U.S. Department of the Interior

U.S. Geological Survey

Characterization of Groundwater Quality Based on

Regional Geologic Setting in the Piedmont and 

Blue Ridge Physiographic Provinces, North Carolina

Scientific Investigations Report 2009–5149

Prepared in cooperation with the North Carolina Department of Environment and 

Natural Resources, Division of Water Quality, Aquifer Protection Section
Front-cover photograph. Walnut Cove Overlook at mile marker 396 along the Blue Ridge Parkway, Buncombe County, North Carolina. 

Photograph by Jung-Jun Lin, Engineer, Tainan Hydraulics Laboratory, National Cheng Kung University, Taiwan. Used with permission.

Back-cover photograph. Groundwater monitoring well cluster at the North Carolina Zoological Park Research Station, Randolph County, 

North Carolina. Photograph by Kristin McSwain, U.S. Geological Survey.
Characterization of Groundwater Quality 

Based on Regional Geologic Setting in the 

Piedmont and Blue Ridge Physiographic 

Provinces, North Carolina

By Stephen L. Harden, Melinda J. Chapman, and Douglas A. Harned

Prepared in cooperation with the North Carolina Department of Environment and 

Natural Resources, Division of Water Quality, Aquifer Protection Section

Scientific Investigations Report 2009–5149

U.S. Department of the Interior

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

KEN SALAZAR, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2009

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Suggested citation:

Harden, S.L., Chapman, M.J., and Harned, D.A., 2009, Characterization of groundwater quality based on regional 

geologic setting in the Piedmont and Blue Ridge Physiographic Provinces, North Carolina: U.S. Geological Survey 

Scientific Investigations Report 2009–5149, 32 p.
iii

Contents

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

Introduction.....................................................................................................................................................1

Previous Studies ...................................................................................................................................3

Purpose and Scope ..............................................................................................................................4

Description of the Study Area ............................................................................................................4

Geozones................................................................................................................................................5

Methods...........................................................................................................................................................7

Well Network.........................................................................................................................................7

Groundwater Sampling........................................................................................................................7

Statistical Analysis of Water-Quality Data .....................................................................................13

Bedrock Groundwater Quality within Regional Geozones ...................................................................14

Ionic Composition ...............................................................................................................................14

Comparison with Water-Quality Standards....................................................................................18

Comparison of Bedrock Groundwater-Quality Characteristics by Regional Geozone ....................22

Summary and Conclusions.........................................................................................................................29

Acknowledgments.......................................................................................................................................30

References Cited..........................................................................................................................................30

Appendix 1. USGS and NC DWQ analytical results of samples collected from 

PMREP wells and NAWQA wells

Appendix 2. Data set used for geozone statistical comparisons

Appendix 3. Percentile data displayed in box plots

Note: Appendixes are available for download as Microsoft® Excel files from 

http://pubs.usgs.gov/sir/2009/5149/
iv

Tables

1. Regional geozones in the Piedmont and Mountains Provinces of North Carolina............5

2. Well information for the North Carolina Department of Environment and 

Natural Resources, Division of Water Quality, Piedmont and Mountains 

Resource Evaluation Program research stations ...................................................................9

3. Well information for U.S. Geological Survey National Water-Quality 

Assessment Program sites used in this study .......................................................................11

4. Water-quality standards applicable to constituents or 

properties measured in groundwater samples .....................................................................19

5. Summary of wells within geozones in the Piedmont and Mountains 

Provinces in North Carolina having chemical constituents 

in groundwater exceeding North Carolina and U.S. Environmental 

Protection Agency drinking-water standards .......................................................................20

6. Summary results of Kruskal-Wallis test and Tukey comparison test 

of water-quality constituent or property and geozone in the 

Piedmont and Mountain Provinces of North Carolina..........................................................24

Figures

1–3. Maps showing—

 1. Study area and locations of groundwater research stations used in the 

Piedmont Mountains Resource Evaluation Program in North Carolina.......................2

 2. Regional geozones in the Piedmont and Mountains Provinces 

of North Carolina ...................................................................................................................6

 3. Distribution of Piedmont and Mountains Resource Evaluation 

Program research stations and National Water-Quality 

Assessment Program well sites in regional geozones 

of the Piedmont and Mountains Provinces of North Carolina .......................................8

4–6. Piper diagrams showing—

 4. Ionic composition of groundwater samples within eight geozones 

in the Piedmont and Mountains Provinces of North Carolina .....................................15

 5. Ionic composition of all samples from the Milton geozone, 

and open-hole samples and individual fracture-zone samples 

for wells RK-232, RK-242, and RK-245 in the Milton geozone 

in the Piedmont and Mountains Provinces of North Carolina .....................................16

 6. Ionic composition of samples from the Felsic intrusive geozone 

in the Piedmont and Mountains Provinces of North Carolina .....................................18

7. Graphs showing distributions of values of specific conductance 

and concentrations of calcium, potassium, sodium, bicarbonate, 

chloride, silica, ammonia, aluminum, and uranium for geozones 

in the Piedmont and Mountains Provinces of North Carolina.............................................26
v

Conversion Factors, Datums, and Abbreviated Units

Multiply By To obtain

Length

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

square mile (mi2) 259.0 hectare (ha)

square mile (mi2) 2.590 square kilometer (km2) 

Radioactivity

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

Vertical coordinate information is referenced to the North American Vertical Datum of 1988 

(NAVD 88).

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83).

Altitude, as used in this report, refers to distance above the 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).
vi

Acronyms and Abbreviations

AMCL alternative MCL

ANOVA analysis of variance

APRASA Appalachian Valley and Ridge, Blue Ridge, and Piedmont 

regional aquifer-system analysis

APS Aquifer Protection Section of the NCDENR DWQ

AWRS Allison Woods research station 

BAT3 Bedrock-Aquifer Transportable Testing Tool

BCRS Bent Creek research station

DO dissolved oxygen

DWQ Division of Water Quality of the NCDENR

E estimated

LPRS Langtree Peninsula research station 

LWRS Lake Wheeler research station 

MCL maximum contaminant level

MDL method detection limit

MMM multimedia mitigation program

N nitrogen

NAWQA National Water-Quality Assessment Program

NCDENR North Carolina Department of Environment and Natural Resources

NWQL National Water Quality Laboratory

PAS Piedmont Aquifer System

PMREP Piedmont and Mountains Resource Evaluation Program

RHRS Raleigh hydrogeologic research station

RL reporting levels

ROS regression on order statistics

SMCL secondary maximum contaminant level

THRS Tater Hill research station

UPRS Upper Piedmont research station

USEPA U.S. Environmental Protection Agency

USGS U.S. Geological Survey

ZPRS Zoological Park research station
Abstract

A compilation of groundwater-quality data collected as 

part of two U.S. Geological Survey studies provides a basis for 

understanding the ambient geochemistry related to geologic 

setting in the Piedmont and Blue Ridge Physiographic Provinces (hereafter referred to as Piedmont and Mountains Provinces) of North Carolina. Although the geology is complex, a 

grouping of the sampled wells into assemblages of geologic 

units described as “geozones” provides a basis for comparison 

across the region. Analyses of these two data sets provide a 

description of water-quality conditions in bedrock aquifers 

of the Piedmont and Mountains Provinces of North Carolina. 

Analyzed data were collected between 1997 and 2008 from a 

network of 79 wells representing 8 regional geozones distributed throughout the Piedmont and Mountains Provinces. This 

area has experienced high rates of population growth and an 

increased demand for water resources. Groundwater was used 

by about 34 percent of the population in the 65 counties of 

this region in 2005. An improved understanding of the quality 

and quantity of available groundwater resources is needed to 

plan effectively for future growth and development. The use of 

regional geologic setting to characterize groundwater-quality 

conditions in the Piedmont and Mountains Provinces is the 

focus of this investigation.

Data evaluation included an examination of selected 

properties and the ionic composition of groundwater in the 

geozones. No major differences in overall ionic chemistry of 

groundwater among the geozones were evident with the data 

examined. Variability in the cationic and anionic composition 

of groundwater within a particular geozone appeared to reflect 

local differences in lithologic setting, hydrologic and geochemical conditions, and(or) land-use effects. The most common exceedances of the drinking-water criteria (in accordance 

with Federal and State water-quality standards) occurred for 

radon, pH, manganese, iron, and zinc. Radon had the most 

exceedances, with groundwater from 61 of the 69 sampled 

wells having activities higher than the U.S. Environmental 

Protection Agency’s proposed maximum contaminant level of 

300 picocuries per liter. Overall, the Milton and the Raleigh 

and Charlotte geozones had the greatest number, eight each, 

of water-quality properties or constituents that exceeded 

applicable drinking-water criteria in at least one well. The 

Eastern Blue Ridge and Felsic intrusive geozones each had 

seven properties or constituents that exceeded criteria, and the 

Carolina slate geozone had six.

Based on limited data, initial results of statistical comparison tests identified statistically significant differences in 

concentrations of some groundwater constituents among the 

geozones. Statistically significant differences in median values 

of specific conductance and median concentrations of calcium, 

potassium, sodium, bicarbonate, chloride, silica, ammonia, 

aluminum, antimony, cadmium, and uranium were identified 

between one or more geozone pairs. Overall, the groundwater 

constituents appear to be influenced most significantly by the 

Inner Piedmont, Carolina slate, and Felsic intrusive geozones.

The study data indicate that grouping and evaluating analytical 

data on the basis of regional geozone setting can be useful for 

characterizing water-quality conditions in bedrock aquifers of 

the Piedmont and Blue Ridge Provinces of North Carolina.

Introduction

Population in the Piedmont and Blue Ridge Physiographic Provinces (hereafter referred to as Piedmont and 

Mountains Provinces) of North Carolina, including all or part 

of 65 counties, has grown from about 2.66 million people 

in 1940 to 6.11 million people in 2000 (Daniel and Dahlen, 

2002). In 2005, the estimated population of the region was 

6.66 million people, an increase of about 9 percent from 2000 

(U.S. Census Bureau, 2005). Associated with this high rate of 

population growth is an increased demand for water resources.

The percentage of the total population in the Piedmont 

and Mountains Provinces supplied by groundwater was about 

47 percent between 1960 and 1980 and then decreased to 

about 32 percent in 1990 (Daniel and Dahlen, 2002). The 

percentage of the population in the region served by groundwater was about 41 percent in 2000 (U.S. Geological Survey, 

2003) and about 34 percent in 2005 (U.S. Geological Survey, 

2008a). These decreases are attributed primarily to the high 

rates of population growth associated with the five metropolitan areas of Raleigh, Durham, Greensboro, Winston-Salem, 

and Charlotte, known collectively as the “Piedmont Crescent” 

(fig. 1; Pippin and others, 2008) that are served primarily by 

Characterization of Groundwater Quality Based on 

Regional Geologic Setting in the Piedmont and 

Blue Ridge Physiographic Provinces, North Carolina

By Stephen L. Harden, Melinda J. Chapman, and Douglas A. Harned
2 Characterization of Groundwater Quality in the Piedmont and Blue Ridge Physiographic Provinces, North Carolina Fall Line BCRS BCRS Charlotte Mooresville Lake Norman Greensboro Winston- Salem Raleigh Durham AWRS LPRS THRS ZPRS UPRS RHRS LWRS AWRS BCRS LWRS LPRS RHRS THRS UPRS ZPRS VIRGINIA SOUTH CAROLINA GEORGIA TENNESSEE 

Blue Ridge

Coastal Plain

84 

36 35 34 

83 82 81 80 79 78 77 76 

0 20 40 60 80 100 MILES

0 20 40 60 80 100 KILOMETERS

Allison Woods Hydrogeologic research stationBent Creek research stationNorth Carolina State University Lake Wheeler Road research stationLangtree Peninsula research stationRaleigh hydrogeologic research stationTater Hill research stationNorth Carolina State University Upper Piedmont research station

North Carolina Zoological Park research stationPiedmont CrescentPhysiographic Province line

EXPLANATION

Research station and identifierIdentifier

Figure 1. Study area amd locations of groundwater research stations used in the Piedmont Mountains Resource Evaluation Program in North Carolina. 

Atlantic

Ocean

Piedmont

Base from digital files of: U.S. Department of Commerce, Bureau of Census,  1990 Precensus TIGER/Line Files-Political boundaries, 1991 U.S. Environmental Protection Agency, River File 3 U.S. Geological Survey, 1:100,000 scale 

Figure 1. Study area and locations of groundwater research stations used in the Piedmont Mountains Resource Evaluation Program in North Carolina.
Introduction 3

surface-water based municipal supplies. The potential for 

future development of surface water may become limited to 

meet increased demands associated with population growth 

(Harned, 1989; Daniel and Dahlen, 2002). Consequently, 

the development of groundwater as a source of supply may 

become more important for meeting future demands in metropolitan areas of the Piedmont, and remains an important 

source of water supply for more than 2.25 million persons 

in the region. Because future drinking-water supplies are a 

primary concern in the Piedmont and Mountains Provinces, 

an improved understanding of the quality and quantity of 

available groundwater resources is needed by State and local 

agencies to plan effectively for growth and development.

Previous Studies

Groundwater quality in the crystalline igneous and 

metamorphic rock aquifers of the Piedmont and Mountains 

Provinces has been examined through various local and 

regional studies. Studies of ambient groundwater quality in 

bedrock wells associated with different rock types have varied 

in geographic extent, covering areas ranging from one or more 

counties to the physiographic-province scale. 

LeGrand (1958) examined the chemical composition 

of water from various wells located in igneous and metamorphic rocks of the North Carolina Piedmont province. He 

noted chemical differences in groundwater between two rock 

groups, felsic rocks (granite, granite gneiss, mica schist, slate, 

and rhyolite flows and tuffs) and mafic rocks (diorite, gabbro, 

hornblende gneiss, and andesite flows and tuffs). Groundwater 

associated with the felsic rocks had low concentrations of dissolved solids, low hardness, a pH less than 7, and about equal 

amounts of sodium and calcium. The mafic rocks yielded 

groundwater that had higher concentrations of dissolved 

solids, greater hardness, a pH greater than 7, and a greater 

amount of calcium than sodium.

Similar results are noted for a groundwater study in 

Orange County, North Carolina, that examined water-quality 

results of groundwater samples from 51 domestic bedrock 

wells that were grouped into felsic rocks and mafic rocks 

(Cunningham and Daniel, 2001). The observed median 

values of dissolved solids (122 milligrams per liter (mg/L)), 

pH (6.66), bicarbonate (100 mg/L), calcium (18 mg/L), 

magnesium (3.9 mg/L), and sodium (8.4 mg/L) for groundwater in the felsic rocks generally were lower than median 

values of dissolved solids (176 mg/L), pH (7.10), bicarbonate (161 mg/L), calcium (34 mg/L), magnesium (5.6 mg/L), 

and sodium (8.1 mg/L) for groundwater in the mafic rocks. 

Although these results are consistent with the findings of 

LeGrand (1958), a statistical evaluation of the Orange County 

data indicated no significant differences in median concentrations for any of the sampled water-quality constituents (including common anions and cations, metals and trace elements, 

nutrients, and radon) between the felsic and mafic rock groups 

(Cunningham and Daniel, 2001). 

Several previous investigations have assessed the distribution of naturally occurring contaminants, such as arsenic 

and radon, in groundwater in the Piedmont and Mountains 

Provinces (Loomis and others, 1987; Pippin, 2005; Campbell, 

2006, 2008; Vinson and others, 2009). Arsenic and radon are 

known to pose a risk to human health. Natural occurrences of 

these contaminants in groundwater associated with bedrock 

aquifers, therefore, is an important environmental issue for 

more than 2.25 million people in the Piedmont and Mountains 

region who use groundwater as a source of water supply.

Pippin (2005) performed a spatial analysis of total arsenic 

in groundwater from wells located throughout the North 

Carolina Piedmont and found that wells in rock bodies of the 

Carolina Slate Belt in the area around Stanly and Union Counties had the greatest probability of containing elevated levels 

of arsenic above a concentration of 0.001 mg/L. Wells in some 

areas of the Kings Mountain Belt, Charlotte Belt, and Inner 

Piedmont Belt also had a relatively high probability of containing elevated arsenic concentrations. Rocks of volcanic or 

volcaniclastic origin associated with the high probability areas 

have the greatest potential for hosting wells with elevated 

arsenic concentrations (Pippin, 2005). 

Concentrations of naturally occurring radon in groundwater of the Piedmont and Mountains Provinces generally are 

higher in metaigneous rocks than in metasedimentary rocks 

(Loomis and others, 1987; Campbell, 2006, 2008; Vinson and 

others, 2009). Loomis and others (1987) examined data from 

96 public water-supply wells and found that groundwater 

radon levels were highest in granites and gneisses within the 

Blue Ridge, Inner Piedmont, and Raleigh Belt regions; lower 

levels were observed for mafic and metavolcanic rocks. 

Similar results were observed in multicounty studies by 

Campbell (2006 and 2008) where groundwater radon concentrations were higher for wells in metaigneous rocks than 

for wells in metasedimentary rocks. Groundwater samples 

from 103 private supply wells in Buncombe, Henderson, 

and Transylvania Counties (Campbell, 2006) had a median 

radon concentration of 7,480 picocuries per liter (pCi/L) for 

wells in metaigneous rocks (including Henderson gneiss, 

Toxaway gneiss, meta-granodiorite, Caesar’s Head granite, 

and amphibolite) compared to a median radon concentration 

of 4,040 pCi/L for wells in metasedimentary rocks (including 

migmatitic biotite gneiss, garnet-mica schist, metagraywacke, 

biotite gneiss, muscovite-biotite gneiss, and rocks of the 

Brevard Fault zone). Groundwater samples from 87 private 

supply wells in 11 western North Carolina counties (Campbell, 2008) had a median radon concentration of 1,810 pCi/L 

for wells in metaigneous rocks (including Caeser’s Head 

granite, Cherryville granite, granitic gneiss, Henderson 

gneiss, porphyroblastic gneiss, and amphibolite) compared 

to a median radon concentration of 1,390 pCi/L for wells 

in metasedimentary rocks (including schists, para-gneisses, 

metagraywacke, and rocks of the Brevard Fault zone). In an 

investigation of 117 private supply wells in fractured crystalline rock in Wake County, Vinson and others (2009) noted 

that groundwater radon activities for wells in the Rolesville 
4 Characterization of Groundwater Quality in the Piedmont and Blue Ridge Physiographic Provinces, North Carolina

granite (monzogranite, granite, granodiorite, and pegmatite 

in places) were an order of magnitude higher compared to 

wells in the Raleigh gneiss (sedimentary and granitic components) and wells in metasedimentary and metavolcanic 

rocks (felsic gneiss, felsic schist, phyllite, meta-tuff, and some 

ultramafic rocks).

As part of the Appalachian Valley and Ridge, Blue Ridge, 

and Piedmont regional aquifer-system analysis (APRASA), 

Briel (1997) compiled and examined available groundwaterquality data spanning 11 eastern states from Alabama to New 

Jersey. A total of 10,564 wells with water-quality data were 

used to characterize the study area, with 25 percent of the 

wells (2,682) located in the Piedmont and Mountains Provinces of North Carolina (Briel, 1997). Regional and local 

variations in groundwater quality were observed among 

the physiographic provinces throughout the study area and 

reflected differences in geologic and hydrologic factors, such 

as lithology, the availability of soluble materials, and the 

degree of exposure of rock to water. 

In a large-scale water-quality investigation of the bedrock 

aquifers of the Piedmont Physiographic Province of the eastern United States, Lindsey and others (2006) subdivided the 

Piedmont Aquifer System (PAS) on the basis of the general 

rock type of the aquifers into three areas for study, including 

crystalline, siliciclastic, and carbonate aquifers. The rocks 

in the Piedmont of North Carolina are primarily igneous and 

metamorphic and were considered as part of the crystalline 

aquifer system. Overall, constituent concentrations of nitrate, 

pesticides, and volatile organic compounds were related to 

land-use factors. The subgrouping of the three aquifer types 

based on the general rock lithology was not considered 

appropriate for analysis of radon where activities varied on the 

basis of variations in mineralogy (Lindsey and others, 2006). 

In general, radon activities were greater in the crystalline 

rock aquifer having felsic mineralogy than in the crystalline 

rock aquifer having mafic mineralogy. As part of their work, 

Lindsey and others (2006) indicated that further study of 

explanatory factors, including such things as spatial density of 

well networks, different land-use conditions, and aquifer rock 

lithologies, was needed for characterizing water-quality conditions in the PAS.

In 1999, the North Carolina Department of Environment 

and Natural Resources Division of Water Quality (NCDENR 

DWQ) Groundwater Section implemented the Piedmont and 

Mountains Resource Evaluation Program (PMREP) to evaluate groundwater resources in the crystalline bedrock aquifers 

of the Piedmont and Mountains Provinces (Daniel and Dahlen, 

2002). The program is supported through a cooperative study, 

which began in 1999 between the NCDENR DWQ Aquifer 

Protection Section (APS), formerly known as the Groundwater 

Section, and the U.S. Geological Survey (USGS) North Carolina Water Science Center. The PMREP supports NCDENR 

DWQ’s mission of preserving and protecting North Carolina’s 

groundwater and surface-water resources by characterizing 

groundwater flow in a variety of hydrogeologic settings, as 

well as documenting the occurrence and mobility of both 

naturally occurring and land-use related contaminants (Daniel 

and Dahlen, 2002).

During the first part of the PMREP from 2000 through 

2008, intensive field data collection and interpretive data 

analyses were conducted at research stations established in 

regionally representative hydrogeologic settings of the Piedmont and Mountains Provinces (Chapman and others, 2005, 

2007; Huffman and others, 2006; Pippin and others, 2008). 

The research stations are located in areas having a range of 

geologic, hydrologic, and land-use characteristics where 

hydrogeologic processes and water-quality conditions have 

been described in diverse geologic regions of the Piedmont 

and Mountains Provinces (fig. 1). In this study, results of 

investigations conducted at selected PMREP research stations

were combined with data from studies conducted as part of 

the USGS National Water-Quality Assessment (NAWQA) 

Program (Hughes and others, 2000; Kozar and others, 2001; 

Lindsey and others, 2006) to characterize water-quality conditions based on regional geologic settings in the Piedmont and 

Mountains Provinces of North Carolina. 

Purpose and Scope

This report presents the results of an evaluation to determine whether grouping groundwater-quality data based on 

regional geologic setting may be a useful approach for characterizing ambient water-quality conditions in bedrock aquifers 

of the Piedmont and Mountains region in North Carolina. 

Water-quality data obtained from bedrock wells sampled as 

part of the PMREP and as part of USGS NAWQA studies were 

grouped into eight regional geologic zones, or “geozones” 

(as identified later in this report), distributed throughout 

the Piedmont and Mountains Provinces of North Carolina. 

Water-quality data (primarily inorganic constituents, including major ions, nutrients, metals, and radiochemical isotopes) 

are compiled and described for each geozone. Differences in 

water-quality constituents among the geozones were examined 

statistically. Study results can be used to characterize the influence of hydrogeologic setting on ambient groundwater quality 

in the North Carolina Piedmont and Mountains.

Description of the Study Area

The study area covers approximately 27,500 square 

miles (mi2) and includes the Piedmont and Mountains Provinces of North Carolina (fig. 1). Groundwater in the Piedmont 

and Mountains Provinces flows in hydrogeologic settings 

composed primarily of metamorphic and igneous rocks that 

range in composition from felsic to ultramafic. Rocks in these 

regions have undergone several episodes of intense metamorphism, folding, faulting, and igneous intrusion. Weathered 

regolith, composed of soil, residuum, saprolite, alluvium, and 

colluvium, overlies the fractured bedrock (Heath, 1980).

Groundwater flow is complex, consisting of an interconnected but distinct two-component groundwater system, with 

the regolith providing storage to underlying fractures in the 
Introduction 5

bedrock (Heath, 1980). A third component, the transition zone, 

commonly is present between the regolith and bedrock (Harned 

and Daniel, 1992). The transition zone generally is considered to be the most transmissive part of the groundwater-flow 

system and likely is susceptible to contamination. Because the 

shallow regolith and underlying bedrock fractures are connected, the aquifer systems of the Piedmont and Mountains 

Provinces are considered to be unconfined, although local confinement may occur where overlain by thick regolith (Chapman 

and others, 2005). Additionally, because the aquifers in these 

provinces are shallow, they are susceptible to contamination 

from activities on land surface (Daniel and Dahlen, 2002).

Geozones

In the Piedmont and Mountains Provinces, the metamorphic and igneous rocks have been historically grouped 

and described as a series of northeast-trending geologic belts 

(North Carolina Geological Survey, 1985). Studies during the 

last two decades indicate that the geology is more complicated, 

and that these belts do not adequately distinguish among 

lithostratigraphic, structural, and metamorphic features (see 

discussion in Horton and Zullo, 1991). Hibbard and others 

(2006) present a modern interpretation of the regional geology with groupings based on lithology and tectonic history. As 

part of ongoing studies, USGS investigators (Wright Horton 

and Bruce Lindsey, U.S. Geological Survey, written commun., 

2008) have modified the lithotectonic units of Hibbard and 

others (2006) into regional “geozones” for use in evaluating 

water-quality characteristics throughout the Piedmont and 

Mountains aquifer system in the eastern United States. The 

resulting geozones for the Piedmont and Mountains Provinces 

are presented in figure 2. The distribution of geozones from 

west to east, with some deviation based on age, includes the 

Grenville gneiss, Western Blue Ridge, Eastern Blue Ridge, 

Inner Piedmont, Milton, Felsic intrusive, Raleigh and Charlotte, Carolina slate, Outer Carolina, and Mesozoic geozones 

(fig. 2). Descriptions of the geozones in relation to lithotectonic 

units of Hibbard and others (2006) as well as geologic belts 

based on the Geologic Map of North Carolina (North Carolina 

Geological Survey, 1985) are provided in table 1.

Table 1. Regional geozones in the Piedmont and Mountains Provinces of North Carolina (modified from North Carolina Geological 

Survey [1985] and Hibbard and others [2006]).

[mi2, square miles; NCGS, North Carolina Geological Survey]

Geozone Areal extent 

(mi2)

Percent 

of total Description

Mesozoic 1,424 5.2 Mesozoic rift basins. Hibbard and others (2006) unit 38a. Mainly alluvial and lacustrine clastic 

sedimentary rocks with local diabase dikes and sills. NCGS (1985): Triassic basins. 

Outer Carolina 975 3.5 Unseparated magmatic and clastic metasedimentary sequences. Hibbard and others (2006) unit 14c. 

NCGS (1985): Eastern slate belt. 

Carolina slate 5,012 18.3 Magmatic-arc rocks east of the Central Piedmont suture, low metamorphic grade. Hibbard and others 

(2006) units 14a and 14b; NCGS (1985): Carolina Slate Belt.

Raleigh and 

Charlotte

2,194 8.0 Infrastructural magmatic-arc/oceanic rocks. Hibbard and others (2006) unit 15: gneiss, schist, 

metavolcanic rocks, and amphibolites, with local mélange and ultramafic bodies. NCGS (1985): 

Raleigh belt.

Felsic intrusive 5,654 20.6 Plutonic (igneous intrusive) rocks of felsic to intermediate composition throughout the Piedmont and 

Mountains. Hibbard and others (2006) unit 37 and parts of 8b, 14, 25, 15, and 34.

Milton 932 3.4 Magmatic-arc and associated rocks east of the Central Piedmont suture. Hibbard and others (2006) 

units 8a and 9. Mainly lower to middle (8a) Ordovician metamagmatic rocks and associated metaclastic rocks of the Chopawamsic volcanic arc; (9) Neoproterozoic to Lower Paleozoic mainly 

clastic metasedimentary rocks, schist, and gneiss locally associated with amphibolite and metaultramafic rocks. NCGS (1985): Milton belt.

Inner Piedmont 3,601 13.1 Inner Piedmont composite terrane including Smith River allochthon. Hibbard and others (2006) 

units 7a, 9, and 10.

Eastern Blue 

Ridge

3,298 12.0 Western Piedmont accretionary complex of Potomac composite terrane. Hibbard and others (2006) 

unit 7: Neoproterozoic to lower Paleozoic mainly clastic metasedimentary rocks, schist, and gneiss 

containing metaclastic mélanges and subordinate amphibolites, and meta-ultramafic rocks. NCGS 

(1985) part of Blue Ridge belt.

Western Blue 

Ridge

2,483 9.0 Neoproterozoic to Cambrian mainly clastic metasedimentary rocks filling rift basins and associated 

magmatism relate to Iapetan rifting. Locally contains fragments of oceanic crust. Hibbard and 

others (2006) units 2, 3, and 4. NCGS (1985): part of Blue Ridge belt.

Grenville 

gneiss

1,901 6.9 Ancestral North American basement and associated rocks of the Blue Ridge Province and Sauratown 

Mountains window. Hibbard and others (2006) unit 1: gneiss, schist, and plutonic rocks affected 

by the Grenville orogeny and associated post-orogenic granitoid bodies. NCGS (1985): part of 

Blue Ridge and Inner Piedmont belts.

Total 27,474 100
6 Characterization of Groundwater Quality in the Piedmont and Blue Ridge Physiographic Provinces, North Carolina Base from digital files of: U.S. Department of Commerce, Bureau of Census, 1990 Precensus TIGER/Line Files-Political boundaries, 1991 U.S. Environmental Protection Agency, River File 3 U.S. Geological Survey, 1:100,000 scale 86 36 35 34 85 84 83 82 81 80 79 78

0 20 40 60 80 100 MILES0 20 40 60 80 100 KILOMETERS

Fall

Line

Blue

Ridge

Grenville gneissWestern Blue RidgeEastern Blue RidgeInner PiedmontMiltonFelsic intrusive

Raleigh and CharlotteCarolina slateOuter CarolinaMesozoicPhysiographic Province line

Piedmont Coastal Plain

TENNESSEE

VIRGINIA

SOUTH

CAROLINA GEORGIA

EXPLANATION

Atlantic

Ocean

Figure 2. Regional geozones in the Piedmont and Mountains Provinces of North Carolina. Figure 2. Regional geozones in the Piedmont and Mountains Provinces of North Carolina.
Methods 7

Methods

This section provides a discussion of the network of 

well sites and sample collection methods used to generate the 

water-quality data set used in this study. Methods used for 

compiling and analyzing the data for use in evaluating waterquality characteristics within and between the geozones also 

are presented.

Well Network

The groundwater-quality data examined in this report 

were collected between 1997 and 2008. The well network 

consists of 79 wells located in different regional geozones distributed throughout the North Carolina Piedmont and Mountain study area (fig. 3). Data for this study were obtained from 

PMREP research stations and the NAWQA Program.

Groundwater-quality data were compiled for 30 bedrock 

wells sampled at 8 PMREP research stations (table 2; fig. 3), 

each of which included several individual wells. Six of the 

research stations (AWRS, LPRS, LWRS, RHRS, UPRS, and 

ZPRS) are located in the Piedmont, and two research stations 

are located in the Mountains (BCRS and THRS). The research 

station wells are located in 7 of the 10 regional geozones 

(fig. 3; table 2). 

The research stations generally consist of transects of 

monitoring wells located parallel to an assumed flow path 

within a conceptual “slope-aquifer” system (LeGrand, 2004) 

from recharge areas to discharge areas. Well clusters installed 

at the research stations are designed to monitor separate zones 

in the groundwater system, including the shallow regolith, 

transition zone, and deep bedrock (Chapman and others, 

2005). In this study, water-quality data were compiled only 

from bedrock wells located at the PMREP research stations, in 

accordance with the purpose of this investigation. In addition 

to the PMREP well sites, water-quality data also were obtained 

from 49 domestic-supply bedrock wells within the North 

Carolina Piedmont and Mountain provinces that were sampled 

as part of NAWQA studies in North Carolina, South Carolina, 

and West Virginia. The NAWQA wells are located in 8 of the 

10 regional geozones (fig. 3; table 3).

Groundwater Sampling

From 2001 to 2008, both the USGS and the NCDENR 

DWQ collected groundwater-quality samples from the 

PMREP research station well sites (table 2) for analysis by 

their respective laboratories. Multiple rounds of sampling 

typically were conducted at each of the PMREP well sites 

although the specific number of samples collected per well 

varied. The NAWQA wells (table 3) generally had one round 

of groundwater samples collected for laboratory analyses. 

Groundwater samples were collected from the NAWQA–NC, 

NAWQA–SC, and NAWQA–WV well sites in 2007, 1998, 

and 1997, respectively.

Groundwater sampling conducted by the USGS at 

PMREP and NAWQA well sites followed established, documented protocols for collecting and processing samples for 

chemical analyses (Koterba and others, 1995; U.S. Geological 

Survey, variously dated; U.S. Geological Survey, written commun., 2008). The NCDENR DWQ also followed established 

protocols for collecting groundwater samples at the PMREP 

research stations (North Carolina Division of Water Quality, 

2008a). Sampling methods included the use of submersible 

pumps and peristaltic pumps. Wells were purged until measured physical properties stabilized. Physical properties were 

measured by using a multiparameter water-quality instrument 

and flow-through chamber to record dissolved oxygen (DO), 

pH, temperature, and specific conductance. Incremental alkalinity titrations were conducted in the field. Additional details 

of groundwater-sampling methods used at PMREP research 

stations are provided by Huffman and others (2006).

Groundwater samples collected by the USGS were 

analyzed by the USGS National Water Quality Laboratory 

(NWQL) in Lakewood, Colorado, and their contract laboratories. Samples collected by the NCDENR DWQ were analyzed 

by the DWQ Laboratory Section in Raleigh, North Carolina. 

Information on the analytical program and services of each 

laboratory can be accessed on the Internet (U.S. Geological 

Survey, 2008b; North Carolina Division of Water Quality, 

2008b). The laboratory results for common ions, nutrients, 

metals, and radiochemical isotopes analyzed in samples collected from the PMREP and NAWQA wells were compiled 

and evaluated to characterize water-quality conditions in the 

geozones of the Piedmont and Mountains study area. The 

complete analytical data set, grouped by geozone, is provided 

as Appendix 1. The nitrate-concentration values presented in 

this report were measured as nitrite plus nitrate, in milligrams 

per liter as nitrogen (N). Because nitrite typically represents 

a small fraction of the total concentration, the reported values 

are presented and discussed as nitrate.

The following discussion notes differences in the analytical results of samples collected and analyzed by the USGS and 

the NCDENR DWQ for PMREP well sites (Appendix 1). The 

number of samples analyzed by the USGS and NCDENR for a 

given PMREP well often varied because of differences in sampling frequency between the two agencies. Filtered samples 

collected by the USGS were analyzed by the NWQL for dissolved concentrations of major ions, nutrients, and trace elements. Most NCDENR samples were unfiltered and analyzed 

by the DWQ laboratory for total concentrations of major ions, 

nutrients, and metals. The analytical reporting levels (RL) 

for constituent concentrations reported by the USGS NWQL 

tend to be lower than those reported by the DWQ laboratory. 

For example, dissolved arsenic concentrations reported by the 

NWQL generally have an RL of 1 or 2 micrograms per liter 

(μg/L), and total arsenic concentrations reported by the DWQ 

laboratory generally have an RL of 5 or 10 μg/L (Appendix 1). 

Because of these differences, an intercomparison of analytical 

results between the USGS NWQL and the DWQ laboratory 

for the PMREP well samples was considered beyond the scope 

of this study.
8 Characterization of Groundwater Quality in the Piedmont and Blue Ridge Physiographic Provinces, North Carolina Base from digital files of: U.S. Department of Commerce, Bureau of Census, 1990 Precensus TIGER/Line Files-Political boundaries, 1991 U.S. Environmental Protection Agency, River File 3 U.S. Geological Survey, 1:100,000 scale 86 36 35 34 85 84 83 82 81 80 79 78

0 20 40 60 80 100 MILES0 20 40 60 80 100 KILOMETERS

Fall

Line

Blue

Ridge

Grenville gneissWestern Blue RidgeEastern Blue RidgeInner PiedmontMiltonFelsic intrusive

Raleigh and CharlotteCarolina slateOuter CarolinaMesozoicPhysiographic Province lineNAWQA well location and numberPMREP research station and number

Piedmont Coastal Plain

TENNESSEE

VIRGINIA

SOUTH

CAROLINA GEORGIA

EXPLANATION

Atlantic

Ocean

Figure 3. Distribution of Piedmont and Mountains Resource Evaluation Program research stations and National Water-Quality Assessment Program well sites in regional geozones of the Piedmont and Mountains Provinces of North Carolina. 

AS-086

AS-085

AS-088

AS-089

THRS AS-087

WT-097

AX-040

AX-039

WT-096

AL-052

AL-053

AL-051

AL-054

MC-106 BK-106

AWRS

CW-348

YD-201

RO-150

CB-155 GS-287

GS-288

LPRS

RO-151

GU-657

GU-658

UPRS CS-146

YD-202

CV-134

CV-135 RU-076

BCRS CH-230

MG-030

MO-243

ZPRS

DS-165

RA-174

MG-031

UN-146

UN-145

UN-143

UN-143

AM-221 CS-145

OR-683

OR-684

PS-092

DR-072

WK-351

WK-350

FR-169

RHRS

WR-154

WK-349

LWRS

LPRS

Figure 3. Distribution of Piedmont and Mountains Resource Evaluation Program research stations and National Water-Quality Assessment Program well sites in regional geozones of the Piedmont and Mountains Provinces of North Carolina.
Methods 9

Table 2. Well information for the North Carolina Department of Environment and Natural Resources, Division of Water Quality, Piedmont 

and Mountains Resource Evaluation Program research stations.—Continued

[USGS, U.S. Geological Survey; ddmmss.s, degrees, minutes, seconds; bls, below land surface; NAVD 88, North American Vertical Datum of 1988]

Local 

field 

name

USGS site number USGS station name County Latitude

ddmmss.s

Longitude

ddmmss.s

Well 

depth 

(feet bls)

Land-surface 

altitude 

(feet above 

NAVD 88)

Regional 

geozone 

designation

Allison Woods research station (AWRS)

IR-163 355429080492303 IR-163 Allison Woods RS 

MW-1D Bed nr Turnersburg

Iredell 355429.7 804923.2 400 900 Inner Piedmont

IR-164 355429080492304 IR-164 Allison Woods RS CH-1 

nr Turnersburg

Iredell 355430.3 804923.6 100 900 Inner Piedmont

IR-167 355428080493003 IR-167 Allison Woods RS 

MW-2D Bed nr Turnersburg

Iredell 355428.3 804930.2 400 870 Inner Piedmont

IR-176 355427080493704 IR-164 Allison Woods RS CH-4 

nr Turnersburg

Iredell 355427.9 804936.9 100 830 Inner Piedmont

Bent Creek research station (BCRS)

BU-070 352840082381003 BU-070 Bent Creek RS MW-1D 

(bedrock well)

Buncombe 352841 823812 221 2,202 Eastern Blue 

Ridge

BU-082 352810082383503 BU-082 Bent Creek RS MW-5D 

(bedrock well)

Buncombe 352810 823835 300 2,305 Eastern Blue 

Ridge

BU-103 352919082383802 BU-103 Bent Creek RS MW-6D 

(bedrock well)

Buncombe 352919 823839 190 2,420 Eastern Blue 

Ridge

Langtree Peninsula research station (LPRS)

IR-132 353135080524203 IR-132 Langtree RS MW-2D 

nr Mt Mourne, NC

Iredell 353136 805242 400 803 Raleigh and 

Charlotte

IR-147 353141080524703 IR-147 Langtree RS MW-1D 

(bedrock well)

Iredell 353141 805247 602 813 Raleigh and 

Charlotte

IR-153 353145080524703 IR-153 Langtree RS MW-4D 

(bedrock well)

Iredell 353145 805247 400 802 Raleigh and 

Charlotte

Lake Wheeler research station (LWRS)

WK-279 354356078403503 WK-279 Lake Wheeler RS 

MW-1D (bedrock well)

Wake 354356 784034 302 339 Raleigh and 

Charlotte

WK-286 354404078403103 WK-286 Lake Wheeler RS 

MW-3D (bedrock well)

Wake 354405 784031 301 376 Raleigh and 

Charlotte

WK-287 354401078403401 WK-287 Lake Wheeler RS PW-1 Wake 354401 784034 302 358 Raleigh and 

Charlotte

Raleigh Hydrogeologic research station (RHRS)

WK-331 354328078295704 WK-331 Raleigh Hydrogeologic 

RS WC-1CH (bedrock)

Wake 354329 782957 90 165 Felsic intrusive

WK-334 354315078300103 WK-334 Raleigh RS WC-2D 

nr Garner, NC (bedrock)

Wake 354315 783002 460 188 Felsic intrusive

WK-335 354315078300104 WK-335 Raleigh RS WC-2CH 

nr Garner, NC (bedrock)

Wake 354316 782959 85 199 Felsic intrusive

WK-338 354305078295803 WK-338 Raleigh Hydrogeologic 

RS WC-3D (bedrock)

Wake 354306 782958 300 210 Felsic intrusive
10 Characterization of Groundwater Quality in the Piedmont and Blue Ridge Physiographic Provinces, North Carolina

Table 2. Well information for the North Carolina Department of Environment and Natural Resources, Division of Water Quality, Piedmont 

and Mountains Resource Evaluation Program research stations.—Continued

[USGS, U.S. Geological Survey; ddmmss.s, degrees, minutes, seconds; bls, below land surface; NAVD 88, North American Vertical Datum of 1988]

Local 

field 

name

USGS site number USGS station name County Latitude

ddmmss.s

Longitude

ddmmss.s

Well 

depth 

(feet bls)

Land-surface 

altitude 

(feet above 

NAVD 88)

Regional 

geozone 

designation

Tater Hill research station (THRS)

WT-098 361646081433001 WT-098 Tater Hill RS ASU-1D 

(bedrock) nr Boone, NC

Watauga 361646 814303 345 4,060 Western Blue 

Ridge

WT-099 361646081430402 WT-099 Tater Hill RS ASU-2D 

bedrock nr Boone, NC

Watauga 361645.9 814303.7 350 4,060 Western Blue 

Ridge

Upper Piedmont research station (UPRS)

RK-229 362334079421603 RK-229 Upper Piedmont RS 

MW-N1D

Rockingham362335 794217 300 673 Milton

RK-232 362331079421603 RK-232 Upper Piedmont RS 

MW-N2D

Rockingham362332 794217 300 672 Milton

RK-234 362328079421702 RK-234 Upper Piedmont RS 

MW-N3D

Rockingham362328 794217 260 770 Milton

RK-236 362323079421202 RK-236 Upper Piedmont RS 

MW-N4D

Rockingham362323 794213 300 840 Milton

RK-238 362240079411802 RK-238 Upper Piedmont RS 

MW-S1D

Rockingham362241 794119 301 803 Milton

RK-242 362231079310804 RK-242 Upper Piedmont RS 

MW-S3D

Rockingham362232 794108 438 705 Milton

RK-245 362226079410103 RK-245 Upper Piedmont RS 

MW-S4D

Rockingham362226 794101 380 660 Milton

Zoological Park research station (ZPRS)

RA-164 353704079451204 RA-164 NC Zoo RS MW-1D 

(bedrock)

Randolph 353704.1 794511.9 300 598 Carolina slate

RA-167 353708079452103 RA-167 NC Zoo RS MW-2D 

(bedrock)

Randolph 353707.7 794520.6 260 639 Carolina slate

RA-170 353712079452703 RA-170 NC Zoo RS MW-3D 

(bedrock)

Randolph 353711.8 794527.3 180 665 Carolina slate

RA-172 353708079452104 RA-172 NC Zoo RS CH-2 

(corehole)

Randolph 353708 794521 98 636 Carolina slate
Methods 11

Table 3. Well information for U.S. Geological Survey National Water-Quality Assessment Program sites used in this study.—Continued

[USGS, U.S. Geological Survey; ddmmss.s, degrees, minutes, seconds; bls, below land surface; NAVD 88, North American Vertical Datum of 1988]

Local 

field name 

(see fig. 3)

USGS site number USGS station name County Latitude

ddmmss.s

Longitude

ddmmss.s

Well 

depth 

(feet bls)

Land-surface 

altitude 

(feet above 

NAVD 88)

Regional 

geozone 

designation

Wells sampled by NAWQA Study Unit in North Carolina

AM-221 361355079261601 AM-221 Piedmont MAS 18 Alamance 361355 792616 240 244 Felsic intrusive

CB-155 352529080380801 CB-155 Piedmont MAS 24 Cabarrus 352529 803808 305 680 Felsic intrusive

CH-230 353756079295501 CH-230 Piedmont MAS 46 Chatham 353756 795955 263 544 Carolina slate

CS-145 361540079123001 CS-145 Piedmont MAS 17 Caswell 361540 791230 182 215 Felsic intrusive

CS-146 362817079231601 CS-146 Piedmont MAS 35 Caswell 362817 792316 322 138 Milton

DR-072 361020078484801 DR-072 Piedmont MAS 42 Durham 361020 784848 125 446 Carolina slate

DS-165 353040080062101 DS-165 Piedmont MAS 47 Davidson 353040 800621 105 162 Carolina slate

FR-169 360345078323201 FR-169 Piedmont MAS 9 Franklin 360345.4 783231.7 125 148 Raleigh and 

Charlotte

GU-657 360923079524201 GU-657 Piedmont MAS 19 Guilford 360923.1 795241.6 165 261 Felsic intrusive

GU-658 355900079521101 GU-658 Piedmont MAS 20 Guilford 355900 795211 400 275 Felsic intrusive

MG-030 352512079494501 MG-030 Piedmont Major 

Aquifer Study Site 50

Montgomery 352512 794945 245 530 Carolina slate

MG-031 351440080000601 MG-031 Piedmont Major 

Aquifer Study Site 52

Montgomery 351440 800006 230 415 Carolina slate

MO-243 352828079313501 MO-243 Piedmont Major 

Aquifer Study Site 49

Moore 352828 793135 308 412 Carolina slate

OR-683 360152079110301 OR-683 Piedmont MAS 43 Orange 360151.8 791102.5 160 649 Carolina slate

OR-684 355302079091201 OR-684 Piedmont MAS 44 Orange 355302.3 790912.2 300 173 Carolina slate

PS-092 362707078495301 PS-092 Piedmont MAS 41 Person 362707 784953 142 — Carolina slate

RA-174 354054079583201 RA-174 Piedmont MAS 45 Randolph 354054 795832 325 143 Carolina slate

RO-150 353127080253401 RO-150 Piedmont MAS 13 Rowan 353127 802534 145 227 Raleigh and 

Charlotte

RO-151 354307080265501 RO-151 Piedmont MAS 22 Rowan 354307 802655 165 255 Felsic intrusive

UN-145 350251080212301 UN-145 Piedmont Major 

Aquifer Study Site 53

Union 350251 802123 255 508 Carolina slate

UN-146 345327080241601 UN-146 Piedmont Major 

Aquifer Study Site 54

Union 345327 802416 250 501 Carolina slate

WK-349 354954078421301 WK-349 Piedmont MAS 12 Wake 354953.6 784213 185 177 Felsic intrusive

WK-350 355448078225901 WK-350 Piedmont MAS 23 Wake 355447.9 782259.3 240 102 Felsic intrusive

WK-351 355212078173501 WK-351 Piedmont MAS 21 Wake 355211.6 781734.7 125 91 Felsic intrusive

WR-154 361805078051001 WR-154 Piedmont MAS 8 Warren 361804.5 780510.3 300 85 Raleigh and 

Charlotte

YD-201 361322080391801 YD-201 Piedmont MAS 37 Yadkin 361322 803918 422 993 Milton

YD-202 360819080281501 YD-202 Piedmont MAS 38 Yadkin 360819 802815 222 877 Milton
12 Characterization of Groundwater Quality in the Piedmont and Blue Ridge Physiographic Provinces, North Carolina

Table 3. Well information for U.S. Geological Survey National Water-Quality Assessment Program sites used in this study.—Continued

[USGS, U.S. Geological Survey; ddmmss.s, degrees, minutes, seconds; bls, below land surface; NAVD 88, North American Vertical Datum of 1988]

Local 

field name 

(see fig. 3)

USGS site number USGS station name County Latitude

ddmmss.s

Longitude

ddmmss.s

Well 

depth 

(feet bls)

Land-surface 

altitude 

(feet above 

NAVD 88)

Regional 

geozone 

designation

Wells sampled by NAWQA Study Unit in South Carolina

AX- 039 354911081161401 AX- 39 Alexander 354910.62 811614.26 155 1,060 Inner Piedmont

AX- 040 355856081130501 AX- 40 Alexander 355857.96 811308.01 80 1,330 Inner Piedmont

BK-106 354252081372501 BK- 106 Burke 354251.84 813725.43 830 1,700 Felsic intrusive

CV-134 352703081391601 CV- 134 Cleveland 352703 813916 205 960 Inner Piedmont

CV-135 351252081363401 CV- 135 Cleveland 351252 813634 300 750 Inner Piedmont

CW-348 353535081133401 CW- 348 Catawba 353535 811334 165 890 Inner Piedmont

GS-287 352222081005001 GS- 287 Gaston 352222 810050 205 830 Felsic intrusive

GS-288 352106081152601 GS- 288 Gaston 352106 811526 80 910 Raleigh and 

Charlotte

MC-106 354154081532201 MC- 106 McDowell 354154 815322 305 1,340 Inner Piedmont

RU- 076 352050081593201 RU- 76 Rutherford 352050 815932 450 1,010 Inner Piedmont

UN-143 345323080423701 UN- 143 Union 345323 804237 160 635 Carolina slate

Wells sampled by NAWQA Study Unit in West Virginia

AL-051 362445081154701 AL-051 Alleghany 362445 811547 700 2,750 Eastern Blue 

Ridge

AL-052 363345080564001 AL-052 Alleghany 363345 805640 146 2,735 Eastern Blue 

Ridge

AL-053 363053081084101 AL-053 Alleghany 363053 810841 273 3,020 Eastern Blue 

Ridge

AL-054 363208081195801 AL-054 Alleghany 363208 811958 500 2,780 Grenville gneiss

AS-085 362739081305001 AS-085 Ashe 362739 813050 205 2,680 Eastern Blue 

Ridge

AS-086 363254081364201 AS-086 Ashe 363254 813642 125 3,480 Western Blue 

Ridge

AS-087 361957081273001 AS-087 Ashe 361957 812730 135 3,040 Eastern Blue 

Ridge

AS-088 362436081254101 AS-088 Ashe 362436 812541 95 2,720 Eastern Blue 

Ridge

AS-089 362339081412601 AS-089 Ashe 362339 814126 144 3,080 Grenville gneiss

WT-096 361638081384401 WT-096 Watauga 361638 813844 305 3275 Eastern Blue 

Ridge

WT-097 361327081354601 WT-097 Watauga 361327 813546 150 3230 Eastern Blue 

Ridge
Methods 13

The following discussion notes differences between 

the data sets compiled from the PMREP and NAWQA wells 

(fig. 3; tables 2, 3) to characterize groundwater quality in the 

regional geozones. The PMREP well set consists of stations 

that are somewhat limited in geographic range but have many 

wells present at each station. The NAWQA well set consists 

of single wells that are spatially diverse throughout the study 

area. In addition, many groundwater samples typically were 

collected and analyzed for the PMREP wells, which are used 

solely for monitoring and are pumped only during sampling 

events. Only one sample, typically, was collected and analyzed 

for the NAWQA wells, which are used for domestic supply 

and are pumped regularly. The data from both programs were 

combined for data analysis to examine water-quality conditions in 8 of the 10 geozones located in the study area. In this 

study, no wells were present in the Mesozoic or Outer Carolina geozones (fig. 3); hence, these geozones are not discussed 

further. Analytical data were available for only two wells 

(AL-054 and AS-089) in the Grenville gneiss geozone and 

three wells (AS-086, WT-098, and WT-099) in the Western 

Blue Ridge geozone (fig. 3; tables 2, 3). 

Statistical Analysis of Water-Quality Data

Evaluation of the chemical data was accomplished in several ways to characterize water-quality conditions in regional 

geozones within the Piedmont and Mountains Provinces 

(fig. 2). Piper diagrams, analysis of variance (ANOVA), and 

box plots were used to analyze the statistical and geochemical 

variability in the water-quality data for the regional geozones.

Within each geozone, water-quality analyses were 

examined using Piper diagrams (Piper, 1944), which display 

overall information on the primary ionic composition of water 

in each geozone. In addition, the analytical results within each 

geozone were compared to available State (North Carolina 

Department of Environment and Natural Resources, 2006) 

and Federal (U.S. Environmental Protection Agency, 2003) 

maximum contaminant levels (MCLs) for drinking-water 

standards to determine if the chemical constituents in samples 

exceeded drinking-water criteria. The analytical results also 

were compared to the U.S. Environmental Protection Agency 

(USEPA) secondary maximum contaminant levels (SMCLs; 

U.S. Environmental Protection Agency, 2003), which typically 

are used for examining constituents that have an aesthetic 

effect on drinking water.

A Kruskal-Wallis one-way ANOVA test (Helsel and 

Hirsch, 1992) on ranked data was used to examine the relation 

between regional geozones and groundwater constituents. The 

test is used for a given constituent to determine if all geozones 

have the same median concentration of the constituent or 

whether at least one median is different. The null hypothesis 

for the Kruskal-Wallis test is that the median concentrations 

of each geozone are identical. The alternate hypothesis is 

that the median concentration in at least one geozone differs 

from the other geozones. The null hypothesis is rejected if the 

p-value computed from the test is smaller than or equal to the 

significance level of α = 0.05, indicating that the geologic setting, or geozone, has a significant influence on the constituent 

concentrations. If the p-value is greater than α = 0.05, the null 

hypothesis is not rejected. Note that the lack of significance 

does not mean that for a given constituent, concentrations do 

not differ among the geozones; it only means that a significant 

difference in medians between the geozones, if any, was not 

large enough to be detected with the given sample size. 

The Kruskal-Wallis test results for a given constituent 

may indicate that the null hypothesis is rejected and that a statistically significant difference in median concentrations of the 

constituent exists among the geozones; however, it does not 

specify which geozones are different. In these cases, a Tukey 

multiple-comparison test (Helsel and Hirsch, 1992) was used 

to pinpoint differences among the geozones for a particular 

constituent. The Tukey multiple-comparison test compares all 

possible pairs of the geozones and determines which comparisons are significantly different at α = 0.05. 

Box plots were used to provide statistical summaries 

of those chemical constituents noted as having significant 

differences among the geozones. Outlier data that were less 

than the 10th percentile or greater than the 90th percentile were 

excluded from the box plots. For many constituents examined in the data set, individual results often were reported as 

a left-censored value (less than [<] the RL), which complicates the process for statistically summarizing the data, and 

hence, obtaining necessary data for constructing the box plots. 

Often times, researchers may substitute the censored result, or 

nondetect, with an arbitrary value that is equal to zero, the RL, 

or one-half of the RL; however, this approach can potentially 

bias the data set and lead to false conclusions (Bonn, 2008).

For constituents having left-censored data with one or 

more reporting levels, summary statistics were estimated using 

regression on order statistics (ROS) methods (Helsel and 

Cohn, 1988; Helsel and Hirsch, 1992). Some of the censored 

data analyzed with ROS contained constituent values that 

were reported by the laboratory as estimated (E), which means 

analytes are considered to be present in the sample at concentrations above the long-term method detection limit (MDL) 

but less than the RL. A conservative approach was used in 

handling E values. In preparing censored data for analysis 

with ROS, the E values were replaced with the RL such that 

these values with high uncertainty were not compared to one 

another, but rather, were ranked equally in the analysis (Bonn, 

2008). Similarly, the Tukey multiple-range test, which is 

used to test for geozone differences for a given constituent, 

often was performed on a constituent data set having “less 

than” censored values, usually with one or more RLs. In these 

cases, the censored data were set to the highest RL such that 

all reported censored values that were less than the RL were 

ranked equally prior to analysis with the Tukey multiplerange test. Additional information on various approaches and 

methods used for understanding and interpreting low-level 

concentration data reported by the USGS NWQL is presented 

by Bonn (2008). Statistical summaries were not estimated with 
14 Characterization of Groundwater Quality in the Piedmont and Blue Ridge Physiographic Provinces, North Carolina

ROS for a given constituent data set when the percentage of 

censored values exceeded 80 percent, there were fewer than 

three uncensored observations, or the uncensored observations 

were all equal. Box plots for constituents having censored data 

were plotted using the percentiles estimated with ROS.

Bedrock Groundwater Quality within 

Regional Geozones

Characterization of the chemical properties of groundwater within the geozones is necessary for a better understanding of the quality of water contained in the fractured bedrock 

aquifers and the suitability of the water for various uses. The 

quantities and types of dissolved chemical constituents and the 

physical and chemical properties of groundwater are a result 

of the bedrock mineralogy and processes of the hydrogeologic 

environment. Various factors influence groundwater quality in 

bedrock aquifers (Briel, 1997; Daniel and Dahlen, 2002; Lindsey and others, 2006; McMahon and Chapelle, 2008; Vinson 

and others, 2009), including (1) the chemical characteristics 

of the water from recharge areas; (2) lithologic composition of 

the crystalline bedrock and overlying regolith materials, which 

contain soluble parent minerals; (3) the rate of flow, which 

controls the residence time between the moving water and the 

aquifer materials; and (4) redox conditions, which influence 

the distribution of natural and anthropogenic contaminants 

along groundwater-flow paths. Identification of the physical 

properties and chemical constituents in groundwater, therefore, 

reveals information about the aquifer materials that contain 

the water as well as whether the water contains contaminants, 

such as radon or nitrate, which are derived from natural rock 

sources and anthropogenic surface sources, respectively. 

Changes in water quality through time within an aquifer are 

often indicative of human activities and are caused either by 

changes in population and land use or changes in pumping for 

water supplies (Daniel and Dahlen, 2002). Changes in quality 

also can occur naturally over time from other factors, such as 

groundwater circulation and geochemical reactions.

Ionic Composition

Overview: The general ionic composition, or water type, 

of groundwater within the geozones was examined by using 

a Piper diagram (fig. 4). The water type(s) present in the 

geozones were classified based on the predominant cations 

and anions dissolved in the bedrock groundwater samples 

(Appendix 1). In determining water type using a Piper diagram (fig. 4), the analytical results are plotted on the central 

quadrilinear diagram. To determine the specific primary 

cations, the plotted data point is projected to the lower leftside trilinear diagram, which shows the percentage of cations 

in water composition. The anions are determined the same 

way on the right-side trilinear diagram. In the middle of the 

quadrilinear part of the Piper diagram, mixed ionic composition is indicated where no cation or anion is dominant. The 

chemical analyses from the PMREP and NAWQA wells in 

each geozone included for analysis with the Piper diagram 

were limited to those samples where the ion balance, or cation 

and anion sums, were determined to be within 10 percent 

of each other. Only dissolved cations (calcium, magnesium, 

sodium, and potassium) and anions (bicarbonate, sulfate, chloride, and nitrate) having concentrations detected above the RL 

were included in the analyses. The water type determined for 

individual well samples plotted for each geozone in figure 4 is 

provided in Appendix 1.

The limited samples available for evaluation of the ionic 

composition of groundwater for the Grenville gneiss (two 

wells) and Western Blue Ridge (three wells) geozones indicate 

a calcium/bicarbonate to calcium-sodium/bicarbonate water 

type (Appendix 1; fig. 4). Water quality in these geozone wells 

appears to reflect ambient conditions with no noticeable influence from local land use.

The ionic composition of groundwater in the Eastern 

Blue Ridge, Inner Piedmont, Milton, Felsic intrusive, Raleigh 

and Charlotte, and Carolina slate geozones is variable with 

no dominant water type (fig. 4). No major differences in 

overall ionic chemistry among these geozones were evident 

with the data examined. The basic ionic compositions of most 

groundwater samples in these geozones generally are either 

a calcium-sodium/bicarbonate or a calcium-magnesium/

bicarbonate water type where calcium is the dominant cation, 

followed by either magnesium or sodium, and bicarbonate is 

the dominant anion (fig. 4; Appendix 1). The more notable 

changes in water type occurred in the anionic composition of 

the groundwater in response to anthropogenic effects, such as 

nutrient inputs from local land use that increased groundwater 

nitrate concentrations.

Observed variability in the cationic and anionic composition of groundwater within a particular geozone appears to 

reflect local differences in lithologic setting, groundwater-flow 

paths, geochemical conditions, and(or) land use. Data from the 

Milton and Felsic intrusive geozones is used to describe some 

of the factors that influence bedrock groundwater chemistry at 

local scales. 

Milton Geozone: The ionic compositions of groundwater 

samples from seven PMREP wells at the Upper Piedmont 

research station (UPRS) and three spatially separated NAWQA 

wells were evaluated for the Milton geozone (fig. 5A; Appendix 1). The UPRS is located on an agricultural research farm 

owned and operated by North Carolina State University and 

the North Carolina Department of Agriculture; consequently, 

groundwater quality at some well locations has been influenced by nutrients, potentially including animal manure 

and inorganic fertilizers. The overall ionic composition for 

samples from the Milton geozone is variable, including a 

calcium-magnesium/bicarbonate type for well RK-232, a 

calcium-sodium-magnesium/nitrate-bicarbonate type for well 

RK-242, and a calcium-sodium/sulfate-bicarbonate type for 

well RK-245. Wells RK-242 and RK-245 are located about 
Bedrock Groundwater Quality within Regional Geozones 15

825 feet (ft) apart along a transect in the southern part of the 

UPRS all within a felsic gneiss rock type (Huffman and others, 2006). Well RK-232 is located about 1.6 miles (mi) away 

in the northern part of the UPRS and taps a mica gneiss and 

schist rock type. 

Groundwater samples from wells RK-232, RK-242, and 

RK-245 (with borehole depths ranging from 300 to 438 ft) 

typically were collected by lowering a pump down the open 

borehole of the well to target a fracture zone at a depth of 

about 130–160 ft. These “open-hole” samples are considered to be a composite of the entire well because water may 

have been pulled from fractures located above and below 

the targeted zone. In 2004, a multifunction Bedrock-Aquifer 

Transportable Testing Tool (BAT3; Shapiro, 2001) was used 

to collect groundwater-quality samples from wells RK-232, 

RK-242, and RK-245. The BAT3 allows discrete intervals 

of a borehole to be isolated hydraulically for water-quality 

sampling by using two inflatable packers that seal against the 

borehole wall and isolate the fracture zone to be sampled.

The groundwater ionic composition differs among wells 

RK-232, RK-242, and RK-245 for both the open-hole samples 

and the individual fracture-zone samples (fig. 5B; Appendix 1), most likely because of variable rock types tapped by 

the wells at the local scale and effects from local agricultural 

practices. A subtle transition in groundwater cationic composition occurs with depth at these well sites. Calcium is the 

dominant cation for these well samples at all depths (fig. 5B; 

Appendix 1). After calcium, the relative proportion of magnesium tends to decrease with fracture depth as the proportion of sodium increases. The proportion of calcium to both 

magnesium and sodium also tends to increase with depth. This 

shift in cationic composition likely reflects differences in the 

amount of ion exchange between the groundwater and rock 

materials where a greater proportion of calcium has dissolved 

80

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0 100 100

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CALCIUM

CATIONS

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MAGNESIUM

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CHLORIDE PLUS NITRATE

ANIONS

0 20 100

 BICARBONATE

80

100

60

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SULFATE

CALCIUM PLUS MAGNESIUM

Carolina slate

Eastern Blue Ridge

Felsic intrusive

Grenville gneiss

Western Blue Ridge

Inner Piedmont

Milton

Raleigh and Charlotte

EXPLANATION

Figure 4. Piper diagram showing ionic composition of groundwater samples within eight geozones in the Piedmont

 and Mountains Provinces in North Carolina.

PERCENT

PERCENT

Figure 4. Piper diagram showing ionic composition of groundwater samples within eight geozones in the Piedmont and 

Mountains Provinces of North Carolina.
16 Characterization of Groundwater Quality in the Piedmont and Blue Ridge Physiographic Provinces, North Carolina

Figure 5. Piper diagrams showing ionic composition of (A) all samples from the Milton geozone, and

 (B) open-hole samples and individual fracture-zone samples for wells RK-232, RK-242, and RK-245

 in the Milton geozone in the Piedmont and Mountains Provinces of North Carolina.

CS-146

RK-229

RK-232

RK-234

RK-236

RK-238

RK-242

RK-245

YD-201

YD-202

EXPLANATION

80

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80

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SULFATE PLUS CHLORIDE PLUS NITRATE

PERCENT

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0 100 100

60 40 20

CALCIUM

CATIONS

100 80 0

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MAGNESIUM

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CHLORIDE PLUS NITRATE

ANIONS

0 20 100

 BICARBONATE

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SULFATE

CALCIUM PLUS MAGNESIUM

PERCENT

PERCENT

PERCENT

RK-232

RK-242

RK-245

Indicates sampled fracture zone

in feet below land surface

Indicates sample collected

 from well borehole

EXPLANATION

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SULFATE PLUS CHLORIDE PLUS NITRATE

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0 100 100

60 40 20

CALCIUM

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100 80 0

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0

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MAGNESIUM

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CHLORIDE PLUS NITRATE

ANIONS

0 20 100

 BICARBONATE

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0

SULFATE

CALCIUM PLUS MAGNESIUM

53-70

283-300

234-251 148-165

77-93

199-340

115-340

77-93

199-340

115-340

77-93

199-340

115-340

81-98

81-98

283-300

234-251

148-165 81-98

283-300

234-251

148-165

81-98

187-204

149-166

53-70

187-204 149-166

53-70

187-204

149-166

A

B

Figure 5. Piper diagram 

showing ionic composition 

of (A) all samples from the 

Milton geozone, and (B) openhole samples and individual 

fracture-zone samples 

for wells RK-232, RK-242, 

and RK-245 in the Milton 

geozone in the Piedmont 

and Mountains Provinces of 

North Carolina.
Bedrock Groundwater Quality within Regional Geozones 17

into older groundwater with longer residence times and flow 

paths that occur in the deeper fractures. Water composition for 

well RK-232 shifts from calcium-magnesium/bicarbonate at 

fracture interval 53–70 ft, to calcium-sodium/bicarbonate at 

fracture interval 149–166 ft, to calcium/bicarbonate at fracture 

interval 187–204 ft. The open-hole sample for well RK-232 

is classified as a calcium-magnesium/bicarbonate type that 

appears to be influenced primarily by groundwater derived 

from the shallow fracture at 53–70 ft.

Dissolved nitrate concentrations for all RK-232 samples 

plotted in figure 5B were less than the RL of 0.06 mg/L 

(Appendix 1). Dissolved chloride and dissolved sulfate concentrations in the same samples were less than 5 mg/L and 

8 mg/L, respectively. The absence of nitrate in well RK-232 

possibly reflects denitrification processes occurring along the 

groundwater-flow path and potential dilution from surfacewater interaction. Well RK-232 is located in a flood plain near 

the groundwater-discharge zone at the stream, downgradient 

from agricultural land use in upgradient recharge areas of the 

groundwater-flow path (Huffman and others, 2006). Conditions that generally promote denitrification in groundwater 

occur when nitrate is present, DO concentrations are low—

less than 0.2 mg/L, and electron donors (such as carbon) are 

present (Korom, 1992; Seitzinger and others, 2006; McMahon 

and Chapelle, 2008). In general, dissolved oxygen is consumed along groundwater-flow paths from recharge areas 

to discharge areas as microorganisms use DO as an electron 

acceptor during microbial degradation of organic carbon. 

As DO is depleted along the groundwater-flow path, nitrate, 

if present, becomes the next most favorable electron acceptor used by microorganisms (Korom, 1992; McMahon and 

Chapelle, 2008). The presence of anoxic redox conditions, as 

indicated by the low DO concentrations of about 0.2 mg/L, 

in older groundwater moving along deeper flow paths at well 

RK-232 suggest that any nitrate that may have been present 

from upgradient areas of the flow path likely has been reduced 

through denitrification processes. 

A more pronounced difference is noted in the anionic 

composition of samples from wells RK-242 and RK-245 as 

compared to RK-232. At well RK-242, the principal anions 

include bicarbonate and nitrate, with a slight shift occurring 

at depth as the relative proportion of bicarbonate to nitrate 

increases. The occurrence of nitrate in groundwater at well 

RK-242 (fig. 5B; Appendix 1) likely reflects surface inputs 

of nitrogen associated with agricultural activities at the 

site. This well is located in a recharge area with downward 

gradients from the shallow regolith to the bedrock. Additionally, hydraulic heads measured during sampling of the four 

bedrock-fracture zones were all similar, within 0.5 ft of each 

other, indicating that the fractures may be interconnected 

and have similar transmissivities; thereby, allowing groundwater nitrate from surface sources to migrate downward into 

deeper bedrock fractures. Dissolved chloride concentrations 

were detected at fairly uniform concentrations in the four 

fracture zones, ranging from 5.44 to 5.96 mg/L. The decrease 

in concentrations of both DO and nitrate with depth in the 

fracture-zone samples at 81–98 ft (DO = 7.0 mg/L, nitrate = 

9.46 mg/L), 148–165 ft (DO = 2.8 mg/L, nitrate = 8.17 mg/L), 

234–251 ft (DO = 0.6 mg/L, nitrate = 8.16 mg/L), and 

283–300 ft (DO = 0.3 mg/L, nitrate = 7.71 mg/L) indicates 

that some denitrification of nitrate may be occurring in deeper 

parts of the bedrock aquifer at well RK-242. 

A difference in anionic composition also is observed for 

the open-hole and fracture-zone samples from well RK-245, 

ranging from a calcium-magnesium/bicarbonate-sulfate type 

in fracture interval 77–93 ft to a calcium/sulfate-bicarbonate 

type in fracture zone 199–340 ft (fig. 5B; Appendix 1). At this 

well, the relative proportion of sulfate to bicarbonate increases 

with depth. In contrast to well RK-242, groundwater redox 

conditions at well RK-245 appear more favorable for enabling 

nitrate reduction through denitrification. Well RK-245, located 

downgradient from well RK-242 in a discharge area next to 

a stream, has an upward vertical hydraulic gradient. Dissolved nitrate concentrations for all RK-245 samples plotted 

in figure 5B were less than the RL of 0.06 mg/L (Appendix 1). 

The anoxic redox conditions present in groundwater at well 

RK-245, as indicated by the low DO concentrations of about 

0.1 mg/L, suggest that any groundwater nitrate moving along 

deep flow paths from upgradient sources likely has been 

reduced through denitrification processes. Dissolved sulfate 

concentrations are higher in the RK-245 samples (range of 

46.2–96.3 mg/L) as compared to concentrations in samples 

from wells RK-232 and RK-242, which are less than 8 mg/L. 

Although local mineralogy and redox processes cannot be 

excluded, the drilling fluids used during well construction 

are the suspected source of the elevated sulfate levels in this 

well. The borehole of well RK-245 is highly fractured, and the 

drilling mud may not have been completely flushed from the 

fractures during well development, thus, representing a potential source of sulfate to groundwater in this well. 

Felsic Intrusive Geozone: The ionic composition of most 

samples from the 14 wells in the Felsic intrusive geozone 

(fig. 6; Appendix 1) cluster around a calcium-sodium/bicarbonate to calcium-magnesium/bicarbonate water type. A 

noticeable shift in ionic composition, however, occurs for 

several sites (WK-331, WK-335, and WK-338) where nitrate 

replaces bicarbonate as the dominant anion. These three wells 

are located at the Raleigh hydrogeologic research station 

(RHRS; fig. 3) where past field applications of municipal 

biosolids from a wastewater treatment plant have resulted in 

nitrate contamination of shallow groundwater in the regolith and transition zones (McSwain and others, 2009). Wells 

WK-338 (depth = 300 ft) and WK-335 (depth = 85 ft) are 

located in recharge areas of the groundwater-flow path, and 

well WK-331 (depth = 90 ft) is located by the discharge area 

near the receiving river. The vertical hydraulic gradients in 

the open borehole of deep well WK-338 and shallow wells 

WK-335 and WK-338 are downward such that surfacederived nitrate in shallow groundwater flows downward into 

deeper parts of the bedrock aquifer. Dissolved nitrate and 

dissolved chloride concentrations in these wells commonly 

exceed 50 mg/L and 12 mg/L, respectively (Appendix 1). 
18 Characterization of Groundwater Quality in the Piedmont and Blue Ridge Physiographic Provinces, North Carolina

DO concentrations in these wells range from 4.8 to 6.3 mg/L, 

indicating oxic conditions where the potential for nitrate 

reduction from denitrification is low.

Interestingly, in well WK-334, also located at the RHRS 

waste application fields, the mean dissolved nitrate concentration (5.6 mg/L) and mean dissolved chloride concentration 

(4.2 mg/L) for the three samples are noticeably lower than 

those observed for wells WK-331, WK-335, and WK-338. 

Well WK-334 is located near WK-335 and is the deepest well 

(depth = 460 ft) along the flow path. DO concentrations (range 

of 0.3–1.0 mg/L) indicate suboxic to anoxic conditions at well 

WK-334. The lower nitrate concentrations at well WK-334 

may reflect geochemical conditions that promote the reduction 

of nitrate through denitrification. Additionally, well WK-334 

has an upward vertical hydraulic gradient within the borehole 

(from higher head in the deeper fractures), which may result 

in a dilution of nitrate concentrations in shallow fractures 

with older groundwater in deeper fractures, having little or no 

nitrate. These data illustrate how hydrogeologic, geochemical, and land-use factors can have a substantial influence on 

groundwater quality at the local scale. The complex nature of 

the fractured bedrock system can allow contaminants at one 

site to be confined to shallow parts of the aquifer (less than 

100 ft in depth), whereas a well located hundreds of feet away 

may have interconnected fractures that allow the contaminants 

to migrate hundreds of feet down into the bedrock where 

vertical hydraulic gradients are downward.

Comparison with Water-Quality Standards

Another approach used to characterize water-quality 

conditions in the regional geozones was comparison of the 

sample analytical results with applicable State and Federal 

water-quality standards to determine if sample constituents 

in the geozones exceeded drinking-water criteria. All of the 

groundwater samples collected at the PMREP research station 

wells (table 2) and the NAWQA wells (table 3) were included 

in the comparison with the water-quality standards. A list of 

water-quality constituents (that have applicable water-quality 

standards) measured in the groundwater samples is provided in 

table 4. Both dissolved and total constituent concentrations in 

AM-221

BK-106

CB-155

CS-145

GS-287

GU-657

GU-658

RO-151

WK-331

WK-334

WK-335

WK-338

WK-349

WK-350

EXPLANATION

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80

60

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0

SULFATE PLUS CHLORIDE PLUS NITRATE

80

100

60

40

20

20

0

40

60

80

0 100 100

60 40 20

CALCIUM

CATIONS

100 80 0

20

40

60

80

100

20

40

60

80

0

20

40

60

80

MAGNESIUM

SODIUM PLUS POTASSIUM

40 60 80

CHLORIDE PLUS NITRATE

ANIONS

0 20 100

 BICARBONATE

80

100

60

40

20

0

SULFATE

CALCIUM PLUS MAGNESIUM

C

Figure 6. Piper diagram showing ionic composition of samples from the Felsic intrusive geozone

 in the Piedmont and Mountains Provinces of North Carolina.

PERCENT

PERCENT

Figure 6. Piper diagram showing ionic composition of samples from the Felsic intrusive geozone in the 

Piedmont and Mountains Provinces of North Carolina.
Bedrock Groundwater Quality within Regional Geozones 19

samples measured by the USGS NWQL and(or) the NCDENR 

DWQ laboratory were compared with the standards. The 

radionuclide radon is not listed in table 4 because no official MCL has been finalized for this constituent; however, 

the USEPA has proposed an MCL of 300 pCi/L for radon 

in drinking water provided by community water systems if 

no state or local multimedia mitigation (MMM) program is 

in place or an alternative MCL (AMCL) of 4,000 pCi/L if a 

USEPA-approved MMM program exists (U.S. Environmental 

Protection Agency, 2007). Radon results for the groundwater 

samples are compared to both proposed standards. The analytical data for all groundwater samples used in this study are 

provided in Appendix 1. 

To compare the analytical results with the drinking-water 

standards, the data were summarized by chemical constituent 

(total and dissolved) to reflect the number of wells (not the 

number of samples) in a geozone that exceeded a particular 

standard. This approach was used to normalize the data set 

because the number of samples (both filtered and unfiltered) 

collected at the PMREP wells varied and only one sample (filtered) typically was collected at each NAWQA well site. This 

method ensures that a well is counted only once if an exceedance for a constituent is observed, regardless of whether the 

constituent exceeded the standard in one sample or all the 

samples from the well. Additional information on exceedances 

for each well sample can be found with the analytical data 

provided for all the samples in Appendix 1.

For each geozone, the number of wells sampled and the 

number of wells exceeding a drinking-water standard were 

summarized by constituent (table 5). Approximately half of 

the constituents measured in all geozone wells had total or 

dissolved concentrations less than the applicable Federal and 

State MCLs (tables 4, 5). The constituents with no exceedances for any geozone include antimony, barium, beryllium, 

boron, cadmium, chloride, chromium, copper, mercury, nickel, 

selenium, silver, and sulfate. Those constituents or properties that exceeded drinking-water standards for at least one 

geozone well include aluminum, arsenic, fluoride, iron, lead, 

manganese, nitrate, pH, uranium, and zinc (table 5). Radon 

activities exceeded the proposed MCL of 300 pCi/L in 61 of 

the 69 geozone wells sampled for radon. The proposed AMCL 

of 4,000 pCi/L was exceeded in 18 of the 69 wells.

Few exceedances were noted for the three wells sampled 

in the Western Blue Ridge geozone and the two wells sampled 

in the Grenville gneiss geozone, likely reflecting the small 

sample size. The pH value for one well in the Western Blue 

Ridge geozone was outside the pH range of 6.5–8.5 standard 

pH units. Radon also exceeded the AMCL of 4,000 pCi/L for 

the one well sampled for that constituent in the Western Blue 

Ridge geozone. Both wells in the Grenville gneiss geozone 

had radon measurements exceeding the MCL of 300 pCi/L.

A limited number of geozone wells had exceedances of 

arsenic, lead, nitrate, and uranium (table 5). The USEPA MCL 

of 0.010 mg/L for arsenic was exceeded in 2 of 7 wells sampled in the Milton geozone and 4 of 17 wells sampled in the 

Carolina slate geozone. For the Milton geozone, exceedances 

Table 4. Water-quality standards applicable to constituents or 

properties measured in groundwater samples.

[MCL, maximum contaminant level; USEPA, U.S. Environmental Protection 

Agency; SMCL, secondary maximum contaminant level; —, not applicable; 

mg/L, milligram per liter]

Chemical 

constituent

North Carolina

MCL (mg/L)a

USEPA

MCL (mg/L)b

USEPA

SMCL (mg/L)b

Aluminum — — 0.05–0.2

Antimony — 0.006 —

Arsenic 0.05 0.010c —

Barium 2.0 2.0 —

Beryllium — 0.004 —

Boron 0.315 — —

Cadmium 0.00175 0.005 —

Chloride 250 — 250

Chromium 0.05 0.1 —

Copper 1.0 1.3d 1.0

Fluoride 2.0 4.0 2.0

Iron 0.3 — 0.3

Lead 0.015 0.015d —

Manganese 0.05 — 0.05

Mercury 0.00105 0.002 —

Nickel 0.1 — —

Nitrate (as nitrogen) 10.0 10.0 —

pH 6.5–8.5e — 6.5–8.5e

Selenium 0.05 0.05 —

Silver 0.0175 — 0.10

Sulfate 250 — 250

Thallium — 0.002 —

Uranium — 0.030f —

Zinc 1.05 — 5

a North Carolina Department of Environment and Natural Resources (2006); 

standard represents maximum allowable total concentration of constituent in 

groundwater used as a source of drinking-water supply.

b U.S. Environmental Protection Agency (2003); standard represents 

maximum allowable concentration of constituent in finished drinking water 

delivered to any user of a public water system. Finished drinking water typically has undergone filtration as part of treatment process.

c As of January 23, 2006.

d Copper and lead regulated by a treatment technique that requires water 

suppliers to control the corrosiveness of their water. Water systems must take 

additional steps if 10 percent of tap samples exceed the 1.3-mg/L action level 

for copper or 0.015-mg/L action level for lead.

e

 Reported in standard pH units.

f

 As of December 8, 2003.
20 Characterization of Groundwater Quality in the Piedmont and Blue Ridge Physiographic Provinces, North Carolina

Table 5. Summary of wells within geozones in the Piedmont and Mountains Provinces in North Carolina having chemical constituents 

in groundwater exceeding North Carolina and U.S. Environmental Protection Agency drinking-water standards. 

[#, number; diss., dissolved; N, nitrogen; >, greater than; pCi/L, picocuries per liter]

Chemical constituent 

or property

Grenville gneiss Western Blue Ridge Eastern Blue Ridge Inner Piedmont

# wells 

sampled

# wells 

exceeded

# wells 

sampled

# wells 

exceeded

# wells 

sampled

# wells 

exceeded

# wells 

sampled

# wells 

exceeded

Aluminum, total

Aluminum, diss.

0

2

0

0

2

3

0

0

3

11

1

0

4

11

2

0

Antimony, diss. 2 0 3 0 10 0 8 0

Arsenic, total

Arsenic, diss.

0

2

0

0

2

3

0

0

3

11

0

0

4

11

0

0

Barium, total

Barium, diss.

0

2

0

0

2

3

0

0

3

11

0

0

4

11

0

0

Beryllium, diss. 2 0 3 0 10 0 8 0

Boron, diss. 0 0 2 0 2 0 1 0

Cadmium, total

Cadmium, diss.

0

2

0

0

2

3

0

0

3

11

0

0

4

11

0

0

Chloride, total

Chloride, diss.

0

2

0

0

2

3

0

0

3

10

0

0

4

8

0

0

Chromium, total

Chromium, diss.

0

2

0

0

2

3

0

0

3

11

0

0

4

11

0

0

Copper, total

Copper, diss.

0

2

0

0

2

3

0

0

3

11

0

0

4

11

0

0

Fluoride, total

Fluoride, diss.

0

2

0

0

2

3

0

0

3

10

0

0

4

8

0

0

Iron, total

Iron, diss.

0

2

0

0

2

3

0

0

3

11

3

2

4

11

2

2

Lead, total

Lead, diss.

0

2

0

0

2

3

0

0

3

11

1

0

0

8

0

0

Manganese, total

Manganese, diss.

0

2

0

0

2

3

0

0

3

11

2

2

4

11

1

1

Mercury, total

Mercury, diss.

0

0

0

0

0

0

0

0

3

3

0

0

0

0

0

0

Nickel, total

Nickel, diss.

0

2

0

0

2

3

0

0

3

11

0

0

4

11

0

0

Nitrate+nitrite (as N), total

Nitrate+nitrite (as N), diss.

0

2

0

0

2

3

0

0

3

10

0

0

4

9

0

0

pH 2 0 3 1 11 7 11 6

Radon-222, total

(>300 pCi/L)

(>4,000 pCi/L)

2

2

2

0

1

1

1

1

10

10

9

3

9

9

9

0

Selenium, total

Selenium, diss.

0

2

0

0

2

3

0

0

3

11

0

0

3

10

0

0

Silver, total

Silver, diss.

0

2

0

0

2

3

0

0

3

11

0

0

4

11

0

0

Sulfate, total

Sulfate, diss.

0

2

0

0

2

3

0

0

3

10

0

0

4

8

0

0

Uranium, diss. 2 0 3 0 10 0 9 0

Zinc, total

Zinc, diss.

0

2

0

0

2

3

0

0

3

11

1

1

4

11

0

0
Bedrock Groundwater Quality within Regional Geozones 21

Chemical constituent 

or property

Milton Felsic intrusive Raleigh and Charlotte Carolina slate

# wells 

sampled

# wells 

exceeded

# wells 

sampled

# wells 

exceeded

# wells 

sampled

# wells 

exceeded

# wells 

sampled

# wells 

exceeded

Aluminum, total

Aluminum, diss.

7

10

2

0

4

11

0

0

4

8

1

0

3

17

0

0

Antimony, diss. 10 0 11 0 9 0 16 0

Arsenic, total

Arsenic, diss.

7

10

2

2

4

14

0

0

5

9

0

0

3

17

0

4

Barium, total

Barium, diss.

8

10

0

0

4

11

0

0

5

9

0

0

3

16

0

0

Beryllium, diss. 10 0 11 0 9 0 16 0

Boron, diss. 10 0 12 0 8 0 15 0

Cadmium, total

Cadmium, diss.

7

10

0

0

4

11

0

0

4

9

0

0

3

17

0

0

Chloride, total

Chloride, diss.

7

10

0

0

4

15

0

0

5

9

0

0

3

16

0

0

Chromium, total

Chromium, diss.

7

10

0

0

4

11

0

0

4

9

0

0

3

17

0

0

Copper, total

Copper, diss.

7

10

0

0

4

11

0

0

4

9

0

0

3

17

0

0

Fluoride, total

Fluoride, diss.

7

10

2

2

4

15

0

0

5

9

0

1

3

16

0

0

Iron, total

Iron, diss.

7

10

4

3

4

15

3

0

5

9

1

0

3

17

0

3

Lead, total

Lead, diss.

7

10

0

0

4

11

0

0

4

9

0

0

3

17

0

0

Manganese, total

Manganese, diss.

7

10

5

5

4

15

1

4

5

9

2

1

3

17

2

8

Mercury, total

Mercury, diss.

0

0

0

0

4

0

0

0

2

3

0

0

0

0

0

0

Nickel, total

Nickel, diss.

7

10

0

0

4

11

0

0

4

9

0

0

3

17

0

0

Nitrate+nitrite (as N), total

Nitrate+nitrite (as N), diss.

7

10

0

0

4

15

4

4

2

9

0

0

3

16

0

0

pH 10 6 15 9 10 6 17 6

Radon-222, total

(>300 pCi/L)

(>4,000 pCi/L)

10

10

10

3

14

14

13

7

9

9

6

3

14

14

11

1

Selenium, total

Selenium, diss.

7

10

0

0

4

11

0

0

4

9

0

0

3

17

0

0

Silver, total

Silver, diss.

7

10

0

0

4

11

0

0

4

9

0

0

3

17

0

0

Sulfate, total

Sulfate, diss.

7

10

0

0

4

15

0

0

5

9

0

0

3

16

0

0

Uranium, diss. 10 0 11 1 9 1 16 0

Zinc, total

Zinc, diss.

7

10

5

4

4

11

1

1

4

9

1

0

3

17

2

2

Table 5. Summary of wells within geozones in the Piedmont and Mountains Provinces in North Carolina having chemical constituents 

in groundwater exceeding North Carolina and U.S. Environmental Protection Agency drinking-water standards.—Continued 

[#, number; diss., dissolved; N, nitrogen; >, greater than; pCi/L, picocuries per liter]
22 Characterization of Groundwater Quality in the Piedmont and Blue Ridge Physiographic Provinces, North Carolina

of total and dissolved arsenic were confined to wells RK-229 

(maximum total concentration of 0.025 mg/L) and RK-234 

(maximum total concentration of 0.014 mg/L) located at the 

UPRS (Appendix 1). Exceedances of dissolved arsenic for the 

Carolina slate geozone occurred in four NAWQA wells, with 

concentrations ranging from 0.0106 to 0.0383 mg/L in wells 

MG-031, RA-174, UN-145, and UN-146 located in the central 

and southern parts of the geozone (fig. 3). This observed distribution of arsenic exceedances is in agreement with the study 

by Pippin (2005) that identified a zone trending northeast from 

Union to Person Counties where groundwater has a high probability of containing ambient concentrations of arsenic above 

the USEPA MCL of 0.010 mg/L, especially in association with 

rocks of the Carolina Slate Belt. 

The concentration of total lead measured in one of three 

wells from the Eastern Blue Ridge geozone exceeded the MCL 

of 0.015 mg/L. Concentrations of dissolved lead measured in 

all 11 wells from the Eastern Blue Ridge geozone were less 

than the MCL of 0.015 mg/L. Both total and dissolved concentrations of nitrate exceeded the MCL of 10 mg/L in four wells 

for the Felsic intrusive geozone. These four wells are WK-331, 

WK-334, WK-335, WK-338, and they are located at the RHRS 

(Appendix 1) where past over-applications of municipal 

biosolids have elevated nitrate concentrations in groundwater. 

No other PMREP wells or NAWQA wells sampled for any of 

the geozones had nitrate concentrations exceeding 10 mg/L. 

Concentrations of dissolved uranium, a naturally occurring 

radionuclide, exceeded the USEPA MCL of 0.030 mg/L in 

2 wells, 1 each from the Felsic intrusive (total of 11 wells) and 

Raleigh and Charlotte (total of 9 wells) geozones.

The most common exceedances of the drinking-water 

criteria occurred for radon, pH, manganese, iron, and zinc 

(table 5). Radon activity levels exceeding the proposed MCL 

of 300 pCi/L occurred in all geozones. Radon occurs naturally 

in groundwater of the Piedmont and Mountains Provinces with 

the highest levels often associated with metaigneous rocks of 

felsic composition (Loomis and others, 1987; Campbell, 2006 

and 2008; Vinson and others, 2009), such as those in the Felsic 

intrusive geozone, which had the largest proportion of wells (7 

of 14) with radon activities that exceeded the proposed AMCL 

of 4,000 pCi/L (table 5; Appendix 1). The Eastern Blue Ridge, 

Milton, and Raleigh and Charlotte geozones also had several 

wells with radon activities greater than 4,000 pCi/L. All nine 

wells in the Inner Piedmont geozone had radon activities 

higher than 300 pCi/L but less than 4,000 pCi/L. All geozones, 

with the exception of the Grenville gneiss geozone, had multiple wells with pH values outside the range of 6.5–8.5, most 

of which were less than 6.5. Manganese exceeded the MCL of 

0.05 mg/L for wells in six geozones, with the highest proportion of exceedances occurring in the Milton and Carolina slate 

geozones. Exceedances of iron and zinc occurred in over half 

the geozones, and most commonly for the Milton or Carolina 

slate geozones. All exceedances of zinc occurred solely for 

samples collected from wells constructed with galvanized steel 

casing, suggesting that the high zinc concentrations may be 

related to leaching from well-casing material and not ambient 

groundwater conditions in the geozones.

Overall, the Milton and the Raleigh and Charlotte 

geozones had the greatest number of constituents, eight each, 

that exceeded applicable drinking-water criteria in at least 

one well (table 5). The Eastern Blue Ridge and Felsic intrusive geozones followed with seven constituents each and the 

Carolina slate geozone with six constituents that exceeded 

drinking-water criteria. With the exception of nitrate and zinc, 

constituents with concentrations exceeding drinking-water 

criteria appeared to reflect ambient groundwater conditions 

in the geozones. Exceedances of nitrate and zinc are considered to reflect contamination from local land use and wellcasing materials, respectively. Radon was the most commonly 

exceeded constituent, with 61 of the 69 sampled wells having 

activities higher than the proposed MCL of 300 pCi/L. The 

presence of radon in groundwater used for public supply is 

of particular environmental concern because of the potential 

human exposure to radon in groundwater through ingestion 

(drinking) or inhalation (showering), which increases the risk 

of developing cancer (U.S. Environmental Protection Agency, 

2007). The trace elements iron, manganese, and zinc were the 

other most common constituents that exceeded drinking-water 

criteria, but the concern with these analytes in drinking water 

generally is associated with aesthetic effects.

Comparison of Bedrock GroundwaterQuality Characteristics by 

Regional Geozone

The relation between regional geozones and groundwater 

constituents was evaluated to determine whether potential 

differences in groundwater-constituent concentrations were 

discernible among the geozone groupings used to represent the 

different regional geologic settings in the Piedmont and Mountains Provinces. This type of information is important because 

it can be used to identify potential areas or type settings that 

may be suitable for withdrawing groundwater as a source of 

drinking-water supply or for avoiding areas where naturally 

occurring constituents may be present at concentrations that 

are hard to remove or treat for water-supply purposes. The 

results of this evaluation are considered to be preliminary 

because of the limited number of wells (2–14) analyzed 

per geozone.

Several steps were taken in compiling the data used 

in statistical comparisons of the constituent concentrations 

among the geozones. The primary steps were aimed at reducing potential bias within the geozones between the PMREP 

research station data (locally concentrated wells with multiple 

samples) and the NAWQA data (spatially separated wells with 

single samples). A median constituent concentration was used 

for each PMREP station, derived from the all samples and 

wells at each station (Appendix 1). These data were combined 

with the NAWQA well data for each geozone. PMREP wells 

deemed to be influenced significantly with nitrate derived from 
Comparison of Bedrock Groundwater-Quality Characteristics by Regional Geozone 23

land-use activities at the PMREP stations were excluded from 

the evaluation. In addition, the samples collected from the 

NAWQA wells were analyzed by the USGS laboratory; hence, 

only USGS laboratory data from the PMREP wells were considered in the evaluation. This process of elimination produced 

a more uniform data set but also reduced the statistical power 

of the data by lowering the overall number of values for a 

given constituent in each geozone. The compiled data set used 

for the geozone statistical comparisons and additional details 

on the processing steps are provided in Appendix 2.

Two statistical methods were used to test for statistical 

differences in median concentrations of constituents among 

the geozones. Results of the Kruskal-Wallis test and Tukey 

comparison test performed on the analytical data presented in 

Appendix 2 are summarized in table 6. The Kruskal-Wallis 

test results indicated that the median concentrations of some 

constituents were significantly different (where the p-value 

is less than α = 0.05) for some geozones. The water-quality 

properties and constituents noted to be influenced by the geologic setting, or geozone, include specific conductance, total 

hardness, calcium, magnesium, potassium, sodium, bicarbonate, chloride, silica, ammonia, aluminum, antimony, cadmium, 

selenium, and uranium. These properties and constituents were 

analyzed further using the Tukey multiple comparison test to 

identify the geozone pairs where the median concentrations 

were different.

With some exceptions, the Tukey test identified one or 

more geozone pairs for each constituent or property having 

a statistical difference in median concentration at the 0.05 

significance level (table 6). The Kruskal-Wallis test indicated 

a significant difference in median concentration for total 

hardness, magnesium, and selenium (p-value of about 0.03). 

Conversely, the pair-wise comparison with the Tukey test did 

not identify any geozone pairs with significant differences for 

total hardness, magnesium, or selenium, thus reflecting possible limitations of the method for examining data sets where 

the sample size is small and variability is high.

Box plots are provided in figure 7 as a visual reference 

for examining those geozone pairs in table 6 having statistically significant differences in median values for specific 

conductance and concentrations of calcium, potassium, 

sodium, bicarbonate, chloride, silica, ammonia, aluminum, 

and uranium. The geozone concentrations for antimony and 

cadmium are not displayed in the box plots because most noncensored detections above the analytical RL for each constituent occurred in one geozone (Appendix 2). The concentration 

percentiles displayed in each box plot for each geozone constituent, along with the number of censored values and levels, 

are provided as Appendix 3. For a given constituent with 

censored data, the box plots include the maximum censoring 

level observed for all the geozones. For example, the maximum RL for ammonia was 0.04 mg/L as N (fig. 7); however, 

a given geozone may not have censored data or there may be 

more than one RL (Appendix 3).

In most cases, the geozones with statistically significant 

differences in median values of a property (specific conductance) or concentrations of a particular constituent were 

limited to one geozone pair, or the median concentration of a 

constituent in one geozone was significantly different when 

compared with several other geozones (table 6). For example, 

significant differences in specific conductance were identified between the Carolina slate and Inner Piedmont geozones. 

The median specific conductance of 278 microsiemens per 

centimeter at 25 degrees Celsius (μS/cm) for the Carolina slate 

geozone is significantly greater than the median of 86 μS/cm 

for the Inner Piedmont geozone (fig. 7). For the remaining six 

geozones, the overall range in specific-conductance values 

may vary among the geozones; however, no other statistically 

significant differences in median values of specific conductance were identified based on the Tukey test (table 6). This 

finding does not mean that specific-conductance values do not 

differ among the geozones in the Piedmont and Mountains 

Provinces; it only means that a significant difference in medians between the geozones, if any, was not large enough to be 

detected with the given sample size.

Potassium is an example of a constituent where the 

median concentration in one geozone is significantly different 

from several other geozones (table 6). The median potassium 

concentration of 0.56 mg/L in the Carolina slate geozone 

is significantly lower than the median concentrations in the 

Grenville gneiss (3.82 mg/L), Eastern Blue Ridge (1.70 mg/L), 

Inner Piedmont (1.62 mg/L), Milton (2.62 mg/L), and Felsic 

intrusive (2.51 mg/L) geozones (table 6; fig. 7). Of all the constituents, sodium had the most diverse combination of geozone 

pairs identified as having statistically significant differences in 

median concentrations (table 6).

Overall, the Inner Piedmont, Carolina slate, and Felsic intrusive geozones had the most significant variations in 

median concentrations of groundwater constituents. The Inner 

Piedmont geozone had 10 constituents with median concentrations significantly different compared with one or more of 

the other geozones (table 6). The Carolina slate and Felsic 

intrusive geozones followed with eight constituents each with 

significantly different median concentrations. Although the 

results of this preliminary examination are based on a data set 

with a limited number of wells and many constituent concentrations near to or less than analytical RLs, the results suggest 

that evaluating analytical data on the basis of geozone setting 

may be a useful approach for characterizing water-quality 

conditions in bedrock aquifers of the Piedmont and Mountains 

Provinces of North Carolina. Further evaluation of a more 

comprehensive data set would be needed to more fully characterize similarities and differences in water quality, as well 

as causal effects, among the geozones. Additional data needed 

include local lithologic information at each site, additional 

well sampling sites in each geozone, and wells in locations 

considered to represent ambient groundwater conditions.
24 Characterization of Groundwater Quality in the Piedmont and Blue Ridge Physiographic Provinces, North Carolina

Table 6. Summary results of Kruskal-Wallis test and Tukey comparison test of water-quality constituent 

or property and geozone in the Piedmont and Mountain Provinces of North Carolina.—Continued

[diss., dissolved; NS, no significant differences between geozones based on Tukey test; *, indicates significance at α = 0.05; 

<, less than]

Chemical constituent 

or property

Kruskal-Wallis test Tukey comparison test

p-value Geozone comparisons 

significant at α = 0.05

Oxygen, diss. 0.2756 NS

pH 0.4843 NS

Specific conductance 0.0135* Carolina slate - Inner Piedmont

Hardness, total 0.0295* NS

Calcium, diss. 0.0138* Carolina slate - Inner Piedmont

Magnesium, diss. 0.0308* NS

Potassium, diss. <0.0001* Carolina slate - Grenville gneiss 

Carolina slate - Eastern Blue Ridge 

Carolina slate - Inner Piedmont

Carolina slate - Milton 

Carolina slate - Felsic intrusive

Sodium, diss. <0.0001* Inner Piedmont - Milton

Felsic intrusive - Western Blue Ridge 

Felsic intrusive - Inner Piedmont

Carolina slate - Western Blue Ridge 

Carolina slate - Eastern Blue Ridge 

Carolina slate - Inner Piedmont

Bicarbonate, diss. 0.0104* Carolina slate - Eastern Blue Ridge

Chloride, diss. 0.0073* Carolina slate - Western Blue Ridge

Fluoride, diss. 0.1374 NS

Silica, diss. 0.0010* Felsic intrusive - Western Blue Ridge 

Felsic intrusive - Eastern Blue Ridge

Felsic intrusive - Inner Piedmont

Sulfate, diss. 0.4126 NS

Ammonia, diss. 0.0027* Inner Piedmont - Eastern Blue Ridge 

Inner Piedmont - Milton

Inner Piedmont - Felsic intrusive 

Inner Piedmont - Raleigh and Charlotte

Nitrite+nitrate, diss. 0.5960 NS

Orthophosphate, diss. 0.0861 NS

Phosphorus, diss. 0.0786 NS
Comparison of Bedrock Groundwater-Quality Characteristics by Regional Geozone 25

Table 6. Summary results of Kruskal-Wallis test and Tukey comparison test of water-quality constituent 

or property and geozone in the Piedmont and Mountain Provinces of North Carolina.—Continued

[diss., dissolved; NS, no significant differences between geozones based on Tukey test; *, indicates significance at α = 0.05; 

<, less than]

Chemical constituent 

or property

Kruskal-Wallis test Tukey comparison test

p-value Geozone comparisons 

significant at α = 0.05

Aluminum, diss. 0.0002* Inner Piedmont - Milton

Inner Piedmont - Felsic intrusive 

Inner Piedmont - Carolina slate

Antimony, diss. 0.0022* Carolina slate - Eastern Blue Ridge 

Carolina slate - Inner Piedmont

Carolina slate - Felsic intrusive 

Carolina slate - Raleigh and Charlotte

Arsenic, diss. 0.0962 NS

Barium, diss. 0.0551 NS

Beryllium, diss. 0.1693 NS

Boron, diss. 0.1370 NS

Cadmium, diss. 0.0037* Raleigh and Charlotte - Eastern Blue Ridge 

Raleigh and Charlotte - Inner Piedmont

Raleigh and Charlotte - Milton 

Raleigh and Charlotte - Felsic intrusive 

Raleigh and Charlotte - Carolina slate

Chromium, diss. 0.1261 NS

Cobalt, diss. 0.1898 NS

Copper, diss. 0.7724 NS

Iron, diss. 0.3128 NS

Lead, diss. 0.0617 NS

Manganese, diss. 0.8692 NS

Molybdenum, diss. 0.5540 NS

Nickel, diss. 0.4116 NS

Selenium, diss. 0.0322* NS

Zinc, diss. 0.5082 NS

Radon-222, total 0.1101 NS

Uranium, diss. 0.0034* Felsic intrusive - Eastern Blue Ridge 

Felsic intrusive - Inner Piedmont
26 Characterization of Groundwater Quality in the Piedmont and Blue Ridge Physiographic Provinces, North Carolina

Figure 7. Distributions 

of values of (A) specific 

conductance and 

concentrations of (B) calcium, 

(C) potassium, (D) sodium, 

(E) bicarbonate, (F) chloride, 

(G) silica, (H) ammonia, 

(I) aluminum, and (J) uranium 

for geozones in the Piedmont 

and Mountains Provinces of 

North Carolina.

Figure 7. Distributions of values of (A) specific conductance, and

 concentrations of (B) calcium, (C) potassium, (D) sodium,

 (E) bicarbonate, (F) chloride, (G) silica, (H) ammonia, (I) aluminum,

DISSOLVED CALCIUM, IN MILLIGRAMS PER LITER80

70

60

50

40

30

20

10

0

2 2 9 8 4 11 6 14

GG WBR EBR IP M FI RC CS

SPECIFIC CONDUCTANCE, IN MICROSIEMENS

PER CENTIMETER AT 25 DEGREES CELSIUS

700

600

500

400

300

200

100

0

2 2 9 8 4 11

NUMBER OF OBSERVATIONS

6 14

GG WBR EBR IP M FI RC CS

6

5

4

3

2

1

0

DISSOLVED POTASSIUM, IN MILLIGRAMS PER LITER

2 2 9 8 4 11 6 14

GG WBR EBR IP M FI RC CS

DISSOLVED SODIUM, IN MILLIGRAMS PER LITER40

30

20

10

0

2 2 9 8 4 11 6 14

GG WBR EBR IP M FI RC CS

A.

B.

C.

D.

GG

WBR

EBR

IP

M

FI

RC

CS

Grenville gneiss

Western Blue Ridge

Eastern Blue Ridge

Inner Piedmont

Milton

Felsic intrusive

Raleigh and Charlotte

Carolina slate

Geozone names

90th Percentile

EXPLANATION

75th Percentile

25th Percentile

10th Percentile

Individual sample points

Median
Comparison of Bedrock Groundwater-Quality Characteristics by Regional Geozone 27

E.

F.

G.

H.

GG WBR EBR IP M FI RC CS

GG

WBR

EBR

IP

M

FI

RC

CS

Grenville gneiss

Western Blue Ridge

Eastern Blue Ridge

Inner Piedmont

Milton

Felsic intrusive

Raleigh and Charlotte

Carolina slate

Geozone names

90th Percentile

EXPLANATION

75th Percentile

25th Percentile

10th Percentile

Individual sample points

Median

NUMBER OF OBSERVATIONS

DISSOLVED CHLORIDE , IN MILLIGRAMS PER LITER60

50

40

30

20

10

0

2 2 9 8 4 11 6 14

DISSOLVED BICARBONATE, IN MILLIGRAMS PER LITER 280

240

200

160

120

80

40

0

2 2 9 1 4 9 5 13

GG WBR EBR IP M FI RC CS

GG WBR EBR IP M FI RC CS

50

40

30

20

10

0

DISSOLVED SILICA, IN MILLIGRAMS PER LITER

2 2 9 8 4 11 6 14

DISSOLVED AMMONIA, IN MILLIGRAMS PER LITER

AS NITROGEN

1

0.1

0.01

2 2 9 8 4 11 6 14

Max censored level = 0.04

GG WBR EBR IP M FI RC CS

Figure 7. Distributions of values of (A) specific conductance,

 and concentrations of (B) calcium, (C) potassium, (D) sodium,

 (E) bicarbonate, (F) chloride, (G) silica, (H) ammonia, (I) aluminum,

d (J) ifithPidt d Mti

Figure 7. Distributions 

of values of (A) specific 

conductance and 

concentrations of (B) calcium, 

(C) potassium, (D) sodium, 

(E) bicarbonate, (F) chloride, 

(G) silica, (H) ammonia, 

(I) aluminum, and (J) uranium 

for geozones in the Piedmont 

and Mountains Provinces of 

North Carolina.—Continued
28 Characterization of Groundwater Quality in the Piedmont and Blue Ridge Physiographic Provinces, North Carolina

GG WBR EBR IP M FI RC CS

Max censored level = 1.0

DISSOLVED URANIUM , IN MICROGRAMS PER LITER100

10

1

0.1

0.01

2 2 9 8 4 11 6 14

DISSOLVED ALUMINUM, IN MICROGRAMS PER LITER25

20

15

10

5

0

2 2 9 8 4 11 6 14

NUMBER OF OBSERVATIONS

Max censored level = 1.6

GG WBR EBR IP M FI RC CS

GG

WBR

EBR

IP

M

FI

RC

CS

Grenville gneiss

Western Blue Ridge

Eastern Blue Ridge

Inner Piedmont

Milton

Felsic intrusive

Raleigh and Charlotte

Carolina slate

Geozone names

90th Percentile

EXPLANATION

75th Percentile

25th Percentile

10th Percentile

Individual sample points

Median

I.

J.

Figure 7. Distributions of values of (A) specific conductance,

 and concentrations of (B) calcium, (C) potassium, (D) sodium,

 (E) bicarbonate, (F) chloride, (G) silica, (H) ammonia, (I) aluminum,

 and (J) uranium for geozones in the Piedmont and Mountains

 Provinces of North Carolina–continued

Figure 7. Distributions 

of values of (A) specific 

conductance and 

concentrations of (B) calcium, 

(C) potassium, (D) sodium, 

(E) bicarbonate, (F) chloride, 

(G) silica, (H) ammonia, 

(I) aluminum, and (J) uranium 

for geozones in the Piedmont 

and Mountains Provinces of 

North Carolina.—Continued
Summary and Conclusions 29

Summary and Conclusions

In this report, grouping and evaluating analytical data 

based on regional geozone setting provided useful information for characterizing the quality of groundwater in bedrock 

aquifers in the North Carolina Piedmont and Blue Ridge 

Physiographic Provinces (Piedmont and Mountains Provinces). 

Analytical data were evaluated from 79 wells representing 8 of 

10 different regional geozones distributed throughout the North 

Carolina Piedmont and Mountains Provinces. Data evaluation 

included an examination of the ionic composition of groundwater in the geozones, a comparison of groundwater constituent 

concentrations to applicable State and Federal drinking-water 

standards, and statistical comparisons to identify significant differences in constituent concentrations among the geozones.

The general ionic composition, or water type, in the 

geozones combined with other chemical and hydrologic 

data provide insight into some of the factors that influence groundwater quality in the bedrock aquifers. The basic 

ionic compositions of bedrock groundwater in the geozones 

generally were classified as a calcium-sodium/bicarbonate or 

calcium-magnesium/bicarbonate water type. No major differences in overall ionic chemistry among the geozones were 

evident based on the data examined. Variability in the cationic 

and anionic composition of groundwater within a particular 

geozone appeared to reflect local differences in lithologic 

setting, geochemical conditions, groundwater-flow paths, 

and(or) land-use effects. At many of the geozone well sites, a 

subtle transition in groundwater cationic composition occurred 

with depth. The shift in cationic composition appeared to 

reflect differences in the amount of ion exchange between the 

groundwater and rock materials, where a greater proportion 

of calcium was dissolved into older groundwater with longer flow paths deeper in bedrock. In some cases, hydrologic 

factors, such as vertical hydraulic gradients and the degree of 

fracture interconnectivity, as well as land use, were found to 

have a significant influence on groundwater quality at the local 

scale. At some sites, surface-derived contaminants were localized to shallow portions of the bedrock aquifer or were present 

in multiple fracture zones distributed over hundreds of feet 

within the bedrock well. The more noticeable changes in water 

type occurred in the anionic composition of the groundwater 

in response to anthropogenic effects, such as nutrient inputs 

from local land use that increased groundwater concentrations 

of nitrate and chloride. 

Analytical results of groundwater samples were compared to applicable State and Federal water-quality standards 

to determine if sample constituents and properties in the 

geozones exceeded drinking-water criteria. For all of the 

geozone wells, concentrations of antimony, barium, beryllium, 

boron, cadmium, chloride, chromium, copper, mercury, nickel, 

selenium, silver, and sulfate were less than drinking-water 

standards. Those constituents and properties that exceeded 

drinking-water standards for at least one geozone well include 

aluminum, arsenic, fluoride, iron, lead, manganese, nitrate, 

pH, uranium, and zinc. The most common exceedances of the 

drinking-water criteria occurred for radon, pH, manganese, 

iron, and zinc. Exceedances of nitrate and zinc are considered 

to reflect contamination from local land use and well-casing 

materials, respectively. The remaining constituents with 

concentrations that exceeded drinking-water criteria appeared 

to reflect ambient groundwater conditions in the geozones. 

Naturally occurring radon was the most commonly exceeded 

constituent, with 61 of the 69 sampled wells having activities higher than the USEPA proposed MCL of 300 pCi/L. The 

Felsic intrusive geozone had the largest proportion of wells (7 

of 14) with radon activities that exceeded the USEPA proposed 

AMCL of 4,000 pCi/L. The presence of radon in groundwater 

used for public supply is of particular environmental concern 

because of the potential for human exposure to radon through 

ingestion or inhalation. Overall, the Milton and the Raleigh 

and Charlotte geozones had the greatest number, eight each, 

of water-quality properties or constituents that exceeded 

applicable drinking-water criteria in at least one well. The 

Eastern Blue Ridge and Felsic intrusive geozones each had 

seven properties or constituents that exceeded drinking-water 

criteria, and the Carolina slate geozone had six.

 A preliminary evaluation was conducted to identify 

statistically significant differences in median concentrations 

of groundwater constituents among the geozones. Significant 

differences were identified for several water-quality properties or constituents, including specific conductance, calcium, 

potassium, sodium, bicarbonate, chloride, silica, ammonia, 

aluminum, antimony, cadmium, and uranium. In most cases, 

statistically significant differences in median values of a 

property or concentrations of a particular constituent were 

limited to one geozone pair, or the median concentration of a 

constituent in one geozone was significantly different when 

compared with several other geozones. Overall, the Inner 

Piedmont, Carolina slate, and Felsic intrusive geozones had 

the most significant variations in median concentrations of 

groundwater constituents.

The preliminary results of the statistical comparison 

tests among the geozones suggest that evaluation of analytical 

data based on geozone setting may be a useful approach for 

characterizing water-quality conditions in bedrock aquifers of 

the Piedmont and Mountains Provinces. Additional data are 

needed for a more thorough analysis of groundwater-quality 

conditions within the different geozone settings to better 

characterize similarities and differences in bedrock aquifers 

throughout the North Carolina Piedmont and Mountains Provinces. Enhanced data are needed with wells in areas that best 

indicate ambient water quality and wells that reflect different 

land-use effects for the geozones. These types of data could 

provide a good baseline for understanding natural variability 

and better ascertaining differences related to land use. 
30 Characterization of Groundwater Quality in the Piedmont and Blue Ridge Physiographic Provinces, North Carolina

Acknowledgments

This report is based on work conducted cooperatively by 

the North Carolina Department of Environment and Natural 

Resources, Division of Water Quality, Aquifer Protection Section and the U.S. Geological Survey. The authors acknowledge 

Bruce Lindsey and Wright Horton, U.S. Geological Survey, 

for compiling and providing coverage information of the 

regional geozones located in the Piedmont and Mountains 

Provinces study area. Appreciation is extended to the private 

landowners who graciously allowed access to their properties for the collection of water samples. The authors thank 

Fred Falls and Mark Kozar, U.S. Geological Survey, for their 

assistance in compiling groundwater data from the NAWQA 

Program. Thanks also to all staff of the North Carolina Department of Environment and Natural Resources, Division of 

Water Quality, and the U.S. Geological Survey who helped 

collect and process water-quality data used during the study.

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Prepared by:

USGS Publishing Network

Raleigh Publishing Service Center

3916 Sunset Ridge Road

Raleigh, NC 27607

For additional information regarding this publication, contact:

Stephen L. Harden, Hydrologist

USGS North Carolina Water Science Center

3916 Sunset Ridge Road

Raleigh, NC 27607

email: slharden@usgs.gov

Or visit the USGS North Carolina Water Science Center Web site at:

http://nc.water.usgs.gov/

Harden and others—Characterization of Groundwater Quality in the Piedmont and Blue Ridge Physiographic Provinces, NC—Scientific Investigations Report 2009–5149