U.S. Department of the Interior

U.S. Geological Survey

Circular 1353

Water Quality in the Northern Atlantic Coastal Plain 

Surficial Aquifer System, Delaware, Maryland, 

New Jersey, New York, North Carolina, 

and Virginia, 1988–2009

National Water-Quality Assessment Program

The Quality of Our Nation’s Waters
Cover. The intersection of suburban development, farmland, forest, and wetlands near the shore of the Chesapeake Bay near 

Crisfield, Maryland. Photograph by Emily Nauman of the Integration and Application Network, University of Maryland Center for 

Environmental Science (IAN, UMCES).
Water Quality in the Northern Atlantic 

Coastal Plain Surficial Aquifer System, 

Delaware, Maryland, New Jersey, New York, 

North Carolina, and Virginia, 1988–2009

By Judith M. Denver, Scott W. Ator, Jeffrey M. Fischer, Douglas C. Harned, 

Christopher Schubert, and Zoltan Szabo 

National Water-Quality Assessment Program

U.S. Department of the Interior

U.S. Geological Survey

The Quality of Our Nation’s Waters

Circular 1353
U.S. Geological Survey, Reston, Virginia: 2014

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Suggested citation:

Denver, J.M., Ator, S.W., Fischer, J.M., Harned, D.C., Schubert, Christopher, and Szabo, Zoltan, 2014, The quality 

of our Nation’s waters—Water quality in the Northern Atlantic Coastal Plain surficial aquifer system, Delaware, 

Maryland, New Jersey, New York, North Carolina, and Virginia, 1988–2009: U.S. Geological Survey Circular 1353, 

88 p., http://dx.doi.org/10.3133/cir1353.

Library of Congress Cataloging-in-Publication Data

Denver, J. M.

The quality of our Nation’s waters---water quality in the Northern Atlantic coastal plain surficial aquifer system, 

Delaware, Maryland, New Jersey, New York, North Carolina, and Virginia, 1988-2009 / by Judith M. Denver, Scott W. 

Ator, Jeffrey M. Fischer, Douglas C. Harned, Christopher Schubert, and Zoltan Szabo.

 pages cm. -- (Circular / U.S. Geological Survey ; 1353)

Includes bibliographical references.

ISBN 978-1-4113-3751-0

1. Water quality--North Atlantic Region. 2. Water quality--East (U.S.) 3. Groundwater--Quality--North Atlantic 

Region. 4. Groundwater--Quality--East (U.S.) I. Ator, Scott W. II. Fischer, Jeffrey M. III. Harned, Douglas. IV. Schubert, 

Christopher E. V. Szabo, Zoltan, 1959- VI. Geological Survey (U.S.) VII. Title. VIII. Series: U.S. Geological Survey 

circular ; 1353.

TD223.13.D46 2014

628.1’140975--dc23

 2014001593

ISSN 1067-084X (print)

ISSN 2330-5703 (online)

ISBN 978-1-4113-3751-0

U.S. Department of the Interior

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director
National Water-Quality Assessment Program

Photograph by Emily Nauman, Integration and Application Network, University of Maryland Center for Environmental Science (IAN, UMCES)

"USGS, through the NAWQA Program, has extensively studied the movement of 

nutrients through groundwater. We look forward to using this information as we 

work with state and local partners to implement practices on the land that have 

the greatest benefit to the quality of our local and Bay waters."

Nick DiPasquale

EPA Director of the Chesapeake Bay Program
iv

Foreword

The United States has made major investments in assessing, managing, regulating, and 

conserving natural resources, such as water and a variety of ecosystems. Sustaining the quality 

of the Nation’s water resources and the health of our diverse ecosystems depends on the 

availability of sound water-resources data and information to develop effective, science-based 

policies. Effective management of water resources also brings more certainty and efficiency 

to important economic sectors. Taken together, these actions lead to immediate and long-term 

economic, social, and environmental benefits that make a difference to the lives of millions of 

people (http://water.usgs.gov/nawqa/applications/).

Two decades ago, Congress established the U.S. Geological Survey’s National Water-Quality 

Assessment (NAWQA) Program to meet this need. Since then NAWQA has served as a primary 

source of nationally consistent information on the quality of the Nation’s streams and groundwater, on ways in which water quality changes over time, and on the natural features and 

human activities affecting the quality of streams and groundwater. Objective and reliable data, 

systematic scientific studies, and models are used to characterize where, when, and why the 

Nation’s water quality is degraded—and what can be done to improve and protect the water for 

human and ecosystem needs. This information is critical to our future because the Nation faces 

an increasingly complex and growing need for clean water to support people, economic growth, 

and healthy ecosystems. For example, NAWQA findings for public-supply wells, which provide 

water to about 105 million people, showed that 22 percent of source-water samples contained 

at least one contaminant at levels of potential health concern. Similarly, 23 percent of samples 

from domestic (or privately owned) wells, which supply untreated water to an additional 

43 million people, also had contaminant levels of potential concern. 

This report is one of a collection of publications that describe water-quality conditions in 

selected Principal Aquifers of the United States (http://water.usgs.gov/nawqa/studies/praq/). 

The collection is part of the series “The Quality of Our Nation’s Waters,” which describes major 

findings of the NAWQA Program on water-quality issues of regional and national concern 

and which provides science-based information for assessing and managing the quality of our 

groundwater resources. Other reports in this series focus on occurrence and distribution of 

nutrients, pesticides, and volatile organic compounds in streams and groundwater, the effects 

of contaminants and streamflow alteration on the condition of aquatic communities in streams, 

and the quality of untreated water from private domestic and public-supply wells. Each report 

builds toward a more comprehensive understanding of the quality of regional and national water 

resources (http://water.usgs.gov/nawqa/nawqa_sumr.html ). All NAWQA reports are available 

online at http://water.usgs.gov/nawqa/bib/.

The information in this series primarily is intended for those interested or involved in resource 

management and protection, conservation, regulation, and policymaking at regional and national 

levels. In addition, the information should be of interest to those at a local level who wish to 

know more about the general quality of streams and groundwater in areas near where they live 

and how that quality compares with other areas across the Nation. We hope this 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.

Jerad Bales

Acting Associate Director for Water

U.S. Geological Survey
v

High Plains aquifer system—Circular 1337

Glacial aquifer system—Circular 1352

Northern Atlantic Coastal Plain 

surficial aquifer system—Circular 1353

Piedmont, Blue Ridge, and Valley and 

Ridge aquifers—Circular 1354

Piedmont and Blue Ridge 

carbonate-rock aquifers

Piedmont and Blue Ridge 

crystalline-rock aquifers

Valley and Ridge siliciclasticrock aquifers

Valley and Ridge carbonaterock aquifers

Early Mesozoic basin aquifers 

Upper Floridan aquifer and overlying

surficial aquifers—Circular 1355

Mississippi embayment–Texas coastal 

uplands aquifer system—Circular 1356

Mississippi River Valley alluvial aquifer

Denver Basin aquifer system—Circular 1357

Southwest Principal Aquifers—Circular 1358

California Coastal Basin aquifers

Central Valley aquifer system

Basin and Range basin-fill aquifers

Rio Grande aquifer system 

Western Volcanics—Circular 1359

Hawaiian volcanic-rock aquifers

Snake River Plain basin-fill and 

basaltic-rock aquifers

Columbia Plateau basin-fill and 

basaltic-rock aquifers

Principal Aquifers and Water-Quality Summary Reports

National summary of groundwater quality in Principal Aquifers—Circular 1360
vi

Contents

Chapter 1: Overview of Major Findings and Implications ....................................................................1

Chapter 2: NAWQA Approach to Assessing Groundwater Quality ....................................................7

Chapter 3: Environmental Setting and Hydrogeology .........................................................................17

Chapter 4: Natural Hydrogeologic and Geochemical Processes That Affect 

Groundwater Quality .............................................................................................................25

Chapter 5: Contaminants of Concern in Drinking Water and in Groundwater Flowing 

to Streams...............................................................................................................................33

Chapter 6: Understanding Where and Why Key Contaminants Occur in Groundwater...............41

Chapter 7: Groundwater as a Source of Contaminants to Streams and Estuaries........................61

References cited..........................................................................................................................................71

Glossary .......................................................................................................................................................76

Appendixes ..................................................................................................................................................81
vii

Introduction to This Report

This report contains the major findings of a regional assessment of groundwater quality in 

Delaware, Maryland, New Jersey, New York, North Carolina, and Virginia. It is one of a series 

of reports by the U.S. Geological Survey (USGS) National Water-Quality Assessment (NAWQA) 

Program that present major findings for Principal Aquifers, other aquifers, and major river basins 

across the Nation. In these reports, water quality is discussed in terms of local, State, regional, 

and national issues. Conditions in the Northern Atlantic Coastal Plain surficial aquifer system 

are compared to conditions found elsewhere and to selected national benchmarks, such as 

those for drinking-water quality.

This report is intended for individuals working with water-resource issues in local, State, 

or Federal agencies, universities, public interest groups, or the private sector. The information 

will be useful in addressing current issues, such as drinking-water quality, source-water protection, and monitoring and sampling strategies. This report will also be useful for individuals who 

wish to know more about the quality of groundwater in areas near where they live and how that 

quality of water compares with the quality of water in other areas across the region and the 

Nation.

Water-quality conditions in the Northern Atlantic Coastal Plain surficial aquifer system that are 

summarized in this report are discussed in greater detail in other reports listed in the references. 

Detailed technical information, data and analyses, sample collection and analytical methodology, and models, graphs, and maps that support the findings presented in this report in addition 

to reports in this series from other Principal Aquifers can be accessed from the NAWQA Program 

Web site (http://water.usgs.gov/nawqa/).
viii

Companion studies of this aquifer system are discussed in the following reports:

A Surficial Hydrogeologic Framework 

for the Mid-Atlantic Coastal Plain

Professional Paper 1680

U.S. Department of the Interior

U.S. Geological Survey

Detailed description of the hydrogeologic framework for the surficial aquifer system of the 

Northern Atlantic Coastal Plain:

A Surficial Hydrogeologic Framework for the Mid-Atlantic Coastal Plain

by Scott W. Ator, Judith M. Denver, David E. Krantz, Wayne L. Newell, and Sarah K. 

Martucci

U.S. Geological Survey Professional Paper 1680 available at

http://pubs.usgs.gov/pp/2005/pp1680/

Detailed description of nitrogen transport for the surficial aquifer system in the North 

Carolina Coastal Plain:

Geochemistry and Characteristics of Nitrogen Transport at a Confined Animal 

Feeding Operation in a Coastal Plain Agricultural Watershed, and Implications for 

Nutrient Loading in the Neuse River Basin, North Carolina, 1999–2002

by T.B. Spruill, A.J. Tesoriero, H.E. Mew, Jr., K.M. Farrell, S.L. Harden, 

A.B. Colosimo, and S.R. Kraemer

U.S. Geological Survey Scientific Investigations Report 2004–5283 available at 

http://pubs.usgs.gov/sir/2004/5283/

Detailed description of natural and human influences on water quality in the surficial aquifer 

system of the Northern Atlantic Coastal Plain:

Natural and Human Influences on Water Quality in a Shallow Regional 

Unconsolidated Aquifer, Northern Atlantic Coastal Plain

by Scott W. Ator

U.S. Geological Survey Scientific Investigations Report 2008–5190 available at 

http://pubs.usgs.gov//sir/2008/5190/

Detailed description of water quality in the surficial aquifer system of the 

Delmarva Peninsula:

Factors Affecting Spatial and Temporal Variability in Nutrient and Pesticide 

Concentrations in the Surficial Aquifer on the Delmarva Peninsula

by Linda M. Debrewer, Scott W. Ator, and Judith M. Denver

U.S. Geological Survey Scientific Investigations Report 2005–5257 available at 

http://pubs.usgs.gov/sir/2005/5257/

Prepared in cooperation with the

New Jersey Department of Environmental Protection 

Occurrence of Radium-224, Radium-226, 

and Radium-228 in Water of the Unconfined 

Kirkwood-Cohansey Aquifer System, 

Southern New Jersey

Scientific Investigations Report 2004-5224

U.S. Department of the Interior

U.S. Geological Survey

Detailed description of radium occurrence in the surficial aquifer system of New Jersey:

Occurrence of Radium-224, Radium-226, and Radium-228 in Water of the 

Unconfined Kirkwood-Cohansey Aquifer System, Southern New Jersey

by Zoltan Szabo, Vincent T. dePaul, Thomas F. Kraemer, and Bahman Parsa

U.S. Geological Survey Scientific Investigations Report 2004–5224 available at 

http://pubs.usgs.gov/sir/2004/5224/
Chapter 1

The surficial aquifer system of the Northern Atlantic Coastal Plain is 

an important renewable source of drinking water, of flow to streams, 

and of recharge to deeper confined aquifers. Millions of people in the 

densely populated areas of southern New Jersey and Long Island, New York, 

rely on the surficial aquifer system of the Northern Atlantic Coastal Plain for 

public and domestic water supply. The surficial aquifer system also is the 

primary source of drinking water in many of the more rural areas throughout 

the region. This region’s population and need for water will only continue to 

increase. The permeable sands and gravels of the surficial aquifer system do 

little to hinder the transport of chemicals released at the land surface down to 

the shallow water table of this unconfined aquifer system. Discharge from the 

aquifer system supplies most of the flow in local streams, and chemicals in that 

discharge contribute to the ecological degradation of coastal waters. Improved 

understanding of the natural and human factors that affect the quality of water 

in the surficial aquifer system supports effective and efficient water-resources 

management for human health and for the sustained availability of groundwater.

Photograph by Emily Nauman, IAN, UMCES

Groundwater from the surficial aquifer system provides most of the flow and also 

carries chemicals from surrounding land uses to local streams in the Northern 

Atlantic Coastal Plain.

Chapter 1: Overview of Major Findings 

and Implications
Shallow

monitoring wells

Domestic

well

NOT TO SCALE

Water

table

Confined aquifers

Public

supply

wells

Surficial aquifer system

(unconfined, freshwater) 

Confining units

-

Number of samples,

in percent

Percentage of samples with 

concentrations above a 

human-health benchmark

25

20

15

10

5

0

Urban Agriculture

EXPLANATION

Dieldrin

Nitrate

The types of chemicals detected

 in groundwater are affected 

by local land use.

Land use

Chemical contaminants 

from human sources, 

including from fertilizers, 

manures, pesticides, and 

volatile organic compounds, 

occur in groundwater in many 

areas of the surficial aquifer 

system, although concentrations are generally less than 

human-health benchmarks. 

Benchmarks, however, 

are not available for all 

compounds or for mixtures of 

compounds. See chapter 5.

Nitrate was detected at concentrations greater than 

1 milligram per liter as nitrogen (mg/L as N)—the level attributed to human-related sources—in water from more than half 

of the wells sampled. Nitrate moves readily in groundwater 

that contains dissolved oxygen (as occurs in New Jersey, Long 

Island, and the Delmarva Peninsula), but is lost to denitrification in areas without dissolved oxygen (as occurs in much of 

North Carolina). Nitrate concentrations are generally highest 

beneath agricultural areas, but are also elevated beneath urban 

areas. See chapter 6.

The surficial aquifer system of the Northern Atlantic Coastal Plain is made up of unconfined aquifers that underlie most of the area. This aquifer system is a critical renewable source of drinking water and is the source of most flow to streams and of recharge to 

underlying confined aquifers. Millions of people rely on the surficial aquifer system for 

public and domestic water supply, in particular in the densely populated areas of Long 

Island, New York, and in southern New Jersey, but also in more rural areas. Because the 

aquifer sediments are permeable and the water table is shallow, the surficial aquifer system 

is vulnerable to contamination from chemicals that are applied to the land surface and 

carried into groundwater with infiltrating rainfall and snowfall.

2 Nitrate is one of the most widespread contaminants in groundwater

Overview of Major Findings for the Northern Atlantic Coastal 

The quality of most groundwater produced 

for public and domestic water supply is 

suitable for drinking, although contaminants 

at concentrations greater than human-health 

benchmarks have been detected in

some places

1

NY

PA

WV

MD

NJ

NC

TN

VA

DE

Nitrate concentration, in 

milligrams per liter as nitrogen30

20

10

0

Agriculture Urban

Maximum

Contaminant 

Level

New 

Jersey 

Long 

Island 

North 

Carolina 

Delmarva

Peninsula 

New 

Jersey 
Shallow

monitoring 

wells

Saltwater

Coastal

estuary

Freshwatersaltwater interface

Public-supply well in 

semiconfined aquifer

Atlantic

Ocean

Radium concentration, in 

picocuries per liter

10.0

6.0

2.0

pH < 5 pH > 5

Maximum 

Contaminant

Level

Concentrations of radium in groundwater increase as pH 

decreases and are commonly greater in areas with agricultural 

land use because the transformation of applied nitrogen 

compounds to nitrate increases the acidity (lowers the pH) of 

groundwater. See chapter 6.

Nitrate from 

groundwater, in particular, 

contributes to the 

ecological degradation of 

many streams and coastal 

water bodies. Pesticides 

and other contaminants 

also are contributed from 

groundwater to streams 

and coastal waters. 

See chapter 7.

If chemicals that enter groundwater are persistent (do not break down), 

they can remain in groundwater for many years after their source has 

been removed or reduced. For example, drinking-water suppliers in new 

suburban areas might encounter groundwater contaminated from activities 

that occurred years ago—such as with nitrate and pesticides from agricultural sources. Some contaminants whose use was banned decades ago, such 

as dieldrin—the pesticide most commonly detected in groundwater at a 

concentration of concern for human health—still are commonly detected in 

groundwater. See chapter 6.

Plain Surficial Aquifer System

Radium occurs commonly in groundwater as a result of the degradation of uranium and 

thorium minerals naturally present in aquifer sediments

3

Groundwater in the surficial aquifer system 

transports contaminants to surface waters

4

Chemicals in groundwater move slowly and can be detected 

in the environment for several decades after they enter the 

surficial aquifer system

5
1Chemicals from human-related sources were widely detected, but 

most groundwater produced for public and domestic water supply 

is suitable for drinking

Agricultural and urban activities are sources of many contaminants to the surficial aquifer 

system. The aquifer system is highly susceptible to human-related contamination in the 

northern part of the study area, in particular, because the soils and aquifer are composed of 

sandy, permeable material that allows contaminants to move readily from the land surface 

into the aquifer. Groundwater in these areas contains dissolved oxygen, which prevents 

many contaminants, such as nitrate, from degrading as they move through the aquifer. 

Nitrate was measured at concentrations attributed to human-related sources in water from 

more than one-half of the wells sampled and was measured above the U.S. Environmental 

Protection Agency Maximum Contaminant Level (MCL) of 10 milligrams per liter as 

nitrogen (mg/L as N) in 7 percent of drinking-water well samples. Pesticide and volatile 

organic compounds (VOCs) were detected in about one-third and one-half, respectively, of 

all groundwater samples although, with the exception of the pesticide dieldrin, concentrations rarely exceeded a human-health benchmark. Dieldrin, an insecticide commonly used 

to control termites until its use was banned in 1985, was measured at concentrations greater 

than its human-health benchmark of 0.002 microgram per liter (μg/L) in almost 20 percent 

of drinking-water wells sampled, most of which were in urban areas.

Where the surficial aquifer system is used for public water supply, the Safe Drinking 

Water Act (Public Law 93–523; 88 Stat. 1660) requires that contaminated groundwater 

that enters the distribution system be treated to reduce concentrations to below regulatory 

levels. Treatment, such as blending or ion exchange and, in some areas, drilling of deep 

wells to access groundwater that is less contaminated, increases the cost of water supplies. 

Millions of people in rural areas of the Northern Atlantic Coastal Plain rely on privately 

owned (domestic) wells that are screened in the surficial aquifer system for their drinking 

water. Routine testing of water from domestic wells is not required, and homeowners are 

responsible for maintaining and monitoring wells and for obtaining treatment to resolve any 

water-quality problems. As a result, there is a need for public education aimed at describing 

where contaminants are likely to occur and what testing and treatment options are available 

for domestic well owners.

2Concentrations of nitrate, one of the most widespread 

contaminants in groundwater, are related to nitrogen use and are 

controlled by reduction/oxidation processes

Nitrate was measured at levels of concern for human health in some samples, 

particularly in samples from oxic (containing dissolved oxygen) groundwater 

Overview of Major Findings and 

Water-Quality Issues for the Northern Atlantic Coastal 

New Jersey, New York, North Carolina, and Virginia

Photographs from top to bottom: USGS; Chris 

Sherwood; USGS; Chesapeake Bay Program 

4 Water Quality in the Northern Atlantic Coastal Plain Surficial Aquifer System, 1988–2009
Chapter 1

underlying agricultural areas. For example, concentrations of nitrate in shallow groundwater on the agricultural Delmarva Peninsula exceeded the MCL in about one-third of 

samples and from agricultural areas of southern New Jersey exceeded the MCL in almost 

two-thirds of samples. Oxic groundwater underlying urban areas also was contaminated 

with nitrate, but to a lesser degree than groundwater underlying agricultural areas. As 

long as water remains oxic, nitrate in shallow groundwater has the potential to move over 

time into deep parts of the surficial aquifer system that are used for drinking water. In 

groundwater without dissolved oxygen (anoxic), nitrate can be reduced to nitrogen gas 

through a process called denitrification, and in parts of the surficial aquifer system where 

groundwater was anoxic, such as coastal North Carolina, nitrate was rarely measured at 

concentrations greater than the MCL.

Nitrate is colorless and odorless in water and therefore its presence in groundwater is 

not obvious. Owners of domestic wells that withdraw drinking water from the surficial 

aquifer system might be unaware of the potential for nitrate contamination in their 

groundwater. This is a concern, particularly in areas with intensive agriculture and with 

well-drained soils and sandy aquifer sediments, which are conditions under which the 

highest concentrations of nitrate have been measured.

3 Radium from geologic sources is a potential human-health 

concern in drinking water

The sediments of the surficial aquifer system contain radium, which is of concern 

because ingesting radium is associated with an increased risk for cancer. Radium that is 

associated with aquifer sediments is released into groundwater if the water is acidic (pH 

less than 7), which generally is the case in the surficial aquifer system (median pH 5.3). 

As a consequence, radium concentrations in samples of surficial aquifer groundwater were 

among the highest in the Nation. Some land-use practices, such as application of nitrogencontaining fertilizers, can lower the pH of the already acidic groundwater and cause radium 

concentrations to increase. Radium exceeded the MCL of 5 picocuries per liter (pCi/L) 

in about one-third of samples from drinking-water wells; the MCL was exceeded most 

commonly in areas where the pH was particularly low, such as in southern New Jersey. 

Analysis of groundwater for radium was limited mostly to the northern part of the study 

area, but because groundwater is acidic throughout the surficial aquifer system, radium is 

likely to occur in unmonitored areas as well.

Basic, inexpensive geochemical measurements of water, such as pH, can help determine 

where water-supply managers and homeowners might consider testing groundwater for 

radium. Treatment or blending of water can reduce concentrations to less than the MCL, 

but these remedies are costly. Understanding the factors that affect radium in groundwater 

can help localities prioritize areas for new groundwater development and reduce treatment 

costs. The potential for radium to occur in water from domestic wells where water testing 

is not always required emphasizes the need for public education aimed at describing where 

radium is likely to occur.

Implications

Plain Surficial Aquifer System, Delaware, Maryland, 

Photographs from top to bottom: USGS; 

USGS; USGS; Ron MacArthur, Cape Gazette

Chapter 1: Overview of Major Findings and Implications 5
4Groundwater provides the majority of streamflow in the Northern 

Atlantic Coastal Plain and is a substantial source of nitrogen in 

streams and estuaries

Ecological problems such as algal blooms and low dissolved oxygen that are caused 

in part by excessive nitrogen are common in estuaries along the coast of the Northern 

Atlantic Coastal Plain, including Chesapeake and Delaware Bays and Albemarle, Pamlico, 

and Long Island Sounds. Much of that nitrogen is delivered to the estuaries by the streams 

that flow into them, and the source of as much as 95 percent of the water in streams that 

originate in the Coastal Plain is groundwater that seeps up through the streambeds. Nitrogen 

in groundwater discharged to streams is mostly in the form of nitrate. Concentrations of 

nitrate in streams in the Northern Atlantic Coastal Plain range from less than the detection 

limit (0.05 mg/L as N) to more than 12 mg/L as N and often exceed the U.S. Environmental 

Protection Agency (USEPA) ecoregion nutrient criterion of 0.71 mg/L as N for these 

streams. The amount of nitrogen contributed by groundwater to streams depends on local 

hydrogeology and land use. Groundwater contributes more than 70 percent of the total 

nitrogen to headwater streams that flow to the Chesapeake Bay from the Eastern Shore of 

Maryland. In contrast, in headwater streams in the Coastal Plain of the Chesapeake Bay 

Western Shore in Maryland, groundwater contributes only 5 percent of the total nitrogen.

Decreasing inputs of nitrate to groundwater should aid management practices aimed 

at improving the quality of water in streams. However, because of the slow movement of 

groundwater, effects of these reductions on surface-water quality will not be immediate. It 

will likely take years to decades before reductions of nitrate concentrations in groundwater 

result in substantial water-quality improvements in streams and estuaries in this area.

5Past land- and chemical-use practices have left a legacy of 

groundwater contamination

Chemicals in groundwater move slowly through the aquifer. If the chemicals are persistent, they can degrade the quality of water withdrawn by drinking-water wells or discharged 

into streams several decades after the chemicals entered the aquifer. An increase in nitrate 

concentrations in groundwater on the Delmarva Peninsula of 2 mg/L as N between 1988 and 

2001 corresponds to increases in fertilizer applications that began in the 1960s. Even though 

fertilizer applications have stabilized, concentrations of nitrate in streams on the Delmarva 

Peninsula are continuing to increase as older groundwater with increased concentrations of 

nitrate reaches streams. Conversely, an increase in concentrations of nitrate in groundwater 

and streams on Long Island during the first part of the 20th century was gradually reversed 

after installation of sewers beginning in the 1950s.

Some contaminants whose use was banned decades ago still are commonly detected in 

groundwater. Dieldrin is the pesticide most frequently detected in groundwater at a concentration of concern for human health, even though its use was banned more than 30 years ago. 

Similarly, although applications of the pesticide aldicarb were discontinued on Long Island 

after 1979, aldicarb degradation products remained detectable in groundwater until well into 

the 1990s.

Groundwater remediation is costly and slow, so management practices that prevent the 

release of contaminants are likely to be the most effective way to improve or maintain the 

quality of groundwater. Because of the slow movement of groundwater, drinking-water 

suppliers in new suburban areas might encounter groundwater contaminated with chemicals—such as with nitrate and pesticides—that are associated with agricultural activities that 

occurred years ago. Continued groundwater monitoring coupled with models of groundwater 

flow and geochemistry will assist in determining when legacy contamination has diminished 

to acceptable concentrations.

Photographs from top to bottom: 

Chesapeake Bay Program; USGS; USGS; 

USGS

6 Water Quality in the Northern Atlantic Coastal Plain Surficial Aquifer System, 1988–2009
Chapter 2

Groundwater studies conducted by NAWQA in the surficial aquifer 

system of the Northern Atlantic Coastal Plain were designed to answer 

broad questions about the occurrence, fate, and transport of contaminants in aquifers used as sources of drinking water. Water from installed monitoring wells, existing domestic wells, and public-supply wells was sampled to 

characterize the quality of water in the surficial aquifer system.

Photograph from Delaware Geological Survey

Chapter 2: NAWQA Approach to Assessing 

Groundwater Quality

Drillers installing a monitoring well.

This chapter summarizes 

the study design used to 

investigate water quality 

in the Northern Atlantic 

Coastal Plain surficial 

aquifer system.
Understanding study results

Important aspects of the NAWQA Principal Aquifer assessments:

•	 Water samples were collected at the wellhead (see sidebar, Anatomy of a well, p. 14) prior to any treatment. They represent 

the quality of the groundwater resource but not necessarily the quality of tap water.

•	 The focus of the assessments is the condition of the total resource, including groundwater in a wide range of hydrologic 

and land-use settings across the Nation, rather than conditions at specific sites with known water-quality concerns.

•	 The assessments are guided by a nationally consistent study design, and all assessments use the same methods of 

sampling and analysis. Findings apply to water quality of a particular aquifer but also contribute to the larger picture of how 

and why water quality varies regionally and nationally. This consistent approach helps to determine if a water-quality issue 

is isolated or widespread. (See http://water.usgs.gov/nawqa/about.html for more information.)

•	 The assessments focused on aquifers used for water supply or on shallow groundwater that underlies an area with 

a particular type of land use. Because the NAWQA groundwater study areas do not cover the full spatial extent of 

the targeted Principal Aquifer, the findings may not represent the effects of the full range of geology, climate, and 

land use present.

•	 Analytical methods used by USGS chemists for assessments of water quality in Principal Aquifers are designed to measure 

constituents at as low a concentration as possible. As a result, constituents frequently are detected at concentrations far 

below human-health benchmarks for drinking water (see sidebar, Human-health benchmarks and other guidelines used 

in this assessment, p. 34). Low-level detections allow scientists to identify and evaluate emerging issues and to track 

contaminant levels over time.

Assessing Water Quality in the Surficial Aquifer 

System of the Northern Atlantic Coastal Plain

How does one go about characterizing the groundwater 

over an area as large as that covered by the surficial aquifer 

system of the Northern Atlantic Coastal Plain, let alone the 

whole United States? The approach the U.S. Geological 

Survey (USGS) has taken is to study selected parts of these 

aquifers to gain a better understanding of how and why water 

quality varies (see sidebar, Understanding study results, 

this page).(1) These groundwater studies are the building blocks 

of NAWQA’s water-quality assessments of Principal Aquifers 

(see sidebar, NAWQA assessments use a wide range of 

geochemical data and site information, p. 9). There were many 

studies in each Principal Aquifer, each with a different focus 

on information needs about and the natural and human-related 

factors that influence groundwater quality (see appendix 1, 

fig. A1–1 and table A1–1). Groundwater studies that were 

designed to broadly assess water-quality conditions in parts 

of aquifers used for drinking-water supply sampled networks 

of randomly located monitoring wells and pumping wells 

(see sidebar, What types of wells were sampled, and how 

might that affect water quality?, p. 11). Agricultural and urban 

land-use studies were designed to characterize and explain the 

quality of recently recharged groundwater in these two landuse settings by sampling mostly shallow domestic wells and 

shallow monitoring (nonpumping) wells that were installed 

as part of the NAWQA Program. Other NAWQA studies in 

the Northern Atlantic Coastal Plain focused on public water 

supplies or groundwater quality along individual flow paths.

8 Water Quality in the Northern Atlantic Coastal Plain Surficial Aquifer System, 1988–2009
Chapter 2

NAWQA assessments use a wide range of geochemical data and site information

Constituents measured in samples from most wells

Constituent group Examples

Water-quality properties pH, specific conductance, dissolved 

oxygen, temperature

Major ions (filtered) Bromide, calcium, chloride, 

magnesium, sodium, sulfate

Trace elements (filtered) Arsenic, boron, iron, manganese, 

selenium, uranium

Nutrients (filtered) Ammonia, nitrate, phosphorus

Pesticides (filtered) Herbicides, insecticides, fungicides

Volatile organic compounds Solvents, gasoline hydrocarbons, 

refrigerants, trihalomethanes, 

fumigants

Organic carbon (filtered)

Additional constituents measured in samples from some wells

Constituent group Examples

Radionuclides Radon

Groundwater age tracers Tritium, chlorofluorocarbons

Stable isotopes Oxygen-18, hydrogen-2

Microorganisms Escherichia coli and total coliforms

Additional site information

Use of well

Well depth

Depth to water

Well-construction data

Principal Aquifer

Land-surface elevation at well

Land use within a 500-meter

(1,640-foot) radius buffer

Estimates of nutrient inputs

Estimates of pesticide use

Chemists at the USGS National Water Quality Laboratory analyze 

groundwater samples using an array of sophisticated techniques.

Additional information (often called “ancillary information”) about 

the well and the surrounding environment complements the 

chemical data measured. This additional information often is key to 

making sense of the chemical data. For example, the information 

might be used to determine that shallow groundwater is more (or 

less) vulnerable to contamination than deep groundwater, that 

domestic wells are more (or less) vulnerable to contamination 

than public-supply wells, or that urban land use is associated with 

different types of groundwater contamination than is agricultural 

land use. Chemical data without accompanying ancillary data 

are much less useful for understanding factors that affect 

groundwater quality. 
Fall Line

PENNSYLVANIA

VIRGINIA

MARYLAND

DELAWARE

NEW 

JERSEY

WEST 

VIRGINIA

NEW 

YORK

CONNECTICUT

NORTH CAROLINA

SOUTH

CAROLINA

EXPLANATION

Well type

Drinking-water study

Drinking-water study in 

 Castle Hayne aquifer

Agricultural land-use study

Urban land-use study

Special study of public 

 supplies

Other studies of surficial 

 aquifer

ATLANTIC OCEAN

Washington, DC

N

Area of enlargement

This Principal Aquifer assessment brings together and 

interprets results from all the NAWQA groundwater studies in 

the Northern Atlantic Coastal Plain (see appendix 1, fig. A1–1 

and table A1–1). Groundwater studies were designed to 

answer the following questions: How does land use affect 

groundwater quality? How does water quality change as it 

moves through an aquifer? and What is the quality of the 

drinking-water resource? The primary focus of these studies 

was the unconfined surficial aquifer system where contamination from human activities is most likely to have an effect on 

water quality (see sidebar, What is a contaminant?, p. 13). 

Most of the samples collected were from wells screened in the 

unconsolidated sedimentary deposits of the Northern Atlantic 

Coastal Plain; samples from one network of drinking-water 

wells in North Carolina came from the Castle Hayne aquifer, 

which is composed of carbonate rocks (fig. 2–1; see sidebar, 

Aquifer, p. 14). The wells are distributed across much of the 

area overlying the aquifers, but because the wells are located 

only in NAWQA groundwater study areas, some parts of the 

surficial aquifer system are not represented.

Figure 2–1. The types of wells 

sampled by the National WaterQuality Assessment (NAWQA) 

Program in the surficial aquifer 

system of the Northern Atlantic 

Coastal Plain include drinkingwater wells and wells monitoring 

shallow groundwater underlying 

agricultural and urban land.

10 Water Quality in the Northern Atlantic Coastal Plain Surficial Aquifer System, 1988–2009
Chapter 2

A

B

What types of wells were sampled, and how might that affect 

water quality?

Two types of wells that supply drinking water were sampled: domestic (private) wells and 

public-supply wells. Domestic wells typically are shallower than public-supply wells and therefore 

pump water that is nearer to sources of manmade contaminants, such as fertilizers and pesticides, 

at the land surface than the deeper, public-supply wells. Domestic wells commonly are in rural 

areas so are more likely than public-supply wells to be vulnerable to contamination from agricultural 

chemicals. Routine testing of water from domestic wells is not required, and homeowners are 

responsible for testing, maintenance, and treatment of the water from their domestic wells. 

Public-supply wells pump water from deep in the surficial aquifer system and commonly are in 

suburban and urban areas so are more likely than domestic wells to be vulnerable to contamination 

from chemicals associated with urban activities. Public-supply wells have larger pumps and longer 

screened intervals than domestic wells and are pumped for longer periods of time. As a result, 

public-supply wells pump much larger volumes of water than domestic wells and so have much 

larger capture zones.(2) Water that is provided to consumers from public-supply wells is required 

to be tested on a routine basis to help assure that the water meets Federal and State waterquality standards.

More than half of the groundwater samples collected from the surficial aquifer system for 

this study were from monitoring wells. Monitoring wells are used for measuring water levels or 

collecting water samples, and those sampled in the NAWQA groundwater studies were installed 

expressly for that purpose. Monitoring wells are not used for drinking water, irrigation, or 

other purposes.

A, USGS setting up to sample a monitoring well, and B, a domestic well.

Chapter 2: NAWQA Approach to Assessing Groundwater Quality 11
Saltwater

Shallow

monitoring wells

Shallow

monitoring wells

Water

table

Confined aquifers

Public

supply

wells

Coastal estuary

Freshwater-saltwater interface

Surficial aquifer system 

(unconfined, freshwater) 

Public-supply well in semiconfined aquifer

Confining units

Atlantic Ocean

Domestic well

2 block

NOT TO SCALE

Figure 2–2. Different types of wells tap groundwater from different parts of the surficial aquifer system. Most monitoring wells were screened 

near the water table and tap groundwater whose quality is affected by the overlying land use. Drinking-water wells generally tap deeper water 

than do monitoring wells, and of those, public-supply wells generally tap deeper groundwater than do domestic-supply wells. 

Throughout this assessment, the results are grouped as 

those that characterize the groundwater used as a drinkingwater supply and those that characterize the quality of groundwater that has recently recharged in either an agricultural or an 

urban land-use setting (fig. 2–2; table 2–1). The groundwater 

used for drinking generally is from relatively deep wells; this 

water recharged the aquifer many years ago and represents 

the effects of past land-use practices on water quality. The 

groundwater used for land-use studies is generally from 

shallow wells. Results of the land-use studies are of interest 

because the information can be used to evaluate the effect of 

recent human activities on shallow groundwater quality and 

can provide an early warning of issues that might adversely 

affect deep drinking-water resources in the future. Land-use 

studies also can provide an indication of recent changes or 

improvements in water quality related to changes in chemical 

use. About 46 percent of the wells sampled were domestic 

drinking-water wells, about 43 percent were monitoring wells, 

and less than 10 percent were public water supply and other 

well types. The wells sampled ranged in depth from 5 to 

612 feet (ft) and most were in unconfined parts of the surficial 

aquifer system; some of the deepest wells, in particular eight 

wells with depths greater than 200 ft, were in the confined part 

of the system (table 2–1).

12 Water Quality in the Northern Atlantic Coastal Plain Surficial Aquifer System, 1988–2009
Chapter 2

Table 2–1. The goals of different studies determined the types of wells sampled in the surficial aquifer system. Drinking-water studies 

sampled wells screened at depths that represent the part of the aquifer used for drinking water, whereas land-use studies sampled 

wells screened at more shallow depths (just below the water table) that represent the part of the aquifer that has received recent 

recharge through a particular land use.

Characteristic Drinking-water studies Agricultural land-use studies Urban land-use studies

Predominant land use Mixed Agricultural Residential and commercial

Number of wells 149 101 95

Types of wells Mostly domestic, some public 

supply and monitoring

Mostly monitoring, some 

domestic Monitoring

Median well depth (range) 80 (20–612) feet 22 (6–288) feet 40 (10–96) feet

Contaminants have a wide range of sources, both manmade and geologic. Most organic chemicals in groundwater that are of 

concern for human health are manmade. In contrast, most inorganic constituents in groundwater have geologic or other natural 

sources, although their concentrations in groundwater may be altered by human activities, such as irrigation and groundwater 

pumping. Some contaminants have both manmade and natural sources. For example, nitrate in groundwater has many natural 

sources, but nitrate concentrations in groundwater underlying agricultural and urban areas commonly are higher than in other areas 

because of contributions from sources associated with human activities.

But what exactly is a contaminant? The word means different things to different people. For example, a contaminant is defined 

by the Safe Drinking Water Act (SDWA) as “any physical, chemical, biological, or radiological substance or matter in water” 

(see http://www.epw.senate.gov/sdwa.pdf). This broad definition of contaminant includes every substance that may be found 

dissolved or suspended in water—everything but the water molecule itself. This is not a very practical definition because this would 

imply that all water is “contaminated.” Pure water that has nothing dissolved in it does not occur naturally—not even rainfall is pure 

water, because it contains, at a minimum, some dissolved gases.

The U.S. Environmental Protection Agency defines a contaminant as “Any physical, chemical, biological, or radiological 

substance or matter that has an adverse effect on air, water, or soil” (see http://epa.gov/region04/superfund/qfinder/glossary.html). 

This definition is more practical and allows both manmade constituents and those with geologic sources in water to be defined 

as contaminants. However, it does not define what “adverse” means, and what may be adverse in one way might be beneficial 

in another. In this circular, a contaminant is defined as any physical, chemical, biological, or radiological substance or matter in 

groundwater that is manmade or that impairs the use of water for its intended purpose. Impairment is determined by comparing a 

measured concentration to benchmarks or guidelines. By this definition, all manmade compounds, such as pesticides and volatile 

organic compounds, are contaminants because they do not occur naturally in groundwater. If a constituent with a geologic source, 

such as arsenic, occurs in drinking water at a concentration above its human-health benchmark, it also is considered a contaminant. 

What is a contaminant?

Chapter 2: NAWQA Approach to Assessing Groundwater Quality 13
Aquifer (aq.ui.fer)—  ̆

When pumped, groundwater flows through the 

screened interval and 

up and out of the well

Unsaturated

Saturated

Water table

Screen

Well bore

Water level

in well

Wellhead

Well casing

Pump

Grout

Gravel pack

Confining layer

Confined aquifer

Unconfined aquifer

Unsaturated zone

Water table Wells

Surface water

Screen

An underground layer of saturated permeable materials (rock, gravel, sand, or silt) that will yield a 

useful quantity of water to a well.

Anatomy of a well

A well is simply a hole in the ground (well bore or borehole)

from which water can be removed. The well bore is lined with 

a well casing, such as a pipe, to prevent the well bore from 

collapsing. The casing, along with a sealant (called grout), also 

prevents water from flowing into the well from the land surface 

or from 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, to allow 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. Some wells 

are cased only near the land surface, allowing water to flow into 

the well from nearly the entire length of the well bore.

The unsaturated zone is the area below the land 

surface and above an aquifer. In addition to soil, 

rocks, and air, the unsaturated zone contains water 

from the land surface (such as rain) that is slowly 

moving downward to the water table of the aquifer.

An unconfined aquifer is bounded at its top by the 

water table, below which water fills all the pore 

spaces in the rock. Water from the land surface can 

move down into an unconfined aquifer.

A confining layer is a layer of material (often clay) 

through which water does not easily flow, creating a 

boundary between aquifers.

A confined aquifer is bounded at its top by a confining 

layer. Water enters or “recharges” confined aquifers 

where the confining layer is not present. Where the 

confining layer is not continuous or is breached (for 

example, by a well), flow between the unconfined and 

confined aquifer can occur.

The pressure within a confined aquifer can be 

greater than that in the overlying unconfined aquifer 

if the source of the water in the confined aquifer is 

at a higher elevation than the unconfined aquifer. 

In that case, water in a well in a confined aquifer 

will rise to a higher level than that in the overlying 

unconfined aquifer.

Sedimentary aquifer 

Groundwater storage and flow 

between grains of sediment

Bedrock aquifer

Groundwater storage

and flow in fractures

Carbonate aquifer

Groundwater storage and flow

in solution cavities or fractures

14 Water Quality in the Northern Atlantic Coastal Plain Surficial Aquifer System, 1988–2009
Chapter 2

Drinking-Water Studies

To assess water-quality conditions of the groundwater 

used as a drinking-water resource, one sample was collected 

from each of 149 randomly selected existing pumping wells or 

deep monitoring wells. In addition, samples from 45 publicsupply wells were collected in New Jersey and Delaware for 

special studies that focused on the quality of public drinking 

water in those areas (fig. 2–1).

Land-Use Studies

To assess the quality of recently recharged groundwater 

(generally less than 10 years old)(1) underlying agricultural 

and urban areas, groundwater samples were collected from 

existing shallow domestic wells or shallow monitoring 

wells in those land-use settings (fig. 2–1; table 2–1). The 

agricultural setting of the Northern Atlantic Coastal Plain 

includes areas with a variety of vegetables and row crops such 

as corn, soybeans, and small grains. Samples were collected 

in agricultural areas from southern New Jersey through North 

Carolina. The urban land-use setting of the Northern Atlantic 

Coastal Plain is similar to that of the Nation as a whole—

commercial and single-family and multifamily residential 

development. For this study, samples were collected from 

three urban areas: Long Island, southern New Jersey, and 

Virginia Beach, Virginia. The median depth of wells sampled 

for agricultural studies was 22 ft, and the median depth of 

wells sampled for urban studies was 35 ft. The difference in 

depths between wells from areas with agricultural land use 

and those from urban areas reflects a difference in the depth to 

the water table from south to north. The water quality of these 

samples is an indication of how the chemicals—nitrogen, 

pesticides, solvents, gasoline—used for crop production and 

in day-to-day life might affect the quality of a future drinkingwater resource.

Top photograph from the Chesapeake Bay Program; bottom photograph by Mark Nardi, USGS

The chemicals used in agricultural and urban or suburban areas 

that enter shallow groundwater might affect the quality of a future 

drinking-water resource in deep parts of the surficial aquifer system.

Chapter 2: NAWQA Approach to Assessing Groundwater Quality 15

T

Chapter 3

he surficial aquifer system is unconfined, has a shallow water table, and 

typically is composed of sandy sediments. Abundant rainfall provides 

aquifer recharge, but can also carry chemicals from the land surface 

to the water table. Sources of potential chemical contamination from human 

activities are widespread—there are heavily urbanized areas in the north and 

areas of intensive agriculture in the south. The surficial aquifer system is a 

major source of public drinking-water supply in heavily populated Long Island 

and southern New Jersey. It also is an important source of drinking water for 

residents of many small towns and rural households with domestic wells. The 

surficial aquifer system provides as much as 95 percent of streamflow that 

originates in the Northern Atlantic Coastal Plain and recharges underlying 

confined aquifers.

Chapter 3: Environmental Setting 

and Hydrogeology 

Photograph by Delaware Geological Survey

Flooded marshland behind Primehook Beach, Delaware.

This chapter summarizes 

background information 

for the Northern Atlantic 

Coastal Plain surficial 

aquifer system and 

provides the context for 

understanding findings 

about water quality in 

this Principal Aquifer. 

The chapter covers 

the environmental and 

hydrogeologic settings, 

including information on 

population, land use, and 

water use.
VIRGINIA

MARYLAND

WEST

VIRGINIA

SOUTH CAROLINA

CONNECTICUT

NEW

JERSEY

NORTH CAROLINA

Washington, D.C.

Chesapeake Bay

NEW YORK

DELAWARE

Long 

Island Sound

Delaware Bay

Albemarle Sound

Pamlico Sound

ATLANTIC OCEAN

 Fall Line

Richmond

Baltimore

Philadelphia

PENNSYLVANIA

New York City

N

Area of enlargement

732

0

EXPLANATION

Elevation, in feet above

 mean sea level

circ1353_ch03fi1 explanation

Figure 3–1. The Northern 

Atlantic Coastal Plain aquifer 

system underlies about 

50,000 square miles in six States. 

Land-surface elevation ranges 

from sea level along the coast to 

more than 700 ft above sea level 

along the Fall Line.

Environmental Setting

The Northern Atlantic Coastal Plain covers about 

50,000 square miles in six States and extends from the 

southern border of North Carolina northward through Long 

Island (fig. 3–1). The study area is bounded on the east by the 

Atlantic Ocean and on the west by the Fall Line (see sidebar, 

What is the Fall Line?, p. 19) and older rocks of the Piedmont 

Physiographic Province. The Coastal Plain sediments form 

a wedge beginning at the Fall Line and the northern edge 

of Long Island, thickening to as much as 14,000 ft along 

the Atlantic coastline.(3–5) The land surface generally slopes 

seaward from elevations greater than 700 ft at the Fall Line 

to sea level along the coast of the Atlantic Ocean and major 

estuaries like Chesapeake and Delaware Bays and Albemarle, 

Long Island, and Pamlico Sounds. Topography is generally flat 

to gently rolling, although the region contains some areas of 

moderate topographic relief.

18 Water Quality in the Northern Atlantic Coastal Plain Surficial Aquifer System, 1988–2009
Chapter 3

circ1353 chapter03_fig 02

Photograph from the Chesapeake Bay Program

Evaporation 

and transpiration

Groundwater recharge

Where rainfall goes

Overland runoff to streams

32%

17%

51%

8 inches

15 inches

25 inches

Figure 3–2. About one-half of the precipitation that falls in the Northern Atlantic Coastal Plain evaporates or is transpired by plants, 

about one-third recharges the shallow groundwater system, and the remainder runs off of the land surface directly to streams.

The Northern Atlantic Coastal Plain has a humid and 

temperate to subtropical climate. Average annual precipitation is abundant, from about 40 inches near Washington, 

D.C., to more than 56 inches in eastern North Carolina.(6) 

About one-third of this water recharges the surficial aquifer 

system(7) (fig. 3–2). Average annual air temperatures range 

from about 62 degrees Fahrenheit (°F) in the south to about 

54 °F in the north. Precipitation is typically distributed fairly 

evenly throughout the year, although rainfall is often most 

intense during tropical weather systems and thunderstorms 

during the warm months.

What is the Fall Line?

The early exploration of the major rivers of the eastern United 

States was impeded by rapids and waterfalls along a low eastfacing cliff that parallels the Atlantic coastline from New Jersey 

through the Carolinas. These waterfalls and rapids form what is 

referred to as the Fall Line and occur where the older consolidated 

rocks of the Piedmont Physiographic Province emerge from 

beneath the sediments of the Coastal Plain Physiographic 

Province. The potential for developing water-wheel-powered mills 

and other industries and the limitation of upstream river travel 

at the Fall Line helped to guide the location of major cities such 

as Baltimore, Maryland; Philadelphia, Pennsylvania; Richmond, 

Virginia; and Washington, D.C.

Photograph by Gary Fleming, Virginia Department of Conservation and Recreation

Chapter 3: Environmental Setting and Hydrogeology 19
Circ1353_ch03 fig03

Public-supply wells

Surficial aquifer system 

(unconfined, freshwater) Saltwater

Water

table

Confined aquifers

Coastal estuary

Freshwater-saltwater interface

Public-supply well in semiconfined aquifer

Confining units

Atlantic Ocean

Shallow

monitoring wells

Shallow

monitoring wells

Domestic well

NOT TO SCALE

Figure 3–3. The surficial aquifer system of the Northern Atlantic Coastal Plain is under water-table conditions (unconfined) and therefore 

is vulnerable to contamination from chemicals that are applied to the land surface and carried into groundwater with recharging rainfall. 

Groundwater in the surficial aquifer system moves along flow paths from the water table in upland recharge areas to local streams and 

estuaries. Regional groundwater flow predominantly is from west to east, except where pumping has altered natural flow directions. The 

surficial aquifer system is underlain by a series of confined aquifers that are largely protected by overlying fine-textured and impermeable 

confining units from contamination by chemicals from human sources.

Hydrogeologic Setting

The surficial aquifer system is made up of unconfined 

parts of the many aquifers that are just below the land surface 

in much of the Northern Atlantic Coastal Plain (see sidebar, 

The surficial aquifer system includes many different local 

unconfined aquifers, p. 21). These aquifers extend from the 

water table down to the first fine-grained sediment layer 

(confining unit) that obstructs the downward movement of 

water (fig. 3–3). The water table is near the land surface in 

flat areas near streams and estuaries, but can be more than 

30 ft deep beneath upland areas with hills and deep stream 

valleys,(8) and is more than 200 ft deep beneath parts of 

Long Island.(3) The relatively coarse and permeable sediments 

that compose the surficial aquifer system mostly are less than 

100 ft thick, but in some areas are much thicker, particularly 

on Long Island and in southern New Jersey (500 to 800 ft).(9)

Most of these sediments were deposited by flowing streams 

and rivers or in tidal waters; unconsolidated glacial deposits 

cover these sediments and form the surface of the surficial 

aquifer system on Long Island. Although predominantly 

sandy, the texture of the sediments of the surficial aquifer 

system includes coarse-textured sand and gravel in some 

areas and fine-textured silt and clay in other areas.(10) A part 

of the surficial aquifer system in the southern part of North 

Carolina, called the Castle Hayne aquifer, is composed of 

consolidated limestone.

The surficial aquifer system of the Northern Atlantic 

Coastal Plain is under water-table conditions (unconfined) and 

therefore is vulnerable to contamination from chemicals that 

20 Water Quality in the Northern Atlantic Coastal Plain Surficial Aquifer System, 1988–2009
Chapter 3

Thick beds of coarse-textured sands and gravels, like those shown 

here, that were deposited by rivers and streams, make up some of the 

most productive parts of the surficial aquifer system and are widely 

used for drinking-water supply. Large amounts of water can be stored 

between the sediment grains and withdrawn by wells that tap these 

deposits. Because these sediments are generally buried, geologists 

take advantage of exposures in sand pits and other excavations to 

study them.

Photograph from Delaware Geological Survey

are applied to the land surface and carried into groundwater 

with recharging rainfall. Groundwater in the surficial aquifer 

system moves along flow paths from the water table in 

upland recharge areas to local streams and estuaries. Regional 

groundwater flow predominantly is from west to east, except 

where pumping has altered the natural direction of flow. The 

surficial aquifer system is underlain by a series of confined 

aquifers that are largely protected by overlying fine-textured 

and impermeable confining units from contamination by 

chemicals from human sources.

Groundwater in the surficial aquifer system is an important source of domestic and (where the aquifer is particularly 

thick) public water supplies in many areas and is the major 

source of freshwater flow to streams. Most groundwater in 

the surficial aquifer system flows along relatively short flow 

paths to discharge to local streams and tidal waters (fig. 3–3); 

groundwater discharge provides 40 to 95 percent of flow in 

Coastal Plain streams.(7, 12) About 2 percent of infiltrating 

groundwater recharges deeper confined aquifers and about 

4 percent is withdrawn by wells.(7)

The surficial aquifer system is underlain by a series of 

alternating confined aquifers and confining units (fig. 3–3). 

In some areas, a part of an otherwise confined aquifer is in 

direct contact with the overlying unconfined surficial aquifer 

sediments and is considered to be part of the surficial aquifer 

system. The confined aquifers are composed of unconsolidated 

to semiconsolidated sand, gravel, or shell material. Confining 

units are composed of fine-textured sediments such as silt 

and clay that are relatively impermeable and do not easily 

transmit water.

On Long Island, the surficial aquifer includes the upper 

glacial aquifer and underlying Magothy aquifer. In New Jersey, 

the outcrop of the Potomac-Raritan-Magothy aquifer system 

near the Fall Line and the Kirkwood-Cohansey aquifer system to 

the east are the principal surficial aquifers. The surficial aquifer 

on the Delmarva Peninsula is commonly called the Columbia 

aquifer and includes the subcrop areas of many otherwise 

confined aquifers, such as aquifers of the Chesapeake Group. In 

Virginia and North Carolina where the sandy surficial sediments 

are generally thinner than to the north, the surficial aquifer is 

unnamed. In Virginia, sediments of the surficial aquifer are in 

hydraulic connection with the Yorktown-Eastover aquifer in many 

parts of the eastern Coastal Plain, and in North Carolina, the 

surficial aquifer system also includes the Castle Hayne aquifer 

where it subcrops beneath the surficial sands in the southeastern 

part of the Coastal Plain. For more information about the aquifers 

of the Northern Atlantic Coastal Plain, see Trapp and Meisler.(6)

The surficial aquifer system includes many different local unconfined aquifers

Chapter 3: Environmental Setting and Hydrogeology 21
circ1353_ch3-fig 04

Fall Line

Delmarva 

Peninsula

DELAWARE

EXPLANATION

Agriculture

Urban

Forest and shrub

Wetlands

Delaware Bay

Albemarle Sound

Pamlico Sound

Long Island Sound

ATLANTIC OCEAN

CONNECTICUT

NEW

JERSEY

NEW

YORK

PENNSYLVANIA

NORTH CAROLINA

MARYLAND

WEST VIRGINIA

VIRGINIA

SOUTH 

CAROLINA

Chesapeake Bay

New York

Philadelphia

Baltimore

Washington, D.C.

Richmond

Virginia Beach

Raleigh

Area of enlargement

N

Photographs from top to bottom: Courtesy of the 

Chesapeake Bay Program; Jane Thomas, IAN, UMES;

IAN, UMES; USGS

Figure 3–4. Diverse human activities across the Northern Atlantic Coastal Plain can affect groundwater 

quality. Intensive agriculture is common on the Delmarva Peninsula and in parts of New Jersey and 

North Carolina. The urban corridor along the Fall Line from the area of Washington, D.C., to the north is 

densely populated.

Land Use and Population

How land is used is important from a water-quality 

standpoint because some land uses—agricultural and 

urban, for example—are accompanied by use of manmade 

chemicals. Much of the Northern Atlantic Coastal Plain is 

forested, but intensive agriculture is common in some areas, 

particularly on the Delmarva Peninsula and in parts of North 

Carolina (fig. 3–4). This region also includes some of the 

most densely populated urban areas of the Nation, supporting 

a growing population of more than 21 million (fig. 3–5). The 

greatest density of development is congregated along the Fall 

Line, near major cities such as Baltimore, New York, Philadelphia, and Washington, D.C. A steadily increasing population 

requires more and more water resources for drinking-water 

supplies and other uses.

22 Water Quality in the Northern Atlantic Coastal Plain Surficial Aquifer System, 1988–2009
Chapter 3

1970 1975 1980 1985 1990 1995 2000 2005

Year

Population in the Northern Atlantic Coastal Plain

Population, in millions

22

21

20

19

18

17

16

circ1353ch03_fig 06

1,000

Groundwater withdrawals, in millions of gallons per day

900

800

700

600

500

400

300

200

100

0

Irrigation

(2000)

Public supply

(2005)

Self-supplied

industrial

(2000)

Self-supplied

domestic

(2005)

Groundwater withdrawals

Figure 3–5. With a population of more than 21 million people (in 2005), the Northern Atlantic Coastal Plain 

region is one of the most populous parts of the Nation. A steadily increasing population requires more and 

more water resources for drinking-water supplies and other uses.

Figure 3–6. Public water supply is the dominant use of 

groundwater from the Northern Atlantic Coastal Plain region. Photograph used with permission from Ron MacArthur, Cape Gazette© 2003

Water Use

The availability of reliable and sustainable sources of 

water for public water consumption, industry, agriculture, and 

natural ecosystems is critical to an area of diverse land uses 

and expanding population like the Northern Atlantic Coastal 

Plain. About half of the population relies on groundwater for 

drinking water, and the remainder relies on surface water. The 

main use of groundwater is for public drinking-water supply, 

but groundwater also is used for domestic supply, irrigation, 

and industrial purposes (fig. 3–6). In areas where the surficial 

aquifer system is thick, such as on Long Island and in southern 

New Jersey, it is a major source of groundwater used for 

drinking water. In areas where the surficial aquifer system 

is thin and is not sufficient for public drinking-water supply, 

underlying confined aquifers are more frequently used for 

public water supply. Public water supply is the dominant 

use of groundwater from the Northern Atlantic Coastal 

Plain region.

About 800 million gallons per day was withdrawn from 

unconfined and confined aquifers in the Northern Atlantic 

Coastal Plain to meet the demands of public water supply for 

about 5 million people in 2005 (Jack Monti, U.S. Geological 

Survey, written commun., 2012; see sidebar, The Northern 

Atlantic Coastal Plain aquifer at a glance, p. 24).(13) On Long 

Island, nearly 3 million people use about 260 million gallons 

of water each day from the unconfined and interconnected 

glacial and Magothy aquifers, which are part of the surficial 

aquifer system, making this aquifer the largest single source 

Chapter 3: Environmental Setting and Hydrogeology 23
40,000,000,000 Gallons per day on average that recharge the aquifer system—

95 percent of which discharges to surface waters

1,200,000,000 Gallons per day pumped for all uses

965,000,000 Gallons per day used for public and domestic water supply

260,000,000 Gallons per day used for public water supply from the surficial 

aquifer system on Long Island alone

150,000,000 Gallons per day used for domestic water supply from private wells

15,000,000 Number of people using groundwater

50–100 Gallons per day used by each person in a household

e Northern Atlantic Coastal Plain aquifer system at a glance

Photograph by Chris Isherwood

of drinking water in the region.(13) Cities near the Atlantic 

coast typically rely primarily on confined aquifers to avoid 

problems with saltwater intrusion, which can occur in 

the surficial aquifer system where extensive groundwater 

pumping has drawn in saline water from nearby coastal areas. 

When saltwater is detected in drinking-water supplies, wells 

commonly are drilled deeper or moved inland. When these 

solutions are not available, water can be imported (as is done 

in Virginia Beach) or brackish water can be desalinated (as is 

done in Cape May, N.J.), but these solutions come with high 

costs, which typically are passed on to the consumer.

More than 2 million people in the Northern Atlantic 

Coastal Plain rely on domestic wells for drinking-water 

supplies. The surficial aquifer system is an important source 

of domestic water supply in urbanized areas of New Jersey 

and eastern Long Island. Groundwater is the sole source of 

domestic drinking water in rural areas, and many of these 

wells are in the unconfined surficial aquifer system.

About half of the Northern Atlantic Coastal Plain’s 

drinking-water supply comes from surface water—streams, 

rivers, and reservoirs—located in the Piedmont Physiographic 

Province to the west of the Fall Line. This includes tap water 

in Baltimore, New York City, and Washington, D.C. Where 

groundwater supplies are insufficient to meet the needs of 

large population centers that are some distance from the 

Fall Line, such as in Virginia Beach, water is imported from 

streams and reservoirs that originate in the Piedmont to the 

west. Low-relief and highly permeable sediments in the 

Coastal Plain are not conducive to reservoir construction, but 

small reservoirs are important water sources in some areas. 

Water from streams also is used for agricultural irrigation and 

industrial supply.

24 Water Quality in the Northern Atlantic Coastal Plain Surficial Aquifer System, 1988–2009
Chapter 4

Farmland with poultry houses in Delaware.

The movement of water and the transport of contaminants through the 

surficial aquifer system are controlled by the local hydrogeologic 

setting and geochemical processes. Groundwater moves from recharge 

areas on the land surface along subsurface flow paths to discharge areas in 

streams, rivers, wetlands, and estuaries. The geologic materials that make up 

the surficial aquifer system determine the residence time of groundwater, the 

geochemical environment within the aquifer, and the susceptibility of groundwater to contamination from natural and human-related sources.

Chapter 4: Natural Hydrogeologic 

and Geochemical Processes That Affect 

Groundwater Quality

This chapter summarizes 

the natural hydrogeologic and geochemical 

processes that affect the 

movement and quality 

of groundwater in the 

surficial aquifer system 

of the Northern Atlantic 

Coastal Plain.
Direction of 

groundwater flow 

Water table

Unsaturated

zone

Confining unit

more than 30 years

20 to 30 years

10 to 20 years

0 to 10 years

NOT TO SCALE

Hydrogeologic Processes

Hydrogeology Affects the Movement of 

Groundwater and Contaminants Into and 

Through the Surficial Aquifer System

The route that a drop of groundwater takes through an 

aquifer is called a flow path. Flow paths start at the water table 

where precipitation infiltrates or recharges the aquifer and end 

where groundwater discharges into streams, rivers, wetlands, 

and coastal water bodies or is withdrawn by a well (fig. 4–1).

Groundwater and any dissolved chemicals it contains 

move readily through those parts of the surficial aquifer 

system where soils and sediments are coarse-grained and 

permeable. Because groundwater is easily pumped from 

permeable parts of the aquifer system, those are the parts 

commonly tapped for drinking water. Groundwater in 

permeable parts of the aquifer system is more vulnerable to 

contamination from chemicals applied on the land surface than 

is groundwater in areas with fine-grained sediments, which 

impede the transport of water and chemicals. Further, groundwater in the surficial aquifer system is particularly vulnerable 

to contamination from the land surface because the overlying 

unsaturated zone typically is thin (less than 30 ft). Infiltration 

through the soil and unsaturated zone is relatively fast in many 

areas, which limits the potential that contaminants will be 

retained in soils or sediments or degraded before they reach 

the water table.

The texture and composition of the sediments in the 

surficial aquifer system, which control whether a chemical 

that enters groundwater remains dissolved or whether it 

adheres to the sediment or is degraded, vary from one part of 

the system to another. Near-surface sediments in the northern 

Figure 4–1. Groundwater in the surficial aquifer system flows from recharge areas at the water table to pumping wells or to discharge areas, 

such as local streams, usually within a few decades. Groundwater in shallow parts of the surficial aquifer system is young and ages over time 

as it moves into deeper parts of the aquifer.

Photograph by Jessica Carpenter, USGS Photograph from Delaware Geological Survey

Groundwater discharge is an important source of water to local 

streams, rivers, and coastal water bodies.

26 Water Quality in the Northern Atlantic Coastal Plain Surficial Aquifer System, 1988–2009
Chapter 4

part of the study area, including Long Island, and south 

through the Delmarva Peninsula are mostly sands and gravels 

washed off of glaciers—these are some of the thickest and 

most permeable sediments in the Northern Atlantic Coastal 

Plain. Water and dissolved chemicals move readily through 

these sediments. In contrast, in low-lying areas near the 

coast, such as in Virginia and North Carolina, the surficial 

aquifer system generally is composed of younger sediments 

that were deposited in marshes, bays, and beaches that have 

considerable amounts of fine-grained silt, clay, and organic 

matter. The movement of water through the sediments in the 

low-lying areas near the coast is slower than through those to 

the north; chemicals tend to adhere to fine-grained particles, 

which deter their movement through the aquifer and can 

increase the potential for some chemicals to degrade, thus 

making these areas less vulnerable to contamination from the 

land surface.

How is groundwater residence time determined?

The age of a groundwater sample, which is used to determine the residence time of water in a flow system, is the time elapsed 

since the water first entered the aquifer at the water table. Groundwater age is estimated by measuring concentrations of some of 

the manmade gases that are dissolved in the groundwater. The gases measured, which include chlorofluorocarbons (CFCs), sulfur 

hexafluoride, and radioactive tritium, entered the aquifer with recharge water. Concentrations of these gases in a groundwater sample 

are compared against atmospheric records to estimate the year that the sampled water recharged the surficial aquifer. For more 

information on using chemical tracers to determine groundwater age, visit the USGS Reston Chlorofluorocarbon Laboratory Web site 

(http://water.usgs.gov/lab/).

Laboratory equipment used for determination of groundwater age.

Most Groundwater in the Surficial Aquifer 

System is Relatively Young

The residence time of groundwater in most parts of the 

surficial aquifer system ranges from a few years to several 

decades, with a median of 10 to 15 years in many places 

(see sidebar, How is groundwater residence time determined?, 

this page).(14–16) Residence times are short—from less than 

1 year to about 20 years—in areas where the surficial aquifer 

is thin and flow paths are short, such as in extensively ditched 

lowland areas near the North Carolina coast. In settings where 

the surficial aquifer is several hundred feet thick, such as in 

New Jersey and Long Island—where there are longer, more 

regional flow paths—the residence time of some groundwater 

in the surficial aquifer is more than 200 years, although most 

groundwater residence times are less than 50 years even in 

those settings.(14)

Chapter 4: Natural Hydrogeologic and Geochemical Processes That Affect Groundwater Quality 27
6

4

2

1

0 1 2 3 4 5 6 7

Dissolved oxygen, in milligrams per liter

6

4

2

1

0 0.5 1 1.5 2 2.5 3 3.5

Nitrate, in milligrams per liter

6

4

2

1

0 500 1,000 1,500 2,000

Iron, in micrograms per liter

6

4

2

1

0 4 5 6 7

pH, in pH units

*Note: Subregions 3, 5, and 7 did not have enough data to create bar graphs.

1

2

3

4

5

6

7

MCP, Middle Coastal Plain; ICP, Inner Coastal Plain

EXPLANATION

Coastal Lowlands—Poor natural drainage because of low elevation 

and relief. Groundwater typically has low dissolved-oxygen 

concentrations.

MCP, Mixed Sediments—Moderate relief. Drainage and redox vary 

with geology.

MCP, Fine Sediments—Fine sediments and moderate relief promote 

runoff and limit infiltration.

MCP, Sand with Overlying Gravels—Coarse sediments promote 

infiltration and oxidation in surficial aquifer.

ICP, Upland Sand and Gravel—Coarse sediments promote oxic 

conditions in shallow groundwater, but are thin and laterally 

discontinuous. Relief promotes runoff over infiltration.

ICP, Dissected Outcrop Belt—Drainage and groundwater redox 

varies with geology. Where unconfined, groundwater is oxic.

Alluvial and Estuarine Valleys—Poor natural drainage is common as 

a result of low relief and elevation, and fine sediments. Groundwater typically has low dissolved-oxygen concentrations, but redox 

condition varies with geology.

ATLANTIC OCEAN

Area of enlargement

Hydrogeologic subregion

NEW YORK

PENNSYLVANIA

WEST

VIRGINIA NEW

JERSEY

CONNECTICUT

MARYLAND DELAWARE

VIRGINIA

NORTH CAROLINA

SOUTH CAROLINA

Washington, DC

N

Wells screened near the water table typically intercept 

recently recharged groundwater (residence time less than 

10 years), and the water quality of samples from these wells 

reflects the influence of recent land-use activities near the 

wells. Wells, such as most public-supply wells, that are deeper 

than those nearer the water table and pump at high rates have a 

large zone of capture and intercept a mix of older and younger 

water that infiltrated the groundwater system over a wide area; 

this water can contain contaminants from a wide variety of 

sources (fig. 4–1).

Seven hydrogeologic subregions were developed on the 

basis of the physiography and the texture and composition 

of surficial sediments. These subregions group general and 

geochemical conditions of the surficial aquifer system of 

the Northern Atlantic Coastal Plain to aid in interpreting and 

predicting the distribution of contaminants in groundwater.(10)

For example, sediments of the Middle Coastal Plain (MCP), Sand 

with Overlying Gravels, and Inner Coastal Plain (ICP), Dissected 

Outcrop Belt subregions are sandy and permeable. The surficial 

aquifer in these subregions is oxic and particularly vulnerable 

to contamination from land-based chemical applications, and 

nitrate contamination of groundwater is common. In contrast, in 

the Coastal Lowlands subregion, sediments are less permeable 

and contain more organic matter, which reacts with and slows 

the movement of many contaminants. The surficial aquifer in this 

subregion commonly is anoxic and less vulnerable to contamination from human sources than it is in other subregions; dissolved 

iron from geologic sources typically is present in groundwater. 

Groundwater is acidic throughout the surficial aquifer system; pH 

typically is lower in areas with higher concentrations of dissolved 

oxygen and nitrate than in areas without dissolved oxygen but 

with high iron concentrations.

How hydrogeologic setting controls geochemical processes

Geochemical Processes

Hydrogeologic Setting Controls Geochemical 

Processes That Affect Groundwater Quality

The capacity of chemicals that enter groundwater to 

remain dissolved and travel through the aquifer is affected 

by a variety of geochemical reactions that are related to the 

hydrogeologic setting through which the water passes. The 

surficial aquifer system has been divided into seven subregions 

28 Water Quality in the Northern Atlantic Coastal Plain Surficial Aquifer System, 1988–2009
Chapter 4

that summarize general characteristics and geochemical 

conditions of the various hydrogeologic settings (see sidebar, 

How hydrogeologic setting controls geochemical processes, 

p. 28).(10) Groundwater residence time also can be an important 

control on geochemical processes because longer residence 

times increase the length of time groundwater interacts with 

aquifer sediments.

The Presence or Absence of Dissolved Oxygen 

in Groundwater Controls the Transformation and 

Transport of Many Contaminants in the Aquifer

Concentrations of dissolved oxygen in groundwater 

are controlled by reduction/oxidation (redox) reactions 

(see sidebar, How do redox reactions work?, this page). 

These processes are important because they control the 

transformation of nitrate to harmless nitrogen gas (see 

sidebar, How does nitrate enter groundwater? How is it lost 

from groundwater?, p. 31) and the degradation of many 

How do redox reactions work?

Reduction/oxidation (redox) processes require one chemical species that donates electrons and another chemical species that 

accepts those electrons. As a chemical species donates electrons it is “oxidized,” and as the other species accepts electrons it is 

“reduced.” Redox processes typically are facilitated by microbes (bacteria), which use the energy produced by the processes. In 

groundwater, organic carbon is the most common electron donor. If dissolved oxygen is present, it is the preferred electron acceptor, 

because reduction of dissolved oxygen produces more energy than reduction of other chemical species that commonly occur in 

groundwater. The atmosphere is the source of the dissolved oxygen, so the redox conditions in an aquifer near where recharge occurs 

usually are oxic (defined here as having a concentration of dissolved oxygen of at least 0.5 mg/L).

As groundwater moves through the aquifer along a flow path, the dissolved oxygen in the groundwater gradually is consumed by 

redox processes. Once all of the dissolved oxygen is consumed, other chemical species can accept electrons and become reduced. 

If nitrate is present, it will become the preferred electron acceptor until it in turn is completely consumed. This pattern continues, with 

manganese, iron, sulfate, and finally carbon dioxide acting as electron acceptors until they are consumed, in that order. This order 

of use of electron acceptors has important implications for the preservation, degradation, and even production of contaminants in 

groundwater. Because redox reactions occur in a sequence, it can take a long time for strongly reducing conditions to develop. For this 

reason, anoxic groundwater commonly is older than oxic groundwater, and, within the anoxic category, strongly reducing groundwater 

commonly is older than mildly reducing groundwater.

From a water-quality perspective, denitrification—the reduction of nitrate to nitrogen gas—is one of the most important redox 

processes that occurs in groundwater. Nitrate is a concern for human health and, where it discharges to surface water, can impair 

aquatic communities. Conversion of nitrate by denitrification to harmless nitrogen gas, the same gas we breathe in the atmosphere, is 

the primary way that nitrate is removed from water.

Oxygen Nitrate Manganese Iron Sulfate Carbon dioxide

Oxic Anoxic

Oxic Mildly

reducing

Strongly

reducing

Very strongly

reducing

Order of consumption of electron acceptors

manmade chemicals, such as solvents, gasoline compounds, 

and pesticides. Redox reactions also can control whether 

trace elements, such as arsenic, iron, and manganese, that 

occur naturally in sediments in most parts of the surficial 

aquifer system dissolve into the groundwater or remain 

bound up in the sediments. The distribution of organic matter 

in soils is a major factor controlling the dissolved-oxygen 

content of water entering the surficial aquifer system and is 

one of the factors that is characterized by the hydrogeologic 

subregions (see sidebar, How hydrogeologic setting controls 

geochemical processes, p. 28). For example, the poorly 

drained soils common to the Coastal Lowlands hydrogeologic 

subregion contain abundant organic matter, and the 

underlying groundwater is commonly anoxic (contains 

little or no dissolved oxygen). In this subregion, dissolved 

iron is likely to be present in groundwater. In contrast, in 

the more well-drained areas, such as the MCP, Sand with 

Overlying Gravels hydrogeologic subregion, groundwater 

is mostly oxic (contains dissolved oxygen), and nitrate is a 

common contaminant.

Chapter 4: Natural Hydrogeologic and Geochemical Processes That Affect Groundwater Quality 29
Oxic Anoxic

1

10

100

1,000

10,000

Iron Manganese

Oxic Anoxic

Secondary

Maximum

Contaminant

Level

Secondary

Maximum

Contaminant

Level

Median concentration of iron and maganese 

in groundwater 

Concentration, in micrograms per liter

Sorption Processes and the pH of Groundwater 

Can Affect the Movement of Dissolved Metals 

and Other Ions in Groundwater

Sorption is a process in which ions or molecules that are 

dissolved in water become attached to the surfaces of solid 

material, such as sediment particles, either temporarily or 

permanently. The attachment of positively charged ions to 

negatively charged surfaces of clay particles and of positively 

charged pesticide compounds to the negatively charged 

surfaces of clay particles and organic matter are examples of 

sorption. Desorption is the release of sorbed chemicals from 

aquifer solids into groundwater. Whether a chemical is sorbed 

or desorbed at a particular point in the aquifer system depends 

on several factors, including the properties of the dissolved 

chemicals and the solid aquifer materials, the presence of other 

ions that compete for sorption sites, redox conditions, and 

groundwater pH (fig. 4–2).

Groundwater in the surficial aquifer system commonly 

is acidic (pH less than 7) because the aquifer sediments lack 

the soluble minerals, such as calcium carbonate, that can 

buffer (neutralize) the natural acidity of rainfall. Groundwater 

pH along groundwater flow paths can change over time 

as minerals dissolve and other chemical reactions occur. 

Decreases in pH can increase the movement of some constituents through the aquifer by increasing their solubility or by 

reducing the degree to which the constituents sorb to aquifer 

materials. For example, the decrease in pH caused by nitrification (see sidebar, How does nitrate enter groundwater? How 

is it lost from groundwater?, p. 31) causes manganese oxides 

to dissolve and radium to desorb from aquifer sediments(17)

(see chapters 5 and 6).

Figure 4–2. Iron and manganese sorb (attach) onto aquifer sediments where groundwater is oxic (dissolved-oxygen concentration of 

at least 0.5 milligram per liter) and the concentrations of iron and manganese therefore are low in oxic groundwater. When groundwater is 

anoxic (dissolved-oxygen concentration less than 0.5 milligram per 

liter), iron and manganese desorb (are released) from sediments 

into groundwater and therefore are present in groundwater at high 

concentrations, commonly greater than the U.S. Environmental 

Protection Agency Secondary Maximum Contaminant Level. 

30 Water Quality in the Northern Atlantic Coastal Plain Surficial Aquifer System, 1988–2009
Chapter 4

How does nitrate enter groundwater? How is it lost from groundwater?

Nitrification

Nitrogen commonly is applied to fields or released from sewage disposal systems in the form of ammonium. Because oxygen 

usually is plentiful in soil, most of the ammonium that reaches groundwater has been transformed in the soil zone to nitrate by nitrification, a microbially driven process that requires dissolved oxygen. During nitrification, ammonium (NH4

+

) in water (H2O) enriched with 

oxygen (O2

) is converted to nitrate (NO3

–

) and hydrogen ions (H+); the release of hydrogen ions into groundwater lowers the pH (makes 

water more acidic).

Denitrification

Nitrate moves with groundwater through the aquifer as long as dissolved oxygen is present, sometimes along the entire length 

of a flow path from aquifer recharge area to discharge area. If no dissolved oxygen is present, most nitrate is reduced to harmless 

nitrogen gas through another microbially driven process called denitrification. During denitrification, nitrate (NO3

–

) reacts with organic 

matter (CH2

O) and is reduced to nitrogen gas (N2). Bicarbonate (HCO3

–

) and carbon dioxide (CO2) also are formed during denitrification, 

raising the pH (makes water more alkaline).

NO3

-

CH2O N2

H2O 5 4 2 2 5 H+

H+

HCO3

-

Denitrification can be an important process for nitrate loss from groundwater in wetlands and riparian zones, in poorly drained soils, 

and in sediments in deep parts of the aquifer where dissolved oxygen is not present.

NH4

+

O2

H2O NO3

- H+ 6

Chapter 4: Natural Hydrogeologic and Geochemical Processes That Affect Groundwater Quality 31
Degradation Affects the Transport and Transformation of Manmade 

Chemicals in Groundwater

Bacteria in soil and groundwater use organic compounds (chemicals that contain carbon), 

including some manmade chemicals, such as pesticides and VOCs, as a source of energy and 

carbon for growth. As a result, the organic compounds are degraded or transformed into other 

organic or inorganic compounds called degradates. This process is referred to as biodegradation. Most of the degradation of manmade organic chemicals that are applied to or spilled on 

the land surface takes place in the soil zone where oxygen is abundant and bacterial activity is 

greatest, especially in soils that contain a lot of organic matter. Some compounds, such as gasoline hydrocarbons, can be used directly by bacteria as food sources and are rapidly degraded. 

Other compounds, such as chlorinated solvents, are not as easily degraded, especially where 

aquifer conditions are oxic. The rate at which an organic chemical biodegrades depends on its 

chemical structure, the environmental conditions, and the types of bacteria present. Biodegradation can result in complete degradation of organic chemicals to produce carbon dioxide, 

water, and other simple products or it can lead to formation of intermediate degradates that can 

be detected in groundwater. Degradates of common crop herbicides, including metolachlor, 

alachlor, and atrazine, typically are detected in groundwater in the surficial aquifer system at 

concentrations as high or higher than their parent compound (see chapter 6).

Photograph by Luke Myers, USGS

Leaks and waste disposal associated with chemical manufacturing plants can be a source of VOCs in groundwater.

32 Water Quality in the Northern Atlantic Coastal Plain Surficial Aquifer System, 1988–2009
Chapter 5

The quality of groundwater used for drinking water is impaired by chemicals from natural and manmade sources throughout the surficial aquifer 

system of the Northern Atlantic Coastal Plain. Ten chemicals were 

detected at a concentration that exceeded a human-health benchmark in at least 

one sample from wells used for drinking water. The chemicals most commonly 

detected in concentrations greater than a benchmark were nitrate and dieldrin, 

which come from human activities, and radium, which has a geologic source. 

Although pesticides and VOCs were widely detected in groundwater, in most 

cases these chemicals were present at concentrations far less than human-health 

benchmarks. The discharge of groundwater that contains elevated concentrations of nitrate to surface waters also is a water-quality concern because 

nutrient enrichment of streams and estuaries has caused widespread degradation 

of aquatic habitats.

This chapter provides a 

summary of constituents 

that were detected at a 

concentration greater 

than or near a humanhealth benchmark in 

water from drinking-water 

supply and monitoring 

wells tapping the surficial 

aquifer system of the 

Northern Atlantic Coastal 

Plain. Results from water 

samples from all types 

of wells are included 

in this chapter because 

manmade chemicals 

detected in shallow 

monitoring wells have the 

potential to travel over 

time to deep parts of the 

surficial aquifer system 

used for drinking water. 

Nitrate in streams also is 

discussed as it relates to 

aquatic health.

Chapter 5: Contaminants of Concern in 

Drinking Water and in Groundwater Flowing 

to Streams
Many chemicals in groundwater, whether their source is geologic or human 

activities, can harm human health if they are present at elevated concentrations in 

drinking water. Chemicals of potential human-health concern in groundwater from 

the surficial aquifer system were identified by comparing the concentrations in water 

samples to guidelines and standards for drinking water that are considered protective of 

human health, referred to collectively as human-health benchmarks (see sidebar, Humanhealth benchmarks and other guidelines used in this assessment, this page) (table 5–1). 

Other chemicals that do not pose health problems but that impart an unpleasant taste 

or smell to water or that cause other undesirable effects also are discussed (table 5–2). 

Not all compounds that were measured in NAWQA studies had human-health benchmarks.

Photograph by Jessica Carpenter, USGS

Human-health benchmarks and other guidelines used in this assessment

Concentrations of constituents measured for this assessment 

were compared to human-health benchmarks to place study 

findings in the context of human health. The benchmarks are 

threshold concentrations in water above which the concentration 

of a contaminant in drinking water could adversely affect human 

health. Human-health benchmarks were available for about 

two-thirds of the 290 constituents and properties measured for 

the Principal Aquifer assessments. Two types of human-health 

benchmarks were used: U.S. Environmental Protection Agency 

(USEPA) Maximum Contaminant Levels (MCLs) (http://water.epa.

gov/drink/contaminants/index.cfm) and U.S. Geological Survey 

(USGS) Health-Based Screening Levels (HBSLs). MCLs are legally 

enforceable drinking-water standards that specify the maximum 

permissible level of a constituent in water that is delivered to any 

user of a public water system(55) (values used in this report were 

current as of February 2012). Although MCLs are used to regulate 

the quality of drinking water only from public-supply sources, they 

also are useful for evaluating the quality of water from domestic 

and monitoring wells. An MCL was available for 53 of the constituents measured. For some constituents for which an MCL has not 

been established, the USGS, in collaboration with the USEPA and 

others, developed non-enforceable HBSLs by using standard 

USEPA methods for establishing drinking-water guidelines and 

current toxicity information(58, 69, 77) (values used in this report were 

current as of February 2012; see http://water.usgs.gov/nawqa/

HBSL). An HBSL was available for 135 constituents measured. 

Radon has neither an MCL nor an HBSL, but two MCLs have been 

proposed. Copper and lead have USEPA action levels rather than 

an MCL.

In addition to human-health benchmarks, non-healthbased guidelines—Secondary Maximum Contaminant Levels 

(SMCLs)—were available for some of the constituents measured 

in this assessment. The SMCLs are non-enforceable guidelines 

for concentrations of “nuisance” constituents in drinking water 

that can cause unwanted cosmetic effects, such as skin or tooth 

discoloration; aesthetic effects, such as unpleasant taste, odor, or 

color; or technical effects, such as corrosion or sedimentation of 

plumbing or reduced effectiveness of water treatment.(78)

Concentrations greater than one-tenth of a human-health 

benchmark were used in this assessment to indicate which 

contaminants occurred, either individually or as mixtures, at 

concentrations that approach those of potential concern for 

human health, and to identify contaminants that might warrant 

additional monitoring and study. The criterion of one-tenth of a 

benchmark is consistent with various State and Federal practices 

for reporting contaminant occurrence in groundwater and for 

identifying contaminants of potential human-health concern (for 

example, see U.S. Environmental Protection Agency;(81) New 

Jersey Department of Environmental Protection(92)).

Screening-level assessments, such as this one, provide 

perspective on the potential relevance of detected contaminants 

to human health and can help in planning future studies.(69) They 

are not designed to evaluate specific effects of contaminants 

on human health and are not a substitute for comprehensive 

risk assessments. It is important to note that occurrence of a 

contaminant at a concentration greater than its benchmark does 

not mean that adverse effects are certain to occur, because the 

benchmarks are conservative (protective) and source-water 

samples were collected prior to any treatment or blending that 

could alter contaminant concentrations in finished drinking water. 

There are water-treatment options, such as charcoal filtration, 

that can be used to lower the concentration of the contaminant to 

below the benchmark before the water is consumed.

34 Water Quality in the Northern Atlantic Coastal Plain Surficial Aquifer System, 1988–2009
Domestic wells—The homeowner’s responsibility

Almost 2 million people in the Northern Atlantic Coastal Plain rely on privately owned 

household wells for drinking water. These wells are not regulated under the Safe Drinking Water 

Act, which applies only to public-supply wells. In many parts of the Northern Atlantic Coastal 

Plain, routine testing of domestic wells is not required after the well is installed, except for some 

areas when the property changes owners. It is the homeowner’s responsibility to monitor the 

quality of the well water and take any actions to make their drinking water safe, such as replacing 

the well or installing and maintaining a treatment system.

Chapter 5

Percentage of samples with concentrations

above the human-health benchmark

Percentage of monitoring wells25

20

15

10

5

0

Urban Agriculture

Land use

Dieldrin

Nitrate

EXPLANATION

Photographs: top left, the Chesapeake Bay Program;top right, USGS

Dieldrin and Nitrate Were the Chemicals From 

Human Sources That Most Frequently Exceeded 

a Human-Health Benchmark

Concentrations of the insecticide dieldrin exceeded 

its human-health benchmark of 0.002 microgram per liter 

(μg/L) in almost one of every five (18 percent) samples from 

drinking-water wells in the surficial aquifer (table 5–1). 

Elevated concentrations of dieldrin can cause problems with 

the liver and central nervous system and can contribute to an 

increased risk of cancer.(18) The surficial aquifer system has 

one of the highest detection rates of dieldrin in the Nation, and 

concentrations were particularly elevated in shallow groundwater underlying urban areas (fig. 5–1). Dieldrin was widely 

used for insect control from 1950 until 1974, when it was 

banned for all uses except termite control. The use of dieldrin 

was banned completely in 1987.

Seven percent of the groundwater samples from drinkingwater wells in the surficial aquifer system had concentrations 

of nitrate greater than its MCL of 10 mg/L as N. More than 

one-half of samples had concentrations of nitrate greater than 

1 mg/L as N (table 5–1), which is one-tenth of the MCL and 

is greater than the maximum concentration in the surficial 

aquifer system that is likely to be from natural sources 

(0.4 mg/L as N).(8) Nitrate was one of the most common 

chemicals detected at concentrations greater than the national 

MCL in groundwater from domestic (1 percent) and public 

(2 percent) sources of drinking water.(19, 20) Concentrations 

greater than the MCL in drinking water can put infants under 

the age of 6 months at risk of serious illness. Elevated concentrations of nitrate result from leaching of excess nitrogen 

in fertilizers and manures applied to crops and in fertilizer 

applied to turf in urban areas as well as from septic-tank and 

sewage discharges. High concentrations were most common in 

groundwater in agricultural areas, including in domestic wells 

(see sidebar, Domestic wells—The homeowner’s responsibility, this page).

Nineteen constituents were detected at a concentration 

greater than their human-health benchmark in groundwater 

from at least one well sampled in the surficial aquifer system 

as part of the NAWQA Program (table 5–1). In addition to 

nitrate and dieldrin from human sources and radium from 

geologic sources, seven other constituents were detected at a 

Figure 5–1. In urban areas, dieldrin was more frequently detected 

at a concentration greater than its human-health benchmark than 

was nitrate, but the reverse was true in agricultural areas. This 

difference is related to where chemicals are used. The pesticide 

dieldrin was widely used in urban areas to control termites until 1987, 

when its use was banned; nitrate is most widely used in agricultural 

areas and leaches into groundwater from fertilizer and manure 

applied to fields.

concentration greater than their human-health benchmark in at 

least one sample from a drinking-water supply well. Ten of the 

19 compounds were detected at concentrations greater than a 

human-health benchmark in groundwater in less than 1 percent 

of the wells sampled.

Although numerous pesticide compounds and VOCs 

were detected in at least one groundwater sample from a 

drinking-water well, no pesticides other than dieldrin and only 

three VOCs were detected at a concentration greater than their 

respective benchmark (table 5–1). The samples containing 

the elevated concentrations of VOCs—the solvents perchloroethene (PCE) and trichloroethene (TCE), and the fumigant 

1, 2-dibromoethane (also known as ethylene dibromide or 

EDB)—were from three public-supply wells.

Chapter 5: Contaminants of Concern in Drinking Water and in Groundwater Flowing to Streams 35
Table 5–1. Nineteen constituents were detected at a concentration greater than their human-health benchmark in groundwater from at least one well 

concentration above their human-health benchmark in at least one sample from a drinking-water supply well. Nitrate and dieldrin, from human sources, 

the 19 constituents were detected above a human-health benchmark in groundwater in less than 1 percent of the wells sampled.

[AMCL, Alternative Maximum Contaminant Level; AL, Action Level; HBSL, U.S. Geological Survey Health-Based Screening Level; MCL, U.S. Environmental Protection Agency 

Constituent

Human-health benchmark* All well types Drinking water 

Number

of wells 

sampled

Frequency of 

concentrations 

greater than 

benchmark 

(percent of wells)

Frequency of 

concentrations 

within 10 percent 

of benchmark 

(percent of wells)

Number 

of wells 

sampled

Value Type

Nutrients 

Fluoride 4 mg/L MCL 422 0.2 4.3 142 

Nitrate 10 mg/L as N MCL 423 9.9 53.4 143 

 

Alachlor 2 μg/L MCL 390 0.3 0.8 137 

Atrazine 3 μg/L MCL 392 0.3 2.6 136 

Dieldrin 0.002 μg/L HBSL low 389 11.6 12.1 136 

Diuron 2 μg/L HBSL low 250 0.4 0.8 50 

1,2-Dibromoethane 0.05 μg/L MCL 341 0.6 0.3 140 

Perchloroethene (PCE) 5 μg/L MCL 356 1.4 3.4 143 

Trichloroethene (TCE) 5 μg/L MCL 370 0.5 1.6 143 

Trace 

Arsenic 10 μg/L MCL 275 1.1 7.3 99 

Barium 2000 μg/L MCL 275 0.4 6.9 99 

Beryllium 4 μg/L MCL 275 0.4 12 99 

Boron 1,000 μg/L HBSL 244 0.4 5.7 92 

Lead 15 μg/L AL 275 1.1 18.5 99 

Nickel 100 μg/L HBSL 275 0.4 9.1 99 

Manganese 300 μg/L HBSL 397 5.5 52.1 138 

Zinc 2,000 μg/L HBSL 275 1.1 8.0 99 

Radio 

Radon-222 4,000 pCi/L Proposed AMCL 196 0 0 90 

300 pCi/L Proposed MCL 196 27 91.8 90 

Radium-226 and radium-228 5 pCi/L† MCL 74 28.4 90.5 59 

*Human-health benchmarks are current as of August 2010. MCL values are from U.S. Environmental Protection Agency;(84) HBSL values are from Toccalino.(69)

†The 5 pCi/L MCL for radium is a combined concentration of radium-226 and radium-228, the most common isotopes of radium. Water samples for radium were not 

collected in all parts of the Northern Atlantic Coastal Plain.

36 Water Quality in the Northern Atlantic Coastal Plain Surficial Aquifer System, 1988–2009
Chapter 5

sampled in the surficial aquifer system as part of the National Water-Quality Assessment (NAWQA) Program. Ten constituents were detected at a 

and radium, from geologic sources, were the constituents most frequently detected at a concentration above a human-health benchmark. Ten of 

(USEPA) Maximum Contaminant Level; mg/L, milligrams per liter; μg/L, micrograms per liter; pCi/L, picocuries per liter; —, not analyzed]

wells—Domestic and public supply Monitoring wells—Urban land use Monitoring wells—Agricultural land use

Frequency of 

concentrations 

greater than 

benchmark 

(percent of wells)

Frequency of 

concentrations 

within 10 percent 

of benchmark 

(percent of wells)

Number 

of wells 

sampled

Frequency of 

concentrations 

greater than 

benchmark 

(percent of wells)

Frequency of 

concentrations 

within 10 percent 

of benchmark 

(percent of wells)

Number 

of wells 

sampled

Frequency of 

concentrations 

greater than 

benchmark 

(percent of wells)

Frequency of 

concentrations 

within 10 percent 

of benchmark 

(percent of wells)

and major ions

0.7 2.8 93 0 1.1 81 0 7.4

7.0 62.2 93 4.3 67.7 81 23.5 54.3

Pesticides 

0 1.5 89 0 0 80 1.3 1.3

0 2.9 91 1.1 1.1 81 0 7.4

18.4 19.1 89 19.1 20.2 80 1.3 1.3

0 2.0 73 1.4 1.4 76 0 0

VOCs

1.4 0.7 94 0 0 32 0 0

2.8 4.9 94 1.1 5.3 42 0 0

1.4 3.5 94 0 1.1 42 0 0

elements

0 4.0 56 1.8 5.4 56 1.8 5.4

0 4.0 56 0 7.1 56 0 7.1

0 11.1 56 0 7.1 56 0 7.1

0 1.1 56 0 7.1 56 0 7.1

1 34.3 56 0 0 56 0 0

0 6.1 56 0 8.9 56 0 8.9

0.7 30.4 93 7.5 48.4 81 11.1 71.6

0 2.0 56 0 0 56 0 0

nuclides

0 0 1.0 0 0 30 0 0

18.9 95.6 1.0 100 100 30 60 100

32.2 91.5 0 — — 3.0 0 100

Chapter 5: Contaminants of Concern in Drinking Water and in Groundwater Flowing to Streams 37
Radium contamination of 

groundwater is a major 

concern in Vineland, New 

Jersey, where 33,000 

people rely on the surficial 

aquifer system for drinking 

water. Construction alone of 

treatment facilities for radium 

removal cost about $1,000,000 

per well, with additional funds 

required annually to maintain 

the systems. 

Figure 5–2. Lead was detected 

in more than 70 percent of 

samples from drinking-water 

wells from the surficial aquifer 

system, but was detected at 

concentrations greater than the 

Occupational Safety and Health 

Administration action level of 

15 micrograms per liter in only 

one sample. Concentrations of 

lead were higher in water that 

had a pH less than 5.5.

Lead, in micrograms per liter

30

20

10

0

3 4 5 6 7 8 9

pH

Action level

EXPLANATION

Well type

Public supply

Domestic

Radium and Radon Were the Chemicals From Geologic Sources That 

Most Frequently Exceeded a Human-Health Benchmark

A relatively small number of samples (59) from drinking-water wells were analyzed for 

radium, (herein the sum of the radium isotopes 226Ra and 228Ra), but more than 30 percent 

of those samples (table 5–1) contained radium at a concentration greater than its MCL of 

5 picocuries per liter (pCi/L). All the samples analyzed for radium were collected in Delaware, 

Maryland, and New Jersey, so these samples may not be representative of groundwater from 

the entire surficial aquifer system. The radioactive minerals uranium and thorium in aquifer 

sediments are the source of radium in groundwater. Ingestion of radium in drinking water 

increases cancer risk, especially risk of bone and sinus cancers.(21, 22)

Radon, an inert gas derived from radium, also was widely detected in groundwater. Radon 

dissolves readily in water, but its primary concern for human health is through inhalation of 

indoor air in living spaces after the radon degasses from water into air.(23) Inhalation of radon 

gas is second only to cigarette smoking as a cause of lung cancer.(24) There were no samples 

from wells used for drinking water where radon was detected at a concentration exceeding the 

proposed alternative MCL of 4,000 pCi/L, but radon was detected at concentrations greater 

than the proposed lower MCL of 300 pCi/L in 19 percent of samples from wells used for 

drinking water (table 5–1). Concentrations of radon in drinking water of the Northern Atlantic 

Coastal Plain generally were lower than those in most other Principal Aquifers where radon 

was measured (see appendix 3).

Trace Elements and Other Constituents Seldom Exceeded Human-Health 

Benchmarks in Groundwater

Trace elements and fluoride were less frequently detected in groundwater from this 

area than in groundwater from other parts of the Nation (table 5–1) because their detection 

is related to the occurrence of minerals that are rare in the sediments of the surficial aquifer 

system.(19) Fluoride, manganese, and lead were detected in groundwater at concentrations 

greater than their human-health benchmarks (4 mg/L, 300 μg/L, and 15 μg/L, respectively) 

in only a very small percentage of samples (only one water sample for each constituent) from 

drinking-water wells (table 5–1). The source of manganese and fluoride is minerals in aquifer 

sediments.(25) The trace element lead could potentially occur in groundwater through leaching 

from old water pipes. Although several other trace elements were detected in drinking water at 

concentrations greater than one-tenth of a human-health benchmark, they were not detected at 

concentrations greater than a benchmark (table 5–1). Like lead, trace elements such as barium, 

beryllium, and nickel tend to dissolve more readily in acidic water than in water with neutral 

pH (fig. 5–2).

See chapter 6 for more 

information on radium 

in groundwater from the 

surficial aquifer system.

Tables of complete 

analyses of groundwater 

samples discussed in this 

report can be found at 

http://dx.doi.org/10.3133/

cir1360.

38 Water Quality in the Northern Atlantic Coastal Plain Surficial Aquifer System, 1988–2009
Chapter 5

Several “Nuisance” Properties and Constituents 

Exceeded Secondary Maximum Contaminant 

Levels

Secondary Maximum Contaminant Levels (SMCLs) are 

guidelines for nuisance constituents or properties in groundwater that can cause undesirable effects, such as unpleasant 

taste, odor, or color, but that do not present a direct concern 

for human health (see sidebar, Human-health benchmarks 

and other guidelines used in this assessment, p. 34). Only 

three of these properties or constituents—pH, iron, and 

manganese—were detected at a concentration exceeding a 

SMCL in more than one sample from a drinking-water well 

(table 5–2). The pH of groundwater in the surficial aquifer 

system was less than the USEPA recommended SMCL of 

6.5 for drinking water in more than 80 percent of samples 

and, with a median of 5.3, is lower than that of groundwater 

in most other Principal Aquifers.(26) Although pH itself is not 

a human-health concern, water with low pH is acidic and 

corrodes or dissolves metals,(27) such as lead in plumbing 

(fig. 5–2), and can cause other constituents, such as radium 

and manganese, that are present in aquifer sediments to be 

released into groundwater.(28) The low pH of groundwater 

in the surficial aquifer system results from the large amount 

of the mineral quartz in aquifer sediments. Quartz does not 

dissolve readily and therefore does little to buffer (neutralize) 

the natural acidity of rainfall that recharges the aquifer. The 

acidity of groundwater also can increase as a result of denitrification, which releases hydrogen ions into groundwater and 

lowers the pH.

Concentrations of iron and manganese, which occur naturally in aquifer sediments, exceeded their SMCLs of 300 μg/L 

and 50 μg/L in 21 and 14 percent of samples of groundwater 

from wells used for drinking, respectively, and in more than 

one-third of all wells sampled (table 5–2). Iron and manganese 

are released from aquifer sediments into the groundwater 

Table 5–2. Secondary Maximum Contaminant Levels (SMCLs) in drinking water were exceeded by some constituents in groundwater 

from the surficial aquifer system of the Northern Atlantic Coastal Plain—most commonly iron and manganese. These constituents 

are primarily from geologic or other natural sources, so their distribution is not related to a particular land use but is related to 

groundwater geochemistry.

[mg/L, milligrams per liter; <, less than; μg/L, micrograms per liter]

Property or 

major ion Value

All well types Drinking water wells—

Domestic and public supply

Monitoring wells—

Urban land use

Monitoring wells—

Agricultural land use

Number 

of wells

Frequency of 

concentration 

greater than 

SMCL (percent)

Number 

of wells

Frequency of 

concentration 

greater than 

SMCL (percent)

Number 

of wells

Frequency of 

concentration 

greater than 

SMCL (percent)

Number 

of wells

Frequency of 

concentration 

greater than 

SMCL (percent)

Total dissolved solids 500 mg/L 366 5.0 143 0.7 93 5.4 81 7.4

pH (standard units)

< 6.5 420 81 144 85.4 89 88.8 81 86.4

> 8.5 420 0 144 0 89 0 81 0

Chloride 250 mg/L 424 2.0 144 0 93 3.2 81 2.5

Fluoride 2 mg/L 422 0 142 1.4 93 0 81 0

Iron 300 μg/L 425 37 144 20.8 93 23.7 81 45.7

Manganese 50 μg/L 397 39 138 14.5 93 37.6 81 55.6

Sulfate 250 mg/L 424 0.2 144 0 93 0 81 1.2

Staining of laundry and plumbing fixtures 

due to iron and manganese deposition is one 

of the most common water-quality problems 

for homeowners in the Northern Atlantic 

Coastal Plain

Iron and manganese in groundwater are a common 

water-quality problem in the Northern Atlantic Coastal Plain. 

When water containing dissolved iron and manganese comes 

into contact with the oxygen in air, the iron and manganese 

precipitate and are deposited as brownish stains on laundry 

and plumbing fixtures. Water 

softeners, which exchange the 

iron and manganese for sodium 

and which work best for the 

acidic water common to the 

surficial aquifer system, can be 

installed by homeowners to solve 

this problem. These systems 

typically cost between $1,500 and 

$3,000 per household.

when the groundwater is anoxic (see chapter 4). Iron can give 

water a bitter, astringent taste and stain laundered clothing and 

plumbing fixtures with a brownish color (see sidebar, Staining 

of laundry and plumbing fixtures due to iron and manganese 

deposition is one of the most common water-quality problems 

for homeowners in the Northern Atlantic Coastal Plain, this 

page). Manganese also stains laundry and plumbing fixtures 

and, at high levels in drinking water, is a concern for human 

health. Because of the unpleasant taste and staining associated 

with dissolved iron and manganese, domestic well water 

is more likely to be treated for these trace metals than for 

contaminants such as nitrate that have greater health risks but 

no taste or odor.

Photograph courtesy of Culligan

Chapter 5: Contaminants of Concern in Drinking Water and in Groundwater Flowing to Streams 39
Percentage of stream samples with nitrate 

concentration exceeding 0.71 milligram per

liter as N in base flow

Delaware Bay

Chesapeake Bay, 

Western Shore

Chesapeake Bay, 

Eastern Shore

Albemarle and 

Pamlico Sounds

All Northern Atlantic 

Coastal Plain watersheds

0 20 40 60 80 100

Figure 5–3. Concentrations of total nitrogen exceeding 

the 0.71 milligram per liter U.S. Environmental Protection Agency 

ecoregional criterion can be detrimental to the health of aquatic life. 

Nitrate commonly exceeds this concentration in Northern Atlantic 

Coastal Plain streams during base flow (when most stream water is 

from groundwater), especially on the Eastern Shore of the Chesapeake Bay. Algae growing in a stream are a sign of excessive nutrient 

concentrations in water that can lead to the reduction in dissolved 

oxygen and fish kills. 

Phosphorus primarily is transported to surface water by 

overland runoff

Phosphorus, along with nitrate, contributes to the ecologic degradation of aquatic systems. 

Unlike nitrate, however, high concentrations of phosphorus are not commonly present in groundwater in most of the Northern Atlantic Coastal Plain surficial aquifer system. Phosphorus from 

groundwater sources therefore is not addressed in this report. Most phosphorus sorbs onto sediment particles and is transported to streams in surface runoff during storms. However, phosphorus 

from phosphate-bearing rocks does affect concentrations of phosphorus in groundwater in some 

parts of the Coastal Plain in North Carolina and can be an important source of phosphorus loading 

to some streams of the Albemarle-Pamlico drainage basin.(31)

Groundwater From the Surficial Aquifer System Contributes to the 

Degradation of Water Quality in Streams and Estuaries

The health of streams and estuaries in the North Atlantic Coastal Plain, such as the 

Chesapeake Bay and the Albemarle and Pamlico Sounds (see fig. 3–1), has been affected by 

high concentrations of the nutrients nitrogen and phosphorus, which can cause large algal 

blooms (see sidebar, Phosphorus primarily is transported to surface water by overland runoff, 

this page). When the algae die, their decomposition consumes dissolved oxygen, lowering the 

concentration of oxygen to levels that critically affect aquatic life. Nitrate from groundwater 

in the surficial aquifer system is a major source of nitrogen in streams. In some parts of the 

Northern Atlantic Coastal Plain where nitrate concentrations in groundwater are high, the 

median concentration of nitrate in streams during base-flow conditions is greater than the 

0.71 mg/L as N USEPA ecoregional criterion for total nitrogen (fig. 5–3).(29, 30) Concentrations 

of total nitrogen, which includes nitrate and other dissolved nitrogen species, greater than this 

level are harmful to the health of aquatic life.

See chapter 7 for more 

information on the 

transport of nitrate and 

other chemicals from 

groundwater to streams.

40 Water Quality in the Northern Atlantic Coastal Plain Surficial Aquifer System, 1988–2009
Chapter 6

Photographs (left column) from top to bottom: USGS, Chesapeake 

Bay Program; USDA Natural Resources Conservation Service

Photographs (right column) from top to bottom: USGS; next two 

from the Chesapeake Bay Program

This chapter describes 

the sources of and 

factors that affect 

concentrations of nitrate, 

pesticides, VOCs, and 

radium in the surficial 

aquifer system of the 

Northern Atlantic 

Coastal Plain.

Sources of nitrate in 

groundwater include manure 

spreading, fertilizer applications, 

livestock waste, and other 

agricultural activities. In urban 

areas, sources of nitrate include 

septic systems, leaking sewer 

lines, and lawn fertilizers.

Nitrate was the most common and widespread human-related contaminant in wells sampled in the surficial aquifer system, but pesticide 

compounds and VOCs also were common contaminants in water 

samples from areas where these compounds are used. Regional differences in 

concentrations of nitrate, pesticides, and VOCs are related to land use and to 

differences in hydrogeologic settings that affect dissolved-oxygen concentrations and other geochemical conditions in groundwater. Radium was the most 

common contaminant from geologic sources in wells sampled. The occurrence 

of radium is related to the naturally acidic conditions in groundwater and to the 

increased acidity that results from nitrification.

Chapter 6: Understanding Where and Why Key 

Contaminants Occur in Groundwater
Modified from Nolan and Hitt(80) Predicted nitrate concentration, 

in milligrams per liter as nitrogen

EXPLANATION

≥1 >1–5 >5–10 >10

DE

NJ

Washington,

D.C.

Model for shallow groundwater

NY

PA

MD

WV

VA

NC

SC

CT

Nitrate in Groundwater

Nitrate was measured at concentrations greater than 1 mg/L as N, the average level 

attributed nationally to human-related sources,(32) in water from more than one-half of all wells 

sampled in the surficial aquifer system and in water from more than 60 percent of drinkingwater wells sampled (see table 5–1). Almost one-fourth of the shallow groundwater samples 

from areas underlying agricultural land use had nitrate concentrations greater than the MCL 

of 10 mg/L as N. High concentrations of nitrate present in shallow groundwater can persist for 

decades and travel over time to deep parts of the surficial aquifer system where water is used 

for drinking.

Where the surficial aquifer system is used for public water supply, nitrate contamination 

necessitates treatment, such as blending or anion exchange and, in some areas, drilling of 

wells into deeper confined aquifers that underlie the surficial aquifer; water treatment and 

drilling of new wells increase the cost of water supplies (see sidebar, Treatment of nitrate, 

p. 43). In contrast, water from domestic wells is not routinely tested and, because nitrate is 

colorless and odorless, many residents are not aware that groundwater from their wells might 

be contaminated.

Nitrogen compounds occur naturally in the environment, but substantial amounts are 

contributed to the land from a variety of human sources, including inorganic fertilizers, animal 

manure, human sewage, and combustion of fossil fuels. Nitrogen compounds in fertilizer and 

manure applied to the land surface are converted by bacteria to nitrate (see sidebar, How does 

nitrate enter groundwater? How is it lost from groundwater?, p. 31), which is highly soluble 

and therefore likely to leach through the soil into groundwater.(33) Many parts of the surficial 

aquifer system are vulnerable to nitrate contamination (fig. 6–1).

Figure 6–1. Concentrations of nitrate in parts of the surficial aquifer system, such as Delaware and 

eastern Maryland, are predicted to exceed the Maximum Contaminant Level of 10 milligrams per liter as 

nitrogen, largely because of abundant nonpoint sources of nitrogen and hydrogeologic and soil conditions 

that promote nitrate transport. Predicted concentrations of nitrate in groundwater were derived from a 

statistical model that used measured concentrations of nitrate from across the Nation to extrapolate to 

unsampled areas.

Concentrations of nitrate 

exceeded the MCL of 

10 mg/L as N in almost 

one-fourth of the shallow 

groundwater samples from 

areas underlying agricultural land use. Nitrate in 

shallow groundwater can 

persist for decades and 

travel over time to deep 

parts of the surficial aquifer 

system that supply water 

used for drinking. 

42 Water Quality in the Northern Atlantic Coastal Plain Surficial Aquifer System, 1988–2009
Chapter 6

Treatment of nitrate

Treatment of nitrate in groundwater used for public and 

municipal water supplies is very expensive, often costing several 

hundreds of thousands to more than a million dollars for the initial 

construction of a treatment plant and about $100,000 per year to 

operate. In some places, water utilities blend high-nitrate water 

from surficial aquifer system wells with low-nitrate water from 

deeper confined wells, which can require drilling deeper wells and 

building additional infrastructure at considerable expense. Water 

treatment using anion exchange (nitrate is exchanged for the 

chloride in potassium chloride) also can be used. In Delaware, the 

cost of water treatment is about 40 percent of the total cost paid by 

the consumer.

Photograph from Can Stock Photo Inc© Photograph from the Chesapeake Bay Program

Corn is one of the most important crops produced in the Northern Atlantic Coastal Plain.

Chapter 6: Understanding Where and Why Key Contaminants Occur in Groundwater 43
Nitrate, in milligrams

 per liter as nitrogen

Land cover, 2001

Urban

Agricultural

Forested

Wetland

Water

>1 to <5

<1.0

>5 to <10

>10

EXPLANATION

Area of enlargement

Maximum

Contaminant

Level

0

10

20

30

Nitrate, in milligrams per liter 

as nitrogen

Agricultural Urban

New Jersey

Delmarva Peninsula

North Carolina

Long Island

New Jersey

All areas

See sidebar, Boxplots, p. 45

Nitrate in shallow groundwater

Forested

N

Chesapeake Bay

Nitrate in groundwater—all sampled wells

Delaware Bay

Albemarle

Sound

Pamlico Sound

Long Island Sound

CONNECTICUT

NEW

JERSEY

PENNSYLVANIA

NEW

YORK

WEST

VIRGINIA

SOUTH 

CAROLINA

Chesapeake Bay

Fall Line

DELAWARE

MARYLAND

ATLANTIC OCEAN

VIRGINIA

NORTH CAROLINA

Washington, D.C.

Nitrate Occurrence in Groundwater Reflects the Distribution of Nitrogen 

Sources and the Hydrogeologic and Geochemical Processes That Affect 

Nitrate Movement

The highest concentrations of nitrate in groundwater were in areas of intensive agriculture 

in the northern part of the surficial aquifer system (fig. 6–2). On the agricultural Delmarva 

Peninsula, nitrate concentrations exceeded the nitrate MCL of 10 mg/L as N in about one-third 

of groundwater samples, and in agricultural areas of southern New Jersey nitrate concentrations 

exceeded the MCL in almost two-thirds of samples. In these areas, well-drained sandy soils 

and aquifer sediments are well-suited for agriculture. Soils and shallow aquifer sediments are 

permeable and weathered and contain minimal amounts of organic matter; in addition, the 

groundwater remains oxic for many decades as it moves along flow paths to discharge at wells 

and streams (fig. 6–3A). These characteristics are favorable for the formation and transport 

of nitrate from fertilizer and manure applications to groundwater and prevent its breakdown 

to harmless nitrogen gas. Concentrations of nitrate generally are much lower in the surficial 

aquifer system in North Carolina. In this area, agricultural sources are similar to those in the 

north, but poorly drained soils and organic-rich aquifer sediments promote suboxic or anoxic 

conditions in groundwater;(16, 31) as a result, nitrate is lost through denitrification as groundwater 

moving down a flow path becomes anoxic (fig. 6–3B).

Figure 6–2. Nitrate occurred in all types of wells in 

the surficial aquifer system throughout the Northern Atlantic Coastal Plain. Concentrations most 

commonly approached or exceeded the MCL of 

10 mg/L as nitrogen in areas of intensive agriculture 

and urban development in the northern part of the 

Northern Atlantic Coastal Plain. In these areas, 

groundwater is oxic, which prevents the breakdown 

of nitrate to nitrogen gas. Nitrate concentrations 

commonly were lower to the south, where, similarly 

to the north, nitrogen is applied to agricultural land, 

but groundwater is predominantly anoxic, which 

facilitates nitrate breakdown. Concentrations were 

lowest in forested areas where there are few nitrogen sources.

Concentrations of nitrate 

in groundwater in the 

surficial aquifer system 

exceeded the MCL most 

frequently in areas of 

intensive agriculture with 

well-drained soils and 

sandy aquifer sediments in 

Delaware, Maryland, and 

New Jersey.

44 Water Quality in the Northern Atlantic Coastal Plain Surficial Aquifer System, 1988–2009
Chapter 6

14 Well-nest location and concentration 

of nitrate, in milligrams per liter 

as nitrogen

General direction of groundwater 

flow

Indicates anoxic water (Dissolved 

oxygen, < 0.5 milligram per liter)

Indicates oxic water (Dissolved 

oxygen, ≥ 0.5 milligram per liter)

EXPLANATION

Elevation, in feet Elevation, in feet

Surficial aquifer

Confining unit

–100

–120

Sea

Level

20

40

–40

–20

–60

–80

20

7.4

14

14

17

14

17

15

14

17

21

1.3

21

4

22

Water table

Confining unit

Confined aquifer

0.12

19.6

<0.06

9.3 9.3

9.4

1.5

26.6 3.4

0.07

45.1

0.06

19.8 Water table

Surficial

aquifer

3.6

17 17

A

B

0.25 0.5 MILE

0 0.25 0.5 KILOMETER

0

10

20

30

40

50

60

70

80

Sea

Level

0.5 1 MILE

0.5 1 KILOMETER

0

0

Figure 6–3. A, Elevated concentrations 

of nitrate from agricultural sources occur 

throughout the surficial aquifer where conditions are oxic, as seen in part of Delaware, 

where the entire aquifer is oxic and concentrations of nitrate more than 20 milligrams per liter 

as nitrogen are present as deep as 100 feet 

below the land surface. B, In contrast, at an 

agricultural site in North Carolina, concentrations of nitrate are high in the shallow part of 

the surficial aquifer where the groundwater is 

oxic, but 10 or more times lower in the deeper 

part of the aquifer where the groundwater is 

anoxic, because denitrification occurs only 

under anoxic conditions.(16, 61)

Boxplots

Boxplots are used to illustrate how results are distributed within a group. The 

“box” ranges from the 25th to the 75th percentile and represents 50 percent of the 

data. The horizontal line in the middle of the box is the median value—one-half of the 

values in the group are greater than the median and one-half are less.

Percentiles describe the percentage of values in a group that are less than the 

given value: 25 percent of the values in a group are less than the 25th percentile; 

75 percent of the values in a group are less than the 75th percentile. The median is 

also the 50th percentile.

If, for example, the 75th percentile for the measured concentration of a 

contaminant in a group of wells is equal to the human-health benchmark for that 

contaminant, then 75 percent, or three-fourths, of the wells have a concentration 

of that contaminant less than the benchmark, and 25 percent, or one-fourth, have a 

concentration greater than the benchmark.

The “whiskers” (vertical lines) in these figures extend to the 10th and 90th 

percentiles; box and whiskers together represent 80 percent of the data. Values 

greater than the 90th or less than the 10th percentile are shown as individual 

points (outliers).

EXPLANATION

25th percentile

10th percentile

75th percentile

50th percentile

 (median)

90th percentile

Outlier

Outlier

50 percent of data

fall within the box 

80 percent of data fall 

 between endpoints of whiskers 

Chapter 6: Understanding Where and Why Key Contaminants Occur in Groundwater 45
Figure 6–4. On the Delmarva 

Peninsula, the median concentration of nitrate in water from 

all types of wells was about 

5 milligrams per liter as nitrogen 

(mg/L as N). Where groundwater 

was oxic, the median concentration of nitrate in samples from 

domestic wells was greater 

than the Maximum Contaminant 

Level of 10 mg/L as N for drinking 

water. Median nitrate concentrations in groundwater that was 

anoxic were below the detection 

level of 0.05 mg/L as N. 

40

30

20

10

 0

29

30

20

17

24

9

12

6

29

Nitrate, in milligrams per liter as nitrogen

Total, 

all wells

Dissolved oxygen

greater than or

equal to

0.5 mg/L

Dissolved 

oxygen

less than

0.5 mg/L

All Oxic Anoxic

Groundwater sample type

Shallow groundwater in agricultural areas

Groundwater used for domestic supply

Groundwater used for public supply 

Number of samples

Maximum

Median

Minimum

23

EXPLANATION

Maximum 

Contaminant 

Level

Nitrate in wells

Modified from Denver and others(52)

Concentrations of nitrate also are commonly elevated in groundwater in areas of dense 

urban and suburban development on Long Island and in New Jersey as a result of septicsystem discharge and other sources of wastewater return flow. Nitrate concentrations in urban 

and suburban areas, however, generally are lower than in predominantly agricultural areas 

(fig. 6–2).(34)

Many public-supply wells in small towns and most domestic wells in rural areas pump 

water from the surficial aquifer system. Where the groundwater is oxic, water from these wells 

commonly contains nitrate from agriculture and other human sources, which can limit its 

potability (fig. 6–4). Public water supplies are tested to ensure that the MCL is not exceeded. 

Because domestic wells commonly are located in agricultural areas where concentrations 

of nitrate in groundwater are more likely to be elevated, contamination by nitrate is a 

concern (fig. 6–4).

Changes in Nitrogen Applications to the Land Surface Might Not Be 

Reflected in Groundwater Quality for Many Years

Changes in land use or land-use practices over time can result in changes in concentrations 

of nitrate in groundwater. Because groundwater flows slowly from where it recharges at the 

land surface to where it discharges at a well or stream, changes in concentrations of nitrate in 

groundwater withdrawn from wells generally lag many years or even decades behind changes 

in nitrogen application at the land surface.(14) For example, fertilizer use has increased greatly 

in the last half century (fig. 6–5). The average applications of nitrogen from fertilizer on corn 

in the United States increased from 72 kilograms per hectare (kg/ha) in 1965 to more than 

150 kg/ha in the 1980s.(33) Increasing concentrations of nitrate in the surficial aquifer system 

on the Delmarva Peninsula between 1988 and 2001 (fig. 6–6) are related to the increase of 

the amount of nitrogen used in various applications that occurred decades earlier;(36) similar 

trends are apparent in agricultural areas in other parts of the surficial aquifer system.(14, 16) In 

contrast, removal of nitrogen sources can cause concentrations of nitrate in groundwater to 

decrease over time, such as occurred in western Long Island following sewering of residential 

areas that had previously been on septic systems (see sidebar, After years of increasing nitrate 

concentrations, improved infrastructure caused nitrate to decrease in groundwater in parts of 

Long Island, p. 48).

Changes in concentrations 

of nitrate in wells can 

lag many years or even 

decades behind changes in 

nitrogen application at the 

land surface.

46 Water Quality in the Northern Atlantic Coastal Plain Surficial Aquifer System, 1988–2009
Chapter 6

Median nitrate concentration in oxic groundwater

Milligrams per liter

as nitrogen

Year

Water table

1 to 3 miles

0

25

50

75

Depth below land surface, in feet

100

Corn Urban

Corn Soybeans or

small grains

Depth of

monitoring

wells

Surficial aquifer

Confining unit

Depth of

public-supply

wells

NOT TO SCALE

Nitrate concentration, in 

milligrams per liter as 

nitrogen

<5

5–10

>10 

EXPLANATION

Direction of groundwater 

flow 

Maximum 

Contaminant

Level

15

10

5

0

15

10

5

0

1988 2001–03 1988 2001–03

Shallow monitoring wells Domestic wells

Depth of 

monitoring wells

Modified from Dubrovsky and others(35)

No data

Nitrate in groundwater

Nitrogen in fertilizer 

 applications

EXPLANATION

Nitrogen in fertilizer applications, in grams 

per square meter per year

Nitrate in groundwater, in milligrams per 

liter as nitrogen

10 milligrams per

liter as nitrogen

60

50

40

30

20

10

0

14

12

10

8

6

4

2

0

1940 1950 1960 1970 1980 1990 2000

Year

Nitrogen in fertilizer applications compared with nitrate in groundwater

Maximum 

Contaminant

Level

Modified from Böhlke and Denver(62)

Figure 6–5. Increases in nitrate 

concentration in groundwater 

samples from shallow monitoring wells lagged behind those in 

nitrogen fertilizer applications 

by almost two decades in an 

agricultural area in Kent County, 

Maryland. About 30 percent of 

the nitrogen applied for agricultural crop production has 

leached into shallow groundwater in this area. Dates shown 

for groundwater represent the 

estimated year that groundwater 

recharged the aquifer (see sidebar, How is groundwater residence time determined?, p. 27).

Figure 6–6. Nitrate moves along flow paths in the surficial aquifer system of the Delmarva Peninsula from recharge areas to where groundwater discharges to wells and surface water. Nitrate concentrations in samples collected from shallow monitoring and domestic wells in the 

1980s and again in the 2000s demonstrate that concentrations of nitrate have increased. These higher nitrate concentrations will continue to 

move through the aquifer toward deeper wells and discharge areas, provided that the groundwater remains oxic.

Chapter 6: Understanding Where and Why Key Contaminants Occur in Groundwater 47
Nitrate, in milligrams per liter

as nitrogen

Population, in thousands

Time, in years

12

10

8

6

4

2

0

1910 1920 1930 1940 1950 1960 1970

Population

50

40

30

20

10

0

Nassau County, sewered Suffolk County, unsewered

Maximum 

Contaminant 

Level

1952–64

Range of years

1965–73 1974–81 1982–97

25

20

15

10

5

0

1952–64 1965–73 1974–81 1982–97

Nitrogen, in milligrams 

per liter as nitrogen

Nitrogen concentration in sewered and unsewered areas

Kings County

Queens County

Nassau 

County

Suffolk County 

Kings County

Queens County

Nassau 

County

Suffolk County 

1920 2000

*Note: Number indicates population density per square mile in each county.

NEW YORK

CONNECTICUT

See sidebar, 

Boxplots,

p. 45

Nitrate in groundwater in Nassau County

Population density, 1920 and 2000

Nitrate

1,494

4,733 18,435

34,247

NEW YORK

CONNECTICUT

118

457 4302

30462

Concentrations of nitrate in shallow 

groundwater on Long Island increased 

substantially with the expansion of population in 

urban centers and to the east beginning in the 

early 1900s. In Nassau County, concentrations of 

nitrate in groundwater increased from less than 

detectible levels in 1910 to about 10 mg/L as N in 

1975 because of increased wastewater disposal 

through cesspools and septic tanks in dense 

residential developments.(63, 64) Groundwater is 

the sole source of freshwater on Long Island. 

Increases in pumping drew nitrate-enriched 

water down to depths as great as 500 ft or more 

in this area, causing concern over the quality of 

this important source of drinking water.

After years of increasing nitrate concentrations, improved infrastructure caused nitrate to 

decrease in groundwater in parts of Long Island

In response to deteriorating groundwater quality, installation of sewer systems in parts of Nassau County began in the 

1950s. The quality of groundwater has since improved,(51, 70) although these improvements lag the installation of sewering 

because of the slow movement of groundwater from the water table to wells. In unsewered areas of Suffolk County, which 

have had slower population growth than Nassau County and have some agricultural land use as well, concentrations of 

nitrate in groundwater have continued to increase slowly.

48 Water Quality in the Northern Atlantic Coastal Plain Surficial Aquifer System, 1988–2009
Chapter 6

Manmade Organic Compounds in Groundwater

Abundant use of pesticides and VOCs and the sandy sediments and oxic conditions of the 

surficial aquifer system have led to widespread occurrence of these chemicals in groundwater. 

As a result, pesticides and VOCs occur widely in the surficial aquifer system, but concentrations, with the exception of those for dieldrin, rarely exceeded established drinking-water 

standards or other human-health benchmarks.

Pesticides

Pesticides occur more frequently in groundwater in agricultural areas in the northern 

part of the surficial aquifer system than in many of the other intensively farmed areas of the 

Nation, including much of the Corn Belt in the Midwest and the Central Valley in California 

(fig. 6–7). This pattern is related to a combination of factors that affect pesticide occurrence in 

groundwater. These factors include where and how much of a pesticide is used, the chemical 

properties of the pesticide, and characteristics of the soil and the aquifer that affect whether a 

pesticide breaks down and whether it adheres to solids or remains dissolved in groundwater.

Pesticide compounds include 

herbicides, insecticides, and 

fungicides used for weed and 

pest control. The most intensively 

used pesticide compounds are 

agricultural herbicides, although 

insecticides and other pesticide 

compounds are heavily used in 

some urban areas as well.

Figure 6–7. In the northern part of the Northern Atlantic Coastal Plain, herbicides are heavily applied 

and can infiltrate readily through sandy soils and aquifer sediments down to the water table. Herbicides 

were detected more frequently in this area than in some of the most intensively farmed areas of the 

United States, including the Corn Belt in the Midwest and the Central Valley in California.(79)

DE

NJ

DC

PA

NY CT

VA

NC

SC

WV

MD

Highest 25 percent of studies

Lowest 25 percent of studies

Highest (greater than 0.406)

Medium (0.090 to 0.406)

Lowest (less than 0.090)

EXPLANATION

Herbicide use, in pounds 

per acre of agricultural land

No reported use

Percentage of samples in a groundwater study 

in which one or more herbicides was detected. 

Each circle represents a groundwater study

Middle 50 percent of studies

3

3

Herbicides in shallow groundwater in agricultural areas

Central

Valley

Corn Belt

Modified from Gilliom and others(79)

Chapter 6: Understanding Where and Why Key Contaminants Occur in Groundwater 49
Wells monitoring shallow groundwater

Agricultural areas

Urban areas

Network type

EXPLANATION

Any >0.1

Concentration, in 

micrograms per liter

Wells used for drinking water

Percentage of wells in which pesticide 

was detected

0 20 40 60 80 100

Percentage of wells in which pesticide 

was detected

0 20 40 60 80 100

n= 89 to 91 Number of samples

Italics indicate pesticide degradate

Public supply

Domestic

Simazine

Deethylatrazine

Metolachlor

Prometon

Dieldrin

Carbofuran

Atrazine

Alachlor

Shallow groundwater, agricultural areas 

n = 80 to 81 

Metolachlor

Prometon

Alachlor

Dieldrin

Atrazine

Shallow groundwater, urban areas 

n = 89 to 91 

Deethylatrazine

Simazine

Carbofuran

Simazine

Deethylatrazine

Metolachlor

Prometon

Alachlor

Dieldrin

Carbofuran

Atrazine

Wells used for drinking water, 

public supply

n = 38 to 68 

Simazine

Deethylatrazine

Metolachlor

Prometon

Alachlor

Dieldrin

Carbofuran

Atrazine

Wells used for drinking water, 

domestic supply

n = 67 to 69 

 

Individual pesticides were detected most commonly in groundwater in those land-use 

settings where they primarily are used (fig. 6–8). For example, metolachlor is a broadspectrum herbicide for control of weeds in farm fields and, not surprisingly, was detected 

more commonly in samples from wells monitoring shallow groundwater in agricultural areas 

than from wells in urban areas. Some pesticides, like atrazine, are used in both cropland and 

urban settings and were detected in wells in both agricultural and urban areas. Simazine and 

prometon, which are used for roadside weed control, were detected most commonly in samples 

from wells monitoring shallow groundwater in urban areas.

Figure 6–8. The pesticide compounds most frequently detected in groundwater of the surficial aquifer system were herbicides, with the 

exception of the insecticides dieldrin and carbofuran. Pesticide occurrence generally reflects broad-use patterns, especially in agricultural 

areas, although pesticides also can reflect local use, especially in urban areas.

50 Water Quality in the Northern Atlantic Coastal Plain Surficial Aquifer System, 1988–2009
Chapter 6

Table 6–1. Of the top 10 pesticides (in terms of pounds applied) used on agricultural crops in the 

Northern Atlantic Coastal Plain in 2007, 9 were herbicides and only 1 was an insecticide. Of these 

10 pesticides, only the herbicides atrazine, S-metolachlor, metolachlor, and alachlor were detected 

frequently in groundwater.

Compound Type Number of counties with 

reported use

Total applied 

(millions of pounds)

Glyphosate Herbicide 125 4.52

Atrazine Herbicide 123 2.27

S-metolachlor Herbicide 125 1.47

Acetochlor Herbicide 57 0.38

Linuron Herbicide 113 0.22

Alachlor Herbicide 98 0.18

Fluometuron Herbicide 40 0.10

Terbufos Insecticide 100 0.10

Trifluralin Herbicide 118 0.08

Metolachlor Herbicide 57 0.07

Figure 6–9. In agricultural areas of the Delmarva Peninsula, pesticide compounds were detected more frequently in samples of groundwater in areas with well-drained, permeable soils that contained little organic matter than in samples of groundwater from areas with 

poorly drained, organic-rich soils.

Chemical characteristics and environmental factors also affect pesticide occurrence. 

Commonly detected compounds, such as atrazine, metolachlor, and their degradates, are more 

water soluble and break down less readily in groundwater than some other compounds that 

also are used frequently but that have chemical properties that limit their movement to and in 

groundwater, such as glyphosate.(37, 38) Although twice as much of the herbicide glyphosate 

is applied in the Northern Atlantic Coastal Plain than atrazine (table 6–1), glyphosate was 

not among the pesticides most frequently detected (fig. 6–8). Organic-rich soils impede the 

movement of pesticides to the water table, and as a result, pesticide concentrations generally 

are lower in groundwater in poorly drained organic-rich areas than in areas with well-drained 

soils (fig. 6–9).

circ1353 ch6_fig9

n=17 Number of samples

2

1.5

1

0.5

0 0

5

10

15

20

25

30

35

40

45

50

55

60

n=17 n=12 n=17 n=12

Concentration, in micrograms per literConcentration, in micrograms per liter

Modified from Debrewer and others(36)

Poorly drained soils

Well-drained soils

Total atrazine Total metolachlor

EXPLANATION

See sidebar, Boxplots, p. 45 

Total atrazine and total metolachlor

 are the sum of parent and 

 degradate compounds

Chapter 6: Understanding Where and Why Key Contaminants Occur in Groundwater 51
0

10

20

30

40

50

0 1 2 or 3 4 or 5 > 5

Number of pesticide compounds detected in a sample

Percentage of water samples

Occurrence of pesticide mixtures in water samples 

Degradates of Some Pesticide Compounds Were Detected in Groundwater 

at Higher Concentrations Than Were the Parent Compounds

If a pesticide degradate is soluble in water and does not break down in the environment, 

it can infiltrate down through the unsaturated zone into groundwater along with any remaining 

parent compound. Degradates of some herbicides, such as alachlor and metolachlor, were 

measured in surficial aquifer system groundwater at concentrations that were 10 to 100 times 

greater than those of their parent compounds (fig. 6–10).(36, 39) Deethylatrazine and other 

atrazine degradates also occur commonly, but concentrations of deethylatrazine generally were 

similar to those of its parent compound atrazine (fig. 6–10).

Figure 6–10. Concentrations of alachlor and metolachlor degradates were much higher than those of their parent compounds in samples 

of shallow groundwater from monitoring wells in agricultural areas of the Delmarva Peninsula. Concentrations of the atrazine degradate, 

deethylatrazine, however, were similar to those of atrazine. (ESA, ethyl sulfonic acid)

Pesticides Commonly Occur in Groundwater as 

Mixtures of Compounds

More than half of samples from the surficial aquifer 

system in which at least one pesticide was detected also 

contained additional pesticides (fig. 6–11). In groundwater 

samples collected in the Maryland Coastal Plain during 

2001 through 2004, 30 of 32 samples in which a pesticide 

was detected also contained at least one other pesticide 

compound, and one sample contained 11 different pesticide 

compounds.(38) Mixtures of pesticides might occur in 

groundwater because pesticides often are applied as mixtures 

of several active ingredients and several different pesticides 

are applied for different purposes in the same area. Also, 

groundwater withdrawn by wells often contains water from 

many different flow paths, which originate in different areas 

and therefore can carry different chemicals.

Figure 6–11. Pesticides commonly occurred as a mixture of multiple compounds in wells sampled throughout the surficial aquifer 

system. More than half of samples in which at least one pesticide 

was detected also contained one or more additional pesticides.

100

10

1

0.1

0.01

0.001

0.0001

Concentration, in micrograms per liter

MCL (2 μg/L) MCL (3 μg/L)

HBSL 

(700 μg/L)

Alachlor

Alachlor

ESA

Atrazine

Deethylatrazine

Metolachlor

Metolachlor

ESA

Concentrations of pesticides and their degradates in shallow 

groundwater in agricultural areas

EXPLANATION

Outlier data value (more than 15 times

the interquartile range (IQR)

outside the quartile) 

75th percentile (75 percent of the data

values are less than this value) 

25th percentile (25 percent of the

data values are less than this value) 

Median

Data value within 1.5 times the interquartile

range (IQR) outside the quartile 

Interquartile range (IQR)

(Bold indicates parent; not bold

indicates degradate)

Modified from Debrewer and others(36)

There are no human-health 

benchmarks for pesticide 

degradates in drinking 

water, and little is known 

about their potential to 

affect human health.

52 Water Quality in the Northern Atlantic Coastal Plain Surficial Aquifer System, 1988–2009
Chapter 6

Figure 6–12. Use of different pesticide compounds changes as some pesticides are phased out and 

new pesticides are registered and begin to be used. Some of these changes are related to new farming 

practices; use of glyphosate for weed control, for example, has increased dramatically with the development of genetically modified corn and soybeans that are glyphosate resistant. The use of alachlor and 

metolachlor has decreased as new herbicides, including acetochlor, glyphosate, and s-metolachlor, 

have entered the market. 

Some Pesticide Compounds Are No Longer Used and Yet Are Still 

Detected in Groundwater

Patterns of pesticide use change over time (fig. 6–12) as a result of changes in regulations, 

monitoring results, effectiveness, cost, and other factors. In many places, herbicides and their 

degradates continue to be detected in the surficial aquifer system as much as several decades 

after their use ceased (see sidebar, The legacy of aldicarb contamination in groundwater on 

eastern Long Island, p. 54).(16, 36) For example, use of dieldrin was discontinued in 1987, but it 

was detected in samples of surficial aquifer system groundwater collected in the late 1990s and 

early 2000s.(37, 38) Degradates of dichlorodiphenyltrichloroethane (DDT), the use of which was 

discontinued in the 1970s, continued to be detected as much as 25 years later.(39)

1960 1970 1980 1990 2000 2010

Increasing use

EXPLANATION

Changes in pesticide use over time

Decreasing use

Registered for use

Atrazine

Alachlor

Metolachlor

S-metolachlor

Glyphosate

Acetochlor

Human-health benchmarks 

have been developed 

for individual pesticide 

compounds, but the humanhealth effects of mixtures 

of pesticides are largely 

unknown.

Chapter 6: Understanding Where and Why Key Contaminants Occur in Groundwater 53
The legacy of aldicarb contamination in groundwater on eastern Long Island

The insecticide aldicarb was used extensively on potato crops on eastern Long Island starting in 1975, but its use was discontinued 

in 1979 because it was detected in groundwater at levels that exceeded the New York State recommended level for drinking water.(65, 67) 

Aldicarb degradates continued to be detected in groundwater for at least two decades after that because of the length of time it takes 

for groundwater to move from the point of recharge to the point of discharge. Although concentrations of these degradates began to 

decrease in shallow groundwater near potato fields by the early 1980s, aldicarb degradates were not detected in water from a deeper 

monitoring well (84 ft) until several years after aldicarb use had been banned. This case study illustrates how changes in groundwater 

quality often lag years or even decades behind changes in chemical use at the land surface.

Potato field

Aldicarb use discontinued

Photograph from Can Stock Photo Inc.©

Aldicarb sulfoxide

EXPLANATION

Aldicarb sulfone

40

30

20

10

0

1978 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 2002 2004

Concentrations of aldicarb degradates in groundwater

Date

Concentration, in micrograms per liter 

54 Water Quality in the Northern Atlantic Coastal Plain Surficial Aquifer System, 1988–2009
Chapter 6

Figure 6–13. Volatile organic compounds (VOCs) were detected most frequently in samples of groundwater from public-supply wells. Most 

public-supply wells sampled in this study are located in urban areas, which is where VOCs are most commonly used. Chloroform, the VOC most 

commonly detected in both public- and domestic-supply wells, forms when water is treated with chlorine for disinfection. VOCs detected in 

groundwater of the surficial aquifer system include compounds that are used commonly and that are relatively stable in oxic groundwater.

Wells monitoring shallow groundwater

Agricultural areas

Urban areas

Network type

EXPLANATION

Any >0.2

Concentration, in 

micrograms per liter

Wells used for drinking water

0 20 40 60 80 100 0 20 40 60 80 100

Public supply

Domestic

Percentage of wells in which VOC 

was detected 

Number of samples

Percentage of wells in which VOC 

was detected

n = 93

Shallow groundwater, urban areas 

Trichloroethene

Methyl tert-butyl ether

1,1-Dichloroethane

1,4-Dichlorobenzene

1,1-Dichloroethene

Chloroform

Tetrachloroethene

1,1,1,-Trichloroethane

Trichloroethene

Methyl tert-butyl ether

1,1-Dichloroethane

1,4-Dichlorobenzene

1,1-Dichloroethene

Chloroform

Tetrachloroethene

1,1,1,-Trichloroethane

Trichloroethene

Methyl tert-butyl ether

1,1-Dichloroethane

1,4-Dichlorobenzene

1,1-Dichloroethene

Chloroform

Tetrachloroethene

1,1,1,-Trichloroethane

Trichloroethene

Methyl tert-butyl ether

1,1-Dichloroethane

1,4-Dichlorobenzene

1,1-Dichloroethene

Chloroform

Tetrachloroethene

1,1,1,-Trichloroethane

n = 93

n = 34 to 49

Shallow groundwater, agricultural areas 

n = 66 to 69

Wells used for drinking water, public supply

n = 67 to 73

Wells used for drinking water, 

domestic supply

VOCs in Groundwater

Most VOCs are used in urban areas because of common urban practices such as water 

treatment using chlorine, the use of solvents for dry cleaning and manufacturing, and the 

storage and use of gasoline hydrocarbons. As a result, most VOC detections, including detections of multiple VOCs in individual water samples, were in urbanized areas of the surficial 

aquifer system where leaks and spills are most likely to occur rather than in rural areas 

(figs. 6–13 and 6–14). VOCs were more frequently detected in samples of groundwater from 

the surficial aquifer system in the highly urbanized northern part of the surficial aquifer system 

than in samples across the Nation as a whole.(40)

Chapter 6: Understanding Where and Why Key Contaminants Occur in Groundwater 55
Photographs from top to bottom: Connie 

J. Ross, USGS; USGS Circular 1292; Luke 

Meyers, USGS

EXPLANATION

Land cover in 2001

Urban

Agricultural

Forested

Wetland

Water

0

1

2 or 3

4 or 5

> 5

Number of VOCs 

 detected in sample 

 (of 83 compounds)

N

Area of enlargement

Delaware Bay

Albemarle 

Sound

Pamlico Sound

Long Island Sound

CONNECTICUT

NEW

JERSEY

NEW PENNSYLVANIA

YORK

NORTH CAROLINA

WEST

MARYLAND

VIRGINIA

VIRGINIA

SOUTH 

CAROLINA

Chesapeake Bay

Fall Line

DELAWARE

ATLANTIC OCEAN

Washington, D.C.

VOCs are a class of organic compounds common in commercial 

and household products such as 

fuels, solvents, paints, refrigerants, and cleaners. VOCs also 

are used in many manufacturing 

applications and some are used 

as pesticides.

Figure 6–14. In the highly urbanized northern part of the Northern Atlantic Coastal Plain, at least one 

volatile organic compound (VOC) was detected in about 80 percent of samples, and three or more VOCs 

were detected in more than 35 percent of samples. In the less urbanized southern part of the Northern 

Atlantic Coastal Plain, VOCs were detected in less than 50 percent of samples, and three or more VOCs 

were detected in less than 25 percent of samples.

VOCs were more frequently 

detected in the highly 

urbanized northern part of 

the surficial aquifer system 

than in Principal Aquifers 

across the Nation as a 

whole.

56 Water Quality in the Northern Atlantic Coastal Plain Surficial Aquifer System, 1988–2009
Chapter 6

The VOCs that were most commonly detected in samples from the surficial aquifer system 

were the disinfection byproduct chloroform, the gasoline oxygenate methyl tert-butyl ether 

(MTBE), and the chlorinated solvents perchloroethene (PCE), 1,1,1-trichloroethane (TCA), 

and trichloroethene (TCE) (fig. 6–13). These VOCs also were among those most frequently 

detected nationally.(40) Chloroform typically is produced when water is disinfected using 

chlorine, a common practice at water treatment plants and in domestic wells, although it also 

can form naturally. The widespread detection of chloroform in samples from monitoring wells 

in urban areas and in drinking-water wells relative to its detection in samples from monitoring 

wells in agricultural areas might indicate leakage of chlorinated water from sewer lines and 

septic systems or treatment of domestic wells to remove bacteria. The presence of MTBE 

in groundwater at elevated concentrations, typically in urban areas, can result from gasoline 

leakage or spills. Its detection in groundwater at low concentrations in less urbanized areas 

might result from equilibration with MTBE in the atmosphere.(40) The chlorinated solvents 

detected in groundwater most likely come from point-source leaks or spills, such as dry 

cleaners, automotive shops, or manufacturing operations. VOCs were detected infrequently in 

agricultural areas of the Northern Atlantic Coastal Plain.

VOCs were detected more frequently in public-supply wells than in domestic wells 

(fig. 6–13), likely because most public-supply wells are in urban areas, whereas many of the 

domestic wells sampled are in rural areas where VOC use is low. Additionally, public-supply 

wells, which have larger capacity than domestic wells, have larger capture zones than domestic 

wells, so they are more likely to intercept water from multiple local sources of contamination 

(see sidebar, What types of wells were sampled, and how might that affect water quality?, 

p. 11). Unlike public-supply wells, routine monitoring of domestic wells is not required, and 

VOC contamination can go undetected.

Characteristics of soil and aquifer sediments and the presence or absence of dissolved 

oxygen can affect VOC occurrence in groundwater. Water can move more rapidly through 

sandy sediments, allowing little time for the VOCs contained in the water to break down. 

Groundwater moves more slowly through finer textured sediments, which allows more time 

for chemical reactions to occur.(40) Many commonly used VOCs, such as chloroform and some 

solvents, are unlikely to break down under oxic conditions and are common in sandy sediments 

of the urbanized northern part of the surficial aquifer system. Chlorinated solvents and MTBE, 

the VOCs most frequently detected, degrade slowly in oxic conditions and can persist for many 

decades in oxic groundwater. Other VOCs, such as gasoline hydrocarbons, degrade much more 

rapidly in oxic conditions; consequently, these VOCs were detected much less frequently, even 

though they are produced and used in great quantities. Although VOCs are used less in rural 

areas than in urban areas, anoxic conditions present in some parts of the surficial aquifer system 

such as coastal North Carolina (fig. 6–14) that are favorable for the degradation of some VOCs 

might contribute to the lower frequency of detection in that area.

The most frequently 

detected VOCs—a 

drinking water disinfection 

byproduct, a gasoline 

additive, and three 

chlorinated solvents—are 

unlikely to degrade in 

the groundwater.

Chapter 6: Understanding Where and Why Key Contaminants Occur in Groundwater 57
EXPLANATION

Land cover

Urban

Agricultural

Forested

Wetland

Water

Radium, in picocuries 

 per liter

≥2 to <5

<2

≥5

Chesapeake Bay

NEW 

JERSEY

VIRGINIA

DELAWARE

ATLANTIC OCEAN

Delaware Bay

PENNSYLVANIA

MARYLAND

Area

of

enlargement

NY

PA

WV MD

NJ

NC

TN

VA

DE

DC

Figure 6–16. Sediments of the Northern Atlantic Coastal Plain 

contain small amounts of the radioactive elements uranium and 

thorium. The natural radioactive decay of these elements produces 

the radium and radon present in groundwater. Here, a U.S. Geological Survey scientist uses a Geiger counter to measure radioactivity 

in sediments.

Radium

Samples collected from the northern part of the surficial aquifer system were analyzed 

for the carcinogenic radionuclide radium. Of the samples from drinking-water wells that were 

analyzed for radium, the concentration of radium exceeded the MCL of 5 pCi/L in one-third of 

those samples (fig. 6–15). In a NAWQA Program study of radium that included the Northern 

Atlantic Coastal Plain surficial aquifer system, the Mid-Continent and Ozark Plateau CambroOrdovician aquifer system was the only other Principal Aquifer where the concentration of 

radium exceeded the MCL in more than one-fourth of samples.(28) Exposure to radium can 

increase the risk of cancer, particularly when the exposure is from ingesting radium dissolved 

in drinking water.(21, 22, 41)

Unstable radioactive elements are found in all rocks, soil, and water at a wide range 

of concentrations. The most common radioactive elements—uranium and thorium, which 

are present in sediments of the surficial aquifer system 

(fig. 6–16)—decay slowly and produce other radioactive 

elements, such as radium and radon, which in turn decay to 

yet other elements (see sidebar, Sequence of radionuclide 

decade, p. 60).(42) Radioactive decay of radium produces radon 

gas, which is also a known carcinogen.

Ra

88

226

Figure 6–15. Concentrations of radium exceeded the U.S. 

Environmental Protection Agency Maximum Contaminant Level 

of 5 picocuries per liter in areas with well-weathered quartz-rich 

sediments and naturally acidic (low pH) groundwater and in areas of 

agricultural and urban land use. In agricultural and urban areas, the 

groundwater can become more acidic as the result of biogeochemical reactions involving applied nitrogen compounds.(42)

58 Water Quality in the Northern Atlantic Coastal Plain Surficial Aquifer System, 1988–2009
Chapter 6

The Acidity of Surficial Aquifer System Groundwater Causes Radium in 

Aquifer Sediments to Be Released to Groundwater

Radium is more soluble in acidic (pH less than 7) groundwater than in alkaline (pH greater 

than 7) groundwater and therefore typically occurs at higher concentrations where groundwater 

pH is low (fig. 6–17). Groundwater in many areas of the surficial aquifer system is naturally 

acidic where aquifer sediments are weathered and composed mostly of relatively insoluble 

quartz sand. Such insoluble aquifer sediments are particularly abundant in parts of the Middle 

and Inner Coastal Plain hydrogeologic subregions of the surficial aquifer system, which occur 

mostly in New Jersey, in the Delmarva Peninsula, and in North Carolina near the Fall Line (see 

sidebar, How hydrogeologic setting controls geochemical processes, p. 28). Some of the highest 

concentrations of radium measured in the surficial aquifer system were in parts of the Maryland 

Coastal Plain where pH was less than 3.9(43) and in parts of the New Jersey Coastal Plain where 

pH was less than about 4.9.(44, 45) Water with low pH that leads to release of radium from aquifer 

sediments also occurs in areas of the surficial aquifer system that have not been sampled for 

radium,(28) indicating that radium could be a broader-scale human-health concern in drinking 

water than is currently documented.

Concentrations of radium commonly are greater in groundwater in agricultural areas than 

in urban areas in the surficial aquifer system.(42) This difference might be related to fertilizer 

applications. Geochemical reactions that convert the nitrogen compounds in agricultural fertilizer to nitrate lower the pH (increase the acidity) of groundwater, causing radium in aquifer 

sediments to be released to groundwater.(43, 45) Lime commonly is applied where nitrogen 

fertilizers are applied, and ion-exchange reactions resulting from the constituents in the lime 

also can enhance release of radium from aquifer sediments into the groundwater.(42) Radium 

concentrations in shallow groundwater are higher than those in deep groundwater; concentrations decrease with depth because acidic groundwater is slowly neutralized (the pH increases) 

as it moves downward along flow paths and reacts with aquifer sediments.

Figure 6–17. Concentrations of radium in groundwater samples 

typically were higher in water with a pH less than 5 than in less acidic 

groundwater, but most samples contained at least some radium.

Radium could be a 

broader-scale humanhealth concern in drinking 

water from the surficial 

aquifer system than is 

currently documented 

because acidic groundwater, which tends to have 

higher concentrations 

of radium than alkaline 

groundwater, occurs 

in parts of the surficial 

aquifer system that have 

not been tested for radium.

10

2.0

4.0

6.0

8.0

Effect of pH on radium

Note: Boxplots are censored at 

2.0 picocuries per liter and 

therefore truncated.

Radium, in picocuries per liter 

Maximum 

Contaminant 

Level

pH 5 < pH >5

See sidebar, Boxplots, p. 45

Chapter 6: Understanding Where and Why Key Contaminants Occur in Groundwater 59
Measurement of Elevated Concentrations of Radium in Groundwater Has 

Led to New Guidelines or Mandates for Testing of Domestic Well Water in 

Some Areas

Under the Clean Water Act (Public Law 92–500, 86 Stat. 816), water that is supplied to 

consumers from public-supply wells is required to be tested on a regular basis for concentrations of nitrate, radium, and other contaminants. Routine testing of water from domestic wells, 

however, is not required. The measurement of radium at concentrations exceeding drinkingwater standards has motivated new guidelines or mandates for monitoring of groundwater 

from domestic wells in some areas of the surficial aquifer system where radium is known 

to occur, including parts of Maryland(46) and New Jersey.(47) Once homeowners are aware 

of the problem, they can lower concentrations of radium to an acceptable level with a wellmaintained water softener (cation-exchange) system(45, 48) or other treatment.

circ1353 ch06 CustomSidebar3

Sequence of radionuclide decay

Ra–226

1,622 y

Beta decay

Time of half-life:

 y, years; d, days; 

 h, hours; m, minutes;

 s, seconds

Alpha decay

Radioactive isotope

EXPLANATION

Uranium 238

Lead Lead

Thorium 232

Radium

Actinium

Uranium Thorium

Protactinium

Thorium

Radium

Radon

Other

radionuclides

with short

half-lives

Pa–234

1.18 m

Th–234

24.1 d

U–234

2.48x105 y

Th–230

7.52x104y

Ra–226

1,622 y

Rn–222

3.825 d

Pb–206

(stable)

Ra–228

5.75 y

Th–232

1.41x1010 y

Ac–228

6.13 h

Th–228

1.91 y

Ra–224

3.64 d

Pb–208

(stable)

Other

radionuclides

with short

half-lives

Modified from Hall and others;

(66) Zapecza and others(68)

U–238

4.49x109 y

Radium, which is radioactive, is formed by the natural radioactive decay of uranium and thorium contained in rocks and soils. 

Radium in turn decays to other radioactive elements, including radon. Radionuclides decay at different rates, as indicated by their 

half-lives. The final product of uranium and thorium decay is lead, which is stable (does not decay).

60 Water Quality in the Northern Atlantic Coastal Plain Surficial Aquifer System, 1988–2009
Chapter 7

This chapter describes 

the transport of contaminants from groundwater 

in the surficial aquifer 

system to surface waters 

of the Northern Atlantic 

Coastal Plain.

Nitrate, pesticides, and other contaminants discharge with groundwater 

to surface waters. In particular, the discharge of nitrate in groundwater 

to Northern Atlantic Coastal Plain streams is a concern because 

nutrient enrichment of streams and estuaries has caused widespread degradation of aquatic habitat. Improving the ecological condition of coastal waters 

will require that nitrogen contributions from groundwater decrease. It will 

take years to decades, however, before reductions in nitrogen inputs at the 

land surface are reflected in lower nitrate concentrations in groundwater that 

discharges to streams and coastal water bodies.

Chapter 7: Groundwater as a Source of 

Contaminants to Streams and Estuaries
circ 1353ch07_fig 01

0 to 10 years

10 to 20 years

20 to 30 years

more than 30 years

Stream

Water table

Unsaturated

zone

Confining unit

Direction of 

groundwater flow 

NOT TO SCALE

Figure 7–1. Most flow in local streams is contributed by groundwater that discharges from the surficial aquifer system. A stream contains a 

mixture of groundwater of different ages that is contributed from many different groundwater flow paths. Because groundwater recharge originates at different points within the stream watershed, stream chemistry reflects many local natural and human influences that are distributed 

throughout the watershed.

Groundwater and the 

nitrate it contains 

discharges to streams and 

directly into estuaries.

Groundwater Provides Most of the Water and Nitrate in Streams That 

Flow to Estuaries

Without groundwater, streams in the Northern Atlantic Coastal Plain would be dry most 

of the time. Groundwater provides 50 to 95 percent of freshwater flow in streams that originate 

in the surficial aquifer system and that flow to estuaries, such as Chesapeake and Delaware 

Bays, and to Long Island, Albemarle, and Pamlico Sounds.(12) Virtually all (about 99 percent) 

of the groundwater that discharges to these streams, which is referred to as base flow, is from 

the surficial aquifer system.(7) Groundwater also discharges directly to estuaries and other 

coastal waters (see sidebar, Groundwater discharge to coastal waters, p. 63).(49, 50) It takes from 

a few years to a few decades for groundwater to move from the water table through most parts 

of the surficial aquifer system and discharge to local surface waters (fig. 7–1). Groundwater 

that moves along deeper flow paths in thicker parts of the surficial aquifer system or through 

confined aquifers can take hundreds of years to discharge to streams and estuaries.(7)

Nitrogen, in the form of nitrate, is transported by groundwater to surface water. The 

primary source of nitrogen in most streams in the Northern Atlantic Coastal Plain is nitrate 

from the surficial aquifer system (fig. 7–2). During late winter and spring 2000, groundwater 

contributed more than 44,000 pounds of nitrogen in the form of nitrate (or 1.9 pounds per 

square mile on average) daily to headwater streams in the Northern Atlantic Coastal Plain 

that flow into ecologically sensitive coastal waters (see sidebar, Nitrate from groundwater 

discharging to surface water contributes to the ecological degradation of Northern Atlantic 

Coastal Plain coastal waters, p. 65(29)).

62 Water Quality in the Northern Atlantic Coastal Plain Surficial Aquifer System, 1988–2009
Groundwater discharge to coastal waters

Nitrate-rich groundwater discharge can contribute substantial 

loads of nitrogen to tidal creeks, estuaries, and the ocean.(51, 75, 86)

However, sediments in many coastal discharge areas, such as salt 

marshes and the bottom sediments of tidal creeks and lagoons, 

have a high organic carbon content,(49) and nitrate in discharging 

groundwater is removed by denitrification (see sidebar, How does 

nitrate enter groundwater? How is it lost from groundwater?, p. 31).

Chapter 7

Northern Atlantic Coastal Plain

EXPLANATION

≤37

> 37 to 54

> 54

Percentage of total nitrate load

 contributed by base flow

SC

DE

Washington,

 D.C.

NY CT

NJ

PA

WV

MD

VA

NC 

Modified from Dubrovsky and others(35)

Figure 7–2. Groundwater discharge contributes a substantial portion of the total amount of nitrate carried by streams in the Northern Atlantic 

Coastal Plain, particularly in the north where nonpoint nitrogen sources are abundant and concentrations of nitrate in groundwater are high. In 

the northern part of the surficial aquifer system, groundwater contributions of nitrate to streams are among the highest in the Nation.(89)

Aerial image of Queenstown, Maryland, along Queenstown 

Creek. Photograph by Jane Thomas, Integration and Application 

Network, University of Maryland Center for Environmental Science 

(ian.umces.edu/imagelibrary/).

Chapter 7: Groundwater as a Source of Contaminants to Streams and Estuaries 63
Chesterville Branch near Crumpton, Maryland, 1996–2001

10

9

8

7

6

5

4

3

2

1

0

0 10 100 1,000 10,000

Streamflow is a mix of 

groundwater and overland 

runoff

Streamflow

is primarily

groundwater

(base flow)

Nitrate concentration, in milligrams per liter as nitrogen

Streamflow, in cubic feet per second

Modified from Ator and others (11)

Concentrations of nitrate in 

streams commonly decrease 

during high-flow periods.

Figure 7–3. In Chesterville Branch near Crumpton, Maryland, nitrate concentrations approached 

10 milligrams per liter as nitrogen during base-flow periods, but decreased substantially during higher flow 

conditions as groundwater discharge was diluted by low-nitrate water from precipitation and runoff. 

Concentrations of nitrate in streams usually are highest during base-flow conditions when 

most of the water is from groundwater, especially in winter, when consumption of nitrogen by 

aquatic organisms is minimal.(11, 53, 54) Although the median concentration of nitrate in streams 

of the Northern Atlantic Coastal Plain during base flow is less than 1 mg/L as N, concentrations greater than 10 mg/L as N have been measured in some streams.(11, 51) During high flow, 

concentrations of nitrate typically are lower because contributions from precipitation and 

overland runoff, which have low concentrations of nitrate, dilute the contributions from nitraterich groundwater (fig. 7–3).(11, 52, 56)

If nitrate-rich groundwater flows through an anoxic zone (dissolved oxygen less than 

0.5 mg/L), such as can occur in the riparian zone or in the fine-grained organic-rich sediments 

of the hyporheic zone, the nitrate can be removed by denitrification (fig. 7–4). The greatest 

potential for denitrification in riparian-zone sediments is where the surficial aquifer is very thin 

and where organic-rich sediments with reducing conditions extend to the base of the aquifer, 

such as in eastern North Carolina.(57, 60) In contrast, at a study site in Maryland with a thick 

sandy aquifer and sandy streambed sediments with little organic matter, nitrate discharged 

relatively unaltered to the stream.(62) Settings similar to those at the Maryland study site are 

most common in the northern part of the surficial aquifer system where the aquifer is thick 

enough that groundwater flows beneath the influence of the riparian zone (fig. 7–4).

64 Water Quality in the Northern Atlantic Coastal Plain Surficial Aquifer System, 1988–2009
Chapter 7

Ditch

Hyporheic 

zone

Riparian zone

Tile drains

Organic

rich soil 

Water table

Deep groundwater with low

concentrations of nitrate

High concentrations

 of nitrate

High concentrations

of nitrate

Modified from Dubrovsky and others NOT TO SCALE (35)

Figure 7–4. Many factors and processes can affect nitrate-rich groundwater as it moves from the point of recharge to discharge 

in streams. Where the aquifer is thick, nitrate-rich water can flow beneath the organic-rich, reducing soils of the riparian zone 

and discharge relatively unaltered to streams through sandy streambed sediments. This nitrate-rich groundwater can be diluted 

by nitrate-poor groundwater that recharged through the riparian zone, or by deep, old groundwater that is nitrate-poor because 

it recharged before the widespread use of fertilizers or other sources of nitrogen. Nitrate can be removed from groundwater in 

various ways before it discharges to streams. Nitrate can be taken up by plants in the riparian zone or in the stream channel. If 

sufficient organic matter is present in the aquifer, in the riparian zone, or in the streambed sediments of the hyporheic zone, nitrate 

can be transformed by denitrification to nitrogen gas. However, groundwater recharge that contains nitrate can be intercepted by 

tile drains or ditches and diverted to surface water before the groundwater encounters reducing conditions.

Nitrate from groundwater discharging 

to surface water contributes to the 

ecological degradation of Northern 

Atlantic Coastal Plain coastal waters

High concentrations of nitrogen (often in the form of 

nitrate from groundwater) and phosphorus from fertilizers, 

manures, and sewage contribute to the fair to poor water 

quality in tidal estuaries and other coastal waters along 

much of the Northern Atlantic Coastal Plain.(83) Ecological 

problems associated with excessive nutrients include 

algal blooms, decreased dissolved oxygen, abundance 

of submerged aquatic vegetation, and degradation of 

aquatic life habitats, leading to declining fisheries. The 

water-quality index is based on a combination of indicators of ecological health that are measured throughout 

the region. These indicators include levels of nitrogen, 

phosphorus, chlorophyll a (in algae), water clarity, and 

dissolved oxygen.

Water-quality index

Chapter 7: Groundwater as a Source of Contaminants to Streams and Estuaries 65
0 20 40 60

Deethylatrazine

Metolachlor ESA

Prometon

Simazine

Alachlor

Alachlor ESA

Alachlor OA

Metolachlor

Metolachlor OA

Atrazine

Acetochlor ESA

Percentage detected

Acetochlor OA

Acetochlor

Percentage of stream base-flow samples in which

contaminant was detected

80 100

EXPLANATION

Percentage detected at 

 concentration 

 >0.1 microgram per liter

(Bold indicates parent; italic

indicates degradate)

Figure 7–5. Pesticide (parent and degradate) compounds are detected frequently in streams of the 

Northern Atlantic Coastal Plain during base-flow conditions, when groundwater discharge is the primary 

source of water. The compounds detected most frequently in streams are among those detected most 

frequently in groundwater (see figure 6–8).

Groundwater Discharge Contributes Pesticides to Streams in the 

Northern Atlantic Coastal Plain

Soluble pesticides and their degradates in groundwater are transported along flow paths 

and discharge to streams (fig. 7–5).(11, 71) Although most pesticides are transported to streams 

in overland flow that happens when there are storms during the pesticide application season, 

groundwater provides a continuous source of pesticide compounds to streams throughout the 

year. During late winter and spring 2000, more than 57 pounds of alachlor, atrazine, metolachlor, and certain degradates (combined) were contributed daily from groundwater to headwater streams of the Northern Atlantic Coastal Plain.(29) Concentrations of individual pesticide 

compounds in base flow generally were less than 0.1 μg/L, but concentrations of some pesticide 

degradates in some streams were more than 3 μg/L. As in groundwater, pesticide compounds in 

stream base flow often occur in mixtures of several compounds; at least five different pesticides 

or degradate compounds were detected in each of the 23 headwater streams sampled on the 

Delmarva Peninsula during base flow in late winter and spring 2000, and one stream contained 

17 different compounds.(52) Pesticide concentrations in Northern Atlantic Coastal Plain streams 

during base flow generally are far below levels likely to harm fish and aquatic invertebrates.(59)

66 Water Quality in the Northern Atlantic Coastal Plain Surficial Aquifer System, 1988–2009
Chapter 7

0

1

2

3

4

0

20

40

60

80

100

Nitrate

Metolachlor (including degradates)

Coastal Plain area of

major watersheds

Coastal Plain

hydrogeologic subregions

Percentage of applied 

metolachlor that enters streams 

with groundwater discharge 

Percentage of total nitrogen load

 in streams from groundwater

B

A

Entire Northern Atlantic

Coastal Plain

Inner Coastal Plain (ICP), 

upland sand and gravels

Coastal lowlands

Middle Coastal Plain (MCP), 

mixed sediments

MCP, fine sediments

MCP, sand with overlying gravels

ICP, dissected outcrop belt

Alluvial and estuarine valleys

Albemarle and Pamlico Sounds

Chesapeake Bay, Eastern Shore

Chesapeake Bay, Western Shore

Delaware Bay

Modified from Ator and Denver(29)

Figure 7–6. Patterns across hydrologic subregions (see sidebar, How hydrogeologic setting controls geochemical processes, p. 28) and major 

watersheds in A, the percentage of nitrate in total nitrogen loads in streams that comes from groundwater are similar to B, the percentage of 

applied metolachlor that enters streams with groundwater discharge. This similarity in patterns likely occurs because the factors that control 

the transport of nitrate through groundwater to surface water also control the transport of metolachlor. Nitrate from groundwater provides much 

of the daily load of nitrogen in streams in some parts of the Northern Atlantic Coastal Plain. About 1 to 3 percent of applied metolachlor reaches 

streams in the Northern Atlantic Coastal Plain through groundwater discharge, mostly in the form of degradate compounds.

Chemical Properties and Local Natural and Human Influences Control the 

Transport of Chemical Contaminants From Groundwater to Streams

Only compounds that dissolve in water and that do not break down in groundwater, such 

as atrazine, metolachlor, and nitrate, will survive the typically years- to decades-long trip along 

groundwater flow paths to streams. Concentrations of nitrate and pesticides in stream base flow 

generally are greatest near areas where they are used and where local soil, hydrogeologic, and 

geochemical conditions promote their movement and stability in groundwater (see chapters 5 

and 6).(53, 73, 74) Nitrate, for example, is stable along groundwater flow paths where groundwater 

is oxic, but is transformed to nitrogen gas where groundwater passes through anoxic areas 

before discharging to streams.(16, 61)

Groundwater on the Delmarva Peninsula is particularly effective at transporting contaminants to streams because contaminant sources at the land surface are abundant and because 

well-drained and well-oxygenated soils and a shallow water table promote the movement of 

nitrate and other chemicals into and through the shallow surficial aquifer (see sidebar, The 

effects of groundwater nitrate on restoration of the Chesapeake Bay, p. 68). The Eastern Shore 

of the Chesapeake Bay, which is on the Delmarva Peninsula, generates more than half of the 

fluxes of base-flow alachlor, atrazine, metolachlor, and nitrate to streams in the entire Northern 

Atlantic Coastal Plain (excluding Long Island, for which data were not available), even though 

it makes up only 9 percent of that area (fig. 7–6).(29)

Chapter 7: Groundwater as a Source of Contaminants to Streams and Estuaries 67
The effects of groundwater nitrate on 

restoration of the Chesapeake Bay

The Chesapeake Bay, which is the largest and one of the 

most biologically diverse estuaries in North America, was listed 

as “impaired” under the Clean Water Act in 2000 partly because 

of excessive concentrations of nitrogen.(72) Most of the nitrogen 

in the Chesapeake Bay comes from tributary streams(76) that 

receive about half of their nitrogen from groundwater.(85) A total 

maximum daily load (TMDL) has been established to help manage 

nitrogen reductions in the bay,(87) and Federal agencies have 

been directed to develop a new strategy to protect and restore 

Chesapeake Bay.(90)

Management activities designed to limit nitrogen loading to 

the Chesapeake Bay will have to address the groundwater contribution of nitrate. They will also have to consider the substantial 

lag times between improvement to shallow groundwater quality 

and the discharge of that groundwater to streams and the estuary. 

The problem is particularly acute on the Eastern Shore, where 

groundwater contributions of nitrate make up 70 percent of the 

total nitrogen load to the bay from that area.

The Eastern Shore Coastal Plain is less than half the size of 

the Western Shore Coastal Plain, but it is intensively agricultural 

and its sandy sediments are favorable for transport of nitrate 

into and by groundwater to surface water. The Western Shore, 

in contrast, which has more fine-grained aquifer sediments, has 

more urban and forested land and less agriculture. As a result, 

Eastern Shore tributaries deliver nearly eight times more nitrate 

(27,300 pounds per day) to the Chesapeake Bay than Western 

Shore tributaries that originate in the Coastal Plain (3,500 pounds 

per day).

EASTERN SHORE

WESTERN SHORE

Chesapeake

Watershed

Chesapeake Bay

Fall Line

3,500 27,300

Washington D.C.

Baltimore

Richmond

MARYLAND

DELAWARE

VIRGINIA

NEW PENNSYLVANIA

JERSEY

Land use

Urban

Agricultural

Forested

Wetland

Water

Other

EXPLANATION

Amount of nitrogen 

 delivered, in pounds 

 per day

It will take years to decades 

for reduction in inputs of 

nitrate and other contaminants to groundwater to be 

reflected in water-quality 

improvements in streams 

and estuaries.

Contributions of Contaminants From Groundwater to Streams Reflect the 

History of Local Land Use, Land Management, and Chemical Applications

Management practices aimed at improving the quality of Northern Atlantic Coastal Plain 

surface water will require lowering inputs of nitrate to groundwater. However, the effect of 

reductions of nitrate to groundwater will not immediately improve surface-water quality. 

Because of the time it takes for groundwater to move from the point of recharge to the point of 

discharge, it will take years to decades before reductions in nitrogen inputs to the water table 

result in substantial water-quality improvements in streams and coastal estuaries. The time 

required will be even longer in areas where the unconfined aquifer is thick and flow paths are 

long (fig. 7–7).(14)

Water quality of stream base flow reflects both recent and past chemical inputs at the land 

surface and other influences. An increase in concentrations of nitrate in the Choptank River on 

the Delmarva Peninsula in recent decades (1980s to 2000s), for example, is related to intensive 

fertilizer applications in previous decades, which are just now being reflected in concentrations 

of nitrate in discharging groundwater.(82, 88) On Long Island, the beneficial effects of sewer 

installation in the 1950s were still being seen into the 1990s, as concentrations of nitrogen in 

surface water receiving groundwater discharge continued to decrease (fig. 7–8).

68 Water Quality in the Northern Atlantic Coastal Plain Surficial Aquifer System, 1988–2009
Chapter 7

circ1353ch07-fig 07

10 to 20 yrs

Less than 10 yrs

Less than

10 yrs

30+ yrs 20 to 30 yrs

Confining unit

Surficial aquifer

Higher

Nitrate concentration

Lower

Travel-time of nitrate through groundwater to streams

14

12

10

8

6

4

2

0

4

2

0

70 75 80 85 90 95 98

Year

Bellmore Creek

(sewered)

Patchogue River 

(unsewered)

21.5

Trends in nitrogen concentrations

B

A

Total nitrogen concentration, in milligrams per liter

Modified from Monti and Scorca(51)

Figure 7–7. For streams that receive much of their nitrate from groundwater discharge, many years or even decades are typically required before results of management actions or land-use changes that reduce nitrogen inputs at the land surface are fully 

reflected in surface-water quality.

Figure 7–8. Trends in nitrogen concentrations in two streams on Long Island, New York, reflect changes 

in groundwater contamination that resulted from contrasting changes in waste-water disposal practices. 

A, Nitrogen concentrations in streams have decreased substantially in an area with intensive development 

where sewers were installed beginning in the 1950s. B, Nitrogen concentrations are slowly increasing in an 

unsewered area where onsite sewage disposal with septic systems is continuing.

Chapter 7: Groundwater as a Source of Contaminants to Streams and Estuaries 69
70 Water Quality in the Northern Atlantic Coastal Plain Surficial Aquifer System, 1988–2009

http://water.usgs.gov/nawqa/pasumm

http://water.usgs.gov/nawqa/bib
References

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86. Dillow, J.J.A., and Greene, E.A., 1999, Ground-water 

discharge and nitrate loadings to the coastal bays of 

Maryland: U.S. Geological Survey Water-Resources 

Investigations Report 99–4167, 8 p., accessed 

February 1, 2001, at http://md.water.usgs.gov/publications/

wrir-99-4167/.

87. U.S. Environmental Protection Agency, 2010, Chesapeake 

Bay compliance and enforcement strategy, May 12, 

2010: U.S. Environmental Protection Agency, accessed 

September 11, 2013, at http://www.epa.gov/reg3wapd/tmdl/

ChesapeakeBay/tmdlexec.html.

88. Hirsch, R.M., Moyer, D.L., and Archfield, S.A., 2010, 

Weighted regressions on time, discharge, and season 

(WRTDS), with an application to Chesapeake Bay 

river inputs: Journal of the American Water Resources 

Association, v. 46, no. 5, p. 857–880, doi:10.1111/j.1752-

1688.2010.00482.x.

89. Spahr, N.E., Dubrovsky, N.M., Gronberg, J.M., Franke, 

O.L., and Wolock, D.M., 2010, Nitrate loads and 

concentrations in surface-water base flow and shallow 

groundwater for selected basins in the United States, 

water years 1990–2006: U.S. Geological Survey Scientific 

Investigations Report 2010–5098, 20 p., accessed June 29, 

2010, at http://pubs.usgs.gov/sir/2010/5098/.

90. Federal Leadership Committee for Chesapeake Bay, 2010, 

Strategy for protecting and restoring the Chesapeake 

Bay watershed: U.S. Environmental Protection Agency 

EPA–903–R–10–003, accessed August 12, 2013, at http://

executiveorder.chesapeakebay.net/file.axd?file=2010%2F5

%2FChesapeake+EO+Strategy%20.pdf.

91. U.S. Geological Survey, [undated], Source water-quality 

assessment (SWQA) program: U.S. Geological Survey, 

accessed August 30, 2013, at http://water.usgs.gov/

nawqa/swqa/.

92. New Jersey Department of Environmental Protection, 2003, 

Susceptibility of source water to community water-supply 

wells in New Jersey to contamination by volatile organic 

compounds: New Jersey Department of Environmental 

Protection, 21 p. (Also available at http://www.state.nj.us/

dep/swap/reports/gw_voc.pdf.)

References Cited 75
Glossary

A

adsorb/adsorption Process in which ions or 

molecules dissolved in groundwater become 

attached to the surfaces of solid materials, such 

as sediment or rock particles in aquifers, either 

temporarily or permanently. See sorption.

anoxic Water with no dissolved oxygen or a 

very low concentration (less than 0.5 milligram 

per liter) of dissolved oxygen.

anthropogenic Associated with, occurring 

because of, or influenced by human activity.

apparent groundwater age The time elapsed 

since the recharge water became isolated from 

the atmosphere. The term "age" is normally 

qualified with the word "apparent" to signify 

that the accuracy of the determined age depends 

on many variables.

aquifer A geologic formation, group of 

formations, or part of a formation that contains 

a sufficient amount of saturated permeable 

material (for example, soil, sand, gravel and (or) 

rock) to yield substantial quantities of water to 

wells and springs.

B

base flow Groundwater seepage into a stream 

or river. The continual contribution of groundwater to streams and rivers is an important 

source of streamflow between rain events.

carbonate rock Rocks, such as limestone 

or dolostone, that are composed primarily 

of minerals, such as calcite and dolomite, 

containing the carbonate ion (CO3

2–).

C

confined aquifer (artesian aquifer) An aquifer 

in which the groundwater is bounded between 

layers of relatively impermeable material, 

such as clay or dense rock. When tapped by a 

well, water in a confined aquifer is forced up, 

sometimes above the land surface, by pressure 

within the aquifer.

confining unit A hydrogeologic unit of 

impermeable or distinctly less permeable 

material within an aquifer or bounding one or 

more aquifers.

constituent A chemical or biological substance 

in water, sediment, or biota that can be measured 

by an analytical (laboratory) method.

contaminant For the purposes of this report, 

any manmade compound at any concentration, 

or any constituent with a geologic source 

measured at a concentration exceeding the 

designated human-health benchmark.

D

degradate A compound formed by the 

transformation of a parent compound, typically 

an organic contaminant or another degradate, 

by chemical, photochemical, or biological 

reactions.

degradation The breakdown of a compound 

to a chemically simpler compound by abiotic or 

biotic processes.

denitrification The bacterial reduction of 

dissolved nitrate to nitrogen gas. Denitrification 

is the primary process by which nitrate can be 

eliminated naturally in groundwater.

desorption The release of a sorbed material 

from the solid to which it was sorbed. Opposite 

process of adsorption.

discharge The rate of flow of surface water 

or groundwater past a given point at a given 

moment, expressed as volume per unit of time. 

Also, the outflow from an aquifer, spring, or 

well or up through a streambed.

domestic well A privately owned well that 

typically serves one home and supplies water for 

human consumption and other homeowner uses.

drinking-water standard or guideline A 

threshold concentration in a public drinkingwater supply designed to protect human health 

or to identify acceptable concentrations of 

constituents that cause unpleasant tastes, odors, 

or colors in the water.

E

eutrophication The enrichment of water by 

nutrients, most commonly phosphorus and 

nitrogen. During eutrophication, respiration 

processes that use organic matter cause a 

marked decline in dissolved-oxygen concentrations of water.

evapotranspiration Loss of water from soil by 

evaporation and plant transpiration combined.

76 Water Quality in the Northern Atlantic Coastal Plain Surficial Aquifer System, 1988–2009
Glossary

F

flow path The route or pathway of water 

flowing through the hydrologic system. Typi-

cally refers to subsurface (groundwater) flow.

fumigant A pesticide in the volatile organic 

compound (VOC) chemical class that is applied 

to soils to reduce populations of plant parasitic 

nematodes (harmful rootworms), weeds, fungal 

pathogens, and other soil-borne microorganisms. G

groundwater Water that exists beneath the 

land surface, but most commonly refers to water 

in fully saturated soils and geologic formations.

groundwater age See apparent groundwater 

age.

groundwater discharge The flow of water 

from the saturated zone, for example, from a 

spring or a well or as seepage to surface water.

groundwater flow path See flow path.

groundwater recharge The infiltration of 

water to the saturated zone. Also refers to water 

that reaches the water table by infiltration of 

precipitation or irrigation water through the 

unsaturated zone or by seepage of water from 

surface-water bodies such as streams and lakes.

groundwater residence time The average 

amount of time it takes for groundwater to move 

from the point where it enters the aquifer to 

a specific point of discharge such as a well or 

stream. H

half-life The time required for the concentra-

tion of a compound in a given environmental 

medium to be reduced to one-half of its original 

value by one or more processes, such as degra-

dation or transport into another environmental 

medium.

headwater stream The upper part of a stream, 

especially a large stream or river, including the 

upper drainage basin.

Health-Based Screening Level (HBSL) An 

estimate of concentration (for a noncarcinogen) 

or concentration range (for a carcinogen) in 

water that (1) may be of potential human-health 

concern, (2) can be used as a threshold value 

against which measured concentrations of 

contaminants in ambient groundwater samples 

can be compared, and (3) is consistent with U.S. 

Environmental Protection Agency Office of 

Water methodologies.

herbicide A chemical pesticide designed to 

control or destroy plants, weeds, or grasses.

human-health benchmark A threshold 

concentration above which the concentration 

of a contaminant in drinking water could have 

adverse effects on human health. Treatment or 

other measures can be used before the water 

is consumed to lower the concentration of the 

contaminant below the benchmark.

hydrogeologic setting A unit with common 

hydrogeologic characteristics and therefore 

common susceptibility to contamination; a 

composite description of all the major geologic 

and hydrologic factors that affect and control the 

movement of groundwater into, through, and out 

of an area.

hydrogeology The geologic and hydrologic 

features that control the movement of water, 

solutes, and small particles through the 

subsurface.

hyporheic zone The region beneath and next 

to a streambed where surface water and shallow 

groundwater mix. I

infiltration Movement of water, typically 

downward, into soil or porous rock.

isotope Each of two or more forms of the 

same element with the same atomic number (the 

same number of protons) but a different number 

of neutrons and therefore different atomic 

weights. L

land-use study A study by the U.S. Geological 

Survey National Water-Quality Assessment 

(NAWQA) Program to assess the effects of a 

specific land-use type (generally agricultural or 

urban) on groundwater quality, in most cases by 

sampling groundwater from monitoring wells 

that tap water from or near the water table. M

major aquifer A regionally extensive subsur-

face geologic formation or group of formations 

that is used, or has the potential to be used, as a 

substantial groundwater resource.

major aquifer study A study by the U.S. 

Geological Survey National Water-Quality 

Assessment (NAWQA) Program that involves 

sampling of water at 20 to 30 domestic and 

(or) public-supply wells that withdraw water 

from major aquifers. The major aquifer studies 

represent a mix of land uses and target water 

that is used for drinking-water supply.

Maximum Contaminant Level (MCL) Maximum 

permissible level of a contaminant in water that 

Glossary 77
is delivered to any user of a public water system. 

MCLs are enforceable standards established by 

the U.S. Environmental Protection Agency.

median The middle or central value in a 

distribution of data ranked in order of magnitude 

such that one-half of the data are higher than the 

median and one-half are lower. The median is 

also called the 50th percentile.

milligrams per liter (mg/L) A unit expressing 

the concentration of a chemical constituent 

as weight (milligrams) of constituent per unit 

volume (liter) of water; equivalent to one part 

per million in most streamwater and groundwater. One thousand micrograms per liter (μg/L) 

is equivalent to 1 mg/L.

monitoring well A well used to measure water 

quality or groundwater levels continuously or 

periodically. Not typically used as a source of 

drinking water. Sometimes referred to as an 

"observation well."

N

nitrate An ion consisting of one nitrogen atom 

and three oxygen atoms (NO3

–

). Nitrate is a 

plant nutrient and is very mobile in soils.

nitrification The formation of nitrates by the 

oxidation of ammonium salts to nitrites followed 

by oxidation of nitrites to nitrates.

nonpoint source A contaminant source that 

is not a discrete point, such as a pipe, ditch, or 

tunnel. Areas of fertilizer and pesticide applications, atmospheric deposition, and stormwater 

runoff are examples of sources of nonpoint 

contamination.

nutrient An element or compound essential 

for animal and plant growth. Common nutrients 

include nitrogen, phosphorus, and potassium, 

such as are found in fertilizer.

O

organic matter Matter resulting from the decay 

of a plant or an animal and containing organic 

carbon compounds. Organic matter is rich in 

nutrients and is an essential component of soils.

organic compound A chemical compound 

containing the element carbon. A few types 

of carbon-containing compounds, including 

oxides of carbon (such as carbon dioxide, or 

CO2

), are not considered to be organic. Organic 

compounds include petroleum products, 

solvents, and many pesticides.

oxic Water with a concentration of 

dissolved oxygen greater than or equal to 

0.5 milligram per liter.

oxidation The loss of electrons by a chemical 

species as a result of transfer to another 

chemical species, typically dissolved oxygen. 

The species donating electrons is "oxidized."

P

permeable Capable of transmitting liquids or 

gases through pores or openings.

pesticide Any substance, organic or inorganic, 

used to kill plant or animal pests.

pH A measure of the acidity (pH less than 7) or 

alkalinity (pH greater than 7) of a solution; a pH 

of 7 is neutral. Formally defined as the logarithm 

of the reciprocal of the hydrogen ion concentration (activity) of a solution.

point source A stationary location or fixed 

facility from which contaminants are discharged, 

for example, a pipe, ditch, ship, ore pit, or 

factory smokestack.

precipitation Any or all forms of water 

particles that fall from the atmosphere, such as 

rain, snow, hail, and sleet. Also, the process in 

which a solid is formed from a fluid supersaturated with dissolved ions.

Principal Aquifer A regionally extensive 

aquifer or aquifer system that has the potential to 

be used as a source of potable water. A Principal 

Aquifer can be composed of one or more major 

aquifers.

public-supply well A privately or publicly 

owned well that provides water for public use 

to (1) a community water system, (2) a transient 

noncommunity water system such as a campground, or (3) a nontransient, noncommunity 

system, such as a school.

R

radioactive decay The spontaneous emission 

of particles (alpha or beta) and gamma rays from 

an atom with an unstable nucleus (radionuclide).

recharge The addition of water to the 

saturated zone naturally by precipitation or 

runoff or artificially by spreading or injection. 

Also, the water that is added.

reduction The gain of electrons by a chemical 

species, typically dissolved oxygen, as a result 

of transfer from another chemical species. The 

species accepting electrons is "reduced." Once 

all of the dissolved oxygen has been reduced, 

other chemical species can accept electrons, 

following the most energetically favorable order.

reduction/oxidation (redox) Chemical reactions that involve the transfer of electrons from 

one chemical species to another, resulting in a 

78 Water Quality in the Northern Atlantic Coastal Plain Surficial Aquifer System, 1988–2009
Glossary

change in the valence state of the species. Redox 

processes in groundwater often are microbially 

facilitated.

residence time The average amount of time 

that a solute, particle, organism, or other entity 

spends within a given environmental medium, 

such as a lake, groundwater, and the atmosphere.

riparian zone Pertaining to or located on the 

bank of a body of water, especially a stream.

runoff Excess rainwater or snowmelt that is 

transported to streams by flow over the land 

surface. S

Secondary Maximum Contaminant Level 

(SMCL) Guidelines set by the U.S. Environ-

mental Protection Agency for concentrations of 

"nuisance" constituents in drinking water that 

may cause unwanted effects, such as unpleasant 

taste, color, or odor; discoloration of skin or 

teeth; or corrosion or staining of plumbing 

fixtures. Public drinking-water systems are 

recommended but not required to comply with 

these guidelines.

siliciclastic rocks Rocks formed by the 

compaction and cementation of quartz-rich 

mineral grains. Sandstone and shale are 

examples of siliciclastic rocks.

sorption The general process by which solutes, 

ions, and colloids become attached to solid 

matter.

study unit A major hydrologic system of 

the United States, geographically defined by 

surface- or groundwater features, in which U.S. 

Geological Survey National Water-Quality 

Assessment (NAWQA) Program sampling 

studies are focused.

subsurface The region of earth materials 

beneath the land surface that encompasses the 

soil and unsaturated and saturated zones. T

tile drain A drain installed to collect subsur-

face water and route it to a drainage ditch, 

stream, or wetland. Originally constructed using 

short segments of clay or cylindrical concrete 

“tiles” and installed manually, modern tile drains 

typically are corrugated, perforated plastic pipes 

installed 3 to 6 feet below the soil surface by 

mechanical trenchers.

trace element An element found in only minor 

amounts (concentrations less than 1 milligram 

per liter) in water or sediment; includes arsenic, 

cadmium, chromium, copper, lead, mercury, 

nickel, and zinc. U

unconfined aquifer An aquifer that has a 

water table; an aquifer containing unconfined 

groundwater.

unsaturated zone A subsurface zone 

containing both water and air. The unsaturated 

zone is limited above by the land surface and 

below by the water table. V

volatile organic compound (VOC) An organic 

chemical that has a high vapor pressure relative 

to its water solubility. VOCs include compo-

nents of gasoline, fuel oils, lubricants, organic 

solvents, fumigants, some inert ingredients in 

pesticides, and some byproducts of chlorine 

disinfection.

vulnerability The tendency or likelihood for 

contaminants to reach a specified position in 

the groundwater system after introduction at 

some location above the uppermost aquifer. 

The vulnerability of a groundwater resource 

to contamination depends both on the intrinsic 

susceptibility of the resource and on the loca-

tions and types of human and geologic sources 

of contaminants, locations of wells, and the 

characteristics of the contaminant(s). W

watershed The land area that drains into a 

particular stream, river, lake, estuary, or coastal 

zone.

water table The upper surface of the saturated 

zone below which all voids (spaces) are filled 

with water.

Glossary 79
Acknowledgments

Special thanks to the following individuals and organizations for their 

contribution:

NAWQA and other USGS personnel in the Northern Atlantic Coastal Plain Principal 

Aquifers study area for their contributions of data and research results.

NAWQA’s many partners in the governmental and non-governmental sectors who have 

help guide scientific efforts and ensure that NAWQA information meets the needs of local, 

State, regional, and national stakeholders.

USGS personnel Jessica Carpenter for preparing illustrations; and Leslie DeSimone, 

Sharon Qi, and Michael Rupert for assistance with preparation of the database, appendices, and 

Web pages.

USGS reviewers:

• Michael Focazio

• John Masterson

• Wayne Lapham

• Barbara Mahler

• Sandra Eberts

80 Water Quality in the Northern Atlantic Coastal Plain Surficial Aquifer System, 1988–2009
Appendixes

Appendix 1. Study Components of the Northern Atlantic Coastal Plain

Surficial Aquifer System 

Appendixes 81

How does land use affect groundwater quality? How does water 

quality change as it moves through an aquifer? What is the quality 

of the drinking-water resource? National Water-Quality Assessment 

(NAWQA) Program groundwater assessments include different types 

of studies, specifically designed to answer questions such as these. 

The following is a brief listing of the various NAWQA studies. The 

general location of these studies and a description of each are shown 

in figure A1–1 and table A1–1.

•	 Land-use studies sampled water from water-table monitoring 

wells installed in urban and agricultural areas to assess the 

effects of these land uses on the quality of the underlying 

groundwater. Although not usually used for drinking, this 

water supplies recharge to deep parts of the aquifer system.

•	 Flow-path studies investigate how water quality changes 

as it moves along a groundwater flow path. Samples 

were collected from wells installed along a groundwater flow path.(1)

•	 Major aquifer studies provide a broad overview of the quality 

of the deep parts of the aquifer system used for drinkingwater supply. Most of the wells sampled were domestic 

wells that were distributed across a large area in a mixture of 

land uses.

•	 Source-water quality assessment studies sampled water 

from public-supply wells to understand occurrence of 

unregulated manmade chemicals in the groundwater 

resources that serve large numbers of people.(91)

Results of these studies were reinforced by locating some of 

the studies within the boundaries of larger studies. For example, 

the recharge areas for flow-path studies were located within the 

boundaries of the land-use studies to provide information on the 

quality of the recharge. In turn, the boundaries of the land-use studies 

were located within the boundaries of the major aquifer studies to 

provide information on how the quality of the recharge affects that 

of the groundwater from the deep parts of the aquifer. Each study 

sampled water from a network of a few to as many as 30 wells. 

Data from these studies were available for the Principal Aquifer 

assessments.
circ1353 appendix1_fig 01

1

2

6

6

17

11

8

9

7

16

14

15

12

10

3

4

13

5

ATLANTIC OCEAN

PENNSYLVANIA

VIRGINIA

MARYLAND

DELAWARE

NEW 

JERSEY

WEST 

VIRGINIA

NEW 

YORK

CONNECTICUT

NORTH CAROLINA

SOUTH

CAROLINA

Washington, D.C.

Fall Line

EXPLANATION

Well type:

Drinking-water study

Drinking-water study in Castle 

Hayne aquifer

Agricultural land-use study

Urban land-use study

Special study of public supplies

Other studies of surficial aquifer

17 Indicates location of study

 component on table A1–1

Area of enlargement

N

82 Water Quality in the Northern Atlantic Coastal Plain Surficial Aquifer System, 1988–2009

Figure A1–1. General location of components of the study of the Northern Atlantic Coastal Plain 

surficial aquifer. 
Appendixes

Table A1–1. Study components of the Northern Atlantic Coastal Plain surficial aquifer system assessment.

[F, field parameters including water temperature, specific conductance, pH, alkalinity, and dissolved oxygen; M, major ions; N, nutrients; T, trace elements; P, 

pesticide compounds; V, volatile organic compounds; R, radon; RA, radium; PWS, public-supply wells; MON, monitoring wells; DOM, domestic wells; ft, feet]

Map 

number Study location Water-quality parameters

Year(s) 

sampled

Number of 

wells Well types

Range 

of well 

depths 

(ft)

Drinking-water studies

1 New Jersey F,M,N,P,V,T 1998, 2002 33 DOM 38–175

2 Delmarva Peninsula F,M,N,P,V,T,R 2001 29 DOM, MON 15–100

3 Virginia, North Carolina F,M,N,P,V,T 1994, 2002 27 MON, DOM 5–75

4 North Carolina, confined wells F,M,N,P,V,T 1994 6 PWS 329–612

5 North Carolina, Castle Hayne aquifer F,M,N,P,V,T 2002–03 30 MON, DOM, PWS 22–240

6 New Jersey, Delaware, Maryland F,M,N,P,V,T 2003–04 24 DOM, MON 41–155

Urban land-use studies

7 Long Island, NY F,M,N,P,V 2006 30 MON 12–96

8 New Jersey, Glassboro area F,M,N,P,V 2005 28 MON 14–71

9 New Jersey, Glassboro area F,M,N,P,V 1996, 1997 23 MON 5–51

10 Virginia Beach, VA F,M,N,P,V 1995 14 MON 35–95 

Agricultural land-use studies

11 New Jersey, Glassboro area F,M,N,P 1996 15 MON 19–61

12 Delmarva Peninsula F,M,N,P 2001 29 MON, DOM 9–38

13 North Carolina F,M,N,P 1994, 2002 49 MON, DOM 7–45

Special drinking-water studies

14 New Jersey, Glassboro area F,M,N,P,V,T 2007 15 PWS 173–775

15 Delaware, public-supply study F,M,N,P,V,RA,R 2000 30 PWS 37–139

Other surficial aquifer studies

16 New Jersey, forested area F,M,N,P 1996 13 MON 9–32

17 Delmarva, NAWQA pilot study F,M,N,P,T,R 1988 38 MON, DOM 29–38

Appendixes 83
Water-quality properties and constituents measured and a summary of data for 1988–2009, including laboratory reporting 

levels and human-health benchmarks for drinking water, are presented only online. A data summary is available for download 

at http://pubs.usgs.gov/circ/1353/ and includes appendix 2 and the table listed below. The complete data archive is included 

with the report that summarizes groundwater quality for many of the Nation’s Principal Aquifers(26) and is available at 

http://pubs.usgs.gov/circ/1360/.

Table A2–1 Comprehensive list of water-quality properties and constituents analyzed in samples collected during 1988–2009 in the 

Northern Atlantic Coastal Plain surficial aquifer system, Delaware, Maryland, New Jersey, New York, North Carolina, and Virginia, 

including laboratory reporting levels and human-health benchmarks for drinking water.

84 Water Quality in the Northern Atlantic Coastal Plain Surficial Aquifer System, 1988–2009

Appendix 2. Water-Quality Properties and Constituents Measured, a 

Summary of Data, and Complete Data Archive for 1988–2009
Appendixes

Appendix 3. Water Quality of the Northern Atlantic Coastal Plain Surficial 

Aquifer System in a National Context

This appendix shows graphical comparisons of chemical concentrations (1991 to 2010) for selected inorganic and organic 

constituents of potential health or aquatic-life concern in selected Principal Aquifers of the United States. For each constituent, 

the concentration data are grouped according to five well types: agricultural land-use study wells (includes shallow agricultural 

monitoring wells), urban land-use study wells (includes shallow urban monitoring wells), major aquifer study wells, domestic 

wells, and public-supply wells. For each well type, the aquifers also are grouped according to aquifer lithology: basalt and 

volcanics,** crystalline, carbonate, sandstone, semiconsolidated sand and gravel, glacial unconsolidated sand and gravel, and 

unconsolidated sand and gravel (nonglacial).

The number of samples for a constituent varies among Principal Aquifers, and data for a particular compound were not plotted 

if there were fewer than 10 samples for a Principal Aquifer. Not all Principal Aquifers for which data are available are shown. 

Note that analytical detection limits vary among the constituents. Boxplots are truncated at common assessment levels and data 

reported as non-detects or analytical values below common assessment levels are not shown. The data used in this appendix and 

boxplots for additional constituents are available at http://pubs.usgs.gov/circ/1360/.

** Note: Two of the Principal Aquifers in this group include limited samples from basin-fill aquifers within the extent of the basaltic aquifer.

circ1358_appendix2 -explanation

Principal Aquifer (number of samples)

10th

percentile

25th

percentile

75th

percentile

Median

(50th percentile)

Lithology

Basalt and volcanics** Crystalline Carbonate Sandstone Semiconsolidated 

 sand and gravel

Glacial unconsolidated sand and gravel Unconsolidated sand and gravel (nonglacial)

MCL Maximum Contaminant Level

U.S. Environmental Protection Agency

 contaminant levels for drinking water 

(NA) Wells of this type were not 

 sampled for this constituent

90th

percentile

Data above

90th percentile

Data below

10th percentile*

SMCL Secondary Maximum Contaminant Level

PMCL Proposed Maximum Contaminant Level

AMCL Proposed Alternative Maximum Contaminant Level

* Data below laboratory reporting limits are not shown.

Appendixes 85
circ1353_denver_appendix2 fig 01

Agricultural land-use wells

MCL = 3 μg/L

0.001 0.01 0.1 1 10

Urban land-use wells

(28)

(NA)

(30)

(27)

(39)

(43)

(26)

(26)

(114)

(NA)

(74)

(128)

(152)

(NA)

(93)

(9)

(NA)

(NA)

(20)

(NA)

(20)

(NA)

(25)

(NA)

(50)

(NA)

(NA)

MCL = 3 μg/L

0.001 0.01 0.1 1 10

(NA)

 (NA)

Domestic wells

Surficial (30)

Mississippi River Valley alluvial (5)

High Plains (314)

Alluvial–Pleistocene terrace deposits (NA)

Alluvial–Denver Basin (7)

Rio Grande (24)

Central Valley (138)

California Coastal Basin (18)

Basin and Range basin-fill (131)

Glacial–West (55)

Glacial–West Central (93)

Glacial–Central (189)

Glacial–East (99)

Texas coastal uplands (49)

Northern Atlantic Coastal Plain (65)

Mississippi embayment (27)

Valley and Ridge siliciclastic (81)

Early Mesozoic (68)

Denver Basin (55)

Cambrian-Ordovician (72)

Valley and Ridge carbonate (117)

Piedmont and Blue Ridge carbonate (29)

Upper Floridan (124)

Castle Hayne (6)

Piedmont and Blue Ridge crystalline (151)

New England crystalline (111)

Hawaiian volcanic (NA)

MCL = 3 μg/L

0.001 0.01 0.1 1 10

Snake River Plain basin fill–basalt rock (149)

Columbia Plateau basin fill–basalt rock (63)

Public-supply wells

(14)

(12)

(31)

(NA)

(NA)

(40)

(15)

(94)

(100)

(11)

(28)

(33)

(68)

(8)

(49)

(55)

(1)

(2)

(15)

(81)

(4)

(NA)

(35)

(5)

(23)

(1)

(24)

MCL = 3 μg/L

0.001 0.01 0.1 1 10

(19)

(34)

Principal Aquifer

Principal Aquifer

Atrazine concentration, in micrograms 

per liter

Atrazine concentration, in micrograms 

per liter

Atrazine concentration, in micrograms 

per liter

Atrazine concentration, in micrograms 

per liter

Atrazine

Mississippi River Valley alluvial (NA)

Alluvial–Pleistocene terrace deposits (NA)

Alluvial–Denver Basin (30)

Rio Grande (91)

Central Valley (140)

California Coastal Basin (NA)

Basin and Range basin-fill (37)

Glacial–West (25)

Glacial–West Central (153)

Glacial–East (54)

Texas coastal uplands (NA)

Northern Atlantic Coastal Plain (83)

Mississippi embayment (NA)

Valley and Ridge siliciclastic (24)

Early Mesozoic (NA)

Denver Basin (NA)

Cambrian-Ordovician (NA)

Valley and Ridge carbonate (93)

Piedmont and Blue Ridge carbonate (30)

Upper Floridan (6)

Castle Hayne (NA)

Piedmont and Blue Ridge crystalline (NA)

New England crystalline (NA)

Hawaiian volcanic (NA)

Snake River Plain basin fill–basalt rock (110)

Columbia Plateau basin fill–basalt rock (128)

Glacial–Central (281)

Surficial (135)

High Plains (85)

86 Water Quality in the Northern Atlantic Coastal Plain Surficial Aquifer System, 1988–2009
Appendixes

circ1353_denver_appendix2 fig 02

(95)

Nitrate concentration, in micrograms 

per liter as nitrogen

Northern Atlantic Coastal Plain (83)

Nitrate concentration, in micrograms 

per liter as nitrogen

MCL = 10 μg/L

MCL = 10 μg/L

0.01 0.1 1 10 100 0.01 0.1 1 10 100

Nitrate concentration, in micrograms 

per liter as nitrogen

Northern Atlantic Coastal Plain (66)

Domestic wells

Nitrate concentration, in micrograms 

per liter as nitrogen 

MCL = 10 μg/L

(35)

Public-supply wells

MCL = 10 μg/L

0.01 0.1 1 10 100 0.01 0.1 1 10 100

Agricultural land-use wells Urban land-use wells

Mississippi embayment (NA)

Valley and Ridge siliciclastic (23)

Early Mesozoic (NA)

Denver Basin (NA)

Cambrian-Ordovician (NA)

Valley and Ridge carbonate (92)

Piedmont and Blue Ridge carbonate (30)

Upper Floridan (6)

Castle Hayne (NA)

Piedmont and Blue Ridge crystalline (NA)

New England crystalline (NA)

Hawaiian volcanic (NA)

Snake River Plain basin fill–basalt rock (111)

Columbia Plateau basin fill–basalt rock (129)

Surficial (138)

Mississippi River Valley alluvial (NA)

High Plains (85)

Alluvial-Pleistocene terrace deposits (NA)

Alluvial-Denver Basin (29)

Rio Grande (119)

Central Valley (140)

California Coastal Basin (NA)

Basin and Range basin-fill (37)

Glacial–West (25)

Glacial–West Central (154)

Glacial–Central (282)

Glacial–East (54)

Texas coastal uplands (NA)

(28)

 (NA)

(30)

 (27)

 (39)

(44)

 (26)

(24)

(116)

(NA)

 (74)

(129)

(153)

(NA)

(10)

(NA)

(NA)

(19)

(NA)

(20)

(NA)

(25)

 (NA)

(50)

(NA)

(NA)

(NA)

(NA)

Surficial (30)

Mississippi River Valley alluvial (5)

High Plains (314)

Alluvial–Pleistocene terrace deposits (NA)

Alluvial–Denver Basin (7)

Rio Grande (25)

Central Valley (137)

California Coastal Basin (18)

Basin and Range basin-fill (123)

Glacial–West (55)

Glacial–West Central (96)

Glacial–Central (210)

Glacial–East (97)

Texas coastal uplands (50)

Mississippi embayment (27)

Valley and Ridge siliciclastic (81)

Early Mesozoic (69)

Denver Basin (75)

Cambrian-Ordovician (71)

Valley and Ridge carbonate (116)

Piedmont and Blue Ridge carbonate (29)

Upper Floridan (127)

Castle Hayne (6)

Piedmont and Blue Ridge crystalline (151)

New England crystalline (113)

Hawaiian volcanic (NA)

(NA)

(12)

(31)

 (NA)

(NA)

(42)

(15)

(93)

(101)

(11)

(28)

(37)

(49)

(8)

(41)

 (1)

(2)

(15)

(80)

(4)

(NA)

(34)

(5)

(8)

(1)

(25)

(19)

(34)

Snake River Plain basin fill–basalt rock (151)

Columbia Plateau basin fill–basalt rock (63)

Principal Aquifer

Principal Aquifer

Nitrate plus nitrite (nitrate)

Appendixes 87
Radon concentration, in picocuries 

per liter

Surficial (29)

Mississippi River Valley alluvial (NA)

High Plains (NA)

Alluvial–Pleistocene terrace deposits (NA)

Alluvial-Denver Basin (NA)

Rio Grande (47)

Central Valley (83)

California Coastal Basin (NA)

Basin and Range basin-fill (35)

Glacial–West (NA)

Glacial–West Central (63)

Glacial–Central (149)

Glacial–East (11)

Texas coastal uplands (NA)

Northern Atlantic Coastal Plain (30)

Mississippi embayment (NA)

Valley and Ridge siliciclastic (21)

Early Mesozoic (NA)

Denver Basin (NA)

Cambrian-Ordovician (NA)

Valley and Ridge carbonate(90)

Piedmont and Blue Ridge carbonate (30)

Upper Floridan (4)

Castle Hayne (NA)

Piedmont and Blue Ridge crystalline (NA)

New England crystalline (NA)

Hawaiian volcanic (NA)

Agricultural land-use wells

PMCL = 

300 pCi/L AMCL = 4,000 pCi/L

Snake River Plain basin fill–basalt rock (81)

Columbia Plateau basin fill–basalt rock (109)

Major aquifer wells

Radon concentration, in picocuries 

per liter

(30)

(54)

(327)

 (NA)

(NA)

(29)

(62)

(110)

(239)

(61)

(80)

(142)

(137)

(43)

 (90)

(44)

(59)

(53)

(59)

(115)

(30)

(NA)

(112)

(30)

(168)

(112)

(30)

PMCL = 

300 pCi/L AMCL = 4,000 pCi/L

(77)

(43)

Radon concentration, in picocuries 

per liter

Surficial (30)

Mississippi River Valley alluvial (5)

High Plains (307)

Alluvial–Pleistocene terrace deposits (NA)

Alluvial-Denver Basin (6)

Rio Grande (24)

Central Valley (111)

California Coastal Basin (17)

Basin and Range basin-fill (112)

Glacial–West (48)

Glacial–West Central (58)

Glacial–Central (153)

Glacial–East (97)

Texas coastal uplands (40)

Northern Atlantic Coastal Plain (55)

Mississippi embayment (17)

Valley and Ridge carbonate (79)

Early Mesozoic (48)

Denver Basin (55)

Cambrian-Ordovician (47)

Valley and Ridge carbonate (112)

Piedmont and Blue Ridge carbonate (29)

Upper Floridan (88)

Castle Hayne (6)

Piedmont and Blue Ridge crystalline (150)

New England crystalline (109)

Hawaiian volcanic (NA)

Domestic wells

Radon concentration, in picocuries 

per liter

(NA)

(12)

(16)

 (NA)

(NA)

(5)

(1)

(77)

(83)

(10)

(1)

(20)

(41)

(NA)

(15)

(27)

(1)

(2)

(3)

(66)

(4)

(NA)

(5)

(5)

(8)

(1)

(25)

Public-supply wells

10 100 1,000 10,000 100,000 1,000,000

PMCL =

300 pCi/L PMCL = AMCL = 4,000 pCi/L 300 pCi/L AMCL = 4,000 pCi/L

10 100 1,000 10,000 100,000 1,000,000

(13)

(34) Snake River Plain basin fill–basalt rock (115)

Columbia Plateau basin fill–basalt rock (59)

Principal Aquifer

Principal Aquifer

10 100 1,000 10,000 100,000 1,000,000 10 100 1,000 10,000 100,000 1,000,000

Radon

88 Water Quality in the Northern Atlantic Coastal Plain Surficial Aquifer System, 1988–2009
Prepared by the USGS Science Publishing Network

Pembroke and Raleigh Publishing Service Centers

Edited by Anna N. Glover and Kay Naugle

Illustrations by Susan Bergin and 

Christine T. Mendelsohn

Layout by Christine T. Mendelsohn

For more information concerning this report, contact:

Chief, National Water-Quality Assessment Program

U.S. Geological Survey

413 National Center

12201 Sunrise Valley Drive

Reston, VA 20192

http://water.usgs.gov/nawqa/
ISSN 1067-084X (print)

ISSN 2330-5703 (online)

http:// dx.doi.org/10.3133/cir1353

Northern Atlantic 

Coastal Plain surficial

aquifer system