U.S. Department of the Interior

U.S. Geological Survey

Circular 1354

Water Quality in the Principal Aquifers of the Piedmont, 

Blue Ridge, and Valley and Ridge Regions, 

Eastern United States, 1993–2009

National Water-Quality Assessment Program

The Quality of Our Nation’s Waters
Cover. Photograph copyright istockphoto.com.
Water Quality in the Principal Aquifers 

of the Piedmont, Blue Ridge, and Valley 

and Ridge Regions, Eastern United States, 

1993–2009

By Bruce D. Lindsey, Tammy M. Zimmerman, Melinda J. Chapman, 

Charles A. Cravotta III, and Zoltan Szabo

National Water-Quality Assessment Program

Circular 1354

U.S. Department of the Interior

U.S. Geological Survey

The Quality of Our Nation’s Waters
U.S. Department of the Interior

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

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

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Suggested citation:

Lindsey, B.D., Zimmerman, T.M., Chapman, M.J., Cravotta, C.A., III, and Szabo, Zoltan, 2014, The quality of our 

Nation’s waters—Water quality in the Principal Aquifers of the Piedmont, Blue Ridge, and Valley and Ridge regions, 

eastern United States, 1993–2009: U.S. Geological Survey Circular 1354, 107 p., http://dx.doi.org/10.3133/cir1354.

Library of Congress Cataloging-in-Publication Data

Lindsey, Bruce D., 1962-

 The quality of our nation’s waters---water quality in the principal aquifers of the Piedmont, Blue Ridge, and Valley 

and Ridge regions, eastern United States, 1993--2009 / by Bruce D. Lindsey, Tammy M. Zimmerman, Melinda J. Chapman, Charles A. Cravotta III, and Zoltan Szabo.

 pages cm. -- (Circular / U.S. Geological Survey ; 1354)

 Includes bibliographical references.

 ISBN 978-1-4113-3752-7

1. Water quality--East (U.S.) 2. Groundwater--Quality--East (U.S.) I. Zimmerman, Tammy M. II. Chapman, M. J. 

(Melinda J.) III. Cravotta, Charles A. IV. Szabo, Zoltan, 1959- V. Geological Survey (U.S.) VI. Title. VII. Series: U.S. 

Geological Survey circular ; 1354.

 TD223.1.L56 2014

628.1’140975--dc23

2014001595

ISSN 1067-084X (print)

ISSN 2330-5703 (online) 

ISBN 978-1-4113-3752-7
iii

National Water-Quality Assessment 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 Water Quality in the Principal Aquifers

of the Piedmont, Blue Ridge, and Valley and Ridge Regions..............................................7

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

Chapter 4: Physical and Geochemical Processes Affect Groundwater Quality...............................27

Chapter 5: Quality of the Groundwater Resource Used for Drinking Water......................................35

Chapter 6: Constituents From Geologic Sources ...................................................................................45

Chapter 7: Contaminants Associated With Human Activities..............................................................57

Chapter 8: Can Discharge of Nutrients From Groundwater Impair the Health of

Streams and Estuaries? ..........................................................................................................83

References Cited .........................................................................................................................................89

Glossary.........................................................................................................................................................93

Appendixes...................................................................................................................................................97
vii

Introduction to This Report

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

Principal Aquifers in the Piedmont, Blue Ridge, and Valley and Ridge regions: the Piedmont and Blue 

Ridge crystalline- and carbonate-rock aquifers, early Mesozoic basin (siliciclastic-rock) aquifers, and 

the Valley and Ridge carbonate- and siliciclastic-rock aquifers. This 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, or 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 Piedmont, Blue Ridge, and Valley and Ridge regions are compared to conditions 

found elsewhere and to select 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, and 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 to 

the quality of water in other areas across the region and the Nation.

Water-quality conditions in the Piedmont, Blue Ridge, and Valley and Ridge regions 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, 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 national NAWQA Web site 

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

Companion studies of these aquifers are in the following reports.

Factors Affecting Occurrence and Distribution of Selected 

Contaminants in Ground Water From Selected Areas in the 

Piedmont Aquifer System, Eastern United States, 1993–2003

By Bruce D. Lindsey, William F. Falls, Matthew J. Ferrari, 

Tammy M. Zimmerman, Douglas A. Harned, Eric M. Sadorf, 

and Melinda J. Chapman

U.S. Geological Survey Scientific Investigations Report 2006–5104 

(also available at http://pubs.usgs.gov/sir/2006/5104/)

Contributions of Phosphorus from Groundwater to Streams 

in the Piedmont, Blue Ridge, and Valley and Ridge Physiographic Provinces, Eastern United States

By Judith M. Denver, Charles A. Cravotta III, Scott W. Ator, and 

Bruce D. Lindsey

U.S. Geological Survey Scientific Investigations Report 2010–5176 

(also available at http://pubs.usgs.gov/sir/2010/5176/)

Factors Affecting Groundwater Quality in the Valley and 

Ridge Aquifers, Eastern United States, 1993–2002

By Gregory C. Johnson, Tammy M. Zimmerman, Bruce D. Lindsey, 

and Eliza L. Gross

U.S. Geological Survey Scientific Investigations Report 2011–5115 

(also available at http://pubs.usgs.gov/sir/2011/5115/)

Naturally Occurring Contaminants in the Piedmont and 

Blue Ridge Crystalline-Rock Aquifers and Piedmont Early 

Mesozoic Basin Siliciclastic-Rock Aquifers, Eastern United 

States, 1994–2008

By Melinda J. Chapman, Charles A. Cravotta III, Zoltan Szabo, and 

Bruce D. Lindsey 

U.S. Geological Survey Scientific Investigations Report 2013–5072 

(also available at http://pubs.usgs.gov/sir/2013/5072/)
Groundwater is an important source of 

water for domestic supply.

Groundwater is an important source 

of water for domestic supply.

The Principal Aquifers of the Piedmont, Blue Ridge, and Valley and 

Ridge regions are a critical water resource, underlying an area with a 

population of more than 40 million people in 10 states. The suburban 

and rural population is large and growing rapidly and increasingly depends 

on groundwater as a source of water supply. The amount of water pumped 

from domestic (private) wells tapping these aquifers—more than 550 million 

gallons per day—is among the largest in the Nation. Some of these aquifers 

are among the most vulnerable in the Nation to contamination from chemicals 

with human-related or geologic sources. Untreated groundwater from nearly 

one-third of the drinking-water supplies sampled failed at least one test 

typically used by health officials to determine if water is safe to drink. The 

quality of groundwater in these aquifers also has ecological importance, as the 

aquifers are a major source of water and contaminants to streams and estuaries 

such as the Chesapeake Bay and the Albemarle-Pamlico Sound.

Chapter 1: Overview of Major Findings 

and Implications

Chapter 1

Photograph copyright istockphoto.com

The population in this region is large and growing rapidly. 

Photograph by Eliza Gross, USGS

Photograph by Tammy Zimmerman, USGS
Valley and Ridge Physiographic Province

Valley and Ridge siliciclastic-rock aquifers

Domestic

well

Domestic

well

 Shallow

monitoring

 well 

Valley and Ridge carbonate-rock aquifers

Water

table Forest

Agriculture Urban

Spring

Sinkhole

Not to scale

Valley and Ridge carbonate- and siliciclastic-rock aquifers, Piedmont and Blue Ridge carbonate- and 

0

10

20

30

40

50

60

70

0

10

20

30

40

50

60

70

Percentage of contaminants exceeding

at least one human-health benchmark

Early

Mesozoic

basin

Valley

and 

Ridge

Valley

and 

Ridge

Piedmont

and 

Blue Ridge

Piedmont

and 

Blue Ridge

Siliciclastic-rock

aquifers

Carbonate-rock 

aquifers

Crystallinerock aquifers

Contaminants from

geologic sources

Contaminants from

human sources

T

Exceedance of human-health benchmark 

he aquifers of the Piedmont, Blue Ridge, and Valley and Ridge regions underlie an area 

with a population of more than 40 million people in 10 states. The suburban and rural 

population is large, growing rapidly, and 

increasingly dependent on groundwater as a source 

of supply, with more than 550 million gallons per 

day withdrawn from domestic wells for household 

use. Water from some of these aquifers does not 

meet human-health benchmarks for drinking water 

for contaminants with geologic or human sources. 

Water from samples in crystalline- and siliciclastic-rock 

aquifers frequently exceeded standards for contaminants 

with geologic sources, and samples in carbonate-rock aquifers 

frequently exceeded standards for contaminants with human sources, 

most often nitrate and bacteria.

NY

PA

OH

WV MD

NJ

NC

SC

AL GA

TN

VA

KY

DE

Water from the crystalline- and siliciclastic-rock aquifers frequently had concentrations of radon, manganese, and arsenic that exceeded human-health benchmarks. Concentrations of radon in nearly one of every four wells used as a source 

of drinking water in the crystalline-rock aquifers exceeded the proposed Alternative 

Maximum Contaminant Level (MCL) of 4,000 picocuries per liter (pCi/L). Concentrations of arsenic exceeded the drinking water standard of 10 micrograms per liter 

only under specific geochemical conditions. See chapter 6.

Many contaminants in groundwater have geologic sources, but 

geochemical conditions control whether or not those contaminants 

dissolve and move through groundwater

1

Photograph by Lester J. Williams, USGS

Domestic well

The aquifers of the Piedmont, Blue Ridge, and Valley and Ridge regions
Concentrations of nitrate and bacteria—the main drinking-water contaminants with human sources—were high in 

carbonate-rock aquifers and frequently exceeded human-health benchmarks

crystalline-rock aquifers, and Early Mesozoic basin aquifers

Large contributions of nitrate and phosphorus from groundwater to streams have a 

negative effect on ecological health of estuaries, such as the Chesapeake Bay and 

Albemarle-Pamlico Sound

3

Blue Ridge 

Physiographic Province

Piedmont Physiographic Province

Forest

Agriculture

Urban Spring

Domestic

well

Piedmont and Blue Ridge

crystalline-rock aquifers Early Mesozoic

basin aquifers

(siliciclastic-rock)

Piedmont and Blue Ridge

crystalline-rock aquifers

Piedmont and Blue Ridge

carbonate-rock aquifers

Not to scale

About half of the nitrogen delivered by streams to the Chesapeake Bay originates from groundwater, with much of this coming from fertilizer and manure. Groundwater moves slowly; therefore, 

better management of nitrogen at the land surface may not result in improved stream-water quality 

for many years and in some cases several decades. In some areas, groundwater contains naturally 

occurring phosphorus from geologic sources at concentrations that exceed recommended in-stream 

criteria for phosphorus. Understanding how geologic sources deliver phosphorus to streams could 

contribute to the ongoing development of nutrient management strategies to limit transport of phosphorus to estuaries. See chapter 8.

Concentrations of nitrate from domestic wells in the carbonate-rock aquifers are the 

highest in the Nation for domestic water supplies, exceeding the Maximum Contaminant Level 

(MCL) of 10 milligrams per liter in water from one of every four wells or springs sampled, 

mainly in agricultural areas. Nitrate concentrations in water from wells in the crystalline-rock 

aquifers also frequently exceeded the nitrate MCL, where sources of nitrogen were high in the 

area around the well. Fecal-indicator bacteria (Escherichia coli) were also 

identified as a contaminant of concern for human health, detected in 

one-half of the samples from drinking-water sources (wells and 

springs) in carbonate-rock aquifers. Karst features, such as sinkholes, make the carbonate-rock aquifers particularly susceptible 

to contamination from the land surface. See chapter 7.

Photograph by William Kochanov,Pennsylvania Geological Survey

2Photograph by Connie Loper, USGS

Application of agricultural 

chemicals.

Sinkhole

Chesapeake Bay
 Overview of Major Findings and 

1 Bedrock geology controls where concentrations of radon, arsenic, and manganese in drinking water are potential

human-health concerns

Crystalline rocks are the principal source of high concentrations of radon in groundwater 

in the Piedmont, Blue Ridge, and Valley and Ridge regions. Radon concentrations in water 

from nearly one of every four drinking-water wells sampled in the Piedmont and Blue Ridge 

crystalline-rock aquifers exceeded the limit of 4,000 picocuries per liter (pCi/L) proposed by 

the U.S. Environmental Protection Agency (USEPA) for public water suppliers,(1) one of the 

highest rates of exceedance in the Nation.(2, 3)

Siliciclastic rocks and their metamorphic (crystalline) equivalents are the principal source 

of arsenic and manganese in groundwater. Although concentrations of these trace elements 

were low in most parts of the study area, arsenic concentrations in groundwater exceeded the 

Maximum Contaminant Level (MCL) of 10 micrograms per liter (μg/L) set by the USEPA 

for public water supplies in localized areas of the Piedmont and Blue Ridge crystalline-rock 

and early Mesozoic basin aquifers (siliciclastic rocks). In these areas, geochemical conditions are such that arsenic is released from the rock into groundwater, and nearly 10 percent 

of the arsenic concentrations measured exceeded the MCL. Most of the exceedances of the 

human-health benchmark for manganese were in water from wells in the Valley and Ridge 

siliciclastic-rock aquifers, where concentrations in 15 percent of samples exceeded the limit 

of 300 μg/L, one of the highest rates of exceedance in the Nation.

Because geology plays such an important role in controlling where groundwater might 

contain contaminants from geologic sources at concentrations of potential concern for human 

health, information such as detailed geologic maps can help local water managers to predict 

where concentrations might exceed the MCL. Basic measures of water chemistry, such as 

pH and dissolved oxygen, also can help managers determine where testing for contaminants 

from geologic sources might be advisable. More than 7 million people in these regions 

rely on domestic wells for their drinking water, but routine testing of water from domestic 

wells is not required, and thus homeowners are responsible for the testing, maintenance, or 

treatment of the water from their domestic wells. The widespread detection of contaminants 

from geologic sources in domestic wells emphasizes the need for public education aimed 

at describing probable locations of areas where groundwater might be contaminated and 

informing the public of available testing and treatment options.

2 A combination of bedrock geology and human activities on the land surface controls where concentrations of bacteria and 

nitrate in drinking water are potential human-health concerns

Carbonate-rock aquifers are highly susceptible to contamination from human-related 

sources because they have features such as sinkholes and conduits that allow contaminants to 

move rapidly from the land surface into the aquifer. The carbonate-rock aquifers commonly 

are overlain by large flat areas well suited for agricultural and urban development. The 

human activities associated with these land uses are sources of many contaminants, including 

bacteria and nitrate. As a result, fecal-indicator bacteria (Escherichia coli) were detected 

in one-half of the drinking-water sources (wells and springs) sampled in the Piedmont and 

4 Water Quality in the Principal Aquifers of the Piedmont, Blue Ridge, and Valley and Ridge Regions, Eastern United States, 1993–2009

Photographs from top to bottom:

Albert Becher, USGS; Eliza Gross, USGS; 

Joseph Ayotte, USGS; and Alan M. Cressler, 

USGS
Implications

Blue Ridge carbonate-rock aquifers and the Valley and Ridge carbonate-rock aquifers, 

and bacterial contamination was present in all of the aquifers. Concentrations of nitrate in 

domestic water sources in these same two carbonate-rock aquifers are the highest in the 

Nation, exceeding the USEPA MCL of 10 milligrams per liter as nitrogen (mg/L as N) in one 

of every four wells and springs sampled.

Crystalline-rock aquifers, although typically overlain by areas of less intense agricultural 

activity than the carbonate-rock aquifers, also are susceptible to contamination related to 

human activities. In areas where agricultural activity in areas overlying the crystalline-rock 

aquifers was greatest, the median concentration of nitrate in groundwater was 3 times higher 

than that in nonagricultural areas. Expansion of high-intensity agricultural activities, such as 

confined animal-feeding operations and increased acreage of corn for biofuels, in areas over 

both carbonate- and crystalline-rock aquifers is likely to result in higher concentrations of 

nitrate and bacteria in groundwater.

As agricultural land is converted to suburban development, homeowners and developers 

are likely to determine that the groundwater is contaminated with nitrate and bacteria associated with the previous use of the land. Continued public outreach and education can increase 

homeowners’ awareness of the need for testing the water in domestic wells and the types of 

contaminants that are likely to be detected in a given area.

3 Groundwater is a source of nitrate and phosphorus to streams and coastal waters, such as the Chesapeake Bay

and Albemarle-Pamlico Sound

Nitrate and phosphorus both can be transported by groundwater to streams. Where 

groundwater contains dissolved oxygen, as is common in the carbonate-rock aquifers and 

some of the crystalline-rock aquifers, nitrate does not break down and moves through the 

aquifer with the groundwater. The source of most of the nitrate is fertilizer and manure. 

Phosphorus transport requires that the groundwater contain little or no dissolved oxygen 

or have a high pH; these conditions are more common in the siliciclastic-rock aquifers and 

some of the crystalline-rock aquifers. The source of the phosphorus is geologic: it comes 

from minerals in these aquifers.

About half of the nitrogen delivered by streams to the Chesapeake Bay originates from 

groundwater. The majority of the streams sampled had nitrate concentrations that exceeded 

the ecological criteria recommended by the USEPA.(4) Because groundwater moves slowly, 

decades might be needed for changes in management practices at the land surface to be 

reflected in the concentrations of nitrate delivered by groundwater to streams. Understanding 

both the amount of nitrogen delivered by groundwater to streams and the associated lag 

times is essential to developing realistic expectations for recovery of the water quality 

in surface-water bodies after implementation of practices aimed at reducing in-stream 

nitrogen concentrations.

More than 20 percent of the phosphorus in streams draining crystalline- and siliciclasticrock aquifers is from groundwater. Nearly half of the streams sampled had concentrations 

of phosphorus that exceeded the ecological criteria recommended by the USEPA.(4) Understanding which streams receive substantial amounts of phosphorus from geologic sources 

will allow development of more effective strategies to limit transport of phosphorus to 

streams and estuaries, such as the Chesapeake Bay and the Albemarle-Pamlico Sound.

Chapter 1

Chapter 1: Overview of Major Findings and Implications 5

Photographs from top to bottom: Copyright 

istockphoto.com; Eliza Gross, USGS; the 

USGS Photo Library; Alan M. Cressler, USGS
“High quality water resources are vital to the health 

and environmental and economic sustainability of our 

communities. NAWQA studies help to inform the EPA’s 

Water Protection Division to reduce threats to our water 

supplies today and to protect our groundwater resources for 

the future.”

Victoria Binetti, Associate Division Director, Water 

Protection Division, U.S. Environmental Protection 

Agency, Region 3

Photograph by Alan M. Cressler, USGS
The Piedmont, Blue Ridge, and Valley and Ridge regions cover a large 

and diverse geographic area. These three physiographic provinces are 

subdivided by bedrock type to form five Principal Aquifers in these 

regions, and thus these Principal Aquifers form a natural framework for a 

study of groundwater quality (table 2–1). Agricultural and urban land uses are 

human activities that influence groundwater quality and thus also are important 

considerations in study design. Groundwater studies therefore are grouped by 

Principal Aquifer and by land use. This grouping allows for a comprehensive 

assessment of water quality in the regions that addresses broad questions about 

the occurrence, fate, and transport of contaminants in aquifers and the resulting 

quality of the water that is used for human consumption or that discharges to 

nearby surface-water bodies.

Chapter 2: NAWQA Approach to Assessing Water 

Quality in the Principal Aquifers of the Piedmont, Blue 

Ridge, and Valley and Ridge Regions

Most groundwater samples 

were collected from 

domestic-supply wells in 

the Piedmont, Blue Ridge, 

and Valley and Ridge 

regions. Additionally, some 

groundwater samples were 

obtained from springs, 

monitoring wells, and 

public-supply wells.

Chapter 2

This chapter 

summarizes the study 

design used to investigate water quality 

in Principal Aquifers 

in the Piedmont, Blue 

Ridge, and Valley and 

Ridge regions.
Assessing Water Quality in the Principal Aquifers of the Piedmont, Blue 

Ridge, and Valley and Ridge Regions 

How does one go about characterizing the groundwater over an area as large as the 

10 states covered by the Piedmont, Blue Ridge, and Valley and Ridge regions, let alone the 

whole United States? The approach taken by the U.S. Geological Survey (USGS) NAWQA 

Program is to use regional groundwater studies to gain a better understanding of how and 

why water quality varies. These groundwater studies are the building blocks of NAWQA’s 

water-quality assessments of Principal Aquifers. Each of the many studies of each Principal 

Aquifer focused on different information needs about groundwater quality and the natural and 

human-related factors that influence it (fig. 2–1; tables 2–1 and 2–2; appendix 1, table A–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 the aquifer? 

What is the quality of the drinking-water resource?

Each study sampled a group of 20 to 30 wells and, in some cases, springs. Some of these 

groundwater studies were designed to assess the quality of water in aquifers that are the most 

important present and future groundwater resources for drinking-water supply and included 

samples from randomly located domestic (private) and public-supply wells. Wells were chosen 

without targeting a specific land use, and land use was mixed in the areas around these wells 

(see sidebar, What types of wells were sampled, and how might the choice of sites affect water 

quality?, p. 12). Agricultural and urban land-use studies were designed to characterize and 

explain the quality of recently recharged groundwater (generally less than 10 years old)(5) in 

these land-use settings. The agricultural land-use studies sampled mostly domestic wells and 

thus determined the quality of drinking water in agricultural areas. The urban land-use studies 

sampled mostly shallow monitoring wells that were installed as part of the NAWQA Program. 

The results of the land-use studies were used to evaluate the effects of recent human activities 

on groundwater quality and can provide an early warning for issues that might adversely affect 

drinking-water resources in similar settings in the future. Land-use studies also can provide an 

indication of recent changes in water quality related to changes in chemical use.

This report assesses water quality in five Principal Aquifers in the Piedmont, Blue Ridge, 

and Valley and Ridge regions. These Principal Aquifers are defined by grouping or splitting 

the three physiographic provinces on the basis of three major bedrock types—crystalline, 

siliciclastic, or carbonate. Grouping the Piedmont and Blue Ridge Physiographic Provinces and 

then splitting by bedrock type defines the Piedmont and Blue Ridge crystalline-rock aquifers, 

the Piedmont and Blue Ridge carbonate-rock aquifers, and the early Mesozoic basin aquifers 

(mostly siliciclastic bedrock); the latter group of aquifers is the exception because it does not 

include the rock type and province in its name. Splitting the Valley and Ridge Physiographic 

Province by bedrock type defines the Valley and Ridge siliciclastic-rock aquifers and the Valley 

and Ridge carbonate-rock aquifers. The names are all plural—aquifers—because, rather than 

representing a single contiguous aquifer, they represent a number of disconnected aquifers with 

common characteristics such as bedrock and physiography.

The bedrock types of the Principal Aquifers have a substantial effect on water quality and 

were used to group the study results for analysis. Within the five Principal Aquifers, the results 

are grouped as those that characterize the quality of groundwater used as a drinking-water 

supply in areas with mixed land use, those that characterize the quality of groundwater used as 

a drinking-water supply in agricultural areas, and those that characterize the quality of shallow 

(recently recharged) groundwater in urban areas (table 2–1).

8 Water Quality in the Principal Aquifers of the Piedmont, Blue Ridge, and Valley and Ridge Regions, Eastern United States, 1993–2009

Some studies focused on 

the effects of specific land 

uses—such as agricultural 

or urban—on the quality 

of groundwater, whereas 

others characterized the 

quality of groundwater used 

as a drinking-water supply. 
Figure 2–1. For this assessment, 642 wells and springs were sampled. Physiographic provinces and aquifer names established the 

framework for the study of this region. The sampling design focused on characterizing the quality of groundwater used for drinking 

water in areas with mixed land use, from domestic wells in areas of agricultural land use, and near the water table underlying 

urban areas.

EXPLANATION

Sampling location

Agricultural area, groundwater used for drinking water

Urban area, groundwater near water table

Mixed land use, groundwater used for drinking water

Five Principal Aquifers

Early Mesozoic basin (siliciclastic rock)

Piedmont and Blue Ridge carbonate rock

Piedmont and Blue Ridge crystalline rock

Piedmont and Blue Ridge

(combined)

Three Physiographic Provinces

Piedmont, Blue Ridge, and Valley and Ridge regions

Valley and Ridge siliciclastic rock

Valley and Ridge carbonate rock 

Chesapeake Bay

AlbemarlePamlico

Sound

INDIANA

OHIO

KENTUCKY

TENNESSEE

ALABAMA

GEORGIA

SOUTH 

CAROLINA

NORTH 

CAROLINA

WEST 

VIRGINIA

VIRGINIA

PENNSYLVANIA

NEW 

JERSEY

DELAWARE

MARYLAND

NEW YORK

Atlantic Ocean

N

Study

area

Valley and Ridge 

Chapter 2

Chapter 2: NAWQA Approach to Assessing Water Quality in the Principal Aquifers of the Piedmont, Blue Ridge, and Valley and Ridge Regions 9
Table 2–1. Characteristics of groundwater studies in the Piedmont, Blue Ridge, and Valley and Ridge regions. 

Land and water use

Mixed land-use— 

Groundwater

used for drinking water

Agricultural area—Groundwater

used for drinking water

Urban area—Groundwater

near water table

Objective of study

Assess quality of water in 

aquifers that are the most 

important present and future 

groundwater resources for 

drinking-water supply in 

mixed land-use areas

Assess quality of recently 

recharged water in aquifers 

used for drinking-water supply 

in agricultural land-use settings

Assess quality of recently 

recharged, shallow 

groundwater in urban 

land-use settings

Predominant use of water from 

sampling site Domestic or public Domestic Unused monitoring well

Number of sites sampled, by Principal Aquifer

Name of aquifer

Mixed land use—

Groundwater

used for 

drinking water 

Agricultural area—

Groundwater

used for 

drinking water

Urban area—

Groundwater

near water table

Total for 

aquifer

Piedmont and Blue Ridge crystalline-rock aquifers 178 24 70a 272

Early Mesozoic basin aquifers (Piedmont and Blue Ridge 

siliciclastic-rock) 74 0 0 74

Valley and Ridge siliciclastic-rock aquifers 62 25 0 87

Piedmont and Blue Ridge carbonate-rock aquifer 0 30 0 30

Valley and Ridge carbonate-rock aquifers 67a 92 20 179

381 171 90 642

Table 2–2. Numbers of sites by study grouping and Principal Aquifer.

[A single sample from each site was used to evaluate water quality. The suffix “a” indicates that the total number of sites includes springs]

10 Water Quality in the Principal Aquifers of the Piedmont, Blue Ridge, and Valley and Ridge Regions, Eastern United States, 1993–2009
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

Aquifer (aq.ui.fer) — ̆

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.

Wells in this report were completed in one of three 

types of bedrock, shown below. Water flows through 

the fractures in the bedrock and into the well.

Bedrock type and aquifer names

crystalline rock siliciclastic rock carbonate rock

Piedmont and Blue Ridge 

crystalline-rock aquifers

Early Mesozoic 

basin aquifers 

(mostly siliciclastic, 

some crystalline)

Valley and

Ridge siliciclasticrock aquifers

Piedmont and 

Blue Ridge 

carbonate-rock 

aquifers

Valley and Ridge 

carbonate-rock 

aquifers

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

a useful quantity of water to a well.

Crystalline rocks are igneous and metamorphic 

rocks that include granitic rocks (shown 

above) along with many other lithologic types.

Siliciclastic rocks are sedimentary 

rocks that include shale and 

sandstone (shown above).

Carbonate rocks include limestone 

(shown above) and dolomite.

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.

Chapter 2

Chapter 2: NAWQA Approach to Assessing Water Quality in the Principal Aquifers of the Piedmont, Blue Ridge, and Valley and Ridge Regions 11
12 Water Quality in the Principal Aquifers of the Piedmont, Blue Ridge, and Valley and Ridge Regions, Eastern United States, 1993–2009

Photograph by Eliza Gross, USGS Photograph by Connie Loper, USGS

Two types of wells were sampled: drinking-water wells and 

monitoring wells. The drinking-water wells included domestic 

(private) wells and public-supply wells. Domestic wells usually 

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. These wells 

commonly are installed in rural areas and so are more likely 

than public-supply wells to be vulnerable to contamination from 

agricultural chemicals. Public-supply wells pump water from 

deeper in the aquifer and commonly are installed in suburban 

and urban areas and 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 do domestic wells and are 

operated for longer periods of time. As a result, public-supply 

wells pump much larger volumes of water than do domestic 

wells and thus draw in water from a much larger area. Therefore, public-supply wells are more vulnerable than domestic 

wells to manmade contamination from distant sources. If the 

amount of water withdrawn is large enough, a public-supply well 

can change the flow direction and velocity of the groundwater, 

which can, in turn, affect the groundwater geochemistry and the 

constituents contained within the groundwater. Routine testing 

of water from domestic wells is not required, and homeowners 

are responsible for testing, maintenance, or treatment of the 

water from their domestic well. Water from public-supply wells 

is required to be tested by the well operator on a routine basis to 

assure that the water provided to consumers meets Federal and 

State water-quality standards.

Monitoring wells sampled in the NAWQA groundwater 

studies were installed solely for assessing water quality. 

Monitoring wells are not pumped regularly—they are used for 

the measurement of water levels or occasionally the collection 

of water samples, but are not used for drinking water, irrigation, 

or other purposes.

Springs were sampled in some areas to represent shallow 

groundwater quality, and in other areas to represent groundwater used as a source of domestic or public supply. Springs, 

although often improved for human use, are natural discharge 

points for groundwater. The area contributing recharge to a 

spring can be very large, and in karst areas (see sidebar, Karst 

features make carbonate-rock aquifers highly susceptible to 

contamination, p. 29) might not reflect chemical characteristics 

of the land use immediately surrounding the spring. The volume 

of discharge from a spring is a general indicator of whether it 

represents recharge from a relatively small area or integrates 

water recharged over a larger area. Springs can be difficult to 

protect from surface contaminants such as bacteria.

What types of sites were sampled, and how might the choice of sites affect water quality?
Groundwater-Quality Assessment Design

A major goal of the NAWQA Program was to assess the quality of water in aquifers that 

are the most important present and future groundwater resources used for drinking-water 

supply. Wells used for drinking water were sampled to assess this groundwater resource, and 

because no specific land use was targeted, these wells are in areas with a mix of land uses 

types. One sample was analyzed from each of 381 randomly selected wells and springs used in 

16 different NAWQA studies—269 domestic wells, 36 public-supply wells, 41 other well types, 

and 35 springs (mixed land use, groundwater used for drinking-water supply; table 2–2). The 

water was sampled prior to any onsite treatment rather than at the tap, but in most cases households had no treatment system, so the quality of the water sampled and the quality of the water 

consumed by the homeowner were essentially the same. The wells are distributed across much 

of the area overlying the aquifer, but, because the wells are only in NAWQA groundwater-study 

areas, groundwater in some parts of the Piedmont, Blue Ridge, and Valley and Ridge regions 

is not represented (fig. 2–1). The type of well selected for each study may have an effect on the 

quality of water from those wells (appendix 2).

To assess the quality of recent groundwater recharge in areas of agricultural land use, 

groundwater samples were collected from 168 domestic wells, 2 stock wells, and 1 well 

supplying an institution (agricultural area, groundwater used for drinking-water supply; 

fig. 2–1; table 2–1). These agricultural land-use studies are unique in the NAWQA Program 

because, although monitoring wells typically were used by the NAWQA Program to assess 

recently recharged water, the relatively rapid movement of groundwater in this area allows 

samples from domestic wells to represent recently recharged water. Thus, the quality of water 

from these wells represents both recently recharged water and the resource used as a household 

drinking-water supply. The primary agricultural setting of the Piedmont, Blue Ridge, and Valley 

and Ridge regions is row crops, hay, and pasture, and most crops are grown without irrigation. 

Corn, a fertilizer- and pesticide-intensive crop, is the predominant crop. Concentrated animalfeeding operations for dairy cattle, hogs, and chickens are common.

To assess the quality of recent groundwater recharge in urban areas, 90 groundwater 

samples were collected from 50 shallow monitoring wells, 10 domestic wells, 11 other well 

types, and 19 springs (urban area, groundwater near water table; fig. 2–1; table 2–2). The urban 

land-use settings in the areas where samples were collected were similar to the settings where 

many Americans work and live—residential and commercial development either in established 

urban areas or recently developed suburbs. The suburbs are the most rapidly changing areas 

near the cities throughout the eastern United States, where land that previously was agricultural 

or forested is being converted to urban land uses. Wells usually were not available for sampling 

in this area, and so shallow monitoring wells were installed for this study. The domestic and 

other types of wells that were sampled most often were shallow, like the monitoring wells. 

Thus, the wells in the urban area most often tapped water that is just below the water table and 

not used directly as a drinking-water supply, but which is expected to move down into deeper 

parts of the aquifers that are used for drinking-water supply. The water quality of these samples 

is an indication of how the chemicals used in day-to-day life—pesticides, solvents, gasoline—

might affect the quality of a future drinking-water resource.

Chapter 2

Chapter 2: NAWQA Approach to Assessing Water Quality in the Principal Aquifers of the Piedmont, Blue Ridge, and Valley and Ridge Regions 13

Water from 82 percent of 

the wells and springs in 

this study was a source 

of drinking water, and 

the depth ranges of wells 

generally were similar.
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 186 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(24)

(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 51 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(48, 58, 59) (values used in this report were current as of February 2012; see http://water.usgs.gov/

nawqa/HBSL). An HBSL was available for 72 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-health-based 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.(71)

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 humanhealth concern (for example, see U.S. Environmental Protection Agency;(60) New Jersey Department of Environmental Protection(76)). 

Screening-level assessments such as this one provide perspective on the potential significance of detected contaminants to human 

health and can help in planning future studies.(59) 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.

Photograph by Joseph Ayotte, USGS

Concentrations of constituents measured for this assessment 

were compared to human-health benchmarks.

14 Water Quality in the Principal Aquifers of the Piedmont, Blue Ridge, and Valley and Ridge Regions, Eastern United States, 1993–2009
Chapter 2

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. 11) 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 might 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 feasible. 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. 14). Low-level detections allow scientists to identify and evaluate emerging issues and to

track contaminant levels over time.

Chapter 2: NAWQA Approach to Assessing Water Quality in the Principal Aquifers of the Piedmont, Blue Ridge, and Valley and Ridge Regions 15
NAWQA assessments use a wide range of geochemical data and site information

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. 

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) Iron and manganese

Radionuclides Radon, 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

Trace elements (filtered) Arsenic, boron, selenium

Radionuclides Radium, alpha activity

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

16 Water Quality in the Principal Aquifers of the Piedmont, Blue Ridge, and Valley and Ridge Regions, Eastern United States, 1993–2009
Photograph by Eliza Gross, USGS

The Piedmont, Blue Ridge, and Valley and Ridge regions have diverse 

natural and manmade features, from pristine mountainous forested areas, 

to broad agricultural valleys, to large cities and sprawling suburbs. 

Unseen beneath the landscapes is a variety of types of bedrock. These bedrock 

types directly affect water quality, but also influence the landforms themselves 

and human activities on the surface. To evaluate groundwater resources of this 

region, it is essential to understand how both the natural features of the region 

and human activities on the land surface affect water quality.

Chapter 3: Environmental and 

Hydrogeologic Setting

Chapter 3

This chapter summarizes background 

information for the 

Principal Aquifers in 

the Piedmont, Blue 

Ridge, and Valley and 

Ridge regions and 

provides the context 

for understanding 

findings about water 

quality in these 

Principal Aquifers.

The chapter covers 

the environmental 

and hydrogeologic 

setting, including 

population, land use, 

and water use.
Figure 3–1. The most densely populated areas are in the 

Piedmont near New York, New York, Philadelphia, Pennsylvania, 

Baltimore, Maryland, Washington, DC, Charlotte, North Carolina, 

Raleigh, North Carolina, Richmond, Virginia, and Atlanta, 

Georgia. Population density is lower in more rural agricultural 

and forested areas in the Blue Ridge and the Valley and Ridge. 

Urban and agricultural land uses are potential sources of nitrate, 

pesticides, and volatile organic compounds in groundwater.

Environmental Setting 

The Principal Aquifers in the Piedmont, Blue Ridge, and Valley and Ridge regions extend 

from Alabama northward to New York, covering 149,000 square miles. The five Principal 

Aquifers in this area(6) (fig. 2–1) generally correspond to the locations of the physiographic 

provinces with the same names, except that the early Mesozoic basin aquifers lie within the 

Piedmont Physiographic Province.(7)

The topography of the Piedmont generally reflects the underlying geology. Rolling hills 

and ridges are in areas underlain by the rocks most resistant to erosion, such as granite or 

sandstone. Flat lowlands are more common in areas underlain by more easily erodible rocks 

such as shale or carbonate rocks. The main topographic feature of the Blue Ridge is a long 

linear ridge that runs north and south and is underlain by crystalline rocks similar to those in 

the Piedmont. The topography of the Valley and Ridge also is characterized by long linear 

features—the valleys and ridges that give the province its name—and as in the Piedmont, the 

landforms reflect the underlying rocks. Relief ranges from about 4,000 feet in Virginia to nearly 

1,000 feet in Alabama. Topography is important for groundwater quality because it affects the 

time taken and distance traveled by groundwater as it moves from the land surface through the 

aquifer, ultimately to discharge to streams.(8)

The climate of the region is humid and temperate, with average annual precipitation in the 

range of 40 to 60 inches. Precipitation is greater at higher elevations, and the temperatures are 

much warmer in the south than in the north. Recharge to the aquifers is highest in areas with the 

greatest precipitation and lowest temperatures.(8)

Land Use and Population Distribution Affect the Distribution of Potential 

Contaminants

The Piedmont, Blue Ridge, and Valley and Ridge regions are overlain by three predominant land uses. The land use overlying the largest area of all five aquifers combined is forest 

(fig. 3–1; table 3–1), and agricultural and urban land uses are the second and third largest 

land-use types. The distribution of these land-use types on 

the landscape affects water quality. For example, agricultural 

land uses are associated with sources of contaminants such 

as nutrients(9) and pesticides,(10) and urban land uses are 

associated with sources of contaminants such as pesticides 

and volatile organic compounds (VOCs). Large tracts of 

developed (agricultural or urban) land are more likely to have 

an adverse effect on water quality than areas where land is 

mostly undeveloped, such as forests.

Imagery from PictometryTM International 

Corporation

OHIO

GEORGIA

VIRGINIA

KENTUCKY

ALABAMA

INDIANA

PENNSYLVANIA

 NORTH

CAROLINA

TENNESSEE

NEW YORK

SOUTH

CAROLINA

WEST

VIRGINIA

MD

NJ

DE

V a l l e y a n d R i d g e

B l u e R i d g e

P i e d m o n t

New York

Philadelphia

Baltimore

Washington DC

Richmond

Charlotte

Raleigh

Atlanta

EXPLANATION

Land use

Agriculture

Urban

Forest

Other

Physiographic Province 

 boundary

Study-area boundary

Study

area

N

Homer and others (56)

18 Water Quality in the Principal Aquifers of the Piedmont, Blue Ridge, and Valley and Ridge Regions, Eastern United States, 1993–2009
Figure 3–2. Total population and the rate of population growth 

are greatest in the area overlying the Piedmont and Blue Ridge 

crystalline- and carbonate-rock aquifers, whereas population 

density is greatest in the area overlying the early Mesozoic basin 

aquifers. Population growth creates an increasing demand for 

high-quality groundwater.

Land use (fig. 3–1) and population are closely related. 

Most of the population in the study area resides in the part 

of the Piedmont that overlies the crystalline- and carbonaterock aquifers—about 24 million in 2005 (fig. 3–2). Rapidly 

growing metropolitan and suburban areas around some 

large northeastern cities are in the Piedmont, although the 

cities themselves (Philadelphia, Baltimore, Richmond, and 

Washington, D.C.) are not. In the Southeast, large cities such 

as Atlanta, Charlotte, and Raleigh are completely within the 

Piedmont. Many of the suburban areas around these cities 

rely on groundwater, although large cities themselves use 

surface water as a source of supply. The population of the part 

of the Piedmont overlying the early Mesozoic basin aquifers 

is nearly 9 million, making this the most densely populated 

area overlying any of the five aquifers. The population of 

the Valley and Ridge (overlying both the carbonate- and 

siliciclastic-rock aquifers) is 5 million.(11)

The size of the rural population is important with respect 

to water use and water supply. Six of the 10 states with the 

largest rural populations in the Nation are in the Piedmont, 

Blue Ridge, and Valley and Ridge regions: Pennsylvania, 

North Carolina, Georgia, Tennessee, Alabama, and Virginia.(12)

As a result, a large number of people living in areas of rural 

activities such as agriculture also rely on groundwater as a 

source of water for domestic supply.

Table 3–1. The Piedmont, Blue Ridge, and Valley and Ridge regions in the early 2000s were predominantly forested.(56) Although 

agricultural and urban areas cover a smaller percentage of the land area than does forested land, contaminants associated with 

agricultural and urban practices, such as nitrate, pesticides, and volatile organic compounds, are important water-quality issues.

Principal Aquifer Percentage of land use

Agricultural Forested Urban Other

Piedmont and Blue Ridge crystalline-rock aquifers 19 71 6 4

Early Mesozoic basin aquifers 35 42 21 3

Valley and Ridge siliciclastic-rock aquifers 21 73 4 3

Piedmont and Blue Ridge carbonate-rock aquifers 52 29 16 2

Valley and Ridge carbonate-rock aquifers 37 53 7 3

All aquifers 22 68 7 3

Chapter 3

0

400

800

1,200

1,600

1970 1975 1980 1985 1990 1995 2000 2005

Population, in millions,

over the aquifer(11)

Population density, in

persons per square mile(11)

0

5

15

10

20

25

 EXPLANATION

Early Mesozoic basin aquifers 

Valley and Ridge carbonate-rock and 

siliciclastic-rock aquifers

Piedmont and Blue Ridge carbonate-rock 

and crystalline-rock aquifers

Population

Population density

Chapter 3: Environmental and Hydrogeologic Setting 19
Figure 3–3. Public and domestic water supplies are the primary 

uses of groundwater from the Principal Aquifers in the Piedmont, 

Blue Ridge, and Valley and Ridge regions.(13, 14) Piedmont and Blue 

Ridge crystalline- and carbonate-rock aquifers (combined) rank as 

the second highest in the Nation for domestic water-supply use. 

0

100

200

300

400

Aquifer

EXPLANATION

Irrigation Public

supply

Self-supplied

industrial

Self-supplied

domestic

Groundwater withdrawals, in millions of gallons per day 

**

*

Early Mesozoic basin

Piedmont and Blue Ridge carbonate rock

Piedmont and Blue Ridge crystalline rock

Piedmont and Blue Ridge crystalline and 

carbonate rock (combined)

Valley and Ridge siliciclastic rock

Valley and Ridge carbonate rock

Valley and Ridge siliciclastic and 

carbonate rock (combined)

**

*

Figure 3–3.

Groundwater withdrawals

[Self-supplied domestic withdrawal data as of 2005 (14) 

and all other withdrawal data as of 2000 (13)]

Water Use is Predominantly for Domestic and 

Public Supply

The total rate of water withdrawals from the five 

Principal Aquifers combined is 1.2 billion gallons per day, 

with about 1 billion gallons per day for domestic and public 

supply alone (fig. 3–3).(13) More people in these regions rely 

on groundwater for domestic supply than in the entire U.S. 

west of the Mississippi River.(14) The Piedmont and Blue Ridge 

crystalline- and carbonate-rock aquifers (combined) rank as 

the second highest in the Nation for domestic water-supply 

use, with the Valley and Ridge siliciclastic- and carbonaterock aquifers (combined) and the early Mesozoic basin 

aquifers ranking seventh and ninth in the Nation, respectively. 

The early Mesozoic basin aquifers have lower total water-use 

volumes, but cover a smaller area with a higher population 

density; as a result, the groundwater resources in this area 

are critical for water supply. Water intended for domestic and 

public supply requires a high level of quality.

More than 7 million people in the Piedmont, Blue Ridge, 

and Valley and Ridge regions live in rural areas and rely on 

groundwater as a source of domestic supply (fig. 3–4). The 

States of Pennsylvania and North Carolina rank third and 

fourth in the Nation for the number of people that rely on 

domestic wells as a source 

of supply, and Virginia 

is among the top 10.(14)

Routine testing of water 

from domestic wells is not 

required, and thus homeowners are responsible for 

testing, maintenance, and 

treatment of the water from 

their domestic well.

20 Water Quality in the Principal Aquifers of the Piedmont, Blue Ridge, and Valley and Ridge Regions, Eastern United States, 1993–2009

More people rely on 

groundwater for domestic 

supply in the Piedmont, Blue 

Ridge, and Valley and Ridge 

regions than in the entire 

United States west of the 

Mississippi River.
Figure 3–4. The number of people per square mile who rely on domestic wells for a source of household supply is high in the 

Piedmont, Blue Ridge, and Valley and Ridge regions relative to other parts of the country. Routine testing of water from domestic 

wells is not required.

Modified from DeSimone(25)

EXPLANATION

Population (1995) supplied by

 domestic wells, in persons per 

 square mile

Greater than or equal to 100

Greater than or equal to 10 

 and less than 100

Greater than or equal to 1

 and less than 10

Greater than or equal to 0.1

 and less than 1

Greater than or equal to 0.01 

 and less than 0.1

Study-area boundary

Photograph by John H. Bowman 

Chapter 3

Aquifers in the Piedmont, Blue Ridge, and Valley and Ridge regions at a glance

11 The number of states underlain by these aquifers

642 The number of wells and springs sampled in these aquifers

139,000 The number of square miles covered by these aquifers

7,000,000 The number of people using these aquifers for drinking water

1,200,000,000 The number of gallons per day used for water supply

Chapter 3: Environmental and Hydrogeologic Setting 21
Figure 3–5. Areas with steep slopes, such as the Blue Ridge, are largely forested, whereas the flatter areas of the Piedmont are 

overlain by more agricultural and urban land. Groundwater flows through relatively small fractures in the Piedmont and Blue Ridge 

crystalline-rock aquifers and in the early Mesozoic basin aquifers. In the Piedmont and Blue Ridge carbonate-rock aquifers, however, 

water can enter the aquifer from the land surface through sinkholes and flow through enlarged fractures and conduits that develop in 

the carbonate rock. This hypothetical cross-section is based on the northern Piedmont. A cross-section of the southern Piedmont(67) 

illustrates an example of where the the carbonate-rock aquifers and early Mesozoic basin aquifers cover a small percentage of the 

overall area or are not present.

Blue Ridge 

Physiographic Province

Piedmont Physiographic Province

Forest

Agriculture

Urban Spring

Domestic

well

Piedmont and Blue Ridge

crystalline-rock aquifers Early Mesozoic

basin aquifers

(siliciclastic-rock)

Piedmont and Blue Ridge

crystalline-rock aquifers

Piedmont and Blue Ridge

carbonate-rock aquifers

Water table

Not to scale

Hydrogeologic Setting Accounts for Many 

Differences in Water Quality

The ease with which water and contaminants move into 

and through an aquifer—called the intrinsic susceptibility—is 

influenced by aquifer properties, such as the size and connections of fractures in the bedrock, the amount and timing of 

recharge, and the time and distance over which the water is in 

contact with the aquifer. These characteristics vary by aquifer 

and bedrock type. In addition, bedrock type affects landforms 

(such as steep or flat terrain), which in turn affect recharge 

area and land use overlying the aquifers. 

Characteristics of Aquifers in the Piedmont and 

Blue Ridge Physiographic Provinces

Piedmont and Blue Ridge Crystalline-Rock Aquifers

Groundwater in the Piedmont and Blue Ridge crystallinerock aquifers moves through a network of well-connected 

fractures. In general, the water travels only a short distance 

before it discharges to the nearest stream (fig. 3–5); groundwater age—the time the water has spent in the aquifer—was 

less than 60 years in all of the wells in this area. In wells 

where groundwater ages were determined, about half of the 

ages were less than 20 years, and the other half of the ages 

were between 20 and 60 years (see chapter 4). Because of the 

short travel distances and young groundwater ages, human 

activities near a well are likely to have a substantial effect on 

the quality of water withdrawn from that well. Although much 

of the area overlying these aquifers is forested, a mix of urban 

and agricultural land that overlies some parts of the aquifers 

provides sources of human-related contaminants.

Piedmont and Blue Ridge Carbonate-Rock Aquifers

The Piedmont and Blue Ridge carbonate-rock 

aquifers are karst aquifers (see sidebar, Karst features make 

carbonate-rock aquifers highly susceptible to contamination, 

p. 29), which are more susceptible to contamination than are

crystalline- or siliciclastic-rock aquifers. The term karst means 

that water moving through the aquifer has dissolved away 

22 Water Quality in the Principal Aquifers of the Piedmont, Blue Ridge, and Valley and Ridge Regions, Eastern United States, 1993–2009
Figure 3–6. Land use overlying the Valley and Ridge siliciclastic-rock aquifers is a mix of forest and agriculture in the valleys and 

forest on the ridges. Water in the siliciclastic-rock aquifers flows through narrow fractures at slower rates than the rates of flow in the 

larger conduits of the carbonate-rock aquifers. A large percentage of the land in the areas overlying the Valley and Ridge carbonaterock aquifers is used for agriculture. The largest urban centers overlie the Valley and Ridge carbonate-rock aquifers as well. The 

carbonate-rock aquifers have sinkholes, enlarged fractures, conduits, and other karst features that allow rapid transport of water and 

contaminants into and through the aquifers.

Valley and Ridge Physiographic Province

Valley and Ridge siliciclastic-rock aquifers

Domestic

well

Domestic

well

 Shallow

monitoring well 

Valley and Ridge carbonate-rock aquifers

Water

table Forest

Agriculture Urban

Spring

Sinkhole

circ1354_ch03_g 6block

Not to scale

some of the rock, enlarging fractures in the bedrock into 

voids or conduits. Contaminants can move rapidly into and 

through these aquifers by way of the voids and conduits. No 

groundwater ages were determined for this aquifer, but it is 

similar to the Valley and Ridge carbonate-rock aquifers, in 

which about 75 percent of the wells had groundwater ages 

of less than 20 years and the other 25 percent between 20 

and 60 years (see chapter 4). Unlike the other fractured-rock 

aquifers, the youngest groundwater ages are known to be on 

the order of a few days because of the rapid transport of water 

through the conduits in the bedrock. The land surface in this 

area is generally flat (fig. 3–5), increasing infiltration of water 

and contaminants. This area has the highest percentage of 

agricultural land use (52 percent) and the lowest percentage 

of forested land use (29 percent) of any of the aquifers, and 

agricultural chemicals are the main potential contaminants.

Early Mesozoic Basin Aquifers

The early Mesozoic basin aquifers mainly consist of siliciclastic rocks but also include minor amounts of crystalline 

rocks (fig. 3–5). Sandstone and shale are the major bedrock 

types. The terrain ranges from flat to hilly. This area has the 

highest percentage of urban land use (21 percent) compared 

to the other aquifers, giving these aquifers a greater potential 

source of urban contaminants. In wells where groundwater 

ages were determined, about half of the ages were less than 

20 years, and the other half of the ages were between 20 and 

60 years (see chapter 4). Areas in North Carolina known as the 

Durham-Sanford basins are siliciclastic-rock aquifers(8) that do 

not appear on figure 2–1, but these areas were not sampled as 

a part of the early Mesozoic basin aquifers.

Characteristics of Aquifers in the Valley and 

Ridge Physiographic Province

Valley and Ridge Carbonate-Rock Aquifers

The Valley and Ridge carbonate-rock aquifers also are 

karst aquifers. The carbonate-rock aquifers typically are in 

a valley floor and are overlain by flat terrain. Groundwater 

generally flows from the adjacent ridges toward the valley 

center (figs. 3–6, 3–7), but because of the influence of 

geologic structures and karst conduits, the actual path 

followed by water is hard to predict. Water that moves through 

the fractures dissolves away some of the rock, enlarging the 

fractures into karst features such as sinkholes and conduits. 

In wells where groundwater ages were determined for the 

Valley and Ridge carbonate-rock aquifers, about 75 percent 

of the ages were less than 20 years, and the other 25 percent 

were between 20 and 60 years (see chapter 4). The youngest 

groundwater ages are known to be on the order of a few days 

Chapter 3

Chapter 3: Environmental and Hydrogeologic Setting 23
Figure 3–7. In carbonate-rock aquifers (top), recharge can enter the aquifer from both the siliciclastic-rock ridges and the carbonaterock valley floor. Water recharged near the ridges is less likely to be affected by contamination from land-based sources because 

the ridges mostly are forested. In both types of aquifers in the Valley and Ridge, the position of a well along the groundwater-flow 

path affects the quality of water because water generally moves from a forested ridge toward a valley overlain by agriculture. As 

water approaches the discharge area (stream), flow paths from deeper parts of the aquifer, which are older and less likely to contain 

contaminants from human sources, begin to converge toward the stream, mixing with shallower water.(22)

Soil

Carbonate-rock aquifers

Siliciclastic-rock aquifers

Fracture

Water table

circ 1354 ch03 Figure 7.

Major

stream 

Sinkhole

A. Groundwater recharge enters the aquifer from the forested ridge or 

the more highly developed valley floor (see land use in figure 3–6). 

In the carbonate-rock valley, water can recharge the aquifer 

directly through sinkholes.

B. Once below the water table, groundwater follows the general path 

shown by the light blue arrows from recharge to discharge areas. 

Groundwater following shallower flow paths and groundwater near 

the ridges is younger than groundwater following deeper flow 

paths and groundwater closer to the valley center.

C. In carbonate-rock aquifers, fractures can be enlarged to form 

conduits and caverns. Water flows much more easily through 

conduits than through fractures. When groundwater enters a 

conduit, it follows the direction of that feature and flows rapidly 

towards the discharge area. 

D. In siliciclastic-rock aquifers, groundwater flows through connected 

networks of fractures—these fractures create a more circuitous 

flow path and may actually slow the movement of groundwater as it 

moves in the direction indicated by the general flow path. 

E. Near the center of a valley, groundwater from the deeper 

part of the aquifer converges toward the stream, and even 

relatively shallow wells in this part of the valley can draw in water 

from deeper flow paths. NOT TO SCALE

EXPLANATION

Shallow

flow path

Shallow

 flow path

Deep flow path

Deep flow path

Generalized flow path

Path through fractures 

 or conduits

Recharge from land surface

Groundwater age

increases with depth

Groundwater age increases toward valley center

A

B

C

D

A

B

E

E

Carbonate-rock aquifer

Siliciclastic-rock aquifer

because of the rapid transport of water through the conduits in 

the bedrock (see sidebar, Karst features make carbonate-rock 

aquifers highly susceptible to contamination, p. 29). A high 

percentage of the land overlying these aquifers is agricultural 

(37 percent), second only to the agricultural land overlying 

the Piedmont and Blue Ridge carbonate-rock aquifers, and 

provides a large potential source of agricultural chemicals.

Valley and Ridge Siliciclastic-Rock Aquifers

In the Valley and Ridge siliciclastic-rock aquifers, 

fractures are not enlarged by dissolution as they are in the 

carbonate-rock aquifers. The siliciclastic-rock aquifers 

therefore generally contain older water and are less susceptible 

to surface contamination than are the carbonate-rock aquifers. 

In wells where groundwater ages were determined, about two 

thirds of the ages were less than 20 years, and ages in the other 

third were between 20 and 60 years (see chapter 4). Sandstone 

is resistant to weathering and forms ridges that are nearly 

always forested. The valleys between the ridges typically 

are underlain by a more easily eroded bedrock type such as 

shale (figs. 3–6, 3–7). Among the five Principal Aquifers, the 

Valley and Ridge siliciclastic-rock aquifers are overlain by 

the highest percentage of forested land (73 percent) and thus 

have fewer potential sources of contaminants from human 

activities. The valleys underlain by siliciclastic bedrock are 

not as large or as flat as those underlain by carbonate bedrock, 

and so the intensity of agricultural activity (numbers of 

animals and density of cropland) overlying the siliciclasticrock aquifers is lower than that overlying the carbonate-rock 

aquifers (fig. 3–7).

24 Water Quality in the Principal Aquifers of the Piedmont, Blue Ridge, and Valley and Ridge Regions, Eastern United States, 1993–2009
Photograph by John H. Bowman

In the Piedmont, major land-use types are distributed evenly across the landscape, as opposed to 

the Valley and Ridge, where the land use follows a more structured, linear pattern. Ridges are mostly 

forested and agriculture is concentrated in the valley floors.

Imagery from Pictometry

TM International Corporation

Chapter 3

Chapter 3: Environmental and Hydrogeologic Setting 25
Aquifers, bedrock type, and lithologic groups in the Piedmont, Blue Ridge, and 

Valley and Ridge regions

In this report, each of the five Principal Aquifers is identified 

with a major bedrock type: crystalline, siliciclastic, or carbonate 

bedrock. The major bedrock types in each of these five Principal 

Aquifers can be further subdivided into different lithologic groups. 

For example, in the Piedmont and Blue Ridge crystalline-rock 

aquifers, five lithologic groups are identified. Groundwater quality 

can differ among the lithologic groups because of differences 

in mineral composition and geologic origin. For example, radon 

concentrations are much higher in the felsic igneous and metamorphosed sedimentary clastic lithologic group than in the mafic 

and ultramafic lithologic group because of differences in uranium 

content. Differences among lithologic groups are especially 

important for assessing contaminants from geologic sources. 

Subdividing the bedrock types into lithologic groups was found to 

be more useful for interpreting water quality in the Piedmont and 

Blue Ridge crystalline-rock aquifers and the early Mesozoic basin 

aquifers than in the other three Principal Aquifers. Geochemically 

important minerals may or may not be present in the vicinity of 

every well in a given lithologic group; therefore, the quality of 

water within a lithologic group may vary from well to well.

Early 

Mesozoic 

basin

Piedmont 

and 

Blue Ridge

Piedmont 

and 

Blue Ridge

Valley

and 

Ridge

Valley 

and 

Ridge

Siliciclastic-rock aquifers Carbonate-rock aquifers 

Principal Aquifer

Bedrock type

Lithologic group

Crystalline-rock aquifers 

The five Principal 

Aquifers consist of one 

of three bedrock types

Lithologic groups are 

categories within a 

bedrock type 

(discussed only for the 

Piedmont and Blue 

Ridge crystalline and 

early Mesozoic 

aquifers in this report)

Felsic

igneous and

metamorphic

Intermediate

igneous and

metamorphic

Metamorphosed

sedimentary

clastic

Quartz-rich

metamorphic

Mafic and

ultramafic

Feldspar-rich

sedimentary

Clastic

sedimentary

Lacustrine

sedimentary

Quartz-rich

sedimentary

Mafic and

ultramafic

(crystalline)

Sandstone

Shale

Limestone

Dolomite

Mixed limestone and dolomite

26 Water Quality in the Principal Aquifers of the Piedmont, Blue Ridge, and Valley and Ridge Regions, Eastern United States, 1993–2009
I

n these regions, landscape features control the flow of groundwater from the 

recharge areas on the ridges and hilltops to the discharge areas at streams 

and springs in low-lying areas. Furthermore, the differences in the bedrock 

types below the surface control important unseen processes, such as how easily 

water enters the aquifer and how long it takes for groundwater to move through 

the aquifer to a well or a discharge area. What minerals dissolve into the water, 

and what chemical transformations are contaminants likely to undergo during 

that journey? The physical and chemical characteristics of water vary among 

the five Principal Aquifers and therefore provide an important background for 

understanding the differences in water quality and answering these questions.

Chapter 4: Physical and Geochemical Processes 

Affect Groundwater Quality

Chapter 4

Photograph by Bruce Lindsey, USGS

This chapter explains 

and discusses the 

natural physical and 

geochemical processes 

that affect the movement and quality of 

groundwater in the 

five Principal Aquifers 

of the Piedmont, Blue 

Ridge, and Valley and 

Ridge regions.
In fractured bedrock, water 

travels through the cracks and 

spaces between the rocks 

rather than through the rock 

itself. The water follows the 

direction of the fractures (blue 

arrows) rather than infiltrating 

straight down. The example 

above is a sedimentary rock 

with well-defined layers, 

typical of siliciclastic-rock 

aquifers in the Valley and 

Ridge and the early Mesozoic 

basin aquifers.

The Physical Framework of the Aquifers Affects 

the Storage and Movement of Water

All five of the Principal Aquifers in the Piedmont, Blue 

Ridge, and Valley and Ridge regions are bedrock aquifers—water flows through and is withdrawn from fractures 

and other openings in the rock (see photograph to the right). 

Water flows from recharge areas toward lower parts of the 

landscape, where it discharges to springs or streams. Water 

travels slowly through these aquifers if fractures are small 

and poorly connected but travels faster if fractures are larger 

and well connected. Some water in carbonate-rock aquifers 

flows through fractures, but some water flows through the 

larger, well-connected voids (conduits) (see sidebar, Karst 

features make carbonate-rock aquifers highly susceptible to 

contamination, p. 29). Karst features greatly increase the rate 

of infiltration from the surface and movement of water and 

contaminants through the aquifer.

Variations in Groundwater Age Affect Water 

Quality

Groundwater age is the time elapsed since the recharged 

water moved below the water table and became isolated 

from the atmosphere. The term “age” commonly is qualified 

with the word “apparent” to signify that the accuracy of the 

determined age depends on many variables; however, the 

term “groundwater age” is used in this report for simplicity. 

Groundwater age can affect the quality of water because 

the chemicals in use and released to the land surface have 

changed over time—for example, groundwater that recharged 

in the 1960s likely was exposed to different contaminants 

than groundwater that recharged in 1900 or 2000. In addition, 

concentrations of contaminants can be affected by groundwater age because older groundwater has had more time for 

human-related contaminants, such as organic chemicals and 

nitrate, to adhere to aquifer materials or to break down. On 

the other hand, older water has spent more time in contact 

Figure 4–1. All of the groundwater in the Principal Aquifers 

in the Piedmont, Blue Ridge, and Valley and Ridge regions was 

young—60 years old or less—compared to groundwater in most 

aquifers of the Nation, but the percentage of samples from the 

Valley and Ridge carbonate-rock aquifers with groundwater 

ages of less than 20 years was larger than in the other aquifers. 

Groundwater with ages in the range of 0–20 years is more likely 

to contain human-related contaminants than is groundwater with 

ages in the range of 20–60 years because of the shorter available 

time for contaminants to break down and changes in chemical 

use over time.(15–20)

Less than 20 years

20 to 60 years

Groundwater age

EXPLANATION

Valley 

and Ridge 

Valley 

and Ridge 

Piedmont and 

Blue Ridge

Early Mesozoic 

basin

Figure 4–1. Groundwater in the 

Siliciclastic-rock aquifers Carbonate-rock

aquifers

Crystalline-rock

aquifers

Groundwater age by aquifer

with the aquifer rock, resulting in increased concentrations 

of constituents from geologic sources. For these reasons, an 

understanding of the distribution of groundwater ages in the 

Principal Aquifers of the Piedmont, Blue Ridge, and Valley 

and Ridge regions is helpful.

Groundwater in the carbonate-rock aquifers typically 

is younger than is groundwater in the crystalline- and 

siliciclastic-rock aquifers (fig. 4–1). Groundwater ages can 

be estimated by measuring compounds and radioactive 

substances that enter the ground with recharge and that travel 

with the groundwater to the sampling point. All of the groundwater measured in aquifers of the Piedmont, Blue Ridge, and 

Valley and Ridge regions was young (less than 60 years old) 

compared to groundwater in most aquifers of the Nation(15–20)

because of abundant recharge 

and relatively short flow paths. 

In this report, groundwater 

ages are divided into two 

categories: less than 20 years 

and 20 to 60 years.

Photograph by William Burton, USGS

28 Water Quality in the Principal Aquifers of the Piedmont, Blue Ridge, and Valley and Ridge Regions, Eastern United States, 1993–2009

Young groundwater 

is likely to contain 

contaminants related to 

human activities, but old 

groundwater is likely to 

contain constituents from 

geologic sources.

Most groundwater in these Principal Aquifers moves 

through networks of interconnected fractures.
Karst features make carbonate rock aquifers highly susceptible to contamination

The Piedmont and Blue Ridge carbonate-rock aquifers 

and the Valley and Ridge carbonate-rock aquifers have unique 

features—referred to as karst features—that form when 

the rocks dissolve. Carbonate rocks, mainly limestone and 

dolomite, are much more soluble than most other bedrock 

types. Although carbonate rocks are fractured in a manner 

similar to the other fractured-bedrock aquifers, in karst systems 

the fractures become enlarged, and in some cases they are 

large enough to become caverns. Karst features include 

sinkholes, caverns, springs, and disappearing streams. Water 

can enter the aquifers directly through sinkholes and travel 

rapidly through cracks, fissures, and conduits. The karst 

features make the system highly susceptible to contamination 

(see photographs below); thus, if a contaminant source is 

present, contaminant levels in groundwater in areas with karst 

features can be higher than in areas where karst features do 

not exist. Moreover, predicting the movement of water and 

changes in concentration over time in an aquifer that includes 

karst features can be more challenging than in other aquifers.

The rapid transport of water through karst conduits in 

the carbonate-rock aquifer contributes to the younger ages 

of water in the carbonate-rock aquifers relative to that in the 

crystalline- and siliciclastic-rock aquifers (fig. 4–1). Some of 

the water in the carbonate-rock aquifers not only could have 

an age of less than 20 years—it could have traveled from the 

recharge area to the well or spring in a matter of days to weeks. 

Dye-tracing studies in these aquifers have shown that water can 

be transported from recharge areas to discharge areas miles 

away in days.(57) The ages shown in figure 4–1 likely pertain to 

the part of the carbonate-rock aquifer that is in the fractured 

rock without conduits, where flow tends to be slower than flow 

in conduits.

Karst conduits may allow turbulent flow that mixes air into 

the water and results in oxic conditions, which controls the 

degradation of some contaminants. Water samples from the 

carbonate-rock aquifers have higher concentrations of dissolved 

oxygen than the other aquifers. In carbonate-rock aquifers, 

areas with high and medium densities of sinkholes have higher 

concentrations of dissolved oxygen than areas with a low 

density of sinkholes. Thus, the presence of karst features helps 

explain both the young groundwater ages and the prevalence of 

oxic conditions in the carbonate-rock aquifers.

Chapter 4

Water moves 

much more 

rapidly through 

conduits than 

through fractures.

Overland runoff

Sinkhole

Photograph by Ann Tihansky, USGS

Photograph by Randy Conger, USGS 

chapter 4 sidebar: Karst features make carbonate rock aquifers system highly susceptible to contamination

Sinkholes are holes or 

depressions in the land 

surface that result from

dissolving carbonate rocks. 

They allow water to bypass 

soil filtration and rapidly 

enter the aquifer. 

Wells that directly intersect a karst 

conduit may draw in water that has 

traveled quickly from the land surface 

and may contain contaminants from 

human activities.

Karst features

Looking down at conduits

in uncased well

Water table Sinkhole

Chapter 4: Physical and Geochemical Processes Affect Groundwater Quality 29
Figure 4–2. Low pH is a common characteristic of water from crystalline-rock aquifers, whereas hardness is a characteristic of water 

from the carbonate-rock aquifers and some parts of the siliciclastic-rock aquifers. Low pH and very low or high hardness can cause 

problems with the use of the water (see chapter 6), but these two basic geochemical properties also control concentrations of many 

other constituents.

Bedrock Type Affects General Properties of Water

Minerals that compose the three major bedrock types affect the geochemical conditions in 

each of the aquifers, which in turn affect the transport of contaminants from both geologic and 

human sources. For example, the highly soluble calcite and dolomite minerals in carbonate-rock 

aquifers that lead to the development of karst features also lead to hard water, high dissolved 

solids concentrations, high alkalinity, and predominantly neutral (6.5–7.5) pH (fig. 4–2). 

Crystalline and siliciclastic rocks typically have less capacity than do carbonate rocks to 

neutralize acidic precipitation, resulting in lower pH. However, siliciclastic rocks in the early 

Mesozoic basin aquifers contain carbonate minerals in the natural cement between the sediment 

grains, and groundwater in these aquifers therefore is harder and has more neutral pH values 

than groundwater in the Valley and Ridge siliciclastic-rock aquifers (fig. 4–2). The pH and the 

concentrations of dissolved solids in groundwater affect the solubility and transport of many 

contaminants from geologic sources.

Minerals that compose the 

three major bedrock types 

affect geochemical properties such as hardness and 

pH, which in turn affect 

the concentrations of other 

contaminants.

pH range

EXPLANATION

pH and hardness by aquifer

Hardness as calcium

carbonate, in 

milligrams per liter

Less than 6.5

6.5 to 7.5

Greater than 7.5

Soft (0 to 75)

Moderately hard

 (greater than 75 to 150)

Hard (greater than 

 150 to 300)

circ1354_ch4_fig2

Piedmont and Blue Ridge Early Mesozic basin Valley and Ridge Piedmont and Blue Ridge Valley and Ridge 

Crystalline-rock aquifers Siliciclastic-rock aquifers Carbonate-rock aquifers

30 Water Quality in the Principal Aquifers of the Piedmont, Blue Ridge, and Valley and Ridge Regions, Eastern United States, 1993–2009
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

Chapter 4

Chapter 4: Physical and Geochemical Processes Affect Groundwater Quality 31
Figure 4–3. Redox conditions for as much as half of the water in the crystalline- and siliciclastic-rock aquifers were 

anoxic or mixed, whereas redox conditions in the carbonate-rock aquifers predominantly were oxic.

Reduction/Oxidation (Redox) Processes 

Affect Which Constituents are Detected in 

Groundwater

Redox processes (see sidebar, How do redox reactions 

work?, p. 31) affect the chemical quality of groundwater, 

and redox conditions vary among the Principal Aquifers 

(fig. 4–3). Redox processes can mobilize or immobilize trace 

elements from aquifer materials, contribute to the breakdown 

or preservation of human-related contaminants, and generate 

undesirable byproducts, such as dissolved iron, manganese, 

hydrogen sulfide, and methane.(21)

Older groundwater is more likely to be anoxic, but 

the onset of anoxic conditions differs among the aquifers. 

All of the water from wells that was less than 20 years old in 

the carbonate-rock aquifers was oxic, and a large percentage 

of the water older than 20 years was oxic as well. Thus, 

conditions in the carbonate-rock aquifers are favorable for 

the transport of contaminants that do not break down in oxic 

conditions, such as nitrate. In the crystalline- and siliciclasticrock aquifers, water from a higher percentage of wells—

even water with an age of less than 20 years—was anoxic 

or showed evidence of a mixture of anoxic and oxic waters 

(mixed redox source) (fig. 4–4). Concentrations of many 

contaminants from geologic sources, such as arsenic, iron, and 

manganese, are likely to be higher in older, anoxic water, such 

as in the crystalline- and siliciclastic-rock aquifers.

EXPLANATION

Oxic

Redox condition

Mixed source

Anoxic

Piedmont and Blue Ridge Early Mesozic basin Valley and Ridge Piedmont and Blue Ridge Valley and Ridge 

Crystalline-rock aquifers Siliciclastic-rock aquifers Carbonate-rock aquifers

Figure 4–3.

Redox condition by aquifer

32 Water Quality in the Principal Aquifers of the Piedmont, Blue Ridge, and Valley and Ridge Regions, Eastern United States, 1993–2009
Figure 4–4. In all three types of aquifers, water with an age of 

less than 20 years was more likely to be oxic than water with an 

age of 20 years or more.(15–20) Nitrate and some other contaminants 

are less likely to break down in oxic water than in anoxic water. 

Trace elements like iron and manganese are more likely to be 

mobile in anoxic water.

Chapter 4

Carbonaterock 

aquifers

Redox condition by bedrock type and groundwater age

Less than

20 years 20–60 years

Siliciclasticrock 

aquifers

Crystallinerock 

aquifers

(Area of pie is proportional to 

number of samples in age range)

0–60 years

EXPLANATION

Figure 4–4.

Oxic

Redox condition

Mixed source

Anoxic

Fracture bearing 

water from oxic zone

Fracture bearing 

water from anoxic zone

Water drawn from this well 

is classified as “mixed”

Water table

Fractured-rock aquifer

A well may draw in water from parts of the aquifer 

that have different geochemical characteristics and 

may thus create a mixture that would not occur under 

normal conditions.

Mixing of oxic and anoxic groundwater

Chapter 4: Physical and Geochemical Processes Affect Groundwater Quality 33
The Topographic Position of a Well on the 

Landscape Affects Water Quality

Chemical characteristics of groundwater can change as 

the water moves from the recharge area through the aquifer to 

a well or discharge area. In the Valley and Ridge aquifers, the 

relative location of the well on the landscape—its topographic 

position—is important because it indicates the position on the 

flow path and therefore groundwater age.(22) The topographic 

position is the location of the well with respect to the highest 

point in the watershed and the major stream draining the valley (fig. 4–5) and can indicate geochemical conditions. For 

example, the few wells in carbonate-rock aquifers with anoxic 

water were either near the ridge, where the water chemistry 

may have been affected by the nearby siliciclastic rocks, or in 

the low-lying valley centers, where the water is oldest. Oxic 

and anoxic water are found in all topographic positions in the 

siliciclastic-rock aquifers. Anoxic water was most common in 

low-lying areas, probably because older, anoxic water from 

deeper parts of the aquifers is moving into the shallower zones 

toward the groundwater-discharge areas.

Figure 4–5. In the Valley and Ridge carbonate-rock aquifers, 

most groundwater samples were oxic regardless of the position 

on the flow path. In the Valley and Ridge siliciclastic-rock 

aquifers, a higher percentage of anoxic water in the center of 

the valley results from older groundwater converging into the 

shallower zone near the stream. Redox conditions control which 

contaminants are likely to be present.

Stream

Mostly oxic

conditions

Oxic

conditions

Mostly oxic

conditions

Anoxic

conditions

Aquifer 

near stream

Aquifer

near ridge

Aquifer

between ridge 

and stream

Oxic 

Anoxic

Redox condition

Siliciclasticrock aquifers

Siliciclastic-rock aquifers

EXPLANATION

Flow path

 Shallow flow path

 Deep flow path

NOT TO SCALE

Well

Figure 4–5.

Soil

Soil

Carbonate-rock aquifers

Water table

Fracture

Residence time generally 

increases with depth

Residence time generally increases along flow path

Topographic section 

 boundary

34 Water Quality in the Principal Aquifers of the Piedmont, Blue Ridge, and Valley and Ridge Regions, Eastern United States, 1993–2009
Drinking water for public and domestic supply is the dominant use of 

groundwater in the Principal Aquifers of the Piedmont, Blue Ridge, 

and Valley and Ridge regions. Does the quality of groundwater support 

these uses? What constituents are present at concentrations that pose a potential 

human-health concern when groundwater is used as a source of drinking water? 

What constituents may adversely affect the use of the water? The chemical 

constituents of concern in these regions originate from geologic sources (radon, 

arsenic, and manganese) and also from human activities (nitrate and fecalindicator bacteria).

Chapter 5: Quality of the Groundwater Resource 

Used for Drinking Water 

Chapter 5

Imagery from Pictometry

TM International Corporation

This chapter identifies and discusses 

constituents that were 

detected at a concentration greater than or 

near a human-health 

benchmark in water 

from drinking-water 

supply wells tapping 

the Principal Aquifers 

in the Piedmont, Blue 

Ridge, and Valley and 

Ridge regions.
Twenty-eight percent of the sites that are used as a source of drinking water and were 

sampled for this assessment contained at least one contaminant at a concentration that exceeded 

its human-health benchmark (see sidebar, Human-health benchmarks and other guidelines 

used in this assessment, p. 14). Of the 14 contaminants with concentrations that exceeded a 

human-health benchmark (table 5–1), 8 have a geologic source, and 17 percent of samples 

contained at least 1 of these 8 contaminants at a concentration exceeding its benchmark. 

Table 5–1. Fourteen contaminants were found at concentrations greater than human-health benchmarks in samples from drinking-water wells 

whose concentrations exceeded their human-health benchmarks in at least one well.

[n, number of samples; E. coli, Escherichia coli; pCi/L, picocuries per liter; μg/L, micrograms per liter; mg/L, milligrams per liter; CFU/100 mL, colony-forming units per 

—, percentage exceeding one-tenth of human-health benchmark not evaluated]

Type of 

drinking-water 

contaminant

Contaminant name

Human-health benchmark All drinking-water samples

Value Type n

Percentage exceeding

Human-health 

benchmark

One-tenth of humanhealth benchmark

Geologic 

sources

*Radon-222

300 pCi/L Proposed MCL

478

83 — 

4,000 pCi/L Proposed AMCL 11 —

†Gross alpha activity 15 pCi/L MCL 119 8.4 — 

Arsenic 10 μg/L MCL 217 4.1 22 

Manganese 300 μg/L HBSL 507 4.3 22 

ǂRadium-226 and 

radium-228 5 pCi/L MCL 120 .83 — 

Uranium 30 μg/L MCL 412 .49 13 

¶Antimony 6 μg/L MCL 196 .51 1.5 

Strontium 4,000 μg/L HBSL 206 .49 14 

Human sources

Nitrate 10 mg/L MCL 507 11 55 

Alpha HCH 0.006 μg/L HBSL 441 .23 .23 

Dieldrin 0.002 μg/L HBSL 522 1.3 1.5 

Acrylonitrile 0.06 μg/L HBSL 259 .39 .39 

Trichloroethene 

(TCE) 5 μg/L MCL 385 .26 1.3 

≈Escherichia coli 1 CFU/100 mL MCL 234 24 — 

* U.S. Environmental Protection Agency proposed AMCL and proposed MCL used for radon.

† The MCL for gross alpha excludes radon and uranium.

ǂ MCL is for the sum of Ra-226 and Ra-228.

¶ Although concentrations in some samples exceeded one-tenth of the benchmark, detection frequencies at one-tenth of the benchmark 

could not be fully determined because the detection limits for a large proportion of the samples were greater than one-tenth of the benchmark.

≈ Detection of a single E. coli colony is considered a violation of the drinking-water standard. 

36 Water Quality in the Principal Aquifers of the Piedmont, Blue Ridge, and Valley and Ridge Regions, Eastern United States, 1993–2009
Four of the 8 are trace elements—arsenic, manganese, strontium, and antimony—and 

4 are radionuclides—radon, gross alpha-particle activity, radium, and uranium. Concentrations 

of the other 6 contaminants that exceeded a human-health benchmark are related to human 

activities, and 23 percent of the samples—almost one-quarter—contained at least 1 of these 

contaminants at a concentration exceeding its human-health benchmark. These 6 contaminants 

are the bacterium Escherichia coli (E. coli), the nutrient nitrate, the pesticides dieldrin and 

alpha-HCH, and the VOCs trichloroethene and acrylonitrile.

in this study. These contaminants are derived from both human-related sources and geologic sources. This table lists the contaminants 

100 milliliters; AMCL, Alternative Maximum Contaminant Level; MCL, Maximum Contaminant Level; HBSL, Health-Based Screening Level; n/a, not applicable; 

Sources of contaminant in drinking water Potential health effects from exposure above the 

human-health benchmark 

Decay of radium-226, part of the uranium-238 decay series, in 

aquifer materials Increased risk of lung cancer 

Decay of radioactive elements, primarily uranium and radon, in 

aquifer materials Increased risk of cancer

Aquifer materials, pesticide application in orchards, some industrial wastes Skin damage, problems with circulatory system, increased risk

of cancer

Aquifer materials, some industrial wastes Neurological effects

Aquifer materials Increased risk of cancer

Aquifer materials Increased risk of cancer, kidney toxicity

Aquifer materials, discharge from petroleum refineries, fire retardants, 

ceramics, electronics, solder Increase in blood cholesterol, decrease in blood sugar

Aquifer materials Abnormal bone development

Fertilizer use, manure, sewage and septic-system effluent, aquifer materials

Infants below the age of 6 months who drink water containing 

nitrate in excess of the MCL could become seriously ill and, if 

untreated, may die; symptoms include shortness of breath and bluebaby syndrome

Hazardous waste sites, insecticide application Increased risk of cancer

Insecticide Liver and central nervous system problems, increased risk of cancer

Industrial discharges, leachate from chemical waste sites Developmental, hematological, neurological, and reproductive effects, 

increased risk of cancer

Discharge from metal degreasing sites and factories Liver problems, increased risk of cancer

Human sewage, animal manure Gastrointestinal illness

Chapter 5

Chapter 5: Quality of the Groundwater Resource Used for Drinking Water 37
Figure 5–1. Patterns of contamination distinctly differed among 

the three major bedrock types. In the crystalline- and siliciclasticrock aquifers, concentrations of contaminants from geologic 

sources—primarily manganese and radon—more commonly 

exceeded a human-health benchmark than did concentrations 

of contaminants associated with human activities. In contrast, 

in the carbonate-rock aquifers, exceedances of benchmarks for 

contaminants associated with human activities, such as nitrate 

and Escherichia coli, were among the highest in the Nation. 

Exceedances of benchmarks for contaminants from a geologic 

source were rare in the carbonate-rock aquifers; however, 

some of the natural constituents that exceeded standards in the 

crystalline- and (or) siliciclastic-rock aquifers—gross alphaparticle activity, radium, and arsenic—were not evaluated in most 

samples from the carbonate aquifers.

Radon exceeded its proposed MCL(23) most frequently, 

with 83 percent of the sites sampled for radon exceeding 

the proposed MCL of 300 pCi/L; however, that frequency 

drops to 11 percent if the proposed Alternative Maximum 

Contaminant Level (AMCL)(23) of 4,000 pCi/L is considered. 

Following radon was E. coli, which exceeded its MCL in 

25 percent of samples, and 

nitrate, which exceeded its 

MCL of 10 milligrams per liter 

as nitrogen (mg/L as N) in 

11 percent of samples.

Exceedances of a 

human-health benchmark were 

unevenly distributed among 

the five Principal Aquifers, 

with clear differences based 

on aquifer bedrock type 

(fig. 5–1; table 5–2). Manganese and radon, contaminants 

from geologic sources, exceeded a benchmark most 

commonly in samples from the crystalline- and siliciclasticrock aquifers, and contaminants from a human-related source 

exceeded a benchmark most commonly in samples from the 

carbonate-rock aquifers. The following chapters will explain 

how the sources and movement of the contaminants vary 

among the five Principal Aquifers, resulting in the observed 

patterns of occurrence and concentrations.

Some water-quality characteristics are not related to 

human health, but instead to the desirability of the groundwater to be used as a source of drinking water (table 5–3). 

The context for these characteristics, such as aesthetic issues 

(taste and odor), cosmetic issues (skin or tooth discoloration), 

or technical issues related to the effects of the water on 

plumbing (scale or corrosion), is provided by USEPA 

Secondary Maximum Contaminant Levels (SMCLs)(71) for 

public water supplies. Hardness does not have an SMCL, so 

concentrations were evaluated with respect to other USEPA 

guidelines. The reasons for the distribution of these contaminants, generally considered to be a nuisance rather than a 

hazard, are discussed in chapter 6.

Distributors of publicwater supplies are required 

to reduce concentrations of 

contaminants to less than 

the MCL before distribution, sometimes at great 

expense.

0

10

20

30

40

50

60

70

0

10

20

30

40

50

60

70

Contaminants from geologic sources

Contaminants from human sources

Exceedance of human-health benchmark 

Early

Mesozoic

basin

Valley

and 

Ridge

Valley

and 

Ridge

Piedmont

and 

Blue Ridge

Piedmont

and 

Blue Ridge

Siliciclastic-rock

aquifers

Carbonate-rock 

aquifers

Crystallinerock aquifers

Figure 5–1. 

Percentage of samples containing at least one constituent at a 

concentration exceeding a human-health benchmark

38 Water Quality in the Principal Aquifers of the Piedmont, Blue Ridge, and Valley and Ridge Regions, Eastern United States, 1993–2009

Only two wells in the 

carbonate-rock aquifers 

contained a constituent 

from a geologic source 

at a concentration that 

exceeded its human-health 

benchmark.
What is a contaminant?

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. 

Chapter 5

Chapter 5: Quality of the Groundwater Resource Used for Drinking Water 39
Table 5–2. Contaminants whose concentrations in groundwater approached or exceeded human-health benchmarks varied by Principal Aquifer.

[n, number of samples; E. coli, Escherichia coli; pCi/L, picocuries per liter; μg/L, micrograms per liter; mg/L, milligrams per liter; CFU/100 mL, colony-forming units per 

Source of 

drinking-water 

contaminant

Contaminant 

name

Human-health benchmark Piedmont and Blue Ridge 

crystalline-rock aquifers Early Mesozoic 

Value Type n

Percentage 

exceeding 

human-health 

benchmark

n

Geologic sources

*Radon

300 pCi/L Proposed MCL

177

90

51

4,000 pCi/L Proposed AMCL 24 

†Gross alpha activity 15 pCi/L MCL 64 9.4 27 

Arsenic 10 μg/L MCL 125 4.0 49 

Manganese 300 μg/L HBSL 179 3.9 71 

ǂRadium 5 pCi/L MCL 65 0 27 

Uranium 30 μg/L MCL 179 1.1 51 

Antimony 6 μg/L MCL 125 0.8 28 

Strontium 4,000 μg/L HBSL 95 0 0 

Human sources

Nitrate 10 mg/L MCL 178 6.2 72 

Alpha HCH 0.006 μg/L HBSL 113 0.88 71 

Dieldrin 0.002 μg/L HBSL 194 0.52 71 

Acrylonitrile 0.06 μg/L HBSL 137 0.73 49 

Trichloroethene (TCE) 5 μg/L MCL 150 0 49 

¶Escherichia coli 1 CFU/100 mL MCL 113 5.3 1 

Percentage of sites 

where at least 

one contaminant 

exceeded 

benchmark

35 

 * U.S. Environmental Protection Agency proposed AMCL and proposed MCL used for radon.

† The MCL for gross alpha excludes radon and uranium. 

ǂ MCL is for combined radium—the sum of Ra-226 plus Ra-228.

¶ Detection of a single E. coli colony is considered a violation of the drinking-water standard.

40 Water Quality in the Principal Aquifers of the Piedmont, Blue Ridge, and Valley and Ridge Regions, Eastern United States, 1993–2009
100 milliliters; AMCL, Alternative Maximum Contaminant Level; MCL, Maximum Contaminant Level; HBSL, Health-Based Screening Level; n/a, not applicable]

basin aquifers Valley and Ridge 

siliciclastic-rock aquifers

Piedmont and Blue Ridge 

carbonate-rock aquifers

Valley and Ridge 

carbonate-rock aquifers

Percentage 

exceeding 

human-health 

benchmark

n

Percentage 

exceeding 

human-health

 benchmark

n

Percentage 

exceeding 

human-health 

benchmark

n

Percentage 

exceeding 

human-health 

benchmark

94

80

68

29

86

141

77

5.9 6.3 0 0

11 28 3.6 0 n/a 0 n/a

8.2 28 0 0 n/a 15 0

2.8 81 15 29 0 147 .7

3.7 28 0 0 n/a 0 n/a

0 83 0 29 0 70 0

0 28 0 0 n/a 15 0

n/a 28 0 0 n/a 83 1.2

0 82 3.7 29 62 146 18

0 82 0 29 0 146 0

2.8 82 0 29 3.5 146 2.1

0 28 0 0 n/a 45 0

2.0 57 0 29 0 100 0

0 28 3.6 0 n/a 92 54

19 25 62 52

Chapter 5

Chapter 5: Quality of the Groundwater Resource Used for Drinking Water 41
Table 5–3. Seven constituents or physical properties were measured at concentrations that exceeded guidelines for aesthetic issues, 

such as taste or odor, or technical effects on plumbing, such as scaling or corrosion. These issues mostly are related to the natural 

characteristics of the water. The table lists any constituent whose concentration exceeded a guideline in at least one well.

[SMCL, Secondary Maximum Contaminant Level; MCL, Maximum Contaminant Level; TDS, total dissolved solids; n, number of samples; mg/L, milligrams 

per liter]

Constituent 

type Constituent Undesirable effect Value of SMCL

All 

aquifers

Piedmont and 

Blue Ridge 

crystallinerock aquifers

Early 

Mesozoic 

basin 

aquifers

Valley and 

Ridge 

siliciclasticrock aquifers

Piedmont 

and Blue 

Ridge 

carbonaterock aquifers

Valley and Ridge 

carbonate-rock 

aquifers

n= * 492 † 164 71 81 29 147

Percentage of samples exceeding value

Trace 

elements

Iron Rusty color, sediment, 

taste, staining 0.3 mg/L 9.8 13 2.8 27 0 2.0

Manganese Staining, taste 0.05 mg/L 19 23 8.5 47 6.9 4.8

Major ions

Fluoride Tooth discoloration ǂ 2 mg/L 0.2 0.6 0 0 0 0

Sulfate Salty taste 250 mg/L 0.4 0 2.8 0 0 0

 Properties

TDS

Hardness, deposits, 

colored water, 

staining, salty taste

500mg/L 3.0 1.2 4.2 1.2 10 4.8

Hardness Encrustation, scale ¶ 150 mg/L 48 8.5 65 19 93 90

pH

Low pH—bitter 

metallic taste, 

corrosion

High pH—slippery 

feel, soda taste, 

deposits

≈ 6.5–8.5 30 68 8.3 22 0 .7

* n = 507 for manganese and 609 for hardness.

† n = 179 for manganese and 242 for hardness.

ǂ Fluoride has an MCL of 4 mg/L, but it was never exceeded.

¶ Hardness does not have an SMCL; criteria are from U.S. Environmental Protection Agency(37).

≈ The pH in all samples that did not meet the criteria was less than 6.5 units with the exception of one sample from the Valley and Ridge siliciclastic-rock aquifers, 

in which the pH exceeded 8.5 units. 

42 Water Quality in the Principal Aquifers of the Piedmont, Blue Ridge, and Valley and Ridge Regions, Eastern United States, 1993–2009
How Findings Compare With Those Across 

the Nation

The percentage of drinking-water wells sampled in the 

Piedmont, Blue Ridge, and Valley and Ridge regions with at 

least one contaminant that exceeded its human-health benchmark (28 percent) was slightly higher than the percentage of 

domestic-supply wells (23 percent) or public-supply wells 

(22 percent) sampled across the Nation.(1, 25) These percentages 

are based on the proposed AMCL for radon of 4,000 pCi/L(23) 

and do not include bacteria, for consistency with national 

values. The percentage of wells used for drinking water in 

which E. coli was detected (25 percent) was much higher 

than the percentage of public- and domestic-supply wells 

sampled nationally in which the bacterium was detected 

(6 percent).(26) The Piedmont and Blue Ridge carbonate- and 

Valley and Ridge carbonate-rock aquifers ranked among 

the highest in the Nation for the percentage of samples with 

concentrations of nitrate that exceeded the MCL of 10 mg/L 

as N. The percentage of wells in the Piedmont and Blue Ridge 

crystalline-rock aquifers with exceedances of the proposed 

MCL and AMCL for radon also was high relative to the 

percentage of wells sampled nationally.

Chapter 5

Contaminants measured in groundwater are either related 

to human activity at the land surface or a geologic source of 

the contaminant in the aquifer. Human-related sources can 

be from accidental spills at a single location—called a point 

source—or from a widespread source, such as fertilizers or 

pesticides that are used over a large area—called nonpoint 

sources. Geologic sources are materials in the aquifer that 

can be released into groundwater under certain geochemical 

conditions and also can be harmful to humans if ingested at 

high enough concentrations. Examples of contaminants from 

geologic sources include arsenic, uranium, and radon.

Sources of contaminants in groundwater

Rock layer containing

geologic contaminant 

source

Geologic source

circ1354 ch5 sidebar fig 1

Custome sidebar: Sources of contaminants in groundwater

Modified from Eberts and others (77)

Contaminant

Contaminant

plume

Human-related point source

Human-related nonpoint source

Leaking chemicalstorage tank

circ1354 sidebar ch5 fig2

Chapter 5: Quality of the Groundwater Resource Used for Drinking Water 43

The Piedmont and Blue Ridge carbonate-rock 

aquifers and the Valley and Ridge carbonate-rock 

aquifers ranked among the highest in the Nation 

for the percentage of samples exceeding the 

nitrate MCL.
A variety of point-of-use household water 

treatments are available

“My home system consists of a filter, a water 

softener, and an ultraviolet (UV) light system. I also have 

a reverse osmosis (RO) system serving the kitchen area. 

The filter, UV light, and RO systems treat for nitrate and 

bacteria and cost about $1,600. The UV light and RO 

filters are changed annually for about $200. The water 

softener is for extremely hard water, which was recently 

replaced for about $1,500 and costs about $100 a year 

for salt.”

—Pennsylvania homeowner with 

domestic-supply well

The nitrate removal plant in Martinsburg, Pennsylvania. 

Example of domestic-well user’s home water-treatment system. 

Photograph by Bruce Lindsey, USGS

Photograph from the Morrisons Cove Herald, used with permission

Cost of water treatment

Even when water is plentiful, it is not truly available for use unless 

the quality is acceptable for the intended use. Both water quality 

and availability are essential to maintaining water supply for 

municipal, domestic, agricultural, aquatic-life, and recreational 

use. Options to reduce or remove harmful constituents from water 

are available; they vary depending on the type of contaminant 

and the specifics of water-supply management, but can cost 

thousands of dollars per household.

Some options for domestic-well users and publicsupply providers

•	 installing and maintaining household treatment devices;

•	 buying drinking water, either in bottles or from another 

supplier;

•	 blending water—diluting a contaminated source with 

water from an uncontaminated source;

•	 locating and developing a new uncontaminated source of 

drinking water; and

•	 building or upgrading and operating a treatment plant to 

remove contaminants.

Public water suppliers are required to reduce 

contaminant concentrations to meet regulatory 

standards, sometimes at great expense 

Groundwater is the major source for municipal water-supply 

systems in much of rural Pennsylvania. The water supply for 

Martinsburg, a south-central borough with a population of about 

2,000 people, regularly has nitrate concentrations higher than the 

MCL of 10 mg/L. With no other water sources available, Martinsburg opened a water-treatment facility in 2011 to remove nitrate. 

The ion-exchange facility is the first of its type in Pennsylvania 

and cost $4.1 million, or more than $2,000 per person (Randy 

Stoltz, Martinsburg Municipal Authority, written commun., 2012). 

Nitrate contamination is also common in the surrounding areas.

44 Water Quality in the Principal Aquifers of the Piedmont, Blue Ridge, and Valley and Ridge Regions, Eastern United States, 1993–2009
Constituents from geologic sources enter the groundwater at various 

depths beneath the land surface. The rocks that contain these constituents are not easily seen on the landscape, but the locations of these 

different rock types have been determined by geologic mapping and are used 

to determine the distribution of these constituents. Water samples from the 

crystalline-rock and siliciclastic-rock aquifers frequently contained one or 

more constituents from geologic sources at concentrations higher than their 

human-health benchmarks, but detections of these constituents were rare in 

samples from the carbonate-rock aquifers. The contaminants from geologic 

sources that most frequently exceeded human-health benchmarks were arsenic, 

manganese, and radon. A substantial percentage of samples from crystallineand siliciclastic-rock aquifers also did not meet non-health-related guidelines 

for pH and manganese.

Chapter 6: Constituents From Geologic Sources

Chapter 6

Why is it important to understand concentrations of contaminants 

from geologic sources?

Nationally, the trace elements arsenic and manganese and the radionuclides uranium, radon, 

and radium are among the contaminants from geologic sources that most frequently exceed 

human-health benchmarks.(1, 2) Domestic-well owners rarely test for these contaminants, but should 

be aware of their potential presence in groundwater and the health risks if their concentrations 

exceed human-health benchmarks. Millions of people in the Piedmont, Blue Ridge, and Valley 

and Ridge regions consume water from domestic wells that is not regulated or monitored in the 

same way as public water supplies and could be a source of exposure. A better understanding of 

the geochemical environments that are associated with high levels of contaminants allows water 

managers to focus on the most important problems in specific geographic areas. The trace-element 

and radionuclide concentration data collected for large-scale regional studies, such as this one, 

could be used to provide assistance to private-well owners in identifying where the potential 

human-health risks are highest and information for prioritization of private wells for sampling.

This chapter describes 

the sources of and 

factors that affect 

constituents from 

geologic sources in 

the Principal Aquifers 

of the Piedmont, Blue 

Ridge, and Valley and 

Ridge regions.
Lithology Controls Where Concentrations of Arsenic and Manganese in 

Groundwater Are Elevated

Groundwater in some parts of the Piedmont, Blue Ridge, and Valley and Ridge regions 

had concentrations of arsenic and manganese that exceeded human-health benchmarks. The 

areas with elevated concentrations of arsenic were in specific lithologic groups in the Piedmont 

and Blue Ridge crystalline-rock and early Mesozoic basin aquifers. Elevated concentrations 

of manganese were common in the Piedmont and Blue Ridge crystalline-rock, early Mesozoic 

basin, and Valley and Ridge siliciclastic-rock aquifers.

Four trace elements—arsenic, antimony, manganese, and strontium—were elevated 

(detected in one or more water samples at concentrations greater than their human-health 

benchmarks; table 5–1). Eight other trace elements—barium, copper, lead, selenium, molybdenum, nickel, thallium, and zinc—were detected at concentrations greater than one-tenth of 

their human-health benchmarks near levels that are of potential concern for human health. 

Concentrations of arsenic and manganese exceeded the human-health benchmarks much more 

frequently than did concentrations of antimony and strontium. For a complete listing of the 

detection frequencies of trace elements and radionuclides, see table A3–1.

Arsenic Concentrations in Groundwater That Exceed Drinking-Water 

Standards Are Associated With Specific Lithologic Groups 

Concentrations of arsenic exceeded the MCL of 10 μg/L 

in only 4 percent of all drinking-water wells sampled in 

the Piedmont, Blue Ridge, and Valley and Ridge regions 

(table 5–1), less frequently than the national rate for domestic 

wells (7 percent)(3) or public-supply wells (10 percent).(2)

However, elevated arsenic concentrations in groundwater 

in the Piedmont, Blue Ridge, and Valley and Ridge regions 

have a predictable distribution geographically with respect to 

geochemical conditions. Concentrations of arsenic exceeded 

the MCL in 9 percent of drinking-water wells in two specific 

lithologic groups: the lacustrine sedimentary lithologic group 

in the early Mesozoic basin aquifers and the metamorphosed 

sedimentary clastic lithologic group in the Piedmont and 

Blue Ridge crystalline-rock aquifers (fig. 6–1; table 5–2; 

see sidebar, Aquifers, bedrock type, and lithologic groups in the Piedmont, Blue Ridge, 

and Valley and Ridge regions, p. 26). These lithologic groups are made up of fine-grained 

sedimentary rocks, such as mudstone, shale, siltstone, their metamorphic equivalents, and 

some metavolcanic rocks.(28)

Lithology can vary greatly within a well bore, as shown by the dark 

and light rocks in the rock core. Because of its origin, the dark 

shale is likely to bear arsenic that could dissolve and contribute 

to arsenic found in a well. Although these lithologies may make 

up a small fraction of the rock, the arsenic may be released 

from the rock into the groundwater under specific geochemical 

conditions in concentrations high enough to be of potential 

human-health concern.

46 Water Quality in the Principal Aquifers of the Piedmont, Blue Ridge, and Valley and Ridge Regions, Eastern United States, 1993–2009

Arsenic is a trace element in 

aquifer materials and also in 

pesticides and industrial waste. 

Long-term exposure to arsenic 

in drinking water is related to 

elevated risks of cancer and skin 

damage.(27)

Arsenic concentrations 

exceeded the MCL in 

groundwater associated 

with only two specific 

lithologic groups in 

two Principal Aquifers, 

indicating that a targeted 

approach to further testing 

for arsenic could be used.

Elevated arsenic concentrations were associated 

with high pH, providing a 

screening tool for further 

investigation of arsenic.

As

33

75
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

See sidebar, Boxplots, this page

1

0.5

10

100

200

Metamorphosed

sedimentary

clastic

All other

lithologic

groups

All other

lithologic

groups

Lacustrine

sedimentary

Early Mesozoic basin 

aquifers

Piedmont and Blue Ridge

crystalline-rock aquifers 

circ1354_ch6_fig01

Arsenic concentration, in micrograms per liter

0.5

1

10

100

200

Arsenic concentration, in micrograms per liter

Piedmont and 

Blue Ridge

Siliciclastic-rock

aquifers

Carbonate-rock 

aquifers

Crystallinerock aquifers

Early

Mesozoic

basin

Valley

and 

Ridge

No samples

Valley

and 

Ridge

Piedmont 

and

Blue Ridge

A. Arsenic concentration by aquifer

B. Arsenic concentration by aquifer and lithology

Common assessment level for arsenic

USEPA Maximum Contaminant Level

(MCL) for arsenic

EXPLANATION Area of graph below

Because of variations in reporting limits, a common assessment level of 

1.0 microgram per liter is used to evaluate concentrations of arsenic. For a 

single compound or element, the common assessment level is typically set 

at the maximum level commonly used to report nondetections.

Figure 6–1. A, The highest concentrations of arsenic were 

measured in samples from the early Mesozoic basin aquifers. 

The early Mesozoic basin aquifers and the Piedmont and Blue 

Ridge crystalline-rock aquifers were the only aquifers in which 

arsenic concentrations exceeded the MCL of 10 micrograms 

per liter in one or more samples. B, Within those two Principal 

Aquifers, concentrations of arsenic exceeded the MCL only in 

samples from the metamorphosed sedimentary clastic lithologic 

group in the Piedmont and Blue Ridge crystalline-rock aquifers 

and the lacustrine sedimentary lithologic group in the early 

Mesozoic basin aquifers.

Chapter 6: Constituents From Geologic Sources 47
Figure 6–3. Nine percent of groundwater samples from both the 

metamorphosed sedimentary clastic and lacustrine sedimentary 

lithologic groups had concentrations of arsenic that exceeded the 

MCL of 10 micrograms per liter. Arsenic concentrations in about 

90 percent of samples in the lacustrine sedimentary lithologic 

group exceeded 1 microgram per liter, whereas concentrations 

in only about 20 percent of the samples in the metamorphosed 

sedimentary clastic group exceeded this level. No wells in any 

other aquifers or in any other lithologic group had concentrations 

of arsenic that exceeded the MCL, and only 5 percent of the 

samples from those areas had concentrations that exceeded 

1 microgram per liter. Identification of these lithologic groups can 

be used to target sampling efforts to those areas most likely to 

have elevated concentrations of arsenic.

Arsenic is not released from rocks—even arsenic-rich 

rocks—into the groundwater except under specific geochemical conditions.(29–32) Although arsenic concentrations in 

groundwater in some settings are influenced by redox conditions, in these aquifers, pH exerts much more control over 

arsenic concentrations than redox conditions (fig. 6–2). The 

lithologic groups that had the highest concentrations of arsenic 

in groundwater (lacustrine sedimentary and metamorphosed 

sedimentary clastic) also had the highest pH values.

Although the health concerns for long-term exposure 

to arsenic in drinking water are well documented,(27) most 

homeowners with domestic-supply wells are unaware if their 

drinking-water supply contains arsenic unless they have 

specifically tested for it. Testing may be prudent,(33) especially 

if the water has high pH and is from one of the bedrock 

types or specific lithologic groups identified here as having 

high potential for elevated arsenic concentrations (fig. 6–3). 

Because of the considerable variation in the composition 

of rocks in these aquifers, however, arsenic concentrations 

also might be elevated in wells not in the areas highlighted 

on figure 6–3.

EXPLANATION

Study-area boundary

Aquifers where arsenic was 

 detected in at least one 

 sample at a concentration 

 greater than 10 micrograms 

 per liter 

Less than or equal to 1

Greater than 1 to 10

Greater than 10

Arsenic concentration, in 

 micrograms per liter

Figure 6–3.

Study

area

0

10

20

30

40

50

60

70

80

Less than 6.5 6.5 to less 

than 7.5

Greater than or

equal to 7.5

pH category

EXPLANATION

Oxic

Redox condition

Anoxic

Figure 6–2.

Percentage of samples with

arsenic concentrations greater

than 1 microgram per liter 

Arsenic detection frequency

Figure 6–2. The highest arsenic concentrations are associated 

with high pH. The largest percentage of samples with a 

concentration of arsenic exceeding 1 microgram per liter was 

from waters with a pH greater than 7.5 regardless of whether the 

redox conditions were oxic or anoxic.

48 Water Quality in the Principal Aquifers of the Piedmont, Blue Ridge, and Valley and Ridge Regions, Eastern United States, 1993–2009
Figure 6–5. Manganese is released from manganese-bearing 

minerals when groundwater is anoxic and has low pH. Anoxic 

conditions have a greater effect on manganese concentration 

than does pH. More than 90 percent of samples that were 

anoxic and had a pH of less than 6.5 contained manganese at a 

concentration exceeding 30 micrograms per liter (one-tenth of 

the human-health benchmark).

EXPLANATION

Oxic

Redox condition

Anoxic

Less than 6.5 6.5 to less 

than 7.5

Greater than or

equal to 7.5

pH category

Figure 6–5.

0

10

20

30

40

50

60

70

80

90

100

Percentage of samples with

manganese concentration

greater than 30 micrograms per liter 

Manganese detection frequency

See sidebar, Boxplots, p. 47

Manganese concentration, in 

micrograms per liter

Piedmont and 

Blue Ridge

Siliciclastic-rock

aquifers

Carbonate-rock 

aquifers

Crystallinerock aquifers

Early

Mesozoic

basin

Valley

and 

Ridge

Valley

and 

Ridge

Piedmont 

and

Blue Ridge

Manganese concentration by aquifer

Figure 6–4.

Common assessment level for manganese

Human-health benchmark for manganese

EXPLANATION

0.1

1

10

100

1,000

10,000

Figure 6–4. Manganese concentrations were highest in samples 

from the Valley and Ridge siliciclastic-rock aquifers, where 

15 percent of the samples exceeded the human-health benchmark 

of 300 micrograms per liter. Manganese concentrations were 

lowest in the carbonate-rock aquifers, where they exceeded the 

benchmark in only a single sample.

Manganese is a trace element 

in aquifer materials and some 

industrial wastes. Manganese 

has a human-health benchmark, 

established because of neurological effects, of 300 μg/L.(58, 59)

Mn

25

55

Manganese Concentrations Exceed Human-Health Benchmarks Most 

Frequently in Wells in the Valley and Ridge Siliciclastic-Rock Aquifers

Overall, only 4 percent of the samples from drinking-water wells contained manganese at 

a concentration greater than the human-health benchmark of 300 μg/L (table 5–1). The highest 

concentrations of manganese were in samples from drinking-water wells in the Valley and 

Ridge siliciclastic-rock aquifers (fig. 6–4), where 15 percent of wells had manganese concentrations that exceeded the benchmark. In contrast, concentrations exceeded the human-health 

benchmark in only a single sample from the carbonate-rock aquifers (table 5–2). In addition 

to posing concerns for human health, manganese also is considered a nuisance constituent in 

groundwater because it stains plumbing fixtures (see nuisance contaminants in chapter 6).

Manganese concentrations in groundwater tend to be elevated in areas where the bedrock 

contains manganese and geochemical conditions in the aquifer are such that manganese is 

released from the rock into the groundwater. High concentrations of manganese were related to 

anoxic redox conditions and, to a lesser extent, to low pH(35) (fig. 6–5), which are more common 

in the Valley and Ridge siliciclastic-rock aquifers (figs. 4–2, 4–3) and the Piedmont and Blue 

Ridge crystalline-rock aquifers than in the other three aquifers. Measures of redox condition and 

pH, however, typically are not available to homeowners, but because the SMCL of 50 μg/L is 

lower than the human-health benchmark of 300 μg/L, visual signs such as staining could be an 

indication of elevated concentrations of manganese. However, manganese can be at high enough 

concentrations to be a potential health concern without any visible evidence; thus testing is the 

best alternative for those who live in areas that tend to have high manganese concentrations.

Chapter 6

Chapter 6: Constituents From Geologic Sources 49

Most of the concentrations of manganese 

exceeding the humanhealth benchmark were 

in the Valley and Ridge 

siliciclastic-rock aquifers 

and were associated 

with low pH and 

anoxic conditions.
Uranium is a radioactive 

element found in aquifer 

materials. Drinking water with 

uranium at concentrations 

above the MCL has been 

associated with increased risk 

of cancer and kidney toxicity.

Groundwater in the Piedmont, 

Blue Ridge, and Valley and Ridge 

regions routinely had concentrations of radon that exceeded 

both the 300 pCi/L proposed MCL 

and the 4,000 pCi/L proposed 

AMCL.(23) The highest concentrations of radon were in water 

from wells in the Piedmont and 

Blue Ridge crystalline-rock 

aquifers, and especially in 

specific lithologic groups in 

these aquifers.

U

92

238

Radon Concentrations Frequently Exceeded Proposed Drinking-Water 

Standards, and Other Radionuclides Exceeded Standards in a Few Cases

Uranium is the parent of many other radioactive elements (see sidebar, Radioactive decay, 

p. 51)—its radioactive decay produces other radionuclides. Although present in trace amounts

in many types of bedrock, uranium is present in higher amounts in some bedrock types, 

including the felsic crystalline rocks that are found in parts of the Piedmont and Blue Ridge 

crystalline-rock aquifers. Uranium, radium, and radon have different geochemical characteristics and are transported by groundwater in different ways; for this reason, groundwater from 

areas underlain by rocks with high uranium content does not always have high concentrations 

of other radionuclides.

Uranium Concentrations Exceed Human-Health Benchmarks in a Few 

Wells and Locations of High Concentrations Are Related to Geology

Concentrations of uranium exceeded the MCL of 30 μg/L in fewer than 1 percent of the 

drinking-water wells sampled (table 5–1). Although the two samples with the highest concentrations of uranium were from the Piedmont and Blue Ridge crystalline-rock aquifers, the 

median uranium concentration was highest for samples from the early Mesozoic basin aquifers, 

followed by those from the Piedmont and Blue Ridge carbonate-rock aquifers (fig. 6–6).

Uranium concentrations are greater than 3 μg/L most commonly in groundwater that has 

a pH greater than 7.5 (fig. 6–7).(28) The sedimentary-rock aquifers of the early Mesozoic basin 

and the Piedmont and Blue Ridge carbonate-rock aquifers have both the highest percentage of 

wells with high pH (fig. 4–2) and the highest concentrations of uranium (fig. 6–6). The highest 

median uranium concentrations were in areas where source rocks had moderate to high uranium 

content and pH greater than 7.5.(28) In contrast, in the Piedmont 

and Blue Ridge crystalline-rock aquifers, although uraniumbearing minerals are abundant, uranium concentrations in 

water generally were low, likely because of the low pH of these 

waters. This result indicates the critical role of groundwater 

geochemistry for controlling uranium in groundwater regardless 

of the uranium content of the surrounding bedrock.

Rn

86

222

Felsic rocks, such as granite (far left), generally are uranium-rich 

because of the presence of uranium in the mineral feldspar, a 

light-colored mineral that is a component of felsic rocks. Where 

uranium is abundant, other radionuclides also can be present. 

Mafic rocks, such as diabase (on the right), contain much less 

feldspar and thus much less uranium, so mafic rocks are less 

likely to be sources of radionuclides.

Photographs by Bruce Lindsey, USGS

50 Water Quality in the Principal Aquifers of the Piedmont, Blue Ridge, and Valley and Ridge Regions, Eastern United States, 1993–2009

Concentrations of uranium 

in groundwater were low 

where pH was low, even in 

the presence of uraniumrich rocks.
Figure 6–6. The median concentration of uranium was higher in 

the early Mesozoic basin aquifers than in the other four Principal 

Aquifers. High pH in the early Mesozoic basin aquifers allows 

uranium to be released from uranium-rich rocks of the aquifer into 

the groundwater.

Figure 6–7. Uranium concentrations greater than 3 micrograms 

per liter (one-tenth of the MCL) were more common where 

samples were anoxic and pH was high.

Radioactive decay

Two of the radionuclides of concern, radium-226 and radon-222, are products of the 

radioactive decay of uranium-238. The other radionuclide of concern, radium-228, is a product 

of the radioactive decay of thorium-232. The USEPA MCL for radium is for the combined total of 

radium-226 and radium-228. In this report, uranium-238 is referred to as uranium; radon-222 is 

referred to as radon; and the combined concentration of radium-226 and radium-228 is referred to as 

radium concentration.

1

10

100

Uranium concentration, in micrograms per liter

USEPA Maximum 

 Contaminant Level 

 for uranium

EXPLANATION

Piedmont and 

Blue Ridge

Siliciclastic-rock

aquifers

Carbonate-rock 

aquifers

Crystallinerock aquifers

Early

Mesozoic

basin

Valley

and 

Ridge

Valley

and 

Ridge

Piedmont 

and

Blue Ridge

Figure 6–6. 

Uranium concentration by aquifer

See sidebar, Boxplots, p. 47

EXPLANATION

Oxic

Redox condition

Anoxic

Less than 6.5 6.5 to less 

than 7.5

Greater than or

equal to 7.5

pH category

Figure 6–7.

0

5

10

15

20

25

30

35

Percentage of samples with

uranium concentration greater

than 3 micrograms per liter 

Uranium detection frequency

Chapter 6

Chapter 6: Constituents From Geologic Sources 51
Figure 6–8. A, The median concentration of radon in 

groundwater samples from each of the five Principal Aquifers 

exceeded the proposed MCL of 300 picocuries per liter.

Concentrations in one or more samples from the crystalline-rock 

aquifers and siliciclastic-rock aquifers exceeded the proposed 

AMCL of 4,000 picocuries per liter. B, Radon concentrations 

in groundwater samples from the Piedmont and Blue Ridge 

crystalline-rock aquifers and the early Mesozoic basin aquifers 

are related to lithologic group. In the lithologic groups from left to 

right, the rocks generally contain less uranium, and thus there is 

less radon in the groundwater.

Radon Exceeds the Proposed Alternative Maximum Contaminant Level in 

About One of Every Four Wells Sampled in the Crystalline-Rock Aquifers

Radon (radon-222) is a water-soluble, radioactive gas that originates from the radioactive decay of radium-226, part of the uranium-238 decay series; thus the source of radon in 

groundwater is aquifer materials. Radon in drinking water is associated with lung cancer and 

stomach cancer.

Concentrations of radon in 83 percent of the wells sampled exceeded the proposed 

MCL of 300 pCi/L, and 11 percent exceeded the proposed AMCL of 4,000 pCi/L for radon 

(table 5–1).(23) In the Piedmont and Blue Ridge crystalline-rock aquifers, one in every four 

samples contained radon at a concentration exceeding the higher of the two proposed MCLs 

(4,000 pCi/L) (fig. 6–8A).

High radon concentrations were very closely linked to bedrock type. Although median 

radon concentrations were greater than the proposed MCL of 300 pCi/L in water samples from 

all five Principal Aquifers (fig. 6–8A), concentrations in some 

samples exceeded the proposed AMCL of 4,000 pCi/L only 

in the Piedmont and Blue Ridge crystalline-rock aquifers, 

the early Mesozoic basin aquifers, and the Valley and Ridge 

siliciclastic-rock aquifers. The geographic distribution of the 

aquifers where radon exceeded 4,000 pCi/L includes a large 

percentage of the Piedmont, Blue Ridge, and Valley and Ridge 

regions (fig. 6–9.) Variations in lithology also affected the 

pattern of concentrations within individual aquifers. Within 

the Piedmont and Blue Ridge crystalline-rock aquifers, the 

mafic lithologic group had notably lower concentrations 

of radon relative to the other lithologic groups in that 

aquifer (fig. 6–8B). 

10

100

1,000

10,000

40,000

Radon concentration, in picocuries per liter

Piedmont and 

Blue Ridge

Siliciclastic-rock

aquifers

Carbonate-rock 

aquifers

Crystallinerock aquifers

Early

Mesozoic

basin

Valley

and 

Ridge

Valley

and 

Ridge

Piedmont 

and

Blue Ridge

10

100

1,000

10,000

40,000

Radon concentration, in picocuries per liter

Proposed

Proposed alternative

USEPA Maximum Contaminant Level for radon

EXPLANATION

Metamorphosed 

sedimentary 

clastic

Felsic

igneous

QuartzrichIntermediate Mafic/

ultramafic*

A. Radon concentration by aquifer

B. Radon concentration by lithology

Piedmont and Blue Ridge crystalline-rock aquifers

See sidebar, Boxplots, p. 47

*Seven of these samples were from early Mesozoic basin aquifers.

Area of 

graph below

52 Water Quality in the Principal Aquifers of the Piedmont, Blue Ridge, and Valley and Ridge Regions, Eastern United States, 1993–2009
Figure 6–9. Wells with radon concentrations that exceeded 

the proposed AMCL of 4,000 picocuries per liter of radon were 

distributed across a large geographic area. 

Radon concentrations that exceeded the proposed AMCL 

of 4,000 pCi/L were measured in samples from wells across 

large areas of the Piedmont, Blue Ridge, and Valley and 

Ridge regions, and concentrations greater than 300 pCi/L 

were likely to be measured almost anywhere throughout 

this area (fig. 6–9). The widespread distribution of elevated 

concentrations of radon indicates the importance of continued 

public education and outreach, particularly because both 

air and water affect an individual’s exposure to radon (see 

sidebar, Radon risk from water, this page).

Radon risk from water

Radon is a naturally occurring radioactive gas and is part of the 

decay series of uranium, an element that is found in trace amounts 

in many aquifer sediments and rocks. Radon is soluble in water and 

was detected in samples of groundwater from all wells in the Principal 

Aquifers of the Piedmont, Blue Ridge, and Valley and Ridge Region. 

USEPA classifies radon as a human carcinogen,(49) and people who 

are exposed to radon, mostly in the air but also through drinking water, 

have an increased risk of getting cancer over their lifetime, especially 

lung cancer.(61) Health effects from radon in drinking water are primarily 

through inhalation, after the gas is released from solution—for example, 

in the shower. Radon gas (represented by red dots) dissolved in groundwater can be pumped from a well and then contained within household 

pipes. Once the water is aerated through a faucet and released into the 

air, the radon can be inhaled when people are close to showerheads or 

drinking directly from faucets.

In 1999, the USEPA proposed two regulatory levels for radon in 

drinking water. The higher level, the AMCL of 4,000 pCi/L, would apply 

to public water systems or states that have programs to reduce radon 

risks from all sources. The lower level, the MCL of 300 pCi/L, would apply 

to systems or states that do not have such programs in place.(23) Both 

proposed MCLs are used as human-health benchmarks in this circular, 

but are not yet formal regulatory benchmarks. For more information on 

radon and how to test for it, see http://www.epa.gov/radon/index.html.

Radon gas (represented by red dots) dissolved in 

groundwater can be pumped from a well and then 

contained within household pipes. Once the water 

is aerated through a faucet and released into the air, 

the radon can be inhaled when people are close to 

showerheads or drinking directly from faucets.

Modified from Otten and others(75)

Chapter 6

EXPLANATION

Study-area boundary

Aquifers where radon was 

 detected in at least one 

 sample at a concentration

 greater than 4,000 picocuries

 per liter (pCi/L)

Less than or equal to 300

Greater than 300 to 4,000

Greater than 4,000

Radon concentration, 

 in pCi/L

Figure 6–9.

Study

area

Chapter 6: Constituents From Geologic Sources 53

Radon concentrations 

were most likely to be 

elevated in groundwater 

in the crystalline-rock 

aquifers.
Radium Exceeds Drinking Water Standards Only in Unique Geologic Areas

Only a single sample contained radium at a concentration that exceeded the MCL for 

radium (greater than 5 pCi/L for radium-226 and radium-228 combined; table 5–1). This 

sample was from a well in the early Mesozoic basin aquifers (table 5–2). The two siliciclasticrock aquifers (Valley and Ridge siliciclastic-rock aquifers and early Mesozoic basin aquifers) 

had higher median concentrations of radium than did the Piedmont and Blue Ridge crystallinerock aquifers. No samples from the Valley and Ridge carbonate-rock or Piedmont and Blue 

Ridge carbonate-rock aquifers were analyzed for radium.

Anoxic conditions and low pH favor the release of radium from minerals into groundwater.(36) Consequently, concentrations of radium exceeded 1 pCi/L more frequently in samples 

with anoxic or mixed redox conditions than in oxic samples. Of the samples in which the 

radium concentration exceeded 1 pCi/L, 85 percent had anoxic or mixed redox conditions or a 

pH value less than 6.5. For oxic samples, radium exceeded 1 pCi/L most frequently when pH 

values were less than 5.5.

High Concentrations of Nuisance Constituents and Properties in 

Groundwater Affect its Desirability for Use

Groundwater in many parts of the Piedmont, Blue Ridge, and Valley and Ridge regions 

did not meet recommended levels for nuisance constituents and properties. Where these 

constituents and properties, such as hardness and pH, are high or low, water treatment may 

be needed to improve the taste or odor or to prevent effects on plumbing, such as corrosion 

or encrustation. Groundwater in the Piedmont and Valley and Ridge carbonate-rock aquifers 

commonly is very hard. Groundwater in the Piedmont and Blue Ridge crystalline-rock aquifers 

commonly is soft and corrosive. Concentrations of dissolved iron and manganese are elevated 

primarily in groundwater from crystalline- and siliciclastic-rock aquifers.

Low pH can lead to corrosion, damaging pipes and eventually 

causing leaks.

Secondary Maximum Contaminant Levels 

(SMCLs) 

SMCLs are referred to as guidelines, because they are not 

enforceable and do not relate to human health. Instead, they 

are intended to prevent scaling; staining of fixtures and laundry; 

corrosion of plumbing; undesirable taste, odor, and color; or other 

undesirable characteristics. Hardness does not have an SMCL 

but is included with the SMCLs because of its potential negative 

effect on water use.

Ra

88

226

54 Water Quality in the Principal Aquifers of the Piedmont, Blue Ridge, and Valley and Ridge Regions, Eastern United States, 1993–2009

Radium is an element that results 

from the radioactive decay of 

uranium in aquifer materials. 

Elevated concentrations of 

radium, like radon, in drinking 

water are associated with an 

increased risk of cancer.

Photograph by William Warner

Water from the carbonaterock aquifers generally 

is hard, whereas water 

from the crystalline-rock 

aquifers has low pH and 

is corrosive.
Differences in the mineralogy of the five Principal Aquifers in the Piedmont, Blue Ridge, 

and Valley and Ridge regions affect the general chemistry of the water. Aquifer minerals such 

as calcite and dolomite that dissolve readily result in groundwater with high alkalinity and 

stable, near-neutral pH (6.5 to 7.5), whereas less soluble silicate minerals generally lead to 

groundwater with lower alkalinity and lower, less stable pH values. Dissolved minerals and 

the pH of the water can adversely affect use of the water (see sidebar, Secondary Maximum 

Contaminant Levels (SMCLs), p. 54).

Low pH and soft water can lead to corrosion, damaging pipes and eventually causing 

leaks. Hard water can cause scaling and clogging of plumbing (encrustation), but soft 

water with low pH can corrode pipes. Water was hard (calcium carbonate equivalent 150 to 

300 mg/L) or very hard (calcium carbonate equivalent greater than 300 mg/L)(37) in about 

one-half of the drinking-water wells sampled, mostly in the carbonate-rock aquifers. About 

one-third of the samples were soft (calcium carbonate equivalent 0 to 75 mg/L) and had pH 

values less than the recommended pH of 6.5—the majority of these were from the Piedmont 

and Blue Ridge crystalline-rock aquifers. More than 90 percent of the wells in the two 

carbonate-rock aquifers and 65 percent of the wells in the early Mesozoic basin aquifers had 

hard water (fig. 4–2; table 5–3). Calcite and dolomite in the rocks of these aquifers dissolves 

easily and leads to groundwater with high concentrations of calcium and magnesium, which 

are the main components of water hardness. Although high hardness is not a major problem in 

the crystalline-rock aquifers, low pH levels and very soft water there can cause corrosion of 

plumbing (table 5–3).

Elevated concentrations of iron and manganese can cause staining of plumbing fixtures 

and laundry and can cause drinking water to have an unpleasant taste. Concentrations of 

iron exceeded the SMCL in 10 percent of the drinking-water samples from all aquifers, and 

manganese concentrations exceeded the SMCL in 19 percent of the drinking-water samples 

from all aquifers. Iron and manganese concentrations most commonly exceeded SMCLs in 

water from the Valley and Ridge siliciclastic-rock aquifers (table 5–3), where groundwater was 

more likely to be anoxic (fig. 4–3) and have low to moderate pH (fig. 4–2; table 5–3). Iron and 

manganese exceeded the SMCLs in nearly a quarter of the samples from the Piedmont and Blue 

Ridge crystalline-rock aquifers. Geochemical conditions typical of carbonate-rock aquifers, 

such as neutral pH and oxic conditions, lead to low concentrations of iron and manganese in 

the water. Substantial costs are associated with onsite treatment for low pH, hardness, or metals 

such as iron and manganese. Homeowners choosing not to treat their water face potential costs 

from corroded pipes (low pH), clogged plumbing fixtures (hard water), additional costs for 

detergents (hard water), or stained laundry (iron and manganese).

Chapter 6

Chapter 6: Constituents From Geologic Sources 55

Photograph courtesy of Culligan

Elevated concentrations 

of iron and manganese 

in wells in the Valley and 

Ridge siliciclastic-rock 

aquifers can cause water 

to have an unpleasant taste 

and to stain laundry and 

plumbing fixtures. 
Concentrations of Contaminants From Geologic Sources Are Controlled by 

Source, pH, and Redox Conditions

Concentrations of all five of the contaminants from geologic sources that were the focus of 

this study—arsenic, manganese, radon, uranium, and radium—are controlled by some combination of the minerals in the rock, pH, and redox conditions. In general, radon concentrations 

greater than 300 pCi/L in groundwater were common in all of the aquifers, but concentrations 

greater than 4,000 pCi/L were found only in areas with uranium-rich rocks (table 6–1). 

Elevated concentrations of manganese in groundwater are associated with more anoxic 

conditions, with pH being a potential secondary contributing factor. Elevated concentrations 

of arsenic and uranium in groundwater both are associated with elevated pH, with arsenic- or 

uranium-rich source rocks being a secondary factor (table 6–1). Elevated concentrations of 

radium in groundwater follow the same pattern as manganese, with elevated concentrations 

being associated with more anoxic conditions, and pH being a potential secondary contributing 

factor. These patterns of concentrations are different from those that would be anticipated 

based solely on abundance of source materials in the rock, illustrating the dominance of 

geochemical controls.

Table 6–1. Factors controlling concentrations of constituents from geologic sources in the 

groundwater of the Piedmont, Blue Ridge, and Valley and Ridge regions.

Constituent Factors that favor elevated constituent concentrations

Radon Uranium-rich aquifer rocks

Manganese Manganese-rich aquifer rocks, low pH, low dissolved oxygen concentrations

Arsenic Arsenic-rich aquifer rocks, high pH

Uranium Uranium-rich aquifer rocks and high pH

Radium Uranium-rich aquifer rocks, high pH, low dissolved oxygen concentrations

56 Water Quality in the Principal Aquifers of the Piedmont, Blue Ridge, and Valley and Ridge Regions, Eastern United States, 1993–2009
Human activities bring about changes that are visible on the land surface, 

such as fields and pastures, residential developments, small towns, 

and large cities. Each of these changes brings sources of potential 

contaminants to groundwater. In the Piedmont, Blue Ridge, and Valley and 

Ridge regions, a number of contaminants that are associated with human activities were detected in groundwater, and some were detected at concentrations 

of potential concern for human health. Samples from the two carbonate-rock 

Principal Aquifers frequently contained contaminants associated with human 

sources at concentrations higher than human-health benchmarks, but very few 

samples from the crystalline-rock and siliciclastic-rock aquifers exceeded these 

benchmarks. Bacteria and nitrate were the contaminants that most frequently 

exceeded their benchmarks. Pesticides and VOCs, although frequently detected, 

rarely were measured at concentrations exceeding human-health benchmarks.

Chapter 7: Contaminants Associated With 

Human Activities

Chapter 7

Photograph by Bruce Lindsey, USGS

This chapter describes 

the sources and factors 

that affect humanrelated contaminants in 

the Principal Aquifers 

in the Piedmont, Blue 

Ridge, and Valley and 

Ridge regions.
Water from 25 percent of 

the wells and springs used 

as sources of drinking 

water in the two carbonaterock Principal Aquifers 

exceeded the MCL for 

nitrate, the highest rate 

of exceedance among 

Principal Aquifers assessed 

in the Nation.

Nitrate Exceeds Drinking-Water Standards in Carbonate-Rock Aquifers 

With Intensive Agricultural Activity

In the Piedmont and Blue Ridge and the Valley and Ridge carbonate-rock aquifers, 

concentrations of nitrate in more than 25 percent of samples from drinking-water wells 

exceeded the MCL of 10 mg/L as N. In contrast, in the Valley and Ridge siliciclastic-rock 

aquifers, early Mesozoic basin aquifers, and Piedmont and Blue Ridge crystalline-rock aquifers, the nitrate MCL was exceeded in fewer than 4 percent of samples. Bedrock type, land use, 

and redox conditions affect the concentrations of nitrate.

Nitrate Concentrations Are Highest in the Carbonate-Rock Aquifers

Nitrate concentrations in groundwater in the Principal Aquifers of the Piedmont, Blue 

Ridge, and Valley and Ridge regions exceeded the MCL of 10 mg/L for nitrate in 11 percent 

of the samples, but the distribution of high nitrate concentrations among the five aquifers was 

dissimilar (fig. 7–1A). Concentrations in the carbonate-rock aquifers were among the highest 

in the Nation (table A–4),(39) in sharp contrast to concentrations in the Piedmont and Blue 

Ridge crystalline-rock aquifers, the early Mesozoic basin aquifers, and the Valley and Ridge 

siliciclastic-rock aquifers, where the rate of exceedance of the MCL for nitrate for domestic 

wells was similar to that for the rest of the Nation.(9) The wide range in nitrate concentrations 

results from differences in aquifer susceptibility and sources of nitrogen.

Nitrogen Sources, Including Agriculture And Other Human Activities, 

Affect Nitrate Concentrations

Differences in sources of nitrogen ranged widely among the Principal Aquifers of the 

Piedmont, Blue Ridge, and Valley and Ridge regions and account for many of the observed 

differences in nitrate concentrations in groundwater. Nitrate concentrations in groundwater 

were highest in areas affected by human activities on the land surface, such as agricultural and 

urban land use that create major sources of nitrogen (fig. 7–1B). Estimates of the annual mass 

of nitrogen applied in the area surrounding each site, including nitrogen from manure, chemical 

fertilizer, and atmospheric deposition, also can help to explain the variability in nitrate concentrations measured. Manure and chemical fertilizer were the primary sources of nitrogen in most 

areas and usually were directly related to high nitrate concentrations (fig. 7–2).(40)

Groundwater from agricultural areas in the Piedmont and Blue Ridge carbonate-rock 

aquifers (fig. 7–1B) ranked as having the highest median nitrate concentrations of any of the 

agricultural areas sampled in the Nation (11 mg/L) (appendix A–4). In this area, 52 percent of 

the total area overlying the Principal Aquifer is agricultural land, and the number of pounds 

per square mile of nitrogen applied to the area surrounding wells in this area ranks among 

the highest in the Nation (fig. 7–2). Lancaster County, Pennsylvania, which overlies the 

Piedmont and Blue Ridge carbonate-rock aquifers and part of the Piedmont and Blue Ridge 

crystalline-rock aquifers, was ranked as the county with the highest application rate of nitrogen 

produced from manure from confined animal-feeding operations in the Nation in 1982, 1987, 

and 1992.(40) The samples from this area were predominantly from domestic-supply wells, so 

the results represent the drinking-water supply as well as recently recharged groundwater in 

agricultural areas.

Sources of nitrate include 

fertilizer, animal manure, and 

human sewage. Excess nitrate in 

drinking water is associated with 

adverse human-health effects, 

including methemoglobinemia, 

which can cause death in 

infants.(38)

58 Water Quality in the Principal Aquifers of the Piedmont, Blue Ridge, and Valley and Ridge Regions, Eastern United States, 1993–2009

Nitrate
Figure 7–1. A, Nitrate concentrations were highest in the 

Piedmont and Blue Ridge carbonate-rock aquifers and the 

Valley and Ridge carbonate-rock aquifers. B, The combination 

of agricultural land use and carbonate or crystalline bedrock 

resulted in higher concentrations of nitrate in groundwater than 

did other aquifer and land-use combinations.

Figure 7–2. The Piedmont and Blue Ridge carbonate-rock 

aquifers underlie an area with some of the highest nitrogen 

inputs in the Nation, mainly as a result of manure application from 

animal-feeding operations.(40) The agricultural areas overlying 

other aquifers also have high inputs of nitrogen.

35

30

25

20

15

10

5

0

35

30

25

20

15

10

5

0

Nitrate concentration, in milligrams per liter as N

A. Nitrate concentration by Principal Aquifer

B. Nitrate concentration by aquifer and land use

EXPLANATION

USEPA Maximum Contaminant Level for nitrate

Valley and 

Ridge 

Early

Mesozoic

basin

Valley and 

Ridge

Piedmont and 

Blue Ridge

Early

Mesozoic

basin

Valley and 

Ridge

Valley and 

Ridge

Piedmont and 

Blue Ridge

Piedmont

and 

Blue Ridge

Piedmont

and 

Blue Ridge

Siliciclastic-rock

aquifers

Carbonate-rock

aquifers

Crystallinerock aquifers

Agriculturalareas

Figure 7–1. Nitrate concentrations

See sidebar, Boxplots, p. 47

Agricultural area—Groundwater used for drinking water

Mixed land use—Groundwater used for drinking water

Urban area—Groundwater near water table

Chapter 7

Land use

Agricultural area, groundwater 

 used for drinking water

Urban area, groundwater

 near water table

Mixed land use, groundwater

 used for drinking water

Valley and

Ridge 

Early

Mesozoic

basin

Valley and 

Ridge

Piedmont and 

Blue Ridge

Piedmont

and 

Blue Ridge

Siliciclastic-rock

aquifers

Carbonate-rock

aquifers

Crystallinerock aquifers

Ag Urb Mix Ag Ag Ag Urb Mix

Ag

Urb

Mix

Mix Mix

Atmospheric deposition

Nitrogen fertilizer, 

 non-farm use

Nitrogen fertilizer, 

 farm use

Animal manure 

Major source

EXPLANATION

0

5,000

10,000

15,000

Nitrogen input, in pounds per square mile,

median for all sources

Figure 7–2. An estimate of nitrogen inputs 

Nitrogen sources by aquifer and land use

Chapter 7: Contaminants Associated With Human Activities 59
Figure 7–3. Nitrogen inputs and redox conditions 

together control concentrations of nitrate. Wells in areas 

with moderate inputs of nitrogen and oxic conditions 

had higher median nitrate concentrations than did wells 

in areas with moderate inputs of nitrogen and anoxic 

conditions. Median nitrate concentrations in the areas 

with anoxic groundwater were 20 to 100 times lower than 

those in areas with similar inputs of nitrogen and oxic 

groundwater. The nitrogen source is a circular area with 

a 1,640-foot radius around each well.(40)

The wells sampled in an agricultural area in the Piedmont and Blue Ridge crystalline-rock 

aquifers also had high concentrations of nitrate (fig. 7–1B). Many of these samples were collected 

in Lancaster County, Pennsylvania. These wells are also domestic-supply wells, and although they 

are not plotted separately in appendix 4 (they are grouped with the domestic wells), the median 

concentrations of nitrate are among the highest in the Nation. Although this area makes up a fairly 

small part of the Piedmont and Blue Ridge crystalline-rock aquifers, these results demonstrate 

that, like the carbonate-rock aquifers, the crystalline-rock aquifers are vulnerable to nitrate 

contamination when nitrogen sources, such as fertilizer and manure, are present.

Redox Processes, Groundwater Age, and Karst Features Affect Transport 

and Fate of Nitrate

Although nitrogen sources help to explain concentrations of nitrate in groundwater, other 

factors play a critical role as well. Where an aquifer does not have conditions that favor nitrate 

persistence or transport, concentrations in groundwater may be low, even if large nitrogen sources 

are present.

Whether or not nitrate remains in groundwater is controlled by denitrification, a process that 

reduces nitrate to harmless nitrogen gas (see sidebar, How do redox reactions work?, p. 31). Denitrification can take place in anoxic conditions but not in oxic conditions (see chapter 4). Nitrate in 

the carbonate-rock aquifers is least likely to be removed by denitrification, because these aquifers 

have the most oxic water (fig. 4–3). In contrast, many of the samples in the Valley and Ridge 

siliciclastic-rock aquifers had anoxic water (fig. 4–3), and the effects of denitrification are evident. 

The agricultural area of the Valley and Ridge siliciclastic-rock aquifers had the fourth highest 

inputs of nitrogen among the 10 aquifer land-use groupings (fig. 7–2), but samples from wells in 

these areas had the lowest median nitrate concentration of any of these groups (fig. 7–3). If wells 

with similar nitrogen inputs are compared, wells with oxic water had nitrate concentrations that 

were much higher than did wells with anoxic water (fig. 7–3). Although nitrate was more likely to 

become degraded in some aquifers than others, any individual well with oxic conditions and large 

nitrogen sources is likely to have high nitrate concentrations, regardless of the aquifer.

0 2,000

0

2

4

6

8

10

12

Moderate nitrogen source, 

most wells have oxic water

Moderate nitrogen source,

most wells have anoxic water

Median nitrate concentration, in 

milligrams per liter as N

Median nitrogen input from all sources, in pounds per square mile

10,000

Nitrate concentration relative to nitrogen input

EXPLANATION

Land use—Shape. Each symbol Aquifer—Color

 represents a group of wells 

 in a single area with the 

 designated land use

Early Mesozoic basin

Piedmont and Blue Ridge 

 crystalline rock

Valley and Ridge siliciclastic rock 

Valley and Ridge carbonate rock 

Piedmont and Blue Ridge 

Agricultural area, groundwater carbonate rock 

 used for drinking water

Urban area, groundwater 

 near water table

Mixed land use, groundwater 

 used for drinking water

4,000 6,000 8,000 12,000 14,000 16,000 18,000

Figure 7–3.

60 Water Quality in the Principal Aquifers of the Piedmont, Blue Ridge, and Valley and Ridge Regions, Eastern United States, 1993–2009

Nitrate is most likely to 

degrade in the siliciclasticrock aquifers, because of 

relatively old groundwater 

ages and anoxic conditions.
Groundwater age is related to nitrate concentrations because older water was recharged 

when nitrogen application rates were lower than at present and thus should contain lower 

concentrations of nitrate than water that recharged more recently. Also, reducing conditions that 

favor denitrification may take several years to develop. Because denitrification can substantially 

affect concentrations of nitrate, older water may have lower concentrations of nitrate than 

younger water, even if the initial nitrate concentrations were the same.

One of the reasons that the carbonate-rock aquifers are susceptible to nitrate contamination 

is that they have high-permeability karst features, such as sinkholes and conduits, which allow 

contaminants from the surface to easily enter the aquifer and to rapidly move through them 

(see sidebar, Karst features make carbonate-rock aquifers highly susceptible to contamination, 

p. 29). As a consequence, much of the water that reaches a well is very young. In addition, the

presence of sinkholes favors oxic conditions,(41) making denitrification unlikely. The combination of direct infiltration of contaminants into the aquifer, rapid movement through the aquifer, 

and oxic conditions within the aquifer, coupled with large nitrogen inputs, explains the high 

nitrate concentrations in samples from the carbonate-rock aquifers.

Chapter 7

Weathered rocks that make up the carbonate-rock aquifer are exposed at the surface and provide direct pathways for water and 

contaminants to enter the aquifer without the filtering effects of soils. Voids, or enlarged holes in carbonate rocks, such as the ones 

shown here, are common in the aquifer. Water moves rapidly through these voids and remains oxygenated. Photograph by William Kochanov, Pennsylvania Geological Survey

Photograph by Eliza Gross, USGS

Chapter 7: Contaminants Associated With Human Activities 61

Karst features in the 

carbonate aquifers 

provide a pathway for 

contaminants to enter the 

aquifer and contribute 

to oxic conditions that 

prevent nitrate from being 

removed by denitrification 

in the aquifer.

Photograph by Eliza Gross, USGS
Bedrock type and agricultural land use are important, but ultimately redox conditions 

control nitrate concentrations

The interaction of bedrock, land use, and redox conditions 

control concentrations of nitrate in groundwater. Nitrate 

concentrations were higher in samples from the carbonaterock aquifers than in samples from the siliciclastic-rock or 

crystalline-rock aquifers, and the areas with more agricultural 

land had higher nitrate concentrations than areas with less 

agricultural land. Within every bedrock-type and land-use 

category, the subsets of samples with oxic conditions had higher 

nitrate concentrations than the corresponding samples with 

anoxic or mixed redox conditions, and, within each bedrock 

type, the median nitrate concentration for the low agricultural 

Custom Sidebar chapter 7_fig 01

Siliciclastic Crystalline Carbonate

0.05

More

than

half

Less

than

half

Anoxic

or

mixed

Oxic

1.0 4.0 0.4 0.8 0.05 0.06 3.2 0.2 1.3 0.4 7.2

Less than 0.5 mg/L and less than 25 percent

Greater than or equal to 0.5 mg/L and less than 25 percent

Greater than or equal to 0.5 mg/L and greater than or equal to 25 percent

Nitrate concentrations and exceedances 

 of the Maximum Contaminant Level

Median nitrate concentration and its percentage of exceedance

EXPLANATION

Bedrock type

Agricultural intensity around well

(percentage of area)

Redox conditions in well

Median nitrate concentration in final category, in milligrams per liter

land-use/oxic category was higher than for the high agricultural 

land-use/anoxic category. In the first category, inputs of nitrogen 

are low, but oxic conditions allow nitrate to persist, whereas in 

the second category, higher inputs of nitrogen are present, but 

anoxic or mixed redox conditions cause nitrate to be transformed 

into harmless nitrogen gas. No groups with anoxic or mixed 

redox conditions had median concentrations of nitrate that were 

greater than 0.5 mg/L, regardless of bedrock type or land use. 

These results illustrate that anoxic conditions result in low nitrate 

concentrations even where aquifers are highly susceptible and 

inputs of nitrogen are large.

62 Water Quality in the Principal Aquifers of the Piedmont, Blue Ridge, and Valley and Ridge Regions, Eastern United States, 1993–2009
In the Valley and Ridge region, the topographic position on the landscape affects 

nitrate concentrations

Nitrate concentrations are related to the topographic 

position of the well on the landscape in the Valley and Ridge 

region.(22) In the area close to the ridge, groundwater is oxic, 

making denitrification unlikely, but nitrate concentrations are low 

because few nitrogen sources are associated with the forested 

land use overlying that part of the aquifer. Midway between the 

ridge and stream, the nitrate concentrations are higher because 

the water is oxic, and agricultural activity provides nitrogen 

sources. Concentrations in groundwater close to the stream are 

lower, despite the high percentage of agricultural land overlying 

this area. Several reasons for this finding are possible. Shallow 

wells in this area may draw in water from the oldest end of a 

shallow flow path or a deep flow path converging toward the 

stream with even older water. Because the water at this point 

on the flow path is oldest, denitrification might have removed 

more of the original nitrate. Older water is also most likely to 

have recharged when nitrogen inputs at the land surface were 

lower (fig. 4–5). Understanding this potential spatial distribution 

of nitrate concentrations across a valley could be useful for 

choosing a source of water supply in these areas. This relation 

between nitrate concentration and topography applies to both 

the carbonate-rock and siliciclastic-rock aquifers of the Valley 

and Ridge region, although nitrate concentrations are higher in 

the carbonate-rock aquifers. 

Stream

Stream

Well

Well

Aquifer 

near stream

Aquifer

near ridge

Aquifer

between ridge 

and stream

Aquifer 

near stream

Aquifer

near ridge

Aquifer

between ridge 

and stream

NOT TO SCALE

Agriculture Agriculture

EXPLANATION

Water table

Fracture

Topography section 

boundary

Flow path

 Shallow flow path

 Deep flow path

Less than 1

1 to 4

Greater than 4

Nitrate concentration,

 in milligrams per liter

Soil Soil

Siliciclastic-rock aquifers

Siliciclasticrock aquifers

Carbonate-rock aquifers

circ1354_ch7Redox_nitrate_sidebar_figure1.ai

Chapter 7

Chapter 7: Contaminants Associated With Human Activities 63
Concentrated animal-feeding operations produce large amounts of manure, which is stored in 

lagoons, as shown here, until it is used to fertilize crops. Areas with large sources of nitrogen from 

animal manure had the highest median nitrate concentrations.

What Do High Concentrations of Nitrate Mean for Drinking-Water 

Availability?

Homeowners with high concentrations of nitrate in their domestic wells have three alternatives: potential human-health problems, treatment, or obtaining alternative sources of domestic 

supply (see sidebar, Cost of water treatment, p. 44). Which alternative is chosen depends on 

awareness, financial resources, and personal preferences of the homeowner. Domestic-well 

owners who are unaware of the issue or unable to afford treatment systems could be consuming 

contaminated water. The USGS did not assess public water systems in those areas of the 

carbonate-rock aquifers that had the highest levels of nitrate, but numerous public-supply 

systems in this aquifer have had to make costly upgrades for nitrate treatment. In Pennsylvania, 

fewer than 2 percent of approximately 18,000 public drinking-water systems that use groundwater as their source need a permanent nitrate-removal system, but most of those that do are 

concentrated in five counties that are underlain by carbonate-rock aquifers. The number of 

systems in those five counties using permanent nitrate treatment increased 13 percent between 

2009 and 2012.

If nitrogen inputs increase substantially in areas overlying the carbonate-rock or crystalline-rock aquifers, concentrations of nitrate in groundwater could increase at some time in the 

future. For example, if the number of concentrated animal-feeding operations or the number 

of acres in corn production increases in areas overlying these susceptible aquifers, increases in 

nitrate concentrations in groundwater should be expected.

Imagery from Pictometry

TM International Corporation

64 Water Quality in the Principal Aquifers of the Piedmont, Blue Ridge, and Valley and Ridge Regions, Eastern United States, 1993–2009
Activities that affect nitrogen inputs are increasing in some states within the study area. Corn 

acreage began to trend downward in the late 1990s, but then increased in Georgia, North Carolina, 

Pennsylvania, and Virginia from 2002 to 2007, possibly from increased corn acreage for ethanol 

production. North Carolina had a large and increasing inventory of hogs during this time period. North 

Carolina is the state with the largest number of hogs in the study area and in 2007 ranked second in 

the Nation in hog production.(72–74) During 1987–2007, the number of hogs quadrupled, but the number 

of hog farms decreased by half because of the concentration of more animals onto a small number 

of very large operations. Georgia similarly had a continuing increase in the number of chickens and 

in 2007 had the largest number of broilers in the Nation. The general implication is that agricultural 

activity related to high concentrations of nitrate is increasing. As of 2012, only a single well network 

in these five Principal Aquifers has been sampled for trends in nitrate concentrations, and no 

statistically significant changes in nitrate concentrations were found.(62) The samples were from 

Pennsylvania and Virginia, however, where the rates of chicken and hog production are relatively 

steady. No samples for the determination of nitrate trends in well water were collected in networks in 

North Carolina or Georgia, where potential sources are increasing.

100

200

300

0

Georgia North Carolina

State

Pennsylvania

1987

1992

1997

2002

2007

0

0.4

0.8

2.0

1.6

Corn, in millions of 

acres harvested

2

0

4

6

8

10

12

Corn

Hogs

Chickens

2.4

2.8

1.2

Virginia

Chicken inventory, 

in millions of 

chickens

Hog inventory, 

in millions of hogs

Nitrogen sources are changing over time

Chapter 7

Chapter 7: Contaminants Associated With Human Activities 65
Figure 7–4. Total coliform bacteria were detected in nearly 

half of the wells in the sampled aquifers, indicating a potential 

pathway exists for microbial pathogens to contaminate 

water supplies. Escherichia coli (E. coli), an indicator of fecal 

contamination, was detected in nearly half of the wells sampled 

in the Valley and Ridge carbonate-rock aquifers. Most of the wells 

sampled were domestic-supply wells; domestic-well owners are 

responsible for monitoring and treating their household water 

supply. Springs were not included in the aquifer analysis.

Total coliform and E. coli are 

found in the feces of warmblooded animals; total coliform 

are also present in soils. Total 

coliform bacteria are typically 

not harmful, but they are used 

as a screening tool to signal 

the possible presence of 

other microbial contaminants. 

E. coli does not always cause 

illness but is an indicator of 

direct fecal contamination. 

Some strains of E. coli can 

cause gastrointestinal illness 

and even more serious 

health effects.

Bacteria Were Detected in Many Wells

In the Piedmont, Blue Ridge, and Valley and Ridge regions, total coliform bacteria were 

detected in all samples from springs and nearly 60 percent of samples from wells. E. coli, 

an indicator of fecal contamination, was detected in 91 percent of samples from springs and 

17 percent of samples from wells. Although detections were frequent in all aquifers and 

settings, the carbonate-rock aquifers had a higher frequency of detection for E. coli and total 

coliform, as did areas overlain by agricultural land use.

Total coliform bacteria were detected in more than half of the wells sampled, regardless of 

aquifer, indicating that wells in fractured bedrock also are vulnerable to microbial contamination. Total coliform bacteria and E. coli were detected most frequently in wells in the Valley 

and Ridge carbonate-rock aquifers (fig. 7–4). Sinkholes, fractures, and rapid flow of water are 

typical of carbonate aquifers and make these aquifers vulnerable to bacterial contamination.

0

50

100

Valley and Ridge

siliciclasticrock aquifers

Piedmont and 

Blue Ridge

crystallinerock aquifers

Piedmont and 

Blue Ridge

carbonaterock aquifers

Valley and Ridge

carbonaterock aquifers

No data

Detection frequency, 

in percent

Total coliform

EXPLANATION

E. coli

Total coliform and E. coli detection 

frequency by Principal Aquifer

Figure 7–4.

66 Water Quality in the Principal Aquifers of the Piedmont, Blue Ridge, and Valley and Ridge Regions, Eastern United States, 1993–2009

One in four of the drinkingwater sources (wells and 

springs) analyzed for 

bacteria showed evidence 

of fecal contamination, 

indicating a potential 

human-health concern.
Figure 7–5. Total coliform and Escherichia coli were detected 

most commonly in samples from wells surrounded by agricultural 

land, likely because of manure application in these areas. The 

relatively high detection frequency of total coliform in all of the 

land-use areas likely is related to its presence in soils and other 

nonfecal environments.

Animal manure applied as a fertilizer is related to the highest 

nitrate concentrations and also introduces total coliform bacteria 

and Escherichia coli to the environment.

Animal Manure and Human Sewage Are Sources of Bacteria

Sources of bacteria vary by land use.(42) Both total coliform and E. coli were detected 

more frequently in wells in agricultural areas than in wells surrounded by other land-use types 

(fig. 7–5), likely because of the high density of livestock and manure application in agricultural 

areas. In some cases, bacteria were detected in a well in an undeveloped area far from intensive 

livestock operations—in these cases, an onsite septic system or wildlife could have been the 

source of the bacteria.

Chapter 7

0

20

40

60

80

100

Frequency of detection of bacteria, in percent

EXPLANATION

Figure 7–5.

Total coliform and E. coli detection frequency by 

aquifer and land use

Total 

coliform

Carbonate-rock aquifers

Valley and Valley and Ridge 

Ridge 

Piedmont and 

Blue Ridge

Piedmont

and 

Blue Ridge

Siliciclasticrock

aquifers

Crystallinerock aquifers

Land use

Agricultural area, groundwater used for drinking water

Mixed land use, groundwater used for drinking water

Escherichia

coli

No data 

Photograph by Tim McCabe, U.S. Department of Agriculture

Chapter 7: Contaminants Associated With Human Activities 67
Figure 7–6. Total coliform bacteria were 

detected in all, and Escherichia coli (E. coli) 

was detected in nearly all, of the 23 springs 

sampled, which are used for drinking-water 

supply. Drinking water that contains E. coli, 

a common indicator of fecal contamination, 

could be a risk to human health.

A loose-fitting or poorly 

maintained well cap, such 

as A, the cracked cap in this 

picture, may not prevent 

bacteria from entering a well. 

B, A sanitary seal has a rubber 

gasket that tightly seals the 

casing when properly installed. 

This seal prevents dirt and 

insects from entering a well 

through the top of the casing 

and has been shown to reduce 

the frequency of detections of 

bacteria in wells.

Karst Features and Well-Construction Practices Affect the Transport and 

Survival of Bacteria

The presence of karst features helps explain why bacteria were detected more 

frequently in wells in carbonate-rock aquifers than in wells in other bedrock types (fig. 7–4). 

Furthermore, total coliform and E. coli were detected much more frequently in samples from 

springs—a typical karst feature—than in samples from wells (fig. 7–6). Springs typically flow 

out of large fractures that allow rapid transport of bacteria without any filtration.

Total coliform bacteria were detected in all, and E. coli was detected in nearly all, of the 

23 springs sampled, which are used for drinking-water supply. Drinking water that contains 

E. coli, a common indicator of fecal contamination, could be a risk to human health. Springs 

can have short, shallow flow paths and are not protected from bacterial sources at the land 

surface. Although wells typically have a casing that prevents surface runoff from entering 

the well, both wells and springs in the carbonate-rock aquifers are highly vulnerable to fecal 

contamination.

Well-construction characteristics are important factors that affect concentrations of 

bacteria, particularly E. coli.

(43) A loose-fitting well cap allows dirt and insects to get underneath and fall into the well. Sanitary well caps minimize the detection frequency of bacterial 

contamination.(43) Lack of a cement-grout seal can allow water and bacteria to travel down the 

annular space outside of the casing and into the well (see sidebar, Anatomy of a well, p. 11). 

Wells with cement-grout seals had a lower detection frequency of bacteria than did wells in 

which soil was used to fill the annular space.(44)

Implications of Fecal Contamination of Water Supplies

Human consumption of bacteria-laden water from domestic-supply wells probably is 

common and is likely to cause a number of unreported illnesses.(45) Education focused on 

homeowner awareness of the benefits of maintaining well integrity, regular water testing, 

and treating water from the domestic-supply well could reduce exposure to bacteria from 

drinking water. In some cases, even those who are aware of bacterial contamination of their 

domestic-water supply choose to drink the water because of the high cost of permanent 

point-of-use treatment or purchasing bottled water. Pathogenic viruses that cause illness—not 

just indicator organisms—are present in some samples of water drawn from wells tapping 

the Piedmont and Blue Ridge crystalline-rock aquifers, the Valley and Ridge carbonate-rock 

aquifers, and the Valley and Ridge siliciclastic-rock aquifers.(46)

Photographs by Bruce Lindsey, USGS

Wells Springs

Total coliform

EXPLANATION

E. coli

0

50

100

Detection frequency, in percent

Total coliform and E. coli detection 

frequency from wells and springs 

in carbonate-rock aquifers

A

B

68 Water Quality in the Principal Aquifers of the Piedmont, Blue Ridge, and Valley and Ridge Regions, Eastern United States, 1993–2009
Pesticides include herbicides, 

insecticides, and fumigants. 

Pesticides analyzed for this study 

include compounds that can 

cause cancer(47) or other adverse 

health effects.(24) 

Pesticides Were Detected Frequently but in Most Cases at Low 

Concentrations

At least one pesticide was detected in 55 percent of samples, and multiple pesticide 

compounds were commonly detected in a single sample. Median atrazine concentrations in 

domestic wells in the Piedmont and Blue Ridge carbonate-rock aquifers and the Valley and 

Ridge carbonate-rock aquifers were higher than those measured in domestic wells nationally. 

However, only 2 of the 47 pesticides analyzed—dieldrin and alpha-HCH—were measured at 

concentrations that exceeded their human-health benchmarks—and these exceedances were 

found in samples from fewer than 2 percent of sites used as sources of drinking water.

Thirty-five of the 47 pesticide compounds measured were detected in at least one sample, 

and at least one pesticide was detected in 55 percent of samples. The most frequently detected 

pesticides were atrazine, deethylatrazine, metolachlor, simazine, prometon, tebuthiuron, 

and dieldrin. Of the 47 pesticides analyzed, only dieldrin and alpha-HCH were measured at 

concentrations that exceeded their human-health benchmarks in samples of drinking water. 

The concentration of dieldrin exceeded its human-health benchmark in water samples from 

1.3 percent of sites, and alpha-HCH exceeded its human-health benchmark in water samples 

from 0.2 percent of sites used as drinking water. Pesticides at concentrations greater than 

one-tenth of a human-health benchmark are approaching levels of potential concern for 

human health.(48) Atrazine exceeded 0.3 μg/L, one-tenth of its MCL of 3.0 μg/L, in 7 percent 

of samples collected from sites used as sources of drinking water. The pesticides cyanazine, 

simazine, and alachlor also were detected at concentrations greater than one-tenth of their 

respective human-health benchmarks in some wells (see appendix 3, table A3–2).

Photograph by Connie Loper, USGS

Chapter 7

Chapter 7: Contaminants Associated With Human Activities 69

Only two pesticide 

compounds were measured 

at concentrations that 

exceeded their humanhealth benchmarks and 

that occurred in fewer 

than 2 percent of the wells 

sampled.
Figure 7–7. Pesticides were 

detected most frequently in 

samples from carbonate- and 

crystalline-rock aquifers in 

areas overlain by agricultural 

or urban land use. Detection 

frequencies in these areas 

are among the highest in 

the Nation.

Assessment levels

Pesticide occurrence is evaluated at two assessment levels: detections at any concentration 

without regard to differences in reporting levels among pesticides, and detections at a concentration greater than 0.1 μg/L. The second option is called a common assessment level, and it facilitates 

comparison of detection frequencies among compounds with different reporting levels.

Bedrock Type and Land Use Affect the Occurrence and Distribution of 

Pesticides

The frequency of detection of at least one pesticide was greater in samples from the 

agricultural and urban areas in carbonate- and crystalline-rock aquifers than in samples from 

the other combinations of Principal Aquifer and land use (fig. 7–7). Detection frequencies in 

those areas were among the highest in the Nation. Almost all of the samples with concentrations 

exceeding one-tenth of any pesticide benchmark were collected from wells in carbonate-rock 

aquifers overlain by agricultural land use.

0

10

20

30

40

50

60

70

80

90

100

*

No detections greater than 0.1 microgram per liter 

 in this setting

Frequency of detection of at least

one pesticide, in percent

EXPLANATION

Pesticide-detection frequency

Any Greater

than 0.1

Concentration, in

micrograms per liter

Carbonate-rock 

aquifers

Valley and

Ridge 

Valley and

Ridge 

Piedmont and 

Blue Ridge

Piedmont

and 

Blue Ridge

Early

Mesozoic

basin

Siliciclastic-rock

aquifers

Crystallinerock aquifers

Figure 7–7.

Land use

Agricultural area, groundwater used for drinking water

Urban area, groundwater near water table

Mixed land use, groundwater used for drinking water

70 Water Quality in the Principal Aquifers of the Piedmont, Blue Ridge, and Valley and Ridge Regions, Eastern United States, 1993–2009

Pesticides were detected 

in more than 90 percent of 

samples of groundwater 

from carbonate-rock 

aquifers in areas overlain 

by urban or agricultural 

land use.
Figure 7–8. Mixtures of pesticides (two or more pesticides in the same sample) were more common in samples from the carbonaterock aquifers (C) than in samples from crystalline- and siliciclastic-rock aquifers (A, B). With the exception of mixtures in the crystallinerock aquifers in areas of agricultural land use, the pattern of pesticide mixtures in crystalline- and siliciclastic-rock aquifers (A, B) was 

similar to that reflected in data for the entire Nation (D).

(10)

Mixtures of pesticides (two or more pesticides detected in the same sample) also were 

detected more frequently in the carbonate-rock aquifers than in crystalline- and siliciclasticrock aquifers (fig. 7–8). In fact, mixtures of pesticides in the Piedmont and Blue Ridge and 

Valley and Ridge carbonate-rock aquifers were much more prevalent than in aquifers nationwide with five or more pesticides detected in more than 40 percent of wells in agricultural areas 

and more than 80 percent of the wells in urban areas. In contrast, the frequency of detection of 

mixtures in crystalline- and siliciclastic-rock aquifers was nearly identical to that seen nationally, with the exception of the crystalline-rock aquifers in areas of agricultural land use. In all 

of the bedrock types, mixtures were usually detected more frequently in water from sites in 

agricultural and urban land-use settings than in water from sites used as a drinking-water supply 

in areas of mixed land use. Human-health benchmarks are not available for most mixtures 

of pesticides, but the detection of multiple pesticide compounds in drinking-water wells is a 

potential health concern, even at concentrations below human-health benchmarks. The reason 

is that the health effects of mixed compounds may be different from the health effects of each 

compound in isolation, and these interactions are not well understood.(2)

Chapter 7

Figure 7–8.

Percentage of samples with equal

or greater numbers of detectable

pesticide compounds 

C. Carbonate-rock aquifers

This study

Number of pesticide compounds in sample

0

20

40

60

80

100

1 2 3 4 5 6 7 8 9 10

D. National dataset

Land use

EXPLANATION

Agricultural

Urban

Mixed land use

A. Crystalline-rock aquifers

1 2 3 4 5 6 7 8 9 10

B. Siliciclastic-rock aquifers

1 2 3 4 5 6 7 8 9 10 1 2 3 4 5 6 7 8 9 10

Chapter 7: Contaminants Associated With Human Activities 71

Detections of multiple 

pesticides in a well were 

much more common in the 

carbonate-rock aquifers 

than in the crystalline- and 

siliciclastic-rock aquifers or 

in other aquifers nationally.
Figure 7–9. Pesticides were detected most frequently in samples of groundwater in the carbonate-rock aquifers in areas overlain 

by agricultural and urban land use. Detection frequencies for metolachlor, simazine, and prometon in urban networks in the 

carbonate-rock aquifers equaled or exceeded the detection frequency in agricultural networks. The agricultural-use herbicides 

deethylatrazine, atrazine, and metolachlor also had high detection frequencies in wells from the crystalline-rock aquifers overlain 

by agricultural land use. These pesticides were not detected in the siliciclastic-rock aquifers overlain by agricultural land use.

EXPLANATION

Carbonate-rock aquifers

Percentage of samples in which pesticide was detected

Crystalline- and siliciclastic-rock aquifers

Dieldrin

Tebuthiuron

Prometon

Simazine

Metolachlor

Atrazine

Deethylatrazine

Dieldrin

Tebuthiuron

Simazine

Prometon

Metolachlor

Atrazine

Deethylatrazine

0 20 40 60 80 100

Dieldrin

Tebuthiuron

Prometon

Simazine

Metolachlor

Atrazine

Deethylatrazine

0 20 40 60 80 100

Agricultural areas, groundwater used for drinking water 

Urban areas, groundwater near water table 

Mixed land use, groundwater used for drinking water 

Land use

Any Greater

 than 0.2

Concentration, in

micrograms per liter

Agricultural area

Urban area

Mixed land use

circ1354 ch07 fig 7-9

Sources of Pesticides Are Related to Land Use and Affect Which 

Pesticides Are Detected

Most pesticides are manmade chemicals, and their presence in the environment is the 

direct result of human activities in the agricultural, industrial, commercial, and home and 

garden environments.(10) Undeveloped areas such as forests include little or no direct pesticide 

sources, and pesticides were rarely detected in samples collected in those areas. Atrazine and 

metolachlor are agricultural pesticides with high application rates in the Piedmont, Blue Ridge, 

and Valley and Ridge regions (appendix 5). Detection frequencies of these two herbicides and 

deethylatrazine (a degradation product of atrazine) were higher in areas with more agricultural 

land use (fig. 7–9). Higher concentrations of dieldrin, a pesticide that was used in urban areas 

and is now banned, were detected in areas with a larger percentage of urban land surrounding 

a well.(8, 22) Simazine, prometon, and tebuthiuron are general-use herbicides that have both 

agricultural and urban uses—these pesticides were detected most frequently in samples from 

urban areas in the carbonate-rock aquifers. Pesticides with highest use—atrazine and metolachlor—were among the most frequently detected pesticides in domestic and public-supply 

wells (fig. 7–10).

72 Water Quality in the Principal Aquifers of the Piedmont, Blue Ridge, and Valley and Ridge Regions, Eastern United States, 1993–2009
Figure 7–10. All seven of the most frequently detected pesticides were detected most often in samples of groundwater 

from domestic wells in the carbonate-rock aquifers. Many of these domestic wells are also in agricultural areas. 

Comparisons show that detection frequencies in public-supply wells in crystalline-rock aquifers are greater than in 

domestic wells in that same aquifer; however, other comparisons to public-supply wells are not possible because of the 

lack of samples.

Percentage of samples from drinking-water wells where pesticide was detected

Carbonate-rock aquifers Crystalline- and siliciclastic-rock aquifers

Public supply

Domestic

Well type

EXPLANATION

Any Greater 

than 0.2

Concentration, in 

micrograms per liter

Insufficient data to 

evaluate public-supply wells

Dieldrin

Tebuthiuron

Prometon

Simazine

Metolachlor

Atrazine

Deethylatrazine

0 20 40 60 80 100

Dieldrin

Tebuthiuron

Prometon

Simazine

Metolachlor

Atrazine

Deethylatrazine

0 20 40 60 80 100

Domestic wells

Public-supply wells

circ1354 ch07-fig7-10

Photograph by Kevin Breen, USGS

Chapter 7

Chapter 7: Contaminants Associated With Human Activities 73
Chemical Properties of Pesticides Affect Their Transport and Persistence

The combination of use and physical and chemical properties influences the likelihood 

of the detection of a pesticide in groundwater. Two aspects of pesticide chemistry of particular 

importance are resistance to degradation—often call “persistence”—and the pesticide’s affinity 

for attaching, or adsorbing, to the surfaces of soil particles. Persistent pesticides remain in the 

environment in their original form for long periods and hence have the potential to be transported once they are released in the environment. A persistent pesticide that does not adsorb to 

soil has a high potential to move down to and with the groundwater and likely will be detected 

more frequently than a nonpersistent pesticide that adsorbs to soil.

Metolachor and atrazine have high and nearly identical application rates in this area 

(appendix 5), and neither is likely to adsorb to soil particles, but metolachor was detected about 

half as often as atrazine, likely because metolachor breaks down more rapidly. Chlorpyrifos 

has a moderately high application rate (appendix 5), but is almost never detected (appendix 3, 

table A3–2) because it adsorbs to particles and breaks down quickly. Dieldrin was banned 

several years before these samples were collected and moves very slowly in groundwater 

because it readily adsorbs to particle surfaces but is still detected because it breaks down 

very slowly.

Atrazine Concentration is Related to Topographic Position

The relations among the combined effects of sources, transport, fate, and frequency 

of detection of pesticides in the environment are illustrated by atrazine concentrations in 

the Valley and Ridge carbonate-rock aquifers. In the topographic position nearest the ridge, 

atrazine concentrations in groundwater increase from the ridge toward the valley. The ridge 

is mostly forested, and the increase in concentrations likely results from the increasing 

intensity of agricultural activity and pesticide use toward the valley (fig. 7–11). In the middle 

topographic section, concentrations do not change substantially. In the area near the stream, 

however, concentrations of atrazine decrease, despite high percentages of urban and agricultural 

land use in the valley center. This decrease could be the result of dilution, as deeper flow paths 

with old, low-atrazine water converge with shallower flow paths toward the discharge point. A 

sample with a larger proportion of old water is likely to have lower concentrations of pesticides 

because the older water was recharged during a time when usage was lower, and any atrazine 

that might have been present may have been degraded.

74 Water Quality in the Principal Aquifers of the Piedmont, Blue Ridge, and Valley and Ridge Regions, Eastern United States, 1993–2009
Figure 7–11. Concentrations of atrazine in groundwater are 

related to the topographic position on the landscape in the 

Valley and Ridge carbonate-rock aquifers.(22) Across the three 

topographic positions within the shallow aquifer, atrazine 

concentrations tended to increase as groundwater moved away 

from the ridge, remained stable between the ridge and the stream, 

and decreased as groundwater approached the stream. This 

pattern is attributed to increases in sources of atrazine along the 

valley floor and degradation of atrazine along the flow path and 

(or) dilution with older water less affected by atrazine use.

Figure 7–11.

Agriculture

Atrazine concentration 

increasing in

shallow aquifer

Atrazine concentration 

not changing in

shallow aquifer

Atrazine concentration

decreasing in

shallow aquifer

because of 

dilution with cleaner,

deeper water

Stream

Well

Aquifer 

near stream

Aquifer

near ridge

Aquifer

between ridge 

and stream

Zone of elevated

 atrazine concentration

Soil

Siliciclastic-rock aquifer

Carbonate-rock aquifer

EXPLANATION

Fracture

Shallow, elevated atrazine 

 concentration 

NOT TO SCALE

Topographic section 

 boundary 

Water table

Shallow, outside the zone of elevated 

 atrazine concentration

Deep, outside the zone of elevated 

 atrazine concentration

Flow path

Pesticides Detected in Drinking-Water Wells and Springs Rarely Exceeded 

Human-Health Benchmarks

The likelihood of a pesticide concentration exceeding a human-health benchmark in a 

public-supply well or domestic well is low on the basis of the results of this assessment. Only 

about 2 percent of the wells sampled had a pesticide concentration that exceeded its benchmark. 

Furthermore, the pesticide that most frequently exceeded its benchmark was dieldrin, which is 

no longer used. The prevalence of mixtures of pesticides in groundwater could be important if 

new information about the human-health effects of mixtures becomes available. However, the 

frequent detection of pesticides indicates that manmade chemicals can be transported to and 

through all five aquifers. The awareness that these aquifers are susceptible to contamination 

from manmade chemicals is important when future aquifer-protection efforts and the need for 

continued monitoring are considered as chemical-use patterns evolve.

Chapter 7

Chapter 7: Contaminants Associated With Human Activities 75

The likelihood that a 

concentration of a pesticide 

in groundwater used as a 

source of drinking water 

will exceed a human-health 

benchmark is very low.
Volatile Organic Compounds Were Frequently Detected in Samples of 

Groundwater From Urban Areas

VOCs were frequently detected in wells in the Principal Aquifers of the Piedmont, Blue 

Ridge, and Valley and Ridge regions, but concentrations generally were below human-health 

benchmarks. Almost 30 percent of the samples from wells had at least one VOC detected at a 

common assessment level of 0.2 μg/L or greater, with chloroform and methyl tert-butyl ether 

(MTBE) having the highest detection frequency. Detection frequency of VOCs was highest in 

urban areas. Only one well used as a source of drinking water had a concentration of a VOC 

(trichloroethene) that exceeded an MCL, and another well had a concentration that exceeded 

the human-health benchmark for acrylonitrile.

Of the 86 VOCs analyzed, only trichloroethene and acrylonitrile had concentrations that 

exceeded human-health benchmarks (see sidebar, Human-health benchmarks and other guidelines used in this assessment, p. 14)(48, 49) in samples collected from sources of drinking water. 

A single sample from a domestic-supply well in the early Mesozoic basin aquifers contained 

trichloroethene (TCE) at a concentration that exceeded its MCL, and a sample of raw water 

(before treatment) from a public-supply well in the Piedmont and Blue Ridge crystalline-rock 

aquifers contained acrylonitrile at a concentration that exceeded its human-health benchmark. 

Eight other VOCs—1,2-dichloroethane, 1,2-dichloropropane, benzene, carbon tetrachloride, 

methylene chloride, naphthalene, tetrachloroethene (PCE), and trichloromethane (chloroform)—were detected at concentrations that were greater than one-tenth of their respective 

human-health benchmarks in samples from sources of drinking water (appendix 3, table A3–3 

lists all VOCs analyzed and their detection frequencies).

Of the 86 VOCs analyzed, 54 were detected in at least one sample. MTBE was the 

most frequently detected VOC in drinking-water supply wells, followed by chloroform, 

1,1,1-trichloroethane, PCE, and TCE (appendix 3, table A3–3). The frequency of detection of 

chloroform at concentrations of greater than 0.2 μg/L in domestic-supply wells in all of the 

aquifers of the Piedmont, Blue Ridge, and Valley and Ridge regions (7 percent; fig. 7–12) was 

only slightly higher than the national rate of (5 percent).(50) Public-supply wells in the crystalline-rock aquifers, however, had detection frequencies of chloroform (19 percent; fig. 7–12) 

that were almost twice as high as the national rate (11 percent).(50) High pumping rates in 

public-supply wells capture water recharged from a larger area than domestic-supply wells, 

increasing the probability of drawing water from an area affected by a leak or spill of VOCs. 

The public-supply wells in the crystalline-rock aquifers are in densely populated areas, where 

there are many potential sources of VOCs. These factors can increase the detection frequency 

of VOCs in public-supply wells.(2) Unlike the other human-related contaminants—nitrate and 

pesticides—differences in detection frequencies of VOCs among domestic wells in carbonate 

and crystalline-rock and siliciclastic-rock aquifers at a common assessment level of 0.2 μg/L 

were negligible.

Volatile organic compounds 

are in gasoline, solvents, and 

other household, commercial, 

and industrial chemicals. VOCs 

analyzed for this study include 

compounds that can cause 

cancer(47) or other adverse health 

effects.(24)

Although concentrations 

of VOCs were well 

below human-health 

benchmarks, detections 

of VOCs in samples from 

about one-third of the 

wells indicated a general 

susceptibility to contamination by VOCs.

Volatile

organic

compounds

76 Water Quality in the Principal Aquifers of the Piedmont, Blue Ridge, and Valley and Ridge Regions, Eastern United States, 1993–2009
Figure 7–12. The highest detection frequencies of the most frequently occurring VOCs were in public-supply wells in the crystalline 

rock aquifers, but public-supply wells were not evaluated in the other aquifers. These wells tend to be in more urban areas, and high 

pumping rates in these wells draw in water from a large area. Comparisons show very low detection frequencies in the carbonate 

bedrock and the other bedrock types and similar patterns of detection frequencies. Domestic wells in all of the aquifers tended to be 

near forested or agricultural areas, where potential VOC sources are few.

Percentage of samples from drinking-water wells where VOC was detected

0 20 40 60 80 100

Trichloroethene

Chloroform

MTBE

Tetrachloroethene

1, 1, 1-Trichloroethane

0 20 40 60 80

Chloroform

100

Trichloroethene

1, 1, 1-Trichloroethane

Tetrachloroethene

MTBE

Insufficient data to 

evaluate public-supply wells

Carbonate-rock aquifers Crystalline- and siliciclastic-rock aquifers

Domestic wells

Public-supply wells

Public supply

Domestic

Well type

EXPLANATION

Any Greater

than 0.2

Concentration, in

 micrograms per liter

cir1354 ch07_fig 7-12

Chapter 7

Chapter 7: Contaminants Associated With Human Activities 77
Figure 7–13. VOCs were 

detected most commonly in 

wells in urban areas overlying 

the carbonate-rock aquifers. 

This detection frequency is 

consistent with the greater 

abundance of potential sources 

of VOCs in urban areas. The 

differences among bedrock 

types, however, were not as 

large as for other contaminants 

related to human activities, 

such as pesticides and nitrate.

Photograph copyright istockphoto.com

Because VOC detections 

are related to urban land 

use, areas that are densely 

populated but rely on 

groundwater for domestic 

supply are at the greatest 

risk of exposure to VOCs.

Volatile Organic Compounds Are Associated Primarily With Urban Land 

Use and Densely Populated Areas

Detection frequencies of VOCs were highly variable by land-use category, ranging 

from 75 percent in groundwater from urban areas of the carbonate-rock aquifers to as low 

as 4 percent in groundwater from agricultural areas of the Valley and Ridge carbonate-rock 

aquifers (fig. 7–13). The higher frequency of detection for VOCs in urban groundwater than in 

other land-use types is consistent with the greater abundance of potential sources of VOCs in 

urban areas. The majority of the detections in urban areas were in shallow monitoring wells not 

used as sources of drinking water. The pattern of detection frequencies for MTBE, chloroform, 

1,1,1-trichloroethane, tetrachloroethene, and trichloroethene with respect to land use (fig. 7–14) 

was similar to the overall detection pattern for VOCs (fig. 7–13), with all five being detected 

more frequently in wells in urban areas than in wells in areas with agricultural land use or 

drinking-water-supply wells in areas of mixed land use.

0

10

20

30

40

50

60

70

80

90

100

Frequency of detection of at least one

VOC compound, in percent

Carbonate-rock 

aquifers

Valley and

Ridge 

Valley and

Ridge 

Piedmont and 

Blue Ridge

Piedmont

and 

Blue Ridge

Early

Mesozoic

basin

Siliciclastic-rock

aquifers

Crystallinerock aquifers

EXPLANATION

VOC detection frequency

Greater 

than 0.2

Concentration, in

micrograms per liter

Figure 7–14

Land use

Agricultural area, groundwater used for drinking water

Urban area, groundwater near water table

Mixed land use, groundwater

78 Water Quality in the Principal Aquifers of the Piedmont, Blue Ridge, and Valley and Ridge Regions, Eastern United States, 1993–2009
Figure 7–14. Differences among detection frequencies for the five most frequently detected VOCs follow the same pattern as the 

overall detection frequencies. Detections were rare in agricultural areas and most frequent in urban areas, and detection frequencies 

were highest in the urban areas underlain by carbonate bedrock. In areas of mixed land use, detection frequencies did not vary 

greatly among the three aquifer bedrock types.

Carbonate-rock aquifers Crystalline- and siliciclastic-rock aquifers

1, 1, 1-Trichloroethane

MTBE

Chloroform

Trichloroethene

Chloroform

MTBE

Tetrachloroethene

1, 1, 1-Trichloroethane

0 20 40 60 80 100

Trichloroethene

0 20 40 60 80

Chloroform

MTBE

Tetrachloroethene

1, 1, 1-Trichloroethane

100

Trichloroethene

Tetrachloroethene

circ1354 ch07 fig 7-13

EXPLANATION

Percentage of samples in which constituent was detected

Agricultural areas, groundwater used for drinking water 

Urban areas, groundwater near water table

Mixed land use, groundwater used for drinking water 

Land use

Any Greater

than 0.2

Concentration, in

micrograms per liter

Agricultural area

Urban area

Mixed land use

Chapter 7

Chapter 7: Contaminants Associated With Human Activities 79
Figure 7–15. As population density(40) increases from rural to suburban to urban areas, the frequency of detection of at least one 

volatile organic compound increases from less than 10 percent to more than 75 percent. 

The frequency of detection of VOCs in groundwater is highest in the most densely 

populated portions of the study area (fig. 7–15). Wells in urban areas and the densely populated 

early Mesozoic basin area had higher detection frequencies of at least one VOC than did wells 

in more rural areas with lower population densities but highly susceptible carbonate lithologies. 

This result is consistent with the increase in potential VOC sources as population density 

increases. Sources of the VOCs analyzed include gasoline, disinfection byproducts (created 

from chlorination of water or wastewater), solvents, and organic synthesis compounds. These 

VOCs enter the environment from leaking storage tanks and accidental spills, which are more 

common in urbanized areas than in agricultural or rural areas. Products that go down household 

drains may also contain VOCs; therefore, septic systems and leaking sewer lines also could be 

a source of VOCs to groundwater. Densely populated areas often use public water supplies, 

which would be required to test and treat for VOCs if concentrations were above MCLs. 

Homeowners in areas that are densely populated but that continue to use domestic-supply wells 

would be most likely to encounter high concentrations of VOCs.

1 10 100 1,000 10,000

Population density, in people per square mile

Rural Suburban Urban

Detection frequency of at least one VOC, in percent

EXPLANATION

Land use—Shape. Each symbol Aquifer—Color

 represents a group of wells 

 in a single area with the 

 designated land use

Early Mesozoic basin

Piedmont and Blue Ridge 

 crystalline rock

Valley and Ridge siliciclastic rock 

Valley and Ridge carbonate rock 

Piedmont and Blue Ridge 

 carbonate rock 

80

70

60

50

40

30

20

10

0

VOC detections by population density

Agricultural area, groundwater 

 used for drinking water

Urban area, groundwater 

 near water table

Mixed land use, groundwater 

 used for drinking water

Figure 7–15.

80 Water Quality in the Principal Aquifers of the Piedmont, Blue Ridge, and Valley and Ridge Regions, Eastern United States, 1993–2009
MTBE and chloroform accounted for nearly 40 percent of all of the detections of VOCs. 

Both MTBE and chloroform are highly soluble, degrade relatively slowly in both oxic and 

anoxic groundwater,(51) and are widely used. MTBE was used from the late 1980s until about 

2006 to increase the octane rating of gasoline, and its presence in groundwater likely is the 

result of leaking storage tanks or other inadvertent releases. Use of MTBE has recently been 

banned in many states as a result of its widespread presence in groundwater; however, its 

use was common during the sampling period for the studies summarized in this report.(50) 

Chloroform has some industrial uses, but it also is a byproduct that is formed when water is 

disinfected with chlorine, one of the most common procedures used at water-treatment plants. 

Chlorinated tap water used for irrigation or leaking sewer lines therefore could be a source of 

chloroform to groundwater. The characteristics of these chemicals coupled with their pervasive 

sources account for their high detection frequency in groundwater.

Chapter 7

Photograph copyright istockphoto.com

Chapter 7: Contaminants Associated With Human Activities 81

Chloroform and MTBE 

accounted for nearly 

40 percent of the detections 

of VOCs.
Properties of individual VOCs and geochemical conditions in the 

aquifers affect transport and persistence

Although VOCs were detected most commonly in urban areas with abundant potential 

sources (fig. 7–13), the properties of each compound also affect whether they are transported with 

groundwater, in turn affecting the patterns of VOCs that are detected. Properties that affect transport 

of VOCs in groundwater include solubility, density, and degradation characteristics. The gasoline 

additive MTBE (methyl tert-butyl ether) and benzene, toluene, ethylbenzene, and xylene (known as 

BTEX compounds), also found in gasoline, provide a good example of how differences in chemical 

properties affect VOC occurrence. The sources of these compounds are similar, since all would be 

present in a leak or spill of gasoline, but the extent to which they dissolve in water and how rapidly 

they break down are very different. The gasoline additive MTBE dissolves readily(63) and resists 

degradation,(50) and it was detected much more frequently in groundwater than were the BTEX 

compounds, which are less soluble(64, 65) and degrade more readily.(50) Therefore both the amount of a 

VOC released to the environment and its chemical characteristics affect whether or not it is detected 

and at what concentration.

Degradation characteristics of individual VOCs affect their occurrence in groundwater, but 

degradation also depends on conditions in the aquifer. For example, chloroform is more persistent 

in oxic groundwater, and carbon disulfide is more persistent in anoxic groundwater—these characteristics are reflected in the frequencies at which they are detected. Thus, if sources are similar, 

detection frequencies of other VOCs in water samples will tend to be higher when redox processes in 

the aquifer favor the persistence of that compound and lower when redox processes favor degradation of that compound.(50)

0

1

1 10 100 1,000 10,000 100,000

2

3

4

5

6

7

8

9

MTBE

Benzene

Toluene

Ethylbenzene

Solubility, in milligrams per liter

Detection frequency, in percent

Readily degrade

Resists

degradation

o-Xylene m- and p-Xylene

Ch07 sidebar Figure 7–19.

VOC detections by solubility

0

10

20

30

40

50

Detection frequency, in percent

EXPLANATION

Oxic

Anoxic

Redox condition

Chloroform Carbon

disulfide 

VOC detections by 

redox condition

82 Water Quality in the Principal Aquifers of the Piedmont, Blue Ridge, and Valley and Ridge Regions, Eastern United States, 1993–2009
Nutrient-laden groundwater that discharges to 

streams can cause algal blooms that consume 

dissolved oxygen, resulting in fish kills.

Photograph by Eliza Gross, USGS

Streamflow in the Piedmont, Blue Ridge, and Valley and Ridge regions 

is sustained by groundwater discharge, but groundwater discharge can 

contribute nitrate and phosphorus to the streams in concentrations that 

affect their ecological health. About 50 percent of the nitrogen in streams in 

this area comes from groundwater. More than 20 percent of the phosphorus 

in streams overlying crystalline-rock and siliciclastic-rock aquifers is from 

groundwater and originates from geologic sources.

Between periods of rainfall, streamflow is sustained by discharge 

of groundwater from the aquifer. Contaminants in the aquifer 

discharge to the stream, providing a large proportion of the total 

contaminant load in the stream.

Chapter 8: Can Discharge of Nutrients From 

Groundwater Impair the Health of Streams 

and Estuaries?

Chapter 8

This chapter describes 

the transport of 

nitrogen and phosphorus from groundwater to surface-water 

bodies in the Piedmont, 

Blue Ridge, and Valley 

and Ridge regions.
Groundwater discharge sustains streamflow

In the water cycle, precipitation can run overland into 

streams, evaporate, transpire, or infiltrate into the aquifer. Water 

that infiltrates into the aquifer may be extracted by wells or 

eventually discharge to streams. This discharge of groundwater 

to streams sustains streamflow between periods of rainfall and 

is known as base flow. As groundwater travels from recharge to 

discharge, contaminants can move with the water and eventually 

discharge to the stream with the water at the end of the flow path 

(modified from Winter and others,fig. 8(66)).

Recharge 

to aquifer

Evaporation and 

transpiration

Direction of Overland streamflow runoff

Discharge from aquifer to stream

(groundwater discharge or base flow)

Sidebar8 ch08 Groundwater discharge 

Nitrogen and phosphorus are essential nutrients for plant growth; however, when these 

nutrients are overabundant in surface water, harmful algae can bloom, causing problems 

such as depletion of oxygen in the water and production of toxins, subsequently harming bay 

grasses, fish, crabs, and oysters. Groundwater discharge (see sidebar, Groundwater discharge 

sustains baseflow, this page) contributes nitrogen and phosphorus to streams and to estuaries 

downstream, such as the Chesapeake Bay(52) and the Albemarle Sound. The Chesapeake Bay is 

typical of estuaries in the Piedmont, Blue Ridge, and Valley and Ridge regions: groundwater 

discharge accounts for about half of the stream water flowing into the Bay. A substantial 

amount of the nitrogen and phosphorus reaching the Bay is delivered by groundwater discharge 

as well. The largest effort ever undertaken by the USEPA to reduce contamination in a watershed was recently established for the Chesapeake Bay.(53)

Groundwater discharge accounts for about 30 to 55 percent of the water 

flowing in streams—this groundwater contribution to streamflow is called base 

flow. About 40 to 70 percent of the nitrogen load in streams in the Piedmont, 

Blue Ridge, and Valley and Ridge regions is from base flow (fig. 8–1).(54)

Carbonate-rock aquifers deliver the highest percentage of flow and the highest 

loads of nitrate to streams, and the early Mesozoic basin aquifers deliver the 

lowest. The close connection, both chemically and physically, between the 

aquifer and the streams is reflected in the similar concentrations of nitrate in 

samples collected from wells and from stream base flow (fig. 8–2).(54) Three 

factors contribute to the high loads of nitrate contributed by the carbonaterock aquifers: the ratio of base flow to total streamflow is higher because 

overland runoff is less; inputs of nitrogen to the groundwater are higher; and 

the groundwater commonly is oxic, so denitrification is less likely in streams 

that receive discharge from these aquifers (fig. 8–3). In contrast, in the areas 

overlying the other aquifers, the proportion of overland runoff is larger, the 

percentage of agricultural land use is lower, and anoxic conditions leading to 

denitrification are more common than in the carbonate-rock aquifers. These 

factors contribute to low nitrogen loads in base flow.

Concentrations of nitrate in stream base flow generally mirror concentrations of nitrate in groundwater discharging to those streams. The majority 

of the streams contained nitrate at concentrations that exceeded the USEPA 

recommended ecological criterion.(4)

Figure 8–1. Nitrogen load in streams is related 

to the amount of streamflow that comes from 

groundwater discharge.

0

20

40

60

80

100

0 20 40 60 80 100

Median percentage of total streamflow 

from groundwater discharge

Median percentage of total nitrogen

load from groundwater discharge

EXPLANATION

Aquifers

Early Mesozoic basin

Piedmont and Blue Ridge 

 crystalline rock

Valley and Ridge siliciclastic rock

Valley and Ridge carbonate rock

Groundwater discharge and nitrogen

84 Water Quality in the Principal Aquifers of the Piedmont, Blue Ridge, and Valley and Ridge Regions, Eastern United States, 1993–2009

Carbonate-rock aquifers 

delivered the highest loads 

of nitrate to streams.

Groundwater discharge contributed 30 to 55 percent of 

flow and 40 to 70 percent of the nitrogen load to streams. 

Concentrations of nitrate exceeded the levels recommended 

by the U.S. Environmental Protection Agency for protection 

of aquatic life and stream health (recommended ecological 

criterion) in the majority of the streams sampled.
Valley 

and Ridge 

Valley 

and Ridge 

Piedmont and 

Blue Ridge

Siliciclastic-rock

aquifers

Carbonate-rock 

aquifers

Crystallinerock aquifers

Early Mesozoic

basin

Piedmont and

Blue Ridge

Piedmont Physiographic Province (0.69 milligram per liter)

Blue Ridge and Valley and Ridge Physiographic Provinces

 (0.31 milligram per liter)

See sidebar, Boxplots, p. 47

U.S. Environmental Protection Agency recommended 

ecological criteria for total nitrogen in flowing streams(4)

Groundwater (wells or springs)

Sample source

Streams under base-flow conditions

EXPLANATION

Nitrate in groundwater and streams

30

25

20

15

10

5

0

Nitrate concentration, in milligrams per liter as N

Figure 8–3. A, Large nitrogen sources provide nitrate to 

recharge to carbonate-rock aquifers, where oxic conditions 

allow the nitrate to move through the aquifer to the stream 

without denitrification. B, Nitrate enters the aquifers from 

sources above the crystalline-rock (and other) aquifers, 

C, but concentrations are likely to be lower by the time they 

reach the stream because of denitrification along the flow 

path or D, because of dilution from water recharging through 

undeveloped areas.

Carbonate-rock aquifer Crystalline-rock aquifer

Nitrate

A B

C

D

Decreasing along flow path

Increasing along flow path

Nitrate-rich recharge entering 

 aquifer from land surface

EXPLANATION

Nitrate-free recharge entering 

 aquifer from land surface

High Medium Low

Nitrate concentration

Concentration—Concentration is the mass of a constituent 

per volume of water, such as milligrams per liter. The concentration is important because it determines the potential ecological 

effects of the nutrients at the place and time the sample is 

collected. The USEPA-recommended criteria for nutrients in 

streams(4) are given as concentrations. These recommended 

criteria represent the maximum level of a contaminant necessary 

to protect designated uses of rivers and streams—such as 

protection of aquatic life and recreation.

Load—Load is the total mass of a contaminant transported 

in a stream or river, calculated by multiplying the concentration 

of the contaminant by the flow rate. It is given as a mass for 

a specified time period, in units such as tons per year. Loads 

are important for evaluating the potential ecological effects 

of nutrients on receiving water bodies downstream and over 

longer time periods. Regulatory limits on nutrient loads are 

site specific.

Concentration and load are related measures of nutrient levels in streams

Chapter 8

Figure 8–2. Concentrations of nitrate in stream base 

flow generally mirror concentrations of nitrate in groundwater discharging to those streams. The majority of the 

streams contained nitrate at concentrations that exceeded 

the U.S. Environmental Protection Agency recommended 

ecological criterion.(4)

Chapter 8: Can Discharge of Nutrients From Groundwater Impair the Health of Streams and Estuaries? 85
Phosphorus: essential nutrient but quantity is key

Phosphorus is an important nutrient but, like nitrogen, it can cause problems in surfacewater bodies when concentrations are too high. The total amount of phosphorus in a sample 

of stream water can include substantial amounts of particle-bound phosphorus, particulate 

organic phosphorus, and dissolved phosphorus; however, the predominant form of phosphorus in 

groundwater is dissolved phosphorus. For this reason, in this discussion, the term “phosphorus” 

is used to refer to sources and loads, but the term “dissolved phosphorus” is used to refer to 

measured concentrations.

Groundwater Can Contribute a Large Amount of Phosphorus to Streams

Elevated concentrations of phosphorus are a concern for stream ecology in the region—

the recommended ecological criteria for total phosphorus in streams (see sidebar, Concentration 

and load are related measures of nutrient levels in streams, p. 85) were exceeded in 42 percent 

of stream base-flow samples and 29 percent of groundwater samples. Groundwater discharge to 

streams contributes about 20 percent of the phosphorus load measured in stream base flow.

(55)

Concentrations of phosphorus in groundwater are controlled by the type of rock that 

makes up the aquifer. Median phosphorus concentrations in samples from the crystalline-rock 

and siliciclastic-rock aquifers—0.02 mg/L—were twice as high as those from the carbonaterock aquifers because some of the minerals that make up the crystalline and siliciclastic rocks 

contain phosphorus (fig. 8–4). Geochemical conditions such low alkalinity, low calcium 

concentrations, and anoxic conditions allow phosphorus to be dissolved from minerals in the 

crystalline-rock and siliciclastic-rock aquifers and to be transported with the groundwater 

(fig. 8–5). Potential geologic contributions of phosphorus to groundwater and streams from 

these aquifers are important watershed-management considerations.

Minerals in the carbonate-rock aquifers contain little phosphorus. In addition, geochemical 

conditions in the carbonate-rock aquifers are such that any phosphorus that enters the aquifer 

in recharge is likely to precipitate or adsorb to mineral surfaces as the groundwater moves 

along flow paths (fig. 8–5). As a result, concentrations of dissolved phosphorus in groundwater 

samples from the carbonate-rock aquifers were very low—more than 75 percent of the samples 

from the carbonate-rock aquifers had phosphorus concentrations less than the reporting limit of 

0.01 mg/L (fig. 8–4). Despite very low concentrations of phosphorus in carbonate-rock aquifers, dissolved phosphorus concentrations in stream base flow commonly were at least twice as 

high as in groundwater (median concentration 0.02 mg/L) in areas underlain by carbonate-rock 

aquifers (fig. 8–4). It is unknown why concentrations are 

higher in stream base flow than in groundwater samples, but 

evidently the source of the dissolved phosphorus in base flow or 

geochemical conditions at groundwater discharge locations are 

different from those of the groundwater that was sampled.

86 Water Quality in the Principal Aquifers of the Piedmont, Blue Ridge, and Valley and Ridge Regions, Eastern United States, 1993–2009

Concentrations of 

dissolved phosphorus 

exceeded recommended 

ecological criteria for total 

phosphorus in streams

in 42 percent of stream 

base-flow samples.

Potential geologic contributions of phosphorus 

to groundwater and 

streams are an important 

watershed-management 

consideration.

Concentrations of 

dissolved phosphorus in 

groundwater samples from 

the carbonate-rock aquifers 

were very low—more than 

75 percent of the samples 

from the carbonate-rock 

aquifers had phosphorus 

concentrations less than 

the laboratory reporting 

limit of 0.01 mg/L.
Figure 8–4. Dissolved phosphorus concentrations were higher in samples of groundwater from crystalline-rock and 

siliciclastic-rock aquifers than in samples from carbonate-rock aquifers. Consequently, contributions of dissolved 

phosphorus in crystalline-rock and siliciclastic-rock aquifers from geologic sources to stream base flow may be a substantial 

fraction of the total amount of phosphorus in streams. In contrast, the relatively low dissolved phosphorus concentrations in 

groundwater in carbonate-rock aquifers cannot explain the high concentrations in base flow in streams in these areas.

Figure 8–5. A, Phosphorus rarely infiltrates through soils 

to groundwater, regardless of land use. B, In aquifers with 

a geologic source of phosphorus, groundwater dissolves 

the phosphorus from the rock. C, Phosphorus moving with 

groundwater into the carbonate-rock aquifers precipitates or 

is adsorbed. D, Water travels long distances in the crystallinerock aquifer with increases in phosphorus concentration along 

the flow path because of the geologic source of phosphorus.

Phosphorus

Decreasing along flow path

Increasing along flow path

EXPLANATION

Recharge entering aquifer 

from land surface

CrystallineCarbonate-rock aquifer rock aquifer

A

B

C

D

Geologic 

source of

phosphorus

Figure 8–5.

Low Medium High

Phosphorus concentration

0.01

0.001

0.1

1

10

Dissolved phosphorus concentration, 

in milligrams per liter as P

Piedmont Physiographic Province (0.037 milligram per liter)

Blue Ridge and Valley and Ridge Physiographic Provinces

 (0.01 milligram per liter)

U.S. Environmental Protection Agency recommended 

 ecological criterion, for phosphorus in flowing streams(4)

Groundwater (wells or springs)

Sample source

Streams under base-flow conditions

Phosphorus in groundwater and streams

EXPLANATION

Valley 

and Ridge 

Valley 

and Ridge 

Piedmont and 

Blue Ridge

Siliciclastic-rock

aquifers

Carbonate-rock 

aquifers

Crystallinerock aquifers

Early Mesozoic

basin

Piedmont and

Blue Ridge

See sidebar, Boxplots, p. 47

Chapter 8

Dissolved phosphorus concentrations were higher in samples of groundwater from 

crystalline-rock and siliciclastic-rock aquifers than in samples from carbonate-rock aquifers. 

Consequently, contributions of dissolved phosphorus in crystalline-rock and siliciclastic-rock 

aquifers from geologic sources to stream base flow may be a substantial fraction of the 

total amount of phosphorus in streams. In contrast, the relatively low dissolved phosphorus 

concentrations in groundwater in carbonate-rock aquifers cannot explain the high concentrations in base flow in streams in these areas.

Chapter 8: Can Discharge of Nutrients From Groundwater Impair the Health of Streams and Estuaries? 87
Photograph copyright istockphoto.com
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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 

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Lindsey, B.D., 2010, Contributions of phosphorus from 

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59. Toccalino, P.L., 2007, Development and application of 

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at http://dx.doi.org/10.3133/cir1360.)

92 Water Quality in the Principal Aquifers of the Piedmont, Blue Ridge, and Valley and Ridge Regions, Eastern United States, 1993–2009
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.

Alternative Maximum Contaminant Level 

(AMCL) The higher of two proposed U.S. 

Environmental Protection Agency regulatory 

levels for radon in drinking water. The higher 

level, the AMCL of 4,000 pCi/L, would 

apply to public water systems or states that 

have programs to reduce radon risks from all 

sources. The lower level of 300 pCi/L would 

apply to systems or states that do not have 

such programs in place. Both proposed MCLs 

are used as human-health benchmarks in this 

circular, but are not yet formal regulatory 

benchmarks.

anoxic Having no dissolved oxygen or a 

very low concentration (less than 0.5 milligram per liter) of dissolved oxygen.

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 significant 

quantities of water to wells and springs. 

B

base flow Streamflow coming from 

groundwater seepage into a stream or river. 

Base flow is the continual contribution of 

groundwater to streams and rivers and is 

an important source of streamflow between 

rainstorms.

bedrock/bedrock type General term for 

consolidated (solid) rock that underlies soils 

or other unconsolidated material. In this report 

bedrock type refers to carbonate, crystalline, 

or siliciclastic bedrock.

C

carbonate bedrock/carbonate-rock aquifer

Rocks (such as limestone or dolostone) that 

are composed primarily of minerals (such as 

calcite and dolomite) containing the carbonate 

ion (CO3

2-).

conduit/conduit flow Groundwater flow 

through pipe-like or channel-like openings 

in the rock. Conduits control the direction of 

water flow and greatly increase the speed at 

which water travels through the aquifer.

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 

Circular, any manmade compound at any 

concentration, or any naturally occurring 

constituent measured at a concentration 

exceeding its human-health benchmark.

crystalline bedrock/crystalline-rock aquifer

Rocks (igneous or metamorphic) consisting 

wholly of crystals or fragments of crystals. 

Granite and schist are examples of crystalline 

rocks.

D

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. It is the most 

important way that nitrate can be eliminated 

naturally in groundwater.

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, for 

example, from a spring, from a well, or up 

through a streambed.

Glossary

Glossary 93
dissolution The process of dissolving a 

solid (mineral) into a homogeneous solution 

(water). Dissolution reactions result in the 

addition of ions to water as minerals react 

with water. Common dissolution reactions 

include dissolution of carbonate rock (limestone or dolomite) and incongruent dissolution of silicate minerals (feldspar) by carbonic 

acid (H2CO3).

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 

drinking-water supply designed to protect 

human health or to identify acceptable 

concentrations of constituents that cause 

unpleasant tastes, odors, or colors in the 

water. 

F

felsic A term applied to igneous rocks, such 

as granite, that contain abundant light-colored 

minerals such as feldspar and quartz. Also, a 

term applied to those minerals as a group.

G

groundwater Any water that exists beneath 

the land surface but most commonly applied 

to water in fully saturated soils and geologic 

formations.

groundwater age See apparent groundwater 

age.

groundwater flow path The route or 

pathway followed by water flowing through 

the hydrologic system. Usually refers to 

subsurface (groundwater) flow.

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 concentration 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 degradation or transport into another 

environmental medium.

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 ground-water samples can be 

compared; and (3) is consistent with USEPA 

Office of Water methodologies.

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 drunk to lower the concentration of the 

contaminant below the benchmark.

I

igneous rocks Rocks that solidified from 

molten or partly molten material. Granite is an 

example of an igneous rock.

intrinsic susceptibility A measure of the 

ease with which a contaminant in water 

enters and moves through an aquifer. It is a 

characteristic of the aquifer and overlying 

material and hydrologic conditions and is 

independent of the chemical characteristics of 

the contaminant and its sources.

K

karst Surface and subsurface terrane that 

is formed on and in soluble rocks, such as 

limestone and gypsum, primarily by dissolution and collapse and that is characterized by 

sinkholes, caves, and underground drainage. 

karst aquifer A body of soluble rock that 

conducts water principally via a connected 

network of tributary conduits formed by 

the dissolution of the rock, which drains a 

groundwater basin and discharges to at least 

one perennial spring.

L

land-use study A study by the U.S. Geological Survey National Water-Quality Assessment (NAWQA) Program used to assess the 

effects of a specific land-use type (generally 

agricultural or urban) on groundwater quality, 

in most cases by sampling groundwater from 

94 Water Quality in the Principal Aquifers of the Piedmont, Blue Ridge, and Valley and Ridge Regions, Eastern United States, 1993–2009
monitoring wells that tap water from or near 

the water table.

lithology/lithologic group The physical 

character of a rock on the basis of color, 

structure, mineralogical composition, grain 

size, and other characteristics. As used in 

this report, it is a subdivision of the major 

bedrock types. M

mafic A term applied to igneous rocks, such 

as basalt, that contain abundant dark-colored 

minerals with a high percentage of magne-

sium and iron. Also, a term applied to those 

minerals as a group.

major-aquifer studies 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 contami-

nant in water that is delivered to any user 

of a public water system. MCLs are 

enforceable standards established by the 

U.S. Environmental Protection Agency.

metamorphic rocks Rock derived from 

mineralogical, chemical, or structural changes 

to preexisting rocks in response to marked 

changes in temperature, pressure, shearing 

stress, and chemical environment, generally 

at depth in the Earth’s crust. Gneiss and schist 

are examples of metamorphic rocks.

monitoring well A well used to measure 

water quality or groundwater levels continu-

ously or periodically. Not typically used as a 

source of drinking water. Sometimes referred 

to as an “observation well.” N

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.

O

oxic Water with a concentration of dissolved 

oxygen greater than or equal to 0.5 milligram 

per liter. P

permeability A measure of the relative ease 

with which a porous or fractured medium 

can transmit groundwater. Rock formations 

that transmit fluids readily are described as 

permeable.

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.

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 emis-

sion of particles (alpha or beta) and gamma 

rays from an atom with an unstable nucleus 

(radionuclide).

recently recharged groundwater

Groundwater that was recharged after 1952, 

as indicated by tritium concentrations greater 

than 0.5 tritium unit.

recharge The addition of water to the 

saturated zone naturally by precipitation or 

runoff or artificially by spreading or injection. 

Also, water that is added.

reduction/oxidation (redox) Chemical 

reactions that involve the transfer of electrons 

from one chemical species to another, 

resulting in a change in the valence state of 

the species. Redox processes in groundwater 

often are microbially facilitated.

Glossary

Glossary 95
S

Secondary Maximum Contaminant Level 

(SMCL) Guidelines set by the U.S. Environmental 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. 

sedimentary rocks Rocks composed of 

particles derived from the erosion or weathering of preexisting rocks or from chemical 

precipitation from water. Sandstone and 

limestone are examples of sedimentary rocks.

siliciclastic bedrock/siliciclastic-rock 

aquifer Rocks formed by the compaction 

and cementation of quartz-rich mineral 

grains. Sandstone and shale are examples of 

siliciclastic rocks.

sinkhole Any closed depression in soil or 

bedrock formed by the erosion and transport 

of earth material from below the land surface, 

which has a closed topographic contour and 

drains to the subsurface. Typically occurs in 

karst terrane.

susceptibility See definition for 

intrinsic susceptibility.

T

tritium unit (TU) A measure of the 

concentration of tritium (3H), equal to one 3

H atom in 1,018 atoms of hydrogen (H), or 

3.24 picocuries per liter.

V

volatile organic compound (VOC)

An organic chemical that has a high vapor 

pressure relative to its water solubility. VOCs 

include components of gasoline, fuel oils, 

lubricants, organic solvents, fumigants, some 

inert ingredients in pesticides, and some 

byproducts of chlorine disinfection.

vulnerable/vulnerability The tendency 

or likelihood for contaminants to reach a 

specified position in the ground-water system 

after introduction at some location above 

the uppermost aquifer. The vulnerability of 

a groundwater resource to contamination 

depends both on its intrinsic susceptibility and 

on the locations and types of geologic sources 

of contamination and contamination associated with human activities, locations of wells, 

and characteristics of the contaminant(s).

W

water table The upper surface of the 

saturated zone, below which all voids (spaces) 

are filled with water.

96 Water Quality in the Principal Aquifers of the Piedmont, Blue Ridge, and Valley and Ridge Regions, Eastern United States, 1993–2009
Appendix Appendixes

Appendixes 1–6
Names used by the NAWQA Program

Network description used 

in this report 

Network 

name Study unit name Network type

 Early Mesozoic basin 

potosus2 Potomac River Basin Major-aquifer study Mixed land uses, groundwater used for drinking water 

delrsus1 Delaware River Basin Major-aquifer study Mixed land uses, groundwater used for drinking water 

linjsus3 Long Island and New Jersey Coastal Drainages Major-aquifer study Mixed land uses, groundwater used for drinking water 

 Piedmont and Blue Ridge 

lsuslusag1 Lower Susquehanna River Basin Land-use study Agricultural areas, groundwater used for drinking water 

 Piedmont and Blue Ridge 

acfblusur1 Apalachicola-Chatahoochee-Flint River Basin Land-use study Urban areas, groundwater near water table 

acfblusur2 Apalachicola-Chatahoochee-Flint River Basin Land-use study Urban areas, groundwater near water table 

albesus8 Albemarle-Pamlico Drainage Basin Major-aquifer study Mixed land uses, groundwater used for drinking water 

kanasus2 Kanawa-New River Basin Major-aquifer study Mixed land uses, groundwater used for drinking water 

*lsussus2 Lower Susquehanna River Basin Major-aquifer study Agricultural areas, groundwater used for drinking water 

Mixed land uses, groundwater used for drinking water 

podldwgs1 Potomac River Basin Source-water quality assessment Mixed land uses, groundwater used for drinking water 

podllusrc1 Potomac River Basin Land-use study Urban areas, groundwater near water table 

podlreffo2 Potomac River Basin Reference network Mixed land uses, groundwater used for drinking water 

potosus1 Potomac River Basin Major-aquifer study Mixed land uses, groundwater used for drinking water 

santdwgs1 Santee River Basin and Coastal Drainages Source-water quality assessment Mixed land uses, groundwater used for drinking water 

santsus3 Santee River Basin and Coastal Drainages Major-aquifer study Mixed land uses, groundwater used for drinking water 

Valley and Ridge 

lsuslusag2 Lower Susquehanna River Basin Land-use study Agricultural areas, groundwater used for drinking water 

lsuslusag3 Lower Susquehanna River Basin Land-use study Agricultural areas, groundwater used for drinking water 

luslusur1 Lower Susquehanna River Basin Land-use study Urban areas, groundwater near water table 

potolusag1 Potomac River Basin Land-use study Agricultural areas, groundwater used for drinking water 

podlreffo1 Potomac River Basin Reference network Mixed land uses, groundwater used for drinking water 

utenspr1 Upper Tennessee River Basin Spring study Mixed land uses, groundwater used for drinking water 

utensus1 Upper Tennessee River Basin Major-aquifer study Mixed land uses, groundwater used for drinking water 

Valley and Ridge 

delrsus2 Delaware River Basin Major-aquifer study Mixed land uses, groundwater used for drinking water 

lsussus1 Lower Susquehanna River Basin Major-aquifer study Mixed land uses, groundwater used for drinking water 

†potolusag2 Potomac River Basin Land-use study Agricultural areas, groundwater used for drinking water 

potolusfo1 Potomac River Basin Land-use study Mixed land uses, groundwater used for drinking water 

* This network was sampled as a single network but regrouped on the basis of land use for this analysis.

† One well from the potolusag2 network was in the Valley and Ridge carbonate-rock aquifers and was grouped with that aquifer for analysis.

Appendix 1. NAWQA Cycle 1 groundwater sampling networks in the Piedmont, Blue 

[Water-use abbreviations: DOM, domestic supply; PUB, public supply; ODW, other drinking-water supply (commercial, institutional); OTH, other (stock,irrigation, fire, dewatering, 

urban land use; urban, greater than 25 percent urban land use and less than 25 percent agricultural land use; undeveloped, less than 25 percent agricultural and less than 5 percent urban 

98 Water Quality in the Principal Aquifers of the Piedmont, Blue Ridge, and Valley and Ridge Regions, Eastern United States, 1993–2009
What data were collected and why

Number 

of sites 

sampled

Water use of sites 

(listed in 

descending order)

Aquifer bedrock

type

Sampling 

period

Predominant land use 

(listed in descending order) 

aquifers

Assess the quality of water in major aquifer 23 DOM, ODW Siliciclastic, 

crystalline 1994 Agriculture, mixed

Assess the quality of water in major aquifer 30 DOM, UN, PUB, ODW Siliciclastic 1999 Agriculture, mixed, urban

Assess the quality of water in major aquifer 21 DOM Siliciclastic, 

crystalline 1998 Agriculture, mixed, urban

carbonate-rock aquifers

Describe effects of agricultural land use on shallow groundwater 30 DOM, OTH Carbonate 1993 Agriculture

crystalline-rock aquifers

Describe effects of urban land use on shallow groundwater 21 DOM, UN, OTH Crystalline 1994–95 Urban

Describe effects of urban land use on shallow groundwater 19 SPRINGS-UNUSED Crystalline 1994–95 Urban

Assess the quality of water in major aquifer 55 DOM, ODW, UN Crystalline 2007 Mixed

Assess the quality of water in major aquifer 30 DOM, ODW, PUB, UN Crystalline 1997 Mixed, agriculture, undeveloped

Describe effects of agricultural land use on shallow groundwater 24 DOM Crystalline 1994 Agriculture

Assess the quality of water in major aquifer 6 DOM Crystalline 1994 Forested

Assess the quality of source water used by community water 

systems dependent on groundwater sources for public supply 15 PUB Crystalline 2003 Mixed, agriculture

Describe effects of urban land use on shallow groundwater 30 UN Crystalline 2003 Urban, mixed

Describe background conditions for water quality near 

podllusrc1 network 2 UN Crystalline 2003–05 Undeveloped

Assess the quality of water in major aquifer 25 DOM, ODW, UN Crystalline 1994 Agriculture, urban, mixed

Assess the quality of source water used by community water 

systems dependent on groundwater sources for public supply 15 PUB Crystalline 2008 Undeveloped, mixed

Assess the quality of water in major aquifer 30 DOM, ODW Crystalline 1998 Undeveloped, mixed

carbonate-rock aquifers

Describe effects of agricultural land use on shallow groundwater 30 DOM, ODW Carbonate 1994 Agriculture

Describe effects of agricultural land use on shallow groundwater 30 DOM, OTH Carbonate 1995 Agriculture

Describe effects of urban land use on shallow groundwater 20 DOM, UN, ODW, OTH Carbonate 1995 Urban, mixed

Describe effects of agricultural land use on shallow groundwater 32 DOM Carbonate 1993 and 

2002 Agriculture, mixed

Describe background conditions for water quality 

near potolusag1 network 2 UN Carbonate 2002 Undeveloped

Assess the quality of water in major aquifer through sampling 

of springs 35 SPRINGS-DOM, UN, PUB Carbonate 1997 Mixed, undeveloped, agriculture

Assess the quality of water in major aquifer 30 DOM, ODW, UN, OTH Carbonate 1998–99 Mixed, undeveloped, agriculture

siliciclastic-rock aquifers

Assess the quality of water in major aquifer 30 DOM, PUB, ODW, UN Siliciclastic 2000 Agriculture, urban, mixed

Assess the quality of water in major aquifer 29 DOM Siliciclastic 1993 Agriculture, undeveloped

Describe effects of agricultural land use on shallow groundwater 25 DOM Siliciclastic 1995 Agriculture, mixed

Describe effects of forested land use on shallow groundwater 3 DOM, ODW Siliciclastic 1995 Undeveloped

Ridge, and Valley and Ridge Principal Aquifers.

industrial); UN, unused monitoring well or spring; land-use definitions for 1,640-ft radius around well: agriculture, greater than 50 percent agricultural land use and less than 5 percent 

land use; mixed, any site not meeting the other three definitions]

Appendixes

Appendixes 99
NAWQA groundwater assessments include different types of studies specifically designed to 

answer questions such as these. 

• Land-use studies sample 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 the deeper 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 (http://pubs.usgs.gov/circ/circ1112/flowpath.html).

• Major-aquifer studies provide a broad overview of the quality of the deeper aquifer system used for drinking-water supply. Most of the wells sampled were domestic wells that were distributed across a large area of mixed land use.

• Vulnerability studies are designed to assess the vulnerability of water delivered by public-supply wells to contamination

from geologic and manmade sources. Samples were collected from multiple depths in public-supply wells to determine

where and how contaminants from different sources enter the wells and how natural processes and human activities affect

water quality (http://oh.water.usgs.gov/tanc/NAWQATANC.htm).

• Source-water-quality assessment studies are designed to determine the detection frequency and concentrations of unregulated manmade chemicals in groundwater from public-supply wells that serve large numbers of people (http://water.usgs.

gov/nawqa/swqa/).

• Results of these studies were reinforced by placing the areas for some of the studies within the boundaries of larger studies. For example, the recharge areas for flow-path studies were placed 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 placed within the

boundaries of the major aquifer studies to provide information on how the quality of recharge affects the quality of the

deeper groundwater.

• 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 assessment.

100 Water Quality in the Principal Aquifers of the Piedmont, Blue Ridge, and Valley and Ridge Regions, Eastern United States, 1993–2009

Appendix 2. 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?
Appendixes

Water-quality properties and constituents measured and a summary of data for

Circular 1354, including laboratory reporting levels and human-health benchmarks for drinking 

water, are presented only online in PDF form. The tab-delimited data also are available for

download at http://pubs.usgs.gov/circ/1354/.

Appendix 3. 
Appendix 4. The Piedmont, Blue Ridge, and Valley and Ridge Regions Principal 

Aquifers in a national context.

circ1354 appendix4_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.

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). Data for a particular compound were 

not plotted if there were fewer than 10 samples for a particular well network in a Principal Aquifer; not all Principal 

Aquifers for which data were available are shown. Note that analytical detection limits varied among the constituents and 

that the number of samples for a constituent can vary greatly between Principal Aquifers. The data used in this appendix 

and boxplots for additional constituents are available at http://pubs.usgs.gov/circ/1360/.

(78)

** Note: Two of the Principal Aquifers in this group include basin-fill aquifers within the extent of the 

basaltic aquifer.

102 Water Quality in the Principal Aquifers of the Piedmont, Blue Ridge, and Valley and Ridge Regions, Eastern United States, 1993–2009
Figure A4–1. Concentrations of arsenic and radon in Principal Aquifers of the Piedmont, Blue Ridge, and Valley and 

Ridge regions in a national context.

Surficial (30)

Mississippi River Valley alluvial (5)

High Plains (313)

Alluvial–Pleistocene terrace deposits (NA)

Alluvial–Denver Basin (7)

Rio Grande (25)

Central Valley (135)

California Coastal Basin (18)

Basin and Range basin-fill (129)

Glacial–West (23)

Glacial–West Central (76)

Glacial–East (68)

Texas coastal uplands (50)

Northern Atlantic Coastal Plain (60)

Mississippi embayment (27)

Valley and Ridge siliciclastic (27)

Early Mesozoic (24)

Denver Basin aquifer (75)

Cambrian-Ordovician (69)

Valley and Ridge carbonate (28)

Piedmont and Blue Ridge carbonate (NA)

Upper Floridan (128)

Castle Hayne (6)

Piedmont and Blue Ridge crystalline (99)

New England crystalline (107)

Hawaiian volcanic (NA)

Snake River Plain basin fill–basalt rock (54)

Columbia Plateau basin fill–basalt rock (NA)

Arsenic in domestic wells

MCL = 10 μg/L

Radon in domestic wells 

(30)

(5)

(307)

(NA)

 (6)

(24)

(111)

(17)

(112)

(69)

(58)

(153)

(97)

(40)

(55)

(17)

(79)

(48)

(55)

(47)

(112)

(29)

(88)

(6)

(150)

(109)

(NA)

(115)

(59)

PMCL = 

300 pCi/L AMCL = 4,000 pCi/L

0.1 1 10 100 1,000 10 100 1,000 10,000 100,000 1,000,000

Glacial–Central (187)

Principal Aquifer

Concentration, in micrograms per liter Concentration, in picocuries per liter

Appendixes

Appendixes 103
Figure A4–2. Concentrations of nitrate plus nitrite in Principal Aquifers of the Piedmont, Blue Ridge, and Valley 

and Ridge regions in a national context

Nitrate plus nitrite in agricultural 

land-use wells

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 (99)

Glacial–West Central (154)

Glacial–Central (282)

Glacial–East (54)

Texas coastal uplands (NA)

Northern Atlantic Coastal Plain (83)

Mississippi embayment (NA)

Valley and Ridge siliciclastic (23)

Early Mesozoic (NA)

Denver Basin Aquifer (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)

Nitrate plus nitrite in urban 

land-use wells

(28)

(NA)

(30)

(27)

 (39)

(44)

(26)

(24)

(116)

(NA)

(74)

(129)

(153)

(NA)

(95)

(10)

(NA)

(NA)

(19)

(NA)

(20)

(NA)

(25)

(NA)

(50)

(NA)

(NA)

(NA)

 (NA)

0.01 0.1 1 10 100 0.01 0.1 1 10 100

Nitrate plus nitrite in domestic wells

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 (76)

Glacial–West Central (96)

Glacial–Central (210)

Glacial–East (97)

Texas coastal uplands (50)

Northern Atlantic Coastal Plain (66)

Mississippi embayment (27)

Valley and Ridge siliciclastic (81)

Early Mesozoic (69)

Denver Basin aquifer (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)

Snake River Plain basin fill–basalt rock (151)

Columbia Plateau basin fill–basalt rock (63)

0.01 0.1 1 1MCL = 10 mg/L 0 100

Concentration, in milligrams per liter as N

Concentration, in milligrams per liter as N

Principal aquifer

MCL = 10 mg/L

MCL = 10 mg/L

MCL = 10 mg/L

Concentration, in milligrams per liter as N

104 Water Quality in the Principal Aquifers of the Piedmont, Blue Ridge, and Valley and Ridge Regions, Eastern United States, 1993–2009
Figure A4–3. Concentrations of atrazine in Principal Aquifers of the of the Piedmont, Blue Ridge, and Valley and 

Ridge regions in a national context.

Principal Aquifers Atrazine in agricultural land-use wells

Principal Aquifers

Surficial (135)

Mississippi River Valley alluvial (NA)

High Plains (85)

Alluvial–Pleistocene terrace deposits (NA)

Alluvial–Denver Basin (19)

Rio Grande (91)

Central Valley (140)

California Coastal Basin (NA)

Basin and Range basin-fill (37)

Glacial–West (25)

Glacial–West Central (153)

Glacial–Central (281)

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 (11)

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)

MCL = 3 μg/L

0.001 0.01 0.1 1 10

Snake River Plain basin fill–basalt rock (110)

Columbia Plateau basin fill–basalt rock (128)

Atrazine in 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)

Atrazine in 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)

Concentration, in micrograms per liter Concentration, in micrograms per liter

Concentration, in micrograms per liter

Appendixes

Appendixes 105
Appendix A5–1. Median estimated pesticide application rates in 1,640-foot radius around wells in the Piedmont, Blue Ridge, and Valley and Ridge 

[Purple shading indicates the compound analyzed for this study as one of the 47 pesticides considered in this report; turquoise shading indicates the compound analyzed as one 

around wells in the entire Piedmont, Blue Ridge, and Valley and Ridge regions(40)]

Pesticide name

Piedmont, 

Blue Ridge, and 

Valley and 

Ridge regions

Piedmont and Blue Ridge crystalline-rock aquifers Early Mesozoic Basin 

aquifers 

Agricultural area, 

groundwater used 

for drinking water

Urban area, 

groundwater near 

water table

Mixed land use, 

groundwater used for 

drinking water

Mixed land use, 

groundwater used for 

drinking water

Atrazine 2.81935 18.1027 0 0.9209 5.8796 

Metolachlor 2.18153 26.2701 0 0.5686 6.4269 

2-4-D 1.45619 1.91145 0.00335 1.47692 1.04367 

Alachlor 0.87990 3.13786 0 0.37758 1.58316 

Chlorpyrifos 0.67702 2.1948 0.00026 0.424 0.46811 

Glyphosate 0.62834 1.57021 0.00038 0.21434 0.93554 

Pendimethalin 0.51646 10.9862 0 0.2215 0.6156 

Methyl-bromide (VOC schedule) 0.47826 2.8023 0 0.3003 0.5172 

Simazine 0.41200 1.49714 0.00008 0.13081 0.71509 

Sulfur 0.34645 3.84947 0.00048 0.1098 1.07934 

Oil 0.34238 1.51694 0.00192 0.11876 0.53809 

Mancozeb 0.27749 1.25224 0.00038 0.16105 0.26724 

Captan 0.25585 1.50632 0.00099 0.07442 0.54124 

Terbufos 0.17474 0.39616 0 0.14621 0.22473 

*Cyanazine 0.17371 1.24328 0 0.05712 0.59418 

Chloropicrin 0.11990 0.6797 0 0.07243 0.12439 

Chlorothalonil 0.11990 0.87618 0 0.03823 0.79568 

Azinphos-methyl 0.07423 0.35149 0.00003 0.01106 0.13877 

Ziram 0.06903 0.32945 0 0.04093 0.12563 

Carbaryl 0.06850 0.13701 0.00084 0.04324 0.07456 

Metiram 0.06003 0.5899 0.00023 0.02528 0.08324 

Dimethoate 0.05397 0.27783 0 0.01506 0.04119 

Methomyl 0.04762 0.21909 0 0.02454 0.14396 

Linuron 0.04250 0.25095 0 0.00928 0.07275 

Methyl-parathion 0.03743 0.20274 0.00007 0.03596 0.04985 

Maneb 0.01464 0.09038 0 0.00233 0.08081 

Acephate 0.01165 0.10206 0 0.00706 0.02394 

Bentazon 0.00958 0.03886 0 0.01896 0.01092 

Maleic-hydrazide 0.00390 0.0446 0 0.00203 0.00117 

Ethalfluralin 0.00108 0.01133 0 0.0008 0.00004 

*The sale and use of Cyanazine was prohibited after September 30, 2002.

106 Water Quality in the Principal Aquifers of the Piedmont, Blue Ridge, and Valley and Ridge Regions, Eastern United States, 1993–2009
regions, in kilograms, in 1997.

of the 86 volatile organic compounds considered in this report. Pesticides are sorted in descending order of the mass of active ingredient applied in the 1,640-foot radius 

Valley and Ridge siliciclastic-rock aquifers

Piedmont and Blue 

Ridge carbonate-rock 

aquifers

Valley and Ridge carbonate-rock aquifers

Agricultural area, 

groundwater used for 

drinking water

Mixed land use, 

groundwater used for 

drinking water

Agricultural area, 

groundwater used for 

drinking water

Agricultural area, 

groundwater used for 

drinking water

Urban area, 

groundwater near 

water table

Mixed land use, 

groundwater used for 

drinking water

1.5856 10.4065 35.2294 19.2818 2.3225 0.7701

1.9519 13.0461 38.3035 22.4624 2.7737 0.2593

1.595 1.24741 2.45768 2.1197 0.22432 0.40262

0.56449 1.91987 5.43722 3.28255 0.37547 0.11135

0.67373 1.23229 5.28276 2.77033 0.28954 0.40395

3.03327 0.8351 2.11997 1.49684 0.16219 0.13244

0.0298 5.5499 20.5867 10.2396 1.3458 0.1917

0 1.6084 10.2111 1.1718 0.1715 0.0115

0.53557 0.7506 2.2998 1.6861 0.17145 0.10222

0.04527 0.60604 1.48427 1.30771 0.15191 0.00159

0.16157 0.82877 0.75578 3.41969 0.18212 0.24109

0.02683 0.73017 2.01281 1.27056 0.08146 0.34223

0.03866 0.73545 0.88334 2.87422 0.17198 0.04703

0.06705 0.24368 0.78097 0.46239 0.04984 0.01306

0.00098 0.76464 2.44832 1.24968 0.15667 0.01799

0 0.42363 2.62315 0.28331 0.04360 0.00222

0 0.43761 2.05981 0.44488 0.06475 0.02243

0.01133 0.16579 0.20357 0.61046 0.035 0.03504

0.04163 0.18866 0.18992 0.77434 0.0388 0.02069

0.0017 0.06184 0.15629 0.11727 0.01079 0.54896

0.04928 0.27555 0.29705 1.70989 0.07641 0

0.13847 0.27813 0.83292 0.58679 0.05841 0.00127

0.00561 0.10352 0.20858 0.26032 0.01974 0.01867

0.0115 0.11491 0.25138 0.16935 0.02721 0

0.01992 0.10107 0.15638 0.31087 0.01883 0.00193

0 0.04218 0.23947 0.05041 0.00633 0.01309

0 0.01217 1.31522 0.01169 0.00273 0.40517

0.00181 0.00555 0.05546 0.02303 0.0014 0.0029

0 0.01357 1.8552 0.00834 0.00208 0.60776

0 0.00470 0.04164 0.0056 0.00121 0

Appendixes

Appendixes 107

Prepared by the USGS Science Publishing Network

Publishing Service Centers

Edited by Mary S. Ashman, Pembroke

Illustrations by Caryl J. Wipperfurth, Raleigh, and 

Christine T. Mendelsohn, Pembroke

Layout by Christine T. Mendelsohn, Pembroke

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/
Principal Aquifers of the Piedmont, 

Blue Ridge, and Valley and Ridge 

regions—includes Piedmont and 

Blue Ridge carbonate- and 

crystalline-rock aquifers, Early 

Mesozoic basin aquifers, and 

Valley and Ridge carbonate- and 

siliciclastic-rock aquifers

ISSN 1067-084X (print)

ISSN 2330-5703 (online)

Printed on recycled Paper http:// dx.doi.org/10.3133/cir1354