Evaluation of Ground-Water Flow and 

Hydrologic Budget for Lake Five-O, 

A Seepage Lake in Northwestern Florida

By J.W. Grubbs

U.S. GEOLOGICAL SURVEY

Water-Resources Investigations Report 94-4145

Prepared in cooperation with the

U. S. ENVIRONMENTAL PROTECTION AGENCY and the 

FLORIDA DEPARTMENT OF ENVIRONMENTAL PROTECTION

Tallahassee, Florida

1995
U.S. DEPARTMENT OF THE INTERIOR

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY

Gordon P. Eaton, Director

For additional information Copies of this report can be

write to: purchased from:

District Chief U.S. Geological Survey

U.S. Geological Survey, WRD Earth Science Information Center

Suite 3015 Open-File Reports Section

227 North Bronough Street Box 25286, MS 517

Tallahassee, FL 32301 Denver Federal Center

Denver, CO 80225

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.
Contents III

CONTENTS

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

Introduction ...................................................................................................................................................................... 1

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

Previous Studies .......................................................................................................................................................... 2

Description of Study Area .......................................................................................................................................... 3

Acknowledgments ...................................................................................................................................................... 3

Ground-Water Flow System near Lake Five-O ................................................................................................................ 3

Local Hydrogeology ................................................................................................................................................... 7

Ground-Water Movement ........................................................................................................................................... 10

Head Distribution and Ground-Water Flow near Lake Five-O ......................................................................... 10

Boundary Conditions ........................................................................................................................................ 12

Lateral Boundary ..................................................................................................................................... 13

Upper Boundary ...................................................................................................................................... 14

Lower Boundary ...................................................................................................................................... 17

Net Ground-Water Flow to Lake 5-O ............................................................................................................... 18

Methods ................................................................................................................................................... 18

Results ..................................................................................................................................................... 19

Simulation of Ground-Water Flow ................................................................................................................................... 22

Model Development .................................................................................................................................................... 22

Calibration of Steady-State Simulations ..................................................................................................................... 25

Calibration of Transient Model ................................................................................................................................... 29

Flow-Path Simulations ............................................................................................................................................... 33

Hydrologic Budget for Lake Five-O ................................................................................................................................. 35

Summary .......................................................................................................................................................................... 39

References ........................................................................................................................................................................ 40

FIGURES

1. Map showing location of Lake Five-O study area..................................................................................................... 2

2. Topographic map of the Lake Five-O study area ...................................................................................................... 4

3. Map showing location of data collection sites in the Lake Five-O study area .......................................................... 5

4. Hydrogeologic section A-A′ through Lake Five-O showing midpoints of screened intervals of observation 

wells used to construct the section............................................................................................................................ 8

5. Schematic of altitude of seismic reflector surface beneath Lake Five-O .................................................................. 9

6. Graph showing water levels at Lake Five-O and selected ground-water monitoring wells, July 1988 to February 

 1991 ........................................................................................................................................................................... 11

7-9. Maps showing:

7. Water-table configuration near Lake Five-O, October 4, 1989 ........................................................................ 12

8. Water-table configuration near Lake Five-O, November 5, 1990 .................................................................... 13

9. Hydrogeologic section A-A′ through Lake Five-O showing vertical head distribution near the lake 

for April 13, 1989.............................................................................................................................................. 14

10. Hydrogeologic section A-A′ through Lake Five-O showing vertical head distribution near the lake 

for January 17, 1990 .................................................................................................................................................. 15
IV Contents

11-13. Graphs showing:

11. Lake Five-O stage and head in Upper Floridan aquifer, December 1988 to January 1991 ............................. 16

12. Computed monthly net ground-water flow to Lake Five-O during 1989-90 ................................................... 20

13. Cumulative daily net precipitation inputs and and cumulative daily lake volume changes for Lake Five-O 

1989- 90 ........................................................................................................................................................... 21

14. Map showing areal discretization and boundary conditions for simulation models of the ground-water flow 

system near Lake Five-O.......................................................................................................................................... 23

15. Map showing vertical discretization, boundary conditions, and calibrated values of hydraulic conductivity

for simulation models of the ground-water flow system near Lake Five-O ............................................................. 24

16. Map showing simulated and observed water levels in the surficial aquifer near Lake Five-O, October 4, 1989 .... 27

17. Graph showing sensitivity of steady-state models of ground-water flow system near Lake Five-O to changes in

horizontal hydraulic conductivity and leakance of layer 6 ...................................................................................... 28

18. Graph showing sensitivity of steady-state models of ground-water flow system near Lake Five-O to changes in 

recharge and anisotropy............................................................................................................................................ 28

19. Graph showing sensitivity of transient model of the ground-water flow system near Lake Five-O to changes 

in aquifer storage properties..................................................................................................................................... 31

20. Graph showing simulated and observed heads for Lake Five-O and adjacent ground water system, 

December 12, 1988 through January 22, 1991......................................................................................................... 32

21. Graph showing simulated and computed monthly net ground-water flow to Lake Five-O, 1989-90 ...................... 33

22. Map showing contributing area to Lake Five-O, as defined by ground-water flow paths of particles that 

discharge to the lake .................................................................................................................................................. 34

23. Map showing particle traces projected on to hydrogeologic section B-B’ near Lake Five-O.................................. 35

24. Graph showing monthly hydrologic budget components for Lake Five-O, 1989-90............................................... 36

TABLES

1. Description of monitoring wells in the Lake Five-O study area..................................................................................... 6

2. Monthly net ground-water flow to Lake Five-O, 1989-90 ............................................................................................. 19

3. Temporal discretization and upper and lower boundary stresses used in the transient model of ground-water 

flow near Lake Five-O................................................................................................................................................... 30
Contents V

Conversion Factors, Vertical Datum, Acronyms, and Additional Abbreviations

Multiply By To obtain

centimeter (cm) 0.3937 inch

meter (m) 3.281 foot

kilometer (km) 0.6214 mile

hectare 2.471 acre

cubic meter (m3) 35.31 cubic foot

centimeter per year (cm/yr) 0.3937 inch per year

meter per day (m/d) 3.281 foot per day

meter per year (m/yr) 3.281 foot per year

square meter per day (m2/d) 10.76 square foot per day

cubic meter per day (m3/d) 35.31 cubic foot per day

newton per cubic meter (N/m3) 0.006366 pound per cubic foot

pascal (Pa) 0.02089 pound per square foot

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

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

1929) - a geodetic datum derived from a general adjustment of the first-order level nets of the United States 

and Canada, formerly called Sea Level Datum of 1929.

Acronyms

ANC acid neutralizing capacity

NOAA National Oceanographic and Atmospheric Administration

Additional abbreviations

microequivalents per liter

°C °F

°F 1.8 = ( ) °C + 32

μeq L⁄
VI
Introduction 1

Evaluation of Ground-Water Flow and Hydrologic

Budget for Lake Five-O, A Seepage Lake in 

Northwestern Florida

By J.W. Grubbs

Abstract

Temporal and spatial distributions of 

ground-water inflow to, and leakage from Lake 

Five-O, a softwater, seepage lake in northwestern 

Florida, were evaluated using hydrologic data and 

simulation models of the shallow ground-water 

system adjacent to the lake. The simulation 

models indicate that ground-water inflow to the 

lake and leakage from the lake to the groundwater system are the dominant components in the 

total inflow (precipitation plus ground-water 

inflow) and total outflow (evaporation plus leakage) budgets of Lake Five-O. Simulated groundwater inflow and leakage were approximately 

4 and 5 times larger than precipitation inputs and 

evaporative losses, respectively, during calendar 

years 1989-90. Exchanges of water between Lake 

Five-O and the ground-water system were consistently larger than atmospheric-lake exchanges. A 

consistent pattern of shallow ground-water inflow 

and deep leakage was also evident throughout the 

study period. The mean time of travel for groundwater that discharges at Lake Five-O (time from 

recharge at the water table to discharge at the 

lake) was estimated to be within a range of 3 to 

6 years. Flow-path evaluations indicated that the 

intermediate confining unit probably has a negligible influence on the geochemistry of groundwater inflow to Lake Five-O. The hydrologic 

budgets and flow-path evaluations provide critical 

information for developing geochemical budgets 

for Lake Five-O, and for improving the understanding of the relative importance of various 

processes that regulate the acid-neutralizing 

capacity of softwater seepage lakes in Florida.

INTRODUCTION

The acidification of lakes through the 

atmospheric deposition of mineral acids has been 

found to adversely affect a number of aquatic organisms and habitats, particularly in the northeastern 

United States and Canada. Florida has a larger 

number and higher percentage of acidic lakes than any 

other major region in the United States (Pollman and 

Canfield, 1991). Although acidic depositional impacts 

are less evident in Florida lakes (Brenner and others, 

1990, p. 376), the low cation-exchange capacities of 

soils in soft-water lake regions suggests that these 

lakes could be vulnerable to further, perhaps harmful, 

levels of acidification. At present, a quantitative 

understanding of the relative importance of the various 

processes that regulate the acid-neutralizing capacity 

(ANC) in Florida lakes is lacking. Improving the 

understanding of ANC regulation is hampered by the 

uncertainty associated with the hydrology of seepage 

lakes, which are the largest class of lakes in Florida 

(Pollman and Canfield, 1991). Seepage lakes, by 

definition, have no surface inlets or outlets, and their 

hydrology is often poorly understood because the 

exchange of water between these lakes and their 

adjacent ground-water systems is difficult to quantify. Developing a quantitative understanding of seepage lake ANC regulation requires a better 

understanding of the magnitude and timing of contributions of water from, and losses to, adjacent groundwater systems, as well as knowledge of the history 

(residence time and contact with various lithologies) 

and chemistry of ground water that discharges to seepage lakes.

In 1988, two parallel, 4-year studies were begun 

to describe the hydrologic and chemical dynamics of 
2 Evaluation of Ground-Water Flow and Hydrologic Budget for Lake Five-O, A Seepage Lake in Northwestern Florida

two soft-water, seepage lakes: Lake Five-O in Bay 

County in northwestern Florida (fig. 1), and Lake 

Barco in north-central Florida (Pollman and others, 

1991). The objective of these studies was to quantify 

the relative importance of-ground water contributions 

of ANC and in-lake transformation processes in the 

regulation of ANC in acidic, seepage lakes (Pollman 

and others, 1991). To satisfy this objective, hydrologic and geochemical budgets were developed for 

both lakes through direct and indirect measurement

of hydrologic, meteorologic, and water-quality 

variables, and through the use of hydrologic and 

geochemical models.

The studies at Lake Five-O and Lake Barco 

were conducted by KBN Engineering and Applied 

Sciences, Tetra Tech, and the U.S. Geological Survey. 

The studies were supported with funding or assistance 

from the U.S. Environmental Protection Agency, the 

U.S. Geological Survey, the Florida Electric Power 

Coordinating Group, the Electric Power Research 

Institute, Southern Company Services, and the Florida 

Department of Environmental Protection. The role 

of the U.S. Geological Survey in the study of Lake 

Five-O was to develop a hydrologic budget for the 

lake and collect samples of lake water, ground water, 

and precipitation for chemical analysis (Andrews and 

others, 1990). The hydrologic budget for Lake Five-O 

was developed using direct measurements of precipitation, energy-budget estimates of lake evaporation 

(Sacks and others, 1994), and calibrated models of the 

ground-water flow system adjacent to the lake.

Purpose and Scope

This report describes (1) the hydrology of the 

ground-water flow system near Lake Five-O, (2) the 

development and calibration of simulation models of 

the ground-water flow system near Lake Five-O, 

and (3) the hydrologic budget of Lake Five-O, as 

determined from model simulation results and data 

collected during 1988-91. The report is based on 

hydrologic and lithologic data that were collected 

from July 1988 to February 1991 at Lake Five-O and 

the immediate vicinity. These data are presented in the 

context of a conceptual model of the ground-water 

flow system, which formed the basis for subsequent 

mathematical models of the flow system. The report 

presents the results of simulations using several 

steady-state models and a transient model and presents 

a monthly hydrologic budget for Lake Five-O for 

calendar years 1989 and 1990. Simulated groundwater flow paths and residence times are also 

presented in the report.

Previous Studies

Several articles and reports which describe 

various aspects of the studies at Lake Barco and Lake 

Five-O have been published. Andrews and others 

(1990) described the study area of Lake Five-O and presented a preliminary evaluation of the hydrogeologic 

setting and water quality of the lake. 

G U L F

O F

M E X I C O

0 10 MILES

0 10 KILOMETERS

30°00′

30°30′

15′

86°00′ 85°15′ 45′ 30′

BAY

COUNTY

STUDY

AREA

WASHINGTON WALTON COUNTY COUNTY

JACKSON

COUNTY CALHOUN COUNTY GULF COUNTY

Panama City

Greenhead

Fountain

20

77

LOCATION OF

BAY COUNTY

Figure 1. Location of Lake Five-O study area.
Ground-Water Flow System Near Lake Five-O 3

Pollman and others (1991) presented preliminary water 

budgets and evaluations of processes controlling alkalinity regulation in lakes Barco and Five-O. Sacks and 

others (1992) described the hydrogeologic setting and 

presented a preliminary data analysis for the hydrologic 

budget at Lake Barco. Sacks and others (1994) presented final lake evaporation estimates 

for lakes Barco and Five-O, and evaluated the various 

factors that influence evaporation and alternative 

techniques for estimating lake evaporation at both lakes.

Description of Study Area

Lake Five-O is in the Crystal Lake Karst area 

of the Dougherty Karst physiographic district (Brooks, 

1981). This area is characterized by coastal terrace 

deposits that have been modified by extensive karst 

development (Brooks, 1981). The Lake Five-O watershed is part of a relict marine terrace of Pleistocene 

age, lying between 21 and 52 m above sea level 

(Schmidt and Clark, 1980). Soils in the region are 

deep, excessively drained, and consist of very permeable, Lakeland series sands (Duffee and others, 1984). 

Vegetation in the area includes various species of pine 

and scrub oak trees, which are common to excessively 

drained sand ridges and terraces in Florida. Residential development in the area is sparse, and pine tree 

cultivation is the dominant land use (Andrews and 

others, 1990). The trees in the watershed were 

harvested to the periphery of the lake basin 

(approximately 50 m from the shoreline) in the spring 

of 1988, before the study began, and the watershed 

was planted with sand pine in the fall of 1988 

(Pollman and others, 1991).

The physiography of the Lake Five-O study area 

is typical of the Crystal Lake Karst region. Lake FiveO and surrounding lakes lie in steep-sided depressions 

(slopes of 8-12 percent) and are surrounded by a 

mostly flat plateau area with land surface altitudes 

ranging from 25 to 30 m above sea level (fig. 2). Lake 

Five-O is relatively deep, with the altitude of the relatively flat bottom ranging from 2.5 m above sea level 

to -0.15 m below sea level (Andrews and others, 

1990). The maximum lake depth ranged from 13.5 to 

15.4 m during the study period, with a mean daily 

value of 14.5 m. The surface area of Lake Five-O 

ranged from 10.4 to 11.3 hectares, with a mean daily 

value of 10.9 hectares. Lake volume ranged from 

9.09 x 105 m3 to 1.11 x 106

 m3, with a mean daily 

value of 1.02 x 106

 m3.

The climate of Bay County is humid subtropical, with an annual average temperature of 21 °C, 

and monthly average temperatures ranging from 12 °C 

in January to 28 °C in July (Schmidt and Clark, 1980). 

Mean-annual precipitation is approximately 160 cm, 

with the wettest months of summer and early fall (June 

through September) typically accounting for nearly 

half of the annual precipitation. Distinct dry periods 

typically occur in the spring months of April and May, 

and fall months of October and November. Mean 

annual lake evaporation is approximately 120 cm, and 

is typically lowest in winter and greatest in summer. 

The large difference between mean annual precipitation and evaporation (approximately 40 cm), accounts 

in large part for the high annual runoff (50-100 cm) in 

this region (Rumenik, 1988). The precipitation total 

for the first full year of the study period, 1989, was 

approximately 25 percent above normal, whereas in 

1990, the total was approximately 25 percent below 

normal.

Acknowledgments

The author gratefully acknowledges the 

Rosewood Timber Company and George H. Eubank, 

Forest Manager, for granting access to Lake Five-O. 

The author is also grateful for technical support 

provided by representatives of the U.S. Environmental 

Protection Agency, the Florida Electric Power Coordinating Group, the Electric Power Research Institute, 

Southern Company Services, and the Florida Department of Environmental Protection. Finally, the author 

would like to acknowledge the work performed by 

Hal Davis, U.S. Geological Survey, in refining the 

initial interpretation of the seismic reflection survey.

GROUND-WATER FLOW SYSTEM NEAR 

LAKE FIVE-O

The following sections describe the water-level 

monitoring network, hydrogeology and ground-water 

flow system near Lake Five-O. The physical extent, 

lithologic characteristics, and estimated values of 

hydraulic properties are presented for each of the 

hydrogeologic units that influence the hydrology of 

Lake Five-O. Descriptions of the total hydraulic head 

(head) distribution, conceptual boundary conditions, 

and preliminary estimates of ground-water flow to and 

from Lake Five-O are also presented to emphasize 

essential features of the ground-water flow system. 
4 Evaluation of Ground-Water Flow and Hydrologic Budget for Lake Five-O, A Seepage Lake in Northwestern Florida

Cedar Creek

Itty Bitty

Pond

Lake

Five-O

Campers Pond

River

Lake

Contour interval 2 meters

Supplementary countour interval 1 meter

Datum is sea level

0 1 KILOMETER

0 1 MILE

85°39′

30°26′

85°41′ 40′

25′

30°24′

Figure 2. Topography of the Lake Five-O study area.
Ground-Water Flow System Near Lake Five-O 5

Collectively, this information defines a conceptual 

model of the ground-water system near Lake Five-O. 

This conceptual model forms the basis for subsequent 

numerical (simulation) models of the ground-water 

flow system.

A water-level monitoring network was 

established at the study site to collect water-level and 

water-quality data from the ground-water system 

adjacent to Lake Five-O. The network consisted of 

55 wells finished at various depths and locations in the 

surficial aquifer, intermediate confining unit, and 

Upper Floridan aquifer, and 6 water-level stations at 

Lake Five-O and 5 surrounding lakes (fig 3). A 

description of each well is given in table 1. Wells near 

Big

Island

Lake

Little

Island

Lake

Lake

Five-O

Rattlesnake

Pond

Campers

Pond

Itty Bitty

Pond

0 1/4 MILE

0 0.25 KILOMETERS

30°25′30′′

30°25′

85°40′ 85°39′30′′

20

A ́

A

w1.5

w1.4

w1.2

w1.3

w1.1

w2.1,w2.2

w3.1,w3.2

w17

w28

w32

w4

w5 pz4.1, pz4.2

w27

w16

w15 w29

w14

w26

w12 w30

w13.3

w13.2

w13.1

w20

pz1.1-pz1.3

lp1,lp2

lp3 pz3.1,pz3.2

w19

w18

w21

w22 w10 w11

w25

w9

w23

w24

w31

w6.1

w6.2

w7

w8

pz2.1

pz2.2

pz2.3

EXPLANATION

A A ́

w6.2

LINE OF

SECTION

UNIMPROVED ROAD

STAFF GAGE

ELEVATION REFERENCE

POINT

WELL AND IDENTIFIER

CLIMATE STATION

CONTINUOUS RECORDER

AND IDENTIFIER

w4

Figure 3. Location of data-collection sites in the Lake Five-O study area.

the lake margin have the prefix “pz” or “lp”, and the 

remaining wells have the prefix “w”. Groups of wells 

that are clustered together but screened at different 

altitudes are referred to as well or piezometer nests, 

and are identified by a decimal number that follows 

one of the above prefixes. The number before the 

decimal indicates the well nest number and the 

number following the decimal indicates the well 

within a particular well nest. For example, well w1.5 

is the fifth well located in well nest 1. Water-level 

(head) data from the wells and lakes were used to 

describe temporal variations in the head distribution 

near Lake Five-O, evaluate patterns of ground-water 

inflow and leakage, and infer hydraulic characteristics 
6 Evaluation of Ground-Water Flow and Hydrologic Budget for Lake Five-O, A Seepage Lake in Northwestern Florida

Table 1. Description of monitoring wells in the Lake Five-O study area 

[SA, surficial aquifer; ICU, intermediate confining unit; UFA, Upper Floridan aquifer]

Well 

identifier Latitude-longitude

Altitude of top 

of well casing, 

in meters above 

sea level

Well depth, 

in meters

Well screen 

length, in 

meters

Hydrogeologic 

unit

w1.1 30°24′58′′ 85°39′47′′ 27.68 11.35 1.52 SA

w1.2 30°24′58′′ 85°39′46′′ 27.50 17.60 1.52 SA

w1.3 30°24′59′′ 85°39′47′′ 27.40 25.96 1.52 ICU

w1.4 30°24′58′′ 85°39′47′′ 27.44 31.54 1.52 ICU

w1.5 30°24′59′′ 85°39′47′′ 27.22 42.28 1.52 UFA

w2.1 30°25′02′′ 85°39′48′′ 26.87 9.30 1.52 SA

w2.2 30°25′02′′ 85°39′48′′ 26.98 18.04 1.52 SA

w3.1 30°25′07′′ 85°39′49′′ 26.80 10.72 1.52 SA

w3.2 30°25′07′′ 85°39′49′′ 26.84 18.40 1.52 SA

w4 30°25′08′′ 85°40′00′′ 25.28 10.58 1.52 SA

w5 30°25′11′′ 85°39′56′′ 26.58 12.28 1.52 SA

w6.1 30°25′15′′ 85°40′04′′ 22.83 9.22 1.52 SA

w6.2 30°25′15′′ 85°40′04′′ 23.07 11.36 1.52 SA

w7 30°25′15′′ 85°40′01′′ 26.88 12.36 1.52 SA

w8 30°25′16′′ 85°40′00′′ 26.78 12.41 1.52 SA

w9 30°25′31′′ 85°40′00′′ 26.91 12.19 1.52 SA

w10 30°25′27′′ 85°39′56′′ 29.06 12.15 1.52 SA

w11 30°25′28′′ 85°39′50′′ 26.87 12.00 1.52 SA

w12 30°25′26′′ 85°39′41′′ 27.01 12.22 1.52 SA

w13.1 30°25′24′′ 85°39′44′′ 27.01 12.41 1.52 SA

w13.2 30°25′24′′ 85°39′44′′ 27.08 18.53 1.52 SA

w13.3 30°25′23′′ 85°39′44′′ 26.61 40.81 1.52 UFA

w14 30°25′20′′ 85°39′29′′ 27.62 10.82 1.52 SA

w15 30°25′15′′ 85°39′40′′ 27.62 12.17 1.52 SA

w16 30°25′03′′ 85°39′30′′ 27.54 11.14 1.52 SA

w17 30°25′10′′ 85°39′50′′ 25.73 28.14 1.52 ICU

w18 30°25′24′′ 85°39′57′′ 22.81 11.14 0.91 SA

w19 30°25′25′′ 85°39′54′′ 23.00 10.75 0.91 SA

w20 30°25′22′′ 85°39′45′′ 22.95 10.52 1.52 SA

w21 30°25′26′′ 85°39′58′′ 27.76 15.50 0.91 SA

w22 30°25′28′′ 85°39′59′′ 27.38 15.51 0.91 SA

w23 30°25′30′′ 85°40′02′′ 27.24 16.58 0.91 SA

w24 30°25′31′′ 85°40′04′′ 22.54 12.08 0.91 SA

w25 30°25′30 85°39′54 27.33 15.54 0.91 SA

w26 30°25′22 85°39′34 27.13 15.75 0.91 SA

w27 30°25′08 85°39′35 27.08 15.68 0.91 SA

w28 30°25′06 85°39′53 25.68 13.12 0.91 SA

w29 30°25′15 85°39′35 28.68 14.70 0.91 SA

w30 30°25′25 85°39′36 27.57 16.29 0.91 SA

w31 30°25′24 85°40′05 26.71 15.71 0.91 SA

w32 30°25′00 85°40′00 25.28 10.57 0.91 SA

pz1-1 30°25′21 85°39′47 16.00 8.34 0.91 SA

pz1-2 30°25′21 85°39′47 15.97 5.94 0.91 SA

pz1-2A 30°25′21 85°39′47 16.23 6.34 1.52 SA

pz1-3 30°25′21 85°39′47 16.17 10.56 1.52 SA

lp1 30°25′21′′ 85°39′46′′ 14.94 3.36 0.91 SA

lp2 30°25′20′′ 85°39′46′′ 14.79 1.67 0.91 SA
Ground-Water Flow System Near Lake Five-O 7

lp3 30°25′23′′ 85°39′54′′ 14.76 1.40 0.91 SA

pz2-1 30°25′16′′ 85°39′57′′ 17.87 6.86 1.52 SA

pz2-2 30°25′16′′ 85°39′57′′ 17.94 9.77 1.52 SA

pz2-3 30°25′16′′ 85°39′57′′ 17.94 12.84 1.52 SA

pz3-1 30°25′24′′ 85°39′54′′ 15.44 11.16 1.52 SA

pz3-2 30°25′24′′ 85°39′54′′ 15.45 3.75 1.52 SA

pz4-1 30°25′12′′ 85°39′49′′ 17.57 5.54 1.52 SA

pz4-2 30°25′12′′ 85°39′49′′ 17.64 12.12 1.52 SA

Table 1. Description of monitoring wells in the Lake Five-O study area—Continued 

[SA, surficial aquifer; ICU, intermediate confining unit; UFA, Upper Floridan aquifer]

Well 

identifier Latitude-longitude

Altitude of top 

of well casing, 

in meters above 

sea level

Well depth, 

in meters

Well screen 

length, in 

meters

Hydrogeologic 

unit

of the hydrogeologic units of interest. Cuttings, sediment cores, and geophysical surveys from the above 

wells were also used to describe the hydrogeology of 

the study area.

Local Hydrogeology

The hydrogeologic setting of the study area 

has been previously discussed by Andrews and others 

(1990). They identified three hydrogeologic units 

that influence the hydrology of Lake Five-O: the surficial aquifer, the intermediate confining unit, and the 

Upper Floridan aquifer. A revised version of their preliminary hydrogeologic section, A-A′, is shown in 

figure 4.

The surficial aquifer consists of an unconsolidated layer of very permeable sands of Pliocene to 

Holocene age which extend from land surface (26-30 m 

above sea level) to an altitude of 3 to 9 m above sea 

level in the plateau of the study area (Andrews and 

others, 1990). The grain-size distribution of the surficial sediments within 2 m of land surface is characterized by fine to coarse sands with a silt-clay fraction of 

less than 5 to 10 percent (Duffee and others, 1984). 

Horizontal hydraulic-conductivity estimates from slug 

tests of 23 wells screened within the surficial aquifer 

ranged from 8 to 75 m/d, with median, and lower and 

upper quartile values of 19, 12 and 23 m/d, respectively (Andrews and others, 1990). This range of 

values is consistent with the range of 13 to 17 m/d 

given by the Soil Conservation Service (Duffee and 

others, 1984) for Lakeland sands, and a range of 5 to 

54 m/d using Hazen’s approach (Fetter, 1988, p. 81). 

Given the well sorted, sandy character of the surficial 

aquifer sediments, the ratio of horizontal to vertical 

hydraulic conductivity (anisotropy) of the surficial 

sediments was assumed to be low, probably within a 

range of 1 to 20. These characteristics also suggest 

that the porosity of the sands in the surficial aquifer is 

probably within a range of 0.25 to 0.50 (Fetter, 1988), 

and specific yield within a range of 0.10 to 0.30 

(Johnson, 1967; Walton, 1970). Based on the sediment compressibility estimates of Freeze and Cherry 

(1979, p. 55) and Domenico and Schwartz (1990, 

p. 111) and the estimate given by Lohman (1979, p. 8), 

specific storage of the surficial sediments is probably 

within a range of 10-6 to 10-3 m-1.

 The surficial aquifer is underlain by the Jackson 

Bluff Formation (intermediate confining unit) of 

Pliocene age, which consists of calcareous, sandy clay 

to clayey sand with large quantities of shell material, 

and seams of fossiliferous sandy limestone (Andrews 

and others, 1990). A thin, dense shelly clay layer, 

approximately 1.5-m thick, occurs at the base of the 

intermediate confining unit in the study area (Andrews 

and others, 1990). In the plateau of the study area, the 

top and bottom of the intermediate confining unit occur 

at altitudes of 3 to 9 m above and 4 to 14 m below sea 

level, respectively. Lithologic data and a reflection surface from a seismic reflection survey indicate that 

nearer the lake, the top of the intermediate confining 

unit dips toward several depressions in the top of the 

unit (figs. 4 and 5). These depressions are most likely 

the result of the collapse of the intermediate confining 

unit and surficial aquifer into one or more voids created 

by dissolution of the underlying limestone aquifer. 

Highly permeable sands from the 
8 Evaluation of Ground-Water Flow and Hydrologic Budget for Lake Five-O, A Seepage Lake in Northwestern Florida

surficial aquifer have probably filled these breaches, 

thus creating a high conductance pathway between the 

surficial aquifer and the Upper Floridan aquifer.

Estimates of the hydraulic properties of the 

intermediate confining unit were based on the lithologic characteristics of the unit, previously published 

data, and limited slug test data. Lithologic data indicate that conductive properties may vary considerably 

within the intermediate confining unit. The 1.5-mthick clay layer at the base of the intermediate confining unit seems to be much less conductive and more 

continuous than the limestone and sandy-clay/clayeysand facies in the overlying intermediate confiningunit sediments (Andrews and others, 1990). Maslia 

and Hayes (1988, pl. 3) presented a highly generalized 

map of leakance (vertical hydraulic conductivity 

divided by thickness) which indicates that the regional 

leakance of the sediments overlying the Upper 

Floridan aquifer is within the range of 5 x 10-5 to 

Lake Five-O

METERS

INTERMEDIATE CONFINING UNIT

UPPER FLORIDAN AQUIFER

SURFICIAL AQUIFER

pz1.2

pz1.1

pz1.3

lp nest w20

w13.1

w13.2

w13.3

pz4.1 w12

pz4.2

w17

w3.1

w3.2

w2.1

w2.2

w1.1

w1.2

w1.3

w1.4

w1.5

24

SEA

LEVEL

28

20

12

16

8

4

4

8

16

12

20

0 50 100

0 250 500 FEET

150 METERS

Vertical exaggeration is x10

w2.2

EXPLANATION

SCREENED INTERVAL

OF WELL

LAKE SURFACE OR WATER TABLE

A A′

Figure 4. Hydrogeologic section A-A′ through Lake Five-O showing midpoints of screened intervals of observation wells 

used to construct the section. (Location of section is shown in figure 3. Hydrogeologic section A-A′ revised from Andrews 

and others, 1990.)

5 x 10-4 d-1 in northern Bay and southern Washington 

Counties. Leakance values for the intermediate 

confining unit are probably smaller than these regional 

estimates, because the regional values are averaged 

over large areas, within which confining unit is often 

breached. Given these regional leakance estimates 

and the lithologic information collected at the study 

site, the vertical hydraulic conductivity of the basal 

clay layer was assumed to be within the range of 10-11

to 10-5 m/d. Determining representative hydraulic 

conductivity values for intermediate confining-unit 

sediments that overlie the basal clay is difficult 

because of the poorly sorted nature and variable 

composition of unconsolidated sediments within the 

unit, and the lack of information on the extent and 

degree of secondary porosity of the limestone beds 

within the unit. A slug test conducted at well w1.3 

(fig. 3) yielded an estimated range in horizontal 

hydraulic conductivity of 0.03 to 0.15 m/d. However, 
Ground-Water Flow System Near Lake Five-O 9

the results of this test are extremely limited because of 

the heterogeneity of the unit. Horizontal hydraulic 

conductivity values for the intermediate confining unit 

sediments in the study area (excluding the basal clay 

layer) were assumed to vary over a much larger range 

(10-3 to 1 m/d) than that indicated by the single slug 

test. The anisotropy of these intermediate confiningunit sediments was assumed to be within 10 to 1,000. 

The porosity of the intermediate confining unit was 

assumed to be in the range of 0.10 to 0.50, and specific 

storage was assumed to be in the range of 10-6 to 10-3

m-1(Freeze and Cherry (1979, p. 55); Lohman (1979, 

p. 8); Domenico and Schwartz (1990, p. 111).

The Floridan aquifer system consists of a thick 

sequence of mostly Paleocene to early Miocene age 

carbonate rocks (Miller, 1986) that underlie Florida, 

and parts of Alabama, Georgia, and South Carolina. 

The permeability of the Floridan aquifer system is 

Figure 5. Altitude of seismic reflector surface beneath Lake Five-O. (Hal Davis, U.S. Geological Survey, written commun., 

1991.)

0

0

500 FEET

100 METERS

LINE OF EQUAL ALTITUDE OF SEISMIC

REFLECTOR SURFACE—In meters below

sea level. Contour interval is 4.1 meters.

EXPLANATION

-10.4

Lake Five-O

-14.5

-14.5

-18.6

-10.4 -6.3

-14.5

largely attributed to secondary porosity resulting from 

the dissolution of the carbonate rock (Maslia and 

Hayes, 1988, p. 20). Miller (1986) subdivided the 

Floridan aquifer system into the Upper and Lower 

Floridan aquifers, which are separated in most areas 

by a middle confining unit. The Upper Floridan 

aquifer influences the hydrology of many of the lakes 

and streams near the study area. In the study area, the 

top of the Upper Floridan aquifer is at depths of 4 to 

14 m below sea level, and the aquifer has a thickness 

of approximately 180 m. The study area lies at the 

southern extent of a belt of high transmissivity in the 

Floridan aquifer system, which has resulted from a 

highly interactive surface-water and ground-water 

flow (Maslia and Hayes, 1988, p. 21). Estimates of 

transmissivity of the Floridan aquifer system in the 

study area range from 9 x 103

 to 6 x 104 m2/d (Maslia 

and Hayes, 1988, pl. 6).
10 Evaluation of Ground-Water Flow and Hydrologic Budget for Lake Five-O, A Seepage Lake in Northwestern Florida

Ground-Water Movement

Hydrologic data collected during the study 

provided the basis for a preliminary description of 

ground-water flow near Lake Five-O. Hydraulic head 

and lithologic information were used to conceptualize 

ground-water flow within and between the hydrogeologic units. The head data were also used to define 

the location and characteristics of boundaries of the 

flow system. Precipitation, evaporation, and lakevolume data were used to compute preliminary 

estimates of ground-water inflow to and leakage from 

Lake Five-O. In this report, the term ground-water 

inflow refers to the flow of water into Lake Five-O 

from the contiguous ground-water system, and the 

term leakage refers to the flow of water from Lake 

Five-O to the contiguous ground-water system.

Head Distribution and Ground-Water Flow near 

Lake Five-O

Head fluctuations during the study period were 

generally consistent with the typical seasonal patterns 

of precipitation in northwestern Florida (wet 

conditions during the summer and dryer conditions 

during the fall and spring). Heads rose during the wet 

summer seasons of 1989 and 1990, and fell during dry 

periods during the spring months of 1989 and 1990, 

and the fall of 1990 (fig. 6). These seasonal head 

changes also occurred in a consistent manner throughout the Lake Five-O flow system (fig. 6), which 

resulted in approximately constant spatial distributions 

of head over the range of meteorologic conditions that 

occurred during the study (figs. 7-10). This indicates 

that patterns of ground-water flow were also reasonably consistent during the study period.

Head data from the monitoring network 

indicated a consistent pattern of ground-water flow 

toward Lake Five-O. With the exception of a small 

area adjacent to the northwest lake margin, water-table 

altitudes were consistently higher than the stage of 

Lake Five-O, and water-table altitudes increased with 

distance from the lake (figs. 7-10). Higher water-table 

altitudes were observed in the southern part of the 

study area and decreased to the northwest. This 

head distribution resulted in a steeper horizontal head 

gradients south of the lake and smaller gradients 

northwest of the lake. This suggests that ground-water 

inflow rates are largest south of the lake and descrease 

to the northwest. This pattern of ground-water flow 

was observed throughout the study period, over a wide 

range of hydrologic conditions, which indicates 

ground-water inflow to Lake Five-O occurred 

around most of the lake perimeter throughout the 

study period.

 The head data also indicated a strong potential 

for downward leakage from Lake Five-O to the Upper 

Floridan aquifer (fig. 9-11). Differences between 

the stage of Lake Five-O and the head in the Upper 

Floridan aquifer (as estimated from heads at wells 1.5 

and 13.3) ranged from 1.3 to 2.2 m (fig. 11). Abrupt 

declines in this head difference were observed during 

the summer (June-August) of 1989, but abrupt 

increases were observed during the summer of 1990 

(fig. 11). This indicates that leakage from Lake Five-O 

decreased during the summer of 1989 and increased to 

a maximum in the summer of 1990.

Changes in this head difference (and therefore 

leakage from Lake Five-O) are probably due to 

regional variations in recharge to the Upper Floridan 

aquifer during the two periods. This hypothesis is 

supported by observed water levels from a long-term 

Upper Floridan aquifer monitoring well in nearby 

Greenhead, Fla. (approximately 9 km north of Lake 

Five-O), and by regional precipitation data. The 

water-level changes in the Greenhead well were 

similar to those observed in the Upper Floridan wells 

at Lake Five-O. Water levels in the Upper Floridan 

aquifer at Lake Five-O rose more rapidly than those in 

wells in the surficial aquifer at the site in the summer 

of 1989, but showed little increase in the summer of 

1990. This suggests that regional recharge to the 

Upper Floridan aquifer was probably greater than 

recharge in the immediate vicinity of Lake Five-O 

during the summer of 1989, and less than recharge in 

the vicinity of Lake Five-O in the summer of 1990. 

Precipitation totals at two climatic stations located 

north of Lake Five-O, in an area where the Upper 

Floridan aquifer crops out, were comparable to those 

measured at Lake Five-O during the summer of 1989, 

but much lower than those measured at Lake Five-O 

during the summer of 1990. Although the hydraulic 

head differences between the lake and the Upper 

Floridan aquifer changed seasonally during the study, 

the stage of Lake Five-O was consistently greater than 

the head in the Upper Floridan aquifer, which indicates 

that the lake leaked throughout the study period.

The head data also indicate that significant 

hydraulic conductivity differences exist within the 

intermediate confining unit and between the surficial 

aquifer and intermediate confining unit. As previously 
Ground-Water Flow System Near Lake Five-O 11

discussed, Andrews and others (1990) indicated that 

the confining properties of the intermediate confining 

unit are probably not evenly distributed within the 

unit, with the dense, shelly clay layer at the base of 

the unit being the most effective confining layer 

within the unit. Their conclusion was based on the 

large head loss (5.2-5.8 m) that occurred across the 

basal clay layer (fig. 9 and 10). The head loss across 

this layer was much greater than that between wells 

1.3 and 1.4 (less than 0.05 m) (fig. 9 and 10), which 

are screened within the unit. An abrupt decrease in 

hydraulic conductivity at the surficial aquifer/interFigure 6. Water levels of Lake Five-O and selected ground-water monitoring wells, July 1988 to February 1991.

20

11

12

13

14

15

16

17

18

19

WATER LEVEL, IN METERS ABOVE SEA LEVEL

J A S ON D J FMAM J J A S ON D J FMAM J J A S ON D JF

1988 1989 1990 1991

WELL W1.1

(SURFICIAL

AQUIFER)

WELL W1.3

(INTERMEDIATE

CONFINING UNIT)

WELL W6.2

(SURFICIAL AQUIFER)

LAKE FIVE-O

WELL W1.5

(UPPER FLORIDAN

AQUIFER)

mediate confining unit contact is indicated by relatively large vertical head losses of 0.43 to 0.61 m 

across this contact at well nest 1 (fig. 9-11). The 

lower hydraulic conductivity of the intermediate confining unit sediments (relative to that of the surficial 

aquifer sediments) limits the downward movement 

of water from the surficial aquifer to the intermediate 

confining unit.

The head data also corroborated the seismic 

reflection interpretation, which indicated that three 

breaches in the intermediate confining unit exist 
12 Evaluation of Ground-Water Flow and Hydrologic Budget for Lake Five-O, A Seepage Lake in Northwestern Florida

beneath Lake Five-O. Vertical head differences 

between the surficial aquifer and Upper Floridan 

aquifer were much smaller near Lake Five-O 

(1.3-2.2 m) than those beyond the lake basin at 

well nests 13 and 1 (5.2-5.8 m) (fig. 9-11). The most 

plausible explanation for this difference is that Lake 

Five-O and the surficial aquifer leak large volumes of 

water through one or more highly conductive breaches 

in the intermediate confining unit under Lake Five-O. 

Downward leakage through these confining unit 

breaches probably represents the dominant sink for 

the shallow ground-water system and is largely 

EXPLANATION

15.44

LINE OF EQUAL WATERTABLE ALTITUDE—In

meters above sea level.

Dashed where inferred.

Contour interval is variable

UNIMPROVED ROAD

WELL LOCATION AND

WATER-TABLE ALTITUDE—In

meters above sea level

16

Big

Island

Lake

Little

Island

Lake

Lake

Five-O

Rattlesnake

Pond

Campers

Pond

Itty Bitty

Pond

0 1/4 MILE

0 0.25 KILOMETERS

30°25′30′′

30°25′

85°40′ 85°39′30′′

20

15.29

17.46

18.65

16.77

17.98

17.71

16.31

17.25

17.93

17.55

16.92

16.60 16.20

15.80

15.51

15.32

15.29

15.35

15.32

15.40

15.38

15.39

15.14

15.14

14.90

14.77

14.85

16.00

18.92

19.55

15.29

17.92

15.97

15.96

15.82

15.40

15.87

15.71

20

19

15.5

LAKE SURFACE ALTITUDE—In

meters above sea level

17

15.35

16

15.35

15.44

18

Figure 7. Water-table configuration near Lake Five-O, October 4, 1989.

responsible for the pattern of horizontal ground-water 

flow toward the lake (figs. 7 and 8).

Boundary Conditions

Three boundaries were used to characterize the 

ground-water flow system near Lake Five-O: (1) a 

lateral, no-flow boundary, (2) an upper free-surface 

boundary defined by the water table and the water 

surface of Lake Five-O, and (3) a lower specified-head 

boundary at the base of the intermediate confining 

unit. This section describes essential features of each 

of these boundaries.
Ground-Water Flow System Near Lake Five-O 13

Lateral Boundary

A lateral, no-flow boundary was used to define 

the perimeter of the shallow ground-water system. 

This boundary restricts the exchange of water between 

the surficial flow system near Lake Five-O and 

surrounding lakes and streams. The existence and 

location of this boundary was inferred from waterBig

Island

Lake

Little

Island

Lake

Lake

Five-O

Rattlesnake

Pond

Campers

Pond

Itty Bitty

Pond

30°25′30′′

30°25′

85°40′ 85°39′30′′

20

0 1/4 MILE

0 0.25 KILOMETERS

14.12

14.33

16.15

17.28

15.52

16.75

16.58

16.08

16.88

15.85

15.22 15.64

14.72

14.41

14.18

14.23 14.19

14.33

14.44

13.89

13.95

14.02

14.65

17.55

18.23

16.83

17.39

14.69 14.76

14.65

14.28

14.91

18

14.5

17

15

14.3

14.3

14.14 14.35

16

EXPLANATION

18.23

UNIMPROVED ROAD

WELL LOCATION AND WATER-TABLE

ALTITUDE—In meters above sea level

LAKE SURFACE ALTITUDE—In

meters above sea level

17.39

LINE OF EQUAL WATER-TABLE

ALTITUDE—In meters above sea

level. Contour interval is variable

LATERAL NO-FLOW BOUNDARY

STAGNATION POINT

16

Figure 8. Water-table configuration near Lake Five-O, November 5, 1990.

table and topographic maps. The no-flow boundary is 

defined by ground-water flowlines that are coincident 

with water-table divides. These flowlines originate 

south of the lake, encircle the lake, and terminate at a 

stagnation point on the water-table surface, northwest 

of Lake Five-O (fig. 8). Although head data is insufficient to define the southern extent of this boundary, 
14 Evaluation of Ground-Water Flow and Hydrologic Budget for Lake Five-O, A Seepage Lake in Northwestern Florida

data from 7.5 minute topographic maps indicate that 

water levels in Bream Lake and Cedar Creek (fig. 2) 

are generally lower than minimum water levels 

observed in wells south of Lake Five-O. This indicates the existence of a no-flow boundary between 

southern limits of the well network and Bream Lake 

and Cedar Creek. Occasionally during the study, 

inflow gradients were observed from Campers Pond 

toward Lake Five-O; however, these gradients were 

generally much smaller than those observed in active 

areas of the surficial flow system, and were similar in 

magnitude to head gradients in the relatively stagnant 

area northwest of Lake Five-O. A lateral no-flow 

boundary between Campers Pond and Lake Five-O 

was considered a reasonable representation of the 

limits of the shallow flow system at Lake Five-O, 

because of the intermittent and weak nature of the 

inflow gradients in this part of the study area.

Figure 9. Hydrogeologic section A-A′ through Lake Five-O, showing vertical head distribution near the lake for April 13, 

1989. (Location of section is shown in fig. 3).

0 50 100

0 250 500 FEET

150 METERS

Vertical exaggeration is x10

METERS

A A′

UPPER FLORIDAN AQUIFER

SURFICIAL AQUIFER

18.04

17.60

17.58

12.26

dry

17.29

dry

15.88

13.96

13.83

N/D

14.01

14.16

dry dry

12.13

13.76

18.05

14.37

14.00

13.98

13

15

17

28

24

20

12

16

8

4

4

SEA

LEVEL

8

16

12

20

18

13.74

14

14

INTERMEDIATE CONFINING UNIT 16

13.74

13.74

Lake Five-O

EXPLANATION

15.88

13.74

DIRECTION OF GROUND-WATER FLOW

LAKE OR WATER TABLE SURFACE

LINE OF EQUAL HYDRAULIC HEAD—In meters above sea level.

Contour interval is variable

LAKE-LEVEL—In meters above sea level

HYDRAULIC HEAD VALUE—In meters above sea level

16

13.74

Upper Boundary

The upper boundary of the ground-water and 

lake flow system was defined by the water table and 

lake surfaces. The location of these boundaries is 

partially determined by the flux of water across these 

boundaries (recharge rates for the water table and 

net-precipitation rates for the lake surface). Values of 

recharge over the water-table boundary were estimated 

from (1) an analysis of the daily water-level 

hydrograph at well 6.2, (2) measurements of chloride 

concentrations of water in the surficial aquifer and 

atmospheric deposition rates of chloride, and (3) previous estimates of recharge for similar areas in northwestern Florida.

The method of computing recharge from wellhydrograph data is based on a water budget for a 

volume of the surficial aquifer at the water table with a 

unit surface area, negligible moisture content above 
Ground-Water Flow System Near Lake Five-O 15

the water table, and lacking any internal sources or 

sinks. Under these conditions, the following conservation equation holds:

(1)

where is the rate of volume change of the 

aquifer per unit area at time ; is the specific yield 

of the surficial aquifer (dimensionless); is the 

rate of change of the altitude of the water table at time 

; is the recharge rate at time , in units of volume 

per unit area per unit time or, equivalently, length per 

unit time; and is the volume per unit area per 

unit time. Equation 1 can be rearranged and integrated 

over a given time interval, , to obtain an estimate of 

recharge that occurred during that interval:

Figure 10. Hydrogeologic section A-A′ through Lake Five-O, showing vertical head distribution near the lake for 

January 17, 1990. (Location of section is shown in fig. 3.)

UPPER FLORIDAN AQUIFER

SURFICIAL AQUIFER

19.47

19.46

19.03

19.03

13.66

18.83

18.81

17.47

17.41

15.85

15.50

15.46 15.35

15.40

15.25

15.53

15.76

15.78 16.12

13.62

15.35

15

16

17

18

19

16

14

METERS

SEA

LEVEL

28

24

20

12

16

8

4

4

8

16

12

20

A A′

0 50 100

0 250 500 FEET

150 METERS

EXPLANATION Vertical exaggeration is x10

16

15.88

13.74

DIRECTION OF GROUND-WATER FLOW

LAKE OR WATER-TABLE SURFACE

LINE OF EQUAL HYDRAULIC HEAD—In meters

above sea level. Contour interval is variable

LAKE LEVEL—In meters above sea level

HYDRAULIC HEAD VALUE—In meters above sea level

15.30

15.30

INTERMEDIATE CONFINING UNIT

Lake Five-O

15.30

V

·

( )t Syh

·

( )t R

·

( )t Q

· = = – o( )t

V

·

( )t

t Sy

h

·

( )t

t R· t

Q

·

o( )t

∆t

(2)

During dry conditions, recharge can be assumed to be 

negligible relative to storage changes, and outflow can 

be estimated using the following equation if a watertable hydrograph and an estimate of specific yield are 

available:

(3)

where is the absolute value of the average 

change in the water-table altitude during a given 

hydrograph recession under suitably dry conditions. 

For a system in which the hydraulic head gradients are 

approximately constant over time, the outflow rate can 

be assumed to be constant, and can be estimated using 

the following equation:

R∆t R

·

( )t dt

∆t

∫ Sy h

·

( )t dt

∆t

∫ Q

·

o( )t dt

∆t

∫ = = +

Q

·

o( )t dt

∆t

∫ Sy h

·

( )t dt

∆t

∫ – Sy = = hrecess∆t

hrecess
16 Evaluation of Ground-Water Flow and Hydrologic Budget for Lake Five-O, A Seepage Lake in Northwestern Florida

(4)

where represents the constant outflow rate. 

Equation 4 implies that . Thus, a 

recharge volume for any period can be obtained by 

combining equations 2 and 4:

(5)

where is recharge over time interval .

 The terms and in equation 5 

were obtained from head data from well 6.2, with the 

latter term computed from data from August 1990 to 

January 1991. Specific yield was varied from 0.10 to 

0.30 to obtain upper and lower limits, respectively, for 

discharge. The analysis of the water-level hydrograph 

1988 1989

D J FMAM J J A S OND J FMAM J J A S OND

1990

1.25

1.30

1.35

1.40

1.45

1.50

1.55

1.60

1.65

1.70

1.75

1.80

1.85

1.90

1.95

2.00

2.05

2.10

2.15

2.20

2.25

LAKE FIVE-O STAGE MINUS UPPER FLORIDAN HEAD, IN METERS

15.2

11.4

15.4

11.6

11.8

12.0

12.2

12.4

12.6

12.8

13.0

13.2

13.4

13.6

13.8

14.0

14.2

14.4

14.6

14.8

15.0

ALTITUDE, IN METERS ABOVE SEA LEVEL

LAKE FIVE-0 STAGE

HEAD IN

UPPER FLORIDAN AQUIFER

NEAR LAKE FIVE-0

HEAD DIFFERENCE BETWEEN

LAKE FIVE-0 AND THE

UPPER FLORIDAN AQUIFER

Figure 11. Comparison of Lake Five-O stage and head in the Upper Floridan Aquifer, December 1988 to January 1991.

Q

·

o( )t dt

∆t

∫ c

Q

·

o

dt

∆t

∫ c

Q

·

o

∆t Sy = == hrecess∆t

c

Q

·

o c

Q

·

o

Sy = hrecess

R∆t Sy h

·

( )t dt

∆t

∫ = + hrecess∆t

R∆t ∆t

h

·

( )t hrecess

at well 6.2 indicated that average annual recharge at 

Lake Five-O for 1989-90 was between 36 and 144 cm 

(20-90 percent of precipitation). The analysis also 

indicated that monthly recharge rates were generally in 

the range of 18 to 166 cm/yr, but were as low as zero 

during dry periods, and as high as 612 cm/yr 

during summer rainy periods.

Average annual recharge was also estimated 

with a method that used precipitation rates and 

chloride concentrations (Vacher and Ayers, 1980) as 

follows:

(6)

where is the average annual recharge, is 

the average annual precipitation, is the 

R P

Clatmos

– [ ]

ClSA

– [ ] = ----------------------

R P

Clatmos

– [ ]
Ground-Water Flow System Near Lake Five-O 17

“effective” chloride concentration due to atmospheric 

loading (wet plus dry deposition), and is the 

average chloride concentration of water in the surficial 

aquifer. The “effective” chloride concentration due to 

atmospheric loading, , was computed as 

the product of the volume weighted mean concentration of chloride in precipitation (19 ) and the 

average ratio of total atmospheric chloride deposition 

(wet plus dry deposition) and wet deposition of 

chloride in Florida, 1.4 (Baker, 1991). The upper and 

lower quartile of chloride concentrations in water from 

the surficial aquifer wells (excluding those measured 

in samples from the wells at the margins of Lake 

Five-O) were 51 and 68 , respectively. Given 

an average annual precipitation rate of 160 cm, the 

above chloride concentrations indicate that average 

annual recharge to the surficial aquifer in the study 

area is within a range from 61 to 81 cm.

The recharge rates calculated in the above well 

hydrograph and chloride analyses were in reasonable 

agreement with rates reported by Vecchioli and others 

(1990), who used a base flow separation technique to 

estimate recharge for a three-county area immediately 

west of Bay County. They reported average annual 

recharge estimates ranging from 76 to 102 cm for 

areas with basin characteristics similar to those at 

Lake Five-O. Rates at Lake Five-O may be smaller 

than those reported by Vecchioli and others, because 

annual precipitation minus potential evaporation (net 

precipitation) is 10 to 15 cm higher in the area they 

studied.

Net-precipitation rates for the lake surface part 

of the upper boundary were estimated as the difference 

between measured precipitation and estimated lake 

evaporation. Long-term average net precipitation was 

estimated using historic monthly precipitation data 

and estimates of historic lake evaporation at Lake 

Five-O. Monthly precipitation totals were obtained 

from a National Oceanic and Atmospheric Administration (NOAA) weather station in Fountain, Fla. 

(approximately 24 km east-northeast of Lake Five-O) 

for the period April 1955 to February 1989, and from 

measurements at Lake Five-O for the period March 

1989 to December 1990. Long-term average lake 

evaporation was based on estimates of monthly lake 

evaporation that were obtained using two methods. 

For the period of October 1962 through May 1989, 

monthly lake evaporation was estimated at Lake Five-O 

using a relation between energy-budget evaporation at 

Lake Five-O and pan evaporation from the nearest 

NOAA pan evaporation site in Milton, Fla. (Sacks 

and others, 1994) which is located about 125 km west 

of Lake Five-O. For the period of June 1989 to 

December 1990, estimates of monthly lake evaporation were made by Sacks and others (1994). These 

estimates of monthly precipitation and evaporation 

were used to compute monthly mean values of precipitation and evaporation at Lake Five-O, which were 

then summed to obtain estimates of average annual 

precipitation and evaporation of 157 cm and 118 cm, 

respectively. Average annual net precipitation at Lake 

Five-O was then computed as the difference between 

these two values, or 39 cm. This estimate of net 

precipitation compares favorably with the range of 30 

to 38 cm reported by Visher and Hughes (1975), which 

was based on lake evaporation for the period 1946-55 

and precipitation for the period 1931-60. During the 

study at Lake Five-O, net precipitation rates generally 

ranged from -4 to 76 cm/yr, and were as low as -153 

cm/yr during the dry fall of 1990, and as high as 510 

cm/yr during the extremely wet summer of 1989. 

Annual net precipitation values during the study 

period were more extreme than any of the values 

from the period of estimated record. The 85 cm of net 

precipitation measured during 1989 was larger than 

any annual estimate available at Lake Five-O, whereas 

the -10 cm measured in 1990 was the smallest value 

among the available data.

Lower Boundary

The lower boundary of the conceptual system 

was defined as the contact between the base of the 

intermediate confining unit and the Upper Floridan 

aquifer, and was characterized by the head distribution 

along this contact. This head distribution was 

assumed to have a value that, although variable with 

time, is the same everywhere on the lower boundary 

(uniform spatial distribution) for a given moment in 

time. The assumption of a uniform spatial distribution 

is supported by the small to negligible difference 

between concurrently measured heads at the two wells 

screened within the Upper Floridan aquifer (wells 

w1.5 and w13.3), and the assumed large transmissivity 

of the Upper Floridan aquifer at Lake Five-O.

ClSA

– [ ]

Clatmos

– [ ]

μeq L⁄

μeq L⁄
18 Evaluation of Ground-Water Flow and Hydrologic Budget for Lake Five-O, A Seepage Lake in Northwestern Florida

Net Ground-Water Flow to Lake Five-0

Net ground-water flow (ground-water inflow 

minus leakage) to Lake Five-O was estimated using a 

water balance approach that used measurements of 

precipitation, lake evaporation, and lake volume 

changes. This net ground-water flow analysis made

it possible to make a qualitative assessment of the 

significance of lake-ground-water exchanges during 

the study period. The analysis also made it possible to 

make quantitative estimates of minimum ground-water 

inflow and leakage rates during the study period and 

for long-term average conditions.

Methods

The method used to compute net ground-water 

flow was virtually the same as that used by Pollman 

and others (1991) and can be described as follows:

The hydrologic budget for Lake Five-O is given as:

 

(7)

where: is the lake volume change during

time interval , is precipitation, is lake evaporation, is ground-water inflow to Lake Five-O, 

 is leakage from Lake Five-O to the contiguous 

ground-water system, and is the 

error associated with the estimate, , of the hydrologic variable (for example, is the estimate of 

the actual amount of precipition, , that occurred during time interval i, and is the error associated with the precipitation estimate, ). The above 

hydrologic budget equation assumes that surface runoff to Lake Five-O is negligible. An analysis of lake 

volume changes that occurred immediately after rainstorms corroborated this assumption.

Equation 7 can be rearranged to solve for net 

ground-water flow: 

(8)

(9)

Thus an estimate of can be computed as:

(10)

Note that and 

With the exception of the error terms, all of the 

terms on the right side of equation 9 were measured 

directly or indirectly. Volume per unit area estimates 

of evaporation were obtained from the energy budget 

estimates and regression relation with the Milton, Fla., 

NOAA station pan data (Sacks and others, 1994), and 

volume per unit area estimates of precipitation were 

given by precipitation measurements at Lake Five-O 

and the Fountain, Fla. NOAA station. Volumetric 

estimates of precipitation and evaporation ( and , 

respectively) were computed from the volume per unit 

area estimates by multiplying total precipitation or 

evaporation (in volume per unit surface area units) 

by lake area. Lake volume changes and lake area were 

estimated using lake-stage measurements and the 

relation between lake volume and stage that was 

developed from a bathymetric survey of Lake Five-O 

(Andrews and others, 1990).

Although the error, , associated with a 

given net ground-water flow estimate, is 

unknown, the uncertainty of can be characterized by computing a confidence interval as follows:

(11)

where is the standard normal deviate for an 

level of significance and is the standard deviation of .

This approach assumes that the error, , 

associated with follows a normal distribution 

with mean equal to zero ( is assumed to be 

unbiased) and a variance equal to .

The standard deviation of the error associated

with the net ground-water flow estimate, , was 

computed as follows (Pollman and others, 1991):

(12)

∆Vi ∆V

ˆ

i ε

∆V

ˆ , i = –

P

ˆ

i ε

P

ˆ, i –     E

ˆ

i ε

E

ˆ i

–     = –

+ Q

ˆ I i , ε

Q

ˆ I i ,

–     Q

ˆ O i , ε

Q

ˆ O i ,

–     –

∆Vi

i Pi Ei

QI i ,

QO i ,

ε

θˆ, i

θˆi θi = –

θˆi

θi P

ˆ i

Pi

ε

P

ˆ, i

P

ˆ = i – P

P

ˆ i

Qnet i , QI i , QO i , – Q

ˆ I i , ε

Q

ˆ I, i –    

Q

ˆ O i , ε

Q

ˆ O, i –     –

= =

Qnet i , ∆V

ˆ

i ε

∆V

ˆ, i –     P

ˆ

i ε

P

ˆ, i –     E

ˆ

i ε

E

ˆ, i –     = – +

Qnet i ,

Q

ˆ net i , ∆Vˆ i Pˆ i Eˆ = – + i

Qnet i , Q

ˆ net i , ε

Q

ˆ net i ,

= –

ε

Q

ˆ net i ,

ε

∆V

ˆ i

ε

P

ˆ

i

– ε

E

ˆ i

= +

P

ˆ

i E

ˆ

i

ε

Q

ˆ net i ,

Q

ˆ net i ,

Q

ˆ net i ,

Q

ˆ net i , zα ⁄ 2σε

Q

ˆ

net i ,

±

zα ⁄ 2 α ⁄ 2

σε

Q

ˆ

net i , ε

Q

ˆ net i ,

ε

Q

ˆ net i ,

Q

ˆ net i ,

Q

ˆ net i ,

σε

Q

ˆ

net i ,

2

σε

Q

ˆ

net i ,

σε

Q

ˆ

net i ,

P

ˆ ( )i CV

P

ˆ

i

( )

2

E

ˆ ( )i CV

E

ˆ

i

( )

2

∆V

ˆ ( )i CV

∆V

ˆ

i

( )

2

= + +

+
Ground-Water Flow System Near Lake Five-O 19

where is the coefficient of variation 

for hydrologic variable . 

Estimates of lake volume change were assumed 

to have a coefficient of variation of 5 percent (Winter, 

1981). Monthly precipitation estimates were assumed 

to have a coefficient of variation of 15 percent (Winter, 

1981) for periods where data was available at Lake 

Five-O, and 30 percent during January and February 

1989, when precipitation at Lake Five-O was 

estimated with data from a nearby NOAA weather

station in Fountain, Fla. This latter value was obtained 

by summing the “at site” coefficient of variation 

(15 percent) and the average of the absolute value of 

the difference between precipitation at Lake Five-O 

and the Fountain station for the 3 months (March, 

April, and May 1989) when concurrent data were 

available for the two sites. Coefficient of variation 

values for the lake evaporation estimates were 

obtained from Sacks and others (1994). The mean, 

lower quartile, and upper quartile of their monthly 

coefficient of variation values were 15, 11, and 17 

percent, respectively. For the months of January 

through May 1989, when lake evaporation was estimated using a regression relation between energy-budget estimates and pan estimates (Sacks and others, 

1994), the evaporation coefficient of variation was 

assumed to be equal to the sum of the standard error of 

regression and the energy budget coefficient of variation for the same month in 1990.

Results

The results of the monthly net ground-water 

flow computations and associated errors are listed 

in table 2 and are shown in figure 12. The temporal 

distribution of net ground-water flow is consistent 

CVθˆi σεθˆ

i θ

ˆi = ⁄

θ

Table 2. Monthly net ground-water flow to Lake Five-O, 1989-90

[All units are in cubic meters, unless otherwise noted; is the standard deviation of the error component of the net 

ground-water flow estimate. Negative values of net ground-water flow indicate that leakage exceeded ground-water inflow.]

Month

Average 

lake 

volume

Change 

in lake 

volume

PrecipitationEvaporation

Net 

groundwater flowin percent of 

net groundwater flow

1989

Jan. 989,700 -26,100 3,800 3,200 -26,700 2,300 9

Feb. 964,000 -21,900 6,900 4,100 -24,700 2,600 10

Mar. 943,300 -13,900 17,700 7,900 -23,700 3,300 14

Apr. 935,200 -8,700 10,600 11,000 -8,300 2,800 34

May 922,500 -14,700 8,900 13,800 -9,900 2,900 29

June 945,600 69,100 56,800 12,000 24,400 9,300 38

July 1,024,800 66,700 26,300 13,300 53,700 5,400 10

Aug. 1,068,400 26,200 20,900 12,300 17,600 3,700 21

Sept. 1,091,800 22,300 21,500 16,800 17,700 3,700 21

Oct. 1,103,400 3,100 15,100 13,000 1,000 2,700 270

Nov. 1,108,200 4,100 13,800 9,400 -300 2,300 770

Dec. 1,102,700 1,000 13,500 7,900 -4,600 2,300 50

1990

Jan. 1,107,000 -3,100 3,800 3,300 -3,500 1,500 43

Feb. 1,103,000 -4,800 5,700 4,800 -5,800 1,300 22

Mar. 1,092,000 -15,500 9,600 8,600 -16,500 2,000 12

Apr. 1,076,100 -17,800 12,000 11,100 -18,700 2,400 13

May 1,058,600 -22,000 9,400 15,100 -16,300 2,400 15

June 1,041,300 -9,100 23,700 14,300 -18,500 4,200 23

July 1,046,400 17,900 31,300 15,000 1,600 5,300 330

Aug. 1,055,600 -2,400 16,000 15,600 -2,800 3,600 130

Sept. 1,033,300 -35,600 3,900 17,900 -21,600 2,900 13

Oct. 998,600 -35,100 5,500 15,400 -25,200 2,800 11

Nov. 963,200 -35,600 800 11,300 -25,100 2,600 10

Dec. 932,000 -28,900 7,000 7,400 -28,600 2,600 9

σε Q

ˆ , net

σ

ε Q

ˆ , net

σ

ε Q

ˆ , net
20 Evaluation of Ground-Water Flow and Hydrologic Budget for Lake Five-O, A Seepage Lake in Northwestern Florida

than the area below the line during the dryer year of 

1990. This indicates that leakage from Lake Five-O 

greatly exceeded ground-water inflow during 1990.

 The significance of ground-water inflow and 

leakage to the water budget of Lake Five-O is illustrated by a plot of cumulative net precipitation inputs 

(precipitation minus evaporation) and cumulative 

changes in lake volume over time in fig. 13). If Lake 

Five-O were completely isolated from the groundwater system, and one assumes that surface runoff is 

negligible and that errors in estimating atmospheric 

fluxes and lake volume changes are reasonably small 

and unbiased, then graphs of cumulative net precipitation inputs and cumulative changes in lake volume 

should be approximately coincident. However, during 

the winter and spring seasons in 1989 and 1990 and 

the fall season of 1990, the volume of Lake Five-O 

decreased much faster than predicted by atmospheric 

with seasonal patterns of precipitation in northwestern 

Florida. The computed monthly net ground-water 

flow peaked during the summer wet season in 1989 

and 1990 (fig. 12). The large, positive peak for the 

summer of 1989 resulted from the historically high 

precipitation during this period. The effect of precipitation on ground-water inflow and leakage is evident 

when the shaded areas above and below the line 

defined by are evaluated (fig. 12). The total 

area above this “zero line” represents the sum of all 

months with positive values of net ground-water flow, 

and the total area below the “zero line” represents the 

sum of all months with negative values of net groundwater flow. During the wet year of 1989, the areas 

above and below the line are approximately equal, 

which indicates that ground-water inflow was approximately equal to leakage during 1989. However, the 

shaded area above the “zero line” was much smaller 

70,000

-40,000

-30,000

-20,000

-10,000

0

10,000

20,000

30,000

40,000

50,000

60,000

NET GROUND-WATER FLOW TO LAKE FIVE-O, IN CUBIC METERS

J F M A M J J A S O N D J F M A M J J A S O ND

1989 1990

MONTHLY NET GROUND-WATER FLOW WITH

99-PERCENT CONFIDENCE INTERVAL ERROR BAR

Figure 12. Computed montly net ground-water flow to Lake Five-O during 1989-90.

Q

ˆ net = 0
Ground-Water Flow System Near Lake Five-O 21

fluxes alone. In addition, the lake volume increased 

much faster than predicted by net atmospheric inputs 

during the summer of 1989, and, to a lesser extent, 

during the summer of 1990. These observations 

indicate that ground-water inflow and leakage are 

significant components of the water budget of Lake 

Five-O.

The net ground-water flow estimates also 

provide some insight into the magnitude of the components of net ground-water flow, ground-water inflow 

and leakage. This is accomplished by noting that 

negative values of net ground-water flow represent 

minimum estimates of leakage, and positive values of 

net ground-water flow represent minimum estimates 

of ground-water inflow (Pollman and others, 1991). 

Thus, the monthly net ground-water flow values 

indicate that monthly inflow and leakage rates of at 

least 5.4 x 104 and 2.9 x 104 m3, respectively, 

occurred during the study period (table 2). Because 

Figure 13. Cumulative daily net-precipitation inputs and cumulative daily lake-volume changes for Lake Five-O, 1989-90.

140,000

120,000

100,000

80,000

60,000

40,000

20,000

0

-20,000

-40,000

-60,000

-80,000

-100,000

CUMULATIVE VOLUME, IN CUBIC METERS

J F M A MJ J A S ON D J FM A M J J A S O N D

1989 1990

LAKE VOLUME CHANGE

NET PRECIPITATION

(PRECIPITATION

MINUS EVAPORATION)

the head difference between Lake Five-O and the 

Upper Floridan aquifer was greater than or near 1.7 m 

for most months in the study period, the large negative 

net ground-water flow values of -26,700 to - 23,700 

m3

observed in January-March 1989 (when the Lake 

Five-O-Upper Floridan aquifer head difference was 

approximately 1.7 m) should represent minimum leakage estimates for most months. This indicates that 

leakage from Lake Five-O was at least 760 to 880 m3/d 

during 1989-1990, which represents at least 68 to 

71 percent of the total outflow (lake evaporation plus 

leakage) from the lake during 1989-90.

Estimates of minimum annual ground-water 

inflow and leakage during the study period can be 

made by summing months with positive net groundwater flow values and assigning these to ground-water 

inflow, and by summing months with negative net 

ground-water flow values and assigning these to 

ground-water outflow (Pollman and others, 1991). 
22 Evaluation of Ground-Water Flow and Hydrologic Budget for Lake Five-O, A Seepage Lake in Northwestern Florida

This procedure results in estimates of minimum 

annual ground-water inflow of 35 percent and 

1 percent of the total inflow (precipitation plus 

ground-water inflow) budget for 1989 and 1990, 

respectively. Minimum annual leakage was estimated 

to be 45 percent and 56 percent of total outflow for 

1989 and 1990, respectively. This approach should 

yield estimates of ground-water inflow and outflow 

that are well below expected values, because it 

assumes that ground-water inflow and leakage do not 

occur simultaneously. Head differences between the 

near-lake wells and Lake Five-O, and between Lake 

Five-O and the Upper Floridan aquifer indicate that 

significant ground-water inflow and leakage occurred 

concurrently, throughout the study period.

An estimate of minimum long-term average 

ground-water inflow to Lake Five-O can be derived 

from estimates of leakage from the above analysis, and 

average annual precipitation minus lake evaporation 

(net precipitation). The equation for this calculation is 

as follows:

, (13)

where is average annual ground-water inflow; 

 is average annual leakage; is average 

annual net-precipitation; is average annual lake 

surface area; and is average annual lake volume 

change, which is assumed to be zero.

Assuming that (1) average annual net precipitation is equal to 39 cm (see discussion of boundary 

conditions), (2) the average annual lake surface area 

equals 10.9 hectares (average area during the study 

period), and (3) the average annual leakage is at least 

880 m3/d (estimated minimum leakage rate for most of 

1989-90), then the minimum average annual groundwater inflow is estimated to be at least 760 m3

/d, or 

62 percent of the estimated average annual total inflow 

to Lake Five-O.

SIMULATION OF GROUND-WATER FLOW

A computer program by McDonald and 

Harbaugh (1988) was used to simulate ground-water 

flow within the surficial aquifer and intermediate 

confining unit near Lake Five-O. The program uses 

a finite difference scheme to integrate the equation for 

three-dimensional, saturated ground-water flow under 

equilibrium (steady-state) or nonequilibrium (transient) conditions. The resulting system of equations 

was solved using either the “strongly implicitly procedure” (McDonald and Harbaugh, 1988, p. 12-1), or the 

“preconditioned conjugate-gradient” method (Hill, 

1990). A subprogram that allows for time-variant 

specified-head boundaries (Leak and Prudic, 1991) 

was also used in conjunction with the main modeling 

program. The following sections describe the development and calibration of the steady-state and 

transient ground-water flow models, and the results of 

flow-path simulations using the calibrated flowfields.

Model Development

Three-dimensional models were developed to 

represent the Lake Five-O ground-water system under 

steady-state and transient conditions. The horizontal 

grid and boundary conditions used in the steady-state 

and transient models are shown in figure 14. The horizontal grid is composed of 81 rows and 57 columns. 

All rows in the grid have a constant width of 20 m. 

Columns 7 through 67 also have a constant width of 

20 m, and columns 4 through 1 progressively increase 

in size westward from 30 to 100 m (expansion factor 

of 1.5). In vertical section, the numerical models 

represent the sediments from the land surface to the 

uppermost part of the Upper Floridan aquifer and are 

discretized into seven horizontal layers of varying 

thickness (fig. 15). Layer 1 was simulated as a watertable layer, whereas layers 2-6 were represented as 

confined layers. Layer 7 was treated as a specifiedhead boundary representing the Upper Floridan 

aquifer.

The model grid described above makes it 

possible to approximate the geometry of the major 

lithologic contacts and aquifer boundaries. The extent 

of the surficial aquifer and intermediate confining unit 

is approximated by assigning a single value or limited 

number of values of hydraulic properties to a section 

of the model domain (parameter zone) representing 

these lithologic units (fig. 15). Several parameter 

zones were also used to represent hydraulic property 

variations within the surficial aquifer and intermediate 

confining unit, or within model layers (fig. 15). A 

basal clay zone is used to represent the dense, lowpermeability clay layer at the base of the intermediate 

confining unit. A plateau zone was introduced during 

the calibration process to represent increased 

anisotropy in the upper 5 to 10 m of the intermediate 

confining unit in the plateau area. Another parameter 

zone (the transition zone) was introduced to more 

Q

ˆ in Qˆ out Pˆ Eˆ ( ) – Aˆ – avg ∆Vˆ = +

Q

ˆ in

Q

ˆ out Pˆ Eˆ –

A

ˆ avg

∆V

ˆ
Simulation of Ground-Water Flow 23

accurately represent the geometry of the contact 

between the surficial aquifer and intermediate confining 

unit where this contact dips toward the breaches in the 

intermediate confining unit. A lakebed sediment zone 

is used to represent lower permeability sediments 

EXPLANATION

INACTIVE AREA OF MODEL GRID

ACTIVE AREA OF MODEL GRID: AQUIFER CELLS

ACTIVE AREA OF MODEL GRID: LAKE CELLS

NO-FLOW BOUNDARY

WELL LOCATION

Little

Island

Lake

Big

Island

Lake

Lake

Five-O

Rattlesnake

Pond

Campers

Pond

Itty Bitty

Pond

0 1/4 MILE

0 0.25 KILOMETERS

30°25′30′′

30°25′

85°40′ 85°39′30′′

20

1

1

10

20

30

40

50

60

70

80

10 20 30 40 50 57

COLUMNS

ROWS

Figure 14. Areal discretization and boundary conditions for simulation models of the groundwater flow system near Lake Five-O.

(relative to the surficial aquifer) in surficial aquifer cells 

contiguous with and beneath the lake in model layers 3 

and 4. The top of the Upper Floridan aquifer, which 

defines the lower boundary of the simulation models, is 

set at a constant altitude of 13.7 m below sea level.
24 Evaluation of Ground-Water Flow and Hydrologic Budget for Lake Five-O, A Seepage Lake in Northwestern Florida

Lake Five-O is represented in the model by a 

zone of highly conductive material in layers 1-4 

(fig. 15). For transient simulations, the storage properties of this lake zone are identical to those of water. 

The physical extent of this lake zone was determined 

with data from the bathymetric survey and by reproducing the relation between lake stage and lake 

volume that was developed from this survey.

Within-layer variations in horizontal hydraulic 

conductivity are accounted for by using “equivalent” 

hydraulic conductivity values (Freeze and Cherry, 

1979, p. 33), which were computed using a weighted 

mean algorithm (Freeze and Cherry, 1979, p. 34). 

For example, the transmissivity of cells in the 

Vertical exaggeration x10

0 100 200 300 FEET

0 50 100 METERS

EXPLANATION

PLATEAU ZONE

BASAL CLAY ZONE {

{

Kv = 3 x 10-5 meters/day (m/d)

Kh = 0.61 meters/day (m/d)

Kv = 0.0012 meters/day (m/d)

TRANSITION

ZONE

LAKE BED

SEDIMENT ZONE {

{

Kh = 18 meters/day (m/d)

Kv = 0.11 meters/day (m/d)

upper half = surf. aquifer

lower half = int. conf. unit

FREE-SURFACE BOUNDARY (RECHARGE FLUX SPECIFIED OVER THE SURFICIAL

AQUIFER; NET PRECIPITATION FLUX SPECIFIED OVER LAKE FIVE-O).

SPECIFIED-HEAD BOUNDARY

A A′

LAYER

1

2

3

4

5

6

7

METERS

12.2

9.1

4.6

SEA

LEVEL

6.1

13.7

RECHARGE RECHARGE RECHARGE RECHARGE RECHARGE NET PRECIP NET PRECIP

UPPER FLORIDAN AQUIFER

LAKE FIVE-O

SURFICIAL AQUIFER { Kh = 18 m/d

Kv = 1.8 m/d

Kh = 1500 m/d

Kv = 11-18 m/d

INTERMEDIATE

CONFINING UNIT { Kh = 0.61 m/d

Kv = 0.0061 m/d

Figure 15. Vertical discretization, boundary conditions, and calibrated values of hydraulic conductivity for simulation 

models of the ground-water flow system near Lake Five-O.

transition zone was computed as follows:

(14)

where is transmissivity, is the layer thickness, 

 is the equivalent horizontal hydraulic conductivity, and and are the horizontal hydraulic 

conductivities for the surficial aquifer and intermediate 

T LK = h eq ,

= L

Kh SA , ( )L

2

--

L ---------------------------

Kh ICU , ( )L

2

--

L

+ -----------------------------

         

= L

Kh SA ,

2 ------------- Kh ICU ,2 + ---------------    

T L

Kh eq ,

Kh SA , Kh IC , U
Simulation of Ground-Water Flow 25

confining unit, respectively. Vertical flow between 

model layers is simulated using the leakance parameter (vertical hydraulic conductivity divided by flow 

path distance). Leakance values were calculated (from 

equivalent vertical hydraulic conductivity values) 

using a harmonic mean algorithm (Freeze and 

Cherry, 1979, p. 34; McDonald and Harbaugh, 1988, 

p. 5-12, equation 49) when vertical hydraulic conductivity varied within a model layer or across contiguous 

model layers.

The boundary conditions for the simulation 

models (figs. 14 and 15) are identical to those 

described in the previous discussion of ground-water 

flow near Lake Five-O. Lateral no-flow boundaries 

encircle the ground-water basin and are coincident 

with water-table divides previously described. Freesurface boundaries are used for the upper surface of 

the models (water-table boundary for the aquifer and a 

lake-surface boundary for Lake Five-O). The use of a 

free-surface boundary at the lake made it possible to 

predict lake stage and volume fluctuations in response 

to changing hydrologic conditions, and also greatly 

simplified calibration of the transient model. 

Recharge and net precipitation functions are used to 

represent the flux of water across the upper surface of 

the models. Net precipitation was computed from 

precipitation and evaporation data, as previously 

discussed. Recharge was determined by model calibration using values within limits indicated by base 

flow, precipitation, well hydrograph, and chloride 

data. Upper Floridan aquifer head values (representing the specified head boundary at the base of the 

model) were obtained by averaging measured heads at 

wells w1.5 and w13.3. Values of recharge, net precipitation, and Upper Floridan aquifer head are constant 

in space and time for the steady-state models, and constant in space, variable with time for the transient 

model. Although heads were not computed by the 

model for layer 7 because of the specified-head 

boundary condition, this layer is active because it can 

vary with time and can act as a source or sink of water 

for the overlying active layers.

Calibration of Steady-State Models

Four steady-state models were calibrated to 

hydrologic conditions observed on four different 

dates: December 12, 1988; May 9, 1989; October 4, 

1989; and August 6, 1990. These models were developed to satisfy one or more of the following 

objectives: (1) refine the “premodeling” or prior 

estimates of hydraulic properties and their spatial 

distribution, (2) provide initial conditions for transient 

simulations, and (3) provide flowfields necessary for 

evaluation of flow paths and residence times of ground 

water that discharges to Lake Five-O (ground-water 

inflow). The model calibrated to December 12, 1988, 

conditions was developed with objectives (1) and 

(2) in mind. The August 6, 1990, steady-state model 

was developed to provide initial conditions for a preliminary transient simulation of the latter part of the 

study period (August 6, 1990 through January 22, 

1991). The rationale for this transient simulation is 

discussed in a later section that describes the calibration of the full transient model (conditions from 

December 12, 1988 through January 22, 1991). 

Finally, steady-state models of May 9, 1989, and 

October 4, 1989, were developed to evaluate groundwater flow paths and residence times for low waterlevel and high water-level conditions, respectively.

The steady-state models were calibrated by 

comparing simulated and observed heads, and by 

comparing the simulated leakage from Lake Five-O 

with the minimum estimate of leakage from the preceding section describing net ground-water inflow to 

Lake Five-O. The head calibration criteria was set at 

 m, which represents approximately 5 percent of 

the total head loss within the shallow ground-water 

flow system near Lake Five-0. Simulated leakage 

from Lake Five-O was also required to be greater than 

880 m3/d for an acceptable calibration.

Although the real ground-water system is never 

at equilibrium, this condition can be approached 

during sustained hydrograph peaks or plateaus, where 

inputs to the system are approximately equal to outputs, and at the end of extended hydrograph recessions, when storage changes over time are small. 

One of these conditions was present during each of 

the above calibration dates (fig. 6). Steady-state 

conditions were most closely approximated for 

December 12, 1988, and two factors indicate that head 

distribution on this date may also be representative of 

long-term average conditions. First, local precipitation data and head data from the long-term Upper 

Floridan aquifer well at Greenhead, Fla. suggest that 

antecedent conditions (as estimated by 1987 and 1988 

data) at Lake Five-O were comparable to long-term 

average conditions. Second, heads in the Lake Five-O 

ground-water system on December 12, 1988, were 

probably similar to their long-term averages. 

±0.3
26 Evaluation of Ground-Water Flow and Hydrologic Budget for Lake Five-O, A Seepage Lake in Northwestern Florida

This conclusion is based, in part, on the similarity of 

heads in the surficial aquifer on December 12, 1988, 

to average heads during the study period. The average 

heads during the study period are assumed to be representative of long-term average conditions because 

study-period precipitation and evaporation rates were 

similar to their respective long-term averages. Additionally, the head in the Greenhead well on December 

12, 1988, was approximately equal to the long-term 

average value, which indicates that the head in the 

Upper Floridan aquifer at Lake Five-O on December 

12, 1988, may have been representative of long-term 

average conditions. The December 12, 1988, model 

was the first of the steady-state models to be calibrated.

During the first attempts at calibrating the 

December 12, 1988, steady-state model, the location 

of the southwestern limit of the lateral no-flow boundary was based on the extrapolated water-table contours 

given by Andrews and others (1990), and was approximately concident with the unimproved road in this 

area. Values of hydraulic conductivity, leakance, and 

recharge were varied within preestablished ranges, but 

all configurations consistently underestimated heads 

observed south of Lake Five-O. This problem of 

underestimating heads was solved by extending the 

southwestern limits of the no-flow boundary into the 

plateau area southeast of Bream Lake (figs. 2 and 8). 

The new location of this boundary was approximately 

coincident with a topographically defined basin 

boundary in this area, and was supported by head data 

from subsequent placement of a shallow well (w32) 

west of well nest 1.

After adjusting the location of the southwestern 

no-flow boundaries, calibration efforts resumed with 

a simple model configuration with uniform distributions of horizontal hydraulic conductivity (Kh = 18 

and 0.06 m/d in the surficial and intermediate confining unit, respectively) and anisotropy (Kh: Kv = 10:1) 

within each unit, and a single value of vertical hydraulic conductivity for the basal clay unit (Kv = 7 x 10-6

m/d). The root mean square error of the head 

differences (head RMSE) was within the calibration 

criteria at this point, but the simulated heads were low 

for the lake (-0.40 m) and high in the intermediate 

confining unit wells, w1.3 and w1.4 (approximately 

0.50 m). Net precipitation was increased over the lake 

to raise the simulated lake stage, but lake stage showed 

little sensitivity to net precipitation. The lake stage 

was more sensitive to anisotropy in the deeper lake 

sediments, and a suitable head match for the lake 

and surficial wells was obtained by increasing the 

anisotropy of the lake sediments from 10:1 to approximately 140:1 in layers 3 and 4.

The final adjustments to the steady-state model 

centered on improving the simulation of the head loss 

that occurred across the contact between the surficial 

aquifer and intermediate confining unit at well nest 1 

(defined by the head difference between wells w1.2 

and w1.3). Some improvement was achieved by 

increasing the horizontal hydraulic conductivity in the 

intermediate confining unit from 0.06 m/d to 0.6 m/d. 

Recharge was then increased to 0.65 m/yr to increase 

heads in the surficial aquifer and lake, which had 

dropped moderately (because of the increase in the 

horizontal hydraulic conductivity in the intermediate 

confining unit). The final model configuration was 

obtained by increasing the anisotropy of the plateau 

parameter zone by a factor of 5 (which reduced the 

leakance between the surficial aquifer and intermediate confining unit in the plateau area by a factor of 5). 

This reduction was also supported by trial runs of the 

transient model, which showed an improved head 

response for wells w1.3 and w1.4. The calibrated 

values of hydraulic conductivity are shown in figure 15.

The head RMSE for the final configuration of 

the December 12, 1988, model was 0.16 m, and simulated heads for this configuration were within -0.13 to 

0.27 m of observed heads for all wells except w3.1 and 

w3.2, which were underestimated by 0.38 and 0.31 m, 

respectively. For this final model configuration, simulated leakage from Lake Five-O to the contiguous 

ground-water system was 1,900 m3

/d, which was 

greater than the estimated lower limit of 800 m3/d. 

Simulated ground-water inflow to Lake Five-O was 

1,800 m3/d.

Calibrations of the other steady-state models 

(for simulating conditions on May 9, 1989, October 

10, 1989, and August 6, 1990) were achieved by 

adjusting the recharge and net precipitation rates only. 

Simulated leakage was greater than the 800 m3

/d minimum in all of these models. For the low-water model 

(May 9, 1989), recharge and net precipitation were set 

at 0.60 and -0.25 m/yr, respectively. The head RMSE 

for this match was 0.18 m, and simulated heads were 

within -0.21 to +0.11 m of observed heads for all wells 

except 28, 3.1 and 3.2, which were underestimated by 

0.66, 0.37 and 0.37 m, respectively. For the highwater model (October 10, 1989), recharge and net precipitation were set at 0.66 and 0.25 m/yr, respectively. The head RMSE for this match was 
Simulation of Ground-Water Flow 27

0.23, and simulated heads were within -0.18 to 

+0.30 m of observed heads for all wells except 28, 3.1, 

and 3.2, which were underestimated by 0.84, 0.39, 

and 0.34 m, respectively. For the simulation of the 

summer 1990 hydrograph peak (August 6, 1990, 

model), values of recharge and net precipitation were 

identical to those used in the simulation of conditions 

on October 10, 1989. The head RMSE for this match 

was 0.22 m, and simulated heads were within -0.52

 to +0.27 m of observed heads for all wells. The 

poorer correspondence of simulated and observed 

heads for this simulation is attributed to the brevity of 

18.65

17.98

0 1/4 MILE

0 0.25 KILOMETERS

30°25′30′′

30°25′

85°40′ 85°39′30′′

20

15.44

17.46

16.77

17.71

16.31

17.25

17.93

17.55

16.92

16.60 16.20

15.80

15.51

15.32

15.29

15.35

15.32

15.40

15.38

15.39

15.14

15.14

14.90

14.77

14.85

16.00

18.92

19.55 17.92

15.97

15.96

15.82

15.40

15.87

15.71

21

17

20

19

18

16

15.29

EXPLANATION

17.92

SIMULATED LINE OF EQUAL WATER-TABLE

ALTITUDE—In meters above sea level.

UNIMPROVED ROAD

WELL LOCATION AND WATER-TABLE

ALTITUDE (HEAD)—In meters above sea level.

LAKE SURFACE ALTITUDE—In

meters above sea level

18.23

16

Lake

Five-O

Big

Island

Lake

Little

Island

Lake

Rattlesnake

Pond

Campers

Pond

Itty Bitty

Pond

Figure 16. Simulated and observed water levels in the surficial aquifer near Lake Five-O, October 4, 1989.

the summer 1990 peak, which provided less time for 

the system to approach steady-state conditions. The 

simulated water table for conditions observed on 

October 4, 1989, is represented by the contour map 

shown in figure 16. Similar water table configurations were evident in the steady-state simulations of 

conditions on December 12, 1988, May 9, 1989, and 

August 6, 1990.

The sensitivity of the steady-state models to 

changes in model inputs was examined by varying 

model input variables, such as recharge and hydraulic 

conductivity, and comparing the model output to 
28 Evaluation of Ground-Water Flow and Hydrologic Budget for Lake Five-O, A Seepage Lake in Northwestern Florida

that of the calibrated model for conditions on 

December 12, 1988. The input variables were varied 

within their probable ranges (established prior to 

modeling); therefore, statements regarding the sensitivity of the models to a given variable are only valid 

for the probable range established for that variable. 

Three model outputs were examined in the sensitivity 

analysis: simulated heads, ground-water inflow to 

Lake Five-O, and leakage from Lake Five-O. The 

head RMSE statistic was the primary means of evaluating the sensitivity of simulated heads (head 

response) to changes in model inputs, and differences 

between simulated heads and heads observed on 

December 12, 1988, were used to compute this statistic.

The head response of the steady-state models 

was most sensitive to changes in recharge, horizontal 

hydraulic conductivity and anisotropy of the surficial 

aquifer, and horizontal hydraulic conductivity of the 

intermediate confining unit (Kh, ICU). The sensitivity 

of the models to these variables is evident in the steep 

slopes of the head sensitivity curves (relations 

between head RMSE and change from calibrated 

value) for these variables (figs. 17 and 18). Despite 

1.4

0

0.2

0.4

0.6

0.8

1.0

1.2

ROOT MEAN SQUARE ERROR OF SIMULATED HEAD,

IN METERS

CHANGE FROM CALIBRATED VALUE,

AS A MULTIPLE OF CALIBRATED VALUE

0.000001 0.00001 0.0001 0.001 0.01 0.1 1 10

HORIZONTAL HYDRAULIC

CONDUCTIVITY,

SURFICIAL AQUIFER

HORIZONTAL HYDRAULIC

CONDUCTIVITY,

INTERMEDIATE

CONFINING UNIT

LEAKANCE, LAYER 6

0

0.2

0.4

0.6

0.8

1.0

ROOT MEAN SQUARE ERROR OF

SIMULATED HEAD, IN METERS

CHANGE FROM CALIBRATED VALUE,

AS A MULTIPLE OF CALIBRATED VALUE

0.001 0.01 0.1 1 10 100

RECHARGE

ANISOTROPY,

SURFICIAL

AQUIFER

ANISOTROPY,

INTERMEDIATE

CONFINING UNIT

ANISOTROPY,

LAKE SEDIMENTS

Figure 17. Sensitivity of steady-state models of groundwater flow system near Lake Five-O to changes in 

horizontal hydraulic conductivity and leakance of layer 6.

Figure 18. Sensitivity of steady-state models of groundwater flow system near Lake Five-O to changes in recharge 

and anisotropy.

the relatively gentle slope of the Kh, ICU head sensitivity curve for values within 0.05 to 1 times the 

calibrated value of Kh, ICU (fig. 17), heads in wells 

screened within the intermediate confining unit were 

quite sensitive to changes in Kh, ICU over this range. 

For example, when the Kh, ICU was set to 0.05 times 

the calibrated value, the difference between the 

simulated and observed head at well w1.4 increased 

from -0.08 to -0.86 m, whereas the head RMSE only 

increased from 0.16 to 0.29 m. The smaller head 

RMSE change (relative to the change in the heads of 

the intermediate confining unit wells) is explained by 

the fact that (1) the head RMSE statistic is more 

heavily weighted toward heads in the surficial aquifer, 

because more wells were constructed in the surficial 

aquifer than in the intermediate confining unit, and 

(2) heads in the surficial aquifer were much less 

sensitive to changes in Kh, ICU than were heads in the 

intermediate confining unit.

The head response of the steady-state models 

was only partially sensitive to changes in the anisotropy of the intermediate confining unit, and insensitive to changes in net-precipitation and the leakance of 
Simulation of Ground-Water Flow 29

the base of the intermediate confining unit (figs. 17 

and 18). Heads in the intermediate confining unit 

changed appreciably when the anisotropy of the intermediate confining unit was increased to 10 times the 

calibrated value (although heads in the surficial 

aquifer, and hence head RMSE, increased only 

moderately). However, the heads did not change 

appreciably when the anisotropy of the intermediate 

confining unit was decreased by a factor of 10 

(fig. 18).

Rates of ground-water inflow to and leakage 

from Lake Five-O were generally insensitive to 

changes in model input variables, with ground-water 

inflow and leakage rates generally within 3 percent 

of the calibrated values. Ground-water inflow and 

leakage rates were most sensitive to changes in 

recharge and lakebed anisotropy. The largest changes 

in ground-water inflow and leakage rates were a 

35 percent increase in ground-water inflow and leakage (resulting from a 25 percent increase in recharge) 

and an 8 percent decrease in ground-water inflow and 

leakage (resulting from an order of magnitude increase 

in lakebed anisotropy).

A final steady-state modeling exercise was 

conducted to evaluate changes in ground-water inflow 

and leakage rates that might occur during several years 

of above average or below average precipitation. 

Estimates of the 90th percentiles of average annual 

precipitation rates, net precipitation rates, and Upper 

Floridan aquifer head were used to represent values 

that might occur during a typical wet period, and 

estimates of the 10th percentiles of these stresses were 

used to represent values that might occur during a 

typical dry period. Recharge rates for the wet or dry 

period simulations were obtained by multiplying the 

wet or dry period precipitation rates by the ratio of the 

recharge rate used in the steady-state simulation of 

conditions during December 12, 1988, and the longterm average precipitation rate. The results of this 

analysis indicate that ground-water inflow and leakage 

rates might increase by approximately 30 percent from 

the values obtained from the December 12, 1988, 

model during extended wet periods, and decrease by 

approximately 30 percent during extended dry periods.

Calibration of Transient Model

Calibration of the transient model consisted of 

determining values of storage properties and developing a function that defined temporal changes in 

recharge rates from December 12, 1988, to January 22, 

1991. The hydraulic conductivity distributions in the 

transient model were identical to those in the calibrated steady-state models. Temporal discretization 

was accomplished by dividing the simulation period 

into 43 stress periods, and assigning recharge rates, net 

precipitation rates, and beginning and ending Upper 

Floridan aquifer head values to each of these periods 

(table 3). During summer wet periods, stress period 

intervals ranged from 1 day to 2 weeks, and were 

defined by periods of little or no precipitation and 

periods of more intense precipitation. During the 

remainder of the study period, when rainfall was more 

moderate, stress period intervals were approximately 

1 month in length. The quality of the transient calibration was determined by comparing the shape of simulated and observed well hydrographs, and by 

comparing simulated and computed monthly net 

ground-water flow volumes.

Calibration of the transient model began with a 

storage property calibration in which values of specific yield in layer 1 and specific storage in layers 2-6 

were systematically adjusted in an effort to simulate 

the slope of well hydrographs during period the of 

August 6, 1990, to January 22, 1991. Recharge was 

assumed to be negligible during most of this period, 

because rainfall was well below normal and the waterlevel hydrograph for well w6.2 showed a steep and 

generally uninterrupted recession (fig. 6). This 

assumption made it possible to calibrate specific 

yield and specific storage independently of recharge. 

The storage properties of lake cells did not require 

calibration: the specific yield for lake cells in layer 

1 was fixed at a value of 1, and the specific storage

of lake cells in layers 2-4 was fixed at a value of 

4.5 x 10-6 m-1, which is equal to the product of the 

compressibility of water and the specific weight of 

water at 22 °C (4.5 x 10-10 Pa-1 and 9.8 x 103 N/m3

, 

respectively). Eight stress periods were used in the 

storage property calibration, and the duration and 

boundary stresses for all but the first of these stress 

periods coincide with those given for stress periods 36 

through 43 in table 3. The first stress period in the 

storage property calibration was 3 days shorter than 

stress period 36, because the simulation period for the 

storage property calibration began 3 days after the 

beginning of stress period 36. Recharge was also set 

to zero in the first stress period of the storage property 

calibration because rainfall amounts were negligible 

and the hydrograph of well w6.2 showed a steady 
30 Evaluation of Ground-Water Flow and Hydrologic Budget for Lake Five-O, A Seepage Lake in Northwestern Florida

Table 3. Temporal discretization and upper and lower boundary stresses used in the transient model of ground-water flow 

near Lake Five-O

[m/d, meters per day; UFA, Upper Floridan aquifer]

Stress 

period Begin date End date Duration, in days

Recharge rate,

in m/d

Net-precipitation 

rate, in m/d

Beginning

UFA head,

in meters

Ending

 UFA head,

in meters

 1 12-12-88 1-12-89 32 0.00000 -0.00117 12.84 12.61

 2 1-13-89 2-08-89 27 .00000 - .00013 12.61 12.38

 3 2-09-89 3-09-89 29 .00000 .00147 12.38 12.18

 4 3-10-89 4-12-89 34 .00249 .00314 12.18 12.20

 5 4-13-89 5-08-89 26 .00133 - .00064 12.20 12.05

 6 5-09-89 5-31-89 23 .00218 - .00164 12.05 12.10

 7 6-01-89 6-09-89 9 .00295 .02068 12.10 12.24

 8 6-10-89 6-14-89 5 .00311 - .00287 12.24 12.36

 9 6-15-89 6-25-89 11 .01509 .02352 12.36 12.71

10 6-26-89 7-02-89 7 .00774 - .00149 12.71 12.95

11 7-03-89 7-06-89 4 .00606 .02043 12.95 13.09

12 7-07-89 7-11-89 5 .00494 - .00280 13.09 13.23

13 7-12-89 7-23-89 12 .00328 .00791 13.23 13.47

14 7-24-89 8-06-89 14 .00288 - .00421 13.47 13.59

15 8-07-89 8-08-89 2 .00166 .02398 13.59 13.61

16 8-09-89 9-05-89 28 .00073 .00358 13.61 13.68

17 9-06-89 10-03-89 28 .00222 .00040 13.68 13.67

18 10-04-89 11-08-89 36 .00220 .00163 13.67 13.67

19 11-09-89 12-12-89 34 .00161 - .00073 13.67 13.62

20 12-13-89 1-16-90 35 .00222 .00169 13.62 13.64

21 1-17-90 2-06-90 21 .00121 - .00094 13.64 13.56

22 2-07-90 3-06-90 28 .00068 .00033 13.56 13.49

23 3-07-90 4-03-90 28 .00041 .00154 13.49 13.41

24 4-04-90 5-01-90 28 .00027 - .00079 13.41 13.21

25 5-02-90 6-05-90 35 .00056 - .00165 13.21 12.95

26 6-06-90 6-10-90 5 .00134 .01065 12.95 12.91

27 6-11-90 6-21-90 11 .00104 - .00168 12.91 12.85

28 6-22-90 6-24-90 3 .00210 .02421 12.85 12.84

29 6-25-90 6-30-90 6 .00108 - .00202 12.84 12.83

30 7-01-90 7-03-90 3 .00135 .00667 12.83 12.82

31 7-04-90 7-06-90 3 .00144 - .00404 12.82 12.82

32 7-07-90 7-14-90 8 .00911 .01807 12.82 12.84

33 7-15-90 7-23-90 9 .00506 .00295 12.84 12.85

34 7-24-90 8-01-90 9 .00263 - .00375 12.85 12.86

35 8-02-90 8-02-90 1 .00000 .01522 12.86 12.86

36 8-03-90 8-16-90 14 .00099 - .00467 12.86 12.82

37 8-17-90 8-22-90 6 .00130 .01120 12.82 12.78

38 8-23-90 9-05-90 14 .00000 - .00212 12.78 12.67

39 9-06-90 9-25-90 20 .00000 - .00607 12.67 12.50

40 9-26-90 11-04-90 40 .00000 - .00226 12.50 12.17

41 11-05-90 12-02-90 28 .00000 - .00329 12.17 11.91

42 12-03-90 1-02-91 31 .00000 .00015 11.91 11.64

43 1-03-91 1-22-91 19 .00000 .00260 11.64 11.55
Simulation of Ground-Water Flow 31

decline during this period. The calibrated head distribution from the steady-state simulation of August 6, 

1990, was used as the initial conditions (initial head 

distribution) for the August 6, 1990, to January 22, 

1991 simulation period.

A suitable match to the August 6, 1990, to 

January 22, 1991, hydrograph recession was obtained 

by using a constant value of specific yield of 0.14 for 

aquifer cells in layer 1, and a constant specific storage 

value of 3 x 10-6 m-1 for aquifer cells in layers 2-6. 

Storativity values for individual model layers (in 

layers 2-6) were then calculated by multiplying layer 

thickness by the calibrated specific storage value of 

3 x 10-6 m-1. Specific yield was determined by initially setting specific storage to 3 x 10-6 m-1 and evaluating the correspondence between simulated and 

measured head declines for specific yield values of 

0.10 to 0.30. The calibrated value of specific yield 

was then used in subsequent model runs in which specific storage was varied within the predefined limits. 

None of the head matches from these runs, however, 

showed significant improvement over those obtained 

with specific storage equal to 3 x 10-6 m-1. The calibration process for specific yield and specific storage 

is illustrated in figure 19, which depicts the RMSE of 

predicted hydrograph slope (slope RMSE) for various 

values of specific yield and specific storage (at an 

abscissa coordinate equal to one, specific yield and 

specific storage are equal to their respective calibrated 

values). Figure 19 indicates that the calibrated values 

of specific yield and specific storage yielded the minimum slope RMSE values, and that the model was 

much more sensitive to changes in specific yield than 

to changes in specific storage.

The final calibration process consisted of 

systematically adjusting the recharge function to 

reproduce well hydrographs and monthly estimates of 

net ground-water flow during 1989 and 1990. Initial 

recharge estimates for each stress period were computed using equation 5 and the calibrated value of 

specific yield (from the first phase of the transient 

calibration). This preliminary recharge function was 

checked for mass balance errors and adjusted during 

the transient calibration to achieve acceptable 

hydrograph and net ground-water flow matches. The 

calibrated recharge function fell within upper and 

lower preliminary estimates (based upon calibrated 

specific yield and minimum and maximum water-table 

recession rates) for all months, with the exception of 

June 1989 and July 1990, when recharge rates were 

highest.

Simulated and measured heads are shown for 

Lake Five-O and selected wells in figure 20. 

Simulated well hydrographs reproduced the shape of 

measured hydrographs for almost all of the wells. 

Simulated heads were generally within 0.30 m of 

observed heads during the study period. Head differences at some wells were as large as 0.43 to 0.52 m 

near the summer 1989 and 1990 hydrograph peaks. 

The largest differences between observed and simulated heads consistently occurred at wells w3.1, w3.2, 

and w28, where heads were generally underestimated 

by 0.36, 0.36, and 0.64 m. Simulated heads at Lake 

Five-O and the near-lake piezometer nests were generally within 0.15 m of measured heads, with maximum 

differences less than 0.30 m.

0

0.01

0.02

0.03

0.04

ROOT MEAN SQUARE ERROR OF SIMULATED WELL-HYDROGRAPH SLOPE

(AUGUST 6, 1990 TO JANUARY 3, 1991), IN METERS PER DAY

CHANGE FROM CALIBRATED VALUE,

AS A MULTIPLE OF CALIBRATED VALUE

0.1 1 10 100 1,000 10,000

SPECIFIC YIELD

SPECIFIC

STORAGE

Figure 19. Sensitivity of transient model of the groundwater flow system near Lake Five-O to changes in aquifer 

storage properties.
32 Evaluation of Ground-Water Flow and Hydrologic Budget for Lake Five-O, A Seepage Lake in Northwestern Florida

Simulated and computed net ground-water flow 

to Lake Five-O is shown in figure 21. Simulated net 

ground-water flow values were within the 99 percent 

confidence intervals of the computed values for 18 of 

the 24 months. The largest absolute differences 

between simulated and computed net ground-water 

flow occurred in January, June, July and September of 

1989. None of these differences were unreasonably HEAD, IN METERS ABOVE SEA LEVEL

HEAD, IN METERS ABOVE SEA LEVEL

19

18

17

16

15

14

13

x

x

x

x

x x

xx x x

x xx x x x

x x x x x

x

xx

x

x

x

x

W4

W5

W19

20

19

18

17

16

15

14

13

W1.2

W2.2

W3.2

PZ4.2

x x x x

x x x x x

x

xx

x

x

xx

x

xx x x

x x x x

x

x

x

20

19

18

17

16

15

14

13

W1.4

W28

W20

19

18

17

16

15

14

13

x xx

x x x x x xx x x xxx x x

x

x

x

x

x

W27

W7

PZ2.2

ND J JFMAMJ J A SOND J FMAMJ J A SOND

1988 1990 91 1989

18

17

16

15

14

13

x

x

x x x x x xx x x xxx x

x

xx

x

x

xx

W29

W8

LAKE FIVE-0

ND J JFMAMJ J A SOND J FMAMJ J A SOND

1988 1990 91 1989

18

17

16

15

14

13

x x x

x

x x x x x x x x x x x x x

x

xxx

x

W26

PZ3.1

x

SIMULATED HEAD FROM TRANSIENT MODEL

OBSERVED HEAD IN INDICATED WELLS

WELL NUMBER

EXPLANATION

W8

Figure 20. Simulated and observed heads for Lake Five-O and adjacent ground-water system, December 12, 1988, through 

January 22, 1991.

large when compared to the magnitude of simulated 

ground-water inflow and leakage. Differences 

between simulated and computed monthly net groundwater flow were within -10 and +14 percent of total 

inflow or total outflow for 20 of the 24 months during 

1989 and 1990, and were within percent of the 

total inflow or outflow for all months in 1989 and 

1990. 

±26
Simulation of Ground-Water Flow 33

Thus, even the largest differences between simulated 

and computed net ground-water flow represented less 

that 30 percent of total inflow to, or total outflow from 

Lake Five-O. The temporal distribution of these differences are further described in a later section 

describing the hydrologic budget for Lake Five-O.

Flow-Path Simulations

A particle tracking program (Pollock, 1989) was 

used to evaluate ground-water pathlines (flow paths) 

and residence times for hypothetical parcels of ground 

water (particles) that discharge to Lake Five-O. 

Flow-path simulations were conducted using the head 

distributions from the December 12, 1988, May 9, 

1989, and October 4, 1989, steady-state simulations 

(medium, low, and high water-level conditions respecJ F M A MJ J A S ON D J FM A M J J A S O N D

1989 1990

70,000

-40,000

-30,000

-20,000

-10,000

0

10,000

20,000

30,000

40,000

50,000

60,000

NET GROUND-WATER FLOW TO LAKE FIVE-O, IN CUBIC METERS

COMPUTED NET GROUND-WATER FLOW WITH

99-PERCENT CONFIDENCE INTERVAL ERROR BAR

SIMULATED NET GROUND-WATER FLOW

Figure 21. Simulated and computed monthly net ground-water flow to Lake Five-O, 1989-90.

tively). Porosity values were assumed to be constant 

within a given lithologic unit (surficial aquifer or 

intermediate confining unit). A range of residence 

times was calculated by conducting a series of flowpath simulations in which porosity values were varied 

within the probable ranges established for the surficial 

aquifer (0.25-0.50) and intermediate confining unit 

(0.10-0.50).

Results of a flow path simulation where 

particles were placed along the entire surface of the 

lakebed and allowed to flow in reverse toward their 

points of recharge to the water table are shown in

figure 22. These results indicate that most of the 

surrounding ground-water basin contributes to Lake 

Five-O. Recharge between the contributing area and 

ground-water basin boundaries bypasses Lake Five-O 

and discharges to the Upper Floridan aquifer. The 

residence times for inflow particles (traveltime from 
34 Evaluation of Ground-Water Flow and Hydrologic Budget for Lake Five-O, A Seepage Lake in Northwestern Florida

recharge at the water table to discharge at the lake) 

generally ranged from 0.6 to 9 years, and showed little 

variation between high and low water conditions. 

Mean residence time was estimated to be within a 

range of 3 to 6 years.

Campers

Pond

Big

Island

Lake

Little

Island

Lake

Lake

Five-O

Rattlesnake

Pond

Itty Bitty

Pond

0 1/4 MILE

0 0.25 KILOMETERS

30°25′30′′

30°25′

85°40′ 85°39′30′′

GROUND-WATER FLOW PATH

EXTENT OF CONTRIBUTING AREA

EXPLANATION

INACTIVE AREA OF MODEL GRID

ACTIVE AREA OF MODEL GRID: AQUIFER CELLS

ACTIVE AREA OF MODEL GRID: LAKE CELLS

NO-FLOW BOUNDARY

Figure 22. Contributing area to Lake Five-O, as defined by ground-water flow paths of particles that 

discharge to the lake.

The vertical character of ground-water flow 

within and between the surficial aquifer and intermediate confining unit is shown in figure 23. The 

flowlines are deflected downward at two points 

(stagnation points) an infinitesimal distance outside 
Hydrologic Budget for Lake Five-O 35

the lakebed where the head in the ground-water flow 

system is the same as the altitude of the lake surface 

(figs. 9, 10, and 23) Ground-water inflow to Lake 

Five-O occurs above these points, and leakage from 

the lake to the contiguous ground-water system occurs 

below these points. The location of these stagnation 

points did not change appreciably from low to high 

water conditions. The simulated flow paths in figure 

23 also indicate that almost all of the ground-water 

flow near Lake Five-O occurs within the surficial 

aquifer, and that ground water that discharges to Lake 

Five-O does not move through the intermediate 

confining unit.

HYDROLOGIC BUDGET FOR LAKE FIVE-O

The expression for the hydrologic budget of 

Lake Five-O was previously given in equation 7. 

Big

Island

Lake

Little

Island

Lake

Lake

Five-O

Rattlesnake

Pond

Campers

Pond

Itty Bitty

Pond

B ́

B

LOCATION OF

HYDROGEOLOGIC SECTION B-B’

NEAR LAKE FIVE-0

Vertical exaggeration is x10 0 100 200 300 FEET

0 50 100 METERS

B B′

SURFICIAL AQUIFER

INTERMEDIATE CONFINING UNIT

GROUND-WATER FLOW

PATH (PARTICLE TRACE)

Stagnation Point

Lake Five-O

Stagnation Point

Figure 23. Particle traces projected onto hydrogeologic-section B-B′ near Lake Five-O.

In this expression, five variables were considered significant in the hydrologic budget of Lake Five-O: lake 

volume change, ground-water inflow to the lake, leakage from the lake to the contiguous ground-water 

system, and precipitation and evaporation over the 

surface of the lake. As previously mentioned, lake 

volume changes, precipitation, and evaporation were 

all measured or estimated before development of the 

ground-water models. The estimates of these variables made it possible to estimate net ground-water 

flow prior to modeling, but not the individual components that comprise net ground-water flow: groundwater inflow and leakage. With the calibration of the 

ground-water flow models, it was possible to calculate ground-water inflow to, and leakage from Lake 

Five-O, using the simulated ground-water flowfields 

from the these models. At this stage of the analysis, a 

complete hydrologic budget for Lake Five-O was 

available which explicitly accounted for each of the 
36 Evaluation of Ground-Water Flow and Hydrologic Budget for Lake Five-O, A Seepage Lake in Northwestern Florida

relevant hydrologic variables. The following section 

describes the hydrologic budget for Lake Five-O 

during the 1989-90 study period. Particular emphasis is 

given to the description of temporal and spatial 

variations in ground-water inflow and leakage because 

these variables represent such a large part of the hydrologic budget of Lake Five-O. The relation between the 

estimates of the hydrologic variables 

during the study period and the long-term average 

values of these variables is also discussed, as are 

changes in the hydrologic budget that might occur in 

response to changing boundary conditions.

The results of the ground-water flow simulations 

indicate that the ground-water system is the 

dominant source of water for Lake Five-O. During 

the 1989-90 study period, simulated ground-water 

inflow was estimated to be approximately 1.2 x 106

 m3 

(average inflow of 1.7 x 103 m3/d), which is approximately 4 times larger than estimated precipitation 

inputs (3.4 x 105

 m3) for this period. The simulation 

results also indicate that the ground-water system is the 

dominant sink for water leaving Lake Five-O. Leakage 

from Lake Five-O was estimated to be approximately 

1.4 x 106

 m3 (1.9 x 103 m3/d), which is approximately 5 

times larger than estimated evaporation losses (2.6 x 

105 m3) for this period. The lake volume decreased by 

8.4 x 104

 m3 or 8 percent from January 1, 1989, to 

December 31, 1990, and varied from a minimum of 

9.1x 105

 m3 to a maximum of 

1.1x 106

 m3.

The temporal distribution of water inputs 

(precipitation and ground-water inflow) to Lake Five-O 

indicates that both precipitation and ground-water 

exhibited a strong seasonal dependence, and that Lake 

Five-O received large volumes of ground water 

throughout the study period under a variety of climatic 

conditions (fig. 24). Monthly precipitation inputs 

ranged from approximately 800 to 5.7 x 104

 m3 during 

1989 and 1990, with the largest values occurring 

during the summer wet periods and the smallest values 

occurring during the typically dryer winter, spring, 

and fall seasons. Monthly ground-water inflows to 

Lake Five-O ranged from approximately 

3.5 x 104 to 8.4 x 104

 m3 (fig. 24). The highest simulated ground-water inflow volumes also occurred 

during summer wet periods in 1989 and 1990; the

lowest values occurred during the relatively dry 

winter-spring seasons of 1989 and 1990, and the fall of 

1990 (fig. 24).

 Simulated ground-water inflows were consistently larger than concurrent precipitation inputs to 

the lake. During the wetter than normal year of 1989 

(precipitation was approximately 25 percent above normal), ground-water inflow was approximately 

3 times greater than precipitation. Although simulated 

ground-water inflow decreased moderately in 1990 

(precipitation was approximately 25 percent below normal), the ratio of simulated ground-water inflow VOLUME, IN CUBIC METERS

100,000

80,000

60,000

40,000

20,000

0

-20,000

-40,000 J F MAMJ J AS ON D J FMA M J J A SO ND

1989 1990

RESIDUAL

LAKE VOLUME CHANGE

VOLUME, IN CUBIC METERS

100,000

80,000

60,000

40,000

20,000

0

-20,000

-40,000

LEAKAGE

EVAPORATION

VOLUME, IN CUBIC METERS

100,000

80,000

60,000

40,000

20,000

0

-20,000

-40,000

GROUND-WATER INFLOW

PRECIPITATION

Figure 24. Monthly hydrologic-budget components for 

Lake Five-O, 1989-90.
Hydrologic Budget for Lake Five-O 37

to precipitation (Qi:P) increased to 4.5:1. Similar 

patterns were evident in the distribution of the 

monthly ground-water inflow and precipitation values 

(fig. 24). The ratio, Qi

:P, was lowest during the 

summer wet periods, but ground-water inflow still 

remained larger than precipitation inputs during these 

periods. The largest Qi

:P values occurred during dry 

winter months in 1989 and 1990 and during the dryer 

than normal fall of 1990.

The above relations between ground-water 

inflow and precipitation indicate several characteristics of the temporal distribution of water inputs to 

Lake Five-O. First, the seasonal distribution of 

ground-water inflow is consistent with the typical 

seasonal distribution of precipitation, with the highest 

inflow rates occurring during the wet summer season 

and the smallest inflow rates occurring during the 

dryer winter, spring, and fall seasons. Second, 

ground-water inflow is consistently greater than 

precipitation over monthly and longer time scales. 

Finally, ground-water inflow to Lake Five-O can 

increase relative to precipitation during dry periods.

Seasonal patterns were evident in the temporal 

distribution of evaporation losses, but were not evident 

in leakage losses from Lake Five-O (fig. 24). Monthly 

evaporation losses from Lake Five-O ranged from 3.2 

x 103

 to 1.8 x 104 m3, and showed a strong seasonal 

dependence with the smallest rates occurring in the 

winter months, and the largest rates occurring 

during the summer months (fig. 24). This seasonal 

dependence in the volumetric estimates of evaporation 

from Lake Five-O follows directly from that observed 

in the volume per unit area evaporation estimates of 

Sacks and others (1994). Examination of the seasonal 

distribution of historic estimates of evaporation from 

Lake Five-O (using the Milton NOAA station regression relations) indicates that the seasonal distribution 

of evaporation during the study is representative of the 

typical seasonal distribution. Simulated monthly leakage from Lake Five-O to the contiguous ground-water 

system ranged from 4.4 x 104

 to 6.6 x 104 m3 (fig. 24). 

The monthly leakage values were directly related to 

the head difference between the lake and Upper 

Floridan aquifer (fig. 11), and exhibited no obvious 

seasonality. Minimum leakage rates occurred during 

the summer of 1989, when the head difference 

between the lake and Upper Floridan aquifer was at 

a minimum (1.3 m). Leakage rates returned to their 

pre-summer 1989 levels during the fall of 1989 

through the spring of 1990, and increased to their maximum levels in the summer and fall of 1990, when the 

head difference between Lake Five-O and the Upper 

Floridan aquifer was a maximum (1.9-2.2 m).

Simulated leakage volumes were consistently 

greater than concurrent evaporative losses throughout 

the study period. The ratio of leakage to evaporation 

(Qo:E) was 5.1:1 in 1989, and increased to 5.3:1 in the 

dryer year of 1990. Simulated leakage from Lake 

Five-O exceeded evaporative losses for all months 

during the study, with maximum values of Qo:E occurring during the winter seasons, when lake evaporation 

was at a minimum (fig. 24). These observations indicate that leakage is consistently greater than lake evaporation over monthly and longer timescales. This 

conclusion is analogous to that observed for groundwater inflow and precipitation, and indicates that 

exchanges of water between the ground-water system 

and Lake Five-O are consistently greater than 

atmospheric-lake exchanges.

The temporal distribution of lake volume 

changes (fig. 24) reflects the temporal distribution of 

total inflow (precipitation plus ground-water inflow) 

to Lake Five-O. Monthly lake volume change ranged 

from -3.6 x 104 to 6.9 x 104 m3. Lake volume changes 

were greatest during the dry periods in the winter and 

early spring of 1989, spring 1990, and fall 1990, and 

during the wet periods of the summer and early fall of 

1989 and summer 1990. Lake volume changes were 

smallest from October 1989 through February 1990. 

Although lake volume changes are, by definition, a 

function of inputs and outputs, almost all of the variability in temporal distribution of lake volume change 

can be explained by variations in input. This dependence exists because the temporal distributions of precipitation and ground-water inflow are much more 

variable than those of evaporation and leakage.

The temporal distribution of the residuals from 

the monthly hydrologic budget (difference between 

simulated and computed net ground-water flow) is 

given in figure 24. The magnitude of the residuals in 

the monthly budget ranged from -1.6 x 104

 to 1.2 x 104

m3

. A moderate inverse relation existed between the 

residuals and lake volume changes, in that larger, positive-value residuals usually occurred when lake volume decreased significantly, and smaller, negativevalue residuals usually occurred when lake volume 

increased significantly. Given the above dependence 

of lake volume changes on total inflow, this indicates 

that ground-water inflow is probably underestimated 
38 Evaluation of Ground-Water Flow and Hydrologic Budget for Lake Five-O, A Seepage Lake in Northwestern Florida

in months with large increases lake volume, and overestimated in months with large decreases in lake 

volume.

The spatial distribution of simulated groundwater inflow and leakage indicated little variation 

between high and low water conditions. High rates 

of simulated ground-water inflow occurred around all 

but the northwestern margin of Lake Five-O, with the 

highest rates occurring along the southern margin. 

Although ground-water inflow rates were greatest at 

shallow depths, the simulations indicate that groundwater inflow occurs at appreciable depths in some 

regions of the lake. Approximately 90 percent of the 

total ground-water flow to Lake Five-O was accounted 

for in the upper 6 m of the lake (layers 1 and 2). Simulated leakage was limited to deeper areas of Lake 

Five-O, with approximately 90 percent of total leakage 

occurring through the bottom third of Lake Five-O 

(depths greater than 10 m). The highest rates were 

located in the vicinity of the confining bed breaches.

Comparison of the depth distribution of groundwater inflow during high- and low-water periods also 

indicated that changes in ground-water inflow were 

primarily a result of increases in the saturated thickness of the surficial aquifer rather than major changes 

in head gradients across most of the lakebed. Groundwater inflow rates at depths greater than 1 to 

3 m (layers 2-4) showed no significant changes 

between high- and low-water conditions, which indicates that head gradients were unchanged in deeper 

lakebed areas that receive ground-water inflow. In 

particular, between May 9, 1989, and October 4, 1990, 

when simulated ground-water inflow increased 

approximately 25 percent, shallow head gradients near 

Lake Five-O decreased slightly. This resulted in less 

ground-water inflow to the lake per unit area of lakebed; however, the increase in the inundated area of the 

shallow lakebed more than offset the effects of 

reduced inflow gradients.

Although the study period included unusually 

wet periods and unusually dry periods, the average 

ground-water inflow and leakage rates during the 

study are probably similar to long-term, average 

inflow and leakage rates. This conclusion is supported 

by the similarity between study period average precipitation and net precipitation rates (158 and 37 cm/yr, 

respectively) and long-term average rates (157 and 

39 cm/yr, respectively). Additionally, local precipitation data and head data from the Upper Floridan 

aquifer well at Greenhead suggest that antecedent 

conditions (as estimated by 1987 and 1988 data) at 

Lake Five-O were comparable to long-term average 

conditions. The average ground-water inflow and 

leakage values from the transient model (1.7 x 103

and 1.9 x 103 m3/d, respectively) were also similar to 

the values obtained from the steady-state model of 

conditions on December 12, 1988, (1.8 x 103 and 

1.9 x 103

 m3/d, respectively), which were considered 

representative of long term, average conditions. Thus, 

long-term, average ground-water inflow to, and leakage from Lake Five-O are expected to be approximately 1.7 x 103

 to 1.8 x 103 m3/d and 1.9 x 103 m3/d, 

respectively (approximately 4 and 5 times precipitation and evaporation, respectively).

Long-term average ground-water inflow and 

leakage rates seem to be relatively insensitive to drawdown in the Upper Floridan aquifer, which could result 

from increased pumpage of the aquifer. Potential 

effects of drawdown were evaluated in a series of 

steady-state simulations in which the head assigned to 

the lower boundary was systematically lowered. 

Average annual recharge and net precipitation rates 

were used in all of these simulations, and the location 

of the lateral (ground-water divide) boundaries were 

assumed to be unaffected by the imposed drawdowns. The results of the simulations indicated that 

drawdowns of up to 5 m below the long-term average 

head in the Upper Floridan aquifer would produce 

only modest declines in ground-water inflow and leakage rates. At the maximum drawdown of 5 m, groundwater inflow and leakage decreased by only 7 percent, 

despite the fact that lake stage and volume decreased 

by approximately 4 m and 50 percent, respectively. 

This small decline in ground-water inflow and leakage 

rates may be explained by two factors. First, the total 

input to the flow system (recharge to the surficial 

aquifer plus net precipitation over Lake Five-O) was 

virtually constant for all of the drawdown scenarios. 

Second, potentially larger reductions in ground-water 

inflow and leakage rates (because of lowered water 

levels and corresponding reductions in the area of the 

lakebed through wich ground-water inflow and leakage occur) were offset by an increase in the head 

gradients between the lake and the ground-water 

system. This increase in the head gradients would 

most likely result from a reduction in the saturated 

thickness (and therefore transmissivity) of the surficial 

aquifer in response to declining water levels.
Summary 39

The significance of the ground-water inflow and 

leakage components of the hydrologic budget of Lake 

Five-O emphasizes the importance of considering the 

interactions between lakes and contiguous groundwater systems when developing hydrologic budgets 

for Florida lakes. When these interactions are poorly 

understood, the exchange of water between the lake 

and the ground-water system may be underestimated, 

because hydrologic budget analyses are often based on 

the assumption that ground-water inflow (or leakage) 

is negligible when net leakage (or net ground-water 

inflow) is indicated. This conclusion is supported by 

the analyses of head data, computed net ground-water 

inflow, and the ground-water flow simulations at 

Lake Five-O, all of which indicated that the lake consistently receives large volumes ground-water inflow 

and leaks large volumes of water to the ground-water 

system. This conclusion is also consistent with recent 

work by several investigators. Based on their preliminary net ground-water flow and geochemical analyses 

of lakes Barco and Five-O, Pollman and others (1991) 

indicated that previous studies had underestimated 

ground-water inflow to some seepage lakes in northwestern Florida. Stauffer and Canfield (1992) used a 

mass-balance analysis (Stauffer, 1985) of silica to 

infer that ground-water inflow rates for seepage lakes 

in northwestern Florida were one to two orders of 

magnitude greater than the rate of 0.01 m3

/m2-yr 

estimated by Baker and others (1988). Similar results 

were obtained for several lakes in ridge provinces in 

peninsular Florida. Stauffer and Canfield (1992) also 

noted that previous work by Deevey (1988) probably 

underestimated ground-water inflow and leakage 

rates, because the hydrologic budget used in that study 

was based on the assumption that ground-water inflow 

and leakage do not occur simultaneously. This negative bias is a particular problem in lakes, such as Lake 

Five-O, that continuously receive ground water and 

leak to the ground-water system simultaneously. 

The analysis of the ground-water flow system and 

hydrologic budget of Lake Five-O underscores the 

importance of physical and chemical ground-water 

data when developing hydrologic budgets for seepage 

lakes.

SUMMARY

As part of a larger study of geochemical 

processes in acidic seepage lakes in Florida, a study 

of the hydrology and hydrologic budget of Lake Five-O 

in northwestern Florida was conducted during 1988-91. 

As with most seepage lakes in karst settings, developing a quantitative understanding of the hydrology of 

Lake Five-O was complicated by the difficulty of 

quantifying exchanges of water between the lake and 

its contiguous ground-water system. A primary objective of the work at Lake Five-O was to improve the 

understanding of the ground-water flow system near 

Lake Five-O, and to develop a hydrologic budget for 

the lake. Quantitative studies of the hydrology of 

seepage lakes, such as Lake Five-O, can provide 

information that is critical to understanding the 

geochemistry of these lakes.

 The hydrogeology near Lake Five-O is characterized by three distinct lithologic units: a very permeable, sandy surficial aquifer; a less permeable, 

calcareous, sandy-clay to clayey-sand intermediate 

confining unit; and, at the base of the system, a 

sequence of highly transmissive carbonate rocks that 

comprise the Upper Floridan aquifer in the study area. 

Seismic surveys indicated that the intermediate confining unit has been breached under the lake, providing a high conductance pathway for the downward 

movement of water from the surficial aquifer and lake 

to the Upper Floridan aquifer.

Head data collected during the study indicated a 

consistent pattern of flow toward Lake Five-O and a 

strong potential for ground-water inflow to and leakage from the lake over a wide range of hydrologic conditions throughout the study period. The head data 

also indicated that ground-water inflow occurs at 

appreciable depths around much of the perimeter of 

Lake Five-O, and that the largest rates of ground-water 

inflow occur along the southern margin of the lake, 

where head gradients are larger. Heads in the surficial 

aquifer, lake, and intermediate confining unit were 

consistently higher than those in the underlying Upper 

Floridan aquifer, indicating a consistent potential for 

downward flow from the shallow system to the Upper 

Floridan aquifer. Head differences between the surficial and Upper Floridan aquifers were much smaller 

near Lake Five-O relative to the surrounding plateau 

area, which is consistent with the interpretation of 

effective breaching of the intermediate confining unit 

under the lake.

Precipitation, evaporation, and lake volume 

data also indicated that the ground-water flow to Lake 

Five-O and leakage from the lake to the groundwater system are significant components in the hydrologic budget of the lake. The volume of Lake Five-O 
40 Evaluation of Ground-Water Flow and Hydrologic Budget for Lake Five-O, A Seepage Lake in Northwestern Florida

increased much faster than predicted by netprecipitation inputs alone, during the summer of 1989 

and, to a lesser extent, during the summer of 1990. 

Conversely, during relatively dry periods in the spring 

of 1989, and the spring and fall of 1990, lake volumes 

decreased much faster than predicted by net atmospheric losses. The precipitation, evaporation, and 

lake volume data were also used to compute estimates 

of net ground-water inflow (ground-water inflow 

minus leakage) and minimum estimates of study 

period and long-term average ground-water inflow 

and leakage rates. The latter analysis indicated that 

long-term average ground-water inflow and leakage 

represent at least 62 percent and 70 percent of the 

total inflow and outflow budgets of Lake Five-O, 

respectively.

Simulation models of the ground-water flow 

system near Lake Five-O indicate that ground-water 

inflow and leakage are the dominant components in 

the inflow and outflow budgets of the lake, and that 

ground-water inflow and leakage are considerably 

larger than the minimum estimates given by the net 

ground-water flow analysis. During 1989-90, groundwater inflow and leakage were estimated to be 

1.2 x 106 and 1.4 x 106 m3, respectively. Groundwater inflow and leakage are approximately 4 and 5 

times larger than precipitation and evaporation, 

respectively. The temporal distribution of simulated 

ground-water inflow and leakage also indicated that 

exchanges of water between the ground-water system 

and lake were consistently larger than atmosphericlake exchanges throughout 1989-90. The relative 

importance of exchanges of water between the 

ground-water system and lake generally increase when 

atmospheric-lake exchanges are at seasonal minimums. Climatic data and model results indicated that 

average rates of ground-water inflow and leakage for 

the study period were probably representative longterm, average rates.

The spatial distributions of simulated groundwater inflow and leakage exhibited little variation 

between dry and wet periods. The highest rates of 

inflow were predicted for the southern margin of Lake 

Five-O. Analysis of the depth distribution of groundwater inflow indicated that temporal changes in 

ground-water inflow were primarily the result of 

changes in the saturated thickness of the surficial 

aquifer, rather than major changes in head gradients or 

reversals of flow along the lakebed. Simulated leakage was limited to deeper areas of the lakebed, with 

the highest leakage rates located in the vicinity of the 

confining bed breaches.

Residence times and flow paths of ground-water 

inflow were evaluated using a particle tracking program and head distributions from steady-state model 

simulations. Mean residence time of ground-water 

discharging to Lake Five-O was estimated between 

3 and 6 years. Flow-path evaluations indicated that 

ground water discharging at the lake has had negligible contact with the sediments in the intermediate 

confining unit.

The dominance of ground water in the hydrologic budget of Lake Five-O is contrary to previous 

studies that suggested that ground-water contributions 

are generally small relative to precipitation inputs in 

seepage lakes. The simulation results at Lake Five-O 

are consistent with recent work that indicates that 

these studies might have significantly underestimated 

ground-water inflow to many seepage lakes. This 

recent work, coupled with the results obtained at Lake 

Five-O, emphasizes the importance of ground water in 

the hydrology of Florida seepage lakes.

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balance of lakes, Water Resources Bulletin, v. 17, 

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42 Evaluation of Ground-Water Flow and Hydrologic Budget for Lake Five-O, A Seepage Lake in Northwestern Florida