U.S. Department of the Interior

U.S. Geological Survey

Water Quality in the Upper Floridan Aquifer

and Overlying Surficial Aquifers, 

Southeastern United States, 1993–2010

Circular 1355

National Water-Quality Assessment Program

The Quality of Our Nation’s Waters
Cover. Wekiwa Spring, Wekiwa Springs State Park, Orange County, Florida. 

Photographs in this report are by Alan M. Cressler, U.S. Geological Survey, unless otherwise indicated.
The Quality of Our Nation’s Waters

Water Quality in the Upper Floridan Aquifer 

and Overlying Surficial Aquifers, 

Southeastern United States, 1993–2010

By Marian P. Berndt, Brian G. Katz, James A. Kingsbury, and Christy A. Crandall

National Water-Quality Assessment Program

Circular 1355

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2014

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Suggested citation:

Berndt, M.P., Katz, B.G., Kingsbury, J.A., and Crandall, C.A., 2014, The quality of our Nation’s waters—Water quality 

in the Upper Floridan aquifer and overlying surficial aquifers, southeastern United States, 1993–2010: U.S. Geological 

Survey Circular 1355, 72 p., http://dx.doi.org/10.3133/cir1355.

Library of Congress Cataloging-in-Publication Data

Berndt, Marian P.

 The quality of our nation’s waters---water quality in the upper Floridan aquifer and overlying surficial aquifers, 

Southeastern United States, 1993--2010 / by Marian P. Berndt, Brian G. Katz, James A. Kingsbury, and Christy 

A. Crandall, National Water-Quality Assessment Program.

 pages cm. -- (Circular, ISSN 1067-084X ; 1355)

 Includes bibliographical references.

 ISBN 978-1-4113-3753-4

1. Water quality--Southern States. 2. Groundwater--Southern States--Quality. 3. Drinking water--Contamination--

Southern States. 4. Floridan Aquifer. I. Katz, Brian G. II. Kingsbury, James A. III. Crandall, Christy A. IV. Geological 

Survey (U.S.) V. Title. VI. Series: U.S. Geological Survey circular ; 1355.

 TD223.5.B47 2014

 363.739’420975--dc23

 2014012827

ISSN 1067-084X (print)

ISSN 2330-5703 (online)

ISBN 978-1-4113-3753-4
iii

National Water-Quality Assessment Program

“The USGS NAWQA program is an important resource to Florida utilities using groundwater as 

a source for potable water. The program gathers vital data on the health of the Floridan Aquifer 

that is a primary source of potable water to the State of Florida. The NAWQA program also 

provides groundwater water quality data and studies on emerging groundwater contaminants 

that further advance our collective understanding of this complex aquifer system.”

 —John Troutt, Environmental Monitoring Manager at Tampa Bay Water

“Monitoring water quality is necessary to gauge the effectiveness of management practices 

that impact water quality. The USGS NAWQA studies allow us to be informed of water 

quality issues in other States and adjust our water quality monitoring needs accordingly. The 

NAWQA data within our State provides useful water-quality information that helps us to better 

understand our complex aquifer systems.”

 —Richard Hicks, Florida Department of Environmental Protection, 

 Ground Water Management Section
iv

Foreword

The United States has made major investments in assessing, managing, regulating, and conserving 

natural resources, such as water and a variety of ecosystems. Sustaining the quality of the 

Nation’s water resources and the health of our diverse ecosystems depends on the availability 

of sound water-resources data and information to develop effective, science-based policies. 

Effective management of water resources also brings more certainty and efficiency to important 

economic sectors. Taken together, these actions lead to immediate and long-term economic, 

social, and environmental benefits that make a difference to the lives of millions of people 

(http://water.usgs.gov/nawqa/applications/).

Two decades ago, Congress established the U.S. Geological Survey’s National Water-Quality 

Assessment (NAWQA) Program to meet this need. Since then, NAWQA has served as a primary 

source of nationally consistent information on the quality of the Nation’s streams and groundwater, 

on ways in which water quality changes over time, and on the natural features and human activities 

affecting the quality of streams and groundwater. Objective and reliable data, systematic scientific 

studies, and models are used to characterize where, when, and why the Nation’s water quality 

is degraded—and what can be done to improve and protect the water for human and ecosystem 

needs. This information is critical to our future because the Nation faces an increasingly complex 

and growing need for clean water to support people, economic growth, and healthy ecosystems. For 

example, NAWQA findings for public-supply wells, which provide water to about 105 million people, 

showed that 22 percent of source-water samples contained at least one contaminant at levels of 

potential health concern. Similarly, 23 percent of samples from domestic (or privately owned) wells, 

which supply untreated water to an additional 43 million people, also had contaminant levels of 

potential concern.

This report is one of a collection of publications that describe water-quality conditions in selected 

Principal Aquifers of the United States (http://water.usgs.gov/nawqa/studies/praq/). The collection 

is part of the series “The Quality of Our Nation’s Waters,” which describes major findings of the 

NAWQA Program on water-quality issues of regional and national concern and which provides 

science-based information for assessing and managing the quality of our groundwater resources. 

Other reports in this series focus on occurrence and distribution of nutrients, pesticides, and volatile 

organic compounds in streams and groundwater, the effects of contaminants and streamflow 

alteration on the condition of aquatic communities in streams, and the quality of untreated water 

from private domestic and public-supply wells. Each report builds toward a more comprehensive 

understanding of the quality of regional and national water resources (http://water.usgs.gov/nawqa/

nawqa_sumr.html). All NAWQA reports are available online at http://water.usgs.gov/nawqa/bib/.

The information in this series primarily is intended for those interested or involved in resource 

management and protection, conservation, regulation, and policymaking at regional and national 

levels. In addition, the information should be of interest to those at a local level who wish to know 

more about the general quality of streams and groundwater in areas near where they live and how 

that quality compares with other areas across the Nation. We hope this publication will provide you 

with insights and information to meet your needs and will foster increased citizen awareness and 

involvement in the protection and restoration of our Nation’s waters.

Jerad Bales 

Acting Associate Director for Water

U.S. Geological Survey
v

http://pubs.usgs.gov/sir/2014/1360

California Coastal Basin aquifers

Central Valley aquifer system

Basin and Range basin-fill aquifers

Rio Grande aquifer system

Snake River Plain basin-fill and 

basaltic-rock aquifers

Columbia Plateau basin-fill and

basaltic-rock aquifers 

Mississippi embayment–Texas coastal

uplands aquifer system—Circular 1356

Mississippi River Valley alluvial aquifer

Denver Basin aquifer system—Circular 1357

Piedmont and Blue Ridge 

carbonate-rock aquifers

Piedmont and Blue Ridge 

crystalline-rock aquifers

Valley and Ridge carbonaterock aquifers

High Plains aquifer system—Circular 1337

Upper Floridan aquifer and overlying

surficial aquifers—Circular 1355

Northern Atlantic Coastal Plain 

surficial aquifer system—Circular 1353

Glacial aquifer system—Circular 1352

Hawaiian volcanic-rock aquifers

Early Mesozoic basin aquifers

Valley and Ridge siliciclasticrock aquifers

Southwest Principal Aquifers—Circular 1358 Piedmont, Blue Ridge, and Valley and 

Ridge aquifers—Circular 1354

Western Volcanics—Circular 1359

Principal Aquifers and Water-Quality Summary Reports

National summary of groundwater quality in Principal Aquifers—Circular 1360

High Plains aquifer system—Circular 1337

Glacial aquifer system—Circular 1352

Northern Atlantic Coastal Plain surficial

aquifer system—Circular 1353

Piedmont, Blue Ridge, and Valley and 

Ridge aquifers—Circular 1354

Piedmont and Blue Ridge 

carbonate-rock aquifers

Piedmont and Blue Ridge 

crystalline-rock aquifers

Valley and Ridge siliciclasticrock aquifers

Valley and Ridge carbonaterock aquifers

Early Mesozoic basin aquifers 

Upper Floridan aquifer and overlying

surficial aquifers—Circular 1355

Mississippi embayment–Texas coastal 

uplands aquifer system—Circular 1356

Mississippi River Valley alluvial aquifer

Denver Basin aquifer system—Circular 1357

Southwest Principal Aquifers—Circular 1358

California Coastal Basin aquifers

Central Valley aquifer system

Basin and Range basin-fill aquifers

Rio Grande aquifer system 

Western Volcanics—Circular 1359

Hawaiian volcanic-rock aquifers

Snake River Plain basin-fill and 

basaltic-rock aquifers

Columbia Plateau basin-fill and 

basaltic-rock aquifers
vi

Contents

Chapter 1: Overview of Major Findings and Implications .......................................................1

Chapter 2: NAWQA Approach to Assessing Groundwater Quality .......................................7

Chapter 3: Environmental and Hydrogeologic Setting...........................................................15

Chapter 4: Natural Processes and Human Activities That Affect

 Groundwater Quality ................................................................................................25

Chapter 5: Quality of the Groundwater Resource Used for Drinking ..................................29

Chapter 6: Understanding Where and Why Key Contaminants Occur

 in Groundwater.........................................................................................................37

References Cited .........................................................................................................................57

Glossary.........................................................................................................................................61

Appendixes...................................................................................................................................65
vii

Introduction to This Report

This report contains the major findings of a regional assessment of water quality in the Upper 

Floridan aquifer of the Floridan aquifer system and overlying surficial aquifers in the Southeastern 

United States. It is one of a series of reports by the U.S. Geological Survey (USGS) National 

Water-Quality Assessment (NAWQA) Program that present major findings for Principal Aquifers, 

other aquifers, and major river basins across the Nation. In these reports, water quality is 

discussed in terms of local, State, regional, and national issues. Conditions in these aquifers are 

compared to conditions found elsewhere and to selected national benchmarks, such as those for 

drinking-water quality.

This report is intended for individuals working with water-resource issues in local, State, or Federal 

agencies, universities, public interest groups, or the private sector. The information will be useful in 

addressing current issues, such as drinking-water quality, source-water protection, and monitoring 

and sampling strategies. This report also will be useful for individuals who wish to know more about 

the quality of groundwater in areas near where they live and how that quality of water compares to 

the quality of water in other areas across the region and the Nation.

Water-quality conditions in the Upper Floridan aquifer of the Floridan aquifer system and overlying 

surficial aquifers summarized in this report are discussed in greater detail in other reports listed in 

the references. Detailed technical information, data and analyses, sample collection and analytical 

methodology, models, graphs, and maps that support the findings presented in this report in addition 

to reports in this series from other Principal Aquifers can be accessed from the national NAWQA 

Web site (http://water.usgs.gov/nawqa/).
viii

Estimating nitrate concentrations in groundwater at selected wells and springs

in the surficial aquifer system and Upper Floridan aquifer, Dougherty Plain 

and Marianna Lowlands, Georgia, Florida, and Alabama, 2002–50

U.S. Geological Survey Scientific Investigations Report 2013–5150

By Christy A. Crandall, Brian G. Katz, and Marian P. Berndt

(Also available at http://pubs.usgs.gov/sir/2013/5150/)

Factors affecting water quality in domestic wells in the Upper Floridan 

aquifer, Southeastern United States, 1998–2005

U.S. Geological Survey Scientific Investigations Report 2009–5147

By Marian P. Berndt and Christy A. Crandall

(Also available at http://pubs.usgs.gov/sir/2009/5147/)

Companion studies of these aquifers are discussed in the following reports.

Water withdrawals and trends from the Floridan aquifer system in the 

Southeastern United States, 1950–2000

U.S. Geological Survey Circular 1278

By Richard L. Marella and Marian P. Berndt

(Also available at http://pubs.usgs.gov/circ/2005/1278/)
Chapter 1

LA

AK

MS AL

FL

GA

TN NC

SC

GULF OF MEXICO

ATLANTIC OCEAN

Pumping of irrigation 

water for agriculture, 

including center pivot 

irrigation systems (shown 

here), accounts for about 

half of the water use from 

the Upper Floridan aquifer. 

Chapter 1: Overview of Major Findings 

and Implications

Photograph by USDA NAIP

The Floridan aquifer system supplies drinking water to about 10 million 

people in the southeastern United States. Withdrawals of water from 

this highly productive carbonate aquifer system have increased by 

more than 500 percent since 1950, as population, tourism, and agricultural 

production in the region have increased. Groundwater from the Floridan aquifer 

system is important to the economies of the States that overlie it—as a source 

of water for agriculture and as the source of large, clear-water springs that are 

recreational and tourist destinations.

 The U.S. Geological Survey (USGS) assessment of water-quality conditions

of the Upper Floridan and surficial aquifers for the period 1993–2010 provides a 

regional assessment of water quality in these important aquifers. This assessment 

describes where and why specific groundwater-quality conditions occur in 

different parts of the region and how contaminants—from both geologic and 

human sources—move into and through the groundwater system. A better 

understanding of the factors that affect water quality supports effective waterresources management for human and ecological health and for the sustainability 

of this critical resource as the population and the need for water increase. 
Southeastern

Coastal Plain

aquifer system Unconfined

Upper Floridan

aquifer

Semiconfined

Upper Floridan

aquifer 

Domestic

well

Spring

Urban

monitoring

wells 

Agricultural

monitoring wells

Public-supply

well

Public-supply

well

Domestic

well 

Fewer than 1 in 20 water samples collected from drinking-water 

wells in the Upper Floridan aquifer contained a constituent at a 

concentration that exceeded a human-health benchmark. Among all 

constituents measured, radon exceeded its human-health benchmark 

(proposed alternative Maximum Contaminant Level of 4,000 picocuries per liter) most frequently. See page 29.

Somewhat surprisingly, nitrate concentrations in 

the Upper Floridan aquifer in many areas have remained 

relatively low, despite widespread sources of nitrogen. 

Of the 252 samples from the Upper Floridan aquifer, only 

one sample had a nitrate concentration greater than the 

Maximum Contaminant Level of 10 milligrams per liter 

(mg/L) as nitrogen. Nearly half of the samples, however, 

had nitrate concentrations greater than the recently 

adopted criterion of 0.35 mg/L. Although these concentrations are not a concern for human health, even small 

increases in groundwater nitrate concentrations might 

have adverse effects on the water quality and aquatic 

species that rely on spring flows for habitat. See page 43.

About 10 million people rely on groundwater from the Upper Floridan 

and surficial aquifers for drinking water. The Upper Floridan aquifer also is 

of primary importance to the region as a source of water for irrigation and as a 

source of crystal clear water that discharges to springs and streams providing 

recreational and tourist destinations and unique aquatic habitats. The reliance 

of the region on the Upper Floridan aquifer for drinking water and for the 

tourism and agricultural economies highlights the importance of long-term 

management to sustain the availability and quality of these resources.

Overview of Major Findings and Implications for the Upper 

The quality of groundwater in the Upper Floridan aquifer 

is among the best in the Nation

Nitrate concentrations commonly exceed the criterion for water discharging from springs

1

2

Photograph by USGS
Confining

layer

Unconfined

surficial

aquifers

Confined

Upper Floridan

aquifer

Domestic well

NOT TO SCALE

Confined condition

Unconfined

Semiconfined

Confined

Percent of samples

0 20 40 60 80 100

0

1

2 or more

Number of pesticides detected

EXPLANATION

Pesticides were detected 

most frequently in 

unconfined areas of the 

Upper Floridan aquifer. 

Karst features, such as sinkholes and large conduits, have developed in those areas where the Upper Floridan aquifer is 

unconfined and rain that infiltrates the soil and recharges the aquifer can dissolve away the carbonate rock. Karst features allow 

contaminants, such as pesticides, to move rapidly from the land surface into the aquifer. These features are common where the 

Upper Floridan aquifer is unconfined in southwestern Georgia and north-central Florida — areas where agricultural activities 

are concentrated. See pages 20 –21, 39, and 44.

The pumping of public-supply wells in the Upper Floridan 

aquifer can move water that contains dissolved oxygen from 

the surficial aquifer into the Upper Floridan aquifer. The 

infusion of oxygen during pumping into areas where the Upper 

Floridan aquifer does not contain oxygen can cause arsenic 

to be released into the groundwater. In addition, this shallow 

water can bring with it elevated concentrations of nitrate or 

other contaminants from urban and agricultural sources at the 

land surface. Understanding how the aquifer geochemistry 

changes as a result of large-volume pumping is vital to the 

sustainability of this important resource. See page 38–39.

Pesticide occurrence

Floridan Aquifer and Overlying Surficial Aquifers

The absence of an upper confining layer and the presence of karst features increase the vulnerability 

of parts of the Upper Floridan aquifer to contamination from human activities 

Alteration of groundwater flow can change 

geochemical conditions and increase 

groundwater vulnerability to contamination

4

3

Photograph by USGS
Top photograph by Matthew O’Malley, St. Johns 

River WMD; second photograph by USGS

1 The quality of groundwater in the Upper Floridan aquifer isamong the best in the Nation.

Fewer than 1 in 20 water samples collected from drinking-water wells in the 

Upper Floridan aquifer contained a constituent at a concentration that exceeded a 

human-health benchmark. This number is much lower than that for the Nation as a whole, 

for which a human-health benchmark was exceeded in more than 1 in 5 groundwater 

samples. Among all constituents measured in samples from the Upper Floridan aquifer, 

radon exceeded its human-health benchmark most frequently—about 3 percent of drinkingwater wells contained radon at a concentration that exceeded the U.S. Environmental 

Protection Agency (USEPA) proposed alternative Maximum Contaminant Level of 

4,000 picocuries per liter. Strontium and arsenic were the only other constituents measured 

at a concentration greater than a human-health benchmark, and these exceedances occurred 

in 1 percent or less of samples. 

2 Nitrate concentrations commonly exceed the criterion for water 

discharging from springs.

Somewhat surprisingly, nitrate concentrations in the Upper Floridan aquifer in many 

areas have remained relatively low, despite widespread sources of nitrogen. Of the 

252 groundwater samples collected from the Upper Floridan aquifer, only one sample had 

a nitrate concentration greater than the Maximum Contaminant Level of 10 milligrams per 

liter (mg/L) as nitrogen. Nearly half of the samples, however, had nitrate concentrations 

greater than the recently adopted criterion of 0.35 mg/L for springs in Florida. Although 

these concentrations are not a concern for human health, even small increases in nitrate 

concentrations might have adverse effects on the groundwater quality. Much of the 

groundwater in the Upper Floridan aquifer discharges as “crystal-clear” spring flow 

that supports a variety of aquatic plants and animals throughout the State of Florida. In 

addition, many large springs are recreational destinations that contribute substantially to 

the economies of the surrounding areas. Over the past several decades small increases 

in nitrate concentrations have degraded groundwater quality, and many spring runs have 

suffered from an increase in nuisance aquatic vegetation, reduced clarity, and harmful 

algal blooms, which can impair the recreational use of springs.

4 Water Quality in the Upper Floridan Aquifer and Overlying Surficial Aquifers, Southeastern United States, 1993–2010

Overview of Major Findings Water- 

Overlying Surficial Aquifers, Southeastern United States
3 The absence of an upper confining layer and the presence of

 karst features increase the vulnerability of parts of the Upper

Floridan aquifer to contamination from human activities.

Karst features, such as sinkholes and large conduits, have developed in areas where the 

Upper Floridan aquifer is unconfined and where rain and soil water can dissolve away the 

carbonate rock. These karst features play a particularly important role in controlling the 

vulnerability of groundwater to contamination because they allow contaminants to move 

rapidly from the land surface into the aquifer. Karst features are common in the Upper 

Floridan aquifer in southwestern Georgia and north-central Florida. Activities associated with 

agriculture in these areas are sources of many contaminants, including nitrate and pesticides. 

In these areas, nitrate concentrations greater than 1 mg/L as nitrogen were measured in 

more than half of the samples collected compared to about 10 percent of samples collected 

in other areas. Similarly, one or more pesticides were detected in more than half of the 

samples in these areas compared to about 20 percent of samples in other areas, although 

pesticide concentrations were low (near their detection levels) throughout the study area. 

A critical aspect of protecting the Upper Floridan aquifer as a source of drinking water 

is the consideration of karst features in the transport of contaminants from land surface. 

Continued public outreach and education can increase awareness of the importance and 

vulnerability of karst water resources.

4 Alteration of groundwater flow can change geochemical conditions

and increase groundwater vulnerability to contamination.

Alteration of groundwater flow and geochemistry, either by high-volume pumping or 

aquifer storage and recovery projects, can have unintended consequences for water quality. 

In some areas, high-volume pumping from the Upper Floridan aquifer for public water 

supply has increased the downward movement of water from the surficial aquifers into 

the Upper Floridan aquifer. This shallow water can bring with it elevated concentrations 

of nitrate and other contaminants from sources at the land surface. Also, as oxic (contains 

oxygen) water from surficial aquifers mixes with water in the underlying Upper Floridan 

aquifer, which typically contains little or no dissolved oxygen, the resulting change in the 

geochemistry in the Upper Floridan aquifer can cause minerals to dissolve and release 

contaminants, such as arsenic, into the groundwater. Aquifer storage and recovery projects, 

which inject oxic surface water into the aquifer during wet periods and withdraw it for use 

during dry periods, also change the aquifer geochemistry and can mobilize arsenic.

As the demand for water grows in areas overlying the Upper Floridan aquifer, changes 

in water quality may occur as more water is pumped from the Upper Floridan aquifer and 

more aquifer storage and recovery projects are undertaken. Understanding how the aquifer 

geochemistry changes as a result of these withdrawals is vital to the sustainability of this 

important resource.

Chapter 1: Overview of Major Findings and Implications 5

Quality Issues for the Upper Floridan Aquifer and 
Photograph by USDA NRCS
Chapter 2

A technician measures specific 

conductance of groundwater prior 

to collecting a sample. Groundwater 

samples were analyzed for many 

chemical constituents by using 

nationally consistent water-quality 

sampling protocols.

Chapter 2: NAWQA Approach to Assessing 

Groundwater Quality

This chapter summarizes 

the study design used to 

investigate water quality 

in the Upper Floridan 

and surficial aquifers.

Groundwater studies conducted in the Upper Floridan and surficial 

aquifers as part of the USGS NAWQA Program were designed to 

answer broad questions about the occurrence and distribution of 

contaminants on a regional scale and the processes controlling the transport 

and fate of those contaminants on a local scale.
Assessing the Water Quality in the Upper 

Floridan and Surficial Aquifers 

How does one go about characterizing the groundwater 

over an area as large as that covered by the Upper Floridan 

aquifer, let alone the whole United States? The approach taken 

by the USGS is to use different types of groundwater studies 

to gain a better understanding of how and why water quality 

varies. These groundwater studies are the building blocks of 

the NAWQA water-quality assessments of Principal Aquifers. 

Many studies were conducted in each Principal Aquifer, each 

with a different focus on information needs about groundwater 

quality and the natural and human-related factors that influence the aquifer (table 2–1; appendix 1). 

Groundwater studies were designed to answer the 

following questions: How does land use affect groundwater 

quality? How does water quality change as it moves through 

the aquifer? What is the quality of the drinking-water 

resource? Groundwater studies designed to broadly assess 

water-quality conditions in aquifers used as a source of 

drinking water focused on sampling existing water-supply 

wells (these studies are called major aquifer studies). 

Agricultural and urban land-use studies designed to 

characterize and explain the quality of recently recharged 

groundwater (generally less than 10 years old) (1) in these 

land-use settings focused on sampling mostly shallow 

monitoring (nonpumping) wells that were installed as 

part of the NAWQA Program. Other studies focused 

on public water supplies or groundwater quality along 

individual flow paths.(2, 3)

This Principal Aquifer assessment brings together and 

interprets results from all of the NAWQA groundwater studies 

in the Upper Floridan and surficial aquifers. Throughout this 

assessment, the results are grouped to characterize groundwater used as a drinking-water resource and to characterize 

the quality of groundwater that has recently recharged in 

urban and agricultural land-use settings. 

Table 2–1. Summary of studies and types of wells sampled in the surficial and Upper Floridan aquifers from 1993 to 2010.

Characteristic Major aquifer

study

Agricultural

land-use study

Urban

land-use study

Public-supply

study

Contaminant

transport study

Study objective Assess quality

of groundwater 

in aquifer used 

for drinkingwater supply

Assess quality

of shallow

groundwater

in agricultural

land-use setting

Assess quality

of shallow 

groundwater

in urban landuse setting

Assess quality

of groundwater

in aquifer used 

for drinkingwater supply

Assess the

vulnerability

of supply wells 

to contamination 

from sources at 

land surface

Land use in study area Mixed Agricultural Residential and 

commercial

Mostly residential 

and commercial

Urban

Type of wells sampled Mostly domestic Monitoring Monitoring Public supply Monitoring and

public supply

Number of wells per 

study (typical)

30 31–47 26–32 15 23

Number of studies in 

surficial aquifers

0 3 2 0 1

Number of studies in

Upper Floridan aquifer

6 0 2 2

(both in Tampa)

1

(in Tampa)

Number of wells sampled 176* 109 58 30 23

Median well depth,

in feet

164 31 38 562 64

*

Includes 22 springs. 

8 Water Quality in the Upper Floridan Aquifer and Overlying Surficial Aquifers, Southeastern United States, 1993–2010

Almost 400 wells were sampled to characterize the water 

quality of the Upper Floridan and surficial aquifers.
Chapter 2

Tampa area

Ocala area

Wells sampled in surficial aquifers Wells sampled in the Upper Floridan aquifer

Figure 2–1.

Tampa area

Ocala area

FLORIDA

ALABAMA GEORGIA

SOUTH 

CAROLINA

MISSISSIPPI

0 100 150 200 KILOMETERS

0 50 100 150 200 MILES

50

N

Northern extent of study area

Northern extent of study area

Floridan aquifer system

Agricultural land-use 

 monitoring

Urban land-use

 monitoring

Domestic

Public supply

Monitoring near

 public-supply well

EXPLANATION

Type of well 

EXPLANATION

Surficial aquifers (includes

 local surficial aquifers and 

 surficial aquifer system) 

Agricultural land use

Urban land use

Type of monitoring well 

Land-use studies allow us to evaluate how our activities 

and the chemicals we use affect groundwater quality, and the 

results can provide an early warning for potential contamination 

of drinking-water resources. To assess the quality of recently 

recharged groundwater, 167 wells in urban or agricultural areas 

were sampled one time each. Most of these wells were shallow 

monitoring wells installed by the USGS in the surficial aquifers 

(fig. 2–1)—about 100 of these wells were in agricultural areas 

(table 2–1). The primary agricultural setting studied was row 

crops, such as cotton, soybeans, and corn. Row crops in this 

region require irrigation, and pesticides typically are applied 

to these crops—two factors that can affect the quality of 

the underlying groundwater. About 60 of the shallow wells 

were in urban land-use settings similar to those where many 

people work and live—single- and multi-family residential 

and commercial developments, mostly in the suburbs of Ocala 

and Tampa, Fla. High-volume pumping for public supply and 

nonpoint-source contamination from urban land-use settings 

can affect the quality of the underlying groundwater. The wells 

from which samples were collected for the agricultural and 

urban land-use studies range from 12 to 250 feet (ft) deep and 

tap groundwater near the water table. The water quality of 

shallow groundwater is an indication of how the chemicals we 

use in day-to-day life—pesticides, solvents, gasoline—might 

someday affect the quality of the drinking-water resource.

To assess the quality of groundwater used as a drinkingwater resource, one sample was analyzed from each of about 

180 randomly selected existing water-supply wells in the 

Upper Floridan aquifer (see sidebar, NAWQA assessments use 

a wide range of geochemical data and site information, p. 13, 

and appendix 1). About 85 percent of the wells sampled were 

drinking-water wells. These wells were distributed across the 

study area, and nearly all of them were domestic (private) 

wells (fig. 2–1). The remaining 15 percent of wells included 

some irrigation and aquaculture wells and other private wells 

that were not used as a source of drinking water. The wells 

sampled ranged in depth from very shallow (38 ft) to quite 

deep (800 ft), but three-fourths of the wells were less than 

about 250 ft deep (table 2–1). Public-supply and monitoring 

wells in the Tampa, Fla., area were sampled to characterize the 

transport of contaminants to the Upper Floridan aquifer and to 

assess the quality of water used for public supply.

Figure 2–1. About 130 shallow monitoring wells tapping surficial aquifers were sampled from 1993 to 2010. About 250 wells tapping 

the Upper Floridan aquifer were sampled during this same time period. Wells tapping the Upper Floridan aquifer were deeper than the 

surficial wells and included wells used for drinking-water supply—domestic and public-supply wells—in addition to some monitoring 

wells in urban and agricultural areas.

Chapter 2: NAWQA Approach to Assessing Groundwater Quality 9
Anatomy of a well

A well is simply a hole in the ground (well bore or borehole) from 

which water can be removed. The well bore is lined with a well 

casing, such as a pipe, to prevent the well bore from collapsing. 

The casing, along with a sealant (called grout), also prevents water 

from flowing into the well from the land surface or from parts of 

the aquifer where the water quality may be less desirable. The 

casing can be open at the bottom or perforated at a specific depth 

with a screen, to allow water to flow into the well where it can be 

pumped to the surface. Coarse sand or gravel (called sand pack or 

gravel pack) can be placed around the well screen to help improve 

the flow of water into the well. Some wells are cased only near the 

land surface, allowing water to flow into the well from nearly the 

entire length of the well bore.

When pumped, groundwater flows through the 

screened interval and 

up and out of the well

Unsaturated

Saturated

Water table

Screen

Well bore

Water level

in well

Wellhead

Well casing

Pump

Grout

Gravel pack

Confining layer

Confined aquifer

Unconfined aquifer

Unsaturated zone Water table

Wells

Surface water

An underground layer of saturated permeable materials (rock, gravel, sand, or silt) that will yield 

a useful quantity of water to a well.

Bedrock aquifer

Groundwater storage

and flow in fractures

Screen

An unconfined aquifer is bounded at its top by the 

water table, below which water fills all the pore spaces 

in the rock. Water from the land surface can move 

down into an unconfined aquifer. 

A confining layer is a layer of material (often clay)

through which water does not easily flow, creating a 

boundary between aquifers.

A confined aquifer is bounded at its top by a confining 

layer. Water enters or “recharges” confined aquifers 

where the confining layer is not present. Where the

confining layer is not continuous or is breached (for

example, by a well), flow between the unconfined and

confined aquifer can occur. 

The pressure within a confined aquifer can be greater

than that in the overlying unconfined aquifer if the

source of the water in the confined aquifer is at a higher

elevation than the unconfined aquifer. In that case, 

water in a well in a confined aquifer will rise to a higher

level than that in the overlying unconfined aquifer. 

The unsaturated zone is the area below the land 

surface and above an aquifer. In addition to soil, 

rocks, and air, the unsaturated zone contains water 

from the land surface (such as rain) that is slowly 

moving downward to the water table of the aquifer. 

Aquifer (aq.ui.fer)—  ̆

Carbonate aquifer

Groundwater storage and flow

in solution cavities or fractures

10 Water Quality in the Upper Floridan Aquifer and Overlying Surficial Aquifers, Southeastern United States, 1993–2010
Understanding study results

Important aspects of the NAWQA Principal Aquifer assessments:

•	 Water samples were collected at the wellhead (see sidebar, Anatomy of a well, p. 10) prior to any treatment.

 They represent the quality of the groundwater resource but not necessarily the quality of tap water. 

•	 The focus of the assessments is the condition of the total resource, including groundwater in a wide 

range of hydrologic and land-use settings across the Nation, rather than conditions at specific sites with 

known water-quality concerns.

•	 The assessments are guided by a nationally consistent study design, and all assessments use the same 

methods of sampling and analysis. Findings apply to water quality of a particular aquifer but also contribute 

to the larger picture of how and why water quality varies regionally and nationally. This consistent 

approach helps to determine if a water-quality issue is isolated or widespread. (See http://water.usgs.

gov/nawqa/about.html for more information.)

•	 The assessments focus on aquifers used for water supply or on shallow groundwater that underlies an 

area with a particular type of land use. Because the NAWQA groundwater study areas do not cover the 

full spatial extent of the targeted Principal Aquifer, the findings might not represent the effects of the full 

range of geology, climate, and land use present.

•	 Analytical methods used by USGS chemists for assessments of water quality in Principal Aquifers are 

designed to measure constituents at as low a concentration as possible. As a result, constituents frequently 

are detected at concentrations far below human-health benchmarks for drinking water (see sidebar, 

Human-health benchmarks and other guidelines used in this assessment, p. 32). Low-level detections 

allow scientists to identify and evaluate emerging issues and to track contaminant levels over time.

Chapter 2

Samples of shallow groundwater 

were collected from the surficial 

and Upper Floridan aquifers 

underlying the cities of Ocala 

and Tampa, Florida. The quality 

of water indicates how chemicals 

that we use in our daily lives 

affect groundwater.

Photograph by Guy H. Means, Florida Geological Survey

Chapter 2: NAWQA Approach to Assessing Groundwater Quality 11
The types of wells sampled and how well type can affect the quality of the water collected

Many of the groundwater samples analyzed for this water-quality assessment were collected from shallow monitoring wells. 

Monitoring wells are not pumped regularly—they are used for measuring water levels or occasionally collecting water samples, 

but they are not used for drinking water, irrigation, or other purposes. Monitoring wells sampled in the NAWQA groundwater 

studies were installed expressly for the purpose of monitoring groundwater quality. Because monitoring wells typically tap 

shallow groundwater that has recently recharged the aquifer and that is close to the land surface where chemicals are applied, 

the water pumped is more likely to contain manmade contaminants than water pumped from deeper wells.

Groundwater samples also were collected from two types of wells that supply drinking water: domestic (private) wells and 

public-supply wells. Both well construction and land-use setting can affect the water quality of the samples collected. Domestic 

wells typically are shallower than public-supply wells, so they pump water that is nearer to sources of manmade contaminants, 

such as fertilizers and pesticides, at the land surface. Domestic wells commonly are in rural areas and thus are more likely than 

public-supply wells to be vulnerable to contamination from agricultural chemicals. Public-supply wells pump water from deeper 

in the aquifer and commonly are in suburban and urban areas, so they are more likely than domestic wells to be vulnerable to 

contamination from chemicals associated with urban activities. Public-supply wells have larger pumps and longer screened 

intervals than do domestic wells and are pumped for longer periods of time. As a result, public-supply wells pump much larger 

volumes of water than do domestic wells and so have much larger capture zones; therefore, public-supply wells are more vulnerable than are domestic wells to manmade contamination from distant sources. If the amount of water pumped is large enough, it 

can change the direction and speed of groundwater flow, which can, in turn, affect the geochemistry of the groundwater. Routine 

testing of water from domestic wells is not required, and so homeowners are responsible for testing, maintenance, or treatment 

of the water from their domestic wells. Water from public-supply wells is required to be tested by the well operator on a routine 

basis to help assure that the water provided to consumers meets Federal and State water-quality standards.

Finally, some groundwater samples were collected from wells that supply water for purposes other than drinking, such 

as irrigation and aquaculture. These wells can have pumping rates similar to or greater than those of public-supply wells; 

they might be pumped every day or only during spring and summer. Depending on their depth, location, and pumping rate, 

these supply wells might be more or less vulnerable than drinking-water wells to contamination associated with agriculture 

or urban activities. 

(Left) Domestic and (right) public-supply wells were the primary type of water-supply wells sampled.

Photographs by USGS

12 Water Quality in the Upper Floridan Aquifer and Overlying Surficial Aquifers, Southeastern United States, 1993–2010
Chemists at the USGS National Water Quality Laboratory analyze 

groundwater samples using an array of sophisticated techniques.

Additional information (often called “ancillary information”) about 

the well and surrounding environment complements the chemical 

data measured. This additional information often is key to making 

sense of the chemical data. For example, the information might 

be used to determine that shallow groundwater is more (or less) 

vulnerable to contamination than deep groundwater, that domestic 

wells are more (or less) vulnerable to contamination than publicsupply wells, or that urban land use is associated with different 

types of groundwater contamination than is agricultural land use. 

Chemical data without accompanying ancillary data are much less 

useful for understanding factors that affect groundwater quality. 

Constituents measured in samples from most wells

Constituent group Examples

Water-quality properties pH, specific conductance, dissolved 

oxygen, temperature

Major ions (filtered) Bromide, calcium, chloride, 

magnesium, sodium, sulfate

Trace elements (filtered) Arsenic, boron, iron, manganese, 

selenium, uranium

Nutrients (filtered) Ammonia, nitrate, phosphorus

Pesticides (filtered) Herbicides, insecticides, fungicides

Volatile organic 

compounds

Solvents, gasoline hydrocarbons, 

refrigerants, trihalomethanes, 

fumigants

Organic carbon (filtered)

Additional constituents measured in samples from some wells

Constituent group Examples

Radionuclides Radon

Groundwater age tracers Tritium, chlorofluorocarbons

Stable isotopes Oxygen-18, hydrogen-2

Microorganisms Escherichia coli and total coliforms

Additional site information

Use of well

Well depth

Depth to water

Well-construction data

Principal Aquifer

Land-surface elevation at well

Land use within a 500-meter 

(1,640-foot) radius buffer

Estimates of nutrient inputs

Estimates of pesticide use

NAWQA assessments use a wide range of geochemical data and site information

Chapter 2

Photographs on this page by USGS

Chapter 2: NAWQA Approach to Assessing Groundwater Quality 13
Monitoring wells installed in agricultural 

and urban areas were sampled to 

characterize shallow groundwater 

quality underlying these land uses. 

14 Water Quality in the Upper Floridan Aquifer and Overlying Surficial Aquifers, Southeastern United States, 1993–2010
Chapter 3

Orlando, Florida, is one of several large cities that overlies the Floridan aquifer 

system and relies on the aquifer for drinking water. The Floridan aquifer system 

is the source of drinking-water supply to approximately 10 million people, most of 

them in Florida, Georgia, and South Carolina. Over 4 billion gallons are withdrawn 

from this aquifer system each day for irrigation, drinking water, and industrial uses.

Photograph by Matthew O’Malley, St. Johns River WMD

Chapter 3: Environmental and 

Hydrogeologic Setting 

This chapter summarizes

background information

for the Upper Floridan 

aquifer and overlying 

surficial aquifers and 

provides the context for 

understanding findings 

about water quality in 

these Principal Aquifers. 

The chapter covers 

the environmental and 

hydrogeologic setting, 

including population, 

land use, and water use.

The clear inland springs that issue from the thick carbonate rocks of 

the Floridan aquifer system appeal to residents and tourists alike, 

and the temperate to tropical climate, abundant rainfall, and plentiful 

groundwater make the area a top producer of oranges, tomatoes, and other 

crops. All of these factors—geology, population growth, climate, and 

agriculture—affect flow and the quality of groundwater in the Upper Floridan 

aquifer and overlying surficial aquifers.
EXPLANATION

Aquifer or aquifer system

Surficial aquifer system

Local surficial aquifers

Coastal lowlands aquifer system

Biscayne aquifer

 Aquifers overlying the Floridan aquifer system

Figure 3–1.

N

Environmental Setting and Climate

The Floridan aquifer system underlies a vast area—

about 100,000 square miles (mi2)—that includes all of 

Florida and parts of Alabama, Georgia, Mississippi, and 

South Carolina (fig. 3–1). Low, gently rolling terrain in 

the northwestern areas transitions to flat topography near 

the coasts—the highest point in Florida is a mere 345 ft in 

elevation. The climate is temperate in the north but subtropical 

in most of Florida and tropical in south Florida. The area 

receives substantial rainfall—the long-term average ranges 

from about 45 inches per year (in/yr) in southeastern Georgia 

to about 68 in/yr in southern Alabama,(4) and in some areas 

rainfall is more than 100 in/yr. The driest areas are in 

southeast Georgia, and the wettest are in southeast Florida, 

the western panhandle of Florida, and south Alabama. The 

wettest times of the year are late winter or early spring and the 

summer—there is a 50 percent chance of rain on any summer 

day. Torrential rain is a common occurrence, particularly along 

the coasts—tropical disturbances or hurricanes can bring 

rainfall in excess of 3 inches and, in extreme cases, 24-hour 

amounts of 10 inches or more. The timing and amount of 

rainfall are key factors that control the amount of water that 

recharges the aquifers and the rate at which groundwater 

moves through the aquifers.

The land overlying the Upper Floridan aquifer is flat and most of it is managed forest, agricultural, or wetlands, as shown here at the 

Ten Thousand Islands National Wildlife Refuge, Collier County, Florida, (left) and a peanut field in Miller County, Georgia (right).

Figure 3–1. Four surficial aquifers or aquifer systems overlie the 

Floridan aquifer system. Water-quality data for the surficial aquifer 

system and local surficial aquifers—referred to in this report as 

“surficial aquifers”—are included in this assessment.

16 Water Quality in the Upper Floridan Aquifer and Overlying Surficial Aquifers, Southeastern United States, 1993–2010

The Upper Floridan aquifer is one of the most 

productive aquifers in the world with 33 springs 

in Florida alone that each discharge more than 

64 million gallons per day.
Chapter 3

Confining

layer 

Unconfined

surficial

aquifer

Semiconfined Upper Floridan aquifer

Confined Upper Floridan aquifer

Unconfined Upper Floridan aquifer

NOT TO SCALE

Figure 3–2.

Hydrogeologic Setting

The Floridan aquifer system, one of the most productive 

aquifers in the world, is composed of a thick sequence of 

carbonate rocks (limestone and dolomite), which lends the 

aquifer system many of its unique hydrogeologic characteristics. The limestone and dolomite that make up the Floridan 

aquifer system can dissolve when they come into contact with 

acidic rainwater or soil water—the landforms and associated 

hydrology that form when these rocks dissolve is called “karst” 

(see sidebar, Karst, p. 20). Karst features that form in the 

aquifer can allow large amounts of water to move to springs 

or wells. For example, in Florida alone, there are 33 springs 

each with flow greater than about 64 million gallons per day 

(Mgal/d).(5) The Floridan aquifer system is overlain across 

most of its extent by the surficial aquifer system or by local 

surficial aquifers (fig. 3–1). In many areas, the Floridan aquifer 

system is separated from an overlying surficial aquifer by a 

confining layer, but in some areas the two aquifer systems are 

in contact (figs. 3–2, 3–3).(6) The Southeastern Coastal Plain 

aquifer system underlies the Floridan aquifer system, and 

these aquifers are connected in southern Georgia.(6)

The Floridan aquifer system consists of as much as 

1,500 ft of carbonate rock—limestone and dolomite.(7) The 

system is divided into two aquifers—the Upper Floridan 

aquifer and the Lower Floridan aquifer. The Upper Floridan 

aquifer is highly permeable and is the primary source of 

drinking water throughout its extent—in most areas the Upper 

Floridan aquifer yields water supplies sufficient for most 

purposes, and there is no need to drill into the Lower Floridan 

aquifer, which is deeply buried and contains poor-quality 

water in many areas. This report focuses only on the Upper 

Floridan aquifer. 

The surficial aquifer system and local surficial aquifers 

consist mostly of beds of unconsolidated sand, shelly sand, 

and shell and, in some areas, limestone.(6) These aquifers 

range in thickness from just tens of feet in parts of westcentral and north-central Florida to more than 300 ft along 

the southeastern Georgia coast and eastern Florida coast. In 

this report, the term “surficial aquifers” is used to refer to the 

surficial aquifer system and the local surficial aquifers together 

(fig. 3–1). The surficial aquifers are unconfined. Most of the 

water that infiltrates the surficial aquifers moves quickly 

along short flow paths to discharge as base flow to streams, 

but some water moves downward into the Floridan aquifer 

system.(6) The presence of contaminants in surficial aquifers is 

of concern to local water users, but also to those who use the 

Upper Floridan aquifer for drinking water.

Figure 3–2. Unconfined surficial aquifers overlie the Upper Floridan aquifer. A confining layer of sand, clay, marl, 

limestone, and dolomite separates these aquifers in many areas. The presence or absence of this confining layer 

is a key factor that controls the vulnerability of the Upper Floridan aquifer to contamination.

Chapter 3: Environmental and Hydrogeologic Setting 17
NOT TO SCALE

Semiconfined area

Confined area

Unconfined area

NOT TO SCALE

NOT TO SCALE

Confined conditions in the Upper Floridan aquifer

N

Figure 3–3.

EXPLANATION

Unconfined

Semiconfined (confining layer 

 <100 feet thick or breached)

Confined

Not mapped

Confined condition for the 

 Upper Floridan aquifer

EXPLANATION

"Can we make the upper Floridan colors in the x-sections the same color as the confined conditions-- that is #1 lightest color?" 

So do you mean unconfined color?

Surficial aquifers

Clay or low permeability 

 confining layer

Upper Floridan aquifer

Hydrogeologic unit

Water

Confined conditions modified from Miller(7)

In most areas, a confining layer composed of sand, clay, 

marl, limestone, and dolomite of low permeability separates 

the surficial aquifers from the underlying Floridan aquifer 

system (figs. 3–2, 3–3). This confining layer ranges in thickness from several feet in parts of southwestern Georgia and 

west-central and north-central Florida to more than 100 ft 

in southeastern Georgia, northeastern Florida, and southern 

Florida. The thickness of the confining layer controls where 

and how much recharge to the underlying Upper Floridan 

aquifer occurs.(4) The Upper Floridan aquifer is characterized 

as being unconfined where the confining layer is absent, 

semiconfined where the confining layer is less than 100 ft 

thick or breached, and confined where the confining layer is 

100 ft or more thick or continuous(7) (fig. 3–3).

Most of the recharge to the Upper Floridan aquifer is 

concentrated in areas where the aquifer is unconfined or 

semiconfined. In some areas, sinkholes in streambeds create a 

direct hydraulic connection between the stream and the aquifer, 

capturing much or all of the streamflow and diverting it down 

into the aquifer. The areas of highest recharge (10 to 25 in/yr) 

are in west-central, northern, and western Florida, and in 

southwestern Georgia.(4, 8) The least amount of recharge (1 in/yr 

or less) occurs in northeastern Florida and southeastern Georgia 

(fig. 3–4), where the confining layer is more than 100 ft thick.

Karst features form in unconfined areas of the Upper 

Floridan aquifer, where the aquifer rocks are exposed or are 

covered by only a thin layer of sand or soil. Karst also forms 

in some semiconfined areas, where sinkholes breach the 

confining layer and allow precipitation to move downward 

into the aquifer. As a result, karstic parts of the Upper Floridan 

aquifer are found most commonly in west-central and northcentral Florida, west Florida, and southwest Georgia. 

Figure 3–3. The major hydrogeologic feature that affects groundwater flow and quality in the Upper Floridan aquifer is the 

thickness of the overlying confining layer. Where present, this layer controls the amount of recharge and contaminants that 

can enter the aquifer.

18 Water Quality in the Upper Floridan Aquifer and Overlying Surficial Aquifers, Southeastern United States, 1993–2010
Chapter 3

<1

1 to <10

10 to 20

 0.1 to 10

EXPLANATION

Estimated predevelopment recharge

 to and discharge from the Upper

 Floridan aquifer, in inches per year

Recharge

Discharge

Modified from Bush and Johnston(4)

Figure 3–4.

 Estimated predevelopment recharge and discharge

N

Figure 3–4. The Upper Floridan aquifer receives recharge 

primarily where the confining layer is less than 100 feet thick. The 

recharge averages more than 4 inches per year, with some areas 

receiving more than 10 inches per year. These large amounts of 

recharge water replenish the aquifer and provide some dilution 

of contaminants.

In many areas the limestone of the Upper Floridan 

aquifer crops out (is exposed) at the land surface, 

forming rocky ledges such as those shown here 

along the Cross Florida Greenway.

Sinkholes such as this are entryways to the aquifer—water from the land surface can reach 

the water table with no filtration from overlying soils.

Chapter 3: Environmental and Hydrogeologic Setting 19
KARST An underground world of rock and water

River

Spring run

Spring

Large amounts of groundwater discharge at crystal-clear springs 

that are used for recreation and provide habitat for aquatic plants 

and animals.

Photograph by USDOT, Federal Highway Administration,National Scenic Byways Program (http://www.fhwa.dot.gov/hep/scenic_byways/)

Photograph by Emma Kingsbury

What is karst? Karst describes the distinctive landforms and hydrology that are created when rocks dissolve away. 

Two common types of rocks—limestone and dolomite—dissolve when they interact with rain or soil water, which are 

weakly acidic. The dissolution of the rocks creates a network of interconnected openings in the rock (called conduits) 

that allow large amounts of water to move rapidly into and through the aquifer.

Why is karst important? Karst aquifers provide water for both people and animals. The large underground openings

and oxygenated groundwater in karst aquifers provide habitat for rare aquatic species, many of which have yet to be

formally identified. Karst aquifers are highly productive, sustainable sources of groundwater, but are vulnerable

to contamination. Because of the unique hydrogeologic characteristics of karst conduits, unconventional 

approaches are required to understand how water and contaminants move through these systems.

The Georgia Blind Salamander is a protected species in 

Georgia and Florida that only lives in cave waters of the 

Floridan aquifer.

How does karst form? The process of creating 

karst, or karstification, starts when water (H2

O) in rainfall 

interacts with carbon dioxide (CO2

) in the atmosphere to 

create carbonic acid (H2

CO3), a weak acid. (The naturally 

occurring presence of carbonic acid in rainwater is why 

rain has a pH of less than 7.) When carbonate rocks—

limestone and dolomite—interact with an acid, a little 

bit of the rock dissolves, releasing calcium ions and CO2

.

Initially, water flows through small cracks in the 

rocks. As the rock along the walls of a crack dissolve, 

the crack becomes wider—over time, what were 

originally small fractures or tiny spaces gradually 

enlarge into openings called conduits.

A carbonate rock 

fizzes when dilute 

hydrochloric acid 

dissolves it.

20 Water Quality in the Upper Floridan Aquifer and Overlying Surficial Aquifers, Southeastern United States, 1993–2010
Chapter 3

Upper Floridan

aquifer

Groundwater

flow direction 

Sinkhole Surficial aquifers

Confining layer

NOT TO SCALE

Over time, small cracks along which water flows grow larger as 

the carbonate rocks dissolve—sometimes forming large conduits 

or caves.

Sinkholes form when the land surface collapses because of the 

dissolution of the underlying carbonate rocks. Sinkholes provide 

direct pathways for water and contaminants at the land surface to 

enter the aquifer.

Photograph by Ann Tihanksy, USGS

A vulnerable resource. In a classic karst landscape, water recharges the aquifer through sinkholes, flows through 

conduits, and discharges from a spring. This type of flow is unique to karst aquifers and has key implications for 

water quality.

•	 Water that recharges the aquifer through sinkholes is not filtered by soils.

•	 Rapid flow through the aquifer allows little time for contaminants to break down or for microorganisms to die off.

•	 Conduits are large enough that flow can be turbulent, allowing groundwater to transport particles and any 

associated contaminants.

•	 Because the location of conduits commonly is unknown, predicting groundwater flow direction and time of 

travel is difficult.

Chapter 3: Environmental and Hydrogeologic Setting 21
Land cover (1990s)

Agriculture

Bare rock, quarry, or transitional

Forest or shrub

Grassland or herbaceous

Open water

Urban

Wetland

EXPLANATION

Figure 3–5.

N

Land Use and Population

About one-third of the land overlying the Floridan aquifer 

system is forest land used for the production of paper products 

and lumber (fig. 3–5). Wetlands cover about 25 percent and 

agricultural land covers about 20 percent of the land area, 

followed by 7 percent urban, 4 percent water, and 10 percent 

other land uses (T.L. Arnold, U.S. Geological Survey, written 

commun., 2006). Field crops (including cotton, peanuts, corn, 

soybeans, and wheat) are grown in the central part of the 

study area, citrus crops are grown in the southern part, and 

vegetables are grown throughout the study area. Dairy and 

Figure 3–5. The predominant land covers overlying the Upper Floridan aquifer include forests, wetlands, and 

the combination of agricultural and urban areas. Agricultural land use is concentrated in southwest Georgia 

and central Florida.

poultry farms are present in central parts of the study area. 

In southwestern Georgia and throughout Florida, agriculture 

and agriculture-related activities play a major role in 

the economy.(9, 10)

The population in areas overlying the Floridan aquifer 

system increased from less than 10 million people in 1970 to 

more than 22 million people in 2005. The highly populated 

areas that rely on the Floridan aquifer system are located in 

northern and coastal parts of Florida and coastal Georgia, 

and include the cities of Savannah, Ga., and Jacksonville, 

Orlando, Tampa, St. Petersburg, Clearwater, Tallahassee, 

and Lakeland, Fla. (fig. 3–6).

Cotton is the predominant 

crop grown in southwestern Georgia. Florida

is the Nation’s primary 

producer of citrus 

fruits and also is a 

major producer of 

fresh vegetables.

Land cover (1990s)

Agriculture

Bare rock, quarry, 

 or transitional

Forest or shrub

Grassland or 

 herbaceous

Open water

Urban

Wetland

EXPLANATION

22 Water Quality in the Upper Floridan Aquifer and Overlying Surficial Aquifers, Southeastern United States, 1993–2010 Data from T.L. Arnold, USGS, written commun., 2006; Homer and others(61) Photograph by USGS
Chapter 3

1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000

4,500

4,000

3,500

3,000

2,500

2,000

1,500

1,000

500

0 

Water withdrawals, in million gallons per day

Year

Public supply

Irrigation

Industrial

Total

EXPLANATION

Water use

Figure 3–7.

Atlanta

Macon

Mobile

Pensacola

Tallahassee

Charleston

Hilton Head

Savannah

Jacksonville

Daytona Beach

Orlando

Tampa

St Petersburg

Clearwater

Gainesville

Lakeland Cocoa

Miami

<10

10 to 50

>50 to 100

>100 to 337

Withdrawals from the 

 Floridan aquifer system

 in 2000, in million 

 gallons per day

From Marella and Berndt(12)

EXPLANATION

Figure 3–6.

N

Water Use 

Surficial aquifers are used primarily for domestic 

or private household water supply or in areas where the 

Floridan aquifer system is nonpotable or too deep to be used 

economically.(11) Total groundwater withdrawals from the 

surficial aquifers in Florida in 2005 were 532 Mgal/d;(11)

water use estimates for the surficial aquifers are not available 

for Georgia and South Carolina. Withdrawals from surficial 

aquifers in most of the counties in Florida were less than 

20 Mgal/d. Groundwater withdrawals from surficial aquifers 

greater than 50 Mgal/d occurred only in southern Florida. In 

these areas, the underlying Floridan aquifer system is not used 

for drinking water (public supply and domestic uses), and the 

surficial aquifers are the primary source of drinking water.

Nearly 10 million people relied on the Floridan aquifer 

system for their drinking-water supplies in 2000; of these, 

about 1.6 million people obtained their drinking water from 

domestic wells.(12) Most of the groundwater withdrawn from 

the Floridan aquifer system for drinking water is from the 

Upper Floridan aquifer. The largest withdrawals are in central 

Florida and along the coast in Georgia and South Carolina 

(fig. 3–6). This important aquifer system also supports 

agriculture, industry, and tourism.

About 90 percent of the water withdrawn from the 

Floridan aquifer system is obtained from the Upper Floridan 

aquifer, which contains potable water in most areas. Localized 

contamination of the Upper Floridan aquifer, however, has 

forced water suppliers in some areas to obtain water from the 

Lower Floridan aquifer. In central and northeastern Florida, 

water from the Lower Floridan aquifer is potable, but in most 

other areas the water does not meet drinking-water standards 

because of high chloride concentrations.(13) In some areas, 

desalination of brackish groundwater and surface water is a 

supplemental source of water supply, providing as much as 

515 Mgal/d.(14)

Use of the Floridan aquifer system has increased 

substantially since the mid-1900s—withdrawals of water 

increased from 630 Mgal/d in 1950 to 4,020 Mgal/d in 

2000 (12) (fig. 3–7). Withdrawals for irrigation account 

for nearly half of that increase—from 90 Mgal/d in 1950 

to 1,950 Mgal/d in 2000. Withdrawals for public supply 

increased from 85 Mgal/d in 1950 to 1,330 Mgal/d in 2000. 

Nearly 60 percent of that increase occurred from 1980 to 2000, 

during which the population served by public supply increased 

from 4.5 million to 8.2 million people. Domestic withdrawals 

increased from 45 Mgal/d in 1950(4) to 166 Mgal/d in 2000.

Figure 3–6. The Floridan aquifer system is the source of 

drinking-water supply for about 10 million people, including the 

cities of Savannah, Georgia, and Jacksonville, Orlando, Tampa, 

St. Petersburg, Clearwater, Tallahassee, and Lakeland, Florida. 

Withdrawals are highest in central Florida where withdrawals 

for both drinking water and irrigation are greatest. 

Figure 3–7. Water withdrawals from the Floridan 

aquifer system increased more than 500 percent 

from 1950 to 2000, largely as a result of increases in 

irrigation and public-supply withdrawals. Irrigation is 

most extensive in Florida and southwestern Georgia.

Modifed from Marella and Berndt(12)

Chapter 3: Environmental and Hydrogeologic Setting 23
4,000,000,000

10,000,000

100,000

>1,000

45

Gallons of water used for water supply each day

People who use the aquifer for drinking water

Square miles covered by the aquifer

Springs

Inches of rainfall per year on average

e Upper Floridan aquifer at a glance

In Georgia, agriculture is most extensive in the southwestern part of the State, and irrigation has increased with the 

introduction of center pivot systems in the 1970s.(15) In 2004, 

irrigation was used on almost 40 percent of the cropland in 

the study area in Georgia, which has greatly increased crop 

productivity and helped agriculture play a major role in the 

regional economy.(9) Crop types planted in the study area have 

changed from a mostly corn and peanut rotation in the 1970s 

to mostly cotton and peanuts.(15)

Large quantities of groundwater withdrawals from 

the Floridan aquifer system have caused extensive waterlevel drawdowns and saltwater intrusion in several areas. 

Near Jacksonville, Fla., and Savannah, Ga., water levels 

have declined at an average rate of up to 0.5 ft per year 

since 1950,(16) and well fields have been relocated farther 

inland or have been supplemented with surface water.(17)

Along the coasts of Georgia and South Carolina, 

high chloride concentrations in groundwater are attributed 

to intrusion of seawater caused by large groundwater 

withdrawals in the Savannah and Hilton Head Island, S.C., 

areas.(16) Other areas have limited withdrawals because of 

elevated chloride concentrations or the potential for saltwater 

intrusion, including the central east and west coasts of 

Florida (13, 16) and the coast of Georgia.(18)

24 Water Quality in the Upper Floridan Aquifer and Overlying Surficial Aquifers, Southeastern United States, 1993–2010
Chapter 4

Confining

layer

Unconfined

surficial

aquifers

Southeastern

Coastal Plain

aquifer system Unconfined

Upper Floridan

aquifer

Confined

Upper Floridan

aquifer

Semiconfined

Upper Floridan

aquifer 

Domestic

well

Spring

Urban

monitoring

wells Domestic well

Agricultural

monitoring wells

Public-supply

well

Public-supply

well

Domestic

well NOT TO SCALE

This chapter summarizes 

and explains the hydrologic and geochemical 

processes and human 

activities that affect 

the movement and 

quality of groundwater 

in the Upper Floridan 

aquifer and overlying 

surficial aquifers.

Idealized geologic diagram showing the confining layer that separates the Upper Floridan and surficial aquifers 

and plays an important role in determining water quality in the Upper Floridan aquifer.

Chapter 4: Natural Processes and Human 

Activities That Affect Groundwater Quality

The absence or presence of a confining layer controls the susceptibility 

of the Upper Floridan aquifer to contamination. Where there is no 

confining layer, water moves through the overlying surficial aquifers 

directly into the Upper Floridan aquifer. Where a confining layer separates 

the surficial aquifers from the Upper Floridan aquifer, the confining layer 

protects the groundwater in the Upper Floridan aquifer from sources of 

contamination at the land surface. High-volume pumping for public water 

supply and agricultural uses can cause water from the surficial and Upper 

Floridan aquifers to mix, changing the geochemical conditions and affecting 

contaminant concentrations.
Karst Features Provide Pathways for Rapid 

Movement of Water and Contaminants

In parts of the Upper Floridan aquifer that are unconfined 

or semiconfined (fig. 3–3), surface water recharges the aquifer 

through karst features, such as sinkholes and disappearing 

streams. The groundwater flows through conduits and caves 

that have formed within the carbonate rocks of the aquifer 

and discharges to wells, springs, and streams. Flow through 

conduits is similar to water flowing through a pipe, and 

large amounts of water can move through the aquifer very 

quickly—in weeks, days, or even hours. Such flow characteristics are unique to karst aquifers and have fundamental 

implications for water quality. Water that recharges the aquifer 

through sinkholes or swallow holes in streambeds enters the 

aquifer directly, without the filtration by soils that normally 

would remove some of the bacteria and other contaminants in 

surface runoff. Once in the aquifer, there is little potential for 

contaminants in groundwater that moves through conduits to be 

removed by sorption (adhesion) to rocks, because the volume 

of water is large relative to the rock surfaces that the water 

contacts. Rapid groundwater flow through the aquifer allows 

little time for contaminants to break down or for bacteria to 

die off—as a result contaminants can be transported quickly 

to wells or springs. In addition, the hydrology of karst is such 

that standard approaches for characterizing groundwater flow 

and contaminant transport commonly cannot be applied. For 

example, the exact location and dimensions of conduits are 

largely unknown, so prediction of where and when contaminated water will discharge from the aquifer is problematic. 

Unconfined Aquifers Are Vulnerable to 

Contamination From Human Activities 

Groundwater typically is young in unconfined aquifers 

with shallow depths to water, such as the surficial aquifers 

that overlie the Upper Floridan aquifer. That is, the water has 

recharged the aquifer within the past 60 years, potentially 

transporting chemicals such as fertilizers and pesticides 

with it. The Upper Floridan aquifer also is vulnerable 

to contamination from these chemicals because, where 

unconfined, the aquifer is hydrologically connected to 

the surficial aquifers. In some areas, the Upper Floridan 

aquifer is directly connected to the land surface by sinkholes 

and disappearing streams. Urban and agricultural runoff, 

application of treated sewage to spray fields, manure 

applications to cropland, and septic tanks all are sources

of nitrate and organic chemicals at the land surface.(19, 20)

The large openings of sinkholes (above) and springs (right) 

provide direct pathways for contaminants to quickly move 

into and through the Upper Floridan aquifer.

26 Water Quality in the Upper Floridan Aquifer and Overlying Surficial Aquifers, Southeastern United States, 1993–2010
Chapter 4

Confined conditions modified from Miller(7)

Semiconfined

area

Confined area

Unconfined area

Figure 4–1.

N EXPLANATION

Mixed

Anoxic

Oxic

Redox condition

Unconfined

Semiconfined (confining layer 

 <100 feet thick or breached)

Confined

Not mapped

Confined condition for Upper 

 Floridan aquifer

Redox conditions in surficial aquifers

Redox conditions in the Upper Floridan aquifer

Surficial aquifers (includes

 local surficial aquifers and 

 surficial aquifer system) 

The Presence or Absence of Oxygen in 

Groundwater Affects the Occurrence of 

Some Contaminants

The amount of oxygen in groundwater determines the 

redox condition of the water, which affects groundwater 

quality (see sidebar, How do redox reactions work? p. 28). 

Most groundwater in the surficial aquifers and unconfined 

parts of the Upper Floridan aquifer is oxic (dissolved-oxygen 

concentration at least 0.5 mg/L) (fig. 4–1). In oxic groundwater, nitrate is stable and is the primary form of nitrogen. 

In contrast, where a confining layer is present, recharge rates 

commonly are slow—as a result groundwater becomes anoxic 

(dissolved-oxygen concentration <0.5 mg/L) as the dissolved 

oxygen gradually is consumed by microbial activity in the 

aquifer. Most groundwater in semiconfined and confined 

areas of the Upper Floridan aquifer is anoxic. In anoxic 

groundwater, nitrate can be converted to harmless nitrogen gas 

through denitrification. The presence or absence of dissolved 

oxygen also is an important control on the solubility of 

arsenic-bearing minerals in the Upper Floridan aquifer. Some 

of these minerals are unstable under oxic conditions and can 

dissolve, releasing arsenic into the groundwater, whereas other 

arsenic-bearing minerals are unstable under anoxic conditions. 

Figure 4–1. The redox condition 

(oxic or anoxic) of the groundwater 

controls the concentrations of some 

contaminants, mainly nitrate and 

arsenic. Groundwater in the surficial 

aquifers and unconfined parts of the 

Upper Floridan aquifer mostly is oxic 

(containing at least 0.5 mg/L dissolved 

oxygen) and in confined parts of 

the Upper Floridan aquifer mostly is 

anoxic (containing less than 0.5 mg/L 

dissolved oxygen). 

Chapter 4: Natural Processes and Human Activities That Affect Groundwater Quality 27
Surficial aquifer 

Conceptual model of flow paths in surficial and Upper Floridan aquifers 

in a semiconfined area during pumping conditions

NOT TO SCALE

Publicsupply

well

Recharge

Water

table Sinkhole

Confining layer (clay)

Upper Floridan aquifer

Older anoxic 

groundwater

Recently recharged

oxic groundwater

containing nitrate or

other contaminants

Mixing zone for oxic and

anoxic groundwater

Figure 4–2. Figure 4–2. When a public-supply well in a semiconfined part of the Upper Floridan

aquifer is pumped, more flow from surficial aquifers reaches the intake to the publicsupply well than would occur where the Upper Floridan aquifer is confined. Because 

groundwater in the unconfined surficial aquifers has been recharged recently, it is oxic 

and potentially contains contaminants from land surface. Large withdrawals for publicsupply wells can cause changes in the redox conditions in the Upper Floridan aquifer and 

also allow contaminants from the surface to move into the public drinking-water supply.

High-Volume Pumping for Water 

Supply Can Alter Geochemical 

Conditions and Increase the 

Vulnerability of Groundwater 

to Contamination

High-volume pumping of groundwater 

for public supply or agricultural use changes 

the direction and rate of groundwater flow 

and may transport contaminants from the 

land surface and overlying surficial aquifers 

into the Upper Floridan aquifer (fig. 4–2). 

High-volume pumping also can change the 

geochemistry of the groundwater near the 

pumping well. An influx of oxic groundwater 

from surficial aquifers into anoxic parts of the 

Upper Floridan aquifer can cause the redox 

conditions of groundwater near the pumping 

well to become mixed. This, in turn, can cause 

some constituents with geologic sources, 

such as arsenic, to be released from minerals 

in the aquifer rocks into the groundwater.

How do redox reactions work? 

Reduction/oxidation (redox) processes require one chemical species that donates electrons and another chemical species that 

accepts those electrons. As a chemical species donates electrons it is “oxidized,” and as the other species accepts electrons it is 

“reduced.” Redox processes typically are facilitated by microbes (bacteria), which use the energy produced by the processes. In 

groundwater, organic carbon is the most common electron donor. If dissolved oxygen is present, it is the preferred electron acceptor, 

because reduction of dissolved oxygen produces more energy than reduction of other chemical species that commonly occur in 

groundwater. The atmosphere is the source of the dissolved oxygen, so the redox conditions in an aquifer near where recharge occurs 

usually are oxic (defined here as having a concentration of dissolved oxygen of at least 0.5 mg/L).

As groundwater moves through the aquifer along a flow path, the dissolved oxygen in the groundwater gradually is consumed by 

redox processes. Once all of the dissolved oxygen is consumed, other chemical species can accept electrons and become reduced. 

If nitrate is present, it will become the preferred electron acceptor until it in turn is completely consumed. This pattern continues, with 

manganese, iron, sulfate, and finally carbon dioxide acting as electron acceptors until they are consumed, in that order. This order of use 

of electron acceptors has important implications for the preservation, degradation, and even production of contaminants in groundwater. 

Because redox reactions occur in a sequence, it can take a long time for strongly reducing conditions to develop. For this reason, anoxic 

groundwater commonly is older than oxic groundwater, and, within the anoxic category, strongly reducing groundwater commonly is 

older than mildly reducing groundwater. 

From a water-quality perspective, denitrification—the reduction of nitrate to nitrogen gas—is one of the most important redox 

processes that occurs in groundwater. Nitrate is a concern for human health and, where it discharges to surface water, can impair 

aquatic communities. Conversion of nitrate by denitrification to harmless nitrogen gas, the same gas we breathe in the atmosphere, 

is the primary way that nitrate is removed from water.

Oxygen Nitrate Manganese Iron Sulfate Carbon dioxide

Oxic Anoxic

Oxic Mildly

reducing

Strongly

reducing

Very strongly

reducing

Order of consumption of electron acceptors

28 Water Quality in the Upper Floridan Aquifer and Overlying Surficial Aquifers, Southeastern United States, 1993–2010
Chapter 5

Chapter 5: Quality of the Groundwater Resource 

Used for Drinking 

This chapter provides a 

summary of constituents 

that were detected at a 

concentration greater 

than or near a humanhealth benchmark from

drinking-water supply 

wells tapping the 

Upper Floridan aquifer 

and nondrinking-water 

wells in the overlying 

surficial aquifer.

Photograph by Emma Kingsbury

The Upper Floridan aquifer is the principal source of drinking water for 

about 10 million people, and the quality of water in the Floridan aquifer 

system is among the best in the Nation. Only 4 percent of drinkingwater wells sampled contained a contaminant that exceeded a human-health 

benchmark. Although few contaminants were detected at concentrations of

potential human-health concern in the Upper Floridan aquifer, elevated concentrations of nitrate and the occurrence of pesticides in parts of the surficial 

aquifers indicate that the underlying Upper Floridan aquifer is vulnerable to 

contamination where it is recharged by water from the surficial aquifers.
Few Contaminants Were Detected at 

Concentrations That Exceeded a HumanHealth Benchmark

Two constituents derived from geologic sources—

radon-222 (radon) and strontium—were measured at a 

concentration greater than their human-health benchmark 

in 1 percent or more of drinking-water wells in the Upper 

Floridan aquifer (table 5–1). Radon concentrations in 

groundwater are determined by the uranium and radium 

content of the rocks that make up the aquifer materials and 

the physical characteristics of those aquifer materials.(21)

When dissolved in water, radon can degas and be inhaled as 

the water flows from taps and showerheads. The inhalation 

of radon increases the risks of lung cancer.(22) There is no 

U.S. Environmental Protection Agency (USEPA) Maximum 

Contaminant Level (MCL) for radon, but USEPA has 

proposed an MCL of 300 picocuries per liter (pCi/L) and 

a proposed alternative MCL (AMCL) of 4,000 pCi/L for 

radon.(23) About half of the samples from drinking-water 

wells contained radon at a concentration greater than the 

proposed MCL; most of these samples were from wells in 

central Florida. Only about 3 percent of samples contained 

radon at a concentration greater than the proposed AMCL. 

Strontium was detected in all samples from the Upper 

Floridan aquifer, but was measured at concentrations that 

exceeded its human-health benchmark of 4,000 micrograms 

per liter (μg/L) in just 1 percent of samples. Strontium 

commonly substitutes for calcium in the minerals that make 

up the limestones and dolomites of the Upper Floridan 

aquifer, and as a result strontium was commonly present 

in groundwater. The primary human-health concern for 

strontium in drinking water is potential adverse effects on 

bone development and growth during early childhood. 

A number of constituents and properties—“nuisance 

constituents”—can adversely affect water quality for reasons 

other than human health. Iron, manganese, hardness, total 

dissolved solids, and several major ions can cause water to 

taste bad, smell bad, stain plumbing fixtures, corrode pipes, 

or have other undesirable effects. The USEPA has issued 

Secondary Maximum Contaminant Levels (SMCLs) as 

nonmandatory water-quality criteria for nuisance constituents. 

Six nuisance constituents were measured at a concentration 

greater than their SMCL in at least one sample from a 

drinking-water well in the Upper Floridan aquifer. Of these, 

iron was the most prevalent, exceeding its SMCL in about 

15 percent of drinking-water wells sampled in the Upper 

Floridan aquifer and in similar percentages of monitoring 

wells in the Upper Floridan and surficial aquifers (table 5 –2).

Table 5–1. Two contaminants from geologic sources, radon and strontium, were measured at concentrations greater than humanhealth benchmarks in 1 percent or more of drinking-water wells sampled in the Upper Floridan aquifer. Three other inorganic constituents

were found at concentrations within one-tenth of their human-health benchmarks in 10 percent or more of drinking-water wells.

[MCLs are U.S. Environmental Protection Agency (USEPA) Maximum Contaminant Levels (MCLs) for public water supplies; HBSLs are U.S. Geological 

Survey Health-Based Screening Levels; <, less than; pCi/L, picocuries per liter; μg/L, microgram per liter; mg/L, milligram per liter]

Contaminant

Human-health

benchmark

Upper Floridan aquifer Upper Floridan aquifer Surficial aquifer

Drinking-water wells All wells All wells

Number 

of wells 

sampled

Frequency 

of concentrations 

greater

than 

benchmark 

(percent

of wells)

Frequency

of concentrations 

within 

10 percent

of benchmark 

(percent

of wells)

Number 

of wells 

sampled

Frequency 

of concentrations 

greater

than 

benchmark 

(percent

of wells)

Frequency

of concentrations 

within 

10 percent

of benchmark 

(percent

of wells)

Number 

of wells 

sampled

Frequency 

of concentrations 

greater

than 

benchmark 

(percent

of wells)

Frequency 

of concentrations 

within 

10 percent 

of benchmark 

(percent

of wells)

Value Type

Radon 4,000 pCi/L

300 pCi/L

Proposed

MCLs

93

93

3

55

45

95

157

157

3

46

41

97

50

50

8

56

62

100

Strontium 4,000 μg/L HBSL 162 1 14 208 2 15 85 0 0

Arsenic 10 μg/L MCL 163 < 1 16 236 < 1 15 100 0 4

Fluoride 4 mg/L MCL 163 0 17 244 0 16 143 0 3

Manganese 300 μg/L HBSL 161 0 9 252 < 1 9 144 < 1 19

Nitrate 10 mg/L MCL 161 0 23 249 < 1 33 144 16 58

Dieldrin 0.002 μg/L HBSL 159 0 0 251 < 1 < 1 138 1 1

30 Water Quality in the Upper Floridan Aquifer and Overlying Surficial Aquifers, Southeastern United States, 1993–2010
What is a contaminant?

Contaminants have a wide range of sources, both manmade and geologic. Most organic chemicals in groundwater that are 

of concern for human health are manmade. In contrast, most inorganic constituents in groundwater have geologic or other natural 

sources, although their concentrations in groundwater may be altered by human activities, such as irrigation and groundwater 

pumping. Some contaminants have both manmade and natural sources. For example, nitrate in groundwater has many natural 

sources, but nitrate concentrations in groundwater underlying agricultural and urban areas commonly are higher than in other 

areas because of contributions from sources associated with human activities.

But what exactly is a contaminant? The word means different things to different people. For example, a contaminant is 

defined by the Safe Drinking Water Act (SDWA) as “any physical, chemical, biological, or radiological substance or matter in 

water” (see http://www.epw.senate.gov/sdwa.pdf). This broad definition of contaminant includes every substance that may be 

found dissolved or suspended in water— everything but the water molecule itself. This is not a very practical definition because 

this would imply that all water is “contaminated.” Pure water that has nothing dissolved in it does not occur naturally—not even 

rainfall is pure water, because it contains, at a minimum, some dissolved gases.

The U.S. Environmental Protection Agency defines a contaminant as “Any physical, chemical, biological, or radiological 

substance or matter that has an adverse effect on air, water, or soil” (see http://epa.gov/region04/superfund/qfinder/glossary.html).

This definition is more practical and allows both manmade constituents and those with geologic sources in water to be defined 

as contaminants. However, it does not define what “adverse” means, and what may be adverse in one way might be beneficial 

in another. In this circular, a contaminant is defined as any physical, chemical, biological, or radiological substance or matter in 

groundwater that is manmade or that impairs the use of water for its intended purpose. Impairment is determined by comparing 

a measured concentration to benchmarks or guidelines. By this definition, all manmade compounds, such as pesticides and 

volatile organic compounds, are contaminants because they do not occur naturally in groundwater. If a constituent with a 

geologic source, such as arsenic, occurs in drinking water at a concentration above its human-health benchmark, it also is 

considered a contaminant.

Chapter 5

Table 5–2. Iron concentrations were greater than the value recommended by USEPA for the aesthetic quality of water in about 

15 percent of drinking-water samples in the Upper Floridan aquifer. Concentrations of chloride, fluoride, iron, manganese, sulfate, 

and dissolved solids were outside the range of values for aesthetic quality of water in less than 5 percent of the drinking-water 

samples. 

[SMCLs are U.S. Environmental Protection Agency (USEPA) Secondary Maximum Contaminant Levels (SMCLs) for public water supplies; mg/L, milligram per liter; μg/L, microgram per liter]

Constituent

Guideline Upper Floridan aquifer Upper Floridan aquifer Surficial aquifer

Drinking-water wells All wells All wells

Value Type

Number 

of wells 

sampled

Frequency of 

concentration 

greater than 

guideline

(percent

of wells)

Number 

of wells 

sampled

Frequency of 

concentration 

greater than 

guideline

(percent

of wells)

Number 

of wells 

sampled

Frequency of 

concentration 

greater than 

guideline

(percent

of wells)

Chloride 250 mg/L SMCL 160 2 244 1 143 0

Fluoride 2 mg/L SMCL 160 2 244 2 143 0

Iron 300 μg/L SMCL 160 15 244 18 144 13

Manganese 50 μg/L SMCL 160 4 244 5 144 12

Sulfate 250 mg/L SMCL 160 1 244 2 143 0

Total dissolved solids 500 mg/L SMCL 160 5 244 6 143 0

Chapter 5: Quality of the Groundwater Resource Used for Drinking 31
Human-health benchmarks and other guidelines used in this assessment 

Concentrations of constituents measured for this assessment were compared to human-health benchmarks to 

place study findings in the context of human health. The benchmarks are threshold concentrations in water above 

which the concentration of a contaminant in drinking water could adversely affect human health. Human-health 

benchmarks were available for about two-thirds of the 285 constituents and properties measured for the Principal 

Aquifer assessments (appendix 2). Two types of human-health benchmarks were used: U.S. Environmental Protection 

Agency (USEPA) Maximum Contaminant Levels (MCLs) (http://water.epa.gov/drink/contaminants/index.cfm) and 

U.S. Geological Survey (USGS) Health-Based Screening Levels (HBSLs). MCLs are legally enforceable drinkingwater standards that specify the maximum permissible level of a constituent in water that is delivered to any user 

of a public water system(59) (values used in this report were current as of February 2012). Although MCLs are used 

to regulate the quality of drinking water only from public-supply sources, they also are useful for evaluating the 

quality of water from domestic and monitoring wells. An MCL was available for 53 of the constituents measured. 

For some constituents for which an MCL has not been established, the USGS, in collaboration with the USEPA 

and others, developed non-enforceable HBSLs by using standard USEPA methods for establishing drinking-water 

guidelines and current toxicity information(56, 57, 58) (values used in this report were current as of February 2012; see 

http://water.usgs.gov/nawqa/HBSL). An HBSL was available for 135 constituents measured. Radon has neither an 

MCL nor an HBSL, but two MCLs have been proposed. Copper and lead have USEPA action levels rather than an MCL.

In addition to human-health benchmarks, non-health-based guidelines—

Secondary Maximum Contaminant Levels (SMCLs)—were available for some of 

the constituents measured in this assessment. The SMCLs are non-enforceable 

guidelines for concentrations of “nuisance” constituents in drinking water that 

can cause unwanted cosmetic effects, such as skin or tooth discoloration; 

aesthetic effects, such as unpleasant taste, odor, or color; or technical effects, 

such as corrosion or sedimentation of plumbing or reduced effectiveness of 

water treatment.(60)

Concentrations greater than one-tenth of a human-health benchmark 

were used in this assessment to indicate which contaminants occurred, either 

individually or as mixtures, at concentrations that approach those of potential 

concern for human health, and to identify contaminants that might warrant 

additional monitoring and study. The criterion of one-tenth of a benchmark is 

consistent with various State and Federal practices for reporting contaminant 

occurrence in groundwater and for identifying contaminants of potential 

human-health concern (for example, see U.S. Environmental Protection 

Agency; (54) New Jersey Department of Environmental Protection(55)).

Screening-level assessments, such as this one, provide perspective on the 

potential relevance of detected contaminants to human health and can help in 

planning future studies.(57) They are not designed to evaluate specific effects of 

contaminants on human health and are not a substitute for comprehensive risk 

assessments. It is important to note that occurrence of a contaminant at a concentration greater than its benchmark 

does not mean that adverse effects are certain to occur, because the benchmarks are conservative (protective) and 

source-water samples were collected prior to any treatment or blending that could alter contaminant concentrations 

in finished drinking water. There are water-treatment options, such as charcoal filtration, that can be used to lower 

the concentration of the contaminant to below the benchmark before the water is consumed.

Iron concentrations that exceed the 

SMCL in household water can result 

in staining of bathtubs, sinks, and 

other plumbing fixtures. Elevated iron 

also can give drinking water a metallic 

taste or unpleasant odor.

Photograph courtesy of Culligan

32 Water Quality in the Upper Floridan Aquifer and Overlying Surficial Aquifers, Southeastern United States, 1993–2010
Photograph by Lynn Betts, USDA NRCS

Chapter 5

0 20 40 60 80 100

Nitrate concentration, in

 milligrams per liter as nitrogen

 (number in parentheses is 

 number of wells sampled) 

EXPLANATION

0.1 to 1

>10

>1 to 10

Nitrate concentration in surficial 

and Upper Floridan aquifers

Percentage of samples

Monitoring

wells (144)

Monitoring and

other nondrinkingwater wells (91)

Drinking-water

wells (158)

Surficial aquifers

Upper Floridan aquifer

< 0.1

Figure 5-1

0 20 40 60 80 100

Nitrate concentration, in

 milligrams per liter as nitrogen

 (number in parentheses is 

 number of wells sampled) 

EXPLANATION

0.1 to 1

>10

>1 to 10

Nitrate concentration in surficial 

and Upper Floridan aquifers

Percentage of samples

Monitoring

wells (144)

Monitoring and

other nondrinkingwater wells (91)

Drinking-water

wells (158)

Surficial aquifers

Upper Floridan aquifer

< 0.1

Figure 5-1

Nitrate is a Potential Contaminant of Concern 

for Drinking Water Because of its Widespread 

Occurrence in Shallow Monitoring Wells in 

Urban and Agricultural Areas

The occurrence of nitrate at concentrations greater 

than the background concentration indicates that the Upper 

Floridan and surficial aquifers are vulnerable to contamination 

from human activities at the land surface. Almost one-fourth 

(23 percent) of the samples from drinking-water wells had 

Figure 5–1. Concentrations of nitrate 

exceeded the human-health benchmark 

of 10 mg/L as N in 16 percent of samples 

from monitoring wells in surficial aquifers, 

but in only one sample from a monitoring 

well in the Upper Floridan aquifer, and in no 

samples from drinking-water wells.

nitrate concentrations greater than 1 mg/L as N (table 5–1; 

fig. 5–1), which is above the background concentration 

of 0.1 mg/L for the Upper Floridan aquifer.(24) The highest 

nitrate concentrations occurred in the unconfined parts of 

the Upper Floridan aquifer in southwest Georgia and northcentral Florida where agriculture is the dominant land use. 

About 32 and 41 percent of samples in urban and agricultural 

monitoring wells, respectively, in the Upper Floridan and 

surficial aquifers had nitrate concentrations greater than 

1 mg/L as N. 

Fertilizer use in urban areas on lawns and turf (above) and in agricultural areas (right) are sources of nitrate to groundwater. 

Photograph by USGS

Chapter 5: Quality of the Groundwater Resource Used for Drinking 33
Figure 5–2.

0 20 40 60 80 100

Domestic wells (128) Public-supply wells (35) 

Upper Floridan aquifer 

Surficial aquifers

Urban land-use monitoring wells (28) 

Urban land-use monitoring wells (25) 

Deisopropylatrazine

Tebuthiuron

Bentazon

Prometon

Metolachlor

Hydroxyatrazine

Atrazine

Deethylatrazine

Deisopropylatrazine

Tebuthiuron

Bentazon

Prometon

Metolachlor

Hydroxyatrazine

Atrazine

Deethylatrazine

Agricultural land-use monitoring wells (97) 

0 20 40 60 80 100

EXPLANATION

At any concentration

Detection (number in parentheses

 is number of wells sampled)

At concentration >0.1 μg/L

(Common assessment level)

0 20 40 60 80 100

0 20 40 60 80 100 0 20 40 60 80 100

Percentage of samples in which pesticide was detected 

Percentage of samples in which pesticide was detected 

Figure 5–2. The herbicide atrazine and two of its degradation products (deethylatrazine and hydroxyatrazine) 

were the pesticide compounds most commonly detected in samples from the surficial and Upper Floridan 

aquifers. Detection frequencies in public-supply wells in the Upper Floridan aquifer were comparable to 

detection frequencies in monitoring wells, but concentrations were lower.

Low-Level Concentrations of Pesticides 

and Volatile Organic Compounds Were 

Detected Frequently

Dieldrin, an insecticide, was the only pesticide or 

volatile organic compound (VOC) detected at a concentration 

greater than a human-health benchmark (table 5 –1). Although 

few benchmarks were exceeded, numerous pesticides 

and VOCs were detected in both the surficial and Upper 

Floridan aquifers, and about 20 percent of the 38 pesticides 

and 18 percent of the 27 VOCs detected were present in 

more than 10 percent of the samples. The occurrence of 

these chemicals—which includes herbicides, insecticides, 

solvents, disinfection by-products, gasoline hydrocarbons 

and oxygenates, refrigerants, and fumigants—highlights the 

diverse industrial, urban, and agricultural land uses that can 

affect groundwater quality. 

The most frequently detected pesticides in samples from 

the surficial and Upper Floridan aquifers were the herbicides 

atrazine and its degradates (deethylatrazine, hydroxyatrazine, 

and deisopropylatrazine), metolachlor, prometon, bentazon, 

and tebuthiuron (fig. 5 –2). Most of these herbicides and 

degradates were detected in about the same percentage of 

agricultural monitoring wells in the surficial aquifers and 

public-supply wells in the Upper Floridan aquifer, but concentrations were lower in public-supply wells. Only tebuthiuron, 

an herbicide with nonagricultural uses, was measured at a 

concentration above 0.1 μg/L in a public-supply well, whereas 

seven herbicide compounds were detected above 0.1 μg/L in 

some agricultural monitoring wells in the surficial aquifers. 

These commonly detected herbicide compounds were 

detected much less frequently in domestic wells than in the 

public-supply wells in the Tampa, Fla., area. This difference 

likely reflects the larger areas that contribute water to publicsupply wells than to domestic wells and greater use of these 

herbicides in the contributing areas of public-supply wells. 

34 Water Quality in the Upper Floridan Aquifer and Overlying Surficial Aquifers, Southeastern United States, 1993–2010
Chapter 5

Surficial aquifers

Agricultural land-use monitoring wells (55) Urban land-use monitoring wells (39) 

0 20 40 60 80 100 0 20 40 60 80 100

Percentage of samples in which pesticide was detected 

Figure 5–3.

0 20 40 60 80 100

Domestic wells (124) Public-supply wells (32) 

Upper Floridan aquifer 

Urban land-use monitoring wells (37) 

0 20 40 60 80 100 0 20 40 60 80 100

Percentage of samples in which pesticide was detected 

Toluene

Tetrachloroethene (PCE)

1,2-dichloropropane

Carbon disulfide

Chloroform

Toluene

Tetrachloroethene (PCE)

1,2-dichloropropane

Carbon disulfide

Chloroform

EXPLANATION

At any concentration

Detection (number in parentheses

 is number of wells sampled)

At concentration >0.2 μg/L

(Common assessment level)

The VOCs most frequently detected in the Upper 

Floridan aquifer were chloroform, carbon disulfide, 

1,2-dichloropropane, and tetrachloroethene (also known 

as perchloroethylene or PCE) (fig. 5 –3). Chloroform is a 

trihalomethane compound that commonly is produced by the 

chlorination of water and wastewater. Other potential sources 

of chloroform and other trihalomethanes that might affect 

water withdrawn from domestic wells are laundry wastewater, 

which contains bleach, and shock chlorination of wells for 

disinfection. Chloroform was detected in samples from 

44 percent of public-supply wells and 24 percent of domestic 

wells in the Upper Floridan aquifer and in 32 percent of 

monitoring wells in urban land-use areas in surficial aquifers. 

Most of the public-supply wells and urban monitoring wells 

sampled were located in the Tampa area. Chloroform and 

carbon disulfide have been detected frequently in groundwater 

in the Tampa area.(25)

Figure 5–3. Chloroform and carbon disulfide were the VOCS most frequently detected in the surficial and Upper 

Floridan aquifers. Chloroform is a by-product of the chlorination of drinking water and its frequent occurrence may 

be related to infiltration of treated water and in some cases shock chlorination of domestic wells. Carbon disulfide 

is a solvent used in industrial applications.

Chapter 5: Quality of the Groundwater Resource Used for Drinking 35

The occurrence of pesticides, solvents, 

disinfection by-products, gasoline 

hydrocarbons, and fumigants in groundwater highlights the diverse industrial, 

urban, and agricultural land uses that 

can affect groundwater quality.
Dissolution along joints and fractures in the carbonate rocks that make up the Upper Floridan aquifer create pathways for water to 

move rapidly into and through the aquifer.

36 Water Quality in the Upper Floridan Aquifer and Overlying Surficial Aquifers, Southeastern United States, 1993–2010
Chapter 6: Understanding Where and Why 

Key Contaminants Occur in Groundwater

Chapter 6

Agricultural and urban land-use practices have the potential to adversely 

affect groundwater quality in the Upper Floridan and surficial aquifers.

Photograph by Guy H. Means, Florida Geological Survey

This chapter describes 

the sources of and factors 

that affect arsenic, 

radon, nitrate, and 

pesticide concentrations 

in the Upper Floridan 

aquifer and overlying 

surficial aquifers.

Few contaminants were measured in groundwater in the Upper Floridan 

and surficial aquifers at concentrations approaching human-health 

benchmarks. Although contaminants related to human activities are 

detected commonly in the surficial aquifers, the presence of an extensive 

confining layer and the enormous quantities of water moving through the 

Upper Floridan aquifer keep concentrations of contaminants low in that 

aquifer in most areas. Human activities in some areas, however, have caused 

arsenic concentrations to increase. In other places, the presence of karst 

features makes the Upper Floridan aquifer vulnerable to contamination.
As

33

75

EXPLANATION EXPLANATION

Surficial aquifers (includes

 local surficial aquifers and 

 surficial aquifer system) 

<1 1 to 10

Arsenic concentration, in μg/L 

<1 1 to 10 >10

Arsenic concentration, in μg/L 

Figure 6–1.

Unconfined

Semiconfined (confining layer 

 <100 feet thick or breached)

Confined

Not mapped

Confined condition for the 

 Upper Floridan aquifer

Arsenic concentrations in surficial aquifers Arsenic concentrations in the Upper Floridan aquifer

N

Confined conditions modified from Miller(7)

Arsenic

Concentrations of arsenic were low—usually less than one-tenth of the MCL 

(10 μg/L)—in most wells in the surficial and Upper Floridan aquifers. Elevated 

concentrations in some wells in the Upper Floridan aquifer are related to high-volume 

pumping, injection of water during aquifer storage and recovery, or other human 

actions. These actions can change the native geochemistry of the groundwater, 

causing arsenic-rich minerals in the aquifer rocks to dissolve.

More than 80 percent of samples in both the surficial and Upper Floridan aquifers had low 

arsenic concentrations—less than 1 μg/L (fig. 6–1). Concentrations of arsenic in groundwater 

samples from the surficial and Upper Floridan aquifers are lower than those in many other 

aquifers sampled nationally (appendix 3). Arsenic concentrations exceeded the MCL of 10 μg/L 

in only one monitoring well and one public-supply well—both wells are in semiconfined parts 

of the Upper Floridan aquifer in the Tampa area. Elevated arsenic concentrations in groundwater used for drinking water are of concern because ingestion of arsenic is associated with 

cancers and other human-health problems.(26) Some studies have reported a risk of developing 

lung and bladder cancers for people that consume drinking water with arsenic concentrations as 

low as 1 μg/L.(27)

Arsenic is a trace element 

found in aquifer materials 

and can also come from 

pesticide application or 

industrial waste. Long-term 

exposure to arsenic in 

drinking water is related 

to elevated risks of cancer 

and skin damage.

Figure 6–1. Elevated concentrations of arsenic (>1 μg/L) were more common in samples from the Upper Floridan aquifer than in 

samples from the surficial aquifers because trace minerals containing arsenic are present in the carbonate rocks that make up the 

Upper Floridan aquifer. High-volume pumping and other human activities that alter groundwater flow can cause arsenic-bearing 

minerals in the rocks of the Upper Floridan aquifer to dissolve and release arsenic into the groundwater.

38 Water Quality in the Upper Floridan Aquifer and Overlying Surficial Aquifers, Southeastern United States, 1993–2010
Chapter 6

NOT TO SCALE

Publicsupply

well

Recharge Water

table

Sinkhole

Confining layer (clay)

Surficial aquifer

Upper Floridan aquifer

Older anoxic 

groundwater

Recently recharged

oxic groundwater

Mixing zone for oxic and anoxic

groundwater where arsenic is mobilized

Figure 6–2. Figure 6–2. High-volume pumping for public supply can induce mixing of oxic water from 

surficial aquifers and anoxic groundwater from the Upper Floridan aquifer. The mixing, in turn, 

causes arsenic-rich minerals to dissolve and release arsenic to the groundwater near the 

public-supply well.

Meeting the increasing demands for water can have unintended consequences 

High-volume pumping of public-supply wells in the Upper Floridan aquifer can cause 

arsenic concentrations in groundwater to increase (fig. 6–2). As pumping withdraws deep, 

anoxic groundwater from the Upper Floridan aquifer, oxic water in the overlying surficial 

aquifers moves downward and into the Upper Floridan aquifer and mixes with the native 

water. Mixing of oxic and anoxic groundwater also can be caused by a water-supply procedure 

called aquifer storage and recovery (see sidebar, Aquifer storage and recovery (ASR) can 

increase arsenic concentrations in the Upper Floridan aquifer, p. 40). The mixing of oxic and 

anoxic water causes some minerals that are stable under anoxic conditions to dissolve. One 

of those minerals, pyrite, is present in some of the limestones of the Upper Floridan aquifer 

and contains as much as 1 percent arsenic.(28) When the pyrite dissolves, it releases arsenic 

into the groundwater.

Arsenic concentrations at a public-supply well near Tampa ranged from 4 to 19 μg/L at 

different depths sampled. This range in arsenic concentrations—from less than half to almost 

twice the MCL of 10 μg/L—reflects the geochemical conditions associated with different 

depths and pumping conditions in the same well. The highest arsenic concentrations likely 

occurred as a result of oxic water from the overlying surficial aquifer moving down into a 

high-flow zone in the Upper Floridan aquifer, which can cause arsenic-rich pyrite to dissolve 

and release arsenic into the groundwater.(25) Under normal operation, the public-supply well 

draws water from several flow zones in the aquifer, and arsenic concentrations are about 

3 μg/L—well below the MCL.

Chapter 6: Understanding Where and Why Key Contaminants Occur in Groundwater 39
Aquifer storage and recovery (ASR) can increase arsenic concentrations in the Upper 

Floridan aquifer

During ASR, the aquifer essentially functions as a “water bank”—water is 

injected into an aquifer during wet periods and is recovered during dry periods. The 

number of ASR facilities in Florida has increased greatly in the past 15–20 years.(29)

By 2011, there were 26 ASR facilities in operation in Florida and more than 15 

sites under development.(30) Most of these facilities are located in coastal areas in 

southern Florida, where excess freshwater is stored in brackish parts of the Floridan 

aquifer system. In response to the projected increase in population and water 

demand over the next several decades in Florida, local and county water utilities 

in southern Florida are planning additional ASR facilities.

Limestone in southwest Florida contains arsenic-rich “framboidal” pyrite, 

so-called because of its raspberry shape (“framboise” is the French word for 

raspberry). Pyrite is stable where groundwater in the Upper Floridan aquifer is 

anoxic, but dissolves in the oxic water injected into the aquifer during the waterstorage phase of ASR. Arsenic has been measured in samples of recovered 

water at concentrations 5 to 10 times greater than the MCL of 10 μg/L.(31) The 

mobilization of arsenic during ASR could result in additional treatment costs to 

remove arsenic from the recovered water. Concentrations of arsenic decrease 

over time as additional cycles of storage and recovery deplete the arsenic in the 

aquifer materials.(31)

Injection of oxygen-rich water into

well and storage in the aquifer 

Modified from Reese and Alvarez-Zarikian(29)

Confining

layer

Confining

layer

Aquifer

Recovery of water 

from well 

CH 6-ASR sidebar

Schematic of an aquifer storage and recovery well

Oxic water in the storage zone

mixes with native, anoxic 

water in the Upper Floridan aquifer. 

Arsenic is released into

groundwater when pyrite in the 

rock that makes up the aquifer 

dissolves under oxic conditions.

Arsenic that has been released

from pyrite in the storage and

mixing zones is pumped out with 

the stored water.

Unaffected zone

Mixing zone

Storage zone

As As

As As

As As As As

As As As As

As

As

As

As As As

As As

As As

As

As

As As

As

As

As

Pyrite containing arsenic 

EXPLANATION

NOT TO SCALE

This image from a scanning electron 

microscope shows raspberry-shaped pyrite 

in the Upper Floridan aquifer. Some of the 

pyrite contains arsenic at concentrations 

greater than 1,000 mg/kg. The arsenic 

can be released into groundwater when 

oxygen-rich water is injected into the 

aquifer during the ASR process.

~20 μm

Photograph by Roy Price

40 Water Quality in the Upper Floridan Aquifer and Overlying Surficial Aquifers, Southeastern United States, 1993–2010
Radon risk from water

Radon is a naturally occurring radioactive gas that is 

part of the decay series of uranium, an element that occurs in 

trace amounts in aquifer sediment and rocks. Radon is soluble 

in water and was detected in groundwater from nearly all of 

the wells sampled in the Upper Floridan aquifer. The USEPA 

classifies radon as a human carcinogen,(32) and people who are 

exposed to radon, mostly in the air but also through drinking 

water, have an increased risk of getting cancer over their 

lifetime, especially lung cancer.(33) Health effects from radon 

in drinking water are primarily through inhalation, after the 

gas is released from solution—for example, in the shower.

In 1999, the USEPA proposed two regulatory levels for 

radon in drinking water. The higher level of 4,000 pCi/L, known 

as the alternative MCL, would apply to public water systems 

or States that have programs to reduce radon risks from all 

sources. The lower level of 300 pCi/L would apply to systems 

or States that do not have such programs in place.(23) Both 

proposed MCLs are used as human-health benchmarks in 

this circular, but are not yet formal regulatory benchmarks. 

For more information on radon and how to test for it, see 

http://www.epa.gov/radon/index.html. For State-specific 

information about radon in Florida, see http://www.doh.

state.fl.us/environment/community/radon/index.html; 

in Georgia, see http://www.dca.ga.gov/development/

EnvironmentalManagement/programs/radon.asp; and in 

South Carolina, see http://www.scdhec.gov/environment/

envserv/radon.htm.

Radon gas (represented by red dots) dissolved in 

groundwater can be pumped from a domestic well 

through household pipes. Once the water is aerated 

through a faucet and released into the air, the radon 

can be inhaled when people are near showerheads 

or are drinking directly from faucets.

Chapter 6

Modified from Otton and others (89) 

Rn

86

222

Modified from Otton and others(53)

Radon is an element that 

results from the radioactive 

decay of uranium in aquifer 

materials. Radon in drinking 

water is associated with lung 

cancer and stomach cancer.

Radon

Radon, a by-product of the radioactive decay of uranium, was detected in nearly 

all samples from drinking-water wells (85 percent)—concentrations exceeded the 

proposed MCL of 300 pCi/L in about half of the samples, but rarely exceeded the 

alternative MCL of 4,000 pCi/L. 

Radon is a colorless, odorless, tasteless gas that emits ionizing radiation—it occurs 

naturally in some rocks, soil, water, and air. Radon that is present in groundwater as a dissolved 

gas is derived from trace amounts of uranium in minerals in the rocks that make up the aquifer. 

Inhalation of excessive amounts of radon can cause lung cancer. Radon primarily enters homes 

from the soil and underlying rocks through cracks or openings in walls or basement floors, but 

also can enter homes as it degases from water. Residents that rely on surface water or public 

groundwater supplies are less likely to be exposed to radon than are residents with domestic 

wells—the large public-supply water distribution systems provide a greater amount of time 

for radon to degas or to decay (radon has a half-life of 3.8 days) before the water reaches a 

residence than do smaller domestic systems. 

A little more than half (56 percent) of the samples from drinking-water wells in the Upper 

Floridan aquifer—most of which were domestic wells—contained radon at a concentration 

that exceeded the proposed MCL of 300 pCi/L, but concentrations in only 3 percent of samples 

exceeded the alternative MCL of 4,000 pCi/L (see sidebar, Radon risk from water, below). 

Routine testing of water from domestic wells is not required, and homeowners are responsible 

for identifying and treating any water-quality problems. The occurrence of radon at elevated 

concentrations in domestic wells emphasizes the need for public education to describe where 

radon is likely to occur and what testing and treatment options are available.

Chapter 6: Understanding Where and Why Key Contaminants Occur in Groundwater 41
Figure 6–4. Some samples from the surficial aquifers in north-central Florida (Ocala area) had relatively high concentrations of radon, 

but samples for analysis of radon primarily were collected in Georgia. Samples from the Upper Floridan aquifer were more widely

distributed, and radon concentrations were highest in north-central Florida. Differences in the trace amounts of uranium in minerals in 

the rocks that make up the aquifer account for the differences in radon concentrations in groundwater. 

Figure 6–3. About 80 percent 

of wells sampled in north-central 

Florida exceeded the proposed 

MCL of 300 pCi/L. Only 3 percent 

of drinking-water wells sampled 

exceeded the alternative MCL of 

4,000 pCi/L.

EXPLANATION EXPLANATION

Surficial aquifers (includes

 local surficial aquifers and 

 surficial aquifer system) 

<300 300 to 4,000 >4,000

Radon concentration, in

 picocuries per liter

<300 300 to 4,000 >4,000

Radon concentration, in 

 picocuries per liter

Figure 6–4.

Unconfined

Semiconfined (confining layer 

 <100 feet thick or breached)

Confined

Not mapped

Confined condition for the 

 Upper Floridan aquifer

Radon concentrations in surficial aquifers Radon concentrations in the Upper Floridan aquifer

Ocala area

N

Confined conditions modified from Miller(7)

Figure 6–3.

Percentage of samples

Radon concentration, in

 picocuries per liter

EXPLANATION

<300

>4,000

300 to 4,000

0 20 40 60 80 100

Radon concentration in drinking-water

wells in the Upper Floridan aquifer

Northcentral

Florida

Southern

Georgia

Southern

South

Carolina

Figure 6–3.

Percentage of samples

Radon concentration, in

 picocuries per liter

EXPLANATION

<300

>4,000

300 to 4,000

0 20 40 60 80 100

Radon concentration in drinking-water

wells in the Upper Floridan aquifer

Northcentral

Florida

Southern

Georgia

Southern

South

Carolina

Radon concentrations that exceeded the proposed MCL of 300 pCi/L were more common 

in samples from drinking-water wells in north-central Florida than in southern Georgia and 

southern South Carolina (fig. 6–3). These differences are related to spatial differences in 

uranium content in the rocks of the Upper Floridan aquifer. Radon concentrations in the central 

part of Florida were higher in samples from the surficial aquifers than from the Upper Floridan 

aquifer (fig. 6–4), but transport of radon from the surficial aquifers to the Upper Floridan 

aquifer is unlikely because of the short half-life of radon. Radon concentrations in samples 

from both the surficial and Upper Floridan aquifers were lower than in most of the other 

Principal Aquifers across the Nation where radon was measured (appendix 3).

42 Water Quality in the Upper Floridan Aquifer and Overlying Surficial Aquifers, Southeastern United States, 1993–2010

Concentrations of radon 

commonly exceeded the 

lower, proposed MCL but 

rarely exceeded the higher, 

alternative MCL.
Chapter 6

Nitrogen in fertilizers, which are applied to agricultural crops and urban landscapes alike, can leach into the surficial aquifers.

Photograph by Jeff Vanuga, USDA NRCSPhotograph by USGS

Nitrate

Nitrate concentrations in shallow monitoring wells in the surficial aquifers 

exceeded the MCL of 10 mg/L as N in about 1 of every 6 wells sampled, but nitrate 

concentrations in the Upper Floridan aquifer exceeded the MCL in only 1 of 249 wells 

sampled. Nitrate from manure and fertilizers applied in agricultural and urban areas 

can leach into the surficial aquifers, which in turn provide recharge to the Upper 

Floridan aquifer. Although nitrate concentrations in the Upper Floridan aquifer 

are not a drinking-water concern, more than half of the samples had concentrations 

greater than the nutrient criterion of 0.35 mg/L as N for springs in Florida. For these 

crystal-clear springs, even small increases in nitrate concentrations can cause algal 

growth that degrades water quality.

Nitrate concentrations are elevated in the surficial aquifers 

The application of fertilizer and animal manure in agricultural areas, such as those in 

southwestern Georgia and southern South Carolina, has contributed to elevated levels of nitrate 

in the surficial aquifers (fig. 6–5). In these agricultural areas, nitrate concentrations exceeded 

the MCL of 10 mg/L as N in about 1 of every 6 samples from wells in the surficial aquifers. 

The median concentration of nitrate in these wells was 3.3 mg/L and the maximum concentration was 35 mg/L. Nitrate concentrations were above the background concentration of about 

0.1 mg/L(24, 34, 35) in more than three-fourths of the wells. 

Nitrate concentrations in shallow groundwater underlying the Ocala and Tampa areas were 

affected by the overlying land use. About one-third of the samples had concentrations above the 

background concentration of 0.1 mg/L. However, fewer samples in these urban areas exceeded 

the MCL than in agricultural areas—only one sample had a concentration (12 mg/L) that 

exceeded the MCL. 

Elevated concentrations of nitrate in the surficial aquifers primarily are a drinking-water 

concern for those who use domestic wells that tap the surficial aquifers. Routine testing of 

water from these wells is not required, and homeowners are responsible for identifying and 

treating any water-quality problems. For those who rely on the Upper Floridan aquifer for their 

drinking water, elevated concentrations of nitrate in the surficial aquifers are a potential concern 

because the surficial aquifers provide recharge to the Upper Floridan aquifer.

Chapter 6: Understanding Where and Why Key Contaminants Occur in Groundwater 43
Nitrate concentrations in surficial aquifers

Tampa area

Ocala area

Percentage of samples

Agricultural

Nitrate concentrations

in the surficial aquifers

by land-use study type

Urban

0 20 40 60 80 100

Pasco County

Hillsborough

Pinellas Co

County

EXPLANATION

Surficial aquifers (includes local surficial

 aquifers and surficial aquifer system) 

< 0.1 0.1 to 1 >1 to 10 >10

< 0.1

0.1 to 1

>1 to 10 

>10

Nitrate concentration, in milligrams

 per liter as nitrogen

Nitrate concentration, 

 in milligrams per liter

 as nitrogen

EXPLANATION

Figure 6–5.

N

Nitrate concentrations in the Upper Floridan aquifer are controlled by land 

use and the presence or absence of a confining layer

Despite four decades of large inputs of nitrogen from manure and fertilizer to agricultural 

and urban land that overlies the Upper Floridan aquifer, nitrate concentrations were near 

background levels of 0.1 mg/L in more than half of the wells sampled in the Upper Floridan 

aquifer. Nitrate concentrations in this aquifer were substantially lower than those in other 

Principal Aquifers across the Nation (appendix 3). Concentrations of nitrate in only one 

sample from the Upper Floridan aquifer—from an agricultural land-use monitoring well in 

southwestern Georgia—exceeded the MCL of 10 mg/L (fig. 6–6). Nitrate concentrations did 

not exceed the MCL in any of the drinking-water wells sampled. The effects of human activities 

were nonetheless evident: concentrations were greater than background levels of 0.1 mg/L in 

about one-fourth (24 percent) of the drinking-water wells. 

Elevated nitrate concentrations in the Upper Floridan aquifer occurred mostly in southwestern Georgia and west-central Florida. Nitrate concentrations exceeded the background 

level of 0.1 mg/L in about 80 percent of the wells sampled in unconfined areas of the Upper 

Floridan aquifer and were greater than 1 mg/L in about 60 percent of these wells because land 

Figure 6–5. In urban and agricultural areas, fertilizer, wastewater, and livestock production are 

sources of nitrate to the surficial aquifers. Nitrate concentrations were greater than the drinking 

water MCL (10 mg/L) in about 20 percent of wells in agricultural areas and in about 3 percent of 

wells in urban areas.

44 Water Quality in the Upper Floridan Aquifer and Overlying Surficial Aquifers, Southeastern United States, 1993–2010
Chapter 6

Confined conditions modified from Miller(7)

Confined conditions in the 

Upper Floridan aquifer

Unconfined Semiconfined Confined 

Distribution of nitrate concentration by confined condition

in the Upper Floridan aquifer 

0.1

0.01

1.0

10

Nitrate concentration, 

in milligrams per liter as N

EXPLANATION

Mixed

Anoxic

Oxic

Redox condition

N

Figure 6–6.

Ocala area

Tampa area

Hillsborough

County

Pasco County

Pinellas

County

EXPLANATION

Unconfined

Semiconfined (confining layer <100 feet thick

 or breached)

Confined

Not mapped

Confined condition for the Upper Floridan aquifer

< 0.1 0.1 to 1 >1 to 10 >10

Nitrate concentration, in milligrams 

 per liter as nitrogen

Nitrate concentrations in the Upper Floridan aquifer

See sidebar, Boxplots, p. 46

use is mostly agricultural, the Upper Floridan aquifer is predominantly unconfined, and the 

groundwater is oxic. In these areas, the aquifer is particularly vulnerable to contamination from 

nitrate. Agricultural activities, such as the application of fertilizers and manure and confined 

animal feeding operations, provide a source of nitrate. The lack of a confining layer allows 

nitrate from the land surface to move down into the aquifer. Sinkholes and other karst features, 

which are common where the Upper Floridan aquifer is unconfined, funnel surface runoff 

directly into the Upper Floridan aquifer (see sidebar, Karst, p. 20). 

Nitrate concentrations in the Upper Floridan aquifer were lowest throughout much of southeastern Georgia, southeastern South Carolina, and northeastern Florida, where the Upper Floridan 

aquifer is confined by a thick clay layer (fig. 6 – 6). Nitrate concentrations exceeded the background 

level of 0.1 mg/L in only about 20 percent of the wells sampled in semiconfined and confined areas 

of the Upper Floridan aquifer and were greater than 1 mg/L in about 10 percent of these wells. 

The confining layer limits the downward movement of water, which contains nitrate and other 

contaminants, from the surficial aquifers to the Upper Floridan aquifer. Further, groundwater 

in these semiconfined and confined areas of the Upper Floridan aquifer commonly is anoxic, 

which facilitates the transformation of nitrate to nitrogen gas by denitrification.

Figure 6–6. Hydrogeology—in particular the presence or absence of a confining layer—has a strong 

influence on concentrations of nitrate in the Upper Floridan aquifer. Where the Upper Floridan aquifer 

is unconfined, groundwater was predominantly oxic, and nitrate concentrations commonly were 

greater than 1 mg/L. In the confined and semiconfined parts of the Upper Floridan aquifer, in contrast, 

groundwater was predominantly anoxic, and most nitrate concentrations were less than 0.1 mg/L.

Chapter 6: Understanding Where and Why Key Contaminants Occur in Groundwater 45
Boxplots 

Boxplots are used to illustrate how results are distributed within a group. The “box” ranges from the 25th to the 75th percentile 

and represents 50 percent of the data. The horizontal line in the middle of the box is the median value—one-half of the values in the 

group are greater than the median and one-half are less. 

Percentiles describe the percentage of values in a group that are 

less than the given value: 25 percent of the values in a group are less than 

the 25th percentile; 75 percent of the values in a group are less than the 

75th percentile. The median is also the 50th percentile.

If, for example, the 75th percentile for the measured concentration 

of a contaminant in a group of wells is equal to the human-health 

benchmark for that contaminant, then 75 percent, or three-fourths, of 

the wells have a concentration of that contaminant less than the benchmark, and 25 percent, or one-fourth, have a concentration greater than 

the benchmark.

The “whiskers” (vertical lines) in these figures extend to the 10th and 

90th percentiles; box and whiskers together represent 80 percent of the data.

EXPLANATION

25th percentile

10th percentile

75th percentile

50th percentile

 (median)

90th percentile

50 percent of data

fall within the box 

80 percent of data fall 

between endpoints of whiskers 

Radium Springs, Dougherty County, Georgia. 

46 Water Quality in the Upper Floridan Aquifer and Overlying Surficial Aquifers, Southeastern United States, 1993–2010
Chapter 6

Nitrate concentrations in groundwater often exceed the criterion for 

springs in Florida 

Groundwater from the surficial and Upper Floridan aquifers discharges as spring flow—

as base flow to rivers and streams, and as freshwater to estuaries—commonly in the form 

of “crystal-clear” water that supports a variety of aquatic plants and animals throughout 

the State of Florida. In addition, many large springs are recreational sites that contribute 

substantially to the economies of the surrounding areas. Human activities over the past 

several decades have degraded groundwater quality, and many spring runs, streams, and rivers 

in Florida have suffered from an increase in nuisance aquatic vegetation, reduced clarity, 

and algal blooms. The algal blooms, which can produce toxins that are harmful to humans, 

animals, and ecosystems, result from elevated concentrations of nutrients (nitrogen and 

phosphorus) that originate from fertilizers, wastewater discharge, urban stormwater runoff, 

and other human sources. 

In response to water-quality degradation from nutrients, the USEPA approved the 

State of Florida’s nutrient criteria in November 2012 for spring vents, lakes, streams, and 

south Florida estuaries. The criterion for nitrate and nitrite is an annual geometric mean of 

0.35 mg/L for spring discharge.(36)

Nitrate concentrations measured in samples from the Upper Floridan aquifer commonly 

exceeded the nitrate criteria for springs—nearly half of the samples had concentrations greater 

than 0.35 mg/L. Despite large inputs of nitrogen in agricultural and urban areas, nitrate 

concentrations in groundwater remain low compared to other Principal Aquifers across the 

Nation (appendix 3). Nevertheless, small increases in groundwater nitrate concentrations can 

have adverse effects on the water quality and aquatic species that rely on spring flows for 

habitat and can limit the use of springs for recreation.

Nitrate and phosphorus in spring discharge can cause increases in the amounts of nuisance aquatic 

plants and algae in spring pools and runs. 

Chapter 6: Understanding Where and Why Key Contaminants Occur in Groundwater 47
Nitrate contamination of springs in Florida and Georgia

The Upper Floridan aquifer in Florida and Georgia sustains one of the largest concentrations of freshwater springs in the world. 

More than 1,000 recorded artesian springs in Florida discharge more than 9 billion gallons per day from the Upper Floridan aquifer.(37)

The exceptionally clear spring flow is a unique recreational resource enjoyed by millions of people for generations. Sites such as Weeki 

Wachee Springs, Wakulla Springs, Ichetucknee Springs, Rainbow Springs, and Silver Springs continue to be popular vacation destinations. 

The springs also provide important base flow to rivers and streams and freshwater to estuaries. The springs offer habitat for diverse 

aquatic life, and the springs and karst support many native species, such as snails and crayfish, at least 30 of which are imperiled.(38)

Springs in northern Florida and southern Georgia serve as early warning systems for the degradation of regional groundwater quality. 

Over the past several decades, human activities have severely affected spring water quantity and quality and associated ecosystems. As 

a result of increased pumping and climatic influences, water levels in the Upper Floridan aquifer have dropped in many places, resulting 

in lower spring discharge. In some cases, springs have stopped flowing.(39) In response to these changes occuring at springs and because 

of the importance of springs to the local economies, the Florida Legislature has provided over $15 million in funding for the Florida Springs 

Wekiwa Springs. Ginnie Springs.

More than 1,000 springs have been 

mapped in the Upper Floridan aquifer 

in Florida and Georgia. Most of the 

springs are located in unconfined 

and semiconfined areas of the aquifer 

and provide water for recreation, 

base flow to rivers, and habitat for 

diverse animal species.

Spring discharging from the 

 Upper Floridan aquifer

EXPLANATION

Unconfined

Semiconfined (confining layer 

 <100 feet thick or breached)

Confined

Confined condition for the Upper

 Floridan aquifer

N

Confined conditions modified from Miller(7)

Circa 1910, photograph source unknown

Photograph by USGS

48 Water Quality in the Upper Floridan Aquifer and Overlying Surficial Aquifers, Southeastern United States, 1993–2010
Data from Suwannee River Water Management District, Florida, written commun., November 5, 2013 

Initiative program, which began in 2001. The program focuses on education and outreach, protection and restoration, and research 

and monitoring of the State’s freshwater springs.

Nitrate concentrations in spring flow have increased over the past several decades in many springs as a result of nitrogen 

inputs from fertilizers, septic tanks, and land application of treated municipal wastewater and biosolids. Springflow contributes 

large loads of nitrogen to surface waters (40–43) and, as a result, nuisance levels of algae and macrophytes have increased in many 

spring-fed rivers in Florida.(39, 44, 45) In the Suwannee River Basin in Florida, which contains the majority of first magnitude springs 

(springflow greater than 100 cubic feet per second) in Florida, fertilizers have been the dominant source of nitrogen since 1940.(46, 47)

Fertilizer use peaked in the area in the 1970s, but spring water responses to changes in fertilizer use have been muted and delayed 

because it takes decades, on average, for water to move through the aquifer and discharge at springs.(48) Photographs by John Moran, Florida Nature Photography

Left, Nitrate concentrations have increased from 

less than 0.5 mg/L in 1950 to greater than 1 mg/L 

in 2005 in spring flow from four springs in the 

Suwannee River Basin. These types of changes 

in nitrate concentrations likely contribute to 

the increase of algae growth in many springs in 

Florida. Below, The clarity of water in Fanning 

and Peacock Springs has decreased over the last 

20 years, in part because of algae. These types 

of changes to springs in Florida can discourage 

tourism and demonstrate how sensitive these 

springs are to nutrient inputs.

0

1950 1960 1970 1980

Year

1990 2000 2010

2

4

6

1

3

5

Concentration, in

milligrams per liter as nitrogen

Fanning

Troy

Manatee

Rock Bluff

EXPLANATION

Nitrate concentration

Spring

Chapter 6

1989

Fanning Springs Peacock Springs

1990

2012 2011

Chapter 6: Understanding Where and Why Key Contaminants Occur in Groundwater 49
0 1 2 3 4 5 6

Concentration of groundwater age tracer 1

0

1

2

3

4

Concentration of groundater age tracer 2

Groundwater age and nitrate concentrations in the surficial 

and Upper Floridan aquifers

Fig 6-7.

Older Younger

Public-supply well

Modified from Katz and others(25)

water water EXPLANATION

Surficial aquifer

Upper Floridan aquifer

Nitrate not detected

Nitrate detected

 >0.1 mg/L

Figure 6–7. In Tampa, Florida, the similarity between groundwater age and 

nitrate concentrations in samples from a public-supply well that taps the 

Upper Floridan aquifer and from the surficial aquifers indicates that highvolume pumping of the public-supply well is causing nitrate-rich groundwater 

to move downward from the overlying surficial aquifer into the Upper Floridan 

aquifer—parts of the Upper Floridan aquifer not affected by pumping had 

older groundwater and lower nitrate concentrations.

Pumping of public-supply wells can increase vulnerability to nitrate 

contamination 

Prolonged, high-volume pumping for water supply can affect the quality of groundwater 

withdrawn from the Upper Floridan aquifer. Groundwater is the source of drinking water for 

nearly 1.6 million people in the Tampa area, where over 325 Mgal/d are pumped from the 

unconfined and semiconfined parts of the Upper Floridan aquifer for drinking water.

(11) In this 

area, the Upper Floridan aquifer is separated from the surficial aquifers by a confining layer. 

Groundwater from most of the wells in the Upper Floridan aquifer was older and had much 

lower nitrate concentrations than did groundwater in the overlying surficial aquifers (fig. 6–7). 

Unexpectedly, the groundwater age and nitrate concentration in a Tampa public-supply well 

drilled into the Upper Floridan aquifer were more similar to the age and concentrations in 

samples from monitoring wells that tap the surficial aquifers than to the age and concentrations 

in monitoring wells that tap the Upper Floridan aquifer. This finding indicates that pumping 

might be causing nitrate-rich water from the surficial aquifers to move down along conduits 

into the parts of the Upper Floridan aquifer tapped by the public-supply well.

50 Water Quality in the Upper Floridan Aquifer and Overlying Surficial Aquifers, Southeastern United States, 1993–2010
It may take decades for nitrate concentrations in groundwater to respond to 

changes in nitrogen inputs

How will nitrate concentrations in groundwater change in the future if nitrogen inputs stay the same, 

gradually decrease, or cease entirely? These questions were addressed using groundwater-flow models used 

to simulate nitrate concentrations on the basis of changes in the amount of nitrogen inputs.(49) Three scenarios 

were investigated: (1) nitrogen inputs remain at the amounts applied in 2001 (the most recent year for which 

estimates of nitrogen input were available); (2) a gradual reduction (4 percent per year) in nitrogen inputs from 

2002 to 2050; and (3) no nitrogen inputs after 2001. Three management scenarios were compared for water 

from one well in an agricultural area in southwestern Georgia that taps the unconfined Upper Floridan aquifer. 

These simulations highlight the lag time between activities at the land surface and changes in the water quality 

of the aquifer. Under the first scenario, nitrate concentrations in groundwater are predicted to increase by 

about 3 mg/L as nitrogen over a period of about 30 years to about 13 mg/L and to remain at that level into the 

future. Under the second scenario, even though inputs do not increase, nitrate concentrations in groundwater 

are predicted to continue to increase for about 30 years before beginning to decline. Under the third scenario, 

nitrate concentrations continue to increase for about 15 years with no nitrogen inputs, and concentrations 

decrease to about half of the current concentration within about 30 years. 

Chapter 6

Simulated nitrate concentration in groundwater, 

in milligrams per liter as nitrogen

6,000

5,000

4,000

3,000

2,000

1,000

1950 1960 1970 1980 1990 2000 2010 2020 2030 2040 2050

0

16

14

12

10

8

6

4

2

0

Estimated average fertilizer nitrogen based on sales,

in kilograms

YEAR

Simulated nitrate concentrations

Fixed nitrogen

inputs

Gradual reduction

in nitrogen inputs No nitrogen inputs

Estimated average fertilizer nitrogen and simulated nitrate concentrations in groundwater in the Upper Floridan aquifer in an agricultural area in southwest Georgia

Fertilizer inputs

Nitrate

Chapter 6: Understanding Where and Why Key Contaminants Occur in Groundwater 51
Pesticides

Pesticides, used to control weeds, insects, and other pests in agricultural and 

urban areas, can leach into the surficial aquifers, which provide recharge to the 

Upper Floridan aquifer. Pesticides are detected most commonly in those parts of the 

Upper Floridan aquifer that are overlain by areas of intensive pesticide use and where 

the Upper Floridan aquifer is unconfined and has numerous karst features. At least 

one pesticide was detected in nearly half of the public-supply wells and 13 percent of 

the domestic wells sampled in the Upper Floridan aquifer, but concentrations of only 

one insecticide—dieldrin—exceeded a human-health benchmark and dieldrin was 

detected in only about 1 percent of samples. Overall, concentrations of pesticides were 

very low (less than 0.1 μg/L) and were well below human-health benchmarks.

Pesticides were detected frequently in wells in the surficial aquifers, 

typically as mixtures of two or more 

At least one pesticide was detected in 55 percent of the wells in the surficial aquifers 

underlying agricultural and urban areas. The herbicide atrazine and its degradate, deethylatrazine, were detected most commonly in about 25 percent of wells (more than any other 

pesticides). Concentrations of almost all pesticides were less than human-health benchmarks, 

with the exception of dieldrin, which exceeded its human-health benchmark of 0.002 μg/L in 

two samples. The occurrence of dieldrin in groundwater more than 20 years after its use was 

banned is evidence of how persistent this chemical is in the environment. 

Two or more pesticides were detected in 42 percent of samples from shallow groundwater 

in the surficial aquifers underlying agricultural areas and 24 percent of samples from shallow 

groundwater underlying urban areas (fig. 6–8). The occurrence of two or more pesticides in 

groundwater from surficial aquifers used for drinking is a potential health concern, even at 

concentrations below human-health benchmarks, because the potential health effects of such 

mixtures are not well understood.(50) The vulnerability of the surficial aquifers to pesticide 

contamination is a concern for households that use domestic wells as a source of drinking water 

and also is a concern because these aquifers provide recharge to the Upper Floridan aquifer.

Similar pesticides were detected in the surficial and Upper Floridan aquifers

Atrazine (and its degradates) and metolachlor each were detected in 10 percent or more of 

samples from both the surficial and Upper Floridan aquifers (fig. 6–9). Atrazine and metolachlor, 

which are used to control weeds in agricultural and urban (residential and commercial) areas, 

respectively, are two of the most heavily used herbicides in the Nation and two of the herbicides 

most commonly detected in groundwater.(51) Bentazon, prometon, and tebuthiuron were detected 

in at least 5 percent of samples from both aquifers. It is not surprising that similar pesticides 

were detected in both aquifers given that the surficial aquifers are an important source of 

recharge to the Upper Floridan aquifer, but it nonetheless confirms the vulnerability of the 

Upper Floridan aquifer to contamination from the overlying surficial aquifers. 

Concentrations of the pesticides detected in the Upper Floridan aquifer were low and none 

exceeded a human-health benchmark. Although atrazine was detected in more than 15 percent 

of samples from the Upper Floridan aquifer, concentrations were lower than those from many 

other Principal Aquifers sampled across the Nation (appendix 3).

52 Water Quality in the Upper Floridan Aquifer and Overlying Surficial Aquifers, Southeastern United States, 1993–2010

Atrazine and its 

degradates were the 

pesticides detected 

most frequently. 
Chapter 6

Pesticide detections in

surficial aquifers by 

land-use study type

Tampa area

Ocala area

Pesticide detections in surficial aquifers

Agricultural

Urban

Percentage of samples

0 20 40 60 80 100

Hillsborough

County

Pasco County

Pinellas

County

EXPLANATION

Surficial aquifers (includes local surficial

 aquifers and surficial aquifer system) 

0 1 2 or more

Number of pesticides detected per sample

Figure 6–8.

0

1

2 or more

Number of pesticides detected

EXPLANATION

N

Figure 6–9.

Atrazine

Deethylatrazine

Hydroxyatrazine

Metolachlor

Deisopropylatrazine

Bentazon

Fluometuron

Desulfinylfipronil

Prometon

Tebuthiuron

Imidacloprid

Hexazinone

Pesticides detected in 

surficial aquifers

0 10 20 30

Detection frequency, in percent

Pesticides detected in 

the Upper Floridan aquifer

0 10 20 30

EXPLANATION

At any concentration

Detection

At concentration >0.1 μg/L

Figure 6– 8. Two or more pesticides were detected frequently in shallow groundwater in the surficial 

aquifers underlying urban and agricultural areas. The vulnerability of the surficial aquifers to pesticide 

contamination is a concern for households that rely on domestic wells as a source of drinking water.

Figure 6–9. Pesticide occurrence 

was similar in samples from the 

surficial and Upper Floridan aquifers, 

but concentrations typically were 

higher in the surficial aquifers. 

Atrazine and metolachlor are among 

the pesticides with the largest use 

in the Nation and were among the 

herbicides most commonly detected 

in the study area.

Chapter 6: Understanding Where and Why Key Contaminants Occur in Groundwater 53
Pesticides used in agricultural and urban areas 

were detected frequently in both the surficial 

and Upper Floridan aquifers.

Photographs: top by Jeff Vanuga, USDA NRCS; bottom by USGS

Hydrogeology and rates of pesticide use affect the vulnerability of the 

Upper Floridan aquifer to pesticide contamination 

The occurrence of pesticides in the Upper Floridan aquifer is related to hydrogeologic 

conditions and intensity of pesticide use. The absence of a confining layer and the presence 

of karst features, such as sinkholes, facilitate the transport of any pesticides used to the Upper 

Floridan aquifer. In areas where the confining layer is absent, one or more pesticides were 

detected in more than 60 percent of the wells sampled (fig. 6–10). Differences in the intensity 

of pesticide use across the study area also are reflected in the occurrence of pesticides in 

groundwater. In southwestern Georgia where agriculture is the predominant land 

use (fig. 3–5), two or more pesticides were detected in more than half of the wells 

sampled (fig. 6–10).

The influence of hydrogeology on groundwater vulnerability to contamination is further illustrated by pesticide occurrence in the Upper Floridan aquifer 

in southwestern and southeastern Georgia. Both of these areas are characterized 

by agricultural land use, but the hydrogeologic characteristics of the Upper 

Floridan aquifer in the two areas differ. In southwestern Georgia, the Upper 

Floridan aquifer is relatively shallow, is unconfined, and has numerous karst 

features. Two or more pesticides were detected in southwestern Georgia in more 

than half of the wells sampled. In contrast, in southeastern Georgia, the Upper 

Floridan aquifer is relatively deep, is confined, and has few karst features. A 

pesticide was detected in only two wells in southeastern Georgia. The lack of 

a confining layer and the presence of karst features — sinkholes and conduits—

facilitate the movement of pesticides into the Upper Floridan aquifer. 

Where hydrogeology is similar, differences in pesticide use contribute to 

pesticide occurrence. The Upper Floridan aquifer is unconfined in both southwest 

Georgia and west-central Florida, and the depths of the wells sampled in these 

areas are comparable. Agricultural land use and pesticide use, however, are much 

more intense in southwest Georgia than in west-central Florida— the amount of 

atrazine applied to crops in areas near wells in southwest Georgia was estimated 

to be more than 10 times greater than in west-central Florida.(52) These patterns 

of use are reflected in the much more common detection of pesticides in wells in 

southwest Georgia than in those in west-central Florida. For example, atrazine 

was detected in about half of the wells sampled in southwest Georgia, but in 

only one well in the west-central Florida area.

Pesticides used in urban areas are a source of contamination to the 

Upper Floridan aquifer 

In the Tampa area, pesticides were detected commonly in shallow parts of the Upper 

Floridan aquifer and also in the deeper parts of the aquifer that are used for public water supply. 

Pesticides are used in residential and commercial areas to control weeds, insects, and other 

pests in lawns, landscaping, parks, and golf courses, and along roads and other rights-of-way.(51)

At least one pesticide was detected in almost 60 percent of samples of shallow groundwater 

from monitoring wells in the Upper Floridan aquifer and in nearly 40 percent of samples 

from public-supply wells in urban areas (fig. 6–11). About 40 percent of the samples in which 

pesticides were detected in shallow groundwater had concentrations above 0.1 μg/L, but all of 

the pesticide concentrations detected in the public-supply wells were less than this common 

assessment level. The occurrence of pesticides indicates that the deep parts of the Upper 

Floridan aquifer used for water supply in the Tampa area likely are vulnerable to contamination 

because the aquifer in this area largely is unconfined and because karst features provide a 

pathway from the land surface directly into the aquifer. 

Pesticides were detected 

in nearly 40 percent of 

public-supply wells in the 

Upper Floridan aquifer.

54 Water Quality in the Upper Floridan Aquifer and Overlying Surficial Aquifers, Southeastern United States, 1993–2010
Chapter 6

Confined conditions modified from Miller(7)

Pesticide detections in the Upper Floridan aquifer

Pesticide detections by 

confined condition in the 

Upper Floridan aquifer

Unconfined

Semiconfined

Confined

N

Figure 6–10.

Ocala area

Tampa area

Hillsborough

County

Pasco County

Pinellas

County

Percentage of samples

0 20 40 60 80 100

EXPLANATION

Unconfined

Semiconfined (confining layer <100 feet thick 

 or breached)

Confined

Not mapped

Confined condition for the Upper Floridan aquifer

0 1 2 or more

Number of pesticides detected per sample

0

1

2 or more

Number of pesticides detected

EXPLANATION

Urban monitoring wells Public-supply wells

Percentage of samples in which at

least one pesticide was detected

Pesticide detections in monitoring and publicsupply wells in the Upper Floridan aquifer

100

80

60

40

20

0

Figure 6–11.

Detected at any

concentration

Detected at a

concentration

>0.1 μg/L

Figure 6–10. Pesticides were detected most commonly in samples from the Upper Floridan aquifer where 

the upper confining layer that limits the downward movement of water and contaminants is absent, but were 

rarely detected where the aquifer is confined. The presence of karst features in areas where the aquifer is 

unconfined contributes to the transport of pesticides from the land surface into the Upper Floridan aquifer.

Figure 6–11. In urban areas, pesticides were 

detected commonly in shallow groundwater 

and in deep groundwater used for drinking. 

Pesticide concentrations, however, were 

higher in shallow than in deep groundwater—

nearly 40 percent of samples of shallow 

groundwater had at least one pesticide at 

a concentration greater than 0.1 μg/L, but 

no samples from public-supply wells had a 

pesticide concentration greater than 0.1 μg/L.

Chapter 6: Understanding Where and Why Key Contaminants Occur in Groundwater 55

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springs basin, Florida: Journal of the American Water 

Resources Association, v. 45, no. 3, p. 607–627.

43. Heffernan, J.B., Liebowitz, D.M., Frazer, T.K., 

Evans, J.M., and Cohen, M.J., 2010, Algal blooms and 

the nitrogen-enrichment hypothesis in Florida springs—

Evidence, alternatives, and adaptive management: 

Ecological Applications, v. 20, no. 3, p. 816–829.

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44. Jones, G.W., Upchurch, S.B., Champion, K.M., 

DeHaven, E.C., Starks, Roberta, Dejongh, Lisa, 

Tomlinson, Chris, Hood, Jason, Anastasiou, Chris, and 

Miller, E.J., 1996, Origin of nitrate in ground water 

discharging from Rainbow Springs, Marion County, 

Florida: Brooksville, Fla., Southwest Florida Water 

Management District, 155 p.

45. Florida Springs Task Force, 2000, Florida’s springs—

Strategies for protection & restoration: Florida Department of Environmental Protection Web site, accessed 

November 21, 2013, at http://www.dep.state.fl.us/

springs/reports/index.htm.

46. Katz, B.G., Hornsby, H.D., Böhlke, J.F., and 

Mokray, M.F., 1999, Sources and chronology of nitrate 

contamination in spring waters, Suwannee River Basin, 

Florida: U.S. Geological Survey Water-Resources 

Investigation Report 99–4252, 54 p.

47. Katz, B.G., 2004, Sources of nitrate contamination and 

age of water in large karstic springs of Florida: Environmental Geology, v. 46, nos. 67, p. 689–706.

48. Katz, B.G., Böhlke, J.K., and Hornsby, H.D., 2001, 

Timescales for nitrate contamination of spring waters, 

northern Florida, USA: Chemical Geology, v. 179, 

nos. 1–4, p. 167–186.

49. Crandall, C.A., Katz, B.G., and Berndt, M.P., 2013, 

Estimating nitrate concentrations in groundwater at 

selected wells and springs in the surficial aquifer 

system and Upper Floridan aquifer, Dougherty Plain 

and Marianna Lowlands, Georgia, Florida, and Alabama, 

2002–50: U.S. Geological Survey Scientific Investigations 

Report 2013–5150, 65 p., accessed November 21, 2013, 

at http://pubs.usgs.gov/sir/2013/5150/.

50. Toccalino, P.L., and Hopple, J.A., 2010, The quality of 

our Nation’s waters—Quality of water from public-supply 

wells in the United States, 1993–2007— Overview of 

major findings: U.S. Geological Survey Circular 1346,

58 p.

51. Gilliom, R.J., Barbash, J.E., Crawford, C.G., Hamilton, P.A.,

Martin, J.D., Nakagaki, Naomi, Nowell, L.H., Scott, J.C.,

Stackelberg, P.E., Thelin, G.P., and Wolock, D.M., 2006, 

The quality of our Nation’s waters—Pesticides in the 

nation’s streams and ground water, 1992–2001: U.S. 

Geological Survey Circular 1291, 172 p. (Also available 

at http://pubs.usgs.gov/circ/2005/1291/.)

52. Katz, B.G., Berndt, M.P., and Crandall, C.A., 2013, 

Factors affecting the movement and persistence of 

nitrate and pesticides in the surficial and Upper Floridan 

aquifers in two agricultural areas in the southeastern 

United States: Environmental Earth Sciences, v. 71,

p. 2779–2795.

53. Otton, J.K., Gundersen, L.C.S., and Schumann, R.R., 

1993, The geology of radon: U.S. Geological Survey 

General Interest Publication, 29 p.

54. U.S. Environmental Protection Agency, 1994, 

Evaluating and identifying contaminants of concern for 

human health: U.S. Environmental Protection Agency 

Hazardous Waste Management Division Region 8 

Superfund Technical Guidance RA–03, 10 p., accessed 

September 23, 2013, at http://www2.epa.gov/sites/

production/files/documents/r8_ra03-cocs.pdf.

55. New Jersey Department of Environmental Protection, 

2003, Susceptibility of source water to community 

water-supply wells in New Jersey to contamination by 

volatile organic compounds: New Jersey Department 

of Environmental Protection, 21 p. (Also available at 

http://www.state.nj.us/dep/swap/reports/gw_voc.pdf.)

56. Toccalino, P.L., Norman, J.E., Phillips, R.H., 

Kauffman, L.J., Stackelberg, P.E., Nowell, L.H., 

Krietzman, S.J., and Post, G.B., 2004, Application of 

health-based screening levels to ground-water quality 

data in a state-scale pilot effort: U.S. Geological Survey 

Scientific Investigations Report 2004–5174, 64 p.

57. Toccalino, P.L., 2007, Development and application of 

health-based screening levels for use in water-quality 

assessments: U.S. Geological Survey Scientific Investigations Report 2007–5106, 12 p.

58. Toccalino, P.L., Norman, J.E., Booth, N.L., Thompson, J.L.,

and Zogorski, J.S., 2012, Health-based screening levels—

Benchmarks for evaluating water-quality data: U.S. 

Geological Survey Web site, accessed April 24, 2013, at 

http://water.usgs.gov/nawqa/HBSL/.

59. U.S. Environmental Protection Agency, [undated], 

National primary drinking water regulations—Drinking 

water contaminants: U.S. Environmental Protection

Agency Web site, accessed May 25, 2012, at 

http://water.epa.gov/drink/contaminants/index.cfm.

60. U.S. Environmental Protection Agency, [undated], 

Secondary drinking water regulations — Guidance for 

nuisance chemicals: U.S. Environmental Protection 

Agency 816 –F–10 – 079, accessed April 24, 2013, at 

http://water.epa.gov/drink/contaminants/

secondarystandards.cfm.

61. Homer, C.H., Fry, J.A., and Barnes, C.A., 2012, The 

National Land Cover Database: U.S. Geological 

Survey Fact Sheet 2012–3020, 4 p. (Also available at 

http://pubs.usgs.gov/fs/2012/3020/fs2012-3020.pdf.)

62. DeSimone, L.A., McMahon, P.B., and Rosen, M.R., 

2014, The quality of our Nation’s waters—Summary of 

water quality in Principal Aquifers of the United States, 

1991–2010: U.S. Geological Survey Circular 1360, 151 p.

(Also available at http://dx.doi.org/10.3133/cir1360.)

References Cited 59
Pitt Spring, Bay County, Florida.
Glossary

Glossary

A

anoxic Water with no dissolved oxygen 

or a very low concentration (less than 

0.5 milligram per liter) of dissolved oxygen.

apparent groundwater age The time 

elapsed since the recharge water became 

isolated from the atmosphere. The term 

“age” is normally qualified with the word 

“apparent” to signify that the accuracy of the 

determined age depends on many variables. 

aquifer A geologic formation, group 

of formations, or part of a formation that 

contains a sufficient amount of saturated 

permeable material (for example, soil, sand, 

gravel and (or) rock) to yield substantial 

quantities of water to wells and springs.

C

carbonate rock Rocks, such as limestone 

or dolostone, that are composed primarily 

of minerals, such as calcite and dolomite, 

containing the carbonate ion (CO3

2–).

common assessment level A single 

concentration threshold used to establish 

an equal basis for comparing detection 

frequencies among multiple chemicals. Use 

of a common assessment level avoids biases 

in detection frequencies caused by one 

compound having a lower detection level 

than another. Also sometimes referred to 

as a “common detection level.” 

conduit/conduit flow Groundwater flow 

through pipe-like or channel-like openings 

in bedrock. Conduits control the direction of 

water flow and greatly increase the speed at 

which water travels through an aquifer. 

confined aquifer (artesian aquifer) An 

aquifer in which the groundwater is bounded 

between layers of relatively impermeable 

material, such as clay or dense rock. When 

tapped by a well, water in a confined aquifer 

is forced up, sometimes above the land 

surface, by pressure within the aquifer.

confining layer Geologic material with little 

or no permeability or hydraulic conductivity. 

Water does not pass through this layer or the 

rate of movement is extremely slow.

D

degradate A compound formed by the 

transformation of a parent compound, 

typically an organic contaminant or another 

degradate, by chemical, photochemical, or 

biological reactions.

denitrification The bacterial reduction 

of dissolved nitrate to nitrogen gas. 

Denitrification is the primary process by 

which nitrate can be eliminated naturally 

in groundwater.

domestic well A privately owned well 

that typically serves one home and supplies 

water for human consumption and other 

homeowner uses.

F

flow path The route or pathway of 

water flowing through the hydrologic 

system. Typically refers to subsurface 

(groundwater) flow.

G

groundwater Water that exists beneath 

the land surface, but most commonly 

refers to water in fully saturated soils and 

geologic formations.

groundwater age See apparent 

groundwater age.

groundwater flow path See flow path.

Glossary 61
H

Health-Based Screening Level (HBSL)

An estimate of concentration (for a 

noncarcinogen) or concentration range 

(for a carcinogen) in water that (1) may 

be of potential human-health concern, 

(2) can be used as a threshold value against 

which measured concentrations of contaminants in ambient groundwater samples can 

be compared, and (3) is consistent with 

U.S. Environmental Protection Agency 

Office of Water methodologies.

human-health benchmark A threshold 

concentration above which the concentration 

of a contaminant in drinking water could have 

adverse effects on human health. Treatment or 

other measures can be used before the water 

is consumed to lower the concentration of the 

contaminant below the benchmark.

hydrogeologic unit A body of rock 

distinguished and characterized by its porosity 

and permeability. Also called a hydrostratigraphic unit.

hydrogeology The geologic and hydrologic 

features that control the movement of 

water, solutes, and small particles through 

the subsurface.

K

karst Surface and subsurface terrane that 

is formed on and in soluble rocks, such as 

limestone and gypsum, primarily by dissolution and collapse, and that is characterized by 

sinkholes, caves, and underground drainage.

L

land-use study A study by the U.S. 

Geological Survey National Water-Quality 

Assessment (NAWQA) Program to assess the 

effects of a specific land-use type (generally 

agricultural or urban) on groundwater quality, 

in most cases by sampling groundwater from 

monitoring wells that tap water from or near 

the water table.

lithology The physical character of a rock

on the basis of color, structure, mineralogical 

composition, grain size, and other characteristics.

M

major aquifer A regionally extensive 

subsurface geologic formation or group of 

formations that is used, or has the potential to 

be used, as a substantial groundwater resource.

major aquifer study A study by the U.S. 

Geological Survey National Water-Quality 

Assessment (NAWQA) Program that involves 

sampling of water at 20 to 30 domestic and 

(or) public-supply wells that withdraw water 

from major aquifers. The major aquifer 

studies represent a mix of land uses and target 

water that is used for drinking-water supply.

Maximum Contaminant Level (MCL)

Maximum permissible level of a contaminant 

in water that is delivered to any user of a 

public water system. MCLs are enforceable 

standards established by the U.S. Environmental Protection Agency.

milligrams per liter (mg/L) A unit expressing 

the concentration of a chemical constituent 

as weight (milligrams) of constituent per 

unit volume (liter) of water; equivalent to 

one part per million in most streamwater and 

groundwater. One thousand micrograms per 

liter (μg/L) is equivalent to 1 mg/L.

monitoring well A well used to measure 

water quality or groundwater levels continuously or periodically. Not typically used as a 

source of drinking water. Sometimes referred 

to as an “observation well.”

O

oxic Water with a concentration of 

dissolved oxygen greater than or equal to 

0.5 milligram per liter.

P

pesticide Any substance, organic or 

inorganic, used to kill plant or animal pests.

pH A measure of the acidity (pH less than 7) 

or alkalinity (pH greater than 7) of a solution; 

a pH of 7 is neutral. Formally defined as the 

logarithm of the reciprocal of the hydrogen 

ion concentration (activity) of a solution.

62 Water Quality in the Upper Floridan Aquifer and Overlying Surficial Aquifers, Southeastern United States, 1993–2010
Glossary

Principal Aquifer A regionally extensive 

aquifer or aquifer system that has the potential 

to be used as a source of potable water. A 

Principal Aquifer can be composed of one 

or more major aquifers.

public-supply well A privately or publicly 

owned well that provides water for public 

use to (1) a community water system, 

(2) a transient noncommunity water system, 

such as a campground, or (3) a nontransient, 

noncommunity system, such as a school. R

recently recharged groundwater Ground-

water that was recharged after 1952, as 

indicated by tritium concentrations greater 

than 0.5 tritium unit.

recharge The addition of water to the 

saturated zone naturally by precipitation or 

runoff or artificially by spreading or injection. 

Also, the water that is added.

reduction-oxidation (redox) Chemical 

reactions that involve the transfer of electrons 

from one chemical species to another, 

resulting in a change in the valence state of 

the species. Redox processes in groundwater 

often are microbially facilitated. S

Secondary Maximum Contaminant Level 

(SMCL) Guidelines set by the U.S. Environ-

mental Protection Agency for concentrations 

of “nuisance” constituents in drinking water 

that may cause unwanted effects, such as 

unpleasant taste, color, or odor; discoloration 

of skin or teeth; or corrosion or staining of 

plumbing fixtures. Public drinking-water 

systems are recommended but not required to 

comply with these guidelines.

semiconfined aquifer An aquifer (or part 

of an aquifer) that is partially covered by 

a confining layer. For the Upper Floridan 

aquifer, a semiconfined aquifer is defined as 

the part of the aquifer where the confining 

layer is less than 100 feet thick or is breached.

sinkhole Any closed depression in soil or 

bedrock formed by the erosion and transport 

of earth material from below the land surface. 

A sinkhole typically has a closed topographic 

contour, drains to the subsurface, and occurs 

in karst terrane.

spring run A body of flowing water or 

stream that originates from a spring. 

spring vent An opening that concentrates 

groundwater discharge at the Earth’s surface. 

Study Unit A major hydrologic system of 

the United States, geographically defined by 

surface- or groundwater features, in which 

U.S. Geological Survey National WaterQuality Assessment (NAWQA) Program 

sampling studies are focused.

swallow hole An opening or hole in 

bedrock into which a stream loses water or 

disappears underground. U

unconfined aquifer An aquifer that has a 

water table; an aquifer containing unconfined 

groundwater. V

volatile organic compound (VOC) An 

organic chemical that has a high vapor 

pressure relative to its water solubility. VOCs 

include components of gasoline, fuel oils, 

lubricants, organic solvents, fumigants, some 

inert ingredients in pesticides, and some 

by-products of chlorine disinfection.

vulnerability The tendency or likelihood for 

contaminants to reach a specified position in 

the groundwater system after introduction at 

some location above the uppermost aquifer. 

The vulnerability of a groundwater resource 

to contamination depends both on the intrinsic 

susceptibility of the resource and on the 

locations and types of human and geologic 

sources of contaminants, locations of wells, 

and the characteristics of the contaminant(s). W

water table The upper surface of the 

saturated zone below which all voids 

(spaces) are filled with water.

Glossary 63
Baltzell Spring, no. 2, Jackson County, Florida.
Appendixes

Appendix 1. NAWQA Studies Design

 How does land use affect groundwater quality? How does water quality change as 

it moves through an aquifer? What is the quality of the drinking-water resource? NAWQA 

groundwater assessments include different types of studies, specifically designed to answer 

questions such as these. 

•	 Land-use studies focus on sampling water from water-table monitoring wells installed 

in urban and agricultural areas to assess the effects of these land uses on the quality of 

the underlying groundwater. Although not typically used for drinking, this water supplies 

recharge to the deeper aquifer system. 

•	 Flow-path studies investigate how water quality changes as it moves along a groundwater flow path. Samples were collected from wells installed along a groundwater flow 

path (http://pubs.usgs.gov/circ/circ1112/flowpath.html).

•	 Major aquifer studies provide a broad overview of the quality of the deeper aquifer 

system used for drinking-water supply. Most of the wells sampled were domestic wells 

that were distributed across a large area in a mixture of land uses. 

•	 Vulnerability studies are designed to assess the vulnerability of water delivered by 

public-supply wells to contamination from natural and manmade contaminants. Samples 

were collected from multiple depths in public-supply wells to determine where and how 

contaminants from different sources enter the wells and how natural processes and 

human activities affect water quality (http://oh.water.usgs.gov/tanc/NAWQATANC.htm).

•	 Source-water quality assessment studies focus on sampling water from public-supply 

wells to understand occurrence of unregulated manmade chemicals in the groundwater 

resources that serve large numbers of people (http://water.usgs.gov/nawqa/swqa/).

 Results of these studies were reinforced by locating some of the studies within the 

boundaries of larger studies. For example, the recharge areas for flow-path studies were 

located within the boundaries of the land-use studies to provide information on the quality of 

the recharge. In turn, the boundaries of the land-use studies were located within the boundaries 

of the major aquifer studies to provide information on how the quality of the recharge affects 

that of the deeper groundwater.

 Each study involved sampling water from a network of a few to as many as 30 wells. Data 

from these studies were available for the Principal Aquifer assessments.

Appendix 1 65
Table A1–1. Study components of the Upper Floridan and surficial aquifers water-quality assessment. 

[F, field parameters, including water temperature, specific conductance, pH, alkalinity, turbidity, and dissolved oxygen; M, major ions; N, nutrients;

D, dissolved organic carbon; TE, trace elements; P, pesticide compounds; V, volatile organic compounds; R, radon; RA, radium; T, tritium; SI, stable 

isotopes of oxygen and hydrogen; A, age tracers other than tritium; MB, microbiological; AC, other anthropogenic organic compounds; TANC, Transport 

of Anthropogenic and Natural Contaminants study]

Study

component Study location Study purpose

Water-quality 

parameters

measured

Year 

sampled

Number 

of wells 

sampled

Well types

Major aquifer study Southwestern 

Georgia

Broadly characterize water 

quality in the unconfined 

Upper Floridan aquifer

F, M, N, D, TE, P, 

V, R,* T,* SI,* 

A,† MB,†, RA,†

1995, 

2002

34 Mostly 

domestic 

wells

Major aquifer 

study‡

Southwestern 

Georgia

Broadly characterize water quality 

in springs in the unconfined 

Upper Floridan aquifer

F, M, N, D, TE, P, 

V, R,* T*

1995 22 Springs

Major aquifer study North-central Florida Broadly characterize water 

quality in the unconfined 

Upper Floridan aquifer

F, M, N, D, TE, P, 

V, R,* T,* SI,* 

MB,† RA†

2002 30 Domestic 

wells

Major aquifer study North-central Florida Broadly characterize water 

quality in the semiconfined 

Upper Floridan aquifer

F, M, N, D, TE, P, 

V, R,* T,* RA†

2002 30 Domestic 

wells

Major aquifer study Southwestern 

Georgia

Broadly characterize water 

quality in the confined 

Upper Floridan aquifer

F, M, N, D, TE, 

P, V, R,* T,* 

MB,† RA†

2005 30 Domestic 

wells

Major aquifer study Southern South 

Carolina

Broadly characterize water 

quality in the semiconfined 

Upper Floridan aquifer

F, M, N, D, TE, P, 

V, R,* T,* A,†

MB,† RA†

1998, 

2006

30 Domestic 

wells

Land-use study Southwestern 

Georgia

Characterize the quality of recently 

recharged water in an agricultural 

setting in the surficial aquifer (six

wells were in Upper Floridan aquifer)

F, M, N, D, TE, P, 

V, R, T, SI, A,†

1993, 

2002– 03

47 Monitoring 

wells

Land-use study South-central

Georgia

Characterize the quality of recently 

recharged water in an agricultural 

setting in the surficial aquifer

F, M, N, D, TE, P, 

V, R, T, SI

1994, 

2002

31 Monitoring 

wells

Land-use study Southern South 

Carolina

Characterize the quality of recently 

recharged water in an agricultural 

setting in the surficial aquifer

F, M, N, D, TE, P, 

V, R, T, SI

1997, 

2007

31 Monitoring 

wells

Land-use study Ocala and Tampa, 

Florida

Characterize the quality of recently 

recharged water in an urban

setting in the surficial aquifer

F, M, N, D, TE, P, 

V, R, T

1995, 

2002

26 Monitoring 

wells

Land-use study Ocala and Tampa, 

Florida

Characterize the quality of recently 

recharged water in an urban setting in the Upper Floridan aquifer

F, M, N, D, TE, P, 

V, R, T

1995, 

2002

32 Monitoring 

wells

Source-water

quality

assessment

Tampa, Florida area Characterize water quality in

public-supply wells in the unconfined Upper Floridan aquifer

F, M, N, D, TE, V, 

R, RA, T, AC

2002 15 Publicsupply 

wells

Source-water

quality

assessment

Tampa, Florida area Characterize water quality in

public-supply wells in the semiconfined Upper Floridan aquifer

F, M, N, D, TE, V, 

R, RA, T, AC

2002 15 Publicsupply 

wells

Transport of

anthropogenic

and natural

contaminants to 

supply wells

(TANC study)‡

Tampa, Florida area Examine factors resulting in

detection of natural and 

anthropogenic contaminants

in a public-supply well

F, M, N, D, P, V, 

T, A , R, SI

2002–05 23:

11 surficial

12 Upper 

Floridan

Clusters of 

monitoring wells 

along 

transects

*Analyzed in samples collected during 1991–2001 only.

†Analyzed in samples collected during 2002–2009 only.

‡Samples from study not included in Principal Aquifers Summary Circular 1360.

66 Water Quality in the Upper Floridan Aquifer and Overlying Surficial Aquifers, Southeastern United States, 1993–2010
Appendixes

Appendix 2. Water-Quality Properties and Constituents 

Measured, a Summary of Data, and Complete Data Archive 

for 1993 –2010

Water-quality properties and constituents measured and a summary of data for 1993–2010, 

including laboratory reporting levels and human-health benchmarks for drinking water, are 

presented only online. The data summary and a complete data archive are available for 

download at http://pubs.usgs.gov/circ/1355/.

Appendix 2 67
This appendix shows graphical comparisons of chemical concentrations (1991 to 2010) for selected inorganic and 

organic constituents of potential health or aquatic-life concern in selected Principal Aquifers of the United States. For 

each constituent, the concentration data are grouped according to five well types: agricultural land-use study wells 

(includes shallow agricultural monitoring wells), urban land-use study wells (includes shallow urban monitoring wells), 

major aquifer study wells, domestic wells, and public-supply wells. For each well type, the aquifers also are grouped 

according to aquifer lithology: basalt and volcanics,** crystalline, carbonate, sandstone, semiconsolidated sand and 

gravel, glacial unconsolidated sand and gravel, and unconsolidated sand and gravel (nonglacial). Data for a particular 

compound were not plotted if there were fewer than 10 samples for a particular well network in a Principal Aquifer; 

not all Principal Aquifers for which data were available are shown. Note that analytical detection limits varied among 

the constituents and that the number of samples for a constituent can vary greatly between Principal Aquifers. The data 

used in this appendix and boxplots for additional constituents are available at http://pubs.usgs.gov/circ/1360/.(62)

Principal Aquifer (number of samples)

10th

percentile

25th

percentile

75th

percentile

Median

(50th percentile)

Lithology

Basalt and volcanics** Crystalline Carbonate Sandstone Semiconsolidated 

 sand and gravel

Glacial unconsolidated sand and gravel Unconsolidated sand and gravel (nonglacial)

MCL Maximum Contaminant Level

U.S. Environmental Protection Agency

 contaminant levels for drinking water 

(NA) Wells of this type were not 

 sampled for this constituent

** Note: Two of the Principal Aquifers in this group include limited samples from basin-fill aquifers within the extent of the basaltic aquifer. 

90th

percentile

Data above

90th percentile

Data below

10th percentile*

SMCL Secondary Maximum Contaminant Level

PMCL Proposed Maximum Contaminant Level

AMCL Proposed Alternative Maximum Contaminant Level

* Data below laboratory reporting limits are not shown.

Appendix 3. Water Quality of the Upper Floridan Aquifer in a National Context

68 Water Quality in the Upper Floridan Aquifer and Overlying Surficial Aquifers, Southeastern United States, 1993–2010
Appendixes

Domestic wells

Public-supply wells

Arsenic concentration, in micrograms per liter

0.1 1.0 10 100 1,000

Principal Aquifer

Surficial (30)

Mississippi River Valley alluvial (5)

High Plains (313)

Alluvial–Pleistocene terrace deposits (NA)

Alluvial–Denver Basin (7)

Rio Grande (25)

Central Valley (135)

California Coastal Basin (18)

Basin and Range basin fill (129)

Glacial–West (23)

Glacial–West Central (76)

Glacial–Central (187)

Glacial–East (68)

Texas coastal uplands (50)

Northern Atlantic Coastal Plain (60)

Mississippi embayment (27)

Valley and Ridge siliciclastic (27)

Early Mesozoic (24)

Denver Basin (75)

Cambrian-Ordovician (69)

Valley and Ridge carbonate (28)

Piedmont and Blue Ridge carbonate (NA)

Upper Floridan (128)

Castle Hayne (6)

Piedmont and Blue Ridge crystalline (99)

New England crystalline (107)

Hawaiian volcanic (NA)

Snake River Plain basin fill–basalt rock (54)

Columbia Plateau basin fill–basalt rock (NA)

Surficial (NA)

Mississippi River Valley alluvial (12)

High Plains (31)

Alluvial–Pleistocene terrace deposits (NA)

Alluvial–Denver Basin (NA)

Rio Grande (42)

Central Valley (15)

California Coastal Basin (94)

Basin and Range basin fill (101)

Glacial–West (11)

Glacial–West Central (15)

Glacial–Central (31)

Glacial–East (49)

Texas coastal uplands (8)

Northern Atlantic Coastal Plain (5)

Mississippi embayment (13)

Valley and Ridge siliciclastic (1)

Early Mesozoic (1)

Denver Basin (15)

Cambrian-Ordovician (65)

Valley and Ridge carbonate (NA)

Piedmont and Blue Ridge carbonate (NA)

Upper Floridan (35)

Castle Hayne (5)

Piedmont and Blue Ridge crystalline (8)

New England crystalline (1)

Hawaiian volcanic (25)

Snake River Plain basin fill–basalt rock (NA)

Columbia Plateau basin fill–basalt rock (22)

MCL=10 μg/L

Arsenic

Appendix 3 69
MCL=3 μg/L

Domestic wells

Public-supply wells

Atrazine concentration, in micrograms per liter

0.001 0.01 0.1 1.0 10

Surficial (30)

Mississippi River Valley alluvial (5)

High Plains (314)

Alluvial–Pleistocene terrace deposits (NA)

Alluvial–Denver Basin (7)

Rio Grande (24)

Central Valley (138)

California Coastal Basin (18)

Basin and Range basin fill (131)

Glacial–West (55)

Glacial–West Central (93)

Glacial–Central (189)

Glacial–East (99)

Texas coastal uplands (49)

Northern Atlantic Coastal Plain (65)

Mississippi embayment (27)

Valley and Ridge siliciclastic (81)

Early Mesozoic (68)

Denver Basin (55)

Cambrian-Ordovician (72)

Valley and Ridge carbonate (117)

Piedmont and Blue Ridge carbonate (29)

Upper Floridan (124)

Castle Hayne (6)

Piedmont and Blue Ridge crystalline (151)

New England crystalline (111)

Hawaiian volcanic (NA)

Snake River Plain basin fill–basalt rock (149)

Columbia Plateau basin fill–basalt rock (63)

Surficial (14)

Mississippi River Valley alluvial (12)

High Plains (31)

Alluvial–Pleistocene terrace deposits (NA)

Alluvial–Denver Basin (NA)

Rio Grande (40)

Central Valley (15)

California Coastal Basin (94)

Basin and Range basin fill (100)

Glacial–West (11)

Glacial–West Central (28)

Glacial–Central (33)

Glacial–East (68)

Texas coastal uplands (8)

Northern Atlantic Coastal Plain (49)

Mississippi embayment (55)

Valley and Ridge siliciclastic (1)

Early Mesozoic (2)

Denver Basin (15)

Cambrian-Ordovician (81)

Valley and Ridge carbonate (4)

Piedmont and Blue Ridge carbonate (NA)

Upper Floridan (35)

Castle Hayne (5)

Piedmont and Blue Ridge crystalline (23)

New England crystalline (1)

Hawaiian volcanic (24)

Snake River Plain basin fill–basalt rock (19)

Columbia Plateau basin fill–basalt rock (34)

Principal Aquifer

Atrazine

70 Water Quality in the Upper Floridan Aquifer and Overlying Surficial Aquifers, Southeastern United States, 1993–2010
Appendixes

Nitrate plus nitrite (nitrate)

Domestic wells

Public-supply wells

Principal Aquifer

Surficial (30)

Mississippi River Valley alluvial (5)

High Plains (314)

Alluvial–Pleistocene terrace deposits (NA)

Alluvial–Denver Basin (7)

Rio Grande (25)

Central Valley (137)

California Coastal Basin (18)

Basin and Range basin fill (123)

Glacial–West (55)

Glacial–West Central (96)

Glacial–Central (210)

Glacial–East (97)

Texas coastal uplands (50)

Northern Atlantic Coastal Plain (66)

Mississippi embayment (27)

Valley and Ridge siliciclastic (81)

Early Mesozoic (69)

Denver Basin (75)

Cambrian-Ordovician (71)

Valley and Ridge carbonate (116)

Piedmont and Blue Ridge carbonate (29)

Upper Floridan (127)

Castle Hayne (6)

Piedmont and Blue Ridge crystalline (151)

New England crystalline (113)

Hawaiian volcanic (NA)

Snake River Plain basin fill–basalt rock (151)

Columbia Plateau basin fill–basalt rock (63)

Surficial (NA)

Mississippi River Valley alluvial (12)

High Plains (31)

Alluvial–Pleistocene terrace deposits (NA)

Alluvial–Denver Basin (NA)

Rio Grande (42)

Central Valley (15)

California Coastal Basin (93)

Basin and Range basin fill (101)

Glacial–West (11)

Glacial–West Central (28)

Glacial–Central (37)

Glacial–East (49)

Texas coastal uplands (8)

Northern Atlantic Coastal Plain (35)

Mississippi embayment (41)

Valley and Ridge siliciclastic (1)

Early Mesozoic (2)

Denver Basin (15)

Cambrian-Ordovician (80)

Valley and Ridge carbonate (4)

Piedmont and Blue Ridge carbonate (NA)

Upper Floridan (34)

Castle Hayne (5)

Piedmont and Blue Ridge crystalline (8)

New England crystalline (1)

Hawaiian volcanic (25)

Snake River Plain basin fill–basalt rock (19)

Columbia Plateau basin fill–basalt rock (34)

Nitrate concentration, in milligrams per liter as nitrogen

0.01 0.1 1 10 100

MCL=10 mg/L

Appendix 3 71
Principal Aquifer

Surficial (30)

Mississippi River Valley alluvial (5)

High Plains (307)

Alluvial–Pleistocene terrace deposits (NA)

Alluvial–Denver Basin (6)

Rio Grande (24)

Central Valley (111)

California Coastal Basin (17)

Basin and Range basin fill (112)

Glacial–West (48)

Glacial–West Central (58)

Glacial–Central (153)

Glacial–East (97)

Texas coastal uplands (40)

Northern Atlantic Coastal Plain (55)

Mississippi embayment (17)

Valley and Ridge siliciclastic (79)

Early Mesozoic (48)

Denver Basin (55)

Cambrian-Ordovician (47)

Valley and Ridge carbonate (112)

Piedmont and Blue Ridge carbonate (29)

Upper Floridan (88)

Castle Hayne (6)

Piedmont and Blue Ridge crystalline (150)

New England crystalline (109)

Hawaiian volcanic (NA)

Snake River Plain basin fill–basalt rock (115)

Columbia Plateau basin fill–basalt rock (59)

Radon concentration, in picocuries per liter

10 100 1,000 10,000 100,000 1,000,000

PMCL=

300 pCi/L AMCL=

4,000 pCi/L

Domestic wells

Radon

72 Water Quality in the Upper Floridan Aquifer and Overlying Surficial Aquifers, Southeastern United States, 1993–2010
Prepared by the USGS Science Publishing Network

Raleigh Publishing Service Center

Edited by Kay P. Naugle

Illustrations and layout by Caryl J. Wipperfurth

For more information concerning this report, contact:

Chief, National Water-Quality Assessment Program

U.S. Geological Survey

413 National Center

12201 Sunrise Valley Drive 

Reston, VA 20192

http://water.usgs.gov/nawqa/
Printed on recycled paper

ISSN 1067-084X (print)

ISSN 2330-5703 (online)

http://dx.doi.org/10.3133/cir1355

Upper Floridan aquifer

Surficial aquifer (overlying

the Floridan aquifer system)