U.S. GEOLOGICAL SURVEY

Water-Resources Investigations Report 

By Nicasio Sepúlveda

Simulation of Ground-Water Flow in the 

Intermediate and Floridan Aquifer Systems 

in Peninsular Florida

Tallahassee, Florida

2002

Prepared in cooperation with the

South Florida Water Management District

St. Johns River Water Management District

02-4009
Copies of this report can be 

purchased from:

U.S. Geological Survey

Branch of Information Services

Box 25286

Denver, CO 80225-0286

888-ASK-USGS

The use of firm, trade, and brand names in this report is for identification purposes only 

and does not constitute endorsement by the U.S. Geological Survey.

For additional information

write to:

District Chief

U.S. Geological Survey

Suite 3015

227 N. Bronough Street

Tallahassee, FL 32301

Additional information about water resources in Florida is available on the 

Internet at http://fl.water.usgs.gov

U.S. DEPARTMENT OF THE INTERIOR

GALE A. NORTON, Secretary

U.S. GEOLOGICAL SURVEY

CHARLES G. GROAT, Director
Contents III

CONTENTS

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

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

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

Previous Studies........................................................................................................................................................... 4

Acknowledgments ....................................................................................................................................................... 4

Description of Study Area ..................................................................................................................................................... 6

Groups of Physiographic Regions ............................................................................................................................... 6

Climate......................................................................................................................................................................... 6

Hydrogeologic Framework.................................................................................................................................................... 6

Ground-Water Flow System .................................................................................................................................................. 15

Altitude of the Water Table of the Surficial Aquifer System ...................................................................................... 18

Potentiometric Surface of the Intermediate Aquifer System....................................................................................... 22

Potentiometric Surface of the Upper Floridan Aquifer ............................................................................................... 25

Water-Level Measurements in the Lower Floridan Aquifer........................................................................................ 27

Chloride Concentrations in Ground Water .................................................................................................................. 27

Recharge to and Discharge from the Upper Floridan Aquifer .................................................................................... 30

Water Use........................................................................................................................................................... 30

Artificial Recharge from Drainage and Injection Wells .................................................................................... 34

Spring Flow ....................................................................................................................................................... 35

Recharge to the Unconfined Upper Floridan Aquifer ....................................................................................... 37

Discharge to Swamps ........................................................................................................................................ 39

Simulation of Ground-Water Flow ........................................................................................................................................ 42

Steady-State Approximation........................................................................................................................................ 42

Conceptual Model........................................................................................................................................................ 46

Boundary Conditions................................................................................................................................................... 55

Recharge and Discharge .............................................................................................................................................. 57

Initial Distribution of Hydraulic Properties from Local Models................................................................................. 60

Calibration of Ground-Water Flow Model .................................................................................................................. 67

Transmissivity of the Intermediate Aquifer System .......................................................................................... 70

Transmissivity of the Upper Floridan Aquifer .................................................................................................. 70

Transmissivity of the Lower Floridan Aquifer .................................................................................................. 71

Leakance of the Upper Confining Unit of the Intermediate Aquifer System.................................................... 74

Leakance of the Intermediate Confining Unit ................................................................................................... 75

Leakance of the Middle Confining and Middle Semiconfining Units .............................................................. 77

Recharge and Discharge Areas of the Upper Floridan Aquifer......................................................................... 77

Spring Flow ....................................................................................................................................................... 86

Potentiometric Surfaces..................................................................................................................................... 91

Ground-Water Flow Budget............................................................................................................................... 93

Sensitivity Analyses........................................................................................................................................... 93

Effects of Projected 2020 Ground-Water Withdrawals ............................................................................................... 93

Projected 2020 Ground-Water Withdrawals and Artificial Recharge................................................................ 97

Simulated 2020 Scenarios and Boundary Conditions ....................................................................................... 99

Projected 2020 Drawdowns............................................................................................................................... 102

Projected 2020 River and Spring Flow.............................................................................................................. 106

Projected 2020 Ground-Water Flow Budget ..................................................................................................... 110

Model Limitations ................................................................................................................................................................. 110

Summary and Conclusions .................................................................................................................................................... 112
IV Contents

References............................................................................................................................................................................. 113

Appendixes............................................................................................................................................................................ 117

A1. Location and extent of simulated areas in the intermediate aquifer system or intermediate confining 

unit compiled from local ground-water flow models ..................................................................................... 118

A2. Location and extent of simulated areas in the Upper Floridan aquifer compiled from local 

ground-water flow models.............................................................................................................................. 120

A3. Location and extent of simulated areas in the Lower Floridan aquifer compiled from local 

ground-water flow models.............................................................................................................................. 122

B. Description of wells equipped with continuous water-level recorders........................................................... 123

C. Description and flow measurements of Upper Floridan aquifer springs........................................................ 128

FIGURES

1-3. Maps showing:

1. Location of study area and Water Management District boundaries.............................................................. 3

2. Groups of physiographic regions.................................................................................................................... 7

3. Measured rainfall at National Oceanic and Atmospheric Administration stations from August 1993 

through July 1994 ........................................................................................................................................... 8

4. Diagram showing stratigraphic units, general lithology, and hydrogeologic units ................................................. 9

5-8. Maps showing:

5. Generalized extent of the unconfined areas of the Upper Floridan aquifer, the intermediate aquifer 

system, and thickness of the intermediate confining unit............................................................................... 11

6. Altitude of the top of the Upper Floridan aquifer........................................................................................... 12

7. Altitude of the base of the Upper Floridan aquifer......................................................................................... 13

8. Extent and areal configuration of the top of the Lower Floridan aquifer....................................................... 14

9. Diagram showing generalized hydrogeologic sections A-A' and B-B' showing conceptualized 

ground-water flow ................................................................................................................................................... 16

10-11. Maps showing:

10. Altitude of the base of the Floridan aquifer system ....................................................................................... 17

11. Lakes, streams, locations of stream gaging stations, and surficial-aquifer wells used to estimate 

the altitude of the water table of the surficial aquifer system......................................................................... 19

12. Diagram showing relations among water table, minimum water table, and land-surface altitude ......................... 20

13-14. Graphs showing:

13. Regressed and measured water-table altitudes for all groups of physiographic regions......................................... 21

14. Correlation between water table, minimum water table, and difference between land-surface altitude 

and minimum water table for group 4 and group 9 of physiographic regions ........................................................ 22

15-23. Maps showing:

15. Estimated altitude of the water table of the surficial aquifer system, average conditions for 

August 1993 through July 1994 ..................................................................................................................... 23

16. Location of wells equipped with continuous water-level recorders ............................................................... 24

17. Estimated potentiometric surface of the intermediate aquifer system, average conditions for 

August 1993 through July 1994 ..................................................................................................................... 26

18. Estimated potentiometric surface of the Upper Floridan aquifer, average conditions for 

August 1993 through July 1994 ..................................................................................................................... 28

19. Estimated altitude of water containing a chloride concentration of 5,000 milligrams per liter in the 

Floridan aquifer system .................................................................................................................................. 29

20. Distribution of recharge and discharge areas of the Upper Floridan aquifer, average conditions for 

August 1993 through July 1994 ..................................................................................................................... 31

21. Locations of Upper Floridan aquifer drainage and injection wells ................................................................ 32

22. Locations of Upper Floridan aquifer springs.................................................................................................. 36

23. Rivers in the unconfined areas of the Upper Floridan aquifer and locations of stream gaging stations ........ 38

24. Graphs showing approximation of base flow and surface runoff from daily recorded discharges at 

Suwannee River gaging stations 02319500, 02320500, and 02323500.................................................................. 40
Contents V

25-27. Maps showing:

25. Estimated minimum and maximum net recharge rates in the unconfined areas of the 

Upper Floridan aquifer ................................................................................................................................... 41

26. Areal extent of swamps in the unconfined areas of the Upper Floridan aquifer ............................................ 42

27. Parts of the uniform grid used to develop the ground-water flow model, shown in the vicinity 

Duval and Orange Counties............................................................................................................................ 44

28. Hydrographs showing water-level fluctuations in wells tapping unconfined areas of the 

Upper Floridan aquifer............................................................................................................................................ 45

29-36. Diagrams showing:

29. Layering scheme and representation of geologic units in local models 1 and 3, and corresponding 

layering scheme in the regional model ........................................................................................................... 47

30. Layering scheme and representation of geologic units in local models 2, 7, 10, and 13, and 

corresponding layering scheme in the regional model ................................................................................... 48

31. Layering scheme and representation of geologic units in local models 4 and 8, and corresponding 

layering scheme in the regional model ........................................................................................................... 49

32. Layering scheme and representation of geologic units in local models 5 and 12, and corresponding 

layering scheme in the regional model ........................................................................................................... 50

33. Layering scheme and representation of geologic units in local model 6, and corresponding layering 

scheme in the regional model ......................................................................................................................... 51

34. Layering scheme and representation of geologic units in local model 9, and corresponding layering 

scheme in the regional model ......................................................................................................................... 52

35. Layering scheme and representation of geologic units in local model 11, and corresponding layering 

scheme in the regional model ......................................................................................................................... 53

36. Layering scheme and representation of geologic units in local model 14, and corresponding 

layering scheme in the regional model ........................................................................................................... 54

37-46. Maps showing:

37. Specified heads along the lateral boundary cells of the Upper Floridan aquifer............................................ 56

38. Average ground-water withdrawal rates from the intermediate aquifer system, August 1993 

through July 1994 ........................................................................................................................................... 57

39. Average ground-water withdrawal rates from and injection rates to the Upper Floridan aquifer, 

August 1993 through July 1994 ..................................................................................................................... 58

40. Average ground-water withdrawal rates from the Lower Floridan aquifer, August 1993 

through July 1994 ........................................................................................................................................... 59

41. Cells where transmissivity of the intermediate aquifer system in some local models is either less 

than one-half or greater than twice the geometric mean of all transmissivities for the same cell.................. 61

42. Cells where transmissivity of the Upper Floridan aquifer in some local models is either less 

than one-half or greater than twice the geometric mean of all transmissivities for the same cell.................. 62

43. Cells where transmissivity of the Lower Floridan aquifer in some local models is either less 

than one-half or greater than twice the geometric mean of all transmissivities for the same cell.................. 63

44. Cells where leakance of the upper confining unit of the intermediate aquifer system in some 

local models is not within one order of magnitude of the geometric mean of all leakances for 

the same cell ................................................................................................................................................... 64

45. Cells where leakance of the intermediate confining unit in some local models is not within one 

order of magnitude of the geometric mean of all leakances for the same cell ............................................... 65

46. Cells where leakance of the middle semiconfining unit in some local models is not within one order 

magnitude of the geometric mean of all leakances for the same cell............................................................. 66

47-48. Graphs showing:

47. Bilinear interpolation used to determine simulated water levels at observation wells from simulated 

water levels at the center of cells............................................................................................................................. 68

48. Comparison of simulated to measured water levels in the intermediate aquifer system, Upper 

Floridan aquifer, and Lower Floridan aquifer for the calibrated model.................................................................. 69

49-65. Maps showing:

49. Transmissivity of the intermediate aquifer system from the calibrated model............................................... 71

50. Transmissivity of the Upper Floridan aquifer from the calibrated model ...................................................... 72

51. Transmissivity of the Lower Floridan aquifer from the calibrated model...................................................... 73
VI Contents

52. Leakance of the upper confining unit of the intermediate aquifer system from the calibrated model........... 74

53. Simulated vertical leakage rates to and from the intermediate aquifer system through the upper

confining unit, average August 1993 through July 1994 conditions .............................................................. 75

54. Leakance of the intermediate confining unit and the lower confining unit of the intermediate

aquifer system from the calibrated model ...................................................................................................... 76

55. Simulated vertical leakage rates to and from the Upper Floridan aquifer, average August 1993

through July 1994 conditions ......................................................................................................................... 78

56. Leakance of the middle confining and middle semiconfining units from the calibrated model .................... 79

57. Simulated vertical leakage rates to and from the Lower Floridan aquifer through the middle

confining and middle semiconfining units, average August 1993 through July 1994 conditions.................. 80

58. Specified stages along the simulated river cells in the unconfined areas of the Upper Floridan aquifer,

average August 1993 through July 1994 conditions....................................................................................... 81

59. Simulated riverbed conductance of river cells in the unconfined areas of the Upper Floridan aquifer ......... 82

60. Simulated lateral flow to and from the Upper Floridan aquifer across model boundaries,

average August 1993 through July 1994 conditions....................................................................................... 84

61. Location of differences between simulated and estimated recharge and discharge cells in the

Upper Floridan aquifer, average August 1993 through July 1994 conditions................................................ 85

62. Simulated recharge rates to the unconfined areas of the Upper Floridan aquifer, average

August 1993 through July 1994 conditions.................................................................................................... 86

63. Simulated potentiometric surface of the intermediate aquifer system, average August 1993

through July 1994 conditions ......................................................................................................................... 91

64. Simulated potentiometric surface of the Upper Floridan aquifer, average August 1993 through

July 1994 conditions....................................................................................................................................... 92

65. Simulated potentiometric surface of the Lower Floridan aquifer, average August 1993 through

July 1994 conditions....................................................................................................................................... 94

66. Diagram showing simulated volumetric flow budget for the intermediate aquifer system, Upper Floridan 

aquifer, and Lower Floridan aquifer, average August 1993 through July 1994 conditions .................................... 95

67. Graphs showing model sensitivity to changes in selected model parameters......................................................... 96

68-73. Maps showing:

68. Projected 2020 ground-water withdrawal rates from the intermediate aquifer system.................................. 99

69. Projected 2020 ground-water withdrawal rates from and injection rates to the Upper Floridan aquifer ....... 100

70. Projected 2020 ground-water withdrawal rates from the Lower Floridan aquifer ......................................... 101

71. Simulated intermediate aquifer system drawdowns from average August 1993 through July 1994

conditions to projected 2020 conditions......................................................................................................... 103

72. Simulated Upper Floridan aquifer drawdowns from average August 1993 through July 1994

conditions to projected 2020 conditions......................................................................................................... 104

73. Simulated Lower Floridan aquifer drawdowns from average August 1993 through July 1994

conditions to projected 2020 conditions......................................................................................................... 105

74. Diagram showing simulated volumetric flow budget for the intermediate aquifer system, Upper Floridan 

aquifer, and Lower Floridan aquifer for projected 2020 conditions for the specified-flux 

boundary conditions ................................................................................................................................................ 111
Contents VII

TABLES

1. Description of ground-water flow models considered in the study area ................................................................ 5

2. Multiple linear regression coefficients for the minimum water table and the difference between 

land-surface altitude and minimum water table...................................................................................................... 21

3. Multiple linear regression coefficients for September 1993 and May 1994 water-level averages in the 

Upper Floridan aquifer for each group of physiographic regions .......................................................................... 26

4. Estimated ground-water withdrawals and uses, by county and by Water Management District, from 

the intermediate aquifer system, Upper Floridan aquifer, and Lower Floridan aquifer, August 1993 

through July 1994 ................................................................................................................................................... 33

5. Estimated discharge rates from free-flowing wells, by county, in the Upper Floridan aquifer in the 

St. Johns River Water Management District, August 1993 through July 1994 ...................................................... 34

6. Estimated recharge rates for drainage wells to the Upper Floridan aquifer, by county, August 1993 

through July 1994 ................................................................................................................................................... 35

7. Surface-runoff rates calculated from hydrograph separation approximations applied to August 1993 

through July 1994 daily discharge data obtained from stream gaging stations in the unconfined areas 

of the Upper Floridan aquifer ................................................................................................................................. 39

8. Geographical information system coordinates of the corners of the ground-water flow model grid..................... 43

9. Differences in water levels measured at wells tapping the unconfined areas of the Upper Floridan aquifer 

and equipped with continuous water-level recorders.............................................................................................. 46

10. Water-level residual statistics for the calibrated model .......................................................................................... 70

11. Estimated and simulated base flow of rivers in the unconfined areas of the Upper Floridan aquifer, 

average August 1993 through July 1994 conditions............................................................................................... 83

12. Comparison of measured or estimated and residual flows from Upper Floridan aquifer springs, 

average August 1993 through July 1994 conditions............................................................................................... 87

13. Projected 2020 ground-water withdrawals and uses, by county and by Water Management District, 

from the intermediate aquifer system, Upper Floridan aquifer, and Lower Floridan aquifer ................................ 98

14. Projected 2020 recharge rates for drainage wells to the Upper Floridan aquifer by county .................................. 102

15. Projected 2020 base flow of rivers in the unconfined areas of the Upper Floridan aquifer for 

specified-flux boundary conditions ........................................................................................................................ 106

16. Comparison of simulated 2020 and simulated August 1993 through July 1994 flow of 

Upper Floridan aquifer springs for specified-flux boundary conditions ................................................................ 107
VIII Contents

CONVERSION FACTORS AND VERTICAL DATUM 

*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.

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

°C=(°F-32)/1.8

Sea level: In this report, “sea level” refers to the National Geodetic Vertical Datum of 1929 (NGVD of 1929)--a geodetic datum

derived from a general adjustment of the first-order level nets of both the United States and Canada, formerly called Sea Level

Datum of 1929.

Horizontal coordinate information (latitude-longitude) is referenced to the North American Datum of 1927 (NAD27).

Multiply By To obtain

Length

inch (in.) 2.54 centimeter

foot (ft) 0.3048 meter

mile (mi) 1.609 kilometer

Area

acre 0.4047 hectare

square mile (mi2

) 2.590 square kilometer 

Flow Rate

cubic foot per second (ft3

/s) 0.02832 cubic meter per second 

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

inch per year (in/yr) 25.4 millimeter per year

Hydraulic Conductivity

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

*Transmissivity

foot squared per day (ft2

/d) 0.09290 meter squared per day 

Leakance

foot per day per foot [(ft/d)/ft] 1.0 meter per day per meter

Acronyms and additional abbreviations used in report:

DEM digital elevation model

FPZ Fernandina permeable zone

FDEP Florida Department of Environmental Protection

FAS Floridan aquifer system

IAS intermediate aquifer system 

ICU intermediate confining unit

LFA Lower Floridan aquifer

MCU middle confining unit

MSCU middle semiconfining unit

mg/L milligrams per liter

MODFLOW U.S. Geological Survey Modular Three-Dimensional Ground-Water Flow Model

NOAA National Oceanic and Atmospheric Administration

ROMP Regional Observation and Monitoring Well Program

RMS root mean square

SJRWMD St. Johns River Water Management District

SFWMD South Florida Water Management District

SWFWMD Southwest Florida Water Management District

SR surface runoff

SAS surficial aquifer system

SRWMD Suwannee River Water Management District

UFA Upper Floridan aquifer 

USGS U.S. Geological Survey

UTM Universal Transverse Mercator
Abstract 1

Simulation of Ground-Water Flow in the Intermediate 

and Floridan Aquifer Systems in Peninsular Florida

By Nicasio Sepúlveda

Abstract

A numerical model of the intermediate and 

Floridan aquifer systems in peninsular Florida was 

used to (1) test and refine the conceptual understanding of the regional ground-water flow system; (2) develop a data base to support subregional 

ground-water flow modeling; and (3) evaluate 

effects of projected 2020 ground-water withdrawals on ground-water levels. The four-layer model 

was based on the computer code MODFLOW-96, 

developed by the U.S. Geological Survey. The top 

layer consists of specified-head cells simulating 

the surficial aquifer system as a source-sink layer. 

The second layer simulates the intermediate aquifer system in southwest Florida and the intermediate confining unit where it is present. The third and 

fourth layers simulate the Upper and Lower Floridan aquifers, respectively. Steady-state groundwater flow conditions were approximated for 

time-averaged hydrologic conditions from August 

1993 through July 1994 (1993-94). This period 

was selected based on data from Upper Floridan 

aquifer wells equipped with continuous waterlevel recorders. The grid used for the ground-water 

flow model was uniform and composed of square 

5,000-foot cells, with 210 columns and 300 rows.

The active model area, which encompasses 

about 40,800 square miles in peninsular Florida, 

includes areas of various physiographic regions 

classified according to natural features. Hydrogeologic conditions vary among physiographic 

regions, requiring different approaches to 

estimating hydraulic properties for different areas. 

The altitudes of water levels for the surficial aquifer system and heads in the Upper Floridan aquifer, for time-averaged 1993-94 conditions, were 

computed by using a multiple linear regression of 

measured water levels in each of the physiographic 

regions.

Ground-water flow simulation was limited 

vertically to depths containing water with chloride 

concentrations less than 5,000 milligrams per liter. 

Water-level altitudes in the Floridan aquifer system beneath which chloride concentrations exceed 

5,000 milligrams per liter were estimated from 

previously developed maps and analytical results 

of ground-water samples. Flow across the interface represented by this chloride concentration 

was assumed to be negligible.

The ground-water flow model was calibrated using time-averaged data for 1993-94 at 

1,624 control points, flow measurements or estimates at 156 springs in the study area, and baseflow estimates of rivers in the unconfined areas of 

the Upper Floridan aquifer obtained by using a 

generalized hydrograph separation of recorded 

discharge data. Transmissivity of the intermediate 

aquifer system, Upper Floridan aquifer, and Lower 

Floridan aquifer; leakance of the upper and lower 

confining units of the intermediate aquifer system, 

the intermediate confining unit, the middle confining unit, and the middle semiconfining unit; spring 

and riverbed conductances; and net recharge rates 

to unconfined areas of the Upper Floridan aquifer 

were adjusted until a reasonable fit was obtained. 
2 Simulation of Ground-Water Flow in the Intermediate and Floridan Aquifer Systems in Peninsular Florida

Root-mean-square residuals between computed 

and simulated heads in the intermediate aquifer 

system, Upper Floridan aquifer, and Lower Floridan aquifer were 3.47, 3.41, and 2.89 feet, respectively. The overall root-mean-square residual was 

3.40 feet. Simulated spring flow was 96 percent of 

the total measured (or estimated) spring flow in the 

study area.

Simulations were made to project waterlevel declines from 1993-94 to 2020 conditions. 

The calibrated flow model was used to simulate 

the potentiometric surfaces of the intermediate 

aquifer system, Upper Floridan aquifer, and Lower 

Floridan aquifer for 2020 using water-use projections provided by the Water Supply Assessment 

plans of the State Water Management Districts. 

Water-use projections for 2020 were based on estimated population growth and 1995 withdrawals. 

Heads in the Upper Floridan aquifer under projected 2020 water-use stresses were simulated for 

two scenarios: (1) assigning interpolated 1993-94 

heads along the lateral boundaries of the Upper 

Floridan aquifer; and (2) assigning 1993-94 simulated flux rates across the same boundaries.

Projected 2020 ground-water withdrawals 

for municipal, industrial, commercial, agricultural, and self-supplied domestic uses was approximately 3,400 million gallons per day, an increase 

of about 36 percent from 1993-94. The largest projected drawdown in the potentiometric surface of 

the Upper Floridan aquifer, for both scenarios, was 

simulated in Orange County, with a drawdown of 

10 feet in the central part of the County. Projected 

drawdowns of 6 feet were simulated in parts of 

Duval and Polk Counties.

INTRODUCTION

The intermediate and Floridan aquifer systems 

are the principal sources of water supply in much of 

peninsular Florida. As the population of the State 

continues to grow, the demand for water continues to 

increase. In some areas of Florida, decreasing water 

levels and increasing mineralization of ground water 

have become problems for local and state water-management officials. Ground-water flow models are 

important tools for assessing the effects of present and 

future ground-water development. For this reason, a 

number of ground-water flow models have been developed for areas within peninsular Florida; however, 

these areally extensive models are of a relatively coarse 

resolution. In addition, because Florida comprises several Water Management Districts, most ground-water 

modeling efforts have been focused within the boundaries of individual Districts, thus reducing the potential 

to simulate inter-District ground-water flow under 

current and projected stresses. This is particularly true 

along the common boundaries of the St. Johns River 

Water Management District (SJRWMD), Southwest 

Florida Water Management District (SWFWMD), 

South Florida Water Management District (SFWMD), 

and the Suwannee River Water Management District 

(SRWMD). Present and projected development of the 

ground-water resources in these Districts require reliable estimates of inter-District ground-water flow. As a 

result, the U.S. Geological Survey (USGS) began a 

5-year project in 1996 in cooperation with SJRWMD 

and SFWMD to develop a fine-resolution, areally 

extensive ground-water flow model of the intermediate 

and Floridan aquifer systems extending from Camden 

and Charlton Counties, Georgia, to just south of Martin 

County in south Florida (fig. 1).

The objective of developing such a model is to 

refine the conceptual understanding of regional 

ground-water flow in the intermediate aquifer system 

and the Floridan aquifer system. The data base of 

hydraulic properties resulting from the model could be 

used as input to an interpolation algorithm that extracts 

specific hydraulic properties and data to obtain appropriate general-head boundaries, specified fluxes, or initial estimates of hydraulic properties needed to develop 

fine-resolution ground-water flow studies on smaller 

scales. Leake and Claar (1999) developed computer 

programs to generate input files for a local model of 

finer resolution from the input files of a regional model 

of coarser resolution.

Hydrologic conditions reflected by the time 

average from August 1993 to July 1994 (1993-94) 

potentiometric-surface maps, ground-water withdrawals, and water-level and spring flow measurements constitute the steady-state approximation referenced in this 

report. The rationale for selecting the 1993-94 period is 

explained later in the report.
Introduction 3

Figure 1. Location of study area and Water Management District boundaries.
4 Simulation of Ground-Water Flow in the Intermediate and Floridan Aquifer Systems in Peninsular Florida

Purpose and Scope

This report presents the results of a numerical 

model used to simulate the regional ground-water flow 

system in peninsular Florida. The model was used to 

(1) refine the conceptual understanding of the intermediate aquifer system and the Floridan aquifer system; 

(2) develop a data base to support subregional groundwater flow modeling; and (3) evaluate relative effects 

of 2020 ground-water withdrawals on ground-water 

levels for which the model was constructed. Hydrogeologic data are presented, including a conceptual model 

of the flow system and applications of a finite-difference flow model based on this conceptualization. The 

flow model simulations are designed to characterize the 

complex four layer aquifer system that underlies the 

study area. Discussions in this report include modeling 

procedures, imposition of boundary conditions, calibration strategies, rationale for a steady-state assumption, sensitivity analyses, volumetric flow estimates 

among hydrogeologic units, and specific areas where 

substantial water-level declines could occur based on 

2020 projected ground-water withdrawals.

Geographical information system data bases or 

coverages (Environmental Systems Research Institute, 

Inc., 1997) were developed to manage spatially distributed information of hypsography and hydrography that 

covered the study area. Digital coverages were projected into the same coordinate system to achieve consistency of coordinate systems among coverages. The 

coordinate system of all coverages was the Universal 

Transverse Mercator (UTM) projection, zone 17 of the 

Florida coordinate system, west zone (Snyder, 1983). 

The 1927 North American Datum was used for all 

coverages generated in this study; the unit length was 

feet (ft).

Multiple linear regressions were used to generate 

the altitude of the water table of the surficial aquifer 

system, and the time-averaged heads in the intermediate aquifer system and the Floridan aquifer system, 

which comprises the Upper Floridan aquifer and the 

Lower Floridan aquifer. The regressions applied to the 

surficial aquifer system allowed for the estimation of 

the altitude of the water table at any point in the study 

area. The regressions applied to the intermediate aquifer system and the Upper Floridan aquifer allowed for 

the generation of the potentiometric surfaces of these 

two aquifers.

Previous Studies

Numerous investigations of ground-water flow 

in the intermediate aquifer system and the Floridan 

aquifer system in peninsular Florida have been conducted. Fourteen flow models, developed from these 

investigations, provided initial estimates of the areal 

distribution of hydraulic properties for the regional 

model discussed in this report (table 1). The extent of 

those flow models encompasses a large part of the 

study area (appendix A). Conceptualizations of the 

ground-water flow system differed among the various 

models.

The surficial aquifer system was simulated as a 

constant-head source-sink bed in 12 of the 14 groundwater flow models listed in table 1. Models developed 

by Hancock and Basso (1993) and Yobbi (1996) simulated the surficial aquifer system as an active layer, and 

calibrated to water levels measured at wells tapping the 

surficial aquifer, stream stages, and lake elevations.

Flow in parts of the intermediate aquifer system 

was simulated in southwest Florida (appendix A1) by 

Ryder (1985), Barcelo and Basso (1993), Metz (1995), 

and Yobbi (1996), and in parts of north-central Florida 

by Groszos and others (1992). The simulated transmissivity value for the intermediate aquifer system in 

north-central Florida was characteristic of a low-permeability confining unit (Motz, 1995). In this study, the 

intermediate aquifer system in north-central Florida is 

simulated as part of the intermediate confining unit.

Ground-water flow in the Upper Floridan aquifer 

was simulated by models developed in all of the studies 

listed in table 1 (appendix A2). The Upper Floridan 

aquifer is the most developed aquifer in the study area 

(Ryder, 1985; Tibbals, 1990). Simulation of groundwater flow in the Lower Floridan aquifer (appendix A3) 

was limited because of a substantially smaller number 

of observation wells tapping the Lower Floridan aquifer.

Acknowledgments

The author would like to thank the SFWMD, 

SJRWMD, and SWFWMD for providing the wateruse, water-quality, and ground-water flow model data 

bases used in this study. The author also wants to thank 

Ronald Ceryak of SRWMD for providing information 

on hydraulic properties of aquifers in the SRWMD area.
Introduction 5

Table 1. Description of ground-water flow models considered in the study area

[Abbreviations used in layering description: SAS, surficial aquifer system; IAS, intermediate aquifer system; UFA, Upper Floridan aquifer; LFA, Lower 

Floridan aquifer; SH, specified head; UT, uniform transmissivity; FPZ, Fernandina Permeable Zone]

Model

number

Authors

(year) General location

Number of

Grid type Layering 

rows columns description

1 Grubb and Rutledge 

(1979)

Parts of Polk, Lake, 

Sumter, Hernando, and 

Pasco Counties

36 40 Uniform cell size of 

5,400 by 6,075 feet

SAS, layer 1, SH

UFA, layer 2

2 Ryder 

(1985)

West-central Florida 49 32 Uniform cell size of 

4 miles by 4 miles

SAS, layer 3, SH

IAS, layer 2

UFA, layer 1

3 Fretwell 

(1988)

Pasco County 38 54 Uniform cell size of 

1 square mile

SAS, layer 1, SH

UFA, layer 2

4 Yobbi 

(1989)

Citrus and Hernando 

Counties

22 18 Uniform cell size of 

2 miles by 2 miles

UFA, one-layer model

5 Tibbals 

(1990)

East-central Florida 50 24 Uniform cell size of 

4 miles by 4 miles

SAS, layer 3, SH

UFA, layer 2

LFA, layer 1

6 Lukasiewicz 

(1992)

Martin and St. Lucie 

Counties

54 53 Uniform cell size of 

1 square mile

SAS, layer 1, SH

UFA, layer 2

LFA, layer 3

LFA, layer 4, SH

7 Barcelo and Basso 

(1993)

Southwest Florida 56 60 Uniform cell size of 

2 miles by 2 miles

SAS, layer 1, SH

IAS, layer 2

UFA, layer 3

8 Blanford and Birdie 

(1993)

Hernando County 33 43 Uniform cell size of 

1 square mile

UFA, one-layer model

9 Hancock and Basso 

(1993)

Parts of Hernando, Pasco, 

Hillsborough, and 

Pinellas Counties

62 69 Variable cell size, 

ranging from 2,640 by 

2,640 feet to 1 square 

mile

SAS, layer 1

UFA, layer 2

UFA, layer 3

 10 Metz 

(1995)

Hardee and DeSoto 

Counties

47 46 Uniform cell size of 

5,390 by 6,050 feet

SAS, layer 1, SH

IAS, layer 2

UFA, layer 3

11 Motz 

(1995)

North-central Florida 53 54 Variable cell size, 

ranging from 5,000 by 

5,000 feet to 15,000 by 

20,000 feet

SAS, layer 1, SH

IAS, layer 2, UT

UFA, layer 3

LFA, layer 4

FPZ, layer 5

12 Murray and Halford 

(1996)

Orange, Seminole, and 

parts of Volusia, Lake, 

and Osceola Counties

40 55 Uniform cell size of 

5,322 by 6,050 feet

SAS, layer 1, SH

UFA, layer 2

LFA, layer 3

13 Yobbi 

(1996)

Parts of Polk, Osceola, 

Hardee, De Soto, Highlands, and Glades 

Counties

86 41 Uniform cell size of 

1 square mile

SAS, layer 1

IAS, layer 2

UFA, layer 3

14 Durden 

(1997)

Northeast Florida 68 35 Variable cell size, 

ranging from 5,222 by 

6,057 feet to 18,280 by 

23,499 feet

SAS, layer 1, SH

UFA, layer 2

LFA, layer 3

FPZ, layer 4
6 Simulation of Ground-Water Flow in the Intermediate and Floridan Aquifer Systems in Peninsular Florida

DESCRIPTION OF STUDY AREA

The study area (fig. 1) extends about 284 miles 

(mi) from Charlton and Camden Counties, Georgia, in 

the north to near the Palm Beach - Martin County line 

in south Florida. The west-to-east extent of the study 

area spans about 200 mi from the Gulf of Mexico to the 

Atlantic Ocean. The study area encompassed about 

40,800 square miles (mi2). Some offshore areas in 

northeast and southwest Florida are considered to be 

part of the study area because hydraulic data suggest 

these areas may affect ground-water flow in the Upper 

Floridan aquifer.

The land-surface altitude in the study area ranges 

from sea level to about 285 ft in an area in south-central 

Polk County. About 75 percent of the land surface in 

the study area has an altitude less than 100 ft, and about 

40 percent has an altitude less than 50 ft.

Groups of Physiographic Regions

The study area is subdivided into 52 distinct 

physiographic regions (White, 1970). Although the 

main physiographic regions of the study area generally 

correspond to distinct hydrogeologic regions, the delineation of physiographic features in an area of low relief, 

such as Florida, can be difficult. The delineations of 

physiographic regions by White (1970), therefore, were 

based on a combination of natural features, rather than 

on single elevation contours. The 52 physiographic 

regions delineated by White (1970) were grouped into 

10 generalized regions based on geomorphology and 

the correlation of water levels between physiographic 

regions (fig. 2). Not all regions are contiguous.

Climate

The climate of the study area is classified as 

subtropical and is characterized by warm, normally 

wet summers and mild, dry winters. Maximum temperatures usually exceed 90 °F during the summer, but 

may fall below freezing for several days in the winter 

in the northern and central parts of the study area 

(fig. 1). The 30-year average (1961-90) annual rainfall 

for the study area, computed from rainfall data collected from National Oceanic and Atmospheric 

Administration (NOAA) stations, is 51.20 inches per 

year (in/yr). Measured rainfall at 53 NOAA stations in 

the study area averaged 53.23 inches (in.) from August 

1993 to July 1994 (fig. 3), which was 4 percent higher 

than the 30-year average. Absolute differences between 

rainfall measured in 1993-94 and the long-term average 

(1961-90) at individual stations were less than 10 in. at 

all but five stations.

HYDROGEOLOGIC FRAMEWORK

Comprehensive descriptions of the hydrogeology in all or in parts of the study area were presented 

by Ryder (1985), Miller (1986), Meyer (1989), Sprinkle (1989), Tibbals (1990), and Arthur and others 

(2001). This report presents a brief description of the 

hydrogeologic framework of the Floridan Aquifer 

system (FAS) and underlying and overlying units, 

including areal variations in thickness and occurrence throughout Florida.

The surficial aquifer system (SAS), which is the 

uppermost water-bearing hydrogeologic unit, includes 

sediments of Holocene, Pleistocene, and Pliocene ages 

(fig. 4). The SAS mostly consists of variable amounts 

of sand, sandy clay, shell beds, silt, and clay. Limestone 

units within the SAS are primarily in southwest Florida. In coastal areas, the SAS consists of cemented 

shell and shelly marl. The SAS extends throughout 

most of the study area, except where the Upper Floridan aquifer (UFA) is unconfined, and is used for water 

supply only in coastal areas where the UFA contains 

brackish water. The SAS is used primarily for individual household water supply in areas where the intermediate aquifer system (IAS) and the FAS are deep or 

contain poor quality water. In some places the clays in 

the SAS are thick and continuous enough to divide the 

SAS into two or three separate layers, but generally the 

aquifer system is undivided. In most parts of central, 

east, and northeast Florida, the SAS either provides 

recharge or receives discharge from the UFA.

Carbonate rocks of Miocene age within the intermediate confining unit (ICU) in southwest Florida are 

permeable enough to form a productive aquifer, the 

IAS. The IAS underlies the SAS and extends throughout most of southwest Florida. The unit consists mainly 

of clastic sediments interbedded with carbonate rocks 

that generally coincide with the Hawthorn Group 

(fig. 4). Although the IAS is less permeable than the 

underlying UFA, the carbonate rock units within the 

IAS are sufficiently permeable and productive to constitute a water-supply source. Confining beds that overlie the UFA and underlie the SAS limit the vertical 

extent of the IAS in west-central Florida. The thickness 

of the IAS varies from about 25 ft in parts of Hillsborough and Polk Counties to about 400 ft in Charlotte 

County (Ryder, 1985). The thickness generally 

increases north to south (Ryder, 1985).
Hydrogeologic Framework 7

Figure 2. Groups of physiographic regions (modified from White, 1970, plate 1).
8 Simulation of Ground-Water Flow in the Intermediate and Floridan Aquifer Systems in Peninsular Florida

Figure 3. Measured rainfall at National Oceanic and Atmospheric Administration stations from August 

1993 through July 1994.
Hydrogeologic Framework 9

Figure 4. Stratigraphic units, general lithology, and hydrogeologic units.
10 Simulation of Ground-Water Flow in the Intermediate and Floridan Aquifer Systems in Peninsular Florida

In areas away from southwest Florida, beds of 

clay and carbonate rocks of Pliocene and Miocene ages 

form the ICU. The hydrogeologic units, ICU and IAS, 

within the Hawthorn Group (fig. 4) are differentiated 

based on the permeability of the rock. In contrast to the 

IAS, the ICU is considerably less permeable. The ICU 

and the IAS coalesce at the boundaries of the IAS. 

The map showing the estimated thickness of the ICU 

generated by Miller (1986) was highly regionalized and 

based on sparse data. The more localized maps of 

Planert and Aucott (1985), Lukasiewicz (1992), and 

Spechler (1995) were used to modify Miller’s (1986) 

map of the ICU thickness in southwest, east-central, 

south, and northeast Florida, respectively. The resulting 

map shows that thickness varies from 0 ft in the northwest part of the study area (fig. 5) to about 750 ft in the 

southeast. The UFA is considered to be unconfined in 

areas where the ICU is absent or very thin, including 

areas along the Withlacoochee River in west-central 

Florida and along parts of the Hillsborough River. The 

thickness of the ICU within the areal extent of the IAS 

represents the cumulative thickness of the IAS and 

overlying and underlying confining beds of the IAS 

(fig. 5). The seaward extent of the IAS coincides with 

those areas where the transmissivity was simulated to 

be higher than 100 feet squared per day (ft2

/d) by 

Barcelo and Basso (1993).

A thick sequence of limestone and dolomitic 

limestone of Oligocene and Eocene ages and having 

variable permeability forms the FAS. The FAS is the 

principal source of ground water in the study area. The 

aquifer system ranges in thickness from about 1,300 ft in 

the northwest part of the study area to about 3,500 ft in 

southwest Florida (Miller, 1986). The FAS is divided 

into two aquifers of relatively high permeability, referred 

to as the UFA and the Lower Floridan aquifer (LFA). 

These aquifers are separated by a less permeable unit 

called the middle confining unit (MCU) in west-central 

Florida and in the northwest part of the study area, and 

the middle semiconfining unit (MSCU) in east-central 

Florida. The altitude of the top of the UFA, defined as the 

first occurrence of vertically persistent, permeable, consolidated carbonate rocks, ranges from 100 ft above to 

850 ft below sea level (fig. 6). The top of the UFA coincides either with the top of the Suwannee Limestone or 

the top of the Ocala Limestone, depending on location 

(fig. 4). The map in figure 6 showing the altitude of the 

top of the UFA, as identified by Miller (1986), was 

refined using the results of more localized studies by 

Knochenmus and Hughes (1976), Buono and Rutledge 

(1979), Shaw and Trost (1984), Planert and Aucott 

(1985), Navoy and Bradner (1987), Schiner and others 

(1988), Phelps (1990), Lukasiewicz (1992), Spechler 

(1993, plate 1), and Bradner (1994).

Lithologic data that establish the base of the UFA 

are less numerous than data defining the top of the UFA. 

A generalized contour map of the base of the UFA 

(fig. 7) was revised from Miller (1986) using additional 

information from Tibbals (1990) and files of the 

SJRWMD (Brian McGurk, SJRWMD, written commun., 1997). The base of the UFA ranges in depth from 

about 250 ft below sea level in parts of Marion and 

Sumter Counties to 2,100 ft below sea level in Charlton 

County, Ga., and in Charlotte County, Fla. (fig. 7). The 

base of the UFA is marked by the top of either the MCU, 

denoted analogously to Miller’s (1986) notation by 

units II, III, and IV in west-central Florida and the 

northwest part of the study area, or the top of the 

MSCU, denoted by unit I in east-central Florida.

Rather than a single low-permeability unit separating the UFA and the LFA, several units of regional 

extent separate the UFA from the LFA (figs. 7 and 8). 

Common boundaries of the MCU and MSCU delineate 

the approximate updip limit of the Lower Floridan 

aquifer (fig. 8). Any of these regionally extensive lowpermeability units may contain thin layers of moderate 

to high permeability. These confining units are not continuous and do not necessarily consist of the same rock 

type everywhere.

In east-central Florida, the UFA and LFA are 

separated by the MSCU, a sequence of somewhat 

permeable, soft, chalky limestone that locally contains 

some gypsum and chert and commonly is partially 

dolomitized (fig. 4). Although the MSCU, zone I in figure 7, is considered to be the leakiest of all confining 

units in the study area (Miller, 1986), the hydraulic 

connection between the MSCU and the LFA varies 

from place to place. This semiconfining unit is thin or 

absent in the northwest part of the study area (fig. 8), 

but is as much as 600 to 800 ft thick in the south-central 

part of the study area (Tibbals, 1990).

In west-central Florida and in the northwest part 

of the study area, the UFA and LFA are separated by the 

MCU, which is composed of gypsiferous dolomite and 

dolomitic limestone of considerably lower permeability than that of the MSCU in east-central Florida 

(Miller, 1986). The MCU, zones II through IV in 

figure 7, limits the movement of ground water between 

the UFA and LFA. At some locations, the confining 

unit is a very fine-grained limestone; in other places the 

MCU is a dolomite with pore spaces filled with anhydrite. The presence of intergranular evaporites in the 

MCU suggests that the exchange of vertical fluxes 

between the UFA and LFA could be higher in east-central Florida than in west-central Florida. The UFA and 

LFA merge vertically into one aquifer where no MCU 

or MSCU is present (fig. 8).
Hydrogeologic Framework 11

Figure 5. Generalized extent of the unconfined areas of the Upper Floridan aquifer, the intermediate 

aquifer system, and thickness of the intermediate confining unit.
12 Simulation of Ground-Water Flow in the Intermediate and Floridan Aquifer Systems in Peninsular Florida

Figure 6. Altitude of the top of the Upper Floridan aquifer.
Hydrogeologic Framework 13

Figure 7. Altitude of the base of the Upper Floridan aquifer.
14 Simulation of Ground-Water Flow in the Intermediate and Floridan Aquifer Systems in Peninsular Florida

Figure 8. Extent and areal configuration of the top of the Lower Floridan aquifer.
Ground-Water Flow System 15

A discontinuity in the surface of the MCU can be 

seen along section in figure 9. Although it is not 

certain whether the MCU in southwest Florida and the 

MSCU in east-central Florida merge, the information 

shown in section does not preclude this possibility 

(fig. 9). Given the contrast in permeability between the 

MCU and the MSCU, ground-water exchange between 

the UFA and LFA could be quite variable in areas near 

this discontinuity (fig. 9). For example, exchange of 

ground water between the UFA and LFA probably is 

higher in east-central Florida than in southwest Florida 

because the permeability of the MSCU is higher than the 

permeability of the MCU.

The altitude of the top of the LFA ranges from 

-400 ft in Marion County to -2,500 ft along the Gulf 

Coast near Sarasota County (Miller, 1986; fig. 8). 

Discontinuities in the configuration of the top of the 

LFA are due to discontinuities in the configurations of 

the MCU and the MSCU in the study area. In northeast 

Florida, the LFA is subdivided into two zones, the upper 

zone of the LFA and the Fernandina permeable zone 

(FPZ). In southeast-central Florida, a localized productive zone called the Boulder zone occurs within the LFA.

A base of generally low-permeability dolomite 

and evaporite beds of Paleocene age form the sub-Floridan confining unit, or the base of the FAS. This base is 

defined as the first occurrence of vertically persistent 

beds of anhydrite or, in their absence, the top of the transition from generally permeable carbonate rocks to 

much less permeable gypsiferous and anhydritic carbonate beds (Miller, 1986). These beds of very low 

permeability serve as the hydraulic base of the FAS 

and, in the study area, range in altitude from about 

-1,200 ft in the northwest part of the study area to 

about -4,100 feet in south Florida (Miller, 1986; fig. 10).

GROUND-WATER FLOW SYSTEM

Average hydrologic conditions from August 1993 

to July 1994 were used as the basis for discussion of the 

general ground-water flow characteristics in the study 

area. Hydrologic conditions may change from one time 

frame to another depending on rainfall amounts, 

ground-water withdrawal patterns, and substantial 

changes in surface-water discharge or recharge patterns. 

The rationale for selecting this period is presented later 

in the report. Unless otherwise specified, the term “average hydrologic conditions” in this report refers to 1993-

94 average conditions.

A-A'

B-B'

The assessment of ground-water flow was 

achieved by estimating the altitude of the water table of 

the SAS, and the potentiometric surfaces of the IAS and 

the UFA. The SAS generally is recharged by rainfall, 

irrigation, and, in areas where the water table is below 

the potentiometric surface of the underlying UFA, by 

diffuse upward leakage from the UFA. The assessment 

of vertical flow between the UFA and the LFA and flow 

within the LFA itself could be conducted only at a few 

sites because of the paucity of water-level measurements available for the LFA.

Ground-water flow in areas where chloride concentration exceeds 5,000 milligrams per liter (mg/L) 

was not considered to be part of the flow system in this 

study, based on the fact that advective flow in such areas 

is considered to be lower than in areas where chloride 

concentrations are lower than 5,000 mg/L. Groundwater density increases associated with higher chloride 

concentrations generally decrease the potential of water 

movement. Thus, transmissivity is lower than would be 

the case if the entire FAS contained freshwater.

Recharge to or discharge from the UFA occurs 

mostly through infiltration to or from the ICU, where 

this unit is present. The leakage rate to the UFA in confined areas is a function of the hydraulic gradient 

between the SAS and UFA and the vertical conductance 

of the ICU. Recharge to the UFA in the northwest part 

of the study area, where the ICU is absent, occurs in the 

form of net aquifer recharge from rainfall infiltration 

(Ryder, 1985; Fretwell, 1988; Yobbi, 1989; Blandford 

and Birdie, 1993; and Motz, 1995).

The spatial variability of transmissivities, as well 

as vertical leakance, in the FAS is quite high. Transmissivities of the UFA, derived from the models listed in 

table 1, range from 2,000 ft2

/d in St. Lucie County 

(Lukasiewicz, 1992) to 13,000,000 ft2/d in Citrus 

County (Yobbi, 1989). Simulated transmissivities in the 

LFA range from 33,000 ft2

/d in parts of east-central 

Florida (Tibbals, 1990) to 780,000 ft2/d in parts of 

northeast Florida (Durden, 1997). Vertical leakance of 

the upper confining unit of the IAS, based on models in 

table 1, range from 1.0 x 10-6 (ft/d)/ft in parts of Hardee 

and De Soto Counties (Metz, 1995) to 1.3 x 10-1 (ft/d)/ft 

in the Gulf of Mexico (Barcelo and Basso, 1993). Similarly, simulated vertical leakance values of the ICU and 

the MSCU range over five orders of magnitude 

(Durden, 1997; Motz, 1995; Lukasiewicz, 1992).
1

6SimulationofGround-WaterFlowintheIntermediateandFloridanAquiferSystemsinPeninsularFlorida

Figure 9. Generalized hydrogeologic sections A-A' and B-B' showing conceptualized ground-water flow (modified from Miller, 1986; refer to figure 5 for locations of 

sections).
Ground-Water Flow System 17

Figure 10. Altitude of the base of the Floridan aquifer system (modified from Miller, 1986).
18 Simulation of Ground-Water Flow in the Intermediate and Floridan Aquifer Systems in Peninsular Florida

Altitude of the Water Table of the 

Surficial Aquifer System

In this study, the altitude of the water table was 

estimated to specify the water levels in the SAS. 

Water levels in the SAS are not actively simulated in 

this model. Instead, the SAS was simulated as a 

source-sink layer with specified heads as one of the 

hydraulic properties determining the leakage rates to 

and from the underlying layer, namely the ICU or the 

IAS.

A commonly used algorithm for generating the 

altitude of the water table in an unconfined aquifer is 

to perform a linear regression between the measured 

water levels and the land-surface altitude. This 

algorithm, however, generally fails to provide reliable estimates in upland areas of low recharge or 

high hydraulic conductivity. In such areas, landsurface altitudes and water levels generally are not 

correlated.

The algorithm presented herein introduces the 

concept of the “minimum water table” to refer to the 

surface interpolated strictly from the measured altitude at drains in the SAS such as streams and lakes. 

This minimum water table becomes one of the variables used in the regression of the water table. To 

account for areas where the water table emulates 

land-surface altitude, a second variable is added to 

the regression: the vertical distance between landsurface altitude and the minimum water table. A limitation of this algorithm is its inability to estimate the 

water table in areas where ground-water mounds are 

formed by perched layers in the SAS.

The altitude of the water table was approximated by using a series of multiple linear regressions 

(one for each group of physiographic regions) among 

the measured levels in SAS wells, the interpolated 

minimum water table, and the difference between 

land-surface altitude and the minimum water table. 

Water-level measurements at SAS wells were compiled from data bases of the SJRWMD, SFWMD, 

SWFWMD, SRWMD, and USGS. The minimum 

water-table surface was interpolated from measured 

or estimated stages at lakes and streams (fig. 11). A 

digital land-surface elevation model was generated 

from digitized hypsography obtained from the 

SJRWMD, SFWMD, SWFWMD, and USGS.

Average lake elevations for 1993-94 were 

computed for 544 gaged lakes in the study area 

(fig. 11). Average river stages for the same period 

were computed for 233 stream gaging stations 

(fig. 11). Gaged rivers were divided in segments 

according to the location of the stream gaging stations. In cases where upstream or downstream end 

nodes of the river segments coincided with a lake, the 

lake elevation was used as the river stage at the node. 

The river stage was computed at all discrete nodes 

located along the meander of each river segment 

through linear interpolation of measured river stages. 

The computed lake elevations and river stages were 

assumed to be representative of the water-table elevation at the same sites. All lake elevations, river 

stages, and water levels were referenced to the 

National Geodetic Vertical Datum of 1929.

The digital representation of hypsography for 

the study area was generated from 5-ft contour interval hypsography digitized by SJRWMD, SFWMD, 

SWFWMD, and SRWMD from 7.5-minute USGS 

topographic quadrangle maps. A digital elevation 

model (DEM) of square cells 100-ft wide was generated by using the digitized hypsography, lake elevations from gaged lakes, and river stages computed 

along the meanderings of gaged rivers. The DEM 

was generated by using quintic splines. Using the 

DEM, the land-surface altitude could be interpolated 

at any point in the study area. The maximum absolute difference between the estimated land-surface 

altitude from the DEM and the land-surface altitude 

at surveyed points was 5.75 ft.

Elevations for ungaged lakes and stages along 

ungaged rivers were interpolated by using the DEM. 

Although some of the ungaged lakes may not be representative of the regional water table (some of the 

lakes may be perched), determining which lakes to 

exclude was beyond the scope of this study.

The minimum water table was generated from 

the interpolation of lake elevations, river stages, and the 

ocean shoreline (which was assigned a water-table altitude of zero feet). The relations among minimum water 

table, water table, and land-surface altitude are shown 

in figure 12. The minimum water table was bounded 

above by land-surface altitude and below by the bottom 

of the SAS, defined to be the sum of the thickness of 

the ICU (fig. 5) and the altitude of the top of the UFA 

(fig. 6).
Ground-Water Flow System 19

Figure 11. Lakes, streams, locations of stream gaging stations, and surficial-aquifer wells used to 

estimate the altitude of the water table of the surficial aquifer system.
20 Simulation of Ground-Water Flow in the Intermediate and Floridan Aquifer Systems in Peninsular Florida

Elevations of the minimum water table at 

1,050 wells tapping the SAS were interpolated from the 

generated minimum water-table surface. Water-table 

measurements at the SAS wells were grouped by the 

physiographic region (fig. 2) in which each well was 

located. Land-surface altitudes at the SAS wells were 

interpolated from the DEM. Multiple linear regressions 

for each group of physiographic regions were computed based on the equation:

, (1)

where

WTi is the water-table measurement at SAS 

well i, in feet,

MINWTi is the minimum water table interpolated at 

SAS well i, in feet,

LSAi is the land-surface altitude interpolated at 

SAS well i, in feet, and

β1 and β2 are the dimensionless regression coefficients of the multiple linear regression.

Multiple linear regressions computed for each group of 

physiographic regions indicated a strong correlation 

between the response variable WTi

 and the regressor 

variables MINWTi

 and LSAi - MINWTi in equation 1 

(table 2). The root-mean-square (RMS) residual 

between measured and linearly regressed water-table 

Figure 12. Relations among water table, minimum water table, and land-surface altitude.

WTi β1 MINWTi β2 ( LSAi - MINWTi = + )

altitudes was computed for all groups of physiographic 

regions, resulting in a weighted average residual of 

3.53 ft, with the difference between the regressed and 

measured water-table altitudes ranging from -17.20 to 

18.49 ft (table 2). The linearly regressed and measured 

water-table altitudes were strongly correlated (fig. 13). 

Although the overall improvement to the RMS residuals resulting from computing a multiple linear regression for each group, instead of one regression for all 

groups, of physiographic regions does not seem considerable, the results show that for some physiographic 

groups, the regression coefficient for variable 

LSAi - MINWTi is not negligible. For groups 3, 4, 5, 

and 7, the same regression coefficient is small (table 

2). Groups 3, 4, and 7 are classified as uplands and 

ridges, whereas group 5 is classified as a valley (fig. 

2). A leaky ICU or a high SAS hydraulic conductivity 

could result in a lack of correlation between the 

water table and the land-surface altitude. The water 

table increasingly emulates land-surface altitude as 

regression coefficient β2 in equation 1 increases. As 

the value of β2 decreases, the water table becomes 

increasingly correlated with the minimum water table.

In areas where the minimum water table, the 

land-surface altitude, and the water table coincide, the 

water table was redefined as the minimum water table 

in order to correct small errors introduced by values of β1
Ground-Water Flow System 21

Table 2. Multiple linear regression coefficients for the minimum water table and the difference 

between land-surface altitude and minimum water table

[Group number refers to figure 2. Refer to equation 1 for the definition of regression coefficients; na, not applicable]

Number 

of 

surficial 

aquifer 

system

wells 

 Regression 

coefficient 

of minimum 

water table 

(β1)

Regression 

coefficient of 

difference 

between digital 

elevation 

and minimum 

water table (β2)

Rootmeansquare 

residual 

(feet)

Range of values 

of difference 

between 

regressed and 

measured water 

table (feet)

Correlation 

coefficient

Group 

number

Multiple linear regressions

1 23 1.00 0.48 3.36 [-6.04, 6.60] 0.97

2 226 1.04 .39 3.29 [-7.68, 18.49] .99

3 94 1.06 .10 4.10 [-12.09, 7.35] .99

4 364 1.09 .04 4.07 [-17.20, 12.76] .99

5 74 1.09 .14 3.88 [-9.68, 11.54] .99

6 140 1.06 .28 2.53 [-7.45, 7.61] .99

7 14 1.05 .06 4.32 [-8.32, 5.74] .99

8 16 1.02 .49 3.72 [-6.59, 7.51] .99

9 37 .96 .85 1.79 [-3.28, 4.29] .99

10 62 1.02 .42 1.83 [-6.28, 4.04] .99

Weighted mean 1,050 na na 3.53 [-17.20, 18.49] na

Regression 

coefficient

 Regressions without grouping

β1 = 0 1,050 0.00 2.27 50.79 [-79.17, 330.31] 0.39

β1 = 1 1,050 1.00 .26 5.68 [-19.26, 41.37] .42

β1 = β2 1,050 .83 .83 13.88 [-28.81, 25.23] .88

β2 = 0 1,050 1.11 .00 4.76 [-24.25, 21.10] .99

β1 ≠ β2 ≠ 0 1,050 1.08 .10 4.27 [-21.88, 19.32] .99

Figure 13. Regressed and measured water-table altitudes 

for all groups of physiographic regions.

in table 2 that deviate from unity. Setting the value of β1

in equation 1 to unity would lower substantially the correlation coefficient of the resulting linear regression. 

Similarly, a nonunity value of β2 is needed to maintain 

a reliable correlation coefficient.

If only one multiple linear regression were computed to approximate the water table, instead of one for 

each group of physiographic regions, then the wrong 

conclusion could be drawn that WTi

 was weakly correlated with LSAi - MINWTi

 over the study area, reducing equation 1 to a linear regression. The correlation 

between variables WTi

 and LSAi - MINWTi tends to 

increase as the correlation between variables WTi

 and 

MINWTi

 decreases; the correlation between variables 

WTi

 and LSAi - MINWTi is variable across the study 

area (fig. 14). The inclusion of only MINWTi

 or 

LSAi - MINWTi

 in equation 1 significantly reduces the 

correlation coefficient of the water-table regression.
22 Simulation of Ground-Water Flow in the Intermediate and Floridan Aquifer Systems in Peninsular Florida

The application of equation 1, and the regression 

coefficients in table 2, resulted in an estimated watertable altitude that ranged from 0 to 240 ft (fig. 15). 

The estimated water table for average 1993-94 conditions was greater than 150 ft in areas of Alachua, Baker, 

Bradford, Clay, Columbia, Polk, and Suwannee Counties (fig. 15). Water-table altitudes generally decrease 

coastward. Water-table altitudes beyond the shoreline 

were assumed to be the equivalent freshwater head of 

the water column obtained from the digitized bathymetry. Areas where the UFA is considered unconfined 

(fig. 5) were excluded from the water-table map 

(fig. 15) because, in general, the SAS is absent in this 

area or its areal extent is minimal.

Potentiometric Surface of the Intermediate 

Aquifer System

A data base of well hydrographs from continuous water-level recorders was generated for the purpose 

of assessing hydrologic conditions in the study area. 

Data from the SJRWMD, SRWMD, and USGS were 

used to generate hydrographs for sites throughout the 

study area (fig. 16; appendix B).

Daily measurements from 11 wells tapping the 

IAS in southwest Florida (fig. 16; appendix B) 

equipped with continuous water-level recorders were 

used to obtain monthly average water levels for 

September 1993 and May 1994 and average water 

levels for the period from August 1993 to July 1994.

Figure 14. Correlation between water table, minimum water table, and difference between land-surface 

altitude and minimum water table for (A) group 4 and (B) group 9 of physiographic regions (refer to figure 2 

for physiographic regions).
Ground-Water Flow System 23

Figure 15. Estimated altitude of the water table of the surficial aquifer system, average conditions for 

August 1993 through July 1994.
24 Simulation of Ground-Water Flow in the Intermediate and Floridan Aquifer Systems in Peninsular Florida

Figure 16. Location of wells equipped with continuous water-level recorders.
Ground-Water Flow System 25

The average heads from August 1993 to July 1994 at 

these 11 wells were linearly regressed with the September 1993 and May 1994 monthly averages according to 

the multiple linear regression:

, (2)

where

is the computed average water-level 

measurement at well i, in feet,

is the computed monthly average for 

September 1993 at well i, in feet,

is the computed monthly average for 

May 1994 at well i, in feet,

βI is the intercept of the multiple linear 

regression, in feet, and

βS and βM are the dimensionless regression coefficients of the multiple linear regression.

Regression coefficients βS and βM represent the 

influence of the September 1993 ( ) and May 1994 

( ) averages on the resulting annual averages .

Regression coefficients βI, βS, and βM, computed 

by using data from the IAS wells equipped with continuous water-level recorders, were 0.54, 0.63, and 0.37, 

respectively. The correlation coefficient of this multiple 

linear regression was 0.99, implying a strong correlation between the annual averages and the monthly 

averages of September 1993 and May 1994 at the 

water-level recorder sites. Lack of sufficient water-level 

data in the IAS precluded the computation of separate 

multiple linear regressions for the groups of physiographic regions within the IAS, namely groups 2, 3, 4, 

5, 9, and 10 (fig. 2).

A total of 107 water levels was measured in IAS 

observation wells during both the months of September 

1993 (Mularoni, 1994a) and May 1994 (Metz and Brendle, 

1994b). Water levels at these sites were compiled to estimate the potentiometric surface of the IAS. The assumption was made that the single measurement made at each 

of the IAS wells in September 1993 and May 1994 was 

representative of the monthly average at the site. Estimated annual average heads at these 107 observation 

wells were computed from equation 2 using the September 1993 and May 1994 water levels and the regression 

coefficients βI, βS, and βM. The 107 estimated annual 

averages and the 11 annual average heads computed 

from continuous water-level data constituted the 118 

control points used to generate the potentiometric-surface map of the IAS (fig. 17).

hi βI βS hSi βM hMi = + +

hi

hSi

hMi

hSi

hMi hi

The estimated average 1993-94 potentiometric 

surface of the IAS shows altitudes of 120 ft in southcentral Polk County and in the area where Polk, Hillsborough, Hardee, and Manatee Counties meet 

(fig. 17). The thickness of the IAS decreases towards 

the northern, eastern, and western boundaries of the 

model. The IAS continues beyond the southern 

boundary of the model area. Ground water flows laterally from areas of potentiometric surface highs 

toward boundaries where the aquifer pinches out. In 

particular, upward leakage in discharge areas in 

parts of Sarasota, De Soto, Highlands, and Glades 

Counties causes ground water to move towards the 

eastern and western boundaries of the aquifer.

Potentiometric Surface of the Upper 

Floridan Aquifer

Daily measurements at UFA wells equipped 

with continuous water-level recorders were used to 

obtain monthly averages for September 1993 and May 

1994 and averages for 1993-94. Regression coefficients βI, βS, and βM for the UFA were obtained from 

the continuous water-level data by making a multiple 

linear regression (equation 2) for each group of physiographic regions (table 3). The regression coefficients 

βI, βS, and βM calculated for the UFA showed that the 

estimated annual averages were influenced more by 

the September 1993 averages than by the May 1994 

averages for physiographic regions 3, 4, 5, 6, 9, and 10 

(table 3). The correlation coefficients for all regressions indicated a strong correlation between the 

annual averages and the September 1993 and May 

1994 averages (table 3).

Water-level measurements made during the 

months of September 1993 and May 1994 in the 

SJRWMD, SWFWMD, and parts of SFWMD to generate potentiometric-surface maps for the UFA (Metz 

and Brendle, 1994a; Mularoni, 1994b; Schiffer and 

others, 1994; and Spechler and others, 1993) were 

assumed to represent monthly averages. Water-level 

measurements made in the SRWMD and the computed annual average heads at UFA wells equipped 

with continuous water-level recorders were used to 

increase the density of water-level measurements. 

Estimated annual average heads at UFA observation 

wells were computed from equation 2 using the compiled September 1993 and May 1994 water-level data 

and the regression coefficients for the respective physiographic regions.
26 Simulation of Ground-Water Flow in the Intermediate and Floridan Aquifer Systems in Peninsular Florida

Table 3. Multiple linear regression coefficients for September 1993 and May 1994 water-level averages in the Upper 

Floridan aquifer for each group of physiographic regions

[Group number refers to figure 2. Refer to equation 2 for the definition of regression coefficient; na, not applicable]

Group 

number

Number of 

Upper Floridan 

aquifer wells 

Intercept 

(feet)

(βI)

Regression 

coefficient of 

September 1993 

(βS)

Regression

coefficient of 

May 1994

(βM)

Root-meansquare 

residual 

(feet)

Range of values of difference between 

regressed and measured heads (feet)

Correlation 

coefficient

1 9 0.77 0.08 0.90 0.60 [-0.80, 0.87] 0.99

2 16 .62 .39 .62 .22 [-0.33, 0.34] .99

3 16 .02 .68 .32 .36 [-0.73, 0.84] .99

4 15 .79 .58 .42 .39 [-0.7l, 0.60] .99

5 16 -.22 .55 .46 .21 [-0.48, 0.35] .99

6 23 .25 .64 .35 .46 [-1.34, 0.78] .99

7 32 -.57 .13 .87 .59 [-1.22, 1.17] .99

8 4 .39 .25 .75 .35 [0.21, 0.54] .99

9 4 -3.42 .77 .29 .25 [-0.19, 0.42] .99

 10 8 -.12 .62 .36 .27 [-0.29, 0.53] .99

Weighted mean 143 na na na .40 [-1.34, 1.17] na

No grouping 143 .22 .54 .46 .85 [-3.18, 4.62] .99

Figure 17. Estimated potentiometric surface of the intermediate aquifer system, average conditions 

for August 1993 through July 1994.
Ground-Water Flow System 27

Several UFA water-level measurements in 

SRWMD were obtained by correlating measured water 

levels of May 1990 (Meadows, 1991) and of May-June 

1995 (Mahon and others, 1997) with average water 

levels for 1993-94 obtained from continuous water-level 

recorders in the area (fig. 16). A multiple linear regression among average water levels for 1993-94, water 

levels of May-June 1995, and water levels of May 1990 

was used to calculate regression coefficients for 

May-June 1995 and May 1990. The computed multiple 

linear regression coefficients were used to estimate 

annual average water levels for 1993-94 at sites without 

continuous recorders for which only May-June 1995 and 

May 1990 measurements were available. The 39 additional annual average water levels estimated through this 

regression were used to improve the definition of the 

1993-94 potentiometric surface of the UFA. At sites 

where measured May 1990, September 1993, May 1994, 

and May-June 1995 water levels were available, annual 

average water levels for 1993-94 were computed using 

both approximations, the May 1990 and May-June 1995 

regression coefficients, and the September 1993 and 

May 1994 regression coefficients listed in table 3. 

A comparison of both approximations showed that the 

maximum absolute difference between the estimated and 

the computed annual average heads was 0.3 ft.

An area of high potentiometric-surface altitude 

for the UFA was documented in Gilchrist County 

(fig. 18), with a steep gradient towards the Santa Fe 

River, the Suwannee River, and eastern Alachua County 

(Col and others, 1997). This potentiometric high is associated with both low aquifer transmissivity and high 

recharge rates (Col and others, 1997). Water-level data 

collected at an additional 28 sites by the Florida 

Geological Survey were used to enhance the resolution 

of the UFA potentiometric surface across the County. 

A total of 1,483 average 1993-94 water levels were used 

to generate a potentiometric-surface map for the UFA 

(fig. 18). Of these points, 23 were outside the study area 

but were used to assess, in part, the heads of the UFA 

near the lateral boundaries of the study area.

Ground-water divides, which can be delineated 

from the potentiometric surface, define the regional direction of ground-water flow. Ground water generally flows 

away from the ground-water divides. A clear example of 

a ground-water divide based on the potentiometric-surface map of the UFA can be seen in southwest Lake 

County extending across central Polk and Highlands 

Counties (fig. 18). This ground-water divide perpendicularly crosses the closed UFA potentiometric- surface contours, ranging from 80 to 125 ft. Analogously, in areas of 

potentiometric highs characterized by closed contours of 

higher altitude than the surrounding area, ground water 

flows perpendicular to the ground-water divide.

Highly transmissive areas in the UFA are characterized by widely spaced potentiometric-surface 

contours, particularly in Suwannee, south-central 

Columbia, northwest Alachua, south-central Marion, 

west-central Citrus, and west-central Lake Counties 

(fig.18). This characterization of the potentiometric 

surface of the UFA also can be observed in parts of 

Osceola and southwest Polk Counties.

Poorly transmissive areas are characterized by 

closely spaced contours, particularly in central Gilchrist, 

southeast Levy, east-central Clay, central Putnam, and 

northeast Marion Counties. Large horizontal hydraulic 

gradients are identified by areas of closely spaced contours in figure 18. Areas of large horizontal hydraulic 

gradients are typically characterized by a zone of low 

transmissivity that causes abrupt changes in water levels.

Water-Level Measurements in the Lower 

Floridan Aquifer

Annual average heads were computed at 24 sites 

tapping the LFA where monthly water-level measurements were available. Monthly values were regressed, 

using equation 2, to compute annual average 1993-94 

heads. The correlation coefficient of this multiple linear 

regression was 0.99. The resulting regression coefficients 

were 0.60 for September 1993 and 0.40 for May 1994. 

These regression coefficients were used to estimate 1993-

94 average heads at 22 LFA wells for which only September 1993 and May 1994 measurements were available. 

The 46 resulting annual average heads were not sufficient 

to generate a potentiometric-surface map for the LFA.

Chloride Concentrations in Ground Water

A data base containing site location, water-sample depth, and chloride concentration of ground-water 

samples from UFA wells was created with water-quality 

data from the SJRWMD, SFWMD, SWFWMD, 

SRWMD, and the USGS. The chloride data were used 

to estimate the altitudes in the FAS where chloride concentrations are approximately 5,000 mg/L. SJRWMD 

generated a map of altitudes where chloride concentrations of 5,000 mg/L are present in the FAS in east-central Florida (Brian McGurk, SJRWMD, written commun., 

1999). Lines of equal chloride concentrations depicted 

in the SJRWMD map were extended across the study 

area (fig. 19) with the aid of the water-quality data base 

and a generalized contour map showing the altitude of 

water having 10,000 mg/L chloride concentration 

generated by Sprinkle (1989).
28 Simulation of Ground-Water Flow in the Intermediate and Floridan Aquifer Systems in Peninsular Florida

Figure 18. Estimated potentiometric surface of the Upper Floridan aquifer, average conditions for 

August 1993 through July 1994.
Ground-Water Flow System 29

EXPLANATION

ALTITUDE CONTOUR -- Shows estimated altitude of ground water with 5,000 milligrams

per liter chloride concentration. Contour interval is variable. Datum is sea level

J OBSERVATION WELL

-200

A A HYDROGEOLOGIC SECTION

Extent of chloride

concentration map

developed by St. Johns

River Water Management

District

ATLANTIC OCEAN

GULF OF MEXICO

BRADFORD

OSCEOLA

BAKER DUVAL

BREVARD

INDIAN

RIVER

ST. LUCIE

MARTIN

OKEECHOBEE

HIGHLANDS

GLADES

HARDEE

DE SOTO

CHARLOTTE

SARASOTA

MANATEE

HILLSBOROUGH

PINELLAS

PASCO

HERNANDO

CITRUS

ALACHUA

LEVY

DIXIE

LAFAYETTE GILCHRIST

UNION

COLUMBIA

SUWANNEE

MADISON

HAMILTON

WARE

CHARLTON CAMDEN

NASSAU

CLAY ST. JOHNS

MARION

PUTNAM

FLAGLER

POLK

SUMTER

LAKE SEMINOLE

ORANGE

VOLUSIA

GEORGIA

FLORIDA

Old

Tampa Bay

Hillsborough

Bay

J

JJ J J J J J J

J J J J J J

J J

J

J J J J J J J J J

J J J J J J J J J J J JJ J

J J J

J J J J J J

J J J

J J J J J JJ J J J J J J

J J J J J J

JJ J J J J J J JJ J J J J J J J J JJ J J

J J J J J J J J J J

J J J J J J

J J J J JJ J J J J J J

J

JJ

J J

J J J J J

J

J

J J

J J

J

J

J

J

J J

J J J

J

J

J J J

J

J J J

J

J J J J J J JJ J J J

J J J J

J J J J J J J J J J

J JJ J J J J J J J J J J J J J J

JJ

J

J J

J

JJJ

J

JJ

JJ

JJ J

J

J J J

J

JJ

J

J

J

JJ

J JJ

JJJ J J JJ J JJ J J

JJ J

JJ J J JJ JJ J J J J J J J JJJ J J J J J J JJ JJ JJ J J JJ JJJ J J J J J J J JJJ J JJ J J J J J J J JJ J J J J J J J

J J J

J J J J

JJ J J J JJJJ J J J

JJ

J J

J J J

JJ J JJ J J J J J J J J J J J J J J J J J J J J J J J J J J J

J J J J J J

-2000

-2200

-1800

-1500

-1500

-1500

-1000

-1000

-1000

-500

-500

-500

-1800

-1800

-2000

-2000

-2200

-2200

-2400

-1600

-1600

-1600

-1600

-1800

-1800

-1400

-1400

-1200

-1200

-1000

-800

-600

-400

-400

-400

-400

-200

-200

-150

-150

-600

-600

-600

-800

-800

-800

-1200

-1000

-1000

-1000

-200

-200

-150

-600

-600

-800

-800

-1400

-1000

0

0 25

25

50 MILES

50 KILOMETERS

  

 

 

	






Base modified from U.S. Geological Survey digital data; 1:100,000, 1985

Universal Transverse Mercator projection, zone 17

A

B

A

B

Suwannee

River

Santa Fe

River

Figure 19. Estimated altitude of water containing a chloride concentration of 5,000 milligrams per liter 

in the Floridan aquifer system (modified from Brian McGurk, St. Johns River Water Management 

District, written commun., 1999; and Sprinkle, 1989).
30 Simulation of Ground-Water Flow in the Intermediate and Floridan Aquifer Systems in Peninsular Florida

The map generated by Sprinkle (1989, p. I50) 

was used to approximate the occurrence of 5,000-mg/L 

chloride concentrations in the northwest part of the 

study area. The data indicated saltwater encroachment 

along the meanderings of the Suwannee and Santa Fe 

Rivers. Chloride concentrations in ground water were 

assumed to decrease away from these rivers.

In west-central Florida, generalizations based on 

chloride trends from east-central Florida were required 

because of the paucity of available data. Chloride data in 

east-central Florida and data published by Hickey 

(1990) provided estimates of the gradients in Polk 

County where chloride concentrations change from east 

to west. Water-level contours of -1,500; -1,800; and 

-2,000 ft are closely spaced in the vicinity of Lake, Polk, 

and Highlands Counties, reflecting the assumption that 

chloride concentrations in ground water increase from 

east-central to west-central Florida (fig. 19).

The purpose of approximating the altitude at 

which 5,000 mg/L of chloride is present in ground 

water was to estimate the base of the ground-water flow 

model; that is, the location beyond which horizontal 

and vertical ground-water flow is attenuated due to the 

density increase caused by the increased chloride concentration. Generally, water levels in the UFA near the 

coastlines of the study area are above sea level and 

decrease seaward, which minimizes the movement of 

seawater inland (Todd, 1980). Lines of equal chloride 

concentration at offshore sites in the Atlantic Ocean 

were estimates based on a few offshore points documented by Scholle (1979), Johnston and others (1982), 

and Johnston (1983). Chloride data from a well in 

Hillsborough Bay (Sinclair, 1979) indicated there is no 

ground water in the bay with chloride concentrations 

less than 5,000 mg/L. The same situation was assumed 

to occur in Old Tampa Bay (fig. 19) based on chloride 

trends obtained from Dames and Moore (1988).

Sulfate concentrations near the base of the UFA in 

southwest Florida tend to increase from east-central to 

west-central Florida (Hickey, 1990). Increases in sulfate 

concentrations in ground water also increase the density 

of ground water. There were insufficient sulfate data to 

assess specific areas where sulfate concentrations would 

cause increases in ground-water density equivalent to a 

chloride concentration of 5,000 mg/L; however, the 

occurrence of high sulfate concentrations in southwest 

Florida would have the equivalent effect of displacing 

the base of ground-water flow farther east (fig. 19).

Recharge to and Discharge from the 

Upper Floridan Aquifer

The areal distribution of recharge and discharge 

areas to the UFA were delineated by comparing the 

altitude of the water table of the SAS and the potentiometric surface of the UFA (fig. 20). Recharge to the 

UFA (and the IAS) occurs mainly by downward leakage 

from the SAS through the ICU (where present) when the 

altitude of the water table is higher than the potentiometric surface of underlying aquifers. The estimated 

recharge areas, based on figures 15, 17, and 18, are areas 

where either the water table has a higher altitude than the 

potentiometric surface or the UFA is unconfined. 

Recharge and discharge areas for the UFA were 

delineated where there were no measured water levels 

in the IAS by comparing the altitude of the water table 

of the SAS and the interpolated heads of the UFA. In 

areas where measured water levels were available for 

the IAS, the altitudes of the potentiometric surfaces of 

the IAS and the UFA were compared to identify areas 

of recharge to and discharge from the UFA.

In areas where the potentiometric surface of the 

UFA (and the IAS) is above the altitude of the water 

table of the SAS, there is a potential for water to discharge from the UFA as upward leakage to the SAS 

through the ICU (fig. 20). This is the case in some 

coastal areas. Discharge from the UFA also occurs as 

ground-water withdrawals, spring flows, and flow from 

the unconfined areas of the UFA to swamps and rivers.

Recharge to the UFA also occurs at discrete 

points, both naturally occurring and anthropogenic. 

Recharge occurs at natural sinkholes such as Alachua 

and Haile Sinks (fig. 21). These sinks were assigned a 

combined recharge rate to the UFA of 10 million gallons 

per day (Mgal/d), based on estimates by Phelps (1987). 

No recharge rates for other natural sinks in the study area 

are available. Artificial recharge to the freshwater part of 

the UFA occurs from drainage wells, injection wells in 

Alachua County, and rapid-infiltration basins.

Water Use

Within the study area, ground-water withdrawals 

from the IAS, UFA, and LFA for public-water supply, 

commercial or industrial (including thermoelectricpower generation and recreational uses), and agricultural purposes were compiled or estimated for 1993-94 

(depending on the water-use type). Most of the groundwater withdrawals were compiled from consumptive 

user permit data bases and water-use data files from 

SJRWMD, SFWMD, SWFWMD, and SRWMD. The 

estimation of other types of discharge from wells, 

such as self-supplied domestic water-use data and 

discharge rates from free-flowing wells, is presented 

in this section.

Total estimated ground-water withdrawals from 

the IAS, UFA, and LFA for public-water supply, commercial and industrial, agricultural, and self-supplied 

domestic uses for 1993-94 were 2,488 Mgal/d (table 4).
Ground-Water Flow System 31

Figure 20. Distribution of recharge and discharge areas of the Upper Floridan aquifer, average 

conditions for August 1993 through July 1994.
32 Simulation of Ground-Water Flow in the Intermediate and Floridan Aquifer Systems in Peninsular Florida

Figure 21. Locations of Upper Floridan aquifer drainage and injection wells.
Ground-Water Flow System 33

Table 4. Estimated ground-water withdrawals and uses, by county and by Water Management District, from the intermediate 

aquifer system, Upper Floridan aquifer, and Lower Floridan aquifer, August 1993 through July 1994

[Source: St. Johns River Water Management District (SJRWMD); South Florida Water Management District (SFWMD); Southwest Florida Water Management District (SWFWMD); Suwannee River Water Management District (SRWMD); and U.S. Geological Survey. All rates are in million gallons per day; 

-- indicates no wells are tapping the aquifer or aquifer is absent]

County

Intermediate aquifer system Upper Floridan aquifer Lower Floridan aquifer

Publicwater 

supply

Industrial

and 

commerciala

a

Includes mining, thermoelectric power generation, recreational, and landscape irrigation uses.

Agricultural

Domestic

Publicwater 

supply

Industrial 

and 

commerciala

AgriculturalDomestic

Publicwater 

supply

Industrial 

and commerciala

Agricultural

Alachua -- -- -- -- 24.62 5.99 34.96 6.93 -- -- -- 

Baker -- -- -- -- .80 .23 .14 1.24 0.31 -- 0.05

Bradford -- -- -- -- 4.08 3.49 2.25 2.08 -- -- -- 

Brevard -- -- -- -- 2.61 1.02 13.75 0.52 -- -- -- 

Charlotte 0.11 0.07 14.29 1.17 .43 .07 4.91 -- -- -- -- 

Citrus -- -- -- -- 10.17 6.26 1.57 6.73 -- -- -- 

Clay -- -- -- -- 2.66 3.87 .77 2.90 7.43 1.77 -- 

Columbia -- -- -- -- 4.22 3.17 13.42 4.36 -- -- -- 

DeSoto .95 .24 6.12 0.52 .64 .16 53.87 .01 -- -- -- 

Dixie -- -- -- -- 1.69 .79 4.29 1.10 -- -- -- 

Duval -- -- -- -- 27.70 14.36 1.35 .08 73.42 12.34 .12

Flagler -- -- -- -- 1.46 .08 6.25 .95 -- -- -- 

Gilchrist -- -- -- -- 2.32 .38 24.62 1.10 -- -- -- 

Glades -- -- 1.27 .01 .44 -- 6.36 -- -- -- .18

Hamilton -- -- -- -- 16.94 14.80 16.54 .78 -- -- -- 

Hardee .04 .08 5.83 .42 1.39 .09 54.83 .01 -- -- -- 

Hernando -- -- -- -- 17.06 22.60 3.71 2.66 -- -- -- 

Highlands .33 .25 17.41 .21 6.91 2.01 96.22 .01 -- -- 18.54

Hillsborough .01 .02 .95 .55 65.41 25.56 72.34 9.48 -- -- -- 

Indian River -- -- -- -- 5.21 .23 31.35 .09 -- -- -- 

Lafayette -- -- -- -- .59 .57 22.08 .71 -- -- -- 

Lake -- -- -- -- 26.01 27.80 29.43 9.55 1.73 .32 .32

Levy -- -- -- -- 3.20 3.85 25.62 3.31 -- -- -- 

Madison -- -- -- -- .28 .26 7.79 .17 -- -- -- 

Manatee .16 .27 2.24 2.42 11.09 1.16 77.39 .02 -- -- -- 

Marion -- -- -- -- 18.54 6.44 8.46 21.76 -- -- .04

Martin -- -- -- -- 1.20 .42 9.65 -- -- -- -- 

Nassau -- -- -- -- 4.46 34.00 .42 .13 -- 1.91 -- 

Okeechobee -- -- -- -- -- .39 24.42 .03 -- -- .34

Orange -- -- -- -- 111.81 7.73 18.43 12.60 60.41 .40 .11

Osceola -- -- -- -- 18.27 2.62 46.12 6.28 .97 -- .02

Palm Beach -- -- -- -- .42 -- -- -- -- -- -- 

Pasco -- -- -- -- 95.89 16.00 20.07 8.07 -- -- -- 

Pinellas -- -- -- -- 40.69 1.87 .68 4.46 -- -- -- 

Polk .30 1.60 3.88 .80 59.39 90.61 127.12 5.69 .88 -- .47

Putnam -- -- -- -- 3.38 7.22 6.58 9.42 -- .98 -- 

St. Johns -- -- -- -- 6.63 .01 21.01 2.12 .02 -- -- St. Lucie -- -- -- -- 1.10 .25 11.42 .43 -- -- -- 

Sarasota 2.94 .83 .94 4.96 15.79 3.16 4.38 .03 -- -- -- 

Seminole -- -- -- -- 50.03 .39 7.90 7.90 6.91 -- -- Sumter -- -- -- -- 2.51 1.20 8.96 3.11 -- -- -- 

Suwannee -- -- -- -- 5.05 3.33 63.69 2.94 -- -- -- 

Union -- -- -- -- 1.11 .09 1.73 .72 -- -- -- 

Volusia -- -- -- -- 47.37 .71 16.53 7.39 -- .12 .01

Camden, Ga. -- -- -- -- 2.43 34.36 .01 -- -- -- -- 

Charlton, Ga. -- -- -- -- .70 -- -- -- -- -- -- 

Total 4.84 3.36 52.93 11.06 724.70 349.60 1,003.39 147.87 152.08 17.84 20.20

Water Management District

SJRWMD -- -- -- -- 289.61 98.91 156.44 69.93 135.09 17.48 0.63

SFWMD 0.01 0.10 11.38 0.25 58.43 10.57 184.50 11.27 16.99 .36 19.57

SWFWMD 4.83 3.26 41.55 10.81 332.24 171.26 452.50 45.92 -- -- --

SRWMD -- -- -- -- 44.42 68.86 209.95 20.75 -- -- --

Intermediate 

aquifer 

system

Upper 

Floridan 

aquifer

Lower 

Floridan 

aquifer

Total 

by 

aquifer

Publicwater 

supply

Industrial 

and 

commercial

Agricultural Domestic

Total 

by 

use

Total 72.19 2,225.56 190.12 2,487.87 881.62 370.80 1,076.52 158.93 2,487.87
34 Simulation of Ground-Water Flow in the Intermediate and Floridan Aquifer Systems in Peninsular Florida

Pumping rates were 72 Mgal/d from the IAS; 2,226 

Mgal/d from the UFA; and 190 Mgal/d from the LFA. 

Corresponding withdrawals were 882 Mgal/d for public-water supply; 371 Mgal/d for commercial and 

industrial; 1,076 Mgal/d for agricultural uses; and 

159 Mgal/d for self-supplied domestic uses. Estimated 

ground-water withdrawals from self-supplied domestic 

wells for 1993-94 were obtained from USGS data 

(Richard L. Marella, written commun., 1998).

Ground-water withdrawals for public-water 

supply and commercial or industrial purposes were 

compiled from the monthly operating reports of 

SJRWMD, SFWMD, SWFWMD, and SRWMD, 

which are based on meter readings. Ground-water 

withdrawals were distributed according to aquifer 

source by using the well locations, penetration depths 

of individual wells in each well field, well status, and 

well capacity from the consumptive user permit data 

base. Pumping rates for Camden, Charlton, and Ware 

Counties, Ga., for public-water supply and industrial 

purposes were obtained from the USGS.

Several assumptions were made to apportion 

total water withdrawals among individual wells in the 

same well field and between aquifers for wells with 

open intervals tapping more than one aquifer. At well 

fields where only the cumulative withdrawal was specified, each active well was assigned a ground-water 

withdrawal equal to the total withdrawn multiplied by 

the ratio of the well capacity to the sum of well capacities for all active wells in the well field. Assigned 

water withdrawals from each aquifer from wells with 

open intervals tapping more than one aquifer were set 

equal to the total withdrawal rate multiplied by the 

ratio of the transmissivity of the interval open to a particular aquifer to the transmissivity of the entire open 

interval of the well.

Ground-water withdrawal data for SRWMD 

were provided by SRWMD (Ronald Ceryak, written 

commun., 1997). Measured and estimated pumping 

rates from wells in SWFWMD were provided by the 

SWFWMD (Tabitha Ostow, written commun., 1999). 

Pumping rates from wells used for agricultural purposes in SJRWMD and SFWMD were estimated from 

the required application rates for each crop type, which 

depend on rainfall, watering needs, and estimated 

evapotranspiration rates. These application rates were 

computed by the Water Management Districts. The 

application rate for each cultivated acre, adjusted by the 

irrigation method, was multiplied by the number of cultivated acres to obtain the estimated pumping rate for a 

specific crop. Many wells pumped water for more than 

one crop type. Total ground-water withdrawals for well 

fields were the sum of pumping rates for all crop types. 

When ground water was not the only source for irrigation, adjustments to pumping rates were made by using 

the estimated ground-water use percentage listed in the 

water-use permit data base.

Estimates of discharge rates from free-flowing 

UFA wells in SJRWMD were obtained from SJRWMD 

(Brian McGurk, written commun., 1997). Discharge 

rates for 1993-94 were estimated to be 6.81 Mgal/d, of 

which more than 50 percent occurred in Brevard 

County (table 5). No information is available about 

free-flowing wells, if any, outside SJRWMD.

Artificial Recharge from Drainage and 

Injection Wells

Artificial recharge to the UFA occurs through 

about 364 active drainage wells and 41 injection wells 

(fig. 21). Of these wells, 242 are concentrated in the 

Orlando metropolitan area and were verified to be 

active (CH2M Hill, 1997). These drainage wells, cased 

to the top of the UFA and then drilled open-hole into 

the UFA, generally are used as a means to dispose of 

street runoff from storm drains and to control lake levels. Total recharge to the UFA through drainage wells 

in the study area was estimated to be 68 Mgal/d 

(table 6) based on total rainfall for 1993-94. The 

recharge rate at drainage wells is strongly correlated to 

the amount of total rainfall for any period. 

 Recharge to the UFA through drainage wells in 

the Orlando metropolitan area for 1993-94 was approximated from recorded rainfall data at nearby NOAA 

stations (fig. 3) and from adjustments to previously 

estimated long-term recharge rates at these wells by 

CH2M Hill (1997) and Bradner (1996). Estimates of 

recharge rates from drainage wells were based on a 

water-budget analysis of the basins containing streetrunoff and lake-level control wells.

Table 5. Estimated discharge rates from free-flowing 

wells, by county, in the Upper Floridan aquifer in the 

St. Johns River Water Management District, August 

1993 through July 1994

[Source: Brian McGurk, St. Johns River Water Management District. Rates are in million gallons per day]

County Discharge

Brevard 3.44

Indian River 2.00

Putnam .03

St. Johns .36

Seminole .87

Volusia .11

Total 6.81
Ground-Water Flow System 35

For street-runoff wells, the first step was to calculate a runoff coefficient for the drainage area. The 

runoff coefficient can be defined as the fraction of the 

total rainfall that becomes runoff over a given time 

period. CH2M Hill (1997) calculated an average runoff 

coefficient of 0.578 for a group of street-runoff drainage wells in Orange County. An average runoff coefficient, weighted by the contributing drainage areas of 

each well, was computed for the same group of streetrunoff drainage wells used by CH2M Hill (1997). The 

resulting area-weighted average runoff coefficient used 

in this study was 0.478. An area-weighted average runoff coefficient lower than the 0.578 value computed by 

CH2M Hill (1997) is attributed to predominantly lower 

runoff coefficients for street-runoff drainage wells with 

larger contributing drainage areas.

The total area drained by street-runoff wells in 

metropolitan Orlando was estimated to be 6,083 acres 

(CH2M Hill, 1997). The 30-year average recharge rate 

to the UFA at street-runoff drainage wells, computed 

from the product of runoff coefficient, total rainfall at 

the Orlando McCoy station (fig. 3), and drainage area, 

was estimated to be 10.40 Mgal/d. The estimated 

recharge rate to the UFA from street-runoff drainage 

wells for 1993-94 was 11.91 Mgal/d, equal to 

10.40 Mgal/d multiplied by the ratio of rainfall for this 

period (fig. 3) to the 30-year rainfall average.

A water-budget analysis of the basins contributing drainage to 10 lake-level control wells (Bradner, 

1996) showed a basin-area weighted recharge rate to 

the UFA of 10.84 in/yr. This recharge rate is lower than 

the non-weighted numerical average of 18.08 in/yr 

computed by CH2M Hill (1997) for the same basins. 

The total area drained by these lake-level control wells 

in metropolitan Orlando was estimated to be 33,031 

acres (CH2M Hill, 1997). The 30-year average 

Table 6. Estimated recharge rates for drainage wells to 

the Upper Floridan aquifer, by county, August 1993 

through July 1994

[Source: CH2M Hill, 1997; Bradner, 1996; Phelps, 1987. Rates, in 

million gallons per day, are rounded to integers]

County Recharge

Alachua 11

Marion 4

Orange 45

Putnam 1

Seminole 2

Suwannee 5

Total 68

recharge rate to the UFA at lake-level control wells, 

computed from the product of drainage area and basinarea weighted recharge rate, was estimated to be 

26.64 Mgal/d. The estimated recharge rate to the UFA 

for 1993-94 at lake-level control wells was 

30.50 Mgal/d, equal to 26.64 Mgal/d multiplied by 

the ratio of rainfall for this period to the 30-year rainfall 

average.

Recharge to the UFA through drainage wells outside the Orlando area but in Orange and Seminole 

Counties for 1993-94 was estimated to be 4.21 Mgal/d. 

Recharge to the UFA from drainage wells outside 

Orange and Seminole Counties (fig. 21), assuming 

all were street-runoff wells, was estimated to be 

11.57 Mgal/d.

Additional recharge to the UFA occurs through 

injection wells monitored by the Florida Department of 

Environmental Protection (FDEP). Recharge rates 

from injection wells (fig. 21) were obtained from 

FDEP. A total of 10.62 Mgal/d recharged the UFA by 

injection wells in Alachua County during the 1993-94 

period. The injection wells in Alachua County were the 

only FDEP wells recharging the freshwater part of the 

UFA.

Spring Flow

More than 150 springs discharge water from the 

UFA in the study area. A major factor in spring flow is 

the net aquifer recharge from rainfall; however, spring 

response is delayed by aquifer-matrix storage. Higher 

spring flows are common in late fall after the rainy season, whereas lower flows occur in late spring when 

rainfall is low. Spring flows from the UFA tend to create depressions in the potentiometric surface, the areal 

extent of which depends on the magnitude of the spring 

flow and aquifer and confining-unit properties in the 

vicinity of the spring. A summary of spring-flow data 

for springs discharging from the UFA and the locations 

of these springs was compiled (fig. 22, appendix C) 

from Rosenau and others (1977), Yobbi (1989, 1992), 

USGS (1994, 1995, 1996, 1997, 1999), and Leel 

Knowles (USGS, written commun., 1999). The springs 

in the study area with the highest flows, based on measured values or estimated flow measurements for 1993-

94, are Silver Springs (number 66 in fig. 22), Rainbow 

Springs (number 72), Crystal River Spring Group 

(number 81), Alapaha Rise (number 2), and Ichetucknee Springs (combined flow of numbers 24 through 29 

in fig. 22). Total spring flow from the UFA was approximately 6,380 cubic feet per second (ft3

/s) or 4,130 

Mgal/d (appendix C).
36 Simulation of Ground-Water Flow in the Intermediate and Floridan Aquifer Systems in Peninsular Florida

Figure 22. Locations of Upper Floridan aquifer springs.
Ground-Water Flow System 37

Of the 156 springs in the study area, only 28 

were measured during 1993-94 (U.S. Geological Survey, 1994, 1995; appendix C). Average flows from the 

remaining 128 springs were estimated by using the following approximations. Spring flow measurements by 

the USGS (1996, 1997, 1999) and Rosenau and others 

(1977) at 87 springs either prior to or after 1993-94 

were used to estimate 1993-94 flows by multiplying the 

measured flow by the ratio of the 1993-94 rainfall to the 

rainfall that occurred during the year in which the measurement was made. Although the ratio between spring 

flow and rainfall is not constant from one year to the 

next, measured flows varied somewhat linearly with 

rainfall in those years when more than one flow measurement was available. Flows at nine unmeasured 

third and fourth magnitude springs were assigned flow 

values based on field reconnaissance (Leel Knowles, 

USGS, written commun., 1999). Flow at Crescent 

Beach submarine spring (spring 41, appendix C) also 

has never been measured; the estimated flow of 30 ft3

/s 

is highly generalized (Rick Spechler, USGS, oral commun., 2000).

Average flows for 1993-94 at 31 springs in 

Citrus, Hernando, and Pasco Counties were estimated from the average May 1988 to April 1989 

flows computed by Yobbi (1992). The average of the 

flow measurements at Weeki Wachee Springs 

(spring 138 in fig. 22) was 185 ft3/s from May 1988 

to April 1989 (Yobbi, 1992), and 129 ft3/s from 

August 1993 to July 1994 (U.S. Geological Survey, 

1994, 1995), or about 70 percent of the average flow 

from May 1988 to April 1989. Because of the lack of 

additional spring-flow measurements from May 1988 

to April 1989 and from August 1993 to July 1994, 

average flows from August 1993 to July 1994 for 

springs in Citrus, Hernando, and Pasco Counties 

were estimated to be 70 percent of the flows determined by Yobbi (1992) from May 1988 to April 

1989. Similarly, the average flow in 1993-94 for 

additional springs listed by Yobbi (1989) was 

assumed to be 70 percent of the average flow for the 

period of record (appendix C).

Recharge to the Unconfined Upper Floridan 

Aquifer

A generalized water budget for unconfined areas 

of the UFA within the study area was used to estimate 

upper and lower bounds for the net aquifer recharge 

rate in these areas. Total rainfall, direct runoff estimates 

derived from separation of discharge hydrographs, and 

evapotranspiration estimates based on physiographic 

regions were used to approximate the net aquifer 

recharge in unconfined areas of the UFA. The estimated net aquifer recharge rate to unconfined areas of 

the UFA for the 1993-94 period was computed from the 

equation:

, (3)

where

NR is net recharge to unconfined areas of the 

UFA, in inches per year,

TR is total rainfall, in inches per year,

ET is evapotranspiration, in inches per year, and

SR is surface runoff, in inches per year.

Equation 3 assumes that changes in storage were negligible, an assumption that is supported by data from 

continuous ground-water level recorders. Rainfall for 

the 1993-94 period was computed from data collected 

at NOAA stations (fig. 3). The unconfined areas of the 

UFA were divided into Thiessen (1911) polygons, 

which were based on the location of rain gages.

Net recharge to unconfined areas of the UFA is 

the subcomponent of the water budget that drives 

ground-water flow through the UFA when applied 

water exceeds evapotranspirative losses and overcomes 

capillary effects in the unsaturated zone. These 

recharge rates vary areally based on topography and 

physiography. Recharge rates to the aquifer are 

expected to be lower near coastal lowlands and swamps 

than in the Northern Highlands (fig. 2) or in other areas 

of high land-surface altitude. In low-lying areas, the 

potentiometric surface of the UFA is close to land surface and evapotranspiration (ET) rates probably are 

high. Recharge rates also are expected to be low near 

rivers, where surface runoff is more likely to discharge 

to rivers rather than recharge ground water. Aquifer 

recharge rates should decrease from the Northern Highlands (fig. 2) towards rivers in the unconfined areas of 

the UFA having the largest flows, namely, the Suwannee, Santa Fe, Steinhatchee, Waccasassa, and Withlacoochee Rivers, and parts of the Hillsborough River 

(fig. 23).

NR = TR - ET - SR
38 Simulation of Ground-Water Flow in the Intermediate and Floridan Aquifer Systems in Peninsular Florida

Several discharge areas along 

the Suwannee River in the unconfined 

areas of the UFA can be identified 

from the UFA potentiometric surface 

(fig. 18). The UFA discharges to the 

Suwannee River in areas where the 

stage at the Suwannee River is lower 

than the surrounding potentiometric 

surface. The Suwannee River 

recharges the UFA in areas where the 

river stage is higher that the surrounding potentiometric surface of the 

UFA. Surface water from the coastal 

plains and swamps discharges to the 

Gulf of Mexico.

Evapotranspiration varies 

areally; a range of ET values was 

obtained from documented minimum 

and maximum measured rates. The 

minimum ET rate was assumed to be 

27 in/yr, measured at a herbaceous, 

successional vegetation in a deforested area of the Lake Wales Ridge 

during the 1993-94 period (Sumner, 

1996). The maximum ET rate was 

assumed to be 46 in/yr, the mean 

evapotranspiration measured at seven 

vegetated sites (rather than open 

water) in the Everglades (fig. 1) from 

January 1996 to December 1997 

(German, 2000).

Estimates of surface runoff 

(SR) were computed by applying a 

generalized hydrograph separation 

approximation to calculate base flow 

and surface runoff from the daily discharge data obtained from 10 USGS 

stream gaging stations (table 7, 

fig. 23). Estimates of SR, normalized 

per unit area of each respective drainage basin, ranged from 0.5 to 8 in/yr 

(table 7). The hydrograph separation 

approximation used in this study 

considered most hydrograph peaks to 

be part of SR (fig. 24). As a first 

approximation, the hydrograph 

separation code HYSEP (Sloto and 

Crouse, 1996) was used with a 

substantially larger drainage area 

Figure 23. Rivers in the unconfined areas of the Upper Floridan aquifer and 

locations of stream gaging stations.
Ground-Water Flow System 39

than the actual area in order to increase the number of 

days after which SR ceases. The resulting surface runoff from HYSEP at each gaging station was adjusted so 

that large areas of the hydrograph peaks were considered to be part of SR. The frequency of storm events 

during the wet season, the large capacity of riverbank 

storage in the Suwannee, Withlacoochee, Waccasassa, 

Steinhatchee, Santa Fe, and Hillsborough Rivers, the 

large hydraulic connection between the UFA and these 

rivers, and the potential for slow overland drainage 

from swamps are some of the reasons why surface runoff would be underestimated if HYSEP was directly 

applied to these hydrographs. Surface runoff to the 

Suwannee River was estimated to be 7 in/yr, the 

rounded mean of the three stations on this river 

(table 7). The lowest surface-runoff rate, 0.5 in/yr, 

occurred at the Withlacoochee River in west-central 

Florida.

Because of the uncertainty of ET and SR and 

rather than computing a specific net recharge rate to 

unconfined areas of the UFA, ranges of values for the 

net recharge rate were estimated. These ranges were 

used to bracket final net recharge rates obtained during 

model calibration. The minimum net recharge rate was

estimated from equation 3 by using the maximum ET 

rate; the maximum net recharge rate was estimated by 

using the minimum ET rate. The application of equation 3 to unconfined areas of the UFA resulted in net 

recharge rates that ranged from 0 to 18 in/yr in some 

areas, and from 13 to 32 in/yr in other areas (fig. 25). 

Estimated net recharge rates generally increased from 

the coastline inland.

Discharge to Swamps

Land-use maps indicate that about 12 percent of 

the unconfined areas of the UFA are swamps. These 

swamps include coastal or bay swamps, cypress and 

mangrove swamps, and river and lake swamps. Swamp 

areas located within the areal extent of the unconfined 

areas of the UFA were delineated to encompass all contiguous areas of swamp adjacent to the coast (fig. 26). 

Inland swamp areas also were delineated. Hydrologically, swamps generally are areas where the potentiometric surface of the UFA is at or near land surface. 

Swamps generally are considered to be discharge 

areas of the UFA because water drains out of the UFA 

into the swamps.

Table 7. Surface-runoff rates calculated from hydrograph separation approximations applied to August 1993 through 

July 1994 daily discharge data obtained from stream gaging stations in the unconfined areas of the Upper Floridan aquifer

[Station number refers to figure 23; USGS, U.S. Geological Survey. Average surface-runoff flow rates are rounded to one significant figure; mi2, square 

miles; ft3

/s, cubic feet per second; in/yr, inches per year]

USGS

station 

number

Station name

Drainage 

area 

(mi2)

Average 

daily

discharge

(ft3/s)

Estimated 

average base 

flow (ft3/s)

Estimated

average 

surface-runoff

 flow (ft3/s)

Average 

surface-runoff 

flow per unit 

area (in/yr)

02303000 Hillsborough River near Zephyrhills 220 123 67 56 3.0

02312000 Withlacoochee River at Trilby 570 47 25 22 .5

02313000 Withlacoochee River near Holder 1,825 319 252 67 .5

02313700 Waccasassa River near Gulf Hammock 480 193 89 104 3.0

02319000 Withlacoochee River near Pinetta 2,120 1,714 621 1,093 7.0

02319500 Suwannee River at Ellaville 6,970 5,961 1,933 4,028 8.0

02320500 Suwannee River at Branford 7,880 6,678 3,269 3,409 6.0

02322500 Santa Fe River near Fort White 1,017 1,067 925 142 2.0

02323500 Suwannee River near Wilcox 9,640 9,410 5,056 4,354 6.0

02324000 Steinhatchee River near Cross City 350 205 36 169 7.0
40 Simulation of Ground-Water Flow in the Intermediate and Floridan Aquifer Systems in Peninsular Florida

Figure 24. Approximation of base flow and surface runoff from daily recorded discharges at Suwannee River 

gaging stations (A) 02319500, (B) 02320500, and (C) 02323500 (refer to figure 3 for rainfall station locations 

and to figure 23 for gaging station locations).
Ground-Water Flow System 41

Figure 25. Estimated minimum and maximum net recharge rates in the 

unconfined areas of the Upper Floridan aquifer.
42 Simulation of Ground-Water Flow in the Intermediate and Floridan Aquifer Systems in Peninsular Florida

Figure 26. Areal extent of swamps in the unconfined areas of the Upper 

Floridan aquifer.

SIMULATION OF GROUNDWATER FLOW

A steady-state ground-water 

flow model of the ground-water flow 

in the IAS and FAS was constructed 

and calibrated to time-averaged data 

for the period August 1993 through 

July 1994. The rationale for 

calibrating the model for this time 

period is explained in this section. 

Estimates of hydraulic properties 

obtained from local and regional 

ground-water flow models (table 1) 

were integrated with the hydrogeologic data discussed in previous sections. The model was developed 

using the USGS finite-difference 

ground-water flow code MODFLOW-96 (Harbaugh and McDonald, 

1996). The calibrated model was used 

to analyze the response of the FAS to 

projected ground-water withdrawals 

in 2020.

A uniformly spaced grid of 

square 5,000-ft cells was used as the 

framework to discretize the groundwater flow system horizontally. The 

UTM coordinates of the grid corners 

given in table 8 facilitate reproduction 

of the grid. Grid spacing (fig. 27) 

resulted from the need to use a finer 

grid resolution than in previous 

models (table 1). The grid consisted of 

300 rows and 210 columns and was 

oriented along a north-south axis for 

simplicity because the majority of 

stresses or boundary conditions were not 

aligned along any particular axis.

Steady-State Approximation

The program MODFLOW-96 

(Harbaugh and McDonald, 1996), used 

to simulate flow in the FAS, solves the 
Simulation of Ground-Water Flow 43

three-dimensional ground-water flow equation using 

finite-difference approximation and a block-centered 

grid. The equation solved by MODFLOW-96 can be 

written, after time averaging, as:

(4)

where

Kxx, Kyy, Kzz are the hydraulic conductivities along 

the columns, rows, and layers, respectively, 

in feet per day,

h is the average head for the time interval 

[t1, t2], in feet,

W is the average flux per unit volume for the 

time interval [t1, t2] and denotes the sink 

or source of water, in (feet/days)/feet,

Ss is the specific storage of the aquifer, 

in feet-1,

x, y, z are the spatial coordinates, in feet,

t is time, in days, and

t1, t2 are times at the beginning and end of the time 

interval, in days.

The term W represents: drainage to springs or swamps; 

aquifer-river interactions; withdrawals from pumping; 

recharge from injection or drainage wells; and net 

recharge rates from rainfall infiltration in the unconfined 

areas of the UFA.

Table 8. Geographical information system 

coordinates of the corners of the ground-water 

flow model grid

[X and Y coordinates refer to Universal Transverse Mercator projection (Snyder, 1983), zone 17]

Grid corner X coordinate

(feet)

Y coordinate

(feet)

 Upper left 900000 11270000

 Upper right 1950000 11270000

 Lower right 1950000 9770000

 Lower left 900000 9770000

∂x

∂ Kxx

∂h

∂x

-----    

∂y

∂ Kyy

∂h

∂y

-----    

∂z

∂ Kzz∂h

∂z

-----   + +   – W

 = Ss

ht2 ht1 ( ) –

t2 t1 – ----------------------, 

A steady-state approximation to equation 4 

implies that the magnitude of the right-hand side, which 

also can be written as Ss ∆h / ∆ t, is small. The expression 

S ∆h/∆ was evaluated instead of t Ss ∆h / ∆ in order to t

approximate the product of specific storage and aquifer 

thickness with a single value. The expression S ∆h/∆ , t

used to assess the error introduced in the model by a 

steady-state approximation, was estimated by using the 

measured UFA heads at the beginning and at the end of 

any 1-year interval, and the estimated storage coefficient S for the UFA. Head data from UFA wells from 

October 1987 to September 1997 were used to evaluate 

the magnitude of S ∆h/∆t. Estimates of this value are 

much smaller in areas where the UFA is confined than 

in areas where the UFA is unconfined because the 

storage coefficient of the confined UFA can be two 

orders of magnitude smaller than the storage coefficient 

of unconfined areas of the UFA.

The 1-year interval having the smallest RMS 

water-level difference from the first to the last day of the 

interval, for 32 wells tapping the unconfined areas of the 

UFA, was August 1, 1993, through July 31, 1994. The 

RMS value for this period was 0.71 ft, with a mean of 

0.22 ft. This RMS water-level difference amounts to a 

value of 0.86 in/yr for S ∆ h/∆ , assuming a storage t

coefficient of 0.10. The RMS water-level difference for 

the same 1-year period in the IAS and in confined areas 

of the UFA were 2.46 and 2.15 ft, respectively, both of 

which resulted in values of approximately 0.03 in/yr for 

S ∆h/∆ assuming a storage coefficient of 0.001 for t

confined aquifers.

Although head fluctuations during this period 

(August 1, 1993, through July 31, 1994) were substantial in some wells in unconfined areas of the UFA wells 

(fig. 28), net head differences ranged from -1.07 to 

1.66 ft (table 9). The locations of wells tapping the 

unconfined areas of the UFA (fig. 16) and the head differences listed in table 9 indicate that positive and negative head differences were uniformly distributed in 

these areas. It was assumed, based on the relatively 

small 0.86 in/yr RMS water-level difference in the 

unconfined areas of the UFA, that for the period from 

August 1, 1993, through July 31, 1994, the FAS was at 

steady-state conditions.
44 Simulation of Ground-Water Flow in the Intermediate and Floridan Aquifer Systems in Peninsular Florida

Figure 27. Parts of the uniform grid used to develop the ground-water flow model, shown in the vicinity 

of (A) Duval and (B) Orange Counties.
Simulation of Ground-Water Flow 45

Figure 28. Hydrographs 

showing water-level 

fluctuations in wells 

tapping unconfined 

areas of the Upper 

Floridan aquifer (gaps 

indicate data not 

available; refer to figure 

16 for well locations).
46 Simulation of Ground-Water Flow in the Intermediate and Floridan Aquifer Systems in Peninsular Florida

Table 9. Differences in water levels measured at 

wells tapping the unconfined areas of the Upper 

Floridan aquifer and equipped with continuous 

water-level recorders

[Map location number refers to figure 16; --, data not available]

Map 

location 

number

Water level

(feet above sea level) Difference 

(feet) August 1, 1993 July 31, 1994

1 52.00 52.90 0.90

 2 24.16 -- --

 3 44.60 45.61 1.01

 4 25.96 27.18 1.22

 5 23.60 25.03 1.43

 6 28.54 28.78 .24

 7 21.59 22.49 .90

 8 33.61 33.57 -.04

 9 21.97 21.84 -.13

10 38.71 39.22 .51

11 33.86 35.52 1.66

12 41.37 41.05 -.32

13 43.18 42.85 -.33

14 2.16 3.56 1.40

15 45.14 45.14 .00

16 45.65 46.27 .62

17 43.32 42.89 -.43

18 10.17 9.60 -.57

19 -- 4.43 --

20 39.28 38.69 -.59

21 5.05 5.11 .06

22 43.18 43.16 -.02

23 31.87 32.13 .26

24 4.10 4.31 .21

25 6.15 6.71 .56

26 26.63 26.30 -.33

27 5.81 6.20 .39

28 14.90 14.98 .08

29 14.35 13.28 -1.07

30 12.97 12.78 -.19

31 10.62 10.60 -.02

32 14.87 14.04 -.83

Conceptual Model

The regional ground-water flow system was simulated as a quasi three-dimensional ground-water flow 

model with four layers. A challenging aspect of model 

development was the synthesis of the layering schemes 

from the local or regional models into a single regional 

model (figs. 29-36). The SAS, IAS (or ICU in areas 

where the IAS is absent), the UFA, and the LFA were 

designated layers 1 through 4, respectively. The SAS 

(layer 1) was simulated as a source-sink layer. Confining layers were simulated by using vertical leakance 

arrays, similar to the approach used in models 1-3, 5-7, 

and 9-14 (table 1). Model-simulated ground-water flow 

occurs horizontally within the aquifers and vertically 

through the confining units.

The IAS (layer 2) in southwest and south-central 

Florida was simulated as a single active aquifer 

bounded above and below by arrays of leakance values. 

Outside the areas where the IAS is a productive aquifer, 

a 1-ft thick layer was used to simulate the transition of 

the IAS into areas where this aquifer becomes the ICU. 

A transmissivity value of 0.01 ft2/d was used to simulate negligible horizontal flow through the ICU. The 

combination of this low transmissivity and a vertical 

leakance between the SAS and the ICU equal to the 

vertical leakance between the ICU and the UFA 

allowed the ICU to be simulated as a single confining 

unit. The transmissivity value of 0.01 ft2/d does not 

necessarily represent the hydraulic conductivity of the 

ICU in the 1-ft thick layer.

Because this model is restricted to simulating the 

movement of freshwater within the aquifers, areas 

where the IAS, the UFA, and the LFA (layers 2-4) contain water with chloride concentrations exceeding 

5,000 mg/L are considered inactive, thus minimizing 

potential errors introduced by simulating aquifer areas 

containing water of variable density. In general, the 

thickness of the UFA (layer 3) is the difference between 

the top (fig. 6) and the bottom of the UFA (fig. 7). 

In areas where chloride concentrations in the UFA are 

greater than 5,000 mg/L (fig. 19), however, the thickness is assumed to be the difference between the top of 

the UFA (fig. 6) and the base of freshwater. The effective freshwater thickness of the LFA (layer 4) is the difference between the top of the LFA (fig. 8) and either 

the altitude at which water with chloride concentrations 

exceeding 5,000 mg/L occur (fig. 19) or the base of the 

LFA (fig. 10), whichever is higher. Some offshore areas 

in the UFA, estimated to have chloride concentrations 

less than 5,000 mg/L, were considered active areas.
Simulation of Ground-Water Flow 47 Figure 29. Layering scheme and representation of geologic units in local models 1 and 3, and corresponding layering scheme in the regional model (refer to table 1 for model numbers).
48 Simulation of Ground-Water Flow in the Intermediate and Floridan Aquifer Systems in Peninsular Florida

Figure 30. Layering scheme and representation of geologic units in local models 2, 7, 10, and 13, and corresponding layering scheme in the regional model (refer to 

table 1 for model numbers).
Simulation of Ground-Water Flow 49 Figure 31. Layering scheme and representation of geologic units in local models 4 and 8, and corresponding layering scheme in the regional model (refer to table 1 for model numbers).
50 Simulation of Ground-Water Flow in the Intermediate and Floridan Aquifer Systems in Peninsular Florida

Figure 32. Layering scheme and representation of geologic units in local models 5 and 12, and corresponding layering scheme in the regional model (refer to table 1 

for model numbers).
Simulation of Ground-Water Flow 51 Figure 33. Layering scheme and representation of geologic units in local model 6, and corresponding layering scheme in the regional model (refer to table 1 for model number).
52 Simulation of Ground-Water Flow in the Intermediate and Floridan Aquifer Systems in Peninsular Florida

Figure 34. Layering scheme and representation of geologic units in local model 9, and corresponding layering scheme in the regional model (refer to table 1 for model 

number).
Simulation of Ground-Water Flow 53 Figure 35. Layering scheme and representation of geologic units in local model 11, and corresponding layering scheme in the regional model (refer to table 1 for model number).
54 Simulation of Ground-Water Flow in the Intermediate and Floridan Aquifer Systems in Peninsular Florida

Figure 36. Layering scheme and representation of geologic units in local model 14, and corresponding layering scheme in the regional model (refer to table 1 for 

model number).
Simulation of Ground-Water Flow 55

In offshore areas, water is discharged from the UFA to 

the SAS, which was simulated with a water table equal 

to the equivalent freshwater head of the sea water 

column. The saltwater part of the FAS was not included 

in the model because the interface is relatively sharp 

and movement of the interface is assumed to have little 

or no effect on simulated heads (fig. 9). The assumption 

was made that a sharp freshwater-saltwater interface 

occurs laterally and that flow across this interface is 

negligible. This sharp interface determined which 

model areas were considered inactive.

Recharge to or discharge from the IAS is 

assumed to occur through the upper or lower confining 

units of the IAS. Physiographic regions identified as 

ridges are simulated with higher leakance values than 

plains and valley because confining units along ridges 

are thinner and more permeable. As a result, leakage 

rates along Lake Wales Ridge, DeLand Ridge, Orlando 

Ridge, and Crescent City Ridge are higher than those of 

the Osceola Plain or De Soto Plain (fig. 2). Flow entering or leaving the IAS across the southern or western 

boundaries of the model is simulated by the difference 

in hydraulic heads between the IAS and UFA and the 

vertical leakance of the confining units. Flow entering 

or leaving the IAS through the northern or eastern 

boundaries is determined by the water exchanges 

between the IAS and ICU along these boundaries.

The MCU was represented by vertical leakance 

values that limit water exchange between the UFA 

(layer 3) and the LFA (layer 4) in west-central and 

southwest Florida, and in the northwest part of the study 

area to a greater degree than in areas where the MSCU 

is present (fig. 9). However, the rate of water exchange 

between the UFA and LFA in areas where the MCU is 

present should not be zero, as suggested by Hickey 

(1990). The MSCU in east-central, southeast, and 

northeast Florida was represented by an array of vertical 

leakance values that allows water exchange between the 

UFA and the LFA. In areas where no MSCU or MCU is 

present, the vertical exchange of water between the 

UFA and the LFA was simulated at a rate that depends 

on the vertical hydraulic conductivity of the two aquifers, which was assumed to be proportional to the 

horizontal hydraulic conductivity of the aquifers.

The River, Drain, and Recharge packages 

(Harbaugh and McDonald, 1996) were used in this 

ground-water flow model. The River package was used 

to simulate the discharge of water to and from rivers in 

unconfined areas of the UFA (fig. 23). The discharge of 

ground water to swamps in unconfined areas of the 

UFA (fig. 26) and the flow from UFA springs located 

outside river cells were simulated by using the Drain 

package. Flow from springs was simulated as discharge 

to drain cells using the measured or estimated spring 

pool altitude as the drain elevation. Flow from UFA 

springs located in river cells was simulated as the base 

flow of the river at the respective river cells. The net 

recharge rates to unconfined areas of the UFA were 

assigned using the Recharge package.

The solution of the ground-water flow equation 

(eq. 4) allows areal variations in transmissivity to simulate regional heterogeneities. No estimates of anisotropy were available, however, so a lateral anisotropy 

ratio of 1:1 was assumed.

Boundary Conditions

Model boundaries were assigned to approximate 

the true ground-water flow system as accurately as 

possible. The SAS (layer 1) was simulated as a layer of 

constant heads. This allows the UFA to discharge to the 

SAS or to receive leakage from the SAS at rates dictated by the relative difference in head between the 

water table and the UFA and the vertical leakance of the 

ICU in areas where this unit exists. The altitude of the 

water table was used to define these constant heads. A 

no-flow boundary condition was applied along all lateral boundaries of layer 2 (the IAS or ICU). Flow entering or leaving cells in layer 2 is assumed to occur as 

either horizontal flow to neighboring cells or vertical 

flow to either the SAS (layer 1) or the UFA (layer 3).

Lateral boundary conditions for the UFA 

(layer 3) and the LFA (layer 4) were either no-flow or 

specified-head. For the UFA (layer 3), a combination of 

no-flow and specified-head boundaries were used. 

Along the Gulf of Mexico in Citrus, Hernando, and 

Pasco Counties, most of the lateral flow in the UFA is 

assumed to be discharged by numerous springs. Based 

on this observation, no-flow conditions were applied to 

the boundary of layer 3 in those areas (fig. 37). For the 

LFA (layer 4) no-flow conditions were applied along all 

lateral boundaries. The eastern and western boundaries 

of the LFA coincided with the location at which the 

chloride concentration in the LFA exceeds 5,000 mg/L. 

Specified heads in the UFA along the northeastern 

boundary of the model, from about 70 mi offshore from 

Camden County, Ga., to about 20 mi offshore from 

St. Johns County (fig. 37), were set equal to the equivalent freshwater head. The application of this boundary 

condition limits water exchange between the simulated 

SAS and the UFA along these boundary cells. Specified 

heads, interpolated or extrapolated from the 1993-94 

potentiometric surface of the UFA, were assigned to the 

remaining lateral boundaries of the UFA.
56 Simulation of Ground-Water Flow in the Intermediate and Floridan Aquifer Systems in Peninsular Florida

Figure 37. Specified heads along the lateral boundary cells of the Upper Floridan aquifer.
Simulation of Ground-Water Flow 57

Recharge and Discharge

Total discharge from the UFA as spring flow for 

average 1993-94 conditions was estimated to be 

approximately 4,130 Mgal/d (appendix C). Additional 

discharge, in the form of ground-water withdrawals 

from the IAS, UFA, and LFA for the 1993-94 time 

period, was estimated to be approximately 2,490 Mgal/d 

(tables 4 and 5). These withdrawals were applied to the 

corresponding model cells of their respective aquifers, 

and the resulting areal distributions are shown in 

figures 38 through 40. Ground-water withdrawals from 

the IAS exceeded 0.5 Mgal/d in grid cells in Charlotte, 

De Soto, Highlands, and Sarasota Counties (fig. 38). 

Ground-water withdrawals from the UFA exceeded 

5 Mgal/d in grid cells in Alachua, Hamilton, Hernando, Hillsborough, Lake, Manatee, Marion, Nassau, Orange, Pasco, Pinellas, and Polk Counties, and in 

Camden County, Ga (fig. 39). The largest ground-water 

withdrawals (greater than 5 Mgal/d) from the LFA were 

in Orange and Duval Counties (fig. 40).

Artificial recharge to the UFA occurs through 

drainage and injection wells and rapid-infiltration 

basins. Drainage and injection wells (fig. 21) were 

simulated as recharge wells at the respective cell 

locations of the various wells. Recharge rates from 

drainage and injection wells to the UFA (layer 3) were 

highest in Orange County (fig. 39). Rapid-infiltration 

basins were simulated by increased leakance values of 

the ICU rather than directly applied recharge or 

injection wells to the UFA.

Figure 38. Average ground-water withdrawal rates from the intermediate aquifer system, August 1993 

through July 1994.
58 Simulation of Ground-Water Flow in the Intermediate and Floridan Aquifer Systems in Peninsular Florida

Figure 39. Average ground-water withdrawal rates from and injection rates to the Upper Floridan 

aquifer, August 1993 through July 1994.
Simulation of Ground-Water Flow 59

Figure 40. Average ground-water withdrawal rates from the Lower Floridan aquifer, August 1993 

through July 1994.
60 Simulation of Ground-Water Flow in the Intermediate and Floridan Aquifer Systems in Peninsular Florida

Initial Distribution of Hydraulic Properties 

from Local Models

The initial estimate of transmissivity and leakance values for the model area was obtained from the 

previously calibrated local models (table 1). The model 

grid developed in this study (fig. 27), referred to as the 

“regional model grid,” was used as a framework for 

storing and analyzing transmissivity and leakance values obtained from the local models. The data set of all 

transmissivity and leakance values from local models 

was used to generate the initial distribution of these 

properties and to identify areas of discrepancy where 

model areas overlap.

The initial distribution of transmissivity and 

leakance values for the regional model was obtained 

from local models by overlaying the set of center points 

and model grid cells of both the regional model and the 

local models. The center points of the regional model 

grid cells were intersected with the local model grid 

cells; transmissivity and leakance assigned by the local 

models at the center points of the regional model grid 

were stored in corresponding regional grid cells. In 

addition, center points of the local model grid cells 

were intersected with the regional model grid cells; 

transmissivity and leakance values assigned by the 

local models at the center points of the local model grid 

cells also were stored in the corresponding regional 

grid cells.

Transmissivity of the UFA and leakance values 

of the ICU from local models 1 and 3 (appendix A2, 

fig. 29) were stored in the regional model grid. Leakance values for the ICU in the regional model in the 

areas of local models 1 and 3 were computed by multiplying by 2 the leakance from those models. A factor 

of 2 was used because the true vertical thickness of the 

ICU was divided into two units of equal thickness (an 

upper unit between the SAS and the ICU and a lower 

unit between the ICU and the UFA). The equivalent 

leakance of each unit is twice the total leakance of the 

ICU based on the definition of vertical leakance. The 

resulting value was applied as the leakance between the 

SAS and ICU and between the ICU and UFA. This process was repeated in all models that simulate vertical 

leakance of the ICU. Consequently, the vertical leakance of the ICU in areas outside the IAS, to be used in 

areas where there is no IAS, should be equal to one-half 

the simulated vertical leakance in the regional model.

Transmissivity of the IAS and UFA and leakance 

of the upper and lower confining units of the IAS from 

local models 2, 7, 10, and 13 (appendixes A1 and A2; 

fig. 30) were stored in the regional model grid. 

Transmissivity of the LFA in these areas, as well as in 

all areas where flow in the LFA was actively simulated 

in the regional model but not in the local models, was 

calculated by multiplying the estimated thickness of the 

LFA (figs. 8 and 10) by the value of hydraulic conductivity computed from adjacent models. 

Transmissivity values for the UFA in west-central Florida, as well as directly applied net recharge 

rates to the UFA, were the hydraulic parameters from 

local models 4 and 8 (appendix A2, fig. 31) stored in 

the regional model grid. Directly applied net recharge 

rates to the UFA were bracketed by the ranges shown in 

figure 25.

Transmissivity values for the UFA and LFA and 

leakance of the ICU and MSCU in east-central Florida 

were the hydraulic properties from local models 5 and 

12 (fig. 32) that were stored in the regional model grid. 

The main difference between the layering schemes of 

models 5 and 12, compared to model 6, was that model 

6 simulated the LFA with two layers, the lower of 

which was a constant-head layer (fig. 33).

Transmissivity values for the UFA and leakance 

of the ICU were the hydraulic properties from local 

model 9 that were stored in the regional model grid 

(fig. 34). The transmissivity values of the UFA in the 

area encompassed by model 9 were the sum of the 

model-derived transmissivities for layers 2 and 3 

(fig. 34). A study of the simulated hydraulic gradients 

between layers 2 and 3 of the UFA in model 9 found 

these gradients to be minimal (Hancock and Basso, 

1993; Sepúlveda, 2001); accordingly, layers 2 and 3 

were treated in the regional model as a single layer and 

the assigned leakance between layers 2 and 3 was not 

used for this study.

The hydraulic properties from local models 11 

and 14 stored in the regional model grid were transmissivity of the UFA and LFA, and leakance from the ICU 

and the MSCU (figs. 35 and 36). The areal extent of the 

IAS in north-central Florida was simulated in the 

regional model as part of the ICU because the simulated 

transmissivity in local model 11 for the IAS (Motz, 

1995) was characteristic of a confining unit. The simulation of the IAS in model 11 coincides with areas 

where the ICU is present (appendix A1). The transmissivity of the FPZ simulated in models 11 and 14 was 

added to the transmissivity of the LFA only in areas 

where this zone contains water with chloride concentrations less than 5,000 mg/L; otherwise, the FPZ was 
Simulation of Ground-Water Flow 61

excluded from the simulation area (figs. 35 and 36). 

The main difference between the layering schemes of 

models 11 and 14 was that model 11 simulated the IAS 

in parts of north-central Florida as a separate layer, 

whereas model 14 simulated only the ICU (with vertical leakance) because the IAS is absent in northeast 

Florida.

A regional model grid cell, in an area of overlapping local models, for which the transmissivity of the 

IAS, UFA, or LFA in a local model was either less than 

one-half or greater than twice the resulting geometric 

mean of transmissivities from local models, was designated as a cell with a transmissivity discrepancy. The 

geometric mean was assumed to give a better estimate 

of the true mean in heterogeneous media than the arithmetic or harmonic mean (Bouwer, 1978, p. 133). Areas 

of transmissivity discrepancy in the IAS were found 

mainly in parts of Charlotte, De Soto, Polk, Manatee, 

Hardee, and Sarasota Counties (fig. 41). Areas of transmissivity discrepancy in the UFA were mostly in parts 

of Marion, Hernando, Lake, Orange, Hillsborough, 

Hardee, Highlands, Okeechobee, Indian River, and 

St. Lucie Counties (fig. 42). Areas of transmissivity 

discrepancy in the LFA were mostly in parts of 

Alachua, Clay, St. Johns, Putnam, Marion, and Orange 

Counties (fig. 43).

Figure 41. Cells where transmissivity of the intermediate aquifer system in some local models is 

either less than one-half or greater than twice the geometric mean of all transmissivities for the same 

cell.
62 Simulation of Ground-Water Flow in the Intermediate and Floridan Aquifer Systems in Peninsular Florida

Figure 42. Cells where transmissivity of the Upper Floridan aquifer in some local models is either less 

than one-half or greater than twice the geometric mean of all transmissivities for the same cell.
Simulation of Ground-Water Flow 63

Figure 43. Cells where transmissivity of the Lower Floridan aquifer in some local models is either less 

than one-half or greater than twice the geometric mean of all transmissivities for the same cell.
64 Simulation of Ground-Water Flow in the Intermediate and Floridan Aquifer Systems in Peninsular Florida

A regional model grid cell, in an area of 

overlapping local models, for which the leakance of 

the upper or lower confining units of the IAS, the 

ICU, or the MSCU in a local model was not within 

one order of magnitude of the resulting geometric 

mean of leakances from local models, was designated 

as a cell with a leakance discrepancy. Areas of 

leakance discrepancy in the upper confining unit of 

the IAS were mostly in parts of Manatee, Hardee, and 

Highlands Counties (fig. 44). Areas of discrepancy 

in the ICU were mainly in parts of Clay, Putnam, 

Marion, Lake, Polk, Hardee, and Highlands Counties 

(fig. 45). Areas of discrepancy in the MSCU were 

mostly in parts of Clay, St. Johns, Orange, Indian 

River, Okeechobee, and St. Lucie Counties (fig. 46).

In cases where more than one transmissivity or 

leakance value occurred in any of the regional grid cells 

(in areas of overlap of the local models), the set of 

parameters from the local model that resulted in 

smaller differences between simulated and measured 

water levels was used to generate the initial distribution 

of transmissivity and leakance values. The following 

preferential order of local models was used to assign 

transmissivity and leakance in model overlap areas: 

14, 11, 12, 13, 10, 7, 4, 9, 3, 1, 6, 5, 2, and 8. Initial estimates for transmissivity and leakance in areas outside 

the boundaries of these local models but inside the 

regional model area were estimated by extrapolation of 

data and were further refined during calibration.

Initial estimates of transmissivity for the IAS 

were obtained from local models 1, 7, 10, and 13, 

whereas initial estimates of the leakance of the upper 

confining unit of the IAS were obtained from models 1, 

5, 7, 10, and 13. These initial estimates were refined 

during calibration. The differences between the lateral 

extent of models 2 and 7 along the Gulf of Mexico and 

the simulated extent of the IAS shown in appendix A1 

can be attributed to excluding from simulation those 

areas in the IAS with a simulated transmissivity in 

model 7 of less than 1 ft2/d (Barcelo and Basso, 1993).

Figure 44. Cells where leakance of the upper confining unit of the intermediate aquifer system in 

some local models is not within one order of magnitude of the geometric mean of all leakances for the 

same cell.
Simulation of Ground-Water Flow 65

Figure 45. Cells where leakance of the intermediate confining unit in some local models is not within 

one order of magnitude of the geometric mean of all leakances for the same cell.
66 Simulation of Ground-Water Flow in the Intermediate and Floridan Aquifer Systems in Peninsular Florida

Figure 46. Cells where leakance of the middle semiconfining unit in some local models is not within 

one order of magnitude of the geometric mean of all leakances for the same cell.
Simulation of Ground-Water Flow 67

The distribution of transmissivity for the UFA 

was initially selected from models 1 through 14 

(appendix A2) and was refined during calibration. All 

local models, except for the one-layer local models 4 

and 8, were used to generate the initial estimates of the 

leakance of the ICU and the lower confining unit of the 

IAS. A large area in the SRWMD was not simulated by 

the ground-water flow models shown in appendix A2. 

The western lateral boundary of the IAS and that of the 

UFA were assumed to coincide along the Gulf of 

Mexico, an assumption made because the lack of 

chloride data precluded mapping the altitude at which 

5,000-mg/L concentrations of chloride are present in 

the Gulf of Mexico. The IAS and UFA share a common 

western boundary, which is the reason for the differences between the lateral extent of the UFA along the 

Gulf of Mexico in models 2 and 7 and that simulated in 

the regional model (appendix A2). Differences 

between the lateral extent of the simulated area in the 

regional model and the lateral extent of models 4, 5, 

and 6 in the Gulf of Mexico and the Atlantic Ocean 

are due to differences between the grid resolution of 

these models and that of the regional model grid.

Local models 5, 6, 11, 12, and 14 (appendix A3) 

were used to generate the initial distribution of transmissivity of the LFA and the leakance of the MSCU. 

Large differences between the lateral extent of models 

5, 6, and 14 along the Atlantic Ocean and the simulated 

extent of the LFA (fig. 40) in the regional model are due 

to the exclusion from simulation of aquifer areas containing water with chloride concentrations greater than 

5,000 mg/L.

Calibration of Ground-Water Flow Model

Computed average heads in the IAS, UFA, and 

LFA for the August 1, 1993, to July 31, 1994, steadystate period were the control points used to calibrate the 

ground-water flow model. The number of control 

points for the IAS, UFA, and LFA respectively were: 

118; 1,460; and 46. The measured or estimated spring 

flows (appendix C) and base-flow estimates of rivers 

in the unconfined areas of the UFA also were used for 

comparison to calibrate the model. No parameter 

estimation technique was used to achieve model 

calibration.

Calibration of the regional ground-water flow 

model was accomplished by adjusting input hydraulic 

parameters within reasonable ranges from the initial 

distribution of values until the model closely approximated observed field conditions based on aquifer 

heads, spring flows, and river flows. The “goodness” or 

improvement of the calibration generally is based on 

the differences between simulated and measured or 

estimated heads, spring flows, and stream discharges. 

Simulated heads and flows from a calibrated, deterministic ground-water model commonly depart from measured heads and flows, even after a diligent calibration 

effort. The difference between model results and what 

actually occurs in the aquifers, referred to as model 

error, is the cumulative result of simplification of the 

conceptual model, grid scale, measurement errors, and 

the difficulty in obtaining sufficient measurements to 

account for all of the spatial variation in hydraulic 

properties throughout the model area. Calibration was 

considered to be achieved when the RMS residual for 

water levels was comparable to the error associated 

with the time average of measurements used to calibrate the model, namely the RMS residual associated 

with the approximation of the altitude of the water table 

of the SAS (3.53 ft, table 2). The calibration criterion 

used for spring flows was to be as close as possible to 

the measured or estimated flow of major springs, as 

long as none of the hydraulic values used deviated substantially from the initial distribution of values.

Hydraulic parameters that were adjusted during 

calibration of the ground-water flow model included: 

the net recharge rate to unconfined areas of the UFA; 

the transmissivity of the IAS, UFA, and LFA; the riverbed conductance (where the UFA is unconfined); the 

leakance of the upper and lower confining units of the 

IAS, ICU, MCU, and MSCU; and the conductance of 

swamps and springs in the UFA.

The calibration process was iterative and consisted of (1) assigning initial hydraulic parameters 

from local models; (2) comparing simulated and average heads and simulated and estimated spring flows for 

the steady-state period of 1993-94; and (3) adjusting 

hydraulic parameters aimed at minimizing the differences between simulated and average heads and 

between simulated and estimated spring flows for 

1993-94. The guiding principle of calibration was that 

the model parameter with the highest sensitivity for a 

given area or aquifer was adjusted first; other less 

sensitive parameters were adjusted only if reasonable 

residuals were not achieved. In cases where two 

parameters were about equally sensitive, each was 

adjusted separately.
68 Simulation of Ground-Water Flow in the Intermediate and Floridan Aquifer Systems in Peninsular Florida

The initial distribution of hydraulic parameters 

from the local model was used to define contiguous 

zones of equal values. The areal extents of the transmissivity zones for the IAS, UFA, and LFA were independent from each other. The areal extents of the leakance 

zones for the upper and lower confining units of the 

IAS, ICU, MCU, and MSCU were also independent 

from each other. The areal extents of these zones were 

modified during calibration.

Because measured water levels rarely coincide 

with the center of a cell, simulated water levels at observation wells were interpolated laterally from simulated 

heads at the centers of the four cells surrounding the 

observation well (fig. 47) to allow for a continuous distribution of water levels. The neighboring cells used in 

the bilinear interpolation were those for which the coordinates of the center of the cells completely encompassed the coordinates of the observation well (fig. 47). 

Vertical interpolation was not necessary because of the 

discontinuity and associated refraction of potential 

fields across zones of differing transmissivities.

Simulated and measured water levels in the IAS, 

UFA, and LFA agree reasonably well throughout most 

of the study area (fig. 48). About 85 percent of simulated water levels are within 5 ft of measured values, 

with RMS values of 3.47, 3.41, and 2.89 ft for the IAS, 

UFA, and LFA, respectively (table 10). The RMS residual for all layers in the model was 3.40 ft. The histogram of residuals for the UFA appears to be normally 

distributed (fig. 48), indicating the error in the model 

also is normally distributed.

Figure 47. Bilinear interpolation used to determine simulated water levels at observation 

wells from simulated water levels at the center of cells.
Simulation of Ground-Water Flow 69

Figure 48. Comparison of simulated to measured water levels in the (A) 

intermediate aquifer system, (B) Upper Floridan aquifer, and (C) Lower 

Floridan aquifer for the calibrated model.
70 Simulation of Ground-Water Flow in the Intermediate and Floridan Aquifer Systems in Peninsular Florida

Transmissivity of the Intermediate Aquifer System

The process of modifying the initial transmissivity distribution of the IAS during calibration included: 

(1) testing all IAS transmissivity values from local 

models in the discrepancy areas shown in figure 41; 

(2) finding values that decreased the absolute value of 

residuals at control points within the discrepancy 

areas; (3) using recent transmissivity estimates for 

Sarasota County (Knochenmus and Bowman, 1998); 

(4) assigning low transmissivity values along the western, eastern, and northern boundaries of the IAS to simulate decreasing aquifer thickness; (5) making limited 

transmissivity changes in and near areas of potentiometric-surface highs (fig. 17) to reduce residuals at 

nearby control points; and (6) making transmissivity 

changes to areas in the IAS where residuals were 

unreasonably large and sensitive to changes in 

transmissivity.

Transmissivity in the IAS from the calibrated 

model ranged from 100 ft2

/d along the northern, eastern, and western boundaries of the IAS to 30,000 ft2/d 

in parts of Sarasota County (fig. 49). All transmissivity 

values for this model were rounded to two significant 

figures to simplify the calibration process because 

more significant figures for transmissivity values generally did not result in meaningful changes in simulated 

water levels. Changes made to the initial distribution of 

transmissivity of the IAS (fig. 49) generally were small 

because heads in the IAS were more sensitive to changes 

in the leakance of the confining units of the IAS.

Transmissivity of the Upper Floridan Aquifer

Substantial changes to the initial distribution of 

transmissivity of the UFA were required to decrease the 

absolute value of differences between simulated and 

computed average heads for the calibration period. 

Procedures in addition to those discussed in the previous section for the IAS were required to modify the 

transmissivity of the UFA during model calibration. 

The simulation of flow from several springs not 

included in any of the local models resulted in a higher 

transmissivity in the vicinity of these springs than the 

initial values. Steep hydraulic gradients in the potentiometric surface of the UFA in some areas needed to be 

simulated with “barriers” of low transmissivity zones 

not included in the initial distribution of transmissivity. 

The effects of replacing specified- or general-head 

boundaries used in local models with internally simulated heads in the regional model generally required 

changes to the initial transmissivity distribution. The 

effects of including ground-water withdrawals in areas 

of the regional model that were not included in local 

models also required changes to the initial transmissivity distribution for the UFA.

Transmissivity of the UFA from the calibrated 

model ranged from 3,000 ft2/d in areas where either 

limestone comprising the aquifer has low permeability 

or where the UFA is thin to 12,000,000 ft2

/d in areas of 

cavernous limestone near springs. Transmissivity values were low in recharge areas in north-central Lafayette County, central Gilchrist County, east-central 

Bradford County, parts of Levy and Marion Counties, 

north Lake County, and north-central Volusia County 

(fig. 50). Areas in the vicinity of springs were given 

higher transmissivity values than the surrounding 

areas, which allowed more water to reach the springs. 

The highest transmissivity values were in the vicinity 

of Alapaha Rise, Silver Springs, Rainbow Springs, and 

Crystal River Spring Group (figs. 22 and 50). These 

springs all had estimated or measured flows in excess 

of 400 ft3/s (appendix C).

Table 10. Water-level residual statistics for the calibrated model

Aquifer

Number of 

water-level 

measurements

Minimum 

residual

(feet)

Maximum 

residual

(feet)

Mean 

residual

(feet)

Root-meansquare 

residual

(feet)

Intermediate aquifer 

system

118 -9.62 6.89 -0.18 3.47

Upper Floridan aquifer 1,460 -9.90 9.90 -.19 3.41

Lower Floridan aquifer 46 -6.90 5.21 .17 2.89

Entire model 1,624 -9.90 9.90 -.18 3.40
Simulation of Ground-Water Flow 71

Transmissivity of the Lower Floridan Aquifer

The distribution of transmissivity of the LFA 

from the calibrated model (fig. 51) was generated, in 

part, by identifying the values that decreased the 

absolute value of residuals at control points within 

the discrepancy areas (fig. 43). Simulated transmissivity values in Orange, Seminole, and Lake Counties 

(Brian McGurk, SJRWMD, 2000, written commun.) 

were evaluated and incorporated in the LFA transmissivity distribution in areas where these values 

decreased the residuals at control points. Transmissivity values in west-central Florida and in the northwest areas of the model depended on the thickness of 

the LFA and the equivalent hydraulic conductivity 

assigned in adjacent models in east-central Florida 

(figs. 29-31).

Figure 49. Transmissivity of the intermediate aquifer system from the calibrated model.

Changes to the initial distribution of transmissivity values for the LFA were made in zones where residuals at control points were unreasonably large; these 

changes were of regional scale because of the paucity of 

water levels available from the LFA. Transmissivity values for the LFA in areally extensive parts of northeast 

Florida agreed with those used by Durden (1997).

Transmissivity values for the LFA and derived 

from the calibrated model ranged from 5,000 ft2

/d 

along parts of the lateral boundaries of the LFA to 

760,000 ft2

/d in northeast Florida. High transmissivity 

values also were simulated in parts of Orange County 

and south Florida (fig. 51). Transmissivities are low 

(5,000 ft2/d) along the lateral boundary of the LFA, 

reflecting a limited movement of freshwater through a 

thin part of the aquifer because of proximity to the freshwater-saltwater interface.
72 Simulation of Ground-Water Flow in the Intermediate and Floridan Aquifer Systems in Peninsular Florida

Figure 50. Transmissivity of the Upper Floridan aquifer from the calibrated model.
Simulation of Ground-Water Flow 73

Figure 51. Transmissivity of the Lower Floridan aquifer from the calibrated model.
74 Simulation of Ground-Water Flow in the Intermediate and Floridan Aquifer Systems in Peninsular Florida

Leakance of the Upper Confining Unit of the 

Intermediate Aquifer System

Prescribing the distribution of the water table of 

the SAS (as discussed previously) rather than using the 

water-table altitudes from local models required modifying the initial distribution of leakances. Simulated 

heads in the IAS were sensitive to simulated leakance 

values of the upper confining unit. Leakance values 

were adjusted so that the simulated leakage rates were 

within the range of values of simulated leakage rates in 

models 7, 10, and 13.

As shown in figure 52, simulated leakance of the 

upper confining unit of the IAS ranged from 

1.0 x 10-6 (ft/d)/ft to 1.4 x 10-3 (ft/d)/ft. These leakance values resulted in simulated leakage rates to the 

IAS that ranged from upward leakage of about 60 in/yr 

in parts of Hardee County to downward leakage of 

about 34 in/yr in parts of Highlands County (fig. 53). 

Most of Hardee County, however, is a recharge area of 

the IAS. The highest leakance values are in the Lake 

Wales Ridge physiographic area (fig. 2), where aquifer 

recharge is highest and confining beds are relatively 

thin and permeable (fig. 53). Simulated leakance values 

within the De Soto Plain physiographic area (fig. 2) 

that were approximately the same magnitude as those 

of the Lake Wales Ridge area, resulted in downward 

vertical leakage rates lower than in the Lake Wales 

Ridge area (fig. 53). This indicates that the vertical 

hydraulic gradients between the SAS and the IAS in the 

Lake Wales Ridge area are higher than those in the De 

Soto Plain. Large areas of the IAS have simulated vertical leakage rates (upward or downward) less than or 

Figure 52. Leakance of the upper confining unit of the intermediate aquifer system from the 

calibrated model.
Simulation of Ground-Water Flow 75

equal to 1.0 in/yr (fig. 53), resulting from a relatively 

low leakance value for the upper confining unit of the 

intermediate aquifer system.

Leakance of the Intermediate Confining Unit

The initial leakance distribution of the ICU was 

modified in some areas to reduce the simulated downward leakage rates to a fraction of the 1993-94 rainfall, 

except in areas of artificial recharge such as rapid-infiltration basins. In such areas, leakance values were 

adjusted so that the simulated leakage rates were within 

the range of simulated leakage rates for models shown 

in appendix A1. The range of simulated leakage rates 

was used to determine the sensitivity of simulated 

heads to the vertical leakance values. If changes in 

leakage rates did not substantially reduce the magnitude of the residuals, then transmissivity was adjusted.

Simulated leakance values of the ICU ranged 

from 1.0 x 10-6 (ft/d)/ft to 7.0 x 10-3 (ft/d)/ft (fig. 54). 

This range of values included simulated leakance 

values of the lower confining unit of the IAS as an 

extension of the ICU throughout the simulated extent 

of the IAS. The largest leakance values were simulated 

in parts of Orange, Pasco, Hillsborough, and Hardee 

Counties. The initial leakance values were increased in 

recharge areas of the UFA to increase simulated water 

levels as needed to match water levels at control points. 

In discharge areas of the UFA, leakance values of the 

Figure 53. Simulated vertical leakage rates to and from the intermediate aquifer system through the 

upper confining unit, average August 1993 through July 1994 conditions.
76 Simulation of Ground-Water Flow in the Intermediate and Floridan Aquifer Systems in Peninsular Florida

Figure 54. Leakance of the intermediate confining unit and the lower confining unit of the intermediate 

aquifer system from the calibrated model.
Simulation of Ground-Water Flow 77

ICU were decreased from initial values to increase simulated heads as needed. Changes in simulated heads in 

the UFA caused by adjustments to transmissivity 

values were assessed in separate runs of the model. 

The main reasons for the spatial variability in leakance 

values of the ICU are the spatial variability in ICU thickness, and in vertical hydraulic gradients between the 

SAS and the UFA and between the IAS and the UFA 

(where the IAS is present).

Upward leakage was as high as 80 in/yr in parts 

of coastal Hillsborough County near Hillsborough 

Bay, whereas downward leakage was as high as 

108 in/yr in southwest Orange County (fig. 55). 

The largest downward leakage rates to the UFA 

were simulated in areas where artificial recharge 

occurs through rapid-infiltration basins (O’Reilly, 

1998). Most of the leakage rates to and from the UFA 

were less than 10 in/yr. High downward leakage rates 

to the UFA also were simulated in parts of physiographic region 4 (fig. 2), particularly along the 

Lake Wales Ridge region (fig. 55). Simulated leakage 

rates to the UFA in SFWMD were consistent with 

previous estimates by Fairbank and Hohner (1995, 

plate VI).

Leakance of the Middle Confining and 

Middle Semiconfining Units

The presence or absence of evaporites is the 

main reason for a marked difference in leakance for 

the MCU as compared to the MSCU (Miller, 1986). 

The simulated leakance of the MCU ranged from 

6.2 x 10-6 to 1.4 x 10-4 (ft/d)/ft (fig. 56). The largest 

simulated leakance of the MSCU was 2.0 x 10-3

(ft/d)/ft. As indicated by Hickey (1990), the leakance 

of the MCU could be as high as 5.0 x 10-4 (ft/d)/ft, 

assuming a uniform thickness of 200 ft for the MCU 

and a vertical hydraulic conductivity of 0.1 ft/d. 

Areas where neither the MCU or the MSCU are 

present were simulated with larger leakance values 

than those simulated for the MCU (fig. 56). Leakance 

values in those areas were derived from the simulated 

transmissivity of the UFA, the estimated freshwater 

thickness of the UFA, and an estimated ratio of 

100:1 between horizontal and vertical hydraulic 

conductivity.

Large leakance values also were simulated in 

areas of strong hydraulic connection between the 

UFA and LFA, indicated by the relatively small 

hydraulic gradients between these two aquifers. 

These areas include parts of Brevard, Duval, Indian 

River, Marion, Nassau, and Orange Counties, and 

Charlton County, Ga. The simulated large leakance 

values in areas of Baker, Nassau, Duval, and Clay 

Counties, and Charlton and Ware Counties, Ga., 

combined with large transmissivities in the LFA for 

the same areas (fig. 51), could explain the source of 

water withdrawn from the LFA in discharge areas of 

Duval County.

The simulated vertical leakage rate in areas where 

the MCU is present was mostly less than 1.0 in/yr 

(fig. 57), a consequence of the low simulated leakance for the MCU. In contrast, high simulated leakage to the LFA occurred in parts of Alachua, Clay, 

Highlands, Lake, Orange, Polk, and Putnam Counties 

(fig. 57). Some parts of Highlands County, adjacent 

to the MCU-MSCU transition zone, had simulated 

downward leakage rates to the LFA that ranged from 

5 to 8 in/yr.

Recharge and Discharge Areas of the 

Upper Floridan Aquifer

Rivers represent a feature where both recharge 

to and discharge from the UFA can occur. Flow 

between the aquifer and rivers in unconfined areas of 

the UFA was simulated using the River package of 

the MODFLOW-96 River Package (Harbaugh and 

McDonald, 1996). Simulated flow between the aquifer and the river is determined by riverbed conductance and the difference between river stage and the 

simulated water level in the river cell. The river stage 

and altitude of the riverbed bottom were assigned by 

using linear interpolation of measured stages at 

gaging stations (fig. 58).

Conductance values assigned to riverbed 

cells depended on the interaction between the UFA 

and the rivers. The potentiometric surface of the 

UFA suggests that river-aquifer interactions along 

the meanderings of the Steinhatchee River, 

Waccasassa River, and parts of the Withlacoochee 

and Hillsborough Rivers in west-central Florida is 

not as strong as the interactions along the Suwannee, Santa Fe, and Ichetucknee Rivers, and the 

Withlacoochee River in Madison County (figs. 18 

and 58). 
78 Simulation of Ground-Water Flow in the Intermediate and Floridan Aquifer Systems in Peninsular Florida

EXPLANATION

VERTICAL LEAKAGE RATE, IN INCHES PER YEAR -- Negative leakage rates indicate

upward leakage and positive leakage rates indicate downward leakage

-19.99

0.01

3.01

10.01

3.00 25.01 - 108.00

10.00

25.00

-

-

-

-

-

-

-10.00 -9.99

-80.00 -20.00

0.00

0

0 25

25

50 MILES

50 KILOMETERS

ATLANTIC OCEAN

GULF OF MEXICO

  

 

 

	






FLORIDA

GEORGIA

BRADFORD

OSCEOLA

BAKER

DUVAL

BREVARD

INDIAN

RIVER

ST. LUCIE

MARTIN

OKEECHOBEE

HIGHLANDS

GLADES

HARDEE

DE SOTO

CHARLOTTE

SARASOTA

MANATEE

HILLSBOROUGH

PINELLAS

PASCO

HERNANDO

CITRUS

ALACHUA

LEVY

DIXIE

LAFAYETTE GILCHRIST

UNION

SUWANNEE

COLUMBIA

MADISON

HAMILTON

WARE

CHARLTON CAMDEN

NASSAU

CLAY ST. JOHNS

MARION

PUTNAM

FLAGLER

POLK

SUMTER

LAKE

SEMINOLE

ORANGE

VOLUSIA

Hillsborough

Bay

Simulated extent of the

intermediate confining unit

within the study area

Simulated

extent of the

intermediate

aquifer system

within the

study area

Base modified from U.S. Geological Survey digital data; 1:100,000, 1985

Universal Transverse Mercator projection, zone 17

Extent of

simulated area

Figure 55. Simulated vertical leakage rates to and from the Upper Floridan aquifer, average August 

1993 through July 1994 conditions.
Simulation of Ground-Water Flow 79

Figure 56. Leakance of the middle confining and middle semiconfining units from the calibrated 

model.
80 Simulation of Ground-Water Flow in the Intermediate and Floridan Aquifer Systems in Peninsular Florida

Figure 57. Simulated vertical leakage rates to and from the Lower Floridan aquifer through the middle 

confining and middle semiconfining units, average August 1993 through July 1994 conditions.
Simulation of Ground-Water Flow 81

Figure 58. Specified stages along the simulated river cells in the unconfined 

areas of the Upper Floridan aquifer, average August 1993 through July 1994 

conditions.
82 Simulation of Ground-Water Flow in the Intermediate and Floridan Aquifer Systems in Peninsular Florida

Figure 59. Simulated riverbed conductance of river cells in the unconfined 

areas of the Upper Floridan aquifer.

A strong hydraulic connection 

between the karstic aquifer and a 

river cell was reflected by a large 

riverbed conductance. For example, 

water levels in the UFA near some 

sections of the Suwannee River are 

nearly the same as the river stage, 

supporting the large riverbed conductance values. The simulated 

riverbed conductance of a cell that 

did not contain a UFA spring generally was lower than that of a cell that 

contained a spring (fig. 59). The 

riverbed conductance of a cell with a 

spring was adjusted based on the 

estimated or measured spring flow 

and the head difference between the 

river stage and the simulated head at 

the river cell.

A comparison of estimated 

and simulated base flows of rivers in 

the unconfined areas of the UFA 

(table 11) shows that the simulated 

base flow generally is lower than the 

estimated base flow. Reasons for this 

include the fact that riverbank storage is not accounted for in the simulated base flow. Also, the drainage 

area for four of the seven rivers is 

larger than the drainage area of the 

unconfined simulated sections of the 

rivers. Finally, the simulated base 

flow does not reflect the hydraulic 

routing of discharge from UFA 

springs and swamps to river cells.

Simulated recharge to and discharge from the UFA occurs across 

lateral boundaries of the model at 

specified-head cells (fig. 37). Simulated hydraulic gradients in the vicinity of the boundaries and simulated 

transmissivity values at the specified 

head cells determine the flux and 

direction across these boundaries. 

Simulated flows leaving or entering 

the model area along the eastern 

and most of the northern, southern 

and western boundaries were less than 

1 ft3

/s per cell (fig. 60). Water enters 

the model area at rates equal to 
Simulation of Ground-Water Flow 83

a

Based on October 1998 through September 1999 discharge data.

b

Includes base flow of Withlacoochee River - SRWMD.

c

Includes base flow of Santa Fe River and Withlacoochee River - SRWMD.

d

Includes base flow of Ichetucknee River.

Table 11. Estimated and simulated base flow of rivers in the unconfined areas of the Upper Floridan aquifer, average 

August 1993 through July 1994 conditions

[Station number refers to figure 23. Simulated base flow is flow captured by river cells in unconfined areas of the Upper Floridan aquifer, which generally 

have a smaller drainage area than the river basins. SRWMD, Suwannee River Water Management District; SWFWMD, Southwest Florida Water Management District; mi2

, square miles; ft3/s, cubic feet per second; --, does not apply or not available]

USGS

station 

number

Station name Drainage area 

(mi2)

Average daily 

discharge

(ft3/s)

Estimated 

average base 

flow (ft3/s)

Simulated 

base flow 

(ft3/s)

02303000 Hillsborough River near Zephyrhills 220 123 67 35

02312000 Withlacoochee River at Trilby 570 47 25 30

02313000 Withlacoochee River near Holder 1,825 319 252 121

02313700 Waccasassa River near Gulf Hammock 480 a193 a89 79

02319000 Withlacoochee River near Pinetta 2,120 1,714 621 6

02319500 Suwannee River at Ellaville 6,970 5,961 b1,933 b683

02320500 Suwannee River at Branford 7,880 6,678 b3,269 b1,430

02322500 Santa Fe River near Fort White 1,017 1,067 925 275

02323500 Suwannee River near Wilcox 9,640 9,410 c5,056 c2,502

02324000 Steinhatchee River near Cross City 350 205 36 17

-- Unconfined sections of Hillsborough River -- -- -- 68

-- Unconfined sections of Withlacoochee River - SWFWMD -- -- -- 92

-- Unconfined sections of Withlacoochee River - SRWMD -- -- -- 90

-- Ichetucknee River -- -- -- 305

-- Unconfined sections of Santa Fe River -- -- -- d 872

-- Unconfined sections of Suwannee River -- -- -- c

2,585

or greater than 2 ft3/s per cell across a few cells on the 

northern and western boundaries, and some cells on the 

southern boundaries in Charlotte County. Flow rates of 

similar magnitude leave the model area across cells on the 

northern boundary and in coastal Pinellas County 

(fig. 60). Flow leaving the model area totaled 462 ft3

/s, 

with 113 ft3/s across the northern boundary, 40 ft3/s 

across the eastern boundary, 53 ft3

/s across the southern 

boundary, and 256 across the western boundary. Flow 

entering the model area totaled 317 ft3/s, with the highest flow rate (151 ft3/s) across the southern boundary 

and the lowest flow rate (18 ft3

/s) across the eastern 

boundary. Flow entering the model area across the 

northern boundary totaled 52 ft3

/s, whereas 96 ft3/s 

entered across the western boundary. entering the 

model area along the Gulf of Mexico (western boundary) totaled 5 ft3

/s. Flows entering the model area 

across cells in the Gulf of Mexico and Atlantic Ocean 

probably are induced by pumping from the UFA near 

these boundaries (fig. 39).

Mapped recharge and discharge areas were 

compared with areas in which vertical recharge to and 

discharge from the UFA were simulated (fig. 20). Discrepancies between the areal distributions of mapped 

and model-simulated recharge and discharge areas of 

the UFA (figs. 20 and 61) are isolated throughout the 

model area and occur mainly at cells where the hydraulic gradient between the SAS and the UFA (or the SAS 

and the IAS) is relatively small. In the unconfined areas 

of the UFA, discrepancies between mapped and simulated recharge and discharge areas occurred at some 

river and swamp cells. Some swamp cells in Sumter 

County, mapped as discharge areas, were identified as 

recharge areas because the simulated net recharge rate 

at each cell exceeded the simulated flux drained out of 

the cell.

Simulated net recharge rates to unconfined areas 

of the UFA were adjusted from a range of values established by the maximum and minimum recharge rates 

presented previously (fig. 25), average water levels, and 

spring flow. Net recharge rates are high near areas 

where the water table is deep, as is the case in the 

Northern Highlands region (fig. 2), and generally 

decrease coastward where evapotranspiration is high. 
84 Simulation of Ground-Water Flow in the Intermediate and Floridan Aquifer Systems in Peninsular Florida

Figure 60. Simulated lateral flow to and from the Upper Floridan aquifer across model boundaries, 

average August 1993 through July 1994 conditions.
Simulation of Ground-Water Flow 85

Figure 61. Location of differences between simulated and estimated recharge and discharge cells in 

the Upper Floridan aquifer, average August 1993 through July 1994 conditions.
86 Simulation of Ground-Water Flow in the Intermediate and Floridan Aquifer Systems in Peninsular Florida

Net recharge rates to the aquifer, 

which ranged from 0 to 24 in/yr 

(fig. 62), were rounded off to integers. 

Cells with specified heads along lateral boundaries of the UFA were not 

assigned recharge rates (fig. 62). 

The highest recharge rate, 24 in/yr, 

was simulated in parts of Suwannee 

and Columbia Counties. The average 

of all net recharge rates was about 

13.6 in/yr for 6,364 cells in the unconfined areas of the UFA (approximately 

5,700 mi2

). Total recharge to unconfined areas of the UFA was about 

5,720 ft3

/s.

Spring Flow

Flow from springs (fig. 22) 

located outside river cells was simulated by drain cells; flow from springs 

in river cells was simulated as the flow 

from the aquifer to the river (fig. 59). 

Spring-pool elevation (table 12) was 

used as the drain-cell elevation. The 

simulated spring-pool elevation of 

springs in river cells was the computed river stage of the cell. Conductance at either drain or river cells 

was adjusted to reduce the difference between estimated or measured 

and simulated spring flow. 

Of the 156 springs in the study 

area, 130 had flows simulated with a 

residual (difference between measured or estimated and simulated 

flow) of less than 5 ft3

/s; 11 spring 

flows were simulated with a residual 

between 5 and 10 ft3

/s; 10 spring 

flows were simulated with a residual 

between 10 and 20 ft3

/s; and 5 had 

residuals greater than 20 ft3

/s. The 

largest residuals occurred at springs 

simulated as river cells. Simulated 

flows at Blue Springs and Alapaha 

Rise (springs 1 and 2, respectively, 

fig. 22) were underestimated by about 

28 and 54 ft3

/s, respectively (table 12). 

Simulated spring flow was 96 percent of the total estimated 6,380 ft3

/s 

spring flow for 1993-94.

Figure 62. Simulated recharge rates to the unconfined areas of the Upper 

Floridan aquifer, average August 1993 through July 1994 conditions.
Simulation of Ground-Water Flow 87

Table 12. Comparison of measured or estimated and residual flows from Upper Floridan aquifer springs, average 

August 1993 through July 1994 conditions

[Spring number refers to figure 22. Row and column refer to model grid. Flows from springs in the same row and column were combined. River cells: 

WNW, Withlacoochee River in northwest Florida; WWC, Withlacoochee River in west-central Florida; SUR, Suwannee River; ICH, Ichetucknee River; 

STR, Steinhatchee River; SFR, Santa Fe River; WAC, Waccasassa River; and HIR, Hillsborough River. Springs simulated as river cells are indicated by the 

name of the nearest river; drain, indicates spring was simulated as drain cell. Residual flow is simulated flow minus measured or estimated flow; ft3

/s, cubic 

feet per second]

Spring 

number Spring name Row Column River

cell

Measured 

or 

estimated 

flow

 (ft3/s)

Residual 

flow

(ft3/s)

Pool 

elevation 

or river 

stage 

(feet)

1 Blue Spring near Madison 41 7 WNW 118.0 -28.2 42.0

2 Alapaha Rise near Fort Union 44 17 SUR 427.0 -54.1 36.2

3 Holton Spring near Fort Union 44 19 drain 12.5 .0 38.3

4 Suwannee Springs near Live Oak 47 27 SUR 9.8 5.2 41.5

5, 6 Suwanacoochee Spring and Ellaville Spring at Ellaville 48 12 SUR 112.0 -.8 33.0

7 Falmouth Spring at Falmouth 49 14 drain 134.0 7.8 32.5

8 White Sulphur Springs at White Springs 52 37 drain 42.3 11.1 50.0

9 Charles Springs near Dell 63 8 drain 4.7 -.1 24.5

10 Allen Mill Pond Spring near Dell 64 7 SUR 12.2 -1.1 23.8

11 Wadesboro Spring near Orange Park 65 103 drain 1.0 .1 24.0

12 Blue Spring near Dell 66 8 SUR 70.0 -16.0 23.6

13 Peacock Springs 67 14 drain 81.1 .4 19.0

14 Telford Spring at Luraville 68 12 SUR 35.8 -3.8 21.6

15 Running Springs (East and West) near Luraville 68 15 SUR 88.0 -.4 19.9

16 Convict Spring near Mayo 69 16 SUR 1.1 .2 19.6

17 Royal Spring near Alton 70 17 SUR 1.9 -.3 19.0

18 Owens Spring 72 19 SUR 43.3 -1.2 17.4

19 Mearson Spring near Mayo 73 20 SUR 51.0 -1.2 17.0

20 Troy Spring near Branford 75 22 SUR 132.0 -18.3 16.0

21 Little River Springs near Branford 76 24 SUR 67.0 -7.8 14.9

22 Ruth Spring near Branford 76 23 drain 7.5 .1 16.5

23 Green Cove Springs at Green Cove Springs 77 106 drain 3.0 .1 21.0

24, 25 Ichetucknee Head Spring near Fort White and Cedar Head 

Spring

 77 37 drain 49.0 6.9 22.0

26-29 Blue Hole, Roaring, Singing, Boiling, Mill Pond, Grassy 

Hole, and Coffee Springs (parts of Ichetucknee Springs)

 78 37 ICH 258.0 .5 19.0

30 Branford Springs at Branford 79 26 SUR 35.8 -5.3 13.2

31 Jamison Spring 81 37 drain 3.0 .0 16.5

32 Hornsby Spring near High Springs 87 48 drain 49.8 1.3 30.5

33, 34 Turtle Spring near Hatchbend and Fletcher Spring 87 29 SUR 61.9 -10.1 9.3

35 Steinhatchee Spring near Clara 87 2 STR .7 .5 21.1

36 Ginnie Spring near High Springs 88 41 SFR 57.1 -.9 23.3

37 Blue Springs near High Springs (including Lilly Springs) 89 42 drain 41.2 .3 24.5

38 Poe Springs near High Springs 89 44 SFR 53.6 -.3 26.3

39 Rock Bluff Springs near Bell 91 27 SUR 33.2 -7.2 9.0

40 Guaranto Spring near Rock Bluff Landing 92 26 SUR 12.0 -1.6 7.9

41 Crescent Beach Submarine Spring 94 135 drain 30.0 12.6 0.5
88 Simulation of Ground-Water Flow in the Intermediate and Floridan Aquifer Systems in Peninsular Florida

42, 43 Lumbercamp Springs and Sun Springs near Wannee 97 26 drain 46.3 -1.7 4.0

44 Hart Springs near Wilcox 100 25 drain 90.8 -2.0 4.2

45 Otter Springs near Wilcox 102 25 drain 16.0 -.3 2.0

46 Whitewater Springs 103 107 drain 1.2 .2 23.5

47 Copper Springs near Oldtown (including Little Copper 

Spring)

104 23 SUR 25.4 -6.8 4.7

48 Bell Springs 105 25 drain 5.1 -.1 2.0

49 Fannin Springs near Wilcox (including Little Fannin 

Spring)

106 26 drain 97.7 -.3 .5

50 Satsuma Spring 111 106 drain 1.1 .0 18.0

51 Blue Springs near Orange Springs 112 94 drain .5 .0 31.0

52 Orange Spring at Orange Springs 112 89 drain 2.0 -.5 54.5

53 Camp Seminole Spring at Orange Springs 113 88 drain .8 -.4 54.5

54 Welaka Spring near Welaka 114 106 drain 1.0 -1.0 11.0

55 Manatee Spring near Chiefland 113 23 drain 187.0 -9.3 2.8

56 Mud Spring near Welaka 116 106 drain 2.3 .2 8.3

57 Blue Spring near Bronson 116 40 WAC 8.0 -.5 38.4

58 Beecher Springs near Fruitland 117 107 drain 6.3 .0 2.0

59 Croaker Hole Spring near Welaka 118 105 drain 90.3 1.5 6.8

60 Tobacco Patch Landing Spring Group near Fort McCoy 118 90 drain 1.0 .0 30.8

61 Wells Landing Springs near Fort McCoy 119 90 drain 5.0 -.1 30.8

62 Salt Springs near Eureka 124 102 drain 79.0 .3 1.8

63 Wekiva Springs near Gulf Hammock 129 43 drain 45.4 -.2 24.5

64 Silver Glen Springs near Astor 132 108 drain 100.0 -21.2 1.9

65 Sweetwater Springs along Juniper Creek 134 106 drain 12.5 2.5 7.0

66 Silver Springs near Ocala 134 81 drain 640.0 -19.7 40.0

67, 68 Morman Branch Seepage into Juniper Creek and Juniper 

Creek Tributary near Astor

136 107 drain 3.0 3.3 14.0

69, 70 Juniper Springs and Fern Hammock Springs near Ocala 136 103 drain 18.8 -10.9 23.5

71 Ponce de Leon Springs near De Land 140 125 drain 24.3 -.6 4.0

72 Rainbow Springs near Dunnellon 142 57 drain 637.0 -16.6 30.8

73 Alexander Springs near Astor 144 112 drain 113.0 -10.6 10.5

74 Mosquito Springs Run, Alexander Springs Wilderness 147 121 drain 2.0 -.6 16.0

75 Wilson Head Spring near Holder 151 64 WWC 1.9 .5 29.4

76 Blue Spring near Holder 151 65 WWC 10.6 .1 29.7

77 Gum Springs near Holder 152 70 drain 67.6 3.5 35.5

78 Camp La No Che Springs near Paisley 153 114 drain 1.0 -.3 41.3

79 Blue Spring near Orange City 153 127 drain 135.0 -8.9 2.5

Table 12. Comparison of measured or estimated and residual flows from Upper Floridan aquifer springs, average 

August 1993 through July 1994 conditions--Continued

[Spring number refers to figure 22. Row and column refer to model grid. Flows from springs in the same row and column were combined. River cells: 

WNW, Withlacoochee River in northwest Florida; WWC, Withlacoochee River in west-central Florida; SUR, Suwannee River; ICH, Ichetucknee River; 

STR, Steinhatchee River; SFR, Santa Fe River; WAC, Waccasassa River; and HIR, Hillsborough River. Springs simulated as river cells are indicated by the 

name of the nearest river; drain, indicates spring was simulated as drain cell. Residual flow is simulated flow minus measured or estimated flow; ft3

/s, cubic 

feet per second]

Spring 

number Spring name Row Column River

cell

Measured 

or 

estimated 

flow

 (ft3/s)

Residual 

flow

(ft3/s)

Pool 

elevation 

or river 

stage 

(feet)
Simulation of Ground-Water Flow 89

80 Blackwater Springs near Cassia 158 117 drain 1.4 -1.4 38.5

81 Crystal River Spring Group 157 46 drain 613.2 50.6 1.5

82 Little Jones Creek Head Spring near Wildwood 159 79 drain 8.0 .1 42.0

83 Green Springs 160 133 drain .3 .0 17.0

84 Gemini Springs near DeBary (all 3) 160 129 drain 10.5 -.8 2.8

85 Little Jones Creek Spring No. 2 near Wildwood 160 79 drain 5.0 .0 42.0

86 Messant Spring near Sorrento 160 117 drain 12.0 -.6 30.0

87 Seminole Springs near Sorrento 161 115 drain 37.0 -22.5 40.0

88 Palm Springs Seminole State Forest 161 120 drain .5 .2 28.5

89 Little Jones Creek Spring No. 3 near Wildwood 161 80 drain 3.0 .0 43.0

90 Droty Springs near Sorrento 162 116 drain .6 -.6 39.0

91 Halls River Head Spring 162 47 drain 4.8 -.2 1.5

92 Island Spring near Sanford 162 122 drain 6.4 .0 16.0

93 Halls River Springs 163 46 drain 102.2 .5 1.5

94-96 Homosassa Springs, Southeast Fork of Homosassa 

Springs, and Trotter Spring at Homosassa Springs

164 47 drain 120.7 .8 1.5

97 Fenney Springs near Coleman, Head Spring of Shady 

Brook Creek

164 82 drain 15.0 -3.0 48.0

98, 99 Shady Brook Creek Springs No. 2 and 3 165 82 drain 5.8 .0 44.5

100 Shady Brook Creek Spring No. 4 166 80 drain 2.9 .1 46.0

101 Sulphur Camp Springs 166 116 drain .6 -.1 34.0

102 Hidden River Springs near Homosassa (including Hidden 

River Head Spring)

166 47 drain 6.7 -.1 2.0

103 Rock Springs near Apopka 167 116 drain 53.0 -1.5 26.5

104 Shady Brook Creek Spring No. 5 167 79 drain 2.9 .2 47.0

105 Bugg Spring at Okahumpka 167 91 drain 8.6 -.1 63.0

106, 108 Blue Springs near Yalaha and Holiday Springs at Yalaha 168 96 drain 6.6 -1.9 65.0

107 Mooring Cove Springs near Yalaha 168 95 drain .4 -.4 68.5

109 Potter Spring near Chassahowitzka (including Ruth 

Spring)

168 46 drain 14.4 -.2 1.5

110 Witherington Spring near Apopka 169 117 drain 1.0 .0 32.0

111 Salt Creek Head Spring 169 47 drain .4 .0 1.5

112 Lettuce Creek Spring 169 48 drain 3.7 -.1 2.5

113, 115 Crab Creek Spring and Chassahowitzka Springs near 

Chassahowitzka

170 48 drain 99.6 -.3 1.5

114, 119 Unnamed Tributary above Chassahowitzka Springs and 

Baird Creek Head Spring near Chassahowitzka

170 47 drain 23.7 .0 1.5

116 Wekiwa Springs in State Park near Apopka 171 119 drain 56.5 -.9 13.5

117 Miami Springs near Longwood 171 120 drain 4.0 -.1 15.0

Table 12. Comparison of measured or estimated and residual flows from Upper Floridan aquifer springs, average 

August 1993 through July 1994 conditions--Continued

[Spring number refers to figure 22. Row and column refer to model grid. Flows from springs in the same row and column were combined. River cells: 

WNW, Withlacoochee River in northwest Florida; WWC, Withlacoochee River in west-central Florida; SUR, Suwannee River; ICH, Ichetucknee River; 

STR, Steinhatchee River; SFR, Santa Fe River; WAC, Waccasassa River; and HIR, Hillsborough River. Springs simulated as river cells are indicated by the 

name of the nearest river; drain, indicates spring was simulated as drain cell. Residual flow is simulated flow minus measured or estimated flow; ft3

/s, cubic 

feet per second]

Spring 

number Spring name Row Column River

cell

Measured 

or 

estimated 

flow

 (ft3/s)

Residual 

flow

(ft3/s)

Pool 

elevation 

or river 

stage 

(feet)
90 Simulation of Ground-Water Flow in the Intermediate and Floridan Aquifer Systems in Peninsular Florida

118 Lake Jesup Spring near Wagner 171 131 drain .6 .6 32.0

120 Clifton Springs near Oviedo 172 133 drain 1.5 .9 30.0

121 Starbuck Spring near Longwood 172 124 drain 12.3 -.2 23.0

122 Beteejay Lower Spring near Chassahowitzka (including 

Beteejay Head Spring)

171 46 drain 7.3 .0 1.8

123, 125 Palm Springs and Sanlando Springs near Longwood 172 123 drain 22.6 .0 23.0

124 Rita Maria Spring near Chassahowitzka 171 47 drain 3.3 .0 2.0

126, 127 Unnamed Spring No. 10, 11, 12, Ryle Creek Lower 

Spring, and Ryle Creek Head Spring near Bayport

172 45 drain 27.3 1.0 1.5

128 Blue Run Head Spring near Chassahowitzka 172 46 drain 4.6 .0 1.2

129 Double Run Road Seepage near Astatula 173 101 drain 2.0 .1 67.0

130 Unnamed Spring No. 8 173 44 drain 4.9 .2 1.5

131 Blind Creek Springs (including unnamed spring No. 7 and 

Blind Creek Head Spring)

174 44 drain 42.7 -15.1 1.3

132 Apopka (Gourdneck) Spring near Oakland 181 105 drain 31.4 -1.2 69.0

133 Unnamed Spring No. 6 182 44 drain 2.8 .0 1.5

134, 135 Salt Spring and Mud Spring near Bayport 182 45 drain 39.3 -1.5 1.5

136, 137 Jenkins Creek Spring No. 5 and Unnamed Spring No. 4 184 44 drain 21.6 -.1 1.5

138 Weeki Wachee Springs near Brooksville 184 48 drain 129.0 1.9 9.8

139 Unnamed Spring No. 2 188 43 drain .7 .0 8.0

140, 142,

 143

Boat Spring, Unnamed Spring No. 1, and Magnolia 

Springs at Aripeka

190 42 drain 7.2 .0 1.5

141 Bobhill Springs 190 43 drain 1.8 .0 8.0

144 Unnamed Spring No. 3 near Aripeka 193 41 drain 17.8 -1.4 1.5

145 Horseshoe Spring near Hudson 193 40 drain 9.7 -3.5 .5

146 Salt Springs near Port Richey 200 38 drain 8.2 2.3 1.0

147 Crystal Springs near Zephyrhills 209 72 HIR 37.0 -5.1 52.0

148 Sulphur Springs at Sulphur Springs 220 55 drain 25.0 -.5 5.0

149 Lettuce Lake Spring 221 61 drain 8.3 -.4 14.5

150, 151 Six-Mile Creek Spring and Eureka Springs near Tampa 221 62 drain 2.6 -.1 14.5

152 Buckhorn Spring near Riverview 230 64 drain 15.0 -3.1 10.0

153, 154 Lithia Springs Minor and Lithia Springs Major near Lithia 232 69 drain 39.1 -5.0 9.5

155 Little Salt Spring near Murdock 289 68 drain .9 .0 25.0

156 Warm Mineral Springs near Woodmere 290 67 drain 6.7 -.1 6.0

Totals 6,383.8 -224.7

Table 12. Comparison of measured or estimated and residual flows from Upper Floridan aquifer springs, average 

August 1993 through July 1994 conditions--Continued

[Spring number refers to figure 22. Row and column refer to model grid. Flows from springs in the same row and column were combined. River cells: 

WNW, Withlacoochee River in northwest Florida; WWC, Withlacoochee River in west-central Florida; SUR, Suwannee River; ICH, Ichetucknee River; 

STR, Steinhatchee River; SFR, Santa Fe River; WAC, Waccasassa River; and HIR, Hillsborough River. Springs simulated as river cells are indicated by the 

name of the nearest river; drain, indicates spring was simulated as drain cell. Residual flow is simulated flow minus measured or estimated flow; ft3

/s, cubic 

feet per second]

Spring 

number Spring name Row Column River

cell

Measured 

or 

estimated 

flow

 (ft3/s)

Residual 

flow

(ft3/s)

Pool 

elevation 

or river 

stage 

(feet)
Simulation of Ground-Water Flow 91

Potentiometric Surfaces

The simulated potentiometric surface of the IAS 

for the average August 1993 through July 1994 conditions generally reflects the main features of the estimated potentiometric surface of the IAS (figs. 17 and 

63). The potentiometric-surface highs in southwest and 

south-central Polk County were reasonably simulated, 

as were the decreasing heads west and south of the 

potentiometric highs. A potentiometric-surface high 

was simulated in northwest Highlands County; there 

were an insufficient number of measured water levels to 

determine whether this high in the estimated potentiometric surface exists. The RMS residual between 

simulated and computed heads in the IAS was 3.47 ft, 

with differences ranging from -9.62 ft in Charlotte 

County to 6.89 ft in Sarasota County. About 85 percent 

of the water-level measurements in the IAS were simulated within 5 ft. The largest residuals in the IAS were 

located in the parts of Charlotte, Sarasota, and De Soto 

Counties (figs. 17 and 63).

The simulated potentiometric surface of the UFA 

for 1993-94 replicated the main features of the estimated 

potentiometric surface (figs. 18 and 64). Both simulated 

and estimated potentiometric surfaces of the UFA show 

the potentiometric highs in Lafayette, Gilchrist, northwest 

Putnam, southeast Levy, central Volusia, east-central and 

west-central Pasco, and north-central Polk Counties.

Figure 63. Simulated potentiometric surface of the intermediate aquifer system, average August 

1993 through July 1994 conditions.
92 Simulation of Ground-Water Flow in the Intermediate and Floridan Aquifer Systems in Peninsular Florida

Figure 64. Simulated potentiometric surface of the Upper Floridan aquifer, average August 1993 

through July 1994 conditions.
Simulation of Ground-Water Flow 93

The simulated surface shows the depressions present in 

numerous areas across the Florida peninsula. Groundwater flow in the UFA was simulated reasonably well by 

the model, including the locations of the main recharge 

and discharge areas of the UFA. The RMS residual 

between simulated and computed heads in the UFA was 

3.41 ft, with differences ranging from -9.90 ft in 

Manatee County to 9.90 ft in Camden County, Ga. 

About 85 percent of water-level measurements in the 

UFA were simulated within 5 ft.

The RMS residual between simulated and computed heads in the LFA for 1993-94 was 2.89 ft, with 

differences ranging from -6.90 ft in Okeechobee County 

to 5.21 ft in Orange County. About 91 percent of the 

water-level measurements in the LFA were simulated 

within 5 ft (fig. 65). Differences in heads between the 

UFA and the LFA (figs. 64 and 65) in areas where the 

MSCU is present are smaller than in areas where the 

aquifers are separated by the MCU, implying that the 

hydraulic gradients between the two aquifers are determined mainly by the permeability of the intervening 

confining unit. Small differences in heads exist 

between the two aquifers in areas where no MCU or 

MSCU is present.

Ground-Water Flow Budget

Volumetric flow rates simulated in the IAS, UFA, 

and LFA were computed to quantify contributions of 

each component of the ground-water flow system 

(fig. 66). Separate net flow rates were computed for the 

unconfined and confined areas of the UFA. Spring flow 

represented the largest discharge flux in the model area 

(4,559 ft3/s simulated from drain cells and 1,600 ft3

/s 

from river cells). The second largest discharge was from 

water withdrawals from all aquifers (3,860 ft3

/s). Total 

recharge to the UFA, from both net recharge to the 

unconfined areas and from leakage through the IASICU layer, was 11,855 ft3/s.

Sensitivity Analyses

Sensitivity analyses were conducted to assess the 

response of simulated heads to specified changes in 

selected model parameters. Those parameters considered for sensitivity analysis included the transmissivities 

of the IAS, UFA, and LFA; the leakances of the upper 

confining unit of the IAS, the ICU, and the MSCU; the 

net recharge rate to unconfined areas of the UFA; and 

the specified heads along segments of lateral boundaries 

of the UFA. Parameters were varied from the values 

used to achieve calibration in order to assess the 

response of simulated heads to changes in model parameters. The RMS errors of the differences between simulated and computed heads were used as the criterion to 

assess the effects of changes made to parameter values 

used in the calibrated model.

The effect each parameter had on simulation 

results was assessed by varying independently from 0.2 

to 2.0 times the values of transmissivity and net recharge 

rates, from 0.5 to 1.5 times the specified heads along lateral boundaries of the UFA, and from 0.01 to 10.0 times 

the leakance values (fig. 67). These ranges of values 

may not include all the uncertainties associated with 

some of the parameters; the ranges were aimed at providing a perspective on parameter sensitivity.

The sensitivity analyses indicated that simulated 

heads were sensitive to changes in specific ranges of 

values for leakance and transmissivity. Simulated heads 

were more sensitive to changes in leakance values of 

the upper confining unit of the IAS, the ICU, and the 

MSCU between 1.0 x 10-5 and 1.0 x 10-3 (ft/d)/ft than 

to changes in leakance values either less than 1.0 x 10-5

or greater than 1.0 x 10-3 (ft/d)/ft (fig. 67). Simulated 

heads were sensitive to all transmissivity value ranges 

tested for the UFA. Simulated heads were more sensitive to changes in transmissivity values of the LFA 

greater than 100,000 ft2/d than to changes 

in transmissivities less than or equal to 100,000 ft2

/d. 

Changes in net recharge rates to unconfined areas of 

the UFA of 10 in/yr or greater caused larger RMS 

residuals than changes to rates less than 10 in/yr. 

Changes in RMS residuals were not sensitive to 

changes in transmissivity values of the IAS or to 

changes in specified heads in the UFA (fig. 67).

Effects of Projected 2020 Ground-Water 

Withdrawals

The calibrated model was used to evaluate the 

potential effects of projected 2020 ground-water withdrawals on water levels in the IAS, UFA, and LFA and 

on spring flow from the UFA. Boundary conditions simulating the greatest and smallest possible drawdowns 

that could occur were used to bracket the effects of 

projected 2020 withdrawals.
94 Simulation of Ground-Water Flow in the Intermediate and Floridan Aquifer Systems in Peninsular Florida

Figure 65. Simulated potentiometric surface of the Lower Floridan aquifer, average August 1993 

through July 1994 conditions.
Simulation of Ground-Water Flow 95

Figure 66. Simulated volumetric flow budget for the intermediate aquifer system, Upper Floridan aquifer, and Lower 

Floridan aquifer, average August 1993 through July 1994 conditions.
96 Simulation of Ground-Water Flow in the Intermediate and Floridan Aquifer Systems in Peninsular Florida

Figure 67. Model sensitivity to changes in selected model parameters.
Simulation of Ground-Water Flow 97

Projected 2020 Ground-Water Withdrawals and 

Artificial Recharge

Projected ground-water withdrawals from 

the IAS, UFA, and LFA for 2020 were estimated 

from the Water-Supply Assessment plans of the 

SJRWMD, SFWMD, SWFWMD, SRWMD 

(St. Johns River Water Management District, 1998; 

South Florida Water Management District, 1998; 

Southwest Florida Water Management District, 

1998; Suwannee River Water Management District, 

1998), water-use data for 1995, and USGS data. 

Measured and estimated pumping rates from wells in 

SWFWMD for 1995 were provided by SWFWMD 

(Tabitha Ostow, written commun., 2000). Water-use 

data from wells in SRWMD for 1995 were provided 

by SRWMD (Ronald Ceryak, written commun., 

1999). Projected 2020 water-use data for municipal 

well fields in SJRWMD and SFWMD were estimated 

by the water-supply utilities, whereas other uses in 

SJRWMD and SFWMD were estimated from compiled 1995 water-use data from files of the 

SJRWMD, SFWMD, and USGS; and the WaterSupply Assessment plans. Projected 2020 withdrawal 

estimates were based on projected population 

increases from 1995 to 2020 and projected changes 

in water use trends.

Projected 2020 ground-water withdrawals for 

public-water supply, industrial and commercial, 

agricultural, and self-supplied domestic uses totaled 

96 Mgal/d from the IAS; 2,989 Mgal/d from the UFA; 

and 308 Mgal/d from the LFA (table 13). These totals 

represent increases from 1993-94 water withdrawals 

of 33, 34, and 62 percent from the IAS, UFA, and 

LFA, respectively (tables 4 and 13). Orange County 

had the largest projected increase in withdrawals, 

from 211 Mgal/d in 1993-94 to 391 Mgal/d in 2020 

(tables 4 and 13). The smallest increase, from 3.65 to 

4.82 Mgal/d, was projected for Union County. Projected self-supplied domestic water use for 2020 was 

calculated based on 1995 estimates by Marella (1999) 

and by using multipliers for each county based on projected population increases. Projected ground-water 

withdrawals from self-supplied domestic wells in the 

IAS and UFA were approximately 17 and 196 Mgal/d, 

respectively (table 13).

Projected increases in withdrawal rates from the 

respective aquifers vary from one part of the study 

area to another. Ground-water withdrawals from the 

IAS are expected to increase from 72 Mgal/d in 1993-

94 to 96 Mgal/d in 2020. The most substantial 

increases are projected in central Glades, northeast 

Charlotte, and west-central Sarasota Counties (figs. 38 

and 68). Smaller increases are anticipated for northcentral Sarasota and west-central Polk Counties. 

Ground-water withdrawals from the UFA are anticipated to increase from 2,226 Mgal/d in 1993-94 to 

2,989 Mgal/d in 2020. In particular, total projected 

withdrawals in Orange, Seminole, and Volusia Counties are estimated to increase from 289 Mgal/d in 

1993-94 to 485 Mgal/d in 2020 (tables 4 and 13). Projected increases in withdrawals from the UFA in St. 

Lucie and Duval Counties were 13 and 27 Mgal/d, 

respectively, from 1993-94 to 2020 rates. No significant increases in ground-water withdrawals are anticipated in southwest, south-central, northwest, or 

north-central Florida (figs. 39 and 69). Ground-water 

withdrawals from the LFA are anticipated to increase 

from 190 Mgal/d in 1993-94 to 308 Mgal/d in 2020 

(tables 4 and 13). Projected withdrawals from the LFA 

in Orange and Duval Counties are expected to 

increase in 2020 by 69 and 21 Mgal/d, respectively, 

from 1993-94 rates (tables 4 and 13); increases in central Orange County and central Duval County are 

particularly apparent (figs. 40 and 70). Smaller 

increases are projected for Lake and Highlands 

Counties.

Recharge to the UFA from drainage wells for 

2020 was estimated, assuming rainfall was equal to the 

1961-90 average at NOAA stations near drainage 

wells (figs. 3 and 21), by using the weighted-average 

runoff coefficient of 0.478, and the procedure presented previously to calculate 1993-94 rates. 

Recharge from drainage wells was estimated to be 

about 62 Mgal/d (table 14). Recharge to the UFA 

through injection wells for 2020 in Alachua County 

was assumed to be 10.62 Mgal/d, the same as for the 

calibration period. The amount of recharge to the UFA 

from drainage wells is largely dependent on rainfall, 

but recharge from injection wells could increase with 

population. No relation, however, has been established that could be used to estimate recharge to the 

UFA from injection wells due to population increases.
98 Simulation of Ground-Water Flow in the Intermediate and Floridan Aquifer Systems in Peninsular Florida

Table 13. Projected 2020 ground-water withdrawals and uses, by county and by Water Management District, from the 

intermediate aquifer system, Upper Floridan aquifer, and Lower Floridan aquifer

[Source: St. Johns River Water Management District (SJRWMD); South Florida Water Management District (SFWMD); Southwest Florida Water Management District (SWFWMD); Suwannee River Water Management District (SRWMD); and US Geological Survey. All rates are in million gallons per day; -- 

indicates no wells are tapping the aquifer or aquifer is absent]

County

Intermediate aquifer system Upper Floridan aquifer Lower Floridan aquifer

Publicwater 

supply

Industrial 

and 

commerciala

a

Includes mining, thermoelectric power generation, recreational, and landscape irrigation uses. 

AgriculturalDomestic

Publicwater 

supply

Industrial 

and 

commerciala

AgriculturalDomestic

Publicwater 

supply

Industrial 

and 

commerciala

Agricultural

Alachua -- -- -- -- 39.97 7.46 24.52 9.16 -- -- --

Baker -- -- -- -- 1.22 .31 .24 1.55 0.55 -- 0.12

Bradford -- -- -- -- 5.33 3.57 2.64 2.54 -- -- --

Brevard -- -- -- -- 8.09 1.02 10.51 .16 -- -- --

Charlotte 0.03 0.16 17.85 1.76 .56 -- 5.47 -- -- -- --

Citrus -- -- -- -- 18.97 7.05 1.39 12.03 -- -- --

Clay -- -- -- -- 4.39 3.68 1.90 3.46 15.40 0.51 --

Columbia -- -- -- -- 7.03 4.43 14.42 6.96 -- -- --

DeSoto 2.03 .59 6.96 .79 1.01 .22 60.43 .03 -- -- --

Dixie -- -- -- -- 2.87 1.46 8.45 1.87 -- -- --

Duval -- -- -- -- 45.80 21.70 2.80 .04 89.76 16.92 .33

Flagler -- -- -- -- 5.00 .11 4.69 .11 -- -- --

Gilchrist -- -- -- -- 4.33 .63 43.20 2.05 -- -- --

Glades -- -- 9.48 .02 .80 -- 13.66 -- -- -- .19

Hamilton -- -- -- -- 18.24 14.93 19.52 1.39 -- -- --

Hardee .17 .12 6.86 .71 2.88 2.28 48.59 .02 -- -- --

Hernando -- -- -- -- 31.74 20.30 2.81 4.71 -- -- --

Highlands .56 .30 15.14 .36 14.16 3.12 120.42 .02 -- .14 25.49

Hillsborough .02 .01 .69 .76 79.38 18.32 76.05 13.00 -- -- --

Indian River -- -- -- -- 20.96 .07 34.94 .06 -- -- --

Lafayette -- -- -- -- 2.61 .42 16.96 1.27 -- -- --

Lake -- -- -- -- 90.11 25.55 40.65 2.25 3.54 .56 .53

Levy -- -- -- -- 5.65 6.03 62.99 5.62 -- -- --

Madison -- -- -- -- .38 .37 11.17 .23 -- -- --

Manatee .31 .39 1.76 3.61 18.52 1.96 93.30 .04 -- -- --

Marion -- -- -- -- 41.73 5.10 10.60 30.92 .01 -- .04

Martin -- -- -- -- 5.06 .58 10.58 -- -- -- --

Nassau -- -- -- -- 11.50 29.24 2.62 .09 -- 1.29 --

Okeechobee -- -- -- -- 1.19 .71 32.68 .05 -- -- .49

Orange -- -- -- -- 207.34 15.85 17.93 19.85 128.81 1.31 .15

Osceola -- -- -- -- 30.44 1.95 54.81 11.32 9.04 -- .02

Palm Beach -- -- -- -- 1.38 -- -- -- -- -- --

Pasco -- -- -- -- 115.58 11.86 18.26 11.07 -- -- --

Pinellas -- -- -- -- 43.28 2.04 .58 6.11 -- -- --

Polk .71 .82 4.11 1.25 103.69 76.27 127.23 8.51 .72 -- .61

Putnam -- -- -- -- 4.24 8.80 11.57 10.27 -- 1.09 --

St. Johns -- -- -- -- 17.49 .02 29.20 1.39 -- -- --

St. Lucie -- -- -- -- 12.58 .39 13.26 .43 -- -- --

Sarasota 8.76 .96 .41 7.40 30.08 4.80 4.76 .06 -- -- --

Seminole -- -- -- -- 86.99 .36 6.71 6.56 10.44 -- --

Sumter -- -- -- -- 4.18 2.92 11.66 5.58 -- -- --

Suwannee -- -- -- -- 8.09 4.55 81.55 4.99 -- -- --

Union -- -- -- -- 1.57 .14 2.11 1.00

Volusia -- -- -- -- 94.79 1.27 18.14 8.94 -- .18 .01

Camden, Ga. -- -- -- -- 3.52 49.82 .20 -- -- -- --

Charlton, Ga. -- -- -- -- 1.01 -- -- -- -- -- --

Total 12.59 3.35 63.26 16.66 1,255.73 361.67 1,176.17 195.71 258.27 22.00 27.98

Water Management District

SJRWMD -- -- -- -- 615.96 103.59 193.62 75.43 196.13 20.58 1.16

SFWMD -- .02 20.90 .40 102.36 16.07 234.40 18.81 62.14 1.42 26.82

SWFWMD 12.59 3.33 42.36 16.26 474.83 151.39 471.92 69.43 -- -- --

SRWMD -- -- -- -- 62.58 90.62 276.23 32.04 -- -- --

Intermediate 

aquifer 

system

Upper 

Floridan 

aquifer

Lower 

Floridan 

aquifer

Total

 by 

aquifer

Publicwater 

supply

Industrial 

and 

commercial

AgriculturalDomestic

Total 

by 

use

Total 95.86 2,989.28 308.25 3,393.39 1,526.59 387.02 1,267.41 212.37 3,393.39
Simulation of Ground-Water Flow 99

Simulated 2020 Scenarios and 

Boundary Conditions

Specified heads interpolated from the estimated 

potentiometric-surface map (fig. 18) were applied 

along the lateral boundaries of the UFA for the 1993-94 

steady-state calibration. Increased withdrawals in 2020 

could potentially affect heads along the lateral boundaries of the model. Fluxes across the lateral boundaries 

may increase due to projected increases in withdrawals.

Consequently, conditions in 2020 were bracketed by simulating two scenarios, one with specified 

1993-94 lateral boundary heads and another with specified 1993-94 lateral boundary fluxes. The 1993-94 

specified-head boundaries allowed as much water as 

was needed to enter the model area to maintain heads at 

the boundaries for projected 2020 conditions, which 

may not be realistic and could have underestimated 

drawdowns. Specified-head boundaries can be 

regarded as the best-case drawdown scenario for 2020 

simulations. On the other hand, the 1993-94 specifiedflux boundaries limited the flow of water across the 

boundary to the rates for 1993-94 conditions. Increased 

withdrawals result in lower water levels because no 

additional flow is allowed to enter the model area 

across the lateral boundaries to supply water needed to 

support additional withdrawals. Specified-flux boundaries can be regarded as the worst-case 

drawdownscenario for 2020 simulations.

Figure 68. Projected 2020 ground-water withdrawal rates from the intermediate aquifer system.
100 Simulation of Ground-Water Flow in the Intermediate and Floridan Aquifer Systems in Peninsular Florida

Figure 69. Projected 2020 ground-water withdrawal rates from and injection rates to the Upper 

Floridan aquifer.
Simulation of Ground-Water Flow 101

Figure 70. Projected 2020 ground-water withdrawal rates from the Lower Floridan aquifer.
102 Simulation of Ground-Water Flow in the Intermediate and Floridan Aquifer Systems in Peninsular Florida

The 1993-94 water-table altitude was used for 

the 2020 simulations by again modeling the SAS as a 

constant-head layer. This assumption allowed for 

induced increases in vertical leakage to the UFA resulting from projected pumping increases. Simulated net 

recharge rates to unconfined areas of the UFA in 2020 

were assumed to be equal to the 1993-94 calibrated 

rates, which is consistent with applying the altitude of 

the SAS from 1993-94 for the 2020 simulations.

As with the 1993-94 simulation, no-flow 

boundary conditions were imposed along the lateral 

boundaries of the IAS or ICU and LFA layers for 2020 

simulations. Similarly, no-flow boundaries in the UFA 

established along coastal Citrus, Hernando, and Pasco 

Counties during model calibration also were applied 

for the 2020 simulations.

Projected 2020 Drawdowns

Simulated 2020 drawdowns (compared to 1993-

94 heads) were computed at each model grid cell for 

each aquifer layer. Very few cells had increases in water 

levels from 1993-94 conditions, and therefore, were not 

shown in the drawdown maps. The greatest water level 

increase was 2 ft.

A projected drawdown of 20 ft in the IAS potentiometric surface for 2020 was simulated in central 

Glades County (fig. 71), regardless of which lateral 

boundary condition was used for the UFA. In that area, 

withdrawals are projected to increase substantially 

from 1993-94 to 2020. Drawdowns of 12 and 14 ft 

were simulated in west-central Sarasota County and 

west-central Polk County, respectively. Very little difference was observed between the simulated IAS heads 

for the two lateral boundary conditions used for the 

UFA (specified-head and specified-flux).

The two scenarios for simulating boundary conditions in the UFA (specified-head and specified-flux) 

had relatively the same effect on simulated water levels 

in some areas of the model. For example, a drawdown 

of 10 ft was simulated in central Orange County using 

either method; however, the extent of the 10-ft drawdown area was greater for the specified-flux condition. 

The same was true for the 6-ft drawdown in Seminole, 

Osceola, and Duval Counties. In western Polk County, 

the maximum drawdown for the specified-flux scenario 

was 6 ft, whereas the maximum for the specified-head 

scenario was 4 ft. Drawdowns of 4 ft were projected in 

an areally extensive part of northeast Florida, and in 

northeast Volusia, west-central Polk, and central 

Manatee Counties. No substantial projected drawdowns were simulated in the northwest and westcentral parts of the model area (fig. 72), due to 

relatively small projected increases in ground-water 

withdrawals from the UFA.

Simulated drawdowns in the southeastern part of 

the model area (St. Lucie and Okeechobee Counties) 

were much more sensitive to the lateral boundary conditions in the UFA. A projected drawdown of 12 ft was 

simulated in the UFA in parts of St. Lucie County with 

the specified-flux boundary, compared to simulated 

drawdowns of only 2 ft with the specified-head boundary (fig. 72). A low transmissivity combined with a 

relatively large increase in ground-water withdrawals, 

as is the case in St. Lucie County, results in larger drawdowns, whereas high transmissivity combined with a 

small increase in ground-water withdrawals results in 

smaller drawdowns, as is the case in Marion County.

The largest projected drawdown in the potentiometric surface of the LFA for 2020 was simulated in 

Orange County (fig. 73) using either of the two lateral 

boundary conditions for the UFA. A drawdown of 10 ft 

extends through the central part of Orange County for 

the specified-flux boundary condition, whereas a maximum drawdown of 8 ft was simulated using the 

specified-head boundary. Drawdowns up to 8 ft also 

extend into parts of Osceola and Seminole Counties for 

both lateral boundary conditions. For both boundary 

conditions, a drawdown of 4 ft extends through a large 

area in Duval County, with a maximum of 6 ft 

simulated for specified-flux conditions. Simulated 

drawdowns in the potentiometric surfaces of the UFA 

and LFA in Orange and Duval Counties were consistent 

with projected increases in ground-water withdrawals 

from these aquifers. No substantial projected 

Table 14. Projected 2020 recharge 

rates for drainage wells to the 

Upper Floridan aquifer by county

[Source: CH2M Hill, 1997; Bradner, 1996; 

Phelps, 1987. Rates, in million gallons per 

day, are rounded to integers]

County Recharge

Alachua 11

Marion 5

Orange 39

Putnam 1

Seminole 1

Suwannee 5

Total 62
Simulation of Ground-Water Flow 103

drawdowns were simulated in the northwest and 

west-central parts of the model area (fig. 73) owing to 

the absence of ground-water withdrawals from the LFA.

Upward flow from the LFA to the UFA is less in 

the southeastern part of the model for the specifiedhead lateral boundary condition in the UFA, as compared to the specified-flux boundary condition. Simulation with the specified-flux lateral boundary results in 

drawdowns of 6 ft extending through a large area in the 

Figure 71. Simulated intermediate aquifer system drawdowns from average August 1993 through July 1994 

conditions to projected 2020 conditions.

southeastern part of the model area. The simulation of 

the specified-head lateral boundary condition results in 

drawdowns of only 2 ft for the same area (fig. 73). 

A no-flow condition along the lateral boundary of the 

LFA, combined with the fact that no water is 

withdrawn from the LFA in the southeast part of the 

model area (fig. 70), indicates that the projected drawdown in the LFA was due solely to increased upward 

flow from the LFA to the UFA.
104 Simulation of Ground-Water Flow in the Intermediate and Floridan Aquifer Systems in Peninsular Florida

Figure 72. Simulated Upper Floridan aquifer drawdowns from average August 1993 through July 1994 

conditions to projected 2020 conditions.
Simulation of Ground-Water Flow 105

Figure 73. Simulated Lower Floridan aquifer drawdowns from average August 1993 through July 1994 

conditions to projected 2020 conditions.
106 Simulation of Ground-Water Flow in the Intermediate and Floridan Aquifer Systems in Peninsular Florida

Projected 2020 River and Spring Flow

The Hillsborough, Waccasassa, Steinhatchee, 

and Withlacoochee Rivers had minor changes in simulated base flow from 1993-94 to 2020 (table 15). The 

reduction in base flow for 2020 in the Suwannee and 

Santa Fe Rivers of about 4 percent compared to 1993-

94 conditions can be attributed solely to projected 

increases in ground-water withdrawals because no 

other hydraulic properties changed from the calibrated 

model for 1993-94 conditions. There were no significant differences between 2020 base flows of rivers in 

unconfined areas of the UFA simulated with specifiedflux lateral boundaries in the UFA (table 15) and the 

base flows simulated with specified-head lateral 

boundaries.

Simulated 2020 spring flows at 145 of the 

156 UFA springs were within 10 ft3/s of the 1993-94 

flows. Simulated flows at Manatee Spring and Silver 

Springs decreased by more than 20 ft3/s relative to 

simulated 1993-94 flows (fig. 22; table 16). This is a 

decrease of 16 percent for Manatee Spring and 8 percent for Silver Springs. Simulated flows at Blue Spring 

near Orange City, Rock Springs, Apopka Spring, and 

Weeki Wachee Springs decreased between 15 and 

17 ft3

/s relative to simulated 1993-94 flows because of 

projected increases in ground-water withdrawals in 2020. 

Total simulated spring flow for the specified-flux 

boundary condition was about 20 ft3

/s less than that 

simulated with the specified-head boundary condition. 

Total simulated spring flow for 2020 for the worst-case 

drawdown scenario was about 5,800 ft3/s, which was 

94 percent of the simulated spring flow for 1993-94 

(table 16). In the confined areas of the UFA, simulated 

spring flow for 2020 decreased 14 percent from the 

1993-94 simulated flow, whereas spring flow 

decreased 5 percent in the unconfined areas.

Table 15. Projected 2020 base flow of rivers in the unconfined areas of the Upper Floridan aquifer for specified-flux 

boundary conditions

[Station number refers to figure 23. Simulated base flow is flow captured by river cells in unconfined areas of the Upper Floridan aquifer, which 

generally have a smaller drainage area than the river basins; difference in base flow is simulated 2020 base flow minus simulated 1993-94 base flow. 

SRWMD, Suwannee River Water Management District; SWFWMD, Southwest Florida Water Management District; mi2, square miles; ft3/s, cubic 

feet per second; --, does not apply or not available]

USGS

station 

number

Station name

Drainage 

area 

(mi2)

Simulated 

2020 

base flow 

(ft3/s)

Simulated 

1993-94 

base flow 

(ft3/s)

Difference 

in

 base flow 

(ft3/s)

02303000 Hillsborough River near Zephyrhills 220 35 35 0

02312000 Withlacoochee River at Trilby 570 32 30 2

02313000 Withlacoochee River near Holder 1,825 115 121 -6

02313700 Waccasassa River near Gulf Hammock 480 77 79 -2

02319000 Withlacoochee River near Pinetta 2,120 5 6 -1

02319500 Suwannee River at Ellaville 6,970 a660 a683 -23

02320500 Suwannee River at Branford 7,880 a

1,386 a1,430 -44

02322500 Santa Fe River near Fort White 1,017 266 275 -9

02323500 Suwannee River near Wilcox 9,640 b2,420 b2,502 -82

02324000 Steinhatchee River near Cross City 350 17 17 0

-- Unconfined sections of Hillsborough River -- 69 68 1

-- Unconfined sections of Withlacoochee River - SWFWMD -- 86 92 -6

-- Unconfined sections of Withlacoochee River - SRWMD -- 83 90 -7

-- Ichetucknee River -- 298 305 -7

-- Unconfined sections of Santa Fe River -- c851 c

872 -21

-- Unconfined sections of Suwannee River -- b

2,494 b2,585 -91

a

Includes base flow of Withlacoochee River - SRWMD.

b

Includes base flow of Santa Fe River and Withlacoochee River - SRWMD.

c

Includes base flow of Ichetucknee River.
Simulation of Ground-Water Flow 107

Table 16. Comparison of simulated 2020 and simulated August 1993 through July 1994 flow of Upper Floridan aquifer springs 

for specified-flux boundary conditions

[Spring number refers to figure 22. Row and column refer to model grid. Flows from springs in the same row and column are combined. River cells: WNW, 

Withlacoochee River in northwest Florida; WWC, Withlacoochee River in west-central Florida; SUR, Suwannee River; ICH, Ichetucknee River; STR, 

Steinhatchee River; SFR, Santa Fe River; WAC, Waccasassa River; and HIR, Hillsborough River. Springs simulated as river cells are indicated by the name 

of the nearest river; drain, indicates spring was simulated as drain cell; flow difference is simulated 2020 flow minus simulated 1993-94 flow; ft3/s, cubic 

feet per second]

Spring 

number Spring name Row Column River 

cell

Simulated

2020 flow

( ft3/s)

Simulated

1993-94 

flow 

(ft3/s)

Flow 

difference

( ft3/s)

1 Blue Spring near Madison 41 7 WNW 85.2 89.8 -4.6

2 Alapaha Rise near Fort Union 44 17 SUR 366.1 372.9 -6.8

3 Holton Spring near Fort Union 44 19 drain 11.9 12.5 -.6

4 Suwannee Springs near Live Oak 47 27 SUR 14.0 14.9 -.9

5, 6 Suwanacoochee Spring and Ellaville Spring at Ellaville 48 12 SUR 108.1 111.2 -3.1

7 Falmouth Spring at Falmouth 49 14 drain 138.2 141.8 -3.6

8 White Sulphur Springs at White Springs 52 37 drain 45.1 53.5 -8.4

9 Charles Springs near Dell 63 8 drain 4.5 4.6 -.1

10 Allen Mill Pond Spring near Dell 64 7 SUR 10.9 11.1 -.2

11 Wadesboro Spring near Orange Park 65 103 drain .0 1.1 -1.1

12 Blue Spring near Dell 66 8 SUR 53.4 54.0 -.6

13 Peacock Springs 67 14 drain 79.8 81.5 -1.7

14 Telford Spring at Luraville 68 12 SUR 31.3 32.0 -.7

15 Running Springs (East and West) near Luraville 68 15 SUR 84.4 87.6 -3.2

16 Convict Spring near Mayo 69 16 SUR 1.3 1.3 .0

17 Royal Spring near Alton 70 17 SUR 1.5 1.6 -.1

18 Owens Spring 72 19 SUR 40.6 42.1 -1.5

19 Mearson Spring near Mayo 73 20 SUR 48.0 49.8 -1.8

20 Troy Spring near Branford 75 22 SUR 110.5 113.7 -3.2

21 Little River Springs near Branford 76 24 SUR 57.5 59.2 -1.7

22 Ruth Spring near Branford 76 23 drain 7.2 7.6 -.4

23 Green Cove Springs at Green Cove Springs 77 106 drain 1.6 3.1 -1.5

24, 25 Ichetucknee Head Spring near Fort White and Cedar Head 

Spring

 77 37 drain 50.6 55.9 -5.3

26-29 Blue Hole, Roaring, Singing, Boiling, Mill Pond, Grassy Hole, 

and Coffee Springs (parts of Ichetucknee Springs)

 78 37 ICH 252.4 258.5 -6.1

30 Branford Springs at Branford 79 26 SUR 29.7 30.4 -.7

31 Jamison Spring 81 37 drain 3.0 3.0 .0

32 Hornsby Spring near High Springs 87 48 drain 48.1 51.1 -3.0

33, 34 Turtle Spring near Hatchbend and Fletcher Spring 87 29 SUR 50.3 51.8 -1.5

35 Steinhatchee Spring near Clara 87 2 STR 1.2 1.2 .0

36 Ginnie Spring near High Springs 88 41 SFR 54.5 56.2 -1.7

37 Blue Springs near High Springs (including Lilly Springs) 89 42 drain 40.4 41.5 -1.1

38 Poe Springs near High Springs 89 44 SFR 51.7 53.3 -1.6

39 Rock Bluff Springs near Bell 91 27 SUR 23.7 26.0 -2.3

40 Guaranto Spring near Rock Bluff Landing 92 26 SUR 9.7 10.4 -.7

41 Crescent Beach Submarine Spring 94 135 drain 37.3 42.6 -5.3

42, 43 Lumbercamp Springs and Sun Springs near Wannee 97 26 drain 42.6 44.6 -2.0

44 Hart Springs near Wilcox 100 25 drain 80.3 88.8 -8.5

45 Otter Springs near Wilcox 102 25 drain 15.0 15.7 -.7

46 Whitewater Springs 103 107 drain .5 1.4 -.9

47 Copper Springs near Oldtown (including Little Copper Spring) 104 23 SUR 15.6 18.6 -3.0

48 Bell Springs 105 25 drain 4.5 5.0 -.5

49 Fannin Springs near Wilcox (including Little Fannin Spring) 106 26 drain 87.1 97.4 -10.3

50 Satsuma Spring 111 106 drain 1.0 1.1 -.1

51 Blue Springs near Orange Springs 112 94 drain .5 0.5 .0

52 Orange Spring at Orange Springs 112 89 drain .9 1.5 -.6

53 Camp Seminole Spring at Orange Springs 113 88 drain .1 .4 -.3

54 Welaka Spring near Welaka 114 106 drain .0 .0 .0

55 Manatee Spring near Chiefland 113 23 drain 149.6 177.7 -28.1
108 Simulation of Ground-Water Flow in the Intermediate and Floridan Aquifer Systems in Peninsular Florida

56 Mud Spring near Welaka 116 106 drain 2.3 2.5 -.2

57 Blue Spring near Bronson 116 40 WAC 6.2 7.5 -1.3

58 Beecher Springs near Fruitland 117 107 drain 6.3 6.3 .0

59 Croaker Hole Spring near Welaka 118 105 drain 90.4 91.8 -1.4

60 Tobacco Patch Landing Spring Group near Fort McCoy 118 90 drain 1.0 1.0 .0

61 Wells Landing Springs near Fort McCoy 119 90 drain 4.6 4.9 -.3

62 Salt Springs near Eureka 124 102 drain 78.8 79.3 -.5

63 Wekiva Springs near Gulf Hammock 129 43 drain 44.4 45.2 -.8

64 Silver Glen Springs near Astor 132 108 drain 78.4 78.8 -.4

65 Sweetwater Springs along Juniper Creek 134 106 drain 14.9 15.0 -.1

66 Silver Springs near Ocala 134 81 drain 570.7 620.3 -49.6

67, 68 Morman Branch Seepage into Juniper Creek and Juniper Creek 

Tributary near Astor

136 107 drain 6.2 6.3 -.1

69, 70 Juniper Springs and Fern Hammock Springs near Ocala 136 103 drain 7.8 8.0 -.2

71 Ponce de Leon Springs near De Land 140 125 drain 23.2 23.7 -.5

72 Rainbow Springs near Dunnellon 142 57 drain 606.6 620.4 -13.8

73 Alexander Springs near Astor 144 112 drain 101.1 102.3 -1.2

74 Mosquito Springs Run, Alexander Springs Wilderness 147 121 drain .9 1.4 -.5

75 Wilson Head Spring near Holder 151 64 WWC 2.2 2.4 -.2

76 Blue Spring near Holder 151 65 WWC 10.4 10.7 -.3

77 Gum Springs near Holder 152 70 drain 67.6 71.1 -3.5

78 Camp La No Che Springs near Paisley 153 114 drain .0 .7 -.7

79 Blue Spring near Orange City 153 127 drain 110.5 126.1 -15.6

80 Blackwater Springs near Cassia 158 117 drain .0 .0 .0

81 Crystal River Spring Group 157 46 drain 650.5 663.8 -13.3

82 Little Jones Creek Head Spring near Wildwood 159 79 drain 7.2 8.1 -.9

83 Green Springs 160 133 drain .0 .3 -.3

84 Gemini Springs near DeBary (all 3) 160 129 drain 7.8 9.8 -2.0

85 Little Jones Creek Spring No. 2 near Wildwood 160 79 drain 4.6 5.0 -.4

86 Messant Spring near Sorrento 160 117 drain 10.4 11.4 -1.0

87 Seminole Springs near Sorrento 161 115 drain 6.1 14.5 -8.4

88 Palm Springs Seminole State Forest 161 120 drain .0 .7 -.7

89 Little Jones Creek Spring No. 3 near Wildwood 161 80 drain 2.8 3.0 -.2

90 Droty Springs near Sorrento 162 116 drain .0 .0 .0

91 Halls River Head Spring 162 47 drain 4.6 4.7 -.1

92 Island Spring near Sanford 162 122 drain 4.9 6.4 -1.5

93 Halls River Springs 163 46 drain 101.0 102.7 -1.7

94-96 Homosassa Springs, Southeast Fork of Homosassa Springs, 

and Trotter Spring at Homosassa Springs

164 47 drain 119.6 121.5 -1.9

97 Fenney Springs near Coleman, Head Spring of Shady Brook 

Creek

164 82 drain 10.7 12.0 -1.3

98, 99 Shady Brook Creek Springs No. 2 and 3 165 82 drain 5.7 5.8 -.1

100 Shady Brook Creek Spring No. 4 166 80 drain 2.8 3.0 -.2

101 Sulphur Camp Springs 166 116 drain .0 .5 -.5

102 Hidden River Springs near Homosassa (including Hidden 

River Head Spring)

166 47 drain 6.5 6.6 -.1

103 Rock Springs near Apopka 167 116 drain 35.7 51.5 -15.8

104 Shady Brook Creek Spring No. 5 167 79 drain 2.9 3.1 -.2

105 Bugg Spring at Okahumpka 167 91 drain 6.2 8.5 -2.3

106, 108 Blue Springs near Yalaha and Holiday Springs at Yalaha 168 96 drain .0 4.7 -4.7

Table 16. Comparison of simulated 2020 and simulated August 1993 through July 1994 flow of Upper Floridan aquifer springs 

for specified-flux boundary conditions--Continued

[Spring number refers to figure 22. Row and column refer to model grid. Flows from springs in the same row and column are combined. River cells: WNW, 

Withlacoochee River in northwest Florida; WWC, Withlacoochee River in west-central Florida; SUR, Suwannee River; ICH, Ichetucknee River; STR, 

Steinhatchee River; SFR, Santa Fe River; WAC, Waccasassa River; and HIR, Hillsborough River. Springs simulated as river cells are indicated by the name 

of the nearest river; drain, indicates spring was simulated as drain cell; flow difference is simulated 2020 flow minus simulated 1993-94 flow; ft3

/s, cubic 

feet per second]

Spring 

number Spring name Row Column River 

cell

Simulated

2020 flow

( ft3/s)

Simulated

1993-94 

flow 

(ft3/s)

Flow 

difference

( ft3/s)
Simulation of Ground-Water Flow 109

107 Mooring Cove Springs near Yalaha 168 95 drain .0 .0 .0

109 Potter Spring near Chassahowitzka (including Ruth Spring) 168 46 drain 14.0 14.3 -.3

110 Witherington Spring near Apopka 169 117 drain .1 1.0 -.9

111 Salt Creek Head Spring 169 47 drain .4 .4 .0

112 Lettuce Creek Spring 169 48 drain 3.5 3.6 -.1

113, 115 Crab Creek Spring and Chassahowitzka Springs near 

Chassahowitzka

170 48 drain 97.9 99.3 -1.4

114, 119 Unnamed Tributary above Chassahowitzka Springs and Baird 

Creek Head Spring near Chassahowitzka

170 47 drain 23.4 23.7 -.3

116 Wekiwa Springs in State Park near Apopka 171 119 drain 43.0 55.6 -12.6

117 Miami Springs near Longwood 171 120 drain 2.9 3.9 -1.0

118 Lake Jesup Spring near Wagner 171 131 drain .0 1.2 -1.2

120 Clifton Springs near Oviedo 172 133 drain .0 2.4 -2.4

121 Starbuck Spring near Longwood 172 124 drain 5.9 12.1 -6.2

122 Beteejay Lower Spring near Chassahowitzka (including 

Beteejay Head Spring)

171 46 drain 7.2 7.3 -.1

123, 125 Palm Springs and Sanlando Springs near Longwood 172 123 drain 11.5 22.6 -11.1

124 Rita Maria Spring near Chassahowitzka 171 47 drain 3.2 3.3 -.1

126, 127 Unnamed Spring No. 10, 11, 12, Ryle Creek Lower Spring, 

and Ryle Creek Head Spring near Bayport

172 45 drain 27.9 28.3 -.4

128 Blue Run Head Spring near Chassahowitzka 172 46 drain 4.6 4.6 .0

129 Double Run Road Seepage near Astatula 173 101 drain .0 2.0 -2.0

130 Unnamed Spring No. 8 173 44 drain 5.1 5.1 .0

131 Blind Creek Springs (including unnamed spring No. 7 and 

Blind Creek Head Spring)

174 44 drain 27.3 27.6 -.3

132 Apopka (Gourdneck) Spring near Oakland 181 105 drain 13.9 30.2 -16.3

133 Unnamed Spring No. 6 182 44 drain 2.8 2.8 .0

134, 135 Salt Spring and Mud Spring near Bayport 182 45 drain 37.2 37.8 -.6

136, 137 Jenkins Creek Spring No. 5 and Unnamed Spring No. 4 184 44 drain 21.0 21.6 -.6

138 Weeki Wachee Springs near Brooksville 184 48 drain 115.6 130.9 -15.3

139 Unnamed Spring No. 2 188 43 drain .6 .7 -.1

 140, 142,

 143

Boat Spring, Unnamed Spring No. 1, and Magnolia Springs 

at Aripeka

190 42 drain 7.0 7.2 -.2

141 Bobhill Springs 190 43 drain 1.6 1.8 -.2

144 Unnamed Spring No. 3 near Aripeka 193 41 drain 16.0 16.4 -.4

145 Horseshoe Spring near Hudson 193 40 drain 6.1 6.2 -.1

146 Salt Springs near Port Richey 200 38 drain 9.8 10.5 -.7

147 Crystal Springs near Zephyrhills 209 72 HIR 31.9 31.9 .0

148 Sulphur Springs at Sulphur Springs 220 55 drain 23.9 24.5 -.6

149 Lettuce Lake Spring 221 61 drain 7.5 7.8 -.3

150, 151 Six-Mile Creek Spring and Eureka Springs near Tampa 221 62 drain 2.4 2.5 -.1

152 Buckhorn Spring near Riverview 230 64 drain 10.1 11.9 -1.8

153, 154 Lithia Springs Minor and Lithia Springs Major near Lithia 232 69 drain 30.0 34.1 -4.1

155 Little Salt Spring near Murdock 289 68 drain .1 .9 -.8

156 Warm Mineral Springs near Woodmere 290 67 drain 5.3 6.6 -1.3

Total 5,796.6 6,159.4 -362.8

Table 16. Comparison of simulated 2020 and simulated August 1993 through July 1994 flow of Upper Floridan aquifer springs 

for specified-flux boundary conditions--Continued

[Spring number refers to figure 22. Row and column refer to model grid. Flows from springs in the same row and column are combined. River cells: WNW, 

Withlacoochee River in northwest Florida; WWC, Withlacoochee River in west-central Florida; SUR, Suwannee River; ICH, Ichetucknee River; STR, 

Steinhatchee River; SFR, Santa Fe River; WAC, Waccasassa River; and HIR, Hillsborough River. Springs simulated as river cells are indicated by the name 

of the nearest river; drain, indicates spring was simulated as drain cell; flow difference is simulated 2020 flow minus simulated 1993-94 flow; ft3

/s, cubic 

feet per second]

Spring 

number Spring name Row Column River 

cell

Simulated

2020 flow

( ft3/s)

Simulated

1993-94 

flow 

(ft3/s)

Flow 

difference

( ft3/s)
110 Simulation of Ground-Water Flow in the Intermediate and Floridan Aquifer Systems in Peninsular Florida

Projected 2020 Ground-Water Flow Budget

A comparison of the projected flow budget for 

2020 to 1993-94 conditions showed a reduction in 

spring flow for 2020 in both confined and unconfined 

areas of the UFA, mainly due to projected increases in 

ground-water withdrawals in the UFA. An increase in 

projected ground-water withdrawals in the confined 

areas of the UFA of 946 ft3

/s induced an increase in net 

downward leakage from the IAS or ICU layer to the 

UFA layer of 937 ft3

/s, almost equal to the increase in 

water withdrawals (figs. 66 and 74). Total recharge to 

the UFA, in the form of both net recharge or net downward leakage through the IAS or ICU layer, was 

12,792 ft3/s, an increase of about 8 percent from 1993-

94 conditions. Changes in net flow across the unconfined-confined boundary of the UFA (fig. 5) were not 

significant because net simulated flow to unconfined 

areas from confined areas decreased only 3 percent in 

2020 from 1993-94 conditions (figs. 66 and 74).

MODEL LIMITATIONS

Some of the limiting factors of the ground-water 

flow model presented in this report are: simplifications 

in the conceptual model, inherent model assumptions, 

lack of water-level and flow measurements in areas 

where spatial variability of hydraulic and hydrologic 

properties is poorly known, and inaccuracies in landsurface altitude measurements. Model simulations are 

based on the assignment of hydraulic properties to grid 

cells, the use of specified heads in the UFA along lateral 

boundaries of the model, the grid resolution, the estimated average water table and Floridan aquifer heads, 

the estimated ground-water withdrawals, and the 

assumption of steady-state conditions in the Floridan 

aquifer for 1993-94. An error in any of these can limit 

the accuracy of model simulations.

Ground-water flow simulations generally are 

based on conceptual models that are simplified representations of complex heterogeneous ground-water 

flow systems. Assumptions such as isotropy, vertical 

homogeneity within each layer, and the absence of 

preferential flow zones are examples of simplified representations that can be sources of error in a groundwater flow model. The lack of sufficient measurements 

to account for the spatial variation of hydraulic properties throughout the model area necessitated these simplifications. Simplifying the model does not invalidate 

model results, although model results should be interpreted at scales larger than the representative grid cell.

The ground-water flow equation solved by the model 

(equation 4) is the continuity equation for flow derived 

from the principal of conservation of mass and the 

assumptions that water is incompressible and of constant viscosity, incorporated with Darcy’s law 

(Bouwer, 1978, p. 202). This equation is valid for 

ground-water flow conditions where the velocity of 

ground water is low and flow is laminar. In karstic 

terrains, it is possible for flow through caverns and 

solution channels to be turbulent. Thus, the equation is 

not valid for the entire Floridan aquifer system. Simplifications made in this study were the assumptions 

that laminar flow was everywhere and that effective 

transmissivity was uniform throughout each grid cell 

of the model such that mass is conserved, along with 

known hydraulic gradients.

Inaccuracies inherent in the algorithm used to 

estimate the water table distribution or the estimated 

potentiometric surface of the UFA, such as lack of data 

and residual errors, could lead to errors in the delineation of recharge or discharge areas. In turn, these inaccuracies can lead to inaccuracies in leakance and 

leakage rates among the SAS, IAS or ICU, and UFA.

The assumption of constant water levels in areas 

where the SAS is present may introduce error into the 

model. Temporal changes in the water table of the SAS 

would bring temporal changes in leakage rates between 

the SAS and the IAS or ICU. In particular, in the vicinity of some cells, induced leakage due to large groundwater withdrawals from the UFA could cause these 

cells to go dry. Simulating active cells in the SAS, however, was beyond the scope of this study. The water 

table of the SAS was generated for a year when total 

rainfall was nearly equal to the overall 30-year average 

rainfall computed from NOAA rainfall stations. The 

estimated water table for the SAS in this study should 

be modified for local or regional ground-water flow 

studies for which simulation periods correspond to 

hydrologic conditions that deviate from average rainfall 

conditions.

The assumption of uniform heads throughout the 

vertical thickness of each grid cell is another possible 

source of error in the simulated heads. Vertical hydraulic gradients between the SAS and the UFA suggest that 

simulated heads in the ICU could represent, at best, a 

value within the range of heads observed throughout 

the thickness of the ICU. The simulated heads in the 

ICU should be interpreted as an interpolation from 

the hydraulic gradients between the SAS and the UFA. 
Model Limitations 111

Figure 74. Simulated volumetric flow budget for the intermediate aquifer system, Upper Floridan aquifer, and Lower 

Floridan aquifer for projected 2020 conditions for the specified-flux boundary conditions (worst-case drawdown 

scenario).
112 Simulation of Ground-Water Flow in the Intermediate and Floridan Aquifer Systems in Peninsular Florida

Simulation of the observed hydraulic gradients in the 

ICU would require additional layering of this hydrogeologic unit, which is beyond the scope of this study.

The simulated transmissivities in the IAS, UFA, 

and LFA and the leakances of the upper confining unit 

of the IAS, ICU, and MSCU in this study may vary 

from values in previously published local ground-water 

flow models. Many of the modifications to transmissivity values from previous ground-water flow models 

were based on additional data available from aquifer 

tests and on the need to reduce differences between 

simulated and measured heads. Areas in the IAS, UFA, 

or LFA where ground-water withdrawals were minimal 

for 1993-94 may require changes to simulated values of 

transmissivity and leakance as future aquifer stresses 

from increased ground-water withdrawals reflect aquifer responses in areas previously not stressed.

Lack of data for the MSCU and LFA precludes a 

reliable estimation of their respective leakance and 

transmissivity values; only 46 LFA control points were 

available during this study. The no-flow boundaries 

specified along the perimeter of active cells in the LFA 

was based on the assumption of minimal ground-water 

flow across the estimated surface beneath which the 

FAS contains water with chloride concentrations 

greater than 5,000 mg/L. The estimated surface shown 

in figure 19 can be updated as more water-quality data 

become available. In addition, flow in the LFA might 

be understood better as the potentiometric surface of 

the LFA becomes better defined by more head measurements as additional wells are drilled. The assumption of a stationary freshwater-saltwater interface also 

could introduce error in the model if ground-water 

withdrawals or lack of recharge induce movement of 

this interface.

In spite of the limitations, the model can indicate 

the general movement of ground water in the study 

area. By providing estimates to hydraulic parameters, 

this model can be used to generate initial estimates of 

the hydraulic properties needed for more localized 

studies. In addition, the model also can provide a good 

assessment of drawdowns in response to projected 

ground-water withdrawals.

SUMMARY AND CONCLUSIONS

A four-layer, finite-difference steady-state 

ground-water flow model of the intermediate aquifer 

system (IAS) and the Floridan aquifer system (FAS) in 

peninsular Florida was developed and calibrated. 

Hydraulic properties were extracted from this model 

for smaller-scale ground-water flow models within the 

study area. The active model area is about 40,800 square 

miles (mi2

) and extends approximately 284 miles (mi) 

from Charlton and Camden Counties, Georgia, to just 

south of the Palm Beach - Martin County line in 

Florida. The west-to-east extent of the study area 

spans about 200 miles from the Gulf of Mexico to the 

Atlantic Ocean.

The hydrogeologic framework of the study area 

includes sediments that form the surficial aquifer 

system (SAS); the less permeable clay and carbonate 

rocks that form the intermediate confining unit (ICU), 

the accumulation of more permeable carbonate rocks 

than those of the ICU that form the IAS in southwest 

Florida; and a thick sequence of carbonate rocks of 

variable permeability that form the FAS. A base of 

generally low permeability dolomite and evaporite 

beds form the sub-Floridan confining unit. In east-central Florida, the Upper Floridan aquifer (UFA) and 

Lower Floridan aquifer (LFA) are separated by the middle semiconfining unit (MSCU), a less permeable, 

commonly partially dolomitized limestone that locally 

contains some gypsum and chert. The MSCU is thin or 

absent in the northwest part of the study area, but is as 

much as 600 to 800 ft thick in the south-central part of 

the study area. In west-central Florida, the UFA and 

LFA are separated by the middle confining unit 

(MCU), a gypsiferous dolomite and dolomitic limestone of considerably lower permeability than the 

MSCU in east-central Florida.

The IAS and FAS were approximately at steadystate conditions for the period August 1, 1993, through 

July 31, 1994. Errors introduced by this steady-state 

approximation were estimated to be minimal. A uniformly spaced grid of 5,000-foot square cells 

(210 columns and 300 rows) was used to discretize the 

hydraulic properties of the SAS, IAS or ICU, UFA, 

and LFA, and to implement the ground-water flow 

model. The flow model was calibrated using average 

heads for the IAS, UFA, and LFA for 1993-94, measured or estimated spring flows, and measured river 

flows in the unconfined areas of the UFA.

The time-averaged altitude of the water table of 

the SAS and time-averaged heads of the IAS, UFA, and 

LFA were generated from computed average heads for 

the August 1993 through July 1994 period. Multiple 

linear regressions among the water-level measurements 

at SAS wells, an interpolated minimum water-table 

surface, and the difference between land-surface 
References 113

altitude and the interpolated minimum water table were 

used to approximate the altitude of the water table in 

the study area where the SAS is present.

The altitude of the water table and the potentiometric surfaces of the IAS and UFA were used to identify the areal extent of recharge and discharge areas 

within the UFA. Recharge to the UFA occurs mainly 

from downward leakage from the SAS through the ICU 

when the water table is higher than the potentiometric 

level of the UFA. Artificial recharge to the UFA occurs 

from drainage wells, injection wells, and rapid-infiltration basins.With the exception of swamps, springs, and 

some river segments in the unconfined areas of the 

UFA, most of the unconfined areas of the UFA is 

recharged by rainfall. Discharge from the UFA occurs 

mainly by upward leakage to the SAS through the ICU 

when the potentiometric surface is higher than the 

water table. Discharge from the UFA also occurs as 

ground-water withdrawals, spring flows, and flow from 

the unconfined areas of the UFA to swamps and rivers.

Boundary conditions varied among the various 

aquifer layers. The SAS was simulated as a layer of 

constant heads. No-flow conditions were imposed 

along the lateral boundaries of the ICU layer. Lateral 

boundaries of the UFA generally were specified heads. 

Areas of the UFA and LFA containing water with chloride concentrations greater than 5,000 milligrams per 

liter (mg/L) were considered inactive and delineated by 

no-flow boundaries. All lateral boundaries of the LFA 

in the model were assumed to be no flow.

Ground-water withdrawals for 1993-94 within 

the study area from the IAS, UFA, and LFA totaled 

2,488 million gallons per day (Mgal/d). Additional discharge from the UFA occurred from spring flow which, 

for 1993-94, was estimated to be about 4,126 Mgal/d. 

Total recharge to the UFA from 364 drainage wells in 

the study area was estimated to be 68 Mgal/d, based on 

rainfall data for 1993-94. Artificial recharge to the UFA 

from injection wells in Alachua County was estimated 

to be 10.62 Mgal/d.

The ground-water flow model was calibrated by 

independently varying the following parameters: net 

recharge rate to unconfined areas of the UFA; transmissivity of the IAS, UFA, and LFA; riverbed conductance 

of rivers in the unconfined areas of the UFA; leakance 

of the upper confining unit of the IAS, the ICU, the 

MCU, and the MSCU; and the conductance of swamps 

and springs in the UFA that were simulated as drain 

cells. Simulated and measured heads at control points 

of the IAS, UFA, and LFA showed a close agreement 

across most of the study area, with residuals of less than 

5 feet (ft) in about 85 percent of the control points. The 

root-mean-square residuals for the IAS, UFA, and LFA 

were 3.47, 3.41, and 2.89 ft, respectively. Simulated 

spring flow was within 5 cubic feet per second (ft3/s) of 

the estimated flow in about 83 percent of the springs. 

The largest spring-flow residuals occurred in springs 

simulated as river cells. Simulated flow was about 

96 percent of an approximate total estimated spring 

flow of 6,380 ft3/s for 1993-94.

The effects of projected increases in groundwater withdrawals for 2020 relative to 1993-94 rates 

were analyzed by using, in two separate simulations, 

specified-head and specified-flux boundary conditions 

for lateral boundaries of the UFA. Projected 2020 

ground-water withdrawals from the IAS, UFA, and 

LFA were estimated to be 3,394 Mgal/d, which represents an increase of about 36 percent from 1993-94. A 

projected drawdown of 20 ft in the IAS potentiometric 

surface for 2020 was simulated in central Glades 

County, resulting from projected increases in groundwater withdrawals. A projected drawdown of 10 ft in 

the UFA potentiometric surface for 2020 was simulated 

in central Orange County using either of the two lateral 

boundary conditions for the UFA, with a projected 8 ft 

drawdown extending to parts of Osceola and Seminole 

Counties. A projected drawdown of 6 ft in the UFA for 

2020 was simulated for Duval County for both lateral 

boundary conditions. A projected drawdown of 10 ft in 

the LFA potentiometric surface for 2020 was simulated 

in Orange County. Simulated drawdowns in the potentiometric surfaces of the UFA and LFA in Orange and 

Duval Counties were consistent with projected 

increases in ground-water withdrawals from these 

aquifers. No substantial projected drawdowns in the 

UFA or LFA were simulated in the northwest and westcentral parts of the model area.

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APPENDIXES
118 Simulation of Ground-Water Flow in the Intermediate and Floridan Aquifer Systems in Peninsular Florida

CLINCH

ECHOLS

LANIER

WARE

CHARLTON CAMDEN LOWNDES

MARTIN GLADES

ST. LUCIE

INDIAN

RIVER

BREVARD

OKEECHOBEE

HIGHLANDS

HARDEE

OSCEOLA

POLK

DE SOTO

CHARLOTTE

SARASOTA

MANATEE

HILLSBOROUGH

PASCO

ORANGE

SEMINOLE

VOLUSIA

LAKE

SUMTER

CITRUS

HERNANDO

LEVY

PUTNAM

NASSAU

DUVAL

CLAY ST. JOHNS

ALACHUA

BRADFORD

SUWANNEE COLUMBIA

MADISON

HAMILTON BAKER

FLAGLER

DIXIE GILCHRIST

LAFAYETTE

PINELLAS

UNION

FLORIDA

GEORGIA

MARION

Estimated extent of

the intermediate

confining unit within

the study area

Estimated extent of

the unconfined areas

in the Upper Floridan

aquifer

Estimated

extent of the

intermediate

aquifer

system

within the

study area

Extent of the

study area

14

Durden

(1997)

5 Tibbals (1990)

1

Grubb

and

Rutledge

2

(1979) Ryder

(1985)

10

Metz

(1995)

0

0 25

25

50 MILES

50 KILOMETERS

83°00′ 82°00′ 81°00′

30°0 ′ 0

31°0 ′ 0

29°00′

28°00′

27°00′

ATLANTIC OCEAN

GULF OF MEXICO

Base modified from U.S. Geological Survey digital data; 1:100,000, 1985

Universal Transverse Mercator projection, zone 17

Appendix A1. Location and extent of simulated areas in the intermediate aquifer system or 

intermediate confining unit compiled from local ground-water flow models (refer to table 1 for general 

description of models).
Appendix A1 119

CLINCH

ECHOLS

LANIER

WARE

CHARLTON CAMDEN LOWNDES

GEORGIA

FLORIDA

MARTIN GLADES

ST. LUCIE

INDIAN

RIVER

BREVARD

OKEECHOBEE

HIGHLANDS

HARDEE

OSCEOLA

POLK

DE SOTO

CHARLOTTE

SARASOTA

MANATEE

HILLSBOROUGH

PASCO

ORANGE

SEMINOLE

VOLUSIA

LAKE

SUMTER

CITRUS

HERNANDO

MARION

LEVY

PUTNAM

NASSAU

DUVAL

CLAY ST. JOHNS

ALACHUA

BRADFORD

COLUMBIA

SUWANNEE

MADISON

HAMILTON

BAKER

FLAGLER DIXIE

GILCHRIST

LAFAYETTE

PINELLAS

UNION

Estimated extent of

the intermediate

confining unit within

the study area

Estimated extent

of the unconfined

areas in the Upper

Floridan aquifer

Estimated

extent of the

intermediate

aquifer

system

within the

study area

Extent of the

study area

7 Lukasiewicz

(1992)

6

Murray and Halford (1996)

12

Yobbi (1996)

13

Barcelo and Basso (1993)

11

Motz (1995)

Hancock

and

Basso (1993)

9

3 Fretwell (1988)

0

0 25

25

50 MILES

50 KILOMETERS

83°00′ 82°00′ 81°00′

30°0 ′ 0

31°0 ′ 0

29°00′

28°00′

27°00′

ATLANTIC OCEAN

GULF OF MEXICO

Base modified from U.S. Geological Survey digital data; 1:100,000, 1985

Universal Transverse Mercator projection, zone 17

Appendix A1. Location and extent of simulated areas in the intermediate aquifer system or 

intermediate confining unit compiled from local ground-water flow models (refer to table 1 for general 

description of models)--Continued.
120 Simulation of Ground-Water Flow in the Intermediate and Floridan Aquifer Systems in Peninsular Florida

0

0 25

25

50 MILES

50 KILOMETERS

83°00′ 82°00′ 81°00′

30°0 ′ 0

31°0 ′ 0

29°00′

28°00′

27°00′

ATLANTIC OCEAN

GULF OF MEXICO

Base modified from U.S. Geological Survey digital data; 1:100,000, 1985

Universal Transverse Mercator projection, zone 17

CLINCH

ECHOLS

LANIER

WARE

CHARLTON CAMDEN LOWNDES

FLORIDA

MARTIN GLADES

ST. LUCIE

INDIAN

RIVER

BREVARD

HARDEE HIGHLANDS

OKEECHOBEE

OSCEOLA

POLK

DE SOTO

CHARLOTTE

SARASOTA

MANATEE

HILLSBOROUGH

PASCO

ORANGE

SEMINOLE

VOLUSIA

LAKE

SUMTER

CITRUS

HERNANDO

MARION

LEVY

PUTNAM

NASSAU

DUVAL

CLAY ST. JOHNS

ALACHUA

BRADFORD

COLUMBIA

SUWANNEE

MADISON

HAMILTON

BAKER

FLAGLER

DIXIE GILCHRIST

LAFAYETTE

PINELLAS

UNION

GEORGIA

5 Tibbals (1990)

14

1

10

3

4

2

Durden

(1997)

Yobbi

(1989)

Ryder

(1985)

Grubb

and

Rutledge

(1979)

Fretwell

(1988)

Metz

(1995)

Estimated extent of the

Upper Floridan aquifer

within the study area

Extent of the

study area

Appendix A2. Location and extent of simulated areas in the Upper Floridan aquifer compiled from local 

ground-water flow models (refer to table 1 for general description of models).
Appendix A2 121

CLINCH

ECHOLS

LANIER

WARE

CHARLTON CAMDEN LOWNDES

FLORIDA

MARTIN GLADES

ST. LUCIE

INDIAN

RIVER

BREVARD

HARDEE HIGHLANDS

OKEECHOBEE

OSCEOLA

POLK

DE SOTO

CHARLOTTE

SARASOTA

MANATEE

HILLSBOROUGH

PASCO

ORANGE

SEMINOLE

VOLUSIA

LAKE

SUMTER

CITRUS

HERNANDO

MARION

LEVY

PUTNAM

NASSAU

DUVAL

CLAY ST. JOHNS

ALACHUA

BRADFORD

COLUMBIA

SUWANNEE

MADISON

HAMILTON

BAKER

FLAGLER

DIXIE GILCHRIST

LAFAYETTE

PINELLAS

UNION

GEORGIA

0

0 25

25

50 MILES

50 KILOMETERS

83°00′ 82°00′ 81°00′

30°0 ′ 0

31°0 ′ 0

29°00′

28°00′

27°00′

ATLANTIC OCEAN

GULF OF MEXICO

Base modified from U.S. Geological Survey digital data; 1:100,000, 1985

Universal Transverse Mercator projection, zone 17

7

8

Lukasiewicz

(1992)

6

Murray and Halford (1996)

12

Yobbi

(1996)

13

Barcelo and Basso (1993)

Blandford and

Birdie (1993)

Hancock and Basso (1993)

9

11

Motz (1995)

Estimated extent of the

Upper Floridan aquifer

within the study area

Extent of the

study area

Appendix A2. Location and extent of simulated areas in the Upper Floridan aquifer compiled from local 

ground-water flow models (refer to table 1 for general description of models)--Continued.
122 Simulation of Ground-Water Flow in the Intermediate and Floridan Aquifer Systems in Peninsular Florida

14

5

Durden

(1997)

Tibbals (1990)

Motz (1995)

11

Lukasiewicz

(1992)

6

Murray and Halford (1996)

12

CLINCH

ECHOLS

LANIER

WARE

CHARLTON CAMDEN LOWNDES

GEORGIA

FLORIDA

MARTIN GLADES

ST. LUCIE

INDIAN

RIVER

BREVARD

OKEECHOBEE

HIGHLANDS

HARDEE

OSCEOLA

POLK

DE SOTO

CHARLOTTE

SARASOTA

MANATEE

HILLSBOROUGH

PASCO

SEMINOLE

VOLUSIA

LAKE

SUMTER

CITRUS

HERNANDO

MARION

LEVY

PUTNAM

NASSAU

DUVAL

CLAY ST. JOHNS

ALACHUA

BRADFORD

COLUMBIA

SUWANNEE

MADISON

HAMILTON

BAKER

FLAGLER

DIXIE GILCHRIST

LAFAYETTE

PINELLAS

UNION

ORANGE

0

0 25

25

50 MILES

50 KILOMETERS

83°00′ 82°00′ 81°00′

30°0 ′ 0

31°0 ′ 0

29°00′

28°00′

27°00′

ATLANTIC OCEAN

GULF OF MEXICO

Base modified from U.S. Geological Survey digital data; 1:100,000, 1985

Universal Transverse Mercator projection, zone 17

Appendix A3. Location and extent of simulated areas in the Lower Floridan aquifer compiled from local 

ground-water flow models (refer to table 1 for general description of models).
Appendix B 123

Appendix B. Description of wells equipped with continuous water-level recorders

[Well number refers to figure 16. Site numbers with 13 digits indicate data source was St. Johns River Water Management District; site numbers 

ending with 85 indicate data source was Suwannee River Water Management District (SRWMD); data source of all other site numbers was 

U.S. Geological Survey (USGS). Hydrogeologic units: CUFA, confined Upper Floridan aquifer; UUFA, unconfined Upper Floridan aquifer; 

SAS, surficial aquifer system; ICU, intermediate confining unit; IAS, intermediate aquifer system; LFA, Lower Floridan aquifer. --, data not available; 

ROMP, Regional Observation and Monitoring Well Program]

Well 

number Site number Well name Latitude Longitude

Well 

depth 

(feet)

Hydrogeologic 

unit

1 302127082475885 Morgan 302127 824758 260 UUFA

2 301458083141985 Advent Christian Village 301458 831419 60 UUFA

3 301610082591585 Brickles 301610 825915 180 UUFA

4 300904083100985 Moore 300904 831009 144 UUFA

5 300629083024185 Suwannee Farms 300629 830241 247 UUFA

6 300403083081785 Koon 300403 830817 -- UUFA

7 300117082554285 Daniel 300117 825542 -- UUFA

8 294928082355385 08S17E03 High Springs City 294928 823553 300 UUFA

9 294654082581085 Georgia Pacific 294654 825810 35 UUFA

10 294458083142885 DOF Hines Lookout Tower 294458 831428 92 UUFA

11 293731083061885 City of Cross City 293731 830618 145 UUFA

12 293619082362385 USGS Alachua County 293619 823623 252 UUFA

13 293252082292385 Straughn 293252 822923 60 UUFA

14 292921082583285 DNR Manatee Springs State Park 292921 825832 99 UUFA

15 292615082272601 ROMP 134 near Williston 292615 822726 1,185 UUFA

16 2921380820616 M-0012 Johnston C.C. 292138 820616 63 UUFA

17 291910082341185 USGS Levy County 291910 823411 86 UUFA

18 291414082560985 DOF Rosewood Tower 291414 825609 254 UUFA

19 290230082412501 ROMP 125 at Crackertown 290230 824125 280 UUFA

20 290312082250801 USGS CE 14 near Dunnellon 290312 822508 190 UUFA

21 290112082371101 USGS CE 5 near Inglis 290112 823711 125 UUFA

22 290133082140901 ROMP 119 near Ocala 290133 821409 502 UUFA

23 285720082201301 ROMP 116 near Tsala Apopka Lake 285720 822013 55 UUFA

24 285414082284201 North Lecanto near Lecanto 285414 822842 335 UUFA

25 285124082245601 ROMP 113 near Inverness 285124 822456 150 UUFA

26 285102082204001 DOT Highway 41 near Inverness 285102 822040 450 UUFA

27 284317082330601 Chassahowitzka 1 near Chassahowitzka 284317 823306 176 UUFA

28 284330082215401 ROMP 109 near Floral City 284330 822154 260 UUFA

29 283201082315601 Weeki Wachee 283201 823156 259 UUFA

30 282742082375901 ROMP TR18-1 near Aripeka 282742 823759 580 UUFA

31 282613082381701 ROMP TR18-3 near Aripeka 282613 823817 622 UUFA

32 282605082345801 ROMP 97 near Aripeka 282605 823458 355 UUFA

33 3032350822035 BA0059 Eddy Fire Tower 1151 303235 822035 20 SAS

34 3032350822035 BA0058 Eddy Fire Tower 1153 303235 822035 40 ICU

35 3032350822035 BA0057 Eddy Fire Tower 1152 303235 822035 360 CUFA

36 302416081522601 D-0348 J-0413 302416 815226 708 CUFA

37 302416081522602 D-0349 J-0414 302416 815226 2,230 LFA

38 302301081295001 DS-522 Ft. Caroline Park 302301 812950 34 SAS

39 302301081295002 DS-523 Ft. Caroline Park 302301 812950 204 ICU

40 301710081323601 DS-520 301710 813236 60 SAS

41 301710081323602 DS-521 301710 813236 120 ICU

42 301710081323603 D-3824 301710 813236 740 CUFA

43 3016180821109 BA0056 Macclenny 1160 301618 821109 40 SAS

44 3016180821109 BA0055 Macclenny 1161 301618 821109 60 ICU

45 3016180821109 BA0054 Macclenny 1162 301618 821109 368 CUFA

46 3005070812727 SJ0030 Durbin Firetower 300507 812727 120 SAS
124 Simulation of Ground-Water Flow in the Intermediate and Floridan Aquifer Systems in Peninsular Florida

47 3005070812727 SJ0029 Durbin Firetower 300507 812727 603 CUFA

48 2958510815553 C-0127 Penney Farms Tower 295851 815553 136 SAS

49 2958510815553 C-0128 Penney Farms Tower 295851 815553 405 CUFA

50 2951160820058 C-0456 Lake Lowery 295116 820058 20 SAS

51 2951160820058 C-0440 Lake Lowery 295116 820058 124 ICU

52 2951160820058 C-0439 Lake Lowery 295116 820058 198 CUFA

53 2949110815726 C-0455 Gold Head 3 294911 815726 42 SAS

54 2949110815726 C-0454 Gold Head 2 294911 815726 110 ICU

55 2949110815726 C-0453 Gold Head 1 294911 815726 375 CUFA

56 2947280820109 C-0444 Chester Moody 294728 820109 87 SAS

57 2947280820109 C-0443 Chester Moody 294728 820109 131 ICU

58 2947280820109 C-0442 Chester Moody 294728 820109 240 CUFA

59 2946110820049 C-0438 Lake Geneva 3 294611 820049 20 SAS

60 2946110820049 C-0437 Lake Geneva 2 294611 820049 85 ICU

61 2946110820049 C-0436 Lake Geneva 1 294611 820049 146 CUFA

62 2937540811219 F-0191 Washington Oaks State Park 293754 811219 36 SAS

63 2937540811219 F-0200 Washington Oaks State Park 293754 811219 148 CUFA

64 2928590813757 P-0409 Fruitland 292859 813757 55 SAS

65 2928590813757 P-0408 Fruitland 292859 813757 148 CUFA

66 2926030810825 F-0177 Bulow Ruins 292603 810825 43 SAS

67 2926030810825 F-0176 Bulow Ruins 292603 810825 120 CUFA

68 2924470813706 P-0778 Marvin Jones Road 292447 813706 50 SAS

69 2924470813706 P-0777 Marvin Jones Road 292447 813706 110 ICU

70 2924470813706 P-0776 Marvin Jones Road 292447 813706 160 CUFA

71 2924180813309 P-0742 Niles Road 292418 813309 27 SAS

72 2924180813309 P-0143 Niles Road 292418 813309 66 ICU

73 2924180813309 P-0705 Niles Road 292418 813309 400 CUFA

74 2922390813137 P-0724 Silver Pond 292239 813137 25 SAS

75 2922390813137 P-0146 Silver Pond 292239 813137 55 ICU

76 2922390813137 P-0696 Silver Pond 292239 813137 400 CUFA

77 2921240813452 P-0734 Middle Road 292124 813452 20 SAS

78 2921240813452 P-0736 Middle Road 292124 813452 100 ICU

79 2921240813452 P-0735 Middle Road 292124 813452 360 CUFA

80 2914580812942 V-0525 West Pierson 291458 812942 14 SAS

81 2914580812942 V-0524 West Pierson 291458 812942 39 ICU

82 2914580812942 V-0068 West Pierson 291458 812942 125 CUFA

83 2914480812749 V-0528 Pierson Airport 291448 812749 23 SAS

84 2914480812749 V-0557 Pierson Airport 291448 812749 98 ICU

85 2914480812749 V-0531 Pierson Airport 291448 812749 210 CUFA

86 2914480812749 V-0530 Pierson Airport 291448 812749 1,060 LFA

87 2913230811912 V-0770 State Road 40 and State Road 11 291323 811912 35 SAS

88 2913230811912 V-0501 State Road 40 and State Road 11 291323 811912 70 ICU

89 2913230811912 V-0769 State Road 40 and State Road 11 291323 811912 440 CUFA

90 2906140811833 V-0744 Lee Airport 290614 811833 36 SAS

91 2906140811833 V-0743 Lee Airport 290614 811833 72 ICU

92 2906140811833 V-0742 Lee Airport 290614 811833 460 CUFA

Appendix B. Description of wells equipped with continuous water-level recorders--Continued

[Well number refers to figure 16. Site numbers with 13 digits indicate data source was St. Johns River Water Management District; site numbers 

ending with 85 indicate data source was Suwannee River Water Management District (SRWMD); data source of all other site numbers was 

U.S. Geological Survey (USGS). Hydrogeologic units: CUFA, confined Upper Floridan aquifer; UUFA, unconfined Upper Floridan aquifer; 

SAS, surficial aquifer system; ICU, intermediate confining unit; IAS, intermediate aquifer system; LFA, Lower Floridan aquifer. --, data not available; 

ROMP, Regional Observation and Monitoring Well Program]

Well 

number Site number Well name Latitude Longitude

Well 

depth 

(feet)

Hydrogeologic 

unit
Appendix B 125

93 283204081544902 Mascotte near Mascotte 283204 815449 30 SAS

94 283204081544901 Mascotte near Mascotte 283204 815449 160 CUFA

95 282202081384602 Lake Oliver near Vineland 282202 813846 30 SAS

96 282202081384601 Lake Oliver near Vineland 282202 813846 318 CUFA

97 275411081372002 ROMP 57 near Lake Wales 275411 813720 140 SAS

98 275411081372001 ROMP 57 near Lake Wales 275411 813720 634 CUFA

99 274240082212702 ROMP 50 near Wimauma 274240 822127 37 SAS

100 274240082212701 ROMP 50 near Wimauma 274240 822127 562 CUFA

101 274240082212703 ROMP 50 Avon Park near Wimauma 274240 822127 1,430 LFA

102 272728081474703 ROMP 30 near Zolfo Springs 272728 814747 15 SAS

103 272728081474701 ROMP 30 Avon Park near Zolfo Springs 272728 814747 1,266 CUFA

104 270959082203003 ROMP 19 near Sarasota 270959 822030 67 SAS

105 270959082203002 ROMP 19 near Sarasota 270959 822030 205 IAS

106 270959082203001 ROMP 19 near Sarasota 270959 822030 425 CUFA

107 270952080135202 M-1183 270952 801352 21 SAS

108 270952080135201 M-1141 270952 801352 109 ICU

109 2933130811352 F-0164 Levitt Development 293313 811352 90 SAS

110 2913530811604 V-0088 Union Camp 291353 811604 20 SAS

111 282210081352601 Disney at tree farm near Vineland 282210 813526 18 SAS

112 281722080543001 USGS OS171 near Deer Park 281722 805430 19 SAS

113 280619080542601 USGS OS179 at Deer Park 280619 805426 18 SAS

114 280132082452803 ROMP TR14-2 near Dunedin 280132 824528 22 SAS

115 275458082464004 ROMP TR13-1A near Largo 275458 824640 20 SAS

116 275430082431403 ROMP TR13-2 near Largo 275430 824314 16 SAS

117 274812081190301 P49 near Frostproof 274812 811903 17 SAS

118 273923080471801 IR 25 USGS near Yeehaw Junction 273923 804718 19 SAS

119 272504081120101 H11A near Lake Placid 272504 811201 16 SAS

120 272258082181701 KME water table 09 near Verna 272258 821817 42 SAS

121 271559081242501 Lake Groves Road near Lake Placid 271559 812425 23 SAS

122 271226081194301 Bairs Den near Lake Placid 271226 811943 35 SAS

123 271007080142101 M-1179 271007 801421 20 SAS

124 270913080284901 M-1255 270913 802849 28 SAS

125 270609080163401 M-1261 270609 801634 20 SAS

126 270157081203101 USGS H15A near Palmdale 270157 812031 23 SAS

127 270124080280202 M-1048 270124 802802 80 SAS

128 265812080053901 PB-565 265812 800539 22 SAS

129 265725080141801 M-1234 265725 801418 18 SAS

130 275314081514202 ROMP 59 Hawthorn near Bartow 275314 815142 142 IAS

131 273851082031502 ROMP 40 Hawthorn near Duette 273851 820315 180 IAS

132 273156081451401 Rowell 273156 814514 267 IAS

133 272258082195301 KME 04 272258 821953 440 IAS

134 272058082143701 Verna T O-2 near Verna 272058 821437 530 IAS

135 271832082064801 Edgeville 3 at Edgeville 271832 820648 600 IAS

136 271757081493003 ROMP 26 Hawthorn near Gardner 271757 814930 180 IAS

137 271118082285301 Osprey 9 271118 22853 255 IAS

138 270952082095901 Carlton 13 near Arca 270952 820959 287 IAS

Appendix B. Description of wells equipped with continuous water-level recorders--Continued

[Well number refers to figure 16. Site numbers with 13 digits indicate data source was St. Johns River Water Management District; site numbers 

ending with 85 indicate data source was Suwannee River Water Management District (SRWMD); data source of all other site numbers was 

U.S. Geological Survey (USGS). Hydrogeologic units: CUFA, confined Upper Floridan aquifer; UUFA, unconfined Upper Floridan aquifer; 

SAS, surficial aquifer system; ICU, intermediate confining unit; IAS, intermediate aquifer system; LFA, Lower Floridan aquifer. --, data not available; 

ROMP, Regional Observation and Monitoring Well Program]

Well 

number Site number Well name Latitude Longitude

Well 

depth 

(feet)

Hydrogeologic 

unit
126 Simulation of Ground-Water Flow in the Intermediate and Floridan Aquifer Systems in Peninsular Florida

139 270137082235301 Manasota 14 270137 822353 305 IAS

140 304942082213801 USGS OK-9 304943 822138 700 CUFA

141 304756081311101 U.S. Navy in Kings Bay 304756 813111 990 CUFA

142 304512081343601 Huntley-Jiffy (Davis) 304510 813438 -- CUFA

143 304213081270801 N-19 Ft. Clinch, Fernandina Beach 304210 812708 710 CUFA

144 3038230812733 N-0190 Fernandina Beach ITT 8 303823 812733 1,020 CUFA

145 303510083054985 Prescott 303510 830549 188 CUFA

146 303425082473685 Cypress Creek 303425 824736 200 CUFA

147 303224083101785 Santa Deas 303224 831017 195 CUFA

148 303158082562985 Stafford Scaff at Jasper 303158 825629 279 CUFA

149 302859083015085 Carter 302859 830150 167 CUFA

150 302833082542985 Deas 302833 825429 105 CUFA

151 302550081331501 D-3840 St. John’s River Power Park 302550 813315 750 CUFA

152 302251082194985 USGS ONF 6A 302251 821949 338 CUFA

153 302243082360285 ONF University of Florida 1 302241 823604 227 CUFA

154 301933082350585 USGS ONF 2V 301939 823526 262 CUFA

155 301423082261185 USGS Ocean Pond 301423 822611 134 CUFA

156 301031082381085 DOT 301031 823810 836 CUFA

157 301006082461785 St. Regis Paper Company 301006 824617 250 CUFA

158 300747082225885 USGS Lake Butler 300747 822258 35 CUFA

159 300706082402285 Vernon Norton 300706 824022 140 CUFA

160 300635082295985 Lulu Community Center 300635 822959 214 CUFA

161 300220082103085 USGS Railford 300220 821030 294 CUFA

162 300101082245285 USGS Lake Butler 300101 822452 254 CUFA

163 295257082045785 USGS Starke 295257 820457 324 CUFA

164 295055082130885 USGS Graham 295055 821308 206 CUFA

165 294920082044585 SRWMD Santa Fe Swamp 294920 820445 208 CUFA

166 294807082020903 Keystone Heights 294807 820209 250 CUFA

167 2947010812633 SJ0317 SKYES 294701 812633 290 CUFA

168 294530082232085 City of Gainsville 294530 822320 284 CUFA

169 294402082262185 DNR San Felasco Hammock 294402 822621 168 CUFA

170 294330082445085 USGS Trenton 294330 824450 103 CUFA

171 294313082024685 USGS Melrose 294313 820246 259 CUFA

172 293857082203985 University of Florida 293857 822039 406 CUFA

173 2937290812212 SJ0115 Countyline 293729 812212 609 CUFA

174 2935290811917 F-0165 ITT-LW-20 293529 811917 140 CUFA

175 2933130811324 F-0158 ITT Palm Coast 293313 811324 284 CUFA

176 2922040820228 M-0052 Fort McCoy 292204 820228 160 CUFA

177 2918180811904 F-0251 Relay Tower 291818 811904 147 CUFA

178 2917480812903 V-0510 Mew 291748 812903 130 CUFA

179 2917400815620 M-0025 C354 Gores Landing 291740 815620 280 CUFA

180 2913440811557 V-0090 Union Camp 291344 811557 151 CUFA

181 2913430812546 V-0089 Jones near Pierson 291343 812546 414 CUFA

182 2911300820150 M-0026 CE47 near Silver Springs 291130 820150 192 CUFA

183 290743082341501 Tidewater 1 near Dunnellon 290743 823415 784 CUFA

184 290455081530401 USGS Moss Bluff Park 290455 815304 225 CUFA

Appendix B. Description of wells equipped with continuous water-level recorders--Continued

[Well number refers to figure 16. Site numbers with 13 digits indicate data source was St. Johns River Water Management District; site numbers 

ending with 85 indicate data source was Suwannee River Water Management District (SRWMD); data source of all other site numbers was 

U.S. Geological Survey (USGS). Hydrogeologic units: CUFA, confined Upper Floridan aquifer; UUFA, unconfined Upper Floridan aquifer; 

SAS, surficial aquifer system; ICU, intermediate confining unit; IAS, intermediate aquifer system; LFA, Lower Floridan aquifer. --, data not available; 

ROMP, Regional Observation and Monitoring Well Program]

Well 

number Site number Well name Latitude Longitude

Well 

depth 

(feet)

Hydrogeologic 

unit
Appendix B 127

185 2901030805519 V-0508 Smith Street and US 1 290103 805519 210 CUFA

186 285745081054001 USGS at Alamana 285745 810540 21 CUFA

187 2856380812031 V-0083 Blue Springs 285638 812031 432 CUFA

188 284842081533001 College Street at Leesburg 284842 815330 245 CUFA

189 284725081361901 Wolf Sink near Sorrento 284725 813619 205 CUFA

190 284147081220201 USGS SEM 125 at Longwood 284147 812202 146 CUFA

191 283314081455501 City of Clermont 283314 814555 525 CUFA

192 283253081283401 USGS OR47 at Orlo Vista 283252 812835 350 CUFA

193 283249081053201 Bithlo 1 at Bithlo 283249 810532 492 CUFA

194 282835081305201 Palm Lake Drive near Windermere 282839 813026 235 CUFA

195 282738081341401 Lake Sawyer near Windermere 282738 813414 178 CUFA

196 282717081553101 ROMP 101 near Bay Lake 282717 815531 404 CUFA

197 282623081153801 Cocoa-P near Taft 282623 811538 439 CUFA

198 282531081095701 USGS Cocoa D near Narcoossee 282531 810957 300 CUFA

199 282528081340901 Bay Lake near Windermere 282528 813409 223 CUFA

200 282434081283102 Sea World Drive near Vineland 282434 812831 239 CUFA

201 282341081040101 USGS Cocoa A near Bithlo 282341 810401 516 CUFA

202 282127082022501 Cumpressco Ranch near Tarrytown 282127 820225 143 CUFA

203 281949082332001 State Highway 52 near Fivay Junction 281949 823320 73 CUFA

204 281926082212901 Junction of 52 and 581 near Darby 281926 822129 113 CUFA

205 281714081093001 Lake Joel near Ashton 281714 810930 750 CUFA

206 281558082264601 Pasco 13 near Drexel 281558 822646 49 CUFA

207 281532081345001 Loughman near Loughman 281532 813450 250 CUFA

208 281222082393401 Seven Springs near Odessa 281222 823934 301 CUFA

209 281202081391701 PO-1 Thornhill near Davenport 281202 813847 151 CUFA

210 281022082400201 Eldr Wild 3 281022 824002 350 CUFA

211 281008081441801 Lake Alfred near Lake Alfred 281008 814418 425 CUFA

212 280734082442101 ROMP TR15-3 near Tarpon Springs 280734 824421 150 CUFA

213 280655082193001 Morris Bridge 3a near Branchton 280659 821943 600 CUFA

214 280652082195101 Morris Bridge WF 3C near Thonotosassa 280652 821951 1,037 CUFA

215 280053082350202 Sheldon Road near Citrus Park 280053 823502 330 CUFA

216 280022082210501 SWFWMD west of Vandenburg Apt. 280022 822105 37 CUFA

217 275959081552501 Sanlon Ranch near Eaton Park 275959 815525 1,220 CUFA

218 275815082440401 Pinellas 665 275815 824404 299 CUFA

219 275429082093901 ROMP 61 near Pleasant Grove 275429 820939 1,000 CUFA

220 274409082015001 Bethlehem Road near Bradley Junction 274409 820150 1,000 CUFA

221 273718082315501 FP&L at Piney Point 273718 823155 950 CUFA

222 272255082172202 KME near Verna 272255 821722 1,200 CUFA

223 272053082320202 City of Sarasota injection well 2 at Sarasota 272053 823202 1,500 CUFA

224 272012081482501 Marshall near Gardner 272012 814825 478 CUFA

225 271938082251801 Sarasota 9 271938 822518 730 CUFA

226 271232081392201 ROMP 15 Avon Park near Arcadia 271232 813922 1,360 CUFA

Appendix B. Description of wells equipped with continuous water-level recorders--Continued

[Well number refers to figure 16. Site numbers with 13 digits indicate data source was St. Johns River Water Management District; site numbers 

ending with 85 indicate data source was Suwannee River Water Management District (SRWMD); data source of all other site numbers was 

U.S. Geological Survey (USGS). Hydrogeologic units: CUFA, confined Upper Floridan aquifer; UUFA, unconfined Upper Floridan aquifer; 

SAS, surficial aquifer system; ICU, intermediate confining unit; IAS, intermediate aquifer system; LFA, Lower Floridan aquifer. --, data not available; 

ROMP, Regional Observation and Monitoring Well Program]

Well 

number Site number Well name Latitude Longitude

Well 

depth 

(feet)

Hydrogeologic 

unit
128 Simulation of Ground-Water Flow in the Intermediate and Floridan Aquifer Systems in Peninsular Florida

Appendix C. Description and flow measurements of Upper Floridan aquifer springs

[Spring number refers to figure 22. If more than one date of measurement is listed, measured flow is an average of measurements; ft3/s, cubic feet per second; 

dates are shown in month-year format]

Spring 

number Spring name Latitude Longitude County

Measured or 

estimated 

flow

(ft3

/s)

Date(s)

of 

measurement(s)

1 Blue Spring near Madison 302849 831440 Madison a 118.0 07-95, 11-95

2 Alapaha Rise near Fort Union 302614 830513 Hamilton a427.0 08-95, 11-95

3 Holton Spring near Fort Union 302615 830327 Hamilton a

12.5 07-95, 11-95

4 Suwannee Springs near Live Oak 302339 825604 Suwannee a 9.8 07-95

5 Suwanacoochee Spring at Ellaville 302309 831018 Madison b 43.0 11-73

6 Ellaville Spring at Ellaville 302303 831021 Suwannee b69.0 11-73

7 Falmouth Spring at Falmouth 302140 830807 Suwannee b

134.0 11-73

8 White Sulphur Springs at White Springs 301947 824540 Hamilton a42.3 06-98

9 Charles Springs near Dell 301002 831350 Suwannee a4.7 07-95, 11-95

10 Allen Mill Pond Spring near Dell 300945 831433 Lafayette a12.2 07-95, 11-95

11 Wadesboro Spring near Orange Park 300925 814320 Clay b1.0 03-72

12 Blue Spring near Dell 300733 831334 Lafayette a

70.0 07-95, 11-95

13 Peacock Springs 300718 830757 Suwannee a81.1 06-98

14 Telford Spring at Luraville 300624 830957 Suwannee a35.8 07-95, 11-95

15 Running Springs (East and West) near Luraville 300615 830659 Suwannee a

88.0 07-95, 11-95

16 Convict Spring near Mayo 300518 830546 Lafayette a

1.1 07-95, 11-95

17 Royal Spring near Alton 300501 830430 Suwannee a1.9 07-95

18 Owens Spring 300244 830229 Lafayette b43.3 09-73

19 Mearson Spring near Mayo 300228 830132 Lafayette a

51.0 07-95, 11-95

20 Troy Spring near Branford 300021 825951 Lafayette a

132.0 07-95, 11-95

21 Little River Springs near Branford 295947 825759 Suwannee a67.0 07-95, 11-95

22 Ruth Spring near Branford 295944 825838 Lafayette a7.5 07-95, 11-95

23 Green Cove Springs at Green Cove Springs 295936 814040 Clay b

3.0 03-72

24 Ichetucknee Head Spring near Fort White 295902 824543 Suwannee c 41.0 07-95, 11-95

25 Cedar Head (or Alligator Hole) Spring 295900 824532 Columbia c8.0 07-95, 11-95

26 Blue Hole (or Jug) Spring 295847 824531 Columbia c77.0 07-95, 11-95

27 Roaring Springs (including Singing Spring) 295835 824531 Columbia c48.0 07-95, 11-95

28 Boiling Spring 295825 824537 Suwannee c

116.0 07-95, 11-95

29 Mill Pond Spring (including Grassy Hole Springs and Coffee 

Spring)

295804 824537 Columbia c17.0 07-95, 11-95

30 Branford Springs at Branford 295717 825544 Suwannee a35.8 05-98, 06-98

31 Jamison Spring 295532 824556 Columbia b3.0 04-77

32 Hornsby Spring near High Springs 295059 823536 Alachua a49.8 07-95, 12-95

33 Turtle Spring near Hatchbend 295055 825324 Lafayette a27.9 07-95, 11-95

34 Fletcher Spring 295048 825334 Lafayette b34.0 11-72

35 Steinhatchee Spring near Clara 295028 831829 Lafayette b0.7 10-72

36 Ginnie Spring near High Springs 295010 824201 Gilchrist a57.1 07-95, 12-95

37 Blue Springs near High Springs (including Lilly Springs) 294947 824059 Gilchrist a41.2 07-95, 12-95

38 Poe Springs near High Springs 294933 823858 Alachua a53.6 07-95, 12-95

39 Rock Bluff Springs near Bell 294756 825508 Gilchrist a33.2 08-95, 12-95

40 Guaranto Spring near Rock Bluff Landing 294646 825624 Dixie b

12.0 11-72

41 Crescent Beach Submarine Spring 294606 811230 offshore d 30.0

42 Lumbercamp Springs 294227 825608 Gilchrist b6.2 11-72

43 Sun Springs near Wannee 294216 825601 Gilchrist a

40.1 08-95, 12-95

44 Hart Springs near Wilcox 294030 825705 Gilchrist a90.8 08-95, 12-95

45 Otter Springs near Wilcox 293840 825636 Gilchrist b16.0 11-72

46 Whitewater Springs 293806 813853 Putnam b1.2 04-72

47 Copper Springs near Oldtown (including Little Copper Spring) 293650 825826 Dixie b25.4 11-75

48 Bell Springs 293550 825630 Gilchrist b5.1 10-72

49 Fannin Springs near Wilcox (including Little Fannin Spring) 293515 825608 Levy a97.7 07-95, 12-95

50 Satsuma Spring 293159 814036 Putnam b1.1 03-72

51 Blue Springs near Orange Springs 293051 815125 Marion a

.5 05-99

52 Orange Spring at Orange Springs 293038 815638 Marion a

2.0 05-99

53 Camp Seminole Spring at Orange Springs 293021 815706 Marion a.8 05-99

54 Welaka Spring near Welaka 292935 814025 Putnam e 1.0

55 Manatee Spring near Chiefland 292922 825837 Levy a187.0 07-95, 12-95

56 Mud Spring near Welaka 292735 813945 Putnam b2.3 06-72

57 Blue Spring near Bronson 292702 824157 Levy b8.0 06-74

58 Beecher Springs near Fruitland 292654 813849 Putnam 6.3 09-93
Appendix C 129

59 Croaker Hole Spring near Welaka 292618 814121 Putnam 90.3 09-93

60 Tobacco Patch Landing Spring Group near Fort McCoy 292542 815526 Marion a

1.0 05-99

61 Wells Landing Springs near Fort McCoy 292521 815512 Marion a5.0 05-99

62 Salt Springs near Eureka 292100 814358 Marion 79.0 09-93, 05-94

63 Wekiva Springs near Gulf Hammock 291649 823923 Levy b45.4 06-67

64 Silver Glen Springs near Astor 291443 813837 Marion 100.0 09-93, 05-94

65 Sweetwater Springs along Juniper Creek 291307 813936 Marion 12.5 09-93, 05-94

66 Silver Springs near Ocala 291257 820311 Marion 640.0 09-93, 05-94

67 Morman Branch Seepage into Juniper Creek near Astor 291129 813858 Marion a

1.0 06-97

68 Juniper Creek Tributary near Astor 291104 813845 Lake a

2.0 06-97

69 Juniper Springs near Ocala 291101 814246 Marion 8.1 09-93, 05-94

70 Fern Hammock Springs near Ocala 291100 814229 Marion 10.7 09-93, 05-94

71 Ponce de Leon Springs near De Land 290802 812147 Volusia 24.3 09-93, 05-94

72 Rainbow Springs near Dunnellon 290608 822616 Marion 637.0 09-93, 05-94

73 Alexander Springs near Astor 290450 813430 Lake 113.0 09-93, 05-94

74 Mosquito Springs Run, Alexander Springs Wildnerness 290220 812604 Lake a2.0 06-97

75 Wilson Head Spring near Holder 285840 821908 Marion b1.9 06-72

76 Blue Spring near Holder 285809 821852 Citrus b

10.6 05-72

77 Gum Springs near Holder 285731 821354 Sumter b67.6 06-72

78 Camp La No Che Springs near Paisley 285702 813224 Lake a1.0 06-97

79 Blue Spring near Orange City 285650 812023 Volusia 135.0 09-93, 05-94

80 Blackwater Springs near Cassia 285318 812952 Lake e

1.4

81 Crystal River Spring Group 285300 823600 Citrus f 613.0

82 Little Jones Creek Head Spring near Wildwood 285208 820541 Sumter e8.0

83 Green Springs 285145 811455 Volusia b.3 04-72

84 Gemini Springs near DeBary (all 3) 285144 811839 Volusia 10.5 09-93, 05-94

85 Little Jones Creek Spring No. 2 near Wildwood 285134 820518 Sumter e5.0

86 Messant Spring near Sorrento 285121 812956 Lake 12.0 09-93, 05-94

87 Seminole Springs near Sorrento 285044 813122 Lake 37.0 09-93, 05-94

88 Palm Springs Seminole State Forest 285038 812701 Lake a

.5 06-97

89 Little Jones Creek Spring No. 3 near Wildwood 285011 820349 Sumter e3.0

90 Droty Springs near Sorrento 284940 813038 Lake a.6 06-97

91 Halls River Head Spring 284935 823450 Citrus g 4.8

92 Island Spring near Sanford 284922 812503 Seminole a6.4 04-97, 08-97

93 Halls River Springs 284804 823610 Citrus f102.2

94 Homosassa Springs at Homosassa Springs 284758 823520 Citrus g72.4

95 Southeast Fork of Homosassa Springs at Homosassa Spring 284751 823523 Citrus f43.1

96 Trotter Spring at Homosassa Springs 284747 823510 Citrus g

5.2

97 Fenney Springs near Coleman, Head Spring of Shady Brook 

Creek

284742 820219 Sumter b15.0 03-72

98 Shady Brook Creek Spring No. 2 284708 820246 Sumter e2.9

99 Shady Brook Creek Spring No. 3 284646 820238 Sumter e2.9

100 Shady Brook Creek Spring No. 4 284612 820420 Sumter e

2.9

101 Sulphur Camp Springs 284612 813034 Orange a.6 06-97

102 Hidden River Springs near Homosassa (including Hidden River 

Head Spring)

284559 823520 Citrus g6.7

103 Rock Springs near Apopka 284521 813004 Orange 53.0 09-93, 05-94

104 Shady Brook Creek Spring No. 5 284515 820501 Sumter e

2.9

105 Bugg Spring at Okahumpka 284507 815406 Lake 8.6 09-93, 05-94

106 Blue Springs near Yalaha 284455 814941 Lake a3.0 09-97

107 Mooring Cove Springs near Yalaha 284452 814954 Lake a

.4 06-97

108 Holiday Springs at Yalaha 284424 814905 Lake a3.6 09-96

109 Potter Spring near Chassahowitzka (including Ruth Spring) 284354 823548 Citrus g14.4

110 Witherington Spring near Apopka 284353 812922 Orange 1.0 05-93

111 Salt Creek Head Spring 284323 823506 Citrus g

.4

112 Lettuce Creek Spring 284308 823437 Citrus g

3.7

113 Crab Creek Spring 284300 823434 Citrus g34.8

114 Unnamed Tributary above Chassahowitzka Springs (including 

Bubba Spring)

284254 823438 Citrus g20.5

Appendix C. Description and flow measurements of Upper Floridan aquifer springs--Continued

[Spring number refers to figure 22. If more than one date of measurement is listed, measured flow is an average of measurements; ft3/s, cubic feet per second; 

dates are shown in month-year format]

Spring 

number Spring name Latitude Longitude County

Measured or 

estimated 

flow

(ft3

/s)

Date(s)

of 

measurement(s)
130 Simulation of Ground-Water Flow in the Intermediate and Floridan Aquifer Systems in Peninsular Florida

115 Chassahowitzka Springs near Chassahowitzka 284254 823435 Citrus g64.8

116 Wekiwa Springs in State Park near Apopka 284243 812736 Orange 56.5 09-93, 05-94

117 Miami Springs near Longwood 284236 812634 Seminole 4.0 09-93, 05-94

118 Lake Jessup Spring near Wagner 284236 811605 Seminole b.6 05-72

119 Baird Creek Head Spring near Chassahowitzka 284230 823440 Citrus g3.2

120 Clifton Springs near Oviedo 284156 811414 Seminole b

1.5 06-72

121 Starbuck Spring near Longwood 284148 812328 Seminole 12.3 09-93, 05-94

122 Beteejay Lower Spring near Chassahowitzka (including Beteejay Head Spring)284131 823535 Citrus g7.3

123 Palm Springs near Longwood 284127 812334 Seminole 4.1 09-93, 05-94

124 Rita Maria Spring near Chassahowitzka 284126 823528 Hernando g3.3

125 Sanlando Springs near Longwood 284119 812345 Seminole 18.5 09-93, 05-94

126 Unnamed Spring No. 10 (including No. 11 and No.12) 284114 823652 Hernando f19.0

127 Ryle Creek Lower Spring near Bayport (including Ryle Creek 

Head Spring)

284113 823650 Hernando g8.3

128 Blue Run Head Spring near Chassahowitzka 284113 823606 Hernando g

4.6

129 Double Run Road Seepage near Astatula 284039 814425 Lake a2.0 06-96

130 Unnamed Spring No. 8 284017 823808 Hernando f4.9

131 Blind Creek Springs (including unnamed spring No. 7 and 

Blind Creek Head Spring)

283932 823806 Hernando f42.7

132 Apopka (Gourdneck) Spring near Oakland 283400 814051 Lake a

31.4 12-92

133 Unnamed Spring No. 6 283254 823737 Hernando f2.8

134 Salt Spring near Bayport 283246 823709 Hernando g22.3

135 Mud Spring near Bayport 283240 823701 Hernando g17.0

136 Jenkins Creek Spring No. 5 283120 823804 Hernando g

15.3

137 Unnamed Spring No. 4 283118 823806 Hernando f6.3

138 Weeki Wachee Springs near Brooksville 283100 823425 Hernando 129.0 09-93, 05-94

139 Unnamed Spring No. 2 282720 823830 Hernando f

.7

140 Boat Spring at Aripeka 282621 823929 Hernando g

.4

141 Bobhill Springs 282607 823834 Hernando g1.8

142 Unnamed Spring No. 1 282600 823926 Hernando f6.3

143 Magnolia Springs at Aripeka 282558 823926 Pasco g.5

144 Unnamed Spring No. 3 near Aripeka 282352 824027 Pasco b

17.8 08-60

145 Horseshoe Spring near Hudson 282350 824121 Pasco b9.7 12-72

146 Salt Springs near Port Richey 281733 824306 Pasco b8.2 12-72

147 Crystal Springs near Zephyrhills 281030 821120 Pasco 37.0 09-93, 05-94

148 Sulphur Springs at Sulphur Springs 280115 822705 Hillsborough 25.0 10-93, 05-94

149 Lettuce Lake Spring 280104 822102 Hillsborough b8.3 05-73

150 Six-Mile Creek Spring 280104 822018 Hillsborough b1.3 05-71, 10-71

151 Eureka Springs near Tampa 280022 822039 Hillsborough b

1.3 05-73

152 Buckhorn Spring near Riverview 275322 821810 Hillsborough b

15.0 06-72

153 Lithia Springs Minor near Lithia 275201 821349 Hillsborough 8.0 09-93, 05-94

154 Lithia Springs Major near Lithia 275158 821352 Hillsborough 31.1 09-93, 05-94

155 Little Salt Spring near Murdock 270430 821400 Sarasota b.9 05-72

156 Warm Mineral Springs near Woodmere 270333 821540 Sarasota b

6.7 05-74

Total 6,383.8

a

Estimated as the product of measured flow and the ratio of August 1993 through July 1994 rainfall that occurred during the year in which actual flow 

measurement(s) were made. b

Estimated as the product of measured flow from Rosenau and others (1977) and ratio of August 1993 through July 1994 rainfall that occurred during 

the year in which actual flow measurement(s) were made. c

Total flow of Ichetucknee Springs is the combined flow, in downstream order, from Ichetucknee Head Spring, Cedar Head Spring, Blue Hole Spring, 

Roaring Springs, Boiling Spring, and Mill Pond Spring. Estimated flow of 307 ft3

/s from Ichetucknee Springs for 1993-94 was obtained as the product of 

combined flow of 287 ft3

/s for 1995 and the ratio of August 1993 through July 1994 rainfall to 1995 rainfall. d

Highly generalized estimate.

e

Estimated flow from Leel Knowles (USGS, written commun., 2000). f

Estimated to be 70 percent of average of flow measurements from Yobbi (1989). g

Estimated to be 70 percent of average of flow measurements from Yobbi (1992).

Appendix C. Description and flow measurements of Upper Floridan aquifer springs--Continued

[Spring number refers to figure 22. If more than one date of measurement is listed, measured flow is an average of measurements; ft3/s, cubic feet per second; 

dates are shown in month-year format]

Spring 

number Spring name Latitude Longitude County

Measured or 

estimated 

flow

(ft3

/s)

Date(s)

of 

measurement(s)