National Water-Quality Assessment Program

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

Scientific Investigations Report 2013–5072

U.S. Department of the Interior

U.S. Geological Survey
Cover: Photo showing Whiteside tonalite (rock types quartz diorite and granodiorite, lithologic group mafic igneous 

 rocks and their metamorphic equivalents, lithochemical subgroup 41). Photograph by William Burton, 

 U.S. Geological Survey
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

National Water-Quality Assessment Program

Scientific Investigations Report 2013–5072

U.S. Department of the Interior

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

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

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

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Suggested citation:

Chapman, M.J., Cravotta, C.A., III, Szabo, Z., and Lindsey, B.D., 2013, 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: U.S. Geological Survey Scientific Investigations Report 2013–5072, 74 p.
iii

Foreword

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

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

effective management of water, biological, energy, and mineral resources (http://www.usgs.

gov/). Information on the Nation’s water resources is critical to ensuring long-term availability 

of water that is safe for drinking and recreation and is suitable for industry, irrigation, and fish 

and wildlife. Population growth and increasing demands for water make the availability of that 

water, measured in terms of quantity and quality, even more essential to the long-term sustainability of our communities and ecosystems.

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

to support national, regional, State, and local information needs and decisions related to 

water-quality management and policy (http://water.usgs.gov/nawqa). The NAWQA Program 

is designed to answer: What is the quality of our Nation’s streams and groundwater? How 

are conditions changing over time? How do natural features and human activities affect the 

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

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

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

assessments and established a baseline understanding of water-quality conditions in 51 of the 

Nation’s river basins and aquifers, referred to as Study Units (http://water.usgs.gov/nawqa/

studies/study_units.html).

In the second decade of the Program (2001–12), a major focus is on regional assessments of 

water-quality conditions and trends. These regional assessments are based on major river 

basins and principal aquifers, which encompass larger regions of the country than the Study 

Units. Regional assessments extend the findings in the Study Units by filling critical gaps in 

characterizing the quality of surface water and groundwater, and by determining water-quality 

status and trends at sites that have been consistently monitored for more than a decade. In 

addition, the regional assessments continue to build an understanding of how natural features 

and human activities affect water quality. Many of the regional assessments employ modeling 

and other scientific tools, developed on the basis of data collected at individual sites, to help 

extend knowledge of water quality to unmonitored, yet comparable areas within the regions. 

The models thereby enhance the value of our existing data and our understanding of the hydrologic system. In addition, the models are useful in evaluating various resource-management 

scenarios and in predicting how our actions, such as reducing or managing nonpoint and point 

sources of contamination, land conversion, and altering flow and (or) pumping regimes, are likely 

to affect water conditions within a region.

Other activities planned during the second decade include continuing national syntheses of 

information on pesticides, volatile organic compounds (VOCs), nutrients, trace elements, and 

aquatic ecology; and continuing national topical studies on the fate of agricultural chemicals, 

effects of urbanization on stream ecosystems, bioaccumulation of mercury in stream ecosystems, effects of nutrient enrichment on stream ecosystems, and transport of contaminants to 

public-supply wells.
iv

The USGS aims to disseminate credible, timely, and relevant science information to address 

practical and effective water-resource management and strategies that protect and restore 

water quality. We hope this NAWQA publication will provide you with insights and information 

to meet your needs, and will foster increased citizen awareness and involvement in the protection and restoration of our Nation’s waters. 

The USGS recognizes that a national assessment by a single program cannot address all waterresource issues of interest. External coordination at all levels is critical for cost-effective management, regulation, and conservation of our Nation’s water resources. The NAWQA Program, 

therefore, depends on advice and information from other agencies—Federal, State, regional, 

interstate, Tribal, and local—as well as nongovernmental organizations, industry, academia, 

and other stakeholder groups. Your assistance and suggestions are greatly appreciated.

William H. Werkheiser

USGS Associate Director for Water
v

Contents

Foreword ........................................................................................................................................................iii

Abstract...........................................................................................................................................................1

Introduction.....................................................................................................................................................2

Description of Study Area ...................................................................................................................2

Purpose and Scope ..............................................................................................................................6

Geologic Setting and Aquifer Descriptions...............................................................................................6

Previous Studies ...................................................................................................................................9

Geological and Geochemical Framework for Interpretations of Water Quality ...............................14

Previous Lithochemical Classifications ..........................................................................................14

Lithologic Groups................................................................................................................................14

Lithochemical Subgroups..................................................................................................................17

Geochemical Controls on Naturally Occurring Trace Elements and Radionuclides 

in Groundwater....................................................................................................................................20

Water-Quality Data and Methods.............................................................................................................23

Graphical and Statistical Analyses..................................................................................................25

Geochemical Modeling......................................................................................................................26

Water-Quality Characteristics of Aquifers, Lithologic Groups, and Lithochemical Subgroups.....27

Exceedances of Drinking Water Criteria ........................................................................................27

Correlations Among Major and Trace Constituents and Environmental Factors ....................41

Geochemical Conditions Associated with Elevated Concentrations of Naturally 

Occurring Constituents......................................................................................................................44

Trace Elements ..........................................................................................................................44

Arsenic................................................................................................................................44

Manganese........................................................................................................................45

Zinc......................................................................................................................................45

Alkalinity, Hardness, and Dissolved Solids ..................................................................47

Antimony, Lead, Barium, and Molybdenum .................................................................49

Radionuclides.............................................................................................................................49

Uranium ..............................................................................................................................51

Radon..................................................................................................................................53

Radium................................................................................................................................57

Gross Alpha-Particle Activity .........................................................................................60

Potential for Detrimental Effects from Trace Elements, Radionuclides, and 

Associated Contaminants ........................................................................................................61

Summary and Conclusions.........................................................................................................................62

Acknowledgments.......................................................................................................................................64

References Cited..........................................................................................................................................64

Appendix 1. Tables (Excel spreadsheets available online at http://pubs.usgs.gov/

sir/2013/5072/)..................................................................................................................................71

Appendix 2. Probability plots of principal components analysis (PCA) scores for groundwater of siliclastic-rock and crystalline-rock aquifers in the Piedmont and Blue Ridge 

Physiographic Provinces: PC1 “Alkalinity-pH”; PC2 “Chloride-Nitrate”; PC3 “ Redox”; 

PC4 “Temperature-Silica”; and “Radon-Potassium”...............................................................72

Appendix 3. Analytical Issues Relating to Defining Radium Occurrence .......................................73
vi

Figures

1. Map showing boundaries for the three principal aquifers within the Piedmont and 

Blue Ridge Physiographic Provinces ........................................................................................3

2. Map showing 2007 land-use data for the Piedmont and Blue Ridge Physiographic 

Provinces and major metropolitan areas .................................................................................5

3. Map showing U.S. Geological Survey National Water-Quality Assessment (NAWQA) 

Program study unit boundaries within the Piedmont and Blue Ridge Physiographic 

Provinces and sample-collection locations for groundwater-quality data collected 

from 1994–2008..............................................................................................................................7

4. Block diagram showing the conceptual groundwater system, aquifers, and typical 

well types included as part of this study within the Piedmont and Blue Ridge 

Physiographic Provinces.............................................................................................................8

5. Map showing distribution of delineated lithologic groups and 1994–2008 sample 

locations within the study area, Piedmont and Blue Ridge Physiographic 

Provinces .....................................................................................................................................18

6. Generalized schematic cross-section diagram across North Carolina physiographic 

provinces showing generalized geologic terranes and common lithologic groups 

delineated as part of this study ...............................................................................................19

7. Chart showing an example division of the aquifers into lithologic groups, 

lithochemical subgroups, and corresponding geologic formations ..................................20

8. Graphs showing equilibrium fractions of initial concentrations of ions that may be 

dissolved or adsorbed on a finite amount of hydrous ferric oxide at 25 degrees 

Celsius as a function of pH. Anions; cations .........................................................................22

9. Chart showing uranium-238 and thorium-232 decay series ...............................................24

10. Graphs showing probability plots of groundwater-quality data for siliclastic-rock 

and crystalline-rock aquifers in the Piedmont and Blue Ridge Physiographic 

Provinces, 1994–2008. Total dissolved solids; specific conductance; alkalinity; 

hardness; calcium; magnesium; sodium; and potassium ....................................................28

11. Graphs showing probability plots of groundwater-quality data for siliclastic-rock 

and crystalline-rock aquifers in the Piedmont and Blue Ridge Physiographic 

Provinces, 1994–2008. Sulfate; chloride; nitrate; phosphate; bromide; fluoride; 

silica; and aluminum...................................................................................................................29

12. Graphs showing probability plots of groundwater-quality data for siliclastic-rock 

and crystalline-rock aquifers in the Piedmont and Blue Ridge Physiographic 

Provinces, 1994–2008. pH; dissolved oxygen; iron; manganese; arsenic; selenium; 

boron; and molybdenum............................................................................................................30

13. Graphs showing probability plots of groundwater-quality data for siliclastic-rock 

and crystalline-rock aquifers in the Piedmont and Blue Ridge Physiographic 

Provinces, 1994–2008. Cadmium; zinc; copper; lead; cobalt; nickel; chromium; 

and vanadium .............................................................................................................................31

14. Graphs showing probability plots of groundwater-quality data for siliclasti-rock 

and crystalline-rock aquifers in the Piedmont and Blue Ridge Physiographic 

Provinces, 1994–2008. Barium; strontium; beryllium; antimony; uranium; tritium; 

radium; and radon.......................................................................................................................32

15. Boxplots showing groundwater-quality data by lithologic groups of siliclastic-rock 

and crystalline-rock aquifers in the Piedmont and Blue Ridge Physiographic 

Provinces, 1994–2008. Specific conductance; sulfate; chloride; nitrate; phosphate; 

silica; sodium; potassium; and magnesium............................................................................33
vii

16. Boxplots showing groundwater-quality data by lithologic group of siliclastic-rock 

and crystalline-rock aquifers in the Piedmont and Blue Ridge Physiographic 

Provinces, 1994–2008. Calcium; alkalinity; pH; dissolved oxygen; iron; manganese; 

arsenic; selenium; and molybdenum.......................................................................................34

17. Boxplots showing groundwater-quality data by lithologic group of siliclastic-rock 

and crystalline-rock aquifers in the Piedmont and Blue Ridge Physiographic 

Provinces, 1994–2008. Barium; copper; lead; nickel; zinc; tritium; uranium; 

radium 226+228; and radon-222................................................................................................35

18. Boxplots showing saturation indices (SI) for selected minerals in groundwater by 

lithologic group of siliciclastic-rock and crystalline-rock aquifers in the Piedmont 

and Blue Ridge Physiographic Provinces. Calcite; dolomite; gypsum; fluorite; 

fluorapatite; manganese phosphate; strengite; ferrihydrite; and siderite ........................36

19. Boxplots showing saturation indices (SI) for selected minerals in groundwater by 

lithologic group of siliclastic-rock and crystalline-rock aquifers in the Piedmont 

and Blue Ridge Physiographic Provinces. Andularia; chlorite; kaolinite; gibbsite; 

chalcedony; barite; anglesite; cerrusite; and smithsonite...................................................37

20. Chart showing redox/pH matrix summarizing groundwater-quality samples by 

lithologic group of sliciclastic-rock and crystalline-rock aquifers in the Piedmont 

and Blue Ridge Physiographic Provinces, 1994–2008. All nine lithologic groups; 

clastic sedimentary; clastic lacustrine/evaporite; quartz-rich sedimentary; 

metamorphosed clastic sedimentary; quartz-rich metamorphic; felsic igneous or 

metamorphic; intermediate igneous or metamorphic; mafic igneous or 

metamorphic; and ultramafic....................................................................................................38

21. Chart showing redox-pH matrix summarizing groundwater-quality samples greater 

than or equal to the highest common reporting level (HCRL) or human health 

benchmark (HHB). Nitrate; manganese; sulfate; iron; phosphate; lead; arsenic; 

zinc; selenium; chromium; molybdenum; nickel; barium; cobalt; uranium; copper; 

radon-222; and radium-226+228 ..............................................................................................39

22. Map showing areal occurrence of lithochemical subgroups having elevated arsenic 

concentrations (based on the Tukey mean rank test, table 6) in groundwater within 

the Piedmont and Blue Ridge Physiographic Provinces......................................................46

23. Boxplots showing concentrations of selected metals and associated constituents 

in groundwater by casing type for wells in siliciclastic- and crystalline-rock aquifers 

in the Piedmont and Blue Ridge Physiographic Provinces .................................................48

24. Map showing areal occurrence of felsic lithologic groups within the Piedmont and 

Blue Ridge Physiographic Provinces ......................................................................................50

25. Map showing the areal occurrence of lithochemical subgroups having elevated 

uranium concentrations in groundwater (based on Tukey mean rank tests, table 6) 

within the Piedmont and Blue Ridge Physiographic Provinces..........................................52

26. Boxplot showing radon concentrations in groundwater samples related to 

composition of lithologic group................................................................................................54

27. Map showing areal occurrence of lithochemical subgroups having elevated 

radon-222 concentrations in groundwater (based on the Tukey mean rank test, 

table 6) within the Piedmont and Blue Ridge Physiographic Provinces ...........................55

28. Map showing an overlay of areal occurrence of lithochemical subgroups having 

elevated radon-222 concentrations and U.S. Geological Survey National Uranium 

Resource Evaluation Program uranium measurements in groundwater within the 

Piedmont and Blue Ridge Physiographic Provinces ............................................................56

29. Graph showing relations between concentrations of Radium-228 and Radium-226 

in samples from bedrock aquifers of the Piedmont and Blue Ridge Physiographic 

Provinces .....................................................................................................................................58
viii

30. Graph showing relations of concentrations of radium-226 plus radium-228 and 

dissolved oxygen for samples from siliclastic-rock and crystalline-rock aquifers of 

the Piedmont and Blue Ridge Physiographic Provinces, 1994–2008 .................................59

31. Graph showing relations of concentrations of radium-226 plus radium-228 and 

uranium for samples from bedrock aquifers of the Piedmont and Blue Ridge 

Physiographic Provinces...........................................................................................................61

Tables

1. NAWQA study units in the Piedmont and Blue Ridge Physiographic Provinces ..............4

2. Published reports from USGS NAWQA studies and other data sources conducted 

in the Piedmont and Blue Ridge Physiographic Provinces categorized by aquifer 

and constituents investigated ..................................................................................................10

3. Lithologic group, major rock types, and lithochemical subgroups for groundwater 

sites within the siliciclastic-rock and crystalline-rock aquifers in the Piedmont and 

Blue Ridge Physiographic Provinces, 1994–2008..................................................................15

4. Summary of wells within siliciclastic and crystalline rock aquifers of the Piedmont 

and Blue Ridge Physiographic Provinces (1994–2008) having chemical constituents in 

groundwater exceeding U.S. Environmental Protection Agency (2010) drinking water 

standards. (Table 4 is a excel spreadsheet available online at http://pubs.usgs.gov/

sir/2013/5072/)

5. Principal components analysis model of major factors controlling the chemistry of 

groundwater from siliciclastic-rock and crystalline-rock aquifers of the Piedmont 

and Blue Ridge Physiographic Provinces, 1994–2008 ..........................................................42

6. Tukey test for difference in mean rank of arsenic concentration, pH, and concentrations of manganese, iron, uranium, and radon among lithochemical subgroups with a 

minimum of five values for arsenic or uranium. (Table 6 is a excel spreadsheet available online at http://pubs.usgs.gov/sir/2013/5072/)

7. Number of samples with specified ranges of uranium and radium-226 concentrations for 99 water samples for which gross alpha-particle activity was determined.....60
ix

Conversion Factors, Datums

Inch/Pound to SI

Multiply By To obtain

Length

foot (ft) 0.305 meter

mile (mi) 1.609 kilometer (km)

Area

square foot (ft2

) 929 square centimeter (cm2)

square foot (ft2

) 0.093 square meter (m2)

square mile (mi2

) 259 hectare (ha)

square mile (mi2

) 2.59 square kilometer (km2)

Volume

cubic foot (ft3

) 28.32 cubic decimeter (dm3)

cubic foot (ft3

) 0.028 cubic meter (m3)

cubic mile (mi3

) 4.168 cubic kilometer (km3)

acre-foot (acre-ft) 1,233 cubic meter (m3

)

acre-foot (acre-ft) 0.001 cubic hectometer (hm3

)

Flow rate

acre-foot per day (acre-ft/d) 0.014 cubic meter per second (m3

/s)

acre-foot per year (acre-ft/yr) 1,233 cubic meter per year (m3

/yr)

acre-foot per year (acre-ft/yr) 0.001 cubic hectometer per year (hm3

/yr)

Radioactivity

picocurie per liter (pCi/L) 0.037 becquerel per liter (Bq/L)

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows:

°F=(1.8×°C)+32

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows:

°C=(°F–32)/1.8

Horizontal coordinate information is referenced to the North American Datum of 1983 

(NAD 83).

Altitude, as used in this report, refers to distance above a vertical datum.

Specific conductance is given in microsiemens per centimeter at 25 degrees Celsius 

(μS/cm at 25 °C).

Concentrations of chemical constituents in water are given either in milligrams per liter (mg/L) 

or micrograms per liter (μg/L).
x

Acronyms and Abbreviations

acfblusur1 Apalachicola-Chattahoochee Flint River Basin urban land-use study

acfblusur2 Apalachicola-Chattahoochee Flint River Basin urban land-use study

Al aluminum

albesus8 Albemarle-Pamlico Drainages major aquifer study

AMCL alternative maximum contaminant level

ANOVA analysis-of-variance

Ba barium

C carbon

Ca calcium

Cl chloride

CLSD clastic sedimentary rocks

CLSDF feldspar-rich clastic sedimentary rocks

CLSDLAC clastic lacustrine/evaporite sedimentary rocks

CLSDMT metamorphosed clastic sedimentary rocks

CLSDQ quartz-rich sedimentary rocks

CZmd Cambrian/Late Proterozoic metamudstone and meta-argillite

CZms Cambrian/Late Proterozoic mica schist

CZph Cambrian/Late Proterozoic phyllite and schist

delrsus1 Delaware River Basin major aquifer study

DO dissolved oxygen

Fe iron

GEOLEX USGS National Geologic Map Database Geologic Names Lexicon 

GIS geographic information system

H hydrogen

HHB human health benchmark

HBSL health-based screening level

HFO hydrous ferric oxide

HCRL highest common reporting limit

IGMTF felsic igneous and metamorphic rocks

IGMTI intermediate igneous or metamorphic rocks

IGMTM mafic igneous and metamorphic rocks

K potassium

KANA Kanawha/New River Basins

Lc critical level of detection
xi

linjsus3 Long Island/New Jersey Study Unit major aquifer study

lsussus2 Lower Susquehanna River Basin agricultural land-use study

MCL maximum contaminant level

Mg magnesium

μg/L micrograms per liter

mg/L milligrams per liter

Mn manganese

MTQ quartz-rich metamorphic rocks

Na Sodium

NAD83 North American Datum of 1983

NAVD88 North American Vertical Datum of 1988

NAWQA U.S. Geological Survey National Water-Quality Assessment Program

NJDEP New Jersey Department of Environmental Protection 

NLCD 1992 National Land Cover Dataset 1992

NURE U.S. Geological Survey National Uranium Resource Evaluation Program

NWQL U.S. Geological Survey National Water Quality Laboratory

OHSU Oregon Health and Science University

O oxygen

P phosphorus

PBR Piedmont and Blue Ridge 

PC principal components

PCA principal components analysis

Ra radium

S sulfur

SC specific conductance

Si silica

SI mineral saturation index

SMCL secondary maximum contaminant level

SSMDC sample-specific minimum-detection concentration

TDS total dissolved solids

ULMAF ultramafic rocks

USEPA U.S. Environmental Protection Agency

USGS U.S. Geological Survey

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 

Abstract

Groundwater quality and aquifer lithologies in the Piedmont and Blue Ridge Physiographic Provinces in the eastern 

United States vary widely as a result of complex geologic 

history. Bedrock composition (mineralogy) and geochemical 

conditions in the aquifer directly affect the occurrence (presence in rock and groundwater) and distribution (concentration 

and mobility) of potential naturally occurring contaminants, 

such as arsenic and radionuclides, in drinking water. To 

evaluate potential relations between aquifer lithology and 

the spatial distribution of naturally occurring contaminants, 

the crystalline-rock aquifers of the Piedmont and Blue Ridge 

Physiographic Provinces and the siliciclastic-rock aquifers of 

the Early Mesozoic basin of the Piedmont Physiographic Province were divided into 14 lithologic groups, each having from 

1 to 16 lithochemical subgroups, based on primary rock type, 

mineralogy, and weathering potential. Groundwater-quality 

data collected by the U.S. Geological Survey (USGS) National 

Water-Quality Assessment (NAWQA) Program from 1994 

through 2008 from 346 wells and springs in various hydrogeologic and land-use settings from Georgia through New Jersey 

were compiled and analyzed for this study. Analyses for most 

constituents were for filtered samples, and, thus, the compiled 

data consist largely of dissolved concentrations. Concentrations were compared to criteria for protection of human health, 

such as U.S. Environmental Protection Agency (USEPA) 

drinking water maximum contaminant levels and secondary 

maximum contaminant levels or health-based screening levels 

developed by the USGS NAWQA Program in cooperation 

with the USEPA, the New Jersey Department of Environmental Protection, and Oregon Health & Science University. Correlations among constituent concentrations, pH, and oxidationreduction (redox) conditions were used to infer geochemical 

controls on constituent mobility within the aquifers. 

Of the 23 trace-element constituents evaluated, arsenic, 

manganese, and zinc were detected in one or more water 

samples at concentrations greater than established human 

health-based criteria. Arsenic concentrations typically were 

less than 1 microgram per liter (μg/L) in most groundwater 

samples; however, concentrations of arsenic greater than 

1 μg/L frequently were detected in groundwater from clastic 

lacustrine sedimentary rocks of the Early Mesozoic basin 

aquifers and from metamorphosed clastic sedimentary rocks 

of the Piedmont and Blue Ridge crystalline rock aquifers. 

Groundwater from these rock units had elevated pH compared 

to other rock units evaluated in this study. Of the nine samples 

for which arsenic concentration was greater than 10 μg/L, six 

were classified as oxic and three as anoxic, and seven had pH 

of 7.2 or greater. Manganese concentrations typically were 

less than 10 μg/L in most samples; however, 8.3 percent of 

samples from the Piedmont and Blue Ridge crystalline-rock 

aquifers and 3.0 percent of samples from the Early Mesozoic 

basin siliciclastic rock aquifers had manganese concentrations 

greater than the 300-μg/L health-based screening level. The 

positive correlation of manganese with iron and ammonia and 

the negative correlation of manganese with dissolved oxygen 

and nitrate are consistent with the reductive dissolution of 

manganese oxides in the aquifer. Zinc concentrations typically 

were less than 10 μg/L in the groundwater samples considered 

in the study, but 0.4 percent and 5.5 percent of the samples had 

concentrations greater than the health-based screening level of 

2,000 μg/L and one-tenth of the health-based screening level, 

respectively. The mean rank concentration of zinc in groundwater from the quartz-rich sedimentary rock lithologic group 

was greater than that for other lithologic groups even after 

eliminating samples collected from wells constructed with 

galvanized casing.

Approximately 90 percent of 275 groundwater 

samples had radon-222 concentrations that were greater 

than the proposed alternative maximum contaminant level 

of 300 picocuries per liter. In contrast, only 2.0 percent 

of 98 samples had combined radium (radium-226 plus 

radium-228) concentrations greater than the maximum 

contaminant level of 5.0 picocuries per liter, and 0.6 percent 
2 Contaminants in Crystalline-Rock Aquifers and Siliciclastic-Rock Aquifers, Eastern United States, 1994–2008

of 310 samples had uranium concentrations greater than the 

maximum contaminant level of 30 μg/L. Radon concentrations 

were highest in the Piedmont and Blue Ridge crystalline-rock 

aquifers, especially in granite, and elevated median concentrations were noted in the Piedmont Early Mesozoic basin 

aquifers, but without the extreme maximum concentrations 

found in the crystalline rocks (granites). Although the siliciclastic lithologies had a greater frequency of elevated uranium 

concentrations, radon and radium were commonly detected 

in water from both siliciclastic and crystalline lithologies. 

Uranium concentrations in groundwater from clastic sedimentary and clastic lacustrine/evaporite sedimentary lithologic 

groups within the Early Mesozoic basin aquifers, which had 

median concentrations of 3.6 and 3.1 μg/L, respectively, 

generally were higher than concentrations for other siliciclastic lithologic groups, which had median concentrations less 

than 1 μg/L. Although 89 percent of the 260 samples from 

crystalline-rock aquifers had uranium concentrations less than 

1 μg/L, 0.8 percent had uranium concentrations greater than 

the 30-μg/L maximum contaminant level, and 6.5 percent had 

concentrations greater than 3 μg/L. 

Introduction

Since its inception in 1991, the U.S. Geological Survey (USGS) National Water-Quality Assessment (NAWQA) 

Program has collected and reported information about waterquality conditions and changes in those conditions over time. 

From 1991 to 2001 (Cycle I), the NAWQA Program was 

focused on describing water-quality conditions within 51 

major river basins across the United States. Interdisciplinary 

assessments of water chemistry, hydrology, land use, stream 

habitat, and aquatic life established a baseline understanding of water-quality conditions within the 51 river basins and 

aquifers, referred to as study units (Gilliom and others, 1995). 

A major focus of the NAWQA Program during its second 

decade (2002–13, Cycle II) is on regional- and national-scale 

assessments of groundwater-quality status and trends in 

about one-third of the 62 principal aquifers identified by the 

USGS Office of Groundwater (U.S. Geological Survey, 2003;

Lapham and others, 2005). A goal of the Cycle II NAWQA 

Regional Assessments of Principal Aquifers is to address the 

effects of natural features on water quality of major aquifers, 

including soil, geology, mineral composition, and geochemistry, especially oxidation-reduction potential (oxygen-reducing, 

nitrate-reducing, manganese-reducing, and iron-reducing 

conditions) (Lapham and others, 2005). 

This report focuses on naturally occurring contaminants 

in groundwater of the Piedmont and Blue Ridge crystallinerock aquifers and the Early Mesozoic basin siliciclastic-rock 

aquifers (fig. 1). The Piedmont and Blue Ridge crystallinerock aquifers are categorized as fractured igneous and metamorphic rock aquifers, and the Early Mesozoic basin aquifers 

are categorized as fractured sandstone aquifers (U.S. Geological Survey, 2003).

As a precursor to this study, Lindsey and others (2006) 

combined groundwater-quality analyses from 11 NAWQA 

Program studies in the Piedmont Physiographic Province to 

present findings on the occurrence of anthropogenic contaminants and naturally occurring radon. Lindsey and others 

(2006) categorized the aquifer lithologies into three groups: 

carbonate rock, crystalline rock, and siliciclastic rock. Compared to the carbonate-rock aquifers, the crystalline-rock and 

siliciclastic-rock aquifers had higher concentrations of radon 

and associated characteristics that implied naturally occurring 

contaminants could be widespread. The current report is a 

continuation of the effort by Lindsey and others (2006), evaluating the effects of lithologies and geochemical environment 

on the occurrence and distribution of radon and other naturally 

occurring contaminants in groundwater from the crystallinerock and siliciclastic-rock aquifers in the Piedmont and Blue 

Ridge Physiographic Provinces. 

Description of Study Area

The Piedmont and Blue Ridge Physiographic Provinces 

extend from Alabama to New York in the eastern United States 

(Fenneman, 1938; Fenneman and Johnson, 1946) and have 

a collective area of 154,000 square miles (mi2) (fig. 1). This 

region includes three principal aquifers: (1) the Piedmont 

and Blue Ridge crystalline-rock aquifers; (2) the Piedmont 

and Blue Ridge carbonate-rock aquifers; and (3) the Piedmont Early Mesozoic clastic sedimentary rock aquifers, also 

referred to as “sandstone” aquifers (U.S. Geological Survey, 

2003). In the study area, groundwater generally occurs in aquifer units that are local in scale because of complex geologic 

and topographic controls, but that are similar on a regional 

scale on the basis of major bedrock type and hydrogeologic 

properties. The aquifers are affected locally by geologic factors such as lithology and structure, particularly features such 

as fractures and joints, which provide secondary permeability 

for water movement within these “fractured-rock” aquifers. 

The study area discussed in this report includes parts of 

14 NAWQA study units (see “Network Code,” table 1) where 

groundwater-quality data were collected for major aquifer, 

land-use, or drinking water studies in the Piedmont and Blue 

Ridge crystalline-rock and Piedmont Early Mesozoic basin 

siliciclastic-rock aquifers (table 1 and fig. 2). Regional and 

national assessments of the effects on water quality from 

natural environmental factors and human activities are possible because of the application of a consistent study design 

and uniform methods of data collection and analysis within 

the NAWQA Program (Gilliom and others, 1995; Lapham and 

others, 2005). The study area includes several large metropolitan areas (Atlanta, Ga., Charlotte, N.C., Raleigh, N.C., 

Richmond, Va., Washington, D.C., Baltimore, Md., Philadelphia, Pa., and Trenton, N.J.), rural or forested areas in the 

southern and mid-Atlantic sections, and agricultural areas in 

the northeast section (fig. 2). The geologic setting of the area is 

complex and includes a history of deposition, metamorphism, 

igneous intrusion, and extensive folding and faulting; a wide 
Introduction 3

40°

35°

Base from U.S. Bureau of the Census, 1990, 1:500,000 to 1:5,000,000 and Statistics Canada digital data, 2006

Albers Equal-Area Conic projection: Standard Parallels 29°30' N and 45°30' N,

Central Meridian 96°00' W, Latitude of Origin 23°00' N

National Elevation Data from U.S. Geological Survey, 1999

Principal Aquifers from U.S. Geological Survey, 2003

TENNESSEE

ALABAMA

GEORGIA

SOUTH CAROLINA

NORTH CAROLINA

WEST VIRGINIA VIRGINIA

MARYLAND

NEW

JERSEY

PENNSYLVANIA

NEW YORK

KENTUCKY

0 50 100 KILOMETERS 25

0 50 100 MILES 25

OHIO

INDIANA

MICHIGAN

DELAWARE

Piedmont

Blue

Ridge

ATLANTIC

OCEAN

EXPLANATION

PRINCIPAL AQUIFER NAMES

Early Mesozoic basin siliclastic-rock aquifers

Piedmont and Blue Ridge carbonate-rock aquifers

Piedmont and Blue Ridge crystalline-rock aquifers

Physiographic boundary between Piedmont and Blue Ridge

Aquifers in the Piedmont and Blue Ridge

85° 80° 75°

Figure 1. Boundaries for the three principal aquifers within the Piedmont and Blue Ridge Physiographic Provinces.
4 Contaminants in Crystalline-Rock Aquifers and Siliciclastic-Rock Aquifers, Eastern United States, 1994–2008 Table 1. NAWQA study units in the Piedmont and Blue Ridge Physiographic Provinces. [PIED-CRY, Piedmont Physiographic Province crystalline-rock aquifer; PIED-EMZ, Piedmont Physiographic Province Early Mesozoic rock aquifer; BR-CRY, Blue Ridge Physiographic Province crystalline-rock aquifer] Study unit name Study unit code Network code Aquifer Study type Number of ground water samples Year(s) sampled State(s) sampled Apalachicola-Chattahoochee- Flint River Basin acfb acfblusur1 PIED-CRY Urban land-use study - shallow regolith wells only 21 1994–1995 Georgia Apalachicola-Chattahoochee- Flint River Basin acfb acfblusur2 PIED-CRY Urban land-use study - springs only 19 1994–1995 Georgia Albemarle-Pamilico Drainages albe albesus8 PIED-CRY Major aquifer study 55 2007 North Carolina; Virginia Deleware River Basin delr delrsus1 PIED-EMZ Major aquifer study 30 1999 New Jersey; Pennsylvania Kanawha/New River Basins kana kanasus2 BR-CRY Major aquifer study 30 1997 North Carolina; Virginia Long-Island/New Jersey Coastal Drainages linj linjsus3 PIED-EMZ Major aquifer study 21 1998 New Jersey Lower Susquehanna River Basin lsus lsussus2 PIED-CRY Agricultural land-use study 30 1994 Maryland; Pennsylvania Potomac River Basin and Delmarva Penninsula podl podldwgs1 PIED-CRY Drinking water study 15 2003 Maryland; Virginia Potomac River Basin and Delmarva Penninsula podl podllusrc1 PIED-CRY Urban land-use study - shallow wells only 30 2003 Maryland; Virginia Potomac River Basin and Delmarva Penninsula podl podlreffo2 PIED-CRY Urban land-use study - reference network shallow wells 2 2003; 2005 Maryland; Virginia Potomac River Basin poto potosus1 PIED-CRY Major aquifer study 25 1994 Maryland; Virginia Potomac River Basin poto potosus2 PIED-EMZ Major aquifer study 23 1994 Maryland; Pennsylvania; Virginia Santee River Basin and Coastal Drainages sant santdwgs1 PIED-CRY Drinking water study 15 2008 South Carolina

Santee River Basin and

Coastal Drainages sant santsus3 PIED-CRY Major aquifer study 30 1998 North Carolina; South Carolina

TOTAL 346
Introduction 5

85° 80° 75°

40°

35°

Base from U.S. Bureau of the Census, 1990, 1:500,000 to 1:5,000,000 and Statistics Canada digital data, 2006

Albers Equal-Area Conic projection: Standard Parallels 29°30' N and 45°30' N,

Central Meridian 96°00' W, Latitude of Origin 23°00' N

Land cover digital data from Nakagaki and others, 2007

TENNESSEE

ALABAMA

GEORGIA

SOUTH CAROLINA

NORTH CAROLINA

VIRGINIA

WEST VIRGINIA

MARYLAND

NEW

JERSEY

PENNSYLVANIA

NEW YORK

KENTUCKY

0 50 100 KILOMETERS 25

0 50 100 MILES 25

OHIO

INDIANA

MICHIGAN

DELAWARE

Piedmont

Blue

Ridge

ATLANTIC

OCEAN

Roanoke

Rocky Mount

Richmond

Trenton

Columbia

EXPLANATION

LAND COVER

Agriculture

Urban

Physiographic boundary between

 Piedmont and Blue Ridge

Aquifers in the Piedmont and Blue Ridge

Forest

Other

Atlanta

Macon

Asheville

Greenville

Charlotte

Winston-Salem

Greensboro Durham

Raleigh

Washington,

D.C.

Baltimore

Wilmington

Philadelphia

New York

Figure 2. 2007 land-use data for the Piedmont and Blue Ridge Physiographic Provinces and major metropolitan areas.
6 Contaminants in Crystalline-Rock Aquifers and Siliciclastic-Rock Aquifers, Eastern United States, 1994–2008

variety of bedrock lithologies ranging from quartz sandstone 

to black shale and from felsic-rich granite to ultramafic metamorphic rocks; and many rock types of intermediate composition, such as siltstones, gneisses, and schists. The weathering 

of minerals within the rock and the geochemical conditions 

within the aquifer directly affect groundwater quality in this 

region. Thus, in order to assess conditions in the aquifer and 

predict areas where specific contaminants may affect drinking water sources, it is important to understand the factors 

that control the release and transport of naturally occurring 

contaminants in groundwater. 

Purpose and Scope

This report contains the results of an evaluation of the 

potential occurrence and distribution of naturally occurring 

contaminants in groundwater in the Piedmont and Blue Ridge 

crystalline-rock aquifers and the Piedmont Early Mesozoic 

basin siliclastic-rock aquifers in the eastern United States 

(fig. 1). Descriptions of the primary rock type and associated 

mineral assemblages for mapped bedrock in the region were 

used to delineate mappable lithologic groups and lithochemical subgroups that could influence groundwater quality. The 

focus is on trace elements, radionuclides, nutrients, and 

major ions in groundwater that have potential for human 

health effects when present at concentrations approaching or 

exceeding drinking water standards or other human health 

benchmarks. 

Water-quality data collected as part of the NAWQA 

Program from 1994 through 2008 from 346 wells and springs 

in various hydrogeologic and land-use settings from Georgia 

through New Jersey were compiled and analyzed for this 

regional assessment. The sampled sites are from the following 

NAWQA study units (table 1): Apalachicola-Chattahoochee 

Flint River Basins (acfblusur1 and acfblusur2), AlbemarlePamlico Drainages (albesus8), Delaware River Basin (delrsus1), Long Island/New Jersey Coastal Drainages (linjsus3), 

Lower Susquehanna River Basin (lsussus2), Potomac River 

Basin and Delmarva Peninsula (podldwgs1, podllusrc1, podlreffo2, potosus1, and potosus2), and Santee Basin and Coastal 

Drainages (santsus3 and sandwgs1) (fig. 3 and table 1).

Graphical and statistical techniques were used to compare 

constituent concentrations in groundwater to human health 

criteria for drinking water and to compare concentrations 

among different lithologies. Interactions between groundwater and lithologies in various settings were inferred through 

comparison of major and trace-ion chemistry. Implications of 

observations and conclusions for regional mapping of risk for 

elevated arsenic, radionuclides, and other naturally occurring 

contaminants on the basis of geologic mapping are discussed. 

Results and interpretations are compared to those from studies 

conducted by other Federal and State agencies throughout 

the Piedmont and Blue Ridge crystalline-rock and Piedmont 

Early Mesozoic siliclastic-rock aquifer systems in the eastern 

United States. 

Geologic Setting and Aquifer

Descriptions

The geologic setting within the Piedmont and Blue Ridge 

(PBR) Physiographic Provinces of the eastern United States 

(fig. 1) is complex as a result of diverse geologic factors 

including rock type, metamorphic and structural history, and 

weathering. Bedrock in the study area has undergone multiple 

periods of metamorphism, deformation, including folding 

and faulting resulting in superimposed fracturing, and igneous intrusion that has altered mineralogy as well as inherent 

bedrock fabric (foliation). These mineralogical and structural 

characteristics directly affect groundwater and surface-water 

flow, recharge to the aquifer system, and geochemical processes within the aquifer (Cranford and others, 1982). 

Regional geologic units in the PBR Physiographic Provinces historically have been grouped into northeast-trending 

“belts” that described common assemblages of rock types 

grouped by metamorphic facies and broad structural features 

(North Carolina Geological Survey, 1985). More recently, 

Hibbard and others (2006) produced a lithotectonic map of the 

Appalachian Orogen in the eastern United States, grouping 

mapped units into “terranes” that better reflect geologic structural history. The geologic setting of the southern Blue Ridge 

and western Piedmont Physiographic Provinces from Virginia 

to Alabama is described in Clark (2008). Major rock types 

are described in terms of their respective geologic history, 

including depositional setting, metamorphism, and structural 

characteristics. Geologic structural terranes and rock assemblages typically strike northeast-southwest.

The Blue Ridge Physiographic Province, in the western 

part of the study area, is characterized by high relief, with 

mountain peaks rising more than 6,000 feet (ft) above the 

North American Vertical Datum of 1988 (NAVD 88) with 

steep slopes, and valleys generally near 2,000 ft in altitude. 

The Blue Ridge primarily is underlain by metamorphic and 

igneous crystalline rocks; however, some sedimentary rocks 

also are present in Maryland, Pennsylvania, Virginia, and 

North Carolina. The Piedmont Physiographic Province in 

the eastern part of the study area is characterized by a more 

subdued topography, with gently rolling hills and valleys 

and land-surface altitudes ranging from about 300 to 1,500 ft 

above NAVD 88. The Piedmont can be subdivided topographically into lowland and upland areas, where lowlands generally 

are underlain by carbonate rocks (Pennsylvania, Maryland, 

and New Jersey), and by clastic sedimentary rocks in the Early 

Mesozoic rift basins, as shown in a conceptual 

diagram (fig. 4). 

Trapp and Horn (1997) describe bedrock aquifers in the 

PBR Physiographic Provinces in North Carolina, Virginia, 

Maryland, Delaware, Pennsylvania, and New Jersey as being 

dense and almost impermeable, yielding groundwater primarily from secondary porosity and permeability provided by 

fractures. Except for the carbonate rocks, which contain solution openings, joints and fractures are the only openings that 
Geologic Setting and Aquifer Descriptions 7

40°

35°

Base from U.S. Bureau of the Census, 1990, 1:500,000 to 1:5,000,000 and Statistics Canada digital data, 2006

Albers Equal-Area Conic projection: Standard Parallels 29°30' N and 45°30' N,

Central Meridian 96°00' W, Latitude of Origin 23°00' N

National Elevation Data from U.S. Geological Survey, 1999

Principal Aquifers from U.S. Geological Survey, 2003

TENNESSEE

ALABAMA

GEORGIA

SOUTH

CAROLINA

NORTH CAROLINA

VIRGINIA

MARYLAND

NEW

JERSEY

PENNSYLVANIA

NEW YORK

KENTUCKY

0 50 100 KILOMETERS 25

0 50 100 MILES 25

DELAWARE

ATLANTIC

OCEAN

EXPLANATION

PRINCIPAL AQUIFER NAMES

STUDY UNIT and NETWORK CODE ABBREVIATIONS

Early Mesozoic basin siliclastic-rock aquifers

Piedmont and Blue Ridge carbonate-rock aquifers

Piedmont and Blue Ridge crystalline-rock aquifers

NAWQA study unit boundaries

Sample locations

OHIO WEST

INDIANA VIRGINIA

ACFB

ALBE

DELR

KANA

LINJ

LSUS

PODL

SANT

albesus8

santdwgs1

Appalachicola-Chattahoochee-Flint River Basins

Albemarle-Pamlico Drainages

Delaware River Basin

Kanawha/New River Basins

Long Island/New Jersey Study Unit

Lower Susquehanna River Basin

Potomac River Basin and Delmarva Peninsula

 (also referred to as POTO - Potomac River Basin)

Santee Basin and Coastal Drainages

Albemarle Piedmont Major Aquifer Study

Santee Drinking Water Study

albesus8

albesus8

albesus8

santdwgs1

ALBE

albesus8

85° 80° 75°

ACFB

(acfblusur1/

acfblusur2)

KANA

(kanasus2)

DELR

(delrsus1)

LSUS

(lsussus2)

PODL

(podldwgs1/

podllusrc1/

podlreffo2/

potosus1/

potosus2)

LINJ

(linjsus3)

SANT

(santsus3)

Figure 3. U.S. Geological Survey National Water-Quality Assessment (NAWQA) Program study unit boundaries within the Piedmont 

and Blue Ridge Physiographic Provinces and sample-collection locations for groundwater-quality data collected from 1994–2008.
8 Contaminants in Crystalline-Rock Aquifers and Siliciclastic-Rock Aquifers, Eastern United States, 1994–2008

Spring

studies

Piedmont and

Blue Ridge

crystalline-rock

aquifers

Piedmont and

Blue Ridge

crystalline-rock

aquifers

Early Mesozoic

basin aquifers Piedmont

carbonate-rock

aquifers

Agricultural

land-use studies

Major aquifer and 

urban land-use studies

Major aquifer and 

urban land-use studies

Major aquifer studies

Domestic and

monitoring wells

Domestic wells

Domestic wells

Domestic and

monitoring wells

Figure 4. The conceptual groundwater system, aquifers, and typical well types included as part of this study within the Piedmont and 

Blue Ridge Physiographic Provinces.

store and transmit water. Despite this description, many wells 

completed in fractured bedrock aquifers of the PBR are sufficiently productive to be the main source of local water supplies. The carbonate rock units of the Piedmont typically are 

the most productive of the bedrock aquifers but are not widely 

distributed in the region, occurring primarily in Maryland, 

Pennsylvania, and New Jersey. The PBR bedrock aquifers in 

northeastern Pennsylvania and northern New Jersey locally 

are covered by glacial deposits, which include productive 

sand and gravel of the surficial aquifer system. Three principal bedrock aquifers underlie the northern PBR Provinces, in 

order of decreasing area: crystalline-rock and undifferentiated 

sedimentary-rock aquifers, clastic sedimentary-rock aquifers 

in Early Mesozoic basins, and carbonate-rock aquifers. 

Miller (1990) describes the geologic setting of the PBR 

as consisting of many different types of metamorphic and 

igneous rocks that are complexly related. Main rock types 

are varieties of gneiss and schist and extremely fine-grained 

rocks such as phyllite and metamorphosed volcanic tuff and 

ash. Most of the metamorphic rocks were originally sediments 

(metasediments), but some were originally igneous or volcanic materials (metagranites, granite gneiss, metavolcanics). 

The degree of heat and pressure to which the original rocks 

were subjected during metamorphism, as well as the degree of 

structural deformation (principally folding, foliation, jointing, 

and shearing) that they have undergone, has determined the 

final texture and mineralogy of the rocks. Most of the rocks 

have undergone several periods of metamorphism. The metamorphic rocks have been intruded by large to small bodies 

of igneous rock that vary in composition from felsic (lightcolored rocks that contain large quantities of silica as quartz 

and potassium as potassium-feldspar) to mafic (dark-colored 

rocks that contain large quantities of ferromagnesian minerals such as pyroxene, amphibole, or ferrous phyllosilicate 

mineral groups). Large igneous intrusions consist of granite, 

quartz monzonite, and gabbro that occur in plutons that cover 

many tens of square miles. Smaller igneous intrusions, such as 

dikes and sills, include felsic and mafic rocks, such as syenite, 

andesite, diabase, and pegmatite. Rocks in this region are 

displaced by several major fault zones, some of which extend 
Geologic Setting and Aquifer Descriptions 9

for hundreds of miles. Shearing along large fracture zones has 

produced siliceous, intensely fractured rocks, such as mylonite 

or phyllonite (Miller, 1990).

Trapp and Horn (1997) describe aquifers in Early Mesozoic basins that primarily are in three major basins—the Newark basin in New Jersey and Pennsylvania is the largest and 

is the basin from which the most groundwater is withdrawn; 

the Gettysburg basin of Pennsylvania and Maryland is second 

largest; and the Culpeper basin of Virginia is third largest. The 

Richmond basin in Virginia and the Dan River/Danville basin 

in Virginia and North Carolina are of intermediate size. Nine 

small early Mesozoic basins are located in Virginia. Sedimentary rocks in the basins consist predominately of interbedded shale, sandstone, and siltstone, all typically red, reddish 

brown, or maroon, but locally gray or black. Conglomerate, 

dolomite, lacustrine black mudstone, and coal are present 

locally. In many places, the sedimentary rocks are interbedded with basalt flows or have been intruded by diabase dikes 

and sills. Thicknesses of Triassic and Jurassic rocks in the 

large basins have been calculated to be more than 20,000 ft. 

Additional basins in North Carolina, including the Durham, 

Sanford, Wadesboro, and Davie County basins (Lindsey and 

others, 2006, fig. 10), were not considered to be important 

aquifers because they are more compact and more tightly 

cemented than those in the basins to the north and do not yield 

sufficient quantities of water to be considered 

principal aquifers. 

Physically, the aquifer system in the PBR can be composed of one (bedrock only), two (regolith-bedrock), or three 

parts (regolith, transition zone, and bedrock), depending upon 

whether or not weathered material (regolith) overlies the bedrock and if a local transition zone has developed between the 

shallow regolith and deeper bedrock. In the Blue Ridge Physiographic Province, very little, if any, regolith may be present, 

and, if present, it is often debris flow, colluvium, or alluvium 

in stream valleys. In the Piedmont Physiographic Province, 

generally there is at least a two-part system with the weathered 

regolith overlying fractured bedrock as described by Heath 

(1984). The regolith consists of soil, residuum, saprolite, and 

possibly colluviums, alluvium, and debris flow. Residuum and 

saprolite are formed from in situ weathering of the bedrock 

and form a blanket of decomposed or partially decomposed 

rock that is usually thick and clayey. Saprolite retains the 

texture and structure of the parent bedrock from which it is 

derived. A third component, a transition zone between the 

regolith and bedrock (Harned and Daniel, 1992), has been 

more commonly delineated in recent years. The regolith is 

more porous and more permeable than the underlying bedrock 

everywhere. Because the crystalline bedrock formed under 

intense heat and pressure during metamorphism and igneous 

intrusion, the bedrock has few primary pore spaces, and the 

porosity and permeability of the unweathered and unfractured 

bedrock is extremely low (Miller, 1990). Groundwater in 

the bedrock is stored in and moves through secondary fractures and other discontinuities, which form the only effective 

porosity in the unweathered rock. Because of the absence of 

substantial water storage (storativity) in the unweathered rock, 

a large amount of groundwater is found in the weathered and 

slightly porous overlying saprolite. Water slowly drains from 

the saprolite to the fractures in the underlying and hydraulically connected bedrock. Although there are considerable differences in the mineralogy, texture, and structure of the rocks 

composing the PBR aquifers, the overall hydraulic characteristics of the aquifers are similar in a regional context 

(Miller, 1990).

Previous Studies

Data on naturally occurring and manmade contaminants 

in groundwater in the PBR have been collected and reported 

through the NAWQA Program and other USGS studies, some 

of which are summarized in this section. Table 2 lists pertinent 

reports published for these studies, constituents analyzed, 

and major findings relevant to this study. Results from a few 

selected studies that are applicable to this report are summarized in this section.

One of the larger NAWQA data compilations for the 

Piedmont Physiographic Province was reported by Lindsey 

and others (2006) in a summary of groundwater-quality data 

collected from 11 of the 14 NAWQA studies listed in table 1 

(all except albesus8, kanasus2, and santdwgs1). Lindsey and 

others (2006) grouped samples from 225 wells and 19 springs 

into 3 aquifer types: crystalline, siliciclastic, and carbonate. 

While the report focused on the detection and occurrence of 

anthropogenic contaminants, selected naturally occurring contaminants also were evaluated. Concentrations of radon were 

higher in areas underlain by felsic crystalline rocks and lower 

in areas underlain by mafic crystalline rocks. Groundwater 

from adjacent siliciclastic-rock aquifers had concentrations of 

radon lower than those in samples from felsic crystalline-rock 

aquifers. Ninety percent of the 205 samples analyzed for radon 

had concentrations that exceeded the proposed maximum 

contaminant level (MCL) of 300 picocuries per liter (pCi/L), 

and 13 percent of samples had concentrations that exceeded 

the proposed alternative maximum contaminant level (AMCL) 

of 4,000 pCi/L (Lindsey and others, 2006). 

Fifty-five domestic wells in the Piedmont crystallinerock aquifers were sampled for a major aquifer study in the 

Albemarle-Pamlico (ALBE) drainages of North Carolina 

and Virginia in 2007–08 (fig. 3 albesus8, table 1). The data 

for NAWQA samples were combined with other USGS data 

collected in the Piedmont and Blue Ridge of North Carolina for comparison of analytical results for 79 groundwater 

samples within the statewide geologic belt units, or geozones 

(Harden and others, 2009) (table 2). Results from this study 

suggest that the cationic and anionic composition of groundwater from within a particular geozone reflected differences 

in lithologic setting, hydrologic and geochemical conditions, 

and (or) land-use effects. Exceedances in Federal and State 

drinking water standards or proposed standards were noted 

for radon, pH, manganese, iron, and zinc. Radon had the most 
10 Contaminants in Crystalline-Rock Aquifers and Siliciclastic-Rock Aquifers, Eastern United States, 1994–2008 Table 2. Published reports from USGS NAWQA studies and other data sources conducted in the Piedmont and Blue Ridge Physiographic Provinces categorized by aquifer and constituents investigated.—Continued [PDX, Piedmont crystalline-rock aquifer; EMZ, Early Mesozoic basin aquifer; BRX, Blue Ridge crystalline-rock aquifer; VOCs, volatile organic compounds; PODL, Potomac River/Delmarva Peninsula; POTO, Potomac River; LSUS, Lower Susquehanna River; LINJ, Long-Island/New Jersey Coastal Drainages; SANT, Santee River Basin and Coastal Drainages; ACFB, Apalachicola-Chattahoochee-Flint River Basin; DELR, Delaware River Basin; KANA, Kanawha-New River Basin; ALBE, Albemarle-Pamlico Drainages; USGS NC WSC, USGS North Carolina Water Science Center; USGS PA WSC, USGS Pennsylvania Water Science Center] Report Constituents studied Aquifers NAWQA study area Data sources Major findings relevant to current study USGS NAWQA Program Ator and Denis (1997) Nitrate, phosphorus PDX, EMZ PODL, POTO USGS NAWQA Ator and others (1998) Radon, nitrate, pesticides, VOCs EMZ POTO, LSUS USGS NAWQA Radon from the EMZ rocks are similar in concentration to the PDX rocks Ayers and others (2000) Arsenic EMZ LINJ USGS NAWQA Arsenic higher in Early Mesozoic rocks Ayotte and others (2007) Radon, uranium PDX, OTERS LINJ, DELR USGS NAWQA Uranium and radon higher in New England crystalline-rock aquifers Carter and others (2010) VOCs, pesticides PDX, OTERS SANT, PODL USGS NAWQA Denver and others (2011) Phosphorus, trace metals PDX, EMZ POTO, LSUS, ACFB, DELR, SANT, LINJ USGS NAWQA Phosphorus from natural sources reported in crystalline-rock aquifers with alkaline pH or iron-reducing conditions. Fischer and others (2004) Nitrate, pesticides, VOCs, radon, trace elements EMZ, OTERS DELR USGS NAWQA Arsenic and radon higher in Early Mesozoic rocks relative to other aquifer units Frick and others (1998) Pesticides, VOCs PDX, OTERS ACFB USGS NAWQA Hughes and others (2000) Nitrate, pesticides, VOCs, radon PDX, OTERS SANT (Santee Circular) USGS NAWQA Radon higher in crystalline-rock aquifers

Lapham and others (2005) Radon, uranium, trace

elements, VOCs,pesticides

PDX, BRX, 

OTERS

USGS NAWQA Uranium and radon higher in New England crystalline-rock aquifers
Geologic Setting and Aquifer Descriptions 11

Table 2. Published reports from USGS NAWQA studies and other data sources conducted in the Piedmont and Blue Ridge Physiographic Provinces categorized by aquifer and constituents investigated.—Continued[PDX, Piedmont crystalline-rock aquifer; EMZ, Early Mesozoic basin aquifer; BRX, Blue Ridge crystalline-rock aquifer; VOCs, volatile organic compounds; PODL, Potomac River/Delmarva Peninsula; POTO, Potomac River; LSUS, Lower Susquehanna River; LINJ, Long-Island/New Jersey Coastal Drainages; SANT, Santee River Basin and Coastal Drainages; ACFB, Apalachicola-Chattahoochee-Flint River Basin; DELR, Delaware River Basin; KANA, Kanawha-New River Basin; ALBE, Albemarle-Pamlico Drainages; USGS NC WSC, USGS North Carolina Water Science Center; USGS PA WSC, USGS Pennsylvania Water Science Center]

Report Constituents studied Aquifers NAWQA study area Data sources Major findings relevant to current study

USGS NAWQA Program—Continued

Kozar and others (2002) Radon PDX, BRX,

OTERS

KANA USGS NAWQA Noted high concentrations of radon in wells near fracture zones of crystalline rocks. 

Lindsey and Ator (1996) Radon EMZ, PDX,

OTERS

POTO, LSUS USGS NAWQA Noted high concentrations of radon in crystalline rocks; differences between concentrations 

in mafic and felsic lithologies.

Lindsey and others (1997) Nitrate PDX,

OTERS

LSUS USGS NAWQA

Lindsey and others (2006) Nitrate, pesticides,

VOCs, radon

EMZ, PDX,

OTERS

POTO, LSUS,

ACFB,DELR, SANT, LINJ

USGS NAWQA Radon higher in crystalline-rock aquifers

Paybins and others (2000) Radon, nitrate,

 pesticides, VOCs

BRX KANA USGS NAWQA Radon higher in crystalline-rock aquifers

Other data sources

Campbell (2006) Radon, radium,

uranium, trace 

elements

BRX, PRX NC DENR Radon elevated in meta-igneous rocks (granites and gneisses) compared to 

meta-sedimentary rocks.

Harden and others (2009) Radon, uranium, 

trace elements

PDBRX ALBE USGS NAWQA;

USGS NCWSC; NC DENR

Correlation of Geozones (geologic belts) with radon, uranium, and trace elements

Pippin (2005) Arsenic PDX NC DENR Elevated arsenic associate with geologic formations in the Carolina Slate Belt in North Carolina. Rock types including meta-mudstone, meta-argillite, phyllite, schist, and mica schist 

of volcanic origin were associated with elevated arsenic concentrations in groundwater.

Senior and Vogel (1992) Radium PDX PAWSC Elevated radium concentrations correlated to pH conditions lower than 4.7.
12 Contaminants in Crystalline-Rock Aquifers and Siliciclastic-Rock Aquifers, Eastern United States, 1994–2008 Table 2. Published reports from USGS NAWQA studies and other data sources conducted in the Piedmont and Blue Ridge Physiographic Provinces categorized by aquifer and constituents investigated.—Continued [PDX, Piedmont crystalline-rock aquifer; EMZ, Early Mesozoic basin aquifer; BRX, Blue Ridge crystalline-rock aquifer; VOCs, volatile organic compounds; PODL, Potomac River/Delmarva Peninsula; POTO, Potomac River; LSUS, Lower Susquehanna River; LINJ, Long-Island/New Jersey Coastal Drainages; SANT, Santee River Basin and Coastal Drainages; ACFB, Apalachicola-Chattahoochee-Flint River Basin; DELR, Delaware River Basin; KANA, Kanawha-New River Basin; ALBE, Albemarle-Pamlico Drainages; USGS NC WSC, USGS North Carolina Water Science Center; USGS PA WSC, USGS Pennsylvania Water Science Center] Report Constituents studied Aquifers NAWQA study area Data sources Major findings relevant to current study Other data sources—Continueed Senior and Sloto (2006) Arsenic EMZ PAWSC Elevated arsenic correlated to pH conditions higher than 8. Serfes (2004); Serfes and others (2010) Arsenic EMZ NJGS Elevated arsenic higher in EMZ black shale members of Lackatong and Passaic formations in New Jersey Sloto and Senior (1998) Radon PDX, EMZ USGS PAWSC Radon highest from areas underlain by a schist, phyllite, and quartzite rock types Sloto (2002) Radon PDX, EMZ USGS PAWSC Radon lowest in ultramafic serpentinite rocks and higher in the Wissahickon Schist. Sloto (2000) Uranium, radium, radon PDX, EMZ USGS PAWSC Radium elevated in the Chickies Quartzite formation. Radon higher in schist and quartzite rock types. 
Geologic Setting and Aquifer Descriptions 13

exceedances, with 61 of 69 wells sampled exceeding the U.S. 

Environmental Protection Agency’s (USEPA’s) proposed MCL 

of 300 pCi/L and with 18 of 69 sampled wells exceeding the 

USEPA’s AMCL (requires treatment) of 4,000 pCi/L of radon 

(U.S. Environmental Protection Agency, 2010). Fifty percent 

of samples collected from the felsic intrusive rock geozone 

had radon concentrations greater than the AMCL. Statistically 

different median concentrations of calcium, silica, ammonia, 

aluminum, antimony, cadmium, and uranium were delineated 

between one or more geozone pairs (Harden and others, 2009). 

The New England and Appalachian Piedmont region 

was highlighted for radon occurrence in groundwater by the 

reconnaissance assessment by the USEPA (U.S. Environmental Protection Agency, 1999). Radon and uranium occurrences 

in crystalline-rock aquifers in New York and New England 

were identified as an issue of concern by Lapham and others 

(2005) and Ayotte and others (2007). Granitic rocks, such as 

two-mica granites and other high-grade metamorphic rocks, 

were reported to be a source of uranium that is soluble under 

predominantly oxic to sub-oxic geochemical conditions. The 

median value of radon reported was 2,122 pCi/L from the New 

York and New England crystalline-rock aquifer group, with 

about 25 percent of samples exceeding the proposed AMCL 

(Ayotte and others, 2007).

Sloto (2000) presented the results of sampling groundwater from domestic wells in the Piedmont Physiographic 

Province in southeastern Pennsylvania for naturally occurring 

radionuclides, including uranium, radium-226, radium-228, 

and radon-222 (table 2). The results were analyzed according 

the underlying bedrock lithology, which included carbonate rock (limestone, dolomite, and marble), crystalline rock 

(gneiss, phyllite, quartzite, and schist), diabase, sedimentary 

rock of Paleozoic age (conglomerate, limestone, sandstone, 

siltstone, and shale), sedimentary rock of Triassic age (conglomerate, sandstone, siltstone, and shale), and unconsolidated sediments (clay, sand, and gravel). Of the more than 

250 samples analyzed for radium isotopes, 46 percent of 

the wells located in the Chickies Quartzite had elevated 

radium activities that exceeded the USEPA MCL of 5 pCi/L 

for combined radium (radium-226 and radium-228 combined). Elevated radium values were correlated with water 

samples having a pH of less than 4.7 (Senior and Vogel, 1992) 

(table 2). Twenty-three percent of the 170 wells sampled in the 

Chickies Quartzite also had gross alpha-particle activities in 

water that exceeded the 15-pCi/L MCL and 46 percent of the 

wells sampled in the Chickies Quartzite had combined radium 

concentrations exceeding the 5-pCi/L MCL (Sloto, 2000). 

Water samples from 33 percent of the wells in the Chickies Quartzite also had gross beta particle activity exceeding 

the 15-pCi/L MCL. Samples from 13 wells in the Chickies 

Quartzite and nearby geologic units contained concentrations 

of radium-224 (a short-lived daughter product of radium-228) 

up to 265 pCi/L and gross alpha-particle activities up to 

1,300 pCi/L (Senior and Sloto, 2000). Radon-222 activities 

differed among rock types, and of the more than 900 samples 

analyzed from the study area in southeastern Pennsylvania, 

the greatest median values were in the schist (2,500 pCi/L) 

and quartzite (2,300 pCi/L) rock types (Sloto, 2000). About 

89 percent of 534 wells sampled in 38 geologic formations 

in the Piedmont Physiographic Province in Chester County, 

southeastern Pennsylvania, had water with radon-222 concentrations greater than the proposed 300-pCi/L MCL that ranged 

up to 53,000 pCi/L (Senior, 1998).

Sloto (2002) described results from the analyses of 

64 well samples collected in the Big Elk Creek Basin in 

Chester County, Pennsylvania, and Cecil County, Maryland, 

from 1925 through 1999 (table 2). Samples were categorized 

based on surface lithologies consisting of the Peters Creek 

Schist, serpentinite, Wissahickon Schist, pegmatite, and pelitic 

schist. The groundwater from wells located in the Wissahickon 

Schist had the lowest median pH of 5.9, while the samples 

from wells in the serpentinite rock type had the highest values 

ranging from 7.8 to 8.5. Groundwater from the serpentinite 

was categorized as magnesium-bicarbonate type, compared 

to samples from the other lithologies, which had no dominant 

cation to correspond with the dominant anion, bicarbonate. 

The three groundwater samples from the serpentinite unit also 

had the lowest concentrations of radon-222, with a maximum 

activity of 392 pCi/L, while wells in the other lithologic units 

were notably higher; samples from the Wissahickon Schist 

had a median value of 2,500 pCi/L. An assessment of groundwater quality and its relation to lithology and land use based 

on analyses of water samples from 82 wells in the Red Clay 

Creek Basin in the Piedmont Physiographic Province of Pennsylvania and Delaware (a hydrogeologic setting similar to that 

of Elk Creek) also indicated that concentrations of barium, 

lithium, and radon-222 differed among lithologies; radon-222 

activities generally were highest (up to 10,000 pCi/L) in water 

from felsic gneiss and schist units and lowest in water from 

mafic gneiss and serpentinite units (Senior, 1996). 

Radon concentrations documented by Kozar and others 

(2001) (table 2) for the Kanawha/New River Basins (KANA) 

study unit in the Blue Ridge Physiographic Province of 

Virginia and North Carolina were similar to those reported by 

Sloto (2000, 2002) for Pennsylvania, Maryland, and Delaware. 

Kozar and others (2001) noted that radon was detected in concentrations exceeding the proposed USEPA 300-pCi/L MCL 

for radon in 26 of 30 (87 percent) wells sampled. In 10 of 30 

(33 percent) samples, radon exceeded the 4,000-pCi/L proposed AMCL. The median radon concentration detected was 

2,080 pCi/L, and the maximum concentration detected was 

30,900 pCi/L. Of 10 wells having radon concentrations greater 

than 4,000 pCi/L, 8 were on or adjacent to faults; this finding 

suggests that fault zones may be areas of uranium enrichment 

and that fault zones may allow radon migration upward along 

the fault (Kozar and others, 2001). 

Pippin (2005) (table 2) presented results from a database 

of more than 10,000 analytical results for arsenic concentrations from groundwater samples collected primarily from 

domestic wells across North Carolina. A probability analysis 

using indicator kriging techniques was applied to the georeferenced dataset. Spatial correlation between the zone having 
14 Contaminants in Crystalline-Rock Aquifers and Siliciclastic-Rock Aquifers, Eastern United States, 1994–2008

the highest probabilities for elevated arsenic concentrations in 

groundwater and rocks of the Carolina Slate Belt was evident. 

Common rock types associated with these areas of elevated 

arsenic were of volcanic origin, with the highest average 

arsenic concentrations estimated for the following lithologies: metamudstone and meta-argillite (CZmd; North Carolina 

Geological Survey, 1985), phyllite and schist (CZph, North 

Carolina Geological Survey, 1985), and mica schist (CZms, 

North Carolina Geological Survey, 1985) . 

Sources, mobilization, and transport of arsenic in groundwater in the Early Mesozoic basin aquifers of the Passaic 

and Lockatong Formations of the Newark basin, New Jersey, 

were documented by Serfes (2004) and Serfes and others 

(2010) (table 2). Elevated arsenic concentrations [greater than 

10 micrograms per liter (μg/L)] correlated with geochemical 

conditions, including dissolved oxygen (DO) less than 3 milligrams per liter (mg/L) and pH from 7.5 to 8.0. For concentrations of arsenic greater than 40 μg/L, DO was suboxic (less 

than 1.0 mg/L) or nearly suboxic. The major source of arsenic 

was determined to be the mineral pyrite (FeS2) within the 

black shale members of the Passaic Formation (Serfes, 2004).

An assessment of arsenic, boron, and fluoride in groundwater in the Newark basin in Pennsylvania included a review 

of available data (Senior and Sloto, 2006). About 10 percent 

of wells completed in the Early Mesozoic basin aquifers had 

water with arsenic concentrations greater than the MCL of 

10 μg/L. For data collected from 46 wells during that study, 

all groundwater samples with pH values greater than 8 had 

arsenic concentrations greater than 10 μg/L; no samples with 

pH below 7 had arsenic concentrations greater than 10 μg/L. 

Geological and Geochemical

Framework for Interpretations of 

Water Quality

Building on previous work linking aquifer lithology to 

the occurrence of radionuclides, arsenic, and other naturally 

occurring contaminants in groundwater, the primary purpose of this study was to determine if primary rock type and 

associated mineral assemblages described for published State 

geologic maps could be organized into mappable lithologic 

groups and lithochemical subgroups and related to the occurrence of natural contaminants in the crystalline-rock aquifers 

of the Piedmont and Blue Ridge Physiographic Provinces and 

the siliciclastic-rock aquifers in the Piedmont Physiographic 

Province. For this study, bedrock aquifers were divided into 

lithologic groups and lithochemical subgroups based on overall bedrock composition with regard to specific mineralogy 

and the potential for similar weathering characteristics. This 

classification of bedrock aquifer types follows the organization of lithochemical groups by McCartan and others (1998) 

and Peper and others (2001) in the Chesapeake Bay region of 

Maryland, Virginia, and the District of Columbia.

Previous Lithochemical Classifications

McCartan and others (1998) related regional geologic 

data (rock type and mineralogical characteristics) from geologic maps of Maryland and northern Virginia in the southern Chesapeake Bay watershed to water-quality data from 

shallow wells and streams collected from the region. The 

rock types within the region were first grouped with the three 

primary classes of rock—sedimentary, igneous, and metamorphic—then by acid-neutralizing capacity and weathering 

characteristics. Interest stemmed from the apparent mitigation 

of high-acidity surface- and groundwater-quality problems 

from contact with carbonate rock types and elevated nitrate 

problems in groundwater and surface water by rocks and 

sediments high in carbon and sulfur (peat and black shale). 

McCartan and others (1998) organized regional geologic 

map data within the southern Chesapeake Bay watershed into 

four groups: (1) “Sedimentary rocks and their metamorphic 

equivalents,” which included carbonate-rich rocks, clastic 

sedimentary rocks, and metamorphosed clastic sedimentary 

rocks; (2) “Igneous rocks and their metamorphic equivalents,” 

which included mafic igneous rocks and their metamorphic 

equivalents, ultramafic rocks, and felsic igneous rocks and 

their metamorphic equivalents; (3) “Unconsolidated sediments,” such as sands, silts, clays, and organic-rich deposits; 

and (4) “Iron-rich sediment,” such as greensand, magnetite 

and ferro-ilmenite beach sand, and bog iron ore. Lithologies 

in the study area were categorized into 30 lithologic-mineralogic equivalent, or “lithogeochemical,” units (McCartan and 

others, 1998). 

Peper and others (2001) modified the organization of 

McCartan and others (1998) to form three main geologic 

groups by including iron-rich sediments under the “Unconsolidated sediments” group, and then by classifying lithologies on 

the basis of potential rock-water interaction. Classes of rock 

types based on water-reactive minerals and their weathering 

reactions were regrouped by Peper and others (2001) into the 

following five classes of lithogeochemical units: (1) carbonate 

rocks and calcareous rocks and sediments, the most acid-neutralizing; (2) carbonaceous-sulfidic rocks and sediments, likely 

to be oxygen-depleting and reducing; (3) quartzofeldspathic 

rocks and siliciclastic sediments, mostly relatively weakly 

reactive with water; (4) mafic silicate rocks and sediments, 

likely to be oxygen consuming and high solute-load delivering; and (5) rare calcareous-sulfidic (carbonaceous) rocks that 

may be neutralizing and reducing.

Lithologic Groups

Fourteen lithologic groups (table 3) were delineated 

within the study area as an expansion of previous work by 

McCartan and others (1998) and Peper and others (2001) to 

southeastern and northeastern areas of the PBR Physiographic 

Provinces [(appendix 1, table 1-1 (appendix 1 available online 

at http://pubs.usgs.gov/sir/2013/5072/)]. The same principal
Geological and Geochemical Framework for Interpretations of Water Quality 15

Table 3. Lithologic group, major rock types, and lithochemical subgroups for groundwater sites within the siliciclastic-rock and crystalline-rock aquifers in the Piedmont and Blue Ridge Physiographic Provinces, 1994–2008.—Continued[Lithologic groups and lithochemical subgroups are described in detail in appendix table 1-1. Red font indicates units that lack groundwater-quality data. Na, not applicable]

Rock 

type Lithologic group Site count Abbreviation Principal lithologies Lithochemical subgroup number(s)

Carbonate Carbonate-rich rocks 0 na Limestone, dolostone, marble 11,12,13

Siliciclastic Clastic sedimentary rocks 9 CLSD Mudstone, shale 21, 21cSiliciclastic Quartz-rich sedimentary rocks 11 CLSDQ Conglomerate, sandstone 22, 22cSiliciclastic Clastic lacustrine/evaporite 

sedimentary rocks

51 CLSDLAC Argillite, fine-grained mixed clastic, mudstone, 

sandstone, shale, siltstone, arkose

22e

Siliciclastic Feldspar-rich clastic sedimentary rocks 0 na Arkose, graywacke 22fSiliciclastic Sulfidic clastic sedimentary rocks 0 na Black shale, coal 23s, 24sCrystalline Metamorphosed clastic sedimentary rocks 96 CLSDMT Slate, mica schist, pelitic schist, phyllite, quartz-feldspar

schist, schist, metasedimentary rock, meta-argillite, paragneiss, gneiss, melange

31, 31s, 32al, 32c, 32g, 32m, 

32s, 32u, 35, 35a, 35c,35gn, 35gns, 35ml, 41bs

Crystalline Quartz-rich metamorphic rocks 17 MTQ Meta-conglomerate, metasedimentary rock, quartzite 33, 33c, 33myCrystalline Felsic igneous rocks and their metamorphic 

equivalents

71 IGMTF Granite, quartz monzonite, tonalite, metamorphic rock, 

felsicmetavolcanic rock, metavolcanic rock, felsic volcanic rock,rhyolite, alkali syenite

61, 61c, 61m, 61mf, 61ml,

61mv, 61v, 62

Crystalline Intermediate igneous rocks and their 

metamorphic equivalents

55 IGMTI Biotite gneiss, gneiss, felsic gneiss 34agn, 34bg, 34f, 34fi, 34i, 

34s
16 Contaminants in Crystalline-Rock Aquifers and Siliciclastic-Rock Aquifers, Eastern United States, 1994–2008 Table 3. Lithologic group, major rock types, and lithochemical subgroups for groundwater sites within the siliciclastic-rock and crystalline-rock aquifers in the Piedmont and Blue Ridge Physiographic Provinces, 1994–2008.—Continued [Lithologic groups and lithochemical subgroups are described in detail in appendix table 1-1. Red font indicates units that lack groundwater-quality data. Na, not applicable] Rock type Lithologic group Site count Abbreviation Principal lithologies Lithochemical subgroup number(s) Crystalline Mafic Igneous rocks and their Metamorphic equivalents 34 IGMTM Amphibolite, meta-basalt, intermediate metavolcanic rock, diabase, gabbro, mafic rock, metamorphic rock, norite,quartz diorite, basalt, diabase, mafic gneiss 41, 41c, 41mv, 41v, 42, 43, 43em, 44 Crystalline Ultramafic rocks 2 ULMAF Metamorphic rock, serpentinite 50c Sediment Unconsolidated sediments 0 na Sand, silt, clay, gravel, terrace (undifferentiated), alluvium, gravel, sand 71c, 72, 73, 74, 75, 76, 78, 79, 80 Sediment Iron-Rich sediments 0 na Greensand, silty in places; magnetite and ferro-ilmenite beach sand; bog iron ore 77 Total number of samples 346
Geological and Geochemical Framework for Interpretations of Water Quality 17

aquifers are covered—the crystalline-rock aquifers of the 

Piedmont and Blue Ridge and the siliciclastic rocks in the 

Early Mesozoic aquifers of the Piedmont. As with McCartan 

and others (1998), the first step for this study was to translate information from each mapped geologic unit within the 

study area to lithologic-mineralogic equivalents based on 

descriptions for each State geologic map (see References 

Cited section and appendix 1, table 1-2). As a new approach 

for this study, further division of the geologic units under 

“Sedimentary rocks and their metamorphic equivalents” 

heading was made within the siliciclastic sedimentary rocks 

sequence by introducing clastic sedimentary (fine-grained), 

clastic lacustrine/evaporite sedimentary, feldspar-rich clastic sedimentary, quartz-rich sedimentary, and sulfidic clastic 

sedimentary as major lithologic groups in the study area 

(table 3 and appendix 1, table 1-1). The metamorphosed 

clastic sedimentary sequence was further divided in the study 

area by introduction of quartz-rich metamorphic and feldsparrich metamorphic lithologic groups. All other major lithologic 

groups used for this study followed McCartan and others 

(1998) (appendix 1, table 1-1). The 14 major lithologic groups 

delineated for this study and a listing of associated major 

rock types compiled from State geologic maps (Dicken and 

others, 2005a, 2005b; Nicholson and others, 2005, 2006) are 

described in table 3, based on classification schemes presented 

in appendix table 1-1. Representative geologic formations are 

listed in appendix table 1-1 and a detailed descriptions of geologic formations (including State abbreviations) are grouped 

by lithologic group and lithochemical subgroup number in 

appendix table 1-2. Abbreviations for the lithologic groups 

used in this report are listed in table 3 to simplify technical discussions and figure/table presentations. For example, 

the felsic igneous rocks and their metamorphic equivalents 

group is abbreviated as “IGMTF” (table 3). For more detailed 

descriptions and formation references, please see appendix 1, 

tables 1-1 and 1-2.

As a result of the classification of the lithologic groups 

for the purposes of this report, the metamorphic and igneous 

lithologic groups are associated primarily with the crystallinerock aquifers in the PBR Physiographic Provinces, and the 

sedimentary or siliciclastic-rock groups are associated with 

the Early Mesozoic basin within the Piedmont Physiographic 

Province (fig. 1). Additionally, crystalline diabase rocks 

locally intrude the primary sedimentary rocks in the Early 

Mesozoic basins. The diabase dikes are mafic rocks with 

geochemical properties likely similar to rock types such as 

amphibolite. The carbonate-rich lithologic group is limited to 

the northeastern area of the Piedmont (fig. 1). 

The distributions of lithologic groups, as delineated for 

this study, are shown in figure 5 along with available USGS 

NAWQA groundwater-quality sample locations. (Note: 

sulfidic clastic sedimentary and iron-rich sediments were 

not delineated in the study area.) For this report, NAWQA 

groundwater sample data were available for 9 of the 14 lithologic groups; samples were not available for the carbonaterich group, the feldspar-rich clastic sedimentary group, the 

sulfide-rich sedimentary group, unconsolidated sediments, 

and iron-rich sediments (table 3). To illustrate the physical 

relations among different lithologic groups, representative 

lithologic groups within mapped geologic terranes in North 

Carolina are shown in figure 6.

Lithochemical Subgroups

As a starting point, rock type designations from the 

geographic information system (GIS) attributes for the State 

maps (appendix 1, table 1-1 “ROCK_TYPE1”) (Dicken and 

others, 2005a, 2005b; Nicholson and others, 2005, 2006) 

were directly assigned to a particular subgroup number as 

listed in McCartan and others (1998). For example, the use 

of subgroup 32 (appendix 1, table 1-1) for schist rock types 

is continued in this study. Because some of the lithologic 

groups delineated as part of this study are large and contain 

several variable rock types, such as the metamorphosed clastic 

sedimentary, the mafic igneous and metamorphic equivalents, 

and the felsic igneous and metamorphic equivalent groups, the 

generalized rock types such as gneiss or schist, for example, 

were further divided to evaluate potential effects of mineralogical distinctions on geochemistry and thereby groundwater 

quality in the aquifers. Detailed descriptions of geologic units 

are available in the geographic information system (GIS) 

attributes for the State maps as compiled by Dicken and others (2005a, 2005b) and Nicholson and others (2005, 2006) 

(appendix 1, table 1-1). Where formation descriptions did 

not include mineralogy in the GIS attribute file, the USGS 

National Geologic Map Database Geologic Names Lexicon 

“GEOLEX” was used to obtain mineralogical descriptions of 

the formations (http://ngmdb.usgs.gov/Geolex/geolex_home.

html; accessed February 2011). Each lithologic group (with the 

exception of the unconsolidated sediments group) is categorized with regard to chemical composition as felsic, intermediate, mafic, carbonaceous, or sulfidic (appendix 1, table 1-1). 

The metamorphosed clastic sedimentary lithologic group 

is subdivided into 16 lithochemical subgroups based on the 

presence of minerals that may affect aquifer geochemistry. 

For example, sulfidic characteristics [31s and 32s, following McCartan and others (1998)], graphitic content (32g and 

35gns), the presence of calcareous minerals or rocks (35c), the 

presence of aluminuous minerals (32al), the presence of mafic 

minerals (chlorite and hornblende, 32m; biotite, 41bs). The 

subgroup “u” designation is simply “undifferentiated” because 

the description of the geologic unit or formation indicated 

mixed rock types. Other distinctions were made with regard to 

rock types that have similar geologic origin, structure, or textural characteristics (35gn and 35ml) (appendix 1, table 1-1). 

Figure 7 shows an example of the further division of lithologic 

groups into lithochemical subgroups and corresponding geologic formations. 

A few lithologic groups have only minor modifications 

from McCartan and others (1998). The quartz-rich metamorphic group has an additional 33my lithochemical subgroup for 

mylonitic rocks associated with major fault zones (Brevard 
18 Contaminants in Crystalline-Rock Aquifers and Siliciclastic-Rock Aquifers, Eastern United States, 1994–2008

85° 80° 75°

40°

35°

TENNESSEE

ALABAMA

GEORGIA

SOUTH CAROLINA

NORTH CAROLINA

VIRGINIA

WEST VIRGINIA

MARYLAND

NEW

JERSEY

PENNSYLVANIA

NEW YORK

KENTUCKY

0 50 100 KILOMETERS 25

0 50 100 MILES 25

OHIO

INDIANA

MICHIGAN

DELAWARE

Piedmont

Blue

Ridge

ATLANTIC

OCEAN

EXPLANATION

LITHOLOGIC GROUPS

Carbonate-rich rocks

Clastic redimentary rocks

Quartz-rich sedimentary rocks

Clastic racustrine/rvaporite redimentary rocks

Feldspar-rich clastic sedimentary rocks

Metamorphosed clastic sedimentary

Quartz-rich metamorphic rocks

Felsic igneous rocks and their metamorphic equivalents

Intermediate igneous rocks and their metamorphic equivalents

Mafic igneous rocks and their metamorphic equivalents

Ultramafic rocks

Unconsolidated sediments

Water

Physiographic boundary between Piedmont

 and Blue Ridge

Sample locations

Base from U.S. Bureau of the Census, 1990, 1:500,000 to 1:5,000,000 and Statistics Canada digital data, 2006

Albers Equal-Area Conic projection: Standard Parallels 29°30' N and 45°30' N,

Central Meridian 96°00' W, Latitude of Origin 23°00' N

Lithologic group data from Dicken and others, 2005a and

2005b and Nicholson and others, 2005 and 2006

85° 80° 75°

40°

35°

Figure 5. Distribution of delineated lithologic groups and 1994–2008 sample locations within the study area, Piedmont and Blue Ridge 

Physiographic Provinces.
Geological and Geochemical Framework for Interpretations of Water Quality 19

granite

gabbro

gness

schist

IGMTI

IGMTF

IGMTF

IGMTM

CLSD

CLSDF

CLSDQ

MTQ

IGMTI

CLSDQ

Conglomerate

sandstone

Granite

gneiss

IGMTF

IGMTI

TRIASSIC

BASIN

Piedmont Plateau

Appalachian Mountains

Blue Ridge front

CHARLOTTE

TERRANE

Unconsolidated

sediments and

limestone

PIEDMONT

ZONE

CAROLINA

TERRANE

6,000

5,000

4,000

3,000

2,000

1,000

–1,000

–2,000

–3,000

–4,000

–5,000

–6,000

–7,000

–8,000

–9,000

–10,000

–11,000

FEET

NAVD 88

A' A

Fall

Line

meta-argillite

meta-mudstone

metavolcanics

graywacke

metasedimentary rocks

slate

gneiss

CLSDMT

IGMTF

Blue Ridge Piedmont Coastal Plain

Cape

Hatteras

Pamlico

Sound

84 ̊

83 ̊ 36 ̊

34 ̊ 34 ̊

35 ̊

36 ̊

35 ̊

82 ̊

81 ̊

84 ̊ 83 ̊

82 ̊

81 ̊

80 ̊ 79 ̊ 78 ̊ 77 ̊ 76 ̊

80 ̊

79 ̊

78 ̊

77 ̊

76 ̊

Greensboro

Raleigh

Charlotte

Greenville

Wilmington

Asheville Blue Ridge

Piedmont

Coastal

Plain

0 20 40 60 80 100 MILES

0 20 40 60 80 100 KILOMETERS

A

A’

EXPLANATION

Geologic section A A’

Figure 6. Generalized schematic cross-section diagram across North Carolina physiographic provinces showing generalized geologic 

terranes and common lithologic groups delineated as part of this study.
20 Contaminants in Crystalline-Rock Aquifers and Siliciclastic-Rock Aquifers, Eastern United States, 1994–2008

Clastic

sedimentary

Clastic

lacustrine/

evaporite

sedimentary

Quartz-rich 

sedimentary

Early Mesozoic

basin aquifers 

Piedmont

 and 

Blue Ridge

Carbonate-rock

aquifers 

Siliciclastic-rock

aquifers 

Primary

rock type

Lithologic groups are

broad categories within a

rock type with similar

characteristics 

Crystalline-rock

aquifers

Feldspar-rich

clastic

sedimentary 

Lithochemical

Subgroup 

Lithologic

group 

Lithochemical subgroups

are a division of the

lithologic groups 

Geologic

Formation 

argillite, arkose, conglomerate,

fine-grained mixed clastic,

mudstone, sandstone, shale,

siltstone 

Brunswick Formation 

Geologic formations are

the smallest mapped

units available for spatial

analysis 

Piedmont

 and 

Blue Ridge

Figure 7. An example division of the aquifers into lithologic groups, lithochemical subgroups, and corresponding geologic formations.

fault zone, appendix 1, table 1-1). The intermediate igneous rocks and their metamorphic equivalents group includes 

distinctions for the more feldspathic subgroups (34f and 34fl) 

compared to the more intermediate composition groups (34i). 

This group also includes some rock type designations, including biotite gneiss (34bg) and augen gneiss (34agn), as well 

as mineralogical distinctions for sulfidic and graphitic rocks 

(34s). The mafic igneous and metamorphic equivalent rocks 

group has seven lithochemical subgroups, most of which follow McCartan and others (1998), with the addition of 41mv 

and 41v for metavolcanic and volcanic origin, respectively. A 

subgroup 44 was added for an undifferentiated mafic gneiss 

rock type. For the ultramafic rocks group, the subgroup 50gs 

was added to distinguish greenstones that did not contain 

carbonates. For the felsic igneous and metamorphic equivalent rocks group, the subgroup 61m is used to distinguish 

metamorphosed intrusive, while 61mf and 61mi distinguish 

compositional differences of felsic and intermediate, respectively. The subgroup 61v follows McCartan and others (1998) 

with the addition of 61mv for metavolcanic rocks. Where 

lithochemical subgroups were important to the interpretation 

and extrapolation of groundwater-quality data from this study 

for human health issues, the correlation of formations across 

State boundaries was reviewed. Geologic formations may be 

categorized differently based on the available geologic formation data and rock and mineral assemblage descriptions on the 

State maps (appendix 1, table 1-1, “ROCK_TYPE1”; Dicken 

and others, 2005a, 2005b; Nicholson and others, 2005, 2006). 

Also, the USGS National Geologic Map Database, Geologic 

Names Lexicon “GEOLEX” searchable database was used to 

obtain mineralogical descriptions of the formations (http://

ngmdb.usgs.gov/Geolex/geolex_home.html; accessed 

October 2012).

Geochemical Controls on Naturally Occurring 

Trace Elements and Radionuclides in 

Groundwater

Dissolved chemicals in groundwater may be derived 

from rock weathering, biological processes, and anthropogenic 

sources. Dissolution of minerals in bedrock and overlying geologic materials commonly release naturally occurring constituents to the groundwater. Major cations (positively charged ions 

such as calcium, magnesium, sodium, and potassium), major 

anions (negatively charged ions such as sulfate, chloride, 

fluoride, and bicarbonate), and nonionic solutes (uncharged 

solutes such as silica and DO typically are present at concentrations greater than 1 mg/L, whereas trace constituents 

typically are present at concentrations less than 1 mg/L (Hem, 

1985). However, dissolved concentrations of trace constituents 

can range widely depending on their occurrence in the rock 

or other source, the solubility of the constituent elements and 

interacting substances, and geochemical conditions such as pH 

and oxidation-reduction (redox) state that affect element form, 

mobility, and transport in the aqueous environment. 

Groundwater from diverse environments may contain 

naturally occurring trace elements such as iron, manganese, 

zinc, lead, copper, nickel, vanadium, molybdenum, arsenic, 

radium, and uranium (Rose and others, 1979). Descriptions of 
Geological and Geochemical Framework for Interpretations of Water Quality 21

the mineral assemblages in mapped geologic units may be useful when identifying potential geologic sources of dissolved 

constituents, although the mere presence of minerals containing such constituents may not lead to elevated concentrations 

in the associated groundwater. Elevated concentrations of trace 

constituents tend to be found locally or are associated with 

specific aquifer settings, particularly (1) under acidic conditions where the solubilities and mobilities of many element 

species are increased (Kirby and Cravotta, 2005; Cravotta, 

2008a,b) or (2) under reducing conditions where the dissolution of ferric iron [Fe(III)] and manganese [Mn(III,IV)] to 

more soluble ferrous Fe(II) and dissolved manganese [Mn(II)] 

can release adsorbed and coprecipitated metals (Langmuir, 

1997, p. 294–296; McMahon and Chapelle, 2008). 

Although the release of trace elements through mineral 

weathering is a natural process, accelerated mineral decomposition that accompanies the development of strongly acidic 

or reducing conditions could be a consequence of human 

activities. For example, acidification can result from the 

excavation of sulfide minerals or the release of gaseous emissions containing sulfur or nitrogen oxides, and reduction can 

result from the disposal of organic wastes or over-fertilization. 

Furthermore, some constituents may originate from industrial 

sources, manmade materials, or land applications. Thus, in 

order to determine constituent concentrations that may have 

been added to groundwater as a consequence of land-use or 

waste-disposal practices, natural background concentrations 

for specific geologic settings need to be established. Additionally, in order to identify geochemical environments where 

elevated concentrations of constituents may be present, waterquality conditions such as pH and redox state and major ion 

composition need to be characterized. 

Whether a dissolved constituent has originated from 

the weathering of rocks or from anthropogenic sources, its 

transport may be affected by its ionic charge, redox state, 

and tendency to interact with other dissolved elements (ion 

complexation) and solid surfaces (surface complexation or ion 

exchange). Redox-sensitive elements that commonly occur 

in more than one valence state under atmospheric conditions 

near the surface of the Earth include carbon (–4, +4), sulfur 

(–2, +6), nitrogen (–3, +3, +5), iron (+2, +3), manganese (+2, 

+3, +4), arsenic (–3, +3, +5), selenium (–2, +4, +6), chromium 

(+3, +6), molybdenum (+4, +6), vanadium (+3, +4, +5), and 

uranium (+3, +4, +5, +6). Although these and many other 

elements can have a positive valence state or core charge, the 

predominant aqueous species may be positively or negatively 

charged ions, depending on the tendency of the charged element to hydrolyze and to form aqueous complexes. Generally, 

the highly positively charged valences are present as cations 

form oxyanions and, less commonly, oxycations (Turner and 

others, 1981; Stumm and Morgan, 1996; Langmuir, 1997; 

Hodge and others, 1998). For example, in a reducing groundwater environment, chromium in the +3 valence state may 

be present as a cation (Cr+3), whereas in a strongly oxidizing 

environment, chromium in the +6 valence state may be present 

as the oxyanion chromate (CrO4

–2). Likewise, under oxidizing 

conditions, arsenic in the +3 or +5 valence states tends to form 

the oxyanions arsenite (AsO3

–3) or arsenate (AsO4–3), respectively, and uranium in the +6 valence state tends to form the 

uranyl (UO2

+2) oxycation. Furthermore, at the typical pH range 

of natural water, the uranyl ion may interact with carbonate 

and bicarbonate ions to form negatively charged carbonate 

complexes (Langmuir, 1997). The formation of such soluble 

ion complexes can increase the concentrations and transport of 

dissolved trace elements (Cravotta, 2008a, 2008b). 

Concentrations of major cations and anions in natural 

waters generally are controlled by acid-base and precipitation-dissolution reactions; however, the concentration 

and mobility of most trace ions generally are controlled by 

surface-complexation (adsorption) reactions on hydrous 

Fe(III) oxides, Mn(III,IV) oxides, and aluminum oxides and 

silicates (Dzombak and Morel, 1990; Bowell, 1994; Stumm 

and Morgan, 1996; Drever, 1997; Langmuir, 1997). Consequently, the concentrations of trace elements in natural waters 

typically are far below the values that would be predicted 

for saturation with respect to a pure mineral phase (Drever, 

1997; Cravotta, 2008b). For example, at the acidic pH range 

(5 to 6.5) of natural groundwater, dissolved oxyanions, such 

as chromate (CrO4

–2), phosphate (PO4–3), selenite (SeO3–2), 

selenate (SeO4

–2), arsenite (AsO3–3), and arsenate (AsO4–3), 

tend to be weakly sorbed and partly immobilized by hydrous 

ferric oxide (HFO) minerals (fig. 8, such as goethite (FeOOH) 

and ferrihydrite [Fe(OH)3]. However, at the alkaline pH range 

(7.5 to 9) of natural water, sorption of these anions generally decreases with increasing pH and is accompanied by 

corresponding increases in their dissolved concentrations 

(fig. 7). In contrast, dissolved cations, such as chromium 

(Cr+3), copper (Cu+2), cadmium (Cd+2), nickel (Ni+2), and zinc 

(Zn+2), tend to be poorly adsorbed and are relatively mobile at 

acidic pH, whereas at alkaline pH, the cation concentrations 

tend to be attenuated by adsorption on HFO and other oxide 

surfaces (fig. 8). 

The apparent opposite sorption behavior of the anions 

and cations (fig. 8) results from a progressive decrease in the 

effective charge on oxide surfaces from positive (attractive to 

anions) to negative (attractive to cations) as the pH increases 

from acidic to alkaline values (Dzombak and Morel, 1990; 

Stumm and Morgan, 1996; Langmuir, 1997). At alkaline pH 

values, negatively charged oxide atoms at mineral surfaces 

attract cations; however, at acidic pH, protons attached to the 

oxide atoms yield an effective positive charge at the mineral 

surface and thus attract anions. In addition, the sorbed cations 

or anions may be displaced by other charged ions such as 

magnesium (Mg+2), chloride (Cl–

), and sulfate (SO4

–2) through 

ion exchange or competition for sorption sites. Thus, concentrations of trace elements in solution may increase with 

concentrations of total dissolved solids, not only because of 

the release of trace constituents with the major ions dissolved 

from minerals, but because of the displacement of trace ions 

from surface sorption sites by the major ions. For the trace 
22 Contaminants in Crystalline-Rock Aquifers and Siliciclastic-Rock Aquifers, Eastern United States, 1994–2008

100

90

80

70

60

50

40

30

20

10

0

Percent dissolved

0

10

20

30

40

50

60

70

80

90

100

100

90

80

70

60

50

40

30

20

10

0

0

10

20

30

40

50

60

70

80

90

100

Percent adsorbed

CrO4-2

VO4-3

SeO4-2

SeO3-2

AsO4-3

AsO3-3

PO4-3

EXPLANATION

UO2+2

Pb+2

Cr+3

Cu+2

Ra+2

Co+2

Zn+2

Cd+2

Mn+2

Ba+2

Ni+2

EXPLANATION

pH

3 4 5 6 7 8 9 10 11

A

B

Figure 8. Equilibrium fractions of initial concentrations of ions that may be dissolved or adsorbed on a finite amount of hydrous ferric 

oxide at 25 degrees Celsius as a function of pH. A, anions; B, cations.

elements that form aqueous complexes, the increase in soluble 

major ion concentrations also increases the likelihood of the 

formation of soluble ion complexes. 

Cations and anions that had been adsorbed or coprecipitated with Fe(III) or Mn(III,IV) compounds may be 

remobilized under anoxic, reducing conditions. The reducing 

conditions must be sufficient to reduce and dissolve iron and 

manganese but not to produce sulfide, which tends to form 

insoluble compounds with many trace cations (Korte, 1991; 

Welch and others, 2000; Kirk and others, 2004). The reductive dissolution of Fe(III) and Mn(III,IV) oxides typically is 

coupled with the oxidation of organic compounds after supplies of dissolved oxygen, nitrate (NO3

–

), and nitrite (NO2

–

) 

have been depleted, but before the development of sulfatereducing conditions (Ehrlich, 1990; Stumm and Morgan, 

1996; Drever, 1997; McMahon and Chapelle, 2008). Under 

such reducing geochemical conditions, the concentrations of 

dissolved iron and manganese and associated sorbed trace 

anions and cations may become elevated. Thus, the presence 

of dissolved iron, manganese, and sulfate in anoxic groundwater that lacks nitrate and nitrite can be interpreted to indicate 

reducing geochemical conditions capable of mobilizing trace 

elements associated with Fe(III) or Mn(III,IV) oxides in the 

aquifer (McMahon and Chapelle, 2008). 

Naturally occurring radionuclides in groundwater include 

isotopes of uranium (U-238), thorium (Th-232), radium 
Water-Quality Data and Methods 23

(Ra-224, Ra-226, Ra-228), and radon (Rn-222). Uranium-238 

is the parent of radium-226 and radon-222; thorium-232 is 

the parent of radium-228 and radium-224 (fig. 9). Uranium, 

thorium, radium, and other radioisotopes in the decay chains 

tend to be present as dissolved ions that are affected by different geochemical speciation, solubility, and sorption processes 

(Ames and others, 1983a, 1983b). Consequently, the presence 

of uranium, thorium, or radium in groundwater requires a 

mineral source and geochemical conditions in the aquifer that 

are conducive to transport of those elements. The presence 

of radon (of which radon-222 is the most abundant isotope) 

in groundwater is directly related to the presence of a decaychain parent, such as uranium, in the aquifer because radon 

is a highly soluble noble gas that generally is not affected by 

chemical reactions. In addition, all radon isotopes have short 

half-lives and generally occur relatively near the parent source 

in the aquifer. 

Water-Quality Data and Methods

Groundwater-quality data collected by the USGS 

NAWQA Program (Gilliom and others, 1995) from wells in 

various hydrogeologic and land-use settings from Georgia 

through New Jersey were compiled to establish a regional 

database on water quality in the study area. The NAWQA 

design is discussed in Gilliom and others (1995) and Lapham 

and others (2005); protocols for collection of water-quality 

data are presented in Koterba and others (1995). The NAWQA 

groundwater sampling protocols specified prolonged flushing 

of the well to remove water stored in the well bore prior to 

sample collection (Koterba and others, 1995). Furthermore, 

field measurements of dissolved oxygen, pH, alkalinity, and 

other unstable constituents were routinely conducted without 

exposing samples to the atmosphere at the time of sample 

collection. Thus, the data used for this study are presumed to 

represent the in situ water-quality characteristics of the aquifer. 

This report includes groundwater-quality data collected 

as part of 14 NAWQA studies across the eastern United 

States covering the Early Mesozoic basin principal aquifer 

and a variety of fractured felsic and mafic crystalline-rock 

aquifers of the Piedmont and Blue Ridge aquifer as described 

by Lapham and others (2005) (figs. 1 and 3). The compiled 

data are not distributed evenly throughout the study area and 

therefore are not spatially representative of all the aquifers. 

The dataset analyzed consists of 346 samples (appendix 1, 

tables 1-3 and 1-4) collected as part of land use, major aquifer, 

and drinking water studies (table 1). Each study involved a 

network of 20 to 30 wells to document and explain the occurrence and distribution of selected chemical compounds in 

groundwater in particular settings (Gilliom and others, 2006) 

(table 1; fig. 3; appendix 1, tables 1-3 and 14). Groundwater 

data from carbonate rock and glacial aquifers are not included 

in this study.

Data for physical characteristics and concentrations of 

dissolved chemical constituents in groundwater were compiled 

for the 346 sampled wells (appendix 1, tables 1-3 and 1-4). A 

variety of crystalline and siliciclastic bedrock types with associated lithologies were sampled at these well locations. Most 

of these data were collected during the late spring through the 

late summer. Although collected only once per site (from 1994 

through 2008; appendix 1, table 1-4), the groundwater-quality 

data are assumed comparable for the purpose of evaluating 

spatial patterns in water quality for this study. Selected data 

from these compilations are presented in this report; all data 

compiled for this report are accessible on the World Wide Web 

at http://infotrek.er.usgs.gov/apex/f?p=NAWQA:HOME:0; 

accessed October 2012.

Analytical results for most samples were obtained for 

major ions, various trace elements (including iron, manganese, aluminum, antimony, arsenic, barium, beryllium, boron, 

cadmium, chromium, cobalt, copper, lead, lithium, mercury, 

molybdenum, nickel, selenium, silver, strontium, thallium, 

vanadium, and zinc), selected nutrients (including phosphorus and nitrogen compounds), dissolved organic carbon, and 

radionuclides [including radon-222 (hereafter referred to as 

radon), radium isotopes, uranium, and tritium] as well as 

temperature, specific conductance (SC), pH, and concentrations of dissolved oxygen (DO). All analyses are for dissolved 

constituents in water samples that were filtered in the field, 

unless otherwise specified. The water temperature, SC, pH, 

and dissolved-oxygen concentrations were measured in the 

field immediately prior to sample collection. Other chemical 

analyses were conducted at the USGS National Water Quality 

Laboratory (NWQL) in Denver, Colorado (Patton and Truitt, 

1992; Brenton and Arnett, 1993; Fishman, 1993; Werner and 

others, 1996). 

To assess the potential for naturally occurring solutes to 

contaminate drinking water, the concentrations of chemical 

constituents are compared to criteria for protection of human 

health such as USEPA (2009) drinking water maximum contaminant levels (MCLs) and secondary maximum contaminant 

levels (SMCLs) or health-based screening levels (HBSLs). 

Because water quality at a given location will vary temporally 

owing to natural hydrologic processes and seasonality, and 

because samples were collected only once from each well, 

constituent concentrations also were compared to a value of 

one-tenth of the relevant human health criteria. Consideration 

of this lower threshold level increases the certainty that constituents will be acknowledged that could have human health 

implications under variable circumstances. 

The reported water-quality data were used to compute 

mineral saturation indices (explained below), hardness, 

and groundwater redox classes. Hardness was calculated as 

the sum of calcium and magnesium concentrations and is 

expressed as calcium carbonate (Fishman, 1993). The redox 

class was determined on the basis of concentrations of dissolved oxygen, nitrate, manganese, iron, and sulfate using 

thresholds of McMahon and Chapelle (2008). The redox classifications used in this report were simplified to consider only 
24 Contaminants in Crystalline-Rock Aquifers and Siliciclastic-Rock Aquifers, Eastern United States, 1994–2008

Alpha decay

Beta decay

EXPLANATION

Uranium-238

Thorium-232

U-238

4.49 x 109

y

U-234

2.48 x 105

y

Th-234

24.1d

Th-232

1.41 x 1010y

Th-228

1.91y

Th-230

8.0 x 104

y

Pa-234

1.18m

Uranium

Protactinium

Thorium

Radium

Radon

Polonium

Bismuth

Lead

Actinium

Thorium

Radium

Radon

Polonium

Bismuth

Lead

Thallium

Ra-226

1622y

Rn-222

3.825d

Po-218

3.05m

Po-214

1.4 x 10–4s

Pb-214

26.8m

Pb-210

22y

Po-210

138.4d

Pb-206

(stable)

Bi-214

19.7m

Bi-210

5.0d

Ra-228

5.75y

Ra-224

3.64d

Rn-220

55s

Po-216

0.16s

Po-212

0.30 x 10–6s

Pb-212

10.64h

Pb-208

(stable)

Bi-212

60.5m

Tl-208

3.10m

Ac-228

6.13h

Figure 9. Uranium-238 and thorium-232 decay series.
Water-Quality Data and Methods 25

three major classes: “anoxic” (DO < 0.5 mg/L, manganese 

≥ 50 μg/L, and iron ≥ 100 μg/L); “mixed” (DO ≥ 0.5 mg/L and 

either manganese ≥ 50 μg/L or iron ≥ 100 μg/L); and “oxic” 

(DO ≥ 0.5 mg/L, manganese < 50 μg/L, and iron < 100 μg/L).

Ancillary geospatial data were compiled to describe the 

physical characteristics of the watersheds within the potential 

contributing areas of 1,640 ft (NAWQA buffer for ancillary 

data including land use, census data, nutrient sources, and 

point sources) of sampled wells. Each groundwater sample 

site was classified on the basis of the physiographic setting, 

bedrock type (crystalline or siliciclastic), lithologic group, 

and lithochemical subgroup (table 3). Land-cover data were 

compiled and used to compute the percentage of four major 

land uses (wetland, forested, agricultural, and urban) within 

1,640 ft of sampled wells (appendix 1, table 1-3). Generalized land cover was modified from the National Land Cover 

Database 1992 (NLCD 1992) using historical land-use and 

land-cover data (Price and others, 2007) (fig. 2).

Graphical and Statistical Analyses

Various graphical and statistical techniques were used in 

this report to compare water-quality data for different aquifers 

or geologic settings, to estimate natural and anthropogenic 

sources of dissolved constituents, and to identify possible factors affecting the occurrence or transport of solutes in the aquifers in the study area. In general, nonparametric, rank-based 

statistical approaches were used to accommodate non-normally distributed and censored data typical of most environmental samples (Helsel and Hirsch, 2002). Data for individual 

continuous variables, such as chemical concentrations, were 

censored to a common level, and censored values were set 

to a common reporting limit before ranks were computed for 

use in statistical tests. Relations between continuous variables 

were evaluated with scatter plots and correlation coefficients 

(Spearman’s rho); distributions of continuous variables were 

compared among different groups (such as lithology) using 

probability plots, boxplots, and rank-sum or rank-transform 

analysis-of-variance (ANOVA) (Helsel and Hirsch, 2002). 

The data reported for trace element concentrations typically included “censored” values that were less than (<) the 

reporting limit. The reporting limit for a given constituent 

generally was not uniform for all samples. For example, 157 

of 253 samples had censored values for arsenic concentration at five different reporting limits, in micrograms per liter, 

with count (n) indicated in parentheses: <0.06 (n = 11), <0.12 

(n = 15), <0.26 (n = 23), <1.0 (n = 106), or <2.0 (n = 2). However, the other 96 samples had reported trace element concentrations ranging from 0.036 to 57 μg/L (appendix 1, tables 1-4 

and 1-5). Only 42 of these reported concentration values were 

greater than 2.0 μg/L, which was the highest common reporting limit (HCRL) for arsenic (appendix 1, tables 1-4 and 1-5). 

For statistical tests and other computations, the censored data 

and reported values less than the HCRL were considered 

equivalent to the HRCL at 0.99 times the HCRL for most 

constituents; however, for several constituents (arsenic, zinc, 

iron, manganese, aluminum), where the HCRL applied to few 

samples and resulted in excessive censoring, the next highest 

common reporting limit was considered. In the case of arsenic, 

the next highest reporting limit is 1.0 μg/L, which is one-tenth 

of the MCL and one-half of USEPA’s health advisory level 

of 2.0 μg/L (U.S. Environmental Protection Agency, 2001) 

(appendix 1, tables 1-4 and 1-5). 

Probability plots and boxplots are used to illustrate univariate distributions for the different aquifers and associated 

lithologic groups or redox classes. Horizontal reference lines 

on the plots indicate the applicable values of drinking water 

or other human health benchmarks. Probability plots indicate 

the frequency (x-axis), or the proportion, of samples within 

the crystalline- and siliciclastic-rock aquifers that exceeded 

the constituent concentration (y-axis) or other plotted parameter values. For parameters without censored data (where all 

reported values exceeded the detection limits), the maximum 

and minimum reported values correspond to the 0.0 and 1.0 

probability values, respectively, and the median value corresponds to the 0.5 probability value. Censored values are 

not displayed, but are counted to estimate the frequency of 

samples that exceeded the reported values. For parameters 

with censored data, the minimum plotted value corresponds to 

the lowest reported value greater than the detection limit. For 

some constituents, such as fluoride, cobalt, lead, and selenium, 

the minimum reported value has a frequency of exceedance 

less than 0.5, which indicates the median is a censored value. 

Boxplots were used to show the water-quality concentration distributions for the three lithologic groups of the 

siliciclastic-rock aquifers and the six lithologic groups of the 

crystalline-rock aquifers that have water-quality data (table 3). 

The boxplots show the percentile distributions of samples 

with concentrations equal to or less than the associated value. 

All censored values were set to a common reporting limit. 

Where the median for a group is greater than the common 

reporting limit, it is displayed as a horizontal line within the 

box that is defined by the 25th and 75th percentiles for that 

group; otherwise, the median is displayed at the reporting 

limit. Along the top of each boxplot, the number of samples 

in each group is shown above a letter symbol. Groups with a 

different letter symbol have mean ranks that are significantly 

different on the basis of the nonparameteric Tukey test (Helsel 

and Hirsch, 2002) (appendix 1, table 1-6). Results were ranked 

and coded sequentially, with the group with the highest mean 

rank coded “A,” the group with the next highest mean rank 

coded “B,” then “C,” and so on; overlapping groups were 

coded with letters for overlapping groups, “BC,” for example, 

or “BD,” representing overlap with groups B and C, and B, C, 

and D, respectively. Only the first and last letters of the range 

of overlapping groups are listed. The mean ranks of groups 

with one or more of the same letters are not significantly different. Although the mean ranks of groups may not differ, data 
26 Contaminants in Crystalline-Rock Aquifers and Siliciclastic-Rock Aquifers, Eastern United States, 1994–2008

values greater than the 75th percentile are of particular interest 

because these values may exceed relevant benchmarks. 

The frequency and number of samples within specified 

pH and redox classes are illustrated using a bivariate matrix. 

These pH-redox matrices are used in this report to indicate 

relations among aquifer lithology, geochemical environment, 

and probability of contaminant occurrence. Each matrix 

considers four general pH classes (x-axis—pH 4.5 to <5.5; 

5.5 to <6.5; 6.5 to <7.5; 7.5 to <8.5) and the three simplified 

redox classes (y-axis—anoxic; mixed; oxic). The pH-redox 

matrices indicate the frequency of occurrence of the pH-redox 

classes by lithologic group and the frequency of occurrence of 

contaminant concentrations in relation to a specified reporting 

limit or human health benchmark for all the groups combined. 

The corresponding reporting limit or benchmark value and the 

total number of samples considered are indicated at the top of 

the matrix. The number of samples counted within each pHredox class is color-coded to highlight those pH-redox classes 

with the greatest frequency of samples exceeding relevant 

limits. 

Principal components analysis (PCA), computed with 

SAS 9.2 (SAS Institute, Inc., 2008), was used to evaluate 

multivariate correlations among the elements in the regional 

groundwater dataset without prior classification. The goal was 

to identify important hydrochemical processes or master variables that could explain element associations and distributions 

(Joreskog and others, 1976; Drever, 1997; Thyne and others, 2004). The Spearman-rank correlation coefficient matrix 

for the groundwater dataset (appendix 1, table 1-7) provided 

the standardized input for the PCA. Because the PCA model 

would exclude the entire record for any sample with a missing value, those constituents that were missing or those that 

were censored in more than 40 percent of the samples, such 

as dissolved aluminum, fluoride, bromide, organic carbon, 

and many trace elements, were excluded. The PCA model 

was optimized with varimax rotation, and only principal 

components with eigenvalues greater than unity, equivalent to 

correlations with a probability greater than or equal to 0.999, 

were retained (Joreskog and others, 1976; Thyne and others, 

2004). Loadings for each constituent included in the PCA 

model are equivalent to the Spearman-rank correlation coefficient between that constituent and the principal component. 

To aid in interpretations, the scores for each principal component in the PCA model were compiled and then evaluated by 

correlation or graphical analysis with additional variables that 

had been excluded from the PCA, such as lithology, land use, 

well depth, and chemical constituents. For simplification of 

displayed results, the loading values and Spearman-rank correlation coefficient values are multiplied by 100 and rounded. 

Significant correlation coefficients for the additional variables 

are displayed beneath the main PCA model results; only correlation coefficients with probability greater than or equal to 

0.999 are considered significant. 

Geochemical Modeling

Geochemical equilibrium models were developed to 

explain the occurrence of solutes in different geochemical environments and the relations among concentrations of 

dissolved constituents in the groundwater samples. Aqueous 

speciation computations with WATEQ4F (Ball and Nordstrom, 

1991) and PHREEQC (Parkhurst and Appelo, 1999) using 

the WATEQ4F database were used to evaluate the potential 

for the concentrations of dissolved constituents to be limited 

by precipitation-dissolution and (or) adsorption-desorption 

processes. The computed mineral saturation index (SI) values 

for various major and trace minerals were used to indicate the 

potential for mineral dissolution and precipitation. If a mineral 

phase is undersaturated in groundwater (SI less than 0), that 

mineral phase (if present) has the potential to be dissolved by 

the groundwater. Likewise, if a mineral is supersaturated in 

groundwater (SI greater than 0), that mineral phase feasibly 

could precipitate, thus limiting the dissolved constituent concentrations. To illustrate potential differences in geochemical 

properties, the SI values for selected minerals were illustrated 

as boxplots by lithologic groups. 

Adsorption and desorption of anions and cations on 

hydrous ferric-oxide-coated surfaces were evaluated using 

a diffuse double-layer modeling approach with PHREEQC 

(Parkhurst and Appelo, 1999), aqueous speciation data from 

WATEQ4F (Ball and Nordstrom , 1991), and surface complexation data from Dzombak and Morel (1990). Supplemental 

thermodynamic data for radium, chromium, cobalt, and vanadium were obtained from the ThermoChimie data base offered 

with PHREEQC (sit.dat), and surface-complexation constants 

for radium were estimated using empirical adsorption data 

presented by Benes and others (1984). For all of the sorption 

models, to be consistent with Dzombak and Morel (1990), 

the hydrous ferric oxide was specified as 90 mg/L, with a 

specific surface area of 600 square meters per gram consisting 

of 5 x 10–6 moles of strong binding sites and 2 x 10–4 moles 

of weak binding sites. Aqueous speciation and adsorption 

distribution for a range of pH values were computed, and the 

percentage of the total concentration distributed between the 

solution and sorbent was plotted as a function of pH. The 

sorption modeling results were illustrated as fractions of initial 

concentrations of ions that may be dissolved or adsorbed on 

a finite amount of hydrous ferric oxide at 25 degrees Celsius 

(°C) as a function of pH. 
Water-Quality Characteristics of Aquifers, Lithologic Groups, and Lithochemical Subgroups 27

Water-Quality Characteristics of 

Aquifers, Lithologic Groups, and 

Lithochemical Subgroups

Descriptions of groundwater quality generally include 

concentrations of major ions, pH, dissolved oxygen, and other 

variables. In this report, these characteristics are described in 

order to explain naturally occurring contaminants in groundwater from different geologic settings and geochemical 

environments. 

The groundwater quality for the PBR crystalline-rock 

aquifers generally differed from that for the Early Mesozoic 

siliciclastic-rock aquifers, considering the major ion and 

other constituent concentrations for the two bedrock types 

(figs. 10–14). Compared to the crystalline-rock aquifers, 

the groundwater from siliciclastic-rock aquifers had higher 

maximum and median concentrations of total dissolved solids, 

hardness, calcium, alkalinity (calcium carbonate), and sulfate 

(figs. 10–11). Although the median concentrations of magnesium, sodium, chloride, and nitrate in groundwater from 

siliciclastic aquifers also were higher than those medians for 

the crystalline-rock aquifers, the maximum concentrations 

of these constituents were present in groundwater from the 

crystalline-rock aquifers (figs. 10–11). Although median concentrations of silica were comparable for the two aquifer rock 

types, the crystalline-rock aquifers had a larger range for silica 

concentrations and larger ranges and higher medians for potassium and aluminum concentrations than the siliciclastic-rock 

aquifers (figs. 10–11).

The PBR crystalline-rock aquifers and the Early Mesozoic siliciclastic-rock aquifers also exhibited differences in 

the frequency distributions of minor and trace constituents in 

the groundwater (figs. 12–14). However, as explained previously and in more detail below, spatial and temporal variations 

in concentrations of these constituents may be attributed to 

variations in lithology within a particular rock type (lithologic group, lithochemical subgroup) and (or) geochemical 

environment (pH and redox conditions) within the aquifer 

(figs. 15–21). For example, the pH of the groundwater samples 

evaluated for this study ranged from 4.7 to 8.2 (fig. 12A, 

appendix 1, table 1-4). Although the highest and lowest pH 

values were recorded for groundwater from the PBR crystalline-rock aquifers, the groundwater from the Early Mesozoic 

siliciclastic-rock aquifers, particularly the clastic sedimentary 

(CLSD) and clastic lacustrine/evaporite sediments (CLSDLAC), generally had higher mean rank pH values than the 

PBR crystalline-rock aquifers, particularly the felsic or intermediate igneous and metamorphic lithologic units (IGMTI) 

(fig. 16C). About 70 percent of groundwater samples from the 

siliciclastic-rock aquifers had pH greater than 7 compared to 

only about 10 percent from the crystalline-rock aquifers.

Although DO concentrations in the groundwater varied 

widely, a majority of samples for all the aquifer lithologies 

could be characterized as oxic, with DO greater than or equal 

to 0.5 mg/L (fig. 12B). Seven redox classes were determined 

on the basis of concentrations of DO, nitrate, manganese, iron, 

and sulfate by using thresholds of McMahon and Chapelle 

(2008). Of the 346 groundwater samples evaluated for this 

study, 65.0 percent were classified as “oxic” (with DO greater 

than or equal to 0.5 mg/L); 18.2 percent were classified as 

“mixed” (with DO greater than or equal to 0.5 mg/L and manganese greater than or equal to 50 mg/L or iron greater than 

or equal to 100 mg/L); 14.2 percent were classified “anoxic” 

(with DO less than 0.5 mg/L); and the remaining 2.6 percent, 

which lacked data for DO, were classified as “unknown.” 

Based on the Tukey analysis, DO concentrations in groundwater did not differ among lithologies (fig. 15). Of the anoxic 

samples, 3.2 percent were suboxic, 1.7 percent were nitratereducing, 1.7 percent were manganese-reducing, 5.8 percent 

were iron-reducing, and 0.3 percent were methanogenic. 

Because few samples could be characterized as strongly reducing, the anoxic samples were considered as a single class for 

evaluation of geochemical environment. 

To evaluate the potential for aquifer lithology to affect 

contaminant concentrations, the water-quality data and saturation indices for selected minerals were considered in relation 

to the major bedrock type, lithologic groups, and lithochemical subgroups [tables 3 and 4 (table 4 available online at 

http://pubs.usgs.gov/sir/2013/5072/), figs. 10–17]. To evaluate potential for geochemical factors to affect the mobility of 

contaminants in the aquifer, the saturation indices for minerals 

that may be present in the aquifers and soil were summarized 

by lithologic group (figs. 18 and 19), and the frequency of 

contaminant detections and exceedances of human health 

benchmarks were evaluated with respect to the groundwater 

pH and redox characteristics (figs. 20 and 21) that may affect 

the adsorption and release of trace constituents by iron and 

manganese oxides. 

Exceedances of Drinking Water Criteria

Constituents of potential concern were identified on 

the basis of drinking water exposure guidelines proposed 

by the USEPA for sources of public drinking water (U.S. 

Environmental Protection Agency, 2009, 2010), including 

MCLs, HBSLs, or other criteria such as SMCLs. HBSLs were 

developed by the USGS, USEPA, New Jersey Department 

of Environmental Protection (NJDEP), and Oregon Health 

and Science University (OHSU) as an interagency pilot effort 

beginning in 1998 to communicate the potential relevance of 

the water-quality findings of the USGS NAWQA Program in a 

human-health context (Toccalino and others, 2012). 

Provisionally promulgated drinking water standards for 

radium (U.S. Environmental Protection Agency, 1976) became 

final with the Radionuclide Rule of 2000 (U.S. Environmental Protection Agency, 2000) when the other proposed standards for radionuclides were also finalized, or in the case of 

uranium, newly promulgated. Uranium, radium, and radon 

are radioactive elements that can increase human cancer risk 
28 Contaminants in Crystalline-Rock Aquifers and Siliciclastic-Rock Aquifers, Eastern United States, 1994–2008 Crystalline EXPLANATION Siliciclastic Crystalline EXPLANATION Siliciclastic Crystalline EXPLANATION Siliciclastic Crystalline EXPLANATION Siliciclastic Crystalline EXPLANATION Siliciclastic Crystalline EXPLANATION Siliciclastic Crystalline EXPLANATION Siliciclastic Crystalline EXPLANATION Siliciclastic

in milligrams per liter solids, d ve ol ss Total di

Frequency concentration was exceeded

A

1,200

1,1001,000

900800700600500400300200100

0

1,4001,3001,200

1,1001,000

900800700600500400300200100

0

Specific conductance, in microsiemens per centimeter

B

500300100

5010

51

Alkalinity, in milligrams per liter as calcium carbonate

C

700500300100

5010

51

Hardness, in milligrams per liter as calcium carbonate

D

E

200100

5010

51

0.50.1

0.07

in milligrams per liter , Calcium

905010

51

0.50.1

0.06

Magnesium, in milligrams per liter

F

905010

531

0.7

Sodium, in milligrams per liter 

G

30

20

10

51

0.50.2

Potassium, in milligrams per liter

H

0 0.5 1.0 0 0.5 1.0 0 0.5 1.0 0 0.5 1.0

Figure 10. Probability plots of groundwater-quality data for siliclastic-rock and crystalline-rock aquifers in the Piedmont and Blue Ridge Physiographic Provinces, 1994–2008. 

A, total dissolved solids; 

B, specific conductance; 

C, alkalinity; 

D, hardness; 

E, calcium; 

F, magnesium; 

G, sodium; and 

H, potassium.
Water-Quality Characteristics of Aquifers, Lithologic Groups, and Lithochemical Subgroups 29

in milligrams per liter , Sulfate

A

Frequency concentration was exceeded

700500100

5010

51

0.50.1

300100

5010

51

0.50.2

Chloride, in milligrams per liter

B

2010

51

0.5

0.005

0.1

Nitrate, in milligrams per liter as nitrogen

C

0.30.20.1

0.050.020.01

Phosphate, in milligrams per liter as phosphorus

D

E

21

0.50.10.5

Bromide

0.001

in milligrams per liter , 

51

0.50.1

0.050.01

Fluoride, in milligrams per liter

F

703010

54321

Silica, in milligrams per liter 

G

200100

5010

51

0.6

Aluminum, in micrograms per liter

H

0 0.5 1.0 0 0.5 1.0 0 0.5 1.0 0 0.5 1.0

Crystalline

EXPLANATION

Siliciclastic

Crystalline

EXPLANATION

Siliciclastic Crystalline

EXPLANATION

Siliciclastic

Crystalline

EXPLANATION

Siliciclastic

Crystalline

EXPLANATION

Siliciclastic Crystalline

EXPLANATION

Siliciclastic

Crystalline

EXPLANATION

Siliciclastic

Crystalline

EXPLANATION

Siliciclastic

Figure 11. Probability plots of groundwater-quality data for siliclastic-rock and crystalline-rock aquifers in the Piedmont and Blue Ridge Physiographic Provinces, 1994–2008. 

A, sulfate; 

B, chloride; 

C, nitrate; 

D, phosphate; 

E, bromide; 

F, fluoride; 

G, silica; and 

H, aluminum.
30 Contaminants in Crystalline-Rock Aquifers and Siliciclastic-Rock Aquifers, Eastern United States, 1994–2008 8.5 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 2010

51

0.50.1

0.050.02

Dissolved oxygen, in milligrams per liter

B

30,00020,00010,000

5,000

500100

1,000

50

52

10

Iron, in micrograms per liter

C

1,000

500

3,000

100

5010

51

0.50.1

0.050.02

Manganese, in micrograms per liter

D

E

805010

51

0.50.1

0.07

in micrograms per liter , Arsenic

6010

51

0.50.1

0.05

Selenium, in micrograms per liter

F

1,000

500200100

5010

52

Boron, in micrograms per liter 

G

5010

51

0.50.1

Molybdenum, in micrograms per liter

H

0 0.5 1.0 0 0.5 1.0 0 0.5 1.0 0 0.5 1.0

Crystalline

EXPLANATION

Siliciclastic

Crystalline

EXPLANATION

Siliciclastic

Crystalline

EXPLANATION

Siliciclastic

Crystalline

EXPLANATION

Siliciclastic

Crystalline

EXPLANATION

Siliciclastic Crystalline

EXPLANATION

Siliciclastic

Crystalline

EXPLANATION

Siliciclastic Crystalline

EXPLANATION

Siliciclastic

units , pH

A

Frequency concentration was exceeded

Figure 12. Probability plots of groundwater-quality data for siliclastic-rock and crystalline-rock aquifers in the Piedmont and Blue Ridge Physiographic Provinces, 1994–2008. 

A, pH; 

B, dissolved oxygen; 

C, iron; 

D, manganese; 

E, arsenic; 

F, selenium; 

G, boron; and 

H, molybdenum.
Water-Quality Characteristics of Aquifers, Lithologic Groups, and Lithochemical Subgroups 31

61

0.50.1

0.050.02

6,0001,000

500100

5010

51

0.7

B

2,0001,000

500

5010

100

5

0.50.2

1 Copper, in micrograms per liter

Zinc, in micrograms per liter

Cadmium, in micrograms per liter

C

10

5

20

1

0.50.1

0.06

Lead, in micrograms per liter

D

E

2010

51

0.50.1

0.050.01

in micrograms per liter , Cobalt

100

50

5

10

1

0.50.1

0.08

Nickel, in micrograms per liter

F

100

5010

51

0.50.1

Chromium, in micrograms per liter 

G

3010

51

0.50.1

Vanadium, in micrograms per liter

0.05

H

0 0.5 1.0 0 0.5 1.0 0 0.5 1.0 0 0.5 1.0

Crystalline

EXPLANATION

Siliciclastic

Crystalline

EXPLANATION

Siliciclastic

Crystalline

EXPLANATION

Siliciclastic Crystalline

EXPLANATION

Siliciclastic

Crystalline

EXPLANATION

Siliciclastic Crystalline

EXPLANATION

Siliciclastic

Crystalline

EXPLANATION

Siliciclastic Crystalline

EXPLANATION

Siliciclastic

A

Frequency concentration was exceeded

Figure 13. Probability plots of groundwater-quality data for siliclastic-rock and crystalline-rock aquifers in the Piedmont and Blue Ridge Physiographic Provinces, 1994–2008. 

A, cadmium; 

B, zinc; 

C, copper; 

D, lead; 

E, cobalt; 

F, nickel; 

G, chromium; and 

H, vanadium.
32 Contaminants in Crystalline-Rock Aquifers and Siliciclastic-Rock Aquifers, Eastern United States, 1994–2008 2,000 1,000 500 1005010

51

5,0001,000

500100

5020

B 5

0.50.1

1

0.050.01

Beryllium, in micrograms per liter

Strontium, in micrograms per liter

Barium, in micrograms per liter

C

10

5

20

1

0.50.1

0.05

Antimony, in micrograms per liter

D

E

9010

51

0.050.01

Uranium

0.0050.001

in micrograms per liter , 

9050

5

10

1

Tritium units

F

10

51

0.50.1

 per liter picocuries Radium 226 + 228, in 

G

50,00010,000

5,0001,000

500100

60

Radon, in picocuries per liter

H

0 0.5 1.0 0 0.5 1.0 0 0.5 1.0 0 0.5 1.0

Crystalline

EXPLANATION

Siliciclastic

Crystalline

EXPLANATION

Siliciclastic

Crystalline

EXPLANATION

Siliciclastic

Crystalline

EXPLANATION

Siliciclastic

Crystalline

EXPLANATION

Siliciclastic

Crystalline

EXPLANATION

Siliciclastic

Crystalline

EXPLANATION

Siliciclastic Crystalline

EXPLANATION

Siliciclastic

A

Frequency concentration was exceeded

Figure 14. Probability plots of groundwater-quality data for siliclasti-rock and crystalline-rock aquifers in the Piedmont and Blue Ridge Physiographic Provinces, 1994–2008. 

A, barium; 

B, strontium; 

C, beryllium; 

D, antimony; 

E, uranium; 

F, tritium; 

G, radium; and 

H, radon.
Water-Quality Characteristics of Aquifers, Lithologic Groups, and Lithochemical Subgroups 33

*

* * * *

*

*

*

*

*

*

* *

*

*

**

**

*

*

*

**

*

*

*

*

* **

*

**

**

* * **

*

*

* *

* **

*

*

*

* *

*

*

*

*

*

*

*

A B C

10,000

1,000

100

10

1

Specific conductance, in

microsiemens per centimeter

1,000

100

10

1

0.1

Sulfate, in milligrams per liter

1,000

100

10

1

0.1

Chloride, in milligrams per liter

9 51 11 96 17 70 55 34 2

A AB CD CDE DE CDE E BC -

9 51 10 91 16 57 51 28 2

A A BC BC C BC BC AB -

9 51 10 91 16 57 51 28 2

A AB ABCABC BC C C ABC -

D E F

100

10

1

0.1

0.01

1

0.1

0.01

Nitrate, in milligrams per liter as Nitrogen

Phosphate, in milligrams per liter as Phosphorus

100

10

1

Silica, in milligrams per liter

9 51 11 90 16 67 55 28 2

A AB AB AB AB B AB AB -

9 50 10 91 16 67 51 28 2

B AB A AB B A AB AB -

9 51 10 91 16 57 51 28 2

B AB B B B A AB A -

G H I

100

1,000

10

1

0.1

1

100

10

0.1

0.01

Sodium, in milligrams per liter 

Potassium, in milligrams per liter

100

1,000

10

1

0.1

0.01

Magnesium, in milligrams per liter

9 51 10 91 16 57 51 28 2

A A AB BC C AB BC BC -

9 51 10 91 16 57 51 28 2

B AB AB AB AB A A AB -

9 51 10 91 16 57 51 28 2

A A BC BC C BC C AB -

1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9

Outlier data value more than

 3 times the interquartile range

 outside the quartile

Outlier data value less than

 or equal to 3 and more than

 1.5 times the interquartile

 range outside the quartile

Data value less than or equal to

 1.5 times the interquartile range

 outside the quartile

75th percentile

25th percentile

Mean

Clastic sedimentary rocks (CLSD)

Clastic lacustrine/evaporite sedimentary rocks (CLSDLAC)

Quartz-rich sedimentary rocks (CLSDQ)

Metamorphosed clastic sedimentary rocks (CLSDMT)

Quartz-rich metamorphic rocks (MTQ)

Felsic igneous rocks and their metamorphic equivalents (IGMTF)

Intermediate igneous rocks and their metamorphic equivalents (IGMTI)

Mafic igneous rocks and their metamorphic equivalents (IGMTM)

Ultramafic rocks (ULMAF)

1

2

3

4

5

6

7

8

9

EXPLANATION

Figure 15. Groundwater-quality data by lithologic groups of siliclastic-rock and crystalline-rock aquifers in the Piedmont and Blue 

Ridge Physiographic Provinces, 1994–2008. A, specific conductance; B, sulfate; C, chloride; D, nitrate; E, phosphate; F, silica; G, sodium; 

H, potassium; and I, magnesium.
34 Contaminants in Crystalline-Rock Aquifers and Siliciclastic-Rock Aquifers, Eastern United States, 1994–2008

*

* **

*

*

*

*

*

*

*

*

* * *

*

*

* ** *

* *

**

**

**

*

*

*

*

**

*

*

**

*

**

*

*

*

*

*

*

*

*

*

*

*

*

*

*

A B C

1,000

100

10

1

0.1

0.01

1,000

100

10

1

Alkalinity, in milligrams per liter

as calcium carbonate

Calcium, in milligrams per liter

9.0

8.5

8.0

7.5

7.0

6.5

6.0

5.5

5.0

4.5

4.0

pH units

9 51 10 91 16 57 51 28 2

A AB C CD D CD D BC -

9 51 11 96 16 70 52 34 2

A A B CD D BCD D BC -

9 51 11 96 17 70 55 34 2

AB A BC DE E CDE DE CD -

D E F

10

100

1

0.1

0.01

0.001 1

100,000

10,000

1,000

100

10

Oxygen, in milligrams per liter

Iron, in micrograms per liter 

Manganese, in micrograms per liter 

10,000

1,000

100

10

0.1

1

9 51 10 94 16 66 55 34 2

A A A A A A A A -

9 51 10 91 16 57 51 28 2

AB B AB AB A AB AB AB -

9 51 10 91 17 67 55 28 2

B B AB A A AB AB AB -

G H I

100

1,000

10

1

0.1

1

10

0.1

Arsenic, in micrograms per liter 

Selenium, in micrograms per liter

100

10

1

0.1

Molybdenum, in micrograms per liter

1 43 7 55 12 61 55 17 2 1 43 7 55 12 61 55 17 2

- A B B B B B B

1 28 5 55 12 59 55 13 2

- - A A A A A A A - - A AB B B AB B B -

*

1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9

Outlier data value more than

 3 times the interquartile range

 outside the quartile

Outlier data value less than

 or equal to 3 and more than

 1.5 times the interquartile

 range outside the quartile

Data value less than or equal to

 1.5 times the interquartile range

 outside the quartile

75th percentile

25th percentile

Mean

Clastic sedimentary rocks (CLSD)

Clastic lacustrine/evaporite sedimentary rocks (CLSDLAC)

Quartz-rich sedimentary rocks (CLSDQ)

Metamorphosed clastic sedimentary rocks (CLSDMT)

Quartz-rich metamorphic rocks (MTQ)

Felsic igneous rocks and their metamorphic equivalents (IGMTF)

Intermediate igneous rocks and their metamorphic equivalents (IGMTI)

Mafic igneous rocks and their metamorphic equivalents (IGMTM)

Ultramafic rocks (ULMAF)

1

2

3

4

5

6

7

8

9

EXPLANATION

Figure 16. Groundwater-quality data by lithologic group of siliclastic-rock and crystalline-rock aquifers in the Piedmont and Blue Ridge 

Physiographic Provinces, 1994–2008. A, calcium; B, alkalinity; C, pH; D, dissolved oxygen; E, iron; F, manganese; G, arsenic; H, selenium; 

and I, molybdenum.
Water-Quality Characteristics of Aquifers, Lithologic Groups, and Lithochemical Subgroups 35

* *

*

*

*

*

*

*

*

*

*

*

***

*

*

*

*

*

*

*

*

* *

*

*

* * *

*

*

*

*

**

* *

* *

*

*

*

*

*

*

A B C

100

1,000

10,000

10

1

0.1

1,000

100

10

1

0.1

10

1

0.1

Copper, in micrograms per liter

Lead, in micrograms per liter

Barium, in micrograms per liter

1 43 7 55 12 61 55 17 2

- A B BC BC BC BC C -

1 28 5 55 12 61 55 13 2

- AB A AB AB B AB AB -

1 43 7 55 12 61 55 17 2

- B A B B B AB B -

1 43 7 55 12 61 55 17 2

- B A B B B B B -

1 43 7 55 12 61 55 17 2

- A A A A A A A -

D E F

10

100

1

0.1

1

0.1

10,000

1,000

100

10

Nickel, in micrograms per liter

Zinc, in micrograms per liter 

Tritium units

1,000

100

10

0.1

1

7 10 4 25 3 8 5 12 2

B AB - AB - AB AB A -

G H I

100

1,000

10

1

0.1

1

100

10

0.1

0.01

0.001

Radium, in picocuries per liter

Uranium, in micrograms per liter

100,000

10,000

1,000

100

10

Radon, in picocuries per liter

9 36 9 91 17 67 55 24 2

A A B B B B B B -

0 27 2 15 3 28 20 3 0

- A - B - AB AB -

9 36 8 76 10 60 55 20 1

- AB A AB A AB A AB A -

*

1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9

Outlier data value more than

 3 times the interquartile range

 outside the quartile

Outlier data value less than

 or equal to 3 and more than

 1.5 times the interquartile

 range outside the quartile

Data value less than or equal to

 1.5 times the interquartile range

 outside the quartile

75th percentile

25th percentile

Mean

Clastic sedimentary rocks (CLSD)

Clastic lacustrine/evaporite sedimentary rocks (CLSDLAC)

Quartz-rich sedimentary rocks (CLSDQ)

Metamorphosed clastic sedimentary rocks (CLSDMT)

Quartz-rich metamorphic rocks (MTQ)

Felsic igneous rocks and their metamorphic equivalents (IGMTF)

Intermediate igneous rocks and their metamorphic equivalents (IGMTI)

Mafic igneous rocks and their metamorphic equivalents (IGMTM)

Ultramafic rocks (ULMAF)

1

2

3

4

5

6

7

8

9

EXPLANATION

Figure 17. Groundwater-quality data by lithologic group of siliclastic-rock and crystalline-rock aquifers in the Piedmont and Blue Ridge 

Physiographic Provinces, 1994–2008. A, barium; B, copper; C, lead; D, nickel; E, zinc; F, tritium; G, uranium; H, radium 226+228; and 

I, radon-222.
36 Contaminants in Crystalline-Rock Aquifers and Siliciclastic-Rock Aquifers, Eastern United States, 1994–2008

*

*

*

*

*

*

*

*

*

*

*

**

*

*

*

** *

*

* *

* *

*

*

**

*

*

*

* *

**

*

*

*

* *

*

*

*

*

*

*

*

A B C

1

0

–1

–2

–3

–4

–5

–6

–7

Saturation indices, calcite

0

–1

–2

–3

–4

–5

–6

–7

Saturation indices, gypsum

2

0

–2

–4

–6

–8

–10

–12

–14

Saturation indices, dolomite

0

–1

–2

–3

–4

–5

–6

–7

Saturation indices, fluorite

1

0

–1

–2

–3

–4

Saturation indices, manganese phosphate

–5

0

–4

–8

–12

–16

–20

–24

–28

–32

Saturation indices, fluorapatite

1

0

–1

–2

–3

–4

–5

Saturation indices, strengite

1

0

–1

–2

–3

–4

–5

–6

Saturation indices, siderite

2

1

0

–1

–2

–3

–4

–5

Saturation indices, ferrihydrite

D E F

G H I

*

1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9

Outlier data value more than

 3 times the interquartile range

 outside the quartile

Outlier data value less than

 or equal to 3 and more than

 1.5 times the interquartile

 range outside the quartile

Data value less than or equal to

 1.5 times the interquartile range

 outside the quartile

75th percentile

25th percentile

Mean

Clastic sedimentary rocks (CLSD)

Clastic lacustrine/evaporite sedimentary rocks (CLSDLAC)

Quartz-rich sedimentary rocks (CLSDQ)

Metamorphosed clastic sedimentary rocks (CLSDMT)

Quartz-rich metamorphic rocks (MTQ)

Felsic igneous rocks and their metamorphic equivalents (IGMTF)

Intermediate igneous rocks and their metamorphic equivalents (IGMTI)

Mafic igneous rocks and their metamorphic equivalents (IGMTM)

Ultramafic rocks (ULMAF)

1

2

3

4

5

6

7

8

9

EXPLANATION

Figure 18. Saturation indices (SI) for selected minerals in groundwater by lithologic group of siliciclastic-rock and crystalline-rock 

aquifers in the Piedmont and Blue Ridge Physiographic Provinces. A, calcite; B, dolomite; C, gypsum; D, fluorite; E, fluorapatite; 

F, manganese phosphate; G, strengite; H, ferrihydrite; and I, siderite.
Water-Quality Characteristics of Aquifers, Lithologic Groups, and Lithochemical Subgroups 37

*

*

*

*

*

*

*

*

*

*

*

*

*

**

*

*

*

*

*

*

*

*

*

*

*

* *

*

*

*

A B C

3

2

1

0

–1

–2

–3

6

4

2

0

–2

–4

Saturation indices, adularia

Saturation indices, chlorite

Saturation indices, kaolinite

2

0

–2

–4

–6

–8

Saturation indices, gibbsite

1

0

–1

–2

–3

–4

Saturation indices, barite

1

0

–1

Saturation indices, chalcedony

–4

–5

–6

–7

–8

Saturation indices, anglesite

Saturation indices, smithsonite

Saturation indices, cerrusite

D E F

G H I

*

1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9

Outlier data value more than

 3 times the interquartile range

 outside the quartile

Outlier data value less than

 or equal to 3 and more than

 1.5 times the interquartile

 range outside the quartile

Data value less than or equal to

 1.5 times the interquartile range

 outside the quartile

75th percentile

25th percentile

Mean

Clastic sedimentary rocks (CLSD)

Clastic lacustrine/evaporite sedimentary rocks (CLSDLAC)

Quartz-rich sedimentary rocks (CLSDQ)

Metamorphosed clastic sedimentary rocks (CLSDMT)

Quartz-rich metamorphic rocks (MTQ)

Felsic igneous rocks and their metamorphic equivalents (IGMTF)

Intermediate igneous rocks and their metamorphic equivalents (IGMTI)

Mafic igneous rocks and their metamorphic equivalents (IGMTM)

Ultramafic rocks (ULMAF)

1

2

3

4

5

6

7

8

9

EXPLANATION

Figure 19. Saturation indices (SI) for selected minerals in groundwater by lithologic group of siliclastic-rock and crystalline-rock 

aquifers in the Piedmont and Blue Ridge Physiographic Provinces. A, andularia; B, chlorite; C, kaolinite; D, gibbsite; E, chalcedony; 

F, barite; G, anglesite; H, cerrusite; and I, smithsonite.
38 Contaminants in Crystalline-Rock Aquifers and Siliciclastic-Rock Aquifers, Eastern United States, 1994–2008

Redox / pH

A. All samples: 9 lithologic subgroups (n=337) F. MTQ: Quartz-rich metamorphic (n=16)

4.5 to <5.5 5.5 to <6.5 6.5 to <7.5 7.5 to <8.5

Anoxic x 2.7 (9) 8.6 (29) 3.3 (11)

Mixed 3.9 (13) 9.8 (33) 4.7 (16) 0.3 (1)

Oxic 9.2 (31) 33.2 (112) 15.1 (51) 9.2 (31)

Redox / pH 4.5 to <5.5 5.5 to <6.5 6.5 to <7.5 7.5 to <8.5

Anoxic x x 12.5 (2) x

Mixed 18.8 (3) 18.8 (3) x x

Oxic 6.3 (1) 37.5 (6) 6.3 (1) x

Redox / pH

D. CLSDLAC: Clastic lacustrine (n=51)

H. IGMTI: Intermediate igneous or metamorphic (n=55)

4.5 to <5.5 5.5 to <6.5 6.5 to <7.5 7.5 to <8.5

Anoxic x x 7.8 (4) 13.7 (7)

Mixed x x 2.01 (1) x

Oxic x 7.8 (4) 27.5 (14) 41.2 (21)

Redox / pH 4.5 to <5.5 5.5 to <6.5 6.5 to <7.5 7.5 to <8.5

Anoxic x 1.8 (1) 7.3 (4) x

Mixed 7.3 (4) 7.3 (4) 9.1 (5) 1.8 (1)

Oxic 18.2 (10) 43.6 (24) x 3.6 (2)

Redox / pH

C. CLSDQ: Quartz-rich sedimentary (n=10)

I. IGMTM: Mafic igneous or metamorphic (n=34)

4.5 to <5.5 5.5 to <6.5 6.5 to <7.5 7.5 to <8.5

Anoxic x x x 10.0 (1)

Mixed 10.0 (1) 10.0 (1) 10.0 (1) x

Oxic x 20.0 (2) 20.0 (2) 20.0 (2)

Redox / pH 4.5 to <5.5 5.5 to <6.5 6.5 to <7.5 7.5 to <8.5

Anoxic x x 17.6 (6) x

Mixed 2.9 (1) 2.9 (1) 2.9 (1) x

Oxic x 38.2 (13) 29.4 (10) 2.9 (1)

Redox / pH

E. CLSDMT: Metamorphosed clastic sedimentary (n=94) J. ULMAF: Ultramafic (n=2)

4.5 to <5.5 5.5 to <6.5 6.5 to <7.5 7.5 to <8.5

Anoxic x 5.3 (5) 4.3 (4) x

Mixed 2.1 (2) 16.0 (15) 4.3 (4) x

Oxic 14.9 (14) 35.1 (33) 13.8 (13) 4.3 (4)

Redox / pH 4.5 to <5.5 5.5 to <6.5 6.5 to <7.5 7.5 to <8.5

Anoxic x x x x

Mixed x x x x

Oxic x 50.0 (1) 50.0 (1) x

Redox / pH

B. CLSD: Clastic sedimentary (n=9) G. IGMTF: Felsic igneous or metamorphic (n=66)

4.5 to <5.5 5.5 to <6.5 6.5 to <7.5 7.5 to <8.5

Anoxic x x 11.1 (1) x

Mixed x x x x

Oxic x x 77.8 (7) 11.1 (1)

Redox / pH 4.5 to <5.5 5.5 to <6.5 6.5 to <7.5 7.5 to <8.5

Anoxic x 4.5 (3) 10.6 (7) 4.5 (3)

Mixed 3.0 (2) 13.6 (9) 6.1 (4) x

Oxic 9.1 (6) 43.9 (29) 4.5 (3) x

No data

Less than 10 percent of samples

in dataset or subset

10 to 20 percent of samples in

dataset or subset

20 to 50 percent of samples in

dataset or subset

Greater than 50 percent of samples in

dataset or subset

EXPLANATION

Figure 20. Redox/pH matrix summarizing groundwater-quality samples by lithologic group of sliciclastic-rock and crystalline-rock 

aquifers in the Piedmont and Blue Ridge Physiographic Provinces, 1994–2008. A, all nine lithologic groups; B, clastic sedimentary; C, 

clastic lacustrine/evaporite; D, quartz-rich sedimentary; E, metamorphosed clastic sedimentary; F, quartz-rich metamorphic; G, felsic 

igneous or metamorphic; H, intermediate igneous or metamorphic; I, mafic igneous or metamorphic; and J, ultramafic.

when ingested (Mays and others, 1985; U.S. Environmental 

Protection Agency, 1999). The risk is presumed to be linearly 

proportional to exposure (amount and duration; U.S. Environmental Protection Agency, 1999) and was used to determine 

conservative standards (MCLs) designed to limit exposure. 

Because the standards for radionuclides are slightly different 

and somewhat more complex than those for other trace elements, they are listed here in detail. The MCLs promulgated 

for radionuclides in 2000 are as follows: gross alpha-particle 

activity (including radium-226 but excluding uranium and 

radon), 15 pCi/L; gross beta-particle activity, 4 millirems per 

year (isotope-specific dose to be evaluated when a sample 

exceeds 50 pCi/L); uranium, 30 μg/L; and for radium [the 

sum of radium-226 and radium-228 (generally termed combined radium, and conveniently abbreviated as “Ra TOT”) in 

selected tables and figures in this report, including table 4], 

5 pCi/L (table 4). Gross alpha-particle activity had also been 

suggested for use as a compliance-monitoring “screen” for 

combined radium (Hess and others, 1985). In addition, radon 

has had a proposed (health-based) MCL of 300 pCi/L and a 
Water-Quality Characteristics of Aquifers, Lithologic Groups, and Lithochemical Subgroups 39

Redox / pH

A. Nitrate: HHB = 10 49.7% (159/320) > 0.1 HHB B. Manganese: HHB = 300 32.1% (106/330) > 0.1 HHB

4.5 to <5.5 5.5 to <6.5 6.5 to <7.5 7.5 to <8.5

Anoxic x 22.2 (9) 10.3 (29) 0 (11)

Mixed 58.3 (12) 42.4 (33) 18.8 (16) 0 (1)

Oxic 66.7 (30) 60.0 (100) 61.2 (49) 66.7 (30)

Redox / pH 4.5 to <5.5 5.5 to <6.5 6.5 to <7.5 7.5 to <8.5

Anoxic x 77.8 (9) 71.4 (28) 36.4 (11)

Mixed 100 (13) 93.9 (33) 100 (16) 100 (1)

Oxic 6.7 (30) 7.0 (100) 4.0 (50) 3.3 (30)

Redox / pH

G. Arsenic: HCRL = 2 17.1% (42/246) > HCRL

F. Lead: HCRL = 1 14.2% (36/253) > HCRL

4.5 to <5.5 5.5 to <6.5 6.5 to <7.5 7.5 to <8.5

Anoxic x 11.1 (9) 25.0 (20) 80.0 (10)

Mixed 0 (10) 0 (28) 7.1 (14) 0 (1)

Oxic 0 (22) 1.3 (75) 20.0 (30) 74.1 (27)

Redox / pH 4.5 to <5.5 5.5 to <6.5 6.5 to <7.5 7.5 to <8.5

Anoxic x 0 (9) 15.0 (20) 20.0 (10)

Mixed 10.0 (10) 0 (28) 7.1 (14) 0 (1)

Oxic 50.0 (22) 14.7 (75) 10.0 (30) 11.1 (27)

Redox / pH

E. Phosphate: HCRL = 0.02 49.8% (159/319) > HCRL

H. Zinc: HCRL = 20 5.5% (14/253) > HCRL

4.5 to <5.5 5.5 to <6.5 6.5 to <7.5 7.5 to <8.5

Anoxic x 44.9 (9) 42.9 (28) 27.3 (11)

Mixed 33.3 (12) 15.2 (33) 37.5 (16) 100 (1)

Oxic 30.0 (30) 63.0 (100) 67.3 (49) 63.3 (30)

Redox / pH 4.5 to <5.5 5.5 to <6.5 6.5 to <7.5 7.5 to <8.5

Anoxic x 44.4 (9) 35.0 (20) 0 (10)

Mixed 40.0 (10) 7.1 (28) 21.4 (14) 0 (1)

Oxic 27.3 (22) 24.0 (75) 23.3 (30) 3.7 (27)

Redox / pH

I. Selenium: HCRL = 1 7.7% (19/246) > HCRL J. Chromium: HCRL = 1 34.6% (84/243) > HCRL

4.5 to <5.5 5.5 to <6.5 6.5 to <7.5 7.5 to <8.5

Anoxic x 11.1 (9) 10.0 (20) 10.0 (10)

Mixed 20.0 (10) 0 (28) 0 (14) 0 (1)

Oxic 0 (22) 5.3 (75) 16.7 (30) 14.8 (27)

Redox / pH 4.5 to <5.5 5.5 to <6.5 6.5 to <7.5 7.5 to <8.5

Anoxic x 33.9 (9) 15.0 (20) 20.0 (10)

Mixed 70.0 (10) 25.0 (28) 7.1 (14) 0 (1)

Oxic 54.5 (22) 55.4 (74) 17.2 (29) 11.5 (26)

Redox / pH

K. Molybdenum: HCRL = 1 21.4% (48/224) > HCRL L. Nickel: HCRL = 1 37.3% (84/225) > HCRL

4.5 to <5.5 5.5 to <6.5 6.5 to <7.5 7.5 to <8.5

Anoxic

Mixed

Oxic

Redox / pH 4.5 to <5.5 5.5 to <6.5 6.5 to <7.5 7.5 to <8.5

Anoxic x 66.7 (9) 26.3 (19) 50.0 (6)

Mixed 70.0 (10) 78.6 (28) 28.6 (14) 0 (1)

Oxic 40.9 (22) 29.7 (74) 24.0 (25) 0 (17)

Redox / pH

M. Barium: HHB = 2,000 12.6% (31/246) > 0.1 HHB N. Cobalt: HCRL = 1 11.1% (25/225) > HCRL

4.5 to <5.5 5.5 to <6.5 6.5 to <7.5 7.5 to <8.5

Anoxic x 0 (9) 15.0 (20) 10.0 (10)

Mixed 20.0 (10) 0 (28) 7.1 (14) 0 (1)

Oxic 0 (22) 5.3 (75) 26.7 (30) 44.4 (27)

x 11.1 (9) 26.3 (19) 100 (6)

0 (10) 3.6 (28) 50.0 (14) 0 (1)

0 (21) 10.8 (74) 36.0 (25) 64.7 (17)

Redox / pH 4.5 to <5.5 5.5 to <6.5 6.5 to <7.5 7.5 to <8.5

Anoxic x 33.3 (9) 5.3 (19) 0 (6)

Mixed 30.0 (10) 46.4 (28) 14.3 (14) 0 (1)

Oxic 9.1 (22) 1.4 (74) 0 (25) 0 (17)

Redox / pH

O. Uranium: HCRL = 1 21.3% (64/301) > HCRL P. Copper: HCRL = 1 64.2% (158/246) > HCRL

4.5 to <5.5 5.5 to <6.5 6.5 to <7.5 7.5 to <8.5

Anoxic x 22.2 (9) 42.9 (28) 71.4 (7)

Mixed 7.7 (13) 3.3 (33) 25.0 (16) 0 (1)

Oxic 3.3 (30) 5.1 (99) 40.0 (45) 75.0 (20)

Redox / pH 4.5 to <5.5 5.5 to <6.5 6.5 to <7.5 7.5 to <8.5

Anoxic x 55.6 (9) 25.0 (20) 10.0 (10)

Mixed 70.0 (10) 46.4 (28) 21.4 (14) 0 (1)

Oxic 95.5 (22) 85.3 (75) 70.0 (30) 66.7 (27)

Redox / pH

Q. Radon 222: HHB = 4,000 18.4% (49/267) > HHB R. Radium TOT: HHB = 5 23.4% (22/94) > 0.2 HHB

4.5 to <5.5 5.5 to <6.5 6.5 to <7.5 7.5 to <8.5

Anoxic x 14.3 (7) 7.4 (27) 0 (7)

Mixed 30.0 (10) 21.1 (19) 16.7 (12) 0 (1)

Oxic 25.0 (28) 28.0 (93) 4.5 (44) 10.5 (19)

Redox / pH 4.5 to <5.5 5.5 to <6.5 6.5 to <7.5 7.5 to <8.5

Anoxic x 66.7 (3) 42.9 (14) 66.7 (3)

Mixed x 40.0 (5) 37.5 (8) 0 (1)

Oxic 0 (3) 15.4 (26) 12.5 (16) 6.7 (15)

Redox / pH

C. Sulfate: HHB = 250 14.9% (46/309) > 0.1 HHB D. Iron: HHB = 300 26.5% (82/309) > 0.1 HHB

4.5 to <5.5 5.5 to <6.5 6.5 to <7.5 7.5 to <8.5

Anoxic x 22.9 24.0 (25) 72.7 (11)

Mixed 15.4 (13) 6.3 (32) 13.3 (15) 0 (1)

Oxic 0 (28) 8.3 (96) 20.4 (49) 20.0 (30)

Redox / pH 4.5 to <5.5 5.5 to <6.5 6.5 to <7.5 7.5 to <8.5

Anoxic x 66.7 (9) 68.0 (25) 36.4 (11)

Mixed 53.8 (13) 78.1 (32) 80.0 (15) 100 (1)

Oxic 0 (28) 7.3 (96) 4.1 (49) 3.3 (30)

No data

All results below highest common reporting

limit (HCRL) or human health benchmark (HHB)

10 percent of samples exceed HCRL or HHB

10 to 20 percent of samples exceed HCRL or HHB

Greater than 20 to 50 percent of samples exceed HCRL or HHB

Greater than 50 percent of samples exceed HCRL or HHB

EXPLANATION

x

(HCRL) or human health benchmark (HHB). A, nNitrate; B, manganese; C, sulfate; D, iron; E, phosphate; F, lead; G, arsenic; H, zinc; 

I, selenium; J, chromium; K, molybdenum; L, nickel; M, barium; N, cobalt; O, uranium; P, copper; Q, radon-222; and R, radium-226+228 

(TOT).

Figure 21. Redox-pH matrix summarizing groundwater-quality samples greater than or equal to the highest common reporting level 
40 Contaminants in Crystalline-Rock Aquifers and Siliciclastic-Rock Aquifers, Eastern United States, 1994–2008

proposed AMCL of 4,000 pCi/L that may be used if airborne 

remediation or risk reduction approaches are implemented 

(U.S. Environmental Protection Agency, 1991); while proposed, the radon standards have yet to be adopted and fully 

implemented.

Five trace-element constituents (arsenic, antimony, iron, 

manganese, zinc) were detected in one or more water samples 

at concentrations greater than established human health-based 

benchmarks, and 10 additional constituents (barium, beryllium, cadmium, copper, lead, selenium, boron, molybdenum, 

nickel, and strontium) were detected at concentrations greater 

than a threshold of one-tenth the established health-based 

levels (table 4; figs. 12–17). Arsenic was detected at concentrations greater than the MCL of 10 μg/L in 9 of 253 samples, 

and antimony was detected at concentrations greater than the 

MCL of 6 μg/L in 1 of 232 samples. Manganese was detected 

at concentrations greater than the HBSL of 300 μg/L in 24 of 

330 samples, and zinc was detected at concentrations greater 

than the HBSL of 2,000 μg/L in 1 of 253 samples. Iron was 

detected at concentrations greater than the SMCL of 300 μg/L 

in 46 of 315 samples. 

The trace elements most frequently detected at concentrations greater than one-tenth of the MCL or HBSL were arsenic 

(47 of 253 samples > 1 μg/L), manganese (106 of 330 samples 

> 30 μg/L), zinc (14 of 253 samples > 200 μg/L), barium (31 

of 253 samples > 200 μg/L), molybdenum (16 of 230 samples 

> 4 μg/L), and lead (23 of 253 samples > 1.5 μg/L). Additionally, iron (84 of 315 samples > 30 μg/L) commonly was 

present at concentrations greater than one-tenth of the SMCL. 

Considering concentrations greater than one-tenth of the 

relevant threshold, the crystalline lithologies of the PBR had 

a larger percentage of samples (by a factor of two) containing 

iron, manganese, or fluoride, whereas the siliciclastic lithologies of the Early Mesozoic basins had a larger percentage of 

water samples containing arsenic, barium, selenium, boron, 

molybdenum, uranium, chloride, or sulfate (table 4). 

Of the radionuclides analyzed, radon-222 frequently 

was detected at concentrations greater than the proposed 

MCL of 300 pCi/L (248 of 275 samples) or the proposed 

AMCL of 4,000 pCi/L (51 of 275 samples) (table 4). Although 

rarely present at concentrations exceeding their respective 

MCLs, uranium (43 of 310 samples > 3 μg/L) and radium 

[Ra-226 plus Ra-228, abbreviated as (RaTOT)] (47 of 

98 samples > 1.0 pCi/L) frequently were detected at concentrations greater than one-tenth and one-fifth of their MCLs. 

Although the siliciclastic lithologies had a greater frequency 

of elevated uranium concentrations, radon and radium were 

commonly detected in groundwater from siliciclastic and 

crystalline lithologies. Only 2.0 percent of 98 samples had 

combined radium [Ra-226 plus Ra-228, abbreviated as 

(RaTOT)] concentrations greater than the MCL of 5.0 pCi/L, 

and these detections were evenly split among siliciclastic 

and crystalline lithologies; 0.6 percent of 310 samples had 

uranium concentrations greater than the MCL of 30 μg/L, all 

from crystalline lithologies. Only 6 percent of 50 samples had 

radon-222 concentrations greater than the proposed AMCL of 

4,000 pCi/L among the siliciclastic lithologies, but 21 percent 

of 225 samples had concentrations greater than the proposed 

AMCL from crystalline lithologies, most commonly from 

granites.

Considering nutrients and major ions that may be derived 

from natural and human-related sources, few samples had 

concentrations of nitrate (12 of 329 samples) greater than the 

MCL of 10 mg/L as nitrogen (N) or sulfate (2 of 315 samples) 

greater than the SMCL of 250 mg/L, and none had concentrations of chloride greater than the SMCL of 250 mg/L (table 4). 

Nevertheless, many samples had concentrations of nitrate 

(161 of 329 samples), sulfate (47 of 315 samples), or chloride 

(56 of 315 samples) greater than one-tenth of the respective 

MCL or SMCL thresholds. Few samples exceeded this lower 

threshold for fluoride (10 of 315 samples) or nitrite (2 of 

331 samples) (table 4). Generally, mineral weathering can be 

an important source of sulfate; however, elevated concentrations of nitrate and nitrite may be attributed to anthropogenic 

contamination. Likewise, although mineral weathering can 

be a source of background concentrations of chloride and 

fluoride, anthropogenic sources also can produce anomalous 

concentrations of these constituents. 

The concentration of dissolved nitrate in groundwater for 

the study ranged from less than 0.1 to 20.5 mg/L as N, with 

a median of 0.96 mg/L as N (fig. 11C). The siliciclastic and 

crystalline lithologies had median concentrations of nitrate of 

2.37 and 0.78 mg/L as N, respectively. Natural levels of nitrate 

in groundwater from rainfall and plant and animal sources 

generally are less than 1 mg/L as N in the eastern United 

States (Peters and Bonelli, 1982; Puckett, 1994; Holloway and 

others, 1998). Concentrations of nitrate that are greater than 

background concentrations are most commonly associated 

with agricultural and turf (lawns, golf courses) fertilizers and 

also with discharges from septic systems or sewage treatment 

plants (Denver and others, 2010). 

The concentration of chloride in groundwater for the 

study ranged from 0.27 to about 250 mg/L with a median of 

7.0 mg/L (fig. 11B). Groundwater from the siliciclastic-rock 

aquifers and crystalline-rock aquifers had median concentrations of chloride of 15.9 and 5.9 mg/L, respectively, which are 

significantly greater than background levels in atmospheric 

precipitation (Peters and Bonelli, 1982). Concentrations of 

chloride that are greater than background concentrations 

may be associated with agricultural applications of “potash” 

or potassium chloride (KCl) and manure and discharges of 

sewage effluent, although highest concentrations are associated with the application of road deicing salts such as sodium 

chloride (NaCl) and calcium chloride (CaCl2

) in urban areas 

of the northern part of the region (Denver and others, 2010). 

Although chloride transport is conservative, nitrate and other 

forms of nitrogen may be attenuated by denitrification (dissimilatory reduction) or biological uptake (assimilation). 

Hence, the relative abundances of nitrate and chloride may be 

useful with other chemical indicators to identify geochemical 
Water-Quality Characteristics of Aquifers, Lithologic Groups, and Lithochemical Subgroups 41

conditions within the aquifer where naturally occurring contaminants may or may not be attenuated or mobilized. 

The saturation index for selected minerals was evaluated 

to indicate the general corrosivity of the groundwater and the 

potential for specific major and trace minerals to dissolve or 

precipitate, thus increasing or decreasing concentrations of 

solutes in the groundwater. Most groundwaters from all lithologies were undersaturated (SI less than 0) with respect to common sulfides (pyrite, FeS2

), sulfates (gypsum, CaSO4·2H2O), 

carbonates [calcite, CaCO3; dolomite CaMg(CO3)2], and 

aluminosilicates [adularia, KAlSiO3; albite, NaAlSiO3; 

anorthite, CaAl2Si2O8; chlorite,Mg5Al2Si3O10(OH)8] (figs. 18 

and 19), indicating that weathering of major rock-forming 

minerals and trace minerals is a likely source of major and 

trace constituents in the groundwater. However, some groundwater samples, particularly those from the clastic sedimentary 

and clastic lacustrine/evaporite sedimentary lithologies of the 

Early Mesozoic basin aquifers, approached saturation or were 

saturated (SI approximately equal to 0) with respect to calcite 

and dolomite (figs. 18 and 19), indicating that their dissolution 

and precipitation could maintain or limit constituent concentrations. Likewise, most groundwater samples were supersaturated or saturated (SI greater than or equal to 0) with respect to 

kaolinite [Al2

Si2O5(OH)4], gibbsite [Al(OH)3], and ferrihydrite 

[amorphous Fe(OH)3], indicating that the precipitation of these 

secondary phases feasibly could limit concentrations of silica, 

aluminum, and iron. Although concentrations of manganese 

and barium frequently were at saturation levels with respect to 

certain solid phases (manganese phosphate, MnHPO4; barite, 

BaSO4), most other trace elements, including arsenic, selenium, uranium, lead, zinc, copper, cadmium, and strontium, 

were undersaturated with respect to pure mineral phases, 

suggesting that mineral precipitation would not be likely to 

limit concentrations of these contaminants. However, some 

trace elements, particularly divalent cations, may substitute for 

major ions such as calcium and magnesium in minerals that 

are supersaturated, and thus be reduced in concentration in 

the aqueous phase compared to the solubility of the pure trace 

mineral phase. 

The overall relations among bedrock lithology and 

water quality in the study area are consistent with different 

water-quality characteristics for major rock types described 

by Drever (1997). Specifically, in crystalline-rock aquifers, 

slow weathering of silicate minerals tends to produce natural 

groundwater with low concentrations of dissolved solids, alkalinity, and hardness (figs. 10–17). Consequently, most major 

carbonate, sulfate, and hydroxide minerals are undersaturated 

in these settings (figs. 18 and 19). In contrast, groundwater in the siliciclastic-rock aquifers commonly has greater 

concentrations of dissolved solids, alkalinity, and hardness 

than groundwater in adjacent crystalline-rock aquifers. The 

siliciclastic lithologies of the Early Mesozoic basins typically 

are cemented by carbonate minerals and in some places are 

cemented by sulfate minerals (Van Houten, 1965), and these 

minerals tend to dissolve rapidly (Van Houten, 1965; Langmuir, 1971). Concentrations of trace elements may increase 

with concentrations of dissolved solids because of the release 

of trace constituents dissolved from major minerals and 

because of the displacement of trace ions from surface sorption sites by major ions. In some instances, natural constituent 

concentrations may exceed thresholds established to protect 

human health. 

Correlations Among Major and Trace 

Constituents and Environmental Factors

Principal component analysis (PCA) provides insight 

on hydrochemical processes affecting groundwater chemistry in the study area by indicating intercorrelations among 

chemical constituents and environmental variables, such as 

land use and well depth. Five principal components (PCs) 

explain nearly 76 percent of the variance in the regional 

groundwater dataset and consist of 18 routinely detected 

constituent loadings (table 5). Associations of additional 

chemical and physical variables excluded from the model are 

indicated by the Spearman-rank coefficient of correlation of 

these variables with the principal component scores (table 5; 

appendix 1, table 1-7). 

PC1 has positive loadings by alkalinity, pH, calcium, 

magnesium, sodium, sulfate, and specific conductance and 

negative loading by dissolved oxygen (table 5). These loadings are related to increasing dissolved solids associated 

with the weathering of carbonate-bearing and sulfur-bearing 

minerals. Scores on PC1 generally were greater for siliciclastic 

lithologies than for crystalline lithologies and were positively 

correlated with agriculture, latitude, well depth, and waterquality constituents excluded from the PCA model, including 

hardness, total dissolved solids, strontium, uranium, boron, 

arsenic, molybdenum, fluoride, and barium; scores were 

negatively correlated with aluminum, chromium, tritium, and 

copper (table 5). Positive correlations with hardness and trace 

constituents, such as strontium and barium, are consistent 

with the weathering of alkaline-earth carbonate and sulfate 

minerals that are present as cements, fracture filling, and clasts 

in siliciclastic rocks. As explained previously (fig. 7) and in 

more detail below, several of the positively correlated trace 

constituents, specifically arsenic, molybdenum, and uranium, tend to be mobile as anions under high-pH conditions 

(Hodge and others, 1998), whereas negatively correlated trace 

constituents, specifically aluminum, chromium, and copper, 

tend to be mobile as cations under low-pH conditions. The 

positive associations of PC1 with well depth and negative 

associations with tritium and dissolved oxygen are consistent 

with increased mineralization and age of groundwater along 

flow paths. Likewise, positive correlations with latitude could 

relate to less extensively weathered bedrock in northern areas 

compared to more deeply weathered (leached) saprolitic and 

lateritic horizons in the southern parts of the study area. 

PC2 has positive loadings by chloride, bromide, sodium, 

magnesium, nitrate, dissolved organic carbon, and specific 

conductance (table 5). Although the constituents could 
42 Contaminants in Crystalline-Rock Aquifers and Siliciclastic-Rock Aquifers, Eastern United States, 1994–2008

Table 5. Principal components analysis model of major factors controlling the chemistry of groundwater from siliciclastic-rock and 

crystalline-rock aquifers of the Piedmont and Blue Ridge Physiographic Provinces, 1994–2008.—Continued

[Varimax rotation factor pattern for rank-transformed data (SAS, 1988); minimum eigenvalue >1; >, greater than; loading and correlation coefficient values 

multiplied by 100 and rounded; *, indicates significant loadings (p <0.001). PC, principal component]

PC1 PC2 PC3 PC4 PC5

CommuAlkalinity- nalities

pH

Chloridenitrate Redox

Temperaturesilica

Radonpotassium

Constituent loadings

Alkalinity 91 * 18 –3 19 –18 0.93

pH 89 * –12 –4 6 –16 0.85

Calcium 88 * 37 –5 1 –6 0.92

Specific conductance 81 * 55 * 1 –3 –5 0.96

Magnesium 69 * 53 * –5 –11 –6 0.78

Sulfate 67 * 35 22 –17 3 0.66

Dissolved oxygen –64 * –2 –46 * –32 2 0.74

Chloride 30 86 * –2 –15 13 0.86

Bromide 26 71 * 6 29 –7 0.67

Nitrate –21 63 * –43 * –34 19 0.78

Sodium 55 * 59 * –4 32 9 0.76

Dissolved organic carbon 20 58 * 10 0 –36 0.51

Manganese (>1) –1 11 90 * –1 –2 0.79

Iron (>6) 1 –11 82 * –1 –12 0.67

Temperature –18 15 8 86 * –11 0.81

Silica 26 –13 77 * 11 0.69

Radon-222 –18 0 –19 –9 81 * 0.72

Potassium –1 1 46 * 25 48 * 0.54

Eigenvalue: 6.61 2.76 1.66 1.57 1.05 13.64

Cumulative percent variance explained: 36.67 51.68 61.28 70.04 75.85

Spearman Correlations (only values significant at p <0.05 are reported):

Latitude 37 -- -- –63 --

Agricultural land-use percentage 27 -- -- –43 --

Urban land-use percentage -- 22 -- -- --

Forested land-use percentage -- –43 -- 35 --

Wetland land-use percentage -- -- -- -- --

Well depth 25 –23 -- -- --

Hardness 85 46 -- -- --

Total dissolved solids 81 53 -- -- --

Strontium 58 46 -- -- --

Uranium 57 -- -- -- --

Boron (>8) 56 43 -- –72 --
Water-Quality Characteristics of Aquifers, Lithologic Groups, and Lithochemical Subgroups 43

Table 5. Principal components analysis model of major factors controlling the chemistry of groundwater from siliciclastic-rock and 

crystalline-rock aquifers of the Piedmont and Blue Ridge Physiographic Provinces, 1994–2008.—Continued

[Varimax rotation factor pattern for rank-transformed data (SAS, 1988); minimum eigenvalue >1; >, greater than; loading and correlation coefficient values 

multiplied by 100 and rounded; *, indicates significant loadings (p <0.001). PC, principal component]

PC1 PC2 PC3 PC4 PC5

CommuAlkalinity- nalities

pH

Chloridenitrate Redox

Temperaturesilica

Radonpotassium

Spearman Correlations (only values significant at p <0.05 are reported):—Continued

Arsenic (>1) 50 26 -- -- --

Molybdenum 47 -- -- -- --

Fluoride 38 -- -- -- --

Barium 28 53 -- –27 --

Chromium –35 -- -- -- --

Copper –47 -- –28 -- --

Tritium –58 -- -- –45 --

Aluminum (>1.6) –62 -- -- -- --

Nickel -- 45 33 -- --

Radium-226 -- -- 54 -- --

Radium-226 plus radium-228 -- -- 41 -- --

Cobalt -- -- 26 -- --

Ammonia -- -- 26 -- --

Phosphate -- -- –37 39 --

Vanadium -- -- –76 -- --

Lithium -- -- -- -- 47

Silver -- -- -- -- --

Beryllium -- -- -- -- --

Cadmium -- -- -- -- --

Lead -- -- -- -- --

Radium-228 -- -- -- -- --

Antimony -- -- -- -- --

Selenium -- -- -- -- --

Thalium -- -- -- -- --

Zinc (>1) -- -- -- -- --

originate from natural sources, their corresponding positive 

associations are interpreted to indicate anthropogenic sources 

of contamination, such as road-deicing salt, fertilizer, and sewage. Scores on PC2 were negatively correlated with well depth 

and forest area and positively correlated with urban area, hardness, total dissolved solids (TDS), barium, strontium, nickel, 

boron, and arsenic. Although PC2 scores ranged more widely 

for crystalline lithologies compared to siliciclastic lithologies, the median score for siliciclastic lithologies was larger 

than that for crystalline lithologies. The land use overlying the 

siliciclastic lithologies tends more frequently to be urban and 

less frequently to be forested than that for the more steep terrains associated with the crystalline-rock lithologies 

(figs. 2 and 4).

PC3 has positive loadings by manganese, iron, and potassium and negative loadings by dissolved oxygen and nitrate 

(table 5). High scores on PC3 are interpreted to indicate isolation from the atmosphere and the development of reducing 
44 Contaminants in Crystalline-Rock Aquifers and Siliciclastic-Rock Aquifers, Eastern United States, 1994–2008

conditions. Scores on PC3 generally were larger for crystalline 

lithologies than siliciclastic lithologies and were not correlated with land use. Scores on PC3 were positively correlated 

with radium-226, combined radium (Ra-226+Ra-228), cobalt, 

nickel, and ammonia and negatively correlated with vanadium, 

phosphate, and copper. Nitrate is stable under oxidizing conditions where iron and manganese concentrations may be limited 

by precipitation of iron and manganese oxides; ammonia is 

stable under reducing conditions. The positive associations 

of potassium and radium on PC3 and potassium and radon on 

PC5 (described below) are consistent with felsic, granitic, or 

arkosic rocks as a source of radioactive elements in groundwater. As explained previously (figs. 8 and 9) and in more detail 

below, radon is a highly mobile noble gas, whereas radium, 

cobalt, and other cations can be adsorbed by iron and manganese oxides. Subsequent dissolution of the iron and manganese 

oxides under reducing conditions will mobilize the sorbed 

constituents, such as radium and cobalt. The negative correlations of vanadium, phosphate, and copper with PC3 could 

indicate greater mobility of these constituents under oxidizing 

conditions, where the concentrations of iron and manganese 

are low. 

PC4 has positive loadings by temperature and silica 

(table 5). Scores on PC4 were positively correlated with 

forest and phosphate and negatively correlated with latitude, 

agriculture, boron, tritium, and barium. Denver and others 

(2010) explained that co-occurrence of phosphate and silica in 

groundwater from forested areas could result from the weathering of common silicate minerals containing phosphorus as a 

trace constituent. Generally, the mean annual temperature of 

groundwater decreases with latitude, and tritium concentration 

decreases with groundwater age. Thus, these associations on 

PC4 may indicate that increased temperature or longer residence time in the aquifer promotes greater rates and extent of 

weathering of silicate minerals. The range of and median PC4 

scores for crystalline lithologies, which predominate in southern latitudes, were larger than those for siliciclastic lithologies 

(appendix 2). 

PC5 has positive loadings by radon and potassium and 

positive correlations with lithium (table 5). Although the range 

of PC5 scores for crystalline lithologies is greater than that for 

siliciclastic lithologies, the median scores for the two lithologies are comparable. As explained in more detail below, these 

associations and distributions are consistent with the emanation of radon from felsic, granitic, or arkosic rocks that tend to 

be rich in uranium, potassium, and lithium (Speer and others, 

1981; Michel, 1984). Radon, potassium, and lithium tend to be 

mobile in a wide range of groundwater environments compared to other constituents that tend to be affected by variations in pH and (or) redox conditions. 

Geochemical Conditions Associated with 

Elevated Concentrations of Naturally Occurring 

Constituents

The associations of certain trace elements and radionuclides in groundwater may be explained by similar rock 

sources and (or) geochemical processes. These constituents are 

discussed separately below, however, because of differences in 

monitoring frequency, reporting protocols, and human-health 

thresholds. 

Trace Elements 

Arsenic, manganese, and zinc were identified as trace elements of concern in groundwater of the study area on the basis 

of their relatively high frequencies of exceedance of human 

health benchmarks for drinking water (table 4). Additionally, 

lead, barium, antimony, and molybdenum were noteworthy 

because of relatively high frequencies of detection of these 

constituents at concentrations greater than one-tenth of their 

respective health-based thresholds. Specific settings and geochemical conditions associated with anomalous concentrations 

of these constituents are described in more detail below. 

Arsenic

Although arsenic concentrations were less than 1 μg/L in 

nearly 80 percent of the 253 groundwater samples analyzed in 

the study, 8.5 percent of sampled wells in the Early Mesozoic 

basin aquifers and 2.4 percent of the sampled wells in the 

Piedmont and Blue Ridge aquifers had arsenic concentrations 

greater than the USEPA MCL of 10 μg/L (table 4). Arsenic 

concentrations greater than 1 μg/L were detected in samples 

from wells from all lithologies in these aquifers, but all 

concentrations greater than the MCL were present in groundwater from clastic lacustrine sedimentary rocks of the Early 

Mesozoic basin aquifers and from metamorphosed clastic 

sedimentary rocks of the PBR aquifers (fig. 16G). Maximum 

arsenic concentrations in water samples from these aquifers 

were 57 and 38 μg/L, respectively (tables 1-3 and 1-4). Both 

lithologic groups are composed of fine-grained sedimentary 

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

Although redox conditions typically control the mobility 

of arsenic in groundwater (Smedley and Kinniburgh, 2002; 

Welch and Stollenwerk, 2003; Serfes and others, 2010), the 

dominant factor controlling arsenic mobility in the study 

area appears to be pH, with elevated arsenic concentrations 

associated with alkaline pH conditions (fig. 21G). Arsenate 

and arsenite, which are the predominant forms of arsenic in 

groundwater, tend to adsorb to iron-oxide surfaces at acidic 

to neutral pH, but not at alkaline pH conditions (fig. 8). Thus, 

arsenic concentrations greater than 1 μg/L were associated 

with a range of oxic to anoxic redox conditions, primarily 

where pH values were greater than 7.5 (fig. 21G). Of the nine 

samples for which arsenic concentrations were greater than 
Water-Quality Characteristics of Aquifers, Lithologic Groups, and Lithochemical Subgroups 45

10 μg/L, six were classified as oxic and three as anoxic, and 

seven had a pH of 7.2 or greater (maximum pH, 8.0). The 

clastic lacustrine/evaporite sedimentary and metamorphosed 

clastic sedimentary lithologic groups and associated lithochemical subgroups that had the highest mean ranks of arsenic 

on the basis of Tukey tests also had the highest mean ranks of 

pH [fig. 16C and G; table 6 (table 6 available online at http://

pubs.usgs.gov/sir/2013/5072/)]. One other factor that supports 

the concept that the arsenic mobility is controlled by sorption 

processes is the consistent correlation of arsenic concentrations with other oxyanions, including boron, uranium, antimony, and molybdenum (appendix 1, table 1-7). The mobility 

of these oxyanions also is primarily controlled by adsorption 

at acidic to neutral pH and desorption at alkaline pH conditions (figs. 8 and 21). 

The spatial distribution pattern for elevated arsenic concentrations indicates locations with elevated levels of arsenic 

in the rock and geochemical conditions favoring transport 

of arsenic. Although 72 percent of the 47 samples from the 

Early Mesozoic basin aquifers had arsenic concentrations 

greater than 1 μg/L, only 6.3 percent of the 206 samples from 

the PBR crystalline-rock aquifers had arsenic concentrations 

greater than 1 μg/L (table 4). However, 46 percent of the 

samples from two crystalline-rock lithologies, metasedimentary rocks and meta-argillite, had concentrations greater than 

10 μg/L. The results for the samples from the Early Mesozoic 

basin aquifers with respect to arsenic obtained in this study are 

consistent with results of previous studies for these aquifers 

(Serfes, 1994, 2004; Lindsey and others, 2006; Senior and 

Sloto, 2006; Harden and others, 2009; Serfes and others, 

2010). Although the overall exposure of the population in the 

study area to arsenic is low, arsenic concentrations are likely to 

be elevated in some areas that are densely populated in Pennsylvania and New Jersey (figs. 2 and 22). Even in these areas 

of possible elevated arsenic concentrations in groundwater, the 

frequency for exceeding the MCL was less than 15 percent.

Manganese

Manganese concentrations were greater than or equal to 

4 μg/L in 75 percent of the 330 groundwater samples analyzed 

in the study; 7.3 percent and 32.1 percent of the samples had 

concentrations greater than the HBSL of 300 μg/L and onetenth of the HBSL, respectively (table 4). Manganese concentrations generally were elevated in groundwater from the 

PBR crystalline-rock aquifers (figs. 12D and 16F) where the 

bedrock and overlying geologic materials contain manganesebearing minerals and geochemical conditions in the aquifer 

facilitate manganese transport. Only the clastic sedimentary rocks and quartz-rich sedimentary rocks of the Early 

Mesozoic basin aquifers had concentrations of manganese 

less than 300 μg/L in all samples (fig. 16F). On the basis of 

Tukey tests, the mean rank manganese concentrations for the 

metamorphosed clastic sedimentary and quartz-rich metamorphic lithologic groups of the PBR crystalline-rock aquifers 

were greater than those for the clastic sedimentary and clastic 

lacustrine/evaporate sedimentary lithologic groups of the 

Early Mesozoic basin aquifers (fig. 16F). The lithochemical 

subgroup 43 of the mafic igneous and metamorphic lithologic 

group had the greatest median concentration of manganese 

(54 μg/L) of all lithochemical subgroups (table 6). Lithochemical subgroup 43 consists of massive mafic rocks including 

diorite, gabbro, monzodiorite, diabase, and basalt (table 3 and 

appendix 1, table 1-1). 

The concentration of manganese in groundwater was 

positively correlated with iron, cobalt, nickel, aluminum 

(censored at 10 μg/L), and ammonia and negatively correlated 

with dissolved oxygen, nitrate, phosphate, and vanadium 

(appendix 1, table 1-7). The relations between dissolved 

oxygen, manganese, iron, nitrate, and ammonia are consistent 

with elevated mobility of the reduced forms of manganese 

(Mn+2) and iron (Fe+2) under reducing conditions (fig. 21B). 

The positive correlations between manganese, iron, nickel, 

and cobalt could be attributed to the reductive dissolution of 

Mn(III-IV) and Fe(III) oxides and the consequent release of 

sorbed cations (Loganathan and Burau, 1973; Kooner, 1993), 

whereas the negative correlations between manganese, phosphate, and vanadium could be attributed to the precipitation of 

Mn(II)-phosphate and associated vanadate compounds under 

reducing conditions as indicated by the saturation indices for 

MnHPO4

 and Mn3(PO4)2 (appendix 1, table 1-7). Hence, the 

frequencies of elevated concentrations of manganese, iron, 

cobalt, and nickel generally are the opposite of those of nitrate, 

phosphate, and vanadium as functions of pH and redox conditions (fig. 21B). 

Although reductive dissolution of Mn(III-IV) oxides 

could be a primary source of dissolved manganese and 

associated trace elements in many groundwater samples, it 

is noteworthy that the highest concentrations of dissolved 

manganese are associated with low to intermediate values 

of pH and high concentrations of aluminum (fig. 21 and 

appendix 1, table 1-7). Low pH can promote the dissolution of 

various potential sources of manganese, including manganese 

oxides, common aluminosilicate minerals, such as chlorite, 

and carbonate minerals, such as calcite and siderite. The carbonate and aluminosilicate minerals, in which Mn+2 and Fe+2 

commonly substitute for magnesium (Mg+2), would tend to be 

stable under reducing conditions, but not under acidic conditions. Because of incongruent dissolution, the weathering of 

chlorite and other aluminosilicates may preferentially leach 

magnesium, iron, manganese, and, to a lesser extent, aluminum, relative to silica (Drever, 1997). 

Zinc

Zinc concentrations were greater than or equal to 20 μg/L 

in 21 percent of the 253 groundwater samples analyzed in the 

study; 0.4 percent and 5.5 percent of the samples had concentrations greater than the HBSL of 2,000 μg/L and one-tenth 

of the HBSL, respectively (table 4). Although the highest zinc 

concentrations were recorded for groundwater samples from 

the PBR crystalline-rock aquifers (figs. 13B and 17E), the 

quartz-rich sedimentary lithologic group of the Early Mesozoic basin siliciclastic-rock aquifers had a higher mean rank 
46 Contaminants in Crystalline-Rock Aquifers and Siliciclastic-Rock Aquifers, Eastern United States, 1994–2008

TENNESSEE

ALABAMA

GEORGIA

SOUTH CAROLINA

NORTH CAROLINA

VIRGINIA

WEST VIRGINIA

MARYLAND

NEW

JERSEY

PENNSYLVANIA

NEW YORK

KENTUCKY

0 50 100 KILOMETERS 25

0 50 100 MILES 25

OHIO

INDIANA

MICHIGAN

DELAWARE

Piedmont

Blue

Ridge

ATLANTIC

OCEAN

EXPLANATION

LITHOCHEMICAL SUBGROUP

Lacustrine/evaporite sedimentary rocks

Metasedimentary and meta-argillite rocks

Physiographic boundary between Piedmont

 and Blue Ridge

Base from U.S. Bureau of the Census, 1990, 1:500,000 to 1:5,000,000 and Statistics Canada digital data, 2006

Albers Equal-Area Conic projection: Standard Parallels 29°30' N and 45°30' N,

Central Meridian 96°00' W, Latitude of Origin 23°00' N

Lithologic group data from Dicken and others, 2005a and

2005b and Nicholson and others, 2005 and 2006

85° 80° 75°

40°

35°

Figure 22. Areal occurrence of lithochemical subgroups having elevated arsenic concentrations (based on the Tukey mean rank test, 

table 6) in groundwater within the Piedmont and Blue Ridge Physiographic Provinces.
Water-Quality Characteristics of Aquifers, Lithologic Groups, and Lithochemical Subgroups 47

zinc concentration than all other lithologic groups (fig. 17E). 

In some parts of the study area, this lithologic unit is a local 

host of zinc ore minerals (Smith, 1977) and, regionally, may 

have elevated background concentrations of zinc. 

Various natural and anthropogenic materials can be 

important sources of dissolved zinc in groundwater. Zinc 

may be present as a trace mineral, such as sphalerite (ZnS) or 

smithsonite (ZnCO3), or as a trace substitution for calcium, 

magnesium, or other divalent cations in common carbonate and silicate minerals in aquifers of the study area (Smith, 

1977; Hanshaw and Back, 1979). Furthermore, dissolved zinc 

concentrations and transport can be affected by sorption reactions with carbonate minerals and iron and aluminum oxides 

(Zachara and others, 1991; Kooner, 1993; Coston and others, 

1995). As described previously, dissolved cations, such as 

Zn+2, tend to be poorly adsorbed and are relatively mobile at 

acidic pH, whereas at alkaline pH, the cation concentrations 

tend to be attenuated by adsorption on HFO and other oxide 

surfaces (fig. 8). Additionally, zinc can be derived from the 

corrosion of galvanized casing and pipes used for well construction and water distribution. 

The concentration of zinc in groundwater was positively 

correlated with concentrations of copper, lead, and aluminum 

and with percentage of forested land use and negatively correlated with pH, concentrations of calcium, magnesium, alkalinity (as calcium carbonate), sulfate, arsenic, and total dissolved 

solids, percentage of urban land use, and saturation index of 

calcite and many other minerals (appendix 1, table 1-7). These 

correlations are consistent with elevated mobility of zinc 

under acidic, corrosive conditions associated with low pH and 

low ionic strength. Samples with oxic to anoxic redox conditions and pH less than 7.5 had a higher frequency of elevated 

zinc concentrations (greater than 10 mg/L) than samples with 

alkaline pH (fig. 21H). Likewise, concentrations of chromium, 

copper, lead, cobalt, and nickel were more frequently detected 

in groundwater samples with low to moderate pH than in 

groundwater samples with alkaline pH conditions (fig. 21). 

Low-pH conditions may mobilize zinc and other metals from 

natural mineral sources or plumbing and inhibit its adsorption 

by Fe(III), Mn(III-IV), and aluminum-oxides in the aquifer.

Zinc, iron, copper, lead, chromium, nickel, and other metals are commonly used materials for the construction of wells 

and associated plumbing systems. Different casing types may 

be used for different hydrogeological conditions. For example, 

steel and galvanized casing typically are used for small bore, 

deep wells in fractured bedrock, whereas concrete and plastic 

casing typically are used for larger bore, shallow wells in 

regolith or unconsolidated materials or where the water may 

be corrosive. 

To evaluate the ability of plumbing materials to contribute dissolved metals to groundwater samples, boxplots were 

generated on the basis of the well-casing type (steel, galvanized, concrete, plastic, other) identified with each groundwater sample, and Tukey tests were conducted to evaluate 

differences among the mean rank concentration values for 

these sample groups (fig. 23). Although concentrations of 

iron and dissolved oxygen were similar for the five different 

casing types, indicating comparable redox conditions among 

the groups, the maximum and mean rank zinc concentrations were greatest for samples from wells constructed using 

galvanized casing, and the mean rank copper concentration 

was greater for samples from wells constructed using concrete casing (fig. 23). Increased concentrations of zinc may 

be related to higher pH in deeper bedrock wells with steel or 

galvanized steel casing, whereas the more acidic pH in shallow wells (generally regolith material) may increase copper 

concentrations (table 5). Furthermore, the pH was higher for 

the samples associated with galvanized casing than for the 

samples associated with concrete casing. These water-quality 

differences suggest that the aquifer characteristics affect the 

type of well construction and (or) that the casing type affects 

the water chemistry. 

To evaluate the apparent effect of galvanized casing on 

zinc concentrations, the 24 samples identified with galvanized 

casing were eliminated from the dataset before recreating 

the boxplots and the Tukey tests for the nine major lithologic 

groups. Although fewer samples were associated with each 

of the lithologic groups, the boxplots and Tukey test results 

did not differ from the original results. Despite eliminating 

samples associated with galvanized well casing, the quartzrich sedimentary lithologic group of the Early Mesozoic basin 

siliciclastic-rock aquifers had a higher mean rank zinc concentration than all other lithologic groups (fig. 17E). The concentration of zinc in groundwater from the ultramafic lithologic 

group also was elevated, but this group only had a sample size 

of two, so the data could not be rigorously evaluated. 

Alkalinity, Hardness, and Dissolved Solids

Calcite clasts, fracture filling, and cements in some 

sedimentary-rock aquifers can dissolve easily and lead to 

high levels of alkalinity, hardness, and TDS. High alkalinity in such aquifers helps to maintain stable, near neutral pH 

(6 to 8). Thus, corrosivity, pH, hardness, and TDS generally 

are related by the calcite saturation index, which is equal to the 

Langelier index of corrosivity (Drever, 1997). In general, the 

samples from the clastic sedimentary and clastic lacustrine/

evaporite sedimentary lithologic units had the highest overall 

saturation indices for calcite, dolomite, and other carbonate 

minerals (fig. 18). 

Only a few wells sampled from the Early Mesozoic 

basin siliciclastic-rock aquifers had groundwater that was 

classified as hard (calcium carbonate equivalent 150 to 

300 mg/L) or very hard (calcium carbonate equivalent greater 

than 300 mg/L) (fig. 10D). Samples of water from the PBR 

crystalline-rock aquifers frequently were characterized as 

soft and had pH values less than the recommended pH of 6.5 

(fig. 12A). Moderate hardness with neutral pH is considered 

desirable because soft water with low pH can corrode plumbing and facilitate the dissolution of various aquifer minerals, 

whereas hard water with high pH can lead to scaling and 

clogged plumbing (encrustation) (Hem, 1985). 
48 Contaminants in Crystalline-Rock Aquifers and Siliciclastic-Rock Aquifers, Eastern United States, 1994–2008

*

**

*

*

*

*

*

*

*

*

*

*

*

*

*

*

*

*

*******

*

**

*

*

**

*

*

*

*

*

*

*

*

*

*

*

*

*

*

*

**

*

*

A B C

10

100

10,000

1,000

1

0.1 1

10

100

1,000

10,000

100,000

Zinc, in micrograms per liter 

Iron, in micrograms per liter 

Copper, in micrograms per liter 

1,000

100

10

0.1

1

90 24 5 92 42

B A B B B

158 22 5 97 33

A A A A A

90 24 5 92 42

B B A AB AB

175 24 5 97 44

AB A B AB AB

172 22 5 97 39

A A A A A

158 22 5 97 33

A AB B AB B

D E F

9.0

8.5

8.0

7.5

7.0

6.5

6.0

5.5

5.0

4.5

4.0

1

100

10

0.1

0.001

0.01

pH, units 

Oxygen, in milligrams per liter

10

1

1,000

100

10,000

Total dissolved solids, in milligrams per liter

Galvanized steel and iron

Concrete or brick

Polyvinyl chloride (PVC)

Other (unknown material)

Steel

Galvanized steel and iron

Concrete or brick

Polyvinyl chloride (PVC)

Other (unknown material)

Steel

Steel

Galvanized steel and iron

Concrete or brick

Polyvinyl chloride (PVC)

Other (unknown material)

Outlier data value more than

 3 times the interquartile range

 outside the quartile

Outlier data value less than

 or equal to 3 and more than

 1.5 times the interquartile

 range outside the quartile

Data value less than or equal to

 1.5 times the interquartile range

 outside the quartile

75th percentile

25th percentile

Mean

EXPLANATION

*

Figure 23. Concentrations of selected metals and associated constituents in groundwater by casing type for wells in siliciclastic- and 

crystalline-rock aquifers in the Piedmont and Blue Ridge Physiographic Provinces.
Water-Quality Characteristics of Aquifers, Lithologic Groups, and Lithochemical Subgroups 49

Antimony, Lead, Barium, and Molybdenum

Antimony was detected in only 11 of 202 samples 

from crystalline-rock aquifers; none of the 30 samples from 

siliciclastic-rock aquifers had detectable concentrations of 

antimony (table 4, fig. 14D). One sample from the metamorphosed clastic sedimentary lithologic group of the crystallinerock aquifers had a concentration of antimony of 10.4 μg/L, 

which exceeded the MCL of 6 μg/L; four samples had concentrations greater than one-tenth of the MCL (table 4). 

None of the samples evaluated for this study had concentrations of lead, barium, or molybdenum greater than 

their respective MCLs (table 4). However, 23 of 253 samples 

analyzed for lead had concentrations greater than one-tenth of 

the 15-μg/L MCL (9.0 percent of samples had concentrations 

greater than 1.5 μg/L); 31 of 253 samples analyzed for barium 

had concentrations greater than one-tenth of the 2,000-μg/L 

MCL (12.3 percent of samples had concentrations greater than 

200 μg/L); and 16 of 230 samples analyzed for molybdenum 

had concentrations greater than one-tenth of the 40-μg/L 

HBSL (7.0 percent of samples had concentrations greater than 

4 μg/L). A greater frequency of elevated barium concentrations was observed for groundwater samples from siliciclasticrock aquifers compared to those from crystalline-rock aquifers 

(table 4, figs. 14A and 17A), whereas lead concentrations were 

more frequently elevated in samples from the crystalline-rock 

aquifers (table 4, figs. 13D and 17C). 

Lead and barium generally occur as divalent cations 

in carbonate, sulfate, and sulfide minerals and associated 

groundwater. These trace cations commonly substitute for 

calcium or magnesium in major carbonate and sulfate minerals in aquifers (Back and others, 1979). The sulfate minerals 

anglesite (PbSO4

) and barite (BaSO4) and the carbonate minerals cerrusite (PbCO3

) and witherite (BaCO3) can be stable in 

surficial environments where concentrations of sulfate or alkalinity (dissolved carbonate) are elevated (Cravotta, 2008b). 

For example, barite saturation index values greater than 0 for 

many samples from the clastic lacustrine/evaporite sedimentary lithologic group and for one or more samples from several 

other lithologic groups indicate potential for barite precipitation to limit the concentration of barium. Generally, samples 

that were saturated with barite had elevated concentrations of 

sulfate and barium (figs. 15 and 17), which were positively 

correlated for the dataset as a whole (appendix 1, table 1-7). 

However, negative values of saturation indices for witherite 

for all samples and for barite for a majority of samples indicate that these minerals feasibly could dissolve, under most 

of the geochemical conditions under which these samples 

were collected, if present in the aquifer (fig. 19). Likewise, 

all the groundwater samples analyzed in this study were 

undersaturated with respect to anglesite, cerrusite, and other 

lead minerals, indicating that such minerals could feasibly 

dissolve (fig. 19). 

Iron oxides can adsorb lead at relatively low pH values, 

with increasing potential for adsorption as pH increases; 

however, iron oxides are not strong adsorbents of barium at 

pH values less than 8 (fig. 8). Consequently, groundwater 

samples with low pH (4.5 to 5.5) had the greatest frequency of 

lead concentrations greater than the 1-μg/L HCRL (fig. 21F), 

whereas groundwater with neutral to alkaline pH had the highest frequency of barium concentrations greater than one-tenth 

of the 2,000-μg/L MCL (fig. 21M). In contrast with lead and 

barium, molybdenum tends to occur in groundwater as the 

oxyanion molybdate, with characteristics similar to chromate 

and tungstate ions (Cotton and others, 1999). As for other 

oxyanions with decreased adsorption at neutral to alkaline pH 

(fig. 8), groundwater with pH greater than or equal to 6.5 had 

the greatest frequency of molybdenum concentrations greater 

than the 1-μg/L HCRL (fig. 21K). 

Radionuclides

The presence of uranium and radium in water requires a 

uranium source (parent of radium) in the host rock (fig. 9) and 

geochemical conditions in the aquifer that are conducive to 

transport of these elements. Radon, however, is highly soluble 

and is related primarily to the abundance of uranium in the 

rock. The variation in uranium content in rocks is generally 

understood, and this provides key background information on 

the potential sources of uranium, and thus radium and radon, 

its progeny. Overall, a comparison of the aquifers in the study 

area suggests that the elevated concentrations of the various 

radionuclides are distributed in ways that are consistent with 

their lithological sources and their chemical and radiological 

properties. Thorium is the source of two isotopes of radium, 

but it is insoluble in common water geochemical types found 

in the natural environment, except in those representing the 

most extreme geochemical environments (strongly acidic or 

strongly alkaline; Langmuir and Herman, 1980) that are not 

found in the aquifers in the study area. 

For some radionuclide contaminants, knowledge of specific lithology (for example, felsic compared to mafic rocks or 

lacustrine compared to quartzose siliciclastic rocks) is needed 

to explain differences in occurrence that gross lithology (siliciclastic compared to crystalline) cannot. The lithogeochemical 

framework developed for the PBR crystalline-rock aquifers 

and the Early Mesozoic basin aquifers discussed in earlier 

sections of this report was used to identify specific lithologic 

groups with greater potential for radiochemical contaminants. 

Uranium is present in trace amounts in all rock types but can 

become enriched relative to background concentrations in certain rock types because of its high solubility in certain water 

types and because of the wide variety of chemical reactions 

in which it can participate (Langmuir, 1978; Turner-Peterson, 

1980; Hodge and others, 1998). 

Water from areas underlain by more felsic crystalline 

bedrock (fig. 24) is assumed to be more likely to contain 

elevated concentrations of these naturally occurring radionuclides; however, differential weathering of specific minerals 

and rock types (Speer and others, 1981; Michel, 1984) may 

control the geochemical environment and thereby the concentrations of radionuclides and associated constituents in 
50 Contaminants in Crystalline-Rock Aquifers and Siliciclastic-Rock Aquifers, Eastern United States, 1994–2008

85° 80° 75°

40°

35°

TENNESSEE

ALABAMA

GEORGIA

SOUTH CAROLINA

NORTH CAROLINA

VIRGINIA

WEST VIRGINIA

MARYLAND

NEW

JERSEY

PENNSYLVANIA

NEW YORK

KENTUCKY

0 50 100 KILOMETERS 25

0 50 100 MILES 25

INDIANA OHIO

MICHIGAN

DELAWARE

Piedmont

Blue

Ridge

ATLANTIC

OCEAN

EXPLANATION

LITHOCHEMICAL SUBGROUP

Clastic sedimentary rocks 

Quartz-rich sedimentary rocks 

Lacustrine/evaporite sedimentary rocks

Feldspar-rich clastic sedimentary rocks 

Metamorphosed clastic sedimentary rocks 

Schist (aluminous)

Mylonite

Gneiss

Quartz-feldspar schist

Granulite, pegmatite, and alkali-granite

Metamorphic rock (intrusives)

Orthogneiss and gneiss

Felsic metavolcanic rock (F), meta-rhyolite,

metavolcanic rock (F-1), and metavolcanic (S)

Dacite, felsic volcanic rock, and rhyolite

Monzonite, nepheline syenite, syenite, anorthosite,

and alkali syenite

Base from U.S. Bureau of the Census, 1990, 1:500,000 to 1:5,000,000 and Statistics Canada digital data, 2006

Albers Equal-Area Conic projection: Standard Parallels 29°30' N and 45°30' N,

Central Meridian 96°00' W, Latitude of Origin 23°00' N

Lithologic group data from Dicken and others, 2005a and

2005b and Nicholson and others, 2005 and 2006

85° 80° 75°

40°

35°

Figure 24. Areal occurrence of felsic lithologic groups within the Piedmont and Blue Ridge Physiographic Provinces.
Water-Quality Characteristics of Aquifers, Lithologic Groups, and Lithochemical Subgroups 51

groundwater. Transport characteristics of uranium, radium, 

and radon differ; thus, areas underlain by rocks with the 

highest uranium content, which are the felsic rocks in the 

PBR crystalline-rock aquifers, do not always have the highest 

concentrations of all three of these radionuclides.

Uranium

Less than 1 percent of the samples from the aquifers 

exceeded the 30-μg/L MCL for uranium (table 4). Although 

the highest single measurement of uranium of 97 μg/L was in 

a sample taken from paragneiss from the metamorphosed clastic sedimentary lithologic group in the PBR crystalline-rock 

aquifers, samples from the Early Mesozoic basin aquifers had 

statistically higher concentrations of uranium than the samples 

from the PBR crystalline-rock aquifers (table 6; fig. 17). Previous studies indicate that these two areas have a combination 

of sources of uranium in or near the aquifer, with conditions 

conducive to mobilization and transport of uranium (TurnerPetersen, 1980; Szabo and Zapecza, 1991). 

Clastic sedimentary and lacustrine clastic sedimentary 

lithologic groups within the Early Mesozoic basin aquifers 

had elevated median concentrations of uranium (greater than 

1 μg/L) (fig. 17). The highest median uranium concentration 

values of 3.6 and 3.1 μg/L were for the clastic sedimentary 

(CLSD group, subgroup 21) and clastic lacustrine (CLSDLAC group, subgroup 22e) lithologic groups, respectively, of 

the Early Mesozoic basin (tables 3, 6, and 1-1). Of 50 water 

samples from Early Mesozoic basin sedimentary-rock aquifers 

analyzed for uranium, the concentrations were greater than 

4 μg/L in about 26 percent (13 samples) and were greater than 

3 μg/L in 46 percent of the samples (appendix 1, tables 1-3 

and 1-4). For these rock types, only 22 percent of the samples 

contained uranium concentrations that were less than 1 μg/L; 

however, only 2 percent of the samples had uranium concentrations that exceeded 10 μg/L, and the maximum uranium 

concentration was 12 μg/L. 

The uranium concentration distribution was more 

bimodal for the water samples from most crystalline-rock 

aquifers, especially the granites, metasediments, biotite 

gneisses, and paragneisses, which have sedimentary origin and had the highest individual uranium concentrations 

(tables 3 and 1-1). A few water samples (1.1 percent) from 

these crystalline-rock aquifers contained uranium concentrations in excess of 10 μg/L, and a few samples (0.8 percent) 

contained uranium concentrations in excess of the 30-μg/L 

MCL; however, the majority of samples (89 percent) contained concentrations less than 1 μg/L (appendix 1, table 1-4). 

Of water samples from the crystalline-rock aquifers, only 

about 6 percent contained uranium concentrations greater than 

4 μg/L, and about half of those samples (7), were from granite 

and granite gneiss. The two highest uranium concentrations, 

97 and 39 μg/L, were detected from paragneiss (sedimentary 

origin, now metamorphosed) and granite, respectively.

The geographic distribution of these clastic sedimentary 

(CLSD group, subgroup 21, appendix 1, tables 1-3 and 1-1) 

and clastic lacustrine sedimentary (CLSDLAC group, subgroup 22e) lithochemical subgroups is shown in figure 25. 

Common lithologies for these siliciclastic groups are argillite, 

arkose, conglomerate, fine-grained mixed clastic, mudstone, 

sandstone, shale, siltstone, and greywacke. Common lithologies for the crystalline groups are granite, biotite gneiss, 

gneiss, and other metasediments. (Note that some clastic sedimentary rocks are mapped in the Blue Ridge Physiographic 

Province; fig. 25). The clastic sedimentary and lacustrine clastic sedimentary lithologic subgroups of the Early Mesozoic 

basin aquifers and the PBR crystalline-rock aquifers have a 

source of uranium and geochemical characteristics conducive 

to transport of uranium.

Uranium in the PBR crystalline-rock aquifers and Early 

Mesozoic basin aquifers occurs most commonly in groundwater that has higher values of pH under oxic geochemical conditions. Of the 29 water samples with the highest uranium concentrations (>4 μg/L), 16 (55 percent) had pH values greater 

than or equal to 7.3. In the grouping of samples in which the 

concentrations of uranium exceeded 1 μg/L, 73.4 percent 

(47 of 64 samples) had pH values that were neutral to alkaline (greater than or equal to 7.0), and 32.8 percent (21 of 

64 samples) had a pH greater than or equal to 7.5. Conversely, 

in samples where uranium concentrations were low (less than 

1 μg/L), the samples were overwhelmingly acidic, with pH 

less than 7.0 in 211 (86.8 percent) of 243 samples with pH 

measurement. For samples with pH less than 7.0, the samples 

with near-neutral pH (6.5–7.0) had the greatest frequencies for 

exceeding 1 μg/L highest common reporting level (fig 21.O). 

The strong correlation between uranium concentrations and 

pH in groundwater is not surprising because of the important 

role pH and alkalinity (carbonate complexation) play in the 

geochemical mobilization of uranium. Uranium concentrations were positively correlated with pH, specific conductance, 

and concentrations of alkalinity, calcium, sodium, sulfate and 

several trace elements. The trace elements having the strongest positive correlations with pH and alkalinity were arsenic, 

boron, and molybdenum, all of which form oxyanions (Hodge 

and others, 1998) that tend to be mobile in alkaline environments (tables 5 and 1-7). 

Higher concentrations of arsenic and uranium tend to 

occur in water with elevated pH conditions; this association is 

indicative of increased weathering processes (PC1, table 5). 

The high concentrations are consistent with the hypothesized 

increase in the mobility of the oxyanions under high-pH 

conditions, as the calculated adsorption/desorption model 

profiles for uranium and arsenic indicate (fig. 8). Laboratory 

experiments have repeatedly shown that while uranium sorbs 

strongly to iron-hydroxides and clays, uranium is readily 

desorbed from iron hydroxides and clays with increasing pH, 

especially in the presence of carbonate alkalinity (Hsi and 

Langmuir, 1985; Echevarria and others, 2001; Logue and others, 2004). Uranium concentrations are negatively correlated 

with concentrations of DO and iron (appendix 1, table 1-7), 

which is a constituent whose occurrence is strongly affected 

by redox conditions. These correlations are consistent with 
52 Contaminants in Crystalline-Rock Aquifers and Siliciclastic-Rock Aquifers, Eastern United States, 1994–2008

TENNESSEE

ALABAMA

GEORGIA

SOUTH CAROLINA

NORTH CAROLINA

VIRGINIA

WEST VIRGINIA

MARYLAND

NEW

JERSEY

PENNSYLVANIA

NEW YORK

KENTUCKY

0 50 100 KILOMETERS 25

0 50 100 MILES 25

OHIO

INDIANA

MICHIGAN

DELAWARE

Piedmont

Blue

Ridge

ATLANTIC

OCEAN

EXPLANATION

LITHOCHEMICAL SUBGROUP

Clastic sedimentary rocks

Lacustrine/evaporite sedimentary rocks

Physiographic boundary between Piedmont

 and Blue Ridge

Base from U.S. Bureau of the Census, 1990, 1:500,000 to 1:5,000,000 and Statistics Canada digital data, 2006

Albers Equal-Area Conic projection: Standard Parallels 29°30' N and 45°30' N,

Central Meridian 96°00' W, Latitude of Origin 23°00' N

Lithologic group data from Dicken and others, 2005a and

2005b and Nicholson and others, 2005 and 2006

85° 80° 75°

40°

35°

Figure 25. Areal occurrence of lithochemical subgroups having elevated uranium concentrations in groundwater (based on Tukey 

mean rank tests, table 6) within the Piedmont and Blue Ridge Physiographic Provinces.
Water-Quality Characteristics of Aquifers, Lithologic Groups, and Lithochemical Subgroups 53

the thermodynamic property of uranium—the more oxidized 

species form oxyanions that are much more soluble than 

the reduced species (Langmuir, 1978), especially with more 

alkaline pH. Uranium concentrations also are negatively correlated with concentrations of aluminum. Aluminum is not a 

redox-sensitive constituent, but its solubility increases with 

decreasing pH (more acidic conditions); uranium solubility is 

optimal in the opposite conditions (with increasing pH or more 

alkaline conditions).

The general lack of correlation between radon and 

uranium concentrations among the water samples, previously 

noted for sediments of the Early Mesozoic basins by Szabo 

and Zapecza (1991), indicates that geochemical environment 

more strongly controls the dissolution of uranium than that 

of radon. A favorable geochemical environment (high pH and 

alkalinity) is critical for mobilizing uranium, whereas radon is 

readily soluble in any type of water (either acidic or alkaline). 

Previous sampling and analysis of the Early Mesozoic basin 

sediments have similarly shown that uranium concentrations 

tend to be highest (uranium is most mobile) in oxidizing 

alkaline water (Szabo and Zapecza, 1991). Previous sampling efforts for granites and gneisses of the Blue Ridge also 

indicate the lack of correlation between radon and uranium 

concentrations but indicate that high pH and alkalinity tend to 

be the favorable geochemical conditions for uranium occurrence (Vinson and others, 2009).

Radon

Concentrations of radon-222 exceeded the proposed 

AMCL of 4,000 pCi/L in 19 percent of the samples from the 

PBR crystalline-rock aquifers and from the Early Mesozoic 

basin siliciclastic-rock aquifers (table 4). Concentrations 

exceeded 3,250 pCi/L in 25 percent of samples, and 30 percent 

of the samples had a concentration greater than 3,000 pCi/L. 

Overall, 90 percent of the samples had radon concentrations 

greater than the proposed MCL of 300 pCi/L. The overall 

median for the entire sample set is 1,800 pCi/L, thereby 

exceeding the proposed MCL by a factor of 6. Median concentrations are substantially greater than the proposed MCL of 

300 pCi/L, except for the those samples from the combined set 

of mafic igneous and metamorphic rocks and ultramafic-rock 

lithologic groups (IGMTM median 355 pCi/L and ULMAF 

median, 240 pCi/L; tables 3 and 1-1; fig. 17). Median radon 

concentrations for all of the lithologic groups of aquifers start 

at about 1,200 pCi/L for the quartz-rich sedimentary and metamorphosed clastic sedimentary lithologic groups and increase 

substantially for other lithologic groups. The maximum radon 

concentration was 38,000 pCi/L in a sample from felsic biotite 

gneiss. Six of the seven wells with the next highest radon concentrations, all greater than 10,000 pCi/L, were from granite. 

Within the PBR crystalline-rock aquifers, the highest 

median concentrations of radon were detected in samples from 

wells completed in the felsic igneous and the metamorphosed 

clastic sedimentary lithologic groups (fig. 26). Although radon

had similar mean rank and median concentrations for many 

lithologic groups in the PBR crystalline-rock aquifer (table 6), 

each of the lithologic groups had large ranges in concentrations. The mafic/ultramafic lithologies had notably lower 

concentrations of radon, had the lowest mean rank, and had 

the lowest median concentration of 240 pCi/L (fig. 17). Half 

of all samples with radon concentrations less than 200 pCi/L 

were from the mafic igneous and metamorphic and ultramafic 

lithologic groups, which is consistent with findings from 

previous studies in areas of the PBR (Senior, 1998; Sloto, 

2000). The median concentrations of 3,530 pCi/L for granite 

(felsic igneous and metamorphic lithogy) and 3,600 pCi/L for 

schist (metamorphosed clastic sedimentary lithology) were 

the highest among all of the rock-aquifer types, and these 

two groups had the statistically highest radon concentrations 

(table 6; fig. 27). Within the felsic igneous and metamorphic 

crystalline-rock lithologic group (IGMTF group, tables 3 and 

A1), samples from areas underlain by a specific rock type 

(granite, subgroup 61, appendix 1, table 1-1) had concentrations of radon exceeding 4,000 pCi/L in 47 percent of the 

samples, a frequency that was twice as great as that from any 

of the other rock types. About 24 percent of samples from the 

gneiss and schist metasediments had concentrations of radon 

exceeding 4,000 pCi/L. 

The Early Mesozoic basin siliciclastic lithologic groups 

had radon concentrations that generally are comparable to 

those of the PBR crystalline-rock aquifer lithologic groups 

(table 6; fig. 17). The Early Mesozoic basin lacustrine siliciclastic lithologic group within the Piedmont also had a median 

value that was about 1,800 pCi/L, and a small percentage of 

samples (about 8 percent) had a concentration greater than 

4,000 pCi/L (table 6; fig. 17), with a maximum of 5,000 pCi/L. 

Concentrations of radon exceeded 8,000 pCi/L in 21 percent 

of samples from granitic aquifers (IGMTF lithologic group, 

lithochemical subgroup 61) and in 6 percent of samples 

from schistose aquifers (CLSDMT group, lithochemical 

subgroup 32u). The remaining Early Mesozoic basin siliciclastic lithologic groups within the Piedmont also had median 

concentrations of radon greater than 1,000 pCi/L, but less than 

3,000 pCi/L (table 6).

Radon is highly soluble and generally is not affected 

by chemical reactions because it is a noble gas. Radon gas 

was ubiquitous, and concentrations exceeded the proposed 

MCL of 300 pCi/L in samples collected under all geochemical conditions encountered during this study, including pH 

ranging from 4.5 to 8.5 and oxic and anoxic conditions. The 

higher proposed AMCL of 4,000 pCi/L was also exceeded 

in samples collected from all types of geochemical conditions (fig. 21Q). Radon concentrations generally were higher 

in water that was classified as oxic as opposed to waters that 

were classified as anoxic. Acidic to neutral groundwater with 

oxic or mixed redox characteristics had the greatest frequency 

of samples with concentrations exceeding the 4,000-pCi/L 

AMCL for radon (fig. 21Q). Radon concentrations were 

highest where water was most corrosive and substantial rock 

(mineral) weathering was likely occurring. These geochemical 
54 Contaminants in Crystalline-Rock Aquifers and Siliciclastic-Rock Aquifers, Eastern United States, 1994–2008

60 76 10 55 21

10

100

1,000

10,000

14,000

Radon concentration, in picocuries per liter

Proposed

Proposed alternate

USEPA maximum 

 contaminant level 

 for radon

EXPLANATION

25th percentile

10th percentile

75th percentile

50th percentile

 (median)

90th percentile

>90th percentile

<10th percentile

222 Number of samples

Metamorphosed

clastic

sedimentary

(CLSDMT)

Felsic igneous

rocks and their

metamorphic

equivalents

(IGMTF)

Quartz-rich

metamorphic

rocks

(MTQ)

Intermediate 

igneous rocks 

and their 

metamorphic 

equivalents

(IGMTI)

Mafic igneous

rocks and their

metamorphic

equivalents

(IGMTM)

Piedmont and Blue Ridge Piedmont,

Blue Ridge,

and Early

Mesozoic basin

Crystalline rocks

Figure 26. Radon concentrations in groundwater samples related to composition of lithologic group.

conditions are consistent with those noted for granites, which 

tend to yield the highest radon concentrations. The positive corresponding loading of radon and potassium together 

in PCA (PC5 in table 5 and 1-7) is also consistent with the 

highest occurrence of radon in felsic rocks, especially granites, which generally contain potassium feldspar and potassium-bearing mica minerals, such as muscovite, biotite, or 

phlogopite.

Because radon is highly soluble in waters of all geochemical types encountered in the region, the occurrence of radon 

is related primarily to the abundance of uranium in the rock. 

Gamma-ray spectral emission maps constructed from measurements during aerial overflights of the United States in the 

late 1970s as part of the USGS National Uranium Resource 

Evaluation (NURE) Program (Duval and Riggle, 1999) reveal 

a broad range of radionuclide contents. These data include the 

equivalent uranium-238 content interpolated on the basis of 

gamma emissions in the upper 25 centimeters of soil, surface 

sediments, or rock, which can indicate the general presence 

of radionuclide-enriched rock as shown in figure 28. Results 

from the Tukey mean rank test suggest that only the granitic 

rocks (61) and undifferentiated schist (32u) lithochemical 

subgroups had statistically higher concentrations of radon 

in groundwater compared to other lithochemical subgroups; 

however, other areas shown in warm colors in figure 28, such 

as the eastern Piedmont of North Carolina, suggest that other 

felsic rocks, including metavolcanics (lithochemical subgroup 

61mv), may also be important sources of radon.

Uranium is present in trace amounts in all the rock types 

from the region but is strongly enriched relative to background 

concentrations in some of the bedrock types. Overall, a comparison of the aquifers in the study area suggests that elevated 

concentrations of uranium in bedrock most commonly were 

noted in the PBR crystalline-rock aquifers, especially granite 
Water-Quality Characteristics of Aquifers, Lithologic Groups, and Lithochemical Subgroups 55

TENNESSEE

ALABAMA

GEORGIA

SOUTH CAROLINA

NORTH CAROLINA

VIRGINIA

WEST VIRGINIA

MARYLAND

NEW

JERSEY

PENNSYLVANIA

NEW YORK

KENTUCKY

0 50 100 KILOMETERS 25

0 50 100 MILES 25

OHIO

INDIANA

MICHIGAN

DELAWARE

Piedmont

Blue

Ridge

ATLANTIC

OCEAN

EXPLANATION

LITHOCHEMICAL SUBGROUP

Schist, and quartz feldspar schist

Granite, granulite, pegmatite, and alkali-granite

Physiographic boundary between Piedmont

 and Blue Ridge

Base from U.S. Bureau of the Census, 1990, 1:500,000 to 1:5,000,000 and Statistics Canada digital data, 2006

Albers Equal-Area Conic projection: Standard Parallels 29°30' N and 45°30' N,

Central Meridian 96°00' W, Latitude of Origin 23°00' N

Lithologic group data from Dicken and others, 2005a and

2005b and Nicholson and others, 2005 and 2006

85° 80° 75°

40°

35°

Figure 27. Areal occurrence of lithochemical subgroups having elevated radon-222 concentrations in groundwater (based on the 

Tukey mean rank test, table 6) within the Piedmont and Blue Ridge Physiographic Provinces.
56 Contaminants in Crystalline-Rock Aquifers and Siliciclastic-Rock Aquifers, Eastern United States, 1994–2008

TENNESSEE

ALABAMA

GEORGIA

SOUTH CAROLINA

NORTH CAROLINA

VIRGINIA

WEST VIRGINIA

MARYLAND

NEW

JERSEY

PENNSYLVANIA

NEW YORK

KENTUCKY

0 50 100 KILOMETERS 25

0 50 100 MILES

Felsic metavolcanic

rocks (61mv)

25

OHIO

INDIANA

MICHIGAN

DELAWARE

ATLANTIC

OCEAN

EXPLANATION

LITHOCHEMICAL SUBGROUP

Schist, and quartz feldspar schist

Granite, granulite, pegmatite, and alkali-granite

Value (gamma counts per minute)

High: 11.2174

Low: –0.144569

Blank where data missing

Base from U.S. Bureau of the Census, 1990, 1:500,000 to 1:5,000,000 and Statistics Canada digital data, 2006

Albers Equal-Area Conic projection: Standard Parallels 29°30' N and 45°30' N,

Central Meridian 96°00' W, Latitude of Origin 23°00' N

Lithologic group data from Dicken and others, 2005a and 2005b and Nicholson and others, 2005 and 2006;

Uranium gamma counts from National Uranium Resource Evaluation (NURE) database

85° 80° 75°

40°

35°

Figure 28. An overlay of areal occurrence of lithochemical subgroups having elevated radon-222 concentrations and U.S. Geological 

Survey National Uranium Resource Evaluation Program uranium measurements in groundwater within the Piedmont and Blue Ridge 

Physiographic Provinces.
Water-Quality Characteristics of Aquifers, Lithologic Groups, and Lithochemical Subgroups 57

and gneiss (metasediment), but high values are also present 

in most of the region underlain by the Early Mesozoic basin 

sediments, especially the lacustrine siliciclastic sediments. The 

variation in uranium content in the rock types of the region 

provides the key background upon which the concentrations 

of radon can be overlain and interpreted. The intersecting 

distribution of elevated uranium concentrations in bedrock and 

elevated radon concentrations in groundwater demonstrates 

that bedrock uranium distribution is the source, and thus is the 

most useful potential explanatory factor for radon concentrations (fig. 28). There is a large difference in the concentration of uranium in the granites relative to concentrations in 

the mafic rock or in some of the quartz-rich sediment. Thus, 

the large differences in concentrations of radon-222 that are 

observed regionally among these rock types is to be expected, 

based on the variable bedrock uranium contents.

Theoretically, the concentration of radon-222 in groundwater is directly proportional to the concentration of uranium 

in the rock-aquifer matrix (Wanty and others, 1992). The 

distribution of radon in water on the basis of the collected 

concentration data from the sampled well sites from the Piedmont Early Mesozoic basin and PBR crystalline-rock aquifers 

is consistent with the distribution of uranium in the bedrock, 

with the most extreme radon values corresponding to the 

most uraniferous lithologic types. The differences in possible 

concentrations of uranium among the various rock types may 

be relatively large, as much as a full order of magnitude or 

more for granites relative to mafic rocks; hence, the lithologic 

control on radon-222 concentrations is predominant in the 

PBR crystalline-rock aquifer. Concentrations of radon-222 in 

groundwater and concentrations of uranium in the rock-aquifer 

matrix were crudely correlated also for felsic crystalline 

rock and for the thin glacial sand and gravel aquifer systems 

in the northeastern United States (Ayotte and others, 2007). 

Many other factors can affect the radon-222 concentrations to 

some degree, however, including porosity and the efficiency 

of radon-222 emanation (Wanty and others, 1992). In the 

crystalline-rock aquifer types, the common occurrence of the 

uranium-bearing minerals along grain margins is well documented (Speer and others, 1981; Michel, 1984; Davis and others, 1987), again emphasizing the importance of granites and 

similar crystalline-rock types for yielding the highest radon 

concentrations. The variability in emanation can be affected 

by geochemistry and other associated factors because the geochemistry may directly affect the fate of the radium-226 parent 

of the radon; for example, sorption of radium to grain surfaces 

may be enhanced or minimized. Thus, variability in emanation 

may have a small effect on the overall radon-222 concentrations in the PBR crystalline-rock aquifer. The emanation effect 

is likely too small to be notable on the regional scale in comparison to the large variability of bedrock uranium concentrations, but may be one minor factor that further explains some 

of the local-area variation noted within a single rock type. 

Further evaluation of such local-area variation is outside of the 

scope of this study. 

Radium

Only 2 percent of the samples analyzed for radium had 

concentrations that exceeded the 5-pCi/L MCL for combined 

radium-226 and radium-228. Three percent of samples from 

the Early Mesozoic basin aquifers had concentrations that 

exceeded the MCL (table 4). [Radium data were not analyzed from some of the lithologic groups, and the number of 

samples per group was often small (fig. 17H).] Of the aquifers 

that had radium data, the siliciclastic-rock lacustrine aquifers had the highest median concentrations of radium. The 

only lithologic groups within the Piedmont that contained 

samples with concentrations greater than the 5-pCi/L MCL 

for combined radium-226 and -228 were the felsic igneous 

and metamorphic lithologic group (IGMTF, tables 3, 4, and 

1-1) in the PBR crystalline-rock aquifers (sample from granite 

with a concentration of 13.19 pCi/L) and the clastic lacustrine/

evaporate sedimentary groups of the Early Mesozoic basin 

aquifers (sample from lacustrine siltstone with a concentration 

of 10.12 pCi/L; tables 1-3 and 1-4). In two additional samples, 

the combined radium concentration exceeded 4 pCi/L; one of 

those samples was from biotite gneiss (IGMTI group, subgroup 34bg), and the other was from granite gneiss, which is 

derived from granite (IGMTF group, subgroup 61). The mean 

rank of concentrations was not significantly different among 

many of the lithologic groups because of the small range in 

concentrations for each lithochemical subgroup, the large 

number of samples for which the concentrations of the radium 

isotopes were low or not detected, and the limited number of 

samples available for each group (appendix 1, table 1-6 and 

fig. 17H). Additional issues with regard to the detection of the 

radium isotopes are briefly discussed in appendix 3.

A few samples collected in areas underlain by uraniumrich sandstone or siltstone in the Early Mesozoic basin aquifers had concentrations of radium-226 that were substantially 

higher than the associated concentrations of radium-228. In 

general, the concentration ratios of radium-226 to radium-228 

were highly variable among the siliclastic lithologic groups; 

however, the concentrations of these two radionuclides were 

nearly equal (about 1:1 ratio) in most samples from the felsic 

igneous and metamorphic crystalline-rock lithologic group 

(fig. 29).

Concentrations of radium isotopes exceeded the 5-pCi/L 

MCL and the 1-pCi/L common detection threshold more frequently in samples with anoxic or mixed geochemical conditions than did concentrations in samples with oxic geochemical conditions, usually with neutral to alkaline pH (fig. 21R). 

The effect of enhanced solubility of radium in the anoxic-type 

waters is the most important factor explaining the occurrence 

of the occasionally elevated concentrations of radium in the 

PBR bedrock aquifers, in terms of statistical significance (as 

illustrated by the correlation with dissolved oxygen; PC3; 

appendix 1, table 1-7 and table 5) and in terms of occurrence 

of the highest concentration values. For the two samples that 

had a combined radium concentration that exceeded the MCL 

of 5 pCi/L, the DO concentration was less than 0.5 mg/L and 
58 Contaminants in Crystalline-Rock Aquifers and Siliciclastic-Rock Aquifers, Eastern United States, 1994–2008

y = 0.9536x + 0.3833

R2 = 0.9122

0.5

–0.5

1.5

2.5

3.5

4.5

5.5

6.5

7.5

–0.5 0.5 1.5 2.5 3.5 4.5 5.5 6.5 7.5 8.5

Radium-228, in picocuries per liter

Radium-226, in picocuries per liter

Radium, crystalline rocks

EXPLANATION

Radium, siliclastic rocks

y = 0.2213x + 0.8523

R2 = 0.8332

Figure 29. Relations between concentrations of radium-228 and radium-226 in samples from bedrock aquifers of the Piedmont and 

Blue Ridge Physiographic Provinces.

the concentration of manganese (an element far more soluble 

in anoxic than in oxic waters) was greater than 0.05 mg/L in 

both. Of the seven samples in which the combined radium 

concentration exceeded the level of 2 pCi/L, five samples had 

DO concentrations less than 0.5 mg/L, and six samples had 

concentrations of manganese greater than 0.05 mg/L. The frequency of occurrence for concentrations of combined radium 

to be greater than or equal to 1 pCi/L was evaluated among 

samples of water from three Ra-bearing rock aquifer groups 

[felsic igneous and metamorphic lithologic group in the PBR 

crystalline-rock aquifers, including granite; the lacustrine clastic sedimentary groups of the Early Mesozoic basins aquifers; 

and biotite gneiss (IGMTI group, subgroup 34bg, tables 3 and 

1-1)]. Combined radium was greatest when DO was less than 

or equal to 1 mg/L (anoxic). Four of the five highest concentrations of radium-226 occurred when DO concentrations were 

less than 0.5 mg/L. 

Concentrations of combined radium are negatively correlated with DO concentration (appendix 1, table 1-7). Concentrations of combined radium with respect to DO are shown 

in figure 30; the relation is nearly hyperbolic, indicating that 

radium concentrations are high when DO is absent and lower 

or absent when DO is abundant. Combined radium-228 and 

radium-226 (total radium) concentrations also have positive 

correlations with iron and manganese concentrations (appendix 1, table 1-7, abbreviated as RaTOT). The indirect correlations of concentrations of radium with concentrations of DO 

coupled with the direct correlations with the concentrations of 

iron and manganese are indicative of the ongoing geochemical 

processes that allow for the most common mobility of radium: 

lack of adsorption to iron- and manganese-hydroxides and 

oxyhydroxides in anoxic environments. Laboratory studies 

have shown that radium is readily adsorbed by clay minerals 

(Ames and others, 1983a), but has an even stronger pattern of 

preferential adsorption to amorphous iron- and manganeseoxyhydroxides (Moore and Reid, 1973; Ames and others, 

1983b; Benes and others, 1984). A consequence of this strong 

adsorption pattern for radium is that natural iron-oxyhydroxide 

samples have shown that they contain much more radium 

than the surrounding rock matrix (Korner and Rose, 1977). 

Reductive dissolution of the iron oxyhydroxides in anoxic 

conditions releases radium to the water as demonstrated by 

dissolving such oxyhydroxides from sediments and observing 

the sharp increase in the amount of radium in solution (Landa 

and others, 1991). The adsorption properties of radium with 

respect to iron oxyhydroxides (hydrous ferric oxide) under 

variable pH conditions are illustrated in figure 8. In neutral 

and alkaline waters, nearly complete adsorption of radium to 
Water-Quality Characteristics of Aquifers, Lithologic Groups, and Lithochemical Subgroups 59

Anoxic/oxic boundary

Maximum contaminant level

14

13

12

11

10

9

8

7

6

5

4

3

2

1

0

0 1 2 3 4 5 6 7 8 9 10

Radium-226 plus Radium-228, in picocuries per liter

Dissolved oxygen, in milligrams per liter

CLSDQ + MTQ

CLSDLAC

CLSDMT

IGMTF

IGMTI

CLSDLAC, ND

CLSDMT, ND

Not detected

EXPLANATION

ND

*

* * *

*

Figure 30. Relations of concentrations of radium-226 plus radium-228 and dissolved oxygen for samples from siliclastic-rock and 

crystalline-rock aquifers of the Piedmont and Blue Ridge Physiographic Provinces, 1994–2008.

iron oxyhydroxides is indicated. In anoxic conditions with 

reductive dissolution of iron oxyhydroxides, the aquifer loses 

a considerable sink for radium from the solid phase, thus 

allowing for a greater amount of radium to be in solution.

The nearly hyperbolic trend among the concentrations 

of radium and dissolved oxygen observed in the Appalachian 

PBR crystalline-rock aquifers is consistent with similar trends 

observed in other principal aquifers of the United States 

(Szabo and others, 2012a, fig. 3). The somewhat greater 

frequency of occurrence of radium in anoxic conditions is the 

most important factor controlling radium in aquifers used for 

drinking water supply in the United States (Szabo and others, 

2012b). The occurrence pattern, whether in the Appalachian 

PBR crystalline-rock aquifers or in the larger setting of the 

continental United States, reflects the critical role that sorption 

processes, specifically to iron (and manganese) oxyhydroxides, play in controlling the concentration of radium in dilute 

groundwaters. 

Data from this study show that among the Appalachian 

PBR siliciclastic-rock and crystalline-rock aquifers, oxic 

groundwater is more common than anoxic groundwater, causing a greater frequency of elevated uranium concentrations 

relative to radium-226. These findings are consistent with 

results from previous studies (Szabo and Zapecza, 1991; Vinson and others, 2009). Dissolved oxygen was usually present 

in detectable amounts for most, though not all, samples that 

contained uranium; however, dissolved oxygen was usually 

not present, or was present in very low amounts, for most 

samples that contained radium. Concentrations of uranium 

correlated directly and strongly with pH, indicating a lithochemical relation in occurrence patterns to rock types with the 

presence of bicarbonate-rich, near-neutral to alkaline waters 

(table 5, PC1).

For samples with oxic characteristics, radium concentrations tended to be low, but exceeded the 1-pCi/L threshold 

most frequently for low-pH conditions. Low concentrations 

of radium generally are consistent with the high potential for 

adsorption of the divalent radium cation by iron oxides under 

neutral to alkaline conditions, but not under acidic conditions 

in which radium tends to desorb (fig. 21R). Groundwater in the 

study area is not usually acidic, but in the adjoining Coastal 

Plain (fig. 6), pH is commonly less than 5.0, and combined 

radium concentrations exceed the MCL in about 20 to 30 

percent of sampled wells (Szabo and others, 2005, 2012b). 

Data from other studies indicate that elevated radium-226 and 

radium-228 concentrations more frequently occur in groundwater in acidic formations in the PBR crystalline-rock aquifers 

(such as the Chickies Quartzite, Senior and Vogel, 1992). 

Because of the limited radium data collected for the current 

study, results of the assessment should not be considered 
60 Contaminants in Crystalline-Rock Aquifers and Siliciclastic-Rock Aquifers, Eastern United States, 1994–2008

strictly representative of the probability of elevated radium in 

the PBR crystalline-rock aquifers in all lithochemical subroups, which have great diversity in lithochemical composition 

and geochemical environments. 

Gross Alpha-Particle Activity

Approximately 8 percent of the samples analyzed for 

gross alpha-particle activity from the entire dataset exceeded 

the MCL of 15 pCi/L. For the measured gross alpha-particle 

activity to exceed the USEPA drinking water MCL, the activity, excluding that from uranium (all isotopes of uranium 

emit alpha particles), must exceed 15 pCi/L. Only 2 percent 

of the samples exceed the gross alpha-particle activity MCL 

if presumed uranium activity is excluded from the measured 

gross alpha-particle activity. Perhaps equally important is the 

concept that gross alpha-particle activity might be of use as a 

compliance-monitoring “screen” for combined radium (Hess 

and others, 1985) or as an indicator for the presence of high 

concentrations of uranium, radium, or both. The measured 

gross alpha-particle activity is further examined in light of 

this concept. 

Most samples with the highest values of gross alphaparticle activity greater than 15 pCi/L (table 7), contained 

elevated concentrations of alpha-particle-emitting radium-226 

or uranium, and on some occasions, both. The MCL for 

combined radium or uranium was exceeded in some, but not 

all, samples with gross alpha-particle activity greater than 

15 pCi/L. The maximum uranium concentration among the 

samples was 97.4 μg/L, and this sample (from paragneiss 

rock type, appendix 1, tables 1-1 and 1-3; CLSDMT group, 

subgroup 35gn) had the highest gross alpha-particle activity 

of 78.3 pCi/L. Similarly, the sample with the second highest uranium concentration (39.3 μg/L) from granite (IGMTF 

subgroup 61; appendix 1, tables 1-1 and 1-3) had the second 

highest gross alpha-particle activity (33.1 pCi/L). The two 

samples with the highest radium-226 concentrations [7.6 

and 6.6 pCi/L, respectively, from the siliciclastic lacustrine/

evaporite sedimentary rock group (CLSDLAC tables 3 and 

appendix 1, table 1-1) and the felsic igneous lithologic group 

(granite rock type; appendix tables 1-1 and 1-3, IGMTF 

group, subgroup 61)] had gross alpha-particle activity of 

about 30 pCi/L. Samples with moderate gross alpha-particle 

activities (5 to 14.99 pCi/L) occasionally contained moderate concentrations of radium-226 (0.95–1.9 pCi/L) or uranium (4–13 μg/L), but the MCL itself was not exceeded 

for either of these radioactive constituents. Of five samples 

with radium-226 concentrations greater than 1 pCi/L, all had 

measured gross alpha-particle activity greater than 5 pCi/L 

(tables 7 and 1-4). Most of these samples with moderate gross 

alpha-particle activities were from the siliciclastic-rock clastic 

lacustrine/evaporite sedimentary lithologic group (CLSDLAC 

tables 3 and 1-1) within the Early Mesozoic basin aquifers 

and from the granite (IGMTF group, subgroup 61) and biotite 

gneiss (intermediate igneous and metamorphic rocks, tables 3 

and 1-1 IGMTI group, subgroup 34bg) PBR crystalline-rock 

aquifers. The samples from these lithologic groups also had 

the highest frequency of occurrence of gross alpha-particle 

activity, radium-226, and uranium, with concentrations above 

their respective MCLs, and had among the highest median 

concentrations for uranium or radium or both. Low gross 

alpha-particle activities (less than 5 pCi/L) corresponded 

with low concentrations of radium-226 (all samples less than 

1 pCi/L) and mostly low concentrations of uranium; less than 

10 percent of the samples had concentrations of uranium that 

were greater than 4 μg/L. However, one sample with 13 μg/L 

of uranium had less than 5 pCi/L of measured gross alphaparticle activity; such a low gross alpha-particle activity in a 

sample with that amount of uranium is unusual and implies an 

atypical uranium isotopic ratio (Osmond and Cowart, 1976) or 

low analyte recovery.

Table 7. Number of samples with specified ranges of uranium 

and radium-226 concentrations for 99 water samples for which 

gross alpha-particle activity was determined. 

[pCi/L, picocuries per liter; μg/L, micrograms per liter; MCL, maximum 

contaminant level; <, less than; >, greater than;--, not detected]

Uranium 

concentration 

(μg/L)

Gross alpha activities, in pCi/L

<5 5–14.99 >15 (MCL)

>30 (MCL) -- -- 2

10–29.99 1 -- --

4.0–9.99 6 7 3

1.0–3.99 18 6 --

<1 52 2 2

Totals 77 15 7

Radium-226 

concentration 

(pCi/L)

Gross alpha activities, in pCi/L

<5 5–14.99 >15 (MCL)

>5 (MCL) -- -- 2

1.0–4.99 -- 3 --

0.5–0.99 4 3 --

0.3–0.49 6 2 --

<0.3 67 7 5

Totals 77 15 7
Water-Quality Characteristics of Aquifers, Lithologic Groups, and Lithochemical Subgroups 61

Maximum contaminant level

14

13

12

11

10

9

8

7

6

5

4

3

2

1

0

0 10 20 30 40 50 60 70 80 90 100

Radium-226 plus radium-228, in picocuries per liter

Uranium, in micrograms per liter

CLSDQ + MTQ

CLSDLAC

CLSDMT

IGMTF

IGMTI

CLSDAC, ND

CLSDMT, ND

Not detected

EXPLANATION

*

**

*

*

*

ND

Figure 31. Relations of concentrations of radium-226 plus radium-228 and uranium for samples from bedrock aquifers of the Piedmont 

and Blue Ridge Physiographic Provinces.

Conventional measurements of the general amount of 

radioactivity in the water, such as measurements of gross 

alpha-particle activity, did not prove to be reliable for identifying specifically where concentrations might exceed the 

combined radium MCL or the uranium MCL. Elevated gross 

alpha-particle activity indicates the presence of at least one or 

the other of these alpha-particle-emitting radioactive constituents in relatively high concentrations, but the identity of which 

of these nuclides is predominant and contributes the most to 

the radioactivity of the water sample cannot be established 

on the basis of the gross alpha-particle activity measurement 

alone. Sample geochemistry (oxic or anoxic) could be helpful 

in estimating the likelihood of which radionuclide is predominant (fig. 21). The geochemistry of radium and uranium differ, 

and either one or the other tends to predominate in most, but 

not all, cases. The adsorption properties of the two elements 

tend to differ, with uranium most likely to desorb at high pH 

and radium most likely to desorb at low pH (fig. 8) or in the 

presence of anoxic water (fig. 21R). The samples with the 

highest concentrations of radium typically had among the lowest concentrations of uranium, and the converse was also true, 

regardless of the lithologic group (fig. 31).

Potential for Detrimental Effects from Trace 

Elements, Radionuclides, and Associated 

Contaminants

The trace elements arsenic, manganese, and zinc and 

the radionuclides uranium, radon, and radium are among the 

most frequently occurring elements that exceed drinking water 

standards in the United States. The frequencies of occurrence 

of uranium and arsenic are 1.7 and 6.8 percent, respectively, 

in samples from domestic wells nationally (DeSimone, 2009). 

For uranium, combined radium, and arsenic, the frequencies 

of occurrence in excess of the respective MCLs are 0.65, 

2.0, and 3.56 percent, respectively, in the samples from the 

wells throughout the PBR crystalline-rock aquifers and Early 

Mesozoic basin aquifers. All of the samples collected from 

the PBR crystalline-rock and Piedmont Early Mesozoic basin 

siliciclastic-rock aquifers that were analyzed in this study 

are generally consistent with the broader national occurrence 

frequency as reported by DeSimone (2009). Although the 

frequencies of occurrence for uranium, radium, and arsenic for 

this region were similar to the national frequencies in general, 
62 Contaminants in Crystalline-Rock Aquifers and Siliciclastic-Rock Aquifers, Eastern United States, 1994–2008

the frequency of occurrence for radon concentrations greater 

than the AMCL was substantially higher for the PBR crystalline-rock aquifers and Early Mesozoic basin aquifers than that 

for the Nation as a whole. The proposed AMCL was exceeded 

more often in the PBR crystalline-rock aquifers than in the 

Early Mesozoic basin aquifers, and granite is the specific rock 

type in the felsic igneous and metamorphic lithologic groups 

in which the highest percentage (47 percent) of the samples 

exceed 4,000 pCi/L. This relation of radon occurrence with 

lithology represents a readily mapable lithologic target that 

could be useful in prioritizing monitoring activity for radon.

Summary and Conclusions

The following key findings are from the analyses of 346 

groundwater samples collected from wells examined as part 

of 14 U.S. Geological Survey National Water Quality Assessment (NAWQA) Program studies conducted from 1994 to 

2008. These key findings are related to lithologic groups and 

lithochemical subgroups delineated as part of this study:

• Certain naturally occurring trace elements (arsenic, 

manganese, zinc) and radionuclides (uranium, radon, 

radium) in groundwater from wells used as domestic water supplies exceeded public drinking water 

standards, and 10 additional constituents (barium, 

beryllium, cadmium, copper, lead, selenium, boron, 

molybdenum, nickel, and strontium) were detected at 

concentrations greater than a threshold of one-tenth of 

the established health-based levels. 

• Naturally occurring contaminants, such as arsenic, 

radon, uranium, and radium, had geographic occurrence patterns that generally were associated with certain mappable lithologic groups (categorized based on 

primary rock type and mineral assemblages). Although 

there was some spatial overlap, some lithologic groups 

associated with the presence of radionuclides were 

different from others affected by arsenic or other 

trace elements.

• Arsenic concentrations typically were less than 1 μg/L 

in groundwater of the study area. Concentrations of 

arsenic exceeding the 10-μg/L drinking water maximum contaminant level were detected in 8.5 percent 

of the sampled wells in the Piedmont Early Mesozoic 

basin siliclastic-rock aquifers and in 2.4 percent of 

sampled wells in the Piedmont and Blue Ridge crystalline-rock aquifers. The elevated arsenic concentrations 

were predominantly in groundwater samples from clastic lacustrine sedimentary rocks and metamorphosed 

clastic sedimentary rocks (meta-mudstone and metaargillite) and frequently were associated with alkaline 

pH (measured values of 7.2 to 8.0), under oxic to 

anoxic geochemical conditions. Although arsenic can 

be attenuated by adsorption on iron oxides at acidic pH 

(less than 6.5), alkaline pH can facilitate the desorption 

and mobilization of arsenic from iron-oxide surfaces. 

• Manganese concentrations typically were less than 

10 μg/L in groundwater of the study area. Concentrations of manganese exceeding the Health-Based 

Screening Level of 300 μg/L were detected in 

7.3 percent of the wells sampled. Elevated concentrations of manganese were associated with groundwaters from crystalline-rock aquifers that contained less 

than 0.5 mg/L of dissolved oxygen and more than 

100 μg/L of dissolved iron. Many anoxic samples 

with elevated concentrations of manganese also had 

acidic pH. Anoxic conditions and (or) low pH can 

promote the dissolution of manganese oxides and common aluminosilicate minerals, such as chlorite, that 

contain manganese.

• A small number of the samples from crystalline-rock 

aquifers contained high uranium concentrations; 

only 2 of 256 samples (0.8 percent) had concentrations exceeding the 30-μg/L Maximum Contaminant 

Level (MCL), but the majority (89 percent) contained 

concentrations less than 1 μg/L. The highest concentration of uranium was 97 μg/L and was detected in 

a groundwater sample from the Piedmont and Blue 

Ridge crystalline-rock aquifers (paragneiss from the 

metamorphosed clastic sedimentary lithologic group). 

Clastic sedimentary and lacustrine clastic sedimentary 

lithologic groups within the Piedmont Early Mesozoic 

basin siliclastic-rock aquifers had elevated median 

concentrations of uranium (greater than the detection 

limit of 1 μg/L). The median uranium concentration 

was 3.6 μg/L for the siliciclastic lacustrine siltstones of 

the Early Mesozoic basin. 

• Radon exceeded proposed drinking water standards 

with a greater frequency than all other naturally occurring constituents, particularly in groundwater from the 

crystalline-rock aquifers. Concentrations in 19 percent 

of the samples from the these aquifers exceeded the 

Alternative Maximum Contaminant Level (AMCL) of 

4,000 pCi/L for radon, and 90 percent of the samples 

had radon concentrations greater than the proposed 

MCL of 300 pCi/L. At least one sample in all of the 

aquifer lithologic groups except for the mafic and 

ultramafic rocks (which are known to have low uranium concentrations in the rock matrix) had elevated 

concentrations of radon. Granites from the felsic igneous lithologic group in the Piedmont and Blue Ridge 

crystalline-rock aquifers had the greatest frequency 

of radon occurrence greater than the AMCL at about 

46.5 percent. 

• Uranium and radium rarely exceeded their respective 

MCLs (about 1 and 2 percent, respectively). Nevertheless, elevated concentrations of uranium, radium, and 

radon followed predictable geologic distribution pat-
Summary and Conclusions 63

terns. The geologic patterns of occurrence were somewhat different for each of these radionuclides despite 

the fact that radium-226 is a progeny of uranium, 

which emphasizes the importance of aquifer geochemistry for occurrence patterns. 

• Radium was typically highest in anoxic waters that 

contained less than 0.5 mg/L of dissolved oxygen and 

more than 50 μg/L of manganese and on occasion 

more than 100 μg/L of iron, but the number of samples 

that exceeded the MCL of 5 pCi/L was small, only 2 

of 98 samples. Both exceedences, along with most of 

the highest concentrations, were from anoxic waters. 

The occurrence of radium was most notable among 

the clastic sedimentary and clastic lacustrine (lakebed 

silt and feldspathic sand) sediments of the Piedmont 

Early Mesozoic basin siliclastic-rock aquifers and 

in the granites of the crystalline-rock aquifers in the 

Piedmont and Blue Ridge, especially in anoxic conditions in each of these aquifers. The number of elevated 

occurrences of radium in the granite and gneiss 

crystalline-rock aquifers (felsic igneous and metamorphic equivalents lithologic group) was small because 

these rocks commonly tended to have oxic waters. 

• Uranium and arsenic generally were elevated in specific aquifers with alkaline pH conditions, especially 

those with clastic sedimentary and clastic lacustrine 

(lakebed silt and feldspathic sand) sediments. Concentrations of uranium were elevated in a few samples 

from the granite and gneiss crystalline-rock aquifers, 

but in about 89 percent of the samples from the crystalline rocks, the concentration of uranium was less than 

or equal to 1 μg/L.

• The geographic distributions of samples with elevated 

concentrations of uranium and of samples with 

elevated concentrations of radium differed except for a 

small number of samples. The radium-226 radionuclide 

was more frequently detected at elevated concentrations in waters from the siliciclastic rocks of the Early 

Mesozoic basin than was the radium-228 radionuclide; 

however, in waters from the granites and gneisses, the 

concentrations of the two radium isotopes were nearly 

equal in almost all cases.

• In addition to their presence in aquifer bedrock and 

associated geologic materials, the occurrence of most 

trace elements and radiochemicals was related to 

the pH and redox characteristics of the groundwater. 

Anoxic conditions and elevated concentrations of iron 

and manganese were correlated with concentrations 

of radium isotopes in nearly all of the lithochemical 

subroups of the study. Alkaline pH was correlated with 

concentrations of uranium and arsenic, and acidic pH 

was correlated with elevated concentrations of zinc.

The identification of geologic and geochemical factors 

affecting trace-element and radionuclide occurrence can be 

useful to owners of domestic wells because these water systems generally are not monitored regularly for trace elements 

or radionuclides. Millions of people in the Piedmont and Blue 

Ridge region consume water from private domestic wells that 

are not regulated or monitored and could pose a source of 

exposure to contaminants such as arsenic or radon. Domestic 

well owners rarely test for these constituents, but should be 

aware of their potential occurrence in groundwater and the 

health risks of exposure to concentrations that exceed drinking water standards. A better understanding of these issues and 

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 data compiled in this and other 

large-scale regional studies or measured locally for publicsupply wells could be informative to private-well owners 

and municipal authorities in identifying where potential risks 

of naturally contaminated groundwater may be highest and 

provide information for prioritization of testing private wells 

in these areas. For example, the general knowledge that water 

from crystalline-rock aquifers tends to have higher levels 

of radon than water from other aquifer types can be further 

focused to a specific geographic area because water from rocks 

in felsic (specifically those of igneous origin and granitic rock 

types) or metamorphic sedimentary lithologic groups (specifically schist rock types) has the highest radon levels within 

the crystalline-rock aquifers. Results of this study also may 

provide information that would be useful in designing future 

studies, such as prioritizing the radium-226 isotope for analysis where water is withdrawn from uranium-rich sandstone or 

siltstone in the Early Mesozoic basins, in order to increase the 

timeliness of results and perhaps minimize analytical costs for 

local monitoring programs. 

Additional testing of domestic wells and continued public 

education are important, particularly considering the large 

areas of potential occurrence of naturally occurring inorganic 

chemicals and radionuclides. Some of the lithologic groups 

and subgroups identified for comparison with the water-quality 

data compiled for this study lacked the corresponding waterquality data. Sampling of these aquifer units, particularly for 

those lithologies with source minerals associated with radionuclide occurrence could fill these data gaps. Of the potential 

greatest concern are the granite and biotite gneiss and other 

intermediate igneous and metamorphic rocks of the Piedmont 

and Blue Ridge crystalline-rock aquifers and the lacustrine 

sedimentary and feldspar-rich (arkosic) sedimentary lithologic 

groups within the Piedmont Early Mesozoic basin siliciclastic 

aquifers, as well as similar metasediments found among the 

Piedmont and Blue Ridge crystalline-rock aquifers. In addition 

to the direct measure of contaminants, additional measurements to indicate redox characteristics and pH would be useful 

to document the geochemical environments of contaminant 

occurrence.
64 Contaminants in Crystalline-Rock Aquifers and Siliciclastic-Rock Aquifers, Eastern United States, 1994–2008

Acknowledgments

The authors wish to thank all of the participating well 

owners in the study area who so graciously allowed access to 

facilitate the collection of groundwater-quality samples as part 

of the U.S. Geological Survey (USGS) National Water-Quality 

Assessment (NAWQA) Program. 

Much appreciation is extended to dedicated USGS field 

personnel who collected the samples according to comprehensive study requirements. Special thanks to Eliza Gross of the 

USGS Pennsylvania Water Science Center for assistance with 

the geographic information data layers and associated report 

figure and table preparation. Guidance from James W. Horton, 

Jr., USGS, Reston, Va., toward the categorization of the lithologic groups and lithochemical subgroups is much appreciated. 

Thanks are extended to Lisa Senior of the USGS Pennsylvania 

Water Science Center and Mark Kozar of the USGS Washington Water Science Center for their reviews and suggestions 

toward the content of this report. 

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Appendixes 1, 2, and 3

Appendix 1. Tables. (Excel spreadsheets available online at http://pubs.usgs.gov/sir/2013/5072/)

Appendix 2. Probability plots of principal components analysis (PCA) scores for groundwater of siliclastic-rock 

and crystalline-rock aquifers in the Piedmont and Blue Ridge Physiographic Provinces: A, PC1 “Alkalinity-pH”; 

B, PC2 “Chloride-Nitrate”; C, PC3 “ Redox”; D, PC4 “Temperature-Silica”; and E, “Radon-Potassium”.

Appendix 3. Analytical Issues Relating to Defining Radium Occurrence.
72 Contaminants in Crystalline-Rock Aquifers and Siliciclastic-Rock Aquifers, Eastern United States, 1994–2008 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

Frequency concentration was exceeded

-3

-2

-1

0

1

2

3

PC1 score

A

1.0

0.00.10.20.30.40.50.60.70.80.9

Frequency concentration was exceeded

-3

-2

-1

0

1

2

3

PC2 score

B

1.0

0.00.10.20.30.40.50.60.70.80.9

Frequency concentration was exceeded

-3

-2

-1

0

1

2

3

PC3 score

C

1.0

0.00.10.20.30.40.50.60.70.80.9

Frequency concentration was exceeded

-3

-2

-1

0

1

2

3

PC4 score

D

1.0

0.00.10.20.30.40.50.60.70.80.9

Frequency concentration was exceeded

-3

-2

-1

0

1

2

3

PC5 score

E

1.0

Crystalline

Siliciclastic

EXPLANATION

Appendix 2. Probability plots of principal component analysis (PCA) scores for groundwater of siliclastic-rock and crystalline-rock 

aquifers in the Piedmont and Blue Ridge Physiographic Provinces: A, PC1 “Alkalinity-pH”; B, PC2 “Chloride-Nitrate”; C, PC3 “Redox”; 

D, PC4 “Temperature-Silica”; and E, PC5 “Radon-Potassium”. The PCA model is described in table 5 and appendix table 1-7.
Appendix 3 73

Appendix 3. Analytical Issues Relating 

to Defining Radium Occurrence

Interpreting radiological results requires understanding the concepts of detectability and quantification unique to 

radiochemistry measurements. For measurements of radionuclides in this study, these concepts primarily are important for 

measurements of radium concentrations. Uranium concentrations were determined by measuring the mass of the uranium-238 isotope present; therefore, issues unique to radiochemistry measurements were not considered. Concentrations 

of radon-222 were so high that issues regarding detectability 

and quantification were again not of consideration. Concentrations of radium (Ra) isotopes, however, while showing considerable ranges, were for many samples at or below detection, 

or if detected, were low enough that guidelines for quantification could not be met. Furthermore, analytical techniques for 

radium isotopes underwent considerable changes during the 

period of study as did reporting requirements. A measured 

(uncensored) concentration value was reported for nearly all 

the samples (tables 4 and 1-4), although issues relating to 

analysis, detection, quantification, and reporting of the measured radium isotope concentrations affect understanding of 

the radium occurrence. Additional information provided with 

many of the analyses can be useful in determining the statistical likelihood of detection and quantification. 

Random radioactive decay, variable radioactive background noise, and other measurement uncertainties cause 

measurement signal to vary. For a background blank or an 

environmental sample, a nonzero signal may be produced 

even when no radionuclide is present. For this reason, the 

laboratory analyzes an instrument background or blank sample 

and subtracts the response signal from the gross signal of 

the measured environmental sample to obtain the net signal. 

The critical level (LC) is the smallest measured concentration that is statistically different from the instrument background (blank). It is the threshold of detection for whether the 

radionuclide is actually present. The statistical computation 

is set to minimize the statistical likelihood of a false detection. Because of the random variability in the background 

signal, the issue of detection of radioactivity in an individual 

sample result is not in all cases best represented by the general 

reporting level, therefore, but rather by comparison with the 

LC. The sample-specific minimum detectable concentration 

(ssMDC) is a slightly higher (concentration) value than the 

LC that is adequate to distinguish the sample result from that 

of the blank with statistical confidence. For each sample, the 

sample-specific parameters such as yield, detector efficiency, 

and sample aliquot size are used in the formula to determine 

the SSMDC. In the truly simplest case, the SSMDC is about 

twice the LC (Currie, 1968). Reported negative values are possible when the result of a sample is less than that of the blank 

representing background; these values represent non-detection 

in the case of a single measurement. The precision estimate 

of the radiological result is the square root of the sum of 

variances of the factors that affect measurement precision. For 

radionuclides, a large component of the measurement precision is defined by the random decay rate of the radionuclide at 

any instant in time. Though the average rate of decay conforms to that associated with the half life (the time in which 

half of the radionuclides would decay), at any instant in time, 

the rate may randomly be greater or lesser than this average 

rate, resulting in imprecision in the measurement. A sample 

result implies true detection if the result is greater than the LC, 

but confidence that the value of the result is statistically significantly different from the background is achieved if the value 

is also greater than the SSMDC, and the sample may still have 

poor precision. For radionuclide samples analyzed for this 

study before mid-2003, sample-specific critical levels (ssLC) 

were not reported (McCurdy and others, 2008). Comparisons 

of detectability are reliant on the assumption that the reported 

SSMDCs were indeed twice the values of the ssLCs (Currie, 

1968). Importantly, the sensitivities of the analyses were such 

that concentrations great enough to be considered of concern 

to human health could readily be detected and quantified.

The concentrations of Ra-226 were greater than the 

individual SSMDCs (when reported, see table 1-4) or LCs 

(stored in USGS databases but not reported in table 1-4) in 

85 samples, and the reported raw values were greater than zero 

in all but one sample. The concentrations of Ra-226 were less 

than the individual SSMDCs in eight samples from the siliclastic lacustrine siltstones of the Early Mesozoic basin sediments, a frequency greater than in the other rock types, even 

though the highest Ra-226 concentration was also detected in 

this rock type. The concentrations of Ra-226 in samples collected from the Early Mesozoic basin aquifer were determined 

by using alpha spectrometry, whereas the radon-emanation 

method was used for analysis of Ra-226 from samples collected from the remaining aquifers. Alpha spectrometry had 

a higher detector background than did the radon-emanation 

method, resulting in the higher SSMDCs (or LCs). This difference in methodology likely explains why so many sample 

results were reported as less than the SSMDC for samples 

collected from the Early Mesozoic basin aquifer, but not from 

samples collected from the remaining aquifers. Of the remaining rock types, the concentrations of Ra-226 were less than 

the individual SSMDCs or ssLCs in only one or two samples, 

except for the metasediments (CLSDMT, table 3) for which 

three samples had concentrations less than the LC. Concentrations of Ra-226 in 53 samples were greater than 0.1 picocurie 

per liter (pCi/L), but of those, only 1 had a concentration 

(0.126 pCi/L) that was less than the corresponding SSMDC 

(0.145 pCi/L) (table 1-4). The high frequency of detection in 

samples relative to that of Ra-228 is partly attributable to the 

great sensitivity of the radon emanation technique used for 

detecting Ra-226. 

The concentrations of Ra-228 were greater than the 

individual SSMDCs (when reported, see table 1-4) or LCs 

(stored in USGS databases but not reported in table 1-4) in 

48 samples, and reported raw values were greater than zero in 

all but 9 samples (table 4). The SSMDCs for the beta-particle 
74 Contaminants in Crystalline-Rock Aquifers and Siliciclastic-Rock Aquifers, Eastern United States, 1994–2008

emitting Ra-228 (maximum and 75th-percentile values, 

0.906 and 0.569 pCi/L, respectively) were higher than those 

for either of the other Ra isotopes, which are alpha-particle 

emitters (table 1-4) and have lower background values. A 

maximum measured Ra-228 concentration of 0.71 pCi/L 

in one sample, however, was still less than the respective 

SSMDC, illustrating the difficulty in interpreting the distribution of Ra-228 concentrations that are less than about 0.7 to 

0.9 pCi/L (representing the maximum SSMDC). The concentrations of Ra-228 were greater than 0.9 and 1 pCi/L in 17 and 

14 samples, respectively, and were generally found at these 

concentrations in both siliclastic and crystalline rock aquifers. 

An additional 22 samples with concentrations ranging from 

0.19 to 0.71 pCi/L were considered as detectable in quantifiable amounts because the concentration was greater than the 

associated SSMDC or LC. The minimum Ra-228 concentration 

that was greater than the associated SSMDC was 0.593 pCi/L 

(table 1-4) and that was greater than the associated LC that was 

0.19 pCi/L.

Ra-224 concentrations were measured in samples only 

from the sedimentary rock aquifers of the Early Mesozoic 

basin and were not present at concentrations greater than 

1 pCi/L in any of the samples analyzed for this isotope. The 

median concentration value of 0.10 pCi/L is uncertain (an 

estimate at best) because it is less than the value of the median 

of the associated SSMDCs (0.24 pCi/L). The concentrations 

of Ra-224 for the samples collected from this aquifer were 

determined by alpha spectrometry, and this technique has 

high enough SSMDCs to make detection in many samples 

questionable. The concentrations of Ra-224 were greater than 

or equal to the individual SSMDCs in 5 samples, and the 

reported raw values were greater than zero in all but 3 samples 

(table 4). A maximum measured Ra-224 concentration of 

0.47 pCi/L in one sample, however, was still less than the 

respective SSMDC, illustrating that alpha spectrometry may 

have performance issues up to a value of about 0.5 pCi/L. 

Concentrations of Ra-226 plus Ra-228 (termed combined 

Ra, the quantity that is specifically considered in drinking 

water regulations) were quantifiable at about the 1 pCi/L level 

and could be compared to the 5 pCi/L standard. The concentrations of both these Ra radionuclides were greater than the 

respective SSMDC or ssLC for 44 percent of the samples, with 

concentrations of combined radium (Ra-226 plus Ra-228) 

for these samples about equal to or greater than 1.0 pCi/L 

(fig. 14). At least one of these two Ra radionuclides was 

detected with respect to the SSMDC or the ssLC for 88 percent 

of the samples; the SSMDC in all cases when it was reported 

was less than 1 pCi/L (table 1-4). All samples had at least one 

of these two Ra radionuclides reported with a value greater 

than zero (table 4).

For purposes of statistical calculation, and for plots 

illustrating population distribution characterization, such as 

probability plots or boxplots (figs. 14, 17, and 21; tables 1-4 

and 1-5), the distribution of the raw reported laboratory results 

were displayed to provide the sense of the distribution of the 

raw (uncensored) measured values. Values less than zero were 

censored for descriptive purposes in table 4. This approach 

eliminated the need for statistical approaches to estimate low 

concentration values (such as those described by Helsel and 

Hirsch, 2002). For scatter diagrams (figs. 29, 30, and 31), 

when only one Ra radionuclide concentration, Ra-226 or 

Ra-228, was detectable or quantifiable with the prescribed 

level of statistical certainty, the concentration of that one 

isotope was used as the concentration to represent the value 

of combined Ra. When neither Ra radionuclide concentration, 

Ra-226 or Ra-228, was detectable or quantifiable with the 

prescribed level of statistical certainty, the lower of the two 

reported concentrations was used to represent the value of the 

combined Ra concentration. As detailed above, all samples 

had at least one of these two Ra radionuclides reported with 

a value greater than zero (table 4). For purposes of graphical 

representation in the latter case, when neither Ra radionuclide 

concentration, Ra-226 or Ra-228, was detectable, the value 

of either the combined Ra concentration or the individual 

Ra radionuclide was represented as an open rather than as a 

closed symbol. For purposes of the simplest description of 

the distribution, the use of 1 pCi/L as an approximate cutpoint 

separating concentrations detected with greatest statistical 

certainty from those concentrations detected with slightly 

lesser statistical certainty or those not detected at all is reasonably justified by the distribution of the ssMDCs or LCs. In 

terms of the human health benchmark (HHB) of 5 pCi/L, this 

value represents the 20th percentile, and can be described as 

0.2 HHB (fig. 21R; table 4).
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For additional information regarding this publication, contact:

Director

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email: dc_nc@usgs.gov

Or visit USGS North Carolina Water Science Center at:

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Chapman and others—Contaminants in Crystalline-Rock Aquifers and Siliciclastic-Rock Aquifers, Eastern United States, 1994–2008—Scientific Investigations Report 2013–5072

Early Mesozoic basin aquifers

Piedmont and Blue Ridge carbonate-rock aquifers

Piedmont and Blue Ridge crystalline-rock aquifers

Valley and Ridge siliciclastic-rock aquifers Valley and Ridge carbonate-rock aquifers