Hydrogeology and Quality of Ground Water in 

Orange County, Florida

By James C. Adamski and Edward R. German

U.S. Geological Survey

Water-Resources Investigations Report 03-4257

Prepared in cooperation with the

City of Orlando

Orange County Utilities

Orlando Utilities Commission

Reedy Creek Improvement District

St. Johns River Water Management District

South Florida Water Management District

Tallahassee, Florida

2004
U.S. DEPARTMENT OF THE INTERIOR

GALE A. NORTON, Secretary

U.S. GEOLOGICAL SURVEY

Charles G. Groat, Director

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 

write to: 

U.S. Geological Survey

2010 Levy Avenue

Tallahassee, FL 32310

Copies of this report can be 

purchased from:

U.S. Geological Survey

Branch of Information Services

Box 25286

Denver, CO 80225-0286

888-ASK-USGS

Additional information about water resources in Florida is available on the internet 

at http://fl.water.usgs.gov
Contents III

CONTENTS

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

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

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

Previous Investigations..................................................................................................................................................... 3

Site-Numbering System ................................................................................................................................................... 4

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

Description of the Study Area.................................................................................................................................................... 4

Environmental Setting...................................................................................................................................................... 4

Population, Land Use, and Water Use ............................................................................................................................. 8

Data Collection and Analysis................................................................................................................................................... 11

Hydrogeology .......................................................................................................................................................................... 17

Hydrogeologic Setting ................................................................................................................................................... 17

Surficial Aquifer System................................................................................................................................................ 19

Intermediate Confining Unit .......................................................................................................................................... 24

Floridan Aquifer System................................................................................................................................................ 28

Upper Floridan Aquifer........................................................................................................................................ 28

Middle Semiconfining Unit and Lower Floridan Aquifer ................................................................................... 35

Recharge and Discharge....................................................................................................................................... 35

Potentiometric Surface and Ground-Water Flow Patterns................................................................................... 39

Long-Term Trends in Water Levels ............................................................................................................................... 41

Surficial Aquifer System...................................................................................................................................... 41

Floridan Aquifer System ...................................................................................................................................... 43

Water Budget.................................................................................................................................................................. 48

Water Quality ........................................................................................................................................................................... 50

Major Ions and Nutrients ............................................................................................................................................... 51

Surficial Aquifer System...................................................................................................................................... 51

Floridan Aquifer System ...................................................................................................................................... 57

Trace Elements, Radon, and Stable Isotopes ................................................................................................................. 67

Pesticides........................................................................................................................................................................ 72

Summary.................................................................................................................................................................................. 74

Selected References ................................................................................................................................................................. 77

Appendix 1—Ground-water sites with water-quality and water-level data............................................................................. 82

Appendix 2—Site information, field measurements, and major ions concentrations for water samples from the 

surficial and Floridan aquifer systems ........................................................................................................... 88

Appendix 3—Nutrient, total organic carbon, and selected trace elements concentrations for water samples from the 

surficial and Floridan aquifer systems ........................................................................................................... 98

Appendix 4—Site information and pesticide concentrations in water samples from the surficial and the Floridan 

aquifer systems............................................................................................................................................. 108

Figures

1. Map showing location of Orange County, Florida ........................................................................................................ 5

2. Diagram showing annual rainfall at Orlando and Sanford, Florida, 1931-2001 ........................................................... 6

3. Map showing physiography and locations of hydrogeologic sections .......................................................................... 7

4. Diagram showing population (1960-2000) and ground-water use (1965-2000) in Orange County, Florida ................ 8

5. Map showing generalized land use in Orange County, Florida, in 1997, showing expansion of urban land use 

since 1977...................................................................................................................................................................... 9
IV Contents

6-7. Maps showing estimated pumping from public-supply wells tapping the:

6. Upper Floridan aquifer in 2000.............................................................................................................................10

7. Lower Floridan aquifer in 2000.............................................................................................................................11

8-9. Maps showing locations of:

8. Wells with periodic and continuous water-level data and geophysical data .........................................................13

9. Wells and springs where water-quality data were collected from 1990 through 2001 .........................................14

10. Graph showing relation between specific conductance and total dissolved solids in ground-water samples 

collected from 1990 through 2001...............................................................................................................................17

11. Chart showing geologic and hydrogeologic units in Orange County, Florida .............................................................18

12. Map showing generalized thickness of undifferentiated sedimentary deposits overlying the intermediate 

confining unit ...............................................................................................................................................................20

13-15. Diagrams showing generalized hydrogeologic sections:

13. A-A' .......................................................................................................................................................................21

14. B-B' .......................................................................................................................................................................22

15. C-C' .......................................................................................................................................................................23

16. Map showing distribution of horizontal hydraulic conductivity values determined from slug tests in the 

surficial aquifer system ................................................................................................................................................24

17. Diagram showing continuous and periodic water levels in observation wells tapping the surficial aquifer 

system and the Upper Floridan aquifer........................................................................................................................26

18. Map showing generalized thickness of the intermediate confining unit ......................................................................27

19. Diagram showing water levels in three wells tapping the surficial aquifer system, the intermediate confining 

unit, and the Upper Floridan aquifer, and rainfall at Orlando, Florida ........................................................................29

20. Map showing generalized altitude of the top of the Upper Floridan aquifer ...............................................................30

21-23. Diagrams showing selected geophysical logs showing hydrogeologic units for:

21. Site I, Orange County, Florida ..............................................................................................................................31

22. Well OR0621, site II, Orange County, Florida......................................................................................................32

23. Well OR0618, site III, Orange County, Florida ....................................................................................................33

24-25. Maps showing location of aquifer tests and model-derived transmissivity values for the:

24. Upper Floridan aquifer..........................................................................................................................................34

25. Lower Floridan aquifer..........................................................................................................................................36

26. Map showing areas of potential recharge to and discharge from the Upper Floridan aquifer .....................................37

27. Map showing simulated recharge rate to the Upper Floridan aquifer by leakage through the intermediate 

confining unit under average steady-state 1988 conditions and average steady-state 1995 conditions.......................38

28-29. Maps showing potentiometric surface of the Upper Floridan aquifer:

28. September 2001.....................................................................................................................................................40

29. July 1961 ...............................................................................................................................................................41

30. Map showing decline in potentiometric surface of the Upper Floridan aquifer from July 1961 to September 2001 ......42

31. Diagrams showing long-term periodic water levels in an observation well tapping the Upper Floridan aquifer 

and annual rainfall at Orlando, Florida ........................................................................................................................43

32. Map showing generalized potentiometric surface of the Lower Floridan aquifer, May 1999, and water-level 

decline since the 1960s ................................................................................................................................................44

33-36. Diagrams showing:

33. Long-term continuous and periodic water levels in observation wells tapping the surficial aquifer system 

and annual rainfall at Orlando, Florida .................................................................................................................45

34. Long-term continuous water levels in observation wells tapping the Upper Floridan aquifer and annual 

rainfall at Orlando, Florida....................................................................................................................................47

35. Discharge at Wekiva and Rock Springs for entire period of record and for wet season, and annual rainfall 

at Orlando, Florida ................................................................................................................................................49

36. Water budget for Orange County, Florida, 1991-2000..........................................................................................50

37-39. Maps showing concentrations of:

37. Major cations and anions in water samples collected from wells tapping the surficial aquifer system, 

1992 through 2001.................................................................................................................................................53

38. Total dissolved solids in water samples collected from wells tapping the surficial aquifer system, 

1990 through 2001.................................................................................................................................................54

39. Chloride in water samples collected from wells tapping the surficial aquifer system, 1990 through 2001..........55
Contents V

40. Boxplots showing distribution of sodium, chloride, sulfate, bicarbonate, total dissolved solids, and phosphorus 

in water samples from the surficial aquifer system, the Upper Floridan aquifer, and the Lower Floridan aquifer, 

1990 through 2001....................................................................................................................................................... 56

41-46. Maps showing concentrations of:

41. Nitrite plus nitrate in water samples collected from wells tapping the surficial aquifer system, 

1990 through 2001 ................................................................................................................................................ 57

42. Major cations and anions in water samples collected from springs and wells tapping the Upper Floridan

aquifer, 1995 through 2001................................................................................................................................... 58

43. Total dissolved solids in water samples collected from springs and wells tapping the Upper Floridan aquifer, 

1990 through 2001 ................................................................................................................................................ 59

44. Sulfate in water samples collected from springs and wells tapping the Upper Floridan aquifer, 

1990 through 2001 ................................................................................................................................................ 60

45. Chloride in water samples collected from springs and wells tapping the Upper Floridan aquifer, 

1990 through 2001 ................................................................................................................................................ 61

46. Chloride in water samples collected from springs and wells tapping the Upper Floridan aquifer, 

1960 through 1969 ................................................................................................................................................ 62

47-49. Diagrams showing:

47. Changes in specific conductance and in concentrations of sulfate and chloride in well at site 58 

tapping the Upper Floridan aquifer....................................................................................................................... 64

48. Concentrations of chloride and sulfate in water samples from wells tapping the Floridan aquifer system ......... 66

49. Specific conductance in ground-water samples collected from Wekiva Springs and Rock Springs, and 

water levels in nearby observation well tapping the Upper Floridan aquifer ....................................................... 67

50-51. Maps showing concentrations of radon in water samples collected from wells tapping the:

50. Surficial aquifer system, 1999 through 2000......................................................................................................... 70

51. Upper Floridan aquifer, 1999 through 2000 .......................................................................................................... 71

52. Boxplots showing distribution of radon in water samples from the surficial aquifer system, the Upper Floridan 

aquifer, and the Lower Floridan aquifer, 1999 through 2000...................................................................................... 71

53. Diagrams showing relations between deuterium and oxygen-18, and deuterium and total dissolved solids in 

ground-water samples collected from the surficial aquifer system, the Upper Floridan aquifer, and the Lower 

Floridan aquifer, 1999 through 2000........................................................................................................................... 72

Tables

1. Summary of borehole geophysical data collected from wells tapping the Upper Floridan aquifer ................................. 12

2. Summary of well construction information, by aquifer, for wells with water-quality records......................................... 14

3. Analytical methods and drinking water standards for inorganic analytes included in the study...................................... 15

4. Well-construction information, aquifer information, and average horizontal hydraulic conductivity for the 

surficial aquifer system..................................................................................................................................................... 25

5. Well construction data, period of record, and results of trend analysis for long-term observation wells tapping the 

surficial and Floridan aquifer systems.............................................................................................................................. 46

6. Summary statistics of water-quality indicators, and major-ion and nutrient constituents in ground-water samples 

from the surficial and Floridan aquifer systems, 1990 through 2001............................................................................... 52

7. Comparison of historical (1979) and recent (1999) water-quality data for site 24 with the range of quality of 

reclaimed water discharged to rapid infiltration basins in southwestern Orange County ................................................ 61

8. Evaluation of trends in specific conductance, chloride, and sulfate for wells having 20 or more years of record........... 63

9. Summary statistics for selected trace elements and radon in ground-water samples from the surficial and Floridan 

aquifer systems, 1990 through 2001................................................................................................................................. 68

10. Well, date of sample collection, and number of pesticide compounds detected in each sample...................................... 73

11. Pesticides detected in ground-water samples, environmental regulations, and associated health guidance 

concentrations ................................................................................................................................................................... 74
VI Contents

CONVERSION FACTORS AND VERTICAL DATUM

*The standard unit for transmissivity is cubic foot per day per square foot times foot of aquifer thickness [(ft3/d)/ft2]ft. In this report, the

mathematically reduced form, foot squared per day (ft2/d), is used for convenience.

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows: °C=(°F-32)/1.8.

Vertical coordinate information is referenced to the National Geodetic Vertical Datum of 1929 (NGVD 29).

Horizontal coordinate information (latitude-longitude) is referenced to the North American Datum of 1927 (NAD 27).

Acronyms and Abbreviations

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 

Mass

pound (lb) 0.4536 kilogram 

pound per acre per year (lb/acre)/yr 0.4536 kilograms per acre per year

Flow Rate

cubic foot per second (ft3

/s) 0.02832 cubic meter per second 

gallon per day (gal/d) 0.04381 meter per day

gallon per minute (gal/min) 0.04381 meter per minute

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

inch per month (in/mo) 25.4 millimeter per month

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

Hydraulic Conductivity

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

Transmissivity*

foot squared per day (ft2

/d) 0.09290 meter squared per day 

Leakance

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

ET evapotranspiration

FEP fluorinated ethylene propylene

GMWL Global Meteoric Water Line

LOWESS locally weighted scatterplot smoothing 

curve

MCL Maximum Contaminant Limit

μg/L micrograms per liter

μm micrometer

μS/cm microsiemens per centimeter

meq/L milliequivalents per liter

mg/L milligrams per liter

NOAA National Oceanic and Atmospheric 

Administration

pCi/L picoCuries per liter

PTFE polytetrafluoroethylene

RIBs rapid infiltration basins

RCID Reedy Creek Improvement District

SFWMD South Florida Water Management District

SJRWMD St. Johns River Water Management District

TDS total dissolved solids

USGS U.S. Geological Survey

VSMOW Vienna Standard Mean Ocean Water
Abstract 1

Hydrogeology and Quality of Ground Water in 

Orange County, Florida

By James C. Adamski and Edward R. German

ABSTRACT

Ground water is the main source of water supply 

in central Florida and is critical for aquatic habitats and 

human consumption. To provide a better understanding 

for the conservation, development, and management of 

the water resources of Orange County, Florida, a study 

of the hydrogeologic framework, water budget, and 

ground-water quality characteristics was conducted 

from 1998 through 2002. The study also included 

extensive analyses of the surface-water resources, 

published as a separate report.

An increase in population from about 264,000 in 

1960 to 896,000 in 2000 and subsequent urban growth 

throughout this region has been accompanied by a 

substantial increase in water use. Total ground-water 

use in Orange County increased from about 82 million 

gallons per day in 1965 to about 287 million gallons 

per day in 2000. The hydrogeology of Orange County 

consists of three major hydrogeologic units: the surficial aquifer system, the intermediate confining unit, 

and the Floridan aquifer system. Data were compiled 

from 634 sites to construct hydrogeologic maps and 

sections of Orange County. Water-level elevations 

measured in 23 wells tapping the surficial aquifer 

system ranged from about 10.6 feet in eastern Orange 

County to 123.8 feet above NGVD 29 in northwestern 

Orange County from March 2000 through September 

2001. Water levels also were measured in 14 wells 

tapping the Upper Floridan aquifer. Water levels fluctuate over time from seasonal and annual variations in 

rainfall; however, water levels in a number of wells 

tapping the Upper Floridan aquifer have declined over 

time. Withdrawal of ground water from the aquifers by 

pumping probably is causing the declines because the 

average annual precipitation rate has not changed 

substantially in central Florida since the 1930s, 

although yearly rates can vary.

A generalized water budget was computed for 

Orange County from 1991 to 2000. Average rates for 

the 10-year period for the following budget components were computed based on reported measurements 

or estimates: precipitation was 53 inches per year 

(in/yr), runoff was 11 in/yr, spring discharge was 

2 in/yr, and net lateral subsurface outflow and exported 

water was 1 in/yr. Evapotranspiration was 39 in/yr, 

which was calculated as the residual of the waterbudget analysis, assuming changes in storage were 

negligible. 

Water-quality samples were collected from 

April 1999 through May 2001 from a total of 26 wells 

tapping the surficial aquifer system, 1 well tapping the 

intermediate confining unit, 24 wells tapping the Upper 

Floridan aquifer, 2 springs issuing from the Upper 

Floridan aquifer, and 8 wells tapping the Lower Floridan aquifer. These data were supplemented with existing water-quality data collected by the U.S. Geological 

Survey and St. Johns River Water Management 

District.

Concentrations of total dissolved solids, sulfate, 

and chloride in samples from the surficial aquifer 

system generally were low. Concentrations of nitrate 

were higher in samples from the surficial aquifer 

system than in samples from the Upper Floridan or 
2 Hydrogeology and Quality of Ground Water in Orange County, Florida

Lower Floridan aquifers, probably as a result of agricultural and residential land use. Water type throughout 

most of the Upper Floridan and Lower Floridan aquifers was calcium or calcium-magnesium bicarbonate, 

probably as a result of dissolution of the carbonate 

rocks. Water type in both the surficial and Floridan 

aquifer systems in eastern Orange County is sodium 

chloride. Concentrations of total dissolved solids, 

sulfate, and chloride in the aquifers increase toward 

eastern Orange County.

Data from 16 of 24 wells in eastern Orange 

County with long-term water-quality records indicated 

distinct increases in concentrations of chloride over 

time. The increases probably are related to withdrawal 

of ground water at the Cocoa well field, causing an 

upwelling of deeper, more saline water. The most 

commonly detected trace elements were aluminum, 

barium, boron, iron, manganese, and strontium. In 

addition, arsenic was detected in 12 of 59 samples, and 

selenium was detected in 19 of 58 samples. Concentrations generally were low; no samples had concentrations that exceeded any of the Maximum Contaminant 

Levels. Radon was detected in all ground-water 

samples in concentrations ranging from 56 to 14,700 

picoCuries per liter.

Pesticide compounds were detected in low 

concentrations in 8 of 16 ground-water samples from 

the surficial aquifer system and in 2 of 14 samples from 

the Upper Floridan aquifer. The most commonly 

detected compounds were atrazine and its degradate, 

deethyl atrazine. Pesticides were present in shallow 

ground water primarily in urban areas around metropolitan Orlando. The source of pesticide compounds in 

ground-water samples probably is lawn-care and other 

household products containing pesticides. Pesticide 

concentrations did not exceed any Maximum Contaminant Levels.

Data from this study indicate that ground water 

is an abundant resource in Orange County, Florida. The 

quality of ground water generally is within the Primary 

and Secondary Drinking Water Regulations established 

by the U.S. Environmental Protection Agency; 

however, water-quality samples for this study were 

collected under drought conditions. Additional study to 

document water quality during various hydrologic 

conditions would be beneficial to understanding the 

occurrence and distribution of constituents such as 

pesticides.

INTRODUCTION

Ground water from the Floridan aquifer system 

is the main source of water supply in central Florida. 

Ground water from the surficial and Floridan aquifer 

systems discharges to surface-water bodies such as 

lakes, springs, and streams, which provide aquatic 

habitat and recreation. An understanding of the quantity and quality of ground-water resources is important 

because of the numerous uses of ground water and the 

effect of ground water on the aquatic environment in 

central Florida.

A comprehensive water-resources study of 

Orange County by the U.S. Geological Survey (USGS) 

was completed by Lichtler and others (1968). Land use 

in Orange County has changed substantially since the 

late 1960s as a result of rapidly increasing population. 

The increase in population and subsequent urban 

growth was accompanied by a substantial increase in 

water use. Therefore, a new study was needed to document current (2001) ground-water conditions and 

assess potential changes in the quantity and quality of 

the water resources within Orange County.

In 1998, the USGS in cooperation with the 

City of Orlando, Orange County Utilities, Orlando 

Utilities Commission, Reedy Creek Improvement 

District, St. Johns River Water Management District 

(SJRWMD), and South Florida Water Management 

District (SFWMD), began a 4-year study to evaluate 

the ground- and surface-water resources of Orange 

County. A summary of the surface-water investigations 

will be published in a separate report. The objectives 

of the ground-water portion of the study were to: 

(1) describe the current conditions of the ground-water 

resources of Orange County; (2) assess long-term 

trends in ground-water resources, particularly with 

respect to changes in land use and (or) increases in 

water use; and (3) determine natural and anthropogenic 

factors affecting ground-water resources. Data 

collected in and around Orange County by the USGS 

and numerous State and local agencies since 1968 were 

compiled and new data were collected to meet the 

objectives of the present study.

Purpose and Scope

The purpose of this report is to present results 

from a comprehensive study of the ground-water 

resources that will be useful for the conservation, 

development, and management of the water resources 
Introduction 3

of Orange County. This report presents a description of 

the hydrogeology and the water quality of the surficial 

and Floridan aquifer systems. Climate, physiography, 

land use, population, and water use of Orange County, 

as well as a description of data-collection and analytical methods, are included. Current (2001) conditions of 

ground-water availability and water quality also are 

compared and contrasted to past conditions. Natural 

and anthropogenic factors that could be affecting the 

availability and quality of ground water are described.

The scope of the report includes a compilation of 

existing data and description of new data on the geology and ground-water hydrology of Orange County. 

Geologic and geophysical data from 634 sites were 

used to construct hydrogeologic maps and sections. 

An additional 18 new wells that tap the surficial aquifer 

system were installed. Slug tests were done on 4 existing and 16 newly installed wells to measure the 

hydraulic properties of the surficial aquifer system. 

Water levels in 37 wells were measured bimonthly 

from March 2000 through September 2001; four of 

the wells also were instrumented with transducers for 

continuous monitoring of water levels. Historical 

water-level data were statistically analyzed for trends. 

During the period from April 1999 through May 2001, 

water-quality samples were collected once from 

26 wells tapping the surficial aquifer system, 1 well 

tapping the intermediate confining unit, 24 wells 

tapping the Upper Floridan aquifer, 2 springs discharging from the Upper Floridan aquifer, and 8 wells 

tapping the Lower Floridan aquifer. These data were 

combined with water-quality data collected from 1990 

through 2000 to assess the water-quality conditions of 

the ground-water resources of Orange County. Limited 

historical (pre-1990) water-quality data also were 

available for statistical trend analysis.

Previous Investigations

The water resources of Orange County have been 

studied for more than 60 years. Stringfield (1936) and 

Unklesbay (1944) described the geology of the Floridan aquifer system and documented water levels prior 

to extensive ground-water development.

Lichtler and others (1968) reported on both 

ground- and surface-water conditions in Orange 

County in the early-to-mid-1960s. Anderson and 

Joyner (1966) described the availability and quality of 

surface water. Lichtler and others (1976) investigated 

the hydrologic connection between ground water and 

three lakes in the Orlando area. Phelps and German 

(1995), Smoot and Schiffer (1985), German (1983), 

Gaggiani and Lamonds (1977), and Pfischner (1968) 

described the hydrology and quality of water in 

selected Orange County lakes. Schiffer (1989) 

described the effects of urban runoff on the water quality 

of wetlands in the metropolitan Orlando area.

Tibbals and Frazee (1976) and Phelps and 

Schiffer (1996) described hydrogeologic and waterquality conditions at the Cocoa well field in east 

Orange County. Kimrey (1978) completed a preliminary appraisal of drainage wells in and around Orlando. 

The effects of drainage wells on the water quality of the 

Upper Floridan aquifer in the Orlando area were 

described by Schiner and German (1983) and Taylor 

(1993). Bradner (1991) described water quality in the 

Upper Floridan aquifer in the vicinity of drainage wells 

and delineated a hydrocarbon plume in the Upper 

Floridan aquifer in downtown Orlando, resulting from 

operation of a coal-gasification plant. A comprehensive 

inventory of drainage wells and their effects on 

recharge in Orange and Seminole Counties was 

completed by CH2M Hill (1997).

The effects of agricultural (citrus) and urban land 

uses on ground-water quality were studied by Rutledge 

(1987) and German (1996). Fate and transport of nutrients in ground water under a rapid-infiltration basin 

was studied by Sumner and Bradner (1996). A study of 

the water quality and isotope geochemistry of springs 

in central Florida included data from Rock and Wekiva 

Springs in Orange County (Toth, 1999).

A seismic-reflection and hydrogeologic study of 

Lake Apopka was completed by Locker and others 

(1988). A study by O’Reilly and others (2002) of the 

hydrogeology and water quality of the Lower Floridan 

aquifer included all of Orange County.

O'Reilly (1998), Murray and Halford (1996), and 

Tibbals (1990) developed ground-water flow models 

that included all or parts of Orange County. Groundwater flow and salt-water intrusion were modeled in 

Orange County (PB Water, 1999). A ground-water flow 

model developed for Seminole County also included 

Orange County (Spechler and Halford, 2001). A model 

of ground-water flow in peninsular Florida included all 

of Orange County (Sepúlveda, 2002). McGurk and 

Presley (2002) developed a model of ground-water 

flow in east-central Florida that included all of Orange 

County.
4 Hydrogeology and Quality of Ground Water in Orange County, Florida

Site-Numbering System

The USGS National Water Data Information 

System (NWIS) uses a 15-digit number (site identification number) based on latitude and longitude, to identify wells. The first six digits denote the degrees, 

minutes, and seconds of latitude; the next seven digits 

denote the degrees, minutes, and seconds of longitude; 

and the last two digits denote a sequential number for a 

site within a one-second grid. Well site identification 

numbers generally end in 01 or 02. For convenience, in 

this report wells and springs also are given a site 

number that refers to locations shown in the figures. 

Surface-water sites generally are designated by an 

8-digit downstream-order number. Appendix 1 lists the 

site numbers and site identification numbers.

Acknowledgments

The authors express their appreciation to various 

public officials of Orange County, the City of Orlando, 

the Orlando Utilities Commission, Reedy Creek 

Improvement District, St. Johns River Water Management District, and South Florida Water Management 

District, whose cooperation and knowledge greatly 

aided this investigation. The authors are particularly 

grateful for water-quality data provided by the City of 

Orlando and St. Johns River Water Management 

District. The authors also are grateful for access to land 

owned by the city of Orlando, Orange County, South 

Florida Water Management District, St. Johns River 

Water Management District, the State of Florida, and 

private land owners.

DESCRIPTION OF THE STUDY AREA

Orange County is located in east-central Florida 

(fig. 1). The county is about 1,003 mi2 in area, 

with about 916 mi2 of land and about 87 mi2 of water 

(Lichtler and others, 1968). The St. Johns River forms 

the boundary with Brevard County, directly to the east. 

Seminole and Lake Counties are north and west, 

respectively, of Orange County, and Osceola County 

is directly south. The northern and eastern parts of 

Orange County are contained within SJRWMD, 

whereas the south-central and southwestern parts of 

Orange County are contained within SFWMD.

Metropolitan Orlando is the major population 

center in the county. This urban area includes the city 

of Orlando as well as adjacent communities such as 

Apopka, Maitland, and Winter Park. Other communities in Orange County include Bithlo, Christmas, 

Oakland, Winter Garden, and Zellwood. Since 1971, 

the southwestern part of Orange County has become a 

major recreational resource with several large theme 

parks, hotel complexes, and golf courses, attracting 

large numbers of international and domestic tourists.

Environmental Setting

Orange County has a subtropical climate with 

relatively short, warm winters, and long, hot summers. 

The average annual temperature at Orlando is 71.5 oF. 

The average air temperatures in January and July are 

50 oF and 90 oF, respectively. Minimum air temperatures during the winter months occasionally drop below 

freezing, but rarely below 20 oF. Maximum air temperatures in the summer months commonly exceed 90 oF.

 Records of annual precipitation were compiled 

for a National Oceanic and Atmospheric Administration (NOAA) rainfall station in south Orlando and 

Sanford, Florida. The periods of record for these 

stations are from 1931 to 2001. The mean annual rainfall for Orlando was 50 in., the minimum was 30.4 in. 

(2000), and the maximum was 68.7 in. (1960). The 

mean annual rainfall for Sanford was 52 in. (fig. 2).

The distribution of daily rainfall in Orlando probably is typical of all of Orange County. The maximum 

daily rainfall recorded at the Orlando station was 8.19 in. 

in July 1960. Days with low rainfall occur relatively frequently yet account for little of the annual accumulation. 

For example, 50 percent of the days with measurable 

rainfall had 0.2 in. or less total rainfall. These relatively 

low rainfall days accounted for only 8.4 percent of the 

total rainfall accumulation for the period of record. Conversely, higher daily rainfall events, though relatively 

infrequent, account for a large portion of the total rainfall 

accumulation. For example, rainfall totals exceeding 

1 in. occurred on about 12 percent of the days with measurable rainfall, yet accounted for about half of the total 

rainfall accumulation.

Seasonally, the wettest month for the period 1931-

2000 was July, with a maximum rainfall of 19.57 in. (in 

1960) and a mean rainfall of nearly 8 in. Several months 

had less than 0.2 in. of rain, with the driest being December 1944 with no recorded rainfall. Most of the rainfall in 

Orange County occurs during the months of June 
Description of the Study Area 5

COCOA

WELL FIELD

SOUTH FLORIDA

WATER MANAGEMENT DISTRICT

ST. JOHNS RIVER

WATER MANAGEMENT DISTRICT

RAPID

INFILTRATION

BASIN

(Reedy Creek

Improvement

District)

Water

Conserv II

Tosohatchee

State

Reserve

528

I-4

Bithlo

Chuluota

Geneva

Sanford

Lake Mary

Oviedo

Orlando

Casselberry

Altamonte

Springs

Zellwood

Winter Garden

Christmas

Lake

Buena

Vista

Apopka

Maitland

Oakland

Taft

Winter Park

POLK

COUNTY

SEMINOLE

COUNTY

OSCEOLA COUNTY

LAKE COUNTY

VOLUSIA COUNTY

BREVARD

COUNTY

28 20 ° ′

40′

30′ 81° ′ 00

0 5 10 KILOMETERS

0 5 10 MILES

81°40′ 20′ 10′ 80°50′

28°50′

30′

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 °′ °′ °′

STUDY AREA

Orlando

ORANGE

COUNTY

Ocala National Forest

Titusville

Kissimmee

Clermont

POLK

COUNTY

ORANGE COUNTY

Rock Springs

Witherington Springs

Wekiva Springs

St. Johns River

near Cocoa

St. Johns River

near Christmas

Econlockhatchee River

Boggy Creek

Little

Wekiva

River

Wekiva

River

Shingle Creek

Reedy

Creek

Apopka-Beauclair

Canal

Howell Creek

Bonnet

Creek

East Lake Tohopekaliga

Lake

Monroe

Lake

Harney

St. Johns River

Apopka

Lake

Wekiva River

Litt el

i

Wekiva R ver

Lake

Jesup

Soldier Creek

Gee Creek

Howell Cre

ke

Little Econ ol ckha

v

tchee Ri er River

Econlockhatchee

Bo gg y C r e e k

Sh in gle Cre ek

Reedy Creek

Reedy

Lake

Ajay-East

Tohopekaliga

Canal Myrtle-Mary Jane

Canal

Reedy

Creek

EXPLANATION

SURFACE WATER SITE AND NAME

RAIN GAGE -- NOAA (National Oceanic

Atmospheric Administration) at Orlando

International airport and Sanford

International airport

through September (commonly referred to as the wet 

season). For each of these 4 months, the average rainfall 

for the 1931-2000 period exceeded 6 in/mo. The remaining 8 months had average rainfall totals of less than 

4 in./mo (commonly referred to as the dry season). 

The average wet-season rainfall for 1931-2000 was 

28.13 in., and the average dry-season rainfall was 

21.84 in. Hence, more than half of the annual total rainfall generally falls during the wet season. Some dry 

seasons can be relatively wet, as in 1997, when total 

precipitation was 34.2 in., compared to only 30.3 in. 

during the wet season for that year. In 1997, a substantial 

portion of the total dry-season precipitation occurred in 

December (12.63 in.), a normally dry month with an 

average of 2.1 in. of rain. This pattern of occurrence 

indicates that the annual rainfall totals can be dominated 

by a few relatively extreme events.

A considerable amount of variation in annual 

rainfall occurs from year to year, making it difficult to 

determine whether annual rainfall rates have changed 

substantially over time. A cycle of wet and dry periods 

is present, as illustrated by the locally weighted scatterplot smoothing curve (LOWESS) in figure 2. Furthermore, the annual variability of rainfall totals at the 

Orlando and Sanford stations was lower during 1961 

through about 1989 than during the years preceding 

or following this period. No consistent linear trend in 

annual rainfall rates is present for the period of record 

1931-2001. Statistical analysis (Kendall tau) also 

indicates that no long-term trend in rainfall has 

occurred at the Orlando station during the period of 

record (E.R. German, U.S. Geological Survey, written 

commun., 2003).

Figure 1. Location of Orange County, Florida.
6 Hydrogeology and Quality of Ground Water in Orange County, Florida

SANFORD

EXPLANATION

ANNUAL RAINFALL

LOCALLY WEIGHTED SCATTERPLOT

SMOOTHING CURVE

(SMOOTHING FACTOR = 0.28)

MEAN = 50 INCHES AT ORLANDO,

52 INCHES AT SANFORD

ANNUAL RAINFALL, IN INCHES

YEAR

1940 1960 1980 2000

30

40

50

60

70

80

1930 1950 1990 1970

30

40

50

60

70

ORLANDO

80

Land-surface altitudes in Orange County range 

from less than 5 ft above NGVD 29 near the St. Johns 

River to more than 200 ft above NGVD 29 in westcentral Orange County; however, about 75 percent of 

the county has a land-surface altitude ranging between 

70 and 120 ft above NGVD 29. The median altitude is 

about 90 ft above NGVD 29. Topography ranges from 

flat lying in eastern Orange County to gently rolling 

hills in northwestern and southwestern Orange County 

(fig. 3). 

Parts of eight physiographic divisions are located 

in Orange County—the Central Valley, the Eastern 

Valley, the Lake Wales Ridge, the Marion Upland, the 

Mount Dora Ridge, the Orlando Ridge, the Osceola 

Plain, and the Wekiva Plain (White, 1970; fig. 3). The 

Central Valley, Eastern Valley, and Osceola Plain are 

characterized by relatively flat-lying land. The Central 

Valley contains Lake Apopka, one of the largest lakes 

in Florida. The Eastern Valley and Osceola Plain 

historically contained numerous wetlands, many of 

which were drained for development. Wetlands remain 

and (or) were restored in many parts of Orange County, 

including Tosohatchee State Reserve (fig. 1). The Lake 

Wales Ridge, Mount Dora Ridge, and Orlando Ridge 

contain gently rolling hills. These hills generally align 

to form ridges that trend in a northwest-southeast direction (White, 1970). The ridges also are characterized by 

numerous lakes, many of which are seepage lakes with 

no external drainage (fig. 3). The Wekiva Plain and 

Marion Upland are present in northwestern Orange 

County.

Orange County is drained by two major stream 

systems—the St. Johns and Kissimmee River systems. 

The St. Johns River flows along the eastern boundary 

of Orange County. Tributaries of the St. Johns River, 

primarily the Econlockhatchee River and Wekiva 

River, drain about 662 mi2 of eastern and northern 

Orange County. Tributaries of the Kissimmee River—

primarily Bonnett, Boggy, Reedy, and Shingle 

Creeks—drain about 341 mi2 of south-central and 

southwestern Orange County (Lichtler and others, 

1968) (fig. 1). Ground-water seepage provides most of 

the base flow to these streams. In addition, discharge 

from two large springs, Rock and Wekiva (fig. 1), form 

the headwaters of the Wekiva River.

The geology of Orange County consists of metamorphic rocks of Precambrian age overlain by a thick 

sequence of sedimentary rocks ranging from Paleozoic 

to Recent age (Arthur and others, 1994). The sedimentary rocks of Paleocene to Recent age are important 

with respect to the hydrogeology. The oldest rocks in 

this sequence are those of the Cedar Keys Formation of 

Paleocene age. This formation consists primarily of 

marine dolomite with abundant evaporites (gypsum and 

anhydrite). Overlying the Cedar Keys Formation is a 

thick sequence of marine limestones and dolomites of 

Eocene age that contain minor amounts of evaporites.

Figure 2. Annual rainfall at Orlando and Sanford, Florida, 

1931-2001.
Description of the Study Area 7

EA

VA

OSCEOLA

PLAIN

ORLANDO

RIDGE

CENTRAL

VALLEY

LAKE

WALES

RIDGE

Lake

Apopka

Ec

e

o n lo c kh a t ch

e

e Ri v r

L ittl

c

e Econ lo k

h

hat c e e

Ri ve r

Bo gg y Cre e k

S

C

h i

r

ng le e ek

Re ed y Cre ek

MARION

UPLAND

MOUNT

DORA

RIDGE

WEKIVA

PLAIN

Source of topographic data: U.S. Geological Survey Digital Elevation Models.

0 5 10 KILOMETERS

0 5 10 MILES

     



  	











C

A

B

O R A N G E C O U N T Y

EXPLANATION

ALTITUDE, IN FEET ABOVE NGVD 29

Hydrogeologic sections (A-A, B-B, and C-C )shown on figures 13 through 15

0 - 50

50 - 100

100 - 150

150 - 200

200 - 250

A

Figure 3. Physiography (modified from White, 1970) and locations of hydrogeologic sections (figures 13 through 15 of this report)
8 Hydrogeology and Quality of Ground Water in Orange County, Florida

2000

POPULATION, IN THOUSANDS OF PEOPLE

100

200

300

400

500

600

700

800

900

1965 1975 1985 1995

YEAR

0

50

100

150

200

1960

250

300

1,000

1,100

1970 1980 1990

TOTAL

GROUND-WATER

USE

POPULATION

PUBLIC SUPPLY

AGRICULTURAL

USE INDUSTRIAL

USE

DOMESTIC

USE

RECREATIONAL

USE

GROUND-WATER USE,

IN MILLION GALLONS PER DAY

These rocks include the Oldsmar Formation, the Avon 

Park Formation, and the Ocala Limestone. The rocks of 

Eocene age can be more than 2,200 ft thick in Orange 

County. These units generally are fossiliferous and have 

undergone a considerable amount of fracturing and 

dissolution.

Dolomite, limestone, clay, and silt of the 

Miocene-age Hawthorn Group overlie the Ocala 

Limestone. The contact is an erosional unconformity. 

In general, the clay and silt layers have a distinctive 

gray to green color.

Sand, clay, silt, and shell layers of Pliocene 

through Recent age overlie the Hawthorn Group. Many 

such layers were deposited in beach or near-beach 

environments when sea levels were higher than under 

present conditions. The Lake Wales and Mount Dora 

Ridges, which are composed of sand, silt, and clay, 

represent ancient beach environments (Arthur and 

others, 1994).

Most of the above-mentioned geologic units are 

not exposed at the surface in Orange County. Surficial 

geology consists primarily of Pleistocene to Recent 

deposits. The Hawthorn Group is exposed in northwestern Orange County near Rock Springs (Scott and 

others, 2001).

Population, Land Use, and Water Use

Orange County has experienced a large population increase since 1968, mostly as a result of an 

increase in the tourism industry. The population of 

Orange County in 1920 was less than 20,000; the population increased to more than 264,000 by 1960. The 

residential population in 2000 was about 896,000 (U.S. 

Census Bureau, 2002) (fig. 4). In addition to the residential population, about 744,000 tourists visit Orange 

County each week (Orlando Chamber of Commerce, 

written commun., July 1998).

Figure 4. Population (1960-2000; U.S. Census Bureau, 2002) and ground-water use (1965-2000; 

R.L. Marella, U.S. Geological Survey, written commun., 2002) in Orange County, Florida.
Description of the Study Area 9

  




 81°

0 5 10 KILOMETERS

0 5 10 MILES


   80°50





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 ′ ° ° ′ ′

URBAN LAND-USE EXPANSION

FROM 1977 TO 1997

WATER

AGRICULTURE

WETLANDS AND PARKS

EXPLANATION

URBAN LAND USE IN 1977

FOREST, SCRUB, AND

RANGELAND

Land use has changed as a result of increased 

population. A comparison of the extent of urban land use 

in 1977 with the extent of urban land use in 1997 (fig. 5) 

shows that the amount of urban area increased considerably in all directions around the Orlando area and in the 

northwestern part of the county. Urban area increased 

from 142 mi2 (14 percent) in 1977 to 221 mi2 (22 

percent) in 1997. The eastern third of the county has 

seen relatively little increase in urbanization between 

1977 and 1997, especially in the southeast. The increase 

in urban land use has occurred in land previously used 

for range and agriculture. Citrus land use in 1970 

accounted for about 65,960 acres, decreasing to 51,170 

acres in 1978, and to about 8,400 acres in 1990 (Marella, 

1992). A series of freezes in 1983, 1985, and 1989 

destroyed most of the citrus groves, which were replaced 

by urban development as the population growth accelerated. Livestock production similarly has decreased in 

Orange County. The number of cattle decreased from 

36,000 in 1969, to 19,000 in 1978, to 16,000 in 1997, 

and to 14,000 in 2002. The number of hogs decreased 

from 7,700 in 1969 to 790 in 1997 (Jeff Geuder, Florida 

Agricultural Statistics Service, written commun., 2002; 

Florida Agricultural Statistics Service, 2002). In addition, cropland decreased from 108,000 acres in 1969 to 

30,000 acres in 1997 (Jeff Geuder, Florida Agricultural 

Statistics Service, written commun., 2002). Former rowcrop agricultural land north of Lake Apopka recently 

was purchased by SJRWMD as part of the plan to restore 

the lake. Much of this land is being restored to more 

natural conditions, so present (2002) row-crop land use 

in Orange County is reduced greatly from 1997. Agricultural chemical (fertilizers and pesticides) use varies by 

crop and year. For example, the average fertilizer use for 

corn in the 1990s was 105 (lbs/acre)/yr for nitrogen, 

97 (lbs/acre)/yr for phosphorus, and 200 (lbs/acre)/yr for 

potassium. The average fertilizer use for oranges in the 

Figure 5. Generalized land use in Orange County, Florida, in 1997 (source: Orange County Growth Management 

Department), showing expansion of urban land use since 1977 (based on land-use classifications described by 

Anderson and others, 1976).
10 Hydrogeology and Quality of Ground Water in Orange County, Florida

Apopka

Lake

POLK

COUNTY

ORANGE COUNTY

SEMINOLE COUNTY

OSCEOLA COUNTY

LAKE COUNTY VOLUSIA

COUNTY

BREVARD

COUNTY

Rock Springs

G

G

G

Witherington

Springs

Wekiva Springs

EXPLANATION

GSPRING

PUMPING, IN MILLION

GALLONS PER DAY

less than 0.5

0.5 and greater

  




   

0 5 10 KILOMETERS

0 5 10 MILES


   





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 ′ ° ° ′ ′

1990s was 198 (lbs/acre/)yr for nitrogen, 46 (lbs/acre)/yr 

for phosphorus, and 199 (lbs/acre)/yr for potassium. 

Atrazine, a broadleaf herbicide, is applied to corn fields 

at a rate between 1.2 and 1.7 (lbs/acre)/yr; atrazine is not 

used in citrus groves. These numbers are based on 

surveys of growers conducted by the Florida Agricultural Statistics Service (2002). Agricultural chemicals 

also are used in residential settings in maintaining lawns 

and gardens; however, pesticide usage is more highly 

variable depending on the homeowner.

Ground-water use has increased substantially in 

Orange County in the last 35 years. Most of the ground 

water used in Orange County is pumped from the Floridan aquifer system. Total ground-water use in Orange 

County was about 287 Mgal/d in 2000 (fig. 4). In 

comparison, total ground-water use in Orange County 

was 82 Mgal/d in 1965. A large percentage of ground 

water pumped in Orange County is used for public 

water supply, which increased from 63 Mgal/d in 1965 

to 212 Mgal/d in 2000 (R.L. Marella, U.S. Geological 

Survey, written commun., 2002) (figs. 6-7). Of the 

212 Mgal/d of water withdrawn for public water 

supply, about 84 Mgal/d (40 percent) was withdrawn 

from the Upper Floridan aquifer, and 128 Mgal/d 

(60 percent) was withdrawn from the Lower Floridan 

aquifer. Most of the ground water pumped for public 

supply is consumed in Orange County; however, about 

12 percent (26 Mgal/d) of the water pumped for public 

supply is transferred to Brevard County for consumption (R.L. Marella, U.S. Geological Survey, written 

commun., 2002).

The relatively large increase in public supply has 

been partially offset by a decline in agricultural use of 

ground water—from 60 Mgal/d in 1980 to 27 Mgal/d in 

2000. The large increase in agricultural ground-water 

use from 1975 to 1980 represents changes in methods 

to estimate that use (R.L. Marella, U.S. Geological 

Survey, written commun., 2002). The decline in agricultural use after 1985 probably is related to changes in 

land use described previously.

Figure 6. Estimated pumping from public-supply wells tapping the Upper Floridan aquifer in 2000 (modified 

from Spechler and Halford, 2001, and Brian McGurk, St. Johns River Water Management District, written 

commun., 2002).
Data Collection and Analysis 11

Apopka

Lake

ORANGE COUNTY

POLK

COUNTY

SEMINOLE COUNTY

OSCEOLA COUNTY

LAKE COUNTY VOLUSIA

COUNTY

BREVARD

COUNTY

Rock Springs

G

G

G

Witherington

Springs

Wekiva Springs

EXPLANATION

GSPRING

PUMPING, IN MILLION

GALLONS PER DAY

less than 0.5

0.5 and greater

  




   

0 5 10 KILOMETERS

0 5 10 MILES


   





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 ′ ° ° ′ ′

DATA COLLECTION AND ANALYSIS

The ground-water resources of Orange County 

were analyzed by using existing data and new geologic, 

geophysical, water-level, and water-quality data. Existing data were obtained and compiled from various 

Federal, State, and local agencies. Collection of new 

data for this study began in 1999 and continued 

through 2001.

Geologic and geophysical information from 

634 sites were used to construct hydrogeologic 

maps and sections. This information, obtained from 

USGS, SJRWMD, and the Florida Geological Survey, 

included borehole geophysical logs and lithologic 

descriptions of well cuttings by geologists and drillers. 

Analysis included quality assurance of the data and 

verification of well locations. Some wells had been 

assigned a different site identifier by the three agencies. 

Duplicate site information was removed. The reported 

location of a well was checked to verify that the well 

plotted in a reasonable location, and that the latitudelongitude location corresponded with the townshiprange location. The reported land-surface altitude was 

checked against the actual altitude of the location. In 

addition, seven observation wells that tap the Upper 

Floridan aquifer were geophysically logged as part of 

this study (table 1). Geophysical logs included temperature, fluid resistivity, caliper, natural gamma, and electric.

Maps were constructed by determining the depth 

to geologic or hydrogeologic contacts at each well 

location. The top of the Upper Floridan aquifer was 

considered to be the contact between Eocene-age limestones (generally, the Ocala Limestone) and overlying 

Miocene-age sediments (primarily, the Hawthorn 

Group). The top of the intermediate confining unit was 

considered to be the top of the Hawthorn Group, and 

any overlying clay units of Pliocene or Pleistocene age, 

if present. These definitions are consistent with those 

Figure 7. Estimated pumping from public-supply wells tapping the Lower Floridan aquifer in 2000 (modified 

from O’Reilly and others, 2002; Spechler and Halford, 2001; and Brian McGurk, St. Johns River Water 

Management District, written commun., 2002).
12 Hydrogeology and Quality of Ground Water in Orange County, Florida

of previous hydrogeologic studies in central Florida 

(Murray and Halford, 1996; Spechler and Halford, 

2001; Knowles and others, 2002).

Natural gamma logs, which measure the natural 

radioactivity of rock units, were useful for determining 

these geologic contacts. The Hawthorn Group contains 

radioactive minerals that produce an inflection or spike 

in the natural-gamma log (Johnson, 1984; Keys, 1988). 

The beginning and end of this spike generally correspond to the upper and lower contacts, respectively, of 

the Hawthorn Group.

A total of 18 wells (app. 1) that tap the surficial 

aquifer system was installed throughout Orange 

County from August 1999 through February 2000. 

Twelve of the wells were installed near existing wells 

that tap the Upper Floridan aquifer to provide a 

comparison of water levels and water quality between 

the two aquifers. Thirteen of the wells were drilled with 

rotary mud methods, three of the wells were bored with 

hollow stem auger, and two wells were installed by 

using hand augers. At 13 sites, a test hole was drilled to 

the top of the intermediate confining unit, and split 

spoon samples were collected every 5 ft for lithologic 

analysis. In general, wells were completed with a 20/30 

U.S. Standard Sieve size sand filter pack to about 5 ft 

above the top of the screen, followed by a 2-ft-thick 

bentonite seal, then the remaining annular space was 

grouted with type I Portland cement. Final depth of the 

wells tapping the surficial aquifer system ranged from 

9 to 75 ft; screen length ranged from 5 to 20 ft. The 

newly installed wells were developed by pumping until 

the water became clear and (or) specific conductance 

stabilized.

From February through March 2001, slug tests 

were performed on 4 existing observation wells and 16 

of the newly installed observation wells to measure the 

horizontal hydraulic conductivity of the surficial aquifer system. The slug tests consisted of rapidly inserting 

and (or) removing a slug from the water column in the 

well and measuring the change of water level in the 

well over time. The slug was a sand-filled polyvinyl 

chloride pipe lowered with a rope. The water level in 

the well would either fall or rise to return to pre-test 

levels, when the slug was inserted or removed, respectively. Two to six tests were performed at each well. 

Water levels in 37 wells (23 wells tapping the 

surficial aquifer system, 2 wells tapping the intermediate confining unit, and 12 wells tapping the Upper 

Floridan aquifer) were measured bimonthly from 

March 2000 through September 2001. Three of the 

wells tapping the surficial aquifer system and one of 

the wells tapping the Upper Floridan aquifer also were 

instrumented with pressure transducers for the continuous monitoring of water levels. All wells in Orange 

County with water-level data are shown in figure 8.

Water samples collected from a total of 121 wells 

(fig. 9; table 2; apps. 2-4) were used to characterize 

ground-water quality in Orange County. These data, a 

subset of the total water-quality samples available, 

were selected to provide a relatively even geographic 

distribution of data. The large number of samples available around the Cocoa well field (apps. 1 and 2), for 

example, would statistically bias the results, and, 

hence, not all available data were used in the analyses.

Table 1. Summary of borehole geophysical data collected from wells tapping the Upper Floridan aquifer

[--, no data; cal, caliper log; gam, natural gamma log; res, fluid resistivity; temp, water temperature; elec, electric logs including 

spontaneous potential and long- and short-normal resistivity; UF, Upper Floridan aquifer]

Site identifier Site 

number Site name

Elevation 

(feet above 

NGVD 29)

Casing 

length 

(feet)

Depth of 

well (feet 

below land 

surface)

Types of 

logs

282434081283102 45 Sea World Replacement 103.16 158 239 cal, gam, res

282640080565901 180 Tosohatchee UF well 22 -- 203 gam

282739081054501 82 Cocoa F 67.29 200 375 cal, gam, res

282835081305201 86 Palm Lake well 152.00 161 235 temp, cal, gam, res, elec

283003081283901 101 Turkey Lake UF well 125 -- 305 cal, gam 

283253081283401 126 OR-47 81.71 328 350 temp, cal, gam, res

284529081301001 170 Rock Springs UF well 71.90 143 365 temp, cal, gam, res, elec
Data Collection and Analysis 13

Apopka

Lake

POLK

COUNTY

ORANGE COUNTY

SEMINOLE COUNTY

OSCEOLA COUNTY

LAKE COUNTY

VOLUSIA

COUNTY

BREVARD

COUNTY

RAPID

INFILTRATION

BASIN

(Reedy Creek

Improvement

District)

Water

Conserv II

130

104

40

67

65 64

66

53 55

106

134

115

131

151

156

127 126

114

94

103

72

63

15

3

6

45

32

50

24

68

9

162

170

122

110

90

82

54

47 27 26 30

87

92

79

96

101

86 99

81

160

175174

105

59

169

123

176

29

172 177

161

149

144

135

125 120

108

100

117

95

31

74

25

17 12

10 4

48

16

93 89

88

124

91

84

83

141

Wekiva

Springs

Rock

Springs

Witherington

Spring

G

G

G

  




   

0 5 10 KILOMETERS

0 5 10 MILES


   





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 ′ ° ° ′ ′

EXPLANATION 17 SURFICIAL AQUIFER SYSTEM WELL

INTERMEDIATE CONFINING UNIT WELL

UPPER FLORIDAN AQUIFER WELL

LOWER FLORIDAN AQUIFER WELL

WELLS WITH CONTINUOUS DATA

(Site numbers refer to appendixes)

40

24

59

G SPRING

Water-quality samples were collected specifically for this project by the USGS from April 1999 

through May 2001 (fig. 9; apps. 1-4). These samples 

were collected to: (1) assess the overall ground-water 

quality throughout Orange County; and (2) determine 

the occurrence and distribution of selected constituents 

such as nutrients and pesticides. Samples were 

collected once from 26 wells tapping the surficial aquifer system, 1 well tapping the intermediate confining 

unit, 24 wells tapping the Upper Floridan aquifer, and 

8 wells tapping the Lower Floridan aquifer. In addition, 

samples were collected once from Wekiva Springs 

(site 1) and twice from Rock Springs (site 2).

Water samples were collected from wells by 

using either a permanently installed pump or a portable 

2-in-diameter stainless steel pump equipped with fluorinated ethylene propylene (FEP) tubing. Samples from 

Rock Springs and Wekiva Springs were collected by 

using a pump equipped with a polytetrafluoroethylene 

(PTFE) diaphragm pump head and FEP tubing. 

At Rock Springs, the tubing was inserted directly into 

the spring orifice; at Wekiva Springs, the water was 

collected directly from the main orifice using a FEP 

bailer, then composited into a churn splitter prior to 

processing. Field measurements (temperature, specific 

conductance, dissolved oxygen, and pH) of ground 

water were made at all well sites by using a flowthrough chamber and following USGS protocols 

(Wilde and Radtke, 1998). Samples were collected 

after purging the well of at least three casing volumes 

and (or) stabilization of the field measurements. Field 

measurements were made at both spring sites by inserting the instrument probes directly into the spring 

orifice.

Samples were collected for laboratory analysis 

of alkalinity, major ions, silica, selected trace elements, 

radon, total organic carbon, and nutrients including 

nitrite as nitrogen, nitrite plus nitrate as nitrogen, 

ammonia as nitrogen, ammonia plus organic nitrogen 

as nitrogen (hereafter referred to as nitrite, nitrate, 

Figure 8. Locations of wells with periodic and continuous water-level data and geophysical data.
14 Hydrogeology and Quality of Ground Water in Orange County, Florida

Apopka

Lake

POLK

COUNTY

ORANGE COUNTY

SEMINOLE COUNTY

OSCEOLA COUNTY

LAKE COUNTY VOLUSIA

COUNTY

BREVARD

COUNTY

Wekiva

Springs (1)

Rock

Springs (2)

165

41

77

49

40

64 61

55

53

104 106

137 136

129

143

146

111

102

98

85

139

121

145

152

109

13

97

81

107

128 127 126

148

158

159

160

163 167

164 162

166 168

171

179

177 178 176 170

150

105

72

36

3

15 19 22

23

39 33

44

63

58 70

73 62

56 57

60

51

5 7 11 18 14 20 21

28 38 37

42 43

46

52 69

71

76 78

75

82

110

112

122

90

132 140

119 118

116

87

180

92

30

147

138

133

24

G

G

168

  




   

0 5 10 KILOMETERS

0 5 10 MILES


   





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 ′ ° ° ′ ′

8384

113

124

141

16

31

95

117

120

135

144

125

108

100

142

149

154 153

155

161

157

80

35 34

10

17

8

74

4

29

88

93

172

G

EXPLANATION

SURFICIAL AQUIFER SYSTEM WELL

INTERMEDIATE CONFINING UNIT WELL

UPPER FLORIDAN AQUIFER WELL

LOWER FLORIDAN AQUIFER WELL

(Site numbers refer to appendixes)

4

41

3

168

UPPER FLORIDAN AQUIFER SPRING

Figure 9. Locations of wells and springs where water-quality data were collected from 1990 through 2001.

Table 2. Summary of well construction information, by 

aquifer, for wells with water-quality records

[Altitude, in feet above sea level; casing length in feet; well depth 

in feet below land surface]

Number 

of 

wells

Minimum Median Maximum

Surficial aquifer system 34

Land surface altitude 14 93 146

Casing length 3.7 20 54

Well depth 8.7 30 74

Upper Floridan aquifer 62

Land surface altitude 14 71 152

Casing length 10 169 620

Well depth 40 357 840

Lower Floridan aquifer 25

Land surface altitude 59 97 145

Casing length 602 1,060 1,428

Well depth 1,200 1,390 2,440

ammonia and ammonia plus organic nitrogen, respectively), phosphorus, and orthophosphate as phosphorus 

(hereafter referred to as phosphate) (table 3). Samples 

from selected sites were collected for analysis of stable 

isotopes (deuterium and oxygen-18) and (or) 46 pesticides and pesticide metabolites (app. 4) (Zaugg and 

others, 1995). All samples were processed according to 

USGS protocols (Wilde and others, 1999). Samples for 

major-ion, nutrient, and trace-element analyses were 

filtered with a 0.45-μm pore-size disposable capsule 

filter. Samples for major-cation and trace-element analyses were acidified with 2 milliliters nitric acid to 

adjust sample pH to less than 2. Samples for pesticide 

analysis were filtered with a 0.7-μm pore-size baked, 

glass-fiber filter in an aluminum filter plate because 

solid phase extraction of the sample is required for the 

low detection limits (Zaugg and others, 1995). Wholewater samples for low-level constituents such as trace 

elements and pesticide could have concentrations that
Data Collection and Analysis 15

Table 3. Analytical methods and drinking water standards for inorganic analytes included in the study

[Abbreviations used for method of analysis: AA, atomic absorption spectrometry; C, colorimetry; G, residue evaporation at 

180 degree Celsius; I, combustion-infrared method; IC, ion-exchange chromatography; ICP, induction-coupled argon plasma 

atomic emission spectrometry; ISE, ion-selective electrode; SC, scintillation; T, end-point titration. Abbreviations used for 

drinking water standards: MCL, Maximum Contaminant Limit (National Primary Drinking Water Regulations); SS, National 

Secondary Drinking Water Regulations; TT, Treatment Technology (National Primary Drinking Water Regulations). mg/L, 

milligrams per liter; μg/L, micrograms per liter; pCi/L, picoCuries per liter]

Constituent

Method 

of 

analysisa

aFishman and Friedman (1985) and Ocala Water Quality and Research Laboratory (2002).

Reporting 

limita

Drinking water 

standardsb

bU.S. Environmental Protection Agency (2002).

Calcium, mg/L ICP 0.02 None

Magnesium, mg/L ICP .03 None

Sodium, mg/L AA .1 None

Potassium, mg/L AA .1 None

Alkalinity, mg/L T 1.0 None

Sulfate, mg/L IC .2 250 (SS)

Chloride, mg/L IC .1 250 (SS)

Fluoride, mg/L ISE .1 4.0 (MCL), 2.0 (SS)

Bromide, mg/L IC .05 None

Silica, mg/L ICP .01 None

Total dissolved solids, mg/L G 1.0 500 (SS)

Nitrite plus nitrate, as nitrogen, mg/L C .02 10 (MCL)

Nitrite, as nitrogen, mg/L C .01 1 (MCL)

Ammonia, as nitrogen, mg/L C .01 None

Ammonia plus organic nitrogen, 

as nitrogen, mg/L

C .2 None

Phosphorus, mg/L C .02 None

Phosphorus, ortho, as phosphorus, mg/L C .01 None

Organic carbon, total, mg/L I .1 None

Aluminum, μg/L ICP 3.0 50–200 (SS)

Arsenic, μg/L ICP 1.0 50 (MCL)c; 10 (MCL)d

cCurrent as of 2003.

dEffective January 23, 2006.

Barium, μg/L ICP .5 2,000 (MCL)

Beryllium, μg/L ICP 1.0 4 (MCL)

Boron, μg/L ICP 2.0 None

Cadmium, μg/L ICP .5 5 (MCL)

Chromium, μg/L ICP 1.0 100 (MCL)

Cobalt, μg/L ICP 1.0 None

Iron, μg/L ICP 2.0 300 (SS)

Lead, μg/L ICP 2.0 15 (TT)

Lithium, μg/L ICP 1.0 None

Manganese, μg/L ICP 1.0 50 (SS)

Selenium, μg/L ICP 1.0 50 (MCL)

Strontium, μg/L ICP .5 None

Vanadium, μg/L ICP 1.0 None

Radon, pCi/L SC 20.0 None
16 Hydrogeology and Quality of Ground Water in Orange County, Florida

differ from filtered samples because some of these 

constituents will sorb onto sediment and colloids in the 

water. All samples were chilled to less than 4 oC and 

shipped overnight to USGS laboratories in Ocala, Fla., 

and Denver, Colo., for analysis.

Sampling equipment was washed with a 

nonphosphatic detergent and rinsed with deionized 

water to prevent cross contamination. Except for the 

stainless steel pump, equipment used for the collection 

and processing of trace-element samples also was 

rinsed with a 5-percent solution of laboratory-grade 

hydrochloric acid and rinsed with deionized water. 

Equipment used for the collection and processing of 

pesticide samples was rinsed with laboratory-grade, 

pesticide-free methanol and rinsed with laboratorygrade, pesticide-free water. A total of five field-blank 

and five replicate (sequential) samples was collected 

for quality assurance purposes. In addition, sourcesolution blank samples were submitted with field-blank 

samples to verify the quality of the blank water. Fieldblank samples were analyzed for nutrients, trace 

elements, and pesticides. In general, concentrations of 

nearly all constituents were less than method detection 

limits in all field-blank samples. In one field-blank 

sample, manganese was detected at a concentration of 

0.4 μg/L, and in a second field-blank sample, barium 

was detected at the method reporting limit of 0.2 μg/L. 

Ammonia also was detected in one field-blank sample, 

and phosphate was detected in two field-blank samples 

at low concentrations (0.02 and 0.03 mg/L), which is 

near the method reporting limit. However, ammonia 

and phosphate also were detected in the associated 

source-solution blank samples at similar concentrations, indicating the contamination was not a result of 

contamination from the sampling equipment.

Data for replicate samples from the same well 

were in good agreement. For example, replicate 

samples from a well (site 13, fig. 9) had similar concentrations of nutrients (ammonia was 0.19 mg/L in both 

samples), trace elements (barium was 12 μg/L and 

boron was 37 μg/L in both samples; iron was 850 and 

865 μg/L, and lithium was 4.7 and 4.6 μg/L in the 

samples), and pesticides (all concentrations were below 

detection limits). Concentrations of radon, an inert 

radioactive gas, exhibited the largest differences 

between replicate samples. In the same well (site 13, 

fig. 9), replicate samples had radon concentrations of 

175 and 167 pCi/L, a difference of 7 pCi/L, which 

represents less than a 5-percent error.

Water-quality data collected for this project were 

supplemented with data from 60 samples, primarily 

field measurements and concentrations of major ions 

and nutrients, compiled from the USGS and SJRWMD 

databases (apps. 1-4). The samples were collected 

from 1990 through 2000 and analyzed with a variety of 

methods, including the collection of whole-water 

samples. Some sites were sampled more than one time 

during the period of record, in which case only the 

most recent and most analytically complete sample 

was included in this report. Sites also were selected to 

establish an even spatial distribution in Orange County. 

Both whole-water and filtered analyses of major ions 

and nitrate were included in the summary because the 

differences between concentrations of those constituents in whole-water and filtered samples generally are 

very slight. Bicarbonate concentrations were calculated from alkalinity. Data reported as nitrite plus 

nitrate as nitrate were converted to concentrations as 

nitrogen, and were merged with the data collected 

during this study.

Regression analysis indicated that total dissolved 

solids (TDS) concentrations of ground-water samples 

were significantly correlated to specific conductance 

(r2 >0.99) (fig. 10). If TDS concentrations were not 

available, TDS was calculated as a function of specific 

conductance by using the equation:

log(TDS) = 1.02*[log(specific conductance)]-0.30, (1)

where 

TDS is in milligrams per liter, and

specific conductance is in microiemens per centimeter.

This equation is valid, however, only for values of 

specific conductance greater than 2 μS/cm. The error is 

the result of the nonlinear relation between low values 

of specific conductance and TDS. The minimum value 

of specific conductance in this study was 50 μS/cm, 

with a TDS concentration of 34 and a predicted TDS 

concentration of 27 mg/L. Hence, for the range of values 

in this study, the equation to estimate TDS concentrations from specific conductance is acceptable.

Data were analyzed by using nonparametric 

statistical tests. The Mann-Kendall (Kendall’s tau) test, 

a nonparametric correlation procedure, and locally 

weighted scatterplot smoothing (LOWESS) were used 

to analyze long-term data for temporal trends in rainfall, water levels in wells, and discharge from springs 
Hydrogeology 17

100 1,000 10,000 10

100

1,000

SPECIFIC CONDUCTANCE,

IN MICROSIEMENS PER CENTIMETER

10,000

TOTAL DISSOLVED SOLIDS,

IN MILLIGRAMS PER LITER

Surficial aquifer system

Upper Floridan aquifer

Lower Floridan aquifer

EXPLANATION

(Helsel and Hirsch, 1992). Spearman’s rho, a rankorder correlation coefficient, was used to assess relations between chemical constituents. The KruskalWallis test, a nonparametric analysis of variance 

performed after ranking the data (Helsel and Hirsch, 

1992), was used to assess differences in concentrations 

of chemical constituents between aquifers. For all 

statistical tests, a probability level of 5 percent (p-value 

<0.05) was the criterion for statistical significance. 

A p-value <0.05 indicated that the probability of a 

correlation or difference occurring by chance is less 

than 5 percent.

Boxplots were used to illustrate the distribution 

of selected chemical concentrations in the aquifers. 

Maps were used to illustrate the spatial distribution of 

selected hydrogeologic properties and water-quality 

constituents. Water-quality data were compared to 

drinking water regulations established by the U.S. 

Environmental Protection Agency. National Primary 

Drinking Water Regulation are enforceable, healthbased standards that limit the concentrations of certain 

chemical constituents (Maximum Contaminant Levels 

(MCL)) in drinking water. National Secondary Drinking Water Regulation (also referred to as secondary 

standards) are nonenforceable guidelines set for 

cosmetic or aesthetic reasons (U.S. Environmental 

Protection Agency, 2002).

HYDROGEOLOGY

 The geology of central Florida affects the occurrence and movement of ground water in Orange County. 

The area is underlain by about 6,500 ft of marine limestones and dolomites that were deposited on a basement complex of crystalline rock (Lichtler and others, 

1968; Arthur and others, 1994). The upper 2,500 ft of 

sediments are important to the ground-water resources 

of the area.

Hydrogeologic Setting

The geologic formations of interest in central 

Florida are, from oldest to youngest: the Cedar Keys 

Formation (of Paleocene age), the Oldsmar Formation, 

the Avon Park Formation, and the Ocala Limestone (all 

of Eocene age), the Hawthorn Group (of Miocene age) 

and undifferentiated post-Miocene sediments (fig. 11). 

Hydrogeologic units in the area include two aquifer 

systems (the Floridan aquifer system and the surficial 

aquifer system) and an intermediate confining unit. 

The Cedar Keys Formation forms the sub-Floridan 

confining unit.

Some differences are noted between the names 

of geologic units described by Lichtler and others 

(1968, table 2) and those used in this report (fig. 11). 

The unit previously called the Hawthorn Formation is 

now recognized by the Florida Geological Survey as 

the Hawthorn Group and the unit called the Ocala 

Group is now called the Ocala Limestone. Miller 

(1986) determined that there was no paleontological 

basis for differentiating the Avon Park and Lake City 

Limestones, so the thickness of the Avon Park Formation (fig.11) includes the combined thickness of both 

units. Also, because dolomites are an important constituent, the unit should be called a formation rather than a 

limestone. Finally, thickness values for units shown in 

figure 11 may differ from values given by Lichtler and 

others (1968) because data from more recently drilled 

wells were incorporated.

Aquifer nomenclature also has been modified 

since the study by Lichtler and others (1968). The 

nonartesian and secondary artesian aquifers in previous 

studies are now considered to be part of the surficial 

aquifer system and the intermediate confining unit, 

respectively, which separates the surficial aquifer 

system from the Floridan aquifer system. The Floridan 

aquifer system has been divided into the Upper and 

Lower Floridan aquifers, separated by the middle 

semiconfining unit.

Figure 10. Relation between specific conductance and total 

dissolved solids in ground-water samples collected from 

1990 through 2001.
18 Hydrogeology and Quality of Ground Water in Orange County, Florida

*Thickness includes saturated and unsaturated undifferentiated sedimentary deposits overlying the intermediate confining unit. QUATERNARY TERTIARY

RECENT

UNDIFFERENTIATED

PLEISTOCENE DEPOSITS

PLIOCENE

MIOCENE

UPPER

MIDDLE

LOWER

PALEOCENE EOCENE

HAWTHORN

GROUP

OCALA

LIMESTONE

AVON PARK

FORMATION

OLDSMAR

FORMATION

CEDAR KEYS

FORMATION

SURFICIAL AQUIFER

SYSTEM

INTERMEDIATE

CONFINING

UNIT

UPPER

FLORIDAN

AQUIFER

MIDDLE

SEMICONFINING

UNIT

LOWER

FLORIDAN

AQUIFER

FLORIDAN AQUIFER SYSTEM

SUB-FLORIDAN

CONFINING UNIT

LESS THAN

50 - 200

*

10 - GREATER

THAN 200

300 - 400

400 - 700

1,000 - 1,300

POORLY

KNOWN

Quartz sand, unconsolidated,

locally can contain clay, chert

shell, and limestone lenses.

Clay, green-gray color with

phosphatic pebbles, silt,

locally can contain sand and

carbonate units.

Limestone, white, tan, or

cream, fossiliferous, soft to

hard, locally dolomitic, highly

porous.

Limestone, light brown to

brown porous, fossiliferous,

locally dolomitic and

carbonaceous; intergranular

gypsum and anhydrite; brown,

crystalline dolomite.

Limestone, white to light

brown, porous, fossiliferous,

interbedded with dolomite,

minor amounts of gypsum

present.

Dolomite, light gray, with

significant amounts of gypsum

and anhydrite.

MIDDLE

CONFINING

UNIT

APPROXIMATE

THICKNESS

(feet)

SYSTEM

HYDROGEOLOGIC

LITHOLOGY UNIT

STRATIGRAPHIC

SERIES UNIT

In an area underlain primarily by carbonate 

(limestone and dolomite) rocks, sinkholes are common, 

resulting in a landform called karst. Rainfall is naturally acidic and, over long time periods, gradually 

dissolves the limestone, resulting in the formation of 

enlarged conduits and cavities. The cavities may 

subside either gradually or suddenly, forming dolines 

or sinkholes. Sinkholes generally form more rapidly in 

areas of active ground-water recharge because the acidity of the water (and thus its ability to dissolve limestone) is greatest when the water first enters the aquifer. 

Ground-water recharge can occur at rapid rates in areas 

where sediments overlying the limestone are thin or 

very permeable. Such conditions are present in Orange 

County, particularly in the western and northwestern 

parts. Sinkholes frequently occur near the end of the 

dry season, when ground-water levels are low and 

withdrawals from the aquifers are at the annual peak 

rate. Sinkholes can be dry or water-filled, internally 

drained depressions. Most of the natural lakes and 

ponds in Orange County probably were formed by this 

process (Lichtler and others, 1968).

A large sinkhole formed in May 1981 in the city 

of Winter Park. The sinkhole was about 100 ft deep and 

nearly 300 ft across. Initially after the collapse of the 

sinkhole, its sides were steep and the water level in the 

sinkhole was at equilibrium with the water level in the 

underlying limestone. Under natural conditions, the 

bottoms of many sinkhole lakes eventually become 

covered with less permeable clay or silt, “plugging” 

Figure 11. Geologic and hydrogeologic units in Orange County, Florida (modified from Tibbals, 1990, and 

O’Reilly and others, 2002).
Hydrogeology 19

the sinkhole’s direct connection to the underlying limestone. In the Winter Park sinkhole (as in other sinkholes that form in urban areas), fill was emplaced in 

the bottom of the sinkhole and after about 14 years, it 

resembled other sinkhole-formed lakes in Orange 

County (Schiffer, 1998).

The features apparent at land surface in central 

Florida are not the only manifestation of the karstification process. Because of the thick sequence of carbonate rocks, at numerous times during the geologic past 

the rocks that were at land surface at any given time 

were eroded into karst landforms. Numerous horizons 

of paleokarst features, such as fractures and large cavities, are evident when drilling into the subsurface 

formations. These features can cause the wide variations in hydraulic properties observed in the Floridan 

aquifer system.

Surficial Aquifer System

The surficial aquifer system generally consists of 

unconsolidated sand ranging in age from Pliocene to 

Recent (fig. 11). Shell material is present in parts of the 

aquifer system in eastern Orange County and layers of 

silt or clay exist locally. A layer of peat was penetrated 

during well drilling in central Orlando, and hardpan 

also is present at some locations. Most of the surficial 

sediments were deposited in beach or nearshore environments when sea level was higher than at present and 

sediments may have been reworked as sea levels fluctuated during the geologic past.

The surficial aquifer system in Orange County 

ranges widely in thickness. Sediments composing the 

surficial aquifer system are less than 50 ft thick in 

many parts of southwestern and northwestern Orange 

County and more than 150 ft thick in parts of central 

and southeastern Orange County (figs. 12 through 15). 

Values shown on figure 12 represent the thickness 

of both saturated and unsaturated, undifferentiated 

sediments overlying the intermediate confining unit. 

The thickness of saturated sediments (and, thus, the 

surficial aquifer system) fluctuates as the water-table 

altitude fluctuates over time.

The thickness of the surficial aquifer system can 

vary widely over short areal distances; hence, the thickness shown in figure 12 is generalized. For example, in 

southeastern Orange County, thickness values range 

from about 35 to 170 ft in a distance of less than 5 mi. 

Some wells, apparently drilled in sinkholes, indicate 

anomalously large thickness values and were not used 

to construct thickness contours, but are plotted on 

figure 12 to illustrate the variability in thickness.

The variability in thickness values probably 

results from the sediments originally being deposited 

upon an uneven paleokarst surface. Additionally, the 

presence of locally discontinuous clay lenses and the 

lithologic gradation of geologic units over short areal 

distances can cause the lower boundary of the surficial 

aquifer system to be at different depths in the stratigraphic section at different locations. For example, a 

sedimentary layer near the bottom of the surficial aquifer system could grade from sand at one location to 

clay at a nearby location. In this case, the layer would 

be included in the surficial aquifer system at the first 

location but considered to be part of the intermediate 

confining unit at the second location.

The horizontal hydraulic conductivity of the 

surficial aquifer system ranges from 0.05 to 30 ft/d, 

with a median of 3 ft/d (fig. 16 and table 4). These 

values are based on 20 slug tests made by the USGS 

(S. Kinnaman, U.S. Geological Survey, written commun., 

2001) and 1 test made by SJRWMD (Wanielista and 

others, 1992) on 21 wells tapping the surficial aquifer 

system. The tests performed by the USGS were analyzed by using the Bouwer and Rice (1976) method. 

These values are in good agreement with estimates 

from other studies conducted in central Florida. Hydraulic conductivity values for the surficial aquifer system 

ranged from 0.5 to 40 ft/d, with a median of 5 ft/d in 

adjacent Seminole County (Spechler and Halford, 

2001), and from 0.2 to 35 ft/d, with a median of 8 ft/d 

in adjacent Lake County and the nearby Ocala National 

Forest (Knowles and others, 2002). The wide range in 

values probably is the result of lithologic variations in 

the aquifer materials. Relatively high horizontal hydraulic conductivity values occur throughout Orange 

County, including 9 ft/d in the northwestern part of 

the county (site 161), 20 ft/d in southeastern Orange 

County (site 29), and 30 ft/d in central Orlando (site 

120; fig. 16). The sites with the lowest values of horizontal hydraulic conductivity (0.05 ft/d at site 100 and 

0.06 ft/d at site 172) are located in western and northwestern Orange County, respectively (fig. 16). The low 

values probably result from the lithology at the sites 

(primarily fine sand with silt and clay).

In general, water in the surficial aquifer system is 

unconfined, and the altitude of water levels in wells 

tapping the aquifer system represents the water table.
20 Hydrogeology and Quality of Ground Water in Orange County, Florida Apopka Lake ORANGE COUNTY

POLK

COUNTY

SEMINOLE COUNTY

OSCEOLA COUNTY

LAKE COUNTY

Rock

Springs

G

GG

Witherington

Springs Wekiva

Springs

C

B

B

A

100

100

50

100

50

50

100

50

50

50

50

50

50

50

50

100

50

100

100

50

100

50

100

50

100

100

50

50

50

50

100

50

150 100

50

50

100

50

100

150

50

100

150100

150

50

100

150

50

100

50

100

A A

50

WELL IN APPARENT SINKHOLE OR COLLAPSEFEATURE, NOT USED TO CONSTRUCT CONTOURS

WELL

EXPLANATION

HYDROGEOLOGIC SECTIONS.A-A, B-B, and C-C shown in figures 3 and 13-15LINE OF EQUAL THICKNESS OF THEUNDIFFERENTIATED SEDIMENTS.CONTOUR INTERVAL IS 50 FEET

0 5 10 KILOMETERS

0 5 10 MIL

Sources of data: geophysical and geological information from the U.S. Geological survey,St. Johns River Water Management District, and Florida Geological Survey


 






Figure 12. Generalized thickness of undifferentiated sedimentary deposits overlying the intermediate confining unit.
Hydrogeology 21 Figure 13. Generalized hydrogeologic section A-A' (trace of section is shown on figures 3 and 12) (thickness of middle semiconfining unit 

modified from O’Reilly and others, 2002). VERTICA

LANDSURFACE

AQUIFER SYSTEM

UPPER FLORIDAN AQUIFER

MIDDLE SEMICONFINING UNIT

LOWER FLORIDAN AQUIFER

SUB-FLORIDAN CONFINING UNIT

Approximate location of250 mg/L isochlor

Econlockhatchee River

Little Econlockhatchee River

INTERMEDIATE CONFINING UNIT SURFICIAL

Approximate location of5,000 mg/L isochlor

FEET 

Source of isochlor information: O’Reilly and others, 2002.

WEST 

A 

NGVD 29

200

1,0002,0002,400

400600800

1,2001,4001,6001,8002,200

0 5 10 KILO

0 5 

200
22 Hydrogeology and Quality of Ground Water in Orange County, Florida

0 5 10 KILOMETERS

0 5 10 MILES

LAND

SURFACE

UPPER FLORIDAN AQUIFER

MIDDLE SEMICONFINING UNIT

LOWER FLORIDAN AQUIFER

SUB-FLORIDAN CONFINING UNIT

Approximate location of

5,000 mg/L isochlor

Approximate location of

250 mg/L isochlor

VERTICAL SCALE GREATLY EXAGGERATED

SURFICIAL AQ T

INTERMEDIATE CONFINING UNIT UIFER SYS EM

200

NGVD 29

200

400

600

800

1,000

1,200

1,400

1,600

1,800

2,000

2,200

200

200

400

600

800

1,000

1,200

1,400

1,600

1,800

2,000

2,200

Source of isochlor information: O’Reilly and others, 2002.

NGVD 29

B B

2,400 2,400

FEET FEET

NORTH SOUTH

Water-level altitudes during this study ranged from 

about 10.6 ft (site 93) in eastern Orange County to 123.8 ft 

above NGVD 29 (site 161) in northwestern Orange 

County. Water in the surficial aquifer system flows 

laterally from areas of higher altitude to discharge at 

lakes or streams in areas of lower elevation. The altitude 

of the water table generally mimics land surface and is 

higher beneath hilltops than in adjacent low-lying areas. 

At streams and lakes, the altitude of such features represents the altitude of the water table at those locations.

At some locations, flow at the water table of the 

surficial aquifer system can be complex. For example, 

the well at site 172, had water levels ranging from 82.5 

to 89.6 ft above NGVD 29. Lake Lerla, about 1.8 mi 

east of the well, has a surface altitude of about 61 ft 

above NGVD 29. Lake Smith, however, is about 1.2 mi 

west of the well and has a surface altitude of about 

143 ft above NGVD 29 (fig. 16). The difference in 

hydraulic gradient between the well and the respective 

lakes could indicate that ground water in the area of 

Lake Smith is perched locally by a discontinuous 

impervious layer, such as a clay lens.

Ground water in the surficial aquifer system is 

recharged by precipitation throughout most of Orange 

County. In eastern Orange County, the surficial aquifer 

system also is recharged by upward leakage of water 

from the Floridan aquifer system. The surficial aquifer 

system is artificially recharged by inflow from septic 

Figure 14. Generalized hydrogeologic section B-B' (trace of section is shown on figures 3 and 12) (thickness of middle 

semiconfining unit and Lower Floridan aquifer modified from O’Reilly and others, 2002).
Hydrogeology 23

Approximate location of

5,000 mg/L isochlor

MIDDLE SEMICONFINING UNIT

UPPER FLORIDAN AQUIFER

LOWER FLORIDAN AQUIFER

SUB-FLORIDAN

CONFINING UNIT

200

NGVD 29

200

400

600

800

1,000

1,200

1,400

1,600

1,800

2,000

2,200

2,400

200

200

400

600

800

1,000

1,200

1,400

1,600

1,800

2,000

2,200

2,400

C

FEET

C

FEET

Source of isochlor information: VERTICAL SCALE GREATLY EXAGGERATED

O’Reilly and others, 2002.

LAND

SURFACE

SURFICIAL

AQUIFER

SYSTEM

INTERMEDIATE

CONFINING UNIT

NGVD 29

0 5 10 KILOMETERS

0 5 10 MILES

NORTH SOUTH

systems and rapid infiltration basins (RIBs) used to 

dispose of treated wastewater effluent. The surficial 

aquifer system also is artificially recharged by irrigation 

of agricultural land or residential areas with either 

treated wastewater or water pumped from the Floridan 

aquifer system. Water discharges from the surficial aquifer system by evapotranspiration, lateral flow to lakes 

and streams, leakage downward to the Floridan aquifer 

system, and by withdrawals from wells.

The altitude of the water table in the surficial 

aquifer system varies seasonally, responding to variations in rates of recharge and discharge, and generally 

is highest after the wet season and lowest after the dry 

season. The water-level fluctuations in some wells 

(sites 10 and 88; fig. 17) indicate that the water table in 

the surficial aquifer system generally rises rapidly, 

probably as a result of rainstorms. The difference in 

responses probably results from variations in depth to 

the water table and permeability and specific yield of 

the sediments. The water table is at or very near land 

surface at site 88, and within about 10 ft below land 

surface at site 10 (table 4). The horizontal hydraulic 

conductivity calculated from slug tests is 1 ft/d at site 

88 and 4 ft/d at site 10 (fig. 16, table 4). Specific yield 

can vary with water-table depth (Duke, 1972) as well as 

the lithology of the sediments (Fetter, 1980).

Figure 15. Generalized hydrogeologic section C-C' (trace of section is shown on figures 

3 and 12) (thickness of middle semiconfining unit and Lower Floridan aquifer modified 

from O’Reilly and others, 2002).
24 Hydrogeology and Quality of Ground Water in Orange County, Florida

Apopka

Lake

Lake Lerla

Lake

Wilkins

Lake Smith

Lake

Grassmere

POLK

COUNTY

ORANGE COUNTY

SEMINOLE COUNTY

OSCEOLA COUNTY

LAKE COUNTY VOLUSIA

COUNTY

BREVARD

COUNTY

  




   

0 5 10 KILOMETERS

0 5 10 MILES


   





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 ′ ° ° ′ ′

Rock Springs

G

G

G

Witherington

Springs

Wekiva

Springs

EXPLANATION

. WELL TESTED FOR

HYDRAULIC CONDUCTIVITY--

First number is hydraulic conductivity,

in feet per day. Number in parentheses

refers to site number in appendixes 1-4

0.2 (16)

G SPRING

.

..

.

.

.

.

.

.

. .

.

.

.

.

.

.

.

.

..

4 (10)

0.2 (16) 20 (29)

0.4 (31)

3 (74)

1(83)

5(84)

1(88)

3 (89)

4 (95) 0.05 (100)

20 (108)

6 (117) 30 (120)

1 (124) 3 (125)

3 (135)

6 (144)

0.06 (172)

9 (161)

6 (141)

Intermediate Confining Unit

The intermediate confining unit consists primarily 

of clay, silt, and carbonate (limestone and dolomite) 

rocks of the Miocene-age Hawthorn Group (fig. 11). 

The intermediate confining unit also can include clay 

layers of early Pliocene age (Murray and Halford, 

1996). Carbonate rocks, which are present primarily in 

the basal units of the Hawthorn Group, have been 

considered part of the intermediate confining unit in 

most studies (Miller, 1986; Murray and Halford, 1996; 

Spechler and Halford, 2001), but modeled as part of the 

Upper Floridan aquifer in at least one study (O’Reilly, 

1998). For the purposes of this study, the basal carbonate units of the Hawthorn Group are considered to be 

part of the intermediate confining unit. The Hawthorn 

Group also is characterized by the presence of phosphatic sediments throughout its thickness, ranging in 

size from fine sand to gravel. These sediments include 

the apatite mineral francolite (Scott, 1988), which can 

contain uranium-238. Uranium-238 decays radioactively, forming radium and, eventually, radon-222 as 

one of the daughter products. Radon is an inert, radioactive gas that can be dissolved in ground water (Arthur 

and others, 1994).

The intermediate confining unit in Orange 

County ranges in thickness from about 10 to more than 

200 ft, with a median thickness of about 100 ft (figs. 13-

15 and 18). In general, the unit is thickest in southeastern Orange County, and thinnest in southwestern and 

northwestern Orange County. As with the surficial 

aquifer system, the thickness of the intermediate 

confining unit ranges widely over short spatial 

distances. The thickness of the unit ranges from less 

than 50 to more than 150 ft in and around Orlando.

Figure 16. Distribution of horizontal hydraulic conductivity values determined from slug tests in the surficial 

aquifer system.
Hydrogeology 25

Table 4. Well-construction information, aquifer information, and average horizontal hydraulic conductivity for the surficial aquifer system[Source of data, U.S. Geological Survey and St. Johns River Water Management District; NA, not available. Well locations shown in figure 16]

Site 

number Site identifier Site name

Land 

surface altitude 

(feet)

Well 

depth 

(feet)

Depth to 

water (feet 

below land 

surface)

Saturated 

well 

penetration 

(feet)

Screen 

length 

(feet)

Saturated 

aquifer 

thickness 

(feet)

Average 

hydraulic 

conductivity 

(feet per day

10 282202081384602 Lake Oliver shallow well 117.06 38 12.58 25.42 5 50 4 16 282214081112802 Moss Park shallow well 69.05 29 14.7 14.3 3 36 .2 29 282348080564301 OR0715 40.28 30 6.79 23.21 10 60 20 31 282352081224401 S. Orange Park surficial well 83 29 7.5 21.5 10 28 .4 74 282631081323301 Tibet Butler surficial well 104 24 8.86 15.14 10 42 3 83 282739081054502 OR0714 66.68 30 5.36 24.64 10 49 1 84 282739081054503 OR0727 66.67 30 5.34 24.66 10 49 5 88 282838080572402 OR0713 21.6 17 5.09 11.91 10 26 1 89 282838080572403 OR0726 21.65 17 2.82 14.18 10 26 3 95 282912081181201 OR0722 97.82 30 11.03 18.97 10 34 4 

100 283003081283801 Turkey Lake surficial well 125 54 30.09 23.91 10 30 .05 108 283033081290301 Turkey Lake surficial well 110 29 12.46 16.54 10 15 20 117 283210081180401 OR0718 95.79 35 6.6 28.4 10 32 6 120 283228081213501 OR0721 76.16 25 5.86 19.14 10 23 30 124 283249081053203 Bithlo-3 63.14 15 9.37 5.63 3 90 1 125 283251081283501 OR0716 81.46 45 28.08 16.92 10 41 3 135 283345081225701 OR0720 80 32 4.1 27.9 10 30 3 141 283517081121501 CFRP surficial well 75 15 NA NA 10 10 6 144 283555081245101 OR0719 93 32 6.34 24.6 10 28 6 161 284230081345302 OR0107 146 40 30.87 9.13 9.13 9 9 172 284604081330301 OR0717 135 75 45.4 29.6 20 35 .06 
26 Hydrogeology and Quality of Ground Water in Orange County, Florida

Site

Upper Floridan aquifer

9

Depth = 318 feet

Casing length = 103 feet

Site 10

S

Depth = 38 feet

Casing length = 13 feet

urficial aquifer system

Site 161

Surficial aquifer system

Depth = 40 feet

Casing length = 40 feet

Site

Upper Floridan aquifer

160

Depth = 395 feet

Casing length = 100 feet

Site

urficial aquifer system

88

S

Depth = 17 feet

Casing length = 8 feet

Site

Upper Floridan aquifer

87

Depth = 120 feet

Casing length = 80 feet

Site 126

Upper Floridan aquifer

Depth = 350 feet

Casing length = 328 feet

Site

urficial aquifer system

125

S

Depth = 45 feet

Casing length = 35 feet

DATE

1/1/1999 1/1/2000 1/1/2001 1/1/2002

WATER LEVEL, IN FEET ABOVE NGVD 29

10

30

50

70

90

110

130

140

120

100

80

60

40

20

Many locations where the thickness of the intermediate 

confining unit is much more than 200 ft probably are 

anomalous points where sediments filled in paleosinkholes. Although shown on figure 18 to illustrate the 

variability in the thickness, these sinkhole locations 

were not used to construct the contours. Thus, the 

thickness of the intermediate confining unit shown in 

figure 18 is generalized.

The thickness of the intermediate confining unit 

also ranges widely because the contact of the Hawthorn 

Group with the underlying limestones of Eocene age 

is an erosional unconformity. Hence, the Hawthorn 

Group is thicker in places where the top of the limestone has been eroded, and thinner where the top of the 

limestone is higher. Furthermore, the Hawthorn Group 

often is identified by the presence of phosphatic 

pebbles and sand. Reworking of Hawthorn Group 

sediments during Pliocene and Pleistocene time can 

make it difficult to determine the contact between 

the Hawthorn Group and overlying sediments. This 

reworking also can cause the inflection in the natural 

gamma log to occur above the top of the Hawthorn 

Group or can result in a gradual change in gamma 

counts rather than a sharp inflection, making it difficult 

to identify the contact depth.

Leakance of the intermediate confining unit 

ranges over several orders of magnitude. Values 

from aquifer tests range from 1x10-4 (ft/d)/ft in eastern 

Orange County to 2x10-2 (ft/d)/ft in northeastern Polk 

County (Tibbals, 1977; 1981). Leakance values 

obtained from calibrating a ground-water flow model 

in Orange County ranged from 1x10-5 to about 8x10-4

(ft/d)/ft (Murray and Halford, 1996).

Figure 17. Continuous and periodic water levels in observation wells tapping the surficial aquifer 

system and the Upper Floridan aquifer (refer to figure 8 and appendixes for site numbers).
Hydrogeology 27

Apopka

Lake

ORANGE COUNTY

POLK

COUNTY

SEMINOLE COUNTY

OSCEOLA COUNTY

LAKE COUNTY

Rock GSprings

GG

Witherington

Springs Wekiva

Springs

C

B

B

A

100

150

100

50

100

50

150

50

100

100

100

100

100

100

150

50

50

100

100

50

50

100

100

50

100

50

100

50 100

100

50

150

150

100

100

150

100 50

150

100

100

50

100

100

50

150

150

200

100

100

100

250

2

150

150

0 5 10 KILOMETERS

0 5 10 MIL

Sources of data: geophysical and geological information from the U.S. Geological survey,St. Johns River Water Management District, and Florida Geological Survey


 






A A

WELL IN APPARENT SINKHOLE OR COLLAPSEFEATURE, NOT USED TO CONSTRUCT CONTOURSWELL

EXPLANATION

HYDROGEOLOGIC SECTIONS.A-A, B-B, and C-C shown in figures 3 and 13-15SEISMIC DATA POINT (Locker and others, 1988)LINE OF EQUAL THICKNESS OF THE INTERMEDIATECONFINING UNIT. CONTOUR INTERVAL IS 50 FEET

50

Figure 18. Generalized thickness of the intermediate confining unit.
28 Hydrogeology and Quality of Ground Water in Orange County, Florida

Leakance values were smallest in places where the 

intermediate confining unit was thickest, such as in 

southeastern Orange County, and largest in places 

where the intermediate confining unit was thin or 

absent, such as in southwestern and northwestern 

Orange County. Lithology of the intermediate confining 

unit also can affect the leakance values.

Few observation wells tap the intermediate 

confining unit, which contains permeable zones called 

the secondary artesian aquifer by Lichtler and others 

(1968). Only three observation wells [sites 59 and 123 

(fig. 8, app. 1) and a well adjacent to site 48] have longterm water-level measurements. A hydrograph for the 

well at site 123 (fig. 19) indicates that the water level is 

intermediate between levels in the surficial aquifer system and the Upper Floridan aquifer. Water levels in the 

other two wells completed in the intermediate confining unit are greatly affected by nearby pumping of the 

Floridan aquifer system. The intermediate confining 

unit generally is not used as a source of water supply in 

Orange County.

Floridan Aquifer System

The Floridan aquifer system consists of the Upper 

and Lower Floridan aquifers, separated by the middle 

semiconfining unit (and, in some areas the middle confining unit) (fig. 11). These hydrogeologic units consist 

primarily of carbonate rocks of Eocene age. The Upper 

Floridan aquifer consists of the Ocala Limestone and the 

upper one-third of the Avon Park Formation; the middle 

semiconfining unit consists of about the middle onethird of the Avon Park Formation; and the Lower Floridan aquifer consists of the lower one-third to one-half of 

the Avon Park Formation, all of the Oldsmar Formation, 

and the upper part of the Cedar Keys Formation (Miller, 

1986; Murray and Halford, 1996; O’Reilly and others, 

2002). The Floridan aquifer system is confined nearly 

everywhere in Orange County by the intermediate confining unit, although local breaches in the intermediate 

confining unit could exist. The sub-Floridan confining 

unit, which consists of anhydrite beds of the Cedar Keys 

Formation, underlies the Floridan aquifer system 

(fig. 11).

Upper Floridan Aquifer

The structure contour map shown in figure 20 

illustrates the surface of the Eocene-age carbonate 

rocks as it would appear if all overlying sediments were 

removed. Elevation of this surface ranges from about 

250 ft below to nearly 50 ft above NGVD 29, with a 

median of about 62 ft below NGVD 29. The altitude of 

the surface is highest in northwestern and southwestern 

Orange County and lowest in southeastern Orange 

County. In and around the city of Orlando, the altitude 

of the surface ranges greatly over small areal distances 

as a result of carbonate rock dissolution and subsequent 

formation of numerous cavities and sinkholes. At a few 

locations, the top of the Upper Floridan aquifer can be 

more than 250 to 300 ft below NGVD 29, probably as a 

result of the presence of a paleosinkhole. These anomalous points were not used to construct the contours 

shown in figure 20. Remnants of rock—peaks and 

pinnacles—remain between the sinkholes, imparting a 

hummocky appearance to the surface. Many of these 

geomorphic features are localized and not expressed in 

the land surface topography because the Hawthorn 

Group and overlying sediments are thicker over the 

sinkholes than over the peaks and pinnacles. This relation implies that the formation of many of the sinkholes 

predates the deposition of the Hawthorn Group and 

overlying sediments. Sinkholes still form at land 

surface in Orange County either by dissolution of 

the carbonate rocks and (or) by collapse of overlying 

sediments into existing cavities.

Thickness of the Upper Floridan aquifer ranges 

from about 300 to 400 ft throughout most of Orange 

County. The Upper Floridan aquifer in central Florida 

has been divided into two water-bearing zones, A and B 

(figs. 21-23) (Spechler and Halford, 2001). Zone A 

generally corresponds with the Ocala Limestone, 

whereas zone B corresponds to the upper one-third of 

the Avon Park Formation (O’Reilly and others, 2002). 

Zone B is the more productive of the two zones. Thickness of zone A ranges widely. Thickness of the Ocala 

Limestone, where present, ranges from less than 1 to 

more than 115 ft, with a median thickness of about 45 

to 50 ft. The Ocala Limestone is absent, possibly as a 

result of erosion, in many parts of Orange County.

Much of the porosity of the Floridan aquifer 

system is secondary, resulting from fracturing and dissolution of the carbonate rocks. As a result, transmissivity 

of the aquifer can be high with a large range in values 

(fig. 24). Results from 20 aquifer tests in and around 

Orange County (fig. 24) indicate that transmissivity of 

the Upper Floridan aquifer ranges from about 4,000 

to 550,000 ft2/d with a median of about 80,000 ft2/d.
Hydrogeology 29

Site 124

Depth = 15 feet

Casing length = 12 feet

Surficial aquifer system

Site 123

Depth = 75 feet

Casing length = 65 feet

Intermediate confining unit

Site 122

Depth = 492 feet

Casing length = 151 feet

Upper Floridan aquifer

40

50

60

70

30

25

1960 1970 1980 1990 2000

YEAR

ANNUAL RAINFALL, IN INCHES WATER LEVEL, IN FEET ABOVE NGVD 29

30

40

50

60

35

45

55

65

EXPLANATION

ANNUAL RAINFALL

LOCALLY WEIGHTED SCATTERPLOT

SMOOTHING CURVE

(SMOOTHING FACTOR = 0.28)

MEAN ANNUAL RAINFALL

(1935 - 2000 = 50 INCHES)

Figure 19. Water levels in three wells tapping the surficial aquifer system, the 

intermediate confining unit, and the Upper Floridan aquifer, and rainfall at Orlando, 

Florida.
30 Hydrogeology and Quality of Ground Water in Orange County, Florida Apopka Lake ORANGE COUNTY

POLK

COUNTY

SEMINOLE COUNTY

OSCEOLA COUNTY

LAKE COUNTY

C

B

B

A

-50

0

-50

-50

-50

-50

-50

-50

0 -50

-100

-50

0

0

0

0

-50

-100

-50

-50

-100

-150

-100

-50 -150

-50

-100

-100 -100 -150

-200

-150

-200

-250

-250

-100

-100

-50

-250

Rock

Springs

G

GG

Witherington

Springs

Wekiva

Springs

0 5 10 KILOMETERS

0 5 10 MIL

Sources of data: geophysical and geological information from the U.S. Geological survey,St. Johns River Water Management District, and Florida Geological Survey


 






A A

50

WELL IN APPARENT SINKHOLE OR COLLAPSEFEATURE -- NOT USED TO CONSTRUCT CONTOURSWELL

EXPLANATION

HYDROGEOLOGIC SECTIONS A-A, B-B, AND C-CSHOWN IN FIGURES 3 AND 13-15

CONTOUR INTERVAL IS 50 FEET.HACHURES INDICATE DEPRESSION

SEISMIC DATA POINT (Locker and others, 1988)

STRUCTURE CONTOUR -- Shows altitude of the top

of the Floridan aquifer system. Datum is NGVD 29

LOCATION OF GEOPHYSICAL LOGS SHOWNIN FIGURES 21 - 23

Figure 20. Generalized altitude of the top of the Upper Floridan aquifer.
Hydrogeology 31

0

100200300400500600700800900

1,0001,1001,2001,3001,4001,5001,6001,7001,800

0 10 20 30 40

0

100200300400500600700800900

1,0001,1001,2001,3001,4001,5001,6001,7001,800

0 600 1,200 1,800 0

100200300400500600700800900

1,0001,1001,2001,3001,4001,5001,6001,7001,800

30 32 34 36 38

0

100200300400500600700800900

1,0001,1001,2001,3001,4001,5001,6001,7001,800

0 250 500 750 1,000 -2,000 2,000 

E

SHORT-NORMAL

RESISTIVITY,

IN OHM-METERS

LONG-NORMAL RESISTIVITY,

IN OHM-METERS

FLUID RESISTIVITY (STATIC),

IN OHM-METERS

FLOW, IN 

PER M

Negindflowind

CALIPER,

IN INCHES

NotFlobor

LAND SURFACE ELEVATION = 150 FEET

0

100200300400600700800900

1,0001,1001,2001,3001,4001,5001,6001,7001,800

0

500

DEPTH BELOW LAND SURFACE, IN FEET

Figure 21. Selected geophysical logs showing hydrogeologic units for site I (refer to figure 20 for location), Orange County, Florida (moothers, 2002).
32 Hydrogeology and Quality of Ground Water in Orange County, Florida CALIPER, IN INCHES FLOW (STATIC), IN REVOLUTIONS PER MINUTE GAMMA, IN COUNTS PER SECOND FLOW (PU

IN REVOL

PER M

0 10 20 30 40 50 0 10 20 30 40 50 40 50 60 70 80 0 200 400 6

LAND SURFACE ELEVATION = 116 FEET

DEPTH BELOW LAND SURFACE, IN FEET

0

200400600800

1,0001,200

1,3001,100

900700500300100

0

200400600800

1,0001,2001,3001,100

900700500300100

0

200400600800

1,0001,2001,3001,100

900700500300100

0

200400600800

1,0001,2001,3001,100

900700500300100

1,400 1,400 1,400 1,400

Figure 22. Selected geophysical logs showing hydrogeologic units for well OR0621, site II (refer to figure 20 for location), Orange CouU.S. Geological Survey, written commun., 2002).
Hydrogeology 33

GAMMA,

IN COUNTS PER SECOND

LONG-NORMAL RESISTIVITY,

IN OHM-METERS

FLOW, IN REV

PER MI

CALIPER,

IN INCHES

LAND SURFACE ELEVATION = 58 FEET

DEPTH BELOW LAND SURFACE, IN FEET

0

200400600800

1,000

0 5 10 15 20 25 0 25 50 75 100 0 100 200 300 400 500 0 100 

100300500700900

1,100

0

200400600800

1,000

100300500700900

1,100

0

200400600800

1,000

100300500700900

1,100

0

200400600800

1,000

100300500700900

1,100

Figure 23. Selected geophysical logs showing hydrogeologic units for well OR0618, site III (refer to figure 20 for location), Orange Cou(A.M. O’Reilly, U.S. Geological Survey, written commun., 2002).
34 Hydrogeology and Quality of Ground Water in Orange County, Florida

Apopka

Lake

POLK

COUNTY

ORANGE COUNTY

SEMINOLE COUNTY

OSCEOLA COUNTY

LAKE COUNTY VOLUSIA

COUNTY

BREVARD

COUNTY

.

.

.

.

.

. . . .

.

. .

.

.

.

.

.

.

.

.

COCOA

WELL FIELD

__38

465

__30

--

17-55 ____

240

__71

333

50__

--

__4

215

59-100 ____

287

___ 80

287 ___ 278

340

___ 326

390

___ 307

517

___ 102

305

354 __

319

_____ 47-74

265

210-750 _____

205

510-550 _____

325 ___ 106

351

___ 40

366

___ 79

673

120 __

--

Wekiva

Springs

Rock

GSprings

G

MODEL

BOUNDARY

Orlando

S t . J o h n s

e

R i v r

S r t. Johns Rive

EXPLANATION

Chloride concentration equal

to or greater than 5,000 mg/L

is present in some part of the

vertical thickness of aquifer

TRANSMISSIVITY FROM AQUIFER AND SPECIFIC

CAPACITY TEST -- Top number is transmissivity in thousands

of feet squared per day. Bottom number is thickness of aquifer

penetrated by pumped well, in feet; -- data not available

WELLS FOR WHICH GEOPHYSICAL LOGS WERE USED TO

DEFINE FLOW ZONES IN THE FLORIDAN AQUIFER SYSTEM

(SEE FIGURES 21 - 23)

.

100,000 to <500,000

500,000 to 1,000,000

< 10,000

50,000 to <100,000

10,000 to <50,000

TRANSMISSIVITY FROM CALIBRATED FLOW MODEL,

IN FEET SQUARED PER DAY

____ 38

465

28 20 ° ′

40′

30′   

0 5 10 KILOMETERS

0 5 10 MILES

81°40′ 20′ 10′ 80°50′

28°50′

3 ′ 0

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 ′ ° ° ′ ′

Transmissivity values from calibrated ground-water 

flow models generally are greater than those calculated 

from aquifer tests because: (1) wells used for aquifer 

tests rarely pentrate the full thickness of the aquifer; 

and (2) transmissivity values calculated from aquifer 

tests are more likely to represent a point value rather 

than the more areally averaged value derived from a 

ground-water flow model. Model-calibrated transmissivity determined for the Upper Floridan aquifer was 

less than 50,000 ft2/d along the St. Johns River in 

southern and southeastern Orange County (fig. 24; 

Spechler and Halford, 2001). Maximum values of 

transmissivity occur in a broad area extending from 

northwestern Orange County, through the city of 

Orlando, and around the Cocoa well field in southeastern 

Orange County (fig 24; Spechler and Halford, 2001).

Figure 24. Location of aquifer tests and model-derived transmissivity values for the Upper Floridan aquifer 

(modified from Murray and Halford, 1996; PB Water, 1999; and Spechler and Halford, 2001).
Hydrogeology 35

Middle Semiconfining Unit and Lower 

Floridan Aquifer

The middle semiconfining unit, which separates 

the Upper and Lower Floridan aquifers, is composed of 

less permeable carbonate units of the Avon Park 

Formation (fig. 11; O’Reilly and others, 2002). The 

contacts between the middle semiconfining unit and 

the Upper and Lower Floridan aquifers are based 

primarily on permeability (determined from geophysical logs) rather than lithology and stratigraphy. The 

middle semiconfining unit is composed of soft limestone, in which secondary porosity is less developed 

than the rest of the Floridan aquifer system. As a result, 

the middle semiconfining unit is identified on geophysical logs by lower apparent resistivity, smooth borehole 

walls as determined by caliper logs, and little increase 

in borehole flow (O’Reilly and others, 2002).

 The top of the middle semiconfining unit ranges 

from about 330 ft below NGVD 29 in northwestern 

Orange County to about 485 ft below NGVD 29 in 

southeastern Orange County (O’Reilly and others, 

2002). The thickness of the unit ranges from about 385 

ft in southwestern Orange County to 635 ft in eastcentral Orange County (O’Reilly and others, 2002). A 

second confining unit, the middle confining unit, underlies the middle semiconfining unit in western Osceola 

County and southern Lake County (O’Reilly and others, 

2002). This unit is composed primarily of anhydritic and 

gypsiferous dolomite and dolomitic limestone, and is 

much less permeable than the middle semiconfining unit 

(Miller, 1986). One well control point in southwestern 

Orange County indicates that the middle confining unit 

is present in that area (O’Reilly and others, 2002), but is 

not present throughout Orange County.

Few data are available describing the hydraulic 

properties of the middle semiconfining unit. Values of 

leakance of the middle semiconfining unit obtained 

from model simulations range from 1x10-5 (ft/d)/ft to 

2x10-3 (ft/d)/ft (Murray and Halford, 1996; Sepúlveda, 

2002). The vertical hydraulic conductivity of the 

middle semiconfining unit was estimated to be less 

than 0.05 ft/d based on results of aquifer tests at the 

Cocoa well field (Phelps and Schiffer, 1996).

The top of the Lower Floridan aquifer ranges 

from about 870 ft below NGVD 29 in northwestern 

Orange County to 1,120 ft below NGVD 29 in southeastern Orange County (O’Reilly and others, 2002). 

Only four wells in Orange County penetrate the full 

thickness of the Lower Floridan aquifer. Based on four 

wells that penetrate the full thickness of the Floridan 

aquifer system in central and southern Orange County, 

the thickness of the Lower Floridan aquifer ranges 

from 1,023 to 1,180 ft (O’Reilly and others, 2002).

As in the Upper Floridan aquifer, transmissivity 

of the Lower Floridan aquifer is enhanced by secondary porosity features, which probably are the primary 

cause of high transmissivity zones (O’Reilly and 

others, 2002). Only 12 aquifer tests, however, were 

reported for wells tapping the Lower Floridan aquifer 

(fig. 25). Transmissivity values for these tests range 

from 82,000 to more than 900,000 ft2/d (O’Reilly 

and others, 2002). High values of transmissivity 

were calculated for a number of wells. Results of a 

test in Orlando indicated a transmissivity of about 

500,000 ft2/d (Lichtler and others, 1968). Drawdown 

in the observation well (900 ft from the pumped well) 

was only about 1 in. after pumping the supply well at 

3,200 gal/min for 10.5 hours. The small drawdown 

allowed only an estimation of the transmissivity (Lichtler and others, 1968). No tests were completed in eastern Orange County, where few wells tapping the Lower 

Floridan aquifer have been drilled (fig. 25).

Calibration of a model of Seminole County and 

surrounding areas, including Orange County, resulted 

in a large range of values for transmissivity of the 

Lower Floridan aquifer (fig. 25). Maximum values 

were in northwestern Orange County; minimum values 

were in eastern Orange County (fig. 25). More recent 

model-derived transmissivity values for the Lower 

Floridan aquifer in Orange County ranged from about 

5,000 to 700,000 ft2/d (Sepúlveda, 2002).

 Recharge and Discharge

Recharge to the Floridan aquifer system occurs 

naturally from downward leakage of water from the 

surficial aquifer system, and directly from precipitation 

in places where the intermediate confining unit is missing. Recharge also can occur artificially from drainage 

wells and disposal of treated wastewater. Discharge 

occurs naturally by upward leakage of water to the 

surficial aquifer system, and directly to streams such as 

the Wekiva River. Discharge also occurs artificially 

from pumping wells.

Recharge to the Upper Floridan aquifer can 

occur in areas where the altitude of the water table is 

higher than the altitude of the potentiometric surface 

of the Upper Floridan aquifer. At these locations, the 

potential exists for ground water to flow downward 
36 Hydrogeology and Quality of Ground Water in Orange County, Florida

Apopka

Lake

POLK

COUNTY

ORANGE COUNTY

SEMINOLE COUNTY

OSCEOLA COUNTY

LAKE COUNTY VOLUSIA

COUNTY

BREVARD

COUNTY

.

p

.

p

.

.

.

.

.

p

.

.

71-107 _____

405

>500 ____

302

460 ___

444

314-936 _____

418

193-202 _____

258

82 ___

500

668

___

498 575 ___

184

430-870 _____

402

278-367 _____

745

365-460 _____

590

>500 ____

311

82 ___

500

460 ___

444

EXPLANATION

TRANSMISSIVITY FROM AQUIFER AND SPECIFIC CAPACITY TEST --

Top number is transmissivity in thousands of feet squared per day.

Bottom number is thickness of aquifer penetrated by pumped well, in feet;

-- data not available

Chloride concentration greater than 5,000 mg/L

throughout the entire thickness of aquifer

< 100,000 (0 where 500,000 to 1,000,000

chloride concentration greater

than 5,000 mg/L)

200,000 to <500,000

100,000 to <200,000

TRANSMISSIVITY OF THE LOWER FLORIDAN AQUIFER

DETERMINED FROM AQUIFER AND SPECIFIC CAPACITY TEST --

Top number is transmissivity in thousands of feet squared per day.

Bottom number is thickness of aquifer penetrated by pumped well,

in feet; -- data not available

TRANSMISSIVITY FROM CALIBRATED FLOW MODEL,

IN FEET SQUARED PER DAY

Chloride concentration equal to or

greater than 5,000 mg/L is present in

some part of the vertical thickness of aquifer

Wekiva

Springs

Rock

GSprings

G

MODEL

BOUNDARY

  




   

0 5 10 KILOMETERS

0 5 10 MILES


   





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 ′ ° ° ′ ′

from the surficial aquifer system, through the intermediate confining unit, and recharge the Floridan aquifer 

system. Most of Orange County is designated as a 

potential recharge area, based on this criterion (fig. 26).

In reality, this recharge occurs slowly, except in locations where the intermediate confining unit is thin or 

very permeable. Rates of recharge to the Upper Floridan 

aquifer in Orange County were estimated with groundwater flow models (O’Reilly, 1998; Murray and 

Halford, 1996). In western Orange County, natural 

recharge rates can be as high as 25 in/yr, whereas in 

much of central Orange County, the rate is less than 

10 in/yr (fig. 27). Recharge rates can exceed 25 in/yr in 

areas where significant artificial recharge occurs (such 

Figure 25. Location of aquifer tests and model-derived transmissivity values for the Lower Floridan aquifer 

(modified from Spechler and Halford, 2001, and O’Reilly and others, 2002).
Hydrogeology 37

Apopka

Lake

St. Johns River

Reedy Creek

POLK

COUNTY

ORANGE COUNTY

SEMINOLE COUNTY

OSCEOLA COUNTY

LAKE COUNTY VOLUSIA

COUNTY

BREVARD

COUNTY

RAPID

INFILTRATION

BASIN

(Reedy Creek

Improvement

District)

Water

Conserv II

73

46

61

42

37

36

66

35

21

33 13

32 59

35

93

50

96

57

92

49 63

36

57

56

99

47

120

49

28

32

111

108

103

102

99

47

32

30 .

.

. .

.

.

.

.

.

.

.

.

.

.

.

. . .

.

POTENTIAL

DISCHARGE AREA

RECHARGE

AREA

UPPER FLORIDAN AQUIFER DRAINAGE WELL

WELL PAIRS TAPPING THE SURFICIAL

AQUIFER SYSTEM AND THE UPPER FLORIDAN

AQUIFER -- Top number is water level in the surficial

aquifer system and bottom number is water level in

the Upper Floridan aquifer in September 2001, in feet

above NGVD 29

EXPLANATION

63

36

.

G

Rock

Springs

G

GG

Witherington

Springs

Wekiva

Springs

SPRING

  




   

0 5 10 KILOMETERS

0 5 10 MILES


   





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 ′ ° ° ′ ′

as Water Conserv II and the RCID RIBs) or where the 

intermediate confining unit is very leaky (fig. 27). The 

thickness and lithology of the intermediate confining 

unit, which is less than 50 ft thick in many parts of 

western Orange County, but 100 to 150 ft thick in most 

of central and eastern Orange County, probably affects 

the recharge rate.

The Upper Floridan aquifer also receives 

recharge artificially from water flowing into drainage 

wells (fig. 26). Estimates for recharge rates from these 

wells range from about 30 Mgal/d (Tibbals, 1990; 

Murray and Halford, 1996) to nearly 46 Mgal/d 

(Spechler and Halford, 2001). Actual recharge to the 

Upper Floridan aquifer from drainage wells can vary 

considerably because of seasonal and annual variations 

in precipitation and lake stages, but probably ranges 

from about 38 to 50 Mgal/d (CH2M Hill, 1997). 

In addition to drainage wells that receive storm runoff, 

the Upper Floridan aquifer historically received effluent from the disposal of both municipal and industrial 

Figure 26. Areas of potential recharge to and discharge from the Upper Floridan aquifer (modified from Boniol 

and others, 1993; CH2M Hill, 1997; and Murray and Halford, 1996).
38 Hydrogeology and Quality of Ground Water in Orange County, Florida River

Wekiva

chee

Econlock

River

hat

St.

River

Johns

Rock Springs

G

G

G

Witherington

Springs

Wekiva Springs

ORANGE COUNTY

POLK

COUNTY

SEMINOLE COUNTY

LAKE COUNTY

VOLUSIA

COUNTY

BREVARD

COUNTY

Reedy Creek

Improvement

District

rapid infiltration

basins

Water

Conserv II

  





       





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 ′ ° ° ′ ′

0 5 10 KILOMETERS

0 5 10 MILES

DISCHARGE AREA

RATE OF RECHARGE TO THE UPPER

FLORIDAN AQUIFER, INCHES PER YEAR

EXPLANATION

0 - <10 10 - 25 > 25

Approximate

boundary of

model by O’Reilly (1998)

wastewater into so-called sanitary wells. At least 18 

sanitary wells, with depths ranging from 231 to 863 ft, 

were located around the city of Orlando (Unklesbay, 

1944). The use of sanitary wells was discontinued due 

to the risk of contamination of the Floridan aquifer 

system (Thomas L. Lothrop, city of Orlando, oral 

commun., 2003).

The hydrologic system in Orange County also 

receives artificial recharge from wastewater, including 

septic tanks, and from irrigation of citrus groves and 

lawns with treated wastewater (reclaimed water), 

as well as from land application of reclaimed water 

at RIBs in the western part of the county (fig. 1). 

Recharge enters the surficial aquifer system and flows 

into the Upper Floridan aquifer in areas where the 

water table is higher than the potentiometric surface. 

Estimating the amount of artificial recharge requires 

some extrapolation from 1990 data, which indicate that 

more than 211,000 homes and residences were served 

by public sewers (U.S. Census Bureau, 2002). The total 

amount of treated wastewater discharged from public 

sewer systems in Orange County in 1990 was 91.67 

Mgal/d (Marella, 1994) or 434 gal/d per household. 

In 2000, the number of housing units in Orange County 

was more than 361,000. Assuming the same amount 

of wastewater discharge per unit in 2000 as in 1990 

Figure 27. Simulated recharge rate to the Upper Floridan aquifer by leakage through the intermediate confining unit 

under average steady-state 1988 conditions (modified from Murray and Halford, 1996) and average steady-state 1995 

conditions (modified from O’Reilly, 1998).
Hydrogeology 39

(434 gal/d per household), then the estimated total 

amount of wastewater discharge (including public 

sewers and septic tanks) in 2000 would be about 

157 Mgal/d. Of the total 157 Mgal/d, about 30 to 

40 Mgal/d was discharged from a wastewater treatment 

plant directly to surface water either in the Little Econlockhatchee River or to wetlands that drain into the 

St. Johns River (fig. 1). The remainder of the wastewater (157-40 = 117 Mgal/d) potentially contributes 

to the surficial aquifer system in Orange County 

through septic tanks, irrigation, or RIBs, although 

some reclaimed water is lost to evapotranspiration.

In a few locations, the altitude of the potentiometric surface of the Upper Floridan aquifer is higher 

than the water table in the surficial aquifer system, 

indicating areas of potential discharge from the Upper 

Floridan aquifer to the surficial aquifer system or to 

lakes and streams. These areas primarily are in eastern 

Orange County along the St. Johns River, in southwestern Orange County along Reedy Creek, in western 

Orange County beneath parts of Lake Apopka, and in 

northwestern Orange County (fig. 26). In areas of low 

land-surface altitude, artesian pressure causes water in 

wells to flow out of the top of the casing. Numerous 

flowing wells (sites 87, 92, 118, and 180; figs. 8 and 9) 

are present in eastern Orange County, most of which 

are capped to prevent water from discharging continuously. Natural discharge also occurs at springs such as 

Rock, Wekiva, and Witherington Springs in the northwestern part of the county (fig. 1). In southwestern 

Orange County, however, Reedy Creek would go dry 

periodically (zero discharge) prior to urban development in the basin, indicating that natural discharge of 

the Upper Floridan aquifer in that area probably 

was minimal.

Ground-water discharge also occurs artificially 

as a result of withdrawal from wells throughout the 

county. In 2000, about 287 Mgal/d of ground water 

was withdrawn (R.L. Marella, U.S. Geological Survey, 

written commun., 2002), which is equivalent to about 

6 in. of water for the year over the area of the county 

(1,003 mi2). Average rainfall in Orlando is about 50 

in/yr; hence, an amount of ground water equivalent to 

about 12 percent of average rainfall was withdrawn 

from the aquifers in 2000. Furthermore, rainfall in 

2000 was only 30.4 in.; the amount of ground water 

withdrawn was about 20 percent of the rainfall during 

that year.

Potentiometric Surface and Ground-Water 

Flow Patterns

The potentiometric surface of the Upper Floridan 

aquifer ranges from about 110 ft above NGVD 29 in 

southwestern Orange County to less than 30 ft above 

NGVD 29 in eastern Orange County (fig. 28). These 

levels indicate lateral ground-water flow in the Upper 

Floridan aquifer in Orange County is from the southwest to the east-northeast. Local depressions in the 

potentiometric surface result from ground water pumping and from discharge of ground water at Wekiva 

Springs. The slope of the surface is steeper in southwestern Orange County and flattens in eastern Orange 

County, probably as a result of an increase in transmissivity from the southwest to the east.

The potentiometric surface of the Upper Floridan 

aquifer declined throughout much of Orange County 

from 1961 to 2001. Water levels in wells measured near 

the end of the wet season in September 2001 (fig. 28) 

were compared with water levels in wells measured in 

July 1961 (fig. 29), which, according to Lichtler and 

others (1968) represented relatively average conditions.

Water-level measurements for both time periods were 

available for 15 wells. Water levels declined in all 

15 wells, as a result of a general decline in the altitude 

of the potentiometric surface. Declines ranged from a 

minimum of 0.8 ft in southwestern Orange County to 

16 ft in south-central Orange County, with a median 

decline of about 3 ft. Declines greater than 10 ft also 

were observed in central and west-central Orange 

County (fig. 30). Water-level declines in central 

Orange County roughly coincide with areas of urban 

land use (fig. 5) and locations of public-supply wells 

(figs. 6 and 7).

In southwestern Orange County, the altitude of 

the Upper Floridan aquifer potentiometric surface is 

affected by artificial recharge at RIBs. The water level 

in an observation well tapping the Upper Floridan aquifer (site 24) generally increased after 1990, about the 

same time the RIBs began receiving reclaimed water 

(fig. 31). The well is located adjacent to RIBs; hence, 

the water level in the well probably is affected by artificial recharge from the RIBs. A previous study indicated 

an increase ranging from 2 to 4 ft in the altitude of the 

potentiometric surface of the Upper Floridan aquifer in 

the vicinity of site 24 as a result of the effects of the 

RIBs (O’Reilly, 1998).
40 Hydrogeology and Quality of Ground Water in Orange County, Florida

Apopka

Lake

POLK

COUNTY

ORANGE COUNTY

SEMINOLE COUNTY

OSCEOLA COUNTY

LAKE COUNTY VOLUSIA

COUNTY

BREVARD

COUNTY

110

105

100

95

90

85

80

75

70

65

60

55

50

45

40

35

30

25

45

40

35

30

25

45

35

110

105

100

. .

.

.

. .

.

.

. . . .

. .

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

. .

.

.

.

.

. .

. 36

32 30

21

51

46

47

59

51

57

44

49

47

57

64 63

79

80

95 86

61

83

103

108

50

48

48

45

47

42

39

35

36

35

35

37

36 36

35

33

32

EXPLANATION

30 POTENTIOMETRIC CONTOUR --

Shows altitude at which water-level

would have stood in tightly cased wells.

Contour interval 5 feet. Datum is NGVD 29

.51 UPPER FLORIDAN AQUIFER WELL --

Number is altitude of water-level

in feet above NGVD 29

G SPRINGS

  




   

0 5 10 KILOMETERS

0 5 10 MILES


   





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 ′ ° ° ′ ′

Wekiva

Springs

Rock

GSprings

G

The potentiometric surface of the Lower Floridan 

aquifer generally mimics the potentiometric surface of 

the Upper Floridan aquifer, with water flowing laterally 

from southwestern Orange County to northeastern 

Orange County (fig. 32) (O’Reilly and others, 2002).

Compared with the Upper Floridan aquifer, however, 

fewer wells are available for constructing potentiometric-surface maps of the Lower Floridan aquifer. 

No wells are available in southwestern or northeastern 

Orange County to evaluate the highest and lowest altitudes of the potentiometric surface. Water levels from 

four wells that were measured during the 1960s also are 

available for May 1999; the water level declined 9 ft in 

three of the wells and 12 ft in the other well.

The potential exists for ground water to flow 

vertically between the Upper Floridan and Lower 

Floridan aquifers. In six of seven well pairs in Orange 

County, the potentiometric surface of the Upper Floridan aquifer is higher than that of the Lower Floridan 

aquifer, indicating a potential for downward flow from 

the Upper Floridan to the Lower Floridan aquifer. 

The difference between the two potentiometric surfaces 

ranged from 0.3 to 2.4 ft in May 1999 (O’Reilly and 

others, 2002). At one well pair (sites 105 and 106, 

fig. 8), the potentiometric surface of the Lower Floridan aquifer was as much as 0.9 ft higher than that of the 

Upper Floridan aquifer, indicating a potential for 

ground water to flow upward from the Lower Floridan 

to the Upper Floridan aquifer. This difference probably 

results from pumping of the Upper Floridan aquifer in 

the area, rather than being indicative of natural hydrogeologic conditions. 

Continuous water levels in two wells that tap the 

Upper and Lower Floridan aquifers (sites 130 and 131, 

fig. 8) respond similarly to rainstorms (Lichtler and 

others, 1968, p. 97); however, flow between the two 

aquifers is difficult to quantify. The well at site 130 is 

cased only 600 ft, which indicates that the borehole is 

open not only to the Lower Floridan aquifer, but also 

open to about 300 ft of the middle semiconfining unit 

Figure 28. Potentiometric surface of the Upper Floridan aquifer, September 2001 (modified from Knowles and 

Kinnaman, 2002).
Hydrogeology 41

Apopka

Lake

POLK

COUNTY

ORANGE COUNTY

SEMINOLE COUNTY

OSCEOLA COUNTY

LAKE COUNTY VOLUSIA

COUNTY

BREVARD

COUNTY

105

100

95

90

85

80

75

70

65

55

50

45

40

35

30

55

50

45

40

35

60

80

94

91

63

64

30

38

48

57

62

59

58 57

59

58

37 36

67 39

32

39

90 88

109

80

59

69

69

76

81 84

70

64

65

54

55

61

35

58

58

57

57 57

55

57

57 58

59

57

58

57

59

49

56

52

28

38

53

EXPLANATION

30 POTENTIOMETRIC CONTOUR --

Shows altitude at which water-level

would have stood in tightly cased wells.

Contour interval 5 feet. Datum is NGVD 29

39 UPPER FLORIDAN AQUIFER WELL --

Number is altitude of water-level

in feet above NGVD 29

SPRINGS

Wekiva

Springs

Rock

GSprings

G

G

  




   

0 5 10 KILOMETERS

0 5 10 MILES


   





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 ′ ° ° ′ ′

(O’Reilly and others, 2002). Therefore, the water level 

in the well may not accurately indicate the water level 

in the Lower Floridan aquifer.

The low leakance values of the middle semiconfining unit noted earlier might preclude much groundwater flow between the two aquifers, at least in the 

eastern part of the county. Phelps and Schiffer (1996) 

determined that pumping from the Upper Floridan 

aquifer at the Cocoa well field (fig. 1) did not cause an 

observable effect on water levels in the Lower Floridan 

aquifer. Chemical and (or) isotopic analyses of water 

samples from both aquifers at the same site could be 

used to better estimate flow through the middle semiconfining unit.

Long-Term Trends in Water Levels

Surficial Aquifer System

The Mann-Kendall test was used to analyze 

long-term water-level data for significant trends. Longterm data for the surficial aquifer system in Orange 

County are sparse; however, more data are available for 

the Floridan aquifer system. Two observation wells 

tapping the surficial aquifer system (sites 10 and 12, 

fig. 33) had continuous water-level data for 30 or more 

years of record. Three additional wells (sites 48, 91, 

and 124) had monthly water-level measurements for 

more than 20 years (fig. 33; table 5). Results of the 

Mann-Kendall trend analysis indicate that water levels 

have not declined significantly in four of the wells 

(sites 10, 12, 48, and 91; fig. 33). Results also indicate 

that water-level altitudes in one well (site 124) declined 

during the period of record, but the trend is weak 

(fig. 33; table 5).

The results of the trend analysis is for the overall 

monotonic trend for the entire period of record. The 

LOWESS line for site 124 indicates that the decline is 

not constant over the period of record. The water level 

in the well (site 124) increased from the beginning of 

the record to about the early 1980s, declined sharply to 

Figure 29. Potentiometric surface of the Upper Floridan aquifer, July 1961 (modified from Lichtler and 

others, 1968).
42 Hydrogeology and Quality of Ground Water in Orange County, Florida

Apopka

Lake

POLK

COUNTY

ORANGE COUNTY

SEMINOLE COUNTY

OSCEOLA COUNTY

LAKE COUNTY VOLUSIA

COUNTY

BREVARD

COUNTY

  




   

0 5 10 KILOMETERS

0 5 10 MILES


   





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 ′ ° ° ′ ′

10

11

8

1 16 6

6

3 2

3 0

2

3

3

15

3

G SPRINGS

3

EXPLANATION

UPPER FLORIDAN AQUIFER WELL –

Number is decline in water level, in feet,

between July 1961 and September 2001

0 to <1 1 to <5 5 to <10 10 to <15 15 or greater

WATER LEVEL DECLINE, IN FEET

Wekiva

Springs

Rock

Springs

the early 1990s, and then increased to the present 

(2002). The LOWESS lines for the other four wells 

also indicate periods of declining or increasing water 

levels. These oscillations in water levels probably are 

affected by the somewhat cyclic variations in rainfall 

over time (fig. 33).

Temporal trends in lake stage also can indicate 

changes in the water table. Lake water levels were statistically analyzed for trends using annual mean water 

levels for lakes with at least 15 years of record. Most 

of these lakes are around Orlando in central Orange 

County. The tests were done for the entire period of 

record and also for the period from 1967-2000. Of the 

345 lakes in the lake water-level database, 70 lakes had 

15 years or more of record since 1967 and 82 lakes had 

15 years or more of record including all years.

From 1967 to 2000, significant trends were indicated in 27 (39 percent) of the 70 lakes tested. Eighteen 

lakes had increasing water levels and 9 lakes had 

decreasing water levels. For the entire period of record, 

trends were indicated in 32 (39 percent) of the 82 lakes 

tested. Of these 32 lakes, 13 had increasing water levels 

and 19 lakes had decreasing water levels. These results 

indicate no consistent overall trend or change in the 

altitude of the water table of the surficial aquifer 

system throughout Orange County. Any apparent waterlevel changes are localized, probably resulting from 

Figure 30. Decline in potentiometric surface of the Upper Floridan aquifer from July 1961 to September 2001.
Hydrogeology 43

Site 24

Depth = 117 feet

Casing length = 92 feet

1980 1990 2000

YEAR

30

40

50

60

70

90

94

98

102

WATER LEVEL, IN FEET ABOVE NGVD 29 ANNUAL RAINFALL, IN INCHES

ORLANDO

106

92

96

100

104 Start Reedy Creek

Improvement District

RIB operation

such activities as drainage changes or ground-water 

pumping. Short-term variations in precipitation distribution probably also affect the water levels in the surficial aquifer system.

Floridan Aquifer System

Water levels in the Upper Floridan aquifer generally fluctuate over time, increasing, sometimes rapidly, 

during the wet season (June through September), and 

decreasing during the rest of the year. For example, the 

water level in an Upper Floridan aquifer observation 

well in southwestern Orange County (site 9, fig. 8) 

increased 5.1 ft during the wet season, from 103.6 ft 

above NGVD 29 on June 14, 2001, to 108.7 ft above 

NGVD 29 on September 16, 2001 (fig. 17). The water 

level in an observation well in eastern Orange County 

(site 87, fig. 8) increased 4 ft during that same period of 

time. The rise in water level in the first well (site 9) 

could reflect nearby recharge to the aquifer, which 

potentially could cause variations in water quality. 

Because the second well (site 87) is in a discharge area 

for the Floridan aquifer system, local recharge of the 

aquifer seems unlikely. The rise in water level in the 

well at site 87 probably results from recharge that 

Figure 31. Long-term periodic water levels in an observation well 

tapping the Upper Floridan aquifer and annual rainfall at Orlando, 

Florida (refer to figure 8 and appendix 1 for site number).
44 Hydrogeology and Quality of Ground Water in Orange County, Florida

Apopka

Lake

POLK

COUNTY

ORANGE COUNTY

SEMINOLE COUNTY

OSCEOLA COUNTY

LAKE COUNTY VOLUSIA

COUNTY

BREVARD

COUNTY

80

25

30

35

40

70

60

50

45

..

. . . .

. .

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

. .

. . .

. . . .

... . . . . . . .

. . .

.

40 32

34

38

52 45

34

43

44 31

65

47

49 39 42

41 37 31

45 42 38 32

41 33

43

41

42

41 47

36

44 46

44

42

39

39

39

42

42 42

41

41

44

41

42

EXPLANATION

POTENTIOMETRIC CONTOUR--Shows altitude at which

water level would have stood in tightly cased wells.

Dashed where approximate and

Contour interval 5 and 10 feet. Datum is NGVD 29

estimated from Upper

Floridan aquifer potentiometric surface maps.

WELL -- Top number is water level in feet above NGVD 29

in May 1999. Bottom number is water level decline from

the 1960s to May 1999

50

.

42

SPRING

__

9

Wekiva

Springs

Rock

GSprings

G

G

__

9

__

__ 9

9

__

12

  




   

0 5 10 KILOMETERS

0 5 10 MILES


   





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 ′ ° ° ′ ′

occurs to the west, increasing pressure throughout the 

aquifer. The collection of ground-water samples for 

chemical and (or) isotopic analysis during the wet 

season would be useful in identifying areas of rapid 

recharge to the Upper Floridan aquifer. 

Water levels in the Upper Floridan aquifer 

also can change from year to year as a result of 

climatic variations. The water levels in the two abovementioned wells (sites 9 and 87) were each about 2 ft 

lower in September 2000 compared with September 

2001 (fig. 17). Rainfall in June-September 2000 was 

about 21 in. (National Oceanic and Atmospheric 

Administration, 2001) compared with 39 in. in JuneSeptember 2001 (National Oceanic and Atmospheric 

Administration, 2002).

Long-term water-level data from 13 observation 

wells tapping the Upper Floridan aquifer and 2 wells 

tapping the Lower Floridan aquifer were statistically 

analyzed for trends (table 5; figs. 8 and 34). The periods of record used for data analysis ranged from 27 to 

58 years. These wells are not evenly distributed throughout Orange County; most of the wells are located in 

eastern Orange County, including seven wells located 

in and around the Cocoa well field (fig. 1).

Figure 32. Generalized potentiometric surface of the Lower Floridan aquifer, May 1999 (modified from O’Reilly 

and others, 2002), and water-level decline since the 1960s.
Hydrogeology 45

CONTINUOUS WATER LEVELS

EXPLANATION

PERIODIC WATER LEVELS

LOWESS--locally weighted scatterplot smoothing

(smoothing factor for rainfall = 0.28;

smoothing factor for water levels = 0.50)

Site 10

Depth = 38 feet

Casing length = 33 feet

Kendall’s tau = 0.05

Site 91

Depth = 8 feet

Casing length = 8 feet

Kendall’s tau = -0.09

Site 12

Depth = 18 feet

Casing length = 18 feet

Kendall’s ta = .09 u

Site 124

Depth = 15 feet

Casing length = 12 feet

Kendall’s tau = -0.19

Site 48

Depth = 10 feet

Casing length = 10 feet

Kendall’s tau = -0.01

LOWESS CURVE

LOWESS CURVE

LOWESS CURVE

LOWESS CURVE

LOWESS CURVE

ORLANDO

DATE

WATER LEVEL, IN FEET ABOVE NGVD 29

ANNUAL RAINFALL,

IN INCHES

104

108

112

116

52

54

56

58

60

30

40

50

60

70

58

60

62

64

68

66

1960 1970 1980 1990 2000

57

59

61

63

93

95

97

99

1960 1970 1980 1990 2000

WATER LEVEL, IN FEET ABOVE NGVD 29

DATE

106

110

114

70

56

58

60

62

64

100

98

96

94

92

62

Figure 33. Long-term continuous and periodic water levels in observation wells tapping the surficial aquifer 

system (refer to figure 8 and appendix 1 for site numbers) and annual rainfall at Orlando, Florida.
46 Hydrogeology and Quality of Ground Water in Orange County, Florida

Results of the statistical analysis indicated that 

water-level altitudes declined in 11 of the 15 wells. 

Three of the wells (sites 130, 131, and 170) had weak 

declines in water levels (absolute value of tau <0.3; table 

5). Seven wells tapping the Upper Floridan aquifer and 

one well tapping the Lower Floridan aquifer had statistically significant water-level declines (p-value <0.05) as 

indicated by strong (absolute value of tau >0.3) negative 

correlations with time (table 5). Five of these wells are 

located in or near the Cocoa well field, one well (site 50) 

is in southwestern Orange County, one well (site 122) is 

near the town of Bithlo, and one well (site 126) is in 

west-central Orlando (fig. 8). Four wells (sites 9, 30, 82, 

and 90; fig. 8 and app. 1) had no statistically significant 

water-level declines (p-value >0.05) (table 5).

Declines in water levels, based on the minimum 

and maximum values of the LOWESS curves (fig. 34), 

ranged from about 4 ft (site 27) to about 12 ft (site 126), 

with a median of about 6 ft. The water-level decline in 

one well (site 126; fig 8) is particularly interesting. 

The water level in the well began declining from the 

beginning of the period of record to the early 1980s, 

and then stabilized. Prior to 1967, land-surface altitude at the site was 72.12 ft above NGVD 29. After 

1967, the land-surface altitude increased to 81.71 ft, 

probably as a result of nearby construction. The water 

level in the well frequently was greater than 72.12 

until the early 1960s, which would seem to indicate 

that the well flowed periodically. The well is not 

located in or near any discharge areas for the Upper 

Floridan aquifer (fig. 26). Unklesbay (1944) does not 

describe the well as flowing periodically; however, the 

original casing is recorded as being 10 ft above land 

surface (Unklesbay, 1944), which may have been 

installed to prevent flowing and to obtain accurate 

water-level measurements during high water.

Table 5. Well construction data, period of record, and results of trend analysis for long-term observation wells tapping 

the surficial and Floridan aquifer systems

[Kendall’s tau values shown in bold face were statistically significant at the 0.05 significance level]

Site 

number Station identifier Station name

Elevation 

(feet above 

NGVD 29)

Depth of 

well (feet 

below land 

surface)

Casing 

length 

(feet)

Period 

of 

record

Kendall’s 

tau

Surficial aquifer system

10 282202081384602 Lake Oliver surficial well 117.06 38 13 1960 to 1969, 

1974 to 2001

0.05

12 282210081352601 Disney surficial well 99.44 18 18 1969 to 2001 .09

48 282510081054502 Cocoa-M well 70.81 10 10 1969 to 2001 - .005

91 282847081013702 Cocoa-K well 60 8 8 1968 to 2001 - .09

124 283249081053203 Bithlo-3 well 63.14 15 12 1960 to 2001 - .19

Upper Floridan aquifer

9 282202081384601 Lake Oliver deep well 117.12 318 103 1959 to 1969, 

1974 to 2001

- .06

27 282341081040101 Cocoa-A well 75.06 516 301 1960 to 2001 - .34

30 282348080564701 Palmetto well 40.62 381 245 1960 to 2001 - .06

50 282528081340901 Bay Lake deep well 97.1 223 104 1966 to 2001 - .55

63 282531081095701 Cocoa-D well 75.91 300 226 1965 to 2001 - .56

67 282533081082206 Cocoa-C (zone 5) well 63.72 1,004 248 1966 to 2001 - .55

72 282623081153801 Cocoa-P well 94.12 439 245 1971 to 2001 - .33

82 282739081054501 Cocoa-F well 67.29 375 200 1966 to 2001 - .07

90 282847081013701 Cocoa-H well 60 495 252 1971 to 2001 .09

122 283249081053201 Bithlo-1 well 63.58 492 151 1961 to 2001 - .35

126 283253081283401 OR-47 well 81.71 350 328 1943 to 2001 - .53

131 283333081233502 Lake Adair 10 well 80.4 400 105 1974 to 2001 - .19 

170 284529081301001 Rock Springs deep well 71.9 365 143 1961 to 2001 - .21

Lower Floridan aquifer

64 282533081082202 Cocoa-C (zone 1) well 63.71 1,357 1,351 1966 to 2001 - .56

130 283333081233501 Lake Adair 9 deep well 80.4 1,281 601 1974 to 2001 - .22
Hydrogeology 47

LOWESS CURVE

Site 27

Depth = 516 feet

Casing length = 301 feet

Kendall’s ta = -0.34 u

Site 63

Depth = 300 feet

Casing length = 226 feet

Kendall’s tau = -0.56

Site 126

Depth = 350 feet

Casing length = 328 feet

Kendall’s tau = -0.53

Site 122

Depth = 492 feet

Casing length = 151 feet

Kendall’s tau = -0.35

Site 50

Depth = 223 feet

Casing length = 104 feet

Kendall’s ta = -0.55 u

ORLANDO

WATER LEVEL, IN FEET ABOVE NGVD 29

WATER LEVEL, IN FEET ABOVE NGVD 29

ANNUAL RAINFALL,

IN INCHES PER YEAR

75

80

85

90

95

25

30

35

40

25

30

35

40

25

30

35

40

50

60

70

80

1940 1950 1960 1970 1980 1990 2000

YEAR

45

55

65

75

85

45

45

45

30

40

50

60

70

1940 1950 1960 1970 1980 1990 2000

YEAR

100

LOWESS--locally weighted scatterplot smoothing

(smoothing factor for rainfall = 0.28;

smoothing factor for water levels = 0.50)

EXPLANATION

CONTINUOUS WATER LEVELS

Figure 34. Long-term continuous water levels in observation wells tapping the Upper Floridan aquifer and annual rainfall 

at Orlando, Florida (refer to figure 8 and appendix 1 for site numbers).
48 Hydrogeology and Quality of Ground Water in Orange County, Florida

Declines in the altitude of the potentiometric 

surface of the Upper Floridan aquifer also have resulted 

in flow decreases from springs in the Orange County area. 

Discharge rates for Rock and Wekiva Springs (fig. 1) have 

decreased, based on LOWESS curves (fig. 35) generated 

for discharge measurements made during or immediately 

following the wet season (June-September). 

Water-level declines generally are not continuous 

throughout the period of record. Although the overall 

water-level trend in the five wells presented in figure 34 

is decreasing (table 5), water levels in some wells 

exhibit periods of stabilization or even increasing water 

levels. For example, the water level in one well (site 50) 

declined from the beginning of record until about the 

mid-1970s, then stabilized until about the early 1980s, 

and then declined until present (fig. 34). The water level 

in another well (site 63) has declined throughout most 

of the period of record, but the rate of decline decreased 

in the early 1990s (fig. 34). The cyclic pattern of rainfall 

over the period of record could be responsible for the 

brief periods of stabilization or the temporary increase 

of water levels in some of the wells. There seems to be 

no long-term trend in rainfall (fig. 35); therefore, longterm declines in water levels (table 5) likely are due 

primarily to long-term increases in ground-water withdrawals (fig. 4).

Changes in the distribution of pumping also 

could account for some variations in water-level trends. 

For example, the decline and stabilization of water 

levels in the well at site 126 could have resulted from 

changes in land use and, ultimately, pumping practices. 

In 1977, more than half of the land use surrounding the 

well (site 126) was agricultural; however, by 1997, 

a major portion of the land use around the well was 

urban (residential and commercial). After several freezing winters in the 1980s, many orange groves in 

Orange County were redeveloped as subdivisions. The 

displacement of agricultural land by urban land probably allowed for the abandonment of nearby irrigation 

wells, as indicated by the significant decrease in water 

use for agricultural purposes between 1980 and 1990.

Water Budget

A water budget summarizes the inputs and outputs 

of a hydrologic system. In general, inputs to a natural 

system include precipitation and lateral inflow (either 

ground or surface water) from an adjacent area. Assuming that there is no change in storage, inputs are balanced by outputs, such as evapotranspiration, runoff, 

lateral ground-water outflow, spring discharge and, in 

some areas, by exportation of water to adjacent watersheds or counties. A generalized water budget for 

Orange County, assuming changes in storage are negligible, can be described by the following equation:

P = ET + QR+ QS + QO , (2)

where

P is precipitation;

ET is evapotranspiration;

QR is runoff (sum of overland runoff and ground-water 

seepage to streams);

QS is spring discharge; and

QO is net lateral subsurface outflow plus exported water.

All units are in inches per year.

A generalized water budget was computed for 

Orange County from 1991 to 2000 (fig. 36). All values 

were rounded to the nearest whole number. A budget 

averaged over a 10-year period (as opposed to a single 

year’s budget) is likely to be more representative of 

long-term conditions. Also, water released from or 

accumulated as storage within the surficial aquifer 

system, which is difficult to quantify, can be assumed 

to be negligible during a 10-year averaging period. 

The major input to the system is precipitation, 

which averaged about 53 in/yr from 1991 to 2000 at five 

NOAA stations (Clermont, Kissimmee, Orlando, 

Sanford, and Titusville; fig. 1). Runoff from streams 

flowing into Orange County and lateral subsurface 

inflow are relatively minor inputs to the hydrologic 

system. These inputs are included in the computation of 

QR and QO, as described below.

Water leaving the county as surface-water runoff 

(QR) averaged about 11 in/yr from 1991 to 2000, based 

on analyses of streamflow records at 10 USGS gaging 

sites that collect runoff from basins that lie entirely (or 

partially) within Orange County (fig. 1). These sites 

include: Econlockhatchee River near Chuluota 

(02233500); St. Johns River near Christmas 

(02232500); St. Johns River near Cocoa (02232400); 

Wekiva River near Sanford (02235000); Shingle Creek 

(02263800); Boggy Creek (02262900); Bonnet Creek 

(02264100); Apopka-Beauclair Canal (02237700); 

Reedy Creek (02266300); and Howell Creek 

(02234308). Estimated runoff from the Ajay-East 

Tohopekaliga Canal is included in the total (after 

subtracting the estimated runoff from the Myrtle-Mary 

Jane Canal). Flows in the Ajay-East Tohopekaliga 
Hydrogeology 49

Canal and the Myrtle-Mary Jane Canal are not 

currently (2000) gaged but were estimated from 

measurements reported by Lichtler and others (1968). 

Runoff from these 11 sites ranged from 3.4 in/yr at the 

Econlockhatchee River near Chuluota to 0.1 in/yr at 

Howell Creek. At sites where runoff is combined from 

Orange County and an adjacent county, the amount of 

runoff contributed solely from Orange County was estimated by multiplying the total gaged discharge by the 

percentage of the basin area contained within the 

county. Runoff from the eastern boundary of Orange 

County into the St. Johns River was calculated as the 

difference between discharge at the Cocoa and the 

Christmas sites, adjusted proportionately for the length 

of the river that forms the county boundary line. 

Other calculated outflows include about 2 in/yr 

of spring discharge (QS), which is based on the average 

discharge at Wekiva and Rock Springs from 1991 to 

INSTANTANEOUS DISCHARGE, IN CUBIC FEET PER SECOND 30

40

50

60

70

80

90

100

30

40

50

60

70

80

90

100

1930 1940 1950 1960 1970 1980 2000 1990

YEAR

1920 2010

1930 1940 1950 1960 1970 1980 2000 1990

30

40

50

60

70

YEAR

ANNUAL RAINFALL,

IN INCHES

1920 2010

INSTANTANEOUS DISCHARGE, IN CUBIC FEET PER SECOND

30

40

50

60

70

80

90

100

30

40

50

60

70

80

90

100

A WEKIVA SPRINGS

B

ROCK SPRINGS

E ORLANDO

C

WEKIVA SPRINGS -- Wet season

(June through October)

D ROCK SPRINGS -- Wet season

(June through October)

LOWESS--locally weighted scatterplot smoothing

(smoothing factor for rainfall = 0.28;

smoothing factor for spring discharge = 0.50)

EXPLANATION

RAINFALL

DISCHARGE

Figure 35. Discharge at Wekiva and Rock Springs for entire period of record (A and B) and for wet season (C and D), and 

annual rainfall at Orlando, Florida (E). (Spring locations are shown on figure 1.)
50 Hydrogeology and Quality of Ground Water in Orange County, Florida

EXPLANATION

P PRECIPITATION

ET EVAPOTRANSPIRATION

QR SURFACE-WATER DISCHARGE

SPRING DISCHARGE

Q O NET LATERAL SUBSURFACE

OUTFLOW PLUS EXPORTED

WATER

All values are in inches per year.

QS

SUB-FLORIDAN CONFINING UNIT

INTERMEDIATE

CONFINING UNIT

SURFICIAL

AQUIFER SYSTEM

(SAS)

FLORIDAN AQUIFER SYSTEM

(FAS)

P = 53 ET = 39

QS = 2 QR = 11

QO = 1

2000; 1 in/yr of net lateral subsurface outflow plus 

exported water (QO), which includes: the net lateral 

(subsurface) outflow of ground water from the Floridan 

aquifer system as simulated by a USGS ground-water 

flow model (Sepúlveda, 2002); potable water pumped 

from the Cocoa wellfield to Brevard County from 1991 

to 2000; and reclaimed water distributed by Water Conserv II for irrigation in Lake County. Net lateral subsurface outflow of water from the surficial aquifer system 

is assumed to be negligible. Evapotranspiration (ET) 

was calculated as the residual of equation 2 and 

accounted for about 39 in/yr of water removed from the 

system. This back-calculated value of ET is a reasonable estimate for Orange County, because it falls within 

the range of values documented in previous USGS 

studies; that is, the value is greater than the 27 in/yr 

reported for a well-drained, deep water-table ridge site 

in western Orange County (Sumner, 1996), but is considerably less than the evaporation rate (56 in/yr) 

reported by Swancar and others (2000) for a 2-year 

period (August 1996-July 1998) at Lake Starr in eastcentral Polk County.

WATER QUALITY

Ground water is an important resource in Orange 

County for consumption and recreation. Ground water 

also is important for aquatic habitat because it discharges into lakes and streams. The quality of ground 

water in the surficial and Floridan aquifer systems with 

respect to major ions and nutrients, trace elements, 

radon and stable isotopes, and pesticides is described in 

the following sections. Locations of the springs and 

wells sampled are shown in figure 9.

Figure 36. Water budget for Orange County, Florida, 1991-2000.
Water Quality 51

Major Ions and Nutrients

Natural waters generally are classified according 

to the relative concentrations of major ions, also called 

water type, which can be indicative of the geochemical 

evolution of the water. Concentrations of major ions 

(calcium, magnesium, sodium, potassium, bicarbonate, 

sulfate, and chloride) in ground water generally result 

from water-rock interactions in the aquifer. Concentrations of major ions in ground water also can be affected 

by lateral and (or) vertical migration of saline water, 

such as modern or relict seawater (Hem, 1989). In 

addition, low concentrations of major ions are present 

in precipitation. In central Florida, the concentration 

of chloride in rain generally ranges between 0.5 and 

1.0 mg/L (National Atmospheric Deposition Program, 

2002). Major ions can be concentrated in ground water 

through evapotranspiration. Finally, land-use activities 

can affect concentrations of major ions in ground 

water. Anthropogenic sources of these constituents 

include irrigation with water from the Floridan aquifer 

system and disposal of wastewater (Adamski and 

Knowles, 2001; Hem, 1989; Sacks and others, 1998).

Concentrations of nutrients (nitrate, nitrite, 

ammonia, organic nitrogen, and phosphorus) in ground 

water can be derived from natural or anthropogenic 

sources. Nitrogen compounds in ground water can 

originate from decomposition of natural organic 

matter, but also can result from fertilizers and animal 

and human wastes (Hem, 1989; Fetter, 1980). Nitrogen 

oxides occur in precipitation as a result of natural 

chemical reactions and the combustion of fossil fuels 

(U.S. Geological Survey, 1999). In central Florida, the 

concentration of nitrate in rain generally is low, less 

than 1.0 mg/L (National Atmospheric Deposition 

Program, 2002). Phosphorus compounds in ground 

water similarly can originate from natural and anthropogenic sources including phosphatic sediments in the 

aquifer, fertilizers, and animal and human wastes.

Concentrations of some major ions and nutrients 

in drinking water can pose health or aesthetic concerns. 

Concentrations of nitrate greater than 10 mg/L as nitrogen can cause methmoglobinemia in small children 

(Hem, 1989). Concentrations of chloride greater than 

about 250 mg/L can impart an objectionable taste to 

water. High concentrations of sulfate in water can have 

a laxative effect (Fetter, 1980).

Current and historical data were used to assess 

the ground-water quality with respect to major ions 

and nutrients. Data from 34 samples collected from 

the surficial aquifer system, 62 samples from the 

Upper Floridan aquifer, and 25 samples from the 

Lower Floridan aquifer were used to assess the current 

water-quality conditions (tables 2 and 6). These 

samples were collected from 1990 through 2001 by 

the USGS and SJRWMD. Data from 24 wells and 

2 springs were used to assess water-quality trends 

of the Floridan aquifer system.

Surficial Aquifer System

Concentrations of major ions and nutrients in the 

ground water of the surficial aquifer system are highly 

variable. This variation probably results from several 

factors including variability in the lithology of the sediments, interaction with the Upper Floridan aquifer 

including the upwelling of water with higher dissolved 

solids concentration, and effects of land use. Water 

type throughout the surficial aquifer system generally 

is mixed ion; however, samples from two wells tapping 

the surficial aquifer system (sites 29 and 83, fig. 37) in 

southeastern Orange County had calcium-bicarbonate 

type water, possibly as a result of dissolution of shell 

material in the aquifer. Water type in the well at site 93 

was sodium chloride. Water type in samples from 

several wells (sites 10, 35, 74, and 80) in southwestern 

Orange County was calcium sulfate or mixed-cation 

sulfate (fig. 37).

 Median concentrations of TDS, chloride, and 

sulfate generally were low (less than 250 mg/L for 

TDS and less than 50 mg/L for chloride and sulfate) 

in samples from the surficial aquifer system (table 6; 

figs. 38 and 39), but increased in eastern and southwestern Orange County. TDS concentrations exceeded 

the National Secondary Drinking Water Regulation 

(also referred to as secondary standards; U.S. Environmental Protection Agency, 2002) of 500 mg/L in two 

samples from eastern Orange County and in one sample near Lake Apopka (fig. 38). The secondary standard for chloride (250 mg/L) was exceeded in one 

sample from a well (site 93) in eastern Orange County 

(fig. 39). Maximum sulfate concentration in samples 

from the surficial aquifer system was 150 mg/L (table 6); 

the secondary standard for sulfate is 250 mg/L. The 

high TDS (greater than 500 mg/L) and chloride concentrations in eastern Orange (figs. 38 and 39, respectively) correspond with locations where the potentiometric surface of the Upper Floridan aquifer is higher 

than land surface (fig. 26), resulting in water with high 

TDS from the Upper Floridan aquifer discharging into 

the surficial aquifer system. Elevated concentrations of 

TDS and chloride in other parts of Orange County 

could result from land-use activities. For example, 

wells at sites 34 and 35 are adjacent to RIBs, which 

could be affecting water quality at those sites.
52 Hydrogeology and Quality of Ground Water in Orange County, Florida Table 6. Summary statistics of water-quality indicators, and major-ion and nutrient constituents in ground-water samples from the surficia

systems, 1990 through 2001 [Data source, U.S. Geological Survey and St. Johns River Water Management District; temperature, degrees Celsius (oC); specific conductance, microsie

ph, standard units; all concentrations in milligrams per liter; No., number; <, less than; --, no data] Surficial aquifer system Upper Floridan aquifer Lowe

Constituent No. of sam- ples Mini- mum 25th per- centile Median 75th per- centile Maxi- mum No. of sam- ples Mini- mum 25th per- centile Median 75th per- centile Maximum No. of sam- ples Mini- mum 25t

per

cent

Temperature 32 20 24 24.6 25.1 28.5 54 22.3 23.8 24.2 24.7 29.8 24 24 24.6 

Specific conductance 34 50 135 215 468 3,860 55 125 318 580 1,150 4,080 25 207 258 Dissolved oxygen 22 < .1 < .1 < .1 .7 6.7 28 < .1 < .1 < .1 < .1 .2 8 < .1 < .1 pH 33 3.9 5.4 5.9 6.8 9.9 52 5.1 7.5 7.7 7.8 8.3 23 7.3 7.7 Calcium 33 .9 6.1 19 49 170 48 6.0 43 77 110 180 24 26 33 Magnesium 33 .2 1.8 6.1 10 60 48 3.4 6.8 12 25 67 24 6.8 8.1 Sodium 33 1.9 5.1 8.4 18 540 48 3.8 8.4 28 110 610 24 3.8 5.6 Potassium 33 < .1 .9 2.3 9.8 26 48 .6 1.3 1.9 3.4 18 24 .6 .9 Bicarbonate 28 <1 7 18 58 455 48 7.3 133 180 259 400 25 49 129 Sulfate 32 < .2 3.8 17 36 150 56 < .2 14 44 150 300 25 1.7 5.5 Chloride 34 5 8.1 14 29 1,100 56 5.9 11 37 200 1,100 25 6.0 8.5 Fluoride 32 < .1 < .1 < .1 .22 1.0 48 < .1 .1 .2 .3 .8 25 < .1 .1 Bromide 25 < .05 < .05 .05 .08 3.2 31 < .05 < .05 .1 .7 17 22 < .05 < .0Silica 30 .7 4.9 8.9 11 39 48 1.5 10 14 21 31 24 1.3 9.7 Total dissolved solids 33 24 88 112 277 2,230 56 45 168 338 537 2,290 25 117 137 Nitrite plus nitrate, 

as nitrogen

32 < .02 < .02 .02 3.5 20 34 < .02 < .02 < .02 < .02 2 8 < .02 < .0

Nitrite, as nitrogen 26 < .01 < .01 < .01 < .01 .02 32 < .01 < .01 < .01 < .01 .01 8 < .01 < .0Ammonia, as 

nitrogen, dissolved

30 < .01 .02 .2 .41 6.2 32 < .01 .1 .3 .5 .8 8 .1 .2 

Ammonia plus 

organic nitrogen, as nitrogen, total

0 -- -- -- -- -- 6 < .2 < .2 < .2 < .2 .4 0 -- -- 

Ammonia plus 

organic nitrogen, as nitrogen, dissolved

28 < .2 < .2 .3 .6 7.3 26 < .2 < .2 .3 .6 1 8 < .2 < .2 

Phosphorus, total 0 -- -- -- -- -- 8 < .02 .03 .04 .09 .35 0 -- -- Phosphorus, 

dissolved

28 < .02 < .02 .04 .08 1.5 32 < .02 < .02 .05 .1 .34 8 .04 .0

Phosphate, as 

phosphorus, dissolved

28 < .01 < .01 .03 .07 1.4 33 < .02 .02 .03 .08 .3 8 .03 .0

Organic carbon, 

total

28 < .1 1.8 3.3 11 26 25 .6 1.2 2 3.3 11 8 .6 1.3 
Water Quality 53 Figure 37. Concentrations of major cations and anions in water samples collected from wells tapping the surficial aquifer s

through 2001.

Apopka

Lake

POLK

COUNTY

ORANGE COUNTY

SEMINOLE

COUNTY

OSCEOLA COUNTY

LAKE COUNTY VOLU

COU

BRE

COU

Rock

Springs

Witherington Springs

Wekiva

Springs

·

1.0 0.0

1.0

0.0

1.0

0.0

1.0

0.0

1.0

0.0

1.0

0.0

1.0

1.0

1.0 1.0

1.0 1.0

1.0 1.0

0.0

1.0 0.0 1.0

0.0 1.0

1.0 0.0 1.0

1.0 1.0

0.0

2.0 2.0 0.0

2.0 2.0

0.0 2.0 2.0

2.0 2.0 0.0

2.0 0.0 2.0

2.0 0.0 2.0

2.0 0.0 2.0

6.0 0.0 

6.0 0.0 6.0

6.0 0.0 

4.0 0.0 4.0

3.0 0.0 3.0

10 0.0 10

1.0 0.0

1.0 0.0 1.0

172

161

149

144

125

135

120

80 74

35

17

10

31

108

100

4

117

95

141

16

124

83

88 

29

0 5 10 KILOMETERS

0 5 10 MILES

28 20 ° ′

40′

81°40′ 30′ 81° ′ 20′ 10′ 

00

28°50′

30′

1.0 0.0 1.0

149

EXPLANATION

STIFF DIAGRAMS

SPRING

WELL AND NUMBER--

Number refers to site numberin appendixes 1 and 2.Concentrations are in milliequivalentsper liter (meq/L)

Sodium+potassium

Chloride+fluoride

Calcium

Magnesium

Bicarbonate

Sulfate

(meq/L)
54 Hydrogeology and Quality of Ground Water in Orange County, Florida

Apopka

Lake

POLK

COUNTY

ORANGE COUNTY

SEMINOLE COUNTY

OSCEOLA COUNTY

LAKE COUNTY VOLUSIA

COUNTY

BREVARD

COUNTY

Rock

GSprings

G

G

Witherington Springs

Wekiva

Springs

____ 570

153

____ 629

88

2,230 ____

93

____ 316

83

____ 277

29

451 ____

157

____ 395

155

____ 313

142

278 ___

34 120 ____

17

103 ____

172

____ 185

154

102 ____

149

51 ____

161

59 ____

135

189 ____

125

____ 112

108

224

____

80 107 ____

74

____ 138

10

199 ____

35 ____ 70

31

____ 76

100

98 ____

120

____ 63

117

____ 90

95

___ 96

4

____ 105

16

88 ____

124

___ 23

8

34 ____

144 26 ____

141

EXPLANATION

SPRING

<50

50 - 250

>250 - 500

> 500

___ 26

141

Top number is concentration, in milligrams per liter;

bottom number refers to site number in appendix 2

CONCENTRATION RANGES

G

  




   

0 5 10 KILOMETERS

0 5 10 MILES


   





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 ′ ° ° ′ ′

Concentrations of nutrients in the surficial aquifer 

system typically were low. About half of the samples 

had nitrate concentrations less than or equal to the 

method reporting limit of 0.02 mg/L as nitrogen; about 

half the samples had ammonia, ammonia plus organic 

nitrogen, phosphorus, and phosphate concentrations 

less than or equal to 0.2, 0.3, 0.04, and 0.03 mg/L, 

respectively (table 6; figs. 40 and 41). 

Locally, the concentrations of nutrients in ground 

water from the surficial aquifer system can be much 

greater, possibly as a result of natural conditions and 

(or) land-use activities. One sample (site 35, fig. 9) had 

a nitrate concentration of 20 mg/L as nitrogen (table 6), 

exceeding the National Primary Drinking Water Regulation or MCL of 10 mg/L as nitrogen (U.S. Environmental Protection Agency, 2002). This well, located 

adjacent to a RIB, has a depth of 35 ft and a casing 

length of 33 ft. One sample (site 172) had a nitrate concentration of 10 mg/L (fig. 41). This well, completed in 

clean, sandy soil just downgradient from a citrus grove, 

had a depth of 74 ft and a casing length of 54 ft. 

In addition, concentrations of nitrate exceeded 3.0 mg/L 

in samples from two wells near Lake Apopka (sites 142 

and 154), one well in western Orlando (site 125), and 

four wells in southwestern Orange County (sites 10, 

17, 35, and 80).

Elevated concentrations of nitrate indicate 

contamination of ground water, possibly from land-use 

practices such as using lawn fertilizers and septic systems. Concentrations of nitrate in ground water underlying pristine areas generally are low, indicating that 

natural sources of nitrate do not contribute significantly 

to concentrations in ground water in central Florida. 

For example, nitrate concentrations in 14 of 15 samples 

from the surficial aquifer system underlying the nearby 

Ocala National Forest (fig. 1) were less than 1.0 mg/L 

(Adamski and Knowles, 2001).

Three samples from the surficial aquifer system 

had ammonia concentrations greater than 1.0 mg/L 

(sites 153, 155, and 157, fig. 9), and five samples had 

ammonia plus organic nitrogen concentrations greater 

than 1.0 mg/L (sites 88, 93, 108, 154, and 157). Four 

Figure 38. Concentrations of total dissolved solids in water samples collected from wells tapping the surficial 

aquifer system, 1990 through 2001.
Water Quality 55

Apopka

Lake

POLK

COUNTY

ORANGE COUNTY

SEMINOLE COUNTY

OSCEOLA COUNTY

LAKE COUNTY VOLUSIA

COUNTY

BREVARD

COUNTY

Rock

GSprings

GG

Witherington Springs

Wekiva

Springs

1,100 ___

93

180 ___

88

100 ___

34

___ 17

172

___ 5.3

161

___ 23

157

___ 42

155

___ 5.0

154

___ 36

153

___ 6.5

102

___ 34

142

___ 8.4

125

___ 5.9

144

___ 22

135

___ 36

120

___ 18

117

___ 5.7

141

___ 12

124

___ 12

83

___ 9.2

29 ___ 38

16

___ 10

113

___ 31

4

___ 20

31

___ 6.3

74

___ 7.6

80

___ 14

35 ___ 8.7

17

___ 20

8

___ 8.1

10

___ 19

108 ___ 7.0

100

___ 14

95

EXPLANATION

SPRING

< 50

50 - 150

>150 - 500

> 500

CONCENTRATION RANGES

G

___ 57

141

WELL -- Top number is concentration,

in milligrams per liter; bottom number refers to

site number in appendix 2

  





       





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 ′ ° ° ′ ′ 0 5 10 KILOMETERS

0 5 10 MILES

of the wells are near the northern edge of Lake Apopka, 

near areas of former row-crop agriculture. The ammonia and organic nitrogen in the ground water at these 

locations possibly result from the long-term use of 

fertilizers on crops such as corn. Ammonia and organic 

nitrogen also could result from decomposition of natural organic material in the soils. Two of the wells are in 

the Tosohatchee State Reserve (fig. 1), a relatively 

pristine area. The ammonia plus organic nitrogen in 

these samples most likely are from organic-rich soils 

and decay of leaf litter.

A total of seven samples had phosphorus and 

(or) phosphate concentrations greater than or equal to 

0.1 mg/L as phosphorus. Two of the samples were from 

wells in the city of Orlando (sites 117 and 120, fig. 9), 

one sample was from a well located adjacent to a RIB 

(site 35), and four samples were from wells near Lake 

Apopka (sites 142, 153, 155, and 157). As with ammonia plus organic nitrogen, the potential sources of phosphorus in ground water could be natural (phosphatic 

sediments in the aquifer) or from local land use. During 

drilling of the two wells in the city of Orlando, cuttings 

indicated no phosphatic sediments at one site (site 

120), and phosphatic sediments at a depth of 44 ft at the 

second site (site 117). The borehole at the second site 

was backfilled and the final depth was only 35 ft. The 

source of phosphorus in ground water from these two 

wells probably is related to local land use, such as 

fertilizing residential lawns and gardens.

The maximum concentration of phosphorus is 

not regulated in drinking water; however, elevated concentrations of phosphorus in surface water can cause 

excessive growth of algae and cyanobacteria. Water 

samples from lakes in Orange County that had concentrations of phosphorus greater than 0.1 mg/L also had 

concentrations of chlorophyll-a, an indicator of the 

amount of algae and cyanobacteria in a water body, 

greater than 10 μg/L (E.R. German, U.S. Geological 

Survey, written commun., 2003). The concentration of 

phosphorus in the surficial aquifer system can have 

important consequences to algal growth in lakes 

because ground water from the surficial aquifer system 

discharges into lakes and streams.

Figure 39. Concentrations of chloride in water samples collected from wells tapping the surficial aquifer system, 

1990 through 2001.
56 Hydrogeology and Quality of Ground Water in Orange County, Florida

1.0

10

100

1,000

10,000

10

100

1,000

10,000

CHLORIDE, IN MILLIGRAMS PER LITER

0.1

1.0

10

100

1,000

SULFATE,

IN MILLIGRAMS PER LITER

SURFICIAL

AQUIFER

SYSTEM

0.1

1.0

10

100

BICARBONATE,

IN MILLIGRAMS PER LITER

10.0

100

1,000

10,000

T

OTAL DISSOLVED SOLID

S,

IN MILLIGRAMS PER LITER

0.01

0.1

1.0

PHOSPHORUS, DISSOLVED,

IN MILLIGRAMS PER LITER

SODIUM,

IN MILLIGRAMS PER LITER

UPPER

FLORIDAN

AQUIFER

LOWER

FLORIDAN

AQUIFER

SURFICIAL

AQUIFER

SYSTEM

UPPER

FLORIDAN

AQUIFER

LOWER

FLORIDAN

AQUIFER

MRL

28 48 25

MRL

32

56

25

48

24

33

34 56

25

56

25

33

27

32

8

MRL

p<0.01

p<0.01

p<0.01

p<0.01

p<0.01

p=0.9

Interquartile

Range (IQR)

P-value - Probability that observed

differences occur by chance. A p-value of 0.05

(5 percent) or less is considered significant.

EXPLANATION

22 Number of samples

75th percentile + 1.5XIQR

75th percentile

50th percentile (median)

25th percentile

25th percentile -1.5XIQR

MRL Method reporting limit

Outliers

Figure 40. Distribution of sodium, chloride, sulfate, bicarbonate, total dissolved solids, and phosphorus in water samples 

from the surficial aquifer system, the Upper Floridan aquifer, and the Lower Floridan aquifer, 1990 through 2001.
Water Quality 57

Apopka

Lake

POLK

COUNTY

ORANGE COUNTY

SEMINOLE COUNTY

OSCEOLA COUNTY

LAKE COUNTY VOLUSIA

COUNTY

BREVARD

COUNTY

Rock

GSprings

G

G

Witherington Springs

Wekiva

Springs

10 ___

172

___ 8.2

125

___ 20

35

____ 5.5

154

7.8 ____

142

____ 3.9

80

____ 3.4

34

____ 6.3

17 4.6 ____

10

____ 2.8

149

____ 1.4

135

<0.02 ____

93

<0.02 ____

88

<0.02 ____

29

____ 0.02

153

____ 0.02

157

<0.02 ____

144

<0.02 ____

117

<0.02 ____

95

<0.02 ____

141

<0.02 ____

100

<0.02 ____

108

0.02 ____

155

0.97 ___

161

<0.02 ____

120

<0.02 ____

31

<0.02 ____

4

<0.02 ____

16

<0.02 ____

74

____ 0.02

124

<0.02 ____

83

EXPLANATION

SPRING

<1.0

1.0 - 3

>3.0 - 8

>8.0 - 20

CONCENTRATION RANGES

G

<0.02 ____

141

WELL -- Top number is concentration,

in milligrams per liter as nitrogen; bottom number

refers to site number in appendix 3

  





       





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 ′ ° ° ′ ′ 0 5 10 KILOMETERS

0 5 10 MILES

Floridan Aquifer System

Water type in the Upper Floridan (fig. 42) and 

Lower Floridan aquifers throughout much of Orange 

County is calcium or calcium-magnesium bicarbonate as 

a result of the dissolution of limestone and dolomite.

The lithology of the aquifer systems affects the concentrations of these constituents. Whereas the surficial aquifer system primarily is composed of relatively inert 

quartz sand, the Floridan aquifer system is composed of 

more soluble carbonate rocks (limestone and dolomite). 

Dissolution of calcium carbonate in these rocks by 

slightly acidic water (carbonic acid) results in an 

increase in the concentration of calcium and bicarbonate 

in the water by the following reaction (Drever, 1988):

CaCO3 + H2O + CO2 = Ca2+ + 2HCO3

-

. (3)

Concentrations of major ions and TDS 

typically were higher in samples from the Upper 

Floridan aquifer than in samples from the surficial 

aquifer system (table 6; fig. 40). Statistical analysis 

using the Kruskal-Wallis test indicates that median 

concentrations of calcium, magnesium, sodium, bicarbonate, chloride, and TDS were significantly different 

(p <0.05) in samples from the surficial aquifer system 

than in samples from the Upper Floridan aquifer. 

The range of selected constituents is shown in figure 

40. Statistical comparison of water quality from the 

Lower Floridan aquifer is difficult because of the 

limited available data. 

Water type in the Upper Floridan aquifer in about 

the eastern one-third of Orange County is sodium chloride (fig. 42). Concentrations of TDS, chloride, and 

sulfate generally are low (less than 500 mg/L for TDS 

and less than 50 mg/L for chloride and sulfate) 

throughout the Upper Floridan aquifer in Orange 

County (figs. 43 through 45). Concentrations of TDS, 

chloride, and sulfate increase toward eastern and southeastern Orange County, and in northwestern Orange 

County near Rock Springs (figs. 43 through 45). In 

addition, the well at site 24 had relatively high concentrations of TDS, chloride, and sulfate. In the Upper 

Figure 41. Concentrations of nitrite plus nitrate in water samples collected from wells tapping the surficial aquifer 

system, 1990 through 2001.
58 Hydrogeology and Quality of Ground Water in Orange County, Florida Apopka Lake

POLK

COUNTY

SEMINOLE

COUNTY

OSCEOLA COUNTY

LAKE COUNTY VO U

COU

L

BRE

COU

Rock

Springs

Witherington Springs

Wekiva

Springs

3.0 0.0 3.0

3.0 0.0 3.0

3.0 0.0 3.0

3.0 0.0 3.0

3.0 0.0 3.0

3.0 0.0 3.0

3.0 0.0 3.0 3.0 0.0 3.0

3.0 0.0 3.0

2.0 0.0 2.0

2.0 0.0 2.0

2.0 0.0 2.0

4.0 0.0 4.0

5.0 0.0 5.0

6.0 0.0 6.0

6.0 0.0 6.0

4.0 0.0 4.0

5.0 0.0 5.0

4.0 0.0 4.0

5.0 0.0 5.0

3.0 0.0 3.0

3.0 0.0 3.0

5.0 0.0 5.0

6.0 0.0 6.0

6.0 0.0 6.0

6.0 0.0 6.0

10 0.0 10

10 0.0 

10 0.0 10

10 0.0 10

10 0.0 10

4.0 0.0 4.0

4.0 0.0 4.0

7.0 0.0 7.0

7.0 0.0 7.0

3.0 0.0 3.0

4.0 0.0 4.0

10 0.0 

4.0 0.0 4.0

(meq/L)

160

164

162

1

150

166

173

2

170

178

171

177

147

128 126

81

97

24

13

3

105

72

122

110

118

87

9

90

30

15

82

75

11

22

63 69 62

42

3

EXPLANATION

STIFF DIAGRAMS

SPRING

WELL AND NUMBER--

Number refers to site numberin appendixes 1 and 2.Concentrations are in milliequivalentsper liter (meq/L)

Sodium+potassium Chloride+ fluoride

Calcium

Magnesium

Bicarbonate

Sulfate

0 5 10 KILOMETERS

0 5 10 MILES

28 20 ° ′

40′

81°40′ 30′ 81° ′ 20′ 10′ 

00

28°50′

30′

Figure 42. Concentrations of major cations and anions in water samples collected from springs and wells tapping the Upp

aquifer, 1995 through 2001.
Water Quality 59

Apopka

Lake

POLK

COUNTY

SEMINOLE COUNTY

OSCEOLA COUNTY

LAKE COUNTY VOLUSIA

COUNTY

BREVARD

COUNTY

ORANGE COUNTY

Rock

Springs

G

G

G

Witherington Springs

Wekiva

Springs

203 ___

168

146 ___

2

G

G

1,242 ____

132

1,420 ____

118

1,490 ____

87 2,290 ____

92

____ 740

30

____ 500

11

____ 960

42

1,480 ____

75

390 ___

178

____ 520

78

752 ___

82

534 ___

57

804 ___

69

686 ___

51

723 ___

62

___ 654

177

658 ___

171

1,740 ____

176

249 ___

110

320 ___

122

495 ___

90

353 ___

22

411 ___

15

273 ___

72 376 ___

63

255 ___

13

346 ___

24

330 ___

159

___ 723

179

145 ___

164

119 ___

170

159 ___

162

183 ___

1

236 ___

166 141 ___

160

196 ___

148 167 ___

150

___ 204

128

___ 94

107

___ 45

127 109 ___

126

150 ___

81

168 ___

97

243 ___

3

167 ___

105

146 ___

147

___ 437

112

____ 273

173

____ 119

163

EXPLANATION

SPRING

<250

250 - 500

>500 - 1,000

>1,000

CONCENTRATION RANGES

G

146 ____

147

WELL -- Top number is concentration,

in milligrams per liter; bottom number refers to

site number in appendix 2

  





       





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 ′ ° ° ′ ′ 0 5 10 KILOMETERS

0 5 10 MILES

Floridan aquifer, sulfate concentrations exceeded the 

secondary standard of 250 mg/L in 2 of 48 samples, 

chloride concentrations exceeded the secondary standard of 250 mg/L in 8 of 48 samples, and TDS concentrations exceeded the secondary standard of 500 mg/L 

in 15 of 56 samples. In the Lower Floridan aquifer, 

sulfate and chloride concentrations exceeded the secondary standards in 7 of 25 samples, and TDS concentrations exceeded the secondary standard in 8 of 25 

samples. The distribution of sample locations, however, 

is much more limited for wells tapping the Lower Floridan aquifer (fig. 9); hence, these data may not accurately represent the water quality of the entire Lower 

Floridan aquifer throughout Orange County. A detailed 

discussion of the water quality of the Lower Floridan 

aquifer also is available in O’Reilly and others (2002).

At several locations in Orange County, samples 

from the Upper Floridan aquifer had mixed-ion or 

sodium-chloride type water and high concentrations of 

TDS. In eastern Orange County, samples from several 

wells had sodium-chloride water type and concentrations of TDS greater than 1,000 mg/L (figs. 42 and 43, 

respectively). These wells coincide with an area where 

the potentiometric surface of the Upper Floridan aquifer 

is higher than land surface, indicating an area of possible 

discharge from the Upper Floridan aquifer and an 

upwelling of deeper ground water, possibly relict seawater. Similarly, in northwestern Orange County, near Rock 

Springs and along the Wekiva River, water type in the 

Upper Floridan aquifer is mixed ion and concentrations 

of TDS are greater than 500 mg/L. This also is an area of 

discharge from the Upper Floridan aquifer, as evidenced 

by the presence of Rock, Wekiva, and Witherington 

Springs, and could allow upwelling of deeper water, 

probably from the Lower Floridan aquifer, with higher 

concentrations of TDS. No wells tap the Lower Floridan 

aquifer at these two locations, so differences in water 

levels and water quality between the Upper and Lower 

Floridan aquifers cannot be quantified to confirm 

upwelling as a cause for the observed differences.

Figure 43. Concentrations of total dissolved solids in water samples collected from springs and wells tapping the 

Upper Floridan aquifer, 1990 through 2001.
60 Hydrogeology and Quality of Ground Water in Orange County, Florida

Apopka

Lake

POLK

COUNTY

SEMINOLE COUNTY

OSCEOLA COUNTY

LAKE COUNTY

VOLUSIA

COUNTY

BREVARD

COUNTY

ORANGE COUNTY

Rock

Springs

G

G

G

Witherington Springs

Wekiva

Springs

G

G

110 ___

173

300 ___

75

___ 291

177

250 ___

92

150 ___

171

180 ___

176 160 ___

178

150 ___

62

160 ___

82

180 ___

42

180 ___

87

150 ___

116

230 ___

118

157 ___

132

___ 79

168

100 ___

57

110 ___

51

120 ___

69

___ 53

140

133 ___

119

___ 81

90

___ 10

164 ___ 43

166

___ 17

2

___ 12

170

___ 3.1

158

___ 14

162 ___ 18

1

___ 17

150

___ 10

167

___ 16

160

___ 28

128 ___ 25

127

___ 0.4

126

5.5 ___

147

___ 18

133

___ 9.6

138

___ 2.1

81

___ 41

24

<0.2 ___

13

___ 14

97

___ 2.0

72

___ 14

112

___ 32

110

___ 13

105

___ 0.4

3

___ 46

15

___ 44

122

___ 25

22

___ 0.2

63

___ 44

78

___ 47

11

___ 40

30

EXPLANATION

SPRING

<50

50 - 150

>150 - 250

>250

CONCENTRATION RANGES

G

157 ___

132

WELL -- Top number is concentration,

in milligrams per liter; bottom number refers to

site number in appendix 2

  





       





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 ′ ° ° ′ ′ 0 5 10 KILOMETERS

0 5 10 MILES

Water from many of the wells in and around the 

Cocoa well field in southeastern Orange County is 

sodium-chloride type with concentrations of TDS 

greater than 500 mg/L. Samples from two wells had 

TDS concentrations of 960 and more than 1,400 mg/L 

(fig. 43). The water quality of the Upper Floridan aquifer in this area probably is affected by pumping of the 

well field, causing the upwelling of deeper, saline 

water, possibly relict seawater. The migration of this 

water probably is vertical, rather than horizontal, 

through conduits and fractures in the aquifer. The 

majority of affected wells are aligned in a north-south 

orientation. A sample from a well east of the well field 

had a TDS concentration of 740 mg/L (fig. 43).

Constituent concentrations in samples from an 

observation well (site 24) in southwestern Orange 

County could indicate that reclaimed water from 

nearby RIBs is infiltrating into the Upper Floridan 

aquifer. Concentrations of sodium, chloride, and sulfate 

are relatively high (53, 90, and 41 mg/L, respectively) 

and are in the range of concentrations for reclaimed 

water (table 7; Sumner and Bradner, 1996). As noted 

previously, water levels in this well have increased in 

response to the mounding of water beneath the RIBs. 

A sample collected from this well in 1979 prior to the 

installation of the RIBs had concentrations of chloride 

and sulfate of 4.0 and 28 mg/L, respectively (table 7). 

Hence, the concentrations of chloride and sulfate 

apparently have increased over time, probably as a 

result of infiltration of reclaimed water from the RIBs.

Changes in water quality of the Floridan aquifer 

system with respect to concentrations of major ions 

were assessed two ways. First, the historical areal 

(1960 through 1969) distribution of chloride concentrations (fig. 46) was compared to recent (1990 through 

2001) distribution of chloride concentrations (fig. 45).

A limitation to this method is that few wells were sampled both during the 1960s and the recent period. Many 

wells sampled in the 1960s have been abandoned or 

destroyed and other wells have been drilled. For a more 

Figure 44. Concentrations of sulfate in water samples collected from springs and wells tapping the Upper 

Floridan aquifer, 1990 through 2001.
Water Quality 61

Apopka

Lake

POLK

COUNTY

SEMINOLE COUNTY

OSCEOLA COUNTY

LAKE COUNTY VOLUSIA

COUNTY

BREVARD

COUNTY

ORANGE COUNTY

___ 550

132

___ 200

177

___ 560

116

640 ___

87

1,100 ____

92

510 ___

75

290 ___

42

330 ___

30

444 ___

118

214 ___

140

196 ___

119

___ 56

122

200 ___

82 ___ 96

78

220 ___

69

150 ___

51

120 ___

62

110 ___

90

___ 82

11

___ 90

24

647 ___

176

220 ___

171 ___ 41

166

___ 7.3

170

___ 8.6

160

___ 11

167

___ 16

128

___ 6.1

126

___ 9.0

127

___ 13

97 ___ 5.9

81

___ 14

13

___ 22

3

___ 17

72

___ 18

150 ___ 11

147

___ 10

133

___ 13

138

___ 11

105

___ 35

110 ___ 17

112

___ 38

15

___ 34

22

___ 11

63

170 ___

57

Rock

Springs

G

GG

Witherington

Springs

Wekiva

Springs

G 173

___ 7.7 G

___ 34

178

___ 6.7

168

___ 14

1

___ 9.8

___ 17 162

158

___ 7.7

164

___ 7.8

2

EXPLANATION

SPRING

<50

50 - 250

>250 - 500

>500

CONCENTRATION RANGES

G

200 ___

177

WELL -- Top number is concentration,

in milligrams per liter; bottom number refers to

site number in appendix 2

  





       





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 ′ ° ° ′ ′ 0 5 10 KILOMETERS

0 5 10 MILES

accurate assessment of changes in ground-water quality, 

data from the same sites should be compared. Second, 

wells with long-term data were analyzed for trends in 

water quality. Wells with long-term water-quality data 

primarily are located in and around the Cocoa well 

field; hence, areas in other parts of Orange County are 

not well represented with long-term data sites.

 A comparison of historical (fig. 46) and recent 

(fig. 45) chloride concentrations indicates that concentrations in the Upper Floridan aquifer throughout most 

of Orange County do not appear to have changed significantly. For example, samples from two wells in 

eastern Orange County (sites 92 and 116) had chloride 

concentrations of 1,120 and 620 mg/L in the 1960s and 

1,100 and 560 mg/L in recent years (figs. 45 and 46). 

Samples from central, western, and northwestern 

Orange County also have similar chloride concentrations in the 1960s and in recent years. Historic and 

recent samples from the same wells in southwestern 

Orange County are not available, and, therefore, 

Figure 45. Concentrations of chloride in water samples collected from springs and wells tapping the Upper 

Floridan aquifer, 1990 through 2001

Table 7. Comparison of historical (1979) and recent (1999) 

water-quality data for site 24 with the range of quality of 

reclaimed water discharged to rapid infiltration basins in 

southwestern Orange County

[All values in milligrams per liter, except specific conductance in 

microsiemens per centimeter at 25 oC, and pH in standard units; 

--, no data. Site 24 refers to figure 9 and appendix 2]

Constituent

 Site 24

Reclaimed 

watera

aData from Sumner and Bradner, 1996.

March 26, 

1979

July 12, 

1999

Specific conductance 258 645 480 -550

pH 7.9 7.4 5.9 - 8.2

Calcium 37 63 25 - 35

Magnesium 4.7 6.3 5.0 - 10

Sodium 3.8 53 45 - 80

Bicarbonate -- 172 52 -150

Sulfate 28 41 --

Chloride 4 90 60 -155
62 Hydrogeology and Quality of Ground Water in Orange County, Florida

Apopka

Lake

POLK

COUNTY

SEMINOLE COUNTY

OSCEOLA COUNTY

LAKE COUNTY VOLUSIA

COUNTY

BREVARD

COUNTY

ORANGE COUNTY

1,650

620

500

1,120

760

670

348

340

340 277

280

220

55

X

72

82

98

115

54

16

9.0

8.0 8.0

7.5

7.0 8.0

5.0

6.0

8.0

5.5

5.0

13

8.0

14 12

13

8.0 6.5 3.0

4.0

8.0

7.5

3.0

4.0

9.0

9.5

18 13

13

13

9.0

12 15

14 4.0

9.5 2.0

2.0

3.8

4.0

6.0 5.5

6.0

5.0 16

32

15

13

8.0 10 12

11 14

11

10

8.0

8.0 8.011

10 19 19

11

9.0 9.5 10

16 9.0

20

11

12 31

12

24

24

38

10 11

9.4

11

13

10

24

12

13

43

13 32

25

23

6.0

9.0

4.0

14

4.0

6.0 3.0

2.0

2.0

2.0

1.0

4.0

7.0 5.0 10

6.0

6.0 1.0

Rock

Springs

G

G

G

Witherington

Springs

Wekiva

Springs

G8.0

EXPLANATION

SPRING

<50

50 - 250

>250 - 500

>500

CONCENTRATION RANGES

13 G

WELL -- Number is concentration,

in milligrams per liter

WELL WITH RECENT (1990 - 2001) DATA 8.0

28 20 ° ′

40′

81°40′ 20′ 10′ 80°50′ 30′   

28°50′

3 ′ 0

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 ′ ° ° ′ ′ 0 5 10 KILOMETERS

0 5 10 MILES

changes are difficult to assess. Nonetheless, chloride 

concentrations in samples collected recently in southwestern Orange County generally are within the range 

of concentrations in samples collected in the 1960s. 

In southeastern Orange County, however, chloride concentrations apparently have increased, especially in the 

vicinity of the Cocoa well field. For example, chloride 

concentration at the well at site 75 increased from 

280 mg/L in 1966 to 510 mg/L in 1999 (figs. 45 and 46).

Long-term data were analyzed to determine 

if the increase in chloride concentrations were statistically significant. A total of 24 wells, each of which 

had more than 20 samples collected during at least a 

20-year period, was included in the trend analysis 

(table 8). Four of the wells are nested in a single borehole (sites 64 through 67). All of the wells are in eastern Orange County, and all but two of the wells are in 

and around the Cocoa well field. Two wells (sites 64 

and 65) exclusively tap the Lower Floridan aquifer, 

1 well taps the middle semiconfining unit (site 66), 

1 well is open to the Upper Floridan aquifer and 

the middle semiconfining unit (site 67), and the 

remaining 20 wells tap the Upper Floridan aquifer. 

In addition, the long-term data are limited primarily to 

major-ion concentrations; trend analysis was limited to 

specific conductance, chloride, and sulfate. Data from 

14 wells indicated a statistically significant increase in 

specific conductance, whereas data from 2 wells indicated a decrease in specific conductance over time. 

Data from 16 of the wells indicated a statistically 

significant increase in chloride concentrations during 

the period of record, whereas data from 2 of the wells 

indicated a decrease in chloride concentrations during 

the period of record. Data from 13 wells indicated a 

statistically significant increase in sulfate concentrations, whereas data from 5 wells indicated a decrease in 

sulfate concentrations over time (table 8). In one well 

(site 58), sulfate concentration decreased while chloride concentration increased (table 8; fig. 47).

Figure 46. Concentrations of chloride in water samples collected from springs and wells tapping the Upper 

Floridan aquifer, 1960 through 1969.
Water Quality 63

Table 8. Evaluation of trends in specific conductance, chloride, and sulfate for 

wells having 20 or more years of record

[Site numbers refer to appendixes 1 and 2. Kendall’s tau values shown in boldface were statistically 

significant at the 0.05 significance level. Well depth, in feet below land surface; μS/cm, microsiemens 

per centimeter at 25 oC; mg/L, milligrams per liter; <, less than]

 Site 

number Site identifier Well 

depth

Period 

of 

record

Kendall’s tau

Specific 

conductance 

(μS/cm)

Chloride 

(mg/L)

Sulfate 

(mg/L)

28 282344081054201 580 1962 - 2001 0.73 0.77 0.74

37 282404081050501 519 1962 - 2000 .11 .05 .12

38 282405081053002 527 1972 - 2001 .59 .67 .7

42 282412081044701 600 1963 - 2000 .23 .3 .39

43 282416081054101 524 1964 - 2001 -.13 .04 -.26

51 282529081073201 710 1962 - 2001 .49 .58 .56

52 282530081054201 490 1957 - 2000 -.19 -.23 -.32

56 282530081084501 702 1965 - 2000 .59 .54 .32

57 282830081091701 600 1964 - 2001 .24 .35 -.32

58 282530081094001 600 1964 - 2001 -.30 .43 -.63

60 282531081075601 509 1963 - 2001 .85 .88 .78

62 282531081082201 761 1966 - 2001 .75 .86 .79

64 282533081082202 1,360 1966 - 2001 .77 .89 .77

65 282533081082204 1,220 1966 - 2001 .13 -.04 .29

66 282533081082205 1,050 1966 - 2001 -.10 -.4 -.31

67 282533081082206 1,000 1966 - 2000 .46 .62 .45

69 282548081054201 496 1957 - 2001 .08 .07 .02

71 282612081054201 617 1966 - 2001 .26 .27 .26

75 282632081054501 640 1966 - 2000 .53 .54 .48

76 282650081054201 525 1957 - 2001 .23 .24 .24

78 282716081054501 506 1964 - 2001 -.07 -.16 -.17

90 282847081013701 495 1961 - 2001 .37 .33 .13

118 283214080583501 200 1953 - 1999 -.28 -.36 -.27

122 283249081053201 492 1960 - 2000 .34 .3 -.07
64 Hydrogeology and Quality of Ground Water in Orange County, Florida

Site

Kendall’s tau = 0.43 (<0.01)

58

Depth = 600 feet

Casing length = 252 feet

Site

Kendall’s tau = -0.63 (<0.01)

58

Depth = 600 feet

Casing length = 252 feet

Site

Kendall’s tau = -0.30 (<0.01)

58

Depth = 600 feet

Casing length = 252 feet

(<0.01)

EXPLANATION

Number in parenthesis is p-value, or probability

that trend occurs by chance. A probability

less than 5 percent (<0.05) is considered significant

1,000

900

800

700

600

500

400

130

110

90

70

50

60

50

40

30

1970 1980 1990 2000

YEAR

1960 1965 1975 1985 1995 2005

CHLORIDE,

IN MILLIGRAMS PER LITER

SULFATE,

IN MILLIGRAMS PER LITER

SPECIFIC CONDUCTANCE,

IN MICROSIEMENS

PER CENTIMETER

150

Figure 47. Changes in specific conductance and in concentrations 

of sulfate and chloride in well at site 58 tapping the Upper Floridan 

aquifer (site number refers to figure 9 and appendix 1).
Water Quality 65

Twelve of the wells with increasing chloride 

concentrations are in or in close proximity to the Cocoa 

well field, indicating that pumping from the well field 

probably is causing upwelling of deeper, more saline 

water (Phelps and Schiffer, 1996). The water apparently is migrating preferentially through conduits and 

fractures in the rock. Data from the four zones at the 

nested well (sites 64 through 67) illustrate this preferential movement. The increase in chloride concentrations is evident in samples from the shallow (site 67) 

and deep wells (site 64), but not from the intermediate 

wells (table 8; fig. 48).

Limited historical data also are available for 

Wekiva Springs and Rock Springs. Data are limited 

primarily to specific conductance and concentrations of 

sulfate and chloride. Values of specific conductance, 

and probably TDS, have increased in samples from 

both springs since about the mid-1960s (fig. 49). These 

results are in agreement with previous studies (Murray 

and Halford, 1996). Discharge at both springs (fig. 35) 

and water levels in an observation well (site 170, fig. 9) 

that taps the Upper Floridan aquifer near Rock Springs 

have declined over the same period of time (Kendall’s 

tau = -0.21, p <0.01). The increase in specific conductance at Wekiva Springs does not seem to be related to 

upwelling of deeper, saline water. Three observations 

wells (site 162) tapping the surficial aquifer system 

(well depth = 60 ft), the Upper Floridan aquifer (well 

depth = 155 ft), and the middle semiconfining unit 

(well depth = 645 ft) are near Wekiva Springs. The 

median values of specific conductance, in samples collected from 1993 through 1998, were 278 μS/cm for 

the surficial aquifer system, 291 μS/cm for the Upper 

Floridan aquifer, and 240 μS/cm for the middle semiconfining unit. Also, chloride concentrations in the 

water from the Upper Floridan aquifer in the area of 

Wekiva and Rock Springs do not seem to have 

increased since the 1960s (figs. 45 and 46).

As with the surficial aquifer system, nutrient 

concentrations in the Floridan aquifer system generally 

are low. Twenty-nine of 31 recent samples from the Upper 

Floridan aquifer have nitrate concentrations less than or 

equal to the reporting limit of 0.02 mg/L. None of the 

samples from the Lower Floridan aquifer had detectable 

concentrations of nitrate (table 6). A sample from one 

well tapping the Upper Floridan aquifer (site 127, fig. 9) 

had a nitrate concentration of 2.0 mg/L as nitrogen. The 

well, located in west-central Orlando, is a drainage well 

for a lake. Water artificially recharging the aquifer from 

the lake could be the source of nitrate and possibly other 

constituents. The sample from the well also had a TDS 

concentration of 45 mg/L, less than half of the concentration in a sample from a nearby well (site 126, fig. 9), 

possibly indicating that the well received lake water prior 

to being sampled. Similarly, other drainage wells on lakes 

and streets (fig. 26) may allow contaminants to be transported to the Upper Floridan aquifer.

Concentrations of nitrate in water from Wekiva 

and Rock Springs have generally increased with time 

(Spechler and Halford, 2001), although the most recent 

values are somewhat lower than historic maximums. In 

Wekiva Spring, nitrate values ranged from less than 

0.02 mg/L in the 1950s and 1960s to a maximum of 

1.98 mg/L in 1988; the concentration was 0.98 mg/L in 

2001. In Rock Springs, values ranged from less than 

0.02 mg/L in 1956 to 1.81 mg/L in 1993; the concentration was 1.4 mg/L in 1999. Increases in specific conductance from these springs could be related to the 

effects of fertilizer or septic-tank leachate on localized 

recharge water into the springs. Toth (1999) investigated the isotopic geochemistry in selected springs to 

determine sources of nitrates. The ratio of 15N/14N of 

nitrate from synthetic fertilizers ranges from about -5 to 

+8 per mil, whereas the ratio of 15N/14N of nitrate from 

animal wastes or sewage is about +8 to more than 20 per 

mil (Heaton, 1986). Results from Toth (1999) indicated 

the ratio of 15N/14N was +5.8 and +8.6 per mil for Rock 

and Wekiva Springs, respectively, values that are roughly 

between the ranges for synthetic fertilizer and wastewater sources. These values could be indicative of the 

change of land use, from agriculture to residential, and 

hence, reflect the change of the source of nitrate from 

synthetic fertilizers to wastewater. Toth (1999), using 

isotopes to date water from Rock and Wekiva Springs, 

indicated that at least a fraction of the water discharging 

from both springs was young (post-1953). Additional 

dating of spring water would be useful in understanding the sources of nitrates, given the rapid changes in 

land-use occurring in Orange County.

Ammonia and ammonia plus organic nitrogen 

were present in about half of the ground-water samples 

from the Floridan aquifer system. Ammonia plus 

organic nitrogen concentrations were greater than 

0.5 mg/L as nitrogen in 11 of 26 samples from the 

Upper Floridan aquifer. Most of these samples were 

from wells in eastern Orange County. Concentrations 

of ammonia plus organic nitrogen in samples from 

three flowing wells (sites 87, 92, and 118) were 

0.65, 0.64, and 0.63 mg/L as nitrogen, respectively.
66 Hydrogeology and Quality of Ground Water in Orange County, Florida

Site

: Lower Floridan aquifer

Kendall’s ta = 0.89 (<0.01)

64

Zone 1

Depth = 1,357 feet

Casing length = 1,351 feet

u

Site 67

Zone 5: Upper Floridan aquifer

Depth = 1,004 feet

Casing length = 248 feet

Kendall’s ta = 0.62 (<0.01) u

Site

Kendall’s ta = -0.40 (<0.01)

66

Zone 4: Middle semi-confining unit

Depth = 1,050 feet

Casing length = 1,044 feet

u

Site

: Middle semi-confining unit

Kendall’s ta = 0.31 (<0.01)

66

Zone 4

Depth = 1,050 feet

Casing length = 1,044 feet

u

Site

: Upper Floridan aquifer

Kendall’s ta = 0.45 (<0.01)

67

Zone 5

Depth = 1,004 feet

Casing length = 248 feet

u

Site

: Lower Floridan aquifer

Kendall’s ta = 0.77 (<0.01)

64

Zone 1

Depth = 1,357 feet

Casing length = 1,351 feet

u

Site

Kendall’s ta = 0.04 (0.53)

65

Zone 3: Lower Floridan aquifer

Depth = 1,224 feet

Casing length = 1,218 feet

u

Site

: Lower Floridan aquifer

Kendall’s ta = 0.29 (<0.01)

65

Zone 3

Depth = 1,224 feet

Casing length = 1,218 feet

u

CHLORIDE, IN MILLIGRAMS PER LITER

SULFATE, IN MILLIGRAMS PER LITER

1960 1970 1980 1990 2000

70

60

50

40

30

95

90

85

80

75

70

120

100

80

60

40

3,200

2,200

1,200

200

1,300

1,100

900

700

500

80

100

120

140

0

10

20

30

40

50

40

60

80

100

1960 1970 1980 1990 2000

YEAR

1965 1975 1985 1995 2005 1965 1975 1985 1995 2005

YEAR

(<0.01)

EXPLANATION

Number in parenthesis is p-value, or probability

that trend occurs by chance. A probability

less than 5 percent (<0.05) is considered significant

Figure 48. Concentrations of chloride and sulfate in water samples from wells tapping the Floridan aquifer 

system (site numbers refer to appendix 1).
Water Quality 67

Because these wells are located in a discharge area for 

the Upper Floridan aquifer, the source of ammonia plus 

organic nitrogen in these samples probably is natural 

constituents in the aquifer. Only one of eight samples 

from the Lower Floridan aquifer had a concentration of 

ammonia plus organic nitrogen greater than 0.5 mg/L as 

nitrogen. Because of the depth and location of the well 

(site 53 in the Cocoa well field, fig. 9), the source of the 

ammonia plus organic nitrogen also probably is natural.

Phosphorus concentrations generally were low in 

ground-water samples from the Floridan aquifer system 

(table 6). Phosphorus was present in all eight samples 

from the Lower Floridan aquifer, but the maximum 

concentration was only 0.07 mg/L as phosphorus. 

Phosphate concentrations were greater than 0.1 mg/L 

as phosphorus in 7 of 33 samples (sites 1, 15, 97, 147, 

159, 160, and 162, fig. 9) from the Upper Floridan 

aquifer, including Wekiva Springs and one sample from 

a public-supply well (site 97). The source of phosphorus could either be from land-use practices or from the 

leaching of phosphatic sediments in the Hawthorn 

Group by recharging ground water. The median 

concentration of phosphorus was not significantly 

different between samples from the surficial aquifer 

system and the Upper Floridan aquifer. This result 

could indicate the Hawthorn Group is the source of 

phosphorus.

Trace Elements, Radon, and Stable 

Isotopes

Trace elements are metallic and nonmetallic 

constituents that occur naturally in water in low concentrations, generally just a few parts per billion or 

micrograms per liter. The source of many of these constituents is from water-rock interactions in the aquifer. 

Some trace elements (table 9) also have industrial and 

agricultural uses. As a result, the occurrence of these 

trace elements in concentrations elevated above natural 

levels can result from and be indicative of anthropogenic sources. The occurrence of some trace elements 

in water also can pose health or aesthetic problems. 

Trace elements such as arsenic and lead are toxic and 

have National Primary Drinking Water Regulations 

established to limit their consumption in drinking water 

(U.S. Environmental Protection Agency, 2002).

Elements such as cadmium and selenium can accumulate in vegetation, and ultimately in people or animals 

that consume the vegetation (Hem, 1989). Iron can 

impart an objectionable taste to water and stain plumbing fixtures. Trace elements generally occur as charged 

ions that readily sorb onto sediment and colloids in the 

water; hence, the concentrations in the filtered samples 

collected during this study could be less than concentrations in whole water.

Wekiva Springs

Site 1

Rock Springs

Site 2

Site 170

160

210

260

310

SPECIFIC CONDUCTANCE,

IN MICROSIEMENS PER CENTIMETER

200

260

280

220

240

28

30

32

34

36

YEAR

WATER LEVEL, IN FEET ABO

1960 1970 1980 1990 2000

VE

NGVD 29

Figure 49. Specific conductance in ground-water 

samples collected from Wekiva Springs (site 1) and Rock 

Springs (site 2), and water levels in nearby observation 

well tapping the Upper Floridan aquifer (site 170). (Site 

numbers shown on figure 9.)
68 Hydrogeology and Quality of Ground Water in Orange County, Florida Table 9. Summary statistics for selected trace elements and radon in ground-water samples from the surficial and Floridan aquifer syste

[Source of data, U.S. Geological Survey and St. Johns River Water Management District; all concentrations dissolved, in micrograms per liter, except where noted; <, les

Consti- tuent Surficial aquifer system Upper Floridan aquifer Low

No. of sam- ples No. of detec- tions Mini- mum 25th per- centile Median 75th per- centile Maximum No. of sam- ples No. of detec- tions Mini- mum 25th per- centile Median 75th per- centile Maximum No. of sam- ples No. of detec- tions Mini- mum 

Aluminum 24 22 <3.0 20 60 265 1,800 22 4 <3.0 <3.0 <3.0 <3.0 10 3 3 7.6 

Arsenic 25 7 <2.0 <2.0 <2.0 2 6.5 26 4 <2.0 <2.0 <2.0 <2.0 4.1 8 1 <2.0 

Barium 24 24 2 7 19 36 410 26 25 6 9 15 29 120 8 8 9 Beryllium 24 2 < .5 < .5 < .5 < .5 .5 26 1 < .5 < .5 < .5 < .5 1 8 0 < .5 Boron 24 24 8.7 16 24 39 95 23 22 13 20 29 52 170 3 3 34 Cadmium 24 3 < .5 < .5 < .5 < .5 1.2 26 0 < .5 < .5 < .5 < .5 < .5 8 0 < .5 Chromium 12 4 <1.0 <1.0 <1.0 1.4 4 4 0 <1.0 <1.0 <1.0 <1.0 <1.0 2 0 <1.0 Cobalt 24 4 <1.0 <1.0 <1.0 <1.0 1.9 26 0 <1.0 <1.0 <1.0 <1.0 <1.0 8 0 <1.0 Iron, total 6 6 21 46 163 320 3,600

Iron,  dissolved

25 25 3.2 95 399 2,100 26,000 34 33 <2 23 38 140 2,000 22 22 4.4 

Lead 24 2 <2.0 <2.0 <2.0 <2.0 4.9 26 0 <2.0 <2.0 <2.0 <2.0 <2.0 8 0 <2.0 Lithium 24 13 < .5 < .5 .7 1.4 12 23 20 < .5 1 3.2 7.7 14 3 3 1.4 Manganese 25 24 <1.0 7.6 26 58 270 26 24 <1.0 2.9 5.1 13 44 19 9 <1.0 Selenium 24 8 <2.0 <2.0 <2.0 2.1 8 26 9 <2.0 <2.0 <2.0 2.2 6.2 8 2 <2.0 Strontium 24 24 8 25 51 95 5,200 38 38 34 140 710 2,000 10,200 23 23 57 Vanadium 24 14 <1.0 <1.0 1 3 4 26 3 <1.0 <1.0 <1.0 <1.0 2 8 0 <1.0 Radon, in picoCuries

 per liter

22 22 71 513 913 2,250 14,700 22 22 56 153 344 975 1,600 8 8 59 
Water Quality 69

Data from water samples collected for this study 

(1999 through 2001) indicate that the most commonly 

detected trace elements in Orange County were aluminum, barium, boron, iron, manganese, and strontium. 

These constituents were detected in nearly every 

sample collected from the surficial aquifer system, the 

Upper Floridan aquifer, and the Lower Floridan aquifer 

(table 9). Of these constituents, only aluminum, iron, 

and manganese have standards for drinking water 

(table 3). Aluminum has a secondary standard of 

200 μg/L, iron has a secondary standard of 300 μg/L, 

and manganese has a secondary standard of 50 μg/L 

(U.S. Environmental Protection Agency, 2002). The 

secondary standards for aluminum and manganese 

were exceeded in more than 25 percent of the samples 

from the surficial aquifer system. The secondary standard for iron was exceeded in more than 50 percent of 

the filtered samples from the surficial aquifer system, 

but only in about 13 percent of the filtered samples 

from the Upper Floridan aquifer, and about 9 percent of 

filtered samples from the Lower Floridan aquifer. Iron 

concentrations were higher in whole-water samples 

than in filtered samples (table 9).

Other trace elements were detected less frequently 

in ground-water samples. Lithium was detected in 36 

of 50 samples (72 percent). Vanadium was detected in 

17 of 58 samples (29 percent). No drinking water standards are set for these two constituents. Arsenic was 

detected in 7 of 25 samples from the surficial aquifer 

system, in 4 of 26 samples from the Upper Floridan 

aquifer, and in 1 of 8 samples from the Lower Floridan 

aquifer. Beryllium was detected in 2 of 24 samples 

from the surficial aquifer system and 1 of 26 samples 

from the Upper Floridan aquifer, but no samples from 

the Lower Floridan aquifer contained beryllium. Selenium was detected in 8 of 24 samples from the surficial 

aquifer system, in 9 of 26 samples from the Upper 

Floridan aquifer, and in 2 of 8 samples from the Lower 

Floridan aquifer (table 9). Of 24 samples collected 

from the surficial aquifer system, cadmium was 

detected in 3, cobalt in 4, and lead in 2, but none of 

these trace elements were detected in any samples from 

the Floridan aquifer system. The drinking-water MCLs 

for these trace elements are: arsenic, 10 μg/L; beryllium, 4 μg/L; cadmium, 5 μg/L; chromium, 100 μg/L; 

lead, 15 μg/L; and selenium, 50 μg/L (U.S. Environmental Protection Agency, 2002). None of these trace 

elements exceeded the MCL standard in ground-water 

samples collected in Orange County (tables 3 and 9).

Sources of these trace elements probably are 

both natural and anthropogenic. Aluminum, iron, and 

manganese are common constituents in rocks and soil. 

Strontium can substitute for calcium in calcite, the 

major mineral present in limestone, and is present in 

ground water after dissolution of limestone. Arsenic is 

present in rocks, but also has many commercial and 

industrial uses, such as an ingredient in pesticides and 

as a wood preservative. The occurrence of arsenic in 

the surficial aquifer system could be anthropogenic; 

however, the occurrence of arsenic in the Lower Floridan aquifer most likely is natural because the aquifer 

generally is not directly affected by surface contamination. The occurrence of cadmium, chromium, cobalt, 

and lead in samples from the surficial aquifer system 

but not in samples from the Floridan aquifer system 

indicates a possible anthropogenic source for these 

constituents. These constituents have many industrial 

uses; however, the concentrations detected in the samples are within ranges of natural waters (Hem, 1989).

Radon, an inert, radioactive gas, is part of the 

uranium decay series, and results from the radioactive 

decay of radium through the emission of an alpha particle. Radon decays to polonium. Radium, radon, and 

alpha particles can cause cancer (Otton and others, 

1995). National Primary Drinking Water Regulations 

have been established for radium (5 pCi/L) and alpha 

particles (15 pCi/L), but not for radon (U.S. Environmental Protection Agency, 2002).

A total of 52 samples was collected from 1999 

through 2001 and analyzed for radon as part of this 

study. Radon was detected in every sample and concentrations ranged from 56 to 14,700 pCi/L (table 9). 

Radon concentrations generally had no apparent 

pattern of geographic distribution in the Upper Floridan 

aquifer, but high concentrations of radon in the surficial 

aquifer system were somewhat more prevalent in the 

central part of Orange County (figs. 50 and 51). Radon 

concentrations also were significantly higher in 

samples from the surficial aquifer system than in 

samples from the Upper Floridan or Lower Floridan 

aquifers (fig. 52).

The primary source of radon in ground water in 

Orange County probably is the uranium-bearing phosphate minerals in the Hawthorn Group. The concentrations of radon are highest in the surficial aquifer system 

either because the gas dissipates upward to the aquifer 

or because reworking of Hawthorn Group sediments 

distributed the phosphate minerals during deposition 

of sediments that form the surficial aquifer system. 
70 Hydrogeology and Quality of Ground Water in Orange County, Florida

Apopka

Lake

POLK

COUNTY

SEMINOLE COUNTY

OSCEOLA COUNTY

LAKE COUNTY VOLUSIA

COUNTY

BREVARD

COUNTY

ORANGE COUNTY

5,100 ___

135

14,700 ______

95

6,457 _____

117

1,324 _____

120

_____ 1,519

31

2,279 _____

124

_____ 1,015

88

_____ 1,157

93

756 ___

161

561 ___

149

4,443 _____

144

632 ___

100

511 ___

3

519 ___

83

___ 810

29

Rock

GSprings

GG

Witherington

Springs

Wekiva

Springs

251 ___

125

71 ___

108

224 ___

74

434 ___

16

153 ___

141

EXPLANATION

SPRING

<500

500 - 1,000

>1,000 - 5,000

>5,000

CONCENTRATION RANGES

G

153 ___

141

WELL --

site number

Top number is concentration,

in picoCuries per liter; bottom number refers to

in appendix 3

  





       





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 ′ ° ° ′ ′ 0 5 10 KILOMETERS

0 5 10 MILES

Because radon has a half life of only 3.8 days, the gas 

generally is not transported far from its source, which 

probably explains the relatively low concentrations in 

the Floridan aquifer system.

As part of this study, 46 water samples also were 

collected from 1999 through 2000 for the analysis of 

deuterium and oxygen-18, naturally occurring stable 

isotopes of hydrogen and oxygen, respectively. The 

results are reported as relative abundances or ratios to 

a standard, the Vienna Standard Mean Ocean Water 

(VSMOW), which can be useful in determining the 

source of ground water. Negative values indicate that 

the sample was lighter or depleted in these two isotopes 

with respect to the VSMOW. Precipitation becomes 

depleted relative to the VSMOW during evaporation. 

As a result, deuterium and oxygen-18 in meteoric water 

are strongly correlated and generally fall along a 

straight line (global meteoric line), which has the 

following equation:

Deuterium = 8 (oxygen-18) + 10 (Craig, 1961). (4)

Locally, the data, and, therefore, the local meteoric 

lines, can vary depending on the latitude and distance 

from the ocean. Linear regression indicated that deuterium and oxygen-18 in ground-water samples from 

Orange County (fig. 53) were related with the following 

equation:

Deuterium = 5.64 (oxygen-18) + 2.70. (5)

The data plot along a line that is roughly parallel 

to the global meteoric line, indicating that the ground 

water in Orange County generally is of meteoric origin. 

However, samples from the Upper and Lower Floridan 

aquifers generally are heavier (less depleted) than 

samples from the surficial aquifer system (fig. 53). 

In addition, deuterium is significantly correlated to 

TDS (Spearmans’s rho=0.63; fig. 53). These relations 

probably result from mixing of a small amount of relict 

Figure 50. Concentrations of radon in water samples collected from wells tapping the surficial aquifer system, 

1999 through 2000.
Water Quality 71

Apopka

Lake

POLK

COUNTY

SEMINOLE COUNTY

OSCEOLA COUNTY

LAKE COUNTY VOLUSIA

COUNTY

BREVARD

COUNTY

ORANGE COUNTY

_____ 1,067

160

1,092 _____

150

_____ 1,119

97

1,022 _____

3

_____ 1,597

87

_____ 1,311

30

794 ___

81

___ 752

72

___ 815

118

___ 832

147

___ 352

162

335 ___

170

211 ___

128

___ 295

24

202 ___

90

Rock

GSprings

GG

Witherington

Springs

Wekiva

Springs

140 ___

126

175 ___

13

145 ___

105

___ 96

122

100 ___

110

___ 56

82

142 ___

92

EXPLANATION

SPRING

<200

200 - 500

>500 - 1,000

>1,000

CONCENTRATION RANGES

G

832 ___

147

WELL -- Top number is concentration,

in picoCuries per liter; bottom number refers to

site number in appendix

  





       





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 ′ ° ° ′ ′ 0 5 10 KILOMETERS

0 5 10 MILES

Figure 51. Concentrations of radon in water samples collected from wells tapping the Upper Floridan aquifer, 

1999 through 2000.

22

8

22

Interquartile

Range (IQR)

EXPLANATION

22 Number of samples

75th percentile + 1.5XIQR

75th percentile

50th percentile (median)

25th percentile

25th percentile -1.5XIQR

Outliers 10,000

RADON, IN PICOCURIES PER LITER

100

1,000

SURFICIAL

AQUIFER

SYSTEM

UPPER

FLORIDAN

AQUIFER

LOWER

FLORIDAN

AQUIFER

Figure 52. Distribution of radon in water samples from the surficial aquifer system, the Upper Floridan aquifer, 

and the Lower Floridan aquifer, 1999 through 2000.
72 Hydrogeology and Quality of Ground Water in Orange County, Florida

D = (5.64 X O) + 2.7 18

VSMOW

D = (8 X O) + 10

(GMWL)

18

Rho = 0.63

EXPLANATION

VSMOW - Vienna Standard Meteoric Ocean Water

GMWL - Global Meteoric Water Line (Craig, 1961)

Surficial aquifer system

Surficial aquifer system

Lower

Floridan

aquifer

Lower Floridan aquifer

Upper Floridan aquifer

Upper Floridan aquifer

-5 -4 -3 -2 -1 0 1 10 100 1,000 10,000

DELTA OXYGEN-18, IN PER MIL TOTAL DISSOLVED SOLIDS,

IN MILLIGRAMS PER LITER

DELTA DEUTERIUM, IN PER MIL

-30

-20

0

-10

-30

-20

0

-10

DELTA DEUTERIUM, IN PER MIL

10 10

seawater with the meteoric water. As noted previously, 

concentrations of TDS, sulfate, and chloride in the 

Upper Floridan aquifer increase in eastern Orange 

County possibly as a result of upwelling of water from 

deeper zones of the aquifer. Assuming it is similar to 

modern seawater, this relict seawater probably is isotopically similar to the VSMOW, and, therefore, heavier 

than precipitation. Hence, mixing with relict seawater 

would impart a heavier ratio than present in modern 

meteoric water, which is represented by the values of 

the surficial aquifer system.

Pesticides

Pesticides are synthetic, organic compounds 

used to control unwanted plants, insects, and fungi. 

The presence of pesticides in ground water and surface 

water indicates anthropogenic sources, and can pose a 

health or ecological risk. Many of the compounds have 

multiple uses, including agriculture and residential pest 

control, aquatic vegetation control, mosquito control, 

and maintenance of road and utility rights-of-way 

(Shahane, 1999). For example, in Florida, atrazine is 

used to control broadleaf weeds in corn and sugarcane 

fields, and also is used to control weeds in residential 

lawns. Furthermore, pesticides can volatilize after 

application and be transported in the atmosphere. 

Atrazine and other pesticide compounds commonly are 

detected in precipitation in midwestern and northeastern states, but few data are available on the concentrations of pesticides in precipitation in Florida (Majewski 

and Capel, 1995). As a result of their widespread use 

and transport, pesticides commonly are detected in 

ground water (Barbash and Resek, 1996), even in areas 

considered relatively pristine (Adamski, 1997).

As part of this study, 34 ground-water samples 

were collected in Orange County from 1999 through 

2000 and analyzed for 47 pesticides and pesticide 

degradates (app. 4). Sixteen samples were collected 

from wells tapping the surficial aquifer system, 14 

samples from wells tapping the Upper Floridan aquifer, 

and 4 samples from wells tapping the Lower Floridan 

aquifer.

Results indicate that some pesticides and degradates are present locally in low concentrations in the 

surficial aquifer system and the Upper Floridan aquifer. 

Pesticides were detected in samples from eight wells 

tapping the surficial aquifer system and from two wells 

tapping the Upper Floridan aquifer (table 10). Pesticides were not detected in samples from the Lower 

Floridan aquifer. A total of 10 compounds was detected, 

including six herbicides, two insecticides, and two 

metabolites. The maximum number of compounds in 

any single sample was three. The most commonly 

Figure 53. Relations between deuterium and oxygen-18, and deuterium and total dissolved solids in groundwater samples collected from the surficial aquifer system, the Upper Floridan aquifer, and the Lower Floridan 

aquifer, 1999 through 2000.
Water Quality 73

detected pesticide compounds were atrazine and 

deethyl atrazine, a metabolite of atrazine. The maximum concentration of any compound detected was 

0.417 μg/L for simazine (table 11), which was from a 

well tapping the surficial aquifer system in southwestern Orange County. Of the 10 compounds detected, 

only atrazine and simazine have National Primary 

Drinking Water Regulations, which were not exceeded 

in any of the ground-water samples. Eight of the 10 

compounds have target levels for cleanup of ground 

water and surface water in brownfields (Florida 

Department of Environmental Protection, 1999). None 

of the concentrations in any of the ground-water samples exceeded the guidance levels for ground-water 

cleanup, but concentrations of chlorpyifos and diazinon 

exceeded the guidance levels for surface-water cleanup 

(table 11).

Some of the pesticide compounds also have 

ecological effects. For example, chlorpyrifos, diazinon, 

and trifluralin are highly toxic to aquatic organisms, 

and chlorpyrifos and diazinon also are highly toxic to 

birds. DDE, a metabolite of DDT, is thought to cause 

the thinning of egg shells of birds (EXTOXNET, 

2002). Finally, concentrations of atrazine as low as 

0.1 μg/L in water has recently been shown to disrupt 

the endocrine systems of male frogs, causing demasculinization (Hayes and others, 2002).

The source of pesticides in these ground-water 

samples is difficult to assess because of the many uses 

of the compounds. In Orange County, the current land 

uses associated with pesticide use include agriculture 

and urban (commercial and residential). Row-crop 

agriculture, such as corn, has not been an important 

land use since 2000; however, some pesticides and 

metabolites are persistent in the environment. Atrazine, 

chlorpyrifos, DCPA, diazinon, prometon, and simazine 

are commonly used to control pests in residential and 

urban areas. Eight (6 samples from the surficial aquifer 

system and 2 samples from the Upper Floridan aquifer) 

of 10 samples with pesticides detected were collected 

from wells in the metropolitan Orlando area; 5 of the 

samples were collected from residential sites that were 

constructed prior to 1977. Therefore, the source of 

pesticides in these samples likely is from lawn maintenance and other household uses. Because DDT was 

banned in 1972, the source of DDE likely is not from 

recent use. Benfluralin and trifluralin, detected in a 

single sample (app. 4), could result from residential 

use, as both of these pesticides are available for 

residential use. Benfluralin also is used for peanut 

production in Florida, and trifluralin is applied to 

cabbage, corn, cotton, and soybeans; however, none 

of these crops are currently grown commercially in 

Orange County (Shahane, 1999). The well from which 

Table 10. Well, date of sample collection, and number of pesticide

compounds detected in each sample

Site 

number Site identifier Well name

Date 

of 

collection

Number 

of 

compounds 

detected

Surficial aquifer system

 74 282631081323301 Tibet-Butler well 12/2/1999 1

 80 282722081371701 Conserv II well 6/23/1999 2

 95 282912081181201 OR0722 2/15/2000 1

108 283033081290301 Turkey Lake well 2/16/2000 1

117 283210081180401 Englewood Park well 10/26/1999 2

120 283228081213501 Langford Park well 10/26/1999 1

125 283251081283501 OR0716 8/26/1999 1

135 283345081225701 Ivanhoe Park well 10/28/1999 2

Upper Floridan aquifer

126 283253081283401 OR47 7/15/1999 2

147 283646081195401 Bradner well 9/1/1999 3
74 Hydrogeology and Quality of Ground Water in Orange County, Florida

the sample was collected was located in a city park 

surrounded by a residential area that was established 

prior to 1977. Pesticides were not detected in any 

samples from wells in relatively pristine areas, but 

atrazine was detected in one sample from a lake in 

Tosohatchee State Reserve (E.R. German, U.S. 

Geological Survey, written commun., 2003).

The results of this study should not be interpreted 

as a comprehensive investigation of the occurrence and 

distribution of pesticides in ground water of Orange 

County. Many more pesticide compounds are used in the 

county than were investigated by this study. For example, 

as many as 30 different herbicides and insecticides are 

registered for use in orange groves. Of these 

30 compounds, only 4 herbicides and 5 insecticides were 

included in these analyses. Similarly, many more pesticides are registered for residential pest control than could 

be included in the analyses of this study. Finally, the 

constraints of this study allowed only one sample to be 

collected from each of the 34 wells, limiting inferences on 

both temporal and spatial variability of pesticide concentrations in ground water. The effects of seasonal application rates and rainfall variations are not known. Additional 

study would be beneficial to better understand the occurrence, distribution, and temporal variations of pesticides in 

ground water of Orange County.

SUMMARY

A study of the hydrogeologic framework and 

ground-water quality characteristics was conducted 

from 1998 through 2002 for the surficial aquifer and 

Floridan aquifer systems underlying Orange County, 

Florida. Ground water is an abundant resource in 

central Florida and the primary source of water for 

aquatic habitats and human consumption. The quality 

of ground water generally is within the Primary and 

Secondary Drinking Water Regulations established by 

the U.S. Environmental Protection Agency. However, 

ground-water levels, spring flows, and ground-water 

quality have declined in some areas since 1968, probably as a result of withdrawal by pumping and an 

increase in urban land use. The population of Orange 

County increased from about 344,000 in 1970 to 

896,000 in 2000. Ground-water use also increased 

from about 82 million gallons per day (Mgal/d) in 1965 

to about 287 Mgal/d in 2000.

The hydrogeology of Orange County consists 

of three major hydrogeologic units—the surficial 

aquifer system, the intermediate confining unit, and the 

Floridan aquifer system—which together are as much as 

2,500 feet (ft) thick in Orange County. The Floridan 

Table 11. Pesticides detected in ground-water samples, environmental regulations, and associated 

health guidance concentrations

[μg/L,micrograms per liter; estimated values are shown in boldface]

Pesticide 

compound Type

Number 

of 

detections

Observed 

range in 

concentrations 

(μg/L)

National 

Primary Drinking Water 

Regulationa

(μg/L)

aU.S. Environmental Protection Agency, 2002.

Florida health guidance 

concentrationsb

bBrownfield Cleanup Targets, Florida Department of Environmental Protection, 1999.

Ground 

water 

(μg/L)

Surface 

water 

(μg/L)

Atrazine herbicide 3 0.003 - 0.029 3 3 1.8

Benfluralin herbicide 1 0.001 none none none

Chlorpyrifos insecticide 1 .005 none 21 .002

DCPA herbicide 1 .008 none 70 310

Deethyl atrazine metabolite 3 .002 - .003 none none none

Diazinon insecticide 1 .005 none .63 .002

DDE, p,p' metabolite 2 .001 - .003 none .1 .0006

Prometon herbicide 2 .025 - .028 none 105 600

Simazine herbicide 1 .417 4 4 5.8

Trifluralin herbicide 1 .001 none .78 .78
Summary 75

aquifer system consists of the Upper and Lower 

Floridan aquifers, which are separated by the middle 

semiconfining unit and are underlain by the sub-Floridan 

confining unit.

The surficial aquifer system consists mostly 

of unconsolidated sand with interbedded silt, clay, 

and shell of Pliocene to Recent age. The surficial aquifer system generally is between 50 and 100 ft thick 

throughout most of Orange County. The thickness can 

vary widely over small spatial distances because of 

deposition of sediments on a highly eroded surface 

and lateral gradation of units from sand to clay. The 

hydraulic conductivity of the surficial aquifer system, 

based on slug tests on 21 wells, ranges from about 0.05 

to 30 feet per day (ft/d) with a median of about 3 ft/d. 

The two lowest values of hydraulic conductivity were 

measured in wells located in western Orange County.

In general, the surficial aquifer system is 

unconfined, and the altitude of the water levels in wells 

represents the water table. Ground water in the surficial 

aquifer system is recharged mostly by precipitation, but 

also artificially by septic systems and rapid infiltration 

basins used for the disposal of treated wastewater 

(reclaimed water). The water table of the surficial aquifer system generally is higher beneath hilltops than 

beneath valleys. Water-level altitudes measured in 

wells tapping the surficial aquifer system ranged from 

about 10.6 ft in eastern Orange to 123.8 ft above 

NGVD 29 in northwestern Orange County.

In most locations in Orange County, the altitude 

of the water table of the surficial aquifer system is 

higher than the altitude of the potentiometric surface 

of the Upper Floridan aquifer, indicating that ground 

water from the surficial aquifer system potentially can 

flow downward to recharge the Upper Floridan aquifer. 

Water from the surficial aquifer system also flows 

laterally and discharges to lakes and streams.

The intermediate confining unit consists of clay, 

silt, limestone, and dolomite of Miocene age, and can 

include overlying clay layers of Pliocene age. Thickness 

of the unit ranges from about 10 to more than 200 ft, 

with a median thickness of about 100 ft. In general, the 

thickest part of the intermediate confining unit is in 

southeastern Orange County, and the thinnest parts are 

in southwestern and northwestern Orange County.

The Floridan aquifer system consists primarily 

of limestone and dolomite of Eocene age. The Upper 

Floridan aquifer consists primarily of the Ocala Limestone (zone A) and about the upper one-third of the 

Avon Park Formation (zone B); the middle semiconfining unit consists of about the middle one-third of the 

Avon Park Formation; and the Lower Floridan aquifer 

consists of the lower one-third to one-half of the Avon 

Park Formation and the Oldsmar Formation. The 

contacts between the Upper Floridan aquifer, middle 

semiconfining unit, and Lower Floridan aquifer are 

based primarily on permeability rather than stratigraphy and lithology. Much of the porosity of the Floridan 

aquifer system is secondary as a result of dissolution of 

the carbonate rock. 

The altitude of the top of the rock forming the 

Upper Floridan aquifer ranges from about 250 ft below 

to 38 ft above NGVD 29 with a median of about 62 ft 

below NGVD 29. In general, the altitude is highest in 

northwestern Orange County and lowest in southeastern Orange County, but can range widely over short 

spatial distances, resulting from the dissolution of the 

carbonate rocks and formation of numerous sinkholes. 

Thickness of the Upper Floridan aquifer is about 300 to 

400 ft throughout most of Orange County.

The thickness of the middle semiconfining unit 

ranges from about 385 to 635 ft. The top of the rock 

forming the Lower Floridan aquifer ranges from about 

870 to 1,120 ft below NGVD 29. Thickness of the 

Lower Floridan aquifer ranges from about 1,023 to 

1,180 ft.

Water recharges the Upper Floridan aquifer 

throughout most of Orange County either by slow leakage from the surficial aquifer system through the intermediate confining unit or artificially through drainage 

wells and rapid-infiltration basins. Recharge through 

drainage wells is estimated to be about 38 to 50 Mgal/d 

on average.

Discharge from the Upper Floridan aquifer 

occurs naturally at springs in northwestern Orange 

County, and artificially as a result of withdrawal from 

wells throughout the county. In a few locations, the altitude of the potentiometric surface is higher than the 

water table of the surficial aquifer system, indicating 

places of potential ground-water discharge from the 

Upper Floridan aquifer to the surficial aquifer system. 

The potentiometric surface of the Upper Floridan aquifer ranges from about 110 ft above NGVD 29 in southwestern Orange County to less than 30 ft above NGVD 

29 in eastern Orange County. Water levels measured in 

observation wells in July 1961 and September 2001 

indicate the altitude of the potentiometric surface of the 

Upper Floridan aquifer has declined 0.8 to 16 ft, with a 

median of 3 ft. In general, the potentiometric surface of 

the Lower Floridan aquifer mimics that of the Upper 
76 Hydrogeology and Quality of Ground Water in Orange County, Florida

Floridan aquifer, but, in places where pairs of wells tap 

each aquifer, water-level altitudes in the Lower Floridan aquifer generally are lower than those of the Upper 

Floridan aquifer.

Long-term water-level data in the surficial 

aquifer system are available in only five wells. Trend 

analysis indicates that water levels have not declined 

significantly in four of the wells. Trend analysis on 

long-term data collected from 15 wells tapping the Floridan aquifer system indicates statistically significant 

declines in water-level altitudes in seven wells tapping 

the Upper Floridan aquifer and one well tapping the 

Lower Floridan aquifer. Declines range from about 4 ft 

in southeastern Orange County to 12 ft in west-central 

Orlando, with a median of about 6 ft. Although the 

overall trend in these wells is decreasing water levels, 

the data indicate the declines are discontinuous, with 

intermittent periods of stabilization and (or) increasing 

water levels. The long-term declines probably are 

related to pumping of the aquifer rather than climatic 

changes. Declines in the potentiometric surface of the 

Upper Floridan aquifer also have resulted in decreased 

discharge from Rock and Wekiva Springs.

A generalized water budget was computed for 

Orange County from 1991 to 2000. Average rates for 

the 10-year period for the following budget components were computed based on reported measurements 

or estimates: precipitation was 53 inches per year 

(in/yr), runoff was 11 in/yr, spring discharge was 2 

in/yr, and net lateral subsurface outflow and exported 

water was 1 in/yr. Evapotranspiration was 39 in/yr, 

which was calculated as the residual of the waterbudget analysis, assuming changes in storage were 

negligible.

Water quality of the surficial aquifer system is 

highly variable, as a result of lithology of the sediments, 

natural and artificial recharge from the Upper Floridan 

aquifer, and effects of land use. Water type generally is 

mixed ion with low concentrations of total dissolved 

solids (TDS), sulfate, and chloride. The National 

Secondary Drinking Water Regulation for TDS 

(500 mg/L) was exceeded in only 3 of 33 samples from 

the surficial aquifer system. The National Secondary 

Drinking Water Regulation for chloride of 250 milligrams per liter (mg/L) was exceeded in only 1 of 31 

samples from the surficial aquifer system. In general, 

nutrient concentrations also were low in samples from 

the surficial aquifer system. About half of the samples 

had nitrate concentrations less than or equal to the 

method reporting limit of 0.02 mg/L. Two samples had 

concentrations greater than or equal to the Maximum 

Contaminant Level of 10 mg/L. The source of nutrients 

in the surficial aquifer system probably is local agricultural and urban land use.

 Water type in the Floridan aquifer system 

generally is calcium or calcium-magnesium bicarbonate, resulting from the dissolution of the carbonate 

rocks, with low concentrations of TDS (less than 

500 mg/L). Water in the Upper Floridan aquifer 

becomes sodium-chloride type with higher 

concentrations of TDS in eastern Orange County, 

possibly as a result of upwelling of deeper, more saline 

ground water. The National Secondary Drinking Water 

Regulation for TDS was exceeded in 15 of 56 samples.

Recent (1999) water samples from an observation 

well in southwestern Orange County had relatively 

high concentrations of sodium (53 mg/L), chloride 

(90 mg/L), and sulfate (41 mg/L), possibly as a result 

of reclaimed water from nearby rapid infiltration basins 

recharging the Upper Floridan aquifer. One sample 

collected from the same well prior to the installation of 

the rapid infiltration basins (1979) had lower concentrations of chloride (4 mg/L) and sulfate (28 mg/L).

A total of 24 wells that tap the Floridan aquifer 

system had historical water-quality data (1953-2001). 

All wells were located in eastern Orange County, 

mostly in and around the Cocoa well field. Data indicated statistically significant increases in sulfate and 

chloride concentrations over time in 13 and 16 wells, 

respectively, which probably relate to pumping, especially from the Cocoa well field where upward migration of deep saline water occurs. The migration appears 

to move through conduits and fractures, rather than 

diffusely through the aquifer medium, as indicated by 

increases of chloride and sulfate in samples from shallow and deep zones in nested observation wells, but not 

in samples from the two intermediate zones.

Specific conductance and TDS concentrations 

have increased in water samples from Rock and 

Wekiva Springs since about the mid-1960s. Based on 

data from nearby wells, the cause is likely related to the 

effects of fertilizer or septic-tank leachate on localized 

recharge water into the springs rather than upwelling of 

saline water from deeper zones of the aquifer.

Concentrations of nitrate and total organic 

carbon generally were low in samples from the Upper 

and Lower Floridan aquifers. A total of 29 of 31 samples 

collected from the Upper Floridan aquifer had concentrations of nitrate less than or equal to the reporting 

limit of 0.02 mg/L. Concentrations of nitrate in 
References 77

samples from Rock and Wekiva Springs, which issue 

from the Upper Floridan aquifer, were 1.4 and 0.98 

mg/L as nitrogen, respectively, which could be a result 

of the agricultural and (or) residential land use in the 

recharge basins.

The most commonly detected trace elements 

in ground-water samples from Orange County were 

aluminum, barium, boron, iron, manganese, and 

strontium. Arsenic was detected in 7 of 25 samples 

from the surficial aquifer system, 4 of 26 samples from 

the Upper Floridan aquifer, and 1 of 8 samples from 

the Lower Floridan aquifer. Selenium was detected in 

8 of 24 samples from the surficial aquifer system, 

9 of 26 samples from the Upper Floridan aquifer, and 

2 of 8 samples from the Lower Floridan aquifer. These 

elements have many commercial and industrial uses; 

however, the low concentrations in ground water in 

Orange County do not exceed Maximum Contaminant 

Levels established by the U.S. Environmental Protection Agency and are within ranges of natural concentrations.

Radon was detected in every ground-water 

sample, and concentrations ranged from 56 to 14,700 

picoCuries per liter. Radon concentrations generally had 

no apparent pattern of geographic distribution, but were 

substantially higher in samples from the surficial aquifer 

system than in samples from either the Upper or Lower 

Floridan aquifer. Radon is a naturally occurring radioactive gas generated by the decay of radium. The source of 

radon probably is uranium-bearing phosphate minerals 

present in the intermediate confining unit. No Maximum 

Contaminant Level has been established for radon in 

drinking water.

Ratios of stable isotopes of hydrogen (deuterium) and oxygen (oxygen-18) indicate that ground 

water in the surficial aquifer system and Floridan aquifer system is meteoric in origin, falling along a line 

parallel to the global meteoric water line. In general, 

samples from the Floridan aquifer system are slightly 

heavier (less depleted) with respect to the Vienna Standard Mean Ocean Water than are samples from the 

surficial aquifer system, indicating that deep saline 

water, possibly relict seawater, is migrating upward and 

mixing in places with water in the Floridan aquifer 

system.

Pesticides were present in low concentrations in 8 

of 16 samples from the surficial aquifer system and in 2 

of 14 samples from the Upper Floridan aquifer; no pesticides were found in the 4 samples from the Lower Floridan aquifer. The most commonly detected pesticide 

compounds were atrazine and deethyl atrazine. The 

maximum number of compounds detected in any one 

sample was three. The maximum detected concentration 

of any compound was 0.417 micrograms per liter for 

simazine. No pesticide concentration exceeded a Maximum Contaminant Level. The source of pesticides is 

difficult to assess, but detections in urban areas could 

originate from commercial and residential use of pesticides. Pesticides are present in shallow ground water 

primarily in urban areas around metropolitan Orlando.

Water-quality samples for this project were 

collected during drought conditions when leaching from 

lawns and runoff from roads was minimal. Additional 

sample collection during other hydrologic conditions 

would be beneficial to assess the variable ground-water 

quality, particularly the occurrence and distribution of 

pesticides. Furthermore, an increase in data collection, 

including continuation of water-level measurements 

in surficial aquifer system wells and periodic collection 

of water-quality samples from both the surficial aquifer 

and Floridan aquifer systems, would be useful in 

documenting future trends.

SELECTED REFERENCES

Adamski, J.C., 1997, Nutrients and pesticides in ground 

water of the Ozark Plateaus in Arkansas, Kansas, Missouri, and Oklahoma: U.S. Geological Survey WaterResources Investigations Report 96-4313, 28 p.

Adamski, J.C., and Knowles, Leel, Jr., 2001, Ground-water 

quality of the surficial aquifer system and the Upper 

Floridan aquifer, Ocala National Forest and Lake 

County, Florida, 1990-99: U.S. Geological Survey 

Water-Resources Investigations Report 01-4008, 51 p.

Anderson, J.R., Hardy, E.H., Roach, J.T., and Witmer, R.E., 

1976, A land use and land cover classification system 

for use with remote sensor data: U.S. Geological Survey 

Professional Paper 964, 281 p.

Anderson, Warren, and Joyner, B.F., 1966, Availability and 

quality of surface water in Orange County, Florida: 

Tallahassee, Florida Geological Survey Map Series 

no. 24, 1 sheet.

Arthur, J.D., Bond, Paulette, Lane, Ed, Rupert, F.R., and 

Scott, T.M., 1994, Florida’s global wandering through 

geological areas, in Lane, Ed, ed., Florida’s geological 

history and geological resources: Tallahassee, Florida 

Geological Survey Special Publication no. 35, 64 p.

Barbash, J.E., and Resek, E.A., 1996, Pesticides in ground 

water: Distribution, trends, and governing factors: 

Chelsea, Michigan, Ann Arbor Press, 588 p.
78 Hydrogeology and Quality of Ground Water in Orange County, Florida

Boniol, Don, Williams, Marvin, and Munch, Doug, 1993, 

Mapping recharge to the Floridan aquifer using a geographic information system: Palatka, St. Johns River 

Water Management District Technical Publication 

SJ93-5, 41 p.

Bouwer, H., and Rice, R.C., 1976, A slug test for determining hydraulic conductivity of unconfined aquifers with 

completely or partially penetrating wells: Water 

Resources Research, v. 12, no. 3, p. 423-428.

Bradner, L.A., 1991, Water quality in the Upper Floridan 

aquifer in the vicinity of drainage wells, Orlando, 

Florida: U.S. Geological Survey Water-Resources Investigations Report 90-4175, 57 p.

CH2M Hill, 1997, Artificial recharge of the Floridan aquifer 

through drainage or injection wells in Orange and 

Seminole Counties: Prepared for St. Johns River Water 

Management District, 103 p.

Craig, H., 1961, Isotopic variations in meteoric waters: 

Science, v. 133, no. 3465, p. 1702-1703.

Drever, J.I., 1988, The geochemistry of natural waters 

(2d ed.): Englewood Cliffs, NJ, Prentice Hall, 437 p.

Duke, H.R., 1972, Capillary properties of soils—Influence 

upon specific yield: Transactions of the American Society of Agricultural Engineers, v. 15, no. 4, p. 688-691.

EXTOXNET, 2002, Pesticide information profiles [cited 

November 2002] http://ace.orst.edu/info/extoxnet/ 

pips/ghindex.html .

Fetter, C.W., Jr., 1980, Applied hydrogeology: Columbus, 

Ohio, Charles E. Merrill Publishing, 488 p.

Fishman, M.J., and Friedman, L.C. eds., 1985, Methods for 

determination of inorganic substances in water and 

fluvial sediments—Techniques of Water-Resources 

Investigations of the United States Geological Survey, 

book 5, chap. A1: U.S. Geological Survey Open-File 

Report 85-495, 709 p.

Florida Agricultural Statistics Service, 2002, [cited 

November 2002] http://www.nass.usda.gov/fl/ .

Florida Department of Environmental Protection, 1999, 

Groundwater and surface water cleanup target levels 

[cited October 2002] http://www.dep.state.fl.us/waste/ 

quick_topics/rules/documents/62-777/777tab1.pdf .

Gaggiani, N.G., and Lamonds, A.G., 1977, Chemical and 

biological quality of Lakes Faith, Hope, and Charity, at 

Maitland, Florida, with emphasis on storm runoff and 

bulk precipitation, 1971-74: U.S. Geological Survey 

Open-File Report 77-491, 94 p.

German, E.R., 1983, Water quality in Lakes Faith, Hope, 

Charity, and Lucien, 1971-79, in an area of residential 

development and highway construction at Maitland, 

Florida: U.S. Geological Survey Water-Resources 

Investigations Report 82-51, 70 p.

——— 1996, Analysis of nonpoint-source ground-water 

contamination in relation to land use: Assessment of 

nonpoint-source contamination in central Florida: U.S. 

Geological Survey Water-Supply Paper 2381-F, 60 p.

Hayes, T.B., Collins, Atif, Lee, Melissa, Mendoza, Magdelena, Noriega, Nigel, Stuart, A.A., and Vonk, Aaron, 

2002, Hermaphroditic, demasculinized frogs after exposure to the herbicide atrazine at low ecologically relevant doses: Proceedings of the National Academy of 

Sciences, v. 99, no. 8, p. 5476 - 5480.

Heaton, T.H.E., 1986, Isotopic studies of nitrogen pollution 

in the hydrosphere and atmosphere—A review: Chemical 

Geology, v. 59, p. 87-102.

Helsel, D.R., and Hirsch, R.M., 1992, Statistical methods 

in water resources: Amsterdam, Elsevier Science Publishers, 522 p.

Hem, J.D., 1989, Study and interpretation of the chemical 

characteristics of natural water (3d ed.): U.S. Geological 

Survey Water-Supply Paper 2254, 263 p.

Hoefs, J., 1987, Stable isotope geochemistry (3d ed.): Berlin, 

Springer-Verlag, 241 p.

Johnson, R.A., 1984, Stratigraphic analysis of geophysical 

logs from water wells in peninsular Florida: Palatka, 

St. Johns River Water Management District Technical 

Publication SJ 84-16, 57 p.

Keys, W.S., 1988, Borehole geophysics applied to groundwater investigations: U.S. Geological Survey Open-File 

Report 87-539, 304 p.

Kimrey, J.O., 1978, Preliminary appraisal of the geohydrological aspects of drainage wells, Orlando area, central 

Florida: U.S. Geological Survey Water-Resources Investigations Report 78-37, 24 p.

Knowles, Leel, Jr., and Kinnaman, S.L., 2002, Potentiometric surface of the Upper Floridan aquifer in the St. Johns 

River Water Management District and vicinity, September 2001: U.S. Geological Survey Open File Report 

02-182, 1 sheet.

Knowles, Leel, Jr., O’Reilly, A.M., and Adamski, J.C., 2002, 

Hydrogeology and simulated effects of ground-water 

withdrawals in Lake County and Ocala National Forest, 

north-central Florida: U.S. Geological Survey WaterResources Investigations Report 02-4207, 140 p.

Lichtler, W.F., Anderson, Warren, and Joyner, B.F., 1968, 

Water resources of Orange County, Florida: Tallahassee, 

Florida Geological Survey Report of Investigations 

no. 50, 150 p.

Lichtler, W.F., Hughes, G.H., and Pfischner, F.L., 1976, 

Hydrologic relations between lakes and aquifers in a 

recharge area near Orlando, Florida: U.S. Geological 

Survey Water-Resources Investigations Report 76-75, 

61 p.
References 79

Locker, S.D., Brooks, G.R., and Doyle, L.J., 1988, Results 

of seismic reflection investigation and hydrogeologic 

implications for Lake Apopka, Florida: Palatka, 

St. Johns River Water Management District Special 

Publication SJ 88-SP5.

Majewski, M.S., and Capel, P.D., 1995, Pesticides in the 

atmosphere: Distribution, trends, and governing factors: 

Chelsea, Michigan, Ann Arbor Press, 214 p.

Marella, R.L., 1992, Water withdrawals, use, and trends in 

Florida, 1990: U.S. Geological Survey Water-Resources 

Investigations Report 92-4140, 38 p.

——— 1994, Estimated discharge of treated wastewater in 

Florida, 1990: U.S. Geological Survey Open-File 

Report 93-364, 53 p.

——— 1999, Water withdrawals, use, discharge, and trends 

in Florida, 1995: U.S. Geological Survey WaterResources Investigations Report 99-4002, 90 p.

McGurk, B., and Presley, P.F., 2002, Simulation of the 

effects of groundwater withdrawals on the Floridan 

aquifer system in east-central Florida: Model expansion 

and revision: Palatka, St. Johns River Water Management District Technical Publication SJ2002-3, 196 p.

Miller, J.A., 1986, Hydrogeologic framework of the Floridan 

aquifer system in Florida and in parts of Georgia, 

Alabama, and South Carolina: U.S. Geological Survey 

Professional Paper 1403-B, 91 p.

Murray, L.C., Jr., and Halford, K.J., 1996, Hydrogeologic 

conditions and simulation of ground-water flow in the 

greater Orlando metropolitan area, east-central Florida: 

U.S. Geological Survey Water-Resources Investigations 

Report 96-4181, 100 p.

National Atmospheric Deposition Program, 2002, Isopleth 

maps [cited November 2002] http://nadp.sws.uiuc.edu/ 

isopleths/.

National Oceanic and Atmospheric Administration, 2001, 

Climatic data, annual summary, Florida, 2000: Department of Commerce, v. 104, no. 13, 24 p.

——— 2002, Climatic data, annual summary, Florida, 2001: 

Department of Commerce, v. 105, no. 13, 28 p.

Ocala Water Quality and Research Laboratory, 2002, 

Parameter list for fiscal year 2003 [cited October 2002] 

http://qwsu.er.usgs.gov/owqrlep03.shtml .

O’Reilly, A.M., 1998, Hydrogeology and simulation of the 

effects of reclaimed-water application in west Orange and 

southeast Lake Counties, Florida: U.S. Geological Survey 

Water-Resources Investigations Report 97-4199, 91 p.

O’Reilly, A.M., Spechler, R.M., and McGurk, B.E., 2002, 

Hydrogeology and water-quality characteristics of the 

Lower Floridan aquifer in east-central Florida: U.S. 

Geological Survey Water-Resources Investigations 

Report 02-4193, 60 p.

Otton, J.K., Gundersen, L.C.S., and Schumann, R.R., 1995, 

The geology of radon [cited November 2002] 

http://energy.cr.usgs.gov/radon/georadon.html .

PB Water, 1999, Groundwater flow and saltwater intrusion 

modeling in support of the consumptive use permit 

application for Orange County Utilities: Prepared for 

Ardaman and Associates, Inc., and Orange County 

Utilities, 130 p.

Pfischner, F.L., 1968, Relation between land use and chemical characteristics of lakes in southwestern Orange 

County, Florida: U.S. Geological Survey Professional 

Paper 600-B, p. 190-194.

Phelps, G.G., and German, E.R., 1995, Water budgets, water 

quality, and analysis of nutrient loading of the Winter 

Park chain of lakes, central Florida, 1989-92: U.S. Geological Survey Water-Resources Investigations Report 

95-4108, 96 p.

Phelps, G.G., and Schiffer, D.M., 1996, Geohydrology and 

potential for upward movement of saline water in the 

Cocoa well field, east Orange County, Florida: U.S. 

Geological Survey Open-File Report 95-736, 38 p.

Rutledge, A.T., 1987, Effects of land use on ground-water 

quality in central Florida--Preliminary results: U.S. 

Geological Survey Toxic Waste—Ground-water contamination program: U.S. Geological Survey WaterResources Investigations Report 86-4163, 49 p.

Sacks, L.A., Swancar, Amy, and Lee, T.M., 1998, Estimating 

ground-water exchange with lakes using water-budget 

and chemical mass-balance approaches for ten lakes in 

ridge areas of Polk and Highlands Counties, Florida: 

U.S. Geological Survey Water-Resources Investigations 

Report 98-4133, 52 p.

Schiffer, D.M., 1989, Effects of highway runoff on the quality of water and bed sediments of two wetland in central 

Florida: U.S. Geological Survey Water-Resources Investigations Report 88-4200, 63 p.

——— 1998, Hydrology of central Florida lakes—A primer: 

U.S. Geological Survey Circular 1137, 39 p.

Schiner, G.R., and German, E.R., 1983, Effects of recharge 

from drainage wells on quality of water in the Floridan 

aquifer in the Orlando area, central Florida: U.S. 

Geological Survey Water-Resources Investigations 

Report 82-4094, 124 p.

Scott, T.M., 1988, The lithostratigraphy of the Hawthorn 

Group (Miocene) of Florida: Tallahassee, Florida 

Geological Survey Bulletin no. 59, 148 p.

Scott, T.M., Campbell, K.M., Rupert, F.R., Arthur, J.D., 

Missimer, T.M., Lloyd, J.M., Yon, J.W., and Duncan, 

J.G., 2001, Geologic map of the State of Florida: Tallahassee, Florida Geological Survey Map Series 146, 

1 sheet.

Sepúlveda, Nicasio, 2002, Simulation of ground-water flow 

in the intermediate and Floridan aquifer systems in 

peninsular Florida: U.S. Geological Survey WaterResources Investigations Report 02-4009, 130 p.
80 Hydrogeology and Quality of Ground Water in Orange County, Florida

Shahane, A.N., 1999, Summary of agricultural pesticide 

usage in Florida: 1995-98: Florida Department of 

Agriculture and Consumer Services, 111 p.

Smoot, J.L., and Schiffer, D.M., 1985, Hydrology of Lake 

Butler, Florida: U.S. Geological Survey WaterResources Investigations Report 84-4163, 1 sheet.

Spechler, R.M., and Halford, K.J., 2001, Hydrogeology, 

water quality, and simulated effects of ground-water 

withdrawals from the Floridan aquifer system, Seminole 

County and vicinity, Florida: U.S. Geological Survey 

Water-Resources Investigations Report 01-4182, 116 p.

Stringfield, V.T., 1936, Artesian water in the Florida 

Peninsula: U.S. Geological Survey Water-Supply Paper 

773-C, p. 115-195.

Sumner, D.M., 1996, Evapotranspiration from successional 

vegetation in a deforested area of the Lake Wales Ridge, 

Florida: U.S. Geological Survey Water-Resources Investigations Report 96-4244, 38 p.

Sumner, D.M., and Bradner, L.A., 1996, Hydraulic characteristics and nutrient transport and transformation 

beneath a rapid infiltration basin, Reedy Creek Improvement District, Orange County, Florida: U.S. Geological 

Survey Water-Resources Investigations Report 95-4281, 

51 p.

Swancar, Amy, Lee, T.M., and O'Hare, T.M., 2000, Hydrogeologic setting, water budget, and preliminary analysis 

of ground-water exchange at Lake Starr, a seepage lake 

in Polk County, Florida: U.S. Geological Survey WaterResources Investigations Report 00-4030, 66 p.

Szell, G.P., 1993, Aquifer characteristics in the St. Johns 

River Water Management District: St. Johns River 

Water Management District Technical Publication 

SJ 93-1, 495 p.

Taylor, G.F., 1993, Inventory of drainage wells and potential 

sources of contaminants to drainage-well inflow in 

southwest Orlando, Orange County, Florida: U.S. 

Geological Survey Water-Resources Investigations 

Report 93-4061, 1 sheet.

Tibbals, C.H., 1977, Availability of ground water in 

Seminole County and vicinity, Florida: U.S. Geological 

Survey Water-Resources Investigations 76-97, 15 p. and 

4 sheets.

——— 1981, Computer simulation of the steady-state flow 

system of the Tertiary limestone (Floridan) aquifer system in east-central Florida: Water-Resources Investigations Open-File Report 81-681, 31 p.

——— 1990, Hydrology of the Floridan aquifer system in 

east-central Florida: U.S. Geological Survey Professional Paper 1403-E, 98 p.

Tibbals, C.H., and Frazee, J.M., Jr., 1976, Ground-water 

hydrology of the Cocoa wellfield area, Orange County, 

Florida: U.S. Geological Survey Open File Report 

75-676, 67 p.

Toth, D.J., 1999, Water quality and isotope concentrations 

from selected springs in the St. Johns River Water Management District: Palatka, St. Johns River Water Management District Technical Publication SJ99-2, 67 p.

Unklesbay, A.G., 1944, Ground water conditions in Orlando 

and vicinity, Florida: Tallahassee, Florida Geological 

Survey Report of Investigations no. 5, 75 p.

U.S. Census Bureau, 2002, Florida population by counties 

by decennial census: 1900 to 1990 [cited April 2002] 

http://www.census.gov/population/cencounts/ 

fl190090.txt .

U.S. Environmental Protection Agency, 2002, Current drinking water standards [cited October 2002] 

http://www.epa.gov/safewater/mcl.html .

U.S. Geological Survey, 1999, The quality of our Nation’s 

waters—Nutrients and pesticides: U.S. Geological 

Survey Circular 1225, 82 p.

Wanielista, M.P., Eaglin, R.D., Bennett, R.L., and Underwood, S.L., 1992, Econlockhatchee hydrologic balance—Maintaining the balance: Palatka, St. Johns River 

Water Management District Special Publication SJ 93-

SP6, 478 p.

White, W.A., 1970, The geomorphology of the Florida 

Peninsula: Tallahassee, Florida Geological Survey 

Bulletin no. 51, 164 p.

Wilde, F.D., and Radtke, D.B., 1998, Field measurements: 

U.S. Geological Survey Techniques of Water-Resources 

Investigations, book 9, chap. A6.

Wilde, F.D., Radtke, D.B., Gibs, Jacob, and Iwatsubo, R.T., 

1999, Chapter A4 Collection of water samples: U.S. 

Geological Survey Techniques of Water-Resources 

Investigations, book 9.

Zaugg, S.D., Sandstrom, M.W., Smith, S.G., and Fehlberg, 

K.M., 1995, Methods of analysis by the U.S. Geological 

Survey National Water Quality Laboratory—Determination of pesticides in water by C-18 solid-phase extraction and capillary-column gas chromatography/mass 

spectrometry with selected-ion monitoring: U.S. 

Geological Survey Open-File Report 95-181, 49 p.