U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2013–5041

Prepared in cooperation with the North Carolina Department of Environment and 

Natural Resources, Division of Water Quality

Hydrogeology, Groundwater Seepage, Nitrate Distribution, 

and Flux at the Raleigh Hydrogeologic Research Station, 

Wake County, North Carolina, 2005–2007
Cover photograph. Neuse River at USGS streamgaging station 0208739677 below SR2555 near Auburn on the 

Raleigh hydrogeologic research station, Wake County, North Carolina. Photo by Kristen Bukowski McSwain, 

U.S. Geological Survey.
Hydrogeology , Groundwater Seepage, 

Nitrate Distribution, and Flux at the Raleigh 

Hydrogeologic Research Station, Wake 

County, North Carolina, 2005–2007

By Kristen Bukowski McSwain, Richard E. Bolich, and Melinda J. Chapman

Prepared in cooperation with the North Carolina Department of Environment and 

Natural Resources, Division of Water Quality

Scientific Investigations Report 2013–5041 

U.S. Department of the Interior

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

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

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

For more information on the USGS—the Federal source for science about the Earth, its natural and living 

resources, natural hazards, and the environment, visit http://www.usgs.gov or call 1–888–ASK–USGS.

For an overview of USGS information products, including maps, imagery, and publications, 

visit http://www.usgs.gov/pubprod

To order this and other USGS information products, visit http://store.usgs.gov

Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the 

U.S. Government.

Although this information product, for the most part, is in the public domain, it also may contain copyrighted materials 

as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner.

Suggested citation:

McSwain, K.B., Bolich, R.E., and Chapman, M.J., 2013, Hydrogeology, groundwater seepage, nitrate 

distribution, and flux at the Raleigh hydrologic research station, Wake County, North Carolina, 2005–2007: 

U.S. Geological Survey Scientific Investigations Report 2013–5041, 54 p.
iii

Contents

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

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

Purpose and Scope ..............................................................................................................................2

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

Hydrogeologic Setting ................................................................................................................7

Nutrient Plume Summary ...........................................................................................................7

Well and Surface-Water Station Numbering System.....................................................................9

Methods of Data Collection .......................................................................................................................10

Geophysics...........................................................................................................................................10

Water-Level Monitoring and Water Quality ...................................................................................10

Multiport Piezometers...............................................................................................................11

Seepage Meters ........................................................................................................................11

Neuse River Stage..............................................................................................................................12

Hydrogeologic Characterization ...............................................................................................................13

Regolith.................................................................................................................................................13

Transition Zone....................................................................................................................................17

Bedrock ................................................................................................................................................17

Lithology ......................................................................................................................................17

Interpretation of Geophysical and Hydraulic Logs ..............................................................18

Neuse River Stage..............................................................................................................................19

Groundwater Levels ...........................................................................................................................26

Seasonal Trends.........................................................................................................................26

Groundwater-Level Altitudes and Relative Vertical Gradients ..........................................26

Water-Quality Data.............................................................................................................................28

Quality of Water in the Regolith, Transition Zone, and Tributary .......................................32

Quality of Bedrock Groundwater ............................................................................................37

Neuse River Resistivity Profiling .............................................................................................40

Characterization of Groundwater Seepage to the Neuse River..........................................................43

Temperature Distribution in Pore Water Beneath the Neuse River...........................................43

Nutrient Distribution...........................................................................................................................45

Vertical Gradients ...............................................................................................................................45

Seepage and Mass Flux.....................................................................................................................48

Horizontal Seepage and Mass Flux ........................................................................................48

Vertical Seepage and Mass Flux.............................................................................................50

Summary........................................................................................................................................................51

Acknowledgments.......................................................................................................................................52

References Cited..........................................................................................................................................53
Figures

1. Map showing research stations selected for investigation as part of the 

cooperative U.S. Geological Survey and North Carolina Division of 

Water Quality Piedmont and Mountains Resource Evaluation Program 

in North Carolina....................................................................................................................3

2. Map showing locations of Raleigh hydrogeologic research station, hydrogeologic 

units in Wake County, and geologic belts delineated in the Piedmont 

Physiographic Province of North Carolina........................................................................5

3. Aerial photograph of the Raleigh hydrogeologic research station, Wake 

County, North Carolina, overlaid with topographic features showing 

locations of well clusters, streamgages, piezometers, areas of seepage 

meter measurements, and lines of section.......................................................................6

4. Diagram showing conceptual components of the piedmont and mountains 

groundwater system.............................................................................................................8

5. Schematic construction diagram of a multiport piezometer installed at the 

Raleigh hydrogeologic research station, Wake County, North Carolina ...................12

6. Generalized hydrogeologic cross section G–G’ at the Raleigh hydrogeologic 

research station, Wake County, North Carolina, with well-construction, 

equal-area stereonets showing orientation of water-bearing fractures, 

altitude of water-bearing fractures and BAT3 packer sample intervals, and 

dissolved nitrate + nitrite as nitrogen concentration ....................................................16

7. Geophysical logs and interpreted fractures, foliations, and water-bearing 

zones in bedrock well WC-1CH at the Raleigh hydrogeologic research 

station, Wake County, North Carolina..............................................................................19

8. Geophysical logs and interpreted fractures, foliations, and water-bearing 

zones in bedrock well WC-2D at the Raleigh hydrogeologic research 

station, Wake County, North Carolina..............................................................................20

9. Geophysical logs and interpreted fractures, foliations, and water-bearing

zones in bedrock well WC-3D at the Raleigh hydrogeologic research 

station, Wake County, North Carolina..............................................................................21

10. Geophysical logs and interpreted fractures, foliations, and water-bearing

zones in bedrock well WC-3CH at the Raleigh hydrogeologic research 

station, Wake County, North Carolina..............................................................................22

11. Rose diagrams of foliations, secondary fractures, water-bearing fractures, 

and sealed fractures in bedrock boreholes in the Raleigh hydrologic 

research station study area, Wake County, North Carolina.........................................23

12. Lower hemisphere equal-area stereonet showing poles to planes of 

foliations and fractures in bedrock boreholes in the Raleigh hydrologic 

research station study area, Wake County, North Carolina.........................................24

13. Map showing locations of selected surface-water gages within the Neuse 

River Basin and linear regression of gage height measured at surfacewater gaging stations 0208739677 and 02087500 in Wake and Johnston 

Counties, North Carolina ....................................................................................................26

iv
14. Graph showing measured and estimated gage height at station 0208739677 

in Wake County, North Carolina........................................................................................28

15. Graph showing periodic and continuous hourly groundwater levels and 

continuous 15-minute stage recorded at the Raleigh hydrogeologic 

research station, North Carolina, in well cluster WC-3 and well clusters 

WC-1 and 2 and surface-water station 0208739677 from May 2005 to 

September 2007 ...................................................................................................................29

16. Graph showing periodic and hourly groundwater levels recorded in well cluster 

WC-1 and hourly stage recorded at surface-water station 0208739677 at the 

Raleigh hydrogeologic research station, North Carolina, from February 

to July 2006 ...........................................................................................................................30

17. Generalized hydrogeologic cross section HH’ at the Raleigh hydrogeologic

research station, Wake County, North Carolina.............................................................31

18. Piper diagrams showing the water chemistry of samples collected October 2005 

and April 2006 from the regolith and transition-zone wells and the tributary 

site (station 0208739670), and bedrock wells at the Raleigh hydrogeologic 

research station, Wake County, North Carolina.............................................................35

19. Stiff diagrams showing major ion milliequivalents in water samples collected 

in October 2005 from well cluster WC-1, well cluster WC-2, a Neuse River 

tributary, and well cluster WC-3 at the Raleigh hydrogeologic research 

station, Wake County, North Carolina..............................................................................36

20. Graphs showing hourly record of water level, temperature, specific 

 conductance, dissolved oxygen, pH in wells WC-2S and WC-2I and 

daily precipitation at the Raleigh hydrogeologic research station, 

North Carolina, December 2005 through September 2007 ...........................................38

21. Graphs showing 15-minute record of temperature, specific conductance, 

pH, and daily precipitation at station 0208739670 Neuse River tributary 

near Auburn, North Carolina, May 5 to June 5, 2006.....................................................39

22. Graphs showing hourly record of water level, temperature, specific 

conductance, dissolved oxygen, pH in well WC-2D and daily precipitation 

at the Raleigh hydrogeologic research station, North Carolina, 

December 2005 through January 2007.............................................................................41

23. Map showing processed continuous resistivity profile inversion and location 

of CRP survey with approximate projected surface location of diabase dikes 

from magnetometer data overlaid on an aerial photograph of the Raleigh

hydrogeologic research station, Wake County, North Carolina..................................42

24. Contour map of pore-water temperature measured at 2.5 feet below the 

Neuse River bed at the Raleigh hydrogeologic research station, Wake 

County, North Carolina........................................................................................................44

25. Graphs showing contoured concentration of nitrite + nitrate, as nitrogen, and 

ammonia beneath the Neuse River at lines of section A-A’ , CC’, and FF ’ 

at the Raleigh hydrogeologic research station, Wake County, North Carolina ........46

26. Map showing area of Neuse River bed sediments investigated with seepage 

meter measurements at the Raleigh hydrogeologic research station, 

Wake County, North Carolina, in August 2006 and December 2007............................49

v
Tables

1. Characteristics of the monitoring wells and the surface-water sites 

at the Raleigh hydrogeologic research station, Wake County, 

North Carolina......................................................................................................................14

2. Sampling locations, constituents, and dates of collection for 

water-quality samples collected at the Raleigh hydrogeologic 

research station, Wake County, North Carolina.............................................................33

3. Temperature, specific conductance, dissolved oxygen, and nutrient 

concentrations in pore-water samples collected from multiport 

piezometers at two locations in the Neuse River, Raleigh hydrogeolgoic

research station, Wake County, North Carolina, December 14, 2007.........................47

4. Summary of Darcian flux and mass flux of nitrite + nitrate, as 

nitrogen, calculated from seepage measurements in August 2006 

and December 2007 near the upgradient and downgradient 

piezometers, Raleigh hydrologic research station, Wake County, 

North Carolina......................................................................................................................50

vi
vii

Conversion Factors and Abbreviations

Inch/Pound to SI

Multiply By To obtain

Length

inch (in) 2.54 centimeter (cm)

inch (in) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

yard (yd) 0.9144 meter (m)

Area

acre 4,047 square meter (m2

)

Volume

quart (qt) 0.9464 liter (L) 

gallon (gal) 3.785 liter (L) 

million gallons (Mgal) 3,785 cubic meter (m3

)

Flow rate

million gallons per day (Mgal/d) 0.04381 cubic meter per second (m3

/s)

Mass

ton, short (2,000 lb) 0.9072 megagram (Mg) 

ton per year (ton/yr) 0.9072 megagram per year (Mg/yr)

Hydraulic conductivity*

foot per day (ft/d) 0.3048 meter per day (m/d)

SI to Inch/Pound

Multiply By To obtain

Length

meter (m) 3.281 foot (ft) 

Volume

liter (L) 0.2642 gallon (gal)

Mass

megagram (Mg) 1.102 ton, short (2,000 lb)

megagram per year (Mg/yr) 1.102 ton per year (ton/yr)

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

°F=(1.8×°C)+32

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

°C=(°F-32)/1.8

Vertical coordinate information is referenced to the North American Vertical Datum of 1988 (NAVD 88).

*Hydraulic conductivity: The standard unit for hydraulic conductivity is cubic foot per day per square 

foot of aquifer cross-sectional area [(ft3/d)/ft3]. In this report, the mathmatically recuced form, feet per 

day (ft/d), is used for convenience. 

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).
viii
Hydrogeology, Groundwater Seepage, Nitrate Distribution, 

and Flux at the Raleigh Hydrogeologic Research Station, 

Wake County, North Carolina, 2005–2007

By Kristen Bukowski McSwain,1 Richard E. Bolich,2 and Melinda J. Chapman1

1

 U.S. Geological Survey.

2

 North Carolina Department of Environment and Natural 

Resources, Division of Water Quality, Aquifer Protection Section, 

Raleigh, North Carolina.

Abstract

From 2005 to 2007, the U.S. Geological Survey and 

the North Carolina Department of Environment and Natural 

Resources, Division of Water Quality, conducted a study to 

describe the geologic framework, measure groundwater quality, characterize the groundwater-flow system, and describe the 

groundwater/surface-water interaction at the 60-acre Raleigh 

hydrogeologic research station (RHRS) located at the Neuse 

River Waste Water Treatment Plant in eastern Wake County, 

North Carolina. Previous studies have shown that the local 

groundwater quality of the surficial and bedrock aquifers at the 

RHRS had been affected by high levels of nutrients. Geologic, 

hydrologic, and water-quality data were collected from 3 coreholes, 12 wells, and 4 piezometers at 3 well clusters, as well 

as from 2 surface-water sites, 2 multiport piezometers, and 80 

discrete locations in the streambed of the Neuse River. Data 

collected were used to evaluate the three primary zones of 

the Piedmont aquifer (regolith, transition zone, and fractured 

bedrock) and characterize the interaction of groundwater 

and surface water as a mechanism of nutrient transport to the 

Neuse River.

A conceptual hydrogeologic cross section across the 

RHRS was constructed using new and existing data. Two 

previously unmapped north striking, nearly vertical diabase 

dikes intrude the granite beneath the site. Groundwater within 

the diabase dike appeared to be hydraulically isolated from the 

surrounding granite bedrock and regolith. A correlation exists 

between foliation and fracture orientation, with most fractures 

striking parallel to foliation. Flowmeter logging in two of the 

bedrock wells indicated that not all of the water-bearing fractures labeled as water bearing were hydraulically active, even 

when stressed by pumping. 

Groundwater levels measured in wells at the RHRS 

displayed climatic and seasonal trends, with elevated groundwater levels occurring during the late spring and declining 

to a low in the late fall. Vertical gradients in the groundwater 

discharge area near the Neuse River were complex and were 

affected by fluctuations in river stage, with the exception of a 

well completed in a diabase dike. 

Water-quality data from the wells and surface-water sites 

at the RHRS were collected continuously as well as during 

periodic sampling events. Surface-water samples collected 

from a tributary were most similar in chemical composition to 

groundwater found in the regolith and transition zone. Nitrate 

(measured as nitrite plus nitrate, as nitrogen) concentrations in 

the sampled wells and tributary ranged from about 5 to more 

than 120 milligrams per liter as nitrogen. 

Waterborne continuous resistivity profiling conducted 

on the Neuse River in the area of the RHRS measured areas 

of low apparent resistivity that likely represent groundwater 

contaminated by high concentrations of nitrate. These areas 

were located on either side of a diabase dike and at the outfall 

of two unnamed tributaries. The diabase dike preferentially 

directed the discharge of groundwater to the Neuse River and 

may isolate groundwater movement laterally. 

Discrete temperature measurements made within the 

pore water beneath the Neuse River revealed seeps of colder 

groundwater discharging into warmer surface water near a 

diabase dike. Water-quality samples collected from the pore 

water beneath the Neuse River indicated that nitrate was 

present at concentrations as high as 80 milligrams per liter as 

nitrogen on the RHRS side of the river. The highest concentrations of nitrate were located within pore water collected from 

an area near a diabase dike that was identified as a suspected 

seepage area. 

Hydraulic head was measured and pore water samples 

were collected from two 140-centimeter-deep (55.1-inch-deep) 

multiport piezometers that were installed in bed sediments on 

opposite sides of a diabase dike. The concentration of nitrate 

in pore water at a suspected seepage area ranged from 42 to 

82 milligrams per liter as nitrogen with a median concentration 

of 79 milligrams per liter as nitrogen. On the opposite side 
2 Hydrogeology, Groundwater Seepage, Nitrate Distribution, and Flux at the Raleigh Hydrogeologic Research Station

of the dike, concentrations of nitrate in pore water samples 

ranged from 3 to 91 milligrams per liter as nitrogen with a 

median concentration of 52 milligrams per liter. At one of 

the multiport piezometers the vertical gradient of hydraulic 

head between the Neuse River and the groundwater was too 

small to measure. At the multiport piezometer located in the 

suspected seepage area, an upward gradient of about 0.1 was 

present and explains the occurrence of higher concentrations 

of nitrate near the sediment/water interface. 

Horizontal seepage flux from the surficial aquifer to the 

edge of the Neuse River was estimated for 2006. Along a 

130-foot flow path, the estimated seepage flux ranged from 

–0.52 to 0.2 foot per day with a median of 0.09 foot per day. 

The estimated advective horizontal mass flux of nitrate along a 

300-foot reach of the Neuse River ranged from –10.9 to 

5 pounds per day with a median of 2.2 pounds per day. The 

total horizontal mass flux of nitrate from the surficial aquifer 

to the Neuse River along the 130-foot flow path was estimated 

to be about 750 pounds for all of 2006. 

Seepage meters were deployed on the bed of the Neuse 

River in the areas of the multiport piezometers on either side 

of the diabase dike to estimate rates of vertical groundwater 

discharge and flux of nitrate. The average estimated daily 

seepage flux differed by two orders of magnitude between 

seepage areas. The potential vertical flux of nitrate from 

groundwater to the Neuse River was estimated at an average 

of 2.5 grams per day near one of the multiport piezometers and 

an average of 784 grams per day at the other. These approximations suggest that under some hydrologic conditions there 

is the potential for substantial quantities of nitrate to discharge 

from the groundwater to the Neuse River. 

Introduction

The groundwater-flow system in the piedmont and mountains of North Carolina is complex, consisting of numerous 

geologic units that form fractured-bedrock aquifers. These 

aquifers are recharged locally and are susceptible to contamination. In order to better protect and manage the groundwater 

resource, the North Carolina legislature established the Piedmont and Mountains Resource Evaluation Program (PMREP) 

to ensure long-term availability, sustainability, and quality of 

groundwater in this area of the State. In 1999, the U.S. Geological Survey (USGS) and the North Carolina Department of 

Environment and Natural Resources (NCDENR), Division of 

Water Quality (DWQ), began a multiyear cooperative study 

to measure ambient groundwater quality and describe the 

groundwater-flow system at selected research stations in the 

Piedmont and Blue Ridge Physiographic Provinces of North 

Carolina (Daniel and Dahlen, 2002). Included in the study was 

an evaluation of spatial and temporal variations of ambient 

groundwater levels and quality across the Piedmont and Blue 

Ridge Physiographic Provinces. A primary goal of the PMREP 

was the investigation of the vulnerability of the groundwater 

system to contamination (Chapman and others, 2005). 

The PMREP was designed to be an intensive field investigation at individual research stations established in representative hydrogeologic settings across the State. Twelve research 

stations have been selected for study in the Piedmont and Blue 

Ridge Physiographic Provinces (fig. 1), and wells have been 

installed at 10 of these research stations. Data collected as 

part of the PMREP provide information to refine the historical conceptual groundwater-flow models for the Piedmont and 

Blue Ridge Physiographic Provinces in North Carolina and 

the southeastern United States. The work conducted as part of 

this study supports the USGS mission of understanding processes in complex groundwater systems to aid water-resource 

managers in the protection and management of the resource. A 

detailed description of the PMREP study design and implementation is discussed in Daniel and Dahlen (2002).

Purpose and Scope

In 2005, the USGS began investigations at the Raleigh 

hydrogeologic research station (RHRS). This report presents 

the results of a 2 1⁄2-year study to describe the hydrogeologic 

framework, water quality, groundwater flow, and groundwater/

surface-water interactions at the RHRS. The hydrogeologic 

framework was described by integrating new geophysical and 

subsurface drilling data with previous studies conducted at the 

RHRS (ENSR Consulting and Engineering, Inc., 2002, 2003). 

To characterize the water quality at the RHRS, groundwater 

from 12 wells and a tributary were sampled for major inorganic ions and nutrients. The interaction of groundwater and 

surface water was evaluated by measuring temperature and 

nitrate concentration at two depths beneath the Neuse River 

as well as nutrient concentrations at a groundwater seep that 

may provide a mechanism for preferential transport to the 

Neuse River. The lateral and vertical distribution of nitrate and 

ammonia in shallow pore water up- and downgradient from 

the seep are described and discussed as evidence of the transport of nutrients from the aquifer to the river. Hydrologic and 

physical properties of the seep are examined as they related 

to the hydrogeology of the overall study area. Additionally, 

horizontal and vertical nitrate flux in groundwater from the 

RHRS to the adjacent Neuse River are discussed, using the 

spatial distribution of nitrate concentrations and the groundwater discharge estimated from seepage measurements and 

groundwater flux calculated from measured hydraulic heads 

and hydraulic conductivities. 
Introduction 

3

2

6

Fall

Line

1

7

4

5 8

3

9

4

10

11

12

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 MILES0 20 40 60 80 100 KILOMETERS

Langtree Peninsula at Lake Norman (LPRS)North Carolina State University Lake Wheeler Road Field Laboratory (LWRRS)North Carolina State University Upper Piedmont Research Station (UPRS)Bent Creek Experimental Forest (BCRS)Town of Highlands; Cullasaja WatershedRaleigh Hydrogeologic Research StationAllison Woods Hydrogeologic Research StationCoweeta Hydrogeologic Research Station

North Carolina Zoological Park Research StationDuke Forest Research StationAppalachian State University, Tater HillTown of Seven DevilsPhysiographic Province line

Research Station and numberWell installation not completed

12 9

101112

345678

Map number Research station

EXPLANATION

Figure 1. Research stations selected for investigation as part of the cooperative U.S. Geological Survey and North Carolina Division of Water QualityPiedmont and Mountains Resource Evaluation Program in North Carolina.

Atlantic

Ocean

Piedmont

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

Charlotte

Greensboro

Raleigh

Asheville

Figure 1. Research stations selected for investigation as part of the cooperative U.S. Geological Survey and North Carolina Division of Water Quality Piedmont and 

Mountains Resource Evaluation Program in North Carolina.
4 Hydrogeology, Groundwater Seepage, Nitrate Distribution, and Flux at the Raleigh Hydrogeologic Research Station

Description of Study Area

The RHRS lies in the eastern part of the Piedmont Physiographic Province within the Raleigh Belt, a litho-tectonic 

terrane, and is located about 9 miles east-southeast of Raleigh 

in Wake County, North Carolina (fig. 2). The RHRS encompasses about 60 acres within the city of Raleigh’s Neuse River 

Waste Water Treatment Plant (NRWWTP) site. The east and 

west edges of the study area are bounded by two unnamed 

tributaries that drain the study area and discharge to the north 

into the Neuse River, which makes up the northern boundary 

of the study area (fig. 3). The Neuse River and the two 

unnamed tributaries that border the study area are separated 

from the agricultural areas by a mature forested riparian 

buffer the width of which generally exceeds stream buffer 

regulations emplaced by the State of North Carolina (North 

Carolina Department of Environment, Health, and Natural 

Resources, 1997) (later State rule 15A NCAC 2B .0238). The 

riparian buffer ranges in width from about 75 to 125 feet (ft) 

near the Neuse River and from 0 to more than 1,000 ft near 

the unnamed tributaries. Land use in the study area consists 

of agricultural areas of seasonally planted row crops, such as 

corn, interspersed with buffers of grassy and wooded areas. 
Introduction 5

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 

SR Tr-DRB Piedmont Coastal Plain

Blue Ridge

34°

35°

36°

84° 83° 82° 81° 80° 79° 78° 77° 76°

Brevard fault zone

TENNESSEE

VIRGINIA

SOUTH CAROLINA

GEORGIA

0 25 50 75 100 MILES

0 25 50 75 100 KILOMETERS

Atlantic

Ocean

Wake

Co.

Murphy

Belt

Blue

Ridge

Belt

Chauga

Belt

Inner

Piedmont

Belt

Charlotte

Belt

Belt

Subbasin

Tr-DCB

Kings

Mtn.

Gold Hill

shear zone

CS

Belt

Milton

Sauratown Mtn.

ant.

CP

Tr-Wadesboro

Carolina

Slate

Sanford

-Durham

CS

CP

CP

Belt

Eastern

Slate

Belt

Raleigh

 

Schist 

Phyllite 

Metavolcanic,

 undifferentiated 

Metaigneous, mafic 

Metaigneous, felsic 

Metaigneous,

 intermediate 

Igneous, felsic intrusive 

Gneiss, felsic 

Argillite 

Metavolcanic, mafic 

Triassic sedimentary 

Fault breccia

Coastal Plain sediments

 Hydrogeologic units 

Physiographic province

 boundary

Raleigh hydrogeologic 

 research station 

0 5 10 MILES

0 8 16 KILOMETERS

EXPLANATION

Falls

Lake

Neuse

River

Raleigh

N

Sauratown Mtn. ant.

SR

Tr-DCB

Tr-DRB

CS

CP

Sauratown Mountains anticlinorium

Smith River allochthon

Davie County Triassic basin

Dan River Triassic basin

Carolina Slate Belt

Western margin of Coastal Plain 

 Physiographic Province

Boundary between Blue Ridge and Piedmont

 Physiographic Provinces

Thrust fault with teeth on upthrown block

EXPLANATION

Figure 2. Locations of Raleigh hydrogeologic research station, hydrogeologic units in Wake County, and geologic belts delineated

in the Piedmont Physiographic Province of North Carolina (map and hydrogeologic units from North Carolina Geological Survey,

1985; Daniel and Payne, 1990; modified from Camp Dresser & McKee, 2003).

Figure 2. Locations of Raleigh hydrogeologic research station, hydrogeologic units in Wake County, and geologic belts delineated 

in the Piedmont Physiographic Province of North Carolina (map and hydrogeologic units from North Carolina Geological Survey, 1985; 

Daniel and Payne, 1990; modified from Camp Dresser and McKee, 2003). 
6 Hydrogeology, Groundwater Seepage, Nitrate Distribution, and Flux at the Raleigh Hydrogeologic Research Station

170

160

150

160

150

150 160

170

180

190

150

160

170

180

190

170

160

160

170

170

160

150

160

150

150 160

170

180

190

150

170

160

160

170

180

200

190

200

230

240

230

210

220

220

210

200

200

190

210

210

220

210

190

190

180

170

220

160

Figure 3. Aerial photograph of the Raleigh hydrogeologic research station, Wake County, North Carolina, overlaid

with topographic features showing locations of well clusters, streamgages, piezometers, areas of seepage meter

measurements, and lines of section.

WC-1

EXPLANATION

Altitude of land surface above

 NAVD 88. Contour interval

 10 feet

Line of section

Diabase dike trace, dashed

 where approximated

Biosolid application area

USGS streamgaging station

 and number

Well and cluster number

Corehole

Piezometer

Multiport piezometer and

 area of seepage meter

 measurements

Base from 1998 photograph from Wake County, 

North Carolina, GIS database

Contours 1993 USGS topographic sheet, 

Garner Quadrangle, 1:24,000 scale

WC-2

WC-1

WC-3

WC-2

0208739670

Neuse

River

0 300 FEET

0 100 METERS

78°30’05”

35°43’30”

35°43’25”

35°43’20”

35°43’15”

35°43’10”

35° 43’05”

78°30’ 78°29’55” 78°29’50”

G'

G

A

A'

B

B'

C

C'

D

D'

E

E'

F

F'

A A'

Seepage A

upgradient

Seepage B

downgradient

Seep B

downgradient

0208739670

Neuse

River

Falls

Lake

Raleigh WAKE

COUNTY

Raleigh

hydrogeologic

research

station

0208739677

190

200

160

170

180

190

180

200

190

200

230

240

230

210

220

220

210

200

200

190

210

210

220

210

190

190

180

170

160

190

200

Figure 3. Aerial photograph of the Raleigh hydrogeologic research station, Wake County, North Carolina, overlaid 

with topographic features showing locations of well clusters, streamgages, piezometers, areas of seepage meter 

measurements, and lines of section.
Introduction 7

Hydrogeologic Setting

Groundwater flows through fractures within metamorphic, igneous, and sedimentary (Triassic basins) bedrock in 

the Piedmont and Blue Ridge Provinces. Weathered regolith, 

composed of soil residuum, saprolite, alluvium, and colluvium, may overlie the fractured bedrock. Groundwater flow 

is complex, consisting of an interconnected, but distinct, 

two-component groundwater system, in which the regolith 

provides storage for the underlying fractures in the bedrock 

(Heath, 1980). The PMREP investigations include a third 

component of the groundwater-flow system, the transition 

zone (fig. 4), an area commonly present between the regolith 

and bedrock (Harned and Daniel, 1992). 

The RHRS (fig. 2) is located in the igneous felsic 

intrusive (IFI) hydrogeologic unit, which represents approximately 5 percent of the area of the Piedmont and Blue Ridge 

Provinces in North Carolina (Daniel and Dahlen, 2002). The 

RHRS was selected to evaluate the effects of felsic intrusive rocks with local shearing and jointing, thickness and 

composition of the regolith, thickness and characteristics of 

the transition zone, and the development and characteristics 

of bedrock fractures on the groundwater-flow system and 

groundwater quality. 

The groundwater system in the study area has two 

primary components: the shallow, weathered regolith (the 

surficial aquifer) and the deeper, unweathered fractured bedrock (the fractured bedrock aquifer). Based on observed rock 

outcrops and the core samples collected during installation of 

wells at the RHRS, the study area is underlain by a regolith 

composed of poorly consolidated saprolite, alluvium, and soil 

residuum, which overlies granite of the Rolesville batholith 

(McSwain and others, 2009). The regolith is the shallow component of the piedmont groundwater system and, in this report, 

refers to all unconsolidated or poorly consolidated materials 

overlying the crystalline bedrock. At the RHRS, the regolith 

ranged in thickness from 28 to 48 ft. The alluvium material in 

the regolith was found primarily near the banks of the Neuse 

River and its tributaries and consisted of a brown silty fine 

sand to coarse gravel with some cobbles, clay, and organic 

matter. The saprolite, formed by weathering of bedrock, typically was characterized as a light yellowish brown silty fine 

to coarse sand with trace amounts of mica. Generally, granitic 

texture from the underlying bedrock was well preserved within 

the saprolite, and grain size increases with depth. The Rolesville batholith is a granitic intrusion that is massive to weakly 

foliated (Hibbard and others, 2002) and commonly intersected 

by diabase dike intrusions (tabular basaltic bodies with a near 

vertical orientation). 

Groundwater occupies pore spaces in the shallow, weathered regolith. The groundwater stored within the regolith is 

then conveyed to numerous fractures within the Rolesville 

batholith. Because the bedrock has little primary porosity or 

permeability, groundwater in the bedrock occupies secondary 

fractures and discontinuities. Groundwater flow within the 

regolith (surficial aquifer) at the RHRS generally followed the 

topography (ENSR Consulting and Engineering, Inc., 2002). 

The channel of the Neuse River was highly incised with 

the river bank on the RHRS side exceeding a vertical height of 

10 ft. The bottom of the Neuse River typically is covered with 

a silty sand that contains little organic matter. Sediment thickness ranged from less than 1 ft near the river’s edge to more 

than 4 ft in the center (McSwain and others, 2009). There were 

no exposed granite or diabase outcrops within the Neuse riverbank in the area of the RHRS, likely because of the presence 

of thick flood-plain sediments built up over many years.

Nutrient Plume Summary

From 1980 to 2002, the NRWWTP disposed of 

approximately 7,000 tons per year of treated biosolids onto 

1,030 acres of fields that surround the plant, including the 

60 acres that make up the RHRS (ENSR Consulting and 

Engineering, Inc., 2003). The application of biosolids was 

halted in 2002 when studies indicated that the local groundwater of the surficial aquifer contained high levels of nutrients 

(ENSR Consulting and Engineering, Inc., 2002 , 2003). 

Groundwater monitoring in wells at the RHRS from 2005 

to 2007 revealed high concentrations of dissolved nitrite plus 

nitrate as nitrogen, ranging from 69 to 119 milligrams per liter 

as nitrogen (mg/L as N) in the shallow groundwater (surficial 

aquifer) and from 4 to 130 mg/L as N in the deep groundwater 

(fractured bedrock aquifer) (McSwain and others, 2009). 

These elevated concentrations do not represent background 

conditions and were a result of calculation errors in biosolid 

application rates that lead to over application (ENSR Consulting and Engineering, Inc, 2003). ENSR Consulting and Engineering, Inc. (2003), estimated nitrogen loading to the Neuse 

River from groundwater discharge across the NRWWTP by 

using a MODFLOW groundwater flow and MT3D nitrogen 

transport model and assuming no further land application of 

nitrogen in biosolids after 2002. The model indicated that 

across the entire NRWWTP site, nitrogen loading would equal 

122,294 pounds per year (lbs/yr) in 2006 and decrease to 

3,743 lbs/yr in 2050 as a result of natural attenuation processes 

(ENSR Consulting and Engineering, Inc., 2003). 

From June through December 2007, Showers (2008) 

calculated a total nitrate flux of 28,651 pounds (lbs) (about 

57,000 lbs/yr) in four primarily groundwater-fed tributaries on the NRWWTP site. Each of the tributaries monitored 

by Showers (2007) discharged directly into the Neuse River 

and had elevated nitrate concentrations in the surface water 

primarily due to the base flow of groundwater. The drainage 

basin for each tributary contained between 43 to 219 acres of 

biosolid application fields, in total covering about 50 percent of 

the NRWWTP site. The nitrate flux in the basin that drains the 

RHRS was 10,945 lbs over the same June to December 2007 

time period (about 22,000 lbs/yr) and was reported as fairly 

constant (Showers, 2007).

Groundwater contributions to the Neuse River at the 

NRWWTP site were investigated by two graduate students 
8 Hydrogeology, Groundwater Seepage, Nitrate Distribution, and Flux at the Raleigh Hydrogeologic Research Station

SOIL 

ZONE

Water table

TRANSITION

ZONE

Regolith

saturated

zone

Regolith

unsaturated

zone

REGOLITH

FRACTURED BEDROCK

WEATHERED

BEDROCK,

BOULDERS

SAPROLITE

UNWEATHERED

BEDROCK

STRESS-RELIEF FRACTURE

BEDROCK 

STRUCTURE

FRACTURE

Figure 4. Conceptual components of the piedmont and mountains groundwater

 system in North Carolina.

Figure 4. Conceptual components of the piedmont and mountains groundwater 

system (from Harned and Daniel, 1992).
Introduction 9

in the North Carolina State University Marine, Earth, and 

Atmospheric Sciences Department under Dr. William Showers. Fountain (2006) installed monitoring well transects to 

determine the effectiveness of nitrate removal as groundwater 

was transported through a riparian buffer. Fountain (2006) 

concluded that efficient denitrification was occurring within 

the riparian buffer zones as nitrate concentrations in shallow 

groundwater were reduced by 99.1 percent, after factoring in 

rainwater dilution. Reyes (2009) investigated the presence of 

diabase dikes crossing the Neuse River on the NRWWTP site 

and mapped the spatial and temporal variability of groundwater discharge and nitrate concentration at a depth of 30 centimeters (cm; 11.8 in), downstream from a diabase dike, over 

four seasons. Reyes (2009) concluded that nitrate concentrations varied seasonally and were limited by the concentration 

of dissolved organic carbon. Over an area of 1,181.25 square 

meters (12,714.9 square feet), Reyes measured discharge 

amounts of nitrate as high as 27 kilograms of nitrate per day. 

Several reaches of the Neuse River above and below the 

NRWWTP site are listed on the North Carolina 2008 303(d)-

Impaired Waters List (North Carolina Department of Environment and Natural Resources, Division of Water Quality, 2009). 

Because groundwater is classified as a nonpoint source for 

surface-water contamination, subsurface discharge of nutrientrich groundwater from the NRWWTP site has the potential to 

affect surface-water quality within the adjacent Neuse River. 

Imbalanced proportions of nitrogen and phosphorus within 

the river can increase algal production and lead to excessive 

growth of marine plant life, such as phytoplankton. 

Well and Surface-Water Station Numbering 

System

Wells and surface-water stations monitored by the USGS 

are given unique identification numbers using geographic location. A latitude-longitude system is used for wells and drive 

points, and a downstream-order system is used for surfacewater stations. The latitude and longitude of each well cluster 

and the surface-water station at the RHRS were determined by 

using a differential global positioning system (DGPS) receiver 

and are considered accurate to within a few feet (Chapman 

and others, 2005). Wells were assigned a 15-digit site number 

based on latitude and longitude. The latitude and longitude 

constitute the first 13 digits, respectively, and are followed by 

a 2-digit sequence number used to distinguish among wells 

clustered closely together. Each well in a cluster has the same 

site-identification number except for the last two digits. Typically, the assigned sequence numbers begin with 01 for the 

shallowest well and progress with well depth at each cluster. 

Thus, the deeper the well, the higher the sequence number 

(Chapman and others, 2005). 

In addition to the standard USGS well-numbering system, 

the wells in this study also were assigned a local identifier, 

which consists of a two-letter North Carolina county code followed by a three-digit sequence number. For example, wells in 

Wake County are identified by the prefix “WK” followed by 

three numbers that are assigned sequentially. The station name 

includes the site identifier (Raleigh hydrogeologic research 

station [RS]), well descriptor, and number. The well descriptors used in this study were “WC” for monitoring well and 

“PZ” for piezometer. Following the well descriptor is a cluster 

number and a letter, which indicates the aquifer section or 

zone that was being monitored: “S” for shallow zone (regolith), “I” for intermediate zone (transition), and “D” for deeper 

zone (bedrock). Corehole borings that were converted into 

bedrock monitoring wells upon the completion of coring have 

the designation “CH.” For example, well WC-1S is a monitoring well in cluster 1 completed in the shallow regolith zone. 

The drive-point locations used to sample pore water 

within the Neuse River bed sediments were assigned a 15-digit 

site number based on the latitude and longitude of the transect anchor point on the south bank (to the right when facing 

downstream) of the Neuse River. The latitude and longitude 

constitute the first 13 digits and are followed by a 2-digit 

sequence number used to distinguish between drive-point 

locations following the same line of transect. Each drive point 

in a transect has the same site identification number except 

for the last two digits, which incrementally increased with 

distance along the transect. Thus, the farther away a drive 

point was along the line of section from the first point in the 

transect, the higher the sequence number. The station name 

includes the site identifier (Raleigh RS), the transect identifier (F–F′), and the distance, in feet, from the right bank of 

the Neuse River where the drive point was inserted. Transect 

samples were collected while moving in an upstream direction, with the first transect sampled (east) designated as A–A′. 

Subsequently sampled transects were labeled in increasing 

alphabetical order with the most upstream (west) transect 

designated F–F′.

The multiport piezometer locations in this study were 

assigned a 15-digit number based on location. The latitude 

and longitude constitute the first 13 digits and are followed 

by a 2-digit sequence number used to distinguish between the 

sampling port depths. Each of the eight ports in a multiport 

piezometer has the same site-identification number except for 

the last two digits, which incrementally increased with depth 

of placement below the riverbed. 

The downstream order number or station number 

assigned to a surface-water station is based on the location of 

the station in the downstream direction along the mainstem 

of the stream. The first 2 digits of the 8- to 10-digit station 

number identify the hydrologic unit (U.S. Geological Survey, 

1974, 1975) used by the USGS to designate the major drainage system. The next six digits indicate the downstream order 

within the major drainage system. An additional two-digit 

number is added at the end of the station number in areas of 

high station density (Chapman and others, 2005).
10 Hydrogeology, Groundwater Seepage, Nitrate Distribution, and Flux at the Raleigh Hydrogeologic Research Station

Methods of Data Collection

From May 2005 through December 2007, an investigation was performed to describe the hydrogeologic framework, 

groundwater quality, groundwater flow, and groundwater/

surface-water interaction at the RHRS. Three monitoring 

well clusters containing a total of 12 wells and 4 piezometers that monitor the shallow regolith, transition zone, and 

deep bedrock were installed at the RHRS along a conceptual 

high to low topographic profile parallel to a flow path from 

recharge to discharge areas (fig. 3) in a “slope-aquifer system” 

(LeGrand, 2004). A continuous soil and bedrock core was collected using wire-line coring methods at each of the monitoring well cluster locations. The coreholes provided continuous 

samples resulting in soil-to-bedrock profiles at each well 

cluster that were used to determine construction requirements 

for the monitoring wells. Borehole and surface geophysical 

methods were used to delineate the hydrogeologic framework. 

Additional characterization of the interaction between groundwater and surface water was conducted by evaluating the 

hydrologic and geochemical properties within the hyporheic 

zone of the Neuse River. 

Geophysics

Geophysical methods provide information about subsurface structure, lithology, and fluid chemistry, by measuring the 

physical and chemical properties of the aquifer material, borehole fluid, and pore fluid. At the RHRS, a suite of conventional 

and advanced borehole geophysical logs was collected in each 

borehole open to the bedrock. Graphs of processed borehole 

geophysical data collected in bedrock wells at the RHRS can 

be found in McSwain and others (2009). In addition, waterborne continuous resistivity profiling was conducted on the 

Neuse River in the vicinity of the study area to measure the 

apparent resistivity distribution of the sediments, bedrock, and 

pore-water fluid beneath the streambed.

Conventional borehole geophysical logs collected 

included natural gamma, caliper, long and short normal 

resistivity, fluid temperature, and fluid conductance. Details 

about conventional borehole-geophysical methods are given 

in Keys (1990). The advanced geophysical logs collected 

included optical-televiewer (OTV) imaging and electromagnetic flowmeter logs (under ambient and pumping conditions). 

Advanced borehole-geophysical logs were used to determine 

the location and orientation of foliation in the bedrock and of 

fractures intersected by the borehole as well as the movement 

of water within the borehole. A brief description of the optical 

televiewer and electromagnetic geophysical tools is provided. 

Optical-televiewer logging records a magnetically oriented, 

360 degree (°) optical image of the borehole wall (Williams and 

Johnson, 2000). The vertical and horizontal sampling intervals 

for the OTV images were 0.01 and 0.008 inch (in.), respectively. Fractures and other planar features nearly as small 

as the sampling interval can be detected. The location and 

orientation of fractures, foliations, and lithologic changes were 

interpreted from OTV data by using computer software. The 

OTV logs were analyzed to confirm lithology and to determine 

the physical characteristics and orientation of foliation and 

fractures. Because the OTV is an optical system, low optical 

contrast features, such as small fractures in dark rocks, can be 

difficult to delineate. Generally, fracture characteristics, such 

as the presence of iron oxidation or fracture infilling, can be 

visually confirmed and provide information on the potential 

of a fracture to carry groundwater. Flowmeter logging is used 

to measure the direction and rate of vertical movement of 

water in a borehole. When used in conjunction with the other 

geophysical logs, individual fractures or fracture zones where 

water enters or exits the borehole can be identified. Differences in hydraulic head between two transmissive fractures 

produce vertical flow in the borehole under ambient conditions. Water enters the borehole at the fracture zone with the 

higher head and flows towards and out of the fracture with the 

lower head. 

Stationary flow measurements were made under ambient 

and stressed (pumping) conditions between fractures or fracture zones that were identified as potentially water bearing in 

the caliper, resistivity, and OTV logs. Flowmeter logging also 

was conducted under low-rate pumping conditions to identify the least transmissive zones, including those with similar 

ambient heads that would not be identified without stressing 

the aquifer. The electromagnetic (EM) flowmeter used in 

this study is capable of resolving vertical flows from 0.05 to 

10 gallons per minute (gal/min). Water levels were recorded 

during pumping, and EM flowmeter measurements were 

made after the borehole reached a near steady state in which 

the amount of water coming out of storage was less than the 

measurement resolution of the tool. In this investigation, the 

fractures in the bedrock were described with the terms “water 

bearing,” “secondary,” and “sealed,” using characterization 

from caliper and resistivity logs, fluid resistivity and temperature logs, flowmeter logs, and oriented OTV images.

Waterborne continuous resistivity profiling was conducted following methods similar to those outlined in 

Day-Lewis and others (2006). Apparent resistivity data 

were collected using an 8-channel resistivity system and an 

electrode streamer with 11 electrodes at a 5-meter (16.4-ft) 

spacing. These apparent resistivity data were inverted using 

software to develop a geomodel of the subsurface structure 

and stratigraphy in terms of the electrical properties (Snyder 

and Wightman, 2002).

Water-Level Monitoring and Water Quality

Monitoring of water levels, stream stage, and water quality was conducted at selected sites at the RHRS. Water-quality 

samples were collected twice from each monitoring well at 

each cluster and from one unnamed tributary to the Neuse 

River (fig. 3). The water-quality constituents analyzed include 

major ions, nutrients, metals, radon 222, radiochemicals, and 
Methods of Data Collection 11

dissolved gases. Of these, only samples for major ions and 

nutrients were collected during both sampling events. Water 

samples for all other constituents were collected intermittently 

or one time only. Temperature measurements and drive-point 

water-quality samples were collected in the riverbed to better 

understand the movement of groundwater to and from the 

Neuse River. A detailed description of the methods used to 

collect the water-level, stream-stage, and water-quality data 

at the RHRS is presented in McSwain and others (2009). The 

USGS National Water Quality Laboratory in Denver, Colorado, analyzed all water-quality samples. 

The nitrate concentration values presented in this report 

were measured as dissolved nitrate plus nitrite in milligrams 

per liter as nitrogen. Because nitrite typically represented a 

small fraction of the total concentration, the reported values 

are presented and discussed as nitrate. All data were collected 

following the standard operating procedures of the PMREP 

(North Carolina Department of Environment and Natural 

Resources, Division of Water Quality, Groundwater Sciences 

Unit, and U.S. Geological Survey, Water Resources Division, 

North Carolina District, 2008). Data collected at the RHRS are 

accessible in the USGS National Water Information System 

(NWIS) database (U.S. Geological Survey, 2006b). Continuous 

groundwater and water-quality data collected at the RHRS 

during water years*

 2005, 2006, and 2007 were published in 

the USGS annual data reports (U.S. Geological Survey, 2006a, 

2007, and 2008).

A multifunction bedrock-aquifer transportable testing 

tool (BAT3) was used in one sampling event to collect waterquality samples in the open-hole wells at the RHRS. The BAT3

allows discrete intervals of a borehole to be isolated hydraulically for geochemical sampling by using two inflatable packers 

that seal against the borehole wall. The spacing between the two 

packers defines the test interval in the borehole. The equipment is configured with a submersible pump located between 

the packers to withdraw water from the test interval in order 

to collect water-quality samples. The length of the test interval 

and the depth at which water-quality samples were collected 

were determined on the basis of the location of the fractures 

intersecting the borehole as identified in the borehole geophysical logs. A complete discussion of the down-hole components 

of the BAT3

 and its operation is given in Shapiro (2001).

Daily rainfall data were calculated for the RHRS site 

by the State Climate Office of North Carolina, using gagecalibrated radar estimates known as multisensor precipitation estimates (MPE) (Mark Brooks, State Climate Office 

of North Carolina, written commun., October 2008). MPE 

combines National Weather Service weather surveillance radar 

88 Doppler (WSR-88D) precipitation estimates with locally 

available hourly surface-precipitation gages. This combination provides the spatial resolution of radar with the increased 

accuracy of surface-gage networks. A study by the State 

Climate Office of North Carolina indicates that MPE compares 

well with the independent daily precipitation-gage network 

over North and South Carolina (Boyles and others, 2006). 

Multiport Piezometers

Pore-water samples from sediments below the river/

riverbed interface can be difficult to collect. In the permeable 

sediments beneath the Neuse River, large changes in the chemical composition of the pore water could, theoretically, occur 

over short distances. In order to consistently collect multiple 

samples from a single location at different depths, two multiport piezometers, each containing eight sampling ports, were 

constructed in a manner similar to that described in Martin and 

others (2003; fig. 5). 

The two multiport piezometers were installed beneath the 

bed of the Neuse River by hydro-jetting (using high-pressure 

water to liquefact the sediment). The general stratigraphy 

observed during jetting was recorded. The piezometers were 

allowed to settle and equilibrate with the streambed sediments 

for 3 months before water-quality samples were collected. 

Locations of the installed piezometers are shown in figure 3. 

To collect samples from the screened piezometer ports, 

each tube was pumped with a peristaltic pump at a rate of 

about 1 milliliter per second (mL/s). The pumping rate differed slightly from port to port based upon the sediment permeability near the port. Pore water was pumped into a 100-mL 

container that was allowed to overflow and was monitored 

continuously for temperature and conductivity by using a 

calibrated portable field meter. Field properties were recorded, 

and the pore-water samples were filtered using an in-line 

0.45 micron filter and decanted into a bottle. 

Seepage Meters

Seepage meters provide a method to measure losses or 

gains associated with groundwater flux across a streambed 

surface. The conventional seepage meters used for this study 

were inexpensive and simple to fabricate. To fabricate a seepage meter for this study, a bottomless cylinder was formed by 

inverting a 5-gallon polyvinyl chloride (PVC) bucket, which 

had been trimmed to leave a wall about 3 in. deep. A fitting 

was installed in the top of the inverted bucket to allow the 

connection of a discharge tube and sample-collection bag. A 

second fitting was installed to allow for gas escape, if present. 

The cross-sectional area of each meter used in this study was 

0.573 square foot (ft2

).

To measure vertical groundwater seepage (flux) to or 

from the Neuse River, the seepage meter was pushed into the 

bed sediment, and a collection bag containing 0.5 liter (L) of 

water was attached to the discharge tube. The collection bag 

was removed after a period of time ranging from 15 minutes 

to 1 hour, a time period long enough for there to be a measurable change in the volume of water contained in the bag. An 

increase in the volume of water contained in the bag indicated 

groundwater discharging to the Neuse River, and a decrease 

*

Water year is the period October 1 through September 30 and is identified 

by the year in which the period ends. For example, the 2005 water year is 

October 1, 2004, through September 30, 2005.
12 Hydrogeology, Groundwater Seepage, Nitrate Distribution, and Flux at the Raleigh Hydrogeologic Research Station

Figure 5. Schematic construction diagram of a multiport piezometer 

installed at the Raleigh hydrogeologic research station, Wake

County, North Carolina (modified from Martin and others, 2003). 

To water

surface

Connectors

Sediment-water

interface

10

30

50

80

110

150

190

230

Cross Section

Port

Length

5.7 cm

Extraction

cable

Depth (cm)

Screened

interval

13.3cm2

Patch

Screening

material

PVC

pipe

PVC

tube

71 cm

Figure 5. Schematic construction diagram of a multiport 

piezometer installed at the Raleigh hydrogeologic research 

station, Wake County, North Carolina (modified from Martin and 

others, 2003).
Hydrogeologic Characterization 13

in the volume of water contained in the bag indicated a loss 

of water from the Neuse River to the underlying sediments. 

Two different seepage meters were used to measure vertical groundwater seepage, and multiple measurements were 

made by relocating each seepage meter to a different area in 

the vicinity of each site. The rate of vertical groundwater flux 

through the area of the streambed was calculated by a relation 

between the area of the streambed enclosed by the seepage 

meter, the volume of water lost or gained from the bag, and 

the length of time of collection as defined in equation 1 below 

(Majcher and others, 2006). 

 qv

 = Q/A, (1)

where

qv is seepage flux (units of length per time),

Q is volume of seepage into or out of the bag 

divided by the time interval of collection 

(units of cubic length per time), and

 A is cross-sectional area of the seepage meter 

(units of square length).

Neuse River Stage

The river stage at USGS station 0208739677 (Neuse River 

below SR 2555 near Auburn, N.C.) was recorded in 15-minute 

intervals from April to June 2006, but the stage recorder was 

discontinued because it was destroyed by Tropical Storm 

Alberto on June 14, 2006. Stage measurements were collected 

using a submersible pressure transducer to the nearest 0.01 ft 

and recorded by a data-collection platform (DCP). The pressure transducers were field checked periodically and corrected 

to measurements read from a staff plate following methods 

described in Freeman and others (2004).

Hydrogeologic Characterization

Groundwater at the RHRS occupies pore spaces in the 

shallow, weathered regolith and fractures and discontinuities below the weathered regolith. The regolith serves as the 

primary storage reservoir and is the source of recharge to the 

bedrock fractures (Chapman and others, 2005). The three wellcluster locations at the RHRS were located geographically to 

provide water-quality and water-level data along a topographic 

transect, spanning areas of recharge and discharge. A total of 

3 coreholes, 12 wells, and 4 piezometers were installed at the 

RHRS during this investigation (table 1). A conceptual hydrogeologic cross section (G–G′; fig. 3) was constructed along the 

well transect from cluster WC-3 (presumed recharge area) to 

cluster WC-1 (presumed discharge area; fig. 6) using descriptions obtained from continuous core borings and geologist’s 

logs collected at each cluster site. The Neuse River incises the 

regolith on the northern edge of the RHRS and also serves as 

a groundwater-discharge area. Detailed core descriptions for 

each core boring at the RHRS can be found in McSwain and 

others (2009). 

Regolith

The regolith at the RHRS is the shallowest component 

of the groundwater system and includes all soil residuum, 

alluvium, and saprolite that overly the transition zone and 

competent crystalline bedrock. Intensive chemical weathering 

of crystalline metamorphic and igneous rocks in the piedmont 

region produces saprolite and soil residuum. Saprolite retains 

much of the fabric of the underlying parent rock, while weathering of feldspars and micas produces clay as residuum.

The composition and thickness of the regolith varies 

along the well transect. About 33 ft of regolith was encountered in corehole WC-1CH in the lowest topographic area, 

the majority of which was described as alluvium consisting 

of grayish brown, reddish brown, or yellowish brown silty 

fine sand to coarse gravel with some cobbles, trace clay, and 

organic matter (McSwain and others, 2009). The thick alluvium encountered at corehole WC-1CH was because of the 

close proximity of the boring to the Neuse River, where scouring and deposition of alluvial material had occurred in the 

past. At coreholes WC-2CH and WC-3CH, about 48 and 33 ft 

of regolith were encountered, respectively (fig. 6). The regolith 

in these two coreholes consists of saprolite characterized as a 

light yellowish to grayish brown fine to very coarse sand with 

trace amounts of silt, vermiculite, and biotite (McSwain and 

others, 2009).

Three shallow wells were completed in the regolith—one 

at each of the three clusters (fig. 6; table 1). These shallow 

(“S”) wells were completed at depths of about 13 to 28 ft 

below land surface in the shallow part of the regolith within 

the alluvium and (or) saprolite. Additionally, four piezometers (“PZ” wells; table 1) were completed at similar depths 

within the regolith at well cluster WC-1 for use in an aquifer 

test. Because of difficulties encountered during drilling, well 

WC-2S and piezometers PZ-3 and PZ-4 were likely screened 

in the regolith or alluvium and uppermost part of the transition 

zone. In the alluvium, soil residuum, and saprolite, groundwater flows through intragranular pore spaces or through relict 

fractures, and these materials typically have high porosity 

values. However, alluvium and soil residuum commonly have 

low to moderate hydraulic conductivity compared to saprolite. 

Slug tests conducted on wells completed within the saprolite at 

the RHRS yielded hydraulic conductivity values of 6 to 7 feet 

per day (ft/d) at wells WC-2S and WC-3S. In contrast, the 

hydraulic conductivity calculated at well WC-1S, which was 

completed primarily in alluvium, was 0.8 ft/d (McSwain and 

others, 2009). 
14 Hydrogeology, Groundwater Seepage, Nitrate Distribution, and Flux at the Raleigh Hydrogeologic Research Station Table 1. Characteristics of the monitoring wells and the surface-water sites at the Raleigh hydrogeologic research station, Wake County, North Carolina. [NAVD 88, North American Vertical Datum of 1988; WC, well cluster; S, shallow regolith; I, intermediate zone regolith; D, deep; CH, core hole; PZ, piezometer; PVC, Schedule 40 polyvinyl chloride cas- ing; Galv. steel, galvanized steel; R, regolith; B, bedrock; T, transition zone; SW, surface-water site; unk, unknown; na, not applicable] Screened interval or Land-surface Top of casing open borehole Casing Site Station Latitude Longitude Construction altitude altitude Casing interval Screen Zone diameter identification name ddmmss.ss ddmmss.ss date (feet above (feet above material (feet below land type monitored (inches) NAVD 88) NAVD 88) surface) From To 354328078295701 WK-328 Raleigh 354328.83 782957.83 3-30-2005 171.24 173.74 PVC 4 13 28 0.01 in. R RS WC-1S slotted PVC 354328078295702 WK-329 Raleigh 354328.86 782957.66 3-29-2005 170.90 173.62 PVC 4 24 39 0.01 in. T RS WC-1I slotted PVC 354328078295703 WK-330 Raleigh 354328.95 782957.3 4-06-2005 169.27 172.07 Galv. 6 82 342 Open B RS WC-1D steel hole 354328078295704 WK-331 Raleigh 354329.12 782956.93 1-12-2005 165.48 167.88 Galv. 6 21 90 Open B RS WC-1CH steel hole 354328078295705 WK-369 Raleigh 354328.38 782958.2 2-14-2005 172.23 174.73 PVC 1 14.5 24.5 0.01 in. R RS PZ-1 slotted PVC 354328078295706 WK-370 Raleigh 354328.52 782957.65 2-15-2005 171.37 173.87 PVC 1 17.5 27.5 0.01 in. R RS PZ-2 slotted PVC 354328078295707 WK-371 Raleigh 354328.74 782956.7 2-15-2005 165.32 167.82 PVC 1 19 29 0.01 in. R RS PZ-3 slotted PVC 354328078295708 WK-372 Raleigh 354329.33 782956.07 2-15-2005 166.94 169.44 PVC 1 16 26 0.01 in. R RS PZ-4 slotted PVC 354315078300101 WK-332 Raleigh 354314.77 783001.87 3-31-2005 189.64 192.33 PVC 4 13.5 28.5 0.01 in. R RS WC-2S slotted PVC 354315078300102 WK-333 Raleigh 354315.74 783001.92 4-05-2005 189.95 192.46 PVC 4 27 42 0.01 in. T RS WC-2I slotted PVC
Hydrogeologic Characterization 15

Table 1. Characteristics of the monitoring wells and the surface-water sites at the Raleigh hydrogeologic research station, Wake County, North Carolina.—Continued[NAVD 88, North American Vertical Datum of 1988; WC, well cluster; S, shallow regolith; I, intermediate zone regolith; D, deep; CH, core hole; PZ, piezometer; PVC, Schedule 40 polyvinyl chloride casing; Galv. steel, galvanized steel; R, regolith; B, bedrock; T, transition zone; SW, surface-water site; unk, unknown; na, not applicable]

Screened interval or

Land-surface Top of casing open borehole Casing Site Station Latitude Longitude Construction altitude altitude Casing interval Screen Zone diameter identification name ddmmss.ss ddmmss.ss date (feet above (feet above material (feet below land type monitored (inches) NAVD 88) NAVD 88) surface)

From To

354315078300101 WK-332 Raleigh 354314.77 783001.87 3-31-2005 189.64 192.33 PVC 4 13.5 28.5 0.01 in. R

RS WC-2S slotted 

PVC

354315078300102 WK-333 Raleigh 354315.74 783001.92 4-05-2005 189.95 192.46 PVC 4 27 42 0.01 in. T

RS WC-2I slotted 

PVC

354315078300103 WK-334 Raleigh 354315.28 783001.91 2-17-2005 188.49 191.28 Galv. 6 59 440 Open B

RS WC-2D steel hole

354315078300104 WK-335 Raleigh 354316.1 782959.35 2-08-2005 199.50 202.22 PVC 4 70 85 0.01 in. B

RS WC-2CH slotted 

PVC

354305078295802 WK-337 Raleigh 354305.59 782958.14 2-09-2005 210.27 212.96 PVC 4 34 49 0.01 in. T

RS WC-3I slotted 

PVC

354305078295803 WK-338 Raleigh 354305.72 782958.13 1-13-2005 209.83 212.58 Galv. 6 40 300 Open B

RS WC-3D steel hole

354305078295804 WK-339 Raleigh 354305.24 782958.67 12-14-2005 206.27 208.69 PVC 4 40 125 Open B

RS WC-3CH hole

0208739670 Neuse River 354316.17 783006.24 4-15-2006 unk. na na na na na na SW

tribuary near Auburn, NC

0208739677 Neuse River 354330.61 782956.52 2-28-2006 142.65 na na na na na na SW

below SR2555 near Auburn, North Carolina

02087500 Neuse River near 353850 782419 8-01-1927 128.41 na na na na na na SW

Clayton
16 Hydrogeology, Groundwater Seepage, Nitrate Distribution, and Flux at the Raleigh Hydrogeologic Research Station

WC-3 Cluster

3S

3I

3D

3CH

2S 2I 2D

WC-2 Cluster

1S1I

WC-1 Cluster

1CH

G G'

1D

225200175150125

75

50

100

25

0

–25

–50

–75

–100

–125

–150

–175

–200–225

–250

–275

225200

175150125

100

75

50

25

0

–25

–50

–75

–100

–125

–150

–175

–200–225

–250

–275

Altitude, in feet above or below NAVD 88

Vertical exaggeration is about

6 times

0 300 FEET

0 100 METERS

Regolith

Transition zone

Bedrock

Diabase

Alluvium

Neuse

River

Figure 6. Generalized hydrogeologic cross section G-G’ at the Raleigh hydrogeologic research station, Wake County, North Carolina, with well-construction, equal-area stereonets showing orientation of water-bearing fractures, altitude of water-bearing fractures and BAT3 packer sample intervals, and dissolved

nitrate + nitrite as nitrogen concentration. (Location of section G-G’ is shown in figure 3). 

Diabase

Bend in section

Diabase

Bend in section

3S 3D

EXPLANATION

Regolith or transition zonewell and identifier 

Bedrock well and identifier 

Well casing Well casingScreened interval Open borehole

BAT3 packer

Water-bearing fracturesample interval

Stereonet of waterbearing fractures andwell identifier

Dissolved nitrite + nitrate concentration as nitrogen, collected in March and April 2006 (mg/L, milligramsper liter)

WC-3D

WC-3CH

WC-1CH

WC-2D

WC-3CH

100 mg/L

125 mg/L

118 mg/L

130 mg/L

98 mg/L

7 mg/L

4 mg/L

98 mg/L

87 mg/L

69 mg/L

3.56 mg/L

4 mg/L

Figure 6. Generalized hydrogeologic cross section G–G’ at the Raleigh hydrogeologic research station, Wake County, North Carolina, with wellconstruction, equal-area stereonets showing orientation of water-bearing fractures, altitude of water-bearing fractures and BAT3 packer sample 

intervals, and dissolved nitrate + nitrite as nitrogen concentration. Location of section G–G’ is shown in figure 3. 
Hydrogeologic Characterization 17

Transition Zone

A zone of slightly weathered granite bedrock and numerous open near-horizontal fractures near the top of the bedrock 

at the RHRS separate the overlying highly weathered regolith from the competent bedrock. Within this transition zone, 

the bedrock is weathered and altered, but not to the degree 

necessary to create substantial amounts of clay minerals. 

At the RHRS, the thickness of the transition zone increases 

with higher altitude along the well transect, with about 11 ft 

encountered at corehole WC-1CH, 22 ft at WC-2CH, and 30 ft 

at WC-3CH. 

The weathered rock in the transition zone at coreholes 

WC-1CH and WC-3CH was characterized as light gray, yellowish brown, and in some places orange granite, weathered 

to fresh, containing biotite, feldspar, and quartz with wellpreserved granitic texture and mineral grain sizes ranging from 

medium to coarse. Quartz-filled and open fractures within 

the core samples were common and were predominantly near 

horizontal in orientation. In corehole WC-2CH, some of the 

rock in the transition zone differed markedly in color and 

composition from that encountered at coreholes WC-1CH and 

WC-3CH. Transition zone material in a portion of corehole 

WC-2CH (48 to 86 ft) was characterized as olive to dark gray 

or dark reddish brown, slightly plastic clayey silt to slightly 

weathered diabase, with poorly preserved relict laminations. 

This portion of corehole WC-2CH transected a diabase dike 

that had intruded through the granite. Iron oxide-infilled 

fractures were common, but the orientation of the fractures 

appeared to be random for the most part.

Three intermediate wells were completed in the transition 

zone—one at each of the three clusters (fig. 6; table 1). These 

“I” wells were completed in the deepest part of the regolith 

across the saprolite and bedrock interface. Groundwater flows 

through pore spaces and open fractures within the saprolite 

and partially weathered bedrock. Slug tests conducted on wells 

completed within the transition zone at the RHRS yielded 

hydraulic conductivity values ranging from 2 ft/d at well 

WC-1I to 5 ft/d at wells WC-2I and WC-3I (McSwain and 

others, 2009).

Bedrock

Bedrock lithology and fractures at the RHRS were 

described from core samples and geophysical logs and images. 

Lithologic interpretations were based on descriptions from 

geologic core and drill cuttings, natural gamma logs, and 

oriented optical televiewer images. Fractures in the bedrock 

were first noted from the core samples and then further characterized by using caliper and resistivity logs, fluid resistivity 

and temperature logs, flowmeter logs, and oriented optical 

televiewer images.

Six wells were completed in the bedrock—two at each 

of the three clusters (fig. 3; table 1). The “D” wells were 

completed as open-hole wells within the bedrock, and the 

“CH” wells were completed as open-hole bedrock wells 

upon completion of the coreholes, with the exception of well 

WC-2CH, which was installed with a screen. Wells WC-1CH, 

WC-3D, and WC-3CH are also open to the bottom part of 

the transition zone. Slug tests conducted on five of the wells 

completed within the bedrock (well WC-1D was not tested) 

yielded hydraulic conductivity values that ranged from 0.4 ft/d 

at well WC-3CH to 10 ft/d at well WC-2D (McSwain and 

others, 2009).

Lithology

Based on observed rock outcrops, bedrock-core samples, 

and geophysical logs from the RHRS, the site is underlain by 

granitic rocks of the Rolesville granite batholith, the largest 

granitic body in the Eastern United States. The Rolesville 

batholith is reported to be composed of at least three distinct 

intrusions, based on geochemical and textural indications 

(Schneider and Samson, 2001). Core borings at WC-1CH 

and WC-3CH predominantly encountered the finer grained 

Rolesville granite “main phase” intrusion, which at the RHRS 

is described as a gray and pink, fine to medium grained biotite 

granite with some megacrystic feldspar. The granitic rock 

outcrops and core specimens from the RHRS show very weak 

foliation that appear to be striking to the northeast and steeply 

dipping to the southeast. 

Tabular basaltic diabase dike intrusions with a near vertical orientation are commonly found in the Rolesville granite 

and several are noted on the North Carolina State Geologic 

Map (North Carolina Geological Survey, 1985). Surface 

geophysical mapping of the RHRS using a magnetometer 

revealed the presence of at least two unmapped diabase dikes 

that appear to strike roughly due north (fig. 3; McSwain and 

others, 2009). The diabase dikes at the RHRS are highly vertically jointed, mafic, fine grained diabase with some chlorite. 

Well WC-1D at well cluster WC-1 and WC-2CH at well cluster WC-2 were completed within one of the diabase dikes. The 

exact dip angle of the diabase dikes is unknown, but appeared 

to be nearly vertical.

Well WC-1D was completed entirely within a diabase 

dike to a depth of 342 ft below land surface. Drilling through 

the weathered diabase while boring WC-1D was difficult 

because of the extreme degree of vertical fracturing that hampered drill progression. Even though the diabase bedrock is 

highly fractured at this location, well WC-1D yielded essentially no water as the fractures within the diabase dike appear 

to be isolated from the surrounding granite bedrock and the 

overlying alluvium. Because of the highly fractured nature of 

the diabase, it was impossible to collect optical televiewer logs 

to determine orientation of fractures even though the well was 

completed with an open hole. 
18 Hydrogeology, Groundwater Seepage, Nitrate Distribution, and Flux at the Raleigh Hydrogeologic Research Station

Well WC-2CH was completed within corehole WC-2CH 

at cluster WC-2, which transected a diabase dike as delineated 

by magnetometer survey, and is located about 300 ft to the 

east of the other wells located in cluster WC-2. The corehole 

at WC-2CH penetrated through the top of the dike at a depth 

of about 48 ft and came out through the bottom of the dike at 

a depth of about 86 ft. The top of the diabase dike in corehole 

WC-2CH was visible within the core samples as a dense, plastic olive-green clay saprolite that was distinct from the light 

colored sandy granitic saprolite on top of the dike. The granite 

encountered below the diabase dike in corehole WC-2CH was 

slightly weathered and highly fractured at the contact with the 

diabase. No diabase or granite saprolite was noted at the lower 

contact between the dike and the granite. Optical televiewer 

logs were not collected in this well because the highly friable 

nature of the diabase in this borehole necessitated completing 

the well with a screen.

Interpretation of Geophysical and Hydraulic Logs

Geophysical logs were collected at all the open-hole 

bedrock wells at the RHRS, with the exception of WC-1D 

because of the unstable nature of the diabase. An integrated 

analysis of geophysical logs was used to identify the location and distribution of fractures within the bedrock of each 

well. Fracture orientations were determined from the OTV 

image of the borehole wall and are plotted as tadpole diagrams 

(figs. 7–10). 

Fractures noted from an integrated analysis of borehole geophysical logs collected in wells WC-1CH, WC-2D, 

WC-3D, and WC-3CH were separated into three groups 

“water bearing,” “secondary,” and “sealed.” Water-bearing 

fractures were delineated on the basis of the presence of visible iron or biological staining on the OTV log and anomalies in the caliper and resistivity logs that may indicate the 

probability of water flowing within the fracture. A total of 36 

water-bearing fractures were identified along with 198 discrete 

separations or discontinuity planes within the bedrock that did 

not appear to be water bearing and were identified as secondary fractures. Seventeen fractures with partings that have been 

partially or completely joined together by the deposition of 

crystalline material were identified as sealed fractures. 

The orientation of planar features interpreted from the 

OTV logs have been plotted in the form of rose diagrams and 

lower hemisphere equal-area stereonets, both of which provide 

a graphical method for assessing the pattern of planar features 

(figs. 11, 12). Rose diagrams are histograms for which the orientation axis is transformed into a circle to give a true angular 

plot. As used here, the strike of a foliation or fracture is plotted 

as pie-shaped segments of a circle in its true orientation, and 

the length of the radius is proportional to the frequency of 

occurrence of that orientation. Although a rose diagram is limited to displaying only one aspect of a planar feature, a stereonet can be used to graphically display both the strike and dip 

of a planar feature. A stereonet reduces each fracture plane to 

a point that represents the intersection of a pole, perpendicular 

to a fracture plane, with the lower hemisphere projected onto 

the equatorial plane of the hemisphere. Stereonets provide a 

graphical method for assessing the clustering or variability of 

the poles to planes. 

 A graphical comparison of the foliations measured in the 

four bedrock wells with OTV logs was made from the rose 

diagrams and stereonets shown in figures 11 and 12. About 

470 foliations were observed in the boreholes. The foliations 

varied in both strike and dip and collectively appear to be 

scattered (figs. 11A, 12A). The orientation of foliations were 

difficult and somewhat subjective to interpret using OTV logs, 

but are presented here for comparison to the fracture orientations. The mean principal orientation of all foliations was 

north 80° east (N. 80° E.) with a dip of 15° south, which was 

seemingly much shallower than the steeply dipping foliations 

observed in the core. The rose diagram (fig. 11A) indicated 

that nearly 25 percent of the foliations measured had a strike 

between N. 60° E. and N. 90° E., which is consistent with the 

observed outcrop measurements of orientation. 

Although the orientation of fractures varies within 

individual boreholes and across the site, some patterns can be 

detected within the different fracture types when examined as 

a whole. The rose diagrams of the water-bearing, secondary, 

and sealed fractures indicate a predominantly northeast to eastnortheast strike (fig. 11B–D). On the stereonet (fig. 12B), poles 

to water-bearing fractures cluster in two sets—one set strikes 

east-northeast and dips shallowly to the south, and the second 

set strikes northeast and dips moderately to steeply to the 

northwest. Stereonets and altitudes of water-bearing fractures 

for individual bedrock wells are shown in figure 6. 

Poles to secondary fractures appear to be scattered but 

cluster in four sets—one set strikes generally east-northeast 

and dips very shallowly to moderately to the south-southeast, 

a second set strikes northwest and dips moderately to the 

southwest, a third set strikes north and dips steeply to the west, 

and a fourth set strikes east-northeast and dips moderate to 

steeply to the north-northwest. Over half of the poles to sealed 

fractures cluster in the middle of the stereonet, indicating a 

very shallow dip. 

A correlation exists between foliation and fracture 

orientation (fig. 11), with most fractures striking parallel to 

foliation. Nearly 60 percent of water-bearing fractures have 

shallow dips (less than 30°) (fig. 12B) and are likely stressrelief fractures caused by unloading of the granite. Within the 

secondary fractures there appears to be a conjugate joint that 

includes fractures with a north strike, but very few waterbearing fractures have similar orientations, which suggests 

little influence of the conjugate set on groundwater flow. There 

is more uncertainty and variability in the poles that plot in 

the center of the stereonet, because of the inherent difficulty 

of determining the direction of strike and dip on shallow 

features (with dips less than 30°). As a nearly vertical borehole is more likely to intersect the shallow dipping fractures 

than the steeply dipping fractures, it is likely that the quantity 

of steeply dipping fractures occurring within the Rolesville 

granite at the RHRS has been under sampled.
Hydrogeologic Characterization 19

Casing Casing

Secondary fracture

Foliation

Alluvium

Regolith

Transition zone

Bedrock

Sealed fracture

EXPLANATION

Water-bearing fracture

0

10

20

30

40

50

60

70

80

90

Optical televiewer image

0 90 180 270 0

N E S W N

0 90

Fracture/foliation

dip angle (degrees)

3 5

Caliper (inch) Water-bearing

zone

17 20

Temperature

(degrees Celsius)

Depth, in feet below land surface

10

0

20

30

40

50

60

70

80

90

N

S

EW

Example of tadpole

showing dip azimuth

Figure 7. Geophysical logs and interpreted fractures, foliations, and water-bearing zones in

bedrock well WC-1CH at the Raleigh hydrogeologic research station, Wake County, North Carolina. 

Figure 7. Geophysical logs and interpreted fractures, foliations, and water-bearing 

zones in bedrock well WC-1CH at the Raleigh hydrogeologic research station, Wake 

County, North Carolina.
20 Hydrogeology, Groundwater Seepage, Nitrate Distribution, and Flux at the Raleigh Hydrogeologic Research Station

Casing Casing

Optical televiewer image

0 90 180 270 0

N E S W N

0 90

Fracture/foliation

dip angle (degrees)

5 16

Caliper (inch)

17 19

Temperature

(degrees Celsius)

0

20

40

60

80

100

120

140

160

180

200

220

240

260

280

300

320

340

360

380

400

420

440

460

0

20

40

60

80

100

120

140

160

180

200

220

240

260

280

300

320

340

360

380

400

420

440

460

Depth, in feet below land surface

N

S

EW

Example of tadpole

showing dip azimuth

Figure 8. Geophysical logs and interpreted fractures, foliations, and water-bearing zones in bedrock well WC-2D 

at the Raleigh hydrogeologic research station, Wake County, North Carolina.

Secondary fracture

Foliation

Sealed fracture

EXPLANATION

Water-bearing fracture

Regolith

Transition zone

Bedrock

Direction of flow

-2.5 2.5

Flow - ambient

EM flowmeter

(gal/min)

Inflow

Inflow

Outflow

Outflow

Measurement under

ambient conditions

Measurement under

pumping conditions

Water-bearing

zone

Outflow

-2.5 2.5

Flow - pumping

EM Flowmeter

(gal/min)

Inflow

Outflow

Outflow

Inflow

Inflow

Inflow

Figure 8. Geophysical logs and interpreted fractures, foliations, and water-bearing zones in bedrock well WC-2D at the Raleigh 

hydrogeologic research station, Wake County, North Carolina.
Hydrogeologic Characterization 21

-2.5 2.5 -2.5 2.5

Flow - ambient Flow - pumping

EM Flowmeter

(gal/min)

EM Flowmeter

(gal/min)

Optical Televiewer Image

0 90 180 270 0

N E S W N

0 90

Fracture/Foliation

Dip angle (degrees)

5 16

Caliper (inch)

17 19

Temperature

(Degrees Celsius)

Casing Casing

0

20

40

60

80

100

120

140

160

180

200

220

240

260

280

300

N

S

EW

Example of tadpole

showing dip azimuth

Depth, in feet below land surface

0

20

40

60

80

100

120

140

160

180

200

220

240

260

280

300

Secondary fracture

Foliation

Sealed fracture

EXPLANATION

Water-bearing fracture

Regolith

Transition zone

Bedrock

Direction of flow

Measurement under

ambient conditions

Measurement under

pumping conditions

Water-Bearing

Zone

Inflow Inflow

Outflow Outflow

Figure 9. Geophysical logs and interpreted fractures, foliations, and water-bearing zones in bedrock well WC-3D

at the Raleigh hydrogeologic research station, Wake County, North Carolina.

Figure 9. Geophysical logs and interpreted fractures, foliations, and water-bearing zones in bedrock well WC-3D at the 

Raleigh hydrogeologic research station, Wake County, North Carolina.
22 Hydrogeology, Groundwater Seepage, Nitrate Distribution, and Flux at the Raleigh Hydrogeologic Research Station

Casing Casing

0

10

20

30

40

50

60

70

80

90

Optical televiewer image

0 90 180 270 0

N E S W N

0 90

Fracture/foliation

sip angle (degrees)

3 5

Caliper (inch)

17 20

Temperature

(degrees Celsius)

Depth, in feet below land surface

10

0

20

30

40

50

60

70

80

90

100

110

120

130

100

110

120

130

N

S

EW

Example of tadpole

showing dip azimuth

 Figure 10. Geophysical logs and interpreted fractures, foliations, and water-bearing zones

 in bedrock well WC-3CH at the Raleigh hydrogeologic research station, Wake County,

 North Carolina.

Secondary fracture

Foliation

Sealed fracture

EXPLANATION

Water-bearing fracture

Regolith

Transition zone

Bedrock

Water-bearing

zone

Figure 10. Geophysical logs and interpreted fractures, foliations, and waterbearing zones in bedrock well WC-3CH at the Raleigh hydrogeologic research 

station, Wake County, North Carolina.
Hydrogeologic Characterization 23

Figure 11. Rose diagrams of (A) foliations, (B) secondary fractures, (C) water-bearing fractures, and (D) sealed fractures in bedrock 

boreholes in the Raleigh hydrogeologic research station study area, Wake County, North Carolina.

n = 466

10 20 30 40 50 60

0°

90°

180°

270°

n = 198

n=36

2 4 6 8

n=17

1 2 3 4 5

A

10 20 30 40 50 60

0°

90°

180°

270°

B

0°

90°

180°

270°

C 0°

90°

180°

270°

D

Figure 11. Rose diagrams of (A) foliations, (B) secondary fractures, (C) water-bearing fractures, and (D) sealed 

fractures in bedrock boreholes in the Raleigh hydrologic research station study area, Wake County, North Carolina.
24 Hydrogeology, Groundwater Seepage, Nitrate Distribution, and Flux at the Raleigh Hydrogeologic Research Station

A B

Figure 12. Lower hemisphere equal-area stereonet showing poles to planes of (A) foliations and (B) fractures

 in bedrock boreholes in the Raleigh hydrogeologic research station study area, Wake County, North Carolina.

Foliation

Secondary fracture

Sealed fracture

WC-1CH water-bearing fracture

WC-2D water-bearing fracture

WC-3D water-bearing fracture

WC-3CH water-bearing fracture

EXPLANATION

Figure 12. Lower hemisphere equal-area stereonet showing poles to planes of (A) foliations and (B) fractures in bedrock 

boreholes in the Raleigh hydrologic research station study area, Wake County, North Carolina.
Hydrogeologic Characterization 25

Flowmeter logging under ambient and stressed conditions 

was conducted in wells WC-2D and WC-3D to determine the 

relative contribution of water from transmissive fractures to 

the well. Individual stereonets displaying the orientation of the 

transmissive fractures are shown on figure 6. The EM flowmeter surveys results are shown in figures 8 and 9. By convention, upward flow in a borehole is designated with a positive 

value and downward flow with a negative value. A change in 

the measured rate of vertical flow in the borehole indicates an 

addition or removal of water between the two measurement 

locations. Because vertical flow in a borehole is controlled by 

the relation of hydraulic heads of groundwater within each 

fracture, the transmissivity of the fractures that intersect the 

borehole, and the variation of hydraulic heads over time, the 

magnitude and direction of ambient flow may vary temporally.

In well WC-2D under ambient conditions, four shallow 

dipping transmissive fractures at a depth of about 248 ft contributed the majority of water to the well (fig. 8). Water flowed 

upward within the borehole until it exited through a steeply dipping transmissive fracture at 86 ft. When pumped at 1 gal/min, 

the contributions of the fractures in well WC-2D were similar 

to ambient conditions, with the exception of a gain of about 

0.4 gal/min from three fractures at a depth of about 64 ft that 

were inactive under ambient conditions. Under both ambient 

and stressed conditions, the higher hydraulic head of the deep 

transmissive fractures within the borehole caused water to 

flow upward to the transmissive fractures with lower hydraulic 

heads that occur at shallower depths. 

Under both ambient and pumping conditions in well 

WC-3D, two shallow dipping transmissive fractures at about 

66 ft contributed about 0.2 gal/min of water to the well (fig. 9). 

Water flowed downward within the borehole and out through 

transmissive fractures at about 158 ft. When pumped at 1.5 

gal/min, the transmissive fractures at 66 ft continued to lose 

water downward at the same rate as under ambient conditions, 

while moderate to steeply dipping fractures at a depth of about 

43 ft began supplying nearly all of the water to the borehole. 

Unlike in well WC-2D, under both ambient and stressed 

conditions in well WC-3D the hydraulic head of the shallower 

transmissive fractures was higher than that of the deeper transmissive fractures. This negative head difference within the 

borehole indicates a downward vertical gradient that can allow 

water to pass from shallower fractures to deeper fractures and 

facilitate the transport of surface contaminants to deeper zones 

within the bedrock. 

Neuse River Stage

Fluctuations in the stage of the Neuse River control 

groundwater movement near well cluster WC-1. In order for 

groundwater to discharge into the Neuse River, the altitude 

of the water table near the river must exceed the stage of 

the river. If the altitude of the Neuse River is lower than the 

altitude of the water table, surface water will seep through 

the river bank and become groundwater. Thus, the altitude of 

the Neuse River stage is necessary to understand the interaction between groundwater and surface water in the area of 

well cluster WC-1. Locations of surface-water gages located 

near the RHRS within the Neuse River Basin are shown in 

figure 13.

The Neuse River at the RHRS (station 0208739677 

Neuse River below SR 2555 near Auburn, NC; hereafter 

called the “Auburn” station) was ungaged for the majority of 

the study. A number of methods can be used to estimate stage 

for an ungaged site by using gaged sites; however, the direct 

measurement of river stage is preferable. About 2 months of 

continuous river stage levels were collected at the Auburn 

station during the study and were used to verify stage estimates 

at the Auburn station that were calculated by using other 

gaged sites. 

Data for two long-term USGS gaging stations above and 

below the RHRS on the Neuse River were used to estimate river 

stage at the Auburn station. Station 02087500, Neuse River 

at Clayton, NC (hereafter called the “Clayton” station), has 

monitored river stage since 1927, and station 02087183, Neuse 

River near Falls, NC (hereafter called the “Falls” station), has 

monitored river stage since 1960. Two other River Net stations 

operated by North Carolina State University that monitor river 

stage and discharge are located upstream (at Auburn-Knightdale 

Road in Garner, North Carolina ) and downstream (at Mial 

Plantation Road in Clayton, North Carolina) from the Auburn 

station (North Carolina State University, 2000). At the time of 

writing of this report, however, data for both of the River Net 

stations were unavailable for comparison purposes.

The Falls station is located about 22 river miles upstream 

from the RHRS, has a drainage area of about 771 square 

miles (mi2

), and is located just downstream from the Falls 

Lake dam. Stream stage measurements at the Falls station 

were not used in calculations because of the artificially controlled nature of the Neuse River near the dam. The Clayton 

station is downstream from the RHRS by about 10 river miles 

and has a drainage area of 1,150 mi2

, which is similar in size 

to that of the Auburn station (1,084 mi2

) making it a better 

choice to use in estimation of river stages near the RHRS. 

There are two major inputs to this reach of the Neuse River 

between the RHRS and the Clayton station, the outfall of the 

NRWWTP and Marks Creek. Both inputs are relatively small 

compared to the Neuse River discharge. 

The Auburn station at the RHRS was monitored every 

15 minutes for river stage from April 19, 2006 through 

June 14, 2006, when it was destroyed by floodwater from 

Tropical Storm Alberto. To estimate the difference in travel 

time between the Auburn and Clayton stations, the 15-minute 

river stage data for both stations were plotted against time. A 

travel time of 4 hours from the Auburn to Clayton stations was 

calculated by correlating river stage peaks that occurred during the data-collection period. River stage data at the Clayton 

station was then time shifted by -4 hours and plotted against 

stage data collected at the Auburn station to allow discrete 

comparison. Linear regression was applied to the data, a coefficient of regression (R2

) was calculated, and a mathematical 
26 Hydrogeology, Groundwater Seepage, Nitrate Distribution, and Flux at the Raleigh Hydrogeologic Research Station

GRANVILLE

Basin boundary

USGS streamgage and number 

River Net gaging station and name

EXPLANATION

3545'

79 78

30'

36

DURHAM

ORANGE

02087500

Mial Plantation

Figure 13. Location of selected surface-water gages within the Neuse River Basin and surface-water gaging stations

0208739677 (Neuse River below SR 2555 near Auburn, NC) and 02087500 (Neuse River at Clayton, NC) in Wake and Johnston

Counties, North Carolina).

Falls

Lake

Lake

Michie

Lake

Orange

Little River

Res.

River

River

Hillsborough Eno

River

Flat

PERSON

Apex

Cary Raleigh

Lake

Wheeler

Lake

Benson

Swift

River

Durham

Little

WAKE

Neuse

River

Basin

Smithfield

Cr.

JOHNSTON

Neuse

02087500

Mial Plantation

Auburn- 0208739677

Knightdale

02087183

Base from digital files of:

U.S. Department of Commerce, Bureau of Census,

1990 Precensus TIGER/Line Files-Political boundaries, 1991

Environmental Protection Agency, River File 3

U.S. Geological Survey, 1:100,000 scale 

0 5 10 KILOMETERS

0 5 10 MILES

LOCATION OF STUDY AREA IN THE NEUSE RIVER BASIN

IN NORTH CAROLINA

Neuse River

Basin















 





























 































































 









































 

































  























































 

































   

























 

 













 













  



















  

































































 



























  























 















































 

















 









0

1

2

3

4

5

6

7

8

Station 0208739677 gage height, in feet above datum

f(x) = 1.558x – 0.973

R2 = 0.957

0 1 2 3 4 5 6

Station 02087500 gage height, in feet above datum

Figure 13. Locations of selected surface-water gages within the Neuse River Basin and linear regression of gage height measured 

at surface-water gaging stations 0208739677 (Neuse River below SR 2555 near Auburn, North Carolina) and 02087500 (Neuse River at 

Clayton, North Carolina) in Wake and Johnston Counties, North Carolina.
Hydrogeologic Characterization 27

relation was determined (fig. 13). The R2 for the plotted stage 

data was 0.957, indicating that stage data collected at the 

Clayton station was highly correlated with stage data collected 

at the Auburn station and could be used to estimate river stage 

at the Auburn station with relative certainty. The measured 

and estimated gage heights for the Auburn station were plotted 

over the same time period to validate that the estimated gage 

heights were similar to those measured (fig. 14). The gage 

height for the Auburn station was then estimated for the entire 

study period, May 2005 to September 2007, by applying the 

equation determined by linear regression to the 15-minute 

gage height data collected at the Clayton station and shifting 

the time by –4 hours (fig. 15). 

Groundwater Levels

From May 2005 to September 2007, groundwater levels 

were continuously monitored in eight wells at well clusters 

WC-1 and WC-2 and recorded periodically in four wells at 

well cluster WC-3. Groundwater levels in the piezometers 

were measured periodically from February to July 2006.

Seasonal Trends

Groundwater levels measured at the RHRS had climatic 

and seasonal trends similar to fluctuations observed across the 

piedmont in the Southeastern United States (U.S. Geological 

Survey, 2007). Elevated groundwater levels generally occurred 

during the late spring or early summer following winter 

precipitation. As a result of increased evapotranspiration (ET) 

rates and reduced precipitation during the summer months, 

groundwater levels declined to a low in late fall. During the 

study period, typical seasonal trends were affected by persistent drought. 

At well cluster WC-1, water levels in all wells and 

piezometers, with the exception of well WC-1D, were affected 

to some degree by their close proximity to the Neuse River 

(fig. 15). The water level in well WC-1D did not fully recover 

until 4 months after drilling was completed. Despite the interaction caused by the Neuse River (storm peaks can be seen 

in groundwater levels), there was a dampened seasonal trend 

in the water levels in the wells at well cluster WC-1, which 

was reflected to a lesser extent in base-flow trends in the river. 

In wells WC-1S and WC-1I, the typical range of seasonal 

groundwater-level fluctuation was about 5 ft, with the groundwater low occurring in November. As the groundwater high 

was directly linked to the stage of the Neuse River, it did not 

necessarily correlate to the late spring or early summer high 

typical for piedmont wells. In December 2006 and January 

2007 the highest groundwater levels were measured in the 

wells at well cluster WC-1 because of a series of winter storm 

events that flooded the upper part of the Neuse River Basin.

Groundwater levels at well clusters WC-2 and WC-3 

showed a more typical piedmont seasonal trend during the 

monitoring period, with the highest groundwater levels 

occurring about late May and the lowest in November 

(fig. 15). At well clusters WC-2 and WC-3, the typical range 

of seasonal groundwater-level fluctuation was about 4 ft. After 

the occurrence of rainfall during the study period, unlike at 

well cluster WC-1, all of the water levels in the continuously 

monitored wells at well cluster WC-2 rose gradually. This 

delayed response was likely due to the high storage capacity 

of the silt and clay present in the shallow regolith, retarding 

rapid recharge, coupled with a more than 15 ft depth to the 

water table at well cluster WC-2. Rainfall events with a long 

duration and moderate intensity had the greatest effect on 

recharge to the groundwater system, especially during the 

winter months when ET rates were low and soil moisture was 

high, allowing more rainfall to infiltrate to the water table. 

This occurred most notably in November 2006 when the area 

received more than 7 in. of rain, which caused a rise in the 

water table (fig. 15). 

Groundwater-Level Altitudes and Relative 

Vertical Gradients

Throughout the study period, the water-level altitudes 

in the regolith, transition zone, and bedrock were variable 

between well clusters. An accurate calculation of vertical 

hydraulic gradients between the regolith, transition zone, 

and bedrock was not possible because of a well spacing of 

20 ft between wells. Because there is generally little altitude 

difference between the wells at each cluster, it is possible to 

examine the position of water-level altitudes in each monitoring zone in relative terms. In comparing water-level altitude 

among the three zones of the groundwater system, well cluster 

WC-3 was consistent with the conceptual slope-aquifer system 

model, but well clusters WC-1 and WC-2 were more complex. 

At well cluster WC-3, located in the upgradient recharge area, 

water levels had lower hydraulic heads with increasing well 

depth (fig. 15). This downward hydraulic gradient supports 

the conceptual model theory that recharge occurs in areas of 

topographic highs.

 Well cluster WC-2 is topographically mid-slope and 

should be in a recharge area based on the conceptual model. 

An accurate calculation of vertical gradients between all the 

wells at the WC-2 well cluster was not possible because well 

WC-2CH is located about 200 ft east of the other wells in the 

cluster and was completed partly within a diabase dike (fig. 3). 

Additionally, well WC-2CH is about 10 ft higher in landsurface altitude than the other wells in well cluster WC-2. This 

vertical and horizontal separation from well WC-2D, coupled 

with its completion in the diabase dike, was likely the reason 

that the water-level altitude in well WC-2CH was consistently 

about 5 ft higher than the other bedrock well in the cluster 

(fig. 15). If vertical gradient comparisons are made between 

the remaining three wells at well cluster WC-2, the bedrock 

has a water-level altitude that was consistently higher than 

those measured in the transition zone and regolith, indicating 

an upward gradient (discharge) from the bedrock aquifer to the 
28 Hydrogeology, Groundwater Seepage, Nitrate Distribution, and Flux at the Raleigh Hydrogeologic Research Station

0

2

4

6

8

10

12

14

16

18

20

April May June

Measured

Calculated

Gage height, in feet above NAVD 88

Figure 14 - Measured and calculated gage height at station 0208739677 (Neuse River below

 SR 2555 near Auburn, NC) in Wake County, North Carolina.

2006

EXPLANATION

Figure 14. Measured and estimated gage height at station 0208739677 (Neuse River below 

SR 2555 near Auburn, North Carolina) in Wake County, North Carolina.

surficial aquifer. Conversely, within the surficial aquifer at the 

WC-2 cluster, the regolith generally had a higher water-level 

altitude than that of the transition zone, indicating a downward 

gradient (recharge). 

Water levels at well cluster WC-1 were affected by interaction with Neuse River water levels and are, therefore, more 

complex than those found at well clusters WC-2 and WC-3 

where the relative position of hydraulic heads to one another 

was relatively consistent. Under typical hydrologic conditions 

at well cluster WC-1, the water-level altitudes of the well and 

the two piezometers completed in the regolith (well WC-1S 

and piezometers PZ-1 and PZ-2) were higher than that of wells 

and piezometers partially or fully completed within the transition zone and granite bedrock (wells WC-1I and WC-1CH, 

and piezometers PZ-3 and PZ-4) (fig. 16), indicating a local 

downward gradient. However, all regolith, transition zone, and 

deeper bedrock aquifer water levels generally were higher than 

the river stage, indicating that groundwater generally has the 

potential to discharge to the Neuse River except during particularly wet periods. When the stage of the Neuse River rises 

as during Tropical Storm Alberto on June 15, 2006, the gradient reverses, and the water levels in wells and piezometers that 

are partially or fully completed within the transition zone are 

higher than those wells and piezometers that are completed 

within the regolith (fig. 17). The concurrent responses of the 

transition zone wells in cluster WC-1 to changes in river stage 

and the approximate altitude of the river sediments indicate 

that the Neuse River has incised through the saprolite, and 

the alluvium is hydraulically connected to the transition zone 

(figs. 6, 17). 

The degree of vertical gradient reversal among the 

regolith, transition zone, and bedrock was dependent upon the 

stage of the Neuse River. The rapid rise and fall of water levels indicated a pressure response as a result of stage changes 

in the Neuse River. These hydraulic head differences also 

affected the horizontal flow of water into and out of the Neuse 

River through bank storage effects (fig. 17). When the stage 

of the Neuse River was low and head in the aquifers exceeds 

the river stage, groundwater can discharge readily to the river. 

Conversely, when the stage of the Neuse River was high 

because of rainfall or releases from the upstream Falls dam, 

surface water moved into the river bank reducing the potential 

for groundwater discharge. 

Water-Quality Data

Water-quality data from the wells and surface-water sites 

at the RHRS were collected continuously and during periodic 
Hydrogeologic Characterization 29

A

2005 2006 2007

Figure 15. Periodic and continuous hourly groundwater levels and continuous 15-minute stage recorded at the 

Raleigh hydrogeologic research station, North Carolina, in (A) well cluster WC-3 and (B) well clusters WC-1 and 2 

and surface-water station 0208739677 from May 2005 to September 2007. 

0

25

50

75

100

125

150

 2005

Well Cluster

WC-1

MayJuneJuly Aug.

Water-level altitude, in feet above NAVD 88

Blank where data are missing

B

172

177

162

167

152

157

142

147

181

WC-2S

WC-2I

WC-2D

WC-2CH

WC-1S

WC-1I

WC-1D

WC-1CH

0208739677 Neuse River

below Secondary Road

2555 near Auburn, NC

Water-level altitude, in feet above NAVD 88 

Water-level altitude,

 in feet above NAVD 88 

EXPLANATION

M J J A S O N D J F M A M J J A S O N D J F M A M J J A S

189

190

191

192

193

194

195

196

WC-3CH

WC-3D

WC-3I

WC-3S

EXPLANATION

Well Cluster WC-3

Well Cluster WC-2

Blank where data are missing

Figure 15. Periodic and continuous hourly groundwater levels and continuous 15-minute stage recorded at the 

Raleigh hydrogeologic research station, North Carolina, in (A) well cluster WC-3 and (B) well clusters WC-1 and 2 

and surface-water station 0208739677 from May 2005 to September 2007. 
30 Hydrogeology, Groundwater Seepage, Nitrate Distribution, and Flux at the Raleigh Hydrogeologic Research Station

Figure 16. Periodic and hourly groundwater levels recorded in well cluster WC-1 and hourly stage recorded

 at surface-water station 0208739677 at the Raleigh hydrogeologic research station, North Carolina, 

 from February to July 2006.

Water-level altitude of WC-1 wells and stage at station 0208739677, in feet above NAVD 88 

WC-1S

WC-1I

WC-1CH

208739677

PZ-1

PZ-2

PZ-3

PZ-4

WC-1D

Hourly groundwater level

Periodic groundwater level

15-minute surface-water level

EXPLANATION

J J J

J

B

142

144

146

148

150

152

154

156

158

160

162

164

166

168

February March April May June July

2006

Blank where data are missing

Figure 16. Periodic and hourly groundwater levels recorded in well cluster WC-1 and hourly stage recorded at surfacewater station 0208739677 at the Raleigh hydrogeologic research station, North Carolina, from February to July 2006.
Hydrogeologic Characterization 31

0208739677

EXPLANATION

Line of sectionUSGS streamgaging station

Well cluster WC-1

and number

Well and numberCorehole and number

Base from 1988 photograph from Wake County,North Carolina, GIS database

1CH

1S

78°30’5”

35°43’30”35°43’25”

78°29’55” 78°29’50” 78°30’

0208739677

1S 1I 1CH

H'

H

H'H

Figure 17. Generalized hydrogeologic cross section H-H’ at the Raleigh hydrogeologic research station, Wake County, North Carolina. 

0208739677

1S 1I 1CH

H

H H'

Distance, in feet

1S 1I

1CH

Alluvium

Transition zone

Bedrock

Water-table at river

flood stage

Water-table at normal

river stage

Station 0208739677

and staff plate

Neuse River

180170160150140130120110

180170160150140130120110

Altitude, in feet above NAVD 88

75.48

feet

0 25 50 75 100 125 150 175 200 225 250 275

0 300 FEET

0 100 METERS

Neuse

River

Riverbed sediments

Figure 17. Generalized hydrogeologic cross section H–H’ at the Raleigh hydrogeologic research station, Wake County, North Carolina.
32 Hydrogeology, Groundwater Seepage, Nitrate Distribution, and Flux at the Raleigh Hydrogeologic Research Station

sampling events. Continuous water-quality monitoring consisted of recording the physical properties of temperature, pH, 

specific conductance, and dissolved oxygen (DO) in three 

wells at well cluster WC-2. Only temperature, specific conductance, and pH were recorded at USGS station 0208739670 on 

an unnamed tributary to the Neuse River near Auburn, North 

Carolina (hereafter called the “tributary” site location shown 

in fig. 3). 

Periodic water-quality samples were collected during 

two sampling events in October 2005 and March-April 2006 

from all wells except WC-3CH and the tributary site. The core 

borings were not intended to be sampled for water quality, but 

the wells completed in the core holes at the WC-1 and WC-2 

well clusters were sampled to investigate groundwater in the 

diabase dikes at these locations. The BAT3 packer system 

was used in March 2006 during the second sampling event 

to isolate individual fracture zones and collect water-quality 

samples in two of the open boreholes at the RHRS (McSwain 

and others, 2009). The EM flowmeter logs collected in wells 

WC-2D and WC-3D (figs. 8, 9) were used to select the BAT3

packer intervals for sampling. Packer intervals and the location of fractures sampled within that interval are shown on 

figure 6. Sampling locations, constituents, and dates of collection for water-quality samples are listed in table 2. 

A graphical analysis of the periodic water-quality 

data using Piper diagrams (Piper, 1953) and Stiff diagrams 

(Stiff, 1951) is presented in figures 18 and 19, respectively. 

Further description of the methods used to create the diagrams 

as well as box plots of the water-quality data are presented in 

McSwain and others (2009). 

The dissolved nitrate concentrations from samples collected during the March and April 2006 samplings are shown 

in figure 6. When examined spatially, the nitrate concentrations affirm that the hydraulic gradient at well cluster WC-3 

was downward from regolith to bedrock, but upward from 

bedrock to regolith in well cluster WC-2. At well cluster 

WC-1, the nitrate concentration at well WC-1D in the diabase 

dike was much less than in the other cluster wells, supporting 

the idea that the diabase dike is hydraulically isolated from the 

granite bedrock. In the three remaining wells at well cluster 

WC-1, the nitrate concentration was lowest in the transition 

zone (well WC-1I) likely because of mixing with water from 

the Neuse River. In whole, the nitrate concentrations measured 

in groundwater during the March and April 2006 samplings 

confirm the general validity of the slope-aquifer concept as 

applied at the RHRS. 

Quality of Water in the Regolith, Transition Zone, 

and Tributary

Constituent concentrations in samples collected from 

wells within the regolith and transition zone showed little 

variability between sampling events, with the exception of 

well WC-1S (fig. 18A). This variation occurs in nitrate and 

chloride sample values, and likely was due to the interaction 

of the shallow groundwater with the Neuse River. In general, 

groundwater in the regolith and transition zone was a calciummagnesium/chloride-nitrate type, with the exception of wells 

at well cluster WC-1, which were calcium-magnesium/bicarbonate-nitrate (fig. 19). The elevated chloride (up to 34 mg/L) 

and nitrate (up to 130 mg/L as N) anion concentrations were a 

result of the historic biosolid applications and do not represent 

background conditions, which would be less than 10 mg/L 

of chloride and less than 1 mg/L as N of nitrate, respectively. 

Magnesium and bicarbonate concentrations increased with distance along the flow path from recharge at well cluster WC-3 

to discharge at well cluster WC-1. 
Hydrogeologic Characterization 33

Table 2. Sampling locations, constituents, and dates of collection for water-quality samples collected at the Raleigh hydrogeologic research station, Wake County, North Carolina.—Continued[ft BLS, feet below land surface; WC, well cluster; S, shallow regolith; I, intermediate zone regolith; D, deep; CH, core hole; CFC, Chloroflourocarbon; na, not applicable; –, not sampled]

Site

identification

number

Station

name

Packer

interval

sampled

(ft BLS)

Bacteria

Major

inorganic

ions

Nutrients CFC Tritium Helium Dissolved gasses Radon Wastewater compounds Trace metals Pharmaceuticals

354328078295701 WC-1S na Oct. 

2005

Oct. 

2005 

Apr. 

2006

Oct. 

2005 

Apr. 

2006

Apr. 

2006

Apr. 

2006

– – – – – –

354328078295702 WC-1I na Oct. 

2005

Oct. 

2005 

Apr. 

2006

Oct. 

2005 

Apr. 

2006

Apr. 

2006

Apr. 

2006

Apr. 

2006

Apr. 2006 – – – –

354328078295703 WC-1D na Oct. 

2005

Oct. 

2005

Oct. 

2005 

Apr. 

2006 

Sept. 

2007

– Apr. 

2006

Apr. 

2006

–C Sept. 

2007

– – –

354328078295704 WC-1CH na Oct. 

2005

Oct. 

2005 

Apr. 

2006

Oct. 

2005 

Apr. 

2006 

Sept. 

2007

Apr. 

2006

Apr. 

2006

Apr. 

2006

Apr. 2006 Sept. 

2007

– – –

354315078300101 WC-2S na Oct. 

2005

Oct. 

2005 

Dec. 

2005 

Apr. 

2006

Oct. 

2005 

Dec. 

2005 

Apr. 

2006 

Sept. 

2007

Apr. 

2006

Apr. 

2006

Apr. 

2006

Apr. 2006 Sept. 

2007

Dec. 2005 Dec. 

2005

Dec. 2005

354315078300102 WC-2I na Oct. 

2005

Oct. 

2005 

Apr. 

2006

Oct. 

2005 

Apr. 

2006 

Sept. 

2007

Apr. 

2006

Apr. 

2006

Apr. 

2006

Apr. 2006 Sept. 

2007

– – –

354315078300103 WC-2D na Oct. 

2005

Oct. 

2005

Oct. 

2005 

Sept. 

2007

– – – – Sept. 

2007

– – –
34 Hydrogeology, Groundwater Seepage, Nitrate Distribution, and Flux at the Raleigh Hydrogeologic Research Station Table 2. Sampling locations, constituents, and dates of collection for water-quality samples collected at the Raleigh hydrogeologic research station, Wake County, North Carolina.—Continued [ft BLS, feet below land surface; WC, well cluster; S, shallow regolith; I, intermediate zone regolith; D, deep; CH, core hole; CFC, Chloroflourocarbon; na, not applicable; –, not sampled] Packer Major Site Station interval inor- Nutri- Dissolved Wastewater Trace identification Bacteria CFC Tritium Helium Radon Pharmaceuticals name sampled ganic ents gasses compounds metals number (ft BLS) ions 354315078300103 WC-2D 59-77 – Mar. Mar. Mar. Mar. – Mar. 2006 – – –

–

2006 2006 2006 2006

354315078300103 WC-2D 244-260 – Mar. Mar. Mar. Mar. – Mar. 2006 – – –

–

2006 2006 2006 2006

354315078300104 WC-2CH na Oct. Oct. Oct. – – – – Sept. – –

–

2005 2005 2005 2007

Sept. 

2007

354305078295801 WC-3S na Oct. Oct. Oct. Apr. Apr. Apr. Apr. 2006 – – –

–

2005 2005 2005 2006 2006 2006

Apr. Apr. 

2006 2006

354305078295802 WC-3I na Oct. Oct. Oct. Apr. Apr. Apr. Apr. 2006

2005 2005 2005 2006 2006 2006

Apr. Apr. 

2006 2006

354305078295803 WC-3D na Oct. Oct. Oct. – – – – – – –

–

2005 2005 2005 

Sept. 

2007

354305078295803 WC-3D 41-70 – Mar. Mar. Mar. Mar. 

– Mar. 2006 – – –

–

2006 2006 2006 2006

354305078295803 WC-3D 148-164 – Mar. Mar. Mar. Mar. 

– Mar. 2006 – – –

–

2006 2006 2006 2006

354305078295804 WC-3CH na – – – – –

–

– – – –

–

0208739670 Neuse River na Oct. Oct. Oct. – –

– – Sept. – –

–

tribuary 2005 2005 2005 2007

near Sept. Auburn, 2007North Carolina
Hydrogeologic Characterization 35

20

40

60

80 80

60

40

20

20

40

60

80

20

40

60

80

80 60 40 20 20 40 60 80

0

100

0

100

0

100

20 20

40 40

60 60

80 80

100

100

0

0

0

100

100

20

40

60

80

0 0

100

20

40

60

80

CATIONS ANIONS

MAGNESIUM

PERCENT

PERCENT

PERCENT

PERCENT

CALCIUM

SODIUM PLUS POTASSIUM

SULFATE PLUS CHLORIDE PLUS NITRATE

CHLORIDE PLUS NITRATE

100 80 60 40 20 0 0 20 40 60 80 100

CATIONS ANIONS

CALCIUM CHLORIDE PLUS NITRATE

SULFATE

CARBONATE PLUS BICARBONATE

CARBONATE PLUS BICARBONATE

20

40

60

80 80

60

40

20

0

0

100

0

20 20

40 40

60 60

80 80

100

0

100

100

20

40

60

80

0 0

100

20

40

60

80 MAGNESIUM

SODIUM PLUS POTASSIUM

SULFATE

CALCIUM PLUS MAGNESIUM

20

40

60

20

40

60

80

100 SULFATE PLUS CHLORIDE PLUS NITRATE

CALCIUM PLUS MAGNESIUM

I

D

D

A

A

D

DAA

A

DD

A

DD

A

DD A A DAAD

DDA

A

D

A

D

A

D

I

WC-1S

WC-2S

WC-3S

0208739670

WC-1I

WC-2I

WC-3I

I

D

A

D

I

DDAA

EXPLANATION

Figure 18. The water chemistry of samples collected October 2005 and April 2006

from the (A) regolith and transition-zone wells and the tributary site (station 0208739670),

and (B) bedrock wells at the Raleigh hydrogeologic research station,

Wake County, North Carolina.

A

B

WC-2D

WC-3D

WC-1CH

WC-2CH

EXPLANATION

C

B

C

CC B

C

B

BB

B

CC

C

BBB BBB

CC BBB B

B

C

Figure 18. The water chemistry of samples collected October 2005 and 

April 2006 from the (A) regolith and transition-zone wells and the tributary 

site (station 0208739670), and (B) bedrock wells at the Raleigh hydrogeologic 

research station, Wake County, North Carolina.
36 Hydrogeology, Groundwater Seepage, Nitrate Distribution, and Flux at the Raleigh Hydrogeologic Research StationFigure 19. Major ion milliequivalents in water samples collected in October 2005 from (A) well cluster WC-1, (B) well cluster WC-2, (C) a Neuse River tributary

 (station 0208739670), and (D) well cluster WC-3 at the Raleigh hydrogeologic research station, Wake County, North Carolina.

A

B

C

CATIONS

MILLIEQUIVALENTS PER LITER

ANIONS

7 7

CATIONS

MILLIEQUIVALENTS PER LITER

ANIONS

7 7

CATIONS

MILLIEQUIVALENTS PER LITER

ANIONS

7 7

D

CATIONS

MILLIEQUIVALENTS PER LITER

ANIONS

7 7

WC-1S

Sodium plus Potassium

Ammonia, as Nitrogen

Calcium

Magnesium

Chloride

Bicarbonate plus Carbonate

Nitrite plus Nitrate

Sulfate

WC-2S

Sodium plus Potassium

Ammonia, as Nitrogen

Calcium

Magnesium

Chloride

Nitrite plus Nitrate

Bicarbonate plus Carbonate

Sulfate

WC-2I

Sodium plus Potassium

Ammonia, as Nitrogen

Calcium

Magnesium

Chloride

Nitrite plus Nitrate

Bicarbonate plus Carbonate

Sulfate

WC-2D

Sodium plus Potassium

Ammonia, as Nitrogen

Calcium

Magnesium

Chloride

Nitrite plus Nitrate

Bicarbonate plus Carbonate

Sulfate

WC-1I

Sodium plus Potassium

Ammonia, as Nitrogen

Calcium

Magnesium

Chloride

Bicarbonate plus Carbonate

Nitrite plus Nitrate

Sulfate

WC-1CH

Sodium plus Potassium

Ammonia, as Nitrogen

Calcium

Magnesium

Chloride

Nitrite plus Nitrate

Bicarbonate plus CarbonateSulfate

WC-2CH

Sodium plus Potassium

Ammonia, as Nitrogen

Calcium

Magnesium

Chloride

Nitrite plus Nitrate

Bicarbonate plus Carbonate

Sulfate

WC-3S

Sodium plus Potassium

Ammonia, as Nitrogen

Calcium

Magnesium

Chloride

Nitrite plus Nitrate

Bicarbonate plus Carbonate

Sulfate

WC-3I

Sodium plus Potassium

Ammonia, as Nitrogen

Calcium

Magnesium

Chloride

Nitrite plus Nitrate

Bicarbonate plus Carbonate

Sulfate

WC-3D

Sodium plus Potassium

Ammonia, as Nitrogen

Calcium

Magnesium

Chloride

Nitrite plus Nitrate

Bicarbonate plus Carbonate

Sulfate

0208739670

Sodium plus Potassium

Ammonia, as Nitrogen

Calcium

Magnesium

Chloride

Nitrite plus Nitrate

Bicarbonate plus Carbonate

Sulfate

Figure 19. Major ion milliequivalents in water samples collected in October 2005 from (

A) well cluster WC-1, (

B) well cluster WC-2, (

C) a Neuse River tributary (station 0208739670), 

and (

D) well cluster WC-3 at the Raleigh hydrogeologic research station, Wake County, North Carolina.
Hydrogeologic Characterization 37

Water-quality data collected from continuous monitors 

installed in the regolith in well WC-2S and in the transition zone 

in well WC-2I displayed seasonal fluctuations in groundwater 

chemistry (fig. 20). The regolith and transition zone had similar 

seasonal hydrographs, with water levels generally peaking in the 

spring and declining to a low in the late fall. Differences in seasonal groundwater chemistry existed between the regolith and 

transition zone, and these differences did not follow the same 

seasonal cycle as the water levels. In the regolith (well WC-2S) 

when the groundwater level was high in June 2006, the water 

temperature and specific conductance were seasonally low. 

Conversely, in the transition zone (well WC-2I) in June 2006, 

the water level was seasonally high and the water temperature 

was seasonally low, but the specific conductance did not reach 

a seasonal low until August 2008. Groundwater in the transition zone displayed a greater range in specific conductance 

than in the regolith. Dissolved oxygen was higher in groundwater within the regolith than in the transition zone. Aerobic 

conditions dominated within the regolith and transition zone 

and were likely a factor in the persistence of high nitrate 

concentrations in the groundwater. The pH in the regolith and 

transition zone groundwater was acidic and increased slightly 

with depth. The inverse response of specific conductance and 

pH between the regolith and the transition zone was likely an 

effect of surface recharge. 

Surface water collected from the tributary site was intermediate in relative ion composition compared to groundwater in 

the regolith and transition zone at well clusters WC-2 and WC-3 

(fig. 18A) but most resembled wells WC-3S and WC-3I in 

major ion concentration (figs. 19C, D). In October 2005, water 

from the tributary site had a nitrate concentration of 74 mg/L 

as N. In comparison, water from well WC-2I in October 2005 

had a nitrate concentration of 80 mg/L as N and at well WC-3I 

the concentration was 114 mg/L as N. Because the tributary is 

topographically downgradient from both well clusters WC-2 

and WC-3, it was likely that the tributary received groundwater discharge from the regolith and transition zones in both 

areas. 

The surface-water quality continuous monitor in the 

tributary site was operated for about a month before it was 

destroyed by Tropical Storm Alberto in June 2006. However, 

some trends can be inferred from the limited data collected 

(fig. 21). The surface-water temperature was highly variable 

and fluctuated diurnally in response to air temperature and 

insolation. Specific conductance was elevated because of the 

high nitrate and chloride concentrations in the discharging 

groundwater and, though generally stable, decreased markedly 

in response to rainfall events. This decrease was most likely due to 

dilution with less conductive runoff caused by the rainfall event. 

The pH was stable and neutral.

Quality of Bedrock Groundwater

Major ion concentrations in the open-borehole bedrock 

wells and individually sampled fractures showed little variability between sampling events but considerable variability 

from well cluster to well cluster (fig. 18B). Elevated alkalinity, calcium, and sulfate concentrations as well as pH values 

above 8 measured in well WC-1D indicate the well likely was 

grout contaminated, and samples from this well were omitted 

from further data analysis. The type of water sampled from 

the open-borehole well generally was calcium-magnesium/

chloride-nitrate type in the bedrock with the exception of 

well WC-2D, which was calcium/bicarbonate (fig. 19). Even 

though they are completed within the same well cluster, the 

groundwater chemistry from well WC-2CH differed from 

WC-2D in that it had a higher calcium concentration, lower 

bicarbonate plus carbonate concentration, and much higher 

nitrate concentration (51 mg/L as N in WC-2CH as opposed 

to 5 mg/L as N in WC-2D). These chemical differences 

likely were due, in part, to the geology near the open hole or 

screened interval of the well, the 200-ft horizontal separation 

of the two wells, and hydraulic differences within the fracture zone tapped by each well. Well WC-2CH was partially 

completed within a diabase dike, whereas well WC-2D was 

completed solely within granite. 

Whole rock analyses on the cores collected at the RHRS 

indicated that the diabase contains about 7 percent calcium 

as opposed to about 1 percent in the granite (McSwain and 

others, 2009), making geologic differences the likely source 

of the disparity in calcium concentrations within the water 

samples. Wells WC-2D and WC-2CH probably intersect different fracture sets, in part because of their 200-ft horizontal 

separation. Because the source of the dissolved nitrate was a 

land-applied biosolid, it is likely that the fractures within well 

WC-2CH tap a shallower, more nitrate-laden fracture set than 

those intersected by well WC-2D. 

In March 2006 the BAT3

 packer system was used to collect samples from discrete fracture zones within wells WC-2D 

and WC-3D. Samples were collected at depths of 59 to 77 ft 

and 244 to 260 ft within well WC-2D and at depths of 41 to 

70 ft and 148 to 164 ft within WC-3D (fig. 6). 

Once the BAT3 system was installed, hydraulic head 

was monitored above, below, and within the isolated fracture zone to establish vertical gradients within the borehole. 

At well WC-2D, the deepest fracture set at 244 to 260 ft had 

a higher head than the shallower fracture set at 59 to 77 ft, 

indicating upward flow within the borehole and confirming 

measurements collected with the borehole flowmeter (fig. 8). 

This upward flow within the borehole retarded the downward 

migration of contaminated groundwater and was likely why 

water-quality samples collected from well WC-2D had nitrate 

concentrations that were much lower than those found in well 

WC-2CH as well as the regolith and transition zone wells in 

the same well cluster (fig. 6). 

Because fractured-rock flow systems are complex, it is 

unknown if this condition of deep upward discharge exists 

throughout the flow system in the area of WC-2 or if it is an 

artifact caused by connecting the fractures within the borehole 

during the drilling process. The opposite hydraulic condition 

existed at well WC-3D where the deepest fracture set at 148 to 

164 ft had a lower head than the shallower fracture set at 
38 Hydrogeology, Groundwater Seepage, Nitrate Distribution, and Flux at the Raleigh Hydrogeologic Research Station

Water temperature,

in degrees Celsius

Water level, in feet

above NAVD 88

Specific conductance,

in microsiemens per

centimeter at

25 degrees Celsius

Dissolved oxygen, in

milligrams per liter pH Precipitation, in inches

2006 2007

Dec. Jan. Feb. Mar. Apr. May June July Aug. Sept. Dec. Jan. Feb. Mar. Apr. May June July Aug Sept. Oct. Nov.

Figure 20. Hourly record of (A) water level, (B) temperature, (C) specific conductance, (D) dissolved oxygen, (E) pH in wells WC-2S and WC-2I and(F) daily precipitation at the Raleigh hydrogeologic research station, North Carolina, December 2005 through September 2007. 

174173172171170

18171615

1,2001,000

800600400

87654

6.05.55.04.54.03.53.02.52.01.51.00.5

0

WC-2S

WC-2S

WC-2S

WC-2S

WC-2S

WC-2I

WC-2I

WC-2I

WC-2I

WC-2I

ABCDEF

Figure 20. Hourly record of (

A) water level, (

B) temperature, (

C) specific conductance, (

D) dissolved oxygen, (

E) pH in wells WC-2S and 

WC-2I, and (

F) daily precipitation at the Raleigh hydrogeologic research station, North Carolina, December 2005 through September 2007.
Hydrogeologic Characterization 39 Figure 21. Fifteen-minute record of (A) temperature, (B) specific conductance, (C) pH, and (D) daily precipitation at station 0208739670 Neuse River tributary near Auburn, North Carolina, May 5 to June 5, 2006. June May 2006

A

B

C

D

Weir infilled with sediment

5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 1 2 3 4 5

Water temperature, in

degrees Celsius

Specific conductance, in

microsiemens per centimeter at

25 degrees Celsius pH Precipitation, in inches

212019181716151413121110

700600500400300200100

0

8.07.57.06.56.02.01.51.00.5

0

Figure 21. Fifteen-minute record of (A) temperature, (B) specific conductance, (C) pH, and (D) daily precipitation at station 0208739670 Neuse River 

tributary near Auburn, North Carolina, May 5 to June 5, 2006. 
40 Hydrogeology, Groundwater Seepage, Nitrate Distribution, and Flux at the Raleigh Hydrogeologic Research Station

41 to 70 ft, indicating downward flow within the borehole 

as was measured by the borehole flowmeter (fig. 9). This 

downward flow allowed the migration of contaminants from 

the shallow fracture set to the deeper fracture set and was 

likely why water-quality samples collected from WC-3D had 

concentrations of nitrate that were greater than those measured within the regolith and transition zone at well cluster 

WC-3. Water-quality samples collected from well WC-3D had 

concentrations of nitrate that were greater than those measured 

within the regolith and transition zone at well cluster WC-3 

(fig. 6). As the RWWTP ceased land application of biosolids 

in 2002 and because well cluster WC-3 is in a recharge area, 

it was likely that rainfall flushed the nitrate deeper into the 

groundwater system. 

In contrast to the groundwater levels, seasonal fluctuations in groundwater chemistry were not measured by the 

continuous water-quality monitor in the bedrock well WC-2D 

(fig. 22). Groundwater temperature and specific conductance 

steadily increased over the 1-year monitoring period, but pH 

remained stable at a neutral 7.5. DO levels were low, indicating anaerobic conditions, but increased markedly because of 

mixing each time the monitor was serviced, generally taking weeks to return to an ambient level. The lack of seasonal 

changes in groundwater chemistry coupled with a neutral pH 

and very low DO levels suggest a longer residence time for 

groundwater in the bedrock near this well than in the regolith 

or transition zone. The groundwater in the bedrock near well 

WC-2D has largely not been affected by the nitrate-laden 

groundwater in the regolith and transition zone. However, the 

slight rise in groundwater temperature and specific conductance suggests that a source of nitrate was affecting at least 

one of the fractures intersecting the open-hole well WC-2D. 

Neuse River Resistivity Profiling

In November 2005, waterborne continuous resistivity 

profiling was conducted on the Neuse River in the area of the 

RHRS to measure the apparent resistivity distribution of the 

sediments, weathered rock, and pore-water fluid beneath the 

streambed (McSwain and others, 2009). Composition of the 

sediment and weathered rock, amount of water in the pore 

space and fractures, ionic concentration of the pore fluid, and 

variations in pore space affect resistivity. A processed inversion of one continuous resistivity profile section of the Neuse 

River near the study area is presented in figure 23. 

Resistivity is a measure of the ability of a fluid to conduct 

electrical current. Because conductivity is the reciprocal of 

resistivity, one interpretation of the continuous resistivity 

profile in figure 23 was that the areas of low resistivity (dark 

blues) were areas beneath the streambed that contain fluids 

with high specific conductance, while the areas of high resistivity (reds) delineated areas with low specific conductance 

fluids or more competent bedrock. In the processed inversion, 

the low resistivity areas to the west and east sides of the profile 

were located where both of the unnamed tributaries discharge 

to the Neuse River. As the unnamed tributaries were reported 

to have base-flow nitrate concentrations ranging from 30 to 

60 mg/L as N in 2005 and 2006 (Fountain, 2006), the low 

resistivity measurements in the area of the unnamed tributaries 

on the continuous resistivity profile most likely represented 

groundwater contaminated by high concentrations of nitrate 

with the potential to discharge into the Neuse River. 

Based on groundwater sampling at well cluster WC-1, 

high concentrations of nitrate were present in the regolith 

and alluvium (greater than 80 mg/L as N), with a much lower 

concentration of nitrate (6 mg/L as N) detected in groundwater 

within the diabase dike. The small, isolated high resistivity 

area in the middle of the profile is likely the diabase dike, 

while the low resistivity areas located on either side of the 

dike represent shallow groundwater contaminated with nitrate. 

It appears that the diabase dike acts as a barrier that preferentially directs the discharge of groundwater to the Neuse River 

and at depth may isolate groundwater movement laterally 

across the dike. 
 41Figure 22. Hourly record of (A) water level, (B) temperature, (C) specific conductance, (D) dissolved oxygen, (E) pH in well WC-2D and (F) daily precipitation at the Raleigh hydrogeologic research station, North Carolina, December 2005 through January 2007. 

2005 2006 2007

Dec. Jan. Feb. Mar. Apr. May June July Aug. Sept. Dec. Jan. Oct. Nov.

Water temperature,

in degrees Celsius

Water level, in feet

above NAVD 88

Specific conductance,

in microsiemens per

centimeter at

25 degrees Celsius

Dissolved oxygen, in

milligrams per liter pH Precipitation, in inches

174173172171

17.016.516.015.515.0

400350300250200150

2.01.51.00.58.58.07.57.06.5

03210

ABCDEF

Figure 22. Hourly record of (A) water level, (B) temperature, (C) specific conductance, (D) dissolved oxygen, (E) pH in well WC-2D, and (F ) daily 

precipitation at the Raleigh hydrogeologic research station, North Carolina, December 2005 through January 2007.
42 Hydrogeology, Groundwater Seepage, Nitrate Distribution, and Flux at the Raleigh Hydrogeologic Research Station

0

12.3

3.1

6.2

9.2

Depth below water

surface, in meters

West East

Unnamed

tributary

0208739677

 Figure 23. Processed continuous resistivity profile (CRP) inversion and location of CRP survey with approximate projected surface location of

diabase dikes from magnetometer data overlaid on an aerial photograph of the Raleigh hydrogeologic research station, Wake County, North Carolina. 

EXPLANATION

Resistivity

1,000

458

210

96

44

Ohm-m

1S

1I

1D

1CH

PZ-1 PZ-2 PZ-3

PZ-4

0208739677

1S

1I

1D

PZ-1 PZ-2 PZ-3

PZ-4

diabase dike

Conductive groundwater Conductive groundwater

bedrock bedrock

Base from: 1988 photograph from Wake County,North Carolina, GIS database

78°30’5”

35°43’30”35°43’25”

78°29’55” 78°29’50” 78°30’

Neuse

River

0208739670

EXPLANATION

Diabase dike–dashed where

approximated

Location of continuous

resistivity profile

USGS streamgaging station

and number

Well cluster WC-1

Well and numberCorehole and numberPiezometer and number

1CH

PZ-1

1S

0 300 FEET

0 100 METERS

Figure 23. Processed continuous resistivity profile (CRP) inversion and location of CRP survey with approximate projected surface location of diabase dikes from magnetometer data overlaid on an aerial photograph of the Raleigh hydrogeologic research station, Wake County, North Carolina.
Characterization of Groundwater Seepage to the Neuse River 43

Characterization of Groundwater 

Seepage to the Neuse River

The extent of discharge from the groundwater beneath 

the RHRS to the Neuse River was determined by the use of 

physical and chemical methods. Areas of preferential groundwater discharge (seeps) were identified using temperature and 

water-quality methods. Seepage meters were used to quantify 

the movement of water vertically between the groundwater/

surface-water interface, while samples of the pore water at two 

locations beneath the Neuse River were used to characterize 

nitrogen concentrations in the seepage. 

Temperature Distribution in Pore Water Beneath 

the Neuse River

Differences in temperature can be an indication of 

groundwater seepage, especially if measurements are taken 

when the contrast between groundwater and surface-water 

temperatures are at their greatest. Water-quality monitoring 

at the RHRS indicated that the groundwater temperatures 

were fairly stable year round at about 16 degrees Celsius (°C; 

figs. 20, 22). Generally the maximum temperature difference 

between groundwater and surface water occurs twice a year, 

during the late summer when surface-water temperatures can 

exceed 25 °C and during the winter when surface-water temperatures approach 0 °C. 

To locate possible seepage of groundwater, 80 instantaneous temperature measurements were made within the pore 

water beneath the Neuse River across six lines of section 

(fig. 3) on August 8, 2006, at depths of 0.5, 2.5, and 4.5 ft. A 

detailed description of the methods used to collect the temperature measurements can be found in McSwain and others 

(2009). Temperature measurements were also collected from 

within the Neuse River immediately above the riverbed to create an upper boundary and for comparison purposes. Because 

the temperature measurements were collected throughout an 

8-hour period during a hot day in August (maximum air temperature of 35 °C), an increase in the temperature measured 

within the Neuse River was to be expected as the surfacewater temperature rose in response to day time heating. However, the pore-water measurements at 2.5 ft or deeper were 

unlikely to be affected by day time heating. 

To examine the distribution of the pore-water temperatures, contour maps were created from the transect data. A 

general trend toward cooler pore-water temperatures occurred 

with increasing depth. A contour map displaying the aerial distribution of pore-water temperature at a depth of 2.5 ft below 

the bed of the Neuse River is shown in figure 24. The north 

bank of the Neuse River opposite the RHRS between lines 

of section A′ and F′ likely was a groundwater discharge area 

based on the cool isotherms that parallel the bank. Additionally, two seeps discharging much colder groundwater were 

noted, one on the south bank of the Neuse River near section 

C–C′ and a second on the north bank of the Neuse River near 

section D–D′ to the northwest of the first seep (fig. 24). When 

the trace of one of the diabase dikes was overlain on the contour map, the seeps occurred on opposite sides of the dike. The 

presence of these seeps supports the concept that the diabase 

dike may act as a subsurface barrier that preferentially directs 

the discharge of groundwater to the Neuse River. Although 

continuous resistivity profiling (CRP) profiling (fig. 22) suggested that shallow groundwater on both the east and west 

sides of the diabase dike was nitrate laden, the temperature 

distribution survey did not locate a groundwater seep on the 

western side of the dike on the south bank of the Neuse River. 

It is probable that a seep did exist at that location at that time, 

but was not identified by temperature signature because it was 

located between sections D–D′ and E–E′. 
44 Hydrogeology, Groundwater Seepage, Nitrate Distribution, and Flux at the Raleigh Hydrogeologic Research Station

-

Figure 24 - Contour map of pore-water temperature measured at 2.5 ft below the Neuse River bed at the Raleigh hydrogeologic research station,

Base from 1998 photograph from Wake County, North Carolina, GIS databaseContours 1993 USGS topographic sheet, Garner Quadrangle, 1:24,000 scale

 Wake County, North Carolina.

A

A′

B

B′

C

C′

D

D′

E

E′

Approximate

Neuse River

boundary

F

F′

WC-1S

WC-1I

WC-1D

WC-1CH

PZ-1 PZ-2

PZ-3

PZ-4

0208739677

River flow

MP-U

MP-D

Neuse

River

0 100 FEET 50

0 25 METERS 15 

Diabase dike, dashed where

approximated

Contour line depicting areas of

equivalent temperature, indegrees Celsius. Hue indicatesmagnitude

Sampling locationsU.S. Geological Survey

streamgaging station

Corehole and numberWell and numberPiezometer and numberMultiport piezometer

A

A′

20

19

18

17

2625242322

EXPLANATION

21

0208739677

MP-D

PZ-4

WC-1CH

WC-1I

A A′

0208739677

MP-D

PZ-4

WC-1CH

WC-1I

Well cluster WC-1

N

Figure 24. Contour map of pore-water temperature measured at 2.5 feet below the Neuse River bed at the Raleigh hydrogeologic research station, Wake County, North Carolina.
Characterization of Groundwater Seepage to the Neuse River 45

Nutrient Distribution

On August 15-17, 2006, water-quality samples were collected from the pore water beneath the Neuse River at a total 

of 47 locations across sections A–A′, C–C′, and F–F′ by using 

a retractable drive-point piezometer (McSwain and others, 

2009). Profiles of the contoured concentrations were generated 

(fig. 25) for each line of section in order to examine the distribution of the nitrate and ammonia concentrations. Elevated 

nitrate concentrations of about 65 mg/L as N near the RHRS 

on section A–A′ occurred downstream from the RHRS on the 

south side of the Neuse River. At section C–C′, groundwater 

with a nitrate concentration of about 80 mg/L as N was present at depth within the seep on the RHRS side of the Neuse 

River. Higher than background concentrations of nitrate (about 

10 mg/L as N) extended to the north into the center of the 

river along section C–C′, indicating the discharge of nitrate by 

deeper flow paths within the aquifer. At section F–F′, upstream 

and to the west of the diabase dike, nitrate with a concentration of about 56 mg/L as N occurred at depth, but there was 

little lateral extent to the plume. Ammonia was present in 

relatively high concentrations in the center of the Neuse River 

at sections A–A′ and F–F′, but was not present at section C–C′. 

To further characterize the nitrate distribution in the pore 

water beneath the Neuse River, two multiport piezometers 

were installed in bed sediments approximately 20 ft off the 

south bank of the river in September 2007 (locations shown 

in fig. 24). Multiport piezometers MP-U and MP-D were 

installed on opposing sides of the diabase dike to determine 

if the quality of water in the seepage area on one side of the 

dike differed from the non-seepage area on the other side 

of the dike. The multiport piezometers were used to assess 

the vertical distribution of nitrate concentrations in the pore 

water of as much as 140 cm (about 4.6 ft) beneath the Neuse 

River bed (table 3). Piezometer MP-U on the western side of 

the diabase dike was completed near line of section E–E′ in 

sediments described as silty sand grading to a silty sand with 

few cobbles. Piezometer MP-D, on the eastern side of the 

diabase dike, was completed near line of section C–C′ within 

the boundary of the previously identified seep in sediments 

described as a clayey silt grading to a silty sand. 

After installation, the multiport piezometers were allowed 

to equilibrate for several months. On December 14, 2007, a 

pore-water sample was collected from each port within both of 

the multiport piezometers and analyzed for nutrients, temperature, and specific conductance. Dissolved oxygen was measured only in water samples collected from the 20-cm (about 

0.3 ft) and 140-cm (about 4.6 ft) sampling ports of piezometers MP-U and MP-D. Samples of the water in the Neuse 

River were collected near both multiport piezometer sites 

from just above the sediment/water interface for comparison 

purposes. Results of the pore-water sample analyses are shown 

in table 3. 

Pore-water samples collected from multiport piezometer 

MP-U had specific conductance and nitrate concentrations 

that gradually increased with depth below the sediment/water 

interface, while ammonia concentrations decreased with depth. 

Water temperature varied, but generally was about 14.1 oC 

(the same as the temperature of the water in the Neuse River), 

with the exception of the two deepest sampling ports, which 

had slightly warmer temperatures. Nitrate concentrations in 

pore water collected from multiport piezometer MP-U ranged 

from about 3 mg/L as N in the 10-cm (about 0.3 ft) deep port 

to 91 mg/L as N in the 140-cm (about 4.6 ft) deep port, with a 

median concentration of about 52 mg/L as N. The nitrate concentration just above the sediment/water interface in the Neuse 

River was about 0.4 mg/L as N. It is likely that the increasing concentration of nitrate with depth was due to mixing of 

groundwater and surface water within the hyporheic zone in 

the area of multiport piezometer MP-U as aerobic conditions 

predominated at and below a depth of 20 cm (about 0.6 ft). 

In contrast to the multiport piezometer MP-U, pore-water 

samples collected from multiport piezometer MP-D located in 

the previously delineated seep, were warmer in temperature 

and had little measurable ammonia. Specific conductance 

and nitrate concentrations rapidly increased with depth below 

the sediment/water interface (table 3). Water temperature 

ranged from 14.7 to 15.6 oC and increased with depth. Nitrate 

concentrations in pore water ranged from about 42 mg/L as N 

in the 10-cm (about 0.3 ft) deep port to about 82 mg/L as N in 

the 40-cm (about 1.3 ft) port, but varied less than 2 mg/L as N 

from the 20-cm (about 0.6 ft) to 140-cm (about 4.6 ft) ports. 

The median pore-water concentration of nitrate in multiport 

piezometer MP-D was about 79 mg/L as N, while the concentration in the Neuse River just above the sediment/pore-water 

interface was about 0.5 mg/L as N. The occurrence of high 

concentrations of nitrate at such a shallow depth beneath the 

sediment/water interface suggests that little denitrification 

or mixing with surface water was occurring in the hyporheic 

zone in the area of multiport piezometer MP-D. 

Vertical Gradients

Hydraulic head was measured in multiport piezometers MP-U and MP-D on December 14, 2007, at the time of 

sampling to determine the vertical gradient between the Neuse 

River and the groundwater during a period of low flow in the 

river. The stage of the Neuse River at the time of the hydraulic 

head measurement was used as a datum and the measurements 

are listed in table 3. In piezometer MP-U, the hydraulic head 

measured to 0.01 ft in all the ports was equivalent to the Neuse 

River stage, indicating that little measurable vertical gradient was present. It is probable that there was a slight gradient, 

but it was too small to quantify. This lack of vertical gradient 

coupled with the gradual downward increase in nitrate concentrations and aerobic conditions indicates that in the area of the 

multiport piezometer, little vertical movement of groundwater 

was occurring at 140 cm (about 4.6 ft) below the sediment/

water interface, and the gradual downward increase of nitrate 

concentrations may have been the result of the lateral mixing 

of surface water with groundwater. However, this condition 
46 Hydrogeology, Groundwater Seepage, Nitrate Distribution, and Flux at the Raleigh Hydrogeologic Research Station

Figure 25. Contoured concentration of nitrate and ammonia as nitrogen beneath the Neuse River at lines of section A-A’ , C-C’, and F-F’ at the Raleigh hydrogeologic research station, Wake County, North Carolina (see figs. 3 and 24 for line of section locations). 

Area depicting nitrate concentration

 range. Hue indicates magnitude

10.0–19.9 mg/L

20.0–29.9 mg/L

30.0–39.9 mg/L

40.0–49.9 mg/L

50.0–59.9 mg/L

60.0–69.9 mg/L

70.0–79.9 mg/L

80.0–89.9 mg/L

Area depicting ammonia concentration

 range. Hue indicates magnitude

1.00–1.99 mg/L

2.00–2.99 mg/L

3.00–3.99 mg/L

4.00–4.99 mg/L

5.00–5.99 mg/L

6.00–6.99 mg/L

EXPLANATION

Location and value of nitrate (black) and ammonia (blue) sample, in milligrams per liter as nitrogen collected August 15–17, 2006, E is estimated

0

1

2

3

4

5

6

7

8

9 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140

Depth below river stage, in feet

Distance from south bank, in feet

0

1

2

3

4

5

6

7

8

9 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140

Depth below river stage, in feet

Distance from south bank, in feet

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140

Distance from south bank, in feet

A A'

Profile of 

river bed

Neuse River Profile of 

river bed

Neuse River

1.3

0.027

65.0

E0.007

66.5

E0.006 33.4

E0.009

<0.060

0.575

<0.060

1.27

E0.032

1.26

<0.060

0.148

<0.060

0.72 <0.060

0.779

<0.060

0.421

<0.060

0.619

<0.069

0.011

0.587

E0.008

<0.060

0.834

<0.060

6.26

76.5

0.015

78.6

0.010

65.1

E0.008

Profile of 

river bed

Neuse River

<0.060

0.287

E0.030

0.858 55.9 0.018 6.4 0.039 <0.060 0.833

<0.060

0.558

<0.060

3.19

<0.060

2.84 <0.060

2.49

<0.060

0.513

0.071

E0.01

0.0600.949

<0.060

0.0840.139

0.345

0.0840.139

C C'

F F'

80.3

E0.008 79.6 E0.008 77.7 E0.008

74.3

E0.007

1.29

<0.010

0.57

0.101

9.72

0.013

9.68

0.026

1.03

E0.008 0.925

E0.01 0.734

E0.005

0.0640.067

0.079

0.06

<0.060

1.29

Figure 25. Contoured concentration of nitrite + nitrate, as nitrogen, and ammonia beneath the Neuse River at lines of section 

A

–

A’ , 

C–

C’, and 

F–

F ’ at the Raleigh 

hydrogeologic research station, Wake County, North Carolina (see figs. 3 and 24 for line of section locations).
Characterization of Groundwater Seepage to the Neuse River 47

Table 3. Temperature, specific conductance, dissolved oxygen, and nutrient concentrations in pore-water samples collected from multiport piezometers at two locations in the Neuse River, Raleigh hydrogeolgoic research station, Wake County, North Carolina, December 14, 2007. Locations of samples shown in figure 24.[ °C, degrees Celsius; mg/L , miligrams per liter; N, nitrogen; μS, microsiemens; P, phosphorus; cm, centimeter; SR, Secondary Road; NC, North Carolina; –, not measured; E, value estimated]

Site identification 

number Station name Sample date Sample time Hydraulic head (feet) Temperature (°C) Nitrite + nitrate as nitrogen (mg/L as N) Specific conduc- tance (μS) Dissolved oxygen (mg/L) Ammonia, as nitrogen (mg/L as N) Nitrite, as nitrogen (mg/L as N) Ortho- phosphate as phos- phorus (mg/L as P) Alkaline persulfate nitrogen (mg/L as N)

Upgradient piezometer MP-U

0208739677 Neuse River 

below SR 2555 near Auburn, NC

Dec. 14, 2007 1208 1.0 14.1 0.375 154 7.3 E0.010 0.008 E0.004 0.72

354330078295801 WK-352 10 cm Dec. 14, 2007 1125 1.0 14.1 2.98 170

– 0.043 0.006 0.022 3.39

354330078295802 WK-353 20 cm Dec. 14, 2007 1131 1.0 14.4 6.41 198 6.4 0.046 0.002 0.017 6.96

354330078295803 WK-354 30 cm Dec. 14, 2007 1136 1.0 13.8 27.0 370

– 0.055 0.01 0.017 29.0

354330078295804 WK-355 40 cm Dec. 14, 2007 1141 1.0 14.1 45.5 540

– 0.023 0.021 0.015 47.6

354330078295805 WK-356 50 cm Dec. 14, 2007 1145 1.0 14.0 58.7 684

– E0.011 0.027 0.024 61.6

354330078295806 WK-357 80 cm Dec. 14, 2007 1149 1.0 14.1 65.8 697

– E0.015 0.017 0.021 67.7

354330078295807 WK-358 110 cm Dec. 14, 2007 1155 1.0 14.4 81.1 808

– E0.012 0.004 0.022 84.3

354330078295808 WK-359 140 cm Dec. 14, 2007 1159 1.0 14.7 91.0 870 5.3 E0.012 0.015 0.021 89.0

Downgradient piezometer MP-D

0208739677 Neuse River 

below SR 2555 near Auburn, NC

Dec. 14, 2007 1329 1.0 14.1 0.508 152 7.3 <0.02 0.008 E0.003 0.91

354330078295601 WK-360 10 cm Dec. 14, 2007 1308 1.0 14.7 41.8 451

– E0.018 0.012 0.01 43.2

354330078295602 WK-361 20 cm Dec. 14, 2007 1252 1.2 15.0 80.7 780 6.3 <0.02 E0.001 0.025 84.2

354330078295603 WK-362 30 cm Dec. 14, 2007 1319 1.2 15.2 79.7 777

– E0.016 E0.001 0.025 82.5

354330078295604 WK-363 40 cm Dec. 14, 2007 1302 1.3 15.2 81.8 774

– E0.014 E0.001 0.022 79.1

354330078295605 WK-364 50 cm Dec. 14, 2007 1325 1.4 15.2 80.4 777

– E0.015 <0.002 0.025 79.4

354330078295606 WK-365 80 cm Dec. 14, 2007 1258 1.4 15.4 79.0 777

– E0.015 <0.002 0.024 81.4

354330078295607 WK-366 110 cm Dec. 14, 2007 1245 1.4 15.6 78.9 777

– <0.02 E0.001 0.026 82.3

354330078295608 WK-367 140 cm Dec. 14, 2007 1241 1.4 15.6 78.9 776 5.7 E0.013 E0.001 0.022 82.4
48 Hydrogeology, Groundwater Seepage, Nitrate Distribution, and Flux at the Raleigh Hydrogeologic Research Station

most likely did not exist at all depths beneath the sediment/

water interface at this river stage in the Neuse River because 

of the highly elevated nitrate concentrations. Groundwater 

movement and subsequent discharge probably was occurring 

along a deeper flow path. 

Similarly, hydraulic head was measured in multiport 

piezometer MP-D, and a vertical gradient was calculated using 

the stage of the Neuse River as a datum. Measurements are 

listed in table 3. In piezometer MP-D, the measured hydraulic 

head in the 140-cm (about 4.6 ft) port was about 0.4 ft higher 

than the Neuse River. This strong upward vertical gradient of 

0.1 and upward flow of groundwater is likely why nitrate concentrations of 80 mg/L as N were found at the shallow depth 

of 20 cm (about 0.6 ft) beneath the sediment/water interface. 

Seepage and Mass Flux

At the RHRS, vertical fluctuations in river stage and the 

resulting horizontal changes in pressure gradients between 

the aquifer and the Neuse River made vertical and horizontal 

seepage velocity and the resulting mass flux of nitrate into the 

river highly temporal. Solutions to groundwater-flow equations are rarely simple and generally require the use of numerical modeling methods to address complex boundary conditions. However, by examining flow in one direction at a time 

and assuming an isotropic and homogeneous medium with no 

dispersion or diffusion of dissolved constituents, it was possible to approximate the rate of groundwater flow (seepage) 

and mass of nitrate transported through the aquifer. 

Horizontal Seepage and Mass Flux

The availability of hourly river stage from the Auburn 

station at the RHRS and corresponding groundwater level in 

the wells of well cluster WC-1 allowed for the estimation of 

horizontal gradients between the aquifer and the river as they 

changed over time. The horizontal gradient, coupled with 

hydraulic conductivity and effective porosity of the aquifer 

sediments, allowed the calculation of a groundwater, horizontal-seepage flux from the aquifer along a tangential flow path 

to the edge of the Neuse River by using equation 2 (Dominico 

and Schwartz, 1990). 

qh = 1/ne

 × Kh × dh/L, (2)

where

qh is horizontal seepage flux (units of length 

per time),

ne is effective porosity (unitless),

Kh is horizontal hydraulic conductivity (units of 

length per time),

dh is the change in hydraulic head along the flow 

path (units of length), and

L is flow path length (units of length).

In order to quantify the horizontal seepage from WC-1I 

to the Neuse River, a flux was estimated for each day of 2006 

using the gradient between the daily-mean water level in well 

WC-1I and the daily-mean stage of the Neuse River measured 

at the Auburn station. Well WC-1I was selected for gradient 

calculation because it responded quickly to changes in river 

stage due to the transition zone’s direct connection with the 

alluvial bed sediments. Similarly, slug tests conducted in well 

WC-1I were used to define the horizontal hydraulic conductivity because it is a conservative approximation in comparison 

to the hydraulic conductivity of the sandy bed sediments. As 

it is not easy to determine the effective porosity of aquifer 

sediments, a laboratory-based infiltration value for loamy sand 

was used (Rawls and others, 1983). A loamy sand texture also 

has a hydraulic conductivity similar to that measured by slug 

testing in well WC-1I. 

Estimated horizontal seepage fluxes along the 130-ft flow 

path from well WC-1I to the Neuse River (fig. 26) ranged 

from a minimum of -0.52 ft/d (June 15, 2006) to 0.20 ft/d 

(December 13, 2006) with a median value of 0.09 ft/d. A 

negative seepage flux indicates surface water infiltrating 

into the aquifer while a positive flux indicates groundwater 

discharge into the Neuse River. On June 15, 2006, the Neuse 

River flooded its bank because of Tropical Storm Alberto, 

causing the gradient to reverse and river water to infiltrate 

into the aquifer. For a total of 35 days in 2006, the daily-mean 

stage of the Neuse River was higher than the daily-mean 

water level in well WC-1I, effectively damming groundwater 

discharge to the Neuse River. Twenty-four of those 35 days 

occurred during a 1-month time period from November 11 to 

December 10, 2006. The maximum seepage flux occurred on 

December 13, 2006, as a direct result of a rapid decrease in 

river stage that allowed a large quantity of stored groundwater 

to quickly discharge to the Neuse River.

Using the estimated horizontal seepage flux and the 

dissolved nitrate concentration in the groundwater, it was 

possible to approximate the potential horizontal advective 

mass flux of nitrate from groundwater to the Neuse River for a 

given cross-sectional area by applying equation 3 (Dominico 

and Schwartz, 1990). 

F = q × A × Xc

 , (3)

where

F is mass flux (units of mass per time),

q is seepage flux (units of length per time),

A is cross-sectional seepage area (units of 

squared length), and

Xc is the concentration of nitrate (units of mass 

per cubed length). 

Equation 3 was used to estimate the potential horizontal 

discharge of nitrate from groundwater to the Neuse River 

along a 300-ft long reach near well cluster WC-1 (fig. 26). The 

daily horizontal mass flux of nitrate for 2006 was estimated by 

incorporating the daily seepage flux estimated from equation 2. 
Characterization of Groundwater Seepage to the Neuse River 49 -

A

A′

B

B′

C

C′

D

D′

E

E′

F

F′

WC-1S

WC-1I

WC-1D

WC-1CH

PZ-1

PZ-2

PZ-3

PZ-4

0208739677

River flow

MP-U

MP-D

Approximate

Neuse River

boundary

Seepage

area A

Seepage

area B

0 100 FEET 50

0 25 METERS 15 

Diabase dike - dashed where

approximated

Sampling locationsTrace of mass flux crosssectional areaAssumed flow pathU.S. Geological Survey

streamgaging station

Well cluster WC-1

Corehole and numberWell and numberPiezometer and numberMultiport piezometerLocation of seepage meter

measurements

EXPLANATION

Figure 26. Area of Neuse River bed sediments investigated with seepage meter measurements at the Raleigh hydrogeologic research station, Wake County, North Carolina in August 2006 and December 2007.

Neuse

River

N

Base from 1998 photograph from Wake County, North Carolina, GIS database

0208739677

MP-D

PZ-4

WC-1CH

WC-1I

A A′ 

Figure 26. Area of Neuse River bed sediments investigated with seepage meter measurements at the Raleigh hydrogeologic research station, Wake County, North Carolina, in August 2006 and December 2007.
50 Hydrogeology, Groundwater Seepage, Nitrate Distribution, and Flux at the Raleigh Hydrogeologic Research Station

For each day, the wet cross-sectional area was determined by 

using the altitude of the bottom of the transition zone (121.5 ft 

as noted from coring in well WC-1CH) as a lower boundary 

and the stage of the Neuse River as the upper boundary along 

a 300-ft reach at the aquifer/river-bank interface (fig. 26). A 

constant nitrate concentration of 0.0035 pound per cubic foot 

as N (55.9 mg/L as N) was applied in the calculation. This 

was the same concentration of nitrate measured in August 

2006 from the drive-point sample located 20 ft off the bank 

in transect F–F′, which was located near the middle of the 

300-ft reach. 

Estimated horizontal mass flux of nitrate along the 300-ft 

reach of the Neuse River (fig. 26) ranged from a minimum of 

–10.9 pounds per day (lbs/d) on June 15, 2006, to 5.0 lbs/d 

on December 13, 2006, with a median value of 2.2 lbs/d. 

A negative mass flux indicates the effect of surface water 

infiltrating into the aquifer and a positive mass flux indicates 

the discharge of a mass of nitrate into the Neuse River. Mass 

flux of nitrate was estimated for each day and summed to 

approximate a total horizontal nitrate flux of about 750 lbs/

year in 2006 for this 300-ft reach of the Neuse River. Because 

area and nitrate concentration were held at constant levels 

throughout the calculations, the estimation of nitrate mass 

flux was highly dependent upon the magnitude of the seepage 

flux. Thus, the dates of minimum and maximum horizontal 

nitrate mass flux corresponded to the dates of minimum and 

maximum horizontal seepage flux, with the greatest release of 

nitrate from the aquifer to the river following a rapid decline 

in river stage. A substantial limitation of this horizontal mass 

flux estimation was that it only accounts for fluid movement to 

the edge of the river and does not quantify the actual discharge 

into the river. This estimation provides an idea of the potential 

mass of nitrate that was available to be discharged if hydrologic and geologic conditions within the river sediments were 

favorable. Although this was an approximation based on many 

assumptions, it does highlight that over a much larger river 

reach there is the potential for a substantial amount of nitrate 

to discharge as a nonpoint source from the groundwater to the 

Neuse River.

Vertical Seepage and Mass Flux

Seepage meters can be used to determine rates of vertical 

groundwater flow to surface-water bodies (Lee, 1977). On 

August 16, 2006, and December 12, 2007, seepage meters 

were deployed in the bed of the Neuse River in two locations 

(seepage areas A and B; figs. 4, 26) in close proximity to multiport piezometers MP-D and MP-U. These specific areas were 

selected to compare vertical seepage flux in an area of probable groundwater discharge (seepage area B near MP-D to the 

east of the diabase dike) to an area where discharge was likely 

negligible (seepage area A near MP-U on the west side of the 

diabase dike) (fig. 26). Vertical seepage flux was estimated 

using equation 1. 

Vertical seepage flux differed markedly from seepage 

area A to B in both August 2006 and December 2007 (table 4). 

The average estimated daily seepage flux at seepage area A 

was about 4.5 ft/d, about 2 orders of magnitude larger than the 

0.022 ft/d average estimated at seepage area B over both collection dates. This large disparity in vertical seepage measurements provides further evidence that seepage area B (located 

near multiport piezometer MP-D to the east of the diabase 

dike) was an area of preferential groundwater discharge to the 

Neuse River. 

Localized approximations of vertical seepage flux in 

fluvial environments can be highly variable. At both seepage 

areas A and B, sources of error were minimized by making 

repeated measurements with more than one meter, and suspect 

measurements were not used in vertical seepage estimations. Only two small areas of the heterogeneous streambed 

were tested with seepage meters. The magnitude of seepage 

measured in this study at seepage area B was on the high end 

of values reported in the literature for vertical seepage velocities measured in riverine environments of North Carolina 

(Kennedy and others, 2008). Although seepage meters are a 

simple, inexpensive tool to approximate vertical seepage flux, 

several sources of error could affect the measurements. As 

seepage meters measure flow across a small area of the river 

bed, measurement heterogeneities in the seepage direction 

Table 4. Summary of Darcian flux and mass flux of nitrite + nitrate, as nitrogen, calculated from seepage measurements 

in August 2006 and December 2007 near the upgradient and downgradient piezometers, Raleigh hydrologic research 

station, Wake County, North Carolina.

[ft/d, feet per day; ft2, square feet; mg/L, milligrams per liter; g/d, grams per day]

Location Date of 

measurements

Number of 

seepage 

measurements 

Average 

seepage flux 

(ft/d)

Seepage 

area (ft2)

Median nitrite + 

nitrate concentration (mg/L)

Mass flux 

(g/d)

Seepage Area A August 16, 2006 4 0.015 78.56 52 1.74

Seepage Area A December 12, 

2007

4 0.028 78.56 52 3.24

Seepage Area B August 16, 2006 5 3.8 78.56 79 667.8

Seepage Area B December 12, 

2007

4 5.1 78.56 79 896.3
Summary 51

and rate in fluvial settings can be caused by local variation in 

sediment hydraulic conductivity and bed topography (Conant, 

2004). Also, because it is difficult to seat a seepage meter in 

an alluvial environment with fast-flowing water, the measurements collected with the seepage meters may be biased high 

because of scouring of the bed around the meter or by water 

flowing past the submerged collection bag and inducing water 

flow into the bag. 

An approximation of the contribution of nitrate from 

vertical groundwater discharge to the Neuse River at seepage areas A and B can made by the calculation of mass flux 

applying equation 3 but orienting all measurements in a vertical direction. As the true aerial extent of seepage areas A and 

B has not been mapped, a circular area with a radius of 5 ft 

(78.5 square feet (ft2

)) centered on MP-U and MP-D was used 

as an approximation for area in equation 3. This corresponds 

roughly to the circular area that the seepage meters were 

moved within to make repeated discharge measurements. For 

seepage area A mass flux calculations, a nitrate concentration 

of 52 mg/L as N (1.47 grams per cubic foot as N) was used for 

Xc

 as it was the median concentration of the eight samples collected in December 2007 (table 3) from multiport piezometer 

MP-U. Similarly for seepage area B, a nitrite concentration of 

79 mg/L as N (0.22 milligram per cubic foot as N) was used 

for Xc

 as it was the median concentration of the eight samples 

collected in December 2007 (table 3) from multiport piezometer MP-D. Because area and nitrate concentration were held 

at constant levels throughout the calculations (similar to the 

horizontal mass flux calculations), the vertical mass flux of 

nitrate was highly dependent on the magnitude of the seepage 

flux. 

Estimated vertical mass flux of nitrate for seepage areas 

A and B are listed in table 4. At seepage area A the vertical 

mass flux of nitrate averaged about 2.5 grams per day (g/d) 

and at seepage area B, it was 784 g/d. These estimations of 

vertical mass flux are an approximation and were specific to 

hydrologic conditions that occurred on August 16, 2006, and 

December 12, 2007, within the area near where the seepage 

measurements were made. They do not account for errors that 

are inherent to data collection using seepage meters or other 

naturally occurring factors that may play a role in regulating 

groundwater discharge throughout the year, such as evapotranspiration, recharge, groundwater level, and stream stage. 

However, these approximations of vertical mass flux of nitrate 

do indicate that under certain hydrologic conditions, the area 

of preferential discharge near MP-D at seepage area B east 

of the diabase dike has the potential to discharge substantial 

quantities of nitrate to the Neuse River.

Summary

Geologic, hydrologic, and water-quality data were collected from 3 core borings, 12 wells, and 4 piezometers at 3 

well clusters, as well as from 2 surface-water sites, 2 multiport 

piezometers, and 80 discrete locations in the streambed of the 

Neuse River from 2005 to 2007. Data collected were used 

to evaluate the three primary zones of the piedmont aquifer 

(regolith, transition zone, and fractured bedrock) and characterize the interaction of groundwater and surface water as a 

mechanism of nutrient transport to the Neuse River.

The RHRS is representative of the igneous felsic intrusive hydrogeologic unit in the Piedmont Physiographic 

Province as it is underlain by the Rolesville granite batholith, a 

massive to weakly foliated granite intersected by diabase dike 

intrusions. The regolith varies in composition from a saprolite that was present across most of the RHRS to an alluvium 

deposited by the Neuse River. A zone of slightly weathered 

granite bedrock with open near-horizontal fractures near the 

top of the bedrock separated the overlying highly weathered 

regolith from the competent bedrock. The thickness of the 

transition zone increases with higher altitude along the well 

transect, ranging from 11 to 30 ft. The granitic rock outcrops 

and core specimens show a very weak steeply dipping foliation with a north-northeast strike. Two previously unmapped 

north-striking nearly vertical diabase dikes intrude the granite 

bedrock. The dikes appear to be hydraulically disconnected 

from the rest of the groundwater system.

Fractures identified within the boreholes were separated into three groups—“water bearing,” “secondary,” and 

“sealed”—on the basis of optical televiewer image logs. A 

correlation existed between foliation and fracture orientation, 

with most fractures striking parallel to foliation. Water-bearing 

fractures clustered in two sets: one set strikes east-northeast 

with a shallow south dip, and the second strikes northeast and 

dips moderately to steeply to the northwest. The majority of 

water-bearing fractures have dips of less than 30° and were 

likely stress-relief fractures caused by unloading of the granite. Secondary fractures had a conjugate joint set with a north 

strike, but few appeared to be water bearing and thus may 

have had little influence on groundwater flow. 

Groundwater flow and vertical gradients at the RHRS 

generally supported historical conceptual models, with 

recharge occurring in areas of topographic high and discharge 

occurring at topographic lows near streams. The vertical gradient direction at well cluster WC-3 supported the concept of 

the downward movement of groundwater in recharge areas. In 

well WC-3D, the hydraulic head of the shallow transmissive 
52 Hydrogeology, Groundwater Seepage, Nitrate Distribution, and Flux at the Raleigh Hydrogeologic Research Station

fractures was higher than that of the deeper transmissive 

fractures, facilitating the transport of surface contaminants 

to deeper zones within the bedrock. In well WC-2D, under 

both ambient and stressed conditions the higher hydraulic 

head of the deep transmissive fractures caused water to flow 

upward to fractures at shallower depths with lower hydraulic 

heads. Vertical gradients in the discharge area near well cluster 

WC-1 were variable and highly dependent on the stage of the 

Neuse River, with gradient reversals common. Continuous 

groundwater-level data collected in the wells at well cluster 

WC-1 generally mimicked the surface-water stage of the 

Neuse River. It appears as though the Neuse River has incised 

through the alluvium and saprolite, hydraulically connecting it 

to the transition zone. 

Elevated nutrient concentrations were detected in all 

of the groundwater samples collected from the wells and 

surface-water sites at the RHRS. Nitrate estimated from nitrite 

plus nitrate concentrations in the sampled wells and tributary ranged from about 5 to more than 120 mg/L as N. The 

distribution of nitrate concentrations within individual bedrock 

fractures isolated using a multifunction bedrock-aquifer 

transportable testing tool indicated that at well cluster WC-2 

little nitrate was detected in the deep bedrock fractures but 

high concentrations of nitrate were present in deep bedrock 

fractures at WC-3. At well cluster WC-1, the nitrate concentration at well WC-1D completed in a diabase dike was much 

less than in the other WC-1 cluster wells, supporting the idea 

that the diabase dike is hydraulically isolated from the granite 

bedrock. The tributary topographically downgradient from 

well cluster WC-2 and WC-3 is a discharge point for regolith 

and transition zone groundwater originating in both areas. 

Seasonality in the groundwater temperature and specific conductance in the regolith and transition zone groundwater was 

not present in the bedrock groundwater. Based on dissolvedoxygen measurements, groundwater in the regolith and transition zone was aerobic, but groundwater in the bedrock tended 

to be anaerobic. 

The extent of preferential discharge from the groundwater beneath the RHRS to the Neuse River was determined by 

the use of physical and chemical methods. Waterborne continuous resistivity profiling conducted on the Neuse River in 

the area of the RHRS indicated areas of low resistivity, likely 

representing groundwater contaminated by high concentrations of nitrate. The low-resistivity areas were concentrated 

beneath tributaries with high base flow nitrate concentrations 

and on either side of a diabase dike that transects the Neuse 

River. Discrete temperature measurements made within the 

pore water beneath the Neuse River revealed seeps of colder 

groundwater discharging into warmer surface water near a 

diabase dike. Water-quality samples collected from the pore 

water beneath the Neuse River indicated that concentrations 

of nitrate exceeding 80 mg/L as N were present in some areas 

on the RHRS side. Multiport piezometers installed in bed 

sediments on opposing sides of the diabase dike were used 

to assess the vertical distribution of nitrate in the pore water. 

Pore-water samples from the multiport piezometer installed in 

a previously identified seep had a median nitrate concentration 

that exceeded 75 mg/L as N and a vertical hydraulic gradient of 0.1. Pore-water samples from the multiport piezometer 

on the opposite side of the diabase dike had a median nitrate 

concentration of 79 mg/L as N and had no measurable vertical gradient. The disparate temperatures, vertical gradients, 

and nitrate distribution on either side of the diabase dike are 

evidence that the diabase dike preferentially directed the 

discharge of groundwater in the area of well cluster WC-1 to 

the Neuse River and isolated groundwater movement laterally 

across the dike. 

Vertical fluctuations in the stage of the Neuse River and 

the resulting horizontal changes in pressure gradients made the 

calculation of vertical and horizontal seepage velocity and the 

resulting mass flux of nitrate highly temporal. The potential 

horizontal seepage and mass flux of nitrate from groundwater 

to the Neuse River was quantified for 2006 along a 300-ft 

river reach near well cluster WC-1. For the majority of 2006, 

the water level in the surficial aquifer was higher than the 

stage of the Neuse River, allowing groundwater to flux into 

the river with a median rate of 0.09 ft/d. The total horizontal 

mass flux of nitrate from the surficial aquifer to the Neuse 

River was estimated to be about 750 lbs for all of 2006. The 

flux of vertical groundwater seepage and mass of nitrate to the 

Neuse River was estimated by the use of seepage meters in the 

areas of the multiport piezometers on either side of the diabase 

dike. Seepage meter measurements were markedly different 

from one side of the diabase dike to the other, with average 

seepage fluxes of 0.02 ft/d and 4.5 ft/d, respectively. The 

estimated average daily vertical mass flux of nitrate at each 

seepage area was 2.5 g/d and 784 g/d over 2 days of measurement. Although these seepage and mass flux approximations 

are based on many assumptions, they highlight that over a 

small reach there is the potential for a large amount of nitrate 

to discharge from the groundwater into the Neuse River as a 

nonpoint source. 

Acknowledgments

The authors thank members of the North Carolina 

Department of Environment and Natural Resources and the 

USGS Resource Evaluation Program who contributed to data 

collection for this report. In addition, the authors thank Tim 

Woody, Superintendent of the NRWWTP, for his support and 

assistance in facilitating field-site access, and Kimberly Null 

of the USGS NC Water Science Center for her assistance with 

construction of the multiport piezometer. We would also like 

to thank Dr. “Skip” Stoddard from North Carolina State University for assistance in locating the diabase dikes at the site. 
References Cited 53

References Cited

Boyles, R., Raman, S., Sims, A., Schwab, S., Horgan, K., 

Brooks, M., and Frazier, A., 2006, Evaluation and applications of NCEP Stage II and Stage IV gage-corrected 

radar precipitation estimates over the Carolinas: American Meterological Society annual meeting, January 29–

February 3, 2006, Atlanta, Georgia, 12 p.

Camp Dresser & McKee, 2003, Wake County comprehensive goundwater investigation: Camp Dresser and McKee, 

147 p.

Chapman, M.J., Bolich, R.E., and Huffman, B.A., 2005, 

Hydrogeologic setting, ground-water flow, and groundwater quality at the Lake Wheeler Road research station, 

2001–03, North Carolina Piedmont and Blue Ridge Mountains Resource Evaluation Program: U.S. Geological Survey 

Scientific Investigations Report 2005–5166, 85 p.

Conant Jr., B., 2004, Delineation and quantifying ground 

water discharge zones using streambed temperatures: 

Groundwater, v. 42, no. 2, p. 243–257.

Daniel, C.C., III, and Dahlen, P.R., 2002, Preliminary hydrogeologic assessment and study plan for a regional groundwater resource investigation of the Blue Ridge and Piedmont Provinces of North Carolina: U.S. Geological Survey 

Water-Resources Investigations Report 02–4105, 60 p.

Daniel, C.C., III, and Payne, R.A., 1990, Hydrogeologic unit 

map of the Piedmont and Blue Ridge Provinces of North 

Carolina: U.S. Geological Survey Water-Resources Investigations Report 90–4035, 1 map sheet, scale 1:500,000.

Day-Lewis, F.D., White, E.A., Johnson, C.D., Lane, J.W., Jr., 

and Belaval, M., 2006, Continuous resistivity profiling to 

delineate submarine groundwater discharge—Examples and 

limitations: The Leading Edge, v. 25, no. 6, p. 724–728.

Dominico, P.A., and Schwartz, F.W., 1990, Physical and 

chemical hydrogeology: New York, John Wiley & Sons, 

824 p.

ENSR Consulting and Engineering, Inc., 2002, Comprehensive 

site assessment report at the Neuse River Waste Water 

Treatment Plant, 62 p.

ENSR Consulting and Engineering, Inc., 2003, Supplemental 

site assessment report at the Neuse River Waste Water 

Treatment Plant, 108 p.

Fishman, M.J., ed., 1993, Methods of analysis by the U.S. 

Geological Survey National Water Quality Laboratory—

Determination of inorganic and organic constituents in 

water and fluvial sediments: U.S. Geological Survey OpenFile Report 93–125, 217 p.

Fountain, M.J., 2006, Nitrate transport in shallow flow 

systems at the Neuse River Waste Water Treatment Plant: 

Raleigh, North Carolina State University School of Marine, 

Earth, and Atmospheric Sciences, Masters Thesis, 118 p. 

Freeman, L.A., Carpenter, M.C., Rosenberry, D.O., Rousseau, 

J.P., Unger, R., and McLean, J.S., 2004, Use of submersible 

pressure transducers in water-resources investigations: U.S. 

Geological Survey Techniques of Water-Resources Investigations, book 8, chap. A3 [variously paged].

Harned, D.A., and Daniel, C.C., III, 1992, The transition zone 

between bedrock and saprolite—Conduit for contamination? 

in Daniel, C.C., III, White, R.K., and Stone, P.A., eds., 

Ground water in the Piedmont of the Eastern United States, 

Proceedings of a Conference on Ground Water in the Piedmont, Charlotte, North Carolina, Oct. 16–18, 1989: Clemson, South Carolina, Clemson University, p. 336–348.

Heath, R.C., 1980, Basic elements of ground-water hydrology 

with reference to conditions in North Carolina: U.S. Geological Survey Open-File Report 80–44, 86 p.

Hibbard, J.P., Stoddard, E.F., Secor, D.T., and Dennis, A.J., 

2002, The Carolina Zone, Overview of Neoproterozoic to 

early Paleozoic peri-Gondwanan terranes along the eastern 

flank of the southern Appalachians: Earth Science Reviews, 

v. 57, no. 3, p. 299–339.

Kennedy, C.D., Genereux, D.P., Mitasova, H., Corbett, D.R., 

and Leahy, S., 2008, Effect of sampling density and design 

on estimation of streambed attributes: Journal of Hydrology, 

v. 355, no. 1-4, p. 164–180.

Keys, W.S., 1990, Borehole geophysics applied to groundwater investigations: U.S. Geological Survey Techniques of 

Water-Resources Investigations, book 2, chap. E2, 150 p.

Lee, D.R., 1977, A device for measuring seepage flux in lakes 

and estuaries: Limnology and Oceanography, v. 22, no. 1, 

p. 140–147.

LeGrand, H.E., 2004, A master conceptual model for hydrogeological site characterization in the Piedmont and Mountain region of North Carolina, A guidance manual: Raleigh, 

North Carolina Department of Environment and Natural 

Resources, Division of Water Quality, Groundwater Section, 

50 p.

Majcher, E.H., Phelan, D.J., Lorah, M.M., and McGinty, A.L., 

2006, Characterization of preferential ground-water seepage from a chlorinated hydrocarbon-contaminated aquifer 

to West Branch Canal Creek, Aberdeen Proving Ground, 

Maryland, 2002–04: U.S. Geological Survey Scientific 

Investigations Report 2006–5233, 191 p., online only 

http://pubs.usgs.gov/sir/2006/5233/.
54 Hydrogeology, Groundwater Seepage, Nitrate Distribution, and Flux at the Raleigh Hydrogeologic Research Station

Martin, J.B., Hartl, K.M., Corbett, D.R., Swarzenski, P.W., 

and Cable, J.E., 2003, A multi-level pore-water sampler for 

permeable sediments: Journal of Sedimentary Research, 

v. 73, no. 1, p. 128–132.

McSwain, K.B., Bolich, R.E., Chapman, M.J., and 

Huffman, B.A., 2009, Water-resources data and hydrogeologic setting at the Raleigh hydrogeologic research station, 

Wake County, North Carolina, 2005–2007: U.S. Geological 

Survey Open-File Report 2008–1377, 48 p. (only online at 

http://pubs.usgs.gov/of/2008/1377/).

North Carolina Department of Environment and Natural 

Resources, Division of Water Quality, Groundwater Sciences Unit, and U.S. Geological Survey, Water Resources 

Division, North Carolina District, 2008, Standard operating procedures for groundwater research stations, North 

Carolina Piedmont and Mountains Groundwater Resource 

Evaluation Program, accessed July 19, 2010, at http://

h2o.enr.state.nc.us/gwp/documents/GWREP_SOP-Draft_

Apr_08.pdf.

North Carolina Department of Environment and Natural 

Resources, North Carolina Division of Water Quality, 

2009 Basinwide Planning Program—Final, Neuse River 

basinwide water quality plan, accessed July 19, 2010, 

at http://h2o.enr.state.nc.us/basinwide/Neuse/2008/

NeuseRiverBasinPlanDRAFT.htm

North Carolina Department of Environment, Health, and Natural Resources, 1997, Report of Proceedings on the Proposed 

Neuse River Basin Nutrient Sensitive Waters (NSW) Management Strategy. Environmental Management Commission Meeting, June 12, 1997: Raleigh, N.C., North Carolina 

Department of Environment and Natural Resources.

North Carolina Geological Survey, 1985, Geologic map of 

North Carolina: Raleigh, North Carolina Geological Survey, 

scale 1:500,000.

North Carolina State University, 2000, RiverNet accessed in 

November 2008 at http://rivernet.ncsu.edu.

Piper, A.M., 1953, A graphic procedure in the geochemical 

interpretation of water analyses: Washington D.C., U.S. Geological Survey, Groundwater Chemistry Notes, no. 12, 14 p.

Rawls, W.J., Brakensiek, D.L., and Miller, N., 1983, Greenampt infiltration parameters from soils data: Journal of 

Hydraulic Engineering, v. 109, no. 1, p. 62–70.

Reyes, M.M., 2009, River nitrate gains from groundwater 

discharge at waste application fields, Neuse River, North 

Carolina: Raleigh, North Carolina State University School 

of Marine, Earth, and Atmospheric Science, Masters Thesis, 

108 p.

Schneider, D.A., and Samson, S.D., 2001, A comparison of 

zircon and monazite U–Pb ages from the Rolesville Batholith, NC: Lessons from misbehaving minerals, presented at 

Geological Society of America Southeastern Section Annual 

Meeting, April 5–6, 2001, Raleigh, North Carolina.

Shapiro, A.M., 2001, Characterizing groundwater chemistry 

and hydraulic properties of fractured- rock aquifers using 

the Multifunction Bedrock-Aquifer Transportable Testing 

Tool (Bat3

): U.S. Geological Survey Fact Sheet FS-075-01, 

4 p.

Showers, W.J., 2007, Evaluation and remediation of nitrate 

flux from biosolid application fields to surface waters in the 

Neuse River Basin, Final report for Section 319 Non-Point 

Source Pollution Control Grant EW07015, 23 p. 

Snyder, D.D., and Wightman, W.E., 2002, Application of 

continuous resistivity profiling to aqueous characterization, 

in Proceedings of the Symposium on the Application of 

Geophysics to Environmental and Engineering Problems 

(SAGEEP), February 10–14, 2002, Las Vegas, Nevada, 

Paper 13 GSL.

Stiff, H.A., Jr., 1951, The interpretation of chemical water 

analysis by means of patterns: Journal of Petroleum Technology, v. 3, p. 15–17. 

U.S. Geological Survey, 1974, Surface water supply of the 

United States, 1966–70, part 2. South Atlantic Slope and 

Eastern Gulf of Mexico Basins, v. 1, Basins from James 

River to Savannah River: U.S. Geological Survey WaterSupply Paper 2104, p. 3–10.

U.S. Geological Survey, 1975, Quality of surface waters of the 

United States, 1970, part 3. Ohio River Basin: U.S. Geological Survey Water-Supply Paper 2153, p. 1–2.

U.S. Geological Survey, 2006a, USGS groundwater data for 

North Carolina; available online at http://nwis.waterdata.

usgs.gov/nc/nwis/gw/levels.

U.S. Geological Survey, 2006b, USGS water data for North 

Carolina; accessible online at http://nwis.waterdata.usgs.

gov/nc/nwis.

U.S. Geological Survey, 2007, North Carolina annual water 

data report series, 2006; available online at http://nc.water.

usgs.gov/reports/WDR/.

U.S. Geological Survey, 2008, Water Use in North Carolina; 

available online at http://nc.water.usgs.gov/infodata/wateruse/.html.

Williams, J.H., and Johnson, C.D., 2000, Borehole-wall 

imaging with acoustic and optical televiewers for fracturedbedrock aquifer investigations, in Proceedings of the 

Seventh International Symposium on Borehole Geophysics 

for Minerals, Geotechnical, and Groundwater Applications, 

October 24–26, 2000, Houston, Texas: Minerals and Geotechnical Logging Society, p. 43–53. 
For additional information regarding this 

publication, contact:

Director

USGS North Carolina Water Science Center

3916 Sunset Ridge Road

Raleigh, NC 27607

(919) 571-4000

email: dc_nc@usgs.gov

Or visit the USGS North Carolina Water 

Science Center Web site at:

 http://nc.water.usgs.gov

Prepared by:

USGS Science Publishing Network

Raleigh Publishing Service Center

3916 Sunset Ridge Road

Raleigh, NC 27607
McSwain and others—Hydrogeology, Groundwater Seepage, Nitrate Distribution, and Flux at the Raleigh Hydrogeologic Research Station—Scientific Investigations Report 2013–5041