicate that most processes controlling water quality in 

public-supply wells are related to a limited number of factors; however, the 

relative importance of any one factor might be different for different wells. 

These differences can have implications for how managers would protect 

the different wells from contamination. To illustrate this point, the pathways 

and processes whereby contaminants reached four study wells—as determined from site-specific information that includes the sources of recharge, 

geochemical conditions, and groundwater-age mixtures for the wells—are 

summarized below. Findings point to management strategies that might 

afford the greatest protection for a particular well.

10 Factors Affecting Public-Supply-Well Vulnerability to Contamination: Understanding Observed Water Quality and Anticipating Future Water Quality

Operation of Public-Supply Well Caused Seasonal 

Increases in Nitrate and Uranium (See pages 70–71, 82–83)

• Nitrate increased in young, shallow groundwater in response to 

agricultural and urban development.

• Uranium increased in shallow groundwater because irrigationinduced changes in aquifer geochemical conditions released 

uranium from sediments.

• Water from the study well had low concentrations of contaminants 

during summers because shallow, contaminated groundwater was 

diluted by older, uncontaminated groundwater as both types of 

water entered and mixed within the well.

• Water from the well had relatively high concentrations during 

winters because of preferential flow down the wellbore—contaminated water pooled in the aquifer around the bottom of the wellbore 

when the well was not pumping and then reentered the well along 

with shallow, contaminated water when the well resumed pumping.

A short-term solution to this problem was less downtime between 

pumping periods during winter so that deep groundwater could 

retain its high quality and continuously dilute contaminants in 

shallow groundwater simultaneously entering the well.

Multi-Aquifer Wells Routed Contaminants to 

Confined-Aquifer Public-Supply Well

(See pages 34–35, 72–73, 86–87)

• Low concentrations of several anthropogenic 

contaminants were detected in water from several 

confined-aquifer public-supply wells.

• Young, contaminated groundwater entered the study 

well beneath older, uncontaminated groundwater, 

indicating that the contaminants had traveled along a 

preferential flow pathway before reaching the well.

• Many irrigation wells in the surrounding area tapped 

both the confined aquifer and an overlying unconfined 

aquifer, creating pathways across the confining unit 

that enabled contaminants to enter the confined 

aquifer and travel to the supply wells.

Rethinking the use of multi-aquifer wells throughout the 

area could lead to improved protection of the confinedaquifer public-supply wells. For example, limiting irrigation wells to a single aquifer or making engineering 

modifications to prevent down-wellbore flow in existing 

wells could remove preferential flow pathways that 

allow anthropogenic contaminants to reach confinedaquifer public-supply wells in this area.

Modesto, California

York, Nebraska
Tampa, Florida

Woodbury, Connecticut

Chapter 1—Major Findings and Implications 11

Young, Oxic Groundwater Enabled Contaminants 

to Persist and Reach Public-Supply Well (See pages 67, 80–81)

• Urban recharge carried a variety of contaminants to the shallow groundwater—contamination was widespread, but concentrations were low.

• Traveltimes through the unsaturated zone were artificially short because dry wells (a type of 

preferential flow pathway) were used to redirect stormwater runoff into the aquifer.

• Many contaminants in the shallow groundwater remained in solution because of oxic conditions.

• Short traveltimes between the water table and the study well (generally less than 10 years) 

enabled the contaminants to easily reach the well.

The many sources and types of contaminants in this area indicate that groundwater-quality 

protection depends on the entire community. If residents and businesses take steps to reduce 

input of anthropogenic contaminants to the groundwater, a positive effect on the study well 

might be seen in less than 10 years because of the short traveltimes between the water table 

and the well.

Pumping at Public-Supply Well Pulled Contaminants 

Through Karst Features (See pages 68–69, 84–85)

• Contaminants, such as nitrate, were commonly detected in shallow, oxic groundwater in 

an unconfined sand aquifer but infrequently detected in monitoring wells in an underlying 

limestone aquifer.

• Water from the study well, which is open solely to the limestone aquifer, was chemically 

more similar to water in the shallow sand aquifer than to water in the limestone aquifer.

• Pumping at the study well pulled the shallow, contaminated groundwater into the deeper 

limestone and eventually into the well itself by accelerating the movement of groundwater 

and contaminants along karst features (sinkholes, conduits, and cavernous zones) that served 

as preferential flow pathways.

Not all public-supply wells are similarly connected to the sand aquifer. Therefore, one way to 

identify the most vulnerable public-supply wells in this area is to determine the degree to which 

the water from each well reflects that of the overlying sand aquifer. Protection efforts could then 

be intensified for the wells producing water most similar to that in the sand aquifer.

NOTE: Shaded areas on the map depict 

selected principal aquifers shown on page 19.
12 Factors Affecting Public-Supply-Well Vulnerability to Contamination: Understanding Observed Water Quality and Anticipating Future Water Quality

Can We See Patterns 

in Public-Supply-Well 

Vulnerability to 

Contamination? 

Study findings demonstrate the existence of spatial patterns in the 

underlying factors affecting public-supply-well vulnerability to contamination. In other words, the input of contaminants to the groundwater, the 

mobility and persistence of contaminants once in the groundwater, the 

intrinsic susceptibility of the groundwater to contamination, and the presence and nature of preferential flow pathways are different for different 

aquifers. Consequently, systematic patterns in public-supply-well vulnerability to a variety of contaminants should be discernible and predictable, 

but vulnerability patterns for many commonly detected drinking-water 

contaminants remain largely unexplored in the United States. Nevertheless, the study described herein produced results that point to differences in 

the relative vulnerability of water from public-supply wells to contamination from volatile organic compounds (VOCs) in several sand and gravel 

aquifers. More specifically, water from public-supply wells in many sand 

and gravel aquifers in the Western United States was found to have low 

vulnerability to the gasoline components benzene, toluene, and MTBE 

(methyl tert-butyl ether) compared to water from wells in glacial aquifers 

in the Eastern United States. The difference is due to the predominantly 

oxic conditions in the western aquifers that promote the degradation of 

these compounds, together with the generally older water produced by the 

western-aquifer wells. The presence of older water indicates that (1) there is 

more time for degradation reactions to proceed before water reaches publicsupply wells in the western aquifers, and (2) older water, which is generally 

free of anthropogenic contaminants, is available to dilute any such contaminants in younger water entering the western wells. (See pages 88–89.) With 

additional study, spatial and temporal patterns in the vulnerability of water 

from public-supply wells to contamination from additional contaminants 

could be uncovered.
Chapter 1—Major Findings and Implications 13

3

4 5

7

6

8

1

2

10

9

3

4 5

7

6

8

1

2

10

9

Manganese, 

iron, or sulfate 

reducing; or 

methanogenic

Oxic or 

nitrate 

reducing

A. Geochemical conditions

B. Age of groundwater

Less than 

60 years

Greater than 

or equal to 

60 years

NOTE: Shaded areas on the 

map depict selected principal 

aquifers shown on page 19.

The vulnerability of each well is unique, but not necessarily random. Spatial 

patterns in the underlying factors affecting public-supply-well vulnerability to 

contamination exist, as illustrated in these figures depicting related measures of 

vulnerability obtained for this study. Individual maps portray the aggregate proportion of water from public-supply wells in each study area that was linked to various 

geochemical conditions and groundwater ages. 

In figure A, public-supply wells in the California (3) and Texas (8) study areas 

generally produced oxic water, whereas wells in glacial aquifers in the Ohio (1) 

and Connecticut (2) study areas frequently produced more reduced water. 

In figure B, public-supply wells in western aquifers withdrew a higher proportion 

of relatively old water than did wells in eastern aquifers. 

Recognizing spatial patterns in the factors affecting public-supply-well vulnerability to contamination provides an opportunity to understand patterns in observed 

water quality and to anticipate which contaminants might show up in publicsupply wells in different principal aquifers across the Nation. 
When groundwater is the source of drinking 

water, a multi-barrier approach for protecting 

drinking water “from source to tap” must take 

into account the vulnerability of the water from 

the public-supply well(s) to contamination from 

chemicals and microorganisms released into 

the groundwater from activities near the land 

surface or from the solid aquifer material.
Introduction 2

More than 100 million people in the United States, or about onethird of the population, receive their drinking water from publicsupply wells (USEPA, 2011). The water from such wells can be 

vulnerable to contamination from anthropogenic and naturally occurring 

chemicals, as well as microorganisms, in the groundwater. For example, 

the USGS analyzed water from more than 900 public-supply wells across 

the United States during 1993–2007. Twenty-two percent of the water 

samples—which were collected prior to treatment—contained at least 

one chemical contaminant at a concentration greater than a benchmark, 

indicating a potential concern for human health. Benchmarks were U.S. 

Environmental Protection Agency (USEPA) Maximum Contaminant Levels

(MCLs) or USGS health-based screening levels (HBSLs) (Toccalino and 

others, 2010). More recently, a study of 383 public-supply wells distributed 

across 35 states showed that mixtures of two or more contaminants were 

present in more than 90 percent of the samples (Toccalino and others, 2012). 

Well-field operators, water-resource managers, drinking-water regulators 

and scientists recognize that such vulnerabilities exist, but it is not always 

evident why some wells are more contaminated than others. Are there 

differences in the sources of recharge water (and contaminants) for the 

wells? Do processes below land surface differ in ways that result in mobilization of contaminants near some wells and attenuation of contaminants 

near others? Is it simply easier for contaminants to travel to some wells than 

to other wells? Do well operation and other water-management decisions 

contribute to differences in the quality of water from different wells? 

Understanding factors that affect the vulnerability of water from publicsupply wells to contamination is an issue that is relevant to both human 

health and economics. Water suppliers must be ever vigilant to ensure that 

the quality of the water they deliver to the public continually meets waterquality standards established to protect human health. However, removing 

contaminants from water intended for drinking is difficult and expensive. Furthermore, the water treatment processes currently used by many 

public water systems were never designed to remove many of the chemical 

contaminants now found in groundwater. For example, a USGS study found 

that concentrations of some herbicides, herbicide degradates, solvents, and 

several other chemicals in finished water were no different than concentrations detected in the source water (Hopple and others, 2009). 

To ensure that high-quality drinking water is supplied to the public, multiple 

barriers to contamination must be in place: a risk prevention barrier, a risk 

management barrier, a risk monitoring and compliance barrier, and an 

individual action barrier (USEPA, 2002). USGS studies on public-supplywell vulnerability and water quality focus on the first line of defense against 

contamination of drinking water—that is, documenting and explaining the 

quality of groundwater that enters into and is pumped out of wells.

“...no water system should 

have to provide more treatment 

than that which is necessary 

to address naturally occurring 

pollutant concentrations...”

G. Tracy Mehan III, 

U.S. Environmental Protection Agency, 

Assistant Administrator for Water,

 February 8, 2003, memo
16 Factors Affecting Public-Supply-Well Vulnerability to Contamination: Understanding Observed Water Quality and Anticipating Future Water Quality

Understanding Water Quality in Public-Supply Wells

In 2001, the USGS NAWQA Program began an intensive study of publicsupply-well vulnerability to contamination (Eberts and others, 2005). 

The purpose of the study was to increase understanding of detections of 

both anthropogenic and naturally occurring drinking-water contaminants 

(also referred to as “natural contaminants”) in water from the Nation’s 

public-supply wells. Study objectives were (1) assess the effects of natural 

processes and human activities on contaminant occurrence in water from 

public-supply wells, (2) identify factors that are important to evaluate when 

assessing public-supply-well vulnerability to contamination, (3) develop 

simple methods and models for screening public-supply wells for vulnerability to contamination, and (4) understand more clearly the potential effects 

of water-resource development and management decisions on the quality of 

water from public-supply wells. (Public-supply-well vulnerability to terrorist 

threats was not investigated.)

Translating Knowledge into Action

Findings from the study of public-supply-well vulnerability to contamination 

are summarized in this circular. The information can be used to help recognize 

which, when, how, and at what concentrations contaminants in an aquifer might 

occur in water pumped from public-supply wells. Several factors and related 

measures for assessing the vulnerability of individual public-supply wells to 

contamination—regardless of aquifer setting—are at the heart of the circular. 

Case-study examples are used to illustrate concepts and draw attention to findings with implications for the Nation’s groundwater sources of drinking water. 

Source water assessment and protection—

A part of the multi-barrier approach for ensuring safe, clean drinking water

The U.S. Environmental Protection Agency (USEPA) sets 

national drinking-water standards for public water systems 

and oversees state implementation of the standards to ensure 

safe drinking water. As outlined in the 1996 Amendments 

to the Safe Drinking Water Act, a comprehensive plan for 

drinking-water protection involves six steps to be taken 

by every community. The first three steps make up the 

assessment phase of a full prevention program and are 

required, whereas the last three steps constitute the actual 

protection phase and are not required. The six steps are 

(1) delineate (map) areas around surface-water intakes or 

public-supply wells to be protected from contamination, (2) 

inventory known and potential sources of contamination in 

these areas, (3) determine the susceptibility of the watersupply system to these contaminants, (4) notify and involve 

the public about identified threats and what they mean 

to their public water system, (5) implement management 

measures to prevent, reduce, or eliminate threats, and (6) 

develop contingency planning strategies to deal with watersupply contamination or service-interruption emergencies 

(USEPA, 2002). 

The USEPA has recommended five criteria for defining the 

area around a public-supply well that, if contaminated, 

could represent a threat to the quality of water from the well. 

These criteria are (1) distance, (2) drawdown (water-level 

decline), (3) time of travel, (4) flow boundaries, including the 

zone of contribution and zone of influence for a well, and 

(5) assimilative capacity (ability of the surrounding aquifer 

to reduce contaminant concentrations). States and local 

communities delineate source water protection areas by 

implementing these criteria, using a variety of methods. 

Depending on the needs and resources of the community, 

methods are at times as simple as computing a fixed radius 

around a well or as complex as applying a computer model 

of contaminant transport (USEPA, 1993; 1994). Ideally, 

protection of the mapped area will prevent contaminants 

from moving through the soil to the groundwater and 

subsequently being drawn into the well. 
Relevant Terms and Definitions

Chapter 2—Introduction 17

UNSATURATED ZONE

Contains both water and air 

and is generally underlain by 

a saturated zone in which all 

interconnected openings or 

pore spaces are full of water

WATER TABLE

Surface that separates the zone that is generally 

unsaturated from the zone that is generally saturated

CONE OF DEPRESSION

A depression in the water table (or potentiometric 

surface) that develops around a pumping well 

AQUIFER SYSTEM

Permeable and relatively 

impermeable sediments 

and rocks that function 

regionally as a wateryielding unit

UNCONFINED AQUIFER

Aquifer with a water table 

as its upper surface

CONFINING UNIT—Lowpermeability layer of sediment

or rock bounding distinctly 

more permeable aquifers

CONFINED AQUIFER

Aquifer bounded above 

by a confining unit

ZOC—ZONE OF CONTRIBUTION1,2

 3-D volume of the aquifer through which 

 water flows to a well from its area 

 contributing recharge (Morrissey, 1989) ZOT—ZONE OF TRANSPORT

3-D volume that contributes water to a well within a specified amount of 

time. Includes only that part of the ZOC from which water can travel to 

the well in the desired amount of time (for example, 10-year time of 

travel). Dashed where projected to land surface 

ACR—AREA CONTRIBUTING RECHARGE1

Surface area where water entering an aquifer system at 

the water table flows to a well (Reilly and Pollock, 1993)

ZOI—ZONE OF INFLUENCE2

The area within which the 

water table (or potentiometric 

surface for a confined aquifer) 

has been changed in reponse 

to pumping

DIRECTION OF GROUNDWATER FLOW

Arrows depict general direction of flow. 

Actual course followed by the groundwater 

is referred to herein as a “flow pathway”

Public -supply well

1 In this circular, the term “contributing area” is used to collectively refer 

to the area contributing recharge and zone of contribution for a well. 

2 For information on how the ZOI and ZOC may differ for confinedaquifer wells or for wells in aquifer systems with preferential flow 

pathways, see USEPA (1993).

AQUIFER

Layer of permeable rock or sediment in the 

saturated zone that contains enough water 

to supply wells or springs

 Anatomy of a Well

A well is simply a hole in the ground (wellbore or borehole) from which water can be removed. The borehole 

is lined with a well casing, such as a pipe, to prevent the 

borehole from collapsing. The casing, along with a sealant 

(called grout), also prevents water from flowing into the 

well from land surface or parts of the aquifer where the 

water quality may be less desirable. The casing can be 

open at the bottom or perforated at a specific depth with 

a screen, allowing water to flow into the well where it can 

be pumped to the surface. Coarse sand or gravel (called 

sand pack or gravel pack) can be placed around the well 

screen to help improve the flow of water into the well. 

When pumped, groundwater flows through the 

screened interval and 

up and out the well

Unsaturated

Saturated

Water table

Screen

Wellbore

Water level

in well

Wellhead

Well casing

Pump

Grout

Gravel pack
18 Factors Affecting Public-Supply-Well Vulnerability to Contamination: Understanding Observed Water Quality and Anticipating Future Water Quality

Although study results demonstrate the existence of spatial patterns in 

vulnerability, neither the study nor this circular provide a comprehensive 

description of all such patterns. Rather, the information, methods, and 

tools presented herein can be used to help fill knowledge gaps related to 

patterns in public-supply-well vulnerability and contaminant occurrence 

across the Nation. 

Methods and tools in this circular also can be used to help update (enhance) 

existing source water assessments. (See box, page 16.) Updated assessments 

can be used by source water protection partners (for example, local officials, 

community-based environmental groups, farmers and businesses, concerned 

citizens, and state source water protection implementing agencies) to more 

effectively prioritize protection efforts and better anticipate and measure 

outcomes. Updating source water assessments is something that has been 

recommended by the USEPA because each state had only 31⁄2 years to 

complete the initial assessments. Consequently, limited local information 

was incorporated into many early assessments (USEPA, 2006).

Appendix Toolbox

This icon is used to flag 

paragraphs introducing 

tools that are discussed 

further in the Appendix

Study Design

Ten areas across the United States were selected for inclusion in this study 

of public-supply-well vulnerability to contamination. The study areas 

are located in regional aquifer systems that together accounted for nearly 

one-half of the groundwater used for public drinking-water supply in the 

United States in 2000 (Maupin and Barber, 2005) and represent diverse 

aquifers and water management practices. Regional-scale investigations 

were conducted in all study areas (Paschke, 2007; Eberts, 2011), and localscale investigations were conducted in six of the study areas (Katz and 

others, 2007; Starn and Brown, 2007; Burow and others, 2008; Clark and 

others, 2008; Jurgens and others, 2008; Brown and others, 2008; Crandall 

and others, 2009; Landon and others, 2008; Bexfield and others, 2011; 

Lindgren and others, 2011; Musgrove and others, 2011; Heywood, 2013). 

Water-quality samples from more than 1,500 public-supply wells were 

examined, along with output from 16 groundwater-flow models. Consistent 

data collection and methods of analysis were implemented across all study 

areas so that data and findings from the different areas could be compared. 

The shallow groundwater was considered to be the source of anthropogenic 

contaminants in most instances; processes within the unsaturated zone were 

not explicitly addressed. 

Regional scale

Regional-scale investigations were conducted to identify aquifer and well 

characteristics that help explain differences in the occurrence of contaminants among wells in different aquifers. Study areas ranged in size from 

tens to thousands of square miles. Existing geologic, water-level, wellconstruction, water-use, and water-quality data were compiled for wells 

with records in USGS, USEPA, state, and local databases. Water samples 

were collected from a subset of the public-supply wells in each area. The 

samples were analyzed for a suite of constituents that included nutrients, 

Drilling a monitoring well.
Chapter 2—Introduction 19

pesticides, pesticide degradation products, compounds found in wastewater, 

VOCs, VOC degradation products, major ions, and trace elements. Waterquality data were used to characterize geochemical conditions (for example, 

pH and redox conditions) in each area. A groundwater-flow model was 

constructed or refined for each study area to characterize (1) the areas that 

contribute recharge to the supply wells and (2) the zones of contribution for 

the supply wells in each location. (See page 17 for an illustration of these 

and other relevant terms.) This characterization was done by simulating 

the movement of water from recharge areas to individual wells by using 

the output from the groundwater-flow models in particle-tracking models. 

Results from particle-tracking models describe more than 150 characteristics of each public-supply-well contributing area, including groundwater 

velocity, traveltimes from recharge points at land surface to the publicsupply well, traveltimes through different pH and redox zones, and potential 

contaminant sources overlying the areas that contribute recharge water 

to the wells. The aquifer and well information, water-quality data, and 

modeling results were stored in a database that was developed for the study 

(Paschke, 2007) so that the varied types of data could be used together in 

a variety of analyses (see for example, Hinkle and others, 2009; Kauffman 

and Chapelle, 2010; Katz and others, 2011). 

Aquifer systems

High Plains

Edwards-Trinity

Central Valley

Floridan

Basin and Range basin-fill

and carbonate-rock

Glacial

Rio Grande

North Atlantic Coastal Plain

1

2

3

4 5 6

7

8

 1 Great Miami River Basin, OH

 2 Pomperaug River Basin, CT 

 3 Northeastern San Joaquin Valley, CA

 4 Eagle Valley and Spanish Springs, NV 

 5 Salt Lake Valley, UT

 6 Eastern High Plains, NE

 7 Middle Rio Grande Basin, NM 

 8 South-Central Texas, TX 

 9 Central-Northern Tampa Bay Region, FL 

 10 Coastal Plain, NJ 

 7 Middle Rio Grande Basin, NM 

 8 South-Central Texas, TX 

 2 Pomperaug River Basin, CT 

 3 Northeastern San Joaquin Valley, CA

 6 Eastern High Plains, NE

 9 Central-Northern Tampa Bay Region, FL 

10

9

 PUBLIC-SUPPLY-WELL STUDY AREAS

Regional-scale investigations Local-scale investigations 

 2001–11 “First group” 2001–7 “Second group” 2006–11
20 Factors Affecting Public-Supply-Well Vulnerability to Contamination: Understanding Observed Water Quality and Anticipating Future Water Quality

Connecticut

California

Florida

Nebraska

• Unconsolidated sediment

• Mostly oxic conditions

• Short well screen

• Most water from the well entered the aquifer 

within 10 years before sampling 

• Unconsolidated sediment

• Oxic conditions

• Long well screen

• Water from the well entered the aquifer 

decades to thousands of years before sampling 

• Layered unconsolidated sediment

• Anoxic conditions in confined aquifer

• Well screen beneath uppermost clay confining 

unit

• Water from the well was a mix of waters that 

entered the aquifer decades before sampling and 

thousands of years before sampling 

• Carbonate rocks

• Oxic and anoxic conditions

• No well screen (open borehole)

• Water from the well was a mix of waters that 

entered the aquifer less than 1 year before 

sampling and decades before sampling

LOCAL-SCALE INVESTIGATIONS

CHARACTERISTICS OF SELECTED STUDY WELLS

Bedrock

Limestone

Sand and gravel Silt and clay

Water table

150

0

Depth,

in feet

250

0

250

0

400

0

Most of the case-study examples in this circular come from four study areas: Connecticut, in the 

glacial aquifer system; California, in the Central Valley aquifer system; Florida, in the Floridan 

aquifer system; and Nebraska, in the High Plains aquifer. These four areas represent a wide 

range of aquifer settings (for example, different sizes and shapes of aquifers, different aquifer 

materials, different geochemical conditions, different ages of groundwater) and public-supplywell characteristics (for example, different screen lengths, different pumping rates, different 

sources of recharge water). Characteristics of the four areas and the study well in each area 

are summarized in the figures (above) and table (opposite page).
Chapter 2—Introduction 21

Local scale

Local-scale investigations were conducted to explore how contaminants 

arrived at and entered a typical public-supply well in six of the study 

areas, four of which are featured extensively in this circular. The six study 

areas were selected from the larger group so that a variety of aquifer types 

and well sizes could be investigated in greater detail. Each selected well 

produced water with low concentrations (below drinking-water standards) 

of both anthropogenic and naturally occurring drinking-water contaminants. The wells also had recharge areas that included several types of land 

use; the recharge areas were determined by using the regional groundwaterflow models. Local-scale groundwater-flow models were constructed and 

calibrated for each study area by using data collected from monitoring 

wells installed for the study. The local-scale models were then used to 

refine the estimated recharge areas for the public-supply wells, to compute 

traveltimes in the aquifer for water entering the wells, and to explore the 

water sources, pumping stresses, and aquifer properties that resulted in the 

specific mixture of water produced by the wells. Sediment and rock samples 

were collected during drilling of the monitoring wells and were analyzed 

for physical properties and chemical characteristics that could affect the 

movement of water and contaminants in the subsurface. Water samples 

were collected from the completed monitoring wells and from the corresponding public-supply well over a period of several years. The samples 

were analyzed for a variety of drinking-water contaminants (nutrients, pesticides, VOCs, and trace elements) and for general groundwater chemistry 

(pH, dissolved oxygen, major ions, and selected stable isotopes). Samples 

also were analyzed for several chemical tracers that can be used to indicate 

groundwater age. Where conditions permitted, water samples were collected 

from the public-supply wells at several depths within their well screens 

(or open boreholes) while their pumps were running to determine where 

different waters and contaminants actually entered the wells. Data and 

models (groundwater flow, particle tracking, and geochemical) for the six 

local-scale investigations were used to corroborate regional-scale findings. 

They also were used to help study-team scientists answer questions such as, 

“How would the water quality at different wells respond to similar changes 

in contaminant input at the water table?”

LOCAL-SCALE INVESTIGATIONS

CHARACTERISTICS OF SELECTED STUDY WELLS

Location Year of

construction

Average 

pumping rate 

(gallons per 

minute)

Screened or 

open interval 

(feet below 

land surface)

Area contributing 

recharge 

(square miles)

Generalized land use in 

area contributing recharge (percent)

Agriculture Urban Forest Wetland Rangeland

Woodbury, Connecticut 1967 72 45 to 60 0.2 4.8 75 13 6.9 <1

Modesto, California 1961 1,600* 91 to 366 1.6 30 67 <1 <1 2.3

Tampa, Florida 1958 700 118 to 174 9.4 4.2 84 1 5.0 5.8

York, Nebraska 1977 500* 141 to 200 2.4 39 45 11 <1 3.7

*

Average summer pumping rate.
22 Factors Affecting Public-Supply-Well Vulnerability to Contamination: Understanding Observed Water Quality and Anticipating Future Water Quality

Public-Supply-Well Vulnerability Primer

Public-supply-well vulnerability to contamination is not the same as 

groundwater vulnerability, although the two are closely related. They are 

different because groundwater vulnerability is not uniform throughout an 

aquifer, and every public-supply well draws in water from only a part of 

an aquifer. The vulnerability of groundwater is affected by the presence of 

contaminant sources (contaminant input), the ability of released contaminants to move with and persist in the groundwater (contaminant mobility 

and persistence), and the overall ease of groundwater movement in the 

subsurface (intrinsic susceptibility). (See box below.) The vulnerability of 

a public-supply well reflects a combination of these three factors—each 

of which can be influenced by the presence of preferential flow pathways—for the specific mixture of water produced by the well. Therefore, 

understanding the interaction between a public-supply well and an aquifer 

is central to understanding the vulnerability of the well to contamination. 

This primer introduces various aspects of groundwater and public-supplywell vulnerability, which are then detailed throughout the remainder of the 

circular for the purpose of understanding water quality in public-supply 

wells.

The vulnerability of a public-supply 

well to contamination depends on 

the vulnerability of the groundwater 

in different parts of the aquifer 

and the specific mixture of that 

groundwater drawn into the well.

How hydrologists define

public-supply-well vulnerability to contamination

Public-supply-well vulnerability to contamination (PSWV) can be defined as a function of groundwater vulnerability (GWV) and the interaction of a well and the aquifer (Frind and others, 2006), 

which depends on the location, design, construction, operation, and maintenance of the well (WELL). 

 PSWV = f (GWV, WELL) (1)

Definitions of “groundwater vulnerability” are somewhat varied in the technical literature (National 

Research Council, 1993; Rao and Alley, 1993; Vowinkel and others, 1996; Focazio and others, 2002; 

Frind and others, 2006; Kauffman and Chapelle, 2010). In general, however, groundwater vulnerability 

is defined as a function of contaminant input (CI) to the groundwater, contaminant mobility and 

persistence (CMP) once in the groundwater, and intrinsic susceptibility (IS) of the groundwater: 

 GWV = f (CI, CMP, IS) (2)

The contaminant mobility and persistence term accounts for the effects of chemical processes (such 

as sorption, dissolution/precipitation reactions, and biodegradation) and physical processes (such as 

advection and dispersion) on contaminant concentrations (Focazio and others, 2002). 

Intrinsic susceptibility, as used here, is not synonymous with vulnerability. Rather, it is the aspect of 

vulnerability that characterizes the ease with which water enters and moves through an aquifer. It 

depends on aquifer stresses (rates of recharge, including travel through the unsaturated zone and 

interactions with surface water, and well discharge) and aquifer properties (permeability, porosity, and 

hydraulic gradients) (Focazio and others, 2002).

Preferential flow pathways (pathways that provide little resistance to flow) are an additional factor 

affecting the vulnerability of groundwater and water from public-supply wells to contamination because 

such pathways influence the relative importance of each of the other factors. 
Chapter 2—Introduction 23

Contaminant Input to the Groundwater

Activities near the land surface frequently release drinking-water contaminants into groundwater. If the contaminants are from many diffuse sources 

or a widespread, difficult-to-trace source, the source of contamination is a 

nonpoint source. (See figure at left.) Nonpoint-source contamination occurs 

when rainfall or snowmelt flowing over the land picks up contaminants. 

Examples of nonpoint-source contamination are excess fertilizers, herbicides, and insecticides in runoff from agricultural land or residential areas; 

oil, grease, and toxic chemicals in runoff from urban land; and bacteria and 

nutrients from livestock, pet wastes, and faulty septic systems. Although 

concentrations of contaminants in groundwater near nonpoint sources can 

be relatively low, such sources might affect a large volume of water. 

If the contaminants are from a discrete and identifiable location, the 

source of contamination is a point source. (See figure at left.) Point-source 

contamination occurs when a chemical, biological, or radiological substance 

is released into the environment from a localized area. Examples of pointsource contamination are industrial solvent spills and leaky underground 

petroleum storage tanks. Concentrations of contaminants near point sources 

are commonly high, but such sources might affect only a small volume of 

water. Concentrations generally decrease away from the source; the pattern 

of contaminated groundwater emanating from a point source is referred to 

as a “contaminant plume.” 

Geologic materials that make up aquifer sediments and rocks also can 

be a source of drinking-water contaminants. Under certain geochemical 

conditions, minerals in the geologic materials will be released into the 

flowing groundwater. Some chemical elements that compose the minerals 

can be harmful to humans if ingested in high enough quantities. Most of 

these harmful elements are regulated in water from public-supply wells. 

Examples of naturally occurring elements that can affect the quality of 

groundwater used for drinking are arsenic, uranium, and radium. Geologic 

materials that release natural contaminants into groundwater can be either 

localized or widespread.

Nonpoint source Widespread or

diffuse contaminants

Point source

Contaminant plume

Natural source

Source of

contaminants

Water that infiltrates the ground beneath different types of land use can carry with it 

different contaminants that reflect the contrasting activities taking place at the land 

surface. Here, water infiltrating underneath agricultural land (orchards left), next to 

roads (center), and below residential development (right) carries with it contaminants 

typical of each type of land use.
Which way does the groundwater flow? 

That depends on the hydraulic heads and 

gradients in the aquifer system

Hydraulic head is an indicator of the total energy available to move groundwater through an 

aquifer. Recharge areas have higher hydraulic heads than discharge areas. 

Hydraulic gradient is the change in hydraulic head with change in distance in a given direction. 

Groundwater and dissolved contaminants flow in the direction of the greatest decrease in hydraulic 

head. The slope of the water table is a measure of the horizontal hydraulic gradient in an unconfined 

aquifer. The slope of a potentiometric surface is a similar measure for a confined aquifer. The 

direction of groundwater movement between an unconfined and a confined aquifer is governed by 

the vertical hydraulic gradient between the aquifers, which can be upward or downward. 

When a well begins pumping water from an aquifer, the water level in the well falls below the 

hydraulic head in the surrounding aquifer, changing the hydraulic gradient and causing water to 

move from the aquifer into the well.

Here, a USGS scientist uses a water-level tape 

to measure the elevation of the standing water 

in a nonpumping well to determine the hydraulic 

head in the aquifer at the depth of the well 

screen. This water-level measurement can be 

combined with other similar measurements for 

the aquifer so that hydraulic gradients and directions of groundwater flow within the aquifer can 

be determined.

Confined aquifer

Well

screen

General direction of groundwater flow

Unconfined aquifer

Confining unit

Sea level

Hydraulic head at the position of a well screen is the vertical distance 

between the water level in the well—when it is not pumping—and sea level.

Hydraulic gradient in an unconfined aquifer is the slope of the 

water table in the direction of the greatest water-level change. 

Water table

Land surface

DISCHARGE

AREA

RECHARGE AREA

24 Factors Affecting Public-Supply-Well Vulnerability to Contamination: Understanding Observed Water Quality and Anticipating Future Water Quality
Chapter 2—Introduction 25

Core samples (aquifer sediments and 

rocks removed from a borehole during well 

drilling) can be collected, boxed, and sent 

to a laboratory to determine whether naturally occurring toxic elements are present 

in the geologic materials that comprise the 

aquifer. If such drinking-water contaminants are present in the subsurface, the 

flowing groundwater can mobilize them 

and transport them to a public-supply well, 

depending on the geochemical conditions 

in the subsurface.

Contaminant Mobility and Persistence 

in the Groundwater

Once in an aquifer, contaminants that dissolve in water will travel with 

the flowing groundwater. The chemical properties of a contaminant and 

the geochemical conditions encountered by the contaminant will affect the 

contaminant’s mobility and persistence in the groundwater; more specifically, contaminant properties combined with geochemical conditions control 

reaction processes such as sorption/desorption, dissolution/precipitation,

ion exchange, or biodegradation that affect contaminant concentrations 

in groundwater. When these reactions occur at rates that are rapid in 

comparison with groundwater-flow rates, the concentration of a contaminant 

can be substantially reduced (or increased) before the contaminant reaches 

a well. The final concentration of a contaminant entering a well from a 

particular flow pathway will depend on the initial concentration of the 

contaminant and the reactions that took place along the pathway. A particularly important reaction involves the degradation of nitrate—one of the most 

widespread anthropogenic contaminants in groundwater. Dissolved nitrate 

is converted to harmless nitrogen gas in a process known as denitrification 

when dissolved oxygen is absent from the groundwater. Consequently, the 

presence or absence of dissolved oxygen in groundwater greatly influences 

whether nitrate released to the groundwater will degrade before reaching a 

public-supply well. 

The physical properties of aquifer materials can affect contaminant concentrations. For example, aquifers composed of an assortment of sediments 

(silt, silty sand, sand, gravel) tend to disperse dissolved contaminants 

because water and contaminants preferentially travel along pathways that 

provide the least resistance to flow, such as interconnected lenses of relatively coarse aquifer material. This dispersion of contaminants can reduce 

contaminant concentrations at a given location within an aquifer. However, 

dispersion will increase the overall size of the contaminated area because, 

once dispersed, the contaminant mass will tend to occupy a larger part 

of an aquifer. The size of the pores or open spaces within an aquifer also 

can affect contaminants in the groundwater. For example, pore size influences whether pathogenic microorganisms will be removed from the water 

through contact with and attachment to the solid aquifer material.

Intrinsic Susceptibility of the Groundwater 

Intrinsic susceptibility is characterized by the physical properties that 

influence how easy it is for water to move through the unsaturated zone 

and, subsequently, through the saturated zone. Intrinsic susceptibility is 

an important aspect of vulnerability because infiltrating water can contain 

contaminants, and subsurface traveltimes can influence whether reactions 

affecting contaminant concentrations can take place before the water is 

discharged from the aquifer.

Intrinsic susceptibility depends on the thickness and hydraulic properties of the unsaturated zone, which affect recharge rates to the aquifer. 

It also depends on aquifer hydraulic properties (permeability, porosity, 

and hydraulic gradients), which affect flow rates within the aquifer and 
26 Factors Affecting Public-Supply-Well Vulnerability to Contamination: Understanding Observed Water Quality and Anticipating Future Water Quality

discharge rates from the aquifer. For example, the presence of a confining 

unit can substantially decrease groundwater-flow rates (groundwater 

velocity) and increase traveltimes. Consequently, a confining unit can 

decrease the intrinsic susceptibility of the groundwater in that part of 

an aquifer system underlying the confining unit. On the other hand, the 

presence of preferential flow pathways can cause water to travel rapidly 

through the subsurface. This is particularly true near a pumping well where 

hydraulic gradients may steepen, allowing water to move more rapidly 

toward the well than if the well were not pumping. Preferential flow pathways can allow water (and contaminants) to actually bypass parts of an 

aquifer system—including confining units—that they would otherwise flow 

through. As a result, preferential flow pathways can increase the intrinsic 

susceptibility of groundwater by substantially increasing groundwater-flow 

rates and decreasing traveltimes. 

Collectively, recharge rates, discharge rates, the ability of the aquifer material to transmit water, and the size and shape of an aquifer contribute to the 

length of time groundwater will spend in an aquifer. Thus, groundwater 

age—the time elapsed since water entered an aquifer system—is a measure 

of intrinsic susceptibility. In general, young groundwater is more susceptible to anthropogenic contamination than older groundwater, particularly 

if the older groundwater recharged the aquifer before the widespread use of 

manmade chemicals. In this circular, “young” groundwater is considered to 

have been recharged after the 1940s.

Aquifer/Well Interaction

Every well collects water from a distinct combination of flow pathways. 

(See figure below.) The pathways that converge on a specific well can 

be attributed to the location, design (for example, screen placement and 

well capacity), construction, and operation and maintenance of the well. 

Well

screen

Well

casing

Public-supply well

Recharge

Cone of depression Monitoring wells

Domestic well

Water table

Groundwater

flow pathways

The vulnerability of every well is unique 

to itself. Thus, the quality of water from a 

public-supply well will be different from the 

quality of water from nearby wells, including 

other public-supply wells. In this figure, the 

public-supply well draws in water (and associated contaminants) that entered the aquifer 

beneath both residential and agricultural 

areas, whereas the monitoring and domestic 

wells draw in water that entered the aquifer 

solely beneath the agricultural area. In 

addition, the flow pathways that converge 

on the public-supply well represent a wider 

range of travel distances (and traveltimes) 

than the other wells. Pathways associated 

with long traveltimes have the potential to 

allow contaminant reactions to progress 

further than pathways associated with short 

traveltimes. Differences such as these can 

lead to differences in the quality of water 

produced by different wells.
Chapter 2—Introduction 27

Consequently, these varied well characteristics together control whether 

the contributing area of the well intersects a particular contaminant source, 

whether the well draws groundwater from parts of the aquifer favoring 

contaminant mobility and persistence, whether the well pumps water that 

entered the aquifer in recent years, and whether the well intercepts water 

moving along preferential flow pathways. In other words, a well affects its 

own vulnerability by drawing water from an aquifer in a way that uniquely 

combines the different aspects of groundwater vulnerability. Thus, the 

vulnerability of every well is unique to itself. This is true even for wells 

within the same aquifer and well field.

Combined Factors

When contaminants do occur in water from a public-supply well, it is 

the result of complex interactions between contaminant inputs, recharge 

rates, contaminant reaction rates, groundwater velocities and traveltimes, 

flow pathways, and the mixing of water and contaminants in the aquifer 

and in the well. The presence of a contaminant source is but one piece of 

the puzzle. Still, preventing groundwater contamination in the first place 

remains important.

USGS scientists, such as the hydrologist 

in this photo, collect water samples from 

public-supply wells to better understand 

why contaminants occur at detectable levels 

in water from such deep, high-capacity 

wells. Regardless of the combined factors 

that affect public-supply-well vulnerability 

to contamination, a contaminant source 

must be present for a chemical or microbial 

contaminant to show up in water from a 

well. In a prior USGS study, the probability 

of detecting VOCs in water from public- or 

domestic-supply wells was strongly associated with the amount of developed land 

within 3,000 feet of the wells—a simple 

measure of contaminant input (Rowe and 

others, 2010). Thus, preventing contaminants 

from being released to the groundwater 

in the first place remains important for the 

prevention of contaminated drinking water.
USGS scientists collect water samples 

from public-supply wells before the 

water is treated for use as drinking 

water. This enables the raw (untreated) 

water from a well to be analyzed for 

different measures of vulnerability. 

Information on the vulnerability of 

the water from a public-supply well 

to contamination can be used to help 

explain and forecast water quality.
Measures of Vulnerability 3

Groundwater vulnerability expresses the tendency or likelihood for 

contaminants to reach a specified location within an aquifer. This 

likelihood must be estimated from information that can be obtained 

from measurement or modeling (National Research Council, 1993). Similarly, public-supply-well vulnerability expresses the likelihood for contaminants to affect the quality of water from a public-supply well and must be 

inferred from information that is measurable. Measures of vulnerability 

can be as simple as a single parameter, such as depth to groundwater, or as 

complex as the output from a highly sophisticated groundwater-flow model 

that has been calibrated to many types of data. Vulnerability measures can 

provide insight into a single aspect of vulnerability (for example, contaminant input) or multiple aspects of vulnerability (contaminant input, contaminant mobility and persistence, and intrinsic susceptibility). 

The choice of which vulnerability measures to use in a given assessment 

should be based on the knowledge that is desired and the scale of the assessment. Complex measures are more practical for assessments involving individual wells than for assessments involving many wells over a broad area. For 

example, groundwater age can serve as a measure of intrinsic susceptibility, 

but it might not be feasible to estimate groundwater age for the large number 

of wells that would be necessary to assess vulnerability at the aquifer scale. In 

contrast, the percentage of well-drained soils—which does not account for as 

many underlying processes as groundwater age—might be a suitable measure 

of intrinsic susceptibility for an aquifer-wide assessment. 

Each vulnerability measure contributes to the uncertainty in a publicsupply-well vulnerability assessment. Trading one measure for another 

might increase the amount of uncertainty in an assessment, but the tradeoff might be necessary to understand vulnerability at the desired scale. 

Regardless of scale, evaluating information related to contaminant input, 

contaminant mobility and persistence, and intrinsic susceptibility for a well 

(or wells) will provide insight into the vulnerability of the water from the 

well(s) to contamination. This insight can be used to help explain observed 

water quality and forecast future water quality in the well(s). 

Much can be learned by analyzing water 

samples from a public-supply well for 

a variety of chemical constituents. For 

example, chloride and bromide can be 

used to help determine where the water 

from the well came from; dissolved oxygen, 

nitrate, manganese, iron, and sulfate can 

be used to determine redox conditions and, 

thus, which contaminants might be persistent in the groundwater; and age tracers, 

such as tritium and sulfur hexafluoride, 

can be used to help determine how long it 

might take contaminated groundwater to 

reach the well.
Results of this study call attention to three measures of vulnerability that are 

particularly useful for understanding the quality of water from individual 

public-supply wells. Taken together, the three measures can help answer the 

question, “Which contaminants in an aquifer might reach a public-supply 

well, and when, how, and at what concentration might they arrive?” 

1. Sources of recharge that contribute water (and associated contaminants) 

to a well—a measure of contaminant input.

2. Geochemical conditions encountered by water pumped from a well—a 

measure of contaminant mobility and persistence. 

3. Groundwater-age mixture of different waters that blend (or mix) in a 

well—a measure of intrinsic susceptibility.

These three measures, how they were determined for this study, and what 

they communicate about public-supply-well vulnerability to contamination 

are the subject of this chapter. Although these measures can account for 

the effects of preferential flow pathways on contaminant input, contaminant mobility and persistence, and intrinsic susceptibility of water from a 

public-supply well, additional information on preferential flow pathways is 

presented in chapter 4 because of the importance of the topic.

30 Factors Affecting Public-Supply-Well Vulnerability to Contamination: Understanding Observed Water Quality and Anticipating Future Water Quality

Groundwater samples from public-supply wells represent a greater range of 

conditions, both in space and time, than samples from monitoring or domestic 

wells do. As a result, they are particularly useful for gaining insight into the 

contaminant history of an aquifer system. Yet, samples from public-supply 

wells can be challenging to interpret. The more information that is available to 

aid in the interpretation of public-supply-well water samples, the better.

High-pressure air line

for releasing dye

Sampling tubing

Motor

Battery

Inline

fluorometer

Storage

Nitrogen

tank

Generator

Control

box

A combined wellbore-flow and depth-dependent sample collection tool 

was constructed for this study to enable downhole data to be collected 

from actively pumping public-supply wells. In this way, information could be 

obtained on where along the well screen contaminants actually entered a 

well, in addition to information on the range of geochemical conditions and 

groundwater ages represented by the water from the well.
Compared to nearby monitoring 

or domestic wells, public-supply 

wells often produce water with 

a wider range of contaminants 

because they integrate more 

sources of recharge.

Sources of Recharge 

Public-supply wells commonly integrate water from multiple sources of 

recharge that contain different drinking-water contaminants. For example, 

water that is recharged beneath urban land is likely to contain more VOCs 

than water that is recharged beneath agricultural land (Zogorski and 

others, 2006). On the other hand, agricultural recharge is likely to have 

higher concentrations of nitrate than urban recharge (Dubrovsky and 

others, 2010). Both urban and agricultural recharge might contain pesticides, but not necessarily the same ones: simazine and prometon are more 

commonly associated with urban recharge, and atrazine and metolachlor 

are more commonly associated with agricultural recharge (Gilliom and 

others, 2006). Information on the sources of recharge for a public-supply 

well provides insight into which contaminants might be in the water that 

will eventually reach the well. For this reason, an estimate of the sources 

of recharge for a well is a useful measure of the vulnerability of the water 

from the well to contamination.

Chapter 3—Measures of Vulnerability 31

Public-supply well

Zone of contribution for well

Urban area Agricultural area 

URBAN SOURCES OF RECHARGE WATER

OTHER SOURCES OF RECHARGE WATER

Water affected

by urban recharge

Water affected by

agricultural recharge

Water table

Predevelopment water

Water affected by

stream recharge

The sources of recharge water for a well, 

the relative contribution from each source 

of water, and the contaminants associated 

with the different water sources can affect 

the quality of water from a well. This figure 

illustrates how the quality of the water from 

a well can be influenced by multiple and 

different recharge sources. 

The well in the upper diagram draws in 

water that was recharged in an urban 

area (orange arrows). Some contaminants 

associated with shallow groundwater in 

urban areas are VOCs, nitrate, and urbanuse pesticides. 

The well in the lower diagram draws in 

water from three additional sources: 

(1) water recharged in an agricultural 

area (green arrow), which might include 

nitrate and agricultural-use pesticides; (2) 

infiltration of water from a stream (lightblue arrow), which might include nitrate, 

pesticides, and wastewater contaminants; 

and (3) water unaffected by recent land use 

(dark-blue arrow), which probably does not 

include anthropogenic contaminants but 

might include naturally occurring drinkingwater contaminants.
Regional differences in the sources of recharge for public-supply wells

Urban

Forest

Other

land use

Agriculture

Surface

water

Unknown

Mountain

front 

3

4 5

7

6

8

1

2

10

9

Sources of recharge for public-supply wells in multiple study areas were identified by using regional-scale groundwaterflow and particle-tracking models combined with land-use data. The pie charts in the figure above are based on results 

from the models, which were aggregated for the network of public-supply wells in each study area. (The percentages 

represent only the small parts of the aquifers simulated by the models.)

Public-supply wells in the Nevada (4), Utah (5), New Mexico (7), Texas (8) and Ohio (1) study areas primarily produced water 

that recharged the aquifers in localized areas—near the edge of a mountain or beneath a surface-water body. On the other 

hand, public-supply wells in the California (3), Nebraska (6) and New Jersey (10) study areas primarily produced water that 

recharged beneath urban or agricultural lands. Wetlands, which are included in the "Other land use" category in the figure 

above, were an important source of recharge for public-supply wells in the Florida (9) study area. These differences can 

influence which contaminants are detected in water from public-supply wells in different parts of the Nation.

(See page 19 for names of aquifers and study areas.)

32 Factors Affecting Public-Supply-Well Vulnerability to Contamination: Understanding Observed Water Quality and Anticipating Future Water Quality

Identifying Sources of Recharge for a Public-Supply Well

There are a number of ways to estimate the area contributing recharge to 

a public-supply well. The best way to estimate recharge areas is through 

simulation of groundwater flow by using computer models (Franke and 

others, 1998). Even estimates obtained in this way, however, are prone to 

uncertainty. As a result, study-team scientists developed several computer 

programs to help quantify the uncertainty associated with model-derived 

estimates for recharge areas (Starn and Bagtzoglou, 2011). (See Appendix, 

pages 108, 110.) 

Groundwater chemistry data also can provide insight into the origins of 

water, and thus dissolved contaminants, entering a well and can be easier to 

obtain than model-derived estimates. Conservative chemical constituents 

in groundwater (for example, chloride and bromide, and the stable isotopes 

Appendix Toolbox
Chapter 3—Measures of Vulnerability 33

deuterium and oxygen-18) were especially useful for identifying the sources 

of recharge water for study wells. This is because conservative constituents 

are not easily removed from groundwater by reactions with aquifer materials—concentrations remain constant during transport from a recharge 

area to a well. In study areas where different sources of recharge had 

distinctly different concentrations (or ratios) of conservative constituents, it 

was possible to use mixing equations to determine whether water from the 

study well was a mixture of the different recharge waters. The proportion 

of the produced water from each recharge source also could be estimated. A 

sample calculation for resolving a two-component mixture by using conservative constituents is given in the Appendix (page 111). Computer programs 

such as PHREEQC (Parkhurst and Appelo, 1999) can be used to resolve 

more complex mixtures (for example, Bexfield and others, 2011). 

Additional insight into the sources of recharge for several study wells was 

gained by sampling the wells at several depths within their screened intervals. This method of sampling (Appendix, page 108) provided information 

on where different waters and contaminants actually entered the wells along 

their well screens.

The case study on the following pages illustrates how conservative chemical 

constituents, mixing equations, and depth-dependent sampling helped reveal 

the sources of recharge water and associated contaminants for a publicsupply well in this study. 

There are many sources of recharge for 

an aquifer and the public-supply wells 

that tap them. Two of the more common 

sources are (1) water that infiltrates 

beneath urban or agricultural land and 

(2) infiltrating surface water.
CASE STUDY

Geochemical methods for identifying sources of recharge water 

helped explain contaminant occurrence in the Nebraska study well

Water from the Nebraska study well had detectable 

concentrations of VOCs and uranium. Although shallow 

groundwater in the study area contained elevated concentrations of nitrate, VOCs, pesticides, pesticide degradation 

products, uranium, and arsenic, the detection of anthropogenic contaminants in water from the study well was 

confounding. This is because a clay confining unit separates 

the contaminated, unconfined aquifer and the aquifer tapped 

by the well; the well is screened in the confined aquifer 

between 140 and 200 feet (ft) below land surface (bls). 

Deuterium (2

H) and oxygen-18 (18O) data for the publicsupply well and nearby monitoring wells provided insight 

into the sources of recharge for the public-supply well, the 

relative contribution of each water source, and the associated contaminants in the water entering the well. The 

ability to distinguish recharge sources for the study well 

was made possible by the presence of isotopically distinct 

water in different parts of the aquifer system; 2

H and 18O 

The distinct geochemical characteristics of different sources of recharge for the Nebraska study well enabled study-team 

scientists to “unmix” the water from the well. Samples from unconfined-aquifer monitoring wells had 2H and 18O values 

that plotted close to the Global Meteoric Water Line (Craig, 1961) (see figure above), which suggests that recharge to the 

unconfined aquifer was primarily from precipitation. Samples from the confined aquifer generally plotted in a small area to 

the right of the Global Meteoric Water Line, suggesting that the 2

H and 18O values for confined-aquifer waters were modified by evaporation sometime after precipitation. Samples from the public-supply well and some confined-aquifer monitoring wells plotted along a line between samples from the unconfined and confined aquifers, indicating that the water 

from these wells was a mixture of waters from both aquifers. On the basis of a relatively simple two-component mixing 

equation, it was estimated that 12 percent of the water from the public-supply well was from the unconfined aquifer.

 MONITORING WELLS

 Confined aquifer

 Unconfined aquifer 

 Agricultural land use

 Urban land use

 PUBLIC-SUPPLY WELL 

 Wellhead sample

 Samples from selected

 depths within screened inverval

 (number following symbol refers 

 to depth in feet below land surface)

−80

−11 −10 −9 −8 −7

−70

−60

−50

Delta oxygen-18, in per mil

Delta

deuterium,

in per mil

Unconfined aquifer

150

170

200

160 140

Global Meteoric Water Line (Craig, 1961)

Mixing line

Confined aquifer 

Potentiometric

surface in 

confined aquifer

Depth,

in feet

250

0

Confining unit

Confined aquifer

Unconfined aquifer

500 gallons per minute

Nebraska study area

Water table
values commonly reflect recharge-area characteris tics, Differences in the relative proportion of unconfined- and 

such as latitude, altitude and season; and processes, such confined-aquifer water among samples from confined-aquifer 

as evaporation. (See Coplen (1993) for more informa- monitoring wells also were observed. These observations led 

tion on using isotopes as environmental tracers.) to a conceptual model of unconfined-aquifer water moving 

into the confined aquifer through locally present preferential Depth-dependent sampling provided information on where flow pathways that cross the confining unit. Further investi- and how water from the different sources entered the public- gation revealed that many irrigation and public-supply wells supply well. The contribution of water from the unconfined in the area are screened in both the unconfined and confined 

aquifer to the well varied across the screened interval aquifers and that these “multi-aquifer wells” allow water in an unexpected way: more water from the unconfined to rapidly move between the two aquifers. This finding is aquifer entered the well through the bottom of the screened supported by several lines of evidence, including results of interval than through the top. This occurred even though groundwater-flow and solute-transport modeling. (See “Pref- the confined aquifer is deeper than the uncon fined aquifer. erential Flow Pathways” on pages 61–77 for more information on preferential flow in this and other aquifer systems.)

The largest fraction of unconfined-aquifer water entered the public-supply well at the midpoint of the screened 

interval (about 170 ft bls). The screen midpoint also was where concentrations of chloride, VOCs, and uranium 

were greatest, and therefore most similar to concentrations in the unconfined aquifer. Monitoring wells in the 

confined aquifer also were sampled for 2H and 18O to determine the groundwater chemistry at selected depths 

within the aquifer system. The majority of monitoring wells, including those installed directly adjacent to the 

public-supply well, produced very low fractions of water from the unconfined aquifer. However, a smaller subset 

of monitoring wells produced large fractions of unconfined-aquifer water. The large fraction of water from the 

unconfined aquifer in some, but not all, of the monitoring wells suggested that the mixing was a result of flow 

from the unconfined aquifer to the confined aquifer along localized flow pathways. 

Unconfined

aquifer

500 gallons

per minute

Confined

aquifer

Confining

unit

0 20 40 60 80 100

200

150

100

50

0

Fraction of water from unconfined aquifer determined

using oxygen-18

Depth,

in feet

below land

surface (bls)

Confined aquifer

Confined aquifer <100 feet 

 from public-supply well

Wellhead sample

Depth-dependent sample

40–50% from the

unconfined aquifer

12–22% from the

unconfined aquifer

MONITORING WELL

PUBLIC-SUPPLY WELL

MORE INFORMATION

Clark and others, 2008 

Jagucki and others, 2008 

Landon and others, 2008 

Landon and others, 2009
Cyclic water-quality variations can indicate changes 

in the sources of recharge for a well

The relative contribution of different sources of recharge for a well can change in response 

to cyclic or long-term variations in hydrologic conditions (Reilly and Pollock, 1995). 

Changing recharge sources can lead to changes in the vulnerability of the water from a 

well to contamination. 

In an area near San Antonio, Texas, atrazine concentrations in water from a monitoring well 

in the karstic Edwards aquifer varied in response to changes in hydrologic conditions—

concentrations were higher during wetter periods and lower during drier periods. The 

variation in water quality occurred because the well produced a mixture of water from 

two sources that had very different atrazine concentrations. The amount of recently 

recharged surface water from losing streams varied with hydrologic conditions. The 

atrazine concentrations were 10 times higher in the recharging surface water than in the 

groundwater. During wet periods, stream infiltration contributed a greater proportion of 

recharge to the well compared with dry periods, bringing with it a greater amount of atrazine. 

In the same study area, nitrate showed the opposite pattern—concentrations in the 

groundwater were generally higher than in the surface water. Nitrate concentrations in the 

monitoring well were lower during wetter conditions because high nitrate concentrations 

in the shallow groundwater were diluted by a greater proportion of stream infiltration, 

which had lower concentrations of nitrate. 

The vortex in this stream is caused by rapid drainage of water 

through a 2-foot-diameter karst feature in the streambed. Streams 

crossing the recharge zone of the Edwards aquifer lose much

of their flow as water infiltrates to the water table along such

features. Changes in hydrologic conditions rapidly affect the 

quantity of infiltrating stream water, which in turn affects the 

quality of the groundwater resource.

MORE INFORMATION

Musgrove and others, 2010

David Johns, City of Austin, used with permission

36 Factors Affecting Public-Supply-Well Vulnerability to Contamination: Understanding Observed Water Quality and Anticipating Future Water Quality
Chapter 3—Measures of Vulnerability 37

Redox Conditions of Groundwater and Water from Wells

Methods for determining redox conditions in water from wells are not as 

straightforward as methods for determining other geochemical conditions 

(for example, pH and alkalinity) (U.S. Geological Survey, variously dated). 

As a result, this section focuses on redox conditions, why they matter, and 

how they can be assessed. 

The redox condition of a groundwater sample is defined by the dominant 

type of redox reaction (redox process) occurring in the sample. Redox 

reactions involve the exchange of one or more electrons between chemical 

constituents that are either electron donors or electron acceptors. In groundwater, organic carbon is a common electron donor, and there are often 

multiple potential electron acceptors. Dissolved oxygen is the preferred 

electron acceptor. When dissolved oxygen is present to participate in redox 

reactions, the water is “oxic”; when it is not present (dissolved oxygen <0.5 

milligrams per liter (mg/L)), the water is “anoxic.” 

Redox reactions are catalyzed by microorganisms that derive their energy 

from the reactions. Therefore, electron acceptors are used in a sequential 

manner on the basis of the amount of energy produced by each reaction. The 

order in which some common electron acceptors are used is as follows:

dissolved oxygen > nitrate > manganese(IV) > iron(III) > sulfate > carbon dioxide

Once one electron acceptor is consumed, the next most favorable electron 

acceptor will be used and the redox condition of the groundwater will 

become more “reducing.” 

The sequential use of electron acceptors causes zones of different redox 

conditions to exist within aquifers, resulting in some spatial variation in 

contaminant occurrence in the groundwater. Redox zonation, combined 

with well location and design, influences which contaminants released to the 

groundwater will show up in which wells in an aquifer. (See figures on the 

following page.) Because public-supply wells commonly draw in water with 

a wider range of redox conditions than nearby monitoring wells, including 

those completed at a similar depth in the same aquifer, it is not sufficient 

Geochemical Conditions

Geochemical conditions—especially redox conditions, pH, and alkalinity—

strongly affect the mobility and persistence of many chemical constituents 

in groundwater, including those that are regulated in drinking water. After a 

chemical is released into groundwater, the geochemical conditions encountered will affect whether the chemical constituent travels with the groundwater, reacts with the aquifer material, or degrades before it reaches a well. 

For example, nitrate, tetrachloroethene (PCE) and trichloroethene (TCE), 

and uranium are chemically stable and can be transported for long distances 

in oxic groundwater. Thus, information on aquifer geochemical conditions 

provides insight into which contaminants in an aquifer will be stable in the 

groundwater and could be transported all the way to a public-supply well. 

For this reason, an estimate for the geochemical conditions associated with 

the groundwater entering a well is a useful measure of the vulnerability of 

the water from the well to contamination.

“... information on using groundwater-chemistry data to help 

assess well vulnerability is a 

good addition to the wellhead 

protection concepts published by 

the USEPA in the early 1990s.”

Mike Wireman, 

 U.S. Environmental Protection Agency, 

National Groundwater Expert, 2011

oung University oung, Brigham Y Andrew Y

Trace elements in groundwater originate 

primarily from the dissolution and weathering of rocks and minerals that make up 

the aquifer solids and the overlying soils. 

The arsenopyrite shown in this photo is a 

sulfide mineral containing arsenic, which 

can dissolve into the groundwater under 

oxic geochemical conditions. 
to sample and analyze water from monitoring wells to determine the redox 

condition of the groundwater entering a public-supply well. Rather, water 

from the public-supply well itself needs to be analyzed.

Why Redox Conditions Matter

Water managers who understand how redox conditions are distributed within 

an aquifer system are in a position to anticipate which chemical constituents 

in the groundwater (for example, nitrate, arsenic, iron, manganese, and 

certain VOCs or pesticides) would (or would not) be expected to occur in 

water from a particular well. In addition, knowledge about redox conditions 

in an aquifer system can help water managers select the most suitable watertreatment methods for the water from their wells. 

Redox conditions of groundwater also are important because the oxidation 

state of some elements affects their toxicity. For example, the oxidized form 

of chromium (Cr6+) is more toxic than the reduced form (Cr3+).

38 Factors Affecting Public-Supply-Well Vulnerability to Contamination: Understanding Observed Water Quality and Anticipating Future Water Quality

Oxic

Anoxic

Public-supply well

Groundwater

flow pathways

Oxic water

The well in this figure is screened in the 

shallow part of an aquifer and produces 

water that is oxic. Under these conditions, nitrate entering the aquifer at the 

water table would persist as it travels to 

the well. However, naturally occurring 

elements that are attached to the solid 

aquifer material under oxic conditions (for 

example, iron, manganese, and arsenic) 

would not be released to the groundwater 

and become a water-quality issue. 

Zone of contribution for well

Water table Anoxic water

The well in this figure is screened in the 

deep part of the aquifer and produces water 

that is anoxic. Under these conditions, 

nitrate entering the aquifer at the water table 

might not reach the well; it will travel with 

the groundwater until it reaches the anoxic 

zone, where it will start to transform into 

nitrogen gas. However, reactions between 

the groundwater and aquifer material in the 

anoxic zone might release naturally occurring elements that are attached to aquifer 

material under oxic conditions. 

Mixed water

The well in this figure is screened in both 

shallow and deep parts of the aquifer and 

produces a mixture of oxic and anoxic 

waters. Wells with open intervals that 

cross redox boundaries can be vulnerable 

to contamination from a wide range of 

chemi cals. In addition, redox boundaries 

provide an ideal habitat for bacteria that 

cause biofouling (Cullimore, 2008) because 

such environments commonly contain a 

diversity of electron donors and acceptors.

California New

Jersey

Nebraska Ohio

8

6

4

2

0

Nitrate

concentration,

in milligrams

per liter as

nitrogen

Regional-scale investigations

Oxic

EXPLANATION

Mixed

Anoxic

75th percentile

Upper whisker1

Lower whisker

1

 Whiskers span the largest 

and smallest values within 

1.5 times the interquartile range

Median

25th percentile

Nitrate concentrations in networks of 

public-supply wells in four study areas 

reflect regional differences in redox conditions. In each area, the land use was more 

than 25 percent agricultural, and the publicsupply wells tapped highly productive sand 

and gravel aquifers. Aggregated waterquality data indicate that nitrate concentrations in the public-supply wells increased as 

the percentage of oxic conditions increased 

and the percentage of anoxic conditions 

decreased (L.J. Kauffman, U.S. Geological 

Survey, written commun., 2010). 
0

20

40

Percentage of

samples with

arsenic concentration

greater than 10 μg/L

Redox conditions

Total number of

samples analyzed

Glacial aquifer system

NO3

reducing

Mn

reducing

Mixed or

suboxic

Fe/SO4

reducing

Methanogenic

Oxic

331 30 73 160 188 51

Anoxic

In the glacial aquifer system, redox conditions 

are a relatively good indicator of the likelihood 

of detecting elevated arsenic concentrations. 

The glacial aquifer system is a heterogeneous 

mix of unconsolidated sediment that extends 

across parts of 26 states. About 9 percent of 

water samples from this aquifer system had 

elevated arsenic concentra tions (greater than 

the drinking-water standard of 10 micrograms 

per liter (μg/L)). Most of the samples with 

elevated arsenic concentrations had redox 

conditions that were highly reducing (see page 

42); almost 20 percent were iron or sulfate 

reducing, and 45 percent were methanogenic. 

The majority of waters sampled were oxic, but 

less than 1 percent of the oxic samples had 

elevated arsenic concentrations. 

Regional differences in redox conditions of groundwater from public-supply wells

Manganese, 

iron, or sulfate 

reducing; or 

methanogenic

Oxic or 

nitrate 

reducing

Proportion of water from 

public-supply wells drawn 

from different redox zones

3

4 5

7

6

8

1

2

Chapter 3—Measures of Vulnerability 39

A redox framework (see page 42) was used to assess redox conditions of groundwater from public-supply wells in the 

regional-scale investigations of this study. Pie charts in the figure above depict the percentage of water from all wells in 

each study area that was characterized as being in one of two redox groups. Oxic and nitrate-reducing conditions were 

prevalent in aquifers in the Western United States, whereas more reducing conditions were prevalent in the glacial aquifer 

system in the Midwest and East. 

A similar regional trend was identified in a study of more than 5,000 domestic wells in principal aquifers of the Nation (McMahon 

and Chapelle, 2007; McMahon and others, 2009). The spatial distribution of redox processes in groundwater and in water 

from the domestic wells was related to differences in geology, climate, hydrology, and other factors. The predominantly oxic 

conditions in the western aquifers are the result of low concentrations of organic carbon—an important electron donor. Where 

little organic carbon is present, dissolved oxygen is not consumed in redox reactions. In contrast, the glacial aquifer system 

contains abundant electron donors in the form of organic carbon and pyrite, which enable redox reactions to progress until 

conditions are more strongly reducing (McMahon and Chapelle, 2007; McMahon and others, 2009). These differences in redox 

conditions influence contaminant detections in water from wells in different parts of the Nation.

 (See page 19 for names of aquifers and study areas. Only the 

study areas that were included in this analysis are shown here.)
Nitrate transport differs among study areas because of

 differences in redox conditions

Nitrate is one of the most common anthropogenic contami­ Despite similarities in nitrate sources and fluxes, 

nants in groundwater. The source, fate, and transport water from the California public­supply well had 

of nitrate were investigated for the public­supply wells substantial concentrations of nitrate (4–10 mg/L), 

studied at the local scale by using chemical, isotopic, and whereas water from the Nebraska public­supply 

groundwater age­tracer data collected from the wells well had very low concentra tions (<0.5 mg/L). 

and from associated networks of monitoring wells. One important difference between the study areas was 

Redox conditions control whether nitrate persists or the distribution of redox conditions within the aquifer 

degrades as it travels from recharge areas to public­ systems. Concentrations of nitrate in the California study 

supply wells. Under oxic conditions, nitrate is stable well were similar to those in the shallow groundwater 

and can be transported with the groundwater. Under because redox conditions throughout the aquifer were 

anoxic conditions, nitrate is degraded by denitrifica­ predominantly oxic. In the Nebraska study well, concention—the microbial reduction of nitrate to nitrogen gas. trations of nitrate were significantly less than those in 

In general, denitrification occurs where nitrate-bearing the shallow groundwater because anoxic conditions 

water comes into contact with aquifer materials such as prevailed in the confined aquifer tapped by the well. 

organic matter, or reduced iron or sulfur minerals, that 

can be oxidized by reaction with the nitrate. This can Overall, the aquifer systems in the Nebraska and Calioccur near contacts between fine-grained and coarse- fornia study areas have similar percentages of finegrained sediment or in other locations where geologic and grained material. However, for Nebraska, the fine-grained 

mineralogic conditions change along groundwater flow material is predominantly in the form of a continuous 

pathways (McMahon, 2001; Böhlke and others, 2002). layer above the water­producing zone, which is a more 

effective reactive barrier to nitrate than the isolated 

Two of the study areas—California and Nebraska— had fined-grained material in the California study area. 

similar sources of anthropogenic contaminants and 

similar proportions of agricultural and urban land. In Although the Nebraska well was screened in the confined 

both areas, agricultural irrigation was widespread, and aquifer, at least 12 percent of the water captured by 

irrigation return flow was a significant component of the well was from the unconfined aquifer, where 

groundwater recharge. Agricultural fertilizer was the nitrate concentrat ions were elevated. Multiple lines of 

predominant source of nitrate, but septic systems also evidence indicate that nitrate in shallow groundwater 

contributed nitrate. In both study areas, nitrate concen­ reached the confined aquifer by moving down multitrations above the drinking­water standard of 10 mg/L aquifer wells. Anoxic conditions in the confined aquifer 

were detected in some samples of shallow groundwater. prevented the nitrate from reaching the study well.

CASE STUDY

Nitrate in public-supply well:

4–10 mg/L 

Nitrate in

shallow

groundwater:

4–21 mg/L 

Predominantly

oxic

Depth,

in feet

Dentrification

rate: 

2% per year

Water table

300

0

California study area

Denitrification occurred gradually across a thick zone (300 ft) 

of aquifer sediments at a rate equivalent to about 2 percent per 

year in the California study area. About 95 percent of the original 

nitrate was able to persist along flow pathways to the study well. 

This occurred because redox conditions were predominantly oxic 

despite the sizable amount of silt and clay within the aquifer sediments (50 percent). The fine-grained sediments exist as lenses 

within the sands and gravels, resulting in very localized anoxic 

conditions (near the contacts between the fine- and coarse-grained 

sediments) that limit the amount of denitrification that can occur.
Unconfined

Confining unit

Confined

Nitrate in

shallow groundwater

5–78 mg/L 

Nitrate in public-supply well: <0.5 mg/L 

Oxic

Anoxic

Denitrification rate: 

10–99.8% per year

Depth,

in feet

250

0

Nebraska study area

Potentiometric

surface in 

confined aquifer

Water table

Denitrification occurred rapidly at the top of the fine-grained confining unit (just below 

the unconfined aquifer) at an estimated rate equivalent to 10–99.8 percent per year in the

Nebraska study area. This denitrification rate was sufficient to degrade all nitrate in the 

time it took the groundwater to travel through the first 10–13 feet of the confining unit.

Nitrate in public-supply well: <0.5 mg/L 

Unconfined

Confining unit

Confined

Nitrate in

shallow

groundwater

5–78 mg/L 

Oxic

Anoxic

Depth,

in feet

Nebraska study area

250

0

Multi-aquifer well

(nonpumping)

Denitrification rate: 

10–99.8% per year—except

where water travels 

down a multi-aquifer well

High-nitrate water

can bypass the zone

of denitrification

Elevated concentrations of nitrate from the unconfined aquifer leaked into the confined 

aquifer along multi-aquifer wellbores. However, the nitrate did not persist in the anoxic 

conditions of the confined aquifer; about 95 percent of the original nitrate degraded by 

the time the affected groundwater reached the public-supply well. PCE and TCE were 

detected at low concentrations in the public-supply well, even though these contaminants 

also degrade in anoxic conditions. The PCE and TCE might have persisted because they 

commonly are released to groundwater from point sources, which can have very high 

source concentrations. High initial concentrations can result in a need for a greater degree 

of degradation before a chemical constituent disappears from the groundwater. PCE and 

TCE also need geochemical conditions that are more reducing (methanogenic) than the 

conditions needed for nitrate to degrade (nitrate reducing).

MORE INFORMATION

Denitrification in 

four study areas

McMahon and others, 2008a

California study area

Burow and others, 2008 

Jurgens and others, 2008

Nebraska study area

Clark and others, 2008 

Landon and others, 2008

The absence of nitrate 

does not equate with the 

absence of vulnerability 

to other anthropogenic 

contaminants.
“Redox reactions are something that all professionals should 

consider when designing, installing and sampling wells.”

Frederick Bloetscher, Associate Professor,

Florida Atlantic University;

and American Water Works Association,

Water Resources Division Trustee, 2011

42 Factors Affecting Public-Supply-Well Vulnerability to Contamination: Understanding Observed Water Quality and Anticipating Future Water Quality

Assessing Redox Conditions of Water from Wells 

Determining the redox condition of the water from a well is an important 

and practical tool for assessing the vulnerability of the well to contamination from a variety of chemicals. The most reliable method for assessing 

redox conditions is to document consumption and production of reactants 

and products of the redox reactions that occur along groundwater flow 

pathways (Chapelle and others, 1995). Unfortunately, flow pathways are not 

always known and (or) some redox reactants and products are not commonly 

measured in water-quality studies. Further complicating matters, publicsupply wells often integrate water from multiple flow pathways that are 

associated with different redox conditions. To overcome these challenges, a 

redox framework for identifying the redox conditions of water from a well 

was developed by study-team scientists. 

The framework is based on threshold concentrations of six water-quality 

constituents that are relatively inexpensive and easy to measure—dissolved 

oxygen, nitrate, manganese, iron, sulfate, and hydrogen sulfide. The 

Framework for identifying redox conditions of groundwater from a well 

[O2, dissolved oxygen; NO3–N dissolved nitrate as nitrogen; Mn, dissolved manganese; Fe, dissolved iron; SO4, dissolved sulfate; H2S, 

hydrogen sulfide; Mn(IV), oxidized manganese; Fe(III), ferric iron; mg/L, milligrams per liter; -, not applicable; >, greater than or equal 

to; <, less than]

General redox 

category

Predominant redox 

process

Threshold concentration (mg/L)

Fe/H2S mass ratio

O2 NO3–N Mn Fe SO4

Oxic O2 reduction ≥0.5 – <0.05 <0.1 – –

Suboxic1 <0.5 <0.5 <0.05 <0.1 – –

Anoxic NO3 reduction <0.5 ≥0.5 <0.05 <0.1 – –

Mn(IV) reduction <0.5 <0.5 ≥0.05 <0.1 – –

Fe(III) or SO4 reduction <0.5 <0.5 – ≥0.1 ≥0.5 –

 Fe(III) reduction <0.5 <0.5 – ≥0.1 ≥0.5 >10

 Mix Fe(III) and

 SO4 reduction <0.5 <0.5 – ≥0.1 ≥0.5 ≥0.3 and ≤10

 SO4 reduction <0.5 <0.5 – ≥0.1 ≥0.5 <0.3

Methanogenesis <0.5 <0.5 – ≥0.1 <0.5 –

Mixed Criteria for more than one redox process are met

1

 Further definition of redox processes is not feasible. 
Accurate data for the concentration of 

dissolved oxygen in groundwater is important for understanding redox conditions in 

the groundwater. Portable water-quality 

instruments, such as the one depicted in 

this photo, can be used at a field site to 

measure dissolved oxygen in groundwater 

as it is pumped from a well.

National Water-Quality Assessment Program

Transport of Anthropogenic and Natural Contaminants (TANC) 

to Public-Supply Wells

An Excel® Workbook for Identifying Redox Processes in 

Ground Water

U.S. Department of the Interior

U.S. Geological Survey

Open-File Report 2009–1004

threshold concentrations were determined on the basis of microbial physiology, results of multiple field studies, and analytical reporting limits. One 

limitation of the method is that dissolved iron and manganese can precipitate out of solution, so measured concentrations might underestimate the 

extent of iron or manganese reduction. Nonetheless, this is a systematic 

and generalized approach that can be applied to water from wells in diverse 

aquifer settings. 

The redox framework is useful even if the input dataset does not include 

all six recommended constituents. If dissolved oxygen data are not available, qualified redox codes will be assigned. If dissolved sulfide data are 

not available, the processes of iron reduction and sulfate reduction will 

be combined into a single classification. As a result, it might be 

possible to use existing data and the redox framework to arrive 

at an initial estimate for the redox conditions of the water from 

a well. The redox framework is available as a Microsoft Excel 

workbook to make it easy to analyze large datasets (Jurgens 

and others, 2009b). (See Appendix, page 108.)

MORE INFORMATION

Explanation and limitations of the redox framework

Chapelle and others, 2009 

McMahon and Chapelle, 2007

Microsoft Excel workbook for applying the redox framework

Jurgens and others, 2009b

Chapter 3—Measures of Vulnerability 43
Core (sediment or rock) samples were collected during drilling for determination of 

physical and chemical properties of the solid aquifer material. Where anoxic conditions existed in an aquifer, USGS scientists processed the samples in a glove bag that 

was continually purged with ultrapure nitrogen gas (see photo). This was done so that 

the sampling process itself did not alter the geochemical conditions of the samples 

before they were shipped to and analyzed at a laboratory.

44 Factors Affecting Public-Supply-Well Vulnerability to Contamination: Understanding Observed Water Quality and Anticipating Future Water Quality

Detections of naturally occurring 

arsenic or uranium in water from 

public-supply wells in this study 

were linked to human activities 

that altered the geochemical 

conditions of the groundwater.

Human Activities Can Alter Geochemical Conditions and 

Affect Contaminant Concentrations

In domestic and public-supply wells, naturally occurring drinking-water 

contaminants (also referred to herein as “natural contaminants”), such as 

radon, arsenic, and uranium, are detected more frequently at concentrations 

of potential concern than are anthropogenic contaminants, such as nitrate, 

VOCs, and pesticides (DeSimone, 2009; Toccalino and others, 2010). 

Natural contaminants are a component of the minerals that make up the 

solid aquifer material, and can be mobilized—that is, released from sediments and rocks to groundwater—under certain geochemical conditions. 

Human activities can cause local- and regional-scale changes in aquifer 

geochemical conditions and indirectly increase (or decrease) concentrations of natural contaminants in groundwater and in water from publicsupply wells. For example, groundwater in the vicinity of a landfill can 

have elevated concentrations of arsenic, yet the source of the arsenic is 

not the contents of the landfill. Rather, the source is geologic—part of the 

solid aquifer material (Delemos and others, 2006). This type of situation 

occurs because microorganisms degrade large amounts of organic carbon 

derived from the waste within the landfills, creating anoxic conditions in 

the groundwater. Arsenic is then released from the solid aquifer material to 

the groundwater under the newly anoxic conditions, thus increasing arsenic 

concentrations in groundwater downgradient from the landfill.

Another way that human activities can affect concentrations of natural 

contaminants in groundwater is by altering groundwater flow so that waters 

with different chemical characteristics mix. For example, in the Florida 
California Department of Water Resources

Irrigation can increase concentrations of 

naturally occurring drinking-water contaminants in shallow groundwater if excess 

irrigation water infiltrates to the water table 

and is chemically distinct from the native 

groundwater. This is because even slight 

changes in subsurface geochemical conditions (for example, redox, pH, or alkalinity) 

can move toxic elements off the solid 

aquifer material and into the groundwater.

Chapter 3—Measures of Vulnerability 45

study area, much of the downward movement of groundwater is along flow 

pathways that follow natural conduits in the limestone bedrock. Highvolume pumping from the study well pulls shallow, oxic and low-pH water, 

which is capable of dissolving arsenic-bearing minerals, into deeper, anoxic 

and high-pH parts of the aquifer system where arsenic can remain in solution. This accelerated mixing of dissimilar waters both mobilizes arsenic 

from the rocks and allows it to remain dissolved in the newly mixed water.

In the Nebraska study area, mixing of shallow oxic water with deeper 

anoxic water is linked to elevated uranium concentrations in the study well. 

Human activities cause mixing in this study area in two ways: (1) extensive 

pumping of deep water for irrigation produces a strong, downward regional 

hydraulic gradient, and (2) multi-aquifer wells act as artificial conduits for 

shallow water to migrate to deeper parts of the aquifer system in response to 

the downward hydraulic gradient. 

In the California study area, human activities have altered recharge and 

discharge rates, groundwater flow patterns, and geochemical conditions 

in the shallow parts of the aquifer system. The result has been the release 

of uranium from the solid aquifer material to the groundwater. Uranium 

concentrations in water from public-supply wells in the area have increased 

over time and are predicted to continue increasing as a result of these 

changes. (See case study on the following pages.)

MORE INFORMATION

Effects of human-induced alteration

of groundwater on natural contaminants in water from wells

Ayotte and others, 2011

Individual study area findings

Burow and others, 2008 

Jurgens and others, 2008

Katz and others, 2007

Landon and others, 2008
CASE STUDY

Widespread, long-term pumping and irrigation linked

to increased uranium concentrations in California study well

Concentrations of uranium in water from the California 

study well have approached the Maximum Contaminant Level (MCL) of 30 micrograms per liter (μg/L). 

Uranium concentrations greater than the MCL have been 

detected in several nearby public-supply wells, so the 

wells are no longer used as a source of drinking water. 

Uranium occurs naturally; it is attached to the aquifer 

sediments. However, elevated concen trations of uranium 

in groundwater are linked to human activities that have 

altered the geochemical conditions, flow pathways, 

and groundwater velocities in the aquifer system. 

The California study area is within one of the most productive agricultural areas in the world. In 2000, agricultural 

irrigation accounted for about 95 percent of the total water 

use, although urban development has expanded into agricultural areas and constitutes about one-half of the study 

area. Groundwater for irrigation and public supply is from 

a thick sequence of alluvial sand and gravel interbedded 

with lenses of silt and clay. Contaminants in the shallow 

groundwater (less than about 165 ft below land surface) are 

linked to human activities: VOC detections in urban areas, 

and pesticide detections and elevated nitrate concentrations 

in urban and agricultural areas. The deep zone (more than 

230 ft below land surface) contains old water unaffected by 

human activities. At intermediate depths, the groundwater 

has some characteristics of both the deep and shallow zones.

The alkalinity of shallow groundwater in the study area, 

primarily in the form of the bicarbonate ion, has increased 

substantially since irrigation began. In arid and semi-arid 

areas, such as the California study area, low soil moisture 

limits plant growth and soil microbial activity. This leads 

to naturally low levels of soil carbon dioxide. However, 

irrigation has increased both plant growth (in the form of 

crops and landscaping) and soil microbial activity, which 

have increased the amount of carbon dioxide in the soil. 

This increase in soil carbon dioxide has accelerated the 

dissolution of carbonate minerals into infiltrating recharge 

water, which has increased the alkalinity of the shallow, 

oxic groundwater. Such conditions favor uranium mobility. 

Long-term pumping and irrigation also have increased 

the vertical hydraulic gradients in the aquifer, causing 

high-alkalinity water to move deeper into the aquifer 

than it would have under natural flow conditions. 

Consequently, alkalinity and uranium concentrations in 

water from the public-supply well will likely increase 

in the future. (See lower figure, opposite page.)

ater Resources California Department of W

MORE INFORMATION

Burow and others, 2008

Jurgens and others, 2008

Jurgens and others, 2009a
0 100 200 300 400 500

0

10

20

30

40

50

Alkalinity,

in milligrams per liter 

as calcium carbonate

Uranium,

in micrograms

per liter MCL

 MONITORING WELLS

 Shallow

 Intermediate

 Deep 

 PUBLIC-SUPPLY WELL

Uranium is strongly sorbed to aquifer sediments in oxic groundwater with low alkalinity. However, where bicarbonate (the 

most common ion making up alkalinity) is available, it reacts 

with uranium attached to aquifer sediments to form calciumuranyl-carbonate complexes. These complexes dissolve into the 

groundwater, resulting in higher uranium concentrations in the 

water. In the California study area, the highest concentrations of 

both alkalinity and uranium were detected in shallow groundwater, which had been altered geochemically by agricultural 

activities. In deeper parts of the aquifer system, alkalinity is relatively low, so geochemical conditions are favorable for uranium 

to remain attached to aquifer sediments. Because the publicsupply well draws in some shallow groundwater, the inadvertent 

increase in dissolved uranium in the shallow groundwater has 

led to an increase in uranium concentrations in water from this 

(and other) public-supply well(s).

1,600 gpm

Water affected

by human activities

Highest alkalinity

and uranium

Highest alkalinity

and uranium

Mixed water

Predevelopment

water

1,600 gallons per minute (gpm)

Uranium = 11–24 μg/L

Uranium may exceed

MCL of 30 μg/L

Conditions

during 2003–04

Simulated conditions

after 100–200 years

About 20 percent of the water produced by the public-supply well is from the shallow part of the 

aquifer containing high concentrations of alkalinity and uranium. Because this is a relatively small 

proportion of the total water from the well, the produced water has uranium concentrations below the 

MCL of 30 μg/L. 

In the future, uranium concentrations in the public-supply well are likely to increase. This is because 

uranium that is present throughout the solid aquifer material will continue to be mobilized as highalkalinity water is transported deeper into the aquifer in response to strong vertical hydraulic gradients 

caused by pumping and irrigation. Over the next 100–200 years, uranium concentrations in the publicsupply well could reach the MCL if the proportion of water containing high alkalinity and uranium 

becomes sufficiently large. More importantly, uranium concentrations could reach levels of concern in 

shallower public-supply wells within a few decades.
Groundwater-Age Mixtures

The water pumped from a public-supply well is a mixture of groundwater 

that entered the surrounding aquifer at different times in the past because 

public-supply wells draw in water from relatively large parts of an aquifer. 

The number of years since water entered an aquifer system at the water table 

(traveltime since recharge) is referred to as the age of the groundwater and 

can influence which contaminants might be present in the water. Groundwater that was recharged hundreds of years ago is less likely to be contaminated with manmade chemicals than groundwater that was recharged 

years or decades ago. This is because many chemicals that are currently 

found in groundwater were not widely used before the 1950s. However, 

water that recharged an aquifer hundreds of years ago might contain high 

concentrations of naturally occurring drinking-water contaminants because 

the groundwater would have been in contact with the solid aquifer material—the source of the contaminants—for a very long time. For example, in 

the glacial aquifer system, arsenic concentrations above the drinking water 

standard (10 micrograms per liter (μg/L)) were most often associated with 

groundwater that recharged the aquifer system prior to the 1950s. Similarly, 

arsenic concentrations in water from public-supply wells in study areas 

in California, Connecticut, Ohio, Nebraska, Nevada, and Utah increased 

with increasing traveltimes to the wells (increasing groundwater age). The 

groundwater-age mixture for a well characterizes the complete range of time 

that it might take contaminants that are released to the groundwater to reach 

a well. For this reason, an estimate for the groundwater-age mixture for a 

well is a useful measure of the intrinsic susceptibility—and thus vulnerability—of the water from the well. 

The groundwater-age mixture for water from different public-supply wells 

can be highly varied. This is partially because the age of the groundwater 

in different aquifer systems is not the same. The age of the water in any 

A single value for the age of the 

groundwater pumped from a 

public-supply well (for example, 

average groundwater age) will not 

correspond to the length of time 

it will take shallow contaminated 

groundwater to reach the well 

because some of the water will be 

younger and some of it older than 

the single-age estimate.

Public well used to supply water to a community in southwestern Ohio.

MORE INFORMATION

Arsenic in the glacial aquifer system

Thomas, 2007

Public-supply well vulnerability to 

arsenic and uranium

Hinkle and others, 2009

48 Factors Affecting Public-Supply-Well Vulnerability to Contamination: Understanding Observed Water Quality and Anticipating Future Water Quality
aquifer system depends foremost on the amount and rate of recharge 

entering the aquifer system at the water table. The size and shape of an 

aquifer system, the ability of the aquifer material to transmit water, and the 

amount of groundwater pumping also influence groundwater age (Vogel, 

1967; Solomon and others, 2006). In addition, public-supply-well construction and operation (screen placement, pumping rates and schedules) can lead 

to differences in the age mixture of the groundwater pumped from different 

wells, including wells within the same aquifer. 

Public-supply well

Zone of

contribution

for well

Water table

Years

Decades

Centuries

Groundwater

flow pathways

Years

Decades

Centuries

Public-supply wells commonly produce 

groundwater with a mix of ages that span 

decades to hundreds or even thousands 

of years. This is because groundwater age 

generally increases with depth in an aquifer, 

and public-supply wells draw in water from 

multiple depths. The groundwater-age 

mixture for any well is simply the mix of 

ages for the groundwater in the surrounding 

aquifer that enters the well. In this figure, 

two wells in the same aquifer draw in water 

with a different range of groundwater ages 

because they are completed at different 

depths. As a result, they are likely to produce 

water containing different types and 

amounts of drinking-water contaminants.

Chapter 3—Measures of Vulnerability 49

150

Depth below

land surface,

in feet

Groundwater

age

Flow into well, in percent

Flow

into well,

in percent Age distribution curve

Flow into screened interval of well Groundwater-age distribution for well

Young

Young Old

Old 

250

350

0

0

5

5

10

10

Groundwater age

Age distribution histogram

Groundwater of different ages is not necessarily drawn into a public-supply well in equal proportions. 

This is because some aquifer units transmit water more readily than others. For example, in this figure 

(left-hand side), a greater percentage of young, shallow water enters the well compared with older, 

deeper water, presumably because the shallower part of the aquifer transmits water more readily 

than the deeper part. The groundwater-age mixture for the well in this figure is illustrated in graphical 

form on the right-hand side of the figure. Such groundwater-age distribution histograms or curves 

were used in this study to help explain the quality of water from public-supply wells.
Regional differences in the groundwater-age mixtures in public-supply wells

Age of water

Less than

60 years

Greater than

or equal to 

60 years

3

4 5

7

6

1

2

10

Groundwater-age mixtures for hundreds of public-supply wells were computed by using regional-scale groundwater flow 

models constructed for this study. Pie charts based on results from the models illustrate the overall proportion of water in 

each study area with traveltimes from the water table to a supply well of less than 60 years, or greater than or equal to 60 

years. (These percentages represent only the small parts of the aquifers simulated by the models.) 

The public-supply wells in the western aquifers collectively produced a higher percentage of relatively old water than 

did the eastern-aquifer wells. Because many potential drinking-water contaminants, including VOCs, pesticides, and 

fertilizers, were not widely used until recent decades, these differences in groundwater age can influence the detection 

of contaminants in water from public-supply wells in different parts of the Nation. 

 (See page 19 for names of aquifers and study areas. Only the 

study areas that were included in this analysis are shown here.)

USGS scientists use computer programs to 

create models of groundwater flow that can 

be used to help estimate the groundwaterage mixture of the water entering and being 

discharged from a well.

50 Factors Affecting Public-Supply-Well Vulnerability to Contamination: Understanding Observed Water Quality and Anticipating Future Water Quality
Why Groundwater-Age Mixtures Matter

An estimate of the complete groundwater-age mixture for a well can address 

many questions related to water quality that cannot be addressed with a single 

value for the age of the groundwater, such as the average age; for example, 

• What is the potential for in-well dilution of anthropogenic contaminants 

in relatively young water entering a well by old, uncontaminated groundwater simultaneously entering the well? 

• What are the shortest traveltimes from the water table to a well? 

• How readily will the quality of the water pumped from a well respond to 

source water protection efforts? 

• How might the quality of the water pumped from a well respond to widespread contaminant input at the water table (nonpoint-source contamination)?

Estimates of groundwaterage mixtures can help answer 

questions about public-supplywell vulnerability to contamination 

that cannot be answered with 

a single value for the age of the 

produced water.

Chapter 3—Measures of Vulnerability 51

Land use in the vicinity of shallow wells can frequently explain the quality of water 

from such wells, but the relation between land use and water quality in public-supply 

wells is not as straightforward. This is because a substantial time lag exists between 

contaminant input at the water table and contaminant arrival at public-supply wells. 

An investigation into the influence of septic systems on the quality of water from 

different types of wells (monitoring, domestic, and public supply) demonstrates the 

importance of traveltime for explaining water quality in public-supply wells. The number 

of nearby houses served by septic systems, such as the ones in this photo, helped 

explain septage-influenced water detected in shallow monitoring and domestic wells. In 

contrast, minimum traveltimes from the water table to the wells best explained septageinfluenced water detected in public-supply wells (Katz and others, 2011). 

Groundwater-Age Mixtures Are Linked To In-Well Dilution Potential

Uncontaminated water (of any age) that enters a well can dilute contaminants entering the well. Although some young groundwater (recharged after 

the 1940s) entering a well might not be contaminated, it is reasonable to 

expect that most or all old groundwater (recharged before the 1950s) will be 

free of anthropogenic contaminants. Therefore, the proportion of old water 

entering a well can be used as a “lower limit” for the amount of in-well dilution that could occur if the shallow groundwater were contaminated. Such 

information on dilution potential is useful for comparing the vulnerability 

of different wells to contamination from anthropogenic sources. Examples 

of ground-water age distributions and in-well-dilution potential from this 

study are presented on the following page.
0

Age, in years

0 10 20

30

Flow into

well, in

percent

0

Age, in years

0 10 20

125

30

Flow into

well, in

percent

Time elapsed between recharge 

and withdrawal: <15 years

Time elapsed between recharge 

and withdrawal: 1 day to 125 years

Groundwater-age mixture for well

Groundwater-age mixture for well

CONNECTICUT FLORIDA

Bedrock

Limestone

Water table

150

0

Depth,

in feet

Depth,

in feet

250

0

In the Connecticut study area, the aquifer is relatively shallow, 

and nearly all of the water produced by the study well was 

recharged during the past 15 years. As a result, the vulnerability 

of the water from the well to contamination from manmade 

chemicals is relatively high, and the potential for in-well dilution 

of such chemical contaminants by old water is low. 

The well in the Florida study area produces water with a wide range 

of ages, from less than 1 day old to about 125 years old. The very 

young water travels to the well through dissolution features such 

as sinkholes and conduits that connect shallow groundwater and 

the well. This fraction of water is highly vulnerable to contamination 

from anthropogenic sources—both chemical and microbial. The well 

also produces old water from the limestone matrix, so there is some 

potential for in-well dilution of manmade chemicals. 

52 Factors Affecting Public-Supply-Well Vulnerability to Contamination: Understanding Observed Water Quality and Anticipating Future Water Quality

1,000s

1,000s

Time elapsed between recharge 

and withdrawal: 10 to 1,000 years

Time elapsed between recharge 

and withdrawal: 10 to 1,000 years

Potentiometric

surface in 

confined aquifer

0

Age, in years

0 100 200

3

Flow into

well, in

percent

0

Age, in years

0 100 200

3

Flow into

well, in

percent

CALIFORNIA NEBRASKA

Depth,

in feet

Depth,

in feet

250

0

400

0

Groundwater-age mixture for well

Groundwater-age mixture for well

Sand and gravel Silt and clay

Wells in the California and Nebraska study areas produce water with a wide range of ages—10 years to 

thousands of years. (Note that the scales for the axes on these graphs are different from the ones on the 

Connecticut and Florida graphs.) No single recharge year contributes more than a few percent of the total 

volume of water from either of these wells. Because a large fraction of water from the wells entered the 

aquifers prior to the widespread use of manmade chemicals, these wells have a substantial potential for 

in-well dilution of such chemicals by the old groundwater entering the wells. 
Whether or not in-well dilution can prevent concentrations of contaminants from exceeding an MCL ultimately depends on the proportion of the 

contaminated and uncontaminated waters entering the well, along with 

the contaminant concentrations. For example, if water with a contaminant 

concentration 10 times higher than an MCL mixes with old, uncontaminated 

water in a well, the final concentration will remain above the MCL unless 

the old fraction makes up more than 90 percent of the produced water. 

However, if the concentration of the contaminated fraction equals the MCL, 

the concentration in the produced water will remain below the MCL if even 

a modest amount of uncontaminated water also enters the well.

Groundwater-Age Mixtures Include the Shortest Traveltimes to a Well

An estimate of the complete groundwater-age mixture for a well can provide 

insight into the shortest traveltimes to the well, potentially revealing a 

hidden vulnerability to contamination from anthropogenic sources. This 

point is illustrated with results from the local-scale investigations in 

Connecticut and Florida. The groundwater-age mixtures for the study wells 

in each area were derived by using groundwater-flow and particle-tracking 

models. The average groundwater age for the water from each well also was 

computed by using the models. 

A comparison of the average ages for the two wells suggests that the 

Connecticut well produces younger water than the Florida well and is potentially more vulnerable to contamination than the Florida well. (See figure 

below.) However, the complete groundwater-age mixtures estimated for the 

wells (curves on graph) reveal a different story. The Florida well actually 

produces a higher percentage of water that is less than 1 year old compared 

with the Connecticut well. Therefore, the Florida well is likely to have a 

greater vulnerability to contamination from pathogenic microorganisms 

(such as bacteria that have limited survival times in aquifers) compared with 

the Connecticut well.

Differences in groundwater-age 

mixtures among wells can be 

substantial, resulting in notable 

differences in the potential for 

in-well dilution of anthropogenic 

contaminants by old, 

unaffected water.

Chapter 3—Measures of Vulnerability 53

Flow to 

well, in 

percent

0

0 5 10

Age, in years

15 20

10

20

30

Florida well

Average age of water 

was 13 years, 20% of 

water was <1 year

Connecticut well

Average age of water

was 6 years, 5% of water 

was <1 year

125

Average groundwater age does not provide 

enough information on public-supply-well 

vulnerability to contamination. Rather, it is 

the age and proportion of the very youngest 

water entering the well that provides the 

most insight into vulnerability to pathogens 

and anthropogenic contaminants. For 

example, on the basis of groundwater-flow 

models, the average groundwater age 

for the Florida study well was 13 years, 

whereas the average groundwater age for 

the Connecticut study well was 6 years. 

However, more than 20 percent of the water 

from the Florida study well recharged the 

aquifer during the past year, whereas only 5 

percent of the water entering the Connecticut study well was this young.

MORE INFORMATION

Connecticut study area Florida study area

Brown and others, 2009 Crandall and others, 2009 

Jagucki and others, 2010 Jagucki and others, 2009b 

Starn and Brown, 2007 Katz and others, 2007
Groundwater-Age Mixtures Can Influence 

Source Water Protection Outcomes

Time of travel (synonymous with groundwater age when time zero is at the 

water table) is one criterion used to define the area around a well within 

which to implement source water protection. However, source water protection based on similar times of travel (for example, 10-year time of travel) 

can result in dissimilar amounts of protection of different wells with vastly 

different groundwater-age mixtures. For example, almost all of the water 

from the Connecticut study well travels from the water table to the well 

within 10 years. (See figure below.) As a result, source water protection 

within a zone of transport bounded by a 10-year time of travel could influence nearly all of the water from this well. In contrast, less than 5 percent 

of the water from the Nebraska and California wells is recharged within 

10 years of travel from the wells. Land-use changes or protection measures 

within a similar zone of transport would have comparatively little effect on 

the quality of water from these wells—more than 95 percent of the water 

would not be influenced by the protection efforts.

The effectiveness of management 

actions can be tied to the 

groundwater-age mixture of the 

water from a well.

54 Factors Affecting Public-Supply-Well Vulnerability to Contamination: Understanding Observed Water Quality and Anticipating Future Water Quality

0

1 10 100

Age, in years

Cumulative

flow to well,

in percent

1000 10,000

25

50

100

75

Connecticut: 95%

Florida: 50%

Nebraska and California: <5%

Connecticut: 95%

Florida: 50%

Nebraska and California: <5%

Percentage of produced water recharged

within an area bounded by a 10-year time of travel A one-size-fits-all source water protection 

strategy can protect very different percentages of water pumped from different wells. 

The percentage of water that could be 

affected by source water protection within 

a zone defined by a 10-year time of travel 

is substantially different for several study 

wells, as shown in this figure. If it were 

necessary to prioritize protection activities, 

wells that produce predominantly young 

water might receive the greatest benefit 

from such activities.

Estimates of Groundwater-Age Mixtures are Useful 

for Forecasting Water Quality 

Some human activities (for example, septic-system use, some agricultural 

practices, lawn care, and stormwater capture by dry wells) can result in 

widespread contamination of shallow groundwater. A persistent nonpoint 

source of contamination will affect greater and greater depths within an 

aquifer over time, unless the contaminant degrades in the subsurface. 

Because groundwater age increases with depth, the change in water quality 

at a well resulting from nonpoint-source contamination will be related to the 

age mixture of the water pumped from the well.

If the groundwater-age mixture for a well has been estimated, it can be 

used to help forecast future water quality. (See Appendix, pages 109, 117.) 

As an example, a water-quality response curve for a public-supply well 

that produces water with groundwater ages ranging from ten to hundreds 

Appendix Toolbox
Contaminated

Young

water

Old

water

Uncontaminated

Water table

Contaminant input*

A B C

Contaminant

input ceases

*The assumption is that 

the contaminant is 

nonreactive and 

uniformly distributed 

across the water table.

Wells that produce water with different groundwater-age mixtures respond differently to the same input of 

nonpoint-source contamination, as illustrated above. (A) Contaminated water appears in the shallow well. (B) All 

flow pathways bring contaminated water to the shallow well, and the concentration in the well equals that of the 

contaminated recharge water. The contaminant appears in the deep well, but the concentration is relatively low 

because a large fraction of old, uncontaminated water enters the well. (C) The contaminant disappears from the 

shallow well because contaminant input has ceased and clean water now recharges the aquifer. Contaminants 

are still detected in the deep well, but they are associated with past land use as opposed to present land use. 

Contaminant concentrations in the deep well never reach the concentration in the recharge water in (A) and (B) 

because the period of contaminant input is not as long as the period of recharge for the water entering the well—

uncontaminated water always enters the well somewhere along the well screen. However, contaminants will 

continue to be detected in water from the deep well long after recharge with contaminated water ceases.

of years is shown on the left. The contaminant input in this example is 25 

years of widespread, contaminated recharge at the water table (gray shaded 

area on graph). Contaminated water appears in the well after a delay of 

10 years—the shortest traveltime from the water table to the well (green 

curve). (Transport across the unsaturated zone is not depicted.) The initial 

contaminant concentration in the well is a fraction of the input concentration because most flow pathways continue to bring uncontaminated water 

to the well, diluting the contaminated water from the faster pathways. Over 

time, more and more flow pathways bring contaminated water to the well 

and the concentration in the well increases. In this example, the concentration continues to increase even after contamination of the shallow groundwater ceases 25 years after it began. This is because a substantial amount 

of contaminant mass in the aquifer system has yet to reach the well, 

owing to the many flow pathways with traveltimes greater than 25 years. 

Although contaminant concentrations continue to increase after cessation 

of contaminant input, the concentration in the well will never reach the 

concentration in the contaminated recharge. As long as the age range of the 

water drawn into a well is greater than the duration of contaminant input, 

some unaffected water will continuously dilute contaminants entering the 

well. The maximum concentration in the well in this example is 40 percent 

of the input concentration because of the very broad range of groundwater 

ages in the water from the well compared with the 25-year period of 

contaminant input. 

Concentration,

as a percent

of input

0

40

80

0 50

Years

100

Contaminant* input 

* Contaminant is nonreactive and 

 uniformly distributed across the 

 water table. 

Contaminant 

 response in well

A water-quality response curve illustrates 

changes in contaminant concentrations 

in a well over time that occur because of 

changes in recharge quality. A tool that 

can be used to generate water-quality 

response curves by combining contaminant input data and estimated age distributions (TracerLPM) was developed for 

this study. (See Appendix, pages 109, 117).

Chapter 3—Measures of Vulnerability 55

MORE INFORMATION

Connecticut study area Florida study area California study area Nebraska study area

Jagucki and others, 2010 Crandall and others, 2009 Burow and others, 2008 Clark and others, 2008

Starn and Brown, 2007 Jagucki and others, 2009b Jagucki and others, 2009a Jagucki and others, 2008
How does water quality at different wells respond to (middle column), and 25 years of contaminated 

similar contamination of the shallow groundwater? recharge followed by clean recharge (right column). 

To investigate this, water-quality response curves for The age mixtures for groundwater in the wells were the study wells in Connecticut, Florida, California, obtained from groundwater-flow models. The contaminant and Nebraska were created for several hypothetical was assumed to be nonreactive and uniformly distributed contaminant input-duration scenarios. The scenarios 

were 5 years of contaminated recharge followed by across the recharge area for each well. Another assumption was that the contaminant was released into the clean recharge (figure below, left column), 10 years shallow groundwater near the water table; differences in of contaminated recharge followed by clean recharge 

CASE STUDY

Differences in groundwater-age mixtures help explain why 

concentrations of nonpoint-source contaminants continue to increase in 

some wells while decreasing in others

Concentration

relative to input

concentration,

in percent

CONNECTICUT

• Rapid response

• Small dilution

• Rapid flush

0

0

40

80

0

40

80

0

40

80

0

40

80

50 100

5 yrs 10 yrs 25 yrs

0 50 100

Time, in years

Duration of input of a nonpoint-source contaminant

0 50 100

FLORIDA

• Rapid response

• Moderate dilution

• Slow flush

CALIFORNIA

• Slow response

• Moderate dilution

• Slow flush

NEBRASKA

• Slow response

• Large dilution

• Slow flush

Contaminant 

input 

Well responses

The groundwater-age mixture for a well can be used to help forecast the response of the well to widespread 

contaminant input at the water table. Comparison of water-quality response curves for multiple wells can help 

identify differences in the potential response of the wells to similar contaminant input scenarios. Model results 

show that wells with young water and narrow age ranges (for example, Connecticut study well) can respond 

rapidly to contaminant input and source removal, but do not benefit much from in-well dilution of contaminants. 

On the other hand, wells that produce groundwater with a wide range of ages can benefit from in-well dilution 

because no single recharge year contributes more than a few percentage of the water pumped from the well 

(for example, Nebraska). Concentrations can continue to go up after contaminant source removal if a substantial amount of the produced water is older than the number of years of contaminant input. 
contaminant transport across the different unsaturated 

zones were not accounted for in the simulations. Given these 

assumptions, the differences in the graphs on the opposite 

page are solely the result of differences in traveltimes 

and, thus, groundwater-age mixtures among the wells. 

The Connecticut well produces mostly young water. As 

a result, contaminant concentrations in these simulations increase rapidly in the Connecticut well after the 

start of contaminant input (rapid response) and decrease 

rapidly after contaminant input ceases (rapid flush). 

Concentrations in groundwater in the well almost reach 

the input concentration after only 10 years of contaminated recharge (figure on opposite page, top row). 

The water quality in the Florida well responds more 

rapidly to the onset of contaminant input compared to the 

Connecticut well because much of the groundwater entering 

the Florida well travels from the water table to the well in 

a very short amount of time (more than 20 percent of the 

water from the Florida well is less than 1 

year old). However, the Florida well also 

produces a substantial fraction of old water.

So, in-well dilution causes the contaminant 

concentration in the well to be less than 

the input concentration. If contaminant 

input ceased after 25 years, concentrations in the Florida well would decrease 

rapidly because the well produces a high 

proportion of water less than 25 years old. 

The California and Nebraska wells produce a large fraction of old water, resulting in simulated initial responses, 

dilution, and flushing that are much different than those 

for the Connecticut and Florida wells. For the 25-year 

contaminant-input scenario, the time delay between the start 

of contami nant input and peak concentration in the wells 

is close to 4 decades. Concentrations in the wells would 

continue to increase for about 15 years after contaminant 

input ceases (figure on opposite page, bottom two rows) 

because much of the contaminated groundwater would still 

be traveling to the wells at the end of the period of input. 

In these scenarios, contaminant concentrations in the wells 

would never reach the input concentration because the 

wells draw in a large amount of water that is older than 

25 years (the length of contaminant input), resulting in 

continuous dilution of contaminants in younger water. 

Some contaminants (such as nitrate) can degrade within 

an aquifer. The response of the water quality in a well 

to this type of contamination depends on the traveltime 

to the well and the amount of degradation that occurs in 

the surrounding aquifer. Results for a scenario in which 

25 years of contaminant input was 

coupled with site-specific denitrification rates are shown in the figure at the 

lower left. Very little denitrification 

occurs in the aquifers surrounding 

the Connecticut and California wells. 

Therefore, the curves for these wells 

are similar to the curves shown on the 

opposite page for the 25-year scenario 

without denitrification. However, a 

notable amount of denitrification takes place in the aquifers 

in the Florida and Nebraska study areas. As a result, the 

maximum predicted concentrations for these wells are less 

than those predicted on the basis of the age mixtures alone.

Concentration,

as percent

of input

WELL RESPONSES

Connecticut

Contaminant input

0

40

60

100

20

80

0 50

Time, in years

100

Florida

California

Nebraska

Differences in the response of wells to the same contaminant input 

may be particularly pronounced where differences exist in both the 

groundwater-age mixtures and the geochemical conditions that 

affect contaminant degradation rates.

Several years or even decades 

of monitoring will not be enough 

to determine long-term trends in 

many public-supply wells because 

of the wide range of groundwater 

ages in the wells. For these wells, 

a coupled monitoring and modeling 

approach is needed.

MORE INFORMATION

Response of public-supply wells 

to land-use change

McMahon and others, 2008b

Groundwater-age distributions 

for evaluating public-supply 

well vulnerability

Eberts and others, 2012
58 Factors Affecting Public-Supply-Well Vulnerability to Contamination: Understanding Observed Water Quality and Anticipating Future Water Quality

Estimating Groundwater-Age Mixtures for Wells

The link between groundwater age and public-supply-well vulnerability 

to contamination has been recognized for some time, particularly the 

relation between young groundwater and the potential for contamination 

by manmade chemicals (Kazemi and others, 2006, p. 68). Hence, some 

states and local communities estimate the age of the water produced by 

their public-supply wells to gage the vulnerability of their water source to 

contamination. Most often, water from a well is analyzed for a chemical 

indicator (tracer) of relatively young groundwater, and a single value for the 

age of the groundwater from the well is obtained. When groundwater age is 

estimated in this way, the age is an “apparent” groundwater age because a 

substantial amount of interpretation is involved. (For a summary of tracerbased methods for dating groundwater, see Plummer and others, 2003.) 

More importantly, a single value for groundwater age does not describe the 

full recharge history for a public-supply well—some of the water entering 

the well will be younger and some will be older than the estimated age. 

Lack of information on the younger and older water can lead to a misdiagnosis of the vulnerability of the well because contaminant input is likely to 

have changed over time. Therefore, it is useful to have tools for estimating 

the groundwater-age mixture in water from wells.

Water

vulnerable to

anthropogenic

contaminants

Young

water

Old

water

Tritium is a commonly used

tracer to indicate young 

(recently recharged) water

One of the more commonly used indicators of young groundwater is tritium. 

Tritium (3

H) is a radioactive form of 

hydrogen that can be part of the water 

molecules that recharge an aquifer. 

Because it is short lived, tritium is most 

frequently detected in water samples 

that contain a component of relatively 

recent recharge. However, the amount of 

tritium in groundwater recharge has been 

declining since the end of aboveground 

nuclear weapons testing—a source of 

tritium in the atmosphere. As a result, 

tritium is becoming less useful for identifying recently recharged groundwater. 

Moreover, a single tritium measurement does not describe the mixture of 

groundwater ages in water pumped from 

a public-supply well and therefore cannot 

be used to forecast the response of the 

well to widespread contaminant input at 

the water table.

Appendix Toolbox

Water is analyzed for tritium in the laboratory by using (clockwise from upper left) distillation, electrolytic enrichment, secondary distillation, and scintillation counting.

The methods and tools that were used in this study to estimate the complete 

distribution of groundwater ages in water from public-supply wells are 

introduced here and discussed in more detail in the Appendix. (See pages 

109, 113–116.) A widely used (and expensive) method combines groundwater-flow modeling with particle-tracking modeling. Data from many 

locations throughout the modeled area are needed for this method so that 

the models are calibrated to real-world conditions before being used for 
age estimation. Ideally, the calibration data include concentrations of age 

tracers in the groundwater. A less resource-intensive method involves fitting 

a curve described by a mixing equation to tracer concentrations in groundwater from a well of interest. For this method, multiple tracer data (multiple 

tracers for a single sample or a single tracer for multiple samples collected 

over a period of years) are needed to narrow down the possible combinations of groundwater ages in the water. A computer program (TracerLPM; 

Jurgens and others, 2012) was developed for this study to simplify the 

curve-matching process. Finally, if no groundwater-flow model or age-tracer 

data are available for a well, it might be possible to obtain a rough estimate 

of the groundwater-age mixture for the well from basic aquifer properties 

and well characteristics (such as aquifer thickness, porosity, recharge rate, 

and well screen depth) and use of equations that solve for traveltime (Cook 

and Böhlke, 2000; Mendizabal and Stuyfzand, 2009).

Limitations of Groundwater-Age Estimates 

for Assessing Well Vulnerability

One limitation of relying on groundwater-age estimates for assessing 

public-supply-well vulnerability is that reliable estimates can be difficult to 

obtain and might require the services of a technical expert. In addition, the 

groundwater-age mixture of the water from a well might vary over time as a 

result of pumping (Masterson and others, 2002).

Another limitation is that some types of contaminants enter aquifers and 

move towards wells without being transported with the flowing groundwater. Consequently, the age of the water might not reflect the actual 

vulnerability of the water to contamination. For example, the solvents PCE 

and TCE are two of the most frequently detected groundwater contaminants 

in the United States (Zogorski and others, 2006). These VOCs are denser 

than water and can enter and move within aquifers by sinking through 

the unsaturated and saturated zones before dissolving and flowing with 

the groundwater. As a result, such compounds are sometimes detected in 

groundwater that recharged an aquifer hundreds of years ago. 

A method for identifying wells with groundwater that is vulnerable to 

contamination from local sources of VOCs—regardless of the age of the 

water—was developed by using data from this study. The method is based 

on analysis of VOCs at extremely low concentrations (parts per quadrillion) 

using a gas chromatograph with an electron-capture detector; this method 

can provide water managers with an early warning that a well is vulnerable 

to contamination. (See Appendix, pages 109, 118.) 

Finally, old water is not without its vulnerability to contamination. Naturally occurring drinking-water contaminants can occur in such water.

Old water

Young

water

Dissolved contaminant—travels 

with the flowing groundwater

Dense non-aqueous phase liquid 

(DNAPL) contaminant— does not 

necessarily travel with the flowing 

groundwater

Because of the release of dense nonaqueous phase liquids to the environment, 

old water sometimes contains contaminants from recent human activity, as 

depicted in the above figure. The photo 

in the figure shows a site in which such 

dense compounds from an old dry-cleaning 

facility were once stored and leaked.

Chapter 3—Measures of Vulnerability 59

Appendix Toolbox

Tracers of young groundwater may not 

detect a vulnerability to contaminants

that do not travel with the flowing 

groundwater or that occur naturally.
Karst features enable the Peace 

River in Florida to flow quickly 

into the subsurface.
Preferential Flow Pathways 4

Preferential flow pathways are pathways that provide little resistance 

to groundwater flow, enabling water and associated contaminants 

to move relatively quickly in the subsurface. If a stress is added to 

an aquifer system (such as an increase in recharge from precipitation or an 

increase in discharge from pumping wells) the movement of the groundwater 

and contaminants along preferential flow pathways will be accelerated more 

so than along other flow pathways. Preferential flow pathways are important 

because they can affect every aspect of groundwater and public-supply-well 

vulnerability. For example, preferential flow pathways can influence whether 

a source of contaminants is linked to a well, whether chemical reactions can 

substantially alter contaminant concentrations before contaminated groundwater enters a well, and whether contaminated groundwater will reach a well 

within a time period of concern for the well owner. For these reasons, it is 

important to recognize whether preferential flow pathways are influencing the 

quality of water from a well.

“Preferential flow pathways are significant in many hydrogeologic settings 

and need to be taken into account by 

those involved in establishing source 

water protection areas for publicwater-supply wells.”

Darcy Campbell,

U.S. Environmental Protection Agency,

Region 8, 2011

Fractures and other openings in carbonate 

rocks (limestone and dolostone) enable 

water to flow through unsaturated zones 

and within aquifers while encountering little 

resistance to flow. Contaminated water 

traveling along such features will strongly 

influence the vulnerability of the water from 

a well completed in this type of aquifer.

This still-frame photo is from a video 

that was taken using a camera 

lowered into a borehole, and it shows 

a conduit (distinctive void space) 

intersecting the borehole. Water 

and contaminants can travel rapidly 

to a well that is intersected by such 

natural features. 
62 Factors Affecting Public-Supply-Well Vulnerability to Contamination: Understanding Observed Water Quality and Anticipating Future Water Quality

Both natural factors and human activities can lead to preferential flow of 

groundwater and contaminants in the subsurface, as illustrated below: 

Clay and silt

Sand

Groundwater 

flow pathway

Fast flow pathways are preferential flow Public-supply well Recharge

pathways that result from natural variations in the water-transmitting properties 

of aquifer materials. (See figure at right.) 

For example, water can move more rapidly 

through interconnected sand lenses than 

through clay or silt. When a public-supply 

well begins pumping, water is preferentially drawn towards the well from the most 

transmissive material in the aquifer, such 

as a sand or gravel.

Public-supply well Recharge

Carbonate rock

Groundwater

flow pathway

Conduits

Bedding plane

Fracture

Cavernous zone

Hydraulic short-circuits are preferential 

flow pathways that cause water and 

contaminants to bypass aquifer material 

through which they would otherwise flow. 

For example, fractures, bedding planes, 

conduits, and cavernous zones in bedrock 

allow fluids to rapidly bypass the aquifer 

matrix. This type of hydraulic “shortcircuiting” occurs because these natural 

features have relatively wide openings, 

providing little resistance to flow. The 

presence and location of naturally occurring hydraulic short-circuits are not usually 

known. Therefore, it can be difficult to 

quantify their effect on groundwater flow 

and contaminant transport to a publicsupply well.

Public-supply well Recharge

Unconfined aquifer

Water table

Potentiometric surface

Confined aquifer

Confining unit

Nonpumping well

Groundwater

flow pathway

Manmade features also can act as 

hydraulic short-circuits in an aquifer 

system. For example, wells with screens 

that connect multiple aquifers (multiaquifer wells) can allow water and contaminants to move rapidly across confining 

units that otherwise would restrict flow. 
Chapter 4—Preferential Flow Pathways 63

Nonpumping well

Water table

Downward hydraulic gradient Upward hydraulic gradient

Groundwater

flow pathways

The rapid movement of water within wellbores 

is not limited to multi-aquifer wells. Any well 

with a screen that is long enough to intersect 

different parts of an aquifer having different 

hydraulic heads will allow water and contaminants to migrate from one location within the 

aquifer to another by way of the wellbore (see 

figure at right). The movement can be downward or upward, depending on the direction 

of the hydraulic gradient. Because pumping 

affects water levels—a measure of hydraulic 

head—the movement of contaminants between 

aquifer units can change under different 

pumping conditions. Consequently, a change 

in pumping can affect the quality of the water 

from a well. (For an example, see pages 70–71.)

Results of modeling scenarios from this study that further explore how 

wellbores act as preferential flow pathways are presented on pages 75–76.

Flow down annular space

due to incorrectly placed well

seal (surface seal or grout)

Flow from aquifer

to well screen

Screen

Wellbore

Well casing

Pump

Grout

Gravel pack

Contaminant flow through 

hole, crack or casing joint

Shortcomings in the construction or maintenance of public-supply wells also can 

lead to hydraulic short-circuiting in the 

vicinity of a well. For example, breaches 

in the well seal or well casing can allow 

contaminated water to rapidly enter the 

well screen. This type of preferential flow 

pathway was not observed during this 

study and is not further discussed.
64 Factors Affecting Public-Supply-Well Vulnerability to Contamination: Understanding Observed Water Quality and Anticipating Future Water Quality

Preferential flow pathways can be 

the most important pathways for 

the transport of contaminants to a 

public-supply well.

 Why Preferential Flow Pathways Matter

• Preferential flow pathways often deliver the youngest, most vulnerable water 

to a well, along with most of the anthropogenic contaminants that reach the 

well.

• Preferential flow pathways make it difficult to know where the water from a 

well has originated. For example, the youngest water from a well might not 

enter the aquifer in the areas that are closest to the well. As a result, it can 

be challenging to determine which areas at the land surface to prioritize for 

reducing water-quality threats. (See page 65.)

• Preferential flow pathways create favorable conditions for the transport of 

pathogenic (disease-causing) microorganisms: rapid traveltimes reduce the 

opportunity for microorganism die-off, and relatively large interconnected 

openings reduce the opportunity for microorganisms to be removed from the 

water by filtration or sorption to sediments or rocks (USEPA, 2008).

• Preferential flow pathways cause contaminants to be transported at relatively 

rapid rates and reduce the effectiveness of natural processes that remove 

chemical contaminants from groundwater where groundwater velocities 

become sufficiently high compared with contaminant degradation rates. For 

example, if groundwater flow is relatively slow, all dissolved nitrate might be 

degraded before contaminated water reaches a public-supply well. However, 

if groundwater flow is relatively fast, contaminated water might reach the 

well before denitrification is complete.

• Preferential flow pathways can cause water and aquifer materials that are 

not in chemical equilibrium to come into contact. This can lead to chemical 

reactions that increase concentrations of naturally occurring drinking-water 

contaminants in the water.

• Preferential flow pathways can transport surprisingly large volumes of water 

to a well. For example, as much as 35 percent of the water from the Nebraska 

study well might have traveled along preferential flow pathways on the way 

to the well. (See pages 72–73.) Where preferential flow pathways are present, 

the mix of water and contaminants entering a well can change rapidly in 

response to changing hydrologic conditions and pumping rates, affecting the 

overall quality of water from the well.

• Preferential flow pathways appear to be pervasive—each study well was 

affected by at least one type of preferential flow pathway. (See pages 66–73.)

USGS Ohio Water Microbiology Laboratory 

E. coli—A bacterial indicator of fecal 

contamination of groundwater

Preferential flow pathways with large interconnected openings are especially important 

for the transport of pathogenic (disease-causing) microorganisms to public-supply wells. 

Pathogenic microorganisms in groundwater that is drawn into a well, however, can be 

difficult to detect. This is because such organisms might appear only intermittently in the 

water. Therefore, groundwater samples often are analyzed for the presence of indicator 

microorganisms—organisms that are consistently present in fecal waste and that occur 

in greater numbers than associated pathogens. 
CASE STUDY

Preferential flow pathways lead to

complicated contributing areas for study wells

Source water protection is a means of preventing contaminants originating near the land surface from moving 

toward and reaching a well that is used as a source of 

public drinking water (USEPA, 1997). Numerous methods 

have been used to delineate the areas around wellheads 

for assessment and protection. The most sophisticated 

and potentially most accurate methods make use of 

three-dimensional groundwater-flow models. Yet, simpler 

methods (for example, delineation using a fixed-radial 

distance from a well) are more common. Regardless of 

method, source water protection typically focuses on the 

areas closest to a public-supply well—the assumption being 

that recharge at the water table near the wellhead will be 

drawn into the well in the shortest amount of time. However, 

preferential flow pathways can invalidate this assumption. 

CITY OF 

TEMPLE TERRACE

Public-supply well

0 1 KM

0 1 MI

FLORIDA STUDY AREA

<6.4

>6.4 to 16

>16 to 26

>26 to 37

>37 to 50

>50 to 66

>66 to 127

Estimated

traveltime from

recharge area to

public-supply

well, in years

Recharge water moving 

through leaky multi-aquifer 

well to public-supply well

Recharge water moving

through upper confining unit

to public-supply well

YORK

Public-supply well

NEBRASKA STUDY AREA

0 1 KM

0 1 MI

0 1 KM

0 1 MI

Public-supply well

CALIFORNIA STUDY AREA

MODESTO

8 to 10

>10 to 20

>20 to 30

>30 to 40

>40 to 50

>50 to 60

>60 to 70

>70 to 80

>80 to 90

>90 to100

>100 

Estimated

traveltime from

recharge area to

public-supply

well, in years

The recharge area for wells that are affected by preferential flow pathways might not be intuitive. This study finding is 

illustrated in the figures above, which depict simulated areas 

contributing recharge to study wells in California, Florida, and 

Nebraska, each of which produces some water that reached 

the well by way of preferential flow pathways.

Widespread groundwater protection makes sense 

because preferential flow pathways make it difficult 

to know which areas at the land surface are most 

likely to contribute water and contaminants to a well.

For example, the local-scale groundwater-flow models for 

the California, Florida, and Nebraska study areas, which 

include enough detail to account for preferential flow, depict 

situations that might seem surprising. Some of the shortest 

traveltimes to the wells are associated with recharge areas 

that are located at great distances from the wells. For the 

California study area, preferential flow results from natural 

variations in the water-transmitting properties of the 

aquifer materials. For the Florida study area, preferential 

flow occurs within natural conduits and cavernous zones 

in the bedrock. For the Nebraska study area, preferential 

flow occurs within the wellbores of multi-aquifer wells 

located upgradient of the modeled public-supply well. 

The models used to compute the contributing areas 

presented here are themselves limited because they 

simulate only one possible configuration of the preferential flow pathways in each study area. In reality, the 

exact locations of features that result in preferential 

flow are rarely known. However, the complex patterns 

of these simulated recharge areas are realistic.

MORE INFORMATION

Models used to simulate

contributing areas

Burow and others, 2008

Clark and others, 2008

Crandall and others, 2009
66 Factors Affecting Public-Supply-Well Vulnerability to Contamination: Understanding Observed Water Quality and Anticipating Future Water Quality

Implications for Monitoring and 

Protecting Public-Supply Wells 

Preferential flow pathways contributed to the occurrence of contaminants 

in each of the public-supply wells selected for detailed investigation, as 

previously noted. The preferential flow pathways were markedly different 

in each case, increasing the vulnerability of the wells in different ways. 

However, an understanding of preferential flow provided an opportunity to 

improve monitoring or protection strategies for each well, as illustrated in 

the examples that follow.

Preferential flow pathways can leave water from 

confined-aquifer wells vulnerable to contamination from 

anthropogenic chemicals

0

20

40

60

Percentage of

source-water

samples

with detections

Unconfined aquifers

Confined aquifers

Mixed or unknown aquifers

Nitrate

>1 mg/L

Volatile

organic

compounds

Pesticide

compounds

Personal-care

products and

other organic

compounds

66 Factors Affecting Public-Supply-Well Vulnerability to Contamination: Understanding Observed Water Quality and Anticipating Future Water Quality

In a nationwide study of water quality in public-supply wells, about one-third 

of the detections of anthropogenic contaminants were in water from confinedaquifer wells (Toccalino and Hopple, 2010). This was unexpected because 

confined aquifers generally are assumed to be protected from anthropogenic 

contamination by an overlying confining unit. These data indicate that preferential flow pathways that allow contaminants to be transported relatively rapidly 

across confining units might be common, and (or) the geologic information 

needed to accurately characterize the extent of confinement in many instances 

is inadequate.

Recognizing the nature and contribution of 

preferential flow pathways can help resource 

managers prioritize the most effective 

measure for protecting groundwater used as 

a source of drinking water.
Chapter 4—Preferential Flow Pathways 67

s

Linking Stormwater Management and Source Water 

Protection is Necessary to Limit Vulnerability

Findings from the Connecticut study area illustrate the importance of joint 

planning for stormwater management and source water protection. Such planning can limit situations that inadvertently increase the vulnerability of the 

water from a public-supply well to contamination.

Stormwater runoff occurs when precipitation flows across the land surface. As it 

flows, the water can pick up a variety of harmful chemicals and (or) pathogenic 

microorganisms. Some communities redirect this stormwater into an underlying 

aquifer by using dry wells (also called stormwater drainage wells). Use of dry 

wells can reduce flooding and erosion, increase groundwater recharge, and limit 

the amount of contaminants entering a surface-water body, the latter of which 

is desirable because discharging contaminated stormwater to surface water is 

regulated under the National Pollutant Discharge Elimination System (NPDES) 

program, under the authority of the Clean Water Act (USEPA, 1997). Dry 

wells, however, can increase contaminant loading to the groundwater because 

unsaturated-zone processes (such as filtration and degradation) that help remove 

contaminants from infiltrating water are bypassed. Although dry wells are regulated by the Underground Injection Control (UIC) Program, under the authority 

of the Safe Drinking Water Act, they are often close to roadways, parking lots, 

and commercial and industrial loading facilities where spills or illicit discharges 

of hazardous substances might occur (USEPA, 1999). 

Dry wells in the Connecticut study area redirect stormwater from a commercial area into the underlying shallow aquifer (less than 15 ft to the water 

table). Results from the local-scale groundwater-flow model of the area 

indicate that 16 percent of the water from the study well recharged the aquifer 

by way of dry wells. This means that 16 percent of the water from this publicsupply well does not benefit from unsaturated-zone processes that can reduce 

contaminant concentrations in recharge water. Thus, the dry wells have the 

potential to increase the vulnerability of the water from the well to contamination—an unanticipated consequence of using dry wells in the area.

Connecticut study area 

Dry wells act as conduits 

for stormwater runoff

Bedrock

150

0

Depth,

in feet

0

840 12 16 20

10

20

Age, in years

Flow into

public-supply

well, in percent

Recharge through dry wells

Dry wells (storm drains with no outlets) in 

a commercial area within the Connecticut 

study area (photo) are located in the 

recharge area for the study well. The water 

that is shunted to the water table by means 

of the dry wells can travel through the 

aquifer and reach the public-supply well 

within 2 to 4 years—some of the shortest 

traveltimes from the water table to the 

well. The presence of the dry wells, which 

circumvent unsaturated-zone processes 

that can remove contaminants from 

infiltrating water, leave the public-supply 

well more vulnerable to contamination than 

if the dry wells were not present.

MORE INFORMATION

Brown and others, 2009 

Jagucki and others, 2010

Starn and Brown, 2007
68 Factors Affecting Public-Supply-Well Vulnerability to Contamination: Understanding Observed Water Quality and Anticipating Future Water Quality

Comprehensive Monitoring Can Be Essential for Detecting 

the Influence of Preferential Flow 

Findings from the Florida study area illustrate why characterizing groundwater chemistry in multiple aquifers (or aquifer units) can be essential for 

identifying preferential flow pathways bringing low-quality water from 

unexpected parts of an aquifer system to a well.

Nitrate was detected at concentrations as high as 6.1 mg/L in water from 

monitoring wells in the surficial (sand) aquifer system in 

the Florida study area. (See figure below.) Conditions in this 

aquifer were oxic and favored the persistence of dissolved 

nitrate. Nitrate was not detected in water from most monitoring wells in the Upper Floridan aquifer. Conditions in this 

limestone aquifer were anoxic and favored denitrification. 

However, nitrate was detected at low concentrations in water 

from the public-supply well, although the well is open solely 

to the limestone aquifer (median concentration, 0.89 mg/L).

Additional groundwater-chemistry data helped reveal why nitrate was 

detected in the public-supply well. Concentrations of several chemical 

constituents in the water from the public-supply well were more similar 

to concentrations in water from sand-aquifer monitoring wells than from 

limestone-aquifer monitoring wells. Exceptions included several limestoneaquifer monitoring wells that were completed in a cavernous zone within 

Surficial aquifer

system (sand)

Confining unit

Upper Floridan

aquifer

(limestone)

Dissolution features in limestone

act as conduits for contaminants

Florida study area

Depth,

in feet

250

0

Nitrate

concentration,

in milligrams

per liter as N

Surficial aquifer

system

(sand)

Public-supply

well

Upper Floridan

aquifer

(limestone)

0

2

4

6

Outlier1

1 Outliers are defined as values outside of the whisker span.

2 The whiskers span the largest and smallest values within 1.5 times the interquartile range. 

Upper whisker2

75th percentile

25th percentile

Median

Detection limit

(0.06 milligram per liter)

Nitrate detected in the study well in Florida had concentrations above 

those detected in most monitoring wells in the same (limestone) 

aquifer. This is because the high rate of pumping at the public-supply 

well caused contaminated water from the overlying sand to travel 

rapidly along dissolution features in the limestone to the public-supply 

well. Thus, the denitrification reaction taking place in the anoxic limestone aquifer could not remove as much nitrate from the groundwater 

reaching the public-supply well compared with the groundwater 

reaching most of the limestone monitoring wells.
Chapter 4—Preferential Flow Pathways 69

0 1 2 3 4 5 6

0

2

3

1

Sulfur hexafluoride,

in parts per trillion by volume

Tritium,

in tritium

units

PUBLIC-SUPPLY WELL—two samples

 MONITORING WELL

 Sand

 Limestone

 Limestone—conduit well

NOTE: Tritium and sulfur hexafluoride 

are chemical tracers of recent recharge.

Environmental tracer concentrations in water from the public-supply well and monitoring wells completed in a cavernous zone within the limestone bedrock were more 

similar to concentrations in water from the overlying sand aquifer system than water 

from the remaining limestone-monitoring wells, as illustrated in this figure. These data 

confirm the finding that dissolution features in the limestone bedrock act as preferential flow pathways causing some limestone-aquifer wells to draw in water from the 

more vulnerable sand aquifer system.

the limestone bedrock. (See figure above.) The monitoring data demonstrated that the cavernous zone provides a hydraulic connection between the 

overlying sand and some of the limestone wells, including the public-supply 

well. Nitrate-contaminated groundwater might have reached the publicsupply well because pumping at the well increased the groundwater velocity 

within the cavernous zone to the extent that very little denitrification could 

take place before the water reached the well. Alternatively, geochemical 

conditions within the cavernous zone might have been unfavorable for denitrification to occur in the first place (too much dissolved oxygen).

No single monitoring well in the Florida study area provided enough 

information to conclude that the sand aquifer, by way of a cavernous zone 

and other karst features, was the source of contaminants in the publicsupply well. Rather, the conclusion regarding the contaminant pathway in 

this study was drawn from a body of evidence that included groundwaterchemistry data for the public-supply well and monitoring wells in multiple 

aquifers. In general, a monitoring well is unlikely to intersect a preferential 

flow pathway bringing contaminated water to a public-supply well, but the 

degree to which the overall water quality at a public-supply well reflects that 

of an adjacent, more vulnerable aquifer is one way of identifying the most 

vulnerable wells in an area.

MORE INFORMATION

Crandall and others, 2009 

Jagucki and others, 2009

Katz and others, 2007
70 Factors Affecting Public-Supply-Well Vulnerability to Contamination: Understanding Observed Water Quality and Anticipating Future Water Quality

Temporal Fluctuations in Water Quality Can Help Water 

Managers Identify Preferential Flow

Findings from the California study area call attention to the importance of 

collecting monitoring data under different hydrologic conditions. Short-term 

water-quality fluctuations in a deep public-supply well can indicate preferential flow.

Nitrate and uranium concentrations in the public-supply well had fluctuated seasonally for several decades. (See figure below.) The fluctuations 

were recorded in the long-term records for the well. Because the well is 

screened from 60 to 365 ft below land surface, it was unlikely that recharge 

of a different quality traveled from the water table to the well screen each 

season. Thus, the very short-term variation in water quality indicated preferential flow, which was confirmed by further investigation.

The local-scale investigation showed that, during times of heavy pumping 

(summer months), about 20 percent of the flow to the well was from shallow, 

contaminated parts of the aquifer (figure on opposite page, left). When the 

well was not pumping, shallow contaminated groundwater moved down 

the wellbore and out into deeper parts of the aquifer because the verticalhydraulic gradient in the aquifer was downward. During times of light or no 

pumping (winter months), a substantial amount of contaminated water accumulated in the aquifer near the base of the well and was detected in samples 

from a nearby, deep monitoring well (figure on opposite page, center). When 

pumping resumed after a relatively inactive period, the contaminated water 

temporarily stored at depth was drawn back into the well. At these times, 

California study area

Wellbore acts as conduit for contaminants

during times of light or no pumping

Depth,

in feet

400

0

0 0

10

20

30

SUMMER

Nitrate

Uranium

WINTER

Nitrate

Uranium

1965 1975 1985 1995 2005

4

6

2

8

10

Year

Nitrate as

nitrogen,

in milligrams

per liter

Uranium,

in picocuries

per liter

Long-term records for a public-supply well 

can contain evidence of preferential flow, 

such as the seasonal differences in water 

quality recorded for the study well in 

California, as illustrated in this figure.

Seasonal or intermittent operation of a public-supply

can lead to undesirable water-quality variations

because public-supply wells create hydraulic shortcircuits in an aquifer that can allow contaminated 

groundwater to alternately enter a well along greater

or lesser lengths of its well screen. 
Chapter 4—Preferential Flow Pathways 71

nearly 45 percent of the flow to the well was made up of contaminated 

water, and contaminant concentrations in water from the well approached 

drinking-water limits (figure above, right).

The differences in water-quality data for summer and winter helped draw 

attention to the way in which the California public-supply well functioned 

as a hydraulic short-circuit. This knowledge enabled the pumping schedule 

for the well to be updated (longer pumping periods during winter months), 

and the quality of the water improved.

Heavy pumping Light or no pumping Resumed pumping

High nitrate, 

uranium,

and alkalinity

Low nitrate, 

uranium,

and alkalinity

Shallow,

contaminated

groundwater

Intermediate

and deep

groundwater

Water-quality changes linked to disease outbreaks

Changes in hydrologic conditions have preceded most of the worst disease outbreaks attributed to drinking water in affluent 

nations in recent years (Hrudey and Hrudey, 2004). Consequently, it is important to understand whether changes in hydrologic 

conditions lead to rapid changes in the quality of water pumped from a public-supply well. Even if contaminant concentrations 

do not vary significantly, variations in the physical properties (temperature, flow, turbidity) and chemistry (redox, pH, alkalinity) 

of water from a public-supply well are important to identify. Such changes might disrupt water-treatment processes that 

function best when conditions are constant or might signal a change in the source(s) of water for a well.

An outbreak attributed to contaminated groundwater sources of drinking water on South Bass Island, Ohio (photo 

above), affected approximately 1,450 people during summer 2004. The outbreak was caused by the transport of microbial 

contaminants from wastewater treatment facilities and septic systems to the underlying carbonate-rock aquifer and to 

Lake Erie after extreme precipitation events in May–July of the same year. Dissolution features in the aquifer facilitated 

the rapid transport of contaminated water to the drinking-water supply wells (Fong and others, 2007).

MORE INFORMATION

Burow and others, 2008

Jurgens and others, 2008
72 Factors Affecting Public-Supply-Well Vulnerability to Contamination: Understanding Observed Water Quality and Anticipating Future Water Quality

Ugradient wellbores act

as conduits for contaminants

Unconfined aquifer

Confined

aquifer

Nebraska study area

Depth,

in feet

250

0

Determining How Contaminants Reach a Well Can Help 

Narrow Protection Options

Findings from the Nebraska study area illustrate how effective protection 

strategies require an understanding of the dominant pathways of contaminant 

transport to a public-supply well.

Many irrigation and other production wells in this study area are completed 

in an unconfined aquifer and the confined aquifer used for public supply. 

When the multi-aquifer wells are not being pumped or are being pumped at 

low rates, water in the unconfined aquifer flows within their wellbores down 

to the confined aquifer. This downward movement occurs because the water 

levels in the unconfined aquifer are higher than those in the confined aquifer, 

owing to the widespread withdrawal of water from the confined aquifer for 

irrigation. The volume of relatively young, contaminated water that reached 

the confined aquifer in this way was substantial because of the high density 

of multi-aquifer wells in the area—approximately 2 to 3 wells per square 

mile. Nearly 25 percent of all water that entered the confined aquifer arrived 

by means of multi-aquifer wells. 

0

0.4

0.6

0.8

0.2

1.0

Through upgradient wellbores

Through overlying confining unit

0 50 100 150 200

Age, in years

Flow to well

Through either wellbores or confining unit

1,000s

1,000s

Age distribution

0

Age, in years

0 100 200

1

2

Flow into

well, in

percent

Nebraska study area

250

0

Depth,

in feet

Unconfined aquifer

Potentiometric surface

Confining unit

Confined

aquifer

Without upgradient multi-aquifer wells acting as hydraulic short-circuits across the regionally extensive confining 

unit, most water from the Nebraska study well would have been decades old and would not have carried with it 

anthropogenic contaminants from recent human activities. The groundwater-flow and particle-tracking models 

for the Nebraska study area were used to compute how much of the water that entered the investigated publicsupply well originally entered the confined aquifer by way of multi-aquifer wells—as much as 35 percent.
Chapter 4—Preferential Flow Pathways 73

A total of 11 confined-aquifer public-supply wells were sampled for this 

study. Of the 11 wells, 8 were affected by anthropogenic contaminants that 

had leaked from the unconfined aquifer to the confined aquifer through 

multi-aquifer wells. Contaminant concentrations differed among the wells 

according to the proportion of confined and unconfined aquifer waters that 

mixed in the wells. 

As much as 35 percent of the water drawn into the selected study well might 

have entered the aquifer by means of upgradient multi-aquifer wells. (See 

figure on opposite page.) However, it was estimated that 100 percent of the 

anthropogenic contaminants (PCE, TCE, and their degradation byproducts) 

that were detected in the produced water traveled down a multi-aquifer well 

before reaching the supply well. 

Knowing that multi-aquifer wells are the source of the anthropogenic 

contaminants in the confined-aquifer public-supply well enables resource 

managers to focus on devising effective measures to prevent or minimize 

flow through multi-aquifer wells. In addition, resource managers might treat 

the confining unit as having been compromised for the purpose of source 

water protection because it is unlikely that all wells breaching a confining 

unit will be accurately identified. This can be accomplished by protecting 

the area at land surface above a zone of transport for a well that is based 

solely on confined-aquifer groundwater traveltimes—as opposed to one 

that is based on combined unconfined aquifer, confining-unit and confinedaquifer traveltimes. This approach to source water protection can prevent 

the water from a well from becoming contaminated if an unidentified 

multi-aquifer well were to shunt groundwater across a confining unit. (See 

Appendix, pages 119–120, for a sample calculation.)

Historically, individual irrigation and other 

high-volume production wells in the High 

Plains aquifer were screened both above and 

below confining units. This type of construction enables water to move rapidly between 

aquifers, carrying with it contaminants that 

might otherwise remain within a single aquifer. 

A high density of multi-aquifer wells (2 to 3 per 

square mile within the Nebraska study area) 

can exacerbate this situation. The density of 

wells is illustrated by the proximity of circular 

crop patterns in the photo to the right.

John A. Kelley, USDA Natural Resources Conservation Service

MORE INFORMATION

Johnson and others, 2011 

Landon and others, 2008
74 Factors Affecting Public-Supply-Well Vulnerability to Contamination: Understanding Observed Water Quality and Anticipating Future Water Quality

Recognizing the Influence of 

Preferential Flow Pathways 

There a number of methods for recognizing the existence of preferential flow 

pathways and their influence on the quality of water from a public-supply 

well. Methods that were used in this study are listed below:

• Compare groundwater samples from a public-supply well with samples 

from monitoring wells in different parts of the aquifer system to determine 

whether the chemistry of the water from the supply well is more similar 

to water in an overlying or underlying aquifer than to water in the aquifer 

tapped by the well itself. (See pages 68–69.) This method includes the use 

of mixing equations to quantify the amount of the well’s water that originates in the different parts of the aquifer system. 

• Review water-quality data collected under a range of hydrologic conditions over an extended period of time to uncover patterns in the data that 

would otherwise be unexpected for a public-supply well in the absence of 

preferential flow. (See pages 70–71.)

• Use chemical tracers of groundwater age to determine whether young 

(post-1940s) water occurs where it is not expected, such as beneath a 

confining unit or beneath older, tracer-free water.

• Characterize redox processes associated with water from a public-supply 

well to determine whether the wellbore functions as a hydraulic shortcircuit, allowing waters with different chemistries to mix. A mixed redox 

diagnosis in itself cannot be used to conclude that preferential flow pathways exist in the surrounding aquifer, but it can confirm other indications. 

• Collect depth-dependent water-quality samples and wellbore-flow data 

from a pumping public-supply well to determine where contaminated 

water actually enters the well (Izbicki and others, 1999; Landon and 

others, 2009).

• Apply groundwater flow and transport models. The influence of preferential flow pathways will be learned only if they are explicitly included in 

a model. However, failure to include such pathways can call attention to 

their existence in settings where models are unable to adequately reproduce field-based observations of flow and chemistry without simulation of 

preferential flow.

Appendix Toolbox

Depth-dependent water-quality and 

borehole flow data were collected from 

study wells under pumping conditions to 

determine where and how contaminants 

entered the wells. Such information 

can be useful for determining whether 

preferential flow pathways are affecting a 

public-supply well. Here, a USGS scientist lowers sampling equipment into a 

public-supply well through a small access 

pipe extending from the wellhead on the 

far side of the well. The access pipe was 

installed for this study.
Chapter 4—Preferential Flow Pathways 75

Nonpumping wells can have a 

surprisingly large influence on the 

movement of water and associated 

contaminants within an aquifer.

Wellbores as Preferential Flow Pathways— 

Scenario Modeling

Potential Effect of Wellbores on the Movement of Water 

in Unconfined Aquifers 

A groundwater-flow and transport model was constructed to assess the 

possibility that wellbore flow can alter the mix of groundwater ages in an 

unconfined aquifer, thus affecting groundwater and well vulnerability. The 

model simulates a hypothetical aquifer in which nonpumping wells with 

long well screens were placed at different locations. 

Results demonstrate that inactive wells can allow young, potentially 

contaminated groundwater to flow where it otherwise would not be 

expected. (See figure below.) Because inactive wells facilitate the rapid 

movement of groundwater, they can allow groundwater to move to parts 

of aquifers where the water is not in chemical equilibrium with the solid 

aquifer material, potentially releasing natural contaminants to the groundwater. Thus, nearby inactive wells can cause both anthropogenic and natural 

contaminants to occur in water from a public-supply well.

Recharge area Discharge

area

200

0

100

200

0

Depth,

in feet

100

200

0

100

0

Pumping well

200

100

1000

Age, in

years

<8

43

>79

Distance, in feet

0

Nonpumping well

Nonpumping well

Before the introduction of nonpumping wells 

to the simulation, the age of the ground water 

increased with depth. 

A nonpumping well near the upgradient end 

of the recharge area allowed relatively young 

water near the top of the well screen to move 

downward into lower parts of the aquifer 

(dark-blue shading).

A nonpumping well in the discharge area 

allowed older water to enter the lower part of 

the well and exit into shallower parts of the 

aquifer (green shading). 

A pumping well in the discharge area near 

the nonpumping well caused the water to 

reverse flow directions and allowed young 

water to move down the nonpumping well and 

enter the pumping well (dark-blue shading).

MORE INFORMATION

Zinn and Konikow, 2007
76 Factors Affecting Public-Supply-Well Vulnerability to Contamination: Understanding Observed Water Quality and Anticipating Future Water Quality

Potential Effect of Multi-Aquifer Wellbores on the 

Downward Leakage of Water to Confined Aquifers

A groundwater-flow model was constructed to assess the effect of downwellbore flow on a confined-aquifer public-supply well under several 

pumping scenarios. The model simulates a hypothetical aquifer system in 

which pumping from a confined aquifer creates a large downward hydraulic 

gradient between an overlying unconfined aquifer and the confined aquifer. 

At the same time, multi-aquifer wells are present, allowing water to flow 

down into the confined aquifer. 

Results of this modeling scenario indicate that even a single multi-aquifer 

well located over one-half mile from a confined-aquifer public-supply well 

can contribute nearly 10 percent of the water produced by the supply well. 

Stated another way, water produced from such a public-supply well could 

have a contaminant concentration that is equal to 10 percent of the concentration in an overlying unconfined aquifer. 

Moreover, model results demonstrate that multi-aquifer wells do not have to 

be inactive (nonpumping) to serve as hydraulic short-circuits. This is because 

downward hydraulic gradients in settings where large volumes of water are 

pumped from a confined aquifer (such as irrigated agricultural settings) can 

be so great that pumping of an individual multi-aquifer well might not be 

able to fully reverse the downward flow of water within the wellbore. As a 

result, water might be moving downward and out the bottom half of a multiaquifer well even as water is being pumped from the wellhead. (See figure at 

left.) In fact, simulations in which all wells were pumping (both multi-aquifer 

and confined-aquifer wells) produced the highest percentage of leaked, 

unconfined-aquifer water in the confined-aquifer public-supply well.

MORE INFORMATION

Johnson and others, 2011

Multi-aquifer well

Unconfined

aquifer

Confined

aquifer

Confining

layer

Some water in a pumping public-supply 

well can move downward and out the 

bottom of the well screen if the pumping 

rate is insufficient to fully reverse the 

downward flow of water within the 

wellbore caused by a downward hydraulic 

gradient in the aquifer.
Chapter 4—Preferential Flow Pathways 77

0 2 4 MILES

0 2 4 KILOMETERS

NEW MEXICO

Map area

ALBUQUERQUE

Grande

Rio

40

40

25

25

Area where upward hydraulic 

gradients in the aquifer are common

Area where downward hydraulic 

gradients in the aquifer are common

Basin

boundary

Public-supply well in which arsenic concentrations do not show clear effects from groundwater 

flow within wellbore when well is not pumping

Public-supply well in which low-arsenic 

groundwater moves from shallow to deep parts 

of the aquifer when well is not pumping. 

Concentrations decrease in the winter

Public-supply well in which high-arsenic 

groundwater moves from deep to shallow parts 

of the aquifer when well is not pumping. 

Concentrations increase in the winter

EXPLANATION

Arsenic concentrations vary spatially and temporally in water from public-supply wells in Albuquerque, New 

Mexico, partly because groundwater with different arsenic concentrations migrates between different parts 

of the basin-fill aquifer within the wellbores of idle supply wells. During times when the wells are not pumping, 

high-arsenic groundwater from deep within the aquifer moves up and out into the shallow parts of the aquifer 

in areas where hydraulic gradients are upward. When pumping resumes, arsenic-laden water enters these 

wells from both shallow and deep parts of the aquifer. Concentrations in the produced water are then elevated 

until the high-arsenic water is purged from the shallow parts of the aquifer.

Wellbore flow leads to patterns in arsenic detections

Because public-supply wells are pumped less frequently in the winter than in the summer, arsenic 

concentrations are highest in winter water samples from the deepest wells in the parts of the aquifer having 

upward hydraulic gradients (orange circles in figure above). Knowing that well construction (depth), well 

operation (duration of pumping), and position within the groundwater-flow system (location with respect to 

vertical hydraulic gradients) affect high arsenic concentrations in water from public-supply wells will enable 

resource managers to better manage concentrations in the produced water by pumping existing wells for 

longer periods during the winter and by installing new supply wells at shallower depths in certain areas (Laura 

Bexfield, U.S. Geological Survey, written commun., 2012).
Public-supply well wellhouse

Monitoring wells

Public-supply-well vulnerability assessments that incorporate monitoringwell data characterizing conditions in different parts of the aquifer system 

and vulnerability measures characterizing water from the public-supply 

well can provide insight into how the different waters and contaminants 

in the aquifer system combine in the well. Such knowledge can draw 

attention to vulnerabilities that might degrade the quality of water from 

the public-supply well.
“...a good vulnerability assessment can help a water system 

focus its limited resources on 

those factors that present the 

greatest threat.”

Mike Ekberg, 

Miami (Ohio) Conservancy District, 2011

I

deally, data and information on each factor affecting public-supply-well 

vulnerability to contamination will be pulled together as part of a vulnerability assessment for a public-supply well. The factors are (1) contaminant 

input, (2) contaminant mobility and persistence, (3) intrinsic susceptibility of 

the water drawn into a well, and (4) the presence of preferential flow pathways, 

which can influence the relative importance of each of the other factors.

Four assessments illustrating the vulnerability of water from individual 

public-supply wells to contamination are summarized in this section. Each 

assessment represents a common aquifer-well combination (for example, 

stratified glacial sediments and a shallow well with a short well screen) 

and depicts vulnerabilities that are likely to be found in similar settings. 

Measures of vulnerability include the sources of recharge, the geochemical 

conditions, and the age mixture of the water pumped from the wells. 

Monitoring-well data characterizing different parts of the aquifer systems 

also are evaluated. Although water from each of the four wells contained 

similar contaminants, the findings from this study suggest that the strategies 

for protecting the different supplies should be different. 

Two assessments comparing the vulnerability of public-supply wells in 

different aquifers to contamination from nitrate or VOCs also are described. 

Results of these assessments demonstrate the existence of systematic 

spatial patterns in public-supply-well vulnerability to common groundwater 

contaminants. Measures of vulnerability for the assessments consist of 

biodegradation rate (as a measure of contaminant mobility and persistence) 

and groundwater velocity (as a measure of intrinsic susceptibility). Neither 

regional assessment uses data on actual contaminant inputs, which can be 

hard to obtain for spatially large areas. Rather, comparisons are based on 

common input concentrations, and results call attention to differences in the 

“relative” vulnerability among the water supplies

Information on land use is important 

for understanding potential sources of 

recharge and associated contaminants for 

a public-supply well. Here, an aerial photo 

is being reviewed so that USGS scientists 

can better understand land use in an area 

of interest.

Vulnerability Assessment Examples 5
80 Factors Affecting Public-Supply-Well Vulnerability to Contamination: Understanding Observed Water Quality and Anticipating Future Water Quality

Vulnerability of Water from Individual Wells

Public-Supply Well in a Glacial Aquifer System

The public-supply well that was investigated in Woodbury, Connecticut, 

is within the glaciated New England Upland in the west-central part of the 

State. The area has a humid climate. Average annual precipitation is about 

46 inches (in.), and depth to groundwater is about 13 ft below land surface 

(bls). The most productive part of the glacial aquifer system in this area is a 

relatively thick section of stratified glacial sediment in the Pomperaug River 

valley. (See figure below.) Water from the well contained 11 VOCs, nitrate, 

pesticides, uranium, and radon during the study. Only TCE, an industrial 

solvent, had a concentration in the raw (untreated) water from the publicsupply well that was above a drinking-water standard.

Contaminant Input

The area contributing recharge to the well was computed to be 0.2 mi2, of 

which 75 percent was urban, 13 percent was forested, and 5 percent was 

agricultural. (See figure on opposite page.) The primary sources of groundwater recharge were precipitation, infiltration of streamflow, and infiltration 

from septic systems and dry wells. Shallow groundwater beneath urban 

land had higher concentrations and more detections of VOCs than did 

groundwater beneath other types of land. Methyl tert-butyl ether (MTBE) 

was the most frequently detected VOC and was from a plume caused by a 

fuel spill at a gasoline station. Chloroform in the shallow groundwater was 

from infiltration of chlorinated water by way of septic-system drainfields or 

leaky distribution pipes that deliver chlorinated water. Nitrate and chloride 

concentrations above background levels were linked to the widespread use 

of septic systems and (or) the application of road salt. Mixing equations 

were used to determine the percentage of nitrate in water from the publicsupply well that came from septic systems—about 15 percent. Uranium and 

radon in the groundwater were from the solid aquifer material.

Connecticut study well; glacial aquifer system

Pomperaug River

Glacial till

(Silt, clay,

sand, gravel)

Fractured bedrock

Generalized

direction of

groundwater

flow

Glacial stratified sediment

(sand and gravel)

Public-supply well

(Open interval: 45–60 ft

Average pumping rate: 72 gal/min) 

Recharge

Water table

Faults
Chapter 5—Vulnerability Assessment Examples 81

Contaminant Mobility and Persistence

Redox conditions were generally oxic, and nitrate was able to persist in 

the groundwater. Arsenic generally was not a contaminant of concern. 

However, locally abundant organic matter or dissolved organic carbon 

from septic-system leachate caused iron, manganese, arsenic, and other 

trace elements to be released from the aquifer material to the groundwater. 

Uranium concentrations in the groundwater were highest where concentrations of bicarbonate were relatively high—conditions that exist in deep 

glacial deposits and surrounding bedrock. 

Intrinsic Susceptibility

Most of the water produced by the public-supply well was less than 10 years 

old, as estimated from age tracers and groundwater-flow modeling, and was 

derived from the stratified glacial deposits in areas of urban land use. Only a 

small percentage of old, bedrock water entered the well.

Preferential Flow Pathways

Dry wells served as hydraulic short-circuits in the upper part of the unsaturated zone, reducing the amount of time it took stormwater runoff carrying 

contaminants from paved areas to enter the aquifer. Sixteen percent of the 

water from the public-supply well entered the aquifer by way of dry wells. 

(See page 67.) In addition to dry wells, thin zones of coarse-grained sediment served as fast flow pathways for water and contaminants to reach the 

well.

Relevance to Source Protection

Groundwater throughout the aquifer is young, making it particularly vulnerable to contamination. As a result, actions of individual residents and local 

businesses can influence groundwater quality in this area as demonstrated, 

for example, by the widespread detection of septic-system-influenced 

shallow groundwater. Therefore, public education about how actions of 

individuals can negatively affect the community’s water source is of great 

importance. Dry wells in the commercial area exacerbate the vulnerability 

of the water from the public-supply well to contamination by reducing the 

amount of time it would take spilled chemicals to reach the well. Arsenic is 

not a problem for the well, but it could be a problem for any new well inadvertently screened through a localized organic-rich zone within the aquifer.

0 1 MI

0 1 KM

Boundary of local-scale investigation

Boundary of

regional-scale

investigation

Area contributing

recharge to well

Monitoring wells

Public-supply well

LAND USE

Urban

Agricultural

Forested

Wetland

Water

Other

N

CONNECTICUT

Map

area

MORE INFORMATION

Brown and others, 2009 

Jagucki and others, 2010

Starn and Brown, 2007
82 Factors Affecting Public-Supply-Well Vulnerability to Contamination: Understanding Observed Water Quality and Anticipating Future Water Quality

Public-Supply Well in the Central Valley Aquifer System

The public-supply well that was investigated in Modesto, California, is 

in the San Joaquin Valley, which has a semiarid climate. Average annual 

precipitation is about 12 in. Depth to groundwater is about 30 ft bls. The 

well has a long well screen (275 ft) and draws in water from a thick section 

of the Central Valley aquifer system. (See figure below.) Water from the 

well contained nitrate, VOCs, pesticides, uranium, and arsenic during the 

study, although none of these constituents were present at concentrations 

exceeding drinking-water standards. 

Human activities have significantly altered groundwater flow in the area. A 

greater percentage of groundwater recharge now comes from infiltration of 

irrigation water (42 percent) than from precipitation (29 percent). Similarly, 

irrigation pumping is the largest component of groundwater discharge (50 

percent). Water is often pumped and reapplied at the surface multiple times.

Contaminant Input

The area contributing recharge to the well was computed to be 1.6 mi2, of 

which 67 percent was urban and 30 percent was agricultural. (See figure 

on opposite page.) Most of the area was sewered, but some older subdivisions relied on septic systems. An extensive network of dry wells facilitated 

subsurface infiltration of stormwater runoff. 

Nitrate concentrations (median value, 12 mg/L) in shallow groundwater 

(less than 164 ft) were indicative of human activities—likely past or present 

use of agricultural fertilizer. Widespread detection of very low concentrations of pesticides and VOCs indicated recharge beneath agricultural and 

urban land, respectively. Uranium and arsenic in the groundwater were from 

natural sources, although human activities caused uranium concentrations 

in shallow groundwater to increase over time. 

California Department of Water Resources

California study well; Central Valley aquifer system

Alluvial fan deposits

(sand and gravel with

lenses of silt and clay)

Unconsolidated

Semiconsolidated

Generalized direction of

groundwater flow

Recharge

Water table

Agricultural

irrigation well

Irrigation

infiltration

Public-supply well

(Open interval: 91–366 ft

Average summer pumping

rate: 1,600 gal/min)
Chapter 5—Vulnerability Assessment Examples 83

Contaminant Mobility and Persistence

Oxic conditions prevailed throughout the aquifer system, allowing nitrate 

and uranium to persist once in the groundwater. Uranium concentrations 

were highest at shallow depths and were linked to an increase in bicarbonate 

concentrations in the shallow aquifer resulting from an increase in irrigation 

over time. (See case study on pages 46–47.) 

Intrinsic Susceptibility

Shallow groundwater was less than 40 years old. Deep groundwater (greater 

than 325 ft) was thousands of years old. The public-supply well produced 

water with a wide range of groundwater ages (approximately 9 years to 

thousands of years old) because of its long well screen. About 75 percent 

of the water from the well was old enough to predate human influences on 

water quality, providing for substantial in-well dilution of anthropogenic 

contaminants entering the well.

Preferential Flow Pathways

The long screened interval of the well enabled shallow, contaminated 

groundwater to migrate down the wellbore and out into deeper parts of the 

aquifer during times of light or no pumping—most notably during winter 

months. Upon reactivation of the pump, contaminated groundwater simultaneously entered the well from both shallow and deep parts of the aquifer, 

temporarily increasing contaminant concentrations in the produced water. 

The quality of water from the well varied seasonally as a result. (See pages 

70–71.)

Relevance to Source Protection 

A short-term solution to the winter water-quality problems at the well was 

to pump for longer periods during the winter months. In the long term, 

nitrate concentrations in the well will likely decrease without intervention because urban land use in the recharge area has increased over time; 

urban recharge in the study area contains less nitrate than does agricultural 

recharge. Regardless, nitrate concentrations in water from the well will not 

become as high as those in the shallow groundwater because a substantial 

proportion of the water from the well was recharged long before the use of 

nitrate fertilizer. In contrast, uranium concentrations will likely increase 

with time as the downward-moving front of irrigation-affected groundwater occupies a larger proportion of aquifer that is tapped by the publicsupply wells.

C O A S T R A N G E S

S I E R R A

N E V A D A

S A N J O A Q U I N V A L L E Y

Monitoring wells Public-supply well

CALIFORNIA

Modesto

LAND USE

Urban

Agricultural

Forested

Wetland

Water

Other

Stockton

0 1 2 MI

0 2 KM

Boundary of N local-scale

investigation

Boundary of

regional-scale

investigation

Area contributing

recharge to well

MORE INFORMATION

Burow and others, 2008

Jagucki and others, 2008

Jurgens and others, 2008 
84 Factors Affecting Public-Supply-Well Vulnerability to Contamination: Understanding Observed Water Quality and Anticipating Future Water Quality

Public-Supply Well in the Upper Floridan Aquifer

The public-supply well that was investigated near the city of Tampa is within 

the coastal lowlands of west-central Florida. The climate is subtropical, and the 

average annual rainfall is 60 in. The well is completed as an open hole in the 

limestone bedrock of the Upper Floridan aquifer, which is separated from an 

overlying surficial aquifer system by a discontinuous confining unit. (See figure 

below.) Depth to groundwater in the surficial aquifer system is generally 10 to 

50 ft bls. Water from the well contained VOCs, nitrate, pesticides, uranium, 

radon-222, and arsenic at concentrations below drinking-water standards, where 

such standards exist. Historically, total coliform bacteria and fecal streptococci 

bacteria were detected in water from the well field that includes this well. A 

current requirement to disinfect all drinking water in the area now protects 

users of the water from health hazards associated with bacterial contamination.

Contaminant Input

The area contributing recharge to the well was computed to be 9.4 mi2, of 

which 84 percent is urban, 5 percent is wetland, and 4 percent is agricultural. 

(See figure on opposite page.) The urban land is predominantly residential 

with some commercial and industrial areas. Precipitation and infiltration of 

streamflow are the primary sources of groundwater recharge. The area is 

sewered, and stormwater runoff is routed to a series of retention basins.

The surficial aquifer system is an important source of water and contaminants 

for the Upper Floridan aquifer. Every monitoring-well sample from this aquifer 

system contained at least one VOC at low concentrations. Most samples also 

had detectable nitrate at concentrations indicative of human activities, although 

no concentrations were above the drinking-water standard of 10 mg/L.

Radon-222, uranium, and arsenic in the groundwater were derived from the 

solid aquifer material. Radon-222 concentrations were higher in the surficial 

aquifer system than in the Upper Floridan aquifer. In contrast, uranium 

concentrations were naturally higher in the Upper Floridan aquifer. Elevated 

arsenic concentrations in water from the public-supply well were associated 

with a cavernous zone within the Upper Floridan aquifer. 

Florida study well; Upper Floridan aquifer

Public-supply well

(Open interval: 118 –174 ft

Average pumping rate: 700 gal/min) Recharge

Generalized direction

of groundwater flow

Cavernous zone

Water table Sinkhole

Intermediate confining unit (clay)

Surficial aquifer system

(sand)

Upper Floridan aquifer

(limestone)
Chapter 5—Vulnerability Assessment Examples 85

Area

contributing

recharge

Monitoring wells

Public-supply well

FLORIDA

Tampa

St. Petersburg

N

Boundary of

local-scale

investigation

Boundary of

regional-scale

investigation

LAND USE

Urban

Agricultural

Forested

Wetland

Water

Other

0 4 MI

0 4 KM

Contaminant Mobility and Persistence

Groundwater in the surficial aquifer system was oxic, whereas it was anoxic 

in the Upper Floridan aquifer. Nitrate was not detected in samples from most 

monitoring wells in the Upper Floridan aquifer, and evidence existed for 

denitrification in the anoxic Upper Floridan aquifer.

Intrinsic Susceptibility

Most water in the surficial aquifer system was recharged within the past 

6 years and was vulnerable to anthropogenic contamination. Water in the 

Upper Floridan aquifer was generally much older (greater than 60 years). 

Groundwater-age data and modeling results, however, indicated that as 

much as 70 percent of the water from the public-supply well was very 

young water that was drawn into the well from the surficial aquifer system. 

Groundwater-chemistry data revealed multiple similarities between the 

water from the public-supply well and the water in the overlying surficial 

aquifer system (for example, similar median concentrations of chloroform).

Preferential Flow Pathways

A cavernous zone that intersects the open interval of the public-supply well 

was the source of the surficial aquifer system water and most anthropogenic 

contaminants that entered the well. (See pages 68–69.) The cavernous zone 

also contributed arsenic to the well. Samples collected over a range of depths 

in the public-supply well showed that arsenic concentrations in the water 

from the cavernous zone were highest when the well was pumping. Rapid 

movement of oxygenated water within the cavernous zone apparently liberated arsenic from the limestone bedrock during periods of heavy pumping. 

Karst features also led to a complex contributing area for the well, making it 

difficult to know where at the land surface water from the well originated.

Relevance to Source Protection

The proportion of contaminated and uncontaminated water entering the well 

can change rapidly in response to changing hydrologic conditions because 

the cavernous zone enables very young water to be drawn into the well. 

Consequently, extra vigilance is warranted during changing hydrologic 

conditions so that water-treatment processes remain effective. Determining 

the degree to which water from public-supply wells reflects that of the 

surficial aquifer system is one way to prioritize monitoring and land-use 

planning in this area so that limited resources can be used to protect the 

most vulnerable wells.

MORE INFORMATION

Crandall and others, 2009

Jagucki and others, 2009b

Katz and others, 2007

Paschke, 2007
86 Factors Affecting Public-Supply-Well Vulnerability to Contamination: Understanding Observed Water Quality and Anticipating Future Water Quality

Public-Supply Well in the High Plains Aquifer

The public-supply well that was investigated in Nebraska is in the eastern 

part of the State in a mostly flat-lying area. The climate is humid continental, 

and the average annual precipitation is 28 in. Depth to groundwater is about 

10 to 80 ft bls. The well is completed in a confined part of the High Plains 

aquifer (see figure below), as are many of the newer public-supply wells in the 

area; however, irrigation, commercial, and some older public-supply wells are 

screened in multiple aquifers. Water from the well contained low concentrations of VOCs, particularly PCE and TCE and their degradation products. 

Uranium and arsenic, which occur naturally in the aquifer material, also were 

detected in water from the well. No contaminant was detected at concentrations above a drinking-water standard.

Contaminant Input

The area contributing recharge to the well was computed to be 2.4 mi2, of 

which 45 percent was urban and 39 percent was agricultural. (See figure on 

opposite page.) Urban land is primarily low-density residential and commercial. Urban areas are mostly sewered, but septic systems are used on the 

outskirts of some residential areas. Agricultural land is primarily irrigated 

cropland (corn, soybeans, grains). 

Most samples from the unconfined aquifer had elevated concentrations of 

nitrate. The principal sources of the nitrate were septic systems (or sewer 

leakage) and agricultural fertilizer. Low concentrations of agricultural 

pesticides or degradation products also were detected in samples from the 

unconfined aquifer. Widespread detections of very low concentrations of 

VOCs were associated with the urban land.

Water from both the unconfined and confined aquifers had widespread 

detections of uranium and arsenic. The source of these substances was the 

aquifer material itself. Uranium concentrations generally decreased with 

depth, but the highest concentrations were detected in samples that were a 

mix of waters from the unconfined and confined aquifers.

Nebraska study well: High Plains aquifer

Unconfined aquifer (sand and gravel)

Loess (silt)

Upper confining unit (clayey silt)

Lower confining unit (clayey silt)

Upper confined aquifer (sand)

Lower confined lenses (sand and silt)

Shale

Areal recharge

Water table

Agricultural

irrigation well

Irrigation

infiltration

Public-supply well

(Open interval: 141–200 ft

Average summer pumping

rate: 500 gal/min)

Generalized direction of

groundwater flow
Chapter 5—Vulnerability Assessment Examples 87

Contaminant Mobility and Persistence 

Water in the unconfined aquifer was mostly oxic. Water in the upper 

confined aquifer that was tapped the by public-supply well was mostly 

anoxic and devoid of anthropogenic contaminants.

Intrinsic Susceptibility

Water in the unconfined aquifer was less than 48 years old based on 

concentrations of age tracers in water from monitoring wells. Water from 

the confined aquifer was mostly old enough to predate the widespread use 

of many chemicals that can contaminant drinking-water sources. Groundwater samples from the public-supply well and from some confined-aquifer 

monitoring wells were a mix of waters from the unconfined and confined 

aquifers. At least 12 percent of the water from the public-supply well was 

young, unconfined-aquifer water that entered the well through the bottom 

half of the well screen. (See case study on pages 34–35.)

Preferential Flow Pathways

Young, contaminated groundwater from the unconfined aquifer entered the 

confined aquifer by moving down one or more of the many multi-aquifer 

wells in the area. Irrigation pumping in the confined aquifer accelerated the 

downward movement of contaminated water. Although some anthropogenic 

contaminants in the unconfined-aquifer water reached the public-supply 

well, nitrate concentrations in the produced water were very low. This was 

partly because the high-nitrate water from the unconfined aquifer was 

diluted within the public-supply by the large fraction of low-nitrate water 

from the confined aquifer that entered the well. In addition, some of the 

nitrate in the water that short-circuited the confining unit degraded in the 

anoxic conditions of the confined aquifer before reaching the well.

Relevance to Source Protection 

It is unlikely that all leaking multi-aquifer wells within the area will be 

accurately identified. Therefore, it is useful to plan for a scenario in which 

the confining unit does not restrict the downward flow of contaminated 

water from the unconfined aquifer to the “confined” aquifer used as a source 

for drinking water. However, the large percentage of very old water entering 

confined-aquifer public-supply wells will dilute any anthropogenic contaminants in water entering the wells. Contaminant concentrations in leaked 

water would have to be much higher than a drinking-water standard before a 

water purveyor would need to worry about treating the water.

NEBRASKA

York

0 2 MI

0 2 KM

N

Boundary of

regional-scale

investigation

Boundary of

local-scale

investigation

Area contributing

recharge

Monitoring

wells

Public-supply well

LAND USE

Urban

Agricultural

Forested

Wetland

Water

Other

MORE INFORMATION

Clark and others, 2008

Jagucki and others, 2008 

Landon and others, 2008
88 Factors Affecting Public-Supply-Well Vulnerability to Contamination: Understanding Observed Water Quality and Anticipating Future Water Quality

Relative Vulnerability Among Wells 

in Different Aquifers

Nitrate

The relative vulnerability of water from public-supply wells to nitrate 

contamination was evaluated in eight aquifer systems. The relation between 

the denitrification rate and the groundwater velocity in the denitrifying zone 

in each aquifer system was computed. The computed ratios were compared 

and served as a measure of the relative amount of nitrate that could be 

degraded in each area. The method is founded on the concept that nitrate 

can persist if groundwater-flow rates are high relative to denitrification 

rates. Conversely, nitrate in shallow groundwater might not reach a well if 

groundwater-flow rates are low compared to denitrification rates. 

Marine shale

(Colorado)

Glacial outwash sand

Marine sandy clay (Minnesota)

(Florida study area)

Confined sandstone

(Kalahari Desert) Unconfined sandstone

(Kansas)

Fluvial sand

(California

study area)

Glaciofluvial sand

(Washington)

Clayey till

(Nebraska

study area)

Increasing average

groundwater velocity

in denitrifying zone

Increasing denitrification rate

102

100

10-2

10-4

10-6

Nitrate Attenuation Efficiency (NAE)

Ratio of denitrification rate to 

average groundwater velocity. NAE 

increases from lower right-hand 

corner to upper left-hand corner of 

graph. Degradation of nitrate is less 

efficient in unconfined fluvial sand 

(sands deposited in riverine 

environments) than in marine shale

Differences in the efficiency of nitrate 

attenuation within active denitrification 

zones of different aquifers are related 

to differences in the depositional 

environment of the different aquifer 

materials. The nitrate attenuation 

framework depicted in this figure is a 

useful way to compare groundwater 

vulnerability to nitrate contamination 

among aquifer systems.

MORE INFORMATION

McMahon and others, 2008

The efficiency of nitrate attenuation (degradation) in the study areas increased 

as follows: fluvial sand aquifers < glacial sand aquifers < glacial/marine clay < 

marine shale. Observed differences are related to differences in the proportions 

of clay and organic carbon in the different aquifer materials. A high proportion 

of clay favors the removal of nitrate from groundwater because clay is associated with low groundwater velocities (groundwater-flow rates). A high amount 

of organic carbon also favors nitrate removal because organic carbon increases 

denitrification rates by acting as an electron donor during redox reactions that 

reduce nitrate concentrations (denitrification). 

VOCs

The relative vulnerability of water from public-supply wells to VOC contamination was evaluated in six study areas. The combined effects of redox conditions, 

contaminant-specific degradation rates, dispersion, groundwater-flow rates and 

traveltimes for flow pathways converging on the public-supply wells were 

considered. The ratio of predicted concentration (at individual wells) to initial 
Chapter 5—Vulnerability Assessment Examples 89

concentration (at the water table) (C/C0) was used to represent the amount of 

contaminant attenuation that could occur along a flow pathway between the 

water table and a well. The overall measure of vulnerability for an individual 

well was the weighted average C/C0 value for all flow pathways converging on 

the well. The vulnerability for an aquifer was represented by the median C/C0

value for all wells in the aquifer. Public-supply wells in aquifers with low C/C0 

values for a particular VOC are less vulnerable to contamination from that 

VOC than wells with higher values, given the same contaminant input.

The relative vulnerability of water from the public-supply wells to several 

commonly detected VOCs varied systematically between wells in the western 

and eastern study areas. (See figure below.) Computed differences in C/C0

values for wells in the different aquifers are attributed to regional variations in 

hydrologic processes and redox conditions among the aquifers. For example, 

the western study areas have longer groundwater-flow pathways, greater 

traveltimes, and more widespread oxic conditions compared with the eastern 

study areas. Because toluene degrades moderately fast in oxic conditions, a 

greater amount of toluene could be degraded in the western study areas, where 

traveltimes in oxic zones are longer. In contrast, more degradation of PCE 

could occur in the eastern study areas because degradation of PCE is moderately fast in the anoxic conditions more prevalent in the East.

TCE

Measured contaminants in wells

−10

−2

−4

 0

PCE

Chloroform

MTBE

Benzene

Toluene

TCE

PCE

Chloroform

MTBE

Benzene

Toluene

−10

−2

−4

 0

California—Central Valley aquifer system

Western Regional Study Areas

Nebraska—High Plains aquifer

Nevada—Basin and Range aquifer system

Utah—Basin and Range aquifer system

Connecticut—Glacial aquifer system

Relative

vulnerability

Relative

attenuation

Lower

Higher Lower

Higher

Eastern Regional Study Areas

Ohio—Glacial aquifer system

Median

log C/Co

Public-supply wells in four western study areas were inherently less vulnerable to 

chloroform, MTBE, benzene, and toluene contamination than wells in the eastern study 

areas. This means that, if sources of these compounds were present, the contaminants 

would be more likely to show up in the eastern study area wells.

MORE INFORMATION

Kauffman and Chapelle, 2010
The quality of the groundwater drawn into a public-supply well is important 

even if it is possible to remove contaminants from the water. In this photo, 

packed-tower air-stripping systems are used to remove volatile organic 

compounds from a groundwater source of drinking water, but the treatment

comes with a cost. For example, in 2002 it was estimated that the cost to 

treat groundwater in this way ranged from $4.00 to $34.00 per 10,000 gallons, 

depending on the size of the site and the difficulty of contaminant removal 

(Van Deuren and others, 2002). For public water systems that serve 100,000 

people, this cost could approach $1,500,000 per year if a typical family of four 

uses 400 gallons of water per day. Therefore, it can benefit a community to 

understand which contaminants in an aquifer might reach their well(s) and 

when, how, and at what concentration they might arrive so that the highest 

quality of raw (untreated) water can be obtained from the local resource—

reducing the risk that contaminated water will be delivered to the public while 

maintaining an economical supply of water.
6 Conclusions—

Opportunities and Challenges

Clean water is necessary for healthy communities and strong economies. Although it might be possible to remove some contaminants from groundwater, the quality of the water drawn into a publicsupply well prior to treatment remains important because a high-quality 

source of water can help eliminate or reduce the risk of delivering contaminated drinking water to the public. The quality of the source water also is 

an economic issue because removing contaminants from groundwater is 

expensive and difficult. 

Understanding what factors affect public-supply-well vulnerability to 

contamination can lead to opportunities to enhance source water assessment and protection efforts. Such understanding also can help source water 

protection partners anticipate and measure protection-related outcomes. In 

general, the vulnerability of the water from public-supply wells to contamination is a function of contaminant input within the area that contributes 

water to a well, the mobility and persistence of a contaminant once released 

to the groundwater, and the ease of groundwater and contaminant movement 

from the point of recharge to the open interval of a well.

Recharge affected

by urban area

Zone of

contribution

for well

Recharge affected

by agricultural area

Years

Centuries

Predevelopment water

Agricultural Urban area area 

Public-supply well

Area contributing recharge

Years

Centuries

Area contributing recharge

Zone of

contribution

for well

If contaminant input, contaminant mobility 

and persistence, and intrinsic susceptibility are different within the contributing 

areas for two wells, the wells can produce 

water of a different quality even if they are 

located in the same setting. For example, 

the shallow well in this figure (top) is more 

strongly influenced by urban activities 

than the deeper well (bottom) because all 

of the water from the shallow well entered 

the aquifer beneath the urban area, and all 

of the water travels from the water table 

to the well in a relatively short amount of 

time. Consequently, source water protection activities in the urban area might 

benefit the shallow well more so than the 

deeper well.
92 Factors Affecting Public-Supply-Well Vulnerability to Contamination: Understanding Observed Water Quality and Anticipating Future Water Quality

“The variety of tools in this circular 

will be valuable to source water 

protection practitioners.”

Chi Ho Sham, 

The Cadmus Group, Inc.; and

American Water Works Association,

Water Resources Division Trustee, 2011

There are many ways to evaluate how these factors affect the vulnerability, 

and thus water quality, of a well. Measures described in this circular are 

particularly useful for indicating which contaminants in an aquifer might 

reach an individual public-supply well and when, how, and at what concentration they might arrive: 

• Sources of recharge—Information on the sources of recharge for a well 

provides insight into contaminants that might enter the aquifer with the 

recharge water and potentially reach the well. 

• Geochemical conditions—Information on the geochemical conditions 

encountered by groundwater traveling to a well provides insight into 

contaminants that might persist in the water all the way to the well. 

• Groundwater-age mixtures—Information on the ages of the different 

waters that mix in a well provides insight into the time lag between 

contaminant input at the water table and contaminant arrival at the well. It 

also provides insight into the potential for in-well dilution of contaminated 

water by unaffected groundwater of a different age that simultaneously 

enters the well. 

Preferential flow pathways—pathways that provide little resistance to 

flow—can influence how all other factors affect public-supply-well vulnerability to contamination. For example, preferential flow pathways can influence whether a contaminant source is physically linked to a well, whether 

contaminant concentrations are substantially altered before contaminated 

groundwater reaches a well, and whether contaminated groundwater can 

arrive at a well within a timeframe of concern to the well owner. Knowing 

how to recognize the influence of preferential flow pathways on the quality 

of water from a public-supply well can provide opportunities to prevent or 

mitigate the deterioration of a water supply. (See page 74.)

A table summarizing the methods and tools used to understand the vulnerability of water from public-supply wells to contamination in this study is 

included in the Appendix to this circular. Some of the methods and tools are 

straightforward to apply; others might require the assistance of a technical 

expert. Many of the tools make use of information that can be collected at a 

public-supply well itself, simplifying the data-gathering process. 

Obtaining the data and information described in this circular might be challenging for some communities. However, even an evaluation of existing data 

in light of the factors that affect public-supply-well vulnerability to contamination, as described herein, might result in substantial new insight into the 

quality of water from a well. For example, review of historical well records 

might reveal water-quality patterns that indicate preferential flow pathways 

are influencing the water quality. In some instances, the preferential flow 

pathway might be the wellbore itself, and the pumping schedule might be 

the underlying cause of the undesirable water-quality fluctuation. Adjustments to pumping schedules might bring about improved water quality in a 

relatively short amount of time, similar to what was achieved for a publicsupply well that was investigated in California. (See pages 70–71.)

Existing groundwater-chemistry data might provide information on the 

origins of the water from a well or the redox condition of the groundwater 

entering the well. Information on redox conditions can be used to anticipate 
Chapter 6—Conclusions—Opportunities and Challenges 93

which contaminants, once in the groundwater, are more or less likely to 

reach a public-supply well. This includes naturally occurring drinkingwater contaminants such as arsenic, uranium, and radium—chemicals not 

typically addressed by source water assessment and protection programs but 

that can prove challenging to communities. A redox framework presented 

in this circular provides a simple and inexpensive way to identify the redox 

condition of the groundwater pumped from a well. (See pages 42–43.) 

Information on the groundwater-age mixture for a well provides an opportunity for a community to forecast the change in water quality at its well(s) 

in response to changes occurring near the land surface. Although obtaining 

estimates for the groundwater-age mixture for a well can be relatively difficult and expensive, it has been shown that available information on aquifer 

physical properties and well characteristics can be useful for generating a 

rough estimate of the groundwater-age mixtures for wells in some settings.

In summary, water from a public-supply well is a mixture—frequently an 

unknown mixture—of water from various parts of the surrounding aquifer. 

Consequently, water-quality samples from public-supply wells require more 

elaborate interpretation than do samples from monitoring and domestic 

wells. However, knowing what water-quality variables to measure, what 

spatial and temporal scales on which to measure them, and how to interpret 

the resulting data makes it possible for samples from public-supply wells to 

provide a broad window into a well’s past and present water quality—and 

possibly future water quality. Such insight can enable resource managers 

to prioritize actions for sustaining a high-quality groundwater source of 

drinking water.

laurenserota.com
94 Factors Affecting Public-Supply-Well Vulnerability to Contamination: Understanding Observed Water Quality and Anticipating Future Water Quality
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Glossary   ﻿ 101

Glossary

Advection The process by which dissolved 

substances are transported by the bulk of flowing 

groundwater. 

Alkalinity The acid-neutralizing capacity of water; in 

most natural waters, this consists of the bicarbonate ion. 

Alluvial fan A fan-shaped deposit of generally coarsegrained material eroded from mountain slopes, transported by streamflow, and deposited on a valley floor. 

Alluvium Bodies of clay, silt, sand, gravel, or other 

particulate material that has been deposited by a 

stream or other form of running water.

Annular space The space between two cylindrical 

objects, such as a well casing and a wellbore. 

Anoxic Having dissolved oxygen concentration less 

than 0.5 milligram per liter, and having concentration 

of nitrate, manganese, iron, and sulfate as noted in the 

redox framework on page 42.

Anthropogenic Related to the presence or activities 

of humans. 

Apparent (tracer-based) groundwater age An 

estimate for groundwater age based on the assumption that the concentration of a chemical tracer used 

to estimate the age has been unaffected by mixing 

either in the aquifer or at the point of discharge (well 

or spring).

Aquifer A water-bearing layer of soil, sand, 

gravel, or rock that will supply a usable quantity of 

water to a well. 

Aquifer material Sediments or rocks that constitute 

the solid material through which groundwater flows.

Aquifer matrix (see Matrix)

Aquifer system A body of permeable and relatively 

impermeable sediment and (or) rock that functions 

regionally as a water-yielding unit. 

Area contributing recharge (ACR) The surface 

area that defines the location where water entering an 

aquifer system at the water table flows to a well. 

Artificial recharge The addition of surface water to 

a groundwater reservoir by human activity. 

Attenuation The process of decreasing contaminant 

concentrations in groundwater resulting from physical 

or geochemical processes such as filtration, biodegradation, dilution, sorption, or volatilization. 

Bentonite A clay material that expands when wet, 

commonly used to fill gaps and seal the annular space 

inside a wellbore.

Bicarbonate A compound containing the HCO3

−

group, such as sodium bicarbonate (NaHCO3), which 

ionizes in water to produce HCO3

−

. 

Biodegradation The metabolic breakdown of materials into simpler components by living organisms. 

Biofouling The gradual accumulation of waterborne 

organisms (such as bacteria and protozoa) on the 

surfaces of well components (such as casing, screens, 

pumps) that contributes to corrosion and leads to a 

decrease in efficiency. 

Borehole A hole drilled or bored into the earth. 

Carbonate rocks Rocks (such as limestone or dolostone) composed primarily of minerals (such as calcite 

and dolomite) containing the carbonate ion (CO3

2−). 

Chlorofluorocarbons (CFCs) Stable, manmade 

volatile organic compounds first introduced into the 

atmosphere in the 1930s. In groundwater, the concentrations of CFCs can be used to interpret the age of 

relatively young water. 

Community water system A public water system 

that supplies water to the same population year round. 

Includes those systems that rely on public-supply wells 

for their source of water.

Complex A dissolved species formed by the association of two or more simpler dissolved species. 

Cone of depression A depression in the water table 

(or potentiometric surface) that develops around a 

pumping well.

Confined aquifer An aquifer that is completely filled 

with water under pressure and that is overlain by lowpermeability sediment or rock (confining unit). 

Confining unit A low-permeability layer of sediment 

or rock that overlies a distinctly more permeable aquifer. 
102 Factors Affecting the Vulnerability of Public-Supply Wells to Contamination: Understanding Observed Water Quality and Anticipating Future Water Quality 

Contamination Impairment of water quality by 

any chemical, physical, biological, or radiological 

substance. The degree of permissible contamination 

depends on the intended use of the water. 

Conservative Refers to a constituent that moves with 

the groundwater and does not undergo chemical reactions or sorption. 

Degradation The breakdown of materials into 

simpler components.

Denitrification A process by which oxidized forms 

of nitrogen such as nitrate (NO3

−) are reduced to harmless nitrogen gas. The process is usually brought about 

by denitrifying bacteria. 

Desorption Process of changing from an adsorbed 

state on a surface to a gaseous or liquid state.

Deuterium A heavy stable isotope of the hydrogen 

ion (2H), which is part of the water molecule.

Discharge The volume of water that passes a given 

point within a given period of time (for example, the 

outflow of groundwater from an aquifer to a well). 

Dispersion The spreading of chemical constituents 

in groundwater. 

Dissolution The process whereby solids dissolve 

into water. Most dissolved constituents in groundwater are ultimately derived from dissolution of solid 

aquifer materials. 

Domestic well A privately owned well that typically serves one home and supplies water for human 

consumption and other homeowner uses. 

Downgradient The direction that groundwater 

flows; equivalent to “downstream” for surface-water 

flows. 

Drinking-water contaminant A physical, chemical, 

biological, or radiological substance or matter that can 

affect the quality of water for the purpose of drinking.

Dry well A large infiltration trench or pit used to 

capture relatively clean runoff. 

Electron acceptor A species that accepts one or 

more electrons (thereby becoming more negatively 

charged) during a redox reaction. In groundwater, 

common electron acceptors are O2, NO3

−

, MnO2(s), 

Fe(OH)3(s), SO4

2−, and CO2(g). 

End-member A term used herein to describe water 

with a distinct chemical composition representing one 

component in a mixture of different waters.

Fast flow pathway Preferential flow pathway 

resulting from natural spatial variations in watertransmitting properties of aquifer materials. 

Finished water Water that has passed through all 

treatment processes but prior to distribution.

Flow pathway Subsurface course followed by 

groundwater. 

Geochemical conditions In reference to groundwater, 

refers to factors or circumstances that affect the chemical 

reactivity of substances dissolved in the water.

Global Meteoric Water Line An equation that 

represents the relative abundance of stable isotopes of 

hydrogen and oxygen in precipitation from multiple 

locations around the world. 

Groundwater In general, any water that exists 

beneath the land surface, but more commonly 

applied to water in fully saturated soils and geologic 

formations.

Groundwater age The duration of time since water 

was isolated from the atmosphere by recharge to the 

saturated zone. 

Groundwater-age mixture The mixture of groundwater ages that exist in a water sample, such as from a 

public-supply well. 

Groundwater-flow model A digital computer model 

that represents a real groundwater flow system by 

means of mathematical equations. 

Groundwater-flow system A system of flow pathways that describe the movement of water in an aquifer 

or aquifer system.

Groundwater recharge Water that infiltrates the 

ground and reaches the saturated zone. 

Groundwater velocity The rate of movement of 

groundwater through openings in rock or sediment, 

measured as the ratio of distance per traveltime. 

Groundwater vulnerability The likelihood that 

a contaminant will reach a specified position (for 

example, the water table or the depths used for publicwater supply) in a groundwater-flow system.
Glossary   ﻿﻿ 103

Health-based screening levels (HBSLs) Nonenforceable “human-health benchmark” concentrations 

of unregulated contaminants in water developed by the 

USGS in collaboration with the U.S. Environmental 

Protection Agency and others.

Hydraulic conductivity A variable describing the 

rate at which water can move through an aquifer or 

other permeable medium. 

Hydraulic gradient The slope of a water table or 

potentiometric surface. Specifically, the change in 

static hydraulic head per unit distance in a given direction, usually the direction of greatest head decrease. 

Hydraulic head An indicator of the total energy 

available to move groundwater 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 sea level. 

Hydraulic short-circuit Natural or manmade preferential flow pathways that allow water and contaminants to bypass aquifer material. 

Infiltration Flow of water from the land surface into 

the subsurface.

Interquartile range A measure of spread in a collection of data that is equal to the 75th percentile minus 

the 25th percentile. 

Intrinsic susceptibility A measure of the ease with 

which a contaminant in water can enter and move 

through an aquifer. It is a characteristic of the aquifer 

and overlying material, and it is independent of the 

contaminant characteristics or source. 

Ion exchange The substitution of one ion for another 

on the surface of a solid. 

Irrigation return flow The part of irrigation water 

applied to the land surface that is not consumed by 

plants or evapotranspiration and that either infiltrates 

downward to an aquifer or discharges to a surfacewater body. 

Isotopes Atoms of the same element that differ in 

mass because of a difference in the number of neutrons 

in the nucleus.

Karst/karstic Characterized by features that result 

from dissolution and collapse of carbonate rocks, such 

as closed depressions, sinkholes, caves, and underground conduits. 

Lumped-parameter mixing model A mixing equation that can be used to describe the mixture of water 

ages at a point of discharge (well or spring) from an 

aquifer system. 

Major ions Constituents commonly present in water 

at concentrations exceeding 1.0 mg/L. As used in 

this circular, major ions include calcium, magnesium, 

sodium, potassium, sulfate, chloride, fluoride, and 

bicarbonate. 

Matrix The solid framework of a porous system.

Maximum Contaminant Level (MCL) The 

maximum permissible level of a contaminant in water 

that is delivered to any user of a public water system. 

MCLs are enforceable standards established by the 

U.S. Environmental Protection Agency.

Mobilize To release a chemical species from the solid 

aquifer material into the groundwater by a process such 

as dissolution, desorption, or ion exchange. 

Monitoring well A well used to obtain water-quality 

samples or measure water levels. 

Mountain-front recharge Water that enters an 

alluvial aquifer near the boundary of the aquifer and 

an adjacent mountainous area as a result of either 

infiltrating streamflow having headwaters in the mountainous area or flowing groundwater having origins in 

an adjacent aquifer within the mountainous area.

Multi-aquifer well A well that is screened in more 

than one aquifer and that penetrates through one or 

more confining units.

Nonpoint-source contaminant A substance from 

diffuse sources (for example, lawn and cropland 

runoff, the atmosphere, and roadways) that pollutes or 

degrades water. 

Nonreactive (see Conservative)

Nutrient Element or compound essential for animal 

and plant growth. Nitrogen and phosphorus are 

common nutrients in fertilizer, but in high concentrations they can degrade water quality.

Open interval (of a well) The length of the 

screened or unscreened interval through which water 

enters a well. 
104 Factors Affecting the Vulnerability of Public-Supply Wells to Contamination: Understanding Observed Water Quality and Anticipating Future Water Quality 

Oxic Having dissolved oxygen concentration greater 

than or equal to 0.5 milligram per liter, and having 

concentrations of nitrate, manganese, iron, and sulfate 

as noted in the redox framework on page 42. 

Oxygen-18 A heavy stable isotope of the oxygen ion 

(

18O), which is part of the water molecule. 

Particle-tracking model A computer program used 

to track the movement of groundwater particles in a 

simulated flow field generated by using a groundwaterflow model.

Pathogen A disease-producing agent; usually 

applied to a living organism. Generally, any virus, 

bacteria, or fungus that causes disease.

Permeability The capacity of sediment or rock to 

transmit water. 

pH A measure of the relative acidity or alkalinity of 

water. Water with a pH of 7 is neutral; lower pH levels 

indicate increasing acidity, whereas pH levels higher 

than 7 indicate increasingly basic solutions.

Point-source contaminant Any substance that 

degrades water quality and originates from discrete 

locations, such as chemical spills or leaky underground 

storage tanks. 

Porosity A measure of the water-bearing capacity of 

sediment or rock based on the ratio of the volume of 

pore space to the bulk volume of the sediment or rock. 

Potentiometric surface A hypothetical surface 

representing the level to which groundwater would rise 

if not trapped in a confined aquifer. The potentiometric 

surface is equivalent to the water table in an unconfined aquifer.

Precipitation (chemical) Process whereby solids 

form from dissolved species. 

Preferential flow pathway A pathway of least 

resistance followed by groundwater, such as a bedrock 

fracture or a nonpumping well. Preferential flow pathways are associated with relatively short groundwater 

traveltimes. 

Probabilistic A situation or model where there are 

multiple possible outcomes, each having a different 

degree of certainty or uncertainty of occurrence.

Public-supply well A privately or publicly owned 

well that provides water for public use to (1) community water systems, (2) transient non-community water 

systems, such as campgrounds, or (3) non-transient, 

non-community systems, such as schools. 

Public-supply well vulnerability The combined 

effect of groundwater vulnerability and the interaction 

between a well and an aquifer on the quality of water 

from a well. 

Recharge Water that infiltrates the ground and 

reaches the saturated zone. 

Recharge area (see Area contributing recharge)

Redox condition As used in this circular, redox 

condition refers to the geochemical status or position 

of a groundwater system on a scale of very oxidizing 

to very reducing. The redox condition of groundwater 

affects the mobility, degradation, and solubility of 

many contaminants. 

Redox reaction An oxidation-reduction reaction, 

which involves the exchange of electrons between an 

electron donor and an electron acceptor. 

Reduced/Reducing Used in this circular to refer to 

anoxic groundwater.

Saturated zone A zone extending from the water 

table downward, in which all pore spaces are filled 

with water under a pressure greater than atmospheric. 

Screened interval (of a well) The depth interval over 

which water can enter a well through the well screen.

Septic system An onsite system designed to treat 

and dispose of domestic waste. A typical septic system 

consists of a settling tank for solids and a system of 

drainage lines to dispose of fluid. 

Solute A substance that is dissolved in another 

substance, thus forming a solution.

Sorption Removal of a solute from the liquid phase 

by concentrating it on the solid phase of a medium; 

encompasses adsorption and absorption. 

Source water Water used as a source for public 

water systems before undergoing treatment at a 

water facility.
Glossary   ﻿﻿ 105

Source water assessment The assessment phase 

of a program designed to prevent contamination of 

drinking water. Assessment steps include (1) delineating areas around surface-water intakes or publicsupply wells to protect from contamination, (2) inventorying known and potential sources of contamination 

in these areas, and (3) determining the susceptibility 

of the water-supply system to these contaminants.

Source water protection area Area around a 

surface-water intake or public-supply well to protect 

from contamination.

Stable isotopes Nonradioactive forms of an element 

that have different molecular weights. The relative 

abundance of isotopes of elements such as hydrogen, 

oxygen, nitrogen, carbon, and sulfur provides information related to sources of water and reactions affecting 

the chemical composition of groundwater. 

Stormwater runoff Water and associated material 

draining into streams, lakes, or sewers as the result of 

a storm. 

Study well A typical public-supply well in each of six 

study areas, selected to help explore how contaminants 

arrived at and entered the well. Each study well produced 

water with low concentrations of anthropogenic and 

naturally occurring drinking-water contaminants and 

had recharge areas that included several types of land 

use. Also referred to as “investigated well.”

Sulfur hexafluoride (SF6) A volatile organic 

compound derived from natural and human sources, 

whose concentration in the atmosphere increased 

significantly in the 1950s because of increased industrial use. 

Susceptibility (see Intrinsic susceptibility) 

Till An unsorted mixture of boulders, gravel, sand, 

silt, and clay deposited by a glacier. 

Time of travel (see Traveltime)

Tracer A chemical constituent in groundwater that 

can be used to infer flow pathways and groundwater age. 

“Environmental” tracers are added by natural processes, 

although they may have been produced by humans.

Traveltime The time required for groundwater to 

travel between two locations. Traveltime is equal to 

groundwater age when the starting location is at the 

point of recharge. 

Tritium A radioactive isotope of hydrogen (3H) that is 

part of the water molecule. The concentration of tritium 

in the atmosphere greatly increased in the 1950s and 

1960s in response to human activities. In groundwater, 

tritium concentrations can be used to interpret the presence or absence of relatively young water.

Tritium/helium-3 The ratio of tritium and helium-3, 

which forms from the radioactive decay of tritium. The 

tritium/helium-3 ratio can be used to interpret the age 

of relatively young groundwater. (Helium-3 also can be 

derived from aquifer materials, and that fraction can 

be estimated by using concentrations of other gases.)

Turbidity Reduced transparency of water resulting 

from suspended solids such as clay, silt, organic 

matter, or microscopic organisms. 

Unconfined aquifer An aquifer with the water table 

as its upper surface. 

Unconsolidated Loose and not cemented together. 

Sand and silt are examples of unconsolidated sediments.

Unsaturated zone A zone that contains both water 

and air and is generally underlain by a saturated zone 

in which all interconnected openings or pore spaces 

are full of water. 

Upgradient In the opposite direction that groundwater 

flows; equivalent to “upstream” for surface water. 

Volatile organic compounds (VOCs) Chemicals 

that are groundwater contaminants of concern because 

of very large environmental releases, human toxicity, 

and a tendency for some compounds to persist in and 

migrate with groundwater to drinking-water supply 

wells. In general, VOCs have high vapor pressures, 

low to medium water solubilities, and low molecular 

weights. Some VOCs may occur naturally in the 

environment, other compounds occur only as a result 

of human activities, and some compounds have both 

origins. 

Vulnerability assessment A process by which 

information relevant to vulnerability is assembled and 

then evaluated. A groundwater or public-supply well 

vulnerability assessment serves to direct groundwater 

or source water protection efforts.

Water quality A term used to describe the chemical, 

physical, and biological characteristics of water, 

usually with respect to the water’s suitability for a 

particular purpose.
106 Factors Affecting the Vulnerability of Public-Supply Wells to Contamination: Understanding Observed Water Quality and Anticipating Future Water Quality 

Water table The surface that separates the zone 

that is generally unsaturated from the zone that is 

generally saturated. 

Well A hole in the ground (wellbore or borehole) 

from which water can be removed.

Wellbore A borehole that has been completed as a well.

Wellhead The top of a well.

Wellhead protection Protection of all or part of the 

area from which water entering an aquifer system at 

the water table flows to a well.

Wellhead protection area The area around a wellhead to protect from contamination. Synonymous with 

source water protection area for public-supply wells.

Well screen A filtering device used to prevent sediment from entering a well. 

Zone of contribution (ZOC) A three-dimensional 

volume of the aquifer through which water flows to a 

well from its area contributing recharge. 

Zone of influence (ZOI) The area surrounding a 

pumping well within which the water table or potentiometric surface has been changed by groundwater 

withdrawal.

Zone of transport (ZOT) The area surrounding a 

pumping well through which water or a contaminant 

can travel and reach the well within a specified amount 

of time. 
Appendix

TOOLBOX

Appendix Toolbox

 107

Selected Tools Overview..........................................................................................................108

Contaminant Input (Sources of Recharge)............................................................................110

Contaminant Mobility and Persistence (Geochemical Conditions)...................................112

Intrinsic Susceptibility (Groundwater-Age Mixtures)..........................................................113

Water-Quality Forecasting .......................................................................................................117

Direct Observation of Vulnerability.........................................................................................118

Preferential Flow Pathways.....................................................................................................119
Selected Tools Overview

Many methods and tools were used in this study to help assess the vulnerability of groundwater from public-supply wells to 

contamination. This appendix summarizes information on selected tools, such as (1) why a tool was used in this study; (2) where to 

find more information on the tool, including where to find downloadable USGS computer programs; and, (3) what can be learned 

by using the tool as illustrated with data and information from this study. Some of the tools are straightforward to use; others might 

require the assistance of a technical expert.

108 Factors Affecting Public-Supply-Well Vulnerability to Contamination: Understanding Observed Water Quality and Anticipating Future Water Quality

Tool Why it was used For more information

Contaminant Input

(Sources of Recharge)

Groundwater-flow and particletracking models coupled 

with land-use data

To estimate the percentage of simulated inflow to publicsupply wells from areas of different land use. In this 

study, the USGS MODFLOW and MODPATH models 

were used for computing the areas at land surface that 

contribute recharge to public-supply wells, and land use 

was based on the enhanced National Land Cover Data 

(NLCDE). (See box on page 32.)

http://water.usgs.gov/nrp/gwsoftware/

modflow.html;

http://pubs.usgs.gov/ds/2006/240/#proc;

http://www.mrlc.gov/index.php

Computer scripts for running 

Monte Carlo simulations 

To estimate the uncertainty in simulated recharge areas for 

public-supply wells. (See page 110.)

Starn and others, 2010;

Starn and Bagtzoglou, 2012;

http://water.usgs.gov/software/

MonteCarloContributingArea/

Mixing equation To estimate the relative proportions of end-member waters 

in a mixture. (See page 111 for a sample calculation for a 

two-component mixture.)

Parkhurst and Appelo, 1999; 

http://wwwbrr.cr.usgs.gov/projects/

GWC_coupled/phreeqc/

Depth-dependent sampling To collect borehole-flow and water-quality data from 

discrete intervals within the producing zone of a publicsupply well in order to determine where and how water 

and contaminants actually enter the well. 

(See case study on page 34.)

Izbicki, 2004;

Izbicki and others, 1999;

Izbicki and others, 2008

Contaminant Mobility and Persistence

(Geochemical Conditions)

Framework for identifying 

redox processes 

To determine redox conditions associated with water from 

wells on the basis of threshold concentrations of six 

water-quality constituents—dissolved oxygen, nitrate, 

manganese, iron, sulfate, and hydrogen sulfide. 

(See box on page 39 and also pages 42–43.)

Chapelle and others, 2009;

McMahon and Chapelle, 2007; 

http://oh.water.usgs.gov/tanc/NAWQATANCRedox.htm

Microsoft Excel® workbook that can be used to apply the 

redox framework to large water-quality datasets.

Jurgens, 2009;

http://pubs.usgs.gov/of/2009/1004/

Geochemical models To simulate chemical reactions and transport processes 

in natural and contaminated groundwater in order to 

evaluate the mobility of chemical constituents in an 

aquifer, especially trace elements. (See page 112.)

Parkhurst and Appelo, 1999; http://

wwwbrr.cr.usgs.gov/projects/GWC_

coupled/phreeqc/
Tool Why it was used For more information

Intrinsic Susceptibility

(Groundwater-Age Mixtures)

Groundwater-flow and particletracking models calibrated to 

tracer concentrations 

To estimate the groundwater-age mixture in water from 

public-supply wells on the basis of monitoring- and 

public-supply-well data that include measured concentrations of chemical indicators (tracers) of groundwater age. 

(See box on page 50 and also pages 113–114.) 

http://water.usgs.gov/nrp/gwsoftware/

modflow.html

Lumped-parameter models 

(TracerLPM computer 

program)

To estimate the groundwater-age mixture in water from an 

individual well on the basis of measured concentrations 

of groundwater-age tracers in the water. 

(See pages 113, 115.) 

Jurgens and others, 2012;

http://ca.water.usgs.gov/user_projects/

TracerLPM/

Groundwater-stratigraphy 

models 

To generate rough estimates for the groundwater-age mixture in water from a well on the basis of aquifer thickness, 

porosity, recharge rate, and depth to the top and bottom of 

the well screen. (See pages 113, 116.) 

Cook and Böhlke, 2000;

Mendizabal and Stuyfzand, 2009

Water-Quality Forecasting

TracerLPM computer program To generate water-quality response curves from information on the groundwater-age mixture for a well and local 

contaminant degradation rates. (See page 117.)

Jurgens and others, 2012;

http://ca.water.usgs.gov/user_projects/

TracerLPM/

Direct Observation of Vulnerability

Low-level VOC analysis To provide an early warning of public-supply-well vulnerability to contamination from a local source of contaminants on the basis of very low-level detection (parts per 

quadrillion) of volatile organic compounds (VOCs) in the 

water. (See page 118.)

Plummer and others, 2008

Preferential Flow Pathways 

Confined-aquifer zone-oftransport calculation To evaluate the potential for nearby multi-aquifer wells to 

influence water quality in a public-supply well in a confined aquifer. (See pages 119–120.) 

Johnson and others, 2011

Multiple different ways to 

collect, analyze and interpret 

water samples to uncover the 

presence of preferential flow 

pathways 

To recognize the influence of preferential flow pathways on 

the quality of water from a public-supply well. (See page 

74 for a list of methods and related examples.)

Multi-node well (MNW) package for the USGS MODFLOW modelTo simulate the effects of hydraulic short-circuiting by 

production wells that are open to multiple aquifers. (See 

page 76 for model results obtained by using the MNW 

package.)

Konikow and others, 2009;

http://water.usgs.gov/nrp/gwsoftware/

modflow2000/MFDOC/index.

html?mnw2.htm

Appendix TOOLBOX 109
Contaminant Input (Sources of Recharge)

Computer Programs for Estimating Recharge-Area Uncertainty

Areas contributing recharge to wells often are delineated by using steady-state groundwater-flow and particletracking models. Simulated recharge areas, however, are only as good as the underlying groundwater-flow models 

from which they are derived. Monte Carlo simulation can be used to evaluate the uncertainty in simulated recharge 

areas. The Monte Carlo approach involves running a model many times with different combinations of model 

parameter values and then combining the results to express the model output 

in terms of probability. The probability that a location at the land surface 

contributes recharge to a well—given the simplifications in the model used 

to represent the natural system—is computed by dividing the number of 

model runs in which water from that location reached the well by the total 

number of model runs. The computer scripts GEN_LHS and MCDRIVER_

LHS were developed for this study to run Monte Carlo simulations for 

generating probabilistic recharge areas to evaluate recharge-area uncertainty. 

110 Factors Affecting Public-Supply-Well Vulnerability to Contamination: Understanding Observed Water Quality and Anticipating Future Water Quality

With prior information and age-tracer data

With prior information; no age-tracer data

No prior information; no age-tracer data

Model boundary

Probability that area

contributes recharge 

to the well, in percent

1–20

20–40

40–60

60–80

80–100

Public-supply well

0 1 MI

0 1 KM 

N

Results from the Connecticut study area help 

illustrate recharge-area uncertainty and the 

importance of calibrating groundwater-flow 

models to different amounts and types of 

data. The local-scale groundwater-flow 

model was originally calibrated to measured 

groundwater levels and groundwater 

discharge to streams. The particle-tracking 

model MODPATH (Pollock, 1994) was used to 

map the recharge area from the flow-model 

output. A Monte Carlo simulation was used 

to evaluate the uncertainty associated with 

the simulated recharge area. (See top figure 

at right.) The process was repeated for 

the Connecticut well after independent (or 

“prior”) information on several model parameters (for example, porosity) was added to 

the calibration dataset (see middle figure at 

right) to help steer the calibration toward 

realistic values. The process was repeated 

again after observed concentrations of 

tritium were added. This last step ensured 

that the model solution was constrained with 

observations relevant to groundwater traveltime. (See bottom figure at right.) Simulated 

recharge areas were more precise (smaller) 

and less uncertain when the additional types 

of measurements were used to help calibrate 

the model.

MORE INFORMATION

Computer programs for generating 

probabilistic recharge areas

Starn and Bagtzoglou, 2012; 

http://water.usgs.gov/software/

MonteCarloContributingArea/

Reducing recharge area uncertainty

Starn and others, 2010
Two-Component Mixing Equation for Investigating Sources of Recharge for a Well

Relatively simple mixing equations can be used to estimate the proportions of two or more waters that combine 

in a well, provided that different parts of the aquifer system tapped by the well contain groundwater with distinct 

concentrations or ratios of conservative constituents. A two-component mixing equation (binary mixing model) 

and example calculation for the Nebraska study area are given below. (For more complex mixtures, a computer 

program such as the USGS program PHREEQC (Parkhurst and Appelo, 1999) might be needed to resolve the 

mixture.)

 X1 = (Cmix– C2 / C1– C2 ) * 100 

 where 

 X1 is the proportion of end-member 1 in the mixture (in percent) 

 C1 is the concentration in end-member 1 

 C2 is the concentration in end-member 2 

 Cmix is the concentration in the water mixture 

Mixing fractions for the Nebraska study well were calculated by using stable isotope data collected throughout the study 

area. End-member values for deuterium (2H) were set equal to the average value for monitoring-well samples from each 

aquifer (−56.89 per mil for water from the shallow, unconfined aquifer and −77.40 per mil for unmixed water from the 

confined aquifer). The 2

H value for the water mixture—the water from the public-supply well—was set equal to the average 

2

H value for water samples from the public-supply well (−74.97 per mil). On the basis of the two-component mixing equation, the percentage of water from the public-supply well that came from the unconfined aquifer was 12 percent. 

Although stable isotope data were used to “unmix” the water from the Nebraska study well, this type of data is not 

always available. However, for some aquifer systems, chloride (Cl) and bromide (Br) data may offer an alternative 

way to investigate sources and mixing of waters in public-supply wells. For the Nebraska study area, a graph of Cl/

Br versus Cl (see figure below) shows patterns similar to those in a graph of 2

H versus 18O (see case study on pages 

34–35): (1) samples plot in distinct groups on the basis of aquifer type and land use, (2) samples from the publicsupply well plot along a mixing line between the confined aquifer and the unconfined aquifer (in urban areas), and (3) 

the highest fraction of unconfined-aquifer water is in the bottom half of the producing interval. 

1 10 100

100

1,000

Chloride, in milligrams per liter 

Chlorideto-bromide

ratio CONFINED AQUIFER 

UNCONFINED AQUIFER (Urban)

UNCONFINED

AQUIFER

(Agricultural)

150 170200160140

 MONITORING WELLS

 Confined aquifer

 Unconfined aquifer 

 Agricultural land use

 Urban land use

 PUBLIC-SUPPLY WELL 

 Wellhead sample

 Samples from selected

 depths within screened 

 inverval (number following 

 symbol refers to depth in 

 feet below land surface)

Concentrations of inexpensive 

and easy to measure conservative constituents, such as 

chloride and bromide, can 

be a good starting point for 

deciphering the sources of 

recharge water for a well, as 

shown for the Nebraska study 

area.

Once the mixture of waters captured by a public-supply well has been estimated, the maximum concentration of a 

chemical constituent in the water from the well can be computed. For example, the maximum chloride concentrations for the unconfined and confined aquifers in the Nebraska study area were 187 mg/L and 32 mg/L, respectively. Using these data, the two-component mixing equation indicates that the maximum future chloride concentration in water from the public-supply well will be about 50 mg/L, assuming that conditions remain unchanged.

Appendix TOOLBOX 111
Contaminant Mobility and Persistence (Geochemical Conditions)

Geochemical Modeling for Evaluating the Mobility of 

Chemical Constituents in an Aquifer

It is sometimes necessary to use a complex set of geochemical equations that account for processes such as aqueous 

speciation, mineral precipitation/dissolution reactions, redox reactions, adsorption behavior, and mixing of different 

waters to explain the mobility and persistence of a chemical constituent in groundwater. Geochemical modeling 

programs, such as PHREEQC (Parkhurst and Appelo, 1999), can be an important tool for understanding the fate 

of chemical constituents in such instances. Trace elements are examples of constituents that frequently require 

geochemical modeling to evaluate their individual mobility and persistence in an aquifer.

In the Connecticut study area, arsenic concentrations were elevated in water from wells completed in glacial 

deposits that were derived from the organic-rich Cass Formation. PHREEQC was used to speciate and model the 

behavior of dissolved arsenic in equilibrium with aquifer sediments. Modeling results indicate that coatings of iron 

oxyhydroxides on aquifer sediments in the study area can be dissolved, releasing arsenic into solution where the 

organic carbon content is sufficient to create reducing conditions in the aquifer.

In the California study area, uranium concentrations exceeded the Maximum Contaminant Level (MCL) of 30 μg/L in 

several public-supply wells and were frequently above one-half the MCL. The uranium content in the soils and aquifer 

sediments in the study area is naturally high. Uranium concentrations in groundwater were strongly correlated to the 

concentration of dissolved inorganic carbon (DIC) (mainly bicarbonate); concentrations of DIC have increased over 

time as a result of agricultural irrigation and crop production. PHREEQC was used to model the effect of different 

concentrations of DIC on the adsorption of uranium to coatings of iron oxyhydroxides on aquifer sediments. The 

surface complexation model predicted uranium concentrations similar to those that were measured in the groundwater. 

Modeling results indicate that the dissolved uranium was controlled by adsorption processes and DIC concentrations 

within the aquifer, rather than by uranium contained in fertilizers and soil amendments.

In the New Mexico study area, the groundwater is predominately old, having carbon-14 ages ranging from about 

4,000 to more than 22,000 years. PHREEQC was used to model the long-term evolution of major ion chemistry 

in the groundwater and its effect on measured carbon-14 concentrations. This was done so that carbon-14 concentrations could be corrected for estimating groundwater ages. Modeling results indicate that the primary reaction 

affecting measured carbon-14 concentrations was the dissolution of organic carbon. In addition, PHREEQC was 

used to help explain the presence of tritium and VOCs—chemical constituents in groundwater recharged after the 

1940s—in water from some monitoring wells that also had low carbon-14 concentrations, indicating the presence of 

very old groundwater. Modeling results indicate that the water from the monitoring wells was a mixture of young 

and old groundwater, with the young fraction ranging from 1.5 to 46 percent.

MORE INFORMATION

Computer program (PHREEQC) for 

geochemical modeling 

Parkhurst and Appelo, 1999

Geochemical modeling for understanding 

dissolved constituents in groundwater

Bexfield and others, 2011

Brown and others, 2009

Burow and others, 2008

Clark and others , 2008

Jurgens and others, 2008

Jurgens and others, 2009a

112 Factors Affecting Public-Supply-Well Vulnerability to Contamination: Understanding Observed Water Quality and Anticipating Future Water Quality
Appendix TOOLBOX 113

Intrinsic Susceptibility (Groundwater-Age Mixtures)

The following pages present three methods that can be used to estimate groundwater-age mixtures in wells. The 

methods are listed in order of decreasing sophistication and expense. An example comparing age estimates obtained 

by using the different methods is given for the California study well.

Three modeling approaches were used to estimate the 

groundwater-age mixture for the California study well. Although 

differences exist among the estimated age mixtures (see 

figure at right), all three approaches describe an age mixture 

in which 30 to 35 percent of the water from the well recharged 

the aquifer before the 1950s (more than 60 years ago). Thus, the 

different models identify a similar potential for in-well dilution 

of anthropogenic contaminants by old (presumably unaffected) 

groundwater entering the well.

Groundwater-flow and particle-tracking models were calibrated 

to data from the study well and from wells completed in the 

surrounding aquifer. Lumped-parameter models were calibrated 

to data from the study well alone. A groundwater-stratigraphy 

model was not calibrated but was based on published values 

for the aquifer and well geometries (effective porosity (n=0.3); 

aquifer thickness (saturated thickness penetrated by the well, 

D=361 ft); groundwater recharge (N=1.97 ft/yr); upper level of 

the well screen below the water table (dU=58 ft); and lower level 

of the well screen below the water table (dL=333 ft)) (Burow and 

others, 2008).

The different modeling approaches might not produce comparable 

results in other settings. However, in settings where it can be 

demonstrated that the approaches produce sufficiently similar 

age estimates, the simpler models could be used to estimate age 

mixtures for wells where more detailed data and sophisticated 

models are lacking.

How groundwater-age mixtures from three modeling approaches compared

0

0 50 100

Time, in years

150 200

40

60

80

20

100

Lumped-parameter

models

Groundwater

stratigraphy model

Groundwater-flow and

particle-tracking models

Cumulative

fraction of

flow to well,

in percent
114 Factors Affecting Public-Supply-Well Vulnerability to Contamination: Understanding Observed Water Quality and Anticipating Future Water Quality

Approach for Calibrating Groundwater-Flow and Particle-Tracking Models 

for Estimating Age Mixtures in Water from Wells

Probably the most familiar method for estimating the groundwater-age mixture in water from a well involves 

using three-dimensional groundwater-flow and particle-tracking models. Groundwater-flow models are computer 

programs that attempt to reproduce a groundwater-flow system with a complex set of mathematical equations. A 

particle-tracking model calculates the pathways that parcels of water would follow through the simulated flow 

system. Distance, traveltime, and velocity for the various pathways also can be obtained by using particle tracking. 

Together, the traveltimes for all pathways to a well describe the age mixture of water from the well (when 

weighted by how much flow each pathway contributes to the well). Computer programs that were used to estimate 

groundwater-age mixtures in this study include MODFLOW-2000 (Harbaugh and others, 2000) and MODPATH 

(Pollock, 1994). 

Because groundwater-flow models greatly simplify flow in an aquifer system, they must be calibrated before they 

can be applied to real-world problems—such as estimating the groundwater-age mixture in a well. Model calibration involves comparing model output with field measurements to demonstrate that the model is capable of reproducing field conditions. During model calibration, model parameter values (such as hydraulic conductivity, recharge 

rate, or porosity) are adjusted until differences between simulated and measured values (such as water levels, flow 

rates, or tracer concentrations) are minimized. At least some of the measurements used for model calibration should 

be related to the types of predictions that will be made in order to reduce uncertainty in the predictions (Reilly and 

others, 1994; Starn and others, 2009). Therefore, models that will be used to estimate groundwater age and age 

mixtures should be calibrated to some measure of groundwater age. Because groundwater ages estimated using 

different chemical tracers in a mixed-age water sample can be different and might not equal the mean age of the 

water (Plummer and others, 2006), it is better to calibrate groundwater-flow models to measured tracer concentrations (for example, sulfur hexafluoride (SF6), chlorofluorocarbons (CFCs), and tritium (3

H)) than to interpreted ages 

derived from tracer data.

Calibrating a groundwater-flow model to tracer concentrations necessitates the computation of simulated concentrations. Simulated concentrations for wells can be computed by recording the traveltime and amount of flow associated with each particle that tracks to a well in a groundwater-flow model and then summing the amount of flow to 

the well in 1-year increments. The volume of flow for each increment is then divided by the total flow from the well 

to determine the fraction of flow corresponding to each year of recharge. Simulated tracer concentrations are then 

obtained by multiplying the simulated fraction of flow to the well for a given year by the concentration of the tracer 

in recharge water from that year and then summing over all years. Tracer input concentrations are determined such 

that they are relevant to the date a well was sampled so that simulated and measured concentrations can be directly 

compared. For example, if a well was sampled in 2000 and a 1-year time increment is used to compute the simulated 

tracer concentrations, all particles with a simulated traveltime of 10 years are assigned a tracer concentration equal 

to the concentration in 1990 recharge.

MORE INFORMATION

Computer programs for computing groundwater traveltimes

Harbaugh and others, 2000

Pollock and others, 1994

Calibration of groundwater-flow models to tracer concentrations

Burow and others, 2008 Crandall and others, 2009 

Clark and others, 2008 Eberts and others, 2012 
Appendix TOOLBOX 115

Computer Program for Estimating Age Mixtures in Water from Wells by Using 

Lumped-Parameter Models

A less resource-intensive method for estimating the groundwater-age mixture in water from a well involves using 

lumped-parameter models—simple mixing equations. An important assumption for this method is that the aquifer 

geometry and groundwater-flow patterns are simple enough for the groundwater-age mixture in a well to be 

described with a single equation. (Spatial variations in aquifer properties are ignored.) Consequently, the modeler 

must choose a mixing equation that has conceptual relevance to the combination of aquifer(s) and well being 

modeled. Two examples are given in the figures below.

Exponential Mixing Model (EMM)

Binary Mixing Model (BMM)

Aquifer—Lines within aquifers represent groundwaterflow pathways. Direction of 

flow is from left to right

Confining unit

For an aquifer where groundwater age increases 

more and more with increasing depth, an equation 

that describes exponential mixing might be used 

to estimate the age mixture in water from a well 

that fully penetrates the aquifer. For example, the 

exponential mixing model would be appropriate for a 

well in an unconfined aquifer of constant thickness 

receiving uniform recharge.

For an aquifer system where groundwater age is 

distinctly different in different parts of the system, 

an equation that describes a binary mixture might be 

used to estimate the age mixture in water from a well 

that draws in groundwater from different parts of the 

system. For example, the binary mixing model would 

be appropriate for a well in a layered-aquifer system. 

A variety of lumped-parameter models have been developed and used to estimate age mixtures for water discharging 

from different types of aquifers (Małoszewski and Zuber, 1982, 1996; Zuber, 1986; Cook and Böhlke, 2000). A 

computer program, TracerLPM, was developed for this study to make it easier to apply the lumped-parameter models 

for estimating groundwater-age mixtures for wells (and springs). Within the computer program, the user selects one 

or more potentially relevant mixing models for the well of interest. Each mixing model has one or two variables that 

represent parameters such as mean age or dispersion. Values for the model parameters are adjusted until the best 

match between simulated tracer concentrations and measured concentrations is achieved. This is done (1) manually, 

while the modeler visually compares curves described by the model equations with measured concentrations on a 

graph or graphs, or (2) automatically, by inverse methods. The approach for computing the simulated tracer concentration for comparison with measured concentrations is fundamentally the same as the approach described for particle 

tracking (see page 114) except that the underlying age mixture is derived from the mixing equations rather than from 

collections of particles and their traveltimes. For public-supply wells with a wide range of ages, it is necessary to 

calibrate the mixing models to multiple tracer data in order to narrow down the possible combinations of groundwater 

ages in the well (traveltimes to the well). The calibration data can include observed concentrations for multiple tracers 

for a single sample and (or) concentrations for a single tracer collected at the well over a sufficient period of time 

(generally years). Both tracers of young groundwater (for example, SF6, CFCs, 3

H, and helium-3) and tracers of old 

groundwater (for example, carbon-14 and helium-4) can be used in the TracerLPM program.

MORE INFORMATION

Computer program for Comparison of age mixtures 

estimating age mixtures using from lumped-parameter and 

lumped-parameter models particle-tracking models

Jurgens and others, 2012 Eberts and others, 2012 

 
116 Factors Affecting Public-Supply-Well Vulnerability to Contamination: Understanding Observed Water Quality and Anticipating Future Water Quality

Groundwater-Stratigraphy Models for Estimating Age Mixtures in Water from Wells

Often, the data and models that are best for estimating the groundwater-age mixture in water from wells are not 

available. However, for simple aquifers with uniformly distributed recharge (meaning no focused recharge from 

streams or runoff from mountain fronts), it might be possible to estimate the groundwater-age distribution in an 

aquifer by using information on aquifer thickness, porosity, and recharge rate (Cook and Böhlke, 2000). This type 

of data is frequently available or readily obtainable.

The relevant models are referred to as groundwater-stratigraphy models. Similar to lumped-parameter (mixing) 

models, they are single equations that describe different field conditions. The major difference between groundwater-stratigraphy models and lumped-parameter models is that groundwater-stratigraphy models are formulated to 

describe variations in groundwater ages within an aquifer on the basis of aquifer geometry (Vogel, 1967; Cook and 

Böhlke, 2000; Solomon and others, 2006; Mendizabal and Stuyfzand, 2009), whereas the lumped-parameter models 

are formulated to describe age mixtures at points of discharge from the aquifer (wells or springs). The choice 

between the two types of models depends on the data that are available—data related to aquifer geometry or tracer 

concentrations for a well or spring. However, by including information on the depth to the top of the well screen and 

the length of the screened interval, it may be possible to generate a rough estimate of the groundwater-age mixture 

for a well by using a groundwater-stratigraphy model (Cook and Böhlke, 2000; Mendizabal and Stuyfzand, 2009). 

A significant limitation of this method is the lack of model calibration to tracer data. Consequently, there is no feedback to indicate whether the selected model and model parameter values are correct for the system. Therefore, the 

technique is best used for wells where recharge, porosity, and the general pattern of groundwater age in the aquifer 

are known.

An example of a groundwater-stratigraphy model for an unconfined aquifer with constant thickness follows: 

Td = nD/N • ln(D/D– d) (1)

where Td is the traveltime, in years, to a given depth on the well screen, n is effective porosity, 

D is aquifer thickness (feet), N is groundwater recharge (feet per year), and d is the depth on the well 

screen in feet below the water table.

The cumulative-frequency distribution of the age of water from a well is calculated by first replacing d in the model with

 d = dU + 0.01Px(dL – dU) (2)

where dL is the lower level of the well screen (feet below water table), dU is the upper level of the 

well screen (feet below water table), and Px is percentile x in the cumulative distribution (0–100 percent),

and then computing Td for a range of percentiles (for example 0, 20, 40, 60, 80, and 100) using the following form of 

the equation (Mendizabal and Stuyfzand, 2009): 

 Td = nD/N • ln(D/D-(dU + 0.01Px(dL – dU))). (3)
Appendix TOOLBOX 117

Water-Quality Forecasting

Computer Program for Generating Water-Quality Response Curves for Wells

Once an estimate for the age mixture of water from a well is obtained, it can be combined with information on 

contaminant input at the water table to forecast the water-quality response at the well to widespread, shallow 

groundwater contamination (nonpoint-source contamination). Forecasts can account for degradation of reactive 

contaminants if reaction rates are uniform and are included in the calculations.

The computer program TracerLPM, which was developed for estimating groundwater-age mixtures in wells by 

using lumped-parameter models (see page 115), has a routine for creating water-quality response curves from 

estimated age mixtures. The program includes an option for importing estimated groundwater-age mixtures 

generated outside the program so that water-quality response curves for individual wells also can be created from 

groundwater-flow and particle-tracking model results. 

A limitation of using information on groundwater-age mixtures at individual wells for creating water-quality 

response curves is the inability to account for spatial variations in contaminant input. This includes contaminant 

input from most point sources. Three-dimensional models are necessary if such additional insight is desired. 

Concentration,

as percent

of input

WELL RESPONSES

Connecticut

Contaminant input

0

40

60

100

20

80

0 50

Time, in years

100

Florida

California

Nebraska

Water-quality response curves were computed 

for four study wells to explore differences in how 

the water from the wells might change over time 

in response to 25 years of nitrate contamination of 

shallow groundwater. Without this type of analysis, 

it would be difficult to appreciate the vastly different 

consequence of nitrate contamination for the four 

communities relying on these public-supply wells.

MORE INFORMATION

Computer program for forecasting 

effect of nonpoint-source contamination 

on water from wells

Jurgens and others, 2012

Response of public-supply wells to 

changes in nitrate input

McMahon and others, 2008b
118 Factors Affecting Public-Supply-Well Vulnerability to Contamination: Understanding Observed Water Quality and Anticipating Future Water Quality

Direct Observation of Vulnerability

Low-Level VOC Analysis for Assessing Vulnerability Independent of Groundwater Age

A classification scheme that can be used to assess the vulnerability of water from an individual well to VOC 

contamination was developed by using data from this study. The classification scheme is based on the ability to 

detect halogenated VOCs in groundwater at very low concentrations (parts per quadrillion)—near those expected 

for water in equilibrium with air. This is accomplished by using gas chromatography with an electron-capture 

detector (GC-ECD). 

Many halogenated VOCs have entered aquifers along with recharge from precipitation since the 1940s. VOCs in 

groundwater with concentrations at or just below air-water equilibrium can be the result of atmospheric (anthropogenic or natural) sources of VOCs or local, anthropogenic sources of VOCs. Higher concentrations, however, 

indicate that a local, anthropogenic source of VOCs (such as a solvent plume) must be linked to the well by way of 

one or more flow pathways. 

The classification scheme is useful for providing an early indication of whether a link exists between a well and a 

local (often unknown) anthropogenic source of contaminants. However, it does not provide information on the longterm threat posed by the contaminant source. Information on the concentrations and proportions of contaminated 

and uncontaminated waters that mix in a well are necessary to gain such insight. An example of how this classification scheme can be used to assess the vulnerability of water from wells to VOC contamination is given below:

Affected by local, anthropogenic source of VOCs

Texas

Nebraska

Florida

Connecticut

New Mexico

Affected by local or atmospheric, 

anthropogenic source of VOCs 

Affected by atmospheric, 

natural source of VOCs

Contaminated

Median number of detections

Total concentration 

of halogenated VOCs, 

in picograms per liter

WELLS

Air-water equilibrium

Public-supply well

USEPA MCL 

for TCE or PCE

1990

1940

100

0 10

Number of halogenated VOCs detected per sample

20 30

1,000

10,000

100,000

1,000,000

10,000,000

100,000,000

NOTE: All wells in the Texas and 

New Mexico study areas are 

public-supply wells. The remaining 

wells are monitoring wells, unless 

otherwise noted.

The classification scheme was used to assess the vulnerability of water from wells in this study to VOC contamination. On the basis of very low-level VOC analysis, samples from every well contained PCE, the chlorofluorocarbons 

CFC-12 and CFC-11, and methyl iodide at low concentrations—indicating that every well has some vulnerability to 

contamination from VOCs originating near the water table. The data also revealed that approximately 20 percent of 

the public-supply wells had concentrations of VOCs high enough to indicate that a local, anthropogenic contaminant 

source had affected the well. Only one of the wells had concentrations above a drinking-water standard. Many VOCs 

were not detected in any of the wells when more conventional, but less sensitive, gas chromatography with mass 

spectrometry was used to analyze the samples.

MORE INFORMATION

Framework for assessing vulnerability 

to VOC contamination

Plummer and others, 2008
Appendix TOOLBOX 119

Confining unit 

Unconfined aquifer

Confined aquifer 

Confined-aquifer ZOT 

Confined-aquifer zone of transport (ZOT)

projected to land surface

Public -supply well Zone of transport (ZOT) based on confined-aquifer 

traveltimes: 

 ZOT = Qt/θH 

where 

Q is pumping rate of well, 

t is user-defined time of travel, 

θ is porosity, and 

H is height (thickness) of confined aquifer.

(NOTE: This is the cylinder or volumetric-flow 

equation (USEPA, 1993)).

Preferential Flow Pathways

Approach for evaluating potential for multi-aquifer wells 

to affect a public-supply well

A simple way to gain insight into the potential for multi-aquifer wells to affect a confined-aquifer public-supply 

well is to estimate a zone of transport (ZOT) for the public-supply well within the confined aquifer and then 

evaluate the likelihood that a multi-aquifer well lies within that zone. For this purpose, the ZOT would be defined 

solely on the basis of confined-aquifer traveltimes—traveltimes through the unconfined aquifer and confining unit 

would not be considered—because multi-aquifer wells anywhere within a confined-aquifer ZOT are capable of 

allowing contaminated water to reach a public-supply well within a timeframe of interest.

Protecting the area at land surface above a confined-aquifer zone 

of transport can help prevent an unidentified multi-aquifer well 

from shunting contaminated groundwater across the confining unit 

and affecting the public-supply well within a timeframe of interest.
An example illustrating how this approach might be used to evaluate the potential for multi-aquifer wells to affect 

water quality at the Nebraska study well is given below. A 40-year ZOT is used in this example because the U.S. 

Environmental Protection Agency suggests a time of travel of 40 years for differentiating semi-confined conditions 

from highly confined conditions (USEPA, 1991).

IRRIGATION

 Fully screened

 Confined

PUBLIC SUPPLY

 Fully screened

 Confined

INDUSTRIAL

 Fully screened

 Confined

ABANDONED—fully screened

TEST HOLE—fully screened

WELL TYPE

York city limits

Study area

0 1

0 1 2 MILES

2 KILOMETERS

Study well

40-year confined-aquifer

zone of transport (ZOT))

The number of multi-aquifer wells potentially affecting the public-supply 

well within a ZOT that is 5 mi2

 is 10 to 15 wells because the density of 

multi-aquifer wells in the area is approximately 2 to 3 per mi2

. Although 

the estimated ZOT does not equate with the area at the water table that 

ultimately contributes water to the well, simple calculations, such as this, 

can be used to alert water managers that many multi-aquifer wells might 

allow unconfined-aquifer water to travel relatively quickly to a confinedaquifer public-supply well. 

Forty-year confined-aquifer ZOT 

for the Nebraska study well:

Q = 49,087 ft3

/d (367,196 gal/d)

 t = 40 years (14,600 days)

θ = 0.15

H = 34 ft

ZOT = 140,523,569 ft2

 ≈ 5 mi2

120 Factors Affecting Public-Supply-Well Vulnerability to Contamination: Understanding Observed Water Quality and Anticipating Future Water Quality

MORE INFORMATION

Johnson and others, 2011
Publishing support provided by the U.S. Geological Survey Science 

Publishing Network, Menlo Park and Columbus Publishing Service 

Centers

For more information concerning the research in this report, contact:

Chief, National Water-Quality Assessment Program

U.S. Geological Survey

413 National Center

12201 Sunrise Valley Drive

Reston, VA 20192
Eberts and others—Factors Affecting Public-Supply-Well Vulnerability to Contamination—Circular 1385