Cover photograph: Great Egret, Clam Creek, Jekyll Island, Georgia 

Photograph by Alan M. Cressler, U.S. Geological Survey, 2000 
Simulation of Ground-Water Flow in 

Coastal Georgia and Adjacent Parts of 

South Carolina and Florida— 

Predevelopment, 1980, and 2000

By Dorothy F. Payne, Malek Abu Rumman, and John S. Clarke

U.S. Department of the Interior

U.S. Geological Survey

Prepared in cooperation with the

Georgia Department of Natural Resources

Environmental Protection Division

Scientific Investigations Report 2005-5089
U.S. Department of the Interior

Gale A. Norton, Secretary

U.S. Geological Survey

Charles G. Groat, Director

U.S. Geological Survey, Reston, Virginia: 2005

This report is a Web-only publication: pubs.water.usgs.gov/sir2005-5089

For more information about the USGS and its products:

Telephone: 1-888-ASK-USGS

World Wide Web: http://www.usgs.gov/

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.

Although this report is in the public domain, permission must be secured from the individual copyright owners to reproduce any copyrighted materials contained within this report.

Suggested citation:

Payne, D.F., Malek Abu Rumman, and J.S. Clarke, 2005, Simulation of ground-water flow in coastal Georgia and adjacent parts of South Carolina and Florida—Predevelopment, 1980, and 2000: U.S. Geological Survey, Scientific Investigations Report 2005-5089, 91 p.
 iii

Contents

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

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

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

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

Previous Investigations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Hydrogeology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Geologic Setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Hydrogeologic Units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Surficial and Brunswick Aquifer Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Floridan Aquifer System. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Ground-Water Flow System. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Predevelopment. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Modern Day: 1980 and 2000. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Recharge and Discharge. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Ground-Water Pumpage. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

Ground-Water-Level Trends. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Saltwater Contamination. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Simulation of Ground-Water Flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Spatial Discretization. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Model Layering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

Hydraulic Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

Boundary Conditions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

Vertical Boundaries. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

Lateral Boundaries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

Offshore Boundary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

Pumpage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

Model Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

Calibration Targets. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

Recharge. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

Steady-State Simulation of Predevelopment Flow System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

Ground-Water Flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

Water Budget . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

Steady-State Simulation of 1980 and 2000 Flow System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

Calibration of Simulated Head . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

Ground-Water Flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

Vertical Distribution of Head . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

Water-Level Change . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

Predevelopment to 2000 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

1980–2000 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

Water Budget . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

Seaward and Landward Flow. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
iv 

Model Sensitivity. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

Composite-Scaled Sensitivities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

Pumping Sensitivity. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58

Fixed-Head Boundary Condition Sensitivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58

Model Limitations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

Summary. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62

Selected References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64

Appendix A. Observed and Simulated Ground-Water Levels, 1980 and 2000 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

Appendix B. Transient Response Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81

Figures

1. Map showing location of 24-county coastal Georgia area, model area, and major 

structural features. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3

2. Generalized correlation of geologic and hydrogeologic units and model layers. . . . . . . . . . . . . . . . . . . . . .5

3. Schematic block diagram showing hydrogeologic units and influence of structural features

on their occurrence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6

4. Schematic diagram showing conceptual model of predevelopment and modern-day (2000)

flow system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9

5–6. Maps showing—

5. Potentiometric surface of the Upper Floridan aquifer, May 1998 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

6. Mean-annual precipitation for selected National Weather Service stations and 

mean-annual baseflow computed using hydrograph-separation analysis in the model 

area, 1971–2000 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

7. Chart showing estimated ground-water pumpage from the Upper and Lower Floridan aquifers, 

1980–2000. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

8–9. Maps showing—

8. Change in water levels in wells completed in the Upper Floridan aquifer, 

May 1980–September 2000. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

9. Model grid and lateral boundary conditions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

10. Schematic diagram showing model layers and boundary conditions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

11. Hydrogeologic sections showing vertical discretization of hydrogeologic units simulated 

by the model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

12. Schematic diagram showing simulated hydraulic-property zones by model layer . . . . . . . . . . . . . . . . . 22

13. Map showing location of tested model boundaries, selected onshore wells, and 

offshore drilling sites; and difference betweensimulated head using fixed-head and

 no-flow boundary conditions in layers 3, 5, and 7 at various locations in the study area. . . . . . . . . . . . 26

14. Chart showing effects of various pumping distributions on simulated head in the 

Upper Floridan aquifer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

15.–17. Maps showing—

15. Distribution of ground-water pumpage by model layer, 1980, layer 3, Brunswick aquifer 

system; layer 5, Upper Floridan aquifer; layer 5, Upper Floridan aquifer (enlarged view); 

layer 7, Lower Floridan aquifer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

16. Distribution of ground-water pumpage by model layer, 2000, layer 3, Brunswick aquifer 

system; layer 5, Upper Floridan aquifer; layer 5, Upper Floridan aquifer (enlarged view); 

layer 7, Lower Floridan aquifer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

17. Simulated predevelopment potentiometric surfaces for the Brunswick aquifer system, 

Upper Floridan aquifer, and Lower Floridan aquifer, and estimated predevelopment 

potentiometric surface for the Upper Floridan aquifer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

18. Schematic diagram showing simulated predevelopment water budget . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
 v

19. Map showing simulated recharge or discharge during predevelopment . . . . . . . . . . . . . . . . . . . . . . . . . . 39

20. Charts showing observed and simulated water levels (residuals) for 1980 and 2000. . . . . . . . . . . . . . . . 41

21.–26. Maps showing—

21. Difference between simulated and observed water levels (residuals) by model layer

for 1980. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

22. Difference between simulated and observed water levels (residuals) by model layer 

for 2000. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

23. Simulated 1980 potentiometric surfaces for the Brunswick aquifer system, Upper Floridan

aquifer, and Lower Floridan aquifer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

24. Simulated 2000 potentiometric surfaces for the Brunswick aquifer system, Upper Floridan 

aquifer, and Lower Floridan aquifer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

25. Simulated and observed water levels and vertical flow directions at well-cluster sites . . . . . . 48

26. Simulated water-level change, between predevelopment and 2000 for the Brunswick 

aquifer system; Upper Floridan aquifer, and Lower Floridan aquifer . . . . . . . . . . . . . . . . . . . . . . . . . 50

27. Simulated water-level change, between1980 and 2000 for the Brunswick aquifer system, 

Upper Floridan aquifer, and Lower Floridan aquifer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

28. Schematic diagram showing simulated water budget for 1980 and 2000. . . . . . . . . . . . . . . . . . . . . . . . . . . 52

29. Chart showing comparison of major components of predevelopment, 1980, and 2000 

simulated water budgets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

30. Map showing simulated recharge or discharge during 2000 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

31. Schematic diagram showing magnitude and direction of simulated horizontal 

 ground-water flow along the coastline for the Upper and Lower Floridan aquifers 

during predevelopment and 2000 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

32. Chart showing composite-scaled sensitivity of selected model parameters . . . . . . . . . . . . . . . . . . . . . . . 58

33.–34. Maps showing—

33. Change in simulated ground-water levels resulting from a 10-percent increase in 2000 

pumping rates for the Brunswick aquifer system, Upper Floridan aquifer, and Lower 

Floridan aquifer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

34. Change in simulated ground-water levels resulting from a 10-foot increase 

in head along the southern fixed boundary for the Brunswick aquifer system, 

 Upper Floridan aquifer, and Lower Floridan aquifer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

A1. Maps showing locations of wells used for 1980 simulation in study area; Chatham, Bryan, 

and Liberty Counties; and McIntosh, Glynn, and Camden Counties. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68

A2. Maps showing locations of wells used for 2000 simulation in study area; Chatham, Bryan, 

and Liberty Counties; and McIntosh, Glynn, and Camden Counties. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71

B1. Chart showing water-level elevation relative to steady-state waterlevel at 2000 pumping rates 

during five 1-year time steps at 2000 pumping rates at five Georgia locations. . . . . . . . . . . . . . . . . . . . . . 81

B2. Map showing percent of steady-state conditions reached in the Upper Floridan aquifer 

(layer 5) during transient simulation after 25 years of pumping using 5-year time steps at 2000 

pumping conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
vi 

Tables

1. Estimated ground-water pumpage from the Upper Floridan aquifer in the coastal area of 

Georgia and adjacent parts of SouthCarolina and Florida, 1980–2000 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

2. Estimated ground-water pumpage from the Lower Floridan aquifer in the coastal area of 

Georgia and adjacent parts of SouthCarolina and Florida, 1980–2000 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

3. Reported and simulated ranges of aquifer transmissivity and confining unit vertical 

hydraulic conductivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

4. Statistics for quantifiable components of observation accuracy. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

5. Simulated predevelopment water budget by model layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

6. Calibration statistics for simulated heads for 1980 conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

7. Calibration statistics for simulated heads for 2000 conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

8. Simulated 1980 water budget by model layer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

9. Simulated 2000 water budget by model layer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

A1. Simulated and observed ground-water levels, 1980 and 2000 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
 vii

Conversion Factors and Datum

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

square foot (ft2) 929.0 square centimeter (cm2)

square foot (ft2) 0.09290 square meter (m2)

square inch (in2) 6.452 square centimeter (cm2)

section (640 acres or 1 square mile) 259.0 square hectometer (hm2)

square mile (mi2) 259.0 hectare (ha)

square mile (mi2) 2.590 square kilometer (km2)

Volume

gallon (gal) 3.785 liter (L)

gallon (gal) 0.003785 cubic meter (m3)

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

Flow rate

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

cubic foot per day (ft3/d) 0.02832 cubic meter per day (m3/d)

gallon per minute (gal/min) 0.06309 liter per second (L/s)

gallon per day (gal/d) 0.003785 cubic meter per day (m3/d)

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

Hydraulic conductivity

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

Hydraulic gradient

foot per mile (ft/mi) 0.1894 meter per kilometer (m/km)

Transmissivity*

foot squared per day (ft2/d) 0.09290 meter squared per day (m2/d)

inch per year per foot [(in/yr)/ft] 83.33 millimeter per year per meter [(mm/yr)/m]

*Transmissivity: The standard unit for transmissivity is cubic foot per day per square foot times foot 

of aquifer thickness [(ft3/d)/ft2

]ft. In this report, the mathematically reduced form, foot squared per 

day (ft2

/d), is used for convenience.
viii 

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

°F = (1.8 x °C) + 32

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

°C = (°F - 32) / 1.8

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

(NAVD 88)

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

Historical data collected and stored as North American Datum 1927 have been converted to 

NAD 83 for this publication.

Altitude, as used in this report, refers to distance above the vertical datum.
Simulation of Ground-Water Flow in Coastal Georgia 

and Adjacent Parts of South Carolina and Florida— 

Predevelopment, 1980, and 2000

By Dorothy F. Payne, Malek Abu Rumman, and John S. Clarke

Abstract

A digital model was developed to simulate steady-state 

ground-water flow in a 42,155-square-mile area of coastal 

Georgia and adjacent parts of South Carolina and Florida. The 

model was developed to (1) understand and refine the conceptual model of regional ground-water flow, (2) serve as a framework for the development of digital subregional ground-water 

flow and solute-transport models, and (3) serve as a tool for 

future evaluations of hypothetical pumping scenarios used to 

facilitate water management in the coastal area.

Single-density ground-water flow was simulated using the 

U.S. Geological Survey finite-difference code MODFLOW-2000

for mean-annual conditions during predevelopment (pre–1900) 

and the years 1980 and 2000. The model comprises seven layers:

the surficial aquifer system, the Brunswick aquifer system, the 

Upper Floridan aquifer, the Lower Floridan aquifer, and the 

intervening confining units. A combination of boundary conditions was applied, including a general-head boundary condition 

on the top active cells of the model and a time-variable fixed-head 

boundary condition along part of the southern lateral boundary.

Simulated heads for 1980 and 2000 conditions indicate a 

good match to observed values, based on a plus-or-minus 

10-foot (ft) calibration target and calibration statistics. The rootmean square of residual water levels for the Upper Floridan 

aquifer was 13.0 ft for the 1980 calibration and 9.94 ft for the 

2000 calibration. Some spatial patterns of residuals were indicated for the 1980 and 2000 simulations, and are likely a result 

of model-grid cell size and insufficiently detailed hydraulicproperty and pumpage data in some areas. Simulated potentiometric surfaces for predevelopment, 1980, and 2000 conditions 

all show major flow system features that are indicated by 

estimated potentiometric maps.

During 1980–2000, simulated water levels at the centers 

of pumping at Savannah and Brunswick rose more than 20 ft 

and 8 ft, respectively, in response to decreased pumping. Simulated drawdown exceeded 10 ft in the Upper Floridan aquifer 

across much of the western half of the model area, with drawdown exceeding 20 ft along parts of the western, northern, and 

southern boundaries where irrigation pumping increased during 

this period.

From predevelopment to 2000 conditions, the simulated 

water budget showed an increase in inflow from, and decrease 

in outflow to, the general-head boundaries, and a reversal from 

net seaward flow to net landward flow across the coastline. 

Simulated changes in recharge and discharge distribution from 

predevelopment to 2000 conditions showed an increase in 

extent and magnitude of net recharge cells in the northern part 

of the model area, and a decrease in discharge or change to 

recharge in cells containing major streams and beneath major 

pumping centers.

The model is relatively sensitive to pumping and the 

controlling head at the fixed-head boundary and less sensitive 

to the distribution of aquifer properties in general. Model limitations include: (1) its spatial scale and discretization, (2) the 

extent to which data are available to physically define the flow 

system, (3) the type of boundary conditions and controlling 

parameters used, (4) uncertainty in the distribution of pumping, 

and (5) uncertainty in field-scale hydraulic properties. The 

model could be improved with more accurate estimates of 

ground-water pumpage and better characterization of recharge 

and discharge.

Introduction

During the last several decades, population growth in the 

coastal area of Georgia, increased tourism, and sustained industrial activity have resulted in an increase in ground-water pumpage. Recent periods of severe drought also have increased 

stresses on the coastal ground-water system. Projected increase 

in coastal population during the next several decades is 

expected to result in increased, competing ground-water 

demands. The principal source of water in the coastal area is the 

Upper Floridan aquifer, an extremely permeable, high-yielding 

aquifer that was first developed in the late 1800s and has been 

used extensively in the area since then. Pumping from the 

Upper Floridan aquifer has resulted in substantial water-level 

decline near Savannah, Georgia, and saltwater intrusion at the 

northern end of Hilton Head Island, South Carolina, and at 

Brunswick, Georgia. This saltwater contamination has constrained further development of the Upper Floridan aquifer in 
2 Simulation of Ground-Water Flow in Coastal Georgia and Adjacent Parts of 

South Carolina and Florida—Predevelopment, 1980, and 2000

the coastal area and created competing demands for the limited 

supply of water. The Georgia Environmental Protection Division (GaEPD) has capped permitted withdrawal from the Upper 

Floridan aquifer at 1997 rates in parts of the coastal area to limit 

further saltwater intrusion, prompting interest in the development of alternative sources of water supply, primarily from the 

shallower surficial and Brunswick aquifer systems. 

In order to develop a strategy to address these problems 

and projected future coastal water resource needs, the GaEPD 

has implemented the Georgia Coastal Sound Science Initiative 

(CSSI), a series of scientific and feasibility investigations 

designed to assess coastal area ground-water resources and 

address issues of saltwater intrusion and resource sustainability. 

The role of the U.S. Geological Survey (USGS) in the CSSI is 

to collect and analyze hydrogeologic data in order to refine the 

conceptual models of ground-water flow and saltwater transport, expand the conceptual model of the ground-water flow 

system to include potential ground-water resources other than 

the Upper Floridan aquifer, and synthesize this information into 

digital models that describe the ground-water flow system. The 

GaEPD will use these digital models to help design a coastal 

area ground-water management strategy.

The digital models developed by the USGS as part of the 

CSSI must satisfy multiple objectives at varying scales. Objectives include simulation of (1) the regional flow system, including the Brunswick aquifer system and the Lower Floridan aquifer, in addition to the Upper Floridan aquifer; (2) subregional 

flow and localized saltwater intrusion in the Savannah, Ga.–

Hilton Head Island, S.C., area; and (3) localized saltwater intrusion at Brunswick, Ga. To satisfy these objectives, the USGS 

has developed a consistent set of ground-water flow and solutetransport models. These models update and expand on earlier 

digital models for the area.

Purpose and Scope

This report documents the USGS digital ground-water 

flow model developed to simulate the coastal Georgia regional 

ground-water flow system, including the Floridan aquifer 

system and the Brunswick aquifer system. This model is used 

to (1) understand and refine the conceptual model of regional 

ground-water flow, (2) serve as a framework for the development of digital subregional ground-water flow and solutetransport models, and (3) serve as a tool for future evaluations 

of hypothetical pumping-distribution scenarios used to facilitate water management in the coastal area. Discussions in this 

report include modeling procedures; boundary condition construction, testing and rationale; rationale for the steady-state 

approximation; calibration approach; sensitivity analyses; 

estimated volumetric flow budgets for predevelopment, 1980, 

and 2000 conditions; and water-level changes from predevelopment to 2000 and from 1980 to 2000. Data acquired as part of 

the CSSI were integrated with available data from USGS databases, publications, and a variety of other sources, to create 

model input and calibration sets that are as current and selfconsistent as practicable.

Description of Study Area

The GaEPD defines the coastal area of Georgia to include 

the 6 coastal counties and adjacent 18 counties, an area of about 

12,240 square miles (mi2) (fig. 1). To account for natural hydrologic boundaries used for model simulation, the study area has 

been expanded to 42,155 mi2 extending inland in Georgia and 

into northeastern Florida and southwestern South Carolina, and 

the adjacent offshore area (see fig. 1).

The 24-county coastal area has been subdivided by 

GaEPD into three subareas—the northern, southern, and central 

subareas—to facilitate implementation of the State’s watermanagement practices (fig. 1). The northern subarea is northwest of the Gulf Trough, a prominent geologic feature that represents a zone of low permeability in the Floridan aquifer system.

The southern subarea lies south of what GaEPD has called the 

“Satilla Line,” a postulated hydrologic boundary based on a 

change in the configuration of the potentiometric surface of the 

Upper Floridan aquifer, and by linear changes depicted on aeromagnetic, aeroradioactivity, gravity, and isopach maps 

(William H. McLemore, Georgia Environmental Protection 

Division, oral commun., January 6, 2000). The central subarea 

lies between the northern and southern subareas, and includes 

the largest concentration of pumping in the coastal area—the 

Savannah, Brunswick, and Jesup pumping centers (fig. 1).

The study area is in the Coastal Plain physiographic province. Topographic relief ranges from low in the central and 

southern subareas to steep in the northern subarea. Altitudes are 

as high as 100 ft (above NAVD 88) in the central and southern 

subareas, and 300 ft in the northern subarea. Land use is largely 

urban in industrial areas and cities such as Savannah and Brunswick; outside of these areas, land use is a mix of forest, grazed 

woodland, cropland with pasture, marsh, and swampland.

The study area has a mild climate with warm, humid summers and mild winters. Mean-annual temperature ranges from 

about 63 degrees Fahrenheit (oF) in Burke County, Ga., to about 

70°F in Glynn County, Ga., for the period 1971–2000 (National 

Oceanic and Atmospheric Administration, 2002). Mean-annual 

precipitation, based on the period 1971–2000, ranges from 

about 47 inches per year (in/yr) at Waynesboro, Ga., to about 

53 in/yr at Folkston, Ga. (Priest, 2004). Rainfall is not evenly 

distributed throughout the year. Maximum rainfall generally 

occurs during the summer months of June, July, and August. 

Estimated evapotranspiration ranges from 31 in/yr in the northern part of the study area to more than 40 in/yr in Charlton and 

Ware Counties, Ga., near the Okefenokee Swamp (Krause and 

Randolph, 1989). Rainfall as a source of recharge to aquifers is 

most important during the nongrowing season, generally 

October through March, when evapotranspiration is lowest. 
?

?

AIKEN

WARE

DIXIE

TAYLOR

BURKE

CLAY

DUVAL

BERKELEY

CLINCH

ALACHUA

COLLETON

PUTNAM

JASPER

BAKER

WAYNE

LAURENS

SUMTER

ORANGEBURG

MADISON

WORTH

RICHLAND

COFFEE

COLUMBIA

NASSAU

BULLOCH

LONG

EMANUEL

CHARLTON CAMDEN

DODGE

SCREVEN

LEXINGTON

TIFT

BRYAN

WILKES

LIBERTY

IRWIN

SUWANNEE

WILLIAMSBURG

JONES

APPLING

ST JOHNS

BROOKS

BIBB

GLYNN

DOOLY

HAMPTON

CLARENDON

COLQUITT

MITCHELL

TELFAIR

ECHOLS

LOWNDES

BERRIEN

BARNWELL

WASHINGTON

JASPER

LAFAYETTE

WILCOX

HAMILTON

GREENE

TATTNALL

HANCOCK

CRISP

THOMAS

PIERCE

JEFFERSON

CHARLESTON

TWIGGS

BRANTLEY

TOOMBS

EDGEFIELD

PUTNAM

JENKINS

DORCHESTER

EFFINGHAM

CHATHAM

WILKINSON

MCINTOSH

CALHOUN

JEFFERSON

COOK

BACON

UNION

MORGAN

BAMBERG

WALTON

MONROE

HOUSTON

ALLENDALE

TURNER

ATKINSON

GILCHRIST

MCCORMICK

WARREN

JOHNSON

OGLETHORPE

WHEELER

RICHMOND

BALDWIN

PULASKI

NEWTON

JEFF DAVIS

COLUMBIA

LINCOLN

EVANS

LANIER

BEN HILL

CANDLER

BRADFORD

MCDUFFIE

GEORGETOWN

OCONEE

PEACH

BUTTS

BLECKLEY

MACON

GWINNETT

TREUTLEN

CRAWFORD

TAYLOR

MONTGOMERY

TALIAFERRO

GLASCOCK

LEE

ROCKDALE

SUMTER

HENRY

BEAUFORT Model boundary

Brunswick

St Marys

Jacksonville

Fernandina

Beach

Jesup

Waycross

Savannah

Valdosta

84°

32°

33°

31°

30°

83° 82° 81° 80°

GEORGIA

GEORGIA

SOUTH

CAROLINA

FLORIDA

Atla

ntic

Ocean

Port Royal Sound

FLORIDA

GEORGIA

Map area

Northern

Central

Southern

24-county coastal Georgia 

 area as identified by the 

 Georgia Environmental 

 Protection Division

0 50 75 MILES 25

0 75 KILOMETERS 25 50

Base from U.S. Geological Survey 1:100,000-scale 

digital data. Structural features modified from 

Applied Coastal Research Laboratory, 2002; 

Weems and Edwards, 2001; Kellam and Gorday, 1990; 

and Paull and Dillon, 1980.

SOUTH

CAROLINA

Introduction 3

Figure 1. Location of 24-county coastal Georgia area, model area, and major structural features.
4 Simulation of Ground-Water Flow in Coastal Georgia and Adjacent Parts of 

South Carolina and Florida—Predevelopment, 1980, and 2000

Previous Investigations

Several ground-water flow investigations of the Floridan 

aquifer system have been conducted in the study area, some of 

which incorporate digital modeling. As part of the USGS 

Regional Aquifer System Analysis (RASA) program, steadystate ground-water flow models of the Floridan aquifer system 

underlying Florida, southern Georgia, and parts of Alabama and 

South Carolina were developed for predevelopment conditions 

(Bush, 1982) and for 1980 conditions (Bush and Johnston, 1988). 

In these models, the Upper and Lower Floridan aquifers were 

simulated as active layers. In the Bush (1982) model, the top 

boundary was simulated as a specified flux and the bottom 

boundary as no-flow. In the Bush and Johnson (1988) model, the 

top boundary (surficial aquifer) and bottom boundary (Fernandina permeable zone) were simulated as specified-head layers.

Also as part of the RASA program, a subregional model 

comprising the area from coastal Georgia to the updip extent of 

the Floridan aquifer system and adjacent parts of Florida and 

South Carolina was developed for predevelopment conditions 

(Krause, 1982), then recalibrated and refined for steady-state 

1980 conditions (Krause and Randolph, 1989). In both models, 

the surficial aquifer was simulated as a source-sink boundary, 

but only in the latter model was a source-sink boundary applied 

to the area of the Fernandina permeable zone. The subregional 

model was updated and recalibrated to 1985 conditions (Randolph and others, 1991) and refined for consistency with subsequently developed, smaller-scale models (Clarke and Krause, 

2000). A smaller subregional model, comprising primarily the 

coastal counties of the RASA subregional model, was developed as part of a multiscale, multimodel ground-water management tool (Randolph and others, 1991). This model is “telescoped” at a finer grid resolution within the area of the RASA 

model. Vertical boundaries are identical to those of the RASA 

model, and lateral boundaries are derived from the RASA model.

Several smaller-scale models have been developed, focusing 

on the Savannah–Chatham County, Ga., area. Counts and Krause 

(1976) developed a model that simulated the “principal artesian 

aquifer” (which incorporates both the Upper and Lower Floridan 

aquifers) as a single layer calibrated for steady-state predevelopment conditions, then for transient conditions during 1956, 1960,

and 1970, using time steps of variable length. A combination of 

source-sink and no-flow boundary conditions was used for both 

lateral and the top boundaries, with a no-flow boundary condition at the bottom. This model was subsequently expanded and 

refined (with modifications to the boundary conditions) and calibrated to 1980 conditions, using a steady-state approximation 

(Randolph and Krause, 1984). These two models were used to 

simulate hypothetical changes in ground-water levels responding to possible changes in pumping distribution. Another model 

for this area was developed to simulate the water-supply potential of both the Upper and Lower Floridan aquifers (Garza and 

Krause, 1996). This model was “telescoped” within the area of 

the larger RASA model of Krause and Randolph (1989). Vertical boundaries are identical to those of the RASA model, and 

lateral boundaries are derived from the RASA model.

Smaller-scale models have been developed for the 

Brunswick–Glynn County, Ga., area. Krause and Counts 

(1975) developed a model for the principal artesian aquifer 

(incorporating both Upper and Lower Floridan aquifers) that 

simulated steady-state predevelopment conditions and transient 

conditions for 1960 and 1970 using time steps of variable 

length. A model based on the subregional RASA model (Krause 

and Randolph, 1989) was developed to simulate ground-water 

flow in both the Upper and Lower Floridan aquifers and to 

evaluate the possible effects of hypothetical changes in local 

pumping (Randolph and Krause, 1990). This model was telescoped within the area of the larger RASA model of Krause and 

Randolph (1989), and used boundary conditions in a similar 

manner as the previously described smaller-scaled models. 

The Randolph and Krause (1990) model was calibrated for 

steady-state conditions during predevelopment and May 1980, 

and included an independent check using May 1985 pumping 

input and water-level observations.

Clarke and Krause (2000) compared hydraulic-property 

data from the Savannah–Chatham County area (Garza and 

Krause, 1996) and Brunswick–Glynn County area (Randolph 

and Krause, 1990) models for consistency with the RASA model

(Krause and Randolph, 1989), revised hydraulic-property data 

where required, and reported revised calibration statistics for the 

three models. The updated models were then used to simulate a 

variety of water-management scenarios for coastal Georgia.

Hydrogeology

Coastal Plain sediments of varying permeability comprise 

the aquifer and confining units in the study area. These sediments have been divided into geologic formations on the basis 

of their geologic characteristics and into aquifers and confining 

units on the basis of their water-bearing characteristics.

Geologic Setting

Coastal Plain strata consist of consolidated to unconsolidated layers of sand and clay, and semiconsolidated to very 

dense layers of limestone and dolomite. These sediments range 

in age from Late Cretaceous to Holocene, and unconformably 

overlie igneous, metamorphic, and sedimentary rocks of 

Paleozoic to Mesozoic age. The sedimentary units generally 

strike southwest-northeast, and dip and thicken to the southeast, 

where they reach a maximum thickness of 5,500 ft in Camden 

County (Wait and Davis, 1986). A generalized correlation of 

geologic and hydrogeologic units and corresponding model 

layers is shown in figure 2. Prominent structural features in the 

area (figs. 1 and 3), such as the Southeast Georgia Embayment, 

Beaufort Arch, and Gulf Trough, influence the thickness of sediments. Figure 3 is a schematic block diagram showing hydrogeologic units and the influence of structural features on their 

occurrence and relative thickness.
Lower Middle Upper

Post-Miocene

Miocene

Eocene

Oligocene

Paleocene

Upper

Cretaceous

Undifferentiated

Suwannee Limestone

Ocala Limestone

Avon Park Formation

Oldsmar Formation

Cedar Keys Formation

Undifferentiated

Water-table zone Upper Floridan aquifer

FLORIDAN AQUIFER SYSTEM

FLORIDAN AQUIFER SYSTEM

Lower Floridan aquifer Fernandina 

permeable 

zone

Confining unit Not

modeled

Lower Floridan

confining unit

Lazaretto Creek Formation Upper Floridan confining unit

Lower Coastal Plain3

Geologic unit4 Hydrogeologic unit

Barnwell Group

Undifferentiated

Santee Limestone

Upper Coastal Plain1

Congaree

Formation

Geologic unit

Snapp Formation

Ellenton Formation

(undifferentiated)

Upper 

Three 

Runs

aquifer

Confining

unit

Gordon

aquifer

Hydrogeologic 

unit

Steel Creek 

Formation

Black Creek Group

(undifferentiated)

Confining unit2

Upper Dublin 

aquifer

1Modified from Falls and others, 1997. 3Modified from Randolph and others, 1991; Clarke and Krause, 2000. 2In local areas includes Millers Pond aquifer. 4Modified from Randolph and others, 1991; Weems and Edwards, 2001.

Confining

unit

Upper waterbearing zone

Upper 

Floridan semiconfining unit

Lower waterbearing zone

Lower Middle Upper

Series Model

layer

Savannah Brunswick

1

GHB

(not

modeled)

2

3

4

5

6

7

Upper

Brunswick 

 aquifer

Lower

Brunswick 

 aquifer

Upper waterbearing zone

Lower waterbearing zone

Tiger Leap Formation

Parachucla Formation

Marks Head Formation

Coosawhatchie Formation

Ebenezer Formation

SURFICIAL

AQUIFER SYSTEM

BRUNSWICK

AQUIFER SYSTEM

Confining

unit

Confining

unit

Hydrogeology 5

Figure 2. Generalized correlation of geologic and hydrogeologic units and model layers 

(GHB, general-head boundary).
6 Simulation of Ground-Water Flow in Coastal Georgia and Adjacent Parts of 

South Carolina and Florida—Predevelopment, 1980, and 2000

The Southeast Georgia Embayment is a shallow east-tonortheast plunging syncline that subsided at a moderate rate 

from the Late Cretaceous until the late Cenozoic (Miller, 1986). 

Thickness of Coastal Plain deposits is greatest near the embayment (fig. 3).

The Beaufort Arch is centered near Hilton Head Island, S.C., 

and trends parallel to the coast. The arch interrupts the regional 

southward dip of the sediments in that area. Within the area influenced by the Beaufort Arch, Coastal Plain deposits thin and are 

at shallower depths than near the Southeast Georgia Embayment.

The Gulf Trough is a zone of relatively thick accumulations of fine-grained clastic sediments and clay-bearing carbonates, in which the permeability of Coastal Plain deposits 

decrease. In this area, ground-water flow is partially impeded 

by the juxtaposition of rocks of higher permeability updip and 

downdip of the trough, with those of lower permeability within 

the trough (Krause and Randolph, 1989).

In addition to the aforementioned geologic features, the 

Satilla Line (fig. 1) is a postulated hydrologic boundary identified by GaEPD based on a change in the configuration of the 

potentiometric surface of the Upper Floridan aquifer, and by 

linear changes depicted on aeromagnetic, aeroradioactivity, 

gravity, and isopach maps (William H. McLemore, Georgia 

Environmental Protection Division, oral commun., January 6, 

2000). This feature may affect ground-water flow in the area; 

however, its geologic origin and nature are unknown.

Hydrogeologic Units

The principal source of water for all uses in the coastal area 

is the Floridan aquifer system, consisting of the Upper and 

Lower Floridan aquifers (Miller, 1986; Krause and Randolph, 

1989). Secondary sources of water include the surficial and 

Brunswick aquifer systems (Clarke, 2003), consisting of sand 

of Miocene to Holocene age. These water-bearing units are 

separated by confining units of relatively low permeability.

Surficial and Brunswick Aquifer Systems

Paleoriver

channel

Altamaha River

Savannah River

Brunswick

Savannah

Port

Royal

Sound

Lower Floridan aquifer

Gordon aquifer

Upper Floridan aquifer

Upper Three Runs aquifer

Brunswick

aquifer system

Surficial aquifer system

N

NOT TO SCALE

Saltwater

?

?

?

?

Confining unit

Fernandina 

permeable zone

SOUTH CAROLINA

GEORGIA

The surficial aquifer system (model layer 1; fig. 2) consists 

of interlayered sand, clay, and thin limestone beds of Miocene 

to Holocene age (Clarke, 2003). The aquifer system includes a 

water-table zone and two confined zones; however, the areal 

extent of the confined zones is unknown. Leeth (1999) reported 

two confined zones in Camden County; and Clarke and others 

(1990) reported one confined zone at Brunswick, Glynn 

County, and at Skidaway Island, Chatham County. Multiple 

confined zones are believed to be present mostly in areas where 

deposits are thick, such as in the Southeast Georgia Embayment 

(figs. 1 and 3). Reported transmissivity of the water-table zone 

ranges from 14 to 6,700 feet squared per day (ft2

/d), and for the 

confined zones ranges from 150 to 6,000 ft2

/d (Clarke, 2003). 

In this study, undifferentiated sediments comprising the conFigure 3. Schematic block diagram showing hydrogeologic units and influence of structural 

features on their occurrence.
Hydrogeology 7

fined zones of the surficial aquifer system are grouped into the 

upper model layer (layer 1).

The surficial aquifer system is separated from the underlying Brunswick aquifer system by a confining unit (model layer 2;

fig. 2) consisting of silty clay and dense, phosphatic limestone 

of Miocene age. Wait and Gregg (1973) reported vertical 

hydraulic conductivity of this unit (determined from laboratory 

analysis of core) at Brunswick, Glynn County, ranges from 

5.3 x 10–5 to 1.3 x 10–4

 feet per day (ft/d).

The Brunswick aquifer system (model layer 3: fig. 2) 

consists of two water-bearing zones—the upper Brunswick 

aquifer and the lower Brunswick aquifer (Clarke, 2003). The 

upper Brunswick aquifer consists of poorly sorted, fine to 

coarse, slightly phosphatic and dolomitic quartz sand and dense 

phosphatic limestone (Clarke and others, 1990; Leeth, 1999). 

The lower Brunswick aquifer consists of poorly sorted, fine to 

coarse, phosphatic, dolomitic sand (Clarke and others, 1990). 

In general, the upper Brunswick aquifer has lower transmissivity than the lower Brunswick aquifer. Reported transmissivity 

of the upper Brunswick aquifer ranges from 15 to 3,500 ft2

/d, 

and that of the lower Brunswick ranges from 25 to 4,700 ft2

/d, 

with highest values for both aquifers within the area of the 

Southeast Georgia Embayment in Glynn County (Clarke, 

2003). Outside and along the margins of the Southeast Georgia 

Embayment (figs. 1 and 3), permeable sediments comprising 

the Brunswick aquifer system are discontinuous, and the aquifer 

system has a higher percentage of low permeability, clayey 

deposits (Clarke, 2003). In this study, sediments comprising the 

upper and lower Brunswick aquifers are considered as a single 

unit, with combined thickness and composite hydraulic properties used for model simulations.

Floridan Aquifer System

The Floridan aquifer system, consisting of the Upper and 

Lower Floridan aquifers (Miller, 1986; Krause and Randolph, 

1989), is composed of carbonate rocks of mostly Paleocene to 

Oligocene age that locally include Upper Cretaceous rocks 

(fig. 2). The Floridan aquifer system extends from the southern 

coastal plain of South Carolina, west across the coastal plain of 

Georgia and Alabama, and south across Florida. Thickness of 

the Floridan aquifer system in the study area ranges from less 

than 100 ft where the aquifer system crops out in South Carolina 

to about 2,800 ft in Brunswick, Ga. (Miller, 1986).

The Upper Floridan aquifer is overlain by a confining unit 

(model layer 4; fig. 2) consisting of layers of silty clay and 

dense phosphatic dolomite of Oligocene age that separate the 

aquifer from overlying permeable units of the Brunswick aquifer 

system (Clarke, 2003). Reported vertical hydraulic conductivity 

of this confining unit, based on laboratory analysis of core, 

ranges from 2.3 x 10–4 to 3.0 ft/d (Clarke and others, 2004).

The Upper Floridan aquifer (model layer 5; fig. 2) is highly 

productive and consists of Eocene to Oligocene limestone and 

dolomite. The aquifer crops out or is near land surface in the 

northwestern part of the study area and near Valdosta in 

Lowndes County, Ga. (fig. 1), where it is unconfined or semiconfined. To the southeast, the aquifer becomes progressively 

more deeply buried and confined. In this report, clastic sediments of the Upper Three Runs aquifer (Falls and others, 1997) 

in the upper Coastal Plain that are hydraulically connected to 

carbonate deposits of the lower Coastal Plain are included as 

part of the Upper Floridan aquifer (figs. 2 and 3). The transition 

from carbonate to clastic deposits generally occurs north of the 

Gulf Trough (figs. 1 and 3). Reported transmissivity of the 

Upper Floridan aquifer ranges from 530 ft2

/d in Beaufort 

County, S.C., to 600,000 ft2/d in Coffee County, Ga. (Clarke 

and others, 2004). Hydraulic properties of the Upper Floridan 

aquifer vary greatly in the study area, because of the heterogeneity (and locally because of anisotropy) of the aquifer and the 

confinement (or lack of confinement) by confining units 

(Krause and Randolph, 1989). A characteristic of the Floridan 

aquifer system, especially the Upper Floridan aquifer, is that in 

many places, zones of very high hydraulic conductivity exist 

within relatively small vertical intervals of the aquifer (Clarke 

and others, 2004).

In some areas, several distinct water-bearing zones have been 

identified within the Upper Floridan aquifer. McCollum and 

Counts (1964) identified five water-bearing zones near the 

Savannah–Hilton Head Island area in strata that would later be 

defined as part of the Floridan aquifer system, the upper two of 

which are part of the Upper Floridan aquifer (Krause and Randolph, 1989). In the Brunswick–Glynn County, Ga., area, Wait 

and Gregg (1973) identified two distinct water-bearing zones 

(fig. 2) in the Upper Floridan aquifer (their “principal artesian 

aquifer”), and estimated that about 70 percent of the total flow 

from wells open to both zones was coming from the upper zone. 

In Beaufort County, S.C., the term middle Floridan aquifer is used

by the State of South Carolina (Ransom and White, 1999) for a 

water-bearing zone approximately 250–550 ft below land surface.

The Upper Floridan aquifer is underlain by a confining 

unit (model layer 6; fig. 2) of dense, recrystallized limestone 

and dolomite of middle to late Eocene that hydraulically separates to varying degrees the Upper Floridan aquifer from the 

Lower Floridan aquifer (fig. 2). Locally in the Brunswick, Ga., 

area, the confining unit is breached by fractures or solution 

openings, which enhance the vertical exchange of water 

between the Upper and Lower Floridan aquifers (Krause and 

Randolph, 1989).

The Lower Floridan aquifer (model layer 7; fig. 2) is composed mainly of dolomitic limestone of early and middle Eocene

age; at Brunswick, Ga., however, it includes highly permeable 

limestone of Paleocene and Late Cretaceous age (Krause and 

Randolph, 1989). In the northwestern part of the study area, the 

clastic Gordon aquifer (Brooks and others, 1985; Falls and others,

1997) is an updip unit that is hydraulically connected to the Lower 

Floridan aquifer (figs. 2 and 3). Reported transmissivity of the 

Lower Floridan aquifer ranges from 170 ft2/d in Barnwell County, 

S.C., to 43,000 ft2/d in Camden County, Ga. (Clarke and others, 

2004). Although no aquifer tests were conducted in wells completed solely in the Lower Floridan aquifer in northeastern 

Florida, it is likely that transmissivity of the aquifer is high— 

possibly exceeding 100,000 ft2/d (Clarke and others, 2004).
8 Simulation of Ground-Water Flow in Coastal Georgia and Adjacent Parts of 

South Carolina and Florida—Predevelopment, 1980, and 2000

The Lower Floridan aquifer includes several waterbearing zones in parts of the study area. In the Savannah–Hilton 

Head area, the lowermost water-bearing zone of McCollum 

and Counts (1964) is included in the Lower Floridan aquifer 

(W.F. Falls, U.S. Geological Survey, written commun., 

August 28, 2003). In southeastern South Carolina, units of 

Paleocene and early Eocene age can contain permeable beds, 

and production wells are commonly screened in these zones 

and in the overlying Santee Limestone (Newcome, 2000). In 

this report, these productive zones, and the Santee Limestone, 

are considered part of the Lower Floridan aquifer. In southeastern Georgia and northeastern Florida, the Lower Floridan 

includes a deeply buried, cavernous and highly permeable, 

saline water-bearing unit known as the Fernandina permeable 

zone (Krause and Randolph, 1989). This unit is the probable 

source of saltwater contamination in the Upper and Lower 

Floridan aquifers at Brunswick, Ga., and Jacksonville, Fla. 

(Krause and Randolph, 1989). The lateral continuity of this 

zone is unknown; however, test drilling conducted as part of the 

CSSI indicates that the unit is present in downtown Brunswick 

and is absent in northern McIntosh County and at St. Simons 

Island in eastern Glynn County (W.F. Falls, U.S. Geological 

Survey, written commun., 2003).

Ground-Water Flow System

Ground-water flow is controlled mainly by rates and distribution of recharge to and discharge from the system, the 

extent and effects of confinement, the ability of the aquifers to 

transmit and store water, ground-water withdrawal, and the dips 

of the aquifer and confining units. A schematic diagram of the 

conceptualized ground-water flow system in the coastal area is 

shown in figure 4.

Recharge to the water-table zone of the surficial aquifer 

system occurs directly from precipitation throughout the study 

area; recharge to confined aquifers from precipitation occurs at 

outcrop areas (mostly north of the Gulf Trough; figs. 1 and 3), 

or from downward leakage through adjacent semiconfining 

units. Natural discharge occurs directly into some stream 

reaches or indirectly through upward leakage into adjacent units.

The extent and subsurface geometry of the Brunswick 

aquifer system is poorly understood—the aquifer system is not 

known to crop out—thus, recharge is believed to be restricted 

to leakage from the overlying surficial aquifer system or underlying Upper Floridan aquifer. Natural discharge from the Brunswick aquifer system is through upward leakage into the surficial aquifer system and streams.

Ground-water flow in the Upper Floridan aquifer is illustrated on a potentiometric-surface map for May 1998 (Peck and 

others, 1999) shown in figure 5. In the updip, northern part of 

the study area (north of the Gulf Trough; fig. 1) where the aquifers are exposed at or near land surface, the Floridan aquifer 

system receives recharge. Because the units are relatively shallow and that area is characterized by greater topographic relief, 

some aquifer discharge is directly to streams, as indicated by 

contours that bend upstream. From these northern areas, ground 

water flows mostly southeastward toward the coast and discharges into overlying units and surface-water bodies—major 

streams, estuaries, and the Atlantic Ocean. As ground water 

flows coastward, low-permeability sediments near the Gulf 

Trough impede ground-water flow and cause a steep potentiometric gradient, as indicated by contours on the potentiometric 

map (fig. 5). 

South of the Gulf Trough, in the downdip part of the study 

area (fig. 4), the Upper Floridan aquifer is deeper and overlain 

by thick confining units and the Brunswick aquifer system. 

Here, water may enter or discharge from the aquifer through 

leaky confining units.

Localized areas of natural recharge to the Upper Floridan 

aquifer are present in Beaufort County, S.C. This recharge 

results from the shallow depth of the aquifer near the Beaufort 

Arch and localized areas of little or no confinement above the 

Upper Floridan aquifer (fig. 1).

Because little is known about regional ground-water flow 

in the Lower Floridan aquifer, the regional flow characteristics 

of the Lower Floridan aquifer are assumed to be similar to those 

of the Upper Floridan aquifer.

Predevelopment

Prior to development during the 1880s, recharge to the 

Floridan aquifer system was roughly offset by natural discharge 

to springs (both on land and offshore), rivers and other surfacewater bodies, diffuse upward leakage, and other discharge areas 

(fig. 4A). The hydraulic head in the Upper Floridan aquifer was 

sufficiently high that earliest wells flowed at land surface 

throughout much of the coastal area, with water levels about 

65 ft above NAVD 88 at Brunswick, and 30–40 ft above 

NAVD 88 at Savannah (Krause and Clarke, 2001). Recharge 

occurred in the northwestern part of the area, and water flowed 

downgradient toward the coast.

During predevelopment, the Floridan aquifer system likely 

contained freshwater throughout most of the coastal area. Saltwater was present near the aquifer system’s northeastern extent 

on Parris Island (north of Hilton Head Island; fig. 1), S.C., in 

wells drilled during 1899 (Landmeyer and Belval, 1996), likely 

along the freshwater-saltwater interface offshore (Krause and 

Clarke, 2001), and in parts of north-central Florida (Stringfield, 

1966). The aquifer system also probably contained saltwater at 

depth that is not derived from seawater, underlying freshwater 

in the system in the lower part of the Lower Floridan aquifer, for 

example, in Brunswick, Ga. (Wait and Gregg, 1973).
Regional flow system

Regional flow system

?

?

?

?

NOT TO SCALE

Gordon aquifer

Upper Three Runs aquifer

Confining unit

Upper Floridan aquifer

Brunswick aquifer system

Lower Floridan aquifer

Saltwater

?

?

?

?

Surficial aquifer system

Wetlands

Water

table

Flowing

Altamaha well

River

Unsaturated zone

Ocmulgee

River

Little

Ocmulgee

River

A. Predevelopment 

Pumping

well

B. Modern day

Hydrogeology 9

Figure 4. Schematic diagram showing conceptual model of (A) predevelopment and (B) modern-day 

(2000) flow system (modified from Priest, 2004). Arrows indicate general direction of ground-water flow.

Modern Day: 1980 and 2000

The modern-day flow system (figs. 4B and 5) reflects 

changes that have occurred as a result of ground-water development (withdrawal). Ground-water withdrawal has lowered 

water levels, induced additional recharge, reduced natural 

discharge, and increased chloride concentration of ground 

water along the coast. Cones of depression have developed in 

the potentiometric surface of the Upper Floridan aquifer in the 

Savannah, Brunswick, Jesup, and St. Marys, Ga.–Fernandina 

Beach, Fla., areas (fig. 5). The most extensive cone of depression is centered in the Savannah, Ga., area, and is likely the 

result of large pumping rates and low transmissivity of the 

thinning aquifer toward the Beaufort Arch area.

The hydraulic gradient has steepened near these cones of 

depression and from the recharge area downgradient toward the 

coast. These steeper gradients have resulted in high groundwater flow velocities and large quantities of water infiltrating 

into the Upper Floridan aquifer, both vertically and laterally. 

The cones of depression have “captured” ground-water flow, 

which prior to development, may have discharged offshore. 

In addition, diffuse upward leakage of water from the Upper 

Floridan aquifer into overlying units, streams, and wetlands 

may have decreased or ceased, and wells no longer flow at land 

surface (fig. 4B).
10 Simulation of Ground-Water Flow in Coastal Georgia and Adjacent Parts of 

South Carolina and Florida—Predevelopment, 1980, and 2000

Data from offshore test wells drilled during the early 1980s 

at Port Royal Sound, off Hilton Head Island, indicated that saltwater having a chloride concentration exceeding 250 milligrams per liter (mg/L) was present within a few thousand feet 

of the shoreline, with some onshore wells containing water of 

elevated chloride concentration near the coastline in the Hilton 

Head Island area. This saltwater appears to enter the aquifer 

through breaches in the overlying Upper Floridan aquifer confining unit and probably does not represent the steady-state 

freshwater-saltwater interface wedge. Test wells drilled during 

the late 1970s in the Upper Floridan aquifer 60–70 miles (mi) 

offshore of Jacksonville, Fla., indicate that the freshwatersaltwater interface occurs far offshore in the southern part of the 

coastal area (Johnston and others, 1982).

Recharge and Discharge

GEORGIA

FLORIDA

l l

l l l l l l

l

l l

l

l l l l l

l l l l l l l l l l l l l l l l l

l l

l

l l l l l l l l l l l

l l l

l l l l

l l l l

l l

l l

l

l

l

l l l l l l l l l l l l l l l l l l l l l l l l l l

l

l l l l l l l l l l l l l l l

l l l l l l l l l

l l l l l l l l l l l l l l l l l l l l l l

l l l l l l l l l l

l l l l l

l l

l l

l

l l l l l

l

l

l l l

l l

l

l

l

l l l l

l l l l l l l

l l l l

l l

l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l

Altamaha River

Savannah

River

Ogeechee River

BACON

GLYNN

BRANTLEY

PIERCE

WARE

CHARLTON CAMDEN

NASSAU

TOOMBS

TATTNALL

EVANS

BRYAN

LONG

WAYNE

APPLING

LIBERTY

MCINTOSH

CHATHAM

EMANUEL

JENKINS

SCREVEN

EFFINGHAM

BULLOCH

CANDLER

BURKE

Doctortown

Jesup

Brunswick

St Marys Fernandina

Beach

Savannah

–50

0

0

50

50

40

30

20

10

100

50

40

30

20

10

150

250

300

200

0

–30

–20

–10

0

10

0

GA

FL

33°

32°

31°

82°

81°

0

0 20 30 KILOMETERS 10

10 20 30 MILES

Atlantic Ocean

EXPLANATION

Base from U.S. Geological Survey 

1:100,000-scale digital data

100

Gulf Trough—Approximately located 

 (Applied Coastal Research Laboratory, 2002)

Potentiometric contour—Shows altitude at which 

 water level would have stood in tightly cased wells 

 during May 1–26, 1998. Hachures indicate depression. 

 Contour interval 10 and 50 feet. Datum is NAVD 88

Recharge to the hydrologic system is from rainfall, which 

varies spatially from an average of about 47 to 53 in/yr based on 

mean-annual precipitation for the 30-year period 1971–2000 

(Priest, 2004). Rainfall is greatest in the southern part of the area 

and least in the northern part of the area (fig. 6). Most of the 

recharge is discharged from shallow, local flow systems into 

small streams or is lost as evapotranspiration. A small percentage 

of recharge infiltrates through clayey confining units and enters 

the deep, confined regional flow system. In the regional flow 

system, some water discharges to major streams and wetlands 

and some flows southward, discharging to the Atlantic Ocean.

Estimates of mean-annual ground-water discharge to 

streams (baseflow) determined using hydrograph-separation 

methods are considered to approximate a large percentage of the 

long-term average recharge to the ground-water flow system

(Clarke and West, 1998). Priest (2004) used hydrograph-separation techniques to estimate average annual baseflow during 

1971–2001 for 14 streamgaging sites in coastal Georgia (fig. 6).

Estimated baseflow at the 14 sites ranged from 4.4 in/yr along 

the Little Satilla River to 10 in/yr along the Altamaha River.

A portion of estimated long-term average recharge, based 

on hydrograph separation, flows into and out of the shallow, 

unconfined aquifer system, allowing only a fraction to recharge 

the regional flow system. Thus, the estimated baseflow values 

are likely substantially larger than recharge to the regional flow 

system and may be considered to be an upper limit to regional 

recharge simulated by the model. Drought estimates of baseflow reported by Priest (2004) range from 0 to 2.4 in/yr and may 

represent a more reasonable estimate of recharge to the regional 

aquifer system.

Williamson and others (1990) estimated recharge to the 

regional ground-water flow system to be generally less than 

3 in/yr for 1980 conditions in the easternmost Gulf Coast 

Coastal Plain region. This area overlaps the westernmost part of 

the southeastern Atlantic Coastal Plain in Alabama. Although 

this area does not coincide with the study area for this report, it 

is proximal to it and has similar topographic, climatic, and 

hydrogeologic characteristics that would suggest that regional 

recharge rates in the coastal Georgia study area may be similar 

to those estimated by Williamson and others (1990).

Figure 5. Potentiometric surface of the Upper Floridan 

aquifer, May 1998 (modified from Peck and others, 1999).
Waynesboro 47

Tifton

47

Tifton

47

Jesup

49

Homerville

53

Brooklet 47

Savannah

50

Folkston

53

Dublin

47

Brunswick 49

Alma

 49

Bamberg 49

02226500

(5.41)

02225000 

(9.02)

02175500

02198000 (9.80)

(9.29)

02202500

(7.83)

02223500

(6.90)

02225500 

(7.15)

02203000

(6.29)

02215500

(10.0)

02228000 

(5.70)

02226000

(9.31)

02227500

(4.45)

02197600

(8.24)

02317500

(6.36)

SOUTH

GEORGIA CAROLINA

FLORIDA

Atlantic Ocean

Study

area

Ogeechee R

Altamaha R

Savannah R

Ocmulgee R

Alapaha R

Satilla R

L Sa tilla RL Sa tilla R

Salkehatchie R

0 60 MILES 30

0 60 KILOMETERS 30

N

Base from U.S. Geological Survey

1:100,000-scale digital data

GA

FL

SC

GA

Model boundary

Line of equal mean-annual precipitation (1971– 2000)—

 Dashed where approximate. Interval 1 inch

53

52

51

50

49

48

47

NWS station, name, and precipitation amount in inches

Streamflow gaging station and number

 (estimated baseflow, in inches per year)

EXPLANATION

02317500

(6.36)

53

Hydrogeology 11

Figure 6. Mean-annual precipitation for selected National Weather Service (NWS) stations and 

mean-annual baseflow computed using hydrograph-separation analysis in the model area, 1971–2000 

(modified from Priest, 2004).
12 Simulation of Ground-Water Flow in Coastal Georgia and Adjacent Parts of 

South Carolina and Florida—Predevelopment, 1980, and 2000

Ground-Water Pumpage

The locations of ground-water pumping centers and quantities of water withdrawn from these centers may affect substantially ground-water levels in the study area. Changes in pumping rates and the addition of new pumping centers may alter the 

configuration of potentiometric surfaces, reverse ground-water 

flow directions, and increase seasonal and long-term waterlevel fluctuations in the aquifers.

County aggregate and site-specific data were used to estimate average annual pumpage for 1980 and 2000 using procedures described by Taylor and others (2003). County aggregate 

pumping data for Florida are from Marella (2004), and for 

Georgia are from Fanning (2003) and Pierce and others (1982). 

W.J. Stringfield provided South Carolina county aggregate data 

(U.S. Geological Survey, written commun., 2002). J.L. Fanning 

provided site-specific data for Georgia (U.S. Geological Survey, written commun., 2002). P. Bristol provided site-specific 

data for South Carolina (South Carolina Department of Health 

and Environmental Control, written commun., 2003). Site-specific pumping estimates for Florida are from Sepúlveda (2002).

Pumping distribution along the Georgia coastal area has 

varied with time. Prior to the 1950s, ground-water withdrawal 

was limited to scattered pumping centers near major towns such 

as Savannah, Ga. As major industries developed and local 

populations increased in coastal Georgia cities, pumpage in the 

study area increased substantially.

The Upper and Lower Floridan aquifers provide the 

largest amount of ground water in the study area, with an average total withdrawal during 2000 of 682 and 133 million gallons 

per day (Mgal/d), respectively. Pumping from the Upper and 

Lower Floridan aquifers during 1980–2000 is summarized in 

tables 1 and 2, respectively, and shown in figure 7. Water use 

from the Brunswick aquifer system is considerably less than 

from the Upper and Lower Floridan aquifers (probably less than 

1 Mgal/d); specific data for this unit, however, are not available 

during this period.

Average daily pumpage from the Upper Floridan aquifer 

and its updip equivalents during 2000 exceeded 10 Mgal/d in 

Duval and Nassau Counties, Fla.; in Beaufort County, S.C.; and 

in Burke, Camden, Chatham, Coffee, Dooly, Glynn, Jefferson, 

Liberty, Pulaski, Screven, Washington, Wayne, and Wilcox 

Counties, Ga. (table 1). The largest withdrawals from the Upper 

Floridan aquifer were in Chatham (68 Mgal/d), Glynn 

(61 Mgal/d), and Wayne (63 Mgal/d) Counties, Ga. In the 

Lower Floridan aquifer and its updip equivalents, average daily 

pumpage during 2000 exceeded 1 Mgal/d in Duval County, 

Fla.; and in Burke, Chatham, Coffee, Crisp, Dooly, Jefferson, 

Laurens, Pulaski, Screven, Washington, and Wilcox Counties, 

Ga. (table 2). The largest withdrawal from the Lower Floridan 

aquifer was in Duval County, Fla., where pumpage exceeded 

95 Mgal/d during 2000 (table 2).

During 1980–2000, total pumpage from the Upper Floridan aquifer increased by 17 percent, from 583 Mgal/d during 

1980 to a peak of 682 Mgal/d during 2000 (table 1, fig. 7). In 

the Lower Floridan aquifer, withdrawal increased by 14 percent 

from a low of 117 Mgal/d during 1980 to a high of 133 Mgal/d 

during 2000 (table 2; fig. 7).

Table 1. Estimated ground-water pumpage from the Upper Floridan aquifer in the coastal area of Georgia and 

adjacent parts of South Carolina and Florida, 1980–2000.

[do., ditto]

State County Pumpage, in million gallons per day

1980 1985 1990 1995 1997 2000

Florida Baker 1.72 2.88 3.68 2.11 2.11 2.11

do. Columbia 3.05 4.79 5.07 6.92 6.57 6.04

do. Duval 53.96 47.44 41.91 43.91 44.83 44.40

do. Hamilton 0.10 0.30 0.44 0.44 0.46 0.49

do. Nassau 44.09 46.76 49.72 46.66 50.19 49.38

Georgia Appling 5.71 2.60 2.10 2.38 2.47 4.17

do. Atkinson 1.89 1.50 0.58 1.58 1.58 2.91

do. Bacon 2.63 2.28 2.11 2.47 2.21 4.04

do. Ben Hill 3.71 4.92 3.34 10.97 10.98 7.57

do. Berrien 2.43 3.26 2.80 4.65 4.66 5.33

do. Bleckley 5.59 4.28 3.29 2.35 2.35 6.66

do. Brantley 1.46 1.63 1.83 1.90 1.94 1.30

do. Bryan 0.67 0.87 1.03 1.06 1.70 1.60

do. Bulloch 3.75 2.71 5.87 7.83 5.05 5.70

do. Burke 10.30 6.34 5.82 8.16 8.22 22.34

do. Camden 37.12 42.98 45.74 47.15 45.83 50.55

do. Candler 1.83 2.57 1.64 1.67 1.70 2.79

do. Charlton 6.50 1.22 1.38 1.45 0.95 1.25

do. Chatham 79.75 78.98 85.54 75.84 70.66 68.15

do. Clinch 0.85 0.72 0.65 1.03 1.04 1.44
Hydrogeology 13

Data sources: County aggregate and site-specific data were used to estimate average annual pumpage for 1980 and 2000 using procedures 

described by Taylor and others (2003). County aggregate pumping data for Florida are from Marella (2004), and for Georgia are from Fanning (2003) 

and Pierce and others (1982). W.J. Stringfield (U.S. Geological Survey, written commun., 2002) provided South Carolina county aggregate data. 

J.L. Fanning (U.S. Geological Survey, written commun., 2002) provided site-specific data for Georgia. Paul Bristol (South Carolina Department of 

Health and Environmental Control, written commun., 2003) provided site-specific data for South Carolina. Site-specific pumping for Florida is 

estimated from Sepúlveda (2002).

Georgia Coffee 12.59 7.98 5.60 7.59 7.52 15.23

do. Crisp 3.16 3.45 5.31 10.28 10.24 8.56

do. Dodge 7.02 3.95 2.40 4.28 4.28 3.96

do. Dooly 6.30 9.45 3.18 9.25 9.25 18.68

do. Echols 0.17 0.18 0.25 1.04 1.77 2.88

do. Effingham 2.26 2.06 4.98 5.98 4.42 4.62

do. Emanuel 7.34 5.30 4.18 4.51 4.53 4.22

do. Evans 0.38 0.31 0.38 0.49 0.46 0.70

do. Glascock 0.73 0.72 0.99 1.34 1.35 1.36

do. Glynn 95.40 77.84 82.02 63.68 61.61 61.14

do. Irwin 1.96 1.86 2.15 5.75 5.75 6.25

do. Jeff Davis 5.11 5.80 4.77 3.09 3.09 3.84

do. Jefferson 4.97 9.90 8.85 7.76 7.62 12.06

do. Jenkins 2.74 2.65 2.45 3.19 3.13 4.03

do. Johnson 1.37 1.81 0.92 1.83 1.83 2.12

do. Lanier 3.07 2.92 1.69 2.02 2.02 1.97

do. Laurens 4.32 4.15 4.23 5.78 5.81 7.94

do. Liberty 13.62 14.58 17.97 15.91 16.10 15.69

do. Long 0.29 0.24 0.23 0.27 0.27 0.69

do. McIntosh 0.70 1.03 0.76 1.07 1.09 0.85

do. Montgomery 0.89 1.51 0.94 2.40 2.40 1.61

do. Pierce 2.64 2.03 1.80 3.24 3.42 6.22

do. Pulaski 6.94 8.27 6.87 8.59 8.53 11.46

do. Screven 7.90 7.19 7.87 6.36 6.93 16.24

do. Tattnall 1.56 1.89 1.77 3.53 3.59 3.66

do. Telfair 3.28 4.62 3.30 6.33 6.33 4.00

do. Tift 1.89 2.19 2.61 3.95 3.80 3.57

do. Toombs 2.87 3.91 3.61 3.65 4.17 6.30

do. Treutlen 0.49 0.54 0.79 1.31 1.31 1.10

do. Turner 1.02 1.00 0.93 2.91 2.92 2.57

do. Ware 6.25 7.25 6.20 5.51 5.97 8.45

do. Washington 10.01 12.24 13.02 14.39 14.88 16.01

do. Wayne 74.54 69.80 69.27 64.89 63.59 63.47

do. Wheeler 1.60 0.83 0.61 2.22 2.22 1.07

do. Wilcox 4.06 9.84 5.40 8.43 8.43 14.74

South Carolina Allendale 7.84 7.84 8.31 9.44 9.85 9.59

do. Bamberg 1.99 1.99 2.09 2.52 4.04 6.32

do. Barnwell 1.15 1.15 3.32 2.91 4.90 7.50

do. Beaufort 0.85 20.80 17.48 19.56 33.58 21.44

do. Colleton 0.00 0.00 0.00 0.00 0.00 0.00

do. Hampton 3.21 3.21 3.95 4.32 5.99 8.63

do. Jasper 1.25 1.16 1.97 1.31 2.13 3.34

Total 582.81 584.49 579.96 603.42 616.62 682.31

Table 1. Estimated ground-water pumpage from the Upper Floridan aquifer in the coastal area of Georgia and 

adjacent parts of South Carolina and Florida, 1980 – 2000.—Continued

[do., ditto]

State County Pumpage, in million gallons per day

1980 1985 1990 1995 1997 2000
14 Simulation of Ground-Water Flow in Coastal Georgia and Adjacent Parts of 

South Carolina and Florida—Predevelopment, 1980, and 2000

Table 2. Estimated ground-water pumpage from the Lower Floridan aquifer in the coastal area of Georgia and 

adjacent parts of South Carolina and Florida, 1980–2000.

[do., ditto]

State County Pumpage, in million gallons per day 

1980 1985 1990 1995 1997 2000

Florida Baker 0.26 0.43 0.55 0.32 0.32 0.32

do. Columbia 0.00 0.00 0.00 0.00 0.00 0.00

do. Duval 92.52 99.13 100.46 95.01 99.48 95.98

do. Hamilton 0.00 0.00 0.00 0.00 0.00 0.00

do. Nassau 2.51 2.16 2.00 2.09 2.18 2.21

Georgia Appling 0.00 0.00 0.00 0.00 0.00 0.00

do. Atkinson 0.00 0.00 0.00 0.00 0.00 0.00

do. Bacon 0.00 0.00 0.00 0.00 0.00 0.00

do. Ben Hill 0.21 0.39 0.38 1.30 1.30 0.59

do. Berrien 0.41 0.53 0.45 0.67 0.67 0.77

do. Bleckley 0.87 0.63 0.41 0.40 0.40 1.00

do. Brantley 0.00 0.00 0.00 0.00 0.00 0.00

do. Bryan 0.00 0.00 0.00 0.00 0.00 0.00

do. Bulloch 0.23 0.20 0.16 0.31 0.32 0.32

do. Burke 1.60 0.92 0.83 1.26 1.27 3.24

do. Camden 0.00 0.00 0.00 0.00 0.00 0.00

do. Candler 0.26 0.34 0.17 0.19 0.19 0.37

do. Charlton 0.00 0.00 0.00 0.00 0.00 0.00

do. Chatham 3.58 3.20 4.13 3.76 3.78 3.23

do. Clinch 0.00 0.00 0.00 0.00 0.00 0.00

do. Coffee 1.49 0.78 0.25 0.47 0.53 1.73

do. Crisp 0.32 0.28 0.78 1.58 1.59 1.30

do. Dodge 1.01 0.52 0.22 0.46 0.46 0.41

do. Dooly 0.96 1.46 0.41 1.29 1.29 2.93

do. Echols 0.00 0.00 0.00 0.00 0.00 0.00

do. Effingham 0.02 0.01 0.03 0.04 0.03 0.03

do. Emanuel 0.85 0.68 0.36 0.52 0.52 0.48

do. Evans 0.05 0.04 0.05 0.06 0.06 0.09

do. Glascock 0.04 0.02 0.02 0.02 0.02 0.02

do. Glynn 0.00 0.00 0.00 0.00 0.00 0.00

do. Irwin 0.25 0.21 0.26 0.87 0.87 0.96

do. Jeff Davis 0.81 0.89 0.66 0.40 0.40 0.47

do. Jefferson 0.69 1.44 1.03 0.76 0.97 1.68

do. Jenkins 0.41 0.37 0.33 0.47 0.46 0.61

do. Johnson 0.17 0.26 0.12 0.27 0.27 0.32

do. Lanier 0.00 0.00 0.00 0.00 0.00 0.00

do. Laurens 0.74 0.62 0.60 0.97 0.95 1.31

do. Liberty 0.00 0.00 0.00 0.00 0.00 0.00

do. Long 0.01 0.01 0.01 0.02 0.02 0.07

do. McIntosh 0.00 0.00 0.00 0.00 0.00 0.00

do. Montgomery 0.11 0.20 0.10 0.33 0.33 0.19

do. Pierce 0.00 0.00 0.00 0.00 0.00 0.00

do. Pulaski 1.11 1.31 1.09 1.31 1.35 1.81

do. Screven 1.18 1.03 0.40 0.66 0.69 2.32

do. Tattnall 0.06 0.09 0.08 0.28 0.28 0.15

do. Telfair 0.48 0.55 0.32 0.83 0.82 0.42

do. Tift 0.33 0.38 0.46 0.69 0.66 0.62

do. Toombs 0.24 0.31 0.20 0.27 0.27 0.69
Hydrogeology 15

Data sources: County aggregate and site-specific data were used to estimate average annual pumpage for 1980 and 2000 using procedures described by 

Taylor and others (2003). County aggregate pumping data for Florida are from Marella (2004), and for Georgia are from Fanning (2003) and Pierce and 

others (1982). W.J. Stringfield (U.S. Geological Survey, written commun., 2002) provided South Carolina county aggregate data. J.L. Fanning (U.S. Geological Survey, written commun., 2002) provided site-specific data for Georgia. Paul Bristol (South Carolina Department of Health and Environmental 

Control, written commun., 2003) provided site-specific data for South Carolina. Site-specific pumping for Florida is estimated from Sepúlveda (2002).

0

100

200

300

400

500

600

700

800

900

19801982198419861988199019921994199619982000

PUMPAGE, IN MILLION GALLONS PER DAY

Lower Floridan aquifer

Upper 

Floridan 

aquifer

Georgia Treutlen 0.06 0.05 0.06 0.12 0.12 0.11

do. Turner 0.17 0.17 0.16 0.50 0.50 0.44

do. Ware 0.00 0.00 0.00 0.00 0.00 0.00

do. Washington 1.52 1.89 1.96 2.16 2.04 2.07

do. Wayne 0.00 0.00 0.00 0.00 0.00 0.00

do. Wheeler 0.21 0.10 0.06 0.34 0.34 0.14

do. Wilcox 0.68 1.69 0.90 1.43 1.43 2.53

South Carolina Allendale 0.00 0.00 0.00 0.00 0.00 0.00

do. Bamberg 0.00 0.00 0.00 0.00 0.00 0.00

do. Barnwell 0.00 0.00 0.00 0.00 0.00 0.00

do. Beaufort 0.00 0.05 0.01 0.01 0.09 0.26

do. Colleton 0.35 0.55 0.56 0.58 0.47 0.51

do. Hampton 0.00 0.00 0.00 0.00 0.00 0.00

do. Jasper 0.01 0.00 0.00 0.00 0.01 0.01

Total 116.77 123.88 121.00 123.00 127.74 132.69

Table 2. Estimated ground-water pumpage from the Lower Floridan aquifer in the coastal area of Georgia and 

adjacent parts of South Carolina and Florida, 1980 – 2000.—Continued

[do., ditto]

State County Pumpage, in million gallons per day 

1980 1985 1990 1995 1997 2000

Figure 7. Estimated ground-water pumpage from the 

Upper and Lower Floridan aquifers, 1980–2000 (see tables 1 

and 2 for county totals and data sources).

study area and results from greater aquifer interconnection as a 

result of discontinuous or leaky confining units.

During 1980–2000, water levels showed a combination of 

rises and declines in response to changing pumping patterns. 

During this period, total ground-water use increased; however, 

the distribution of withdrawal changed—decreases exceeding 

10 Mgal/d occurred in Chatham and Glynn Counties, and 

increases exceeding 10 Mgal/d occurred in Burke, Camden, 

Dooly, and Wilcox Counties, Ga.; and in Beaufort County, S.C. 

(tables 1 and 2; fig. 7). 

To determine water-level trends during 1980–2000, water 

levels in the Brunswick aquifer system and Upper and Lower 

Floridan aquifers were compared and differences computed 

(appendix A; fig. 8). Water-level data for the Brunswick aquifer 

system are sparse; water levels in one well in Charlton County 

(30E002) declined 9 ft during 1980–2000 (appendix A, fig. 1A, 

table A1). In the Upper Floridan aquifer, water levels rose 

almost 10 ft near the Savannah and from 10 to 20 ft near the 

Brunswick pumping center, and declined in most of the rest of 

the study area (fig. 8, appendix A). Water levels also rose near 

Jesup in Wayne County by more than 10 ft. The largest 

declines, greater than 20 ft, were in the western part of the area 

and near the Gulf Trough. These declines correspond to a general increase in pumpage from the Upper Floridan aquifer in 

these areas. In the Lower Floridan aquifer, water levels in five 

wells at Brunswick were about the same to 5.2 ft lower during 

2000 than during 1980 (appendix A).

Ground-Water-Level Trends

Ground-water levels are affected by precipitation, evapotranspiration, and pumpage. Water levels generally are highest in 

the winter-early spring when precipitation is greatest, evapotranspiration is lowest, and irrigation withdrawals are minimal; 

water levels are lowest during summer and fall when evapotranspiration and pumpage are greatest. In parts of the study area, 

water levels may respond to pumpage from an adjacent aquifer. 

This response is most pronounced in the northern part of the 
16 Simulation of Ground-Water Flow in Coastal Georgia and Adjacent Parts of 

South Carolina and Florida—Predevelopment, 1980, and 2000

Saltwater Contamination

0 60 MILES 30

0 60 KILOMETERS 30

N

Base from U.S. Geological Survey

1:100,000-scale digital data

GA

FL

SC

GA

Model boundary

Change in periodic or 

 mean-monthly water 

 level in feet, 1980–2000

EXPLANATION

10 to 20

0 to 10

–10 to 0

–20 to –10

Decline Rise

Less than –20

SOUTH

GEORGIA CAROLINA

FLORIDA

Atlantic Ocea

Study n

area

Saltwater contamination has been documented at Hilton 

Head Island, S.C., and Brunswick, Ga. (Counts and Donsky, 

1963; Gregg and Zimmerman, 1974; Hayes, 1979; Hughes and 

others, 1989; Krause and Randolph, 1989; Landmeyer and 

Belval, 1996; Wait, 1965). Sources of chloride contamination 

are different in the two areas. 

Offshore of the Savannah–Hilton Head Island area, the 

position of the saltwater wedge is uncertain, as is the degree to 

which the wedge contributes to saltwater intrusion. Near Hilton 

Head Island, possible sources of saltwater contamination 

include modern seawater encroachment or remnant ancient 

seawater as a result of incomplete flushing of the aquifer. 

The most likely source of contamination is seawater entering 

the Upper Floridan aquifer in areas where the overlying 

confining unit is thin or absent, and where hydraulic gradients 

are favorable for the migration of seawater into the aquifer. 

Erosion has partially or completely removed the confining unit 

overlying the Upper Floridan aquifer in the area offshore of 

Hilton Head Island, in Calibogue Sound, on Pinckney Island, 

and in the Colleton River (Foyle and others, 2001) exposing the 

aquifer to seawater or brackish water. Withdrawal of water from 

the Upper Floridan aquifer in the Savannah area since the late 

1800s has resulted in the development of a regional cone of 

depression on the potentiometric surface that extends from 

Figure 8. Change in water levels in wells completed in the Upper Floridan aquifer, May 1980–September 2000.
Simulation of Ground-Water Flow 17

Savannah northeastward across Hilton Head Island. This situation, combined with pumping on Hilton Head Island, has 

resulted in reversal of the normally seaward hydraulic gradients. These conditions may allow seawater to enter the aquifer 

and flow laterally downgradient toward pumping centers. 

Specific-conductance data for the Upper Floridan aquifer in this 

area indicate that saltwater is moving southward from points on 

the northern end of Hilton Head Island, Pinckney Island, and 

the Colleton River (Camille Ransom III, South Carolina 

Department of Health and Environmental Control, oral 

commun., 2004).

Beneath downtown Brunswick, the occurrence of saltwater in the Upper Floridan aquifer has been known for several 

decades (Wait, 1965). Water at about 2,400 ft below land surface

in the lower part of the Lower Floridan aquifer (Fernandina 

permeable zone) has chloride concentrations greater than 

30,000 mg/L, indicating that this is connate water and a likely 

source of saltwater in the Upper Floridan aquifer at Brunswick 

(Krause and Randolph, 1989). The presence of steeply-dipping 

fractures and zones of abundant solution features in the Floridan 

aquifer system in one of these wells (Maslia and Prowell, 1990) 

suggests that saltwater is transported vertically upward into the 

Upper Floridan aquifer from depth. Isochlor maps indicate that 

there may be as many as four points of saltwater intrusion in the 

Brunswick area (Jones, 2001); the geometry and distribution of 

possible conduits, however, are poorly defined in the area.

In previous conceptual models of regional offshore 

ground-water chemistry, an offshore saltwater wedge has been 

inferred to extend from the coastline near Port Royal Sound, 

S.C., to 85 mi offshore of the Georgia – Florida border (Krause 

and Clarke, 2001). The configuration of this feature is based on 

sparse offshore water-chemistry data, an inferred extension of 

the onshore chloride distribution (Sprinkle, 1982), and application of the Ghyben-Herzberg principle (Reilly and Goodman, 

1985). Data collected offshore of the Georgia-Florida border 

indicate that if such a feature exists, then it is relatively far offshore (Johnston and others, 1982), and likely does not contribute to saltwater intrusion in the southernmost coastal counties of 

Georgia or in northeastern Florida. 

Simulation of Ground-Water Flow

A single-density, digital ground-water flow model was 

developed for the coastal area of Georgia, Florida, South 

Carolina, and adjacent offshore area using the USGS finitedifference code MODFLOW-2000 (Harbaugh and others, 2000). 

The model was used to characterize the confined ground-water 

flow system, simulate ground-water management scenarios, 

and create a framework for local-scale solute-transport models. 

A three-dimensional approach was used that allows for simulation of flow in both aquifers and confining units and incorporates vertical and horizontal hydraulic properties of hydrogeologic units. This approach allows the model to be more directly 

and consistently translated into related variable-density solutetransport models, or amended for other purposes. Available 

data are limited to horizontal properties in aquifers and vertical 

properties in confining units, and no conclusions regarding the 

anisotropy of units can be made. Thus, for this study, hydraulic 

properties within each layer are assumed to be isotropic.

The model was designed to simulate major components of 

the confined and regional ground-water flow system, as 

depicted on the schematic diagram shown in figure 4, specifically to characterize flow in the Brunswick and Floridan aquifer 

systems. Recharge to, and discharge from, the confined portion 

of the surficial aquifer system is applied using a general-head 

boundary throughout the study area; recharge to regional confined aquifers is applied using a general-head boundary at outcrop areas (mostly north of the Gulf Trough; figs. 1 and 3), or 

from downward leakage through overlying semiconfining units. 

Because the extent and subsurface geometry of the Brunswick 

aquifer system is poorly understood—the aquifer system is not 

known to crop out—simulated recharge is restricted to leakage 

from the overlying surficial aquifer system or underlying Upper 

Floridan aquifer. Natural discharge from the Brunswick aquifer 

system is through upward leakage into the surficial aquifer 

system and streams. Because little is known about regional 

ground-water flow in the Lower Floridan aquifer, it is assumed 

that the regional flow characteristics of the Lower Floridan 

aquifer are similar to those of the Upper Floridan aquifer. 

Thus, simulated lateral boundary conditions for the Upper and 

Lower Floridan aquifers are identical. Regional discharge is 

simulated as diffuse leakage through confining units in downdip 

areas and offshore, and along a southern specified-head boundary. Model design, including layering, hydraulic properties, and 

boundary conditions are described in greater detail in subsequent sections of this report.

The model was calibrated to simulate steady-state flow 

and mean-annual conditions for predevelopment (pre–1900) 

and the years 1980 and 2000. The years 1980 and 2000 were 

chosen because of the relative abundance and distribution of 

ground-water-level and pumpage data for simulated aquifers. 

Pumping data are less reliable for the year 1980 than 2000, but 

water-level measurement coverage is better, particularly in the 

updip (north of Gulf Trough) area. Because predevelopment 

data are lacking, the model was not calibrated to match heads at 

a specific well. Instead, simulated water levels were qualitatively compared to a map showing the estimated potentiometric 

surface of the Upper Floridan aquifer during predevelopment 

(Johnston and others, 1980).

Steady-state simulation was performed because the purpose of the model is to simulate the ultimate effect on the flow 

system resulting from changes in mean-annual pumping, and 

not to determine how quickly the system responds to these 

changes. To determine whether time-dependent processes 

would affect overall changes in water levels due to pumping 

within the temporal scope of the model (mean-annual conditions), the model’s transient response was tested (appendix B). 

Results showed that relatively extreme changes in stress are 

required to affect a transient response; and for the purpose of 

this model, the steady-state approximation is appropriate.
18 Simulation of Ground-Water Flow in Coastal Georgia and Adjacent Parts of 

South Carolina and Florida—Predevelopment, 1980, and 2000

Spatial Discretization

The finite-difference technique used by MODFLOW-2000 

(Harbaugh and others, 2000) requires that the simulated area be 

divided into discrete blocks or cells. The model encompasses 

42,155 mi2 and is constructed with 119 rows and 108 columns, 

of which a maximum of 10,417 cells are active per layer (fig. 9). 

The flow system is horizontally discretized using irregular grid 

spacing, with cell sizes ranging from approximately 4,000 x 

5,000 ft (0.7 mi2) to 16,500 x 16,500 ft (9.8 mi2). A greater grid 

density at Savannah and Brunswick was chosen to enable 

simulation of steeper head gradients near cones of depression 

and to facilitate linkage with smaller-scale solute-transport 

models being developed in those areas. Graphical grid-generation tools from the graphical user interface ARGUSTM enabled 

visual adjustment of grid position and density. The model is 

oriented such that columns covering Savannah and Brunswick 

are aligned to enable greater grid density in those areas. Columns are oriented approximately parallel to the shoreline and 

the continental shelf escarpment to facilitate application of the 

southeastern (offshore) boundary.

EDGE OF MODEL—NO-FLOW BOUNDARY (EM)

EM

EM

Base from U.S. Geological Survey

1:100,000-scale digital data

N

0 50 MILES 25

0 25 50 KILOMETERS

SOUTH

GEORGIA CAROLINA

FLORIDA

Model

area

Model cells

EXPLANATION

Active

Fixed-head boundary

 (layers 5–7);

 No-flow boundary

 (layers 1–4)

No-flow boundary

 (all layers)

Figure 9. Model grid and lateral boundary conditions.
Simulation of Ground-Water Flow 19

Model Layering

The model was vertically discretized with one layer of 

model cells per hydrogeologic layer, and variable thickness 

depending on the thickness of the layer. There are seven 

actively simulated aquifer and confining unit layers in the 

model (figs. 2 and 10). The aquifers include:

• the confined upper and lower water-bearing zones of 

the surficial aquifer system grouped together as layer 1,

• the upper and lower Brunswick aquifers grouped 

together to form the Brunswick aquifer system (layer 3), 

• the Upper Floridan aquifer (layer 5), and

• the Lower Floridan aquifer (layer 7).

In addition, confining units between these units are 

actively simulated (layers 2, 4, and 6). The Fernandina permeable zone of the Lower Floridan aquifer (layer 7) is not simulated as a distinct zone in the model, because of uncertainty 

about its extent, hydraulic properties, and lack of data with 

which to calibrate those properties.

The surficial aquifer system is divided into two zones—

a water-table zone, which serves as general-head boundary; 

and the underlying confined upper and lower water-bearing 

zones, which are grouped together to form layer 1 of the model. 

Although the surficial aquifer system is actively simulated, the 

spatial discretization of the model is insufficient to simulate 

accurately unconfined flow-system characteristics. Simulated 

flow in the confined surficial aquifer system is used primarily as 

a means to move water into and out of the deeper confined aquifers, 

and not to provide detailed characterization of flow in the unit.

Maps showing the altitude of the top of each layer were 

contoured and digitized based on published literature (Brooks 

and others, 1985; Charm and others, 1969; Clarke and others, 

1990; Hathaway and others, 1981; Kellam and Gorday, 1990; 

Miller, 1986; Scholle, 1979; Steele and McDowell, 1998) and 

unpublished data (A. Foyle, Georgia Southern University, 

Applied Coastal Research Laboratory, written commun., 2002; 

J. Gellici, South Carolina Department of Natural Resources, 

written commun., 2002; H. Gill, Jordan Jones and Goulding, 

written commun., 2001), and modified using new well information collected as part of the CSSI (Falls and others, 2001; Foyle 

and others, 2001). The altitude of the top of each layer was 

adjusted where necessary and justifiable in order to ensure that 

the surfaces did not intersect one another.

The MODFLOW-2000 (Harbaugh and others, 2000) code 

requires that model layers are continuous across the entire 

model area. A schematic diagram (fig. 10) and hydrogeologic 

sections (fig. 11) along the approximate strike and dip of geologic formations illustrate how model layers are discretized. 

To simulate the “pinchout” or absence of the Brunswick aquifer 

system (layer 3) in parts of the study area, layer 3 was assigned 

a nominal thickness and hydraulic properties representative of 

the average of adjacent layers 2 and 4 (figs. 10 and 11). In areas 

where layer 3 is absent, the Upper Floridan aquifer (layer 5) 

is separated from the surficial aquifer system by a composite 

confining unit consisting of layers 2, 3, and 4.

Hydraulic Properties

Hydraulic-property input into the model consists of vertical 

and horizontal hydraulic conductivity for all layers. Initial 

hydraulic conductivities were assigned on the basis of available 

field data including aquifer test, specific-capacity, and laboratory 

permeability data (Clarke and others, 2004; Golder Associates, 

Inc., 2003). In addition, information on potentiometric-head 

gradients and geologic setting (lithologic descriptions, depositional environment, and structural features) was used to estimate 

an approximate distribution of hydraulic conductivity (fig. 12).

Available data are limited to horizontal properties in aquifers 

and vertical properties in confining units, and no relation can be 

established regarding the anisotropy of units. Thus, for this study, 

hydraulic properties within each layer are assumed isotropic.

For aquifer layers (1, 3, 5, and 7) data used as initial 

hydraulic conductivities were derived mostly from aquifer-test 

transmissivity values (Clarke and others, 2004) and then 

divided by aquifer thickness to determine hydraulic conductivity. For confining units (layers 2, 3, 4, and 6), initial hydraulic 

conductivities were derived largely from laboratory permeability data. In layers 1, 6, and 7, the conductivity of each was 

assumed to be homogenous because of limited data for the 

surficial aquifer system, the Lower Floridan confining unit, and 

the Lower Floridan aquifer, respectively. The hydraulic conductivity for layers 2, 3, 4 and 5 were distributed into zones, as 

shown in figure 12. During calibration, values were modified 

where appropriate and supported by hydrogeologic information 

to improve model results. Final calibrated hydraulic conductivity values for each layer are shown in figure 12; ranges of 

reported and calibrated transmissivity for aquifers and vertical 

hydraulic conductivity for confining units are shown in table 3. 

Reported transmissivity values vary by several orders of 

magnitude even across small areas (Clarke and others, 2004), 

allowing some flexibility in calibrated values.

The range of calibrated transmissivity of aquifer units 

varies generally within one order of magnitude of reported 

ranges of values. For confining units, reported vertical hydraulic conductivity is limited to core permeameter data that are 

sparse and poorly distributed in the study area. In addition, these 

data are representative of local conditions where the core was 

recovered, and do not represent a large portion of the unit, and 

typically represent matrix permeability (not secondary). For 

these reasons, adjustments to the vertical hydraulic conductivity 

arrays during calibration were allowed a considerably greater 

degree of variation than the reported ranges and may be several 

orders of magnitude different that reported ranges.
EXPLANATION

General-head boundary—Represents water table or equivalent 

 freshwater head that provides flow to underlying confined aquifers. 

 Flow is restricted by a conductance term, which is equivalent to 

 hydraulic conductivity

No-flow boundary

No-flow or fixed-head boundary

Offshore area—Equivalent freshwater head 

 of overlying saltwater column

Onshore area—Water-table head

Aquifer

Confining unit

Onshore Offshore

Layer 3

Layer 2

CONFINED ZONE OF SURFICIAL AQUIFER SYSTEM Layer 1

BRUNSWICK

AQUIFER SYSTEM

UPPER FLORIDAN AQUIFER

LOWER FLORIDAN AQUIFER

Layer 4

Layer 5

Layer 6

Layer 7

20 Simulation of Ground-Water Flow in Coastal Georgia and Adjacent Parts of 

South Carolina and Florida—Predevelopment, 1980, and 2000

Figure 10. Schematic diagram showing model layers and boundary conditions.
SOUTH

GEORGIA CAROLINA

FLORIDA

Model

area

Atlantic Ocean

Port

Royal

C C' Sound

0 60 MILES

VERTICAL SCALE GREATLY EXAGGERATED

30

0 60 KILOMETERS 30

A'

C'

A C

B

B'

Brunswick

peninsula

Low hydraulic-conductivity zone in model

corresponding to Gulf Trough

Brunswick

peninsula

Hilton

Head

Island

500

NAVD 88

500

1,000

1,500

2,000

2,500

3,000

3,500

Feet

NAVD 88

500

1,000

1,500

2,000

2,500

3,000

3,500

Feet

B

NAVD 88

500

1,000

1,500

Feet

B'

A A'

1 Surficial aquifer system

2 Brunswick aquifer system 

 confining unit

3 Brunswick aquifer system 

4 Upper Floridan confining unit

5 Upper Floridan aquifer

6 Lower Floridan confining unit

7 Lower Floridan aquifer

Layer and unit

Line of section

Model grid

Contact

EXPLANATION

A A'

Simulation of Ground-Water Flow 21

Figure 11. Hydrogeologic sections showing vertical discretization of hydrogeologic units simulated by the model.
SOUTH

GEORGIA CAROLINA

FLORIDA

Model

area

F3

F4

F5

F6

F1

F2

F10

F12

F11

F8

F7

F9

C4

C1

B1

C2

C3

C1

C2

C3

C5

C4

C1

C2

C3

C4

C5

1 N/A 70

2,4 C1 0.00017

 C2 0.2

 C3 0.00001

 C4 0.0001

 C5 0.0001

3 B1 50

 C1 0.00017

 C2 0.2

 C3 0.00001

 C4 0.0001

5 F1 34

 F2 2

 F3 100

 F4 70

 F5 394

 F6 2,819

 F7 150

 F8 2,727

 F9 100

 F10 56

 F11 94

 F12 25

6 N/A 0.02

7 N/A 10

Layer Zone

Hydraulic

conductivity

(feet per day)

Upper Brunswick confining unit

(layer 2)

Surficial aquifer system

(layer 1)

Lower Floridan confining unit (layer 6)

Lower Floridan aquifer (layer 7)

Upper Floridan confining unit

(layer 4)

Brunswick aquifer system

(layer 3)

Upper Floridan aquifer

(layer 5)

N

FL

GA

SC

GA

Hydraulic-property zone

Model-calibrated hydraulic 

 conductivity, in feet per day—

 For purposes of simulation,

 vertical and horizontal 

 conductivities are equal

EXPLANATION

0.00001 to 0.00017

0.02 to 0.2

2 to 94

100 to 394

2,500 to 3,200

C1

22 Simulation of Ground-Water Flow in Coastal Georgia and Adjacent Parts of 

South Carolina and Florida—Predevelopment, 1980, and 2000

Figure 12. Schematic diagram showing simulated hydraulic-property zones by model layer.
Table 3. Reported and simulated ranges of aquifer transmissivity and confining unit vertical hydraulic conductivity.

[Yellow, Golder Associates, Inc., 2003; Clarke, 2003; blue, Clarke and others, 2004; brown, Clarke and others, 1990]

Transmissivity, in feet squared per day

Unit Layer Reported Simulated

Minimum Maximum Minimum Maximum

Surficial aquifer system 1 540 14,000 350 19,100

Brunswick aquifer system 3 10 4,700 250 13,350

Upper Floridan aquifer 5 530 600,000 60 3,020,000

Lower Floridan aquifer 7 170 43,000 50 28,900

Vertical hydraulic conductivity, in feet per day

Unit Layer Report Simulated

Minimum Maximum Minimum Maximum

Upper Brunswick confining unit 2 0.000053 0.00013 0.00017 0.2

Upper Floridan confining unit 4 0.000232 3.01896 0.00017 0.2

Lower Floridan confining unit 6 0.000004 0.16 0.02 0.02

Simulation of Ground-Water Flow 23

Layer 2 is the confining unit overlying the Brunswick aquifer system and is represented by five zones. The Brunswick 

aquifer system comprises one of five zones in layer 3, zone B1; 

and zones C1 through C4 represent the lower permeability confining units beyond the aquifer system extent. With the exception of zone B1 in layer 3, layers 2, 3, and 4 are zoned identically 

such that the three layers represent and function as a single confining unit separating the surficial aquifer system (layer 1) from 

the Upper Floridan aquifer (layer 5). Although there are few 

data to define zones or calibrate these confining unit zones specifically, model results indicate that heads in the Upper Floridan 

aquifer (layer 5) were sensitive to the spatial distribution of 

hydraulic conductivity in the overlying confining units. Zones in 

these layers were used to regulate distribution of recharge to the 

Upper Floridan aquifer (layer 5), which resulted in a better calibration of simulated head. In zone C2, in the coastal zone of 

South Carolina, higher hydraulic conductivity was assigned to 

layer 2 to enable recharge of water into the Upper Floridan aquifer near mounds or highs on the potentiometric-surface map of 

the Upper Floridan aquifer (Ransom and White, 1999).

Hydraulic properties for the Upper Floridan aquifer (layer 5) 

were designated based on an abundance of published data (Clarke

and others, 2004) and are divided into 12 zones (fig. 12). Zones 

F1 and F10 represent updip clastic equivalent units of the Upper 

Floridan aquifer in the northern Coastal Plain (north of Gulf 

Trough), and generally are assigned lower hydraulic conductivity than coastal zones (zones F3–F9, F11), which represent 

more carbonate lithologies. Zone F2 represents the low-permeability Gulf Trough feature, as indicated by published data (Clarke

and others, 2004) and by a steepening of contours on the Upper 

Floridan aquifer potentiometric surface (Peck and others, 1999).

Zones F5 through F8 and F11 represent highly transmissive 

parts of the Upper Floridan aquifer either in or adjacent to the 

Southeast Georgia Embayment. Data in the area of zone F6 are 

sparse; a low hydraulic gradient, however, is indicated on the 

Upper Floridan aquifer potentiometric surface (Peck and others, 

1999), indicating a high permeability. Zone F7 has a relatively 

lower permeability than the adjacent zone F8 and corresponds to 

an observed change in the configuration of the Upper Floridan 

potentiometric surface referred to as the Satilla Line. Hydraulic 

conductivity in zone F11 was assigned a lower value than 

surrounding areas to account for a pronounced hydraulic gradient and cone of depression caused by years of heavy pumping 

in the St. Marys–Fernandina Beach area (Peck and others, 

1999). Zone F5 represents a transitional area between the 

Southeast Georgia Embayment and the Beaufort Arch, where 

the potentiometric gradient increases, but is still outside of the 

area of the cone of depression centered at Savannah. Zone F5 

was assigned a hydraulic conductivity between those of adjacent 

zones F6 and F4.

Zone F3 represents a part of the Upper Floridan aquifer that 

is in or around the Beaufort Arch where the aquifer thins and 

becomes relatively shallow. In zone F3, the hydraulic gradient 

is lower than in adjacent areas, and there are some highs on the 

potentiometric surface that are considered to be recharge zones 

for the Upper Floridan aquifer (Ransom and White, 1999). The 

hydraulic conductivity of this zone was assigned a relatively 

higher value than that assigned to adjacent zones.

Zones F4 and F12 include the large cone of depression 

centered in the Savannah area. During initial model development, zone F4 comprised the entire area of the cone of depression; however, simulations indicated a distinct and consistent 

spatial bias in the residuals between observed and simulated 

heads for this area. Consequently, zone F12 was added to the 

model and assigned a lower hydraulic conductivity to improve 

the spatial bias in residuals.

Zone F9 represents the offshore area, where there are few 

available data. The model, however, is relatively insensitive to 

the hydraulic conductivity of this zone; consequently, values 

assigned here had little effect on the calibration.
24 Simulation of Ground-Water Flow in Coastal Georgia and Adjacent Parts of 

South Carolina and Florida—Predevelopment, 1980, and 2000

Boundary Conditions

Boundary conditions generally are based on natural hydrologic boundaries where available; where unavailable, artificial 

boundaries were constructed. A schematic diagram of model 

layers and vertical boundary conditions is shown in figure 10.

Vertical Boundaries

The lowermost model boundary represents no-flow conditions. Throughout the model area, this boundary corresponds to 

the contact between the Lower Floridan aquifer and underlying 

low-permeability sediments of Paleocene age and older (figs. 2 

and 10).

The uppermost boundary simulates a general-head boundary condition, with different controlling heads and conductance 

terms (equivalent to hydraulic conductivity) for onshore and 

offshore areas (fig. 10). In the onshore area, this boundary condition was applied to the top active aquifer cell in the model, 

which may be in layer 1, 2, or 5, depending on which unit crops 

out at land surface (figs. 10 and 11). This type of boundary condition was chosen because a reliable spatial distribution of 

recharge could not be calculated within the scope of the study.

Initially, a fixed-head boundary was applied at the top of 

the model; however, this resulted in unreasonable simulated 

values of recharge to the regional flow system per model cell. 

A reasonable calculated per-cell recharge is estimated from 

baseflow calculations (Priest, 2004) and recharge rates determined for similar hydrologic settings (Williamson and others, 

1990). By using a general-head boundary with resistance in the 

form of a conductance term, the recharge may be effectively 

limited to reasonable amounts. For the onshore area, the controlling head is the estimated water table (Peck and Payne, 

2003), which is set at land-surface altitude at major streams, 

assuming that streams represent the intersection of the water 

table with land surface. The conductance term is a function of 

variable cell thickness, an assumed spatially constant hydraulic 

conductivity (which is indirectly estimated during model calibration), and the hydraulic conductivity of the active cell to 

which the boundary condition is applied. Conceptually, this 

boundary condition represents a source-sink boundary in the 

unconfined portion of the surficial aquifer that recharges to and 

discharges from the confined, regional ground-water system.

For the offshore area, a general-head boundary condition 

was applied to the top active cells, all of which are in layer 1. 

The controlling head for the offshore part of the model area is 

the freshwater equivalent of the saltwater head, and the conductance is assumed to be constant everywhere for simplicity. For 

the offshore area, conductance is not made a function of thickness because control of the thickness of the top layer is limited, 

and the estimated thickness of this layer in the offshore cells 

generally is only a few feet. These simplifications should not 

have a large affect on simulated results because flow from the 

confined system in the offshore area is assumed to be controlled 

predominantly by the hydraulic properties of layers 2 through 4, 

which primarily are confining units. Conceptually, this boundary condition represents a source-sink boundary in the ocean 

that recharges to and discharges from the confined, regional 

ground-water system.

Lateral Boundaries

Lateral boundary conditions for the ground-water flow 

model (figs. 9 and 10) were selected to coincide as closely as 

possible with assumed no-flow boundaries or ground-water 

divides. With the exception of the Floridan aquifer system 

(layers 5, 6, and 7), lateral boundaries for all layers are 

designated as no-flow.

Simulated flow in the Floridan aquifer system is bounded 

laterally by a combination of no-flow and fixed-head boundaries. The northwestern boundary approximately follows the 

updip extent of the Floridan aquifer system or its equivalent, as 

defined by Miller (1986), and is defined in the model as a noflow boundary. The onshore part of the northeastern boundary 

was assigned a no-flow boundary because it is approximately 

parallel to estimated flow lines as shown on the potentiometric 

surface of the Upper Floridan aquifer (Ransom and White, 

1999). This boundary was projected offshore and connected to 

the southeastern (seaward) no-flow boundary by the easternmost offshore boundary. To the southwest and south of the 

model area, there are no proximal natural hydrologic boundaries for the Floridan aquifer system, as it extends west beneath 

Alabama and south beneath Florida. Additionally, a no-flow 

boundary parallel to estimated flow lines is not an appropriate 

boundary condition because potentiometric-surface maps of the 

Upper Floridan aquifer indicate that water levels and estimated 

flowpaths change with time (Bradner, 1999; Clarke, 1987; 

Johnston and others, 1980 and 1981; Peck and others, 1999). 

In these areas, a time-variable, fixed-head boundary condition 

was applied from the top of the Upper Floridan aquifer to the 

bottom of the Lower Floridan aquifer (layers 5, 6, and 7) to 

enable simulation of changing ground-water levels. The controlling head varies spatially along the boundary according to 

potentiometric-head distributions derived from published maps 

for May 1980 (Johnston and others, 1981), May 1998 (Peck and 

others, 1999), and September 2000 (Peck and McFadden, 2004).

Offshore Boundary

In single-density models, a regional-scale freshwatersaltwater interface usually is represented as a no-flow boundary, assuming that the system is at steady state and the interface 

is sufficiently far from stressed areas to function as if at steady 

state. In reality, the aquifers in this study area are highly 

stressed near the coast, and the location of the regional freshwater-saltwater interface is unknown. Thus, the type and location of boundary condition chosen for the offshore area must be 

considered. If the location and type of boundary condition affect 

the calculated heads within the time frame of interest, then it 

must be constructed carefully; if instead, the model is relatively 

insensitive to variation in reasonable boundary conditions, then 

the simplest approach can be used.
Simulation of Ground-Water Flow 25

To determine the appropriate type of lateral boundary for 

the offshore area, a series of tests was conducted, whereby the 

position of and type of boundary were varied and changes in 

simulated head determined at selected onshore locations. Two 

types of boundaries were tested: (1) a fixed-head boundary for 

which the controlling head was set to the freshwater equivalent 

of NAVD 88, and (2) a no-flow boundary representing the 

freshwater-saltwater interface. Both types of boundaries were 

applied along a vertical plane and positioned at several locations relative to the continental shelf margin: (1) at the shelf 

margin (the shelf-margin test boundary), (2) about 30 mi inland 

from the shelf margin (the intermediate test boundary), and 

(3) about 60 mi inland from the shelf margin (the near-shore test 

boundary). Locations of the tested boundaries and results of the 

evaluation are shown in figure 13.

The shelf-margin test boundary is approximately at the 

Florida–Hatteras slope (fig. 1), which represents the farthest 

location of a natural freshwater-saltwater interface. The nearshore test boundary is the closest boundary to the coast and 

includes a small onshore area where it intersects the northern 

and southern boundaries. This boundary represents an unlikely 

location of the regional freshwater-saltwater interface, because 

offshore drilling indicates that freshwater is present in the 

Upper Floridan aquifer 60–70 mi offshore of Fernandina 

Beach, Fla. (JOIDES J-1 site, Johnston and others, 1982). 

In addition, although saltwater contamination of the Upper 

Floridan aquifer has occurred near Hilton Head Island, S.C., 

freshwater is present in deeper parts of the aquifer, and saltwater appears to be entering through breaches in the Upper 

Floridan aquifer confining unit, rather than horizontal movement of the regional interface.

The difference in simulated head using the two types of 

boundary conditions at the three locations is shown in figure 

13B. The tests indicate that the type of boundary had less effect 

on simulated head when positioned closer to the shelf margin. 

Using the shelf-margin test boundary and the intermediate test 

boundary, the difference in simulated head using the two types 

of boundaries was about 2 ft or less. The largest difference in 

simulated head between no-flow and fixed-head boundary 

conditions results from using the near-shore test boundary, with 

a maximum difference of about 3.5 ft in the Upper and Lower 

Floridan aquifers at Brunswick. Although the different types of 

boundaries affect onshore simulated head, these variations are 

within an acceptable error margin for the model when positioned near the intermediate or shelf-margin test-boundary locations, which are more likely locations for an offshore freshwater-saltwater interface. Following these tests, the decision 

was made to position the offshore boundary along the shelf margin and to utilize the simpler, no-flow boundary condition.

Pumpage

County aggregate and site-specific data were used to estimate average annual pumpage for 1980 and 2000 using procedures described by Taylor and others (2003). For layers 3, 5, 

and 7, the sum of site-specific and nonsite-specific pumping 

rates for 1980 and 2000 were assigned to the model grid cells. 

Pumpage was not assigned to layer 1, the surficial aquifer 

system, because there is too much uncertainty whether the 

pumping would be in the unconfined portion, which is not 

actively simulated, or the confined portion, which is actively 

simulated. In addition, there is not sufficient data to calibrate 

properly the confined surficial aquifer.

Site-specific data generally include permitted industrial and 

public-supply systems, and consist of withdrawal data, permit 

information, and well locations. These data typically include 

information on the aquifer utilized or well-construction information 

that may be used to help determine the aquifer utilized. For some 

multiwell permits, well-specific pumping data were acquired or 

estimated from data provided by the permittee. Nonsite-specific 

data consist of the remainder of county aggregate pumping after 

the sum of site-specific pumping for that county had been subtracted; these data may comprise agricultural, domestic, commercial, or other categories of water use for unpermitted wells. 

Because nonsite-specific data do not include specific withdrawal 

locations or aquifer being utilized, it is important to evaluate how 

best to assign this pumpage to the model.

A series of tests was conducted using a preliminary version 

of the model to determine the best way to distribute estimated 

nonsite-specific pumpage for the model. The tests evaluated the 

response of simulated head in the Upper Floridan aquifer at 

selected locations in the study area to a variety of pumping distributions for the year 2000. Sites selected for evaluation were 

located in areas representative of the variable hydrogeology, 

topography, and land use of the study area. Each simulation 

utilized site-specific data for 2000, and the following nonsitespecific pumping distributions:

• Distribution A: nonsite-specific data are not utilized.

• Distribution B: all nonsite-specific data are assigned to 

the Upper Floridan aquifer.

• Distribution C: nonsite-specific data are equally distributed among the surficial aquifer system, the Brunswick aquifer system, the Upper Floridan aquifer, and 

the Lower Floridan aquifer.

• Distribution D: nonsite-specific data are divided 

among the surficial aquifer system, the Brunswick 

aquifer system, and the Upper Floridan and Lower 

Floridan aquifers based on an estimated percentage distribution of wells with assigned aquifer designations in the 

USGS National Water Information System database using 

procedures described by Taylor and others (2003).

The simulated head in the Upper Floridan aquifer 

resulting from the test simulations at seven locations is shown 

in figure 14. Results indicate that simulated head was higher 

(on average 32 ft) for simulations conducted without nonsitespecific pumping data (distribution A) than for simulations in 

which it isincluded (distributions B–D). When the nonsitespecific pumping was attributed entirely to the Upper Floridan 

aquifer (distribution B), simulated heads generally were lower 

than for test distributions C and D, on average by 4 ft.
SOUTH

GEORGIA CAROLINA

FLORIDA

Model

area

GA

FL

SC

GA

Joides J-1

Joides J-2

Joides J-2

Tenneco

 Coastal 

 Sound

 Science 

Initiative

N

0 50 MILES 25

0 50 KILOMETERS 25

Base from U.S. Geological Survey

1:100,000-scale digital data

0

0.5

1

1.5

2

2.5

3

3.5

4

DIFFERENCE BETWEEN SIMULATED HEAD

USING A FIXED-HEAD BOUNDARY CONDITION

AND A NO-FLOW BOUNDARY CONDITION,

IN FEET

 OBSERVATION LOCATION

Brunswick

St Marys

Bulloch County

Wayne County

Savanna

Brunswick

h

Hilton Head 

 Island

St Marys

Bulloch County

Wayne County

Savannah

Brunswick

Hilton Head 

 Island

St Marys

Shelf-margin test boundary

Intermediate test boundary

Near-shore test 

boundary

A.

B.

Brunswick

St Marys

St Marys

Bulloch

County

Wayne

County

Savannah

Hilton

Head

Island

Shelf margin

Intermediate

Near shore

Brunswick aquifer system

 (layer 3)

Location of model

 test boundary

EXPLANATION

Offshore well and name

Simulated head comparison

 location and name

EXPLANATION

Lower Floridan aquifer

 (layer 7)

Upper Floridan aquifer

 (layer 5)

Model boundary

26 Simulation of Ground-Water Flow in Coastal Georgia and Adjacent Parts of 

South Carolina and Florida—Predevelopment, 1980, and 2000

Figure 13. (A) Location of tested model boundaries, selected onshore wells, and offshore drilling sites; and (B) difference 

between simulated head using fixed-head and no-flow boundary conditions in layers 3, 5, and 7 at various locations in the 

study area. Positive values indicate that simulated head using a fixed-head boundary is consistently higher than that using a 

nonfixed-head boundary. Higher values simulated for the near-shore test boundary indicate the model is more sensitive to 

the type of boundary condition used when the boundary is located closer to shore (and thus closer to onshore simulatedhead comparison locations).
Simulation of Ground-Water Flow 27

Brunswick aquifer system for 1980–2000, it is highly likely 

that some pumpage occurred during this period. Intuitively, the 

distribution among aquifers based on the percentage of wells 

completed in an aquifer in a given county would be more reasonable than an equivalent distribution among aquifers, because 

the former is based on existing well information, and the latter 

may apply too much pumpage to units that generally are used 

less, such as the surficial aquifer system and Lower Floridan 

aquifer. For these reasons, distribution D was chosen as the 

means to assign nonsite-specific pumping data to the model.

Because the nonsite-specific pumping comprises an estimated remainder of total pumping in a county and is at an 

unknown location, this pumping was assumed to be distributed 

equally across each county, based on a preliminary model-grid 

cell size of 15.54 mi2. Other, more complex distributions were 

tested—for example, distributing agricultural pumping according to land use determined using a geographic information 

system—but the results indicated that there was little effect on 

overall distribution of stresses in the ground-water model 

(Taylor and others, 2003).

Simulated ground-water pumpages for the Brunswick aquifer 

system and the Upper and Lower Floridan aquifers are shown in 

figure 15 for the 1980 simulation and in figure 16 for the 2000 simulation. There is an apparent decrease in pumping in the Brunswick aquifer system between 1980 and 2000. Because of 

improved water-use reporting, a greater fraction of estimated 

total pumpage is attributed to site-specific, and a lesser fraction 

to nonsite-specific, pumping for 2000 than for 1980. This results 

in an apparent decline in nonsite-specific pumpage and results 

in an apparent decline in pumpage for the Brunswick aquifer 

system in some counties during this period—for example, 

Glynn County and Liberty County. It is more likely that pumpage increased during this time period, but there are no data to 

substantiate this. Ultimately, the total amount of pumping is 

small, there are few observations with which to calibrate and 

many other sources of uncertainty, so the effect on the model 

results is insignificant. Note that county totals, described earlier, 

may be larger than pumpage applied to the model in some 

counties that are split along model boundaries.

Model Calibration

0

50

–50

100

–100

150

–150

200

250

300

EXPLANATION

Simulated pumping distribution

SIMULATED HEAD, IN FEET ABOVE

OR BELOW (–) NAVD 88

Laurens County

Montgomery County

Camden County

Glynn County

Long County

Chatham County

Beaufort County

A — Nonsite-specific data not included

B — All nonsite-specific data applied 

 to Upper Floridan aquifer

C — Nonsite-specific data applied equally 

 to all simulated aquifers

D — Nonsite-specific data divided according 

 to percentage of wells completed 

 in aquifer in a given county

GEORGIA SOUTH

CAROLINA

The model was calibrated by adjusting hydraulic properties 

and boundary conditions to match observed water levels. Other 

factors considered during calibration include: (1) matching 

simulation results to the conceptual model of the ground-water 

flow system, including distributions of recharge and discharge 

areas, and directions of flow among aquifers; (2) adhering to 

the geologic and hydrogeologic framework; (3) maintaining 

reasonable values of hydraulic properties as defined by field 

data (Clarke and others, 2004); and (4) maintaining reasonable 

values of aquifer recharge in cells supplied by the general-head 

boundary when compared to baseflow estimates reported by 

Priest (2004) and other estimates of regional recharge (Williamson and others, 1990).

Figure 14. Effects of various pumping distributions on 

simulated head in the Upper Floridan aquifer.

Simulated head for pumping distributions C and D are intermediate relative to those for distributions A and B. When pumping 

was equally divided among the aquifers (distribution C), simulated heads generally were higher than when distribution D was 

applied by an average of 10 ft. This difference results because 

distribution C assigns a greater percentage of pumping to the 

shallow surficial and Brunswick aquifer systems than distribution D, which is based on the proportion of wells completed in 

the various aquifers in a given county. This distribution reduces 

the amount of pumpage designated for the Upper Floridan, 

resulting in higher simulated head.

Nonsite-specific pumping is an important consideration 

for model simulations because it comprises a substantial portion of total pumpage applied to some model layers; without 

these data, no pumping would be attributed to the Brunswick 

aquifer system. Although there are no permitted wells in the 
28 Simulation of Ground-Water Flow in Coastal Georgia and Adjacent Parts of 

South Carolina and Florida—Predevelopment, 1980, and 2000

The model was calibrated using the following procedure:

1. The model initially was calibrated to 2000 stress 

conditions using head data.

2. Hydraulic-characteristics arrays derived from 

calibration of the 2000 period were used as initial 

conditions for simulation of 1980 conditions.

3. The additional observations for 1980 allowed 

additional refinement of the aquifer 

characteristics, particularly in the updip area.

4. These modified characteristics then were used to 

refine calibration of 2000 conditions.

This iterative process continued until the model was calibrated for both 1980 and 2000 conditions. Upon completion of 

calibration to 1980 and 2000 conditions, a simulation of predevelopment conditions was performed by removing all pumping 

from the model and comparing these results to an estimated 

predevelopment potentiometric surface for the Upper Floridan 

aquifer (Krause and Randolph, 1989).

Improvement in the quality of the 1980 and 2000 steadystate simulations between successive model runs was evaluated 

by comparisons of the following:

• residuals (differences) between observed head and 

simulated head; and the mean, root-mean square, and 

standard deviation of the residuals; and

• percentage of wells whose residual met established 

calibration criteria (see discussion below).

The mean of the residuals indicates whether the mean difference between computed and observed water levels is skewed 

positive or negative in magnitude. The root-mean square is the 

square root of the average deviation of the residuals from zero.

Aquifer and confining-unit properties were initially based 

on available data and were adjusted using trial-and-error 

parameter estimation during calibration. Automated parameter 

estimation techniques were attempted, but there are not enough 

different types of observation data for these techniques to be 

successful (Hill, 1998).

Calibration Targets

In calibrating the model, two types of water-level observations (appendix A) were used: (1) synoptic water-level measurements from the Upper Floridan aquifer during May 1980 

and September 2000, and (2) mean-monthly water levels for 

sites with continuous recorder data.

An accuracy analysis of water-level data provides a calibration target of acceptable margin of error (table 4). Most 

observations for both 1980 and 2000 are for the Upper Floridan 

aquifer (layer 5). Because too few observations exist for either 

the Brunswick aquifer system (layer 3) or Lower Floridan 

aquifer (layer 7) to be analyzed separately, observations from 

all layers (3, 5, and 7) are analyzed as a single set. Because 

observation data differ by site and season for the two calibration 

years, a separate accuracy analysis is provided for each year.

SOUTH

GEORGIA CAROLINA

FLORIDA

Model area

Map

area

Approximate extent

of Brunswick

aquifer system

A.

0.02

0.02

0.16

0.13

0.02

BRYAN

LIBERTY

MCINTOSH

GLYNN

CAMDEN

Base from U.S. Geological Survey

1:100,000-scale digital data

N

0 10 20 MILES

0 10 20 KILOMETERS

EXPLANATION

Total estimated pumping per county—

 In million gallons per day, layer 3 

 (Brunswick aquifer system)

Less than 0.025

0.025 to 0.1

0.1 to 0.15

Greater than 0.15

Figure 15. Distribution of ground-water pumpage by model 

layer, 1980, (A) layer 3, Brunswick aquifer system (values 

rounded to 0.01 million gallons per day [10,000 gallons per 

day], values do not agree with values shown in table 8 and 

figures 28A and 29 because of rounding; (B) layer 5, Upper 

Floridan aquifer; (C) layer 5, Upper Floridan aquifer (enlarged 

view); (D) layer 7, Lower Floridan aquifer.
SOUTH

GEORGIA CAROLINA

FLORIDA

Model

area

Model boundary

0 to 0.5

0.5 to 1

1 to 5

5 to 10

10 to 51

Pumping rates per grid cell— 

 In million gallons per day, 

 during 1980, layer 5 

 (Upper Floridan aquifer)

Base from U.S. Geological Survey

1:100,000-scale digital data

N

0 50 MILES 25

0 25 50 KILOMETERS

B.

EXPLANATION

Simulation of Ground-Water Flow 29

Figure 15. Distribution of ground-water pumpage by model layer, 1980, (A) layer 3, Brunswick aquifer system; (B) layer 5, 

Upper Floridan aquifer; (C) layer 5, Upper Floridan aquifer (enlarged view); (D) layer 7, Lower Floridan aquifer—continued.
Area

of map

above

Area of map

at right

SOUTH

GEORGIA CAROLINA

FLORIDA

Model area

Model boundary

0 5 10 15 20 MILES

0 5 10 15 20 KILOMETERS

EXPLANATION

Pumping rates per grid cell— 

 In million gallons per day, 

 during 1980, layer 5 

 (Upper Floridan aquifer)

0 to 0.5

0.5 to 1

1 to 5

5 to 10

10 to 51

N

Base modified from U.S. Geological Survey 

1:100,000-scale digital data

C.

Savannah

Jacksonville

Fernandina Beach

Brunswick

St Marys

30 Simulation of Ground-Water Flow in Coastal Georgia and Adjacent Parts of 

South Carolina and Florida—Predevelopment, 1980, and 2000

Figure 15. Distribution of ground-water pumpage by model layer, 1980, (A) layer 3, Brunswick aquifer system; (B) layer 5, 

Upper Floridan aquifer; (C) layer 5, Upper Floridan aquifer (enlarged view); (D) layer 7, Lower Floridan aquifer—continued.
0 5 10 15 20 MILES

0 5 10 15 20 KILOMETERS

EXPLANATION

Pumping rates per grid cell— 

 In million gallons per day, 

 during 1980, layer 7 

 (Lower Floridan aquifer)

0 to 0.5

0.5 to 1

1 to 5

5 to 10

10 to 51

N

Base modified from U.S. Geological Survey 

1:100,000-scale digital data

D.

Area

of map

above

Area of map

at right

SOUTH

GEORGIA CAROLINA

FLORIDA

Model area

Model boundary

Savannah

Jacksonville

Fernandina Beach

Brunswick

St Marys

Simulation of Ground-Water Flow 31

Figure 15. Distribution of ground-water pumpage by model layer, 1980, (A) layer 3, Brunswick aquifer system; (B) layer 5, 

Upper Floridan aquifer; (C) layer 5, Upper Floridan aquifer (enlarged view); (D) layer 7, Lower Floridan aquifer—continued.
Table 4. Statistics for quantifiable components of 

observation accuracy.

Factor

Year

1980 2000

Altitude accuracy

Number of observations 297 176

Range (feet) 0.1 to 10 0.1 to 10

Average (feet) 4.5 4.1

Standard deviation (feet) 3.5 3.3

Percent of observations without accuracy data 29 18

Seasonal variation

Number of observations 67

Range (feet) 0.4 to 7.1

Average (feet) 2.2

Standard deviation (feet) 1.3

Sum of standard deviations for altitude

accuracy and seasonal variation 4.8 4.6

32 Simulation of Ground-Water Flow in Coastal Georgia and Adjacent Parts of 

South Carolina and Florida—Predevelopment, 1980, and 2000

Quantifiable components of water-level observation accuracy analysis include the accuracy of the land-surface altitude, 

annual variation in water levels, and accuracy of the water-level 

measurement. Water-level measurement error is considered to 

be insignificant. Land-surface altitude accuracy differs depending on the method used for determination. For values determined from topographic maps, one-half of the contour interval 

(generally 10 ft) is considered to be the altitude accuracy. For 

values determined using global-positioning instrumentation or 

surveying, the accuracy generally is within 1 ft; however, these 

data are sparse in the study area. Annual water-level variability 

was determined using data from wells equipped with continuous recorders. The data used for this analysis include all years 

from the period of record through 2002 for which 10 or more 

months of mean-monthly data exist.

Observation accuracy consists of the sum of the standard 

deviations of land-surface altitude accuracy and annual variability of water-level accuracy, and is 4.8 ft for 1980 and 4.6 ft for 

2000 (table 4). Because of other potential errors in the observation data, as well as errors in data used for model input or in 

development of the conceptual model, Kuniansky and others 

(2003) suggested that a good calibration target for fit of simulated-to-observed water levels would be two times the standard 

deviation of observation accuracy. Thus, the final calibration 

targets for simulated ground-water levels are 9.6 ft for 1980 and 

9.2 ft for 2000. For the purpose of simplicity, these values are 

rounded to 10 ft for both the 1980 and 2000 simulations. An 

additional statistic used to evaluate model calibration is computed by dividing the standard deviation of the residuals by the 

range of water-level variation. Generally, if the range of waterlevel data is large, the standard deviation of residual errors also 

is large. This dimensionless statistic generally should be less 

than one; a good fit of the data would be reflected if the ratio 

was approximately 0.1 or less indicating that the residuals are 

generally less than 10 percent of the range in altitude of the 

observations (Kuniansky and others, 2003).

SOUTH

GEORGIA CAROLINA

FLORIDA

Model area

Map

area

Approximate extent

of Brunswick

aquifer system

A.

0.02

0.06

0.11

0.04

0.01

BRYAN

LIBERTY

MCINTOSH

GLYNN

CAMDEN

Base from U.S. Geological Survey

1:100,000-scale digital data

N

0 10 20 MILES

0 10 20 KILOMETERS

EXPLANATION

Total estimated pumping per county—

 In million gallons per day, layer 3 

 (Brunswick aquifer system)

Less than 0.025

0.025 to 0.1

0.1 to 0.15

Greater than 0.15

Figure 16. Distribution of ground-water pumpage by model 

layer, 2000, (A) layer 3, Brunswick aquifer system (values 

rounded to 0.01 million gallons per day [10,000 gallons per 

day], values do not agree with values shown in table 9 and 

figures 28B and 29 because of rounding; (B) layer 5, Upper 

Floridan aquifer; (C) layer 5, Upper Floridan aquifer (enlarged 

view); (D) layer 7, Lower Floridan aquifer.

Period of record
Model boundary

SOUTH

GEORGIA CAROLINA

FLORIDA

Model

area

0 to 0.5

0.5 to 1

1 to 5

5 to 10

10 to 51

Pumping rates per grid cell— 

 In million gallons per day, 

 during 2000, layer 5 

 (Upper Floridan aquifer)

N

Base from U.S. Geological Survey

1:100,000-scale digital data

0 50 MILES 25

0 25 50 KILOMETERS

B.

EXPLANATION

Simulation of Ground-Water Flow 33

Figure 16. Distribution of ground-water pumpage by model layer, 2000, (A) layer 3, Brunswick aquifer system; (B) layer 5, 

Upper Floridan aquifer; (C) layer 5, Upper Floridan aquifer (enlarged view); (D) layer 7, Lower Floridan aquifer—continued.
0 5 10 15 20 MILES

0 5 10 15 20 KILOMETERS

EXPLANATION

Pumping rates per grid cell— 

 In million gallons per day, 

 during 2000, layer 5 

 (Upper Floridan aquifer)

0 to 0.5

0.5 to 1

1 to 5

5 to 10

10 to 51

N

Base modified from U.S. Geological Survey 

1:100,000-scale digital data

C.

Area

of map

above

Area of map

at right

SOUTH

GEORGIA CAROLINA

FLORIDA

Model area

Model boundary

Savannah

Jacksonville

Fernandina Beach

Brunswick

St Marys

34 Simulation of Ground-Water Flow in Coastal Georgia and Adjacent Parts of 

South Carolina and Florida—Predevelopment, 1980, and 2000

Figure 16. Distribution of ground-water pumpage by model layer, 2000, (A) layer 3, Brunswick aquifer system; (B) layer 5, 

Upper Floridan aquifer; (C) layer 5, Upper Floridan aquifer (enlarged view); (D) layer 7, Lower Floridan aquifer—continued.
Area

of map

above

Area of map

at right

SOUTH

GEORGIA CAROLINA

FLORIDA

Model area

Model boundary

Savannah

Jacksonville

Fernandina Beach

Brunswick

St Marys

0 5 10 15 20 MILES

0 5 10 15 20 KILOMETERS

EXPLANATION

Pumping rates per grid cell— 

 In million gallons per day, 

 during 2000, layer 7 

 (Lower Floridan aquifer)

0 to 0.5

0.5 to 1

1 to 5

5 to 10

10 to 51

N

Base modified from U.S. Geological Survey 

1:100,000-scale digital data

D.

Simulation of Ground-Water Flow 35

Figure 16. Distribution of ground-water pumpage by model layer, 2000, (A) layer 3, Brunswick aquifer system; (B) layer 5, 

Upper Floridan aquifer; (C) layer 5, Upper Floridan aquifer (enlarged view); (D) layer 7, Lower Floridan aquifer—continued.
36 Simulation of Ground-Water Flow in Coastal Georgia and Adjacent Parts of 

South Carolina and Florida—Predevelopment, 1980, and 2000

Recharge

During model calibration, the amount of recharge allowed 

into the regional system was constrained by stream baseflow 

data (Priest, 2004) and estimates of regional recharge in a 

proximal and hydrologically similar area (Williamson and 

others, 1990). The model simulates the regional flow system 

and does not attempt to simulate specifically the shallow unconfined flow system. Estimated recharge to the regional system is 

not likely to exceed long-term average stream baseflow estimates from hydrograph separation, and is more likely to be on 

the order of drought baseflow estimates. The maximum allowable recharge into any model cell was limited to 1–10 inches. 

Stream baseflow data (Priest, 2004) were used indirectly in 

model calibration as a control on recharge into the system. 

Because the scope of the study does not include a detailed 

evaluation of stream-aquifer interaction, the model was not 

designed to simulate ground-water discharge to streams. 

Instead, the baseflow estimates are interpreted as a maximum 

amount of recharge that could occur in any grid cell, as calculated from the flux of the general-head boundary condition into 

the active area of the model. Long-term average stream baseflow estimates for 14 basins in the model area range from 4 to 

10 in/yr (fig. 6) (Priest, 2004). Assuming that stream discharge 

approximately equals recharge in an unstressed steady-state 

system, the maximum allowable recharge into any model cell is 

limited to approximately 10 in/yr.

Recharge/discharge rate in inches per year is calculated as:

x conversion factor Q K(h2–h1)

A L =

where

Q is the discharge rate in cubic feet per day, 

A is area of the cell in square feet,

K is the hydraulic conductivity of the general-head boundary cell in feet per day,

h2 is the controlling head (water-table altitude) in feet, 

h1 is the calculate head in the topmost active cell in feet, 

L is one-half the active cell thickness (the distance 

between the grid-cell centroid where head is 

calculated and the imposed boundary condition at 

the top of the cell), and

conversion factor converts units of feet per day to inches 

per year.

When the controlling head altitude (h2) is greater than that 

of the adjacent active model cell, the calculated rate is positive, 

recharge occurs, and water enters the modeled system from the 

general-head boundary. When h2 is lower than that of the adjacent active model cell, discharge occurs, and water exits the 

system to the general-head boundary.

Steady-State Simulation of 

Predevelopment FlowSystem

The predevelopment flow system was simulated by eliminating all pumping from the calibrated 1980–2000 model. 

Although a quantitative evaluation using calibration statistics is 

not possible because of sparse water-level observations, the 

simulation can be evaluated qualitatively by comparing to an 

estimated predevelopment potentiometric-surface map for the 

Upper Floridan aquifer (Krause and Randolph, 1989).

Ground-Water Flow

Although the simulated and estimated predevelopment 

potentiometric-surface maps for the Upper Floridan aquifer 

do not match exactly, they do show similar features, including 

an upstream deflection of potentiometric contours along major 

streams in the northwestern part of the area, a steepening of 

the potentiometric gradient in the area of the Gulf Trough, a 

flattening of the potentiometric gradient in the southwestern 

part of the model area, and flow toward a potentiometric low in 

coastal South Carolina near Hilton Head Island and Port Royal 

Sound (fig. 17) (see locations, fig. 1). Simulated predevelopment flow in the Lower Floridan aquifer (layer 7) is similar to 

flow in the Upper Floridan (layer 5), with nearly identical 

ground-water levels.

In the Brunswick aquifer system, simulated predevelopment flow generally is to the southeast, with part of the flow 

northeastward toward a potentiometric low in eastern Chatham 

County (fig. 17). A more gentle hydraulic gradient in the 

southeastern model area reflects the greater thickness and 

transmissivity of the aquifer system near the Southeast Georgia 

Embayment. Simulated water levels in the Brunswick aquifer 

system, where present, generally were higher than in the Upper 

Floridan aquifer in the northwestern part of the area, indicating 

a potential for downward flow into the Upper Floridan. In the 

southeastern part of the area, simulated water levels in the 

Brunswick aquifer system were lower than in the Upper Floridan aquifer, indicating a potential for upward flow into the 

Brunswick aquifer system.

Water Budget

The simulated predevelopment water budget includes the 

following components of inflow and outflow to the groundwater flow system: (1) recharge from the general-head boundary, (2) inflow across lateral specified-head boundaries, 

(3) discharge to the general-head boundary, and (4) outflow 

across lateral specified-head boundaries (fig. 18; table 5). 

Predevelopment flow was characterized using the MODFLOW 

postprocessor ZONEBUDGET (Harbaugh, 1990). Flow was 

summarized by model layer (table 5) and by dividing the model 

into several zones and computing discharge into and out of the 

zone. Zones were designated to summarize discharge to and 

from fixed-head and general-head boundaries, between adjacent layers, and along the coastline (fig. 18).
SOUTH

GEORGIA CAROLINA

FLORIDA

Model

area

Approximate

 limit of Brunswick 

 aquifer system

GA

FL

SC

GA

40

80

20

40

120

160

140

100

60

60

80

100

140

120

40

40

40

160

180

200

220

240

260

20

20

280 300

320

340

360

280

300

200

280

260

180

220

260

240

300

60

60

80 100

120

140

40

40

40

160

180

200

220

240

260

20

20

280 300

320

340

360

280

300

160

200

280

260

180

220

260

240

300

60

20

20

60

140

40

20

60

80

100

150

200

250

Model boundary

0 50 MILES 25

0 50 KILOMETERS 25

Base from U.S. Geological Survey

1:100,000-scale digital data

N

EXPLANATION

Simulated potentiometric contour—

 Shows simulated potentiometric surface, 

 in feet, during predevelopment. Hachures 

 indicate depression. Contour interval 20 feet. 

 Datum is NAVD 88

60

Estimated potentiometric contour—

 Shows estimated potentiometric surface, 

 in feet, during predevelopment (Krause and 

 Randolph, 1989). Contour interval, in feet, 

 is variable. Datum is NGVD 29

60

A. Brunswick aquifer system

 (layer 3)

C. Lower Floridan aquifer

 (layer 7)

B. Upper Floridan aquifer

 (layer 5)

Figure 17. Simulated predevelopment potentiometric 

surfaces for the (A) Brunswick aquifer system, 

(B) Upper Floridan aquifer, and (C) Lower Floridan 

aquifer, and estimated predevelopment potentiometric surface for the Upper Floridan aquifer.

Simulation of Ground-Water Flow 37
Onshore Offshore

199

172

8.42

42.0

.584

34.4

17.1

18.9

69.1

66.4

192

174

.001

0

1.51

14.1

1.51

14.1

62.9

35.3

80.6

88.6

79.7

87.7

79.7

87.7

57.5

44.9

57.5

44.9

Surficial aquifer system (layer 1)

Ground-water flow, in million gallons per day, and budget component

EXPLANATION

General-head 

 boundary

Interunit

 leakage

Lateral fixed-head 

 boundary

69.1

66.4 44.9

57.5

Brunswick aquifer system (layer 3)

Upper Floridan confining unit (layer 4)

Upper Floridan aquifer (layer 5)

Lower Floridan confining unit (layer 6)

Lower Floridan aquifer (layer 7)

Brunswick aquifer system 

 confining unit (layer 2)

Table 5. Simulated predevelopment water budget by model layer.

[Values reported to three significant digits and may not sum to totals because of independent rounding; —, not applicable.]

Inflow, in million gallons per day Outflow, in million gallons per day

Model 

layer

Hydrogeologic 

unit

Inflow from 

general-head 

boundary

Inflow across 

lateral 

boundaries

Total

Outflow to 

general-head 

boundary

Outflow across 

lateral 

boundaries

Total

1 Surficial aquifer system 208.0 — 208.0 214.0 — 214.0

2 Confining unit 17.1 — 17.1 18.9 — 18.9

3 Brunswick aquifer system — — — — — —

4 Confining unit — — — — — —

5 Upper Floridan aquifer 69.1 192 261 66.4 174 240

6 Confining unit — 0.000 0.000 — 0.001 0.001

7 Lower Floridan aquifer — 1.51 1.51 — 14.1 14.1

Total all layers 294 193 487 299 188 487

Percentage of flow 60.3 39.7 100 61.4 38.6 100

38 Simulation of Ground-Water Flow in Coastal Georgia and Adjacent Parts of 

South Carolina and Florida—Predevelopment, 1980, and 2000

Figure 18. Schematic diagram showing simulated predevelopment water budget.
Model boundary

EXPLANATION

2 to 3

1 to 2

0 to 1

0 to 1

1 to 2

2 to 3

3 to 4

Simulated recharge or 

 discharge per model cell, 

 in inches per year

Recharge

0 50 MILES 25

0 50 KILOMETERS 25

Base from U.S. Geological Survey

1:100,000-scale digital data

N

Discharge

SOUTH

GEORGIA CAROLINA

FLORIDA

Model

area

Simulation of Ground-Water Flow 39

Figure 19. Simulated recharge or discharge during predevelopment.

Based on the total simulated inflow of 487 Mgal/d, 

60.3 percent (294 Mgal/d) is contributed by leakage from the 

general-head boundary, and 39.7 percent (193 Mgal/d) is 

contributed as inflow from lateral specified-head boundaries in 

layers 5 and 7 (table 5). Based on the total 487 Mgal/d outflow, 

ground-water discharge to the general-head boundary accounts 

for 61.4 percent of the outflow, with 38.6 percent attributed to 

outflow at lateral specified-head boundaries. Along the lateral 

specified-head boundary, the net flow is about 18 Mgal/d into 

the model area in the Upper Floridan aquifer and 12.6 Mgal/d out 

of the model area in the Lower Floridan aquifer.

Flow from the general-head boundary represents recharge 

from the water table to deeper confined aquifers. A map showing 

the areal distribution of recharge and discharge in the onshore 

area is shown in figure 19. The total simulated recharge to the 

ground-water system in the onshore area is 285 Mgal/d (fig. 18), 

which is equivalent to an average of about 0.21 in/yr across the 

entire onshore area. Simulated recharge per model cell was 

4 inches or less throughout the area, which was within the 

10-in/yr limit based on stream baseflow (Priest, 2004) and other 

estimates of regional recharge (Williamson and others 1990). 

The simulated recharge-discharge map (fig. 19) indicates that 

recharge generally occurs in interstream areas and discharge 

occurs in stream valleys and lowland areas near the coast. Areas 

with the highest recharge rates generally are found in interstream 

areas at higher altitudes than adjacent areas.

Simulated discharge from the ground-water flow system 

in the onshore area was 257 Mgal/d (fig. 18). Computed rates 

of interaquifer leakage indicate there is a dominant upward 

component of flow in the study area. Most of this flow occurs 

south of the Gulf Trough (see location, fig. 1), where vertical 

flow gradients are upward in low-lying areas along the coast. 

Highest rates of simulated discharge to the general-head boundary (fig. 19) occur along major rivers north of the Gulf Trough, 

including the Savannah (see locations, figs. 1 and 6) and the 

Ocmulgee Rivers (see location, fig. 6).
40 Simulation of Ground-Water Flow in Coastal Georgia and Adjacent Parts of 

South Carolina and Florida—Predevelopment, 1980, and 2000

A total of 199 Mgal/d recharge the surficial aquifer system 

(layer 1) in the onshore area, with 172 Mgal/d discharged to 

streams or wetlands, leaving 27 Mgal/d available to recharge 

deeper units (fig. 18). The Brunswick aquifer system (layer 3) is 

not known to crop out in the study area, and the only mechanism 

for water to enter the aquifer is through leakage from adjacent units 

(fig. 18). Simulated leakage to the Brunswick aquifer system is provided mostly from layer 4 (87.7 Mgal/d) and layer 2 (80.6 Mgal/d).

Simulated recharge to the Upper Floridan aquifer (layer 5) 

is provided by direct recharge from the general-head boundary, 

by flow from lateral fixed-head boundaries, and from interaquifer leakage (fig. 18). During predevelopment, most of the 

flow to the Upper Floridan aquifer was from lateral fixed-head 

boundaries (192 Mgal/d) along the southern part of the simulated area. Simulated recharge from the general-head boundary 

was 69.1 Mgal/d, and leakage to the Upper Floridan aquifer was 

79.7 Mgal/d from overlying units and 44.9 Mgal/d from underlying units (fig. 18). Net inflow from the lateral fixed-head 

boundary, approximately 18 Mgal/d, was greater than net 

inflow from the general-head boundary, and net flow to adjacent layers 4 and 6 was 8.0 and 12.6 Mgal/d, respectively.

The Lower Floridan aquifer (layer 7) is recharged mostly by 

leakage from layer 6 and by flow from lateral boundaries (fig. 18). 

During predevelopment, total flow to the Lower Floridan aquifer 

was 59.0 Mgal/d, of which 97 percent was leakage from layer 6.

To assess the movement of water offshore and along the 

coastline, several zones were designated and flow rates summarized using ZONEBUDGET (Harbaugh, 1990) (fig. 18). During 

predevelopment conditions, water both entered and discharged 

from the surficial aquifer system into the general-head boundary in the offshore area, with discharge exceeding recharge by 

33.6 Mgal/d.

Steady-State Simulation of 1980 

and 2000 Flow System

Mean-annual conditions for 1980 and 2000 were simulated 

using the steady-state approximation. Simulated head and water 

budget are summarized and compared.

Calibration of Simulated Head

Ground-water conditions during 1980 were calibrated on 

the basis of water-level measurements in 297 wells (table 6; 

fig. 20; appendix A): 3 are completed in the Brunswick aquifer 

system (layer 3), 285 are completed in the Upper Floridan 

aquifer (layer 5), and 9 are completed in the Lower Floridan 

aquifer (layer 7). For the Upper Floridan aquifer wells, the 

residual, or difference between simulated minus observed 

ground-water levels, ranged from –37.0 to 44.2 ft, with a mean 

of –0.470 ft and a root-mean square of 13.0 ft (table 6, fig. 20). 

For the three Brunswick aquifer system wells, residuals ranged 

from –14.0 to 18.9 ft; and for the nine Lower Floridan aquifer 

wells, residuals ranged from –5.20 to 20.1 ft. Residuals for the 

Upper Floridan aquifer were normally distributed, with 70 percent of the simulated values within the 10-ft calibration target 

of observed values. Dividing the standard deviation of the residuals for the Upper Floridan aquifer by the range of observed 

water-levels yields a calibration fit of 0.031, indicating a good 

fit of the data (Kuniansky and others, 2003).

Ground-water conditions during 2000 were calibrated on 

the basis of water-level measurements in 175 wells: 10 are 

completed in the Brunswick aquifer system (layer 3), 154 are 

completed in the Upper Floridan aquifer (layer 5), and 11 are 

completed in the Lower Floridan aquifer (layer 7) (table 7; 

fig. 20; appendix A). For the 10 Brunswick aquifer system wells,

residuals ranged from –7.67 to 13.3 ft, with a mean of 1.79 ft 

and a root-mean square of 5.91 ft. For the Upper Floridan aquifer wells, water-level residuals ranged from –44.4 to 36.4 ft, with 

a mean of –0.843 ft and a root-mean square of 9.94 ft. For the 

Lower Floridan aquifer wells, residuals ranged from –3.62 to 

21.5 ft, with a mean of 5.20 ft and a root-mean square of 9.15 ft. 

Residuals for all layers were normally distributed; simulated 

values were within the 10-ft calibration target of observed values 

for 80 percent of the Brunswick aquifer system wells, 79 percent of the Upper Floridan wells, and 73 percent of the Lower 

Floridan wells (table 7). Dividing the standard deviation of the 

residuals by the range of water-level variation yields a calibration fit of 0.142 for the Brunswick aquifer system, 0.031 for the 

Upper Floridan aquifer, and 0.056 for the Lower Floridan aquifer, indicating a good fit of the data (Kuniansky and others, 2003).

Table 6. Calibration statistics for simulated heads for 1980 conditions.

[Residual equals simulated minus observed head; –, minus; —, not calculated because less than 10 values]

Calibration statistic Brunswick aquifer system 

(layer 3)

Upper Floridan aquifer 

(layer 5)

Lower Floridan aquifer 

(layer 7)

Number of observations 3 285 9

Range of observations (feet) 37.2 414 253

Minimum residual (feet) –14.0 –37.0 –5.20

Maximum residual (feet) 18.9 44.2 20.1

Mean residual (feet) –2.25 –0.470 4.98

Standard deviation of residuals (feet) — 13.0 —

Root-mean square residual (feet) — 13.0 —

Percentage of simulated values within 10-foot error criteria 0 70 67

Calibration fit: Standard deviation of residuals divided by 

range of observed values (Kuniansky and others, 2003) — 0.031 —
A. 1980 Calibration

–200

–100

0

100

200

300

–200 –100 0 100 200 300

SIMULATED WATER LEVEL, IN FEET ABOVE OR BELOW (–) NAVD 88

–200 –100 0 100 200 300

OBSERVED WATER LEVEL, IN FEET ABOVE OR BELOW (–) NAVD 88

B. 2000 Calibration

–100

–200

0

100

200

300

0

20

40

60

80

–40 –30 –20 –10 0 10 20 30 40

–40 –30 –20 –10 0 10 20 30 40

0

20

40

60

80

WATER-LEVEL RESIDUALS, IN FEET

WATER-LEVEL RESIDUALS, IN FEET

NUMBER OF MEASUREMENTS NUMBER OF MEASUREMENTS

Upper Floridan R2 = 0.966

Brunswick aquifer system

Upper Floridan aquifer

Lower Floridan aquifer

Upper Floridan R2 = 0.975

Brunswick aquifer system

Upper Floridan aquifer

Lower Floridan aquifer

Simulation of Ground-Water Flow 41

Figure 20. Observed and simulated water levels (residuals) for 1980 and 2000.

Table 7. Calibration statistics for simulated heads for 2000 conditions.

[Residual equals simulated minus observed head; –, minus]

Calibration statistic Brunswick aquifer system 

(layer 3)

Upper Floridan aquifer 

(layer 5)

Lower Floridan aquifer 

(layer 7)

Number of observations

Range of observations (feet)

Minimum residual (feet)

Maximum residual (feet)

Mean residual (feet)

Standard deviation of residuals (feet)

Root-mean square residual (feet)

Percentage of simulated values within 10-foot error criteria

Calibration fit: Standard deviation of residuals divided by 

range of observed values (Kuniansky and others, 2003)

10 154 11

54.1 319 142

– 7.67 – 44.4 – 3.62

13.3 36.4 21.5

1.79 – 0.843 5.20

7.65 9.94 7.89

5.91 9.94 9.15

80 79 73

0.142 0.031 0.056
42 Simulation of Ground-Water Flow in Coastal Georgia and Adjacent Parts of 

South Carolina and Florida—Predevelopment, 1980, and 2000

The spatial distribution of residuals provides some indication of potential bias in the model. Water-level residuals for the 

Brunswick aquifer system (layer 3) and Upper and Lower Floridan aquifers (layers 5 and 7) are shown for 1980 in figure 21 and 

for 2000 in figure 22. Generally, for layers 3 and 7, there are too 

few observations to discern spatial patterns of the residuals. For 

layer 5, however, there is an observed correlation in the magnitude of residuals with physiography for both the 1980 and 2000 

simulations—in the northwestern part of the model area (north 

of the Gulf Trough), residuals are largest and show the greatest 

variability; in the coastal area, residuals are mostly of smaller 

magnitude. A likely reason for this correlation is greater topographic relief and a larger degree of stream-aquifer interaction 

north of the Gulf Trough, which result in a more irregular potentiometric surface. The model has a poorer fit in these areas, 

because the model cell size is not sufficient to depict this scale 

of heterogeneity.

A more subtle feature is the clustering of positive and negative residuals in parts of the updip area. It is possible that there 

are subregional scale variations in hydraulic conductivity that 

are not accounted for in the model because of a lack of data. An 

additional source of error may be inaccuracy of pumping data in 

the northwestern area, particularly for the 1980 simulation. 

Because much of the withdrawal in this area is for irrigation, 

pumpage estimates have a large margin of error and may not be 

accurately accounted for in the simulations.

Another spatial pattern of the residuals is evident for the 

1980 simulation near the cone of depression at Savannah. Here, 

residuals generally are negative (simulated heads lower than 

observed) and probably reflect pumping data inaccuracy in this 

heavily stressed area.

Ground-Water Flow

Simulated 1980 and 2000 potentiometric surfaces (figs. 23 

and 24, respectively) for the Upper Floridan aquifer have similar prominent features to those shown on potentiometric-surface 

maps for May 1980 (Johnston and others, 1981), May 1998 

(Peck and others, 1999), September 1998 (Ransom and White, 

1999), and September 2000 (Peck and McFadden, 2004). These 

features include large cones of depression in the Savannah, Ga., 

and Jacksonville, Fla., areas; smaller cones of depression at 

Jesup, Brunswick, and the St. Marys, Ga.–Fernandina Beach, 

Fla., areas; a steepening of the potentiometric gradient in the 

area of the Gulf Trough; flattening of the potentiometric gradient 

in the southwestern part of the model area; and potentiometric 

highs north of Port Royal Sound, S.C. (see locations, fig. 1).

The simulated potentiometric surfaces for the Lower Floridan aquifer (layer 7) are similar to those for the Upper Floridan 

for 1980 and 2000 conditions, indicating interaquifer leakage 

through layer 6 (figs. 23 and 24). Differences in simulated water 

levels for the Upper and Lower Floridan aquifers result from the 

vertical hydraulic conductivity of the intervening confining unit 

(layer 6), the generally lower transmissivity of the Lower Floridan aquifer, and the distribution of pumping from the Lower 

Floridan aquifer. Simulated water levels in the Lower Floridan 

aquifer are lower in the Savannah, Ga., and Fernandina Beach 

and Jacksonville, Fla., areas, where the aquifer is utilized for 

water supply. Although the general similarity of simulated 

water levels suggests an interaquifer leakage response, data are 

sparse for the Lower Floridan aquifer (layer 7) and the overlying confining unit (layer 6). Additional information on the 

hydraulic properties of the Lower Floridan aquifer and overlying confining unit are necessary to more accurately simulate 

interaction between the Upper and Lower Floridan aquifers.

The simulated potentiometric surfaces are similar (figs. 23 

and 24) for the Brunswick aquifer system (layer 3) for 1980 and 

2000. Flow generally is to the southeast, with part of the flow 

captured by a cone of depression in the Savannah area. Because 

there is no known pumpage from the Brunswick aquifer system 

in this area, this cone of depression results from interaquifer 

leakage to the heavily pumped Upper Floridan aquifer. A lower 

hydraulic gradient in the southeastern model area reflects the 

greater thickness and transmissivity of the aquifer system near 

the Southeast Georgia Embayment.

Vertical Distribution of Head

To provide an indication of how well the model simulates 

vertical-head relations among aquifers, simulated water levels 

for 2000 were compared to observed data from selected well 

clusters in the coastal area (fig. 25). These clusters consist of 

two or more wells completed in several hydrogeologic units.

Some of the well clusters were constructed after the year 

2000 and, thus, have water levels that reflect changes that 

occurred after the model was calibrated. With the exception of 

head relations between the Upper and Lower Floridan aquifers, 

which have similar values of head in parts of the area, waterlevel changes after 2000 do not affect the relative difference in 

head between layers. For example, if the difference in head 

between two layers indicated upward flow during 2000, that 

same relation would be apparent during 2001, 2002, or 2003, 

despite changing water levels. To minimize the effect of variations in vertical gradients because of seasonal changes, meanmonthly water levels for September 2003—a period when 

water levels were available for most of the well clusters—were 

selected for analysis where available. In some instances meanmonthly data were not available and discrete water-level 

measurements were used.
Simulation of Ground-Water Flow 43

Simulated and observed water levels show similar vertical 

flow potential in most of the area. In the northern part of the 

area, simulated and observed water levels at the Hopeulikit and 

Bulloch South well clusters both show downward gradients, 

reflecting aquifer recharge (fig. 25).

Near the Savannah area cone of depression, water levels in 

the Upper Floridan aquifer are depressed as a result of heavy 

pumping and flow gradients mostly are downward from the 

surficial aquifer system to the Upper Floridan aquifer for both 

simulated and observed water levels. This downward gradient is 

indicated at the Springfield, Pineora, Hutchinson Island, Tybee 

Island, Skidaway, Fort Pulaski, and Richmond Hill well clusters. In parts of this area—at the Springfield, Fort Pulaski, and 

Tybee Island well clusters—simulated water levels indicate 

slightly upward gradients from the Lower Floridan aquifer to 

the Upper Floridan aquifer; observations at the Fort Pulaski and 

Tybee Island well clusters, however, indicate that the flow 

gradient mostly is downward in these areas. This discrepancy 

probably reflects the model’s simplification of the hydrologic 

system because of a lack of hydrogeologic data for the Lower 

Floridan and its overlying confining unit.

At the Gardi well cluster, observed water levels indicate a 

downward gradient from the surficial aquifer system to the 

Upper Floridan aquifer. Ground-water pumpage at Jesup, about 

10 mi north of the site, lowered water levels in the Upper Floridan aquifer and created a downward flow gradient. Simulated 

water levels at the site also show a downward flow gradient 

from the surficial aquifer system to the Brunswick aquifer system;

however, simulated water levels indicate upward flow from the 

Upper Floridan into the Brunswick aquifer system. This discrepancy with observed conditions may result from a lower 

simulated hydraulic conductivity assigned to the Brunswick 

aquifer system, which resulted in lower simulated water levels.

In the Brunswick–Glynn County area, observed and 

simulated vertical-head profiles reflect the complexity of the 

flow system in that area. In the downtown Brunswick area, flow 

gradients generally are upward from the Brunswick aquifer 

system to the surficial aquifer system, as indicated by the simulated and observed vertical-head profiles at the Coffin Park and 

Georgia Pacific South well clusters. Although there are no 

observed data, this relation also is indicated by simulated head 

at the Georgia Ports Authority well cluster at Brunswick.

Head relations among the Brunswick aquifer system and 

the Upper and Lower Floridan aquifers in the downtown Brunswick area are complex and variable. Simulated-head profiles in 

this area indicate that the Upper Floridan aquifer has the lowest 

water levels relative to surrounding units and, thus, is a “hydrologic sink” that receives flow from both overlying and underlying units. Partial profiles based on observations in this area 

indicate that head gradients generally are upward. Well clusters 

in that area, however, do not provide observations for all of the 

units; thus, the characterization of flow gradients is incomplete. 

Despite the lack of observations in some of the units, the simulated-head profiles seem reasonable because ground-water 

pumpage from the Upper Floridan aquifer has lowered groundwater levels and created flow gradients toward the aquifer.

Near the Altamaha River at the Ebenezer Bend well cluster, observed water levels indicate that the upper Brunswick 

aquifer has the lowest water levels relative to surrounding units 

and is a hydrologic sink, receiving flow from the underlying 

lower Brunswick aquifer and overlying surficial aquifer system. 

The reason for this condition is unclear; however, ground-water 

pumpage from the Brunswick aquifer system is occurring 

within 2.25 mi of the site, which may have lowered water levels 

relative to adjacent units. Simulated head at this location indicates that flow is downward from the surficial aquifer system to 

the Upper Floridan aquifer. This apparent discrepancy results 

because the model did not simulate flow in both the upper and 

lower Brunswick aquifers, but grouped these units together into 

a single layer (3) and, thus, could not provide an indication of 

flow within the Brunswick aquifer system.

At St. Marys, Camden County, the Upper Floridan aquifer 

has the lowest simulated water levels relative to surrounding 

units and is a hydrologic sink, receiving flow from the underlying Lower Floridan aquifer and overlying Brunswick aquifer 

system. Although there are no observations in the Upper Floridan aquifer at this site, data from nearby wells indicate that 

water levels in the aquifer are lowered as a result of pumping for 

industrial and public supply (Peck and others, 2005). During 

2000, the aquifer supplied 36.7 Mgal/d (Fanning 2003), and 

potentiometric surface maps (Peck and McFadden, 2004) 

indicate the presence of a cone of depression during 2000.

Water-Level Change

To assess the effect of development on the ground-water 

system, maps were developed showing simulated changes in 

ground-water level. Water-level changes from predevelopment 

to 2000 conditions are shown in figure 26, and from 1980 to 

2000 in figure 27.

Predevelopment to 2000

From predevelopment to 2000, ground-water levels 

declined in each of the simulated layers, with the most 

pronounced declines occurring in the Upper Floridan aquifer. 

In the Upper Floridan aquifer, simulated drawdown from 

predevelopment to 2000 exceeded 20 ft throughout most of the 

area and was greatest near the center of the cone of depression 

at Savannah, where water levels declined more than 100 ft 

(fig. 26). Simulated drawdown exceeded 60 ft in a large area 

that includes the Savannah cone of depression and adjacent 

pumping centers in Bryan, Liberty, and Long Counties; in the 

Brunswick area; in the Jacksonville–Fernandina Beach, Fla., 

and St. Marys, Ga., areas; and in the area surrounding Sandersville in Washington County. Most of these declines resulted 

from direct pumping from the Upper Floridan; however, 

declines in the Jacksonville, Fla., area likely are due to interaquifer leakage through layer 6 in response to pumping from 

the Lower Floridan aquifer. At Jacksonville, Fla., the Lower 

Floridan aquifer is used for water supply; estimated withdrawal 

during 2000 was 96 Mgal/d (see Duval County, table 2, fig. 16D) 

(see locations, fig. 1).
SOUTH

GEORGIA CAROLINA

FLORIDA

Model

area

SC

GA

GA

FL

Model boundary

0 50 MILES 25

0 50 KILOMETERS 25

A. Brunswick aquifer system

 (layer 3)

0 50 MILES 25

0 50 KILOMETERS 25

C. Lower Floridan aquifer

 (layer 7)

Water-level residuals, in feet—

 Represents difference of

 simulated minus observed 

 water levels

EXPLANATION

B. Upper Floridan 

 aquifer

 (layer 5)

Base from U.S. Geological Survey

1:100,000-scale digital data

Base from U.S. Geological Survey

1:100,000-scale digital data

N

30 to 40

40 to 46

20 to 30

10 to 20

10 to –10

–10 to –20

–20 to –30

–30 to –38

44 Simulation of Ground-Water Flow in Coastal Georgia and Adjacent Parts of 

South Carolina and Florida—Predevelopment, 1980, and 2000

Figure 21. Difference between simulated and observed water levels (residuals) by model layer for 1980.
SC

GA

GA

FL

SOUTH

GEORGIA CAROLINA

FLORIDA

Model

area

Model boundary

0 50 MILES 25

0 50 KILOMETERS 25

A. Brunswick aquifer system

 (layer 3) 

0 50 MILES 25

0 50 KILOMETERS 25

C. Lower Floridan aquifer

 (layer 7)

Base from U.S. Geological Survey

1:100,000-scale digital data

Base from U.S. Geological Survey

1:100,000-scale digital data

Water-level residuals, in feet—

 Represents difference of

 simulated minus observed 

 water levels

EXPLANATION

N

30 to 38

20 to 30

10 to 20

10 to –10

–10 to –20

–20 to –28

B. Upper Floridan 

 aquifer

 (layer 5)

Simulation of Ground-Water Flow 45

Figure 22. Difference between simulated and observed water levels (residuals) by model layer for 2000.
SOUTH

GEORGIA CAROLINA

FLORIDA

Model

area

Model boundary

GA

FL

SC

GA

l l l l l l l l l l l l l

l

l l

l l

l l l l l l l l l l l l l

l l l l

l l l l l

l

l l l l l l

l l l l l l l l l l

l l l l l l

l l l l l l l l l l l

0

40

60

80

100

120

140

–20

–40

–60

60

40

80

20

20

100

120

140

0

180

200

220 240

260

280 300

320

340

–80

–100

240

180

140

200

280

220

260

280

240

260

160

160

280

40

240

0

0

–20

–40

–60

0

0

60

40

40

80

20

20

20

100

120

140

160

0

180

200

220

240

260

280 300

320

340

240

240

160

260

60

0

280

280

280

200

180

260

240

220

–20

–20

–40

–60

–80

–100

20

20

0

0 50 MILES 25

0 50 KILOMETERS 25

Base from U.S. Geological Survey

1:100,000-scale digital data

EXPLANATION

Simulated potentiometric contour—

 Shows simulated potentiometric surface 

 during 1980, in feet. Hachures indicate 

 depression. Contour interval 20 feet. 

 Datum is NAVD 88

60

A. Brunswick aquifer system

 (layer 3)

C. Lower Floridan aquifer

 (layer 7)

B. Upper Floridan aquifer

 (layer 5)

Figure 23. Simulated 1980 potentiometric surfaces 

for the (A) Brunswick aquifer system, (B) Upper

Floridan aquifer, and (C) Lower Floridan aquifer.

Approximate

 limit of Brunswick 

 aquifer system

N

46 Simulation of Ground-Water Flow in Coastal Georgia and Adjacent Parts of 

South Carolina and Florida—Predevelopment, 1980, and 2000
SOUTH

GEORGIA CAROLINA

FLORIDA

Model

area

N

GA

FL

SC

GA

l l

l l l

l l l

l l l l l l l l l l l l l l l l l l l l

l l l l

l

l l l l l l l l l l l l

l l l l l

l l l l l l l l l l l l l l l l l l l l l l

A. Brunswick aquifer system

 (layer 3)

C. Lower Floridan aquifer

 (layer 7)

B. Upper Floridan aquifer

 (layer 5)

40

40

60

80

20

20

100

120

0

0

140

160

180

200

240 220

260 280

–20

300

320

–40

340

160

260

260

260

200

220

180

280

280

240

240

0

20

80

100

120

–20

140

60

40

40

40

60

80

20

20

100

120

140

0

160

180

200

220

260 240

280

–20

300

320

–40

340

0

260

280

180160

260

260

240

220

280

200

240

220

200

–60

–80

0

0

200

220

–80

–60

–40

–60

–20

–40

–20

0

0

0

0

0 50 MILES 25

0 50 KILOMETERS 25

Base from U.S. Geological Survey

1:100,000-scale digital data

Model boundary

Figure 24. Simulated 2000 potentiometric surfaces 

for the (A) Brunswick aquifer system, (B) Upper

Floridan aquifer, and (C) Lower Floridan aquifer.

Approximate

 limit of Brunswick 

 aquifer system

EXPLANATION

Simulated potentiometric contour—

 Shows simulated potentiometric surface 

 during 2000, in feet. Hachures indicate 

 depression. Contour interval 20 feet. 

 Datum is NAVD 88

60

Simulation of Ground-Water Flow 47
WAYNE

BULLOCH

LONG

CAMDEN

BRYAN

LIBERTY

GLYNN

CHATHAM

MCINTOSH

EFFINGHAM

Hopeulikit, Bulloch County 

(September 2000) 

NS

102.3

102.0

168.8

146.2

Bulloch South, Bulloch County 

(September 2003) 

NS

10.1

10.1

96.3

34.3

Fort Pulaski, Chatham County 

(September 2003) 

Tybee Island, Chatham County 

(September 2003) 

Coffin Park, Glynn County 

(September 2000) 

Georgia Pacific South,

Glynn County (September 2000) 

Georgia Port Authority Brunswick,

Glynn County (September 2003)

***Fernandina permeable zone

*Low permeability unit beneath Lower Floridan aquifer

* *

NS

Observed

1

3

5

7

Simulated

Ground-water level,

 in feet above or

below (–) NAVD 88

Model

layer

NS NS

Observed

1

3

5

7

Simulated

Ground-water level,

 in feet above or

below (–) NAVD 88

Model

layer

St. Marys, Camden County 

(September 2000) 

–2.1

3.6

1.0

7.8

3.7

10.9

—

—

—

—

Ebenezer Bend, Glynn County

(September 2003) 

NS

8.4

8.5

9.2

8.9

NS

Observed

1

3

5

7

Simulated

Ground-water level,

 in feet above or

below (–) NAVD 88

Model

layer

–1.3

10.6

8.7

—

—

—

Gardi, Wayne County 

(September 2000) 

NS

10.4

10.4

36.0

9.5

NS

Observed

1

3

5

7

Simulated

Ground-water level,

 in feet above or

below (–) NAVD 88

Model

layer

28.8

15.8

8.2

—

—

—

109.6

34.9

—

—

—

—

NS

Observed

1

3

5

7

Simulated

Ground-water level,

 in feet above or

below (–) NAVD 88

Model

layer

116.5

112.0

—

—

—

— NS

Observed

1

3

5

7

Simulated

Ground-water level,

 in feet above or

below (–) NAVD 88

Model

layer

Pineora, Effingham County 

(January 2003) 

NS

Observed

1

3

5

7

Simulated

Ground-water level,

 in feet above or

below (–) NAVD 88

Model

layer

NS

42.2

2.6

–1.7

–1.6

57.2

14.1

—

—

—

—

Springfield, Effingham County 

(September 2003) 

NS

Observed

1

3

5

7

Simulated

Ground-water level,

 in feet above or

below (–) NAVD 88

Model

layer

NS

13.6

4.1 5.7

6.5

54.8

31.0

—

—

—

NS

1

3

5

7

Ground-water level,

 in feet above or

below (–) NAVD 88

Model

layer

Hutchinson Island, Chatham County 

(September 2000) 

NS

Observed

1

3

5

7

Simulated

Ground-water level,

 in feet above or

below (–) NAVD 88

Model

layer

NS

–99.2

–64.8

–101.4

–107.2

2.5

–36.7

—

—

—

—

—

NS

NS NS NS NS

–26.9

–2.9

–5.7

–47.4

–25.9

–25.7

–2.3

–5.9 —

—

—

***

NS

Observed

1

3

5

7

Simulated

Ground-water level,

 in feet above or

below (–) NAVD 88

Model

layer

NS

Observed Simulated

–26.9

–1.4

–3.1

–30.0

–17.6

–17.0

0.2

–3.9

—

—

NS

1

3

5

7

Ground-water level,

 in feet above or

below (–) NAVD 88

Model

layer

Observed Simulated

Skidaway Island, Chatham County 

(September 2000)

NS

1

3

5

7

Ground-water level,

 in feet above or

below (–) NAVD 88

Model

layer

Observed Simulated

NS

–46.0

2.4

–47.2

–47.1

1.7

–20.0

—

—

—

—

NS

11.7

4.1

5.9

7.4

9.3

0.8

9.9

—

—

—

NS

1

3

5

7

Ground-water level,

 in feet above or

below (–) NAVD 88

Model

layer

Observed Simulated

NS

24.7

25.7

9.1

11.0

0.3

10.1

—

—

—

—

—

NS

8.4

3.5

4.2 5.8

10.1

1.7

10.2

—

—

—

Richmond Hill, Bryan County 

(September 2003) 

NS

1

3

5

7

Ground-water level,

 in feet above or

below (–) NAVD 88

Model

layer

Observed Simulated

NS

–17.4

–17.7

–14.8

–14.9

10.8

–8.5

—

—

—

—

Gardi

Pineora

St Marys

Ft Pulaski

Springfield

Hopeulikit

Bulloch

South Hutchinson

Island

Coffin Park

Tybee Island

Skidaway

Island

Georgia 

Port Authority 

Brunswick

Figure 25. Simulated and observed water levels and vertical flow directions (arrows) at well-cluster 

sites (NS, not simulated; —, no observed value; Upper Brunswick; Upper Brunswick aquifer; Lower Brunswick; Lower 

Brunswick aquifer; Upper Floridan, Upper Floridan aquifer; Lower Floridan, Lower Floridan aquifer; PZ, permeable zone). 

Richmond

Hill

Ebenezer

Bend

Georgia

Pacific

South

GEORGIA

NS

1

3

5

7

Water-bearing

unit

(observed and

simulated)

Model layer

Surficial

Brunswick

Floridan

Water-table zone

Upper Brunswick

Upper Floridan

Lower Floridan

Fernandina PZ**

**Brunswick area

Lower Brunswick

Confined zone

EXPLANATION

Aquifer

system

48 Simulation of Ground-Water Flow in Coastal Georgia and Adjacent Parts of 

South Carolina and Florida—Predevelopment, 1980, and 2000
Simulation of Ground-Water Flow 49

At Sandersville, Ga., withdrawal from the Upper Floridan 

aquifer was relatively low at 16 Mgal/d (see Washington 

County, table 1, fig. 16B), and the large decline may have 

resulted from hydraulic properties assigned to model layer 5. 

Here, the aquifer is known to be thin, largely clastic and has low 

transmissivity. Another reason for the large simulated decline 

may be the proximity of the pumping center to the northern noflow boundary of the model.

Simulated drawdown in the Lower Floridan aquifer from 

predevelopment to 2000 also occurred across a wide area and 

was similar in magnitude to the Upper Floridan aquifer (fig. 26). 

In the Jacksonville, Fla., area, the Lower Floridan aquifer 

supplied an estimated 96 Mgal/d during 2000, which produced 

a simulated drawdown exceeding 80 ft near the centers of 

pumping and exceeding 60 ft across the surrounding area. 

Elsewhere in the study area, the Lower Floridan aquifer 

provides considerably less water (generally less than 4 Mgal/d 

per county), and the similarity of drawdown patterns to that in 

the Upper Floridan aquifer suggests an interaquifer leakage 

response to pumping from the Upper Floridan aquifer.

Simulated drawdown in the Brunswick aquifer system 

from predevelopment to 2000 exceeded 20 ft throughout most 

of its area of occurrence (fig. 26). The largest declines of greater 

than 80 ft were near the center of pumping in Savannah. 

Because the Brunswick aquifer system is not widely used in the 

area (withdrawal during 2000 was 0.24 Mgal/d), the simulated 

decline is largely due to interaquifer leakage in response to 

pumping from the Upper Floridan aquifer.

1980–2000

During 1980–2000, simulated water levels showed a 

combination of rises and declines in response to changing 

pumping patterns (fig. 27; appendix A). During this period, 

total ground-water use increased; however, the distribution of 

withdrawal changed—decreases exceeding 10 Mgal/d occurred 

in Chatham and Glynn Counties, and increases exceeding 

10 Mgal/d occurred in Burke, Camden, Dooly, and Wilcox 

Counties, Ga., and in Beaufort County, S.C. (tables 1–2; fig. 7) 

(see locations, fig. 1).

Simulated drawdown exceeded 10 ft in the Upper Floridan 

aquifer across much of the western half of the model area, with 

drawdown exceeding 20 ft along parts of the western, northern, 

and southern boundaries (fig. 27; appendix A). These declines 

correspond to a general increase in pumpage from the Upper 

Floridan aquifer in these areas and match observed water-level 

trends in the area (fig. 8). In the Savannah area, simulated water 

levels in the Upper Floridan aquifer at the center of pumping 

rose more than 20 ft in response to decreased pumping, whereas 

observed water levels rose by a maximum of about 9 ft during 

the same period (figs. 8 and 27; appendix A). A less pronounced 

rise in water levels occurred in the Brunswick area in response 

to decreased pumping—simulated water levels rose as much as 

8 ft and observed water levels rose by as much as 14 ft during 

1980–2000.

Water levels in the Lower Floridan aquifer and Brunswick 

aquifer system also showed a combination of rises and declines 

during 1980–2000. Because these aquifers were not widely utilized in the coastal area during 1980–2000, most of the simulated changes were in response to changes in pumping patterns 

in the Upper Floridan aquifer, which induced a leakage response

in the two aquifers. Water-level rises during 1980–2000 were 

simulated in both the Brunswick aquifer system and Lower 

Floridan aquifer at the center of the cone of depression at 

Savannah (fig. 27). Simulated water-level rises exceeded 20 ft 

in these aquifers; however, there are no water-level 

observations that span this period to confirm these changes. It is 

possible that some of the simulated rise in the Lower Floridan 

aquifer at Savannah may result from decreased pumpage from 

the aquifer in that area; however, estimated pumpage from the 

aquifer showed only a minor decrease of less than 0.4 Mgal/d 

during 1980–2000 (table 2).

Simulated and observed water levels in the Lower Floridan 

aquifer in the Brunswick area during 2000 showed little change, 

and were mostly within +/– 5 ft of 1980 levels (fig. 27). In the 

northern and western parts of the area, simulated water levels in 

the Lower Floridan aquifer showed a similar pattern of change, 

as that in the Upper Floridan aquifer; however, there are no waterlevel observations that span this period to confirm these changes.

Simulated water levels in the Brunswick aquifer system 

during 2000 were mostly within +/– 5 ft of 1980 water levels 

across most of the area, with the exception of the Savannah area 

(described above), and an area along the northwestern limit of 

the aquifer system, where water-level declines exceed 5–10 ft 

(fig. 27). Most of the simulated changes were in response to 

changes in pumping patterns in the Upper Floridan aquifer, 

which induced a leakage response in the aquifers. There are no 

water-level observations, however, that span this period to confirm these changes.

Water Budget

The water budgets are similar for the 1980 and 2000 simulations. Pumpage captures a large percentage of the flow that 

had left the study area via lateral boundaries in the predevelopment simulation. The simulated water budgets for the 1980 and 

2000 simulations are illustrated in figure 28 and summarized in 

tables 8 and 9, respectively.

For the 1980 simulation, of the total simulated inflow rate 

of 1,082 Mgal/d, 37.3 percent (404 Mgal/d) is contributed by 

leakage from the general-head boundary, and 62.7 percent 

(679 Mgal/d) is contributed as inflow from lateral specifiedhead boundaries in layers 5 and 7 (table 8). Based on the total 

1,082 Mgal/d outflow rate, 17.5 percent is attributed to groundwater discharge to the general-head boundary, 18.6 percent to 

outflow at lateral specified-head boundaries, and 63.9 percent 

as discharge to wells. Along the lateral specified-head boundary, the net flow is about 462 Mgal/d into the model area in the 

Upper Floridan aquifer and 15.8 Mgal/d into the model area in 

the Lower Floridan aquifer.
SOUTH

GEORGIA CAROLINA

FLORIDA

Model

area

GA

FL

SC

GA

B. Upper Floridan aquifer

 (layer 5)

0 50 MILES 25

0 50 KILOMETERS 25

A. Brunswick aquifer system

 (layer 3)

Base from U.S. Geological Survey

1:100,000-scale digital data

Approximate

 limit of Brunswick 

 aquifer system

C. Lower Floridan aquifer

 (layer 7)

N

EXPLANATION

–20

0

Model boundary

–40

–60

–80

–100

Change in simulated head 

 between predevelopment

 conditions and 2000, in feet

Decrease in

water level

Figure 26. Simulated water-level change, between

predevelopment and 2000 for the (A) Brunswick 

aquifer system, (B) Upper Floridan aquifer, and 

(C) Lower Floridan aquifer.

50 Simulation of Ground-Water Flow in Coastal Georgia and Adjacent Parts of 

South Carolina and Florida—Predevelopment, 1980, and 2000
SOUTH

GEORGIA CAROLINA

FLORIDA

Model

area

GA

FL

SC

GA

B. Upper Floridan aquifer

 (layer 5)

0 50 MILES 25

0 50 KILOMETERS 25

A. Brunswick aquifer system

 (layer 3)

Base from U.S. Geological Survey

1:100,000-scale digital data

Approximate

 limit of Brunswick 

 aquifer system

C. Lower Floridan aquifer

 (layer 7)

Model boundary

EXPLANATION

15

20

10

5

–5

–10

Change in simulated head in 

 the Brunswick aquifer system 

 between 1980 and 2000, in feet

Increase in

water level

Decrease in

water level

EXPLANATION

30

20

10

–10

–20

–30

–40

–50

Change in simulated head in the 

 Upper and Lower Floridan aquifers 

 between 1980 and 2000, in feet

Increase in

water level

Decrease in

water level

N

Simulation of Ground-Water Flow 51

Figure 27. Simulated water-level change, between1980 and 2000 for the (A) Brunswick aquifer system, (B) Upper Floridan aquifer, 

and (C) Lower Floridan aquifer.
A. Onshore Offshore 1980

B. 2000

253

127

18.3

18.3

34.8

6.65

97.9

37.4

659

197

0

.004

19.5

3.72

135

8.28

23.4

177

23.9

177

23.9

168

70.3

168

70.2

0.341 (Layer 3)

177

114 (Layer 7)

578 (Layer 5)

286

116

23.8

15.5

7.38

8.45

11.3

4.61

11.3

4.61

46.6

3.62

141

22.3

712

268

0

.004

15.5

2.32

15.5

2.32

176

4.42

234

12.6

234

12.6

234

13.1

234

13.1

178

62.1

178

62.1

0.241

129

669

Ground-water flow, in 

 million gallons per day, 

 and budget component

EXPLANATION

General-head 

 boundary

Interunit leakage

Lateral fixed-head 

 boundary

Pumpage 669

Surficial aquifer system (layer 1)

Brunswick aquifer system (layer 3)

Upper Floridan confining unit (layer 4)

Upper Floridan aquifer (layer 5)

Lower Floridan confining unit (layer 6)

Lower Floridan aquifer (layer 7)

Brunswick aquifer system 

 confining unit (layer 2)

52 Simulation of Ground-Water Flow in Coastal Georgia and Adjacent Parts of 

South Carolina and Florida—Predevelopment, 1980, and 2000

Figure 28. Schematic diagram showing simulated water budget for (A) 1980 and (B) 2000. (Values rounded to three 

significant digits. Values do not agree with those shown in figures 15A and 16A because of rounding.)
Table 8. Simulated 1980 water budget by model layer.

[Values reported to three significant digits and may not sum to totals due to independent rounding; well discharge for the Brunswick aquifer system does not 

match total pumpage on figure 15A because of rounding; well discharges for Upper and Lower Floridan aquifers do not agree with totals listed in tables 1 and 2 

because some counties listed in these tables are partially outside of the simulated area and, thus, are not accounted for by the model; —, not applicable]

Inflow, in million gallons per day Outflow, in million gallons per day

Model 

layer

Hydrogeologic 

unit

Inflow from 

general-head 

boundary

Inflow across 

lateral 

boundaries

Total

Outflow to 

general-head 

boundary

Outflow 

across lateral 

boundaries

Discharge 

to wells Total

1 Surficial aquifer system 271 — 271 145 — — —

2 Confining unit 34.8 — 34.8 6.65 — — —

3 Brunswick aquifer system — — — — — 0.341 0.341

4 Confining unit — — — — — — —

5 Upper Floridan aquifer 97.9 659 757 37.4 197 578 578

6 Confining unit — 0.004 0.004 — 0.000 — —

7 Lower Floridan aquifer — 19.5 19.5 — 3.72 114 114

Total all layers 404 679 1,082 189 201 692 1,082

Percentage of flow 37.3 62.7 100 17.5 18.6 63.9 100

Table 9. Simulated 2000 water budget by model layer.

[Values reported to three significant digits and may not sum to totals because of independent rounding; well discharge for the Brunswick aquifer system does not 

match total pumpage on figure 16A because of rounding; well discharges for Upper and Lower Floridan aquifers do not agree with totals listed in tables 1 and 2 

because some counties listed in these tables are partially outside of the simulated area and, thus, are not accounted for by the model; —, not applicable]

Model 

layer

Hydrogeologic 

unit

Inflow, in million gallons per day Outflow, in million gallons per day

Inflow from 

general-head 

boundary

Inflow across 

lateral 

boundaries

Total

Outflow to 

general-head 

boundary

Outflow 

across lateral 

boundaries

Discharge 

to wells Total

1 Surficial aquifer system 310 — 310 132 — — 132

2 Confining unit 46.6 — 46.6 3.62 — — 3.62

3 Brunswick aquifer system — — — — — 0.241 0.241

4 Confining unit — — — — — — —

5 Upper Floridan aquifer 141 712 854 22.3 268 669 959

6 Confining unit — 0.004 0.004 — 0.000 — 0.000

7 Lower Floridan aquifer — 15.5 15.5 — 2.32 129 131

Total all layers 498 728 1,226 157 270 798 1,226

Percentage of flow 40.6 59.4 100 12.8 22.0 65.1 100

Simulation of Ground-Water Flow 53
100

73.4

58.3

67.5

64.2

80.1

10.0

50 50 100

2000

1980

Predevelopment

0

FLOW IN UPPER FLORIDAN AQUIFER

(LAYER 5) ALONG COASTLINE

Offshore Landward Seaward

recharge

Onshore 

recharge

Offshore 

discharge

Pumpage

from

layer 7 Pumpage

from

layer 5

Pumpage

from

layer 3

Onshore 

discharge

Flow into model

from fixed-head

boundaries

Flow out of 

model at fixed-head

boundaries

EXPLANATION

Predevelopment flow 1980 flow 2000 flow

Simulated flow, in

million gallons per day

Simulated flow, in million gallons per day

8.42

285

188 193

257

42.0 18.3

385

679

0.341

578

114

201

171

18.3 23.8

474

728

0.241

669

129

270

142

15.5

54 Simulation of Ground-Water Flow in Coastal Georgia and Adjacent Parts of 

South Carolina and Florida—Predevelopment, 1980, and 2000

Figure 29. Comparison of major components of predevelopment, 1980, and 2000 simulated water budgets. (Values 

rounded to three significant digits. Values do not agree with those shown in figures 15A and 16A because of rounding.)

For the 2000 simulation, of the total simulated inflow rate 

of 1,226 Mgal/d, 40.6 percent (498 Mgal/d) is contributed by 

leakage from the general-head boundary, and 59.4 percent 

(728 Mgal/d) is contributed as inflow from lateral specifiedhead boundaries in layers 5 and 7 (table 9). Based on the total 

1,226 Mgal/d outflow rate, 12.8 percent is attributed to groundwater discharge to the general-head boundary, 22.0 percent to 

outflow at lateral specified-head boundaries, and 65.1 percent 

as discharge to wells. Along the lateral specified-head boundary, the net flow is about 444 Mgal/d into the model area in the 

Upper Floridan aquifer and 13.2 Mgal/d into the model area in 

the Lower Floridan aquifer.

Major components of the simulated water budgets for predevelopment, 1980, and 2000 are compared and summarized in 

figure 29. As ground-water pumpage increased in the area, 

water-level declines occurred in the Upper Floridan aquifer 

resulting in changes in the flow rate into, out of, and within the 

ground-water system. These changes include (1) increased 

inflow from and decreased outflow to the general-head boundaries in both the onshore and offshore areas; (2) increased 

outflow to, and increased inflow from, lateral fixed-head 

boundaries; and (3) increased landward and seaward flow rates, 

with net flow along the coastline increasing landward. 

Fixed-head and general-head boundaries provide additional water to the flow system in response to pumping stresses. 

Most of the increase in flow occurs in the Upper Floridan 

aquifer along the southern fixed-head boundary—more water 

is entering the system through this boundary than through all 

other boundaries combined. This is not unreasonable because 

the Upper Floridan aquifer extends far beyond that boundary 

throughout Florida (Miller, 1986), and potentially could be a 

substantial source of ground water into the modeled area.

Simulated recharge (inflow from the general-head boundary in the onshore area) showed increases between predevelopment, 1980, and 2000 (fig. 29). Total simulated recharge in the 

onshore area during predevelopment was 285 Mgal/d (0.21 in), 

during 1980 was 385 Mgal/d (0.28 in), and during 2000 was 

474 Mgal/d (0.35 in). These increases correspond to declining
Simulation of Ground-Water Flow 55

water levels in the Upper Floridan aquifer, which produced a 

downward hydraulic gradient and induced increased flow from 

the surficial aquifer system into deeper units. Pumpage also 

reduces simulated discharge to the source-sink boundary in the 

offshore area. 

A map showing simulated recharge and discharge for 2000 

(fig. 30) indicates a similar areal distribution of simulated 

recharge and discharge as the predevelopment map (fig. 19), 

with the following exceptions: 

1. Areas of recharge north of the Gulf Trough have 

increased in extent and magnitude of recharge.

2. Discharge rates to major streams have decreased 

throughout the area, with flow gradients beneath 

downstream reaches of the Savannah and 

Altamaha Rivers reversing, so that these areas 

now recharge underlying aquifers.

3. A similar reversal in flow gradient beneath the 

major pumping center in the Savannah area, in 

Beaufort and Jasper Counties, S.C., and in the 

central part of the model area has induced 

simulated downward flow into underlying 

confined aquifers.

Seaward and Landward Flow

In the offshore area and along the coastline, simulated 

quantities of flow have changed since predevelopment and may 

have important implications for saltwater intrusion. The simulated quantity of water discharging from the surficial aquifer 

layer into the Atlantic Ocean has decreased since predevelopment, whereas flow has increased (fig. 29) from the ocean into 

the surficial aquifer layer. Along the coastline, the ratio of seaward to landward flow within the Upper Floridan aquifer has 

changed since predevelopment, when more of the flow in the 

aquifer was seaward; whereas during 1980 and 2000, more of 

the flow was landward. 

The direction and magnitude of horizontal onshore-offshore flow in the Upper and Lower Floridan aquifers during 

predevelopment and 2000 conditions at various locations along 

the coastline are shown in figure 31. The onshore-offshore 

interface was subdivided into zones on the basis of designated 

hydraulic-property zones for the Upper Floridan aquifer (fig. 12).

Simulated predevelopment flow was from onshore to offshore for all zones in both aquifers, with the greatest flow rates 

in the southern part of the simulated area (fig. 31A). In the 

Upper Floridan aquifer, the largest simulated offshore flow 

rate was in zone F8 in the Camden County area, and is probably 

the result of relatively high hydraulic conductivity of the aquifer 

in that area (zone F8, fig. 12). Lower Floridan flow rates are 

less than those for the Upper Floridan because of its lower 

hydraulic conductivity.

The simulated flow direction for 2000 pumping conditions 

in both aquifers generally is landward, as a result of onshore 

pumping (fig. 31B). An exception is along the coast in zone F8 

in Camden County area, where flow in both aquifers is predominantly offshore similar to predevelopment. This area is 

characterized by a high hydraulic conductivity zone (F8, 

fig. 12) located between two relatively lower hydraulic 

conductivity zones (F7 and F11), which contain major pumping 

centers at Brunswick and St. Marys, Ga.–Fernandina Beach, 

Fla., respectively. During predevelopment conditions, the 

regional potentiometric gradient in the area of Glynn, Camden, 

and Nassau Counties (zones F7, F8 and F11) is low relative to 

present-day conditions, so the offshore flux in the area is lower 

than during 2000. Pumping at the two major pumping centers 

has resulted in the development of a large regional potentiometric gradient and even larger localized gradients around 

cones of depression in the potentiometric surface of the Upper 

Floridan aquifer (fig. 5). Thus, even though the water levels are 

lower during 2000 than during predevelopment, the potentiometric gradient is steeper, resulting in higher flow rates in the 

offshore direction. These larger fluxes channel through the 

higher hydraulic conductivity zone in Camden County (zone 

F8) toward the offshore area and curl back landward toward the 

pumping centers in the adjacent zones (F7 and F11).

Model Sensitivity

The sensitivity of the calibrated steady-state model was 

evaluated to determine the relative importance of several model 

parameters on simulated ground-water levels and flow rates. 

Ground-water models are variably sensitive to model input used 

to describe the physical aspects of a system. This includes aquifer properties and well pumping rates; structural features of the 

model, such as grid resolution, model layering, and temporal 

discretization; and location and type of model boundaries. For 

the parameters for which the model is most sensitive, small 

variations in those parameters will result in large differences in 

simulated water levels. Furthermore, if the model is more sensitive to one parameter than to others, the degree of uncertainty 

of that parameter will have a greater effect on the uncertainty of 

the model results than that of other parameters.

To evaluate the sensitivity of the coastal ground-water 

flow model, it was assumed that the model layering and types 

of model boundaries are set, and that the grid discretization has 

been optimized for the distribution of input data and observations. Because the model simulates steady-state conditions, 

temporal discretization is not considered as part of the analysis. 

The sensitivity analysis includes composite-scaled 

sensitivities of comparable parameters calculated using the 

sensitivity equation method described in MODFLOW-2000 

(Hill and others, 2000), as well as from the perturbation method. 

The composite-scaled sensitivity as described in Hill (1998) is 

a dimensionless measure of the change in calculated head with 

respect to the value of a parameter and is independent of the 

actual values of the observations. A larger composite-scaled 

sensitivity indicates a relatively larger sensitivity of the model 

to a given parameter. Composite-scaled sensitivities were used 

to evaluate the relative sensitivities of pumping rate, vertical 

and horizontal hydraulic conductivity, and the conductance of 

the general-head boundary. 
Model boundary

0 50 MILES 25

0 50 KILOMETERS 25

Base from U.S. Geological Survey

1:100,000-scale digital data

N

EXPLANATION

1 to 2

0 to 1

0 to 1

1 to 2

2 to 3

3 to 4

Simulated recharge or 

 discharge per model cell, 

 in inches per year

Discharge

Recharge

SOUTH

GEORGIA CAROLINA

FLORIDA

Model

area

56 Simulation of Ground-Water Flow in Coastal Georgia and Adjacent Parts of 

South Carolina and Florida—Predevelopment, 1980, and 2000

Figure 30. Simulated recharge or discharge during 2000.

The perturbation method was used to examine model 

sensitivity to pumping and to the fixed-head boundary condition. Because the composite-scaled sensitivity analysis 

indicated a high sensitivity to pumping rates, a simulation 

was conducted, whereby 2000 pumping was increased by 

10 percent, and the resulting decline in water levels mapped. 

An additional simulation was conducted to test the sensitivity 

of the southern fixed-head boundary, which contributes a substantial portion of water to the flow system. Because the value 

of the controlling head at this boundary cannot be compared 

using composite-scaled sensitivity, the head at this boundary 

was increased by 10 ft, and the resulting rise in simulated water 

levels was mapped.

Composite-Scaled Sensitivities

Composite-scaled sensitivities for pumping rate, vertical 

and horizontal hydraulic conductivity for each zone, and the 

conductance of the general-head boundary for both the 1980 

and 2000 simulations are shown in figure 32. The general pattern of relative sensitivity is similar for the 1980 and 2000 simulations, with minor differences in the order of sensitivities. 

The sensitivity of vertical hydraulic conductivity for the aquifer units (layers 1, 3, 5, and 7) and horizontal hydraulic conductivity for the confining units (layers 2, 4, and 6) was negligible 

and are not shown on figure 32.

The model is most sensitive to pumping by a wide margin. 

Of the remaining parameters, the model is most sensitive to 

the horizontal hydraulic conductivity of the Upper Floridan 

aquifer (layer 5) in the coastal area (zones UF6Kh, UF4Kh, 

UF5Kh, and UF7Kh) and in the area north of the Gulf Trough 

(zone UF1Kh), and to the vertical hydraulic conductivity of the 

Upper Floridan confining unit (layers 2, 3, and 4, zone CU1Kv). 

The model is less sensitive to other hydraulic conductivity 

zones, either because of fewer water-level observations (for 

example, BWKh for layer 3, or LFKh for layer 7) or to a lesser 

influence on overall model results. The model is relatively 

insensitive to the conductance of the general-head boundary.
SOUTH

GEORGIA CAROLINA

FLORIDA

Model

area

FL

GA

GA SC

FL

GA

GA SC

A. Predevelopment 

B. 2000

N

Figure 31. Schematic diagram showing magnitude, in million gallons per day, 

and direction of simulated horizontal ground-water flow along the coastline 

for the Upper and Lower Floridan aquifers during (A) predevelopment, and 

(B) 2000. Multicolored indicators along the coast correspond to the interface 

between the aquifers underlying the onshore and offshore areas. Each color 

indicates a hydraulic conductivity zone in layer 5 (Upper Floridan aquifer).

4.93

33.5

1.3

7.32

4.25

1.08

1.10

.309

.146

.260

.432

.432

.259

33.7

53.2

7.32

4.12

11.2

4.50

5.49

.437

.907

.196

.968

.410

Lower Floridan aquifer

Upper Floridan aquifer

Lower Floridan aquifer

Upper Floridan aquifer

F11

F8

F7

F5

F4

F3

F8 Hydraulic-property zone

General direction of ground-water 

 flow and magnitude, in million 

 gallons per day

EXPLANATION

F11

F8

F7

F5

F4

F3

Simulation of Ground-Water Flow 57
Parameter Parameter value, 

in feet per day Description Composite-scaled sensitivity 

2000 1980

Wells variable* Pumping rates for calibration years 44.2577 49.52356

UF6Kh 2,819 Layer 5 horizontal conductivity, zone F6 10.84692 12.35307

UF4Kh 70 Layer 5 horizontal conductivity, zone F4 10.68336 14.87876

CU1Kv 0.00017 Layers 2, 3, 4, vertical conductivity, zone 1 9.727391 9.196414

UF5Kh 394 Layer 5 horizontal conductivity, zone F5 8.594508 11.02104

UF7Kh 150 Layer 5 horizontal conductivity, zone F7 5.846392 8.382938

UF1Kh 34 Layer 5 horizontal conductivity, zone F1 5.534489 4.971471

UF8Kh 2,727 Layer 5 horizontal conductivity, zone F8 4.62915 4.281414

UF10Kh 56 Layer 5 horizontal conductivity, zone F10 4.506208 2.847728

ghb variable General head conductance 4.072358 3.574101

LFKh 10 Layer 7 horizontal conductivity 3.539038 4.127525

UF12Kh 25 Layer 5 horizontal conductivity, zone F12 3.374115 4.644035

UF11Kh 94 Layer 5 horizontal conductivity, zone F11 4.124236 1.467137

CU5Kv 0.0001 Layers 2, 3, 4, vertical conductivity, zone 5 2.038849 2.186209

UF2Kh 2 Layer 5 horizontal conductivity, zone F2 1.078982 1.702882

UF3Kh 100 Layer 5 horizontal conductivity, zone F3 0.89406 0.99214

BWKh 50 Layer 3 horizontal conductivity, zone B1 0.719744 0.728282

CU3Kv 0.00001 Layers 2, 3, 4, vertical conductivity, zone 3 0.657379 0.395817

SURFKh 70 Layer 1 horizontal conductivity 0.416764 0.42627

CLFKv 0.02 Layer 6 vertical conductivity 0.413864 0.353178

UF9Kh 100 Layer 5 horizontal conductivity, zone F9 0.268043 0.310139

CU4Kv 0.0001 Layers 2, 3, 4, vertical conductivity, zone 4 0.115305 0.223505

CU2Kv 0.2 Layers 2, 3, 4, vertical conductivity, zone 2 8.75E-02 5.72E-02

* cubic feet per day

0

10

20

30

40

50

60

Wells

UF6KhUF4Kh

CU1Kv

UF5KhUF7KhUF1KhUF8Kh

UF10Kh

ghb

LFKh

UF12Kh

UF11Kh

CU5Kv

UF2KhUF3Kh

BWKh

CU3Kv

SURFKh

CLFKv

UF9Kh

CU4KvCU2Kv

PARAMETER

COMPOSITE-SCALED SENSITIVITY

2000

1980

58 Simulation of Ground-Water Flow in Coastal Georgia and Adjacent Parts of 

South Carolina and Florida—Predevelopment, 1980, and 2000

Figure 32. Composite-scaled sensitivity of selected model parameters.

Pumping Sensitivity

To provide an illustration of how a change in pumping 

would affect simulated head, a 10-percent increase during 2000 

pumping rates was simulated and the resulting change in water 

levels mapped (fig. 33). A value of 10 percent was used 

because pumping rates could have a margin of error at least this 

large in some parts of the model area. This increase in pumpage 

resulted in widespread decline in water level greater than 2 ft 

across most of the simulated area that diminished toward the 

southern boundary and into South Carolina. As would be 

expected, the largest differences in simulated head are in areas 

where pumping is most concentrated and rates are highest; 

water-level declines exceeded 10 ft the Upper and Lower 

Floridan aquifers (layers 5 and 7) in the Savannah, Ga., and at 

least 4 ft in Jacksonville, Fla., areas. Simulated head also 

decreased substantially in an area near the northwestern noflow boundary where the Upper and Lower Floridan aquifers 

are relatively thin and have low hydraulic conductivities. A 

reduction in pumping rates of 10 percent would show rises in 

water level of similar distribution and magnitude.

Fixed-Head Boundary Condition Sensitivity

To evaluate the sensitivity of the southern fixed-head 

boundary, a simulation was conducted, whereby the head along 

the boundary was increased by 10 ft, and the difference in simulated water levels was mapped (fig. 34). A 10-ft change was 

considered a reasonable variation, based on water-level changes 

observed in some wells in the area. As would be expected, 

water-level rises are greatest close to the boundary and diminish with distance from the boundary. In the Upper Floridan 

aquifer, the lateral extent of a water-level rise greater than 8 ft 

is dependent on the hydraulic conductivity of the aquifer—the 

higher the hydraulic conductivity, the further the rise is propagated in the aquifer (zones F6, F8, and F11, and part of F7, 

fig. 12). The Lower Floridan aquifer shows a similar pattern of 

water-level response as the Upper Floridan aquifer; the Lower 

Floridan aquifer, however, is assigned uniform hydraulic properties; thus, the rise is a result of an interaquifer leakage response. 

The simulated water-level rise is subdued toward the low-permeability Gulf Trough (zone F2, fig. 12), and in the relatively 

lower permeability Chatham County area (zones F4 and F12, 

fig. 12). A reduction of fixed-head values along the southern 

boundary of the same magnitude would result in a similar distribution and magnitude of simulated water-level decline.
SOUTH

GEORGIA CAROLINA

FLORIDA

Model

area

GA

FL

SC

GA

Model boundary

EXPLANATION

–2

0

–4

–6

–8

–10

Change in simulated head resulting 

 from 10 percent increase in 

 pumping rates relative to 2000 

 pumping conditions, in feet

Decrease in

water level

0 50 MILES 25

0 50 KILOMETERS 25

Base from U.S. Geological Survey

1:100,000-scale digital data

A. Brunswick aquifer system

 (layer 3)

C. Lower Floridan aquifer

 (layer 7)

B. Upper Floridan aquifer

 (layer 5)

Approximate

 limit of Brunswick 

 aquifer system

Figure 33. Change in simulated ground-water levels 

resulting from a 10-percent increase in 2000 pumping 

rates for the (A) Brunswick aquifer system, (B) Upper

Floridan aquifer, and (C) Lower Floridan aquifer.

Simulation of Ground-Water Flow 59
SOUTH

GEORGIA CAROLINA

FLORIDA

Model

area

GA

FL

SC

GA

Model boundary

EXPLANATION

8

6

4

2

0

Change in simulated head resulting 

 from 10-foot increase in head along 

 southern fixed-head boundary relative 

 to 2000 boundary conditions, in feet

Increase in

water level

0 50 MILES 25

0 50 KILOMETERS 25

Base from U.S. Geological Survey

1:100,000-scale digital data

A. Brunswick aquifer system

 (layer 3)

C. Lower Floridan aquifer

 (layer 7)

B. Upper Floridan aquifer

 (layer 5)

Figure 34. Change in simulated ground-water levels 

resulting from a 10-foot increase in head along the 

southern fixed boundary for the (A) Brunswick 

aquifer system, (B) Upper Floridan aquifer, and 

(C) Lower Floridan aquifer.

Approximate

 limit of Brunswick 

 aquifer system

60 Simulation of Ground-Water Flow in Coastal Georgia and Adjacent Parts of 

South Carolina and Florida—Predevelopment, 1980, and 2000
Simulation of Ground-Water Flow 61

Model Limitations

A ground-water flow model is a simplified representation 

of natural processes and properties of a hydrologic system and, 

as such, is subject to limitations. These limitations affect the 

degree to which the model accurately represents the system and 

the reliability of the model to predict system characteristics at 

any location or time. Limitations are inherent in the theoretical 

and computational aspects of the model (such as the equation of 

flow used), as well as in the specifics of model construction, 

including spatial and temporal model scales and discretization, 

types of boundary conditions applied, and data used to describe 

the physical characteristics of the system. Most of the data are 

not analyzed for error or uncertainty, limiting the ability to 

quantify resultant model uncertainty with any reliability. 

Emphasis herein is placed on limitations of the model construction, because it is not within the scope of this report to analyze 

limitations of numerical models of Darcian flow. In general, an 

attempt is made to discuss model limitations qualitatively and 

analyze the contributing causes of model uncertainty.

The ability of the model to address specific aspects of the 

ground-water flow system is limited by its spatial scale and discretization. This model encompasses a large area and is 

designed to simulate the regional-scale flow system characteristics; it is not designed to simulate more localized features. The 

scale and discretization of this model limit its ability to simulate 

accurately the surficial aquifer system and related surfacewater/ground-water interactions. A more appropriately scaled 

model to simulate the shallow flow system would consist of grid 

cells that are small enough to capture local-scale changes in 

topography and flow gradients near streams.

Appropriate discretization is important for calibration of 

the model and depicting spatial distribution of physical properties and stresses; it is limited, however, by the extent to which 

data are available to define the flow system. In the study area, 

the discretization of aquifer and confining units limits the interpretation of interaquifer flow. For example, although the Brunswick aquifer system is simulated as a single, laterally continuous unit, there is uncertainty in the lateral extent and continuity 

of permeable deposits that compose the aquifer system. Thus, in 

the model, pumping in one area may affect simulated head in 

another area of the aquifer, when in reality the two areas may be 

isolated from each other by zones of low permeability or aquifer 

thinning. There also is some uncertainty in the consistency with 

which aquifers are defined; for example, the permeable zones 

that define the Upper and Lower Floridan aquifers. If inconsistently or incorrectly defined, this may result in the model incorrectly interpreting response in the Upper Floridan aquifer to 

stresses in the Lower Floridan aquifer, or vice versa.

This model is designed to provide a regional approximation of the ground-water system over time on the scale of tens 

of years and is not intended to simulate short-term annual or 

seasonal changes. These longer time-scale changes were simulated with successive steady-state approximations for predevelopment, 1980, and 2000. Transient response testing indicates 

that the steady-state approximation is reasonable for these time 

periods and hydrogeologic conditions (appendix B). To simulate short-term transient response in the flow system would 

require transient simulation and greater temporal resolution of 

stresses, boundary conditions, and observation data.

Boundary conditions are another area of uncertainty in the 

model because they are difficult to quantify. Reliable estimates 

of recharge require information on climatic factors, such as 

rainfall and evapotranspiration rates; soil characteristics 

(permeability and thickness); and streamflow (used to compute 

runoff and baseflow). This model simulates recharge to the 

regional flow system using a general-head boundary as a simplification of the flow system. Thus, increased recharge is 

induced by increased pumpage. It is possible that recharge is 

more (or less) limited than that allowed by this boundary 

condition. This type of boundary does not account for all of the 

aforementioned factors controlling recharge, but was used as a 

control to limit the quantity of water entering the flow system 

within or below estimates derived from stream baseflow data 

(Priest, 2004).

In the absence of a physical hydrologic boundary and sufficient data to quantify flow, a fixed-head boundary based on 

observed water levels was used to define the southern boundary 

of the model. This type of boundary serves as a source-sink 

boundary, whereby water is allowed to flow into and out of the 

model area in response to head gradients. Because fixed-head 

cells provide an unlimited supply of water, simulated head near 

this boundary may be overpredicted (high). Increased pumpage 

near this boundary, in excess of that used to calibrate the model, 

may induce unrealistic quantities of flow into the model and 

result in an underprediction of drawdown. Furthermore, the 

proportions of water entering the system from this boundary 

and the general-head boundary are uncertain and constrained 

only by imprecise limits on regional recharge. It is possible that 

a different proportion of water entering from these boundaries 

could result in an acceptable solution to these simulations, but a 

different solution for other scenarios.

Sensitivity testing indicates that the model is very sensitive 

to the rate and distribution of pumpage. There is uncertainty in 

the rate and distribution of nonsite-specific and site-specific 

pumping, even though the locations of site-specific stresses are 

known. Also, estimates of earlier pumping rates and distributions are even more uncertain; for example, there is less certainty in the values and distribution of pumping during 1980 

than during 2000.

Pumping uncertainty results from limited metering in the 

model area, errors in self-reporting by some users, errors in 

countywide estimates for several categories of water use, uncertainty about which aquifer each well is tapping, and errors in 

estimating annual rates for a seasonally varying value (such as 

irrigation). Because the model is calibrated to simulate average 

annual conditions, errors in annual pumping rate and distribution can lead to errors in the calibration. For example, if the 

estimated pumping is substantially lower than actual pumping, 

the calibrated hydraulic conductivity may be too low, and the 

model may predict a greater response to future stresses than the 

system may actually demonstrate.
62 Simulation of Ground-Water Flow in Coastal Georgia and Adjacent Parts of 

South Carolina and Florida—Predevelopment, 1980, and 2000

The model also is limited by uncertainty in available 

hydraulic-property data and in the application of these data to 

the model. The availability of field hydraulic-property data is 

highly variable, both spatially within a unit and among different 

units. In areas or units of high data density, there may be considerable variation in field values; for example, hydraulic conductivity of the Upper Floridan aquifer varies by two orders of 

magnitude in the Brunswick area. This variation may be due to 

measurement error, error in estimation of transmissivity or 

hydraulic conductivity, or localized heterogeneity in aquifer 

properties. Because the objective of the model is to describe the 

regional flow system characteristics, the hydraulic conductivity 

of each unit is subdivided only into subregional-sized zones. It 

is possible that in some areas, however, aquifer properties may 

vary at the model-grid cell scale, especially for units that have 

heterogeneous aquifer properties such as the carbonate portion 

of the Floridan aquifer system.

Although zoning hydraulic properties on a cell-by-cell 

basis may give a better match of calculated heads with observed 

heads, it may not represent a better or more accurate physical 

model and, thus, may not always be justifiable. In areas of low 

data density, for example confining units, initial hydraulicproperty values are only approximations and may be incorrect 

by several orders of magnitude.

Another source of uncertainty is a nonunique distribution 

of hydraulic properties that results in acceptable model calibration. These uncertainties may limit the ability of the model to 

predict the system response to stress changes. For example, if 

the calibrated hydraulic conductivity is too high in one area, the 

model may underestimate response to stress changes there; 

alternatively, if the calibrated hydraulic conductivity is too low, 

the model may overestimate response to stress changes. As 

another example, if the calibrated hydraulic conductivity is too 

high near a source-sink boundary, this may allow too much flow 

across that boundary, resulting in an overall incorrect budget 

and flow system. Uncertainty in confining-unit hydraulic conductivity also may result in incorrect estimates of flow among 

units and a misrepresentation of interaquifer leakage.

It is likely that the greatest sources of uncertainty in the 

model are the boundary conditions, including pumping. In areas 

that are sufficiently removed from model boundaries, and for 

which the location of most pumping is known and for which the 

rate is accurately estimated, the model likely represents the 

characteristics of flow reasonably well and can provide a 

reasonable assessment of effects of changing stresses. Such 

areas include the coastal areas between Camden County, Ga., 

and Beaufort County, S.C.

Despite the limitations of the model, one of its most useful 

characteristics is the illustration of those aspects of uncertainty 

that, with improved data density or quality, could best improve 

the model and the general understanding of the flow system in 

the region. Because the sensitivity analysis indicated that pumpage is the most sensitive parameter of the calibrated model, 

more accurate estimates of ground-water pumpage and distribution could improve substantially the model simulation. 

Improved characterization of boundary conditions would also 

improve model simulation—more accurate recharge and discharge estimates would provide more reasonable constraints on 

flow into and out of the flow system.

Sensitivity analysis indicates that the model could be 

improved with additional hydraulic-property information. 

In particular, information on the hydraulic conductivity of 

the Upper Floridan aquifer in zones F4, F5, and F6 (fig. 12) 

may help improve model accuracy. Additional data on the 

vertical hydraulic conductivity of the Upper Floridan confining 

unit (layer 4 and parts of layers 2 and 3) also would improve 

the calibration.

Summary

This report documents a digital ground-water flow model 

used to simulate the regional ground-water flow system in 

coastal Georgia and adjacent parts of Florida and South Carolina, including the Floridan aquifer system and the Brunswick 

aquifer system. The model was used to update and refine previous regional-scale flow models for the region, and to (1) understand and refine the conceptual model of regional ground-water 

flow, (2) create a framework for the development of digital 

subregional ground-water flow and solute-transport models, 

and (3) evaluate hypothetical pumping distribution scenarios.

The single-density ground-water flow model was developed using the U.S. Geological Survey finite-difference code 

MODFLOW-2000, and encompasses a 42,155-square-mile 

area in coastal Georgia, Florida, and South Carolina, including 

the adjacent offshore area. A steady-state approximation was 

used to simulate ground-water flow and mean-annual conditions for predevelopment (pre–1900) and the years 1980 and 

2000. The steady-state approximation is appropriate because 

the purpose of the model is to simulate the ultimate effect on the 

flow system resulting from changes in average annual pumping, 

and because unreasonably extreme changes in stress are 

required to affect a transient response.

The model was calibrated by adjusting hydraulic properties and boundary conditions to get a satisfactory match with 

observed water levels, and to agree with spatial distributions 

of recharge and discharge areas and directions of flow between 

aquifers, while maintaining “realistic” values of hydraulic 

properties as defined by field data. Final calibration targets for 

simulated ground-water levels are 10 feet (ft) for both the 1980 

and 2000 simulations, based on observation accuracy and 

observed seasonal variations in ground-water levels. The 

maximum allowable recharge into any model cell is limited to 

approximately 10 inches per year (in/yr).

The simulated and estimated predevelopment potentiometric-surface maps for the Upper Floridan aquifer show a pronounced interaction with streams in the northwestern part of the 

area, a steepening of the potentiometric gradient in the area of 

the Gulf Trough, a flattening of the potentiometric gradient in 

the southwestern part of the model area, and flow toward a 

potentiometric low in coastal South Carolina near Hilton Head 
Summary 63

Island and Port Royal Sound. Sixty percent of the total simulated inflow is contributed by leakage from the general-head 

boundary, and 40 percent is contributed as inflow from lateral 

specified-head boundary. Thirty-nine percent of the total outflow discharges to the general-head boundary, and 61 percent is 

attributed to outflow at the lateral specified-head boundary. 

Total simulated recharge averages about 0.21 in/yr across the 

entire onshore area. Along the coastline, there is a net flow of 

ground-water seaward from onshore toward offshore.

Ground-water conditions during 1980 were calibrated on 

the basis of water-level measurements in 297 wells, mostly 

completed in the Upper Floridan aquifer. Residuals for the 

Upper Floridan aquifer were normally distributed, with a rootmean square of 13.0 ft, and 70 percent of the simulated values 

within the 10-ft calibration target of observed values. Dividing 

the standard deviation of the residuals for the Upper Floridan 

aquifer by the range of observed water-level variation yielded a 

good calibration fit of 0.031.

Ground-water conditions during 2000 were calibrated on 

the basis of water-level measurements in 175 wells, mostly 

completed in the Upper Floridan aquifer. The root-mean square 

of residuals was 5.91 ft for the Brunswick aquifer system, 

9.94 ft for the Upper Floridan aquifer, and 9.15 ft for the Lower 

Floridan aquifer. Residuals for all layers were normally distributed; simulated values were within the 10-ft calibration target of 

observed values for 80 percent of the Brunswick aquifer system 

wells, 79 percent of the Upper Floridan aquifer wells, and 

73 percent of the Lower Floridan aquifer wells. Dividing the 

standard deviation of the residuals by the range of water-level 

variation yielded a calibration fit of 0.142 for the Brunswick 

aquifer system, 0.031 for the Upper Floridan aquifer, and 

0.056 for the Lower Floridan aquifer.

Residuals in the Upper Floridan aquifer show a correlation 

in the magnitude of residuals with physiography and geology 

for both the 1980 and 2000 simulations—north of the Gulf 

Trough, residuals are largest and show the greatest variability; 

in the coastal area, residuals are mostly of smaller magnitude. 

The model cell size is not sufficient to capture the greater topographic relief and a larger degree of stream-aquifer interaction 

north of the Gulf Trough; thus, larger residuals are expected 

there. Clustering of positive and negative residuals in parts of 

the updip area are possibly a result of subregional scale variations in hydraulic conductivity that are not accounted for in the 

model either from lack of data or inaccuracy of pumping data. 

For the 1980 simulation near the cone of depression at Savannah, residuals generally are negative, reflecting site-specific 

pumping data inaccuracy in this heavily stressed area.

Simulated 1980 and 2000 potentiometric surfaces for the 

Upper Floridan aquifer have similar prominent features to those 

shown on estimated 1980 and later potentiometric-surface 

maps. These features include large cones of depression in the 

Savannah, Ga., St. Marys, Ga.–Fernandina Beach, Fla., and 

Jacksonville, Fla., areas; smaller cones of depression at Jesup 

and Brunswick, Ga.; a steepening of the potentiometric gradient 

in the area of the Gulf Trough; flattening of the potentiometric 

gradient in the southwestern part of the model area; and potentiometric highs north of Port Royal Sound, S.C. The simulated 

potentiometric surfaces for the Lower Floridan aquifer generally mimic those for the Upper Floridan for 1980 and 2000 conditions, indicating interaquifer leakage through the confining 

unit. The simulated potentiometric surfaces also are similar for 

the Brunswick aquifer system for 1980 and 2000.

Simulated and observed water levels show similar vertical 

flow potential in most of the study area. Where the simulated 

and observed vertical flow potentials do not agree, the likely 

causes are lack of detailed data to define local hydraulic properties, and also the simplification of the conceptual system.

From predevelopment to 2000, simulated drawdown of the 

Upper Floridan aquifer exceeded 20 ft throughout most of the 

area and was greatest near the center of the major pumping center at Savannah, where water levels declined more than 100 ft. 

Simulated drawdown in the Lower Floridan was of similar 

magnitude. In the Brunswick aquifer system, simulated drawdown exceeded 20 ft throughout most of its extent.

During 1980–2000, water levels in all aquifers showed a 

combination of rises and declines. Simulated drawdown 

exceeded 10 ft in the Upper Floridan aquifer across much of the 

western half of the model area, with drawdown exceeding 20 ft 

along parts of the western, northern, and southern boundaries. 

At the Savannah and Brunswick pumping centers, simulated 

water levels in the Upper Floridan aquifer rose more than 20 ft 

and 8 ft, respectively, in response to decreased pumping in those 

areas. Water-level changes in the Lower Floridan aquifer and 

Brunswick aquifer system were mostly in response to changes 

in pumping patterns in the Upper Floridan aquifer.

The water budgets for the 1980 and 2000 simulations are 

similar and show changes from the predevelopment flow budget, including: (1) increased inflow from and decreased outflow 

to the general-head boundaries in both the onshore and offshore 

areas; (2) increased outflow to, and inflow from, lateral fixedhead boundaries; and (3) increased landward and decreased 

seaward flow rates, with net flow along the coastline changing 

from seaward to landward. 

Simulated recharge and discharge for 2000 conditions differ from those for predevelopment conditions: (1) areas of 

recharge north of the Gulf Trough have increased in areal extent 

and magnitude of recharge; (2) discharge rates in model cells 

containing major streams have decreased throughout the area, 

with some of the flow reversing to recharge; and (3) reversal 

from discharge to recharge conditions has occurred in all cells 

beneath the major pumping centers.

Composite-scaled sensitivities indicate the model is very 

sensitive to pumping and moderately sensitive to hydraulic 

conductivities in the Upper Floridan and overlying confining 

unit. A 10-percent increase in pumping rates results in widespread decline in water level of greater than 2 ft across most of 

the simulated area, with declines exceeding 10 ft in areas of 

concentrated pumping. A 10-ft increase in head along the southern model boundary resulted in an increase in simulated head 

extending from this boundary northward to Chatham County, Ga.

Model limitations include: (1) the model’s spatial scale 

and discretization, (2) the extent to which data are available to 
64 Simulation of Ground-Water Flow in Coastal Georgia and Adjacent Parts of 

South Carolina and Florida—Predevelopment, 1980, and 2000

physically define the flow system, (3) the type of boundary 

conditions and controlling parameters used, (4) uncertainty in 

the distribution of pumping, and (5) uncertainty in field-scale 

hydraulic properties. The model could be improved with more 

accurate estimates of ground-water pumpage and better 

characterization of recharge and discharge.

Selected References

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Georgia Geologic Survey Project Report 48, 15 p.

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Survey Open-File Report 99-100, 1 sheet, scale 1:100,000.

Brooks, Rebekah, Clarke, J.S., and Faye, R.E., 1985, Hydrogeology of the Gordon aquifer system of east-central Georgia: 

Georgia Geologic Survey Information Circular 75, 41 p.

Bush, P.W., 1982, Predevelopment flow in the Tertiary limestone aquifer, southeastern United States; A regional analysis 

from digital modeling: U.S. Geological Survey WaterResources Investigations Report 82-905, 41 p.

Bush, P.W., and Johnston, R.H., 1988, Ground-water hydraulics, regional flow, and ground-water development of the 

Floridan aquifer system in Florida, and in parts of Georgia, 

South Carolina, and Alabama: U.S. Geological Survey Professional Paper 1403-C, 80 p.

Charm, W.B., Nesteroff, W.D., and Valdes, Sylvia, 1969, 

Detailed stratigraphic description of the JOIDES cores on the 

continental margin off Florida: U.S. Geological Survey Professional Paper 581-D, 13 p.

Clarke, J.S., 1987, Potentiometric surface of the Upper Floridan 

aquifer in Georgia, 1985, and water-level trends, 1980–85: 

Georgia Geologic Survey Hydrologic Atlas 16, 1 sheet, scale 

1:1,000,000.

Clarke, J.S., 2003, The surficial and Brunswick aquifer 

systems—alternative ground-water resources for coastal 

Georgia, in Proceedings of the 2003 Georgia Water 

Resources Conference, April 23–24, 2003, The University of 

Georgia, Kathryn J. Hatcher, ed., Institute of Ecology, the 

University of Georgia, Athens, Georgia, CD–ROM.

Clarke, J.S., Hacke, C.M., and Peck, M.F., 1990, Geology and 

ground-water resources of the coastal area of Georgia: 

Georgia Geologic Survey Bulletin 113, 106 p.

Clarke, J.S., and Krause, R.E., 2000, Design, revision, and 

application of ground-water flow models for simulation of 

selected water-management scenarios in the coastal area of 

Georgia and adjacent parts of South Carolina and Florida: 

U.S. Geological Survey Water-Resources Investigations 

Report 00-4084, 93 p.

Clarke, J.S., Leeth, D.C., Taylor-Harris, DáVette, Painter, J.A., 

and Labowski, J.L., 2004, Summary of hydraulic properties 

of the Floridan aquifer system in coastal Georgia and adjacent parts of South Carolina and Florida: U.S. Geological 

Survey Scientific Investigations Report 2004-5264, 52 p.

Clarke, J.S., and West, C.T., 1998, Simulation of ground-water 

flow and stream-aquifer relations in the vicinity of the Savannah River Site, Georgia and South Carolina, predevelopment 

through 1992: U.S. Geological Survey Water-Resources 

Investigations Report 98-4062, 134 p.

Counts, H.B., and Krause, R.E., 1976, Digital model analysis of 

the principal artesian aquifer, Savannah, Georgia area: U.S. 

Geological Survey Water Resources Investigations 76-133, 

4 sheets.

Counts, H.B., and Donsky, Ellis, 1963, Salt-water encroachment, geology, and ground-water resources of the Savannah 

area, Georgia and South Carolina: U.S. Geological Survey 

Water-Supply Paper 1611, 100 p.

Falls, W.F., Baum, J.S., Harrelson, L.G., and Brown, L.H., 

1997, Geology and hydrogeology of Cretaceous and Tertiary 

strata, and confinement in the vicinity of the U.S. Department 

of Energy Savannah River Site, South Carolina and Georgia: 

U.S. Geological Survey Water-Resources Investigations 

Report 97-4245, 125 p.

Falls, W.F., Harrelson, L.G., Conlon, K.J., and Petkewich, M.D., 

2001, Hydrogeology and water quality of the Lower Floridan 

aquifer, coastal Georgia, 1999–2000: in Proceedings of the 

2001 Georgia Water Resources Conference, March 26–27, 

2001, The University of Georgia, Kathryn J. Hatcher, ed., 

Institute of Ecology, The University of Georgia, Athens, 

Georgia, p. 652–655.

Fanning, J.L., 2003, Water use in Georgia by county for 2000 

and water-use trends for 1980–2000: Georgia Geologic 

Survey Information Circular 106, 176 p.

Foyle, A.M., Henry, V.J., and Alexander, C.R., 2001, The Miocene

aquitard and the Floridan aquifer of the Georgia/South Carolina coast: Geophysical mapping of potential seawater intrusion sites: Georgia Geologic Survey Bulletin 132, 61 p.

Garza, Reggina, and Krause, R.E., 1996, Water-supply potential of major streams and the Upper Floridan aquifer in the 

vicinity of Savannah, Georgia: U.S. Geological Survey 

Water-Supply Paper 2411, 36 p.

Golder Associates, Inc., 2003, Reports on the Brunswick aquifer studies for Bryan, Chatham, Effingham, and Glynn Counties, and Ebenezer Bend: Georgia Geologic Survey Report 

CD-4, CD–ROM. 

Gregg, D.O., and Zimmerman, E.A., 1974, Geologic and 

hydrologic control of chloride contamination in aquifers at 

Brunswick, Glynn County, Georgia: U.S. Geological Survey 

Water-Supply Paper 2029-D, 44 p.
Selected References 65

Harbaugh, A.W., 1990, A computer program for calculating 

subregional water budgets using results from the U.S. Geological Survey modular three-dimensional finite-difference 

ground-water flow model: U.S. Geological Survey Open-File 

Report 90-392, 46 p.

Harbaugh, A.W., Banta, E.R., Hill, M.C., and McDonald, M.G., 

2000, MODFLOW-2000, The U.S. Geological Survey 

modular ground-water model – User guide to modularization 

concepts and the ground-water flow process: U.S. Geological 

Survey Open-File Report 00-92, 121 p.

Hathaway, J.C., Schlee, J.S., Poag, W.C., Valentine, P.C., 

Weed, E.G.A., Bothner, M.H., Kohout, F.A., Manheim, F.T., 

Schoen, Robert, Miller, R.E., and Schultz, D.M., 1981, Summary of the 1976 Atlantic margin coring project of the U.S. 

Geological Survey: U.S. Geological Survey Open-File 

Report 81-0239, 217 p.

Hayes, L.R., 1979, The ground-water resources of Beaufort, 

Colleton, Hampton, and Jasper Counties, South Carolina: 

South Carolina Water Resources Commission Report No. 9, 

91 p.

Hill, M.C., 1998, Methods and guidelines for effective model 

calibration: U.S. Geological Survey Water-Resources 

Investigations Report 98-4005, 90 p.

Hill, M.C., Banta, E.R., Harbaugh, A.W., and Anderman, E.R., 

2000, MODFLOW-2000, the U.S. Geological Survey modular ground-water model—User guide to the observation, 

sensitivity, and parameter-estimation processes and three 

post-processing programs: U.S. Geological Survey OpenFile Report 00-184, 209 p.

Hughes, W.B., Crouch, M.S., and Park, A.D., 1989, Hydrogeology and saltwater contamination of the Floridan Aquifer in 

Beaufort and Jasper Counties, South Carolina: South Carolina Water Resources Commission Report Number 158, 52 p.

Johnston, R.H., Healy, H.G., and Hayes, L.R., 1981, Potentiometric surface of the Tertiary limestone aquifer system, 

southeastern United States, May 1980: U.S. Geological 

Survey Open-File Report 81-486, 1 sheet, scale.

Johnston, R.H., Krause, R.E., Meyer, F.W., Ryder, P.D., 

Tibbals, C.H., and Hunn, J.D., 1980, Estimated potentiometric surface for the Tertiary limestone aquifer system, southeastern United States, prior to development: U.S. Geological 

Survey Open-File Report 80-406, 1 sheet.

Johnston, R.H., Bush, P.W., Krause, R.E., Miller, J.A., and 

Sprinkle, C.L., 1982, Summary of hydrologic testing in Tertiary limestone aquifer, Tenneco offshore exploratory well, 

Atlantic OCS, Lease-Block 427 (Jacksonville NW 17-5): 

U.S. Geological Survey Water-Supply Paper 2180, 15 p.

Jones, L.E., 2001, Saltwater contamination of the Upper Floridan aquifer at Brunswick, Georgia: in Proceedings of the 

2001 Georgia Water Resources Conference, March 26–27, 

2001, The University of Georgia, Kathryn J. Hatcher, ed., 

Institute of Ecology, The University of Georgia, Athens, 

Georgia, p. 644–647.

Kellam, M.F., and Gorday, L.L., 1990, Hydrogeology of the 

Gulf Trough–Apalachicola Embayment area, Georgia: 

Georgia Geologic Survey Bulletin 94, 73 p.

Krause, R.E., 1982, Digital model evaluation of the predevelopment flow system of the Tertiary limestone aquifer, southeast 

Georgia, northeast Florida, and southern South Carolina: 

U.S. Geological Survey Water-Resources Investigations 

Report 82-173, 27 p.

Krause, R.E., and Clarke, J.S., 2001, Coastal ground water at 

risk – Saltwater contamination at Brunswick, Georgia and 

Hilton Head Island, South Carolina: U.S. Geological Survey 

Water Resources Investigations Report 01-4107, 1 sheet.

Krause, R.E., and Counts, H.B., 1975, Digital model analysis of 

the principal artesian aquifer, Glynn County, Georgia: U.S. 

Geological Survey Water Resources Investigations 1-75 

Open-File Report, 4 sheets.

Krause, R.E., and Randolph, R.B., 1989, Hydrology of the 

Floridan aquifer system in southeast Georgia and adjacent 

parts of Florida and South Carolina: U.S. Geological Survey 

Professional Paper 1403-D, 65 p.

Kuniansky, E.L., Gomez-Gomez, Fernando, and Torres-Gonzalez, Sigfredo, 2003, Effects of aquifer development and 

changes in irrigation practices on ground-water availability 

in the Santa Isabel area, Puerto Rico: U.S. Geological Survey 

Water-Resources Investigations Report 03-4303, 56 p.

Landmeyer, J.E., and Belval, D.L., 1996, Water-chemistry and 

chloride fluctuation in the Upper Floridan aquifer in the Port 

Royal Sound area, South Carolina, 1917–93: U.S. Geological 

Survey Water-Resources Investigations Report 96-4102, 106 p.

Leeth, D.C., 1999, Hydrogeology of the surficial aquifer in the 

vicinity of a former landfill, Naval Submarine Base Kings 

Bay, Camden County, Georgia: U.S. Geological Survey 

Water-Resources Investigations Report 98-4246, 28 p.

Marella, R.L., 2004, Water withdrawals, use, discharge, and 

trends in Florida, 2000: U.S. Geological Survey Scientific 

Investigations Report 2004-5151, 136 p.

Maslia, M.L., and Prowell, D.C., 1990, Effects of faults on fluid 

flow and chloride contamination in a carbonate aquifer 

system: Journal of Hydrology, v. 115, nos. 1–4, p. 1–49.

McCollum, M.J., and Counts, H.B., 1964, Relation of salt-water 

encroachment to the major aquifer zones, Savannah area, 

Georgia and South Carolina: U.S. Geological Survey WaterSupply Paper 1613-D, 26 p.

Miller, J.A., 1986, Hydrogeologic framework of the Floridan 

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Paper 1403-B, 91 p.

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Monthly station normals of temperature, precipitation, and 

heating and cooling degree days 1971–2000: Asheville, N.C., 

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Appendix A 67

Appendix A. Observed and Simulated Ground-Water Levels, 1980 and 2000
FLORIDA

SOUTH

GEORGIA CAROLINA

Model

boundary

Map

area

32°

33°

31°

83°

82°

81°

Well and name

EXPLANATION

28D001

0 50 MILES 25

0 50 KILOMETERS 25

Base from U.S. Geological Survey

1:100,000-scale digital data

Duval

Colleton

Jasper

Nassau

Hampton

Barnwell

Bamberg

Allendale

Beaufort

Jefferson

Dodge

Wilcox

Crisp

Turner

Ben

Hill

Coffee Bacon

Glynn

Brantley Pierce

Ware Berrien Atkinson

Lanier

Clinch Charlton Camden

Irwin

Wheeler

Toombs Tattnall

Evans

Bryan

Long

Wayne

Appling Jeff Davis

Telfair

Liberty

McIntosh

Chatham

Montgomery

Johnson

Emanuel

Treutlen

Laurens

Jenkins

Screven

Bulloch Effingham

Candler

Burke

SC

GA

GA

FL

HAM83

BW654

BW653

BW652

BW154

BW151

30U002

20R003

20N005 16P004

COL-97

BFT787

BFT786

BFT453

BFT444

BFT439

BFT429

BFT304

BFT121

36S004

34U008 33U023

33U021

33S010

32V007

32M009

32M001

32L004

31Z085

31W025 31W010

31T010

31S008 31R001

31M034

31M033

31M024

31L001

31H005

30V002

30P003

30N002

30M011

30M007

30M005

30L012

30L011 30L003

30K004

30H005

30H003

29V001

29T009

29R001

29N003

29M004

29M002

29L005

29K002

29H002

29F001

28W004

28T001

28S003

28R001

28P001

28K001

28H004

28D001

See part C

See part C

See part C

See part B

See part B

See part B

27W001

27S002

27S001

27R005

27Q005

27N005

27M001

27G006

27E003

27E002

26X005

26T001

26M003

26M002

25U003

25S004

25S003

25S002

25R001

25Q001

25P002

25N003

25D001

24V002

24T002

24S001

24L004

24J001

24H001

23U008

23N001

23J007 23J006

23G001

22T001

22Q004

22N003

22N001

22M003

22F001

21T001

21S001

21P002

21P001

21N002

21L001

21K001

21J001

20L002

19Q001

19P007

19P006

19P003

19P001

19N002

19L001

18Q002

18P002

18P001

18N003

17P001

17M009

17M007

30E002

27G005

BFT315_2

BFT315_1

BW650

BW647

BW646

BW644

31M022

31M006

33M004

A.

68 Simulation of Ground-Water Flow in Coastal Georgia and Adjacent Parts of 

South Carolina and Florida—Predevelopment, 1980, and 2000

Figure A1. Locations of wells used for 1980 simulation in (A) study area; (B) Chatham, Bryan, and Liberty Counties (enlarged); and 

(C) McIntosh, Glynn, and Camden Counties (enlarged).
B.

FLORIDA

SOUTH

GEORGIA CAROLINA

FLORIDA

SOUTH

GEORGIA CAROLINA

Model

boundary

Map

area

Well and name

BRYAN CHATHAM

LIBERTY

EXPLANATION

0

0

Base from U.S. Geological Survey

1:100,000-scale digital data

N

5 10 KILOMETERS

5 10 MILES

34M075

36P093 36P091

36N002

36M018

35P057

35N059

35N021

34N091

34N089

34M056

34M054

34M049

37Q034

37P086

37P013

37P009 37P006

37P005

36Q020 36Q287

36Q019

36P087

35Q043

35P085

35P071

34P014

34P012

33M003

33P019

33N091

33N089

33N085 32N013

32N012

32N010

37Q090

37Q066

37Q033 37Q043

37Q016

37Q015 37Q012

37Q006

36Q300

36Q011

36Q008

35R018

39Q003

38Q006

38Q002

38Q001

37Q160

36Q020

Appendix A 69

Figure A1. Locations of wells used for 1980 simulation in (A) study area; (B) Chatham, Bryan, and Liberty Counties 

(enlarged); and (C) McIntosh, Glynn, and Camden Counties (enlarged)—continued.
FLORIDA

SOUTH

GEORGIA CAROLINA

Model

boundary

Map

area

34H399

34H391

34G036

33H188

34H469

34H424

34H401

34H400

34H393

34H392

34H372

34H366

34H363

34H355

34H345

34H344

34H133

34H128

34H122

34H117

34H112

34H085

34H062

34G002

33H180

33H141

33H133

33H130

33H110

33G008 33G002

34H373

GLYNN

MCINTOSH

CAMDEN

Brunswick

0 1 2 MILES

0 1 2 KILOMETERS

N

0

0 10 KILOMETERS 5

5 10 MILES

Base from U.S. Geological Survey

1:100,000-scale digital data

N

35K069 Well and name

EXPLANATION

C.

35H044

35H037

34H383

34H381 34H358

34H328

34H204

34K095

34K084

33J044 34J051

34J029

34J021

34J009

34H410 34H408

34H357

33J034 33J028

33J026

33H190

33H079

33H052

33H038

34K082 34K081

34K073

33K019

33K005

32H001

32G015

32G007

32G004

32F048 32F008

33H177

33H164

33G006

33G005

34G020

34G009

32E031

31F022

30F004

33E023

33E007

33E004

33D004

32E033

31E001

33E002

33F001

33H100

35K069

34L048

34K085

34K083 34K012

33L027

33K016

35L068

35L067

34L061 33L010

35M013 34M076 34M070

34L060

33H139

70 Simulation of Ground-Water Flow in Coastal Georgia and Adjacent Parts of 

South Carolina and Florida—Predevelopment, 1980, and 2000

Figure A1. Locations of wells used for 1980 simulation in (A) study area; (B) Chatham, Bryan, and Liberty Counties (enlarged); and 

(C) McIntosh, Glynn, and Camden Counties (enlarged)—continued.
FLORIDA

SOUTH

GEORGIA CAROLINA

Model

boundary

Map

area

32°

33°

31°

83°

82°

81°

0 50 MILES 25

0 50 KILOMETERS 25

Base from U.S. Geological Survey

1:100,000-scale digital data

Well and name

EXPLANATION

28D001

A.

Duval

Colleton

Jasper

Nassau

Hampton

Barnwell

Bamberg

Allendale

Beaufort

Jefferson

Dodge

Wilcox

Crisp

Turner

Ben Hill

Coffee Bacon

Glynn

Brantley

Pierce

Ware

Berrien Atkinson

Lanier

Clinch Charlton

Camden

Irwin

Wheeler

Toombs

Tattnall

Evans

Bryan

Long

Wayne

Jeff Davis Appling

Telfair

Liberty

McIntosh

Chatham

Montgo

mery

Johnson

Emanuel

Laurens Treutlen

Jenkins Screven

Effingham Bulloch

Candler

Burke

See part C

See part C

See part C

See part B

See part B

See part B

N-62

HAM83

32Y033

D-3840

BFT429

35T003

33V021 34V004

33V020

33U023

33U019 33U021

33U009

33M004

32W006

32U005

32R002

32L015

32L004

31V014

31T010

31S008

31R001

31H005

30X003

30V002

30U005

30R005

30P003

30N002

30M007

30M003

30L012

30L011

30L003

30H003

30H005

30E007

29W002

29V001

29T009

29R003

29R001

29Q001

29N003

29M002

29L005

28W002

28T001

28S004

28S003

28R001

28K001

28D001

27S002

27R004

27R003

27Q002

27M001

27G006

27E004

26R001

26Q002

26M003

25Q001

21T001

35S008

32L016

30E002

BFT1813

BFT1810

31V008

GA

FL

SC

GA

Appendix A 71

Figure A2. Locations of wells used for 2000 simulation in (A) study area, (B) Chatham, Bryan, and Liberty Counties (enlarged), and 

(C) McIntosh, Glynn, and Camden Counties (enlarged).
FLORIDA

SOUTH

GEORGIA CAROLINA

FLORIDA

SOUTH

GEORGIA CAROLINA

Model

boundary

Map

area

Well and name

EXPLANATION

0

0

Base from U.S. Geological Survey

1:100,000-scale digital data

N

5 10 KILOMETERS

5 10 MILES

BRYAN CHATHAM

LIBERTY

B.

36M018

39Q024

39Q026

39Q003

38Q002

37Q185

37Q033 37Q043

37Q006

37P114

37P006

37P005

36Q300

36Q020

36Q019

36Q008

35R018

39Q028

36N012

35P109

34R039

34N089

33N089

32N012 32N010

36Q020

72 Simulation of Ground-Water Flow in Coastal Georgia and Adjacent Parts of 

South Carolina and Florida—Predevelopment, 1980, and 2000

Figure A2. Locations of wells used for 2000 simulation in (A) study area; (B) Chatham, Bryan, and Liberty Counties 

(enlarged); and (C) McIntosh, Glynn, and Camden Counties (enlarged)—continued.
Appendix A 73

Table A1. Simulated and observed ground-water levels, 1980 and 2000.

[Observed values for 1980 are for May; observed values for 2000 are for September; values in bold are for wells that have continuous recorders 

and water levels that are mean-monthly values for either May 1980 or September 2000; —, data]

Well 

number

Model 

layer

Simulated and observed ground-water levels 

and difference, in feet

Water-level change, 

in feet

1980 calibration 2000 calibration 1980–2000

Simulated Observed Difference Simulated Observed Difference Observed Simulated

27G005 3 86.22 67.34 18.88 — — — — —

30E002 3 25.27 36.90 –11.63 20.20 27.87 –7.67 –9.03 –5.07

32L016 3 — — — 9.51 15.79 –6.28 — —

33D071 3 — — — 10.68 –2.07 12.75 — —

33E002 3 16.09 30.11 –14.02 — — — — —

33G028 3 — — — 10.58 16.75 –6.17 — —

34H437 3 — — — 9.93 5.88 4.05 — —

34J078 3 — — — 9.09 3.71 5.39 — —

35S008 3 — — — 17.80 12.09 5.71 — —

36N012 3 — — — –12.89 –26.18 13.29 — —

39Q026 3 — — — –3.86 –2.31 –1.54 — —

39Q028 3 — — — –3.86 –2.21 –1.65 — —

17M007 5 236.73 230.14 6.59 — — — — —

17M009 5 237.20 230.86 6.34 — — — — —

17P001 5 274.50 266.56 7.94 — — — — —

18N003 5 237.20 206.06 31.14 — — — — —

18P001 5 257.95 213.76 44.19 — — — — —

18P002 5 241.62 206.43 35.19 — — — — —

18Q002 5 228.40 208.87 19.53 — — — — —

19L001 5 199.20 218.68 –19.48 — — — — —

19N002 5 211.22 177.56 33.66 — — — — —

19P001 5 203.65 170.59 33.06 — — — — —

19P003 5 206.37 164.02 42.35 — — — — —

19P006 5 203.99 170.46 33.53 — — — — —

19P007 5 198.75 165.22 33.53 — — — — —

19Q001 5 211.82 198.00 13.82 — — — — —

20L002 5 182.96 206.14 –23.18 — — — — —

21J001 5 65.62 63.73 1.89 — — — — —

21K001 5 131.87 102.05 29.82 — — — — —

21L001 5 147.85 179.85 –32.00 — — — — —

21N002 5 178.20 167.43 10.77 — — — — —

21P001 5 181.46 159.31 22.15 — — — — —

21P002 5 180.43 161.11 19.32 — — — — —

21S001 5 213.34 220.69 –7.35 — — — — —

21T001 5 226.20 230.68 –4.48 217.71 220.19 –2.48 –10.49 –8.49

22F001 5 71.92 71.33 0.59 — — — — —

22M003 5 135.86 144.38 –8.52 — — — — —

22N001 5 162.49 163.21 –0.72 — — — — —

22N003 5 160.87 171.10 –10.23 — — — — —

22Q004 5 170.37 160.09 10.28 — — — — —

22T001 5 214.99 226.57 –11.58 — — — — —

23G001 5 63.23 53.76 9.47 — — — — —

23J006 5 59.72 54.05 5.67 — — — — —

23J007 5 60.25 61.12 –0.87 — — — — —

23N001 5 109.14 88.30 20.84 — — — — —

23U008 5 224.52 218.42 6.10 — — — — —
74 Simulation of Ground-Water Flow in Coastal Georgia and Adjacent Parts of 

South Carolina and Florida—Predevelopment, 1980, and 2000

24H001 5 59.17 60.42 –1.25 — — — — —

24J001 5 59.37 74.64 –15.27 — — — — —

24L004 5 56.83 59.50 –2.67 — — — — —

24S001 5 179.37 143.45 35.92 — — — — —

24T002 5 194.39 173.58 20.81 — — — — —

24V002 5 237.86 222.23 15.63 — — — — —

25D001 5 60.29 55.81 4.48 — — — — —

25N003 5 78.01 41.80 36.21 — — — — —

25P002 5 125.30 128.09 –2.79 — — — — —

25Q001 5 122.89 116.25 6.64 108.53 96.75 11.78 –19.50 –14.35

25R001 5 144.80 129.70 15.10 — — — — —

25S002 5 163.09 131.95 31.14 — — — — —

25S003 5 163.89 138.65 25.24 — — — — —

25S004 5 167.93 143.58 24.35 — — — — —

25U003 5 202.63 202.61 0.02 — — — — —

26M002 5 50.92 56.07 –5.15 — — — — —

26M003 5 52.47 54.89 –2.42 40.97 35.84 5.13 –19.05 –11.51

26Q002 5 — — — 63.79 49.95 13.84 — —

26R001 5 — — — 108.60 103.85 4.75 — —

26T001 5 177.24 177.28 –0.04 — — — — —

26X005 5 246.41 283.45 –37.04 — — — — —

27E002 5 56.08 50.90 5.18 — — — — —

27E003 5 56.16 48.17 7.99 — — — — —

27E004 5 — — — 44.47 40.54 3.93 — —

27G006 5 52.83 54.99 –2.16 41.76 40.48 1.28 –14.51 –11.07

27M001 5 47.09 51.05 –3.96 36.22 33.07 3.15 –17.98 –10.87

27N005 5 42.85 53.84 –10.99 — — — — —

27Q002 5 — — — 62.15 44.97 17.18 — —

27Q005 5 88.74 77.40 11.34 — — — — —

27R003 5 — — — 103.75 90.67 13.08 — —

27R004 5 — — — 85.61 92.42 –6.81 — —

27R005 5 108.66 99.07 9.59 — — — — —

27S001 5 135.16 130.15 5.01 — — — — —

27S002 5 134.62 124.64 9.98 118.90 99.21 19.69 –25.43 –15.72

27W001 5 213.67 221.81 –8.14 — — — — —

28D001 5 52.64 46.18 6.46 41.20 37.79 3.41 –8.39 –11.43

28H004 5 49.48 59.50 –10.02 37.98 — — — —

28K001 5 48.36 51.70 –3.34 — 44.39 –44.39 –7.31 –48.36

28P001 5 34.02 56.23 –22.21 — — — — —

28R001 5 91.67 84.13 7.54 73.69 59.40 14.29 –24.73 –17.97

28S003 5 130.03 130.54 –0.51 115.72 106.51 9.21 –24.03 –14.31

28S004 5 — — — 110.12 88.15 21.97 — —

28T001 5 148.44 140.83 7.61 139.72 103.29 36.43 –37.54 –8.72

28W002 5 — — — 196.48 202.54 –6.06 — —

28W004 5 209.75 208.00 1.75 — — — — —

29F001 5 49.02 51.37 –2.35 — — — — —

Table A1. Simulated and observed ground-water levels, 1980 and 2000.—Continued

[Observed values for 1980 are for May; observed values for 2000 are for September; values in bold are for wells that have continuous recorders 

and water levels that are mean-monthly values for either May 1980 or September 2000; —, data]

Well 

number

Model 

layer

Simulated and observed ground-water levels 

and difference, in feet

Water-level change, 

in feet

1980 calibration 2000 calibration 1980 – 2000

Simulated Observed Difference Simulated Observed Difference Observed Simulated
Appendix A 75

29H002 5 47.60 48.89 –1.29 — — — — —

29K002 5 39.90 44.62 –4.72 — — — — —

29L005 5 35.55 38.22 –2.67 26.34 27.08 –0.74 –11.14 –9.22

29M002 5 36.41 49.69 –13.28 26.70 36.04 –9.34 –13.65 –9.70

29M004 5 27.55 38.78 –11.23 — — — — —

29N003 5 30.01 40.41 –10.40 20.57 32.16 –11.59 –8.25 –9.44

29Q001 5 — — — 19.88 29.59 – 9.71 — —

29R001 5 49.15 49.50 –0.35 36.39 33.05 3.34 –16.45 –12.76

29R003 5 — — — 35.70 34.98 0.72 — —

29T009 5 138.94 136.97 1.97 125.67 113.32 12.35 –23.65 –13.27

29V001 5 162.85 170.40 –7.55 154.32 160.00 –5.68 –10.40 –8.53

29W002 5 — — — 179.74 167.92 11.82 — —

30E007 5 — — — 36.09 32.30 3.79 — —

30F004 5 41.77 43.84 –2.07 32.45 34.70 –2.25 –9.14 –9.32

30H003 5 44.34 37.93 6.41 34.88 27.58 7.30 –10.35 –9.46

30H005 5 44.10 40.88 3.22 34.68 37.35 –2.67 –3.53 –9.42

30K004 5 32.45 29.39 3.06 — — — — —

30L003 5 20.68 25.03 –4.35 13.08 13.95 –0.87 –11.08 –7.60

30L011 5 35.71 32.78 2.93 26.62 20.91 5.71 –11.87 –9.09

30L012 5 26.18 34.04 –7.86 18.01 19.53 –1.52 –14.51 –8.17

30M003 5 — — — 11.62 11.03 0.59 — —

30M005 5 16.36 23.36 –7.00 — — — — —

30M007 5 24.02 15.30 8.72 15.58 19.26 –3.68 3.96 –8.44

30M011 5 23.38 29.21 –5.83 — — — — —

30N002 5 24.63 33.26 –8.63 15.97 21.25 –5.28 –12.01 –8.66

30P003 5 21.90 30.86 –8.96 12.44 5.12 7.32 –25.74 –9.46

30R005 5 — — — 13.67 23.74 –10.07 — —

30U005 5 — — — 112.32 114.98 –2.66 — —

30V002 5 140.56 151.63 –11.07 126.01 138.79 –12.78 –12.84 –14.55

30X003 5 — — — 163.49 161.82 1.67 — —

31E001 5 39.09 41.58 –2.49 29.66 30.10 –0.44 –11.48 –9.43

31F022 5 39.43 42.98 –3.55 30.12 32.77 –2.65 –10.21 –9.31

31H005 5 41.15 38.89 2.26 32.19 32.70 –0.51 –6.19 –8.97

31L001 5 26.80 25.30 1.50 — — — — —

31M006 5 6.30 10.26 –3.96 — — — — —

31M022 5 8.46 10.52 –2.06 — — — — —

31M024 5 7.69 0.43 7.26 — — — — —

31M033 5 10.48 11.21 –0.73 — — — — —

31M034 5 12.44 14.49 –2.05 — — — — —

31R001 5 21.91 39.73 –17.82 12.34 23.64 –11.30 –16.09 –9.57

31S008 5 23.30 44.63 –21.33 13.99 41.96 –27.97 –2.67 –9.30

31T010 5 74.62 111.25 –36.63 63.76 54.95 8.81 –56.30 –10.86

31V008 5 — — — 97.60 122.83 –25.23 — —

31V014 5 — — — 99.60 118.09 –18.49 — —

31W010 5 129.83 146.92 –17.09 — — — — —

31W025 5 149.24 169.30 –20.06 — — — — —

Table A1. Simulated and observed ground-water levels, 1980 and 2000.—Continued

[Observed values for 1980 are for May; observed values for 2000 are for September; values in bold are for wells that have continuous recorders 

and water levels that are mean-monthly values for either May 1980 or September 2000; —, data]

Well 

number

Model 

layer

Simulated and observed ground-water levels 

and difference, in feet

Water-level change, 

in feet

1980 calibration 2000 calibration 1980 – 2000

Simulated Observed Difference Simulated Observed Difference Observed Simulated
76 Simulation of Ground-Water Flow in Coastal Georgia and Adjacent Parts of 

South Carolina and Florida—Predevelopment, 1980, and 2000

31Z085 5 158.61 134.69 23.92 — — — — —

32E031 5 36.85 41.44 –4.59 27.33 29.56 –2.23 –11.88 –9.52

32E033 5 34.52 38.59 –4.07 — — — — —

32E038 5 — — — 26.37 25.46 0.91 — —

32F008 5 37.19 43.01 –5.82 27.96 32.28 –4.32 –10.73 –9.23

32F048 5 36.82 38.24 –1.42 — — — — —

32G004 5 36.78 37.64 –0.86 — — — — —

32G007 5 37.12 40.11 –2.99 28.02 27.09 0.93 –13.02 –9.09

32G015 5 35.78 38.06 –2.28 26.90 29.95 –3.05 –8.11 –8.87

32H001 5 29.87 24.53 5.34 22.77 21.56 1.21 –2.97 –7.10

32J003 5 — — — 19.55 23.34 –3.79 — —

32L004 5 18.01 19.51 –1.50 11.22 10.55 0.67 –8.96 –6.78

32L015 5 — — — 10.24 8.22 2.02 — —

32M001 5 9.99 14.47 –4.48 — — — — —

32M009 5 11.01 12.67 –1.66 — — — — —

32N010 5 7.77 23.10 –15.33 0.40 10.14 –9.74 –12.96 –7.38

32N012 5 8.63 17.96 –9.33 0.98 4.49 –3.51 –13.47 –7.65

32N013 5 10.36 12.18 –1.82 — — — — —

32R002 5 — — — 10.17 18.01 –7.84 — —

32U005 5 — — — 70.85 96.50 –25.65 — —

32V007 5 104.53 121.32 –16.79 — — — — —

32W006 5 7.02 — — 105.44 98.95 6.49 — —

33D004 5 — 4.26 –4.26 –1.80 1.91 –3.71 –2.35 –1.80

33D069 5 — — — 0.60 2.99 –2.39 — —

33E004 5 31.31 36.32 –5.01 — — — — —

33E007 5 22.76 22.92 –0.16 13.26 15.26 –2.00 –7.66 –9.50

33E009 5 — — — 22.45 30.97 –8.52 — —

33E023 5 32.36 41.13 –8.77 — — — — —

33E027 5 — — — 20.55 23.51 –2.96 — —

33F001 5 31.30 36.36 –5.06 — — — — —

33F003 5 — — — 22.40 26.15 –3.75

33G002 5 28.69 20.27 8.42 — — — — —

33G003 5 — — — 21.81 22.86 –1.05 — —

33G005 5 31.19 36.25 –5.06 — — — — —

33G006 5 31.29 41.02 –9.73 — — — — —

33G008 5 28.35 20.90 7.45 20.64 18.72 1.92 –2.18 –7.71

33H038 5 11.43 15.40 –3.97 — — — — —

33H052 5 9.03 10.98 –1.95 — — — — —

33H079 5 2.34 7.88 –5.54 — — — — —

33H100 5 –5.63 –3.75 –1.88 — — — — —

33H110 5 –13.80 –15.75 1.95 — — — — —

33H120 5 — — — –3.55 –2.99 –0.56 — —

33H130 5 –10.55 –15.22 4.67 –2.46 –4.59 2.13 10.63 8.08

33H133 5 –6.74 –11.73 4.99 –0.65 –0.89 0.24 10.84 6.09

33H139 5 28.13 17.20 10.93 — — — — —

33H141 5 1.21 3.92 –2.71 4.75 7.24 –2.49 3.32 3.54

Table A1. Simulated and observed ground-water levels, 1980 and 2000.—Continued

[Observed values for 1980 are for May; observed values for 2000 are for September; values in bold are for wells that have continuous recorders 

and water levels that are mean-monthly values for either May 1980 or September 2000; —, data]

Well 

number

Model 

layer

Simulated and observed ground-water levels 

and difference, in feet

Water-level change, 

in feet

1980 calibration 2000 calibration 1980 – 2000

Simulated Observed Difference Simulated Observed Difference Observed Simulated
Appendix A 77

33H164 5 24.36 16.96 7.40 18.53 15.08 3.45 –1.88 –5.84

33H174 5 — — — 6.74 9.33 –2.59 — —

33H177 5 32.23 27.94 4.29 23.68 21.16 2.52 –6.78 –8.55

33H180 5 –7.06 –8.57 1.51 0.19 –1.15 1.34 7.42 7.24

33H190 5 4.31 7.94 –3.63 5.29 8.03 –2.74 0.09 0.97

33H207 5 — — — 4.37 4.16 0.21 — —

33J026 5 15.09 18.19 –3.10 — — — — —

33J027 5 — — — 11.66 14.31 –2.65

33J028 5 12.89 15.32 –2.43 — — — — —

33J034 5 11.46 12.69 –1.23 — — — — —

33K005 5 16.57 16.32 0.25 — — — — —

33K016 5 13.69 11.40 2.29 — — — — —

33K019 5 13.55 11.72 1.83 — — — — —

33L010 5 11.76 11.31 0.45 — — — — —

33L027 5 14.46 16.80 –2.34 8.15 6.98 1.17 –9.82 –6.30

33M003 5 4.96 2.73 2.23 — — — — —

33M004 5 9.71 11.45 –1.74 3.21 0.40 2.81 –11.05 –6.50

33N085 5 6.84 10.43 –3.59 — — — — —

33N089 5 7.92 8.45 –0.53 0.77 –5.48 6.25 –13.93 –7.15

33N091 5 3.37 5.65 –2.28 — — — — —

33P019 5 2.40 8.37 –5.97 — — — — —

33S010 5 23.15 38.96 –15.81 — — — — —

33U009 5 — — — 53.31 67.18 –13.87 — —

33U019 5 — — — 48.46 73.11 –24.65 — —

33U021 5 68.47 76.68 –8.21 53.17 71.15 –17.98 –5.53 –15.30

33U023 5 63.36 86.89 –23.53 48.21 75.31 –27.10 –11.58 –15.15

33V020 5 — — — 62.20 76.70 –14.50 — —

33V021 5 — — — 78.25 98.46 –20.21 — —

34G002 5 20.83 15.96 4.87 15.34 17.62 –2.28 1.66 –5.49

34G009 5 29.77 40.53 –10.76 20.64 35.50 –14.86 –5.03 –9.13

34G016 5 — — — 18.65 23.69 –5.04 — —

34G020 5 28.97 30.68 –1.71 20.07 26.18 –6.11 –4.50 –8.90

34H062 5 2.43 –11.84 14.27 3.72 0.26 3.46 12.10 1.28

34H085 5 8.22 1.06 7.16 7.35 4.23 3.12 3.17 –0.87

34H097 5 — — — 13.10 11.78 1.32 — —

34H112 5 10.11 2.08 8.03 8.58 6.62 1.96 4.54 –1.53

34H117 5 8.14 –0.81 8.95 7.51 4.54 2.97 5.35 –0.63

34H122 5 7.30 0.03 7.27 — — — — —

34H125 5 — — — 6.46 4.45 2.01 — —

34H128 5 4.22 –4.73 8.95 — — — — —

34H133 5 –0.24 –8.62 8.38 — — — — —

34H204 5 21.27 23.63 –2.36 — — — — —

34H328 5 13.96 12.18 1.78 9.23 8.78 0.45 –3.40 –4.74

34H344 5 3.73 –10.61 14.34 4.63 3.25 1.38 13.86 0.90

34H345 5 7.77 –0.13 7.90 — — — — —

34H355 5 2.58 –4.00 6.58 4.36 3.88 0.48 7.88 1.78

Table A1. Simulated and observed ground-water levels, 1980 and 2000.—Continued

[Observed values for 1980 are for May; observed values for 2000 are for September; values in bold are for wells that have continuous recorders 

and water levels that are mean-monthly values for either May 1980 or September 2000; —, data]

Well 

number

Model 

layer

Simulated and observed ground-water levels 

and difference, in feet

Water-level change, 

in feet

1980 calibration 2000 calibration 1980 – 2000

Simulated Observed Difference Simulated Observed Difference Observed Simulated
78 Simulation of Ground-Water Flow in Coastal Georgia and Adjacent Parts of 

South Carolina and Florida—Predevelopment, 1980, and 2000

34H357 5 9.59 9.78 –0.19 6.99 8.74 –1.75 –1.04 –2.60

34H358 5 15.08 12.77 2.31 — — — — —

34H363 5 13.14 8.36 4.78 — — — — —

34H366 5 9.23 2.10 7.13 — — — — —

34H371 5 — — — 10.70 10.83 –0.13 — —

34H372 5 11.80 1.83 9.97 — — — — —

34H373 5 1.29 –11.08 12.37 3.67 –1.07 4.74 10.01 2.38

34H381 5 17.30 14.08 3.22 — — — — —

34H383 5 15.35 13.23 2.12 — — — — —

34H392 5 –0.86 –2.24 1.38 2.70 3.33 –0.63 5.57 3.56

34H393 5 12.67 7.69 4.98 9.98 8.53 1.45 0.84 –2.69

34H400 5 0.67 –9.04 9.71 — — — — —

34H401 5 –1.25 –10.04 8.79 — — — — —

34H403 5 — — — 10.36 10.12 0.24 — —

34H408 5 –0.53 3.54 –4.07 3.01 2.25 0.76 –1.29 3.54

34H410 5 6.67 6.23 0.44 5.76 7.85 –2.09 1.62 –0.90

34H424 5 –3.02 –9.26 6.24 0.98 1.04 –0.06 10.30 3.99

34H469 5 –6.85 –9.45 2.60 –0.75 5.56 –6.31 — —

34J009 5 12.61 5.05 7.56 — — — — —

34J021 5 10.74 10.44 0.30 — — — — —

34J029 5 12.84 12.41 0.43 8.30 8.09 0.21 –4.32 –4.55

34J051 5 11.47 12.31 –0.84 8.13 9.65 –1.52 –2.66 –3.34

34K012 5 12.75 12.47 0.28 — — — — —

34K073 5 13.29 10.51 2.78 7.68 1.97 5.71 –8.54 –5.60

34K081 5 13.24 14.82 –1.58 — — — — —

34K082 5 13.47 11.22 2.25 — — — — —

34K083 5 12.36 12.85 –0.49 — — — — —

34K084 5 12.80 9.01 3.79 — — — — —

34K085 5 12.21 13.63 –1.42 — — — — —

34K095 5 13.20 9.01 4.19 7.72 1.78 5.94 –7.23 –5.48

34L048 5 10.58 12.52 –1.94 4.76 8.06 –3.30 – 4.46 –5.81

34L060 5 7.54 6.65 0.89 1.68 –3.00 4.68 –9.65 –5.86

34L061 5 10.38 13.96 –3.58 4.49 4.17 0.32 –9.79 –5.89

34M049 5 1.95 –0.16 2.11 — — — — —

34M054 5 1.61 –1.88 3.49 — — — — —

34M056 5 3.01 3.58 –0.57 — — — — —

34M070 5 7.14 5.27 1.87 1.07 –6.32 7.39 –11.59 –6.07

34M075 5 4.56 1.43 3.13 –1.24 –9.49 8.25 –10.92 –5.81

34M076 5 5.99 4.45 1.54 0.11 –4.20 4.31 –8.65 –5.88

34N089 5 –1.36 –1.39 0.03 –7.02 –14.29 7.27 –12.90 –5.66

34N091 5 2.76 1.71 1.05 — — — — —

34P012 5 –11.32 –3.94 –7.38 — — — — —

34P014 5 –4.92 0.91 –5.83 — — — — —

34R039 5 — — — –8.91 –5.48 –3.43 — —

34U008 5 48.24 52.00 –3.76 — — — — —

34V004 5 — — — 66.19 74.60 –8.41 — —

Table A1. Simulated and observed ground-water levels, 1980 and 2000.—Continued

[Observed values for 1980 are for May; observed values for 2000 are for September; values in bold are for wells that have continuous recorders 

and water levels that are mean-monthly values for either May 1980 or September 2000; —, data]

Well 

number

Model 

layer

Simulated and observed ground-water levels 

and difference, in feet

Water-level change, 

in feet

1980 calibration 2000 calibration 1980 – 2000

Simulated Observed Difference Simulated Observed Difference Observed Simulated
Appendix A 79

35H037 5 20.44 20.93 –0.49 — — — — —

35H044 5 17.90 16.43 1.47 11.54 9.94 1.60 –6.49 –6.35

35K069 5 11.50 6.79 4.71 5.74 –0.79 6.53 –7.58 –5.75

35L067 5 8.38 3.18 5.20 — — — — —

35L068 5 7.32 7.97 –0.65 1.62 –2.83 4.45 –10.80 –5.70

35M013 5 2.61 –1.43 4.04 –2.82 –11.87 9.05 –10.44 –5.42

35N021 5 –4.56 –8.19 3.63 — — — — —

35N059 5 –2.84 –7.34 4.50 — — — — —

35P057 5 –3.38 –3.82 0.44 — — — — —

35P071 5 –18.16 –6.84 –11.32 — — — — —

35P085 5 –40.49 –20.15 –20.34 — — — — —

35Q043 5 –47.45 –23.57 –23.88 — — — — —

35R018 5 –29.06 –3.68 –25.38 –27.84 –18.07 –9.77 –14.39 1.22

35T003 5 — — — 6.35 31.40 –25.06 — —

36M018 5 –1.50 –6.81 5.31 –6.61 –16.89 10.28 — —

36N002 5 –4.20 –5.68 1.48 — — — — —

36P087 5 –48.71 –26.50 –22.21 — — — — —

36P091 5 –23.76 –19.13 –4.63 — — — — —

36P093 5 –21.44 –16.92 –4.52 — — — — —

36Q008 5 –114.15 –99.60 –14.55 –85.35 –90.48 5.13 9.12 28.80

36Q011 5 –114.76 –103.95 –10.81 — — — — —

36Q019 5 –56.58 –29.33 –27.25 –49.58 –38.06 –11.52

36Q020 5 –54.46 –29.11 –25.35 –48.73 –42.17 –6.56 –13.06 5.73

36Q287 5 –66.24 –45.92 –20.32 — — — — —

36Q300 5 –71.64 –45.51 –26.13 –58.33 –54.33 –4.00 — —

36S004 5 –21.17 –8.22 –12.95 — — — — —

37P005 5 –56.64 –45.12 –11.52 –52.38 –52.52 0.14 –7.40 4.26

37P006 5 –65.30 –51.62 –13.68 –59.10 –54.44 –4.66 –2.82 6.20

37P009 5 –53.78 –42.50 –11.28 — — — — —

37P013 5 –50.55 –37.22 –13.33 — — — — —

37P086 5 –41.40 –28.32 –13.08 — — — — —

37P114 5 — — — –47.38 –46.02 –1.36 — —

37Q006 5 –130.28 –130.13 –0.15 — — — — —

37Q012 5 –95.37 –94.44 –0.93 — — — — —

37Q015 5 –91.60 –91.14 –0.46 — — — — —

37Q016 5 –87.52 –85.82 –1.70 –82.00 — — — —

37Q033 5 –85.97 –77.95 –8.02 –74.13 –75.59 1.46 2.36 11.84

37Q034 5 –67.97 –56.07 –11.90 — — — — —

37Q043 5 –62.57 –51.92 –10.65 –59.72 –56.99 –2.73 –5.07 2.85

37Q066 5 –74.41 –73.62 –0.79 — — — — —

37Q090 5 –115.00 –108.92 –6.08 — — — — —

37Q160 5 –59.04 –48.53 –10.51 — — — — —

37Q185 5 — — — –98.41 –99.22 0.81 — —

38Q001 5 –25.14 –23.16 –1.98 — — — — —

38Q002 5 –25.48 –23.60 –1.88 –25.99 –30.36 4.37 –6.76 –0.51

38Q006 5 –43.90 –37.28 –6.62 — — — — —

39Q003 5 –16.97 –18.94 1.97 –17.84 –27.90 10.06 –8.96 –0.87

Table A1. Simulated and observed ground-water levels, 1980 and 2000.—Continued

[Observed values for 1980 are for May; observed values for 2000 are for September; values in bold are for wells that have continuous recorders 

and water levels that are mean-monthly values for either May 1980 or September 2000; —, data]

Well 

number

Model 

layer

Simulated and observed ground-water levels 

and difference, in feet

Water-level change, 

in feet

1980 calibration 2000 calibration 1980 – 2000

Simulated Observed Difference Simulated Observed Difference Observed Simulated
80 Simulation of Ground-Water Flow in Coastal Georgia and Adjacent Parts of 

South Carolina and Florida—Predevelopment, 1980, and 2000

BFT–121 5 8.72 23.19 –14.48 — — — — —

BFT–1810 5 — — — –1.95 –0.08 –1.87 — —

BFT–1813 5 — — — –3.97 –3.50 –0.47 — —

BFT–304 5 –6.80 –14.47 7.67 — — — — —

BFT–315-1 5 0.21 0.17 0.04 — — — — —

BFT–315-2 5 0.24 –0.83 1.07 — — — — —

BFT–429 5 –2.31 –2.81 0.50 –6.21 –5.90 –0.31 –3.09 –3.90

BFT–439 5 –2.86 –13.09 10.23 — — — — —

BFT–444 5 –0.25 –7.04 6.79 — — — — —

BFT–453 5 4.74 –2.44 7.18 — — — — —

BFT–786 5 0.39 1.96 –1.57 — — — — —

BFT–787 5 0.39 –2.42 2.81 — — — — —

BW–151 5 196.97 221.75 –24.78 — — — — —

BW–154 5 195.42 225.82 –30.40 — — — — —

BW–644 5 191.21 209.76 –18.55 — — — — —

BW–646 5 191.02 222.28 –31.26 — — — — —

BW–647 5 191.05 222.27 –31.22 — — — — —

BW–650 5 191.11 199.59 –8.48 — — — — —

BW–652 5 191.14 219.96 –28.82 — — — — —

BW–653 5 191.16 220.25 –29.09 — — — — —

BW–654 5 190.99 210.46 –19.47 — — — — —

COL–_97 5 59.22 45.46 13.76 — — — — —

D–3840 5 — — — –18.30 –13.25 –5.05 — —

HAM–83 5 37.98 9.97 28.01 31.56 4.40 27.16 –5.57 –6.42

N–62 5 — — — –5.67 –17.86 12.19 — —

16P004 7 275.72 255.58 20.14 — — — — —

20N005 7 194.48 181.66 12.82 — — — — —

20R003 7 207.76 211.03 –3.27 — — — — —

30U002 7 130.47 135.66 –5.19 — — — — —

32Y033 7 — — — 131.78 110.27 21.51 — —

33D073 7 — — — 6.81 3.58 3.23 — —

33H188 7 20.20 17.72 2.47 15.73 12.46 3.27 –5.26 –4.47

33H206 7 — — — 9.52 8.41 1.12 — —

33J044 7 13.61 17.63 –4.02 10.27 13.89 –3.62 –3.74 –3.35

34G036 7 23.93 23.62 0.31 17.39 20.09 –2.70 –3.53 –6.55

34H391 7 15.35 9.01 6.34 12.11 9.18 2.92 0.17 –3.25

34H399 7 18.20 3.03 15.17 13.77 1.76 12.01 –1.27 –4.43

34H436 7 — — — 9.56 11.70 – 2.14 — —

35P109 7 — — — –14.41 –21.83 7.42 — —

39Q024 7 — — — –17.46 –31.67 14.22 — —

Table A1. Simulated and observed ground-water levels, 1980 and 2000.—Continued

[Observed values for 1980 are for May; observed values for 2000 are for September; values in bold are for wells that have continuous recorders 

and water levels that are mean-monthly values for either May 1980 or September 2000; —, data]

Well 

number

Model 

layer

Simulated and observed ground-water levels 

and difference, in feet

Water-level change, 

in feet

1980 calibration 2000 calibration 1980 – 2000

Simulated Observed Difference Simulated Observed Difference Observed Simulated
Appendix B 81

To determine whether a transient calibration would be 

required to represent the ground-water flow system accurately, 

a preliminary version of the calibrated model was used to 

examine the rate of response of the simulated system to a 

change in stress using a reasonable value for storage coefficient, 

based on available data. If tests indicate that the flow system 

responds rapidly to changes in stress during representative time 

periods and, thus, behaves like a steady-state system, then transient simulation is neither efficient nor useful. Transient simulation would increase the complexity of the modeling effort by 

requiring the calibration of additional time-dependent parameters and multiple sets of observations. Because the purpose of 

the model is to examine the ultimate effect of pumping stress on 

the flow system, and not to depict how the system responds to 

these changes over time, steady-state simulation would be a 

more effective means to meet project objectives.

Some storage coefficient data are available for the study 

area, primarily for the Upper Floridan aquifer, but these data 

vary by several orders of magnitude and show no discernible 

spatial pattern. For the following tests, a constant storage 

coefficient value of 0.0004 was used based on the median value 

of available data for the region (Clarke and others, 2004). The 

median value is within the limits of suggested values for confined aquifers (Freeze and Cherry, 1979).

A test was designed to determine how rapidly a steadystate condition was approached at several locations. For this 

test, the initial condition was predevelopment steady state. 

Pumping conditions for 2000 were applied for five 1-year 

transient time steps, and the calculated heads at several locations were compared with the steady-state heads during 2000 

conditions. Figure B1 shows the difference between calculated 

head in layer 5, the Upper Floridan aquifer, at each time step and 

at the steady-state solution. At each of the locations shown, the 

calculated head after a 5-year transient simulation was very 

close to the calculated steady-state head.

Appendix B. Transient Response Testing

Another test was designed to determine whether the system 

is likely to behave in a transient or steady-state manner during 

the 1980–2000 simulation period, assuming reasonable hydraulic properties and using a time step reasonable for available 

pumping data. To simplify the model, the initial condition was 

predevelopment steady state, and four 5-year time steps followed, each at 2000 pumping conditions, although this represents a more extreme change than if initial conditions were set 

to 1980 steady state. The fraction of drawdown achieved relative to total steady-state drawdown is calculated as follows:

(predevelopment_head) – (transient_head)

(predevelopment_head) – (steady-state_head)

0

20

40

60

80

100

120

Initial

condition

1 2 3 45

YEARS

WATER LEVEL NORMALIZED TO 2000

STEADY-STATE WATER LEVEL, IN FEET

Chatham County

McIntosh County

Wayne County

Glynn County

Camden County

Figure B1. Water-level elevation relative to steady-state 

water level at 2000 pumping rates during five 1-year time steps 

at 2000 pumping rates at five Georgia locations. Value at 0 years 

is water level under steady-state predevelopment conditions 

relative to water level under steady-state 2000 conditions.
SOUTH

GEORGIA CAROLINA

FLORIDA

Model

area

GA

FL

SC

GA

70

80

90

EXPLANATION

Percent of steady-state 

 drawdown in the Upper 

 Floridan aquifer (layer 5) 

 reached during transient 

 simulation

Base from U.S. 

Geological Survey

1:100,000-scale digital data

Model boundary

80

0 25 50 MILES

0 50 KILOMETERS 25

N

82 Simulation of Ground-Water Flow in Coastal Georgia and Adjacent Parts of 

South Carolina and Florida—Predevelopment, 1980, and 2000

Figure B2. Percent of steady-state conditions reached in the Upper 

Floridan aquifer (layer 5) during transient simulation after 25 years of 

pumping using 5-year time steps at 2000 pumping conditions.

For example, a fractional drawdown value of 0.9 

indicates that 90 percent of total steady-state drawdown was achieved by the end of the 20-year transient 

simulation. Figure B2 shows that fractional drawdown 

in layer 5, the Upper Floridan aquifer, is greater than 

0.9 for almost the entire model area. For more realistic 

changes in stress, the transient response may be even 

more subdued. Near the northeastern offshore boundary, the fractional drawdown is less than 0.7 across a 

small offshore area, although maximum drawdown is 

less than 4 feet. A possible cause of the more transient 

response in this small area might be the thinness of the 

layer, which in combination with the constant storage 

coefficient results in a large specific storage value. 

The smaller fractional drawdown, however, was considered insignificant, and a steady-state approximation for the system was considered reasonable, 

because (1) total drawdown in this area is small, 

(2) data control on the hydrologic-unit thickness or 

hydraulic properties is limited, and (3) there are no 

potentiometric data. No further testing was warranted.

The rate of pumping change in these simulations 

is considerably more than what realistically occurs in 

the model area, except perhaps in localized areas; for 

example, agricultural areas where seasonal pumping 

rates may fluctuate substantially. These tests demonstrate that under extreme rates of pumping change, 

simulated heads for most of the model area very 

closely approach the steady-state heads within 5–20 

years. For this reason, the steady-state approximation 

is considered a reasonable way to simulate the flow 

system.

Reference Cited

Clarke, J.S., Leeth, D.C., Taylor-Harris, DáVette, 

Painter, J.A., and Labowski, J.L., 2004, Summary 

of hydraulic properties of the Floridan aquifer 

system in coastal Georgia and adjacent parts of 

South Carolina and Florida: U.S. Geological 

Survey Scientific Investigations Report 2004-5264, 

52 p.

Freeze, R.A., and Cherry, J.A., 1979, Groundwater: 

Prentice-Hall Inc., Englewood Cliffs, N.J., 604 p.

Prepared by USGS Georgia Water Science Center, 

Atlanta, Georgia.

Figure design and production by Caryl J. Wipperfurth 

and Bonnie J. Turcott.

Editing and page layout by Patricia L. Nobles.