Hydrogeology and Results of Tracer 

Tests at the Old Tampa Well Field in 

Hillsborough County, with Implications 

for Wellhead-Protection Strategies in 

West-Central Florida

By James L. Robinson

U.S. Geological Survey

Water-Resources Investigations Report 93-4171

Prepared in cooperation with the

SOUTHWEST FLORIDA WATER MANAGEMENT DISTRICT

Tallahassee, Florida 

1995
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY 

GORDON P. EATON, Director

For additional information Copies of this report can be

write to: purchased from:

District Chief U.S. Geological Survey

U.S. Geological Survey Earth Science Information Center

Suite 3015 Open-File Reports Section

227 N. Bronough Street Box 25286, MS 517

Tallahassee, FL 32301 Denver Federal Center

	Denver, CO 80225
CONTENTS

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

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

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

Description of the Study Site........................................................................................................................................ 2

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

Approach....................................................................................................................................................................... 4

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

Regional Hydrogeologic Setting ............................................................................................................................................ 4

Hydrogeology of the Old Tampa Well Field.......................................................................................................................... 6

WeUNetwork......................................................^ 6

Geophysical Surveys .................................................................................................................................................... 6

Hydrogeologic Units..................................................................................................................................................... 12

Hydraulic Properties of the Upper Floridan Aquifer.................................................................................................... 14

Rock Core Analyses............................................................................................................................................ 15

Aquifer Tests....................................................................................................................................................... 16

Conceptual Hydrogeologic Model......................................................................................................................................... 20

Numerical Model.................................................................................................................................................................... 22

Model Grid and Boundary Conditions ......................................................................................................................... 22

Input Parameters........................................................................................................................................................... 25

Steady-State Calibration............................................................................................................................................... 27

Transient Calibration.................................................................................................................................................... 28

Analysis of'Model Water Budgets................................................................................................................................ 31

Sensitivity Analysis...................................................................................................................................................... 32

Limitations of the Model Analysis............................................................................................................................... 33

Tracer Tests............................................................................................................................................................................ 34

Techniques of Tracer Testing....................................................................................................................................... 34

Preliminary Tests.......................................................................................................................................................... 34

Deep Test...................................................................................................................................................................... 35

Shallow Test................................................................................................................................................................. 37

Analysis of Tests........................................................................................................................................................... 38

Volumetric Displacement Method...................................................................................................................... 39

Curve-Matching Method..................................................................................................................................... 39

Tracer-Dilution Method...................................................................................................................................... 40

Simuation of Tracer Test with Particle-Tracking Program.................................................................................................... 41

Strategies for Wellhead Protection......................................................................................................................................... 43

Alternative Well Construction...................................................................................................................................... 43

Alternative Well-Field Design...................................................................................................................................... 44

Summary and Conclusions..................................................................................................................................................... 46

References Cited..................................................................................................................................................................... 48

Appendix.--Apparent Secondary Porosity Logs from Borehole Video Survey..................................................................... 51

Contents III
FIGURES

1. Map showing location of the old Tampa well field................................................................................................... 3

2. Generalized hydrogeologic framework of Hillsborough County .............................................................................. 5

3. Map showing locations of wells at the old Tampa well field.................................................................................... 7

4. Map showing locations of photolineaments and line of surface geophysical survey at the old Tampa well field.... 9

5. Profile showing ground-penetrating radar record near well 17 and hydrogeologic interpretation............................ 10

6. Diagrams showing electromagnetic geophysical records at the old Tampa well field and hydrogeologic

interpretation.............................................................................................................................................................. 11

7. Geologic section A-A at the old Tampa well field..................................................................................................... 12

8. Diagram showing flowmeter logs for wells at the old Tampa well field, 1990......................................................... 12

9. Diagram showing identification of effective secondary porosity in well 15 A using borehole geophysical

and video logs............................................................................................................................................................. 13

10. Diagram showing effective secondary porosity logs for wells 15, ISA, and the Ocala monitor well and a

conceptual log............................................................................................................................................................ 14

11. Map showing thickness of the intermediate confining unit in the vicinity of the old Tampa well field................... 15

12-15. Graphs showing:

12. Relation between pumping rate and drawdown in the Upper Floridan aquifer at the old Tampa

well field, 1990................................................................................................................................................... 18

13. Distance-drawdown plot for Upper Floridan aquifer wells at the old Tampa well field, 1990,

after 1 minute, 20 minutes, and 48 hours of pumping well 15 at 980 gallons per minute.................................. 18

14. Time-drawdown data for Upper Floridan aquifer wells 19 and 20 at the old Tampa well field, 1990.............. 19

15. Time-drawdown plot for well 15 at the old Tampa well field, 1990.................................................................. 19

16. Diagram showing the hydrogeologic framework at the old Tampa well field.......................................................... 21

17. Diagram showing the hydrogeologic framework of the numerical model of the ground-water flow system

at the old Tampa well field......................................................................................................................................... 23

18. Diagram showing node locations of wells in the numerical model of the old Tampa well field.............................. 24

19. Map showing orientation of the numerical model grid to the local potentiometric surface of the Upper Floridan 

aquifer, July 1990....................................................................................................................................................... 25

20. Graph showing simulated drawdowns in permeable unit two of the Upper Floridan aquifer for the uniform 

transmissivity model and for models using increased and decreased transmissivity in nodes simulating 

a fracture zone............................................................................................................................................................ 29

21. Graph showing measured and model-simulated water-level drawdowns at the old Tampa well field for

aquifer tests of July and December 1990................................................................................................................... 30

22. Diagram of the tracer injection and withdrawal system at the old Tampa well field................................................ 35

23. Graph showing dye-concentration curve for well 15 during the Rhodamine WT dye test ending

November 18, 1990.................................................................................................................................................... 36

24. Diagram showing correlation between effective secondary porosity and tracer concentration in well 11

at the old Tampa well field......................................................................................................................................... 37

25. Graph showing dye-concentration curve for well 15 during the Rhodamine WT dye test ending April 6, 1991..... 38

26. Graph showing analysis of the dye-concentration data for well 15 for the test ending November 18, 1990,

using the Laplace transform solution for radially convergent flow ........................................................................... 39

27. Diagram of the MODPATH particle-tracking simulation of the matrix flow for the dye test ending

November 18, 1990, assuming a uniform effective porosity of 21 percent for the Upper Floridan aquifer.............. 42

28-31. Maps showing simulated area of contribution encompassed:

28. By the 10-year time of travel from permeable unit one to a well completed in permeable units

one and two of the Upper Floridan aquifer......................................................................................................... 44

29. By the 30-year time of travel from permeable unit one to a well completed in permeable units

two and three of the Upper Floridan aquifer....................................................................................................... 44

30. By the 30-year time of travel from permeable unit one to two wells completed in permeable units 

two and three of the Upper Floridan aquifer and aligned 

parallel to the regional ground-water flow ......................................................................................................... 45

31. By the 30-year time of travel from permeable unit one to two wells completed in permeable units 

two and three of the Upper Floridan aquifer and aligned 

perpendicular to the regional ground-water flow ............................................................................................... 45

IV Hydrogeology and Results of Tracer Tasts Implications for Wellhead-Protection Strategies in West-Central Florida
TABLES

1. Construction data for wells at the old Tampa well field............................................................................................ 8

2. Types of borehole geophysical logs available for Upper Floridan aquifer wells at the old Tampa well field.......... 8

3. Reported hydraulic properties of the Upper Floridan aquifer at the old Tampa well field........................................ 15

4. Analyses of rock cores collected from the Upper Floridan aquifer in Hillsborough, Pasco, Pinellas, and

Polk Counties, Florida................................................................................................................................................ 16

5. Total drawdown in selected wells at the old Tampa well field during aquifer tests of July and

December 1990........................................................................................................................................................... 17

6. Input parameters for model of the old Tampa well field............................................................................................ 26

7. Measured and model-simulated drawdowns in the Upper Floridan aquifer at the old Tampa well field for

steady-state calibration of two aquifer tests............................................................................................................... 27

8. Water budgets for the Upper Floridan aquifer for steady-state and transient simulations of 50-hour and

168-hour aquifer tests at the old Tampa well field in June and December 1990....................................................... 31

9. Results of model sensitivity analysis based on simulation of the 50-hour aquifer test.............................................. 33

10. Horizontal hydraulic conductivity estimates based on tracer dilution in the injection well...................................... 41

11. Areas of contribution simulated for four wellhead-protection strategies................................................................... 46

Contents V
CONVERSION FACTORS, VERTICAL DATUM, ACRONYMS, AND ADDITIONAL ABBREVIATIONS

Multiply By To obtain

inch (in.) 25.4 millimeter

foot (ft) 0.3048 meter

foot per hour (ft/h) 0.3048 meter per hour

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

foot per day per foot [(ft/d)/ft] 1.3048 meter per day per meter

mile (mi) 1.609 kilometer

foot squared (ft2 ) 0.09290 meter squared

foot squared per day (ft2/d) 0.09290 meter squared per day

square mile (mi2) 2.590 square kilometer

acre 0.4047 hectare

cubic foot (ft3) 0.02832 cubic meter

cubic foot per day (ft3/d) 0.02832 cubic meter per day

gallon per minute (gal/min) 0.06308 liters per second

million gallons per day (Mgal/d) 3,785 cubic meters per day

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

°C = 5/9 x (°F-32).

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.

Acronyms

EM = electromagnetic 

FDEP = Florida Department of Environmental Protection

GPR = ground-penetrating radar 

ROMP = Regional Observation Monitoring Program 

SWFWMD = Southwest Florida Water Management District

USGS = U.S. Geological Survey 

WCRWSA = West Coast Regional Water Supply Authority

Additional abbreviations

ft/ft = foot per foot 

kHz= kilohertz 

MHz= megahertz

min = minute

ppb = parts per billion

VI Hydrogeology and Results of Tracer Tests Implications for Wellhead-Protection Strategies in West-Central Florida
Hydrogeology and Results of Tracer Tests at the Old Tampa 

Well Field in Hillsborough County, with Implications for 

Wellhead-Protection Strategies in West-Central Florida

By James L. Robinson

Abstract

The Upper Floridan aquifer of west-central 

Florida, composed of a thick sequence of primarily 

Eocene age carbonate rocks, is the primary source 

of public water supply in Hillsborough County, 

Florida. The aquifer is at risk from sources of 

contamination at land surface over much of the 

area, because the sediments that overlie the aquifer 

do not prevent downward movement of water. An 

evaluation of wellhead-protection strategies for 

the Upper Floridan aquifer was conducted using a 

40-acre site in northeastern Hillsborough County 

as a test case. Carbonate rocks were penetrated 

from about 50 feet below land surface to a depth of 

800 feet. Limestone is the predominant rock above 

550 feet and dolomite the predominant rock below 

that depth.

The results of drilling and aquifer testing at the 

site indicate that the Upper Floridan aquifer 

consists of layers of permeable and less permeable 

units. The upper 400 feet of the aquifer responded 

to pumping as an equivalent porous medium for a 

range of discharge rates from 450 to 1,000 gallons 

per minute. Transmissivity and storage coefficient 

values estimated for the upper 400 feet of the 

aquifer were on the order of 23,000 feet squared 

per day and 10"4, respectively. Effective porosity 

values of rock cores collected at the site ranged 

from 21 to 46 percent.

A numerical aquifer-simulation model of the 

Upper Floridan aquifer was constructed and 

calibrated using the results of two aquifer tests

performed at the site. Results of numerical modeling support the proposed hydrogeologic model of 

a layered aquifer with semiconfining units retarding vertical movement of water. These results 

indicate that the transmissivity of the upper 400 

feet of the Upper Floridan aquifer is about 20,000 

feet squared per day, the total transmissivity is on 

the order of 60,000 feet squared per day, and the 

storage coefficient is on the order of 10"4.

Two tracer tests were performed using salt and 

two were performed using a fluorescent dye 

injected into the open-hole interval of a well and 

then measuring the movement of the tracer 

induced by pumping a nearby well at a rate of 980 

gallons per minute. Movement of water spiked 

with salt was monitored with fluid resistivity 

logging. Results of tracer tests using salt indicated 

a direct hydraulic connection between the pumped 

well and the injected well 25 feet away. Fluorescent dye was injected into a well 200 feet from the 

pumped well at depths of 40 to 340 feet below land 

surface. Concentration of the fluorescent dye in the 

water discharged from the pumped well was 

measured continuously with a fluorometer. A dye 

breakthrough occurred about 4 hours after pumping began, and concentration peaked after 15 hours 

of pumping. A second dye breakthrough occurred 

after 36 days, and dye concentration peaked after 

48 days. The bimodal distribution of tracer arrival 

indicates a dual porosity ground-water flow 

system. Analysis of tracer test results indicate an 

effective porosity of 25 percent and a longitudinal 

dispersivity of 1.3 feet for the aquifer matrix.

Abstract 1
The particle-tracking program MODPATH 

was used to simulate the ground-water travel times 

measured during tracer tests. A range of effective 

porosity of 0.3 to 1.5 percent produced a travel 

time of 15 hours, approximating the observed 

15-hour time to peak concentration of the first 

tracer arrival. An effective porosity of 21 percent 

produced a travel time of 49 days, closely approximating the observed time to peak concentration of 

the second tracer arrival.

A wellhead-protection strategy, utilizing the 

less permeable units in the Upper Floridan aquifer 

as a flow barrier between sources of contamination 

at land surface and the pumped zone of the aquifer, 

was evaluated. The particle-tracking program 

MODPATH was used to delineate the areas of 

contribution from the aquifer matrix for hypothetical well and well-field designs. Results of 

model runs indicate that limiting withdrawal of 

water to permeable units below less permeable 

units, pumping at a rate of less than 980 gallons 

per minute, and the orientation of multiple wells in 

a line perpendicular to the local ground-water 

gradient reduced the total area of contribution 

from overlying layers to the pumped well(s), 

thereby reducing the potential for contamination 

from sources at land surface.

The results of this study demonstrate the 

heterogeneity of the Upper Floridan aquifer. An 

effective porosity value too low to be representative of the matrix of the Upper Floridan aquifer 

was necessary for the equivalent porous media 

model to simulate the first arrival of a dye tracer, 

supporting the hypothesis of ground-water flow 

through secondary porosity. However, the use of 

uniform porosity models and particle-tracking to 

simulate the results of tracer tests successfully 

simulated ground-water flow through the aquifer 

matrix. Vertical movement of water in the Upper 

Floridan aquifer was dominated by matrix flow.

INTRODUCTION

The principal source of municipal water supply in 

west-central Florida is the Upper Floridan aquifer. In 

northern Hillsborough and Pasco Counties, the top of 

the aquifer is about 50 ft below land surface. The aquifer

is at risk from sources of contamination at land surface 

because the sediments that overlie the aquifer do not 

prevent downward movement of water (Swancar and 

Hutchinson, 1992). The Florida Department of Environmental Protection (FDEP) has enacted regulations 

that limit the amount and type of land use near well 

fields to protect public water supplies. The FDEP has 

not mandated a methodology for delineation of 

wellhead-protection zones; that responsibility has been 

delegated to the counties.

The U.S. Geological Survey (USGS), in cooperation with the Southwest Florida Water Management 

District (SWFWMD), initiated a study in 1989 to 

collect the hydrogeologic data needed to evaluate 

ground-water flow near wells pumping from the Upper 

Floridan aquifer. The data were used to evaluate 

wellhead-protection strategies for west-central Florida. 

The study site is the old Tampa well field, an area of 

eastern Hillsborough County in which hydrogeologic 

conditions are representative of those in west-central 

Florida. The old Tampa well field is inactive and has 

several wells completed in the Upper Floridan aquifer.

Purpose and Scope

This report presents the results of a study to 

evaluate the hydrogeology of a carbonate aquifer 

system in west-central Florida using photolineament 

analysis, geophysical surveys, test drilling, rock-core 

analyses, aquifer tests, and tracer tests. The type and 

distribution of secondary porosity in the Upper 

Floridan aquifer at the test site is described in this 

report. Areas of contribution to hypothetical wells open 

to the Upper Floridan aquifer are delineated using a 

calibrated, steady-state, numerical, aquifer-simulation 

model of the ground-water flow system at the old 

Tampa well field and a particle-tracking program. 

Results of numerical modeling and particle-tracking 

are used to evaluate wellhead-protection strategies for 

west-central Florida.

Description of the Study Site

The old Tampa well field is a 40-acre pasture 14 mi 

east of the city of Tampa in northeastern Hillsborough 

County (fig. 1). The major drainage feature at the study 

site is Pemberton Creek, which flows west-northwest 

along the southern boundary of the site and drains into 

Lake Thonotosassa about 4 mi from the site. Land 

surface ranges from about 67 to 75 ft above sea level.

Hydrogeology and Results of Tracer Tests Implications for Wellhead-Protection Strategies in West-Central Florida
82° 45 

28° 15

45 r

27° 30

HILLSBOROUGH COUNTY

28° -JS

Figure 1. Location of the old Tampa well field.

Water levels in wells open to the Upper Floridan 

aquifer at the site ranged from 47 to 72 ft above sea 

level for the period from 1958 to 1992. Water levels 

typically fluctuated about 10 ft between the wet and dry 

seasons. The site was evaluated for possible use as a 

well field by the City of Tampa, who owns the property, 

but was never used.

Rainfall in Hillsborough County usually occurs 

between May and October, and a pronounced dry 

season occurs between October and May. Average 

annual rainfall is about 50.8 in., but has varied from 

32.0 to 76.6 in. since 1900 (Southwest Florida Water 

Management District, 1988). Ground-water use in 

northeastern Hillsborough County varies seasonally; 

the greatest demand is for irrigation of strawberries 

during the dry season. Intense pumping also occurs 

during short periods of freezing temperatures to protect 

crops from frost damage.

Previous Investigations

Results of previous investigations of the study area 

are documented in reports published by the USGS, the 

SWFWMD, the Florida Geological Survey, and publications in technical journals. Reports of investigations 

by the USGS that present hydrogeologic data for the 

study area include those by Sinclair (1973; 1982), 

Ryder and others (1980), Wolansky and Corral (1985), 

and Miller (1986). The USGS reports that present data 

on ground-water tracer studies include reports by Reeder 

and others (1976) and Mull and others (1988). Vecchioli 

and others (1989) presented an evaluation of the methodology for delineation of wellhead-protection zones 

in west-central Florida. McDonald and Harbaugh 

(1988) documented the USGS modular ground-water 

flow model (MODFLOW), and Pollock (1989) 

documented a particle-tracking program (MODPATH)

Introduction
that uses the output of MODFLOW to calculate 

ground-water pathlines and time of travel.

Publications of the SWFWMD that present hydrogeologic data for the site include reports by Gilboy 

(1985), Jones (1985), Bengtsson and others (1986), and 

a ground-water resource availability inventory for 

Hillsborough County (Southwest Florida Water Management District, 1988). Bengtsson and others (1986) 

developed a numerical aquifer-simulation model of the 

Upper Floridan aquifer for the general vicinity of the 

study site.

Several FGS reports include information about 

the test site. Menke and others (1961) described the 

hydrology of Hillsborough County and presented geologic data, well-construction records, and aquifer-test 

data for the old Tampa well field. Scott (1988) 

redefined the Miocene sediments of Florida in that the 

Hawthorn Formation and Tampa Limestone of previous works were assigned group and member status, 

respectively. Andrews (1990) presented a map of the 

transmissivity and well yields of the Upper Floridan 

aquifer in Florida.

Recent papers in technical journals have presented 

the concept of ground-water flow in doubly porous 

aquifers, the characteristics of solute transport in 

aquifers, and the use of particle-tracking programs to 

delineate areas of wellhead-protection. Hickey (1984) 

described a simple test to check the validity of the 

assumption of Darcian flow in an aquifer. Smith and 

Vaughan (1985) presented the results of aquifer withdrawal tests in nonradial flow regimes. Gordon (1986) 

discussed the dependency of effective porosity on 

fracture continuity. Moench (1989) presented a 

Laplace transform solution for aquifer tracer testing 

that can be used to determine longitudinal dispersivity 

and effective porosity of a uniformly porous aquifer. 

Sabatini and Austin (1991) discussed the adsorption 

characteristics of fluorescent dyes used as groundwater tracers. Bair and others (1991) and Buxton and 

others (1991) presented the results of particle-tracking 

analyses of ground-water flow.

Approach

The study was organized into four phases of 

investigation: (1) surface and borehole geophysical 

surveys, (2) exploratory drilling and aquifer testing, 

(3) numerical modeling, and (4) tracer testing and 

particle tracking. Ground-penetrating radar (GPR) and 

electromagnetic (EM) surface geophysical surveys

were performed to detect subsurface cavities, fracture 

zones, and other karst features in the Upper Floridan 

aquifer. Borehole geophysical surveys were performed 

in the Upper Floridan aquifer wells to correlate hydrogeologic units across the site. Ten shallow wells were 

installed to determine the thickness and composition of 

the unconsolidated surficial sediments. Analyses of 

rock cores from the Upper Floridan aquifer and the 

results of aquifer tests and tracer tests were used to 

estimate the hydraulic properties of the Upper Floridan 

aquifer. A numerical aquifer-simulation model of the 

site was calibrated using the results of the aquifer withdrawal tests. Water movement in the Upper Floridan 

aquifer was measured using fluorescent dye and table 

salt as tracers. A particle-tracking program was used to 

simulate ground-water travel times measured with tracers and to estimate ground-water travel times and flow 

path in the matrix of the karstic Upper Floridan aquifer 

for different combinations of pumped well(s), withdrawal rates, pumped interval within the aquifer, and 

well placement. The results of the particle-tracking 

simulations were used to evaluate wellhead-protection 

strategies for west-central Florida.

Acknowledgments

The author thanks the City of Tampa for allowing 

access to the site, the West Coast Regional Water 

Supply Authority, and Schreuder and Davis, Inc., for 

supplying geophysical logs of Upper Floridan aquifer 

wells at the old Tampa well field, drilling a 400-ft deep 

test well, and providing samples of rock cores collected 

from the Upper Floridan aquifer. The author also 

thanks Mark Stewart, Department of Geology, University of South Florida; Warren Hogg, West Coast 

Regional Water Supply Authority; and Cathleen 

Beaudoin and Glen Rubis, Schreuder and Davis, Inc., 

for their suggestions concerning the hydrogeology of 

the old Tampa well field.

REGIONAL HYDROGEOLOGIC SETTING

The lithology, structure, and geologic history of the 

sediments underlying Hillsborough County control the 

movement of ground water in the area. The geology of 

southwest Florida is known from studies of formations 

exposed at land surface, from examination of drill 

cuttings and cores, and from borehole surveys. Hillsborough County lies within the Florida Plateau, a structurally stable, partially submerged, carbonate platform

4 Hydrogeology and Results of Tracer Tests Implications for Wellhead-Protection Strategies in West-Central Florida
overlain by deposits of Tertiary and Cretaceous age 

having a gentle homoclinal dip to the south-southwest 

(Southwest Florida Water Management District, 1988). 

The thickness of the sediments underlying Hillsborough 

County ranges from less than 8,000 ft in the northeastern part of the county to more than 13,000 ft in the 

southwestern part (Applin, 1951). These sediments 

were deposited on rhyolite and volcanic agglomerates 

(Applin, 1951) and range in age from Late Cretaceous 

to Holocene. These sediments are subdivided into three 

facies: (1) unconsolidated sand, clay, and marl of 

middle Miocene and younger age; (2) limestone and

dolomites of early Miocene to late middle Eocene age; 

and (3) gypsiferous limestone and dolomites of early 

middle Eocene and older age. Facies 2 encompasses 

the Upper Floridan aquifer, and facies 1 and 3 constitute the overlying and underlying confining units and 

aquifers, respectively.

The ground-water system in Hillsborough County 

(fig. 2) consists of three distinct hydrogeologic units: 

(1) the surficial aquifer system; (2) the intermediate 

confining unit; and (3) the Floridan aquifer system 

(Southeastern Geological Society, 1986). The surficial 

aquifer system is the permeable hydrogeologic unit

Series

Holocene 

and 

Pleistocene

Pliocene

Miocene

Oligocene

Eocene

Paleocene

Stratigraphic 

unit 1

Surficial sands

Bone Valley 

Formation

Hawthorn 

Group

Tampa 

Member

Suwannee 

Limestone

Ocala 

Limestone

Avon Park 

Formation

Oldsmar 

Formation

Cedar Keys 

Formation

General 

lithology

Sand and beach 

deposits

Sand and clay

Phosphatic 

sand, clay, 

and marl

Limestone, 

sandy, clay

Limestone, 

fossiliferous

Limestone, 

chalky, 

foraminiferal

Limestone, 

dolomite, 

gypsum

Dolomite, 

limestone, 

gypsum

Dolomite, 

limestone, 

gypsum, and 

anhydrite

Hydrogeo]j>gic unit''

Surficial aquifer 

system

Intermediate 

confining unit

F 

L 

0 

R 

I 

D 

A 

N

A 

Q 

U 

I 

F 

E 

R

S 

Y 

S

T 

E 

M

Upper 

Floridan 

aquifer

Middle 

confining 

unit

Lower 

Floridan 

aquifer

Sub-Floridan 

confining unit

Approximate 

thickness at 

study site 3 

(feet)

10-25

10-40

1,100

400

1,800

4 ?

 Based on nomenclature of Miller (1986). 

2 Based on nomenclature of Southeastern Geological Society (1986).

3 Thickness at study site estimated from data collected during onsite drilling 

and from Miller (1986),

Total thickness of the sub-Floridan confining unit beneath Hillsborough County 

is not known.

Figure 2. Generalized hydrogeologic framework of Hillsborough County.

Regional Hydrogeologic Setting 5
exposed at land surface and includes unconsolidated to 

poorly indurated clastic sediments; it also can include 

indurated carbonate rocks other than those of the 

Floridan aquifer system. The intermediate confining 

unit includes interbedded sand, clay, and marl that lie 

between, and collectively retard the exchange of water 

between, the overlying surficial aquifer system and the 

underlying Floridan aquifer system. The intermediate 

confining unit thickens in southern Hillsborough 

County and permeable beds within it locally constitute 

aquifers (Vecchioli and others, 1989). The Floridan 

aquifer system includes all or part of the Paleocene to 

early Miocene Series. The top of the aquifer system 

generally coincides with the absence of elastics and 

with the top of the vertically persistent permeable 

carbonate section. In peninsular Florida, the base of the 

Floridan aquifer system coincides with anhydrite beds 

that lie near the top of the Cedar Keys Formation.

The Floridan aquifer system is divided into the 

Upper and Lower Floridan aquifers in southwestern 

Florida by a middle confining unit of low permeability 

(Miller, 1986). The Upper Floridan aquifer can be 

further subdivided based on permeability. Two distinct 

water-bearing zones have been described by the Southwest Florida Water Management District (1988). A 

shallow permeable zone in the aquifer is near land 

surface in northern Hillsborough County and is about 

200 ft deep in the southern part of the county. The most 

permeable layers within this shallow zone occur at the 

contacts between the Tampa Member and the Suwannee Limestone and the contact between the Suwannee 

Limestone and the Ocala Limestone (Southwest Florida Water Management District, 1988). The middle to 

lower part of the Ocala Limestone acts as a semiconfining unit between the upper and lower permeable zones. 

The deep, permeable zone of the Upper Floridan aquifer consists of fractured dolomite within the Avon Park 

Formation and ranges in depth from about 500 to 

1,200 ft below land surface from northern to southern 

Hillsborough County (Southwest Florida Water 

Management District, 1988).

HYDROGEOLOGY OF THE OLD TAMPA 

WELL FIELD

The hydrogeology of the ground-water flow 

system at the old Tampa well field was determined by 

test drilling, surface and borehole geophysical surveys, 

analysis of rock cores, and aquifer tests. Surface 

geophysical surveys were used to investigate the

hydrogeologic significance of photolineaments at the 

old Tampa well field. Borehole geophysical surveys 

were used to determine the type and distribution of 

porosity and permeability in the Upper Floridan aquifer 

and to correlate geologic and hydrogeologic units 

between wells. The hydraulic coefficients of the Upper 

Floridan aquifer were estimated using rock-core 

analyses and aquifer tests.

Well Network

Twenty-three wells have been installed at the old 

Tampa well field (fig. 3), 13 of which are completed in 

the Upper Floridan aquifer (table 1). Seven 4-in.-diameter and two 16-in.-diameter wells were drilled in 1957 

by the City of Tampa in an attempt to locate a water 

supply. These wells were completed with casing set 

from 39 to 74 ft below land surface and ranged in depth 

from 250 to 800 ft. Geophysical logging in 1990 indicated that the borehole in well 23 was blocked at 92 ft 

below land surface, and well 22 was blocked at 412 ft 

below land surface. Subsequent soundings with a 

weighted line, however, indicated that both boreholes 

were open to the reported depths of 256 and 800 ft, 

respectively. The West Coast Regional Water Supply 

Authority (WCRWSA) initiated the drilling of four 

Upper Floridan aquifer wells at the site in March 1990: 

(1) well 15 A; (2) the Ocala monitor well (OMW); 

(3) the Avon Park monitor well (APMW); and (4) the 

Avon Park production well (APPW). The WCRWSA 

also modified well 22 by reaming it to 800 ft and setting 

additional casing to 480 ft below land surface. All 

Upper Floridan aquifer wells at the site are completed 

with open-hole construction. A trailer-mounted power 

auger rig was used during this study to install ten 

1.25-in.-diameter wells (table 1) in the unconsolidated 

sediments. Four wells were completed in the surficial 

aquifer system and six wells were completed in the 

intermediate confining unit.

Geophysical Surveys

Surface and borehole geophysical methods were 

used to collect geologic and hydrologic information 

about the subsurface at the old Tampa well field. 

Surface geophysical methods included GPR and EM 

surveys. Borehole geophysical logs were run in all 

accessible wells (table 2), and borehole video tapes 

were made of six of the Upper Floridan aquifer wells at 

the site (appendix).

6 Hydrogeology and Results of Tracer Tests Implications for Wellhead-Protection Strategies in West-Central Florida
s s

23 Q 

15A0

BO 

OMW 

APPW 

APMW

EXPLANATION

LINE OF GEOLOGIC SECTION

LINE OF GROUND-PENETRATING RADAR PROFILE

WELL COMPLETED IN INTERMEDIATE CONFINING UNIT

WELL COMPLETED IN UPPER FLORIDAN AQUIFER

WELL COMPLETED IN SURFICIAL AQUIFER SYSTEM

OCALA MONITOR WELL

AVON PARK PRODUCTION WELL

AVON PARK MONITOR WELL 23d*

STAFFORD

13

..21

APMW

ROAD

22 r

CREEK

500 FEET

SO 100 METERS

Figure 3. Locations of wells at the old Tampa well field.

The GPR surveys used the reflection of a 10- to 

1,000-MHz energy pulse to map subsurface layers. The 

depth of investigation is generally less than 100 ft and 

is limited by many factors, including signal frequency 

and lithology. The EM surveys used an induced EM 

field to measure contrasts in the electrical conductance 

of subsurface material and respond to good conductors 

at shallow depths. The depth of investigation of an EM 

survey is limited by instrument power, interferences 

such as power lines and underground cables, and attenuation of the signal by good electrical conductors at 

shallow depths. The maximum depth of investigation 

varies with instrument and power source, but usually is 

less than 300 ft with portable hand units. Very low 

frequency EM techniques that use 5- to 25-kHz radio 

transmissions to induce the EM field, as well as high 

frequency techniques that use a paired transmitterreceiver coil system to induce the EM field, were used 

to perform geophysical surveys at the site.

Surface geophysical surveys were used to study the 

hydrogeologic significance of photolinear traces at the 

test site. Four photolineaments in the vicinity of the old 

Tampa well field (fig. 4) were mapped by Mark T. 

Stewart (University of South Florida, written commun., 1990). Photolineaments have been linked to 

fracture zones in the Upper Floridan aquifer in Florida 

by Vernon (1951) and to faults by Culbreth (1988). 

Fracture zones and faults are believed to influence 

ground-water movement, stream orientation, sinkhole 

development, and water quality.

Profiling of GPR across photolinear traces A, B, C, 

and D did not detect any change in the subsurface 

structure near the photolineaments. It is possible that 

the source of the photolineaments lies below the depth 

of penetration of the radar signal. The GPR record in 

the vicinity of shallow well 17 (fig. 3), however, 

indicated sloping reflectors. These are interpreted as 

sediments draped over a collapse feature (fig. 5). This

Hydrogeology of the Old Tampa Well Field
Table 1 . Construction data for wells at the old Tampa well field

[Depth is well depth below land surface; casing is casing depth below land surface. LSD, altitude of land surface, in feet 

above sea level, rounded to the nearest foot; OMW, Ocala monitor well; APMW, Avon Park Monitor well; APPW, Avon 

Park production well]

Surficial aquifer system

Well 

number 

(fig. 3)

15

17

A

B

Depth Casing . __ 

(feet) (feet

11.2 1.2 69

17.0 7.0 70

15.2 5.2 67

9.0 0.0 72

Intermediate confining unit

Well 

number 

(fig- 3)

11

16

19

20

22

23

Depth 

(feet)

26.5

31.0

23.6

30.0

40.0

20.5

Casing 

(feet)

6.5

11.0

3.6

0.0

20.0

10.5

LSD

70

74

72

70

74

71

Upper Floridan aquifer

Well 

number 

(fig. 3)

11

13

15

15A

16

19

20

21

22

23

OMW

APMW

APPW

Depth 

(feet)

'750

345

415

410

250

400

2400

256

800

3 256

450

800

800

Casing 

(feet)

74

64

67

58

50

78

39

92

480

86

250

565

550

LSD

70

68

70

70

74

72

72

74

73

72

75

75

75

'Well plug set at 422 feet below land surface for tracer tests.

Blocked at 340 feet below land surface; well plug set at 90 feet below land surface for shallow tracer test. 

Blocked at 92 feet below land surface.

Table 2. Types of borehole geophysical logs available for Upper Floridan aquifer wells at the old Tampa well field 

[X, logged under nonpumping conditions; P, logged under pumping conditions; OMW, Ocala monitor well; APMW, Avon Park monitor 

well; APPW, Avon Park production well]

Well 

numbar 

(fig. 3)

11

13

15

15A

16

19

20

22

23

OMW

APMW

APPW

Natural 

gamma

X

X

X

X

X

X

X

X

X

X

X

Caiiper

X

X

X

X

X

X

X

X

X

X

Flow

X,P

X,P

X,P

p

p

X,P

X,P

Electric 

log

X

X

X

X

X

X

X

X

Resistivity

Focused 

(guard)

X

X

X

X

Fluid

X,P

X

X,P

X,P

X

X

p

X,P

X,P

X,P

Acoustic 

velocity

X

X

X

X

Temperature

X,P

X

X,P

X,P

X

X

X

p

X,P

X,P

X,P

SI, - VUeo "\ .. porosity

X XX

X X X,P

X

X XX

X

X

8 Hydrogeology and Results of Tracer Tests implications for Wellhead-Protection Strategies in West-Central Florida
EXPLANATION

LINE OF GEOPHYSICAL SURVEY

PHOTOLINEAR TRACE 

UPPER FLORIDAN AQUIFER WELL 

SURFICIAL AQUIFER SYSTEM WELL 

OCALA MONITOR WELL

AVON PARK PRODUCTION 

WELL

AVON PARK MONITOR WELL

50 100 METERS

Figure 4. Locations of photolineaments and line of surface geophysical survey at the old Tampa well field.

interpretation is supported by a test boring at the 

location of shallow well 17. The boring reached a depth 

of 310 ft without penetrating limestone (Menke and 

others, 1961). The GPR profile shows that the surface 

of the water table is not depressed at the probable sinkhole, indicating poor hydraulic connection between the 

surficial aquifer system and the underlying Upper 

Floridan aquifer.

An alternating sequence of conductive and resistive 

anomalies was indicated by EM surveys across the 

traces of photolineaments A and B, north-northwest of 

well 15 (fig. 6). A zone of limestone pinnacles and 

troughs is one hydrogeologic interpretation of the 

anomalies (Mark T. Stewart, University of South 

Florida, written commun., 1990). The photolineament 

northwest of the well field is interpreted as the surface 

expression of a fracture zone. Wells completed within

a fracture zone are likely to respond to pumping in a 

non-Theis manner. No EM surveys of photolineaments 

C and D were performed.

Borehole geophysical and video logs were used to 

correlate geologic and hydrogeologic units between the 

Upper Floridan aquifer wells. The natural gamma, 

focused resistivity, and spontaneous potential logs and 

the lithologic descriptions of the material penetrated 

during drilling were used to determine formation 

boundaries based on the criteria described by Gilboy 

(1985). A geologic section (fig. 7) shows that the strata 

under the study site are nearly horizontal. The formations include the Tampa Member of the Hawthorn 

Group, the Suwannee Limestone, the Ocala Limestone, 

and the Avon Park Formation. Miocene and Pliocene 

sediments younger than the Tampa Member are not 

differentiated.

Hydrogeology of the Old Tsmpa Well Field
111

Sigaffl rip20

13=S 2S

Il2|30

8--^

<|^3S

Cu'-'z40

CD <

45

GROUND-PENETRATING RADAR PROFILE

LiCC

0 

S

10

15

ii- 20 p"

fij 30

t 

LU 

Q

35

40

45

WATER TABLE x

4YDROGEOLOGIC INTERPRETATION 

0 50 FEET

15 METERS

Figure 5. Ground-penetrating radar record near well 17 

(fig. 3) and hydrogeologic interpretation.

Flowmeter and fluid resistivity logs were used to 

determine the depth intervals at which water enters 

wells and to define the distribution of permeable and 

semiconfining units in the Upper Floridan aquifer. 

Wells were pumped at rates of 300 to 1,000 gal/min 

during flowmeter logging (fig. 8). The flowmeter logs 

were normalized to compensate for borehole diameter. 

A zone of extensive borehole caving from 500 to 300 ft 

below land surface was penetrated during the drilling 

of the Avon Park monitor well (fig. 8). The flowmeter 

log of this well could be inaccurate in this zone because 

the borehole diameter measurements are estimated.

This is the probable cause of the loss of flow indicated 

in this zone by the log for the Avon Park monitor well 

(fig. 8). The flowmeter log for well 15 indicates that 

about 50 percent of the water entering the well originates from the depth interval of 410 to 380 ft below 

land surface, and about 45 percent of the water entering 

the well originates from the depth interval of 250 to 

70 ft below land surface (fig. 8). The flowmeter log of 

well 15A indicates that about 25 percent of the water 

entering the well originates from the depth interval of 

400 to 380 ft below land surface, and about 70 percent 

of the water entering the well originates from the depth 

interval of 250 to 70 ft below land surface. Additional 

flowmeter surveys in the Ocala monitor well and the 

Avon Park production well indicate that the strata from 

425 to 550 ft below land surface does not yield significant quantities of water to the wells, but the interval 

from 550 to 800 ft below land surface does yield 

significant quantities (fig. 8).

The vertical distribution of effective secondary 

porosity in open boreholes at the study site was determined using techniques described by Safko and Hickey 

(1992). Apparent secondary porosity for a specified 

depth, identified using video logs (appendix), is considered to be effective if water enters the well at that depth. 

The identification of effective secondary porosity in 

well 15A using borehole geophysical logs and video 

logs is illustrated in figure 9. The effective secondary 

porosity logs for wells 15, 15A, and the Ocala monitor 

well were compared, and a conceptual log (fig. 10) was 

prepared to depict the approximate vertical distribution, and type, of effective secondary porosity, which 

has significant lateral extent within the upper 400 ft of 

the Upper Floridan aquifer. Additional zones of effective secondary porosity probably exist at depths greater 

than those shown in figure 10, but there is insufficient 

data from the study site to perform an analysis for these 

depths.

The descriptive terminologies used for porosity 

features are "fracture" and "vug" porosity. Fracture 

porosity is used in this report to describe cracks in the 

rocks that are caused by tectonic deformation. Vug 

porosity is used to describe pores that are large enough 

to be seen in a borehole television survey, and the term 

applies to both nonfabric selective porosity and to 

fabric selective porosity, such as moldic porosity.

The data indicate that the Upper Floridan aquifer at 

the old Tampa well field is a layered aquifer system 

with at least three permeable units. The first two permeable units constitute the shallow permeable zone and

10 Hydrogeology and Results of Tracer Tests Implications for Wellhead-Protection Strategies in West-Central Florida
WELL 15

DC 

111 I111 CO

52

h- CO

t cc to

LLJ

g

2

EM-34 SURVEY: 20 m HORIZONTAL COILS

2,500 FEET

40

20

PHOTOLINEAR 

TRACES

DEEPER BEDROCK PINNACLE 

AND TROUGH STRUCTURE

SHALLOW ROCK 

OR SAND COLUMN

VERY LOW FREQUENCY SURVEY

CO

LLJ 

LLJ 

DC 

O 

LLJ 

Q

2,500

20

FEET

-20

PHOTOLINEAR 

TRACES

^TERNATING CONDUCTIVITY 

AND RESISTIVE ANOMALIES

TILT ANGLE

POSSIBLE HYDROGEOLOGIC INTERPRETATION

I

111

m

LLJ 

111 

LL.

z

LU

111 UL

2,500 FEET

100-130 -

Figure 6. Electromagnetic geophysical records at the old Tampa well field and hydrogeologic 

interpretation. (See fig. 4 for location of section C-C.)

Hydrogeology of the Old Tampa Well Field 11
UNDIFFERENTIATED SURFICIAL 

SEDIMENTS

LAND 

SURFACE

200 

UNDIFFERENTIATED MIOCENE 

" AND PLIOCENE SEDIMENTS -

TAMPA MEMSER (2)

SUWANNEE LIMESTONE

OCALA LIMESTONE

AVON PARK FORMATION

0_____100 FEET 

0 25 METERS 

VERTICAL EXAGGERATION X1.7

(1) AVON PARK PRODUCTION WELL

(2) NOMENCLATURE FROM SCOTT (1988)

Figure 7. Geologic section A-A' at the old Tampa well field. 

(See fig. 3 for location of section.)

the third permeable unit corresponds to the deep 

permeable zone (Southwest Florida Water Management District, 1988). In this report, permeable units 

one, two, and three refer to the water-producing zones 

from about the top of the limestone to 250, 380 to 410, 

and 550 to 800 ft below land surface, respectively, that 

were identified using flowmeter and fluid resistivity 

logs. Figure 10 indicates that zones of effective secondary porosity with significant lateral extent are layered 

throughout the upper 400 ft of the Upper Floridan aquifer. Permeable units have little relation to formation 

boundaries. Therefore, the aquifer system is correctly 

conceptualized using hydrogeologic boundaries rather 

than formation boundaries.

Hydrogeologic Units

The surficial aquifer system at the old Tampa well 

field consists of fine to medium grained quartz sand 

that grades downward to marl and clay of the intermediate confining unit. Quartz sandstone cemented with 

clay also is present. Thickness of the surficial aquifer 

system ranges from less than 50 to more than 100 ft and 

can be highly variable over short distances because of

wiE5LL wfiu "fit °r,]r W2E2LL A (")w A <s7

HI 

HI

UJ

U- 300 - 

DC

W

5

O 500-

LU 

CO

Q. 

UJ 

Q

CASING CASING

0 50 100

PERCENT 

FLOW

(1) OCALA MONITOR WELL

(2) AVON PARK MONITOR WELL

(3) AVON PARK PRODUCTION WELL

0 SO 1000 SO 1000 50 10

PERCENT PERCENT PERCENT

FLOW FLOW FLOW

Figure 8. Flowmeter logs for wells at the Old Tampa well field, 1990.

12 Hydrogeology and Results of Tracer Tests Implications for Wellhead-Protection Strategies in West-Central Florida
LU

2 CC LLJ

O O ^

[I 0. O

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UJ O 5 

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Hydrogeology of the Old Tampa Well Field 13
WELL 15 WELL 15A

OCALA

MONITOR

WELL CONCEPTUAL

UJ 

HI 

LL.

LU

o

cc

CO 

Q

z

I

UJ

m

a.

UJ 

Q

100

300

CASING

oooooooooo

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-

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xxxxxxxxxxx

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xxxxxxxxxxx

xxxxxxxxxxx

CASING

xxxxxxxxxxx 

xxxxxxxxxxx 

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CASING

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xxxxxxxxxxx

EXPLANATION 

X FRACTURE POROSITY 

O VUG POROSITY

Figure 10. Effective secondary porosity logs for wells 15, 

15A, and the Ocala monitor well and a conceptual log.

the irregular surface of the underlying limestone. The 

horizontal hydraulic conductivity of permeable units of 

the surficial aquifer system in Hillsborough County 

ranges from less than 1 to more than 100 ft/d and the 

storage coefficient ranges from 0.05 to 0.3 (Southwest 

Florida Water Management District, 1988). The 

average range of values of hydraulic conductivity is 5 

to 25 ft/d (Freeze and Cherry, 1979) for the size and 

type of sediments at the study site.

The surficial aquifer system is underlain by the 

intermediate confining unit. The intermediate confining unit at the old Tampa well field is composed of 10 

to 50 ft of clay, sandy clay, and marl. Reported values 

of the leakance coefficient of the intermediate confining unit in southwest Florida range from 0.0001 to 

0.001 (ft/d)/ft (Wolansky and Corral, 1985). A thickness map of the intermediate confining unit (fig. 11) 

was prepared using the well completion records of 67

wells within a 2-mi radius of the old Tampa well field. 

The thickness of the intermediate confining unit ranges 

from 10 to 60 ft (fig. 11). The irregular shape of the 

contours indicates that the unit might have been deposited on a surface of variable elevation, or the unit was 

subjected to erosion after deposition, or both. The 

confinement of the underlying Upper Floridan aquifer 

could be highly variable because of the varying thickness and composition of the intermediate confining 

unit.

The surface of the Upper Floridan aquifer at the 

test site is irregular and is interpreted as a paleokarst 

surface. Test drilling and surface geophysical surveys 

support this interpretation. Limestone was first penetrated at considerably different depths in adjacent wells 

during drilling. The drilling log of well 11 indicates 

limestone at 43 ft below land surface. A borehole 

drilled 125 ft west of well 11 had not penetrated limestone at a depth of 140 ft. The borehole designated well 

17 by Menke and others (1961) was drilled to 310 ft 

below land surface without penetrating limestone. The 

middle confining unit of the Floridan aquifer system in 

the vicinity of the old Tampa well field has not been 

penetrated, but is estimated to be about 1,100 ft below 

land surface (Ryder and others, 1980).

Hydraulic Properties of the Upper Floridan 

Aquifer

The hydraulic properties of the Upper Floridan 

aquifer at the old Tampa well field were estimated from 

published values, rock-core analyses, and aquifer tests 

and are summarized in table 3. Estimates of the transmissivity of the Upper Floridan aquifer to a depth of 

800 ft range from 29,000 to 37,000 ft2/d. Estimates of 

the storage coefficient range from 5xlO~ 5 to2xlO~ 3 . 

Aquifer tests indicate that the aquifer is stratified, with 

the deeper rocks being more permeable than the shallow ones. For example, doubling the depth of well 15 

increased the estimated transmissivity by a factor of 

about three (Mehke and others, 1961), Doubling the 

depth of well 22 increased the estimated transmissivity 

by a factor of about four (Menke and others, 1961) or 

five (Wolansky and Corral, 1985).

Based on the reported compressibility of carbonates (Freeze and Cherry, 1979, p. 55), a storage coefficient range of 1.1 x 10 4 to 1.1 x 10~2 was calculated for 

the Upper Floridan aquifer at the study site. A simple 

method to estimate storage coefficients, as described 

by Lohman (1979, p. 53), yields a storage coefficient

14 Hydrogeology and Results of Tracer Tests Implications for Wellhead-Protection Strategies in West-Central Florida
82« 13

28° 03

28' 02 -

28° 01 -

EXPLANATION

THICKNESS OF THE

INTERMEDIATE CONFINING 

UNIT

GREATER THAN 50 FEET

k\N 25 TO 50 FEET 

I __ | LESS THAN 25 FEET 

 _ _ APPROXIMATE BOUNDARY OF STUDY SITE

/E«<^ WELL LOCATION AND 

(5U)» THICKNESS OF THE

INTERMEDIATE CONFINING

UNIT. IN FEET

Figure 11. Thickness of the intermediate confining unit in the 

vicinity of the old Tampa well field.

of 2 x 10 - 3 for the depth interval from 70 to 250 ft below 

land surface (permeable zone one), 4 x 10' 4 for the depth 

interval from 380 to 425 ft below land surface (permeable 

zone two), and about 2.5 x 10 ~ 3 for the depth interval 

from 550 to 800 ft below land surface (permeable zone 

three). These estimates fall within the range of values estimated using the reported compressibility of carbonates.

Rock-Core Analyses

Analyses of rock cores in southwest Florida indicate 

the ratio of horizontal to vertical hydraulic conductivity 

reported for the Suwannee and Ocala Limestones in 

Hillsborough County spanned five orders of magnitude 

(table 4). At one site, this ratio varied by three orders of 

magnitude within a depth interval of 432 to 500 ft below 

land surface in the Ocala Limestone. Reported values for 

effective porosity range from 17 to 46 percent for the 

Suwannee and Ocala Limestones in Hillsborough 

County. These data support the interpretation of the 

aquifer as a layered system.

Two 10-ft rock cores were collected at depths of 170 

to 180 and 300 to 310 ft below land surface during the 

drilling of well 15 A. These depth intervals were sampled 

because they coincide with permeable unit one and the 

semiconfining unit separating permeable units one and 

two of the Upper Floridan aquifer. The core collected 

from 170 to 180 ft below land surface was composed of 

fossiliferous, granular limestone with abundant visible 

porosity. The core collected from 300 to 310 ft below land 

surface was composed of chalky, dense limestone with 

only minor visible porosity. Representative samples of 

the cores were analyzed to determine effective porosity 

and vertical and horizontal hydraulic conductivity (table 4).

Table 3. Reported hydraulic properties of the Upper Floridan aquifer at the old Tampa well field 

[ft/d, foot per day; ft2/d, foot squared per day; (ft/d)/ft, foot per day per foot; APPW, Avon Park production well]

Pumped 

well 

(fig. 3)

15 

15 

22 

22 

22 

APPW

Open hole 

interval of 

pumped well 

(feet)

68-227 

68-413 

72-420 

72-800 

72-810 

550-800

Hydraulic 

conductivity 

(ft/d)

30 

40 

20 

40 

50 

120

Transmissivity Storage 

(ft2/d) coefficient

4,800 

14,000 

7,000 

29,000 

37,000 

30,000

0.00005 

.0003 

.0007 

.002 

.0015 

.0023

Leakance 

coefficient Source of data 

[<ft/d)/ft]

0.004 

.0027 

.00013 

.00027 

< .00032 

.00048

Menke and others (1961) 

Menke and others (1961) 

Menke and others (1961) 

Menke and others (1961) 

Wolansky and Corral (1985) 

Schreuder and Davis, Inc. (1991)

Hydrogeology of the Old Tampa Well Field 1 5
Table 4. Analyses of rock cores collected from the Upper Floridan aquifer in Hillsborough, Pasco, Pinellas, and Polk

Counties, Florida

[ft, feet; ft/d, feet per day; NR, not reported; , no data]

Formation

Ocala Limestone

Suwannee Limestone

Ocala Limestone

Avon Park Formation

Suwannee Limestone

Ocala Limestone

Ocala Limestone

Avon Park Formation

T c"'

360 Pasco

400

450

72 Polk

269 Polk

282

317

447 Polk

519

170-180 Hillsborough

300-310 Hillsborough

432 Hillsborough

445

500

505

550

628 Pinellas

906

710

Hydraulic 

conductivity

Vertical Horizontal 

(ft/d) (ft/d)

0.005

.79

.01

.007

.12

.16

.27

.00001

1.61

.54

.20

2.8

.22

5.8

.008

.03

1.8

1.2

.09

0.002

.16

.01

.01

.134

.16

.54

.00008

1.47

.51

.37

.118

.06

.01

.01

8.0

NR

NR

NR

Ratio of 

horizontal to 

vertical hydraulic 

conductivity 

(dimensionless)

0.4

.2

1.0

1.4

1.1

1.0

2.0

8.0

.9

.9

1.8

.04

.3

.002

1.25

266.7

_

Effective 

porosity Source 

(pecent)

37

38

39

32

44

27

44

18

30

21

46

44

45

17

32

21

48

21

29

CH2MHill, Inc. (1990)

CH2MHill, Inc. (1990)

CH2MHill, Inc. (1990)

Stewart(1966)

Stewart(1966)

Ste wart (1966)

Ste wart (1966)

Ste wart (1966)

Stewart(1966)

This study

This study

CH2MHill, Inc. (1990)

CH2MHill, Inc. (1990)

CH2MHill, Inc. (1990)

CH2MHill, Inc. (1990)

CH2MHill, Inc. (1990)

Hickey (1977)

Hickey(1977)

Hickey (1979)

The results of the analyses indicate that the sample of 

the rock core collected from 170 to 180 ft below land 

surface has a ratio of about 1:1 with respect to horizontal to vertical hydraulic conductivity and an effective 

porosity of 21 percent. The sample of rock collected 

from 300 to 310 ft below land surface has a ratio of 

about 2:1 and an effective porosity of 46 percent. These 

values fall within the range of values reported for 

analyses of rock cores from Pasco, Pinellas, Polk, and 

Hillsborough Counties (table 4).

Aquifer Tests

Two Upper Floridan aquifer tests were performed 

at the study site in 1990. The first test was conducted in 

July by the USGS and consisted of pumping well 15 

with a 20-horsepower submersible electric pump at a 

rate of 980 gal/min for 50 hours. The open-hole interval 

of the pumped well, 67 to 404 ft below land surface, 

penetrated permeable units one and two of the Upper 

Floridan aquifer. This test differed from previous tests 

in that observation wells in the surficial aquifer system,

the intermediate confining unit, and all three permeable 

units of the Upper Floridan aquifer were monitored 

during the test. Water levels were measured in two 

Upper Floridan aquifer Regional Observation Monitornng program (ROMP) wells 4 mi northwest of the 

study site. A rising water-level trend of 0.006 ft/h was 

measured in well 15A onsite and in the ROMP wells 4 

mi away. The drawdown data for the onsite Upper 

Floridan aquifer wells were corrected for this trend. A 

rising water-level trend of 0.01 ft/h was measured in 

onsite wells completed in the intermediate confining 

unit. The drawdown data for intermediate confining 

unit wells were corrected for this trend. Water levels in 

wells completed in the surficial aquifer showed no consistent trend, and drawdown data for these wells were 

not corrected. A rain gage installed by the WCRWSA 

measured no rainfall during the test. Atmospheric pressure at the site during the test was not monitored, but 

water-level fluctuations caused by atmospheric pressure 

changes probably were small compared to water-level 

fluctuations as a result of pumping during most of the test.

16 Hydrogeology and Results of Tracer Tests Implications for Wellhead-Protection Strategies in West-Central Florida
Table 5. Total drawdown in selected wells at the old Tampa well field during aquifer tests of July and 

December 1990

[gal/min, gallons per minute; ft, feet; OMW, Ocala monitor well; APMW Avon Park monitor well; APPW, Avon Park 

Production well]

Test Pumped 

well (fig. 3)

Discharge

rate 

(gal/min)

Observation 

well (fig. 3)

Total

observed

drawdown1 (ft)

Hydrogeologic

unit penetrated

by observation well

U.S. Geological Survey, 

July 1990. Duration, 50 hours

15 980 15 

17 

A

0.4 

.0 

.0

Surficial aquifer system

11

16

19

20

22

23

16

21

23

OMW

15 

15a

19

20

APMW

3.2 

2.4 

2.9 

1.5 

2.0 

.5

3.3 

3.9 

1.4

6.2

10.5 

6.2

4.3

4.4

2.7

Intermediate confining unit

Permeable unit of Upper 

Floridan aquifer:

Unit one

Unit two

Units one and two

Unit three

West Coast Regional Water APPW 2,150

Supply Authority . Duration,

168 hours

16

22

23

16

23

OMW

15

22

APMW

APPW

2.7

1.5

.15

3.6

3.0

6.0

6.0

6.4

6.5

7.7

Intermediate confining unit

Permeable unit of Upper

Floridan aquifer:

Unit one

Unit two

Units one and two

Unit three

All observed drawdowns corrected for regional trends except data for the surficial aquifer system during the U.S. Geological Survey test of July 1990. 

Schreuder and Davis, Inc. (1991).

A second aquifer test, conducted in December 

1990 by Schreuder and Davis, Inc. (1991), used the 

Avon Park production well as the pumped well. The 

well was pumped at a rate of 2,150 gal/min, and drawdown was measured in nine observation wells. The 

open-hole interval, 550 to 800 ft below land surface, 

corresponds to permeable unit three of the Upper 

Floridan aquifer. Total drawdown measured in selected 

wells at the old Tampa well field at the end of pumping 

for both aquifer tests is presented in table 5.

Results of the USGS test indicated that water levels 

in the shallow wells completed in the surficial aquifer 

system began to decline between 5 and 24 hours after 

pumping of well 15 began. The drawdown in the wells 

ranged from 0.0 to 0.4 ft after 50 hours of pumping. 

Drawdown of water levels was greatest in the wells 

closest to the pumped well. Water levels in the intermediate confining unit wells began to decline between 

30 and 60 min after pumping of well 15 began at a rate 

of 980 gal/min. The drawdown in the wells completed

Hydrogeology of the Old Tampa Well Field 17
in the intermediate confining unit ranged from 0.5 to 

3.2 ft after 50 hours of pumping. There was no pattern 

of drawdown in the intermediate confining unit. Some 

wells farther away from the pumped well had greater 

drawdowns than wells closer to well 15, which 

indicates that leakage to the Upper Floridan aquifer 

through the intermediate confining unit is variable. 

The rate of drawdown in wells completed in the Upper 

Floridan aquifer decreased to less than 0.01 ft/h after 

44 hours of pumping, indicating that the aquifer was 

approaching steady-state conditions.

Analytical methods used to estimate aquifer 

hydraulic coefficients assume laminar flow in response 

to pumping. Because the aquifer at the site has well 

developed secondary porosity, graphical analyses were 

made to test the assumption of laminar flow.

Two analyses were made of pumping rate and 

water-level decline. First, an analysis was made of 

pumping rate and drawdown data collected prior to the 

aquifer test of July 1990. The relation between drawdown in wells 15 A and 19 and the pumping rate from 

well 15 after 1 hour of pumping is shown in figure 12.

jr 

O

u

O 

cc in

z 

O

Q

cc

Q

D WELL 15A - 103 FEET FROM 

PUMPED WELL 15 (fig.3}

WELL 18 - 238 FEET FROM 

PUMPED WELL 15

PUMPING RATE, IN GALLONS PER MINUTE

Figure 12. Relation between pumping rate and drawdown in 

the Upper Floridan aquifer at the old Tampa well field, 1990.

A nearly linear relation exists between drawdown and 

pumping rate within the limits of measurement error 

for the pumping rate. A second analysis involved plotting drawdown against distance from the pumped well

for all observation wells open to the same depth interval 

as the pumped well for early-time and late-time data 

(fig. 13). The plot shows that drawdown decreased with 

distance from the pumped well, as would be expected for 

laminar flow conditions. These tests indicate that flow is 

laminar and that Darcy's equation can be used to estimate 

hydraulic properties of the aquifer.

D

- A WELL 15A:

X WELL 20:

O WELL 19.

O 

Q

CC 

Q

I I I I I II I | 

EXPLANATION

WELL 11: 25 FEET FROM 

PUMPED WELL

103 FEET FROM 

PUMPED WELL

200 FEET FROM 

PUMPED WELL

239 FEET FROM 

PUMPED WELL

10 100

DISTANCE FROM PUMPED WELL, IN FEET

Figure 13. Distance-drawdown plot for Upper Floridan 

aquifer wells at the old Tampa well field, 1990, after 1 minute, 

20 minutes, and 48 hours of pumping well 15 at 980 gallons 

per minute.

A graphical analysis was performed to test the 

assumption that the aquifer responds to pumping as an 

isotropic media. The presence of effective secondary 

porosity in the form of isolated vertical fractures can 

create an anisotropic, nonradial flow response in aquifers 

(Smith and Vaughan, 1985). A semilog plot was made of 

time-drawdown data for wells 19 and 20, which are 

nearly equidistant from and form a 90-degree angle with 

the pumped well (fig. 14). The data points overlie one 

another indicating that, beyond about a 200-ft radius 

from the pumped well, the aquifer probably had a radial 

isotropic response to pumping during the 50-hour aquifer 

test. If wells 19 and 20 are on rays with the principle axes 

of transmissivity, they could have similar drawdowns 

even in an anisotropic system. However, because anisotropy cannot be confirmed, the simpler isotropic model is 

assumed to be correct.

18 Hydrogeology and Results of Tracer Tests Implications for Wellhead-Protection Strategies in West-Central Florida
A graphical analysis of drawdown data was made to 

test the assumption of equivalent porous media behavior. 

A semilog plot of drawdown against time was made for 

wells 19 and 20 (fig. 14) and for the pumped well (fig. 15). 

The plots show a straight-line trend for the time interval 

from 20 to 200 min after pumping began. Early-time and 

late-time drawdowns do not plot on the straight-line trend. 

The time-drawdown data from 20 to 200 min after pumping began are most representative of the time interval during which the aquifer responded to pumping as a confined, 

equivalent porous medium. The early-time drawdown 

data could indicate the effects of secondary porosity or 

pump surging. If secondary porosity influences the timedrawdown data, figures 14 and 15 indicate that a large 

enough volume of aquifer has been stressed after 20 min 

of pumping so that the system response resembles that of a 

homogeneous porous media. The late-time drawdown data 

indicate the influence of leakage from the overlying and 

underlying hydrogeologic units.

These analyses support the assumption that the upper 

400 ft of the Upper Floridan aquifer at the old Tampa well 

field responds to pumping as a semiconfined, horizontally 

isotropic, equivalent porous medium after approximately 

20 min of pumping. Flow seems to be laminar for the 

range of discharge rates tested. The hydraulic effects of 

effective secondary porosity, if any, are indistinguishable 

from the total aquifer response after about 20 min of 

pumping at a rate of approximately 1,000 gal/min.

5

4 

K

UJ 

UJ

X '

\WDOWN,

ro

OC 

O

1

0 

0

 1 1 1 1 1 1 III 1 1 1 1 1 1 1 1[ 1 I 1 1 1 1 1

WELL DISTANCE TO PUMPED WELL

20 e 200 FEET Transmissivily = 

feel squared per 

19 A 239 FEET Transmissivily = 

feel squared per

-

WELL 20

'I I 1 1 I 1 1 1!) 1 1 1

21.000 AS

da * ,9 

19.000 £A

day ^

****

< -

WELL 19

A^ TRANSMISSIVITY - 0.183 (Q)

o 

A

o

o ^&* WELL 15

,,..,.f ??.*....,

1 1 10

DRAWDOWN/ 

LOG CYCLE -

PUMPING RATE (Q)- 188, 660

CUBIC FEET PER DAY 

.1 .... ,,.,1 ...

100 1,000

TIME SINCE PUMPING BEGAN, IN MINUTES

Figure 14. Time-drawdown data for Upper Floridan aquifer wells 

19 and 20 at the old Tampa well field, 1990.

Transmissivity of the pumped interval was estimated 

from the plot of time-drawdown data and for wells 19 and 

20 (fig. 14) and for the pumped well (fig. 15). The timedrawdown data before 20 min and after 200 min after pumping began were not used. The open-hole interval of the 

pumped well fully penetrates the two upper permeable 

zones of the Upper Floridan aquifer and is separated from

I 

o 

o

rr 

o

TRANSMISSIVITY= 0.183 |Q)

DRAWDOWN/LOO CYCLE

WELL 15 PUMPING RATE <Q)= 188.860 CUBIC FEET PER DAY 

TRANSMISSIVITY = 23,000 FEET SQUARED PER DAY

10 100

TIME SINCE PUMPING BEGAN, IN MINUTES

Figure 15. Time-drawdown plot for well 15 at the old Tampa well field, 1990.

Hydrogeology of the Old Tampa Well Field 19
the lower permeable zone by a semiconfining unit. 

Pumping flowmeter logs indicate that the semiconfining unit does not transmit measurable quantities of 

water to wells. Therefore, the pumped well was 

assumed to fully penetrate the tested zones and no correction for partial penetration was necessary. A 

straight-line solution (Lohman, 1979, p. 23) yielded 

estimated transmissivities of the tested interval in the 

pumped well, well 19, and well 20 of 23,000, 19,000, 

and 21,000 ft2/d, respectively. The transmissivity estimate is representative of the cumulative transmissivity 

of the individual units penetrated.

CONCEPTUAL HYDROGEOLOGIC 

MODEL

A conceptual hydrogeologic model of the groundwater flow system at the old Tampa well field was formulated using the results of onsite drilling, the geologic 

section (fig. 7), the distribution of permeability in the 

Upper Floridan aquifer as determined using flowmeter 

and fluid resistivity logs (figs. 8 and 9), and the results 

, of aquifer tests. A water budget was calculated for the 

study area using the configurations of the water table in 

the surficial aquifer system and the potentiometric % 

surface in the Upper Floridan aquifer and the estimated 

hydraulic coefficients of these hydrogeologic units.

In the conceptual hydrogeologic model of the old 

Tampa well field, the surficial aquifer system and the 

intermediate confining unit rest unconformably on the 

variable paleokarst surface of the Upper Floridan aquifer (fig. 16). The Upper Floridan aquifer is layered; at 

least three permeable zones occur from 50 to 250, 380 

to 425, and 550 to 800 ft below land surface and are 

separated by two semiconfining units occurring from 

250 to 380 and 425 to 550 ft below land surface. 

Results of analyses of aquifer test data indicate that the 

Upper Floridan aquifer at the test site responds to 

pumping as a horizontally isotropic, equivalent porous 

medium. The total thickness of the Upper Floridan 

aquifer at the site is estimated to be 1,100 ft (Miller, 

1986). The middle confining unit of the Floridan aquifer 

system, about 1,100 to 1,200 ft below land surface 

(Ryder and others, 1980; Miller, 1986), underlies the 

Upper Floridan aquifer in this conceptual model. The 

Lower Floridan aquifer was not modeled at the study 

site.

The surficial aquifer system was estimated to have 

a horizontal hydraulic conductivity of about 10 ft/d. 

The sands of the surficial aquifer system grade downward to marl and clay of the intermediate confining 

unit. The intermediate confining unit was estimated to 

have a vertical hydraulic conductivity of about 0.01 ft/d. 

For the measured thickness of the intermediate confining unit at the site, this value yields leakance coefficient 

values ranging from 0.001 to 0.0002 (ft/d)/ft, which is 

within the range of reported leakance coefficient values 

for this unit (Wolansky and Corral, 1985).

The Upper Floridan aquifer, based on the results of 

aquifer tests, was estimated to have a transmissivity on 

the order of 60,000 ft2/d. The transmissivity value 

calculated for well 15 was divided proportionally 

between permeable units one and two of the Upper 

Floridan aquifer, and the semiconfining unit that separates them, using pumping flowmeter surveys of well 

15. The estimated transmissivities are 10,350 ft2/d for 

permeable unit one, 1,150 ft2/d for the intervening 

semiconfining unit, and 11,500 ft2/d for permeable unit 

two. The transmissivity of that part of permeable unit 

three from 550 to 800 ft below land surface was calculated to be approximately 30,000 ft2/d (Schreuder and 

Davis, Inc., 1991). The total transmissivity of the lower 

part of the Upper Floridan aquifer (550-1,150 ft below 

land surface) was estimated to be on the order of 

40,000 ft2/d.

Water levels in wells completed in the surficial 

aquifer system and in wells completed in different 

permeable units of the Upper Floridan aquifer were 

measured to determine the vertical ground-water head 

gradient at the study site in June 1990. The water level 

in the surficial aquifer was about 5 ft higher than the 

water level in the Upper Floridan aquifer. There was no 

measurable difference between the head in permeable 

units one and two of the Upper Floridan aquifer, but the 

head in permeable unit three was about 0.5 ft less than 

in the overlying units of the Upper Floridan aquifer. 

Water levels in wells completed in the surficial aquifer 

system and the intermediate confining unit declined in 

response to pumping the Upper Floridan aquifer. This 

decline indicates hydraulic connection between the 

surficial aquifer system and the Upper Floridan aquifer; 

therefore, the Upper Floridan aquifer is best described 

as semiconfined.

Ground-water withdrawals in northeastern 

Hillsborough County vary seasonally. Average annual 

permitted withdrawal for agricultural use in the vicinity 

of the study site is 26 Mgal/d, with a maximum daily 

permitted withdrawal for agricultural use of 286 

Mgal/d (Bengtsson and others, 1986). In 1983 and 

1985, the water level in well 15 declined about 18 ft

20 Hydrogeology and Results of Tracer Tests Implications for Wellhead-Protection Strategies in West-Central Florida
WELL 1 1

LU 

LU

LU 

O

LL

cr 

co

D

Z 

< 

_l

g

HI

m

I iQ_ 

LU 

D

100 -

200 -

300 .

400

500 -

600 -

700 .

800 -

900 -

1,000 -

1,100 -

1,

X

0

(1)

(2)

(3)

FRACTURE POROSITY

VUG POROSITY

OCALA MONITOR WELL

AVON PARK MONITOR WELL

AVON PARK PRODUCTION WELL

Figure 16. Hydrogeologic framework at the old Tampa well field.

Conceptual Hydrogeologic Model 21
during 2 days as a result of heavy pumping to protect 

crops from frost damage. Similar events occurred in 

1977 and 1981, but the water-level declines were not as 

great. Rapid water-level declines can result in the formation of sinkholes and can interrupt the water supply 

to residents with shallow wells (Bengtsson and others, 

1986). A water budget for the study site was estimated 

for July 1990, assuming steady-state conditions during 

the season of low pumpage. The distribution of the 

water table in the surficial aquifer system and the 

potentiometric surface in the Upper Floridan aquifer in 

July 1990 and the hydraulic coefficients estimated from 

aquifer tests were used to calculate the volume of water 

entering the Upper Floridan aquifer from the surficial 

aquifer system and the volume of water entering the 

study site by regional flow through the Upper Floridan 

aquifer. Vertical leakage from the surficial aquifer system amounted to approximately 62,000 ft3/d. Regional 

flow in the Upper Floridan aquifer through the site was 

estimated to be about 690,000 ft3/d.

The complexity of the ground-water flow system 

makes it difficult to use analytical methods to determine the hydraulic coefficients of the Upper Floridan 

aquifer. A numerical model of the ground-water flow 

system at the study site was constructed and calibrated 

using the results of the two aquifer tests performed in 

1990. The hydraulic coefficients estimated with 

analytical methods were compared to those necessary 

to calibrate the numerical model. This comparison 

provided a way to determine if the estimated hydraulic 

coefficients were reasonable approximations for the 

Upper Floridan aquifer at the site.

NUMERICAL MODEL

A finite-difference numerical model was used to 

simulate the ground-water flow system at the old 

Tampa well field. The objectives of the modeling were 

to: (1) provide additional estimates of the hydraulic 

coefficients of the Upper Floridan aquifer; (2) test the 

conceptual model of the ground-water flow system; 

and (3) generate the velocity-vector field required by 

the particle-tracking program to simulate the groundwater travel times measured with tracer tests. The 

USGS modular model "MODFLOW" (McDonald and 

Harbaugh, 1988) was selected because of its ability to 

simulate layered aquifers and because the particletracking program "MODPATH" (Pollock, 1989) uses 

the output from the modular model. The MODFLOW 

model can use the quasi-three-dimensional approach to

simulate flow between multiaquifer systems (twodimensional (x,y) horizontal flow through aquifers and 

vertical (z) flow through confining units using leakance 

terms and ignoring storage) or the three-dimensional 

approach by specifying a model layer for each hydrogeologic unit. Steady-state and transient simulations 

were made.

Model Grid and Boundary Conditions

The conceptual model of the ground-water flow 

system at the old Tampa well field was represented 

using a 6-layer numerical model with a 28-column by 

28-row variable grid simulating a 6,112 by 6,112 by 

1,200-ft volume of aquifer system (fig. 17). Model 

layer 1 represents the surficial aquifer system. The 

intermediate confining unit is simulated with a vertical 

leakance term using the quasi-three-dimensional 

approach. The remaining hydrogeologic units are modeled with the three-dimensional approach by assigning 

individual model layers to each hydrogeologic unit. 

Model layers 2,4, and 6 represent permeable units one, 

two, and three of the Upper Floridan aquifer, respectively. Model layers 3 and 5 represent the semiconfining units of the Upper Floridan aquifer. The model grid 

locations of the 12 wells completed in the Upper Floridan aquifer that were used as control points for model 

calibration are shown in figure 18.

The heads in the surficial aquifer system, after 

drawdown due to pumping had stabilized, were used as 

a specified head boundary in model layer 1 for the 

steady-state simulation. For the transient simulations, 

the heads in the surficial aquifer system before pumping began were used as initial conditions in model layer 

1, and the layer was activated so that drawdown could 

occur. The model layers representing the Upper Floridan aquifer were bounded by general head boundaries 

on all sides. A conductance term was calculated for 

each general head grid cell based on a specified head at 

a distance of l"mi from the model bpundary, or about 

2.2 mi from the pumped well, and the transmissivity 

value determined for the hydrologic unit represented 

by that model layer using the results of onsite aquifer 

withdrawal tests. Based on the assumption of horizontally isotropic conditions, one transmissivity value was 

used to calculate the conductance terms for the general 

head boundary cells for each layer. The distance 

between the general-head boundary and the model 

boundary was set at 1 mi for two reasons: (1) the distance to zero drawdown was about 1 mi based on the

22 Hydrogeology and Results of Tracer Tests Implications for Wellhead-Protection Strategies in West-Central Florida
26 COLUMNS (e.112 FEET)

LAND 

SURFACE

100

200

UJ 

UJ

u.

UJ

O

&

DC3

CO 

Q

z

I

UJ 

CD

Q. 

UJ 

Q

UJ

DC 

Q. 

Q.

300

400

500

600

700

800

900

1,000

1,100

1,200

-

-

LAYER 2 T = 9,700

S - 0.0003

VERTICAL CONu'JCTANCE-0.0

LAYER-3 T-430 

S-0.0002 

VERTICAL CONDUCTANCE-0.01

LAYER-4 T- 10.500 * 

S-0.0002

34

)5

JVERTICAL CONDUCTANCE-0.05

LAYER S, T-1.200

S-0.0002

VERTICAL CONDUCTANCE-0.025

-

-

LAYER 6 T- 40. 000

5=0.0001

//////////////////

EXPLANATION

O - PUMPING RATE. IN CUBIC 

FEET PER DAY

K = HORIZONTAL HYDRAULIC 

CONDUCTIVITY, IN FEET 

PER DAY

K v - VERTICAL HYDRAULIC 

CONDUCTIVITY, IN FEET 

PER DAY

S - STORAGE COEFFICIENT

T - TRAMSMISSIVITY. IN FEET 

SQUARED PER DAY

Figure 17. Hydrogeologic framework of the numerical model of the ground-water flow system at the old Tampa 

well field.

Numerical Model 23
relatively small withdrawal rate of the pumped well 

and the estimated permeability and storage properties 

of the Upper Floridan aquifer; and (2) the projection of 

the estimated hydraulic coefficients at the test site 

beyond the vicinity of the old Tampa well field cannot 

be justified without additional detailed hydrogeologic 

data. The lower boundary of the model was a no-flow 

boundary, simulating the middle confining unit of the 

Floridan aquifer system.

Initial water-level conditions for model simulations were established using onsite and regional waterlevel measurements. Estimated water-level altitudes

were used as the starting head values in model nodes 

that do not contain wells. The configuration of the 

potentiometric surface within the study area remained 

unchanged between May 1988 (Lewelling, 1989), 

September 1988 (Barr, 1989), and September 1989 

(Knochenmus and Barr, 1990), but did fluctuate about 

10 ft. Based on the history of water-level fluctuations 

within the study area, the potentiometric surface for 

July 1990 was estimated by subtracting 4 ft of altitude 

from the potentiometric-surface altitudes of September 

1989 (Knochenmus and Barr, 1990). Water levels in 

wells at the old Tampa well field were used as starting

COLUMNS [(C) 4-25] 

10 15 20

EXPLANATION

OCALA MONITOR WELL

AVON PARK PRODUCTION WELL

AVON PARK MONITOR WELL

NUMERICAL MODEL NODE

OMW 

APPW 

APMW

LOCATIONS AND WELL NUMBER

25

Figure 18. Node locations of wells in the numerical model of the old Tampa well field 90 METERS

24 Hydrogeology and Results of Tracer Tests Implications for Wellhead-Protection Strategies in West-Central Florida
N

J

\

EXPLANATION

POTENTIOMETRIC CONTOUR-Shows 

altitude of the potenliomelric 

surface. Contour interval 10 feet. 

Datum is sea level

Figure 19. Orientation of the numerical model grid to the local potentiometric 

surface of the Upper Floridan aquifer, July 1990.

heads for the model nodes in which the wells were 

located. The model grid was oriented to the northeastsouthwest so that the model boundaries were approximately parallel or orthogonal to the regional potentiometric-surface contours of the Upper Floridan aquifer 

(fig. 19).

Input Parameters

Input parameters for the numerical model were 

derived from field tests and from the model calibration 

process (table 6). The transmissivities of model layers 

were based on results of aquifer tests, core analyses, 

and pumping flowmeter logs. Transmissivities estimated from aquifer tests conducted in 1961, 1985, and 

1990 were used as starting values for the model 

hydraulic parameters and adjusted (table 6) until the 

simulated drawdowns approximated the observed 

drawdowns at the end of the aquifer test (table 7). The 

storage coefficients used for model layers were estimated using ranges of values for similar sediments and 

from the compressibility of limestone (Freeze and 

Cherry, 1979).

There are several disparities between the hydraulic 

coefficients determined for rock cores (table 4) and 

hydraulic coefficients listed in table 6. These disparities 

are due to the difference between the part of the aquifer 

represented by rock cores and the part of the aquifer 

represented by a model layer. The rock-core analyses 

(table 4) are representative of small vertical intervals of 

the aquifer matrix. Results of the analyses indicate a 

ratio of about 1:1 between horizontal to vertical 

hydraulic conductivity for the aquifer matrix and a 

maximum horizontal hydraulic conductivity of 8 ft/d. 

Hydraulic coefficients used in the numerical model, 

however, represent entire hydrologic units that include 

both relatively permeable and impermeable layers of 

the aquifer matrix and permeability resulting from 

effective secondary porosity. As a result, the equivalent 

horizontal hydraulic conductivities for the transmissivity values used in the model and the ratios of horizontal 

to vertical hydraulic conductivity used to estimate the 

vertical-conductance values in the model are significantly higher than the values reported for individual 

rock-cores in table 4.

Numerical Model 25
Table 6. Input parameters for model of the Old Tampa well field 

[ft/d, feet per day]

Parameter Layer Adjusted 

value Source

Altitude of starting heads, 

in feet

Altitude of bottom of 

layer 1, in feet

Horizontal hydraulic 

conductivity, in feet 

per day

Transmissivity, in feet 

squared per day

Vertical conductance 

between layers, in 

feet per day per foot

Storage coefficient 

(dimensionless)

Pumping rate, in gallons 

per minute

1

2

3-5 

6

2

3

4

60-64 

55-63 

55-63 

55-63 

55-63 

55-63

Measured onsite and estimated using September 1989 Upper 

Floridan aquifer potentiometric surface map (Knochenmus and 

Barr, 1990).

-50-(-40) Estimated from well completion records obtained during well 

inventory and records of material penetrated by onsite 

exploratory drilling.

10 Selected from a range of values for similar rock types (Freeze and 

Cherry, 1979; Wolansky and Corral, 1985).

9,700 Estimated from U.S. Geological Survey aquifer test,

July 1990. 

430 Estimated from U.S. Geological Survey aquifer test,

July 1990. 

10,500 Estimated from U.S. Geological Survey aquifer test,

July 1990. 

1,200 Estimated from aquifer test, December 1990, (Schreuder and

Davis, Inc., 1991). 

40,000 Estimated from aquifer test, December 1990, (Schreuder and

Davis, Inc., 1991).

0.0002-0.001 Calculated using estimated vertical hydraulic conductivity value 

of 0.01 ft/d for the intermediate confining unit divided by the 

thickness of the intermediate confining unit in each model 

node, equation 53 (McDonald and Harbaugh, 1988).

0.004 Calculated using vertical hydraulic conductivities from rock cores 

and unit thickness with equation 51 (McDonald and Harbaugh, 

1988).

0.005 Calculated using vertical hydraulic conductivities from rock core 

analyses and unit thickness, equation 51 (McDonald and 

Harbaugh, 1988).

0.05 Calculated using vertical hydraulic conductivities from rock core 

analyses and unit thickness, equation 51 (McDonald and 

Harbaugh, 1988).

0.025 Calculated using vertical hydraulic conductivities from rock core 

analyses and unit thickness, equation 51 (McDonald and 

Harbaugh, 1988).

0.25 Ranges of values for similar sediments (Freeze and Cherry, 1979).

0.0003

0.0002

0.0001

Estimated from the compressibility of limestone (Freeze and 

Cherry, 1979; Lohman, 1979).

430 Discharge rate during the July 1990 aquifer test conducted by U.S.

20 Geological Survey proportioned to individual hydrogeologic

->30 units based on flowmeter survey of well 15.

2,150 Reported discharge rate during December 1990 aquifer test 

(Schreuder and Davis, Inc., 1991).

26 Hydrogeology and Results of Tracer Tests Implications for Wellhead-Protection Strategies in West-Central Florida
Table 7. Measured and model-simulated drawdowns in the Upper Floridan aquifer at the old Tampa well field for steadystate calibration of two aquifer tests

[gal/min, gallons per minute; ft, feet; APMW, Avon Park monitor well; OMW, Ocala monitor well. Residuals were computed by subtracting the 

model-simulate drawdown from the measured drawdown. A positive value indicates that the model-simulated drawdown is less than the 

measured drawdown, and the reverse is indicated by a negative residual]

Pumped 

Test well 

(fig- 3)

U.S. Geological 15

Survey calibration

(50 hours)

Schreuder and APPW

Davis, Inc. (1991)

(168 hours)

Dis- ,. . 

charge Well rtMeasVred 

rate Ifia 3) drawdown

(galin) ^ * '" -' W

980 ISA

16

19

20

21

23

APMW

2,150 15

16

22

OMW

APMW

6.2

3.3

4.3

4.4

3.9

1.4

2.7

6.0

3.6

6.4

6.0

6.5

Uniform 

transmissivity model

Simulated _ . . , 

. . Residual drawdown .ft

at node (ft) ( '

5.5

3.3

3.8

4.3

3.6

2.8

1.9

5.0

3.1

5.9

5.2

7.3

0.7

.0

.5

.1

.3

-1.4

.8

1.0

.6

.5

.8

- .8

Transmissivity x10 

simulating fractures

Simulated 

drawdown 

at node (ft)

5.2

3.0

3.3

4.0

2.8

2.5

1.8

4.7

3.1

5.1

4.8

6.8

Residual 

(ft)

1.0

.3

1.0

.4

1.1

-1.1

.9

1.3

^5

1.3

1.2

- .3

Transmissivity x 0.1 

simulating fractures

Simulated _ . . , 

. . Residual drawdown

at node (ft) ( '

6.1

3.9

4.5

4.8

4.8

3.3

2.1

5.7

3.3

7.0

5.9

8.3

0.1

- .6

- .2

- .4

- .9

-1.9

.6

.3

.3

- .6

.1

-1.8

Additional disparity exists between the transmissivities estimated using analytical methods and those 

necessary to calibrate the numerical model. The transmissivity of the semiconfining unit separating permeable units one and two (model layer 3) was estimated to 

be about 1,150 ft2/d using the results of aquifer tests 

and flow-meter surveys, but a transmissivity of 430 

ft2/d was necessary to calibrate the model. The difference between the two is within the measurement error 

of the methods used to calculate the value and illustrates the difficulty in determining accurate numbers 

for hydraulic coefficients of complex ground-water 

flow systems.

The intermediate confining unit was simulated 

using the quasi-three-dimensional approach. This 

approach assumes that the confining unit makes no 

contribution to the storage capacity of the aquifer 

(McDonald and Harbaugh, 1988). Values of vertical 

conductance used for layer 1 were calculated using 

equation 53 of McDonald and Harbaugh (1988), 

which is the equation for the leakage coefficient. For 

layer 1, vertical conductance ranged from 0.0002 to 

0.001 (ft/d)/ft, and values were assigned to model 

nodes using the estimated thickness of the intermediate 

confining unit in that model node (fig. 11) and a 

uniform vertical hydraulic conductivity of 0.01 ft/d.

Steady-State Calibration

The results of the 50-hour aquifer test indicated 

that the Upper Floridan aquifer approached steadystate conditions within 44 hours after pumping began at 

a rate of 980 gal/min. A steady-state calibration to the 

50-hour aquifer test was achieved when the drawdown 

in model layers 2,4, and 6 approximated the drawdown 

measured in seven wells completed in permeable units 

one, two, and three of the Upper Floridan aquifer 

within an error criteria of plus or minus 1 ft. The 

simulated drawdown in model layer 2 was calibrated to 

observed drawdown in wells completed in permeable 

unit one of the Upper Floridan aquifer, and the simulated drawdown in model layer 4 was calibrated to 

observed drawdown in wells open to both permeable 

units one and two of the Upper FLoridan aquifer. The 

simulated drawdown in model layer 6 was calibrated to 

observed drawdown in the Avon Park monitor well, 

completed in permeable unit three of the Upper 

Floridan aquifer.

The calibration of the steady-state simulation was 

evaluated by computing the residual difference 

between the observed water-level drawdowns in seven 

observation wells and the simulated drawdowns at the 

corresponding model nodes (table 7). Simulated 

drawdowns matched the observed drawdowns within 

a range of 0.8 ft below to 1.4 ft above the observed

Numerical Model 27
drawdown; however, excluding well 23, the simulated 

drawdowns matched the observed drawdowns within 

the error criteria of plus or minus 1 ft. The water budget 

for this simulation indicated that 42 percent of the 

pumpage (79,000 ft3/d) was derived as leakage from 

model layer 1 and 58 percent (110,000 ft3/d) was from 

the general head boundaries.

The steady-state calibration was tested by simulating the 168-hour aquifer test conducted at the site in 

December 1990 and comparing measured drawdowns 

in five observation wells completed in the Upper Floridan aquifer with simulated drawdowns in the corresponding model nodes. The general head boundary 

conductance terms were reused, but the starting heads 

in the model layers and the general head boundary 

nodes were modified to reflect site conditions at the 

time of the test. The model simulated the measured 

drawdowns in five Upper Floridan aquifer observation 

wells within a range of 1.0 ft below to 0.8 ft above the 

observed drawdowns (table 7). The water budget for 

this simulation indicates that 20 percent of the pumpage (84,000 ft3/d) was derived from leakage from 

model layer 1 and 80 percent (330,000 ft3/d) was from 

the general head boundaries. Additional simulations of 

the aquifer tests were made to evaluate the possibility 

that photolineaments at the site represent a fracture 

zone. Photolineaments A and B were simulated as a 

fracture zone of increased permeability resulting from 

secondary permeability and as a fracture zone of 

decreased permeability resulting from recrystallization 

of the limestone or infilling on the fractures with clay 

and sand or both. Photolineaments C and D were not 

evaluated because data were not available to determine 

the hydrogeologic significance of C and D.

Transmissivity values for all model layers were 

increased one order of magnitude and then decreased 

one order of magnitude for model nodes representing 

the area through which photolineaments A and B were 

traced (fig. 4). The vertical conductance between model 

layers was not increased or decreased. Borehole videos 

of wells at the site indicated very few high angle fractures longer than 2 to 5 ft; therefore, the probability of 

a semiconfining layer being breached by fractures and 

effective secondary porosity is low. Additionally, aquifer test data indicated that the semiconfining units 

retarded the flow of water between more permeable 

units.

Changes in the transmissivity of the model nodes 

simulating photolineaments A and B had measurable 

but relatively little effect on simulated drawdowns,

possibly because they are oriented approximately parallel to the direction of flow through the model. Results 

of simulating possible fracture zones are shown in 

figure 20 and listed in table 7.

Simulated drawdowns in the model nodes 

corresponding to observation wells decreased when 

photolineaments A and B were modeled as a fracture 

zone of increased transmissivity (table 7). Simulated 

drawdowns in the model nodes corresponding to observation wells generally were increased when photolineaments A and B were modeled as a fracture zone of 

decreased transmissivity (table 7). Comparison of the 

simulated drawdowns for the uniform transmissivity 

model to the fracture transmissivity models indicates 

that the calibration of the model to the observed data 

was either not improved or slightly worse using 

increased and decreased transmissivities. The results of 

model simulations neither confirmed nor precluded the 

existence of fractures at the test site, and the uniform 

transmissivity model was accepted as a representative 

model of the ground-water flow system at the old 

Tampa well field.

Transient Calibration

A transient simulation of the 50-hour aquifer test 

was made to obtain an additional estimate of the storage coefficient of the Upper Floridan aquifer. Calibration of the transient model was achieved by adjusting 

the storage coefficients of the layers simulating the 

Upper Floridan aquifer. The vertical-conductance values and transmissivities used in the calibrated steadystate model were not changed. Initially, the storage 

coefficients from onsite aquifer tests (table 3) were 

used; however, the storage coefficient estimated using 

the compressibility of limestone (10~4) produced a better calibration. A specific yield of 0.25, representative 

of unconfined sand, was assigned to the surficial aquifer system.

The first 6 hours of pumping were simulated with 

two 3-hour time steps, and the next 6 hours were simulated with a single time step. The remaining aquifer test 

was simulated with two 12-hour and one 14-hour time 

steps. The total simulated time of pumping was 50 

hours. The simulated stressed interval of the Upper 

Floridan aquifer was the depth interval from about 50 

to 425 ft below land surface. Observed and modelsimulated hydrographs for seven wells completed in 

the Upper Floridan aquifer are shown in figure 21. The 

water budget at the end of the last time step indicated

28 Hydrogeology and Results of Tracer Tests Implications for Wellhead-Protection Strategies in West-Central Florida
COLUMNS 3-26

H

EXPLANATION

NODES SIMULATED AS FRACTURE ZONE ON BASIS OF PHOTOLINEAR 

TRACE FROM FIGURE 4

SIMULATED DRAWDOWN. IN FEET. USING UNIFORM TRANSMISSIVITY

SIMULATED DRAWDOWN, IN FEET, USING INCREASED TRANSMISSIVITY 

(X10) IN NODES SIMULATING FRACTURE ZONES

SIMULATED DRAWDOWN, IN FEET. USING DECREASED TRANSMISSIVITY 

(X0.1) IN NODES SIMULATING FRACTURE ZONES

SIMULATED DRAWDOWNS AFTER 50 HOURS OF PUMPING WELL 15 AT 

AT A RATE OF 980 GALLONS PER MINUTE, WHICH IS 188.660 CUBIC 

FEET PER DAY

Figure 20. Simulated drawdowns in permeable unit two of the Upper Floridan 

aquifer for the uniform transmissivity model and for models using increased and 

decreased transmissivity in nodes simulating a fracture zone.

that about 42 percent of the pumpage (79,000 ft3/d) was 

derived from leakage from the surficial aquifer system, 

about 56 percent (106,000 ft3/d) was from boundary 

flow, and less than 2 percent (4,000 ft3/d) was derived 

from storage.

A transient simulation also was conducted of a 

168-hour aquifer test in which the Avon Park production well was pumped. The first 3 hours of pumping 

were simulated with a single time step, the next 9 hours 

with a single time step, and the next 48 hours with two

24-hour time steps. The remaining aquifer test was 

simulated with two 40-hour and a single 28-hour time 

step. The simulated stressed interval of the Upper 

Floridan aquifer was the depth interval from 550 to 

1,150 feet below land surface. The water budget at the 

end of the last time step indicated that about 20 percent 

of the water withdrawn from the well (84,000 ft3/d) was 

from leakage, less than 1 percent (400 ft3/d) was from 

storage, and about 80 percent (330,000 ft3/d) was from 

boundary flow.

Numerical Model 29
AQUIFER TEST, JULY 1990 

PUMP WELL 15 AT 980 

GALLONS PER MINUTE

AQUIFER TEST, DECEMBER 1990 

PUMP APPW AT 2,150 GALLONS PER MINUTE

PERMEABLE UNIT ONE 

OF THE UPPER 

FLORIDAN AQUIFER

PERMEABLE UNIT TWO 

OF THE UPPER 

FLORIDAN AQUIFER

PERMEABLE UNIT THREE 

OF THE UPPER 

FLORIDAN AQUIFER

WELL 23

*"]* 

10 30 SO

WELL 16 0- 

2-

O-i

2-

*° 

 

» 0 J

i i° 4

10 30 50 

WELL 21

«- P>8 i 

. l  1l 

10 30 50

WELL 19

2-

4-

0- 

2- 

4-

0-

2- 

4.

6-

"*« 8 8 J

10 30 50

» . . i

10 30 SO 

WELL ISA

&8 8 8 (

t'n t'n Kn

APMW

2- °°2 8 8 f

10 30 50

TIME SINCE PUMPING BEGAN, 

IN HOURS

 MEASURED DRAWDOWN 

o MODEL-SIMULATED DRAWDOWN

WELL 16

0 .... . ... .

!- 0 J

50 100 ISO

WELL 15

6 : 88 »8i

50 100 ISO

WELL 22

e- 8 « 8 8 8 !

0 

SO 

III

100 150 

TIME SINCE PUMPING BEGAN, IN HOURS

VERTICAL SCALE REPRESENTS DRAWDOWN, IN FEET

Figure 21. Measured and model-simulated water-level drawdowns at the old Tampa well field for aquifer tests of 

July and December 1990.

Simulation of the aquifer tests at the site with the 

transient numerical model indicates that the modeled 

equivalent porous medium response to pumping differs 

significantly from the observed response during the 

first 1 to 2 hours after pumping began, but matched the 

remaining time response reasonably well. The probable 

reason is that early-time drawdown in the aquifer is 

affected by ground-water flow through, secondary 

porosity features, such as vugs and fractures, that were 

not modeled. However, as the cone of depression 

developed around the pumped well, secondary porosity 

features represented smaller and smaller percentages of 

the total stressed volume of aquifer, and the total system responded regionally as an equivalent porous 

medium as simulated in the model. The transient calibration of the model indicates that the ground-water 

flow system at the old Tampa well field can be modeled 

as layered and horizontally homogeneous and isotropic 

on a local scale. The time required for the aquifer to 

respond t,o pumping as an equivalent porous medium 

probably depends on the percentage of effective secondary porosity in the stressed interval, the withdrawal 

rate, and the permeability and storage properties of the 

stressed interval. It was concluded that the model simulation of water-level declines in response to pumping 

was reasonably accurate (fig. 21).

Numerical modeling of ground-water flow tested 

the conceptualized hydrogeologic model of the Upper 

Floridan aquifer at the old Tampa well field. Results of 

steady-state and transient simulations of onsite aquifer

30 Hydrogeology and Results of Tracer Tests Implications for Wellhead-Protection Strategies in West-Central Florida
Table 8. Water budgets for the Upper Floridan aquifer for steady-state and transient simulations of 50-hour and 

168-hour aquifer tests at the old Tampa well field in June and December 1990 

[ft3/d, cubic feet per day]

Steady-state conditions 

with no pumping

June 1990

Inflow

Storage

Head dependant boundaries

Leakage from surficial aquifer system

Total

Outflow

Storage

Head dependant boundaries

Pumpage

Total

December 1990

Inflow

Storage

Head dependant boundaries

Leakage from surficial aquifer system

Total

Outflow

Storage

Head dependant boundaries

Pumpage

Total

tf/d 

(X1.000) Percent

Steady-state Transient Simulation 

simulation (end of last time step)

tf/d 

(xl.OOO) Percent (x1 ^

50-hour aquifer test, pump well 1 5,

0

632

44

675

0

676

0

575

0

94

6

100

0

100

0 TOO"

0

709

79 7SB"

0

599

189

7S8

0

90

10

TOO

0

76

24

TOO

168-hour aquifer test, pump APPW ,

0

637.5

33.5

WT

0

671

0

67T

0

95

5

TOO

0

100

0

Too

0

828

84

912

0

498

414

9T2

0

91

9

TOO

0

55

45

TOO

Percent

at 189,000 ftrVd Inflow

4

707

79

790

0

601

189

790

0.5

89.5

10

TOT

0

76

24

100

at 41 4,000 frVd Inflow

0.4

828

84

9TZ4"

0

498.4

414

9123

0

91

9

TOO

0

54.6

45.4 roo~~

tests indicate that the transmissivity of the Upper 

Floridan aquifer at the study site is about 60,000 ft2/d 

with the upper 400 ft of the aquifer accounting for 

about 20,000 ft2/d of the total. The storage coefficient 

is on the order of 10~4 . Additional information on the 

hydraulic properties of the upper 400 ft of the Upper 

Floridan aquifer was collected using tracer tests.

Analysis of Model Water Budgets

Water budgets for the Upper Floridan aquifer for 

the steady-state and transient-model simulations were 

analyzed to determine the sources of water for the 

pumpage (table 8), to determine if the sources of water 

indicated by the water budgets of the model were consistent with the conceptual hydrogeologic model and 

were supported by observed data, and to check the 

volumetric calibration of the model. The water budget 

of the steady-state numerical model, with no pumping,

was compared to the water budget of the Upper 

Floridan aquifer at the study site estimated using flownet analysis. The model calculated about 44,000 ft3/d of 

water entering the Upper Floridan aquifer by vertical 

leakage through the quasi-three-dimensional confining 

unit. About 632,000 ft3/d of water entered and about 

676,000 ft3/d left the model through general head 

boundaries. This compares well with the water budget 

of the Upper Floridan aquifer estimated using flow-net 

analysis in which flow through the Upper Floridan 

aquifer was estimated to be about 690,000 ft3/d; 

leakage from the surficial aquifer system accounts for 

approximately 62,000 ft3/d of that total. These 

estimates were based on a 5-ft head difference between 

the surficial aquifer system and the Upper Floridan 

aquifer, a leakance coefficient of 0.00033 for the intermediate confining unit, a transmissivity of 63,000 ft2/d 

for the Upper Floridan aquifer, and a head gradient of 

0.0016 ft/ft across the area.

Numerical Model 31
The water budgets of the steady-state model with 

no pumpage for the conditions in June and December 

1990 and of the model for the 50-hour and 168-hour 

aquifer tests were analyzed to determine the effects of 

withdrawal of water from the model layers simulating 

the Upper Floridan aquifer on leakage from the model 

layer simulating the surficial aquifer system. Withdrawal of water from model layers 2 and 4 (50-hour 

test) induced an additional 35,000 ft3/d of downward 

leakage from model layer 1 through the quasi-threedimensional confining unit. Leakage from layer 1 

accounted for about 42 percent of the pumpage. 

Withdrawal of water from model layer 6 (168-hour 

test) induced an additional 50,000 ft3/d of downward 

leakage from model layer 1 through the quasi-threedimensional confining unit. Leakage from layer 1 

accounted for about 20 percent of the pumpage.

The water in the Upper Floridan aquifer in Hillsborough County is ultimately derived by leakage, but 

the model budgets indicate that the deeper test had less 

effect on leakage within the modeled area than did the 

shallow test. The distribution of sources of water indicated by the model simulations is supported by the data 

collected during aquifer tests at the site. The water level 

in shallow well 16, completed in the intermediate confining unit and located about 700 ft from well 15, 

declined 2.4 ft after 48 hours of pumping well 15 at a 

rate of 980 gal/min; however, the water level in shallow 

well 16, located about 75 ft from the Avon Park production well, declined only 1.4 ft after 48 hours of pumping the Avon Park production well at a rate of 2,150 

gal/min.

The differences between model water budgets for 

the transient simulations of the two aquifer tests indicate the effects of pumping different model layers, with 

different hydraulic properties, at different rates. Model 

layers 2, 3, and 4 collectively have about half the transmissivity of model layer 6, slightly higher storage coefficient values, and a better hydraulic connection to 

model layer 1. As a result, a greater percentage of the 

pumpage is supplied by leakage from layer 1 when 

model layers 2, 3, and 4 are pumped, there is greater 

withdrawal from storage, and there are fewer contributions from the general head boundaries than when 

pumping model layer 6.

Analysis of the model water budgets for steadystate and transient simulations of the same aquifer test 

(table 8) indicates the transient simulation had 

approached steady-state conditions by the end of the 

last time step. By the end of the last time step for the

50-hour and 168-hour tests, less than 1 percent of the 

water budget for the Upper Floridan aquifer was being 

supplied by withdrawal from storage. Examination of 

the water budgets for the various simulations indicates 

that the source of water to the model layers representing the Upper Floridan aquifer is primarily the general 

head boundary cells, indicating that ground-water flow 

through the study site is predominately horizontal. The 

sources of water indicated by the water budgets of the 

various simulations are consistent with the conceptual 

hydrogeologic model. It was concluded that the model 

provides a reasonable approximation of ground-water 

flow through the Upper Floridan aquifer at the study 

site.

Sensitivity Analysis

A sensitivity analysis of the numerical model of the 

ground-water flow system at the old Tampa well field 

was made using the drawdown calculated for the Upper 

Floridan aquifer by the calibrated steady-state model 

simulation of the 50-hour aquifer test. This analysis 

provided information on the relative sensitivity of the 

model to changes in values of the estimated hydraulic 

parameters. The sensitivity of the model was tested by 

making tenfold changes in the leakance coefficients, 

twofold changes in transmissivity of the Upper Floridan aquifer and general head boundary conductance, 

and conversion to no-flow and specified-head boundary 

conditions. One parameter at a time was changed, noting the effect on the drawdown simulated in the Upper 

Floridan aquifer. The model was considered to be sensitive to a parameter if the change in the parameter produced more than 1 ft of change in the mean drawdown 

over the 784 model nodes, or increased the range of 

drawdown values more than 5 ft^ or both. Results of 

sensitivity analyses are summarized in table 9.

Analyses indicated that the model is relatively 

sensitive to changes in the transmissivity of the Upper 

Floridan aquifer and the boundary conditions. Flow 

through the model layers is predominately horizontal; 

therefore, the model is relatively insensitive to changes 

in the vertical conductance values of the model layers 

representing the leakage coefficients of the intermediate confining unit and the Upper Floridan aquifer. The 

vertical conductance terms used for model layers 4, 5, 

and 6 cannot be adequately tested with this method, 

which is consistent with the interpretation of the computed water budgets of the model simulations of the 

aquifer tests.

32 Hydrogeology and Results of Tracer Tests Implications for Wellhead-Protection Strategies in West-Central Florida
Table 9. Results of model sensitivity analysis based on simulation of the 50-hour aquifer test 

[NA, not applicable]

Input 

parameter

Steady-state calibration

Vertical conductance of 

layer 1

Vertical conductance of 

layers 2 through 5

Transmissivity of layers 

2 through 6

General head boundary 

conductance

Factor 

changa in 

parametar

NA

(0.1) 

(10.0)

(0.1) 

(10.0)

(0.5) 

(2.0)

(0.5) 

(2.0)

Range and mean of drawdown in 784 model nodes,

Model layer 2

Range

1.4-10.4

1.9-11.1 

0.3-8.4

1.8-10.9 

1.4-9.6

1.2-18.6 

1.6-6.1

2.5-11.4 

0.8-9.7

Mean

3.4

4.0 

1.7

3.8 

2.9

4.7 

2.6

4.4 

2.7

Modal layer 4

Ranga

1.4-10.3

1.7-10.7 

0.8-9.5

1.8-11.7 

1.4-9.2

1.4-18.3 

1.5-6.2

2.6-11.6 

0.7-9.7

Mean

2.9

3.2 

2.1

3.8 

2.6

3.9

2.4

4.1 

2.2

, in faat

Model layar 6

Range

1.4-2.0

1. 4-2.3 

'0.8-1.3

1.0-1.4 

1.4-2.6

1.3-2.7 

1.4-1.7

2.6-3.2 

0.6-1.3

Mean

1.8

2.1 

1.1

1.3 

2.1

2.2 

1.6

3.0 

1.1

No-flow boundary for NA 17.8-26.2 19.5 20.5-29.1 21.9 20.7-21.0 20.9 

layers 2 through 6

Specified head boundary NA 0.1-8.9 2.1 0.02-8.8 1.6 0.01-0.4 0.3 

for layers 2 through 6__ __ __

The sensitivity analyses indicated that estimates of 

the transmissivity of the Upper Floridan aquifer were 

reasonable approximations of the hydraulic conditions 

at the site. However, the leakance coefficient of the 

intermediate confining unit could be in error by as 

much as an order of magnitude. Sensitivity analyses 

indicated that using no-flow boundary conditions for 

the Upper Floridan aquifer will cause the model to 

greatly overestimate the drawdown and using specified 

head boundaries for the Upper Floridan aquifer results 

in measurable underestimates of the drawdown.

Limitations of the Model Analysis

The numerical model is a simplified mathematical 

approximation of the hydrogeologic model, which is in 

turn a simplified conceptual approximation of a finite 

volume of the ground-water flow system at the old 

Tampa well field. A numerical model will not provide 

accurate predictions on a scale finer than the separation 

of the data points used to build the model. The model 

should be used only for the finite area where the hydrogeology has been defined.

Knowledge of the spatial variation of aquifer characteristics is usually unknown or limited in extent, and 

it is common to assume uniform properties by default. 

The individual layers of the aquifer were assumed to be 

uniformly porous, although field observations indicate

otherwise. This assumption is justified because aquifer 

response to pumping was shown to approximate that of 

a uniformly porous medium within 20 min after pumping began. Generally, a simple model is preferred over 

a more complex one.

The upper 550 ft of the Upper Floridan aquifer was 

divided into two permeable units (model layers 2 and 

4) and two semiconfining units (model layers 3 and 5). 

The remaining thickness of the Upper Floridan aquifer 

was not differentiated into permeable and less permeable layers. The aquifer is probably stratified throughout its thickness, but there are no data to use as a basis 

for further subdivision of the aquifer for depths more 

than 800 ft below land surface. The model, therefore, 

might not accurately represent the lower 550 ft of the 

Upper Floridan aquifer.

The leakance coefficient of the semiconfining unit 

between permeable units two and three of the Upper 

Floridan aquifer is not well quantified at the old Tampa 

well field. The vertical hydraulic conductivities used to 

calculate the vertical conductance for model layers 4,5, 

and 6 were not estimated from core samples collected 

onsite as were those for model layers 2 and 3.

This model is not intended as a management tool. 

The accuracy of the model simulation of aquifer 

response to stress at the old Tampa well field is limited 

to the conditions under which the model was calibrated. The model might not provide accurate simulation results under greatly different conditions.

Numerical Model 33
TRACER TESTS

Test drilling and borehole geophysical data 

confirmed the presence of secondary porosity in the 

Upper Floridan aquifer at the study site. Tracer tests 

were conducted to investigate the importance of 

ground-water flow through secondary porosity and to 

collect additional hydrogeologic information about the 

aquifer. Salt (sodium chloride) and Rhodamine WT 

fluorescent dye were used as tracers. Salt was used for 

short-term tests over small horizontal distances (less 

than 24 hours and less than 25 ft). Rhodamine WT 

fluorescent dye was used for long-term tests over horizontal distances of up to 200 ft because it is detectable 

at concentrations as low as 0.1 ppb.

Techniques of Tracer Testing

Tracer tests were conducted by injecting salt or dye 

into an open-hole interval of an Upper Floridan aquifer 

well. The movement of the ground water induced by 

pumping well 15 at a rate of about 980 gal/min was 

then measured. Tracers were injected using two different methods: (1) circulation of water in the injection 

well to achieve an even mixture of tracer and (2) pulse 

injection using a drop pipe to disperse tracer throughout a specific depth interval or at a specific depth below 

land surface.

The rate and direction of water movement in the 

aquifer were determined by measuring the change in 

tracer concentration in the water. Initial tests involved 

injecting salt into well 11. Static and pumping fluidresistivity logs were run in the injection well and in the 

pumped well to establish baseline conditions. A 

fluid-resistivity log was run in well 11 after the injection of salt to determine the placement of the tracer in 

the well bore. Multiple fluid-resistivity logs were run in 

wells 11 and 15 during the pumping of well 15 to monitor the movement of salty water from the injection 

well to well 15. Subsequent tests involved injecting 

Rhodamine WT into wells 11 and 20.

The movement of ground-water spiked with 

Rhodamine WT was determined by measuring the dye 

concentration in water in the injection well, in well 11 

between the injection well and the pumped well, and in 

the water discharged from the pumped well. Groundwater samples were collected from the injection well at 

discrete depth intervals for 48 hours after the test began 

to determine the rate of depletion of dye and from well 

11 every 2 to 3 days to measure the vertical distribution 

of dye concentration in the upper 400 ft of the Upper

Floridan aquifer. Dye concentration in water discharged 

from the pumped well was measured continuously 

using a fluorometer fitted with a flow-through chamber 

and was recorded every 5 min by an onsite computer 

interfaced with the fluorometer.

The fluorometer used to measure the concentration 

of Rhodamine WT was calibrated before each test with 

standards of 0-, 1-, and 10-ppb concentration. The fluorometer is sensitive to fluctuations in ambient air temperature, which ranged from about 21 to 35 °C during 

the tests. A higher instrument temperature results in a 

higher indicated tracer concentration. Inspection of 

temperature data and dye concentration indicated that 

temperature fluctuations of less than 5 °C did not measurably alter indicated dye concentration. Test data 

were interpreted using dye concentrations that were 

measured when the instrument temperature was within 

a range of 2 °C to compensate for the daily fluctuation 

of temperature.

Preliminary Tests

Several short-term tracer tests were used to determine where water enters the borehole of the pumped 

well and to determine the direction of flow in well 11, 

25 ft east of the pumped well. Flowmeter logs of well 

11, which was drilled to 750 ft and cased to a depth of 

74 ft, indicated upward flow in the borehole from permeable unit three as a result of pumping well 15, which 

is 415 ft deep with 67 ft of casing and open only to permeable units one and two. A well plug was installed at 

a depth of 422 ft in well 11 to eliminate this flow before 

the long-term tracer tests were conducted. This was 

done so that all of the water pumped from well 15 

would come from the section of the aquifer in which 

the tracer was placed, which was necessary for accurate 

calculations of the porosity of the aquifer matrix using 

tracer travel time to the pumped well. Subsequent flowmeter logging detected no vertical flow in the bottom of 

well 11 when well 15 was pumped,, but tracer tests indicated that upward flow of water still occurred in well 11 

when well 15 was pumped.

Rhodamine WT or salt was injected into well 11 at 

specific depths below land surface during the tracer 

tests. In the first test, Rhodamine WT was injected into 

well 11 at a depth of 80 ft below land surface. Well 15 

was then pumped at a rate of 980 gal/min. The arrival 

of dye at the pumped well through a single 1- to 2-in.- 

diameter vug was visible with a borehole video camera. 

The dye traveled between the two wells in about 60

34 Hydrogeology and Results of Tracer Tests Implications for Wellhead-Protection Strategies in West-Central Florida
200 FEETI 2S I l-FEET-j

DISCHARGE=980 

GALLONS PER MINUTE

PUMPED WELL 

15

WELL 11

ui 

I

DC 

CO

i

ffi

I

O

o

o

o

UI

<

INJECTION £ 

WELL jg 

20 O

100 

200 

300 

400 

500 

TO 

DE 

410

casing

open 

hole

TAL 

3TH 

FEET

 SAMPLE DEPTH 80 FEET

 SAMPLE DEPTH 120 FEET

 SAMPLE DEPTH 180 FEET 

 SAMPLE DEPTH 220 FEET 

 SAMPLE DEPTH 260 FEET

 SAMPLE DEPTH 300 FEET 

 SAMPLE DEPTH 340 FEET BLOCKED AT 

SAMPLE DEPTH 380 FEET 34 ET

INJECTION ZONES^11 ^

PACKER AT TOTAL 

422 FEET DEPT1 

400 FEE

1

<

=T

iS5

-s

1

1

1

I 1

rh

"-H

v

 i

SAND 

CLAY

LIMESTONE

Figure 22. Tracer injection and withdrawal system at the old Tampa well field.

seconds, which indicates a direct hydraulic connection 

between wells 11 and 15 at about 82 ft below land surface. Subsequent tests using salt as the tracer confirmed 

the inflow of water to the pumped well at 82 ft below 

land surface.

Additional tracer tests were conducted by injecting 

salt in well 11 at a depth of 380 ft below land surface 

and monitoring the upward movement of salty water in 

the well bore as well 15 was pumped. Salty water from 

a depth of 380 ft in well 11 did not rise above a depth 

of 80 ft below land surface. Fluid resistivity logging 

indicated that salty water from well 11 entered well 15 

at 82 ft below land surface within 30 min after pumping 

of well 15 began. These data indicate that, as a result of 

preferential flow at a depth of about 80 ft, water flows 

upward from the bottom of well 11 to the zone of preferential flow and then laterally to the pumped well.

Deep Test

The first long-term tracer test at the old Tampa well 

field involved injecting 1.32 L of 20-percent solution 

(314 g) Rhodamine WT dye into well 20 that is 200 ft 

east of the pumped well and 175 ft east of well 11. The

borehole of well 20, originally drilled to a depth of 400 

ft, was blocked at 340 ft below land surface. Dye was 

circulated in the open borehole by pumping groundwater through a drop pipe from a depth of 340 ft, spiking the water with dye, and injecting the spiked water 

into well 20 through another drop pipe inserted to a 

depth of 40 ft below land surface. The injection process 

was terminated when the water pumped from 340 ft 

below land surface contained dye. Pumping of well 15 

was then begun. Simultaneous measurements of water 

levels in the injection well and the pumped well indicated that the drawdown cone became steady-shape 

within 20 min of the start of pumping. Well 15 was to 

be pumped continuously until the dye had been completely purged from the aquifer; however, a pump malfunction interrupted the test after about 50 hours and 

periodically thereafter throughout the 124-day test. 

Well 15 was pumped at a rate of about 980 gal/min for 

70 percent of the time (87 out of 124 days) during the 

period from July 18, 1990, to November 18, 1990.

Movement of dye was monitored in three wells 

(fig. 22). Dye concentration was measured periodically 

in samples collected with a thief sampler from well 20 

at depths of 80, 120, 180, 260, and 290 ft below land

Tracer Tests 35
surface to calculate the rate of dye depletion in the 

injection well. The dye concentration in samples collected from drop pipes in well 11 at depths of 80, 120, 

180, 220, 260, 300, 340, and 380 ft below land surface 

was measured periodically to determine the vertical 

distribution of dye at a point between the injection well 

and the pumped well. Dye concentration in water discharged from the pumped well was measured continuously to determine arrival time, peak concentration, 

persistence time, and total mass recovered.

The concentration of dye in water pumped from 

well 15 as a function of time is illustrated in figure 23. 

The pump did not operate continuously for the entire 

test; therefore, the periods of pump shutdown were discounted when calculating dye travel time. The discontinuous pumping probably had two effects on the dye 

arrival: (1) it delayed the arrival time and the time to 

peak concentration of secondary dye peaks, and (2) it 

lowered the peak concentration of later arrivals by dispersing the dye. As a result, computed porosity values 

based on arrival time of those peak dye concentrations 

could be overestimated. The first arrival time of the dye 

was 3.6 hours after pumping began, and the time to

peak dye concentration was 15 hours. Persistence time 

of the dye concentration greater than 0.3 ppb was 50 

hours. A second dye arrival occurred after 36 days of 

pumping. Time to peak dye concentration for the second 

arrival was 48 days. Persistence time following the second arrival was 18 days. Third and fourth dye arrivals 

occurred after 70 and 79 days of pumping, respectively. 

The persistence times of the third and fourth dye arrivals 

were 1 to 2 days; however, it was difficult to determine 

these persistence times because the relative increase in 

dye concentration was small. The persistence time for 

the entire test was greater than the 87 days of pumping. 

About 69 g (22 percent) of the dye was recovered.

Dye concentration in ground-water samples 

collected from drop pipes in well 11 also was measured 

periodically during the test. Dye was detected in samples collected from 380 ft below land surface, 40 ft 

below the depth of the open-hole interval of the injection well. The presence of dye at this depth indicates 

that the injection well is blocked but not plugged at 340 ft 

below land surface, or that there is significant leakage 

through the semiconfining unit separating permeable 

units one and two.

20 30 40 SO 60

PUMPING TIME, IN DAYS

70

Figure 23. Dye-concentration curve for well 15 during the Rhodamine 

WT dye test ending November 18, 1990.

36 Hydrogeology and Results of Tracer Tests Implications for Wellhead-Protection Strategies in West-Central Florida
WELL 15 PUMPING LOGS

(PUMPING RATE 980 

GALLONS PER MINUTE)

FLOWMETER FLUID 

LOG RESISTIVITY

COMPOSITE 

EFFECTIVE

SECONDARY 

POROSITY

TRACER

CONCENTRATION 

IN WELL 11, 22 DAYS 

AFTER TEST BEGAN

Hi 

m

w

Q

HI 

CD

O. 

HI 

Q

100 -

200 

300 -

400 -

67 FEET

^

i

«

OF 10 INCH CASING

I

7

I

J>

r

i

3.080 3,084 

1 1

vvvv FRACTURE XXXX poROSITY

0000 POROSITY

XXXXXXXXXXXXXX

ooooooooooooo 

ooooooooooooo

oooooooocoooo

ooooooooooooo

xxxxxxxxxxxxxx

ooooooooooooo

xxxxxxxxxxxxxx

ooooooooooooo

xxxxxxxxxxxxxx 

xxxxxxxxxxxxxx

74 FEET OF 

4 INCH CASING

{ 1

V

\

J/

/

0.5 1.0 1.5 2.0

1 1 1 1

25 50 75 

PERCENT OF 

TOTAL FLOW

OHMS PARTS PER BILLION

Figure 24. Correlation between effective secondary porosity and tracer 

concentration in well 11 at the old Tampa well field.

A correlation exists between elevated dye concentration in samples collected from drop pipes in well 11 

and effective fracture porosity (fig. 24). There are two 

possible explanations for the increased dye concentration 

in zones of effective fracture porosity. The first is that 

ground water in the zones of effective fracture porosity 

moves along a more direct path than ground water in 

the aquifer matrix. The dye would be less dilute as a 

result. Another explanation is that less sorption of the 

dye occurs in zones of effective fracture porosity 

because the surface area of the aquifer exposed to the 

water is smaller, which would result in a larger relative 

concentration of dye in conduit flow zones.

Shallow Test

A second test using well 20 as the injection well 

was initiated after the Rhodamine WT concentration in 

water discharged from well 15 decreased to 0.08 ppb, 

which was near the background level of 0.05 ppb. The

test was conducted to determine how a contaminant 

introduced into the upper permeable unit of the Upper 

Floridan aquifer might move to a pumped well. First, a 

plug was installed in the injection well at a depth of 90 ft, 

making the open-hole interval 39 to 90 ft below land 

surface. Pumping of well 15 began several days prior to 

dye injection, not only to further purge the previously 

introduced dye from the aquifer, but also to establish a 

steady-state flow field in the vicinity of the injection 

well. For the shallow test, 1.9 L of 20-percent solution 

(452 g) Rhodamine WT dye was introduced into well 

20 from 39 to 90 ft below land surface using the pulse 

method. This method involved inserting a drop pipe to 

90 ft below land surface in the injection well. The dye 

was poured into the drop pipe and followed with the 

volume of water required to place the slug of dye in the 

drop pipe from 39 to 90 ft below land surface. Removing the drop pipe left a column of dye in the open-hole 

interval of the injection well.

Tracer Tests 37
Well 15 was pumped at a rate of about 980 gal/min. 

The pump stopped periodically, as it did during the first 

test. From the startup on February 7 to shutdown on 

April 6, the pump ran 89 percent of the time (51 out of 

57 days). The concentration of dye as a function of time 

in water pumped from well 15 is shown in figure 25. 

The first arrival time of dye was 5 hours. Dye concentration stabilized after 9 days, but did not decline. A 

second dye arrival was indicated at 15 days, and a third 

arrival occurred after 43 days of pumping. The peak 

concentration and persistence time could not be determined because the pump failed before the dye concentration had peaked. About 50 g (11 percent) of the dye 

was recovered.

20 30 40

PUMPING TIME, IN DAYS

Figure 25. Dye-concentration curve for well 15 during the 

Rhodamine WT dye test ending April 6, 1991.

Analysis of Tests

Analysis of the dye test data was complicated by 

the low recovery percentage and the multiple dye arrivals. Recent investigations of sorption of Rhodamine 

WT indicate that it is not totally conservative. Dye 

sorption tests performed by Sabatini and Austin (1991) 

indicate that the level of adsorption of Rhodamine WT 

dye increases with increasing valence and concentration of background cations. Adsorption of Rhodamine 

WT also increases in the presence of calcium chloride 

(Sabatini and Austin, 1991). The use of salt as a 

ground-water tracer during preliminary tests might 

have increased the sorption of the Rhodamine WT. 

Sorption probably had a significant role in attenuating 

the dye and reducing the amount of dye recovered; 

however, the shape of the tracer concentration curve as 

a function of time is more important than the percent 

recovery.

The multimodal distribution of dye arrival during 

the deep tracer test is representative of ground-water 

flow through different flow media. The rapid peak and 

decline of the initial dye arrival for the deep tracer test 

are consistent with flow through secondary porosity 

(Mull and others, 1988). The initial dye arrival represents the travel time and transport properties of the part 

of the aquifer having interconnected secondary porosity (solution conduits or fractures), possibly through 

the zone of effective vug porosity identified at 82 ft 

below land surface in wells 15 and 11 during preliminary tracer tests and through the zone of effective fracture porosity identified at depths of 250 and 266 ft 

below land surface in wells 15 and ISA using geophysical logs and borehole videos (figs. 9 and 10). The 

secondary dye arrival is probably representative of the 

matrix flow arrival. The third and fourth dye arrivals 

(fig. 23) during the deep test probably do not represent 

significant components of flow in the aquifer because 

of the small increase in dye concentration and the short 

persistence time.

Interpretation of the shallow tracer test is complicated by the poor separation of the dye arrivals into distinct events. The rapid dye arrival at the pumped well 

can only be explained by ground-water flow through a 

conduit or fracture. The lack of a peak-and-decline 

cycle of dye concentration for the first dye arrival indicates that secondary porosity probably is a more important mechanism for ground-water flow in this part of 

the aquifer than it is in the aquifer as a whole.

Vugs are the dominant form of effective secondary 

porosity in the first 200 ft of the Upper Floridan aquifer 

(fig. 10). Fracture porosity occurs primarily below 200 

ft below land surface. The rapid rise and decline of dye 

concentration during the first dye arrival during the 

deep tracer test could be attributed to ground-water 

flow through fracture porosity. The shape of the dyeconcentration curve for the shallow tracer test might be 

more representative of ground-water flow through vug 

porosity.

The tracer test data were evaluated using the volumetric displacement method, a Laplace transform solution curve-matching method, and a tracer-dilution 

method to estimate the porosity, longitudinal dispersivity, and hydraulic conductivity of the aquifer. Each 

method assumes a uniformly porous aquifer. The 

results of the analyses are representative of only the 

aquifer matrix and could misrepresent conditions in a 

doubly porous aquifer.

38 Hydrogeology and Results of Tracer Tests Implications for Wellhead-Protection Strategies in West-Central Florida
Volumetric Displacement Method

The effective porosity of the injected zone was 

estimated based on the time required to remove the 

volume of water in the aquifer between the injection 

well and the pumped well at the specified discharge rate. 

This is expressed as (Florida Department of Environmental Regulation, 1987):

tQ

n p = 100, (1)

where

n is the effective porosity, in percent; 

t is the time required to replace the volume of water 

between the pumped well and the injection well (the 

time to peak tracer concentration), in days; 

Q is the discharge rate, in cubic feet per day; 

r is the radius from the pumped well to the injection well,

in feet; and 

h is the thickness of the injected zone, in feet.

The effective porosity of the aquifer for the deep tracer 

test, based on a 15-hour time to peak concentration, a discharge rate (Q) of 980 gal/min, a straight-line travel distance (r) of 200 ft, and an injected thickness (h) of 300 ft, is 

0.3 percent. This porosity value is representative of the 

effective secondary porosity, possibly fracture porosity. 

Based on the time to the second tracer peak of 48 days, the 

discharge rate, and the distance to the injection well, the 

effective porosity of the aquifer matrix is 24 percent. This 

value is within the range of porosities measured in limestone cores collected from the Upper Floridan aquifer in 

southwestern Florida (table 4).

A similar analysis was performed for the shallow 

tracer test using a discharge rate equal to the volume of 

water flowing to the pumped well from the section of the 

aquifer injected, or Q = 440 gal/min x '/4 of the thickness 

of permeable unit one = 110 gal/min, an injected thickness 

(h) of 51 ft, and r = 200 ft. The effective secondary porosity of the part of the aquifer tested is 0.3 percent, based on 

a 21-hour time to peak concentration for the first arrival. 

The estimated effective porosity of the aquifer matrix is 14 

percent, based on the arrival time of 43 days for the last 

tracer arrival. Because this estimate is made using an 

arrival time rather than a time to peak concentration, the 

porosity estimate represents a minimum value.

Curve-Matching Method

A Laplace transform solution for the injection of 

tracer in a well situated in a homogeneous aquifer with an 

existing steady-state, horizontal, radially convergent flow 

field (Moench, 1989) was used to estimate the effective 

porosity and longitudinal dispersivity of the aquifer using

the results of the deep tracer test. The advection-dispersion 

equation for plane radial flow in a porous medium is used 

as the controlling equation (Moench, 1989).

= R , 

dt

- (2) [ ^1 _ ^

r 3r L L 3rJ V 3r 

where 

r is the radial distance from the center of the pumped well to

the point of interest, in feet; 

DL is the coefficient of longitudinal hydrodynamic dispersion,

in feet squared per day; 

C is the tracer concentration in the aquifer at the time of

interest averaged over the vertical surface of area 2rc rh, 

in parts per billion; where h is the injected thickness, in 

feet;

v is the particle velocity, in feet per day; 

R is the tracer retardation factor, dimensionless; and 

t is the time from the start of the tracer test, in days.

Estimates of aquifer matrix porosity and dispersivity are 

made using theoretical arrival curves for different peclet 

numbers, in the same way that type curves are used to analyze aquifer test data (fig. 26). Effective porosity is estimated 

using the matchpoints from the type curve and equation 3:

t

nh -rw2 )'td/Q' (3)

where

ne is effective porosity, in percent; 

t is the match point time, in days; 

h is the injected thickness, in feet; 

rL is the radius to the injected well, in feet; 

rw is the radius of the withdrawal well, in feet; 

td is the dimensionless match point time; and 

Q is the withdrawal rate, in cubic feet per day.

TIME (t), IN DAYS

1 10 100 1.0

o

O

O 

O

_ 

2 

O 

W 

2 

LU

Q

1 U

o a

0.6

0.4

0.2

0 

0

........

-

MATCH POINT

ld=02 

1=10 DAYS '

, , ..,.,., . , , J

 TYPE CURVE

PECLET NUMBER- 150

»- DATA PLOT

.1 V 1 1 1 1 1 Mil 1 1 1 1 1 ll.l

01 0.1- 1 10 10 °

DIMENSIONLESS TIME (t d )

Figure 26. Analysis of the dye-concentration data for well 15 for 

the test ending November 18,1990, using the Laplace transform 

solution for radially convergent flow.

Tracer Tests 39
The longitudinal dispersivity is estimated using 

equation 4:

where

(4)

Js the longitudinal dispersivity, in feet; 

rL is the radius to the injection well, in feet; and 

Pe is the Peclet number of the type curve that the field data

best fit, dimensionless.

Based on this method, the effective porosity of the 

aquifer was estimated to be 25 percent when the second 

dye arrival of the deep tracer test was analyzed, which 

agrees well with estimates of the effective porosity of 

the aquifer matrix based on rock-core analysis (21 

percent) and observed arrival time (24 percent).

The aquifer dispersivity also was estimated using 

the Laplace transform solution described by Moench 

(1989). The flow medium through which each tracer 

arrival moved was treated separately. Longitudinal 

dispersivity for the matrix flow tracer arrival was 

calculated by matching the shape of the tracer timeconcentration curve of the deep injection test to type 

curves. No estimate was made for the part of the aquifer 

through which the first dye arrival moved because flow 

conditions in the secondary porosity might not be 

Darcian. The longitudinal dispersivity of the aquifef 

matrix calculated using this method is 1.3 ft. This value 

is representative for diffuse flow through porous media 

where advection is the dominant mechanism of solute 

transport.

Tracer-Dilution Method

Ground-water samples were collected with a thief 

sampler at specific depths from well 20 during the first 

48 hours of the deep tracer test. The rate of decline of 

tracer concentration in the samples from the injection 

well was analyzed using a method described by Sun 

(1976). The horizontal hydraulic conductivity of the 

aquifer is calculated using the equations

(C-CG)I(CO-CG) = EXP(-ALvt/V), (5)

K = v/7, (6) 

where

C is the tracer concentration at time t, in parts per billion; 

CG is the background tracer concentration (equal to 0 for

this test);

CO is the initial tracer concentration, in parts per billion; 

A is twice the borehole diameter for open-hole wells, in

feet;

L is the length of the sampled interval (equal to 1 ft for 

the thief sampler);

v is the flow velocity in the well, in feet per day; 

t is the elapsed time, in days; 

V is the volume of water in the sampled interval (equal to

7tL(A/4)2), in cubic feet; 

K is the horizontal hydraulic conductivity of the aquifer

matrix, in feet per day; and

I is the ground-water gradient (dimensionless) between 

the injection well and the pumped well.

The horizontal hydraulic conductivity for 80 ft 

below land surface is calculated using equations 5 and 

6 and the data in table 10 as follows:

(C-CG)/(CO-CG) = EXP (-ALvt/V)

(90-0)7(160-0) = EXP[(-(1 ft)(l ft) v (0.17 d)/(rcL(A/4)2]

0.56 = EXP[-(0.17 ft27d) ft)(0.0625 ft2))]

0.56 = EXP[-(0.17 ft2/d) v/0.20 ft3 ] 

ln(0.56) = -0.85 d/ft(v) 

-0.58 = 0.85 d/ft(v) 

v = 0.68 ft/d.

K = v/I

K = 0.68/0.043

K= 16 ft/d.

The hydraulic conductivity was calculated at five 

depths in the upper 300 ft of the Upper Floridan aquifer 

(permeable unit one and part of the semiconfining unit) 

(table 10). The calculated hydraulic conductivities 

range from 16 to 34 ft/d and average 26 ft/d. The equivalent range in transmissivity values is from 4,800 to 

10,200 ft2/d, and the average is 8,000 ft2/d. These 

values agree reasonably well with the range of reported 

values of 7,000 to 14,000 ft2/d (table 3) for the upper 

300 ft of the aquifer and with the value estimated from 

the 50-hour aquifer test and the numerical model 

(1 0,500 ft2/d).

The theoretical arrival time of the tracer mass was 

calculated using the matrix porosity and hydraulic 

conductivity calculated with the methods described in 

Freeze and Cherry (1979, p. 71) by

V = (KI x 100)/ne (7)

t = r/V, (8)

where

V is the average linear velocity, in feet per day; 

K is the horizontal hydraulic conductivity, in feet per day; 

I is the ground-water gradient, dimensionless (foot per

foot);

ne is the effective porosity of the aquifer matrix, in percent; 

t is the arrival time of the tracer mass, in days; and 

r is the radial distance from the pumped well to the 

injection well, in feet.

40 Hydrogeology and Results of Tracer Tests Implications for Wellhead-Protection Strategies in West-Central Florida
Table 10. Horizontal hydraulic conductivity estimates based on tracer 

dilution in the injection well

[A, twice the bore hole diameter for open hole wells, in feet; V, volume of water in 

the sampled interval (equal to jiL(A/4)2), in cubic feet; t, elapsed time, in days; 

C, tracer concentration at time t, in parts per billion; CO, initial tracer 

concentration, in parts per billion; I, the ground-water gradient, dimensionless, 

between the injection well and the pumped well; v, flow velocity in the well, in feet 

per day; K, horizontal hydraulic conductivity of aquifer matrix, in feet per day; 

 , no data]

Sample depth .. 

(feet) c/co

80

120

180

260

290

1.0 0.2

0.17

.47

1.0 .2

.17

.47

.84 .14 

.17

.47

.76 .11

.17

.46

.76 .11

.42

160

90

16

465

135

13

370

80

6.9

300

80

7.3

198

7.8

_

0.56

.10

.29

.028

.216

.0186

.266

.024

_

.0394

_

0.043

.042

.043

.042

.043

.042

.043

.042

_

.0417

_

0.68

.71

1.46

1.09

1.47

1.01

1.16

1.01

_

1.12

_

16

17

34

26

34

24

27

24

_

27

The calculated theoretical arrival time of the tracer mass 

is 46 days when using an average hydraulic conductivity of 26 ft/d and a pumping ground-water gradient (I) 

of 0.042 ft/ft between the injection well and the pumped 

well and assuming a uniform distribution of porosity 

(ne) of 25 percent. The close agreement between the 

estimated and observed tracer arrival times during the 

deep tracer test indicates that the ratio of hydraulic 

conductivity to effective porosity is accurate.

The results of tracer tests at the old Tampa well 

field indicate the heterogeneity of the Upper Floridan 

aquifer. The multiple dye arrivals indicate groundwater flow through a dual porosity aquifer. Analysis 

of tracer concentration data representative of groundwater flow through the aquifer matrix indicates an 

effective porosity of about 25 percent, a longitudinal 

dispersivity of 1.3 ft, and an average hydraulic 

conductivity of 26 ft/d.

SIMULATION OF TRACER TEST WITH 

PARTICLE-TRACKING PROGRAM

The particle-tracking program MODPATH 

(Pollock, 1989) was used to simulate the ground-water 

travel times measured during the deep tracer test. The 

objective of the particle-tracking simulation was to 

compare ground-water travel times in an equivalent 

porous media model of the ground-water flow system 

with observed ground-water travel times. Successful

simulation of the assumed matrix flow arrival time of 

the tracer would support the assumption of equivalent 

porous media properties for the aquifer matrix. A 

particle-tracking simulation of the ground-water travel 

times measured during the deep tracer test was made 

because the test was performed in that zone of the 

aquifer most likely to be contaminated and also the 

zone most likely to be utilized for water supply.

The MODPATH program computes particle 

locations and travel times in three dimensions based on 

advective flow in a uniformly porous medium. 

MODPATH can track particles forward in time and 

space in the direction of ground-water flow, or backward from model cells toward ground-water recharge 

locations. Mixing of chemical constituents by hydrodynamic dispersion is not considered; therefore, the 

model cannot be used to predict solute concentrations. 

However, it can be used to estimate the area of contribution and the time of travel to a well or a well field.

The cell-by-cell flow terms from the steady-state 

MODFLOW model simulation of the USGS aquifer 

test were used as input to MODPATH. MODPATH was 

used to simulate the deep tracer test that used well 20 

as the injection well. The Upper Floridan aquifer was 

modeled using a uniform value of effective porosity for 

each hydrogeologic unit represented with a model 

layer. The effective porosity of the aquifer matrix 

calculated from tracer tests was about 25 percent, but

Simulation of Tracer Test with Particle-Tracking Program 41
RECOVERY 

WELL 15

h- 200 FEET INJECTION 

WELL 20

IIII 111

uZ

llf

tr

w

Q

z

0 i

200 

400 

600 

3

in

CO

OL 

111 

Q

800 

1,000 

[ SURFICIAL AQUIFER SYSTEM ]

49-DAY 

ARRIVAL

9-DAY 

ARRIVAL

»

3

4

5

R MODEL 

0 LAYERS

_L

Particle location at 2-day time steps 

N. effective porosity of n=0.21 or

\

~\_

*

Permeable

using 

21 percent

Permeable

 . Unit Two

Permeable

Unit Three

Middle Confining Unit of

Floridan Aquifer System

Figure 27. MODPATH particle-tracking simulation of the matrix flow for the dye test ending November 18, 

1990, assuming a uniform effective porosity of 21 percent for the Upper Floridan aquifer.

this value is probably too high as a result of the discontinuous pumping. An effective porosity of 21 percent, 

determined from analyses of rock cores, was used for 

model layers 2 and 4 representing permeable units one 

and two of the Upper Floridan aquifer, respectively.

Results of simulating the deep tracer test are shown 

in figure 27. A travel time of 49 days was calculated for 

layer 2, and 9 days for layer 4. Ground-water samples 

collected at 380 ft below land surface from well 11 contained tracer, indicating that the tracer was present in 

permeable unit two of the Upper Floridan aquifer. The 

8- to 10-day arrival time predicted by MODPATH for 

model layer 4 was not observed in the dye concentration data collected from well 15, but that arrival could 

have been masked by the trailing edge of the preceding 

dye peak. These results agree well with the observed 

dye arrival time of 48 days and dye arrival times 

estimated with analytical equations.

A MODPATH simulation of a hypothetical tracer 

test was used to estimate the effective porosity value 

needed to generate a 15-hour travel time from the injection well to the pumped well at the given discharge rate. 

An effective porosity of 0.3 percent for model layer 2, 

and 1.5 percent for model layer 4 was necessary to produce the observed 15-hour time to peak tracer concentration. This test supports the assumption of flow 

through secondary porosity at the old Tampa well field, 

because these effective porosity values are about one to 

two orders of magnitude lower than the range of effective porosity values reported for the matrix of the 

Upper Floridan aquifer (table 4), and they are equivalent to the effective porosity values calculated using the 

time to peak tracer concentration for the first tracer 

arrival and the volumetric displacement method.

42 Hydrogeology and Results of Tracer Tests Implications for Wellhead-Protection Strategies in West-Central Florida
STRATEGIES FOR WELLHEAD 

PROTECTION

Previous strategies for wellhead protection were 

based on the assumption of a uniform distribution of 

porosity in the Upper Floridan aquifer. Wellhead-protection zones, delineated from contributing areas to 

wells, were determined in 1988 for 336 wells and 8 

well fields permitted for average daily withdrawal rates 

of 100,000 gal or more per day (Vecchioli and others, 

1989) using the existing FDEP guidelines. The vertical 

travel time from the water table to the top of the Upper 

Floridan aquifer and the radius of the protection zone 

were calculated using simple analytical equations. 

Vecchioli and others (1989), acknowledging the limitations of the assumptions, evaluated the effects of 

hydrogeologic factors such as the natural slope of the 

potentiometric surface, hydrologic boundaries, porosity variations, and differences in aquifer thickness on 

the size and shape of the protection zones delineated by 

using the FDEP guidelines. The establishment of a 

hierarchy of wellhead protection on the basis of the calculation of vertical travel time from the water table to 

the Upper Floridan aquifer, however, does not consider 

direct connection of the water table to the Upper Floridan aquifer through sinkholes, excavations, and 

macropores.

The data collected at the old Tampa well field 

indicate that the assumption of a uniform distribution 

of effective porosity in the Upper Floridan aquifer is 

inappropriate. Tracer tests performed at the study site 

indicate that the Upper Floridan aquifer could be modeled best as a dual porosity, ground-water flow system. 

Flow takes place through two media: effective secondary porosity and the aquifer matrix. Also, the results of 

tracer tests indicate that the time required for displacement of water in a specified volume of aquifer, assuming a uniform distribution of effective porosity, is not 

the minimum travel time between two points within the 

aquifer. Ground-water travel times for distances of a 

few hundred feet through the aquifer matrix are on the 

order of months or years, but travel times in the effective secondary porosity are measured in hours or days. 

However, the results of borehole video surveys, aquifer 

tests, and numerical modeling indicate that vertical 

movement of water across less-permeable units of the 

Upper Floridan aquifer is primarily through the matrix 

of the aquifer. For this reason, simulation of groundwater flow through the matrix of the Upper Floridan 

aquifer need to be considered in the implementation of 

wellhead-protection strategies.

The objective of wellhead-protection strategies is to 

limit contributions to the pumped interval from zones 

that could be contaminated. It is advantageous to conduct borehole tests such as flowmeter logging, video 

surveys, and geophysical logging to determine the distribution and nature of porosity and permeability in the 

aquifer. The first permeable unit below the intermediate 

confining unit of the Upper Floridan aquifer is the most 

likely to be contaminated, and pumping from that unit 

could induce leakage from the surficial aquifer system. 

Alternative well-field designs that incorporate more production wells, deeper casings, and moderate pumping 

rates should reduce leakage from the overlying sediments in the immediate vicinity of the pumped well(s).

Computer simulation of the effects of different 

stresses on the ground-water flow system provides 

standards for comparison that can be used to develop 

wellhead-protection strategies. The wellhead-protection strategy presented here is based on limiting the use 

of shallow permeable units of the Upper Floridan 

aquifer because those units could have well developed 

zones of effective secondary porosity and are the most 

vulnerable to contamination. Withdrawal of water from 

shallow permeable units of the Upper Floridan aquifer 

also can induce leakage from the surficial aquifer system, thereby increasing the potential for contamination. 

For this study, the effectiveness of four wellheadprotection strategies was evaluated using the 

calibrated, steady-state, MODFLOW model and 

MODPATH.

Alternative Well Construction

Two particle-tracking simulations were run to 

demonstrate how the area of contribution to a pumped 

well might change when the well casing is deepened. 

The simulation of aquifer response to a production well 

having 50 ft of casing and open to the upper 375 ft of the 

Upper Floridan aquifer (permeable units one and two 

and the intervening semiconfining unit, model layers 2, 

3, and 4) is used as a standard for comparison (fig. 28). 

This is compared to the simulated response to a pumped 

well with 380 ft of casing (fig. 29) open to the lower 770 

ft of the Upper Floridan aquifer (permeable units two 

and three and the intervening semiconfining unit, model 

layers 4,5, and 6). The pumping rate is 1 Mgal/d for each 

test. A MODPATH particle-tracking analysis was 

performed for each simulation by backtracking 100 to 

400 particles from the uppermost model layer stressed 

by the pumped well toward the recharge locations.

Stategies for Wellhead Protection 43
The area encompassed by the 10-year time of 

travel in model layer 2 to the single well was 0.18-mi2 . 

Forty-five percent of the total pumpage was derived 

from model layer 2 (fig. 28). The area of contribution 

encompassed by the 30-year time of travel in model 

layer 2 to the single well includes the model boundaries 

and may not be accurate. Therefore, the 0.18-mi2 area 

encompassed by the 10-year time of travel in model 

layer 2 was used as the standard for comparison.

The area of contribution from layer 2 encompassed 

by the 10-year time of travel was zero for the second 

simulation. The area of contribution from layer 2 

encompassed by the 30-year time of travel was 0.08 mi2

in the second simulation and supplied 6 percent of the 

pumpage (fig. 29). Thus, deepening the well casing so 

that the well withdrew water directly from permeable 

units two and three approximately halved the area of 

contribution from the layer most likely to be contaminated and increased the time of travel to the well.

Alternative Well-Field Design

A particle-tracking analysis also was made to compare the area of contribution for a single deep well with 

380 ft of casing pumped at a rate of 1 Mgal/d (fig. 29) to 

that of two wells with 380 ft of casing pumped at a rate

DIRECTION OF REGIONAL 

GROUND-WATER FLOW

EXPLANATION

LINE ENCOMPASSES AREA OF CONTRIBUTION 

FROM PERMEABLE UNIT ONE OF THE UPPER 

FLORIDAN AQUIFER AFTER 10 YEARS 

OF PUMPING \

0 1,000 FEET

DIRECTION OF REGIONAL 

GROUND-WATER FLOW

EXPLANATION

LINE ENCOMPASSES AREA OF CONTRIBUTION 

FROM PERMEABLE UNIT ONE OF THE UPPER 

FLORIDAN AQUIFER AFTER 30 YEARS 

OF PUMPING

1,000 FEET i I

200 METERS 200 METERS

CONDITIONS SIMULATED CONDITIONS SIMULATED

Pumping rate 

Pumped interval

Well depth 

Casing depth

Depth to top of permeable unit one

Depth to bottom of permeable

unit one

Production rate from permeable 

unit two

Number of particles

Area of contribution from permeable

unit one of Upper Floridan aquifer

- 1 million gallons per day

- model layers 2, 3, 4; permeable units 

one and two; and intervening 

semiconfining unit

- 425 feet

- 50 feet

- 50 feet

- 250 feet

- 450,000 gallons per day

- 100 placed in permeable unit one

- 0.18 square mile

Figure 28. Simulated area of contribution encompassed by 

the 10-year time of travel from permeable unit one to a well 

completed in permeable units one and two of the Upper 

Floridan aquifer.

Pumping rate 

Pumped interval

Well depth 

Casing depth

Depth to top of permeable unit one

Depth to bottom of permeable

unit one

Production rate from permeable 

unit two

Number of particles

Area of contribution from permeable

unit one of Upper Floridan aquifer

1 million gallons per day

model layers 4, 5, 6; permeable units

two and three; and intervening

semiconfining unit

1,100 feet

380 feet

50 feet

250 feet

60,000 gallons per day

400 placed in permeable unit one 

0.08 square mile

Figure 29. Simulated area of contribution encompassed by 

the 30-year time of travel from permeable unit one to a well 

completed in permeable units two and three of the Upper 

Floridan aquifer.

44 Hydrogeology and Results of Tracer Tests Implicetions for Wellheed-Protection Strategies in West-Central Florida
of 0.50 Mgal/d each (fig. 30). All wells withdrew water 

from model layers 4, 5, and 6, simulating permeable 

units two and three and the intervening semiconfining 

unit of the Upper Floridan aquifer. The analysis was performed by seeding 400 particles in layer 4 at each well 

and backtracking them toward the recharge locations.

The areas of contribution from model layer 2 were 

delineated for two production wells aligned parallel to 

the regional ground-water flow. The area encompassed 

by the 30-year travel time to the wells from model layer 

2 was 0.097 mi2 (fig. 30), which is slightly larger than 

the 0.08-mi2 area encompassed by the 30-year travel

time to a single well (fig. 29). This area supplied 10 

percent of the total pumpage. The orientation of wells 

parallel to the regional ground-water gradient causes 

the cones of depression of the individual wells to overlap one another. The overlap induces greater leakage 

from overlying layers, which produces an additional 

area of contribution from model layer 2 between the 

two pumped wells (fig. 30).

A similar analysis was made to show the areas of 

contribution from model layer 2 to two wells pumped at 

a rate of 0.5 Mgal/d each when the wells are aligned perpendicular to the regional ground-water flow (fig. 31).

DIRECTION

OF REGIONAL

GROUND-WATER

FLOW

EXPLANATION

LINE ENCOMPASSES AREA DF CONTRIBUTION 

FROM PERMEABLE UNIT ONE OF THE UPPER 

FLORIDAN AQUIFER AFTER 30 YEARS 

OF PUMPING \

1,000 FEET

AREA OF 

CONTRIBUTION

DIRECTION OF REGIONAL 

GROUND-WATER FLOW

EXPLANATION

LINE ENCOMPASSES AREA OF CONTRIBUTION 

FROM PERMEABLE UNIT ONE DF THE UPPER 

FLORIDAN AQUIFER AFTER 30 YEARS 

OF PUMPING \

1,000 FEET

200 METERS 200 METERS

CONDITIONS SIMULATED CONDITIONS SIMULATED

Pumping rate 

Pumped interval

Well depth 

Casing depth

Depth to top of permeable unit one

Depth to bottom of permeable

unit one

Production rate from permeable 

unit two

Number of particles

Area of contribution from permeable

unit one of Upper Floridan aquifer

- 1 million gallons per day

- model layers 4, 5, 6; permeable units 

two and three; and intervening 

semiconfining unit

- 1,100 feet

- 380 feet

- 50 feet

- 250 feet

- 60,000 gallons per day

- 400 placed in permeable unit one

- 0.097 square mile

Figure 30. Simulated area of contribution encompassed by 

the 30-year time of travel from permeable unit one to two wells 

completed in permeable units two and three of the Upper 

Floridan aquifer and aligned parallel to the regional groundwater flow.

Pumping rate 

Pumped interval

Well depth 

Casing depth

Depth to top of permeable unit one

Depth to bottom of permeable

unit one

Production rate from permeable 

unit two

Number of particles

Area of contribution from permeable

unit one of Upper Floridan aquifer

- 0.5 million gallons per day

- model layers 4, 5, 6; permeable units 

two and three; and intervening 

semiconfining unit

- 1,100 feet

- 380 feet

- 50 feet

- 250 feet

- 60,000 gallons per day

- 400 placed in permeable unit one

- 0.06 square mile

Figure 31. Simulated area of contribution encompassed by 

the 30-year time of travel from permeable unit one to two wells 

completed in permeable units two and three of the Upper 

Floridan aquifer and aligned perpenticular to the regional 

ground-water flow.

Stategies for Wellhead Protection 45
Table 11 . Areas of contribution simulated for four wellheadprotection strategies

[Mgal/d, million gallons per day; ft, feet; mi2 , square miles]

Percent 

Number Well Pumped Total area of of

of discharge depth Time contribution pumpage 

pumping rate interval (years) from layer 2 derived 

wells (Mgal/d) (ft) (mi2) from

layer 2

1

1

2

2

1.0

1.0

.5

.5

50-425

380-800

380-800

380-800

10

30

30

30

0.18

.08

.097

.06

45

6

10

6

The total area of contribution to the wells from layer 2, 

encompassed by the 30-year time of travel to the wells 

from layer 2, was 0.06 mi2 and supplied 6 percent of the 

total pumpage. This area was the smallest calculated by 

the four computer simulations.

The results of the particle-tracking simulations 

indicate that deepened well casings and decentralized 

withdrawal decreased the leakage and total area of contribution from the overlying layers to the pumped interval, thus reducing the potential for contamination of 

wells. The use of multiple wells as a protection strategy 

is most effective when wells are oriented perpendicular 

to the ground-water flow, thereby reducing the potential for overlapping cones of depression. The wellheadprotection strategies reduced the area of contribution 

from layer 2 by 47 to 67 percent, and the contribution 

from layer 2 decreased by 78 to 87 percent (table 11). 

Simultaneously, the time related area of contribution 

from the aquifer matrix to the well was increased 200 

percent, theoretically increasing the response time to a 

contaminant spill. The conclusions drawn from the 

particle-tracking simulations can be applied to any 

vertically layered aquifer system.

SUMMARY AND CONCLUSIONS

The Upper Floridan aquifer is the primary source 

of water supply for west-central Florida. The aquifer is 

at risk over much of the area from sources of contamination at land surface. An evaluation of wellheadprotection strategies for the Upper Floridan aquifer was 

made using the 40-acre old Tampa well field in northeastern Hillsborough County as a test site.

Three hydrogeologic units were identified in the 

ground-water flow system at the old Tampa well field: 

the surficial aquifer system, the intermediate confining 

unit, and the Floridan aquifer system. The surficial 

aquifer system and the intermediate confining unit 

overlie and collectively confine the Floridan aquifer 

system.

The top of the Floridan aquifer system at the study 

site was penetrated by wells at different depths. The 

results of test drilling and surface geophysical surveys 

support the interpretation of the top of the aquifer as a 

paleokarst surface. The Floridan aquifer system is 

divided into an upper and lower aquifer by a middle 

confining unit of lower permeability. The Upper Floridan aquifer at the old Tampa well field can be further 

subdivided into at least three permeable units separated 

by semiconfining units of lower permeability. Zones of 

effective secondary porosity are layered throughout the 

upper 400 ft of the aquifer. Rock cores from the Upper 

Floridan aquifer at the site have effective porosity 

values of 21 to 46 percent.

The results of an aquifer test in July 1990 indicate 

that the Upper Floridan aquifer at the well field 

responds to long-term pumping as an equivalent porous 

medium. The transmissivity of the upper 400 ft of the 

Upper Floridan aquifer was calculated to be about 

23,000 ft2/d. The transmissivity estimate is representative of the cumulative transmissivity of two permeable 

units within the aquifer and an intervening semiconfining unit. The transmissivity of the individual units was 

estimated by dividing the total transmissivity proportionally among the units based on pumping flowmeter 

logs. The estimated transmissivities are 10,350 ft2/d for 

permeable unit one, 1,150 ft2/d for the semiconfining 

unit, and 11,500 ft2/d for permeable unit two. It is difficult to use analytical models to estimate the hydraulic 

coefficients of the Upper Floridan aquifer because the 

ground-water flow system is complex.

Uniform porosity numerical models were used to 

simulate flow in the Upper Floridan aquifer at the old 

Tampa well field. The results of the aquifer test of July 

1990 were used to calibrate a numerical aquifer simulation model of the ground-water flow system at the 

site. The model calibration was tested by simulating the 

results of an aquifer test conducted in December 1990. 

Results of numerical modeling indicate that the transmissivity of the upper 400 ft of the Upper Floridan 

aquifer at the site is about 20,000 ft2/d and the storage 

coefficient is about 10'4 .

46 Hydrogeology and Results of Tracer Tests Implications for Wellhead-Protection Strategies in West-Central Florida
Ground-water tracer tests were performed in the 

upper 300 ft of the Upper Floridan aquifer by injecting 

a tracer into the open-hole interval of a well completed 

in the aquifer. Movement of tracer was induced by 

pumping a well 200 ft from the injection well and open 

to the same depth interval. Multiple tracer arrivals at 

the pumped well are interpreted as ground-water flow 

in two media: (1) interconnected secondary porosity 

where conduit flow occurs, and (2) the aquifer matrix 

where diffuse flow occurs. Tracer arrival times on the 

order of hours for the 200-ft horizontal distance 

between the injection and pumped wells are interpreted 

as ground-water movement through interconnected 

secondary porosity. A tracer arrival after 36 days of 

pumping, with a peak concentration after 48 days of 

pumping, closely approximates the theoretical tracer 

arrival time calculated using uniform porosity assumptions and volumetric displacement. This was interpreted as the matrix flow arrival.

The effective porosity and longitudinal dispersivity 

of the aquifer matrix, based on the tracer test data, were 

estimated to be 25 percent and 1.3 ft, respectively. 

Calculated horizontal hydraulic conductivity values of 

the upper 300 ft of the Upper Floridan aquifer, based on 

dilution of the tracer in the injection well, ranged from 

16 to 34 ft/d, equivalent to a transmissivity of about 

4,800 to 10,200 ft2/d. The estimated transmissivity of 

that part of the Upper Floridan aquifer is approximately 

10,500 ft2/d, based on aquifer tests.

The use of uniform porosity and permeability 

models to predict ground-water travel time between 

two points in the aquifer is inappropriate because of 

ground-water flow through effective secondary porosity. The distribution and degree of connection of the 

secondary porosity determines the minimum travel 

time between two points within the aquifer. Because 

the aquifer responds to long-term pumping as an equivalent porous medium, ground-water travel times in the 

aquifer matrix can be estimated with uniform porosity 

models. Ground-water travel times through the aquifer 

matrix are on the order of months or years, but travel 

times in the interconnected secondary porosity are 

measured in hours or weeks. The results of borehole 

video surveys, aquifer tests, and numerical modeling 

indicate that vertical movement of water across lesspermeable units of the Upper Floridan aquifer is primarily through the matrix of the aquifer. Thus, consideration of ground-water flow through the matrix of the 

Upper Floridan aquifer is relevant to the evaluation of 

wellhead-protection strategies for west-central Florida.

The wellhead-protection strategy presented in this 

study is based on limiting the use of shallow permeable 

units of the Upper Floridan aquifer. The uppermost permeable unit of the Upper Floridan aquifer has effective 

secondary porosity and is the unit most vulnerable to a 

contaminate spill at the surface. Withdrawal of water 

from this unit induces leakage from the surficial aquifer 

system and increases the possibility of contamination.

The calibrated steady-state numerical model and a 

panicle-tracking program were used to simulate 

ground-water flow in the aquifer matrix. The particletracking model calculated ground-water travel times of 

48 to 50 days between the injection well and the 

pumped well when using an effective porosity of 21 

percent. These results closely approximate the 

observed time to peak tracer concentration measured 

during the deep tracer test. The particle-tracking model 

was then used to estimate the time of travel and areas 

of contribution to hypothetical wells to evaluate wellhead-protection strategies.

Wellhead-protection strategies were judged to be 

effective if they reduced the withdrawal of water from 

the upper permeable unit of the Upper Floridan aquifer. 

Results of particle-tracking analyses indicate that, by 

limiting the withdrawal of water to the second permeable unit and by allowing only moderate pumping 

rates, the leakage and total area of contribution from 

the overlying layers would be reduced. The orientation 

of multiple wells to the local ground-water gradient is 

an important consideration for wellhead-protection 

strategies. Locating wells in a line perpendicular to the 

local ground-water gradient reduces the potential for 

overlapping cones of depression. Overlapping cones of 

depression increase leakage from overlying layers, 

potentially increasing the risk of contamination.

Protective strategies that limit recharge to the 

pumped interval from the overlying sediments would 

provide the most protection for pumped wells. The 

upper permeable unit of the Upper Floridan aquifer is 

the unit most likely to be contaminated by direct 

recharge through sinkholes or excavations. Pumping 

from that unit also may induce leakage from the surficial aquifer system. Alternative well-field designs that 

incorporate more production wells, deeper casings, and 

moderate pumping rates reduce the risk of contamination of public-supply wells from nearby surface 

sources.

Summary and Conclusions 47
REFERENCES CITED

Andrews, W.J., 1990, Transmissivity and well yields of the 

Upper Floridan aquifer in Florida: Florida Geological 

Survey Map Series 132, 1 sheet.

Applin, P.L., 1951, Preliminary report on buried pre-Mesozoic rocks in Florida and adjacent states: U.S. Geological Survey Circular 91, 28 p.

Bair, E.S., Springer, A.E., and Roadcap, G.S., 1991, Delineation of traveltime-related capture areas of wells using 

analytical flow models and particle tracking analysis: 

Ground Water, v. 29, no. 3, p. 387-397.

Barr, G.L., 1989, Potentiometric surface of the Upper Floridan aquifer, west-central Florida, September 1988: U.S. 

Geological Survey Open-File Report 88-730, 1 sheet.

Bengtsson, T.O., Downing, H.C., and Geurink, J.S., 1986, 

The hydrologic effects from intense groundwater 

pumpage in east-central Hillsborough County: Southwest Florida Water Management District, 11 p.

Buxton, H.T., Reilly, T.E., and Smolensky, D.A., 1991, 

Particle tracking analysis of recharge areas on Long 

Island, New York: Ground Water, v. 29, no. 1, p. 63-71.

CH2M Hill, Inc., 1990, Drilling and testing of wells 2-D, 

3-D, and 4-D, Northwest Hillsborough Water 

Resources Project: Consultant's report prepared for the 

Southwest Florida Water Management District Project 

No. P00610, app. F, Rock Core Analysis.

Culbreth, M.A., 1988, Geophysical investigation of lineaments in South Florida: Masters Thesis, Department of 

Geology, University of South Florida, 97 p.

Florida Department of Environmental Regulation, 1987, 

Water quality standards: Chapter 17.3, in Florida 

Administrative Code.

Freeze, R.A., and Cherry, J.A., 1979, Groundwater: Englewood Cliffs, N.J., Prentice-Hall, 604 p.

Gilboy, A.E., 1985, Hydrogeology of the Southwest Florida 

Water Management District: Southwest Florida Water 

Management District Technical Report 85-01, 18 p.

Gordon, M.J., 1986, Dependence of effective porosity on 

fracture continuity in fractured media: Ground Water, 

v. 24, no. 4, p. 446-452.

Hickey, J.J., 1977, Hydrogeologic data for the McKay 

Creek subsurface waste-injection test site, Pinellas 

County, Florida: U.S. Geological Survey Open-File 

Report 77-802, 94 p.

 1979, Hydrogeologic data for the South Cross Bayou 

subsurface-injection test site, Pinellas County, Florida: 

U.S. Geological Survey Open-File Report 78-575, 87 p.

 1984, Field testing the hypothesis of darcian flow

through a carbonate aquifer: Ground Water, v. 22, no. 5, 

p. 544-547.

Jones, K.C., 1985, Aquifer characteristics within the Southwest Florida Water Management District: Southwest 

Florida Water Management District.

Knochenmus, L.A., and Barr, G.L., 1990, Potentiometric 

surface of the Upper Floridan aquifer, west-central 

Florida, September 1989: U.S. Geological Survey 

Open-File Report 90-133, 1 sheet.

Lewelling, B.R., 1989, Potentiometric surface of the Upper 

Floridan aquifer, west-central Florida, May 1988: U.S. 

Geological Survey Open-File Report 88-461, 1 sheet.

Lohman, S.W, 1979, Ground-water hydraulics: U.S. Geological Survey Professional Paper 708, 70 p.

McDonald, M.G., and Harbaugh, A.W, 1988, A modular 

three-dimensional finite-difference ground-water flow 

model: U.S. Geological Survey Techniques of WaterResources Investigations, book 6, chap. Al, 576 p.

Menke, C.G., Meredith, E.W, and Wetterhall, W.S., 1961, 

Water resources of Hillsborough County, Florida: Florida Geological Survey Report of Investigations no. 25, 

101 p.

Miller, J.A., 1986, Hydrogeologic framework of the Floridan aquifer system in Florida and in parts of Georgia, 

Alabama, and South Carolina: U.S. Geological Survey 

Professional Paper 1403-B, 91 p.

Moench, A.F., 1989, Convergent radial dispersion: A 

Laplace transform solution for aquifer tracer testing: 

Water Resources Research, v. 25, no. 3, p. 439-447.

Mull, D.S., Smoot, J.L., and Libermann, T.D., 1988, Dye 

tracing techniques used to determine ground-water flow 

in a carbonate aquifer system near Elizabethtown, 

Kentucky: U.S. Geological Survey Water-Resources 

Investigations Report 87-4147, 95 p.

Pollock, D.W., 1989, Documentation of computer programs 

to compute and display pathlines using results from the 

U.S. Geological Survey modular three-dimensional 

finite-difference ground-water flow model: U.S. Geological Survey Open-File Report 89-381, 188 p.

Reeder, H.O., Wood, W.W., Ehrlich, G.G., and Sun, R.J., 

1976, Artificial recharge through a well in fissured carbonate rock, West St. Paul, Minnesota: U.S. Geological 

Survey Water-Supply Paper 2004, 80 p.

Ryder, P.O., Johnson, D.M., and Gerhart, J.M., 1980, Model 

evaluation of the hydrogeology of the Morris Bridge 

well field and vicinity in west-central Florida: U.S. 

Geological Survey Water-Resources Investigations 

80-29, 92 p.

Sabatini, D.A., and Austin, T.A., 1991, Characteristics of 

Rhodamine WT and Fluorescein as adsorbing groundwater tracers: Ground Water, v. 29, no. 3, p. 341-349.

Safko, P.G., and Hickey, J.J., 1992, A preliminary approach 

to the use of borehole television surveys for characterizing secondary porosity in the Floridan aquifer system: 

U.S. Geological Survey Water-Resources Investigations Report 91-4168.

Schreuder and Davis, Inc., 1991, Pemberton Creek site well 

construction and aquifer testing: Consultant's report 

prepared for and in the files of the West Coast Regional 

Water Supply Authority, 63 p.

48 Hydrogeology and Results of Tracer Tests Implications for Wellhead-Protection Strategies in West-Central Florida
Scott, T.M., 1988, The lithostratigraphy of the Hawthorn 

Group (Miocene) of Florida: Florida Geological Survey 

Bulletin no. 59, 148 p.

Sinclair, W.C., 1973, Hydrogeologic characteristics of the 

surficial aquifer in northwest Hillsborough County, 

Florida: U.S. Geological Survey Open File Report 

73-023, 97 p.

 1982, Sinkhole development resulting from ground 

water withdrawal in the Tampa area, Florida: U.S. Geological Survey Water-Resources Investigations 81-50, 

19 p.

Smith, E.D., and Vaughan, N.D., 1985, Aquifer test analysis 

in nonradial flow regimes: A case study: Ground Water, 

v. 23, no. 2, p. 167-175.

Southeastern Geological Society, 1986, Hydrogeological 

units of Florida: Florida Geological Survey Special 

Publication no. 28, 9 p.

Southwest Florida Water Management District, 1988, 

Ground-water resource availability inventory, Hillsborough County, Florida: Southwest Florida Water 

Management District Resource Management and Planning Departments, 203 p.

Stewart, H.G., 1966, Ground-water resources of Polk

County, Florida: Florida Geological Survey Report of 

Investigations no. 44, 170 p.

Sun, R.J., 1976, Hydrodynamic dispersion and movement 

of injected water, p. 52-75, in Reeder, H.O., Wood, 

W.W., Ehrlich, G.G., and Sun, R.J., Artificial recharge 

through a well in fissured carbonate rock, West 

St. Paul, Minnesota: U.S. Geological Survey WaterSupply Paper 2004, 80 p.

Swancar, Amy, and Hutchinson, C.B., 1992, Chemical and 

isotopic composition and potential for contamination of 

water in the Upper Floridan aquifer, west-central Florida, 1986-89: U.S. Geological Survey Open-File Report

92-47,47 p.

Vecchioli, John, Hunn, J.D., and Aucott, W.R., 1989, Evaluation of methodology for delineation of protection 

zones around public-supply wells in west-central Florida: U.S. Geological Survey Water-Resources Investigations Report 88-4051, 36 p.

Vernon, R.O., 1951, Geology of Citrus and Levy Counties, 

Florida: Florida Geological Survey Bulletin no. 33, 

256 p.

Wolansky, R.M., and Corral, M.A., 1985, Aquifer tests in 

west-central Florida, 1952-76: U.S. Geological Survey 

Water-Resources Investigations Report 84-4044, 127 p.

References Cited 49
APPENDIX

Apparent Secondary Porosity Logs from Borehole Video Survey

Borehole television logs of wells at the old Tampa well field were viewed to compile logs of apparent secondary 

porosity. The apparent secondary porosity log is divided into two sections: (1) description of notable discontinuous 

features in the borehole wall and (2) description of the continuous borehole wall characteristics. The terminology 

used to describe the secondary porosity in the boreholes is that of Safko and Hickey (1992) and is summarized 

below.

Vug porosity appears as a void space in the rock, visible on only part of the borehole. Cavern porosity appears as 

a void space in the rock where the entire borehole wall disappears from view. Fracture porosity appears as cracks 

or breaks in the rock. The size of a vug porosity feature has been standardized to the grain-size classification of 

pebble (0.01-0.21 ft), small cobble (0.21-0.42 ft), and large cobble (0.42-0.84 ft) according to J.J. Hickey (U.S. 

Geological Survey, oral commun., 1991). The size of cavern porosity is standardized to small (0.84-2.00 ft) and 

large (>2.00 ft). Fracture porosity was not classified as to size. The descriptions of the character of the borehole 

walls use common adjectives that refer to the degree of uniformity of the hole. Smooth, rough, uneven, irregular, 

blocky, pitted, grainy, and round are used with the normal meaning. Descriptions of the character of the borehole 

wall take the following general form: shape of borehole, relative smoothness of wall surface, general appearance 

of density of rock, and visual estimate of the relative porosity of the matrix based on the previous characteristics.

Because the visible secondary porosity could have been created during drilling, it is termed "apparent secondary 

porosity." Apparent secondary porosity is termed effective secondary porosity if water enters the well bore from 

that feature. The identification of effective secondary porosity requires the use of flowmeter, fluid resistivity, and 

temperature logs and the driller's log containing comments on bit drops, dredging zones, and lost circulation. This 

exercise was performed for those wells for which the necessary data were collected, including wells 15, 15A, and 

the Ocala monitor well, and is summarized in the main body of the report as figure 10. Detailed descriptions of the 

"apparent secondary porosity" observed in wells at the old Tampa well field are presented here.

Appendix-Apparent Secondary Porosity Logs from Borehole Video Survey 51
Well 11

Well 11 was drilled using a cable-tool drill rig. Much of the borehole has an oblong shape and clearly shows groves 

worn in the side of the borehole by the up and down motion of the drill stem. Well 11 was logged while pumping 

well 15, located 25 ft due west of well 11, at a rate of 980 gal/min.

Notable discontinuous features

Depth

interval Observation

(feet)______________________________

74 Bottom of casing

78 Particles moving laterally across field of view

79 Small vug

81 Medium vug

84 Enlarged bore, apparent cavity

94 Medium vug, upward flow of water

105-108 Small vugs with horizontal flow

112-114 Fracture

115 Enlarged bore

132 Fracture

134 Small vugs

144 Small vug

158 Medium vug

165-166 Small to medium vugs

169 Borehole enlargement, medium vugs

172 Small to medium vug

174 Small to medium vug

175 Medium to large vug

180-185 Fracture

183 0.5 ft bed of dark strata

186-196 Fracture

192 Small to medium vug

198 Small to medium vug

216 Fracture

221 Small vugs

222 Fracture

232-236 Small vugs

254 Fracture

255-257 Fracture, irregular bore

264 Small vug

267-268 Hole enlargement, possible fracture, small vugs

275-276 Small vugs

282-284 Fracture

331-379 Small fractures

379 Small vug

390 Small vug

395-397 Small vugs

52 Hydrogeology and Results of Tracer Tests Implications for Wellhead-Protection Strategies in West-Central Florida
Depth 

interval

(feet)

Observation

400 Short elongated vug

403-409 Fractures

444 Fracture

444-448 Small vugs

496-497 Fracture, nodules of dark rock

505-522 Fracture

527 Small to medium vugs

528-534 Fractures

544 Small cavity

586 Small vugs

591 Small vug

592 Fracture

595 Medium vug

599 Large vug

615 Medium vugs

618 Small cavity

618 Small vugs, short fracture

622-624 Fracture

626-633 Fractures

638-641 Medium to large vugs

641 Intersecting, high angle fractures (X)

641 At this depth, the camera operator decided to discontinue logging because of borehole conditions.

Summary of Borehole Wall Characteristics

Notable continuous features

Depth 

interval

(feet)

Observation

74 Bottom of casing

74-82 Rounded to irregular, pitted, porous

82-105 Rounded, smooth, numerous very small vugs, low matrix porosity

105-119 Irregular to oblong, blocky, smooth, dense, low matrix porosity

119-125 Oblong, pitted to slightly pitted, porous

125-144 Oblong, smooth to slightly pitted, low matrix porosity

144-161 Oblong, pitted, grainy, porous

161-251 Oblong to rounded, blocky to smooth, dense, low matrix porosity, thin beds of more porous rock

251-260 Rounded, pitted, grainy, small vugs porous

260-335 Rounded, smooth to pitted, low matrix porosity, small vugs, thin beds of more porous rock

335-350 Rounded, smooth to pitted, low to moderate porosity, small vugs

350-357 Rounded, smooth, dense, low matrix porosity

357-370 Rounded, pitted, small vugs, porous

370-395 Rounded, smooth, dense, small vugs, low matrix porosity

395-399 Rounded, smooth, pitted, vuggy, porous

399-456 Rounded, smooth, dense, small vugs, low matrix porosity, thin beds of very vuggy rock

456-460 Rounded, grainy, porous

Appendix-Apparent Secondary Porosity Logs from Borehole Video Survey 53
Depth 

interval Observation

(feet)

460-465 Rounded, smooth, dense, low matrix porosity

465-469 Rounded, grainy, porous

469-586 Rounded, smooth, dense, thin beds of porous rock, low matrix porosity

586-589 Rounded, blocky, pitted, low matrix porosity

589-605 Rounded, pitted, vuggy, porous

605-615 Rounded, smooth, dense, low matrix porosity

615-622 Rounded, pitted, vuggy, porous

622-628 Rounded, smooth, slightly pitted, low matrix porosity

628-641 Rounded, pitted, vuggy, porous

641 Total depth logged

Well 13

Well 13 is approximately 2,000 ft west of well 15.

Notable discontinuous features

Depth

interval Observation

(feet)__________________________________

63 Bottom of casing

63-65 Small vugs

67 Enlarged bore

69 Medium vug, enlarged bore

72-73 Small vugs

74 Small vugs

78 Medium vug, enlarged bore

83-85 Small to medium vugs, enlarged bore, possible fracture

88-91 Small to medium vugs

93-94 Small vugs

101 Enlarged bore

101-104 Fracture

105-106 Fracture, small vugs

108-113 Small vugs

115-117 Small vugs, enlarged bore, possible fracture

121 Medium vug

124-126 Small vugs

142-144 Small to medium vugs

148 Medium vug

163-164 Medium vug, fracture

166-167 Medium to small vugs

177-179 Small to medium vugs

185 Medium vug, enlarged bore

187 Medium to large vug

54 Hydrogeology and Results of Tracer Tests Implications for Wellhead-Protection Strategies in West-Central Florida
Depth 

interval Observation

(feet)

190 Small to medium vug

196-200 Small vugs

203 Small vugs

206 Small vugs

207 Small to medium vugs

213 Fracture

214-216 Small to medium vugs

222-225 Small to medium vugs, short fracture at 224 ft

238-242 Small vugs

242 Fracture

245 Small vug

264-265 Small vugs, fracture, enlarged bore

268 Fracture

269 Small vugs

274-276 Small vugs

280-281 Fracture, small vug

285-286 Fracture

291 Small vugs

295 Small vugs

302 Small vugs

304-308 Small to medium vugs

334-336 Small vugs

336 Total depth

Summary of Borehole Wall Characteristics

Notable continuous features

Depth 

interval Observation

(feet)

63 Bottom of casing

63-72 Rounded, smooth, dense, low matrix porosity

72-74 Rounded, rough, grainy, porous

74-77 Rounded, smooth, dense, small vugs, low matrix porosity

77-84 Rounded, rough, pitted, dense, low matrix porosity

84-96 Rounded to irregular, blocky, smooth, dense, low matrix porosity

96-98 Rounded, rough, pitted, porous

98-101 Rounded, smooth, dense, pitted, low matrix porosity

101-103 Rounded, smooth, dense, low matrix porosity

103-106 Rounded, rough, pitted, vuggy, porous

106-126 Rounded, smooth, dense, pitted, low matrix porosity

126-135 Rounded, rough, grainy, porous

135-144 Rounded, smooth, dense, low matrix porosity

144-145 Rounded, rough, uneven, porous

145-163 Rounded, smooth, dense, pitted, low matrix porosity

163-167 Rounded, rough, uneven, porous

167-170 Rounded, smooth, dense, low matrix porosity

Appendix-Apparent Secondary Porosity Logs from Borehole Video Survey 55
Depth 

interval Observation

(feet)

170-179 Rounded, rough, uneven, pitted, porous

179-194 Rounded, smooth, pitted, low matrix porosity

194-196 Rounded, grainy, pitted, porous

196-257 Rounded to irregular, smooth to slightly pitted, dense, low matrix porosity

257-264 Rounded, grainy, porous

264-265 Irregular, broken, rough, uneven, low matrix porosity

265-274 Rounded, grainy to pitted, rough, porous

274-327 Rounded, smooth, dense, low matrix porosity

327-329 Rounded to irregular, rough, uneven, pitted, porous

329-332 Rounded, pitted, dense, low matrix porosity

332-334 Rounded to uneven, irregular, rough, pitted, low matrix porosity

334-336 Rounded, smooth, pitted, dense, low matrix porosity

Well 15

Well 15 was logged while it was being pumped at a rate of 980 gal/min.

Notable discontinuous features

Depth

interval Observation

(feet)__________________________________________________

67 Bottom of casing

70 Bore enlargement

80 Small vug

83 Small vug

87 Small vug

101 Small vug

105-108 Vugs

165-169 Large cavity

169 Laminated bedding

179 Medium vug

181 Medium vugs

201 Small vug

232 Small vugs

236-239 Dark nodules (chert?)

250-252 Enlarged borehole diameter one one side, possible high angle fracture

257-259 Medium vugs

264 Possible fracture

318-321 Fracture

326-329 Fracture

373-381 Fracture filled with dark deposit

375 Small vugs

404 Well ends in large intersecting vertical fractures appears as (X), rock appears blocky

404 Total depth logged

56 Hydrogeology and Results of Tracer Tests Implications for Wellhead-Protection Strategies in West-Central Florida
Summary of Borehole Wall Characteristics

Notable continuous features

Depth 

interval Observation

(feet)

67 Bottom of casing

67-71 Rounded, pitted, dense, low matrix porosity

71-74 Irregular, blocky, pitted, dense, low matrix porosity

74-82 Rounded to slightly irregular, pitted, vuggy, moderate porosity

82-102 Rounded, pitted, smooth, dense, low matrix porosity

102-109 Rounded, blocky, pitted, vuggy, porous

109-116 Rounded, smooth, dense, low matrix porosity

116-125 Rounded, pitted, grainy, porous

125-128 Rounded, smooth, dense, low matrix porosity

128-129 Rounded, pitted, grainy, porous

129-134 Rounded, pitted, dense, low matrix porosity

134-141 Rounded, pitted, porous

141-150 Rounded, smooth, dense, low matrix porosity

150-165 Rounded, pitted, grainy, porous

165-169 Large cavity, bore walls not visible

169-201 Rounded, blocky, dense, low matrix porosity

201-203 Rounded, pitted, porous

203-208 Rounded, smooth, dense, low matrix porosity

208-214 Rounded, pitted, grainy, porous

214-222 Rounded, smooth, pitted, dense, low matrix porosity

222-223 Rounded, pitted, porous

223-230 Rounded, smooth, slightly pitted, low matrix porosity

230-240 Rounded, pitted, porous

240-249 Rounded, smooth, pitted, dense low matrix porosity

249-269 Rounded, pitted, grainy, vuggy, porous, interlayered beds of smooth, dense, low porosity rock

269-336 Rounded, smooth, slightly pitted, dense, low matrix porosity

336-345 Rounded to slightly irregular, pitted, vuggy, porous

345-380 Rounded, smooth, slightly pitted, dense, low matrix porosity

380-386 Rounded to slightly irregular, blocky, pitted, moderate porosity

386-393 Rounded, pitted, dense, low matrix porosity

393-397 Rounded, pitted, vuggy, porous

397-401 Rounded, pitted, dense, low matrix porosity

401-404 Irregular, blocky, dense, enlarged hole, low matrix porosity

Appendix-Apparent Secondary Porosity Logs from Borehole Video Survey 57
Well 15A

Well 15A was logged while pumping well 15, located 103 ft due west, at a rate of 980 gal/min. Much of the borehole wall was covered by what appears to be iron bacterial slime.

Notable discontinuous features

Depth 

interval

(feet)

Observation

57 Bottom of casing

65 Fracture

69 Fracture

79-80 Fracture

84 Medium vug

98 Small vugs

101 Medium vug

104 Medium vug

141 Medium vug

158 Small to medium vug

162 Small vug

164 Small vug

166 Fracture

168 Small vug

169 Small to medium vug with possible high angle fracture

177 Fracture

181 Medium vug

182 Fracture and enlarged bore hole

185 Large to medium vug

189-204 Small vugs

213 Small to medium vugs with possible high angle fracture

214 Fracture

217 Small vugs

226 Small vugs

238-241 Medium vugs

250-251 Medium vugs

259 Small vugs

266 Fracture

273 Medium vugs

323-331 Fracture

335-343 Fractured zone

352 Small vug

357 Small vugs

369 Small vug

371-378 Small vugs

382 Small to medium vugs

388 Small vug

397 Small to medium vug

400 Small vug

402 Fracture

405 Slime covers camera

405 Total depth logged

58 Hydrogeology and Results of Tracer Tests Implications for Wellhead-Protection Strategies in West-Central Florida
Summary of Borehole Wall Characteristics

Notable continuous features

Depth

interval Observation 

(feet)

57-63 Rounded, irregular, blocky, rough, dense, low matrix porosity

63-95 Rounded, grainy, pitted, porous

95-105 Rounded, pitted, grainy, dense, less porous

105-110 Rounded, smooth, pitted, dense, low matrix porosity

110-119 Rounded, grainy, pitted, very porous

119-122 Rounded, pitted, less porous

122-127 Rounded, smooth, pitted, low matrix porosity

127-131 Rounded, pitted, grainy, porous

131-136 Rounded, pitted, dense, less porous

136-142 Rounded, smooth, dense, low matrix porosity

142-158 Rounded, pitted, grainy, porous

158-160 Rounded, smooth to slightly pitted, dense, low matrix porosity

160-163 Rounded, pitted, porous

163-214 Rounded, smooth, slightly pitted, dense, low matrix porosity, with thin beds of porous rock

214-248 Rounded, irregular, blocky, smooth, dense, low matrix porosity

248-250 Rounded, pitted, grainy, porous

250-335 Rounded, smooth, slightly pitted, dense, low matrix porosity

335-347 Irregular, blocky near fractures, pitted, porous

347-378 Rounded, slightly pitted, moderate to low matrix porosity

378-385 Rounded, pitted, porous

385-392 Rounded, slightly pitted to smooth, dense, low matrix porosity

392-395 Rounded, pitted, porous

395-405 Slime covers borehole wall

405 Slime covers camera

Well 20

Well 20 was logged while pumping well 15, located 200 ft due west, at a rate of 980 gal/min.

Notable discontinuous features

Depth

interval Observation 

(feet)_________________________________________________

38 Bottom of casing

39 Short fracture with sediment flowing into bore when camera gets stuck in casing

56 Medium vugs

65-80 Several zones of small vugs, some so solutionized that they are almost cavities.

81-83 Small to medium vugs

84-86 Short fracture, enlarged bore

87 Small vug

102-118 Small vugs

Appendix-Apparent Secondary Porosity Logs from Borehole Video Survey 59
Depth 

interval Observation

(feet)

147 Medium vug

147-149 Short fracture

165-167 Small vugs

174 Medium vug

177-186 Layered zones of small vugs

322 Hole blocked

322 Total depth logged

Summary of Borehole Wall Characteristics

Notable continuous features

Depth 

interval Observation

(feet)

39 Bottom of casing

39-57 Rounded, smooth, dense, low matrix porosity

57-66 Rounded to irregular, rough, pitted, porous

66-72 Rounded, smooth, low matrix porosity

72-80 Rounded, smooth, vuggy, porous

80-89 Rounded, smooth to irregular, blocky, dense, low matrix porosity

89-91 Rounded, smooth, dense, low matrix porosity

91-96 Rounded to irregular, pitted, porous

96-100 Rounded, smooth, slightly pitted, low matrix porosity

100-104 Rounded, pitted, vuggy, porous

104-108 Rounded, smooth, dense, low matrix porosity

108-109 Rounded, pitted, grainy, porous

109-121 Rounded, smooth, slightly pitted, dense, low matrix porosity

121-123 Rounded to irregular, blocky, rough, porous

123-150 Rounded, smooth, pitted, dense, low matrix porosity

150-160 Rounded, rough, pitted, porous

160-165 Rounded, smooth, pitted, dense, low matrix porosity

165-174 Rounded, irregular, blocky, dense, low matrix porosity

174-184 Rounded, smooth to uneven, dense, thin beds of vuggy, porous rock, low matrix pprosity

184-186 Rounded, uneven, blocky, low matrix porosity

186-239 Rounded, smooth, pitted, dense, low matrix porosity

239-245 Rounded, rough, vuggy, pitted, porous

245-253 Rounded to irregular, pitted, vuggy, porous

253-254 Rounded, smooth, dense, low matrix porosity

254-267 Rounded to irregular, blocky, pitted, porous

267-322 Rounded, smooth, dense, low matrix porosity

322 Hole blocked

60 Hydrogeology and Results of Tracer Tests Implications for Wellhead-Protection Strategies in West-Central Florida
Avon Park Monitor Well

This well was logged while pumping well 15, located 867 ft to the southwest.

Notable discontinuous features

Depth

interval

(feet)

Observation

557 Bottom of casing

558-600 Particles in water, difficult to see, unremarkable borehole wall

570-572 Small vugs

577 Small to medium vug

582-599 Small vugs

601-609 Small vugs

613-617 Small vugs

618-619 Fracture

619-624 Small vugs

624-626 Fracture

627 Ledge, enlarged bore possibly due to fracture

632-635 Fracture, enlarged bore

635-639 Small vugs

639 Fracture

642 Fracture

646 Fracture

651-652 Fracture

654-658 Fracture, enlarged bore

660 Fracture, medium vug

664-667 Fracture, enlarged bore

668-669 Enlarged bore

670-672 Fracture

673 Enlarged bore

674-676 Fracture

678-679 Medium to large vugs, fracture

680-683 Small vugs, fracture

683-686 Fracture

689-692 Fracture

692-697 Small vugs

697-701 Fracture

702-710 Fractures, small vugs

709 Intersecting fractures

710-715 Small vugs

716-717 Fracture

721-725 Fracture, enlarged bore

728-732 Fracture

735-763 Enlarged bore, fractures, angular, broken rock.

762-764 Yellowish tint to water, fracture

768 Enlarged bore, fracture

771 Bottom hole filled with sediment

771 Total depth logged

Appendix-Apparent Secondary Porosity Logs from Borehole Video Survey 61
Summary of Borehole Wall Characteristics

Notable continuous features

Depth 

interval Observation

(feet)

557 Bottom of casing

557-581 Rounded, slightly irregular, smooth to rough, dense, thin layers of small vugs, low matrix porosity

581-599 Rounded, smooth, pitted, vuggy, porous, with beds of smooth, dense, low porosity rock

599-609 Rounded, smooth to rough, vuggy, porous

609-612 Rounded, smooth, dense, low matrix porosity

612-621 Rounded, pitted, porous

621-623 Rounded, smooth, dense, low matrix porosity

623-625 Rounded, smooth, pitted, vuggy, porous

625-631 Rounded, smooth, dense, low matrix porosity

631-634 Enlarged, irregular bore, blocky, dense, low matrix porosity

634-642 Rounded to irregular, blocky, rough to smooth, dense, low matrix porosity

642-648 Rounded to irregular, rough, blocky, smooth, vuggy, porous

648-651 Rounded, smooth, dense, low matrix porosity

651-653 Rounded, rough, vuggy, porous

653-659 Rounded to irregular, smooth to rough, dense, enlarged bore at 654, blocky, low matrix porosity

659-660 Rounded to irregular, rough, vuggy, porous

660-692 Rounded, blocky, smooth to rough, dense, low matrix porosity

692-695 Rounded, smooth to rough, vuggy porous

695-710 Rounded, smooth to rough, dense, low matrix porosity

710-725 Rounded, rough, pitted, vuggy, porous

725-736 Rounded, rough to smooth, dense, low matrixp porosity

736-771 Irregular, broken, blocky, rough, dense, low matrix porosity

771 Bottom of hole

Ocala Monitor Well

This well was logged while pumping well 15, located 219 ft away, at a rate of 980 gal/min.

Notable discontinuous features

Depth

interval Observation

(feet)______________________

247 Bottom of casing

247-254 Cement in bore

254 Small vugs, rough, pitted surface

260-262 Medium vugs

268 Fracture

277-278 Fracture

279-281 Medium to small vugs

297-298 Small to medium vugs

306 Medium vugs

324 Small vugs

62 Hydrogeology and Results of Tracer Tests Implications for Wellhead-Protection Strategies in West-Central Florida
Depth 

interval

(feet)

Observation

333

340

341

383-397 

403-406

407-412

408-412

418-422

421-425

432

444

446

Small vugs

Medium vug

Small vug

Medium to small vugs

Fracture, small vugs

Small vugs

Fracture

Small and medium vugs

Fracture

Small vug

Small vugs

Total depth

Summary of Borehole Well Characteristics

Notable continuous features

Depth 

interval

(feet)

Observation

247 Bottom of casing.

249-253 Rounded, smooth, dense, low matrix porosity

253-257 Rounded to slightly irregular, smooth to slightly pitted, dense, occasional small vugs, low matrix porosity

257-277 Rounded, rough, pitted, dense, vuggy, porous

277-335 Rounded, smooth, dense, small vugs, low matrix porosity

335-352 Rounded, rough, pitted, porous

352-362 Rounded, smooth, dense, low matrix porosity

362-372 Rounded, rough, low matrix porosity

372-382 Rounded, smooth, dense, low matrix porosity

382-394 Rounded, irregular to regular, pitted, porous

394-410 Rounded, smooth, dense, low matrix porosity, irregular, blocky, and pitted around fractures

410-418 Rounded, rough, pitted, porous

418-425 Irregular, blocky, vuggy, fractured, low matrix porosity

425-445 Rounded, slightly rough, low matrix porosity, thin layers that are pitted, very porous

445-446 Round, smooth, low matrix porosity

446 Bottom of hole

Appendix-Apparent Secondary Porosity Logs from Borehole Video Survey 63