Assessment of the Fresh- and Brackish-Water 

Resources Underlying Dunedin and Adjacent 

Areas of Northern Pinellas County, Florida

By Lari A. Knochenmus and E.S. Swenson

U.S. Geological Survey

Water-Resources Investigations Report 96-4164

Prepared in cooperation with the

CITY OF DUNEDIN and the

SOUTHWEST FLORIDA WATER MANAGEMENT DISTRICT

Tallahassee, Florida 

1996
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY 

Gordon P. Eaton, Director

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

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

For additional information Copies of this report can be

write to: purchased from:

District Chief U.S. Geological Survey

U.S. Geological Survey Branch of Information Services

Suite 3015 Box 25286

227 N. Bronough Street Denver, CO 80225

Tallahassee, FL 32301
CONTENTS

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

Introduction......................................................................................................._^

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

Description of the Study Area..........................................................................................................................................2

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

Hydrogeologic Framework and Hydraulic Properties in the Study Area..................................................................................4

Surficial Aquifer System..................................................................................................................................................4

Intermediate Confining Unit ............................................................................................................................................4

Floridan Aquifer System..................................................................................................................................................4

Zone A....................................................................................................................................................................7

Semiconfining Unit Between Zones A and B........................................................................................................ 7

Zone B........................................................................................................................................................^

Semiconfining Unit Between Zones B and C........................................................................................................ 7

Delineation of the Hydrogeologic Units and Producing Zones Underlying the Study Area........................................... 7

Occurrence of Ground Water and Ground-Water Development in the Study Area................................................................. 10

Water Levels and Dunedin Well-Field Pumpage........................................................................................................... 11

Ground-Water Flow Patterns.......................................................................................................................................... 11

Ground-Water Quality in Dunedin........................................................................................................................................... 13

Lateral Variations............................................................................................................_^ 13

Vertical Variations.......................................................................................................................................................... 16

Temporal Variations..............................................................................................._^ 18

Brackish Production-Well Aquifer Test...................................................................................................................................22

Background Data............................................................................................................................................................22

Aquifer-Test Design and Implementation......................................................................................................................22

Limitations of Analytical Aquifer-Test Analyses.......................................................................................................... 24

Numerical Simulation of the Fresh- and Brackish-Water Resources Underlying Dunedin .................................................... 25

Numerical Methods........................................................................................................................................................25

Conceptual Model..........................................................................................................................................................25

Spatial Discretization............................................................^^

Temporal Discretization.................................................................................................................................................2^

Boundary Conditions Used to Simulate the Aquifer Test.............................................................................................. 26

Boundary Conditions Used to Simulate Water-Quality Changes.................................................................................. 28

Input Parameters ............................................................................................................................................................28

Wells...............................................................................................................................................................................29

Model Analysis of the Aquifer Test.........................................................................................................................................30

Results............................................................................................................................................................................31

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

Model Analysis of Historical Chloride-Concentration Change............................................................................................... 35

Results............................................................................................................................................................................35

Sensitivity Analysis........................................................................................................................................................37

Model Analysis of Long-Term Brackish-Water Withdrawals ................................................................................................. 37

Results............................................................................................................................_^

Case 1-Single Well Open to Lower Zone A........................................................................................................ 39

Case 2-Single Well Open to Upper and Lower Zone A Discharging Equal Volumes from Each Zone............. 39

Case 3-Single Well Open to Upper and Lower Zone A Discharging Unequal Volumes from Each Zone......... 39

Summary and Conclusions ......................................................................................................................................................42

Selected References............................................................................................................................................................^

Appendix Construction data for selected wells...................................................................................................................... 45

Contents III
FIGURES

la. Map showing location of the study area and selected U.S. Geological Survey network wells.................................. 3

Ib. Map showing location of the existing production wells and observation wells, new wells drilled as part

of the study, generalized water plant boundary, and historic downtown district Dunedin, Florida........................... 4

2. Chart showing generalized stratigraphic and hydrogeologic section underlying the study area................................ 6

3. Map showing location of selected wells, well names, and traces of the hydrogeologic sections A-A' and B-B' ....... 8

4-8 Graphs showing:

4. Hydrogeologic sections A-A' and B-B' showing natural-gamma log traces correlated to permeable

zones and semiconfining units........................................................................................................................... 9

5. Hydrogeologic units underlying the aquifer-test site showing natural-gamma log traces correlated to

permeable zones and semiconfining units......................................................................................................... 10

6. Daily water levels for Pinellas 665 and Garden Street Triangle wells, 1954-94............................................... 12

7. Average annual pumpage and year that production wells went online in the Dunedin well field..................... 12

8. Pumpage and monthly water levels in production well 14, 1983-91................................................................. 13

9-11. Maps showing:

9. Potentiometric surface of the Upper Floridan aquifer, northern Pinellas County, May 1994........................... 14

10. Potentiometric surface of the Upper Floridan aquifer, northern Pinellas County, September 1994 ................. 15

11. Chloride concentrations, in milligrams per liter, in selected wells penetrating zone A, 1993, Dunedin, 

Florida...................................................................................................................................................^

12-16. Graphs showing:

12. Specific-conductance profile during well drilling.............................................................................................. 18

13. Stiff diagrams of major ions from selected depths in the brackish monitor and brackish production wells..... 19

14. Chloride concentrations in water from production wells 16, 15, 14, 13, 9, and 6, 1970-94 .............................21

15. Water levels from observation wells illustrating aquifer response in a multilayered aquifer system, 

April 5-26, 1995...............................................^

16. Aquifer-test data from the brackish monitor and production 14 observation wells .......................................... 24

17. Chart showing model grid, simulated open-hole interval, and boundary conditions................................................. 27

18-23. Graphs showing:

18. Measured and model-simulated water levels in the brackish monitor and production 14 wells....................... 31

19. Velocity-vector field in a radial section at the end of the 3-day aquifer test and at the end of the

25-year history match........................................................................................................................................ 33

20. Sensitivity of simulated water levels to changes in intrinsic permeabilities of selected hydrogeologic units.. 34

21. Comparison of measured and simulated chloride concentrations from the water-plant wells, 1970-94........... 36

22. Sensitivity of simulated chloride concentration to changes in intrinsic permeabilities of selected

hydrogeologic units............................................................................................................................................ 38

23. Model simulated chloride-concentration changes using various pumping scenarios from upper and lower

zone A............................................................................................................................................................40-41

Tables

1. Records of the city of Dunedin production and observation wells and trend analysis results of chloride 

concentration.............................................................^

2. Model-input values and model-derived hydraulic parameters................................................................................ 29

IV Contents
VERTICAL DATUM, ACRONYMNS, AND ABBREVIATIONS

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.

mi2 = square mile 

ft = feet 

ft2 = square feet 

ft/d = feet per day 

(ft/d)/ft = feet per day per foot 

ft2/d = feet squared per day

in = inch

Mgal/d = million gallons per day 

gal/min = gallons per minute 

mg/L = milligrams per liter

DS = dissolved solids 

lb/in2 = pounds per square inch 

SAS = surficial aquifer system 

ICU = intermediate confining unit 

UFA = Upper Floridan aquifer 

LFA = Lower Floridan aquifer 

UZA = upper zone A 

LZA = lower zone A 

SCU = semiconfming unit

ROMP = Regional Observation and Monitoring Well 

Program

SWFWMD = Southwest Florida Water 

Management District

USGS = United States Geological Survey 

HST3D = Heat and Solute Transport in 3 Dimensions

Contents V
VI Contents
Assessment of the Fresh- and Brackish-Water 

Resources Underlying Dunedin and Adjacent Areas of 

Northern Pinellas County, Florida

By Lari A. Knochenmus and E.S. Swenson

Abstract

The city of Dunedin is enhancing their potable 

ground-water resources through desalination of 

brackish ground water. An assessment of the 

fresh- and brackish-water resources in the Upper 

Floridan aquifer was needed to estimate the 

changes that may result from brackish-water 

development. The complex hydrogeologic framework underlying Dunedin and adjacent areas of 

northern Pinellas County is conceptualized as a 

multilayered sequence of permeable zones and 

confining and semiconfining units. The permeable 

zones contain vertically spaced, discrete, waterproducing zones with differing water quality. 

Water levels, water-level responses, and water 

quality are highly variable among the different 

permeable zones.

The Upper Floridan aquifer is best characterized as a local flow system in most of northern 

Pinellas County. Pumping from the Dunedin well 

field is probably not influencing water levels in the 

aquifer outside Dunedin, but has resulted in localized depressions in the potentiometric surface surrounding production-well clusters.

The complex geologic layering combined with 

the effects of production-well distribution probably contribute to the spatial and temporal variability in chloride concentrations in the Dunedin well 

field. Chloride concentrations in ground water 

underlying the Dunedin well field vary both vertically and laterally. In general, water-quality rapidly changes below depths of 400 feet below sea 

level. Additionally, randomly distributed waterproducing zones with higher chloride concentrations may occur at shallow, discrete intervals 

above 400 feet. A relation between chloride concentration and distance from St. Joseph Sound is 

not apparent; however, a possible relation exists 

between chloride concentration and productionwell density. Chloride-concentration data from 

production wells show a consistently increasing 

pattern that has accelerated since the late 1980's. 

Chloride-concentration data from 15 observation 

wells show increasing trends for 6 wells, decreasing trends for 3 wells, and no trend for 6 wells.

The current and future, fresh- and brackishwater resources were evaluated using a numerical 

ground-water flow and solute-transport model. 

Simulation results indicate that the hydraulic conductivity of the uppermost permeable zone (upper 

zone A) of the Upper Floridan aquifer is four times 

greater than the two underlying permeable zones 

(lower zone A and zone B). The simulated 

hydraulic conductivities of the semiconfining units 

are four orders of magnitude less than the permeable zones. Simulation results show the importance of semiconfining units as a mechanism for 

retarding the vertical movement of higher salinity 

ground water. Simulation results indicate that 

pumping from the brackish-water zone does not 

negatively influence the chloride-concentration 

trends in the overlying fresh-water zone; however, 

chloride changes in the fresh-water zone will continue to occur due to the continuation of current 

fresh-water withdrawals. Chloride changes in the 

brackish-water zone will occur from pumping 

brackish water.

Abstract
INTRODUCTION

The city of Dunedin, and other coastal communities in Florida face special problems in developing and 

sustaining fresh ground-water resources. Historically, 

the city of Dunedin has obtained potable water from 

wells penetrating the uppermost part of the Upper 

Floridan aquifer underlying the city. The Dunedin well 

field provides potable water for inhabitants living in a 

12-square-mile (mi2) area. As population density 

increased, production in the well field also increased. 

Public supply demands for Dunedin are expected to 

increase from the 1994 average of 4.9 to 7.2 million 

gallons per day (Mgal/d) in the year 2020. Although 

ample supplies of ground water continue to be available, the ground-water quality is changing. These 

changes, caused by concentrated withdrawals within 

the well field, persist today. One method to enhance 

ground-water resources is through desalination of 

brackish ground water. To continue to provide potable 

water, Dunedin plans to develop the brackish-water 

resources underlying the city. Evaluation of the brackish-water resources required the delineation of the 

water-producing and water-quality zones within the 

hydrogeologic units underlying Dunedin.

In this report, the Upper Floridan aquifer is divided 

into three water-quality zones that generally coincide 

with the permeable zones: the fresh-water zone (upper 

zone A), the brackish-water zone (lower zone A), and 

the saline-water zone (zone B). The zones are defined 

by a range in concentration for dissolved solids (DS) 

and chlorides. The ranges in DS concentrations for the 

fresh-water, brackish-water, and saline-water zones are 

0-500,500-10,000, and greater than 10,000 milligrams 

per liter (mg/L), respectively. The ranges in chloride 

for the fresh-water, brackish-water, and saline-water 

zones are 0-250, 250-5,200, and greater than 5,200 

mg/L, respectively.

Purpose and Scope

This report presents the results of a study initiated 

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

cooperation with the city of Dunedin and the Southwest Florida Water Management District (SWFWMD), 

to assess the fresh- and brackish-water resources in the 

Upper Floridan aquifer underlying the city of Dunedin 

and adjacent areas of northern Pinellas County (figs, la 

and Ib). The report includes: descriptions of the 

hydrogeologic framework and geologic controls affecting the distribution of hydraulic characteristics and

water quality in the heterogeneous, multilayered, multidensity aquifer system and discussions of the occurrence of ground water and aquifer testing in the study 

area. Simulation results are presented from a numerical ground-water flow and solute-transport model estimating current and future, fresh- and brackish-water 

resources near the Dunedin water plant.

Information presented in this report was obtained 

from data collected during this investigation, from 

unpublished reports on file at the USGS, and from published USGS, SWFWMD, and consultants' reports. 

Information collected during this investigation 

included lithologic and geophysical logs, water-quality, water-level, and aquifer-test data. Producing zone 

locations and their associated water quality were delineated using lithologic and geophysical logs and 

geochemical data collected during drilling of the brackish production and brackish monitor wells and from 

other Dunedin production and monitor wells. A conceptual model of the ground-water system and waterquality distribution was developed to: (1) organize the 

concepts of the physical and chemical system behavior, 

(2) provide an understanding of the ground-water flow 

and the water-quality changes resulting from simultaneous withdrawals from both the fresh-water producing zone (upper zone A) and brackish-water producing 

zone (lower zone A), and (3) relate these concepts to 

the framework of the numerical model.

Description of the Study Area

The 156 mi2 study area includes most of northern 

Pinellas County and encompasses the Dunedin well 

field located near the center of the study area (fig. la). 

Pinellas County is a peninsula in west-central Florida 

situated between the Gulf of Mexico and Tampa Bay. 

The brackish production and brackish monitor wells, 

drilled as part of this study, are located at the Dunedin 

water plant (fig Ib). The area is predominately urban; 

the population density of Dunedin is approximately 

4,750 persons per mi2 .

The study area is part of the Gulf Coastal Lowlands 

physiographic province comprised of scarps and terraces created during various Pleistocene sea-level 

stands. Heath and Smith (1954) recognized three of 

these terraces in the study area. The topographic surface ranges from sea level at the coast to 65 feet (ft) 

above sea level on the eastern boundary of Dunedin.

Assessment of the Fresh- and Brackish-Water Resources Underlying Dunedin and Areas of Northern Pinellas County, Fla.
82°51'

28°04' 

49'

28°00' 

27°54

0 0.5 1.0 MILE 

0 0.51.0 KILOMETER

58' 

56

Base from Southwest Florida Water Management District digital data, 1:250,000,1992 

Universal Transverse Mercator projection, Zone 17

Figure 1a. Location of the study area and selected U.S. Geological Survey network wells. (Location and name.)

Introduction
82°47' 82°45'

28°03' 

V6

n59/60

EXPLANATION

GENERAL WATER PLANT 

BOUNDARY

PRODUCTION WELL 

AND NUMBER

OBSERVATION WELL

AND NUMBER

WELL CLUSTER (including 

the Brackish Production, Brackish 

Monitor, and Production 14 wells)

0 0.5 1.0 KILOMETER

28°00' 

Base from Southwest Florida Water Management District digital data, 1:250,000, 1992 

Universal Transverse Mercator projection, Zone 17

Figure 1 b. Location of the existing production wells and observation wells, new wells drilled as part of 

the study, generalized water plant boundary, and historic downtown district Dunedin, Florida.

4 Assessment of the Fresh- and Brackish-Water Resources Underlying Dunedin and Areas of Northern Pinellas County, Fla.
Acknowledgments

The author recognizes the invaluable support and 

cooperation from the city of Dunedin. A special thanks 

to Steve Rogers, a Dunedin water plant operator and 

field geologist, for his technical assistance in all data collection and monitoring phases of the investigation. Special thanks also to a dedicated student employee, Rachel 

Brewer, for her assistance during the aquifer test.

HYDROGEOLOGIC FRAMEWORK AND 

HYDRAULIC PROPERTIES IN THE STUDY 

AREA

Water-bearing Cenozoic-age formations in westcentral Florida consist of a thick sequence of sedimentary units, deposited during various transgressive and 

regressive sea-level events, that form a multilayered 

sequence of higher- and lower-permeability units. The 

hydrogeologic framework underlying the study area is 

composed of the surficial and Floridan aquifer systems 

separated by the intermediate confining unit. The aquifer systems contain one or more permeable zones separated by lower-permeability units (fig. 2). The term 

semiconfining unit will be used in this report to designate the lower-permeability carbonate units that do not 

yield large quantities of water to wells. General 

descriptions of the hydrogeologic units and hydraulic 

properties were compiled from previous studies. Identification of the permeable zones and semiconfining 

units within the Upper Floridan aquifer are based on 

lithologic and geophysical-log data collected as part of 

this and previous studies. These hydrogeologic units 

are described in the following sections of the report.

Surficial Aquifer System

In the study area, the surficial aquifer system 

(SAS) is the uppermost water-bearing unit but it is seldom used for potable water supplies because of its limited thickness (10 to 50 ft). The water table is within 

10 ft of land surface and fluctuates less than 5 ft annually. The water table fluctuates seasonally, with lower 

levels occurring during the dry season (October 

through May) and higher levels occurring during the 

wet season (June through September). Recharge to the 

SAS is from rainfall.

The hydraulic properties of the SAS, in the study 

area, are highly variable due to the nature of deposition 

affecting the physical characteristics of grain size, sorting, and thickness. Values of horizontal hydraulic conductivity, vertical hydraulic conductivity, and porosity, 

reported by previous investigators, range from 13 to 33 

feet per day (ft/d) (Cherry and Brown, 1974; Sinclair, 

1974), 0.36 to 13 ft/d (Sinclair, 1974; Hutchinson and 

Stewart, 1978), and 29.2 to 32.2 percent (Hutchinson 

and Stewart, 1978), respectively.

Intermediate Confining Unit

The low permeability intermediate confining unit 

(ICU) occurs between the SAS and Upper Floridan 

aquifer (UFA). The ICU is comprised of interbedded 

elastics and carbonates that reflect a variety of depositional environments. In the vicinity of Dunedin, the 

ICU is approximately 60 ft thick and is predominantly 

composed of clay that retards the vertical movement of 

ground water between the SAS and UFA.

The hydraulic properties of the ICU are highly 

variable. The leakance values, reported by previous 

investigators, range from 0.00001 to 0.015 feet per day 

per foot ((ft/d)/ft) (Black, Crow, and Eidness, Inc., 

1978; Seaburn and Robertson, Inc., 1983; M.P. Brown 

and Associates, Inc., 1986; Ryder, 1982, 1985). The 

most plausible values of leakance, in the vicinity of 

Dunedin, are the values ranging from 0.00001 to 

0.0001 (ft/d)/ft reported by Ryder (1982).

Floridan Aquifer System

The Floridan aquifer system is a thick sequence of 

Tertiary-age limestones, dolomites, and evaporites and 

includes the UFA, the middle confining unit, and 

Lower Floridan aquifer (LFA). This report is concerned only with the UFA which includes the following 

geologic units: the Tampa Member of the Arcadia Formation of the Hawthorn Group, the Suwannee Limestone, the Ocala Group, and the Avon Park Formation 

(fig. 2). Hickey (1982) subdivided the UFA into four 

permeable zones separated by semiconfining units. 

The zones were designated as A, B, C, and D and are 

used in this report. Only zones A and B contain water 

that has DS concentrations less than 35,000 mg/L; further discussions will be limited to these permeable 

zones.

Zone A

In Dunedin, zone A comprises the Tampa Member 

and the uppermost part of the Suwannee Limestone, 

and is the shallowest and freshest of the permeable 

zones. The top of zone A occurs at about sea level and 

the bottom at about 250 ft below sea level. The values 

for transmissivity, reported by previous investigators,

Hydrogeologic Framework and Hydraulic Properties in the Study Area
e 

s

W 

W

oo

3

U 

PQ

1 \J\J

o100-

200- 

300- 

400- 

500- 

*;nnSERIES

PLEISTOCENE

MIOCENE

OLIGOCENE

EOCENE

GEOLOGIC 

UNIT

/"HAWTHORN GROUP 

NDIFFERNTIATED / 

»ANDS &/ 

CLAY/ 

UNDIFFERENTIATED 

ARCADIA 

FORMATION

V^

TAMPA 

MEMBER 

OF THE 

ARCADIA 

FORMATION

SUWANNEE 

LIMESTONE

OCALA 

GROUP

AVON PARK 

FORMATION

HYDROGEOLOGIC 

UNIT

/FLORIDAN AQUIFER SYSTEM a

/ / *-! 

SURFICIAL 

AQUIFER /

SYSTEM/

TERMEDIATE

CONFINING 

UNIT /

UPPER 

FLORIDAN 

AQUIFER

LITHOLOGY

SAND

SAND, 

CLAY, AND 

LIMESTONE

LIMESTONE, 

DOLOMITE, 

AND 

DOLOMITIC 

LIMESTONE

DOLOMITE

PERMEABLE 

ZONE AND 

(SEMI)CONFININGUNIT

SURFICIAL

gffKgft^vpgg&^t^Z

UPPER 

PERMEABLE 

ZONE A

LOWER 

PERMEABLE 

ZONE A

IH

PERMEABLE 

ZONEB

MMiSSSmmmimmim

Illlllil^^^ll^ll :ll

iPilillliliiiii

:::^:-:V:V:::::^:::^:-:::;:>i:!:i;:::::gx:^::;i:!:£;;::s::::;-::

PERMEABLE 

ZONEC

MODEL

UNIT

LAYERS

LAYER 7

LAYER 6

LAYER 5

LAYER 4

LAYERS

LAYER 2

LAYER 1

NM1

NM1 Not modeled

Figure 2. Generalized stratigraphic and hydrogeologic section underlying the 

study area.

6 Assessment of the Fresh- and Brackish-Water Resources Underlying Dunedin and Areas of Northern Pinellas County, Fla.
range from 10,000 to 40,000 feet squared per day (ft2/d) 

(Hickey, 1982; Seaburn and Robertson, Inc., 1983; and 

M.P. Brown and Associates, Inc., 1986).

Semiconfining Unit Between Zones A and B

Underlying permeable zone A is the lower Suwannee Limestone that acts as a semiconfining unit (SCU 

2). This semiconfining unit averages 80 ft in thickness 

and is composed of carbonates that do not yield large 

quantities of water. Cores taken from the semiconfining units, indicate that closed fractures predominate 

and, therefore, primary porosity rather than secondary 

porosity controls permeability in the semiconfining 

units (Hickey, 1982). The published values of vertical 

hydraulic conductivity of the semiconfining unit range 

from 0.0013 to 2 ft/d (Hickey, 1982).

Zone B

Permeable zone B is comprised of the lowermost 

section of the Suwannee Limestone and upper Ocala 

Group. The unit consists of thin beds of dolomite, 

dolomitic limestone, and limestone. Zone B averages 

60 ft in thickness and is characterized by rapidly changing salinity with depth. Salinity in zone B ranges from 

1,500 to 10,000 mg/L DS. Zone B is less transmissive 

(5,000 ft2/d) than zones A (10,000 to 40,000 ft2/d) or C 

(900,000 to 1,200,000 ft2/d) (Hickey, 1982, p. 17).

Semiconfining Unit Between Zones B and C

Underlying permeable zone B is the lower Ocala 

Group that acts as a semiconfining unit (SCU 3). This 

semiconfining unit averages 140 ft in thickness and is 

composed of carbonates that do not yield large quantities of water. Cores from this semiconfining unit indicate that the beds have closed fractures; consequently, 

the unit retards the vertical movement of water 

between zones B and C (Hickey, 1982, p. 18). Calculated vertical hydraulic conductivities range from 0.1 

to 1 ft/d (Hickey, 1982, p. 19-20).

Delineation of the Hydrogeologic Units and 

Producing Zones Underlying the Study Area

Geophysical logs from eight wells were used to 

further delineate the distribution of zones A, B, and 

intervening semiconfining units in the study area. 

These eight wells were selected based on penetration 

depth and areal distribution. The locations and identification names or numbers of the eight wells are shown

in figure 3; well-construction data are listed in the 

appendix. Three generalized hydrogeologic sections, 

one oriented north-south, one oriented east-west, and 

another at the water plant test site were constructed 

using these eight wells. The distribution of permeable 

zones and semiconfining units correlated from gamma 

log traces is shown in figures 4 and 5.

Vertically spaced, discrete water-producing zones 

are present within zone A (figs. 4 and 5). The vertical 

position of these producing intervals within zone A is 

highly variable suggesting that fluid flow occurs 

through a network of randomly distributed permeable 

pathways. Generally, the producing zones occur in the 

upper and lower parts of zone A. These producing 

zones are separated by semiconfining unit 1 (SCU1). 

The presence of SCU1 is delineated on gamma logs by 

a zone of elevated gamma radiation, indicating that 

interbedded clays exist throughout the study area (fig. 

4). Therefore, zone A was partitioned into upper zone 

A (UZA) and lower zone A (LZA)~ the two being separated by SCU 1.

The geologic unit that is the primary source of 

ground water to wells in the Dunedin well field is the 

Tampa Member, designated as upper zone A. However, many of the wells penetrate both the upper and 

lower zone A and some water may be contributed from 

the producing zone in lower zone A. The average 

open-hole interval is 162 ft. The term open-hole interval is the depth between the bottom of the casing and 

the bottom of the well. The wells in the southern part 

of Dunedin are deeper than those in the downtown 

area.

Two test wells (the brackish production and brackish monitor wells) were drilled at the Dunedin water 

plant (fig. Ib) to define the vertical distribution of producing zones. The first test well (the brackish monitor 

well) was drilled to 550 ft and cased to 60 ft. Interpretations of geophysical logs and packer-test results indicated the existence of three producing zones (fig. 5). 

The uppermost producing zone, that contributes the 

majority of water to wells in the Dunedin area, occurs 

in upper zone A, a second zone occurs in lower zone A, 

and a third minor producing zone occurs in zone B. 

The first test well was later cased to 400 ft and back 

plugged to 465 ft (340-405 ft below sea level). The 

source of water to the well, following final construction, is zone B.

The second test well (the brackish production well) 

was drilled to 400 ft and cased to 70 ft. Interpretations 

of geophysical logs indicated the existence of two

Hydrogeologic Framework and Hydraulic Properties in the Study Area
82°5T

28°04' 

49' 47' 82°39'

ROMP 

TR14-2 

WELL

CLEARWATER

EXPLANATION

A' LINE OF HYDROGEOLOGIC SECTION

 31 WELL LOCATION AND NAME

0 0.5 1.0 MILE 

0 0.51.0 KILOMETER

WELL CLUSTER-Including the 

brackish production, brackish 

monitor, and production 14 wells)

Base from Southwest Florida Water Management District digital data, 1:250,000,1992 

Universal Transverse Mercator projection, Zone 17

Figure 3. Location of selected wells, well names, and traces of the hydrogeologic sections A-A' and B-B.

8 Assessment of the Fresh- and Brackish-Water Resources Underlying Dunedin and Areas of Northern Pinellas County, Fia.
A'

PRODUCTION

31 100

w

PQ 100

g H 200

PQ h-f

on 

W 300

400

500

BRACKISH ROMP 

MONITOR TR14-2

PRODUCTION

33

Intermediate Confining Unit 

___- 

Upper Permeable Zone A

100

100

200

300

400

500

0.5 1.0 KILOMETER

B

B'

i

w

PQ

ROMP

TR14-2

100

100

<

W 

> 

O 

PQ

« 200

300

5" P U

< 400

500

ROMP

TR14-3

Permeable Zone B

0.5 1.0 MILE

0 0.5 1.0 KILOMETER

EXPLANATION 

 PRODUCING ZONES 

- INFERRED LINE

ROMP

TR14-1

100

100

200

300

400

500

Figure 4. Hydrogeologic sections A-A and B-B showing natural-gamma log traces correlated to permeable zones and 

semiconfining units.

Hydrogeologic Framework and Hydraulic Properties in the Study Area 9
e

w

100

100

ow 20°

300

HHH

< 400 

500

BRACKISH 

MONITOR

BRACKISH 

PRODUCTION

PRODUCTION

14

Surficial Aquifer System

Semiconfining Unit 2 

Permeable Zone B

Semiconfining Unit 3

EXPLANATION 

I PRODUCING ZONES 

- INFERRED LINE

100

100

200

300

400

500

HORIZONTAL DISTANCE HORIZONTAL DISTANCE 

IS 68 FEET IS 62 FEET

Figure 5. Hydrogeologic units underlying the aquifer-test site showing natural-gamma log traces correlated 

to permeable zones and Semiconfining units.

major producing zones (fig. 5). The uppermost producing zone, that contributes the majority of water to wells 

in the Dunedin area, occurs in upper zone A and a second zone occurs in lower zone A. The second test well 

was later cased to 220 ft. The source of water to the 

well, following final construction, is predominantly 

from the interval between 220 and 260 ft (160-200 ft 

below sea level). Additional producing zones were 

detected from spinner flowmeter measurements at 

discharge rates of 300, 600, and 1,000 gal/min. As the 

pumping rate was increased, water to the well was contributed from deeper depths in the borehole.

OCCURRENCE OF GROUND WATER AND 

GROUND-WATER DEVELOPMENT IN THE 

STUDY AREA

The occurrence of freshwater in the Upper Floridan 

aquifer is best characterized as a lens underlain by brackish and saline water. The freshwater portion of the 

ground-water flow system underlying the city of 

Dunedin appears to be a local flow system that functions 

independently of the regional ground-water flow system. 

A localized flow system is indicated from water-level 

hydrographs and potentiometric-surface maps.

10 Assessment of the Fresh- and Brackish-Water Resources Underlying Dunedin and Areas of Northern Pinellas County, Fla.
Water Levels and Dunedin Well-Field 

Pumpage

Water-level changes resulting from ground-water 

development have occurred in the study area. Prior to 

urban development, the primary land use in the study 

area was for citrus production. The USGS Dunedin 

7 1/2-minute quadrangle map for 1943 shows the land 

use as citrus groves; in 1952, land use was still predominantly citrus groves. Also during this period, wellfield pumpage was 10 times greater from the Clearwater well field than from the Dunedin well field 

(Black and Associates and Bridley, Wild and Associates, Inc., 1952). Hydrographs for Pinellas 665 and 

Garden Street Triangle wells demonstrate long-term 

and seasonal water-level patterns (fig. 6). These wells 

were selected as indicators of water-level response to 

pumpage in the study area. The hydrographs indicate 

that water levels decreased during the period 1957-75, 

stabilized during the period 1975-81, and increased 

since 1981. Maximum water-level declines in Pinellas 

665 and Garden Street Triangle wells, from 1957 -75, 

were 6 and 4 ft, respectively. Water levels in these 

wells may be associated with changes in ground-water 

withdrawals in the study area. Since 1981 the lack of 

correlation between increases in Dunedin well-field 

pumpage and increases in water levels in the two observation wells indicates that water-level changes in these 

wells are probably the result of decreases in groundwater withdrawals in the Clearwater well field and cessation in irrigation pumpage due to increased urban 

development and loss of citrus groves. These trends 

indicate that pumpage in the Dunedin well field is probably not affecting water levels in the aquifer outside of 

Dunedin.

Historical Dunedin well-field withdrawals are 

shown in figure 7. The construction and production of 

the wells occurred in stages. Wells 1, 2, 3, 4, 5, 6, and 

50 began production in 1958, and well 9 in 1961. Well 

11 began production in late 1964, was utilized in 1964, 

1965, 1991, and has been in production since 1993. 

Consequently, well 11 is not shown in figure 7. Well 12 

began production in 1965. Wells 13, 14, 15, and 16 

began production between 1970-72. Wells 28, 29, and 

30 began production in 1985 and were used intermittently until 1988-89. Wells 31 and 33 began production 

in 1990 and 1992, respectively. The production wells 

are clustered and are designated as the "downtown 

wells" (wells 1, 2, 3, and 4), the "water plant wells" 

(wells 6, 9, 13, 14, 15 and 16), and the "southern area 

wells" (wells 5, 28, 29, 30, and 31). In addition, three

more wells are located in northern Dunedin (wells 11, 

12, and 33) (fig. Ib). Current (1994) pumpage from the 

city of Dunedin is approximately 4.9 Mgal/d. 

Although total well-field pumpage has increased, withdrawal rates from many of the wells has decreased due 

to the construction of new wells.

Evaluation of water levels and pumpage records 

from the water plant wells, supplied by Dunedin personnel for the period 1983-91, indicate that although 

individual well pumpage has decreased, water levels 

have not discernibly changed. An example of this relation is shown in figure 8. Production well 14 was 

selected because it is closest to the centroid of pumping 

at the water plant and to the test wells drilled as part of 

this study. Water levels in individual wells generally 

are not directly affected by changes in individual well 

pumpage except during 1985 when pumping from production well 14 was significantly reduced (fig. 8).

Ground-Water Flow Patterns

Changes from the historical potentiometric surface 

do not appear to be substantial, possibly due to the fact 

that water levels in the study area are controlled by 

local, rather than regional, conditions. Several investigators, including Heath and Smith (1954, p. 29), have 

substantiated the occurrence of a local flow system in 

the study area. Prior to the mapping of the potentiometric surface as part of their 1954 study, it was 

thought that the water in the Upper Floridan aquifer 

underlying the study area was derived from recharge in 

counties to the north (Pasco) and east (Hillsborough) of 

Pinellas County; however, detailed mapping of the 

potentiometric surface revealed that in the Dunedin and 

Clearwater areas of Pinellas County, all of the freshwater recharging the Upper Floridan aquifer is from local 

precipitation (Heath and Smith, 1954, p.29-31). Freshwater recharge, originating from precipitation, percolates through the hydraulically restrictive intermediate 

confining unit. Most of the local recharge apparently 

occurs along the topographic mound near the eastern 

boundary of Dunedin and Clearwater, where the potentiometric surface is elevated. Rainfall appears to be the 

only source of freshwater recharge to the Upper Floridan aquifer. Results of another study (Rodney N. 

Cherry and others, written commun., 1974) indicated 

the occurrence of the ground-water mound and determined the location of the ground-water divide using 

flow-net analysis. The analysis indicated that practically no freshwater enters the Pinellas County

Occurrence of Ground Water and Ground-Water Development in the Study Area 11
20

00

W

1 £H h I l~~l

W

ES

i i i i r

Garden Street Triangle Well

0 i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i 

1954 1964 1974 1984 1994

YEAR 

Figure 6. Daily water levels for Pinellas 665 and Garden Street Triangle wells, 1954-94.

wS

Q

0-, 04 § w

sii £<

< j

§ <

^^ ^^

S§O *~i

w §

< ^ ^ KH

4.5

4.0

3.5

3.0

2.5

2.0

1.5

1 0

: ' ' ' ;

0 m i Hi -

5- i 1 ! :

CN ri m

1 (X (X . 

OO

^ ? ' "-; I 

SH ] o ^ B

^" (X fflH an

S 2 * : U^ OH

^ 1

: B " ! "

^ P12 Production well number

: ^ :

", , , :

1955 1960 1970 1980 

YEAR

1990 1995

Figure 7. Average annual pumpage and year that production wells went online in the 

Dunedin well field.

12 Assessment of the Fresh- and Brackish-Water Resources Underlying Dunedin and Areas of Northern Pinellas County, Fla.
14

12

10

§ 4

§ °

g 12

SPU 10

no data

1983 1984 1985 1986 1987 1988 1989 1990 1991

^ 1983 1984 1985 1986 1987 1988 1989 1990 1991

YEAR

Figure 8. Pumpage and monthly water levels in production well 14, 

1983-91.

peninsula as lateral flow in the Floridan aquifer. 

Hutchinson (1983, p. 19-20) presents two maps showing flow lines drawn on the potentiometric surface of 

the Upper Floridan aquifer for May and September 

1978 in the Tampa Bay area. Both maps show that 

ground-water flow from the potentiometric-surface 

high is easterly toward Tampa Bay and westerly toward 

St. Joseph Sound and the Gulf of Mexico. The depression in the potentiometric surface around Tampa Bay is 

evidence of the natural discharge of ground water to the 

bay.

Potentiometric-surface maps were constructed as 

part of this study. The May and September 1994 potentiometric surfaces of the Upper Floridan aquifer in the 

study area are shown in figures 9 and 10. Three small, 

circular depressions in the potentiometric surface are 

the result of production-well pumpage in the Clearwater and Dunedin well fields. Water levels in eastern 

Dunedin and Clearwater create a dome-shaped feature 

on the potentiometric surface. Historically, this potentiometric-surface mound has been a common feature. 

The mound creates a ground-water divide east of the 

city of Dunedin indicating that the only source of freshwater recharge to the Upper Floridan aquifer is local 

recharge.

GROUND-WATER QUALITY IN DUNEDIN

The Upper Floridan aquifer underlying Dunedin 

contains a thin lens of fresh-water, maintained by 

recharge from rainfall, that is underlain by brackish and 

saline ground water. The spatial water-quality distribution is governed by the fact that ground water underlying 

Dunedin is a mixture of fresh-water and saltwater that 

varies both laterally and vertically. This mixture ranges 

in quality from fresh to saline and has been defined by 

previous investigators as a transitional type water 

(Causseaux and Fretwell, 1983). Transitional type 

waters have chloride concentrations ranging from 25 to 

19,000 mg/L. Spatially, the region occupied by this 

mixture is designated as the transition zone. In general, 

chloride and DS concentrations increase with depth; 

however, vertical variations in salinity may occur in the 

uppermost part of the Upper Floridan aquifer.

Lateral Variations

Average 1993 chloride concentrations in water 

from wells tapping part or all of zone A were mapped 

to determine the lateral variations. Chloride concentrations in water from the production wells ranged from 

21 to 242 mg/L. Chloride concentrations in water from

Ground-Water Quality in Dunedin 13
82°51' 49' 47' 45' 43' 41' 82°39'

28°04

02

28°00

58'

56'

27°54'

0 0.5 1.0 MILE 

0 0.5 1.0 KILOMETER

6 POTENTIOMETRIC CONTOUR-- 

Shows altitude of the potentiometric 

surface. Contour interval 2 feet. 

Datum is sea level 

WELL LOCATION AND WATER 

LEVEL IN WELL, IN FEET

Figure 9. Potentiometric surface of the Upper Floridan aquifer, northern Pinellas County, May 1994.

14 Assessment of the Fresh- and Brackish-Water Resources Underlying Dunedin and Areas of Northern Pinellas County, Fla.
82°51' 49' 47' 45' 43' 41' 82°39'

28°04

02'

28°00'

58'

56'

27°54'

0 0.5 1.0 MILE 

0 0.5 1.0 KILOMETER

r\_

EXPLANATION

POTENTIOMETRIC CONTOUR-- 

Shows altitude of the potentiometric 

surface. Contour interval 2 feet. 

Datum is sea level

6.6 WELL LOCATION AND WATER 

> LEVEL IN WELL, IN FEET

Figure 10. Potentiometric surface of the Upper Floridan aquifer, northern Pinellas County, September 1994.

Ground-Water Quality in Dunedin 15
the observation wells ranged from 35 to 570 mg/L 

(fig. 11). In general, highest chloride concentrations 

occur where production wells are clustered rather than 

in the vicinity of St. Joseph Sound, indicating the 

source of chloride may be water from deeper zones 

rather than lateral intrusion of modern seawater. 

Elevated chloride concentrations occur in some of the 

observation wells. These observation wells (including 

10M, 18M, 19M, and 21M) were originally production 

wells and it is probable that ground-water withdrawal 

has had a localized effect on the water quality in these 

wells. The highest chloride concentration (570 mg/L) 

was in water from observation well 51. Well 51 is 

located near the coast and near Curlew Creek. Curlew 

Creek has incised the limestone of the Upper Floridan 

aquifer. This breach in the intermediate confining unit 

may allow direct movement of saline water from St. 

Joseph Sound into the aquifer; however, statistical 

analysis indicates that chloride concentrations in water 

from well 51 have been decreasing at an average rate of 

69 mg/L per year for the last 10 years (Dann K. Yobbi, 

written commun., 1996). This trend does not tend to 

support lateral movement of modern seawater into the 

freshwater zones of the Upper Floridan aquifer.

In general, chloride concentrations are highest 

where pumping is concentrated; however, the chloride 

concentrations in water from adjacent wells commonly 

are different even though the wells are similarly constructed. The reasons for this may include: (1) wells 

penetrate flow zones with different chloride concentrations; (2) the hydraulic gradient between flow zones 

changes with time affecting the relative contribution 

from discrete flow zones with differing water quality; 

and (3) changes with time in water quality of individual 

flow zones. The chloride concentration in water from 

each well represents a mixture of water from all producing zones penetrated by the well.

Vertical Variations

Delineation of the vertical water-quality changes in 

the uppermost part of the Upper Floridan aquifer was 

accomplished by evaluating the distribution of selected 

chemical constituents from water samples collected 

during well construction and packer testing.

Values of specific conductance were plotted to 

show the vertical variations within the Upper Floridan 

aquifer (fig. 12). The data show that water with elevated specific conductance occurs in both upper and 

lower zone A and water with lower specific conductance occurs in semiconfining unit 2. Water from wells 

that penetrate zone B show a dramatic increase in specific conductance in this zone. Data from the brackish 

monitor and TR14-2 wells indicate that saline water 

occurs deeper than 400 ft below sea level.

As part of this study, water-quality samples were 

collected at selected intervals from both the brackish 

monitor and brackish production wells to characterize 

the vertical distribution of chemical constituents in 

water from discrete producing zones. Water samples 

were collected after the test wells were drilled using a 

thief sampler, a Fultz pump with a 1/4-inch drop pipe, 

and a submersible pump during packer testing. The 

depth intervals sampled correspond to locations of producing zones indicated by geophysical logs and spinner 

flowmeter measurements.

Stiff diagrams showing major ion concentrations 

of water samples from selected depths in the brackish 

monitor well illustrate the vertical distribution of water 

types and salinity in the Upper Floridan aquifer 

(fig. 13). The shape of the diagram indicates the water 

type; for example, a "T" shape indicates sodium chloride type water and a "diamond" shape indicates a calcium bicarbonate type water. The size of the diagram 

indicates concentration (the larger the diagram the 

higher the DS). Stiff diagrams for water from the 

brackish monitor well indicate a sodium chloride type 

water throughout the interval sampled. These "T" 

shaped stiff diagrams and the trend of increasing size 

(concentration) with depth are likely the result of 

upward flow of higher salinity water from zone B, 

rather than natural conditions within zone A. The stiff 

diagrams for water samples collected from the brackish 

production well are somewhat anomalous. Above a 

depth of 334 ft below sea level, the water is a sodium 

chloride type; however, the size (concentration) 

decreases with depth. The sodium and chloride concentrations decrease throughout the interval between 

286 and 334 ft below sea level. A calcium bicarbonate 

type water was present at the bottom of the well. This 

trend also was supported by interpretation of the fluid 

resistivity log.

The layering of higher salinity water over lower 

salinity water in the uppermost Upper Floridan aquifer, 

as observed in wells in the vicinity of Dunedin (figs. 12 

and 13), could result from: (1) past seawater encroachment into zones of higher permeability during the 

higher Pleistocene sea-level stands and incomplete 

flushing of the seawater by the present-day flow system; or (2) the occurrence of structural features, such as

16 Assessment of the Fresh- and Brackish-Water Resources Underlying Dunedin and Areas of Northern Pinellas County, Fla.
82°47' 82°45'

28°03' 

 PRODUCTION WELL 

O OBSERVATION WELL

28°00' 

Base from Southwest Florida Water Management District digital data, 1:250,000,1992 

Universal Transverse Mercator projection, Zone 17

Figure 11. Chloride concentrations, in milligrams per liter, in selected wells penetrating zone A, 1993, 

Dunedin, Florida. (Wells are identified in figure 1b.)

Ground-Water Quality in Dunedin 17
WEST BRACKISH 

MONITOR

ROMP

TR14-2

ROMP

TR14-3

ROMP 

TR14-1

H 

W

E

zHH 

J

W

>

W

J 

<

W

W 

PQ 

W

Q 

D H

5

EAST

0

-50

-100

-150

-200

-250

-300

-350

-400

-450

-500

-550

0 50,000 0 50,000 0 2,000 0 2,000 

SPECIFIC CONDUCTANCE, IN MICROSIEMENS PER CENTIMETER

UPPER ZONE A

EXPLANATION 

LOWER ZONE A ZONEB

Figure 12. Specific-conductance profile during well drilling.

fractures, that provide hydraulic connection to sources 

of saline water. Available data indicate that the relatively shallow zones of higher salinity correspond to 

zones of relatively higher transmissivity in the Upper 

Floridan aquifer. Deeper in the Upper Floridan aquifer 

(within and below zone B), the water changes rapidly 

from fresh to saline.

Temporal Variations

To understand the dynamic changes in the flow 

system, temporal variations in water quality, as indicated by chloride, were evaluated. Chloride-concentration data from the Dunedin production wells have 

been tabulated since 1970. As part of another USGS 

study, water-quality data were statistically analyzed for 

trends (Dann K.Yobbi, written commun., 1996). Table 1 

presents the statistical trend-analysis of chloride concentration in the production and observation wells in 

Dunedin. In general, the quality of water from production wells has changed between 1970-95. Increasing 

chloride-concentration trends are occurring in water 

from all production wells except production well 4. 

Statistical trend-analysis of data from the past 10 years 

indicates that chemical changes also are occurring at 

production well 4. The rate of change in chloride concentration is not uniform throughout the well field 

(table 1); however, the greatest rate of change at most 

of the wells has occurred since 1988. Long-term chloride concentrations for the six water plant production 

wells are shown in figure 14. Combined pumpage from 

these six wells increased from 1 Mgal/d in 1970 to 1.9 

Mgal/d in 1980 and then decreased to 1.05 Mgal/d in 

1990. Total well-field pumpage increased from 2.4 

Mgal/d in 1970 to 4.9 Mgal/d in 1994. Although total 

well-field pumpage has increased, individual well 

pumpage has decreased; however, decreases in individual production-well withdrawal have not altered the 

chloride-concentration trend.

18 Assessment of the Fresh- and Brackish-Water Resources Underlying Dunedin and Areas of Northern Pinellas County, Fla.
BRACKISH MONITOR WELL 

CATIONS AN IONS

400 300 200 100 0 100 200 300 

'COMPOSITE

-89 FEET

-154 FEET

-244 FEET

-329 FEET

-354 FEET

400

--402 FEET

-464 FEET

BRACKISH PRODUCTION WELL 

CATIONS , ANIONS

15 10 0 10 15 

COMPOSITE

-84 FEET

20

-174 FEET

-224 FEET

-254 FEET

-286 FEET

-334 FEET

EXPLANATION

Sodium + Potassium_____Chloride

Calcium <^ __JZ^ Bicarbonate + Carbonate 

Magnesium Sulfate

-224 FEET IS THE WATER SAMPLE COLLECTION DEPTH BELOW SEA LEVEL 

ANION AND CATION SCALE IN MILLIEQUIVALENTS PER LITER

Figure 13. Stiff diagrams of major ions from selected depths in the brackish 

monitor and brackish production wells.

Ground-Water Quality in Dunedin 19
Table 1. Records of the city of Dunedin production and observation wells and trend analysis results of 

chloride concentration

[n, sample size; p, significance level (positive value indicates increasing trend, negative value indicates decreasing 

trend); HS, highly significant (p< 0.01); S, significant (0.01 < p< 0.05); MS, marginally significant (0.05 < p <0.10); *, not 

significant (p > 0.10); mg/L, milligrams per liter; ND, means less than 10 years of data available; same, means same 

data set for all available data and 1985-94 water years]

Well name 

or number

1

2

3

4

5

6

9

10M

11

12

13

14

15

16

18M

19M

21M

28

29

30

31

33

34

50

51

52

53

54

55

56

57

58

59

60

Station number

2800450824722

2800460824709

2800490824704

2800120824715

2800320824625

2801120824537

2801140824543

2801390824548

2802290824513

2802290824534

2801230824546

2801230824541

2801250824540

2801250824531

2801230824459

2801240824449

2801320824500

2800280824545

28000110824621

2800100824610

2800190824606

2802150824510

2801260824455

2801020824632

2802490824643

2802150824639

2801140824707

2800150824711

2802170824544

2802170824544

2800500824534

2800500824534

2800200824607

2800200824607

All available data

Period of 

record

1970-95

1970-95

1970-95

1970-92

1970-95

1971-95

1970-95

1970-95

1987-95

1970-95

1971-95

1970-92

1971-95

1971-95

1977-95

1978-95

1977-95

1985-95

1985-95

1985-95

1985-94

1991-94

1992-95

1987-95

1992-95

1992-95

1992-95

1992-95

1992-95

1992-95

1992-95

1992-95

1992-95

1992-95

n

848

936

942

639

849

887

913

484

137

852

923

685

833

865

450

498

426

344

410

404

178

63

39

87

38

40

40

40

39

37

37

34

38

35

P

HS

HS

HS

*

HS

HS

HS

S

S

HS

HS

HS

S

HS

S

HS

*

HS

HS

HS

HS

*

*

*

MS

*

MS

MS

*

MS

S

S

*

*

Trend 

(mg/L)

2.3

4.0

0.8

*

2.0

2.7

2.0

3.6

18.7

0.8

3.3

2.2

1.9

4.8

5.6

14.5

*

12.7

20.5

16.5

30.3

*

*

*

-69.4

*

-6.1

-43.6

*

3.0

2.5

3.2

*

*

Data for 1985-1994 water years

n

515

610

600

314

516

567

576

147

ND

511

592

356

521

552

210

258

182

same

same

same

ND

ND

ND

ND

ND

ND

ND

ND

ND

ND

ND

ND

ND

ND

P

*

*

HS

HS

HS

HS

HS

*

ND

HS

HS

*

HS

HS

*

HS

*

same

same

same

ND

ND

ND

ND

ND

ND

ND

ND

ND

ND

ND

ND

ND

ND

Trend 

(mg/L)

*

*

1.7

5.9

8.4

6.2

6.3

*

ND

1.6

6.3

*

7.6

8.3

*

19.6

*

same

same

same

ND

ND

ND

ND

ND

ND

ND

ND

ND

ND

ND

ND

ND

ND

20 Assessment of the Fresh- and Brackish-Water Resources Underlying Dunedin and Areas of Northern Pinellas County, Fla.
400

300

200

100

PRODUCTION WELL 16

Depth = 175 ft 

Casing = 65 ft

i i i i i i i i i i i i i i i i i i i i i i i i i

0. 

00

OI-H 

 J

400

300

200

100

W 

U

§ o

U

a

04

O 400

ffi 

U

300

200

100

PRODUCTION WELL 14

Depth = 260 ft 

Casing = 59 ft

;\

PRODUCTION WELL 9

Depth = 332 ft 

Casing = Not available

400

300

200

100

PRODUCTION WELL 15

Depth = 220 ft 

Casing = 68 ft

PQ

IX 

00

04

O

g 

O

>o o v~> o >o

r- oo oo o\ o\

CT\ CT\ O^ CT\ CT\

400

300

200

04 100

w

U

o °

U

w

PRODUCTION WELL 13

Depth = 340 ft 

Casing = 60 ft

CT\ CT\ <y\ O\

O\ O\

04 

O

U

400

300

200

100

PRODUCTION WELL 6

Depth = 175 ft 

Casing = 76 ft

M H -

O >0 O >O

OO OO O\ O^

CT\ CT\ CT\ ^\ CT\ CT\ 00 00CT\ CT\

Figure 14. Chloride concentrations in water from production wells 16, 15, 14, 13, 9, and 6, 1970-94.

Ground-Water Quality in Dunedin 21
The localized effect of pumping on chloride concentration in water from the Dunedin observation wells 

is indicated by the results of the statistical trend analysis (table 1). The annual changes in chloride concentration range from -69.4 to 14.5 mg/L per year. Wells 

with decreasing chloride-concentration trends occur 

near the coast (wells 51, 53, and 54); one possible reason for this trend is the cessation of irrigation pumping 

from nearby wells in the Dunedin Country Club. Wells 

with increasing chloride-concentration trends were 

originally production or irrigation wells (wells 10M, 

18M, 19M, 55/56, 57/58, and 59/60). Ground-water 

withdrawal has likely had a local effect on water quality from these wells.

The observed increasing chloride-concentration 

trend in water from Dunedin production wells is probably the result of three factors. The most important factor is the relation between the position of waterproducing zones in the well and increasing chloride 

concentration with depth in the aquifer; wells receiving 

water from deeper producing zones may allow direct 

pumping of water with elevated chloride concentrations. A second factor is the upward movement of 

water with elevated chloride concentrations caused by 

well pumpage. A third factor is lateral movement of 

water with elevated chloride concentrations. This factor is less important because lateral chloride-concentration changes occur over a distance of miles, whereas 

vertical changes occur over a distance of hundreds of ft.

BRACKISH PRODUCTION-WELL 

AQUIFER TEST

Aquifer testing is a method for quantifying hydraulic properties of aquifers. Typically, a well is pumped 

at a constant rate and the drawdown and recovery of 

water levels in one or more observation wells are measured. The drawdown and recovery data are analyzed 

using analytical equations and various type-curve 

matching techniques. A 3-day aquifer test was conducted as part of this study.

Background Data

Water levels were measured continuously at the 

following wells: brackish production, brackish monitor, production 14, ROMP TR14-2 Tampa and Ocala, 

18M, 10M, and 57. Tide stage of the Gulf of Mexico 

was measured continuously at the Dunedin Marina. 

Data were collected for about a year and spanned the 

period of 5 months prior to and 7 months after the aquifer test. The data were collected using electronic

water-level monitoring equipment including Campbell 

Basic Data Recorders (BDR 301), Design Analysis 

pressure transducers, or Handar shaft encoders with 

float/weight water-level-measurement devices.

Hydrographs were constructed to better understand 

the water-level response in a multilayered aquifer. 

Examples of the hydrograph data for the period April 

5-26, 1995, are shown in figure 15. The hydrographs 

indicate that water levels in wells are tidally affected. 

In addition, water levels in some of the observation 

wells (10M and production 14) are affected by all 

pumping rate changes within the Dunedin well field. 

Water levels in other wells (18M and 57) are affected 

by maximum pumping rates corresponding to permitted watering days. Water levels in some wells (TR14- 

2 Ocala and Tampa and brackish monitor) do not 

appear to be affected by well-field pumpage. This variability in water-level response to well-field pumpage 

indicates that the Upper Floridan aquifer is a multilayered system. The average seasonal fluctuation is about 

2 ft and ranges from 1 to 3 ft.

Differences in water levels among the wells indicate that head increases with depth, indicating that in 

the Upper Floridan aquifer the ground-water flow 

potential is upward. An apparent exception to this 

trend is the ROMP TR14-2 Ocala well. After the water 

levels were corrected for salinity, the equivalent freshwater head was 8.77 ft higher than the measured head. 

After the salinity corrections were made, it was apparent that water levels throughout the Upper Floridan 

aquifer increase with depth, and the upward head gradient likely is the dominant mechanism for chlorideconcentration changes.

Aquifer-Test Design and Implementation

The USGS conducted an aquifer test at the study 

site in August 1994. As pumping commenced at the 

brackish production well, water levels were measured 

in seven observation wells, including the brackish 

monitor, production 14, ROMP TR14-2 Tampa, 

ROMP TR14-2 Ocala, 18M, 10M, and 57. The purpose of the test was to determine the hydraulic properties of upper and lower zone A, zone B, and the 

intervening semiconfining units (SCU1 and SCU2). 

The pumped well and the observation wells penetrate 

to varying degrees, upper and lower zone A, zone B, 

and the semiconfining units. Well construction information is listed in the appendix. Water levels were measured at the pumped well and seven observation wells.

22 Assessment of the Fresh- and Brackish-Water Resources Underlying Dunedin and Areas of Northern Pinellas County, Fla.
PRODUCTION 14 WELL

10M WELL

5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26

H

PQPQ

H-f

£

PQ J

£on 

W

1

57 WELL

567 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26

3.0

PQ

> 2.5

PQ

1.5

ROMP TR14-2 OCALA WELL

5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26

BRACKISH MONITOR WELL

ROMP TR-2 TAMPA WELL

5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26

April 1995

Figure 15. Water levels from observation wells illustrating aquifer response in a multilayered aquifer 

system, April 5-26, 1995. (Note that the production well 14 is consistently higher than the 10M well. 

The 57 well is consistently higher than the 18M well.)

Brackish Production-Well Aquifer Test 23
The test began at 0910 hours on August 16,1994, with 

an average pumping rate of 670 gal/min; unfortunately, 

the pump stopped at 2330 hours on August 16, 1994. 

The test was restarted at 0200 hours on August 17, 

1994, with an average pumping rate of 690 gal/min. 

The test ended at 0910 hours on August 19, 1994.

Water levels in three of the seven observation wells 

exhibited a noticeable response to pumping the brackish production well. Because water levels in 10M and 

production 14 behave identically, only data for the 

brackish monitor and production 14 wells were analyzed. The data were corrected for tidal effects. The 

calculated tidal efficiency is 7 percent. The tidal efficiency may be calculated and drawdown data may be 

adjusted for tide-stage changes occurring during an 

aquifer test by measuring stage fluctuations prior to and 

during the test and applying the equation presented by 

Ferris and others (1962).

The tidally corrected drawdown data are shown in 

figure 16. Additional drawdown measured in production 14, 200 minutes into the test, is believed to have 

resulted from increased pumpage from the nearby production wells. Attempts were made to analytically analyze the test; the results are discussed in the following 

section of the report.

Limitations of Analytical Aquifer-Test 

Analyses

Conducting careful, controlled, and successful 

aquifer tests of the hydrogeologic units of the Upper 

Floridan aquifer is difficult because the aquifer has a 

layered and non-uniform permeability distribution. In 

addition, characterizing hydraulic properties is more 

difficult in heterogeneous, fractured rock than in homogeneous, granular rock. At small scales, on the order of 

inches (in) to ft, significant variations in hydraulic conductivity arise from the presence or absence of fractures. At larger scales, on the order of tens to hundreds 

of ft, variations in hydraulic conductivity arise from the 

presence of zones of numerous, open well-connected 

fractures versus sparse, tight, poorly connected fractures. Consequently, hydraulic properties quantitatively determined at a particular location in the aquifer 

may not be representative of properties at an adjacent 

location (Paul A. Hsieh, written commun., 1992).

Several analytical models were utilized in an 

attempt to quantify the hydraulic properties of the permeable zones and semiconfining units. Results of the 

analyses were highly variable and precluded the determination of reliable hydraulic properties of the hydrogeologic units underlying the Dunedin well field.

_ O

H

w

E

I

O 

Q 

£

<

e* Q

o In

o 

N>

0.05

0.02

0.01

 - Production 14

- - Brackish Monitor

10 100 1,000 

TIME SINCE PUMPING BEGAN, IN MINUTES

10,000

Figure 16. Aquifer-test data from the brackish monitor and production 14 observation wells.

24 Assessment of the Fresh- and Brackish-Water Resources Underlying Dunedin and Areas of Northern Pinellas County, Fla.
Possible reasons for why the analytical models failed 

include the less than optimal distribution of observation wells, pumping interference, and the heterogeneous nature of the multilayered Upper Floridan 

aquifer.

The drawdown data were provided at the request of 

the cooperator (city of Dunedin) to their consultant 

Leggette, Brashears, and Graham. Results of aquifertest analysis by Leggette, Brashears, and Graham 

(1995), using the USGS water-level data, were also 

highly variable. Calculated transmissivity values 

ranged over three orders of magnitude (6,000 to 

240,000 ft2/d) using various analytical methods. Storativity values were greater than 1 for five of the 10 

analyses. The final conclusion of the consultant's 

report (1995) is that the calculation of aquifer properties in the study area when using simple analytical 

equations is ambiguous.

An alternative approach to analytical analysis of 

aquifer-test data is numerical analysis. Hutchinson and 

Trommer (1991) showed that, in contrast to analytical 

approaches, numerical analysis was not constrained by 

multiple phases of discharge and recovery, length of 

testing, assumptions concerning storage within a semiconfining unit, or number of layers in the hydrogeologic system. The 3-day aquifer test was analyzed 

using the Heat and Solute Transport in Three-Dimensions (HST3D) numerical code to derive hydraulic 

properties of the multilayered aquifer underlying 

Dunedin and is discussed in the following sections of 

the report.

NUMERICAL SIMULATION OF THE 

FRESH- AND BRACKISH-WATER 

RESOURCES UNDERLYING DUNEDIN

A digital modeling approach was used to simulate 

density-dependent, ground-water flow and advectivedispersive transport of a conservative ground-water 

solute (chloride ion) using geologic, hydrologic, and 

chemical data collected as part of this study. Two of the 

tasks of this study were to derive hydraulic properties 

of the multilayered aquifer underlying Dunedin and to 

estimate changes in water quality in response to development of the brackish ground-water resources. 

A cylindrical ground-water flow and solute-transport 

model was developed to test and refine the conceptualization of the hydrogeologic system underlying Dunedin. The model was utilized as an interpretative tool in 

which simulated water levels and water quality could

be tested against observed water levels and water quality to derive reasonable estimates of the hydraulic 

properties of and water-quality distribution in the 

uppermost Upper Floridan aquifer. Numerical simulation was a two-phase process. The initial phase was to 

simulate the 3-day aquifer test; the second phase was to 

simulate long-term water-quality changes.

The following sections of the report include descriptions of the numerical methods, the conceptual model, 

and the selection of parametric coefficients to represent 

aquifer characteristics based on data acquired at the 

brackish well test site. Determination of the hydraulic 

characteristics was accomplished by replicating field 

data acquired during the aquifer test at the Dunedin 

water plant. Determination of the chemical characteristics was accomplished by replicating chloride-concentration changes in water from the six water plant wells 

over a 25-year period (1970-94). Estimates of chlorideconcentration changes resulting from the addition of 

brackish ground-water development were made.

Numerical Methods

The USGS computer code HST3D (Kipp, 1987), 

which solves the equations for ground-water flow and 

solute transport using a finite-difference approximation, was used. HST3D was the selected simulator 

because it can simulate variable-density ground-water 

flow. Backward-in-time and backward-in-space finitedifference equations were used to solve the groundwater flow and solute-transport equations in the simulator code. The reader is referred to Kipp (1987) for a 

discussion of the numerical methods used in the simulator code.

Conceptual Model

The hydrogeologic system underlying Dunedin is 

conceptualized as containing multiple permeable zones 

separated by leaky semiconfining units. Ground-water 

quality changes from fresh to saline with depth. Eight 

layers were delineated to represent the hydrogeologic 

framework underlying Dunedin. These eight layers 

include the portion of the aquifer containing water with 

less than 35,000 mg/L DS and correspond to the 20-ft 

thick surficial aquifer system (layer 8), the 60-ft thick 

intermediate confining unit (layer 7), the 80-ft thick 

upper zone A (layer 6), the 60-ft thick semiconfining 

unit 1 (layer 5), the 80-ft thick lower zone A (layer 4), 

the 100-ft thick semiconfining unit 2 (layer 3), the 60- 

ft thick zone B (layer 2), and the 140-ft thick semiconNumerical Simulation of the Fresh- and Brackish-Water Resources Underlying Dunedin 25
fining unit 3 (layer 1) (fig. 17). Evidence from geophysical logs and water-level and water-quality differences 

support this conceptualization and zonation of the multilayered, variable density aquifer underlying Dunedin.

Due to the complexity of the hydrogeologic system, lack of consistent pressure (water level/head) and 

concentration (chloride) data, and multiple stresses 

(tidal and pumping) on the ground-water flow field, a 

numerical model with cylindrically symmetric coordinates was constructed to emphasize the vertical features of the system. Characteristically, chloride 

concentrations will have a greater rate of change in the 

vertical direction than in the lateral direction. It is often 

advantageous to use simplified models to conceptualize the hydrologic system and to assess uncertainty 

associated with aquifer properties. The model represents a 600-ft thick section underlying the Dunedin 

water plant, because this site is the test site for the first 

brackish-water well and relatively more data exist for 

this region.

The conceptual model incorporates several major 

assumptions about the hydrogeologic framework and, 

therefore, only approximates the actual aquifer. One of 

the major assumptions is that the aquifer behaves as an 

equivalent porous medium; however, field data indicate that the aquifer may be highly heterogeneous and 

anisotropic due to fracturing and dissolution of the carbonate rocks. Compounding this problem is the 

unavailability of data for some of the hydrogeologic 

units and inconsistency of data within hydrogeologic 

units. Therefore, imprecise estimations of parameter 

values are likely during the process of matching simulated to observed data.

Spatial Discretization

The test site was simulated using a cylindricalcoordinate grid of 97 columns and 30 rows. The spatial 

discretization consists of 97 variably spaced vertical 

columns and 30 evenly spaced horizontal rows (fig. 

17). The origin of the grid is the bottom left corner. 

Radial spacing of the columns expands logarithmically 

from 0.14 ft (near the well) to a maximum of 50 ft. 

This discretization produces a fine grid near the 

pumped well where fluid pressure and concentration 

gradients are expected to be large. Vertical spacing of 

the rows was 20 ft. This discretization was selected to 

minimize the number of rows while approximately 

positioning nodal locations to adequately represent the 

eight hydrogeologic units of the multilayered aquifer. 

The simulated area is 3,000 ft in radius and 600 ft in

depth. Greater radial extents of 6,000 and 10,000 ft 

also were simulated. The greater model extents did not 

affect the pressures and concentrations at the lateral 

boundary, because of the type of boundary condition 

selected; therefore, the smaller model region was 

selected for further numerical analysis. Boundary conditions will be discussed in a later section of the report.

Temporal Discretization

Two different temporal discretizations were used. 

One was used for the aquifer-test simulations and the 

other for the water-quality change simulations.

The 72-hour aquifer test and 48-hour recovery 

period were divided into four separate stress periods. 

Stress period 1 corresponds to the initial 14.33 hours of 

brackish-water withdrawal prior to the unexpected cessation of pumping. Ten equal time steps of 1.43 hours 

were simulated. Stress period 2 corresponds to the 2.5 

hours when the pump was off. Two equal time steps of 

1.25 hours were simulated. Stress period 3 corresponds to the remaining 55 hours of brackish-water 

withdrawal during the aquifer test. Twenty equal time 

steps of 2.75 hours were simulated. Stress period 4 corresponds to the 48 hours of recovery following the 72- 

hour brackish-water withdrawal. Twelve 4-hour time 

steps were simulated. These time-step scenarios were 

selected to generate temporal output to match field data 

collected during the aquifer test.

Temporal discretization for simulating water-quality changes was achieved by using the automatic time 

stepping capability of the HST3D numerical code. The 

minimum time-step length was set at 0.1 day to accommodate temporal changes in the boundary conditions 

and pumping changes. The maximum time-step length 

was set at 14 days to generate temporal output to match 

field data. The actual time increment used for each 

time step is automatically selected by the model, within 

the specified range, to reach the target pressure and 

concentration limits specified by the user.

Boundary Conditions Used to Simulate the 

Aquifer Test

The extent of the model area, delineated by the 

boundaries, represents a region that extends beyond the 

radial distance where water levels in observation wells 

were affected by pumping the brackish production well 

during the 3-day aquifer test. Four types of boundary 

conditions were implemented.

The top and bottom of the model were designated 

as specified pressure boundaries corresponding to the 

top of the surficial aquifer system and the base of semiconfining unit 3. For the purposes of simulating the

26 Assessment of the Fresh- and Brackish-Water Resources Underlying Dunedin and Areas of Northern Pinellas County, Fla.
Specified Pressure Boundary

SURFICIAL AQUIFER SYSTEM

INTERMEDIATE CONFINING UNIT

600

0 1,000 2,000

Specified Pressure Boundary

RADIAL DISTANCE FROM MODEL ORIGIN, IN FEET

I SIMULATED PRODUCTION AND OBSERVATION OPEN HOLE INTERVALS

3,000

Figure 17. Model grid, simulated open-hole interval, and boundary conditions.

aquifer test, it was assumed that no background vertical 

leakage occurred, therefore hydrostatic pressure conditions were deemed appropriate. A specified hydrostatic pressure of 0.0 pound per square inch (lb/in2) was 

assigned to the upper boundary. This boundary type for 

the top of the model was deemed reasonable because 

the water table in the surficial aquifer system (1) did not 

change during the aquifer test; and (2) has not changed, 

on average, during the period 1987-95 in the vicinity of 

Dunedin. A specified hydrostatic pressure of 259.34 

lb/in2, equivalent to the pressure exerted from a 600-ft 

column of freshwater in the overlying formations, was 

assigned to the lower boundary. The pressure was calculated using the density of freshwater, because waterquality changes did not occur during the aquifer test.

This boundary type for the base of the model was 

deemed reasonable, because the semiconfining unit 3 

has a hydraulic conductivity that is three orders of magnitude less than the overlying permeable zone.

The inner cylindrical boundary has two types of 

boundary conditions. The brackish production well 

was simulated at the radial origin corresponding to 

rows 17 and 18 and represents the producing interval of 

lower zone A. The rows on the inner cylindrical 

boundary that represent the cased interval of the well 

were assigned a no-flow condition.

The outer cylindrical boundary is defined by a transient flow, aquifer-influence function that utilizes the 

Carter-Tracy approximation to compute flow rates 

between the inner, discretized aquifer region and an 

infinite homogeneous outer region where aquifer propNumerical Simulation of the Fresh- and Brackish-Water Resources Underlying Dunedin 27
erties are only generally known. The primary benefit 

of using the aquifer-influence function boundary condition is the reduction in discretized area while simulating an infinite aquifer. This option reduces the 

computer storage and computation time needed for 

simulation (Kipp, 1987, p. 59).

Boundary Conditions Used to Simulate WaterQuality Changes

The model area, delineated by the boundaries, 

encompasses a region that extends beyond the radial 

distance affected by production well withdrawals at the 

Dunedin water plant. Historical water-level and chloride-concentration data indicate that well-field pumpage results in localized drawdowns (potentiometricsurface depressions) and circular isochlors extending 

small radial distances from production well clusters. 

Therefore, the extent of the model area was chosen to 

be the same as the aquifer-test simulation.

Four types of boundary conditions were implemented. The simulated vertical extent of the model area 

was the same as for the aquifer-test simulation. A specified-pressure and specified-solute concentration 

boundary was designated at the top of the model, 

because average water levels and water quality in the 

surficial aquifer system do not appear to have changed 

in the vicinity of Dunedin. For the purposes of simulating water-quality changes due to production well 

pumping, the dynamic effects of leakage and multidensity flow preclude the use of hydrostatic pressures; 

therefore, density-dependent pressures were calculated 

as input for the lower specified-pressure boundary. 

The upper boundary was assigned a constant pressure 

of 0.0 lb/in2 and a constant-solute concentration value, 

represented as a mass fraction, was set at 0.0 corresponding to a 25-mg/L chloride concentration, which is 

the background chloride concentration. The bottom 

boundary was assigned a constant pressure and solute 

concentration corresponding to the base of semiconfining unit 3. The pressure on the bottom boundary was 

set at 243.51 lb/in2 . This value was calculated using the 

water-level and chloride-concentration data at the base 

of semiconfining unit 3 and is different from 259.34 

lb/in2 (used to simulate the aquifer test) due to the consideration of the density differences in the aquifer. 

A constant-solute concentration value, represented as a 

mass fraction, was set at 0.63 corresponding to a chloride concentration of 12,000 mg/L. A mass fraction 

value of 1.0 corresponds to a chloride concentration of 

19,000 mg/L. The base of the model coincides with the

approximate location of saline water below which the 

aquifer has 100 percent saltwater.

The inner cylindrical boundary has two types of 

boundary conditions. Various pumping scenarios were 

simulated by changing the input well definition representing the different open-hole intervals. The rows on 

the inner cylindrical boundary that represent the cased 

interval of the well were assigned a no-flow condition.

The outer cylindrical boundary is characterized as 

a transient flow, aquifer-influence function that utilizes 

the Carter-Tracy approximation to compute flow rates 

between the inner, discretized aquifer region and an 

infinite homogeneous outer region where aquifer properties are only generally known. The assigned pressure 

distribution along the outer cylindrical boundary was 

calculated using water-level and chloride-concentration data collected as part of this study. The primary 

benefit of using the aquifer-influence function boundary condition is the reduction in simulated area size. 

This option reduces the computer storage and computation time needed for simulation.

Input Parameters

The validity of selected input parameters used in 

the simulations and presented in this report were evaluated using sensitivity analysis. The purpose of the 

sensitivity analysis was (1) to independently assess the 

effect of varying initial hydraulic parameter values and 

(2) to provide a match between simulated and observed 

water-level and chloride-concentration data.

The initial simulations incorporated the best estimates of hydraulic properties derived from aquifer-test 

analyses, laboratory core analyses, and geophysical log 

interpretations. Data were compiled from previous 

studies in Pinellas County (Hickey, 1982; Knochenmus 

and Thompson, 1991) and collected as part of this 

study. The sensitivity analysis involved systematically 

adjusting the initial input parameters and assessing 

their effects on model results. These effects were evaluated by comparing simulated and observed waterlevel changes during the brackish production well aquifer test. Input parameters used to simulate the aquifer 

test were those input parameters that resulted in a reasonable water-level match. Selected model input 

parameters are listed in table 2.

The initial chloride-concentration distribution used 

in the water-quality-change simulations were estimated 

from (1) water-quality data collected as part of this 

study and (2) historical chloride-concentration data 

compiled by Dunedin personnel as part of their wateruse permit requirements. Sensitivity analysis was used

28 Assessment of the Fresh- and Brackish-Water Resources Underlying Dunedin and Areas of Northern Pinellas County, Fla.
to generate an input data set by systematically adjusting initial parameter values. The sensitivity analysis 

focused on matching simulated and observed waterlevel and chloride-concentration data for the period 

1970-94. Input parameter values used to simulate this 

25-year period are listed in table 2.

Wells

The pumped well simulated in the aquifer test 

model is the brackish production well. The well is 

located at the radial origin of the model. The open-hole 

interval of the well is defined in terms of row numbers. 

The simulated open-hole length was 40 ft corresponding to rows 17 and 18 and is equivalent to a depth interval from 240 to 280 ft below land surface. This interval 

corresponds to the approximate location of the major

producing zone in lower zone A. Variable discharge 

rates were used to simulate the aquifer test. The discharge rates for the four stress periods were 670, 0, 

690, and 0 gal/min, respectively. These correspond to 

changes in withdrawal rates during the aquifer test. 

The model distributes the total simulated discharge 

among the cells based on cell location, hydraulic conductivity, and well-completion factor. The well-completion factor defines the amount of flow to the well 

from individual cells in response to pressure differences between the simulated well bore and aquifer. 

A well-completion factor of 1 was assigned to the two 

pumped cells.

The simulated open-hole interval of the pumped 

well for the 25-year chloride concentration history 

match approximates the location of the water-producing 

zone in upper zone A in the vicinity of the water plant.

Table 2. Model-input values and model-derived hydraulic parameters

[Constant parameters: water temperature, 75 degrees Fahrenheit; viscosity, 0.8904 centipoise; molecular diffusivity, 9.3 

x 10'5 ; porosity, 0.3; matrix compressibility, 6.2 x 10'6 cubic inches per pound; longitudinal/transverse dispersivity, 

12.5/2.5 feet; --, no data; ft2 , square foot; ft/d, feet per day; d'1 , per day; Ib/ft3 , pounds per cubic foot; mg/L, milligrams per 

liter; Ib/in2 , pounds per square inch; shaded columns or * represent input values used for brackish-water simulations.]

Intrinsic |

 y !^SeC^°!!C permeability (ft2)/ Storage Leakance I unit and top/bottom , , ,. «- ,, K 

, , . hydraulic coefficient (d' 1 ) boundaries , . r /f , conductivity (ft/d)

Top specified -- \ 

pressure boundary

Surficial aquifer 3.9xlO-]1 /10 3.0x10-' -- |

Intermediate 3.9 x 10' 16/ 0.0001 1.9 x 10'4 1.7 x 1Q-6 I 

confining unit

Upper zone A 1.6 x lO'09 / 400 2.0 x 10'4 -- \

Semiconfining 7.8 x 1Q- 13 /0.2 1.3 xlO'4 3.3 x 10~3 j 

unit 1 |

Lower zone A 3.9 x lO' 10 / 100 2.0 x 10'4 - |

Semiconfining 7.8 x 1Q- 13 /0.1 3.1 x 10"4 2.0 x 10'3 

unit 2 *(0.2)

ZoneB 3.9 x lO' 10 / 100 1.6 xlO'4 -- j

Semiconfining 3.9 x 10-13 /0.1 3.9 x 1Q-4 7.1 x 10'3 

unit 3

Bottom specified 

pressure boundary

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Numerical Simulation of the Fresh- and Brackish-Water Resources Underlying Dunedin 29
The well is located at the radial origin of the model. The 

open-hole interval of the well is defined in terms of row 

numbers. The simulated open-hole length was 40 ft corresponding to rows 24 and 25 and is equivalent to a 

depth interval from 120 to 160 ft below land surface. 

This interval corresponds to the approximate location of 

the major producing zone in upper zone A. Discharge 

rates were varied yearly (25 times) during the simulation 

period (1970-94) corresponding to the average annual 

rate from the six water plant production wells. The 

pumpage data were provided by the city of Dunedin and 

the SWFWMD. The averaged discharge rates ranged 

from 0.088 to 0.314 Mgal/d. A well-completion factor 

of 1 was assigned for the two pumped cells.

In all scenarios, the pumped well is located at the 

radial origin of the model. Open-hole intervals for the 

well tapping upper and lower zone A are the same as 

described above. For the simulations used to estimate 

water-quality changes in response to brackish-water 

development, the following well definitions were utilized:

1. Case 1, long-term (30 years) brackish-water withdrawals from lower zone A to approximate aquifer response to brackish-water production located 

away from freshwater production-well clusters;

2. Case 2, long-term, simultaneous withdrawal from 

both the freshwater (upper zone A) and brackishwater (lower zone A) producing zones at equal 

rates; and

3. Case 3, long-term, simultaneous withdrawal from 

both the freshwater and brackish-water producing zones at unequal rates.

Case 1:

A steady discharge rate of 300 gal/min was used to 

simulate 30 years of brackish-water withdrawal from 

lower zone A, because it is the expected average permitted withdrawal rate. A well-completion factor of 1 

was assigned to the two cells representing the openhole interval in lower zone A.

Case 2:

Two discharge rates of 300 and 600 gal/min 

(150 and 300 gal/min from each producing zone) were 

independently tested. Equal discharge from upper and 

lower zone A was simulated to assess the effects of 

combined fresh- and brackish-water withdrawals. 

The well bore used to simulate combined withdrawals 

from upper and lower zone A was approximated by 

designating two open-hole intervals in a single well.

The allocation of flow to the well from upper and lower 

zone A was accomplished by specifying a well-completion factor for each node representing the open-hole 

intervals. This procedure was necessary because only 

one pumping well can be simulated in a cylindrical 

coordinate system. To achieve equal discharge rates 

from upper and lower zone A, well completion factors 

of 0.25 and 1.0 were assigned to the cells representing 

upper and lower zone A, respectively. Assignment of 

these well completion factors to upper and lower zone 

A was an adjustment for the differences in permeability 

values, because upper zone A is four times more permeable than lower zone A.

Case 3:

Steady discharge rates of 300 and 600 gal/min 

(at unequal volumes from the two producing zones) 

were selected for simulating water-quality changes in 

response to different pumping scenarios. Well-completion factors were assigned a value of 1 or less in the 

cells representing the open-hole interval in upper zone 

A to adjust the allocation of flow for the various pumping scenarios.

Observation Wells:

Observation wells also were simulated. The locations of observation wells approximate the locations of 

the production 14 and brackish monitor observation 

wells. The observation wells were located 61 (column 

32) and 70 ft (column 33), respectively, from the brackish production well. These simulated observation wells 

have 20-ft completion intervals corresponding to row 25 

and 9, respectively. For the water-quality change simulations, observation wells penetrated upper zone A, 

lower zone A, and zone B and were located 5 ft from the 

pumped well. The simulated open-hole intervals are 

rows 25, 18, and 9, respectively.

MODEL ANALYSIS OF THE AQUIFER TEST

Numerical simulation of the aquifer test data provided an alternative method for calculating hydraulic 

properties of the permeable zones and semiconfining 

units by matching simulated and observed water-level 

changes in the pumped well (lower zone A) and observation wells above (upper zone A) and below (zone B). 

However, emphasis was not placed on obtaining an 

exact water-level match but rather on gaining an understanding of the dynamics of the flow system. Hydro30 Assessment of the Fresh- and Brackish-Water Resources Underlying Dunedin and Areas of Northern Pinellas County, Fla.
static pressure conditions were assumed. Under 

hydrostatic conditions, initial water levels in the aquifer are assumed to be at equilibrium and, therefore, 

simulated pressure changes are only in response to 

brackish production well withdrawals. This approach 

is used for "drawdown" simulations. In this type of 

simulation, changes in head are of interest, rather than 

absolute values of head. The principle of superposition 

is used to justify calculation of drawdown from an arbitrary horizontal datum that represents the initial head 

distribution. This initial head distribution represents 

dynamic average steady-state conditions (Anderson 

and Woessner, 1992, p. 199-200). The advantage of 

superposition is that the effects of the aquifer test can 

be evaluated without accounting for other unknown 

stresses acting on the ground-water system (Reilly and 

others, 1987). Model results are discussed in the following section of the report.

Results

Model results were evaluated by comparing simulated and observed water levels and by a flow field 

analysis using plots of vector fields. Observed and 

simulated water-level changes in the brackish monitor 

(zone B) and production 14 (upper zone A) wells are 

depicted in figure 18. To get these results, only slight 

changes were made to the initial input data. Modelsimulated changes in zone B parallel those observed in 

the brackish monitor well during all four phases of the 

aquifer test. Model-simulated changes in upper zone A 

match best during the recovery phase of the test. The 

observed water levels are apparently influenced by 

changes in (1) tide stage at the coast and (2) pumping 

rates in the well field. Response to tide-stage changes 

are indicated by the cyclic rise and fall of water levels 

in both the production 14 and the brackish monitor

si

>

W

wC/3

w

8.0 

7.5 

7.0 

6.5 

6.0 

5.5 

5.0 

4.5 

4.0 

3.5 

3.0 

2.5 

2.0

Brackish Monitor Well 

Zone B

Production Well 14 

Upper Zone A

EXPLANATION

OBSERVED WATER LEVEL 

SIMULATED ZONE B WATER LEVEL 

SIMULATED UPPER ZONE A WATER LEVEL

16 17 18 19 20

AUGUST 1994

21 22

Figure 18. Measured and model-simulated water levels in the brackish monitor and 

production 14 wells.

Model Analysis of the Aquifer Test 31
wells (fig. 18). The effects of pumping-rate changes in 

the nearby production wells are indicated by: (1) the 

increased drawdown (falling water levels) in production well 14 at 200 minutes into the test (fig. 16) corresponding to a 200 gal/min increase in discharge from 

the water plant production wells; (2) the rapid rise in 

water levels in production 14 and brackish monitor 

wells about 14 hours into the test when the pump in the 

brackish production well unexpectedly stopped; and 

(3) the rapid rise in water levels at the end of the 3-day 

aquifer test. As indicated in figures 16 and 18, the 

water level in production well 14 responds to pumping 

changes in both upper and lower zone A.

Simulated water-level changes in the brackish production well compare favorably with observed 

changes. Observed water levels indicate rapid 

response and stabilization. Twenty ft of drawdown 

occurred within the first minute of the test. This drawdown was about 80 percent of the total drawdown (25 

ft) during the test. Simulated water levels in the 

pumped well also exhibited a rapid response to pumping and a simulated total drawdown of about 24 ft. 

Model derived hydraulic conductivities, storage coefficients, and leakance of the hydrogeologic units are 

listed in table 2.

The flow field generated by the model at the end of 

the simulated 3-day pumping period is depicted in figure 19. The figure supports a favorable numerical solution of the flow field because vectors indicate relatively 

uniform flow and point in the direction of the known 

vertical head gradients in the study area. Flow was 

nearly lateral in the permeable zones and nearly vertical in the semiconfining units. Near the simulated 

pumped well in lower zone A, fluid movement is 

downward from upper zone A and upward from zone B 

as a result of lowering the head 24 ft in lower zone A. 

Inflow occurs along the upper, lower, and outer boundaries. The velocity-vector field in the surficial aquifer 

system and intermediate confining unit is not shown 

because the velocities are low. Simulations indicated 

that the selection of hydraulic conductivity assigned to 

the hydrogeologic layers above and below the pumped 

zone affect water-level responses in the pumped zone 

and overlying and underlying permeable zones. These 

changes in water-level responses are discussed in the 

following section of the report.

Sensitivity Analysis

Sensitivity analysis was the tool used to gain an 

understanding of the role of each parameter in generating model results in response to changes in individual 

input parameters. Changes in model results provide 

insight as to the degree that changes in parameters may 

affect the flow field. Input parameters including porosity, bulk matrix compressibility, and intrinsic permeability were individually varied over a reasonable 

range of values to determine the sensitivity of the 

model to these parameters.

The input values for porosity and bulk matrix compressibility had little effect on the model results. Sensitivity analysis of the bulk matrix compressibility 

indicates that an order of magnitude reduction or 

increase in value only affected simulated water levels 

immediately following changes in the simulated withdrawal rates. A reduction in bulk matrix compressibility caused a more sluggish water-level response. An 

increase in bulk matrix compressibility caused a masking of the water-level response to pumping-rate 

changes.

The model was most sensitive to intrinsic permeability. Hydraulic conductivity (K) and intrinsic permeability (k) values are listed in table 2. Hydraulic 

conductivity (K) can be defined in terms of intrinsic 

permeability (k) using a relation presented in Freeze 

and Cherry (1979, p. 29). The relation is

K =

where, MH is dynamic viscosity of the fluid in units M/LT,

p is fluid density in units M/L3 ,

g is gravitational acceleration in units L/T2,

k is intrinsic permeability in units L2, and

K is hydraulic conductivity in units L/T.

The following discussion will use the term hydraulic conductivity as a surrogate for intrinsic permeability 

because hydraulic conductivity is the commonly used 

parameter of interest. The effects of hydraulic conductivity on simulated water levels are illustrated in figure 

20. Water levels in both upper zone A (production 14) 

and zone B (brackish monitor) were least affected by 

changes in hydraulic conductivity in the intermediate 

confining unit and in semiconfining unit 3. The greatest divergence of simulated from measured water levels in upper zone A occurred from changes in hydraulic 

conductivity in upper zone A and semiconfining unit 1.

32 Assessment of the Fresh- and Brackish-Water Resources Underlying Dunedin and Areas of Northern Pinellas County, Fla.
Velocity-Vector Field at the End of the 3-Day Aquifer Test

LLJ LLJ

LJJ

cc

=>

CO 

Q

I 

LJJ 

CD

UJ 

D

UJ 

UJ

UJ

cc

ID 

CO 

D

I

UJ 

CD

UJ 

Q

100

200

300

400

500

600

100

200

300

400

500

600

LI

LI

LI

Velocity-Vector Field at the End of the 25-Year History Match

AAAAAAAAAAAAAAAAAAAAA

W A A A A A A A N N N N f\ N N f\l\ N [\ [\ N Nf\ f\ Nf\ M^

1,000 2,000 3,000

RADIAL DISTANCE FROM MODEL ORIGIN, IN FEET

Figure 19. Velocity-vector field in a radial section at the end of the 3-day aquifer test and at the end 

of the 25-year history match. (A schematic of the model grid and hydrogeologic layers corresponding 

to the gridding on this figure is shown in figure 17.)

Model Analysis of the Aquifer Test 33
LEVEL ABOVE WATERSEA LEVEL, IN FEET

5

7 

6 

5 

4 

3 

2 

8 

7 

6 

5 

4 

3 

2 

8 

7 

6 

5 

4 

3 

2 

8 

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6 

5 

4 

3

T

Brackish Monitor Well ^ r*~^ ;

Production Well 14 

INTERMEDIATE CONFINING UNIT

16 17 18 19 20 21 22

Brackish Monitor Well

Production Well 14 

SEMICONFINING UNIT 1

16 17 18 19 20 21 22

Brackish Monitor Well

" gteWJO'iA/^Afvu^AJvvYwvy$^*^^ *<M>^®, ^,

- ^^^^rrrrmrn^m^

-

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Production Well 14 

SEMICONFINING UNIT 2

16 17 18 19 20 21 22

Brackish Monitor Well

^bfid^MMMtitoM 0^!<ife3**»W/XA/

Production Well 14 

1- SEMICONFINING UNIT 3

5

7 

6 

5 

4 

3 

2 

8 

7 

6 

5 

4 

3 

2 

8 

7 

6 

5 

4 

3 

2

o

A

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Brackish Monitor Well

/M^JJT./\ y^^/^V

^ff^^SHs^H^

Production Well 14 

UPPER ZONE A

16 17 18 19 20 21 22

Brackish Monitor Well

" ^^I^^^S3^^^^at'^>9M>^ ~~

'~^^sssaa^^^^ '

Production Well 14 

LOWER ZONE A

16 17 18 19 20 21 22

Brackish Monitor Well

- ifeftlljI^gg-Y^-rrp-rTTT^ ~°~

~ A A. majfflffiftKjSftrfpflimiili*0'^^

^^M^ni * <^5r"T -"- 

Production Well 14 

ZONEB

16 17 18 19 20 21 22 

AUGUST 1994

EXPLANATION

SIMULATED WATER LEVEL FOR TWICE 

THE SELECTED INTRINSIC PERMEABILITY

SIMULATED WATER LEVEL FOR THE 

SELECTED INTRINSIC PERMEABILITY

SIMULATED WATER LEVEL FOR HALF THE 

SELECTED INTRINSIC PERMEABILITY

jre 20. Sensitivity of simulated water levels tc

16 17 18 19 20 21

AUGUST 1994

22 changes in intrinsic permeabilities of selected 

hydrogeologic units.

34 Assessment of the Fresh- and Brackish-Water Resources Underlying Dunedin and Areas of Northern Pinellas County, Fla.
The greatest divergence of simulated from measured 

water levels in zone B occurred from changes in 

hydraulic conductivity in zone B and semiconnning 

unit 2. Divergence of simulated from measured water 

levels in both upper zone A and zone B occurred from 

changes in hydraulic conductivity in lower zone A. 

This implies that small changes in hydraulic conductivity, with the exception of the intermediate confining 

unit and semiconfining unit 3, result in a significant 

divergence of simulated from measured water levels. 

Model results indicate that the hydrogeologic system 

underlying the city of Dunedin water plant was reasonably simulated because only a narrow range of hydraulic conductivity values mimic the observed hydraulic 

response in the multilayered aquifer.

MODEL ANALYSIS OF HISTORICAL 

CHLORIDE-CONCENTRATION CHANGE

The purpose for simulating changes in chloride 

concentration in ground water during the 25-year simulation period (1970-94) was to assess how well model 

results compared to historically observed chloride-concentration changes in water from the Dunedin water 

plant production wells. If a reasonable match can be 

accomplished, then the model can be used to analyze 

the response of the aquifer to past and present groundwater withdrawals and to estimate the responses of the 

aquifer to future ground-water development. The 

model-derived hydraulic parameters obtained from the 

aquifer-test simulation were used as the initial distribution of the hydraulic characteristics of the permeable 

and semiconfining units comprising the uppermost 

Upper Floridan aquifer underlying Dunedin. However, 

one modification to the input parameters was necessary. An intrinsic permeability value of 7.8x10" 13 ft2 

(hydraulic conductivity value of 0.2 ft/d) was utilized 

as input for semiconfining unit 2 after chloride concentration trends could not be adequately simulated using 

a value of 3.9xlO~13 ft2 . Several attempts were made to 

resolve this modification including rerunning the aquifer-test simulation; however, when using the larger 

value the observed drawdown in the zone B could not 

be duplicated. This may be due to differences in model 

results for a 3-day versus 25-year simulation period.

Approximately 25 years of record (1970-94) for 

pumpage, water levels, and chloride concentrations in 

upper zone A were available for comparison. However, historical chloride-concentration data for deeper 

zones in the aquifer had to be estimated, because no

data exist for the hydrogeologic layers below upper 

zone A. The initial chloride-concentration values for 

the deeper zones, computed as a mass fraction distribution, were set equal to the estimated chloride-concentration distribution in native waters, assumed to have 

existed in 1970.

The annual pumping rate used in the simulation 

was the average rate of the six water plant wells (numbers 6, 9, 13, 14, 15, and 16). The rationale for using 

an average value from the six wells is that observed 

water-level and water-quality data from individual 

wells are probably influenced by the combined effects 

of several adjacent pumped wells in production well 

clusters. This phenomena was indicated by evaluating 

simulated chloride changes using only production well 

14. During the period 1970-94, this well was out of 

service for several years. During the out-of-service 

periods, simulated chloride-concentration values stabilized while observed concentrations continued to rise; 

thus, the rationale for using an average pumping rate 

from the six water plant wells.

Results

Model results were evaluated by comparing simulated to observed water-level and water-quality 

changes in upper zone A. In addition, a flow field analysis using plots of velocity vectors was assessed. For 

ease of model evaluation and comparison, the simulated pressures and mass fraction values were converted to water levels and chloride concentrations, 

respectively. Because water levels remained fairly 

constant throughout the period 1970-94, discussion of 

the model results focuses on comparisons between 

observed and simulated chloride concentrations. The 

simulated chloride-concentration change was compared to the observed chloride-concentration change in 

each of the six production wells (fig. 21).

In general, simulated and observed water levels 

and chloride concentrations compared favorably; however, comparisons were slightly different among the 

water plant wells. The most favorable comparisons 

were for production wells 13 and 14. These two wells 

are located nearest the centroid of pumping for the 

water plant production-well cluster.

Comparisons for production wells 15 and 16 were 

poorest during the period 1970-79. Simulated chloride 

concentrations tended to be substantially lower (less 

than 60 mg/L) than observed chloride concentrations 

during this period. These two wells are located north

Model Analysis of Historical Chloride Concentration Change 35
400

300

200

100

ftj o

I I I I I I I I I I I T I I I I I \ I I I I I I

PRODUCTION 16

i i i i i i i i i i i i i i i i i i i i i i i i

1970 1980 1990

§ 400

Z

o

300

u

g 200

U

100

o _Jffi

PRODUCTION 14

i i i i i i i i i i i i i i i i i i i i i i i i

400

300 -

1970 1980 1990

400

300

200

100

PRODUCTION 13

i i i i i i i i i i i i i i i i i i i i i i i

1970 1980 1990 1970 1980 1990

p 400

o 

Q

Z

oo

§

300 -

ill i i i i i i i i

1970 1990

400

300 -

1970 1990

EXPLANATION 

OBSERVED CHLORIDE CONCENTRATION FROM WATER PLANT PRODUCTION WELLS

SIMULATED CHLORIDE CONCENTRATION USING AVERAGE WITHDRAWALS FROM 

THE SIX WATER PLANT PRODUCTION WELLS

Figure 21. Comparison of measured and simulated chloride concentrations from the water-plant wells, 1970-94.

36 Assessment of the Fresh- and Brackish-Water Resources Underlying Dunedin and Areas of Northern Pinellas County, Fla.
of the brackish-water test site. It should be noted that 

the earliest observed chloride concentrations are elevated relative to other production wells in the well 

field. These elevated chloride concentrations are probably the result of measurement error, because the values are an order-of-magnitude higher during this period 

than elsewhere in the well field.

Comparisons for production wells 6 and 9 were 

poorest during the period 1983-94. Simulated chloride 

concentrations tended to be substantially higher 

(greater than 50 mg/L) than observed chloride concentrations during this period. These two wells are located 

south of the brackish-water test site and farthest from 

the centroid of pumping. The poor duplication 

between simulated and observed chloride-concentration changes in production wells 6 and 9 is because 

these wells are less affected by well interference due to 

their distance from the centroid of pumping.

Simulated chloride-concentration data from all of 

the water plant production wells show a break-in-slope 

in the late 1980's corresponding to a break-in-slope in 

the observed chloride-concentration data. In general, 

the chloride-concentration trends are similar and a 

comparison between simulated and observed concentrations is reasonably good. This favorable comparison 

lends some confidence in using the transport model to 

assess the association between ground-water withdrawals from and chloride changes in upper zone A.

Flow was nearly lateral in the upper zone A and 

zone B permeable zones (fig. 19). Fluid movement is 

downward from the surficial aquifer system and 

upward from all units below upper zone A. Inflow 

occurs along the upper and lower boundaries. Outflow 

occurs along the outer boundary in upper zone A. 

A flow divide occurs in upper zone A at a radial distance of about 1,600 ft. This may be the result of the 

circulation or circular convection caused by pumping a 

multilayered, multidensity aquifer. Circular convection has been shown to be a major process influencing 

the distribution of solute in space and time (Hickey, 

1989, p. 1494). The simulated flow divide appears to 

be consistent with the observed localized potentiometric-surface depressions that exist in the Dunedin well 

field.

Sensitivity Analysis

Sensitivity analysis was the tool used to determine 

the sensitivity of the model-predicted chloride changes 

to model input and to assess the uncertainty of estimated aquifer hydraulics and transport properties. The 

analysis provides a means to identify the most important aquifer characteristics. Input parameters that were 

tested include intrinsic permeability (hydraulic conductivity), dispersivity, and mass fraction (chloride 

concentration) distribution. These parameters were 

individually varied over a reasonable range of values to 

determine the sensitivity of the model results to these 

parameters. The model results were not sensitive to 

changes in parameter values for dispersivity.

Model results indicated that the selection of 

hydraulic conductivity was the most important factor 

and produced the greatest changes in the simulated 

chloride concentrations; therefore, the discussion of the 

sensitivity analysis emphasized this parameter. The 

effects of intrinsic permeability (hydraulic conductivity) on simulated chloride concentrations are shown in 

figure 22. Divergence of simulated from measured 

chloride concentrations in upper zone A occurred from 

changes in hydraulic conductivity in the three semiconfining units. Greatest divergence occurred from 

changes in hydraulic conductivities of semiconfining 

units 1 and 2. Small divergence occurred from changes 

in hydraulic conductivity in zone B and semiconfining 

unit 3. No measurable divergence occurred from 

changes in hydraulic conductivity in upper and lower 

zone A. These findings imply that small changes in 

simulated hydraulic conductivity for the semiconfining 

units result in a significant divergence of simulated 

from observed chloride concentrations. The importance of the semiconfining units as a mechanism for 

retarding the vertical movement of higher salinity 

ground water is significant. The model results show 

that the derived hydraulic parameters and water-quality 

distribution are reasonable estimates for the hydrogeologic system underlying Dunedin.

MODEL ANALYSIS OF LONG-TERM 

BRACKISH-WATER WITHDRAWALS

Numerical simulation also was used to estimate 

future trends in chloride concentrations in the uppermost Upper Floridan aquifer resulting from brackishwater development. To learn more about the effects of 

future pumping on chloride-concentration and waterlevel changes, the model was run for 30 years using 

three different pumping scenarios. A 30-year time 

period was selected for economic purposes (typical 

amortization period) and, more importantly, because 

historical chloride trends in water from upper zone A

Model Analysis of Long-Term Brackish-Water Withdrawals 37
400

300

200

100

00

o

UPPER ZONE A

1970 1980 1990

400

300 -

1970 1980 1990

O

H

400

300

200

g 100 §U

LOWER ZONE A

Qh H 

C*

O

_]

K 

U

Q

oo

r\ 

1970 1980 1990

400

1970 1980 1990 

YEAR

400

300

200

100

SEMICONFINING UNIT 2

1970 1980 1990

400

300

200

100

0

SEMICONFINING UNIT 3

1970 1980 1990 

YEAR

EXPLANATION

O SIMULATED CHLORIDE CONCENTRATION IN UPPER ZONE A

+ CHLORIDE CONCENTRATION IN UPPER ZONE A FROM HALVING INTRINSIC PERMEABILITY

^ CHLORIDE CONCENTRATION IN UPPER ZONE A FROM DOUBLING INTRINSIC PERMEABILITY

Figure 22. Sensitivity of simulated chloride concentration to changes in intrinsic 

permeabilities of selected hydrogeologic units.

38 Assessment of the Fresh- and Brackish-Water Resources Underlying Dunedin and Areas of Northern Pinellas County, Fla.
indicated that a break-in-slope did not occur until after 

18 years of pumping. Field data and model analysis of 

historical water-level and chloride-concentration 

changes (discussed in previous sections) indicate that 

the pressure field rapidly responds to ground-water 

withdrawals (on the order of minutes); whereas, the 

concentration field may take years to equilibrate.

The effects of brackish-water development on 

water-level and chloride-concentration changes in 

upper and lower zone A and zone B were evaluated 

using three different pumping scenarios, including 

brackish-water withdrawals away from fresh-water 

production well clusters and simultaneous pumping 

from upper and lower zone A. These scenarios are designated as cases. In case 1, a single well in lower zone 

A was simulated to assess the effects of pumping only 

brackish water at a rate of 300 gal/min. In case 2, a single well open to upper and lower zone A was simulated 

to assess the effects of pumping equal volumes of 

freshwater and brackish water at total rates of 300 and 

600 gal/min. In case 3, a single well open to upper and 

lower zone A was simulated to assess the effects of 

pumping unequal volumes of freshwater and brackish 

water from upper and lower zone A at total rates of 300 

and 600 gal/min and to assess if an optimal pumping 

scenario exists that may limit these changes. The initial pressure and mass-fraction distribution used in 

these simulations is the resultant distribution after the 

25-year simulation and approximates the conditions for 

1995, the year that the first brackish production well 

was completed.

Results

Model results are discussed in terms of possible 

changes that could occur in the Upper Floridan aquifer 

from the addition of brackish-water withdrawals. For 

ease of model evaluation and comparison, the simulated pressures and mass-fraction values were converted to water levels and chloride concentrations, 

respectively.

Case 1-Single well open to lower zone A

Simulation results from the 300 gal/min pumping 

rate from lower zone A during the 30-year simulation 

period indicated that brackish-water development does 

not influence the water quality in upper zone A. Chloride concentrations stabilize in upper zone A when 

lower zone A is pumped; however, chloride concentrations rapidly rise in lower zone A and may exceed

4,000 mg/L after 30 years of brackish-water withdrawals (fig. 23). Pumping from lower zone A results in a 

lower head in lower zone A than in either upper zone A 

or zone B. The head difference is greater between zone 

B and lower zone A (7 ft) than between upper zone A 

and lower zone A (4 ft). A slight break-in-slope in simulated chloride concentrations occurs after about 18 

years. This break-in-slope may indicate that the hydrologic system is reaching an equilibrium between 

recharge of freshwater from upper zone A and recharge 

of higher salinity water from zone B. The chloride concentration also increased in lower zone B, probably as 

the result of the upward flow from the underlying semiconfining unit.

Case 2-Single well open to upper and lower zone 

A discharging equal volumes from each zone

The second pumping scenario was the simulation 

of equally distributed withdrawals from upper and 

lower zone A during a 30-year period (fig. 23). Two 

withdrawal rates, totaling 300 and 600 gal/min were 

simulated. Only slight differences in chloride-concentration trends for the three permeable zones occurred as 

the result of raising the total pumping rate from 300 to 

600 gal/min. Simulated chloride concentrations in 

upper zone A initially decreased. After 10 years, however, chloride concentrations rose more rapidly. Chloride changes in upper zone A are probably the result of 

the continuation of current freshwater withdrawals 

from upper zone A rather than from pumping lower 

zone A. The chloride concentrations rapidly rose in 

lower zone A and exceeded 4,000 mg/L after 30 years 

of simultaneous fresh- and brackish-water withdrawal. 

The simulated chloride concentrations in lower zone A 

rose more rapidly in case 2 than in case 1. This more 

rapid rise is probably the result of reduced recharge 

from upper zone A. The simulated chloride-concentration trend in zone B is the same as in case 1.

Case 3- Single well open to upper and lower zone 

A discharging unequal volumes from each zone

The third pumping scenario was the simulation of 

unequally distributed withdrawals from upper and 

lower zone A during a 30-year period (fig. 23). Two 

withdrawal rates, totaling 300 and 600 gal/min were 

simulated. More water was withdrawn from lower 

zone A (56 and 70 percent) than from upper zone A. 

Again, doubling the total pumping rate from 300 to 600 

gal/min resulted in only slight changes in the chlorideconcentration trends in the three permeable zones.

Model Analysis of Long-Term Brackish-Water Withdrawals 39
UJ 

H

IVI

MILLIGRAMS PE] iIN 

>£H

0

^lt

NCENTR^

CO: [DE 

\TED CHLOR]

D

HH

00

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800

600

400

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3,000

2,000 

1,000

0 

1<

12,000 

10,000 

8,000

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1«

_ CASE 1 Only Pumping Lower Zone A

Response in UZA

-

-

-

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rPmiiii 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 n 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

70 1980

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1990 2000 2010 2020

i i i i i i i i i i i i i i i i i i i i i

+++++

++++

i

-

irt*^

L CASE I Only Pumping Lower Zone A J

; Response in LZA

' i i i i i i i i i i i i i i i i i i i i i i i i i i i i i

)95 2000

i i i i i i i i

'-*#**

i "

2005 2010 2015 2020 202

^ ^^-

-

CASE 1 Only Pumping Lower Zone A

_ Response in ZB

i i i i i i i i i i i i i i i i i i i i i i i i i i i i i

?95 2000

i

2005 2010 2015 2020 202

YEAR

UZA: Upper Zone A

LZA : Lower Zone A

ZB : Zone B

CASE 2~Pumping Equal Volumes

from Upper and Lower Zone

Response in UZA

1970 1980 1990 2000 2010 2020 

5,000 r-r

4,000

3,000

2,000

1,000

CASE 2 Pumping Equal Volumes

from Upper and Lower Zone A 

Response in LZA

i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i 0

1995 2000 2005 2010 2015 2020 2025

12,000

10,000

8,000

6,000

4,000

2,000

0

1995 2000 2005 2010 2015 2020 2025

YEAR

EXPLANATION

O 25-YEAR HISTORY MATCH

+ TOTAL WITHDRAWAL 300 GPM

A TOTAL WITHDRAWAL 600 GPM

CASE 2~Pumping Equal Volumes

from Upper and Lower Zone A 

Response in ZB

Figure 23. Model simulated chloride-concentration changes using various pumping scenarios 

from upper and lower zone A.

40 Assessment of the Fresh- and Brackish-Water Resources Underlying Dunedin and Areas of Northern Pinellas County, Fla.
UJ 

H

0-

00

O

O

H

U

gU

g

U 

Q

1,000

800

600

400

200

5,000

4,000

3,000

2,000

1,000

CASE 3 Pumping Unequal Volumes^ 

from Upper (44%) and Lower 

(56%) Zone A 

- Response in UZA

1,000

800

600

400

200

CASE 3 Pumping Unequal Volumes 

from Upper (30%) and Lower 

(70%) Zone A 

- Response in UZA

1980 1990 2000 2010 2020

CASE 3 Pumping Unequal Volumes 

from Upper (44%) and Lower 

(56%) Zone A

Response in LZA

0

1995

1970 1980 1990 2000 2010 2020

0

CASE 3- Pumping Unequal Volumes 

from Upper (30%) and Lower 

(70%) Zone A

Response in LZA

12,000

10,000

8,000

6,000

4,000

2,000

0

CASE 3 Pumping Unequal Volumes 

from Upper (44%) and Lower 

(56%) Zone A

Response in ZB

2000 2005 2010 2015 2020 2025 1995 2000 2005 2010 2015 2020 2025

12,000

10,000

8,000

6,000

4,000

2,000

CASE 3- Pumping Unequal Volumes 

from Upper (30%) and Lower 

(70%) Zone A

Response in ZB

1995 2000 2005 2010 2015 2020 2025 

YEAR

0

1995 2000 2005 2010 2015 2020 2025

YEAR

EXPLANATION

UZA: Upper Zone A Q 25-YEAR HISTORY MATCH 

LZA : Lower Zone A + TOTAL WITHDRAWAL 300 GPM 

ZB : Zone B ^ TOTAL WITHDRAWAL 600 GPM

Figure 23.--Continued Model simulated chloride-concentration changes using various pumping 

scenarios from upper and lower zone A.

Model Analysis of Long-Term Brackish-Water Withdrawals 41
Pumping a larger percentage of the total pumpage from 

lower zone A resulted in flattening of the chloride trend 

in upper zone A. Increasing the pumpage to 70 percent 

from lower zone A resulted in a decrease in the final 

chloride concentration in upper zone A by about 25 

percent. Simulated chloride-concentration trends in 

lower zone A do not appear to be affected by increasing 

withdrawal from the zone. The simulated chlorideconcentration trend in zone B is the same as in the previous two cases.

Model results for case 1 indicate that pumping 

from lower zone A does not appear to negatively influence the chloride-concentration trends in upper zone A. 

This evaluation is reasonable because the head gradient 

between upper zone A and lower zone A is reversed 

when lower zone A is pumped. Water levels in lower 

zone A declined as much as 25 ft at a pumping rate of 

600 gal/min. Therefore, development of the brackishwater resources should not adversely influence the 

freshwater zone (upper zone A) of the Upper Floridan 

aquifer. Water-quality changes in lower zone A may 

restrict development of large quantities of brackish 

water.

SUMMARY AND CONCLUSIONS

This report presents results of a study conducted to 

assess the fresh- and brackish-water resources in the 

uppermost Upper Floridan aquifer underlying Dunedin, Florida. The aquifer contains a lens of freshwater 

that is maintained by recharge from rainfall. This lens 

is relatively thin and is underlain by brackish and saline 

ground water. Historically, Dunedin has obtained 

enough ground water to provide potable water for its 

inhabitants. Although ample quantities of water are 

available, the water quality has changed. The effect on 

ground-water quality from concentrated withdrawals 

persists. Desalination of brackish ground water is one 

method used to enhance the ground-water resources. 

Evaluation of the fresh- and brackish-water resources 

required the delineation of producing zones and the 

water-quality distribution in the hydrogeologic units 

underlying Dunedin. A conceptual model of the hydrogeologic system was developed and tested using 

numerical simulation.

Geophysical, lithologic, water-level, and waterquality data that were collected as part of this study 

were used to assess the distribution of flow zones 

within the uppermost Upper Floridan aquifer underlying Dunedin. Through correlation of data, the hydrogeologic framework underlying the study area was 

conceptualized as a multilayered sequence of permeable zones and confining and semiconfining units. The 

presence of vertically spaced, discrete water-producing 

zones within these permeable zones was indicated. 

Generally, these producing zones occurred in the upper 

part of zone A (between 0 and 60 ft below sea level), 

the lower part of zone A (between 160 and 200 ft below 

sea level), and in zone B (between 350 and 400 ft below 

sea level). Based on the differences in water levels and 

water quality between producing zones in upper and 

lower zone A, the unit was divided into two permeable 

zones separated by a semiconfining unit. Upper zone 

A is the primary production zone in the Dunedin well 

field; however, many of the production wells penetrate 

both upper and lower zone A.

Evaluation of water-level data and potentiometricsurface maps indicated that, in northern Pinellas County, 

the Upper Floridan aquifer is best characterized as a 

local flow system. Pinellas 665 and Garden Street Triangle water-level data were used to evaluate water-level 

declines in response to municipal pumping in Dunedin 

and Clearwater and irrigation pumping in citrus groves. 

Water levels declined during the period 1957-75, stabilized during the period 1975-81, and have increased 

slightly since 1981. Pumping from the Dunedin well 

field increased from 1.2 to 4.9 Mgal/d during the period 

1957-94, indicating that pumpage in the well field is 

probably not influencing water levels in the aquifer outside of Dunedin. Changes from the historical potentiometric surface in the study area do not appear to be 

substantial. Potentiometric-surf ace features such as the 

ground-water mound (divide) east of Dunedin and 

Clearwater and small radial depressions surrounding 

well clusters in these two cities have been common.

As part of this study eight water-level-collection 

sites were established in Dunedin. Water-level 

responses were highly variable among the observation 

wells, indicating a multilayered aquifer. These wells 

penetrate the different permeable zones in the aquifer.

The chloride concentrations in ground water 

underlying the study area are indicative of transitional 

type water and vary both vertically and laterally. 

Below lower zone A, water quality rapidly changes and 

saline water may occur at depths of 400 ft below sea 

level. In addition, zones of higher salinity water occur 

at shallow, discrete depths within zone A. Although 

delineation of the sources of chloride in the aquifer are 

difficult to quantify, it is highly unlikely that the only 

mechanism of transport is by lateral intrusion of mod42 Assessment of the Fresh- and Brackish-Water Resources Underlying Dunedin and Areas of Northern Pinellas County, Fla.
ern seawater. A relation between chloride concentration and distance from St. Joseph Sound is not 

apparent. Upper zone A is the freshwater zone; lower 

zone A is the brackish-water zone; and zone B is the 

saline-water zone.

The water quality of samples from production 

wells has changed during the period 1970-94. Greatest 

changes have occurred since the late 1980's. Elevated 

chloride concentrations are probably the result of direct 

pumping of water with elevated chloride concentrations, the upward movement of water with elevated 

chloride concentrations induced by well pumpage 

(upconing), or from lateral movement of water from 

the coast. Direct pumping and upconing of higher 

salinity waters likely are the important factors influencing water-quality changes, because lateral chlorideconcentration changes occur over a distance of miles, 

whereas vertical changes occur of a distance of hundreds of ft.

A numerical-modeling approach was used to simulate density-dependent, ground-water flow and transport. The model was designed to be an interpretative 

tool used to gain an understanding of the current and 

future fresh- and brackish-water resources underlying 

Dunedin.

Numerical simulation of the aquifer-test data provided a method to estimate hydraulic properties of the 

permeable zones and semiconfming units by comparing simulated and observed water-level changes in the 

multilayered aquifer. Numerical methods have advantages over analytical solutions because numerical 

methods are not constrained by multiphase discharge, 

storage within confining units, or number of layers in 

the hydrogeologic system. The derived hydraulic conductivities of upper zone A, lower zone A, and zone B 

were 400, 100, and 100 ft/d, respectively. The hydraulic conductivities in the semiconfining units ranged 

from 0.1 to 0.2 ft/d and the overlying intermediate confining unit was 0.0001 ft/d.

The purpose for simulating changes in chloride 

concentration in water during the 25-year simulation 

period (1970-94) was to assess how well model results 

compared to historically observed chloride-concentration changes in water from the Dunedin water plant 

production wells. Approximately 25 years of pumpage, water-level, and chloride-concentration data in 

upper zone A were available for comparison. Comparisons were made between the simulated and observed 

chloride-concentration changes in the six water plant 

wells. In general, simulated and observed chloride 

concentrations compared favorably, especially for production wells 13 and 14. These wells are located closest to the centroid of the water plant production-well 

cluster. The simulated chloride-concentration trend 

changed significantly in the late 1980's, corresponding 

to a similar trend in measured chloride concentrations.

Numerical simulation also was used to estimate 

future trends in chloride concentration in the uppermost Upper Floridan aquifer resulting from brackishwater development. Three different pumping scenarios 

were simulated including a single brackish-water well 

away from current production wells, combined withdrawals from both upper and lower zone A in equal 

volumes, and combined withdrawals from both upper 

and lower zone A in unequal volumes. The model 

results indicate that pumping from lower zone A (the 

brackish-water zone) does not appear to negatively 

influence the chloride-concentration trends in upper 

zone A (case 1). However, chloride-concentration 

changes in upper zone A, indicated in cases 2 and 3 

(combined fresh-water/brackish-water withdrawals), 

will likely occur due to the continuation of current 

fresh-water withdrawals from upper zone A. Simulated 30-year chloride-concentration changes in lower 

zone A do not appear to be affected by changes in 

pumping rates or volumes. However, chloride-concentration changes in lower zone A will likely occur from 

pumping brackish water from lower zone A.

SELECTED REFERENCES

Anderson, M.P., and Woessner, W.W., 1992, Applied

groundwater modeling: Simulation of flow and advective transport: San Diego, Calif., Academic Press, 381 p.

Black and Associates and Bridley, Wild and Associates, 

Inc., 1952, Development of a water supply for Pinellas 

County, Florida: Clearwater, Fla., Consultant's report 

in the files of Pinellas County Water Department.

Black, Crow, and Eidness, Inc., 1978, Drilling and testing of 

the monitoring and injection wells at the Southwest 

Wastewater Treatment Plant for the city of St. Petersburg, Florida: Brooksville, Fla., Consultant's report in 

the files of the Southwest Florida Water Management 

District.

Causseaux, K.W., and Fretwell, J.D., 1983, Chloride concentrations in the coastal margin of the Floridan aquifer, southwest Florida: U.S. Geological Survey WaterResources Investigations Report 82-4070, 33 p.

Cherry, R.N., and Brown, D.P., 1974, Hydrogeologic

aspects of a proposed sanitary landfill near Old Tampa 

Bay, Florida: Tallahassee, Florida Bureau of Geology 

Report of Investigations no. 68, 25 p.

Selected References 43
Ferns, J.G., Knowles, D.B., Brown, R.H., and Stallman, 

R.W., 1962, Theory of aquifer tests: U.S. Geological 

Survey Water-Supply Paper 1536-E, 174 p.

Freeze, R.A., and Cherry, J.A., 1979, Groundwater: Englewood, N.J., Prentice-Hall, 604 p.

Heath, R.C., and Smith, P.C., 1954, Ground water resources 

of Pinellas County, Florida: Tallahassee, Florida Geological Survey Report of Investigations no. 12, 139 p.

Hickey, J.J., 1982, Hydrogeology and results of injection 

tests at waste-injection test sites in Pinellas County, 

Florida: U.S. Geological Survey Water-Supply Paper 

2183, 42 p.

 1989, Circular convection during subsurface injection 

of liquid waste, St. Petersburg, Florida: Water 

Resources Research, v. 25, no. 7, p. 1481-1494.

Hutchinson, C.B, 1983, Assessment of the interconnection 

between Tampa Bay and the Floridan aquifer, Florida: 

U.S. Geological Survey Water-Resources Investigations Report 82-54, 55 p.

Hutchinson, C.B., and Stewart, J.W, 1978, Geohydrologic 

evaluation of a landfill in a coastal area, St. Petersburg, 

Florida: U.S. Geological Survey Water-Resources 

Investigations Report 77-78, 40 p.

Hutchinson, C.B., and Trommer, J.T., 1991, Model analysis 

of hydraulic properties of a leaky aquifer system, Sarasota County, Florida in Subitzky, Seymor, ed., Selected 

Papers in the Hydrologic Sciences: U.S. Geological 

Survey Water-Supply Paper 2340, p. 49-62.

Kipp, K.L., Jr., 1987, HST3D: A computer code for simulation of heat and solute transport in three-dimensional 

ground-water flow systems: U.S. Geological Survey 

Water-Resources Investigations Report 86-4095, 519 p.

Knochenmus, L.A., and Thompson, T.H., 1991, Hydrogeology and simulated development of the brackish 

ground-water resources in Pinellas County, Florida: 

U.S. Geological Survey Water-Resources Investigations Report 91-4026, 20 p.

Leggette, Brashears, and Graham, Inc., 1994, City of Dunedin aquifer monitoring and testing program: Dunedin, 

Fla., Consultant's report in the files of the city of Dunedin, 105 p.

 1995, Evaluation of Brackish Production well number 

1 pumping test: Dunedin, Fla., Consultant's report in 

the files of the city of Dunedin, 8 p.

M.P. Brown and Associates, Inc., 1986, Hydrogeological 

investigation for the development and management of 

the Floridan aquifer, City of Dunedin, Pinellas County, 

Florida: Dunedin, Fla., Consultant's report in the files 

of the city of Dunedin.

Reilly and others, 1987, The principle of superposition and 

its application in ground-water hydraulics: Techniques 

of Water-Resources Investigations of the U.S. Geological Survey, v. 3, chap. B6, 28 p.

Ryder, P.O., 1982, Digital model of predevelopment flow in 

the tertiary limestone (Floridan) aquifer system in westcentral Florida: U.S. Geological Survey WaterResources Investigations Report 81-54, 61 p.

 1985, Hydrology of the Floridan aquifer system in 

west-central Florida: U.S. Geological Survey Professional Paper 1403-F, 78 p.

SDI, Inc., 1993, Groundwater in the Vicinity of the City of 

Clearwater: Clearwater, Fla., Consultant's report in the 

files of the city of Clearwater, 75 p.

Seaburn and Robertson, Inc., 1983, Results of the hydrologic testing program for the city of Clearwater water 

supply investigation: Clearwater, Fla., Consultant's 

report in the files of the city of Clearwater.

Sinclair, W.C., 1974, Hydrogeologic characteristics of the 

surficial aquifer in northwest Hillsborough County, 

Florida: Tallahassee Florida Bureau of Geology Information Circular no. 86, 98 p.

44 Assessment of the Fresh- and Brackish-Water Resources Underlying Dunedin and Areas of Northern Pinellas County, Fla.
Appendix

Appendix 45
Appendix. Construction data for selected wells

Well name or number

1

2

3

4

5

6

9

10M

11

12

13

14

15

16

18M

19M

21M

28

29

30

31

32

33

34

Latitude and 

longitude

280045/824722

280046/824709

280049/824704

280012/824715

280032/824625

280112/824537

280114/824543

280139/824548

280229/824513

280229/824534

280123/824546

280123/824541

280125/824540

280125/824531

280123/824459

280124/824449

280132824500

280028/824545

280011/824621

280010/824610

280019/824606

280031/824532

280215/824510

280126/824455

Elevation at top of 

casing (feet above 

sea level)

13.57

18.38

27.33

17.40

37.63

62.16

57.30

53.61

42.12

55.49

58.80

58.34

58.99

58.05

48.31

65.48

53.69

57.91

38.64

54.14

42.49

48.53

37.65

68.02

Total depth (feet )

146

153

149

160

255

175

332

88

250

105

340

260

220

175

243

300

290

300

300

302

239

201

105

Unknown

Casing depth 

(feet)

Unknown

56.5

Unknown

55.5

64.7

76.5

Unknown

55.5

48.6

64.5

60.2

59.2

68.2

64.9

53.0

Unknown

Unknown

63.5

60.0

53.0

51.0

48.0

42.0

Unknown

46 Assessment of the Fresh- and Brackish-Water Resources Underlying Dunedin and Areas of Northern Pinellas County, Fla.
Appendix. Construction data for selected wells (Continued)

Well name or number

50

51

52

53

54

55

57

59

Brackish Monitor

Brackish Production

ROMPTR14-1

(Tampa/Suwannee)

ROMP TR14-2 

(Tampa/Ocala)

ROMP TR14-3 

(Tampa/Suwannee)

East Bay Country Club

St. Catherine

Cove Cay

Clearwater-Dunedin 31

Pinellas Deep Well 665

Mission Hills

Garden Street Triangle

Regency Oaks

USGS Deep 27

Clearwater-Dunedin 27

Clearwater 67

Latitude and 

longitude

280102/824632

280249/824643

280215/824639

280114/824707

280015/824711

280217/824544

280050/824534

280020/824607

280124/824543

280123/824543

280002/824126

280132/824528

280118/824345

275430/824607

275521/824443

275604/824317

275753/824353

275815/824404

275842/824303

275843/824742

275949/824424

280254/824416

275753/824337

280022/824229

Elevation at top of 

casing (feet above 

sea level)

34.10

9.22

12.95

22.4

24.19

55

51.14

34.10

59.36

60.16

20.18/20.19

55.22/55.37

97.61/97.61

10.78

19.80

9.42

56.49

36.61

58.27

33.44

92.50

68.53

51.06

69.69

Total depth (feet )

205

30

80

85

75

115

189

262

465

400

170/284

218/460

176/319

245

200

160

283

299

385

208

305

405

560

297

Casing depth 

(feet)

66.0

22.0

45.0

40

38

Unknown

Unknown

51

400

220

70/264

213/440

125/299

Unknown

150

80

Unknown

81

47

54

102

Unknown

523

92

Appendix 47