Lake County Water Authority

St. Johns River Water Management District

Southwest Florida Water Management District

U.S. GEOLOGICAL SURVEY

Water-Resources Investigations Report 02-4207

Prepared in cooperation with:
U.S. GEOLOGICAL SURVEY

Water-Resources Investigations Report 

Hydrogeology and Simulated Effects of 

Ground-Water Withdrawals from the 

Floridan Aquifer System in Lake County 

and in the Ocala National Forest and 

Vicinity, North-Central Florida

Prepared in cooperation with the

Lake County Water Authority

St. Johns River Water Management District

Southwest Florida Water Management District

02–4207

By Leel Knowles, Jr., Andrew M. O’Reilly, and James C. Adamski

Tallahassee, Florida

2002
For additional information 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

Use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply 

endorsement by the U.S. Geological Survey.

Additional information about water resources in Florida is available on the World Wide Web at 

http://fl.water.usgs.gov

write to:

District Chief

U.S. Geological Survey, WRD

Suite 3015

227 North Bronough Street

Tallahassee, FL 32301

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

Background.................................................................................................................................................................. 4

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

Previous Studies........................................................................................................................................................... 5

Acknowledgments ....................................................................................................................................................... 5

Description of Study Area ..................................................................................................................................................... 5

Physiography ............................................................................................................................................................... 7

Drainage....................................................................................................................................................................... 7

Climate......................................................................................................................................................................... 9

Land and Water Use..................................................................................................................................................... 9

Data-Collection Network............................................................................................................................................. 11

Hydrogeology ........................................................................................................................................................................ 11

Stratigraphy.................................................................................................................................................................. 17

Structure....................................................................................................................................................................... 17

Surficial Aquifer System ............................................................................................................................................. 18

Intermediate Confining Unit........................................................................................................................................ 20

Floridan Aquifer System.............................................................................................................................................. 20

Occurrence of Brackish (or Saline) Water......................................................................................................... 23

Hydraulic Characteristics................................................................................................................................... 28

Recharge ............................................................................................................................................................ 28

Discharge ........................................................................................................................................................... 29

Potentiometric Surface of the Upper Floridan Aquifer ............................................................................................... 29

Long-Term Trends ....................................................................................................................................................... 30

Rainfall and Lake Stage..................................................................................................................................... 30

Ground-Water Levels ........................................................................................................................................ 34

Upper Floridan Aquifer Springs ........................................................................................................................ 34

Water Budget ..................................................................................................................................................... 39

Simulation of Ground-Water Flow ........................................................................................................................................ 43

Model Design............................................................................................................................................................... 43

Model Layers and Grid ...................................................................................................................................... 43

Boundary Conditions ......................................................................................................................................... 45

Aquifer and Confining Unit Properties.............................................................................................................. 47

Aquifer Stresses ................................................................................................................................................. 51

Recharge and Evapotranspiration ............................................................................................................ 51

Streams and Lakes ................................................................................................................................... 60

Wetlands................................................................................................................................................... 61

Springs ..................................................................................................................................................... 62

Pumping and Drainage Wells .................................................................................................................. 62

Calibration ................................................................................................................................................................... 66

Inverse Model .................................................................................................................................................... 66

Observations ...................................................................................................................................................... 67

Calibration Procedure ........................................................................................................................................ 68

Parameter Values from Calibrated Model ......................................................................................................... 69

Parameter Uncertainty ....................................................................................................................................... 75

Model Fit............................................................................................................................................................ 78

Simulated 1998 Water Levels and Flows..................................................................................................................... 84
IV Contents

Effects of Projected 2020 Ground-Water Withdrawals............................................................................................... 90

Projected Boundary Conditions......................................................................................................................... 90

Projected Water Use.......................................................................................................................................... 91

Predicted Water Levels and Flows.................................................................................................................... 91

Lake and Wetland Water Levels ....................................................................................................................... 101

Spring and Well-Field Contributing Areas ....................................................................................................... 103

Effects of Parameter Uncertainty on Model Predictions................................................................................... 108

Model Limitations....................................................................................................................................................... 112

Summary ............................................................................................................................................................................... 114

Selected References .............................................................................................................................................................. 117

Appendixes............................................................................................................................................................................ 123

A. Index to stream-gaging and climatological data-collection sites ........................................................................... 124

B. Index to lake-gaging and surficial aquifer system well data-collection sites ........................................................ 126

C. Index to Floridan aquifer system well and spring data-collection sites ................................................................. 130

FIGURES

1-3. Maps showing:

1. Location of study (model) area....................................................................................................................... 3

2. Model boundary delineated using the May 1998 potentiometric surface of the Upper Floridan aquifer ...... 6

3. Topography, physiographic regions, and locations of hydrogeologic sections .............................................. 8

4. Diagram showing distribution of water withdrawals from the Floridan aquifer system......................................... 10

5-7. Maps showing:

5. Locations of stream-gaging and climatological data-collection sites............................................................. 12

6. Locations of lake-gaging and surficial aquifer system well data-collection sites .......................................... 13

7. Locations of springs and Floridan aquifer system well sites.......................................................................... 14

8. Chart showing geologic units, hydrogeologic units, and equivalent layers and boundary conditions 

used in the ground-water flow model...................................................................................................................... 15

9. Diagram showing hydrogeologic sections A-A', B-B', and C-C' ............................................................................ 16

10-16. Maps showing:

10. Generalized altitude of the base of the surficial aquifer system..................................................................... 19

11. Generalized thickness of the intermediate confining unit .............................................................................. 21

12. Generalized altitude of the top of the Floridan aquifer system ...................................................................... 22

13. Generalized altitude of the base of the Upper Floridan aquifer ..................................................................... 24

14. Generalized thickness of the middle semiconfining and confining units....................................................... 25

15. Generalized altitude of the base of the Floridan aquifer system .................................................................... 26

16. Generalized depth to water containing 5,000 milligram per liter chloride concentration in 

the Floridan aquifer system ............................................................................................................................ 27

17. Graph showing cumulative daily rainfall distribution for climatological sites in the study area 

December 1997 - November 1998 .......................................................................................................................... 30

18-22. Hydrographs showing:

18. Cumulative departure from average rainfall (A) and lake-stage time series (B) for selected sites 

in the study area.............................................................................................................................................. 32

19. Lake-stage duration for selected lakes............................................................................................................ 33

20. Long-term and 1997-98 water levels in selected wells tapping the surficial and the 

Upper Floridan aquifer systems in the study area .......................................................................................... 35

21. Annual rainfall and springflow (1935-2000) of first-magnitude Upper Floridan 

aquifer springs in the study area..................................................................................................................... 38

22. Annual springflow of selected second- and third-magnitude Upper Floridan aquifer springs

in the study area, 1929-2000 .......................................................................................................................... 40

23. Diagram showing water budget for Lake County, the Ocala National Forest, and vicinity within 

the model area, 1998 ............................................................................................................................................... 41
Contents V

24-28. Maps showing:

24. Finite-difference grid showing active and inactive model cells ..................................................................... 44

25. Lateral boundary conditions for the Upper and Lower Floridan aquifers and drainage-well

recharge rates for the Upper Floridan aquifer ................................................................................................ 46

26. Internal boundary conditions representing streams........................................................................................ 48

27. Internal boundary conditions representing lakes or wetlands ........................................................................ 49

28. Horizontal hydraulic conductivity of the Upper Floridan aquifer used to initiate model calibration ............ 50

29. Diagram showing simulated natural net recharge at the water table as a function of simulated water-table 

altitude..................................................................................................................................................................... 55

30-33. Maps showing:

30. Artificial net recharge rates from rapid infiltration basins and spray fields, average 1998 conditions .......... 58

31. Areas in which septic tank leakage and domestic self-supplied ground-water withdrawals from 

the Upper Floridan aquifer were estimated to occur ...................................................................................... 59

32. Ground-water withdrawal rates for the Upper Floridan aquifer specified in the model, 

average 1998 conditions ................................................................................................................................. 64

33. Ground-water withdrawal rates for the Lower Floridan aquifer specified in the model, 

average 1998 conditions ................................................................................................................................. 65

34. Graph showing composite scaled sensitivities for initial and final parameter values ............................................. 70

35-37. Maps showing:

35. Transmissivity of the Upper Floridan aquifer based on aquifer thickness and horizontal hydraulic 

conductivity from the calibrated model.......................................................................................................... 73

36. Leakance of the intermediate confining unit based on confining unit thickness and vertical hydraulic

conductivity from the calibrated model.......................................................................................................... 76

37. Leakance of the middle semiconfining and middle confining units based on confining unit 

thickness and vertical hydraulic conductivity from the calibrated model ...................................................... 77

38-39. Graphs showing:

38. Comparison of weighted residuals to weighted simulated values.................................................................. 79

39. Normal probability plot of weighted residuals............................................................................................... 80

40-42. Maps showing:

40. Simulated water table and water-level residuals for the surficial aquifer system, average 

1998 conditions .............................................................................................................................................. 81

41. Simulated potentiometric surface and water-level residuals for the Upper Floridan aquifer, 

average 1998 conditions ................................................................................................................................. 82

42. Simulated potentiometric surface and water-level residuals for the Lower Floridan aquifer, 

average 1998 conditions ................................................................................................................................. 83

43. Diagram showing simulated volumetric water budget for the aquifer system in the model area, 

average 1998 conditions.......................................................................................................................................... 86

44-50. Maps showing:

44. Simulated rate of net recharge or discharge at the water table, average 1998 conditions .............................. 88

45. Simulated rate of leakage through the intermediate confining unit, average 1998 conditions....................... 89

46. Ground-water withdrawal rates for the Upper Floridan aquifer specified in the model, 

projected 2020 conditions............................................................................................................................... 92

47. Ground-water withdrawal rates for the Lower Floridan aquifer specified in the model, 

projected 2020 conditions............................................................................................................................... 93

48. Simulated drawdown in the surficial aquifer system from average 1998 conditions as a result 

of projected 2020 conditions .......................................................................................................................... 94

49. Simulated drawdown in the Upper Floridan aquifer and simulated decrease in flow from 

selected Upper Floridan aquifer springs from average 1998 conditions as a result of 

projected 2020 conditions............................................................................................................................... 95

50. Simulated drawdown in the Lower Floridan aquifer from average 1998 conditions as a result 

of projected 2020 conditions .......................................................................................................................... 96
VI Contents

51-52. Diagrams showing:

51. Simulated volumetric water budget for the aquifer system in Lake County, average 1998 and 

projected 2020 conditions............................................................................................................................... 99

52. Simulated volumetric water budget for the aquifer system in the Ocala National Forest, 

average 1998 and projected 2020 conditions.................................................................................................. 100

53-57. Maps showing:

53. Contributing areas for selected springs and public-supply well fields based on particle-tracking 

analyses of simulated steady-state 1998 conditions ....................................................................................... 105

54. Contributing areas for selected springs and public-supply well fields based on particle-tracking 

analyses of simulated steady-state projected 2020 conditions ....................................................................... 106

55. Half-length of linear 95-percent confidence intervals on the simulated drawdown in the surficial 

aquifer system from average 1998 conditions as a result of projected 2020 conditions................................ 109

56. Half-length of linear 95-percent confidence intervals on the simulated drawdown in the 

Upper Floridan aquifer from average 1998 conditions as a result of projected 2020 conditions................... 110

57. Half-length of linear 95-percent confidence intervals on the simulated drawdown in the 

Lower Floridan aquifer from average 1998 conditions as a result of projected 2020 conditions .................. 111

TABLES

1. Summary statistics of springflows.......................................................................................................................... 36

2. Domestic self-supplied ground-water withdrawal rates and septic tank leakage rates, average 1998 

and projected 2020 conditions................................................................................................................................ 57

3. Initial parameter values specified in the model and final parameter values estimated with the inverse 

model to calibrate the model................................................................................................................................... 69

4. Coefficients of variation and linear 95-percent confidence intervals for the parameters estimated with 

the inverse model .................................................................................................................................................... 75

5. Water-level residual statistics for the calibrated model .......................................................................................... 78

6. Average and maximum drawdowns from average 1998 conditions as a result of projected 2020 

conditions for two predictive scenarios simulated by the model............................................................................ 97

7. Simulated discharge from selected Upper Floridan aquifer springs, projected 2020 conditions ........................... 102
Contents VII

CONVERSION FACTORS AND VERTICAL DATUM

*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.

Altitude: In this report, altitude refers to distance above or below sea level.

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

Acronyms and additional abbreviations:

ET evapotranspiration

FAS Floridan aquifer system

ICU intermediate confining unit

LCWA Lake County Water Authority

LFA Lower Floridan aquifer

MCU middle confining unit

MSCU middle semiconfining unit

mg/L milligrams per liter

Mgal/d million gallons per day

NOAA National Oceanic and Atmospheric Administration

Ocala NF Ocala National Forest

RASA Regional Aquifer Systems Analysis

SJRWMD St. Johns River Water Management District

SFWMD South Florida Water Management District

SWFWMD Southwest Florida Water Management District

SAS surficial aquifer system

TDEM time domain electromagnetic measurements

USFS U.S. Department of Agriculture, Forest Service

USGS U.S. Geological Survey

UFA Upper Floridan aquifer

Multiply By To obtain

Length

inch (in.) 2.54 centimeter

foot (ft) 0.3048 meter

mile (mi) 1.609 kilometer

Area

square foot (ft2) 0.0929 square meter

square mile (mi2) 2.590 square kilometer

square mile (mi2) 259 hectare

Flow

cubic foot per second (ft3/s) 0.02832 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

Leakance

foot per day per foot [(ft/d)/ft] 1.000 meter per day per meter

*Transmissivity

 foot squared per day (ft2/d) 0.0929 meter squared per day
VIII Contents

List of Symbols

Roman

A Area of sediments constituting the riverbed, [L2]

AR Artificial recharge from agricultural and golf-course irrigation and reclaimed water application, in inches

BD Boundary leakage or net outflow crossing study-area boundary, in inches

BF Total base flow, in inches

BFFAS Total base flow from the Floridan aquifer system, in inches

BFSAS Total base flow from the surficial aquifer system, in inches

Criv Conductance of sediments constituting the riverbed specified in the River package, [L2/T] 

De Extinction depth, [L]

El Lake evaporation, [L/T]

ET Evapotranspiration, [L/T]

ETa Evapotranspiration extracted from artificial recharge before the water percolates to the water table, [L/T]

ETn Natural evapotranspiration, which is evapotranspiration in the absence of artificial recharge, [L/T]

ETn,ex Excess natural evapotranspiration exceeding ETn,min, [L/T]

ETn,min Minimum natural evapotranspiration, [L/T]

h Simulated water-table altitude, [L]

Hriv Stream or lake water-level altitude specified in the River Package, [L]

Kh,sa Horizontal hydraulic conductivity of surficial aquifer system, [L/T]

Kh,lf Horizontal hydraulic conductivity of Lower Floridan aquifer, [L/T]

Kh,uf Multiplier for horizontal hydraulic conductivity of Upper Floridan aquifer, dimensionless

Kv,ic1 Vertical hydraulic conductivity of intermediate confining unit, less than or equal to 50 feet thick, [L/T]

Kv,ic2 Vertical hydraulic conductivity of intermediate confining unit, greater than 50 feet thick, [L/T]

Kv,ms Multiplier for vertical hydraulic conductivity of middle semiconfining unit and middle confining unit, 

Upper Floridan aquifer, [L/T] 

Kv,riv Vertical hydraulic conductivity of sediments constituting the riverbed, [L/T]

L Length unit

m Thickness of sediments constituting the riverbed, [L]

N Net recharge, [L/T]

Na Artificial net recharge, [L/T]

Ncbl Net recharge at closed-basin lakes, [L/T]

Nn Natural net recharge, [L/T]

Nn,max Maximum natural net recharge, [L/T]
Contents IX

Nn,min Minimum natural net recharge, [L/T]

no Number of water-level and flow observations

np Number of parameters estimated with inverse model

NSAS Net recharge to the surficial aquifer system, in inches

NUFA Net recharge to the Floridan aquifer system, in inches

Ocbl Overland runoff to a closed-basin lake, [L/T]

Os Overland runoff to a surface-water body other than a closed-basin lake, [L/T]

Osa Overland runoff to a surface-water body as a result of artificial recharge, [L/T]

Osn Natural overland runoff, which is overland runoff in the absence of artificial recharge, to a surface-water 

body other than a closed-basin lake, [L/T]

Osn,ex Excess natural overland runoff exceeding Osn,min, [L/T]

Osn,min Minimum natural overland runoff, [L/T]

P Precipitation, [L/T]

Qriv Volumetric flow rate simulated by the River Package, [L3/T]

Ra Artificial recharge, [L/T]

Rex,max Maximum rate of combined excess evapotranspiration and excess overland runoff, [L/T]

RF Rainfall, in inches

RO Overland runoff, in inches

SE Standard error of regression

SP Springflow, in inches

Sy Specific yield of the aquifer in which the water table is located or equal to 1 for a closed-basin lake, 

dimensionless

SSWR Sum of squared, weighted residuals, dimensionless

T Time unit

VANlf Vertical anisotropy of Lower Floridan aquifer, dimensionless

VANsa Vertical anisotropy of surficial aquifer system, dimensionless

VANuf Vertical anisotropy of Upper Floridan aquifer, dimensionless

W Pumpage from the Floridan aquifer system, in inches

wk Weight for the kth water level, [L-2], or flow, [(L3/T)-2]

yk kth observed water level, [L], or flow, [L3/T]

kth simulated water level, [L], or flow, [L3/T]

zbot Altitude of the bottom of sediments constituting the riverbed specified in the River Package, [L]

zls Mean land-surface altitude in model cell, [L]

yˆ

k
X Contents

Greek

∆H Change in water-table altitude or closed-basin lake level, [L/T]

∆ S Total net change in storage, in inches

∆ Sal Change in storage in the aquifer in which the water table is located and in closed-basin lakes, [L/T]

∆ Saq Change in storage in the aquifer in which the water table is located, [L/T]

∆ Scbl Change in closed-basin lake storage, [L/T]

∆ SFAS Net change in storage of the Floridan aquifer system, in inches

∆ SSAS Net change in storage of the surficial aquifer system, in inches
Abstract 1

Hydrogeology and Simulated Effects of Ground-Water 

Withdrawals from the Floridan Aquifer System in 

Lake County and in the Ocala National Forest and 

Vicinity, North-Central Florida

By Leel Knowles, Jr., Andrew M. O’Reilly, and James C. Adamski

ABSTRACT

The hydrogeology of Lake County and the 

Ocala National Forest in north-central Florida was 

evaluated (1995-2000), and a ground-water flow 

model was developed and calibrated to simulate 

the effects of both present day and future groundwater withdrawals in these areas and the surrounding vicinity. A predictive model simulation 

was performed to determine the effects of projected 2020 ground-water withdrawals on the 

water levels and flows in the surficial and Floridan aquifer systems.

The principal water-bearing units in Lake 

County and the Ocala National Forest are the surficial and Floridan aquifer systems. The two aquifer systems generally are separated by the 

intermediate confining unit, which contains beds 

of lower permeability sediments that confine the 

water in the Florida aquifer system. The Floridan 

aquifer system has two major water-bearing zones 

(the Upper Floridan aquifer and the Lower Floridan aquifer), which generally are separated by 

one or two less-permeable confining units.

The Floridan aquifer system is the major 

source of ground water in the study area. In 1998, 

ground-water withdrawals totaled about 

115 million gallons per day in Lake County and 

5.7 million gallons per day in the Ocala National 

Forest. Of the total ground water pumped in Lake 

County in 1998, nearly 50 percent was used for 

agricultural purposes, more than 40 percent for 

municipal, domestic, and recreation supplies, and 

less than 10 percent for commercial and industrial 

purposes.

Fluctuations of lake stages, surficial and 

Floridan aquifer system water levels, and Upper 

Floridan aquifer springflows in the study area are 

highly related to cycles and distribution of rainfall. Long-term hydrographs for 9 lakes, 8 surficial aquifer system and Upper Floridan aquifer 

wells, and 23 Upper Floridan aquifer springs 

show the most significant increases in water levels 

and springflows following consecutive years with 

above-average rainfall, and significant decreases 

following consecutive years with below-average 

rainfall. Long-term (1940-2000) hydrographs of 

lake and ground-water levels and springflow show 

a slight downward trend; however, after the early 

1960’s, this downward trend generally is more 

pronounced, which corresponds with accumulating rainfall deficits and increased development.

The U.S. Geological Survey three-dimensional ground-water flow model MODFLOW2000 was used to simulate ground-water flow in 

the surficial and Floridan aquifer systems in Lake 

County, the Ocala National Forest, and adjacent 

areas. A steady-state calibration to average 1998 

conditions was facilitated by using the inverse 

modeling capabilities of MODFLOW-2000. 

Values of hydrologic properties from the 
2 Hydrogeology and Simulated Effects of Ground-Water Withdrawals from the Floridan Aquifer System in Lake County and in the 

Ocala National Forest and Vicinity, North-Central Florida

calibrated model were in reasonably close agreement with independently estimated values and 

results from previous modeling studies. The 

calibrated model generally produced simulated 

water levels and flows in reasonably close agreement with measured values and was used to 

simulate the hydrologic effects of projected 2020 

conditions.

Ground-water withdrawals in the model 

area have been projected to increase from 

470 million gallons per day in 1998 to 704 million 

gallons per day in 2020. Significant drawdowns 

were simulated in Lake County from average 

1998 to projected 2020 conditions: the average 

and maximum drawdowns, respectively, were 

0.5 and 5.7 feet in the surficial aquifer system, 

1.1 and 7.6 feet in the Upper Floridan aquifer, and 

1.4 and 4.3 feet in the Lower Floridan aquifer. 

The largest drawdowns in Lake County were simulated in the southeastern corner of the County 

and in the vicinities of Clermont and Mount Dora. 

Closed-basin lakes and wetlands are more likely 

to be affected by future pumping in these large 

drawdown areas, as opposed to other areas of 

Lake County. However, within the Ocala National 

Forest, drawdowns were relatively small: the 

average and maximum drawdowns, respectively, 

were 0.1 and 1.0 feet in the surficial aquifer 

system, 0.2 and 0.8 feet in the Upper Floridan 

aquifer, and 0.3 and 0.8 feet in the Lower Floridan 

aquifer.

Projected 2020 withdrawals from the Floridan aquifer system caused decreases from average 

1998 conditions in the following simulated flows: 

combined rates of excess evapotranspiration and 

excess overland runoff (which represent evapotranspiration and overland runoff that occur in 

excess of their assumed minimum rates); groundwater discharge to streams, lakes, and wetlands; 

and springflow. The largest simulated flow 

decreases for first- or second-magnitude springs 

in Lake County were at Apopka (28 percent), 

Seminole (12 percent), and Bugg Springs (9 percent). The largest simulated flow decrease for 

first- or second-magnitude springs in the Ocala 

National Forest was at Juniper Springs (4 percent).

Particle-tracking analyses were used to 

delineate areas that contribute recharge to selected 

springs. Based on average 1998 conditions, the 

contributing area for Apopka Spring covers 

approximately 30 square miles and has an average 

contributing recharge flux of 15 inches per year, 

and the contributing area for Alexander Springs 

covers approximately 76 square miles and has an 

average contributing recharge flux of 18 inches 

per year. The contributing area for Alexander 

Springs changed little as a result of projected 

2020 conditions because relatively little pumping 

exists in the vicinity of the spring’s contributing 

area. However, the size of the contributing area 

for Apopka Spring decreased to 26 square miles 

and the average contributing recharge flux 

decreased to 13 inches per year as a result of projected 2020 conditions.

INTRODUCTION

The Floridan aquifer system (FAS), particularly the Upper Floridan aquifer (UFA), is the primary source of ground water in peninsular Florida. 

The FAS is being increasingly stressed as a result of 

increased pumping and below-average rainfall. 

Future water-use projections indicate as much as a 

100 percent increase in the demand for ground water 

across central Florida from 1995 to 2020. Much of 

this increased demand for ground water results from 

the urban expansion of the Orlando and south-central Marion County areas of central Florida.

The study (model) area is about 4,800 square 

miles (mi2) and includes Lake County, the Ocala 

National Forest (Ocala NF), and the surrounding 

adjacent counties in north-central Florida (fig. 1). 

Lake County (1,150 mi2) and the Ocala NF 

(690 mi2) are characterized by diverse landforms 

including numerous lakes, streams, wetlands, and 

springs. Urban development in Lake County is 

increasing rapidly, whereas little development has 

occurred or is expected to occur within the Ocala 

NF. Lake County contains numerous communities 

and small towns and is located 25 miles (mi) or more 

west and northwest of Orlando, which accounts for 

some of the population increase in recent years. The 

Ocala NF, although relatively undeveloped, contains 

several small communities and private lands (about 

75 mi2) including Astor, Salt Springs, and Lynne.
Introduction 3

Juniper

Alexander Creek

Black

Water Creek

River

Ocklawaha

St. Johns

S

i

t. Johns R ver

Pa al t al kaha River

R iver

Wek vi a River

Creek

Withlacoochee River

Kissimmee River

Headwaters of

Lake

Griffin

Lake Ocklawaha

Orange

Lake

Lake

Weir

Lake

Bryant

Lake

Eaton

Lake

Jumper

Mill Dam

Lake

Halfmoon

Lake

Lake

Dora

Lake Minneola

Lake

Lake

Louisa

Lake

Harris

Lake

Eustis

Lake

Norris

Lake

Dorr

Lake

George

Lake

Kerr

Lake

Yale

Lake Minnehaha Johns

Lake

Lake

Apopka

Lake

Jessup

Lake

Monroe

Butler

Lake

Panasoffkee

28°20′

40′

29°00′

20′

29°40′

82°10′ 50′ 30′ 81°10′

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

Universal Transverse Mercator projection, zone 17

Rock Ridge

Dade City

Eva

Mascotte

Groveland

Clermont

Astor

Astor Park

Orange

Springs

Orlando

Apopka

Wildwood

Leesburg

Sanford

Mt. Dora

Umatilla

Altoona

Ocala

Eustis

Lady Lake

Tavares

Lynne

Paisley

CLAY

COUNTY

PASCO

COUNTY

ALACHUA

COUNTY

MARION

COUNTY

ST. JOHNS

COUNTY

FLAGLER

COUNTY

PUTNAM COUNTY

VOLUSIA

COUNTY

LAKE COUNTY

SEMINOLE

COUNTY

ORANGE COUNTY

OSCEOLA

POLK COUNTY

COUNTY

HERNANDO

COUNTY

CITRUS

COUNTY

SUMTER

COUNTY

G

G

GG

GG

G GGGG

G

G G

GG GG

G G

G

G

G

G

G

GG G

G G

G

GG

GG G

GG

G

GG

G

GGG

G

GG

G

GG

G

GGG

G

G

GG G

G

G

Silver Glen

Springs

Alexander

Springs

Silver

Springs

Salt

Springs

Blue

Springs

OCALA

NATIONAL

FOREST

“OVERLAP”

0 5 10 KILOMETERS

0 5 10 MILES

STUDY AREA

(MODEL)

BOUNDARY

STUDY AREA

EXPLANATION

OCALA NATIONAL FOREST

PROCLAMATION BOUNDARY

LAKE COUNTY G SPRING

WATER MANAGEMENT

DISTRICT BOUNDARY

SFWMD

SWFWMD

SWFWMD

SFWMD

301

301

17 92

27

441

441

441

441

301

17

92

27

301

315

326

314

75

4

4

4

75

40

40

42

44

50

50

33

33

48

48

46

19

19

19

436

415

44

Figure 1. Location of study (model) area.
4 Hydrogeology and Simulated Effects of Ground-Water Withdrawals from the Floridan Aquifer System in Lake County and in the 

Ocala National Forest and Vicinity, North-Central Florida

These communities generally lie along the Ocala NF 

outer perimeter, hereafter called the Ocala NF Proclamation “Boundary.” The area of the Ocala NF that 

encompasses the northeastern part of Lake County is 

hereafter called the “overlap.” Also, urban expansion 

in the Ocala area and southwestern Marion County has 

extended toward the Ocala NF and currently has 

reached areas near the western and southwestern edges 

of the Ocklawaha River-Ocala NF boundary (fig. 1).

Background

Since 1960, the rates of ground-water pumping 

for municipal (public supply), domestic, and agricultural use have increased steadily in the central Florida 

peninsula. Demands for public supply are increasing 

mainly in response to continuing growth in population. 

Population in central Florida (which includes Lake, 

Marion, Volusia, Orange, Seminole, Oceola, and 

Brevard Counties) has increased by about 260 percent 

from about 683,270 in 1960 (University of Florida, 

1976) to 2,469,200 in 1995 (University of Florida, 

1996). Population for the same area is expected to 

increase by at least 57 percent from 1998 to 3,884,700 

by 2020 (Smith and Nogle, 1999). Population in Lake 

and Marion Counties increased nearly 300 percent, 

from 109,000 in 1960 to more than 401,500 in 1995. 

An additional 100 percent increase in population is 

expected in Lake and Marion Counties from 1995 to 

2020. Meanwhile, water levels of lakes, streams, the 

surficial aquifer system (SAS), and the FAS in the central Florida peninsula generally have decreased. 

Although much of the decline in water levels can be 

attributed to persistent below-average rainfall, 

increased pumping probably also is a factor in the general decline of water levels throughout the central 

Florida area. Annual ground-water withdrawals from 

the FAS in Lake and Marion Counties increased from 

about 61 million gallons per day (Mgal/d) in 1965 to 

about 166 Mgal/d in 1998 (Richard Marella, U.S. Geological Survey, written commun., 1999). Pumping 

rates for agricultural uses declined slightly during the 

1980’s, mainly as a result of reductions in irrigated 

citrus acreage and reduced citrus processing following 

several tree-killing freezes. During the 1990’s, water 

use generally shifted away from citrus irrigation to 

public supply, recreation, turf irrigation, and domestic 

use. Ground water withdrawn from the UFA in Lake 

and Marion Counties is expected to increase by more 

than 50 percent from 166 Mgal/d in 1998 to more than 

250 Mgal/d in 2020 (Brian McGurk, St. Johns River 

Water Management District; and Lou Motz, University 

of Florida, written commun., 1999).

Prior to the 1990’s, ground-water pumping generally occurred from low-capacity wells widely distributed through Lake and Marion Counties; however, 

as public-supply and domestic use continues to 

increase, the tendency could be to concentrate more 

pumping into smaller areas. Consequently, effects 

from pumping will be more pronounced and localized, 

particularly in central parts of Lake and Marion Counties. In contrast, pumping in the Ocala NF has been 

and is expected to remain relatively small and stable. 

However, increased ground-water pumping from adjacent areas in Lake and Marion Counties could affect 

ground-water flow conditions within the Ocala NF in 

the future. The need for better conservation and management of ground-water resources in Lake County is 

increasing because of the rapid population growth in 

the county. An understanding of the water resources of 

the area, including the responses of lake stages, aquifer 

water levels, and springflows to ground-water pumping in Lake County and in the Ocala NF vicinity is 

needed so that water managers can develop strategies 

to meet the growing water-supply demands.

To address the water-resource needs, the Lake 

County Water Authority (LCWA), St. Johns River 

Water Management District (SJRWMD), Southwest 

Florida Water Management District (SWFWMD), and 

the U.S. Department of Agriculture, Forest Service 

(USFS), are participating in cooperative programs 

with the U.S. Geological Survey (USGS) to collect 

hydrologic data. Included in these programs are surface-water data-collection networks with the LCWA, 

SJRWMD, and USFS, as well as semiannual measurements of ground-water levels in the SAS and FAS with 

the SJRWMD and SWFWMD. In 1995, the USGS, 

with the aforementioned cooperators, began a 5-year 

study (1995-2000) to evaluate the ground-water 

resources of Lake County and the Ocala NF and to 

determine the potential effects of increased groundwater withdrawals on those resources. Most data for 

this study were collected from September 1997 

through December 1998.

Purpose and Scope

This report presents a description of the hydrogeology of Lake County and the Ocala NF and quantifies the effects of future ground-water withdrawals 
Description of Study Area 5

from the Floridan aquifer system. Ground-water level, 

surface-water stage and discharge, springflow, and 

water-use data are presented. Well log interpretations 

were used to determine the lithology and better understand the hydrogeology. Water-quality data collected 

during the study are presented by Adamski and 

Knowles (2001). A numerical model of the groundwater flow system was constructed and used to evaluate the effects of projected increases in ground-water 

withdrawals on water levels in the surficial and Floridan aquifer systems and on springflow from the Upper 

Floridan aquifer. Although the primary focus of this 

study was in Lake County and the Ocala NF, groundwater withdrawals in adjacent central Florida counties 

necessitated expansion of the model area to include a 

more regional system (fig. 1).

Previous Studies

Hydrologic data were collected systematically 

from 1966-70 for a summary report of water resources 

for east-central Florida, including Lake County, by 

Lichtler (1972). Knochenmus and Hughes (1976) provided a detailed description of the surface- and 

ground-water flow system in Lake County and presented the results of a 3-year investigation to assess the 

availability and quality of surface- and ground-water 

resources, and the effects of anthropogenic stresses on 

the hydrologic system in Lake County. Faulkner 

(1973) provided a detailed report of the stratigraphy, 

structural geology, and ground-water flow system of 

the proposed Cross-Florida Barge Canal right-of-way, 

with emphasis in the Ocala area. Tibbals (1975) conducted aquifer tests on the upper 100 feet (ft) of the 

FAS in the barge canal right-of-way to provide information about the potential exchange of water between 

the proposed canal and the aquifer. A quantitative 

appraisal of east-central Florida’s water resources, 

inclusive of all but extreme western Lake County, also 

was done by Tibbals (1990) as part of the Regional 

Aquifer Systems Analysis (RASA). Lithologic and 

borehole data for the Green Swamp area of south Lake 

County are provided by Grubb and others (1978). 

Adamski and Knowles (2001) presented the results of 

a water-quality study for 217 water samples taken 

from the SAS and UFA in Lake County and the Ocala 

NF. O’Reilly and others (2002) described the hydrogeologic and water-quality characteristics of the 

Lower Floridan aquifer, in east-central Florida, including parts of Lake County. Hydrogeology and results of 

simulating the ground-water flow system surrounding 

and including parts of the study area are presented by 

Motz (1995), Murray and Halford (1996), O’Reilly 

(1998), Spechler and Halford (2001), Sepúlveda 

(2002), and McGurk and Presley (in press).

Acknowledgments

The authors wish to thank the U.S. Department 

of Agriculture, Forest Service, for access and assistance in the Ocala NF and the many residents and 

public officials who contributed information useful to 

the investigation; granted permission to drill new 

wells; and permitted access to wells, lakes, and springs 

to collect water-level and flow measurements and 

water samples. The authors also are grateful to 

Mr. Joe M. Branham for supplying flow measurements 

and hydrogeologic information for Bugg Spring; 

Mr. Joseph A. Bishop, Lake Forestry Station, Florida 

Department of Agriculture, for access and gage 

installation in the Seminole State Forest; and 

Mr. Walter D. Wood of Lake County Water Resources 

Management for well access and for providing historic 

hydrologic data from wells located on county lands.

DESCRIPTION OF STUDY AREA

The study area, which includes Lake County 

and the Ocala NF, is located 25 mi or more west and 

northwest of Orlando in north-central Florida (fig. 1). 

Lake County is predominantly rural with a few, mostly 

widely scattered, but steadily growing towns such as 

Leesburg, Tavares, Eustis, Clermont, and Lady Lake. 

While the Ocala NF remains mostly undeveloped, 

urban expansion in Marion County is approaching the 

southwestern and western edges of the Forest. The 

study area was defined by delineating estimated 

ground-water flow divides of the UFA around Lake 

County and the Ocala NF where possible (fig. 2). 

Specified-head boundaries were used to define the 

ground-water model study area in the southeastern 

section across northern Osceola and western Orange 

Counties, and in the southwestern section across western Sumter and southwestern Marion Counties. The 

model boundary is described in detail later in this 

report.
6 Hydrogeology and Simulated Effects of Ground-Water Withdrawals from the Floridan Aquifer System in Lake County and in the 

Ocala National Forest and Vicinity, North-Central Florida

Juniper

Alexander Creek

Black

Water Creek

River

Ocklawaha

St. Johns

S

i

t. Johns R ver

Pa al t al kaha River

R iver

Wek vi a River

Creek

Withlacoochee River

Lake

Griffin

Lake Ocklawaha

Orange

Lake

Lake

Weir

Lake

Bryant

Lake

Eaton

Lake

Jumper

Mill Dam

Lake

Halfmoon

Lake

Lake

Dora

Lake Minneola

Lake

Lake

Louisa

Lake

Harris

Lake

Eustis

Lake

Norris

Lake

Dorr

Lake

George

Lake

Kerr

Lake

Yale

Lake Minnehaha Johns

Lake

Lake

Apopka

Lake

Jessup

Lake

Monroe

Butler

Lake

Panasoffkee

40′

29°00′

20′

82°10′ 50′ 30′ 81°10′

29°40′

28°20′

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

Universal Transverse Mercator projection, zone 17

CLAY

COUNTY

PASCO

COUNTY

ALACHUA

COUNTY

MARION

COUNTY

ST. JOHNS

COUNTY

FLAGLER COUNTY

PUTNAM COUNTY

VOLUSIA

COUNTY

LAKE COUNTY

SEMINOLE

COUNTY

ORANGE COUNTY

OSCEOLA

COUNTY

POLK

COUNTY

HERNANDO

COUNTY

CITRUS

COUNTY

SUMTER

COUNTY

0 5 10 KILOMETERS

0 5 10 MILES

30

85

85

80

75

70

50

6560 55

40 45

3530

25

5

30

20

15

10

15

15

10

5

10

50

45

10

15

60

55

80

75

70

65

20

25

30

55

75

65

95

125

40

50

60

80

90

70

80

80

90

100

105

35

45

15

10

35

85

10

35

25

65

30

25

20

15

25

20

25 30

35

40

35

110

120

65

60

50 55

70

75

80

90

95

100

110 105

15

15

40

115

20

15

20

50

25

85

NO-FLOW

MODEL

BOUNDARY

G

G

GG

GG

G GGGG

G

G G

GG GG

G G

G

G

G

G

G

GG G

G G

G

GG

GG G

GG

G

GG

G

GGG

G

GG

G

GG

G

GGG

G

G

GG G

G

G

OCALA

NATIONAL

FOREST

EXPLANATION

50 POTENTIOMETRIC CONTOUR -- Shows altitude at which water

level would have stood in tightly cased wells. Hachures indicate

depressions. Contour intervals 5 and 10 feet. Datum is sea level

OCALA NATIONAL FOREST

PROCLAMATION BOUNDARY

LAKE COUNTY

G SPRING

UPPER FLORIDAN AQUIFER GROUND-WATER DIVIDE

SPECIFIED

HEAD

BOUNDARY

SPECIFIED

HEAD

BOUNDARY

Figure 2. Model boundary delineated using the May 1998 potentiometric surface of the Upper 

Floridan aquifer (modified from Adamski, 1998).
Description of Study Area 7

Physiography

Lake County and the Ocala NF are in the 

Central Highlands topographic division described by 

Cooke (1945). Within this division, Puri and Vernon 

(1964) described 25 landforms, 8 of which occur in 

Lake County and 4 in the Ocala NF (fig. 3). In general, 

the landforms are of three basic types: ridges, valleys, 

and uplands. All of these landforms show strong lineation parallel with the present-day Atlantic Coast, 

which implies a coastal origin. Land-surface elevations in the study area range from near sea level to 

more than 300 ft above sea level.

Ridges characterize land areas with the highest 

altitudes. Two of these ridge systems—the Mount 

Dora Ridge and Lake Wales Ridge—extend into and 

across Lake County and the Ocala NF. These ridges 

are a series of gently rolling hills that trend northwestsoutheast and include up to 200 ft or more of surficial 

sands. The Lake Wales Ridge is, in general, the higher 

of the two, with hilltop elevations ranging from 200 to 

more than 300 ft above sea level. The highest point in 

peninsular Florida is Sugarloaf Mountain (312 ft) 

located at the northern end of the Lake Wales Ridge 

west of Lake Apopka in Lake County (fig. 3). Ridges 

also are characterized by closed-basin lakes, watertable depths greater than 100 ft, and subsurface drainage, but surface-drainage features generally are absent. 

The Mount Dora Ridge contains some of the highestaltitude lakes in peninsular Florida.

The Central Valley, a second common landform, 

terminates the northern end of the Lake Wales Ridge 

and offsets the ridge from the Mount Dora Ridge to the 

east. The Central Valley is characterized by large lakes 

(the Ocklawaha chain), flat terrain, and relatively high 

surface runoff. In the Central Valley, water levels in 

SAS wells are at or only a few feet below land surface, 

and water flows above land surface from many UFA 

artesian wells.

The third common landform is the upland, of 

which there are three in the study area: Lake Upland, 

Marion Upland, and Sumter Upland. Uplands are 

characterized by moderate land-surface elevations and 

relief, numerous closed-basin lakes that become connected during high water, shallow lakes, and moderate 

water-table depths. Drainage from the uplands generally is poor, so that surface water can take as long as 

many weeks to drain following extended periods of 

heavy rainfall.

Drainage

Lake County and the Ocala NF are drained by 

both surface and subsurface drainage systems. Surface 

drainage mainly is by the Palatlakaha River, Ocklawaha River, and other tributaries to and including the 

St. Johns River (fig. 1). Small areas in the southwestern and southeastern parts of Lake County are drained 

by the Withlacoochee River and the headwaters of the 

Kissimmee River, respectively. Annual surface runoff 

for the Ocala NF likely is much smaller in comparison 

to Lake County because drainage generally is poor, 

and surface runoff to the St. Johns and Ocklawaha Rivers 

primarily is received by tributaries draining lands surrounding the Ocala NF.

Prior to about 1960, the natural surface-drainage 

system generally was not well developed and stream 

channels existed only as high-water connectors 

between lakes or wetlands. During the 1960’s, many 

channels were deepened and improved to facilitate 

drainage as the area developed. Flow through the 

Ocklawaha chain-of-lakes is regulated by control 

structures.

The St. Johns River, Wekiva River, and Black 

Water Creek drain the northeastern part of Lake 

County (fig. 1). Black Water Creek flows southeast to 

its confluence with the Wekiva River, which then 

flows northeast to the St. Johns River. Black Water 

Creek and the St. Johns River have nearly parallel 

alignments, but flow in opposite directions. Very little, 

if any, surface drainage occurs in the high ridge areas 

as evidenced by the absence of surface-water features. 

Rather, nearly all of the precipitation infiltrates to the 

SAS or directly to the UFA, or is lost by evapotranspiration (ET).

The Ocklawaha River, Alexander Creek, and 

Juniper Creek drain the western and eastern parts of 

the Ocala NF. Most of the flow in these streams is supplied by springs and diffuse ground-water discharge 

from the UFA, which is controlled by stream stage. 

High stream stages whether caused by rainfall, tidal 

influences, or control structures suppress the groundwater contribution to streams. This condition can be 

observed along Lake Ocklawaha, which is regulated to 

maintain a water level approximately 15-20 ft above 

the natural stream stage. Discharge from submerged 

springs in Lake Ocklawaha increased by a magnitude 

of 10 or more when the regulated stage was lowered 

by approximately 9 ft during the winter of 1998-99, 

based on measurements made by the USGS.
8 Hydrogeology and Simulated Effects of Ground-Water Withdrawals from the Floridan Aquifer System in Lake County and in the 

Ocala National Forest and Vicinity, North-Central Florida

EXPLANATION

Less than 25

25 to 49

50 to 74

75 to 99

100 to 150

Greater than 150

LAKES AND STREAMS

LAKE COUNTY

ALTITUDE OF LAND SURFACE, IN FEET ABOVE SEA LEVEL

PHYSIOGRAPHIC REGION

BOUNDARY

LINE OF HYDROGEOLOGIC

SECTION (see figure 9)

OCALA NATIONAL FOREST

A A

28°20′

40′

29°00′

20′

29°40′

82°10′ 50′ 30′ 81°10′

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

Universal Transverse Mercator projection, zone 17

C

B

B

A

A

C

R

ATLANTIC BEACH RIDGE

ATLANTIC BARRIER CHAIN

ATLANTIC COAST

ESPANOLA

AL RIDGE

HILL

EASTERN VALLEY

CRESCENT CITY RIDGE

ST. JOHNS RIVER OFFSET

MOUNT

DORA

RIDGE

MOUNT DORA RIDGE

MARION UPLAND

DELAND RIDGE

WEKIVA PLAIN

GENEVA HILL

ORLANDO RIDGE

LAKE UPLAND

LAKE WALES RIDGE

OSCEOLA PLAIN

WESTERN VALLEY

POLK UPLAND

BROOKSVILLE RIDGE

TSALA APOPKA PLAIN

SUMTER UPLAND

CENTRAL VALLEY

COTTON

PLANT

RIDGE

OCALA

HILL

FAIRFIELD HILLS

MARTEL HILL

TEASDALE HILL

SAN MATEO PALATKA HILL HILL

WELAKA HILL

DUVAL FLORAHOME UPLAND

NORTHERN HIGHLANDS

VALLEY

Sugarloaf

Mountain Lake

Apopka

MODEL

BOUNDARY

0 5 10 KILOMETERS

0 5 10 MILES

Figure 3. Topography, physiographic regions (modified from White, 1970), and locations of 

hydrogeologic sections.
Description of Study Area 9

 A substantial amount of ground water discharges from the UFA as springs and seeps (65 of 

which are identified in this report). Most springs are 

located along or near the Ocklawaha, St. Johns, and 

Wekiva Rivers, but a few small springs are located in 

central Lake County on the south shore of Lake Harris 

and extending to the west shore of Lake Apopka. 

Silver Springs, the largest spring in the study area and 

located just west of the Ocala NF, near the city of 

Ocala, has an annual mean flow of 788 cubic feet per 

second (ft3

/s) for 1932-2000. Other springs, with average discharges greater than 100 ft3

/s, include Blue, 

Silver Glen, and Alexander Springs (fig.1).

From the early 1900’s to the 1960’s, drainage 

wells were drilled into the UFA, particularly in the city 

of Ocala and in western Orange County. Some drainage wells were drilled in partially plugged sinkholes or 

excavated retention ponds to augment the natural internal drainage as storm runoff increased with urban 

expansion. Others were drilled near lakes to control 

lake stages and prevent flooding of roads or residential 

areas. Many drainage wells have been plugged and are 

no longer in use. State regulations now prohibit the 

drilling of any additional drainage wells.

Climate

Climate in the study area is subtropical and typically characterized by warm and humid, rainy summers and temperate, dry winters. Summer daily 

maximum air temperatures typically exceed 90 degrees 

Fahrenheit (o

F), and occasionally exceed 100 oF. Winter daily minimum air temperatures generally are mild 

with occasional freezes, mainly from December to 

March each year. The mean annual air temperatures at 

Ocala and Clermont are 70.8 o

F and 72.0 oF, respectively, for 1971-2000 (National Oceanic and Atmospheric Administration, 1935-99 and 2000). Mean 

monthly air temperatures range from 58.1 o

F and 

59.8 oF at Ocala and Clermont, respectively, in January 

to 81.7 oF at Ocala in July and 81.7 oF at Clermont in 

July and August. Mean annual precipitation at Ocala 

and Clermont is about 50 inches (in.) for 1971-00 with 

50-55 percent typically falling during the summer.

Substantial rainfall events generally are associated with diurnal summer thunderstorms, summer or 

autumn tropical systems, and wet autumn, winter, or 

spring frontal systems (Winsberg, 1990). Typically, the 

frequency of high-intensity thunderstorms increases 

each year during late May and early June, peaking in 

July and August before becoming less frequent after 

early October. April and November typically are the 

driest months. Patterns of temperature and rainfall, 

however, vary within the study area and are influenced 

by large lakes that are slower in response to change in 

temperature than the surrounding land areas. Tropical 

systems can affect the study area during the summer 

and generate copious amounts of rainfall, which provides significant ground-water recharge. Occasionally, 

tropical systems affecting the area extend well into 

autumn. Rains associated with tropical and wet frontal 

systems generally are more widespread than those 

associated with summer thunderstorms. Recharge to 

the ground-water system can be substantial during wet 

winter months especially because evapotranspiration 

(ET) and pumping rates are at their annual minimum 

(Knowles, 1996, and R. Marella, U.S. Geological Survey, 1999, oral commun.).

Land and Water Use

Land use in Lake County in 1998 consisted of 

agriculture (30 percent), predominantly citrus and 

plant nurseries; National/State Forests and preserves 

(18 percent); rangeland (7 percent); urban (7 percent); 

open water and wetlands (36 percent); and some localized sand, rock, and peat mining (2 percent). From 

1977 to 1994, agricultural land use declined from 

about 43 to 30 percent with freeze-killed citrus groves 

being replaced mostly by urban development.

USFS land covers about 89 percent of the area 

within the Proclamation Boundary of the Ocala NF 

and includes several recreation sites, transportation 

corridors, and lands designated for other special uses 

(Richard B. Shellfer, U.S. Department of Agriculture, 

Forest Service, 1999, oral commun.). The communities of Astor, Paisley, Lynne, and Salt Springs are 

located along or within the Boundary. Of the Ocala 

NF, 63 percent is designated as land suitable for timber 

management or harvest, 11 percent is private residential, 6.5 percent is wilderness, about 1 percent is for 

U.S. Navy bombing practice, and less than 1 percent is 

recreational sites. The remaining 18 percent is used as 

transportation corridors and access roads (2,300 mi). 

Although mining currently is not practiced in the 

Ocala NF, a few small claypits are used for road maintenance within the forest area.

Ground water withdrawn from the FAS in Lake 

County is used for numerous purposes (fig. 4). In 

1998, Lake County pumped about 115 Mgal/d 

(110 Mgal/d from the UFA and 5 Mgal/d from the 

LFA)—less than 48 percent for agricultural use; about
10 Hydrogeology and Simulated Effects of Ground-Water Withdrawals from the Floridan Aquifer System in Lake County and in 

the Ocala National Forest and Vicinity, North-Central Florida

1998 TOTAL WATER

WITHDRAWN

IN LAKE COUNTY (115) 1998 TOTAL WATER WITHDRAWN

IN STUDY AREA (470)

COMMERCIAL AND INDUSTRIAL

DOMESTIC (SELF-SUPPLIED)

AGRICULTURE

RECREATION

DOMESTIC (PERMITTED)

PUBLIC SUPPLY

2020 PROJECTED TOTAL WATER WITHDRAWN

IN STUDY AREA (704)

2020 PROJECTED TOTAL

WATER WITHDRAWN

IN LAKE COUNTY (173)

EXPLANATION

61.1 NUMBER IS PERCENT OF TOTAL WATER WITHDRAWN

(430) NUMBER IN PARENTHESES IS THE TOTAL WATER

WITHDRAWN IN MILLION GALLONS PER DAY

1.2 (5)

4.7 (22)

8.3 (39)

11.7 (55)

46.3 (218)

27.8 (131)

9.8 (11)

7.5 (9)

2.4 (3)

1.7 (2)

31.0

(35)

47.6 (55)

61.1 (430)

19.1 (134) 7.9 (56)

6.5 (46)

4.4 (31)

1.0 (7)

5.7 (10)

2.0 (3)

1.6 (3)

1.2 (2)

31.7 (55)

57.8 (100)

Figure 4. Distribution of water withdrawals from the Floridan aquifer system.
Hydrogeology 11

45 percent for municipal, domestic, and recreation 

use; and more than 7 percent for commercial or industrial use. Domestic water use is subdivided into subcategories: self-suppled and permitted. Self-supplied 

water use generally consists of water withdrawn from 

private (homeowner) wells with casings 4 inches or 

less in diameter. Permitted water use consists of larger 

private and community wells with casings 6 inches or 

larger in diameter. Total water pumped in Lake County 

is projected to be 173 Mgal/d (162 Mgal/d from the 

UFA and 11 Mgal/d from the LFA) by 2020—less than 

63 percent municipal, domestic, and recreation; about 

32 percent agriculture; and less than 6 percent commercial or industrial (Brian McGurk, St. Johns River 

Water Management District, 1999, written commun.). 

Currently, about 15 percent of all water pumped from 

the UFA in Lake County is returned to the groundwater system as artificial recharge by land application, 

mostly in the southeastern and central areas of the 

County; and by septic systems distributed throughout 

the county (Richard Marella, U.S. Geological Survey, 

1999, oral commun.).

Nearly all water withdrawn in the Ocala NF is 

from the UFA and is used for domestic and recreation 

purposes at scattered communities, recreation sites, 

and private residences. In 1998, water use totaled 

5.7 Mgal/d, with about 60 percent being returned as 

artificial recharge to the ground-water system by septic 

systems (Richard Marella, U.S. Geological Survey, 

1999, oral commun.), mainly in communities and at 

recreation sites in the Ocala NF. Additional analysis of 

1998 water-use data since the publication of Adamski 

and Knowles (2001) resulted in an updated value 

(from about 2 Mgal/d to about 5.7 Mgal/d) for water 

use in the Ocala NF. The magnitude and type of water 

use in the Ocala NF is not expected to change significantly by 2020.

Data-Collection Network

Data-collection sites were inventoried based on 

a review of existing data in the study area. Additional 

sites were added to define better the characteristics of 

the hydrologic system and the relation between water 

levels in the SAS and the potentiometric surface of the 

UFA. Data include rainfall; stream stage and discharge; lake stage; and ground-water levels (figs. 5, 6, 

and 7; app. A, B, and C). Additional hydrologic, lithologic, and water-use data were acquired from the 

USGS, SJRWMD, LCWA, USFS, SWFWMD, 

SFWMD, Lake County Water Resources Management, Florida Geological Survey, and National 

Oceanic and Atmospheric Administration (NOAA) 

computer and paper files.

The lake and ground-water monitoring network 

(figs. 6 and 7; apps. B and C) includes existing sites 

measured by the USGS, SJRWMD, or SWFWMD; 

10 new lake staff gages (6 in the Ocala NF and 4 in 

Lake County); 27 new wells tapping the SAS (12 each 

in Lake County and the Ocala NF, and 3 in the overlap 

area); and 1 new well tapping the UFA in the Ocala 

NF. Several of the new SAS wells were constructed 

adjacent to UFA wells to determine the local head difference between the aquifers. Synoptic water-level 

data were collected bi-monthly during the 16-month 

period, September 1997 to December 1998, to provide 

“instantaneous” measurements. Continuous waterlevel recorders were installed at two of the paired-well 

sites.

More than 1,300 lithologic, geophysical, and 

geological logs were analyzed to determine the stratigraphy and lithology of the SAS, the intermediate 

confining unit (ICU), and the FAS in the study area. 

Four additional UFA wells were logged and core 

samples were collected from 35 SAS wells and 

boreholes drilled during the study. Altitudes and thicknesses of lithologic units and control points interpreted 

from these logs were interpolated using a minimum 

curvature method to construct generalized lithologic 

maps of the study area.

HYDROGEOLOGY

The principal water-bearing units in the study 

area, the SAS and FAS (fig. 8), are separated by the 

ICU, which contains beds of lower permeability that 

confine the FAS. The FAS has two major water-bearing zones, the UFA and the Lower Floridan aquifer 

(LFA), which generally are separated by the less permeable middle semiconfining unit (MSCU) or the 

middle confining unit (MCU). Underlying the FAS is 

the sub-Floridan confining unit, which contains lowpermeability limestone, dolomite, and anhydrite. The 

bottom of the freshwater flow system is defined either 

by the sub-Floridan confining unit or the depth at 

which the chloride concentration of the water is 

greater than 5,000 milligrams per liter (mg/L) in the 

FAS, whichever is shallower. Generalized hydrogeologic sections based on well logs are shown in figure 9.
12 Hydrogeology and Simulated Effects of Ground-Water Withdrawals from the Floridan Aquifer System in Lake County and in 

the Ocala National Forest and Vicinity, North-Central Florida

River

Ocklawaha

St. Johns

S

i

t. Johns R ver

Pa al t al kaha River

R iver

Wek vi a River

Withlacoochee River

LAKE COUNTY

Lake

Griffin

Lake Ocklawaha

Orange

Lake

Lake

Weir

Lake

Bryant

Lake

Eaton

Mill Dam

Lake

Halfmoon

Lake

Lake

Dora

Lake Minneola

Lake

Lake

Louisa

Lake

Harris

Lake

Eustis

Lake

Norris

Lake

Dorr

Lake

George

Lake

Kerr

Lake

Yale

Lake Minnehaha Johns

Lake

Lake

Apopka

Lake

Jessup

Lake

Monroe

Butler

Lake

Panasoffkee

Lake

Jumper

MODEL

BOUNDARY

28°20′

40′

29°00′

20′

29°40′

82°10′ 50′ 30′ 81°10′

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

Universal Transverse Mercator projection, zone 17

CLAY

COUNTY

PASCO

COUNTY

ALACHUA

COUNTY

MARION

COUNTY

ST. JOHNS COUNTY

FLAGLER

COUNTY

PUTNAM COUNTY

VOLUSIA

COUNTY

SEMINOLE

COUNTY

ORANGE COUNTY

OSCEOLA COUNTY

POLK

COUNTY

HERNANDO

COUNTY

CITRUS

COUNTY

SUMTER

COUNTY

0 5 10 KILOMETERS

0 5 10 MILES

OCALA

NATIONAL

FOREST

p

p

p p

p

p p

p

p

p

pp p

p p p p p p p

p p p p

p p

p

p

p pp p

p

p

p pp p

p p

p pp pp

pp p p

p pp p p p

p p p

p

16 15

11 10

13

19

23 22

26

27

36

35

44

53

57

60

61

74

81

90

85

64

21

28

25

33

40

39

32

37

42

41

46

47

49 50

54

55

87

91 92

68

70

71

75

79

83

88

69

82

84

77 76 73

72

63 67

93

3

5

8

4

43

¤

¤

¤

¤

¤

¤ ¤ ¤

¤

¤

¤

¤

¤ ¤ ¤

¤

¤ ¤

¤

¤

7

9

14

18

20

34 31

38

29

58 56 59

78

86 89

62

80

52

45

51

30

£

24

£

2

£

1

£

6

12£

£17

£

48

66 £ 65

£

£

G

GG

GG

G GGGG

G

G G

GG GG

G G

G

G

G

G

G

GG G

G G

G

GG

GG G

GG

G

GG

G

GGG

G

GG

G

GG

G

GGG

G

G

GG G

G

G

G

EXPLANATION

OCALA NATIONAL

FOREST PROCLAMATION

BOUNDARY

STREAM-GAGING

SITE AND NUMBER p

COMPLETE

CLIMATOLOGICAL

SITE AND NUMBER

¤

3

7

G SPRING

£1

RAINFALL

CLIMATOLOGICAL

SITE AND NUMBER

LAKE COUNTY

Figure 5. Locations of stream-gaging and climatological data-collection sites (numbers refer 

to appendix A).
Hydrogeology 13

Lake

Griffin

Lake Ocklawaha

Orange

Lake

Lake

Weir

Lake

Bryant

Lake

Eaton

Lake

Jumper

Mill Dam

Lake

Halfmoon

Lake

Lake

Dora

Lake Minneola

Lake

Lake

Louisa

Lake

Harris

Lake

Eustis

Lake

Norris

Lake

Dorr

Lake

George

Lake

Kerr

Lake

Yale

Lake Minnehaha Johns

Lake

Lake

Apopka

Lake

Jessup

Lake

Monroe

Butler

Lake

Panasoffkee

River

Ocklawaha

St. Johns

S

i

t. Johns R ver

Pa al t al kaha River

R iver

Wek vi a River

Withlacoochee River

MODEL

BOUNDARY

0 5 10 KILOMETERS

0 5 10 MILES

OCALA

NATIONAL

FOREST

p

p pp

pp p

p p p p

p p

p p p p p pp p

p

p p

p

p

p p p p p p p p p pp p p p p

p p p p p

p p p p p p

p p pp p

pp p p p p p

p p

p p

p p p p p

p p

p p p p p

p

p p

p p

162 161

152

154

156

146

144

147

150

151

149

168

141

138

132

135 130

136

139

142

115

116 122

117

124

109

111

112

101

107 108

96

118

123

100

91

73

70

84

94

65 69

71

78 79 81

86

95

98 103 104

77

85

90

62

72 66

51

57

74

93

48

47

34

36

39 37

40

15

16 17

26

30

19

23

24 25

29 33 35

42

5

13 14

167

.

. . .

. . . . . . .

. . . .

. . . .

. .

. . . .

. . . . . . . . . . . . . . . . . .

. . . . . .

. . . . .

. . . . . .. . . . . . . .

. . . .

.

. .

. .

. . .

. . .

. .

1

2

3

4

6

7

10 9 12

113 110

158157

153 155

148

145 143

140

134

137

129

120 121

126

125

128

114

131

119

102 99 97

92 87

76

63

80

64

53 54

61

56

49

44

43 41

45

46

32

31 28 27

21 20 18 22

11

8

55 58

50

68

59

75 67

60

52

38

82

89

83

106

88

105

127

133

159

163 160

164 165

165

169

170

CLAY

COUNTY

PASCO

COUNTY

ALACHUA

COUNTY

MARION

COUNTY

ST. JOHNS COUNTY

FLAGLER

COUNTY

PUTNAM COUNTY

VOLUSIA

COUNTY

SEMINOLE

COUNTY

ORANGE COUNTY

OSCEOLA

POLK COUNTY

COUNTY

HERNANDO

COUNTY

CITRUS

COUNTY

SUMTER

COUNTY

LAKE

COUNTY

28°20′

40′

29°00′

20′

29°40′

82°10′ 50′ 30′ 81°10′

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

Universal Transverse Mercator projection, zone 17

EXPLANATION

LAKE-GAGING

SITE AND NUMBER

SURFICIAL AQUIFER

SYSTEM

SITE AND NUMBER

p

.

161

165

OCALA NATIONAL

FOREST PROCLAMATION

BOUNDARY

LAKE COUNTY

Figure 6. Locations of lake-gaging and surficial aquifer system well data-collection sites 

(numbers refer to appendix B).
14 Hydrogeology and Simulated Effects of Ground-Water Withdrawals from the Floridan Aquifer System in Lake County and in 

the Ocala National Forest and Vicinity, North-Central Florida

River

Ocklawaha

St. Johns

S

i

t. Johns R ver

Pa al t al kaha River

R iver

Wek vi a River

Withlacoochee River

CLAY

COUNTY

PASCO

COUNTY

ALACHUA

COUNTY

MARION

COUNTY

ST. JOHNS COUNTY

FLAGLER

COUNTY

PUTNAM COUNTY

VOLUSIA

COUNTY

SEMINOLE

COUNTY

ORANGE COUNTY

OSCEOLA

POLK COUNTY

COUNTY

HERNANDO

COUNTY

CITRUS

COUNTY

SUMTER

COUNTY

0 5 10 KILOMETERS

0 5 10 MILES

Lake

Griffin

Lake Ocklawaha

Orange

Lake

Lake

Weir

Lake

Bryant

Lake

Eaton

Lake

Jumper

Mill Dam

Lake

Halfmoon

Lake

Lake

Dora

Lake Minneola

Lake

Lake

Louisa

Lake

Harris

Lake

Eustis

Lake

Norris

Lake

Dorr

Lake

George

Lake

Kerr

Lake

Yale

Lake Minnehaha Johns

Lake Lake

Apopka

Lake

Jessup

Lake

Monroe

Butler

Lake

Panasoffkee

LAKE COUNTY

MODEL

BOUNDARY

OCALA

NATIONAL

FOREST

. . . . .

. . . .

.

.

. . .

. . .

.

. .

. .

.

25

32

42 39

53

62 72

55

33

59

106

127

101

89

223

109

172

95

82

94

219

180

281

.

. . . .

.. . .

. . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . .. . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . .. . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . .. . . .. . . .. . . . . . .. . . .. . . . . .. . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . .. .. . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . .

.

.

21

218

36

44 51

54 60

61

77

75

70

71

64

23

34

49

48

57 58

50

46

45

52

43

65 68 67

399 400

397 398

396 395

385 386

388 389 384

1

2 3 4

6

8

9

108

407

406

401

402 405

404

403

383 381 382

372 376

373

369

378

375

365 367 366 364

363

362 359

358 360 357 356 361

354

350

355

353 351

347 349 348 344 346

343 345 342

340 339

341

338

13

10 11

5

7

87

69 66

47

41

84

74

37

73

120 134

30 31 27

16 15

12

17 18

22

19

24 20

26

28

29 35

40

38

56

14

330

315

292

272

262 261

244 249

224

200

167

155

138

76 78 79

98

86

93 96 104

91

107 124

135

131

119 112

118 121

116

139

151

152

123

111

103 114

125

141

148 149

157

156

143

150

329 328

327

323

90

97

181

161 165 169 160

176 173 170 162

122

130 128

144

146

132

171 177

147

203

196

201

216

83

85

80

81

335

325 320

332334 331

336 337

304

290

298 291

285

279

268

226

239

168

185

186

179 188 191

209 212

202 194

217 222 208

206

221

251 243

260

254 255

263 257 264

225

228

231

230

166

210 213

207

199

232

236 233 237 241

250 256

270

273

289

296

302

299 300

288

275

277

303 307 305

316

322

319 313 301

317 314

312

310

295 297

282

274

306

309

294

278

280

286

284

287

283

267

271

248

246

266 269

245

253 252

247

258

259

240 242

220

215

214

227

229

175 174

183

182

G

GG

GG

G GGGG

G

G G

GG GG

G G

G

G

G

G

G

GG G

G G

G

GG

GG G

GG

G

GG

G

GGG

G

GG

G

GG

G

GGG

G

G

GG G

G

G

G

391 393

390

394

392

387

368

371 370

377

374

380

379

352

333 324 326

321

311 318

308

293

276

265

235

204

197

187

164

159

158 154

142

234

140

198

211 205

190 195 193 192

189

184 178

163

153

145

126

115 117

113 110

105

102

63

88

92

99 100

129

133 137

136

28°20′

40′

29°00′

20′

29°40′

82°10′ 50′ 30′ 81°10′

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

Universal Transverse Mercator projection, zone 17

EXPLANATION

UPPER FLORIDAN

AQUIFER WELL AND

NUMBER

LOWER FLORIDAN

AQUIFER WELL AND

NUMBER

.281

.

291

G394 SPRING

OCALA NATIONAL

FOREST PROCLAMATION

BOUNDARY

LAKE COUNTY

Figure 7. Locations of springs and Floridan aquifer system well sites (numbers refer to 

appendix C).
Hydrogeology 15

SPECIFIED-HEAD

OR NO-FLOW

BOUNDARY

SPECIFIED-HEAD

OR NO-FLOW

BOUNDARY

SUB-FLORIDAN CONFINING UNIT

(Extremely low permeability)

(NO-FLOW BOUNDARY)

SURFICIAL AQUIFER

SYSTEM

(LAYER 1)

UPPER FLORIDAN

AQUIFER

(LAYER 2)

LOWER FLORIDAN

AQUIFER

(LAYER 3)

SERIES LITHOLOGY

GEOLOGIC UNITS

HYDROGEOLOGIC UNITS,

EQUIVALENT LAYERS IN COMPUTER

MODEL, AND BOUNDARY CONDITIONS

AQUIFER STRATIGRAPHIC

UNIT

THICKNESS

(feet)

0 - 150

0 - 200

0 - 300

0 - 200

600 - 1,600

300 - 1,350

500 - 2,200

UNDIFFERENTIATED

DEPOSITS

HAWTHORN

GROUP

OCALA

LIMESTONE

AVON PARK

FORMATION

OLDSMAR

FORMATION

CEDAR KEYS

FORMATION

HOLOCENE

PLEISTOCENE

PLIOCENE

MIOCENE

PALEOCENE

UPPER

MIDDLE

LOWER

EOCENE

FLORIDAN AQUIFER SYSTEM SURFICIAL AQUIFER

SYSTEM

Alluvium, freshwater marl, peats and muds

in stream and lake bottoms. Also, some

dunes and other windblown sand.

Mostly quartz sand. Locally can contain

organic deposits and thin beds of clay.

Interbedded deposits of sand, silty sand

and clay, shell fragments; phosphatic clay

sometimes present at base of formation.

Interbedded quartz, sand, silt, shell, and

clay; green to gray and white, phosphatic,

cemented alluvial conglomerate, sandstone,

dolostone, and limestone; typically silicified

and fractured near base of formation.

White to cream to tan, soft to hard, granular,

porous, marine foraminiferal limestone, often

dolomitic; sometimes sand, sandy or cherty

limestone at top of formation.

Light brown to brown, soft to hard, porous to

dense, granular to chalky, fossiliferous

limestone and brown, crystalline dolomite.

Alternating beds of light brown to white,

chalky, porous, fossiliferous limestone and

porous crystalline dolomite; minor amounts

of anhydrite and gypsum.

Dolomite, with considerable anhydrite and

gypsum, some limestone.

NO-FLOW

BOUNDARY

INTERMEDIATE CONFINING UNIT

(LEAKANCE)

MIDDLE SEMICONFINING UNIT

OR MIDDLE CONFINING UNIT

(LEAKANCE)

SPECIFIED-FLUX OR HEAD-DEPENDENT

BOUNDARY

LATE

QUATERNARY

TERTIARY

CENOZOIC

SYSTEM ERA

OLIGOCENE SUWANEE LIMESTONE 0 - 140 Limestone, sandy limestone, fossiliferous

EARLY

Figure 8. Geologic units, hydrogeologic units, and equivalent layers and boundary conditions used in the ground-water flow model.
16 Hydrogeology and Simulated Effects of Ground-Water Withdrawals from the Floridan Aquifer System in Lake County and in 

the Ocala National Forest and Vicinity, North-Central Florida

SURFICIAL AQUIFER SYSTEM

INTERMEDIATE

CONFINING UNIT

Blue

Springs,

Marion Co.

Wekiwa

Springs

PUTNAM CO.

MARION CO.

Salt

Springs

Ocklawaha River Lake

Kerr Sweetwater

Springs

Hopkins

Prairie

MARION CO.

LAKE CO.

Mosquito

Springs AlexanderSprings

LAKE CO.

ORANGE CO.

ORANGE CO.

POLK CO.

Blackwater Creek

C C

SEA

LEVEL

SEA

LEVEL

500 500

1,000 1,000

2,000 2,000

1,500 1,500

2,500 2,500

500 500

B B MARION CO. LAKE CO. VOLUSIA CO.

SURFICIAL AQUIFER

SYSTEM

Silver

Springs

Silver

Glen

Springs Lake

George

Ocklawaha

River

Land

surface

SEA

LEVEL

SEA

LEVEL

500 500

1,000 1,000

2,000 2,000

1,500 1,500

500 500 SECT. C-C’

SEA

LEVEL

SEA

LEVEL

500 500

1,000 1,000

1,500 1,500

2,000 2,000

2,500 2,500

500 500

Land

surface

SECT. C-C’

Apopka

Spring

A A

Wekiwa

Springs

Lake

Apopka

SUMTER CO.

LAKE CO.

LAKE CO.

ORANGE CO.

ORANGE CO.

SEMINOLE CO.

SEMINOLE CO.

VOLUSIA CO.

Wekiva

River St. JohnsRiver

ESTIMATED BOUNDARY BETWEEN

FRESHWATER AND UNDERLYING WATER

WITH CHLORIDE CONCENTRATION GREATER

THAN 5,000 MILLIGRAMS PER LITER

UPPER FLORIDAN

AQUIFER

SURFICIAL AQUIFER

SYSTEM

MIDDLE CONFINING UNIT

INTERMEDIATE CONFINING UNIT

MIDDLE SEMICONFINING UNIT

LOWER FLORIDAN AQUIFER

NOT SIMULATED

IN MODEL

SUB-FLORIDAN CONFINING UNIT

UPPER

FLORIDAN

AQUIFER INTERMEDIATE

CONFINING UNIT

MIDDLE

SEMICONFINING UNIT

MIDDLE

CONFINING

UNIT LOWER FLORIDAN AQUIFER

NOT SIMULATED

IN MODEL

SUB-FLORIDAN CONFINING UNIT

UPPER FLORIDAN AQUIFER

MIDDLE

SEMICONFINING UNIT

NOT SIMULATED IN MODEL

LOWER FLORIDAN AQUIFER

SUB-FLORIDAN CONFINING UNIT

0 10 20 MILES

0 10 20 KILOMETERS

SECT. B-B’

SECT. A-A’

EXPLANATION

Figure 9. Hydrogeologic sections A-A', B-B', and C-C' (locations of sections shown in fig. 3).
Hydrogeology 17

Stratigraphy

The Florida Plateau is a large, tectonically stable 

carbonate platform that has accumulated thick deposits of Paleocene and Eocene limestones, dolomites, 

some evaporites, and a comparatively thin post-Eocene 

sequence of sand and clay (Faulkner, 1973). These 

sedimentary deposits overlie the post-Paleozoic 

Coastal Plain Floor and are about 4,000 ft thick, of 

which 1,500 to 2,500 ft are pre-Tertiary in age and the 

remainder is Tertiary and younger (fig. 8). Oligoceneage and older sediments were deposited while most of 

the carbonate platform was submerged in a shallow 

sea environment. Evaporites were deposited in shallow, land-locked marine basins on emergent areas of 

the platform during lower sea-level stands.

The basal Tertiary unit is the Cedar Keys Formation of late Paleocene age. The Cedar Keys Formation 

generally has extremely low permeability and thus 

functions as the sub-Floridan confining unit at the base 

of the FAS. Conformably overlying the Cedar Keys 

are about 600-800 ft of the lower Eocene Oldsmar 

Formation, composed mostly of limestone with some 

interbedded dolomite and minor amounts of evaporites, anhydrite, and gypsum (fig. 8). The Avon Park 

Formation, a thick sequence of marine limestone and 

dolomite, conformably overlies the Oldsmar Formation and is characterized by alternating layers of soft to 

hard, fossilferous brown, crystalline dolomite and 

light-brown to tan limestone (fig. 8). The Avon Park 

Formation typically is highly fractured and vugular.

An erosional unconformity separates the Avon 

Park Formation from the overlying Ocala Limestone 

of late Eocene age. The Ocala Limestone has eroded 

entirely in some parts of Marion County and southwestern Lake County. In these areas, the Avon Park 

Formation is present at or relatively near land surface 

(less than 50-ft depth); however, in other parts of the 

study area, the top of the Ocala Limestone ranges from 

near land surface to more than 250 ft below land surface. The Ocala Limestone is composed of white to 

cream or tan limestone, which usually is fossilferous 

and soft to hard (fig. 8). Geologists’ logs indicate that 

some Ocala Limestone contains chert, either as irregular masses, thin layers, or localized caps. Differential 

erosion of the limestone surface has caused the formation of pinnacles and a wide variation in the altitude of 

the surface of the limestone, resulting in a mature karst 

terrain featuring rolling hills and numerous sinkhole 

depressions. Overlying the Ocala Limestone in some 

parts of Florida is the Suwannee Limestone of Oligocene age. The Suwannee Limestone has undergone 

extensive erosion and is present only in the southwestern part of the study area (Miller, 1986).

The Hawthorn Group of Miocene age unconformably overlies the Ocala Limestone and, where 

present, ranges in thickness from less than 5 ft to 

150 ft (fig. 8). The thickest units are associated with 

fill deposits in paleosinks that typically are not 

expressed at the surface. The Hawthorn Group consists 

mostly of marine sand interbedded with clay, sandy 

phosphatic clay, cemented alluvial conglomerate, and 

in some places, basal units of hard dense or fractured 

limestone or dolomite. In much of western Marion and 

southwestern Volusia Counties, much of the sediments 

of the Hawthorn Group have eroded and the remaining 

deposits commonly cap the hilltops. Elsewhere in the 

study area, the Hawthorn Group is more continuous.

Overlying the Hawthorn Group across much of 

the study area is a variety of mostly siliclastic sediments of Pliocene to Holocene age that range in thickness from 0 to 200 ft (fig. 8). Undifferentiated 

Pliocene to Holocene sediments that overlie the 

Hawthorn Group typically include phosphatic, nonmarine clayey sands, marine and lacustrine sand, shell 

marl, and sandy clay. Thin beds of Pleistocene clays 

on the Mount Dora Ridge (fig. 3) locally support lakes 

and small water-filled sinks.

Structure

The carbonate rocks of the Florida Plateau were 

deposited in shallow transgressive and regressive seas 

over the southeastward-plunging Peninsula Arch, 

which probably originated during the late Paleozoic or 

early Mesozoic Era when crustal stresses caused a 

gentle upward warping of the Coastal Plain Floor 

(Faulkner, 1973). The Peninsular Arch is the primary 

structural control for sediments deposited during 

Mesozioc and early Eocene time (Faulkner, 1973, 

p. 24). The axis of the subsurface arch trends northwest to southeast through the Ocala NF in eastern 

Marion County, northeastern Lake County, northwestern Orange County, and southwestern Volusia County. 

During middle-late Eocene time, the Ocala Uplift 

formed in western Marion and Sumter Counties, 

approximately parallel to the trend of the Peninsula 

Arch. As the Ocala Uplift developed, tensional 

stresses caused a series of extensive vertical and lateral 
18 Hydrogeology and Simulated Effects of Ground-Water Withdrawals from the Floridan Aquifer System in Lake County and in 

the Ocala National Forest and Vicinity, North-Central Florida

fracture systems in the rock strata at the top and along 

the sides of the Uplift. Many of these fractures that 

border the Ocala NF to the north and west have vertical displacements of up to 100 ft or more (Faulkner, 

1973, p. 40).

 Numerous cavern systems exist in the Ocala 

Limestone, most of which are oriented along fracture 

systems of the Ocala Uplift. Flow of acidic water 

through the fracture systems resulted in further dissolution of the rock surrounding the original fractures, 

increasing the size and innerconnection of the fractures. Millions of years ago, extensive cavern systems 

were formed when the Ocala Limestone was closer to 

the land surface. Presently, the same processes are still 

occurring.

The fracture system also influenced the locations of the Ocklawaha River, Silver Springs, and 

many springs in the St. Johns River Basin (fig. 1). 

Faulkner (1973) concluded that the Lower Ocklawaha 

River valley and the area to the east of it have been 

structurally lowered. As a result, low-permeability 

sediments of the Hawthorn Group are present east of 

the river, but have been eroded from the higher areas 

of the Mount Dora Ridge. The apparent relative downward displacement of the sediments east of the river 

resulted in these low-permeability beds of the Hawthorn Group blocking eastward flow of ground water 

in the underlying limestone (fig. 9, sections A-A' and 

B-B'). As a result, ground water is forced upward and 

has surfaced in this region at what is now Silver 

Springs (Faulkner, 1973). Highly interconnected 

secondary porosity has developed in the UFA (Ocala 

Limestone) in and around these springs, where transmissivity values locally can be on the order of 106 feet 

squared per day (ft2

/d) (Tibbals, 1990). Other firstmagnitude springs located near the St. Johns River and 

smaller springs in Lake Ocklawaha probably developed by similar processes.

 Surficial Aquifer System

The SAS is the uppermost, unconfined waterbearing unit in the study area and is composed principally of fine- to coarse-grained quartz sand, silt, and 

interbedded clay, peat, marl, and shell. These deposits 

generally are discontinuous, and the lithology and texture of the deposits can vary considerably locally both 

vertically and laterally. The upper 5-15 ft of the unit 

consist predominately of fine-grained, well-sorted 

sand of Holocene age overlying Pleistocene-age 

sediments, which range from 0 to 150 ft thick. 

For purposes of this study, the base of the SAS was 

defined by the first occurrence of persistent beds of 

Pliocene or Miocene age containing at least 50 percent 

silt, clay, limestone, or dolomite. In the study area, the 

altitude of the base of the SAS generally ranges from 

200 ft below to greater than 100 ft above sea level 

(fig. 10). Thickness of the SAS is highly variable and 

ranges from 0 ft (SAS absent) in parts of the Lake 

Upland, Sumter Upland, and the DeLand Ridge to 

nearly 300 ft in parts of the Central Valley, Mount 

Dora and Lake Wales Ridges, and the Marion Upland 

(figs. 3 and 10). Along the St. Johns and Wekiva Rivers, the SAS generally is 10-20 ft thick. Elsewhere, the 

thickness of the SAS generally is about 50-60 ft thick.

The vertical hydraulic conductivity of samples 

from the SAS reported by Knochenmus and Hughes 

(1976) was found to be highly variable and ranged 

from 0.03 to 160 feet per day (ft/d). In some areas, 

discontinuous and relatively low-permeability beds of 

reddish-brown to gray hardpan are present within a 

few feet of the land surface. These layers of hardpan 

are composed of slightly to well-indurated, organic, 

iron-oxide cemented sand and clay.

The unconfined SAS serves as a filter bed and 

reservoir for storing precipitation that eventually 

recharges the underlying FAS. However, the SAS has 

little potential as a major source of ground water compared to the FAS, although wells completed in the 

Pleistocene-age sands are pumped for irrigation and 

mining in some areas along the Mount Dora Ridge.

The upper boundary of the SAS is defined by 

the water table. In swampy lowlands and flatlands, the 

water table generally is at or near land surface 

throughout most of the year. Generally, the water table 

follows the shape of land-surface topography in a 

subdued manner, but can be as much as 150 ft or more 

below land surface in some areas. In addition to the 

influence of topography, the slope of the water table 

varies depending on hydrologic conditions, such as 

rainfall and ET rates; subsurface drainage to lakes and 

streams; and vertical leakage of water to the UFA. 

During wet periods when infiltration exceeds ET, the 

slope of the water table towards lakes or streams steepens as the storage of water in the SAS increases. 

During dry periods, the slope flattens as water drains 

from storage and is lost to ET or to seepage to lakes 

or streams.
Hydrogeology 19

Figure 10. Generalized altitude of the base of the surficial aquifer system.

River

Ocklawaha

St. Johns

S

i

t. Johns R ver

Pa al t al kaha River

R iver

Wek vi a River

Withlacoochee River

MODEL

BOUNDARY

28°20′

40′

29°00′

20′

29°40′

82°10′ 50′ 30′ 81°10′

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

Universal Transverse Mercator projection, zone 17

CLAY

COUNTY

PASCO

COUNTY

ALACHUA

COUNTY

MARION

COUNTY

ST. JOHNS COUNTY

FLAGLER

COUNTY

PUTNAM COUNTY

VOLUSIA

COUNTY

SEMINOLE

COUNTY

ORANGE COUNTY

OSCEOLA

POLK COUNTY

COUNTY

HERNANDO

COUNTY

CITRUS

COUNTY

SUMTER

COUNTY

LAKE

COUNTY

0 5 10 KILOMETERS

0 5 10 MILES

OCALA

NATIONAL

FOREST

.

. .

.

. .

.

. . .

.

.

.

.

.

. . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . ... . . . . . . . . . . .. . . .. . .. . . . . . .

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.. . . . . . . . . . . . . .. . . . . . . . . .. . . . .. . .. .

.. . . . .. .. . . .. .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . .. . . .. .. . . . . . . . . . . . . .. . .. . . . . . . . . . . . . . . . . .. . . . . .. . . . ... . .. . . . . . . . . . . . . . . . . . .

. . . .. .. . . . . . .. .. . . . .. . .. . . . . . . . . . . . . .. . . . . . . ... . . . . . .. . . . . . . . .. . . . . . . . . . . . . . . . .. .. . . . . . . . . . .. . .. . . . . . . . . . . .. . . ..

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... . . . . . ..

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. ..... . . . . . .. . . . .. . . . . . . . . . . . . . . . .

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.. . . . . . . . .. . . . . . .

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..

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..

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..... . . . . . . . . . . . . . . .. . . . . .. .

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.... . .

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. . . . . . . ... ... . ..

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.

.

.

. .

.

Absent

EXPLANATION

51 to100

1 to 50

-50 to 0

-100 to -51

-200 to -101

LAKE COUNTY

OCALA NATIONAL FOREST

PROCLAMATION BOUNDARY

Greater

than 100

. WELL CONTROL POINT

ALTITUDE OF THE BASE OF THE

SURFICIAL AQUIFER SYSTEM, IN FEET

ABOVE OR BELOW (-) SEA LEVEL
20 Hydrogeology and Simulated Effects of Ground-Water Withdrawals from the Floridan Aquifer System in Lake County and in 

the Ocala National Forest and Vicinity, North-Central Florida

The water-bearing properties of the SAS vary 

considerably across the study area and depend largely 

upon aquifer thickness, lithology, grain-size distribution, sorting, packing, and cementation of the sediments within the aquifer. Horizontal hydraulic 

conductivity values of the SAS were determined from 

slug tests performed on 30 SAS wells in Lake County 

and the Ocala NF. The values ranged from 0.2 to 35 ft/d, 

having a geometric mean of 8 ft/d, with the highest 

values in areas of the Mount Dora Ridge, Lake Wales 

Ridge, and Marion Upland. These values are in agreement with results of 21 slug tests from Seminole 

County, where horizontal hydraulic conductivity of the 

SAS ranged from 0.5 to 40 ft/d with a geometric mean 

of 8 ft/d (Spechler and Halford, 2001). CH2MHill 

(1989) reported horizontal hydraulic conductivity of 

the SAS ranged from 25 to 160 ft/d based on slug tests 

conducted in the Lake Wales Ridge in extreme southwestern Orange County.

Intermediate Confining Unit

The ICU, which underlies the SAS, generally 

consists of the Hawthorn Group of late-to-middle 

Miocene age and, locally, low-permeability beds of 

Pliocene age. Throughout most of the study area, the 

ICU serves as a confining layer (except where 

breached by sinkholes) that restricts the vertical movement of water between the overlying SAS and the 

underlying UFA. The unit is composed principally of 

an interbedded mixture of marine sediments including 

locally highly phosphatic sand and pebbles, clay and 

limestone; montmorillonitic clay and shell; and basal 

lenses of sandy, dolomitic, phosphatic limestone. 

Lower zones of the ICU, which are composed of shell 

or high-permeability carbonate rocks, can yield a limited supply of potable water. Some basal units of the 

Hawthorn Group consist of high-permeability carbonate rocks that generally are not in direct hydraulic connection with the UFA in the study area, except locally 

where these rocks are highly fractured. The base of the 

ICU, interpreted from well logs, is defined where the 

downward occurrence of phosphatic sediments is absent.

The thickness of the ICU typically is considered 

to be the thickness of the Hawthorn Group. However 

in some areas, particularly in western Volusia and 

northern Marion Counties, the upper part of the ICU 

includes mostly Hawthorn-Group phosphatic sands, so 

that the actual confining part of the unit is thinner than 

the total thickness of the Hawthorn Group. In many 

areas, low-permeability Pliocene clays and silts overlie 

the Hawthorn Group. Therefore, the thickness of the 

ICU as defined in this report will refer to the sediments 

that collectively confine the FAS, which includes all or 

part of the Hawthorn Group and part or none of the 

overlying Pliocene-age sediments.

 Well logs indicate that the ICU ranges in thickness from 0 (where the UFA is unconfined) to 150 ft 

thick in the study area (fig. 11). Locally, the unit can 

be thin or absent, especially across parts of Sumter and 

western Marion Counties, along the northern extension of the Mount Dora Ridge, and in parts of western 

Volusia County east of the St. Johns River. The ICU is 

thickest in the Central Valley, the Lake Upland, the 

southern extension of the Mount Dora Ridge, and the 

St. Johns River Offset (figs. 3 and 11). The vertical 

hydraulic conductivity of the ICU is highly variable 

across the study area, ranging from 0.01 to 0.53 ft/d 

(Knochenmus and Hughes, 1976). Tibbals (1990) 

reported leakance values ranging from 1x10-6 to 

6x10-4 feet per day per foot ((ft/d)/ft). The diverse 

lithology and range of hydraulic conductivity values 

of the ICU reflects the variety of depositional environments that occurred during the Pliocene and 

Miocene Epochs.

Floridan Aquifer System

The FAS is a thick sequence of carbonate rocks 

(limestones, dolomitic limestones, and dolomite) of 

Eocene to Oligocene age that are generally high in 

permeability and hydraulically connected in varying 

degrees. The FAS in the study area ranges in thickness from 900 to 2,000 ft (Miller, 1986, plate 27), 

and is subdivided on the basis of the vertical occurrence of two zones of relatively high permeability, 

the UFA and the LFA, which generally are separated 

by the MSCU or MCU.

The top of the youngest pre-Miocene sediments is considered to be the top of the UFA. The 

Suwannee Limestone has been removed by past erosional processes in most of the study area (Miller, 

1986). The Ocala Limestone also is absent in some 

areas, particularly in areas of southwestern Lake 

County and the adjacent counties, as a result of past 

erosional processes. The top of the UFA in these 

areas is defined by the dolomitic limestones of the 

Avon Park Formation. A generalized map of the altitude of the top of the UFA is shown in figure 12. The 

surface of the UFA is irregular and paleokarstic.
Hydrogeology 21

MODEL

BOUNDARY

28°20′

40′

29°00′

20′

29°40′

82°10′ 50′ 30′ 81°10′

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

Universal Transverse Mercator projection, zone 17

CLAY

COUNTY

PASCO

COUNTY

ALACHUA

COUNTY

MARION

COUNTY

ST. JOHNS COUNTY

FLAGLER COUNTY

PUTNAM COUNTY

VOLUSIA

COUNTY

LAKE COUNTY

SEMINOLE

COUNTY

ORANGE COUNTY

OSCEOLA

POLK COUNTY

COUNTY

HERNANDO

COUNTY

CITRUS

COUNTY

SUMTER

COUNTY

0 5 10 KILOMETERS

0 5 10 MILES

OCALA

NATIONAL

FOREST

. .

. . . . . . . . . . . . .

.

. . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . .. . . . . . .. . . .. . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . .. . . . ... . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . .. . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . .. . . . . . .. . . . .

. .. . . . . . . .. . . . .. . . .. . . . . ... . . .. .. . . . . . . .. . . . . . .

. . . . . . . . . . . . . . . . . . . . .

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.. .. . . . .

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. . .. .. . . . .

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. .

. . . ...

.

... . . . . . . .. . . . . . . .

.

. .

. . .

Absent

EXPLANATION

1 to 25

26 to 50

51 to 100

THICKNESS OF THE

INTERMEDIATE CONFINING

UNIT, IN FEET

101 to 150

OCALA NATIONAL FOREST

PROCLAMATION

BOUNDARY

LAKE COUNTY

. WELL CONTROL POINT . LAKE APOPKA

CONTROL POINT -- based

on seismic reflection data.

From Locker and

others, 1988

.

. .

.

. .

. . . .

Figure 11. Generalized thickness of the intermediate confining unit.
22 Hydrogeology and Simulated Effects of Ground-Water Withdrawals from the Floridan Aquifer System in Lake County and in 

the Ocala National Forest and Vicinity, North-Central Florida

MODEL

BOUNDARY

28°20′

40′

29°00′

20′

29°40′

82°10′ 50′ 30′ 81°10′

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

Universal Transverse Mercator projection, zone 17

CLAY

COUNTY

PASCO

COUNTY

ALACHUA

COUNTY

MARION

COUNTY

ST. JOHNS COUNTY

FLAGLER COUNTY

PUTNAM COUNTY

VOLUSIA

COUNTY

LAKE COUNTY

SEMINOLE

COUNTY

ORANGE COUNTY

OSCEOLA

POLK COUNTY

COUNTY

HERNANDO

COUNTY

CITRUS

COUNTY

SUMTER

COUNTY

0 5 10 KILOMETERS

0 5 10 MILES

OCALA

NATIONAL

FOREST

51 to 100

EXPLANATION

1 to 50

-50 to 0

-100 to -51

ALTITUDE OF THE TOP OF THE

FLORIDAN AQUIFER SYSTEM,

IN FEET ABOVE

OR BELOW (-) SEA LEVEL

-150 to -101

-250 to -151

Less than -250

OCALA NATIONAL FOREST

PROCLAMATION BOUNDARY

LAKE COUNTY

. WELL CONTROL POINT

GSPRING

. LAKE APOPKA

CONTROL POINT -- based

on seismic reflection data.

From Locker and others, 1968

. .

. . . . . . . . . . . .

.

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. .. . .. . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .. . . . . . . . . .. .. . . . . . . . . . . . . . . . . . . . .. . . . . . . .. . . .. . . . . . .. . . . . . . . .. . . . . . . . . . .. . .

. .. . . .. . . . . ... . . . .

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.. .. . . . . .. . . . . . . . . . .. . . . . .. . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . .. . .. . . . . . . . . . . . .. . . . . . . ... . . .. .. . . . . . . . . . . . . . . .. . . . . . . . . . .. .. . . . .. . . .. . . . . . . . . . .. . . . . .

. . . . . . .. . . . . . . . .. . . . . . . . . . . . . . . . .. . . . . . . . .. . . . .

. . . . . . . . . . . . . . . . . ... . . . . . . . . . .. . . . . . .. . . . . . .. . . . .

.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. .. . . . . . . . . . . .. . . . . . . . . . . . . . . . . . ... . . .. . . . . .. . . . .. . . . . . . . . . . . .. .. . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . .. . . . . . . . . . . . . .. . . . . . . . . . . . . ... . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . .. . .

. . . . .. .. . . . . . . . .. . . . . .. . .. . . . . . . . . . . . . . . . . . .. . . . . . . . .. . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . .. . .. . . . . . .. . . . .. . . . . .. .

. . . . . . . . ... . . . . . . . . . . . . .. . . . . . . . . ..

. . . . . . . . . . . .

. . .. . . . . . . . . . . . . . . . . . . . . .

. .

. . . ...

.

... . . . . . .. . . . . . . .

.

. .

. .

.

.

G

GG

GG

G GGGG

G

G G

GG GG

G G

G

G

G

G

G

GG G

G G

G

GG

GG G

GG

G

GG

G

GGG

G

GG

G

GG

G

GGG

G

G

GG G

G

G

G

.

. .

.

.

. . ..

.

Figure 12. Generalized altitude of the top of the Floridan aquifer system.
Hydrogeology 23

The altitude of the UFA ranges from nearly 100 ft 

above sea level in northwestern Polk County, southern 

and eastern Sumter County, and in areas of western 

Marion County to more than 250 ft below sea level 

southeast of Salt Springs and in north-central Marion 

County. The top of the UFA is highest along the crest 

of the Ocala Uplift, which extends through the western 

part of the study area, and is lowest along large dissolution features and sinkholes in and near the St. Johns 

and Ocklawaha River basins surrounding the eastern 

and northern edges of the Ocala NF. As shown in figure 12, the deeply eroded surface of the pre-Miocene 

carbonate sediments trends northwest-southeast and 

extends from Osceola County, across western Orange 

County and central Lake County, then along the 

Ocklawaha River to the St. Johns River; and also 

extends along the entire St. Johns River in the study 

area. Sinkhole-type depressions on the surface of the 

UFA are common; however, many of these features are 

small and localized, and are not shown in figure 12. 

The thickness of the UFA averages about 300 ft 

through most of the study area, and ranges from 200 ft 

thick or less in the southwestern part of the Ocala NF 

and in extreme northwestern Lake County to more 

than 400 ft thick in the extreme northeastern part of the 

Ocala NF and southwestern Lake County. The base of 

the UFA is shown in figure 13.

Throughout most of the study area, the MSCU 

or MCU separates the UFA and LFA (fig. 8). Miller 

(1986, p. B56-59) describes the MSCU and MCU as 

two, separate, distinct units. The MSCU is composed 

of beds of relatively less permeable limestone and 

dolomitic limestone and extends across most of the 

study area (fig. 14). In the western one-third of the 

study area, the MCU is composed primarily of gypsiferous dolomite and dolomitic limestone, which forms 

a much less leaky confining unit than the MSCU and 

functions hydraulically as the base of the freshwater 

flow system in the FAS in west-central Florida (Tibbals, 1990, p. E14). Because the MSCU and MCU are 

two distinct units, a discontinuity exists in the base of 

the UFA along the western edge of the study area 

(fig. 13). For the purpose of this study, the MSCU and 

the MCU (or the combined layers where overlapped) 

are hereafter referred to as the MSCU/MCU.

The MSCU/MCU ranges in thickness from 

more than 1,000 ft in northern Polk County and 

extreme southern Lake County to less than 100 ft in 

west-central Marion and southeastern Alachua Counties where it becomes thin or absent. In Lake County, 

the thickness of the MSCU/MCU averages about 

250 ft across northern parts of the county and about 

750 ft across southwestern parts of the county. The 

MCU is not known to exist in the Ocala NF; the 

MSCU is fairly uniform in thickness, ranging from 

about 100 ft along the Ocklawaha River to more than 

300 ft in extreme northeastern Lake County.

The LFA, which underlies the MSCU/MCU, 

includes about the bottom one-third of the Avon Park 

Formation and all of the Oldsmar Formation. The 

aquifer is highly productive and is composed of 

alternating beds of limestone and fractured dolomite. 

Permeability within this aquifer is related mostly to 

secondary porosity developed along bedding planes, 

joints, and fractures (Miller, 1986). The top of the LFA 

dips from northwest to southeast across the study area 

with altitudes ranging from 400 ft below sea level in 

central Marion County and the Ocala NF to more than 

1,000 ft below sea level in areas to the south and west 

of, and including, southern Lake County (Miller, 1986, 

plate 31). The LFA averages about 1,400 ft in thickness across the study area, ranging from about 1,200 ft 

in the northern part of the Ocala NF to 1,500 ft or more 

in western Lake County and much of northern Sumter 

County. The altitude of the base of the LFA also dips 

from northwest to southeast and ranges from about 

1,500-1,800 ft below sea level in central Marion 

County to about 2,500 ft below sea level in southwestern Lake County (fig. 15). The base of the LFA is 

underlain by the sub-Floridan confining unit.

Occurrence of Brackish (or Saline) Water

The northern and eastern parts of Lake County 

and the Ocala NF (although well inland) contain 

naturally occurring brackish or moderately saline 

ground water in the UFA at relatively shallow depths 

along the St. Johns River and its tributaries. Saline, 

connate ground water (particularly enriched in 

chloride and sulfate) moves upward from the LFA into 

lower parts of the UFA in discharge areas along the 

lower Wekiva River near the Lake-Seminole County 

line, in the St. Johns River along the Lake-MarionVolusia-Putnam County line, across the northern part 

of the Ocala NF, and along the lower Ocklawaha 

River, particularly Lake Ocklawaha. Springs in these 

areas (fig. 7) discharge brackish water mixed with the 

fresher water circulating in the upper parts of the UFA 

(Adamski and Knowles, 2001). The approximate 

depth to water containing 5,000 mg/L chloride 

concentration in the FAS is shown in figure 16.
24 Hydrogeology and Simulated Effects of Ground-Water Withdrawals from the Floridan Aquifer System in Lake County and in 

the Ocala National Forest and Vicinity, North-Central Florida

MODEL

BOUNDARY

28°20′

40′

29°00′

20′

29°40′

82°10′ 50′ 30′ 81°10′

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

Universal Transverse Mercator projection, zone 17

CLAY

COUNTY

PASCO

COUNTY

ALACHUA

COUNTY

MARION

COUNTY

ST. JOHNS COUNTY

FLAGLER COUNTY

PUTNAM COUNTY

VOLUSIA

COUNTY

LAKE COUNTY

SEMINOLE

COUNTY

ORANGE COUNTY

OSCEOLA

POLK COUNTY

COUNTY

HERNANDO

COUNTY

CITRUS

COUNTY

SUMTER

COUNTY

0 5 10 KILOMETERS

0 5 10 MILES

OCALA

NATIONAL

FOREST

EXPLANATION

ALTITUDE OF THE BASE OF THE

UPPER FLORIDAN AQUIFER, IN

FEET BELOW (-) SEA LEVEL

Western extent

of middle

semiconfining unit

OCALA NATIONAL FOREST

PROCLAMATION BOUNDARY

LAKE COUNTY

GSPRING

-350 to -200

-500 to -351

-650 to -501

-800 to -651

-950 to -801

G

GG

GG

G GGGG

G

G G

GG GG

G G

G

G

G

G

G

GG G

G G

G

GG

GG G

GG

G

GG

G

GGG

G

GG

G

GG

G

GGG

G

G

GG G

G

G

G

Figure 13. Generalized altitude of the base of the Upper Floridan aquifer (modified from McGurk 

and Presley, in press; and Miller, 1986).
Hydrogeology 25

Western

extent of

middle

semiconfining

unit

MODEL

BOUNDARY

28°20′

40′

29°00′

20′

29°40′

82°10′ 50′ 30′ 81°10′

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

Universal Transverse Mercator projection, zone 17

CLAY

COUNTY

PASCO

COUNTY

ALACHUA

COUNTY

MARION

COUNTY

ST. JOHNS COUNTY

FLAGLER COUNTY

PUTNAM COUNTY

VOLUSIA

COUNTY

LAKE COUNTY

SEMINOLE

COUNTY

ORANGE COUNTY

OSCEOLA

POLK COUNTY

COUNTY

HERNANDO

COUNTY

CITRUS

COUNTY

SUMTER

COUNTY

0 5 10 KILOMETERS

0 5 10 MILES

OCALA

NATIONAL

FOREST

G

GG

GG

G GGGG

G

G G

GG GG

G G

G

G

G

G

G

GG G

G G

G

GG

GG G

GG

G

GG

G

GGG

G

GG

G

GG

G

GGG

G

G

GG G

G

G

G

Absent

EXPLANATION

1 to 50

51 to 100

101 to 250

THICKNESS OF THE MIDDLE

SEMICONFINING AND

CONFINING UNITS, IN FEET

251 to 500

501 to 1,000

Greater than

1,000

Eastern extent of

middle confining unit

OCALA NATIONAL FOREST

PROCLAMATION BOUNDARY

LAKE COUNTY

GSPRING

Figure 14. Generalized thickness of the middle semiconfining and confining units (modified from 

McGurk and Presley, in press; and Miller, 1986).
26 Hydrogeology and Simulated Effects of Ground-Water Withdrawals from the Floridan Aquifer System in Lake County and in 

the Ocala National Forest and Vicinity, North-Central Florida

MODEL

BOUNDARY

28°20′

40′

29°00′

20′

29°40′

82°10′ 50′ 30′ 81°10′

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

Universal Transverse Mercator projection, zone 17

CLAY

COUNTY

PASCO

COUNTY

ALACHUA

COUNTY

MARION

COUNTY

ST. JOHNS COUNTY

FLAGLER COUNTY

PUTNAM COUNTY

VOLUSIA

COUNTY

LAKE COUNTY

SEMINOLE

COUNTY

ORANGE COUNTY

OSCEOLA

POLK COUNTY

COUNTY

HERNANDO

COUNTY

CITRUS

COUNTY

SUMTER

COUNTY

0 5 10 KILOMETERS

0 5 10 MILES

OCALA

NATIONAL

FOREST

G

GG

GG

G GGGG

G

G G

GG GG

G G

G

G

G

G

G

GG G

G G

G

GG

GG G

GG

G

GG

G

GGG

G

GG

G

GG

G

GGG

G

G

GG G

G

G

G

EXPLANATION

-1,500

to -1,750

-1,751

to -2,000

-2,001

to -2,250

ALTITUDE OF THE BASE OF

FLORIDAN AQUIFER SYSTEM,

IN FEET BELOW SEA LEVEL

Less than

-2,500

-2,251

to -2,500

OCALA NATIONAL FOREST

PROCLAMATION BOUNDARY

LAKE COUNTY

GSPRING

Figure 15. Generalized altitude of the base of the Floridan aquifer system (from Miller, 1986).
Hydrogeology 27

MODEL

BOUNDARY

CLAY

COUNTY

PASCO

COUNTY

ALACHUA

COUNTY

MARION

COUNTY

ST. JOHNS COUNTY

FLAGLER COUNTY

PUTNAM COUNTY

VOLUSIA

COUNTY

LAKE COUNTY

SEMINOLE

COUNTY

ORANGE COUNTY

OSCEOLA

POLK COUNTY

COUNTY

HERNANDO

COUNTY

CITRUS

COUNTY

SUMTER

COUNTY

0 5 10 KILOMETERS

0 5 10 MILES

G

GG

GG

G GGGG

G

G G

GG GG

G G

G

G

G

G

G

GG G

G G

G

GG

GG G

GG

G

GG

G

GGG

G

GG

G

GG

G

GGG

G

G

GG G

G

G

G

OCALA

NATIONAL

FOREST

28°20′

40′

29°00′

20′

29°40′

82°10′ 50′ 30′ 81°10′

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

Universal Transverse Mercator projection, zone 17

EXPLANATION

100 to 500

501 to 1,000

1,001 to 1,500

DEPTH TO WATER CONTAINING

5,000 MILLIGRAM PER LITER

CHLORIDE CONCENTRATION,

IN FEET BELOW LAND SURFACE

1,501 to 2,000

2,001 to 2,500

OCALA NATIONAL FOREST

PROCLAMATION BOUNDARY

LAKE COUNTY

GSPRING

Figure 16. Generalized depth to water containing 5,000 milligram per liter chloride concentration in 

the Floridan aquifer system (modified from McGurk and others, 1998; and Sprinkle, 1989).
28 Hydrogeology and Simulated Effects of Ground-Water Withdrawals from the Floridan Aquifer System in Lake County and in 

the Ocala National Forest and Vicinity, North-Central Florida

The map is based primarily on chloride-concentration 

data collected from monitor wells and test drilling in 

the study area; and to a lesser degree, time domain 

electromagnetic measurements (TDEM) that were 

collected in the mid- to late-1980’s (McGurk and 

others, 1998; and Nicasio Sepúlveda, U.S. Geological 

Survey, 1999, written commun.). The depth to water 

containing at least 5,000 mg/L chloride concentration 

ranges from less than 200 ft along the St. Johns River 

to nearly 2,500 ft below land surface in southern 

Sumter and Lake Counties. The thickest section of 

freshwater in the FAS in the study area is located 

across much of southern and central Lake County, 

extending into the southwestern part of the Ocala NF.

Hydraulic Characteristics

The transmissivity of the UFA varies widely 

across the study area and is a function of both primary 

and secondary porosity. Secondary porosity features 

that enhance permeability result from dissolution of 

the limestone aquifer matrix along bedding planes, 

joints, and fractures, but because of the irregular 

distribution and innerconnection of such features, 

transmissivity of the aquifer can vary widely. The 

transmissivity of the UFA varies throughout the study 

area and ranges from 3,750 to 2,000,000 ft2/d, generally increasing northward from Lake County to central 

Marion County. Pride and others (1966) estimated the 

transmissivity of the UFA in the Green Swamp area of 

southern Lake County to be 3,750 to 39,200 ft2/d. 

Knochenmus and Hughes (1976) estimated that the 

transmissivity of the UFA averages about 40,000 ft2/d 

in Lake County. The highest transmissivity values in 

the study area occur locally around many of the larger 

UFA springs. Faulkner (1973) reported an average 

transmissivity of greater than 2,000,000 ft2/d in the 

vicinity of Silver Springs and Ocala in central 

Marion County.

The storage coefficient of most confined carbonate aquifers ranges from about 1x10-5 to 1x10-3

(Lohman, 1972). Storage coefficient values for the UFA 

are more uniform than values of transmissivity, and 

typically range from 5x10-4 to 1x10-3 (Tibbals, 1990).

Few data are available to quantify the hydraulic 

properties of the MSCU that separates the UFA and 

LFA. The leakance of the MSCU, as estimated by 

Tibbals (1990) and Murray and Halford (1996), was a 

uniform value of about 5x10-5 (ft/d)/d except in spring 

areas along the St. Johns River. There, the leakance of 

the MSCU likely is very high because of a deep 

fracture system that provides a good hydraulic connection between the UFA and LFA.

The transmissivity of the LFA varies throughout 

the study area and is less well known than that of the 

UFA. Tibbals (1981) estimated transmissivity values 

of about 30,000 ft2/d in most of the Ocala NF and 

southern Lake County, about 60,000 ft2

/d in central 

and northern Lake County, and about 130,000 ft2/d in 

western Orange County. Murray and Halford (1996) 

reported an aquifer test conducted near Apopka in 

northwestern Orange County (fig. 1) yielded a transmissivity value of 632,000 ft2

/d. Additional transmissivity values for the LFA from 10 aquifer tests in 

Orange County were reported by O’Reilly and others 

(2002).

Estimates of storage coefficient for the LFA are 

based on sparse information, however, the storage 

coefficient values for the LFA are likely similar to 

those for the UFA. Values of storage coefficient calculated from aquifer tests of the LFA in Orange County 

ranged from 4x10-3 to 2x10-4 (Tibbals, 1990).

Recharge

Recharge areas of the UFA cover much of the 

study area and include well-drained and poorly 

drained soils, swamps, closed-basin lakes, and sinkholes. The UFA is recharged by the downward movement of water through the SAS and, where present, the 

ICU. The rate of recharge varies with the vertical 

hydraulic conductivity and thickness of the SAS and 

ICU, and the magnitude of the downward head gradient. The annual recharge rate to the UFA averages 

about 7 inches per year (in/yr) in Lake County 

(Knochenmus and Hughes, 1976). Recharge rates 

range from as high as 20-30 in/yr or greater on the 

Lake Wales and Mount Dora Ridges to 0 in/yr in the 

St. Johns River Offset areas of Lake County and the 

Ocala NF (O’Reilly, 1998; Murray and Halford, 

1996). Less than 1 in/yr of lateral ground-water inflow 

is estimated to enter the UFA in Lake County from 

Polk County (Knochenmus and Hughes, 1976). An 

indeterminate amount of additional lateral groundwater inflow enters the UFA in the Ocala NF from 

western Marion and northern Lake Counties. Additional recharge also occurs through drainage wells 

drilled into the UFA to dispose of excess surface water 

in Ocala and western Orange County. Recharge to the 

SAS, and consequently to the UFA, is augmented 

locally by artificial recharge—wastewater land application, rapid-infiltration basins, and septic systems.
Hydrogeology 29

Discharge

Discharge from the UFA to the SAS occurs 

where the potentiometric surface of the UFA is higher 

than the water table of the SAS. In some areas, the 

potentiometric surface of the UFA is above land surface. Wells that tap the UFA in these areas are known 

as flowing artesian wells. Discharge from the UFA in 

Lake County, the Ocala NF, and vicinity generally 

occurs through numerous springs, a few flowing wells, 

and as diffuse ground-water discharge along the 

St. Johns River, Wekiva River, Ocklawaha River, the 

south shore of Lake Harris, and the western shore of 

Lake Apopka.

Springflow from the UFA in the study area for 

1998 was estimated at nearly 1,300 Mgal/d (6 in/yr), 

of which 236 Mgal/d (4.3 in/yr) was from Lake 

County and 227 Mgal/d (6.9 in/yr) was from the Ocala 

NF. Silver Springs, the largest spring in the study area, 

has an annual mean flow of 788 ft3/s (1932-2000); 

Alexander Springs, the largest in Lake County, has an 

annual mean flow of 106 ft3

/s (1931-2000); and Silver 

Glen Springs, the largest in the Ocala NF, has an 

annual mean flow of 102 ft3

/s (1931-2000) (fig.7). 

Spring discharge (from as many as 20 springs reported 

in the Lake Ocklawaha area) and ground-water discharge from the UFA in and along the lower Ocklawaha River are substantially suppressed by elevated 

river stages regulated at the outlet of Lake Ocklawaha.

Potentiometric Surface of the Upper 

Floridan Aquifer

Potentiometric surface maps of the UFA in the 

northern and central Florida peninsula are published 

semiannually by the USGS in cooperation with various State and local agencies. The potentiometric 

surface of the UFA represents the altitude to which 

water levels will rise in tightly cased wells. Area-wide 

potentiometric-surface maps of the LFA commonly 

are not constructed because of the sparsity of wells 

tapping only the LFA. Many wells that tap the LFA 

also tap the UFA and are not hydraulically isolated 

from the UFA by well casing. O’Reilly and others 

(2002) present potentiometric-surface maps of the 

LFA in east-central Florida, including parts of Lake 

County, for September 1998 and May 1999. A potentiometric-surface map of the UFA in the study area was 

constructed using water-level measurements made by 

the USGS in approximately 292 wells in May 1998, 

including 30 new wells identified for the purposes of 

this study (fig. 2).

Water levels in the UFA respond to seasonal 

variations in rainfall on a regional scale and to pumping on a local scale. Generally, the potentiometric 

surface of the UFA in the study area is lowest during 

May or June near the end of the dry season and highest 

in September or October near the end of the wet 

season. During 1998, however, the potentiometric 

surface was highest during March and lowest during 

December. This was attributed to extremely wet conditions during the (El Niño) winter of 1997-98, followed 

by a 4-month-long drought, and then drier than 

average conditions during the summer wet season. 

Still, the potentiometric surface of the UFA across the 

study area generally was slightly higher in 1998 than 

most years during the 1990’s. Increased pumping 

during the spring and early summer drought locally 

increased declines in water levels.

The potentiometric surface of the UFA in 

May 1998 ranges from greater than 125 ft above sea 

level in northern Polk County to less than 5 ft above 

sea level at Lake George (fig. 2). In the study area, 

water in the UFA flows from potentiometric-surface 

highs in northern Polk and southwestern Clay and 

Putnam Counties to potentiometric-surface lows 

located in discharge areas. Ground water in Lake 

County generally flows in a north to northeast direction. In the Ocala NF, ground water generally flows 

eastward. Water-level measurements from UFA wells 

in May 1998 generally were about the average of the 

water-level measurements made during 1998.

Widely spaced potentiometric-surface contours 

can indicate highly transmissive areas of the UFA, 

such as the southwestern part of the Ocala NF and 

areas of northeastern Lake County south of the overlap 

(fig. 2). In contrast, closely spaced contours can indicate low transmissivity areas, such as areas directly 

west of the St. Johns River in Lake County and along 

the Ocklawaha River (fig. 2). There are two possible 

explanations for this area of low transmissivity. One is 

a change in lithology, possibly related to depositional 

environments. Corings from UFA wells in northernmost Lake County indicate that the top of the Ocala 

Limestone is cherty in some places and sandy in other 

places. Many of the UFA wells in this area also have 

partially collapsed holes, indicating a weakened, 

fractured, or sandy substructure. Another possible 

explanation is the presence of subsurface structural 

features. If such features exist, they could result in the 

juxtaposition of permeable zones in the west against 

less permeable zones in the east, as described by 

Faulkner (1973). As ground water flows laterally 

toward the St. Johns River, less permeable layers could 
30 Hydrogeology and Simulated Effects of Ground-Water Withdrawals from the Floridan Aquifer System in Lake County and in 

the Ocala National Forest and Vicinity, North-Central Florida

act as barriers, forcing water to the surface at numerous springs and seeps, and as diffuse ground-water 

discharge where the ICU is breached, thin, or absent.

Long-Term Trends

In the early 1930’s, a large data-collection network was developed by State and Federal agencies in 

cooperation with the USGS to record rainfall, lake 

stage, ground-water levels, and springflow. Data from 

this network were used to identify hydrologic trends of 

sites in the study area.

Rainfall and Lake Stage

Rainfall in the study area is characterized by 

variation of rainfall amounts from place to place, as 

well as from day to day and year to year. On an annual 

basis, however, there are cycles of wet seasons (June 

through September) and dry seasons (October through 

May). This variation in rainfall results in seasonal 

trends in surface- and ground-water levels and springflow. In the study area, the long-term average annual 

rainfall is about 51 in., with 55-60 percent falling 

during the 4-month wet season and 40-45 percent falling during the 8-month dry season (National Oceanic 

and Atmospheric Administration, 1935-99). This average is based on 65 years of record from four climatological sites in the study area. An additional 25 sites 

were used to analyze rainfall data for the study period, 

December 1997-98 (fig. 5).

Cumulative daily rainfall distribution based 

on the 29 climatological sites in the study area for 

December 1997-November 1998 is shown in 

figure 17. The total measured rainfall for the study 

period was 57.02 in., or slightly above the average 

annual rainfall of about 51 in. A wetter than usual 

winter during 1997-98 was followed by a prolonged 

4-month period with little or no rain, thereby 

shortening the wet season rainfall by about a month. 

The greatest recharge to the SAS occurs when the 

rising slope of the cumulative rainfall curve is 

steepest—such as during the periods of December 1997 

through March 1998 and July 1998 through September 1998.

CUMULATIVE RAINFALL, IN INCHES

0

10

20

30

40

50

60

DEC JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV

CUMULATIVE MEASURED

DAILY RAINFALL

TOTAL MEASURED RAINFALL = 57.02 INCHES

1997 1998

CUMULATIVE AVERAGE

MONTHLY RAINFALL

(Ocala, Lisbon and Clermont sites;

sites 12, 24, and 65, respectively, fig. 5)

AVERAGE ANNUAL

RAINFALL

50.94 INCHES

(Ocala, Lisbon

and Clermont sites;

sites 12, 24, and 65,

respectively, fig. 5)

Figure 17. Cumulative daily rainfall distribution for climatological sites in the study area December 1997 - 

November 1998.
Hydrogeology 31

Long-term, multiyear cycles in rainfall are difficult to discern primarily because of the lack of available long-term rainfall record within the study area. 

The longest rainfall records available in the study area 

begin in 1935 and include Clermont, Lisbon, Ocala, 

and Bushnell (sites 65, 24, 12, and 48, respectively) 

(fig. 5 and app. A). Graphs of cumulative departure 

from average rainfall at these sites from 1935 to 2000 

are shown in figure 18A. A rising slope indicates 

above-average rainfall (excess), whereas a declining 

slope shows below-average rainfall (deficit). For 

example, the long-term record at Lisbon shows that 

from 1935 to 1961 there was a total excess of about 

84 in., an average excess of 3.1 in/yr. However, from 

1961 to 1991 (a period dominated by drought conditions), the rainfall deficit totaled about 127 in., an 

average deficit of 4.2 in/yr. In contrast, the record at 

Clermont shows a slight deficit for 1939-69 and a 

slight surplus since 1970, indicating that rainfall trends 

are not necessarily the same at all sites in the study 

area.

 Responses of lake stage are highly related to 

cycles and distribution of rainfall (fig. 18B). The most 

significant rises and falls in lake stage follow consecutive years with above-average and below-average rainfall, respectively. For example, the lowest (1995) and 

highest (1961) annual mean lake stages of Lake Weir 

coincide with the maximum cumulative rainfall deficit 

and excess, respectively, at both Ocala and Lisbon. For 

the period, 1935-2000, lake stages generally were 

highest during 1936-38, 1946, 1950, 1960, 1984, and 

1996; and lowest during 1957, 1981, and 1994. The 

long-term (1940-2000) hydrograph of Lake Harris 

(fig. 18B) shows no discernible trends, whereas 

Lakes Griffin, Weir, and Bryant show a slight downward trend. After the early 1960’s, declines in lake 

stages generally are more pronounced, which corresponds well with the accumulating rainfall deficits of 

the same time period.

The magnitude of lake water-level fluctuations 

is related to the leakance of the lake bottom, but also to 

the hydraulic gradients beneath and surrounding the 

lake. Lakes located where a large, downward hydraulic 

gradient exists between the SAS and the UFA typically 

fluctuate more than those lakes located where the 

downward gradient is very small or reversed. For 

example, Deerhaven Lake (high downward gradient) 

has fluctuated nearly 7 ft over 18 years, whereas, 

Lake Dorr (little or no downward gradient) has fluctuated by less than 1 ft over 34 years.

Lakes are designated as either closed-basin 

(seepage) or flow-through (drainage). Closed-basin 

lakes are internally drained and have no surface inlet 

or outlet, whereas, flow-through lakes have a surface 

inlet and outlet, and surface water flows through the 

lakes, which often are regulated by structures along 

streams interconnecting these lakes. Lake water either 

evaporates, infiltrates through the lake bottom, or is 

pumped for irrigation.

Lake water-level duration curves show the time 

that given values of lake water-surface elevations were 

exceeded for the period of record (fig. 19), and provide 

a frame of reference to determine the water levels in 

lakes relative to the long-term water levels (represented by the curves). For example, the water level in 

Lake Bryant, a closed-basin lake, can be expected to 

be below 53 ft above sea level, 65 percent of the time, 

based on available record (500 observations made 

1936-2000). Conversely, lake water levels will exceed 

53 ft above sea level, 35 percent of the time. Annual 

mean lake water level in 1998 for Lake Bryant was 

51.26 ft above sea level (based on 4 measurements) 

which, based on 64 years of record, was exceeded 

about 83 percent of the time (fig. 19). Similarly for the 

remaining closed-basin lakes, annual mean lake water 

level for Lake Weir in 1998 was 55.32 ft above sea 

level (based on 52 measurements) and for Lake Dorr 

was 43.28 ft above sea level (based on 51 measurements) in 1998, which was exceeded about 80 percent 

of the time (1942-99) and 81 percent of the time 

(1965-2000), respectively.

For the flow-through lakes, annual mean lake 

water levels for Lakes Louisa, Harris, and Griffin were 

94.79 ft (based on 52 measurements), 62.86 ft (based 

on 362 measurements), and 58.53 ft above sea level 

(based on 364 measurements) in 1998, respectively, 

which were exceeded 80 percent of the time (1957-

2000), 70 percent of the time (1936-99), and 75 percent of the time (1944-99), respectively. The curves 

shown in figure 19 indicate that, in general, the water 

levels of the closed-basin lakes fluctuate slightly less 

than those of the flow-through lakes; however, the 

closed-basin lakes (selected based on the longevity of 

data record) also are located in areas where the downward gradient between the SAS and the UFA is nearly 

zero. Therefore, these closed-basin lakes may not represent the magnitude of the water-level fluctuations 

that occur in other closed-basin lakes located where a 

much larger downward gradient exists between the 

SAS and the UFA in the study area.
32 Hydrogeology and Simulated Effects of Ground-Water Withdrawals from the Floridan Aquifer System in Lake County and in 

the Ocala National Forest and Vicinity, North-Central Florida CUMULATIVE DEPARTURE FROM AVERAGE RAINFALL, IN INCHES

LISBON (50.28)

CLERMONT (50.69)

OCALA

(53.11)

BUSHNELL

(52.12)

AVERAGE FOR

STUDY AREA (51.43)

100

90

80

70

60

50

40

30

20

10

0

-10

-20

-30

-40

-50

100

90

80

70

60

50

40

30

LAKE LOUISA (95.62)

Breaks in time series are missing record

(95.62) Number in parenthesis is mean annual stage for period

of record, in feet above sea level

1935 1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000

LAKE HARRIS (61.62)

LAKE DORR (43.17)

LAKE BRYANT (52.03)

LAKE GRIFFIN (58.43) LAKE WEIR

(56.50)

SELLARS LAKE (45.04)

WILDCAT LAKE (42.79)

DEERHAVEN LAKE (38.07)

ANNUAL MEAN LAKE STAGE, IN FEET ABOVE SEA LEVEL

EXPLANATION

1935 1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000

EXPLANATION

Number in parenthesis is the average annual rainfall,

in inches 1935-2000

(50.28)

(A)

(B)

Solid lines indicate annual average.

Dashed line indicates long-term average.

Solid lines indicate annual average

Dashed lines indicate long-term average

Figure 18. Cumulative departure from average rainfall (A) and lake-stage time series (B) for selected sites in the 

study area (climatological sites refer to appendix A and lake sites to appendix B).
Hydrogeology 33

. . . . . . . . . . . . . ..

.. . . . . . . . . . . . . .

.

.

. . . . . . .

..

. . . . .

. .

......

. .. . ... . . . .

.

. . . . . . . . . . . . . . . ... ..

. . . . . . . . . .

LAKE LOUISA

(1957-2000; 8,416)

LAKE WEIR (1942-99; 20,110)

LAKE BRYANT

(1936-2000; 500)

LAKE HARRIS (1936-99; 15,668)

LAKE GRIFFIN (1944-99; 16,468)

LAKE DORR

(1965-2000; 2,248)

0.1 10 20

TIME INDICATED STAGE WAS EQUALED OR EXCEEDED, IN PERCENT

40

45

50

55

60

65

70

75

80

85

90

95

100

30 40 50 60 70 80 90 99.0 99.9

LAKE STAGE, IN FEET ABOVE SEA LEVEL

1.0

EXPLANATION

Numbers in parenthesis

following lake name are years of record

and the number of observations

(1957-2000; 8,416)

CLOSED-BASIN LAKE

. FLOW-THROUGH LAKE

.

Figure 19. Lake-stage duration for selected lakes.
34 Hydrogeology and Simulated Effects of Ground-Water Withdrawals from the Floridan Aquifer System in Lake County and in 

the Ocala National Forest and Vicinity, North-Central Florida

Ground-Water Levels

Water levels measured in SAS and UFA wells 

respond to rainfall variations in the same manner that 

lake stages respond. Daily mean water levels for eight 

continuous-record wells in the study area for the 

period 1935-2000 are shown in figure 20A. Continuous water-level data were available prior to 1960 only 

for well 291115081592501 (Sharpes Ferry Marion 

DOT5). The daily mean water level for 1936-2000 at 

the Sharpes Ferry well is 47.96 ft above sea level and 

ranges from 43.20 ft in 1957 to 55.42 ft above sea 

level in 1960.

The daily mean water levels in wells with continuous record from September 1997 through 1998 

(field data-collection period for this study) are shown 

in figure 20B. Except for well 291204081564801 

(Ocala NF SAS well near Lynne), which reached a 

maximum during early October 1998, water levels in 

SAS and UFA wells reached maximums during March 

1998 following an unusually wet (El Niño) winter.

Rainfall events, particularly during the (El Niño) 

winter of 1997-98, generally caused short-term peaks 

in the hydrographs of SAS wells and more subdued 

and delayed peaks in the hydrographs of UFA wells 

(fig. 20B). Water levels in SAS wells located where 

the unsaturated zone is relatively thin respond quickly 

to rainfall events, generally peaking within a day or 

two following the event. In areas where the unsaturated zone is thick (100 ft or more), water levels in SAS 

wells can take weeks before responding to rainfall 

events. Persistent, longer-term rainfall events over a 

period of months are needed to rewet surficial sands 

sufficiently to allow water to move downward to 

recharge the SAS, and ultimately, the UFA. Waterlevel rises in wells tapping the UFA will occur first as 

a response to increased pressure resulting from the 

addition of infiltrating water to the overlying water 

table. The amount of time for recharge water to reach 

the UFA can be long, particularly if the ICU has a 

small vertical hydraulic conductivity, large thickness, 

or both. In the study area, water levels in the UFA can 

take up to several weeks to respond to rainfall events, 

but generally water levels respond within a few days of 

a rainfall event where the UFA is near the land surface, 

unconfined or thinly confined, and recharge rates are 

high (for example, in well 283204081544901 at Mascotte in western Lake County). Water-level fluctuations in the UFA are greatest in recharge areas, 

generally about 5 ft during 1997-98. Periodic measurements of the UFA potentiometric surface in high 

recharge areas of Sumter and western Lake Counties 

indicated a range as much as 15-20 ft during 1997-98, 

which possibly was a result of nearby pumping or 

rerouting of surface water for mining that would have 

otherwise recharged the aquifer locally.

Upper Floridan Aquifer Springs

Sixty-five springs that discharge water from the 

UFA were inventoried during the study (table 1). 

Springflow occurs at discrete points (vents and boils) 

or as diffuse ground-water discharge over broader 

areas where the potentiometric surface of the UFA is 

above land surface and where the ICU overlying the 

Floridan aquifer has been breached.

Total calculated springflow for the study area in 

1998 was 1,979 ft3/s (1,279 Mgal/d). In Lake County 

during 1998, 22 springs accounted for a total springflow of 258 ft3/s (167 Mgal/d); and in the Ocala NF, 

14 springs accounted for a total springflow of 348 ft3/s 

(225 Mgal/d). The overlap area contains three springs 

with a total springflow of 112 ft3

/s (72.6 Mgal/d). Diffuse ground-water discharge, estimated using stream 

base flow measurements made by the USGS in 1981 

and 1997, is about 102 ft3/s (66.0 Mgal/d) along Juniper and Alexander Creeks. Diffuse ground-water discharge estimated by using a 30-day sliding average of 

minimum flows, is about 27 ft3

/s (17.4 Mgal/d) in 

Lake Ocklawaha. Diffuse ground-water discharge also 

is likely in the St. Johns River, south of Lake George, 

but the discharge rate is indeterminate because of 

nearly flat gradients and a large storage capacity of the 

river in and downstream of the lake.

Springs are categorized by their long-term 

mean discharges: first-magnitude springs have mean 

discharges of 100 ft3/s or more, second-magnitude 

springs have mean discharges of greater than 10 to 

100 ft3/s, and third-magnitude springs have mean 

discharges of greater than 1 to 10 ft3/s (Rosenau and 

others, 1977). The study area has 4 first-magnitude, 

18 second-magnitude, and 43 third-magnitude or 

less springs.

The largest (first-magnitude) springs, based 

on long-term discharge (1931-2000), are Silver 

Springs in Marion County, Blue Springs in Volusia 

County, Silver Glen Springs in the Ocala NF in 

Marion County, and Alexander Springs in the Ocala 

NF in Lake County. Together these springs account 

for 1,152 ft3/s (745 Mgal/d), or about 59 percent of 

the total springflow in the study area (fig. 21). 
Hydrogeology 35

1935 1945 1950 1955 1960 1940

40

50

60

70

80

90

100

110

UFA 290133082140901 (44.90)

1965 1970 1975 1980 1985 1990 1995 2000

DAILY MEAN WATER LEVEL, IN FEET ABOVE SEA LEVEL

UFA 291115081592501 (47.96)

UFA 282127082022501 (90.85)

UFA 283204081544901 (99.97)

UFA 283314081455501 (82.99)

UFA 284842081533001 (64.29)

EXPLANATION

SAS 283204081544902 (100.58) Aquifer, well number, and mean daily water level for

period-of-record, in feet above sea level

SAS 283204081544902 (100.58)

UFA 290133082140901 (47.08)

UFA 291115081592501

(48.79)

UFA 290628081425301 (48.28)

UFA 282127082022501 (91.07)

SAS 283204081544902

(100.95) UFA 283204081544901 (100.28)

UFA 283314081455501 (83.58)

10

20

30

40

50

60

70

80

90

100

110

SEPT OCT NOV DEC

1997 1998

JAN FEB MAR APR MAY JUNE JULY AUG SEPT OCT NOV DEC

SAS 291204081564801 (75.48)

UFA 284634081262004 (30.18) SAS 292817081483601 (29.56)

UFA 282717081553101 (98.42)

UFA 284842081533001 (66.35)

UFA 290950081315501 (12.99)

SAS 284634081262001 (30.98)

UFA 292817081483602 (22.02)

UFA 282717081553101 (97.75)

SAS Surficial aquifer system

UFA Upper Floridan aquifer

LONG-TERM HYDROGRAPH

STUDY PERIOD HYDROGRAPH

(A)

(B)

Solid lines indicate daily mean Dashed lines indicate long-term average

Figure 20. Long-term and 1997-98 water levels in selected wells tapping the surficial and the Upper 

Floridan aquifer systems in the study area (well sites referenced in appendixes B and C). 
36 Hydrogeology and Simulated Effects of Ground-Water Withdrawals from the Floridan Aquifer System in Lake County and in 

the Ocala National Forest and Vicinity, North-Central Florida

Table 1. Summary statistics of springflows

[ft3/s, cubic foot per second; NA, not available. Abbreviation for accuracy of annual springflow: E, estimate determined from measurements not made 

during 1998 or derived from baseflow of receiving stream; G, estimate based on reconnaissance; M, measurements made during 1998]

USGS 

site 

identification

number

Station name County

Date 

of 

first 

measurement

 Mean 

springflow 

for period 

of record 

(ft3/s)

Mean 

springflow

for 1998 

(ft3/s)

Accuracy 

of 

springflow 

measurements

283400081405100 Apopka (Gourd Neck) Spring near Oakland Lake 1971 38 33 M

283844081422300 Wolf’s Head Spring along railroad grade near Astatula Lake 1997 .1 .1 E

283903081430100 Bear Spring near Astatula Lake NA NA 2 G

284038081443201 Double Run Road Seepage (into Little Lake Harris) 

near Astatula

Lake 1997 2.0 2.4 E

284047081441501 Seepage Run (into Little Lake Harris), County Road 

561 near Astatula

Lake 1997 .1 .1 E

02237400 Holiday Springs at Yalaha Lake 1946 3.8 4.5 M

284437081491700 Sun Eden Spring near Yalaha Lake 1997 .2 .2 E

284452081495400 Mooring Cove Springs near Yalaha Lake 1997 .4 .4 E

284455081494100 Blue Springs, Park Drive near Yalaha Lake 1972 3.0 2.8 M

02237322 Bugg Spring near Okahumpka Lake 1943 11 12 M

284740081251701 Wekiva Falls Resort flowing borehole Lake 1975 20 20 E

284922081250300 Island Spring, Wekiva River Lake 1982 6.4 6.6 E

284940081303800 Droty Springs near Sorrento Lake 1997 .7 .8 E

285038081270100 Palm Springs, Seminole State Forest Lake 1997 .5 .6 E

02235250 Seminole Springs near Sorrento Lake 1931 35 40 E

285102081263900 Blueberry Spring, Seminole State Forest Lake 1997 .1 .1 E

285105081263800 Moccasin Springs, Seminole State Forest Lake 1997 .3 .3 E

02235255 Messant Spring near Sorrento Lake 1946 15 18 E

285224081262400 Shark’s Tooth Spring, Seminole State Forest Lake 1997 .1 .2 E

285318081295200 Blackwater Springs near Cassia Lake NA NA .2 G

285702081322400 Camp La-No-Che Springs near Paisley Lake 1954 .9 1.1 E

290220081260400 Mosquito Springs Run, Alexander Springs Wilderness Lake 1997 1.9 2.3 M

02236095 Alexander Springs Lake 1931 106 104 M

291136081381000 Juniper Creek South Tributary Seepage near Astor Lake 1980 6.1 6.1 E

02236160 Silver Glen Springs near Astor Marion 1931 102 102 M

02236132 Fern Hammock Springs near Ocala Marion 1935 13 13 M

02236130 Juniper Springs near Ocala Marion 1931 10 11 M

02236152 Morman Branch (Upper), State Road 19 Marion 1929 2.8 2.8 E

291200081390601 Morman Branch Seepage (into Juniper Creek) near 

Astor

Marion 1980 4.6 4.6 E

02239500 Silver Springs near Ocala Marion 1932 788 930 M

02236147 Sweetwater Springs along Juniper Run Marion 1980 13 14 M

02236205 Salt Springs Marion 1929 81 84 M

292521081551200 Wells Landings Springs Marion 1999 9.9 5.0 E

292540081552400 Tobacco Patch Landing Spring Group 1a (Group 1 run 

inflow)

Marion 1999 .7 .5 E

292542081552600 Tobacco Patch Landing Spring Group 1 Marion 1999 2.8 .5 E
Hydrogeology 37

293021081570600 Camp Seminole Spring, Girl Scout Camp, Orange 

Springs

Marion 1999 .8 1.0 E

293038081563800 Orange Spring near Orange Springs Marion 1972 4.8 2.5 E

02243550 Blue Springs near Orange Springs Marion 1935 5.7 .5 E

284241081281800 Barrel Springs, Wekiwa Springs State Park Orange 1997 .2 .2 M

02234600 Wekiwa Springs, Wekiwa Springs State Park Orange 1932 69 66 M

02234620 Witherington Springs, Wekiwa Springs Park near 

Apopka

Orange 1945 3.9 4.7 E

02234610 Rock Springs near Apopka Orange 1931 60 58 M

284612081303401 Sulphur (Camp) Springs near Mt. Plymouth Orange 1995 .6 .7 E

292618081412100 Croaker Hole Spring near Welaka Putnam 1981 86 94 M

02236220 Beecher Springs near Fruitland Putnam 1960 9.9 12 M

292725081393500 Forest Springs near Welaka Putnam 1972 .4 .4 E

292735081394500 Mud Spring near Welaka Putnam 1972 1.9 1.7 E

02244022 Welaka Spring Putnam 1972 1.0 1.0 E

02244020 Nashua Spring near Welaka Putnam 1946 .3 .5 E

293159081403600 Satsuma Spring near Satsuma Putnam 1956 1.5 1.3 E

02234991 Sanlando Springs near Longwood Seminole 1941 20 22 M

02234996 Palm Springs near Longwood Seminole 1941 7.2 5.3 M

02234997 Starbuck Spring near Longwood Seminole 1944 14 15 M

02234650 Miami Springs near Longwood Seminole 1945 4.9 5.1 M

284515082050100 Shady Brook Spring #5 (South Panasoffkee Spring 

Group) near Lake Panasoffkee

Sumter NA NA 3.0 E

284612082042000 Shady Brook Spring #4 (South Panasoffkee Spring 

Group) near Lake Panasoffkee

Sumter NA NA 3.0 E

284646082023800 Shady Brook Spring #3 (South Panasoffkee Spring 

Group) near Coleman

Sumter NA NA 3.0 E

284708082024600 Shady Brook Spring #2 (South Panasoffkee Spring 

Group) near Coleman

Sumter NA NA 3.0 E

284742082021900 Fenney Spring (headspring of Shady Brook to Lake 

Panasoffkee) near Coleman

Sumter 1946 41 43 E

285011082034900 Little Jones Creek Spring #3 (North Panasoffkee 

Spring Group) near Wildwood

Sumter NA NA 3.0 E

285134082051800 Little Jones Creek Spring #2 (North Panasoffkee 

Spring Group) near Wildwood

Sumter NA NA 5.0 E

285208082054100 Little Jones Creek Headspring (North Panasoffkee 

Spring Group) near Wildwood

Sumter NA NA 8.0 E

285144081183900 Gemini Springs near DeBary Volusia 1972 11 11 M

02235500 Blue Springs near Orange City Volusia 1932 156 157 M

02236110 Ponce De Leon Springs near DeLand Volusia 1929 28 22 M

Total 1,979 ft3/s

Table 1. Summary statistics of springflows--Continued

[ft3/s, cubic foot per second; NA, not available. Abbreviation for accuracy of annual springflow: E, estimate determined from measurements not made 

during 1998 or derived from baseflow of receiving stream; G, estimate based on reconnaissance; M, measurements made during 1998]

USGS 

site 

identification

number

Station name County

Date 

of 

first 

measurement

 Mean 

springflow 

for period 

of record 

(ft3/s)

Mean 

springflow

for 1998 

(ft3/s)

Accuracy 

of 

springflow 

measurements
38 Hydrogeology and Simulated Effects of Ground-Water Withdrawals from the Floridan Aquifer System in Lake County and in 

the Ocala National Forest and Vicinity, North-Central Florida

£

. £

£

Silver Springs near Ocala (788)

Alexander Springs (106)

Silver Glen Springs (102)

Blue Springs near Orange City (156)

FIRST-MAGNITUDE SPRINGS

(contribute 59 percent of total springflow)

.

£. £

. . . .

.

10,000

MEAN ANNUAL SPRINGFLOW,

IN CUBIC FEET PER SECOND

100

1,000

1935 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 1940

10

324 (788) Site number, number in parenthesis is the mean annual springflow

for period of record, in cubic feet per second

EXPLANATION

Solid lines indicate annual average

Dashed lines indicate long-term average

CUMULATIVE DEPARTURE

FROM AVERAGE RAINFALL,

IN INCHES

LISBON (50.28)

CLERMONT (50.69)

OCALA

(53.11)

BUSHNELL

(52.12)

AVERAGE FOR

STUDY AREA (51.43)

100

90

80

70

60

50

40

30

20

10

0

-10

-20

-30

-40

-50

1935 1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000

EXPLANATION

Number in parenthesis is the average annual rainfall,

in inches 1935-2000

(50.28)

Solid lines indicate annual average.

Dashed line indicates long-term average.

Figure 21. Annual rainfall and springflow of first-magnitude Upper Floridan aquifer springs in the study area, 

1935-2000 (spring sites refer to appendix C).
Hydrogeology 39

The average discharge from Silver Springs and 

Blue Springs for 69 years of record is 788 ft3

/s 

(509 Mgal/d) and 156 ft3/s (101 Mgal/d), respectively. 

Long-term trends in spring discharges from Silver 

Springs and Blue Springs (fig. 21) closely follow 

trends in rainfall at Ocala (fig. 18A); however, the 

response of spring discharge from Blue Springs occasionally is suppressed by river stage in the St. Johns 

River. In the long term, springflows have followed the 

same slight downward trend as that of most lake stages 

and ground-water levels, particularly since the early 

1960’s.

Second- and third-magnitude springs account 

for about 517 ft3

/s (334 Mgal/d), or 26 percent of the 

total springflow in the study area—about 142 ft3/s 

(92 Mgal/d) in Lake County and 234 ft3

/s (151 Mgal/d) 

in the Ocala NF, with about 13 ft3

/s (8 Mgal/d) in the 

overlap area (fig. 22). For convenience, the Lake 

County total includes the 20 ft3

/s (13 Mgal/d) of discharge from a flowing borehole at the Wekiva Falls 

Resort. Long-term data for the second- and third-magnitude springs are not as complete as for the first-magnitude springs, but the long-term trends appear to be 

similar. Many of the second- and third-magnitude 

springs also are affected by stages in receiving 

streams, particularly submerged springs located in 

lakes or streams. For example, spring discharge from 

Blue Springs (Marion County) and Wells Landing 

Springs in Lake Ocklawaha are suppressed by regulated stage (elevated 15-18 ft above the natural flow 

condition) maintained by Rodman Dam at the outlet of 

Lake Ocklawaha. Spring discharge from Blue Springs 

was nearly 11 ft3

/s (6.9 Mgal/d) in 1935 before stage 

regulation. When the regulated stage was lowered by 

approximately 11 ft in March 1998, discharge was 

estimated at about 6.0 ft3

/s (3.9 Mgal/d), and when the 

stage was returned to the regulated 20-ft level in May 

1998, discharge was measured at 0.5 ft3

/s (0.3 Mgal/d). 

The remaining 15 percent of the total springflow is 

from diffuse ground-water discharge and springs 

smaller than third-magnitude.

Water Budget

A generalized water-budget analysis of the 

study (model) area was made for 1998 by using measured or estimated values of rainfall, artificial 

recharge, springflow, streamflow, pumpage, storage 

changes, and net boundary leakage. An annual average 

estimate of ET was calculated as the residual of the 

water budget. A summary of the water budget is 

shown in figure 23. This water budget was compiled 

for comparison to the water budget from the groundwater flow model simulation. 

A generalized water-budget equation used for 

the study area to solve for ET, as shown in figure 23, 

is:

ET = RF + AR - RO - SP - BF - ∆S - W - BD, (1)

where;

BF = BFFAS + BFSAS,

∆S = ∆SFAS + ∆SSAS, and

ET is evapotranspiration;

RF is rainfall;

AR is artificial recharge from agricultural and golfcourse irrigation and reclaimed water 

application;

RO is overland runoff;

SP is springflow;

BF is total base flow;

BFFAS is base flow from the FAS;

BFSAS is base flow from the SAS;

∆S is total net change in storage;

∆SFAS is net change in storage of the FAS;

∆SSAS is net change in storage of the SAS;

W is total pumpage from the FAS;

BD is net boundary leakage, or net outflow within 

the FAS crossing study-area boundary;

and all components are in inches.

The largest output component in the water budget, ET, was about 33 in., or about 58 percent of the 

sum of the rainfall (RF) and artificial recharge (AR) 

components in 1998. Open-water surface (lake, wetlands, and stream) evaporation, which can be comparable to other estimates of ET, was estimated from 

pan-evaporation data by applying pan-to-lake coefficients (Sacks and others, 1994), and potential ET from 

the Gainesville (site 2) and Lisbon (site 24) climatological sites (fig. 5 and app. A). Potential ET for the 

study area was estimated to be about 51 in. in 1998. 

The open-water surface area is about 345 mi2

, or about 

7 percent of the study area. Partitioning the ET component into lake- and land-evaporation (7 percent lake 

and 93 percent land coverage) quantities yields a lakeevaporation of about 1.5 in. and a land-evaporation of 

slightly less than 32 in. for the study area in 1998.
40 Hydrogeology and Simulated Effects of Ground-Water Withdrawals from the Floridan Aquifer System in Lake County and in 

the Ocala National Forest and Vicinity, North-Central Florida

£

£

. .

.

.

.

£

£ £

£

£

£

£ £

.

.

£

. . .

.

.

£

£

£

£

£

£

£

.

. .

£

£

£

£

£

£

£

£

£

£

£

£

£

£

£

£

£

£ £

Holiday Springs (3.8)

SELECTED SECOND- AND THIRD-MAGNITUDE SPRINGS

(south-central Lake and Orange Counties)

Wekiwa Springs (69)

Rock Springs (60)

Messant

Springs (15)

Seminole Springs (35)

Wekiva Falls Resort flowing borehole (20)

Bugg Spring (11)

Blue Springs at Yalaha (3.0)

Fern Hammock

(13)

SECOND- AND THIRD-MAGNITUDE SPRINGS

(contributing 26 percent of total springflow)

Apopka Spring (38)

£ £ £

Croaker Hole (86)

Ponce DeLeon Springs (28)

Morman Branch and

Juniper Creek South Tributary (13.5)

Sweetwater Springs

(13)

Mosquito Springs (1.9)

Juniper Springs (10)

SELECTED SECOND- AND THIRD-MAGNITUDE SPRINGS

(Ocala National Forest and vicinity)

Wells Landing Springs (9.9)

Salt Springs (81)

Blue Springs (5.7)

MEAN ANNUAL SPRINGFLOW, IN CUBIC FEET PER SECOND

1930 1935 1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000

1

1925

10

100

10

100

1,000

1

1925 1930 1935 1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000

117 (69) Site number, number in parenthesis is the mean annual

springflow for period of record, in cubic feet per second

EXPLANATION

Solid lines indicate annual average

Dashed lines indicate long-term average

Figure 22. Annual springflow of selected second- and third-magnitude Upper Floridan aquifer springs in the 

study area, 1929-2000 (spring sites refer to appendix C).
Hydrogeology 41

SURFICIAL AQUIFER SYSTEM

NSAS

W BF NUFA FAS SP

BFSAS

UPPER FLORIDAN AQUIFER BD

LOWER FLORIDAN AQUIFER

FLORIDAN AQUIFER

SYSTEM

EXPLANATION

NET BOUNDARY LEAKAGE

∆S

All values are in inches.

CONTROL

VOLUME FOR

ENTIRE

MODEL AREA

N = RF + AR - ET - RO SAS

N = + W + SP + BD UFA BF + S FAS ∆ FAS

GENERALIZED WATER-BUDGET EQUATION

FOR ENTIRE MODEL CONTROL VOLUME

BFSAS

NSAS

TOTAL BASE FLOW

BASE FLOW FROM

SURFICIAL AQUIFER SYSTEM

TOTAL NET CHANGE

IN STORAGE

ARTIFICIAL RECHARGE EVAPOTRANSPIRATION

NET RECHARGE TO

SURFICIAL AQUIFER SYSTEM

NET RECHARGE TO

UPPER FLORIDAN AQUIFER

TOTAL PUMPAGE FROM

FLORIDAN AQUIFER SYSTEM

RAINFALL

OVERLAND RUNOFF

SPRINGFLOW FROM

FLORIDAN AQUIFER SYSTEM

BD

BF

AR ET

NUFA

W

RF

RO

SP

RO

LAND SURFACE

ET

RF + AR

(18) (57) (1) (33) (7)

ET = RF + AR - RO S - W - BD - SP - BF - ∆

(33) (57) (1) (7) (6) (1) (2) (6) (3)

where

BF = BF + BF , S = S + FAS SAS FAS and ∆ ∆ ∆SSAS

NSAS =BF + N SAS UFA + S∆ SAS

(18) (5) (12) (1)

∆SSAS

∆SFAS

(12) (2) (0) (1) (6) (3)

BFFAS

BASE FLOW FROM

FLORIDAN AQUIFER SYSTEM

∆SFAS NET CHANGE IN STORAGE OF

FLORIDAN AQUIFER SYSTEM

∆SSAS NET CHANGE IN STORAGE OF

SURFICIAL AQUIFER SYSTEM

TO

LAND

SU

RFA

CE

TO

LAND

SU

RFA

CE

TO

LAND

S

U

RFACE

Figure 23. Water budget for Lake County, the Ocala National Forest, and vicinity within the model area, 1998.
42 Hydrogeology and Simulated Effects of Ground-Water Withdrawals from the Floridan Aquifer System in Lake County and in 

the Ocala National Forest and Vicinity, North-Central Florida

Rainfall (RF) and artificial recharge (AR) from 

agricultural and golf-course irrigation and reclaimed 

water application were about 57 in. and 1 in., respectively, representing the total available input in 1998. 

An areal estimate of rainfall was determined using the 

Theissen method (Fetter, 1980), which adjusts for nonuniform site distribution by applying a weighting factor for each site. This factor was based on the size of 

the area within an irregular polygon constructed 

around and closest to each site. Twenty-nine sites 

(with an average area of 166 mi2 per site) were used in 

the analysis. Rainfall varied considerably across the 

study area, ranging from about 45 in. at Tavares in 

Lake County to nearly 70 in. at Jumper Lake in the 

Ocala NF. Rainfall exceeded 60 in. across central Lake 

County and west-central Ocala NF. Rainfall was less 

than 50 in. mostly across southern Lake County. AR 

was estimated using reclaimed water, agricultural, and 

golf-course water-use data.

Overland runoff (RO) was about 7 in., and was 

estimated by subtracting the base flow from the net 

streamflow in the model area (fig. 5). Net streamflow 

was 18.6 in. in 1998; 11.4 in. of the streamflow was 

base flow and was estimated from 30-day sliding minimum flows, which included springflow and diffuse 

ground-water discharge. Net streamflow was computed by subtracting streamflow exiting the study area 

from streamflow entering the area.

Springflow (SP) was about 6 in. in 1998 and was 

estimated by using annual mean discharges based on 

bi-monthly discharge measurements and daily mean 

discharge data for all springs in the study area. 

Although springflow contributes to base flow, it was 

considered as a separate component in this analysis.

Total base flow (BF), excluding measured 

springflow (SP), was about 6 in. with base flow from 

the FAS to streams (BFFAS) estimated to be about 1 in. 

BFFAS was determined by using end-of-dry season 

base flows (when contribution from the SAS is at a 

minimum) of contributing tributaries to the St. Johns 

River and seepage run measurements made in 1981 

(Tibbals, 1990) and during this study. Therefore, the 

base flow from the SAS (BFSAS) was determined to be 

about 5 in. by subtracting BFFAS from BF.

Total net change in aquifer storage (∆S) was 

estimated by adding the net change in storage of the 

SAS (∆SSAS) and the net change in storage of the FAS 

(∆SFAS). The total net change in storage is zero for a 

steady-state period; however, a 12-month time period 

could not be selected so that ∆S would be zero for the 

study area. Therefore, a 12-month period was selected 

during which ∆SSAS was minimal and was determined 

by averaging the changes in lake- and ground-water 

levels measured bi-monthly. Based on 46 wells measured bi-monthly, water levels in the SAS increased by 

0.70 ft on average. Using a specific yield of 0.15, 

∆SSAS was computed to be 0.11 ft (1.3 in.). Based on 

118 wells measured bi-monthly, water levels in the 

UFA and LFA increased by 0.12 ft and decreased by 

1.29 ft, respectively, on average. Using a specific yield 

of 0.001, ∆SFAS was computed to be less than 0.01 in. 

and therefore was assumed to be zero. ∆S was slightly 

more than zero across eastern Lake County and the 

Ocala NF, and ranged from a 1-in. loss in southern and 

central Lake County to a nearly 2-in. gain in southcentral and southwestern areas of the Ocala NF.

Total pumpage (W) from the FAS was about 

2 in. in 1998 and was estimated from annual water-use 

data.

Net boundary leakage (BD), or net flow crossing the model boundaries, was estimated to be nearly 

3 in. for the FAS (boundary leakage for the SAS was 

assumed to be negligible.) This estimate was made by 

constructing flow nets (Fetter, 1980) in areas where 

water in the UFA crossed the model boundary based 

on the observed potentiometric surface. This estimate 

was based on the assumption that horizontal gradients 

in both the LFA and UFA were the same, as there were 

no LFA water-level data available. Therefore, the 

actual boundary leakage could be more or less than 

3 in. depending on the horizontal gradients across 

these boundaries in the LFA.

Net recharge to the SAS in 1998, as shown in 

figure 23, is:

NSAS = RF + AR- ET- RO = 18 in., (2)

where;

NSAS is net recharge to the SAS, in inches; and

other terms are as previously defined.

A generalized water-budget equation used for 

the SAS, as shown in figure 23, is:

NSAS = BFSAS + NUFA + ∆SSAS = 18 in., (3)
Simulation of Ground-Water Flow 43

where;

NUFA is net recharge to the FAS, in inches; and

other terms are as previously defined.

Rearranging equation 3 and solving for NUFA

yields:

NUFA = NSAS - ∆SSAS - BFSAS = 12 in., (4)

where all terms are as previously defined.

A generalized water-budget equation used for 

the FAS, as shown in figure 23, is:

NUFA = BFFAS + W + SP + BD + ∆SFAS = 12 in., (5)

where all terms are as previously defined.

SIMULATION OF GROUND-WATER FLOW

The conceptual model (fig. 8) and hydrologic 

information presented in the previous sections were 

used to construct a numerical ground-water flow 

model of the SAS and FAS. The model simulates 

steady-state, ground-water flow both for average 1998 

conditions (December 1997 to December 1998) and 

for projected 2020 ground-water withdrawals. For 

simplicity, conditions during the time period from 

December 1997 to December 1998 will hereafter be 

referred to as average 1998 conditions. Particle-tracking analyses were used to identify the areas that contribute recharge to selected springs and well fields 

under both average 1998 and projected 2020 conditions.

Model Design

The USGS three-dimensional ground-water 

flow model code MODFLOW-2000 (version 1.1) was 

used to simulate the flow system (Harbaugh and 

others, 2000). MODFLOW-2000 incorporates significant revisions over previous versions (McDonald and 

Harbaugh, 1988; and Harbaugh and McDonald, 1996), 

most notably the capability to solve other types of 

equations in addition to the ground-water flow equation. One of the additional capabilities used in this 

study is the inverse model component that uses formal 

sensitivity and parameter estimation methods to 

facilitate model calibration (Hill and others, 2000). 

For simplicity, MODFLOW-2000 will hereafter be 

referred to as “MODFLOW.”

Model Layers and Grid

Vertical discretization of the model was based 

on hydrogeologic maps (figs. 10-15). Three layers 

were used to represent the SAS, UFA, and LFA 

(fig. 8). The resistance to flow between adjacent layers 

was assumed to be controlled by the leakance of the 

intervening ICU or MSCU/MCU (fig. 8). The leakance of a confining unit is calculated as the vertical 

hydraulic conductivity of the unit divided by its thickness. The large contrast in hydraulic conductivity 

between the confining units and adjacent aquifers, 

typically at least one-hundred times less in the confining unit, indicates that flow is nearly vertical in the 

confining units. Therefore, the confining units were 

simulated by using their respective vertical hydraulic 

conductivities and thicknesses, rather than as separate 

layers. Where the ICU does not exist (fig. 11), the 

thickness of the unit was assumed to be 1 ft and the 

base of the SAS was specified at an altitude 1 ft above 

the top of the UFA. Likewise, where the MSCU/MCU 

does not exist (fig. 14), the thickness was assumed to 

be 1 ft and the base of the UFA was specified at an 

altitude of -499 ft, which is the approximate altitude 

of the midpoint of the MSCU/MCU in adjacent areas 

where the unit does exist. Conceptualization of the 

MSCU/MCU can be difficult because of the existence 

of two distinct and disconnected confining units that 

collectively represent the MSCU/MCU in the model 

(figs. 8 and 9). For modeling purposes, the MSCU/MCU 

was assumed to extend from the top of the shallower 

confining unit, the MSCU (middle confining unit I as 

described by Miller (1986, p. B56)), to the bottom of 

the deeper confining unit, the MCU (middle confining 

unit II as described by Miller (1986, p. B56)) (fig. 14).

Horizontal discretization of each layer was 

oriented along a north-south axis and consisted of 

220 rows and 140 columns with a uniform cell size of 

2,500 by 2,500 ft (fig. 24). Of the 60,487 active cells, 

19,855 represented the SAS; 21,414 represented the 

UFA; and 19,218 represented the LFA (fig. 24). The 

active model area represented a surface area of 

approximately 4,800 mi2

.
44 Hydrogeology and Simulated Effects of Ground-Water Withdrawals from the Floridan Aquifer System in Lake County and in 

the Ocala National Forest and Vicinity, North-Central Florida

82°20′ 82°00′ 81°10′

29°40′

29°00′

28°20′

0

0

5 10 MILES

5 10 KILOMETERS

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

Universal Transverse Mercator projection, zone 17

LAKE

COUNTY

ORANGE COUNTY

SEMINOLE

COUNTY

VOLUSIA

COUNTY

OSCEOLA COUNTY

POLK COUNTY

PASCO

COUNTY

HERNANDO

COUNTY

SUMTER

COUNTY

CITRUS

COUNTY

MARION COUNTY

ALACHUA COUNTY

PUTNAM COUNTY

ST. JOHNS COUNTY

FLAGLER

COUNTY

MODEL

BOUNDARY

OCALA

NATIONAL

FOREST

BOUNDARY

EXPLANATION

ACTIVE AND INACTIVE MODEL CELLS

ACTIVE: Surficial aquifer system,

Upper Floridan aquifer, and

Lower Floridan aquifer

ACTIVE: Upper Floridan aquifer

and Lower Floridan aquifer

INACTIVE: Surficial aquifer

system

ACTIVE: Surficial aquifer system

and Upper Floridan aquifer

INACTIVE: Lower Floridan aquifer

Figure 24. Finite-difference grid showing active and inactive model cells.
Simulation of Ground-Water Flow 45

Boundary Conditions

Vertical boundaries were based on the geologic, 

water-quality, and flow-system characteristics of the 

aquifer system in the model area. A combination of 

specified fluxes and head-dependent boundaries 

representing net recharge served as the upper boundary condition, which was located at the altitude of the 

water table; how net recharge was simulated by the 

model is described in more detail in a later section. 

The lower boundary condition was specified as 

no-flow. The very low permeability of the sub-Floridan confining unit functions as a physical barrier to 

ground-water flow. However, highly mineralized water 

exists within the FAS above the sub-Floridan confining unit (fig. 9). If the transition from fresh to mineralized water and the consequent density disparity occurs 

abruptly and in the absence of any large nearby stress 

such as a pumping well, the interface between the 

freshwater and mineralized water flow systems can be 

approximated as a no-flow boundary (Reilly, 2001, 

p. 11). The altitude at which the chloride concentration 

of water is equal to 5,000 mg/L generally can be 

assumed to be the base of the freshwater flow system 

in east-central Florida based on the following reasoning: (1) the 5,000 mg/L chloride concentration 

approximately represents the boundary between 

moderately brackish water and very brackish to saline 

water; and (2) the thickness of the transition zone 

between the 5,000 and 10,000 mg/L chloride concentrations is relatively small (McGurk and Presley, in 

press). Therefore, the no-flow lower boundary 

condition for the model was established by the location of the 5,000 mg/L chloride concentration (fig. 16) 

or the top of the sub-Floridan confining unit (base of 

the FAS, fig. 15), whichever occurred at a shallower 

depth.

Lateral boundary conditions for each model 

layer were based on the geologic, water-quality, and 

flow-system characteristics of each aquifer, as well as 

the need to locate boundaries relatively far from the 

areas of interest (Lake County and the Ocala NF). In 

the SAS, a no-flow condition was specified along all 

lateral boundaries because relatively little regional lateral flow occurs in the SAS, based on reported values 

of hydraulic conductivity and aquifer thickness, and 

the SAS is thin or absent in parts of the model area. In 

areas where the SAS and ICU are thin or absent, the 

UFA probably functions as an unconfined aquifer. For 

modeling purposes, model cells in the SAS were specified as inactive where the saturated thickness of the 

SAS was less than 20 ft and the ICU was less than 5 ft 

thick (fig. 24). In the areas where the SAS was inactive, interaction of the ground-water system with the 

surface environment, such as stream leakage or net 

recharge, was applied to the UFA.

Lateral boundary conditions for the UFA and 

LFA consisted of no-flow and specified-head boundaries. No-flow lateral boundaries for the UFA (fig. 25) 

were established perpendicular to potentiometric contour lines from the May 1998 UFA potentiometric-surface map (fig. 2). No-flow lateral boundaries for the 

LFA were coincident with those of the UFA (fig. 25) 

based on the assumption that the LFA potentiometric 

surface is similar in configuration to that of the UFA. 

Specified-head boundaries were established from 

southwestern Marion to west-central Sumter Counties 

and across central Orange County (fig. 25). All specified-head cells were simulated using the MODFLOW 

Time-Variant Specified-Head Package (Leake and 

Prudic, 1991). Specified-head values for the UFA from 

southwestern Marion to west-central Sumter Counties 

and across central Orange County and for the LFA in 

central Orange County were interpolated from an 

arithmetic average of data collected in May and 

September 1998. No water-level data were available 

for the LFA from southwestern Marion to west-central 

Sumter Counties; consequently, specified-head values 

were estimated based on the overlying UFA water 

levels and an assumed head difference. Aucott (1988) 

indicated that the area along the Withlacoochee River 

downstream of State Road 48, including Lake Panasoffkee, is a discharge area for the UFA (fig. 1). Along 

the model boundary in this area the specified-head 

values for the LFA were assumed to be 1 ft above 

those of the UFA and 2 ft below those of the UFA 

elsewhere along the boundary. Additional no-flow 

boundaries were specified for the LFA at the interface 

of freshwater and water with chloride concentrations 

greater than 5,000 mg/L. A model cell was specified as 

inactive if less than 3 percent of the layer thickness 

was above the freshwater/saltwater interface. The 

small criterion of 3 percent was chosen so the thinning 

of the aquifer between the point where the freshwater/

saltwater interface rises above the bottom of the aquifer to the point where the interface intersects the top of 

the aquifer (fig. 8) would be accurately represented in 

the model. Numerous LFA cells met this criterion, 

generally in the vicinity of the St. Johns River, functioning as a no-flow boundary for the adjacent active 

LFA and overlying UFA cells (fig. 24).
46 Hydrogeology and Simulated Effects of Ground-Water Withdrawals from the Floridan Aquifer System in Lake County and in 

the Ocala National Forest and Vicinity, North-Central Florida

82°20′ 82°00′ 81°10′

29°40′

29°00′

28°20′

0

0

5 10 MILES

5 10 KILOMETERS

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

Universal Transverse Mercator projection, zone 17

LAKE

COUNTY

ORANGE COUNTY

SEMINOLE

COUNTY

VOLUSIA

COUNTY

OSCEOLA COUNTY

POLK COUNTY

PASCO

COUNTY

HERNANDO

COUNTY

SUMTER

COUNTY

CITRUS

COUNTY

MARION COUNTY

ALACHUA COUNTY

PUTNAM COUNTY

ST. JOHNS COUNTY

FLAGLER

COUNTY

NO-FLOW

BOUNDARY

NO-FLOW

BOUNDARY

NO-FLOW

BOUNDARY

OCALA

NATIONAL

FOREST

BOUNDARY

EXPLANATION

LATERAL BOUNDARIES

SPECIFIED-HEAD CELL

DRAINAGE-WELL RECHARGE,

IN MILLION GALLONS PER DAY

LESS THAN 0.25

0.25 - 0.75

GREATER THAN 0.75

Figure 25. Lateral boundary conditions for the Upper and Lower Floridan aquifers and drainage-well 

recharge rates for the Upper Floridan aquifer.
Simulation of Ground-Water Flow 47

Internal boundary conditions were established 

to represent interaction of the ground-water system 

with streams, lakes, or wetlands (figs. 26 and 27) 

because MODFLOW does not simulate surface-water 

flow. Streams, flow-through lakes, and wetlands were 

simulated using the MODFLOW River Package, as 

described in more detail in a later section. Closedbasin lakes were simulated as variable-head cells in 

the SAS where the area of the lake exceeded 50 percent that of the cell. In order to effectively represent 

the absence of aquifer materials in these lakes, the 

hydraulic conductivity of the SAS was specified to be 

1,000 times that of the surrounding SAS. This value 

was of sufficient magnitude to allow the simulation of 

a flat water table across a lake; likewise, other investigators have reported favorable results using high 

hydraulic conductivity nodes to represent closed-basin 

lakes (Lee, 1996; O’Reilly, 1998, p. 37; and Anderson 

and others, 2002). Although closed-basin lakes are 

surface-water features, MODFLOW is able to simulate 

closed-basin lakes because the lack of surface-water 

flow makes them a surface expression of the groundwater system (Reilly, 2001, p. 6).

Aquifer and Confining Unit Properties

In order to be generally consistent with the 

known hydrogeology in the model area, the SAS was 

simulated as an unconfined aquifer and both the UFA 

and LFA were simulated as confined aquifers. A notable exception is the UFA, which is unconfined in some 

areas where the water level is below the top of the 

aquifer and the ICU does not exist. Simulation of the 

UFA as confined in these areas can result in a calculated transmissivity that is too large. The error resulting from this simplifying assumption is small because 

the simulated water level averages only 11 ft below the 

specified top of the aquifer in these areas, which is 

only 3 percent of the average thickness of the UFA.

Initial values of the horizontal hydraulic conductivity of each aquifer were based on data collected 

during this study, data reported by previous investigators, or both. All cells in the SAS initially were 

assigned a horizontal hydraulic conductivity of 6 ft/d 

with the exception of cells representing closed-basin 

lakes, which were assigned a value of 6,000 ft/d. The 

value of 6 ft/d represents the geometric mean of results 

from 30 slug tests performed in the SAS for this study 

and reported results of 30 additional slug tests or aquifer performance tests (CH2M Hill, 1989; Szell, 1993; 

and Spechler and Halford, 2001, p. 16). The initial distribution of horizontal hydraulic conductivity of the 

UFA was based on data from previous models. As a 

part of a regional modeling effort covering most of 

peninsular Florida, Sepúlveda (2002) compiled a grid 

consisting of square 5,000-ft cells representing transmissivity values for the UFA from 14 different models. 

The cells of that grid are coincident with the cells of 

the present model: one 5,000-ft cell of the transmissivity grid covers exactly four 2,500-ft model cells. 

The portion of the transmissivity grid that is within the 

boundary of the present model contains data from previously calibrated models by Grubb and Rutledge 

(1979); Ryder (1985); Tibbals (1990); Blanford and 

Birdie (1993); Motz (1995); and Murray and Halford 

(1996). Where previously calibrated models overlap, 

the value of transmissivity used is the value from the 

model that provided the best fit between measured and 

simulated water levels (Sepúlveda, 2002, p. 64). The 

horizontal hydraulic conductivity of the UFA was calculated by dividing the values in the transmissivity 

grid by the thickness of the aquifer for each model cell 

(fig. 28). The initial value of horizontal hydraulic conductivity of the LFA was specified as a uniform value 

of 200 ft/d, based on an approximate average of transmissivities from previous models and 10 aquifer tests 

conducted in Orange County (Spechler and Halford, 

2001, p. 26) and the approximate aquifer thickness in 

the model area.

Relatively few data exist on aquifer anisotropy 

in the model area. Vertical anisotropy is defined as the 

ratio of horizontal to vertical hydraulic conductivity. 

Vertical anisotropy is common in many rocks and 

unconsolidated sediments, and vertical anisotropy of 

5 or 10 is not unusual (Bouwer, 1978, p. 56). Camp 

Dresser and McKee, Inc. (1984) and Sumner and 

Bradner (1996, p. 21) reported horizontal and vertical 

hydraulic conductivities with vertical anisotropy values of 1 to 3 for the SAS in western Orange County. 

Results of a long-term aquifer performance test indicate a vertical anisotropy of 39 for the SAS in eastern 

Orange County (Bush, 1979, p. 32). Available data 

based on analyses of rock cores from the FAS indicate 

highly variable vertical anisotropy values of 0.002 to 

267 for the Ocala Limestone and 0.9 to 8 for the Avon 

Park Formation in Hillsborough County (Robinson, 

1995, p. 16) and 0.5 to 38 for the Avon Park Formation 

and 1.2 for the Oldsmar Formation in north-central 

Polk County (Navoy, 1986, p. 29). Based on these disparate results, a uniform initial value of 10 for vertical 

anisotropy was assigned to the SAS, UFA, and LFA.
48 Hydrogeology and Simulated Effects of Ground-Water Withdrawals from the Floridan Aquifer System in Lake County and in 

the Ocala National Forest and Vicinity, North-Central Florida

82°20′ 82°00′ 81°10′

29°40′

29°00′

28°20′

0

0

5 10 MILES

5 10 KILOMETERS

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

Universal Transverse Mercator projection, zone 17

LAKE

COUNTY

ORANGE COUNTY

SEMINOLE

COUNTY

VOLUSIA

COUNTY

OSCEOLA COUNTY

POLK COUNTY

PASCO

COUNTY

HERNANDO

COUNTY

SUMTER

COUNTY

CITRUS

COUNTY

MARION COUNTY

ALACHUA COUNTY

PUTNAM COUNTY

ST. JOHNS COUNTY

FLAGLER

COUNTY

MODEL

BOUNDARY

OCALA

NATIONAL

FOREST

BOUNDARY

EXPLANATION

INTERNAL BOUNDARIES

GAGED STREAMS

UNGAGED STREAMS

GAGED AND UNGAGED

STREAMS IN SAME CELL

Figure 26. Internal boundary conditions representing streams (boundary conditions are applied to the SAS 

except where the SAS is inactive, in which case they are applied to the UFA).
Simulation of Ground-Water Flow 49

82°20′ 82°00′ 81°10′

29°40′

29°00′

28°20′

0

0

5 10 MILES

5 10 KILOMETERS

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

Universal Transverse Mercator projection, zone 17

LAKE

COUNTY

ORANGE COUNTY

SEMINOLE

COUNTY

VOLUSIA

COUNTY

OSCEOLA COUNTY

POLK COUNTY

PASCO

COUNTY

HERNANDO

COUNTY

SUMTER

COUNTY

CITRUS

COUNTY

MARION COUNTY

ALACHUA COUNTY

PUTNAM COUNTY

ST. JOHNS COUNTY

FLAGLER

COUNTY

MODEL

BOUNDARY

EXPLANATION

INTERNAL BOUNDARIES

GAGED FLOW-THROUGH

LAKE

UNGAGED FLOW-THROUGH

LAKE

CLOSED-BASIN LAKE

WETLAND

WETLAND AND GAGED

FLOW-THROUGH LAKE

OCALA

NATIONAL FOREST

BOUNDARY

Figure 27. Internal boundary conditions representing lakes or wetlands (boundary conditions are applied to 

the SAS except where the SAS is inactive, in which case they are applied to the UFA).
50 Hydrogeology and Simulated Effects of Ground-Water Withdrawals from the Floridan Aquifer System in Lake County and in 

the Ocala National Forest and Vicinity, North-Central Florida

82°20′ 82°00′ 81°10′

29°40′

29°00′

28°20′

0

0

5 10 MILES

5 10 KILOMETERS

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

Universal Transverse Mercator projection, zone 17

LAKE

COUNTY

ORANGE COUNTY

SEMINOLE

COUNTY

VOLUSIA

COUNTY

OSCEOLA COUNTY

POLK COUNTY

PASCO

COUNTY

HERNANDO

COUNTY

SUMTER

COUNTY

CITRUS

COUNTY

MARION COUNTY

ALACHUA COUNTY

PUTNAM COUNTY

ST. JOHNS COUNTY

FLAGLER

COUNTY

MODEL

BOUNDARY

EXPLANATION

HYDRAULIC CONDUCTIVITY,

IN FEET PER DAY

10 to 50

50 to 100

100 to 500

500 to 1,000

1,000 to 10,000

Greater than 10,000

OCALA

NATIONAL

FOREST

BOUNDARY

Figure 28. Horizontal hydraulic conductivity of the Upper Floridan aquifer used to initiate model calibration 

(final values from calibrated model are 20 percent higher).
Simulation of Ground-Water Flow 51

Horizontal anisotropy is defined in MODFLOW as the 

ratio of hydraulic conductivity along model columns 

to that along model rows, because the model grid is 

assumed to be aligned with the principal directions of 

hydraulic conductivity (Harbaugh and others, 2000, 

p. 39). Although some investigators have postulated 

that fractures and displacements may affect the 

regional flow of ground water in the FAS (Faulkner, 

1973, p. 43), no data are available within the model 

area to define horizontal anisotropy. Therefore, a horizontal anisotropy of 1 was assigned to the SAS, UFA, 

and LFA.

Limited data exist for the vertical hydraulic conductivity of the confining units in the model area. 

Available data based on analyses of cores from the 

ICU indicate highly variable vertical hydraulic conductivity values of 0.01 to 0.53 ft/d in Lake County 

(Knochenmus and Hughes, 1976) and 3.2x10-6 to 

4.0x10-1 ft/d from various locations in the SJRWMD 

(Boniol and others, 1993). An initial value of 0.02 ft/d 

was used for the vertical hydraulic conductivity of the 

ICU. Estimates of the vertical hydraulic conductivity 

of the MSCU are available from aquifer performance 

tests conducted in the vicinity of the model area and 

include values of: 1 to 3 ft/d in south-central Orange 

County (Boyle Engineering Corp., 1995); less than 

0.05 ft/d in eastern Orange County (Phelps and 

Schiffer, 1996, p. 31); and 0.02 to 2 ft/d in central 

Osceola County (Post, Buckley, Schuh, and Jernigan, 

Inc., 1990). The only data available for the vertical 

hydraulic conductivity of the MCU are those reported 

by Navoy (1986, p. 29) from laboratory analyses of 

rock cores obtained from a test well in north-central 

Polk County: vertical hydraulic conductivities ranged 

from 2.4x10-5 to 0.81 ft/d, with a geometric mean of 

5.5x10-3 ft/d, based on samples from 12 different 

depths. These data, combined with lithologic considerations, indicate that the vertical hydraulic conductivity 

of the MCU probably is considerably less than that of 

the MSCU. Therefore, it was necessary to account for 

this large difference in hydraulic conductivities 

between the two units, despite the fact that the MSCU 

and the MCU are represented in the model by only one 

confining unit. A vertical hydraulic conductivity of 

0.07 ft/d for the MSCU/MCU initially was assigned to 

all cells. This value was modified by a multiplier 

unique to each model cell to account for the spatial 

variations in vertical hydraulic conductivity as follows.

• Where only the MSCU exists, a multiplier of 1 was 

used so the initial value of vertical hydraulic conductivity was 0.07 ft/d.

• Where only the MCU exists, a multiplier of 0.01 

was used because field data indicate that, on average, the vertical hydraulic conductivity of the 

MCU is about one-hundredth that of the MSCU. 

Therefore, the initial value of vertical hydraulic 

conductivity was 7x10-4 ft/d. 

• Where both the MSCU and the MCU exist, the multiplier was calculated as 0.01(B/Bmcu) where B is 

the thickness of the MSCU/MCU specified in the 

model (extending from the top of the MSCU to the 

bottom of the MCU) and Bmcu is the thickness of 

the MCU. This effectively means that only the 

thickness of the MCU is considered when 

MODFLOW calculates the vertical conductance 

between the UFA and LFA. This is an acceptable 

simplification because the vertical hydraulic conductivity of the MCU is much less than that of 

either the MSCU or the permeable zone between 

the two units.

• Where neither the MSCU nor the MCU exist, a 

multiplier of 100 was used so the initial value of 

vertical hydraulic conductivity was 7 ft/d. This 

combined with a specified thickness of 1 ft effectively provides no resistance to flow between the 

UFA and LFA in such areas.

Aquifer Stresses

Aquifer stresses were estimated for calibration 

of a steady-state model to simulate average 1998 conditions. Where data were available at discrete times 

during 1998, average annual values were calculated as 

an arithmetic average of these data.

Recharge and Evapotranspiration

The aquifer system is recharged when sufficient 

water is applied to overcome evapotranspirative losses 

and capillary effects in the unsaturated zone and 

remaining water percolates across the water table. 

When precipitation or artificial recharge rates exceed 

the infiltration capacity of the soil, some water continues to move downward, while excess water is rejected 

and becomes overland runoff. Of the water that 

reaches the water table, recharge is the fraction that is 

not quickly extracted by ET and that moves downgradient. However, other processes also can contribute 

recharge to the aquifer system. In a karst environment, 
52 Hydrogeology and Simulated Effects of Ground-Water Withdrawals from the Floridan Aquifer System in Lake County and in 

the Ocala National Forest and Vicinity, North-Central Florida

such as exists in the study area, closed-basin lakes are 

common and precipitation falling on these lakes must 

either evaporate or recharge the aquifer. In a karst 

environment the possibility exists that some fraction of 

overland runoff provides recharge. Overland runoff to 

a surface-water body other than a closed-basin lake 

does not recharge the ground-water system; whereas 

overland runoff to a closed-basin lake provides water 

to the lake, which might subsequently recharge the 

aquifer. In addition, overland runoff that is captured by 

a land-surface depression, such as might be created by 

a sinkhole, can subsequently infiltrate and percolate 

toward the water table. Lastly, water released from 

storage in aquifer pore spaces or in closed-basin lakes 

as a result of a falling water level in the aquifer or lake 

can be mathematically represented as a flux occurring 

over the time period during which the drop in water 

table or lake level was measured. Considering all these 

factors, recharge to the aquifer system was simulated 

as a net recharge (N) in the model as described by

, (6)

, (7)

where

P is precipitation, [L/T];

Ra is artificial recharge, [L/T];

Ocbl is overland runoff to a closed-basin lake, [L/T];

ET is evapotranspiration, [L/T];

Os is overland runoff to a surface-water body 

other than a closed-basin lake, [L/T];

∆Sal is change in storage in the aquifer in which the 

water table is located and in closed-basin 

lakes, [L/T];

Sy is the specific yield of the aquifer in which the 

water table is located or equal to 1 for a 

closed-basin lake, [dimensionless];

∆H is change in water-table altitude or closedbasin lake level, [L/T]; and

L and T denote length and time units, respectively.

Conceptually, the inclusion of ∆Sal as a component of 

net recharge can be interpreted as follows: (1) an 

increase in storage is equivalent to a reduction in net 

recharge because a rise in the water-table or closedbasin lake level is produced by water that otherwise 

would have been available to move downgradient 

through the aquifer or out of the closed-basin lake 

or (2) a decrease in storage is equivalent to an increase 

in net recharge because a drop in the water-table or 

closed-basin lake level releases water that can move 

downgradient through the aquifer or out of the closedbasin lake. Because of the small storage coefficients 

typical of confined aquifers, the change in storage in 

the confined FAS during 1998 was assumed negligible; therefore, a correction for storage changes was not 

considered for either the UFA (where it is confined) or 

the LFA. Net recharge was applied to the SAS, except 

where the SAS was inactive (fig. 24), in which case it 

was applied to the UFA.

On an average annual basis, net recharge might 

not be greatly affected by spatial variations in precipitation, because ET and overland runoff probably have 

some moderating effect on net recharge. That is, ET 

and overland runoff probably are greater where precipitation is greater, and ET and overland runoff probably 

are smaller where precipitation is smaller; therefore, 

net recharge could be of similar magnitude in both situations. Sumner (2001, p. 46) reported data for Tiger 

Bay watershed in central Volusia County that might 

help support this hypothesis, but on a temporal basis; 

the ratio of annual ET to annual precipitation was 0.74 

and 0.77 for 1998 and 1999, respectively, even though 

precipitation in 1998 was 12 percent less than that in 

1999. Based on the results reported by Sumner (2001, 

p. 46) and the lack of data concerning the actual spatial 

distribution of ET or overland runoff, model-wide 

average values were used to estimate several terms in 

equation 6, as will be discussed later in this section.

The complex process through which net 

recharge occurs was greatly simplified in the model. 

Net recharge (eq. 6) was assumed to be divisible into 

two components: natural net recharge and artificial 

net recharge. Natural net recharge is net recharge that 

results from precipitation; whereas artificial net 

recharge is net recharge from artificial means.

Natural net recharge can occur at the water table 

or a closed-basin lake. Natural net recharge at the 

water table (Nn) can be defined by noting that Ocbl and 

Ra of equation 6 will be zero:

, (8)

, (9)

, (10)

N PRa Ocbl – ET Os – ∆Sal = + + –

∆Sal S = y∆H

Nn P ETn – Osn – ∆Saq = –

ETn ETn min , ETn ex , = –

Osn Osn min , Osn ex , = –
Simulation of Ground-Water Flow 53

where

∆Saq is change in storage in the aquifer in which the 

water table is located, [L/T];

ETn is natural evapotranspiration, which is evapotranspiration in the absence of artificial 

recharge, [L/T];

ETn,min is minimum natural evapotranspiration, [L/T];

ETn,ex is excess natural evapotranspiration exceeding ETn,min, [L/T];

Osn is natural overland runoff, which is overland 

runoff in the absence of artificial recharge, 

to a surface-water body other than a closedbasin lake, [L/T]; 

Osn,min is minimum natural overland runoff, [L/T]; 

and

Osn,ex is excess natural overland runoff exceeding 

Osn,min, [L/T].

Substituting equations 9 and 10 into equation 8 and 

rearranging yields:

(11)

ET and overland runoff are not well known and are 

difficult to account for in MODFLOW; therefore, the 

components of ETn and Osn will be considered 

together as denoted by the brackets in equation 11. 

This acknowledges the difficulty in differentiating the 

individual effects of ET or overland runoff on net 

recharge. Equation 11 was applied at each model cell 

of the SAS, or the UFA where the SAS was inactive 

(fig. 24), except at cells representing closed-basin or 

flow-through lakes (fig. 27). However, four of the 

terms of equation 11 (P, ETn,min, Osn,min, and ∆Saq) 

were assumed not to vary spatially and model-wide 

average values were used. In addition, some of the 

terms of equation 11 will be assumed to be zero 

depending on whether the simulated water table is 

above or below the extinction depth. Extinction depth 

is the maximum depth below land surface from which 

water contributing to ET can be extracted and is 

dependent primarily on plant root depth and soil 

hydraulic characteristics. Areas of the model where 

the simulated water table is below the extinction depth 

are referred to as upland zones, and areas of the model 

where the simulated water table is equal to or above 

the extinction depth are referred to as transitional 

zones. Extinction depth was specified to be 13 ft 

Nn P ETn min , Osn min , – [ ] +

ETn ex , Osn ex , – [ ] + ∆Saq – .

=

(Tibbals, 1978, p. 10; Tibbals, 1990, p. 10) in all 

model cells. Lichtler and others (1968, p. 86) reported 

that the maximum depth of tree roots in Orange 

County is about 15 ft, which is in reasonable agreement with an extinction depth of 13 ft.

High rates of net recharge often have been 

considered to exist in areas where the water table is 

deep, the water table is below the root zone of the local 

vegetation, and the surficial sands are very permeable 

(Lichtler and others, 1968, p. 86; Knochenmus and 

Hughes, 1976, p. 32; and Yobbi, 1996, p. 22). In an 

upland zone, therefore, ET and overland runoff probably are near their minimum values, and natural net 

recharge probably is at its maximum value. Therefore, 

ETn,ex and Osn,ex in equation 11 can be set equal to 

zero, which yields an expression defined as maximum 

natural net recharge (Nn,max):

. (12)

ETn,min is assumed to be 27 in/yr based on a value of 

ET reported by Sumner (1996, p. 30) for nonirrigated, 

herbaceous vegetation with a deep water table (below 

the root zone). Knochenmus and Hughes (1976, p. 32) 

reported that annual overland runoff for the Lake 

Wales Ridge in Lake County (fig. 3) is 0 to 4 inches; 

Osn,min was assumed to be zero for the purposes of an 

initial estimate of Nn,max . ∆Saq was considered to be 

negligible because the average change in water level 

measured in 79 SAS wells during 1998 was only 

1.0 inch. Therefore, in an upland zone natural net 

recharge initially was specified to be a uniform value 

of 30 in/yr (P = 57 in/yr and ETn,min = 27 in/yr) and 

was simulated using the MODFLOW Recharge 

Package.

In a transitional zone, ET and overland runoff 

are assumed to exceed their minimum values by an 

amount represented by ETn,ex and Osn,ex, respectively. 

Excess ET is assumed to occur as a result of a shallower water table (equal to or above the extinction 

depth) in the transitional zone than in the upland zone, 

and thus water is more accessible to plant roots and 

direct evaporation. Excess overland runoff is assumed 

to occur as the simulated water table rises above the 

extinction depth. The capacity to store water in the 

unsaturated soil pores decreases as the thickness of the 

unsaturated zone decreases, and the presence of the 

fully saturated capillary fringe further reduces this 

storage capacity. In the ground-water flow model, only 

Nn max , P ETn min , Osn min , – [ ] + ∆Saq = –
54 Hydrogeology and Simulated Effects of Ground-Water Withdrawals from the Floridan Aquifer System in Lake County and in 

the Ocala National Forest and Vicinity, North-Central Florida

the depth of the water table below the average land 

surface can be simulated for each model cell. In reality, the simulated water table could be above land surface in some fraction of a model cell and below land 

surface in the remainder of the model cell, because 

land surface altitude can vary considerably within a 

single model cell. If this situation occurred, overland 

flow should be greater than if the water table were 

below land surface throughout the model cell. Therefore, the assumption that overland runoff likely will 

increase as the water table rises above the extinction 

depth probably is reasonable.

Natural net recharge in a transitional zone is 

defined by combining equations 11 and 12:

. (13)

Therefore, natural net recharge always is less in a transitional zone than in an upland zone. The quantity in 

brackets in equation 13 represents the combined 

effects of excess ET and excess overland runoff; this 

quantity was simulated by using the MODFLOW 

Evapotranspiration Package. The important premise is 

that the sum of excess ET and excess overland runoff 

can be approximated as a linear function of water-table 

altitude; however, either excess ET or excess overland 

runoff may not individually follow this linear relation. 

This approximation is believed to be reasonable 

because both excess ET and excess overland runoff are 

assumed to be proportional to water-table altitude. The 

validity of this approximation was tested during model 

calibration. Substituting the mathematical function 

used by the Evapotranspiration Package (McDonald 

and Harbaugh, 1988) for the term [ETn,ex + Osw,ex] of 

equation 13 yields the following expression for natural 

net recharge in a transitional zone:

, (14)

where

Rex,max is maximum rate of combined excess ET and 

excess overland runoff, [L/T];

zls is mean land-surface altitude in model cell, 

[L];

h is simulated water-table altitude, [L]; and

De is extinction depth, [L].

The minimum rate of natural net recharge (Nn,min) 

occurs in a transitional zone when the simulated watertable is above land surface:

. (15)

Nn,max of equations 14 and 15 is identical to that specified for an upland zone; therefore, Nn,max was calculated using equation 12 for model cells in transitional 

zones as well as upland zones and was simulated using 

the MODFLOW Recharge Package. The initial value 

of the maximum rate of excess ET was estimated to be 

24 in/yr (potential ET of 51 in/yr based on 1998 pan 

evaporation data minus ETn,min of 27 in/yr). The maximum rate of excess overland runoff is unknown; its initial value was assumed to be equal to the average rate 

of total overland runoff of 7 in/yr (RO, eq. 1 and fig. 

23). Therefore, the initial value of Rex,max was specified 

to be a uniform value of 31 in/yr in all model cells. The 

altitude at which the maximum rate of ET was simulated by the Evapotranspiration Package was set equal 

to mean land-surface altitude in each model cell (zls, 

eq. 14). Mean land-surface altitude was determined 

from a digital elevation model interpolated from 5-ft 

topographic contours from USGS 7.5-minute quadrangles. Therefore, in a transitional zone initial values of 

natural net recharge varied from -1 to 30 in/yr, depending on the position of the simulated water table.

Figure 29 graphically depicts how equations 12, 

14, and 15 and the Recharge and Evapotranspiration 

Packages were used to simulate natural net recharge at 

the water table. Where the simulated water table is 

above land surface, natural net recharge is at its minimum value (eq. 15); where the simulated water table is 

below the extinction depth, natural net recharge is at 

its maximum value (eq. 12); and between these two 

extremes, natural net recharge varies linearly (eq. 14). 

Using the combined fluxes simulated by the Recharge 

and Evapotranspiration Packages to simulate natural 

net recharge eliminates the need to delineate upland 

and transitional zones. Whether a model cell is in an 

upland or transitional zone depends only on the simulated position of the water table relative to the specified extinction depth. Furthermore, this allows the 

model to simulate possible future changes in natural 

net recharge that might occur as a result of a change in 

stress, such as future increases in ground-water withdrawals. In addition, changes in the spatial distribution 

of natural net recharge can be easily effected by 

adjusting only three parameters in the model: Nn,max, 

Rex,max, and De (fig. 29).

Nn Nn max , ETn ex , Osn ex , = – [ ] +

Nn Nn max , Rex max , 1

zls – h

De

= – – --------------

zls De h zls ( ) – ≤ ≤ ,

Nn min , Nn max , Rex max , = –
Simulation of Ground-Water Flow 55

Nn Nn,max - Rex, max = 1 zls - h

De

Nn, min Nn,max - Rex, max =

Nn,max P - ETn,min + Osn,min - Saq = 

TRANSITIONAL ZONE

Recharge and Evapotranspiration

Packages active

UPLAND ZONE

Recharge Package active

zls - De

zls

SIMULATED WATER-TABLE ALTITUDE

Nn, min Nn,max

SIMULATED NATURAL NET RECHARGE

EXPLANATION

De

ETn,min

h

Nn

Nn,max

Nn, min

Osn,min

P

Rex, max

Saq

zls



Extinction depth

Minimum natural evapotranspiration (that is, evapotranspiration in the

absence of artificial recharge)

Simulated water-table altitude

Natural net recharge

Maximum natural net recharge

Minimum natural net recharge

Minimum natural overland runoff (that is, overland runoff in the absence of

artificial recharge) to a surface-water body other than a closed-basin lake

Precipitation

Maximum rate of combined excess evapotranspiration and excess overland runoff

Change in storage in the aquifer in which the water table is located

Mean land surface altitude in model cell

Figure 29. Simulated natural net recharge at the water table as a function of simulated water-table altitude.
56 Hydrogeology and Simulated Effects of Ground-Water Withdrawals from the Floridan Aquifer System in Lake County and in 

the Ocala National Forest and Vicinity, North-Central Florida

Net recharge was specified at closed-basin lakes 

(fig. 27) because water levels in these lakes were simulated by the model; net recharge was specified at zero 

at all flow-through lakes (fig. 27) because these lakes 

were simulated by internal boundary conditions. Net 

recharge at a lake is equivalent to natural net recharge 

because artificial recharge is assumed to be zero at 

lakes. Net recharge at a closed-basin lake (Ncbl) can be 

defined as follows (based on eq. 6):

, (16)

where

El is lake evaporation, [L/T]; and

∆Scbl is change in closed-basin lake storage, [L/T].

Because closed-basin lakes generally are located in 

areas where the surficial sands are permeable, the infiltration capacity of the soil probably is relatively great 

and overland runoff probably is correspondingly 

small. Therefore, overland runoff to closed-basin 

lakes was assumed to be very small and was set equal 

to zero. Annual lake evaporation in central Florida 

often has been reported to be slightly less than annual 

precipitation and is commonly based on pan-evaporation data (Lichtler and others, 1968; Knochenmus and 

Hughes, 1976; O’Reilly, 1998). Farnsworth and others 

(1982) reported a value of 48 in. for annual free-water 

surface evaporation in peninsular Florida, which 

closely approximates lake evaporation from a shallow 

lake with negligible heat storage. A study by Swancar 

and others (2000), however, indicates that annual lake 

evaporation for a closed-basin lake can actually 

exceed annual precipitation. Swancar and others 

(2000) reported that annual lake evaporation (measured by the energy-budget method from August 1996 

through July 1998) at Lake Starr, a 134-acre closedbasin lake in central Polk County, exceeded annual 

precipitation by 13 percent the first year (El

 = 57.08 in. 

and P = 50.68 in.) and 3 percent the second year 

(El = 55.88 in. and P = 54.04 in.). Actual measurements of lake evaporation were not available for any 

lakes in the model area in 1998; consequently, lake 

evaporation was assumed to be equal to potential ET 

(51 in/yr) estimated from 1998 pan-evaporation data. 

The change in closed-basin lake storage was estimated to be 5 in/yr, which was the average change in 

lake level measured at 90 lakes in the model area. 

Therefore, net recharge at closed-basin lakes initially 

was specified to be 1 in/yr (P = 57, El

 = 51, and 

∆Scbl = 5 in/yr) and was simulated using the MODFLOW Recharge Package.

Artificial recharge applied above the water table 

consisted of land application of reclaimed water from 

wastewater treatment plants and effluent leakage from 

septic tanks. The fraction of artificial recharge that 

actually percolates to the water table is referred to as 

artificial net recharge. Artificial net recharge (Na) can 

be defined based on equation 6 and noting that P and 

Ocbl are zero, and assuming that artificial recharge 

causes a negligible change in aquifer storage:

, (17)

where

Ra is artificial recharge, [L/T];

ETa is evapotranspiration extracted from artificial 

recharge before the water percolates to the 

water table, [L/T]; and

Osa is overland runoff to a surface-water body as a 

result of artificial recharge, [L/T]; overland 

runoff to a closed-basin lake as a result of 

artificial recharge is assumed to be zero.

Reclaimed water can be applied at the land surface by 

rapid infiltration basins, spray fields, or some other 

form of irrigation (agricultural, golf course, or landscape irrigation). Artificial recharge rates were based 

on 1998 gaged or estimated reclaimed water flow rates 

(D. Boniol, St. Johns River Water Management 

District, written commun., 2000). Artificial net 

recharge from rapid infiltration basins was assumed to 

be 100 percent of the gaged or estimated flow to the 

basin. Because the basins are small in area and generally have a high infiltration capacity, it is reasonable to 

assume a negligible amount of water is lost to ET; 

also, the nature of basin design and operation ensures 

that no water is lost to overland runoff. The other 

forms of reclaimed water application involve irrigation 

over relatively large areas. As a result of the increase 

in available water from irrigation, ET will be greater 

than ETn (eq. 8) in irrigated areas, resulting in a significant reduction in the amount of water that reaches the 

water table. In addition, overland runoff from irrigation can occur if the application rate exceeds the 

infiltration capacity of the soil. The magnitude of overland runoff is unknown, but can occur as a result of, 

for example, excessive landscape irrigation. Average 

reclaimed water application rates for 1998 at spray 

fields, agricultural crops, golf courses, and landscaped 

Ncbl P Ocbl El – ∆Scbl = + –

Na Ra ETa – Osa = –
Simulation of Ground-Water Flow 57

areas were 84, 20, 32, and 24 in/yr, respectively. 

Because of the relatively small agricultural, golf 

course, and landscape irrigation rates, all of the 

reclaimed water was assumed to be lost to increases in 

ET and overland runoff. However, the large application rates typical at spray fields precluded this assumption, and artificial net recharge at these locations was 

assumed equal to the application rate reduced by the 

quantity of water representing the increase in ET. This 

additional ET resulting from spray-field application of 

reclaimed water (ETa, eq. 17) was assumed to equal 

the difference between potential ET (51 in/yr) and 

minimum ET (27 in/yr), or 24 in/yr. This assumes that 

the spray field was located in an upland zone where 

ET previously was at its minimum value. Likewise, the 

assumption that most agricultural, golf course, and 

landscape irrigation is lost to ETa also is primarily 

based on the assumption that the irrigation occurs in 

an upland zone. If reclaimed water irrigation facilities 

were located in a transitional zone, where ET is 

simulated to exceed its minimum value, ETa should be 

less than 24 in/yr. This possible discrepancy was not 

formally accounted for in the model for three reasons: (1) the delineation of upland and transitional 

zones was based on the simulated depth of the water 

table relative to extinction depth, and assignment of a 

reclaimed water irrigation facility to one zone or the 

other and the calculation of ETa would have to be 

performed in an iterative manner; (2) overland runoff 

could account for some fraction of the discrepancy; 

and (3) the total amount of reclaimed water applied in 

the model area by irrigation is only 0.2 in/yr averaged 

over the model area, which is very small compared to 

other water budget components (fig. 23). Artificial net 

recharge at rapid infiltration basins and spray fields 

was simulated using the MODFLOW Well Package 

(fig. 30). The areal distribution of individual spray 

fields was unknown; therefore, artificial net recharge 

at these locations was evenly distributed over a circular area based on the reported acreage and location of 

each spray field.

Artificial recharge resulting from septic tank 

leakage was estimated from domestic self-supplied 

ground-water withdrawal rates. Artificial recharge 

resulting from septic tank leakage is equivalent to the 

rate of leakage from septic tank drain fields for the following two reasons: (1) ET induced by the additional 

water provided by septic tank drain fields (ETa, eq. 17) 

is assumed to be negligible, because a septic tank drain 

field probably would increase the moisture content of 

the unsaturated zone only in the immediate vicinity of 

the drain field and probably would not significantly 

increase the amount of soil moisture available to be lost 

to ET; and (2) overland runoff (Osa, eq. 17) is zero 

because septic tank drain fields are located below land 

surface. Domestic self-supplied withdrawals represent 

ground-water withdrawn by individual households and 

small commercial, industrial, and public supply systems 

with a daily average pumpage of less than 0.01 Mgal/d 

or fewer than 400 users (Marella, 1999, p. 17). Domestic self-supplied withdrawals and septic tank leakage 

were assumed to occur only in land-use areas classified 

as low- or medium-density residential that were not 

served by a public water supply utility (fig. 31). Domestic self-supplied withdrawal rates generally are compiled by county, and average 1998 withdrawal rates 

for each county in the model area (table 2) were estimated from 1995 data provided by R.L. Marella 

(U.S. Geological Survey, written commun., 2000).

Table 2. Domestic self-supplied ground-water withdrawal 

rates and septic tank leakage rates, average 1998 and 

projected 2020 conditions

[mi2, square miles; Mgal/d, million gallons per day; in/yr, inches per year]

County Area1

(mi2)

1This represents only the portion of the county within the model area 

where domestic self-supplied withdrawals and septic tank leakage are estimated to occur.

Domestic selfsupplied withdrawals2

(Mgal/d)

2This represents only the withdrawals that are estimated to occur in the 

portion of the county within the model area, that is, the county-wide withdrawal rate is greater than this value if the county does not lie completely 

within the model area.

Septic tank 

leakage

(in/yr)

1998 2020 1998 2020

Alachua 5.0 0.65 0.71 1.6 1.8

Hernando 3.3 .51 .76 1.9 2.9

Lake 41 11.2 2.0 3.4 .60

Marion 103 22.3 29.8 2.7 3.6

Orange 5.8 6.7 7.3 15 16

Osceola 1.3 .38 .56 3.6 5.4

Pasco 1.5 .33 .38 2.9 3.3

Polk .67 .10 .13 1.9 2.4

Putnam 35 6.9 6.2 2.5 2.2

Seminole .00095 .0010 .00066 13 8.8

Sumter 15 3.8 5.7 3.2 4.7

Vousia 5.1 2.0 2.0 4.9 5.0
58 Hydrogeology and Simulated Effects of Ground-Water Withdrawals from the Floridan Aquifer System in Lake County and in 

the Ocala National Forest and Vicinity, North-Central Florida

82°20′ 82°00′ 81°10′

29°40′

29°00′

28°20′

0

0

5 10 MILES

5 10 KILOMETERS

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

Universal Transverse Mercator projection, zone 17

LAKE

COUNTY

ORANGE COUNTY

SEMINOLE

COUNTY

VOLUSIA

COUNTY

OSCEOLA COUNTY

POLK COUNTY

PASCO

COUNTY

HERNANDO

COUNTY

SUMTER

COUNTY

CITRUS

COUNTY

MARION COUNTY

ALACHUA COUNTY

PUTNAM COUNTY

ST. JOHNS COUNTY

FLAGLER

COUNTY

MODEL

BOUNDARY

EXPLANATION

ARTIFICIAL

NET RECHARGE RATE,

IN INCHES PER YEAR

0 to 10

10 to 25

25 to 75

Greater than 75

OCALA

NATIONAL

FOREST

BOUNDARY

Figure 30. Artificial net recharge rates from rapid infiltration basins and spray fields, average 1998 conditions.
Simulation of Ground-Water Flow 59

82°20′ 82°00′ 81°10′

29°40′

29°00′

28°20′

0

0

5 10 MILES

5 10 KILOMETERS

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

Universal Transverse Mercator projection, zone 17

LAKE

COUNTY

ORANGE COUNTY

SEMINOLE

COUNTY

VOLUSIA

COUNTY

OSCEOLA COUNTY

POLK COUNTY

PASCO

COUNTY

HERNANDO

COUNTY

SUMTER

COUNTY

CITRUS

COUNTY

MARION COUNTY

ALACHUA COUNTY

PUTNAM COUNTY

ST. JOHNS COUNTY

FLAGLER

COUNTY

MODEL

BOUNDARY

EXPLANATION

AREA OF SEPTIC TANK LEAKAGE

AND DOMESTIC SELF-SUPPLIED

GROUND-WATER WITHDRAWALS

OCALA

NATIONAL

FOREST

BOUNDARY

Figure 31. Areas in which septic tank leakage and domestic self-supplied ground-water withdrawals from 

the Upper Floridan aquifer were estimated to occur (septic tank leakage rates and domestic self-supplied 

ground-water withdrawal rates are listed in table 2).
60 Hydrogeology and Simulated Effects of Ground-Water Withdrawals from the Floridan Aquifer System in Lake County and in 

the Ocala National Forest and Vicinity, North-Central Florida

Septic tank leakage as a percentage of domestic selfsupplied withdrawals was estimated to range from 

33 to 140 percent, with an average of approximately 

60 percent, for the counties in the model area 

(R.L. Marella, U.S. Geological Survey, written commun., 2000). Because of the difficulty in accurately 

estimating these data, average 1998 septic tank leakage for each county was assumed to equal 60 percent 

of the respective domestic self-supplied withdrawal 

rate. Using these estimated septic tank leakage rates, a 

flux was calculated based on the area of each county 

where domestic self-supplied withdrawals and septic 

tank leakage were assumed to occur (fig. 31). This 

yields an average flux representing the artificial 

recharge resulting from septic tank leakage for each 

county (table 2). Artificial recharge resulting from septic tank leakage was simulated using the MODFLOW 

Well Package.

Streams and Lakes

Leakage of water from the ground-water system 

to streams and lakes, or from streams and lakes to the 

ground-water system (figs. 26 and 27), was simulated 

by using the MODFLOW River Package. Only flowthrough lakes were simulated by using the River Package. Closed-basin lakes were simulated as variablehead cells. Streams and lakes simulated with the River 

Package were in hydraulic connection with the SAS, 

except where the SAS was inactive (fig. 24); streams 

and lakes in such areas were simulated to be in hydraulic connection with the UFA. Flow between an aquifer 

and a stream or lake is simulated by the River Package 

using the following equations (McDonald and Harbaugh, 1988): 

, (18)

, (19)

, (20)

where

Qriv is volumetric flow rate simulated by the River 

Package, [L3

/T];

Criv is conductance of sediments constituting the 

riverbed specified in the River Package, 

[L2/T];

Hriv is stream or lake water-level altitude specified 

in the River Package, [L2

/T];

h is simulated water-table altitude, [L];

zbot is altitude of the bottom of sediments constituting the riverbed specified in the River Package, [L];

Kv,riv is vertical hydraulic conductivity of sediments 

constituting the riverbed, [L/T];

A is area of sediments constituting the riverbed, 

[L2

]; and

m is thickness of sediments constituting the riverbed, [L].

Under typical conditions, the flow between an aquifer 

and a stream or lake is dependent on the head difference between the aquifer and the stream or lake 

(eq. 18). However, if the simulated water table is 

below the bottom of the streambed or lakebed, then 

flow between an aquifer and a stream or lake is no 

longer dependent on the altitude of the simulated water 

table: a constant flow to the aquifer is simulated 

because an unsaturated zone now exists under the 

stream or lake (eq. 19). Gaged streams and lakes were 

treated slightly differently than those that were 

ungaged because additional data existed to better simulate the gaged stream or lake.

For gaged streams, streambed conductance 

(eq. 20) was based on an estimated vertical hydraulic 

conductivity for streambed material of 0.3 ft/d, the 

product of streambed width measured during discharge measurements and total stream length in the 

model cell (A, eq. 20), and an assumed streambed 

thickness of 1 ft. No measurements of the vertical 

hydraulic conductivity of streambed material were 

available and the spatial variability is unknown; therefore, only a single value of vertical hydraulic conductivity was used. The initial value of 0.3 ft/d was 

calculated as one-half the vertical hydraulic conductivity of the SAS (horizontal hydraulic conductivity of 

6 ft/d and vertical anisotropy of 10). Streambed 

conductance for gaged streams was adjusted during 

model calibration by varying Kv,riv (eq. 20). Values of 

stream water level, streambed width, and streambed 

bottom altitude were assigned by linear interpolation 

based on measured values at gaging stations using the 

RIVGRID program developed by Leake and Claar 

(1999).

Qriv Criv Hriv ( ) – h h zbot = , >

Qriv Criv Hriv zbot ( ) – h zbot = , ≤

Criv

Kv riv , A

m = -----------------
Simulation of Ground-Water Flow 61

No data existed for ungaged streams except their 

locations, which were obtained from USGS 

1:100,000-scale topographic quadrangle maps. Each 

ungaged stream generally is relatively small. Considered collectively, however, ungaged streams can have a 

significant impact on the ground-water system because 

they are a significant surface-water feature in many 

parts of the model area (fig. 26). Whether all ungaged 

streams perennially contain water is not known; in particular, some small streams and the headwaters of 

some streams may have been dry during 1998. Therefore, streambed bottom altitude was set equal to 

stream water level so the river nodes representing 

ungaged streams would never be recharging the SAS 

or UFA (Qriv equals zero when the simulated water 

table is less than or equal to streambed bottom altitude, 

eq. 19). Ungaged stream water level was estimated by 

adding 3 ft to the median land-surface altitude along 

the stream. Median land-surface altitude along each 

stream in each model cell was obtained by using a geographic information system to: (1) delineate a 200-ft 

buffer polygon around all ungaged streams; (2) intersect this polygon with a digital elevation model; and 

(3) calculate the median land-surface altitude of all 

digital elevation model nodes that fell within the buffer 

polygon for each model cell. Because little is known 

about ungaged streams, the stream water level and 

streambed bottom altitude were specified at values 

such that the river nodes representing ungaged streams 

do not become active until the simulated water table is 

well above (3 ft) the likely stream water level. At this 

higher water level, ungaged streams probably are more 

likely to contain water and flow. Streambed conductance was based on an initial streambed material vertical hydraulic conductivity of 0.3 ft/d, the area of the 

200-ft buffer polygon contained in the model cell, and 

an assumed streambed thickness of 1 ft. Streambed 

conductance for ungaged streams was adjusted during 

model calibration by varying Kv,riv (eq. 20).

For flow-through lakes, lakebed conductance 

was based on an initial lakebed material vertical 

hydraulic conductivity of 0.3 ft/d, the area of the lake 

contained in the model cell, and an assumed lakebed 

thickness of 1 ft. Lakebed conductance was adjusted 

during model calibration by varying Kv,riv (eq. 20). 

Lake stage was obtained from field measurements for 

gaged flow-through lakes. The water level for each 

ungaged flow-through lake was estimated from a digital elevation model to be the median value of all digital 

elevation model nodes that fell within the shoreline of 

the lake. The water level that had been assigned to 

lakes in the digital elevation model was equal to the 

lowest interpolated land-surface altitude along the lake 

shore; this approximately represents the lake water 

level depicted on the USGS 7.5-minute topographic 

quadrangles. A comparison of water levels estimated 

from the digital elevation model with water levels 

obtained from field measurements for 27 flow-through 

lakes indicated that the digital elevation model provided a reasonably accurate estimate of lake water 

level: water levels estimated from the digital elevation 

model averaged 0.7 ft less than gaged lake water levels 

and varied from 2.2 ft less than to 3.2 ft greater than 

gaged lake water level. Most lakes in the model area 

are relatively shallow. Danek and others (1991) 

reported average depths from 7 to 19 ft for Lakes Harris, Beauclair, Dora, Eustis, Yale, Griffin, and Weir. 

Lakebed bottom altitude was uniformly specified to be 

15 ft below the lake water level to reflect the relatively 

shallow depth assumed for all lakes in the model area.

Wetlands

The numerous wetlands within the model area 

typically have perennial water levels that are above or 

very near land surface. Many wetlands are adjacent to 

streams; consequently, ground-water discharge to wetlands can augment stream flow if the water level in the 

wetland rises above land surface and overland flow to 

the stream occurs. Because of this important groundwater and surface-water interaction, wetlands were 

included in the model (fig. 27) and were simulated 

using the MODFLOW River Package (eqs. 18, 19, and 

20). Wetlands were simulated to be in hydraulic connection with the SAS, except where the SAS was inactive (fig. 24); wetlands in these areas were simulated to 

be in hydraulic connection with the UFA. Only wetlands that occupied greater than 50 percent of a model 

cell were included. Conductance of the sediments representing the bed materials of the wetland was based 

on an initial vertical hydraulic conductivity of 0.3 ft/d, 

the area of the wetland contained in the model cell, 

and a thickness of 1 ft. Conductance was adjusted during model calibration by varying Kv,riv (eq. 20). The 

water level in the wetland was estimated using the digital elevation model and adding 3 ft to the median 

land-surface altitude of the portion of the wetland in 

each model cell. This water level was used as the river 
62 Hydrogeology and Simulated Effects of Ground-Water Withdrawals from the Floridan Aquifer System in Lake County and in 

the Ocala National Forest and Vicinity, North-Central Florida

node stage. The altitude of the riverbed bottom was set 

equal to the river node stage so the river nodes representing wetlands would never be recharging the SAS 

or UFA (Qriv equals zero when the simulated water 

table is less than or equal to riverbed bottom altitude, 

eq. 19). Using this formulation means that river nodes 

representing wetlands are inactive except where the 

simulated water table is greater than 3 ft above the 

median land-surface altitude in the wetland. The 

important assumptions are (1) on an average annual 

basis, a wetland will receive a net gain of water from 

the ground-water system; and (2) if the water level in a 

wetland were 3 ft deep, overland flow to nearby 

streams would occur, representing a loss of water from 

the ground-water system. These assumptions probably 

are not always valid, but the use of river nodes to represent wetlands as described above is a reasonable 

approximation and preferable to not explicitly simulating any ground-water interaction with wetlands.

Springs

All 65 UFA springs documented for this study 

were included in the model (fig. 7 and table 1), representing a total springflow of 1,279 Mgal/d (about 

5.6 in/yr averaged over the model area). During model 

calibration each spring was simulated with a single 

UFA well discharging an amount of water equal to the 

measured or estimated 1998 springflow (table 1). The 

flow from some springs is derived in part from zones 

of saline water in the UFA. Some of these zones might 

contain water with chloride concentrations exceeding 

5,000 mg/L; consequently, some fraction of the springflow would be derived from the saline water flow system, which is not simulated by the model. However, no 

adjustments were made to the springflows specified in 

the model to account for this possibility because of the 

lack of data available to make this determination. The 

MODFLOW Drain Package was used to simulate the 

springs in the final calibrated model. This allowed 

simulated springflow to vary as the head difference 

between the specified spring pool altitude and the 

simulated UFA water level changed during predictive 

simulations.

Based on the assumption that the hydraulic 

properties of the FAS are constant and that spring pool 

altitude will change relatively little over time, impacts 

on springflow from future stresses on the groundwater system primarily are a function of changes in the 

UFA water level at the spring. The change in flow at a 

spring simulated by the Drain Package is largely 

dependent on the initial simulated head difference at 

the spring. For example, if the actual head difference 

at a spring is 10 ft and the simulated head difference is 

5 ft, a simulated 1 ft drawdown at the spring would 

result in a 20 percent decrease in simulated springflow. 

However, if the initial simulated head difference were 

equal to its actual value of 10 ft, the simulated 

decrease in springflow would be only 10 percent for 

the same 1 ft drawdown. Considering this reasoning 

when converting spring “wells” to “drains,” a drain 

elevation (representing an adjusted spring pool altitude) was calculated so that the simulated 1998 head 

difference was equal to the measured or estimated 

head difference. The head difference at each spring 

was calculated as the altitude of the average UFA 

water level in the model cell containing the spring 

(interpolated from May 1998 (fig. 2) and September 

1998 (Bradner, 1999) potentiometric maps) minus the 

measured or estimated 1998 spring pool altitude. 

Spring pool altitudes measured during 1998 were 

available for 18 springs, representing 83 percent of 

total spring flow; spring pool altitudes for the remaining 47 springs were estimated from land-surface altitudes obtained from USGS 7.5-minute topographic 

quadrangles. The spring conductance was calculated 

as the 1998 spring discharge divided by the head difference. This formulation acknowledges the fact that 

in any calibration effort discrepancies exist between 

simulated and measured water levels. By forcing the 

simulated head difference at each spring to equal its 

measured or estimated value, a change in springflow 

from 1998 conditions as a result of projected future 

conditions will not be affected by an incorrectly simulated 1998 head difference.

Pumping and Drainage Wells

Average 1998 ground-water withdrawals for the 

FAS from pumping wells within the model area were 

estimated to total approximately 470 Mgal/d (about 

2.1 in/yr averaged over the model area). This total 

included water used for public supply, agriculture, 

commercial or industrial purposes, recreation, and 

domestic self-supply. Ground-water withdrawals from 

the SAS, if occurring, are small and were not included 
Simulation of Ground-Water Flow 63

in the model. Pumping rates for 1998 were available 

for wells in the SWFWMD (Jim Waylon, Southwest 

Florida Water Management District, written commun., 

1999); however, 1998 rates were not available for 

many wells in the SJRWMD and the South Florida 

Water Management District (SFWMD), and, therefore, 

were estimated based on 1995 rates obtained from 

Brian McGurk (St. Johns River Water Management 

District, written commun., 2000) and L.H. Motz (University of Florida, written commun., 1999). Countywide average pumping rates for 1998 were acquired 

from R.L. Marella (U.S. Geological Survey, written 

commun., 2000), and were used to estimate 1998 

pumping rates for individual wells by using the percent change in the county-wide average pumping rate 

from 1995 to 1998 (for wells with reported rates) for 

each water-use category. Estimated 1998 pumping 

rates for individual wells were calculated so that the 

total pumpage in each county, including wells with 

reported 1998 pumping rates, equaled the 1998 

county-wide pumpage provided by R.L. Marella (U.S. 

Geological Survey, written commun., 2000). Reported 

1998 pumping rates represented approximately 35 percent of the total pumpage in the model area; the 

remaining 65 percent of the total pumpage was estimated from 1995 data. Well location and construction 

data were obtained from Brian McGurk (St. Johns 

River Water Management District, written commun., 

2000), Jim Waylon (Southwest Florida Water Management District, written commun., 1999), and L.H. Motz 

(University of Florida, written commun., 1999), with 

the exception of domestic self-supply wells.

Where available, total well depth and well 

casing depth were used in combination with hydrogeologic maps to determine whether a well was withdrawing water from either the UFA or LFA or both. Where 

no well construction data were available, 100 percent 

of the well pumpage was assigned to the UFA. Where 

well construction data indicated a well was withdrawing water from both the UFA and LFA, total well 

pumpage was divided between the two aquifers based 

on the thickness of each aquifer open to the well and 

an assumed ratio of UFA to LFA horizontal hydraulic 

conductivity of 0.5. The ratio of 0.5 is based on an 

average transmissivity of 80,000 ft2

/d for the UFA and 

500,000 ft2

/d for the LFA, based on results of aquifer 

performance tests reported by Szell (1993) and 

Spechler and Halford (2001, p. 24 and 26), and an 

average aquifer thickness of 400 ft for the UFA and 

1,300 ft for the LFA, calculated from aquifer and confining unit base altitudes specified in the model. Very 

few wells withdraw water from both the UFA and 

LFA, and pumping from these wells represents less 

than 3 percent of total ground-water withdrawals in 

the model; therefore, the error resulting from this 

approximation probably has little impact on model 

results. The final distribution of ground-water withdrawal rates, excluding domestic self-supplied withdrawals, for average 1998 conditions used in the 

model for the UFA and LFA are shown in figures 32 

and 33, respectively.

Domestic self-supplied ground-water withdrawals were based on estimated 1998 rates (table 2) and 

distributed evenly county-by-county over low- and 

medium-density residential land-use areas that were 

not served by a public water supply utility (fig. 31). 

The 1998 estimate of domestic self-supplied water use 

was 20 percent greater than the 1995 values provided 

by R.L. Marella (U.S. Geological Survey, written commun., 2000), and was based on the 20 percent increase 

in total public-supply water-use change over the same 

time period (1995-1998). All domestic self-supply 

wells were assumed to be withdrawing from only the 

UFA. All pumping wells were simulated using the 

MODFLOW Well Package.

Average 1998 recharge rates from drainage 

wells within the model area were estimated to total 

approximately 13 Mgal/d (about 0.06 in/yr averaged 

over the model area). Average drainage-well recharge 

rates for a particular period of time are strongly correlated to the amount of precipitation during that period 

(Sepúlveda, 2002, p. 34). Sepúlveda (2002, p. 34-35) 

estimated average drainage-well recharge rates for the 

period August 1993 through July 1994 based on a reanalysis of data reported by CH2M Hill (1997) and 

Bradner (1996) for wells in the Orlando metropolitan 

area, and extrapolated these data to wells outside the 

Orlando metropolitan area. Estimated drainage-well 

recharge rates specified in the present model (fig. 25) 

were based on those reported by Sepúlveda (2002, 

p. 34-35), but those estimates were adjusted to reflect 

1998 conditions by multiplying by the ratio of 

precipitation during 1998 to that during the period 

August 1993 through July 1994. All drainage wells 

were simulated using the MODFLOW Well Package.
64 Hydrogeology and Simulated Effects of Ground-Water Withdrawals from the Floridan Aquifer System in Lake County and in 

the Ocala National Forest and Vicinity, North-Central Florida

82°20′ 82°00′ 81°10′

29°40′

29°00′

28°20′

0

0

5 10 MILES

5 10 KILOMETERS

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

Universal Transverse Mercator projection, zone 17

LAKE

COUNTY

ORANGE COUNTY

SEMINOLE

COUNTY

VOLUSIA

COUNTY

OSCEOLA COUNTY

POLK COUNTY

PASCO

COUNTY

HERNANDO

COUNTY

SUMTER

COUNTY

CITRUS

COUNTY

MARION COUNTY

ALACHUA COUNTY

PUTNAM COUNTY

ST. JOHNS COUNTY

FLAGLER

COUNTY

MODEL

BOUNDARY

EXPLANATION

UPPER FLORIDAN

AQUIFER PUMPAGE,

IN MILLION GALLONS

PER DAY

0 to 0.1

0.1 to 0.25

0.25 to 0.75

0.75 to 2

Greater than 2

OCALA NATIONAL

FOREST

BOUNDARY

Figure 32. Ground-water withdrawal rates for the Upper Floridan aquifer specified in the model, average 

1998 conditions.
Simulation of Ground-Water Flow 65

82°20′ 82°00′ 81°10′

29°40′

29°00′

28°20′

0

0

5 10 MILES

5 10 KILOMETERS

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

Universal Transverse Mercator projection, zone 17

LAKE

COUNTY

ORANGE COUNTY

SEMINOLE

COUNTY

VOLUSIA

COUNTY

OSCEOLA COUNTY

POLK COUNTY

PASCO

COUNTY

HERNANDO

COUNTY

SUMTER

COUNTY

CITRUS

COUNTY

MARION COUNTY

ALACHUA COUNTY

PUTNAM COUNTY

ST. JOHNS COUNTY

FLAGLER

COUNTY

MODEL

BOUNDARY

EXPLANATION

LOWER FLORIDAN

AQUIFER PUMPAGE,

IN MILLION GALLONS

PER DAY

0 to 0.1

0.1 to 0.25

0.25 to 0.75

0.75 to 2

Greater than 2 OCALA

NATIONAL

FOREST

BOUNDARY

Figure 33. Ground-water withdrawal rates for the Lower Floridan aquifer specified in the model, average 

1998 conditions.
66 Hydrogeology and Simulated Effects of Ground-Water Withdrawals from the Floridan Aquifer System in Lake County and in 

the Ocala National Forest and Vicinity, North-Central Florida

Calibration

Calibration is the attempt to reduce the difference between model results and observed data by 

adjusting model hydrologic parameters within reasonable ranges. In MODFLOW, a parameter is defined as 

“a single value that is given a name and determines the 

value of a variable used in the finite-difference 

ground-water flow equation at one or more model 

cells” (Harbaugh and others, 2000, p. 4). For the 

purposes of the present model, the numerical value of 

a parameter is defined as a “parameter value;” the 

model cell or group of model cells to which the parameter applies is defined as a “parameter zone;” and the 

areal distribution of parameter zones for all parameters 

is defined as the “parameter zonation” of the model. 

Simulated values usually depart from observed values, 

even after a diligent calibration effort. The differences 

between model results and observations usually result 

from the simplifications inherent in the conceptual 

model, grid scale, and the difficulty in obtaining 

sufficient measurements to account for all of the 

spatial variation in hydrologic properties and stresses 

throughout the model area.

The steady-state ground-water flow model was 

calibrated to water-level and flow data collected from 

December 1997 to December 1998 (average 1998 

conditions). The rationale for developing the steadystate calibration to average 1998 conditions was as 

follows: model results will be used to ascertain the 

long-term hydrologic effects of future ground-water 

withdrawals, not the relatively short-term transient 

variations that may occur as a result of, for example, 

a period of below-average precipitation. The SAS is 

strongly influenced by temporal variations in precipitation, ET, and the movement of water in the unsaturated zone. However, sufficient data do not exist to 

simulate unsaturated-zone hydraulics on a regional 

scale. In addition, a transient simulation would have 

introduced at least three more variables, SAS, UFA, 

and LFA storage coefficients, which are not well 

known. December 1997 to December 1998 was the 

longest period during the 16 months of data collection 

for this study that the SAS and FAS were considered 

reasonably near steady-state based on measured water 

levels. Calibration to historical data (that is, data 

collected prior to the data-collection period for this 

study) was not feasible because relatively few data 

existed prior to this study, especially for the SAS. 

Finally, the bimonthly frequency of data collection 

during the study period was sufficient for calculation 

of representative annual average values.

Inverse Model

Model calibration was facilitated by using the 

Observation, Sensitivity, and Parameter Estimation 

Processes of MODFLOW (Hill and others, 2000). 

These capabilities of MODFLOW, which use nonlinear, least-squares regression and associated statistics, 

constitute the inverse model that was used to automatically calculate parameter values that produce the 

smallest differences, or residuals, between observed 

water levels and flows, or observations, and simulated 

water levels and flows. The fit between observed and 

simulated water levels and flows is quantified by the 

following weighted least-squares objective function 

(Hill, 1998, p. 4), the value of which is referred to as 

the sum of squared, weighted residuals (SSWR):

, (21)

where

yk is kth observed water level, [L], or flow, [L3/T]; 

is kth simulated water level, [L], or flow, 

[L3/T];

wk is the weight for the kth water level, [L-2], or 

flow, [(L3

/T)-2]; and

no is the number of water-level and flow observations.

The weights in equation 21 account for measurement 

errors incurred in acquiring the observation in the field 

and normalize the residuals to account for the difference in units between water-level and flow observations. The best fit between observed and simulated 

values, and therefore, the set of optimal parameter 

values, is achieved by minimizing the objective function (eq. 21). The minimum of the objective function 

generally is considered to be attained at the iteration 

when the greatest change in any parameter value is 

less than 1 percent (Hill and others, 2000, p. 79); 

this was the convergence criterion used in the present 

model.

Parameter values estimated by an inverse model 

are optimal only for the given conceptual model, 

parameter zonation, and observations. That is, different conceptual models, parameter zonations, or observations could yield a different set of optimal parameter 

values. Observations generally will not change for a 

given calibration effort once errors (for example, a 

water-level observation assigned to the incorrect 

SSWR wk yk yˆ

k ( ) – 2

k = 1

no

= ∑

yˆ

k
Simulation of Ground-Water Flow 67

aquifer) have been identified and resolved. Therefore, 

the primary focus of model calibration with an inverse 

model is to identify the conceptual model and parameter zonation that best match the available observations. 

A statistic that can be used to compare various conceptual models and parameter zonations is the standard 

error of the regression (SE):

, (22)

where np is number of estimated parameters and the 

other variables are as defined in equation 21. A smaller 

value of SE is preferred because it generally indicates 

a model that both more closely matches the observations and has fewer parameters. Other qualities of a 

more accurate model include: reasonable parameter 

values; narrower confidence intervals on the estimated 

parameters; weighted residuals that are independent 

and normally distributed; and weighted residuals that 

exhibit no consistent spatial patterns or trends (Hill, 

1998, p. 49 and 53). An inverse model can provide the 

statistics to make these evaluations. Testing alternative 

conceptual models and parameter zonations is a trialand-error process that can be significantly guided by 

the use of an inverse model and its related statistics, 

but this process must be constrained by more qualitative hydrologic knowledge and actual field data. Additional details on the theory and application of inverse 

modeling to ground-water flow problems are presented by Carrera and Neuman (1986a, 1986b, 1986c), 

Yeh (1986), Cooley and Naff (1990), Poeter and Hill 

(1997), Hill and others (1998), and Hill (1998).

Observations

The inverse model included water-level observations from 54 closed-basin lakes (assumed to be representative of the SAS water table), 79 SAS wells, 

251 UFA wells, and 20 LFA wells. Multiple measurements for a well or lake were reduced to a single representative 1998 water-level measurement by calculating 

an arithmetic average.

The inverse model also included one flow observation representing the total ground-water discharge, 

or base flow, (excluding spring discharge) to all 

streams and all lakes and wetlands that drain to 

streams in the model area. This quantity was independently estimated for the entire model area by examining 1998 discharge records of gaging stations located 

where major streams entered or exited the model area. 

Base flow was estimated by applying a 30-day sliding 

minimum to the total gaged discharge record; this is 

similar to the sliding-interval method described by 

Sloto and Crouse (1996), except the width of the 

sliding interval used by Sloto and Crouse (1996) is 

dependent on stream drainage area. This analysis 

resulted in an estimate of approximately 7.2 in/yr for 

overland runoff and 11.4 in/yr for base flow. Of this 

11.4 in/yr, 5.6 in/yr is springflow, which leaves 

5.8 in/yr (1.78 x 108 ft3/d) of base flow that was used 

as an observation. This method of estimating base flow 

is simple and may contain considerable error, but it is 

believed to be a reasonable approximation considering 

the large degree of uncertainty inherent in all streamflow hydrograph-separation techniques. Measured or 

estimated springflows were not included in the inverse 

model as observations because the conductance term is 

poorly known and typically has to be individually 

estimated for each spring. In addition, most large 

springs are believed to have reliable flow measurements for 1998 (90 percent of total springflow was 

based on measurements made in 1998 as shown in 

table 1); therefore, specifying these flows with the 

MODFLOW Well Package is less problematic than if 

considerable error were known to exist in these 

measurements.

The weight (wk of equation 21) assigned to each 

observation was equal to the inverse of the variance of 

the estimated measurement error (Hill, 1998, p. 45). 

The measurement errors for each observation were 

assumed to be normally distributed. For water-level 

observations, it was assumed that there was a 95-percent probability that the true water level was within 

+2 ft of the observed water level; this yields a standard 

deviation of measurement error of 1 ft and a weight of 

1 ft-2. For the flow observation, it was assumed that 

there was a 95-percent probability that the true base 

flow was within +20 percent of the estimated base 

flow; this yields a coefficient of variation of measurement error of 10 percent and a weight of approximately 3.1x10-15 (ft3

/d)-2. The large error specified for 

the flow observation acknowledges the large uncertainty in this observation. Assignment of weights is 

subjective, although Hill (1998, p. 48) states that the 

method described above generally works well in practice. In addition, the inverse model was relatively 

insensitive to the weights used; for example, increasing the weights for all water-level observations to 4 ft-2

resulted in relatively little change in the final parameter values estimated by the inverse model.

SE SSWR

no np – = ------------------
68 Hydrogeology and Simulated Effects of Ground-Water Withdrawals from the Floridan Aquifer System in Lake County and in 

the Ocala National Forest and Vicinity, North-Central Florida

Calibration Procedure

A three-step iterative procedure was used to 

calibrate the model:

1. Adjust parameter values to minimize the differences 

between observed data and the corresponding 

simulated values for a given conceptual model and 

parameter zonation. For example, change net 

recharge to better match observed water levels in 

an aquifer.

2. Adjust the parameter zonation, followed by repeating step 1 for a given conceptual model. For 

example, assume that the hydraulic conductivity of 

an aquifer in the western part of a model area is 

different from that in the eastern part and estimate 

separate values for each.

3. Adjust the conceptual model, followed by repeating 

step 1 and, possibly, step 2. For example, simulate 

the possible presence of a confining unit not 

included in the original conceptual model.

Step 1 was performed automatically by the inverse 

model, and steps 2 and 3 were performed in a trialand-error manner. This three-step procedure was 

continued until a combination of a conceptual model, 

parameter zonation, and parameter values were 

attained that best matched both the quantitative 

observed data and the modeler’s qualitative knowledge 

of the hydrologic system.

The calibration procedure relied heavily upon 

the principal of parsimony. Hill (1998, p. 37) explains 

that, “Using the principal of parsimony, the model is 

kept as simple as possible while still accounting for 

the system processes and characteristics evident in the 

observations and while respecting other information 

about the system.” Hill (1998, p. 38) further states that 

use of a simple model generally will produce a wellposed inverse model that will converge to an optimal 

set of parameter values, given reasonable starting 

parameter values. Using the inverse model and the 

principal of parsimony, the three-step model calibration procedure was continued until a model was 

obtained that best met the following criteria: small 

standard error; narrow confidence intervals on 

estimated parameter values; weighted residuals that 

were random, independent, and normally distributed; 

and realistic parameter values. This process is an 

objective analysis of the information provided by the 

observations.

Calibration started with a very simple model 

and complexity was added as warranted by statistics 

calculated by the inverse model. Composite scaled 

sensitivity and parameter correlation coefficient are 

two statistics that are particularly useful for determining which parameters could likely be uniquely estimated with the available observations (Hill and others, 

1998, p. 523). The composite scaled sensitivity for a 

parameter is a measure of the total amount of information provided by all the observations for the estimation 

of that parameter. The larger the composite scaled 

sensitivity, the more sensitive the model is to that 

parameter. The inverse model generally will have difficulty converging if the composite scaled sensitivity for 

any parameter is less than 0.01 times the largest composite scaled sensitivity (Hill, 1998, p. 38). The 

parameter correlation coefficient is a measure of the 

linear dependence of one parameter on another. Two 

parameters with a correlation coefficient exceeding 

0.95 generally cannot be uniquely estimated (Hill and 

others, 1998, p. 523).

During the calibration process the simulated 

water level in the SAS was below the bottom of the 

aquifer in some areas, primarily in western Marion and 

northern Sumter Counties. For the purposes of this 

model, the altitude of the base of the SAS was adjusted 

to maintain a minimum aquifer thickness of 20 ft 

throughout the model area. The altitudes of the bases 

of all the underlying confining units and aquifers also 

were adjusted to maintain their original thicknesses. 

This was reasoned to be an acceptable solution to this 

problem, as opposed to adjusting parameter values in 

the immediate vicinities of these isolated areas, for the 

following reasons: First, no SAS wells, lakes, or 

streams exist in these areas to provide data on the 

altitude of the water table that could be used to guide 

the adjustment of parameter values in these areas. 

Also, regional interpolation of lithologic data collected 

only at isolated locations might have led to the erroneous interpretation that an areally continuous SAS 

exists in these areas. Faulkner (1973, p. 35) and Phelps 

(1994, p. 5) reported that the Hawthorn Group has 

been removed by erosion in western Marion County 

and any remnants commonly form isolated caps on 

hilltops. This degree of detail generally was not 

portrayed on the lithologic maps compiled for this 

study (figs. 10 and 11), which depict a more generalized lithology. Therefore, additional areas where the 

SAS possibly should have been simulated as inactive 

could not be delineated because of the lack of more 

detailed lithologic data. Finally, nearly all the net 

recharge applied at the water table in these areas 

reaches the UFA as leakage through the ICU, because 
Simulation of Ground-Water Flow 69

very little lateral flow is simulated in the SAS in these 

areas. This is essentially equivalent to specifying the 

SAS as inactive in these areas and applying net 

recharge directly to the UFA, as was done in areas 

where the SAS was known to be absent. Therefore, the 

error introduced by actively simulating the SAS in 

these isolated areas, where perhaps the SAS does not 

exist, probably is negligible. After adjustments were 

made to the aquifer and confining unit base altitudes, 

additional calibration was performed to attain the final 

calibrated model.

Parameter Values from Calibrated Model

Thirteen parameters constitute the parameter 

zonation used in the final model and collectively 

define the hydrologic properties of the ground-water 

flow system in the model area (table 3). Zonation of 

the model into areas of homogeneous hydrologic properties (each zone characterized by one constant parameter value) minimizes nonuniqueness problems, which 

commonly are caused by estimating too many parameters. A nonunique solution is not desirable because it 

indicates that different sets of parameters can be estimated from the same set of observations (Carrera and 

Neuman, 1986b, p. 212). All parameters representing 

hydraulic conductivity or anisotropy were log-transformed in MODFLOW using the natural logarithm; 

this can promote convergence of the inverse model by 

making the ground-water flow equation less nonlinear 

and prevent actual parameter values from becoming 

negative (Carrera and Neuman, 1986b, p. 214).

Eight parameters were estimated with the 

inverse model for the final calibrated model (table 3). 

The remaining five parameters were specified in the 

ground-water flow model but were not estimated using 

the inverse model (table 3). The sensitivity of the 

model to the various parameters did not change greatly 

during the calibration process, as evidenced by the 

generally similar composite scaled sensitivities 

between the initial and final parameter values (fig. 34). 

Table 3. Initial parameter values specified in the model and final parameter values estimated with 

the inverse model to calibrate the model

[ET, evapotranspiration; in/yr, inches per year; ft/d, foot per day; <, less than or equal to; >, greater than; --, not applicable]

Parameter description Parameter 

symbol Initial value Final value

Maximum rate of natural net recharge for entire model 

area excluding lakes (in/yr)

Nn,max 30 27

Net recharge at closed-basin lakes1 (in/yr) Ncbl 1.0 --

Maximum rate of combined excess ET and excess 

overland runoff1

 (in/yr)

Rex,max 31 --

Horizontal hydraulic conductivity (ft/d):

Surficial aquifer system Kh,sa 6.0 22

Lower Floridan aquifer Kh,lf 200 66

Multiplier for horizontal hydraulic conductivity of 

Upper Floridan aquifer (dimensionless)

Kh,uf 10.

2,3 12.2,3

Vertical anisotropy (dimensionless):

Surficial aquifer system1 VANsa 10 --

Upper Floridan aquifer1 VANuf 10 --

Lower Floridan aquifer1 VANlf 10 --

Vertical hydraulic conductivity (ft/d):

Riverbed Kv,riv .30 .084

Intermediate confining unit, < 50 ft thick Kv,ic1 .020 .028

Intermediate confining unit, > 50 ft thick Kv,ic2 .020 .0060

Multiplier for vertical hydraulic conductivity of middle semiconfining unit and middle confining unit 

(dimensionless)

Kv,ms .070.

2 .56.2

1

This parameter was specified and not estimated with the inverse model. 2

The value of the hydraulic property represented by this parameter is the product of the unique value assigned to 

each model cell (representing the spatial variability of the hydraulic property) and this parameter value. 3

The spatially variable values of horizontal hydraulic conductivity of the Upper Floridan aquifer (fig. 28) were 

decreased by a factor of 10 in order to ensure that Kh,uf would not equal 1. The composite scaled sensitivity of a log-transformed parameter is calculated as zero by the inverse model when the parameter equals 1.
70 Hydrogeology and Simulated Effects of Ground-Water Withdrawals from the Floridan Aquifer System in Lake County and in 

the Ocala National Forest and Vicinity, North-Central Florida

1.32

9.41 8.67

0.32

11.22

15.31

3.27

0.09

6.13

0.40 0.09 0.02

17.51

0

5

10

15

20

25

30

PARAMETER

COMPOSITE SCALED SENSITIVITY

(a)

6.19

9.66

6.10

0.93

9.27

20.30

0.27 0.06

5.18

0.13 0.08 0.07

16.38

0

5

10

15

20

25

30

Nn,max

Kh,sa

Kh,uf

Kh,lf

Kv,riv

Kv,ic1

Kv,ic2

Kv,ms

Ncbl

Rex,max

VANsa

VANuf

VANlf

ESTIMATED WITH

THE INVERSE MODEL

NOT ESTIMATED WITH

THE INVERSE MODEL

(b)

Nn,max

Kh,sa

Kh,uf

Kh,lf

Kv,riv

Kv,ic1

Kv,ic2

Kv,ms

Ncbl

Rex,max

VANsa

VANuf

VANlf

EXPLANATION

Maximum rate of natural net recharge for entire model area excluding lakes

Horizontal hydraulic conductivity of surficial aquifer system

Multiplier for horizontal hydraulic conductivity of Upper Floridan aquifer

Horizontal hydraulic conductivity of Lower Floridan aquifer

Vertical hydraulic conductivity of sediments constituting the riverbed

Vertical hydraulic conductivity of intermediate confining unit, less than or equal to 50 feet thick

Vertical hydraulic conductivity of intermediate confining unit, greater than 50 feet thick

Multiplier for vertical hydraulic conductivity of middle semiconfining unit and middle confining unit

Net recharge at closed-basin lakes

Maximum rate of combined excess evapotranspiration and excess overland runoff

Vertical anisotropy of surficial aquifer system

Vertical anisotropy of Upper Floridan aquifer

Vertical anisotropy of Lower Floridan aquifer

Nn,max =

Kh,sa =

Kh,uf =

Kh,lf =

Kv,riv =

Kv,ic1 =

Kv,ic2 =

Kv,ms =

Ncbl =

Rex,max =

VANsa =

VANuf =

VANlf =

Figure 34. Composite scaled sensitivities for (a) initial and (b) final parameter values.
Simulation of Ground-Water Flow 71

The calibrated model was most sensitive to the vertical 

hydraulic conductivity of the ICU where the ICU is 

greater than 50 ft thick (Kv,ic2), maximum rate of natural net recharge (Nn,max), multiplier for horizontal 

hydraulic conductivity of the UFA (Kh,uf), and vertical 

hydraulic conductivity of the ICU where the ICU is 

less than or equal to 50 ft thick (Kv,ic1); moderately 

sensitive to the horizontal hydraulic conductivity of 

the SAS (Kh,sa), horizontal hydraulic conductivity of 

the LFA (Kh,lf), and maximum rate of combined excess 

ET and excess overland runoff (Rex,max); and relatively 

insensitive to the vertical hydraulic conductivity of the 

riverbed (Kv,riv), multiplier for vertical hydraulic conductivity of the MSCU/MCU (Kv,ms), vertical anisotropy of the SAS (VANsa), vertical anisotropy of the 

UFA (VANuf), vertical anisotropy of the LFA (VANlf), 

and net recharge at closed-basin lakes (Ncbl) (fig. 34). 

Parameter correlation coefficients were similar 

between the initial and final parameter values, the largest of which was only 0.75 for the final values of 

parameter pairs Kv,ms and VANlf and Nn,max and 

Rex,max. Composite scaled sensitivities and correlation coefficients can vary significantly among 

different parameter values as a result of the effects of 

model nonlinearity and scaling by the parameter 

value (Hill, 1998, p. 41-42). This was not the case for 

the present model, and these statistics were well 

suited for testing alternative conceptual models and 

parameter zonations.

Natural net recharge was represented by three 

parameters in the calibrated model (table 3): Nn,max, 

Ncbl, and Rex,max. Simulated values of natural net 

recharge at the water table were adjusted by varying 

the values of parameters Nn,max and Rex,max and the 

extinction depth (fig. 29). Net recharge to closed-basin 

lakes was directly simulated by the value of Ncbl. The 

large composite scaled sensitivity for Nn,max allowed 

this parameter to be estimated with the inverse model 

(fig. 34). The small composite scaled sensitivity for 

Ncbl precluded the estimation of this parameter with 

the inverse model (fig. 34); Ncbl was held constant at 

its initial value of 1 in/yr, which is consistent with 

independent evidence. Rex,max has a composite scaled 

sensitivity of moderate magnitude (fig. 34), but this 

parameter was not estimated with the inverse model 

for reasons that will be discussed later in this section.

In upland zones (areas where the simulated 

water table is below the extinction depth), natural net 

recharge is equal to Nn,max and its value from the 

calibrated model is 27 in/yr. This value represents the 

highest natural net recharge flux simulated in the 

model area and is consistent with independent evidence discussed in a previous section. The 3-in/yr 

discrepancy between the initial and final values of 

Nn,max may be explained by the fact that minimum 

natural overland runoff (Osn,min, eq. 12) probably is 

not zero as was initially assumed; also, this discrepancy may be explained by errors in the initially 

estimated values of precipitation, minimum natural 

ET, or change in aquifer storage (P, ETn,min, and ∆Saq, 

respectively, eq. 12) or in the specified value of Rex,max.

In transitional zones (areas where the simulated 

water table is equal to or above the extinction depth), 

natural net recharge is equal to the sum of the fluxes 

simulated by the MODFLOW Recharge Package 

(equal to Nn,max) and Evapotranspiration Package. 

The flux simulated by the Evapotranspiration Package 

represents the combined effects of excess ET and 

excess overland runoff, and is determined by the 

simulated water-table altitude, Rex,max, and the extinction depth (fig. 29). Rex,max was specified at its initial 

value of 31 in/yr for the calibrated model; therefore, in 

transitional zones natural net recharge simulated by 

the calibrated model equals -4 in/yr where the simulated water table is above land surface and linearly 

varies from -4 in/yr, where the simulated water table is 

equal to land surface, to 27 in/yr, where the simulated 

water table is equal to the extinction depth.

As previously mentioned, Rex,max could not be 

estimated with the inverse model even though the 

ground-water flow model was fairly sensitive to this 

parameter. The inverse model was unable to converge 

to the specified convergence criterion when attempts 

were made to estimate Rex,max in addition to the other 

eight parameters. When a larger convergence criterion 

was specified, the inverse model converged with a 

smaller standard error (eq. 22), but with unrealistic 

values for Nn,max (42 in/yr) and Rex,max (92 in/yr). 

Many different conceptual models and parameter 

zonations were tested in an attempt to devise a method 

to estimate natural net recharge with the inverse model 

that did not require the specification of Rex,max, such as 

dividing Nn,max into two parameters based on suspected areas of low or high overland runoff. However, 

no alternative models yielded both a good fit to the 

observations and realistic parameter values. Therefore, 

the current conceptual model and parameter zonation 

were considered to be the most appropriate because 

both yielded a reasonably good fit to the observations; 

parameter values were realistic and could be supported 

by independent evidence; and six of the parameters to 

which the model is most sensitive were estimated with 

the inverse model (the seventh being Rex,max, fig. 34). 

In addition, all alternative models fit the observations 
72 Hydrogeology and Simulated Effects of Ground-Water Withdrawals from the Floridan Aquifer System in Lake County and in 

the Ocala National Forest and Vicinity, North-Central Florida

only slightly better (standard error was no more than 

4 percent smaller), had one or more unrealistic parameter values, and had more complex parameter zonations.

MODFLOW does not allow for the incorporation of extinction depth as a parameter to be estimated 

with the inverse model, but the value specified for 

extinction depth does affect how natural net recharge 

is simulated with the model (fig. 29). Therefore, the 

sensitivity of the model to extinction depth and the 

possible optimum value of extinction depth were 

investigated in a trial-and-error manner. Eleven simulations of the inverse model were performed using the 

same parameter zonation as the calibrated model; one 

of the following extinction depths was used for each 

simulation: 3, 6, 9, 13, 16, 20, 25, 30, 35, 40, and 

45 ft. Parameter values from the calibrated model 

(table 3) were used as the initial values for each 

inverse model simulation. The standard error (eq. 22) 

varied over a small range from 8.87 to 9.12 among the 

11 simulations; the minimum standard error occurred 

for an extinction depth of 35 ft and gradually increased 

for depths above and below this value. These results 

indicate that the model was relatively insensitive to the 

value of extinction depth; consequently, the optimum 

value of extinction depth cannot be determined with 

much confidence using results from only the present 

model. An extinction depth of 13 ft was specified for 

the calibrated model because this represents the 

extinction depth with the smallest standard error for 

which supporting independent evidence also exists.

The values of 31 in/yr for Rex,max and 13 ft for 

extinction depth also can be justified based on a comparison of the combined rate of ET and overland runoff simulated by the Evapotranspiration Package and 

that estimated for the independent water budget 

(fig. 23). First, natural ET (ETn, eq. 9) is assumed to 

be equal to the rate of ET from land surface areas 

(excluding about 7 percent of the model area which is 

lakes or streams) because ET extracted from artificial 

recharge (ETa, eq. 17) is very small on a model-wide 

average basis. ET from land surface areas was estimated to be about 32 in/yr based on the independent 

water budget, as described in a previous section. Next, 

natural overland runoff (Osn, eq. 10) is assumed to be 

equal to the rate of overland runoff because overland 

runoff resulting from artificial recharge (Osa, eq. 17) is 

very small on a model-wide average basis. Overland 

runoff was estimated to be 7 in/yr based on the independent water budget (fig. 23). Therefore, the combined rate of ET and overland runoff estimated for the 

independent water budget is 39 in/yr and is equal to 

the sum of natural ET and natural overland runoff. 

Finally, the same quantity (sum of ETn and Osn) can be 

calculated based on results from the calibrated model. 

The combined rate of excess ET and excess overland 

runoff (ETn,ex (eq. 9) and Osn,ex (eq. 10), respectively) 

simulated by the Evapotranspiration Package was 

12.5 in/yr. The combined rate of minimum ET and 

minimum overland runoff (ETn,min (eq. 9) and Osn,min

(eq. 10), respectively) was assumed to be 27 in/yr, as 

described in a previous section. Therefore, the combined rate of ET and overland runoff simulated by the 

model is 40.3 in/yr. The combined rate of ET and 

overland runoff simulated by the model is only 

1.3 in/yr greater than that estimated for the independent water budget; this is within the error expected for 

these data. In conclusion, the values of 31 in/yr for 

Rex,max and 13 ft for extinction depth produce simulated fluxes in good agreement with fluxes based on 

observed data.

The final value of Kh,sa was 22 ft/d, larger than 

its initially estimated value but well within the range 

of reported values from slug tests and aquifer performance tests. In fact, an examination of a histogram of 

60 measurements of horizontal hydraulic conductivity, 

within or in the vicinity of the model area, indicate that 

the largest number of values (19 measurements) are in 

the range 2.5 to 6.3 ft/d but the second largest number 

of values (13 measurements) are in the range 16 to 

40 ft/d. In addition, 10 of the 60 measurements have 

values less than or equal to 1 ft/d, and probably can be 

excluded because they are not regionally representative values based on the typical lithology of the SAS.

The parameter Kh,uf is a multiplier for the spatially variable distribution of values representing the 

horizontal hydraulic conductivity of the UFA (table 3). 

The final value of this parameter indicates that the horizontal hydraulic conductivity of the UFA is 20 percent 

greater than its initial estimate in all model cells. Transmissivity was calculated as the product of horizontal 

hydraulic conductivity from the calibrated model and 

UFA thickness to facilitate comparison with results 

from previous models and aquifer performance tests. 

Transmissivity ranges from 5,900 to 11,000,000 ft2

/d 

with a geometric mean of 160,000 ft2

/d (fig. 35). 

Because the initial distribution of the horizontal 

hydraulic conductivity of the UFA was based on results 

from previous models (fig. 28), the final UFA transmissivity matches the trends depicted in previous models, 

but generally is slightly greater in magnitude. A meaningful comparison with results from aquifer performance tests is more difficult because relatively few 

tests have been performed within the model area.
Simulation of Ground-Water Flow 73

82°20′ 82°00′ 81°10′

29°40′

29°00′

28°20′

0

0

5 10 MILES

5 10 KILOMETERS

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

Universal Transverse Mercator projection, zone 17

LAKE

COUNTY

ORANGE COUNTY

SEMINOLE

COUNTY

VOLUSIA

COUNTY

OSCEOLA COUNTY

POLK COUNTY

PASCO

COUNTY

HERNANDO

COUNTY

SUMTER

COUNTY

CITRUS

COUNTY

MARION COUNTY

ALACHUA COUNTY

PUTNAM COUNTY

ST. JOHNS COUNTY

FLAGLER

COUNTY

MODEL

BOUNDARY

EXPLANATION

TRANSMISIVITY,

IN FEET SQUARED PER DAY

Less than 40,000

40,000 to 100,000

100,000 to 250,000

250,000 to 500,000

500,000 to 1,000,000

Greater than 1,000,000

OCALA

NATIONAL

FOREST

BOUNDARY

Figure 35. Transmissivity of the Upper Floridan aquifer based on aquifer thickness and horizontal hydraulic 

conductivity from the calibrated model.
74 Hydrogeology and Simulated Effects of Ground-Water Withdrawals from the Floridan Aquifer System in Lake County and in 

the Ocala National Forest and Vicinity, North-Central Florida

Results from 126 aquifer performance tests with 

reported values for UFA transmissivity were compiled 

for this study (Szell, 1993; Spechler and Halford, 

2001, p. 24). One-half of these lie outside, but still in 

the vicinity of, the model boundaries. Of the 63 tests 

performed in the model area, only 10 were in Lake 

County and none in the Ocala NF. Significant discrepancies exist between model-derived and aquifer performance test-derived values of transmissivity—in 

some instances greater than two orders of magnitude. 

However, transmissivity values derived from model 

calibration were greater than those from 54 of the 

63 aquifer performance tests. Many factors may 

explain this trend, but two of the more likely explanations are: (1) most of the wells used in the aquifer 

performance tests do not fully penetrate the UFA and 

consequently the calculated transmissivity may be 

underestimated (of the 49 tests where well construction data were available, 65 percent of the wells were 

open to less than 50 percent of the full aquifer thickness); and (2) model-derived transmissivity probably 

is more representative of a regional value that takes 

into account larger scale features, such as fractures and 

dissolution features, which contribute to a larger transmissivity than would be measured at the smaller scale 

of an aquifer performance test.

Because the hydraulic properties and potentiometric surface of the LFA are not well known, the 

horizontal hydraulic conductivity of the LFA was 

represented by one constant parameter Kh,lf (table 3). 

Transmissivity, calculated as the product of horizontal 

hydraulic conductivity from the calibrated model and 

LFA thickness, ranges from 2,300 to 99,000 ft2/d with 

a geometric mean of 56,000 ft2

/d. The smallest values 

are the result of a thinner aquifer caused by the presence of water containing 5,000 mg/L chloride concentration. These values are less than transmissivity 

values derived from aquifer performance tests in 

Orange County (Spechler and Halford, 2001, p. 26), 

but generally are comparable in average magnitude to 

transmissivity values from previous models (Tibbals, 

1990; Motz, 1995; and Murray and Halford, 1996). 

The spatial distribution of LFA transmissivity is considerably different in the present model compared to 

the previous models. However, the spatial distribution 

of LFA transmissivity based on Kh,lf fit the observations better than using LFA transmissivity values from 

previous models.

The vertical anisotropy of each of the three aquifers (SAS, UFA, and LFA) was specified at a value of 10 

and not estimated with the inverse model. The composite scaled sensitivities of the vertical anisotropy parameters were very small (less than one hundredth of that for 

the parameter with the largest composite scaled sensitivity) for both the initial and final sets of parameters values (fig. 34). Therefore, sufficient information was not 

present in the observations to justify adjustment of these 

parameters during model calibration.

The parameter Kv,riv represents the vertical 

hydraulic conductivity of sediments constituting the 

riverbed simulated by the MODFLOW River Package 

(table 3). River nodes are used to simulate streams, 

flow-through lakes, and wetlands; the conductance of 

the bed material for each of these features was formulated such that conductance is proportional to the area 

of the bed material. Therefore Kv,riv functions as a 

multiplier that uniformly adjusts the spatially variable 

values of riverbed conductance. In interpreting the 

value of Kv,riv, it is important to realize that while Kv,riv

is conceptualized to represent vertical hydraulic conductivity, its final value could be compensating for 

errors in the specified riverbed area, the assumed riverbed thickness of 1 ft, or the scale of the model grid. 

Nevertheless, the final value of 0.084 ft/d for Kv,riv

probably is a reasonable value, because Kv,riv generally would be expected to be less than the measured 

horizontal hydraulic conductivity of SAS sediments 

(the smallest of which was 0.2 ft/d).

Composite scaled sensitivities indicate that the 

model is quite sensitive to the vertical hydraulic conductivity of the ICU (parameters Kv,ic1 and Kv,ic2, 

fig. 34), but very little information exists on which to 

base a logical parameter zonation. Field measurements 

of leakance or vertical hydraulic conductivity are 

sparse and detailed descriptions of the lithology of the 

ICU, while more abundant, probably are fewer than 

would be required to represent the heterogeneity in 

confining unit properties. Heterogeneity in aquifer and 

confining unit properties is expected in any geologic 

environment. In the mantled karst environment common in the model area, however, a large degree of 

heterogeneity in hydrologic properties, resulting from, 

for example, sinkhole activity, is typical rather than 

exceptional. Results from previous models can have 

limited transfer value because for a given leakage rate 

between two aquifers, leakance is inversely and completely correlated with the simulated head difference 

between the aquifers. That is, if the head difference 

between the SAS and the UFA is simulated incorrectly, the leakance could be adjusted during calibration to compensate for this error. A large error in 

simulated head difference is likely because the altitude 

of the SAS water table is unknown in many areas and 

can vary considerably over small distances.
Simulation of Ground-Water Flow 75

Considering all of these uncertainties, numerous 

parameter zonations of the vertical hydraulic conductivity of the ICU were tested, taking into account such 

things as total thickness of overburden (depth to the 

top of the UFA below land surface) and surface geology (Scott and others, 2001). The final parameter 

zonation used two parameters, Kv,ic1 and Kv,ic2, to represent the vertical hydraulic conductivity of the ICU 

where the unit is less than or equal to 50 ft thick and 

greater than 50 ft thick, respectively (table 3). This 

was based on the reasoning that the confining unit is 

more likely to be discontinuous or breached by sinkhole activity where it is relatively thin. Consequently, 

the average vertical hydraulic conductivity of the ICU 

in such areas would be greater than in areas where it is 

relatively thick and less prone to sinkhole activity. 

This difference in vertical hydraulic conductivity is 

evident in the final parameter values: Kv,ic1 is nearly 

5 times larger than Kv,ic2, and both values are within 

the range of previously reported values. Leakance of 

the ICU (fig. 36) was calculated based on vertical 

hydraulic conductivity from the calibrated model and 

confining unit thickness (fig. 11). Leakances of the ICU 

generally agree, on average, with those from previous 

models, although some variations in spatial distribution 

are present. Considerable error might exist in the leakances derived from this model, especially at a local 

scale, but the simple parameter zonation presented here 

was determined to be the most suitable given the 

regional scale of this model and the available data.

The parameter Kv,ms is a multiplier for the 

spatially variable distribution of values representing 

the vertical hydraulic conductivity of the MSCU/MCU 

(table 3). The spatial variability accounts only for the 

different lithology between the MSCU and the MCU. 

More specifically, vertical hydraulic conductivity is a 

uniform value equal to Kv,ms where only the MSCU 

exists; vertical hydraulic conductivity is a uniform 

value equal to one-hundredth of Kv,ms where only the 

MCU exists and where the MSCU and the MCU overlap; and vertical hydraulic conductivity is a uniform 

value equal to one hundred times Kv,ms where no 

MSCU exists. The final value of Kv,ms of 0.56 ft/d is 

consistent with previously reported values for the 

MSCU. The final vertical hydraulic conductivity of the 

MCU is 0.0056 ft/d, which also is consistent with previously reported values. Leakance of the MSCU/MCU 

(fig. 37) was calculated based on vertical hydraulic 

conductivity from the calibrated model and confining 

unit thickness (fig. 14). To be consistent with the 

assumption that the MCU is much less leaky than the 

MSCU, only the thickness of the MCU was used to 

calculate leakance where the MSCU and the MCU 

overlap. The final leakances of the MSCU generally 

are greater than those from previous models; whereas 

final leakances of the MCU generally are less than 

those from previous models.

Parameter Uncertainty

The parameter variance-covariance matrix calculated by the inverse model was used to quantify the 

reliability of parameter estimates. Parameter coefficients of variation and confidence intervals on parameter values can be useful for comparing the reliability of 

different parameters from the same run of the inverse 

model (Hill, 1992, p. 58). Coefficients of variation and 

linear 95-percent confidence intervals on parameter 

values were calculated for the parameters estimated 

with the inverse model (table 4). Parameters Nn,max, 

Kh,uf, Kv,ic1, and Kv,ic2 have relatively small coefficients of variation and narrow confidence intervals 

(when considered as a fraction of the respective final 

parameter value), indicating they are estimated more 

reliably than parameters Kh,sa, Kh,lf, Kv,riv, and Kv,ms. 

This is because the observations contain more information from which these parameters can be estimated, 

as indicated by their larger composite scaled sensitivities (fig. 34); however, the linear 95-percent confidence 

intervals in table 4 should be interpreted only as rough 

measures of parameter uncertainty because the model is 

nonlinear and weighted residuals are not normally distributed. Nevertheless, linear 95-percent confidence 

intervals indicate the relative reliability of the final 

parameter values (Yager and others, 2001, p. 52-53).

Table 4. Coefficients of variation and linear 95-percent

confidence intervals for the parameters estimated with

the inverse model

Parameter1

1

See table 3 for parameter symbol definition.

Final 

value

Coefficient 

of 

variation

Linear 95-percent

confidence interval

Lower 

limit

Upper 

limit

Nn,max 27 0.051 24 30

Kh,sa 22 .43 11 48

Kh,uf 12..

2

2The value of the hydraulic property represented by this 

parameter is the product of the unique value assigned to each 

model cell (representing the spatial variability of the hydraulic 

property) and this parameter value.

.16 8.7.

2 16..2

Kh,lf 66 .43 31 140

Kv,riv .084 14 .0033 2.1

Kv,ic1 .028 .23 .018 .043

Kv,ic2 .0060 .18 .0043 .0084

Kv,ms .56.

2 2.2 .077.2 4.1.2
76 Hydrogeology and Simulated Effects of Ground-Water Withdrawals from the Floridan Aquifer System in Lake County and in 

the Ocala National Forest and Vicinity, North-Central Florida

82°20′ 82°00′ 81°10′

29°40′

29°00′

28°20′

0

0

5 10 MILES

5 10 KILOMETERS

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

Universal Transverse Mercator projection, zone 17

LAKE

COUNTY

ORANGE COUNTY

SEMINOLE

COUNTY

VOLUSIA

COUNTY

OSCEOLA COUNTY

POLK COUNTY

PASCO

COUNTY

HERNANDO

COUNTY

SUMTER

COUNTY

CITRUS

COUNTY

MARION COUNTY

ALACHUA COUNTY

PUTNAM COUNTY

ST. JOHNS COUNTY

FLAGLER

COUNTY

MODEL

BOUNDARY

EXPLANATION

LEAKANCE, IN FEET PER DAY

PER FOOT

Less than 0.8 x 10

INTERMEDIATE

CONFINING UNIT NOT

SIMULATED BECAUSE

SURFICIAL AQUIFER

SYSTEM INACTIVE

-4

0.8 x 10 -4 to 5 x 10-4

5 x 10 -4 to 10 x 10 -4

10 x 10-4 to 30 x 10 -4

Greater than 30 x 10 -4

OCALA

NATIONAL

FOREST

BOUNDARY

Figure 36. Leakance of the intermediate confining unit based on confining unit thickness and vertical hydraulic 

conductivity from the calibrated model.
Simulation of Ground-Water Flow 77

82°20′ 82°00′ 81°10′

29°40′

29°00′

28°20′

0

0

5 10 MILES

5 10 KILOMETERS

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

Universal Transverse Mercator projection, zone 17

LAKE

COUNTY

ORANGE COUNTY

SEMINOLE

COUNTY

VOLUSIA

COUNTY

OSCEOLA COUNTY

POLK COUNTY

PASCO

COUNTY

HERNANDO

COUNTY

SUMTER

COUNTY

CITRUS

COUNTY

MARION COUNTY

ALACHUA COUNTY

PUTNAM COUNTY

ST. JOHNS COUNTY

FLAGLER

COUNTY

MODEL

BOUNDARY

EXPLANATION

LEAKANCE, IN FEET PER DAY

PER FOOT

MIDDLE SEMICONFINING

UNIT NOT SIMULATED

BECAUSE LOWER FLORIDAN

AQUIFER INACTIVE

Less than 0.2 x 10-4

5 x 10 -4 to 20 x 10-4

20 x 10-4 to 50 x 10-4

Greater than 50 x 10 -4

0.2 x 10-4 to 5 x 10-4

MIDDLE SEMICONFINING

ABSENT, USED LARGE

LEAKANCE OF 56 OCALA

NATIONAL

FOREST

BOUNDARY

WESTE

FIN

RN

NT F MI EXTE O

T

DD

E

SEMIC L ON ING

UNI

EASTERN

EXTENT

OF

MIDDLE

CONFINING

UNIT

Figure 37. Leakance of the middle semiconfining and middle confining units based on confining unit 

thickness and vertical hydraulic conductivity from the calibrated model.
78 Hydrogeology and Simulated Effects of Ground-Water Withdrawals from the Floridan Aquifer System in Lake County and in 

the Ocala National Forest and Vicinity, North-Central Florida

A parameter might be precisely estimated based 

on its confidence interval, but still not represent its true 

value as a result of parameter correlation or local minima in the objective function. Correlation coefficients 

indicate that all 13 parameters are not highly correlated. A test was performed to determine if the eight 

final parameter values for the calibrated model were 

the result of a global, rather than local, minimum in 

the objective function as suggested by Hill and others 

(2000, p. 18). The inverse model was run twice, using 

a different set of initial parameter values for each 

run: the lower and upper limits of the linear 95-percent confidence intervals (table 4). Final parameter 

values from each of these two runs were very close to 

those for the calibrated model, with the exception of 

Kh,sa and Kv,riv. Using the upper limit of the linear 95-

percent confidence intervals from the final calibrated 

model as the initial parameter values, the values estimated by the inverse model for Kh,sa and Kv,riv were 

13 and 0.10 ft/d, respectively, and linear 95-percent 

confidence intervals were 5.6 and 30 ft/d for Kh,sa and 

0.0030 and 3.6 ft/d for Kv,riv. These linear 95-percent 

confidence intervals significantly overlap the confidence intervals on the corresponding final values for 

the calibrated model (table 4). Therefore, the eight 

final parameter values for the calibrated model can be 

considered unique for the observations used in the 

inverse model.

Model Fit

An evaluation of the model fit was made for the 

calibrated model using statistics and results generated 

by the inverse model in order to determine how well 

simulated water levels and flows matched observed 

values. The calibrated model has a standard error 

(eq. 22) of 8.95, which is less than or only slightly 

greater than the value for each of the alternative models tested during calibration. The standard error will 

equal 1 if the model fit is consistent with the weights 

assigned to the observations, but typically will exceed 

1 as a result of model error, such as discretization 

effects (Poeter and Hill, 1997, p. 254). An examination 

of water-level residuals grouped by aquifer indicated 

that the majority of the model error is associated with 

the SAS (table 5). In fact, the root-mean-square error 

(RMSE) is nearly three times larger for the SAS waterlevel observations than for the UFA or LFA observations. The calculation of RMSE is identical to that for 

standard error except the number of estimated parameters is excluded from equation 22 and only water-level 

observations and no weights are included in the calculation of SSWR (eq. 21). Even though RMSE does not 

account for the number of estimated parameters, this is 

of no consequence when comparing results from models with the same parameter zonation. Even though the 

water-level residuals are quite large for the SAS, these 

errors are a relatively small percentage of total model 

response because the RMSE is less than 9 percent of 

the simulated head range in the aquifer (table 5). In 

addition, the SAS water table is a more complex surface than the potentiometric surface of either the UFA 

or LFA, and, therefore, a more accurate simulation of 

the SAS water table would require a smaller grid size 

than is reasonable for the present regional-scale 

model. Simulated base flow in streams and in lakes 

and wetlands that drain to streams closely matches 

observed base flow, considering the error inherent in

Table 5. Water-level residual statistics for the calibrated model

[RMSE, root mean square error; ft, feet; %, percent]

Aquifer

Number 

of 

water-level 

observations

Minimum 

residual1

(ft)

Maximum 

residual1

(ft)

Average 

residual1

(ft)

RMSE

(ft)

RMSE as 

percent of 

head range2

(%)

Surficial aquifer system 133 -46.70 50.50 -1.78 13.9 8.9

Upper Floridan aquifer 251 -14.90 17.10 .53 4.72 3.0

Lower Floridan aquifer 20 -4.00 9.99 2.33 4.60 2.9

Entire model 404 -46.70 50.50 -.14 8.87 5.7

1Residual is observed minus simulated value.

2

Head range is the maximum simulated water level minus the minimum simulated water level for all active model cells in 

each aquifer.
Simulation of Ground-Water Flow 79

estimating actual base flow. Simulated base flow is 

4.5 in/yr, 1.3 in/yr less than the value used as an 

observation, and its weighted residual is quite small. 

Considering the 404 water-level observations and the 

1 flow observation, the correlation coefficient 

between the weighted observations and the weighted 

simulated values is 0.95, indicating that most of the 

weighted simulated values are close to the weighted 

observations. 

Weighted residuals should be randomly distributed about a value of zero for all weighted, simulated 

values (Hill, 1994, p. 3), and they should be independent and normally distributed (Hill, 1994, p. 6). The 

weighted residuals for the calibrated model do not 

show any trend with weighted simulated values and 

the average weighted residual is -0.14 (fig. 38). However, the weighted residuals are not independent and 

normally distributed, because the plotted values do not 

fall along a straight line on a normal probability plot 

(fig. 39); this is particularly true for SAS observations. 

Additional analysis indicated that the curvilinear 

nature of the normal probability plot (fig. 39) probably 

can be attributed to the nonlinearity of the groundwater flow model. Most of the nonlinear functions (for 

example, the MODFLOW River and Evapotranspiration Packages) used in the model are applied to the 

SAS, with some applied to the UFA. The weighted 

residuals for the LFA water levels do appear to be 

normally distributed (fig. 39).

WEIGHTED SIMULATED VALUE

0 20 40 60 80 100 120 140

-60

-50

-40

-30

-20

-10

0

10

20

30

40

50

60

-20

WEIGHTED RESIDUAL

NOTE: Residual defined as

observed minus simulated value

SURFICIAL AQUIFER SYSTEM

WATER LEVEL

UPPER FLORIDAN AQUIFER

WATER LEVEL

LOWER FLORIDAN AQUIFER

WATER LEVEL

BASE FLOW

EXPLANATION

Figure 38. Comparison of weighted residuals to weighted simulated values.
80 Hydrogeology and Simulated Effects of Ground-Water Withdrawals from the Floridan Aquifer System in Lake County and in 

the Ocala National Forest and Vicinity, North-Central Florida

Model linearity was tested by calculating a statistic referred to as the modified Beale’s measure (Hill, 

1994, p. 45) using BEALE-2000 (Hill and others, 

2000), a post-processing program for MODFLOW2000. The modified Beale’s measure for the calibrated 

model is 0.53, with critical values of 0.045 and 0.50. If 

the modified Beale’s measure is less than the smaller 

critical value, the model is effectively linear for parameter values close to the final parameter values; if the 

modified Beale’s measure is greater than the larger 

critical value, the model is highly nonlinear for parameter values close to the final parameter values. Therefore, the calibrated model is highly nonlinear. A closer 

examination reveals that parameter Kv,riv causes most 

of the nonlinearity: the modified Beale’s measure for 

the calibrated model excluding Kv,riv is 0.24, which is 

midway between the two critical values, 0.043 and 

0.48, indicating that the model is only moderately nonlinear. Head-dependent flow boundaries, such as simulated by the MODFLOW River Package and Kv,riv, can 

be a major source of nonlinearity when the simulated 

water level fluctuates above and below the specified 

riverbed bottom altitude (eqs. 18 and 19). The fact that 

the model is nonlinear and that the weighted residuals 

are not normally distributed have important implications for interpretation of some statistics calculated by 

the inverse model, such as linear confidence intervals 

(table 4). Linear confidence intervals on estimated 

parameter values, predicted water levels, and predicted 

flows can be valuable indicators of uncertainty. However, when the model is nonlinear and weighted residuals are not normally distributed, linear confidence 

intervals must be considered only as rough measures 

of simulation uncertainty.

Another important characteristic of an accurate 

model is the spatially random distribution of weighted 

residuals (Poeter and Hill, 1997, p. 254). Model-wide 

trends in the spatial distribution of water-level residuals 

for each aquifer are not apparent (figs. 40, 41, and 42).

-60 -40 -20 0 20 40 60

WEIGHTED RESIDUAL

-4

-3

-2

-1

0

1

2

3

4

STANDARD NORMAL STATISTIC

SURFICIAL AQUIFER SYSTEM

WATER LEVEL

UPPER FLORIDAN AQUIFER

WATER LEVEL

LOWER FLORIDAN AQUIFER

WATER LEVEL

BASE FLOW

EXPLANATION

NOTE: Residual is observed

minus simulated value

Figure 39. Normal probability plot of weighted residuals.
Simulation of Ground-Water Flow 81

82°20′ 82°00′ 81°10′

29°40′

29°00′

28°20′

0

0

5 10 MILES

5 10 KILOMETERS

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

Universal Transverse Mercator projection, zone 17

LAKE

COUNTY

ORANGE COUNTY

SEMINOLE

COUNTY

VOLUSIA

COUNTY

OSCEOLA COUNTY

POLK COUNTY

PASCO

COUNTY

HERNANDO

COUNTY

SUMTER

COUNTY

CITRUS

COUNTY

MARION COUNTY

ALACHUA COUNTY

PUTNAM COUNTY

ST. JOHNS COUNTY

FLAGLER

COUNTY

MODEL

BOUNDARY

EXPLANATION

ALTITUDE OF SIMULATED

WATER TABLE, IN FEET

SURFICAL AQUIFER SYSTEM

NOT SIMULATED

Less than 25

50 to 75

75 to 100

Greater than 100

25 to 50

OBSERVATION WELL OR

LAKE GAGE -- Number

represents water-level

residual (observed minus

simulated value, in feet)

J39

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 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 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 J

J

J J J JJ

J J J J J J J J

J

J J

J 26

-47 39

-8 -3

-39

-5 -10-1

-23

-5

-9

-11 -8

23

-13

20 -7 -3 -5

-1

-8

-1 -2

-1

10

-5 15 2

13

-7 3 -34 -11

-8

-3 -13 -1 -8 -5

27

-5

-19 51

2 -6

-4 -11 3 -1

6 -2 -14

16 -27 -4

3 -13

-8 -11 -26 -1 -1

12 -1 -1 -23

-9

-3 -3

-10

-2

-9

-1

-10

-18 0 36

-4 -7

49 3 7 -12 3

8 -10 1

8 -2 -7 12 -5 7

-14 3

7 -2

4 27

-1

-13 -3

-3

-4 -15 -12

1 -4

0 -10

4 2

0

6

4

2 -17

0

-20

-24

1 1 0 3

8 6 0 1 0

-1 1

15

OCALA

NATIONAL

FOREST

BOUNDARY

Figure 40. Simulated water table and water-level residuals for the surficial aquifer system, average 1998 

conditions.
82 Hydrogeology and Simulated Effects of Ground-Water Withdrawals from the Floridan Aquifer System in Lake County and in 

the Ocala National Forest and Vicinity, North-Central Florida

82°20′ 82°00′ 81°10′

29°40′

29°00′

28°20′

0

0

5 10 MILES

5 10 KILOMETERS

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

Universal Transverse Mercator projection, zone 17

LAKE

COUNTY

ORANGE COUNTY

SEMINOLE

COUNTY

VOLUSIA

COUNTY

OSCEOLA COUNTY

POLK COUNTY

PASCO

COUNTY

HERNANDO

COUNTY

SUMTER

COUNTY

CITRUS

COUNTY

MARION COUNTY

ALACHUA COUNTY

PUTNAM COUNTY

ST. JOHNS COUNTY

FLAGLER

COUNTY

MODEL

BOUNDARY

EXPLANATION

20

.2

SIMULATED

POTENTIOMETRIC

CONTOUR-- Shows altitude

at which water level would

have stood in tightly cased

wells. Hachures indicate

depression. Contour interval

10 feet. Datum is sea level

OBSERVATION WELL--

Number represents

water-level residual

(observed minus simulated

value, in feet)

OCALA NATIONAL

FOREST BOUNDARY

J

.

. .. . .

. . . . .

. . . . . . . . . .

. . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . .

. . . .

3

1 -3 8 0

-6

12 6

-2 13

3 -1

1

-10 2 -5

-8 -7 -2 -3 -6

-3 -1 -11 -4 -3

-7 3 5

-7 -4 -2 -1 -9 -4

1 1

-11

-4 -2

2

1 -10

1 0 -7 -8 1 -8 -2 4

3

2 1 -1 9 5

1 4 2 1 5 3 1 -1

2 -1

-2 9 2 -10 -4 17 1 1 4 1 9 -2 -2

-6

3 6 10 1 -2 -3 3 -7

2 0 -2 -4 2 -15 3 2

1 -11

1

13 0 -2 3 -2 -1 4 0 -1 0 -1

1 -11 0 9

-1 0 0 0 2

2

-2 6 0 -1 -5 8 2 0 -1 -1 -1 2 5 -3 1 1 5 0 -5 2 6 8 2 7

6 1 6 5 1 1 -4 0 -2 3 -1

3 5 11

0 -3 2 5 8 -1 -5 2 -2

6 0 11 10 -6 -1 2 -1 3 -2 6 -3

4 -4 -7 -2

0

3 10 6

2 -3 3 0 4 8 1 4 3

2 1 3

1

-2 1

1 -5

3 -5 -3 2 1

0 -4

-1 -6 -3

0 -3

7 4

1

-2 5

-2 1

1

6 -2

-1 -2

0 0 2 8

9 9 1 4 0 -2 4 0 -3 1

0 -2 0

1 1

0

1

-3

90

80

70

10

30

30

60

20

50

40

10

0

30

10

20

20

20

30

30

40

40

50

50

50

60

60

90

80

70

70

110

100

120

20

Figure 41. Simulated potentiometric surface and water-level residuals for the Upper Floridan aquifer, 

average 1998 conditions.
Simulation of Ground-Water Flow 83

82°20′ 82°00′ 81°10′

29°40′

29°00′

28°20′

0

0

5 10 MILES

5 10 KILOMETERS

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

Universal Transverse Mercator projection, zone 17

LAKE

COUNTY

ORANGE COUNTY

SEMINOLE

COUNTY

VOLUSIA

COUNTY

OSCEOLA COUNTY

POLK COUNTY

PASCO

COUNTY

HERNANDO

COUNTY

SUMTER

COUNTY

CITRUS

COUNTY

MARION COUNTY

ALACHUA COUNTY

PUTNAM COUNTY

ST. JOHNS COUNTY

FLAGLER

COUNTY

OCALA

NATIONAL

FOREST

BOUNDARY

90

80

70

30

60

20

20

50

20

20

30

30

20

20

30

40

40

50

50

60

60

70

80

90

20

. .

. .

. .

. . . .

.

. . . .

. .

.

. .

0 -3

1 6

3 8

7 3 10

1

-1

5 3

-3

4

2 1

7

-4

-3

MODEL

BOUNDARY

EXPLANATION

20

J3

SIMULATED

POTENTIOMETRIC

CONTOUR-- Shows altitude

at which water level would

have stood in tightly cased

wells. Contour interval

10 feet. Datum is sea level

OBSERVATION WELL--

Number represents

water-level residual

(observed minus simulated

value, in feet)

LOWER FLORIDAN AQUIFER

NOT SIMULATED

Figure 42. Simulated potentiometric surface and water-level residuals for the Lower Floridan aquifer, 

average 1998 conditions.
84 Hydrogeology and Simulated Effects of Ground-Water Withdrawals from the Floridan Aquifer System in Lake County and in 

the Ocala National Forest and Vicinity, North-Central Florida

Groupings of positive or negative water-level residuals 

do exist on a more local scale, however, such as within 

the Ocala NF (fig. 40). The “runs statistic” (Hill, 1998, 

p. 22), calculated by the MODFLOW Observation 

Process, can be used to indicate the presence of groupings of residuals or lack of randomness. A “run” is 

defined as a sequence of numbers of the same sign, 

and the runs statistic is a measure of the number of 

runs, or changes in sign, in the set of residuals. The 

number of runs in a given set of residuals is dependent 

on the order in which the residuals are listed. Waterlevel observations were ordered by layer, row, and 

column in the MODFLOW Head-Observation Package input file; that is, for each model layer, water-level 

observations were listed in order according to their 

spatial location: from north to south (increasing row 

number) and from west to east (increasing column 

number). Because the water-level observations were in 

a logical spatial order, the runs statistic for the 

calibrated model can be used as a measure of the 

spatial randomness of the residuals. The value of the 

runs statistic approaches zero as the actual number of 

runs approaches the expected number of runs. The 

runs statistic value of -2.4 indicates there are too few 

runs in the water-level residuals, indicating spatial 

groupings of residuals of the same sign exist in the 

same aquifer; consequently, the residuals are less randomly distributed than would be ideal. This is in 

agreement with the groupings of residuals as previously noted on maps of water-level residuals (figs. 40, 

41, and 42). More randomly distributed residuals 

possibly could be obtained by adjusting parameter 

values at a more local scale or on a node-by-node 

basis, but this would add considerable complexity to 

the model that is not likely to be justified by the existing data. The present parameter zonation is considered 

to be an acceptable compromise between possibly 

greater model accuracy achieved through more 

complex parameter zonations and generally greater 

reliability (that is, less model uncertainty) resulting 

from more parsimonious parameter zonations.

Simulated 1998 Water Levels and Flows

The simulated water table of the SAS (fig. 40) is 

considerably different than the observed water table. 

The largest residuals (table 5 and fig. 38) and the least 

normal (fig. 39) and least random distributions of 

residuals (fig. 40) all occur in the SAS. Reasons for the 

relatively poor match between simulated and observed 

water levels are as follows: (1) the hydrogeology of 

the SAS is very heterogeneous; (2) large spatial variations in ET directly affect net recharge (the largest 

stress on the aquifer); (3) large variations in the leakance of the ICU directly affect the exchange of water 

with the UFA (the second largest stress on the SAS); 

and (4) the SAS generally is in direct hydraulic 

connection with numerous surface-water features 

(streams, lakes, and wetlands), resulting in complex 

interactions between ground water and surface water 

than can vary significantly from one surface-water 

feature to another. In addition, the SAS is much more 

affected than the FAS by the slow dissipation of 

transient effects, which are not accounted for by a 

steady-state simulation, because of the relatively small 

diffusivity of the SAS (the ratio of transmissivity to 

storage coefficient). All these factors are responsible 

for the complex configuration of the water table and 

are some of the least well known characteristics of the 

SAS. Additionally, the model grid resolution probably 

is not fine enough to duplicate the complex water-table 

surface. A relatively large amount of error was 

accepted for the SAS in order to use a simpler 

parameter zonation that reflects this lack of information, while still representing what is known about the 

aquifer on a more regional scale.

The simulated water table of the SAS (fig. 40) 

generally reflects the regional trend of the UFA potentiometric surface (fig. 41), with generally higher 

altitudes in the northern and southern parts of the 

model area and lower altitudes along the S. Johns 

River. However, mounds in the SAS water table occur 

in many areas, particularly along the Mount Dora 

Ridge (figs. 3 and 40), creating flow systems that are 

more local than those in the UFA. The direct interaction of the SAS with the surface environment and the 

relatively low horizontal hydraulic conductivity of the 

SAS contribute to the formation of numerous local 

flow systems. In addition, the configuration of the 

SAS water table and, in particular, the locations of 

water-level mounds are strongly influenced by variations in the leakance of the ICU (compare figures 36 

and 40).

The simulated potentiometric surface of the 

UFA (fig. 41) generally conforms to observed data 

(fig. 2). The water-level residuals for the UFA generally are small (fig. 38), have a RMSE of 4.72 ft 

(table 5), and are fairly normally distributed (fig. 39). 

The runs statistic for the 251 UFA observations is 

-0.56, indicating that relatively few spatial trends exist 
Simulation of Ground-Water Flow 85

in water-level residuals (fig. 41). The observed potentiometric surface of the UFA can be reproduced by the 

model more accurately than the water table of the 

SAS, because the spatial variation of the UFA potentiometric surface is more regional in scale and a relatively large amount of data is available describing its 

configuration. The largest water-level residuals in the 

UFA generally are in northern Lake County near the 

St. Johns River, where water-level observations indicate very steep hydraulic gradients (fig. 2). These 

gradients may be the result of local areas of low 

transmissivity, or perhaps they are the result of vertical 

variations in water level in the UFA. Most large 

residuals were relatively isolated and no attempt was 

made to adjust hydrologic properties on a node-bynode basis to produce a closer match to these observations. Large mounds in the simulated potentiometric 

surface of the UFA do not exist where they are present 

in the simulated water-table surface for the SAS. This 

is the result of the high transmissivity of most of the 

UFA (fig. 35), which allows water to move very easily 

through the aquifer (compared to the SAS) to points of 

discharge, such as springs and wells.

The water-level residuals for the LFA generally 

are small (fig. 38), have a RMSE of 4.60 ft (table 5), 

are normally distributed (fig. 39), and show few spatial 

trends (fig. 42). The most noticeable deficiency 

regarding the simulation of the LFA is the lack of data. 

In most of the model area, no LFA observations are 

available (fig. 42). The majority of observations are in 

Orange and Seminole Counties; many of these are near 

the specified-head boundary in Orange County 

(fig. 25), which reduces the sensitivity of the model to 

these observations. The lack of information leads to a 

simple parameter zonation. The simulated potentiometric surface of the LFA (fig. 42) closely resembles 

that of the UFA (fig. 41). The most notable exception 

is in the southwestern part of the model area where the 

MCU exists. In extreme southern Lake County and 

northern Polk County, the simulated water level in the 

LFA is greater than 25 ft below that of the UFA. This 

large head difference is the direct result of the low 

leakance simulated for the MCU (fig. 37).

A comparison was made between the independent water budget compiled for the model area 

(fig. 23) and the simulated water budget (fig. 43). 

Average annual quantities that can be obtained from 

the independent water budget and that are directly 

comparable to simulated values are natural net 

recharge at the water table, net recharge to the UFA, 

base flow, springflow, and boundary leakage. Base 

flow is accounted for as an observation in the inverse 

model and springflow was specified during calibration. Boundary leakage was not formally included as 

an observation in the inverse model because of the 

large uncertainty involved with the independent estimation of this quantity. Natural net recharge at the 

water table and net recharge to the UFA could not be 

formally included as observations in the inverse 

model, but were checked during calibration to ensure 

reasonable simulated values. Natural net recharge at 

the water table, which excludes artificial recharge and 

includes storage effects, was estimated to be approximately 16 in/yr in 1998 (the sum of precipitation 

(57 in/yr), ET (-33 in/yr), overland runoff (-7 in/yr), 

and net change in storage (-1 in/yr)) from the independent water budget (fig. 23); whereas natural net 

recharge at the water table is simulated to be 12.6 in/yr 

(the sum of the recharge and ET components for the 

SAS and UFA, fig. 43). Net recharge to the UFA was 

estimated to be approximately 12 in/yr in 1998 

(fig. 23); simulated net recharge to the FAS was 

8.9 in/yr (the sum of the recharge and ET components 

for the UFA and net leakage through the ICU, fig. 43). 

The discrepancies of about 3 in/yr between the 

estimated and simulated values primarily are the result 

of boundary leakage for the FAS, which is simulated 

to be about 2 in/yr less than its estimated value. 

Considering the margin of error within which boundary leakage and other components of the water budget 

can be independently estimated, the simulated water 

budget agrees reasonably well with the independent 

water budget.

An examination of the simulated water budget 

reveals some important characteristics of the groundwater flow system in the model area. The total flow 

through the SAS and FAS together is 13.3 in/yr, 

which is equal to the sum of inflows to or outflows 

from the entire aquifer system (fig. 43). However, in 

calculating the total flow, the net differences 

between the recharge and ET components (fig. 43)

were counted as net inflows because they collectively represent natural net recharge. Accordingly, 

natural net recharge for the SAS is 11.4 in/yr, averaged over the entire model area, or 12.3 in/yr averaged over only the 19,855 active SAS model cells 

(fig. 24); natural net recharge for the UFA is 1.2 in/yr, 

averaged over the entire model area, or 16.5 in/yr 

averaged over only the 1,559 model cells where the 

UFA is active and the SAS is inactive (fig. 24). 
86 Hydrogeology and Simulated Effects of Ground-Water Withdrawals from the Floridan Aquifer System in Lake County and in 

the Ocala National Forest and Vicinity, North-Central Florida

SURFICIAL AQUIFER

SYSTEM

Total flowc=15.3

Rechargea

23.2

ETb

11.8

Artificial net

recharge

0.3

Discharge to

surface water

4.0

11.3

3.6

ETb

0.7

Rechargea

1.9

Discharge to

surface water

0.5

Spring flow

5.6

Boundary

inflow

0.3

UPPER FLORIDAN AQUIFER

Total flowc=14.0

1.7

1.1

Boundary

outflow

0.8

Boundary

inflow

0.0

LOWER FLORIDAN AQUIFER

Total flow =1.7

Boundary

outflow

0.3

Well

withdrawals

1.8

Well

withdrawals

0.3

Artificial net

recharged

0.1

EXPLANATION

Represents the maximum rate of natural net recharge.

Simulated using the MODFLOW Recharge Package.

Represents the combined effects of excess evapotranspiration

and excess overland runoff. Simulated using the MODFLOW

Evapotranspiration package.

The sum of all inflows or outflows for the aquifer. The net

difference between the recharge and evapotranspiration

components is counted as a single inflow representing natural

net recharge.

ALL VALUES ARE IN INCHES PER YEAR AVERAGED OVER

THE MODEL AREA (approximately 4,800 square miles)

a

b

c

Includes drainage-well recharge. d

Figure 43. Simulated volumetric water budget for the aquifer system in the model area, average 1998 conditions.
Simulation of Ground-Water Flow 87

Natural net recharge is the source of about 95 percent 

of the 13.3 in/yr of total flow through the SAS and 

FAS; the remaining 5 percent comes from boundary 

inflow and artificial recharge. Most of the 13.3 in/yr 

circulated through the SAS and the UFA rather than 

the LFA. In addition, simulated net leakage through 

the ICU was 7.7 in/yr downward, representing 50 and 

55 percent of the total flows in the SAS and the UFA, 

respectively. These results are in agreement with the 

high leakance characteristics of the ICU. Consequently, hydrologic conditions in the SAS can have 

relatively significant effects on conditions in the UFA; 

likewise, hydrologic conditions in the UFA can have 

relatively significant effects on conditions in the SAS. 

A limited amount of flow from the UFA reaches the 

LFA (about 12 percent of the total flow in the UFA). 

Outflows from the LFA are relatively small, so the 

ground-water flow system in the LFA generally is 

relatively sluggish in comparison to the UFA.

The ground-water flow system can be characterized as relatively vigorous in the SAS and the UFA; 

however, the scale of the flow systems differs greatly 

between the two aquifers. That is, the SAS is characterized by local flow systems with some water moving 

laterally from recharge areas to nearby surface-water 

features, but with most water moving from recharge 

areas vertically downward as leakage to the UFA. 

Nearly three times the amount of water leaks downward to the UFA as discharges to surface-water 

features in the SAS (fig. 43). A comparison of the 

simulated spatial distributions of net recharge at the 

water table (fig. 44) and leakage through the ICU 

(fig. 45) shows numerous similarities, which further 

indicates that water primarily moves vertically in the 

SAS. The UFA is characterized by more regional flow 

systems, with water generally moving laterally from 

recharge areas (sometimes over great distances) to discharge areas, such as springs, well fields, and areas of 

upward leakage to the SAS. Springflow, well withdrawals, boundary outflow, and discharge to surface-water 

features account for 62 percent of the total outflow 

from the UFA; these budget components are partially 

responsible for inducing lateral flow in the aquifer.

The distribution of net recharge or discharge at 

the water table (fig. 44) was calculated from the fluxes 

simulated by the MODFLOW Recharge and Evapotranspiration Packages (fig. 29) and the artificial 

recharge fluxes (eq. 17) specified in the MODFLOW 

Well Package. Simulated discharge occurs in areas 

where the simulated water table is just below land 

surface or above land surface, because parameter 

Rex,max is greater than Nn,max (table 3; fig. 29). This simulated discharge generally occurs in the vicinity of 

streams (fig. 44) and can be considered as representing 

additional base flow that might actually be occurring in 

riparian areas, but is not otherwise accounted for in the 

model. The spatial variation in net recharge at the water 

table primarily is the result of the varying depth of the 

simulated water table below land surface. Therefore, 

simulated net recharge generally is greatest in the ridge 

areas, where the simulated water table typically is far 

below land surface, and lowest in valleys or plains, 

where the simulated water table typically is near land 

surface (compare figs. 3 and 44). This distribution of net 

recharge is consistent with the conceptual model of the 

aquifer system. Net recharge can exceed 30 in/yr (fig 44) 

where artificial recharge occurs; these high rates generally occur at rapid infiltration basins or spray fields.

The spatial distribution of leakage through the 

ICU varies considerably (fig. 45), depending on the 

leakance of the ICU and the head difference between 

the SAS and UFA. A good example of this variation 

occurs in an area of the Ocala NF just west of Juniper 

Springs (site number 311, fig. 7), where the simulated 

leakance of the ICU is low, but adjacent to areas where 

the simulated leakance is high (fig. 36). A mound in 

the SAS water table ranging in altitude from 50 to 

100 ft exists in this area (fig. 40); however, the UFA 

potentiometric surface drops only about 10 ft across 

the area of the SAS mound (fig. 41), indicating a large 

variation in head difference between the two aquifers. 

Leakage is low where the product of leakance and 

head difference is low, such as near the center of the 

mound where leakage varies from 0 to 10 in/yr; leakage is high where the product of leakance and head 

difference is high, such as along the “toe” of the 

mound where leakage rates exceed 30 in/yr. Leakage 

rates through the ICU can exceed net recharge at the 

water table in the SAS. Most leakage rates exceeding 

30 in/yr (fig. 45) are supported by lateral flow in the 

SAS, generally from areas of lower leakance such as 

described in the preceding example. This lateral flow 

generally occurs over relatively short distances, thus 

supporting local rather than regional flow systems. 

Numerous areas of high leakance exist, perhaps even 

more than are simulated in the model. In addition, 

lateral flow that is not intercepted by areas of high 

leakance is likely to discharge at local surface-water 

features or to be extracted by ET before forming 

regional flow systems in the SAS.
88 Hydrogeology and Simulated Effects of Ground-Water Withdrawals from the Floridan Aquifer System in Lake County and in 

the Ocala National Forest and Vicinity, North-Central Florida

82°20′ 82°00′ 81°10′

29°40′

29°00′

28°20′

0

0

5 10 MILES

5 10 KILOMETERS

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

Universal Transverse Mercator projection, zone 17

LAKE

COUNTY

ORANGE COUNTY

SEMINOLE

COUNTY

VOLUSIA

COUNTY

OSCEOLA COUNTY

POLK COUNTY

PASCO

COUNTY

HERNANDO

COUNTY

SUMTER

COUNTY

CITRUS

COUNTY

MARION COUNTY

ALACHUA COUNTY

PUTNAM COUNTY

ST. JOHNS COUNTY

FLAGLER

COUNTY

MODEL

BOUNDARY

EXPLANATION

RECHARGE OR

DISCHARGE RATE,

IN INCHES PER YEAR--

Positive values indicate

recharge and negative

values indicate discharge

Less than 0

0

0 to 10

10 to 20

20 to 30

Greater than 30

OCALA

NATIONAL

FOREST

BOUNDARY

Figure 44. Simulated rate of net recharge or discharge at the water table, average 1998 conditions.
Simulation of Ground-Water Flow 89

82°20′ 82°00′ 81°10′

29°40′

29°00′

28°20′

0

0

5 10 MILES

5 10 KILOMETERS

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

Universal Transverse Mercator projection, zone 17

LAKE

COUNTY

ORANGE COUNTY

SEMINOLE

COUNTY

VOLUSIA

COUNTY

OSCEOLA COUNTY

POLK COUNTY

PASCO

COUNTY

HERNANDO

COUNTY

SUMTER

COUNTY

CITRUS

COUNTY

MARION COUNTY

ALACHUA COUNTY

PUTNAM COUNTY

ST. JOHNS COUNTY

FLAGLER

COUNTY

MODEL

BOUNDARY

EXPLANATION

Less than 0

INTERMEDIATE CONFINING

UNIT NOT SIMULATED

BECAUSE SURFICIAL

AQUIFER SYSTEM INACTIVE

0 to 10

10 to 20

20 to 30

Greater than 30

LEAKAGE RATE,

IN INCHES PER YEAR--

Negative values indicate upward

leakage and positive values indicate

downward leakage

OCALA

NATIONAL

FOREST

BOUNDARY

Figure 45. Simulated rate of leakage through the intermediate confining unit, average 1998 conditions.
90 Hydrogeology and Simulated Effects of Ground-Water Withdrawals from the Floridan Aquifer System in Lake County and in 

the Ocala National Forest and Vicinity, North-Central Florida

The leakage of water between the UFA and LFA 

is determined by the leakance of the MSCU/MCU and 

the head difference between the aquifers. The presence 

of the MCU and its low simulated leakance have 

important implications concerning recharge to the 

LFA. The LFA is recharged only by leakage of water 

from the overlying UFA. In the portion of the model 

area where the MCU exists, the average simulated rate 

of leakage to the LFA is 0.8 in/yr; whereas in the 

remainder of the model area where only the MSCU 

exists or both confining units are absent, the average 

simulated rate of leakage to the LFA is 4.2 in/yr. 

Therefore, the majority of water in the LFA in the 

model area originates as leakage from the UFA east of 

the eastern extent of the MCU.

Effects of Projected 2020 Ground-Water 

Withdrawals

The ground-water flow model simulated the 

SAS and FAS water levels and flows under average 

1998 conditions reasonably well. This calibrated 

model was used to evaluate the potential effects of 

projected ground-water withdrawals in 2020 on 

steady-state water levels and flows in the aquifer 

system. Particle-tracking analyses were used to identify the areas that contribute recharge to selected 

springs and well fields under both average 1998 and 

projected 2020 conditions. Finally, the effects of 

parameter uncertainty on model predictions were 

examined.

Projected Boundary Conditions

Boundary conditions specified in the calibrated 

model were based on average 1998 conditions and 

might not be valid under future conditions. Most 

boundary conditions were not changed for the predictive simulations. The net recharge upper boundary 

condition was not changed because the objective of 

predictive simulations was to evaluate the effects of 

future withdrawals rather than changes in climatic 

conditions, such as a drought, which might affect net 

recharge. Internal boundary conditions representing 

streams or flow-through lakes (figs. 26 and 27) were 

left unchanged because any change in ground-water 

discharge to these features likely would have very little 

impact on their stage, which generally is primarily 

controlled by the surface-water flow system. Internal 

boundary conditions representing wetlands (fig. 27) 

were not changed because the MODFLOW River 

Package was formulated to allow the simulation of 

potential decreases in ground-water discharge to wetlands (where the simulated water table is greater than 

3 ft above the median land-surface altitude in the wetland) and the simulation of potential drawdown of the 

water table in wetlands. The lateral no-flow boundary 

condition in the SAS was not changed because the 

SAS probably will continue to be dominated by downward leakage to the UFA and by local flow systems. 

The lateral no-flow boundary conditions for the UFA 

and LFA (fig. 25) were not changed because the 

general configuration of their respective potentiometric surfaces is not expected to change significantly. 

Historical maps of the UFA potentiometric surface 

indicate that this probably is a good assumption. The 

UFA potentiometric surface generally has decreased in 

altitude more or less uniformly over large areas as 

withdrawals have increased; this is consistent with a 

high-transmissivity aquifer, for which drawdowns tend 

to be relatively small and spread over relatively large 

areas. No attempt was made to account for movement 

of the interface between the freshwater and mineralized water flow systems that might occur under future 

conditions.

The specified-head boundaries in the UFA and 

LFA (fig. 25) were revised because the average 1998 

water levels at these boundaries are not valid under 

future conditions. The specified-head boundaries in 

Marion and Sumter Counties are less problematic 

because projected increases in withdrawals are relatively small in this area; although some of the largest 

increases in withdrawals are projected to occur in 

Orange County and are expected to have a relatively 

large effect at the model boundary in this area. All 

specified-head boundaries were converted to specified 

flow for predictive simulations as follows: simulated 

flows into or out of the specified-head cells were 

extracted from the calibrated model; specified-head 

cells were converted to variable-head cells; the 

MODFLOW Well Package was used to input the corresponding specified flows into each of the former 

specified-head cells; and a comparison between model 

results for the specified-head and specified-flow 

boundaries indicated that the largest discrepancy in 

simulated water level was only 0.001 ft. A specifiedflow boundary is equivalent to assuming that the 

hydraulic gradient at the boundary will remain 
Simulation of Ground-Water Flow 91

unchanged between average 1998 and projected future 

conditions. Although this formulation is an approximation of the unknown future boundary, it probably is 

relatively good: in Orange County where the largest 

drawdowns are expected, future withdrawals are projected to increase east of the model boundary in a 

pattern similar to that specified in the model west of 

the boundary, thereby maintaining a hydraulic gradient 

probably similar to that for average 1998 conditions. 

Furthermore, any error that might be caused by the 

approximation of future boundary conditions will be 

of lesser magnitude in the area of interest (Lake 

County and the Ocala NF) than near the model 

boundaries.

Projected Water Use

Withdrawals from the FAS within the model 

area in 2020 are projected to be 704 Mgal/d (about 

3.1 in/yr averaged over the model area), a 50 percent 

increase from 1998 rates. Nearly all of this increase is 

attributed to additional water withdrawn for public 

supply (fig. 4).

Pumping rates at existing wells and projected 

future wells in 2020 were provided by Brian McGurk 

(St. Johns River Water Management District, written 

commun., 2000), Jim Waylon (Southwest Florida 

Water Management District, written commun., 1999), 

and L.H. Motz (University of Florida, written commun., 1999). Domestic self-supplied ground-water 

withdrawals in 2020 (table 2) were estimated in a manner identical to that for 1998 rates based on projected 

rates of increase or decrease reported by SJRWMD 

(1998), SFWMD (1998), SWFWMD (1998), and 

Suwannee River Water Management District (1998). 

Additional uncertainty exists in projected domestic 

self-supplied withdrawals because the future locations 

of these withdrawals are unknown and were assumed 

to be at the same locations as in 1998. Withdrawals 

from UFA wells (fig. 46) are projected to increase 

from 1998 to 2020 by 44 percent to 576 Mgal/d. 

Withdrawals from LFA wells (fig. 47) are projected 

to increase from 1998 to 2020 by 73 percent to 

128 Mgal/d.

Artificial recharge resulting from septic tank 

leakage in 2020 (table 2) was estimated to be 60 percent of the projected 2020 rates for domestic self-supplied withdrawals in order to be consistent with the 

assumptions used in the calibrated model. This 

resulted in only a 1 percent increase in septic tank 

leakage from average 1998 conditions for the model 

area as a whole, but more significant changes were 

projected for individual counties (table 2). Facilities 

involved in the land application of reclaimed water 

probably will discharge at higher rates and will 

increase in number in the future, but these rates and 

locations are not well known. Therefore, artificial 

recharge from rapid infiltration basins and spray fields 

was assumed constant at 1998 rates. Recharge to the 

UFA by drainage wells was assumed constant at 1998 

rates because the objective of the predictive simulations was not to evaluate changes in climatic conditions that would affect drainage-well recharge rates.

The changes in simulated water levels and flows 

indicated by predictive simulations are unique to a 

particular set of pumping locations and rates. If actual 

2020 well locations or pumping rates differ from those 

described above, then corresponding simulations 

would be required to evaluate the revised 2020 

conditions.

Predicted Water Levels and Flows

The predicted water levels and flows for projected 2020 conditions demonstrate the effects of 

changes in withdrawals from the FAS and artificial 

recharge from septic tank leakage. Septic tank leakage 

generally has a small impact on the results of predictive simulations because the magnitude of and change 

in septic tank leakage is relatively small (table 2) and 

the total volume of water resulting from septic tank 

leakage is a small component of the water budget 

(about 0.14 in/yr averaged over model area). Therefore, in the following discussion the differences in 

simulated water levels and flows between 1998 and 

2020 can be attributed almost entirely to changes in 

the amount of ground water withdrawn from the FAS.

Drawdowns were calculated for all three aquifers (SAS, UFA, and LFA) by subtracting the simulated water level for average 1998 conditions from the 

predicted 2020 conditions (figs. 48, 49 and 50). Drawdowns generally were smallest in magnitude and areal 

extent in the SAS and greatest in the LFA. Average 

and maximum drawdowns in Lake County, the Ocala 

NF, and the entire model area are listed in table 6. The 

largest simulated drawdowns in each aquifer occurred 

in Orange County, where withdrawals generally are 

greatest in both the UFA and LFA. Significant drawdowns were simulated in Lake County, but in the 

Ocala NF drawdowns were relatively small.
92 Hydrogeology and Simulated Effects of Ground-Water Withdrawals from the Floridan Aquifer System in Lake County and in 

the Ocala National Forest and Vicinity, North-Central Florida

82°20′ 82°00′ 81°10′

29°40′

29°00′

28°20′

0

0

5 10 MILES

5 10 KILOMETERS

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

Universal Transverse Mercator projection, zone 17

LAKE

COUNTY

ORANGE COUNTY

SEMINOLE

COUNTY

VOLUSIA

COUNTY

OSCEOLA COUNTY

POLK COUNTY

PASCO

COUNTY

HERNANDO

COUNTY

SUMTER

COUNTY

CITRUS

COUNTY

MARION COUNTY

ALACHUA COUNTY

PUTNAM COUNTY

ST. JOHNS COUNTY

FLAGLER

COUNTY

MODEL

BOUNDARY

EXPLANATION

UPPER FLORIDAN AQUIFER

PUMPAGE, IN MILLION

GALLONS PER DAY

0 to 0.1

0.1 to 0.25

0.25 to 0.75

0.75 to 2

OCALA Greater than 2

NATIONAL

FOREST

BOUNDARY

Figure 46. Ground-water withdrawal rates for the Upper Floridan aquifer specified in the model, projected 

2020 conditions.
Simulation of Ground-Water Flow 93

82°20′ 82°00′ 81°10′

29°40′

29°00′

28°20′

0

0

5 10 MILES

5 10 KILOMETERS

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

Universal Transverse Mercator projection, zone 17

LAKE

COUNTY

ORANGE COUNTY

SEMINOLE

COUNTY

VOLUSIA

COUNTY

OSCEOLA COUNTY

POLK COUNTY

PASCO

COUNTY

HERNANDO

COUNTY

SUMTER

COUNTY

CITRUS

COUNTY

MARION COUNTY

ALACHUA COUNTY

PUTNAM COUNTY

ST. JOHNS COUNTY

FLAGLER

COUNTY

MODEL

BOUNDARY

EXPLANATION

LOWER FLORIDAN AQUIFER

PUMPAGE, IN MILLION

GALLONS PER DAY

0 to 0.1

0.1 to 0.25

0.25 to 0.75

0.75 to 2

Greater than 2 OCALA

NATIONAL

FOREST

BOUNDARY

Figure 47. Ground-water withdrawal rates for the Lower Floridan aquifer specified in the model, projected 

2020 conditions.
94 Hydrogeology and Simulated Effects of Ground-Water Withdrawals from the Floridan Aquifer System in Lake County and in 

the Ocala National Forest and Vicinity, North-Central Florida

Lake

Apopka

SWAMP

GREEN

82°20′ 82°00′ 81°10′

29°40′

29°00′

28°20′

0

0

5 10 MILES

5 10 KILOMETERS

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

Universal Transverse Mercator projection, zone 17

LAKE

COUNTY

ORANGE COUNTY

SEMINOLE

COUNTY

VOLUSIA

COUNTY

OSCEOLA COUNTY

POLK COUNTY

PASCO

COUNTY

HERNANDO

COUNTY

SUMTER

COUNTY

CITRUS

COUNTY

MARION COUNTY

ALACHUA COUNTY

PUTNAM COUNTY

ST. JOHNS COUNTY

FLAGLER

COUNTY

MODEL

BOUNDARY

EXPLANATION

DRAWDOWN, IN FEET

1 to 2

Less than 1

2 to 5

5 to 10

Greater than 10

SURFICIAL

AQUIFER SYSTEM

NOT SIMULATED

OCALA

NATIONAL

FOREST

BOUNDARY

Figure 48. Simulated drawdown in the surficial aquifer system from average 1998 conditions as a result of 

projected 2020 conditions.
Simulation of Ground-Water Flow 95

Lake

Apopka

SWAMP

GREEN

82°20′ 82°00′ 81°10′

29°40′

29°00′

28°20′

0

0

5 10 MILES

5 10 KILOMETERS

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

Universal Transverse Mercator projection, zone 17

LAKE

COUNTY

ORANGE COUNTY

SEMINOLE

COUNTY

VOLUSIA

COUNTY

OSCEOLA COUNTY

POLK COUNTY

PASCO

COUNTY

HERNANDO

COUNTY

SUMTER

COUNTY

CITRUS

COUNTY

MARION COUNTY

ALACHUA COUNTY

PUTNAM COUNTY

ST. JOHNS COUNTY

FLAGLER

COUNTY

MODEL

BOUNDARY

EXPLANATION

DRAWDOWN, IN FEET

1 to 2

2 to 5

5 to 10

Greater than 10

DECREASE IN SPRING FLOW,

IN PERCENT

G

L

N

0 to 10

10 to 20

20 to 35

Less than 1

G G

G

G

G G

GG

G

G

G

G G

G

G

N

N

N

LL

L

N

Silver

Springs

Beecher

Springs

Croaker

Hole

Spring

Salt

Springs

Silver Glen Springs

Sweetwater Springs

Juniper Springs

Fern Hammock Springs

Alexander Springs

Ponce de Leon

Springs

Blue Spring

Gemini Springs Messant

Spring

Seminole

Springs

Wekiva

Falls

flowing

borehole

Rock Springs

Wekiwa Springs Starbuck

Spring

Sanlando

Springs

Apopka

Spring

Bugg

Spring

Fenney

Spring

OCALA

NATIONAL

FOREST

BOUNDARY

Figure 49. Simulated drawdown in the Upper Floridan aquifer and simulated decrease in flow from selected 

Upper Floridan aquifer springs from average 1998 conditions as a result of projected 2020 conditions.
96 Hydrogeology and Simulated Effects of Ground-Water Withdrawals from the Floridan Aquifer System in Lake County and in 

the Ocala National Forest and Vicinity, North-Central Florida

SWAMP

GREEN

Lake

Apopka

82°20′ 82°00′ 81°10′

29°40′

29°00′

28°20′

0

0

5 10 MILES

5 10 KILOMETERS

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

Universal Transverse Mercator projection, zone 17

LAKE

COUNTY

ORANGE COUNTY

SEMINOLE

COUNTY

VOLUSIA

COUNTY

OSCEOLA COUNTY

POLK COUNTY

PASCO

COUNTY

HERNANDO

COUNTY

SUMTER

COUNTY

CITRUS

COUNTY

MARION COUNTY

ALACHUA COUNTY

PUTNAM COUNTY

ST. JOHNS COUNTY

FLAGLER

COUNTY

MODEL

BOUNDARY

EXPLANATION

DRAWDOWN, IN FEET

1 to 2

Less than 1

2 to 5

5 to 10

Greater than 10

LOWER FLORIDAN

AQUIFER NOT

SIMULATED

OCALA

NATIONAL

FOREST

BOUNDARY

Figure 50. Simulated drawdown in the Lower Floridan aquifer from average 1998 conditions as a result of 

projected 2020 conditions.
Simulation of Ground-Water Flow 97

The simulated drawdown in the SAS (fig. 48) 

primarily is a function of the depth of the simulated 

water table below land surface, the leakance of the 

ICU (fig. 36), and the simulated drawdown in the UFA 

(fig. 49). In areas where the 1998 simulated water 

table is shallower than the specified extinction depth, 

drawdown will be smaller than if the simulated water 

table were below the extinction depth, because natural 

net recharge actually increases by an amount equal to 

the reduction in flux simulated by the MODFLOW 

Evapotranspiration Package (fig. 29). For example, in 

Orange County east of Lake Apopka the simulated 

drawdown of the water table varies spatially even 

though the leakance of the ICU is relatively uniform 

(fig. 36) and the variation in drawdown in the UFA is 

fairly small (fig. 49). Greater drawdown occurs in 

areas where the 1998 simulated water table was below 

the extinction depth, such as in ridge areas (compare 

figure 48 with areas where land-surface altitude 

exceeds 150 ft shown in figure 3); smaller drawdown 

occurs where the 1998 simulated water table was 

between land surface and the extinction depth. The 

degree to which drawdown in the UFA is propagated 

to the SAS is controlled by the leakance of the ICU. 

Drawdown in the SAS (fig. 48) most closely matches 

that in the UFA (fig. 49) where the leakance of the 

ICU is greatest (fig. 36). This clearly demonstrates the 

importance of the ICU on the hydrology of the SAS.

Drawdowns simulated in the UFA (fig. 49) 

primarily are a function of the magnitude of the 

increase in ground-water withdrawals from the UFA 

(compare figures 32 and 46), the transmissivity of the 

UFA (fig. 35), the leakance of the MSCU (fig. 37), and 

the simulated drawdowns in the LFA (fig. 50). Drawdowns will be relatively large where the UFA transmissivity is small and the projected increase in 

withdrawals is large, such as in the southeastern corner 

of Lake County (fig. 49). However, drawdowns will be 

relatively small if transmissivity is large even though 

the projected increase in withdrawals is large, such as 

in the northwestern corner of Lake County (fig. 49). 

Drawdown in the LFA can significantly impact the 

UFA in areas where the leakance of the MSCU is 

large. In Orange County east of Lake Apopka, 

moderate increases in withdrawals from the UFA are 

projected to occur, but even greater increases in withdrawals are expected from the LFA; therefore, the 

large drawdowns in the LFA (fig. 50) resulting from 

increased pumping from the LFA and the large 

leakance of the MSCU produce relatively large drawdowns in the UFA (fig. 49). Transmissivity controls 

not only the magnitude of drawdown, but also the 

spatial distribution of drawdown. The effects of 

pumping at a well will propagate further from the well 

if the transmissivity of the aquifer is greater; likewise, 

drawdown will be confined closer to the well if the 

Table 6. Average and maximum drawdowns from average 1998 conditions as a 

result of projected 2020 conditions for two predictive scenarios simulated by the model

[ft, feet; NF, National Forest; SAS, surficial aquifer system; UFA, Upper Floridan aquifer; LFA, 

Lower Floridan aquifer]

Area Aquifer

Predictive scenario

Ground-water withdrawals 

and septic tank leakage 

adjusted to

projected 2020 values in 

entire model area

Ground-water withdrawals 

and septic tank leakage 

adjusted to

projected 2020 values in 

Lake County and

Ocala NF only

Average 

drawdown

(ft)

Maximum 

drawdown

(ft)

Average 

drawdown

(ft)

Maximum 

drawdown

(ft)

Lake County SAS 0.5 5.7 0.3 5.1

UFA 1.1 7.6 .7 6.8

LFA 1.4 4.3 .7 2.9

Ocala NF SAS .1 1.0 .1 .9

UFA .2 .8 .1 .5

LFA .3 .8 .2 .5

Entire model SAS .4 10.3 .1 5.1

UFA .9 11.8 .3 6.8

LFA 1.2 19.1 .3 2.9
98 Hydrogeology and Simulated Effects of Ground-Water Withdrawals from the Floridan Aquifer System in Lake County and in 

the Ocala National Forest and Vicinity, North-Central Florida

transmissivity of the aquifer is small. The relatively 

low transmissivity of the UFA in southern Lake 

County is the primary reason why drawdown from 

withdrawals in southeastern Lake County does not 

extend further west into the Green Swamp (fig. 49).

Drawdowns simulated in the LFA (fig. 50) 

primarily are a function of the magnitude of the 

increase in ground-water withdrawals from the LFA 

(compare figures 33 and 47), the transmissivity of the 

LFA, the leakance of the MSCU (fig. 37), and the simulated drawdown in the UFA (fig. 49). Spatial variation in LFA transmissivity is relatively small because 

the aquifer’s horizontal hydraulic conductivity is 

assumed to be constant. Drawdown in the LFA generally matches, in a qualitative manner, the distribution 

and magnitude of the increase in withdrawals from the 

aquifer. Drawdown in the UFA can significantly 

impact the LFA in areas where the leakance of the 

MSCU is large. For example, in northwestern Seminole County significant drawdowns in the LFA are 

simulated to occur even though there are no withdrawals from the LFA aquifer in this area. Increases in 

withdrawals from the UFA reduce the rate of downward leakage through the MSCU or even reverse the 

direction of leakage, resulting in drawdowns in the 

LFA. A similar situation exists in southern Lake 

County, but the driving mechanism is somewhat different. The majority of the drawdown within this area is 

caused by eastward lateral flow in the LFA induced by 

the larger drawdowns in Orange County. The low leakance of the MCU (fig. 37) precludes much leakage 

between the UFA and LFA in southern Lake County.

The simulated water budget for the entire model 

area for projected 2020 conditions indicates that withdrawals from the FAS increased by 1 in/yr, or 50 percent, compared to average 1998 conditions. Because 

all predictive simulations were for steady-state conditions, any increase in withdrawals must be balanced by 

an equal increase in inflow from or decrease in outflow to other budget components. Comparing simulated 2020 budget components to those for average 

1998 conditions (fig. 43), the 1 in/yr increase in well 

withdrawals can be accounted for as follows: outflow 

to ET nodes representing the combined effects of 

excess ET and excess overland runoff decreased 

0.4 in/yr; inflow from river nodes representing streams 

or flow-through lakes increased 0.1 in/yr, while outflow to river nodes representing streams, flow-through 

lakes, or wetlands decreased 0.2 in/yr, for a net 

decrease of 0.3 in/yr in base flow; and outflow to drain 

nodes representing springflow decreased 0.3 in/yr.

Simulated water budgets for average 1998 and 

projected 2020 conditions also were compiled for 

Lake County and for the Ocala NF (figs. 51 and 52, 

respectively). The flow needed to meet projected 2020 

conditions in Lake County is expected to increase 

1.3 in/yr from that for average 1998 conditions as 

follows: withdrawals from the FAS are projected to 

increase 1.1 in/yr, artificial recharge resulting from 

septic tank leakage is projected to decrease 0.1 in/yr, 

and boundary inflow is simulated to decrease 0.1 in/yr 

because of increases in withdrawals from outside of 

Lake County. This 1.3 in/yr is simulated to be balanced by the following changes in other budget components (fig. 51): outflow to ET nodes representing 

the combined effects of excess ET and excess overland 

runoff decreases 0.5 in/yr; inflow from river nodes 

representing streams or flow-through lakes increases 

0.1 in/yr while outflow to river nodes representing 

streams, flow-through lakes, or wetlands decreases 

0.3 in/yr for a net decrease of 0.4 in/yr in base flow; 

outflow across county boundaries decreases 0.1 in/yr; 

and outflow to drain nodes representing springflow 

decreases 0.3 in/yr. Simulated flow changes in the 

Ocala NF are minimal because changes in withdrawals 

from the FAS in the Ocala NF are negligible. The 

Ocala NF is slightly affected by withdrawals from surrounding areas. Boundary inflow decreases 0.2 in/yr 

from average 1998 to projected 2020 conditions; this 

is simulated to be balanced by a 0.1 in/yr decrease in 

outflow to ET nodes representing the combined effects 

of excess ET and excess overland runoff and a 0.1 in/yr 

decrease in outflow to drain nodes representing springflow (fig. 52).

The decrease in springflow from average 1998 

to projected 2020 conditions that is simulated to occur 

partially accounts for the additional water removed 

from the ground-water flow system by increased withdrawals from the FAS. Spring pool altitudes were 

assumed not to change as a result of projected 2020 

conditions. The percent change in flow for any individual spring is equal to the percent change in the head 

difference (aquifer water level minus spring pool altitude) at the spring. For example, if the simulated head 

difference under 1998 conditions were 2 ft and the 

drawdown in the UFA 1 ft, simulated springflow 

would decrease 50 percent; however, if the simulated 

head difference were 10 ft the simulated springflow 

would decrease only 10 percent for the same 1 ft 

drawdown.
Simulation of Ground-Water Flow 99

SURFICIAL AQUIFER

SYSTEM

Total flowc=13.6/13.8

Rechargea

24.5/24.5

ETb

14.2/13.7

Artificial net

recharge

0.3/0.2

Discharge to

surface water

3.9/3.5

9.6/10.2

2.9/2.7

ETb

0/0

Rechargea

0/0

Discharge to

surface water

0/0

Spring flow

2.9/2.6

Boundary

inflow

3.8/3.7

UPPER FLORIDAN AQUIFER

Total flowc=14.2/14.7

1.5/1.6

0.8/0.8

Boundary

outflow

4.9/4.8

Boundary

inflow

0.2/0.2

LOWER FLORIDAN AQUIFER

Total flow =1.7/1.8

Boundary

outflow

0.8/0.8

Well

withdrawals

2.0/3.0

Well

withdrawals

0.1/0.2

Boundary

outflow

0.1/0.1

3.9/3.5

Boundary

inflow

0.1/0.1

Artificial net

recharge

0/0

EXPLANATION

Represents the maximum rate of natural net recharge.

Simulated using the MODFLOW Recharge Package.

Represents the combined effects of excess evapotranspiration

and excess overland runoff. Simulated using the MODFLOW

Evapotranspiration Package.

The sum of all inflows or outflows for the aquifer. The net

difference between the recharge and evapotranspiration

components is counted as a single inflow representing natural

net recharge.

ALL VALUES ARE IN INCHES PER YEAR AVERAGED OVER THE

AREA OF LAKE COUNTY (approximately 1,150 square miles)

a

b

c

FLOW -- First number represents average 1998 conditions;

second number represents projected 2020 conditions

Figure 51. Simulated volumetric water budget for the aquifer system in Lake County, average 1998 and 

projected 2020 conditions.
100 Hydrogeology and Simulated Effects of Ground-Water Withdrawals from the Floridan Aquifer System in Lake County and in 

the Ocala National Forest and Vicinity, North-Central Florida

SURFICIAL AQUIFER

SYSTEM

Total flowc=21.1/21.2

Rechargea

26.3/26.3

ETb

9.5/9.4

Artificial net

recharge

0.1/0.1

Discharge to

surface water

5.1/5.1

15.6/15.7

4.1/4.1

ETb

0/0

Rechargea

0/0

Discharge to

surface water

0.5/0.5

Spring flow

6.9/6.8

Boundary

inflow

3.9/3.7

UPPER FLORIDAN AQUIFER

Total flowc=21.5/21.4

1.3/1.3

2.0/2.0

Boundary

outflow

8.5/8.5

Boundary

inflow

1.0/1.0

LOWER FLORIDAN AQUIFER

Total flow = 2.3/2.3

Boundary

outflow

0.3/0.3

Well

withdrawals

0.2/0.2

Well

withdrawals

0/0

Boundary

outflow

0.4/0.4

Boundary

inflow

0.1/0.1

Artificial net

recharge

0/0

EXPLANATION

ALL VALUES ARE IN INCHES PER YEAR AVERAGED OVER THE

AREA OF THE OCALA NATIONAL FOREST (approximately 690 square miles)

9.5/9.4

Represents the maximum rate of natural net recharge.

Simulated using the MODFLOW Recharge Package.

Represents the combined effects of excess evapotranspiration

and excess overland runoff. Simulated using the MODFLOW

Evapotranspiration Package.

The sum of all inflows or outflows for the aquifer. The net

difference between the recharge and evapotranspiration

components is counted as a single inflow representing natural

net recharge.

a

b

c

FLOW -- First number represents average 1998 conditions;

second number represents projected 2020 conditions

Figure 52. Simulated volumetric water budget for the aquifer system in the Ocala National Forest, average 1998 

and projected 2020 conditions.
Simulation of Ground-Water Flow 101

Simulated percent decreases in springflow vary 

considerably among the springs in the model area. The 

simulated 2020 flows for all first, second, and thirdmagnitude springs are shown in table 7, and the range 

in percent decreases in flow of all first- and secondmagnitude springs is shown in figure 49. The greatest 

decreases occurred for springs having small head differences (table 7) and located in areas with large drawdowns in the UFA (fig. 49). Of all the first- or secondmagnitude springs, each of the following springs had a 

simulated flow decrease greater than 10 percent: Seminole, Rock, Wekiwa, Starbuck, Sanlando, and Apopka 

(fig. 49 and table 7). Of these six springs, Starbuck, 

Apopka, and Sanlando Springs had the largest simulated decreases in flow of 25, 28, and 33 percent, 

respectively. The largest simulated flow decreases for 

first- or second-magnitude springs in Lake County 

were at Bugg (9 percent), Seminole (12 percent), and 

Apopka Springs (28 percent). The largest simulated 

flow decreases for first- or second-magnitude springs 

in the Ocala National Forest were at Alexander (1 percent), Fern Hammock (1 percent), and Juniper Springs 

(4 percent). Some third-magnitude and smaller springs 

experienced larger percentage decreases in springflow; 

for example, simulated springflow at Bear and Holiday 

Springs in Lake County decreased 44 and 69 percent, 

respectively (table 7).

Ground-water levels within Lake County and 

the Ocala NF are affected by ground-water withdrawals in adjacent areas. More than one-half of the total 

increase in withdrawals from 1998 to 2020 conditions 

is projected to occur in Orange, Seminole, and Marion 

Counties. The effects of this pumping propagate into 

Lake County, the Ocala NF, and surrounding counties. 

An additional predictive simulation was performed by 

adjusting ground-water withdrawals and septic tank 

leakage to projected 2020 conditions only in Lake 

County and the Ocala NF. All other stresses in the 

remainder of the model area were maintained at average 1998 levels. Average and maximum drawdowns 

resulting from this predictive scenario are listed in 

table 6 and can be compared to drawdowns simulated 

under the first predictive scenario. Drawdowns in the 

SAS in Lake County generally were similar in spatial 

distribution to those in figure 48 but were smaller in 

magnitude. Drawdowns in the UFA in Lake County 

exceeded 2 ft in the vicinity of Clermont and in the 

southeastern corner of the County. Drawdowns in the 

LFA in Lake County exceeded 1 ft in the vicinities of 

Lady Lake, Leesburg, Eustis, Mount Dora, and 

Clermont. Drawdowns were relatively small in the 

Ocala NF (table 6). All these drawdowns are less than 

those considering projected 2020 withdrawals within 

the entire model area (figs. 48, 49, and 50; and 

table 6). These results demonstrate the regional scale 

of ground-water flow in the FAS and the importance of 

considering the regional ground-water system surrounding the area of interest when assessing the effects 

of future water-use needs.

Lake and Wetland Water Levels

The interaction between lakes or wetlands and 

the ground-water flow system is quite complex and 

difficult to simulate. The response of a lake or wetland 

to hydrologic stresses can be significantly affected by 

local hydrologic and lithologic conditions. Such variations can be large in the mantled karst environment 

that exists throughout much of the model area. Using 

high-resolution seismic-reflection data, Tihansky and 

others (1996) confirmed the sinkhole origin of four 

lakes along the Lake Wales Ridge in Polk and Highlands Counties. They concluded that the lithology and 

distribution of geologic materials under and in the 

immediate vicinity of a lake can be highly variable, yet 

important in determining the response of the lake to 

changes in stress. Most of the lakes in the model area 

probably are similar to these four lakes; likewise, wetlands where the water level is above land surface probably behave similarly to these lakes. Therefore, the 

effects of increased withdrawals from the FAS can 

differ considerably between any two lakes or wetlands, 

even those in close proximity to each other.

The hydrologic behavior of individual lakes 

cannot be accurately simulated by a regional-scale 

model, such as the one presented in this report. The 

present model should be used only in a more qualitative manner to determine areas where lakes are more 

likely to be affected by increased withdrawals from 

the FAS. A qualitative interpretation of lake-level 

declines in closed-basin lakes can be estimated from 

figure 48. Because net recharge at closed-basin lakes 

(parameter Ncbl, table 3) is assumed to remain constant from average 1998 to projected 2020 conditions, simulated drawdown of closed-basin lake 

levels primarily is controlled by the leakance of 

the ICU and the simulated drawdown in the UFA.
102 Hydrogeology and Simulated Effects of Ground-Water Withdrawals from the Floridan Aquifer System in Lake County and in 

the Ocala National Forest and Vicinity, North-Central Florida

Table 7. Simulated discharge from selected Upper Floridan aquifer springs, projected 2020 conditions

[USGS, U.S. Geological Survey; ft, feet; cfs, cubic feet per second]

USGS 

site 

identification 

number

Station name County

Head 

difference 

at spring1

(ft)

1

Head difference was calculated as the altitude of the average 1998 Upper Floridan aquifer water level (interpolated from May 1998 (fig. 2) and 

September 1998 (Bradner, 1999) potentiometric maps) in the model cell containing the spring minus the measured or estimated 1998 spring pool altitude.

Simulated 

discharge, 

2020 

(cfs)

Percent 

change in 

discharge, 

1998 - 2020

284740081251701 Apopka (Gourd Neck) Spring near Oakland Lake 7.5 24 -28

284612081303401 Bear Spring near Astatula Lake 3.0 1.1 -44

02234620 Double Run Road Seepage (into Little Lake Harris) near Astatula Lake 2.2 1.2 -51

02236205 Holiday Springs at Yalaha Lake 1.0 1.4 -69

02243550 Blue Springs, Park Drive near Yalaha Lake 5.2 2.3 -17

285224081262400 Bugg Spring near Okahumpka Lake 6.3 11 -9.5

285038081270100 Wekiva Falls Resort flowing borehole Lake 2.4 16 -18

291136081381000 Island Spring, Wekiva River Lake 9.8 6.1 -7.2

02237322 Seminole Springs near Sorrento Lake 2.2 35 -12

02235250 Messant Spring near Sorrento Lake 11.0 18 -2.7

292725081393500 Camp La-No-Che Springs near Paisley Lake 8.8 1.0 -5.4

02234610 Mosquito Springs Run, Alexander Springs Wilderness Lake 0.7 2.0 -14

283903081430100 Alexander Springs Lake 9.5 103 -1.1

02236160 Juniper Creek South Tributary Seepage near Astor Lake 15.4 6.1 -.7

284038081443201 Silver Glen Springs near Astor Marion 5.4 102 -.5

02235255 Fern Hammock Springs near Ocala Marion 6.7 13 -1.3

285702081322400 Juniper Springs near Ocala Marion 2.0 11 -4.3

284241081281800 Morman Branch Headwater Seepage at SR19 Marion 1.0 2.5 -11

02236147 Morman Branch Seepage (into Juniper Creek) near Astor Marion 11.5 4.6 -.8

283400081405100 Silver Springs near Ocala Marion 3.7 889 -3.4

285105081263800 Sweetwater Springs along Juniper Run Marion 9.1 14 -.8

02237400 Salt Springs Marion 7.9 84 -.3

291112081400400 Wells Landings Springs Marion 7.9 5.0 -.3

292735081394500 Camp Seminole Spring, Girl Scout Camp, Orange Springs Marion 6.9 1.0 -.8

02234600 Orange Spring near Orange Springs Marion 17.6 2.5 -.3

284437081491700 Wekiwa Springs, Wekiwa Springs State Park Orange 11.3 55 -17

02239500 Witherington Springs, Wekiwa Springs Park near Apopka Orange 9.6 3.1 -33

284452081495400 Rock Springs near Apopka Orange 8.0 45 -19

284047081441501 Croaker Hole Spring near Welaka Putnam 7.3 94 -.1

285318081295200 Beecher Springs near Fruitland Putnam 11.4 12 .0

292618081412100 Mud Spring near Welaka Putnam 7.2 1.7 -.2

02244022 Welaka Spring Putnam 11.7 1.0 -.1

02236220 Satsuma Spring near Satsuma Putnam 17.2 1.3 .1

284922081250300 Sanlando Springs near Longwood Seminole 9.4 15 -33

02236132 Palm Springs near Longwood Seminole 13.2 4.0 -24

285102081263900 Starbuck Spring near Longwood Seminole 13.1 11 -25

02236130 Miami Springs near Longwood Seminole 12.7 4.0 -21

292521081551200 Shady Brook Spring #5 (South Panasoffkee Spring Group) near 

Lake Panasoffkee

Sumter 9.4 3.0 -.7

292540081552400 Shady Brook Spring #4 (South Panasoffkee Spring Group) near 

Lake Panasoffkee

Sumter 1.5 2.9 -4.1

292542081552600 Shady Brook Spring #3 (South Panasoffkee Spring Group) near 

Coleman

Sumter 1.0 2.8 -7.2

293021081570600 Shady Brook Spring #2 (South Panasoffkee Spring Group) near 

Coleman

Sumter 3.0 2.9 -2.7

284455081494100 Fenney Spring (headspring of Shady Brook to Lake Panasoffkee) 

near Coleman

Sumter 2.0 45 -1.2

293038081563800 Little Jones Creek Spring #3 (North Panasoffkee Spring Group) 

near Wildwood

Sumter 1.0 2.6 -12

291200081390601 Little Jones Creek Spring #2 (North Panasoffkee Spring Group) 

near Wildwood

Sumter 1.7 4.5 -9.4

02236095 Little Jones Creek Headspring (North Panasoffkee Spring Group) 

near Wildwood

Sumter 4.2 7.5 -5.8

290220081260400 Gemini Springs near DeBary Volusia 17.8 10 -6.0

283844081422300 Blue Spring near Orange City Volusia 5.8 159 -2.8

284940081303800 Ponce De Leon Springs near DeLand Volusia 11.7 22 -1.0
Simulation of Ground-Water Flow 103

The most definitive conclusion that can be drawn from 

the regional-scale model concerning impacts on lake 

levels caused by withdrawals from the FAS is that 

lake-level declines are more likely to be greater in 

areas where the magnitude of the drawdown in the 

UFA is greater (fig. 49), the leakance of the ICU is 

greater (fig. 36), and no surface-water inflow to or outflow from the lake exists. Applying these criteria to the 

areas of primary interest, Lake County and the Ocala 

NF, closed-basin lakes are most likely to experience 

the greatest decreases in lake levels under projected 

2020 conditions in the following areas: southern Lake 

County, generally east and south of the Clermont 

chain-of-lakes, and east-central Lake County, generally in the vicinity of Mount Dora. However, anywhere 

the ICU has a high leakance, such as many areas in the 

Ocala NF, the potential exists for relatively large 

declines in lake levels if large drawdowns also occur in 

the UFA.

The effects of projected 2020 ground-water 

withdrawals on wetlands is simulated differently by 

the model depending on the position of the simulated 

water table. Where the simulated 1998 water table is 

greater than 3 ft above the median land-surface altitude in a wetland, an increase in 2020 withdrawals 

from the FAS can cause a decrease in simulated 

ground-water discharge to the wetland. Generally, little drawdown of the simulated water table will occur 

because the decrease in simulated ground-water discharge to the wetland occurs instead. Where the simulated 1998 water table is less than or equal to 3 ft 

above the median land-surface altitude in a wetland, an 

increase in 2020 withdrawals from the FAS can cause 

drawdown of the simulated water table in the wetland. 

Because ground-water discharge to the wetland is simulated to be zero in this case, simulated drawdown 

generally will be larger than if ground-water discharge 

were simulated to occur. The hydrologic behavior of 

individual wetlands cannot be accurately simulated by 

a regional-scale model, such as the one presented in 

this report. The present model should be used only in a 

more qualitative manner to determine areas where 

wetlands are more likely to be impacted by increased 

withdrawals from the FAS. A qualitative interpretation 

of water-table declines in the vicinity of wetlands can 

be estimated from figure 48. Factors affecting the 

magnitude and location of water-table declines in wetlands are the same as those described for lake levels. 

Water-table declines in wetlands are more likely to be 

greater in areas where the magnitude of the drawdown 

in the UFA is greater (fig. 49), the leakance of the ICU 

is greater (fig. 36), and no surface-water inflow to or 

outflow from the wetland exists. Likewise, in the areas 

of primary interest, Lake County and the Ocala NF, 

wetlands are most likely to experience the greatest 

decreases in water levels under projected 2020 conditions in the same areas as those described for closedbasin lakes: southern Lake County, generally east and 

south of the Clermont chain-of-lakes, and east-central 

Lake County, generally in the vicinity of Mount Dora. 

However, anywhere the ICU has a high leakance, such 

as many areas in the Ocala NF, the potential exists for 

relatively large water-table declines in wetlands if 

large drawdowns also occur in the UFA.

Spring and Well-Field Contributing Areas

A contributing area for a spring or discharging 

well is defined as the surface area that delineates the 

location of water entering the ground-water system, 

either at the water table or by induced infiltration from 

surface-water bodies, that eventually flows to a spring 

or well and discharges (Reilly and Pollock, 1993, 

p. 2). Often the objective of defining a contributing 

area is to delineate areas where regulatory or management measures may be applied to minimize the likelihood of contaminating source water to springs or 

wells. A contributing area for a spring or well is not 

the same as its cone of depression: sources of water to 

a spring or well can be located considerable distances 

beyond the area of influence of the feature; likewise, 

source water can originate from only a small area 

within the area of influence. Consequently, the configuration of a contributing area can be quite complex 

and generally is best delineated by using a threedimensional ground-water flow model.

The USGS particle-tracking program MODPATH 

(Pollock, 1994) was used with ground-water flow 

model results to delineate contributing areas. Particletracking techniques can only simulate the advective 

transport of solutes and cannot be used to calculate 

solute concentrations in ground water because these 

techniques do not consider dispersion, degradation, or 

retardation processes. Nevertheless, particle-tracking 

techniques are well suited for defining contributing 

areas because the configuration of a contributing area 

is dependent on the hydraulics of the ground-water 

flow system, not the transport or geochemistry of a 

solute.
104 Hydrogeology and Simulated Effects of Ground-Water Withdrawals from the Floridan Aquifer System in Lake County and in 

the Ocala National Forest and Vicinity, North-Central Florida

Effective porosity values for each aquifer and 

confining unit in the model are the only hydrologic 

data required, in addition to standard MODFLOW 

input and output, for a steady-state particle-tracking 

analysis. Effective porosity is the fraction of interconnected pore space in a volume of rock or unconsolidated sediment. An effective porosity value of 0.40 

was used for the SAS and ICU and 0.20 was used for 

the UFA, MSCU/MCU, and LFA. The value of effective porosity only affects particle traveltime and has no 

effect on particle paths calculated by MODPATH; 

therefore, error in the values used for effective porosity 

will not affect the configuration of contributing areas.

Particle starting locations were based on volumetric flow rates; one particle represented approximately 1,000 ft3

/d (7,480 gal/d) of springflow or 

100 ft3

/d (748 gal/d) of well discharge. Particles were 

located on each inflow face of the model cell containing the spring or well; the number of particles were 

proportional to the simulated flow though each cell 

face and were distributed in an array proportional to 

the dimensions of each cell face. For example, if the 

inflow through the face of a cell containing a spring 

were 500,000 ft3/d and the cell face were 2,500 ft wide 

and 400 ft high, particle starting locations were distributed in an array of 56 particles in the horizontal direction and 9 particles in the vertical direction. Particles 

moved through the ground-water system in a backward-tracking manner until they stopped at the water 

table. Because of the proportional arrangement of 

particle starting locations, the density of particle 

ending locations at the water table is proportional to 

the contributing recharge rate. The contributing 

recharge rate is defined in this report as the flow quantity that represents the amount of net recharge at the 

water table or induced infiltration from surface-water 

bodies that eventually flows to the spring or well of 

interest, as opposed to other downgradient discharge 

locations. Contributing recharge rates might vary 

considerably throughout a contributing area; for example, the majority of the water discharged by a spring or 

well might originate in only a small part of its contributing area.

Two springs and two UFA well fields were 

selected for particle-tracking analyses. Apopka Spring 

and Alexander Springs were selected because although 

both are large-discharge springs, each is affected by 

different hydrologic stresses. Apopka Spring is located 

in southern Lake County, where effects from projected 

2020 withdrawals are expected to be relatively large; 

conversely, Alexander Springs is located in the Ocala 

NF, where effects from projected 2020 withdrawals 

are expected to be minimal. Several wells in east-central Lake County, collectively referred to as the 

“Central well field,” and several wells in the southeastern 

corner of Lake County, collectively referred to as the 

“South well field,” were selected because they are 

located in areas expected to experience significant 

impacts from projected 2020 withdrawals.

An attempt was made to select springs and well 

fields that were not located in weak sinks. A weak sink 

is a model cell that contains internal sinks that do not 

discharge at a rate large enough to consume all the 

water entering the cell (Pollock, 1994, p. 2-17); by 

comparison, a strong sink is a model cell in which all 

the water entering the cell is discharged by the internal 

sinks. This problem is exacerbated by large cell size 

and high transmissivity, both of which are common in 

regional ground-water flow models of the FAS. 

Apopka and Alexander Springs and the Central and 

South well fields are all contained in weak sink cells 

under average 1998 conditions, projected 2020 conditions, or both. Springs and well fields that were 

contained in strong sink cells under both average 1998 

and projected 2020 conditions and also were located in 

geographic areas of interest were not available in the 

present model. The degree to which a weak sink 

affects the delineation of a contributing area varies and 

will be discussed in the following paragraphs for each 

of the selected springs or well fields.

The contributing areas depicted in figures 53 

and 54 demonstrate configurations that are typical in a 

complex hydrogeologic environment. The contributing 

area for Apopka Spring covers approximately 30 mi2

based on average 1998 conditions; a flux of approximately 15 in/yr distributed evenly over the contributing area would be required to supply the total 

springflow of 33 ft3/s (21 Mgal/d). Most of the water 

that discharges from Apopka Spring originates as 

recharge at the water table north of Lake Louisa; a 

lesser amount originates as recharge at the water table 

south of Lake Louisa; and a small amount (less than 

4 percent) originates as leakage from Lake Louisa 

and other surface-water bodies (fig. 53). The contributing area for Alexander Springs covers approximately 76 mi2 based on average 1998 conditions; a 

flux of approximately 18 in/yr distributed evenly 

over the contributing area would be required to 

supply the total springflow of 104 ft3/s (67 Mgal/d).
Simulation of Ground-Water Flow 105

Lake

Apopka

82°20′ 82°00′ 81°10′

29°40′

29°00′

28°20′

0

0

5 10 MILES

5 10 KILOMETERS

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

Universal Transverse Mercator projection, zone 17

LAKE

COUNTY

ORANGE COUNTY

SEMINOLE

COUNTY

VOLUSIA

COUNTY

OSCEOLA COUNTY

POLK COUNTY

PASCO

COUNTY

HERNANDO

COUNTY

SUMTER

COUNTY

CITRUS

COUNTY

MARION COUNTY

ALACHUA COUNTY

PUTNAM COUNTY

ST. JOHNS COUNTY

FLAGLER

COUNTY

MODEL

BOUNDARY

EXPLANATION

CONTRIBUTING RECHARGE RATE,

IN MILLION GALLONS PER DAY

Less than 0.025

0.025 to 0.05

0.05 to 0.1

0.1 to 0.2

0.2 to 0.3

Greater than 0.3

G

.

UPPER FLORIDAN AQUIFER

SPRING

UPPER FLORIDAN AQUIFER

WELL FIELD

G

G

J

J FLOW TO

SOUTH

WELL FIELD

FLOW TO

APOPKA

SPRING

FLOW TO

CENTRAL

WELL FIELD

FLOW TO

ALEXANDER

SPRINGS

OCALA

NATIONAL

FOREST

BOUNDARY

Figure 53. Contributing areas for selected springs and public-supply well fields based on particle-tracking 

analyses of simulated steady-state 1998 conditions.
106 Hydrogeology and Simulated Effects of Ground-Water Withdrawals from the Floridan Aquifer System in Lake County and in 

the Ocala National Forest and Vicinity, North-Central Florida

SWAMP

GREEN

Lake

Dora

Lake

Lake

Louisa

Lake

Harris

Apopka

82°20′ 82°00′ 81°10′

29°40′

29°00′

28°20′

0

0

5 10 MILES

5 10 KILOMETERS

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

Universal Transverse Mercator projection, zone 17

LAKE

COUNTY

ORANGE COUNTY

SEMINOLE

COUNTY

VOLUSIA

COUNTY

OSCEOLA COUNTY

POLK COUNTY

PASCO

COUNTY

HERNANDO

COUNTY

SUMTER

COUNTY

CITRUS

COUNTY

MARION COUNTY

ALACHUA COUNTY

PUTNAM COUNTY

ST. JOHNS COUNTY

FLAGLER

COUNTY

MODEL

BOUNDARY

EXPLANATION

CONTRIBUTING RECHARGE RATE,

IN MILLION GALLONS PER DAY

Less than 0.025

0.025 to 0.05

0.05 to 0.1

0.1 to 0.2

0.2 to 0.3

Greater than 0.3

G

.

UPPER FLORIDAN AQUIFER

SPRING

UPPER FLORIDAN AQUIFER

WELL FIELD

G

G

.

. FLOW TO

SOUTH

WELL FIELD

FLOW TO

APOPKA

SPRING

FLOW TO

CENTRAL

WELL FIELD

FLOW TO

ALEXANDER

SPRINGS

OCALA

NATIONAL

FOREST

BOUNDARY

Figure 54. Contributing areas for selected springs and public-supply well fields based on particle-tracking 

analyses of simulated steady-state projected 2020 conditions.
Simulation of Ground-Water Flow 107

The majority of water that discharges from Alexander 

Springs originates as recharge at the water table southwest of the spring in the south-central part of the Ocala 

NF (fig. 53). Water originating in the part of the contributing area that extends into central Lake County 

(fig. 53) accounts for less than 6 percent of the total 

springflow; this source water moves along very long 

flow paths, with most of the water passing through the 

LFA before moving back up into the UFA and discharging at the spring. Only Apopka Spring under 

average 1998 conditions is a strong sink. Apopka 

Spring under projected 2020 conditions and Alexander 

Springs under both average 1998 and projected 2020 

conditions are weak sinks. However, the effect on contributing areas probably is insignificant because the 

flow that passes through these weak sinks, as opposed 

to discharging at the spring, is negligible (less than 

0.5 percent of the respective springflow).

Changes in contributing areas from average 

1998 to projected 2020 conditions were relatively 

small for both springs (figs. 53 and 54). This was 

expected for Alexander Springs because springflow is 

simulated to decrease only 1 percent to 103 ft3/s 

(67 Mgal/d) and relatively little pumping exists in the 

vicinity of the spring contributing area. However, simulated flow at Apopka Spring under projected 2020 

conditions is 24 ft3/s (16 Mgal/d), a decrease of 

28 percent; yet the size of the contributing area 

decreases a relatively small amount from 30 to 26 mi2

. 

The decrease in springflow is balanced by decreases in 

contributing recharge rates in addition to the decrease 

in contributing area size (figs. 53 and 54). A flux of 

approximately 13 in/yr distributed evenly over the 

contributing area would be required to supply the 

reduced flow from Apopka Spring under projected 

2020 conditions.

The configuration of and changes in contributing areas for wells are governed by the same processes 

as those for springs. The Central well field consists of 

four wells located within the same model cell. The 

total pumpage from these wells was 2.4 Mgal/d 

(3.8 ft3/s) in 1998; this is projected to nearly double to 

4.6 Mgal/d (7.1 ft3

/s) in 2020. The Central well field 

was simulated as a weak sink under average 1998 conditions, but as a strong sink with the increase in pumping projected under 2020 conditions. A flow of 

approximately 0.3 Mgal/d (0.5 ft3/s) passes through 

the weak sink containing the Central well field and 

exits from the east face of the cell under average 1998 

conditions. Therefore, the contributing area (fig. 53) 

actually is the contributing area for the model cell, 

which has a total flow of 2.7 Mgal/d (4.2 ft3

/s); the 

contributing area for only the Central well field would 

be smaller, would have lower contributing recharge 

rates, or both. Nevertheless, the actual contributing 

area for the Central well field probably is not greatly 

different from that shown in figure 53. The South well 

field consists of four wells located in the same model 

cell. The total pumpage from these wells was only 

0.060 Mgal/d (0.1 ft3

/s) in 1998, but this is projected 

to increase greatly to 4.5 Mgal/d (7.0 ft3/s) in 2020. 

Well field pumpage was so small in 1998 (about 9 percent of the total flow in the cell) that the contributing 

area for the well field is much too small to be delineated with a regional flow model. The contributing 

area shown in figure 53 is for the 0.7 Mgal/d (1.1 ft3

/s) 

of total flow in the model cell. Under projected 2020 

conditions the South well field is a weak sink because 

a small amount of downward leakage through the 

MSCU/MCU is simulated to occur. This downward 

leakage, however, represents only 0.3 percent of the 

projected 2020 well field pumpage and is not of sufficient magnitude to have a significant effect on the 

delineation of the contributing area for the South well 

field.

Changes in contributing areas from average 

1998 to projected 2020 conditions are significant for 

the Central and South well fields (figs. 53 and 54) 

because large percentage increases in withdrawals are 

projected to occur at these wells. For the Central well 

field, the contributing recharge area increased in size 

from 9.2 mi2

 to 16 mi2, although the average contributing recharge flux decreased slightly from 6.3 in/yr to 

6.0 in/yr. This is a direct result of the relatively low 

rates of net recharge at the water table in the vicinity of 

the Central well field contributing area (fig. 44); consequently, the size of the contributing area must 

increase in order to capture enough water to meet the 

increased demand. In fact, approximately 38 percent 

of the water withdrawn by the Central well field under 

projected 2020 conditions is simulated to originate as 

net recharge at the water table south of Lakes Dora and 

Harris (fig. 54). The South well field is located just 

east of the Green Swamp where rates of net recharge at 

the water table also are relatively low (fig. 44). The 

contributing area for the South well field under projected 2020 conditions covers approximately 13 mi2

and extends west and southwest into the Green Swamp 

over a relatively large area. Contributing recharge rates 

are relatively low in this area (fig. 54), and the average 

contributing recharge flux is 7.4 in/yr.
108 Hydrogeology and Simulated Effects of Ground-Water Withdrawals from the Floridan Aquifer System in Lake County and in 

the Ocala National Forest and Vicinity, North-Central Florida

Effects of Parameter Uncertainty on Model 

Predictions

The uncertainty that exists in parameter values 

from the calibrated model (table 4) also affects the 

reliability of model results. For example, if the vertical hydraulic conductivity of the MSCU/MCU could 

have been more accurately estimated during model 

calibration, then the simulated drawdowns in the 

UFA and LFA might have been significantly different 

from those derived from the calibrated model. Additional analyses were performed in order to illustrate 

the possible effects of parameter uncertainty on model 

predictions.

Linear 95-percent confidence intervals were 

calculated for simulated drawdowns using YCINT-2000 

(Hill and others, 2000), a post-processing program for 

MODFLOW-2000. The length of a linear 95-percent 

confidence interval on a simulated water-level difference, or drawdown, is dependent on the change in sensitivities of the simulated water level with respect to 

each parameter between a base simulation and a predictive simulation (subject to different hydrologic 

stresses or boundary conditions) and on the parameter 

variance-covariance matrix for calibration conditions 

(Hill, 1994, p. 36). The length of linear 95-percent 

confidence intervals will be small in areas where the 

changes in sensitivities are small for the parameters 

with a small variance and small covariances; simulated 

drawdowns in such areas should be interpreted as 

being more reliable. Likewise, the length of linear 

95-percent confidence intervals will be large in areas 

where the changes in sensitivities are large for the 

parameters with a large variance and large covariances; simulated drawdowns in these areas should be 

interpreted as being less reliable.

Linear 95-percent confidence intervals were 

calculated for simulated drawdowns in all 60,487 

active model cells; the half-lengths of the confidence 

intervals are depicted in figures 55, 56, and 57. Because 

these are linear confidence intervals, the lower and 

upper limits are symmetric about the simulated drawdown and can be estimated by subtracting from and 

adding to the simulated drawdown (figs. 48, 49, and 

50) the half-length of the linear 95-percent confidence 

interval. Simultaneous, as opposed to individual, confidence intervals were used to portray uncertainty over 

the entire model area, rather than just at a few specific 

locations. All 13 model parameters were considered in 

the calculation of the linear 95-percent confidence 

intervals on drawdown. The five parameters that were 

held constant during inverse modeling (table 3) might 

be important under predictive conditions, and it is 

important to acknowledge the uncertainty presented by 

these parameters.

The mean half-length of linear 95-percent 

confidence intervals on drawdown is similar among all 

three aquifers: 7.0 for the SAS, 7.0 for the UFA, and 

7.2 ft for the LFA. Throughout most of the model area 

simulated drawdowns generally are equally reliable, 

but uncertainty still exists even where simulated drawdowns are small. The half-length of linear 95-percent 

confidence intervals on drawdown is larger than 6.8 ft 

in all model cells. This minimum level of uncertainty 

results from the uncertainty inherent in the parameter 

values from the calibrated model (table 4), as well as 

the five parameters held constant during inverse modeling, and from the fact that simulated water levels 

throughout the model area are sensitive (to some 

degree) to the increase in ground-water withdrawals 

projected to occur under 2020 conditions. The simulated drawdowns generally are less reliable in western 

Orange and western Seminole Counties, where confidence intervals are largest (figs. 55, 56, and 57). The 

large projected increases in withdrawals from the FAS 

in these areas contribute to this uncertainty by increasing the sensitivity of simulated water levels to one or 

more model parameters. For example, withdrawals 

from the LFA at a well field in northwestern Orange 

County are projected to increase from 3.1 Mgal/d in 

1998 to 21.2 Mgal/d in 2020. Because of such a large 

change in stress, the simulated LFA water level at the 

well field is much more sensitive to Kh,lf under projected 2020 conditions than under average 1998 

conditions: a 100-percent increase in Kh,lf results in a 

14-ft increase in simulated water level under projected 

2020 conditions but only a 2.2-ft increase under 1998 

conditions. This large change in sensitivity, coupled 

with the relatively large variance of the final value of 

Kh,lf (as indicated by the relatively large coefficient of 

variation for Kh,lf, table 4), results in a linear 95-percent confidence interval on drawdown with a halflength of 27 ft at the well field (fig. 57). Consequently, 

simulated drawdown at the well field should be considered less reliable than the simulated drawdown in 

most other areas.
Simulation of Ground-Water Flow 109

Black

Water Creek

Wek vi a River

Rock Springs Run

L v ittle Weki a River

82°20′ 82°00′ 81°10′

29°40′

29°00′

28°20′

0

0

5 10 MILES

5 10 KILOMETERS

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

Universal Transverse Mercator projection, zone 17

LAKE

COUNTY

ORANGE COUNTY

SEMINOLE

COUNTY

VOLUSIA

COUNTY

OSCEOLA COUNTY

POLK COUNTY

PASCO

COUNTY

HERNANDO

COUNTY

SUMTER

COUNTY

CITRUS

COUNTY

MARION COUNTY

ALACHUA COUNTY

PUTNAM COUNTY

ST. JOHNS COUNTY

FLAGLER

COUNTY

MODEL

BOUNDARY

EXPLANATION

HALF-LENGTH

CONFIDENCE INTERVAL,

IN FEET

6.8 to 7.3

7.3 to 8.8

8.8 to 13

13 to 31

SURFICIAL AQUIFER

SYSTEM NOT

SIMULATED

OCALA

NATIONAL

FOREST

BOUNDARY

Figure 55. Half-length of linear 95-percent confidence intervals on the simulated drawdown in the surficial 

aquifer system from average 1998 conditions as a result of projected 2020 conditions.
110 Hydrogeology and Simulated Effects of Ground-Water Withdrawals from the Floridan Aquifer System in Lake County and in 

the Ocala National Forest and Vicinity, North-Central Florida

Black

Water Creek

Wek vi a River

Rock Springs Run

L v ittle Weki a River

82°20′ 82°00′ 81°10′

29°40′

29°00′

28°20′

0

0

5 10 MILES

5 10 KILOMETERS

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

Universal Transverse Mercator projection, zone 17

LAKE

COUNTY

ORANGE COUNTY

SEMINOLE

COUNTY

VOLUSIA

COUNTY

OSCEOLA COUNTY

POLK COUNTY

PASCO

COUNTY

HERNANDO

COUNTY

SUMTER

COUNTY

CITRUS

COUNTY

MARION COUNTY

ALACHUA COUNTY

PUTNAM COUNTY

ST. JOHNS COUNTY

FLAGLER

COUNTY

MODEL

BOUNDARY

EXPLANATION

HALF-LENGTH

CONFIDENCE INTERVAL,

IN FEET

6.8 to 7.3

7.3 to 8.8

8.8 to 13

13 to 31

OCALA

NATIONAL

FOREST

BOUNDARY

Figure 56. Half-length of linear 95-percent confidence intervals on the simulated drawdown in the Upper 

Floridan aquifer from average 1998 conditions as a result of projected 2020 conditions.
Simulation of Ground-Water Flow 111

Black

Water Creek

Wek vi a River

Rock Springs Run

L v ittle Weki a River

82°20′ 82°00′ 81°10′

29°40′

29°00′

28°20′

0

0

5 10 MILES

5 10 KILOMETERS

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

Universal Transverse Mercator projection, zone 17

LAKE

COUNTY

ORANGE COUNTY

SEMINOLE

COUNTY

VOLUSIA

COUNTY

OSCEOLA COUNTY

POLK COUNTY

PASCO

COUNTY

HERNANDO

COUNTY

SUMTER

COUNTY

CITRUS

COUNTY

MARION COUNTY

ALACHUA COUNTY

PUTNAM COUNTY

ST. JOHNS COUNTY

FLAGLER

COUNTY

MODEL

BOUNDARY

EXPLANATION

HALF-LENGTH

CONFIDENCE INTERVAL,

IN FEET

6.8 to 7.3

7.3 to 8.8

8.8 to 13

13 to 31

LOWER FLORIDAN

AQUIFER NOT OCALA SIMULATED NATIONAL

FOREST

BOUNDARY

Figure 57. Half-length of linear 95-percent confidence intervals on the simulated drawdown in the Lower 

Floridan aquifer from average 1998 conditions as a result of projected 2020 conditions.
112 Hydrogeology and Simulated Effects of Ground-Water Withdrawals from the Floridan Aquifer System in Lake County and in 

the Ocala National Forest and Vicinity, North-Central Florida

A large degree of uncertainty in simulated drawdowns does not exist only where large drawdowns also 

are simulated. Changes in simulated fluxes resulting 

from changes in ground-water withdrawals also can 

cause uncertainty in simulated drawdowns, even 

where drawdowns are relatively small. Simulated base 

flow to the Little Wekiva River near its confluence 

with the Wekiva River decreased 33 percent from average 1998 to projected 2020 conditions, leading to a 

significant increase in the sensitivity of the simulated 

SAS water level to several model parameters. Linear 

95-percent confidence intervals with half-lengths 

exceeding 13 ft along the lower Little Wekiva River 

(fig. 55) indicate that the uncertainty in simulated SAS 

drawdown caused by this decrease in simulated base 

flow, which itself was caused by the increase in FAS 

withdrawals, is relatively large. This relatively large 

degree of uncertainty exists even though drawdowns in 

each aquifer in the area were smaller than in surrounding areas (figs. 48, 49, and 50). The leakance of the 

ICU in the area is quite high (fig. 36) and is the mechanism through which pumping from the FAS can have 

such a large effect on the SAS. Consequently, simulated drawdowns in the UFA in the area also are less 

reliable (fig. 56) because of a 33-percent decrease in 

simulated upward leakage through the ICU that occurs 

along with the 33-percent decrease in simulated base 

flow, both of which are caused by the increase in FAS 

withdrawals.

Uncertainty in simulated drawdown can also be 

caused by nonlinear head-dependent boundaries, such 

as those simulated by the MODFLOW Evapotranspiration, River, and Drain Packages. Head-dependent 

boundaries can be nonlinear because the boundary 

condition can change from head-dependent to specified flux, or specified flux to head-dependent, depending on the simulated head in the model cell containing 

the boundary. Many cells in this model contain headdependent boundaries simulated by the MODFLOW 

Evapotranspiration and River Packages, especially in 

the SAS. Many of the large linear 95-percent confidence intervals on drawdown in the SAS (fig. 55) are 

the result of a head-dependent boundary converting 

between specified flux and head-dependent boundary 

conditions. Evapotranspiration nodes change from a 

specified-flux boundary under average 1998 conditions to a head-dependent flux boundary under projected 2020 conditions as the simulated water level 

declines from above land surface to below land surface. For example, one cell in the Ocala NF has a linear 95-percent confidence interval on drawdown with 

a half-length of 18 ft (fig. 55). Drawdown of the SAS 

water table is only 0.12 ft, but this was sufficient to 

lower the simulated water table under projected 2020 

conditions below land surface and change the boundary condition from specified flux to head-dependent 

flux. The sensitivity of the simulated water table to 

each model parameter significantly decreased with the 

head-dependent boundary, resulting in a large linear 

95-percent confidence interval on drawdown. River 

nodes change from a head-dependent flux boundary 

under average 1998 conditions to a specified-flux 

boundary under projected 2020 conditions as the water 

level drops below the riverbed bottom altitude. For 

example, where a river node converts to a specifiedflux boundary with zero flow, which can occur in this 

model at river nodes representing ungaged streams or 

wetlands, the simulated SAS water level is no longer 

constrained by the river node and consequently is 

much more sensitive to all parameters. Such large 

changes in sensitivities lead to linear 95-percent confidence intervals on drawdown with half-lengths 

exceeding 13 ft in eastern Lake County between 

Blackwater Creek and Rock Springs Run (fig. 55).

The model is nonlinear and the weighted residuals are not normally distributed. Under these circumstances, linear confidence intervals do not accurately 

portray the true uncertainty in simulated drawdowns. 

In addition, the confidence intervals presented in this 

section reflect only the uncertainty of the 13 model 

parameters, not the additional uncertainty presented by 

possible alternative conceptual models or parameter 

zonations. Therefore, the half-length of a linear 95-

percent confidence interval should be considered only 

as a rough indicator of the effect of parameter uncertainty on simulated drawdown, based on the assumption that other aspects of the model accurately depict 

the true ground-water flow system. Nevertheless, figures 55, 56, and 57 provide an approximate measure of 

the reliability of model predictions that could be useful 

when assessing the effects of future water-use needs.

Model Limitations

Results derived in this study are based primarily 

on ground-water flow model simulations. Consequently, these results are subject to the assumptions 

and limitations inherent in the model. Model results 

are limited by simplifications of the conceptual model, 

grid scale, the lack of sufficient hydrologic data to 
Simulation of Ground-Water Flow 113

account for all of the spatial variation in hydrologic 

properties and stresses throughout the model area, and 

the distribution and accuracy of data used for calibration and predictive simulations.

The conceptual model used to construct the 

ground-water flow model is a highly simplified representation of the true ground-water system. Simplifications in the conceptual model, which are needed to 

represent extremely complex natural systems, are the 

most likely source of error in the ground-water flow 

model. Also, the aquifer systems are neither isotropic 

nor homogeneous. Varying lithology produces preferential flow zones, and in the FAS these probably are 

enhanced by dissolution features. The actual relation 

between the combined effects of excess ET and excess 

overland runoff and the depth of the water table below 

land surface probably is more complex than that simulated by using the MODFLOW Evapotranspiration 

Package; this might affect model results significantly, 

especially the simulated 2020 drawdowns. Lateral 

boundaries were located as far outside of Lake County 

and the Ocala NF as practical because inaccuracies in 

assigned boundary conditions can adversely affect 

model results, especially near model boundaries. As a 

result, the model should not be used for analysis near 

any of the lateral boundaries. If sufficient data were 

available, transient calibration and model simulations 

would have more accurately portrayed the groundwater system; however, steady-state analyses are sufficient for simulating the average long-term effects of 

future ground-water withdrawals.

The horizontal and vertical discretization 

required by a finite-difference approximation assumes 

that hydrologic properties and stresses do not vary 

within a model cell. Because this rarely is the case in 

natural hydrologic systems, any variations at the scale 

of a model cell must be represented by an appropriate 

average value. The adequacy of the discretization is a 

function of how well an average value of the property 

or stress represents the effects of the actual, spatially 

variable values. Significant variations in hydrologic 

properties at a scale smaller than the model cell (2,500 

by 2,500 ft) are common in the mantled karst environment in the model area. These small-scale variations 

might significantly affect larger scale average values. 

In addition, the location of stresses (for example, well 

withdrawals or artificial recharge) is distorted to some 

degree by discretization effects. The vertical discretization of one model layer for each aquifer does not 

allow for simulation of vertical flow within each aquifer. However, most vertical flow within the FAS probably occurs through the MSCU; significant vertical 

flow may occur within the SAS, but available data are 

not sufficient for the calibration of a multilayer SAS. 

In addition, much of the vertical flow that occurs 

within the SAS probably is associated with local flow 

systems, which cannot be adequately simulated with a 

regional-scale model. Consequently, the discretization 

used in the present model is adequate for fulfilling the 

objective of modeling ground-water flow on a regional 

scale.

Because the objective of the model is to simulate ground-water flow on a regional scale, the model 

should not be used to interpret local-scale phenomena. 

Site-specific questions are better addressed with a sitespecific study. This is particularly true when interpreting the simulated effects of projected 2020 groundwater withdrawals on individual lakes and wetlands.

Parameter values and predictions from the 

present model differ, to some degree, with results from 

other models (Spechler and Halford, 2001; Sepúlveda, 

2002; and McGurk and Presley, in press). Differences 

among results from models covering the same area do 

not necessarily invalidate the respective models. Each 

model’s results are a product of the particular hydrologic data, assumptions, and methodology used in constructing and calibrating the model. Each model is an 

alternative way of representing the ground-water system that, given the available data, may be equally 

valid. In complex, natural systems, selecting only one 

model as superior to all others often is difficult. It may 

be necessary to accept with equal confidence the range 

of results provided by the various models.

Parameter values and distributions from the calibrated model are dependent not only on the accuracy 

of hydrologic data but also on the spatial distribution 

of those data, which include water levels and flows to 

which the model was calibrated, as well as stresses 

such as artificial recharge or well withdrawals. Model 

results are more likely to be accurate in areas where 

there are large known stresses and corresponding 

water-level or flow observations indicating aquifer 

response to those stresses. Model results should be 

interpreted cautiously in areas where there was either 

no stress or a poorly known stress on the aquifer system or no water-level or flow observations. In general, 

if additional data were available, perhaps collected 

under different hydrologic conditions, a recalibration 

of the model might yield significantly different results.
114 Hydrogeology and Simulated Effects of Ground-Water Withdrawals from the Floridan Aquifer System in Lake County and in 

the Ocala National Forest and Vicinity, North-Central Florida

Simulated drawdowns are affected by the accuracy of 2020 water-use projections and are calculated 

relative to average 1998 conditions, which do not represent long-term average conditions. The magnitude 

and distribution of future ground-water withdrawals 

may be different from those based on present-day projections. Error in projected 2020 withdrawals used in 

the model would have a direct affect on simulated 

drawdowns: if actual 2020 withdrawals were greater 

than projected 2020 withdrawals, then actual drawdowns in 2020 would be greater than simulated drawdowns; likewise, if actual 2020 withdrawals were less 

than projected 2020 withdrawals, then actual drawdowns in 2020 would be less than simulated drawdowns. If simulated drawdowns relative to long-term 

average conditions, or any condition other than average 1998 conditions, were desired, an additional simulation based on estimated conditions during the 

desired time period would be required.

Predictive simulations presented in this report 

pertain primarily to changes in ground-water withdrawals and, to a much lesser degree, changes in septic 

tank leakage, but not to changes in climatic conditions, 

such as a drought. Effects of a drought on hydrologic 

conditions, such as aquifer water levels or springflows, 

could be of similar or greater magnitude compared to 

effects from projected 2020 withdrawals. Assessing 

the affects of changes in climatic conditions on the 

ground-water flow system in the model area was 

beyond the scope of this study. However, the predictive simulations do fulfill the model objective of adequately simulating the effects of the major 

anthropogenic stresses on the hydrologic system.

SUMMARY

The hydrogeology and water resources of Lake 

County and the Ocala National Forest (Ocala NF) 

were evaluated during a 5-year study (1995-2000) and 

the effects of future water-use needs on those 

resources were simulated. Lake County is predominantly rural, with several widely scattered, but steadily 

growing urbanized areas. Urban expansion also is 

steadily expanding into the County, mainly from the 

Orlando area southeast of Lake County. The Ocala NF 

remains mostly undeveloped; however, urban expansion in Marion County is steadily moving towards the 

southwestern and western edges of the Ocala NF.

The study area of about 4,800 square miles 

(mi2

), which includes Lake County (1,150 mi2) and 

the Ocala NF (690 mi2

), was defined by delineating 

estimated ground-water flow divides of the Upper 

Floridan aquifer (UFA) around Lake County and the 

Ocala NF. Lake County and the Ocala NF are drained 

by surface and subsurface drainage systems. Surface 

drainage mainly is by the Palatlakaha River, Ocklawaha River, and other tributaries to and including the 

St. Johns River. Subsurface drainage occurs through 

karst features in much of the study area. A substantial 

amount of ground water discharges from the UFA to 

these drainage basins as base flow to streams or 

springs and seeps.

The principal hydrogeologic units in Lake 

County and the Ocala NF are the surficial aquifer 

system (SAS) and the Floridan aquifer system (FAS). 

The two aquifer systems generally are separated by the 

intermediate confining unit (ICU), which contains 

beds of lower permeability sediments that confine the 

water in the FAS. The bottom of the freshwater flow 

system is defined by either the sub-Floridan confining 

unit, which comprises low permeable limestone, dolomite, and anhydrite, or the depth at which the chloride 

concentration of the water is greater than 5,000 milligrams per liter (mg/L) in the FAS, whichever is 

shallower.

The SAS is up to 300 feet (ft) thick where 

present, unconfined, and composed principally of fineto coarse-grained quartz sand, silt, and interbedded 

clay, peat, marl, and shell. The SAS serves as a reservoir and filter bed for purifying water before it 

recharges the underlying FAS. The base of the SAS 

was defined by the first occurrence of persistent beds 

of Miocene or Pliocene age containing at least 50 percent silt, clay, limestone, or dolomite.

The ICU underlies the SAS and generally consists of the Hawthorn Group composed principally of 

an interbedded mixture of marine sediments including 

phosphatic sand, clay, and limestone; and low permeable silt and clay beds of younger age. Throughout 

most of the study area, the ICU restricts the vertical 

movement of water between the SAS and the UFA 

(except where breached by sinkholes). Thickness of 

the ICU is highly variable because of past erosional 

processes and sinkhole formation; thickness ranges 

from 0 to 150 ft in the study area.

The FAS is a thick sequence of carbonate rocks 

that are generally highly permeable and hydraulically 

connected in varying degrees. The FAS in the study 

area ranges in thickness from 900 to 2,000 ft and is 

subdivided on the basis of the vertical occurrence of 
Summary 115

two zones of relatively high permeability, the UFA and 

the Lower Floridan aquifer (LFA), which generally are 

separated by either the middle semiconfining unit or 

middle confining unit. The thickness of the UFA averages about 300 ft through most of the study area, and 

ranges from 200 ft thick or less in the southwestern 

part of the Ocala NF and in the extreme northwestern 

part of Lake County to more than 400 ft thick in the 

extreme northeastern part of the Ocala NF and southwestern Lake County. The LFA averages about 

1,400 ft thick throughout most of the study area, 

ranging from about 1,200 ft in the northern part of 

the Ocala NF to 1,500 ft or more in western Lake 

County and much of northern Sumter County.

The FAS is the major source of ground water in 

the study area. In 1998, about 115 million gallons per 

day (Mgal/d) were pumped in Lake County and about 

5.7 Mgal/d were pumped in the Ocala NF. Agricultural 

use accounted for nearly 50 percent; municipal, 

domestic, and recreation use accounted for more than 

40 percent; and commercial and industrial use 

accounted for less than 10 percent of the total water 

pumped. Total water pumped from the UFA is projected to be about 173 Mgal/d in Lake County by 

2020, and is projected to remain at about 5.7 Mgal/d in 

the Ocala NF.

The UFA in the northern and eastern parts of 

Lake County and the Ocala NF contains naturally 

occurring brackish or moderately saline ground water 

at relatively shallow depths along the St. Johns River 

and its tributaries. The depth to water containing at 

least 5,000 mg/L chloride concentration ranges from 

less than 200 ft along the St. Johns River to nearly 

2,500 ft below sea level in southern Sumter and Lake 

Counties. The thickest section of freshwater in the 

FAS in the study area is located across much of southern and central Lake County, extending into the southwestern part of the Ocala NF.

Recharge areas of the UFA cover much of the 

study area and include well-drained and poorly 

drained soils, swamps, closed-basin lakes, and sinkholes. The UFA is recharged by the downward movement of water through the SAS and, where present, the 

ICU. Additional recharge also occurs through drainage 

wells drilled into the UFA to dispose of excess surface 

water in Ocala and western Orange County. Recharge 

to the SAS, and consequently to the FAS, is augmented locally by artificial recharge—wastewater 

land application, rapid-infiltration basins, and septic 

systems.

Discharge from the UFA generally occurs 

through numerous springs, a few flowing wells, and as 

diffuse ground-water discharge along the St. Johns, 

Wekiva, and Ocklawaha Rivers; the south shore of 

Lake Harris; and the west shore of Lake Apopka. Total 

estimated springflow in 1998 was 1,979 cubic feet per 

second (ft3

/s) (1,279 Mgal/d). Springflow occurs at 

discrete points (vents and boils) or as diffuse groundwater discharge over broader areas where the potentiometric surface of the UFA is above land surface and 

where the ICU overlying the FAS has been breached. 

In Lake County, 22 springs account for a total springflow of 258 ft3/s (167 Mgal/d); and in the Ocala NF, 

14 springs account for a total springflow of 348 ft3

/s 

(225 Mgal/d).

Fluctuations of lake stage and ground-water levels are highly related to cycles and distribution of rainfall. The most significant rises and declines in water 

levels follow consecutive years with above-average 

and below-average rainfall, respectively. The longterm (1940 to 2000) lake and ground-water hydrographs generally show a slight downward trend. After 

the early 1960’s, declines in water levels are more pronounced, which corresponds with the accumulating 

rainfall deficits and increased ground-water withdrawals. Water levels of closed-basin lakes fluctuate 

slightly less than those of flow-through lakes; however, the closed-basin lakes (selected based on the 

longevity of data record) also are located in areas 

where the downward gradient between the SAS and 

the UFA is nearly zero. Therefore, these closed-basin 

lakes may not represent the magnitude of the waterlevel fluctuations that occur in other closed-basin lakes 

located where a much larger downward gradient exists 

between the SAS and the UFA.

A generalized water-budget analysis of the 

study area for 1998 was performed by using measured 

values of springflow, streamflow, pumpage, and storage changes. An annual average estimate of evapotranspiration (ET) was calculated as the residual of the 

water budget. In 1998, total available input was about 

58 inches (in.), representing 57 in. of rainfall and 1 in. 

of artificial recharge; springflow was about 6 in.; 

streamflow (including overland runoff and base flow 

without measured springflow) was about 13 in., representing 7 in. of overland runoff and 6 in. of base flow; 

total pumpage from the FAS was about 2 in.; net 

change in storage was about 1 in.; boundary leakage 

was nearly 3 in.; and ET, computed as the residual, 

was about 33 in.
116 Hydrogeology and Simulated Effects of Ground-Water Withdrawals from the Floridan Aquifer System in Lake County and in 

the Ocala National Forest and Vicinity, North-Central Florida

The USGS three-dimensional ground-water 

flow model MODFLOW-2000 was used to simulate 

ground-water flow in the SAS and FAS in Lake 

County, the Ocala NF, and adjacent areas. A steadystate calibration to average 1998 conditions was facilitated by using the inverse modeling capabilities of 

MODFLOW-2000. Values of hydrologic properties 

from the calibrated model were in reasonably close 

agreement with independently estimated values and 

results from previous modeling studies. The calibrated 

model generally produced simulated water levels and 

flows in reasonably close agreement with measured 

values and was used to simulate the hydrologic effects 

of projected 2020 conditions.

Projected 2020 conditions simulated by the 

model consisted of a 44 and 73 percent average 

increase in withdrawals from the UFA and LFA, 

respectively, and a 1 percent average increase in artificial recharge resulting from septic tank leakage. Drawdowns from average 1998 conditions as a result of 

projected 2020 conditions generally were smallest in 

magnitude and areal extent in the SAS and greatest in 

the LFA. The average and maximum drawdowns, 

respectively, in the model area were 0.4 and 10.3 ft in 

the SAS, 0.9 and 11.8 ft in the UFA, and 1.2 and 

19.1 ft in the LFA. Significant drawdowns were simulated in Lake County: the average and maximum 

drawdowns, respectively, were 0.5 and 5.7 ft in the 

SAS, 1.1 and 7.6 ft in the UFA, and 1.4 and 4.3 ft in 

the LFA. The largest drawdowns in Lake County were 

simulated in the southeastern corner of the County and 

in the vicinities of Clermont and Mount Dora. Closedbasin lakes and wetlands are more likely to be affected 

by future pumping in these large drawdown areas, as 

opposed to other areas of Lake County. However, 

within the Ocala NF, drawdowns were relatively small: 

the average and maximum drawdowns, respectively, 

were 0.1 and 1.0 ft in the SAS, 0.2 and 0.8 ft in the 

UFA, and 0.3 and 0.8 ft in the LFA. Projected 2020 

withdrawals from the FAS caused decreases from 

average 1998 conditions in the following simulated 

flows: combined rates of excess evapotranspiration 

and excess overland runoff (which represent evapotranspiration and overland runoff that occur in excess 

of their assumed minimum rates); ground-water discharge to streams, lakes, and wetlands; and springflow.

Simulated percent decreases in springflow from 

average 1998 to projected 2020 conditions vary considerably among the springs in the model area. The 

greatest decreases occurred for springs that had small 

differences between simulated UFA water levels and 

spring pool altitudes in 1998 and were in close proximity to large drawdowns in the UFA. Of all the firstor second-magnitude springs, each of the following 

springs had a simulated flow decrease greater than 

10 percent: Seminole, Rock, Wekiwa, Starbuck, Sanlando, and Apopka. The largest simulated flow 

decreases for first- or second-magnitude springs in 

Lake County were at Apopka Springs (28 percent), 

Seminole (12 percent), and Bugg (9 percent). The 

largest simulated flow decreases for first- or secondmagnitude springs in the Ocala NF were at Juniper 

Springs (4 percent), Alexander (1 percent), and Fern 

Hammock (1 percent). Some third-magnitude and 

smaller springs experienced percentage decreases in 

simulated springflow exceeding 33 percent.

The USGS particle-tracking program MODPATH 

was used with ground-water flow model results to 

delineate areas that contribute recharge to selected 

springs and well fields under both average 1998 and 

projected 2020 conditions. The contributing area for 

Apopka Spring covers approximately 30 mi2

 and has 

an average contributing recharge flux of 15 inches per 

year (in/yr) based on average 1998 conditions. The 

contributing area for Alexander Springs covers 

approximately 76 mi2

 and has an average contributing 

recharge flux of 18 in/yr based on average 1998 conditions. The contributing area for Alexander Springs 

changed little as a result of projected 2020 conditions 

because relatively little pumping exists in the vicinity 

of the spring’s contributing area. However, the size of 

the contributing area for Apopka Spring decreased to 

26 mi2 and the average contributing recharge flux 

decreased to 13 in/yr because a significant decrease in 

flow from Apopka Spring was simulated for projected 

2020 conditions.

Changes in contributing areas from average 

1998 to projected 2020 conditions are significant for 

the two selected well fields because large percentage 

increases in pumping rates are projected to occur at 

these wells. For the well field in east-central Lake 

County, the contributing recharge area increased in 

size from 9.2 to 16 mi2, while the average contributing 

recharge flux decreased slightly from 6.3 to 6.0 in/yr. 

The pumping rate from the well field in southeast Lake 

County was small in 1998, but the contributing area 

for the well field under projected 2020 conditions covered approximately 13 mi2

 with an average contributing recharge flux of 7.4 in/yr.
Selected References 117

Linear 95-percent confidence intervals were calculated for simulated drawdowns using YCINT-2000, 

a post-processing program for MODFLOW-2000, in 

order to evaluate the effects of parameter uncertainty 

on model predictions. Uncertainty in simulated drawdowns results from the uncertainty present in all 

parameter values and from the fact that all simulated 

water levels are sensitive (to some degree) to the 

increase in ground-water withdrawals projected for 

2020 conditions. Simulated drawdowns generally are 

equally reliable throughout most of the model area; 

however, simulated drawdowns generally are less 

reliable in western Orange and western Seminole 

Counties. The large increases in withdrawals from the 

FAS in these areas contribute to this uncertainty by 

increasing the sensitivity of simulated water levels to 

one or more model parameters.

Results derived in this study were based primarily on ground-water flow model simulations. Consequently, these results are subject to the assumptions 

and limitations inherent in the model. Oversimplification of the conceptual model used to construct the 

ground-water flow model, which is needed to represent extremely complex natural systems, probably is 

the most likely source of error. However, the hydrologic properties and simulation results derived from 

the ground-water flow model and particle-tracking 

analyses were within realistic and previously referenced limits.

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122 Hydrogeology and Simulated Effects of Ground-Water Withdrawals from the Floridan Aquifer System in Lake County and in 

the Ocala National Forest and Vicinity, North-Central Florida
Appendixes 123

Appendixes
124 Hydrogeology and Simulated Effects of Ground-Water Withdrawals from the Floridan Aquifer System in Lake County and in 

the Ocala National Forest and Vicinity, North-Central Florida

Appendix A. Index to stream-gaging and climatological data-collection sites

[Site numbers refer to figure 5. Abbreviation for data type: D, stream stage and discharge; E, evaporation; R, rainfall; S, stream stage. Abbreviation 

for frequency: C, daily; M, monthly or bimonthly; W, weekly. Abbreviation for source of data: LCWA, Lake County Water Authority; NOAA, 

National Oceanographic and Atmospheric Administration; SJRWMD, St. Johns River Water Management District; USFS, U.S. Forest Service; 

USGS, U.S. Geological Survey]

Site 

number

USGS site 

identification 

number

Station name Data 

type

Frequency

Source

of 

data 

1 294500081320001 Federal Point near Palatka R C NOAA

2 294100082160001 Gainesville Municipal Airport R, E C NOAA

3 02244040 St. Johns River, Buffalo Bluff near Satsuma D C USGS

4 02243000 Orange Creek, Orange Springs D C USGS

5 02243960 Ocklawaha River, Rodman Dam D C USGS

6 292600081310001 Crescent City R C NOAA

7 292403081482201 Ocala Sand Pine Seed Orchard, Ocala NF near Kerr City R C USFS

8 02240500 Ocklawaha River, Eureka D C USGS

9 291311081514301 Jumper Lake, Ocala NF, private boat ramp R W USFS

10 02240000 Ocklawaha River near Conner D C USGS

11 02239500 Silver Springs, Silver River near Ocala D C USGS

12 291200082050001 Ocala R C NOAA

13 02236125 St. Johns River, Astor D C USGS

14 290633081375201 Camp Ocala, Ocala NF R C USFS

15 02238500 Ocklawaha River DSS, Moss Bluff S C USGS

16 02238499 Ocklawaha River USS, Moss Bluff D C USGS

17 290100081180001 DeLand 1 SSE R C NOAA

18 290047081382801 Pittman Work Center near Altoona, Ocala NF R C USFS

19 02236000 St. Johns River near DeLand D C USGS

20 285503081550601 Lady Lake R C LCWA

21 02235200 Black Water Creek near Cassia D C USGS

22 02238001 Haines Creek DSS, Lisbon S C USGS

23 02238000 Haines Creek USS, Lisbon D C USGS

24 285200081470001 Lisbon R, E C NOAA

25 02234500 St. Johns River near Sanford D C USGS

26 02312720 Withlacoochee River Wysong Dam, Carlson D C USGS

27 02312700 Lake Panasoffkee Outlet River at Panacoochee Retreats D C USGS

28 02235000 Wekiva River near Sanford D C USGS

29 284809081441401 Tavares, LCWA Office R C LCWA

30 284800081140001 Sanford Experimental Station R C NOAA

31 284749081353901 Round Lake near Sorrento R C LCWA

32 02237734 Wolf Branch at FCRR near Mt. Dora D C USGS

33 022349993 Wekiva River RXR Bridge near Sanford D C USGS

34 284725081320001 Musselburg, CR 435, Mt. Plymouth R C LCWA

35 02312667 Shady Brook Creek D C USGS

36 02312600 Withlacoochee River near Floral City D C USGS

37 02237293 Palatlakaha River at M-1 near Okahumpka D C USGS

38 284439081522202 M-1 Structure near Okahumpka R C LCWA

39 02237701 Apopka-Beauclair Canal DSS near Astatula S C USGS

40 02237700 Apopka-Beauclair Canal USS near Astatula D C USGS

41 02237207 Palatlakaha River DSS M-4 near Okahumpka S C USGS

42 02237206 Palatlakaha River USS M-4 near Okahumpka S C USGS

43 02234998 Little Wekiva River near Longwood D C USGS

44 02312640 Jumper Creek D C USGS

45 284122081534401 Groveland Tower, SR 33 south of Okahumpka R C SJRWMD

46 02237051 Palatlakaha River DSS M-5 near Okahumpka S C USGS

47 02237050 Palatlakaha River USS M-5 near Okahumpka S C USGS

48 284000082050001 Bushnell 2 E R C NOAA
Appendix A 125

49 02237011 Palatlakaha River DSS M-6 near Mascotte S C USGS

50 02237010 Palatlakaha River USS M-6 near Mascotte S C USGS

51 283654081515601 Villa City R C LCWA

52 283544081505001 Lake Lucy near Groveland R C SJRWMD

53 02312500 Withlacoochee River, Croom D C USGS

54 02236900 Palatlakaha River USS Cherry Lake near Groveland D C USGS

55 02236901 Palatlakaha River DSS Cherry Lake near Groveland S C USGS

56 283439081345701 Winter Garden Street R C SJRWMD

57 02312200 Little Withlacoochee River, Rerdell D C USGS

58 283400081321201 Starke Lake near Ocoee R W SJRWMD

59 283355081411701 Site 38 Turnpike, Waits Junction R C SJRWMD

60 02312180 Little Withlacoochee River near Tarrytown D C USGS

61 02312000 Withlacoochee River at Trilby D C USGS

62 282837081511101 Erie Lake near Groveland R C SJRWMD

63 02236700 Little Creek near Clermont D C USGS

64 02312140 Bayroot Slough Headwaters near Bay Lake D M USGS

65 282700081450001 Clermont 7 S R C NOAA

66 282700081190001 Orlando WSO McCoy R C NOAA

67 02236500 Big Creek near Clermont D C USGS

68 02266291 Lateral 405, S-405A near Dr. Phillips D C USGS

69 02263869 South Lake Outlet at S-15 near Vineland D C USGS

70 02266025 Reedy Creek at S-46 near Vineland D C USGS

71 02266205 Whittenhorse Creek at S-411 near Vineland D C USGS

72 02264000 Cypress Creek, Vineland D C USGS

73 02264003 Cypress Creek Canal at S-103A near Vineland D C USGS

74 02311700 Dade City Canal near Dade City D M USGS

75 02266200 Whittenhorse Creek near Vineland D C USGS

76 02264051 Black Lake Outlet at S-101A, Buena Vista D C USGS

77 02264060 Lateral 101 at S-101 near Buena Vista D C USGS

78 282207081413801 CR 474, 0.5 mi. west of US 27 R C LCWA

79 02266295 Lateral 410 at S-410 near Vineland D C USGS

80 282127082022501 Cumpressco Ranch ROMP-89, RNF 308 near Tarrytown R C SWFWMD

81 02311500 Withlacoochee River near Dade City D C USGS

82 02264100 Bonnet Creek near Vineland D C USGS

83 02266300 Reedy Creek near Vineland D C USGS

84 02263130 C-2 Canal near Vineland D C USGS

85 02310947 Withlacoochee River near Cumpressco D C USGS

86 281841081544301 ROMP 88 near Rock Ridge R C SWFWMD

87 02236350 Green Swamp Run near Eva D C USGS

88 02263800 Shingle Creek at Airport near Kissimmee D C USGS

89 281709081461401 Brown Shinn near Polk City R C LCWA

90 02311000 Withlacoochee-Hillsborough Overflow near Richland D C USGS

91 02266480 Davenport Creek near Loughman D C USGS

92 02266500 Reedy Creek near Loughman D M USGS

93 02234990 Little Wekiva River near Altamonte Springs D M USGS

Appendix A. Index to stream-gaging and climatological data-collection sites--Continued

[Site numbers refer to figure 5. Abbreviation for data type: D, stream stage and discharge; E, evaporation; R, rainfall; S, stream stage. Abbreviation 

for frequency: C, daily; M, monthly or bimonthly; W, weekly. Abbreviation for source of data: LCWA, Lake County Water Authority; NOAA, 

National Oceanographic and Atmospheric Administration; SJRWMD, St. Johns River Water Management District; USFS, U.S. Forest Service; 

USGS, U.S. Geological Survey]

Site 

number

USGS site 

identification 

number

Station name Data 

type

Frequency

Source

of 

data 
126 Hydrogeology and Simulated Effects of Ground-Water Withdrawals from the Floridan Aquifer System in Lake County and in 

the Ocala National Forest and Vicinity, North-Central Florida

Appendix B. Index to lake-gaging and surficial aquifer system well data-collection sites

[Site numbers refer to figure 6. Abbreviation for data type: Gs, surficial aquifer system well water level; Ls, lake stage; Q, water quality. Abbreviation 

for frequency: C, daily; M, monthly or bimonthly; Q, quarterly; S, semiannual; W, weekly. Abbreviation for source of data: FLDEP, State of Florida 

Department of Environmental Protection; LCWA, Lake County Water Authority; PBG&S, Parsons Brinckerhoff; SJRWMD, St. Johns River Water 

Management District; SSC, Silver Springs Citrus; SWFWMD, Southwest Florida Water Management District; USFS, U.S. Forest Service; USGS, 

U.S. Geological Survey]

Site 

number

USGS site 

identification 

number

Station name Data 

type

Frequency

Source

 of 

data

1 281440081431701 USGS 6” shallow, Sweet Hill Road, 1.95 mi S of Dean Still Road Gs S USGS

2 281753081574001 Hart Hammock 2” near Rock Ridge Gs M USGS

3 281836081430401 Green Bay Ranch Cabin 2” Gs M USGS

4 281951082012001 Green Swamp LLMS Gs M USGS

5 02263776 Lake Bryan near Vineland Ls Q USGS

6 282152082011201 Green Swamp L11KS near Dade City Gs M USGS

7 282202081384601 Lake Oliver 4” near Vineland Gs C USGS

8 282210081352601 Disney 6”, Tree Farm near Vineland Gs C USGS

9 282241081443901 Sand Mine (L-0050) near Horsehead Pond Gs M SJRWMD

10 282245081492601 Eva 6” Gs M USGS

11 282249081365601 RIBS II 4”, Hartzog Road near Whittenhorse Creek Gs M USGS

12 282318081544003 Hayes Grubb 2” (LK756W) Gs M USGS

13 02263868 South Lake near Vineland Ls C USGS

14 02263850 Bay Lake near Vineland Ls C USGS

15 282507081423001 Keene Lake near Clermont Ls C SJRWMD

16 282650081475701 Kirkland Lake near Clermont Ls W SJRWMD

17 02266239 Trout Lake near Clermont Ls W USGS

18 282706081412601 RCID 4” Gs M USGS

19 282710081490901 Pretty Lake near Clermont Ls D SJRWMD

20 282717081554401 USGS 2”, SWFWMD (former Warden’s House site) near Bay Lake Gs M USGS

21 282740082012101 Green Swamp L12BS 2” near Bay Lake Gs M USGS

22 282801081390802 Conserv II, RIBS V Piez 8004 WNW of Avalon Lookout Tower Gs W PBG&S

23 282837081511101 Erie Lake near Groveland Ls D SJRWMD

24 282911081493701 Pine Island Lake, Florida Boy’s Ranch Road Ls M LCWA

25 02263900 Lake Butler, Windermere Ls W USGS

26 282949081440201 Lake Louisa near Clermont Ls C SJRWMD

27 283114081503701 Bernard DuFrene 4” (L-0697), SR 33 S of Groveland Gs M USGS

28 283204081544902 Mascotte 6” near Mascotte (L-0041) Gs C USGS

29 02237540 Johns Lake, Oakland Ls W USGS

30 283238081470501 Lake Minnehaha, Clermont Ls C SJRWMD

31 283239081523502 Florida Select Citrus West Sprayfield 2” (MW-8) near Groveland Gs Q FLDEP

32 283355081411702 Site 38 Turnpike, Waits Junction 4” (L-0044) Gs C USGS

33 283400081321201 Starke Lake near Ocoee Ls W SJRWMD

34 02312670 Lake Catherine, Groveland Ls W USGS

35 283436081351301 Lake Apopka, Winter Garden Ls C SJRWMD

36 02236900 Cherry Lake, Palatlakaha River near Groveland Ls C USGS

37 283534081460801 Lake Minneola, Clermont Ls C SJRWMD

38 283537081545201 USGS 4” (L-0693), Tuscanooga Road 1.4 mi N of SR 50 Gs M USGS

39 283544081505001 Lake Lucy near Groveland Ls C SJRWMD

40 02236860 Apshawa Lake near Minneola Ls C USGS

41 283624081434401 Camp Lake Abandoned 4” Gs M USGS

42 283713081303601 Trout Lake near Clarcona Ls W SJRWMD

43 283804081470601 Clerbrook RV Resort 2” (MW-3) Gs Q FLDEP

44 283806081444801 S. Duncan 4”, CR561, Quail Hollow Arabian Farms near Horseshoe 

Lake

Gs M USGS
Appendix B 127

45 283817081483502 Sunshine Parkway, STP (MW-1) Gs Q FLDEP

46 283825081521101 Sand Mine 2”, M-6A Structure Road near Palatlakaha River Gs M USGS

47 02237370 Church Lake near Groveland Ls W USGS

48 02237660 Lake Francis near Plymouth Ls M USGS

49 284020081461901 T. McCormack 4” near Howey in the Hills Gs M USGS

50 284122081534402 Groveland Tower 4” (L-0096), SR 33 S of Okahumpka Gs C SJRWMD

51 284147081512301 Turkey Lake, US 27, 7 mi SE of Leesburg Ls M LCWA

52 284149081570001 Sunshine Peat 2” (Claypit), 4 mi N of Center Hill Gs M USGS

53 284230081345301 Plymouth Tower 4” (OR0107) Gs, Q M SJRWMD

54 284322081410301 Apopka Lock (Field Station) 4” (L-0287) Gs M USGS

55 284329081502901 Silver Springs Citrus 2” Background NW-8, New Sprayfield near 

Yalaha

Gs W SSC

56 284330081481002 Silver Springs Citrus 2” Background P-2, Old Sprayfield near Howey 

in the Hills

Gs W SSC

57 284409081361201 Lake Maggiore near Zellwood Ls W SJRWMD

58 284418081532801 Ogden Martin Systems (MW-4) 2”, Okahumpka Gs Q FLDEP

59 284430081552701 (City of) Leesburg, Plantation at SR 470 Spraysite #2, 2” (MW-2A) 

near Okahumpka

Gs Q FLDEP

60 284456082053102 ROMP LP-5 Avon Park Gs M SWFWMD

61 284504081441501 Astatula 1 Landfill 4” (8A) Gs Q USGS

62 284617081341001 Smith Lake near Bay Ridge Ls W SJRWMD

63 284634081262004 Rock Springs Run State Reserve Cluster 4” (OR0650), Shell Mound 

Road

Gs C USGS

64 284733081362401 Wolf Sink (LCWA) 6” Gs M USGS

65 284746081383901 Lake Dora, Mt. Dora Ls C SJRWMD

66 284756081514901 Lake Harris, Leesburg Ls C SJRWMD

67 284759082054102 ROMP LP-6 2”, Coleman Gs M SWFWMD

68 284817081563501 Zeb Teeter 2”, Flying Bar Z Ranch near Airstrip, S Whitney Road near 

Leesburg

Gs M USGS

69 284819081413001 Lake Saunders near Eustis Ls C SJRWMD

70 284827081541801 Dyches Lake, Leesburg Ls W SJRWMD

71 284828081393801 Lake Gertrude, Heim Road near Mt. Dora Ls W LCWA

72 02235260 Mt. Plymouth Lake, Mt. Plymouth Ls W USGS

73 02312698 Lake Panasoffkee Ls C USGS

74 284908081255001 Bear Pond, Seminole State Forest Ls M USGS

75 284914081540602 Allied Universal Corp 2” (MW-1), Leesburg Gs Q FLDEP

76 284923081234802 Yankee Lake 4” (S-1310) Gs M USGS

77 284943081382601 Loch Leven, Loch Leven Dr., Mt. Dora Ls M LCWA

78 02237753 West Crooked Lake near Eustis Ls W USGS

79 285009081384701 Lake Joanna, Eustis Ls C SJRWMD

80 285012081343801 Cardinal Ln. (Extension) R/W 4” (L-0701) Gs M USGS

81 285106081412901 Lake Eustis, Eustis Ls C SJRWMD

82 285144081475001 Leesburg Fire Tower 4” (L-0289) Gs M USGS

83 285147081571001 Lake Co School Board 4” (L-0700), CR 466A near Fruitland Park Gs M USGS

84 285148081513101 Lake Griffin, Picciola Island Trailer Park Ls C SJRWMD

85 285208081354101 Lake Seneca near Eustis Ls D SJRWMD

Appendix B. Index to lake-gaging and surficial aquifer system well data-collection sites--Continued

[Site numbers refer to figure 6. Abbreviation for data type: Gs, surficial aquifer system well water level; Ls, lake stage; Q, water quality. Abbreviation 

for frequency: C, daily; M, monthly or bimonthly; Q, quarterly; S, semiannual; W, weekly. Abbreviation for source of data: FLDEP, State of Florida 

Department of Environmental Protection; LCWA, Lake County Water Authority; PBG&S, Parsons Brinckerhoff; SJRWMD, St. Johns River Water 

Management District; SSC, Silver Springs Citrus; SWFWMD, Southwest Florida Water Management District; USFS, U.S. Forest Service; USGS, 

U.S. Geological Survey]

Site 

number

USGS site 

identification 

number

Station name Data 

type

Frequency

Source

 of 

data
128 Hydrogeology and Simulated Effects of Ground-Water Withdrawals from the Floridan Aquifer System in Lake County and in 

the Ocala National Forest and Vicinity, North-Central Florida

86 02238200 Lake Yale, Grand Island Ls C USGS

87 285300081265302 Boggy Creek Gang 2” (MW-1), Cassia Gs Q FLDEP

88 285357081472801 Cabbage Hammock 4” (L-0703) Gs M USGS

89 285416081555701 Lady Lake Landfill 4” (LLC7D) Gs S USGS

90 285422081371701 Lake Dalhousie, Recreation Area near Umatilla Ls M USGS

91 285422082001801 Lake Miona (STA 253) Boat Ramp near Oxford Ls M SWFWMD

92 285425081323401 USGS 2” (L-0378), Lake Norris Road Gs M USGS

93 285444081251601 Gaiter Lake near Pine Lakes Ls D SJRWMD

94 02312694 Lady Lake near Lady Lake Ls M USGS

95 02237865 Lake Umatilla, Umatilla Ls W USGS

96 285554081294301 Gourd Lake, Dragonwood Farms near Pine Lakes Ls M USGS

97 285605081353501 Coates Tree Farm 2”, NE corner of CR 439 & Will Murphy Road near 

Umatilla

Gs M USGS

98 02238180 Holly Lake near Umatilla Ls W USGS

99 285618081380801 Umatilla Landfill 2” (C-2) Gs S USGS

100 285643081563601 Paradise Lake, Residential Park near Lady Lake Ls M USGS

101 285658081321601 Lake Norris near Paisley Ls D SJRWMD

102 285659081470901 USGS 4” (L-0699), Emeralda Road & CR 452 near Lake Yale Gs M USGS

103 285703081394701 South Twin Lake near Umatilla Ls D SJRWMD

104 285703081425001 Ella Lake near Umatilla Ls C SJRWMD

105 285709081530801 USGS 4” (L-0696), Marion County Road near Lady Lake Gs M USGS

106 285725081584301 Spruce Creek South Perc Pond (MW-3) 2” Gs Q FLDEP

107 285841081331601 Clearwater Lake, Ocala NF near Paisley Ls M USFS

108 285909081314601 Lake Lulu, Paisley Ls D SJRWMD

109 02238330 Big Bass Lake, Ocala NF near Campground Ls M USFS

110 285934081262502 LCFD (District 2 Station 2) 4” (L-0695), Lack Mack Rd Gs M USGS

111 02238170 Nicotoon Lake near Altoona Ls D SJRWMD

112 02235150 Lake Dorr near Altoona Ls W USGS

113 290025081244801 Ocala NF 2” near River Forest (pumphouse), FSR 541 Gs, Q M USGS

114 290155081332401 Bunch Ground Pond 4” (L-0702), Paisley Road & Shockley Cemetery 

Road, Ocala NF

Gs, Q M USGS

115 02238830 Bowers Lake near Ocklawaha Ls D SJRWMD

116 02238800 Lake Weir, Ocklawaha Ls W SJRWMD

117 290224081491801 Doe Lake Camp, Ocala NF Ls M USFS

118 290236081280701 Deerhaven Lake, Ocala NF Ls M USFS

119 290300081391801 Ocala NF 2”, FSR 573, 1.1 mi W of SR 19 Gs, Q M USGS

120 290300081420901 Ocala NF 2”, FSR 573, 0.4 mi E of FSR 566 Gs, Q M USGS

121 290300081471701 Ocala NF 2”, FSR 573, 0.8 mi W of FSR 523 Gs, Q M USGS

122 02238820 Smith Lake near Candler Ls W SJRWMD

123 290350081401101 Blue Sink, Ocala NF Ls M USFS

124 290423081495801 Lake Mary, Ocala NF boat ramp Ls M USFS

125 290425081350801 Ocala NF 2” near Alexander Springs, Paisley Road Gs, Q M USGS

126 290436081383101 Railroad Grade Road 2”, FSR 550 Gs, Q M USGS

127 290447081530101 Moss Bluff Field Station 4” (M-0377) Gs, Q M USGS

128 290452081320001 Ocala NF 2”, FSR 552 & 552A & 552B near Alexander Springs 

Wilderness

Gs, Q M USGS

Appendix B. Index to lake-gaging and surficial aquifer system well data-collection sites--Continued

[Site numbers refer to figure 6. Abbreviation for data type: Gs, surficial aquifer system well water level; Ls, lake stage; Q, water quality. Abbreviation 

for frequency: C, daily; M, monthly or bimonthly; Q, quarterly; S, semiannual; W, weekly. Abbreviation for source of data: FLDEP, State of Florida 

Department of Environmental Protection; LCWA, Lake County Water Authority; PBG&S, Parsons Brinckerhoff; SJRWMD, St. Johns River Water 

Management District; SSC, Silver Springs Citrus; SWFWMD, Southwest Florida Water Management District; USFS, U.S. Forest Service; USGS, 

U.S. Geological Survey]

Site 

number

USGS site 

identification 

number

Station name Data 

type

Frequency

Source

 of 

data
Appendix B 129

129 290531081423101 Ocala NF 4” (M-0412), FSR 566/595, S of US Naval Bombing Range Gs, Q M USGS

130 290615081402801 Farles Lake Prairie, Ocala NF Recreation Area Boat Ramp Ls M USFS

131 290624081483901 Ocala NF 4” (M-0411), FSR 595/579 Gs M USGS

132 290636081375201 Sellers Lake, Camp Ocala near Alexander Springs Ls C SJRWMD

133 290646081442801 Ocala NF 2”, FSR 584 W of US Bombing Range Gs, Q M USGS

134 290647081342101 Alexander Springs 4” (L-0456) Gs, Q M SJRWMD

135 290802081371001 South Grasshopper Lake, Ocala NF boat ramp near Astor Park Ls M USFS

136 02240200 Lake Bryant near Silver Springs Ls M SJRWMD

137 290835081383001 Ocala NF 2”, FSR 562-1 & FSR 524 near US Naval Tracking Station Gs, Q M USGS

138 290914081361101 Crooked Lake, Camp McQuarrie near Astor Park Ls M USFS

139 290932081491801 Halfmoon Lake, Ocala NF near Lynne Ls M USFS

140 291002081330603 Astor 4” (L-0460) Gs, Q M SJRWMD

141 291011081373801 Wildcat Lake, Ocala NF Boat Ramp Ls M USFS

142 02240210 Mill Dam Lake near Lynne Ls M USFS

143 291117081540502 Redwater Lake 4” (M-0045) Gs, Q M SJRWMD

144 291150081532701 Redwater Lake, Ocala NF near Lynne Ls M USFS

145 291204081564801 Ocala NF 2”, NE125 Terr Road (cemetary) near Ocala NF Boundary Gs, Q M USGS

146 291311081514301 Jumper Lake, Ocala NF, private boat ramp Ls M USFS

147 291335081543101 Lake Charles, Ocala NF boat ramp Ls M USFS

148 291440081384801 Ocala NF 2”, SR19 entrance to Silver Glen Springs Gs, Q M USGS

149 291514081445001 Hughes Island Sink, Ocala NF Ls M USFS

150 291519081515501 Lake Eaton, Recreation Area dock Ls M USFS

151 291621081550401 Fore Lake, Ocala NF Recreation Area Ls M USFS

152 291628081413801 Hopkins Prairie near Salt Springs Ls M USFS

153 291657081461501 Ocala NF 4” (M-0408), FSR 88B & FSR 88 Gs, Q M USGS

154 02236210 Lake George near Salt Springs Ls C USGS

155 291751081414301 Ocala NF 4” (M-0413), FSR 90 1.5 mi W of SR 19 Gs, Q M USGS

156 02236200 Lake Kerr, Ocala NF near Salt Springs Ls C SJRWMD

157 292139081512501 Ocala NF 4” (M-0409), FSR 97A08 (former Eureka Lookout Tower 

site)

Gs M USGS

158 292210081524001 Ft. McCoy Gs M USGS

159 292403081422901 Ocala NF 2”, FSR 47 & FSR 43 Gs, Q M USGS

160 292447081441402 SR 19 near Frontier 6” (P-0820) Gs M SJRWMD

161 292515081455401 Lake Delancy, Ocala NF, private dock Ls C SJRWMD

162 02242450 Orange Lake, Orange Lake Ls C SJRWMD

163 292543081483201 Ocala NF 2”, FSR 75 & FSR 88 Gs, Q M USGS

164 292817081483601 Ocala NF 2”, FSR 88 & FSR 31 Gs, Q M USGS

165 292824081443301 Johnson’s Field 4” (P-0473) near Welaka Gs, Q S USGS

166 292948081503002 USGS (P-0738), Road 77 & 77-G Gs M SJRWMD

167 02242400 Lochloosa Lake, Lochloosa Ls C SJRWMD

168 02243958 Lake Ocklawaha near Orange Springs Ls C USGS

169 293556082043401 Hawthorne Tower 4” (A-0436) Gs M SJRWMD

170 293733081474801 Hollister Workctr 4” (P-0511) Gs M SJRWMD

Appendix B. Index to lake-gaging and surficial aquifer system well data-collection sites--Continued

[Site numbers refer to figure 6. Abbreviation for data type: Gs, surficial aquifer system well water level; Ls, lake stage; Q, water quality. Abbreviation 

for frequency: C, daily; M, monthly or bimonthly; Q, quarterly; S, semiannual; W, weekly. Abbreviation for source of data: FLDEP, State of Florida 

Department of Environmental Protection; LCWA, Lake County Water Authority; PBG&S, Parsons Brinckerhoff; SJRWMD, St. Johns River Water 

Management District; SSC, Silver Springs Citrus; SWFWMD, Southwest Florida Water Management District; USFS, U.S. Forest Service; USGS, 

U.S. Geological Survey]

Site 

number

USGS site 

identification 

number

Station name Data 

type

Frequency

Source

 of 

data
130 Hydrogeology and Simulated Effects of Ground-Water Withdrawals from the Floridan Aquifer System in Lake County and in 

the Ocala National Forest and Vicinity, North-Central Florida

Appendix C. Index to Floridan aquifer system well and spring data-collection sites

[Site numbers refer to figure 7. Abbreviation for hydrologic unit: LFA, Lower Floridan aquifer; UFA, Upper Floridan aquifer; SP, Upper Floridan aquifer 

spring. Abbreviation for data type: D, ground-water discharge; Q, water quality; W, ground-water level; --, no data. Abbreviation for frequency of flow and 

water-level data: C, daily; I, intermittently; IA, currently inactive; M, monthly or bimonthly; Q, quarterly; S, semiannual; W, weekly. Abbreviation for source 

of data: FLDEP, State of Florida Department of Environmental Protection; LC, Lake County; LO, Land Owner, PBG&S, Parsons Brinckerhoff; SJRWMD, 

St. Johns River Water Management District; SWFWMD, Southwest Florida Water Management District; USGS, U.S. Geological Survey]

Site

 number

USGS 

site 

identification 

number

Station name

Hydrologic

unit

Data 

type

Frequency 

of flow 

and 

water-level 

data

Source 

of 

data

1 281440081431701 USGS 6”, Sweet Hill Road UFA Q, W S USGS 

2 281532081493001 USGS (PK7410) 6” near Polk City UFA W S USGS 

3 281536081324801 Florida Power 6” (SRK01) near Intercession City UFA W S USGS 

4 281559081260701 Shingle Creek 4” at SR 531A UFA W S USGS 

5 281835081552201 Green Swamp Check Station 4”, Main Grade (Tanic 

Road)

UFA Q I USGS

6 281836081430401 Green Bay Ranch Cabin 4” UFA W M USGS

7 281841081544301 ROMP 88 8” near Rock Ridge UFA Q, W C SWFWMD, USGS

8 281951082012001 Green Swamp L11MD UFA W M SWFWMD, USGS

9 282121082071101 Cummer Office UFA W S USGS

10 282127082022501 Cumpressco Ranch ROMP-89 near Tarrytown UFA Q, W C SWFWMD, USGS

11 282152082011201 Green Swamp L11KD near Dade City UFA W M SWFWMD, USGS

12 282202081384601 Lake Oliver 6” (OR0072) near Vineland UFA W C USGS

13 282221082103001 Collura #1 6” UFA W S USGS

14 282241081443901 Sand Mine (L-0051) near Horsehead Pond UFA Q, W M SJRWMD, USGS

15 282245081492601 Eva 6” UFA Q, W M USGS

16 282318081544003 (L-0555) Green Swamp 4” (Hayes Grubb-LK751W/

LK7425)

UFA Q, W M USGS

17 282331081370801 USGS 4” (OR0009), Hartzog Road near Whittenhorse 

Creek

UFA Q, W S USGS

18 282354081313001 RCID Obs 4” #1, Disney World Buena Vista Blvd UFA Q, W S USGS

19 282434081283102 Sea World Dr 4” replacement near Vineland UFA W C USGS

20 282502081422301 Lykes Bros. replacement (S. Burger) 4” near Keene Lake UFA Q, W S USGS

21 282511081271701 Orangewood #4 UFA W S USGS

22 282528081340901 Bay Lake 8” near Windermere UFA W C USGS

23 282532081511801 J. M. Barry 8” UFA W S USGS

24 282543081385801 J. Mathias 4”, Lake Hickory Nut UFA W S USGS

25 282650081262502 Sand Lake Road MW LFA W S USGS

26 282706081412601 RCID 12” UFA W M USGS

27 282717081553101 ROMP 101 (L-0056) near Bay Lake UFA Q, W C USGS

28 282729081443301 Lake Louisa State Park 4” (L-0053) UFA Q, W M SJRWMD, USGS

29 282738081341401 Lake Sawyer 4” near Windermere UFA Q, W C USGS

30 282740082012101 Green Swamp L12BD 2” near Bay Lake UFA W M SWFWMD, USGS

31 282741081585701 Withlacoochee State Forest 3” (Center/South Grades) near 

Bay Lake

UFA Q, W M USGS

32 282745081283501 Southwest #3 (P-2) LFA W S USGS

33 282758081392801 Conserv II 1W-2 LFA W S USGS

34 282823081500401 D. Patton 6” near Eva Firetower UFA Q, W S USGS

35 282835081305201 Palm Lake Dr. 4” near Windermere UFA W C USGS

36 282838081391401 RIBS II 5-F1A WNW of Avalon Lookout Tower UFA Q, W W PBG&S

37 282851082035301 Boyette 3” UFA W S USGS

38 282923081282801 Ivey’s Nursery 4”, Turkey Lake Road UFA W S USGS

39 282931081285901 Hidden Springs #4 LFA W S USGS

40 282936081340201 Ross, Lake Butler UFA W S USGS

41 283001082064702 Richloam Fire Tower 4” UFA W S USGS
Appendix C 131

42 283006081274101 Kirkman #3 LFA W S USGS

43 283007081575902 GWSI #5 Tower Camp UFA Q I FLDEP

44 283017081391301 (17830) Davenport Road 4” near Oakland UFA Q, W S USGS

45 283019081455701 LCFD District 9 Station 1 4” near Crescent Lake, 

Clermont

UFA Q, W S USGS

46 283028081454701 Country Garden Nursery (S. Tyner) 4”, Clermont UFA Q, W M USGS

47 283036082105502 Ridge Manor 10” UFA W S USGS

48 283111081502201 B. DuFrene 10” near Groveland UFA W S USGS

49 283114081503401 B. DuFrene 2” near Groveland UFA Q M USGS

50 283116081442301 Rings Pond near Clermont UFA W S USGS

51 283128081404701 Johns Lake 4” (L-0052) UFA Q, W M SJRWMD, USGS

52 283204081544901 Mascotte 6” (L-0062) near Mascotte UFA Q, W C USGS

53 283216081320901 Ocoee South #1 (OR0559) Lower LFA W S USGS

54 283232081394101 83213902 22S26E25 Edgewater Bch 4” near East Johns 

Lake

UFA Q, W S USGS

55 283236081290901 Oak Meadows #4 Lower LFA W S USGS

56 283253081283401 OR0047 6”, Orlo Vista UFA Q, W C USGS

57 283307081435301 Jacks Lake 2” UFA W S USGS

58 283314081455501 Clermont (L-0001) UFA W C USGS

59 283322081415401 Clermont Greater Hills North LFA W S USGS

60 283325081374001 Oakland 12” #1 UFA Q, W S USGS

61 283355081411701 Site 38 Turnpike, Waits Junction 4” (L-0199) UFA Q, W C SJRWMD, USGS

62 283357081272201 Pine Hills #1 Lower LFA W S USGS

63 283400081405100 Apopka (Gourd Neck) Spring near Oakland SP D, Q Q SJRWMD, USGS

64 283432081530601 Mascotte 8” UFA Q M USGS

65 283445081573201 H. Fender 6”, SR 50 near Lake/Sumter County Line UFA W M USGS

66 283510082133701 Croom RR Siding 4” near Croom UFA W S USGS

67 283530081514501 Dr. Phillips & Sons (Minute Maid Co.) 4” near Lake Lucy UFA Q, W S USGS

68 283535081545201 G. Barton 4”, 16845 Tuscanooga Road UFA Q, W M USGS

69 283537082151501 ROMP 103 8” near Brooksville UFA W M USGS

70 283540081402401 Montverde School 3” intermittent flow UFA Q M USGS

71 283549081401701 Montverde School 6” freeflow UFA Q, W M USGS

72 283555081300801 Ocoee Forest Oaks #3 LFA W S USGS

73 283638082025702 Webster 8” #2 UFA Q, W M USGS

74 283638082081501 JC48B SCL RxR UFA W S USGS

75 283655081412701 Freeflow 4”, Ferndale UFA Q, W M USGS

76 283718081580201 T. Iley 8”, CR 469 near Center Hill UFA W S USGS

77 283720081421801 Ferndale Baptist Church 4” UFA Q M USGS

78 283737081445501 Coca Cola Foods 3”, CR 561 N of Turnpike UFA W M USGS

79 283804081470601 Clerbrook RV 4” (MW-4) UFA W Q FLDEP

80 283809081324801 Magnolia Park 6” near Ocoee UFA Q M USGS

81 283813081325701 UF Agriculture Research Center (St. Foliage) 4” UFA W S USGS

82 283818081291201 West Regional MW LF-1 LFA W S USGS

83 283828081535701 Sunset Lakes Ski Center 4”, N of Mascotte UFA Q M USGS

84 283829082123701 JC47 Jumper Creek 4” near Doke UFA W S USGS

Appendix C. Index to Floridan aquifer system well and spring data-collection sites--Continued

[Site numbers refer to figure 7. Abbreviation for hydrologic unit: LFA, Lower Floridan aquifer; UFA, Upper Floridan aquifer; SP, Upper Floridan aquifer 

spring. Abbreviation for data type: D, ground-water discharge; Q, water quality; W, ground-water level; --, no data. Abbreviation for frequency of flow and 

water-level data: C, daily; I, intermittently; IA, currently inactive; M, monthly or bimonthly; Q, quarterly; S, semiannual; W, weekly. Abbreviation for source 

of data: FLDEP, State of Florida Department of Environmental Protection; LC, Lake County; LO, Land Owner, PBG&S, Parsons Brinckerhoff; SJRWMD, 

St. Johns River Water Management District; SWFWMD, Southwest Florida Water Management District; USGS, U.S. Geological Survey]

Site

 number

USGS 

site 

identification 

number

Station name

Hydrologic

unit

Data 

type

Frequency 

of flow 

and 

water-level 

data

Source 

of 

data
132 Hydrogeology and Simulated Effects of Ground-Water Withdrawals from the Floridan Aquifer System in Lake County and in 

the Ocala National Forest and Vicinity, North-Central Florida

85 283830081534901 Sunset Lakes 8”, N of Mascotte UFA W M USGS

86 283840081485001 Novelty Crystal Fire, O’Brien Road 0.1 mi N of US 27 UFA W M USGS

87 283840082154801 Barnhart (CE-25) 6”, Nobleton UFA W M USGS

88 283844081422300 Wolf’s Head Spring along RxR Grade near Astatula SP D I USGS

89 283848081221301 Altamonte Springs #2 Lower #11 LFA W S USGS

90 283849081273401 Ecolog Utilities 6”, 2808 Oranole Way near Orange/Seminole County LineUFA W M USGS

91 283855082003301 Central Packing Processing UFA Q I FLDEP

92 283903081430100 Bear Spring near Astatula UFA -- IA USGS

93 283905081485401 Novelty Crystal 4”, O’Brien Road 0.1 mi S of Florida’s 

Turnpike

UFA Q I USGS

94 283906081290001 Apopka, Sheeler Oaks LFA W S USGS

95 283917081254501 Altamonte Springs #5 Lower #14 LFA W S USGS

96 283937081422101 Lake Apopka Restoration Area (SJRWMD) 4” freeflow, 

Clay Island

UFA Q, W M USGS

97 284001081303001 OR0661 UFA Q I FLDEP

98 284003081461401 T. McCormack Nursery 4” near Howey in the Hills UFA W M USGS

99 284038081443200 Double Run Road Seepage (into Little Lake Harris) near 

Astatula

SP D I USGS

100 284047081441500 Seepage Run (into Little Lake Harris), CR 561 near 

Astatula

SP D I USGS

101 284052081212605 Charlotte/North St. (S-1024) Lower LFA W S USGS

102 284119081234500 Sanlando Springs near Longwood SP D, W S USGS

103 284120081331701 Minute Maid/Plymouth 12 UFL #1 UFA Q M USGS

104 284122081534401 Groveland Tower 4 (L-0095) near Okahumpka UFA Q, W C SJRWMD, USGS

105 284127081233400 Palm Springs near Longwood SP D, W S USGS

106 284128081320901 Apopka Grossenbacher #4 Lower (OR0554) LFA W S USGS

107 284131082002101 Stuart Ranch 4” near Center Hill UFA W S USGS

108 284134081564201 Sunshine Peat (prev Hi Acres) 4” near Carson UFA Q, W M USGS

109 284135081565501 Sunshine Peat (prev Hi Acres) 12” Lower, near Carson LFA W S USGS

110 284148081232800 Starbuck Spring near Longwood SP D, W S USGS

111 284230081345301 Plymouth Tower 4” (OR0106) UFA Q, W M SJRWMD, USGS

112 284233081442801 West Astatula 8” UFA W S USGS

113 284236081263400 Miami Springs near Longwood SP D, W S USGS

114 284238081275803 Wekiwa Springs State Park 4” (OR0548) UFA Q, W C SJRWMD, USGS

115 284241081281800 Barrel Springs, Wekiwa Springs State Park SP D I SJRWMD

116 284241081402601 Keen Ranch 12” near Lake Jem UFA W S USGS

117 284243081273600 Wekiwa Springs, Wekiwa Springs State Park SP D, Q, W M USGS

118 284253081441101 Astatula (J. Swaffer Park) 4” UFA Q I USGS

119 284258081495701 Grey 4”, Punkin Center UFA W M USGS

120 284317082142601 Wynnhaven Camp 4” near Wahoo UFA W S USGS

121 284320081410701 Apopka-Beauclair Canal 4” (L-0139) at SJRWMD Field 

Office

UFA Q, W S USGS

122 284328081515901 Creek Farms 8” UFA Q, W S USGS

123 284330081360501 World Foliage Resource, Inc., 8” near Zellwood UFA Q, W S USGS

Appendix C. Index to Floridan aquifer system well and spring data-collection sites--Continued

[Site numbers refer to figure 7. Abbreviation for hydrologic unit: LFA, Lower Floridan aquifer; UFA, Upper Floridan aquifer; SP, Upper Floridan aquifer 

spring. Abbreviation for data type: D, ground-water discharge; Q, water quality; W, ground-water level; --, no data. Abbreviation for frequency of flow and 

water-level data: C, daily; I, intermittently; IA, currently inactive; M, monthly or bimonthly; Q, quarterly; S, semiannual; W, weekly. Abbreviation for source 

of data: FLDEP, State of Florida Department of Environmental Protection; LC, Lake County; LO, Land Owner, PBG&S, Parsons Brinckerhoff; SJRWMD, 

St. Johns River Water Management District; SWFWMD, Southwest Florida Water Management District; USGS, U.S. Geological Survey]

Site

 number

USGS 

site 

identification 

number

Station name

Hydrologic

unit

Data 

type

Frequency 

of flow 

and 

water-level 

data

Source 

of 

data
Appendix C 133

124 284330081464201 Howey in the Hills 14” #3 (L-0591) UFA Q, W I FLDEP, USGS

125 284340081305101 J. Owens 4”, 132 W Ponkan Road near Apopka UFA Q M USGS

126 284353081292200 Witherington Springs, Wekiwa Springs State Park near 

Apopka

SP D IA USGS

127 284407081321601 Apopka Northwest #1 Lower LFA W S USGS

128 284418081532801 Ogden Martin Systems (MW-5) 4”, Okahumpka UFA W Q FLDEP

129 284424081490500 Holiday Springs at Yalaha SP D, Q S USGS

130 284430081552701 Plantation at Leesburg, SR 470 Spraysite #2, 4” (MW-2B) 

near Okahumpka

UFA W Q FLDEP

131 284433081483201 LCFD (Dist 8, Stat 1) 4”, CR 48, Yalaha UFA W S USGS

132 284435082011701 Brentwood, CR 526 near Sumterville UFA W S USGS

133 284437081491700 Sun Eden Spring near Yalaha SP D, Q I USGS

134 284439082131401 Trail's End Fish Camp 6” near Floral City UFA W S USGS

135 284445081462101 Lake Yale Groves 8” (L-0043) near Tavares UFA W M SJRWMD, USGS

136 284452081495400 Mooring Cove Springs near Yalaha SP Q I USGS

137 284455081494100 Blue Springs, Park Dr near Yalaha SP Q S USGS

138 284456082053102 ROMP LP-5 Avon Park UFA W M SWFWMD

139 284504081441501 Astatula 1 Landfill 4” (8B) UFA Q, W Q LC, USGS

140 284507081540600 Bugg Spring near Okahumpka SP D, Q, W M LO, USGS

141 284513081310601 Blue Sink, Orange Co. near Rock Springs UFA Q M USGS

142 284515082050100 Shady Brook Spring #5 (South Panasoffkee Spring 

Group) near Lake Panasoffkee

SP D IA USGS

143 284516081224001 S. P. Griffin 4”, 30 Windsor Isle Road near Lake Mary UFA W M USGS

144 284516081570701 Asphalt Production Corp. 4”, SR 470 near Okahumpka UFA W M USGS

145 284520081295800 Rock Springs near Apopka SP D, Q, W M USGS

146 284523082000201 Jones-Hrs 30” UFA Q I FLDEP

147 284528081530201 Church of God of Prophecy 3” near Okahumpka UFA Q, W S USGS

148 284529081301001 Rock Springs 4” UFA Q, W S USGS

149 284541081265201 Rock Springs Run State Reserve Anderson’s Pasture 2” 

freeflow (OR0068)

UFA Q, W S USGS

150 284553081204801 S-0972 Lake Mary #2 UFA Q I FLDEP, USGS

151 284555081414201 C. Givens Farm 4” near Ellsworth UFA W M USGS

152 284602081391901 Trimble Park Ranger’s House UFA W M USGS

153 284612081303400 Sulphur (Camp) Springs near Mt. Plymouth SP D, Q I USGS

154 284612082042000 Shady Brook Spring #4 (South Panasoffkee Spring 

Group) near Lake Panasoffkee

SP D IA USGS

155 284619082035101 ROMP 111 Thompkins Park 8” UFA Q, W M USGS

156 284634081262004 Rock Springs Run State Reserve (Cluster) 6” (OR0662) UFA Q, W C USGS

157 284635081280601 FL DER Hidden 2” (OR0463) near Rock Springs UFA W S USGS

158 284646082023800 Shady Brook Spring #3 (South Panasoffkee Spring 

Group) near Coleman

SP D IA USGS

159 284708082024600 Shady Brook Spring #2 (South Panasoffkee Spring 

Group) near Coleman

SP D IA USGS

160 284725081361901 Wolf Sink (LCWA) 6” (L-0600) near Sorrento UFA Q, W M USGS

161 284728081322201 Florida Central Academy 6”, Mt. Plymouth UFA W S USGS

162 284732081495301 Frog Leg Ln 4” near Cisky Park, Leesburg UFA Q, W M USGS

Appendix C. Index to Floridan aquifer system well and spring data-collection sites--Continued

[Site numbers refer to figure 7. Abbreviation for hydrologic unit: LFA, Lower Floridan aquifer; UFA, Upper Floridan aquifer; SP, Upper Floridan aquifer 

spring. Abbreviation for data type: D, ground-water discharge; Q, water quality; W, ground-water level; --, no data. Abbreviation for frequency of flow and 

water-level data: C, daily; I, intermittently; IA, currently inactive; M, monthly or bimonthly; Q, quarterly; S, semiannual; W, weekly. Abbreviation for source 

of data: FLDEP, State of Florida Department of Environmental Protection; LC, Lake County; LO, Land Owner, PBG&S, Parsons Brinckerhoff; SJRWMD, 

St. Johns River Water Management District; SWFWMD, Southwest Florida Water Management District; USGS, U.S. Geological Survey]

Site

 number

USGS 

site 

identification 

number

Station name

Hydrologic

unit

Data 

type

Frequency 

of flow 

and 

water-level 

data

Source 

of 

data
134 Hydrogeology and Simulated Effects of Ground-Water Withdrawals from the Floridan Aquifer System in Lake County and in 

the Ocala National Forest and Vicinity, North-Central Florida

163 284740081251700 Wekiva Falls Resort flowing borehole SP D, Q I USGS

164 284742082021900 Fenney Spring (Shady Brook headspring to Lake 

Panasoffkee) near Coleman

SP D IA USGS

165 284757081320701 L. Knowles 4”, Mt. Plymouth UFA Q I USGS

166 284757081543002 C. R. Williams 4”, Caballo Road, Leesburg UFA Q, W S USGS

167 284759082054102 ROMP LP-6 6”, Coleman UFA Q, W M SWFWMD

168 284802081211101 Hartstock 4”, Wilson Ave. near Wilson Corner UFA W M USGS

169 284802081242101 V. Hermosa 4”, Longwood-Markham Rd UFA W M USGS

170 284808081432801 Tavares 12” #8 UFA W S USGS

171 284810082004001 Hogeye Sink 6” near Wildwood UFA W S USGS

172 284822081520601 Leesburg #14 Lower LFA W S USGS

173 284827081403501 D. Bartholow 4” near Tavares UFA W S USGS

174 284830081522401 Leesburg, #6, Canal St. (L-0592) UFA Q I FLDEP

175 284842081533001 Leesburg 12”, College St. (L-0054) UFA W C USGS

176 284856081383001 Mt. Dora 20” #3 UFA Q, W S USGS

177 284857081570901 J. Alibrandi 6”, SR 44 W of Leesburg near county line UFA W S USGS

178 284922081250300 Island Spring, Wekiva River SP D, Q I USGS

179 284923081234801 Yankee Lake Lower (S-1225) LFA W M SJRWMD, USGS

180 284923081234802 Yankee Lake 4” (S-1230) UFA W M USGS

181 284929081294901 Abandoned 10” freeflow off SR 46A near Sorrento UFA Q, W S USGS

182 284934081474801 Lake-Sumter Community College 8” #1 near Leesburg UFA W S USGS

183 284936081475501 Lake-Sumter Community College QW 2” near Leesburg UFA Q I USGS

184 284940081303800 Droty Springs near Sorrento SP D, Q I USGS

185 284954081201101 Anderson 8”, Missouri St. at St. Johns River Estates near 

Lake Monroe

UFA W M USGS

186 285002081215101 S. Cain 3” near Astor Farms UFA W M USGS

187 285011082034900 Little Jones Creek Spring #3 (North Panasoffkee Spring 

Group) near Wildwood

SP D IA USGS

188 285028081253301 Seminole State Forest 4” #1 (L-0037) UFA Q, W M USGS

189 285038081270100 Palm Springs, Seminole State Forest SP D, Q I USGS

190 285044081312200 Seminole Springs near Sorrento SP D, Q, W IA USGS

191 285057081321301 Simpson’s Horse Training Center 8” (L-0365) near 

SR 44/46A

UFA W S USGS

192 285102081263900 Blueberry Spring, Seminole State Forest SP D, Q I USGS

193 285105081263800 Moccasin Springs, Seminole State Forest SP D, Q I USGS

194 285118081391001 Eustis 16” (L-0593), CR 44A UFA Q I FLDEP, USGS

195 285121081295600 Messant Spring near Sorrento SP D, Q, W IA USGS

196 285129081541002 Fruitland Park (abandoned) 6” UFA W S USGS

197 285134082051800 Little Jones Creek Spring #2 (North Panasoffkee Spring 

Group) near Wildwood

SP D IA USGS

198 285144081183900 Gemini Springs near DeBary SP D, Q, W M SJRWMD, USGS

199 285144081475002 Leesburg Fire Tower (L-0290) UFA Q, W M FLDEP, USGS

200 285150082044001 Wildwood Fire Tower CE3 (JC58) 2”, CR 44A UFA Q, W S USGS

201 285152081542901 Fruitland Park (Cales Memorial Recreation Complex) UFA Q I FLDEP, USGS

202 285156081372501 B. Stewart Ranch near Eustis UFA W M USGS

Appendix C. Index to Floridan aquifer system well and spring data-collection sites--Continued

[Site numbers refer to figure 7. Abbreviation for hydrologic unit: LFA, Lower Floridan aquifer; UFA, Upper Floridan aquifer; SP, Upper Floridan aquifer 

spring. Abbreviation for data type: D, ground-water discharge; Q, water quality; W, ground-water level; --, no data. Abbreviation for frequency of flow and 

water-level data: C, daily; I, intermittently; IA, currently inactive; M, monthly or bimonthly; Q, quarterly; S, semiannual; W, weekly. Abbreviation for source 

of data: FLDEP, State of Florida Department of Environmental Protection; LC, Lake County; LO, Land Owner, PBG&S, Parsons Brinckerhoff; SJRWMD, 

St. Johns River Water Management District; SWFWMD, Southwest Florida Water Management District; USGS, U.S. Geological Survey]

Site

 number

USGS 

site 

identification 

number

Station name

Hydrologic

unit

Data 

type

Frequency 

of flow 

and 

water-level 

data

Source 

of 

data
Appendix C 135

203 285207082014501 Wildwood JC57 12”, Masters Ave. UFA Q, W M USGS

204 285208082054100 Little Jones Creek Headspring (North Panasoffkee Spring 

Group) near Wildwood

SP D IA USGS

205 285224081262400 Shark’s Tooth Spring, Seminole State Forest SP D, Q I USGS

206 285230081242201 Lower Wekiva River State Preserve 2” freeflow #2 

(South)

UFA Q, W M USGS

207 285244081471401 Lisbon Baptist Church Grove 8” UFA W M USGS

208 285257081434201 J. Eichelberger 10” (L-0373), Grand Island UFA W S USGS

209 285301081285401 Reese 4” near Cassia UFA Q, W S USGS

210 285310081524101 R. Peters 4”, Lake Griffin UFA W M USGS

211 285318081295200 Blackwater Springs near Cassia SP D IA USGS

212 285318081340601 Eustis Sand Co. 12” (L-0375) UFA Q, W S USGS

213 285357081472801 Cabbage Hammock 6” (L-0620) UFA W S USGS

214 285416081555701 Lady Lake Landfill 4” (LLD06) UFA W M USGS

215 285419081552801 Lady Lake (L-0594) UFA Q I SJRWMD, FLDEP

216 285422082001901 Katz 4”, Lake Miona boat ramp near Oxford UFA W S USGS

217 285426081380901 A. B. Marshall (L-0379) near Umatilla UFA W S USGS

218 285442081181401 Orange City Tower 4” (V-0196) UFA Q, W M SJRWMD

219 285442081181402 Orange City Tower Lower (V-0780) LFA W S USGS

220 285452081563201 R. P. Rowley 5” near Lady Lake UFA Q, W S USGS

221 285454081241201 Lower Wekiwa River State Preserve 2” intermittent flow 

#1 (North)

UFA Q, W M USGS

222 285504081405901 Austin Groves 8” (L-0380), Umatilla UFA W S USGS

223 285524081132401 Galaxy Lower (V-0774) LFA W S USGS

224 285536082044001 G. N. Smith 6”, CR 466 near Oxford UFA W S USGS

225 285539081262901 South Pine Lakes 4”, SR 44 UFA W S USGS

226 285543081133803 V-0772 UFA Q I FLDEP

227 285549081530601 Carlton Village Park 10”, Clearview Lake UFA Q I USGS

228 285551081293601 B. Rogers 4”, Gourd Lake UFA Q, W M USGS

229 285600081530001 Carlton Village Park irrigation 6”, Clearview Lake UFA W M USGS

230 285602081344301 K. Kruckenberg 4” intermittent flow, Will Murphy Road 

near Paisley

UFA Q, W M USGS

231 285606081353601 Coates Tree Farm 4” irrigation UFA Q M USGS

232 285618081491101 G. Davis 4”, Emeralda Island UFA Q, W S USGS

233 285628081400501 Umatilla Blanding (L-0595) UFA Q I FLDEP

234 285638081202400 Blue Springs near Orange City SP D, Q, W M USGS

235 285702081322400 Camp La-No-Che Springs near Paisley SP D, Q I USGS

236 285707081441101 J. F. Irvine Estate 4” (L-0385) near Lake Yale UFA Q, W S USGS

237 285743081390201 Altoona Post Office 4”, US19 UFA W S USGS

238 285810081234101 Lower Wekiva River St Preserve 4” freeflow UFA Q, W M USGS

239 285813081142402 V-0777 UFA Q I FLDEP

240 285820081580301 Steeplechase (Stonecrest) Perc Pond Irrigation 4” (MW-1) 

near Sunset Harbor

UFA Q I FLDEP

241 285827081331401 P. Shokley 6” near Johnson’s Corner, Paisley UFA W S USGS

242 285831081580401 Abandoned 4” near Sunset Harbor UFA W M USGS

Appendix C. Index to Floridan aquifer system well and spring data-collection sites--Continued

[Site numbers refer to figure 7. Abbreviation for hydrologic unit: LFA, Lower Floridan aquifer; UFA, Upper Floridan aquifer; SP, Upper Floridan aquifer 

spring. Abbreviation for data type: D, ground-water discharge; Q, water quality; W, ground-water level; --, no data. Abbreviation for frequency of flow and 

water-level data: C, daily; I, intermittently; IA, currently inactive; M, monthly or bimonthly; Q, quarterly; S, semiannual; W, weekly. Abbreviation for source 

of data: FLDEP, State of Florida Department of Environmental Protection; LC, Lake County; LO, Land Owner, PBG&S, Parsons Brinckerhoff; SJRWMD, 

St. Johns River Water Management District; SWFWMD, Southwest Florida Water Management District; USGS, U.S. Geological Survey]

Site

 number

USGS 

site 

identification 

number

Station name

Hydrologic

unit

Data 

type

Frequency 

of flow 

and 

water-level 

data

Source 

of 

data
136 Hydrogeology and Simulated Effects of Ground-Water Withdrawals from the Floridan Aquifer System in Lake County and in 

the Ocala National Forest and Vicinity, North-Central Florida

243 285859081191001 McGreggor Road 4”, SW DeLand UFA W S USGS

244 285900082072001 CE36 6” (M-0031), Pedro UFA Q, W M SJRWMD

245 285902081551901 MCFD (District 19, Station 27) 4”, Weirsdale UFA Q, W M USGS

246 285908081470101 Big Bass Lake Recreation Area 4” (M-0046) UFA Q, W M USGS

247 285920081490501 Nelson’s Fish Camp 6” Mar-48 (M-0320) near 

Ocklawaha River

UFA W M USGS

248 285930081430901 Ocala Forest Campground 6”, SR 42 near Altoona UFA Q, W S USGS

249 285930082022001 W. Sweetz 4”, US301 near Summerfield UFA W M USGS

250 285933081324001 Paisley Fire Tower UFA Q I USGS

251 285934081262501 LCFD (District 2 Station 2) 4”, Lack Mack Road UFA Q I USGS

252 285935081501201 M-0375 UFA Q I FLDEP

253 285940081522001 K. Scales, Jr., 6” near Weirsdale UFA W M USGS

254 290026081244701 River Forest, FSR 541 near Forest Hills UFA Q I USGS

255 290043081232801 River Forest 2” (L-0059) near Crows Bluff UFA W M SJRWMD

256 290045081295701 Lake Kathryn Recreation Club 4”, Pennsylvania St. UFA W M USGS

257 290047081232501 River Forest 3” (L-0400) near Crows Bluff UFA Q, W S USGS

258 290047081382801 Pittman Work Center (abandoned) 6” near Altoona UFA Q, W S USGS

259 290050081381201 Lake Dorr Recreation Area 4” UFA Q I USGS

260 290052081271201 Central Baptist Youth Camp 4” UFA W M USGS

261 290130082082001 CE35 4” (M-0042) near Pedro UFA W M SJRWMD

262 290133082140901 ROMP 119 Marion Oaks 8” near Ocala UFA W C USGS

263 290138081203201 V-0114 4” UFA Q I FLDEP

264 290208081250201 St. Francis 2 freeflow near Crows Bluff UFA Q, W M USGS

265 290220081260400 Mosquito Springs Run, Alexander Springs Wilderness SP D, Q I USGS

266 290220081485001 Doe Lake Camp 6” UFA Q, W M USGS

267 290228081382301 LCFD (District 4, Station 6) 4”, SR 19 near Altoona UFA Q, W M USGS

268 290230081123401 USGS 3” on I-4 (V-0118) UFA W S USGS

269 290237081550501 MCFD (Old Station 12) 4”, Ocklawaha UFA W M USGS

270 290244081302601 Alexander Springs Creek 4”, FSR 552B near boat ramp UFA W S USGS

271 290300081452001 Big Scrub Camp 6” UFA Q, W M USGS

272 290400082091001 CE33 4” (M-0041) near Ocala UFA Q, W M SJRWMD

273 290420081311701 Amoco 4” 1A (L-0123), FSR 552B UFA W S USGS

274 290427081582801 D. Labagh 4”, Candler UFA W M USGS

275 290445081344001 Alexander Springs Recreation Area Supply 4” UFA Q I USGS

276 290450081343000 Alexander Springs SP D, Q, W M SJRWMD, USGS

277 290451081344401 Alexander Springs 4” (L-0066) UFA W C SJRWMD

278 290455081530401 Moss Bluff 8” (M-0013) UFA Q, W M SJRWMD

279 290512081213601 Glenwood 4” (V-0156) UFA Q, W M SJRWMD

280 290526081493701 L. Schrimsher 4”, near Moss Bluff UFA Q, W M USGS

281 290541081132903 USGS 05 Lower (V-0012) near DeLand LFA W S USGS

282 290552082044701 Wolf Sink CE81 4” (M-0058) near Santos UFA W M SJRWMD

283 290554081390501 Buck Lake Recreation Area handpump 4” UFA Q I USGS

284 290613081402901 Farles Lake Recreation Area handpump 4” UFA Q I USGS

285 290614081183301 V-0742 UFA Q I FLDEP

Appendix C. Index to Floridan aquifer system well and spring data-collection sites--Continued

[Site numbers refer to figure 7. Abbreviation for hydrologic unit: LFA, Lower Floridan aquifer; UFA, Upper Floridan aquifer; SP, Upper Floridan aquifer 

spring. Abbreviation for data type: D, ground-water discharge; Q, water quality; W, ground-water level; --, no data. Abbreviation for frequency of flow and 

water-level data: C, daily; I, intermittently; IA, currently inactive; M, monthly or bimonthly; Q, quarterly; S, semiannual; W, weekly. Abbreviation for source 

of data: FLDEP, State of Florida Department of Environmental Protection; LC, Lake County; LO, Land Owner, PBG&S, Parsons Brinckerhoff; SJRWMD, 

St. Johns River Water Management District; SWFWMD, Southwest Florida Water Management District; USGS, U.S. Geological Survey]

Site

 number

USGS 

site 

identification 

number

Station name

Hydrologic

unit

Data 

type

Frequency 

of flow 

and 

water-level 

data

Source 

of 

data
Appendix C 137

286 290628081425301 Ocala NF Lookout Tower Bombing Range (Abandoned 

Tower #1) 4”

UFA Q, W S USGS

287 290633081375201 Camp Ocala 6” (L-0407), Sellers Lake UFA Q, W S USGS

288 290647081342101 Alexander Springs 4” (L-0040) UFA Q M SJRWMD

289 290650081314001 Johnson, Levy Grant near Astor UFA Q S USGS

290 290708081233101 SJRWMD 4” intermittent flow (V-0280) near Ponce De 

Leon Springs

UFA W S USGS

291 290748081184201 Thomas 5” near DeLeon Springs UFA W S USGS

292 290752082121401 J. Downing (Church) 4”, College Road UFA W M USGS

293 290802081214700 Ponce De Leon Springs near DeLand SP D, Q, W M SJRWMD, USGS

294 290805081540801 D. Craft 4”, Tomahawk Lake near Lynne UFA Q, W M USGS

295 290815082025701 CE-40 replacement 3” (M-0269) UFA W M USGS

296 290820081305001 Alco Fish Camp (Frank Saul) 2” freeflow near Astor UFA Q, W S USGS

297 290820082032001 USGS CE37 4” (M-0037) near Ocala UFA Q, W M SJRWMD

298 290828081215101 V-1028 UFA Q I FLDEP

299 290910081360001 Camp McQuarrie (4-H Club Foundation) 6” UFA Q, W S USGS

300 290950081315501 Astor (A.G. Edwards) 6” (L-0045) UFA W C USGS

301 290953082031301 CE79 4” (M-0038) near Silver Springs UFA W M SJRWMD

302 291002081330601 Astor 6” (L-0455) UFA Q, W M SJRWMD

303 291015081385001 DOT 49 6” (M-0049), SR 19/40 UFA Q, W M SJRWMD

304 291025081263601 V-0506 UFA Q I FLDEP

305 291035081461201 Ocala NF Central Lookout Tower 4” (M-0112) near 

Lynne

UFA W M USGS

306 291051081495701 Mill Dam Lake Recreation Area Supply 4” UFA Q I USGS

307 291057081424301 Juniper Springs Recreation Area Supply #2 (M-0115) UFA Q I USGS

308 291100081422900 Fern Hammock Springs near Ocala SP D, Q, W M SJRWMD, USGS

309 291100081502001 Mill Dam Lake 6” (SCE-123) UFA Q, W M USGS

310 291100082010001 USGS CE76 6”, Highway 314 (M-0028) UFA Q, W M SJRWMD

311 291101081424600 Juniper Springs near Ocala SP D, Q, W M SJRWMD, USGS

312 291105082005901 FL DEP 4”, Highway 314 UFA W M USGS

313 291110082060001 CE44 6” (M-0032), Ocala UFA W M SJRWMD

314 291115081592501 Sharpes Ferry Marion DOT5 (Silver Springs 6” Freeflow) 

near Ocala

UFA W C USGS

315 291115082102901 CE31 replacement 4” (M-0321) UFA W M SJRWMD

316 291117081540501 Redwater Lake 4” (M-0044) UFA Q, W M SJRWMD

317 291130082015001 CE47 6” (M-0026) near Silver Springs UFA W M SJRWMD

318 291136081380001 Juniper Creek South Tributary Seepage near Astor SP D I USGS

319 291140082052701 CE80 4”, Silver Springs UF W M SJRWMD

320 291150081282501 Harper’s 8”, Murphy Road UFA W S USGS

321 291200081390600 Morman Branch Seepage (into Juniper Creek) near Astor SP D I USGS

322 291216082051501 Ocala (M-0322) UFA Q I FLDEP

323 291230081594001 Silver Run 3” freeflow (SCE-154) near Silver Springs UFA Q, W M USGS

324 291257082031100 Silver Springs near Ocala SP D, Q, W C USGS

325 291258081313701 Ezel’s 4”, SE Lake George UFA W S USGS

326 291307081393600 Sweetwater Springs along Juniper Creek SP D, Q, W M SJRWMD, USGS

Appendix C. Index to Floridan aquifer system well and spring data-collection sites--Continued

[Site numbers refer to figure 7. Abbreviation for hydrologic unit: LFA, Lower Floridan aquifer; UFA, Upper Floridan aquifer; SP, Upper Floridan aquifer 

spring. Abbreviation for data type: D, ground-water discharge; Q, water quality; W, ground-water level; --, no data. Abbreviation for frequency of flow and 

water-level data: C, daily; I, intermittently; IA, currently inactive; M, monthly or bimonthly; Q, quarterly; S, semiannual; W, weekly. Abbreviation for source 

of data: FLDEP, State of Florida Department of Environmental Protection; LC, Lake County; LO, Land Owner, PBG&S, Parsons Brinckerhoff; SJRWMD, 

St. Johns River Water Management District; SWFWMD, Southwest Florida Water Management District; USGS, U.S. Geological Survey]

Site

 number

USGS 

site 

identification 

number

Station name

Hydrologic

unit

Data 

type

Frequency 

of flow 

and 

water-level 

data

Source 

of 

data
138 Hydrogeology and Simulated Effects of Ground-Water Withdrawals from the Floridan Aquifer System in Lake County and in 

the Ocala National Forest and Vicinity, North-Central Florida

327 291308082024101 Christian Conference Center near Silver Springs UFA W M USGS

328 291310082045001 CE45 4” (M-0039), Silver Springs UFA W M SJRWMD

329 291341082003401 Abandoned, CR315 near Lynne UFA W M USGS

330 291341082142301 Golden Ocala 6”, Ocala UFA W M USGS

331 291343081254601 R. Jones Fernery 6” (V-0089) near Pierson UFA W C SJRWMD

332 291351081292502 SJRWMD (V-0577), Shell Harbor Road UFA W M SJRWMD

333 291443081383700 Silver Glen Springs near Astor SP D, Q, W M SJRWMD, USGS

334 291445081274001 T. De Witt Taylor School (V-0569) UFA Q I FLDEP

335 291448081381601 Juniper (Hunt) Club 4” near Silver Glen Springs UFA Q, W S USGS

336 291458081294201 V-0068 4” near Pierson UFA W C SJRWMD

337 291508081302801 V-0065 UFA Q I FLDEP

338 291514081515401 Lake Eaton Recreation Area handpump 4” UFA Q, W M USGS

339 291600081550001 Fore Lake Recreation Area CE55 4” (M-0036) UFA Q, W M SJRWMD

340 291620081415001 Hopkin’s Prairie Recreation Area handpump 4” UFA Q, W M USGS

341 291624082090301 Marion Co. Sherriff’s Dept. 4” (M-0419), US301 N of 

Ocala

UFA Q I FLDEP

342 291640081320901 V-0523 UFA Q I FLDEP

343 291728081390501 Ponderosa Club CE30A 2” freeflow (M-0317) near Lisk 

Point, Lake George

UFA Q, W S USGS

344 291740081562001 Gores Landing CE54 (M-0025) 6” near Ocala UFA Q, W C SJRWMD

345 291748081290301 JC Mew Replacement 4” (V-0510) UFA W C SJRWMD, USGS

346 291750081494001 CE-56 6”, CR 314 near Salt Springs UFA W S USGS

347 291835081324201 USCE 426 Pine Island 6” freeflow (V-0155), Stoughton 

House near Seville

UFA W S USGS

348 291849081411401 Lake George 4” (M-0021) near Salt Springs UFA Q, W M SJRWMD, USGS

349 291941081294201 V-0184 Seville Firetower 4” UFA W M SJRWMD

350 292019082064201 CE-66 Sparr replacement 4” (M-0063) UFA Q, W M SJRWMD

351 292056081440901 Salt Springs Civic Association 4” UFA W M USGS

352 292100081435800 Salt Springs SP D, Q, W M SJRWMD, USGS

353 292124081345202 Middle Road Upper (P-0736) UFA W M SJRWMD

354 292138082061601 Johnston C.C. (Black Sink) (M-0012) UFA W C SJRWMD

355 292143081374601 Drayton Island 2” freeflow (P-0423) UFA W S USGS

356 292200081510001 CE84 6” (M-0024) near Salt Springs UFA Q, W M SJRWMD, USGS

357 292204082022801 Fort McCoy Fire Tower 4” (M-0052) UFA Q, W C SJRWMD

358 292207082115001 Friendship Baptist Church UFA W M USGS

359 292218081333101 Union Camp (P-0410) UFA Q I FLDEP

360 292225082053601 Black Sink 10” (M-0284) UFA W M SJRWMD

361 292240081483101 Grassy Pond Recreation Area handpump 4” (M-0153) UFA Q, W M USGS

362 292257081353001 P-0469 UFA Q I FLDEP

363 292418081330902 Unknown (P-0705) UFA W M SJRWMD

364 292440081342601 P-0144 UFA Q I FLDEP

365 292443082003701 MCFD (Stat 2) 4”, Citra UFA W M USGS

366 292447081370601 Marvin-Jones Road 6” (P-0776) UFA W C SJRWMD

367 292447081441401 SR 19 near Frontier 4” (P-0427) UFA Q, W M SJRWMD

368 292521081551200 Wells Landings Springs, Ocklawaha River SP D, Q I USGS

Appendix C. Index to Floridan aquifer system well and spring data-collection sites--Continued

[Site numbers refer to figure 7. Abbreviation for hydrologic unit: LFA, Lower Floridan aquifer; UFA, Upper Floridan aquifer; SP, Upper Floridan aquifer 

spring. Abbreviation for data type: D, ground-water discharge; Q, water quality; W, ground-water level; --, no data. Abbreviation for frequency of flow and 

water-level data: C, daily; I, intermittently; IA, currently inactive; M, monthly or bimonthly; Q, quarterly; S, semiannual; W, weekly. Abbreviation for source 

of data: FLDEP, State of Florida Department of Environmental Protection; LC, Lake County; LO, Land Owner, PBG&S, Parsons Brinckerhoff; SJRWMD, 

St. Johns River Water Management District; SWFWMD, Southwest Florida Water Management District; USGS, U.S. Geological Survey]

Site

 number

USGS 

site 

identification 

number

Station name

Hydrologic

unit

Data 

type

Frequency 

of flow 

and 

water-level 

data

Source 

of 

data
Appendix C 139

369 292528081383501 P-0270 UFA Q, W I FLDEP

370 292540081552400 Tobacco Patch Landing Spring Group 1a (Group 1 run 

inflow), Ocklawaha River

SP D, Q I USGS

371 292542081552600 Tobacco Patch Landing Spring Group 1, Ocklawaha River SP D, Q I USGS

372 292546081513301 CE67 6” (M-0159), FSR 97/75 near Eureka UFA W M SJRWMD, USGS

373 292548081471201 Lake Delancy Recreation Area handpump 4” UFA Q, W M USGS

374 292618081412100 Croaker Hole Spring near Welaka SP D, Q I USGS

375 292622082131801 Huff (M-0367), McIntosh UFA W M SJRWMD

376 292628081385501 Welaka Fish Hatchery 6” freeflow, Fruitland (P-0396) UFA W S USGS

377 292654081384900 Beecher Springs near Fruitland SP D S SJRWMD, USGS

378 292656082125001 Sportsman Cove 2” (M-0351) UFA W M SJRWMD

379 292725081393500 Forest Springs near Welaka SP D S SJRWMD

380 292735081394500 Mud Spring near Welaka SP D S SJRWMD

381 292817081483602 USGS 6” (M-0410), FSR 88/31 UFA Q, W M USGS

382 292824081341501 Thunderbird Airpark (Col. Sauls) 4” (P-0246) near Lake 

Como

UFA W S USGS

383 292824081443301 Johnson’s Field 4” (P-0472) near Welaka UFA Q, W S USGS

384 292859081375701 Highway 308B 4” (P-0408), Fruitland UFA Q, W M USGS

385 292909082095101 (Previously the) Yearling Rest. 4”, Cross Creek UFA W S USGS

386 292929081572201 MCFD, Orange Springs UFA Q I USGS

387 292935081402500 Welaka Spring SP D IA USGS

388 292948081503001 USGS (P-0450), Road 77/77-G UFA Q, W M SJRWMD

389 293004081443601 Caravell Ranch 4” (P-0585) UFA W S USGS

390 293021081570600 Camp Seminole Spring, GSC, Orange Springs SP D I USGS

391 293038081563800 Orange Spring near Orange Springs SP D, Q I USGS

392 293048081403600 Nashua Spring near Welaka SP D IA USGS

393 293051081512500 Blue Springs near Orange Springs SP D, Q I USGS

394 293159081403600 Satsuma Spring near Satsuma SP D S SJRWMD

395 293113081370301 Pomona Park 6” (P-0382) UFA W S USGS

396 293234081424101 Rodeheaver Boys Ranch 4” intermittent flow (P-0280) UFA W S USGS

397 293253082055701 Driscoll 4”, Lake Jeffords UFA W S USGS

398 293300081523901 CE60 8” freeflow (P-0306) UFA Q, W M SJRWMD, USGS

399 293420081415601 A. M. Thomas 4” freeflow (P-0462), Stokes Landing UFA W S USGS

400 293439081524201 3” Freeflow (P-0017) Creek Hwy 315 UFA W S USGS

401 293556082043401 Hawthorne Tower 4” (A-0071) UFA W M SJRWMD

402 293720081595301 Chesser 6” (P-0008), Putnam Hill UFA W C USGS

403 293733081474801 Hollister Workctr 4” (P-0510) UFA Q, W M SJRWMD

404 293744081541601 P-0461, Grassey Lake UFA W S USGS

405 293806081544901 P-0016 Keller No. 11 Putnam Hill UFA W M SJRWMD

406 294012081525701 Lake Grandin 6” (P-0772) UFA Q, W S USGS

407 294308082002201 Swan Lake 10” (P-0001) near Melrose UFA Q, W M SJRWMD

Appendix C. Index to Floridan aquifer system well and spring data-collection sites--Continued

[Site numbers refer to figure 7. Abbreviation for hydrologic unit: LFA, Lower Floridan aquifer; UFA, Upper Floridan aquifer; SP, Upper Floridan aquifer 

spring. Abbreviation for data type: D, ground-water discharge; Q, water quality; W, ground-water level; --, no data. Abbreviation for frequency of flow and 

water-level data: C, daily; I, intermittently; IA, currently inactive; M, monthly or bimonthly; Q, quarterly; S, semiannual; W, weekly. Abbreviation for source 

of data: FLDEP, State of Florida Department of Environmental Protection; LC, Lake County; LO, Land Owner, PBG&S, Parsons Brinckerhoff; SJRWMD, 

St. Johns River Water Management District; SWFWMD, Southwest Florida Water Management District; USGS, U.S. Geological Survey]

Site

 number

USGS 

site 

identification 

number

Station name

Hydrologic

unit

Data 

type

Frequency 

of flow 

and 

water-level 

data

Source 

of 

data
140 Hydrogeology and Simulated Effects of Ground-Water Withdrawals from the Floridan Aquifer System in Lake County and in 

the Ocala National Forest and Vicinity, North-Central Florida