Prepared in cooperation with the

SOUTHWEST FLORIDA WATER MANAGEMENT DISTRICT

GULF OF MEXICO

0 5 10 MILES

0 5 10 KILOMETERS

ROMP 12

ROMP 26

ROMP 17

ROMP 9

ROMP 9.5

ROMP 5

ROMP 13

SARASOTA CO.

Venice

Sarasota Myakka City

Punta Gorda

Port Charlotte

Englewood

MANATEE CO. HARDEE CO.

DE SOTO CO.

CHARLOTTE CO. Charlotte

Harbor

STUDY

AREA

Hydrogeologic Framework and

Geochemistry of the Intermediate Aquifer

System in Parts of Charlotte, De Soto, and

Sarasota Counties, Florida

U.S. Department of the Interior

U.S. Geological Survey

Water-Resources Investigations Report 01-4015
Hydrogeologic Framework and Geochemistry 

of the Intermediate Aquifer System in Parts of 

Charlotte, De Soto, and Sarasota Counties, 

Florida

By A.E. Torres, L.A. Sacks, D.K. Yobbi, L.A. Knochenmus, 

and B.G. Katz

U.S. Geological Survey

Water-Resources Investigations Report 01–4015

Prepared in cooperation with the

SOUTHWEST FLORIDA WATER MANAGEMENT DISTRICT

Tallahassee, Florida

2001
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 Copies of this report can be 

write to: purchased from:

District Chief U.S. Geological Survey

U.S. Geological Survey Branch of Information Services

Suite 3015 Box 25286

227 N. Bronough Street Denver, CO 80225

Tallahassee, FL 32301 888-ASK-USGS

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

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

CONTENTS

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

Introduction ........................................................................................................................................................................... 3

Purpose and Scope....................................................................................................................................................... 3

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

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

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

Methods of Study................................................................................................................................................................... 5

Hydrogeology .............................................................................................................................................................. 5

Geochemistry............................................................................................................................................................... 6

Water Sampling ................................................................................................................................................. 6

Rock Sampling .................................................................................................................................................. 6

Geochemical Modeling...................................................................................................................................... 7

Geologic Framework ............................................................................................................................................................. 7

Depositional History.................................................................................................................................................... 8

Stratigraphy Underlying the Study Area ..................................................................................................................... 9

Avon Park Formation, Ocala Limestone, and Suwanne Limestone.................................................................. 9

Hawthorn Group................................................................................................................................................ 9

Arcadia Formation................................................................................................................................... 10

Nocatee and Tampa Members ....................................................................................................... 11

Peace River Formation ............................................................................................................................ 11

Bone Valley Member of the Peace River Formation..................................................................... 11

Tamiami Formation ........................................................................................................................................... 11

Surficial Deposits .............................................................................................................................................. 12

Hydrogeologic Units and Hydraulic Properties..................................................................................................................... 12

Surficial Aquifer System ............................................................................................................................................. 12

Intermediate Aquifer System....................................................................................................................................... 14

Upper Floridan Aquifer ............................................................................................................................................... 17

Confining Units ........................................................................................................................................................... 18

Ground-Water Flow Patterns ....................................................................................................................................... 18

Hydraulic Connection Between Hydrogeologic Units ................................................................................................ 23

Hydraulic Characteristics at the ROMP 9.5 Site ................................................................................................................... 25

Core Analysis .............................................................................................................................................................. 25

Specific-Capacity Tests ............................................................................................................................................... 25

Aquifer Tests................................................................................................................................................................ 26

Test Design and Implementation ....................................................................................................................... 26

Analytical Analysis of Aquifer Tests................................................................................................................. 31

Hantush (1960) Modified Leaky Method................................................................................................ 31

Jacob (1946) Leaky Artesian Method ..................................................................................................... 31

Neuman and Witherspoon (1972) Ratio Method..................................................................................... 33

Numerical Analysis of Aquifer Tests ................................................................................................................ 33

Limitations of Aquifer Test Analysis ................................................................................................................ 39

Geochemistry of Aquifer Systems......................................................................................................................................... 39

Chemical Composition of Ground Water .................................................................................................................... 39

Vertical Profiles of Chloride and Sulfate........................................................................................................... 43

Saturation State of Ground Water...................................................................................................................... 45

Isotopic Composition of Ground Water....................................................................................................................... 46

Stable Isotopes................................................................................................................................................... 46

Age of Ground Waters....................................................................................................................................... 48

Mineralogical and Isotopic Composition of Aquifer System Material ....................................................................... 50

Geochemical Evolution of Ground Water Along Flow Paths ..................................................................................... 51
IV Contents

Defining Flow Paths .......................................................................................................................................... 51

Description of Geochemical Model and Assumptions ...................................................................................... 51

Upper Arcadia Zone (PZ2) ...................................................................................................................... 53

Lower Arcadia Zone (PZ3)...................................................................................................................... 55

Importance of Geochemical Modeling in Assessing Ground-Water Flow........................................................ 56

Summary ............................................................................................................................................................................... 57

References ............................................................................................................................................................................. 59

Appendices: A. Depth of geologic formations determined by the Florida Geological Survey at the ROMP 5, 9, 

9.5, 12, 13, and 17 sites. ............................................................................................................................ 64

 B. Hydrogeologic, lithostratigraphic, and borehole geophysical data collected at the ROMP 5, 9,

9.5, 12, 13, and 17 sites ............................................................................................................................. 65

 C. Classification criteria used to describe the hydrogeologic units at the ROMP 5, 9, 9.5, 12, 13, 

and 17 sites ................................................................................................................................................ 71

Figures

1. Map showing location of study area, Regional Observation and Monitor-Well Program (ROMP) sites, 

and hydrogeologic sections, west-central Florida ..................................................................................................... 4

2. Chart showing sea-level fluctuations in South Florida.............................................................................................. 8

3. Graph showing relation of stratigraphic and hydrogeologic units ............................................................................ 10

4. Maps showing lithofacies and hydraulic conductivity of the surficial aquifer system, west-central Florida ........... 13

5. Hydrogeologic and lithostratigraphic cross section A-A’ through ROMP sites 9, 9.5, and 17................................. 15

6. Hydrogeologic and lithostratigraphic cross section B-B’ through ROMP sites 5, 12, and 13.................................. 16

7. Graph showing range of leakance values estimated for confining units at the ROMP 5, 9, 9.5, and 12 sites .......... 19

8-9. Maps showing:

8. Potentiometric surface of the Upper Arcadia zone (PZ2) and Lower Arcadia zone (PZ3) 

of the intermediate aquifer system, and Upper Floridan aquifer, southwest Florida, September 1998 ................... 20

9. Head differences among aquifers, area of artesian flow, and thickness of confining

units, September 1998 ............................................................................................................................................... 22

10. Graphs showing water levels in wells at the ROMP 5, 9, 9.5, 12, 13, and 17 sites from May 1997 through 

May 2000................................................................................................................................................................... 24

11. Graph showing hydraulic data collected during coring and testing at the ROMP 9.5 site........................................ 26

12. Map showing location of study area, plan view, and description of wells at the ROMP 9.5 test site....................... 28

13-26. Graphs showing:

13. Water levels in selected observation wells for background, withdrawal, and recovery periods 

for the Lower Arcadia zone (PZ3) aquifer test at the ROMP 9.5 site ................................................................ 29

14. Water levels in selected wells for background, withdrawal, and recovery periods of the Upper Floridan

aquifer test at the ROMP 9.5 site........................................................................................................................ 30

15. Analysis of data from the Lower Arcadia zone (PZ3) and Upper Floridan aquifer tests at the ROMP 9.5

site using the Hantush (1960) method ................................................................................................................ 32

16. Analysis of data from the Lower Arcadia zone (PZ3) aquifer test at the ROMP 9.5 site using the 

Jacob (1946) method........................................................................................................................................... 33

17. Analysis of data from the Lower Arcadia zone (PZ3) aquifer test at the ROMP 9.5 site using the

Neuman-Witherspoon (1972) method ................................................................................................................ 34

18. Type curves of s′/s versus t′

d .............................................................................................................................. 35

19. Radial-model grid used for the simulation of hydraulic properties of the aquifer systems at the

ROMP 9.5 site .................................................................................................................................................... 36

20. Simulated and measured drawdown in selected confining unit wells at the ROMP 9.5 site.............................. 37

21. Simulated and measured drawdown in selected Lower Arcadia zone (PZ3) and Upper Floridan 

aquifer wells at the ROMP 9.5 site ..................................................................................................................... 38

22. Dissolved-solids concentrations and Stiff diagrams showing chemical composition of water from wells

at the ROMP 5, 9, 9.5, 13, and 17 sites that tap the surficial aquifer system, Upper Arcadia zone

(PZ2), Lower Arcadia zone (PZ3), and Upper Floridan aquifer......................................................................... 40

23. Concentrations of chloride and sulfate in samples collected during drilling of the exploratory boreholes

and during sampling of wells at the ROMP 13, 9.5, 17, 9, and 5 sites............................................................... 44
Contents V

24. Relation between delta deuterium (δD) and delta oxygen-18 (δ18O) values in water from wells at the

ROMP 5, 9, 9.5, 13, and 17 sites that are completed in the surficial aquifer system, intermediate 

aquifer system, and Upper Floridan aquifer ....................................................................................................... 47

25. Relation between delta carbon-13 (δ13C) and carbon-14 (14C) values in water from wells at the ROMP 5, 9, 

9.5, 13, and 17 sites that are completed in the surficial aquifer system, intermediate aquifer system, 

and Upper Floridan aquifer................................................................................................................................. 48

26. Tritium concentration in rainfall in Ocala, Florida............................................................................................. 49

27. Map showing ground-water flow paths simulated in the intermediate aquifer system, Upper Arcadia 

zone (PZ2) and Lower Arcadia zone (PZ3), using the geochemical model NETPATH ........................................... 52

Tables

1. Summary of aquifer test results from selected ROMP sites ......................................................................................... 17

2. Information on selected observation wells open to the Upper Arcadia zone (PZ2) and Lower Arcadia zone (PZ3) 

within the intermediate aquifer system......................................................................................................................... 21

3. Porosity and hydraulic conductivity of confining material in selected hydrogeologic units at the ROMP 9.5 

exploratory borehole, De Soto County, Florida............................................................................................................ 27

4. Specific capacity of selected hydrogeologic units estimated by packer tests at the ROMP 9.5 exploratory 

borehole, De Soto County, Florida ............................................................................................................................... 27

5. Summary of aquifer test results for the ROMP 9.5 site using analytical analysis........................................................ 31

6. Summary of aquifer test results for the ROMP 9.5 site using numerical analysis ....................................................... 39

7. Chemical and isotopic data for water collected from wells at the ROMP 5, 9, 9.5, 13, and 17 sites 

during 1996 and 1997 ................................................................................................................................................... 41

8. Saturation state of ground water at the ROMP 5, 9, 9.5, 13, and 17 sites with respect to mineral phases and 

calculated partial pressure of carbon dioxide ............................................................................................................... 46

9. Adjusted carbon-14 age of ground water at the ROMP 5, 9, 9.5, 13, and 17 sites from the intermediate aquifer 

system and the Upper Floridan aquifer......................................................................................................................... 49

10. Mineralogy, carbon content, and isotopic composition of organic and inorganic carbon from selected samples of 

the core at the ROMP 9.5 site, De Soto County, Florida.............................................................................................. 50

11. NETPATH geochemical modeling results along selected flow paths within the intermediate aquifer system ............ 54

Conversion Factors, Vertical Datum, Acronyms, and Additional Abbreviations

Temperature in degrees Celsius (oC) can be converted to degrees Fahrenheit (oF) as follows: oF = (1.8 x oC) + 32

Temperature in degrees Fahrenheit (oF) can be converted to degrees Celsius (oC) as follows: o

C = (oF - 32) / 1.8

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

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

Level Datum of 1929.

Multiply By To obtain

inch (in.) 2.54 centimeter

foot (ft) 0.3048 meter

mile (mi) 1.609 kilometer

acre 4,047 square meter

acre 0.4047 hectare

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

inch per month (in/mo) 2.54 centimeter per month

inch per year (in/yr) 2.54 centimeter per year
VI Contents

Acronyms and Additional Abbreviations

Ca calcium

CaCO3 calcium carbonate

Cl chloride

14C carbon-14 isotope

CH2O organic matter

CO2 carbon dioxide

CU confining unit

δ delta

δ13C delta carbon-13

δ13CDIC δ carbon-13 isotope of dissolved inorganic carbon

δ18O delta oxygen-18

δD delta deuterium

FeS2 pyrite

ft/d foot per day

FGS Florida Geological Survey

gal/min/ft gallons per minute per foot

GMWL global meteoric water line

K’/S’s hydraulic diffusivity

Kr equilibrium reaction constant

Kv vertical hydraulic conductivity

in/yr inches per year

m meter

meq/L milliequivalent per liter

μS/cm microsiemens per centimeter

Mg magnesium

mg/L milligrams per liter

mmol/L millimoles per liter

mL milliliter

MW monitor well

N nitrogen

Na sodium

16O oxygen-16 isotope

18O oxygen-18 isotope

pmc percent modern carbon (analyzed)

PMC Percent Modern Carbon (normalized)

P phosphorus

PCO2 partial pressure of carbon dioxide

PZ permeable zone

PZ1 Tamiami/Peace River permeable zone 1

PZ2 Upper Arcadia permeable zone 2 

PZ3 Lower Arcadia permeable zone 3

RMSE root-mean-square error

ROMP Regional Observation and Monitor-Well Program

SAS surficial aquifer system

SI saturation index

SO4 sulfate

SO4ex excess sulfate concentration

SS sum of squared residuals

SWFWMD Southwest Florida Water Management District

td dimensionless time factor

TU tritium units

UFA Upper Floridan aquifer

USGS U.S. Geological Survey
Abstract 1

Hydrogeologic Framework and Geochemistry of the 

Intermediate Aquifer System in Parts of Charlotte, 

De Soto, and Sarasota Counties, Florida

By A.E. Torres, L.A. Sacks, D.K. Yobbi, L.A. Knochenmus, and B.G. Katz

Abstract

The hydrogeologic framework underlying 

the 600-square-mile study area in Charlotte, De 

Soto, and Sarasota Counties, Florida, consists of 

the surficial aquifer system, the intermediate aquifer system, and the Upper Floridan aquifer. The 

hydrogeologic framework and the geochemical 

processes controlling ground-water composition 

were evaluated for the study area. Particular 

emphasis was given to the analysis of hydrogeologic and geochemical data for the intermediate 

aquifer system. Flow regimes are not well understood in the intermediate aquifer system; therefore, hydrogeologic and geochemical information 

were used to evaluate connections between permeable zones within the intermediate aquifer system 

and between overlying and underlying aquifer systems. Knowledge of these connections will ultimately help to protect ground-water quality in the 

intermediate aquifer system. The hydrogeology 

was interpreted from lithologic and geophysical 

logs, water levels, hydraulic properties, and water 

quality from six separate well sites. Water-quality 

samples were collected from wells located along 

six ground-water flow paths and finished at different depth intervals. The selection of flow paths 

was based on current potentiometric-surface 

maps. Ground-water samples were analyzed for 

major ions; field parameters (temperature, pH, 

specific conductance, and alkalinity); stable isotopes (deuterium, oxygen-18, and carbon-13); and 

radioactive isotopes (tritium and carbon-14).

The surficial aquifer system is the uppermost aquifer, is unconfined, relatively thin, and 

consists of unconsolidated sand, shell, and limestone. The intermediate aquifer system underlies 

the surficial aquifer system and is composed of 

clastic sediments interbedded with carbonate 

rocks. The intermediate aquifer system is divided 

into three permeable zones, the Tamiami/Peace 

River zone (PZ1), the Upper Arcadia zone (PZ2), 

and the Lower Arcadia zone (PZ3). The Tamiami/Peace River zone (PZ1) is the uppermost zone 

and is the thinnest and generally, the least productive zone in the intermediate aquifer system. The 

Upper Arcadia zone (PZ2) is the middle zone and 

productivity is generally higher than the overlying 

permeable zone. The Lower Arcadia zone (PZ3) is 

the lowermost permeable zone and is the most productive zone in the intermediate aquifer system. 

The intermediate aquifer system is underlain by 

the Upper Floridan aquifer, which consists of a 

thick, stratified sequence of limestone and dolomite. The Upper Floridan aquifer is the most productive aquifer in the study area; however, its use 

is generally restricted because of poor water quality. Interbedded clays and fine-grained clastics 

separate the aquifer systems and permeable zones.

The hydraulic properties of the three aquifer 

systems are spatially variable. Estimated transmissivity and horizontal hydraulic conductivity 

varies from 752 to 32,900 feet squared per day and 

from 33 to 1,490 feet per day, respectively, for the 

surficial aquifer system; from 47 to 5,420 feet 

squared per day and from 2 to 102 feet per day, 
2 Hydrogeologic Framework and Geochemistry of the Intermediate Aquifer System in Parts of Charlotte, De Soto, and Sarasota 

Counties, Florida

respectively, for the Tamiami/Peace River zone 

(PZ1); from 258 to 24,633 feet squared per day 

and from 2 to 14 feet per day, respectively, for the 

Upper Arcadia zone (PZ2); from 766 to 44,900 feet 

squared per day and from 10 to 201 feet per day, 

respectively, for the Lower Arcadia zone (PZ3); 

and from 2,350 to 7,640 feet squared per day and 

from 10 to 41 feet per day, respectively, for the 

Upper Floridan aquifer. Confining units separating 

the aquifer systems have leakance coefficients 

estimated to range from 2.3 x 10-5 to 5.6 x 10-3 feet 

per day per foot. Strata composing the confining 

unit separating the Upper Floridan aquifer from 

the intermediate aquifer system are substantially 

more permeable than confining units separating 

the permeable zones in the intermediate aquifer 

system or separating the surficial aquifer and intermediate aquifer systems.

In Charlotte, Sarasota, and western De Soto 

Counties, hydraulic head generally increases with 

depth indicating an upward flow potential. Elsewhere, head decreases with depth indicating a 

downward flow potential. During September 

1998, relatively small head differences occurred 

between the Upper Floridan aquifer and Lower 

Arcadia zone (PZ3) in the intermediate aquifer 

system (up to 5 feet) whereas relatively larger head 

differences occurred between permeable zones of 

the intermediate aquifer system and the surficial 

aquifer system (as much as 8 feet).

The hydraulic connection between the 

surficial aquifer system, the intermediate aquifer 

system and associated permeable units, and the 

Upper Floridan aquifer is variable in the study 

area. Clay beds within the confining units can limit 

the degree of hydraulic connection between aquifer systems and permeable zones; however, facies 

changes within the units may result in local 

hydraulic connection with overlying and underlying aquifers. Generally, better hydraulic connection exists between the Upper Floridan aquifer and 

the Lower Arcadia zone (PZ3) than exists between 

the permeable zones of the intermediate aquifer 

system and the surficial aquifer system. 

Several important findings about flow patterns in the intermediate aquifer system have been 

supported by combining geochemical modeling 

with hydraulic head data. Vertical flow from 

underlying aquifers is significant in the chemical 

evolution of water in the intermediate aquifer system. Flow patterns derived only from potentiometric-surface maps may be misleading because flow 

paths are presumed to be lateral. Recent flow patterns delineated strictly based on potentiometricsurface maps do not represent predevelopment 

ground-water flow paths. The chemical composition of intermediate aquifer waters most likely 

reflects predevelopment conditions. Areas where 

geochemical models indicate large amounts of 

upward flow may actually be areas where discontinuity of the permeable zone exists, thereby limiting lateral flow.

Water in the intermediate aquifer system 

varies widely in chemical composition, but generally fits into one of two categories. At inland sites, 

water is a mixed ion or mixed cation-bicarbonate 

type. Sites closer to the coast have a sodiumchloride or mixed cation-chloride type water. 

Water within the same permeable zone of the intermediate aquifer system does not have a distinct 

chemical composition throughout the study area. 

Water in the surficial aquifer system, which is a 

calcium bicarbonate type, is more dilute than 

water from underlying aquifers. The chemical 

composition of water from the Upper Floridan 

aquifer is variable in the study area with no 

dominant cation present. 

Most water from the surficial aquifer system 

has isotopically lighter deuterium and oxygen-18 

values than water from the intermediate aquifer 

system or the Upper Floridan aquifer. Water from 

the surficial aquifer system most likely represents 

a mixture of meteoric water, with an isotopically 

light composition, and ground water that has been 

recharged by water that has undergone evaporation, with an enriched isotopic composition. Water 

from the intermediate aquifer system and the 

Upper Floridan aquifer may be the result of 

recharge that occurred under different climatic 

conditions than present conditions. Water from the 

three aquifer systems have isotopically distinct 

carbon-13 signatures of dissolved inorganic carbon, which is related to the evolution of inorganic 

carbon.
Intoduction 3

Water from the intermediate aquifer system 

and Upper Floridan aquifer is probably greater 

than 10,000 years old. Age dating indicates that 

water in some parts of these aquifers may be 

greater than 20,000 years old. Thus, the aquifer 

systems may have been recharged under different 

hydraulic conditions than currently observed.

INTRODUCTION

Ground-water withdrawals in southwest Florida 

are expected to increase and may result in water-level 

declines and water-quality degradation. The sources of 

ground water in southwest Florida are the surficial 

aquifer system, the intermediate aquifer system, and 

the Upper Floridan aquifer. The ground-water 

resources in this area are geographically limited by the 

quantity or quality of water in each aquifer. Generally, 

ground-water resources are small to moderate in the 

surficial aquifer system, moderate in the intermediate 

aquifer system, and abundant in the Upper Floridan 

aquifer. In parts of southwest Florida, the ground-water 

resources of the surficial aquifer system and the intermediate aquifer system are in greater demand because 

the ambient quality of the water is typically less mineralized than water in the Upper Floridan aquifer. 

The intermediate aquifer system is an interbedded group of permeable zones and confining units 

deposited under highly varied environmental conditions that form a complex, heterogeneous aquifer system. Several major permeable zones exist within the 

intermediate aquifer system, however, the regional distribution, areal extent, and hydraulic character of permeable zones and confining units and their occurrences 

have not been well defined due to the complexity of the 

system. For ground-water resources in the intermediate 

aquifer system to be adequately protected from waterquality degradation, it is essential to understand the 

hydrogeologic framework, ground-water flow patterns, 

and variations in water quality. As demand for water 

increases in southwest Florida, more information is 

needed to efficiently develop and manage the intermediate aquifer system as a water-supply source.

In 1996, the U.S. Geological Survey (USGS) in 

cooperation with the Southwest Florida Water Management District (SWFWMD) initiated an investigation to evaluate the hydrogeology and geochemistry of 

the intermediate aquifer system in parts of Charlotte, 

De Soto, and Sarasota Counties in southwest Florida 

(fig. 1). Although the majority of this study is focused 

on the intermediate aquifer system, data were collected from the surficial aquifer system and the Upper 

Floridan aquifer to provide an understanding of the 

interaction between aquifers overlying and underlying 

the intermediate aquifer system. 

Purpose and Scope

The purpose of this report is threefold: (1) to 

describe the hydrogeologic framework and geochemistry of the aquifer systems (particularly the intermediate 

aquifer system) in the study area (fig. 1), (2) to evaluate 

the geochemical evolution of ground water to understand the flow regimes in the intermediate aquifer 

system, and (3) to determine the lateral continuity of 

flow within permeable zones of the intermediate aquifer system and the vertical connection between the 

intermediate aquifer system and overlying and underlying aquifer systems. Hydrogeologic and geochemical 

characteristics of the intermediate aquifer system were 

determined at six Regional Observation and MonitorWell Program (ROMP) sites in Charlotte, De Soto, and 

Sarasota Counties. Information presented in this report 

was obtained from data collected during this study and 

from published USGS, Florida Geological Survey 

(FGS), and SWFWMD reports. The stratigraphic and 

hydraulic units composing the hydrogeologic framework were delineated using lithologic and geophysical 

logs, water levels, water quality, and hydraulic characteristics from five existing ROMP sites and one new 

ROMP test site constructed in De Soto County during 

this study. Each ROMP site consists of numerous 

monitor wells that penetrate various depth intervals in 

the surficial aquifer system, the intermediate aquifer 

system, and the Upper Floridan aquifer. The geochemical composition of water was determined from waterquality samples collected at five of the ROMP sites. 

Water was sampled for major ions, field parameters 

(temperature, pH, specific conductance, dissolved 

oxygen, and alkalinity), stable isotopes (deuterium, 

oxygen-18, and carbon-13), and radioactive isotopes 

(tritium and carbon-14). The water-quality data provided the input for geochemical mass-balance modeling. The geochemical evolution of ground water was 

simulated for six apparent flow paths. The flow paths 

were delineated using September 1998 potentiometricsurface maps of the intermediate aquifer system. 
4 Hydrogeologic Framework and Geochemistry of the Intermediate Aquifer System in Parts of Charlotte, De Soto, and Sarasota 

Counties, Florida

Description of the Study Area

The study area encompasses about 600 square 

miles (mi2

) and includes parts of Charlotte, De Soto, 

and Sarasota Counties (fig. 1). The study area lies 

within the physiographic subdivisions of the De Soto 

Plain, the Gulf Coast Lowlands, and the Caloosahatchee Incline (White, 1970). The gently sloping De 

Soto Plain has land surface elevations ranging between 

about 30 and 100 feet (ft). The Gulf Coast Lowlands is 

low with land surface elevations less than 40 ft. The 

Caloosahatchee Incline has land elevations ranging 

from about 50 to 60 ft. All three subdivisions are poorly 

drained broad flatlands containing many sloughs and 

marshes, including some that have been drained by 

ditches and canals. The study area is bisected by the 

Peace River (fig. 1), one of the largest rivers in southwest Florida. Four smaller streams, Horse Creek, 

Joshua Creek, Prairie Creek, and Shell Creek drain the 

eastern and north-central parts of the study area.

Previous Investigations

 Although many reports describe the groundwater resources of southwest Florida, few reports 

focus predominantly on the hydrogeology of the intermediate aquifer system. Current conceptualization of 

the intermediate aquifer system is based on data 

compiled from numerous small area or site specific 

investigations published in USGS, SWFWMD, other 

government agencies, and consultants’ reports. 

Reports by SWFWMD have been particularly informative because they provide detailed lithologic, 

hydraulic, and chemical data from the intermediate 

aquifer system at specific sites in southwest Florida. 

Reports by Clark (1964), Sutcliffe (1975), Joyner and 

Sutcliffe (1976), Sutcliffe and Thompson (1983), 

Wolansky (1983), Gilboy (1985), Duerr and others 

(1988), Duerr and Enos (1991), Hutchinson (1992), 

Broska and Knochenmus (1996), Barr (1996), and 

Knochenmus and Bowman (1998) have helped 

GULF OF MEXICO

0 5 10 MILES

0 5 10 KILOMETERS

Base from SWFWMD digital data, 1:250,000, 1992

Albers Equal-Area Conic projection

Standard Parallels   and 	 
 central meridian






   
 


R12

R26

R17

R9

R9.5

R5

R13

SARASOTA CO.

A

B

B ́

A ́ Venice

Well

Field

Mabry Carlton

Reserve Well

Field

Venice

Sarasota Myakka City

Punta Gorda

Port Charlotte

Venice Gardens

Utility Plantation

Utility

Englewood

Englewood

Well Field 3

Englewood

Well Field 2

Englewood

Well Field 1&4

Peace River

Peace River

Joshua Creek

Prairie Creek

Horse Creek

Shell

MANATEE CO. HARDEE CO.

DE SOTO CO.

CHARLOTTE CO. Charlotte

Harbor

EXPLANATION

LINES OF SECTION Shown in figures 5 and 6

ROMP WELL SITE LOCATION AND NUMBER

MAJOR PUMPING CENTER Well field —

or utility near study area

LOCATION OF STUDY AREA

—

R5

A A ́

Creek

Figure 1. Location of study area, Regional Observation and Monitor-Well Program (ROMP) sites, and hydrogeologic 

sections, west-central Florida.
Methods of Study 5

improve the current state-of-knowledge concerning 

the intermediate aquifer system regional hydrogeology.

The chemical and isotopic composition of intermediate aquifer system water and the interaction and 

mixing of waters among aquifer systems have been 

described in various reports. Berndt and Katz (1992) 

related differences in major-ion composition of waters 

in the intermediate aquifer system to variations in the 

chemical composition and amount of water leaking 

downward from the surficial aquifer system and 

upward from the Upper Floridan aquifer. Sacks and 

Tihansky (1996) characterized the chemical and stable 

isotopic composition of water from the intermediate 

aquifer system in parts of southwest Florida, focusing 

on wells with discrete open-hole intervals; waters in the 

intermediate aquifer system influenced by upward flow 

from the Upper Floridan aquifer were chemically and 

isotopically distinct from areas not so influenced. 

Water quality of the discrete production zones of the 

intermediate aquifer system in Sarasota County was 

reported by Knochenmus and Bowman (1998). 

Kauffman and others (1998) and Hobbie (1993) attributed vertical differences in the chemical composition 

of water in permeable zones of the intermediate aquifer 

system to the influence of confining units and upward 

flow from the Upper Floridan aquifer. 

Acknowledgments

The authors are extremely grateful for the cooperation of SWFWMD, especially the Geohydrologic 

Data Section personnel and Michael T. Gates who provided invaluable help with data collection at the ROMP 

9.5 site. The authors thank Paulette Bond of the Florida 

Geological Survey (FGS) for extracting rock material 

from cores, and Edward T. Furlong and Ronald W. 

Brenton of the USGS for modifying analytical protocols to determine the carbon content of cores. Thanks 

also to Tyler B. Coplen of the USGS for analysis of 

stable isotopes. The authors also thank Blair F. Jones 

and Daniel M. Webster of the USGS for providing 

mineralogic identifications and clay-fraction determinations of cores. Finally, the authors would like to 

express their appreciation to Lynn Barr of the USGS 

for all of his contributions during the course of the 

investigation. 

METHODS OF STUDY

Hydrogeologic and geochemical data were collected from test holes at six ROMP sites (5, 9, 9.5, 12, 

13, and 17) in Charlotte, De Soto, and Sarasota Counties. The ROMP 9.5 site was established during this 

study with a total of 18 monitor wells constructed at 

discrete depths. Hydrogeologic data were collected by 

SWFWMD personnel in accordance with ROMP 

sampling protocol. 

Hydrogeology 

Hollow-stem auger, wire-line coring, and mud 

rotary drilling methods were used to collect lithologic 

and water samples at depth. At each ROMP site, a test 

hole was drilled and continuous cores were collected 

from land surface to variable depths in the Upper 

Floridan aquifer. Data collected during the coring of 

the test hole included water levels, water quality, 

geophysical logs, and specific capacities. Water levels 

were measured while coring and during packer testing. 

A qualitative measure of permeability obtained by the 

visual inspection of cores was used to test discrete 

borehole intervals for hydraulic properties. Specific 

capacity of discrete intervals was determined during 

packer tests. The hydraulic properties of discrete stratigraphic units from the intermediate aquifer system and 

Upper Floridan aquifer were determined from fallinghead permeameter tests. The hydraulic properties of 

permeable zones and confining units were determined 

from aquifer test analysis by SWFWMD and USGS 

personnel. Aquifer tests were evaluated using analytical methods (Jacob, 1946; Hantush, 1960; and Neuman 

and Witherspoon, 1972). Aquifer tests also were evaluated using numerical methods at the ROMP 9.5 site 

(McDonald and Harbaugh, 1988; and Halford, 1992). 

The delineation of the stratigraphic units composing 

the geologic framework was based on stratigraphic 

picks by SWFWMD and FGS personnel. The hydrogeologic framework was delineated using a qualitative 

evaluation of core permeability conducted by 

SWFWMD personnel and by concurrent evaluation of 

borehole data. The geologic and hydrogeologic frameworks were linked using water-level, water-quality, 

geophysical logs, and specific-capacity data (apps. A 

through C). It should be recognized that, for hydraulically complex carbonate strata, all hydrogeologic 

identifications are tentative until detailed flowmeter 

measurements can be collected.
6 Hydrogeologic Framework and Geochemistry of the Intermediate Aquifer System in Parts of Charlotte, De Soto, and Sarasota 

Counties, Florida

Geochemistry

Water and rock samples were collected for chemical and isotopic analysis. Results from the chemical 

and isotopic analysis were used to evaluate geochemical processes influencing the chemical and isotopic 

composition of ground water in the study area. Rock 

mineralogy from selected cores was analyzed using 

x-ray diffraction. The age of ground water in different 

aquifer systems was estimated from isotopic signatures. Geochemical models were used to examine 

reactions and mixing along apparent flow paths in the 

intermediate aquifer system.

Water Sampling

Vertical variability in the composition of ground 

water was determined from water-quality samples 

collected by SWFWMD personnel during exploratory 

drilling of test holes at ROMP sites. Typically, samples 

were collected every 20 to 40 ft from a discrete interval 

at the base of the borehole, with the drill stem acting as 

a temporary casing. Because upper zones were not 

effectively sealed, mixing with water from overlying 

sediments was possible. At the ROMP 9.5 site, samples 

were collected less frequently and only from the uppermost part of the Upper Floridan aquifer. An inflatable 

packer was used to isolate the bottom of the test hole, 

allowing samples to be collected from discrete zones.

Water was collected from wells completed at 

different depth intervals in the surficial aquifer system, 

the intermediate aquifer system, and the Upper 

Floridan aquifer. Existing water-quality data collected 

from the ROMP 17 site for a previous study also was 

utilized (Sacks and Tihansky, 1996). Standard techniques were used to collect water-quality samples 

(Wood, 1976) and to conduct water-quality analyses 

(Skougstad and others, eds., 1979). Water samples 

were collected from the wells using a submersible or 

peristaltic pump, or by using discharge tubes from 

flowing wells. Field measurements of pH, specific 

conductance, temperature, and dissolved oxygen were 

made using a closed flow chamber to prevent atmospheric contact with water samples. 

Water samples were collected after three well 

casing volumes had been removed and field measurements of pH, specific conductance, and temperature 

had stabilized. Alkalinity was measured in the field by 

incremental titration using sulfuric acid. Sulfide concentrations were measured at the ROMP 9.5 site using 

a field spectrophotometer and reagent solutions (Hach 

Company, 1989). Major ions and selected trace 

element concentrations were determined by the USGS 

Water Quality Laboratory in Ocala, Florida. 

Isotope ratios of deuterium (D/H) and oxygen-18 

(

18O/16O) were determined by the USGS stable isotope 

laboratory in Reston, Virginia, using techniques 

described by Coplen and others (1991) and Coplen 

(1994). Tritium (3H), carbon-14 (14C), and carbon-13 

(

13C/12C) were determined by USGS contract laboratories. Standard δ (delta) notation (Gonfiantini, 1981) 

was used to express concentrations of the stable isotopes (deuterium, oxygen-18, and carbon-13), in units 

of parts per thousand (per mil). Standards are Vienna 

Standard Mean Ocean Water (VSMOW) for deuterium 

and oxygen-18 and Pee Dee Belemnite (PDB) for 

carbon-13 (Coplen, 1994). Oxygen and hydrogen 

isotope results are normalized on scales such that the 

oxygen and hydrogen isotopic values of Standard Light 

Antarctic Precipitation (SLAP) are -55.5 and -428 parts 

per thousand (per mil), respectively (Coplen and 

others, 1991, and Coplen, 1994); the 2σ (standard deviation) precision for δ18O, δD, and δ13C are 0.2, 2.0, and 

0.2 per mil, respectively. Water samples for tritium 

were collected and analyzed according to methods 

described by Michel (1989). Tritium activity is 

reported in tritium units (TU; 1 TU is equal to 1 tritium 

atom in 1018 hydrogen atoms, and 7.1 disintegrations 

per minute per gram of water), with a 1σ precision of 

less than 10 percent for waters containing more than 

2 TU (Michel, 1989). Carbon-14 was analyzed using 

gas-stripping and accelerator mass spectrometry methods (Beukens, 1992). All 14C determinations are 

reported in Percent Modern Carbon (PMC) normalized 

to a δ13C value of -25 per mil and the 1950 oxalic acid 

standard from the National Institute of Standards and 

Technology (Stuiver and Polach, 1977; Wigley and 

Muller, 1981). The reported analytical precision is 

1 PMC for 14C values less than 10 PMC and 2 PMC for 

values greater than 10 PMC (A. Mullin, USGS, written 

commun., 1998).

Rock Sampling

Analysis of rock mineralogy was performed on 

eight cores collected from the test hole at the ROMP 

9.5 site. Cores were selected to represent permeable 

zones and confining units of the intermediate aquifer 

system and the upper part of the Upper Floridan aquifer. Bulk sediments and ultra fine (less than 1 millimeter (mm)) clays from these zones were analyzed using 

X-ray diffraction techniques (Brindley and Brown, 
Geologic Framework 7

eds., 1980; Moore and Reynolds, 1989). Bulk samples 

were scanned as amyl acetate smear mounts. The clays 

were analyzed untreated, glycol solvated, and heated at 

550 o

C for 1 hour or longer. Clay minerals were identified based on the response of their (001) reflections 

following the established procedure described above.

Additionally, selected cores from the ROMP 9.5 

test hole were analyzed to estimate the fraction of 

organic carbon in the rock material and the δ13C content of the inorganic and organic carbon. Two methods 

were used to estimate the fraction of organic carbon: 

traditional combustion/gasometric methods and a 

persulfate-oxidation method. For δ13C of organic 

carbon, the inorganic carbon fraction was removed by 

dissolving the sample in hydrochloric acid (Barrie and 

Prosser, 1996). For δ13C of inorganic carbon, 20 milligrams (mg) of carbonate material was reacted with 

2 milliliter (mL) of 100-percent phosphoric acid. The 

liberated carbon dioxide was dried and its 13C content 

was determined using a dual-inlet isotope ratio mass 

spectrometer (Barrie and Prosser, 1996). 

Geochemical Modeling

The geochemical model NETPATH (Plummer 

and others, 1994) was used to determine the dominant 

processes (including chemical reactions and mixing of 

waters) controlling ground-water evolution along six 

apparent flow paths. Input to the model included the 

chemical and isotopic composition of the ground water, 

the saturation state of ground water with respect to 

mineral phases, the known mineralogy of the aquifer 

systems, and the isotopic composition of solid phase 

materials. Input also required defining flow paths 

within the aquifer system and knowledge of vertical 

head differences between aquifers so that likely mixing 

scenarios between aquifers could be hypothesized.

NETPATH also was used to calculate plausible 

ages for ground water using 14C. In the calculations, 

the analyzed 14C values, in percent modern carbon 

(pmc), rather than the normalized values reported by 

the laboratory, in PMC, were used (L.N. Plummer, 

USGS, written commun., 1998). The Fontes and 

Garnier (1979) adjustment model was used to calculate 

the initial 14C activity (Ao) in ground water at each site 

by considering a two-stage evolution of recharge 

waters that includes dissolution and isotopic exchange 

of carbonate minerals with CO2 in the unsaturated zone 

and isotopic exchange with the carbonate rocks in the 

saturated zone. The inorganic carbon system was used 

for computing age corrections; dissolved organic 

carbon and methane were not analyzed for this study, 

and thus, the assumption was made that their concentrations were negligible or their 14C concentrations 

were 0 pmc. All models with 14C parameters used the 

following constraints to calculate Ao: 

(1) a user-defined δ13C of soil gas CO2 of -20 to 

-25 per mil (Deines and others, 1974),

(2) 14C soil gas = 100 pmc, 

(3) 14C of calcite and dolomite = 0 pmc, and 

(4) the δ13C value used is the δ13C composition of 

the dissolved inorganic carbon of the initial 

water. 

The sensitivity of analytical uncertainties in the 

14C value on the computed ages also was evaluated.

GEOLOGIC FRAMEWORK

Defining the geologic framework in the study 

area is problematic. The stratigraphy comprising the 

geologic framework is mired in contradictory and confusing nomenclature because of inconsistent application of the North American Stratigraphic Code (code) 

and the limitations of the code with respect to Florida’s 

rock record (Randazzo, 1997, p. 49). The code relies 

upon definition of lithostratigraphic units to name formations. Carbonate rocks in Florida do not easily lend 

themselves to lithostratigraphic definition because of 

limited exposures in outcrops; as a result, scientists rely 

on key fossils to name stratigraphic units. The code, 

however, does not recognize the practice of using fossils to name formations. Furthermore, identification of 

stratigraphic units is complicated by complex sediment 

facies patterns and phosphorite sedimentation and 

diagenetic alteration of sediments obliterating fossils. 

In addition, correlating geologic units with hydrogeologic units is difficult because of wide variations in 

depositional environment occurring simultaneously 

within the study area. Complex facies changes can 

occur at the boundaries of predominately carbonate 

rocks and siliciclastic rock deposition. Additionally, 

the same rock type may recur at several horizons in the 

lithostratigraphic (geologic) section because the exact 

depositional and diagenetic conditions that produced 

the rocks were repeated several times (Miller, 1986, 

p. B7). The formation names used in this report are 

based upon the geologic definitions of Scott (1988) and 

those used by the FGS.
8 Hydrogeologic Framework and Geochemistry of the Intermediate Aquifer System in Parts of Charlotte, De Soto, and Sarasota 

Counties, Florida

Depositional History

The depositional history of the Florida Platform 

resulted in a thick sequence of evaporite, carbonate and 

siliciclastic facies. These sediments began to accumulate on the Florida Platform with the development of 

the Gulf of Mexico Basin, probably during the middle 

Jurassic (Randazzo, 1997, p. 39). Beginning in the late 

Cretaceous, a stable tectonic period allowed the accumulation of a thick sequence of sedimentary rocks in 

peninsular Florida. The primary force affecting the 

type of sediments deposited was changing sea levels 

throughout the Cenozoic Era and especially during the 

Tertiary. Throughout the Tertiary, the Florida Platform 

was subjected to numerous sea-level fluctuations, and 

variations in sea level were most dramatic from the late 

Oligocene through Pliocene Epochs (Scott, 1988, 

p.112). Periods of rising levels are defined as transgressions and falling sea levels are defined as regressions. 

Sea-level fluctuations, relative to the present sea-level 

stand since the Oligocene Epoch are shown in figure 2. 

On the Florida Platform, transgressions or high sealevel stands, indicated as rising water levels (fig. 2), 

resulted in carbonate and evaporite deposition whereas 

regressions or low sea-level stands, indicated as falling 

water levels (fig. 2), resulted in increased siliciclastic 

sedimentation that suppressed carbonate sedimentation. The depositional history of the Hawthorn Group is 

directly related to the Miocene-age sea-level fluctuations and can be observed in the rock record as episodes 

of cyclic sedimentation that resulted in interbedded 

layers of peloidal and skeletal limestone, mudstone 

GLOBAL CHART OF RELATIVE

COASTAL ONLAP1 GEOCHRONOLOGIC

UNITS

STANDARD

AGES

EPOCHS

SOUTH FLORIDA

RISING FALLING

1 0.5 0

CALABRIAN

PIACENZIAN

TABIANIAN

MESSINIAN

TORTONIAN

LANGHIAN

BURDIGALIAN

AQUITANIAN

CHATTIAN

SERRAVALLIAN

PRESENT

SEA LEVEL

Notes:

(VAIL & MITCHUM, 1979)

GIRONDIAN

LATE

OLIGOCENE MIOCENE PLIO- CENE PLEIST- OCENE

EARLY MIDDLE LATE

CESSOLIAN CASTLE- LANIAN ROSSEL LIAN

UNITS OF PLEISTOCENE AGE

TAMIAMI FM.

PEACE RIVER FM.

ARCADIA FM.

ARCADIA FM. NOCATEE MBR.

TAMPA

MBR.

SUWANNEE LS.

BONE VALLEY

MBR.

BONE VALLEY MBR.

HAWTHORN GROUP

?

?

? ?

?

?

1. The horizontal scale showing amplitude of relative rises and falls is calibrated to a relative scale normalized on the

maximum range of sea level positions of the curve. The highest position of sea level, occurring at the end of the

Cretaceous, is set at 1.0 and the lowest position, at the mid-Oligocene, is set at 0.0

2. Shaded areas schematically show unconformities between formations

Figure 2. Sea-level fluctuations in South Florida (modified from Vail and Mitchum, 1979).
Geologic Framework 9

(carbonate mud), clay (siliciclastic) and quartz sand. In 

addition to influencing the type of sediments deposited, 

sea-level changes interrupted sediment deposition, 

removed sedimentary units by erosion, and altered the 

units post depositionally. Subaerial exposure resulted 

in karstification and erosional sequence-boundary 

surface development (Randazzo, 1997, p. 47).

The Avon Park Formation is a carbonate muddominated peritidal sequence. Open-marine, shallow 

water, and shelf deposits prevailed during the late 

Eocene, which was a period of restricted-flow and open 

marine deposition. During the Oligocene, deposition 

was predominately open-marine. Sea-level fluctuations 

after the Oligocene were a primary factor controlling 

the distribution of depositional environments during 

the Miocene and Pliocene. The frequent sea-level 

fluctuations during the Miocene spread sediments over 

the Florida Platform but left an intermittent stratigraphic record. Carbonate deposition on the southern 

part of the platform persisted until the middle to late 

Miocene. In the southernmost peninsula, deposition of 

the Arcadia Formation continued until the Pliocene. 

Thin beds of siliciclastic sediments were deposited 

nearly platform-wide (Scott, 1997, p. 61) throughout 

the early (lower) Miocene. A major eustatic regression 

occurred during the middle Pliocene (Vail and others, 

1977) creating a disconformity separating younger 

Pliocene- and Pleistocene-age sediments from the 

underlying older Pliocene- and Miocene-age sediments. 

Stratigraphy Underlying the Study Area

The stratigraphy underlying the study area was 

determined from cores collected at the six ROMP sites 

and reported by SWFWMD and FGS (Decker, 1988; 

Gates, 1997a,b; Peterman, 1997; Thompson, 1997; 

Baldini, 1998; Gates, 1998a; Baldini, 1999; Clayton, 

1999; and Jon Arthur, FGS, written commun., 2000). 

Where formation (or member) boundaries differed 

between the reporting sources of geologic data, the 

FGS designations were used. The stratigraphy and 

dominant lithology for the six sites are presented in 

figure 3 and appendices A through C. Test wells 

penetrated the undifferentiated surficial deposits, the 

heterogeneous marine deposits comprising the 

Tamiami Formation and Hawthorn Group, and the 

persistent carbonates comprising the Suwannee Limestone, Ocala Limestone, and Avon Park Formation.

Avon Park Formation, Ocala Limestone, and 

Suwannee Limestone

The 1,200-ft thick, middle Eocene-age Avon 

Park Formation is the oldest stratigraphic unit exposed 

in Florida (Miller, 1986). The Avon Park Formation is 

a carbonate mud, pervasively dolomitized in places and 

undolomitized in others with mostly benthic fossils 

indicating a lack of faunal diversity (Randazzo, 1997, 

p. 50). Lithologically, the formation consists predominately of cream, tan, or light-brown, soft to wellindurated limestone that is mostly pelletal but is locally 

micritic. The limestone may be interbedded with darkbrown, highly fractured sucrosic dolostone. The top of 

the Avon Park ranges from about 1,000 to 1,230 ft 

below sea level in the study area. 

Overlying the Avon Park Formation is the 200- 

to 300-ft thick, late Eocene-age Ocala Limestone. The 

Ocala can contain two distinct lithologic units that 

include a basal dense dolostone and an upper relatively 

pure, porous limestone. The basal dense dolostone is 

similar to the Avon Park dolostone described above. 

The limestone unit has been described as fossiliferous, 

white, soft, friable, and porous, and includes fossils of 

foraminifera, bryozoan fragments, and echinoid 

remains loosely bound by a matrix of micritic cement. 

The basal dense dolostone unit occurs in the Ocala 

Limestone at the ROMP 5, 9, 12, and 17 sites. The top 

of the Ocala ranges from about 700 to about 1,100 ft 

below sea level. 

The boundary between the Ocala and Suwannee 

Limestone can be difficult to identify because of their 

similar lithologic appearance (Randazzo, 1997, p. 50). 

The 70- to 400-ft thick, Oligocene-age Suwannee 

Limestone is a granular, fossilifereous limestone that 

occurs about 600 ft below sea level. The limestone can 

be interbedded with calcilutite (calcareous mud, 

greater than 50-percent silt or clay-size limestone) and 

calcarenite (calcareous sand, greater than 50-percent 

sand-size limestone) units, and the basal unit can be a 

fine-grained dolosilt. Beds of unconsolidated quartz 

sand were noted at the ROMP 5 site. The Suwannee 

Limestone generally is distinguished from the overlying Hawthorn Group by the lack of phosphatic sand 

content, however, the contact becomes obscure in 

Charlotte and Lee Counties (Randazzo, 1997). 

Hawthorn Group

Based on the work of Scott (1988), the Mioceneand early Pliocene-age sediments were reclassified as a 

group. The reclassification of the Hawthorn to group 
10 Hydrogeologic Framework and Geochemistry of the Intermediate Aquifer System in Parts of Charlotte, De Soto, and Sarasota 

Counties, Florida

status is justified by the presence of areally extensive 

and mappable lithologic units (Scott, 1988, p. xii). In 

the study area, the 400- to 600-ft thick, Miocene- and 

early Pliocene-age Hawthorn Group contains multiple 

distinct lithologic units. Sea-level fluctuations during 

the Miocene are reflected in the interbedded layering of 

the siliciclastic and carbonate units that compose the 

Hawthorn Group. The sediments consist of highly 

variable mixtures of clay, silt, sand, and carbonate. 

Limestone is the dominant carbonate phase in southern 

Florida. Additionally, the Hawthorn Group sediments 

contain unique minerals including phosphorite ranging 

from trace amounts to about 50 percent. Phosphogenic 

episodes typically occur during the transition from 

carbonate to siliciclastic deposition. Associated with 

phosphogenesis is the formation of palygorskite and 

sepiolite (magnesium-rich clay) and dolomite. Apatite 

(phosphate grains) is virtually ubiquitous in the Hawthorn sediments underlying peninsular Florida. 

Palygorskite, sepiolite, and dolomite can occur statewide. In southern peninsula Florida, the Hawthorn 

Group includes, in ascending order, the Arcadia and 

Peace River Formations (Scott, 1988, p. 17). 

Arcadia Formation

The Arcadia Formation is the basal unit of the 

Hawthorn Group. The Arcadia Formation may contain 

up to two named members, in ascending order, the 

Nocatee and Tampa Members. Where both of the 

named members cannot be identified, the Arcadia 

Confining unit

Upper Floridan aquifer

Surficial aquifer system

Undifferentiated sand with

some limestone and

shell beds.

Sand, limestone, and shell beds.

Thick clay near top.

Mostly limestone, sandy limestone

and sand. Phosphatic in part.

Dolomite beds common. Clayey

in middle and lower parts.

Limestone, sandy limestone

and sand. Clay beds in upper

and lower parts.

Granular, fossiliferous limestone,

with trace amounts of sand and

clay in the upper portions. Dense

dolostone and indurated limestone,

mostly pelletal

Hawthorn Group

Intermediate aquifer system

Suwannee Limestone

Ocala Limestone

Avon Park Formation

Undifferentiated surficial

deposits (including the

Caloosahatchee Formation

and the Tamiami Formation)

Undifferentiated

Arcadia Formation

Peace River

Formation

Lower Arcadia zone

(PZ3)

Upper Arcadia zone

(PZ2)

Tamiami/Peace

River zone

(PZ1)

Tampa

Member

Nocatee

Member

Confining unit

Confining unit

Confining unit

Miocene

Pliocene

Series Stratigraphic unit Hydrogeologic unit Lithology

Holocene

and

Pleistocene

Oligocene

Eocene

Figure 3. Relation of stratigraphic and hydrogeologic units (modified from Duerr and Wolansky 1986; Southeastern 

Geological Society 1986; Scott 1988; Barr 1996; and Knochenmus and Bowman, 1998).
Geologic Framework 11

Formation is designated the undifferentiated Arcadia 

Formation. The Arcadia Formation is composed predominately of carbonate rocks with varying amounts of 

included and interbedded siliciclastics. The predominate carbonate rock type is dolostone. The undifferentiated Arcadia Formation consists predominantly of 

limestone and dolostone containing varying amounts 

of quartz sand, clay, and phosphate grains. Thin beds of 

quartz sand and clay are present sporadically throughout the section. These thin sand and clay beds are 

generally dolomitic and phosphatic, less than 5 ft thick, 

and of limited areal extent (Scott, 1988, p. 58). Chert, 

which can occur in updip areas, such as in parts of 

Hardee, Hillsborough, Manatee, and Polk Counties, 

and appears to be silicified clays and dolosilts.

Nocatee and Tampa Members

The basal Nocatee Member has the highest content of siliciclastic sediments in the Arcadia Formation. 

The Nocatee Member is a complexly interbedded 

sequence of quartz sands, clays, phosphates, and minor 

carbonates. The Nocatee Member has been designated 

previously as the sand and clay unit of the Tampa 

Formation. The Nocatee Member ranges in thickness 

from about 50 to 200 ft in the study area.

The Tampa Member of the Arcadia Formation 

was formerly referred to as the Tampa Limestone or 

Tampa Formation. The new name reflects a change in 

nomenclature and status of the Tampa lithologic unit, 

which has long been a problematic unit due to facies 

changes and apparent gradational contacts with overlying and underlying units. In areas where the intermediate aquifer system is absent, the Tampa Member 

directly overlies the Suwannee Limestone and is considered part of the Upper Floridan aquifer. Where both 

the Tampa and Nocatee Members are present in the 

stratigraphic section, the Tampa Member overlies the 

Nocatee Member in the study area. The Tampa 

Member occurs between the upper and lower parts of 

the undifferentiated Arcadia Formation. A diagnostic 

characteristic that can be used to separate the Tampa 

Member from the rest of the Arcadia Formation is the 

lack of phosphate content in the Tampa Member. The 

Tampa Member of the Arcadia Formation consists predominantly of limestone with subordinate dolostone, 

sand, and clay and contains little or no phosphate. The 

Tampa Member ranges from zero to less than 200 ft 

thick in the study area. 

Peace River Formation

The Peace River Formation is the upper unit of 

the Hawthorn Group and unconformably overlies the 

Arcadia Formation. Siliciclastics are the predominate 

lithology in the Peace River Formation, comprising 

greater than 66 percent of the rock material. Clay beds 

are common in the formation and phosphate, in varying 

amounts, generally is present. The carbonate content 

increases near the base of the formation. In some areas, 

the contact of the Peace River Formation with the 

underlying Arcadia Formation is delineated by a 

phosphatic rubble zone (Scott, 1988). 

Bone Valley Member of the Peace River Formation

The Bone Valley Formation of former usage has 

been reclassified to member status within the Peace 

River Formation. The reduction in status is due to the 

limited areal extent, gradational relations with supraand subjacent units, and lithologic similarities to the 

undifferentiated Peace River Formation. The Bone 

Valley Member is a clastic unit consisting of pebble- to 

gravel-size phosphate fragments and grains in a matrix 

of quartz sand and clay (Scott, 1988, p. 86-87). Phosphate concentrations are highest in the Bone Valley 

Member. Clays can occur as matrix material but also as 

discrete beds of relatively pure clay referred to as “bed 

clay” by the phosphate industry. These clay beds delineate the base of the Bone Valley Member. A carbonate 

rubble, consisting of basal gravels below the bed clay, 

has been described as delineating the contact between 

the Bone Valley Member and the undifferentiated 

Peace River Formation. When the basal gravels are 

absent, locating the contact with the Peace River 

Formation becomes arbitrary (Scott, 1988, p. 88). The 

Bone Valley Member has not been identified in the 

study area.

Tamiami Formation

The Tamiami Formation occurs over much of 

southern Florida and consists of sand, clay, carbonate, 

and reef facies. At least nine subsurface facies of the 

Tamiami Formation have been mapped by various 

investigators in southwest Florida but only one to four 

of the facies occur in the stratigraphic section at a given 

locality (Missimer, 1992, p. 69). The formation contains 

thick carbonate sequences in southwestern Florida and 

grades into siliciclastic sediments to the north and east. 

In Charlotte, Sarasota, and most of Lee Counties, the 

base of the Tamiami Formation occurs at the top of the 
12 Hydrogeologic Framework and Geochemistry of the Intermediate Aquifer System in Parts of Charlotte, De Soto, and Sarasota 

Counties, Florida

first major green dolosilt/sand unit that marks the top of 

the Peace River Formation (Missimer, 1992, p. 65). In 

the study area, the middle to late Pliocene-age Tamiami 

Formation is thin and is designated stratigraphically as 

part of the undifferentiated surficial deposits (Jon 

Arthur, FGS, oral commun., 2000).

Surficial Deposits

The stratigraphy of the deposits overlying the 

Hawthorn Group (or Tamiami Formation where 

present) is a complex of Plio-Pleistocene to Holoceneage, highly fossiliferous siliciclastics designated the 

undifferentiated surficial deposits. The surficial deposits generally have been separated into three units: 1) the 

Caloosahatchee Marl of Pliocene and Pleistocene age 

consisting of shell beds, shelly, sandy, or silty marl, 

marl, and sandy limestone, 2) Pleistocene-age deposits 

consisting of medium yellow-orange sand with clay 

and shell, and 3) Holocene-age deposits consisting of 

fine-grained quartz sand referred to as undifferentiated 

or surficial sand. Surficial deposits occur throughout 

De Soto and Hardee Counties but thin greatly near 

creeks and rivers. The upper sand unit is composed of 

a fine sand that blankets the area and is about 25 ft thick 

in the study area (Wilson, 1977, p.25). In most of 

De Soto County, the underlying shell and sand unit 

comprises the Caloosahatchee Marl. In southern and 

southeastern De Soto County, the marl contains fine 

sand with clay and large marine shells. Along Prairie 

Creek near the ROMP 12 site (fig. 1), the unit is almost 

entirely composed of marine shells. In northern 

De Soto and Hardee Counties, the basal unit of the 

undifferentiated surficial deposits consists of phosphate 

deposits. In the study area, the middle to late Plioceneage Caloosahatchee Formation is thin and is designated 

stratigraphically as part of the undifferentiated surficial 

deposits (Jon Arthur, FGS, oral commun., 2000). 

HYDROGEOLOGIC UNITS AND 

HYDRAULIC PROPERTIES

The hydrogeologic units underlying the study area 

consist of the surficial aquifer system, the intermediate 

aquifer system, and the Upper Floridan aquifer. The 

corresponding chronostratigraphic, lithostratigraphic, 

and hydrogeologic units underlying the study area are 

shown in figure 3. Deposits overlying the Tamiami Formation or the Hawthorn Group comprise the surficial 

aquifer system. Deposits of the Hawthorn Group and the 

Tamiami Formation form the intermediate aquifer 

system, and the underlying Oligocene and older carbonate rocks compose the Floridan aquifer system. The 

Floridan aquifer system consists of the Upper and Lower 

Floridan aquifers that are separated by a middle confining unit (Miller, 1986). Each of these aquifer systems 

include one or more water-producing zones separated by 

less-permeable units. The hydrogeologic framework 

described in this report is based on data from the six 

ROMP sites located in the study area (fig.1). 

Surficial Aquifer System

The surficial aquifer system comprises Pliocene 

to Holocene-age, unconsolidated to poorly indurated, 

clastic sediments, and is defined as a permeable unit 

contiguous with land surface (Southeastern Geological 

Society, 1986, p.4). Because of the interbedded nature 

of the clastics forming the surficial aquifer system, 

more than one water-producing zone separated by beds 

of lower permeability may occur in the surficial aquifer 

system. The water-bearing capacity of the surficial 

aquifer system is largely dependent on the grain size, 

sorting, and saturated thickness of the sediments. 

Results by Vacher and others (1992), indicate a relation 

between sediment type and hydraulic properties in the 

surficial aquifer system based on mapped regional 

distribution of selected lithofacies and published 

hydraulic properties for the surficial aquifer system 

(fig. 4). The lithofacies map was constructed from 

trilinear analysis of three general lithologies (end members) that include (1) clay (clayey sand), (2) shell (limestone, shelly sand), and (3) sand (fig. 4a). In the study 

area, the surficial aquifer system consists of relatively 

thin, unconsolidated sand, shell, and limestone that 

generally yield small volumes of water to wells. The 

thickness of the surficial aquifer system ranges from 

19 ft at the ROMP 13 site to 69 ft at the ROMP 5 site 

(apps. A through C). The lithofacies in the study area 

span the range from dirty sand (substantial clay content/low water yield) in parts of De Soto County to 

clean sand (moderate water yield) in southeastern and 

southwestern De Soto County to transition (interbedded/wide range water yield) to shell bed (substantial 

limestone and shell content/high water yield) in southwestern Sarasota and most of Charlotte Counties where 

significant shell facies occur (Vacher and others, 1992). 

A map of published hydraulic conductivities (fig. 4b) 

shows an increase in hydraulic conductivities from 

north to south (Vacher and others, 1992).
Hydrogeologic Units and Hydraulic Properties 13

Hydraulic properties of the surficial aquifer 

system have been estimated from aquifer tests conducted at three of the ROMP sites in the study area. The 

aquifer tests were conducted at the ROMP 5 site in 

north-central Charlotte County, at the ROMP 9 site in 

southeastern Sarasota County, and at the ROMP 12 site 

in south-central De Soto County. The test at the ROMP 

5 site evaluated an 85-ft thick sequence of unconsolidated material including interbedded sand, silt, clay, 

shell, and limestone (Gates, 1997a). At the ROMP 9 site 

a 20-ft thick sequence of sand and shell was tested 

(Thompson, 1997), and at the ROMP 12 site a 15-ft 

thick sequence of sand and shell was tested. The 

reported values for transmissivity, horizontal hydraulic 

conductivity, and specific capacity were 2,780, 32,900, 

and 752 feet squared per day (ft2

/d); 33, 1,490, and 

50 feet per day (ft/d); and 14, 160, and 2 gallons per 

minute per foot of drawdown (gal/min/ft) for ROMP 5, 

9, and 12, respectively. These hydraulic property values 

for the surficial aquifer system generally agree with the 

lithofacies/hydraulic properties delineated by Vacher 

and others (1992) (fig. 4).

Sarasota

De Soto

Hardee

Manatee

Hillsborough Polk

Charlotte

Pinellas

Sarasota

De Soto

Hardee

Manatee

Hillsborough

Polk

Charlotte

Pinellas

Highlands

Highlands

LOCATION OF STUDY AREA

LINE OF EQUAL HYDRAULIC

CONDUCTIVITY In feet per day.

Contour interval variable

—

CLEAN SAND

DIRTY SAND

SHELL

TRANSITION

—Clay percentage 10%; shell

ratio 0.1

—Clay percentage 10%; shell

ratio 0.1

—Clay percentage 40%; shell ratio 10

—Shell ratio between 10 and 1

(a) Lithofacies (b) Hydraulic conductivity

LITHOFACIES

13

13

13

40

40

260

40

260

EXPLANATION

40

Figure 4. Lithofacies (a) and hydraulic conductivity (b) of the surficial aquifer system, westcentral Florida (modified from Vacher and others, 1992).
14 Hydrogeologic Framework and Geochemistry of the Intermediate Aquifer System in Parts of Charlotte, De Soto, and Sarasota 

Counties, Florida

Intermediate Aquifer System

The intermediate aquifer system includes all rock 

units that lie between the overlying surficial aquifer system and underlying Upper Floridan aquifer, and generally coincide with the stratigraphic unit designated as 

the Hawthorn Group. Additionally, the stratigraphy that 

corresponds to the Tamiami Formation is included in 

the intermediate aquifer system either as the upper confining unit or as part of the uppermost water-producing 

zone. Generally, the intermediate aquifer system consists of (a) an upper sandy clay, clay, and marl confining 

unit that separates the upper permeable zones in the 

Tamiami and Peace River Formations from the surficial 

aquifer system; (b) a group of up to three water-producing zones separated by confining units and composed 

primarily of carbonate and sandy carbonate rocks 

(Tamiami, Peace River and Arcadia Formations); and 

(c) a lower sandy clay to clayey sand confining unit 

overlying the Upper Floridan aquifer (Nocatee Member 

or Undifferentiated Arcadia Formation). 

The occurrence of multiple, discrete permeable 

zones within the stratigraphy composing the intermediate aquifer system has been documented by previous 

investigators (Sutcliffe, 1975; Joyner and Sutcliffe, 

1976; Wilson, 1977; Wolansky, 1983; Duerr and 

Wolansky, 1986; Miller, 1997; Barr, 1996; and 

Knochenmus and Bowman, 1998). The hydraulic properties of these zones vary depending on (1) the original 

texture of the sediments and (2) post-depositional 

processes such as dolomitization, recrystallization, 

fracturing, and dissolution. Additionally, interpretations of borehole geophysical-log data indicate that 

water-producing zones are of limited vertical extent 

and occur at different depths in adjacent boreholes 

(Broska and Knochenmus, 1996). On a regional scale, 

the heterogeneous distribution of carbonate and siliciclastic sediments makes delineation of water-producing 

zones within the intermediate aquifer system difficult 

to characterize, both in quality and quantity. In the past, 

the zones have been numbered in descending order, 

however, problems arise because equivalent waterbearing lithologies may not be areally persistent. In this 

report the stratigraphic name, as well as permeable 

zone (PZ) number will be used to designate the discrete 

zones in the intermediate aquifer system because the 

stratigraphic name helps to locate the zone vertically in 

the geologic section. 

Currently, the delineation of the permeable zones 

(apparent water-producing zones) and lower-permeability units separating these zones has been based 

predominately on visual inspection of rock cores 

collected during test drilling of test holes at the ROMP 

sites. Additional data collected during test drilling 

including water levels, water quality, and specific 

capacity have been used to characterize the hydraulic 

characteristics of the intermediate aquifer system. 

Based on interpretation from test-hole data, two apparent water-producing zones were delineated at the 

ROMP 5 and 9.5 sites and three apparent water-producing zones were delineated at the ROMP 9, 12, 13, and 

17 sites (apps. B and C). These zones occur in the (a) 

Tamiami/Peace River Formations, referred herein as 

the Tamiami/Peace River zone (PZ1), (b) Upper Arcadia Formation, referred herein as the Upper Arcadia 

zone (PZ2), and (c) Tampa Member or Nocatee Member of the Lower Arcadia Formation, referred herein as 

the Lower Arcadia zone (PZ3). The Tampa Member 

exists in the northern section (ROMP 9, 9.5 and 17 

sites) but not in the southern section (ROMP 5, 12, and 

13 sites) (figs. 5 and 6). Generally, the carbonate units 

yield substantial volumes of water to wells compared to 

the siliciclastic units. The water-bearing capacity 

ranges over several orders of magnitude. Values for 

transmissivity, horizontal hydraulic conductivity, 

storage, leakance, and specific capacity, are reported in 

Gates (1997b), Thompson (1997), Baldini (1999), 

Clayton (1999), and Gates (SWFWMD, written 

commun., 2000), and listed in table 1.

The Tamiami Formation is present in much of 

southern Florida. The Tamiami Formation is an important hydrostratigraphic unit where carbonate rocks are 

the dominant lithology and comprise all or part of the 

Tamiami/Peace River zone (PZ1). Generally, PZ1 is the 

thinnest and least productive zone of the intermediate 

aquifer system. The thickness of PZ1 (where it exists) 

ranges from 24 ft at the ROMP 9 site to 49 ft at the 

ROMP 12 site (apps. B and C). Hydraulic properties of 

PZ1 have been estimated from aquifer tests conducted 

at two of the ROMP sites in the study area. The aquifer 

tests were conducted at the ROMP 9 site in southeastern Sarasota County and at the ROMP 12 site in southcentral De Soto County. The test at the ROMP 9 site 

evaluated a 24-ft thick sequence of sand and limestone 

(Thompson, 1997), and the test at the ROMP 12 site 

evaluated a 49-ft thick sequence of unconsolidated 

material including interbedded dolosilt and dolostone 

(Clayton, 1999). Reported values for transmissivity, 

horizontal hydraulic conductivity, and specific capacity 

were 47 and 5,420 ft2/d; 2 and 102 ft/d; and 2 and 

160 gal/min/ft for ROMP 9 and 12, respectively 

(Thompson, 1997, Clayton, 1999) (table 1). 
Hydrogeologic Units and Hydraulic Properties 15Figure 5. Hydrogeologic and lithostratigraphic cross section A-A’ through ROMP sites 9, 9.5, and 17.
16 Hydrogeologic Framework and Geochemistry of the Intermediate Aquifer System in Parts of Charlotte, De Soto, and Sarasota 

Counties, Florida

Figure 6. Hydrogeologic and lithostratigraphic cross section B-B’ through ROMP sites 5, 12, and 13.
Hydrogeologic Units and Hydraulic Properties 17

The Upper Arcadia zone (PZ2) occurs throughout the study area and consists predominately of limestone and dolostone. The thickness of PZ2 ranges from 

27 at the ROMP 9.5 site to 131 ft at the ROMP 12 site 

(apps. B and C). PZ2 is separated from PZ1 by a clay 

bed within the Peace River Formation. The clay bed is 

thinner in the northern part of the study area (fig. 5) 

than in the southern part (fig. 6). The clay ranges in 

thickness from about 20 to about 60 ft (app. B and C). 

Hydraulic properties of PZ2 vary accordingly with 

lithology and with solution development within the 

limestone and dolomite, more so than to variation in 

thickness (Wolansky, 1983). Hydraulic properties of 

PZ2 were estimated from aquifer tests conducted at 

four of the ROMP sites in the study area (ROMP 5, 9, 

12, and 13). The reported ranges for transmissivity, 

horizontal hydraulic conductivity, and specific capacity 

were 258 to 24,633 ft2/d; 2 to 14 ft/d; and 1 to 

47 gal/min/ft, respectively (table 1). 

The third and lowermost water-producing zone, 

the Tampa Member or Nocatee Member of the Lower 

Arcadia Formation, occurs throughout the study area 

and consists of limestone and dolostone with varying 

amounts of interbedded siliciclastics. Both the Tampa 

and Nocatee Members occur stratigraphically below 

the first occurrence of the Arcadia Formation. The 

thickness of the Lower Arcadia zone (PZ3) ranges from 

57 at the ROMP 13 site to 234 ft at the ROMP 12 site 

(apps. B and C). The hydraulic properties of PZ3 are 

more variable than overlying zones, and probably are 

related to the degree of solution development within 

the limestone and dolomite beds. Typically, PZ3 is the 

most productive zone in the intermediate aquifer system. Hydraulic properties of PZ3 were estimated from 

aquifer tests conducted at five of the ROMP sites. The 

reported ranges for transmissivity, horizontal hydraulic 

conductivity, and specific capacity were 766 to 

44,900 ft2

/d; 10 to 201 ft/d; and 3 to 49 gal/min/ft, 

respectively (table 1.)

Upper Floridan Aquifer

The Upper Floridan aquifer consists of a thick 

carbonate sequence that includes all or part of the 

Paleocene- to Oligocene-age rocks. The Upper 

Floridan aquifer contains one or more water-producing 

zones separated by less-permeable units. Generally, the 

permeability of the Upper Floridan aquifer is very high 

in parts of the Avon Park Formation, somewhat lower 

in the Suwannee Limestone, and lowest in the Ocala 

Limestone. Hydraulic properties have been estimated 

from aquifer tests conducted at five of the ROMP sites 

Table 1. Summary of aquifer test results from selected ROMP sites

[From Gates, 1997; Thompson, 1997; Baldini, 1999; Clayton, 1999; and Gates, SWFWMD, written 

commun., 2000][SAS, surficial aquifer system; PZ2, permeable zone 2 in the Upper Arcadia Formation; PZ3, permeable zone 3 in the Lower Arcadia Formation; UFA, Upper Floridan aquifer; Kh, horizontal hydraulic conductivity; T, transmissivity; S, storage coefficient; L, leakance; Q/s, specific 

capacity; ft, feet; bls, below land surface; ft/d, feet per day; ft2/d, feet squared per day; gpm, gallons 

per minute; --, not determined]

ROMP

site

Hydrogeologic

 unit

Interval

tested

(ft bls)

T

(ft2/d)

Kh

(ft/d) S L

(ft/d/ft)

Q/s

(gal/min/

ft)

ROMP 5 SAS 5 - 85 2,780 33 -- -- 14

PZ2 130 - 230 2,789 14 2.1 x 10-3 3.7 x 10-4 6

PZ3 450 - 600 2,970 20 -- -- 49

UFA 720 - 970 2,610 10 4.1 x 10-1 1.8 x 10-3 6

ROMP 9 SAS 7 - 27 32,900 1,490 -- -- 160

PZ1 40 - 65 47 2 -- 2.3 x 10-5 2

PZ2 122 - 165 24,633 5 2.6 x 10-4 8.2 x 10-4 42

PZ3 190 - 320 6,374 23 2.8 x 10-4 1.7 x 10-3 3

UFA 545 - 860 7,260 23 2.8 x 10-4 5.6 x 10-3 24

ROMP 9.5 PZ3 205 - 331 14,000 111 2.2 x 10-4 3.7 x 10-5 21

UFA 505 - 801 4,870 16 3.0 x 10-4 1.2 x 10-3 12

ROMP 12 SAS 12 - 27 752 50 2.5 x 10-4 2.2 x 10-2 2

PZ1 60 - 110 5,420 102 8.0 x 10-5 5.8 x 10-4 160

PZ2 280 - 409 5,420 9 2.3 x 10-5 1.4 x 10-3 47

PZ3 487 - 710 44,900 201 5.4 x 10-5 -- 32

UFA 725 - 909 7,640 41 3.2 x 10-4 -- 15

ROMP 13 PZ2 282 - 417 258 2 7.6 x 10-5 -- 1

PZ3 510 - 592 766 10 1.1 x 10-4 -- 5

UFA 671 - 786 2,350 21 8.6 x 10-2 -- 3
18 Hydrogeologic Framework and Geochemistry of the Intermediate Aquifer System in Parts of Charlotte, De Soto, and Sarasota 

Counties, Florida

(ROMP 5, 9, 9.5, 12, and 13). Aquifer test data are 

presented for wells that penetrated only the Suwannee 

Limestone. Reported ranges for transmissivity, horizontal hydraulic conductivity, and specific capacity 

were 2,350 to 7,640 ft2

/d; 10 to 41 ft/d; and 3 to 

24 gal/min/ft, respectively (table 1). The relatively low 

hydraulic property values reported for the Suwannee 

Limestone indicate that a substantial water-producing 

zone was not penetrated in the Suwannee Limestone.

Confining Units

In the study area, confining units separating 

water-producing zones and aquifers consist of clay and 

low permeability carbonates (app. C). Hydraulic properties of the confining units range more than two orders 

of magnitude and vary with lithology and thickness. 

Leakance of the confining units was estimated from 

aquifer tests conducted by SWFWMD at the ROMP 5, 

9, 9.5, and 12 sites, and is affected by heterogeneity and 

anisotropy of the aquifer system. The leakance values 

published by SWFWMD (Gates, 1997; Thompson, 

1997; Baldini, 1999; Clayton, 1999; and Gates, 

SWFWMD, written commun., 2000), expressed in 

units of feet per day per foot (ft/d/ft) are:

Generally, the largest and smallest values of 

leakance are for the confining unit separating the Upper 

Floridan aquifer from the lowermost permeable zone in 

the intermediate aquifer system, and the confining unit 

separating the surficial aquifer system from the uppermost permeable zone in the intermediate aquifer 

system, respectively. Leakance of the confining units 

was calculated using analytical models with limiting 

assumptions. Many of these assumptions may be 

violated when applied to the heterogeneous aquifer 

systems. Therefore, reported leakance values may not 

accurately quantify flow across confining units, 

especially where the aquifer is known to receive leakage from underlying, overlying, and/or internal 

sources. 

A check was made using a form of Darcy’s Law 

to test the reasonableness of the leakance values for the 

confining units computed from analytical analysis of 

aquifer test data. Darcy’s Law was formulated as:

L = (q/365)/h,

where

L is leakance of the confining bed, in feet per 

day per foot (ft/d/ft),

q is annual leakage across confining bed, in feet 

per year (ft/yr), and

h is head difference across confining bed, in 

feet (ft).

Figure 7 shows a graph of the empirical relations 

between leakance, head differences, and annual leakage 

(leakage values expressed in units of inches per year 

(in/yr)). The shaded area of the graph corresponds to the 

observed head differences in September 1998 (less than 

1 to 12 ft) and the range in leakage (less than 1 to 3 in/yr) 

between hydrogeologic units derived from numerical 

simulations of ground-water flow of the study area 

(Wolansky, 1983, Ryder, 1985, and SWFWMD, 1994). 

Also shown on figure 7 are the leakance values of the 

confining units derived for each ROMP site using analytical methods and the leakance values derived for 

ROMP 9.5 using numerical analysis. The graph shows 

that most of the leakance values derived from analytical 

analysis of aquifer test data are in excess of 8 in/yr and 

more than twice the rate (less than 1 to 3 in/yr) indicated 

from numerical simulation results. This test using 

Darcy’s Law indicates that most of the analytically 

derived leakance values are probably too high. 

Ground-Water Flow Patterns

Ground-water flow patterns in the intermediate 

aquifer system were evaluated using water-level data 

collected from a network of wells distributed across the 

study area and adjacent areas. Ground-water levels are 

measured and mapped semi-annually in May and September, to depict the seasonal changes in head distribution and ground-water flow patterns in the intermediate 

aquifer system and the Upper Floridan aquifer. Vertical 

flow potentials were evaluated using water-level data 

collected from wells open to discrete depths at a single 

monitor well site. Generally, lateral head differences are 

relatively small compared to vertical head differences.

1UFA:PZ3

(ft/d/ft) 

1

 Confining unit between Upper Floridan aquifer (UFA) and Lower 

Arcadia zone (PZ3)

2PZ3:PZ2

(ft/d/ft)

2

 Confining unit between Lower Arcadia zone (PZ3) and Upper 

Arcadia zone (PZ2)

3PZ2:PZ1

(ft/d/ft)

3 Confining unit between Upper Arcadia zone (PZ2) and Tamiami/Peace River zone (PZ1)

4PZ1:SAS

(ft/d/ft)

4

 Confining unit between Tamiami/Peace River zone (PZ1) and surficial aquifer system (SAS)

na - not available

ROMP 5 1.8 x 10-3 na na 3.7 x 10-4

ROMP 9 5.6 x 10-3 1.7 x 10-3 8.2 x 10-4 2.3 x 10-5

ROMP 9.5 1.2 x 10-3 3.7 x 10-5 1.4 x 10-3 5.5 x 10-4

ROMP 12 na 1.4 x 10-3 5.8 x 10-4 2.2 x 10-2
Hydrogeologic Units and Hydraulic Properties 19

Potentiometric-surface maps of the Upper 

Arcadia zone (PZ2) and the Lower Arcadia zone (PZ3) 

(figs. 8a, b; table 2), and the Upper Floridan aquifer 

(fig. 8c) were prepared using data collected in September 1998. Regional ground-water flow in the intermediate aquifer system is generally westerly toward the 

Gulf of Mexico. Regional flow patterns in PZ3 of the 

intermediate aquifer system are similar to patterns in 

the Upper Floridan aquifer (fig. 8). Flow patterns in 

PZ2 of the intermediate aquifer are generalized due to 

a lack of spatial resolution and are more westerly than 

those in PZ3 or the Upper Floridan aquifer. In the study 

area, ground-water inflow to the intermediate aquifer 

system primarily occurs as upward leakage from the 

underlying Upper Floridan aquifer rather than lateral 

flow through the intermediate aquifer system or 

recharge from the surficial aquifer system. The relative 

contribution of water from each of the aquifer systems 

is supported by geochemical data and discussed in later 

sections of this report. Only minor quantities of inflow 

are contributed from upgradient inland areas, and little 

if any ground-water inflow to the intermediate aquifer 

system is contributed from the surficial aquifer system.

HEAD DIFFERENCE, IN FEET

0.00001

(10 )-6

500

EXPLANATION

R5

Range of leakance computed assuming a leakage (q) of less than 3 inches per year (in/yr)

and head differences of less than 12 feet

ROMP site number

Leakance value reported by U.S. Geological Survey (USGS) using numerical solutions

Leakance value reported by Southwest Florida Water Management District (SWFWMD)

using analytical solutions

Leakage across confining units, in inches per year (in/yr)

Confining unit between the surficial aquifer system (SAS) and the Tamiami/Peace River zone (PZ1)

Confining unit between the Tamiami/Peace River zone (PZ1) and the Upper Arcadia zone (PZ2)

Confining unit between the Lower Arcadia zone (PZ3) and the Upper Arcadia zone (PZ2)

Confining unit between the Upper Floridan aquifer (UFA) and the Lower Arcadia zone (PZ3)

q

UFA:PZ3

PZ1:PZ2

PZ3:PZ2

SAS:PZ1

LEAKANCE, IN FEET PER DAY PER FOOT

0.0001

(10 )-5

0.001

(10 )-4

0.01

(10 )-3

100

10

1

0.1

0.01

q=20 in/yr

q=8 in/yr

q=4 in/yr

q=3 in/yr

R9.5 (UFA:PZ3)

R12 (PZ1:SAS)

R9 (PZ2:PZ1) R9 (PZ3:PZ2)

R12 (PZ2:PZ1)

R5 (UFA:PZ3)

R9 (UFA:PZ3)

R9.5 (UFA:PZ3)

R9 (PZ1:SAS)

R9.5 (PZ3:PZ2)

R9.5 (PZ3:PZ2)

R5 (PZ2:SAS)

q=0.1 in/yr

q=1 in/yr

q=2 in/yr

Figure 7. Range of leakance values estimated for confining units at the ROMP 5, 9, 9.5, and 12 sites.
20 Hydrogeologic Framework and Geochemistry of the Intermediate Aquifer System in Parts of Charlotte, De Soto, and Sarasota 

Counties, Florida

GULF OF MEXICO

0 10 MILES0 10 KILOMETERS

GULF OF MEXICO

0 10 MILES0 10 KILOMETERS

GULF OF MEXICO

0 10 MILES0 10 KILOMETERS

Base maps from SWFWMD digital data, 1:250,000, 1992Albers Equal-Area Conic projectionStandard Parallels and central meridian

SARASOTA MANATEE HARDEE

DE SOTO

CHARLOTTE

SARASOTA MANATEE HARDEE

DE SOTO

CHARLOTTE

SARASOTA MANATEE HARDEE

DE SOTO

CHARLOTTE

20

30

40 50

40

30

20

50

40

50

30

50

30

20

20

40

20

10

20

30

40

50

EXPLANATION

STUDY AREAPOTENTIOMETRIC CONTOUR Shows altitude at which water would have

stood in tightly cased wells. Dashed where approximate. Contour intervalis 10 feet. Datum is sea level

OBSERVATION WELL Number is altitude of water level in feet above sea

level, September 1998 (Table 2 shows observation wells in PZ2 and PZ3)

ROMP WELL AND NUMBER

—

—

—

FLOW PATH Shows direction of selected ground-water flow path

20

R5

(c) Upper Floridan aquifer (UFA)

(modified from Torres and others (1999))

(b) Lower Arcadia zone (PZ3) (a) Upper Arcadia zone (PZ2)

R12

R26

R17

R9

R9.5

R5

R13

R26

R9.5

R9

R5

R13

R12

R26

R17

R9

R9.5

R5

R13

49

52 44

43

46

43

51

49

31

18

47

16

8

21

31

20

20

24 9

16

33

50

52

43

46

48

52

32

38

26

52

36 36

24

49

13

R12

R17

41

46

13

30

52

Figure 8. Potentiometric surface of the (a) Upper Arcadia zone (PZ2) and (b) Lower Arcadia zone (PZ3) of the intermediate aquifer system, and (c) Upper Floridan 

aquifer, southwest Florida, September 1998.
Hydrogeologic Units and Hydraulic Properties 21

Table 2. Information on selected observation wells open to the Upper Arcadia zone (PZ2) and Lower Arcadia zone (PZ3) 

within the intermediate aquifer system

[PZ2, permeable zone 2 in the Upper Arcadia Formation; PZ3, permeable zone 3 in the Lower Arcadia Formation; CHA, Charlotte; DES, De Soto; HAR, 

Hardee; SAR, Sarasota; --, unknown]

Site

identification

number

Name Hydrogeologic

unit County Casing(feet)

Depth

(feet)

Water level

September 98

(feet)1

265004081581901 Ch-311 Mid Htrn Well PZ2 CHA -- 220 26.08

265017082153701 USGS Htrn Well 8 At Placida PZ3 CHA 346 413 17.90

265531082194804 Romp TR 3-3 Htrn Well Nr Englewood PZ3 CHA 370 410 20.00

265531082194805 Romp TR 3-3 Upper Hawthorn Nr Lemon High School PZ2 CHA 155 175 13.00

265638082130705 Romp TR 3-1 Htrn Well Nr El Jobean PZ3 CHA 380 400 30.81

265644081483303 Romp 5-MW 3-IAS-LPZ PZ3 CHA 450 600 50.86

265644081483304 Romp 5-MW2-IAS-UPZ PZ2 CHA 130 230 35.53

265646081554501 USGS Tuh Well C-5 Nr Punta Gorda PZ2 CHA 194 280 23.80

265837081561101 Romp 11 Htrn Well Nr Punta Gorda PZ2 CHA 220 335 35.90

271757081493003 Romp 26 Htrn Well Nr Gardner PZ2 DES 140 180 46.09

270225081443304 Romp 12-INT-L Arcadia Well near Arcadia (Well #4) PZ2 DES 280 409 48.65

270225081443304 Romp 12-INT-L Nocatee Well #5 near Arcadia PZ3 DES 505 705 49.40

270418081365803 ROMP 13-MW3 IAS LPZ PZ3 DES 514 592 52.27

270418081365804 ROMP 13-MW2 - INT-UPZ PZ2 DES 282 417 51.47

270540082001102 Gen. Dev. Htrn Well T-2 Nr Ft, Ogden PZ3 DES 393 496 45.61

270737082025001 ROMP 9.5 - MW18 PZ2 DES 60 77 38.17

270737082025002 ROMP 9.5 - MW2 PZ3 DES 205 330 43.43

271026081583604 Romp 17 Htrn Well Nr Arcadia PZ3 DES 200 240 43.15

271115081462702 Romp 16 Htrn Well Nr Arcadia PZ3 DES 300 340 49.13

271308081522601 City Of Arcadia Htrn Well 2 At Arcadia PZ3 DES 263 372 46.97

272038081530701 Limestone Land Int Well At Limestone PZ2 HAR 126 235 41.02

272108081582601 Hollingsworth Int Well Nr Limestone PZ3 HAR 146 335 31.17

272714081545902 Romp 31 Htrn Well Nr Ona PZ3 HAR 130 350 43.19

272728081474702 Romp 30 Tamp Well Nr Zolfo Springs PZ3 HAR 280 316 47.86

273435081444001 R. Henderson Htrn Well Nr Wauchula PZ3 HAR 105 293 64.26

273555081403001 John White Htrn Well Nr Wauchula PZ3 HAR 63 270 89.19

265710082205101 Englewood Wd HTRN PZ2 SAR 152 310 12.86

270137082235301 Manasota Deep Well 14 Nr Englewood PZ3 SAR 263 305 20.02

270420082230503 Venice Gardens Htrn Well Nr Venice PZ3 SAR 200 400 20.11

270432082085709 Romp 9 (MW4) - Lower Intermediate Monitor,

near Northport, Sarasota 

PZ3 SAR 194 320 43.65

270808082270503 Romp TR 5-1 Htrn Well At Laurel PZ3 SAR 275 289 9.20

270919082234203 Romp TR 5-2 Lower Htrn Well Nr Laurel PZ3 SAR 245 365 23.93

271134082092201 Big Slough Tuh Well Nr Arcadia PZ2 SAR 78 100 29.69

271137082284502 Romp 20 LHtrn Well At Osprey PZ3 SAR 50 370 16.15

271137082284503 Romp 20 TUH Well PZ2 SAR 75 125 0.40

271601082330501 Romp TR 6-1 Htrn Well Nr Sarasota PZ3 SAR 300 315 8.15

271813082201303 Romp 22 - Lower INT Well ,Hi Hat Ranch

near Fruitville

PZ3 SAR 230 260 20.62

1

Altitude above mean sea level.
22 Hydrogeologic Framework and Geochemistry of the Intermediate Aquifer System in Parts of Charlotte, De Soto, and Sarasota 

Counties, Florida

 The study area is primarily an area of upwardflow potential except near the ROMP 13 site in the 

southeastern part of De Soto County (fig. 9), where a 

downward head difference occurs between the aquifer 

systems. Areas of discharge from the Upper Floridan 

aquifer occur in the river valleys and in areas of low 

topographic relief. In river valleys, upward discharge is 

the result of decreased heads in the shallower aquifers. 

In areas of low topographic relief, upward discharge is 

affected by subsurface stratigraphy. Five of the six 

ROMP sites are located in discharge areas where water 

levels increase with depth and the flow potential is 

upward from the Upper Floridan aquifer, through the 

intermediate aquifer system, to the surficial aquifer 

system. At the ROMP 5 site, an anomalous condition 

was observed where a downward head difference 

0 5 10 MILES

0 5 10 KILOMETERS

Base from SWFWMD digital data, 1:250,000, 1992

Albers Equal-Area Conic projection

Standard Parallels 	
 
 and 	
 	

 central meridian

SARASOTA

HARDEE

DE SOTO

CHARLOTTE

Gulf

of

Mexico STUDY

AREA

















 
 	
 




 


HEAD DIFFERENCE AND CONFINING UNIT THICKNESS between adjacent aquifers, in feet. Parenthetic

number is thickness of the intervening confining unit, in feet. An "e" means value was estimated, "na" means value is not available

Head difference and thickness between the surficial aquifer system and the Tamiami/Peace River zone (PZ1) or the

Upper Arcadia zone (PZ2)

Head difference and thickness between the Upper Arcadia zone (PZ2) and the Lower Arcadia zone (PZ3)

Head difference and thickness between the Lower Arcadia zone (PZ3) and the Upper Floridan aquifer

Note: Negative numbers indicate an upward head gradient and positive numbers indicate a downward head gradient.

—Bold number is head difference

ROMP 12

ROMP 17

ROMP 9

ROMP 9.5

ROMP 5

ROMP 13

Port Charlotte

River

River

Joshua Creek

Prairie

Horse

Shell

SARASOTA

COUNTY DE SOTO

COUNTY

CHARLOTTE

COUNTY

Charlotte Harbor

Creek

Peace

Peace

Creek

Creek

EXPLANATION

AREA OF ARTESIAN FLOW–Estimated using topographic and potentiometric surface altitudes

WELL LOCATION AND NAME

3

-15

-1

(59)

(249)

(98)

-4

-12

1

(48)

(30)

(225)

-3

-5

-3

(36)

(111)

(124)

na

-5e

-5

(64)

(50)

(140)

-4

-1

-1

(201)

(59)

(20)

8

0

0

(131)

(124)

(137)

ROMP 5

3

-15

-1

(59)

(249)

(98)

MANATEE

Figure 9. Head differences among aquifers, area of artesian flow, and thickness of confining units, September 1998.
Hydrogeologic Units and Hydraulic Properties 23

occurs between the surficial aquifer system and PZ2, 

while an upward head difference occurs for the other 

hydrogeologic units.

Head differences generally were small between 

the Upper Floridan aquifer and PZ3, and moderate 

between PZ3 and PZ2, and between PZ2 or PZ1 and the 

surficial aquifer system (fig. 9). Localized head-difference reversals, however, can occur in response to local 

ground-water withdrawals. Aquifer test data collected 

at the ROMP sites indicate that relatively large drawdowns in pumped wells (20 to 50 ft) can occur when the 

intermediate aquifer system is stressed, resulting in 

localized head-difference reversals between the Upper 

Floridan aquifer and the intermediate aquifer system.

Although a regional flow system can be mapped 

for the intermediate aquifer system, the depth and extent 

of ground-water flow paths within each of the permeable zones in the intermediate aquifer system are not 

well understood or known, and locally, flow may be 

opposite of the regional flow pattern. The juxtaposition 

of heterogeneous sediments within the intermediate 

aquifer system results in lateral discontinuities (permeable zone pinches out or is discontinuous) and variable 

locations of water-producing zones that probably produce multi-scale flow fields within the aquifer system. 

For example, at the same geographic location, water 

levels in wells penetrating the surficial aquifer system, 

the intermediate aquifer system, and the Upper Floridan 

aquifer may reflect different hydrologic conditions such 

as recharge, flow through, or discharge conditions. 

Freeze and Cherry (1979) have shown that stratigraphic 

layering of sediments with permeability contrasts (geologic heterogeneity) can have profound effects on the 

regional flow system. Subsurface variations in horizontal hydraulic conductivity can create multi-scale flow 

fields and affect the relation between local and regional 

ground-water flow, areas of recharge and discharge, and 

the quantities of flow discharged through the aquifer 

systems (Freeze and Cherry, 1979, p. 197). 

Hydraulic Connection Between 

Hydrogeologic Units

Drilling and testing indicate that the hydraulic 

connection between the surficial aquifer system, permeable zones of the intermediate aquifer system, and 

the Upper Floridan aquifer is variable in the study area. 

Water levels in some wells respond rapidly to pumping 

from overlying and underlying aquifers whereas other 

wells seem to have little or no response to pumping 

when one or more aquifers are stressed. The degree of 

hydraulic connection is related to the presence and 

thickness of clay and dolosilt beds composing the confining units; however, facies changes and karstification 

may result in local hydraulic connection with overlying 

and underlying aquifers. Effective hydraulic coupling 

is indicated by small head differences among aquifers, 

high leakance values, and rapid drawdowns in adjacent 

aquifers when one or the other is stressed. 

Generally, a hydraulic connection exists between 

the Upper Floridan aquifer and the Lower Arcadia zone 

(PZ3). The two zones are probably connected locally 

by fractures or other zones of preferential flow. Thickness of the intervening confining unit averages about 

125 ft and ranges from 20 ft at the ROMP 12 site to 

225 ft at the ROMP 9 site (fig. 9; apps. B and C). 

Relatively small head differences are observed 

between the two zones (fig. 10). Head differences 

between the two hydrogeologic units averaged about 

2 ft in September 1998 and ranged from less than 1 ft at 

the ROMP 5, 9, and 13 sites to 5 ft at the ROMP 17 site 

(fig. 10). Leakage across the intervening confining unit 

is low and ranges from less than 1 to 3 in/yr (Wolanksy, 

1983, Ryder, 1985, and Southwest Florida Water Management District, 1994). Aquifer testing conducted on 

PZ3 and on the Suwannee Limestone of the Upper 

Floridan aquifer indicated, generally, a hydraulic 

response when either unit was stressed, indicating 

hydraulic connection between units. One exception is 

at the ROMP 9 site, where no water-level response was 

recorded in observation wells in the overlying or underlying unit when one or the other unit was stressed. The 

confining unit separating these units is thickest at the 

ROMP 9 site and consists of a sequence of low-permeability units (apps. B and C) that probably restrict the 

hydraulic connection between zones at this site. A second exception is at ROMP 9.5, where no water-level 

response was recorded in the Upper Floridan aquifer 

observation wells when PZ3 was pumped; however, a 

delayed response was recorded in the PZ3 observation 

wells when the Upper Floridan aquifer was stressed. At 

the ROMP 12, 13, and 17 sites, monitor wells are open 

to both the intermediate aquifer system and the Upper 

Floridan aquifer (figs. 5 and 6). The open-hole intervals 

penetrate a contiguous sequence of carbonates that 

comprises the Nocatee Member/Lower Arcadia Formation and Suwannee Limestone, and the hydrogeologic 

units are probably hydraulically coupled at these sites. 
24 Hydrogeologic Framework and Geochemistry of the Intermediate Aquifer System in Parts of Charlotte, De Soto, and Sarasota 

Counties, Florida

55

50

45

40

35

30

25

ROMP 5

1997 1998 1999 2000

ROMP 12

e

e

dry dry

e

e

ROMP 13

e

e

ROMP 9

ROMP 9.5

ROMP 17

55

50

45

40

35

30

65

60

55

50

45

40

50

45

40

35

30

25

20

15

50

45

40

35

30

25

20

15

50

45

40

35

30

25

20

15

WATER LEVEL, IN FEET ABOVE SEA LEVEL

WATER LEVEL, IN FEET ABOVE SEA LEVEL

1997 1998 1999 2000

e

Lower Arcadia zone (PZ3)

Lower Hawthorn/UFA (Suwannee)

UFA (Suwannee)

Avon Park

Estimated value

Surficial aquifer system

Tamiami/Peace River zone (PZ1)

Upper Arcadia zone (PZ2)

EXPLANATION

Figure 10. Water levels in wells at the ROMP 5, 9, 9.5, 12, 13, and 17 sites from May 1997 through May 2000.
Hydraulic Characteristics at the ROMP 9.5 Site 25

Hydraulic data indicate little hydraulic connection between PZ3 and PZ2, probably due to the thickness and low permeability of sediments separating the 

two zones. Thickness of the intervening confining unit 

averages more than 100 ft and ranges from 30 ft at the 

ROMP 9 site to 249 ft at the ROMP 5 site (fig. 9; apps. 

B and C). Moderate head differences are observed 

between the two units (fig. 10). Head differences 

between the two hydrogeologic units averaged about 

5 ft in September 1998 and ranged from less than 1 ft at 

the ROMP 13 site to 12 ft at the ROMP 9 site (fig. 10). 

Leakage across the intervening confining unit is low, 

about 1 in/yr (Wolanksy, 1983). Aquifer testing conducted on PZ3 and PZ2 indicated little or no waterlevel response when one or the other zone is stressed, 

suggesting hydraulic separation between the units. 

Little hydraulic connection exists between the 

surficial aquifer system and either PZ1 or PZ2, probably because of a relatively thick clay unit separating the 

two zones. Thickness of the intervening confining unit 

averages about 100 ft, ranging from 36 ft at the ROMP 

9.5 site to 201 ft at the ROMP 12 site (fig. 9; apps. B 

and C). Head differences between the two hydrogeologic units averaged about 4 ft in September 1998, 

ranging from about 3 ft at the ROMP 5 and 9.5 sites to 

about 8 ft at the ROMP 13 site (fig. 10). Leakage across 

the intervening confining unit is low, in the range of 

1 in/yr (Wolanksy, 1983). Aquifer test results indicated 

no water-level response in the monitored unit when the 

surficial aquifer system, PZ1 or PZ2 were pumped, 

suggesting hydraulic separation between the units. 

HYDRAULIC CHARACTERISTICS AT THE 

ROMP 9.5 SITE

Hydraulic characteristics of permeable zones 

and confining units at the ROMP 9.5 site were quantified using permeameter tests on cores, short-term 

withdrawal tests (specific-capacity tests), and longterm withdrawal tests (aquifer tests). In addition, an 

initial quantification of permeability was determined 

from visual inspection of core samples. In consolidated 

materials and/or rock cores, low porosity is presumed 

to be an indicator of low permeability whereas high 

porosity is presumed to be associated with high 

permeability. This qualitative indication is consistent 

with observations of the core. Most cores obtained 

from the ROMP 9.5 site exhibit some visual degree of 

permeability. 

Core Analysis

Based on visual inspection, 10 cores, each 3-

6 inches long, were selected for laboratory testing of 

hydraulic properties. The cores were selected to be 

representative of both permeable and confining units 

within the intermediate aquifer system and the Upper 

Floridan aquifer (Gates, 1998a). Values for porosity 

and vertical hydraulic conductivity (Kv) were determined by the FGS and are shown in figure 11 and 

table 3. 

Laboratory analyses indicate that hydraulic 

properties determined from cores are highly variable, 

indicative of the variable nature of the sedimentary 

units. The vertical hydraulic conductivities and 

porosity values are generally highest in the confining 

units and lowest in the permeable units. Bulk porosity 

values were highly variable within confining materials 

in selected hydrogeologic units ranging from 28 to 52 

percent (fig. 11, table 3). Kv ranged from 1.124 x 10-4

to 3.818 x 10-2 feet per day (ft/d) (fig. 11; table 3). 

The vertical hydraulic conductivity values determined from cores are much lower than values determined from standard aquifer tests, and are related to the 

difference in scale between core samples and aquifer 

matrix. Fractures, cavities, and preferential pathways 

play a major role in conducting water to a pumped well, 

whereas only the properties of the rock for a discrete 

interval itself were measured in the laboratory. Therefore, the vertical hydraulic conductivity determined 

from the laboratory data should be used cautiously and 

only in a relative sense. 

 Specific-Capacity Tests

Specific capacities of selected intervals of the 

intermediate aquifer system and the Upper Floridan 

aquifer at the ROMP 9.5 site were determined during 

drilling of the test hole. Specific capacity can be used 

to evaluate the productivity of various intervals. Packer 

equipment was used to isolate and pump 12 bottomhole intervals as drilling progressed. Values of specific 

capacity ranged from 0.02 to 0.55 (gal/min)/ft (table 4), 

indicating that productivity of specific zones within the 

intermediate aquifer system is relatively low and 

highly variable. Generally, the highest specific capacity 

is associated with producing zones and the lowest 

specific capacity is associated with confining units. 
26 Hydrogeologic Framework and Geochemistry of the Intermediate Aquifer System in Parts of Charlotte, De Soto, and Sarasota 

Counties, Florida

Aquifer Tests

Two aquifer tests were conducted at the ROMP 

9.5 site to estimate hydraulic properties of the intermediate aquifer system and the Upper Floridan aquifer. 

The first test was conducted on the Lower Arcadia zone 

(PZ3) from January 12-15, 1998, for 2.77 days 

(3,990 minutes). The second test was conducted on the 

Suwannee Limestone (Upper Floridan aquifer) from 

February 2-3, 1998, for 1 day (1,440 minutes). Aquifer 

test data were analyzed using both analytical and 

numerical techniques. 

Test Design and Implementation

A plan view and construction records of the production and observation wells for the aquifer tests conducted at the ROMP 9.5 site are shown in figure 12. 

Water levels were measured continuously in selected 

wells for background, withdrawal, and recovery periods of the tests. The data were collected using pressure 

transducers connected to a digital data logger. Additional water-level measurements were made manually 

to verify data logger water-level data. 

0

-100

-200

-300

-400

-500

-600

-4 -2 0 2 4 6

WATER LEVEL, IN FEET ABOVE

OR BELOW LAND SURFACE

POROSITY, IN PERCENT

0.0 0.2 0.4 0.6

SPECIFIC CAPACITY,

IN GALLONS PER MINUTE PER FOOT

0.001 0.01 0.1 0.6

VERTICAL HYDRAULIC CONDUCTIVITY,

IN FEET PER DAY

DEPTH BELOW LAND SURFACE, IN FEET

PZ2

PZ3

UFA

SAS

EXPLANATION

SAS

PZ2

PZ3

UFA

0

ESTIMATED TRANSMISSIVITY,

IN FEET SQUARED PER DAY

Confining unit

Tested interval

0

-100

-200

-300

-400

-500

-600

20 30 40 50 60

80 160

0

-100

-200

-300

-400

-500

-600

0

-100

-200

-300

-400

-500

-600

DEPTH BELOW LAND SURFACE, IN FEET

PZ2

PZ3

UFA

SAS

PZ2

PZ3

UFA

SAS

PZ2

PZ3

UFA

SAS

0.005 0.05

Surficial aquifer system

Upper Arcadia zone (PZ2)

Lower Arcadia zone (PZ3)

Upper Floridan aquifer

Figure 11. Hydraulic data collected during coring and testing at the ROMP 9.5 site.
Hydraulic Characteristics at the ROMP 9.5 Site 27

Table 3. Porosity and hydraulic conductivity of confining material in selected hydrogeologic units at the ROMP 9.5 

exploratory borehole, De Soto County, Florida

[Bulk porosity calculations and falling-head permeameter tests were conducted on cores collected from the exploration borehole between June and 

August, 1997, by the Florida Geological Survey, Tallahassee, Florida; PZ2, permeable zone 2 in the Upper Arcadia Formation; PZ3, permeable zone 3 in 

the Lower Arcadia Formation; UFA, Upper Floridan aquifer; --, no data]

Core interval below 

land surface 

(feet)

Hydrogeologic unit

Porosity, 

bulk

(percent)

Hydraulic

conductivity 

(feet/day)

Comments for permeameter tests 

(hydraulic conductivity)

64.0 - 64.8 Upper Arcadia zone (PZ2),

confining material

33 9.402 x 10-4 Average of three tests

100.7 - 101.2 Confining unit between PZ2 

and PZ3

52 -- Sample fractured. Test not run

148.7 - 149.1 Confining unit between PZ2 

and PZ3

40 1.672 x 10-4 One test. No flow

195.7 - 198.0 Confining unit between PZ2 

and PZ3

29 1.682 x 10-4 One test. Saturated, but no flow

257.6 - 258.0 Lower Arcadia zone (PZ3),

confining material

28 1.751 x 10-4 One test. No flow

337.3 - 337.7 Confining unit between PZ3 

and UFA

34 2.474 x 10-2 Average of three tests

361.5 - 361.9 Confining unit between PZ3 

and UFA

39 3.095 x 10-2 Average of three tests

388.3 - 388.7 Confining unit between PZ3 

and UFA

33 1.124 x 10-4 One test. No flow

464.2 - 464.6 Confining unit between PZ3 

and UFA

38 3.818 x 10-2 Average of three tests. Minor fractures 

developed. About 1-centimeter thick 

core broken off

500.8 - 501.1 Upper Floridan aquifer, 

confining material

33 3.333 x 10-4 One test. No flow

Table 4. Specific capacity of selected hydrogeologic units estimated by packer tests at the ROMP 9.5 exploratory 

borehole, De Soto County, Florida

[PZ2, permeable zone 2 in the Upper Arcadia Formation; PZ3, permeable zone 3 in the Lower Arcadia Formation; UFA, Upper Floridan aquifer]

Open-hole interval below 

land surface

(feet)

Hydrogeologic unit

Specific capacity

(gallons per 

minute/foot)

Discharge (gallons per 

minute)/

drawdown (feet) 1

Duration of test 

(minutes)

80 - 128 Confining unit between PZ2 and PZ3 0.16 5/31 60

148 - 168 Confining unit between PZ2 and PZ3 2

 0.10 1/90 28

188 - 208 Confining unit between PZ2 and PZ3 0.07 5/70 20

205 - 228 Lower Arcadia zone (PZ3) 0.33 5/15 10

245 - 268 Lower Arcadia zone (PZ3) 0.28 4/14 10

293 - 308 Lower Arcadia zone (PZ3) 0.50 5/10 8

331 - 348 Confining unit between PZ3 and UFA 0.12 3/25 8

348 - 368 Confining unit between PZ3 and UFA 0.02 3/140 50

388 - 398 Confining unit between PZ3 and UFA 0.02 3/108 50

413 - 468 Confining unit between PZ3 and UFA 0.09 4/42 20

448 - 468 Confining unit between PZ3 and UFA 0.11 5/42 15

493 - 513 Upper Floridan aquifer 0.55 5/9 5

1

Discharge decreased during pumping; estimated value is maximum discharge at start of pumping.

2

Specific capacity unreliable, but low permeability is estimated.
28 Hydrogeologic Framework and Geochemistry of the Intermediate Aquifer System in Parts of Charlotte, De Soto, and Sarasota 

Counties, Florida

470/475

205/330

180/190

340/350

340/350

205/330

180/190

60/77

N

MW1 270737082025101

Well name Hydrogeologic unit

Casing depth/

well depth

(feet)

Casing

diameter

(inches)

Distance

from

production

well MW1

(feet)

Distance

from

production

well MW2

(feet)

Upper Floridan aquifer 500/800 12 -- 11.7

10/38 4 32.3 20.7

2/8 4 28.6 16.8

500/800 2 98.7 98.5

205/330 8 11.7 --

Upper Floridan aquifer

Confining unit

Confining unit

Confining unit

Confining unit

Confining unit

Confining unit

Upper Floridan aquifer

Surficial aquifer system

Surficial aquifer system

Lower Arcadia zone (PZ3)

Lower Arcadia zone (PZ3)

Lower Arcadia zone (PZ3)

Lower Arcadia zone (PZ3)

Lower Arcadia zone (PZ3)

Lower Arcadia zone (PZ3)

Upper Arcadia zone (PZ2)

MW2 270737082025102

MW3 270737082025103

MW4 270737082025104

MW5 270737082025201

MW6 270737082025202

MW7 270737082025203

MW8 270737082025204

MW9 270737082025205

MW10 270737082025301

MW11 270737082025302

MW13 270737082025501

MW12 270737082025303

MW14 270737082025901

MW15 270736082025101

MW16 270736082025102

MW17 270736082025103

MW18 270737082025001

2 98.9 99.9

2 100.6 102.5

2 109.2 110.3

2 199.1 199.2

2 109.3 109.5

205/330 2 198.5 198.1

205/330 2 398.2 397.7

205/330 2 798.1 798.4

340/350 2 87.7 99.1

2 85.6 97.2

2 87.3 99.1

4 97.5 104.9

180/190 2 199.9 200.5

MW18

MW13

MW14

(NOT TO SCALE)

MW12

MW10

MW11

MW7

MW8 MW9

MW2 (PZ3 production well)

MW1 (UFA production well)

MW3 MW4

MW16

MW17 MW15

MW6

MW5

Relative position of wells at ROMP 9.5 site

SARASOTA

HARDEE

DE SOTO

CHARLOTTE

Gulf

of

Mexico STUDY

AREA













 
 	
 
 MANATEE

ROMP

9.5

Station

identification

Figure 12. Location of study area, plan view, and description of wells at the ROMP 9.5 test site.
Hydraulic Characteristics at the ROMP 9.5 Site 29

The production wells were pumped using a 

shaft-driven turbine pump powered by a diesel engine. 

Water from the production wells was discharged to 

connecting PVC piping through a totalizing flow-thru 

meter, collapsible hose, and finally through a pipeorifice plate manometer apparatus. All water was 

discharged into a shallow ditch about 100 ft from the 

production wells. Flow rates were recorded using a 

flow-thru meter and orifice-plate manometer 

apparatus. 

The Lower Arcadia zone (PZ3) was stressed at 

an average discharge rate of 425 gallons per minute 

(gal/min) and water levels were measured in the production well and 16 observation wells during the test 

and during a 7-day recovery period. Water levels for 

background, withdrawal, and recovery periods of the 

test for 7 of the 16 observation wells (MW1, MW3, 

MW4, MW6, MW7, MW8, and MW9) are shown in 

figure 13. Diurnal water-level fluctuations of less than 

0.2 foot were observed in the production and observation wells leading into both the PZ3 and Upper 

Floridan aquifer tests. The diurnal fluctuations were 

considered negligible and no water-level corrections 

were made. 

The Suwannee Limestone of the Upper Floridan aquifer was stressed at an average discharge rate 

of 797 gal/min, and water levels were measured in the 

production well and seven observation wells during 

the test and during a recovery period. Water levels for 

background, withdrawal, and recovery periods of the 

test for the pumped well (MW1) and the seven observation wells (MW2, MW5, MW6, MW7, MW15, 

MW17, and MW18) are shown in figure 14. At about 

110 minutes after pumping began, a discharge control 

valve briefly closed and a temporary recovery of 

about 40 ft occurred in the production well. Also, 

WATER LEVEL, IN FEET ABOVE SEA LEVEL

DATE

Upper Floridan aquifer well MW1

Pump on:

1/12/98 @1400 to

1/15/98 @0830

Upper Floridan aquifer well MW6

Confining unit between

PZ2 and PZ3 (Well MW9)

Lower Arcadia zone (PZ3)

Well MW8

Confining unit between PZ3

and UFA (Well MW7)

Surficial aquifer system

well MW4

Surficial aquifer system

well MW3

48

44

40

36

32

1/9/98 1/11/98 1/13/98 1/15/98 1/17/98 1/19/98

Figure 13. Water levels in selected observation wells for background, withdrawal, and recovery periods 

for the Lower Arcadia zone (PZ3) aquifer test at the ROMP 9.5 site.
30 Hydrogeologic Framework and Geochemistry of the Intermediate Aquifer System in Parts of Charlotte, De Soto, and Sarasota 

Counties, Florida

28 29 30 31 1 2 3 4

WATER LEVEL CHANGE, IN FEET

WATER LEVEL CHANGE, IN FEET

TIME, IN DAYS

Upper Floridan aquifer

(Production well)

MW1 MW7

MW6

MW5

MW18 MW2

MW15

Open-hole interval:

500 to 800 feet

Confining unit between

Lower Arcadia zone PZ3

and UFA

(Observation well)

Upper Floridan aquifer

(Observation well)

Upper Floridan aquifer

(Observation well)

Upper Arcadia zone (PZ2)

(Observation well)

Lower Arcadia zone (PZ3)

(Observation well)

Confining unit between

Lower Arcadia zone PZ3

and UFA

(Observation well)

Confining unit between

Upper Arcadia zone (PZ2)

and Lower Arcadia zone (PZ3)

10

0

-10

-30

-50

-70

MW17

0

-4

-8

-12

0

-4

-8

-12

-16

-18

1

0

-2

-4

-6

0.5

0

-0.5

-1

-1.5

-2

0.6

0.4

0

-0.4

-0.8

-1

0.5

0

-0.5

-1.5

-2.5

0.5

0

-0.5

-1.5

-2.5

JAN, 1998 FEB, 1998

28 29 30 31 1 2 3 4

TIME, IN DAYS

JAN, 1998 FEB, 1998

Open-hole interval:

470 to 475 feet

Open-hole interval:

500 to 800 feet

Open-hole interval:

60 to 77 feet

Open-hole interval:

340 to 350 feet

Open-hole interval:

340 to 350 feet

Open-hole interval:

180 to 190 feet

Open-hole interval:

205 to 330 feet

Pumping period (2/2/98 at 1150 through 2/3/98 at 1150)

(Observation well)

Figure 14. Water levels in selected wells for background, withdrawal, and recovery periods of the Upper Floridan aquifer test 

at the ROMP 9.5 site.
Hydraulic Characteristics at the ROMP 9.5 Site 31

offsite pumpage apparently affected water levels in 

PZ2 well MW18, in PZ3 well MW2, and in the confining unit wells MW7, MW15, and MW17. The location 

and pumping rate of the offsite well or wells are 

unknown, and therefore, estimation of the hydraulic 

properties of the confining units was not attempted for 

this test. This unknown pumpage from an offsite 

well(s) did not produce drawdowns in the Upper 

Floridan aquifer observation wells.

Analytical Analysis of Aquifer Tests

Three analytical methods were used to analyze 

the tests conducted at the ROMP 9.5 site. The methods 

selected to analyze the tests are: (1) the Hantush (1960) 

modified leaky method, (2) the Jacob (1946) steadystate method, and (3) the Neuman and Witherspoon 

(1972) ratio method. The methods use the concept of an 

artesian aquifer overlain by leaky confining units and 

assume a homogeneous isotropic aquifer of uniform 

thickness and infinite areal extent. Many of the inherent assumptions of the methods are not met, or were 

only partially satisfied at the ROMP 9.5 site because 

the aquifer system at this site is heterogeneous and 

anisotropic. All of the methods apply curve-matching 

techniques. Detailed procedures for application of the 

analytical methods are described in Kruseman and de 

Ridder (1991). Even though aquifer test data can be fit 

to a type curve from one of the idealized models, there 

is no guarantee that the aquifer system satisfies the 

assumption on which the type curve was developed 

(Wolansky and Corral, 1985). 

Hantush (1960) Modified Leaky Method

The Hantush (1960) modified leaky method can 

be applied to aquifers with leaky confining units. The 

analytical solution accounts for water leaking into the 

aquifer being pumped from confining beds, and for 

the effects of water coming out of storage in the confining beds. Mass plots of drawdown data are plotted 

on logarithmic paper in relation to time since pumping started divided by the square of the radial distance 

between the observation well and the pumped well 

(fig.15). The resulting drawdown curves are superimposed on the family of type curves of the function 

W (u, B) verses 1/u. Parameter estimation was facilitated by the use of an automatic curve matching program AQTESOLV (HydroSOLVE, Inc., 1996). The 

program uses a nonlinear weighted least-squares 

parameter estimation algorithm to match type curves 

to time-drawdown data. Results of the test analysis 

are shown in figure 15 and listed in table 5.

Jacob (1946) Leaky Artesian Method

The Jacob (1946) leaky artesian method also can 

be applied to aquifers with leaky confining units. The 

application of the method assumes steady-state flow 

near a well discharging at a constant rate for a leaky 

confined aquifer, and uses a graphical method for 

determining transmissivity of the aquifer and the 

leakance of the confining bed. Data from the Lower 

Arcadia zone (PZ3) test indicate that a relatively 

steady-state condition had been reached by the end of 

the withdrawal phase of the test. For this test, the 

steady-state drawdown in each observation well is plotted on logarithmic paper in relation to distance of the 

observation well to the pumped well (fig. 16). The data 

then are matched to the Bessel function logarithmic 

type curve Ko (x) versus x with its tabular values given 

in Ferris and others (1962, table 4). The common match 

points and resulting aquifer and confining unit hydraulic coefficients are shown in figure 16 and listed in 

table 5. 

Table 5. Summary of aquifer test results for the ROMP 9.5 

site using analytical analysis 

[PZ3, permeable zone 3 in the Lower Arcadia Formation; UFA, Upper 

Floridan aquifer; T, transmissivity; S, storage coefficient; K′Ss′, leakance 

characteristic of confining bed; K′/b′, leakance coefficient of confining 

bed; K′/Ss′, vertical hydraulic diffusivity of confining beds; d, days; ft, feet; 

ft/d, feet per day; ft2/d, feet squared per day; --, not determined

Method of 

analysis

Aquifer characteristics Confining unit characteristics

T

(ft2/d) S

1K′Ss′ 

(d-1) 

1

Leakance values, derived from the PZ3 aquifer test, incorporate the 

effects of upward and downward leakage.

1K′/b′ 

(ft/d/ft)

K′/Ss′

(ft2/d)

Hantush 

(1960)

 PZ3 10,000 1.3 x 10-4 1.1 x 10-6 -- --

 UFA 4,300 2.8 x 10-4 4.6 x 10-6 -- --

Jacob (1946)

 PZ3 11,800 -- -- 1.9 x 10-4 --

 UFA -- -- -- --

Neuman and 

Witherspoon 

(1972)

-- -- -- -- 2130 to 274 

2

Value is for the interval from 190 to 206 feet below land surface.
32 Hydrogeologic Framework and Geochemistry of the Intermediate Aquifer System in Parts of Charlotte, De Soto, and Sarasota 

Counties, Florida

T = 10,000 ft /d

S = 1.3 x 10

B/r = 2.3 x 10 ft

2

-4

-4 -1

Well r B

MW 8 102.5 0.023

MW16 97.2 0.021

MW11 198.1 0.045

MW13 397.7 0.090

MW14 798.4 0.181

= [4(0.00023)] (10,000) (0.00013)

= 1.1 x 10 d

2

-6 -1

K’S’s = [4 (B/r)] T S 2

MW8

MW16

MW11

MW14

MW13

EXPLANATION

T = 4,300 ft /d

S = 2.8 x 10

B = 4.9 x 10

2

-4

-2

K’S’s = [4 (B/r)] T S 2

Well r

MW5 100

= [4(0.049/100)] (4,300) (0.00028)

= 4.6 x 10 d

2

-6 -1

10-7 10-6 10-5 10-4 10-3 10-2 10-1 100 101

102 103 104

10-6 10-5 10-4 10-3 10-2 10-1 100

0.001

0.01

0.1

1

10

100

0.01

0.1

1

10

DRAWDDOWN, IN FEET

t/r , IN MINUTES PER FOOT SQUARED 2

Intermediate aquifer system

PZ3 aquifer test

Upper Floridan aquifer test

Hantush type curve W(u,B)

Transmissivity in feet squared per day (ft /d)

Storage coefficient

Dimensionless leakance parameter

Distance in feet to observation well from

pumped well

Vertical hydraulic diffusivity of confining bed

2 T

S

B

r

K’S’s

DRAWDDOWN, IN FEET

t/r , IN MINUTES PER FOOT SQUARED 2

Figure 15. Analysis of data from the Lower Arcadia zone (PZ3) and Upper Floridan aquifer tests at the ROMP 9.5 site using 

the Hantush (1960) method.
Hydraulic Characteristics at the ROMP 9.5 Site 33

Neuman and Witherspoon (1972) Ratio Method

Neuman and Witherspoon (1972) developed a 

technique for determining the hydraulic diffusivity 

(K′/S′s) of the confining unit by analyzing the hydraulic response measured in a confining unit to that in the 

aquifer at the same time and at the same radial distance 

from the pumped well. A ratio of the drawdowns in the 

confining unit to the pumped aquifer (s′/s) is plotted on 

logarithmic paper in relation to time (t) since pumping 

started (fig 17). The appropriate ratio method tD type 

curve (fig.18) to which the ratio data are matched is 

determined, as shown in the figures, by using the estimates of aquifer transmissivity and storage coefficient. 

Results of the test analysis are shown in figure 17 and 

table 5.

Numerical Analysis of Aquifer Tests

Numerical simulation of the aquifer test data 

provided an alternative method for determining 

hydraulic properties of the intermediate aquifer system 

and the Upper Floridan aquifer. The USGS model, 

MODFLOW (McDonald and Harbaugh, 1988), was 

used in a radial mode to simulate the water-level 

changes in observation wells. MODFLOW uses a 

block-centered finite-difference approach to solve 

partial differential equations governing ground-water 

flow. Parameter estimation was facilitated by a parameter-estimation program (Halford, 1992). The parameter-estimation procedure uses a minimization of the 

sum of squared residuals (SS) between observed and 

simulated heads; the procedure is based on a modified 

Gauss-Newton method (Gill and others, 1981). The SS 

is defined as:

SS(x) = [(hks–hkm)]2, (1)

 where

hks is the kth simulated water level, in feet;

hkm is the kth measured water level, in feet; and

n is the number of water-level comparisons. 

Although the SS serves as the objective function 

(measure of model fit), the root-mean-square error 

(RMSE) is reported because it is more directly comparable to actual values and serves as a composite of the 

average and the standard deviation of a set. RMSE is 

related to the SS by

 RMSE = (SS/n)0.5, (2)

The hydrogeologic system underlying the ROMP 

9.5 site is conceptualized as a multi-layered system containing discrete water producing zones separated by 

leaky confining units. Hydraulic properties of the 

hydrogeologic units at the ROMP 9.5 site were estimated from drawdown data by using the numerical 

model to match simulated water-level changes to measured water-level changes in observation wells, above, 

below, and within the pumped zones. Data from the two 

aquifer tests, the 2.77-day test of the Lower Arcadia 

zone (PZ3), and the 1-day test of the Upper Floridan 

aquifer were combined into one simulation routine to 

increase overall confidence in the optimization results. 

The system was allowed to recover 97 days after cessation of the initial 2.77-day pumping scenario to eliminate residual drawdown effects from the initial test. 

Hydraulic properties of the surficial aquifer system, the 

uppermost confining unit in the intermediate aquifer 

system, and the Upper Arcadia zone (PZ2) were specified and not estimated. These specified hydraulic 

property values were obtained from aquifer test results 

conducted at other sites. The model, however, was 

found to be insensitive to these parameters.

T = (Q/6.28 s) Ko(r/B) = [81,813 / 6.28(1.1)] 1.0 = 11,843 ft /d2

K’/b’ = T(r/B) / r = (11,843) (0.01) /(80) = 1.9 x 10 ft/d/ft 2 2 2 2 -4

Match point coordinates

Ko (r/B) = 1.0

r/B = 0.01

s = 1.1 feet

r = 80 feet Match point

Bessel function curve

100

10

1

10 100 1,000

DISTANCE (r) FROM PUMPED WELL, IN FEET

STEADY-STATE DRAWDOWN (s), IN FEET

EXPLANATION

Ko (r/B)

K’/b’

T

Q

Modified Bessel function of the second kind

and of zero order

Leakance coefficient of confining bed

Transmissivity in feet squared per day

Pumpage in cubic feet per day

Figure 16. Analysis of data from the Lower Arcadia zone 

(PZ3) aquifer test at the ROMP 9.5 site using the Jacob 

(1946) method.

k = 1

n

∑
34 Hydrogeologic Framework and Geochemistry of the Intermediate Aquifer System in Parts of Charlotte, De Soto, and Sarasota 

Counties, Florida

0.01 0.1 1 10

1

0.1

0.01

0.001

t = Tt / Sr D

2

= 10,000 (0.06) / 0.0002 (110.3)2

= 288; use t curve = 10 D

2

t’D s = K’ t/S’ z2

K’/S’ = t’ z /t s D

2

= 0.075 (16) / 0.07 2

= 274 ft /d2

Well MW9

r = 110.0 ft

z = 16 ft

t = 0.07 d

t’D = 0.075

t = Tt / Sr D

2

= 10,000 (0.087) / 0.0002 (99.1)2

= 433; use t curve = 10 D

2

t’D s = K’ t/S’ z2

K’/S’ = t’ z /t s D

2

= 0.06 (16) / 0.087 2

= 177 ft /d2

Well MW17

r = 99.1 ft

z = 16 ft

t = 0.087 d

t’D = 0.06

TIME (t) SINCE PUMPING STARTED, IN DAYS (d)

DRAWDOWN RATIO , DIMENSIONLESS (s’/s)

t = Tt / Sr D

2

=10,000 (0.148) / 0.0002 (200.5)2

=184; use t curve = 10 D

2

t’D s = K’ t/S’ z2

K’/S’ = t’ z /t s D

2

= 0.075 (16) / 0.148 2

= 130 ft /d2

Well MW12

r = 200.5 ft

z = 16 ft

t = 0.148 d

t’D = 0.075

EXPLANATION

1

0.1

0.01

0.001

0.01 0.1 1 10

1

0.1

0.01

0.001

0.01 0.1 1 10

Distance to observation well from pumped well

Distance from bottom of confining bed piezometer to top of

aquifer

Transmissivity of aquifer

Storage coefficient of aquifer

Dimensionless time factor for aquifer (Tt / Sr )

Dimensionless time factor for confining unit (K’t/S’ z )

Vertical hydraulic diffusivity of confining bed

Drawdown in aquifer

Drawdown in confining bed

Vertical hydraulic conductivity of confining beds

Storage coefficient of confining beds

2

2

s

r

Z

T

S

t

t’

K’/S’

s

s’

K’

S’

D

D

Figure 17. Analysis of data from the Lower Arcadia zone 

(PZ3) aquifer test at the ROMP 9.5 site using the NeumanWitherspoon (1972) method.
Hydraulic Characteristics at the ROMP 9.5 Site 35

The numerical model was constructed using a 

radial-model grid with seven layers to represent the 

hydrogeologic framework underlying ROMP 9.5 

(fig. 19). The model consisted of 28 variable-width rows 

in the vertical direction and 85 variable-width columns 

in the radial direction. The model dimensions represent 

500,000 ft horizontally and 800 ft vertically. Figure 19 

shows only the model grid for the first 1,000 ft from the 

pumping wells. The top, bottom, outer radial edge, and 

the cased interval above the simulated production wells 

were assigned no-flow boundaries. The radial grid spacing is accomplished beginning with a 0.5-ft wide ring at 

the production well, with successive rings being 

1.187 times wider than the previous ring. 

t =10 D

5

10

10

10

10

10

0

-1

-2

-3

-4

10-2

102

10

0.2

0.5

1.0

2.0

k t ́ t =D ́ , DIMENSIONLESS

S zs

2

 ́

s / s, DRAWDOWN RATIO, DIMENSIONLESS  ́

OBS

WELL

OBS

WELL

PUMPED

WELL

CONFINING BED

(k’)

CONFINING BED (k’)

AQUIFER

(k)

Q

r r

z

z

b

t =D = kt Tt

Sr2 S rs

2

t =D ́ k t ́

S zs

2

 ́

101

10-1 100101

Figure 18. Type curves of s′/s versus t′

d (from Neuman and Witherspoon, 1972).
36 Hydrogeologic Framework and Geochemistry of the Intermediate Aquifer System in Parts of Charlotte, De Soto, and Sarasota 

Counties, Florida

0

100200300400500600700800

1

RADIAL DISTANCE FROM PUMPED WELLS, IN FEET

ACE, IN FEET DEPTH BELOW LAND SURF MW2 MW1 MW9 MW8 MW7 MW5 MW12 MW11 MW10 MW13 MW14 MW6

MW5 Well name and open-hole interval

Surficial aquifer system

Confining unit

Confining unit

Upper Floridan aquifer

Upper Arcadia zone (PZ2)Lower Arcadia zone (PZ3)

Confining unit

EXPLANATION

100 200 300 400 500 600 700 800 900 1,000

Figure 19. Radial-model grid used for the simulation of hydraulic properties of the aquifer systems at the ROMP 9.5 site (wells MW15, MW16, and MW17 are not 

shown, but are open to the same intervals as wells MW10, MW11, and MW12).
Hydraulic Characteristics at the ROMP 9.5 Site 37

The measured and simulated (optimized) drawdown curves for PZ3 and the Upper Floridan aquifer 

tests are shown in figures 20 and 21. Overall, simulated 

water-level changes compare favorably with measured 

changes; however, model-simulated changes do not 

parallel the observed hydrographs during the late 

phases of the tests. As indicated previously, the 

measured water levels were influenced by additional 

stresses near the test area. Additionally, some degree of 

horizontal anisotropy exists in PZ3 as evidenced by the 

variation in measured and simulated drawdown curves 

for wells MW8 and MW16 (fig. 21). The RMSE for the 

simulation was 0.30 ft. Results of the numerical analysis are presented in table 6.

CONFINING UNIT WELLS Confining unit between Upper

Arcadia zone (PZ2) and Lower Arcadia zone (PZ3)

CONFINING UNIT WELLS Confining unit between Lower

Arcadia zone (PZ3) and Upper Floridan aquifer (UFA)

SIMULATED DRAWDOWN

MEASURED DRAWDOWN

WELL IDENTIFIER Casing

depth/well depth, in feet below land surface, shown in

parenthesis

–

–

–Bold characters are well name.

6

4

2

0

DRAWD

OWN, IN FEET

DRAWDOWN, IN FEET

0.01 1.0 3.0 0.1

MW9

(180/190)

MW12

(180/190)

MW17

(180/190)

TIME, IN DAYS TIME, IN DAYS

0.0001 0.001 0.01 0.1 1.0 3.0

MW7

(340/350)

MW10

(340/350)

MW15

(340/350)

EXPLANATION

6

4

2

0

6

4

2

0

6

4

2

0

6

4

2

0

6

4

2

0

MW17

(180/190)

Figure 20. Simulated and measured drawdown in selected confining unit wells at the 

ROMP 9.5 site.
38 Hydrogeologic Framework and Geochemistry of the Intermediate Aquifer System in Parts of Charlotte, De Soto, and Sarasota 

Counties, Florida

0.0001 0.001 0.01 0.1 1.0 3.0

0.0001 0.001 0.01 0.1 1.0 3.0

0.0001 0.001 0.01 0.1 1.0

MW5

(500/800)

MW6

(470/475)

MW8

(205/330)

MW11

(205/330)

MW16

(205/330)

MW13

(205/330)

MW14

(205/330)

TIME, IN DAYS

LOWER ARCADIA ZONE (PZ3)

WELLS

UPPER FLORIDAN AQUIFER

(UFA) WELLS

SIMULATED DRAWDOWN

MEASURED DRAWDOWN

WELL IDENTIFIER

Casing depth/well depth, in feet

below land surface, shown in

parenthesis

–Bold

characters are well name.

6

4

2

0

DRAWDOWN, IN FEET

EXPLANATION

6

4

2

0

6

4

2

0

6

4

2

0

16

12

8

4

0

6

4

2

0

16

12

8

4

0

TIME, IN DAYS

DRAWDOWN, IN FEET

MW6

(470/475)

Figure 21. Simulated and measured drawdown in selected Lower Arcadia zone (PZ3) 

and Upper Floridan aquifer wells at the ROMP 9.5 site.
Geochemistry of Aquifer Systems 39

Limitations of Aquifer Test Analysis 

Analysis of aquifer test data at the ROMP 9.5 site 

is difficult because the aquifer system is layered and 

has non-uniform permeability distribution. In addition, 

characterizing hydraulic properties is more difficult in 

heterogeneous, fractured rock than in homogenous, 

granular rock. At small scales, on the order of inches to 

feet, contrasts in horizontal hydraulic conductivity 

could result from the presence or absence of fractures. 

At larger scales, on the order of tens to hundreds of feet, 

contrasts in horizontal hydraulic conductivity values 

could arise from differences between zones of numerous, open, well-connected fractures rather than sparse, 

tight, poorly connected fractures in which wells may be 

completed. Consequently, hydraulic properties determined with quantitative analytical methods at a particular location in the aquifer may not be representative of 

properties at an adjacent location (P.A. Hsieh, USGS, 

written commun.,1992).

The hydraulic properties determined from 

numerical analysis of the ROMP 9.5 aquifer test data 

are considered more realistic for the heterogeneous and 

complex ground-water flow system at the ROMP 9.5 

site in contrast to analytical methods that have inherent 

limitations. In contrast to analytical methods, numerical analysis is not constrained by multiple phases of 

discharge and recovery, length of testing, assumptions 

concerning storage within a confining unit, or the number of layers in the hydrogeologic system. 

GEOCHEMISTRY OF AQUIFER SYSTEMS 

Water-quality samples from discrete producing 

zones of the intermediate aquifer system at five of the 

ROMP sites (5, 9, 9.5, 13, and 17; fig. 1) were collected 

and analyzed for major ions and selected isotopes. 

Water from the surficial aquifer system and Upper 

Floridan aquifer also was collected at each site to 

understand the chemical and isotopic character of 

water potentially entering the system from above or 

below. The main objectives of the geochemical analysis were: (1) to evaluate mixing from underlying or 

overlying aquifers, (2) to better understand chemical 

reactions that influence ground-water composition, and 

(3) to estimate the age of the ground water. Isotopes of 

hydrogen, oxygen, and carbon were used to understand 

the sources and age of ground water. Geochemical 

modeling was used to evaluate mixing between aquifers and to quantify geochemical reactions.

Chemical Composition of Ground Water

Water from the surficial aquifer system was a 

calcium bicarbonate type, except at the ROMP 13 site, 

which was a sodium bicarbonate type water (fig. 22a). 

Generally, surficial aquifer system water was more 

dilute than water from the intermediate aquifer system 

at all of the sites. Water from the ROMP 13 site was the 

most dilute, and thus, appears as a straight line in 

figure 22a. At each ROMP site, chloride, sulfate, magnesium, potassium, and strontium concentrations were 

lower in the surficial aquifer system than in the intermediate aquifer system (table 7). Alkalinity was higher 

in the surficial aquifer system than in the intermediate 

aquifer system at all sites except at the ROMP 13 site. 

The high calcium and bicarbonate concentrations are 

probably from dissolution of calcium carbonate in the 

surficial deposits, enhanced by high partial pressure of 

carbon dioxide (PCO2) levels from the soil zone. 

Lower magnesium concentrations are due to the lack of 

dolomite in the surficial deposits.

Table 6. Summary of aquifer test results for the ROMP 9.5 site using numerical analysis

[PZ2, permeable zone 2 in the Upper Arcadia Formation; PZ3, permeable zone 3 in the Lower Arcadia Formation; UFA, Upper 

Floridan aquifer; CU, confining unit; b, thickness of unit; Kh, horizontal hydraulic conductivity; T, transmissivity; S, storage 

coefficient; Ss′, specific storage; Kv, vertical hydraulic conductivity; L, leakance; ft, feet; ft/d, feet per day; ft2/d, feet squared 

per day ;--, not determined]

Hydrogeologic unit b

(ft)

Kh

(ft/d)

T

(ft2/d) S Ss′

(1/ft)

Kv

(ft/d)

L

(ft/d/ft)

PZ3 124 80.2 9,945 2.0 x 10-4 1.6 x 10-6 5.09 --

UFA 346 17.9 6,193 2.8 x 10-4 8.0 x 10-7 0.38 --

CU between PZ2 and PZ3 111 0.5 56 3.0 x 10-3 2.7 x 10-5 0.01 9.0 x 10-5

CU between PZ3 and UFA 124 1.2 149 1.1 x 10-4 9.0 x 10-7 0.04 3.2 x 10-4
40 Hydrogeologic Framework and Geochemistry of the Intermediate Aquifer System in Parts of Charlotte, De Soto, and Sarasota 

Counties, Florida

WELL Bold Number next to dot is ROMP well

number. Upper name is ROMP monitor well intable 7. Lower number is dissolved solidsconcentration, in milligrams per liter

—

(a) Surficial aquifer system (b) Upper Arcadia zone (PZ2)

EXPLANATION

MW5

674 R5

(c) Lower Arcadia zone (PZ3) (d) Upper Floridan aquifer

ChlorideSulfate

Sodium + Potassium

Calcium

Magnesium

Bicarbonate

OW13

1,270

MW2

702

MW2

360

468

MW18

OW15

1,560

MW1

806

UFA

791

1,100

S2

MW4

416

OW14

1,400

MW2

414

IAS LPZ

542

MW3

320

MW3

1,850

MW1

420

MW1

58

MW4

432

SAS

492

MW5

674

GULF OF MEXICO

0 10 MILES0 10 KILOMETERS

GULF OF MEXICO

0 10 MILES0 10 KILOMETERS

GULF OF MEXICO

0 10 MILES0 10 KILOMETERS

GULF OF MEXICO

0 10 MILES0 10 KILOMETERS

Base from SWFWMD digital data, 1:250,000, 1992Albers Equal-Area Conic projectionStandard Parallels and central meridian

R9

R9.5

R5

R13

SARASOTA MANATEE HARDEE

DE SOTO

CHARLOTTE

R17

R9

R9.5

R5

R13

SARASOTA MANATEE HARDEE

DE SOTO

CHARLOTTE

R17

R9

R9.5

R5

R13

SARASOTA MANATEE HARDEE

DE SOTO

CHARLOTTE

R17

R9

R9.5

R5

R13

SARASOTA MANATEE HARDEE

DE SOTO

CHARLOTTE

MILLIEQUIVALENTS PER LITER

0 15 15

LOCATION OF STUDY AREA

Figure 22. Dissolved-solids concentrations and Stiff diagrams showing chemical composition of water from wells at the ROMP 5, 9, 9.5, 13, and 17 sites that tap the (a) surficial aquifer system, (b) Upper Arcadia zone (PZ2), (c) Lower Arcadia zone (PZ3), and (d) Upper Floridan aquifer.
Geochemistry of Aquifer Systems 41

Water from the intermediate aquifer system had 

a wide range in chemical composition, but generally fit 

into two categories. At the sites furthest inland (ROMP 

9.5, 13, and 17), water from PZ2 and/or PZ3 was a 

mixed ion or mixed cation-bicarbonate type (fig. 22b 

and 22c). Sites closer to the coast (ROMP 9 and 5) had 

a sodium-chloride or mixed cation-chloride type water. 

Water within the same permeable zone of the intermediate aquifer system did not have a distinct chemical 

composition throughout the study area. For the Upper 

Arcadia zone (PZ2), lowest chloride and sulfate concentrations were at the ROMP 13 site, and highest chloride and sulfate concentrations were at the ROMP 9 site 

(table 7). Similarly, lowest chloride and sulfate concentrations for PZ3 were at the ROMP 13 site. The highest 

chloride concentration in PZ3 was at the ROMP 5 site, 

and the highest sulfate concentration was at the ROMP 

9 site (table 7). Water from both PZ2 and PZ3 at the 

ROMP 9 site and from PZ3 at the ROMP 5 site had 

chloride concentrations greater than 250 mg/L (secondary drinking water standard; Florida Department of 

Environmental Protection, 1994). Water from PZ3 at 

the ROMP 9 site was the only intermediate aquifer 

system site with a sulfate concentration greater than the 

secondary drinking water standard of 250 mg/L.

At four of the sites (ROMP 5, 9, 9.5, and 13), 

wells completed in both PZ2 and PZ3 were sampled. 

Very few consistent trends were apparent in comparing 

the composition of water between permeable zones at 

all sites. Chloride concentrations were about 300 percent higher in PZ3 at the ROMP 5 site than they were 

in PZ2 (800 mg/L in PZ3 and 210 mg/L in PZ2), but at 

the other three sites, chloride concentrations were less 

than 10 percent higher in PZ3 than in PZ2. Sulfate concentrations were higher in PZ3 compared to PZ2 at 

three sites (ROMP 5, 9, and 13), but at the ROMP 9.5 

site, sulfate was lower in concentration in PZ3 compared to PZ2 (43 mg/L in PZ3 and 73 mg/L in PZ2). At 

the ROMP 9 and 13 sites, the chemical character of 

water in both permeable zones was similar (fig. 22), 

which may indicate leakage between these zones.

The chemical composition of water from the 

Upper Floridan aquifer was variable at the five sites 

(fig. 22d, table 7). None of the Upper Floridan aquifer 

waters had a dominant cation (no cation greater than 

Table 7. Chemical and isotopic data for water collected from wells at the ROMP 5, 9, 9.5, 13, and 17 sites during 1996 and 1997

[

0

C, degrees Celsius; mS/cm, microsiemens per centimeter at 250C; mg/L, milligrams per liter; mg/L, micrograms per liter; δD, delta deuterium; permil, parts 

per thousand; pc/L, picocurries per liter; δ13C, delta carbon-13; 14C, carbon-14; DIC, dissolved inorganic carbon; PMC, Percent Modern Carbon (normalized); δ18O, delta oxygen-18; ROMP, Regional Observation and Monitor-Well Program; MW, monitor well; OW, observation well; SAS, surficial aquifer system; IAS, intermediate aquifer system; LPZ, lower permeable zone; PZ2, permeable zone 2 in the Upper Arcadia Formation; PZ3, permeable zone 3 in the 

Lower Arcadia Formation; UFA, Upper Floridan aquifer; SWNN, Suwannee; <, less than; --, no data]

Well name Station identification

number

Depth of 

casing/total 

depth below 

land surface 

(feet)

Principal

hydrogeologic

unit

Sample

collection

date

Temperature

(

0

C)

Specific 

conductance

(μS/cm)

pH

(standard 

units)

Alkalinity

(mg/L as 

CaCO3)

ROMP 5 MW5 265644081483305 5/85 SAS 9-25-96 26.2 1,160 7.03 314

ROMP 5 MW2 265644081483304 130/230 PZ2 9-24-96 26.6 1,140 7.15 187

ROMP 5 MW3 265644081483303 450/600 PZ3 9-24-96 30.6 3,040 7.66 105

ROMP 5 MW4 265643081483301 720/970 UFA 9-25-96 28.5 1,870 7.85 105

ROMP 9 MW1 270432082085707 7/27 SAS 8-26-96 26.2 675 7.08 256

ROMP 9 OW13 270432082085704 122/165 PZ2 8-27-96 26.8 2,070 7.51 153

ROMP 9 OW14 270432082085703 190/320 PZ3 8-27-96 24.6 2,260 7.51 147

ROMP 9 OW15 270432082085702 545/860 UFA 8-26-96 27.1 2,450 7.18 148

ROMP 9.5 MW4 270737082025104 2/8 SAS 8-20-97 25.8 703 6.83 266

ROMP 9.5 MW18 270737082025001 61/77 PZ2 8-20-97 28.1 800 7.73 209

ROMP 9.5 MW2 270737082025102 205/331 PZ3 8-19-97 27.1 749 7.69 202

ROMP 9.5 MW1 270737082025101 505/801 UFA 8-19-97 26.4 1,140 7.55 148

ROMP 13 MW1 270418081365805 7/22 SAS 9-26-96 26.9 74 5.53 17

ROMP 13 MW2 270418081365804 282/417 PZ2 9-26-96 26.4 587 7.61 246

ROMP 13 MW3 270418081365803 510/592 PZ3 9-26-96 26.8 544 8.05 200

ROMP 13 MW4 270418081365802 671/786 UFA 9-27-96 26.0 716 8.24 123

ROMP 17 SAS 271026081583605 8/18 SAS 8-28-96 25.3 751 7.04 304

ROMP 17 IAS LPZ 271026081583604 200/240 PZ3 10-2-96 25.7 853 7.63 185

ROMP 17 UFA 271026081583603 395/470 UFA 8-28-96 26.3 973 7.40 225

1ROMP 17 SWNN 271026081583602 620/670 UFA 5-27-92 27.7 1,148 7.17 157

1Data from Sacks and Tihansky (1996).
42 Hydrogeologic Framework and Geochemistry of the Intermediate Aquifer System in Parts of Charlotte, De Soto, and Sarasota 

Counties, Florida

50 percent of the total in milliequivalent per 

liter (meq/L)). The dominant anion was chloride at the 

ROMP 5 and 9 sites. Sulfate was the dominant anion at 

the ROMP 9.5 and 17 sites, and anions were mixed at 

the ROMP 13 site. Similar to the Lower Arcadia zone 

(PZ3) of the intermediate aquifer system, chloride concentrations were greater than 250 mg/L for the sites 

closest to the coast (ROMP 5 and 9). Sulfate concentrations were greater than 250 mg/L at the ROMP 9, 9.5, 

and 17 sites. At some sites, chloride concentrations 

were higher in the Upper Floridan aquifer than in PZ3 

(ROMP 9 and ROMP 13), but at other sites chloride 

concentrations were lower in the Upper Floridan aquifer compared to PZ3 (ROMP 5, 9.5, and 17 sites) 

(table 7). The chemical character of water between PZ3 

and the Upper Floridan aquifer was very similar at 

some sites (ROMP 5 and 9), but at other sites, the 

waters were distinctly different (ROMP 9.5) (figs. 22c 

and 22d). Similarity in the chemical composition of 

waters from both aquifers may indicate a good hydraulic connection between aquifers. 

Sources of elevated sulfate concentrations in the 

Upper Floridan aquifer are from dissolution of gypsum 

in deep parts of the Upper Floridan aquifer and from 

mixing with saline water (Sacks and Tihansky, 1996). 

Sulfate from gypsum dissolution can be distinguished 

from that of seawater mixing by computing the amount 

of excess sulfate relative to conservative mixing. Calculations were made assuming mixing between a dilute 

upgradient water (surficial aquifer system water from 

the ROMP 13 site) and modern seawater (Hem, 1985). 

The fraction of seawater (x) was computed using 

chloride as the conservative tracer: 

x = (Clgw - Clug)/(Clsw - Clug), (3)

where Cl is the chloride concentration of the sampled 

ground water (gw), the upgradient ground water (ug), 

and the seawater (sw). An expected sulfate concentration was computed based on conservative mixing of the 

two end-member waters. The difference between the 

sulfate concentration (SO4) in the sampled ground 

Table 7. Chemical and isotopic data for water collected from wells at the ROMP 5, 9, 9.5, 13, and 17 sites during 1996 and 

1997 (Continued)

[0C, degrees Celsius; mS/cm, microsiemens per centimeter at 250C; mg/L, milligrams per liter; mg/L, micrograms per liter; dD, delta deuterium; permil, 

parts per thousand; pc/L, picocurries per liter; d13C, delta carbon-13; 14C, carbon-14; DIC, dissolved inorganic carbon; PMC, Percent Modern Carbon (normalized); d18O, delta oxygen-18; ROMP, Regional Observation and Monitor-Well Program; MW, monitor well; OW, observation well; SAS, surficial aquifer system; IAS, intermediate aquifer system; LPZ, lower permeable zone; PZ2, permeable zone 2 in the Upper Arcadia Formation; PZ3, permeable zone 3 

in the Lower Arcadia Formation; UFA, Upper Floridan aquifer; SWNN, Suwannee; <, less than; --, no data]

Well name

Principal 

hydrogeologic

unit

Hardness

(mg/L as

CaO3)

Dissolved

solids

(mg/L)

Calcium,

dissolved

(mg/L as

Ca)

Magnesium,

dissolved

(mg/L as

Mg)

Sodium,

dissloved

(mg/L

as Na)

Potassium,

dissolved

(mg/L 

as K)

Chloride, 

dissolved

(mg/L as 

Cl)

Sulfate

dissolved

(mg/L

as SO4)

Sulfide,

dissolved

(mg/L

as S)

1Excess

sulfate

(mg/L

as SO4)

ROMP 5 MW5 SAS 330 674 130 2 110 0.9 190 2 -- -26

ROMP 5 MW2 PZ2 310 702 72 31 100 3 210 43 -- 12

ROMP 5 MW3 PZ3 600 1,850 110 70 320 12 800 220 -- 105

ROMP 5 MW4 UFA 410 1,100 73 48 180 9.3 430 170 -- 108

ROMP 9 MW1 SAS 290 420 100 10 29 1.1 53 12 -- 3

ROMP 9 OW13 PZ2 530 1,270 110 59 220 7.7 430 220 -- 158

ROMP 9 OW14 PZ3 570 1,400 110 67 250 9 460 280 -- 213

ROMP 9 OW15 UFA 620 1,560 120 71 260 8.2 500 360 -- 288

ROMP 9.5 MW4 SAS 290 432 95 13 30 1.5 58 5 <0.01 -5

ROMP 9.5 MW18 PZ2 280 468 64 29 52 2.6 86 73 1.3 59

ROMP 9.5 MW2 PZ3 230 414 40 31 61 2.6 93 43 2.4 28

ROMP 9.5 MW1 UFA 490 806 104 53 47 4.4 69 359 1.5 348

ROMP 13 MW1 SAS 15 58 4 1 8 0.5 7 2 -- 0

ROMP 13 MW2 PZ2 220 360 33 31 36 6.8 31 24 -- 18

ROMP 13 MW3 PZ3 150 320 20 21 56 4.6 32 33 -- 27

ROMP 13 MW4 UFA 200 416 39 22 58 3 100 78 -- 62

ROMP 17 SAS SAS 370 492 110 22 25 0.9 40 38 -- 31

ROMP 17 IAS LPZ PZ3 340 542 63 42 40 3.9 74 150 -- 138

ROMP 17 UFA UFA 370 628 64 48 65 4.9 110 120 -- 103

ROMP 17 SWNN UFA -- 791 110 58 38 4.9 64 380 2.3 370

1

Negative value indicates SO4 is depleted in sample, relative to conservative mixing.
Geochemistry of Aquifer Systems 43

water and the expected concentration, is the “excess 

sulfate” concentration (SO4ex): 

SO4ex = SO4gw - [SO4sw x + SO4ug (1-x)]. (4)

At each ROMP site, water from the Upper Floridan aquifer had higher excess sulfate concentrations 

than water from the intermediate aquifer system and 

surficial aquifer system (table 7). The excess sulfate 

concentration is because gypsum, the mineral source of 

sulfate, occurs in rocks in the Upper Floridan aquifer. 

Excess sulfate concentration was lowest in the Upper 

Floridan aquifer at the ROMP 13 site, probably 

because heads in the Upper Floridan decrease with 

depth at this site, which limits upward movement of 

sulfate-rich water that dissolved gypsum deeper in the 

aquifer. In contrast, excess sulfate concentrations were 

highest at the ROMP 9, 9.5, and 17 sites (greater than 

250 mg/L), and these sites are in an area where upward 

flow in the Upper Floridan aquifer is enhanced by discharging conditions near the Peace River.

Vertical Profiles of Chloride and Sulfate

Profiles of sulfate and chloride concentrations in 

the intermediate aquifer system and the upper part of 

the Upper Floridan aquifer are illustrated in figure 23. 

Differences in trends in sulfate and chloride profiles are 

indicators of differences in mixing of recharge water, 

saline water, and sulfate-rich water from the Upper 

Floridan aquifer. Differences in the profiles illustrate 

the complexity of delineating water sources in the 

intermediate aquifer system. Chloride and sulfate 

concentrations during drilling typically correspond to 

concentrations in the monitor wells finished to the 

same zones. The only major discrepancy was that 

sulfate concentrations were much lower during drilling 

in the intermediate aquifer system at the ROMP 17 site 

than when the wells were sampled. This site was drilled 

about 5 years earlier than the other sites, and perhaps 

sampling methods during drilling were problematic. 

Alternatively, water quality may have degraded over 

time, resulting in higher sulfate concentrations in the 

intermediate aquifer system.

Profiles at the ROMP 13, 9.5, and 17 sites are 

characterized by relatively low concentrations of 

Table 7. Chemical and isotopic data for water collected from wells at the ROMP 5, 9, 9.5, 13, and 17 sites during 1996 and 

1997 (Continued)

[

0C, degrees Celsius; mS/cm, microsiemens per centimeter at 250C; mg/L, milligrams per liter; mg/L, micrograms per liter; δD, delta deuterium; permil, 

parts per thousand; pc/L, picocurries per liter; δ13C, delta carbon-13; 14C, carbon-14; DIC, dissolved inorganic carbon; PMC, Percent Modern Carbon (normalized); δ18O, delta oxygen-18; ROMP, Regional Observation and Monitor-Well Program; MW, monitor well; OW, observation well; SAS, surficial aquifer system; IAS, intermediate aquifer system; LPZ, lower permeable zone; PZ2, permeable zone 2 in the Upper Arcadia Formation; PZ3, permeable zone 3 

in the Lower Arcadia Formation; UFA, Upper Floridan aquifer; SWNN, Suwannee; <, less than; --, no data]

Well name

Principal 

hydrogeologic

unit

Strontium,

dissolved

(μg/L 

as Sr)

NO2+NO3

dissolved 

(mg/L as N)

Phosphorus, 

dissolved 

(mg/L as P)

Fluoride, 

dissolved 

(mg/L as F)

Silica,

dissolved

(mg/L

as SiO2)

δD

(per mil)

Tritium

(TU)

δ13C

of DIC 

(per mil)

14C

of DIC

(as PMC)

δ18O

(per mil)

ROMP 5 MW5 SAS 580 <0.02 5.3 0.1 11 -7.8 10 -10 62.4 -1.86

ROMP 5 MW2 PZ2 1,900 <0.02 7.6 1.2 55 -1.5 <0.3 -7.8 2.03 -1.51

ROMP 5 MW3 PZ3 29,310 <0.02 <0.02 0.8 17 -3.3 <0.3 -5.4 2.67 -1.54

ROMP 5 MW4 UFA 26,000 <0.02 <0.02 0.9 18 -3.4 <0.3 -5.75 5.23 -1.55

ROMP 9 MW1 SAS 600 <0.02 <0.02 0.2 12 -12.1 9.9 -17.2 83.4 -2.48

ROMP 9 OW13 PZ2 14,000 <0.02 <0.02 1.2 17 -4.0 <0.3 -9.8 1.83 -1.41

ROMP 9 OW14 PZ3 20,000 <0.02 <0.02 1.4 18 -3.7 <0.3 -4.4 0.89 -1.27

ROMP 9 OW15 UFA 25,000 <0.02 <0.02 1.6 22 -4.4 <0.3 -3.2 1.8 -1.25

ROMP 9.5 MW4 SAS 310 <0.02 0.16 0.6 17 -2.8 10.56 -11.4 45.58 -0.91

ROMP 9.5 MW18 PZ2 1,700 <0.02 0.08 0.6 22 -3.5 -- -9.68 10.93 -1.51

ROMP 9.5 MW2 PZ3 5,100 <0.02 0.06 0.1 20 -4.5 -- -5.39 2.24 -1.76

ROMP 9.5 MW1 UFA 11,380 <0.02 0.06 2.0 27 -7.6 -- -4.6 0.83 -1.89

ROMP 13 MW1 SAS 30 <0.02 0.74 0.2 7 -13.8 11 -17 112 -2.95

ROMP 13 MW2 PZ2 8,300 <0.02 <0.02 2.1 47 -6.8 <0.3 -4.7 0.41 -1.89

ROMP 13 MW3 PZ3 8,600 <0.02 <0.02 1.8 24 -5.5 <0.3 -6.6 0.55 -1.67

ROMP 13 MW4 UFA 13,000 <0.02 <0.02 0.4 16 -1.6 <0.3 -5.76 0.98 -1.18

ROMP 17 SAS SAS 590 <0.02 0.63 1.0 25 -12.9 11 -13.1 70.3 -2.60

ROMP 17 IAS LPZ PZ3 9,300 <0.02 <0.02 2.1 25 -6.5 <0.3 -6.4 1.37 -2.08

ROMP 17 UFA UFA 10,000 <0.02 <0.02 1.8 23 -6.7 <0.3 -6 0.84 -2.00

ROMP 17 SWNN UFA 16,000 <.002 -- 2.0 24 -8.0 -- -5.8 -- -2.05
44 Hydrogeologic Framework and Geochemistry of the Intermediate Aquifer System in Parts of Charlotte, De Soto, and Sarasota 

Counties, Florida

chloride. At the ROMP 13 site, sulfate and chloride concentrations are generally less that 100 mg/L in both the 

intermediate aquifer system and upper part of the Upper 

Floridan aquifer (fig. 23). Chloride concentrations in the 

intermediate aquifer system were generally about 

30 mg/L, except for higher concentrations in the upper 

confining unit (between about 60 and 80 mg/L). Chloride concentrations in the Upper Floridan aquifer were 

slightly higher (typically between 50 and 100 mg/L) 

than in the intermediate aquifer system. Sulfate concentrations were lowest in the Upper Arcadia zone (PZ2) 

and increased slightly in the Lower Arcadia zone (PZ3), 

as well as in the Upper Floridan aquifer. Excess sulfate 

concentrations were low in both PZ2 and PZ3 (table 7).

At the ROMP 9.5 site, chloride concentrations 

were relatively low and uniform throughout the 

intermediate aquifer system (PZ2 and PZ3) and 

upper part of the Upper Floridan aquifer (below or 

about 100 mg/L), but sulfate had a wider range in 

concentration (fig. 23). Sulfate concentrations were 

about twice as high in PZ2 and the middle confining 

unit (about 70 mg/L) than in PZ3 (about 40 mg/L); 

sulfate concentrations increased in the lower confining unit and were the highest in the Upper Floridan 

aquifer (about seven times higher than in PZ3). 

Similarly, excess sulfate concentrations were much 

higher in the Upper Floridan aquifer than in PZ3 

(table 7), suggesting less of a hydraulic connection 

between these hydrogeologic units. 

Although chloride concentrations were relatively low at the ROMP 17 site, concentrations were 

somewhat elevated in the lower confining unit of the 

intermediate aquifer system and in the upper part of the 

Upper Floridan aquifer (about 130 mg/L). Deeper than 

CONCENTRATION, IN MILLIGRAMS PER LITER

ROMP 5

SAS

PZ3

UFA

PZ2

ROMP 9

SAS

PZ3

UFA

PZ2

PZ1

ROMP 9.5

SAS

PZ2

PZ3

UFA

ROMP 13

Sea

Level

100

-100

-200

-300

-400

-500

-600

-700

-800

100

UFA

PZ2

PZ3

SAS

ROMP 17

PZ3

PZ2

UFA

PZ1

SAS

EXPLANATION

PZ1

SAS

PZ2

PZ3

UFA

ALTITUDE ABOVE OR BELOW SEA LEVEL, IN FEET

0 0 500 0 500 500 1,000 200 0 500 1,000 0

Surficial aquifer system

Tamiami/Peace River zone

Upper Arcadia zone

Lower Arcadia zone

Upper Floridan aquifer

Chloride concentration of water sampled from well, plotted at the open-hole

interval of the well

Sulfate concentration of water sampled from well, plotted at the open-hole

interval of the well

Chloride concentration of water in borehole collected during drilling, plotted at

sampling interval

Sulfate concentration of water in borehole collected during drilling,

plotted at sampling interval

Figure 23. Concentrations of chloride and sulfate in samples collected during drilling of the exploratory boreholes and 

during sampling of wells at the ROMP 13, 9.5, 17, 9, and 5 sites.
Geochemistry of Aquifer Systems 45

about 600 ft below sea level, chloride concentrations in 

the Upper Floridan aquifer decreased substantially (to 

about 70 mg/L), and sulfate concentrations increased to 

greater than 350 mg/L (fig. 23). Excess sulfate concentrations in PZ3 at the ROMP 17 site were higher than 

in PZ3 at the ROMP 9.5 site (table 7) which may indicate a greater hydraulic connection between the Upper 

Floridan aquifer and PZ3 at the ROMP 17 site compared to the ROMP 9.5 site. 

In contrast, chloride concentrations were much 

higher in the profiles at the ROMP 9 and 5 sites than at 

the other three sites (fig. 23). At the ROMP 9 site, chloride concentrations were almost always greater than 

250 mg/L in the intermediate aquifer system and Upper 

Floridan aquifer. In the intermediate aquifer system, 

sulfate and chloride concentrations generally increased 

and decreased the same, with both concentrations 

decreasing slightly in the middle intervals of PZ3. In 

the Upper Floridan aquifer, chloride and sulfate concentrations were fairly uniform to 800 ft below sea 

level. Excess sulfate concentrations were highest in the 

Upper Floridan aquifer, but also were elevated in PZ2 

and PZ3 (table 7), indicating that a hydraulic connection exists between the Upper Floridan aquifer and both 

permeable zones of the intermediate aquifer system at 

the ROMP 9 site.

At the ROMP 5 site, chloride concentrations 

were greater than 150 mg/L throughout the profile, but 

were substantially higher in PZ3 (greater than 600 

mg/L) than in PZ2 and the surficial aquifer system 

(about 200 mg/L). In the upper part of the Upper Floridan aquifer, chloride concentrations decreased to about 

half that in PZ3. Sulfate concentrations were low in 

PZ2 and the middle confining unit (less than 20 mg/L), 

but were consistently higher by about an order of magnitude (about 200 mg/L) in PZ3, the lower confining 

unit, and the Upper Floridan aquifer (fig. 23). This 

increase in sulfate concentration with depth is related to 

seawater mixing. Excess sulfate concentrations in PZ3 

and the Upper Floridan aquifer were nearly the same 

(105 and 108 mg/L, respectively, table 7); however, 

excess sulfate concentration in PZ2 was much lower, 

indicating that PZ2 may be more isolated hydraulically 

from PZ3 than PZ3 is to the Upper Floridan aquifer. 

The isolation of PZ2 from PZ3 is consistent with the 

estimated hydraulic connection from aquifer tests, 

which indicated little hydraulic connection between 

PZ2 and PZ3. Excess sulfate is lower throughout the 

ROMP 5 profile than in the ROMP 9 profile (table 7), 

which illustrates the importance of seawater mixing on 

sulfate concentrations at the ROMP 5 site compared to 

the ROMP 9 site.

Saturation State of Ground Water

Information on the saturation state of water 

with respect to mineral phases can help identify probable reactions considered for geochemical modeling, 

which is discussed later in the report. Saturation 

indexes were computed using WATEQF (integrated as 

part of geochemical mass-balance model NETPATH; 

Plummer and others, 1994) and WATEQ4F (Ball and 

Nordstrom, 1991). The saturation index (SI) is a measure of the departure from equilibrium of the water 

with respect to mineral phases. An SI value of zero, 

with an associated range of uncertainty, indicates the 

water is in equilibrium or saturated with respect to the 

mineral phase, a value less than zero indicates undersaturation (mineral dissolution is possible), and a value 

greater than zero indicates supersaturation (mineral 

precipitation is possible). 

Ground water in the intermediate aquifer system 

and Upper Floridan aquifer is near saturation or supersaturated with respect to calcite (saturation index (SI) 

between -0.17 and 0.5; table 8). Saturation for calcite is 

assumed to be between -0.15 and 0.15, based on uncertainties in chemical analyses and pH determinations. 

Most waters from the surficial aquifer system were 

close to saturation with respect to calcite (SI between 

-0.16 and 0.20), except for water from the ROMP 13 

site, which was highly undersaturated (SI of -3.80).

For dolomite, there is an uncertainty in the exact 

value of the equilibrium reaction constant (Kr), and so 

the SI for both "crystalline” dolomite (Kr = -17.09) and 

“disordered” dolomite (Kr = -16.54) are reported here 

(Nordstrom and others, 1990). Previous investigators 

concluded that disordered dolomite is the form dissolving in the Upper Floridan aquifer (Hsu, 1963; Hanshaw 

and others, 1971; Plummer, 1977). Waters from the 

intermediate aquifer system and Upper Floridan aquifer 

were typically supersaturated with respect to crystalline 

dolomite (SI range between -0.34 to 1.11; table 8; saturation for dolomite is assumed to be between -0.30 and 

0.30). However, most waters from the intermediate 

aquifer system and Upper Floridan aquifer were near 

saturation or undersaturated with respect to disordered 

dolomite (SI range between -0.88 and 0.57). Water from 

the surficial aquifer system was undersaturated for both 

crystalline and disordered dolomite (table 8). 
46 Hydrogeologic Framework and Geochemistry of the Intermediate Aquifer System in Parts of Charlotte, De Soto, and Sarasota 

Counties, Florida

All waters were undersaturated with respect to 

gypsum, celestite, and amorphous silica, and all waters 

were supersaturated with respect to quartz and sepiolite. 

The saturation state of water with respect to most clay 

minerals could not be assessed because water samples 

were not analyzed for aluminum. Waters from the intermediate aquifer system and Upper Floridan aquifer in 

the general study area, reported in Sacks and Tihansky 

(1996), were supersaturated with respect to calcium 

montmorillinite, illite, and kaolinite. The SI of clay 

minerals, however, reflects much uncertainty because 

of variability in stoichiometry and uncertainty in thermodynamic properties of aluminous clays (Fritz, 1985).

Isotopic Composition of Ground Water

Water samples were analyzed for isotopes of the 

water molecule (tritium, deuterium, and oxygen-18) 

and dissolved inorganic carbon (carbon-13 and carbon14). The radioactive isotopes tritium and carbon-14 

were used to estimate the age of the water, whereas the 

stable isotopes of water (deuterium and oxygen-18) 

and inorganic carbon (carbon-13) provided insights 

into the source of water, mixing between waters, and 

the chemical evolution of the water.

Stable Isotopes

Values of δD and δ18O typically are plotted on a 

diagram showing δD versus δ18O, and are related to a 

global meteoric water line (GMWL), representing rainfall from around the world (Craig, 1961). The stable 

isotope composition of waters relative to the GMWL 

can indicate important information on ground-water 

recharge patterns, waters that have undergone evaporation, recharge during different climatic conditions, and 

mixing of ground water and surface water. Waters sampled from the surficial aquifer system for this study 

were isotopically heavy compared to the composition of 

modern rainwater and were offset from the GMWL 

(fig. 24). Most surficial aquifer system waters were 

isotopically lighter than waters from the intermediate 

aquifer system (PZ2 and PZ3) and from the Upper 

Floridan aquifer at the same site; however, water from 

the surficial aquifer system at the ROMP 9.5 site was 

Table 8. Saturation state of ground water at the ROMP 5, 9, 9.5, 13, and 17 sites with respect to mineral phases and 

calculated partial pressure of carbon dioxide

[PCO2, partial pressure of carbon dioxide; SAS, surficial aquifer system; PZ2, permeable zone 2 in the Upper Arcadia Formation; PZ3, permeable 

zone 3 in the Lower Arcadia Formation; UFA, Upper Floridan aquifer]

Well name

 Principal

hydrogeologic

 unit

Saturation index 

Log

Calcite PCO2 Crystalline 

dolomite11

Disordered

dolomite 2 Gypsum Celestite

ROMP 5 MW5 SAS 0.20 -1.00 -1.55 -3.05 -3.68 -1.48

ROMP 5 MW2 PZ2 -0.17 -0.34 -0.88 -2.05 -1.90 -1.83

ROMP 5 MW3 PZ3 0.22 0.64 0.10 -1.38 -0.23 -2.59

ROMP 5 S2 UFA 0.26 0.72 0.17 -1.55 -0.28 -2.78

ROMP 9 MW1 SAS 0.10 -0.44 -0.99 -2.37 -2.90 -1.60

ROMP 9 OW13 PZ2 0.22 0.53 -0.02 -1.33 -0.49 -2.28

ROMP 9 OW14 PZ3 0.15 0.42 -0.12 -1.24 -0.26 -2.32

ROMP 9 OW15 UFA -0.13 -0.12 -0.66 -1.12 -0.09 -1.97

ROMP 9.5 MW4 SAS -0.16 -0.82 -1.36 -2.80 -3.57 -1.33

ROMP 9.5 MW18 PZ2 0.46 0.96 0.42 -1.84 -1.69 -2.34

ROMP 9.5 MW2 PZ3 0.21 0.68 0.13 -2.24 -1.42 -2.32

ROMP 9.5 MW1 UFA 0.22 0.49 -0.05 -1.08 -0.33 -2.33

ROMP 13 MW1 SAS -3.80 -7.86 -8.40 -4.12 -4.56 -1.17

ROMP 13 MW2 PZ2 0.14 0.62 0.08 -2.55 -1.43 -2.15

ROMP 13 MW3 PZ3 0.28 0.96 0.41 -2.59 -1.24 -2.68

ROMP 13 MW4 UFA 0.50 1.11 0.57 -1.98 -0.73 -3.10

ROMP 17 SAS SAS 0.14 -0.07 -0.61 -1.90 -2.46 -1.50

ROMP 17 IAS LPZ PZ3 0.24 0.65 0.11 -1.57 -0.69 -2.31

ROMP 17 UFA UFA 0.11 0.45 -0.09 -1.68 -0.77 -1.99

3 ROMP 17 SWNN UFA 0.10 -0.11 -0.65 -1.05 -0.17 -1.92

1 Equilibrium reaction constant (Kr) = -17.09.

2 Kr = -16.53.

3 Data from Sacks and Tihansky (1996).
Geochemistry of Aquifer Systems 47

isotopically heavier than water from the other aquifers. 

Surficial aquifer system water at the ROMP 9.5 site also 

was offset from the GMWL, indicating that the water 

had undergone evaporation prior to recharging the surficial aquifer system. Much of the study area is low lying 

and evaporation of standing water at the surface prior to 

recharge probably resulted in the enrichment of δD and 

δ18O at the ROMP 9.5 site. Waters from the surficial 

aquifer system plot along a line described by the expression δD= 5.7 δ18O + 2.4 (r2

 = 0.99), and most likely 

represent a mixture of meteoric water with an isotopically light composition and ground water that has been 

recharged by water that has undergone evaporation with 

an enriched isotopic composition.

The stable isotope composition of waters from 

the Lower Arcadia zone (PZ3) is quite similar to that 

for the Upper Floridan aquifer at the ROMP 17, 9, and 

5 sites, suggesting that water moved upward from the 

Upper Floridan aquifer into PZ3. In contrast, the different isotope compositions for waters from PZ2, PZ3, 

and the Upper Floridan aquifer at the ROMP 9.5 site 

indicate complex mixing of waters from overlying and 

underlying aquifer systems. Trend lines connecting isotopically enriched waters from the intermediate aquifer 

system (δD= 5.0 δ18O + 3.8; r2 = 0.56) and Upper 

Floridan aquifer (δD= 6.5 δ18O + 5.3; r2

 = 0.82) also 

indicate mixing between waters with distinct isotopic 

compositions; the poorer relation for the intermediate 

aquifer system suggests that the mixing of water in the 

intermediate aquifer system is more complex than a 

mixing model between two end members. However, 

the enriched δ18O and δD composition of ground water 

from the intermediate aquifer system and Upper Floridan 

aquifer may be the result of recharge that occurred under 

climatic conditions that differed from present conditions. Plummer and others (1993) found stable isotope 

enrichment in the confined part of the Upper Floridan 

aquifer in southern Georgia in waters that were 

recharged during the last glacial maximum. Some 

enrichment in the stable isotope composition of waters 

from the intermediate aquifer system and Upper Floridan aquifer may result from mixing of enriched paleowaters with somewhat depleted younger meteoric water.

Water from the three aquifer systems had 

isotopically distinct carbon-13 signatures of dissolved 

inorganic carbon (δ13CDIC) (table 7). For example, 

δ13CDIC values were isotopically lighter, or more 

depleted in carbon-13, in the surficial aquifer system 

(-10.0 to -17.2 per mil) compared to the intermediate 

aquifer system and Upper Floridan aquifer. In addition, 

δ13CDIC values for water from PZ2 were lighter (-9.8 

to -4.7 per mil) than for water from PZ3 (-6.6 to -4.4 per 

mil). Water from the Upper Floridan aquifer typically 

had the heaviest δ13CDIC values (-6.0 to -3.2 per mil). 

Sacks and Tihansky (1996) also noted that water in the 

intermediate aquifer system typically had lighter 

δ13CDIC values than water in the Upper Floridan 

aquifer. The difference in δ13CDIC is related to the evolution of inorganic carbon in the aquifer systems. Water 

from the surficial aquifer system was most recently in 

contact during recharge with soil-zone CO2, which is 

isotopically light (usually between -20 and -25 per mil; 

Deines and others, 1974). This recharging water dissolved different amounts of calcite in the shallow surficial aquifer system, resulting in high bicarbonate 

concentrations (greater than 250 mg/L as CaCO3) in 

most surficial aquifer system waters and a wide range 

of δ13CDIC values reflecting different carbon sources. 

Waters in deeper aquifer systems evolve to a heavier 

δ13CDIC composition because they are isolated from 

soil-zone CO2 and because the dissolution of isotopically heavy dolomite results in an enrichment of 

δ13CDIC. Dolomite dissolution is illustrated by higher 

concentrations of magnesium and waters closer to 

saturation with respect to dolomite in the deeper 

aquifers compared to the surficial aquifer system.

EXPLANATION

-4 -3 -1 0 -2

0

-10

-20

GMWL

D (per mil)

18O (per mil)

ROMP 9.5

Water from surficial aquifer system wells

Water from Upper Arcadia zone (PZ2) wells

Water from Lower Arcadia zone (PZ3) wells

Water from Upper Floridan aquifer wells

Global meteoric water line [ D = 8( O)+10]   18 GMWL

Figure 24. Relation between delta deuterium (δD) and 

delta oxygen-18 (δ18O) values in water from wells at the 

ROMP 5, 9, 9.5, 13, and 17 sites that are completed in the 

surficial aquifer system, intermediate aquifer system, and 

Upper Floridan aquifer.
48 Hydrogeologic Framework and Geochemistry of the Intermediate Aquifer System in Parts of Charlotte, De Soto, and Sarasota 

Counties, Florida

An inverse relation exists between δ13CDIC and 

14C values in ground water from the surficial aquifer 

system and PZ2 of the intermediate aquifer system 

(fig. 25). This relation is further evidence of the evolution of inorganic carbon from recharge waters (with 

higher 14C activity and lighter δ13CDIC values from 

soil gas) toward deeper ground water (with lower 14C 

activity and heavier δ13CDIC values from dissolution of 

calcite and dolomite in the aquifer). A similar relation 

between δ13CDIC and 14C was observed for ground 

water in South Carolina (Landmeyer and Stone, 1995). 

Differences in the δ13C composition of the originating 

organic matter result in different δ13CDIC signatures in 

incoming CO2 and organic carbon in ground water 

(McMahon and others, 1990). 

Age of Ground Waters

Tritium is useful for age dating relatively young 

ground water (less than 50 years old) because of its short 

half life (12.4 years), whereas 14C can be used for estimating ages of older ground water (500-40,000 years 

old) because of its longer half life (5,730 years). All 

waters sampled from the surficial aquifer system had 

tritium present at concentrations ranging between 

about 10 and 11 TU, indicating that the water is 

“young” and was probably recharged between 30 and 

40 years ago (based on tritium concentrations measured in rainfall in Ocala, Florida between 1961 and 

1991, adjusted for radioactive decay; fig. 26). Presentday rainfall in Florida has a much lower tritium concentration (about 5 TU) than rainfall between the late 

1950's and early 1960's, even after considering radioactive decay of rainfall from the 1960's. Tritium was not 

detected in waters from the intermediate aquifer system 

and Upper Floridan aquifer, indicating that these 

waters are “old” (greater than 50 years old, based on the 

detection limit of 0.3 TU and considering radioactive 

decay). Thus, it appears that little, if any, present-day 

recharge is moving from the surficial aquifer system 

into the intermediate aquifer system in the study area. 

This apparent lack of recharge is supported by upward 

head differences between the intermediate aquifer 

system and the surficial aquifer system in much of the 

study area, inhibiting recharge from the surficial 

aquifer system to the intermediate aquifer system.

Age was estimated for waters from the intermediate aquifer system and from the Upper Floridan aquifer using 14C, and was corrected for reactions with 

inorganic carbon (table 9). Uncertainties in computed 

ages are due to analytical precision of 14C, the 14C 

adjustment models, and input parameters to the models. Most waters had 14C values less than 10 PMC, with 

an analytical uncertainty of 1 PMC. As the measured 

14C concentration decreases, the uncertainty in the age 

increases. For example, water with a 14C value of 

5 PMC can have an age uncertainty of plus or minus 

2,000 years, whereas a water with 14C value of less 

than 0.6 PMC can have an uncertainty range of plus or 

minus 10,000 years. Values assumed for input parameters to the correction models also can contribute uncertainty to the computed age. For example, changing the 

13C value for soil CO2 from -25 to -20 per mil resulted 

in computed ages that were about 2,000 years older 

than those in table 9.

Several trends are apparent in the computed ages 

of the water, after considering uncertainties in the ages. 

All intermediate aquifer system and Upper Floridan 

aquifer waters are probably greater than about 

10,000 years old, and many are greater than 20,000 years 

old (table 9). Thus, both aquifer systems may have been 

recharged under different hydraulic conditions than 

currently observed. 

0 30 60 90 120

0

-10

-20

13 C (per mil PDB)

14C percent modern carbon

EXPLANATION

Water from surficial aquifer system wells

Water from Upper Arcadia zone (PZ2) wells

Water from Lower Arcadia zone (PZ3) wells

Water from Upper Floridan aquifer wells

Figure 25. Relation between delta carbon-13 (d13C) and 

carbon-14 (14C) values in water from wells at the ROMP 5, 

9, 9.5, 13, and 17 sites that are completed in the surficial 

aquifer system, intermediate aquifer system, and Upper 

Floridan aquifer.
Geochemistry of Aquifer Systems 49

At the ROMP 9.5 site, water in the Upper 

Arcadia zone (PZ2) is younger than water in the Lower 

Arcadia zone (PZ3), considering the uncertainty ranges 

(table 9). This younger age may indicate the influence 

of recharge from the surficial aquifer system in PZ2. 

Water in PZ3 may be on a longer flow path moving 

laterally from an upgradient recharge area, or may be a 

mixture of younger water from PZ2 and older water in 

the Upper Floridan aquifer (table 9). Water in the 

Upper Floridan aquifer at the ROMP 9.5 site probably 

evolved over long flow paths from recharge areas to the 

north and east (Hanshaw and others, 1965; Plummer 

and others, 1983; Sacks and Tihansky, 1996). At the 

ROMP 13 and 17 sites, similar ages were computed for 

water in both the intermediate aquifer system and the 

Upper Floridan aquifer (table 9).

In contrast, waters at the ROMP 5 and 9 sites 

decrease in age between the intermediate aquifer 

system and Upper Floridan aquifer (table 9). Water in 

the Upper Floridan aquifer at these sites is about 

10,000 years younger than water sampled from the 

Upper Floridan aquifer at the other sites. These waters 

also had about four times higher chloride concentrations than the other Upper Floridan aquifer waters. The 

younger apparent age at these sites is contradictory to 

the predevelopment flow direction in the Upper 

Floridan aquifer, which originates to the north and east 

of the study area (Ryder, 1982; Plummer and others, 

1983). Because the ROMP 5 and 9 sites are downgradient of the other ROMP sites6, the Upper Floridan 

aquifer water would be expected to be older than water 

at the other ROMP sites. The hypothesis was tested that 

the saline end member for these waters is young, and 

mixing with the older Upper Floridan aquifer water 

results in a younger apparent age of water in these 

Upper Floridan aquifer wells. Assuming linear mixing 

between modern seawater (as the most extreme end 

member) and upgradient water (ROMP 9.5), a mixture 

of 40-percent seawater and 60-percent upgradient 

Upper Floridan aquifer water would account for the 

younger water at the ROMP 9 site. Such a mixture, 

however, would have a chloride concentration of about 

7,600 mg/L, which is more than an order of magnitude 

greater than the observed chloride concentration at the 

ROMP 9 site (500 mg/L). If the age of the seawater is 

VOLUME-WEIGHTED MEAN CONCENTRATION IN RAINFALL,

OCALA, FLORIDA; DATA BEFORE 1961 AND AFTER 1991

RECONSTRUCTED USING AN OTTAWA CORRELATION FOR

1954-1994 (MICHEL, 1989)

CORRECTED FOR DECAY TO 1997 FOR CURRENT STUDY

1950 1960 1970 1980 1990

1,000

100

10

1

TRITIUM CONCENTRATION, IN TRITIUM UNITS

YEAR

1945 1995

Figure 26. Tritium concentration in rainfall in Ocala, 

Florida.

Table 9. Adjusted carbon-14 age of ground water at the 

ROMP 5, 9, 9.5, 13, and 17 sites from the intermediate 

aquifer system and the Upper Floridan aquifer

[MW, monitoring well; SUW, Suwannee; OW, observation well; 

PZ2, permeable zone 2 in the Upper Arcadia Formation; PZ3, permeable zone 3 in the Lower Arcadia Formation; IAS, intermediate 

aquifer system; LPZ, lower permeable zone; CU(PZ3:UFA), confining unit between PZ3 and UFA; UFA, Upper Floridan aquifer]

Well

name

Principal

hydrogeologic

unit

Adjusted 

age 1

(years)

1 Adjusted using Fontes and Garnier (1979) correction model, which 

assumes carbon-14 activity of carbonate minerals and soil CO2 to be equal to 

0 and 100 percent modern carbon, respectively, and delta carbon-13 of carbonate minerals and soil CO2 to be equal to 0 and -25 per mil, respectively; 

age values rounded to nearest 1,000 years.

Range in ages 

due to analytical 

uncertainty2

(years) 

2

For carbon-14 values greater than 10 percent modern carbon, analytical uncertainty is 2 percent modern carbon; for carbon-14 values less than 

10 percent modern carbon, analytical uncertainty is 1 percent modern carbon; for carbon-14 values less than 1 percent modern carbon, analytical 

uncertainty is assumed to be 0.5 percent modern carbon.

ROMP 5 MW2 PZ2 22,000 19,000-28,000

ROMP 5 MW3 PZ3 17,000 14,000-21,000

ROMP 5 SUW2 UFA 12,000 10,000-14,000

ROMP 9 OW13 PZ2 25,000 22,000-32,000

ROMP 9 OW14 PZ3 24,000 18,000-31,000

ROMP 9 OW15 UFA 15,000 11,000-22,000

ROMP 9.5 MW18 PZ2 10,000 9,000-11,000

ROMP 9.5 MW2 PZ3 18,000 15,000-23,000

ROMP 9.5 MW1 UFA 25,000 18,000-33,000

ROMP 13 MW2 PZ2 31,000 21,000-43,000

ROMP 13 MW3 PZ3 32,000 23,000-46,000

ROMP 13 MW4 UFA 26,000 20,000-32,000

ROMP 17 IAS LPZ PZ3 24,000 19,000-35,000

ROMP 17 UFA UFA 27,000 21,000-35,000
50 Hydrogeologic Framework and Geochemistry of the Intermediate Aquifer System in Parts of Charlotte, De Soto, and Sarasota 

Counties, Florida

older, then even higher chloride concentrations would 

be computed for the ROMP 9 site. Therefore, the 

younger age in the Upper Floridan aquifer at the 

ROMP 5 and 9 sites is not strictly a mixture between 

younger seawater and upgradient ground water, and 

may indicate mixing with another, perhaps shallower, 

freshwater end member.

Mineralogical and Isotopic Composition of 

Aquifer System Material

The mineralogy and isotopic composition of 

rocks that compose the aquifer system were evaluated 

to assist in interpreting the chemical and isotopic composition of the ground water. These mineralogic and 

isotopic composition data also were used as input 

parameters to the geochemical model described in the 

following section. Mineralogy was defined by x-ray 

diffraction for eight intervals of the core from the 

ROMP 9.5 site (table 10) and from the literature. The 

intermediate aquifer system is a heterogeneous unit, 

with calcite, dolomite, and quartz being the most 

abundant minerals. The exact stoichiometry of calcite 

and dolomite in the intermediate aquifer system is 

unknown. Clay minerals in the aquifer system include 

Table 10. Mineralogy, carbon content, and isotopic composition of organic and inorganic carbon from selected samples of the 

core at the ROMP 9.5 site, De Soto County, Florida

[Identifications, by x-ray diffraction, are ranked from highest to lowest intensity of the principle peak for each mineral; <1μm, less than one micrometer; per 

mil, parts per thousand; δ13C, delta carbon-13; ROMP, Regional Observation and Monitor-Well Program; PZ2, permeable zone 2 in the Upper Arcadia Formation; PZ3, permeable zone 3 in the Lower Arcadia Formation; UFA, Upper Floridan aquifer; ?, uncertain identification; --, no data; NA, not applicable]

Sample 

depth 

below land 

surface 

(feet)

Hydrogeologic 

unit Bulk contents Ultrafine clays (<1μm)

Organic carbon (traditional 

method)

(percent) 1

Organic carbon 

(persulfate 

method)

(percent) 2

Mean 

persulfate 

(percent)

Organic δ13C 

(per mil)

Inorganic 

δ13C 

(per mil)

68 PZ2 Quartz, dolomite, calcite, and total clayPalygorskite smectite, 

kaolinite (trace), and 

illite (trace ?)

1.5 2.16

1.14

1.65 -20.56 -0.24

188 confining unit 

between PZ2 

and PZ3

Dolomite, quartz, 

total clay, and 

calcite

Palygorskite, smectite, 

sepiolite, and kaolinite

2.1 1.99

1.74

1.86 -20.46 --

239 PZ3 Calcite, dolomite, 

quartz, and total 

clay

Palygorskite, smectite, 

sepiolite, and kaolinite

1.5 0.099

2.22

1.64 -22.38 --

280 PZ3 Quartz, calcite, potassium feldspar, 

dolomite, plagioclase (trace ?), and 

total clay (?)

Smectite, sepiolite, 

palygorskite, and 

kaolinite

1.4 2.01

1.02

1.09

1.37 -21.11 -0.16

320 PZ3 Calcite, quartz, potassium feldspar, plagioclase (?), total 

clay (?), and dolomite (trace ?)

Smectite, plagioclase, 

sepiolite (?), and 

kaolinite (trace)

1.3 0.11

0.05

0.42

0.19 -25.16 --

340 confining unit 

between PZ3 

and UFA

Calcite, quartz, plagioclase, dolomite 

(trace ?), and 

potassium feldspar

Smectite, chain-structure 

clays (?), and kaolinite (trace ?)

1.7 0.12

0.30

0.21 -22.11 --

480 UFA Calcite, quartz (trace), 

and total clay 

(trace)

Smectite, chain-structure 

clays (?), and kaolinite (?)

2.2 0.29

0.10

0.10

0.16 -21.29 --

520 UFA Calcite, quartz, cristobalite (?), and total 

clay

Palygorskite, smectite 

(trace) and kaolinite 

(trace)

2.1 0.09

0.10

0.87

0.35 -23.44 --

Standard 

reference 

material 3

NA NA NA 2.0 1.90

1.85

0.19 -- --

1 Total organic carbon content determined by a combustion method (Wershaw and others, 1987); inorganic carbon content determined by 

a gasometric method (Wershaw and others, 1987); organic carbon content determined by subtracting inorganic carbon from organic carbon. 2 Organic carbon content determined by a modified persulfate-oxidation method (Sandstrom and others, 1986).

3 Analyzed using methods by Wershaw and others, (1987), and Sandstrom and others, (1986).
Geochemistry of Aquifer Systems 51

illite, smectite, sepiolite, palygorskite, and kaolinite; 

other minor minerals include phosphatic minerals 

(apatite [Ca5(PO4)3(F,Cl,OH)], fluorapatite, and francolite [(Ca,Na)5(PO4,CO3)3(F,OH)]), pyrite (FeS2), 

and celestite (SrSO4) (Scott, 1988; McCartan, Duerr, 

and Hawkins, 1992; McCartan, Plummer, and others, 

1992; Florida Geological Survey, written commun., 

1993; Kauffman, 1994; McCartan and others, 1995). 

Gypsum has not been found in the intermediate aquifer 

system.

The organic carbon content of the rock material 

in the intermediate aquifer system and Upper Floridan 

aquifer was determined from eight intervals of the core 

collected at the ROMP 9.5 site. The organic carbon content was relatively uniform, ranging from 1.3 to 

2.2 percent of the bulk material. In the intermediate 

aquifer system, the lowest amounts of organic carbon 

were in the permeable zones, with slightly higher 

amounts of organic carbon found in the confining units. 

The persulfate method provided similar results for samples from the upper zones of the core; however, results 

for samples from deeper zones were an order of magnitude lower than results using the traditional method, 

indicating incomplete recovery of organic carbon. It is 

probable that organic carbon in deeper zones is more 

resistant to oxidation than carbon in shallower zones. 

Selected intervals of the ROMP 9.5 core were 

analyzed for δ13C of the organic and inorganic carbon 

of the rock material (table 10). Values of δ13C of the 

organic carbon ranged from -20.46 to -25.16 per mil, 

with an average value of -22.1 per mil (table 10). These 

values are similar to those for organic carbon from a 

carbonate aquifer in South Carolina (McMahon and 

others, 1990). In the upper part of the intermediate 

aquifer system, δ13C of organic carbon was isotopically heavier than in the deeper part of the intermediate 

aquifer system and in the upper part of the Upper Floridan aquifer. For inorganic carbon, the two samples had 

δ13C values near 0 per mil, indicating little change 

from seawater composition. Minerals were not distinguished in the analysis, but were observed to be both 

calcite and dolomite in the rock core. Other values of 

δ13C of calcite in the intermediate aquifer system range 

from -7.1 to 1.9 per mil (Sacks and Tihansky, 1996). 

Values of δ13C in the Floridan aquifer system range 

from -6.4 to 3.1 per mil for calcite and from -7.5 to 

3.1 per mil for dolomite (Hanshaw and Back 1972; 

Sprinkle, 1989; Cander, 1991; Sacks and Tihansky, 

1996).

Geochemical Evolution of Ground Water 

Along Flow Paths 

For ground-water resources in the intermediate 

aquifer system to be adequately protected from waterquality degradation for future use, it is important to 

understand how water moves laterally within permeable zones and vertically between overlying and underlying permeable zones and aquifer systems. The 

geochemical model NETPATH was used (1) to evaluate the lateral continuity of flow within permeable 

zones, (2) to determine the extent of mixing between 

overlying and underlying aquifer systems, and (3) to 

understand important geochemical reactions occurring 

within the aquifer.

Defining Flow Paths

Potentiometric-surface maps of the Upper 

Arcadia zone (PZ2) and the Lower Arcadia zone (PZ3) 

were used to establish preliminary flow paths (fig. 27). 

These paths assume that water in PZ2 and PZ3 moves 

laterally within the permeable zone between sites. 

Modifications were made to flow paths if no valid 

models were found. In PZ2, flow is from east-northeast 

to the west-southwest; in PZ3, flow is generally from 

east-southeast to west-northwest. Flow paths were 

modeled from upgradient to downgradient sites, but are 

inherently limited by well distribution. The flow paths 

were used to understand the chemical evolution of 

water in PZ2 at the ROMP 9.5, 9, and 5 sites (fig. 27a), 

and in PZ3 at the ROMP 17, 9.5, and 9 sites (fig. 27b). 

Besides lateral flow paths within a permeable 

zone, vertical mixing also was considered. Mixing 

directions were based on measured head differences 

between permeable zones and aquifer systems. Thus, 

reactions and mixing were typically computed between 

several initial wells and a final well along the flow path. 

Description of Geochemical Model and 

Assumptions

The geochemical model NETPATH (Plummer 

and others, 1994) computes a series of chemical reactions between initial and final waters, given a set of 

constraints (elements, isotopes, or electron balance) 

and phases (including mineral dissolution or precipitation, ion exchange, or gas exchange). The model also 

can compute the isotopic composition of the final 

water, provided that the isotopic composition of the initial water(s), and dissolving phases and fractionation 

factors of precipitating phases are defined. Although 

computed models cannot be validated, they can be 
52 Hydrogeologic Framework and Geochemistry of the Intermediate Aquifer System in Parts of Charlotte, De Soto, and Sarasota 

Counties, Florida








	
 











 
 



 
 	
 

 


GULF OF MEXICO

0 10 MILES

0 10 KILOMETERS

GULF OF MEXICO

0 10 MILES

0 10 KILOMETERS

Base from SWFWMD digital data, 1:250,000, 1992

Albers Equal-Area Conic projection

Standard Parallels 	
 
	
 	

 and , central meridian

SARASOTA MANATEE HARDEE

DE SOTO

CHARLOTTE

SARASOTA MANATEE HARDEE

DE SOTO

CHARLOTTE

EXPLANATION

37

20

R5

STUDY AREA

POTENTIOMETRIC CONTOUR Shows altitude at which water would have

stood in tightly cased wells. Dashed where approximate. Contour interval is

10 feet. Datum is sea level

PRESUMED FLOW PATH USED IN NETPATH MODELING

OBSERVATION WELL Number is altitude of water level in feet above sea

level, September 1998

ROMP WELL AND NUMBER ALONG PATH

—

—

40

50

30

30

50

20

20

40

10

20

20 30

40

50

52

32

38

36 36

24

26

49

13

R26

R17

R9

R9.5

R5

R13

41

46

13

30

52

44

49

46

43

43

51

49

31

18

47

R17

R9

R9.5 R13

16

8

21

31

20

20

9 24

16

(b) Lower Arcadia zone (PZ3)

(a) Upper Arcadia zone (PZ2)

R5

Figure 27. Ground-water flow paths simulated in the intermediate aquifer system, (a) 

Upper Arcadia zone (PZ2) and (b) Lower Arcadia zone (PZ3), using the geochemical 

model NETPATH.
Geochemistry of Aquifer Systems 53

rejected when they violate thermodynamic principles 

(for example, dissolution of a mineral from a supersaturated water) or when large discrepancies occur 

between the observed and computed isotopic composition of the final water. Models were constrained by 

calcium, magnesium, carbon, sulfur, electron balance 

(redox state), sodium, and in most cases, chloride. 

Mixing ratios were typically computed from chloride 

concentrations. Electron balance considerations were 

required to model redox reactions, such as oxidation of 

organic matter. 

Reaction phases that were considered include 

stoichiometric calcite and dolomite, organic matter 

(represented as CH2O in the models), CO2 gas, gypsum, pyrite (FeS2), and cation exchange (Ca for Na and 

Mg for Na). Because of wide uncertainties in the stoichiometry and saturation state of clay minerals, it is 

difficult to assess the validity of clay mineral precipitation or dissolution models. Thus, the simplification was 

made that modeled cation exchange reactions also represent clay mineral dissolution and precipitation that 

involve the mass transfer of calcium, magnesium, and 

sodium. Most waters were saturated or supersaturated 

with respect to calcite, and so only calcite precipitation 

was considered. Dissolution was considered if initial or 

final waters were undersaturated with respect to disordered dolomite (the prevalent form in the Upper Floridan aquifer in the study area); dolomite precipitation 

was not considered to be a realistic geochemical process in this freshwater aquifer. Gypsum has not been 

identified in the intermediate aquifer system, and the 

depositional environment during the Miocene was not 

conducive to the formation of permanent evaporite 

minerals. Minor amounts of gypsum dissolution, however, were sometimes considered to balance sulfate. All 

waters from the intermediate aquifer system had a sulfide odor, indicating sulfate-reducing conditions; at the 

ROMP 9.5 site, sulfide concentrations were greater 

than 1 mg/L (table 7). Pyrite precipitation was considered as a phase because of reducing conditions (where 

pyrite oxidation cannot occur). Iron concentrations typically are low, but measurable (5-500 μg/L) in the intermediate aquifer system (Southwest Florida Water 

Management District, 1991; Sacks and Tihansky, 

1996), although iron was not analyzed in this study. 

The choice of carbon phases (CO2 gas and CH2O) were 

selected on the basis of geochemical modeling of the 

Upper Floridan aquifer (Plummer, 1977; Plummer and 

others, 1983); CH2O was only allowed to oxidize representing microbial degradation of organic matter.

Sometimes several models were computed for a 

given flow path. Mixing ratios were usually similar, 

with models typically differing by the amount of mass 

transfer of CO2 and CH2O, and carbonate mineral and 

cation exchange reactions. Models with calculated 

δ13C values similar to observed values were considered 

more realistic than those with large discrepancies. 

Models without gypsum dissolution were favored over 

those containing gypsum dissolution, particularly 

when mixing could be an explanation for an increase in 

sulfate. Many models contained CO2 outgassing. The 

exact mechanism of carbon exchange (CO2 ingassing, 

fermentation, microbial respiration), however, is not 

well understood and is beyond the scope of the present 

study. Minor charge imbalances also can influence the 

mass transfer of neutral compounds like CH2O or CO2

(Plummer and others, 1994, p. 15).

Upper Arcadia Zone (PZ2)

Water in the Upper Arcadia zone (PZ2) at the 

ROMP 9.5 site was modeled along a presumed flow 

path from an upgradient well in northern De Soto 

County, which is outside the immediate study area 

(ROMP 26; data from Sacks and Tihansky, 1996) 

(fig. 27a). The assumption was made that this water is 

chemically similar to upgradient water in the study 

area. Mixing with water from the Lower Arcadia zone 

(PZ3) was considered because heads were greater in 

PZ3 than in PZ2 along most of this path (figs. 8, 9, 10, 

and 27). Major ions increase in concentration along this 

path. A model with plausible reactions and similar 

computed and observed δ13C values had 12 percent of 

the water flowing laterally in PZ2 (initial 1 well, table 

11) and 88 percent of the water moving upward from 

PZ3 (initial 2 well, table 11). In that model, dominant 

reactions (greater than 0.20 millimoles per liter 

(mmol/L)) were CH2O oxidation, CO2 outgassing, 

pyrite precipitation, and cation exchange (also a surrogate for clay mineral reactions); no carbonate mineral 

reactions were computed. This model indicates that lateral flow from the upgradient part of the aquifer is 

minor compared to upward flow from PZ3 in the evolution of water in PZ2 at the ROMP 9.5 site.

Water in PZ2 at the ROMP 9 site was modeled 

on a flow path originating at the ROMP 9.5 site 

(fig. 27a). Mixing with water from PZ3 was considered 

because heads are higher in PZ3 than in PZ2. All major 

ions increase along this path, except for bicarbonate, 

which decreases slightly. Although a lateral head difference exists from the ROMP 9.5 site to the ROMP 9 

site, model results indicate that limited lateral flow 

occurs in PZ2 between these two sites (less than 

10 percent), and the source of most of the water in PZ2 
54 Hydrogeologic Framework and Geochemistry of the Intermediate Aquifer System in Parts of Charlotte, De Soto, and Sarasota 

Counties, Florida

Table 11. NETPATH geochemical modeling results along selected flow paths within the intermediate aquifer system[Units in millimoles per liter (mmol/L) unless otherwise noted; positive mass transfer indicates dissolution or ingassing; negative mass transfer indicates precipitation or outgassing; CH2O, represents organic 

matter; CO2, carbon dioxide; Ca, calcium; Na, sodium; Mg, magnesium; δ13C, delta carbon-13; calc, calculated; obs, observed; Cl, chloride concentration; mg/L, milligrams per liter; R, ROMP; PZ2, permeable zone 2 in the Upper Arcadia Formation; PZ3, permeable zone 3 in the Lower Arcadia Formation; Swnn, Suwannee; --, not in model or less than 0.01 mmol/L; n/a, not applicable; for precipitating phases, 

δ13C or δ34S computed by model based on Rayleigh calculations and fractionation factors defined at the final water (X=1.0)]

Initial 1

(fraction) well

Initial 2

(fraction) well

Initial 3

(fraction) well

Final 

well Calcite Dolo- mite CH2O CO2 Pyrite Ca/Na exchange Mg/Na exchange

δ13C calc

(per mil)1

1 δ13C or inorganic carbon, assuming the following δ13C values for dissolving phases: dolomite = 0 per mil; CH2O = -22 per mil; CO2 = -25 per mil.

δ13C 

obs (per 

mil)

Calc 

age

(years)2

2 Using carbon-14, adjusted using Fontes and Garnier (1979) correction model, assuming carbon-14 activity of carbonate minerals and soil CO2 are 0 and 100 percent modern carbon, respectively, and 

δ13C of carbonate minerals and soil CO2 are 0 and -25 per mil, respectively; age values rounded to nearest 1,000 years.

Cl

calc

(mg/L)

Cl

obs

(mg/L)

(0.12) R26 PZ2 (0.88) R17 PZ3 R9.5 PZ2 -- -- 0.98 -0.41 -0.36 -- 0.42 -9.5 -9.7 n/a 66 64

(0.08) R9.5 PZ2 (0.92) R9 PZ3 n/a R9 PZ2 -0.42 -- 0.68 -0.22 -0.23 -0.51 0.20 -8.0 -9.8 17,000 n/a n/a

(0.07) R9 SAS (0.93) R9 PZ3 n/a R9 PZ2 -0.44 -- 0.63 -0.27 -0.21 -0.45 0.16 -8.6 -9.8 20,000 n/a n/a

(0.44) R13 PZ2 (0.32) R13 SAS (0.24) R5 PZ3 R5 PZ2 0.36 -- 0.36 -0.73 -0.11 0.09 -- -6.2 -7.8 19,000 n/a n/a(0.33) R13 PZ3 (0.44) R17 UFA3

3 Well open to the deepest part of the intermediate aquifer system and the shallowest part of the Upper Floridan aquifer (fig. 5). 

(0.23) R17 

Swnn

R17 PZ3 -0.26 -- -- -0.20 -- -0.33 -0.02 -6.0 -6.4 n/a n/a n/a

(0.86) R13 PZ3 (0.14) R9 PZ3 n/a R9.5 PZ3 -0.46 0.14 0.42 -- -0.11 -0.49 -- -7.6 -5.4 18,000 n/a n/a(0.09) R13 PZ3 (0.91) R9 UFA n/a R9 PZ3 -0.21 0.17 0.81 -1.22 -0.27 -- 0.16 -4.9 -4.4 19,000 n/a n/a
Geochemistry of Aquifer Systems 55

at the ROMP 9 site is upward flow from PZ3 (table 11). 

An alternate scenario for evolution of water at the 

ROMP 9 site had no lateral flow occurring within PZ2 

and a minor fraction (7 percent) of localized recharge 

from the surficial aquifer system mixing with deeper 

water from PZ3 (table 11). Surficial aquifer system 

water is not recharging the intermediate aquifer system 

under current hydraulic conditions, but this recharge 

scenario does indicate that water in PZ2 at the ROMP 

9 site could have evolved in a very localized system, 

isolated from upgradient PZ2 waters. Dominant reactions computed for either scenario include calcite precipitation, CH2O oxidation, CO2 outgassing, and 

pyrite precipitation.

The last flow path modeled in PZ2 was from the 

ROMP 13 site to the ROMP 5 site (fig 27a). Along this 

path, recharge from the surficial aquifer system at the 

ROMP 13 site was considered because this site is in a 

recharge setting (fig. 9). Upward flow from PZ3 at the 

ROMP 5 site also was considered because the head in 

PZ3 is substantially greater than the head in PZ2 at this 

site (figs. 9 and 10). Most major ions increase along 

this path, except for bicarbonate, which decreases 

slightly, and magnesium, which remains the same. A 

plausible model for this flow path had 44 percent of 

water moving laterally, 32 percent recharging from the 

surficial aquifer system, and 24 percent moving 

upward from PZ3 (table 11). This path has less upward 

flow than the other flow paths modeled in PZ2, which 

is consistent with a low leakance value expected 

between PZ2 and PZ3 at the ROMP 5 site. Dominant 

chemical reactions along this path are calcite dissolution (water from surficial aquifer system at ROMP 13 

is undersaturated with calcite), CH2O oxidation, and 

CO2 outgassing.

Lower Arcadia Zone (PZ3)

Two upgradient sites were tested as the initial 

water for lateral flow in the Lower Arcadia zone (PZ3): 

the ROMP 13 and ROMP 5 sites. Water from PZ3 at the 

ROMP 5 site had the highest chloride concentration of 

all waters sampled, and models were not realistic with 

large decreases in chloride along the path (for example, 

substantial amounts of recharge would need to occur 

from the surficial aquifer system, which is contrary to 

head differences). Thus, water from the ROMP 13 site 

was considered to be the upgradient end member for 

lateral flow in PZ3.

The first PZ3 flow path modeled was from the 

ROMP 13 site to the ROMP 17 site (fig. 27b). Mixing 

with water from the Upper Floridan aquifer was considered because heads are greater in the Upper Floridan 

aquifer than in PZ3 at the ROMP 17 site (figs. 9 and 

10). Most major ions increase in concentration in PZ3 

between the ROMP 13 and ROMP 17 sites; sodium and 

bicarbonate concentrations decrease slightly. The composition of water in PZ3 at the ROMP 17 site can be 

modeled by mixing 33 percent of water moving laterally from PZ3 at the ROMP 13 site with 67 percent 

moving upward from the deepest part of the intermediate aquifer system and the Upper Floridan aquifer 

(fig. 5). At the ROMP 17 site, these NETPATH modeling results indicate that mixing with water from the 

Upper Floridan aquifer is important in the evolution of 

water in PZ3. Dominant reactions for this path were 

calcite precipitation and cation exchange (table 11).

The next path modeled in PZ3 was between the 

ROMP 13 and ROMP 9.5 sites (fig. 27b). Mixing with 

water from the Upper Floridan aquifer was considered 

for this path because heads are greater in the Upper 

Floridan aquifer than in PZ3. Major ions increase or are 

similar in concentration along this path. No plausible 

models were computed for this flow path from the 

ROMP 13 site to the ROMP 9.5 site when mixing from 

the Upper Floridan aquifer was considered. Computed 

δ13C values were not close to observed values, and 

some models computed gypsum precipitation, which is 

not likely because waters are undersaturated with 

respect to gypsum. A model with improved δ13C values was computed when considering mixing with 

another lateral source of water from PZ3 (ROMP 9), 

with no mixing from the Upper Floridan aquifer. This 

model has 86 percent of the water coming from PZ3 at 

the ROMP 13 site and 14 percent from PZ3 at the 

ROMP 9 site (table 11). Major reactions include calcite 

precipitation, CH2O oxidation, and cation exchange. 

Although the low sulfate concentration in PZ3 at the 

ROMP 9.5 site seems anomalous, it indicates that this 

zone is isolated chemically from the Upper Floridan 

aquifer, which has a higher sulfate concentration 

(fig. 23). This NETPATH modeling conclusion is contrary to aquifer test results and computed values of leakance, which suggest that a hydraulic connection exists 

between PZ3 and the Upper Floridan aquifer at the 

ROMP 9.5 site. Reasons for this apparent contradiction 

are unresolved and warrant further investigation. It is 

possible that computed leakance from the aquifer test 

could have been overestimated. Alternatively, low 
56 Hydrogeologic Framework and Geochemistry of the Intermediate Aquifer System in Parts of Charlotte, De Soto, and Sarasota 

Counties, Florida

ground-water withdrawals from PZ3 could have 

resulted in water in PZ3 that is chemically isolated 

from the Upper Floridan aquifer. If withdrawals 

increase, leakage from the Upper Floridan aquifer may 

result in degraded water quality in PZ3 (particularly 

increased sulfate concentrations).

The last flow path modeled in PZ3 was between 

the ROMP 13 and ROMP 9 sites (fig. 27b). Mixing 

with water from the Upper Floridan aquifer was again 

considered because heads are greater in the Upper 

Floridan aquifer than in PZ3 at the ROMP 13 site (but 

not presently at the ROMP 9 site). At present, the small 

downward head difference observed at the ROMP 9 

site between PZ3 and the Upper Floridan aquifer can 

not be explained. The chemical evolution of water in 

the intermediate aquifer system and the Upper Floridan 

aquifer at the ROMP 9 site presumably predates 

ground-water pumpage in the area (for example, carbon-14 modeled ages were greater than 10,000 years 

old; table 9). All major ions increase in concentration 

along this path from the ROMP 13 to ROMP 9 sites, 

except for bicarbonate, which decreases in concentration. A plausible model for this path had 9 percent of 

water moving laterally mixing with 91 percent moving 

upward from the Upper Floridan aquifer. This model 

indicates that the evolution of water in PZ3 at the 

ROMP 9 site is dominated by water from the Upper 

Floridan aquifer, and lateral flow is essentially insignificant. Dominant reactions along this path include 

calcite precipitation, CH2O oxidation, CO2 outgassing, 

and pyrite precipitation.

Importance of Geochemical Modeling in 

Assessing Ground-Water Flow

Several important findings about flow patterns in 

the intermediate aquifer system have been corroborated 

by combining geochemical modeling with hydraulic 

head data. Geochemical modeling results suggest that 

vertical flow from underlying aquifers is important in 

the evolution of water in the intermediate aquifer system. Flow patterns derived only from potentiometricsurface maps may be misleading because the maps are 

based on an assumption of a homogenous, isotropic 

system where lateral flow is the dominant groundwater flow component. When vertical flow is incorporated with lateral flow paths, however, a more realistic 

assessment of ground-water flow patterns is possible. 

The study area is primarily an area of discharge from 

the Upper Floridan aquifer to the intermediate aquifer 

system, and upward head differences dominate (fig. 9). 

The potential for downward flow between permeable 

zones and recharge from the surficial aquifer system 

probably occurs in the intermediate aquifer system to 

the north and in the eastern part of the study area 

(ROMP 13, fig. 9). The downward head difference 

from the surficial aquifer system to PZ2 at the ROMP 5 

site is anomalous and cannot be explained at this time. 

Results from NETPATH geochemical modeling 

suggest that recent flow patterns delineated strictly on 

the basis of potentiometric-surface maps do not accurately represent predevelopment ground-water flow 

paths in PZ3. For example, water from the ROMP 5 site 

does not appear to influence the chemical composition 

of water to the northeast, along presumed flow paths. 

Present-day flow paths in PZ3 probably have been 

modified by ground-water withdrawals, particularly in 

the Upper Floridan aquifer to the north and west of the 

study area. The chemical composition of intermediate 

aquifer system waters sampled during this study most 

likely reflect predevelopment conditions. 

Besides mixing, two dominant reactions that 

control the composition of water in the intermediate 

aquifer system are oxidation of organic matter (most 

likely microbially mediated; McMahon and others, 

1990) and cation exchange (and other clay-mineral 

reactions). Calcite precipitation also was consistently 

modeled, but mass transfer of dolomite was minimal. 

Although geochemical modeling indicates areas 

where upward flow strongly influences the chemical 

evolution of ground water in the intermediate aquifer 

system, it does not provide information on the mechanism, location, and timing of this upward flow. For 

example, it is likely that upward flow has occurred as 

slow diffuse flow through confining units, although 

preferential flow through fractures or paleokarst features may locally reduce confinement. In addition, 

areas where large amounts of upward flow are modeled 

may actually indicate that the permeable zone is not 

continuous, thereby limiting lateral flow. An alternative mechanism for mixing deeper ground water into 

shallower zones is flow through wells open to both the 

Upper Floridan aquifer and intermediate aquifer system (Metz and Brendle, 1996). The location of mixing 

along a flow path also is not known, and models are 

inherently limited by availability and distribution of 

wells. 
Summary 57

SUMMARY 

The hydrogeologic framework and the geochemical processes controlling ground-water composition in 

parts of Charlotte, De Soto, and Sarasota Counties, 

Florida, were evaluated. Particular emphasis was given 

to the analysis of hydrogeologic and geochemical data 

for the intermediate aquifer system. 

The geologic framework that forms the aquifer 

systems in the study area consists of undifferentiated 

surficial deposits, the heterogeneous marine deposits 

comprising the Hawthorn Group, and the persistent 

carbonates comprising the Suwannee Limestone, 

Ocala Limestone, and Avon Park Formation. The 

stratigraphic and hydraulic units forming the hydrogeologic framework were delineated using lithologic and 

geophysical logs, water levels, water quality, and 

hydraulic characteristics from five existing Regional 

Observation and Monitor-Well Program (ROMP) sites 

and one new ROMP test site constructed in De Soto 

County during this study. 

The regional ground-water system underlying 

the study area consists of a sequence of aquifers and 

confining units, each containing discrete zones of varying permeabilities. The principal hydrogeologic units 

that underlie the study area are the surficial aquifer 

system, the intermediate aquifer system, and the Upper 

Floridan aquifer. Aquifer heterogeneity results in vertical and areal variability in hydraulic and water-quality 

properties.

The surficial aquifer system is the uppermost 

aquifer and consists of relatively thin, unconsolidated 

sand, shell, and limestone and is unconfined. The thickness of the surficial aquifer system ranges from 19 ft at 

the ROMP 13 site to 69 ft at the ROMP 5 site. Hydraulic properties are variable because of the large range of 

horizontal hydraulic conductivity for the lithologic 

units that make up the aquifer. Transmissivity and 

horizontal hydraulic conductivity range from 752 to 

32,900 ft2

/d and from 33 to1,490 ft/d, respectively.

The intermediate aquifer system is a confined 

system, having as many as three permeable zones. It is 

composed of clastic sediments interbedded with 

carbonate rocks. Interbedded clay and finer grained 

clastics separate the permeable zones. On a regional 

scale, the heterogeneous distribution of carbonate and 

siliciclastic sediments makes delineation of waterproducing zones within the intermediate aquifer system 

difficult to characterize, both in quality and quantity. 

The hydraulic properties of these zones vary depending 

on (1) the original texture of the sediments and (2) postdepositional processes such as dolomitization, recrystallization, fracturing, and dissolution. Based on interpretation from test-hole data, two apparent waterproducing zones were delineated at the ROMP 5 and 

9.5 sites and three were delineated at the ROMP 9, 12, 

13, and 17 sites.

The uppermost permeable zone of the intermediate aquifer system is designated as the Tamiami/Peace 

River zone (PZ1), which is the thinnest and least 

productive zone. The thickness of the Tamiami/Peace 

River zone (where it exists) ranges from 24 ft 

(ROMP 9) to 49 ft (ROMP 12). The reported values 

for transmissivity, horizontal hydraulic conductivity, 

and specific capacity were 47 and 5,420 ft2

/d; 2 and 

102 ft/d; and 2 and 160 gal/min/ft for the ROMP 9 and 

12 sites, respectively. 

The second permeable zone of the intermediate 

aquifer system is designated as the Upper Arcadia zone 

(PZ2). Productivity in this zone is moderate but higher 

than the overlying permeable zone. The thickness of 

the Upper Arcadia zone ranges from 53 ft (ROMP 9) to 

131 ft (ROMP 12). The Upper Arcadia zone is separated from the Tamiami/Peace River zone by a clay bed 

within the Peace River Formation. The clay bed is 

thinner in the northern study area than in the southern 

study area. The clay ranges in thickness from about 20 

to about 60 ft. Hydraulic properties of PZ2 were estimated from aquifer tests conducted at four of the 

ROMP sites in the study area. The reported ranges for 

transmissivity, horizontal hydraulic conductivity, and 

specific capacity were 258 to 24,633 ft2

/d; 2 to 14 ft/d; 

and 1 to 47 gal/min/ft, respectively.

The third and lowermost permeable zone of the 

intermediate aquifer system is designated as the Lower 

Arcadia zone (PZ3), and is typically the most productive zone. The thickness of PZ3 ranges from 57 ft 

(ROMP 13) to 234 ft (ROMP 12). The hydraulic properties of PZ3 are more variable than overlying zones 

and are probably related to the degree of solution 

development within the limestone and dolomite beds. 

Hydraulic properties of PZ3 were estimated from aquifer tests conducted at five of the ROMP sites. The 

reported ranges for transmissivity, horizontal hydraulic 

conductivity, and specific capacity were 766 to 

44,900 ft2

/d; 10 to 201 ft/d; and 3 to 49 gal/min/ft, 

respectively.

The Upper Floridan aquifer is the lowermost 

aquifer included in this study, and consists of a thick, 

stratified sequence of limestone and dolomite. The 

Upper Floridan aquifer is the most productive aquifer 
58 Hydrogeologic Framework and Geochemistry of the Intermediate Aquifer System in Parts of Charlotte, De Soto, and Sarasota 

Counties, Florida

in the study area; however, use of the aquifer is generally restricted because of poor water quality. Generally, 

permeability of the Upper Floridan aquifer is very high 

in parts of the Avon Park Formation, somewhat lower 

in the Suwannee Limestone, and lowest in the Ocala 

Limestone. Hydraulic properties have been estimated 

from aquifer tests conducted at five of the ROMP sites 

in the study area. Aquifer test data are presented for 

wells that only penetrated the Suwannee Limestone. 

The reported values for transmissivity, horizontal 

hydraulic conductivity, and specific capacity were 

2,350 to 7,640 ft2

/d; 10 to 41 ft/d; and 3 to 24 ga/min/ft, 

respectively. The relatively low hydraulic properties 

reported for the Suwannee Limestone indicate that a 

substantial water-production zone was not tested in the 

Upper Floridan aquifer. 

Confining units separating permeable zones and 

aquifers in the study area consist of clays and low 

permeable carbonates. Variation in hydraulic properties of the confining units vary according to lithology 

and thickness. Confining units separating the aquifer 

systems have leakance coefficients estimated to range 

from 2.3 x 10-5 to 5.6 x 10-3 (ft/d/ft). Strata composing 

the confining unit separating the Upper Floridan 

aquifer from the intermediate aquifer system is substantially more permeable than the overlying confining 

units.

Heads within the hydrogeologic units generally 

increase with aquifer depth except in the eastern part of 

the study area. Five of the six ROMP sites are located 

in discharge areas where the flow potential is upward. 

At the ROMP 5 site, an anomalous mixed head difference exists and a downward head was observed 

between the surficial aquifer system and PZ2.

This study used hydrogeologic and geochemical 

information to evaluate the hydraulic connection 

between permeable zones within the intermediate 

aquifer system and between overlying and underlying 

aquifer systems. Knowledge of these connections will 

ultimately help to protect ground-water quality in the 

intermediate aquifer system. Hydraulic connection 

between the hydrogeologic units is variable in the study 

area. The degree of hydraulic connection probably 

depends on the presence and thickness of clay beds 

within the confining units; however, many facies 

changes within the confining units result in local 

hydraulic connection with overlying and underlying 

aquifers. Generally, a hydraulic connection exists 

between the Upper Floridan aquifer and PZ3. Relatively small head differences (up to 5 ft) exist and water 

levels in wells finished in either aquifer respond to 

pumping from above or below. Little hydraulic connection exists between PZ3 and PZ2. Moderate head differences (up to 12 ft) exist and hydraulic separation is 

evidenced by little or no water-level response when one 

or the other zone is pumped. Little hydraulic connection also exists between the surficial aquifer system and 

either PZ1 or PZ2. Relatively larger head differences 

are observed (up to 8 ft) and aquifer testing on the 

zones shows no response when one or the other is 

pumped. 

Water in the surficial aquifer system was chemically more dilute than water in underlying aquifers and 

generally was a calcium bicarbonate type. Water in the 

intermediate aquifer system had a wide range in chemical composition, but generally fit into two categories. 

At the more inland sites, water was a mixed ion or 

mixed cation-bicarbonate type. Sites closer to the coast 

had a sodium-chloride or mixed cation-chloride type 

water. Water within the same permeable zone of the 

intermediate aquifer system did not have a distinct 

chemical composition throughout the study area. Water 

from the Upper Floridan aquifer also was variable in 

the study area and did not have a dominant cation. 

The chemical character of water between PZ3 and the 

Upper Floridan aquifer was quite similar at some sites 

(ROMP 5 and ROMP 9), but was distinctly different at 

other sites (ROMP 9.5). Water from the Upper Floridan 

aquifer had higher excess sulfate concentrations (equation 5) than water from overlying aquifers.

Most water from the surficial aquifer system had 

lighter δD and δ18O values than waters from the intermediate aquifer system or the Upper Floridan aquifer. 

Water from the surficial aquifer system most likely represents a mixture of meteoric water having an isotopically light composition and ground water that has been 

recharged by water that has undergone evaporation and 

may be the result of an enriched isotopic composition. 

Waters from the intermediate aquifer system and the 

Upper Floridan aquifer may be the result of recharge 

that occurred under different climatic conditions than 

those under present conditions. Waters from the three 

aquifer systems have isotopically distinct δ13C signatures of dissolved inorganic carbon, which is related to 

the evolution of inorganic carbon (soil CO2 and reactions with carbonate minerals).

Based on data for 14C of dissolved inorganic 

carbon, all intermediate aquifer system and Upper 

Floridan aquifer waters are probably greater than 

10,000 years old, and many waters are greater than 
References 59

20,000 years old. Thus, both aquifer systems may have 

been recharged under different hydraulic conditions 

than currently observed.

Several important findings about flow patterns in 

the intermediate aquifer system have been supported 

by combining geochemical modeling with hydraulic 

head data. Vertical flow from underlying aquifers is 

important in the evolution of water in the intermediate 

aquifer system. Flow patterns derived only from potentiometric-surface maps may be misleading because the 

maps presume lateral flow paths. Recent flow patterns 

delineated strictly by potentiometric-surface maps also 

do not represent predevelopment ground-water flow 

paths. The chemical composition of intermediate aquifer system waters most likely reflects predevelopment 

conditions. Besides vertical mixing, two dominant processes that control the composition of water in the 

intermediate aquifer system are oxidation of organic 

matter and cation exchange. Areas where geochemical 

models indicate large amounts of upward flow may 

actually indicate discontinuity of the permeable zone, 

thereby limiting lateral flow.

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APPENDICES
64 Hydrogeologic Framework and Geochemistry of the Intermediate Aquifer System in Parts of Charlotte, De Soto, and Sarasota 

Counties, Florida

Appendix A. Depth of geologic formations determined by the Florida Geological Survey (FGS) at the ROMP 5, 9, 9.5, 12, 13, 

and 17 sites 

[ft, feet; lsd, land surface datum; msl, mean sea level; na, not available]

Site

Well

identification

number

Land

surface

datum1

Lithologic unit

Thickness

in ft

Top 

below 

lsd in ft

Bottom 

below

lsd in ft

Top 

above

or below

msl in ft

Bottom

above

or below

msl in ft

ROMP 5 W-16913 40 Undifferentiated Surficial Deposits 69 0 69 40 -29

Peace River Formation 59 69 128 -29 -88

Undifferentiated Arcadia Formation (upper unit) 380 128 508 -88 -468

Nocatee Member 53 508 561 -468 -521

Undifferentiated Arcadia Formation (lower unit) 150 561 711 -521 -671

Suwannee Limestone 278 711 989 -671 -949

Ocala Limestone 281 989 1,270 -949 -1230

Avon Park Formation na 1,270 na -1230 na

ROMP 9 W-17056 25 Undifferentiated Surficial Deposits 28 0 28 25 -3

Peace River Formation 81 28 109 -3 -84

Undifferentiated Arcadia Formation (upper unit) 89 109 198 -84 -173

Tampa Member 119 198 317 -173 -292

Nocatee Member 100 317 417 -292 -392

Undifferentiated Arcadia Formation (lower unit) 128 417 545 -392 -520

Suwannee Limestone 320 545 865 -520 -840

Ocala Limestone 284 865 1,149 -840 -1124

Avon Park Formation na 1,149 na -1124 na

ROMP9.5 W-17597 38 Undifferentiated Surficial Deposits 34 0 34 38 4

Peace River Formation 19 34 53 4 -15

Undifferentiated Arcadia Formation (upper unit) 171 53 224 -15 -186

Tampa Member 174 224 398 -186 -360

Nocatee Member 56 398 454 -360 -416

ROMP 12 W-16578 41 Undifferentiated Surficial Deposits 40 0 40 41 1

Peace River Formation 136 40 176 1 -135

Undifferentiated Arcadia Formation (upper unit) 230 176 406 -135 -365

Nocatee Member 196 406 602 -365 -561

Undifferentiated Arcadia Formation (lower unit) 117 602 719 -561 -678

Suwannee Limestone 186 719 905 -678 -864

Ocala Limestone 227 905 1,132 -864 -1,091

Avon Park Formation 958 1,132 2,090 -1091 -2,049

ROMP 13 W-17396 60 Undifferentiated Surficial Deposits 19 0 19 60 41

Peace River Formation 188 19 207 41 -147

Undifferentiated Arcadia Formation (upper unit) 322 207 529 -147 -469

Nocatee Member 136 529 665 -469 -605

Undifferentiated Arcadia Formation (lower unit) 34 665 699 -605 -639

Suwannee Limestone 78 699 777 -639 -717

Ocala Limestone 284 777 1,061 -717 -1001

Avon Park Formation na 1,061 na -1001 na

ROMP 17 W-15303 22 Undifferentiated Surficial Deposits 26 0 26 22 -4

Peace River Formation 53 26 79 -4 -57

Undifferentiated Arcadia Formation (upper unit) 100 79 179 -57 -157

Tampa Member 113 179 292 -157 -270

Nocatee Member 147 292 439 -270 -417

Suwannee Limestone 401 439 840 -417 -818

Ocala Limestone 275 840 1,115 -818 -1093

Avon Park Formation na 1,115 na -1093 na
Appendix 65Appendix B. Hydrogeologic, lithostratigraphic, and borehole geophysical data collected at the ROMP 5, 9, 9.5, 12, 13, and 17 sites.
66 Hydrogeologic Framework and Geochemistry of the Intermediate Aquifer System in Parts of Charlotte, De Soto, and Sarasota 

Counties, Florida

Appendix B. Hydrogeologic, lithostratigraphic, and borehole geophysical data collected at the ROMP 5, 9, 9.5, 12, 13, and 17 sites.--Continued
Appendix 67Appendix B. Hydrogeologic, lithostratigraphic, and borehole geophysical data collected at the ROMP 5, 9, 9.5, 12, 13, and 17 sites.--Continued
68 Hydrogeologic Framework and Geochemistry of the Intermediate Aquifer System in Parts of Charlotte, De Soto, and Sarasota 

Counties, Florida

Appendix B. Hydrogeologic, lithostratigraphic, and borehole geophysical data collected at the ROMP 5, 9, 9.5, 12, 13, and 17 sites.--Continued
Appendix 69Appendix B. Hydrogeologic, lithostratigraphic, and borehole geophysical data collected at the ROMP 5, 9, 9.5, 12, 13, and 17 sites.--Continued
70 Hydrogeologic Framework and Geochemistry of the Intermediate Aquifer System in Parts of Charlotte, De Soto, and Sarasota 

Counties, Florida

Appendix B. Hydrogeologic, lithostratigraphic, and borehole geophysical data collected at the ROMP 5, 9, 9.5, 12, 13, and 17 sites.--Continued
Appendix 71

Appendix C. Classification criteria used to describe the hydrogeologic units at the ROMP 5, 9, 9.5, 12, 13, and 17 sites 

[SAS, surficial aquifer system; CU, confining unit of the intermediate aquifer system; PZ1, permeable zone 1 in the Tamiami/Peace River 

Formation; PZ2, permeable zone 2 in the Upper Arcadia Formation; PZ3, permeable zone 3 in the Lower Arcadia Formation; UFA, Upper 

Floridan aquifer; BH, borehole data; GP, geophysical data; WQ, water-quality data]

Site Hydrogeologic unit

Depth below 

land surface, 

in feet

Depth above 

(+) or 

below (-) sea 

level, in feet

Criteria

ROMP 5 SAS 0-69 +40-29 Lithology: unconsolidated quartz sand and shell beds with porosity ranging from 

15-50 percent; possibly high permeability from 49-64 ft bls (app. B); GP: gamma 

peak at about 64 ft

CU 69-128 -29-88 Lithology: persistent low permeability clay and phosphatic sediments

PZ2 128-184 -88-144 Lithology: visual observation of permeable sediments from 144 to 159 ft; BH: 

water-level rise below 128 ft; GP: higher formation resistivity from about 128 to 

190 ft

CU 184-433 -144-393 Lithology: persistent low permeability sediments, although some permeable sediments are contained within; BH: decrease in flow below 180 ft

PZ3 433-613 -393-573 BH: water-level rise below 450 ft; end of production on flow log; WQ: rise in 

specific conductance, chloride, and sulfate below 450 ft (fig. 23)

CU 613-711 -573-671 Lithology: persistent low permeability sediments, although some permeable 

sediments are contained within

UFA 711 -671 Lithology: top of Suwannee Limestone picked at 711 ft; 

GP: gamma radiation decreases significantly below 711 ft; 

WQ: increase in chloride concentration below 740 ft 

ROMP 9 SAS 0-28 +25-3 Lithology: unconsolidated, high permeability quartz sand and shell beds; GP: 

gamma peak at 28 ft bls (app. B)

CU 28-40 -3-15 Lithology: persistent low permeability clay and chert sediments

PZ1 40-64 -15-39 Unnamed permeable zone within the Peace River Formation, probably discontinuous; Lithology: bottom of chert bed; top of high permeability limestone and sand 

sediments; BH: water-level rise below 40 ft; WQ: field specific conductance, chloride, and sulfate different than in underlying PZ2 (fig. 23)

CU 64-112 -39-87 Lithology: top of persistent low permeability sediments, although some permeable 

sediments are contained within

PZ2 112-165 -87-140 Lithology: high permeability limestone and sand sediments; BH: water-level rise 

below 119 ft; increase in flow below 120 ft; GP: gamma kick at bottom; WQ: 

increase in field conductance, chloride, and sulfate at 124 ft

CU 165-195 -140-170 Lithology: persistent low permeability carbonate and increase clay content; BH: no 

significant change in water level or flow; GP: low formation resistivity between 

165 and 195 ft; gamma kick at bottom

PZ3 195-320 -170-295 Lithology: interbedded high and low permeability sediments; BH: rise in water 

level below 195 ft; GP: high resistivity between 195 and 320 ft; significant rise in 

flow below 195 ft (may be related to change in borehole diameter)

CU 320-545 -295-520 Lithology: persistent low permeability sediments; BH: no significant change in 

water level; GP: low formation resistivity between 320 and 545 ft

UFA 545 -520 Lithology: top of Suwannee Limestone picked at 545 ft; interbedded high and low 

permeability sediments below 545 ft; BH: rise in flow between 500 and 555 ft ;

GP: gamma radiation decreases below 545 ft
72 Hydrogeologic Framework and Geochemistry of the Intermediate Aquifer System in Parts of Charlotte, De Soto, and Sarasota 

Counties, Florida

ROMP 9.5 SAS 0-34 +38+4 Lithology: unconsolidated quartz sand with porosity ranging from 20 to 30 percent; 

CU 34-68 +4-32 Lithology: low permeability clay and low porosity carbonate sediments; BH: low 

specific capacity; GP: gamma peak at 37 ft (app. B); low formation resistivity from 

37 to 68 ft

PZ2 68-95 -32-59 Lithology: interbedded high and low permeability sediments; 

BH: rise in water level below 74 ft; higher specific capacity than overlying interval; 

GP: high formation resistivity between 68 and 95 ft

CU 95-206 -59-170 Lithology: persistent low permeability sediments;

BH: decline in water level; low specific capacity;

GP: low formation resistivity

PZ3 206-330 -170-294 Lithology: interbedded high and low permeability sediments; 

BH: rise in water level; higher specific capacity than overlying CU; 

GP: high formation resistivity between 206 and 330 ft; 

CU 330-454 -294-418 Lithology: persistent low permeability sediments;

BH: no significant change in water level; lower specific capacity than overlying 

PZ3; GP: low formation resistivity

WQ: increase in chloride and sulfate concentration at top (fig. 23)

UFA 454 -418 Lithology: top of Suwannee Limestone picked at 454 ft; interbedded high and low 

permeability sediments below 454 ft; BH: rise in water level below 458 ft; high 

specific capacity in 493 to 513 ft interval; GP: gamma radiation decreases significantly below 454 ft; rise in borehole water temperature and fluid conductance, and 

decline in fluid resistivity below 475 ft; WQ: rise in field specific conductance, 

hardness, dissolved solids, magnesium, sulfate, and strontium in the 500 to 800 ft 

interval.

ROMP 12 SAS 0-40 +41+1 Lithology: unconsolidated, high to low permeability quartz sand, shell, limestone, 

and phosphate beds (app. B)

CU 40-57 +1-16 Lithology: persistent low permeability sediments

PZ1 57-106 -16-65 Unnamed permeable zone within the Peace River Formation, probably discontinuous; Lithology: interbedded high and low permeability sediments; BH: rise in water 

level at about 70 ft.

CU 106-275 -65-266 Lithology: persistent low permeability sediments;

GP: gamma radiation increases below 125 ft

PZ2 275-406 -266-397 Lithology: interbedded high and low permeability sediments;

GP: gamma radiation decreases at about 275 ft

CU 406-465 -397-456 Lithology: persistent low permeability sediments;

BH: decline in water level at 410 ft;

GP: gamma radiation decreases at about 405 ft

PZ3 465-699 -456-690 Lithology: interbedded high and low permeability sediments;

BH: rise in water level at about 470 ft

CU 699-719 -690-710 Lithology: persistent low permeability sediments;

GP: gamma radiation increases at about 699 ft

UFA 719 -710 Lithology: top of Suwannee Limestone picked at 719 ft; interbedded high and low 

permeability sediments below 719 ft;

GP: small decrease in gamma radiation at 719 ft

Appendix C. Classification criteria used to describe the hydrogeologic units at the ROMP 5, 9, 9.5, 12, 13, and 17 sites 

(Continued)

[SAS, surficial aquifer system; CU, confining unit of the intermediate aquifer system; PZ1, permeable zone 1 in the Tamiami/Peace River 

Formation; PZ2, permeable zone 2 in the Upper Arcadia Formation; PZ3, permeable zone 3 in the Lower Arcadia Formation; UFA, Upper 

Floridan aquifer; BH, borehole data; GP, geophysical data; WQ, water-quality data]

Site Hydrogeologic unit

Depth below 

land surface, 

in feet

Depth above 

(+) or 

below (-) sea 

level, in feet

Criteria
Appendix 73

ROMP 13 SAS 0-19 +62+43 Lithology: unconsolidated quartz sand with porosity ranging between 25 and 35 

percent; relatively permeable beds (ROMP 13 Tech. Pub., 1998, Phase 2, fig 7); 

GP: gamma peak at 19 ft (app. B)

CU 19-99 +43-37 Lithology: relatively impermeable beds (ROMP 13 Tech. Pub., 1998, Phase 2, 

fig. 7); WQ: increase in chloride at about 26 ft (fig. 23)

PZ1 99-111 -37-40 Unnamed permeable zone within the Peace River Formation, probably discontinuous; Lithology: interbedded high and low permeability sediments; GP: increase in 

formation resistivity below 99 ft

CU 111-129 -40-58 Lithology: persistent low permeability sediments; GP: decrease in formation resistivity between 111 and 129 ft

 PZ1 129-144 -58-73 Unnamed permeable zone within the Peace River Formation, probably discontinuous; Lithology: interbedded high and low permeability sediments

CU 144-275 -73-204 Lithology: persistent low permeability sediments; GP: gamma radiation increases 

at about 144 ft

 PZ2 275-390 -204-319 Lithology: relatively permeable beds (ROMP 13 Tech. Pub., 1998, Phase 2, fig. 7); 

BH: decline in water level below 260 ft;

GP: high formation resistivity between 275 and 390 ft

CU 390-505 -319-443 Lithology: relatively impermeable beds (ROMP 13 Tech. Pub., 1998, Phase 2, 

fig. 7); BH: decline in water level below 420 ft;

GP: low formation resistivity; WQ: increase in sulfate at about 420 ft

PZ3 505-562 -443-500 Lithology: relatively permeable beds (ROMP 13 Tech. Pub., 1998, Phase 2, fig. 7); 

GP: high formation resistivity;

WQ: slightly higher chloride and sulfate concentration than in PZ2

CU 562-699 -500-637 Lithology: relatively impermeable beds (ROMP 13 Tech. Pub., 1998, Phase 2, 

fig. 7); GP: low formation resistivity;

WQ: increase in chloride and sulfate concentration at about 680 ft

UFA 699 -637 Lithology: top of Suwannee Limestone picked at 699 ft; interbedded high and low 

permeability sediments below 699 ft;

GP: gamma radiation decreases significantly below 699 ft

Appendix C. Classification criteria used to describe the hydrogeologic units at the ROMP 5, 9, 9.5, 12, 13, and 17 sites 

(Continued)

[SAS, surficial aquifer system; CU, confining unit of the intermediate aquifer system; PZ1, permeable zone 1 in the Tamiami/Peace River 

Formation; PZ2, permeable zone 2 in the Upper Arcadia Formation; PZ3, permeable zone 3 in the Lower Arcadia Formation; UFA, Upper 

Floridan aquifer; BH, borehole data; GP, geophysical data; WQ, water-quality data]

Site Hydrogeologic unit

Depth below 

land surface, 

in feet

Depth above 

(+) or 

below (-) sea 

level, in feet

Criteria
74 Hydrogeologic Framework and Geochemistry of the Intermediate Aquifer System in Parts of Charlotte, De Soto, and Sarasota 

Counties, Florida

ROMP 17 SAS 0-26 +23-3 Lithology: unconsolidated quartz sand with porosity ranging between 10 and 

30 percent; low porosity and low permeability limestone (Tamiami Fm.) from 18 to 

26 ft, not an economical water-bearing unit (Decker, 1988, SWFWMD)

PZ1 26-45 -3-22 Unnamed permeable zone within the Peace River Formation (app.B), probably discontinuous; Lithology: water-bearing beds from 26 to 31 and 35 to 45 ft (Decker, 

1988, SWFWMD)

CU 45-109 -22-86 Lithology: low permeability clay and carbonate sediments (Decker, 1988, 

SWFWMD)

PZ2 109-167 -86-141 Lithology: major water-bearing interval 109-167 ft described by Decker 

(1988);bottom of interval overlies clayey dolostone;

GP: high formation resistivity units within the interval

CU 167-214 -141-191 Lithology: Dolostone with 10-20 percent clay from 167-179 ft;

GP: increase in gamma radiation at about 167 ft; low formation resistivity

PZ3 214-299 -191-276 Lithology: described by Decker (1988) as a major water-bearing interval; previous 

pick of bottom by Duerr (1986); this is the base of the Tampa LS; BH: rise in water 

level below 204 ft;

GP: high formation resistivity between 205 and 250 ft;

WQ: increase in chloride and sulfate concentration at about 275 ft (fig. 23)

CU 299-439 -276-416 Lithology: interval from 363 to 390 ft appears to have confining properties (Decker, 

1988); previous pick by Duerr (1986, and 1991 work notes); BH: decline in water 

level below 335 ft;

GP: low formation resistivity in interval from 335 to 390 ft

UFA 439 -416 Lithology: top of Suwannee Limestone picked at 439 ft (app. B); interbedded high 

and low permeability sediments below 390 ft;

BH: rise in water level below 384 ft; GP: gamma radiation decreases significantly 

below 439 ft; high formation resistivity below 390 ft; WQ: changes in chloride and 

sulfate concentrations at about 600ft (fig. 23)

Appendix C. Classification criteria used to describe the hydrogeologic units at the ROMP 5, 9, 9.5, 12, 13, and 17 sites 

(Continued)

[SAS, surficial aquifer system; CU, confining unit of the intermediate aquifer system; PZ1, permeable zone 1 in the Tamiami/Peace River 

Formation; PZ2, permeable zone 2 in the Upper Arcadia Formation; PZ3, permeable zone 3 in the Lower Arcadia Formation; UFA, Upper 

Floridan aquifer; BH, borehole data; GP, geophysical data; WQ, water-quality data]

Site Hydrogeologic unit

Depth below 

land surface, 

in feet

Depth above 

(+) or 

below (-) sea 

level, in feet

Criteria