SIMULATION OF GROUND-WATER FLOW IN THE

COASTAL PLAIN AQUIFER SYSTEM OF 

I NORTH CAROLINA

PROFESSIONAL PAPER 1404-M
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Simulation of Ground-Water Flow in the 

Coastal Plain Aquifer System of North Carolina

ByG.L. GIESEJ.L. EIMERS, andRW. COBLE

REGIONAL AQUIFER-SYSTEM ANALYSISNORTHERN ATLANTIC COASTAL PLAIN

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1404-M
U.S DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary

U.S GEOLOGICAL SURVEY 

Gordon P. Eaton, Director

Any use of trade, product, or firm names in this publication is for

descriptive purposes only and does not imply endorsement by the

U.S. Government.

Library of Congress Cataloging in Publication Data

Giese, G.L.

Simulation of ground-water flow in the Coastal Plain aquifer system of North Carolina / by G.L. Giese, J.L. Eimers, and R.W.

Coble.

p. cm. (Regional aquifer-system analysis) (U.S. Geological Survey professional paper; 1404-M) 

Includes bibliographical references. 

1. Ground-water flow North Carolina Computer simulation. I. Eimers, Jo Leslie. II. Coble, Ronald W. (Ronald Wimmer).

III. Title. IV. Series. V. Series: U.S. Geological Survey professional paper; 1404-M. 

GB1197.7.G54 1997

551.49'09756 dc20 92-46991

CIP

ISBN 0-607-88305-7

For sale by U.S. Geological Survey, Information Services 

Box 25286, Federal Center, Denver, CO 80225
FOREWORD

THE REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM

The Regional Aquifer-System Analysis (RASA) Program represents a 

systematic effort to study a number of the Nation's most important aquifer 

systems, which, in aggregate, underlie much of the country and which represent an important component of the Nation's total water supply. In general, 

the boundaries of these studies are identified by the hydrologic extent of each 

system and, accordingly, transcend the political subdivisions to which investigations have often arbitrarily been limited in the past. The broad objective for 

each study is to assemble geologic, hydrologic, and geochemical information; 

to analyze and develop an understanding of the system; and to develop predictive capabilities that will contribute to the effective management of the 

system. The use of computer simulation is an important element of the RASA 

studies to develop an understanding of the natural, undisturbed hydrologic 

system and the changes brought about in it by human activities, and to provide a means of predicting the regional effects of future pumping or other 

stresses.

The final interpretive results of the RASA Program are presented in a 

series of U.S. Geological Survey Professional Papers that describe the geology, 

hydrology, and geochemistry of each regional aquifer system. Each study 

within the RASA Program is assigned a single Professional Paper number 

beginning with Professional Paper 1400.

Gordon P. Eaton 

Director

CONTENTS

Foreword......................................................................................

Abstract........................................................................................

Introduction.................................................................................

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

Methods of Investigation.................................................

Previous Investigations...................................................

Acknowledgments.............................................................

Conceptualization of the Aquifer System................................

Hydrogeologic Framework.......................................................

Surficial Aquifer................................................................

Yorktown Aquifer and Overlying Confining Unit .......

Pungo River Aquifer and Overlying Confining Unit..

Castle Hayne Aquifer and Overlying Confining Unit.

Beaufort Aquifer and Overlying Confining Unit .........

Peedee Aquifer and Overlying Confining Unit............

Black Creek Aquifer and Overlying Confining

Unit......................................................................

Upper Cape Fear Aquifer and Overlying Confining 

Unit......................................................................

Lower Cape Fear Aquifer and Overlying Confining 

Unit......................................................................

Lower Cretaceous Aquifer and Overlying Confining 

Unit......................................................................

Simulation of Ground-Water Flow...........................................

Equation of Ground-Water Flow....................................

Page Page

III Simulation of Ground-Water Flow Continued

Ml Digital Model of Ground-Water Flow...................................... M27

1 Grid Design........................................................................ 30

4 Model Boundaries............................................................. 30

4 Model Input........................................................................ 30

4 Ground-Water Recharge........................................ 30

5 Streambed Leakance.............................................. 32

5 TVansmissivity of Aquifers.................................... 34

8 Storage Coefficient of Aquifers............................ 34

9 Leakance of Confining Units................................. 34

14 Ground-Water Withdrawals and Time

14 Discretization............................................ 54

14 Model Calibration........................................................................ 74

18 Procedures......................................................................... 74

18 Results................................................................................ 75

	Saltwater Reentrant in the Cape Fear Arch................ 83

23 Sensitivity Analysis.................................................................... 83

	Method of Analysis ........................................................... 84

23 Results................................................................................ 84

	Hydrologic Analysis of the Aquifer System...................................... 86

27 Water Budget.............................................................................. 86

	Potentiometric Surfaces............................................................. 89

27 Vertical Flow............................................................................... 110

27 Summary and Conclusions................................................................... 139

27 Selected References............................................................................. 140

ILLUSTRATIONS

	Page

FIGURE 1. Map showing the study area of the North Carolina Coastal Plain aquifer system........................................................................... M2

2. Map showing the Northern Atlantic Coastal Plain study area............................................................................................................ 3

3. Sketch showing typical annual water budget for the North Carolina Coastal Plain hydrogeologic system................................ 6

4. Idealized hydrogeologic cross section showing features of the ground-water flow system............................................................ 7

5-9. Hydrogeologic section:

5. A-A' from Richmond County to Brunswick County............................................................................................................. 9

6. B-B' from Johnston County to Pamlico County.................................................................................................................... 10

7. C-C' from Southampton County, Va., to Currituck County, N.C........................................................................................ 11

8. D-U from Brunswick County to Onslow County.................................................................................................................. 12

9. ZX-.D" from Onslow County to Chowan County..................................................................................................................... 13

10-21. Maps showing areal extent of the 

10. Yorktown aquifer and underlying aquifers or basement rock............................................................................................. 15

11. Pungo River aquifer and overlying aquifers.......................................................................................................................... 16

12. Castle Hayne aquifer and overlying aquifers......................................................................................................................... 17

13. Castle Hayne aquifer and underlying aquifers...................................................................................................................... 19

14. Beaufort aquifer and overlying aquifers................................................................................................................................. 20

15. Beaufort aquifer and underlying aquifers............................................................................................................................... 21

16. Peedee aquifer and overlying aquifers.................................................................................................................................... 22

17. Black Creek aquifer and overlying aquifers........................................................................................................................... 24
VI CONTENTS

Page

FIGURES 10-21. Maps showing areal extent of the Continued

18. Upper Cape Fear aquifer and overlying aquifers................................................................................................................ M25

19. Upper Cape Fear aquifer and underlying aquifers or basement rock.............................................................................. 26

20. Lower Cape Fear aquifer and overlying aquifers ............................................................................................................... 28

21. Lower Cape Fear aquifer and underlying aquifers or basement rock............................................................................. 29

22. Finite-difference grid for the flow model of the North Carolina Coastal Plain aquifer system.................................................... 31

23. Schematic representation of model input requirements..................................................................................................................... 32

24. Map showing estimated recharge to surficial aquifer from precipitation........................................................................................ 33

25. Map showing distribution of estimated streambed leakance............................................................................................................. 35

26-34. Maps showing transmissivity used in model simulations for the 

26. Yorktown aquifer...................................................................................................................................................................... 36

27. Pungo River aquifer................................................................................................................................................................. 37

28. Castle Hayne aquifer....................................................................................... 38

29. Beaufort aquifer........................................................................................._^ 39

30. Peedeeaquifer.......................................................^ 40

31. Black Creek aquifer..........................................................................^ 41

32. Upper Cape Fear aquifer........................................................................................................................................................ 42

33. Lower Cape Fear aquifer........................................................................................................................................................ 43

34. Lower Cretaceous aquifer....................................................................................................................................................... 44

35-43. Maps showing total effective thickness of the 

35. Yorktown confining unit........................................................................................................................................................... 45

36. Pungo River confining unit...................................................................................................................................................... 46

37. Castle Hayne confining unit.................................................................................................................................................... 47

38. Beaufort confining unit............................................................................................................................................................ 48

39. Peedee confining unit............................................................................................................................................................... 49

40. Black Creek confining unit...................................................................................................................................................... 50

41. Upper Cape Fear confining unit............................................................................................................................................. 51

42. Lower Cape Fear confining unit............................................................................................................................................. 52

43. Lower Cretaceous confining unit........................................................................................................................................... 53

44. Schematic diagram illustrating aquifer or eonfining-unit pinchout as applied to model structure.............................................. 54

45-53. Maps showing model-derived leakance of the 

45. Yorktown confining unit........................................................................................................................................................... 55

46. Pungo River confining unit...................................................................................................................................................... 56

47. Castle Hayne confining unit.................................................................................................................................................... 57

48. Beaufort confining unit............................................................................................................................................................ 58

49. Peedee confining unit............................................................................................................................................................... 59

50. Black Creek confining unit...................................................................................................................................................... 60

51. Upper Cape Fear confining unit............................................................................................................................................. 61

52. Lower Cape Fear confining unit............................................................................................................................................. 62

53. Lower Cretaceous confining unit........................................................................................................................................... 63

54. Graph showing ground-water pumpage in the North Carolina Coastal Plain, 1900-80, for users of more than 100,000

gallons per day............................................................................................................................................................................... 64

55-63. Maps showing locations of withdrawals greater than 100,000 gallons per day in 1980, for the 

55. Surficial aquifer ............................................................................ 65

56. Yorktown aquifer.........................................................................................................................^ 66

57. Pungo River aquifer................................................................................................................................................................. 67

58. Castle Hayne aquifer.......................................................................................................................................................^ 68

59. Beaufort aquifer.........................................................................................«^ 69

60. Peedee aquifer..................................................................................................................................................................^ 70

61. Black Creek aquifer..................................................................» 71

62. Upper Cape Fear aquifer........................................................................................................................................................ 72

63. Lower Cape Fear aquifer........................................................................................................................................................ 73

64. Diagram showing method of computing heads at off-node locations................................................................................................ 76

65-69. Hydrographs showing observed and simulated water levels in the 

65. Yorktown and Pungo River aquifers..................................................................................................................................... 78

66. Castle Hayne aquifer at the Creswell observation well and DEHNR Arapahoe research station observation

weUu5........................................................................................................................................................» 79

67. Beaufort and Peedee aquifers................................................................................................................................................. 80

68. Black Creek aquifer at USGS observation wells NC-30 and NC-44............................................................................... 81

69. Upper Cape Fear and lower Cape Fear aquifers................................................................................................................ 82

70. Graphs showing head differences in Cretaceous sand aquifers due to changes in group 1 transmissivity, group 1

leakance, and model-wide storage coefficient........................................................................................................................... 87

71. Map showing simulated prepumping (1900) water table for the surficial aquifer........................................................................... 90
CONTENTS VII

	Page

FIGURES 72-80. Maps showing simulated prepumping (1900) potentiometric surface for the 

72. Yorktown aquifer......................................................................._ M91

73. Pungo River aquifer............................................................................................................................................................... 92

74. Castle Hayne aquifer.............................................................................................................................................................. 93

75. Beaufort aquifer...................................................................................................................................................................... 94

76. Peedee aquifer......................................................................................................................................................................... 95

77. Black Creek aquifer ............................................................... 96

78. Upper Cape Fear aquifer....................................................................................................................................................... 97

79. Lower Cape Fear aquifer...................................................................................................................................................... 98

80. Lower Cretaceous aquifer..................................................................................................................................................... 99

81. Map showing simulated 1980 water table for the surficial aquifer.................................................................................................... 100

82-90. Maps showing simulated 1980 potentiometric surface for the 

82. Yorktown aquifer.................................................................................................................................................................... 101

83. Pungo River aquifer ........................................................................ 102

84. Castle Hayne aquifer...................................................................._ 103

85. Beaufort aquifer...................................................................................................................................................................... 104

86. Peedee aquifer........................................................................._^^ 105

87. Black Creek aquifer......................................................._ 106

88. Upper Cape Fear aquifer....................................................................................................................................................... 107

89. Lower Cape Fear aquifer...................................................................................................................................................... 108

90. Lower Cretaceous aquifer..................................................................................................................................................... 109

91. Map showing simulated year 2000 water table for the surficial aquifer, assuming 3-percent annual increase in pumpage..... Ill

92-100. Maps showing simulated year 2000 potentiometric surface during 1980-2000, assuming 3-percent annual increase in 

pumpage, for the 

92. Yorktown aquifer.................................................................................................................................................................... 112

93. Pungo River aquifer............................................................................................................................................................... 113

94. Castle Hayne aquifer.............................................................................................................................................................. 114

95. Beaufort aquifer ..................................................................................................................................... 115

96. Peedee aquifer....................................................................._ 116

97. Black Creek aquifer .......................................................................................................................................................... 117

98. Upper Cape Fear aquifer....................................................................................................................................................... 118

99. Lower Cape Fear aquifer...................................................................................................................................................... 119

100. Lower Cretaceous aquifer..................................................................................................................................................... 120

101-109. Maps showing net vertical flow of water, simulated for predevelopment (1900) conditions, through the 

101. Yorktown confining unit......................................................................................................................................................... 121

102. Pungo River confining unit.................................................................................................................................................... 122

103. Castle Hayne confining unit.................................................................................................................................................. 123

104. Beaufort confining unit........................................................................................................................................................... 124

105. Peedee confining unit............................................................................................................................................................. 125

106. Black Creek confining unit.................................................................................................................................................... 126

107. Upper Cape Fear confining unit........................................................................................................................................... 127

108. Lower Cape Fear confining unit........................................................................................................................................... 128

109. Lower Cretaceous confining unit.......................................................................................................................................... 129

110-118. Maps showing net vertical flow of water, simulated for 1980 conditions, through the 

110. Yorktown confining unit......................................................................................................................................................... 130

111. Pungo River confining unit.................................................................................................................................................... 131

112. Castle Hayne confining unit.................................................................................................................................................. 132

113. Beaufort confining unit..................................................................» 133

114. Peedee confining unit............................................................................................................................................................. 134

115. Black Creek confining unit.................................................................................................................................................... 135

116. Upper Cape Fear confining unit........................................................................................................................................... 136

117. Lower Cape Fear confining unit........................................................................................................................................... 137

118. Lower Cretaceous confining unit.......................................................................................................................................... 138
VIII CONTENTS

TABLES

TABLE 1. Hydrogeologic units for North Carolina, South Carolina, and Virginia Coastal Plain ..................

2. North Carolina ground-water pumpage by aquifer in 1980 ...............................................................

3. Ranges of vertical hydraulic conductivity of confining units for the calibrated model..................

4. Summary of differences between computed and observed water levels for December 31,1980.

5. Range of perturbed values for transmissivity and leakance..............................................................

6. Range of perturbed values for storage coefficient ..............................................................................

7. Results of sensitivity analysis.................................................................................................................

8. Summary of model-computed water budgets for the ground-water flow system of the

	North Carolina Coastal Plain, 1900-2000..................................................................................

M8

74

75

77

84

84

85

CONVERSION FACTORS AND VERTICAL DATUM

Multiply inch-pound unit By To obtain SI unit

inch (in.)

foot (ft)

mile (mi)

foot per mile (ft/mi)

square foot (ft2) 

square mile (mi2)

cubic yard (yd3) 

acre-foot

cubic foot per second (ft3/s) 

cubic foot per second per square mile

gallon per day (gal/d) 

million gallons per day (Mgal/d)

foot per second (ft/s) 

mile per hour (mi/hr)

pound (Ib avoirdupois) 

ton (short, 2,000 Ibs)

Length

25.4

0.3048

1.609

0.1894

Area

0.0929

2.590

4,047

Volume

0.7646

1,233

Flow

0.02832

0.01093

3.785

0.04381

Velocity

0.3048

1.609

Mass

0.4536

0.9072

millimeter (mm)

meter (m)

kilometer (km)

meter per kilometer (m/km)

square meter (m2) 

square kilometer (km2) 

square meter (m2)

cubic meter (m3) 

cubic meter (m3)

cubic meter per second (m3/s) 

cubic meter per second per square

kilometer [(m3/s)/knr] 

liter per day (L/d) 

cubic meter per second (nrVs)

meter per second (m/s) 

kilometer per hour (km/hr)

kilogram (kg) 

metric ton

Sea level: In this report, "sea level" refers to the National Geodetic Vertical Datum of 1929 

(NGVD of 1929) a geodetic datum derived from a general adjustment of the first-order level nets of both 

the United States and Canada, formerly called Sea Level Datum of 1929.
REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHERN ATLANTIC COASTAL PIAIN

SIMULATION OF GROUND-WATER FLOW IN THE 

COASTAL PLAIN AQUIFER SYSTEM OF NORTH CAROLINA

By G.L. GIESE, J.L. EIMERS, and R.W. COBLE

ABSTRACT

A three-dimensional finite-difference digital model was used to 

simulate ground-water flow in the 25,000-square-mile aquifer system of 

the North Carolina Coastal Plain. The model was developed from a 

hydrogeologic framework that is based on an alternating sequence of 10 

aquifers and 9 confining units, which make up a seaward-thickening 

wedge of sediments that form the Coastal Plain aquifer system in the 

State of North Carolina.

The model was calibrated by comparing observed and simulated 

water levels. The model calibration was achieved by adjusting model 

parameters, primarily leakance of confining units and transmissivity of 

aquifers, until differences between observed and simulated water levels were within acceptable limits, generally within 15 feet. The maximum transmissivity of an individual aquifer in the calibrated model is 

200,000 feet squared per day in a part of the Castle Hayne aquifer, 

which consists predominantly of limestone. The maximum value for 

simulated vertical hydraulic conductivity in a confining unit was 2.5 feet 

per day, in a part of the confining unit overlying the upper Cape Fear 

aquifer. The minimum value was 4.1X10"6 feet per day, in part of the 

confining unit overlying the lower Cape Fear aquifer. Analysis indicated the model is highly sensitive to changes in transmissivity and 

leakance near pumping centers; away from pumping centers, the model 

is only slightly sensitive to changes in transmissivity but is moderately 

sensitive to changes in leakance.

Recharge from precipitation to the surficial aquifer ranges from 

about 12 inches per year in areas having clay at the surface to about 20 

inches per year in areas having sand at the surface. Most of this 

recharge moves laterally to streams, and only about 1 inch per year 

moves downward to the confined parts of the aquifer system. Under 

predevelopment conditions, the confined aquifers were generally 

recharged in updip interstream areas and discharged through streambeds and in downdip coastward areas. Hydrologic analysis of the flow 

system using the calibrated model indicated that, because of groundwater withdrawals, areas of ground-water recharge have expanded and 

encroached upon some major stream valleys and into coastal areas. 

Simulations of pumping conditions indicate that by 1980 large parts of 

the former coastal discharge areas had become areas of potential or 

actual recharge.

Declines of ground-water level, which are the result of water 

taken from storage, are extensive in some areas and minimal in others. 

Hydraulic head declines of more than 135 feet have occurred in the 

northern Coastal Plain since 1940 primarily due to withdrawals in the 

Franklin area in Virginia. Declines of ground-water levels greater than 

110 feet have occurred in aquifers in the central Coastal Plain due to 

combined effects of pumpage for public and industrial water supplies.

Water-level declines exceeding 100 feet have occurred in the Beaufort 

County area because of withdrawals for a mining operation and water 

supplies for a chemical plant. Head declines have been less than 10 feet 

in the shallow surficial and Yorktown aquifers and in the updip parts of 

the major confined aquifers distant from areas of major withdrawals. In 

1980, contribution from aquifer storage was 14 cubic feet per second, 

which is about 4.8 percent of pumpage and about 0.05 percent of 

ground-water recharge.

A water-budget analysis using the model simulations indicates 

that much of the water removed from the ground-water system by 

pumping ultimately is made up by a reduction in water leaving the 

aquifer system, which discharges to streams as base flow. The reduction 

in stream base flow was 294 cubic feet per second in 1980 and represents about 1.1 percent of the ground-water recharge. The net reduction to streamflow is not large, however, because most pumped ground 

water is eventually discharged to streams. In places, such as at rock 

quarries in Onslow and Craven Counties, water is lost from streams to 

recharge the water-table aquifer. In simulations for the period 1980- 

2000, assuming a 3-percent increase in pumpage per year since 1980, 

such induced infiltration increased about sevenfold from about 6 to 42 

cubic feet per second.

INTRODUCTION

The North Carolina Coastal Plain (fig. 1) covers an 

area of 25,000 square miles (mi2) in the eastern part of 

the State. This area includes about 47 percent of the 

State and encompasses all or parts of 47 counties.

The western boundary of the study area coincides 

roughly with the boundary between Fenneman's (1938) 

Coastal Plain and Piedmont Province and is delineated 

by the Fall Line. The northern and southern boundaries 

coincide with the Virginia and South Carolina borders, 

respectively. The eastern boundary, though not shown in 

figure 1, is defined as the seaward limit of ground water 

that has a dissolved chloride concentration of 10,000 milligrams per liter (mg/L).

The North Carolina Coastal Plain is an eastward-dipping and thickening sequence of sand, silt, clay, 

and limestone. Beds primarily consisting of sand or limestone compose aquifers, and beds largely consisting of

Ml
M2 REGIONAL AQUIFER-SYSTEM ANALYSIS NORTHERN ATLANTIC COASTAL PLAIN

k oG^xT' C a\V/< W ' Nt

y\ / /, I

Base modified from U.S. Geological Survey digital data 

1:2,000.000.1972

n i r 

0 50 100 KILOMETERS 34-V/

VEXPLANATION

Study area

Cope Hatteras

Boundary of physiographic province Physiographic 

boundaries adapted from Fenneman (1938), Stuckey 

(1965), and Heath (1980)

FIGURE 1. Study area of the North Carolina Coastal Plain aquifer system.

clay and silt are confining units. This sequence of aquifers and confining units is considered to function as a single aquifer system.

Water derived from aquifers of the North Carolina 

Coastal Plain aquifer system constitutes the major 

source of water for municipal and domestic water supplies; it is also an important source of water for industrial 

and agricultural purposes. Total ground-water withdrawal in the North Carolina Coastal Plain for 1980 is 

estimated at more than 250 million gallons per day (Mgal/ 

d). However, this pumpage of ground water, both in the 

North Carolina Coastal Plain and in adjacent States, has 

led to several hydrogeologic problems. These problems 

include lowering of ground-water levels in some areas, 

reduction of base flow to streams, creation of the potential for land subsidence, and movement of saltwater and 

contaminated water into the areas used for water 

supplies.

Effective planning for use of ground water in the 

Coastal Plain is needed to provide for future watersupply needs while minimizing potentially adverse 

effects of this development. Such planning requires a 

detailed qualitative and quantitative understanding of 

the functioning of the ground-water flow system and the 

response of the aquifer system to existing and proposed 

pumping stresses. To help develop such understanding, a 

subregional investigation of the aquifer system of the 

North Carolina Coastal Plain was begun in 1979 as part 

of a larger regional study of the Northern Atlantic

Coastal Plain aquifer system (fig. 2). Both studies 

are part of the U.S. Geological Survey's Regional 

Aquifer-System Analysis (RASA) program. The larger 

regional RASA study covers about 50,000 mi2 of the 

Coastal Plain areas of New York, New Jersey, Delaware, 

Maryland, Virginia, and North Carolina. Other subregional RASA studies were conducted in each of the 

other States, and all were coordinated at State lines. The 

broad objectives for all of the studies were similar to 

assemble geologic, hydrologic, and geochemical information, to analyze and develop an understanding of the 

aquifer system, and to develop capabilities that will be 

used to aid in the effective management of ground-water 

resources.

The final results of the Northern Atlantic Coastal 

Plain RASA study are described in U.S. Geological Survey Professional Paper 1404. This report is chapter M of 

Professional Paper 1404. Other chapters that relate 

directly to this chapter are listed below: 

Chapter A. "The Regional Aquifer System Underlying 

the Northern Atlantic Coastal Plain in 

Parts of North Carolina, Virginia, Maryland, Delaware, New Jersey, and New 

York Summary."

Chapter D. "The Occurrence and Geochemistry of Salty 

Ground Water in the Northern Atlantic 

Coastal Plain."

Chapter F. "Conceptualization and Analysis of GroundWater Flow System in the Coastal Plain
INTRODUCTION M3

80° 79° 78° 77° 76° 75° 74° 73° 72°

41 c

40C

39°

0

0

*

^

*

38°

37°

36°

35°

34°

NORTH \«* CAROLINA

SOUTH 

_ CAROLINA

EXPLANATION

Northern Atlantic Coastal 

Plain aquifer system

Axis of uplift Central arrow 

points in direction of plunge

Physiographic boundary

50 100 MILES

I ' 

0 50 100 KILOMETERS

Base modified from U.S. Geological Survey digital data, 

1.2.000.000.1972

FIGURE 2. The Northern Atlantic Coastal Plain study area (modified from Meisler, 1986).
M4 REGIONAL AQUIFER-SYSTEM ANALYSIS NORTHERN ATLANTIC COASTAL PLAIN

[Aquifers] of Virginia and Adjacent Parts 

of Maryland and North Carolina."

Chapter G. "Hydrogeologic Framework of the Northern 

Atlantic Coastal Plain in Parts of North 

Carolina, Virginia, Maryland, Delaware, 

New Jersey, and New York."

Chapter I. "Hydrogeologic Framework of the North 

Carolina Coastal Plain."

Chapter K. "Geohydrology and Simulation of GroundWater Flow in the Northern Atlantic 

Coastal Plain Aquifer System."

Chapter L. "Geochemistry of the Northern Atlantic 

Coastal Plain Aquifer System."

PURPOSE AND SCOPE

The purpose of this report is to describe the simulation of ground-water flow in the Coastal Plain of North 

Carolina for conditions prior to and after development of 

ground water. A finite-difference ground-water flow 

model was applied to 10 aquifers and intervening confining units, each of which covers all or part of the State's 

25,000-mi2 Coastal Plain. Discussions include modeling 

procedures and boundary conditions, calibration, sensitivity analysis, steady state for prepumping conditions, 

and transient simulations for 1900-80 conditions.

Results of the simulations are discussed with 

respect to changes in ground-water flow as shown by 

changes in the water budget, potentiometric surface of 

each aquifer, and direction of ground-water flow vertically through the confining units. Results are also presented for a simulation of the year 2000 using an assumed 

constant increase in pumping of 3 percent per year from 

1980 to 2000 at all 1980 pumping locations.

METHODS OF INVESTIGATION

This study was conducted in three stages. The first 

stage consisted of the development of a conceptual model 

of the ground-water flow system for the North Carolina 

Coastal Plain. This conceptual model is based on the 

hydrogeologic framework of Winner and Coble (1989, 

1996), estimates of aquifer and confining-unit properties, 

and general concepts as to the functioning of the system including patterns of ground-water movement, 

nature of the system boundaries, and rates and locations 

of recharge and discharge.

The second stage of the study involved the mathematical representation of the aquifer system. Elements 

of the conceptual model of the aquifer system were translated into numerical data sets suitable for manipulation 

in a mathematical-numerical model, which is a combination of a system of mathematical equations representing 

the behavior of the ground-water flow system and

numerical procedures for solving the equations. A computer program described by Trescott (1975) and modified 

by Leahy (1982) was used to solve the governing equations of ground-water flow.

Simulated water levels resulting from model trials 

were compared with measured levels during the period 

1900-SO to determine how realistically the digital model 

reproduced actual water levels. Model parameters were 

varied in order to improve the match between the 

observed and simulated water levels and to reduce 

errors in parameter estimates. When the model reproduced observed water levels within an acceptable range, 

the model was considered to be calibrated. For each trial 

set of model parameters, a series of three simulations 

was made (discussed in detail in the "Model Calibration" 

section of this report).

A sensitivity analysis was then performed on the calibrated model to determine the general degree of 

improvement of model parameter values as a result of 

the model calibration. The analysis consisted of evaluating the effect on model results caused by changing each 

of the model parameters. If model results showed little 

variation over a broad range of variation of a given 

parameter, then the model was said to be insensitive to 

that parameter, and it is not probable that the calibration 

process improved the estimates of the parameter. Conversely, if model results showed great sensitivity to a 

given parameter, then changes made in that parameter 

to improve agreement of model results with observed 

water levels likely resulted in improved estimates of that 

parameter. A computer printout of the model parameters 

used in the calibrated model is on file at the District 

Office, U.S. Geological Survey, P.O. Box 30728, 

Raleigh, NC 27607.

The third stage of the study was hydrologic analysis 

of the aquifer system for predevelopment and development conditions. The analysis involved comparison of 

water levels in aquifers and vertical flow through confining units for the years 1900 (predevelopment) and 1980 

(development). A predictive analysis was also done for 

the year 2000, assuming a 3-percent annual increase in 

pumping from 1980.

These three stages were largely, but not entirely, 

consecutive in time. For example, work done in digital 

model calibration at times revealed a need to revise previous concepts of the hydrogeologic framework. This, in 

turn, led to a revised digital model and affected the 

results of the hydrologic analysis.

PREVIOUS INVESTIGATIONS

Winner and Coble (1989, 1996) described the hydrogeologic framework for the North Carolina Coastal Plain, 

which is used and briefly described in this report. Sev-
CONCEPTUALIZATION OF THE AQUIFER SYSTEM M5

eral of the previous hydrogeologic investigations that 

have contributed significantly to knowledge of the hydrogeology of North Carolina Coastal Plain sediments are 

mentioned below; however, the reader is referred to the 

Winner and Coble reports for a more complete review of 

these investigations.

Clark and others (1912) reported on the first comprehensive survey of the geology and ground-water 

resources of the North Carolina Coastal Plain. LeGrand 

(1964) presented a broad review of the hydrogeology of 

the Atlantic and Gulf Coastal Plains and discussed important hydrologic concepts related to the functioning of the 

ground-water reservoir throughout this large region. 

Brown and others (1972) identified 17 chronostratigraphic units that are more or less continuous throughout the Coastal Plain from New York to North Carolina. 

On the basis of geophysical logs, driller logs, and well cuttings, lithofacies and indices of intrinsic permeability for 

each unit were mapped and estimated.

Other discussions of ground-water hydrology of multicounty areas in the North Carolina Coastal Plain are 

contained in Mundorff (1946), Brown (1959), LeGrand 

(1960), Schipf (1961), Nelson (1964, 1976), Blankenship

(1965), Nelson and Barksdale (1965), Harris and Wilder

(1966), DeWiest and others (1967), Floyd and Peace 

(1974), Peek (1977), and Narkunas (1980); these are listed 

in the "Selected References" section at the end of this 

report.

Publications describing simulation of ground-water 

flow in the Coastal Plain of North Carolina are limited. 

Cosner (1975) simulated ground-water flow in the Lower 

Cretaceous aquifer in southeastern Virginia and parts of 

northeastern North Carolina, utilizing a finite-difference 

model. Sherwani (1973) developed a two-dimensional 

finite-difference model of the Castle Hayne aquifer in the 

Lee Creek area in Beaufort County, primarily as a tool 

for evaluating the effects of dewatering operations for 

phosphate mining on the potentiometric surface. LayneWestern Company (1983) developed a steady-state electric-analog model to simulate flow in the Cretaceous 

aquifer system of southeastern Virginia and northeastern North Carolina.

ACKNOWLEDGMENTS

Data relating to the development of the hydrogeologic framework and conceptualization of the flow system 

were furnished by personnel in the Groundwater Section, Division of Environmental Management, in the 

North Carolina Department of Environment, Health, 

and Natural Resources (DEHNR). The specific individuals deserving special thanks are Perry F. Nelson, William 

Jeter, Carl Bailey, Richard Shiver, Edward Berry, William Bright, and L.A. Register.

CONCEPTUALIZATION OF THE 

AQUIFER SYSTEM

In this report, the conceptual model of the 

ground-water flow system of the North Carolina Coastal 

Plain is presented in a partly qualitative fashion. General 

ideas are discussed concerning the functioning of the 

aquifer system, particularly the ways recharge and discharge occur, how water moves through the system, an 

understanding of the physical environment through 

which flow occurs, and the nature of the system boundaries.

The source of water for the aquifer system is 

recharge from precipitation to the surficial aquifer, 

which, for the most part, is under water-table conditions. 

Figure 3 illustrates a generalized water budget for a 

typical location in the North Carolina Coastal Plain 

(Wilder and others, 1978). The budget shown in figure 3 

represents predevelopment (steady-state) conditions in 

which water entering the aquifer system is equal to the 

water leaving it. Precipitation is about 50 inches per year 

(in/yr). Of this amount, about 33 inches (in.) is lost 

through evapotranspiration, about 5 in. is lost through 

overland runoff, and about 12 in. infiltrates to the water 

table (fig. 3). Of this 12 in., about 11 in. moves laterally to 

streams. In most places, about 1 in. or less moves vertically down through confining units to the confined 

ground-water flow system. Water generally moves down 

into the confined aquifers in interstream areas. In downdip coastward areas (fig. 4), it generally moves up from 

the confined aquifers into streambeds of large rivers and 

into sounds and the ocean.

For modeling purposes, flow through confining units 

is considered to be primarily vertical, as shown in figure 

4, and flow through aquifers is considered to be horizontal. The unconsolidated Coastal Plain aquifer system is 

underlain by crystalline basement rocks of low permeability. It is assumed that there is little significant 

exchange of water between the basement rocks and the 

Coastal Plain aquifers; therefore, the top of the basement 

rocks is considered as a no-flow boundary for the aquifer 

system.

Fresh ground water in Coastal Plain aquifers usually grades into saltwater near the coastline. At some 

point in each aquifer, the effect of saltwater density balances the freshwater head potential so that 

ground-water movement becomes insignificant. In this 

study, this is assumed to occur at the point where the 

chloride concentration in ground water is 10,000 mg/L. 

The seaward limit of ground water containing less than 

10,000 mg/L of chloride, as given by Meisler (1981), is 

considered as a no-flow boundary.

The hydrogeologic framework of Winner and Coble 

(1989,1996) for the Coastal Plain of North Carolina, out-
M6 REGIONAL AQUIFER-SYSTEM ANALYSIS NORTHERN ATLANTIC COASTAL PLAIN

TOTAL PRECIPITATION 

50 INCHES 

PER YEAR

EVAPOTRANSPIRATION 

33 INCHES 

PER YEAR

OVERLAND RUNOFF

TO STREAMS

5 INCHES

PER YEAR

LAND \ 

SURFACE\

GROUND-WATER SEEPAGE

TO STREAMS 

UNCONFINED SURFICIAL AQUIFER 11 INCHES PER YEAR

SEEPAGE TO LARGE RIVERS,

SOUNDS, OR OCEAN

1 INCH PER YEAR

CONFINING UNIT

CONFINED AQUIFER

CONFINING UNIT

DEEP PERCOLATION 

1 INCH PER YEAR

FIGURE 3. Typical annual water budget for the North Carolina Coastal Plain hydrogeologic 

system (modified from Wilder and others, 1978).
CONCEPTUALIZATION OF THE AQUIFER SYSTEM M7

-General area of recharge- General area of dischargeetr/c surface of confined aquifer 

Sea level

NOT TO SCALE

FIGURE 4. Idealized hydrogeologic cross section showing features of the ground-water flow system. Arrows indicate general direction of

ground-water flow.
M8 REGIONAL AQUIFER-SYSTEM ANALYSIS NORTHERN ATLANTIC COASTAL PLAIN

lined in the following section of the report, served as the 

primary basis for the initial numerical representation of 

the flow system in the digital model. Other aspects of the 

conceptual model of the flow system, including recharge 

and discharge, movement of water through the system, 

and the nature of the system boundaries are discussed 

later in the report as they relate to the digital flow 

model.

HYDROGEOLOGIC FRAMEWORK

The hydrogeologic framework presented by Winner 

and Coble (1989, 1996) consists of 10 aquifers and 9 confining units (table 1). However, these hydrogeologic units 

do not occur everywhere in the North Carolina Coastal 

Plain. Equivalent hydrogeologic units for neighboring 

States are also shown in table 1, although some of these

TABLE 1. Hydrogeologic units for North Carolina, South Carolina, and Virginia Coastal Plain 

[Modified from Winner and Coble, 1989,1996]

Virginia hydrogeologic units 1

Columbia aquifer

Yorktown confining unit

Yorktown-Eastover aquifer

St. Marys confining unit

St. Marys-Choptank aquifer

Calvert confining unit

Chickahominy-Piney Point aquifer

Nanjemoy-Marlboro Clay confining unit

Aquia aquifer

Brightseat confining unit4

Brightseat aquifer4

Hydrogeologic units equivalent to North 

Carolina units not present in Virginia

Virginia Beach confining unit5

Virginia Beach aquifer5

Upper Potomac confining unit

Upper Potomac aquifer

Middle Potomac confining unit

Middle Potomac aquifer

Lower Potomac confining unit

Lower Potomac aquifer

North Carolina

Hydrogeologic units

Surficial aquifer

Yorktown confining unit

Yorktown aquifer

Pungo River confining unit

Pungo River aquifer

Castle Hayne confining unit

Castle Hayne aquifer

Beaufort confining unit

Beaufort aquifer

Peedee confining unit

Peedee aquifer

Black Creek confining unit

Black Creek aquifer

Upper Cape Fear confining unit

Upper Cape Fear aquifer

Lower Cape Fear confining unit

Lower Cape Fear aquifer

Lower Cretaceous confining unit6

Lower Cretaceous aquifer6

Model layer 

number

A10

CU9

A9

CU8

A8

CUT

A7

CU6

A6

CUB

A5

CU4

A4

CU3

A3

CU2

A2

GUI

Al

South Carolina hydrogeologic units2

Surficial aquifer

Hydrogeologic units equivalent to 

North Carolina units not present 

in northeastern South Carolina3

Black Creek aquifer

Unnamed confining unit

Middendorf aquifer

Unnamed confining unit

Cape Fear aquifer

'Meng and Harsh (1988).

Southeastern Coastal Plain aquifer system (Aucott and others, 1987).

3Tertiary equivalents (Floridan and Tertiary sand aquifers) are present in the central and southern South Carolina Coastal Plain (Aucott and others, 1987).

4 Restricted to northern Virginia; not present along North Carolina-Virginia boundary.

5 Hamilton and Larson (1988).

6Restricted to northern North Carolina; not present along North Carolina-South Carolina boundary.
HYDROGEOLOGIC FRAMEWORK M9

0 5 10 KILOMETERS

VERTICAL SCALE GREATLY EXAGGERATED

FIGURE 5. Hydrogeologic section A-A from Richmond County to Brunswick County (modified from Winner and Coble, 1989).

are not present at the respective borders with North 

Carolina.

For the convenience of constructing a digital model 

of the ground-water flow system, the hydrogeologic units 

are sometimes abbreviated using a letter and number as 

shown in table 1. Aquifers (A) are numbered from the 

lowest unit upward; confining unit (CU) designations are 

taken from the aquifers they overlie. Thus, aquifer Al is 

the lowermost aquifer, and the confining unit overlying it 

is GUI. These designations are referred to frequently in 

this report.

Selected hydrogeologic sections were adapted from 

Winner and Coble (1989) to show the distribution of aquifers and confining units throughout the North Carolina 

Coastal Plain (figs. 5-9). The idealized sections show the 

thickening, thinning, and pinchouts typical of these 

hydrogeologic units in the study area. Confining units 

are considered to terminate at the limit of the underlying 

aquifer. These relations are built into the modeling process, and the reader is referred to Winner and Coble

(1989) for detailed geologic and hydrologic descriptions. 

Brief descriptions of each hydrogeologic unit are presented in this section.

SURFICIAL AQUIFER

The surficial aquifer (A10) overlies all of the North 

Carolina Coastal Plain (fig. 1) and consists of fine sand, 

silt, clay, shell, soil, residuum, and peat beds. Scattered 

deposits of coarser grained sediments in the unit occur in 

relict beach ridges or in alluvium. Throughout the western and central parts of the Coastal Plain, the thickness 

of the surficial aquifer ranges from a few feet to about 30 

feet (ft); however, the aquifer thickens eastward and is 

more than 200 ft thick in the vicinity of the Outer Banks. 

The sediments of the surficial aquifer are primarily of 

post-Yorktown age, but are not restricted to a single geologic unit in terms of age or lithology (Winner and Coble, 

1989).
M10 REGIONAL AQUIFER-SYSTEM ANALYSIS NORTHERN ATLANTIC COASTAL PLAIN

0 50 100 MILES 

0 50 100 KILOMETERS

Yorktown B'

i Pungo River confining

Surficial aquifer (A10) §ik confining unit (CU9) 

Castle Hayne confining unit (CU7) '-gi,1 Pungo River unit(CU8) Yorktown \ 

f\_ rSib aquifer (A8) \ aquifer (A9)

1.700

FIGURE 6. Hydrogeologic section B-B' from Johnston County to Pamlico County (modified from Winner and Coble, 1989).
HYDROGEOLOGIC FRAMEWORK Mil

0 50 100 KILOMETERS

C'

FIGURE 7. Hydrogeologic section C-C' from Southampton County, Va., to Currituck County, N.C. 

(modified from Winner and Coble, 1989).
M12 REGIONAL AQUIFER-SYSTEM ANALYSIS NORTHERN ATLANTIC COASTAL PLAIN

0 50 100 KILOMETERS

D 5 10 KILOMETERS - 

I-' > VERTICAL SCALE GREATLY EXAGGERATED

FIGURE 8. Hydrogeologic section D-Lf from Brunswick County to Onslow County (modified from Winner and

Coble, 1989).
NORTH

CAROLINA

0 50 100 MILES

0 50 100 KILOMETERS

Land surface Yorkt9wn confining unit (CJJ9) ^Surficial aquifer (A10) Surficial aquifer (A10)

Castle Hayne confining unit (CU7)

Castle Hayne aquifer (A7) Pungo River aquifer (A8)

Beaufort confining unit (CU6)

Peedee aquifer (A5) Beaufort aquifer (A6)

Peedee confining unit (CU5)

Black Creek aquifer (A4)

Upper Cape Fear confining unit (CU3)

pe Fear aquifer (A3)

Lower Cape Fear confining unit <CU2)

0 5 10 KILOMETERS . _ x _. - x ; VERTICAL SCALE GREATLY EXAGGERATED

Pd 

O f* O Ol tO

H

I

FIGURE 9. Hydrogeologic section //-£>" from Onslow County to Chowan County (modified from Winner and Coble, 1989).
M14 REGIONAL AQUIFER-SYSTEM ANALYSIS NORTHERN ATLANTIC COASTAL PLAIN

The surficial aquifer (A10) directly overlies most of 

the confined aquifers at one place or another and 

exchanges water with them either directly or through an 

intervening confining bed. The surficial aquifer receives 

direct recharge from precipitation and is the source of 

water for the deeper confined aquifers and base flow to 

streams. The amount of recharge from precipitation varies areally from about 12 to 20 in/yr, depending on the 

clay content of the soils. Winner and Coble (1989; 1996, 

table 4) estimated the average horizontal hydraulic conductivity of the surficial aquifer to be 29 feet per day 

(ft/d). A more detailed description of movement of water 

within the surficial aquifer is given in the section entitled 

"Model Input."

YORKTOWN AQUIFER AND OVERLYING CONFINING UNIT

The Yorktown aquifer (A9) is equated with the older 

beds of the Pliocene Yorktown Formation of Clark and 

others (1912) and extends throughout the northern half 

of the Coastal Plain (fig. 10) from the Fall Line, where it 

overlies crystalline rocks similar to those in the Piedmont, eastward to beyond the coast. The Yorktown aquifer largely consists of fine sand, silty and clayey sand, 

sand with shells and shell beds, some limestone, and 

some coarse sand beds. In the western Coastal Plain, the 

aquifer is relatively thin, less than 20 ft thick in many 

places, and has been cut into or eroded away by the 

larger streams flowing across the area. In Dare County, 

the Yorktown aquifer attains its maximum thickness of 

more than 300 ft.

The Yorktown aquifer (A9) does not extend into the 

southern half of the Coastal Plain, except for a small area 

in Robeson County (fig. 10), which is the largest of a 

number of outliers shown by Brown and others (1972, 

pi. 21). Figure 10 also shows the areal extent of aquifers 

that underlie the Yorktown aquifer. The surficial aquifer 

(A10) overlies the Yorktown aquifer everywhere.

The estimated horizontal hydraulic conductivity of 

the Yorktown aquifer ranges from 19 to 33 ft/d and averages about 22 ft/d, based on aquifer tests and lithologicand geophysical-log data from 52 wells and test holes 

(Winner and Coble, 1989,1996).

The Yorktown confining unit (CU9) overlying the 

Yorktown aquifer consists of the youngest clay beds of 

the Yorktown Formation in most places, but locally may 

be clay beds of Pleistocene or Holocene age. Its thickness 

averages about 25 ft, ranging from less than 10 ft up to 

70 ft thick. It is composed largely of clay and sandy clay 

and local beds of fine sand or shell. The Yorktown confining unit generally is considered to extend only as far as 

the Yorktown aquifer, even though stratigraphically 

equivalent beds may continue beyond the aquifer limits.

PUNGO RIVER AQUIFER AND OVERLYING CONFINING UNIT

The Pungo River aquifer (A8) consists of the permeable part of the Pungo River Formation of early and middle Miocene age, described in detail by Kimrey (1965). 

The Pungo River aquifer is composed of fine to medium 

marine sands having considerable phosphate content. 

The average horizontal hydraulic conductivity, based on 

analysis of geophysical logs, is estimated to be 33 ft/d, 

according to Winner and Coble (1989). Shells and other 

fossils are present throughout the aquifer; occasionally, 

beds of limestone and coarse sand are found.

The Pungo River aquifer (A8) is thinnest near its 

western and northern limits, where its thickness averages about 15 ft. The aquifer dips eastward and thickens 

to more than 200 ft in the vicinity of the Outer Banks, 

where the top is deeper than 700 ft below sea level. The 

aquifer is overlain everywhere by aquifers A9 or A10 

(fig. 11), except where it is exposed in an open-pit phosphate mine in Beaufort County. The Pungo River aquifer 

is underlain everywhere by the Castle Hayne aquifer 

(A7).

The Pungo River confining unit (CU8) is formed by 

the upper clay beds of the Pungo River Formation and 

contiguous clays of the lowermost Yorktown Formation. 

The confining unit ranges in thickness from less than 10 

ft near the western margin to about 150 ft beneath Currituck County, and the average thickness is nearly 55 ft. 

For most of the area, the confining unit is composed of 

nearly uniform clay containing less than 10 percent sand.

CASTLE HAYNE AQUIFER AND 

OVERLYING CONFINING UNIT

The Castle Hayne aquifer (A7) is delineated as those 

calcareous sediments of Eocene age that are equated 

with the Castle Hayne Limestone and the Trent Formation of former usage of Clark and others (1912). Also part 

of this aquifer are the rocks of Oligocene age, now designated River Bend Formation, overlying the Castle 

Hayne (Brown and others, 1972), which are lithologically 

identical and hydraulically connected to the Castle 

Hayne Limestone. The basal part of the aquifer may consist of older contiguous permeable units in local areas. 

The areal extent of this aquifer is shown in figure 12, 

which also shows the extent of overlying aquifers.

The Castle Hayne aquifer (A7) consists of limestone, 

sand, and minor amounts of clay deposited under marine 

conditions. Limestone may occur as shell limestone, dolomitic limestone, and sandy limestone ranging from 

loosely consolidated to hard and recrystallized. Along the 

western margin, the aquifer occurs near land surface 

from New Hanover County to Craven County. Eastward, 

the aquifer thickens to more than 950 ft in Carteret 

County and to nearly 1,200 ft beneath Cape Hatteras 

(Brown, 1958, fig. 4). In the area north of Albemarle
HYDROGEOLOGIC FRAMEWORK M15

79° 77°

NORTH. 

^ I CARJOUNA

7

VIRGINIA

-*£& v^**4

1

i

^

\-£

^>

^

SOUTH "V"? 

CAROLINA \^

#

^

c

Cape Fear f

\ /

T

, /

0 50 MILES

^a y

50 KILOMETERS

Base modified from U.S. Geological Survey digital data.

1:2.000.000.1972

EXPLANATION

Yorktown aquifer not present

Limit and designation of aquifer and confining unit, 

or basement rock, directly underlying the 

Yorktown aquifer

A8 Pungo River aquifer and confining unit A4^ Black Creek aquifer and confining unit

A7 Castle Hayne aquifer and confining unit A3 Upper Cape Fear aquifer and confining unit

 ~^A6. Beaufort aquifer and confining unit h^. ^ Lower Cape Fear aquifer and confining unit

A5 Peedee aquifer and confining unit - ' B /] Basement rock

FIGURE 10. Areal extent of the Yorktown aquifer (A9) and underlying aquifers or basement rock (modified from Winner and Coble, 1989).
M16 REGIONAL AQUIFER-SYSTEM ANALYSIS NORTHERN ATLANTIC COASTAL PLAIN

Base modified from U S. Geological Survey digital data, 

1:2,000,000,1972

EXPLANATION

Pungo River aquifer not present

 Limit and designation of aquifer and confining unit 

directly overlying the Pungo River aquifer

A10 Surficial aquifer 

A9 Yorktown aquifer and confining unit 

FIGURE 11. Areal extent of the Pungo River aquifer (A8) and overlying aquifers (modified from Winner and Coble, 1989).
HYDROGEOLOGIC FRAMEWORK M17

NORTH

CARbLfN'A \

! > " V

34°-

___I__________________^______I 

Base modified from U S Geological Survey digital data.

1:2,000,000.1972

EXPLANATION

Castle Hayne aquifer not present

 Limit and designation of aquifer and confining unit 

directly overlying the Castle Hayne aquifer

A10 Surficial aquifer

A9 j Yorktown aquifer and confining unit

A8 Pungo River aquifer and confining unit

FIGURE 12. Areal extent of the Castle Hayne aquifer (A7) and overlying aquifers (modified from Winner and Coble, 1989).
M18 REGIONAL AQUIFER-SYSTEM ANALYSIS NORTHERN ATLANTIC COASTAL PLAIN

Sound, limestone beds are thin to nonexistent, and the 

sediments contain more clay. The thickness of the unit 

averages about 50 ft between Bertie and Currituck 

Counties but reaches a maximum of 164 ft in Currituck 

County.

The Castle Hayne aquifer (A7) is the most productive aquifer in North Carolina due to its thickness and 

high percentage of permeable limestone and sand. On the 

basis of aquifer tests and lithologic- and geophysical-log 

data, the hydraulic conductivity of the Castle Hayne 

aquifer ranges from about 15 ft/d, where it is composed of 

fine sand, to about 200 ft/d, where the bulk of the aquifer 

is porous limestone.

The thickness of the Castle Hayne confining unit 

(CUT) averages only about 10 ft; it exceeds 25 ft only in 

Gates County along the Virginia border, in eastern Pamlico and Carteret Counties, and in two small areas along 

the western limit of the Castle Hayne aquifer. The confining unit consists of beds of clay, sandy clay, and clay 

with sandy streaks that are part of the Pungo River Formation, the Yorktown Formation, or younger clays. The 

confining unit is missing in several stream valleys south 

of Craven County and in two areas in the northeastern 

Coastal Plain. In addition to being thinner than most of 

the other confining units, CUT contains more sand; thus, 

it is relatively permeable and allows significant vertical 

leakage between the Castle Hayne aquifer and overlying 

aquifers.

Throughout most of their northern and eastern 

areas (fig. 12), the Castle Hayne aquifer (AT) and confining unit (CUT) are directly overlain by the Pungo River 

aquifer (A8), whereas to the south, the Castle Hayne 

aquifer is overlain by the Yorktown and surficial aquifers 

(A9 and A10). The Castle Hayne aquifer is directly 

underlain by the Beaufort aquifer (A6) and confining unit 

(CU6) northeast of Jones and Onslow Counties and by 

the Peedee aquifer (A5) and confining unit (CU5) southwest of these counties (fig. 13).

BEAUFORT AQUIFER AND OVERLYING CONFINING UNIT

The dark-green and gray sands and clays that were 

identified as the Beaufort Formation of Paleocene age by 

Brown (1959) and later identified as rocks of Midwayan 

age by Brown and others (19T2) make up the bulk of the 

Beaufort aquifer (A6), the extent of which is shown in 

figure 14. As with the other hydrogeologic units, the definition of this aquifer was not restricted to a single geologic formation; the Beaufort aquifer may include parts 

of rock units that are older than the Beaufort Formation 

(Winner and Coble, 1989,1996).

The Beaufort aquifer (A6) consists of fine to medium 

glauconitic sands, clayey sands, and clay beds of marine 

origin. Shell and limestone beds are present but are less 

than 6 ft thick. The thickness of the Beaufort aquifer

ranges from less than 10 ft along its western limit to 

more than 150 ft in the northern part of its eastern limit. 

In Camden and Currituck Counties, the aquifer thins 

toward the east or northeast. The average horizontal 

hydraulic conductivity of the aquifer was estimated to be 

about 35 ft/d, on the basis of lithologic and geophysical 

logs; lower than average values occurred in the northern 

and easternmost parts of the aquifer, where the sand is 

finer and the aquifer contains more clay.

The Beaufort confining unit (CU6) consists of the 

uppermost sediments of the Beaufort Formation and 

possibly some younger clay, silt, and sandy clay. 

Throughout most of the area, the confining unit shows a 

gradation from sandy clay to clay, but in places it contains distinct clay beds interlayered with fine sand or silt. 

The thickness of the confining unit ranges from zero to 

80 ft and averages about 24 ft. The thicker parts of the 

confining unit are in and northeast of Washington 

County.

The Beaufort aquifer (A6) is covered entirely by 

younger rocks (fig. 14). About 90 percent of the Beaufort 

aquifer and confining unit are overlain by the Castle 

Hayne aquifer (AT). The remainder is overlain by the 

Yorktown aquifer (A9) from Lenoir County northward 

(fig. 14). The upper Cape Fear, the Black Creek, and the 

Peedee aquifers and confining units underlie the Beaufort aquifer. The Peedee aquifer and confining unit (A5 

and CU5) underlie about 80 percent of the area of the 

Beaufort aquifer (fig. 15).

PEEDEE AQUIFER AND OVERLYING CONFINING UNIT

The Peedee aquifer (A5) is named for the Upper 

Cretaceous Peedee Sand of Clark and others (1912, p. 

145) and the Peedee Formation of Stephenson and Rathbun (1923, p. 11), of which the aquifer is largely composed. The areal extent of the Peedee aquifer and 

overlying units is shown in figure 16.

The Peedee aquifer (A5) consists of fine to medium 

sands interbedded with clays and silts. Thin beds of consolidated calcareous sandstone and impure limestone are 

interlayered among the sands in some places, particularly in the southeastern North Carolina Coastal Plain 

area. Shells are common throughout the unit. The top of 

the Peedee aquifer dips eastward at an average rate of 

about 24 feet per mile (ft/mi), but the dip varies from 

about 10 ft/mi in the western part to more than 32 ft/mi 

in the more deeply buried part along the coast. The aquifer thickness ranges from zero along its western limit to 

more than 300 ft along the coast from southern Onslow 

County to the South Carolina border. Northeast of 

Onslow County, the maximum thickness of the aquifer 

is less than 200 ft, and the eastern one-third of the unit 

contains saltwater. The average horizontal hydraulic 

conductivity of the Peedee aquifer was estimated from
HYDROGEOLOGIC FRAMEWORK M19

SOUTH X 1 

CAROLINA

Base modified from U.S. Geological Survey digital data. 

1.2.000.000.1972

EXPLANATION 

Castle Hayne aquifer not present

Limit and designation of aquifer and confining unit 

directly underlying the Castle Hayne aquifer

A6 . Beaufort aquifer and confining unit

A5 Peedee aquifer and confining unit

FIGURE 13. Areal extent of the Castle Hayne aquifer (A7) and underlying aquifers (modified from Winner and Coble, 1989).
M20 REGIONAL AQUIFER-SYSTEM ANALYSIS NORTHERN ATLANTIC COASTAL PLAIN

^\4K V1KU1IN1A

Base modified from U S. Geological Survey digital data. 

1-2,000,000,1972

EXPLANATION

Beaufort aquifer not present

 Limit and designation of aquifer and confining unit 

directly overlying the Beaufort aquifer

A9 Yorktown aquifer and confining unit 

A7 Castle Hayne aquifer and confining unit

FIGURE 14. Areal extent of the Beaufort aquifer (A6) and overlying aquifers (modified from Winner and Coble, 1989).
HYDROGEOLOGIC FRAMEWORK M21

Base modified from U.S. Geological Survey digital data, 

1-2,000,000,1972

EXPLANATION

Beaufort aquifer not present

Limit and designation of aquifer and confining unit 

directly underlying the Beaufort aquifer

_A5 _ Peedee aquifer and confining unit

A4 Black Creek aquifer and confining unit

A3 Upper Cape Fear aquifer and confining unit

FIGURE 15. Areal extent of the Beaufort aquifer (A6) and underlying aquifers (modified from Winner and Coble, 1989).
M22 REGIONAL AQUIFER-SYSTEM ANALYSIS NORTHERN ATLANTIC COASTAL PLAIN

34° -

 I i^_________L

Base modified from U.S. Geological Survey digital data 

1-2,000,000.1972

EXPLANATION 

Peedee aquifer not present

Limit and designation of aquifer and confining unit 

directly overlying the Peedee aquifer

A10 Surficial aquifer

A9 Yorktown aquifer and confining unit

A7 Castle Hayne aquifer and confining unit

A6 Beaufort aquifer and confining unit

FIGURE 16. Areal extent of the Peedee aquifer (A5) and overlying aquifers (modified from Winner and Coble, 1989).
HYDROGEOLOGIC FRAMEWORK M23

geophysical logs to be about 34 ft/d by Winner and Coble 

(1989).

The Peedee confining unit (CU5) is composed of clay, 

silty clay, and sandy clay. Winner and Coble (1989,1996) 

did not identify the confining unit with a particular geologic unit, but the unit consists primarily of sediments at 

the Cenozoic-Mesozoic boundary. The average thickness 

of the Peedee confining unit is about 25 ft. In the eastern 

part of the confining unit, thickness may reach 60 ft but 

commonly does not exceed 30 to 35 ft.

BLACK CREEK AQUIFER AND 

OVERLYING CONFINING UNIT

The Black Creek aquifer (A4) contains Upper Cretaceous sediments of both the Black Creek and underlying 

Middendorf Formations (Winner and Coble, 1989, 1996). 

The Black Creek Formation consists mainly of thinly 

laminated gray to black clay, interbedded with gray to 

tan sands. Outcrops also exhibit sand- or clay-dominated 

lenses. The Middendorf Formation consists mainly of fine 

to medium sand, interbedded with silty clay, coarser 

channel sand, and thinly laminated sand and clay. The 

horizontal hydraulic conductivity of the Black Creek 

aquifer, as estimated by Winner and Coble (1989, 1996), 

ranges from about 15 to 50 ft/d, based on geophysical logs 

and aquifer tests.

The Black Creek aquifer and confining unit (A4 and 

CU4) are overlain by the Peedee (A5 and CU5), Beaufort 

(A6 and CU6), and Yorktown (A9 and CU9) aquifers and 

confining units and by the surficial aquifer (A10) (fig. 17). 

The Peedee aquifer overlies the eastern two-thirds of the 

Black Creek aquifer and confining unit. The surficial 

aquifer overlies the Black Creek aquifer and confining 

unit from the Fall Line to the updip limit of the Peedee 

aquifer in the southern North Carolina Coastal Plain, 

except for a small area in Robeson County, where they 

are overlain by the Yorktown aquifer. The Yorktown and 

Beaufort aquifers overlie the Black Creek aquifer and 

confining unit along their western limits in the northern 

North Carolina Coastal Plain.

The Black Creek confining unit (CU4) is primarily 

composed of the uppermost beds of the Black Creek Formation and consists of clay, silty clay, and sandy clay. In 

the northern part of the North Carolina Coastal Plain, it 

may also contain clay of the lower parts of the Beaufort 

or Yorktown Formations. In the deeper subsurface, clay 

in the lower part of the Peedee Formation may also be a 

part of this confining unit. In the Sand Hills area (figs. 1, 

17), where the Black Creek aquifer (A4) is composed of 

the Middendorf Formation, confining unit CU4 consists 

of the uppermost clay of the Middendorf Formation. In 

the highly dissected Sand Hills, clay of the Middendorf 

Formation is cut through in many places by streams; 

thus, the Black Creek confining unit does not exist at

these locations. Farther east, the channels of larger 

streams, such as the Cape Fear and Neuse Rivers, also 

have cut through the confining unit to allow direct 

hydraulic connection between the streams and the Black 

Creek aquifer. The thickness of the confining unit averages about 45 ft, but it may be more than 165 ft thick in 

places in the eastern part of the Coastal Plain.

UPPER CAPE FEAR AQUIFER AND 

OVERLYING CONFINING UNIT

Winner and Coble (1989, 1996) recognized that the 

Upper Cretaceous Cape Fear Formation could be separated into two distinct hydrogeologic units, largely on 

the basis of effective confining beds between the upper 

and lower parts in Brunswick and adjacent counties near 

the South Carolina border. The sediments of the upper 

Cape Fear aquifer are alternating beds of sand and clay. 

The individual beds are commonly 3 to 5 ft thick but 

range up to 15 ft in thickness.

The upper Cape Fear aquifer (A3) varies in thickness from about 10 ft along its western edge to nearly 500 

ft in central Tyrrell County. The average thickness of the 

aquifer is slightly more than 100 ft. The aquifer is thickest beneath the Albemarle-Pamlico Peninsula east of 

Beaufort and Washington Counties. The horizontal 

hydraulic conductivity of the aquifer was estimated from 

geophysical logs to average about 30 ft/d, its values ranging from 10 to 70 ft/d (Winner and Coble, 1989).

As described by Winner and Coble (1989), the upper 

Cape Fear confining unit (CU3) consists of nearly continuous clay, silty clay, and sandy clay beds belonging either 

to the Middendorf Formation in the Sand Hills area or to 

the Black Creek Formation. The thickness of the confining unit averages about 48 ft (Winner and Coble, 1989) 

but may exceed 100 ft in places near the South Carolina 

border, near the southeastern corner of Duplin County, 

and in Dare County. Along the western limit of the upper 

Cape Fear aquifer (A3), the Cape Fear and Neuse Rivers 

have cut through the confining unit; the same is true 

along the Tar and Roanoke Rivers.

The upper Cape Fear aquifer and confining unit (A3 

and CU3) are overlain by the Black Creek aquifer (A4) in 

about 90 percent of their area (fig. 18). The Yorktown 

aquifer (A9) overlies the upper Cape Fear aquifer in the 

northwest, and the Beaufort aquifer (A6) overlies it in 

parts of Gates, Camden, and Hertford Counties. A small 

patch of the surficial aquifer (A10) overlies the upper 

Cape Fear aquifer and confining unit in Wayne, Wilson, 

and Johnston Counties. The lower Cape Fear aquifer and 

confining unit (A2 and CU2) underlie the upper Cape 

Fear aquifer (A3) in about three-fourths of its area (fig. 

19). Elsewhere, the upper Cape Fear aquifer is in contact 

with basement rocks.
M24 REGIONAL AQUIFER-SYSTEM ANALYSIS NORTHERN ATLANTIC COASTAL PLAIN

Base modified from U.S. Geological Survey digital data, 

1:2.000,000,1972

EXPLANATION 

Black Creek aquifer not present

 Limit and designation of aquifer and confining unit 

directly overlying the Black Creek aquifer

A10 Surficial aquifer

A9 Yorktown aquifer and confining unit

A6 Beaufort aquifer and confining unit

A5 Peedee aquifer and confining unit

FIGURE 17. Areal extent of the Black Creek aquifer (A4) and overlying aquifers (modified from Winner and Coble, 1989).
HYDROGEOLOGIC FRAMEWORK M25

35°-

J. NORTH

CAROLINA

Base modified from U S Geological Survey digital data, 

1.2.000.000.1972

EXPLANATION

Upper Cape Fear aquifer not present

Limit and designation of aquifer and confining unit 

directly overlying the upper Cape Fear aquifer

r

A10 Surficial aquifer 

A9 . Yorktown aquifer and confining unit 

- 1_A^TI| Beaufort aquifer and confining unit 

f _A4_-j Black Creek aquifer and confining unit 

FIGURE 18. Areal extent of the upper Cape Fear aquifer (A3) and overlying aquifers (modified from Winner and Coble, 1989).
M26 REGIONAL AQUIFER-SYSTEM ANALYSIS NORTHERN ATLANTIC COASTAL PLAIN

-t^f y\ vrf* - ^iL> 5

fc^o4^jc. -^

Base modified from U.S. Geological Survey digital data. 

1:2.000.000.1972

EXPLANATION

Upper Cape Fear aquifer not present

Limit and designation of aquifer and confining unit, 

or basement rock, directly underlying the upper 

Cape Fear aquifer

A2 Lower Cape Fear aquifer and confining unit 

\ /^ 

 .P - Basement rock

FIGURE 19. Areal extent of the upper Cape Fear aquifer (A3) and underlying aquifers or basement rock (modified from Winner and Coble, 1989).
SIMULATION OF GROUND-WATER FLOW M27

LOWER CAPE FEAR AQUIFER AND 

OVERLYING CONFINING UNIT

The lower Cape Fear aquifer (A2) consists mostly of 

older sand beds of the Cape Fear Formation, lithologically similar to those beds described for the upper Cape 

Fear aquifer (A3). In the southern North Carolina 

Coastal Plain, Winner and Coble (1989) viewed these sediments largely as a regressive phase of the Cape Fear 

Formation having a separate hydrologic identity from 

overlying Cape Fear sand units.

The lower Cape Fear aquifer (A2) strikes northeast 

and dips southwest at a rate of 15 to 35 ft/mi. Its extent is 

shown in figure 20. Its thickness ranges from a few feet 

along its western margin to more than 400 ft in the northeastern North Carolina Coastal Plain. The average 

hydraulic conductivity of the aquifer was estimated by 

Winner and Coble (1989, 1996) from geophysical logs to 

be about 34 ft/d. Individual values ranged from 20 to 75 

ft/d.

The lower Cape Fear confining unit (CU2) is composed of clay and sandy-clay beds that belong largely to 

the Cape Fear Formation. The average thickness of the 

confining unit is about 50 ft. The confining unit exceeds 

75 ft in thickness throughout the eastern quarter of the 

North Carolina Coastal Plain and in parts of Bertie and 

Halifax Counties and is more than 100 ft thick in parts or 

all of Pasquotank, Camden, Currituck, Columbus, and 

Brunswick Counties.

The lower Cape Fear aquifer and confining unit (A2 

and CU2) are overlain by the upper Cape Fear aquifer 

(A3) everywhere except for a small area near the Fall 

Line in Northampton County, where the Yorktown aquifer (A9) overlies them (fig. 20). The Lower Cretaceous 

aquifer and confining unit (Al and GUI) underlie the 

lower Cape Fear aquifer in the eastern half of the North 

Carolina Coastal Plain (fig. 21). Elsewhere, the lower 

Cape Fear aquifer lies on basement rocks.

LOWER CRETACEOUS AQUIFER AND 

OVERLYING CONFINING UNIT

Sediments below the Cape Fear Formation are 

regarded by most investigators as belonging to the 

Lower Cretaceous Series and possibly include older 

rocks (Winner and Coble, 1989, 1996). Thus, the name 

Lower Cretaceous aquifer (Al) was chosen to designate 

this hydrogeologic unit, which is the lowermost aquifer 

defined in this study. The extent of aquifer Al is shown in 

figure 21. Various investigators have established that the 

updip beds of the Lower Cretaceous aquifer are largely 

nonmarine in origin, but the incidence of beds of marine 

origin increases downdip toward the coast. The nonmarine beds are shales, sands, and gravel. Marine beds 

are chiefly limestones that may be sandy or dolomitic.

Data on horizontal hydraulic conductivity of the Lower 

Cretaceous aquifer are lacking in North Carolina, but 

Winner and Coble (1989) inferred from work by Brown 

and Cosner (1974) that the hydraulic conductivity of this 

unit nearby in Virginia ranged from 10 to 40 ft/d, and in 

North Carolina may range from 20 to 30 ft/d.

The thickness of the Lower Cretaceous aquifer (Al) 

ranges from about 15 ft near its western limit to more 

than 2,000 ft in Currituck County. The average thickness 

of the Lower Cretaceous aquifer is about 500 ft.

The Lower Cretaceous confining unit (GUI) consists 

of clay and sandy clay beds that are of Early Cretaceous 

or Late Cretaceous age. The thickness of the unit averages about 45 ft but is nearly 70 ft in Camden and Currituck Counties. The Lower Cretaceous aquifer and 

confining unit are overlain everywhere by the lower 

Cape Fear aquifer (A2) and are underlain everywhere by 

crystalline basement rocks (Winner and Coble, 1989, 

1996).

SIMULATION OF GROUND-WATER FLOW

EQUATION OF GROUND-WATER FLOW

The basic equation of ground-water flow used in this 

report is of the form

 3* T3* 3/ii

Jdh (1)

in which, using units of time (T) and length (L):

TJCX= principal component of the transmissivity tensor

in the x direction (L2?^1 ); 

Tyy= principal component of the transmissivity tensor

in the y direction (L2?^1 ); 

^zr= principal component of the hydraulic conductivity

tensor in the z direction (LT~l)\ 

h= hydraulic head (L); 

S= storage coefficient (dimensionless); 

b= thickness of hydraulic unit (L); 

r=time(r);

x, y, i- space coordinates (L); 

W(x, y, z, t)= volumetric flux per unit volume (T^1 ).

The derivation of this equation will not be explained 

here, but discussions of its development may be found in 

Trescott (1975) and Trescott and Larson (1976).

DIGITAL MODEL OF GROUND-WATER FLOW

The digital model used in this study to solve a 

finite-difference form of the preceding ground-water 

flow equation (eq. 1) is described by Leahy (1982). 

Leahy's model is a modified version of a three-dimen-
M28 REGIONAL AQUIFER-SYSTEM ANALYSIS NORTHERN ATLANTIC COASTAL PLAIN

Base modified from U.S. Geological Survey digital data, 

1:2,000,000,1972

EXPLANATION 

Lower Cape Fear aquifer not present

 Limit and designation of aquifer and confining unit 

directly overlying the lower Cape Fear aquifer

A9 Yorktown aquifer and confining unit

A3 Upper Cape Fear aquifer and confining unit

FIGURE 20. Areal extent of the lower Cape Fear aquifer (A2) and overlying aquifers (modified from Winner and Coble, 1989).
SIMULATION OF GROUND-WATER FLOW M29

/X\^rv/-l o\ \-> ! ^* i

^RfgJJ«^ gj DAS^

\'ci>^'^' ^v_ ^'^r.' S KaP

**feSf-?;

34° -

Base modified from U.S. Geological Survey digital data, 

1:2.000.000.1972

EXPLANATION 

Lower Cape Fear aquifer not present

Limit and designation of aquifer and confining unit, 

or basement rock, directly underlying the lower 

Cape Fear aquifer

Al Lower Cretaceous aquifer and confining unit 

.3 ^ , Basement rock

FIGURE 21. Areal extent of the lower Cape Fear aquifer (A2) and underlying aquifers or basement rock (modified from Winner and Coble, 1989).
M30 REGIONAL AQUIFER-SYSTEM ANALYSIS NORTHERN ATLANTIC COASTAL PLAIN

sional finite-difference flow model program described by 

Trescott (1975) and Trescott and Larson (1976). The 

model assumes that all flow in the aquifers is horizontal and all flow in the confining units is vertical. 

Leahy's modification reduces computer-memory requirements and better simulates confining-unit and aquifer 

pinchouts.

GRID DESIGN

The model area is divided into a rectangular grid 

having 11 layers, 60 rows, and 48 columns. The grid spacing is variable. The smallest rectangle is 12.25 mi2 and 

the largest is 56.25 mi2 (fig. 22). This grid is designed to 

be compatible with the regional model grid described by 

Leahy and Martin (1993) intended for simulating the 

entire Northern Atlantic Coastal Plain aquifer system, in 

that each rectangle of the regional grid is divided into 

four small, local (North Carolina) grids.

Each layer of the North Carolina model has 2,880 

rectangular blocks, which total 31,680 blocks for the 11 

layers. The model nodes are located at the center of each 

block. Each input value assigned to a node is considered 

to be an average value for the entire block. Likewise, 

output values (hydraulic head and drawdown) are also 

average values for that block. An assumption inherent in 

this approach is that the Cartesian coordinate axes x and 

y are aligned with the principal components of the transmissivity tensor T^ and T.

MODEL BOUNDARIES

The confined aquifers (A9-A1) thin to extinction at 

their westernmost limit. Therefore, the westernmost 

limit of each aquifer is treated as a no-flow boundary. The 

westernmost limit of the surficial aquifer (A10) coincides with the westernmost limit of the North Carolina 

Coastal Plain study area and is also treated as a no-flow 

boundary.

The eastern limit of the fresh ground- water flow system is assumed to be at an equal concentration line 

where the water contains 10,000 mg/L of chloride, and 

the line is also treated as a no-flow boundary. The place 

where this concentration line occurs in each aquifer 

within the model area is referred to as the freshwater-saltwater boundary. It is important to note that this 

assumption is considered valid only in areas where there 

are negligible drawdowns due to pumping at or near the 

freshwater-saltwater boundaries. Where a layer's freshwater-saltwater boundary occurs outside the model area, 

the eastern model boundary for that layer is a specified-flux boundary, the fluxes being supplied by the 

regional model. In this report, the eastern limits of the

aquifers are shown only if they occur on land, although 

the aquifers were actually modeled to their freshwatersaltwater boundaries or to the boundary of the model. 

The upper boundary of each confined aquifer is the bottom of its associated confining unit and is treated in the 

model algorithm as if it were a head-dependent flux 

boundary.

The bottom boundary of each aquifer is either a 

head-dependent flux boundary (where another aquifer 

and confining unit are present below) or a no-flow boundary (where bedrock or a freshwater-saltwater boundary 

underlie the aquifer). Where the modeled area of an aquifer is bounded by the Virginia or South Carolina State 

borders, these borders are treated as specified-flux 

boundaries to simulate the movement of water across the 

State lines. Again, the fluxes are supplied from the 

regional model and simulate not only natural flow, but 

also induced flow due to the effects of pumping inside and 

outside the North Carolina study area. The uppermost 

active model node at any location represents a stream- or 

lake-surface elevation and is treated as a constant-head 

node; the lowermost active node at any given location 

represents a freshwater-saltwater boundary or bedrock 

and is treated as a no-flow boundary.

MODEL INPUT

The following sections discuss how aspects of the 

conceptual model of the aquifer system were represented 

numerically in the input requirements for the flow model. 

The model input requirements are shown schematically 

in figure 23. Printouts of model input for the calibrated 

model are available from the U.S. Geological Survey, P.O. 

Box 30728, Raleigh, NC 27607. Many of the items discussed under "Model Input" also relate to the water-budget aspect of the conceptual model of the aquifer system 

(fig. 3).

GROUND-WATER RECHARGE

On a nodal basis, recharge to the surficial aquifer 

(A10) was estimated to range between 12 and 20 in/yr 

over the North Carolina Coastal Plain and to average 

nearly 14 in. Each water-table node was given a recharge 

value of 12,14,16, or 20 in/yr, based on (1) soil characteristics from U.S. Soil Conservation Service maps (Taut 

and others, 1974), (2) estimates of recharge to thick 

sandy soils by Heath (1980), and (3) estimates of base 

flow to the North Carolina Coastal Plain streams from 

hydrograph separation using techniques developed by 

Rorabaugh (1964) and Daniel (1976), as described by 

Wilder and Simmons (1982). Maximum recharge rates 

(fig. 24) occur in the Sand Hills area, and minimum rates
SIMULATION OF GROUND-WATER FLOW M31

Base modified from U.S. Geological Survey digital data, 

1:2,000.000.1972

FIGURE 22. Finite-difference grid for the flow model of the North Carolina Coastal Plain aquifer system.
M32 REGIONAL AQUIFER-SYSTEM ANALYSIS NORTHERN ATLANTIC COASTAL PLAIN

Unconfined aquifer © © © ®

Confining unit

First confined aquifer ©©©©

Confining unit CH30

Lowest confined aquifer

EXPLANATION

Model input requirements

Stream-surface or lake-surface altitude

Water-table altitude

Recharge to water table (QRE)

Stream channel leakance (TK)

Transmissivity of aquifer (T)

Specific yield or storage coefficient of aquifer (S)

Deep percolation (DP)

Effective thickness of confining unit (b)

Vertical hydraulic conductivity of confining unit (KJ

Water-level data required for calibration

Ground-water pumpage

Flow direction

FIGURE 23. Schematic representation of model input requirements.

occur generally in the east, where surficial soils have a 

high clay content.

STREAMBED LEAKANCE

The exchange of water between the surficial aquifer 

and a stream through the streambed can be expressed 

for steady-state conditions and on a unit-area basis by 

the following equation, which is an expression of Darcy's 

law:

= SL(ha-hs) (2)

where

BF= stream base flow (ground-water contribution to 

streamflow), in cubic feet per day per square foot 

[(ft3/d)/ft2];

SL= streambed leakance factor, computed as the ratio 

of the vertical hydraulic conductivity (Kv) of the 

streambed material, in (ft3/d)/ft2, or ft/d, to the 

thickness of the streambed material (6), in feet, 

yielding units of (ft/d)/ft, or per day; 

ha= estimated altitude of water table in the surficial

aquifer, in feet; 

hs= estimated altitude of stream surface, in feet.

The average water-table altitude and lowest stream 

altitude within each appropriate block were estimated 

from U.S. Geological Survey topographic maps at scales 

of 1:24,000 and 1:62,250. The lowest stream altitude for 

each block was chosen because it was considered the 

"controlling" elevation governing flow from the surficial 

aquifer (A10) to the stream system. Base flow was determined from the following equation, which is for steadystate conditions:

BF=QRE±DP (3)

where 

BF= stream base flow, in cubic feet per day per square

foot; 

QRE= ground-water recharge to the surficial aquifer, in

feet per second;

DP=deep percolation into or flow out of the underlying confined aquifer system, in feet per day. 

Ground-water recharge (QRE) was determined for 

and input into each block, as described above in the 

"Ground-Water Recharge" section of this report. Deep 

percolation (DP) was calculated for each block from simulations for predevelopment steady-state conditions in 

which the water table was treated as a constant-head 

boundary. Average base flow for each block could then be 

calculated from equations above, and the resulting 

base-flow value substituted into equation 2 to solve for 

streambed leakance (SL). A constant-head value equal to
SIMULATION OF GROUND-WATER FLOW M33

34° -

Base modified from U.S. Geological Survey digital data, 

1:2.000.000.1972

EXPLANATION

Area where recharge was not estimated 

Area of estimated recharge

Recharge 12 inches per year

Recharge 14 inches per year

 Recharge 16 inches per year

H Recharge 20 inches per year

FIGURE 24. Estimated recharge to surficial aquifer (A10) from precipitation (modified from Winner and Coble, 1989).
M34 REGIONAL AQUIFER-SYSTEM ANALYSIS NORTHERN ATLANTIC COASTAL PLAIN

the stream altitude value was then assigned to each node 

overlying the water table. This procedure allowed fairly 

realistic simulation of water-table behavior during transient simulations. For example, as the water table is 

drawn down due to pumping from a confined aquifer 

below, base flow to streams is reduced; if the water table 

is drawn down below stream level, induced infiltration 

takes place, and the stream "recharges" the surficial 

aquifer.

The mechanics of simulations involving streams and 

streambed leakance are discussed in more detail in the 

"Model Calibration" section of this report; the important 

points here are the mechanics of calculating streambed 

leakance and the recognition that the streambed leakances are not reflective of leakances of real streambeds, 

but represent an "effective" streambed for an entire 

block. Thus, "model" streambed leakances (fig. 25) are 

much less than "real" streambed leakances. The distribution of streambed leakance values used as model input is 

shown in figure 25.

TRANSMISSIVTTY OF AQUIFERS

Transmissivity (T) is equal to Khb, where Kh is the 

horizontal hydraulic conductivity of the aquifer and b is 

the thickness of the aquifer. The model employs the units 

of feet squared per second for transmissivity, but it is 

reported in units of feet squared per day (ft2/d) in the 

text and figures.

Initial estimates of transmissivity for model input 

were determined primarily from examination of geophysical logs. The character of the permeable material within 

each aquifer was interpreted from the logs, and a horizontal hydraulic conductivity (K^) value of the aquifer 

material was assigned to the unit, roughly according to 

guidelines given in Morris and Johnson (1967, table 5). 

Average values of K^ determined in this manner ranged 

from 22 ft/d in the Yorktown aquifer (A9) to 65 ft/d in the 

Castle Hayne aquifer (A7) (Winner and Coble, 1989, 

1996). The hydraulic conductivity was then multiplied by 

the thickness of permeable material within each aquifer 

to arrive at a transmissivity value. Clay and silt layers 

were not included as part of aquifer thickness in computing T. The character of the permeable material was verified from geologists' logs where available. Where 

available and appropriate, existing aquifer-test data 

were used to confirm the transmissivity values.

Maps showing transmissivity used in model simulation after calibration for aquifers A9 through Al are 

shown in figures 26 through 34. Transmissivities diminish 

to zero at the western limits of all units except the surficial aquifer (A10). Maximum transmissivities of slightly 

more than 175,000 ft2/d occur in the Castle Hayne aquifer 

(A7) (fig. 28). Maximum T values in other aquifers seldom

exceeded 10,000 ft2/d. The rationale for varying transmissivity during the calibration process is discussed 

under "Model Calibration."

STORAGE COEFFICIENT OF AQUIFERS

In this study, a storage coefficient value of IxlO"4 

was assigned to all active nodes in confined aquifers; for 

unconfined conditions, values ranging from lxlO~l to 

1.5x10" * were used. Results from aquifer tests in the 

North Carolina Coastal Plain indicate that IxlO"4 is of 

the correct order of magnitude for a storage coefficient 

value for a confined aquifer.

LEAKANCE OF CONFINING UNITS

In the model used for this study, confining units 

were not represented as separate layers. Rather, the 

effects of vertical flow through confining units were 

incorporated in the vertical components of flow in the 

adjacent aquifers by use of a leakance term (TK) defined 

as the ratio of Kv/b for each confining unit, where Kv is 

the vertical hydraulic conductivity of the confining unit 

and b is its effective thickness.

The effective thickness of confining units was commonly less than the total thickness because most confining units contain thin layers of permeable material. The 

thickness of permeable layers within each confining unit 

was subtracted from the total confining-unit thickness to 

arrive at an effective thickness. Winner and Coble (1989) 

gave an effective thickness for each confining unit identified in the North Carolina Coastal Plain.

As previously mentioned, the model assumes all flow 

in aquifers to be horizontal and all flow in confining units 

to be vertical. It was recognized, however, that in many 

places within zones designated as aquifer material, the 

thickness of clay is significant, and this clay often functions as an impediment to vertical flow. A more realistic 

simulation of the flow system was obtained by incorporating the effects of clay within aquifers into the leakance 

terms. Accordingly, total effective thickness (6) values 

for each confining unit were generated (figs. 35-43), 

which represented the sum of (1) effective confining-unit 

thickness, (2) one-half the thickness of clay beds in the 

aquifer above the confining unit, and (3) one-half the 

thickness of clay beds in the aquifer below the confining 

unit. At some locations shown in figures 35 through 43, 

where the confining unit is designated as not present 

(particularly in stream valleys), the clay-within-aquifers 

concept was used to assign an effective clay thickness for 

modeling purposes.

Text continues on p. M54.
SIMULATION OF GROUND-WATER FLOW M35

Base modified from U.S. Geological Survey digital data, 

1:2,000,000,1972

EXPLANATION

Area where streambed leakance 

was not determined

Streambed leakance, per day

Less than 0.00001

0.00001 to 0.00009 

| 0.0001 to 0.0004 

| 0.0005 to 0.0025 

I More than 0.0025

FIGURE 25. Distribution of estimated streambed leakance.
M36 REGIONAL AQUIFER-SYSTEM ANALYSIS NORTHERN ATLANTIC COASTAL PLAIN

'j Transmissivity \ 

< is less than 500 N 

v.feet squared ^ 

\ Per day ,/,

&

$<pk c

>

V

SOUTH {t 

CAROLINA

Cape Fear

n

yo 50 MILES

\

50 KILOMETERS

EXPLANATION

Yorktown aquifer not present

Line of equal transmissivity Interval, 

in feet squared per day, is variable

Base modified from U.S. Geological Survey digital data, 

1-2,000,000,1972

FIGURE 26. Transmissivity of the Yorktown aquifer (A9) used in model simulations.
SIMULATION OF GROUND-WATER FLOW M37

79°

SOUTH 

CAROLINA

50 MILES

50 KILOMETERS

EXPLANATION 

Pungo River aquifer not present

 T,O<JO- Line of equal transmissivity Interval, 

in feet squared per day, is variable

Base modified from U.S. Geological Survey digital data, 

1:2,000,000,1972

FIGURE 27, Transmissivity of the Pungo River aquifer (A8) used in model simulations.
M38 REGIONAL AQUIFER-SYSTEM ANALYSIS NORTHERN ATLANTIC COASTAL PLAIN

EXPLANATION

Castle Hayne aquifer not present

Line or equal transmissivity Interval 

in feet squared per day, is variable

Base modified from U.S. Geological Survey digital data, 

1:2,000,000,1972

FIGURE 28. Transmissivity of the Castle Hayne aquifer (A7) used in model simulations.
SIMULATION OF GROUND-WATER FLOW M39

78°

"T-"

^

NORTH'

CAROLINA, 

4_^. . . .-«

^

'I, 1 Cape 

Hatter as

^

$

SOUTH "* ^ 

CAROLINA

^-A

i

EXPLANATION 

Beaufort aquifer not present

Aquifer transmissivity not determinedWater in aquifer contains 10,000 

milligrams per liter chloride, or more

Line of equal transmissivity Interval, 

in feet squared per day, is variable. 

Hachured where transmissivity is less 

than that in surrounding area

Base modified from U S. Geological Survey digital data, 

1:2,000,000,1972

FIGURE 29. Transmissivity of the Beaufort aquifer (A6) used in model simulations.
M40 REGIONAL AQUIFER-SYSTEM ANALYSIS NORTHERN ATLANTIC COASTAL PLAIN

NORTH 

CAROLINA

EXPLANATION 

Peedee aquifer not present

CAROLINA Aquifer transmissivity not determined 

Water in aquifer contains 10,000 

milligrams per liter chloride, or more

Line of equal transmissivity Interval, 

in feet squared per day, is variable. 

Hachured where transmissivity is less 

than that in surrounding area

Base modified from U.S. Geological Survey digital data, 

1:2,000,000,1972

FIGURE 30. Transmissivity of the Peedee aquifer (A5) used in model simulations.
SIMULATION OF GROUND-WATER FLOW M41

VIRGINIA

NORTH

CAROLINA / o-1 /^^^/ :..'te

° Af? _O5& . HYriE^ . . *J "=-

^4- / o c-< ^ Cr * ^v * ^ ^^i<%%;V> -: f*1 . 5/s-. ^^X^^-* ,^ "W^TiV^ft, '** (S^1 ,.^ ^,,

f\-^|;>^;C/o

EXPLANATION

Black Creek aquifer not present

CAROLINA \\ Aquifer transmissivity not determined 

Water in aquifer contains 10,000 

milligrams per liter chloride, or more

Line of equal transmissivity Interval, 

in feet squared per day, is variable. 

Hachured where transmissivity is less 

than that in surrounding area

Base modified from U.S. Geological Survey digital data, 

1:2,000,000,1972

FIGURE 31. Transmissivity of the Black Creek aquifer (A4) used in model simulations.
M42 REGIONAL AQUIFER-SYSTEM ANALYSIS NORTHERN ATLANTIC COASTAL PLAIN

EXPLANATION 

Upper Cape Fear aquifer not present

V '\

SOUTH \. 1

CAROLINA (

Aquifer transmissivity not determined 

Water in aquifer contains 10,000 

milligrams per liter chloride, or more

Line of equal transmissivity Interval, 

in feet squared per day, is variable. 

Hachured where transmissivity is less 

than that in surrounding area

Base modified from U.S Geological Survey digital data. 

1:2,000,000,1972

FIGURE 32. Transmissivity of the upper Cape Fear aquifer (A3) used in model simulations.
SIMULATION OF GROUND-WATER FLOW M43

EXPLANATION 

Lower Cape Fear aquifer not present

SOUTH 

CAROLINA

 ^ ,. T >| Aquifer transmissivity not determined 

Water in aquifer contains 10,000 

milligrams per liter chloride, or more 

 7,000 Line of equal transmissivity Interval, 

in feet squared per day, is variable

34° -

Base modified from U.S. Geological Survey digital data. 

1-2,000,000,1972

FIGURE 33. Transmissivity of the lower Cape Fear aquifer (A2) used in model simulations.
M44 REGIONAL AQUIFER-SYSTEM ANALYSIS NORTHERN ATLANTIC COASTAL PLAIN

ientarle Sound ,

NORTH

CARJOLJLNA

^WilmjngsoftP;

SOUTH x , 

CAROLINA \\

EXPLANATION

Lower Cretaceous aquifer not present

*fc ,. T >^ Aquifer transmissivity not determined 

Water in aquifer contains 10,000 

milligrams per liter chloride, or more 

7,000 Line of equal transmissivity Interval, 

in feet squared per day, is variable

Base modified from U.S. Geological Survey digital data, 

1:2.000.000,1972

FIGURE 34. Transmissivity of the Lower Cretaceous aquifer (Al) used in model simulations.
SIMULATION OF GROUND-WATER FLOW M45

Total effe 

% thicknes 

less tha 

50 feet

EXPLANATION

Yorktown confining unit not present

Line of equal total effective thickness 

Interval 100 feet Hachured where unit 

is thinner than in surrounding area

Base modified from U.S. Geological Survey digital data, 

1^.000.000,1972

FIGURE 35 Total effective thickness of the Yorktown confining unit (CU9).
M46 REGIONAL AQUIFER-SYSTEM ANALYSIS NORTHERN ATLANTIC COASTAL PLAIN

78°

S > 

/ H 

\J

iSO MILES -700-

EXPLANATION 

Pungo River confining unit not present

Line of equal total effective thickness 

Interval 100 feet.

Base modified from U.S. Geological Survey digital data, 

1:2,000,000,1972

FIGURE 36. Total effective thickness of the Pungo River confining unit (CU8).
SIMULATION OF GROUND-WATER FLOW M47

SOUTH 

CAROLINA V

°^ /

\ /

/^

/'I

O h 

^ o

50 MILES

^

i 

50 KILOMETERS

-700-

EXPLANATION 

Castle Hayne confining unit not present

Line of equal total effective thickness 

Interval, in feet, is variable

Base modified from U.S. Geological Survey digital data. 

1:2.000,000,1972

FIGURE 37. Total effective thickness of the Castle Hayne confining unit (CUT).
M48 REGIONAL AQUIFER-SYSTEM ANALYSIS NORTHERN ATLANTIC COASTAL PLAIN

SOUTH 

CAROLINA

>'C 

AV

/ ,

/

 ^

50 MILES

________I "~1 

50 KILOMETERS

-50-

EXPLANATION

Beaufort confining unit not present

Confining unit thickness not determined 

Water in Beaufort aquifer contains 10,000 

milligrams per liter chloride, or more

Line of equal total effective thickness 

Interval, in feet, is variable. Hachured 

where unit is thinner than in surrounding

Base modified from U.S. Geological Survey digital data, 

1:2.000,000,1972

FIGURE 38. Total effective thickness of the Beaufort confining unit (CU6).
SIMULATION OF GROUND-WATER FLOW M49

EXPLANATION

Peedee confining unit not present

SOUTH 

CAROLINA

Confining unit thickness not determined 

Water in Peedee aquifer contains 10,000 

milligrams per liter chloride, or more

 700 Line of equal total effective thicknes

Interval, in feet, is variable

34° -

Base modified from U.S. Geological Survey digital data, 

1-2,000,000,1972

FIGURE 39. Total effective thickness of the Peedee confining unit (CU5).
M50 REGIONAL AQUIFER-SYSTEM ANALYSIS NORTHERN ATLANTIC COASTAL PLAIN

CAROLINA

1 'i » . £>' ^ H » t ,.

W&&

EXPLANATION

SOUTH Black Creek confining unit not present

CAROLINA Confining unit thickness not determined 

Water in Black Creek aquifer contains 

10,000 milligrams per liter chloride, or 

more

Line of equal total effective thickness 

Interval 50 feet. Hachured where unit is 

thinner than in surrounding area

Base modified from U.S. Geological Survey digital data. 

1:2,000,000,1972

FIGURE 40 Total effective thickness of the Black Creek confining unit (CU4).
SIMULATION OF GROUND-WATER FLOW M51

200^^ --TOS ^

F**V'J¥ K ^ ^/o 

 ** f^>.s: * * f^-.?? v

EXPLANATION 

SOUTH Upper Cape Fear confining unit not present

CAROLINA T » Confining unit thickness not determined 

Water in upper Cape Fear aquifer contains 

10,000 milligrams per liter chloride, or

700 Line of equal total effective thickness 

Interval 100 feet Hachured where unit 

is thinner than in surrounding area

Base modified from U.S. Geological Survey digital data, 

1:2,000,000,1972

FIGURE 41. Total effective thickness of the upper Cape Fear confining unit (CU3).
M52 REGIONAL AQUIFER-SYSTEM ANALYSIS NORTHERN ATLANTIC COASTAL PLAIN

_«- __. -v ^^w^y ..MM,.,.,, gm., ,-- -T T-. -

* : j/ : / ^^ r^ X "\. -^...^AM^OO^ VNORTH

'CARJOLIXA-,

^$&^

EXPLANATION 

Lower Cape Fear confining unit not present

T > Confining unit thickness not determined 

Water in lower Cape Fear aquifer contains 

10,000 milligrams per liter chloride, or

100 Line of equal total effective thickness 

Interval 100 feet. Hachured where unit 

is thinner than in surrounding area

Base modified from U.S. Geological Survey digital data, 

1:2,000,000,1972

FIGURE 42. Total effective thickness of the lower Cape Fear confining unit (CU2).
SIMULATION OF GROUND-WATER FLOW M53

EXPLANATION 

Lower Cretaceous confining unit not present

Confining unit thickness not determined 

Water in Lower Cretaceous aquifer contains 

10,000 milligrams per liter chloride, or

Line of equal total effective thickness 

Interval 100 feet

Base modified from U.S. Geological Survey digital data, 

1:2,000,000,1972

FIGURE 43. Total effective thickness of the Lower Cretaceous confining unit (CUI).
M54 REGIONAL AQUIFER-SYSTEM ANALYSIS NORTHERN ATLANTIC COASTAL PLAIN

Black Creek aquifer (A4)

Upper Cape Fear confining unit {CU3)

_~- - 

Upper Cape Fear aquifer (A3)

" - _ 

tower Cape Fear confining unit (CU2)

Lower Cape Fear aquifer (A2)

CASE A

Pungo River aquifer (A8)

Castle Hayne confining unit (CU7>

Castle Hayne aquifer (A7)

CASEB

FIGURE 44. Schematic diagram illustrating aquifer or confining-unit 

pinchout as applied to model structure.

Initial values for Kv, representing the vertical 

hydraulic conductivity of confining-unit material, were 

based on appraisals of confining-unit effectiveness 

derived from geophysical logs, chemical analyses, differences in head between aquifers above and below the confining unit, and values of vertical hydraulic conductivity 

for various materials given by Morris and Johnson (1967, 

table 6). These initial values ranged from lxlO~3 ft/d to 

4xlO-6 ft/d.

At many locations, two or more confining units need 

to be considered for modeling purposes as occurring 

between two nonsequential aquifers (case A in fig. 44). 

For example, the lower Cape Fear aquifer (A2) may be 

present, and directly above it the lower Cape Fear confining unit (CU2) and the Black Creek aquifer (A4) may 

be present. The upper Cape Fear aquifer (A3) and upper 

Cape Fear confining unit (CU3) are missing in this example. In this situation the lower Cape Fear confining unit 

(CU2) is considered as being composed of two confining 

units: half of the effective thickness belongs to the lower 

Cape Fear confining unit (CU2) and the other half 

belongs to the upper Cape Fear confining unit (CU3), 

even though the upper Cape Fear confining unit (CU3) is 

not present. In general, the confining unit is divided into

N+l equal parts, where N is the number of missing aquifers. Although the transmissivity of the missing aquifers 

is zero at these locations and no horizontal flow can occur, 

the modified version of the Trescott flow model used in 

this study (described in detail by Leahy (1982)) allows for 

vertical movement of water at these locations.

The opposite situation is where a confining unit is 

missing and two aquifers are in direct contact (case B in 

fig. 44). This condition was simulated by arbitrarily 

assigning small confining-unit thicknesses ranging from 

0.01 to 0.001 ft. This occurs mostly in stream valleys 

where the surficial aquifer (A10) is in direct contact with 

an underlying aquifer and results in high leakance values. A notable exception is in the western Pamlico Sound 

area, where the Pungo River aquifer (A8) is in direct contact with the underlying Castle Hayne aquifer (A7).

The leakance (TK) values for the confining units from 

the calibrated model are shown in figures 45 through 53. 

The rationale for varying leakance values during calibration is discussed in the "Model Calibration" section later 

in this report.

GROUND-WATER WITHDRAWALS AND TIME DISCRETIZATION

In order to simulate the behavior of the North Carolina Coastal Plain aquifer system through time, it was 

necessary to develop pumpage histories for major 

ground-water users; that is, users who withdraw 100,000 

gallons per day (gal/d) or more. This information was 

gathered primarily from interviews with personnel of 

public-supply systems and self-supplied industries during 1982-83 and was supplemented by reports and data 

of the U.S. Geological Survey (Robison (1977) and Robison and Mann (1977)), the North Carolina Department of 

Environment, Health, and Natural Resources, formerly 

called the North Carolina Department of Natural 

Resources and Community Development (1983), and the 

North Carolina Department of Human Resources.

Withdrawals by more than 115 public supplies and 

industries in the survey exceeded 195 Mgal/d during 

1980, doubling about every decade from 1940 to 1970 (fig. 

54). The largest single user is a phosphate-mining and 

chemical-production operation in Beaufort County, which 

began withdrawals in 1965 at a rate of 31 Mgal/d. By 

1980, withdrawals for this operation had leveled off at a 

rate of about 64 Mgal/d. Sharp declines in pumpage in 

1944, 1945, 1961, and 1964 are entirely due to decreases 

in pumpage at two rock quarries in Onslow and Craven 

Counties; sharp increases in 1945 and 1958 are due to 

pumpage increases at these same two quarries.

Text continues on p. M64.
SIMULATION OF GROUND-WATER FLOW M55

EXPLANATION

Yorktown confining unit not present 

Leakance values, per day

0.001 to 0.00001

Less than 0.00001

Base modified from U.S. Geological Survey digital data, 

1:2.000,000,1972

FIGURE 45. Model-derived leakance (TK) of the Yorktown confining unit (CU9).
M56 REGIONAL AQUIFER-SYSTEM ANALYSIS NORTHERN ATLANTIC COASTAL PLAIN

NORTH

CAROLINA

EXPLANATION

Pungo River confining unit not present 

Leakance values, per day

0.1 to 0.001 

0.001 to 0.00001 

Less than 0.00001

SOUTH 

CAROLINA

Base modified from U.S. Geological Survey digital data, 

1:2,000,000.1972

FIGURE 46. Model-derived leakance (TK) of the Pungo River confining unit (CU8).
SIMULATION OF GROUND-WATER FLOW M57

NORTH

CARiOLINA^

^,^_ r<s'

EXPLANATION

Castle Hayne confining unit not present 

Leakance values, per day

0.1 to 0.001 

0.001 to 0.00001 

Less than 000001

SOUTH 

CAROLINA \

v

Base modified from U.S. Geological Survey digital data. 

1:2.000.000,1972

FIGURE 47. Model-derived leakance (TK) of the Castle Hayne confining unit (CUT).
M58 REGIONAL AQUIFER-SYSTEM ANALYSIS NORTHERN ATLANTIC COASTAL PLAIN

79° 78° 77° 76°

SOUTH \, 1 

CAROLINA \\

n\^ 8

\ /

//

/ 0

>,! o

50 MILES

_______|

50 KILOMETERS

EXPLANATION

. j Beaufort confining unit not present

, . Leakance not determined Water in 

Beaufort aquifer contains 10,000 

milligrams milligrams per liter chloride, or moi

Leakance values, per day

0.001 to 0.00001 

Less than 0.00001

^"

Base modified from U.S. Geological Survey digital data. 

1:2,000,000,1972

FIGURE 48. Model-derived leakance (TK) of the Beaufort confining unit (CU6).
SIMULATION OF GROUND-WATER FLOW M59

NORTHi

CAROLINA

EXPLANATION

Peedee confining unit not present

» * » Leakance not determined ^Water in 

Peedee aquifer contains 10,000 

SOUTH milligrams per liter chloride, or more

CAROLINA Leakance values, per day

0.1 to 0.001 

0.001 to 0.00001 

Less than 0.00001

34° -

Base modified from U.S. Geological Survey digital data. 

1-2,000,000,1972

FIGURE 49. Model-derived leakance (TK) of the Peedee confining unit (CU5).
M60 REGIONAL AQUIFER-SYSTEM ANALYSIS NORTHERN ATLANTIC COASTAL PLAIN

SOUTH \^ '

\\ t>' \ COLICS- ^Ilmingt3'

>' // \ " r^ ®

\ \p/ \ / BPUNSWiCK' iL*

CAROLINA

Cape Fear

jv^

V>

//°y

^ n ^\1 °

50 MILES

________| ~T

50 KILOMETERS

EXPLANATION

Black Creek confining unit not present

Leakance not determined Water in 

Black Creek aquifer contains 10,000 

milligrams per liter chloride, or more

Leakance values, per day

0.001 to 0.00001 

Less than 0.00001

Base modified from U.S. Geological Survey digital data. 

1 2,000.000.1972

FIGURE 50. Model-derived leakance (TK) of the Black Creek confining unit (CU4).
SIMULATION OF GROUND-WATER FLOW M61

NORTH

CARJQLtNA, ^

' ^ V-

£^- y *

^^4f%>>f 

^&.%^

EXPLANATION

Upper Cape Fear confining unit not present

V // \ "-'rS.'' -"I

\t/ \ . ./ ' BRUNSWICK'. .|l

*^ > » > Leakance not determined Water in upper 

Cape Fear aquifer contains 10,000 

milligrams per liter chloride, or more

Leakance values, per day

0.001 to 0.00001

Base modified from U.S Geological Survey digital data, 

1:2.000.000.1972

FIGURE 51. Model-derived leakance (TK) of the upper Cape Fear confining unit (CU3).
M62 REGIONAL AQUIFER-SYSTEM ANALYSIS NORTHERN ATLANTIC COASTAL PLAIN

fcafcigh.^ 1$ 

/ .NORTH j

^ ! CARlOLlNiA i - ' i,.y'^ 

EXPLANATION 

Lower Cape Fear confining unit not present

,. » Leakance not determined Water in lower 

Cape Fear aquifer contains 10,000 

milligrams per liter chloride, or more

CAROLINA Leakance values, per day

0.1 to 0.001 

0.001 to 0.00001 

Less than 0.00001

Base modified from U.S. Geological Survey digital data, 

1:2,000,000,1972

FIGURE 52. Model-derived leakance (TK) of the lower Cape Fear confining unit (CU2).
SIMULATION OF GROUND-WATER FLOW M63

78°

SOUTH 

CAROLINA

50 MILES

50 KILOMETERS

EXPLANATION 

Lower Cretaceous confining unit not present

Leakance not determined Water in Lower 

Cretaceous aquifer contains 10,000 

milligrams per liter chloride, or more

Leakance values, per day

2] 0.001 to 0.00001

 ^-]

Less than 0.00001

Base modified from U.S Geological Survey digital data, 

1:2,000,000,1972

FIGURE 53. Model-derived leakance (TK) of the Lower Cretaceous confining unit (GUI).
M64 REGIONAL AQUIFER-SYSTEM ANALYSIS NORTHERN ATLANTIC COASTAL PLAIN

150

2 100

1900 1910 1960 1970 1980

FIGURE 54. Ground-water pumpage in the North Carolina Coastal Plain, 1900-80, for users of more than

100,000 gallons per day.

To simulate the flow system over time, the period 

from January 1,1900, to January 1,1981, was divided into 

10 time periods; within each period, pumpage is treated 

as a constant. The 10 time periods chosen are as follows:

Time period Inclusive dates Length of time period (years)

10

1900-20 

1921-39 

1940-45 

1946-52 

1953-57 

1958-64 

1965-67 

1968-72 

1973-77 

1978-SO

21

20

6

7

5

7

3

5

5

3

The locations of withdrawals greater than 100,000 gal/d 

are shown for all aquifers above the Lower Cretaceous 

aquifer (Al) in figures 55 through 63. (There are none in 

the Lower Cretaceous aquifer within North Carolina.) 

Table 2 shows the 1980 pumpages by aquifer.

The pumpage survey conducted as a part of this 

study is considered to be fairly complete for large public 

and industrial ground-water users, but no irrigation 

water-use data were included in the survey. Ground 

water used for irrigation represents a small but rapidly 

increasing component of total water use in the North

Carolina Coastal Plain, and only one published inventory 

presently exists (North Carolina Department of Natural 

Resources and Community Development and U.S. Water 

Resources Council, 1983). For calibration, wells were 

selected that were unaffected by irrigation pumping. In 

addition, much of the ground water used for irrigation 

purposes is withdrawn from the surficial aquifer, and 

such withdrawals have little effect on the deeper confined aquifer system. Therefore, it is believed that the 

lack of irrigation pumpage data does not affect the values 

for transmissivity and confining-unit leakance arrived at 

through the calibration process.

Pumpage records prior to 1975 are incomplete and 

were estimated partly on the basis of population. Prior to 

the 1950's, many ice plants in and around the major cities 

of the North Carolina Coastal Plain used large amounts 

of ground water (1 Mgal/d or more); however, these 

plants have been closed since the 1950's. The effects of 

these and other unmeasured withdrawals on 1980 water 

levels are judged to be small, largely because higher 

recharge rates induced by these withdrawals have 

replaced much of the water withdrawn decades earlier.

Text continues on p. M74.
SIMULATION OF GROUND-WATER FLOW M65

80° 79° 78°

36°

)* ^ N ''08-UMBUS -^V,hrinMMi«'" ( -. ,- x . I vviimmgtora -

SOUTH 

CAROLINA

\>J

7. 50 MILES

n

50 KILOMETERS

EXPLANATION

Location of withdrawal greater 

than 100,000 gallons per day 

from aquifer

4^

Base modified from U.S Geological Survey digital data, 

1:2,000,000,1972

FIGURE 55. Locations of withdrawals greater than 100,000 gallons per day for the surficial aquifer (A10) for 1980.
M66 REGIONAL AQUIFER-SYSTEM ANALYSIS NORTHERN ATLANTIC COASTAL PLAIN

EXPLANATION

Yorktown aquifer not present

O Location of withdrawal greater

than 100,000 gallons per day 

from aquifer

Base modified from U.S. Geological Survey digital data, 

1:2,000,000,1972

FIGURE 56. Locations of withdrawals greater than 100,000 gallons per day for the Yorktown aquifer (A9) for 1980.
SIMULATION OF GROUND-WATER FLOW M67

V4," ' ^ <fem^

/WASHINGTON/ TYRRELL

NORTH.

CAROLINA

SOUTH 

CAROLINA EXPLANATION 

Pungo River aquifer not present

 Location of withdrawal greater

than 100,000 gallons per day 

from aquifer

34° -

Base modified from U.S. Geological Survey digital data, 

1:2.000.000.1972

FIGURE 57. Locations of withdrawals greater than 100,000 gallons per day for the Pungo River aquifer (A8) for 1980.
M68 REGIONAL AQUIFER-SYSTEM ANALYSIS NORTHERN ATLANTIC COASTAL PLAIN

NORTH

CAROLINA

 

^ V^^i; V. *ttv-ev A* -57

EXPLANATION

Castle Hayne aquifer not present

O Location of withdrawal greater

than 100,000 gallons per day 

from aquifer

Base modified from U.S. Geological Survey digital data, 

1:2,000,000,1972

FIGURE 58. Locations of withdrawals greater than 100,000 gallons per day for the Castle Hayne aquifer (A7) for 1980.
SIMULATION OF GROUND-WATER FLOW M69

CARJOEINA

CAROLINA EXPLANATION 

Beaufort aquifer not present

O Location of withdrawal greater

than 100,000 gallons per day 

from aquifer

Base modified from U.S. Geological Survey digital data, 

1:2,000,000,1972

FIGURE 59. Locations of withdrawals greater than 100,000 gallons per day for the Beaufort aquifer (A6) for 1980.
M70 REGIONAL AQUIFER-SYSTEM ANALYSIS NORTHERN ATLANTIC COASTAL PLAIN

fyrs" """^v^SC "

EXPLANATION

Peedee aquifer not present

O Location or withdrawal greater

than 100,000 gallons per day 

from aquifer

Base modified from U.S. Geological Survey digital data, 

1:2,000,000,1972

FIGURE 60. Locations of withdrawals greater than 100,000 gallons per day for the Peedee aquifer (A5) for 1980.
SIMULATION OF GROUND-WATER FLOW M71

VIRGINIA

NGTCh/TYRRELL (. (

V NORTH,

^ CAR*OLINA

CAROLINA EXPLANATION 

Black Creek aquifer not present

O Location of withdrawal greater

than 100,000 gallons per day 

from aquifer

Base modified from U.S. Geological Survey digital data. 

1.2.000.000.1972

FIGURE 61. Locations of withdrawals greater than 100,000 gallons per day for the Black Creek aquifer (A4) for 1980.
M72 REGIONAL AQUIFER-SYSTEM ANALYSIS NORTHERN ATLANTIC COASTAL PLAIN

WASHINGT2N/TYRPELL

SOUTH 

CAROLINA EXPLANATION 

Upper Cape Fear aquifer not present

O Location of withdrawal greater

than 100,000 gallons per day 

from aquifer

34° -

Base modified from U.S. Geological Survey digital data, 

1:2.000,000,1972

FIGURE 62. Locations of withdrawals greater than 100,000 gallons per day for the upper Cape Fear aquifer (A3) for 1980.
SIMULATION OF GROUND-WATER FLOW M73

%!-/ 

fe « to* 

^ -H rJ> ^>

v, V 

SOUTH V

CAROLINA x s

\ /

"\,7 n 50 MILES

w/ ^\I o

^> **jr i

i

50 KILOMETERS

EXPLANATION

Lower Cape Fear aquifer not present

O Location of withdrawal greater

than 100,000 gallons per day 

from aquifer

Base modified from U.S. Geological Survey digital data, 

1:2.000.000.1972

FIGURE 63. Locations of withdrawals greater than 100,000 gallons per day for the lower Cape Fear aquifer (A2) for 1980.
M74 REGIONAL AQUIFER-SYSTEM ANALYSIS NORTHERN ATLANTIC COASTAL PLAIN

TABLE 2. North Carolina ground-water pumpage by aquifer in 1980

Model 

layer 

number

AID

A9

A8

A7

A6

A5

A4

A3

A2

Al

North Carolina aquifer

Surfieial

Yorktown

Pungo River

Castle Hayne

Beaufort

Peedee

Black Creek

Upper Cape Fear

Lower Cape Fear

Lower Cretaceous

1980 pumpage 

(millions of gallons 

per day)

0.3

2.9

1

136

.1

3.5

36.6

12.3

2.9

0

Total pumpage 195.6

MODEL CALIBRATION

PROCEDURES

Calibration of the North Carolina Coastal Plain 

ground-water flow model was a trial-and-adjustment 

procedure whereby model input was varied, and the 

resulting model output was compared with observed values. The process was repeated to minimize the difference 

between computed and observed values, until the computed results agreed with observed values to within 

some acceptable degree of accuracy. Generally, calibration was considered acceptable for purposes of this study 

when computed 1980 water levels were within 15 ft of 

water levels measured in 1980.

A number of calibration simulations were made to 

determine how well the North Carolina Coastal Plain 

ground-water flow model was able to simulate actual 

records of water levels in both pumped and unpumped 

areas and to indicate where further adjustments to 

model parameters might be needed. Where calibration 

was judged inadequate, hydrogeologic, hydraulic head, 

and water-quality data were analyzed to determine the 

cause or causes. Often, poor initial estimates of model 

parameters were suspected of being the primary cause of 

poor water-level matches. Adjustments of the model 

parameters were made, and new simulations performed. 

Again, computed water levels were compared with 

observed values. Thus, an adjustment process involving 

several repetitions was employed in model calibration. 

The parameter most often adjusted was leakance of confining units (TK), followed by transmissivity of the aquifers (T). The other major hydraulic property represented 

in the model, storage coefficient (S), was not varied during the calibration process because the model was found 

not to be sensitive to changes in the values of the storage 

coefficient. Other model parameters that were varied 

during the course of model calibration were the altitude

of the water table, model-boundary fluxes, location of 

freshwater-saltwater boundaries, and pumping rates.

Large changes in leakance values from initial estimates (an order of magnitude or more) were considered 

justified because the vertical conductivity of confining 

units is not well known. However, changes in transmissivity were limited to no more than three times as much 

or no less than one-third as much as initial estimates 

because probable errors in initial estimates were much 

smaller than probable errors in estimates of leakance.

As a result of sensitivity analyses performed early in 

model calibration, it was found that changing transmissivity values produced large changes in computed heads 

near pumping centers, but away from pumping centers, 

changes were small. Changes in leakance of confining 

units also produced large changes in computed heads 

near pumping centers, but the changes away from pumping centers were often significant, too. If the computed 

water levels were too high or too low very near the 

pumping centers, but not elsewhere, a transmissivity 

problem was suspected. If computed water levels were 

found to be generally too high or too low in a given layer 

over a large area, a leakance problem was suspected. The 

vertical flow system was then examined to estimate what 

changes in model parameters would be most likely to 

result in better water-level matches, not only in the layer 

in question, but also in the underlying and overlying 

layers.

In selecting wells with water-level records for calibration purposes, areas were avoided that have experienced water-level declines very close to freshwatersaltwater boundaries (10,000 mg/L chloride concentration). One model assumption is that the freshwater-saltwater boundary is a no-flow boundary. This assumption 

is not valid in areas where pumping may have induced 

flow of saltwater toward the pumping well, and it was 

felt that attempts to match hydrographs in such areas 

would be unrealistic. Full consideration of movement of 

saltwater requires a ground-water flow model capable of 

simulating waters of varying density and is beyond the 

scope of this study.

The assumption of no flow across the freshwatersaltwater boundaries is of particular concern in parts of 

aquifers Al through A4 and part of A6, where simulated 

drawdowns near the freshwater-saltwater boundary 

commonly exceeded 10 ft in 1980. In these areas, actual 

drawdowns would likely be less than those simulated by 

the model. As a result, the confidence in model results 

(calibrated parameters) and predictions is less in these 

areas. The reader is referred to Leahy and Martin (1993) 

for a detailed discussion of the effect of the no-flow 

assumption for the freshwater-saltwater boundaries on 

the results of the regional ground-water flow model of 

the Northern Atlantic Coastal Plain.
SIMULATION OF GROUND-WATER FLOW M75

When testing model parameters during the course of 

model calibration, the usual procedure was to make three 

simulations. The first simulation was for steady-state 

prepumping conditions with water-table heads fixed at 

constant values, indicative of average long-term natural 

conditions. Boundary fluxes were provided by the 

regional model, which was frequently updated with 

refined flux values derived from each State subregional 

model.

Model output from this first simulation included flux 

between the water table and the topmost confined aquifer. This flux can be thought of as the deep percolation 

(DP) term of equation 3, which is the total ground-water 

recharge (QRE) minus base flow of streams (BF).

The quantity QRE for each node was determined as 

described earlier in the "Model Input" section of the 

report. Subtracting or adding DP to this value (depending on the direction of vertical flow) gave BF for each 

node. When values were known also for the altitude of 

the water table, ha, and the lowest stream altitude in 

each node, hs, equation 2 could be solved for streambed 

leakance (SL). An llth (artificial) layer representing 

streams was then added to the model. The head at each 

stream node was set at the constant head, hs, and the calculated value of SL was used as the TK value between 

the stream layer and the water-table layer. Recharge 

(QRE) was added to the surficial aquifer (A10), and a 

steady-state nonpumping simulation was made. The 

advantage of adding a layer representing streams is that, 

in later stressed simulations, the water levels are free to 

fluctuate within the surficial aquifer due to stresses 

either in the aquifer itself or in underlying aquifers. This 

results in a more realistic simulation of the surficial aquifer response to pumping stresses than in simulations 

where the water table is held constant.

A pumping simulation was then made, designed to 

simulate aquifer response to pumpage over the 10 pumping periods discussed above in the section "GroundWater Withdrawals and Time Discretization." Simulated 

hydrographs were compared with observed hydrographs 

for both prepumping and pumping conditions. A significant feature of the simulated hydrographs is that discretization error was reduced by computing heads at well 

locations, most of which are not at grid centers. This 

method of head computation involves solution of the common three-point problem, as shown in figure 64.

RESULTS

As previously mentioned, leakance (TK) of confining 

units, was adjusted more often and to a greater degree 

than any other model parameter. Computed heads were 

found to be significantly sensitive to changes in this

parameter. At the same time, large leakance adjustments 

of as much as two orders of magnitude could be justified 

in many instances because estimated initial values of this 

parameter were subject to more uncertainty than any 

other model parameter. Part of this uncertainty has to do 

with the difficulty in distinguishing silt from clay on geophysical logs, and part has to do with the discontinuity of 

some confining units, which often cannot be estimated 

from interpolation between geophysical logs collected at 

widely spaced locations.

Trial-and-adjustment model calibration indicated a 

general tendency for vertical hydraulic conductivity of 

confining units, Kv, to decrease with increasing depth 

and with increasing distance downdip. The tendency for 

the decrease with depth is reflected by minimum Kv values in table 3 and is thought to be partly related to 

greater compaction of the silts and clays due to the 

greater weight of overlying sediments and the greater 

time for compaction. The tendency for decreasing Kv 

with increasing distance downdip follows from this 

because, downdip, sediments of a given aquifer are generally more deeply buried, and there may also be a general downdip change from coarse clastic to fine clastic 

and marine facies (LeGrand, 1961). A counter-tendency to 

this decrease of Kv with increasing depth and distance 

downdip may occur where nonmarine sediments are 

present, such as in large parts of the lowermost confining 

units GUI, CU2, and CU3. There, beds that make up the 

confining units are more likely to be discontinuous and, 

therefore, provide less resistance to vertical movement 

of water than suggested from the thickness of confining-unit material indicated on geophysical logs. Hence, 

model values of Kv may be higher there (and elsewhere) 

than might be expected for silt or clay in order to reflect 

discontinuities.

Transmissivity (T) was varied less often than leakance during model calibration. Initial estimates of T 

were changed for three aquifers during the course of 

model calibration the Castle Hayne aquifer (A7), the

TABLE 3. Ranges of vertical hydraulic conductivity of confining units 

for the calibrated model

[Confining units defined in table 1 increase in depth from layer 9 to layer 1]

Model 

layer 

number

Vertical hydraulic conductivity of confining 

units Kv (feet per day)

Maximum values Minimum values

CU9 

CU8 

CUT 

CU6 

CU5 

CU4 

CU3 

CU2 

GUI

6.98xlO-3

7.94xlO-3

9.07X10-4

5.88x10^

3.89X10-4

2.77X10-4

2.52

4.96X10-1

2.88X10-4

6.82x10-3 

3.90x10-3

1.46x10^ 7.71xlO-5 

3.42xlO-5 

6.84X10-6 

4.13X10-6 

4.04xlO-5
M76 REGIONAL AQUIFER-SYSTEM ANALYSIS NORTHERN ATLANTIC COASTAL PLAIN

EXPLANATION

1. Locate three computed heads (h) at grid nodes that form a triangle (ABC) 

enclosing an off-node point of interest (D).

2. Find the direction of ground-water flow and hydraulic gradient in the plane 

defined by the apexes of the triangle:

 Locate the triangle apex that has the 

intermediate water level (B).

 By linear interpolation, find the point 

on the side opposite (E) that has the 

same water level as B.

 Draw line BE. This is a line of equal 

water level, 35 ft (feet) in this case.

 Draw a line perpendicular to the equal 

water-level line (BE) that passes through 

either the apex of the higher or lower 

water levels. The resulting line (CF) 

represents the direction of the hydraulic 

gradient. Calculation of the gradient is 

as follows:

hr hf

distance CF

or

en ou ?R J0 ft n

= 0.00085 foot per foot

17,556ft

3. Draw a line parallel to the direction of the gradient that passes through the 

point (D) at which the head is to be computed.

4. Measure distance from a point of known head (G) to point at which head is to 

be calculated (line DG).

5. Knowing the hydraulic gradient (0.00085 foot per foot), the head at point G

(35 ft), and the distance along flow path at DG (8,316 ft), the head at 

point D (hD ) may be calculated:

hD - 35 ft

 - = 0.00085; hD = 42.07 ft 

8,316ft

FIGURE 64. Method of computing heads at off-node locations.
SIMULATION OF GROUND-WATER FLOW M77

TABLE 4. Summary of differences between computed and observed water levels for December 31,1980 

[Values represent computed minus observed differences]

Average difference in water levels (feet)

Aquifer name

Yorktown

Pungo River 

Castle Hayne 

Beaufort

Peedee

Black Creek

Upper Cape Fear 

Lower Cape Fear 

Lower Cretaceous

Model layer 

number

A9

A8 

A7 

A6

A5

A4

A3 

A2

Al

All differences

Number of 

values

27

3

47 

5

28

34

30 

16 

1

Average 

difference

+1.4

+4.2 

+2.4

+2.7

+0.2

-1.0

+7.3 

-5.0 

+14.8

Positive differences only

Number of 

values

16

3 

32

4

12

14

21 

5 

1

Average 

difference

+4.5

+4.2 

+7.3 

+5.2

+11.2

+15.1

+13.5 

+12.3

+14.8

Negative differences only

Number of 

values

11

0 

15 

1

16

20

9 

11 

0

Average 

difference

-3.4

-7.8 

-7.2

-8.1

-12.4

-7.3 

-12.9

Black Creek aquifer (A4), and the upper Cape Fear aquifer (A3). Initial values of transmissivity for the Castle 

Hayne aquifer in Beaufort County, which were estimated 

from geophysical logs, gave computed heads that were 

tens of feet lower than observed heads. When transmissivity values were increased by a factor of 2.5 in that 

area, matches were much closer and were in better 

agreement with aquifer tests reported by DeWiest and 

others (1967). No attempt was made to calibrate for 

transmissivity values in the surficial aquifer (A10).

For the block representing the Castle Hayne aquifer 

(A7) at Lee Creek (row 28, column 37), modeled transmissivity was 41,200 ft2/d for an aquifer thickness of 

about 340 ft (inferred horizontal hydraulic conductivity is 

about 121 ft/d). Values derived by DeWiest and others 

(1967, p. 94) from aquifer-test data indicate a higher 

transmissivity near Lee Creek of 52,100 ft2/d for an 

assumed formation thickness of 300 ft (inferred hydraulic 

conductivity of 174 ft/d). A limitation of the test reported 

by DeWiest and others (1967) is that the pumped well 

used in the test was open only to the top 44 ft of the Castle Hayne aquifer. Such a test may not yield hydraulic 

conductivities representative of the entire aquifer thickness. Thus, the lesser hydraulic conductivity calibrated 

for the Castle Hayne aquifer near Lee Creek is not 

unreasonably low compared with values reported by 

DeWiest and others. Also, the transmissivity value for 

the Castle Hayne aquifer (A7) at Lee Creek derived from 

the model is very close to that derived by Sherwani 

(1973, p. 58) for the Lee Creek area (40,100 ft2/d) for use 

in his digital model.

A 50-percent reduction in T from initial estimates 

everywhere in both the Black Creek aquifer (A4) and the 

upper Cape Fear aquifer (A3) resulted in generally better matches of computed heads with observed water levels. Apparently, silty clays interbedded with coarse sands

in the deeply buried nonmarine and marginal-marine 

sediments of these aquifers were more effective in reducing the horizontal hydraulic conductivity, #/,, of the aquifer material than was first thought from examination of 

the geophysical logs (M.D. Winner, Jr., U.S. Geological 

Survey, oral commun., September 1985). Thus, initial 

estimates of #/, were higher than final calibrated values.

A layer-by-layer summary of differences between 

observed and computed water levels for December 31, 

1980, is given in table 4. The average difference between 

computed and observed water levels for 191 observation 

wells was slightly less than +1.6 ft. Detailed comparisons 

of computed and observed values over time were also 

made. Nearly 240 computed hydrographs were compared 

with observed hydrographs. Figures 65 through 69 show 

selected hydrograph comparisons made during the 

course of model calibration. Those shown were chosen on 

the basis of length of record and a balanced geographic 

and aquifer distribution.

A number of poor matches of computed and 

observed water levels were attributed to discretization 

scale rather than to poor estimates of hydrologic parameters. For example, drawdowns due to pumpage are 

treated by the model as if water levels are drawn down 

evenly over an entire block, rather than as a cone of 

depression around a pumped well, and the three-point 

method illustrated in figure 64 does not completely correct for this where grid size is large. The coarse model 

grid makes errors from this source significant, particularly when computed values for a node are compared 

with values for an observation well located very close to 

a pumping well.

The scale of discretization is probably also responsible for several poor hydrograph matches in the Sand 

Hills area (fig. 1). Topographic relief and water-table gradients are greater in the Sand Hills region than in any
M78 REGIONAL AQUIFER-SYSTEM ANALYSIS NORTHERN ATLANTIC COASTAL PLAIN

39

38

37

36

35

34 -

33

A. Gaskins observation well in the Yorktown aquifer (A9). 

Model node: row 32, column 31 (see fig. 22).

Simulated

Observed

3 -

2 -

B. Sadler observation well in the Pungo River aquifer (A8). 

Model node: row 28, column 41 (see fig. 22).

Simuiated Observed

1

1900 1910 1920 1930 1940 1950 1960 1970 1980

FIGURE 65. Observed and simulated water levels in the (A) Yorktown and (B) Pungo River aquifers.

1990
SIMULATION OF GROUND-WATER FLOW M79

LLJ 

CO 

LLJ

O

m 3

A. Creswell observation well in the Castle Hayne aquifer (A7). 

Model node: row 16, column 36 (see fig. 22).

Simulated

Observed

14

12

>

LLJ

10 -

8 -

Simulated

B. DEHNR Arapahoe research station observation well u5 

in the Castle Hayne aquifer (A7). Model node: row 33, 

column 40 (see fig. 22).

Observed

1900 1910 1920 1930 1940 1950 1960 1970 1980 1990

FIGURE 66. Observed and simulated water levels in the Castle Hayne aquifer at (A) the Creswell observation well and (B) 

DEHNR (North Carolina Department of Environment, Health, and Natural Resources) Arapahoe research station observation

well u5.
M80 REGIONAL AQUIFER-SYSTEM ANALYSIS NORTHERN ATLANTIC COASTAL PLAIN

22

20

18

16

> 14

LLJ 

C/) 

LU

O

DQ 12

Simulated

Observed

A. USGS observation well NC-58 in the Beaufort aquifer (A6). 

Model node: row 13, column 28 (see fig. 22).

84

82

80

78

76

Simulated

B. USGS observation well NC-51 in the Peedee aquifer (A5). 

Model node: row 42, column 29 (see fig. 22).

Observed

1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 

FIGURE 67. Observed and simulated water levels in the (A) Beaufort and (B) Peedee aquifers. USGS, U.S. Geological Survey.
SIMULATION OF GROUND-WATER FLOW M81

A. USGS observation well NC-30 in the Black Creek aquifer (A4) 

Model node: row 11, column 28 (see fig. 22).

50

40 -

30

20

I

QJ 10

CD 

CC

o

uj 0

gCD

LJJ 

LJJ 

LU

Z -20

-30

-40

-50

-60

1900

Simulated

Observed

B. USGS observation well NC-44 in the Black Creek aquifer (A4). 

Model node: row 36, column 33 (see fig. 22).

1910 1920 1930 1940 1950 1960 1970 1980 1990

FIGURE 68.-Observed and simulated water levels in the Black Creek aquifer at USGS (U.S. Geological Survey) observation

wells (A) NC-30 and (5)
M82 REGIONAL AQUIFER-SYSTEM ANALYSIS NORTHERN ATLANTIC COASTAL PLAIN

120

110

100

nn 

90

80

70

~ 60

50

40

30

20

Simulated

A. Composite record from 7 observation wells 

at Clinton in the upper Cape Fear aquifer (A3) 

Model node: row 48, column 22 (see fig. 22).

50

40

30

20

CD

O 10

LLJ 

O

OQ

-10

-20

-30

-40

B. Magette observation well in the lower Cape Fear aquifer (A2). 

Model node: row 16, column 23 (see fig. 22).

Observed

1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 

FIGURE 69. Observed and simulated water levels in the (A) upper Cape Fear and (B) lower Cape Fear aquifers.
SIMULATION OF GROUND-WATER FLOW M83

other part of the North Carolina Coastal Plain, but the 

model grid is not fine enough to adequately reflect this 

variation. Thus, matches of computed with observed 

water levels were generally poor in this area, and the 

degree to which the model can be considered calibrated is 

much less for the Sand Hills region than for other parts 

of the North Carolina Coastal Plain. One practical consequence of this situation is that the accuracy of estimates 

of model parameters given by the flow model for the 

Sand Hills area is more uncertain than elsewhere. 

Future ground-water modeling studies of the Sand Hills 

region could obtain better results by utilizing a much 

finer model grid than that used in this study.

One measure of the degree to which the model can 

be considered calibrated is the density of calibration 

points that were used. By this criterion, the Castle 

Hayne aquifer (A7) would be considered the best calibrated layer (particularly near the large cone of depression centered just north of Aurora in Beaufort County), 

whereas the Lower Cretaceous, Beaufort, and Pungo 

River aquifers (Al, A6, and A8) would be the least well 

calibrated.

SALTWATER REENTRANT IN THE CAPE FEAR ARCH

An unusual landward reentrant of water containing 

high chloride concentrations (equal to or greater than 

10,000 mg/L) has been delineated in the Wilmington area 

by Winner and Coble (1989,1996) in the upper and lower 

Cape Fear aquifers (A3 and A2). This reentrant is shown 

in this report in figures 32 and 33 by the northwestward 

bulge or point in the 10,000 mg/L chloride concentration 

line, which is considered to be a no-flow boundary in this 

study. The existence of the reentrant is supported by the 

work of LeGrand (1955), who described brackish water 

springs more than 20 ft above sea level as far inland as 

Bladen County, and by Meisler (1989), who mapped the 

position of the saltwater in aquifers of the Northern 

Atlantic Coastal Plain. The reentrant overlies the Cape 

Fear arch, which is a broad southeast-trending uplift of 

both bedrock and the overlying sedimentary deposits of 

the Atlantic Coastal Plain (fig. 8). The axis of the uplift is 

marked approximately by the course of the lower Cape 

Fear River (fig. 2).

Trial simulations performed during model calibration indicated that some barrier to flow was needed to 

sustain the unusually high heads in the Cretaceous aquifers in extreme southeastern North Carolina (Peek and 

Register, 1975) and even higher heads further south 

along the South Carolina coast (Aucott and Speiran, 

1985). A barrier might result from the aquifers having a 

lower transmissivity in the Cape Fear arch area than in 

surrounding areas. The juxtaposition of such a barrier

over the arch suggests that the arch might have affected 

depositional patterns in the area during Cretaceous time, 

resulting in deposition of sediments of extremely low 

transmissivity, either with low hydraulic conductivity or 

near zero thickness, in the stratigraphic position of the 

upper and lower Cape Fear aquifers. However, data from 

the few wells in the arch area listed by Winner and Coble 

(1989, 1996) give indication of only small reductions in 

transmissivity of these two aquifers.

The lower Cape Fear aquifer (A2), which exhibits 

the highest heads, is slightly thinner on the northeast 

side of the Cape Fear arch than on the southwest side 

(fig. 8), but does not thin enough to cause a major change 

in transmissivity at this location. The upper Cape Fear 

aquifer (A3) has lower transmissivity on the northeast 

side of the arch than on the southwest side; this results 

from lower hydraulic conductivity in this area (Winner 

and Coble, 1989), not a decrease in thickness, as might be 

the case for the lower Cape Fear aquifer (A2). The trial 

simulations revealed that in order to sustain the high 

heads in the lower Cape Fear aquifer (A2) southwest of 

the arch, a no-flow or nearly no-flow boundary had to be 

present in the general area occupied by the saltwater 

reentrants shown in figures 32 and 33. Adjustment of 

transmissivity values of the lower Cape Fear aquifer 

(A2) over the arch to 50 percent or less of estimated values (fig. 33) resulted in little head buildup southwest of 

the arch.

Because there is no direct hydrogeologic evidence 

indicating a drastic change in the hydraulic conductivity 

or thickness of the upper and lower Cape Fear aquifers, 

it may be that the no-flow boundary results instead from 

the presence of dense water containing at least 10,000 

mg/L chloride over the Cape Fear arch. The confirmation 

of either idea (or other ideas) concerning the nature of 

the flow barrier (low-transmissivity sediments or saltwater) awaits further data and analyses for support.

SENSmVITY ANALYSIS

Two types of applications of sensitivity analyses are 

appropriate in modeling studies. The first application is 

logically performed before model calibration to aid in 

data collection. If this analysis shows that the model is 

not sensitive to changes in certain parameters in part of 

the modeled area, efforts to improve parameter estimates there would not improve the simulation capability 

of the model. Conversely, if the initial sensitivity analysis 

shows that the model is sensitive to changes in a particular parameter in an area, data-collection activities and 

analyses to better define or verify the parameter values 

in that area could result in improved simulation 

capability.
M84 REGIONAL AQUIFER-SYSTEM ANALYSIS NORTHERN ATLANTIC COASTAL PLAIN

TABLE 5. Range of perturbed values for transmissivity and leakance

Value Group 1 Group 2 Group 3

Transmissivity (feet squared per day)

Mean

10 percent of mean

190 percent of mean

2,994

299

5,688

1,758

176

3,340

33,030

3,303

62,757

Leakance (per day)

3.8xlO-5 

3.8X10-6

2.3x10-2 4.8xlO-3

10 percent of mean S.SxlQ-6 2.3xlO"3 4.8X10"4 

190 percent of mean 7.2xlO~5 4.37xlO~2 9.12xlO~3

A second application is performed after calibration 

to evaluate the relation between parameter variability 

and model response. This is an indication of the extent to 

which calibration is likely to have improved parameter 

estimates and may be a guide in the design of parameter-estimation efforts for future modeling studies. The 

following sections discuss this second application of the 

sensitivity analysis during this study.

METHOD OF ANALYSIS

The model response investigated in the sensitivity 

analysis was hydraulic head. The parameters selected for 

testing were transmissivity, storage coefficient of aquifers, and leakance of confining units. Transmissivity and 

leakance values were varied by plus and minus 50 percent and plus and minus 90 percent of their estimated 

values. Storage coefficient was varied by plus and minus 

90 percent of its estimated value.

To minimize computer time required for the analysis 

of transmissivity and leakance, steady-state simulations 

utilizing 1980 pumpage values were made. However, as in 

transient simulations performed for calibration purposes, 

streams were simulated as a constant-head layer, and the 

surficial aquifer (A10) was simulated as a free surface 

receiving recharge. Tests showed that heads computed in 

this manner were similar to 1980 heads computed by 

transient simulations, indicating that steady-state simulations made as a part of the sensitivity analysis would 

provide results similar to transient simulations. However, transient simulations (1900-80) were made to analyze the effects of varying storage coefficient because 

these results are more time dependent.

To simplify the sensitivity analysis, the aquifers and 

confining units were lumped into three groups of hydrologically similar flow systems. Group 1 consisted of the 

Peedee (A5), Black Creek (A4), upper Cape Fear (A3), 

lower Cape Fear (A2), and Lower Cretaceous (Al) aquiTABLE 6. Range of perturbed values for storage coefficient

Value All groups

Confined aquifer (dimensionless)

Mean

10 percent of mean

190 percent of mean

l.OxlO-4 

l.OxlO-5 

1.9X10-4

Unconfined aquifer (dimensionless)

Mean

10 percent of mean

190 percent of mean

1.5x10-2 

2.8X10-1

fers, which are the sand aquifers in Cretaceous rocks. 

Group 2 contained the Yorktown (A9), Pungo River (A8), 

and Beaufort (A6) aquifers, sand aquifers in Tertiary 

rocks, and, when analyzing aquifer parameters, the surficial aquifer (A10). When analyzing a confining-unit 

parameter, the surficial aquifer (A10) was not included. 

Group 3 was solely the Castle Hayne aquifer (A7), which 

is primarily limestone and sand of Tertiary age. Parameters were perturbed and analyzed for each group. The 

exception was storage coefficient; for this parameter, all 

model layers were perturbed and analyzed simultaneously.

RESULTS

Tables 5 and 6 list the mean values (calibrated values) and maximum and minimum values of the parameter 

variations for the three aquifer groups for transmissivity, 

leakance, and storage coefficient. Within each of the 

three aquifer groups, the results of the sensitivity analysis were categorized, and statistics were generated separately for each category. These categories are (1) all 

active nodes, (2) nodes where pumpage occurred (pumpage nodes), (3) nodes with no flow in a laterally adjacent 

node, (4) nodes with constant flux in a laterally adjacent 

node, and (5) nodes with constant head in a vertically 

adjacent node (category 5 is found only in the surficial 

aquifer (A10) in group 2). If a given node fell into more 

than one category, the node was omitted from the analysis in all but the active-node category. The overall results 

of the sensitivity analysis are summarized in table 7.

Table 7 indicates a general tendency for the Cretaceous sand aquifers (group 1) to exhibit a greater degree 

of head variation for a given percentage change in model 

parameter than the other groups. This is mainly attributed to group 1 nodes representing the deep Cretaceous 

aquifers with the least hydraulic contact with the surficial aquifer (A10). The surficial aquifer is resistant to
SIMULATION OF GROUND-WATER FLOW M85

TABLE 7. Results of sensitivity analysis

Head change for indicated percent change in parameters (feet)

Parameter

Transmissivity 

(T)

Leakance 

(TK)

Storage 

coefficient (S)

Category of 

nodes tested

All active 

nodes

Pumpage 

node

Adjacent to 

no-flow node

Adjacent to 

constantflux node

Vertically 

adjacent to 

constanthead node

All active 

nodes

Pumpage 

node

Adjacent to 

no-flow node

Adjacent to 

constantflux node

Vertically 

adjacent to 

constanthead node

All active 

nodes

Pumpage 

node

Adjacent to 

no-flow node

Adjacent to 

constantflux node

Vertically 

adjacent to 

constanthead node

Group 1

Max. (+) 

Mean 

Max. (-)

Max. (+) 

Mean 

Max. (-)

Max. (+) 

Mean 

Max. (-)

Max. (+) 

Mean 

Max. (-)

Max. (+) 

Mean 

Max. (-)

Max. (+) 

Mean 

Max. (-)

Max. (+) 

Mean 

Max. (-)

Max. (+) 

Mean 

Max. (-)

Max. (+) 

Mean 

Max. (-)

Max. (+) 

Mean 

Max. (-)

Max. (+) 

Mean 

Max. (-)

Max. (+) 

Mean 

Max. (-)

Max. (+) 

Mean 

Max. (-)

Max. (+) 

Mean 

Max. (-)

Max. (+) 

Mean 

Max. (-)

-90 

percent

-50 

percent

105 18.1 

-12.6 -5.05 

-907 -138

6.01 2.74 

-91.8 -16.7 

-907 -138

71.9 

-6.85 

-415

105 

-38.4 

-344

37.2 

-78.1 

-361

.19 

-79.8 

-284

16.7 

-79.4 

-361

26.8 

-94.7 

-361

0.01 

-.67 

-7.37

0 

-.68 

-3.53

.01 

-.62

-2.47

0

-.49 

-1.42

18.1 

-3.54 

-59.5

15.1 

-11.0 

-63.5

13.2 

-11.7 

-57.3

.08 

-14.8 

-44.9

5.70 

-10.7 

-57.2

11.8 

-12.9 

-57.3

+50 +90 

percent percent

53.3 77.6 

3.53 5.75 

-12.0 -19.2

53.3 77.6 

7.59 11.5 

-2.07 -3.17

22.1 32.2 

2.70 4.43 

-12.0 -19.2

27.6 41.2 

6.60 10.5 

-6.58 -10.7

 

22.6 32.9 

4.36 6.30 

-6.80 -10.2

18.0 27.0 

6.50 9.71 

-.03 -.06

22.6 32.9 

3.68 5.21 

-105 -7.25

22.6 32.9 

4.41 6.26 

-6.66 -10.2

 

 5.57 

 1.15 

 -.04

 3.13 

 .91 

 .07

 4.15 

 1.15 

 -.04

 2.75 

 .94 

 .01

 

-90 

percent

12.0 

-.01 

-18.0

.76 

-.87 

-4.53

4.52 

.11 

-3.05

4.33 

-.27 

-5.85

10.0 

.09 

-3.76

12.9

-4.28 

-38.2

0 

-6.27 

-19.4

4.90 

-3.77 

-29.5

2.92 

-4.92 

-38.2

0.12 

-.02 

-3.54

.02 

-.01 

-.06

0 

-.01 

-.09

.001 

-.02 

-.18

.12 

-.02 

-3.54

Group 2

-50 

percent

5.24 

.004 

-6.24

.36 

-.36 

-1.33

2.13 

-.05 

-1.56

2.49 

-.10 

-2.26

5.24 

-.05 

-2.03

7.00 

-.47 

-8.59

.11 

-.94 

-4.45

2.11 

-.41 

-6.51

.65 

-.75 

-8.59

+50 

percent

3.70 

.001 

-4.52

.67 

.26 

-.27

1.31 

-.05 

-1.70

1.70 

.07 

-2.25

1.87 

-.04 

-4.52

4.28 

.15 

-5.08

1.99 

.35 

-.05

3.08 

.15 

-.89

4.24 

.30 

-.39

+90 

percent

5.83 

.002 

-7.70

1.09 

.42 

-.46

2.22 

-.08 

-2.78

2.78 

.11 

-3.86

3.25 

-.08 

-7.70

6.54 

.21 

-8.34

2.95 

.51 

-.08

4.58 

.21 

-1.30

6.47 

.44 

-.61

3.14 

.03 

-.04

-.12 

.03 

-.01

.15 

.02 

-.01

.38 

.05 

-.01

3.14 

.02 

-.03

Group 3

-90 -50 +50 

percent percent percent

15.7 

-.50 

-465

11.7 

-18.3 

-465

2.24 

.14 

-2.68

1.31 

-.09 

-2.49

4.06

-8.75 

-114

1.57 

-11.1 

-114

2.82 

-3.79 

-19.4

1.30 

-6.21 

-20.2

0.11 

-.01 

-.28

.11 

-.02 

-.22

.002 

-.01 

-.09

0 

-.01 

-.04

5.59 

-.32

-75.7

4.28 

-3.17 

-75.6

1.14 

.06 

-1.32

.65 

-.18 

-.96

1.96 

-1.61 

-18.5

.96 

-2.17 

-18.5

1.00 

-.36 

-3.74

.44 

-.81 

-4.19

28.6 

.18 

-24.2

28.6 

1.30 

-2.46

1.02 

-.06 

-1.03

.78 

.26 

-.52

6.70 

.64 

-1.25

6.70 

.92 

-.72

1.53 

.10 

-.57

1.75 

.20

-.28

+90 

percent

32.4 

.42 

-4.15

32.4 

1.56 

-2.82

1.20 

-.06 

-1.23

1.05 

.36 

-.62

9.60 

.94 

-1.99

9.60 

1.38 

-1.16

2.19 

.14 

-.83

2.55 

.25

-.44

0.32 

.02 

-.03

.21 

.03 

-.01

.07 

.01 

0

.08 

.02 

.001
M86 REGIONAL AQUIFER-SYSTEM ANALYSIS NORTHERN ATLANTIC COASTAL PLAIN

changes in head for several reasons. First, being unconfined, it has a much larger storage coefficient than the 

lower confined aquifers. Second, in the various simulation schemes described earlier, it either receives direct 

recharge in large amounts or the water table is treated 

as a constant-head surface. Normally, most of this 

recharge goes to stream nodes and only a small amount 

recharges the deeper aquifers. However, if the water levels in the surficial aquifer start to decline, less of this 

recharge will go to streams, and more is available to sustain the water level in the surficial aquifer and recharge 

the deeper aquifers. Further, if the water level is drawn 

down below stream levels, then the streams recharge the 

surficial aquifer. Thus, the surficial aquifer and those 

aquifers in close hydraulic contact with it tend to show 

the least sensitivity to changes in model parameters. 

Table 7 also indicates that pumpage nodes are the most 

sensitive to parameter changes. This indicates that 

future studies on ground-water flow modeling could best 

concentrate parameter estimation efforts in areas being 

pumped.

Generally, the model was found to be highly sensitive to changes in transmissivity and leakance near 

pumping centers. Away from pumping centers, the model 

was only slightly sensitive to transmissivity changes but 

moderately sensitive to changes in leakance (compare 

mean values for head changes for all active nodes for 

transmissivity and leakance in table 7).

The greatest head difference in the sensitivity analysis was for transmissivity in the Cretaceous aquifers 

(group 1), where a decrease in head of more than 900 ft 

occurred at one pumping node in the Black Creek aquifer 

(A4) when transmissivity was reduced by 90 percent (fig. 

70A). Increases in transmissivity and leakance (figs. 7QA 

and 70B) produced smaller head changes than similar 

percentage decreases in these parameters, whereas 

increases in storage coefficient (fig. 70C) produced similar or sometimes greater changes in head than corresponding decreases.

Results of sensitivity analyses on other subregional 

models in the Northeastern Atlantic Coastal Plain were 

similar to the type of model responses reported here. 

Some effects of parameter changes studied in the sensitivity analyses of Virginia and New Jersey were not 

studied in North Carolina, such as confining-unit storage, 

position of the freshwater-saltwater no-flow boundary, 

boundary fluxes, and recharge (Harsh and Laczniak, 

1990; Martin, 1990, in press). However, because of the 

similarity of the models, similar responses might be 

expected for the North Carolina model.

In New Jersey and Virginia, confining-unit storage 

was shown to be important to simulation of the system 

when the response of confining units is in the range of 

inelastic deformation. The Virginia and New Jersey models were not sensitive to the position of the freshwater-saltwater no-flow boundary. However, unlike 

North Carolina, these States have no significant pumpage affecting areas near the freshwater-saltwater 

no-flow boundary. The New Jersey model was sensitive 

to boundary flux near the boundaries but was not sensitive to boundary fluxes two or three nodes away from 

the boundaries. The Virginia and New Jersey models' 

insensitivity to recharge was similar to that for the 

North Carolina flow model. Results indicate that all the 

models were not sensitive to changes in recharge rate to 

the surficial aquifer in transient simulations, because 

streambed leakance was increased or decreased to move 

more or less water into streams to balance increases or 

decreases in recharge (refer to "Streambed Leakance" 

section of report under "Model Input").

HYDROLOGIC ANALYSIS OF THE AQUIFER 

SYSTEM

WATER BUDGET

Most aspects of the overall ground-water flow system have not changed significantly from predevelopment 

conditions in 1900 to development conditions in 1980. The 

only major change on the system during that time interval has been initiation and continued increase of 

ground-water withdrawals. Pumpage in North Carolina 

and in nearby southeastern Virginia has altered the overall water budget very little, although local effects are significant. Pumpage from large areas in North Carolina 

increased from zero before 1900 to 302 cubic feet per second (ft3/s) (195 Mgal/d) in 1980, which is only about 1.2 

percent of the average recharge to the ground-water system of 25,703 ft3/s (table 8).

Simulation results indicate that pumpage from the 

ground-water system over time is ultimately compensated for largely by reduced flow to streams. Reduction 

in flow to streams between 1900 and 1980 over the entire 

North Carolina Coastal Plain, shown as constant-head 

discharge from the system in table 8, was 297 ft3/s and, 

although not large in relation to the total of the average 

flow of all the Coastal Plain streams, could locally be a 

significant percentage of dry-weather streamflow in 

some streams.

Major changes in the amount of water withdrawn 

from wells (fig. 54 and table 8) have resulted in changes 

in contributions from ground-water storage as favorable 

hydraulic gradients are established toward pumping centers. When these gradients are established and if there is 

sufficient recharge, a new equilibrium condition will be 

reached in which contributions from storage to pumpage 

will be negligible. Significant withdrawals from the sand 

aquifers in rocks of Cretaceous age began during 1940-
HYDROLOGIC ANALYSIS OF THE AQUIFER SYSTEM M87

-1,000

-100

100

-100

-100

-50 0 50 

A. PERCENT CHANGE IN TRANSMISSIVITY

MAXIMUM

MINIMUM

-50 0 50 

B. PERCENT CHANGE IN LEAKANCE

100

i i | i i i r

100

6

4

2

0

-2

-4

-6

-8

-100 -50 0 50 

C. PERCENT CHANGE IN STORAGE COEFFICIENT

100

FIGURE 70. Head differences in Cretaceous sand aquifers (group 1) due to changes in (A) group 1 

transmissivity, (B) group 1 leakance, and (O model-wide storage coefficient.
M88 REGIONAL AQUIFER-SYSTEM ANALYSIS NORTHERN ATLANTIC COASTAL PLAIN

TABLE 8. Summary of model-computed water budgets for the ground-water flow system of the North Carolina Coastal Plain, 1900-2000

Flow rates to (+) and from (-) flow system at end of pumping period (cubic feet per second)

Pumping 

period

Prepumping

1

2

3

4

5

6

7

8

9

10

Date

Pre-1900

January 1, 1900- 

December 31, 1920

January 1, 1921- 

December 31, 1939

January 1, 1940- 

December 31, 1945

January 1, 1946- 

December31, 1952

January 1, 1953- 

Deeember31, 1957

January 1, 1958- 

December 31, 1964

January 1, 1965- 

December 31, 1967

January 1, 1968- 

December 31, 1972

January 1, 1973- 

December 31, 1977

January 1, 1978- 

December31, 1980

Simulated year 2000

Change 

in storage

0

-.1

+.2

+4

+6

+7

+6

+20

+14

+14

+14

+26

Specified flux

Recharge

+25,703

+25,703

+25,703

+25,703

+25,703

+25,703

+25,703

+25,703

+25,703

+25,703

+25,703

+25,703

To system

+10

+10

+8

+8

+7

+7

+7

+7

+8

+8

+8

+8

From system

-15.0

-15.3

-15.3

-32.9

-18.4

-18.8

-20.7

-34.5

-28.8

-36.7

-36

-21

Pumping

0

-4.7

-11.8

-16.1

-62.6

-77.2

-125.3

-197.5

-242.2

-275.3

-294

-531

Constant head

To system

+0.5

+.5

+.5

+.5

+1.5

+2.1

+14.9

+3.6

+4.0

+5.7

+5.9

+42.1

From system

-25,701

-25,697

-25,688

-25,670

-25,641

-25,627

-25,588

-25,504

-25,460

-25,422

-25,404

-25,229

Difference 

between 

inflow and 

outflow

-2.5

-3.6

-3.4

-3.5

-4.5

-3.9

-3.1

-2.4

-2.0

-3.3

-3.1

-1.9

45. At that time, contributions from storage amounted to 

several cubic feet per second. During 1965-67, withdrawals from the Castle Hayne aquifer (A7) increased, and 

storage contributions increased to 20 ft3/s. During 1968- 

80, the contributions from storage resulting from withdrawals from the Castle Hayne aquifer were small as 

equilibrium was again established, but increased withdrawals elsewhere, primarily in sand aquifers in rocks of 

Cretaceous age, resulted in total contributions from storage of about 14ft3/s.

Pumpage has changed ground-water movement 

within the system, but change in the recharge to and discharge from the system has been slight. The average 

recharge rate of 25,703 ft3/s was calculated as a constant 

over the simulation period. Natural discharge from the 

system as base flow to streams and discharge to the 

sounds decreased slightly from 25,701 to 25,404 ft3/s from 

pre-1900 to 1980, a decrease of 297 ft3/s, or less than 1.2 

percent of the total recharge. Pumpage was mainly supplied by this diversion. The change in natural discharge 

of 297 ft3/s is very close to the rate of pumping during the 

1978-80 pumping period (294 ft3/s). The excess decrease 

in natural discharge of 3 ft3/s is mostly due to effects of 

pumpage in adjacent States, primarily Virginia.

The principal change in the ground-water flow system of the North Carolina Coastal Plain primarily 

involves vertical movement of water from one layer in 

the system into another in and around those areas where 

the major pumping is taking place. The changes in direction of vertical flow are shown by a series of maps presented in a following section of this report.

Loss of ground-water storage is sensitive to the rate 

of pumping increase. Rate of loss of storage was greatest 

(20 ft3/s) during 1965-67, when withdrawals increased an 

average of about 24.1 ft3/s per year. Since then, the contribution from storage has remained at 14 ft3/s as withdrawal increases have been fairly constant, averaging 

about 7.4 ft3/s.

Simulated ground-water flow across the Virginia 

and South Carolina borders changed significantly from 

1900 to 1980. The most dramatic change was across the 

Virginia line, where 3.8 ft3/s flowed from Virginia to 

North Carolina in 1900; by 1980, ground-water flow was 

from North Carolina to Virginia at a rate of 24.7 ft3/s. 

This change occurred in the lower Cape Fear aquifer 

(A2) and, to a lesser extent, in the upper Cape Fear (A3) 

and Lower Cretaceous aquifers (Al). The change results 

from pumpage mainly from the lower Cape Fear aquifer
HYDROLOGIC ANALYSIS OF THE AQUIFER SYSTEM M89

(A2) and Lower Cretaceous aquifer (Al) at Franklin, Va., 

which is about 25 mi north of Ahoskie, N.C.

Flow from North Carolina to South Carolina was 7.1 

ft3/s in 1900, and this decreased slightly to 6.4 ft3/s in 

1980. This decrease is attributed to pumpage in the 

southern Coastal Plain of North Carolina from the Black 

Creek (A4) and lower Cape Fear (A2) aquifers. A slight 

lowering of the potentiometric surfaces of these two 

aquifers in North Carolina decreased the hydraulic gradient toward South Carolina.

A model simulation was made for the year 2000 

under the assumption that pumping would increase by 3 

percent per year from 1981 to 2000. Under these conditions, contributions from storage would nearly double, 

from 14 to 26 ft3/s (table 8), as cones of depression in the 

sand aquifers of Cretaceous age would continue to 

increase in size, and contributions from streams (induced 

infiltration) would increase more than sevenfold from 

those of 1980 from 5.9 to 42.1 ft3/s. Ground-water discharge to streams would continue but at a very slightly 

decreased rate.

POTENTIOMETRIC SURFACES

Although changes in the overall water budget for 

the Coastal Plain aquifers due to pumping were minor 

during 1900-80, pumping has caused large local 

decreases in the potentiometric surfaces of several aquifers. In order to show the changes in the potentiometric 

surfaces of the 10 aquifers, maps are presented for the 

simulated prepumping conditions (1900), for 1980 conditions, and for assumed pumping conditions in the year 

2000.

Simulated prepumping potentiometric surfaces are 

shown for the 10 aquifers in figures 71 through 80. The 

figures also show available observed values of water levels for the late 1800's and early 1900's in unpumped areas. 

Many of these water-level observations were taken from 

Clark and others (1912) and from unpublished drillers' 

records from that era. In some instances, modern water 

levels are shown in these figures and were assumed to 

represent prepumping conditions in aquifers unaffected 

by pumping, such as for the lower Cape Fear aquifer 

(A2) (fig. 79) in the southern part of the Coastal Plain. 

The potentiometric surfaces for the surficial aquifer 

(A10) and for the updip (western) parts of all the other 

aquifers are high in the interstream divide areas and low 

in the valleys of perennial streams. This distribution indicates that for these aquifers in these areas, most 

ground-water movement is within local flow systems, 

and most ground-water discharge is to the nearby 

streams. In the downdip areas of confined aquifers, the 

Yorktown through Lower Cretaceous aquifers (A9-A1), 

the potentiometric surfaces have a gentle coastward gradient to the east-southeast.

Simulated potentiometric surfaces for 1980 (figs. SI90), when compared with 1900 prepumping potentiometric surfaces (figs. 71-80), indicate lowered water levels in 

parts of most aquifers due to pumping. Notably, drawdowns of more than 135 ft and 90 ft occurred in parts of 

the Lower Cretaceous (Al) and lower Cape Fear (A2) 

aquifers, respectively, near the Virginia border. These 

drawdowns were due largely to pumpage at Franklin, 

Va. (compare fig. 80 with fig. 90 and fig. 79 with fig. 89). 

Drawdowns of more than 60 ft, 125 ft, and 110 ft occurred 

in parts of the lower Cape Fear (A2), upper Cape Fear 

(A3), and Black Creek (A4) aquifers, respectively, in the 

central Coastal Plain in and around Greenville, Kinston, 

and Jacksonville, N.C. These drawdowns were due to 

several large withdrawals in the area; the large cone of 

depression associated with these withdrawals is most 

evident in figure 87.

Drawdowns of more than 30 ft and 60 ft occurred in 

parts of the Beaufort (A6) and Castle Hayne (A7) aquifers, respectively, in the Beaufort County area. Here, the 

drawdowns were caused in large part by withdrawals 

from the Castle Hayne aquifer (A7) in connection with 

open-pit phosphate-mining operations and a chemical 

plant near Aurora (fig. 58); the cone of depression associated with these withdrawals is most evident in figure 84. 

A noteworthy aspect of the drawdown in the Beaufort 

aquifer (A6) is that no water was withdrawn directly 

from this aquifer at the mine and chemical plant. Withdrawals from the overlying Castle Hayne aquifer (A7) 

caused water to move upward through the Beaufort confining unit (CU6) in such quantities as to create a large 

area of depressed potentiometric levels in the Beaufort 

aquifer (A6) (fig. 85).

The simulated 1980 water level in the Castle Hayne 

aquifer (A7) near Aurora was slightly more than 50 ft 

below sea level, whereas measured water levels in some 

wells were greater than 100 ft below sea level in the 

withdrawal area. This difference in simulated and measured water levels results from the large block size used 

in the flow model. There, the simulated average water 

level over the 12.25-mi2 block area was at least 50ft 

higher than measured water levels in the center of the 

cone of depression.

The Peedee aquifer (A5) also showed a decline of 

about 15 ft between 1900 and 1980 in the Aurora area 

(figs. 76 and 86). As with the Beaufort aquifer (A6), the 

Peedee aquifer has no major withdrawals in this area, 

and its water-level decline is similarly attributed to the 

effect of the large cone of depression in the overlying 

Castle Hayne aquifer (A7).

Text continues on p. Ml 10.
M90 REGIONAL AQUIFER-SYSTEM ANALYSIS NORTHERN ATLANTIC COASTAL PLAIN

Albem<f * Sound '

:£~£^

NORTH

CAROLINA

SOUTH 

CAROLINA V

EXPLANATION

 SO Water-table contour Shows altitude of 

simulated water table, 1900. Contour 

interval, in feet, is variable. Datum is 

sea level

Base modified from U.S. Geological Survey digital data, 

1:2,000,000,1972

FIGURE 71. Simulated prepumping (1900) water table for the surficial aquifer (A10).
HYDROLOGIC ANALYSIS OF THE AQUIFER SYSTEM M91

*r XT' - ^J a

y 2, *x\s. MvriP ^. -3 / " / ^.^jqf i,K"r'^ ier<r

^ ^¥^-1 C/

EXPLANATION 

Yorktown aquifer not present

Potentiometric contour Shows altitude at 

which simulated water level would have 

stood in tightly cased wells, about 1900. 

Contour interval, in feet, is variable. Datum 

is sea level

o18 Well Number is altitude of observed water level 

for about 1900, in feet above sea level

Base modified from U.S. Geological Survey digital data, 

1-2,000.000,1972

FIGURE 72. Simulated prepumping (1900) potentiometric surface for the Yorktown aquifer (A9).
M92 REGIONAL AQUIFER-SYSTEM ANALYSIS NORTHERN ATLANTIC COASTAL PLAIN

I NORTH r

r ARiOLINA

JacksonviUe ' i~ ' *

EXPLANATION

Pungo River aquifer not present

50 Potentiometric contour Shows altitude at 

which simulated water level would have 

stood in tightly cased wells, about 1900. 

Contour interval, in feet, is variable. Datum 

is sea level

O14 Well Number is altitude of observed water level 

for about 1900, in feet above sea level

34° -

Base modified from U.S Geological Survey digital data. 

1:2,000,000,1972

FIGURE 73. Simulated prepumping (1900) potentiometric surface for the Pungo River aquifer (A8).
HYDROLOGIC ANALYSIS OF THE AQUIFER SYSTEM M93

?/:^5fc?48' ^w^ ^QiL>^ /

i X. ^^^^ry- - "- ».. >* «£**?, <~r*

SOUTH X^l

CAROLINA f',\" ~-\

Cape Fear

"* 

\J

50 MILES

50 KILOMETERS

-50 

EXPLANATION

Castle Hayne aquifer not present

Potentiometric contour Shows altitude at 

which simulated water level would have 

stood in tightly cased wells, about 1900. 

Contour interval, in feet, is variable. 

Datum is sea level

Well Number is altitude of observed water level 

for about 1900, in feet above sea level

Base modified from U.S. Geological Survey digital data. 

1-2,000,000,1972

FIGURE 74. Simulated prepumping (1900) potentiometric surface for the Castle Hayne aquifer (A7).
M94 REGIONAL AQUIFER-SYSTEM ANALYSIS NORTHERN ATLANTIC COASTAL PLAIN

r£~

tov*\?. <'

> \ <£'' // \X

\ s

SOUTH 

CAROLINA

Capf Fear

EXPLANATION 

Beaufort aquifer not present

» ..» . Potentiometric surface not determined 

* ~~ ^ Water in aquifer contains 10,000

milligrams per liter chloride, or more

-so Potentiometric contour Shows altitude at 

which simulated water level would have 

stood in tightly cased wells, about 1900. 

Contour interval, in feet, is variable. 

Datum is sea level

O Well Number is altitude of observed water level 

for about 1900, in feet above sea level

Base modified from U.S. Geological Survey digital data, 

1:2.000,000,1972

FIGURE 75. Simulated prepumping (1900) potentiometric surface for the Beaufort aquifer (A6).
HYDROLOGIC ANALYSIS OF THE AQUIFER SYSTEM M95

NORTH . 

1 CA'RlOLtNA

EXPLANATION

Peedee aquifer not present

»fc ^* M Potentiometric surface not determined 

~~ Water in aquifer contains 10,000

milligrams per liter chloride, or more SOUTH V 1

50 Potentiometric contour Shows altitude at 

which simulated water level would have 

stood in tightly cased wells, about 1900. 

Contour interval, in feet, is variable. Datum 

is sea level

O Well Number is altitude of observed water level 

for about 1900, in feet above sea level

Base modified from U.S. Geological Survey digital data. 

1:2.000,000,1972

FIGURE 76. Simulated prepumping (1900) potentiometric surface for the Peedee aquifer (A5).
M96 REGIONAL AQUIFER-SYSTEM ANALYSIS NORTHERN ATLANTIC COASTAL PLAIN

78°

VIRGINIA

: £w

^

Franklin

»%. 

 Ahoskie^

T

9&t^P»-

V V *«

vv VJ«

"T"

^>. '? * -

finer I1 O-es

Rait

NOI^TH .

eA.Rk)LINA i

-If

SAl

^3G7

s.-

fail' vM

VWISOBX

ISO. I GREE

! y ^B4--^1 'MAR^N

,V

PITT ^

* * >

3.':^ -"T".

^TWAYhJfxS 

'^.«0 \

-40 ^

&135

i <-X' ' 9>

\

>

'<*, 72*

-4MPSON} D:JPL|N

132

.#]

JONES

, ^ 

/^ ONSLOW 

Jacksonville

m-^efn-l^ / i- , j - 

JPfMtlCC^ ,N^ «v^C "fc^L^i-^j,^ ^v^^

B130

s^

^

c

0

c&$

lf \-

COL

SOUTH 

CAROLINA

FENDER

;<

Cape Fear

I 0 50 MILES

V 50 KILOMETERS

EXPLANATION

Black Creek aquifer not present

'*» * 

» ^ T » Potentiometric surface not determined 

* ~~ Water in aquifer contains 10,000

milligrams per liter chloride, or more

 50 Potentiometric contour Shows altitude at 

which simulated water level would have 

stood in tightly cased wells, about 1900. 

Contour interval, in feet, is variable. Datum 

is sea level

O Well Number is altitude of observed water level 

for about 1900, in feet above sea level

Base modified from U.S Geological Survey digital data, 

1:2,000,000,1972

FIGURE 77. Simulated prepumping (1900) potentiometric surface for the Black Creek aquifer (A4).
HYDROLOGIC ANALYSIS OF THE AQUIFER SYSTEM M97

EXPLANATION

Upper Cape Fear aquifer not present

»t_* >. T J\ Potentiometric surface not determined 

Water in aquifer contains 10,000 

milligrams per liter chloride, or more

 so Potentiometric contour Shows altitude at 

which simulated water level would have 

stood in tightly cased wells, about 1900. 

Contour interval, in feet, is variable. Datum 

is sea level

O65 Well Number is altitude of observed water level 

for about 1900, in feet above sea level

34°-

Base modified from U.S. Geological Survey digital data. 

1.2,000.000.1972

FIGURE 78. Simulated prepumping (1900) potentiometric surface for the upper Cape Fear aquifer (A3).
M98 REGIONAL AQUIFER-SYSTEM ANALYSIS NORTHERN ATLANTIC COASTAL PLAIN

NORTH !

CARJGL

EXPLANATION

Lower Cape Fear aquifer not present

» ,. T » Potentiometric surface not determined 

Water in aquifer contains 10,000 

milligrams per liter chloride, or more SOUTH 

CAROLINA

50 Potentiometric contour Shows altitude at 

which simulated water level would have 

stood in tightly cased wells, about 1900. 

Contour interval, in feet, is variable. Datum 

is sea level

O85 Well Number is altitude of observed water level 

for about 1900, in feet above sea level

Base modified from U.S. Geological Survey digital data. 

1-2.000,000,1972

FIGURE 79. Simulated prepumping (1900) potentiometric surface for the lower Cape Fear aquifer (A2).
HYDROLOGIC ANALYSIS OF THE AQUIFER SYSTEM M99

%* » ,r«Vv - ;//.-r^V>

EXPLANATION

Lower Cretaceous aquifer not present

,.» »| Potentiometric surface not determined 

Water in aquifer contains 10,000 

milligrams per liter chloride, or more

CAROLINA

Potentiometric contour Shows altitude at 

which simulated water level would have 

stood in tightly cased wells, about 1900. 

Contour interval, in feet, is variable. 

Datum is sea level

Base modified from U.S. Geological Survey digital data, 

1:2.000.000.1972

FIGURE 80. Simulated prepumping (1900) potentiometric surface for the Lower Cretaceous aquifer (Al).
M100 REGIONAL AQUIFER-SYSTEM ANALYSIS NORTHERN ATLANTIC COASTAL PLAIN

 ^ BF^HS J ' V? tlr^ p^-^S*

r~_.~ i* I \ v_\v >- : o ? T» «***

/E^GE^dE t-iov7y;^ r 

rV L \ ^V^^fe^ X1 ^ °« ^v. f ^ 3^-Qkv~

^ ^ X I :=> WilsonV/ ^^

v ^ XJ<HsTOr -V^S^icsig^r --VV"

^X Pall -U^Y^ Y, x \ CteSSflfc

CAROLINA VK-Jt ,o^. ,:

GrecnyiHe y»^S - " ^ HXgE^ >

^ Ql> /^

^^-^l^v ^ >fs^ ^fV

<ure^i^^^ So^

iwAYN^SS^K ^j^^fe:' *~TT -its t ' i 1K^^JK&?D<^^ 'ctiMbff^JJtf.JD)

/ C f c ft' »tt .>^" 11 .>

V7^S» A A\^^r

 : C1 ;^r^ V ;Lfi

EXPLANATION

Water-table contour Shows altitude of 

simulated water table, 1980. Contour 

interval, in feet, is variable. Datum is 

sea level

Base modified from U.S. Geological Survey digital data, 

1:2,000,000.1972

FIGURE 81. Simulated 1980 water table for the surficial aquifer (A10).
HYDROLOGIC ANALYSIS OF THE AQUIFER SYSTEM M101

36°

78° 77°

SOUTH 

CAROLINA \V

50 MILES

I

I 

50 KILOMETERS

Yorktown aquifer not present

 uu Potentiometric contour Shows altitude 

at which simulated water level would 

have stood in tightly cased wells, 1980. 

Contour interval, in feet, is variable. 

Datum is sea level

O75 Well Number is altitude of observed water 

level for 1980, in feet above sea level

Base modified from U.S. Geological Survey digital data, 

1:2.000,000.1972

FIGURE 82. Simulated 1980 potentiometric surface for the Yorktown aquifer (A9).
M102 REGIONAL AQUIFER-SYSTEM ANALYSIS NORTHERN ATLANTIC COASTAL PLAIN

78°

50 MILES

I

50 KILOMETERS

-^

Pungo River aquifer not present

Potentiometric contour Shows altitude at 

which simulated water level would have 

stood in tightly cased wells, 1980. Contour 

interval, in feet, is variable. Datum is sea 

level. Hachures indicate water levels are 

lower than in surrounding area

O8 Well Number is altitude of observed water 

level for 1980. in feet above sea level

Base modified from U S. Geological Survey digital data, 

1:2.000.000.1972

FIGURE 83. Simulated 1980 potentiometric surface for the Pungo River aquifer (A8).
HYDROLOGIC ANALYSIS OF THE AQUIFER SYSTEM M103

77°

Cape 

Haneras

S v \ V*

V

x V

SOUTH v^ 

CAROLINA \ \

'-A

0£&

£

Ov

s

Cap? Fear

j ;

r c

50 MILES

«*a " ^5l

KILOMETERS

,32

EXpLANATION j

Castle Hayne aquifer not present B

Potentiometric contour Shows altitude at i 

which simulated water level would have 

stood in tightly cased wells, 1980. Contour 

interval, in feet, is variable. Datum is sea 

level. Hachures indicate water levels are 

lower than in surrounding area

Well Number is altitude of observed water 

level for 1980, in feet above sea level

Base modified from U S. Geological Survey digital data, 

1:2,000,000,1972

FIGURE 84. Simulated 1980 potentiometric surface for the Castle Hayne aquifer (A7).
M104 REGIONAL AQUIFER-SYSTEM ANALYSIS NORTHERN ATLANTIC COASTAL PLAIN

79°

*v;

^

X S

SOUTH VO 

CAROLINA \V s

'\Uminforif- rf

\l

 ~~4/

Cape Feat'

50 MILES

1\1 0 

^ I

50 KILOMETERS

EXPLANATION 

Beaufort aquifer not present

* » »i Potentiometric surface not determined 

"* ~ Water in aquifer contains 10,000

milligrams per liter chloride, or more

 50 Potentiometric contour Shows altitude at 

which simulated water level would have 

stood in tightly cased wells, 1980. Contour 

interval, in feet, is variable. Datum is sea 

level. Hachures indicate water levels are 

lower than in surrounding area

®22 Well Number is altitude of observed water level 

for 1980, in feet above sea level

Base modified from U S Geological Survey digital data 

1:2.000.000.1972

FIGURE 85. Simulated 1980 potentiometric surface for the Beaufort aquifer (A6).
HYDROLOGIC ANALYSIS OF THE AQUIFER SYSTEM M105

Peedee aquifer not present

SOUTH 

CAROLINA /

X /

/-I 50 MILES

ft 0

I 

50 KILOMETERS

-t-

» Potentiometric surface not determined 

Water in aquifer contains 10,000 

milligrams per liter chloride, or more

-so Potentiometric contour Shows altitude at 

which simulated water level would have 

stood in tightly cased wells, 1980. Contour 

interval, in feet, is variable. Datum is sea 

level. Hachures indicate water levels are 

lower than in surrounding area

-J1 Well Number is altitude of observed water level 

for 1980, in feet above sea level

Base modified from U.S. Geological Survey digital data, 

1:2,000,000,1972

FIGURE 86. Simulated 1980 potentiometric surface for the Peedee aquifer (A5).
M106 REGIONAL AQUIFER-SYSTEM ANALYSIS NORTHERN ATLANTIC COASTAL PLAIN

NORTH

CAROLINA

EXPLANATION 

Black Creek aquifer not present

» » »! Potentiometric surface not determined 

Water in aquifer contains 10,000 

milligrams per liter chloride, or more SOUTH 

CAROLINA Potentiometric contour Shows altitude at 

which simulated water level would have 

stood in tightly cased wells, 1980. Contour 

interval, in feet, is variable. Datum is sea 

level. Hachures indicate water levels are 

lower than in surrounding area

O44 Well Number is altitude of observed water level 

for 1980, in feet above sea level

Base modified from U.S. Geological Survey digital data, 

1.2.000.000.1972

FIGURE 87. Simulated 1980 potentiometric surface for the Black Creek aquifer (A4).
HYDROLOGIC ANALYSIS OF THE AQUIFER SYSTEM M107

NORTH

CAROLINA

EXPLANATION

Upper Cape Fear aquifer not present

Potentiometric surface not determined 

Water in aquifer contains 10,000 

milligrams per liter chloride, or more SOUTH 

CAROLINA Potentiometric contour Shows altitude at 

which simulated water level would have 

stood in tightly cased wells, 1980. Contour 

interval, in feet, is variable. Datum is sea 

level. Hachures indicate water levels are 

lower than in surrounding area

Well Number is altitude of observed water level 

for 1980, in feet above sea level

34° -

Base modified from U.S. Geological Survey digital data, 

1.2,000.000,1972

FIGURE 88. Simulated 1980 potentiometric surface for the upper Cape Fear aquifer (A3).
M108 REGIONAL AQUIFER-SYSTEM ANALYSIS NORTHERN ATLANTIC COASTAL PLAIN

78°

ONFLOW V« ll^^^.^V-V" 

Jacksonville. , 'V^ ', ' ' ' ''vW & ^

* T"? i * 

- 

." ' - a 

vk. - " . lr*~ ?-~>JeJ< ." -^>! {A X^rf,&

SOUTH 

CAROLINA

Cape Fear

"W

/ 

0 50 MILES

^ o

^" I

50 KILOMETERS

EXPLANATION

Lower Cape Fear aquifer not present

+ Potentiometric surface not determined 

Water in aquifer contains 10,000 

milligrams per liter chloride, or more

 Potentiometric contour Shows altitude at 

which simulated water level would have 

stood in tightly cased wells, 1980. Contour 

interval, in feet, is variable. Datum is sea 

level. Hachures indicate water levels are 

lower than in surrounding area

O Well Number is altitude of observed water level 

for 1980, in feet above sea level

-50-

Base modified from U.S. Geological Survey digital data, 

1.2,000,000,1972

FIGURE 89. Simulated 1980 potentiometric surface for the lower Cape Fear aquifer (A2).
HYDROLOGIC ANALYSIS OF THE AQUIFER SYSTEM M109

EXPLANATION

Lower Cretaceous aquifer not present

Potentiometric surface not determined 

Water in aquifer contains 10,000 

milligrams per liter chloride, or more SOUTH 

CAROLINA Potentiometric contour Shows altitude at 

which simulated water level would have 

stood in tightly cased wells, 1980. Contour 

interval, in feet, is variable. Datum is sea 

level. Hachures indicate water levels are 

lower than in surrounding area

O" Well Number is altitude of observed water level 

for 1980, in feet above sea level

Base modified from U.S Geological Survey digital data, 

1-2,000,000, 1972

FIGURE 90. Simulated 1980 potentiometric surface for the Lower Cretaceous aquifer (Al).
MHO REGIONAL AQUIFER-SYSTEM ANALYSIS NORTHERN ATLANTIC COASTAL PLAIN

Although the calibrated flow model is not detailed 

enough for many management purposes, simulated 

potentiometric surfaces were generated (figs. 91-100) for 

the year 2000 (by assuming a uniform 3-percent annual 

increase in pumpage for 1980-2000 at the 1980 pumpage 

sites) to assess the effect of future pumping on potentiometric surfaces. Actually, several major ground-water 

pumpages are not expected to increase at this rate during this period, notably at Franklin, Va., and at the phosphate-mining operations in Beaufort County, N.C. Thus, 

in these areas, the year 2000 simulations may not accurately reflect what is likely to occur.

Considerable areas of most aquifers exhibit drawdowns of less than 10 ft for the 1900^80 and 1980-2000 

periods. Comparison of the water-table maps of 1900, 

1980, and 2000 (figs. 71, 81, and 91) shows little or no 

change in water levels in the surficial aquifer (A10). The 

same is true for the potentiometric surfaces of the Yorktown aquifer (A9), as seen in figures 72, 82, and 92. 

Except for the immediate vicinity of major withdrawals, 

potentiometric surfaces in the updip (western) parts of 

the Castle Hayne (A7), Beaufort (A6), Peedee (A5), 

Black Creek (A4), and upper Cape Fear (A3) aquifers 

declined 10 ft or less. The areas of little change for these 

confined aquifers are where each one is directly overlain 

by either the surficial aquifer (A10) or the Yorktown 

aquifer (A9). (Compare figs. 73-78, figs. 83^88, and figs. 

93-98.)

By 1980, water levels in confined aquifers in the 

North Carolina Coastal Plain had not declined below the 

tops of the aquifers. When and if this happens, the 

aquifers will change from confined to unconfined conditions, and the aquifer storage coefficients will increase by 

several orders of magnitude (from about 10"4 to about 

IxHH). Subsequent drawdowns will occur by dewatering rather than by expansion of the water and compaction of aquifer and confining-unit sediments. Drawdowns 

will then proceed at a slower rate than under the previous confined conditions. Thus, model predictions of year 

2000 water levels for such situations might tend to overestimate drawdowns regionally; however, locally they 

may underestimate drawdowns near the pumping 

centers.

VERTICAL FLOW

As a generalization, the downward vertical movement of ground-water recharge to confined aquifers 

under natural conditions occurs mainly in interstream 

areas, and ground-water discharge upward from the confined aquifer system occurs mainly in stream valleys and 

in downdip (coastward) areas. This general pattern of

recharge and discharge has been significantly altered in 

several areas of the North Carolina Coastal Plain, where 

pumping from several of the aquifers has reduced the 

hydraulic head in these aquifers over large areas and has 

thus disturbed the prepumping head relations that 

existed between the aquifers. When those head relations 

are changed, the potential for water to move from one 

aquifer to another through the intervening confining 

unit(s) is changed and often reversed. The changes in 

flow direction due to pumping are evident from a comparison of prepumping conditions of 1900 in figures 101 

through 109 with those of 1980 in figures 110 through 

118.

In the northeastern North Carolina Coastal Plain, 

during prepumping conditions, vertical ground-water 

flow through the Black Creek (CU4), upper Cape Fear 

(CU3), lower Cape Fear (CU2), and Lower Cretaceous 

(GUI) confining units (figs. 106-109) was upward in 

many parts of the area. By 1980, vertical flow gradients 

had been reversed across large parts of these confining 

units (figs. 115-118). Vertical flow by 1980 was downward 

in response to large pumpages from the Lower Cretaceous (Al) and lower Cape Fear (A2) aquifers in 

Franklin, Va.

Extensive pumping from the Black Creek (A4) and 

upper Cape Fear (A3) aquifers in the central North 

Carolina Coastal Plain area has resulted in reversal of 

flow from upward to downward through the Peedee 

(CU5), Black Creek (CU4), and upper Cape Fear (CU3) 

confining units (figs. 105-107 and 114-116). However, 

flow through the upper Cape Fear confining unit in the 

area between Kinston and New Bern is still upward in 

response to pumping from the overlying Black Creek 

aquifer (A4), which is pumped more heavily than the 

upper Cape Fear aquifer (A3). A large area of the Black 

Creek confining unit (CU4) in the southern North Carolina Coastal Plain has experienced a change in vertical 

flow from upward to downward in response to moderate 

but widespread pumping (figs. 106 and 115).

In the Beaufort County area, vertical flow under 

prepumping conditions was generally upward, this being 

a discharge area for most of the North Carolina Coastal 

Plain aquifers present there. After large-scale pumping 

began in 1965 in the Castle Hayne aquifer (A7), vertical 

flow was toward the Castle Hayne aquifer; that is, 

upward vertical flow across confining units CU6 and 

CU5 was greater than before, and vertical flow through 

the Yorktown (CU9) through Castle Hayne (CU7) confining units reversed direction from upward to downward in 

the vicinity of the large-scale pumping areas.

Text continues on p. Ml39.
HYDROLOGIC ANALYSIS OF THE AQUIFER SYSTEM Mill

36°

SOUTH 

CAROLINA !\{

~W

/ o 50 MILES

M I 

50 KILOMETERS

EXPLANATION

-50 - Water-table contour Shows altitude of 

simulated water table, 2000. Contour 

interval, in feet, is variable. Datum is 

sea level

Base modified from U S. Geological Survey digital data, 

1:2,000,000.1972

FIGURE 91. Simulated year 2000 water table for the surficial aquifer (A10), assuming 3-percent annual increase in pumpage during 1980-2000.
M112 REGIONAL AQUIFER-SYSTEM ANALYSIS NORTHERN ATLANTIC COASTAL PLAIN

NX>"RTH

"CAROllNA

EXPLANATION

Yorktown aquifer not present

Potentiometric contour Shows altitude at 

which simulated water level would stand in 

tightly cased wells, 2000. Contour interval 

in feet, is variable. Datum is sea level. 

Hachures indicate water levels are lower 

than in surrounding area

Base modified from U.S. Geological Survey digital data, 

1:2,000,000.1972

FIGURE 92. Simulated year 2000 potentiometric surface for the Yorktown aquifer (A9), assuming 3-percent annual increase in pumpage during

1980-2000.
HYDROLOGIC ANALYSIS OF THE AQUIFER SYSTEM M113

79°

} NORTHJ

CAROLINA

EXPLANATION

CAROLINA Pungo River aquifer not present

Potentiometric contour Shows altitude at 

which simulated water level would stand in 

tightly cased wells, 2000. Contour interval, 

in feet, is variable. Datum is sea level. 

Hachures indicate water levels are lower 

than in surrounding area

Base modified from U.S Geological Survey digital data, 

1:2,000,000,1972

FIGURE 93. Simulated year 2000 potentiometric surface for the Pungo River aquifer (A8), assuming 3-percent annual increase in pumpage during

1980-2000.
M114 REGIONAL AQUIFER-SYSTEM ANALYSIS NORTHERN ATLANTIC COASTAL PLAIN

SOUTH 

CAROLINA

50 MILES

I

i

50 KILOMETERS

EXPLANATION

Castle Hayne aquifer not present

Potentiometric contour Shows altitude at 

which simulated water level would stand in 

tightly cased wells, 2000. Contour interval, 

in feet, is variable. Datum is sea level. 

Hachures indicate water levels are lower 

than in surrounding area

Base modified from U.S. Geological Survey digital data, 

1.2,000,000,1972

FIGURE 94. Simulated year 2000 potentiometric surface for the Castle Hayne aquifer (A7), assuming 3-percent annual increase in pumpage

during 1980-2000.
HYDROLOGIC ANALYSIS OF THE AQUIFER SYSTEM M115

^y"-:\*St

NORTH

CAROLINA

EXPLANATION

Beaufort aquifer not present

SOUTH 

CAROLINA

..» .. Potentiometric surface not determined 

Water in aquifer contains 10,000 

milligrams per liter chloride, or more

Potentiometric contour Shows altitude at 

which simulated water level would stand in 

tightly cased wells, 2000. Contour interval 

in feet, is variable. Datum is sea level. 

Hachures indicate water levels are lower 

than in surrounding area

34° 

Base modified from U.S. Geological Survey digital data, 

1-2.000.000. 1972

FIGURE 95. Simulated year 2000 potentiometric surface for the Beaufort aquifer (A6), assuming 3-percent annual increase in pumpage during

1980-2000.
Ml 16 REGIONAL AQUIFER-SYSTEM ANALYSIS NORTHERN ATLANTIC COASTAL PLAIN

">"'e>\ * * V c "^ -I r^/Y^i^ 

J x^ * * _ CN * :: 

/.:*.*. 

i _ » A*"'

CAROLINA l^vHYbKr-V/* / '

EXPLANATION 

Peedee aquifer not present

^ * »i Potentiometric surface not determined 

Water in aquifer contains 10,000 

milligrams per liter chloride, or more

CAROLINA

so Potentiometric contour Shows altitude at 

which simulated water level would stand in 

tightly cased wells, 2000. Contour interval, 

in feet, is variable. Datum is sea level. 

Hachures indicate water levels are lower 

than in surrounding area

Base modified from U.S. Geological Survey digital data, 

1:2.000.000,1972

FIGURE 96. Simulated year 2000 potentiometric surface for the Peedee aquifer (A5), assuming 3-percent annual increase in pumpage during

1980-2000.
HYDROLOGIC ANALYSIS OF THE AQUIFER SYSTEM M117

EXPLANATION

Black Creek aquifer not present

,. T_,] Potentiometric surface not determined 

Water in aquifer contains 10,000 

milligrams per liter chloride, or more

Potentiometric contour Shows altitude at 

which simulated water level would stand in 

tightly cased wells, 2000. Contour interval 

in feet, is variable. Datum is sea level. 

Hachures indicate water levels are lower 

than in surrounding area

34° -

Base modified from U.S Geological Survey digital data. 

1:2.000.000.1972

FIGURE 97. Simulated year 2000 potentiometric surface for the Black Creek aquifer (A4), assuming 3-percent annual increase in pumpage during

1980-2000.
Ml 18 REGIONAL AQUIFER-SYSTEM ANALYSIS NORTHERN ATLANTIC COASTAL PLAIN

NGR'CH

CARJOLINA

EXPLANATION

Upper Cape Fear aquifer not present

SOUTH \ 

CAROLINA

» ^ » » Potentiometric surface not determined 

" Water in aquifer contains 10,000

milligrams per liter chloride, or more

50 Potentiometric contour Shows altitude at 

which simulated water level would stand in 

tightly cased wells, 2000. Contour interval, 

in feet, is variable. Datum is sea level. 

Hachures indicate water levels are lower 

than in surrounding area

Base modified from U.S. Geological Survey digital data, 

1:2,000,000,1972

FIGURE 98. Simulated year 2000 potentiometric surface for the upper Cape Fear aquifer (A3), assuming 3-percent annual increase in pumpage

during 1980-2000.
HYDROLOGIC ANALYSIS OF THE AQUIFER SYSTEM M119

VIRGINIA

-JK&fc-'

>-3- Ssrffc

EXPLANATION 

Lower Cape Fear aquifer not present

..* Potentiometric surface not determined 

~ Water in aquifer contains 10,000

milligrams per liter chloride, or more

CAROLINA \\

50 Potentiometric contour Shows altitude at 

which simulated water level would stand in 

tightly cased wells, 2000. Contour interval, 

in feet, is variable. Datum is sea level. 

Hachures indicate water levels are lower 

than in surrounding area

34° -

Base modified from U.S. Geological Survey digital data, 

1:2,000,000,1972

FIGURE 99. Simulated year 2000 potentiometric surface for the lower Cape Fear aquifer (A2), assuming 3-percent annual increase in pumpage

during 1980-2000.
M120 REGIONAL AQUIFER-SYSTEM ANALYSIS NORTHERN ATLANTIC COASTAL PLAIN

EXPLANATION

Lower Cretaceous aquifer not present

Potentiometric surface not determined 

Water in aquifer contains 10,000 

milligrams per liter chloride, or more

Potentiometric contour Shows altitude at 

which simulated water level would stand 

in tightly cased wells, 2000. Contour 

interval 25 feet. Datum is sea level. 

Hachures indicate water levels are lower 

than in surrounding area

Base modified from U.S Geological Survey digital data, 

1:2,000,000,1972

FIGURE 100. Simulated year 2000 potentiometric surface for the Lower Cretaceous aquifer (Al), assuming 3-percent annual increase in pumpage

during 1980-2000.
HYDROLOGIC ANALYSIS OF THE AQUIFER SYSTEM M121

NORTH

CAROLINA

EXPLANATION

Yorktown confining unit not present 

Recharge area Flow is downward 

Discharge area Row is upward

CAROLINA N

34° 

Base modified from U.S. Geological Survey digital data, 

1:2,000,000,1972

FIGURE 101. Net vertical flow of water through the Yorktown confining unit (CU9) simulated for predevelopment (1900) conditions.
M122 REGIONAL AQUIFER-SYSTEM ANALYSIS NORTHERN ATLANTIC COASTAL PLAIN

SOUTH 

CAROLINA V'\ EXPLANATION

Pungo River confining unit not present 

Recharge area Flow is downward 

Discharge area Flow is upward

Base modified from U.S Geological Survey digital data, 

1:2,000,000,1972

FIGURE 102. Net vertical flow of water through the Pungo River confining unit (CU8) simulated for predevelopment (1900) conditions.
HYDROLOGIC ANALYSIS OF THE AQUIFER SYSTEM M123

NORTH

"CARlOLINA

SOUTH 

CAROLINA EXPLANATION

Castle Hayne confining unit not present 

Recharge area Flow is downward 

Discharge area Row is upward

34° -

Base modified from U.S. Geological Survey digital data. 

1:2,000.000.1972

FIGURE 103. Net vertical flow of water through the Castle Hayne confining unit (CUT) simulated for predevelopment (1900) conditions.
M124 REGIONAL AQUIFER-SYSTEM ANALYSIS NORTHERN ATLANTIC COASTAL PLAIN

Y^£ / GATES (\ 1 ° I & \*>

^ \^

EXPLANATION 

Beaufort confining unit not present

Flow direction not determined Water 

in Beaufort aquifer contains 10,000 

milligrams per liter chloride, or more

Recharge area Flow is downward 

Discharge area Flow is upward

Base modified from U.S. Geological Survey digital data. 

1:2.000.000,1972

FIGURE 104. Net vertical flow of water through the Beaufort confining unit (CU6) simulated for predevelopment (1900) conditions.
HYDROLOGIC ANALYSIS OF THE AQUIFER SYSTEM M125

^, \

EXPLANATION

Peedee confining unit not present

Flow direction not determined Water 

in Peedee aquifer contains 10,000 

milligrams per liter chloride, or more

Recharge area Flow is downward 

Discharge area Flow is upward

34° -

Base modified from U.S. Geological Survey digital data, 

1.2.000.000.1972

FIGURE 105. Net vertical flow of water through the Peedee confining unit (CU5) simulated for predevelopment (1900) conditions.
M126 REGIONAL AQUIFER-SYSTEM ANALYSIS NORTHERN ATLANTIC COASTAL PLAIN

NORTH

'CAROLINA"

-"M^. ,Afb^

EXPLANATION 

SOUTH Black Creek confining unit not present

CAROLINA Flow direction not determined Water in 

Black Creek aquifer contains 10,000 

milligrams per liter chloride, or more

Recharge area Flow is downward 

Discharge area Flow is upward

Base modified from U.S. Geological Survey digital data. 

1:2,000,000,1972

FIGURE 106. Net vertical flow of water through the Black Creek confining unit (CU4) simulated for predevelopment (1900) conditions.
HYDROLOGIC ANALYSIS OF THE AQUIFER SYSTEM M127

V %.'Bou^

EXPLANATION

Upper Cape Fear confining unit not present

Flow direction not determined Water in 

upper Cape Fear aquifer contains 10,000 

milligrams per liter chloride, or more

Recharge area Flow is downward 

Discharge area Flow is upward

34° -

Base modified from U.S. Geological Survey digital data, 

1:2.000.000.1972

FIGURE 107. Net vertical flow of water through the upper Cape Fear confining unit (CU3) simulated for predevelopment (1900) conditions.
M128 REGIONAL AQUIFER-SYSTEM ANALYSIS NORTHERN ATLANTIC COASTAL PLAIN

'- Aibem«»-?e Smm<!

\ °} "'if

\ j BRUNSWICK ,'\U EXPLANATION 

Lower Cape Fear confining unit not present

Flow direction not determined Water in 

lower Cape Fear aquifer contains 10,000 

milligrams per liter chloride, or more

Recharge area Flow is downward 

Discharge area Flow is upward

Base modified from U.S. Geological Survey digital data, 

1:2.000.000.1972

FIGURE 108. Net vertical flow of water through the lower Cape Fear confining unit (CU2) simulated for predevelopment (1900) conditions.
HYDROLOGIC ANALYSIS OF THE AQUIFER SYSTEM M129

-- ^.^"fiNiJ O^v »: innfcyL*lj '{ » ^ * » A ::

} \ » V^*^~8 *" *" » *5*^ 1^ » i

EXPLANATION 

SOUTH Lower Cretaceous confining unit not present

CAROLINA Flow direction not determined Water in 

Lower Cretaceous aquifer contains 10.000 

milligrams per liter chloride, or more

Recharge area Flow is downward 

Discharge area Flow is upward

Base modified from U.S Geological Survey digital data 

1-2.000,000,1972

FIGURE 109. Net vertical flow of water through the Lower Cretaceous confining unit (GUI) simulated for predevelopment (1900) conditions.
M130 REGIONAL AQUIFER-SYSTEM ANALYSIS NORTHERN ATLANTIC COASTAL PLAIN

34° -

' 

VtLSON X

.. T" X Grecmi

SOUTH X " 

CAROLINA v \

X^,~A

inv

/

50 MILES

50 KILOMETERS

EXPLANATION 

Yorktown confining unit not present 

Recharge area Flow is downward 

Discharge area Flow is upward

Base modified from U.S. Geological Survey digital data, 

1:2,000,000,1972

FIGURE 110. Net vertical flow of water through the Yorktown confining unit (CU9) simulated for 1980 conditions.
HYDROLOGIC ANALYSIS OF THE AQUIFER SYSTEM M131

^ VIRGINIA

y ' NORTH ,.

s CAROLINA

SOUTH XI 

CAROLINA VEXPLANATION

Pungo River confining unit not present 

Recharge area Flow is downward 

Discharge area Flow is upward

Base modified from U.S. Geological Survey digital data, 

1.2,000,000.1972

FIGURE 111. Net vertical flow of water through the Pungo River confining unit (CU8) simulated for 1980 conditions.
M132 REGIONAL AQUIFER-SYSTEM ANALYSIS NORTHERN ATLANTIC COASTAL PLAIN

NORTH f

CARJOLINA

EXPLANATION

Castle Hayne confining unit not present 

Recharge area Flow is downward 

Discharge area Flow is upward

Base modified from U.S. Geological Survey digital data, 

1:2,000,000,1972

FIGURE 112. Net vertical flow of water through the Castle Hayne confining unit (CUT) simulated for 1980 conditions.
HYDROLOGIC ANALYSIS OF THE AQUIFER SYSTEM M133

V NORTH f

CAROl

EXPLANATION

SOUTH Beaufort confining unit not present

CAROLINA Flow direction not determined Water 

in Beaufort aquifer contains 10,000 

milligrams per liter chloride, or more

Recharge area Flow is downward

Discharge area Flow is upward

34° -

Base modified from U.S. Geological Survey digital data, 

1:2.000.000.1972

FIGURE 113. Net vertical flow of water through the Beaufort confining unit (CU6) simulated for 1980 conditions.
M134 REGIONAL AQUIFER-SYSTEM ANALYSIS NORTHERN ATLANTIC COASTAL PLAIN

^v v } § !

EXPLANATION 

SOUTH Peedee confining unit not present

CAROLINA Flow direction not determined Water 

in Peedee aquifer contains 10,000 

milligrams per liter chloride, or more

Recharge area Flow is downward 

Discharge area Flow is upward

Base modified from U.S. Geological Survey digital data, 

1:2,000,000,1972

FIGURE 114. Net vertical flow of water through the Peedee confining unit (CU5) simulated for 1980 conditions.
HYDROLOGIC ANALYSIS OF THE AQUIFER SYSTEM M135

7 Al\>etnftrle Sound

EXPLANATION 

SOUTH Black Creek confining unit not present

CAROLINA Flow direction not determined Water in 

Black Creek aquifer contains 10,000 

milligrams per liter chloride, or more

Recharge area Flow is downward

Discharge area Flow is upward

Base modified from U.S. Geological Survey digital data, 

1-2.000.000.1972

FIGURE 115. Net vertical flow of water through the Black Creek confining unit (CU4) simulatec for 1980 conditions.
M136 REGIONAL AQUIFER-SYSTEM ANALYSIS NORTHERN ATLANTIC COASTAL PLAIN

,,-i I * * *<J } .f - -*- « R

10 - . >L, ' * *,-' \ '' - '1bzJ~

^y N v COLUMBU

EXPLANATION

Upper Cape Fear confining unit not present

Flow direction not determined Water in 

upper Cape Fear aquifer contains 10,000 

milligrams per liter chloride, or more

Recharge area Flow is downward 

Discharge area Flow is upward

Base modified from U.S. Geological Survey digital data, 

1:2.000,000,1972

FIGURE 116. Net vertical flow of water through the upper Cape Fear confining unit (CU3) simulated for 1980 conditions.
HYDROLOGIC ANALYSIS OF THE AQUIFER SYSTEM M137

36°

-£L Cape

Hatierax

CAR;Ot,INA

EXPLANATION 

SOUTH Lower Cape Fear confining unit not present

CAROLINA \\ Flow direction not determined Water in 

lower Cape Fear aquifer contains 10,000 

milligrams per liter chloride, or more

Recharge area Flow is downward 

Discharge area Flow is upward

Base modified from U.S. Geological Survey digital data, 

1:2,000.000.1972

FIGURE 117. Net vertical flow of water through the lower Cape Fear confining unit (CU2) simulated for 1980 conditions.
M138 REGIONAL AQUIFER-SYSTEM ANALYSIS NORTHERN ATLANTIC COASTAL PLAIN

-NOKTH-i

CAROLINA

EXPLANATION

Lower Cretaceous confining unit not present

Flow direction not determined Water in 

Lower Cretaceous aquifer contains 10,000 

milligrams per liter chloride, or more

Recharge area Flow is downward

Base modified from U.S. Geological Survey digital data, 

1-2.000,000,1972

FIGURE 118. Net vertical flow of water through the Lower Cretaceous confining unit (CU1) simulated for 1980 conditions.
SUMMARY AND CONCLUSIONS M139

SUMMARY AND CONCLUSIONS

A three-dimensional finite-difference digital flow 

model was developed to simulate ground-water flow in 

the Coastal Plain aquifer system of North Carolina, covering an area of 25,000 mi2 . The 10-layer model was 

developed from a hydrogeologic framework made up of 

an alternating sequence of aquifers and confining units 

that dip and thicken in the seaward direction. The model 

code used was a three-dimensional finite-difference 

ground-water flow model, and the modeled area was 

divided into a rectangular grid for each of the 10 layers 

with varied spacing representing areas ranging from 

12.25 mi2 to 56.25 mi2. The grid and boundary conditions 

of the mathematical-numerical model were designed to 

be compatible with a regional model of the entire Northern Atlantic Coastal Plain.

Western and eastern boundaries for each of the 

model layers were treated as no-flow boundaries. The 

eastern boundaries were assumed to be along lines of 

equal concentrations of 10,000 mg/L of dissolved chloride. 

Boundaries at the State lines, Virginia to the north and 

South Carolina to the south, were specified-flux boundaries, the fluxes being provided by the regional model. 

The upper boundary of each confined aquifer was the 

bottom of its associated confining unit, which was treated 

as a head-dependent flux boundary.

For the model as a whole, the upper boundary was 

the layer representing streams (a constant-head boundary), and the lower boundary was either bedrock or a 

saltwater-bearing aquifer (no-flow boundary). The treatment of the line representing the 10,000 mg/L chloride 

concentration as a no-flow boundary was valid in areas 

not affected by pumping, but was not valid where effects 

of pumping extended to the boundary, as in parts of the 

Beaufort (A6), Black Creek (A4), upper Cape Fear (A3), 

lower Cape Fear (A2), and Lower Cretaceous (Al) aquifers. In these areas, the model will tend to predict 

greater drawdown than would actually occur.

Model input consisted of transmissivity and storage 

coefficient of the aquifers, confining-unit and streambed 

leakance, ground-water recharge, water-table and 

stream-surface altitudes, boundary-flux rates, and pumpage. For transient simulations, 10 pumping periods starting in 1900 and running through 1980 were used. 

Pumping periods ranged in length from 21 years (first 

pumping period) to 3 years (last pumping period).

A number of calibration simulations were made for 

the purpose of testing how well the model was able to 

reproduce measured water levels and as an aid to indicate where adjustments to model parameters were 

needed. The primary parameters adjusted were leakance 

of the confining units and, to a lesser extent, transmissivity of the aquifers.

During the calibration procedures, about 240 computed hydrographs were compared with observed hydrographs. Also, computed potentiometric surfaces for 

various pumping periods were checked with actual 

water-level data. Adjustments to model parameters 

were made after careful consideration of whether the 

adjustments could be justified based on data on the 

hydrogeologic and ground-water flow systems. The largest changes were in estimates of leakance of confining 

units and were considered reasonable because the initial 

values of this parameter were known with less certainty 

than any other parameters.

For the calibrated model, the maximum transmissivity was about 200,000 ft2/d in a part of the Castle Hayne 

aquifer (A7). The smallest vertical hydraulic conductivity 

of a confining unit for the calibrated model was about 

4.1xlO~6 ft/d in a part of the lower Cape Fear confining 

unit. Simulations indicate that a barrier to flow exists in 

the vicinity of a landward reentrant of water containing 

10,000 mg/L of dissolved chloride in the upper and lower 

Cape Fear aquifers (A3 and A2) over the Cape Fear arch 

in the Wilmington area. The barrier is probably the cause 

of the high hydraulic heads in these two aquifers southwest of the arch in southeastern North Carolina and 

northeastern South Carolina. The nature of the barrier 

needs further research but is likely due either to diminished hydraulic conductivity in the area or to the presence of salty water, or both.

The relatively large block size made it difficult to 

accurately simulate the configuration of the water table 

in the Sand Hills area. Thus, the model is probably less 

accurate in this area than in any other area in the North 

Carolina Coastal Plain.

Sensitivity analysis indicated that the model is especially sensitive to changes in transmissivity in areas 

where pumping occurs, but regionally the model is more 

uniformly sensitive to leakance through the confining 

units. Sensitivity of the model to changes in storage coefficient of the confined aquifers is slight and, as with 

transmissivity, is greatest near the pumping centers.

Under predevelopment (1900) conditions, average 

recharge from precipitation to the surficial aquifer varied 

areally from about 12 in/yr in areas with clay soils to 20 

in/yr in areas with sandy soils. Most of this moved 

directly to streams and only about 1 in/yr moved downward into the confined aquifer system. Generally, 

recharge to the confined aquifer system took place 

mostly in updip interstream areas, and discharge 

occurred in streams and stream valleys and in downdip 

coastal areas. Hydrologic analysis of the aquifer system 

based on simulations indicates that the overall water 

budget has changed little from predevelopment conditions in 1900 to development conditions in 1980. Pumpage 

from large users increased from zero to 302 ft3/s during
M140 REGIONAL AQUIFER-SYSTEM ANALYSIS NORTHERN ATLANTIC COASTAL PLAIN

that period, which amounts to about 1.2 percent of the 

recharge to the ground-water system of 25,703 ft3/s. Simulation results indicate that water lost to the aquifer system through pumping is largely offset by reductions in 

base flow of streams and, to a lesser extent, by reductions in aquifer storage. By 1980, contribution from aquifer storage was about 14 ft3/s, or about 4.8 percent of 

pumpage.

Although the effect of pumping on the overall water 

budget is relatively small, water levels in the North 

Carolina Coastal Plain aquifer system have declined dramatically in parts of some aquifers since 1900 (more than 

135 ft in some locations) due to pumping stresses. The 

three principal areas of long-term decline in water level 

are in the Lower Cretaceous (Al) and lower Cape Fear 

(A2) aquifers in northeastern North Carolina in response 

to withdrawals at Franklin, Va.; in the lower Cape Fear 

(A2), upper Cape Fear (A3), and Black Creek (A4) aquifers in the central North Carolina Coastal Plain, where 

pumping occurs at several sites; and in the Beaufort (A6) 

and Castle Hayne (A7) aquifers in Beaufort County, 

where there are large withdrawals for open-pit phosphate mining and chemical manufacturing.

Since 1900, water levels have declined less than 10 ft 

in the surficial (A10) and Yorktown (A9) aquifers; this is 

also true of the updip parts of most of the other aquifers 

where major withdrawals were not occurring in 1980. 

Simulations to the year 2000 with a 3-percent annual 

increase in withdrawals at 1980 pumping locations gave 

virtually the same results as the 1980 simulations of the 

same two shallow aquifers and updip parts of the other 

aquifers. The year 2000 simulations indicated significant 

local water-level declines in lower confined aquifers, but 

on a cell basis, not below the tops of the aquifers.

Directions of vertical flow through confining units 

have been altered in many areas since 1900. In 1900, vertical ground-water flow through confining units was generally downward in interstream areas and upward in 

major stream valleys and in coastal areas. Ground-water 

pumpage substantially altered that pattern in several 

areas. The most widespread change caused ground water 

to move downward into pumped aquifers in areas where 

it had moved upward in predevelopment times. By 1980, 

areas of downward movement commonly occurred in 

major stream valleys. Areas of downward movement also 

extended farther coastward in 1980 than in 1900 in confining units overlying extensively developed aquifers.

SELECTED REFERENCES

Aucott, W.R., Davis, M.E., and Speiran, G.K., 1987, Geohydrologic framework of the Coastal Plain aquifers of South

Carolina: U.S. Geological Survey Water-Resources Investigations Report 85-4271.

Aucott, W.R., and Speiran, G.K., 1985, Ground-water flow in the 

Coastal Plain aquifers of South Carolina: Ground Water, v. 

23, no. 6, p. 736-745.

Beck, M.B., and van Straten, Gerrit, eds., 1983, Uncertainty 

and forecasting of water quality: New York, Springer-Verlag, 386 p.

Blankenship, R.R., 1965, Reconnaissance of ground-water 

resources of the Southport-Elizabethtown area, North 

Carolina: North Carolina Department of Water Resources 

Ground-Water Bulletin 6,47 p.

Brown, D.L., and Cosner, 0 J., 1974, Ground-water conditions 

in the Franklin area, southeastern Virginia: U.S. Geological Survey Hydrologic Investigations Atlas HA--538, 3 

sheets.

Brown, P.M., 1958, Well logs from the Coastal Plain of North 

Carolina: North Carolina Department of Conservation and 

Development Bulletin 72,68 p.

 1959, Geology and ground-water resources in the Greenville area, North Carolina: North Carolina Department of 

Conservation and Development Bulletin 73,87 p.

Brown, P.M., Miller, J.A., and Swain, F.M., 1972, Structural and 

stratigraphic framework and spatial distribution of the 

permeability of the Atlantic Coastal Plain, New York to 

North Carolina: U.S. Geological Survey Professional 

Paper 796,79 p.

Clark, W.B., Miller, B.L., Stephenson, L.W, Johnson, B.L., and 

Parker, H.N., 1912, The Coastal Plain of North Carolina: 

North Carolina Geological and Economic Survey Bulletin 

3,372 p.

Coble, R.W, Giese, G.L., and Winner, M.D., Jr., 1987, Application of regional aquifer-system analysis study results to 

ground-water management in North Carolina, in Vecchioli, 

John, and Johnson, A.I., eds., Regional aquifer systems of 

the United States, Aquifers of the Atlantic and Gulf 

Coastal Plain: American Water Resources Association 

Monograph Series 9, p. 39-49.

Cosner, 0 J., 1975, A predictive computer model of the Lower 

Cretaceous aquifer, Franklin area, southeastern Virginia: 

U.S. Geological Survey Water-Resources Investigations 

Report 51-74,62 p.

Daniel, J.F., 1976, Estimating evapotranspiration from streamflow records: Water Resources Research, v. 12, p. 360-364.

DeWiest, R J.M., Sayre, AN., and Jacob, C.E., 1967, Evaluation of potential impact of phosphate mining on 

ground-water resources of eastern North Carolina: Board 

of Consultants Report to North Carolina Department of 

Water Resources, 167 p.

Eder, B.K., Davis, J.M., and Robinson, P.J., 1983, Variations in 

monthly precipitation over North Carolina: University of 

North Carolina Water Resources Research Institute 

Report 83-185,50 p.

Emsellem, Y, and deMarsily, G., 1971, An automatic solution 

for the inverse problem: Water Resources Research, v. 7, 

no. 5, p. 1264-1283.

Fenneman, N.M., 1938, Physiography of eastern United States: 

New York, McGraw-Hill, 714 p.
SELECTED REFERENCES M141

Floyd, E.G., and Peace, R.R., 1974, An appraisal of the 

ground-water resources of the upper Cape Fear River 

basin, North Carolina: North Carolina Department of 

Water and Air Resources Ground-Water Bulletin 20,17 p.

Franke, O.L., Reilly, T.E., and Bennett, G.D., 1984, Definition of 

boundary and initial conditions in the analysis of saturated 

ground-water flow systems An introduction: U.S. Geological Survey Open-File Report 84-458, 26 p.

Giese, G.L., Wilder, H.B., and Parker, G.G., Jr., 1985, Hydrology 

of major estuaries and sounds of North Carolina: U.S. Geological Survey Water-Supply Paper 2221,108 p.

Hamilton, PA., and Larson, J.D., 1988, Hydrogeology and analysis of the ground-water flow system in the Coastal Plain of 

southeastern Virginia: U.S. Geological Survey WaterResources Investigations Report 87-4240,175 p.

Harris, W.H., and Wilder, H.B., 1966, Geology and groundwater resources of the Hertford-Elizabeth City area, 

North Carolina: North Carolina Department of Water 

Resources Ground-Water Bulletin 10,89 p.

Harsh, J.F., and Laczniak, RJ., 1990, Conceptualization and 

analysis of ground-water flow system in the Coastal Plain 

[aquifers] of Virginia and adjacent parts of Maryland and 

North Carolina: U.S. Geological Survey Professional 

Paper 1404-F, 100 p.

Heath, R.C., 1980, Basic elements of ground-water hydrology 

with reference to conditions in North Carolina: U.S. Geological Survey Water-Resources Investigations Open-File 

Report 80-44, 86 p.

Hitchon, B., 1969, Fluid flow in the western Canada sedimentary basin, 1, Effect of topography: Water Resources 

Research, v. 5, no. 1, p. 186-195.

Kimrey, J.O., 1965, Description of the Pungo River Formation 

in Beaufort County, North Carolina: North Carolina 

Department of Conservation and Development, Division of 

Mineral Resources Bulletin 79,131 p.

Layne-Western Company, Inc., 1983, Virginia/North Carolina 

analog model study: Kansas City, Kansas, v. 1,131 p.

Leahy, P.P., 1982, A three-dimensional ground-water flow 

model modified to reduce computer memory requirements 

and better simulate confining bed and aquifer pinchouts: 

U.S. Geological Survey Water-Resources Investigations 

Report 82^023, 59 p.

Leahy, PP., and Martin, Mary, 1993, Geohydrology and simulation of ground-water flow in the northern Atlantic Coastal 

Plain aquifer system: U.S. Geological Survey Professional 

Paper 1404-K, 81 p., 22 pis.

LeGrand, H.E., 1955, Brackish water and its structural implications in the Great Carolina Ridge, North Carolina: American Association of Petroleum Geologists Bulletin, v. 39, 

no. 10, p. 2020-2037.

 1960, Geology and ground-water resources of the Wilmington-New Bern area: North Carolina Department of 

Water Resources Ground-Water Bulletin 1, 80 p.

 1961, Summary of geology of Atlantic Coastal Plain: 

American Association of Petroleum Geologists Bulletin, v. 

45, no. 9, p. 1557-1571.

 1964, Hydrogeologic framework of the Gulf and Atlantic 

Coastal Plain: Southeastern Geology, v. 5, no. 4, p. 177-194.

 1976, Hydrologic implications of the 1886 Charleston

earthquake [abs.]: Geological Society of America Abstracts

with Programs, v. 8, no. 2, p. 216. 

Lohman, S.W., 1972, Ground-water hydraulics: U.S. Geological

Survey Professional Paper 708, 70 p. 

Martin, Mary, 1990, Ground-water flow in the New Jersey

Coastal Plain: U.S. Geological Survey Open-File Report

87-528,182 p.

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Plain: U.S. Geological Survey Professional Paper 1404-H.

Meisler, Harold, 1981, Preliminary delineation of salty ground 

water in the Northern Atlantic Coastal Plain: U.S. Geological Survey Open-File Report 81-71,25 p.

 1986, Northern Atlantic Coastal Plain regional aquifer-system study, in Sun, R J., ed., Regional aquifer-system 

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of projects, 1978-84: U.S. Geological Survey Circular 1002, 

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water in the Northern Atlantic Coastal Plain: U.S. Geological Survey Professional Paper 1404-D, 51 p., 6 pis.

Meng, A.A., III, and Harsh, J.F., 1988, Hydrogeologic framework of the Virginia Coastal Plain: U.S. Geological Survey 

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Morris, D.A., and Johnson, A.I., 1967, Summary of hydrologic 

and physical properties of rock and soil materials, as analyzed by the Hydrologic Laboratory of the U.S. Geological 

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Paper 1839-D, 42 p.

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Carolina: North Carolina Department of Conservation and 

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Narkunas, James, 1980, Groundwater evaluation in the central 

Coastal Plain of North Carolina: Raleigh, N.C., North 

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Nelson, P.F., 1964, Geology and ground-water resources of the 

Swanquarter area: North Carolina Department of Water 

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 1976, Interim report on ground-water conditions in 

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ground-water resources of the Kinston area, North Carolina: North Carolina Department of Water Resources 

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of Water Resources publication, 42 p.

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1983, Survey of agricultural irrigation 1982 Lower Cape 

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northeastern North Carolina: North Carolina Department 

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Report of Investigations 15,29 p.
M142 REGIONAL AQUIFER-SYSTEM ANALYSIS NORTHERN ATLANTIC COASTAL PLAIN

Peek, H.M., and Register, L.A., 1975, A preliminary report on 

anomalous pressure in deep artesian aquifers in southeastern North Carolina: North Carolina Department of Natural and Economic Resources Report of Investigations 10, 

20 p.

Robison, T.M., 1977, Public water supplies of North Carolina; 

Part 4, Northern Coastal Plain: Raleigh, N.C., North Carolina Department of Natural and Economic Resources, 

218 p.

Robison, T.M., and Mann, L.T., Jr., 1977, Public water supplies 

of North Carolina; Part 5, Southern Coastal Plain: Raleigh, 

N.C., North Carolina Department of Natural Resources 

and Community Development, 333 p.

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Schipf, R.G., 1961, Geology and ground-water resources of the 

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Report 75,126 p.

Steinhorst, R.K., Hurt, H.W, Innis, G.S., and Haydock, K.P., 

1978, Sensitivity analysis of the ELM model, in Innis, G.S., 

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resources: Raleigh, N.C., North Carolina Department of 

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Tomovic, R., 1962, Sensitivity analysis of dynamic systems: 

New York, McGraw-Hill, 142 p.

Trescott, PC., 1975, Documentation of finite-difference model 

for simulation of three-dimensional ground-water flow: 

U.S. Geological Survey Open-File Report 75-438,48 p.

Trescott, PC., and Larson, S.P., 1976, Supplement to Open-File 

Report 75-438, Documentation of finite-difference model 

for simulation of three-dimensional ground-water flow: 

U.S. Geological Survey Open-File Report 76-591,21 p.

Trescott, PC., Pinder, G.F., and Larson, S.P, 1976, Finite-difference model for aquifer simulation in two dimensions with 

results of numerical experiments: U.S. Geological Survey 

Techniques of Water-Resources Investigations, Book 7, 

chap. Cl, 116 p.

Wilder, H.B., Robison, T.M., and Lindskov, K.L., 1978, Water 

resources of northeast North Carolina: U.S. Geological 

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Wilder, H.B., and Simmons, C.E., 1982, Program for evaluating 

stream quality in North Carolina: U.S. Geological Survey 

Water-Supply Paper 2185-A, 15 p.

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U.S. Geological Survey Open-File Report 87-690,155 p.

 1996, Hydrogeologic framework of the North Carolina 

Coastal Plain: U.S. Geological Survey Professional Paper 

1404-1,106 p., 24 pis.

U.S. GOVERNMENT PRINTING OFFICE: 1997-547-740
SELECTED SEMES OF U.S. GEOLOGICAL SURVEY PUBLICATIONS

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"Publications of the Geological Survey, 1962-1970" may 

be purchased by mail and over the counter in paperback book form 

and as a set of microfiche.

"Publications of the U.S. Geological Survey, 1971-1981" 

may be purchased by mail and over the counter in paperback book 

form (two volumes, publications listing and index) and as a set of 

microfiche.

Supplements for 1982, 1983, 1984, 1985, 1986, and for subsequent years since the last permanent catalog may be purchased 

by mail and over the counter in paperback book form.

State catalogs, "List of U.S. Geological Survey Geologic 

and Water-Supply Reports and Maps For (State)," may be purchased by mail and over the counter in paperback booklet form 

only.

"Price and Availability List of U.S. Geological Survey 

Publications," issued annually, is available free of charge in 

paperback booklet form only.

Selected copies of a monthly catalog "New Publications of 

the U.S. Geological Survey" are available free of charge by mail 

or may be obtained over the counter in paperback booklet form 

only. Those wishing a free subscription to the monthly catalog 

"New Publications of the U.S. Geological Survey" should write to 

the U.S. Geological Survey, 582 National Center, Reston, VA 

20192.

Note Prices of Government publications listed in older catalogs, announcements, and publications may be incorrect. Therefore, the prices charged may differ from the prices in catalogs, 

announcements, and publications.