Water-Resources Potential of the 

Freshwater Lens at Key West, Florida

By Donald J. McKenzie

U.S. GEOLOGICAL SURVEY

Water-Resources Investigations Report 90-4115

Prepared in cooperation with the

SOUTH FLORIDA WATER MANAGEMENT DISTRICT

Tallahassee, Florida 

1990
U.S. DEPARTMENT OF THE INTERIOR

MANUEL LUJAN, JR., Secretary

U.S. GEOLOGICAL SURVEY

Dallas L. Peck, Director

For additional information Copies of this report

write to: can be purchased from

District Chief U.S. Geological Survey

U.S. Geological Survey Books and Open-File Reports

Suite 3015 Federal Center, Building 810

227 N. Bronough Street Box 25425

Tallahassee, Florida 32301 Denver, Colorado 80225
CONTENTS

Abstract 1 

Introduction 1

Purpose and scope 2

Methods and procedures 2 

Description of area 3

Geohydrologic setting 6 

Characteristics of the freshwater lens 6 

Rainfall recharge 6

Water-table fluctuations 7

Configuration of the freshwater lens and transition zone 8

Ground-water quality 14

Major inorganic constituents and physical properties 14

Nutrients 15 

Trace elements 19 

Synthetic organic compounds 21 

Bacterial analyses 23

Summary and conclusions 23 

Selected references 23

FIGURES

1. Map showing location of Key West and the Florida Keys 2

2. Map showing the island of Key West: (A) as it appeared in about 1850, and (B) as it appeared in 1988 4

3. Graph showing monthly rainfall for 1986-88, and long-term average monthly rainfall at Key West 5

4. Map showing location of wells, ground-water level recorders, and cored well sites at Key West 5

5. Map showing water-table elevations for the highest ground-water levels measured during the investigation, 

December 8,1986 8

6. Map showing water-table elevations for the lowest ground-water levels measured during the investigation, 

June 24,1987 9

7. Hydrograph of water-table fluctuations at wells MO-147 and MO-152 and the tide stages at Key West, 

December 10-13,1986 9

8. Hydrograph of well MO-152 showing the times of specific conductance profile measurements in four wells, 

October 22-23,1987 10

9-12. Depth profiles of specific conductance and associated concentrations of chloride at wells:

9. MO-149 10

10. MO-152 11

11. MO-153 11

12. MO-155. 11

13. Hydrologic cross section of Key West along line A-A' showing chloride concentrations in ground water, 

October 21-22,1986 12

14-18. Maps showing:

14. Lines of equal chloride concentration at a depth of 4 feet below the water table, October 21,1986 13

15. Lines of equal chloride concentration at a depth of 4 feet below the water table, April 1,1987 14

16. Surface-geophysical survey sites and lines of equal resistivity, November 20,1986 15

Contents III
17. Average concentrations of nitrate nitrogen in water from wells at depths of 2 to 15 feet below the water 

table at Key West, October 1986 through January 1988 19

18. Average concentrations of lead and iron in water from wells at depths of 2 to 5 feet below the water 

table at Key West, October 1986 through September 1987 21

TABLES

1. Lithologic log of wells MO-155 and MO-158 at Key West 7

2. Chemical and physical characteristics of Key West ground water at depths of 2 to 5 feet below the water table, 

October 21-22,1986 16

3. Nutrient concentrations in Key West ground water at various well depths, October 1986 through 

January 1988 17,18

4. Trace-element concentrations in Key West ground water at depths of 2 to 5 feet below the water table, October 

1986 through April 1988 20

5. Synthetic organic compounds for which gas chromatograph/mass spectrometer analyses were determined 22

METRIC CONVERSION FACTORS

The inch-pound units used in this report may be converted to metric (International System) units by the following factors:

Multiply inch-pound unit

inch (in.)

foot (ft)

mile (mi)

foot per mile (ft/mi) 

square mile (mi ;

million gallons (Mgal)

million gallons per day (Mgal/d)

By

25.4

0.3048

1.609

0.1894 

2.590

3,785

0.04381

To obtain metric unit

millimeter (mm)

meter (m)

kilometer (km)

meter per kilometer (m/km) 

square kilometer (km )

cubic meter (m )

cubic meter per second (m /s)

ADDITIONAL FACTORS

Multiply milliliter (mL) by 0.001 to obtain liter (L).

Temperature in degrees Celsius (°C) can be converted to degrees Fahrenheit (°F) as follows: °F = 1.8(0C) + 32

Sea level: In this report "sea level" refers to the National Geodetic Vertical Datum of 1929 (NGVD of 1929) a geodetic datum 

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

of 1929.

ADDITIONAL ABBREVIATIONS

micrograms per liten=|ig/L

microsiemens per centimeter=uS/cm

milligrams per Hter=mg/L

Contents IV
Water-Resources Potential of the Freshwater Lens 

at Key West, Florida

By Donald J. McKenzie

Abstract

The island of Key West lies at the end of the Florida 

Keys, about 150 miles southwest of Miami. The publicwater supply for the island is provided by the Florida Keys 

Aqueduct Authority Well Field near Miami. However, there 

are many privately owned wells on the island that tap the 

local fresh ground-water lens for potable and nonpotable 

water supply. The number of people who use water from 

the wells for drinking purposes is unknown.

From 1985 to 1988, the U.S. Geological Survey, in 

cooperation with the South Florida Water Management 

District, conducted an investigation to characterize the Key 

West freshwater lens. Observation wells were drilled to 

determine the extent of the lens and to characterize the 

water quality. Previous well logs and well-core data 

collected during the investigation showed the aquifer to be 

a highly permeable, porous, solution-riddled, oolitic 

limestone that allows rainfall recharge to quickly seep into 

the ocean and saltwater to easily intrude the aquifer.

The small freshwater lens (250 milligrams per liter of 

chloride concentration, or less) averages 5 feet in thickness 

below the center of the western half (Old Town) of the 

island. The lens contains about 20 million gallons of freshwater during the dry season and about 30 million gallons 

during the wet season. Underlying the freshwater lens is a 

transition zone of freshwater-saltwater mix that extends to 

the saltwater interface (19,000 milligrams per liter of 

chloride concentration), which is about 40-feet deep at the 

center of the lens. The water table fluctuates and the 

configuration of the lens constantly changes, largely as a 

result of tidal effects. Others events, such as rainfall, 

pumping, and evapotranspiration, are masked by the tidal 

effects.

The freshwater lens is a calcium bicarbonate water 

that grades to a sodium chloride type near the saltwater 

interface. Elevated concentrations of nitrate nitrogen were 

found in water samples from wells in the Old Town district. 

However, concentrations generally were not above the 

maximum contaminant level of 10 milligrams per liter for 

drinking water established by the Florida Department of 

Environmental Regulation. Water samples near an old landfill in the eastern half of the island had concentrations of 

iron (600-1,900 micrograms per liter) and lead (40-800 

micrograms per liter) that exceeded maximum contaminant 

levels of 300 and 50 micrograms per liter. These

trace-element concentrations generally decreased with 

distance from the landfill.

Although the freshwater lens is a potential source of 

water for additional nonpotable water needs in Key West, 

any large-scale pumping could quickly exhaust the 

freshwater lens, and saline water could be rapidly drawn 

through the porous limestone aquifer. Water-quality data 

indicate that the lens is an unlikely source of potable water.

INTRODUCTION

Before 1940, the island of Key West received its 

drinking water from rainfall cisterns, domestic wells that 

tapped a freshwater lens below the island, and from 

"springs,** which were solution holes in the limestone that 

gave access to the freshwater lens. Parker (1941, p. 6) 

reported that hundreds of shallow wells were used for 

potable water supply. During World War n, the U.S. Navy 

developed large parts of the island, and with the subsequent 

need for more freshwater constructed an aqueduct (18-inch 

diameter pipe) from the mainland. The water was pumped 

from a well field and treatment plant at Florida City (fig. 1) 

to Key West, and provided water to the other islands along 

the way. As of 1988, the Florida Keys Aqueduct Authority 

(FKAA) still used the same well field and treatment plant but 

used a larger (24- to 36-inch diameter) aqueduct pipe to 

provide 11 Mgal/d (million gallons per day) of water for the 

system. During the winter tourist season, Key West uses 

about 5 Mgal/d of that total. A reverse-osmosis plant on 

Stock Island, adjacent to the eastern part of Key West, 

produces desalinated water from wells for auxiliary use.

Although the local ground water is considered 

nonpotable by the Monroe County Health Authority (C.D. 

Pierce, Monroe County Public Works Department, oral 

commun., 1987), many residents use it in a dual water-supply 

system. The aqueduct water is used for cooking, bathing, and 

drinking, and the well water is used for such purposes as 

flushing toilets, washing clothes, and lawn irrigation. The 

number of people currently using well water for drinking 

purposes is unknown.

Introduction
26*00'

81*30' 81*00'

25*00' -

24*30'

Figure 1 . Location of Key West and the Florida Keys.

From 1985 to 1988, the U.S. Geological Survey, in 

cooperation with the South Florida Water Management 

District, conducted an investigation to evaluate the 

water-resources potential of the freshwater lens at Key West. 

Long-term ground-water data were not available for Key 

West, and a detailed evaluation of changes over time was not 

possible in this investigation. This investigation focused on 

the occurrence and quality of freshwater on the island and 

established a data base for future intensive studies of the 

water quality and hydraulics of the freshwater lens.

concentration data to further define the configuration of the 

lens. Results of this investigation are presented herein.

The Florida Secondary Drinking Water Regulations 

(Florida Department of Environmental Regulation, 1983) 

establishes a recommended limit of 250 mg/L (milligrams 

per liter) chloride concentration for potable water, which was 

used in this report to define freshwater. However, because 

domestic use of water from privately owned wells could 

include water of much higher chloride concentration, water 

sampling was extended into the transition zone, which underlies the freshwater lens.

Purpose and Scope

This report presents the results of an investigation to 

evaluate the water-resources potential of the freshwater lens 

at Key West by: (1) determining the areal extent, thickness, 

and volume of the lens; and (2) characterizing the water 

quality of the lens.

The work included: test drilling with examination of 

drill cuttings to refine the geologic knowledge of the area; 

measuring water levels to depict the water-table gradient and 

determine the effect of rainfall recharge; collecting specific 

conductance and chloride concentration data to depict the 

configuration of the lens and the zone of freshwater-saltwater 

diffusion; constructing lines of equal chloride concentration 

to delineate the areal extent, thickness, and volume of the 

freshwater lens; and performing a surface-geophysical 

survey to augment the specific conductance and chloride

Methods and Procedures

Observation wells were drilled to determine the 

general water quality and to describe the horizontal and 

vertical configuration of the lens. The wells were drilled 

using only local ground water and were cased only through 

the upper few feet above the water table. Polyvinyl chloride 

(PVC) casing was seated in the borehole, and the annular 

space was grouted. The top of the well casing was capped 

just below land surface, and the installation was protected by 

a water meter box set level with the ground surface.

After the wells were constructed, the elevations of the 

casing tops above sea level were measured using transit and 

rod standard leveling techniques. National geodetic surveys 

and local benchmarks of first-order accuracy were used. All 

ground-water level measurements are referenced to the 

casing tops.

2 Water-Resources Potential of the Freshwater Lens at Key West, Florida
The wells were left uncased below the water table so 

that a specific conductance probe (temperature compensated 

to 25 °C [degrees Celsius]) could be lowered into the well to 

profile the water column. The equipment used for the 

profiling was a Yellow Springs Temperature and 

Conductivity Meter 30001 . The meter was calibrated in the 

laboratory and in the field using standard solutions supplied 

by the U.S. Geological Survey Quality of Water Service Unit 

in Ocala, Fla. The specific conductance probe was lowered 

down the well, and measurements were taken at major 

changes in specific conductance. As the probe was retrieved, 

readings were taken again and comparisons made with the 

descending measurements to determine any changes due to 

disturbance of the water column.

After the profiling of specific conductance, water 

samples were collected at specific conductance horizons, and 

chloride concentrations were determined using the Mohr 

titration method (Brown and others, 1970, p. 69). Water 

samples were collected by use of a low-volume peristaltic 

pump with a 1/4-inch Tygon tube lowered to the desired 

depth. This resulted in minimal disturbance of the water 

column.

A surface-geophysical resistivity survey was 

conducted to augment the chloride concentrations and 

specific conductance measurements and help define the configuration of the freshwater lens and the transition zone. The 

electromagnetic-16R (EM-16R) very low frequency (VLF) 

meter was used because of its ability to operate in an urban 

area with numerous electrical interferences.

Samples for inorganic chemical and physical elements 

were collected in the same manner as the chloride samples. 

The U.S. Geological Survey Quality of Water Service Unit in 

Ocala provided the analyses, using analytical methods 

described by Fishman and Friedman (1985).

Samples for synthetic organic compound analyses 

were collected with a peristaltic pump and Teflon hose. 

Samples for base-neutral and acid-extractable compounds 

were placed in glass containers (supplied by the Quality of 

Water Service Unit in Ocala), preserved with mercuricchloride as a bacteriacide, and packed in ice as recommended 

by the U.S. Environmental Protection Agency (1980). 

Samples to be analyzed for volatile organic compounds were 

collected in 40-mL glass vials with a Teflon seal. Samples 

were shipped in ice for overnight delivery. Samples were 

analyzed using either the gas chromatograph/flame 

ionization detection (GC/FID) method or the gas 

chromatograph/mass spectrometer (GC/MS) method by 

laboratories of the U.S. Geological Survey. The analytical 

methods for the determination of organic substances are 

described by Wershaw and others (1983).

Coliform samples were collected by use of a peristaltic 

pump with a Tygon hose; a new length of hose was used for

1 Use of brand names in this report is for identification purposes 

only and does not constitute endorsement by the U.S. Geological Survey.

each well. The samples were placed in sterile glass bottles, 

and within 2 hours, using the membrane filter technique, the 

necessary dilutions were made and the samples placed in 

portable incubators. Final incubation and bacterial colony 

counts were conducted at a U.S. Geological Survey 

laboratory.

Two continuous water-level recorders were installed, 

one near the northwestern coast adjacent to well MO-147 and 

another near the center of the island adjacent to well MO152. The instruments were Stevens model 7000 digital tape 

recorders that automatically punch the ground-water levels 

on paper tape at hourly increments. The punched tapes were 

later translated to generate a ground-water hydrograph.

A tidal hydrograph was constructed from tide 

prediction tables published by the National Oceanic and 

Atmospheric Administration (1986), and comparison made 

of the tidal hydrograph with ground-water hydrographs of 

inland wells. The tidal range at Key West is about 1.5 feet. 

There is no significant difference between the tidal ranges on 

the Gulf of Mexico side and the Atlantic Ocean side of the 

island. The predicted tides differed from the actual tides by 

no more than about 0.09 foot during the investigation. The 

accuracy is within an acceptable range for comparison of the 

ground-water level and tide.

Discrete ground-water level measurements in all the 

wells were made monthly using a hand-held steel tape. Rainfall data provided by the National Oceanic and Atmospheric 

Administration (NOAA) station at Key West were used to 

describe the response of the lens to rainfall recharge.

Description of Area

The island of Key West lies at the end of the Florida 

Keys, about 150 miles southwest of Miami on the Overseas 

Highway (fig. 1). At a latitude of 24°34' north, the island lies 

adjacent to the tropical Straits of Florida between the Atlantic 

Ocean and the Gulf of Mexico. The island is about 4 miles 

long and 1.5 miles wide, encompassing a residential area of 

about 4 mi2 (square miles). The size and shape of the island 

have changed considerably since it was ceded by Spain to the 

United States in 1821. The island as it appeared in about 

1850 and as it appeared in 1988 is shown in figure 2. (The 

original map is in the archives of the Key West Library.)

The ground elevation typically is 4 to 5 feet above sea 

level with a maximum elevation of 14 feet near the western 

end of the island. The average annual temperature is 24.6 °C, 

and the average annual rainfall is 39.4 inches. Most of the 

rainfall (75-80 percent) generally occurs during the wet 

season from May to November as shown in figure 3 

(Sherwood and others, 1973, p. 8).

Key West had a permanent population of about 28,000 

people in 1987-88, including U.S. Naval personnel 

(Rosemary Vogenet, Key West Planning Department, oral 

commun., 1989). In addition, about 1.5 million tourists visit

Introduction
24°34'30"

24°33'30"

STOCK ISLAND

0 1/4 1/2 KILOMETER

24°34'30" 

24°33I30" 

81°48' 81 "47' 81 °46' 81'45'

KEY WEST

INTERNATIONAL

AIRPORT

81°48' 81°47' 81°46'

Figure 2. The island of Key West: (A) as it appeared in about 1850, and (B) as it appeared in 1988.

81°45'

4 Water-Resources Potential of the Freshwater Lens at Key West, Florida
11

10

9

8

INCHES -J

Z 6

RAINFALL

4 

3 

2

1

0

-

X (0 OS < w <

 » * a

MONTHLY RAINFALL

LONG TERM MONTI

AVERAGE RAINFA \

rl

:| _ j:| |

-hi j

£ 5 * D a S 6 < a § * > < « o

DURING STUDY

-

-

IVf

LL

3

n n ^ ^ ^

ri r -8 j i s j i

|H<|*£|||D

-

; i

1 | |-

S E-i > O SB 

0 5 H < 

H 0 Z 0 ? 00

1986 1987 1988

Figure 3. Monthly rainfall for 1986-88, and long-term average 

monthly rainfall at Key West.

the island each year, averaging about 4,100 individuals per 

day with a slight seasonal variation. The greatest population 

density is on the western part of the island where wooden 

single family residences predominate, many built during the 

19th century. The area is locally referred to as "Old Town." 

Renovation and construction on much of the island is 

ongoing to accommodate an increase in tourism and new 

residents.

The observation wells drilled for this investigation 

(fig. 4) were concentrated in the western part of Key West, 

which is the largest area of natural land (oolitic limestone). 

Much of the eastern part of the island consists of large areas 

of artificially created land (fig. 2A-2B) with extensive 

areas of saltwater mangrove ponds; the freshwater lens is 

small or nonexistent there. The fill material generally was 

sand that was pumped from the adjacent shallow seabed, 

whereas some small inland areas were filled with domestic 

refuse. At present, a large sanitary landfill on Stock Island 

in the eastern part of Key West (fig. 4) receives the city's 

refuse.

Untreated domestic sewage is conveyed to the ocean 

through an outfall pipe to a deep trench, about 5,100 feet 

from the southwestern end of the island. The daily volume of 

sewage is 5.5 to 9.0 Mgal, depending on the amount of 

rainfall and seepage that enters the sewer pipes (Daniel J. 

Lagosky, City of Key West Sewer Department, oral 

commun., 1986). Some commercial developments, such as

MO-152

®

I

EXPLANATION

LINE OF HYDROLOGIC SECTIONSection shown in figure 13

WELL AND U.S. GEOLOGICAL 

SURVEY NUMBER

WATER-LEVEL RECORDER 

CORED WELL SITE

GUL F MEXICO

1/4 1/2 KILOMETER

81*46'

Figure 4. Location of wells, ground-water level recorders, and cored well sites at Key West.

Introduction
condominium buildings, have individual package sewagetreatment plants and onsite effluent injection wells to a depth 

of 80 feet. The only known septic tanks are on the eastern 

part of the island near the Key West International Airport and 

on the northern part of the island near the Garrison Bight 

Yacht Basin (fig. 4). A sewage-treatment plant for the entire 

island is being constructed and is scheduled for completion 

in about 1990.

surface is tree to rise and fall. The freshwater head is greater 

in the center of the island where land-surface elevations are 

higher. Where the aquifer rock is a highly permeable oolitic 

limestone, as is the case at Key West, ground-water gradients 

typically are low. Ground water moves from the center of the 

lens to discharge along the beaches and salt ponds. 

Underlying the freshwater lens is a zone of mixed 

freshwater-saltwater, the transition zone that extends to the 

saltwater interface (19,000 mg/L of chloride).

GEOHYDROLOGIC SETTING

The lower Florida Keys, from Big Pine Key to Key 

West, are a lithologic extension of the oolitic limestone that 

underlies Miami and other southeastern Florida coastal 

cities. The Miami Oolite of Pleistocene age was described by 

Parker and others (1955, p. 102) as honeycombed with 

solution holes, giving the rock an exceedingly high 

permeability. Parker and others further stated that many of 

the solution holes probably connect with channels open to 

the sea, allowing rainwater to rapidly escape to the sea and 

seawater to easily infiltrate the oolitic limestone of the island.

To substantiate and augment Parker's description of 

the oolitic lithology for Key West, two wells were cored to 

the saltwater interface well MO-155 (40 feet deep) in the 

center of the western part of the island and well MO-158 (25 

feet deep) on the eastern part of the island (fig. 4). Lithologic 

descriptions were made from microscopic examination and 

chemical testing of the well cuttings and cores. The lithologic 

logs for wells MO-155 and MO-158 are presented in table 1.

The oolitic limestone in Key West is pale orange, 

crystalline, porous to cavernous, with porosity increasing 

with depth, and contains an abundance of corals and other 

marine fossils indicating deposition in a marine environment. 

The oolite was the only rock type found in the cores of the 

two wells. Descriptions of the oolitic rock in the Lower Keys, 

similar to those of Parker and others (1955, p. 100), have 

been reported by Hoffmeister and others (1967, p. 186) and 

Mitchell-Tapping (1980, p. 118).

A lithologic log from the U.S. Geological Survey file 

for well MO-117 (W-265), drilled in 1938, indicates that 

oolite was penetrated to 200 feet Beneath that was a white 

fossiliferous limestone with fades of gray shale, quartz sand, 

and loosely cemented coquina occurring to the bottom of the 

well at 2,000 feet The exact location at Key West was not 

recorded. Another well (W-137), drilled in the 1920's, also 

has a lithologic log on file with the Survey that indicates a 

hard oolitic limestone, commonly with coral, to a total depth 

of 75 feet The well was constructed on the western part of 

the island near well MO-148.

Fresh ground water (less than or equal to 250 mg/L of 

chloride) on islands, such as Key West, occurs in the form of 

a lens which floats on saltwater, the result of density 

differences. The lens is underwater-table conditions, and the

CHARACTERISTICS OF THE 

FRESHWATER LENS

The highly permeable oolitic limestone extends 

throughout Key West and reaches a depth of about 200 feet. 

It is of sufficient extent at shallow depths in the western half 

of the island to contain a freshwater lens that is tapped by 

many domestic wells. The eastern half of the island has large 

areas of artificially created land and lacks sufficient aquifer 

material for a measurable freshwater lens.

Rainfall Recharge

Ground-water levels indicate that rainfall recharge 

quickly reaches the water table and rapidly seeps to the ocean 

(the amount of mixing in the transition zone is unknown). 

The greatest rainfall recorded during the investigation was 

6.66 inches at the Key West International Airport on 

December 8, 1986. At well MO-152 near the center of the 

island, the water table rose 7.0 inches and then receded to the 

prepeak level about 1 hour after the peak was reached. The 

coastal well recorder (well MO-147) showed no effects from 

the rainfall of December 8.

The next greatest rainfall was 5.26 inches at the airport 

on March 6, 1987. The water table at well MO-152 rose 

about 85 inches and receded in 4 hours. The water-level 

recorder near well MO-147 recorded a 0.15-inch rise with a 

recession in 3 hours. The effect of the rainfall recharge on the 

water table is dependent on the volume and duration of the 

rainfall. This accounts for the differences between rise and 

recession rates of the ground-water levels during the storms 

of December 8,1986, and March 6,1987.

There were five other major rainfall events recorded 

during the investigation; rainfall varied from 3.47 to 1.47 

inches. None of these events produced noticeable changes in 

recorded ground-water levels at wells MO-147 or MO-152.

Surface runoff and evapotranspiration affect the 

amount of rainfall that recharges the aquifer and 

subsequently influences the ground-water levels. There is 

little surface runoff to the ocean from unpaved areas because 

the island generally has low relief and the surface rock is 

highly permeable. About two-thirds of the stormwater runoff

6 Water-ReMtircM Potential of the Freshwater Lem at Key Wert, Florida
(Daniel J. Lagosky, City of Key West Sewer Department, 

oral commun., 1966) from paved areas (about 20 percent of 

total area) is channeled into French drains (dry wells), and 

the rest is conducted by a separate system of stormwater 

sewers to an ocean outfall.

Water-Table Fluctuations

Fluctuations in the water table were monitored at two 

sites by automatic, continuous ground-water level recorders. 

Water-table fluctuations at well MO-152 near the center of

Table 1 . Lithologic log of wells MO-155 and MO-158 at Key West

Description

Well MO-155

Limestone, very pale orange (10 YR 8/2); oosparite; moderately to well cemented; 

porous, vugs, and interparticle.

Limestone, very pale orange (10 YR 8/2); sparse oosparite, mollusks, framework 

organisms and serpulids; well cemented; porous to very porous, vugs.

Limestone, very pale orange (10 YR 8/2); fossiliferous, poorly washed oosparite; 

well cemented; porous to very porous, vugs.

Limestone, very pale orange (10 YR 8/2); biolithite; well cemented; porous, framework, at about 14 to 16 feet sparse oosparite; well cemented; very porous, vugs.

Cavern.

Limestone as in 12 to 16 feet.

Limestone, very pale orange (10 YR 8/2); oolite-bearing sparse to packed oosparite, 

abundant corals, serpulids and worm borings, and mollusks; well cemented; moderately 

porous to porous at 22 to 24 feet, framework and vugs.

Limestone as above; porous and cavernous at about 24 to 25 feet

Limestone, very pate orange (10 YR 8/2); sparse to packed poorly washed biosparite, 

abundant corals; well cemented; very porous, vugs are coated with iron mineral.

Limestone, very pale orange (10 YR 8/2); sparse poorly washed oosparite to packed 

oosparite toward the base, adundant corals including Monastrea sp.; well cemented; 

porous to cavernous in places.

Limestone, very pale orange (10 YR 8/2); oolite-bearing packed sparite to packed 

sparite, abundant corals; well cemented; porous to cavernous.

Cavern. 

Wtell MO-158

Limestone, very pate orange (10 YR 8/2); sparse poorly washed oosparite, mollusks; 

moderately to well cemented; moderately porous to porous in places; vugs and interparticle.

Limestone as above; grades to a fossiliferous oosparite toward base; moderately 

porous (vugs) to porous and slightly porous at about 5 to 5.5 feet.

Limestone, very pale orange (10 YR 8/2); fossiliferous oosparite; well cemented; 

slightly porous to porous at about 14 to 15 feet, vugs.

Limestone, very pale orange (10 YR 8/2); sparse oosparite, benthic foraminifers 

and mollusks; well cemented; porous (vugs) to slightly porous at about 19.5 to 20 feet.

Limestone, very pale orange (10 YR 8/2); intraclastic-bearing sparse oosparite; well

Thickness 

(feet)

4

4

4

4

4

0.5

3.5

3

1

4

4

4 

2.5

5

5

5

5

Depth 

(feet below 

land surface)

0 -

4 -

8 -

12 -

16 -

20 -

20.5 -

24 -

27 -

28 -

32 -

36 - 

2.5 -

5 -

10 -

15 -

20 -

4

8

12

16

20

20.5

24

27

28

32

36

40 

5

10

15

20

25

cemented; moderately porous to porous, vugs.

Charactertetic* of the Freshwater Lent
Old Town ranged from 0.8 foot above sea level on June 16, 

1987, to 2.4 feet above sea level on March 6,1987. Near the 

coast at well MO-147, the range was from 0.4 foot above sea 

level on June 16, 1987, to 2.2 feet above sea level on 

December 30, 1986. The high ground-water levels were of 

short duration and were the product of storms (discussed 

earlier) and tidal effects and were not representative of 

average conditions.

Ground-water levels in all of the observation wells 

were measured monthly with a hand-held steel tape. 

Measurements were completed in as short a period as 

possible to minimize the rapid changes in the ground-water 

levels caused by the semidiurnal tides. The highest watertable measurements were recorded on December 8, 1986, 

following a 6.66-inch rainfall. During the time required to 

make the steel-tape-measurements, the two automatic 

ground-water level recorders indicated a mean decrease of 

0.25 foot. The lowest monthly measurements were made on 

June 24, 1987. During the hour required to make the tape 

measurements, there was a mean decrease of 0.08 foot 

indicated by the ground-water level recorders. Figures 5 and 

6 depict the water table on these dates when the gradients 

were, respectively, about 4 and 1 ft/mi (foot per mile).

The close relation between water-table fluctuations at 

wells MO-147 (about 0.2 mile from the coast) and MO-152 

(about 0.7 mile from the coast) and the tide stages is shown 

in figure 7. A comparison indicates the tidal signal typically 

reaches well MO-147 in about 30 minutes and well MO-152 

in about 1.5 hours. The range in magnitude of water-table

fluctuations decreases from the coast to the inland area, a 

manifestation of decreasing tidal effects with increasing 

distance from the coast. The short lag time reflects the high 

permeability of the aquifer.

Configuration of the Freshwater Lens and 

Transition Zone

The constantly changing configuration of the 

freshwater lens and the transition zone of freshwatersaltwater mix is principally a manifestation of tidal effects. 

To appraise the magnitude of changes that occur in the lens 

and the transition zone, the water columns in four wells 

(MO-149, MO-152, MO-153, and MO-155) were measured 

for specific conductance and sampled for chloride concentrations during a tide cycle on October 22-23, 1987. The data 

were collected at high, low, and intermediate stages of the 

tide-influenced ground-water levels. Figure 8 shows the 

ground-water hydrograph of well MO-152 and the times of 

data collection at the four observation wells. Profiles of 

specific conductance and associated concentrations of 

chloride (A, high ground-water levels; B,1ow ground-water 

levels; and C, intermediate ground-water levels) for each of 

the four observation wells are shown in figures 9 to 12.

A well-defined relation between specific conductance 

and chloride concentration was established at Key West 

during the investigation and is given in the table below. 

Salinity, describing general dissolved salt content, is used in

EXPLANATION

 2.S WATER-TABLE CONTOUR Shows

altitude of water table. Dashed where 

approximately tocated.Contour interval 

0.5 foot-Datum is sea level

24*34'30"

 MO-152 WELL AND U.S. GEOLOGICAL 

SURVEY NUMBER

GULF MEXICO

81*48' 81*47' 81*46' 81*45'

Figure 5. Water-table elevations for the highest ground-water levels measured during the investigation, December 8,1986.

8 Water-Resources Potential of the Freshwater Lens at Key West, Florida
EXPLANATION

 O.S WATER-TABLE CONTOUR Shows 

altitude of water table. Contour 

interval 0.5 foot. Datum is sea level

 MO-152 WELL AND U.S. GEOLOGICAL 

SURVEY NUMBER

GUL F OF MEXICO

81*48' 81*46'

Figure 6. Water-table elevations for the lowest ground-water levels measured during the investigation, June 24,1987.

81*46'

0600 1200 1800 2400 0600 1200 1800 2400 0600 1200 1800 2400 0600 

DBC.10 DEC. 11 DEC. 12 DEC. 13

Figure 7. Water-table fluctuations at wells MO-147 and MO-152 and the tide stages at Key West, December 10-13,1986.

Characteristics of the Freshwater Lens 9
2.5

UJ

J 2.0

1.5

1.0

0.5

2400 0600 1200

OCTOBER 22

1800 2400 0600 1200 

OCTOBER 23

1800 2400

Figure 8. Hydrograph of well MO-15 2 showing the times of spedfic conductance profile measurements in four wells, 

October 22-23,1987.

1.500

CHLORIDE CONCENTRATION,IN MILLIGRAMS PER LITER

5,000 19,000

0 5,000 10.000 15,000 20.000 25,000 55,000 

SPECIFIC CONDUCTANCE .IN MICROSIEMENS PER CENTIMETER AT 25 DEGREES CELSIUS

Figure 9. Depth profiles of specific conductance and associated concentrations of chloride at well MO-149.

10 Water-Resources Potential of the Freshwater Lens at Key West, Florida
CHLORIDE CONCENTRATION .IN MILLIGRAMS PER UTBR 

1.500 5.000

PROFILE A : OCTOBER 22 .1987 AT 1350 

PROFILE B : OCTOBER 23 . 1987 AT 0810 

PROFILE C : OCTOBER 23 ,1987 AT 1050

5.000 10.000 15.000 20.000 25.000 ' 55.000 

SPECIFIC CONDUCTANCE .IN MICROSIBMBNS PER CENTIMETER AT 25 DEGREES CELSIUS

Figure 10. Depth profiles of specific conductance and associated concentrations of chloride at well MO-152.

CHLORIDE CONCENTRATION .IN MILLIGRAMS PER LITER 

1.500 5.000

PROFILE A : OCTOBER 22 .1987 AT 1330

PROFILE B : OCTOBER 23 . 1987 AT 0730

PROFILE C : OCTOBER 23 .1987 AT 1030

5.000 10,000 15,000 20,000 25.000 55,000 

SPECIFIC CONDUCTANCE .IN MICROSIEMENS PER CENTIMETER AT 25 DEGREES CELSIUS

Figure 11. Depth profiles of specific conductance and associated concentrations of chloride at well MO-153.

CHLORIDE CONCENTRATION.IN MILLIGRAMS PER LITER 

1,500 5,000

PROFILE A : OCTOBER 22 .1987 AT 1350 

PROFILE B : OCTOBER 23 . 1987 AT 0810 

PROFILE C : OCTOBER 23 .1987 AT 1050

10.000 15.000 20,000 25.00 

SPECIFIC CONDUCTANCE .IN MICROSIEMENS PER CENTIMETER AT 25 DEGREES CELSIUS

Figure 12. Depth profiles of specific concuctance and associated concentrations of chloride at well MO-155.

Characteristics of the Freshwater Lens 11
the table to relate specific conductance and chloride 

concentration to water type. This information is also shown 

in the well water profiles (figs. 9-12).

Specific conductance 

Type of water (microsiemens per 

centimeter)

Fresh 0 - 1,400 

Very slightly saline 1,400 - 1,900 

Slightly saline 1,900 - 5,000 

Moderately saline 5,000 - 17,000 

Very saline 17,000 - 52,000 

Seawater > 52,000

Chloride 

(milligrams 

per liter)

0 - 250 

250 - 400 

400 - 1,500 

1,500 - 5,000 

5,000 - 19,000 

> 19,000

Wells MO-152, MO-153, and MO-155 are near the 

center of the lens in the vicinity of Old Town, whereas well 

MO-149 is in the western part of Key West in the vicinity of 

Fort Taylor, which is an artificially constructed land area. 

Because of the proximity of well MO-149 to the coast, water 

in this well is more saline at shallower depths than is water in 

the other three wells (fig. 9).

The distinctive characteristic of all four wells is that 

the freshwater and salinity zones measured by chloride and 

specific conductance constantly change with water-table 

fluctuations that are influenced by tides. The salinity of water 

in these wells (figs. 9-12) indicates that wide variations in 

specific conductance may occur within short time spans at 

each depth. For example, at well MO-153 on October 23, 

1987, at a depth of 10 feet, the specific conductance was 

about 1,500 (iS/cm (microsiemens per centimeter) at 

0730 hours; but 3 hours later, the specific conductance had 

increased to almost 13,000 fiS/cm (fig. 11).

All four wells show some similarity in 

specific-conductance profiles. Profile C shows the thinnest 

freshwater zone and was measured midway between high 

and low ground-water levels. Profile B was measured at low 

ground-water levels and has the thickest freshwater zone. 

Profile A was measured at high ground-water levels, and the 

specific conductance falls somewhere between the other two 

extremes, except at well MO-152 on October 22, 1987, at 

1710 hours (fig. 10). This profile actually coincided with a 

low ground-water level (fig. 8) and reflects a thick freshwater 

lens like the B profiles which were also measured at low 

ground-water levels.

Although there probably is a precise relation among 

tide, ground-water levels, and lens thickness, it was not fully 

defined during these three measurements. The correlation of 

lens thickness with semidiurnal tidally induced fluctuation in 

ground-water levels, occurring in a solution-riddled, fissured 

aquifer, is beyond the scope of this investigation.

Because the water-table fluctuations and the 

configuration of the freshwater lens and transition zone are 

constantly changing, vertically and areally, only a 

generalized depiction of the lens and diffusion zones could 

be made. The specific conductance and chloride-concentration data collected during seven sampling events were used 

to construct a generalized cross section of the lens and 

transition zone along the length of the island (fig. 13). The 

measurements were made in May, August, and October 

1986, and March and October 1987 (three sampling events in 

October), during high and low stages of the tide and 

ground-water levels.

The water table generally was 1 to 2 feet above sea 

level at the center of the lens. The freshwater lens ranged in

SEA 

LEVEL

EXPLANATION

LDH Or EQUAL CHLORIDE

CONCENTRATION lateral, ta 

 UU«r»m ftr liter. U nrteMt

-A* Loa or MOTION SHOWN IN nauu 4

Figure 13. Hydrologic cross section of Key West along line A-A' showing chloride concentrations in ground water, 

October 21-22,1986.

12 Water-Resources Potential of the Freshwater Lens at Key West, Florida
thickness from less than 1 foot to 9 feet, with an average 

thickness of 5 feet at the center of the lens near Old Town. 

The lens quickly thinned toward the coast. The very slightly 

saline zone, (chloride concentration of 250-400 mg/L) was 

usually about twice as thick as the overlying freshwater lens, 

ranging in thickness from 7 to 34 feet. The depth to the 

saltwater interface (chloride concentration of 19,000 mg/L) 

near the center of the island was about 40 feet.

To illustrate the areal extent and thickness of the freshwater lens and estimate the volume, lines of equal chloride 

concentration were constructed at 2-foot depth intervals to 

the bottom of the 250-mg/L chloride concentration zone. The 

surface area for each depth was calculated; a 0.20 storage 

coefficient, commonly used for the Miami Oolite (Parker and 

others, 1955, p. 281), was used; and the approximate volume 

was determined. The volume of the freshwater lens on 

October 21,1986, near the end of the wet season, was about 

30 Mgal and on April 1,1987, near the end of the dry season, 

it was about 20 Mgal. Lines of equal chloride concentration 

at a depth of 4 feet below the water table for the two 

respective dates are shown in figures 14 and 15.

The volume of freshwater is affected by the amount of 

rainfall, seepage to the ocean, evapotranspiration, and withdrawal. Determining the volume of freshwater is further 

complicated by the constantly changing configuration of the 

lens and transition zone that is caused by the effects of the 

semidiurnal tide.

Surface-geophysical resistivity measurements were 

made to augment specific conductance and chlorideconcentration data at the observation well sites. The EM-16R 

VLF system was selected because of its operational adaptability to a dense urban area with many magnetic 

interferences, such as overhead electrical wires and metal 

fences. The principal site requirement was that the surface be 

unpaved and soft enough to accept manually inserted probes.

VLF radio signals are transmitted from stations throughout the world. Each VLF radio signal induces electric and 

magnetic fields in the Earth that are related to its electrical 

properties. In surface-geophysical resistivity measurements, an 

instrument measures the ratio of the horizontal components of 

the electric and magnetic fields of a selected signal. It is 

calibrated to read directly the apparent resistivity and the phase 

angle. Because the phase angle measured at all stations was 

45°, it is assumed that the apparent resistivity is equal to the 

true bulk resistivity of the Earth.

The apparent resistivity, measured in ohm-meters and 

equal to bulk resistivity, reflects the entire depth of penetration 

and not a particular depth as did the chloride concentration and 

specific conductance measurements. Because the shape of the 

lens and transition zone constantly changes, a general 

configuration is best reflected by the resistivity measurements: 

the lens is centered under the Old Town area and grades rapidly 

to saline and ocean water.

EXPLANATION

2SO LINE OF CHLORIDE CONCENTRATION 

Interval, in milligrams per liter, 

is variable

24'34'30"

MO-152

180

WELL Upper number is U.S. Geological 

Survey well number.Lower number is 

chloride concentration, in milligrams 

per liter

GUL F OF MEXICO

24"33'30"

1/2 KILOMETER

81*48'

Figure 14. Lines of equal chloride concentration at a depth of 4 feet below the water table, October 21,1986.

Characteristics of the Freshwater Lens 13
24'33'30"

EXPLANATION |

25O LINE OP EQUAL CHLORIDE

CONCENTRATION Interval in 

milligrams per liter, is variable

 MO-152 WELL AND U.S. GEOLOGICAL 

SURVEY NUMBER

1/2 KILOMETER

81*45'

Figure 15. Lines of equal chloride concentration at a depth of 4 feet below the water table, April 1,1987.

The surface-geophysical resistivity measurements 

using the EM-16R VLF meter were made during November 

20-21, 1986. Measurements were recorded at 22 sites 

concentrated in the western half of the island. The 

approximate limits of the freshwater lens as defined by the 10 

ohm-meter line of equal resistivity, located under the central 

part of the western half of the island, are shown in figure 16. 

The highest reading (13.5 ohm-meters) was obtained near the 

site of a former freshwater spring. The zones of freshwater 

and salinity defined by the resistivity lines in figure 16 and 

the chloride concentration lines in figures 14 and 15 are in 

general agreement.

GROUND-WATER QUALITY

The quality of ground water can be described in terms 

of its chemical constituents and physical and biological 

characteristics. The quality is usually defined according to its 

intended use: potable, recreational, agricultural, or industrial. 

The water is considered to be of poor quality or contaminated 

if a particular constituent exceeds the level or limit set for the

intended use. The contamination may be a health hazard, an 

esthetic degradation, or a threat to the aquatic environment. 

Possible sources of ground-water contamination at 

Key West include many of those common to a modern 

urbanized society, including landfill disposal of household 

and industrial refuse (Stock Island); pesticide and fertilizer 

applications; stormwater runoff; and industrial enterprises 

(such as maintenance of airplanes and fuel storage tanks).

Major Inorganic Constituents and 

Physical Properties

The shallow ground water was sampled at depths of 2 to 

5 feet below the water table on October 21-22,1986, for major 

inorganic ions and related characteristics (table 2). The sampling depth was selected on the basis of salinity (or chloride 

concentration) at the time of collection; the objective was to 

obtain samples of the freshest water, the most likely to be 

withdrawn for domestic use. Sampling was conducted during 

various phases of the tide cycle and at various ground-water 

stages.

14 Water-Resources Potential of the Freshwater Lens at Key West, Florida
24*33'30"

EXPLANATION

 2 LINE OF EQUAL RESISTIVITY 

Interval in ohm-meters, is variable,

 MO-152 WELL AND U.S. GEOLOGICAL

SURVEY NUMBER 

A 10 GEOPHYSICAL SURVEY SFTE--

Number is resistivity, in ohm-meters

GUL F OF MEXICO

81*48' si MsFigure 16. Surface-geophysical survey sites and lines of equal resistivity, November 20,1986.

The analytical results of this sampling 

reconnaissance indicate that concentrations of major 

inorganic constituents generally increase with increases in 

chloride concentration. The chloride concentration is a 

good indicator of mixing in that chloride ions enter into few 

chemical or biochemical reactions, and are retained in 

solution through most of the processes which separate 

other ions (Hem, 1985, p. 120). A relation between chloride 

concentrations and other major constituent concentrations 

indicates that the major inorganic composition throughout 

the well water column likely results from a simple mixing 

of freshwater and saltwater. As mentioned earlier, the 

maximum contaminant level (MCL) for chloride is 250 

mg/L. However, this is a consideration for taste rather than 

for human health. People and vegetation can tolerate even 

higher concentrations of chloride (National Academy of 

Sciences, National Academy of Engineering, 1974, p. 328; 

U.S. Environmental Protection Agency, 1986b, p. 206). As 

a result, water with chloride concentrations exceeding 250 

mg/L is pumped frorn many domestic wells in Key West 

that tap various depths and areas of the ground water for 

household and lawn irrigation (Rosemary Vogenet, Key 

West Planning Department, oral commun., 1989).

Wells MO-148, MO-152, and MO-155, sampled 

during various phases of the tide cycle, yielded water with 

chloride concentrations ranging from 45 to 150 mg/L, which 

were among the lowest concentrations measured. The water 

in these wells was a calcium bicarbonate water (table 2). 

However, as the saltwater content increased and chloride 

concentrations exceeded 1,000 mg/L (as shown in some of 

the other well water samples), the water became a sodium 

chloride water with large amounts of magnesium, sulfate, 

and potassium.

Water in the Key West freshwater lens is similar to that 

in the Biscayne aquifer (Sonntag, 1987, p. 18) of adjacent 

Bade County in that these waters have similar major 

constituent concentrations. The freshwater lens has a slightly 

higher specific conductance and greater concentrations of 

dissolved solids and sulfate than water in the Biscayne 

aquifer.

Nutrients

Wells MO-147 to MO-158 were sampled in October 

1986, March 1987, and September 1987, for organic

Ground-Water Quality 15
Table 2. Chemical and physical characteristics of Key West ground water at depths of 2 to 5 feet below the water table, 

October 21-22,1986

[Concentrations are shown in milligrams per Uter, except where noted; sampling depth is shown in feet below die water table; <, less than the value. Specific 

conductance for well MO-156 is a Geld value]

WeH 

number

MO-148

MO-149

MO-151

MO-152

MO-153

MO-154

MO-155

MO-156

MO-157

Sampling 

depth 

(feet)

4

2

4

5

2

2

2

4

2

Well 

number

MO-148

MO-149

MO-151

MO-152

MO-153

MO-154

MO-155

MO-156

MO-157

Calcium 

(Ca)

90

130

200

130

140

180

110

450

450

Mag- Sulnesium Sodium Chloride fate Silica 

(Ma) (Na) (d) (S04) (SiOi)

11 29

34 320

240 1,900

5.2 77

73 570

200 1,600

17 65

1,400 12,000

1,400 12,000

Sampling Alkalinity 

depth (asCaCOs) 

(feet)

4

2

4

5

2

2

2

4

2

200

247

270

289

339

250

232

250

45

530

3,700

130

1,000

2,900

150

20,900

21,000

Dissolved 

solids

408

1,990

7,490

672

2,440

6,240

620

41,199

41,500

48

150

460

70

180

440

45

2,699

2,900

Chemical 

oxygen 

demand

<10

55

32

44

36

73

<10

250

390

6.0

8.3

9.3

7.7

6.8

7.3

6.4

1.9

1.7

Specific 

conductance 

(microsiemens 

per centimeter)

701

2^00

11,600

971

3,980

9,680

1,000

52,000

54,200

Potas- Fluosium ride 

(K) (F)

3.3

39

78

2.2

23

75

5.2

500

490

0.5

1.1

1.6

.7

.6

1.0

.5

1.6

1.6

pH 

(standard 

units)

7.9

7.8

7.7

7.6

7.8

7.6

7.4

7.4

Bicarbonate 

(HC03)

244

301

330

352

412

305

183

Color 

(platinumcobalt 

units)

<5

10

<5

<5

20

5

20

10

Hardness 

(as 

CaCOa)

273

-

1,501

351

654

1,282

348

6,899

6,899

nitrogen, ammonia, nitrate, nitrite, phosphorus, and total 

organic carbon (TOC). Wells MO-148, MO-149, MO-153, 

and MO-154 were sampled a fourth time in January 1988 

because of relatively high nitrate-nitrogen concentrations in 

the earlier samples. Wells MO-147 and MO-150 were 

sampled at a depth of 2 feet below the water table. The 

remaining wells were sampled at depths of 5 to 15 feet 

(5-foot depth intervals) below the water table, depending on 

salinity. The shallower part of the lens that is most likely to 

be tapped for domestic wells was emphasized for sampling. 

Table 3 presents the results of the nutrient analyses of 

the samples collected at various well depths during the four 

sampling events. Of the nutrients analyzed, nitrate nitrogen 

was the most abundant with most samples having concentrations exceeding 1 mg/L; some samples had concentrations 

exceeding 5 mg/L. Nitrate-nitrogen concentrations that 

exceed 1 mg/L usually signify man's influence (National 

Academy of Sciences, National Academy of Engineering, 

1974, p. 73), indicating sources such as fertilizer or domestic 

sewage. For comparison, nitrate-nitrogen concentrations in 

the Biscayne aquifer generally are less than 0.10 mg/L (Irwin 

and Healy, 1978, p. 35; Sonntag, 1987, p. 32). The average 

nitrate-nitrogen concentrations for each well site in Key West 

are shown in figure 17.

In the western half of the island, average 

concentrations of nitrate nitrogen in water samples from 

wells MO-147 to MO-155 ranged from 0.52 to 6.06 mg/L. 

The maximum nitrate-nitrogen concentration detected in any 

well was 11 mg/L (MO-149), which exceeded the MCL (10 

mg/L) of the Primary Drinking Water Regulations 

established by the Florida Department of Environmental 

Regulation (1983). In the eastern part of the lens, nitratenitrogen concentrations were low, ranging from less than 

0.01 mg/L to 034 mg/L in wells MO-156, MO-157, and 

MO-158. A water sample from well MO-158 had a total 

nitrogen concentration of 7.53 mg/L (primarily ammonia) in 

October 1986. The high ammonia concentration likely was a 

result of decomposing organic matter in a drainage ditch near 

the well.

No relation exists between nitrate nitrogen and the 

salinity of the sampled water (table 3), indicating that 

saltwater intrusion contributes little to the nitrate 

concentration. For example, in October 1986, well MO-148 

had a nitrate-nitrogen concentration of 6.50 mg/L and a 

chloride concentration of 45 mg/L. At the same time, well 

MO-154 had a nitrate-nitrogen concentration of 5.35 mg/L 

and a chloride concentration of 2,900 mg/L.

16 Wafer-Resource* Potential of the Freshwater Un* at Key West, Florida
Table 3. Nutrient concentrations in Key West ground water at various well depths, October 1986 through January 1988

[Concentrations shewn in milligrams per liter; , not sampled; <, lew than the value]

Well 

number

MO-147

MO-148

MO-149

MO-150

MO- 151

MO- 152

MO-153

Sampling 

depth 

(feet)

2

5

10

5

10

15

2

5

15

5

10

15

5

Date of 

sampling

Oct. 1986

March 1987

Sept. 1987

Jan. 1988

Oct. 1986

March 1987

Sept. 1987

Jan. 1988

Oct. 1986

March 1987

Sept. 1987

Jan. 1988

Oct. 1986

March 1987

Sept. 1987

Jan. 1988

Oct. 1986

March 1987

Sept. 1987

Jan. 1988

Oct. 1986

March 1987

Sept. 1987

Jan. 1988

Oct. 1986

March 1987

Sept. 1987

Jan. 1988

Oct. 1986

March 1987

Sept. 1987

Jan. 1988

Oct. 1986

March 1987

Sept. 1987

Jan. 1988

Oct. 1986

March 1987

Sept. 1987

Jan. 1988

Oct. 1986

March 1987

Sept. 1987

Jan. 1988

Oct. 1986

March 1987

Sept. 1987

Jan. 1988

Oct. 1986

March 1987

Sept. 1987

Jan. 1988

Ammonia 

nitrogen

0.04

.04

.03

.03

.02

.03

.03

<.01

.02

.04

.01

<.01

.02

.03

.01

_

.03

.04

.01

_

.04

.71

.03

Si

.02

.02

.03

.63

.02

.04

.03

.02

.02

2.80

1.00

.42

2.20

.70

.43

.02

.02

.02

.02

Organic 

nitrogen

0.34

.26

.47

.75

.45

Si

.79

.33

<.20

.28

.40

.54

.68

.23

.64

.60

.62

.66

_

.44

.99

1.97

.88

.38

.25

.34

.67

31

.41

.73

.45

.35

.40

.60

.68

.20

.50

SJ

.50

.25

.26

.56

Nitrite 

nitrogen

0.16

.05

<.01

<.01

<.01

<.01

.01

<.01

<.01

<.01

<.01

<.01

<.01

<.01

.01

<.01

.09

.01

_

.01

<.01

.01

<.01

<.01

<.01

<.01

.21

.02

.04

<.01

<.01

<.01

<.01

.04

.02

<.01

.12

.02

<.01

.01

<.01

.03

Nitrate 

nitrogen

3.24

2.45

6.6

6.50

7.4

5.8

4.10

6.70

4.4

9.4

4.00

5.00

11

.92

9.00

.20

931

8.20

_

3.40

.02

.49

<.01

1.80

.28

1.10

1.79

138

1.96

.95

1.7

1.1

.02

.30

.43

<.02

31

.43

5.00

2.69

4.6

337

Orthophosphate

0.04

.05

.02

.02

.02

.01

.01

.01

.02

.02

.01

.02

.03

.13

.01

_

.02

.02

.02

_

.04

.02

.08

.03

.02

.05

.02

.02

.02

.02

.02

.01

.02

.03

.01

.01

.02

.01

.01

.02

.01

.02

.02

Phosphorus 

asPO4

8.18

.15

.06

.12

.06

.03

.03

.123

.06

.06

.03

.123

.09

.40

.03

_

.06

.06

.06

_

.12

2.48

.24

.09

.184

.15

.06

.153

.06

.06

.153

.03

.06

.15

.03

.03

.092

.03

.03

.092

.03

.06

.06

Total 

phosphorus

0.06

.06

.03

.04

.03

.03

.04

.04

.03

.03

.04

.04

.03

.14

.02

_

.02

.03

.03

_

.04

.81

.18

.11

.06

.06

.08

.05

.02

.03

.05

.02

.05

.05

.02

.05

.03

<.02

.04

.03

<.02

.04

.05

Chloride

7,440

5,108

45

120

275

4,390

500

580

600

1,550

600

4,820

3,400

2,080

950

1360

2,965

2,614

160

180

310

340

310

400

400

3,675

Total 

organic 

carbon

3.1

5.0

5.0

2.0

6.2

6.0

2.0

5.0

8.0

8.0

7.0

_

11.0

_

6.8

33.0

6.0

7.1

9.0

6.5

6.8

6.8

2.9

7.0

5.0

6.8

7.2

3.2

6.0

3.8

6.0

4.5

Ground-Water Qualify 17
Table 3. Nutrient concentrations in Key West ground water at various well depths, October 1986 through January 1988 Continued

Well 

number

MO-154

MO-155

MO-156

MO-157

MO-158

Sampling 

depth Date of 

(feet) sampling

10 Octl986

March 1987

Sept 1987

Jan. 1988

Jan. 1988

15 Octl986

Match 1987

Sept 1987

Jan. 1988

5 Octl986

Match 1987

Sept 1987

Jan. 1988

10 Oct 1986

Match 1987

Sept 1987

Jan. 1988

15 Oct 1986

March 1987

Sept 1987

Jan. 1988

5 Oct 1986

Match 1987

Sept 1987

Jan. 1988

10 Oct 1986

Match 1987

Sept 1987

Jan. 1988

15 Oct 1986

March 1987

Sept 1987

Jan. 1988

5 Oct 1986

March 1987

Sept 1987

Jan. 1988

5 Oct 1986

Match 1987

Sept 1987

Jan. 1988

10 Oct 1986

March 1987

Sept. 1987

15 Oct 1986

Match 1987

Sept 1987

Jan. 1988

5 Oct 1986

March 1987

Sept 1987

Jan. 1988

Ammonia 

nitrogen

.03

.02

.03

.02

.02

.02

.04

.06

.02

0.02

.03

.04

.01

.13

.04

.05

.02

.02

.02

.02

.03

<.01

.02

.04

.01

58

.03

56

.46

.28

.48

.46

.28

.42

Jan. 1988

_

33

.47

6.50

1.80

Organic 

nitrogen

35

35

37

56

.40

.25

.43

.47

.60

035

1.27

1.16

.56

34

33

.27

.33

.27

.63

1.18

<.17

21

.41

.26

.26

.01

.88

.40

.54

.82

.92

.26

52

.98

_

1.67

1.53

1.00

.80

Nitrite 

nitrogen

.03

.01

.01

.03

.03

.02

.05

.01

.02

035

<.01

50

.01

.22

<.01

.15

.01

.01

<.01

<.01

.01

<.01

<.01

<.01

<.01

.01

<.01

.01

.01

<.01

.01

<.01

<.01

.01

__

<.01

.01

<.01

<.01

Nitrate 

nitrogen

257

2.79

3.99

337

2.97

258

135

3.19

2.88

535

.14

3.1

1.60

5.48

3.40

3.95

3.80

4.39

.84

.52

53

1.40

.66

1.10

1.40

.01

.34

.02

.02

.01

.01

.02

.04

.01

_

<.01

.01

.03

<.01

Orthophosphate

.02

.01

.02

.02

.02

.02

.03

.03

.02

0.02

.02

.03

.02

.03

.03

.04

.03

.04

.02

.02

.02

.02

.01

.02

.02

.03

.12

.04

.02

.01

.04

.04

.01

.03

_

.01

.04

.08

.06

Phosphorus 

asP04

.092

.03

.06

.06

.06

.123

.09

.09

.06

0.092

.06

.09

.06

.184

.09

.12

.09

.12

31

.06

.06

.123

.03

.06

.123

37

.12

.06

.03

.12

.12

.03

.09

 

_ __

.03

.12

.18

.18

Total 

phosphorus

.03

<.02

.07

.05

.03

.04

.05

.04

.03

0.03

.06

.06

.02

.06

.04

.05

.04

.04

.10

.29

.06

.04

.02

.04

.04

.06

.14

.06

.10

.09

.12

.08

.07

.23

_

.10

.13

.06

.07

Chloride

834

5,120

_

853

5,300

2,900

1,140

6,603

9,413

11500

11,784

2,268

500

2,270

500

2,270

540

2,278

679

12,800

1,060

19,000

19,120

19,000

14,120

19,000

19,000

Total 

organic 

carbon

2.0

6.2

_

2.0

7.0

7.0

12.0

12.0

4.0

5.8

25

6.0

3.8

25

6.0

3.8

2.0

8.0

2.0

45

14.0

3.8

25

6.2

 

_

<1.0

19.0

1.0

-

18 Water-Resource* Potential of the Freshwater Lent at Key West, Florida
24*34-30"

24*33'30-

EXPLANATION

A MO 1%

058 WELL Upper number is U.S. 

Geological Survey well number. 

Lower number is nitrate nitrogen 

concentration, in milligrams per liter

81*48' 81*47' 81*48' 81*45'

Figure 17. Average concentrations of nitrate nitrogen in water from wells at depths of 2 to 15 feet below the water table at Key West, 

October 1986 through January 1988.

Concentrations of the other nutrients sampled in the 

well waters were low in comparison to nitrate nitrogen. 

Nitrite nitrogen, organic nitrogen, and ammonia nitrogen 

concentrations were less than 1 mg/L in nearly all the 

samples, and total phosphorus concentrations averaged 0.06 

mg/L. TOC concentrations ranged from less than 1.0 to 33.0 

mg/L, with an average for all samples of 6.5 mg/L.

Trace Elements

Trace-element concentrations were determined for well 

waters at depths of 2 to 5 feet below the water table in October 

1986, March 1987, September 1987, and April 1988. Trace 

elements that were analyzed are those commonly found in 

stormwater runoff or those that can be hazardous to human 

health when found in concentrations exceeding the MCUs of 

the Primary Drinking Water Regulations established by the 

Florida Department of Environmental Regulation (1983). The 

elements that were analyzed included arsenic, cadmium, chromium, iron, lead, mercury, selenium, and silver. Although iron 

is not considered hazardous, it was included in the analyses 

because it is a common and often undesirable constituent of

potable water. A MCL of 300 jxg/L (micrograms per liter) for 

iron is included in the Secondary Drinking Water Regulations 

(Florida Department of Environmental Regulation, 1983). 

Table 4 presents the results of analyses of trace elements in 

unfiltered samples of Key West ground water.

All trace-element concentrations were less than the 

MCL's, except for total lead at wells MO-155 to MO-157 and 

total iron at wells MO-150 and MO-154 to MO-157. Lead 

concentrations equaled the MCL (50 \ig/L) at well MO-155 

in March 1987, and exceeded the MCL at well MO-156 

(70 ng/L) in October 1986 and at well MO-157 (800 and 60 

Hg/L) in October 1986 and March 1987, respectively. Lead 

concentrations did not exceed the MCL in additional samples 

from wells MO-155 and MO-156 (table 4). In contrast, 

samples from well MO-157 had high lead concentrations 

during each sampling event; this well also had the highest 

iron concentrations of any of the wells sampled. The high 

concentrations of lead and iron in ground water at well 

MO-157 may be due to the fact that the well is located near 

an old landfill. Concentrations generally decreased with 

increasing distance west of the landfill site. Average 

concentrations of lead and iron at each well site for October 

1986 through September 1987 are plotted in figure 18.

Ground-Water Quality 19
Table 4. Trace-element concentrations tn Key West ground water at depths of 2 to 5 feet below the water table, October 1986 

through April 1988

[Total concentrationi shown in micrograms per liter; , not sampled; <, less than the value; MCL, nummmn contaminant levels Cor all the trace elements are included in the Primary Drinking TOter Regulations, except for iron which is included in the Secondary Drinking Wrter Regulations (Florida Department of Environmental Regulation, 1983)]

Well 

number

MO-147

MO-148

MO-149

MO-150

MO-151

MO-152

MO-153

MO-154

MO-155

MO-156

MO-157

MCL

 3!*1

(feet) Date of sampling

2 October 1986

March 1987

September 1987

April 1988

5 October 1986

March 1987

September 1987

April 1988

5 October 1986

March 1987

September 1987

April 1988

2 ' October 1986

March 1987

September 1987

April 1988

5 October 1986

March 1987

September 1987

April 1988

5 October 1986

March 1987

September 1987

April 1988

5 October 1986

March 1987

September 1987

April 1988

5 October 1986

March 1987

September 1987

April 1988

5 October 1986

March 1987

September 1987

April 1988

5 October 1986

March 1987

September 1987

April 1988

5 October 1986

March 1987

September 1987

April 1988

Arsenic 

(As)

_

4.0

5.0

16

1.0

1.0

1.0

1.0

<1.0

<1.0

2.0

3.0

1.0

 

1.0

1.0

50

Cadmium Chromium Iron 

(Cd) (Cr) (Fe)

_ _ _

1.0 4.0 70

 

<1.0 1.0 90

1.0 <1.0 90

<1.0 <1.0 80

 

 

<1.0 <1.0 70

<1.0 2.0 30

 

 

 

<1.0 <1.0 960

 

<1.0 1.0 160

 

<1.0 <1.0 50

 

<1.0 <1.0 70

2.0 <1.0 100

<1.0 <1.0 120

 

<1.0 1.0 110

<1.0 <1.0 80

1.0 <1.0 70

 

<1.0 .2 130

 

<1.0 2.0 360

 

<1.0 3.0 330

<1.0 <1.0 320

1.0 <1.0 120

 

2.0 5.0 740

 

<1.0 1.0 350

 

<1.0 2.0 1,900

1.0 <1.0 1,500

1.0 <1.0 600

 

10 50 300

Lead 

(Pb)

_

<5

10

20

10

 

20

10

<5

10

<5

6

10

20

20

10

<5

20

30

30

50

9

70

10

800

60

40

50

Mercury 

(Hg)

__

0.2

.1

5

.1

.5

.1

.1

.2

.1

.1

_

.2

.2

<.l

.5

.1

.1

.1

<.l

1.1

.1

.90

.1

__

.8

.1

2.0

Selenium 

(Se)

_

2.0

4.0

6.0

<1.0

1.0

5.0

<1.0

 

2.0

2.0

2.0

2.0

1.0

3.0

1.0

<1.0

<1.0

<1.0

<1.0

<1.0

<1.0

1.0

<1.0

<1.0

<1.0

<1.0

10.0

Silver 

(Ag)

__

1.0

<1.0

2^0

1.0

<1.0

1.0

1.0

 

<1.0

1.0

<1.0

LO

1.0

<1.0

<1.0

1.0

1.0

1.0

1.0

1.0

1.0

1.0

1.0

2.0

2.0

1.0

50.0

20 Water-Resource* Potential of the Freshwater Lent at Key West, Florida
24«34'30"

EXPLANATION 

MO-152 WELL AND U.S. GEOLOGICAL

SURVEY NUMBER Upper number of 

fraction is lead concentration and lower 

number is iron concentration.in 

micrograms per liter, 'asterisk indicates 

only one sample collected.<less than 

indicated value.Blank where no data 

collected.

GULF OF MEXICO

1/2 MILE 

1/2 KILOMETER

81*48' 81*47' 81*46'

Figure 18. Average concentrations of lead and iron in water from wells at depths of 2 to 5 feet below the water table at Key West, 

October 1986 through September 1987.

Mercury concentrations in all the well waters ranged 

from less than 0.1 {ig/L to as much as 1.1 \igfL. Although these 

concentrations do not exceed the MCL(2 jig/L) of the Primary 

Drinking Water Regulations (Florida Department of 

Environmental Regulation, 1983), the range of concentrations 

in Key West ground water was greater than the 0.0 to 03 {ig/L 

range reported for the Biscayne aquifer in Dade County (Irwin 

and Healy, 1978; Irwin and Hull, 1979; Sonntag, 1987).

Synthetic Organic Compounds

On September 1, 1987, water samples were collected 

from wells MO-147 through MO-156 for GC/FID scan 

analysis. The GC/FID analysis, or FID scan, is a screening 

technique for the determination of synthetic organic 

compounds in water. The FID scan analysis provides semiquantitative data without identification of specific 

compounds. The sensitivity of the FID scan is about 10 times 

greater for most compounds than the GC/MS method. 

Results of the FID scan indicated that all the samples 

contained detectable, but very low, concentrations of organic 

compounds. The highest relative magnitude of organic

compounds was found in water samples from wells MO-148 

and MO-152 near the center of the freshwater lens.

A subsequent set of samples collected at the 

monitoring wells in December 1987 had the greatest 

occurrence of measurable organic compounds. Water 

samples were collected from wells MO-152, MO-153, 

MO-154, and MO-148 for a GC/MS analysis of selected 

pesticides and synthetic organic contaminants (commonly 

termed priority pollutants). The priority pollutants are 

compounds designated by the U.S. Environmental Protection 

Agency (1980) as a human health concern for the general 

public. The compounds include volatiles, acid and 

base-neutral extractables, and pesticides. A list of the 

compounds for which analyses were made and the laboratory 

analytical detection limits is given in table 5. The list also 

includes Florida Department of Environmental Regulation 

standards for seven pesticides.

Pesticides analyzed included carbamate insecticides, 

chlorophenol acid herbicides, organochlorine compounds, and 

organophosphorus insecticides. The only pesticides detected 

were two organochlorine compounds: DDT (0.003 \ig/L) and 

its metabolite DDE (0.002 |ig/L) at well MO-148. These 

concentrations only slightly exceeded the analytical detection

Ground-Water Qualify 21
Table 5. Synthetic organic compounds for which gas chromatograph/mass spectrometer analyses were determined

[Compounds were analyzed using laboratory standard solution of known chemical composition. All concentrations are in micrograms per liter. Numbers in parenlfaeses are waterquality standards from the Florida Department of Environmental Regulation, 1963]

Type and name 

of chemical

\Watiles

Benzene

Bromofortn

Carbon tetrachloride

Chloroenzene

Chlorodibromomethanc

Chloroethane

2-Chioroetfaylvinyl ether

Chloroform

1 *> rWlia»t«M«li«illMl* i,z-LnDrocaemyMne 

1,2-trans Dichlorethylene

DicbJorobromomemane

Dicnlorodifluorometnane

1,2-Di chloroethane

1,1-Dichloroetfaylene

1,2-Dichloropropane

1,3-Dichloropropene

cts-irJ-Lncnioropropene

trans-l,3-Dichloropropeoe

Ethylenzene

Methyl bromide

Methyl chloride 

Methylene chloride 

1,1,2, -Tetrachloroethane

Tetrachloroethylene

Toluene

1,1,1-Trichkxoethane

1, 1,2-Trichloroethane

Trichloroethylene

Trichtorofluormethane

Vinyl chloride

Mixtures

Pdchtorinated naphthalenes

Pdychlorinated biphenyls

Base-neutral extractables

Acenaphtheoe

Acenphythyiene

Anthracene

Ijenz(a)antnracene

Deozofajpyreue

Benzo(g,h,i)pefylene

BenzoQrtfluorautheue

Benzo(b)fluorantbene

4-Bromophenyl phenyl etber 

N-Butylbenzyl phthalate 

87Sbis (2Odoroelhoxy>nethane

bis(2-Chloroerbyl)ether

bis?2-Chtaroisopropy])ether

2-Chloronaphthalene

4-CMorophenyl phenyl enter

Cbrysene

1^5,6-Dibenzo(a>h)Bnthracene

Di-n-butyl phthalate 

1,3-Dicnlorobenzene

1,4-Ehchlorobenzene

1,2-DichIorobenzene

Didhyl phthalate

Dimethyl phthalate 

2,6-Diiutrotduene

2,4-Dinitrotduene

Di-n-octylphthalate

bis(2-E%lhexyl)phtha]ate

Fluorantfaene

Fluorene

Hexachlorobenzene

Hexachlorobutadiene

Hexchtorocyclopentadiene

Detection 

limit

0.20

.20

.20

.20

.20

.20

.20

.20

*V\ .ZU

.20

.20

.20

.20

.20

.20

.20

.20

.20

.20

.20

.20 

.20 

.20

.20

.20

.20

.20

.20

.20

.20

.1

.1

5.0

5.0

5.0

10.0

10.0

10.0

10.0

10.0

5.0 

5.0 

5.0

5.0

5.0

5.0

5.0

10.0

10.0

5.0 

5.0

5.0

5.0

5.0

5.0 

5.0

5.0

10.0

5.0

5.0

5.0

5.0

5.0

5.0

Type and name 

of chemical

Base- neutral extractables

HencUoroethane

lndeoo(U23-«Opyrene

Isophorone

Naphthalene

Nitrobenzene

N-Nitrosodi-N-propylamine

N-Nitrosodimetnylamine

N-Nitrosodiphenylamine

Pbenanthrene 

Pyrene

1,2,4-TVichlorobemene

Acid extractables

4-CbJoro-3-methylphenol

2-Chlorophenol

2,4-Dichkjropheool

2,4-Dimethylpbenol

4,6-Dinitro-2-methylpbenol

4,6-Dinitro-orthocread

2,4-Dinitrophenol 

2-Nitropbenol 

4-Nitrophenol

ParacbJorometacresol

Pentachloropbenol

Phenol

2,4,6-Trichloropheod

Pesticides

Aldrin

Ametryne

Atrazine ^*t_i- ^ j-_ _ Cnlordane

Cyanazine

ODD

DDE

DOT

Dieldrin

Diazinon

EDB

Endosulfan

Endrin

Ethion

Ethyl trirhion

Heptachlor

Heptachlor epoxide 

Lindane 

Malathion

Methoxychlor(100)

Methyl parathion

Methyl trirhion

Metbomyl

Mirex

Parathion

Perthane 

Proroetone

Prometryne

Propazine

Propham

Sevin 

Sirvex

Simazine

Simetryne

Styrene

Toxaphene

Trithion

2,4-D

2,4-DP

2.V-T

Xykoe

Detection 

limit

5.0

10.0

5.0

5.0

5.0

5.0

5.0

5.0

5.0 

5.0

5.0

30.0

5.0

5.0

5.0

30.0

30.0

20.0 

5.0 

30.0

30.0

30.0

5.0

20.0

.001

.1

.1 

.1

.1

.001

.00

.001

.001

.01

(0.02) .0

.001

(0.2) .001

.01

.01

.001

.001 

(4) .00,

.01

.01

.01

2.00

.01

.01 .1 .1.1.1

2.0

2.0

(10) .01

.1

.1

.2

(5) 1.0

.01

(100) .01

.01

.01

.2

limit. Of the other organic compound groups, only 

dichlorobromomethane (1.8 ng/L) and chloroform (12 ng/L) 

at well MO-152 were detected. Both concentrations exceeded 

the analytical detection limit of 0.20 ng/L. There are no

drinking water MCL's for these four contaminants, but the U.S. 

Environmental Protection Agency (1986a; 1986b) 

recommends that ambient water concentrations be below 

detection for optimum defense of human health.

22 WateMtetourcet Potential of the Freshwater Lens at Key West, Florida
Bacterial Analyses

The high concentrations of nitrate nitrogen found in 

ground water during the sampling reconnaissance indicated 

the possible presence of sewage, which commonly contains 

high concentrations of nutrients and bacteria. It is possible 

that raw sewage may have been leaking from the network of 

underground pipes that transport the waste to the central 

outfall pipe. To assess this possibility, on January 27,1988, 

wells MO-148, MO-152, MO-154, and MO-155, which had 

high nitrate-nitrogen concentrations and were located near 

the center of the lens, were sampled for the presence of total 

coliform, fecal coliform, and fecal streptococci bacteria. 

Water samples were taken at depths of 5, 10, and 15 feet 

below the water table.

The samples at a depth of 5 feet below the water table 

from wells MO-148, MO-152, and MO-153 were freshwater. 

The other water samples ranged from very slightly saline to 

slightly saline. Sodium chloride has an inhibitory influence 

on coliform bacteria, but the concentrations found in the 

sampled waters probably had little or no toxic effect on the 

bacteria (Salle, 1954, p. 221; U.S. Environmental Protection 

Agency, 1976, p. 45).

The results were inconclusive. The samples from well 

MO-154 showed the presence of fecal coliform at the 5-foot 

depth, but there was no indication of total coliform. The 

water samples from the 10-foot depth at well MO-148 

produced a growth of unidentified bacteria. Coliform and 

fecal streptococci bacteria were not detected in any of the 

other samples.

SUMMARY AND CONCLUSIONS

Key West is underlain by oolitic limestone (Miami 

Oolite) that extends to a depth of about 200 feet. The oolitic 

limestone has a high permeability and is honeycombed with 

solution holes that allow rainwater to rapidly escape to the 

sea and seawater to easily infiltrate. There is a substantial 

amount of freshwater only in the western half of the island.

The freshwater floats in a lens-shaped configuration on 

the saltwater as a result of density differences. The 

freshwater lens (chloride concentration of 250 mg/L or less) 

is thickest, averaging about 5 feet, in the center of the Old 

Town area. Underlying the freshwater is the transition 

zone a mix of freshwater and saltwater that increases in 

salinity vertically until the saltwater interface is reached at a 

depth of about 40 feet.

As a result of tidal effects, the water table fluctuates 

constantly. During the investigation, it ranged from 0.8 to 2.4 

feet above sea level at the center of the island and from 0.4 to 

2.2 feet above sea level near the coast. Tidal effects cause 

continual change, both vertically and are ally, in the

configuration of the freshwater lens and salinity of the 

transition zone.

The amount of freshwater varies with rainfall, seepage 

to the ocean, evapotranspiration, and withdrawal. The effect 

of evaporation and withdrawals on water levels is largely 

masked by tidal influence. During the investigation, the 

estimated freshwater volumes varied from 30 million gallons 

in the wet season to 20 million gallons in the dry season. The 

water-table gradient varied from about 4 to 1 ft/mi.

The freshwater lens is a calcium bicarbonate water. 

Below the freshwater lens, the chloride concentration 

increases with depth, and the water becomes a sodium 

chloride water with large amounts of magnesium, sulfate, 

and potassium. Concentrations of most major inorganic 

constituents increase with salinity. The chemistry of the 

major inorganic constituents in the freshwater lens and 

transition zone is characterized by a simple mixing of 

freshwater and saltwater.

Nitrate-nitrogen concentrations ranged from less than 

1 mg/L to as much as 11 mg/L, indicating the possible 

presence of sewage and the possibility of related bacterial 

contamination. Bacterial analyses were inconclusive, but 

fecal coliform was found in one of the four wells tested. The 

highest concentrations of nitrate nitrogen were found in the 

center and western part of the lens.

Lead concentrations equaled or exceeded the 

maximum contaminant level (50 (Ag/L) in water from three 

observation wells, and iron concentrations exceeded the 

maximum contaminant level (300 pg/L) in water from five 

wells. The highest concentrations of lead and iron were 

found in the eastern part of the lens near an old landfill, but 

the concentrations decreased with increasing distance west 

of the site.

A one-time sampling of well water for pesticides and 

synthetic organic compounds indicated the presence of DDT 

(0.003 ng/L), DDE (0.002 ng/L), chloroform (1.2 ng/L), and 

dichlorobromomethane (1.8 pg/L), all slightly exceeding the 

analytical detection limits. The compounds were found near 

the center of the lens.

Because the freshwater lens is very thin, from less than 

1 foot near the edge to an average of about 5 feet near the 

center, any heavy pumping might quickly exhaust the freshwater supply and draw in saline water through the highly 

porous rock. The reconnaissance water-quality sampling also 

indicated that the lens water does not meet State drinking 

water standards for several constituents.

SELECTED REFERENCES

Brown, Eugene, Skougstad, M.W., and Fishman, M.J., 1970, 

Methods for collection and analysis of water samples for 

dissolved minerals and gases: U.S. Geological Survey Techniques of Water-Resources Investigations, Book 5, Chap. Al, 

160 p.

Selected References 23
Causaras, C.R., 1965, Geology of the surficial aquifer system, 

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 1987, Geology of the surficial aquifer system in Dade 

County, Florida: U.S. Geological Survey Water-Resources 

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Rshman, M J., and Brown, Eugene, 1976, Selected methods of the 

U.S. Geological Survey for the analysis of waste water: U.S. 

Geological Survey Open-File Report 76-177,87 p.

Fishman, JJ., and Friedman, L.C., eds., 1985, Methods for the 

determination of inorganic substances in water and fluvial 

sediments: U.S. Geological Survey Techniques of WaterResources Investigations, Book 5, Chap. Al, 709 p.

Florida Department of Environmental Regulation, 1983, Water 

quality standards: Chapter 17-3, in Florida Administrative 

Code.

Geonics, Ltd., 1979, EM-16 and EM-16R operating manuals: 

Mississauga, Canada, Geonics, Ltd.

Goeriitz, C.F., and Brown, Eugene, 1972, Methods for analysis of 

organic substances in water: U.S. Geological Survey 

Techniques of Water Resources Investigations, Book 5, Chap. 

A3,40 p.

Hem, J.D., 1985, Study and interpretation of the chemical 

characteristics of natural water: U.S. Geological Survey 

Water-Supply Paper 2254,263 p.

Hoffmeister, R.H., Stockman, K.W., and Multer, H.G., 1967, 

Miami Limestone of Florida and its recent Bahamian 

counterpart: Geological Society of America Bulletin, v. 78, 

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Irwin, G.A., and Healy, H.G., 1978, Chemical and physical quality 

of selected public water supplies in Florida, August-September 

1976: U.S. Geological Survey Water-Resources Investigations 

78-21,200 p.

Irwin, G.A., and Hull, R.W., 1979, Chemical, physical, and 

radiological quality of selected public water supplies in 

Florida, November 1977-February 1978: U.S. Geological 

Survey Water- Resources Investigations 79-50,169 p.

Mitchell-Tapping, H J., 1980, DepositionaJ history of the oolite of 

the Miami Limestone formation: New Orleans, Louisiana, 

Amoco Production Company, Florida Scientist, v. 43, no. 2, 

p. 116-125.

National Academy of Sciences, National Academy of Engineering, 

1974, Water-quality criteria, 1972: Washington, D.C., U.S. 

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National Oceanic and Atmospheric Administration, 1986, Tide 

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America: Rockville, Md., National Ocean Service, 269 p.

Parker, G.G., 1941, Memorandum on ground water of Key West

and Big Pine Key, Florida: U.S. Geological Survey Open-File

Report FL-41001, lip. 

Parker, G.G., Ferguson, G.E., Love, S.K., and others, 1955, Water

resources of southeastern Florida, with special reference to the

geology and ground water of the Miami area: U.S. Geological

Survey Water-Supply Paper 1255,965 p. 

Salle, AJ., 1954, Fundamental principles of bacteriology: New

York, McGraw Hill Book Company, Inc., 782 p. 

Sherwood, C.B., McCoy, H.J., and Galliher, C.F., 1973, Water

resources of Broward County, Florida: Florida Bureau of

Geology Report of Investigations 65,141 p. 

Skougstad, M.W., Fishman, M J., Friedman, L.C., Erdmann, D.E.,

and Duncan, S.S., eds., 1979, Methods for determination of

inorganic substances in water and fluvial sediments: U.S.

Geological Survey Techniques of Water-Resources

Investigations, Book 5, Chap. Al, 626 p. 

Sonntag, W.H., 1987, Chemical characteristics of water in the

surficial aquifer system, Dade County, Florida: U.S.

Geological Survey Water-Resources Investigations 87-4080,

42 p., 2 plates. 

Stewart, Mark, and Bretnall, R., 1986, Interpretation of VLF

resistivity data for ground-water contamination surveys:

Groundwater Monitoring Review, v. G., no. 1, p. 71-75. 

U.S. Environmental Protection Agency, 1976, Washington, D.C.,

July 1976,256 p.

 1980, Rules and regulations, National Primary Drinking 

Water Regulations-synthetic organic chemicals: Federal 

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 1966a, Fact sheet, drinking water regulations under 1986 

amendments to SDWA (safe drinking water act): Washington, 

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Water, WH-550D.

1966b, Quality criteria for water 1986: Washington, D.C.

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Wershaw, R.L., Fishman, M J., Grabbe, R.R., and Lowe, L.E., 

1983, Methods for the determination of organic substances in 

water and fluvial sediments: U.S. Geological Survey 

Techniques of Water-Resources Investigations, Book 5, 

Chap. A3,173 p.

24 Water-RMOUKM Potential of the Freshwater Lem at Key West; Florida