Assessing the Vulnerability of Public-Supply Wells to Contamination 

from Urban, Agricultural, and Natural Sources

In 2001, the U.S. Geological Survey’s 

National Water-Quality Assessment 

(NAWQA) Program began an intensive study to assess the vulnerability of 

public-supply wells to contamination 

from a variety of compounds. 

The study builds on previous 

NAWQA studies from 1991 to 2001 

that found low levels of mixtures of 

contaminants in ground water near the 

water table in urban areas across the 

Nation (in about 90 percent of monitoring wells) and, less frequently, in 

deeper ground water typically developed for public supply (Hamilton 

and others, 2004). Data from more 

than 1,000 public-supply wells within 

major water-supply aquifers are being 

evaluated in this study, and data from 

more aquifers and wells are scheduled 

to be added in 2009 (see map, p. 4). 

This NAWQA study is focusing on 

the transport and chemical breakdown 

of selected anthropogenic contaminants from urban and agricultural 

sources, as well as contaminants from 

natural sources, within that part of 

the ground-water system contributing water to public-supply wells. 

Scientists are investigating how the 

linkage between contaminant sources 

and public-supply wells is affected 

by processes that occur below land 

surface—whereby contaminants are 

mobilized, dispersed, diluted, volatilized, adsorbed, and (or) degraded. 

Scientists are also investigating how 

the operation of public-supply wells 

can affect their vulnerability to contamination. 

are limitations in the databases and 

models used to perform the assessments (National Research Council, 

1993). An additional challenge is 

the need to strike a balance between 

complex, costly assessments and those 

that are oversimplified (Focazio and 

others, 2002). As a result, groundwater vulnerability has been assessed 

using many different methods (Nolan, 

1998); most of the previous assessBecause subsurface processes and 

management practices differ among 

aquifers and public-water systems, 

public-supply wells in different parts 

of the Nation are not equally vulnerable to contamination, even where 

similar contaminant sources exist. The 

study is identifying these important 

differences, as well as similarities, in a 

complementary set of aquifer systems, urban settings, and public-water 

systems.

A national priority, a scientific 

challenge

About one-third of the U.S. population 

gets drinking water from public-supply 

wells. The occurrence of contaminants 

in these wells is highly variable (U.S. 

Environmental Protection Agency, 

1999). To safeguard public health, we 

need a better understanding of how 

these wells can become contaminated. 

Understanding public-supply well 

contamination is also an economic 

issue because cleaning up contaminated ground water is expensive and 

difficult. Drinking water from publicsupply wells must meet U.S. Environmental Protection Agency (USEPA) 

and (or) State water-quality standards. 

Vulnerability assessments based on 

sound science will help decision-makers predict which wells are vulnerable 

to contamination and design strategies to prevent future contamination, 

thereby sustaining the water supply. 

Vulnerability assessments, however, 

are inherently uncertain. Scientists 

do not fully understand contaminant 

behavior in the subsurface, and there 

U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2005-3022

April 2005 Printed on recycled paper

By Sandra M. Eberts, Martha L. Erwin, and Pixie A. Hamilton

Contaminants assessed in 

this study

• Anthropogenic contaminants, 

including nitrate, pesticides and their 

breakdown products (such as atrazine 

and deethylatrazine), compounds 

found in wastewater; and volatile 

organic compounds (such as MTBE 

and disinfection by-products)

• Naturally occurring contaminants, 

including arsenic, uranium, radon, 

and radium

• Fecal contamination, using indicators such as E. coli and total coliform 

(bacteria) and coliphage (a group of 

viruses)

What are the most important factors controlling contamination of public-supply wells, and how can we do a better job 

of predicting their vulnerability to contamination?
Ground-water vulnerability—the 

likelihood that contaminants will 

reach a specified reference location 

in a ground-water system (the water 

table, deep within the aquifer, a public-supply well, the interface between 

ground and surface water) (National 

Research Council, 1993). 

Area contributing recharge—the 

surface area at the water table or a 

surface-water body from which water 

entering the ground-water system 

eventually flows to the well. Estimates of areas contributing recharge 

to public-supply wells are made in 

order to target ground-water protection practices (Franke and others, 

1998). 

Ground-water age—the time elapsed 

(ranging from days to millennia) 

since water reached the water table 

during recharge. Young ground 

water tends to be more susceptible to 

contamination from current sources 

at the land surface than older ground 

water (Focazio and others, 2002). 

Water discharging from a well is usually a mixture of waters of different 

ages.

Ground-water sustainability—the 

development and use of groundwater resources in a manner that can 

be maintained indefinitely without 

unacceptable consequences (Alley 

and Leake, 2004). Water quality and 

water quantity are equally critical for 

the long-term sustainability of the 

Nation’s water supply (U.S. Geological Survey, 2002). 

ments have focused on the transport of 

contaminants to the water table rather 

than to public-supply wells. 

 In the current study, we address 

the challenges of vulnerability assessments by collecting and analyzing 

similar data within a variety of settings, including unique data collected 

using new tools. We are developing a 

library of site-specific models to help 

sort out the most important factors to 

include in vulnerability assessments in 

different settings at both regional and 

local scales. 

Study results will provide a foundation for assessing the vulnerability 

of the Nation’s public-supply wells 

to a variety of contaminants, and will 

help those involved in well siting and 

water-quality protection anticipate 

the response of different systems to 

changes in management practices. 

The results also will be useful to those 

involved in planning and implementing State source-water assessment and 

protection programs, as guided by the 

USEPA (U.S. Environmental Protection Agency, 1997).

General objectives of the 

study

• Identify the dominant contaminants 

and sources of those contaminants 

in public-supply wells in representative water-supply aquifers across 

the Nation

• Assess the effects of natural processes (such as degradation) and 

human activities (such as irrigation) 

on the occurrence of contaminants 

in public-supply wells in representative aquifers

• Identify the factors that are most 

important to incorporate into 

public-supply well vulnerability 

assessments in different settings 

and at different spatial scales

• Develop simple methods and 

models for screening public-supply 

wells for vulnerability to contamination in unstudied areas and from 

newly emerging contaminants

• Increase understanding of the 

potential effects of water-resource 

development and management 

decisions on the quality of water 

from public-supply wells

Unique characteristics of 

the study

Sampling at different depths

The screened or open intervals of 

public-supply wells are commonly 

from tens to hundreds of feet in length; 

therefore, water from these wells is 

generally a mixture of waters of different ages that enter the well at different 

depths and are associated with different potential sources of contamination. 

The graphic on this page illustrates a 

situation where recharge to public-supply wells reflects urban and agricultural land-use activities. Specifically, 

water recharges the aquifer in the 

urban area containing urban-related 

contaminants, such as volatile organic 

compounds, and enters the well 

screens above water that has traveled 

from the more distant agricultural area 

where recharge water may contain 

contaminants such as agricultural 

pesticides.

Using a USGS-developed sampler 

(Izbicki, 2004), we are collecting 

samples at multiple depths in pumping 

public-supply wells to ascertain where 

and how contaminants from different 

An aquifer system and public-water system in an urban setting. The water entering the well 

screens of the public-supply wells is of different ages and from different areas because of their 

long screened intervals, which commonly make public-supply wells vulnerable to contamination 

from multiple sources. In this example, sources of contaminants may include those associated with 

urban and agricultural land-use activities. Aquifer materials may also serve as sources of natural 

contaminants such as arsenic.

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Inflow at different depths within a public-supply well. The aerial photo shows an approximately 63-square-mile area near the well. Water 

entering the well screen is associated with different potential sources of contaminants because of the different land-use activities in the areas 

contributing recharge to various intervals along the well screen, as well as the different aquifer materials through which water flows between 

the recharge areas and the well. The amount of contamination that might be contributed by any given interval is related to the volume of water 

that flows into the well along the interval and the concentration of any associated contaminants. Depth-dependent samples are a composite of 

all intervals beneath the sampling point; these samples are being analyzed for chemical quality and ground-water age and then compared to 

samples collected from the wellhead. 

sources enter the wells. For example, 

samples collected from public-supply 

wellheads and analyzed for concentrations of multiple contaminants are 

being “dated” to determine groundwater age and compared to samples 

and ages of water entering the wells at 

various depths (see graph below). This 

analysis is helping to evaluate the usefulness of ground-water age samples 

from wellheads for predicting the risk 

of contamination.

Evaluating multiple settings and 

scales

Consistent methods are being used to 

collect and analyze data, and investigations are being conducted at both 

regional (tens to thousands of square 

miles) and local scales (less than 

10 square miles). We can therefore 

compare and contrast results and 

identify the most important processes 

to include in vulnerability assessments 

applied at different scales and in a 

variety of water-supply aquifers. For 

example, nitrate is detected in ground 

water in most participating study 

areas. The distribution and concentration of nitrate between the water table 

and public-supply wells are controlled 

predominantly by dilution in some 

areas and by dispersion or degradation 

in others. Using models developed for 

each study area, we are exploring how 

these differences in subsurface processes affect the response of different 

aquifer systems to common management practices.

Exploring the consequences of 

uncertainty

To make informed decisions about 

activities at a particular location, decision-makers need to know whether 

the location is contributing recharge 

to public-supply wells. They also 

need information about traveltimes 

between potential contaminant sources 

and public-supply wells. Because 

this information cannot be measured 

directly, decision-makers must rely on 

estimates that are inherently uncertain 

(due to limitations in the methods). We 

are exploring the consequences of this 

uncertainty, and helping decision-makers understand these consequences, by 

comparing estimates from traditional 

and probabilistic modeling approaches 

with actual water-quality data from 

public-supply wells. 

How this information can 

be used

Study results, models, and other decision-support tools will apply to broad 

classes of contaminants, including 

newly identified, emerging contaminants, and will help water managers 

and scientists:

• Better understand how and why 

contamination of public-supply 

wells occurs

• Improve assessments of the vulnerability of ground water and public-supply wells to contamination, 

even in unmonitored areas

• Choose new sites for water supply 

and develop and prioritize monitoring programs

• Evaluate various resource-development and management scenarios.
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The NAWQA Program

The study of public-supply well vulnerability is one of five national priority topics 

being addressed by the NAWQA Program

in its second decade, which began in 2001. 

Other topics include effects of urbanization 

on stream ecosystems; ecological effects 

of nutrient enrichment; mercury in stream 

ecosystems; and sources, transport, and 

fate of agricultural chemicals. In addition, anthropogenic organic contaminants 

in source waters for many of the Nation’s 

largest community water systems are being 

assessed; concentrations in source waters 

are being compared to concentrations in 

finished waters. During the Program’s first 

decade, NAWQA scientists assessed surface- and ground-water chemistry, stream 

hydrology, habitat, and biological communities in 51 major river basins (“Study 

Units”; see map at http://water.usgs.gov/

nawqa). Baseline assessments of pesticides, 

nutrients, VOCs, trace elements, dissolved 

solids, and radon, and of the condition of 

aquatic habitats and fish, insect, and algal 

communities are described in hundreds of 

reports, available at the Web site above. 

Reassessments planned in 42 of the Study 

Units in the Program’s second decade will 

determine trends at many of the streams 

and ground-water sites; fill critical gaps in 

the characterization of water quality; and 

build upon findings that show how natural 

features and human activities affect water 

quality and aquatic ecosystems.

Locations of regional-scale studies of public-supply well vulnerability to contamination from urban, agricultural, and natural sources.

Studies began in 2001 in eight States, in Texas and New Mexico in 2005, and are scheduled for Illinois and New Jersey or New York in 2009. 

Contacts

Sandra Eberts, Team Leader

(614) 430-7740, smeberts@usgs.gov 

Mary Ann Thomas, Ohio 

(614) 430-7736 mathomas@usgs.gov

Craig Brown, Connecticut 

(860) 291-6766 cjbrown@usgs.gov

Karen Burow, California 

(916) 278-3087 krburow@usgs.gov

Michael Rosen, Nevada 

(775) 887-7683 mrosen@usgs.gov

Bernard Stolp, Utah 

(801) 908-5061 bjstolp@usgs.gov

Suzanne Paschke, Colorado 

(303) 236-4882 ext 352 spaschke@usgs.gov

Matthew Landon, Nebraska 

(402) 328-4125 landon@usgs.gov

Laura Bexfield, New Mexico 

(505) 830-7972 bexfield@usgs.gov

Lynne Fahlquist, Texas 

(512) 927-3508 lfahlqst@usgs.gov

Christy Crandall, Florida

(850) 942-9500 ext 3030 crandall@usgs.gov

References 

Alley, W.M., and Leake, S.A., 2004, The journey 

from safe yield to sustainability: Ground Water, 

v. 42, no. 1, p. 12-16.

Focazio, M.J., Reilly, T.E., Rupert, M.G., and 

Helsel, D.R., 2002, Assessing ground-water 

vulnerability to contamination–Providing scientifically defensible information for decision 

makers: USGS Circular 1224, 33 p.

Franke, O.L., Reilly, T.E., Pollock, D.W., and 

LaBaugh, J.W., 1998, Estimating areas contributing recharge to wells–Lessons from previous 

studies: USGS Circular 1174, 14 p.

Hamilton, P.A., Miller, T.L., and Myers, D.N., 

2004: Water quality in the Nation’s streams and 

aquifers–Overview of selected findings, 1991-

2001: USGS Circular 1265, 20 p.

Izbicki, J.A., 2004, A small-diameter sample 

pump for collection of depth-dependent 

samples from production wells under pumping 

conditions: USGS Fact Sheet 2004-3096, 2 p.

National Research Council, 1993, Ground-water 

vulnerability assessment–Predicting relative 

contamination potential under conditions of 

uncertainty: Washington, D.C., National Academy Press, 204 p.

Nolan, B.T., 1998, Modeling approaches for 

assessing the risk of nonpoint-source contamination of ground water: USGS Open File 

Report 98-531, 15 p.

U.S. Environmental Protection Agency, 1997, 

State source water assessment and protection 

programs–Final guidance: USEPA Office of 

Water, EPA 816-R-97-009.

U.S. Environmental Protection Agency, 1999, A 

review of contaminant occurrence in public 

water systems: USEPA Office of Water, EPA 

816-R-99-006, 78 p.

U.S. Geological Survey, 2002, Report to Congress, concepts for national assessment of 

water availability and use: USGS Circular 

1223, 34 p.

U.S. Geological Survey, 2003, Principal aquifers 

of the 48 conterminous United States, Hawaii, 

Puerto Rico, and the U.S. Virgin Islands, 

available online at http://www.nationalatlas.

gov/mld/aquifrp.html .