Hydrogeology and Simulation of the Effects of 

Reclaimed-Water Application in West Orange 

and Southeast Lake Counties, Florida

By Andrew M. O’Reilly

U.S. Geological Survey

Water-Resources Investigations Report 97–4199

Prepared in cooperation with the

REEDY CREEK IMPROVEMENT DISTRICT

ORANGE COUNTY and the

CITY OF ORLANDO

Tallahassee, Florida

1998
U.S. DEPARTMENT OF THE INTERIOR

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY

Thomas J. Casadevall, Acting Director

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

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

For additional information Copies of this report can be 

write to: purchased from:

District Chief U.S. Geological Survey

U.S. Geological Survey Branch of Information Services

Suite 3015 Box 25286

227 N. Bronough Street Denver, CO 80225

Tallahassee, FL 32301 Phone: 800-USA-MAPS
Contents III

CONTENTS

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

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

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

Data Collection ................................................................................................................................................................ 4

Description of Reclaimed-Water Application.................................................................................................................. 4

Previous Studies............................................................................................................................................................. 10

Acknowledgments.......................................................................................................................................................... 12

Hydrologic Setting................................................................................................................................................................... 12

Climate........................................................................................................................................................................... 12

Hydrogeologic Framework ............................................................................................................................................ 14

Ground-Water Flow-System Characteristics ................................................................................................................. 20

Surface Water................................................................................................................................................................. 26

Water Budget.................................................................................................................................................................. 29

Description of Model ............................................................................................................................................................... 35

Boundary Conditions ..................................................................................................................................................... 37

Hydrologic Input Data ................................................................................................................................................... 41

Aquifer and Confining Unit Properties................................................................................................................ 41

Recharge to the Surficial Aquifer System ........................................................................................................... 41

Stream Leakage.................................................................................................................................................... 43

Wetland Discharge ............................................................................................................................................... 43

Ground-Water Withdrawals ................................................................................................................................. 43

Calibration...................................................................................................................................................................... 46

Parameter Estimation........................................................................................................................................... 48

Sensitivity Analyses............................................................................................................................................. 64

Hydrologic Effects of Reclaimed-Water Application.............................................................................................................. 68

Effects under Current Conditions .................................................................................................................................. 69

Effects under Proposed Future Conditions .................................................................................................................... 82

Model Limitations.................................................................................................................................................................... 86

Summary .................................................................................................................................................................................. 87

References................................................................................................................................................................................ 89

FIGURES

1-3. Maps showing:

1. Location of study area, model area, and reclaimed-water application sites........................................................... 3

2. Locations of surface-water and climatological data-collection sites...................................................................... 5

3. Locations of ground-water data-collection sites..................................................................................................... 6

4. Graph showing monthly average reclaimed-water application rates, July 1994 through February 1996 ................... 11

5. Map showing generalized topography and physiographic regions in the study area .................................................. 13

6. Graph showing cumulative daily rainfall, 1994 and 1995........................................................................................... 14

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

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

8. Diagram showing hydrogeologic sections A-A′ and B-B′ .......................................................................................... 16
IV Contents

9-11. Maps showing:

9. Altitude of the top of the intermediate confining unit and locations of hydrogeologic sections..........................18

10. Thickness of the intermediate confining unit........................................................................................................19

11. Water table in the surficial aquifer system, average 1995 conditions...................................................................21

12. Graphs showing cumulative rainfall, surficial aquifer system water levels, and discharge at Reedy 

Creek, 1995 ..................................................................................................................................................................22

13. Map showing potentiometric surface of the Upper Floridan aquifer, average 1995 conditions..................................24

14. Graphs showing changes observed in surficial aquifer system and Upper Floridan aquifer water levels at selected 

monitoring wells, 1995.................................................................................................................................................25

15. Map showing surface-water features in the study area ................................................................................................27

16. Graph showing water levels in Island Lake and nearby surficial aquifer system monitoring wells, September 1994 

through February 1996 .................................................................................................................................................28

17. Graph showing discharge and chloride concentrations in water sampled from Whittenhorse Creek, January 1967 

through January 1997 ...................................................................................................................................................31

18. Diagram showing hydrologic budget of the surficial aquifer system within the model area, 1995 ............................32

19. Map showing distribution of evapotranspiration regions in the model area................................................................33

20. Graph showing rainfall and estimated rates of evapotranspiration in the study area, 1995.........................................36

21-30. Maps showing:

21. Finite-difference grid showing active and inactive model cells in each layer......................................................38

22. Boundary conditions for the surficial aquifer system ...........................................................................................39

23. Boundary conditions for the Upper and Lower Floridan aquifers........................................................................40

24. Transmissivity of the Lower Floridan aquifer specified in the model..................................................................42

25. Artificial recharge rates used to calculate effective recharge, average 1995 conditions ......................................44

26. Effective recharge rates specified in the model, average 1995 conditions ...........................................................45

27. Ground-water withdrawal rates for the Upper Floridan aquifer specified in the model, average 1995 

conditions..............................................................................................................................................................47

28. Calibrated hydraulic conductivity of the surficial aquifer system and delineation of zones used for 

parameter estimation.............................................................................................................................................50

29. Calibrated transmissivity of the Upper Floridan aquifer and delineation of zones used for 

parameter estimation.............................................................................................................................................52

30. Calibrated leakance of the intermediate confining unit and delineation of zones used for 

parameter estimation.............................................................................................................................................54

31. Graph showing comparison of simulated to measured water levels for the calibrated model.....................................56

32-37. Maps showing:

32. Simulated water table and water-level residuals for the surficial aquifer system, 1995.......................................57

33. Simulated water table and water-level residuals for the surficial aquifer system at the Water Conserv II RIB 

sites 5 and 6, 1995 ................................................................................................................................................58

34. Simulated water table and water-level residuals for the surficial aquifer system at the Water Conserv II 

RIB sites 7 and 9, 1995.........................................................................................................................................59

35. Simulated water table and water-level residuals for the surficial aquifer system at the RCID RIB site, 1995 ....60

36. Simulated potentiometric surface and water-level residuals for the Upper Floridan aquifer, 1995 .....................61

37. Simulated potentiometric surface of the Lower Floridan aquifer, 1995 ...............................................................63

38. Diagram showing simulated volumetric water budget of the aquifer system in the model area, 1995 .......................64

39. Graphs showing model sensitivity to independent changes in selected model calibration parameters .......................65

40-46. Maps showing:

40. Simulated change in the surficial aquifer system water table from assumed pre-application conditions 

as a result of steady-state 1995 reclaimed-water application rates.......................................................................70

41. Simulated change in the Upper Floridan aquifer potentiometric surface from assumed pre-application 

conditions as a result of steady-state 1995 reclaimed-water application rates .....................................................72
Contents V

42. Simulated rate of leakage through the intermediate confining unit based on steady-state 1995 conditions........ 73

43. Areas where reclaimed water is expected to be present in the surficial aquifer system based on 

particle-tracking analyses of simulated steady-state 1995 conditions ................................................................. 75

44. Traveltime of reclaimed water from the surficial aquifer system water table to the top of the 

Upper Floridan aquifer based on particle-tracking analyses of simulated steady-state 1995 conditions............. 76

45. Areas of discharge of reclaimed water from the surficial aquifer system to surface-water features at the 

RCID RIB site and vicinity based on particle-tracking analyses of simulated steady-state 

1995 conditions .................................................................................................................................................... 77

46. Areas where reclaimed water is expected to be present in the Upper Floridan aquifer and pathlines 

depicting directions and rates of movement of reclaimed water based on simulated steady-state 

1995 conditions .................................................................................................................................................... 78

47. Diagram showing projected cross-sectional view of simulated pathlines depicting directions and rates of 

movement of reclaimed water from several RIBs at the RCID RIB site based on simulated steady-state 

1995 conditions............................................................................................................................................................ 80

48. Diagram showing projected cross-sectional view of simulated pathlines depicting directions and rates of 

movement of reclaimed water from several RIBs at Water Conserv II RIB site 5 based on simulated 

steady-state 1995 conditions........................................................................................................................................ 81

49. Map showing simulated change in the surficial aquifer system water table from 1995 conditions as a 

result of steady-state proposed future reclaimed-water application rates.................................................................... 84

50. Map showing simulated change in the Upper Floridan aquifer potentiometric surface from 1995 conditions 

as a result of steady-state proposed future reclaimed-water application rates............................................................. 85

TABLES

1. Surface-water and climatological data-collection sites....................................................................................................... 7

2. Ground-water data-collection sites ..................................................................................................................................... 8

3. Average annual stream discharge into and out of model area, 1995................................................................................. 30

4. Characteristics of evapotranspiration regions delineated in figure 19.............................................................................. 30

5. Monthly average pan evaporation, evapotranspiration, pan coefficients, and citrus crop coefficients during 1995........ 35

6. Initial and calibrated values of parameters estimated to calibrate the model ................................................................... 49

7. Selected correlation coefficients between parameters from the calibrated model............................................................ 55

8. Water-level error statistics for the calibrated model ......................................................................................................... 56

9. Calibrated and alternative values of parameters using alternative effective recharge values........................................... 67

10. Discharge of reclaimed water from model based on particle-tracking analyses of simulated steady-state 1995 

conditions .......................................................................................................................................................................... 82

11. Discharge of reclaimed water from model based on particle-tracking analyses of simulated steady-state proposed 

future conditions................................................................................................................................................................ 86
VI Contents

CONVERSION FACTORS, VERTICAL DATUM, ABBREVIATIONS AND ACRONYMS 

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.

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

[(ft3

/d)/ft2]ft. In this report, the mathematically reduced form, foot squared per day (ft2/d), is used for convenience.

.

Multiply By To obtain

Length

inch (in) 2.54 centimeter

foot (ft) 0.3048 meter

mile (mi) 1.609 kilometer

Area

acre 0.4047 hectare

square mile (mi2

) 2.590 square kilometer 

Flow Rate

cubic foot per second (ft3/s) 0.02832 cubic meter per second 

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

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

Hydraulic Conductivity

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

Transmissivity*

foot squared per day (ft2

/d) 0.09290 meter squared per day 

Leakance

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

Acronyms and additional abbreviations used in report

AAS = Alternate Application Site

ET = Evapotranspiration

MODFLOW = U.S. Geological Survey Modular Three-Dimensional 

Ground-Water Flow Model

NOAA = National Oceanic and Atmospheric Administration

PVC = Polyvinyl chloride

RCID = Reedy Creek Improvement District

RIB = Rapid Infiltration Basin

USGS = U.S. Geological Survey
Abstract 1

Abstract

Wastewater reclamation and reuse has become 

increasingly popular as water agencies search for 

alternative water-supply and wastewater-disposal 

options. Several governmental agencies in central 

Florida currently use the land-based application of 

reclaimed water (wastewater that has been treated 

beyond secondary treatment) as a management 

alternative to surface-water disposal of wastewater. Water Conserv II, a water reuse project 

developed jointly by Orange County and the City 

of Orlando, began operation in December 1986. 

In 1995, the Water Conserv II facility distributed 

approximately 28 Mgal/d of reclaimed water for 

discharge to rapid-infiltration basins (RIBs) and 

for use as agricultural irrigation. The Reedy Creek 

Improvement District (RCID) began operation of 

RIBs in September 1990, and in 1995 these RIBs 

received approximately 6.7 Mgal/d of reclaimed 

water. Analyses of existing data and data collected 

during the course of this study were combined 

with ground-water flow modeling and particle-tracking analyses to develop a processoriented evaluation of the regional effects of 

reclaimed water applied by Water Conserv II and 

the RCID RIBs on the hydrology of west Orange 

and southeast Lake Counties.

The ground-water flow system beneath the 

study area is a multi-aquifer system that consists of 

a thick sequence of highly permeable carbonate 

rocks overlain by unconsolidated sediments. 

The hydrogeologic units are the unconfined surficial aquifer system, the intermediate confining unit, 

and the confined Floridan aquifer system, which 

consists of two major permeable zones, the Upper 

and Lower Floridan aquifers, separated by the less 

permeable middle semiconfining unit. Flow in the 

surficial aquifer system is dominated regionally by 

diffuse downward leakage to the Floridan aquifer 

system and is affected locally by lateral flow systems produced by streams, lakes, and spatial variations in recharge. Ground water generally flows 

laterally through the Upper Floridan aquifer aquifer 

to the north and east. Many of the lakes in the study 

area are landlocked because the mantled karst environment precludes a well developed network of 

surface-water drainage.

The USGS three-dimensional ground-water 

flow model MODFLOW was used to simulate 

ground-water flow in the surficial and Floridan 

aquifer systems. A steady-state calibration to 

average 1995 conditions was performed by using a 

parameter estimation program to vary values of 

surficial aquifer system hydraulic conductivity, 

intermediate confining unit leakance, and Upper 

Floridan aquifer transmissivity. The calibrated 

model generally produced simulated water levels in 

close agreement with measured water levels and 

was used to simulate the hydrologic effects of 

reclaimed-water application under current (1995) 

and proposed future conditions.

Hydrogeology and Simulation of the Effects of 

Reclaimed-Water Application in West Orange 

and Southeast Lake Counties, Florida

By Andrew M. O’Reilly
2 Hydrogeology and Simulation of the Effects of Reclaimed-Water Application in West Orange and Southeast 

Lake Counties, Florida

In 1995, increases of up to about 40 ft in the 

water table and less than 5 ft in the Upper Floridan 

aquifer potentiometric surface had occurred as a 

result of reclaimed-water application. The largest 

increases were under RIB sites. An average traveltime of 10 years at Water Conserv II and 7 years at 

the RCID RIBs was required for reclaimed water to 

move from the water table to the top of the Upper 

Floridan aquifer. Approximately 67 percent of the 

reclaimed water applied at the RCID RIB site 

recharged the Floridan aquifer system, whereas 

33 percent discharged from the surficial aquifer system to surface-water features; 99 percent of the 

reclaimed water applied at Water Conserv II 

recharged the Floridan aquifer system, whereas 

only 1 percent discharged from the surficial aquifer 

system to surface-water features. The majority of 

reclaimed water applied at both facilities probably 

will ultimately discharge from the Floridan aquifer 

system outside the model boundaries.

Proposed future conditions were assumed to 

consist of an additional 11.7 Mgal/d of reclaimed 

water distributed by the Water Conserv II and RCID 

facilities. Increases of up to about 20 ft in the water 

table and 2 ft in the potentiometric surface of the 

Upper Floridan aquifer were simulated. The directions of reclaimed water movement through the 

ground-water system generally were similar to 

those under 1995 conditions. However, the greater 

reclaimed-water application rate at the RCID RIBs 

caused approximately half of the RCID reclaimed 

water to discharge to surface-water features and half 

to recharge the Floridan aquifer system.

INTRODUCTION

Wastewater reclamation and reuse has become 

increasingly popular as water agencies search for alternative water-supply and wastewater-disposal options. 

Land-based application of reclaimed water (wastewater that has been treated beyond secondary treatment) 

is being used as an alternative to the discharge of 

treated wastewater to lakes or streams (Metcalf & 

Eddy, Inc. 1991). Reclaimed water can be used in 

place of freshwater previously withdrawn for nonpotable uses, such as agricultural or landscape irrigation. 

The conservation of valuable freshwater resources 

makes the application of reclaimed water an attractive 

management choice. Several governmental agencies 

(Orange County, City of Orlando, and the Reedy 

Creek Improvement District (RCID)) in central 

Florida currently use the land-based application of 

reclaimed water as a management alternative to 

surface-water disposal of wastewater.

Operation of Water Conserv II, a water reuse 

project developed jointly by Orange County and the 

City of Orlando, began in December 1986. In 1995, 

the Water Conserv II facility distributed approximately 

28 Mgal/d of reclaimed water received from two 

wastewater-treatment plants serving the Orlando 

metropolitan area. About 40 percent of the reclaimed 

water was used for agricultural irrigation and 60 

percent was discharged to rapid-infiltration basins 

(RIBs). Areas where several RIBs are concentrated are 

referred to as RIB sites in this report.

The RCID began discharging reclaimed water to 

RIBs in September 1990. In 1995, these RIBs received 

approximately 6.7 Mgal/d of reclaimed water from a 

wastewater-treatment plant serving the Walt Disney 

World theme parks and resorts. The combined flow of 

the Water Conserv II and RCID facilities in 1995 was 

34.7 Mgal/d, but based on design capacities as much 

as 65 Mgal/d might be directed to these facilities in the 

future.

The Water Conserv II and RCID reclaimed-water 

application sites are located in west Orange and 

southeast Lake Counties, Florida (fig. 1). Reclaimed 

water applied to the land surface is either lost to the 

atmosphere through evapotranspiration or percolates to 

the water table of the surficial aquifer system. Once 

within the surficial aquifer system, water can move 

laterally and discharge at an adjacent stream, lake, or 

wetland or be extracted by evapotranspiration where the 

water table is near land surface; or water can move 

downward to recharge the underlying Floridan aquifer 

system, a very transmissive limestone aquifer and the 

major source of freshwater in central Florida.

The Water Conserv II and RCID facilities have 

extensive programs for monitoring the effects of 

reclaimed-water application on the local ground-water 

levels and water quality; however, a process-oriented 

evaluation of the ground-water system was required for 

a more comprehensive and regional appraisal of these 

effects. In 1993, the U.S. Geological Survey (USGS) in 

cooperation with the RCID, Orange County, and the 

City of Orlando, began a 4-year study of the combined 

effects of Water Conserv II and the RCID RIBs on the 

hydrology of west Orange and southeast Lake Counties.
Introduction 3

Figure 1. Location of study area, model area, and reclaimed-water application sites.
4 Hydrogeology and Simulation of the Effects of Reclaimed-Water Application in West Orange and Southeast 

Lake Counties, Florida

Purpose and Scope

This report presents the results of a 

ground-water flow model of west Orange and 

southeast Lake Counties that includes areas where 

reclaimed water is used for irrigation and also is 

directed to RIBs (fig. 1). The model addresses the 

more regional effects of reclaimed-water application 

and not the local effects of an individual RIB or small 

irrigation site. Data collected during the course of the 

study are described and analyzed in the report to help 

characterize the hydrogeology of the study area. 

A hydrologic budget for the surficial aquifer system is 

presented that includes estimated rates of evapotranspiration from the study area, recharge to the Floridan 

aquifer system, and surface-water outflow. Hydrologic 

effects resulting from current (1995) and proposed 

future reclaimed-water application rates are quantified 

based on results of the ground-water flow model and 

particle-tracking analyses.

Data Collection

Data-collection sites were inventoried based on 

a review of existing data in the study area. Additional 

data-collection sites were installed to further define 

the characteristics of the hydrologic system. Data 

collection included rainfall, evaporation, lake stage, 

stream stage and discharge, and ground-water levels 

(figs. 2 and 3, tables 1 and 2). Most data were 

collected from August 1994 through February 1996.

A standard U.S. Class A evaporation pan was 

installed at the RCID RIB site in January 1995 and 

operated through January 1996 (fig. 2). Pan water 

level, pan water temperature (0.25 in. below water 

surface), air temperature (3.3 ft above land surface), 

windspeed (1.6 ft above land surface), and relative 

humidity (3.3 ft above land surface) were measured at 

20-minute intervals. These data were used for estimation of evapotranspiration.

Seven staff gages and 16 surficial aquifer 

system monitoring wells were installed in areas where 

existing water-level data were not available. The wells 

were constructed by the mud-rotary technique, to the 

depth typically required to place the bottom of the well 

at least 20 ft below the water table. Each well 

consisted of 2.5 ft of polyvinyl chloride (PVC) slotted 

screen and 2- or 4-in. diameter threaded flush-joint 

PVC casing from the screen to approximately 3 ft 

above land surface. The borehole annulus was 

backfilled with clean sand to at least 3 ft above the 

screen, followed by cement grout to the land surface. 

Each well was developed with compressed air after 

installation.

Lithologic data were also collected and 

analyzed. Split-spoon cores were collected at 5-ft 

intervals during construction of four of the new wells 

to provide additional data on surficial aquifer system 

lithology. Geophysical logs (caliper and natural 

gamma) were collected at 16 wells to supplement 

existing data on the altitude of the top of the intermediate confining unit and the top of the Upper Floridan 

aquifer.

Description of Reclaimed-Water Application

Water Conserv II is the largest water reuse 

project in the world that combines agricultural irrigation and RIBs (Estow, 1996). Reclaimed water is 

provided free-of-charge to privately owned citrus 

groves and commercial nurseries based on a 20-year 

agreement among the grower, Orange County, and the 

City of Orlando. In 1995, approximately 4,800 acres 

of citrus and 100 acres of nursery were being served.

Water beyond that required for irrigation 

typically is directed to 46 RIBs. Clusters of 3 to 35 

RIBs are located at four separate RIB sites numbered 

5, 6, 7, and 9 (fig. 1). The RIBs were excavated in 

native sands with no soil profile modification. Each 

RIB consists of up to five adjacent cells that are 

connected with buried pipes. The average RIB cell 

bottom surface area is 1.3 acres; the interior 

side-slopes are covered with geotextile liner and the 

exterior slopes are grass-covered. Water is discharged 

to the RIB through a vertical pipe in the center of each 

cell and allowed to fill no more than 2- to 3-ft deep. 

RIBs typically are loaded for 1 week and allowed to 

rest for 1 to 2 weeks while the local ground-water 

mound dissipates. RIBs are loaded on a rotating basis: 

while one set of RIBs is in its resting period, another 

set is being loaded. RIB bottoms are tilled periodically 

to disrupt the thin algal layer that commonly forms 

after extended loading periods and to destroy any 

weeds.

Alternate application sites (AASs) are used 

during extreme and prolonged wet periods when 

irrigation demand is minimal and the RIBs are 

operating at full capacity. Most AASs are natural land 

depressions where reclaimed water can be directed and 

allowed to infiltrate in a manner similar to a RIB. 

Some AASs utilize overhead sprinkler irrigation of 

natural herbaceous vegetation.
Introduction 5

Figure 2. Locations of surface-water and climatological data-collection sites (site information in table 1). 
6 Hydrogeology and Simulation of the Effects of Reclaimed-Water Application in West Orange and Southeast 

Lake Counties, Florida

Figure 3. Locations of ground-water data-collection sites (site information in table 2). 
Introduction 7

Table 1. Surface-water and climatological data-collection sites

[Sites are located by site numbers in figure 2. Sites in close proximity to each other have the same site number. Abbreviation for data 

type: R, rainfall; E, evaporation; S, surface-water stage; D, stream stage and discharge. Abbreviation for frequency: M, data collected 

monthly; W, data collected weekly; C, data collected at least daily but typically hourly. Abbreviation for source of data: M&E, Metcalf 

& Eddy Services, Inc.; OCSMD, Orange County Stormwater Management Department; RCES, Reedy Creek Energy Services, Inc.; 

SJRWMD, St. Johns River Water Management District; USGS, U.S. Geological Survey. --, not applicable]

Site 

number Station number Station name Data 

type Frequency Source of 

data

1 -- Lake Apopka S C SJRWMD

2 -- Lake Minneola S C SJRWMD

3 -- Johns Lake S M OCSMD

4 02236840 Lake Minnehaha S C USGS

5 -- Black Lake S M OCSMD

6 -- Lake Roberts S M OCSMD

7 -- Lake Avalon S M OCSMD

8 -- Crescent Lake S M OCSMD

9 283013081360800 Stream at Tilden Road S M USGS

10 -- Cawood Pond West S M OCSMD

11 -- Flat Lake S M OCSMD

12 -- Lake Butler S M OCSMD

13 -- Lake Speer S M OCSMD

14 -- Water Conserv II HA-5 S W M&E

15 -- Water Conserv II HA-1 S W M&E

16 02236820 Lake Louisa S W USGS

17 -- Lake Hartley S M OCSMD

18 -- Water Conserv II JR-2 S W M&E

19 282826081423000 Hi-Acres pond near Shell Pond Road S M USGS

20 -- Pretty Lake S C SJRWMD

21 282754081402700 Hi-Acres pond near Five Mile Road S M USGS

22 -- Water Conserv II HA-3 S W M&E

23 -- Lake Ingram S M OCSMD

24 02236700 Little Creek D C USGS

25 -- Water Conserv II JR-3 S W M&E

26 -- Water Conserv II HA-6 S W M&E

27 -- Water Conserv II SP-5 S W M&E

28 -- Water Conserv II SP-2 S W M&E

29 -- Water Conserv II HA-4 S W M&E

30 -- Water Conserv II DM-1 S W M&E

31 -- Lake Tibet S M OCSMD

32 -- Lake Hancock S M OCSMD

33 02266239 Trout Lake S W USGS

34 02236500 Big Creek D C USGS

35 -- Kirkland Lake S W SJRWMD

36 -- Island Lake, Water Conserv II HA-2 S W M&E

37 -- Lake Needham S M OCSMD

38 -- Lake Sawgrass S M OCSMD

39 -- Pike Lake, Water Conserv II HA-8 S W M&E

40 -- Huckleberry Lake S M OCSMD

41 282616081405500 Sawgrass Lake S M USGS

42 02266291 L-405 above S-405A D C USGS

42 02266292 L-405 below S-405A S C USGS

43 -- Hickorynut Lake S M OCSMD

44 -- Pocket Lake S M OCSMD

45 02263868 South Lake S C USGS

46 02263850 Bay Lake S C USGS

47 282452081370200 Reedy Lake S M USGS

48 02263870 South Lake outlet below S-15 S C USGS

48 02263869 South Lake outlet above S-15 D C USGS

49 -- Water Conserv II CH-1 S W M&E

50 -- Little Fish Lake S M OCSMD

51 282431081371400 RCID pond near Hartzog Road S M USGS

52 02266026 Reedy Creek below S-46 S C USGS

52 02266025 Reedy Creek above S-46 D C USGS

53 -- Water Conserv II AT-1 S W M&E

54 282351081381600 Bear Bay at SR 545 S M USGS

55 02266205 Whittenhorse Creek above S-411 D C USGS

56 02264000 Cypress Creek D C USGS

57 02266200 Whittenhorse Creek near Vineland D C USGS

58 02264051 Black Lake outlet above S-101A D C USGS

59 02264060 L-101 above S-101 D C USGS

60 02266295 L-410 above S-410 D C USGS

60 02266296 L-410 below S-410 S C USGS
8 Hydrogeology and Simulation of the Effects of Reclaimed-Water Application in West Orange and Southeast 

Lake Counties, Florida

Table 1. Surface-water and climatological data-collection sites--Continued

Table 2. Ground-water data-collection sites

[Sites are located by site numbers in figure 3. Sites in close proximity to each other have the same site number. Abbreviation 

for data type: Gs, surficial aquifer system ground-water level; Gf, Floridan aquifer system ground-water level. Abbreviation for

frequency: B, data collected bimonthly; M, data collected monthly; W, data collected weekly; C, data collected at least daily 

but typically hourly. Abbreviation for source of data: M&E, Metcalf & Eddy Services, Inc.; RCES, Reedy Creek Energy 

Services, Inc.; USGS, U.S. Geological Survey. --, not applicable]

61 02264100 Bonnet Creek D C USGS

62 02266300 Reedy Creek near Vineland D C USGS

63 -- Water Conserv II Rain Gage 6-1 R C M&E

64 -- Water Conserv II Rain Gage 6-2 R C M&E

65 -- Water Conserv II Rain Gage 6-4 R C M&E

66 -- Water Conserv II Rain Gage 6-3 R C M&E

67 -- Water Conserv II Rain Gage 5-1 R C M&E

68 -- Water Conserv II Rain Gage 7-1 R C M&E

69 -- Water Conserv II Rain Gage 7-2 R C M&E

70 -- Water Conserv II Rain Gage 9-1 R C M&E

71 -- Rain Gage at RCID RIB site R C RCES

72 -- Class A Pan Evaporation Station E C USGS

73 02236900 Palatlakaha River at Cherry Lake outlet D C USGS

74 02236350 Green Swamp Run near Eva D C USGS

Site 

number Station number Station name Data type Frequency Source of data

1 283540081402401 LK031 Gf B USGS

2 283417081331401 OR059 Ocoee Drain Well Gf B USGS

3 283422081480401 LK028 Sand Mine Gf B USGS

4 283359081411501 LK027 FDAWPC Well Gf B USGS

5 283325081374001 OR053 City of Oakland #2 Gf B USGS

6 283314081455501 LK025 Clermont Deep Replacement Gf C USGS

7 283307081435301 LK024 Jacks Lake Well Gf B USGS

8 283232081394101 LK023 Edgewater Beach Gf B USGS

9 283121081311601 OR043 Lake Olivia Drain Well Gf B USGS

10 283128081404701 LK020 L-0052 Johns Lake Gf B USGS

11 283116081442301 LK019 Rings Pond Gf B USGS

12 283047081392401 SW-9 Fischer Marsh Road Gs C USGS

13 283017081391301 OR040 Davenport Road Gf B USGS

14 283019081455701 LK101 LCFD District 9 Station 1 Gf B USGS

15 283011081360002 OR038 West Orange Country Club Gf B USGS

16 -- Water Conserv II 6-IW Gf W M&E

16 282939081365701 Water Conserv II 6-F1 Gf C USGS

16 282939081365702 Water Conserv II P-40 Gs C USGS

17 282936081340201 OR036 Ross Home Well Gf B USGS

18 282904081415701 SW-1 Hi-Acres Summit Gs M USGS

19 -- Water Conserv II MW11-01 Gs W M&E

19 282901081380301 Water Conserv II EW11-04 Gs C USGS

20 -- Water Conserv II MW11-02 Gs W M&E

21 282853081403801 Water Conserv II HA7-F Gf B USGS

21 282854081403701 Water Conserv II HA7-2 Gs M USGS

22 282848081384901 SW-10 Ross Grove on Rex Road Gs M USGS

23 282835081305201 OR028 Palm Lake Drive Gf C USGS

24 282844081403701 Water Conserv II HA7-1 Gs M USGS

25 282844081420201 Water Conserv II HA5-1 Gs M USGS

25 282848081420101 Water Conserv II HA5-F Gf B USG5

26 282839081383501 Water Conserv II JR2-F Gf B USGS

27 -- Water Conserv II 5-F1A Gf W M&E

28 282839081410501 Water Conserv II HA1-F Gf B USGS

29 -- Water Conserv II EW12-03 Gs W M&E

29 -- Water Conserv II EW12-04 Gs W M&E

29 -- Water Conserv II MW12-01 Gs W M&E

29 -- Water Conserv II EW12-02 Gs W M&E

30 282827081380401 SW-11 Phillips Grove Gs M USGS

31 282827081385001 Water Conserv II JR2-1 Gs M USGS

31 282835081384901 Water Conserv II JR2-3 Gs M USGS

Site 

number Station number Station name Data type Frequency Source of data
Introduction 9

Table 2. Ground-water data-collection sites--Continued

32 282825081395601 SW-2 Fabry Grove Gs M USGS

33 282823081500401 LK014 D D Gaffney Gf B USGS

34 282749081315801 OR027 Butler Groves Gf B USGS

35 282738081341401 OR025 Lake Sawyer Well Gf C USGS

36 282733081372501 Water Conserv II 2W-1 Gf B USGS

37 282734081404301 SW-3 Hi-Acres Five Mile Road Gs M USGS

38 282732081423901 Water Conserv II HA6-1 Gs M USGS

38 282738081423702 Water Conserv II HA6-3 Gs M USGS

38 282738081423801 Water Conserv II HA6-F Gf B USGS

39 282731081410801 Water Conserv II HA4-F Gf B USGS

40 282728081425501 SW-5 Clonts Grove Gs C USGS

41 282729081443301 LK013 L-0053 Lake Louisa State Park Gf B USGS

42 282724081373401 Water Conserv II DM2-2 Gs M USGS

43 282724081382601 SW-13 Ross Grove on SR 545 Gs M USGS

43 282730081381701 Water Conserv II JR3-1 Gs M USGS

44 282722081371701 Water Conserv II SP5-1 Gs M USGS

45 282715081422501 RCID 4-inch PVC #1 Gs M USGS

46 282706081412601 RCID 12-inch Irrigation Gf C USGS

46 282706081412602 SW-4 RCID Shell Pond Road Gs C USGS

47 282705081430701 LK103 Trout Lake Well Gf B USGS

48 282702081393501 RCID 4-inch PVC #4 Gs M USGS

49 -- Water Conserv II EW16-04 Gs W M&E

49 -- Water Conserv II EW16-01 Gs W M&E

49 -- Water Conserv II GC-10 Gs W M&E

50 282656081383001 SW-12 Ford Avalon Grove Gs M USGS

51 282654081412002 Water Conserv II HA2-5 Gs M USGS

51 282654081412401 Water Conserv II HA2-F Gf B USGS

52 282652081400601 RCID 4-inch PVC #3 Gs M USGS

53 -- Water Conserv II GC-01 Gs W M&E

53 -- Water Conserv II EW13-16 Gs W M&E

54 -- Water Conserv II GC-09 Gs W M&E

54 -- Water Conserv II EW16-02 Gs W M&E

55 -- Water Conserv II EW14-01 Gs W M&E

55 -- Water Conserv II EW14-07 Gs W M&E

56 -- Water Conserv II GC-02 Gs W M&E

56 -- Water Conserv II EW14-02 Gs W M&E

57 -- Water Conserv II EW13-06 Gs W M&E

57 -- Water Conserv II EW13-08 Gs W M&E

58 282642081421901 Water Conserv II HA8-F Gf B USGS

59 -- Water Conserv II EW14-06 Gs W M&E

59 -- Water Conserv II EW14-08 Gs W M&E

60 -- Water Conserv II GC-04 Gs W M&E

60 -- Water Conserv II EW14-04 Gs W M&E

61 -- Water Conserv II GC-03 Gs W M&E

62 -- Water Conserv II EW13-20 Gs W M&E

62 -- Water Conserv II GC-08 Gs W M&E

63 282628081375001 Water Conserv II 9-F1 Gf C USGS

63 282628081375002 SW-6 at Water Conserv II RIB site 9 Gs C USGS

64 -- Water Conserv II GC-05 Gs W M&E

64 -- Water Conserv II EW15-01 Gs W M&E

65 282624081424601 Water Conserv II HA8-FB Gf B USGS

65 282624081424602 Water Conserv II HA8-1 Gs M USGS

66 282623081411001 RCID 4-inch PVC #2 Gs M USGS

66 282626081411101 SW-7 RCID Cook Road Gs M USGS

67 282611081381401 SR 545 at Old YMCA Road Gs M USGS

68 -- Water Conserv II GC-06 Gs W M&E

68 -- Water Conserv II EW15-04 Gs W M&E

69 282544081370501 RCID LW-18 Gs M USGS

70 282543081385801 OR019 Hickorynut Lake Well Gf B USGS

71 -- Water Conserv II GC-07 Gs W M&E

72 282528081340901 OR016 Bay Lake Deep Gf C USGS

73 282532081511801 LK008 Jack Barry Gf B USGS

74 282520081371501 RCID LW-17 Gs M USGS

75 282508081371301 RCID LW-16 Gs M USGS

76 282502081374201 RCID LW-15 Gs M USGS

77 282500081380301 RCID LW-14 Gs M USGS

78 282451081375201 Water Conserv II CH1-F Gf B USGS

79 282445081370901 SW-17 RCID Reedy Lake Gs M USGS

80 282445081391001 Water Conserv II 3W-4 Gf B USGS

81 262442081390001 SW-14 Austin Grove Gs M USGS

Site 

number Station number Station name Data 

type Frequency Source of 

data
10 Hydrogeology and Simulation of the Effects of Reclaimed-Water Application in West Orange and Southeast 

Lake Counties, Florida

Table 2. Ground-water data-collection sites--Continued

Previous Studies

Numerous reports on investigations of the 

hydrology and geology of the general study area are 

available. Cooke (1945) and White (1958, 1970) 

described the geology and geomorphology of central 

Florida. A detailed description of the hydrology of the 

Green Swamp area in central Florida is presented in 

Pride and others (1966); Grubb and others (1978) 

presented numerous lithologic and geophysical data 

within the Green Swamp area, and Grubb and 

Rutledge (1979) modeled ground-water flow in the 

area. Lichtler and others (1968) described the 

hydrology of Orange County, and Knochenmus and 

Hughes (1976) described the hydrology of Lake 

County. Johnson (1979) described the geology of the 

Ocklawaha River basin. Putnam (1975) and German 

(1986) investigated and summarized the hydrologic 

conditions and effects of development within the 

entire RCID. German (1990) described the lithology 

and hydrology of the surficial aquifer system as a part 

of a larger study to determine the water quality effects 

of spray irrigation of treated wastewater in the RCID. 

Ground-water flow models of the Floridan aquifer 

82 282443081425201 LK007 Lykes Groves Gf B USGS

83 282415081391001 Water Conserv II AT1-1 Gs M USGS

84 282354081313001 OR012 Disney World Gf B USGS

85 282330081371101 SW-15 Hartzog Road Shallow Gs C USGS

85 282331081370801 OR009 Hartzog Road Deep Gf C USGS

86 282317081364601 RCID 2-inch PVC #3 Gs M USGS

86 282318081364401 RCID 2-inch PVC #2 Gs M USGS

87 282311081365501 RCID TW-1 Gs M USGS

88 282312081405801 Citrus Valley #2 Gf B USGS

89 282249081365601 SW-16 Fischer Grove on Hartzog Road Gs C USGS

90 282245081492601 LK003 Eva Deep Gf C USGS

91 282241081443901 L-0051 Sand Mine Deep Gf M USGS

91 282241081443902 L-0050 Sand Mine Shallow Gs M USGS

92 282210081352601 RCID Tree Farm Gs C USGS

93 282202081384601 OR005 Lake Oliver Deep Gf C USGS

93 282202081384602 Lake Oliver Shallow Gs C USGS

94 282145081365601 OR003 Britt Groves Gf B USGS

95 282126081403901 LK002 Gf B USGS

96 281536081324801 OS026 FPC Well Gf B USGS

97 281532081345001 PK033 Loughman Deep Gf B USGS

98 281532081493001 PK034 near Polk City Gf B USGS

99 281511081393101 PK032 Gf B USGS

100 -- Water Conserv II P-085 Gs W M&E

101 -- Water Conserv II MW6-28 Gs W M&E

102 -- Water Conserv II P-062 Gs W M&E

103 -- Water Conserv II P-104 Gs W M&E

104 -- Water Conserv II MW5-1 Gs W M&E

105 -- Water Conserv II MW5-4 Gs W M&E

106 -- Water Conserv II MW7-9 Gs W M&E

107 -- Water Conserv II MW7-8 Gs W M&E

108 -- Water Conserv II MW9-7 Gs W M&E

109 -- RCID P8-2D Gs C RCES

110 -- RCID P10-3D Gs C RCES

111 -- RCID P6-4D Gs C RCES

Site 

number Station number Station name Data type Frequency Source of data

The 85 RCID RIBs are similar in construction 

and operation to the Water Conserv II RIBs, except 

that each RCID RIB consists of only one cell with 

approximately 1 acre of bottom surface area. In 

addition, the RCID RIBs are all located at one RIB 

site (fig. 1) and typically operate on a rotating 1-week 

loading period followed by a 4-week resting period.

Monthly reclaimed water application rates vary 

significantly at both facilities. For example, the 

highest application rates in 1995 occurred during the 

July through October rainy season (fig. 4). Wet 

weather and the subsequent increase in ground-water 

levels can cause significant inflow and infiltration into 

wastewater collection systems that produce higher 

flow rates at the wastewater treatment plant (Metcalf 

& Eddy, Inc., 1991). The higher flow rates at the 

wastewater treatment plants serving the Water 

Conserv II and RCID facilities correspond to the 

higher reclaimed water application rates. The variation 

of irrigation rates relative to RIB application rates at 

Water Conserv II (fig. 4) is due to variable crop water 

requirements as influenced by rainfall and crop 

growth characteristics.
Introduction 11

system were constructed for parts of Osceola, Orange, 

and Brevard Counties by Planert and Aucott (1985); 

for east-central Florida by Tibbals (1990); and for the 

greater metropolitan Orlando area by Murray and 

Halford (1996). Sumner (1996) measured and 

modeled evapotranspiration from successional 

vegetation in a deforested area in west Orange County.

Several studies specifically addressed the Water 

Conserv II or RCID RIBs. A description of the hydrogeology of the four Water Conserv II RIB sites was 

presented by Camp Dresser and McKee, Inc. (1984) 

and included ground-water flow modeling of both the 

local and regional impacts of RIB operation on the 

hydrologic system. CH2M Hill (1989) described the 

hydrogeology of the RCID RIB site and presented the 

results of a ground-water flow model simulating the 

impact of the RIBs on the local surficial aquifer system. 

A description of the hydrogeology of a site adjacent to 

and north of the present RCID RIB site was presented 

by CH2M Hill (1993). Sumner and Bradner (1996) 

examined the unsaturated and saturated zone hydraulics 

and nutrient transport and fate at the individual RIB 

scale at the RCID RIB site.

Figure 4. Monthly average reclaimed-water application rates, July 1994 through 

February 1996. 
12 Hydrogeology and Simulation of the Effects of Reclaimed-Water Application in West Orange and Southeast 

Lake Counties, Florida

Acknowledgments

The author expresses his appreciation to Preston 

Merrick, Reedy Creek Improvement District, for 

providing information on the RCID RIBs; Ted W. 

McKim, Reedy Creek Energy Services, Inc., and his 

staff for their maintenance of the pan evaporation 

station and contribution of data on the RCID RIBs; 

Gabor L. Delneky, Orange County Public Utilities 

Division, and Thomas L. Lothrop, City of Orlando 

Environmental Services Department, for providing 

information on the Water Conserv II facility; Phillip F. 

Cross, Metcalf & Eddy Services, Inc., and his staff for 

their assistance with field data collection and contribution of data on the Water Conserv II facility; David F. 

MacIntyre, PB Water, and his staff for providing data 

on the Water Conserv II facility; and the numerous 

landowners who granted permission for the construction of wells or staff gages on their property.

HYDROLOGIC SETTING

The majority of the study area lies within three 

physiographic regions: Lake Wales Ridge, Lake 

Upland, and Osceola Plain (fig. 5). However, nearly 

all of the Water Conserv II and RCID reclaimed-water 

application sites are within the Lake Wales Ridge 

physiographic region, which is characterized by 

relatively high altitudes (exceeding 200 ft in some 

areas), large hills, numerous sinkholes, deep water 

table (greater than 100 ft below land surface in some 

areas), and internal drainage. The karst topography of 

the Lake Wales Ridge precludes significant 

surface-drainage features in many areas. The Lake 

Upland, which includes the Green Swamp west of the 

Lake Wales Ridge, and the Osceola Plain to the east of 

the ridge are relatively flat with numerous wetlands 

separated by low ridges and hills. The water table is at 

or near land surface throughout most of the Lake 

Upland and Osceola Plain except beneath isolated 

ridges and hills.

Much of the study area was planted with citrus 

that was killed or severely damaged during a series of 

freezes in the 1980s; as a result, many citrus groves 

were abandoned. At the time of the study, many of the 

abandoned groves remained uncultivated and had been 

succeeded by natural herbaceous vegetation; however, 

some citrus groves were present along with a few 

areas of scrub oaks and pines.

Climate

Long-term average annual rainfall in central 

Florida is 51 in., based on 80 years of record 

(1913–92) for Orlando and Sanford (Murray and 

Halford, 1996). However, annual rainfall varies from 

year to year; for example, at rain gage site 67 (fig. 2), 

annual rainfall was 80 percent higher in 1994 (70 in.) 

than in 1993 (39 in.). Annual rainfall also varies 

spatially within the study area, but the spatial differences generally are less significant than are the 

temporal differences. For example, 1994 annual 

rainfall at rain gage site 70 was 60 in., about 

15 percent less than rain gage site 67.

Rainfall in central Florida follows seasonal 

trends. Plots of cumulative daily rainfall for 1994 and 

1995 indicate that 69 and 77 percent of the total annual 

rainfall, respectively, occurred during the 5-month 

period from June through October (fig. 6). This 

seasonal variation is corroborated by long-term 

monthly rainfall data and leads to significant seasonal 

trends in surface- and ground-water levels and stream 

discharge.

Although rainfall represents the largest input of 

water to the study area, the largest water loss is 

through evapotranspiration (ET). Conceptually, evapotranspiration is a combination of two processes: 

evaporation of water directly from surface-water 

bodies, plants, and soil; and the transpiration of water 

extracted from soil moisture by plant roots. In practice 

it is difficult to separate the two processes; consequently, they are typically treated as the one quantity 

of ET. In the context of this report, ET is meant to 

represent actual ET, not potential ET, which is the 

amount of water loss which occurs when there is sufficient water available to meet plant needs. Potential ET 

represents an upper limit; a lesser amount of water is 

evaporated or transpired as limited by the actual water 

available. ET varies considerably across the study 

area. This spatial variation is primarily the result of 

differences in vegetation (citrus compared to herbaceous vegetation) and water availability (wetland 

compared to nonirrigated upland). A strong temporal 

variation in ET is due primarily to plant growth 

characteristics and climatological variables such as 

rainfall, solar radiation, windspeed, and humidity.
Hydrologic Setting 13

Figure 5. Generalized topography and physiographic regions in the study area (physiographic regions modified from White 

(1970) and Knochenmus and Hughes (1976)).
14 Hydrogeology and Simulation of the Effects of Reclaimed-Water Application in West Orange and Southeast 

Lake Counties, Florida

Hydrogeologic Framework

The ground-water flow system beneath the 

study area is a multi-aquifer system that consists of a 

thick sequence of carbonate rocks overlain by unconsolidated deposits of sand, silt, and clay. The hydrogeologic units are the surficial aquifer system, the 

intermediate confining unit, and the Floridan aquifer 

system (fig. 7). Beneath the Floridan aquifer system is 

marine dolomite that contains considerable gypsum 

and anhydrite of low permeability and defines the 

bottom of the freshwater flow system in the study area. 

Selected hydrogeologic sections are shown in figure 8.

The surficial aquifer system is the uppermost 

water-bearing unit in the study area. The system is 

unconfined and consists mainly of undifferentiated 

deposits of marine sand, silt, clay, and crushed shell of 

late Pliocene to Recent age (fig. 7). The upper 

boundary of the system is defined by the water table, 

which in most areas is a subdued reflection of 

land-surface topography. The base of the surficial 

aquifer system is defined by the first persistent bed 

containing a significant increase in silt and clay of 

Miocene or Pliocene age. Thickness of the system 

averages about 50 ft but ranges from less than 25 ft in 

low-lying areas around Reedy Creek and the Palatlakaha River to greater than 200 ft at and near collapse 

features located along karstic ridge areas.

The fine- to medium-grained sand that characterizes much of the surficial aquifer system contains 

relatively small amounts of fine-grained sediments 

and is quite permeable. Grain-size analyses of core 

samples collected during drilling operations indicate 

Figure 6. Cumulative daily rainfall, 1994 and 1995 (from average of rainfall measured at site numbers 63 

through 70, fig. 2). 
Hydrologic Setting 15 Figure 7. Geologic units, hydrogeologic units, and equivalent layers and boundary conditions used in ground-water flow model (modified from Murray and Halford, 1996, fig. 4). 
16 Hydrogeology and Simulation of the Effects of Reclaimed-Water Application in West Orange and Southeast 

Lake Counties, Florida

Figure 8. Hydrogeologic sections A-A′ and B-B′ (location of sections shown in fig. 9). Based on

interpolation of data shown in figs. 9 and 10 and Miller (1986).

200

100

SEA

LEVEL

100

200

300

400

A′

SEA

LEVEL

1,000

2,000

0 2 4 MILES

0 2 4 KILOMETERS

0 2 4 KILOMETERS

0 2 4 MILES

A

OSCEOLA CO.ORANGE CO.

RCID RIB site

Water Conserv II

RIB site 7

SECT. A-A′

Water Conserv II

RIB site 6

SR 545

Johns

Lake

US 50

Lake

Apopka

Lake HancockLAKE CO. ORANGE CO.

SR 545

SECT. B-B′

Water Conserv II

RIB site 5

US 27

Lake Louisa

Lake

Speer Lake

Butler

Land surface

Land surface

SURFICIAL AQUIFER SYSTEM

SURFICIAL AQUIFER

INTERMEDIATE

CONFINING UNIT

MIDDLE

SEMICONFINING UNIT

UPPER FLORIDAN AQUIFER

MIDDLE

SEMICONFINING UNIT

UPPER FLORIDAN AQUIFER

INTERMEDIATE

CONFINING UNIT

UPPER FLORIDAN AQUIFER

LOWER FLORIDAN AQUIFER

SUB-FLORIDAN CONFINING UNIT

MIDDLE SEMICONFINING UNIT

B B′

SYSTEM

Water table

Water

table

WATER TABLE REPRESENTS

AVERAGE 1995 CONDITIONS

Lake Nellie

B′

200

100

SEA

LEVEL

100

200

300

400

B
Hydrologic Setting 17

that the upper half of the surficial sediments generally 

are poorly graded and contain less than 10 percent 

(by weight) silt- and clay-sized particles. Finer-grained 

cores generally contained 10 to 30 percent (by weight) 

silt- and clay-sized particles. Most of the finer-grained 

sediments are near the base of the system, although 

isolated beds of clayey to silty sand are present in upper 

parts as well. However, these shallower beds tend to be 

thin, discontinuous, and only locally affect the 

ground-water flow system.

The intermediate confining unit separates the 

surficial and Floridan aquifer systems throughout the 

study area, except where breached by sinkholes, and 

retards the vertical exchange of water between these 

systems (fig. 8). The unit consists of bedded clay, silt, 

sand, crushed shell, and phosphatic limestone of 

Miocene age (Hawthorn Group) and, locally, low 

permeability beds of early Pliocene age (fig. 7). Strata 

of these various lithologies are not present in all areas 

and where present they usually occur in varying proportions; therefore, the overall lithology of the intermediate 

confining unit is quite variable. Beds of basal Hawthorn 

limestone that are in direct contact with the limestone of 

the Upper Floridan aquifer generally are not considered 

to be part of the Floridan aquifer system because the 

beds occur locally and the hydraulic conductivity of the 

Hawthorn limestone is at least an order of magnitude 

less than that of the Floridan limestone (Murray and 

Halford, 1996). However, in the vicinity of Water 

Conserv II RIB site 5, Camp Dresser and McKee, Inc. 

(1984) reported that the Hawthorn limestone is 100- to 

200-ft thick and contains a major cavernous zone. In 

addition, water levels in a monitoring well (site number 

27, fig. 3) open only to this unit reflect a potentiometric 

surface in agreement with that of the regional Upper 

Floridan aquifer. Therefore, the Hawthorn limestone 

was included as part of the Upper Floridan aquifer in 

this area.

The altitude of the top and thickness of the intermediate confining unit are highly variable, owing to 

the effects of past erosional processes and sinkhole 

formation. Where present, the top of the unit generally 

ranges from less than 50 ft below to greater than 75 ft 

above sea level across the study area (fig. 9). Intermediate confining unit thickness ranges from less than 

25 ft in the southwestern part of the study area and 

localized areas in the center of the study area, to 

greater than 150 ft in the northeastern part (fig. 10). 

The unit is relatively thin or absent within many of the 

sinkhole chimneys present across west Orange and 

southeast Lake Counties. These chimneys were often 

backfilled with more permeable surficial sand shortly 

after collapse and provide avenues for water from the 

surficial aquifer system to recharge the underlying 

Floridan aquifer system. The contours for the top and 

thickness of the unit (figs. 9 and 10) are highly generalized and are based on lithologic and geophysical data 

collected at the indicated sites. Actual tops and thicknesses can vary considerably from those shown in 

figures 9 and 10 because of the presence of local 

erosional and collapse features in the underlying 

limestone.

The Floridan aquifer system is composed of a 

sequence of highly permeable Tertiary carbonate rocks 

of Eocene age that average about 2,300 ft in thickness 

across the study area (Tibbals, 1990, fig. 10). The top of 

the system is defined by the porous Ocala limestone of 

upper Eocene age and the base of the system is defined 

by the first occurrence of relatively impermeable, persistent beds of anhydrite associated with the Cedar Keys 

Formation of Paleocene age (fig. 7). The system has 

been subdivided into two major permeable zones, the 

Upper and Lower Floridan aquifers, separated by the 

less permeable middle semiconfining unit (Miller, 

1986). The Upper Floridan aquifer consists of the Ocala 

limestone, where present, and the dolomitic limestone of 

about the upper one-third of the Avon Park Formation 

and ranges in thickness from about 200 to 400 ft across 

the study area (fig. 8). The Ocala limestone is absent in 

some areas as a result of past erosional processes 

(Lichtler and others, 1968). Because of past erosional 

and dissolution processes, the top of the Upper Floridan 

aquifer is highly irregular but generally ranges from an 

altitude of about 50 ft in northern Polk County to about 

-50 ft just southeast of Lake Apopka.

The middle semiconfining unit underlies the 

Upper Floridan aquifer and consists of the less permeable, micritic limestone and dense dolomitic limestone 

found in about the middle one-third of the Avon Park 

Formation. The lithologic character of the Avon Park 

limestone varies considerably with depth and spatially 

across the study area. The top of the middle semiconfining unit slopes from west to east, from an altitude of 

about -300 to -350 ft. The thickness of the unit beneath 

the study area averages about 600 ft (Miller, 1986). 

The Lower Floridan aquifer underlies the middle 

semiconfining unit and includes about the bottom 

one-third of the Avon Park Formation and all of the 

Oldsmar Formation. The Lower Floridan aquifer dips 

from north to south across the study area and averages 

1,300 ft in thickness (Miller, 1986).
18 Hydrogeology and Simulation of the Effects of Reclaimed-Water Application in West Orange and Southeast 

Lake Counties, Florida

Figure 9. Altitude of the top of the intermediate confining unit and locations of hydrogeologic sections.
Hydrologic Setting 19

Figure 10. Thickness of the intermediate confining unit.
20 Hydrogeology and Simulation of the Effects of Reclaimed-Water Application in West Orange and Southeast 

Lake Counties, Florida

Ground-Water Flow-System Characteristics

The surficial aquifer system is recharged by 

rainfall, irrigation, septic-tank effluent, reclaimed 

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

potentiometric surface of the underlying Upper 

Floridan aquifer, by diffuse upward leakage from the 

Floridan aquifer system. Water is discharged from the 

surficial aquifer system through ET in areas with 

relatively shallow water tables, by lateral seepage to 

lakes and streams, and by downward leakage to the 

Floridan aquifer system in areas where the potentiometric surface of the Upper Floridan aquifer is below 

the water table. Little water is pumped from the 

surficial aquifer system, because well yields are 

typically low (compared to the Floridan aquifer 

system) and the water often contains high concentrations of dissolved iron and can be highly colored 

(Tibbals, 1990).

Water levels measured monthly (or more 

frequently) during 1995 at selected surficial aquifer 

system monitoring wells and lakes (figs. 2 and 3, 

tables 1 and 2) were averaged to construct a 

water-table contour map (fig. 11). The localized 

effects of reclaimed-water application were greatest at 

Water Conserv II RIB site 5 where the altitude of the 

mounded water table was greater than 170 ft, or about 

40 ft greater than the levels prior to reclaimed-water 

application. However, the water table is relatively flat 

throughout most of the model area. Where there is 

little relief in the water table, the 10-ft contour interval 

depicted in figure 11 limits the inferences that can be 

made about lateral flow in the surficial aquifer system. 

Numerous local flow systems produced by streams, 

lakes, and spatial variations in recharge probably 

would be apparent if data were available that allowed 

a finer resolution of water-table contours. The thin 

saturated thickness of the surficial aquifer system and 

the relatively high rates of leakage through the intermediate confining unit also reduce the possibility of 

regional lateral flow in the system.

The water table does not always represent a 

subdued reflection of land-surface topography. 

A comparison of figures 5 and 11 illustrates this to be 

especially true under many hills in the Lake Wales 

Ridge, excluding areas near RIB sites. Pride and 

others (1966) and Knochenmus and Hughes (1976) 

reported similar findings in the Lake Wales Ridge in 

Polk and central Lake Counties, respectively. This 

probably is a combination of the effects of a relatively 

high surficial aquifer system hydraulic conductivity 

and a very leaky intermediate confining unit.

Seasonal changes in water-table altitude 

measured during 1995 were typically 2 to 6 ft in areas 

outside the vicinity of reclaimed-water application 

sites and up to about 15 ft at Water Conserv II RIB site 

5. However, given the same recharge flux exiting the 

rooting zone (that is, beneath the depth that plants can 

extract water) the water table will respond differently 

depending primarily upon the thickness and lithology 

of the unsaturated zone. The unsaturated zone ranges 

in thickness from 0 ft at lakes to greater than 100 ft 

under many hills. Where the unsaturated zone is very 

thick, the water table varies smoothly in response to 

monthly or longer term trends in rainfall; although 

where the unsaturated zone is thinner, the water table 

responds more abruptly to daily rainfall events (fig. 12). 

The times of hydrograph minima and maxima also 

differ significantly, and can be delayed up to a month 

or more at a deep water table (fig. 12). Local beds of 

clay and silt in the unsaturated zone can cause an even 

more muted and delayed response to recharge. Where 

such great variability exists in unsaturated thickness, 

the spatial distribution of recharge to the water table is 

not uniform (Winter, 1983). For example, the rise in 

water table under hills where the depth to the water 

table is great is caused not only by vertical infiltration 

through the unsaturated zone, but also by lateral flow 

from water-table mounds that appear earlier at the toe 

of the hill where the water table is near land surface 

(Winter, 1983).

The Upper Floridan aquifer is recharged by 

downward leakage from the surficial aquifer system 

and upward leakage from the Lower Floridan aquifer 

in areas where the potentiometric surface of the Upper 

Floridan aquifer is below that of the Lower Floridan 

aquifer. Excluding recharge from reclaimed water, 

recharge rates from the surficial aquifer system to the 

Upper Floridan aquifer have been estimated to range 

from 3 to 20 in/yr across most of the study area 

(Murray and Halford, 1996). The aquifer is discharged 

by pumping wells, Apopka Spring, diffuse upward 

leakage to the surficial aquifer system (primarily 

beneath Reedy Creek and Lake Apopka), and by 

downward leakage to the Lower Floridan aquifer in 

any areas where the potentiometric surface of the 

Upper Floridan aquifer is above that of the Lower 

Floridan aquifer. Nearly all of the water withdrawn by 

wells in the study area is from the Upper Floridan 

aquifer.
Hydrologic Setting 21

Figure 11. Water table in the surficial aquifer system, average 1995 conditions.
22 Hydrogeology and Simulation of the Effects of Reclaimed-Water Application in West Orange and Southeast 

Lake Counties, Florida

Figure 12. Cumulative rainfall, surficial aquifer system water levels, and discharge at Reedy Creek, 1995 

(depth to water indicates depth of the surficial aquifer system water table below land surface on January 1, 

1995). 
Hydrologic Setting 23

Water levels in Upper Floridan aquifer monitoring wells that were measured bimonthly (or more 

frequently) during 1995 were averaged to construct a 

potentiometric-surface map (fig. 13). Flow in the 

Upper Floridan aquifer primarily is regional because 

the high transmissivity of the Upper Floridan aquifer 

precludes the formation of significant mounds or 

depressions in the potentiometric surface that would 

induce local flow systems. Accordingly, mounds in the 

potentiometric surface from reclaimed-water application are not apparent. In contrast to the surficial 

aquifer system water table, a 10-ft contour interval 

probably accurately represents all the significant variations in the Upper Floridan aquifer potentiometric 

surface. Consequently, referring to figure 13 and 

noting that ground water moves from high-to-low 

head and generally perpendicular to potentiometric 

contours, regional flow in the Upper Floridan aquifer 

can be inferred to occur from northern Polk County, 

where water levels exceed 120 ft, to the north and east, 

where altitudes as low as 70 ft were measured.

Seasonal changes in the Upper Floridan aquifer 

potentiometric surface that were measured during 

1995 were typically 3 to 5 ft. Variations in potentiometric surface generally are similar to those in the 

surficial aquifer system water table, but usually are of 

lesser magnitude (fig 14). Pumpage from the Upper 

Floridan aquifer causes short-term changes in the 

potentiometric surface that usually are not reflected by 

the water table. Fluctuations that more closely mimic 

those in the water table generally indicate a better 

connection between the surficial and Upper Floridan 

aquifers.

No data exist within the study area on the potentiometric surface of the Lower Floridan aquifer, 

although what little data exist elsewhere indicate that 

the potentiometric surface generally is similar to that 

of the Upper Floridan aquifer. Murray and Halford 

(1996) reported model simulated head differences 

between the Upper and Lower Floridan aquifers of 

1 to 3 ft downward in central Orange County. In 

September 1996, measured potentiometric surfaces of 

the Upper and Lower Floridan aquifers indicated 

downward head differences of 1 ft near Polk City and 

0.5 ft near Orlando (G.G. Phelps, USGS, oral 

commun., 1996). A few pumping wells in the study 

area may withdraw water from both the Upper and 

Lower Floridan aquifers; however, insufficient data 

were available regarding well construction or aquifer 

lithology to make this distinction.

Hydraulic conductivity of the surficial aquifer 

system has been estimated by previous investigators. 

Slug tests performed at the RCID RIB site yielded 

horizontal hydraulic conductivity values between 25 

and 160 ft/d (CH2M Hill, 1989); additional tests 

conducted just north of the RCID RIB site yielded 

values of 35 to 67 ft/d (CH2M Hill, 1993). Sumner and 

Bradner (1996) reported that a horizontal hydraulic 

conductivity of 150 ft/d was required to calibrate a 

numerical flow model designed to analyze a loading test 

conducted at one of the RCID RIBs. Camp Dresser and 

McKee, Inc. (1984) reported hydraulic conductivity 

values generally between 20 and 80 ft/d based on 

laboratory analyses of numerous cores collected at each 

of the Water Conserv II RIB sites. This range of values 

was consistent with those used to calibrate a 

ground-water flow model of the Water Conserv II area 

(Camp Dresser and McKee, Inc., 1984).

The leakance of the intermediate confining unit is 

highly variable across the study area and depends on the 

vertical hydraulic conductivities and thicknesses of the 

individual strata of the unit from one location to the 

next. The leakance of any vertically homogeneous 

geologic unit is equal to its vertical hydraulic conductivity divided by its thickness. Field-derived estimates 

of leakance are sparse; however, two Upper Floridan 

aquifer tests conducted in the Water Conserv II project 

area yielded leakance values of 2x10-4 to 9x10-4 (ft/d)/ft 

(Camp Dresser and McKee, Inc., 1984). Leakance 

values calibrated in a more regional flow model ranged 

from 4x10-5 to 8x10-4 (ft/d)/ft (Murray and Halford, 

1996). Results from this calibrated model indicate that 

the highest leakance values are in the southwestern part 

of the study area where the confining unit is thinnest. 

Lowest leakance values are in the northeastern part, just 

southeast of Lake Apopka, where the unit is thickest. 

Fewer similarities in fluctuations of the surficial aquifer 

system water table and the Upper Floridan aquifer 

potentiometric surface in adjacent wells as well as 

increasing head differences between the two aquifers 

also indicate that intermediate confining unit leakance 

decreases from south to north across the study area 

(fig. 14). However, the extremely variable lithology and 

thickness of strata in mantled karst environments, such 

as is characteristic of much of the study area, make 

inferring leakance values (except in a very general 

manner) from unit thicknesses unreliable. It would be 

very difficult to collect data at such a fine resolution as 

to represent this variability in its true complexity. Yet it 

is the very local variations (for example, where the unit 

is breached by sinkholes or composed primarily of silty 
24 Hydrogeology and Simulation of the Effects of Reclaimed-Water Application in West Orange and Southeast 

Lake Counties, Florida

Figure 13. Potentiometric surface of the Upper Floridan aquifer, average 1995 conditions. 
Hydrologic Setting 25

Figure 14. Changes observed in surficial aquifer system and Upper Floridan aquifer water levels at selected 

monitoring wells, 1995. Water levels have been offset for each well by a constant value equal to the water-level 

altitude for the respective well on January 1, 1995. 
26 Hydrogeology and Simulation of the Effects of Reclaimed-Water Application in West Orange and Southeast 

Lake Counties, Florida

sand) that control the leakage rate between the surficial 

and Floridan aquifer systems. Therefore, the areal average leakance of the intermediate confining unit could 

be high in an area where it is breached by several sinkholes, even though the confining unit is composed of 

thick clay.

Results of aquifer tests indicate that the transmissivity of the Upper Floridan aquifer generally is 

lowest in the Green Swamp and increases toward the 

northeast with values of between 3,400 and 

130,000 ft2

/d (Pride and others, 1966; Grubb and 

Rutledge, 1979; Camp Dresser and McKee, Inc., 

1984). Preferential flow zones occur in the Upper 

Floridan aquifer as a result of variations in lithology 

and solution-enlarged cavities in the limestone. 

Likewise, zones of reduced transmissivity exist where 

sinkhole collapses have caused the filling of cavities 

with a mixture of overlying sand and clay.

No data exist within the study area concerning 

the leakance of the middle semiconfining unit and 

transmissivity of the Lower Floridan aquifer. Based on 

regional ground-water flow models, Tibbals (1990) 

and Murray and Halford (1996) reported a value of 

5x10-5 (ft/d)/ft for leakance of the middle semiconfining unit and 30,000 to 130,000 ft2

/d for transmissivity 

of the Lower Floridan aquifer in the study area.

Surface Water

Approximately 16 percent of the surface area of 

the model consists of lakes (fig. 15). The few lakes for 

which bathymetry data are available indicate that these 

lakes are relatively shallow. Based on data by Kenner 

(1964), average depth is approximately 6 ft for Lake 

Apopka, 10 ft for Lakes Louisa and Minnehaha, and 

15 ft for Lake Butler. Many lakes in the study area are 

internally drained and even more have surface-water 

inflow or outflow only during prolonged wet periods. 

Lake level represents a surface expression of the water 

table and the lakes generally are gaining water from 

the surficial aquifer system (for example, Flat Lake 

and Lake Ingram, fig. 11). However, it is not unusual 

for the water table to be sloping away from a lake and 

inducing the flow of lake water into the surficial 

aquifer system. A comparison of the water levels in 

well HA2-5 and Island Lake shows that flow probably 

is out of the lake and into the surficial aquifer system 

(fig. 16), at least along the northern shore.

Ground- and surface-water interaction can be 

largely dependent on the water-table configuration 

near a lakeshore. Winter (1983) reported that transient 

ground-water mounds can occur near a lakeshore due 

to rapid recharge to a shallow water table. Lee and 

others (1991) observed this phenomenon at Lake 

Lucerne in Polk County, a lake which probably is 

hydrologically similar to many lakes in the study area. 

Where these transient water-table mounds exist, they 

could induce a significant amount of ground-water 

inflow to the lake or prevent a significant amount of 

ground-water outflow from the lake, consequently 

having a direct effect on lake level.

The majority of the lakes are within the Lake 

Wales Ridge physiographic region; the mantled karst 

environment of this region is prone to sinkhole development which probably formed most of these lakes. 

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 and concluded that the 

lithology and distribution of geologic materials under 

and in the immediate vicinity of a lake is highly 

variable, yet it is very important in determining the 

response of the lake to changes in stress. Most of the 

lakes in the study area probably are similar to these; 

therefore, the effects on lake level of reclaimed-water 

application can differ considerably, even between 

lakes in close proximity to each other.

A comparison of the water levels in well SW-4 

and Island Lake (fig. 16) indicates that there might be 

an area of high intermediate confining unit leakance 

north of the lakeshore which could induce the 

movement of ground water northward from Island 

Lake and southward from SW-4. These data illustrate 

the effect that leakance of the intermediate confining 

unit can have on the movement of ground water near 

lakes and in the surficial aquifer system in general.

Approximately 18 percent of the surface area of 

the model consists of wetlands (fig. 15). The majority 

of the wetlands are located in the Lake Upland and 

Osceola Plain physiographic regions because the water 

table generally is near land surface in these areas. 

Within the wetlands, the water table is often at or 

above land surface. Fluctuations in wetland water 

levels are generally small. For example, the water 

level at site 54, which is located at the edge of a 

wetland (fig. 2), rose only 1.92 ft from May to October 

1994 even though this was an unusually wet year 

(fig. 6). An ET rate often equal to potential ET and 

drainage by overland flow and streams preclude large 

fluctuations in water levels. Most of the wetlands are 

drained by streams in the Reedy Creek and Palatlakaha River basins (fig. 15).
Hydrologic Setting 27

Figure 15. Surface-water features in the study area. 
28 Hydrogeology and Simulation of the Effects of Reclaimed-Water Application in West Orange and Southeast 

Lake Counties, Florida

Figure 16. Water levels in Island Lake and nearby surficial aquifer system monitoring wells, September 

1994 through February 1996. 
Hydrologic Setting 29

As a result of the karst topography, much of the 

study area is internally drained, especially along the 

Lake Wales Ridge. Therefore, nearly all of the welldeveloped streams are within the Lake Upland and 

Osceola Plain physiographic regions. Whittenhorse, 

Cypress, Bonnet, and Reedy Creeks are part of the 

headwaters of the Kissimmee River; and Big Creek, 

Little Creek, and the Palatlakaha River are part of the 

headwaters of the Ocklawaha River (fig. 15). Annual 

1995 stream discharge was calculated for each major 

stream flowing across a model boundary (table 3). 

The net annual average 1995 stream discharge out of 

the model area was approximately 5.4 in/yr.

Stream discharge is greatly influenced by 

surficial aquifer system water levels and rainfall 

(fig. 12). As rainfall increases in frequency and 

intensity during the summer, stream discharge and 

shallow water levels increase accordingly. The greater 

rainfall increases the amount of overland runoff to 

streams as indicated by sharp, discharge hydrograph 

peaks. If a hypothetical stream discharge hydrograph 

were compiled with these peaks removed, it would 

look similar in shape to the hydrographs of wells 

EW11-4 and SW-16 in the shallow surficial aquifer 

system (fig. 12). This represents the base-flow 

contribution of this system to stream discharge.

There are few streams or wetlands near 

reclaimed-water application sites, with the exception 

of the RCID RIB site. This site is surrounded by 

wetlands to the west, northeast, and east; Whittenhorse 

Creek to the south; and the RCID Perimeter and C-4 

Canals to the east (fig. 15). Chloride concentrations 

have increased significantly in Whittenhorse Creek 

since the start of RCID RIB operation, even as 

discharge appears to have increased slightly (fig. 17). 

Typical chloride concentration of RCID reclaimed 

water is approximately 100 mg/L (J. Hubbard, Reedy 

Creek Energy Services, Inc., oral commun., 1993). 

A plot of chloride concentration and daily discharge 

indicates that water samples taken at periods of low 

flow (that is, when discharge was assumed to be 

primarily base flow) had significantly greater chloride 

concentration after RIB operation began than before 

(fig. 17). No other significant changes in land use have 

taken place in the stream basin since 1990; therefore, 

some reclaimed water probably discharges from the 

surficial aquifer system to Whittenhorse Creek and 

Bear Bay (an area of wetlands immediately to the west 

of the RCID RIB site that drains into Whittenhorse 

Creek, fig. 15).

Chloride concentration also has increased significantly in the Perimeter Canal, which runs along much of 

the eastern boundary of the RCID RIB site (fig. 15). 

Water samples collected from the canal and analyzed by 

RCID since 1980 (near site 52, fig. 2) show an abrupt 

increase in chloride concentration of approximately 

25 mg/L from October 1991 to January 1992. The 

minimum, maximum, and average chloride concentrations were 10, 28, and 19 mg/L, respectively, for the 

period 1980-91 and 33, 89, and 57 mg/L, respectively, 

for the period 1992-96. Therefore, reclaimed water 

probably discharges from the surficial aquifer system to 

the Perimeter Canal.

Water Budget

A water budget for the surficial aquifer system 

within the model area was used to estimate leakage to 

the Floridan aquifer system. The water budget was 

compiled for the 1995 calendar year (fig. 18) using the 

following equations:

P + Ra – ET – Lf

 – Q = ∆S, (1)

∆S = Sy ∆Hs, (2)

where

P is precipitation, [L/T];

Ra is artificial recharge, [L/T], which is the sum of

RIB recharge (RRIB), reclaimed-water irrigation

and AAS recharge (Rri), and nonreclaimedwater irrigation (Rnri);

Lf

 is net leakage between the surficial and Floridan

aquifer systems, [L/T];

Q is stream discharge, [L/T], which is the sum of 

surface runoff (QS), and base flow (QB);

ET is evapotranspiration, [L/T];

∆S is change in storage in the surficial aquifer system,

[L/T];

Sy is specific yield of the surficial aquifer system,

[dimensionless]; and

∆Hs is time rate of change of the water level in the 

surficial aquifer system, [L/T].

All components of the water budget were based on measured or estimated data with the exception of leakage to 

the Floridan aquifer system, which was set equal to the 

value required to balance the water budget. Flow crossing model boundaries was assumed to be negligible 

because, as previously explained, there probably is little 

lateral flow in the surficial aquifer system. P was calculated as an average of data collected at eight rain gages 
30 Hydrogeology and Simulation of the Effects of Reclaimed-Water Application in West Orange and Southeast 

Lake Counties, Florida

(fig. 6). Ra was based on measured values for RRIB

and Rri and an estimated value for Rnri. Rnri was 

assumed to only consist of citrus grove irrigation water 

that was pumped from the Upper Floridan aquifer. 

An irrigation rate of 9.3 in/yr was calculated from measured data for other citrus groves in Lake, Orange, and 

Polk Counties (citrus irrigation rate data provided by V. 

Singleton, St. Johns River Water Management District, 

written commun., 1996). Q was based on measured discharge of streams crossing model boundaries (table 3). 

Measured data indicated that there was a change in 

surficial aquifer system storage for 1995. That is, a rise 

in the water table for 1995 represented an increase in 

water stored in the pore spaces of the surficial aquifer 

system; similarly, a drop in the water table represented 

a decrease in storage. ∆Hs was interpolated from point 

values of net change in water level measured at wells 

and lakes yielding an average value representing a rise 

of 0.39 ft in 1995. A uniform value of 0.35 was used for 

Sy throughout the model area (including lakes and wetlands). Because no data were available on Sy in the 

model area, Sy was estimated based on reported values 

of the total porosity of surficial sediments of 0.36 to 

0.51 (Camp Dresser and McKee, Inc., 1984; Sumner 

and Bradner, 1996) minus a specific retention of poorly 

graded surficial sand of 0.06 cm3/cm3 (Sumner and 

Bradner, 1996). ET was evaluated in detail because it is 

the largest loss in the water budget and one of the most 

difficult to estimate accurately. In addition, ET is an 

important process in determining the fraction of P and 

Ra that reaches the water table to recharge the surficial 

aquifer system.

In order to more accurately estimate the spatial 

variability of ET, the study area was divided into five 

regions having similar ET characteristics (fig. 19 and 

table 4). These regions were defined considering both 

the characteristics of the local vegetation and the 

amount of water available to that vegetation. Areas 

considered to have a sufficient supply of water to meet 

the needs of the local vegetation (regions 1 and 2) 

were assigned maximum ET rates characteristic of that 

vegetation. Lowland areas (region 1) were defined 

where the water table was 5 ft or less below land 

surface, which included lakes and wetlands. Depth to 

the water table below land surface was estimated 

based on measured surficial aquifer system water 

levels and a digital elevation model interpolated from 

5-ft topographic contours from USGS 7.5-minute 

quadrangles. Cultivated citrus groves (region 2) were 

located by field reconnaissance. Regions 3, 4, and 5 

(also located by field reconnaissance) were assumed to 

be moisture deficient for a significant part of the year 

as a result of having a deep water table (greater than 

5 ft below land surface) and not being irrigated. In 

addition, the sandy, rapidly drained soils typical in the 

study area often do not provide a residual moisture 

content great enough to prevent plant moisture stress. 

Table 3. Average annual stream discharge into and out of model area, 1995

[Sites are located by site numbers in figure 2. ft3/s, cubic foot per second; in/yr, inch per year averaged over model area. 

Abbreviation for flow direction: I, stream flows into model area; O, stream flows out of model area]

Site

number

Station 

number Station name Flow direction

Gaged 

discharge

(ft3/s)

Estimated discharge 

crossing model

 boundary

ft3/s in/yr

74 02236350 Green Swamp Run near Eva I 21.5 21.5 1.0

56 02264000 Cypress Creek at Vineland I 13.5 7.51 .4

73 02236900 Palatlakaha River at Cherry Lake 

outlet near Groveland

O 24.5 24.5 1.2

61 02264100 Bonnet Creek near Vineland O 44.7 44.7 2.1

62 02266300 Reedy Creek near Vineland O 74.3 74.3 3.5

1

Approximately 55 percent of stream basin lies outside model area.

Table 4. Characteristics of evapotranspiration regions 

delineated in figure 19

[<, less than or equal to; >, greater than] 

Region 

number Dominant land cover

Fraction 

of 

model 

area

Depth 

to 

water 

table, 

in feet

Average 

1995 

evapotranspiration,

in inches

1 Lake, wetland, and various 

vegetation

0.55 < 5 47

2 Cultivated citrus .03 > 5 45

3 Upland woodland, primarily 

oaks and pines

.01 > 5 38

4 Uncultivated citrus .01 > 5 36

5 Herbaceous vegetation .40 > 5 27
Hydrologic Setting 31

Figure 17. Discharge and chloride concentrations in water sampled from Whittenhorse Creek (site number 57, fig. 2), 

January 1967 through January 1997. 
32 Hydrogeology and Simulation of the Effects of Reclaimed-Water Application in West Orange and Southeast 

Lake Counties, Florida

Figure 18. Hydrologic budget of the surficial aquifer system within the model area, 1995. All values are fluxes 

averaged over the model area.

P = 51.6 in/yr

Rri = 0.9 in/yr

Rnri = 0.1 in/yr

RRIB = 0.5 in/yr

RRIB = 1.1 in/yr

Water Conserv II

RCID

Q = 5.4 in/yr ET = 38.3 in/yr

∆S = 1.6 in/yr

Lf = 8.9 in/yr

SURFICIAL AQUIFER SYSTEM

INTERMEDIATE CONFINING UNIT

INFLOW

P Precipitation

Ra Artificial recharge, sum of

RRIB RIB recharge

Rri Reclaimed water for 

irrigation and alternate 

application sites

Rnri Nonreclaimed water for 

citrus irrigation

OUTFLOW

ET Evapotranspiration

Lf Leakage to Floridan

aquifer system

Q Stream base flow +

surface runoff

∆S Change in storage

WATER

TABLE

BOUNDARY FLOW

ASSUMED NEGLIGIBLE

BOUNDARY FLOW

ASSUMED NEGLIGIBLE

NOTE: FIGURE NOT TO SCALE
Hydrologic Setting 33

Figure 19. Distribution of evapotranspiration regions in the model area. 
34 Hydrogeology and Simulation of the Effects of Reclaimed-Water Application in West Orange and Southeast 

Lake Counties, Florida

Fetter (1988) indicated that 0.05 cm3/cm3 is a typical 

wilting point moisture content for sand. Sumner and 

Bradner (1996) reported a residual moisture content of 

0.06 cm3

/cm3 for a poorly graded sand at the RCID 

RIB site. At a site near Water Conserv II RIB site 6, 

Sumner (1996) reported moisture content in the upper 

1-ft rooting zone of herbaceous vegetation can be as 

low as 0.02 cm3

/cm3 during dry periods and often 

drops rapidly to below 0.05 cm3

/cm3 after summer 

rainfall events.

ET was quantified for each region based on a 

combination of field data and literature values. Using 

data collected at the Class A pan evaporation station, 

monthly average free-water surface evaporation 

(assumed equal to lake evaporation and used for 

region 1) was calculated using the following equation 

from Kohler and others (1955):

where

Efws is free-water surface evaporation, in inches 

per day;

Ep is pan evaporation, in inches per day;

P is barometric pressure, in inches of mercury;

αp is ratio of advected energy used in evaporation

to total energy advected from the pan, 

[dimensionless];

Up is wind movement over pan measured 6 in. above

rim of pan, in miles per day;

T0 is average pan water temperature, in degrees

Fahrenheit; and

Ta is average air temperature, in degrees Fahrenheit.

Free-water surface evaporation is the maximum 

rate of evaporation from a shallow water body that 

does not store an appreciable amount of heat. Lake 

evaporation may be assumed to be equal to free-water 

surface evaporation if energy advected into the lake is 

equal to a corresponding change in energy storage and 

the pan exposure is similar to that of the lake. Based on 

data collected at Lake Lucerne in Polk County, Lee and 

Swancar (1997) reported that annual Efws was about 6 

percent less than annual evaporation calculated from an 

energy budget analysis and it might be a reasonable 

predictor of actual lake evaporation for periods of at 

least a month. The climatological and hydrologic 

variables affecting Lake Lucerne are probably similar 

to those in the study area. Barometric pressure was 

assumed to be constant at 30.11 inches of mercury 

(1995 annual mean as measured at the NOAA Orlando 

WSO McCoy weather station). The ratio αp was taken 

from a plot of αp in relation to pan water temperature 

and daily wind movement (Kohler and others, 1955, 

fig. 5). For comparison to previous studies, monthly 

pan coefficients (kp) were also calculated:

. (4)

Farnsworth and others (1982) determined a kp

for central Florida for the period May through October 

to be 0.75; Lee and Swancar (1997) reported an 

average kp of 0.73 for a 1-year period starting October 

1985 at Lake Lucerne. These compare well with the 

calculated values of 0.72 to 0.75 obtained during this 

study.

Cultivated citrus ET was estimated based on 

Efws as calculated above and monthly crop coefficients 

reported by Rogers and others (1983) for 

well-irrigated citrus with grass cover:

, (5)

where

ETcit is citrus evapotranspiration, [L/T]; and

kc is crop coefficient, [dimensionless].

Sumner (1996) reported daily ET for nonirrigated, herbaceous vegetation in a deforested area near 

Water Conserv II RIB site 6 for the 1-year period 

starting September 15, 1993. The daily data were 

based on a model calibrated to micrometeorological 

measurements of ET. Monthly average ET was 

estimated from data reported by Sumner (1996) and 

used as estimates of herbaceous vegetation ET (EThbv) 

for region 5.

ET for region 3 (upland woodland) and region 4 

(uncultivated citrus) is not well known. Upland 

woodland ET probably is somewhat greater than that 

of uncultivated citrus as a result of a deeper root 

system and the typically poor condition of the uncultivated citrus. ET rates for both these regions were 

assumed to fall between those values for citrus and 

nonirrigated, herbaceous vegetation:

, (6)

, (7)

where

ETucc is uncultivated citrus evapotranspiration, 

[L/T]; and

ETupw is upland woodland evapotranspiration, 

[L/T].

kp

Efws

Ep

= ---------

ETcit k = cEfws

ETucc 0.5 ETcit EThbv = ( ) – + EThbv

ETupw 0.6 ETcit EThbv ( ) – ET = + hbv

Efws = 0.7[Ep + 0.00051Pαp(0.37 + 0.0041Up) (T0 – Ta)

0.88] , (3)
Description of Model 35

ET as calculated above has been summarized 

for 1995 in table 5 and figure 20. The January 1995 

value of Efws was assumed equal to that measured in 

January 1996. As expected during the wet period of 

June through October, all ET values were nearer Efws 

than during drier months.

The uncertainty associated with ET data 

depends on the method used to collect and analyze the 

data. Sumner (1996) reported an error of ±10 percent 

among three models calibrated to ET measured with 

micrometeorological methods; the pan evaporation 

method has a typical error of ±15 percent, depending 

on pan location (Doorenbos and Pruitt, 1977). Error 

may also be introduced in the application of these data 

to an inappropriate area. As described earlier, ET 

varies with plant growth characteristics and water 

availability; therefore, incorrect estimation of these 

can be another source of error.

Based on the data discussed above and application of equations 1 and 2, leakage to the Floridan 

aquifer system was 8.9 in/yr in 1995 (fig. 18). It was 

the second largest outflow from the surficial aquifer 

system and represented about 16 percent of the total 

flow through the hydrologic system. Leakage between 

the surficial and Floridan aquifer systems is variable 

and depends upon the thickness and vertical hydraulic 

conductivity of the intermediate confining unit and the 

head difference between the two aquifer systems.

DESCRIPTION OF MODEL

The conceptual model and hydrologic data 

discussed in the previous section were used to construct 

a numerical ground-water flow model of the surficial 

and Floridan aquifer systems. The model simulates 

steady-state, ground-water flow as affected by both 

current (average 1995) and proposed future reclaimedwater application conditions. Particle-tracking analyses 

were used to identify rates and directions of reclaimed 

water movement as well as to locate points where 

reclaimed water might exit the ground-water system.

The USGS three-dimensional ground-water 

flow model code MODFLOW (McDonald and 

Harbaugh, 1988) was used to simulate the flow system. 

MODFLOW uses a set of simultaneous finite-difference 

equations to approximate the partial differential 

equation that describes the response of ground-water 

flow to hydrologic stresses and boundary conditions. 

This requires vertical and horizontal discretization of 

the total model volume into cells delineated by layers, 

rows, and columns. Therefore, values of aquifer and 

confining-unit hydraulic properties and hydrologic 

stresses can be assigned to the center of each cell, 

defined as a node.

Table 5. Monthly average pan evaporation, evapotranspiration, pan coefficients, and citrus 

crop coefficients during 1995

[in/d, inch per day; Ep, pan evaporation; kp, pan coefficient; Efws, free-water surface evaporation; kc, citrus crop 

coefficient; ETcit, cultivated citrus evapotranspiration; ETupw, upland woodland evapotranspiration; ETucc, 

uncultivated citrus evapotranspiration. kc from data reported by Rogers and others (1983)]

Month Ep

(in/d) kp

Efws

(in/d) kc

ETcit

(in/d)

ETupw

(in/d)

ETucc

(in/d)

January1 0.128 0.74 0.095 0.90 0.086 0.061 0.055

February .152 .75 .114 .90 .103 .074 .067

March .188 .74 .139 .90 .125 .095 .088

April .222 .73 .162 .90 .146 .118 .112

May .299 .72 .215 .95 .204 .163 .153

June .209 .73 .153 1.00 .153 .139 .136

July .237 .73 .172 1.00 .172 .153 .148

August .157 .74 .116 1.00 .116 .116 .116

September .138 .74 .102 1.00 .102 .099 .099

October .121 .74 .089 1.00 .089 .081 .079

November .138 .74 .102 1.00 .102 .079 .073

December .098 .75 .073 1.00 .073 .055 .050

1

Data for January 1995 assumed equal to that measured in January 1996.
36 Hydrogeology and Simulation of the Effects of Reclaimed-Water Application in West Orange and Southeast 

Lake Counties, Florida

Figure 20. Rainfall and estimated rates of evapotranspiration in the study area, 1995. 
Description of Model 37

Ground-water flow was simulated by the model 

by using three layers to represent the surficial, Upper 

Floridan, and Lower Floridan aquifers (fig. 7). The 

resistance to flow between adjacent layers was 

assumed to be controlled by the leakance of the intervening intermediate confining unit or middle semiconfining unit (fig. 7). The leakance is calculated as the 

harmonic mean of the vertical hydraulic conductivities 

of the aquifer or confining unit material between 

nodes (weighted by aquifer or confining unit thickness 

between nodes) divided by the vertical distance 

between corresponding nodes in adjacent model 

layers. The large contrast in hydraulic conductivity 

between the confining units and adjacent aquifers, 

typically at least 100 times less in the confining unit, 

indicates that the flow is nearly vertical in the 

confining units. Therefore, the confining units were 

simulated using leakance values, not as separate 

layers. The model implicitly assumed vertically 

isotropic conditions, because each aquifer was 

simulated by only one layer. Because of the absence of 

any hydrologic data concerning the middle semiconfining unit and Lower Floridan aquifer in the model 

area, it might be more appropriate to consider these 

units collectively as representing a leaky lower 

boundary condition.

Horizontal discretization of each layer consisted 

of 79 rows and 57 columns with variable cell sizes 

ranging from 656 by 656 ft to 3,936 by 3,936 ft 

(fig. 21). Smaller cells were used in areas of interest 

with significant stresses, such as RIB sites; larger cells 

were used in areas of less interest and relatively small 

hydrologic stresses, such as near model boundaries. 

Of the 13,509 cells, 12,996 were active and represented an area of approximately 285 mi2.

Boundary Conditions

Vertical and lateral boundaries were chosen 

where possible to coincide with physical hydrologic 

barriers. The upper boundary condition, located at the 

water table, was a specified flux represented by an 

effective recharge array that is described in more detail 

in the following section. The very low permeability 

sub-Floridan confining unit served as the no-flow 

lower boundary condition (fig. 7). Lateral boundaries 

for all three layers were coincident but varied in type. 

These boundaries were located far enough away from 

reclaimed-water application areas so as to minimize 

their effects in these areas. Lateral boundaries 

generally coincided with a topographic low or lake 

where a local minimum in the water table would most 

likely exist. Therefore, in the surficial aquifer system a 

no-flow condition was specified along all lateral boundaries (fig. 22). This condition is especially appropriate 

because lateral movement of ground water in the 

surficial aquifer system typically is minimal, except in 

very localized flow systems. In the Upper Floridan 

aquifer, a no-flow condition was established where 

potentiometric contour lines (fig. 13) were perpendicular 

to model boundaries; a specified head condition was 

applied elsewhere (fig. 23). Specified- head values were 

interpolated from an arithmetic average of Upper 

Floridan aquifer water-level data measured during 1995 

(fig. 13). Lower Floridan aquifer lateral boundary 

conditions were assumed identical to those of the Upper 

Floridan aquifer. Specified-head values were set 

uniformly 0.75 ft below those used for the overlying 

Upper Floridan aquifer boundary, based on an average 

of previously described data collected outside the study 

area.

Internal boundaries were established at streams, 

wetlands, lakes, and Apopka Spring, because 

MODFLOW does not simulate surface-water flow 

(figs. 22 and 23). Streams and wetlands were modeled 

with the MODFLOW River and Drain Packages, as 

described in more detail in the following section. 

Lakes affected by stream inflow or outflow were 

represented by specified-head cells. Specified-head 

values were based on measured lake-level data where 

available (fig. 2), otherwise values were estimated 

from USGS 7.5-minute topographic quadrangles. 

Lake-level data estimated from topographic maps 

were adjusted to reflect trends indicated by measured 

data in nearby lakes. Landlocked lakes (lakes lacking 

stream inflow or outflow) were simulated as variablehead cells. In order to effectively represent the absence 

of aquifer materials in these lakes, the hydraulic 

conductivity was specified at 1,000 times that of the 

surrounding aquifer. Even though landlocked lakes are 

surface-water features, MODFLOW was able to 

simulate them because of the lack of surface-water 

flow. The upper boundary condition of specified 

effective recharge accounted for the effects of precipitation and evaporation, and the variable-head cells 

allowed the simulation of ground-water flow into or 

out of the lake. Apopka Spring was simulated by the 

Well Package (fig. 23). Discharge at Apopka Spring 

varies considerably depending upon the head difference between the potentiometric surface of the Upper 

Floridan aquifer and the stage of Lake Apopka. No 

discharge data were available during the study;
38 Hydrogeology and Simulation of the Effects of Reclaimed-Water Application in West Orange and Southeast 

Lake Counties, Florida

Figure 21. Finite-difference grid showing active and inactive model cells in each layer.
Description of Model 39

Figure 22. Boundary conditions for the surficial aquifer system (model layer 1). 
40 Hydrogeology and Simulation of the Effects of Reclaimed-Water Application in West Orange and Southeast 

Lake Counties, Florida

Figure 23. Boundary conditions for the Upper and Lower Floridan aquifers (model layers 2 and 3, respectively). 
Description of Model 41

therefore, discharge was estimated based on historical 

head difference and discharge data. The head difference between the potentiometric surface of the Upper 

Floridan aquifer (measured at site 4, fig. 3) and the stage 

of Lake Apopka was approximately 8.1 ft in 1995. Data 

collected during 1988 indicated a discharge of approximately 42 Mgal/d (65 ft3/s) and a head difference of 

about 8 ft. Apopka Spring was simulated by an Upper 

Floridan aquifer well discharging 42 Mgal/d at the 

spring location.

All specified-head cells were simulated by the 

MODFLOW Time-Variant Specified-Head Package 

(Leake and Prudic, 1991). Starting head values were 

set equal to ending head values to comply with the 

steady-state assumption used in the model.

Hydrologic Input Data

Aquifer and confining unit properties and 

hydrologic stresses were initially estimated for 

calibration of a steady-state model to simulate average 

1995 conditions. Average annual 1995 values of 

time-variant data (specified-head values, effective 

recharge, stream stage, and well pumpage) were 

calculated as an arithmetic average of data collected 

monthly or more frequently.

Aquifer and Confining Unit Properties

Aquifer properties required for calibration of 

the model included hydraulic conductivity and base 

altitude of the surficial aquifer system, transmissivity 

of both the Upper and Lower Floridan aquifers, and 

lateral anisotropy of all three aquifers. All cells in the 

surficial aquifer system initially were assigned a 

hydraulic conductivity value of 50 ft/d with the 

exception of cells representing landlocked lakes, 

which were assigned a value of 50,000 ft/d. Surficial 

aquifer system base altitude was established at the top 

of the intermediate confining unit and interpolated 

from measured point values (fig. 9). Using these data, 

MODFLOW calculated surficial aquifer system transmissivity as the difference between the altitudes of the 

simulated water table and aquifer base multiplied by 

hydraulic conductivity. Model-calibrated values of 

transmissivity for both the Upper and Lower Floridan 

aquifers reported by Murray and Halford (1996) were 

used as initial estimates for these parameters. Approximately the western one-fifth of the model area was 

outside of the area modeled by Murray and Halford 

(1996); transmissivity values for these cells were 

extrapolated by continuing the trend of adjacent cells. 

The transmissivity of the Lower Floridan aquifer was 

not changed from its initial values (fig. 24) during 

model calibration, because no field data exist on the 

hydrologic properties of the Lower Floridan aquifer in 

the model area. All three aquifers were assigned a 

lateral anisotropy of 1; that is, aquifer hydraulic properties were assumed to be equal in the model row and 

column directions. No data exist within the model area 

on lateral anisotropy, and the assumption of isotropic 

conditions is consistent with previous models (Grubb 

and Rutledge, 1979; Camp Dresser and McKee, Inc., 

1984; Planert and Aucott, 1985; Tibbals, 1990; CH2M 

Hill, 1993; Murray and Halford, 1996)

Leakance values were required for the intermediate confining unit and the middle semiconfining unit. 

A uniform leakance of 1×10-4 (ft/d)/ft was used as an 

initial value for the intermediate confining unit (approximate average of values from Murray and Halford 

(1996)). Middle semiconfining unit leakance was 

assigned a uniform value of 5×10-5 (ft/d)/ft (Tibbals, 

1990; Murray and Halford, 1996).

Recharge to the Surficial Aquifer System

The surficial 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 surface runoff. Of the water that 

crosses the water table, recharge is the fraction not 

immediately extracted by ET and that moves downgradient. In addition, water released from storage in the 

surficial aquifer system pore spaces as a result of a 

falling water table can be mathematically represented as 

a flux occurring over the time period during which the 

drop in water table was measured. Considering all these 

factors, except surface runoff, surficial aquifer system 

recharge was simulated as an effective recharge. 

Surface runoff was assumed to be negligibly small 

because of the karst environment and highly permeable 

surficial sand characteristic in the model area. Surface 

runoff may be significant where the water table is near 

land surface, such as wetlands, as will be addressed in a 

later section. Effective recharge (N) represented the net 
42 Hydrogeology and Simulation of the Effects of Reclaimed-Water Application in West Orange and Southeast 

Lake Counties, Florida

EXPLANATION

TRANSMISSIVITY, IN FEETSQUARED PER DAY

 

	


		


1 2 3 4 5MILES

0 1 2 3 4 5KILOMETERS

0

ORANGE COUNTY

OSCEOLA COUNTY

Big Creek

Little Cr.

FLORIDA'S TURNPIKE

Lake Apopka

Johns

Lake

Lake

Minneola

Lake

Minnehaha

Starke

Lake

Hickorynut

Lake

Lake

Sheen

Lake

Louisa

W

r

hittenho se

Lake

Nellie Pretty

Lake

Sumner

Lake

Kirkland

Lake

Bay

Lake

Lake

Butler

Lake

Speer

Lake

Hancock

Lake

Down

Lake

Tibet

WALT DISNEY WORLD

Flat

Lake

Sawgrass

Lake

Lake

Davenport

LAKE COUNTY

POLK COUNTY

Hancock

Boggy Lake

Marsh

Lake

Avalon

Bonnet Cr.

Reedy Cr.

Palatlakaha River

Creek

I-4

192

27

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

Albers Equal-Area Conic projection

Standard parallels 29 30 and 45 30 , central meridian -83 00   

BOUNDARY OF

MODEL AREA

RIB site 6

RIB site 5

RIB site 7

RIB site 9

RCID

RIB site

60,000

Greater than 60,000 - less than 130,000

130,000

Figure 24. Transmissivity of the Lower Floridan aquifer specified in the model. 
Description of Model 43

effects of precipitation, artificial recharge, evapotranspiration, and change in surficial aquifer system storage 

as described by

N = P + Ra – ET – ∆S, (8)

The components of N in equation 8 are identical to 

those defined for equations 1 and 2 for the surficial 

aquifer system water budget and were calculated as 

previously explained. However, equation 8 was applied 

at every active model cell; therefore, N consisted of an 

array of values that varied spatially. P was assumed to 

be a uniform value (fig. 18). The array of values used 

for Ra are shown in figure 25. ET was based on 

area-weighted averages of data from figure 19 and 

table 4. Because steady-state conditions assume zero 

change in aquifer storage, a correction was made to 

account for the change in surficial aquifer system storage measured in 1995 by reducing the estimated 

recharge by ∆S where there was a rise in the water table 

and increasing the estimated recharge by ∆S where 

there was a drop in water table. Conceptually, this 

representation of ∆S can be interpreted as follows: 

(1) an increase in storage is equivalent to a reduction in 

recharge, because a rise in water-table altitude is produced by recharge that otherwise would have been 

available to move downgradient; or (2) a decrease in 

storage is equivalent to an increase in recharge, because 

a drop in water-table altitude releases water that otherwise would not have been available to move downgradient. Because of the small storage coefficients typical 

of confined aquifers, the change in storage in the Floridan aquifer system during 1995 was assumed negligible; therefore, a correction for storage changes was not 

made for the Upper and Lower Floridan aquifers.

Effective recharge was calculated exterior to 

MODFLOW and input to the model using the 

MODFLOW Well and Recharge Packages. At artificial recharge sites the subcomponent of effective 

recharge, (P+Ra-ET), was simulated by the Well 

Package by specifying surficial aquifer system wells 

injecting a volume rate of water equal to the product of 

(P+Ra-ET) and recharge site area (RIB bottom area or 

irrigated area). ∆S at artificial recharge sites and N in 

all other areas not containing artificial recharge sites 

were simulated by the Recharge Package as a flux 

array. Figure 26 shows the effective recharge rates 

specified in the model.

Stream Leakage

Leakage of water from the surficial aquifer 

system to streams, or from streams to the aquifer, was 

simulated by the MODFLOW River Package (fig. 22). 

Flow between the aquifer and the stream is controlled by 

the conductance of the streambed and the hydraulic 

gradient between stream stage and aquifer head. 

Streambed conductance was based on a streambed 

material hydraulic conductivity of 10 ft/d, total stream 

length in the model cell, streambed width measured 

during discharge measurements, and streambed thickness 

of 1 ft. Using these values yields a streambed conductance 

of such magnitude that the hydraulic conductivity of the 

adjacent surficial aquifer system primarily controls stream 

leakage. Values of stream stage and stream bottom 

altitude were assigned by linear interpolation based on 

measured values at gaging stations.

Wetland Discharge

The numerous wetlands within the model area 

typically have perennial water levels that are above land 

surface or are less than 2 ft below land surface. For 

convenience of extracting model-simulated leakage to 

streams, wetlands which drained into streams upstream 

from key gaging stations (site numbers 24, 34, 57, and 

62, fig. 2) were simulated by the MODFLOW River 

Package (fig. 22). In these areas, an arbitrarily high 

conductance of 10,000 ft2/d was used and stream stage 

was set equal to land-surface altitude. The stream 

bottom altitude was set equal to stream stage so the river 

nodes representing wetlands would never be recharging 

the surficial aquifer system. All other wetlands were 

simulated by the MODFLOW Drain Package (fig. 22). 

Drain conductance was specified at an arbitrarily high 

value of 10,000 ft2

/d and drain altitude was set equal to 

land-surface altitude.

Both of these representations of wetlands are mathematically identical. Conceptually, where the water level of 

the surficial aquifer system is higher than land-surface 

altitude, the river and drain nodes represent the effects of 

higher surface runoff rates in these areas. Where the 

aquifer’s water level is lower than or equal to land-surface 

altitude, the river and drain nodes have no effect.

Ground-Water Withdrawals

Average 1995 ground-water withdrawals from 

wells within the model area totaled approximately 

23 Mgal/d (about 1.7 in/yr averaged over the model 

area). All water withdrawn for municipal, commercial, 

industrial, or agricultural use was assumed to be pumped 

from wells tapping only the Upper Floridan aquifer. 
44 Hydrogeology and Simulation of the Effects of Reclaimed-Water Application in West Orange and Southeast 

Lake Counties, Florida

Figure 25. Artificial recharge rates used to calculate effective recharge, average 1995 conditions. 
Description of Model 45

Figure 26. Effective recharge rates specified in the model, average 1995 conditions. 
46 Hydrogeology and Simulation of the Effects of Reclaimed-Water Application in West Orange and Southeast 

Lake Counties, Florida

Well locations and ground-water withdrawal rates for 

municipal, commercial, and industrial users were 

obtained from Consumptive Use Permits and monthly 

operating reports compiled by St. Johns River Water 

Management District and South Florida Water Management District. Additional data were obtained from 

R.L. Marella (USGS, written commun., 1996), Reedy 

Creek Energy Services, Inc., and the Florida Department of Environmental Protection. Agricultural water 

use, which consisted solely of citrus irrigation, was 

estimated based on an irrigation rate of approximately 

9.3 in/yr (as described in an earlier section) and irrigated acreage (estimated from field reconnaissance). 

Ground-water withdrawals were simulated by the 

MODFLOW Well Package (fig. 27).

Calibration

Calibration is the attempt to reduce the difference between model results and measured data by 

adjusting model hydrologic parameters within reasonable ranges. A reduction in the differences between 

simulated and measured ground-water levels and 

flows indicates improvement of the calibration. The 

process of running the model, evaluating improvement 

of the model calibration, and adjusting model parameters accordingly is continued iteratively until an 

acceptable calibration criterion is met. Simulated 

water levels and stream discharges usually depart from 

measured values, even after a diligent calibration 

effort. The differences between model results and 

measurements (model error) 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 average water-level data collected during 

1995 by adjusting values of surficial aquifer system 

hydraulic conductivity, intermediate confining unit 

leakance, and Upper Floridan aquifer transmissivity. 

A steady-state calibration to average 1995 conditions 

was considered suitable for several reasons:

1. Model results will be used to ascertain the long-term 

hydrologic effects of reclaimed-water application, not the relatively short-term transient variations that may occur as a result of, for example, 

a year of unusually heavy rainfall.

2. The surficial aquifer system is strongly influenced by 

temporal variations in precipitation, evapotranspiration, and movement of water in the unsaturated 

zone. Sufficient data does not exist to even grossly 

simulate unsaturated zone hydraulics on a regional 

scale. In addition, a transient simulation would 

have introduced at least two more unknowns, 

Upper and Lower Floridan aquifer storage 

coefficients, which are not well known.

3. The 1995 calendar year was the longest period 

during the 19 months of data collection for this 

study that the surficial and Floridan aquifer 

systems were considered reasonably near 

steady-state based on measured water levels.

4. Measured annual precipitation and potential 

evapotranspiration in 1995 were very close to 

long-term average values. Consequently, natural 

hydrologic stresses (all stresses excluding artificial recharge and well pumpage) were probably 

near long-term values and can be used as representative values for predictive simulations.

5. The frequency of data collection was sufficient for 

calculation of representative annual average 

values.

Calibration improvement was determined by decreases in sum-of-squares error (SS) which is defined by

, (9)

where

is kth simulated water level, [L];

hk is kth measured water level, [L]; and

n is number of water-level comparisons.

Although the sum-of-squares error serves as the 

objective function, root-mean-square error (RMSE) is 

reported because RMSE is more directly comparable 

to actual values and serves as a composite of the 

average and the standard deviation of a set. 

Root-mean-square error is related to the 

sum-of-squares error by

. (10)

Because locations of measured water levels 

rarely coincide with cell nodes, simulated water levels 

were interpolated laterally to points of measurement 

from the nodes of surrounding cells. Simulated water 

levels were interpolated because they were assumed to 

be part of a continuous distribution. Vertical interpolation was not considered because of the discontinuity 

and associated refraction of potential fields from an 

aquifer across a confining unit. 

SS hˆ

k hk ( ) – 2

k = 1

n

= ∑

hˆ

k

RMSE SS

n = -----
Description of Model 47

Figure 27. Ground-water withdrawal rates for the Upper Floridan aquifer specified in the model, average 

1995 conditions. 
48 Hydrogeology and Simulation of the Effects of Reclaimed-Water Application in West Orange and Southeast 

Lake Counties, Florida

The model was calibrated using water-level 

measurements from 21 landlocked lakes, 65 surficial 

aquifer system wells, and 36 Upper Floridan aquifer 

wells. The calibration criterion was to minimize the 

objective function (eq. 9). Multiple measurements for a 

well or lake were reduced to a single representative 

1995 water-level measurement by calculating a 

time-weighted arithmetic average. Water-level measurements from additional wells were available but were not 

used for calibration, because these measurements were 

affected by factors that the model did not simulate, such 

as a transient ground-water mound in the immediate 

vicinity of a heavily loaded individual RIB or AAS. 

Therefore, only water-level measurements representative of the regional flow field were used. In addition, 

where dense clusters of wells were present at a RIB site, 

only a few wells representative of the RIB site as a 

whole were used so as not to overly bias the SS, and 

consequently the model calibration, toward these areas.

Stream discharge measurements were not 

formally used during model calibration because of the 

difficulties of accurately determining the base flow, or 

ground-water discharge, component of total gaged 

stream discharge. The upper limit of base flow is equal 

to the gaged stream discharge, although annual average 

base flow probably is less than the gaged discharge. 

Another problem is determining which wetlands 

contribute discharge to which streams. Ground-water 

that discharges into wetlands and subsequently flows 

into a stream is measured as stream discharge at the 

gaging station. If the simulated wetland discharge is not 

attributed to the appropriate stream, the total stream 

discharge simulated by the model as a whole would be 

correct but the relative fraction attributed to each stream 

would be incorrect. Simulated ground-water discharge 

to streams was only considered to assure that it was less 

than gaged stream discharge.

Parameter Estimation

Model calibration was facilitated by a parameter 

estimation program (Halford, 1992). The parameter 

estimation process is initialized by using the model to 

establish the initial differences between simulated and 

measured water levels. These differences, or residuals, 

are then minimized by the parameter estimation 

program. To implement parameter estimation, the sensitivity coefficients (derivatives of simulated water-level 

change with respect to parameter change) are calculated 

by the influence coefficient method (Yeh, 1986) using 

the initial model results. Each parameter is changed a 

small amount and MODFLOW is used to compute new 

water levels for each perturbed parameter. The current 

arrays of sensitivity coefficients and residuals are used 

by a quasi-Newton procedure (Gill and others, 1981, 

p. 137) to compute the parameter changes that should 

improve the model. The model is updated to reflect the 

latest parameter estimates and a new set of residuals is 

calculated. The entire process of changing a parameter 

in the model, calculating new residuals, and computing 

a new value for the parameter is continued iteratively 

until model error or model-error change is reduced to a 

specified level or until a specified number of iterations 

are made (Halford, 1992). Additional details on the 

theory and application of parameter estimation 

techniques to ground-water flow modeling are 

presented by Yeh (1986), Hill (1992), and Poeter and 

Hill (1997).

Twenty-eight parameters (table 6) were used as 

multipliers that modified the value of surficial aquifer 

system hydraulic conductivity, intermediate confining 

unit leakance, or Upper Floridan aquifer transmissivity 

by a fixed amount in the zone assigned to each parameter. Zonation of the model into areas of homogeneous 

hydrologic properties (each zone characterized by one 

constant parameter value) is a way to reduce the number 

of parameters estimated and minimize the nonuniqueness problems associated with overparameterization. 

A nonunique solution is not desirable because it is 

dependent on the initial estimates of parameters.

The zonation method of parameter identification 

was used for parameterizing surficial aquifer system 

hydraulic conductivity (parameters Ks1 and Ks2) which 

yielded the calibrated distribution of hydraulic conductivity (fig. 28). Initially, one parameter was used for 

hydraulic conductivity. However, this resulted in the 

simulation of water-table gradients at the RCID RIB site 

that were too steep. After examining other possible 

explanations (such as a transmissivity that was too low 

resulting from an aquifer base altitude that was too 

high), the surficial aquifer system hydraulic conductivity was assumed to be different in the vicinity of the 

RCID RIB site (see Ks2 zone, fig. 28). Addition of the 

Ks2 parameter produced a much better fit to measured 

water levels. The relatively high value of 150 ft/d is 

substantiated by independent estimates of hydraulic 

conductivity at the RCID RIB site made by CH2M Hill 

(1989) and Sumner and Bradner (1996). The calibrated 

value of Ks1 of 30 ft/d (fig. 28) is within the range of 

values reported by Camp Dresser and McKee, Inc. 

(1984) and CH2M Hill (1989, 1993).
Description of Model 49

Table 6. Initial and calibrated values of parameters estimated to 

calibrate the model

[Parameter units shown in parentheses; in/yr, inch per year; ft/d, foot per day;

ft/d/ft, foot per day per foot; ft2

/d, foot squared per day; --, not applicable]

Parameter description Parameter 

symbol

Initial 

value

Calibrated 

value

Effective recharge to surficial 

aquifer systema (in/yr)

N 1b --

Surficial aquifer system 

hydraulic conductivity (ft/d):

Zone 1 Ks1 50 30

Zone 2 Ks2 50 150

Intermediate confining unit 

leakance (ft/d/ft):

Zone 1 V1 1x10-4 4.1x10-4

Zone 2 V2 1x10-4 13x10-4

Zone 3 V3 1x10-4 47x10-4

Zone 4 V4 1x10-4 42x10-4

Zone 5 V5 1x10-4 53x10-4

Zone 6 V6 1x10-4 21x10-4

Zone 7 V7 1x10-4 6.9x10-4

Zone 8 V8 1x10-4 0.55x10-4

Zone 9 V9 1x10-4 72x10-4

Zone 10 V10 1x10-4 18x10-4

Zone 11 V11 1x10-4 69x10-4

Zone 12 V12 1x10-4 54x10-4

Zone 13 V13 1x10-4 1.0x10-4

Zone 14 V14 1x10-4 1.2x10-4

Zone 15 V15 1x10-4 2.3x10-4

Zone 16 V16 1x10-4 8.3x10-4

Zone 17 V17 1x10-4 38x10-4

Zone 18 V18 1x10-4 0.66x10-4

Zone 19 V19 1x10-4 0.32x10-4

Zone 20 V20 1x10-4 2.1x10-4

Zone 21 V21 1x10-4 2.0x10-4

Zone 22 V22 1x10-4 1.0x10-4

Zone 23 V23 1x10-4 4.1x10-4

Zone 99 V99 1x10-4 40x10-4

Upper Floridan aquifer 

transmissivity (ft2/d):

Zone 1 Tu1 1b 0.30b

Zone 2 Tu2 1b 2.0b

Middle semiconfining unit 

leakancea (ft/d/ft)

Vms 5x10-5 --

Lower Floridan aquifer 

transmissivitya (ft2

/d)

Tlf 1b --

a

 This parameter was specified and not varied during model calibration.

b This value is a multiplier for the spatially variable parameter values.
50 Hydrogeology and Simulation of the Effects of Reclaimed-Water Application in West Orange and Southeast 

Lake Counties, Florida

Figure 28. Calibrated hydraulic conductivity of the surficial aquifer system and delineation of zones used 

for parameter estimation. 
Description of Model 51

The zonation method of parameter identification 

was also used for parameterizing Upper Floridan 

aquifer transmissivity (parameters Tu1 and Tu2) which 

yielded the calibrated distribution (fig. 29). Tu1 and 

Tu2 served as multipliers for the initial Upper Floridan 

aquifer transmissivity distribution. Two parameters 

were required because the use of one global Upper 

Floridan aquifer transmissivity parameter produced an 

Upper Floridan aquifer potentiometric surface that was 

too low in the southwestern part of the model area. 

Inclusion of the Tu1 parameter (fig 29) produced a 

much better match to measured water-level data. Low 

Upper Floridan aquifer transmissivity values in the 

southwestern part of the model area (fig. 29) generally 

are in agreement with previously reported data from 

aquifer tests and model simulations (Pride and others, 

1966; Grubb and Rutledge, 1979; Planert and Aucott, 

1985; and Tibbals, 1990). High transmissivity values 

in the north and northeastern parts of the model area 

(fig. 29) generally are higher than those from previous 

models of the study area (Grubb and Rutledge, 1979; 

Camp Dresser and McKee, Inc., 1984; Planert and 

Aucott, 1985; Tibbals, 1990; and Murray and Halford, 

1996). This might result from a higher rate of leakage 

from the surficial aquifer system to the Upper Floridan 

aquifer because, as will be discussed later, the 

calibrated leakance of the intermediate confining unit 

generally is higher than in previous models. Consequently, with a greater amount of water flowing 

through the Upper Floridan aquifer, a higher transmissivity is required to maintain approximately the same 

potentiometric surface.

It should be emphasized that the calibrated 

values of Upper Floridan aquifer transmissivity are 

dependent on the assumed properties of the middle 

semiconfining unit and Lower Floridan aquifer. Future 

hydraulic testing of the Floridan aquifer system could 

determine aquifer or confining unit properties that 

differ from those modeled. For example, if the 

leakance of the middle semiconfining unit is 

increased, the Upper Floridan aquifer transmissivity 

would decrease because less water would be flowing 

through the aquifer and more would move through the 

middle semiconfining unit and into the Lower 

Floridan aquifer. If the middle semiconfining unit 

were very leaky, the Floridan aquifer system would 

function more as one vertically continuous aquifer 

rather than as separate upper and lower permeable 

zones. Because transmissivity is a function of aquifer 

hydraulic conductivity and thickness, the high Upper 

Floridan aquifer transmissivity in the northern half of 

the model area may be indicative of a much thicker 

aquifer. In this case, these high transmissivities might 

seem more reasonable for an aquifer 2,300-ft thick 

rather than 300-ft thick.

A slight variation of the zonation method was 

used for parameterizing intermediate confining unit 

leakance (parameters V1 through V23 and V99, table 6). 

As a result of the highly variable lithology and 

thickness of the intermediate confining unit, zones of 

uniform leakance could not be accurately delineated 

based only on lithologic and thickness data. An attempt 

was made to weight the leakance distribution by intermediate confining unit thickness (fig. 10); that is, 

leakance would be greater where the unit is thinner and 

less where the unit is thicker. However, this weighting 

approach did not yield satisfactory results. An attempt 

to weight the leakance distribution on sinkhole density 

also proved unsatisfactory. Sinkhole locations were 

inferred from land-surface depressions with the aid of a 

digital elevation model; leakance was assumed to be 

greater where there were a greater number of sinkholes 

and less where there were fewer sinkholes. However, a 

sinkhole might have been “plugged” by filling with low 

permeability sediments yet still leave a land-surface 

depression; likewise, a “buried sinkhole” that produced 

a breach in the intermediate confining unit could exist 

where there is no longer an obvious land-surface 

depression. In a mantled karst environment such as 

exists in the study area, it would be extremely difficult 

to collect data at such a fine resolution as to accurately 

represent the true heterogeneity of leakance. Alternatively, model-calibrated leakance values reported by 

Murray and Halford (1996) were used, although these 

also produced significant discrepancies with measured 

water-level data in some areas. One possible solution 

would be to determine leakance values and leakance 

zonation simultaneously, an approach referred to as 

data-driven zonation (Eppstein and Dougherty, 1996). 

This approach avoids overparameterization and the 

need to know zonation structure prior to calibration. 

A simplified procedure based on the concept of 

data-driven zonation was used to estimate leakance of 

the intermediate confining unit.

Forty-five zones initially were used to represent 

intermediate confining unit leakance. A denser spacing 

of leakance zones was established where more 

water-level data were available. The parameter estimation routine was run for several iterations in order to 

obtain correlation coefficients between all parameters. 

Next, adjacent leakance zones with correlation coefficients greater than 0.7 or to which the model was 
52 Hydrogeology and Simulation of the Effects of Reclaimed-Water Application in West Orange and Southeast 

Lake Counties, Florida

Figure 29. Calibrated transmissivity of the Upper Floridan aquifer and delineation of zones used for 

parameter estimation. 
Description of Model 53

relatively insensitive (that is, the main diagonal value 

of the covariance matrix was less than 1 percent of the 

maximum main diagonal value) were combined into 

one zone. This two-step process was repeated until all 

adjacent leakance zones had correlation coefficients 

less than 0.7 and a main diagonal value greater than 

1 percent of the maximum main diagonal value. 

The zonation structure resulting from this iterative 

process was further adjusted to account for other leakance information (such as areas of higher leakance as 

inferred from estimated water-table altitude, for example parameter V99) to yield the final intermediate confining unit leakance distribution (fig. 30).

Calibrated leakance values generally are greater 

than those reported in previous studies (Grubb and 

Rutledge, 1979; Planert and Aucott, 1985; Tibbals, 

1990; Murray and Halford, 1996) with the exception 

of those reported by Camp Dresser and McKee, Inc. 

(1984). The discrepancy between calibrated values in 

this study and those from earlier studies may be the 

result of several factors. The present model used a 

more detailed set of geologic, water-level, and evapotranspiration data for model construction and calibration. Only the present model and the model by Murray 

and Halford (1996) included the effects of reclaimed 

water applied at Water Conserv II and the RCID RIBs. 

Reclaimed-water application is a large stress on the 

local aquifer system, and the magnitude of this stress 

is relatively well known. This combination of known 

stress and aquifer response is a valuable data set for 

model calibration. However, Murray and Halford 

(1996) simulated the surficial aquifer system as a 

specified-head layer. Only the present model and the 

model constructed by Camp Dresser and McKee, Inc. 

(1984) actively simulated the surficial aquifer system 

with variable-head cells. Variable-head cells are a 

more realistic representation of the aquifer system, and 

thus might have contributed to the more accurate 

estimation of parameter values. The finer discretization of the present model and the model by Camp 

Dresser and McKee, Inc. (1984) (regularly spaced 

square cells 1,000 ft on a side) may also have contributed to the more accurate estimation of parameter 

values.

Parameters generally are not highly correlated, 

as indicated by small correlation coefficients (most 

less than 0.60, table 7); parameters that are highly 

correlated are not desirable because they cannot be 

independently estimated. The most highly correlated 

parameter pairs are Ks1 and V15 and Tu1 and Tu2, with 

correlation coefficients of 0.82 and -0.75, respectively. 

V15 is adjacent to several other zones with considerably 

greater leakances (V10, V11, and V99, table 6 and fig. 

30), which contributes to the relatively high correlation 

between Ks1 and V15. For example, the water table 

could be lowered by increasing Ks1, thereby causing 

more water to move laterally and leak to the Upper 

Floridan aquifer through the adjacent high leakance 

zones. Alternatively, an increase in V15 would lower the 

water table by increasing the vertical leakage to the 

Upper Floridan aquifer directly through the V15 zone. 

The fairly high correlation between Tu1 and Tu2 

primarily is a result of the large transmissivities typical 

in both zones.

Effective recharge (N), leakance of the middle 

semiconfining unit (Vms), and transmissivity of the 

Lower Floridan aquifer (Tlf) were specified in the model 

and were not adjusted during calibration. N was 

specified because more data were available on the 

spatial distribution and magnitude of N relative to other 

parameters. In addition, as will be explained in a later 

section, N was highly correlated to Upper Floridan 

aquifer transmissivity. The high correlation coefficients 

associated with Vms and Tlf (table 7) also precluded 

independent estimation of them. The absence of Lower 

Floridan aquifer potentiometric-surface measurements 

contributed to these high correlation coefficients.

The calibrated model generally produced 

simulated water levels in close agreement with 

measured water levels (table 8 and fig. 31). Approximately 83 percent of the simulated water levels are 

within ±3 ft of the measured water levels. No spatial 

trends in the distribution of water-level residuals are 

apparent (figs. 32, 33, 34, 35, and 36).

The simulated water table (fig. 32) generally 

conforms to that based on measured data (fig. 11). 

The largest differences typically occur in areas of little 

or no water-table control; for example, the northwestern 

part of the model area. The simulated water table differs 

markedly from measured data in several areas. For 

example, the simulated water table is about 25 ft lower 

than measured water levels south of Water Conserv II 

RIB site 6 (near site numbers 19 and 20, fig. 3) and 15 ft 

lower than measured water levels south of Flat Lake 

near Water Conserv II AAS HA-7 (near site numbers 

21 and 24, fig. 3). These discrepancies probably are 

the result of localized heterogeneity in hydrologic 

properties or long-term transient effects that are not 

adequately represented by a steady-state simulation, 

or both. Consequently, no attempt was made to 

match these anomalous measurements during model 

calibration.
54 Hydrogeology and Simulation of the Effects of Reclaimed-Water Application in West Orange and Southeast 

Lake Counties, Florida

Figure 30. Calibrated leakance of the intermediate confining unit and delineation of zones used for 

parameter estimation. 
Description of Model 55

Table 7. Selected correlation coefficients between parameters from the calibrated model

[mi2, square mile; --, no correlation coefficient greater than 0.60]

Parameter1 Zone area 

(mi2)

Number of 

water-level 

measurements2

Normalized 

main 

diagonal3

Correlated parameters and (correlation coefficents)

Most highly 

correlated

Correlation coefficents greater than 0.60

2nd most 

correlated

3rd most 

correlated

4th most 

correlated

Ks1 280.17 73 0.948 V15 (0.82) -- -- --

Ks2 4.54 13 .070 V4 (0.54) -- -- --

V1 54.21 7 .088 V7 (0.22) -- -- --

V2 49.62 6 .037 Tlf (-0.67) Vms (-0.66) -- --

V3 4.73 3 .029 Tu2 (-0.74) -- -- --

V4 4.54 15 .077 Ks2 (0.54) -- -- --

V5 7.14 8 .043 Tu2 (-0.66) V18 (0.63) -- --

V6 2.22 3 .035 V7 (0.24) -- -- --

V7 2.13 19 .332 V8 (0.52) -- -- --

V8 .42 1 .012 V12 (0.52) -- -- --

V9 2.13 4 .021 V14 (0.37) -- -- --

V10 2.13 4 .082 V21 (0.53) -- -- --

V11 1.90 4 .046 Tu2 (0.38) -- -- --

V12 1.71 7 .256 V8 (0.52) -- -- --

V13 33.73 5 .158 V19 (0.28) -- -- --

V14 5.00 2 .028 V9 (0.37) -- -- --

V15 2.54 6 .458 Ks1 (0.82) -- -- --

V16 1.87 2 .246 V19 (0.49) -- -- --

V17 3.69 7 .079 V18 (-0.26) -- -- --

V18 53.18 4 .016 Tu2 (-0.66) V5 (0.63) Tu1 (0.61) --

V19 .37 2 .026 V20 (0.61) -- -- --

V20 2.22 2 .304 V19 (0.61) -- -- --

V21 20.52 5 .129 V10 (0.53) -- -- --

V22 9.78 2 .279 V13 (0.11) -- -- --

V23 14.50 3 .202 V19 (0.19) -- -- --

V99 4.42 1 .061 V15 (0.55) -- -- --

Tu1 100.18 7 .130 Tu2 (-0.75) V18 (0.61) -- --

Tu2 184.53 29 1.000 Tlf (0.83) Vms (0.75) Tu1 (-0.75) V3 (-0.74)

Vms 284.71 36 .044 Tlf (0.97) Tu2 (0.75) V2 (-0.66) --

Tlf 284.71 0 .014 Vms (0.97) Tu2 (0.83) V2 (-0.67) --

1 See table 6 for parameter symbol definitions.

2

 For hydraulic conductivity and transmissivity parameters, the number of water-level measurements is the number of 

measurements within a parameter’s zone in the respective aquifer. For leakance parameters, the number of water-level measurements 

is the number of measurements within a parameter’s zone in both the adjacent overlying and underlying aquifers. 3 Normalized main diagonal of covariance matrix, that is the matrix main diagonal value divided by the maximum main 

diagonal value. This is a rough estimate of the relative sensitivity of the model to a parameter.
56 Hydrogeology and Simulation of the Effects of Reclaimed-Water Application in West Orange and Southeast 

Lake Counties, Florida

Table 8. Water-level error statistics for the calibrated model

[All residual and error values in feet]

Aquifer

Number of 

water-level 

measurements

Minimum 

residual

Maximum 

residual

Average 

error

Root mean 

square 

error

Surficial aquifer system 86 -9.50 5.39 -0.29 2.50

Upper Floridan aquifer 36 -3.72 4.52 .61 1.90

Entire model 122 -9.50 5.39 - .02 2.34

Figure 31. Comparison of simulated to measured water levels for the calibrated model. 
Description of Model 57

Figure 32. Simulated water table and water-level residuals for the surficial aquifer system, 1995 

(insets A, B, and C shown in figs. 33, 34, and 35, respectively). 
58 Hydrogeology and Simulation of the Effects of Reclaimed-Water Application in West Orange and Southeast 

Lake Counties, Florida

Figure 33. Simulated water table and water-level residuals for the surficial aquifer system at the Water 

Conserv II RIB sites 5 and 6, 1995 (inset A, fig. 32). 
Description of Model 59

Figure 34. Simulated water table and water-level residuals for the surficial aquifer system at the Water 

Conserv II RIB sites 7 and 9, 1995 (inset B, fig. 32). 
60 Hydrogeology and Simulation of the Effects of Reclaimed-Water Application in West Orange and Southeast 

Lake Counties, Florida

Figure 35. Simulated water table and water-level residuals for the surficial aquifer system at the RCID 

RIB site, 1995 (inset C, fig. 32).
Description of Model 61

Figure 36. Simulated potentiometric surface and water-level residuals for the Upper Floridan aquifer, 1995. 
62 Hydrogeology and Simulation of the Effects of Reclaimed-Water Application in West Orange and Southeast 

Lake Counties, Florida

Because the model is not intended to represent 

the effects of individual RIBs, the simulated water table 

at the RCID and Water Conserv II RIB sites is somewhat generalized and shows the effects of a cluster of 

RIBs or the RIB site as a whole (figs. 33, 34, and 35). 

The effects of the RIBs on the water table primarily is a 

function of the reclaimed-water application rate and the 

hydraulic properties of the surficial aquifer system and 

the intermediate confining unit. The calibrated surficial 

aquifer system hydraulic conductivity is 30 ft/d at all 

four Water Conserv II RIB sites; therefore, differences 

in water-table configuration among the four sites is the 

result of different reclaimed-water application rates and 

variations in the leakance of the intermediate confining 

unit. Average 1995 reclaimed-water-application rates at 

Water Conserv II RIB sites 5, 6, and 7 were 3.6, 4.4, and 

7.3 Mgal/d, respectively; no reclaimed water was 

applied to the RIBs at Water Conserv II RIB site 9 during 1995. The largest water-table mound occurred at 

RIB site 5 (fig. 33), even though the reclaimed-water 

application rates were greater at RIB sites 6 and 7. 

A comparison of model-calibrated intermediate confining unit leakance (fig. 30) at the three RIB sites shows 

that leakance generally is highest at RIB site 7 and lowest at RIB site 5. That is, a higher intermediate confining unit leakance at RIB site 7 allows water in the 

surficial aquifer system to more easily move to the 

Upper Floridan aquifer, thereby reducing water-table 

altitude (fig. 34). The spatial distribution of RIBs 

also affects the height of water-table mounds. The 

4.4 Mgal/d applied at RIB site 6, when considered on a 

flow per unit area basis, is less than 3.6 Mgal/d applied 

over the much smaller area of RIB site 5; consequently, 

even if hydrologic properties at the two sites were identical a higher water-table mound would exist at RIB site 

5. The simulated water table at the RCID RIB site 

(fig. 35) is considerably flatter than that at Water Conserv II RIB sites 5, 6, or 7. The model-calibrated surficial aquifer system hydraulic conductivity of 150 ft/d 

(compared to 30 ft/d) allows water to more easily move 

laterally in the surficial aquifer system and subsequently 

discharge to local surface-water features. In addition, 

the intermediate confining unit leakance is relatively 

high and the RIBs are located over a fairly large area; 

both of these factors, in combination with the high surficial aquifer hydraulic conductivity, contribute to small 

water-table mounds at the RCID RIB site.

The simulated potentiometric surface of the 

Upper Floridan aquifer (fig. 36) generally agrees with 

that based on measured data (fig. 13). Local variations 

in the measured potentiometric surface (such as southeast of Lake Louisa, fig. 13) may be the result of 

reduced transmissivity caused by sand-filled cavities 

associated with sinkholes. No attempt was made to 

adjust hydrologic properties on a node-by-node basis to 

produce a closer match to these measurements. Large 

mounds in the simulated potentiometric surface of the 

Upper Floridan aquifer do not exist where they were 

present in the surficial aquifer system. This is the result 

of the high calibrated transmissivity of most of the 

Upper Floridan aquifer (fig. 29) which allows water to 

very easily move through the aquifer (compared to the 

surficial aquifer system) to points of discharge, such as 

wells and specified-head boundaries.

The simulated potentiometric surface of the 

Lower Floridan aquifer is shown in figure 37. The 

potentiometric surface of the Lower Floridan aquifer 

probably should more closely resemble that of the 

Upper Floridan aquifer; however, no data exist within 

the model area for confirmation. 

The volumetric water budget for the calibrated 

model is shown in figure 38. Of the 18 in/yr of flow 

through the surficial and Floridan aquifer systems (sum 

of inflows or outflows shown in “bubbles” in figure 38), 

about 75 percent consisted of effective recharge to the 

surficial aquifer system and the remaining 25 percent 

was inflow from specified-head boundaries. Most of the 

18 in/yr circulated through the surficial aquifer system 

and the Upper Floridan aquifer. In addition, simulated 

net leakage between the surficial and Upper Floridan 

aquifer systems was 11.6 in/yr downward and represented 76 and 89 percent of the total flows in the surficial aquifer system and the Upper Floridan aquifer, 

respectively. These results are in agreement with the 

high leakance characteristics of the intermediate confining unit. Consequently, hydrologic conditions in the 

surficial aquifer system can have relatively significant 

effects on conditions in the Upper Floridan aquifer; 

likewise, hydrologic conditions in the Upper Floridan 

aquifer can have relatively significant effects on conditions in the surficial aquifer system. Little water from 

the Upper Floridan aquifer (about 5 percent of its total 

flow) reached the Lower Floridan aquifer. The small 

leakance assigned to the middle semiconfining unit 

(about 20 times smaller than the average leakance of the 

intermediate confining unit) inhibits the exchange of 

water between the Upper and Lower Floridan aquifers. 

Again, if data were available concerning the hydrologic 

properties of the middle semiconfining unit and the 

Lower Floridan aquifer, the resulting simulated flow 

system of the Floridan aquifer system might be 

different.
Description of Model 63

Figure 37. Simulated potentiometric surface of the Lower Floridan aquifer, 1995. 
64 Hydrogeology and Simulation of the Effects of Reclaimed-Water Application in West Orange and Southeast 

Lake Counties, Florida

The simulated water budget (fig. 38) compares 

reasonably well with the surficial aquifer system water 

budget discussed earlier in the report (fig. 18). 

Because effective recharge was specified in the model, 

the value of 13.6 in/yr is equal to the sum of its 

component values (eq. 8) shown in figure 18, with the 

exception of 0.7 in/yr of effective recharge that was 

applied to specified-head lakes and consequently was 

not accounted for by the model. The simulated stream 

discharge was less than the measured discharge in 

Little, Big, Whittenhorse, and Reedy Creeks (site 

numbers 24, 34, 57, and 62, fig. 2) because 

MODFLOW only simulates the ground-water 

discharge (base flow) component of stream discharge. 

The simulated net leakage to the Upper Floridan 

aquifer was approximately 30 percent greater than that 

estimated from the surficial aquifer system water 

budget. However, this difference probably is within 

the margin of error to which leakage to the Upper 

Floridan aquifer can be independently estimated.

Sensitivity Analyses

In order to determine how varying model 

parameters affected simulation results, each parameter, 

except N, was varied independently from 0.1 to 1 

times its calibrated value (in increments of 0.1) and 

from 1 to 10 times its calibrated value (in increments 

of 1). Model sensitivity was described in terms of 

RMSE. The sensitivity of the model to changes in one 

parameter while all others were held at their calibrated 

values is shown in figure 39. The model is most 

sensitive to Tu2 and Ks1; an increase of 2 times (or 

decrease of 0.5 times) the calibrated value of either 

parameter more than doubles RMSE to approximately 

Surficial aquifer system

Upper Floridan aquifer

Lower Floridan aquifer

Effective Recharge1

13.6 Wetland

Discharge2

0.4

Stream

Discharge3

2.0

Upward Leakage

0.3

Downward Leakage

11.9

Boundary

Inflow

1.2 Boundary

Outflow

7.3

Boundary

Inflow

1.8

Boundary

Outflow

2.5

Upward Leakage

0.0

Downward Leakage

0.7

Specified-head

Lake Inflow

1.4

Specified-head

Lake Outflow

1.0

Pumpage and

Apopka Spring

4.8

Total Flow = 15.3

Total Flow = 13.1

Total Flow = 2.5

1

Reduced by 1.6 to account for increase in surficial aquifer system storage.

2

Wetlands not draining to key gaging stations. Simulated by the MODFLOW Drain Package (fig. 22).

3

All streams plus wetlands draining to key gaging stations. Simulated by the MODFLOW River Package (fig. 22).

Figure 38. Simulated volumetric water budget of the aquifer system in the model area, 1995. All values are in 

inches per year averaged over the model area. 
Description of Model 65

EXPLANATION

ICU leakance, zone 4

SAS hydraulic

conductivity, zone 2

ICU leakance, zone 99

ICU leakance, zone 11

MSU leakance

0.1 10 0.2 0.5 1 2 5

MULTIPLIER OF CALIBRATED VALUE

2.3

2.7

2.4

2.5

2.6

0.1 10 0.2 0.5 1 2 5

2.2

3.6

2.4

2.6

2.8

3.0

3.2

3.4

ROOT MEAN SQUARE ERROR, IN FEET

0.1 10 0.2 0.5 1 2 5

MULTIPLIER OF CALIBRATED VALUE

2.30

2.55

2.35

2.40

2.45

2.50

2.55

ROOT MEAN SQUARE ERROR, IN FEET

0.1 10 0.2 0.5 1 2 5

2.3

2.8

2.4

2.5

2.6

2.7

0.1 10 0.2 0.5 1 2 5

0

20

5

10

15

ROOT MEAN SQUARE ERROR, IN FEET

EXPLANATION

UFA transmissivity, zone 2

SAS hydraulic

0.1 10 0.2 0.5 1 2 5

2

6

3

4

5

EXPLANATION

conductivity, zone 1

ICU leakance, zone 15

ICU leakance, zone 7

ICU leakance, zone 20

ICU leakance, zone 22

ICU leakance, zone 12

ICU leakance, zone 16

ICU leakance, zone 23

ICU leakance, zone 13

EXPLANATION

UFA transmissivity, zone 1

ICU leakance, zone 21

ICU leakance, zone 1

ICU leakance, zone 10

ICU leakance, zone 17

EXPLANATION

ICU leakance, zone 5

ICU leakance, zone 2

ICU leakance, zone 6

ICU leakance, zone 3

ICU leakance, zone 14

EXPLANATION

ICU leakance, zone 19

ICU leakance, zone 9

ICU leakance, zone 18

LFA transmissivity

ICU leakance, zone 8

Figure 39. Model sensitivity to independent changes in selected model calibration parameters (SAS, 

surficial aquifer system; ICU, intermediate confining unit; UFA, Upper Floridan aquifer; MSU, middle 

semiconfining unit; LFA, Lower Floridan aquifer). 
66 Hydrogeology and Simulation of the Effects of Reclaimed-Water Application in West Orange and Southeast 

Lake Counties, Florida

5 ft. For many of the parameters to which the model is 

least sensitive (for example, V18 and Tlf), an increase of 

10 times (or decrease of 0.1 times) the calibrated value 

causes less than a 5 percent increase in RMSE.

Several other observations can be made 

concerning figure 39. The “kinks” in the sensitivity 

plots for Ks1, V15, and V20 are most likely the result of 

surficial aquifer system cells going dry as hydraulic 

conductivity or leakance increases. RMSE could 

decrease as cells, where water-level measurements are 

located, go dry and are assigned a residual value of 

zero by the parameter estimation program. Many of 

the sensitivity plots minima do not occur at a multiplier value of 1. This only occurs in the less sensitive 

parameters (for example, V3), and means that a 

slightly better RMSE could have been achieved had 

the parameter estimation routine been allowed to run 

through more iterations. However, notice that the 

absolute minimum RMSE could only be reduced by 

about 0.01 ft (from 2.34 to 2.33 ft). This small 

reduction in RMSE is insignificant and is well within 

the range of uncertainty of even the water-level 

measurements.

The magnitude of the main diagonal of the 

covariance matrix is a rough estimate of the relative 

sensitivity of the model to a parameter. Table 7 lists 

the normalized main diagonal value (that is, the main 

diagonal value divided by the maximum main 

diagonal value) for each parameter. The larger the 

value of the normalized main diagonal for a particular 

parameter, the more sensitive the model, as a whole, is 

to that parameter.

In considering model sensitivity to a particular 

parameter it is important to note both the areal size of 

its zone (relative to total model area) and the number 

of water-level measurements most influenced by the 

parameter (such as the number of measurements 

within its zone). For example, the model as a whole is 

very insensitive to the value of V8 (table 7; fig. 39)— 

its area is only 0.1 percent of the total model area and 

there is only one water-level measurement within its 

zone. However, the altitude of the surficial aquifer 

system water table in the V8 zone and its immediate 

vicinity is very sensitive to V8. Zone areas and number 

of water-level measurements within a zone are 

included in table 7 for each parameter. This information along with the magnitude of the normalized main 

diagonal value and the sensitivity plots in figure 39 

should all be considered when assessing, in a qualitative manner, the relative sensitivity of the model 

(either as a whole or locally) to each parameter.

A different type of sensitivity analysis was 

performed for effective recharge which entailed the 

recalibration of the model to two different sets of 

effective recharge values. Effective recharge was a 

specified, not a calibrated, parameter. An incorrectly 

specified effective recharge matrix would affect the 

values, to some degree, of all other calibrated parameters. Therefore, based on the uncertainty associated 

with the estimation of the component values of N

(eq. 8) a low N matrix and a high N matrix were 

compiled to serve as lower and upper bounds on a 

range of most likely N matrix values. For the low N

matrix, ET was increased by 15 percent and Sy was 

increased to 0.45; similarly, for the high N matrix, ET 

was decreased by 15 percent and Sy was decreased to 

0.25. The calibrated parameter values for the model 

using the average 1995 N matrix were used as initial 

values for recalibration to the two alternative N

matrices. However, to prevent surficial aquifer system 

cells from drying during recalibration to the low N

alternative, all initial values of intermediate confining 

unit leakance parameters were divided by 2.

Model results using each of the three N matrices 

indicate that most parameters did not change significantly, with the exception of Tu1, Tu2, and some 

leakance parameters (table 9). Even though some 

individual leakance parameters changed significantly, 

the zone-area-weighted average leakance of the intermediate confining unit changed only slightly for the 

two alternative N matrices. A 46 percent increase in 

Upper Floridan aquifer transmissivity 

(zone-area-weighted average of Tu1 and Tu2 for the 

high N alternative) caused by a 45 percent increase in 

N indicates a high correlation between these parameters. In fact, the model calibrated to the high N alternative has a slightly smaller RMSE than the model 

calibrated to the average 1995 N alternative. However, 

such a small reduction in RMSE is not enough to 

discount the average 1995 N values, which are better 

supported by independent data. The low N alternative 

probably is not an accurate representation because the 

average error could never be reduced to 0 or nearly 0; 

consequently, the RMSE was greater, though not 

drastically greater. The alternative models show that a 

relatively small RMSE alone does not assure a well 

calibrated model and either some stresses or hydraulic 

conductivities must be known. 

Lateral flow probably occurs across model 

boundaries in some areas of the surficial aquifer 

system (fig. 11); however, the locations of lateral flow 

are not well known because few measured water-level
Description of Model 67

Table 9. Calibrated and alternative values of parameters using 

alternative effective recharge values

[in/yr, inch per year; ET, evapotranspiration averaged over model area; ∆S, change in 

surficial aquifer system storage averaged over model area; N, effective recharge averaged 

over model area; ICU, intermediate confining unit; RMSE, root mean square error]

Parametera

Calibrated parameter value

Average 1995 

effective 

recharge

Low effective 

recharge

High effective 

recharge

ET = 38.30 in/yr

∆S = 1.63 in/yr

N = 14.27 in/yr

ET = 44.01 in/yr

∆S = 2.10 in/yr

N = 8.06 in/yr

ET = 32.53 in/yr

∆S = 1.17 in/yr

N = 20.47 in/yr

Ks1 30 29 31

Ks2 

b 150 150 150

V1 4.1x10-4 2.7x10-4 6.2x10-4

V2 13x10-4 26x10-4 11x10-4

V3 47x10-4 41x10-4 53x10-4

V4 42x10-4 55x10-4 41x10-4

V5 53x10-4 41x10-4 55x10-4

V6 21x10-4 13x10-4 16x10-4

V7 6.9x10-4 4.8x10-4 8.3x10-4

V8 0.55x10-4 0.24x10-4 0.51x10-4

V9 72x10-4 20x10-4 70x10-4

V10 18x10-4 15x10-4 19x10-4

V11 69x10-4 58x10-4 56x10-4

V12 54x10-4 70x10-4 48x10-4

V13 1.0x10-4 0.37x10-4 1.9x10-4

V14 1.2x10-4 0.40x10-4 1.3x10-4

V15 2.3x10-4 2.4x10-4 2.2x10-4

V16 8.3x10-4 7.8x10-4 8.5x10-4

V17 38x10-4 29x10-4 39x10-4

V18 0.66x10-4 0.49x10-4 0.64x10-4

V19 0.32x10-4 0.15x10-4 0.33x10-4

V20 2.1x10-4 1.7x10-4 2.4x10-4

V21 2.0x10-4 1.6x10-4 2.4x10-4

V22 1.0x10-4 0.63x10-4 1.4x10-4

V23 4.1x10-4 2.8x10-4 5.5x10-4

V99 40x10-4 45x10-4 46x10-4

Tu1 0.30c 0.24c 0.50c

Tu2 2.0c 1.4c 2.7c

Average 

ICU

leakanced

9.3x10-4 10x10-4 9.8x10-4

Average 

error

0.02 -0.26 0.07

RMSE 2.34 2.75 2.31

a

 See table 6 for parameter symbol definitions.

b Ks2 was not allowed to exceed 150 ft/day. A higher value would have yielded a 

smaller RMSE but probably is not realistic. c This value is a multiplier for the spatially variable parameter values.

d

 Average ICU leakance weighted by zone area from table 7.
68 Hydrogeology and Simulation of the Effects of Reclaimed-Water Application in West Orange and Southeast 

Lake Counties, Florida

data were available near model boundaries. Therefore, 

in order to test model sensitivity to surficial aquifer 

system boundary conditions, all cells adjacent to the 

lateral model boundary were assigned specified-head 

values (interpolated from fig. 11). A comparison of 

model results from two model simulations, one using 

the specified-head surficial aquifer system boundary 

condition and one using the no-flow boundary condition, was made for all interior model cells (that is, 

excluding all cells adjacent to model boundaries in all 

three layers). Only interior model cells were compared 

because MODFLOW does not calculate flows between 

specified-head cells (for example, a specified-head 

surficial aquifer system cell overlying a specified-head 

Upper Floridan aquifer cell). Model results for the 

specified-head boundary condition compared to the 

no-flow boundary condition indicate the following: 

(1) the average simulated water table was 0.14 ft higher 

(less than 0.01 ft higher at reclaimed-water application 

sites), (2) the average rate of leakage through the intermediate confining unit decreased 0.01 in/yr, and 

(3) the average flux in the surficial aquifer system from 

interior model cells to lateral boundary cells increased 

0.05 in/yr. These results are in agreement with the 

hypothesis that regional lateral flow in the surficial 

aquifer system is minimal. Consequently, the model is 

insensitive to changes in the surficial aquifer system 

boundary condition.

HYDROLOGIC EFFECTS OF RECLAIMEDWATER APPLICATION

The ground-water flow model was used in 

conjunction with particle-tracking analyses and 

analyses of existing data to form a process-oriented 

evaluation of the hydrologic effects of reclaimed water 

applied by the Water Conserv II and RCID facilities. 

Specifically, this evaluation includes an appraisal of 

the following under current (1995) and proposed 

future conditions: changes in surface-water and 

ground-water levels, directions and rates of reclaimed 

water movement through the surficial and Floridan 

aquifer systems, and locations and magnitudes of 

reclaimed water discharge from the surficial and 

Floridan aquifer systems.

MODPATH (Pollock, 1994), a USGS particle-tracking program, was used to supplement 

ground-water flow model results. The program tracks 

“particles” of water based on output from MODFLOW 

simulations. Particle-tracking can only simulate the 

advective transport of solutes and cannot be used to 

calculate solute concentrations in ground water 

because it does not consider dispersion, degradation, 

or retardation processes.

Aquifer and confining unit top altitudes, base 

altitudes, and effective porosities are the only hydrologic data required, in addition to standard 

MODFLOW input and output, for a steady-state particle-tracking analysis. Aquifer and confining unit top 

and base altitudes were based on interpolation of data 

collected during this study as well as data reported by 

Miller (1986), as previously discussed (for example, 

fig. 8). Inaccuracies in aquifer or confining unit top or 

base altitudes can affect particle-tracking results. For 

example, particle traveltime through a confining unit 

simulated by a leakance array is directly proportional 

to confining unit thickness. The mantled karst environment in the study area makes accurately defining 

aquifer and confining unit top and base altitudes very 

difficult. Consequently, interpolated data are necessarily generalized and can differ significantly from actual 

values at any particular site.

Effective porosity is the fraction of interconnected pore space in a volume of rock or unconsolidated sediment. Particle traveltimes are proportional to 

the effective porosity estimates; if effective porosity 

estimates are doubled, ground-water velocities will be 

halved and traveltimes will double. No data exist in 

the study area on effective porosity of the aquifers or 

confining units. Measurements of the total porosity of 

the surficial aquifer system range from 0.36 to 0.51 

(Camp Dresser and McKee, Inc., 1984; Sumner and 

Bradner, 1996); an effective porosity value of 0.40 

was used for the surficial aquifer system and intermediate confining unit. The choice of a representative 

effective porosity value for the Floridan aquifer 

system is complicated by the dual porosity characteristic of karst limestone. That is, total porosity can 

consist of primary or rock porosity, which is characteristic of an unfissured volume of rock, and secondary 

porosity, which is the result of fissures and solution 

openings. Robinson (1995) performed particle-tracking analyses to simulate ground-water traveltimes 

measured during tracer tests conducted in the Upper 

Floridan aquifer in Hillsborough County, Florida. 

Effective porosity values of 0.003 to 0.015 were 

required to reproduce the traveltime for the first peak 

in tracer concentration, whereas a value of 0.21 was 

required to reproduce the traveltime for the second 

peak in tracer concentration. Robinson (1995) 

indicated that this bimodal distribution of tracer arrival 
Hydrologic Effects of Reclaimed-Water Application 69

time probably was the result of conduit flow through 

secondary porosity producing the first peak and 

diffuse flow through the rock matrix (primary 

porosity) producing the second peak. Laboratory 

measurements of effective porosity reported by 

Knochenmus and Robinson (1996) for rock cores from 

wells in Hillsborough, Pasco, and Pinellas Counties, 

Florida, were 0.17 to 0.49 for the Ocala Limestone and 

0.02 to 0.25 for the Avon Park Formation. Given these 

uncertainties, a uniform effective porosity value of 

0.20 was used for the Upper and Lower Floridan 

aquifers and the middle semiconfining unit.

Particle starting locations were based on 

reclaimed water volumetric flow rates so that one 

particle represented approximately 1,000 gal/d of 

applied reclaimed water. For example, a model cell 

completely containing one RIB which receives 

64,000 gal/d of reclaimed water would have 64 evenly 

spaced particles assigned to that cell. Particles were 

allowed to move through the ground-water system in a 

forward-tracking manner until they exited the model.

In interpreting the particle-tracking results 

discussed in the following sections, it is important to 

remember that a steady-state analysis does not allow 

for temporal variations in hydrologic stresses. For 

example, reclaimed water has not always been applied 

at the same rates and locations as it was in 1995. If the 

historical hydrologic stresses had been applied in a 

transient analysis, reclaimed water traveltimes and 

directions could have been different than those 

discussed below. For example, a transient simulation 

and particle-tracking analysis where the spatial distribution of reclaimed water remained constant over time 

but the application rate increased from 10 to 

20 Mgal/d over a 10-year period probably would 

produce slower ground-water velocities and shallower 

reclaimed water flow paths compared to a steady-state 

analysis that assumes a constant 20 Mgal/d application 

rate. Similarly, spatial variations in application rate 

over time probably would cause movement of 

reclaimed water in different directions as well as with 

different traveltimes under transient conditions as 

compared to steady-state conditions where spatial 

variations remain constant over time.

Effects Under Current Conditions

Reclaimed-water application (2.5 in/yr) 

accounted for approximately 14 percent of the total 

simulated flow (18 in/yr, fig. 38) through the surficial 

and Floridan aquifer systems in the model area in 

1995. This result is significant considering the RIBs and 

irrigation sites combined area comprises only about 3 

percent of the total 285 mi2

 model area. Reclaimed 

water is not spread uniformly across the model area; in 

areas such as RIB sites, reclaimed-water application 

dominates the hydrologic system, exceeding natural net 

recharge rates by 10 times or more. Consequently, 

reclaimed-water application probably has altered the 

hydrologic system to some degree.

Actual pre-application conditions (for example, 

during 1985 before either facility started operation) 

could not be accurately determined because few historical data are available on aquifer stresses. Therefore, an 

assumed pre-application condition was simulated by 

removing all reclaimed-water application stresses and 

leaving all other stresses unchanged from their 1995 

values, except ET at reclaimed-water irrigation sites 

which were assigned ET values for herbaceous vegetation (table 4). A new effective recharge array (eq. 8) 

was calculated using the revised ET values and 

excluding reclaimed water components, RRIB and Rri, 

of Ra. Consequently, differences between simulation 

results from the assumed pre-application conditions and 

the 1995 conditions are the result of only two 

factors—the absence of reclaimed-water application 

and the reduction in associated ET. It is important to 

note that water levels at specified-head boundaries 

(figs. 22 and 23) are not simulated and will show no 

change in the figures referenced in the following 

discussion.

Reclaimed-water application has caused the 

surficial aquifer system water table to increase in 

altitude over much of the model area (fig. 40). The 

increase has been slight (average about 2 ft) in the 

Water Conserv II irrigation areas, which probably is 

within the range of uncertainty associated with the 

assumed pre-application condition. In 1995, the average 

citrus grove using Water Conserv II reclaimed water 

was irrigated at a rate of 20 to 30 in/yr. With average 

precipitation and citrus ET of 52 and 45 in/yr, respectively, average net recharge would be about 32 in/yr; 

alternatively, with herbaceous vegetation ET of 27 in/yr, 

and no irrigation, average net recharge would be 25 

in/yr. That is, much of the irrigation water is lost to the 

higher ET of the irrigated citrus, causing only a 

relatively small rise in the water table. This effect of ET 

on net recharge also explains the slight decrease in 

water-table altitude south of Johns Lake (fig. 40). 

A relatively small amount of reclaimed water was 

applied during 1995 to the citrus groves south of Johns 

Lake. This produced an average net recharge of only 
70 Hydrogeology and Simulation of the Effects of Reclaimed-Water Application in West Orange and Southeast 

Lake Counties, Florida

Figure 40. Simulated change in the surficial aquifer system water table from assumed pre-application 

conditions as a result of steady-state 1995 reclaimed-water application rates. 
Hydrologic Effects of Reclaimed-Water Application 71

about 10 in/yr in 1995, compared to 25 in/yr under 

assumed pre-application conditions. Significant 

water-table mounds exist under all RIB sites (except 

Water Conserv II RIB site 9, which is rarely used). 

Based on pre-application water-level data reported by 

Camp Dresser and McKee, Inc. (1984) the average 

increase in water-table altitude from pre-application to 

1995 has been approximately 40, 15, and 25 ft at Water 

Conserv II RIB sites 5, 6, and 7, respectively. CH2M 

Hill (1989) reported an average pre-application 

water-table altitude of 99 ft at the RCID RIB site; 1995 

measured data indicate about a 5-ft increase in the 

water-table altitude as a result of RIB operation. These 

data probably have a margin of error of about 5 ft, 

because they do not represent average annual values 

and were collected in years that may have been hydrologically different from 1995 (for example, greater 

rainfall or less citrus irrigation). Nevertheless, these 

measured rises in water-table altitude generally agree 

with figure 40, with the exception of Water Conserv II 

RIB site 5. Model simulation indicates approximately a 

70-ft increase at this RIB site. This discrepancy could 

be caused by the pre-application condition assumptions. The relatively small simulated increase in 

water-table altitude at the RCID RIB site is the result of 

high intermediate confining unit leakance, high surficial aquifer system hydraulic conductivity, and the 

presence of nearby surface-water features that receive 

ground-water discharging from the surficial aquifer 

system.

The accuracy of simulated increases in lake 

levels is difficult to ascertain and can be affected by a 

number of factors. The response of a lake to hydrologic 

stresses can be significantly affected by very 

local-scale hydrologic and lithologic conditions. Lee 

and Swancar (1997) reported that leakage through a 

lakebed to the Upper Floridan aquifer decreased 

dramatically as the surficial aquifer system vertical 

anisotropy (the ratio of horizontal to vertical hydraulic 

conductivity) increased, based on ground-water flow 

modeling of Lake Lucerne in Polk County. The model 

presented in this report represents an isotropic 

ground-water flow system on a more regional scale and 

generally does not account for variations at the scale of 

an individual lake; such variations are likely to exist 

but their magnitude and spatial distribution are 

unknown. Nevertheless, a qualitative interpretation of 

water-level changes in landlocked lakes can be 

estimated from figure 40. As previously explained, 

specified-head lakes will show no change in figure 40 

(for example, Lake Hancock). In areas of high intermediate confining unit leakance, qualitative water-level 

changes in specified-head lakes might be inferred from 

simulated changes in the Upper Floridan aquifer potentiometric surface (fig. 41). Lakes Ingram, Flat, Reedy, 

Needham, Sawgrass (south of Hancock), Avalon, 

Hancock, and Hartley (fig. 40) are most likely to have 

had a rise in lake level induced by reclaimed-water 

application. Stream outflow might have mitigated 

lake-level rises in Lakes Hancock and Hartley. That is, 

an increase in lake level would be minimal if the 

increase in ground-water discharge to the lake caused 

by reclaimed-water application were balanced by a 

corresponding increase in stream outflow.

Increases in the Upper Floridan aquifer potentiometric surface caused by 1995 reclaimed-water application rates were less than 5 ft and were greatest under 

Water Conserv II RIB site 7 and the RCID RIB site 

(fig. 41). Water-level data collected since 1979 at an 

Upper Floridan aquifer well (site number 85, fig. 3) 

support the simulated increase in the potentiometric 

surface at the RCID RIB site. Measured water levels 

rose in this well from May 1990 to September 1991 

(RCID RIB operation started September 1990) and 

have remained about 4 ft higher than pre-1990 water 

levels since 1991. The relatively small increase in the 

potentiometric surface under most of the 

reclaimed-water application sites, which is less than 

the typical seasonal variations due to rainfall, should 

not be interpreted to mean that little reclaimed water 

reaches the Upper Floridan aquifer. The high transmissivity of the Upper Floridan aquifer precludes the 

formation of large potentiometric-surface highs under 

average 1995 stresses, even though most of the 

reclaimed water is simulated to reach the aquifer.

Simulated leakage rate through the intermediate 

confining unit provides a better indication of the 

quantity and spatial distribution of surficial aquifer 

system water (which includes reclaimed water) that 

recharges the Floridan aquifer system (fig. 42). 

The highest rates of recharge to the Floridan aquifer 

system typically occur under reclaimed-water application sites, especially RIB sites. The simulated net 

leakage to the Floridan aquifer system under the 

assumed pre-application condition was 10.4 in/yr 

(averaged over the model area); therefore, the 

11.6 in/yr simulated under 1995 conditions represents 

about a 10 percent increase in recharge to the Floridan 

aquifer system. The distribution of leakage rates 

generally were similar to effective recharge rates
72 Hydrogeology and Simulation of the Effects of Reclaimed-Water Application in West Orange and Southeast 

Lake Counties, Florida

Figure 41. Simulated change in the Upper Floridan aquifer potentiometric surface from assumed 

pre-application conditions as a result of steady-state 1995 reclaimed-water application rates. 
Hydrologic Effects of Reclaimed-Water Application 73

Figure 42. Simulated rate of leakage through the intermediate confining unit based on steady-state 1995 

conditions. 
74 Hydrogeology and Simulation of the Effects of Reclaimed-Water Application in West Orange and Southeast 

Lake Counties, Florida

(compare figs. 26 and 42), because the surficial aquifer 

system is dominated by diffuse downward leakage to 

the Floridan aquifer system in the study area. The largest differences were in areas where leakance of the 

intermediate confining unit is low or the potentiometric 

surface of the Upper Floridan aquifer was above the 

surficial aquifer system water table. Low leakance of 

the intermediate confining unit induces water in the 

surficial aquifer system to flow laterally until it discharges to surface-water features, is extracted by ET, or 

enters an area of higher leakance where it may more 

easily reach the Upper Floridan aquifer. Upward leakage from the Upper Floridan aquifer to the surficial 

aquifer system occurs where the potentiometric surface 

is above the water table (for example, along Reedy 

Creek and around Lake Apopka, fig. 42). Effective 

recharge to the surficial aquifer system in these areas is 

lost to ET or stream discharge and does not reach the 

Floridan aquifer system.

Reclaimed-water application has caused 

increases in ground-water levels and Floridan aquifer 

system recharge rates over much of the model area. 

However, reclaimed water is not necessarily present in 

the aquifer system everywhere it has induced a change 

in ground-water levels. For example, a local water-table 

mound produced by reclaimed-water application can 

cause an increase in the regional water-table altitude 

upgradient from the mound (even though no reclaimed 

water is present upgradient) by acting like a hydraulic 

dam to regional ground-water flow and decreasing the 

regional water-table gradient. Particle-tracking analyses 

were performed to help determine where reclaimed 

water could be present in the surficial and Upper 

Floridan aquifer systems, how long it takes reclaimed 

water to travel through the ground-water system, and in 

what quantities and areas reclaimed water discharges 

from the ground-water system.

Figure 43 depicts the maximum lateral extent of 

reclaimed water in the surficial aquifer system based 

on particle tracking analyses of simulated steady-state 

1995 conditions. Reclaimed water in the surficial 

aquifer system generally does not travel far beyond 

application site boundaries because there is very little 

lateral flow in the surficial aquifer system. The large 

area of reclaimed water along Reedy Creek east of the 

RCID RIB site is not caused by lateral flow in the 

surficial aquifer system (fig. 43). Rather, some 

reclaimed water at the RCID RIB site moves through 

the intermediate confining unit, travels east through 

the Upper Floridan aquifer, moves upward back 

through the intermediate confining unit, and finally 

discharges from the surficial aquifer system to Reedy 

Creek and adjacent wetlands. Very little interaction 

occurs between Water Conserv II and RCID reclaimed 

water in the surficial aquifer system, except in a small 

area northwest of the RCID RIB site at some citrus 

groves which use Water Conserv II reclaimed water.

The traveltime of reclaimed water from the water 

table to the top of the Upper Floridan aquifer varies 

greatly (fig. 44), depending largely upon the 

reclaimed-water application rate. That is, traveltime is 

shorter in a vigorous flow system (for example, Water 

Conserv II RIB site 5 or the RCID RIB site) and longer 

in a more sluggish flow system (for example, citrus 

groves south of Johns Lake). Based on simulated 

steady-state 1995 conditions, the minimum and average 

traveltimes to the top of the Upper Floridan aquifer are 

approximately 1 and 10 years, respectively, for Water 

Conserv II reclaimed water and approximately 2 and 

7 years, respectively, for RCID RIB reclaimed water. 

Water Conserv II RIB sites generally have shorter 

traveltimes than the RCID RIB site because the application flux rate generally is higher; the 10 year average 

traveltime for Water Conserv II reclaimed water is 

skewed somewhat by the longer traveltimes for the 

irrigation reclaimed water.

Not all reclaimed water exits the surficial 

aquifer system by leakage to the Floridan aquifer 

system—some of the water discharges to lakes, 

streams, and wetlands. The only area of significant 

discharge of reclaimed water to surface-water features 

is in the vicinity of the RCID RIB site. Based on 

simulated steady-state 1995 conditions, RCID 

reclaimed water is discharging to Bear Bay and other 

wetlands, Whittenhorse Creek, Perimeter and C-4 

Canals, and Reedy Creek (fig. 45). This conclusion is 

supported by the increase in chloride concentration 

measured in Whittenhorse Creek (fig. 17) and the 

Perimeter Canal, as previously described.

Reclaimed water that has traveled through the 

intermediate confining unit next enters the Upper 

Floridan aquifer and flows primarily laterally in a 

direction perpendicular to potentiometric-surface 

contour lines. Consequently, reclaimed water could 

potentially be present within the Upper Floridan 

aquifer over a much larger area than in the surficial 

aquifer system (fig. 46). As depicted by the traveltime 

markers on selected pathlines shown in figure 46, it 

would take over 100 years for the reclaimed water to 

reach its maximum lateral extent in the model area. 

Traveltime of reclaimed water in the Upper Floridan 

aquifer is quite variable, and like the surficial aquifer
Hydrologic Effects of Reclaimed-Water Application 75

Figure 43. Areas where reclaimed water is expected to be present in the surficial aquifer system based on 

particle-tracking analyses of simulated steady-state 1995 conditions. 
76 Hydrogeology and Simulation of the Effects of Reclaimed-Water Application in West Orange and Southeast 

Lake Counties, Florida

Figure 44. Traveltime of reclaimed water from the surficial aquifer system water table to the top of the 

Upper Floridan aquifer based on particle-tracking analyses of simulated steady-state 1995 conditions. 
Hydrologic Effects of Reclaimed-Water Application 77

Figure 45. Areas of discharge of reclaimed water from the surficial aquifer system to surface-water 

features at the RCID RIB site and vicinity based on particle-tracking analyses of simulated steady-state 

1995 conditions. 
78 Hydrogeology and Simulation of the Effects of Reclaimed-Water Application in West Orange and Southeast 

Lake Counties, Florida

Figure 46. Areas where reclaimed water is expected to be present in the Upper Floridan aquifer and 

pathlines depicting directions and rates of movement of reclaimed water based on simulated steady-state 

1995 conditions. 
Hydrologic Effects of Reclaimed-Water Application 79

system it is largely dependent upon the local flow 

system. For example, reclaimed water that is within the 

capture area of Apopka Spring moves relatively rapidly 

compared to reclaimed water moving under Reedy and 

Bonnet Creeks. Similar to the surficial aquifer system, 

there is only a small area of possible mixing of Water 

Conserv II and RCID RIB reclaimed water in the 

Upper Floridan aquifer (fig. 46).

Reclaimed water is not necessarily uniformly 

distributed over the vertical extent of the surficial and 

Upper Floridan aquifers. Figures 47 and 48 show 

cross-sectional views of the directions and rates of 

movement of reclaimed water from two RIB sites. 

Because these figures depict three-dimensional 

pathlines that have been projected into two dimensions, 

the pathlines do not always lie within the vertical plane 

of the section line, which can produce apparent crossing 

of pathlines within a single model layer (see Upper 

Floridan aquifer, fig. 48). These figures demonstrate 

some of the major differences between the hydrologic 

characteristics of the RCID and Water Conserv II facilities. Several typical flow paths of RCID RIB reclaimed 

water that discharge to surface-water features are 

depicted in figure 47 by pathlines which terminate in 

Bear Bay, Perimeter and C-4 Canals, Reedy Creek, and 

other wetlands. At Water Conserv II, the lack of a well 

developed surface-water drainage network in close 

proximity to application sites allows very little 

discharge of reclaimed water to surface-water features. 

In addition, the RCID RIB site generally is surrounded 

by areas of low recharge to or discharge from the Upper 

Floridan aquifer (fig. 42). As a result, reclaimed water 

can more easily penetrate the entire depth of the aquifer. 

In contrast, Water Conserv II RIB site 5 is surrounded 

by areas of high recharge which constrain the reclaimed 

water to a smaller flow path within the Upper Floridan 

aquifer (fig. 48). Higher recharge also contributes to the 

typically shorter traveltimes of the Water Conserv II 

reclaimed water (evidenced by comparison of traveltime markers on pathlines in figs. 47 and 48).

Vertical anisotropy could change particle 

pathlines and traveltimes by increasing the resistance 

to vertical flow through the aquifer system. Therefore, 

the effects of aquifer vertical anisotropy on particle-tracking results were investigated by running the 

model under anisotropic conditions. In the following 

discussion, the magnitude of vertical anisotropy is 

represented by the result of dividing horizontal by 

vertical hydraulic conductivity. Vertical anisotropy is 

common in many rocks and unconsolidated sediments, 

and it is not unusual to find vertical anisotropy of 5 or 

10 (Bouwer, 1978). Camp Dresser and McKee, Inc. 

(1984) reported horizontal and vertical hydraulic 

conductivities with vertical anisotropy values of 1 to 3 

for the surficial aquifer system at Water Conserv II RIB 

site 6. Sumner and Bradner (1996) reported a vertical 

anisotropy of 3.3 for the surficial aquifer system at the 

RCID RIB site based on model-calibrated hydraulic 

conductivities. Available data based on analyses of rock 

cores from the Upper Floridan aquifer 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 (Robinson, 1995).

In order to incorporate vertical anisotropy into the 

present model, the surficial and Upper Floridan aquifers 

were subdivided into 3 and 5 layers, respectively. 

The uppermost layer of the surficial aquifer system 

extended 10 ft below the water table and model layers 2 

and 3 were evenly divided between the bottom of layer 1 

and the top of the intermediate confining unit. All 

surficial aquifer system boundary conditions (fig. 22) 

and stresses from the calibrated model were applied to 

layer 1. Layers 2 and 3 had only no-flow lateral 

boundary conditions and no internal boundary conditions or stresses. Layer 1 hydraulic conductivities were 

assigned values identical to the calibrated model (fig. 

28). Transmissivities for layers 2 and 3 were calculated 

based on hydraulic conductivities from the calibrated 

model and respective layer thickness. Leakance values 

between surficial aquifer system layers (vertical 

hydraulic conductivity divided by vertical distancebetween model cell nodes) were calculated so that the 

vertical anisotropy was 10. Model layers 4, 5, 6, 7, and 8 

represented the Upper Floridan aquifer with thicknesses 

representing the following fractions of total aquifer 

thickness: respectively. 

Transmissivity values were calculated as the product of 

respective fractional layer thickness and calibrated 

Upper Floridan aquifer transmissivity (fig. 29). 

Ground-water withdrawal rates at wells and Apopka 

Spring were calculated for each layer as the product of 

respective fractional layer thickness and total withdrawal 

rate. Boundary conditions and specified-head values for 

layers 4 through 8 were identical to those for layer 2 of 

the calibrated model (fig. 23). Leakance values between 

layers were calculated so that the vertical anisotropy was 

100. The intermediate confining unit, middle semiconfining unit, and Lower Floridan aquifer were modeled 

identically to the calibrated model. No attempt was made 

to recalibrate the nine-layer (anisotropic) model.

1/8, 1/8, 1/8, 1/8, and 1/2,
80 Hydrogeology and Simulation of the Effects of Reclaimed-Water Application in West Orange and Southeast 

Lake Counties, Florida

0 5,000 10,000 FEET

0 3,000 METERS

SR 545

Bear Bay Perimeter Canal

RCID

RIBs Reedy Creek

C-4 Canal Water table

Bonnet Creek

Land surface

SURFICIAL

SYSTEM AQUIFER

UPPER

FLORIDAN

AQUIFER

MIDDLE SEMICONFINING UNIT

UNIT

CONFINING

INTERMEDIATE

EXPLANATION

PATHLINE-Delineates flow path of 

particle of reclaimed water. Points 

indicate location of particle at 20-year 

traveltime intervals

SPECIFIED-HEAD BOUNDARY

A A′

100

SEA

100

200

300

400

LEVEL

Figure 47. Projected cross-sectional view of simulated pathlines depicting directions and 

rates of movement of reclaimed water from several RIBs at the RCID RIB site based on 

simulated steady-state 1995 conditions (location of section shown in fig. 46). 
Hydrologic Effects of Reclaimed-Water Application 81

Schofield

Road

Lake

Ingram

Water Conserv II 

RIB site 5

Water

table

Land surface

0 5,000 10,000FEET

0 3,000 METERS

Johns

Lake

US 50

Lake

Apopka

MIDDLE SEMICONFINING UNIT

EXPLANATION

PATHLINE-Delineates flow path of 

particle of reclaimed water. Points 

indicate location of particle at 20-year 

traveltime intervals

SURFICIAL AQUIFER SYSTEM

INTERMEDIATE CONFINING UNIT

UPPER

FLORIDAN

AQUIFER

SPECIFIED-HEAD BOUNDARY

100

SEA

100

200

300

400

LEVEL

200

B B′

Figure 48. Projected cross-sectional view of simulated pathlines depicting directions and 

rates of movement of reclaimed water from several RIBs at Water Conserv II RIB site 5 based 

on simulated steady-state 1995 conditions (location of section shown in fig. 46).
82 Hydrogeology and Simulation of the Effects of Reclaimed-Water Application in West Orange and Southeast 

Lake Counties, Florida

A comparison of particle-tracking results from 

the isotropic and anisotropic models indicates that 

vertical anisotropy generally has little effect on 

reclaimed water discharge locations and magnitudes, 

even though the vertical anisotropy values used were 

relatively large (table 10). The greatest effect is an 

increase of 8 percent in reclaimed water discharge 

from the surficial aquifer system at the RCID RIB site. 

Because the surficial and Upper Floridan aquifers 

probably are anisotropic to some degree, the average 

percentages listed in table 10 probably are closer to 

their true values. No generalization can be made on 

changes in reclaimed water traveltimes induced by 

increasing vertical anisotropy, because traveltime is 

dependent not only on ground-water velocity but also 

on the distance over which movement is measured. 

Increasing the resistance to vertical flow by decreasing 

vertical hydraulic conductivity (therefore, increasing 

vertical anisotropy) would cause flow velocities to 

decrease; however, flow paths would also change, 

possibly becoming shorter. Depending on the relative 

magnitude of the changes in velocity and flow path, 

the result could be either a shorter or longer traveltime.

Based on the averaged results from the isotropic 

and anisotropic models under simulated 1995 conditions, approximately 67 percent of the reclaimed water 

applied at the RCID RIB site recharges the Floridan 

aquifer system, whereas 33 percent discharges from 

the surficial aquifer system to surface-water features; 

99 percent of the reclaimed water applied at Water 

Conserv II recharges the Floridan aquifer system, 

whereas only 1 percent discharges to surface-water 

features (table 10). Discharge percentages of 

reclaimed water within the Floridan aquifer system 

(table 10) are more uncertain (compared to the partitioning of water between the surficial and Floridan 

aquifer systems) because of the (1) unknown accuracy 

of the representation of the middle semiconfining unit 

and Lower Floridan aquifer in the model, (2) assumption that wells penetrate the entire thickness of the 

Upper Floridan aquifer, and (3) assumed absence of 

preferential flow zones within the Upper and Lower 

Floridan aquifers. Nevertheless, most of the reclaimed 

water probably ultimately discharges from the 

Floridan aquifer system outside the model boundaries, 

because slightly over half of the reclaimed water is 

simulated to discharge at specified-head boundaries 

(table 10).

Effects Under Proposed Future Conditions

As both measured data and modeling simulations have indicated, current (1995) reclaimed-water 

application rates have produced noticeable changes in 

the ground-water and, to a lesser degree, surface-water 

systems in west Orange and southeast Lake Counties. 

However, reclaimed-water application rates at Water 

Conserv II and the RCID RIBs probably will increase 

in the future, and hydrologic effects under proposed 

future conditions are unknown. Consequently, model 

simulations are used to predict possible future hydrologic effects of reclaimed-water application.

Table 10. Discharge of reclaimed water from model based on particle-tracking analyses of simulated steady-state 

1995 conditions

[Mgal/d, million gallons per day; RCID, Reedy Creek Improvement District. Average 1995 flow rate for the RCID RIB site was 6.67 Mgal/d; average 

1995 flow rate for Water Conserv II was 28.1 Mgal/d]

Location of reclaimed water 

discharge from model

Percent of average 1995 flow rate

Isotropic1 Anisotropic2 Average

RCID Water 

Conserv II RCID Water Conserv II RCID Water Conserv II

Lakes, streams, and/or wetlands 29 1 37 1 33 1

Upper Floridan aquifer wells 15 25 13 25 14 25

Apopka Spring 0 20 0 21 0 20

Upper Floridan aquifer boundaries 54 51 50 51 52 51

Lower Floridan aquifer boundaries 2 3 0 2 1 3

1

 Ratio of horizontal to vertical hydraulic conductivity was 1:1 for the entire model.

2 Ratio of horizontal to vertical hydraulic conductivity was 10:1 for the surficial aquifer system, 100:1 for the Upper Floridan aquifer, and 1:1 for 

the Lower Floridan aquifer.
Hydrologic Effects of Reclaimed-Water Application 83

The calibrated (isotropic) ground-water flow 

model was used with proposed future reclaimed-water 

application rates for a steady-state simulation of 

proposed future conditions. All other stresses (for 

example, ground-water withdrawal rates) were left at 

their 1995 values. Future irrigation rates at Water 

Conserv II were assumed equal to 1995 rates. Average 

annual daily flow at the Water Conserv II RIBs was 

near the permitted capacity of about 16 Mgal/d in 1995; 

therefore, future application rates at the existing Water 

Conserv RIBs were left at their 1995 values. Several 

additional RIBs have been constructed adjacent to 

Water Conserv II RIB sites 5 and 6 since 1995. 

Combined average flow at these additional RIBs is 

expected to be approximately 0.9 Mgal/d 

(D.F. MacIntyre, PB Water, oral commun., 1996). 

The Orange County National Golf Center, a large golf 

course complex consisting of two full-length 18-hole 

courses and a short 9-hole course, will be constructed 

between Water Conserv II RIB sites 7 and 9. In 

addition, 15 new RIBs will be interspersed in and 

adjacent to the golf course. An estimated 2 Mgal/d of 

Water Conserv II reclaimed water will be applied for 

golf course irrigation and 3 Mgal/d will be applied at the 

golf course RIBs (D.F. MacIntyre, PB Water, oral 

commun., 1996). The estimated future 5.9 Mgal/d of 

reclaimed water makes the total Water Conserv II 

reclaimed-water application rate 34.0 Mgal/d. New 

effective recharge rates were calculated for the model 

using a potential ET rate (equal to Efws) at the golf 

course. Additional RIBs may be constructed in the 

future at Water Conserv II, but definite locations have 

not been established (D.F. MacIntyre, PB Water, oral 

commun., 1996); consequently, no attempt was made to 

include these in the model. Additional RIBs probably 

will not be constructed by the RCID in the near future 

(T.W. McKim, Reedy Creek Energy Services, Inc., oral 

commun., 1997). The permitted capacity of the RCID 

RIB site is 12.5 Mgal/d; therefore, considerable 

capacity still exists at this site. For proposed future 

conditions, all 1995 RCID RIB flows were multiplied 

by 1.87 to yield a total reclaimed-water application rate 

of 12.5 Mgal/d for the RCID RIB site.

The greatest simulated increase (about 20 ft) in 

1995 water-table altitude as a result of proposed future 

conditions was at the Orange County National Golf 

Center (fig. 49). Increases of up to 4 and 10 ft were 

simulated at the new RIBs near Water Conserv II RIB 

sites 5 and 6, respectively. The high surficial aquifer 

system hydraulic conductivity and nearby 

surface-water features are largely responsible for the 

relatively small 4-ft increase in water-table altitude at 

the RCID RIB site. Subject to the same limitations 

previously discussed concerning interpretation of 

lake-level changes under 1995 conditions, the following 

lakes are most likely to experience a rise in water level 

as a result of proposed future reclaimed-water application rates: Ingram, Sawgrass (south of Hancock), 

Needham, Huckleberry, Reedy, Hancock, and Hickorynut (fig. 49). Stream outflow might mitigate lake-level 

rises in Lakes Hancock and Hickorynut.

The greatest simulated increase in the 1995 

Upper Floridan aquifer potentiometric surface as a 

result of proposed future conditions was approximately 

2 ft in the northern two-thirds of the RCID RIB site 

(fig. 50). This is the result of a greater increase in application rate and a lower transmissivity than at Water 

Conserv II application sites. Average net leakage from 

the surficial aquifer system to the Floridan aquifer system 

was 11.9 in/yr, 0.3 in/yr greater than under 1995 

conditions.

Under proposed future conditions, the maximum 

lateral extent of reclaimed water in the surficial and 

Upper Floridan aquifer systems within the model area 

probably would be similar to that under 1995 conditions 

(figs. 43 and 46). The areas of increased water levels 

under proposed future conditions (figs. 49 and 50) are 

well within those areas simulated under 1995 conditions 

(figs. 40 and 41). The greatest difference would be in 

the surficial aquifer system in the vicinity of the Orange 

County National Golf Center where reclaimed water 

was not applied in 1995. Future traveltimes through the 

surficial aquifer system generally would decrease in 

areas where reclaimed-water application rates were 

increased. However, future traveltimes could increase if 

the greater reclaimed-water application rates caused 

longer flow paths. Future traveltimes through the 

Floridan aquifer system are more difficult to generalize, 

because greater reclaimed-water application rates 

would cause higher ground-water velocities but also 

might cause reclaimed water to move deeper through 

the aquifer along longer flow paths.

Particle-tracking analyses were performed for 

proposed future conditions using the three-layer 

isotropic model and the nine-layer anisotropic model 

(table 11). As under 1995 conditions, vertical anisotropy had the greatest effect at the RCID RIB site in the 

partitioning of reclaimed water between surficial 

aquifer system discharge and Floridan aquifer system 

recharge. At the RCID RIB site, vertical anisotropy 

caused an increase in surficial aquifer system discharge 

under 1995 and proposed future conditions of 8 and 6 
84 Hydrogeology and Simulation of the Effects of Reclaimed-Water Application in West Orange and Southeast 

Lake Counties, Florida

Figure 49. Simulated change in the surficial aquifer system water table from 1995 conditions as a result of 

steady-state proposed future reclaimed-water application rates. 
Hydrologic Effects of Reclaimed-Water Application 85

Figure 50. Simulated change in the Upper Floridan aquifer potentiometric surface from 1995 conditions as a result of 

steady-state proposed future reclaimed-water application rates. 
86 Hydrogeology and Simulation of the Effects of Reclaimed-Water Application in West Orange and Southeast 

Lake Counties, Florida

percent, respectively. Therefore, as the reclaimedwater application rate increases, the effects of aquifer 

vertical anisotropy become less significant.

Based on an average of isotropic and anisotropic 

model results under proposed future conditions, 99 

percent of the reclaimed water applied at Water 

Conserv II recharged the Floridan aquifer system, 

whereas only 1 percent discharged from the surficial 

aquifer system to surface-water features (table 11). 

At the RCID RIB site, however, the greater reclaimedwater application rate of 12.5 Mgal/d caused approximately half of the reclaimed water to discharge to 

surface-water features and half to recharge the 

Floridan aquifer system (table 11). The increase in 

reclaimed-water application contributed to a simulated 

increase in ground-water discharge to Whittenhorse 

and Reedy Creeks. Under the proposed future conditions (compared to 1995 conditions), surficial aquifer 

system discharge to Bear Bay and Whittenhorse Creek 

(upstream from the gaging station, site number 57, 

fig. 2) increased 140 percent and discharge to Reedy 

Creek (upstream from the gaging station, site number 

62, fig. 2) increased 30 percent. The partitioning of 

reclaimed water discharge within the Floridan aquifer 

system under proposed future conditions is subject to 

the same limitations as previously discussed for 1995 

conditions. However, the majority of reclaimed water 

that recharges the Floridan aquifer system probably 

ultimately discharges from the ground-water system 

outside the model boundaries. 

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 in the conceptual model, 

grid scale, the difficulty in obtaining sufficient 

measurements to account for all of the spatial variation 

in hydrologic properties and stresses throughout the 

model area, and the distribution and quality of data 

used for calibration.

The conceptual model used to construct the 

ground-water flow model is a highly simplified representation of the true ground-water system. The simplifications in the conceptual model, necessitated by the 

limited ability to model extremely complex natural 

systems, represent the most likely source of error in 

the ground-water flow model. The aquifer systems are 

neither isotropic nor vertically homogeneous. Varying 

lithology produces preferential flow zones, and in the 

Floridan aquifer system these probably are magnified 

by dissolution features. The lack of data on the middle 

semiconfining unit and Lower Floridan aquifer might 

have caused an incorrect representation of these units; 

therefore, the leakance and transmissivity reported for 

these units should not be considered as calibrated 

values. Lateral boundaries were located well outside 

of reclaimed-water application sites because inaccuracies in assigned boundary conditions can adversely 

effect 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 

Table 11. Discharge of reclaimed water from model based on particle-tracking analyses of simulated steady-state 

proposed future conditions

[Mgal/d, million gallons per day; RCID, Reedy Creek Improvement District. Proposed future flow rate for the RCID RIB site was 12.5 Mgal/d; 

proposed future flow rate for Water Conserv II was 34.0 Mgal/d]

Location of reclaimed water 

discharge from model

Percent of proposed future flow rate

Isotropic1 Anisotropic2 Average

RCID Water

Conserv II RCID Water Conserv II RCID Water Conserv II

Lakes, streams, and/or wetlands 47 1 53 1 50 1

Upper Floridan aquifer wells 12 27 11 26 12 26

Apopka Spring 0 17 0 18 0 18

Upper Floridan aquifer boundaries 37 52 35 52 36 52

Lower Floridan aquifer boundaries 4 3 1 3 2 3

1

 Ratio of horizontal to vertical hydraulic conductivity was 1:1 for the entire model.

2 Ratio of horizontal to vertical hydraulic conductivity was 10:1 for the surficial aquifer system, 100:1 for the Upper Floridan aquifer, and 1:1 for 

the Lower Floridan aquifer.
Summary 87

were available, transient calibration and model 

simulations would have more accurately portrayed the 

ground-water system; however, steady-state analyses 

are sufficient for simulating the average long-term 

effects of reclaimed-water application.

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 is rarely 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 smallest model 

cell (656 by 656 ft) are common in the mantled karst 

environment in the study area. These small-scale variations might significantly affect larger scale average 

values. In addition, the location of stresses (for 

example, well pumpage or RIB application rate) is 

distorted somewhat 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, the majority of vertical 

flow probably occurs through the intermediate 

confining unit and middle semiconfining unit, and not 

within the surficial aquifer system, Upper Floridan 

aquifer, or Lower Floridan aquifer. Measured data 

indicate that the vertical head gradients across the intermediate confining unit generally are much greater than 

those in the surficial aquifer system. For example, the 

vertical head gradient in the surficial aquifer system 

near Island Lake was 0.001 on January 4, 1995 (see 

wells HA2-4 and HA2-5, fig. 16), whereas the vertical 

head gradient across the intermediate confining unit 

was 0.04 (surficial aquifer system well HA2-4 and 

Upper Floridan aquifer well HA2-F, site number 51, 

fig. 3). For comparison, Sumner and Bradner (1996) 

indicated that a surficial aquifer system vertical head 

gradient of about 0.06 existed directly under a heavily 

loaded RIB. However, model simulation demonstrated 

that flow was predominately radial (that is, a very small 

vertical head gradient) 30 ft beyond the edge of the 

flooded area within the RIB (Sumner and Bradner, 

1996). Consequently, the discretization used in the 

present model is adequate for fulfilling the objective of 

modeling ground-water flow on a more regional scale, 

such as in the vicinity of a RIB site or a cluster of RIBs 

rather than an individual RIB.

Calibrated parameter values and distributions 

(figs. 28, 29, and 30) are dependent not only on the 

accuracy of measured data but also on the spatial distribution of these data, which include water levels to 

which the model was calibrated as well as stresses, such 

as reclaimed-water application rate or well pumpage. 

Model results are more likely to be accurate in areas 

where there are large known stresses and corresponding 

water-level measurements indicating aquifer response 

to those stresses. Model results in areas where there was 

no stress on the aquifer system or no water-level 

measurements should be interpreted cautiously. For 

example, it would be incorrect to conclude that the only 

area of high surficial aquifer system hydraulic conductivity in the model area is in the vicinity of the RCID 

RIB site (fig. 28). If a stress of similar magnitude with 

corresponding water-level measurements had existed 

elsewhere in the model area, another area of different 

aquifer properties might have become apparent. In other 

words, if additional data were available, perhaps 

collected under different hydrologic conditions, a recalibration of the model might yield significantly different 

results. However, in the areas of interest, many 

water-level measurements exist and the major stress 

(reclaimed-water application) is relatively well known.

Sensitivity analyses indicated that several model 

parameters, especially Upper Floridan aquifer transmissivity, were highly dependent on effective recharge. In 

addition, the model was relatively insensitive to many 

parameters. Consequently, different combinations of 

values for the specified effective recharge array and 

model parameters could yield the same head distribution. Nevertheless, after an extensive calibration effort, 

the hydrologic properties and simulation results derived 

from the ground-water flow model were within realistic 

and previously referenced limits.

SUMMARY

Wastewater reclamation and reuse has become 

increasingly popular as water agencies search for 

alternative water-supply and wastewater-disposal 

options. Several governmental agencies (Orange 

County, City of Orlando, and the Reedy Creek 

Improvement District (RCID)) in central Florida 

currently use the land-based application of reclaimed 

water (wastewater that has been treated beyond 

secondary treatment) as a management alternative to 

surface-water disposal of wastewater. Water Conserv II, 

a water reuse project developed jointly by Orange 

County and the City of Orlando, began operation in 
88 Hydrogeology and Simulation of the Effects of Reclaimed-Water Application in West Orange and Southeast 

Lake Counties, Florida

December 1986. In 1995, the Water Conserv II facility 

distributed approximately 28 Mgal/d of reclaimed 

water for discharge to rapid-infiltration basins (RIBs) 

and for use as agricultural irrigation. The RCID began 

operation of RIBs in September 1990 and in 1995 

these RIBs received approximately 6.7 Mgal/d of 

reclaimed water. In the future, as much as 65 Mgal/d 

might be directed to the Water Conserv II and RCID 

facilities. Analyses of existing data and data collected 

during the course of this study were combined with 

ground-water flow modeling and particle-tracking 

analyses to develop a process-oriented evaluation of 

the regional effects of reclaimed water applied by 

Water Conserv II and the RCID RIBs on the 

hydrology of west Orange and southeast Lake 

Counties.

The ground-water flow system beneath the 

study area is a multi-aquifer system that consists of a 

thick sequence of carbonate rocks overlain by unconsolidated sediments. The hydrogeologic units are the 

surficial aquifer system, the intermediate confining 

unit, and the Floridan aquifer system. The surficial 

aquifer system is unconfined and consists mainly of 

undifferentiated deposits of marine sand, silt, clay, and 

crushed shell of late Pliocene to Recent age. The intermediate confining unit separates the surficial and 

Floridan aquifer systems throughout the study area, 

except where breached by sinkholes, and retards the 

vertical exchange of water between these systems. 

The unit consists of (in varying proportions) bedded 

clay, silt, sand, crushed shell, and phosphatic 

limestone of Miocene age (Hawthorn Group) and, 

locally, low permeability beds of early Pliocene age. 

Extreme variations exist in the lithology and thickness 

of the intermediate confining unit as a result of the 

mantled karst environment in the study area; consequently, only very general trends in intermediate 

confining unit properties can be indicated on a 

regional scale. The Floridan aquifer system is 

composed of a sequence of highly permeable Tertiary 

carbonate rocks of Eocene age. The system has been 

subdivided into two major permeable zones, the Upper 

and Lower Floridan aquifers, separated by the less 

permeable middle semiconfining unit. The marine 

dolomite, gypsum, and anhydrite of the sub-Floridan 

confining unit form the bottom of the freshwater flow 

system in the study area.

Flow in the surficial aquifer system is 

dominated regionally by diffuse downward leakage to 

the Floridan aquifer system and is affected locally by 

lateral flow systems produced by streams, lakes, and 

spatial variations in recharge. Ground water generally 

flows laterally through the Upper Floridan aquifer to 

the north and east. Little data exist on the hydrologic 

characteristics of the middle semiconfining unit or 

Lower Floridan aquifer. Therefore, results presented in 

this report concerning the middle semiconfining unit 

or Lower Floridan aquifer should be interpreted with 

caution.

Approximately one-third of the surface area of 

the model is covered by lakes or wetlands. Many of 

the lakes are landlocked because the mantled karst 

environment precludes a well developed network of 

surface-water drainage. Response of a lake to a hydrologic stress is highly dependent on the local lithology 

of the surficial aquifer system and intermediate 

confining unit and the local variations in recharge 

reaching the water table.

A water budget compiled for the surficial 

aquifer system based on 1995 conditions indicated that 

rainfall and ET were the largest inflow and outflow, 

respectively. Rainfall was approximately 52 in/yr, 

whereas ET averaged 38 in/yr. ET ranged from about 

27 in/yr in areas containing natural herbaceous vegetation and a deep water table to about 47 in/yr where the 

water table was near land surface. Variations in ET 

primarily are the result of spatial differences in vegetation type and water availability combined with 

seasonal changes in plant growth characteristics and 

climatological variables.

The USGS three-dimensional ground-water 

flow model MODFLOW was used to simulate 

ground-water flow in the surficial and Floridan aquifer 

systems. A steady-state calibration to average 1995 

conditions was performed by using a parameter estimation program to vary values of surficial aquifer 

system hydraulic conductivity, intermediate confining 

unit leakance, and Upper Floridan aquifer transmissivity. The calibrated model generally produced simulated water levels in close agreement with measured 

water levels and was used to simulate the hydrologic 

effects of reclaimed-water application under current 

(1995) and proposed future conditions.

Based on simulated and measured data, 

increases of up to about 40 ft in the water table and 

less than 5 ft in the Upper Floridan aquifer potentiometric surface have occurred as a result of 1995 

reclaimed-water application rates. The largest 

increases were under RIB sites. Changes in lake levels 

were more difficult to determine because lake levels 

are influenced by local flow systems and surficial 

aquifer system anisotropy which were not simulated 
References 89

by the model. The lakes most likely to have experienced increases in water levels are those nearest RIB 

sites. Reclaimed water that reached the water table 

generally moved vertically through the surficial 

aquifer system and intermediate confining unit and 

into the Floridan aquifer system. An average traveltime of 10 years at Water Conserv II and 7 years at the 

RCID RIBs was required for reclaimed water to move 

from the water table to the top of the Upper Floridan 

aquifer. Elevated chloride concentrations measured in 

Whittenhorse Creek and Perimeter Canal indicate that 

reclaimed water from the RCID RIB site probably is 

discharging to adjacent surface-water features. 

Approximately 67 percent of the reclaimed water 

applied at the RCID RIB site recharged the Floridan 

aquifer system, whereas 33 percent discharged from 

the surficial aquifer system to surface-water features; 

99 percent of the reclaimed water applied at Water 

Conserv II recharged the Floridan aquifer system, 

whereas only 1 percent discharged from the surficial 

aquifer system to surface-water features. The majority 

of reclaimed water applied at both facilities probably 

will ultimately discharge from the Floridan aquifer 

system outside the model boundaries.

Proposed future conditions were assumed to 

consist of an additional 5.9 Mgal/d of reclaimed water 

distributed by Water Conserv II to discharge to new 

RIBs and to irrigate the new Orange County National 

Golf Center and an additional 5.8 Mgal/d of reclaimed 

water discharged to RCID RIBs. Compared to 1995 

conditions, increases of up to about 20 ft in the water 

table and 2 ft in the potentiometric surface of the 

Upper Floridan aquifer were simulated. The lakes 

nearest the Orange County National Golf Center and 

the RCID RIB site were most likely to experience an 

increase in water level. The directions of reclaimed 

water movement through the ground-water system 

were similar to those under 1995 conditions. Future 

traveltimes through the surficial aquifer system 

generally decreased in areas where reclaimed-water 

application rates were increased. At Water Conserv II, 

99 percent of the reclaimed water recharged the 

Floridan aquifer system, whereas 1 percent discharged 

from the surficial aquifer system to surface-water 

features. The greater reclaimed-water application rate 

at the RCID RIBs caused approximately half of the 

reclaimed water to discharge to surface-water features 

and half to recharge the Floridan aquifer system. 

The increase in reclaimed-water application contributed to increases of 140 and 30 percent in the 

simulated ground-water discharge to Whittenhorse 

Creek and Reedy Creek, respectively, compared to 

simulated 1995 values. The majority of reclaimed water 

that recharges the Floridan aquifer system probably will 

ultimately discharge from the ground-water system 

outside the model boundaries.

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, necessitated by the extreme complexity of 

the natural system and transient phenomena, 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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