Factors Affecting Water Quality in 

Domestic Wells in the Upper Floridan 

Aquifer, Southeastern United States, 

1998-2005

By Marian P. Berndt and Christy A. Crandall

National Water-Quality Assessment Program

Scientific Investigations Report 2009–5147

U.S. Department of the Interior

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

KEN SALAZAR, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2009

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Suggested citation:

Berndt, M.P., and Crandall, C.A., 2009, 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, 39 p.

ISBN 0-607-97815-5
iii

Foreword

The U.S. Geological Survey (USGS) is committed to providing the Nation with reliable scientific information 

that helps to enhance and protect the overall quality of life and that facilitates effective management of water, 

biological, energy, and mineral resources (http://www.usgs.gov/). Information on the Nation’s water resources 

is critical to ensuring long-term availability of water that is safe for drinking and recreation and is suitable for 

industry, irrigation, and fish and wildlife. Population growth and increasing demands for water make the availability of that water, measured in terms of quantity and quality, even more essential to the long-term sustainability of our communities and ecosystems.

The USGS implemented the National Water-Quality Assessment (NAWQA) Program in 1991 to support national, 

regional, State, and local information needs and decisions related to water-quality management and policy 

(http://water.usgs.gov/nawqa). The NAWQA Program is designed to answer: What is the quality of our Nation’s 

streams and groundwater? How are conditions changing over time? How do natural features and human activities affect the quality of streams and groundwater, and where are those effects most pronounced? By combining information on water chemistry, physical characteristics, stream habitat, and aquatic life, the NAWQA 

Program aims to provide science-based insights for current and emerging water issues and priorities. From 1991 

to 2001, the NAWQA Program completed interdisciplinary assessments and established a baseline understanding of water-quality conditions in 51 of the Nation’s river basins and aquifers, referred to as Study Units (http://

water.usgs.gov/nawqa/studyu.html). 

In the second decade of the Program (2001–2012), a major focus is on regional assessments of water-quality 

conditions and trends. These regional assessments are based on major river basins and principal aquifers, 

which encompass larger regions of the country than the Study Units. Regional assessments extend the findings 

in the Study Units by filling critical gaps in characterizing the quality of surface water and groundwater, and 

by determining water-quality status and trends at sites that have been consistently monitored for more than a 

decade. In addition, the regional assessments continue to build an understanding of how natural features and 

human activities affect water quality. Many of the regional assessments employ modeling and other scientific 

tools, developed on the basis of data collected at individual sites, to help extend knowledge of water quality to 

unmonitored, yet comparable areas within the regions. The models thereby enhance the value of our existing 

data and our understanding of the hydrologic system. In addition, the models are useful in evaluating various 

resource-management scenarios and in predicting how our actions, such as reducing or managing nonpoint and 

point sources of contamination, land conversion, and altering flow and (or) pumping regimes, are likely to affect 

water conditions within a region.

Other activities planned during the second decade include continuing national syntheses of information on 

pesticides, volatile organic compounds (VOCs), nutrients, trace elements, and aquatic ecology; and continuing 

national topical studies on the fate of agricultural chemicals, effects of urbanization on stream ecosystems, 

bioaccumulation of mercury in stream ecosystems, effects of nutrient enrichment on stream ecosystems, and 

transport of contaminants to public-supply wells.

The USGS aims to disseminate credible, timely, and relevant science information to address practical and effective water-resource management and strategies that protect and restore water quality. We hope this NAWQA 

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. 

The USGS recognizes that a national assessment by a single program cannot address all water-resource 

issues of interest. External coordination at all levels is critical for cost-effective management, regulation, and 

conservation of our Nation’s water resources. The NAWQA Program, therefore, depends on advice and information from other agencies—Federal, State, regional, interstate, Tribal, and local—as well as nongovernmental 

organizations, industry, academia, and other stakeholder groups. Your assistance and suggestions are greatly 

appreciated.

 Matthew C. Larsen

Associate Director for Water
iv
v

Contents

Foreword ........................................................................................................................................................iii

Acknowledgments........................................................................................................................................ix

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

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

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

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

Hydrogeologic Setting ................................................................................................................3

Land Use, Population, and Water Use......................................................................................4

Methods...........................................................................................................................................................5

Sample Collection and Processing....................................................................................................5

Quality Assurance and Quality Control .............................................................................................5

Water-Quality Benchmarks for Human Health ................................................................................6

Geochemical Analysis..........................................................................................................................6

Statistical Analysis ...............................................................................................................................6

Compilation of Ancillary Data .............................................................................................................7

Characteristics of Sampling Networks ......................................................................................................7

Groundwater Geochemistry.......................................................................................................................10

Dissolved Solids, Dissolved Oxygen, Major Ions, and pH............................................................10

Mineral Saturation Indices ...............................................................................................................16

Trace Elements and Radon................................................................................................................18

Occurrence and Distribution of Selected Contaminants ......................................................................20

Nitrate ..................................................................................................................................................20

Occurrence and Distribution of Nitrate .................................................................................20

Factors Affecting Nitrate Concentrations .............................................................................21

Volatile Organic Compounds.............................................................................................................25

Occurrence and Distribution ...................................................................................................25

Comparison to Water-Quality Benchmarks for Human Health ..........................................26

Factors Affecting Volatile Organic Compound Detections .................................................28

Pesticides.............................................................................................................................................29

Occurrence and Distribution ...................................................................................................29

Comparison to Water-Quality Benchmarks for Human Health ..........................................30

Factors Affecting Pesticide Detections .................................................................................30

Summary........................................................................................................................................................32

References Cited..........................................................................................................................................33

Appendix 1. Volatile organic compounds analyzed in water samples................................................37

Appendix 2. Pesticides analyzed in water samples...............................................................................38

Figures

1. Map showing extent of the Floridan aquifer system in the southeastern 

United States .................................................................................................................................2

2. Map showing unconfined, semiconfined, and confined areas of the Upper Floridan 

aquifer and locations of domestic wells sampled...................................................................3
vi

3. Cross sections showing the general hydrogeology in the unconfined, semiconfined, 

and confined areas of the Upper Floridan aquifer..................................................................3

 4–5. Maps showing—

 4. Generalized land use and land cover overlying the Floridan aquifer system, 

 1992-98.....................................................................................................................................4

 5. Population by county in areas overlying the Floridan aquifer system, 2000................4

 6. Water withdrawals by county from the Floridan aquifer system, 2000 ........................5

7. Boxplot showing distribution of depths of domestic wells by sampling network..............8

8. Map showing spatial distribution of dissolved-solids concentration in domestic 

wells in the Upper Floridan aquifer..........................................................................................11

9. Piper diagram showing major-ion composition of water in domestic wells in the 

Upper Floridan aquifer...............................................................................................................14

10. Map showing spatial distribution of major-ion water types from domestic wells in 

the Upper Floridan aquifer ........................................................................................................15

11. Graphs showing molar concentrations of calcium plus magnesium compared to 

bicarbonate in domestic wells in the Upper Floridan aquifer for unconfined areas, 

semiconfined areas, and confined areas; and molar concentration of bicarbonate 

to sodium in semiconfined areas in South Carolina .............................................................16

12. Boxplots showing distribution of calcite saturation index values, dolomite 

saturation index values, and calcium to magnesium molar ratios by 

sampling network .......................................................................................................................17

13. Map showing spatial distribution of calcium to magnesium molar ratios in 

domestic wells in the Upper Floridan aquifer........................................................................17

14. Map showing spatial distribution of radon concentrations in domestic wells in the 

Upper Floridan aquifer...............................................................................................................20

15. Map showing spatial distribution of nitrate concentration in domestic wells in 

the Upper Floridan aquifer ........................................................................................................20

16. Graph showing percentage of nitrate concentrations at selected concentration 

levels in the entire data set and by sampling network.........................................................21

17. Boxplots showing concentrations of nitrate and dissolved oxygen concentrations 

in domestic wells in the Upper Floridan aquifer by confinement category ......................22

18. Boxplots showing distribution by sampling network of nitrate, dissolved oxygen, and 

dissolved organic carbon concentrations; and estimated nitrogen contributions in the 

500-m-radius area surrounding each well from fertilizer, animal manure, and septic 

tanks..............................................................................................................................................22

19–27. Graphs showing—

 19. Detection frequencies for samples with one to five volatile organic 

compound detections per sample ....................................................................................25

 20. Detection frequencies for samples with volatile organic compound 

detections in each sampling network..............................................................................26

 21. Detection frequencies for volatile organic compounds detected in more 

than 3 percent of samples..................................................................................................26

 22. Concentrations and benchmark quotient values for volatile organic 

compounds and pesticides detected in 3 percent or more of samples .....................27

23. Detection frequencies for chloroform, carbon disulfide, and 1,2-dichloropropane 

by confinement category and land-use category .................................................................28

24. Detection frequencies for samples with one to five pesticide detections per 

sample ..........................................................................................................................................28

25. Detection frequencies for samples with pesticide detections in each sampling 

network.........................................................................................................................................30
vii

26. Detection frequencies for pesticides detected in 1 percent or more of samples ...........30

27. Detection frequencies for atrazine, deethylatrazine, metolachlor, and alachlor by 

confinement category and land-use category ......................................................................31

Tables

1. Classification strategy for the dominant land use in the 500-meter radius area 

surrounding each domestic well site in the Upper Floridan aquifer....................................6

2. Description of sampling networks of domestic wells in the Upper Floridan aquifer ........8

3. Median values for land use, nitrogen sources, population density, and soil 

properties in the areas surrounding domestic wells for all data, the three 

confinement categories, and the five sampling networks.....................................................9

4. Summary statistics of well depth, temperature, pH, major ions, and selected 

chemical constituents in domestic wells in the Upper Floridan aquifer...........................11

5. Summary statistics by sampling network of well depth, temperature, pH, 

major ions, and selected chemical constituents in domestic wells in the 

Upper Floridan aquifer...............................................................................................................12

6. Comparison of pH and concentrations of major ions in domestic wells in the 

Upper Floridan aquifer to human-health benchmarks and non-health guidelines 

for drinking water .......................................................................................................................15

7. Descriptive statistics of concentrations of trace elements in domestic wells in 

the Upper Floridan aquifer ........................................................................................................18

8. Comparison of concentrations of trace elements in domestic wells in the 

Upper Floridan aquifer to human-health benchmarks and non-health guidelines 

for drinking water .......................................................................................................................19

9. Median nitrate concentrations in domestic wells in the Upper Floridan aquifer 

and results of the Peto-Prentice test to determine differences in concentration 

among the three confinement categories ..............................................................................21

10. Results of the Peto-Prentice test to determine differences in nitrate concentration 

in domestic wells in the Upper Floridan aquifer between individual confinement 

categories using pairwise comparisons.................................................................................21

11. Median nitrate concentration in domestic wells in the Upper Floridan aquifer 

and results of the Peto-Prentice test to determine differences in concentration 

among the five sampling networks..........................................................................................23

12. Results of the Peto-Prentice test to determine differences in nitrate concentration 

in domestic wells in the Upper Floridan aquifer between individual sampling 

networks using pairwise comparisons ...................................................................................23

13. Results of correlation tests between nitrate concentration and selected 

explanatory variables using Kendall’s tau for censored data.............................................24

14. Results of the Wilcoxon rank-sum test comparing median values for selected 

chemical and anthropogenic factors between samples with chloroform 

detections and samples without chloroform detections .....................................................29

15. Results of the Wilcoxon rank-sum test comparing median values for selected 

chemical and anthropogenic factors between samples with carbon disulfide 

detections and samples without carbon disulfide detections ............................................29

16. Results of the Wilcoxon rank-sum test comparing median values for selected 

chemical and anthropogenic factors between samples with atrazine detections 

and samples without atrazine detections...............................................................................31
viii

Conversion Factors and Abbreviations

Multiply By To obtain

 Length

meter (m) 3.281 foot (ft) 

kilometer (km) 0.6214 mile (mi)

meter (m) 1.094 yard (yd) 

 Area

square kilometer (km2) 0.3861 square mile (mi2)

 Mass

gram (g) 0.03527 ounce, avoirdupois (oz)

kilogram (kg) 2.205 pound avoirdupois (lb)

 Flow rate

cubic meter per second (m3/s) 35.31 cubic foot per second (ft3/s)

cubic meter per day (m3/d) 0.000264 million gallons per day (gal/d) 

 Radioactivity

picocurie per liter (pCi/L) 0.037 becquerel per liter (Bq/L) 

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

°F=(1.8×°C)+32

Specific conductance is given in microsiemens per centimeter at 25 degrees Celsius (μS/cm at 

25°C).

Concentrations of chemical constituents in water are given either in milligrams per liter (mg/L) 

or micrograms per liter (μg/L).

Concentrations of radioisotopes in water are given in picocuries per liter (pCi/L).

Acronyms and Additional Abbreviations

CFC-12 Dichlorodifluoromethane

HBSL Health-Based Screening Levels

kg/lb conversion of kilograms to pounds

kg/yr kilogram per year

lb/yr pound per year

lb/yr-person pound per year per person

MCL Maximum Contaminant Levels

mg/L milligram per liter

NLCD National Land Cover Data

NAWQA National Water-Quality Assessment Program

people/km2 People per square kilometer

PCE Tetrachloroethene

SI Saturation Index

SMCL Secondary Maximum Contaminant Level

STATSGO State Soil Geographic Data Base

USEPA U.S. Environmental Protection Agency

USGS U.S. Geological Survey

VOC Volatile Organic Compounds
ix

Acknowledgments

Appreciation is extended to all the homeowners in Florida, Georgia, South Carolina, and 

Alabama who allowed the USGS to sample their domestic wells.

Gratitude is expressed to the many USGS field personnel in Florida, Georgia, and South Carolina 

who collected the samples. Thanks also to Terri L. Arnold, USGS Illinois Water Science Center, 

for providing the land-use data and map for the Floridan aquifer system.

Photograph courtesy of H. Means, Florida Geological Survey.
x
Abstract 

The Floridan aquifer system is a highly productive 

carbonate aquifer that provides drinking water to about 

10 million people in Florida, Georgia, and South Carolina. 

Approximately 1.6 million people rely on domestic wells 

(privately owned household wells) for drinking water. 

Withdrawals of water from the Floridan aquifer system have 

increased by more than 500 percent from 630 million 

gallons per day (2.38 cubic meters per day) in 1950 to 

4,020 million gallons per day (15.2 cubic meters per day) 

in 2000, largely due to increases in population, tourism, and 

agriculture production. 

Water samples were collected from 148 domestic wells 

in the Upper Floridan aquifer in Florida, Georgia, South 

Carolina, and Alabama during 1998-2005 as part of the U.S. 

Geological Survey (USGS) National Water-Quality 

Assessment Program. The wells were located in different 

hydrogeologic settings based on confinement of the Upper 

Floridan aquifer. Five networks of wells were sampled consisting of 28 to 30 wells each—two networks were in 

unconfined areas, two networks were in semiconfined areas, 

and one network was in the confined area. Physical properties 

and concentrations of major ions, trace elements, nutrients, 

radon, and organic compounds (volatile organic compounds 

and pesticides) were measured in water samples. Concentrations were compared to water-quality benchmarks for human 

health, either U.S. Environmental Protection Agency (USEPA) 

Maximum Contaminant Levels (MCLs) for public water 

supplies or USGS Health-Based Screening Levels (HBSLs). 

The MCL for fluoride of 4 milligrams per liter (mg/L) was 

exceeded for two samples (about 1 percent of samples). A 

proposed MCL for radon of 300 picocuries per liter was 

exceeded in about 40 percent of samples. 

Nitrate concentrations in the Upper Floridan aquifer 

ranged from less than the laboratory reporting level of 

0.06 to 8 mg/L, with a median nitrate concentration less than 

0.06 mg/L (as nitrogen). Nitrate concentrations did not exceed 

the MCL of 10 mg/L. Statistical comparisons indicated that 

median nitrate concentrations were significantly different 

by degree of confinement where the highest median nitrate 

concentration was 1.46 mg/L for 58 samples from unconfined 

areas, and by network, where the highest median nitrate concentration was 2.43 mg/L in 28 samples from unconfined areas 

in southwestern Georgia. Nitrate concentrations in unconfined 

areas were positively correlated to: (1) the percentage of 

agricultural land use around the well, (2) the amount of 

nitrogen fertilizer applied, and (3) the dissolved oxygen 

concentrations in groundwater.

Volatile organic compounds (VOCs) were detected in 

about 63 percent of all samples. Chloroform, carbon disulfide, 

and 1,2-dichloropropane were the most frequently detected 

VOCs. Chloroform, a byproduct of water chlorination, was 

most frequently detected in unconfined urban areas. Carbon 

disulfide, a solvent, was most frequently detected in confined 

areas in southeastern Georgia. Pesticides were detected in 

about 21 percent of all samples, but were detected in about 

69 percent of the 28 samples from unconfined areas in southwestern Georgia. The herbicides atrazine, deethylatrazine, and 

metolachlor were the most frequently detected pesticides. 

Introduction

The Floridan aquifer system is the primary source 

of drinking water for nearly 10 million people in the southeastern United States. Of the 10 million people, an estimated 

8.2 million people obtained drinking water from publicwater supplies in 2000, and an estimated 1.6 million obtained 

drinking water from domestic wells (privately owned 

wells primarily used for household drinking water) in 2000 

(Marella and Berndt, 2005). This aquifer system also 

supports agriculture, industry and tourism. 

Factors Affecting Water Quality in Domestic Wells 

in the Upper Floridan Aquifer, 

Southeastern United States, 1998-2005

By Marian P. Berndt and Christy A. Crandall
2 Factors Affecting Water Quality in Domestic Wells in the Upper Floridan Aquifer, Southeastern United States, 1998-2005 

The Floridan aquifer system consists of up to 

1,000 meters (m) of carbonate rock (limestone and dolomite) 

(Miller, 1990). The porous nature of the limestone and its 

proximity to the surface in some areas results in a groundwater 

resource that is particularly vulnerable to contamination from 

various land-use activities. This vulnerability, in some areas, is 

due in part to the presence of numerous karst features, such as 

sinkholes, which facilitate the movement of water from 

surface features and overlying hydrogeologic units to the 

uppermost unit of this aquifer system, the Upper Floridan 

aquifer.

The Floridan aquifer system is one of the principal 

aquifers in the United States studied as part of the National 

Water-Quality Assessment (NAWQA) Program of the U.S. 

Geological Survey (USGS) (Lapham and others, 2005) to 

increase the understanding of how natural features and human 

activities affect groundwater quality. Because of the large 

number of people relying on the Upper Floridan aquifer for 

water supply from domestic wells and the relative vulnerability of this aquifer to contamination, an assessment of the 

relation between geochemical, hydrogeologic and anthropogenic (land use and population density) factors can help 

water-resource managers assess the vulnerability of this 

important drinking-water source to contamination.

Purpose and Scope

This report describes the factors affecting water quality 

in domestic wells in the Upper Floridan aquifer of the 

Floridan aquifer system in the southeastern United States. 

Water samples were collected from 148 domestic wells in 

Florida, Georgia, South Carolina, and Alabama during 1998-

2005 as part of the NAWQA Program’s assessment of groundwater quality in principal aquifers. The five networks of 28 to 

30 wells each were sampled in areas classified on the basis of 

the confinement conditions of the Upper Floridan aquifer—

two networks were in unconfined areas of southwestern 

Georgia and northern Florida, two networks were in 

semiconfined areas of northern Florida and southern South 

Carolina, and one network was in confined areas in southeastern Georgia. The network in the unconfined area in 

southwestern Georgia includes 2 wells in Alabama and 3 wells 

in Florida; the network in the semiconfined area in northern 

Florida includes 2 wells in Georgia; and the network in the 

confined area in southeastern Georgia includes 4 wells in 

Florida.

Samples from the domestic wells were analyzed for 

physical properties and concentrations of major ions, trace 

elements, nutrients, radon, volatile organic compounds 

(VOCs) and pesticides.

The quality of groundwater is described for the entire 

data set and for each sampling network. The geochemical and 

anthropogenic factors affecting the occurrence and distribution 

of contaminants were determined by comparing concentrations 

for nitrate and detection frequencies for pesticides and VOCs 

in the sampling networks, confinement categories, and major 

land-use categories. The land-use categories were based on the 

predominant land use in the area surrounding each well, which 

were derived from national-scale data sets for land use.

For this assessment, measured concentrations were 

compared to water-quality benchmarks for human health, U.S. 

Environmental Protection Agency Maximum Contaminant 

Levels (MCLs) and USGS Health-Based Screening Levels 

(HBSLs), to describe the potential importance of water-quality 

results for human health. Measured concentrations were 

also compared to secondary maximum contaminant levels 

(SMCLs), which are non-enforceable guidelines (established 

for non-health issues such as taste, color, odor, or corrosion), 

to describe the suitability of the water for household supply.

Description of Study Area

The Floridan aquifer system underlies an area 

encompassing about 260,000 km2 in southern Alabama, 

southern Georgia, southeastern Mississippi, southern South 

Carolina, and all of Florida (fig. 1). The study area for this 

report covers approximately 60 percent of the areal extent of 

the Floridan aquifer system and includes most of the areas 

where this aquifer system is a primary source of drinking 

water.

FLORIDAN AQUIFER

SYSTEM

EXPLANATION

0 100 200 MILES

0 100 200 KILOMETERS

Birmingham

Mobile

Pensacola

Tallahassee

Atlanta

Macon

Charleston

Savannah

Jacksonville

Daytona

Beach

Orlando

Tampa St. Petersburg

Miami

Clearwater

Gainesville

Lakeland Cocoa

MISSISSIPPI

GEORGIA

ALABAMA

SOUTH CAROLINA

FLORIDA

Figure 1. Extent of the Floridan aquifer system in the southeastern 

United States.
Introduction 3

Hydrogeologic Setting

The Floridan aquifer system is composed of a thick 

sequence of highly permeable and hydraulically connected 

carbonate rocks (principally limestone and dolomite) that 

generally range in age from late Paleocene to early Miocene. 

The rocks vary in thickness from a thin edge where they crop 

out at land surface to more than 1,000 m thick where the 

aquifer system is deeply buried in southern Florida (Miller, 

1990). In most places, the Floridan aquifer system can be 

divided into three units: (1) the Upper Floridan aquifer, (2) the 

middle confining unit, and (3) the Lower Floridan aquifer. The 

middle confining unit restricts the movement of groundwater 

between the Upper and Lower Floridan aquifers. Both the 

Upper and Lower Floridan aquifers are defined on the basis of 

permeability, and their boundaries locally do not coincide with 

those for either formation boundaries or time-stratigraphic 

units (Miller, 1986). In places where no middle confining unit 

exists, the aquifer system is highly permeable throughout its 

vertical extent and is known as the Upper Floridan aquifer 

(Miller, 1986). The Upper Floridan aquifer is the primary 

source of drinking water.

A sequence of sand, clay, marl, limestone, and dolomite 

of low permeability overlies much of the Upper Floridan 

aquifer and forms a confining unit, which ranges in thickness 

from a few meters in parts of southwestern Georgia and westcentral and north-central Florida to more than 100 m in southeastern Georgia, northeastern Florida, southeastern Florida, 

and the western part of the panhandle of Florida. The lithology, thickness, and integrity of these low-permeability units 

have a controlling effect on the development of permeability 

and local groundwater flow (Bush and Johnston, 1988). The 

sand layers are generally referred to as the surficial aquifer 

system, whereas the clay and limestone components comprise 

the intermediate aquifer system or the intermediate confining 

unit. Both the surficial and intermediate aquifer systems act as 

confining units to the Upper Floridan aquifer. The unconfined 

areas of the Upper Floridan aquifer are located in the western 

parts of north-central Florida and in southwestern Georgia 

(fig. 2). Three confinement categories were defined by Miller 

(1986): (1) unconfined (confining units are absent), 

(2) semiconfined (confining unit less than 30 m thick or 

breached), and confined (confining unit more than 30 m thick) 

(fig. 2). Generalized hydrogeologic sections of the unconfined, semiconfined, and confined areas of the Upper Floridan 

aquifer are shown in figure 3. The carbonate rocks of the 

Upper Floridan aquifer are readily dissolved in unconfined 

areas (Miller, 1990). In these areas, dissolution of limestone is 

greatest because water is able to enter and move through the 

aquifer quickly. Water also discharges to surface water quickly 

through numerous springs. 

MISSISSIPPI

GEORGIA

ALABAMA

SOUTH CAROLINA

FLORIDA CONFINEMENT OF THE

UPPER FLORIDAN AQUIFER

UNCONFINED

SEMICONFINED–Confining unit

thin or breached in places

CONFINED

NOT MAPPED

UNCONFINED NETWORK–acfbsus1

UNCONFINED –gaflsus2

SEMICONFINED –gaflsus3

CONFINED –gaflsus4

NETWORK

NETWORK

SEMICONFINED NETWORK–santsus2

NETWORK

0 100 200 MILES

0 100 200 KILOMETERS

EXPLANATION

DOMESTIC WELL SAMPLED IN:

(Modified from Miller, 1986)

"

Figure 2. Unconfined, semiconfined, and confined areas of the 

Upper Floridan aquifer and locations of domestic wells sampled. 

(Confinement map modified from Miller, 1986.)

Figure 3. Conceptual cross sections showing the general 

hydrogeology in A, unconfined, B, semiconfined, 

and C, confined areas of the Upper Floridan aquifer.

EXPLANATION

SAND

CLAY–Low permeability layer

UPPER FLORIDAN AQUIFER–Limestone and dolomite

A Unconfined

B Semiconfined

C Confined

Not to scale
4 Factors Affecting Water Quality in Domestic Wells in the Upper Floridan Aquifer, Southeastern United States, 1998-2005 

Land Use, Population, and Water Use

Forest, wetland, and agricultural areas constitute the 

majority of the land overlying the Floridan aquifer system 

(fig. 4). The land area overlying the Floridan aquifer system is 

34-percent forest, 25-percent wetland, 20-percent agricultural, 

7-percent urban, 5-percent rock quarry or transitional, 

5-percent grassland/herbaceous, and 4-percent water (T.L. 

Arnold, U.S. Geological Survey, written commun., 2006). 

The highly populated areas relying on the Floridan aquifer 

system are located in northern and coastal parts of Florida 

and in the coastal parts of Georgia (fig. 5), and include the 

major cities of Gainesville, Jacksonville, Orlando, St. 

Petersburg, Tallahassee, and Tampa, Florida, and Savannah, 

Georgia (figs. 1 and 5). 

An estimated 4,020 Mgal/d (15,230,000 m3/d) of water 

was withdrawn from the Floridan aquifer system in 2000 

(Marella and Berndt, 2005) with the highest use occurring 

in the most densely populated areas along the eastern coast 

and central Florida (fig. 6). The greatest withdrawals occurred 

in Florida (3,125 Mgal/d; 11,840,000 m3/d), whereas lesser 

amounts were withdrawn in Georgia (825 Mgal/d; 

3,120,000 m3/d), South Carolina (63 Mgal/d; 240,000 m3/d), 

and Alabama (7 Mgal/d; 27,000 m3/d) (Marella and Berndt, 

2005). About 90 percent of the water withdrawn from the 

MISSISSIPPI

ALABAMA GEORGIA

SOUTH CAROLINA

FLORIDA

0

0 100 200 MILES

100 200 KILOMETERS

AGRICULTURE

BARE ROCK, QUARRY, OR TRANSITIONAL

FOREST OR SHRUB

GRASSLAND OR HERBACEOUS

OPEN WATER

URBAN

WETLAND

EXPLANATION

Figure 4. Generalized land use and land cover overlying 

the Floridan aquifer system, 1992-98. (Data from T.L. Armold, 

U.S. Geological Survey, 2006, written commun.)

EXPLANATION

2000 POPULATION

0 - 50,000

>50,000 - 100,000

>100,000 - 500,000

>500,000

MISSISSIPPI

GEORGIA

ALABAMA

SOUTH CAROLINA

FLORIDA

0 100 200 MILES

0 100 200 KILOMETERS

Figure 5. Population by county in areas overlying the Floridan 

aquifer system, 2000. (Data from U.S. Bureau of the Census, 

2000a.)

Floridan aquifer system was obtained from the Upper Floridan 

aquifer, which is closer to land surface and contains potable 

water in most areas. Irrigation (1,949 Mgal/d; 7,383,000 m3/d) 

and public supply (1,329 Mgal/d; 5,034,000 m3/d) were the 

two largest categories of withdrawal from the Floridan aquifer 

system, with lesser amounts withdrawn for industrial 

(576 Mgal/d; 2,180,000 m3/d) and domestic self-supplied 

uses (166 Mgal/d; 629,000 m3/d). 

Withdrawals of water from the Floridan aquifer system 

increased by more than 500 percent between 1950 and 2000 

from 630 Mgal/d (2,386,000 m3/d) (Bush and Johnston, 1988) 

to 4,020 Mgal/d (15,230,000 m3/d) (Marella and Berndt, 

2005). The increase in withdrawals is attributed to increases 

in population, tourism, and agricultural production throughout the southeastern United States. Withdrawals for irrigation 

accounted for nearly half of the increase—irrigation withdrawals increased from 90 Mgal/d (340,000 m3

/d) in 1950 

to 1,950 Mgal/d (7,386,000 m3/d) in 2000. Withdrawals for 

public supply increased from 85 Mgal/d (320,000 m3/d) in 

1950 to 1,330 Mgal/d (5,038,000 m3/d) in 2000. The population served by public supply increased from 4.5 million people 

in 1980 to 8.2 million in 2000. The number of domestic wells 

within Florida increased 23 percent between 1990 and 2000 

(Marella, 2004).
Methods 5

Methods

The following sections describe the methods used to 

collect and analyze data for this report. Methods described 

include sample collection and processing, quality assurance 

and quality control, comparison of results to water-quality 

benchmarks for human health, compilation of ancillary data 

about sampling sites, and geochemical and statistical analysis 

methods.

Sample Collection and Processing

Water samples were collected and processed using 

methods designed to yield samples that were representative of 

environmental conditions, minimally affected by contamination (Koterba and others, 1995). The samples were collected 

directly from the wells before any water treatment and, in 

most instances, before any pressure or holding tanks. Field 

measurements were made of pH, specific conductance, 

temperature, alkalinity, and dissolved oxygen. 

Samples were collected for laboratory analysis of major 

ions, nutrients, dissolved organic carbon, trace elements, 

radon, volatile organic compounds (VOCs), and pesticides. 

Water samples were analyzed at the USGS National Water 

Quality Laboratory (NWQL) in Lakewood, Colorado. VOC 

and pesticide analytes are listed in Appendixes 1 and 2, 

respectively.

Quality Assurance and Quality Control

Procedures to ensure data quality include written 

protocols for collection of field and laboratory quality-control 

samples. Quality-control samples were collected to provide 

information on possible sample contamination and measurement variability associated with the data-collection process 

(Koterba and others, 1995). These quality-control samples 

consisted of field blank and sequential replicate samples 

that were collected, processed, and analyzed using methods 

similar to those used for the environmental samples. Field 

blanks were collected to ensure that equipment was adequately 

cleaned during sample collection to minimize potential cross 

contamination and that collection and processing did not result 

in contamination. Sequential replicate samples were collected 

immediately after environmental samples were collected, 

using the same equipment. Comparison of the results for the 

replicate sample to the results for the environmental sample 

aids in determining the variability of chemical analyses and 

the consistency of sample collection and processing. 

The examination of VOC detections in field blanks, 

without comparison to the subsequently collected groundwater 

samples, overstates the potential for random contamination 

(associated with field protocols) in subsequently collected 

groundwater samples for some VOCs (J.S. Zogorski, U.S. 

Geological Survey, written commun., 2008). The water used 

to process field blanks (blank water) occasionally contains 

some VOCs, and the analytical results for field blanks can 

reflect this contamination source. Field blank contamination 

is most often (but not always) reduced to non-detectable levels 

prior to collection of a groundwater sample, thus eliminating this contamination source in most groundwater samples. 

The additional rinsing of the equipment and sample lines with 

well water prior to the collection of a groundwater sample is 

the probable cause for the elimination of contaminants. This 

native water rinsing has been shown to be highly effective in 

reducing the potential carryover of contaminants from equipment and sample lines to groundwater samples (Taglioli and 

others, 2001). For these reasons, examination of VOC detections in field blanks, without comparison to the subsequently 

collected groundwater samples, is considered inadequate to 

describe the extent of random contamination of groundwater 

samples.

Analysis of NAWQA data from paired blank and 

groundwater samples during the collection of water samples 

from domestic and public-supply wells (during 1997-2008) 

indicates that only two VOCs require an assessment level to 

address potential random contamination associated with the 

method of collection for groundwater samples. 

EXPLANATION

WITHDRAWALS FROM THE

FLORIDAN AQUIFER SYSTEM IN 2000,

IN MILLION GALLONS PER DAY

0 - 1

>1 - 10

>10 - 50

>50 - 100

MISSISSIPPI

GEORGIA

ALABAMA

SOUTH CAROLINA

FLORIDA

>100 - 337

0 100 200 MILES

0 100 200 KILOMETERS

Figure 6. Water withdrawals by county from the Floridan aquifer 

system, 2000. (Data from Marella and Berndt, 2005.)
6 Factors Affecting Water Quality in Domestic Wells in the Upper Floridan Aquifer, Southeastern United States, 1998-2005 

The recommended interim assessment levels are 0.03 

μg/L (micrograms per liter) for toluene and 0.05 μg/L for 

1,2,4-trimethylbenzene (J.S. Zogorski, U.S. Geological 

Survey, written commun., 2008). At these concentrations, 

the maximum probability of contamination in groundwater 

samples due to random sample contamination is about 

1 percent for both compounds (J.S. Zogorski, U.S. Geological 

Survey, written commun., 2008). Detections of toluene and 

1,2,4-trimethylbenzene below these recommended levels were 

treated as non-detections.

Water-Quality Benchmarks for Human Health

Concentrations of water-quality constituents are 

compared to drinking water MCLs and Secondary Maximum 

Contaminant Levels (SMCLs) for regulated compounds 

(U.S. Environmental Protection Agency, 2006), and to USGS 

HBSLs concentrations for unregulated compounds. These 

HBSLs are estimates of benchmark concentrations used to 

evaluate water-quality data in a human-health context and 

were developed collaboratively by the USGS, USEPA, New 

Jersey Department of Environmental Protection, and Oregon 

Health & Science University (Toccalino and others, 2003; 

2004). Although HBSLs are not regulatory standards and are 

not enforceable, they can be used as planning tools to 

prioritize contaminants that merit further study or monitoring. 

These HBSLs can provide an early indication of contaminant 

concentrations before they reach the level of human-health 

concern in water resources (Toccalino and others, 2005). In 

this study, measured concentrations were compared to their 

respective benchmark concentration by calculating a benchmark quotient value to identify compounds of potential 

human-health concern. The benchmark quotient value is the 

ratio of a measured concentration of a detected compound to 

its MCL (for a regulated compound) or HBSL (for an unregulated compound). A benchmark quotient value greater than or 

equal to 1.0 was used to identify concentrations of potential 

human-health concern. A benchmark quotient value greater 

than or equal to 0.1 was used to identify compounds that may 

warrant inclusion in a low-concentration, trends-monitoring 

program. Such monitoring may provide an early indication of 

contaminant levels that approach human-health benchmarks, 

and consequently, concentrations of potential concern.

Geochemical Analysis

The geochemical program Aquachem was used to 

determine major-ion water types by converting concentrations 

of major ions into equivalents per liter (Waterloo Hydrogeologic Inc., 2005). This computer program was also used 

to construct Piper diagrams that show the relative amounts of 

cations and anions in each water sample. The computer 

program PHREEQCI was used to determine calcite and 

dolomite saturation indices (Charlton and Parkhurst, 2002; 

Parkhurst and Appelo, 1999).

Statistical Analysis

Descriptive and nonparametric statistics were used to 

summarize and compare well depth and concentrations of 

field properties, major ions and trace elements, nitrate, and 

dissolved organic carbon. The nonparametric Spearman rank 

correlation test was used to test for correlation between well 

depth and concentrations of major ions and field parameters 

within a group of data. The nonparametric Wilcoxon rank-sum 

test was used to test for differences in values or concentrations between two groups of data (SAS Institute, Inc., 1989a; 

1989b; Helsel and Hirsch, 1992). The alpha value used for 

these tests was 0.05, which means there is a 95-percent confidence that the differences among groups are not due to random 

chance. The p-value from the statistical tests must be less than 

the alpha value of 0.05 for the differences to be significant. 

Table 1. Classification strategy for the dominant land use in the 500-meter-radius area surrounding 

each domestic well site in the Upper Floridan aquifer.

[Land-use data described in Vogelmann and others, 2001, and Nakagaki and Wolock, 2005; land-use categorization 

based on Gilliom and others, 2006]

Land-use classification Buffer area land-use criteria

Agricultural > 50 percent agricultural land and ≤ 5 percent urban land

Urban > 25 percent urban land and ≤ 25 percent agricultural land

Undeveloped ≤ 5 percent urban land and ≤ 25 percent agricultural land

Mixed All other combinations of urban, agricultural, and undeveloped land 
Methods 7

The nonparametric Peto-Prentice test and multiple 

comparison procedures (Helsel, 2005) were used to determine 

if differences in median concentrations of nitrate were 

statistically significant between the samples in the three 

confinement categories and five sampling networks. The 

Peto-Prentice test was designed to handle data censored at 

multiple reporting levels and was used for the nitrate data 

because about half of the 148 samples had nitrate concentrations less than analytical reporting levels of either 0.06 or 

0.05 mg/L. The multiple comparison procedures included 

a series of individual comparisons between two groups at a 

time. For each individual comparison of two groups, the alpha 

value of 0.05 was divided by the number of groups to obtain 

an individual comparison level (Helsel, 2005). Thus, for each 

pair-wise comparison among the three confinement categories 

(such as comparison of confined to unconfined), the alpha 

value of 0.05 is divided by 3, and the p-value must be less than 

0.017 for the differences to be significant, For each pair-wise 

comparison among the five sampling networks, the alpha 

value of 0.05 is divided by 5, and the p-value must be below 

0.010 for the differences to be significant (Helsel, 2005).

The Kendall’s tau test for censored data (Helsel, 2005) 

was used to assess the significance of correlations between 

nitrate concentration and selected factors such as land use, 

nitrogen sources, population density, and soil characteristics. 

This nonparametric method uses ranks of the data to determine 

a monotonic relation between the explanatory variable (or 

factor) and the response variable (nitrate concentration). 

A p-value less than 0.05 indicates a 95-percent confidence 

level that a monotonic relation exists between the explanatory 

variable and the response variable. 

Compilation of Ancillary Data

Ancillary data used in this report are derived from 

ancillary spatial-data layers that were developed to characterize NAWQA sampling sites for national synthesis of waterquality data. Estimates were generated for percentages of land 

within major land-use categories, population density, septic 

tank density, nitrogen and pesticide inputs, and soil properties 

for a 500-m-radius area surrounding wells sampled as part 

of the NAWQA Program (K.J. Hitt, U.S. Geological Survey, 

written commun., 2006). Although, the 500-m-radius area is 

not intended to represent the contributing area to the well, the 

information for this area does provide information about land 

use and potential sources of contamination near the sampled 

well. The national and county level estimates were compiled 

by various authors (listed below). Estimates for each data 

source were then computed for the 500-m-radius area (K.J. 

Hitt, U.S. Geological Survey, written commun., 2006).

Land-use data, population density, septic tank density, 

nitrogen and pesticide inputs, and soil properties were derived 

from several sources. The land-use data set was derived from 

an enhanced version of the USGS 1992 National Land Cover 

Data (NLCD) (Vogelmann and others, 2001; Price and others, 

2007). For population density, the data set consists of 1990 

Census of Population and Housing data, which includes 1990 

population density, percentage of housing units using domestic 

wells, and percentage of housing units with septic tanks (U.S. 

Bureau of the Census 1991; 2000a). Estimates of atmospheric 

deposition of nitrogen, nitrogen from manure, and nitrogen 

from fertilizer were obtained from Ruddy and others (2006). 

Estimates of nitrogen from septic systems were made by using 

census data and equation (1) (Lindsey and others, 2009):

N-Septic = POPDENS × (SEWSEP/100) × 0.785 km2

 × 6.72 lb/yr × 0.454 kg/lb, (1)

where

N-Septic = the estimated nitrogen input from septic 

systems, in kilograms per year,

POPDENS = the 1990 population density in the 500-m

radius, in people per square kilometer

(U.S. Bureau of Census, 1990; 1991),

SEWSEP = the percent of the housing units on septic

systems (U.S. Bureau of Census, 1992)

0.785 km2 = the area of the 500-m radius surrounding 

each well 

6.72 lb/yr = the estimated mass of nitrogen per

person introduced to a septic system 

(U.S. Environmental Protection Agency, 

2002), and

0.454 kg/lb = the conversion from pounds to kilograms.

Estimates of pesticide application amounts for atrazine 

and metolachlor applications were compiled by Nakagaki 

and Wolock (2005). Soils data were obtained from the State 

Soil Geographic (STATSGO) Data Base (U.S. Department of 

Agriculture, 1994) and compiled by Wolock (1997). 

STATSGO includes characteristics such as permeability, 

organic-matter content, particle size, and hydrologic groups. 

Hydrologic groups are ordered from A to D, with group A 

having the lowest runoff potential and group D having the 

highest runoff potential (U.S. Department of Agriculture, 

2007). 

A land-use classification was assigned to each well 

based on the percentages of selected land-use categories in 

the 500-m-radius areas. Sites were assigned a land-use 

classification of agricultural, urban, undeveloped or mixed 

using the classification system described in table 1. Results 

were used to compare the median nitrate concentrations and 

the occurrence of VOCs and pesticides among the land-use 

classifications.
8 Factors Affecting Water Quality in Domestic Wells in the Upper Floridan Aquifer, Southeastern United States, 1998-2005 

Characteristics of Sampling Networks

Samples were collected from 148 domestic wells in 

Florida, Georgia, South Carolina, and Alabama as part of a 

NAWQA regional assessment of water quality in the Floridan 

aquifer system. Wells tapping the Upper Floridan aquifer were 

sampled in networks of 28 to 30 wells from 1998 to 2005. 

Wells were selected to represent groundwater being utilized 

for drinking water or other domestic uses. Most wells sampled 

are used by individual homeowners, but a few are used by 

small groups of homes, churches, or other small communities. 

Samples were collected in 1998 from 30 wells in semiconfined areas in southern South Carolina (network santsus2); 

in 2002 from 28 wells in unconfined areas in southwestern 

Georgia (acfbsus1), 30 wells in unconfined areas in northern 

Florida (gaflsus2), and semiconfined areas in northern Florida 

(gaflsus3); and in 2005 from 30 wells in confined areas in 

southeastern Georgia (gaflsus4) (table 2; fig. 2). It should be 

noted that the network in the unconfined area in southwestern 

Georgia includes 2 wells in Alabama and 3 wells in Florida; 

the network in the semiconfined area in northern Florida 

includes 2 wells in Georgia; and the network in the confined 

area in southeastern Georgia includes 4 wells in Florida.

Well depths were variable among the sampling networks 

with the deepest wells in confined areas of the Upper 

Floridan aquifer (fig. 7). Median well depths were 50 m or less 

for the four networks in unconfined and semiconfined areas 

and ranged from a median depth of 28.0 m for wells in the 

network in unconfined areas in northern Florida (gaflsus2) to 

53.3 m for wells in the semiconfined area in southern South 

Carolina (santsus2). The median depth for wells in the network in the confined area in southeastern Georgia (gaflsus4) 

was 132 m. 

Selected characteristics of the areas surrounding the 

sampled wells, including potential contaminant sources such 

as land use, nitrogen sources, and population density were 

compiled to use as variables to explain water-quality results. 

In addition to potential sources of contaminants, soil 

properties were compiled for the areas surrounding the 

UNCONFINED SOUTHWESTERN

GEORGIA–acfbsus1

U

gaflsus2

SEMICONFINED NORTHERN

FLORIDA–gaflsus3

SEMICONFINED SOUTHERN

SOUTH CAROLINA–santsus2

CONFINED SOUTHEASTERN

GEORGIA–gaflsus4

NCONFINED NORTHERN

FLORIDA–

U-GA U-FL S-FL S-SC

WELL DEPTH, IN METERS

0

100

150

200

250

58

90th PERCENTILE

75th PERCENTILE

NUMBER OF

SAMPLES

MEDIAN

25th PERCENTILE

10th PERCENTILE

28 30 29 30

U-GA

U-FL

S-FL

S-SC

C-GA

C-GA

29

EXPLANATION

50

Figure 7. Boxplot showing distribution of depths of 

domestic wells by sampling network.

Table 2. Description of sampling networks of domestic wells in the Upper Floridan aquifer. 

Upper Floridan aquifer 

confinement category

Network Number 

of wells

Year 

sampled

Network 

code

National Water-Quality 

Assessment (NAWQA) code

Unconfined Southwestern 

Georgia1

28 2002 U-GA acfbsus1

Unconfined Northern Florida 30 2002 U-FL gaflsus2

Semiconfined Northern Florida 30 2002 S-FL gaflsus3

Semiconfined Southern 

South Carolina

30 1998 S-SC santsus2

Confined Southeastern 

Georgia2

30 2005 C-GA gaflsus4

1

Network includes some wells in southeastern Alabama.

2

Network includes some wells in central Georgia and northern Florida.

sampled wells because these data are related to transport of 

contaminants through the aquifer. Estimated values for these 

source and transport factors for the areas surrounding each 

sampled well were used in the analysis of the contaminant 

occurrence in groundwater. Median values for these source 

and transport factors are given for the entire data set and the 

five sampling networks in table 3 and are discussed in 

subsequent paragraphs.

Agricultural and urban land uses provide potential 

sources of contaminants. For the entire data set, the median 

percentage of agricultural land use in areas surrounding wells 

was 40.6 percent (table 3). The median percentage of agricultural land use was greater than 50 percent for three of the 
Characteristics of Sampling Networks

9

Table 3. Median values for land use, nitrogen sources, population density, and soil properties in the areas surrounding domestic wells for all data, the three confinement categories, and the five sampling networks.[people/km2; people per square kilometer]

All data

Samples 

from 

unconfined 

areas

Samples from 

semiconfined 

areas

Unconfined 

southwestern 

Georgia

Unconfined 

northern 

Florida

 Semiconfined 

northern 

Florida

 Semiconfined 

South 

Carolina

Confined

southeastern

Georgia

acfbsus1 gaflsus2 gaflsus3 santsus2 gaflsus4

Number of samples 148 58 60 28 30 30 30 30Land use, percentage of land area1Agriculture 40.6 56.7 29.5 58.4 55.4 35.8 26.2 53.3Urban 0.7 0.7 3.5 0.6 1.9 7.5 0.3 0Forest 26.2 22.7 30.4 30.6 20.4 19.0 35.9 24.7Wetland 6.1 2.1 10.1 2.2 2.1 9.6 10.5 5.9Nitrogen sources, in kilograms per yearAgricultural fertilizer2 1,274 1,337 1,004 2,574 911 1,210 878 1,899Animal manure2 259 769 301 389 1232 530 158 0Atmospheric deposition2 304 289 300 270 355 352 296 321.0Septic tanks4 27.1 25 33.0 17.6 30.1 60.8 24.9 15.8Population and water use3Housing units using domestic wells, percent 76.4 76.7 69.3 70.6 88.2 69.1 69.3 79.9Housing units using septic systems, percent 91.7 92.1 89.5 89.8 96.5 89.7 87.0 94.1Population density, people/km2 14.3 11.4 17.8 9.3 14.7 27.9 14.6 8.3Soil properties, percent of soil materials5Hydrologic Group A 12.6 23.3 11.1 20.9 67.0 26.4 3.7 0.5Hydrologic Group B 9.8 27.5 5.4 61.0 2.0 0 12.7 10.2Organic matter 0.6 0.4 0.7 0.3 0.5 0.6 0.8 0.6Clay 18.5 16.4 19.6 27.7 9.7 8.5 22.8 19.4Silt 16.0 12.3 17.0 17.2 7.3 5.6 21.2 16.2Sand 64.5 69.8 62.9 55.0 82.5 86.1 55.6 63.5

1 Vogelmann and others (2001) and Price and others (2007)2 Ruddy and others (2006).3 U.S. Bureau of the Census (1990; 1991; 1992).4 Equation 1; U.S. Environmental Protection Agency (2002). U.S. Bureau of the Census (1990; 1991)5 U.S. Department of Agriculture (1994) and Wolock (1997).
10 Factors Affecting Water Quality in Domestic Wells in the Upper Floridan Aquifer, Southeastern United States, 1998-2005 

sampling networks—the networks in the unconfined areas in 

southwestern Georgia (acfbsus1) and northern Florida 

(gaflsus2), and the network in the confined area in southeastern Georgia (gaflsus4). For the entire data set, the median 

percentage of urban land use was 0.7 percent. The median 

percentage of urban land use was highest (7.5 percent) in the 

network in the semiconfined area in northern Florida. The 

median percentage of urban land use was less than 1 percent 

in three of the sampling networks. The median percentage of 

forest land use was highest in the network in the semiconfined 

area in southern South Carolina (35.9 percent). The median 

percentage of land area in wetlands was highest (10.5 percent) 

in the network in the semiconfined area in southern South 

Carolina (table 3).

Population density and water use by household showed 

that most housing units used domestic wells and had a septic 

tank system. The median percentage of housing units using 

domestic wells ranged from about 70 to 88 percent, and the 

median percentage of housing units with septic tanks ranged 

from about 87 to 96 percent (table 3). The median population density in 1990 varied from about 8 people per square 

kilometer (people/km2) in the network in the confined area 

in southeastern Georgia (gaflsus4) to about 28 people/km2

for the network in the semiconfined area in northern Florida 

(gaflsus3). 

Nitrogen associated with agricultural land-use sources 

comprised the largest component of the potential sources of 

nitrogen. The median amount of nitrogen from agricultural 

fertilizer was 1,274 kg/yr (kilogram per year) for the entire 

data set, and median values for sampling networks ranged 

from about 878 kg/yr in the semiconfined area in southern 

South Carolina to 2,574 kg/yr in the unconfined area in southwestern Georgia (table 3). The median amount of nitrogen 

from animal manure was 259 kg/yr for the entire data set, and 

median values for sampling networks ranged from 0 kg/yr in 

the confined area in southeastern Georgia to 1,232 kg/yr in the 

unconfined area in northern Florida. The median amounts of 

nitrogen from atmospheric deposition and septic tanks were 

much lower ranging from 270 to 355 kg/yr for atmospheric 

deposition and 15.8 to 60.8 kg/yr for septic tanks.

For the soil characteristics, sampling networks in the 

unconfined areas in northern Florida and southwestern 

Georgia had the highest percentage of soils in soil hydrologic 

groups A and B (the two categories comprising well drained 

and moderately well drained soils) (U.S. Department of 

Agriculture, 2007). Other networks had less than 27 percent of 

soils in either of these two categories. The percentage of soil 

materials comprised of organic matter was low, with median 

values ranging from 0.3 to 0.8 percent. The percentage of soil 

materials comprised of clay ranged from about 8 to 28 percent, 

and the percentage comprised of silt ranged from about 7 to 

21 percent. The percentage of soil materials comprised of sand 

showed the greatest variability among the sampling networks, 

ranging from about 55 to 86 percent (table 3), with the highest 

median percentages occurring in the networks in the unconfined and semiconfined areas in northern Florida. 

Groundwater Geochemistry

The dissolution of aquifer minerals, such as calcite and 

dolomite in carbonate aquifers, is an important geochemical 

process that controls the major-ion composition and pH of 

groundwater. Examination of pH values and concentrations of 

dissolved solids, major ions, and dissolved oxygen spatially 

and with depth in the aquifer can provide a means to assess 

the importance of groundwater residence time as a controlling 

factor on major-ion geochemistry. Few health guidelines have 

been established for major ions, but a MCL of 4 mg/L has 

been established for fluoride. The SMCLs have been 

established for pH, dissolved solids, chloride, sulfate and 

fluoride (U.S. Environmental Protection Agency, 2006). 

Analysis of dissolved oxygen concentrations in groundwater 

aid in determining the oxidation and reduction (redox) 

reactions that serve an important role in the transformation 

and biodegradation of anthropogenic compounds such as 

nitrate, pesticides, and VOCs. Assessment of major-ion water 

types and ion ratios can provide quantitative information 

about mineral-water interactions. In addition, determination of 

saturation indices with respect to calcite and dolomite using 

thermodynamic models is useful to gain an improved 

understanding of the factors that influence groundwater 

geochemistry (Stumm and Morgan, 1981).

Dissolved Solids, Dissolved Oxygen, 

Major Ions, and pH

Concentrations of dissolved solids, dissolved 

oxygen, major ions, and values of pH were assessed for 

the 148 samples collected from domestic wells (table 4). 

Dissolved solids concentrations ranged from 91 to 

5,370 mg/L, with a median concentration of 179 mg/L 

(fig. 8). Concentrations of dissolved oxygen ranged from 

0.1 to 9.7 mg/L with a median concentration of 0.7 mg/L. 

Calcium concentrations ranged from 1.5 to 302 mg/L and 

magnesium concentrations ranged from 0.35 to 90.6 mg/L. 

Median calcium and magnesium concentrations were 42.3 and 

4.67 mg/L, respectively. The median bicarbonate concentration was 154 mg/L, and concentrations ranged from 54 to 

567 mg/L. Sulfate concentrations ranged from 0.7 to 

858 mg/L, with a median concentration of 3.4 mg/L, chloride 

concentrations ranged from 2.21 to 2,580 mg/L with a median 

concentration of 5.47 mg/L, and fluoride concentrations 

ranged from 0.1 to 5.5 mg/L with a median concentration of 

0.17 mg/L. The median pH was 7.5, and pH values ranged 

from 6.3 to 9.0.

Median concentrations of dissolved solids, several major 

ions, and dissolved oxygen varied between sampling 

networks based on confinement categories for the Upper 

Floridan aquifer, although values of pH varied little between 

sampling networks (table 5). The median dissolved solids 

concentrations ranged from 147 mg/L for the unconfined area 

in southwestern Georgia to 235 mg/L for the semiconfined 
Groundwater Geochemistry 11

Table 4. Summary statistics of well depth, temperature, pH, major ions, and selected chemical constituents in domestic 

wells in the Upper Floridan aquifer.

[Concentrations in milligrams per liter, except as noted; μS/cm, microsiemens per centimeter]

Variable Number of 

observations Median Minimum

25th 

Percentile

75th 

Percentile Maximum

Well depth, meters 145 144.5 6.1 25.6 79.2 244

Temperature, degrees Celsius 148 22.4 13.5 20.6 24.0 29.9

Specific conductance, μS/cm 148 291 141 244 374 9,180

Dissolved solids 148 179 91 146 229 5,370

Dissolved oxygen 147 0.7 0.1 0.2 4.1 9.7

pH, standard units 148 7.5 6.3 7.3 7.7 9.0

Bicarbonate 148 154 54 132 195 567

Calcium 148 42.3 1.5 31.5 56.3 302

Magnesium 148 4.67 0.35 1.43 10.6 90.6

Sodium 148 4.50 1.36 2.56 11.0 1,760

Potassium 148 .73 .06 .34 1.97 59.4

Sulfate 148 3.4 1 .7 8.0 858

Chloride 148 5.47 2.21 3.88 8.99 2,580

Silica, as SiO2 148 12.8 3.58 7.49 30.5 65.1

Fluoride 148 0.17 .1 .1 .32 5.52

Bromide 138 .03 .01 .02 .07 9.50

Organic carbon 148 .40 .16 .29 .99 17.1 MISSISSIPPI

GEORGIA

ALABAMA

SOUTH CAROLINA

FLORIDA CONFINEMENT OF THE

UPPER FLORIDAN AQUIFER

UNCONFINED

SEMICONFINED–Confining unit

thin or breached in places

CONFINED

NOT MAPPED

LESS THAN 250

250 - 500

GREATER THAN 500

EXPLANATION

DISSOLVED-SOLIDS

CONCENTRATION, IN

MILLIGRAMS PER LITER

(Modified from Miller, 1986)

0 100 200 MILES

0 100 200 KILOMETERS

Figure 8. Spatial distribution of dissolved solids concentration in 

the Upper Floridan aquifer. (Modified from Miller, 1986.)

area in South Carolina. Median bicarbonate concentrations 

were lowest in the unconfined area in southwestern Georgia 

(133 mg/L) and highest in the semiconfined area in South 

Carolina and the unconfined area in northern Florida (about 

190 mg/L in each network). The median magnesium 

concentration was lowest in the unconfined area in southwestern Georgia, 0.96 mg/L, and was highest, 11.9 mg/L, in 

the confined area in southeastern Georgia. Median chloride 

concentrations ranged from 3.97 mg/L in the unconfined area 

in southwestern Georgia to about 7 mg/L in the unconfined 

and semiconfined areas in northern Florida. Median pH by 

network ranged from 7.3 in the unconfined area in northern 

Florida to 7.6 in the unconfined area in southwestern Georgia 

and the semiconfined areas in northern Florida and South 

Carolina.

Effects of aquifer confinement on groundwater 

geochemistry are seen in the higher concentrations of 

dissolved oxygen in groundwater in unconfined areas 

compared to semiconfined or confined areas. Dissolved 

oxygen in groundwater originates from the atmosphere and is 

depleted as groundwater moves through the flow system. In 

the unconfined areas, water recharges more readily and moves 

through the aquifer quickly. This recently recharged groundwater in unconfined areas often contains more dissolved 

oxygen than older, deeper groundwater. Median concentrations 
12 Factors Affecting Water Quality in Domestic Wells in the Upper Floridan Aquifer, Southeastern United States, 1998-2005 

Table 5. Summary statistics by sampling network of well depth, temperature, pH, major ions, and selected chemical constituents in 

domestic wells in the Upper Floridan aquifer.

[Concentrations are expressed as milligrams per liter, except as noted]

Variable Number of 

observations Median Minimum 25th Percentile

75th 

Percentile Maximum

Unconfined network, southwestern Georgia (acfbsus1)

Well depth, meters 27 42.1 19.5 30.5 66.4 113

Temperature, degrees Celsius 28 21.2 18.8 20.5 22.0 23.5

Specific conductance, μS/cm 28 252 162 223 277 336

Dissolved solids 28 147 98 132 169 243

Dissolved oxygen 28 5.7 .5 4.0 6.4 8.5

pH, standard units 28 7.6 7.1 7.5 7.8 8.0

Bicarbonate 28 133 87 113 151 171

Calcium 28 45.4 30.3 38.6 51.7 68.0

Magnesium 28 .96 .35 .56 1.68 9.77

Sodium 28 2.0 1.4 1.8 2.5 5.0

Potassium 28 .27 .06 .16 .48 .92

Sulfate 28 .66 .21 .46 .83 8.84

Chloride 28 3.97 2.55 3.07 5.17 10.1

Silica, as SiO2 28 7.12 5.62 6.32 8.43 22.4

Fluoride 28 .11 .06 .10 .17 .17

Bromide 28 .03 .02 .02 .03 .05

Organic carbon 28 .3 .2 .2 .3 .4

Unconfined network, northern Florida (gaflsus2)

Well depth, meters 30 28.0 11.6 22.9 31.4 73.2

Temperature, degrees Celsius 30 24.0 20.7 23.5 24.5 26.9

Specific conductance, μS/cm 30 328 164 263 407 1,750

Dissolved solids 30 191 94.5 146 228 1,560

Dissolved oxygen 30 2.2 .2 .8 4.1 9.7

pH, standard units 30 7.3 6.3 6.9 7.5 8.1

Bicarbonate 30 193 70 148 215 309

Calcium 30 55.2 26.7 41.4 70.5 302

Magnesium 30 4.1 .52 1.33 10.7 90.6

Sodium 30 3.69 1.71 2.52 4.43 11.1

Potassium 30 .33 .09 .21 .48 2.06

Sulfate 30 5.44 .14 2.30 9.96 858

Chloride 30 6.98 2.42 4.18 8.41 35.5

Silica, as SiO2 30 6.94 3.58 5.55 9.54 14

Fluoride 30 .10 .07 .10 .16 .81

Bromide 30 .03 .02 .02 .03 .43

Organic carbon 30 .4 .2 .3 1.2 17

Semiconfined network, northern Florida (gaflsus3)

Well depth, meters 29 45.7 21.9 36.6 65.5 108

Temperature, degrees Celsius 30 24.3 21.5 22.8 25.6 29.9

Specific conductance, μS/cm 30 272 141 230 358 1,430

Dissolved solids 30 168 91 136 233 824

Dissolved oxygen 30 .9 .1 .4 2.6 8.8

pH, standard units 30 7.6 6.4 7.4 7.7 8.1

Bicarbonate 30 150 54 126 168 284

Calcium 30 43.5 17.6 34.5 61.2 86.13
Groundwater Geochemistry 13

Variable Number of 

observations Median Minimum 25th Percentile

75th 

Percentile Maximum

Semiconfined network, northern Florida (gaflsus3)—Continued

Magnesium 30 6.48 .98 3.90 8.76 23.5

Sodium 30 4.89 1.85 3.73 6.89 166

Potassium 30 .70 .10 .45 .91 6.18

Sulfate 30 3.97 .07 1.18 14.8 164

Chloride 30 7.0 3.8 5.3 13 321

Silica, as SiO2 30 12 6.8 9.7 15.7 53.3

Fluoride 30 .2 .1 .1 .2 1.1

Bromide 30 .03 .02 .02 .05 1.00

Organic carbon 30 .6 .2 .4 1.1 4.4

Semiconfined network, South Carolina (santsus2)

Well depth, meters 30 53.3 24.4 42.7 91.4 177

Temperature, degrees Celsius 30 18.5 13.5 17.0 19.5 22.0

Specific conductance, S/cm 30 365 189 285 546 9,180

Dissolved solids 30 235 125 191 351 5,370

Dissolved oxygen 30 .2 .1 .1 .2 5.9

pH, standard units 30 7.6 6.5 7.4 8.3 9.0

Bicarbonate 30 193 59 161 288 567

Calcium 30 34.9 1.47 10.8 49.4 68.3

Magnesium 30 3.06 .92 1.55 7.91 73.8

Sodium 30 17.9 2.21 5.95 99.5 1,763

Potassium 30 4.88 .48 1.87 10.1 59.4

Sulfate 30 3.86 .10 1.24 7.16 185

Chloride 30 5.44 2.57 3.95 20.24 2.580

Silica, as SiO2 30 28.8 6.74 23.5 36.2 55.0

Fluoride 30 .38 .10 .14 1.16 5.52

Bromide 30 .03 .01 .02 .08 9.50

Organic carbon 30 1.2 .2 .6 1.5 4.0

 Confined network southeastern Georgia (gaflsus4)

Well depth, meters 29 132 33.2 91.7 186 244

Temperature, degrees Celsius 30 22.5 19.9 22.0 23.8 25.4

Specific conductance, μS/cm 30 293 201 251.00 332.00 766.00

Dissolved solids 30 190 130 168 217 502

Dissolved oxygen 29 .1 .1 .1 .2 5.8

pH, standard units 30 7.4 6.8 7.3 7.5 7.7

Bicarbonate 30 154 123 1,378 183 412

Calcium 30 33.9 24.1 28.6 42.4 67.1

Magnesium 30 11.9 .79 5.28 14.9 40.2

Sodium 30 10.7 1.76 4.05 17.1 47.6

Potassium 30 1.81 .44 1.28 2.35 5.14

Sulfate 30 5.80 .20 2.30 24.1 159

Chloride 30 5.54 2.21 3.80 10.6 48.7

Silica, as SiO2 30 36.1 12.3 26.4 40.5 65.1

Fluoride 30 .40 .1 .2 .6 1.1

Bromide 19 .13 .11 .12 .15 .27

Organic carbon 30 .3 .2 .2 .7 9.7

Table 5. Summary statistics by sampling network of well depth, temperature, pH, major ions, and selected chemical constituents in 

domestic wells in the Upper Floridan aquifer.—Continued

[Concentrations are expressed as milligrams per liter, except as noted]
14 Factors Affecting Water Quality in Domestic Wells in the Upper Floridan Aquifer, Southeastern United States, 1998-2005 

of dissolved oxygen were highest in samples from unconfined 

areas in southwestern Georgia (5.7 mg/L) and northern Florida 

(2.2 mg/L). Median concentrations were less than 1 mg/L for 

samples from the three networks in semiconfined or confined 

areas (table 5).

Comparison of dissolved-solids and major-ion 

concentrations and pH values to MCLs or SMCLs (U.S. 

Environmental Protection Agency, 2006) indicated that few 

samples exceeded these standards for pH, chloride, fluoride, 

sulfate, or dissolved solids. Fluoride is the only major ion for 

which an MCL has been established. Fluoride concentrations 

in two samples (about 1 percent of samples) exceeded the 

MCL of 4 mg/L (table 6). These two occurrences were in the 

semiconfined area in South Carolina. Dissolved-solids concentrations exceeded the SMCL of 500 mg/L in about 

5 percent of wells. For pH, about 4 percent of samples had 

values outside the SMCL range of 6.5 to 8.5. The SMCLs 

for chloride (250 mg/L), fluoride (4 mg/L), and sulfate 

(250 mg/L) were exceeded in less than 3 percent of wells 

(table 6).

Major-ion chemistry was assessed by determining the 

major-ion water types based on concentrations of major ions 

in equivalents. Most water samples had dominant ions of calcium, bicarbonate, and magnesium (fig. 9). For the entire data 

set, the most commonly occurring major-ion water types were 

calcium-bicarbonate (57 percent of the samples) and calciummagnesium-bicarbonate (26 percent) (fig. 10). The dominant 

major ion water type for most samples in unconfined areas 

was calcium-bicarbonate (figs. 9 and 10), whereas water types 

for samples from networks in semiconfined and confined areas 

Figure 9. Piper diagram showing major-ion composition of water in domestic wells in the Upper Floridan aquifer.

EXPLANATION

UNCONFINED–Southwestern Georgia

UNCONFINED–Northern Florida

SEMICONFINED–Northern Florida

SEMICONFINED–Southern South Carolina

CONFINED–Southeastern Georgia

"

20

40

80

60

100

0

20

40

80

60

100

0

60

20

40

0

80

100

60

20

40

0

80

100

PERCENT

PERCENT

PERCENT

SULFATE PLUS CHLORIDE

CALCIUM PLUS MAGNESIUM

CARBONATE PLUS BICARBONATE

CHLORIDE

SULFATE

SODIUM PLUS POTASSIUM

MAGNESIUM

CALCIUM

100

80

60

40

20

0

0

20

40

60

80

100

80

60

20

0

100

80

60

20

40

0

100

80

60

40

20

100

0

20

40

80

60

100

0

40

"

"

"

"

"

"

" " "" "

"

" "

" " ""

"

" "

"

" " "

"""

"

"

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"

"

"

" "

""

" "

" "

"

"

""

"

"

" "

" "

"

"

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"

" "

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"

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Groundwater Geochemistry 15

Constituent Benchmark or guideline Wells exceeding benchmarks 

or guideline

Human-health 

benchmark

Non-health 

guideline

Human-health 

benchmark

Non-health 

guideline

Value Type Value Type Value Percent Value Percent

Physical properties

pH, in standard units1 -- 6.5 - 8.5 SMCL -- -- 6 4.1

Major ions, in mg/L

Chloride -- -- 250 SMCL -- -- 2 1.4

Fluoride 4 MCL 2 SMCL 2 1.4 4 2.7

Sulfate -- -- 250 SMCL -- -- 1 0.7

Total dissolved solids -- -- 500 SMCL -- -- 7 4.7

1

Samples with pH less than 6.5 or pH greater than 8.5 exceed the SMCL.

Table 6. Comparison of pH and concentrations of major ions in domestic wells in the Upper Floridan aquifer to humanhealth benchmarks and non-health guidelines for drinking water.

[All constituents in milligrams per liter, except as noted; MCLs and SMCLs are U.S. Environmental Protection Agency (USEPA) Maximum 

Contaminant Levels (MCLs) and Secondary Maximum Contaminant Levels (SMCLs) for public-water supplies; --, not applicable]

CONFINEMENT OF THE

UPPER FLORIDAN AQUIFER

UNCONFINED

SEMICONFINED–Confining unit

thin or breached in places

CONFINED

NOT MAPPED

Calcium-Bicarbonate

Calcium-Magnesium-Bicarbonate

Mixed

Sodium-Chloride

Sodium-Bicarbonate

MISSISSIPPI

GEORGIA

ALABAMA

SOUTH CAROLINA

FLORIDA

EXPLANATION

MAJOR ION WATER TYPE

0 100 200 MILES

0 100 200 KILOMETERS

Figure 10. Spatial distribution of major-ion water types in water 

from domestic wells in the Upper Floridan aquifer.

were somewhat variable (figs. 9 and 10). For the semiconfined 

area in South Carolina, sodium-bicarbonate was the major-ion 

water type in 8 of 30 samples (fig. 10). Katz (1992) hypothesized that cation exchange is the probable cause of this water 

type in the Upper Floridan aquifer. Sprinkle (1989, p. 31) also 

reported sodium-bicarbonate waters and suggested that “the 

large area of apparent undersaturation in east-central Georgia 

may be related to local cation exchange of sodium for calcium, 

which reduces calcium concentration and brings groundwater 

below saturation with calcite.” Sprinkle (1989) continued to 

state that another possible explanation was “that underlying 

Cretaceous and Tertiary sand aquifers are discharging dilute 

sodium-bicarbonate water into the Upper Floridan aquifer.” 

Molar concentrations of calcium plus magnesium were 

plotted against molar concentration of bicarbonate to assess 

whether calcite and dolomite dissolution is the major 

geochemical process that controls the major-ion 

composition. If calcite and dolomite dissolution is the 

dominant geochemical process, the molar concentration of 

bicarbonate (in millimoles per liter) should be two times 

the concentration of the sum of the molar concentrations of 

calcium and magnesium. Most water samples plot along a line 

with a 2:1 slope (bicarbonate to calcium plus magnesium), 

except for eight samples in the semiconfined area in South 

Carolina and a few samples in the semiconfined area in 

northern Florida and the confined area in southeastern 

Georgia (figs. 11A-C). The eight samples in South Carolina 

show excess bicarbonate concentrations relative to calcium 

and magnesium (fig. 11B). If the molar concentration of 

bicarbonate is plotted against the molar concentration of 

sodium, most of the eight samples plot on a 1:1 slope line, 

indicating that sodium provides the molar equivalent for the 

excess bicarbonate in these samples (fig. 11D). Because the 

increase in sodium is not accompanied by an increase in 

chloride, cation exchange rather than saltwater is the 

probable the source of the sodium. 
16 Factors Affecting Water Quality in Domestic Wells in the Upper Floridan Aquifer, Southeastern United States, 1998-2005 

C Confined

A Unconfined

B Semiconfined

2:1 Bicarbonate to

Calcium + Magnesium

2:1 Bicarbonate to

Calcium + Magnesium

2:1 to

Calcium + Magnesium

Bicarbonate

1:1 Bicarbonate to Sodium

D Semiconfined--southern South Carolina (santsus2)

SOUTHWESTERN GEORGIA–acfbsus1

NORTHERN FLORIDA–gaflsus2

NORTHERN FLORIDA–gaflsus3

SOUTHERN SOUTH

CAROLINA–santsus2

CALCIUM PLUS MAGNESIUM,

IN MILLIMOLES PER LITER

0 5 10 15 20

0 20 40 60 80

BICARBONATE, IN MILLIMOLES PER LITER

0

2

4

6

8

10

0

2

4

6

8

10

0

2

4

6

8

10

0

2

4

6

8

10

BICARBONATE, IN MILLIMOLES PER LITER

SODIUM, IN MILLIMOLES PER LITER

Figure 11. Molar concentrations of calcium plus magnesium compared to 

bicarbonate in domestic wells in the Upper Floridan aquifer for A, unconfined 

areas, B, semiconfined areas, C, confined areas, and D, molar concentration of 

bicarbonate to sodium in semiconfined areas in South Carolina.

sodium (1,763 mg/L), chloride (2,576 mg/L), and sulfate 

(1,855 mg/L). Trace elements were not analyzed in this sample 

collected in 1998; however, in a sample collected from this 

well in 2005, concentrations of several trace elements 

typically present in seawater also had elevated concentrations 

including boron (3.4 mg/L), bromide (9.5 mg/L) and strontium 

(8.0 mg/L).

Mineral Saturation Indices 

The computer program PHREEQC 

(Parkhurst and Appelo, 1999) was used to 

calculate the distribution and activities of 

dissolved species and the saturation state of 

groundwater with respect to the carbonate 

minerals calcite and dolomite. Comparison of 

the results for groundwater samples from the 

five sampling networks can be used to determine 

if the chemical composition of groundwater and 

the aquifer materials varies among networks in 

unconfined, semiconfined and confined areas. 

Assumptions for this program are that all 

dissolved species are at equilibrium, which 

allows the use of thermodynamic models to 

calculate the distribution of dissolved species. 

The saturation index (SI) is a measure of the 

departure from equilibrium and is expressed as:

 SI = log (IAP/KT), (2)

where 

IAP = the ion activity product of the 

components of the solid or 

gaseous phase, and 

KT = the solid or gaseous phase 

solubility equilibrium product at 

the specified temperature 

(at the time of sampling). 

When SI is equal to 0, the solid or gaseous phase 

is in equilibrium with the groundwater. When 

SI is less than 0, the solid or gaseous phase is 

undersaturated, which means that the phase has 

the thermodynamic potential to dissolve. When 

SI is greater than 0, the solid or gaseous phase is 

supersaturated, and the phase has the potential to 

precipitate or volatilize. 

Most SI values with respect to calcite are 

within the -0.2 to 0.2 range considered to be at 

equilibrium with respect to calcite (fig. 12A). The 

lowest median calcite SI was in the confined area 

in southeastern Georgia where nearly half of the 

samples in this network were undersaturated with 

respect to calcite. Most SI values with respect to 

Only one sample collected for this study had a 

sodium-chloride water type, indicating possible mixing of 

saltwater with groundwater. The sample was from the semiconfined area in South Carolina and showed other evidence of 

saltwater mixing, including elevated concentrations of 

dissolved solids (5,370 mg/L), bicarbonate (567 mg/L), 
Groundwater Geochemistry 17

dolomite are less than -0.2 indicating that most samples are 

undersaturated with respect to dolomite (fig. 12B). The highest median SI with respect to dolomite was for samples in the 

semiconfined area in South Carolina where nearly half the 

samples were at equilibrium with respect to dolomite 

(fig. 12B).

The ratio of calcium to magnesium concentration was 

computed based on molar concentrations in groundwater 

samples to determine the effects of dolomite in aquifer 

sediments on the major-ion chemistry of groundwater. The 

lowest values for the calcium to magnesium molar ratio 

in groundwater were in samples from the network in the 

confined area in southeastern Georgia (median ratio was 1.5) 

(figs. 12C and 13). Median calcium to magnesium molar 

ratios were also low in the samples from semiconfined areas 

in southern South Carolina (median ratio 2.1) and in northern Florida (median ratio 3.3) (fig 12C). Molar ratios of 

calcium to magnesium near 1.0 probably reflect the effects 

of dolomite in the aquifer sediments. In contrast, the median 

calcium-magnesium ratio for samples from the unconfined 

area in southwestern Georgia was about 32, indicating that 

groundwater in that area had predominantly been in contact 

with calcite. Additionally, in this area samples were at 

equilibrium with respect to calcite (fig. 12A) and undersaturated with respect to dolomite (fig. 12B). The presence 

of dolomite in aquifer sediments in the confined area may 

explain why the SI values with respect to calcite for samples 

from this area were lower than in the samples from the other 

areas (fig. 12A).

Figure 12. Boxplots showing distribution of A, calcite saturation index values, B, dolomite saturation index values, and 

C, calcium to magnesium molar ratios by sampling network.

CONFINEMENT OF THE

UPPER FLORIDAN AQUIFER

UNCONFINED

SEMICONFINED–Confining unit

thin or breached in places

CONFINED

NOT MAPPED

MISSISSIPPI

GEORGIA

ALABAMA

SOUTH CAROLINA

FLORIDA

EXPLANATION

CALCIUM TO MAGNESIUM

MOLAR RATIO

LESS THAN 5

5.0 - 10.0

GREATER THAN 10

0 100 200 MILES

0 100 200 KILOMETERS

Figure 13. Spatial distribution of calcium to magnesium molar 

ratios in domestic wells in the Upper Floridan aquifer.

-1

0

1

CALCITE SATURATION INDEX

-3

-2

-5

-4

-3

-2

-1

0

1

DOLOMITE SATURATION INDEX

0

10

20

30

40

50

60

CALCIUM TO MAGNESIUM

MOLAR RATIO

U-GA

U-FL

S-FL

S-SC

C-GA

A B C

EXPLANATION

AT EQUILIBRIUM–With respect to mineral

UNCONFINED SOUTHWESTERN GEORGIA–acfbsus1

UNCONFINED NORTHERN FLORIDA–gaflsus2

SEMICONFINED NORTHERN FLORIDA–gaflsus3

SEMICONFINED SOUTHERN SOUTH CAROLINA–santsus2

CONFINED SOUTHEASTERN GEORGIA–gaflsus4

U-GA U-FL S-FL S-SC C-GA U-GA U-FL S-FL S-SC C-GA U-GA U-FL S-FL S-SC C-GA

90th PERCENTILE

75th PERCENTILE

MEDIAN

25th PERCENTILE

10th PERCENTILE
18 Factors Affecting Water Quality in Domestic Wells in the Upper Floridan Aquifer, Southeastern United States, 1998-2005 

Trace Elements and Radon

Trace element concentrations generally are low in 

samples collected from the domestic wells in the Upper 

Floridan aquifer, and no MCLs were exceeded (tables 7 and 

8). SMCLs were exceeded for 12.8 percent of samples for 

iron and 4 percent of samples for manganese (table 8). For 

19 of the 23 trace elements analyzed, more than 50 percent of 

samples had concentrations less than minimum reporting 

limits of 1 μg/L, and all concentrations were less than 

minimum reporting limits for five trace elements (beryllium, 

cadmium, cobalt, silver, and thallium) (table 7). Minimum 

reporting limits were assigned at 1.0 μg/L, except for aluminum (1.6 μg/L), boron (8.0 μg/L) and iron and strontium 

(10 μg/L). Iron and manganese concentrations were generally low, with iron concentrations ranging from less than 10 

to 3,640 μg/L and manganese concentrations range from less 

than 1 to 107 μg/L (table 7). The maximum iron concentration 

of 3,640 μg/L was found in a sample from the unconfined area 

in northern Florida. The maximum manganese concentration, 

107 μg/L, was measured in a sample from the confined area 

in southeastern Georgia. Concentrations of trace elements do 

not appear related to confinement category, but are probably 

related to spatial differences in lithology in the aquifer 

materials or the overlying materials.

Radon concentrations were highly variable in the Upper 

Floridan aquifer, ranging from below the reporting level 

of 10 picocuries per liter (pCi/L) to about 6,000 pCi/L. 

Table 7. Descriptive statistics of concentrations of trace elements in domestic wells in the Upper Floridan 

aquifer. 

[Concentrations are in micrograms per liter; <, less than. Network: santsus2, semiconfined area in South Carolina; gaflsus3, semiconfined area in northern Florida; gaflsus4, confined area in southeastern Georgia; acfbsus1, unconfined southwestern Georgia; and 

gaflsus2, unconfined area in northern Florida] 

Trace 

element

Number 

of analyses

Minimum 

reporting 

limit

Percent of 

samples less 

than minimum 

reporting limit

Maximum 

concentration

Network with 

maximum 

concentration

Aluminum 149 1.6 68 13.3 santsus2

Antimony 148 1.0 99 1.1 gaflsus3

Arsenic 149 1.0 88 9.9 gaflsus3

Barium 149 1.0 11 452 gaflsus4

Beryllium 148 1.0 100 <1.0 --

Boron 119 8.0 22 69 gaflsus3

Cadmium 148 1.0 100 <1.0 --

Chromium 149 1.0 59 9 gaflsus4

Cobalt 148 1.0 100 <1.0 --

Copper 148 1.0 75 10.6 acfbsus1

Iron 149 10.0 58 3,640 gaflsus2

Lead 148 1.0 90 5.1 gaflsus3

Lithium 119 1.0 50 13.6 santsus2

Manganese 148 1.0 50 107 gaflsus4

Molybdenum 148 1.0 78 18.2 gaflsus2

Nickel 148 1.0 58 9.8 gaflsus2

Selenium 149 1.0 89 10.8 gaflsus2

Silver 133 1.0 100 <1.0 --

Strontium 119 10.0 0 5,050 gaflsus4

Thallium 119 1.0 100 <1.0 --

Uranium 148 1.0 92 16.0 gaflsus3

Vanadium 119 1.0 56 43.3 gaflsus3

Zinc 149 1.0 21 175 gaflsus4

 
Groundwater Geochemistry 19

The median radon value for all samples in the Upper Floridan 

aquifer was 198 pCi/L. About 40 percent of radon values (55 

of 136 samples) were greater than the USEPA proposed MCL 

of 300 pCi/L (U.S. Environmental Protection Agency, 1999) 

(fig. 14). The highest radon values were in the unconfined 

area in northern Florida, where 25 of 30 samples (about 

83 percent) had radon concentrations greater than 300 pCi/L 

and 14 samples (47 percent) had radon greater than 

1,000 pCi/L (fig. 14). In the four other sampling networks, 

4 to 64 percent of samples had radon concentrations greater 

than 300 pCi/L. Radon concentrations in groundwater are 

dependent upon the uranium and radium content of the rocks 

comprising the aquifer materials and the physical characteristics of the aquifer materials (Cecil and Green, 2000). The 

higher radon values in the unconfined area in Florida are 

probably not related to confinement, but are related to spatial 

differences in uranium content in the aquifer materials and 

overlying sands and clays or to differences in physical aquifer 

properties. In a recent study in Tampa, Florida, Katz and 

others (2008) reported that radon concentrations were higher 

in the water samples from wells installed in the overlying clay 

and sand of the surficial aquifer than in samples from wells in 

the underlying Upper Floridan aquifer. 

Table 8. Comparison of concentrations of trace elements in domestic wells in the Upper Floridan aquifer to human-health benchmarks 

and non-health guidelines for drinking water. 

[MCLs and SMCLs are U.S. Environmental Protection Agency (USEPA) Maximum Contaminant Levels (MCLs) and Secondary Maximum Contaminant Levels 

(SMCLs) for public-water supplies; HBSLs are health-based screening levels developed by the U.S. Geological Survey using USEPA toxicity data and methods 

(Toccalino and Norman, 2006); μg/L, microgam per liter; --, not applicable] 

Trace element Benchmark of guideline Samples exceeding benchmarks or non-health guidelines

Human-health benchmark Non-health guideline Human-health benchmark Non-health guideline

Value Type Value Type Value Percent Value Percent

Aluminum -- -- 50 to 200 SMCL -- -- 0 0.0

Antimony 6 MCL -- -- 0 0.0 -- --

Arsenic 10 MCL -- -- 0 0.0 -- --

Barium 2,000 MCL -- -- 0 0.0 -- --

Beryllium 4 MCL -- -- 0 0.0 -- --

Boron 1,000 HBSL -- -- 0 0.0 -- --

Cadmium 5 MCL -- -- 0 0.0 -- --

Chromium 100 MCL -- -- 0 0.0 -- --

Cobalt -- -- -- -- -- -- -- --

Copper1 1,300 Action level 1,000 SMCL 0 0.0 0 0.0

Iron -- -- 300 SMCL 0 0.0 19 12.8

Lead1 15 Action level -- -- 0 0.0 -- --

Lithium -- -- -- -- -- -- -- --

Manganese 300 HBSL 50 SMCL 0 0.0 6 4.1

Molybdenum 40 HBSL -- -- 0 0.0 -- --

Nickel 100 HBSL -- -- 0 0.0 -- --

Selenium 50 MCL -- -- 0 0.0 -- --

Silver 100 HBSL 100 SMCL 0 0.0 0 0.0

Strontium 4,000 HBSL -- -- 3 2.5 -- --

Thallium 2 MCL -- -- 0 0.0 -- --

Uranium 30 MCL -- -- 0 0.0 -- --

Vanadium -- -- -- -- -- -- -- --

Zinc 2,000 HBSL 5,000 SMCL 0 0.0 0 --

1

Lead and copper are regulated by a Treatment Technique that requires systems to control the corrosiveness of their water. If more than 10 percent of tap 

water samples exceed the action level, water systems must take additional steps. The action levels are 1.3 μg/L for copper and 0.015 μg/L for lead 

(U.S. Environmental Protection Agency, 2006).
20 Factors Affecting Water Quality in Domestic Wells in the Upper Floridan Aquifer, Southeastern United States, 1998-2005 

Nitrate

Nitrogen is widely used as a fertilizer for agricultural 

activities and is also present in human and animal waste; 

therefore, the land surface is exposed to nitrogen from various 

sources. Nitrate is a highly soluble form of nitrogen that can 

readily leach into the groundwater and can also be denitrified 

to the form of nitrogen gas. The concentrations of nitrate in 

groundwater are a concern because of the potential humanhealth effects, specifically methemoglobinemia in infants, 

which has led the USEPA to establish the MCL for nitrate at 

10 mg/L as nitrogen (U.S. Environmental Protection Agency, 

2006). The widespread occurrence of nitrate, in combination 

with its potential for human-health effects, makes distribution 

of nitrate concentrations in groundwater an important issue to 

understand.

Occurrence and Distribution of Nitrate

Nitrate concentrations in the Upper Floridan aquifer 

ranged from less than the laboratory reporting level of 

0.06 mg/L to 8 mg/L, with a median nitrate concentration less 

than 0.06 mg/L (as nitrogen). Nitrate concentrations did not 

exceed the MCL for drinking water of 10 mg/L. The median 

nitrate concentration for the samples in this study is similar 

CONFINEMENT OF THE

UPPER FLORIDAN AQUIFER

UNCONFINED

SEMICONFINED–Confining unit

thin or breached in places

CONFINED

NOT MAPPED

MISSISSIPPI

GEORGIA

ALABAMA

SOUTH CAROLINA

FLORIDA

EXPLANATION

RADON CONCENTRATION,

IN PICOCURIES PER LITER

<100

100 - 300

>300 - 1,000

>1,000

0 100 200 MILES

0 100 200 KILOMETERS

CONFINEMENT OF THE

UPPER FLORIDAN AQUIFER

UNCONFINED

SEMICONFINED–Confining unit

thin or breached in places

CONFINED

NOT MAPPED

MISSISSIPPI

GEORGIA

ALABAMA

SOUTH CAROLINA

FLORIDA

EXPLANATION

<0.10

0.10 - 1.0

>1.0

NITRATE CONCENTRATION,

IN MILLIGRAMS PER LITER

0 100 200 MILES

0 100 200 KILOMETERS

Figure 14. Spatial distribution of radon concentrations in domestic 

wells in the Upper Floridan aquifer.

Figure 15. Spatial distribution of nitrate concentration in domestic 

wells in the Upper Floridan aquifer.

Occurrence and Distribution of 

Selected Contaminants 

Samples from 148 domestic wells completed in the Upper 

Floridan aquifer were analyzed for a number of constituents 

that are considered anthropogenic contaminants: nitrate, 

VOCs, and pesticides. Results show that water in this aquifer 

is of good quality and no drinking-water standards or humanhealth guidelines were exceeded for any of the anthropogenic 

contaminants. This section describes the occurrence and 

distribution of these contaminants in groundwater as well as 

an assessment of the relative importance of factors affecting 

the concentrations of nitrate, VOCs, and pesticides in domestic 

wells in the Upper Floridan aquifer. Nitrate concentrations are 

described and compared by sampling network and by confinement and land-use categories. The occurrence and distribution 

of VOCs and pesticides are compared by confinement and 

land-use categories. For comparisons by confinement category, 

the samples from the unconfined area include the 58 samples 

from the unconfined areas in southwestern Georgia and northern Florida and the samples from semiconfined areas include 

the 60 samples from the semiconfined areas in northern 

Florida and southern South Carolina.
Occurrence and Distribution of Selected Contaminants 21

the confined area in southeastern Georgia (fig. 15). Overall, 

about 30 percent of samples had concentrations greater than 

1 mg/L, a level which is indicative of human influence in 

many parts of the United States (Nolan and Hitt, 2003). 

However, the percentage of samples with concentrations 

greater than 1 mg/L varied among the sampling networks, 

with the highest percentages in the two networks in unconfined areas (fig. 16). The percentage of samples with concentrations greater than 1 mg/L ranged from 86 and 40 percent 

for samples in unconfined areas in southwestern Georgia and 

northern Florida, respectively, to 3 percent of samples in the 

semiconfined area in South Carolina (figs. 15 and 16). For 

the semiconfined area in northern Florida, about 17 percent 

of samples had nitrate concentrations greater than 1 mg/L. 

For the confined area in southeastern Georgia, 7 percent of 

samples had nitrate concentrations greater than 1 mg/L and 

nitrate was not detected in 27 samples (90 percent). 

Factors Affecting Nitrate Concentrations

Comparison of nitrate concentrations among the three 

confinement categories shows that nitrate concentrations are 

significantly different (p-value less than 0.001) among the 

three categories (table 9), and that median nitrate concentrations are significantly higher in samples from the unconfined 

category compared to the semiconfined and confined 

categories (p-values less than 0.001) (table 10). The median 

nitrate concentration is 1.46 mg/L for 58 samples from the 

unconfined category, compared to median concentrations less 

ALL

SAMPLES

DETECTION FREQUENCY, IN PERCENT

0

20

40

60

80

100

U-GA

(Unconfined

southwestern

Georgia,

acfbsus1)

U-FL

(Unconfined

northern

Florida,

gaflsus2)

S-FL

(Semiconfined

northern

Florida,

gaflsus3)

S-SC

(Semiconfined

southern

South Carolina,

santsus2)

C-GA

(Confined

southeastern

Georgia,

gaflsus4)

NITRATE CONCENTRATION,

IN MILLIGRAMS PER LITER

<0.10

0.10 - 1.0

>1.0

Figure 16. Percentage of nitrate concentrations at selected 

concentration levels in the entire data set and by 

sampling network.

Table 9. Median nitrate concentrations in domestic wells in the Upper Floridan aquifer and results of the 

Peto-Prentice test to determine differences in concentration among the three confinement categories.

[p-values less than alpha value of 0.05 indicate significant difference; <, less than]

Unconfined Semiconfined Confined p-value

Number of samples 58 60 30

Median nitrate concentration 1.46 <0.06 <0.06 <0.001

Table 10. Results of the Peto-Prentice test to determine differences 

in nitrate concentration in domestic wells in the Upper Floridan aquifer 

between individual confinement categories using pairwise comparisons.

[p-values less than alpha value of 0.017 indicate significant difference; 

NS, correlation not significant at an alpha value of 0.017; <, less than]

Unconfined Semiconfined Confined

p-value p-value

Unconfined <0.001 <0.001

Semiconfined 0.0171 (NS)

Confined

 

to the median nitrate concentration of 0.05 mg/L for samples 

from a previous study that included results from nearly 

800 wells (monitoring, domestic, public supply, and other 

types of wells) in the Upper Floridan aquifer in southern 

Georgia and northern Florida (Berndt, 1996).

Nitrate concentrations varied spatially in the Upper 

Floridan aquifer. Concentrations were highest in the 

unconfined area in southwestern Georgia and were lowest in 
22 Factors Affecting Water Quality in Domestic Wells in the Upper Floridan Aquifer, Southeastern United States, 1998-2005 

than 0.06 mg/L in the semiconfined and confined categories 

(fig. 17A; table 9). The median dissolved oxygen concentration 

is also higher in the unconfined category (fig. 17B). The higher 

nitrate concentrations in the unconfined category indicates the 

greater susceptibility of unconfined areas of aquifers to contamination from land-surface activities, especially in carbonate 

aquifers where the thickness and type of sediments that overlie 

the aquifer (or the lack thereof) affect the development of 

secondary porosity, permeability, recharge rate, and groundwater residence time (Brahana and others, 1988). 

Comparison of nitrate concentrations among the five 

sampling networks shows that nitrate concentrations are 

significantly different (p-value less than 0.001) (table 11), and 

that the median nitrate concentration is highest (2.43 mg/L) 

for samples from the unconfined area in southwestern Georgia. 

Median nitrate concentrations in the other sampling networks 

are 0.54 mg/L in the unconfined area in northern Florida 

and less than 0.1 mg/L for each of the remaining three 

sampling networks in semiconfined and confined areas 

(fig. 18A; table 11). The median dissolved oxygen is also 

highest in the unconfined area in southwestern Georgia 

(fig. 18B). Comparison of nitrate concentrations between the 

two networks in unconfined areas showed that nitrate concentrations are significantly different (table 12), implying that 

factors in addition to confinement are affecting nitrate concentrations in these two unconfined areas. One potential factor is 

fertilizer sources of nitrogen—the median amount of nitrogen 

applied as fertilizer was about twice as high in the network 

in the unconfined area in southwestern Georgia than in the 

unconfined area in northern Florida (fig. 18D; table 3). For 

the two networks in semiconfined areas, nitrate concentrations 

are not significantly different from one another (table 12). 

Because of the significant difference in nitrate concentrations 

between the two unconfined networks, examination of factors 

in addition to confinement, such as well depth, soil hydrologic 

properties, and sources of nitrogen can provide information 

NITRATE, IN

MILLIGRAMS

PER LITER

A

B

0

2

4

6

8

6

4

2

0

UNCONFINED SEMICONFINED CONFINED

DISSOLVED OXYGEN,

IN MILLIGRAMS

PER LITER

UNCONFINED SEMICONFINED CONFINED

58

90th PERCENTILE

75th PERCENTILE

NUMBER OF SAMPLES

MEDIAN

25th PERCENTILE

10th PERCENTILE

EXPLANATION

58

60

30

58

60

30

Figure 17. Concentrations of A, nitrate and 

B, dissolved oxygen concentrations in domestic 

wells in the Upper Floridan aquifer by confinement 

category.

A

0

2

4

8

8

6

4

2

0

U-GA U-FL S-FL S-SC C-GA

U-GA U-FL S-FL S-SC C-GA

0

2,000

4,000

6,000

8,000

0

2

4

8

6

0

2,000

4,000

6,000

0

50

100

150

200

U-FL S-FL S-SC C-GA

U-FL S-FL S-SC C-GA

U-GA

U-GA

NITRATE, IN

MILLIGRAMS PER LITER

DISSOLVED OXYGEN, IN

MILLIGRAMS PER LITER

DISSOLVED ORGANIC CARBON,

IN MILLIGRAMS PER LITER

NITROGEN FROM

FERTILIZER,

IN KILOGRAMS

NITROGEN FROM

ANIMAL MANURE,

IN KILOGRAMS

NITROGEN FROM

SEPTIC TANKS,

IN KILOGRAMS

6

B

C

U-GA

U-FL

S-FL

S-SC

C-GA

EXPLANATION

UNCONFINED SOUTHWESTERN GEORGIA–acfbsus1

UNCONFINED NORTHERN FLORIDA–gaflsus2

SEMICONFINED NORTHERN FLORIDA–gaflsus3

SEMICONFINED SOUTHERN SOUTH CAROLINA–santsus2

CONFINED SOUTHEASTERN GEORGIA–gaflsus4

U-GA U-FL S-FL S-SC C-GA U-GA U-FL S-FL S-SC C-GA

90th PERCENTILE

75th PERCENTILE

MEDIAN

25th PERCENTILE

10th PERCENTILE

D

E

F

8,000

Figure 18. Distribution by sampling network of A, nitrate, B, dissolved oxygen, 

and C, dissolved organic carbon concentrations; and estimated nitrogen 

contributions in the 500-m-radius area surrounding each well from D, fertilizer,

E, animal manure, and F, septic tanks.
Occurrence and Distribution of Selected Contaminants 23

about the relative significance of factors affecting nitrate 

concentration in groundwater. 

Correlations of nitrate concentrations in groundwater 

with selected explanatory variables potentially related to 

sources of nitrate and the transport of nitrate to groundwater show that numerous factors were significantly correlated 

with nitrate concentration when all of the 148 samples were 

included (12 factors), but fewer were significant for individual 

sampling networks (3 or less factors for each) (table 13). 

As reported earlier, because nitrate concentrations were less 

than reporting levels in 90 percent of the 30 samples in the 

network in the confined area, correlation analyses were not 

performed for that sampling network. Correlation analyses 

were performed for the factors given in table 3 along with 

well depth, dissolved oxygen concentration, and organic 

carbon concentration, but only those with a significant 

correlation in one of the categories or networks are given in 

table 13. The large number of factors with significant 

correlations for the entire data set and the few significant 

correlations within sampling networks probably indicates that 

variations in nitrate concentration and some explanatory 

factors were greater between the networks than within 

individual networks. As previously discussed, nitrate 

concentrations were significantly different between some 

sampling networks. Additionally, several of the explanatory 

factors showed a large range in values within sampling 

networks (table 3).

For the entire data set of 148 samples, several 

factors related to land use and sources of nitrogen in the areas 

surrounding wells were significantly correlated with nitrate 

concentrations. Nitrate concentrations were significantly 

correlated with the percentage of agricultural land use 

(correlation coefficient, tau = 0.196), nitrogen from fertilizer 

(tau = 0.177), and nitrogen from animal manure (tau = 0.261) 

(table 14). The significant correlations between nitrate 

concentration and the percentage of agricultural land use 

and the agricultural sources of nitrogen (fertilizer and 

animal manure) indicate that higher nitrate concentrations are 

associated with increasing agricultural activities. In samples 

from the unconfined category, significant correlations are also 

seen between nitrate concentrations and the percentage of 

agricultural land use and agricultural fertilizer sources of 

nitrogen. These two factors are not significantly correlated 

with nitrate concentrations in the samples from the semiconfined category (table 13), implying that nitrate concentrations are more related to agricultural sources in unconfined 

areas than in semiconfined areas. Additional factors that 

are significantly correlated with nitrate concentration in the 

unconfined category include the soil properties of percentage 

clay, silt, and sand (inverse correlation). These significant 

Table 11. Median nitrate concentration in domestic wells in the Upper Floridan aquifer and results of the Peto-Prentice test to 

determine differences in concentration among the five sampling networks. 

[p-value less than alpha value of 0.05 indicate significant difference; <, less than]

 

Unconfined Unconfined Semiconfined Semiconfined Confined p-value

acfbsus1 gaflsus2 gaflsus3 santsus2 gaflsus4

Number of 

samples

28 30 30 30 30

Median nitrate 

concentration

2.43 0.54 0.08 <0.06 <0.06 <0.001

Table 12. Results of the Peto-Prentice test to determine differences in nitrate concentration in domestic wells in the Upper 

Floridan aquifer between individual sampling networks using pairwise comparisons.

[p-value less than alpha value of 0.010 indicate significant difference; NS, Correlation not significant at an alpha value of 0.010; <, less than]

 

Unconfined Unconfined Semiconfined Semiconfined Confined

acfbsus1 gaflsus2 gaflsus3 santsus2 gaflsus4

Unconfined acfbsus1 <0.001 <0.001 <0.001 <0.001

Unconfined gaflsus2 0.0263 (NS) <0.001 <0.001

Semiconfined gaflsus3 0.149 (NS) 0.005

Semiconfined santsus2 0.115 (NS)

Confined gaflsus4
24 Factors Affecting Water Quality in Domestic Wells in the Upper Floridan Aquifer, Southeastern United States, 1998-2005 Table 13. Results of correlation tests between nitrate concentration and selected explanatory variables using Kendall’s tau for censored data. [Correlations could not be performed for the confined network because nitrate was not detected in 90 percent of samples. Tau, Kendall’s tau; level of significance of p-value = 0.05; NS, not significant (95-percent confidence level, p-value less than or equal to 0.05)] All Data Samples from un- confined networks Samples from semiconfined networks Unconfined south- western Georgia acfbsus1 Unconfined northern Florida gaflsus2 Semiconfined, north- ern Florida gaflsus3 Semiconfined, southern Carolina santsus2 Number of samples 148 58 60 28 30 30 30 Explanatory factor Tau p-value Tau p-value Tau p-value Tau p-value Tau p-value Tau p-value Tau p-value Land use Agricultural 0.196 <0.001 0.197 0.029 NS NS NS NS NS Forest -0.116 0.033 NS NS NS NS NS NS Wetland -0.238 <0.001 -0.255 0.005 NS NS -0.361 0.005 NS NS Nitrogen sources Agricultural fertilizer 0.177 0.001 0.367 <0.001 NS 0.280 0.038 NS NS NS Animal manure 0.261 <0.001 NS 0.198 0.010 NS NS NS NS Atmospheric deposition -0.24 0.003 NS -0.418 0.001 NS NS NS NS Septic tanks NS NS NS NS NS NS 0.377 0.009 Population and water use Population density NS NS NS NS NS NS 0.352 0.014 Soil properties Hydrologic Group A 0.169 0.002 NS NS NS NS NS NS Hydrologic Group B 0.199 <0.001 0.290 0.001 NS NS NS NS NS Organic matter -0.249 <0.001 NS NS NS NS NS NS Clay NS 0.252 0.005 NS NS NS NS NS Silt NS 0.356 <0.001 NS NS NS NS NS Sand NS -0.299 <0.001 NS NS NS NS NS Physical or chemical data for water sample Well depth -0.198 <0.001 0.201 0.028 NS NS NS NS NS Dissolved oxygen 0.505 <0.001 0.434 <0.001 0.314 <0.001 NS 0.515 <0.001 0.416 <0.001 0.239 0.007 Dissolved organic carbon -0.331 <0.001 -0.353 <0.001 -0.318 <0.001 NS -0.352 0.006 -0.398 0.001 NS
Occurrence and Distribution of Selected Contaminants 25

correlations were not observed for any of the other sample 

groups (table 13), which may indicate the importance of soil 

properties in unconfined areas of aquifers.

For the entire data set, significant inverse correlations 

are noted between nitrate concentrations and the percentage 

of land in forest and wetland categories as well as nitrogen 

from atmospheric deposition, thus indicating that increasing 

amounts of these factors are associated with lower nitrate 

concentrations. This correlation might be expected between 

nitrate concentration and percentage of land in forest and wetland because these two land-use settings are not expected to be 

sources of nitrogen, but the inverse correlation between nitrate 

concentration and nitrogen from atmospheric deposition is 

problematic. Inverse correlations of nitrate with nitrogen from 

atmospheric deposition were also seen for the 60 samples from 

the semiconfined category. These correlations could be related 

to the very low nitrate concentrations in the groundwater 

samples in the semiconfined area in southern South Carolina 

(median nitrate concentration was less than 0.06 mg/L) such 

that very small differences in nitrate concentration (less 

than 0.1 mg/L) could be related to increasing amounts of 

atmospheric deposition of nitrogen (tables 3 and 13).

For the entire data set, and in all but one network, nitrate 

and dissolved oxygen concentrations are significantly 

correlated. The network in the unconfined area in southwestern Georgia was the only network where the correlation 

between dissolved oxygen and nitrate concentrations was 

not significant. The relation between dissolved oxygen and 

nitrate concentrations for the entire data set may be related to 

the denitrification of nitrate in locations where groundwater 

is anoxic (dissolved oxygen concentration less than 0.5 mg/L; 

McMahon and Chapelle, 2008). Nitrate and dissolved-oxygen 

concentrations were not significantly correlated in the unconfined area in southwestern Georgia because the groundwater 

is oxic (dissolved oxygen concentration greater than or equal 

to 0.5 mg/L). Under oxic conditions, nitrate would not be 

reduced, and nitrate concentrations would likely be affected 

by proximity to recharge containing sources of nitrogen.

For the entire data set, nitrate concentrations were 

inversely correlated with well depth, dissolved organic carbon 

concentrations, and percent of soil organic matter. In contrast 

to the inverse correlation for the entire data set, for the 

58 samples in the unconfined category, nitrate concentration 

was positively correlated to well depth. Nitrate concentrations 

were not correlated with well depth in the semiconfined 

category or in the four sampling networks (table 13). Well 

depth is often used as an indication of relative residence time 

for groundwater, with shorter residence times expected in 

shallow wells compared to deeper wells. Deeper wells imply a 

greater depth to the water table. Increases in nitrate 

concentration with increasing well depth would not be 

expected if higher nitrate concentrations are associated with 

shorter residence times (or more recent recharge) or to 

shallow depths to water. Nitrate concentrations were inversely 

correlated with dissolved organic carbon concentration in the 

samples from the unconfined and semiconfined categories, 

and in the networks in the unconfined area in northern Florida 

and the semiconfined area in northern Florida. Organic carbon 

is necessary for the microbial processes that cause denitrification, so waters with elevated dissolved organic carbon 

concentrations would be less likely to have elevated nitrate 

concentrations. 

Volatile Organic Compounds 

The results of VOC analyses are analyzed with respect 

to the occurrence and distribution of VOCs, water-quality 

benchmarks for human health, and factors affecting VOC 

detections. Appendix 1 lists all the VOCs analyzed for 

this study, and the minimum reporting level for each VOC 

analyzed.

Occurrence and Distribution

Volatile organic compounds were detected in 93 of 

148 samples (about 63 percent) collected from domestic wells 

in the Upper Floridan aquifer. These compounds are 

associated with many products, including plastics, adhesives, 

paints, gasoline, fumigants, refrigerants, and dry-cleaning 

fluids. Fifty-eight samples (about 39 percent) contained 

1 VOC detection per sample, 24 samples (about 16 percent) 

contained 2 VOC detections, 6 samples (about 4 percent) 

contained 3 VOC detections, 3 samples (2 percent) contained 

4 VOC detections, and 2 samples contained 5 VOC detections 

(fig. 19). The percentage of samples with one or more 

VOC detections in each sampling network ranged from 

53 percent for the network in the semiconfined area in northern Florida to 70 percent for the network in confined areas 

Figure 19. Detection frequencies for samples 

with one to five volatile organic compound 

detections per sample.

1 2 3 4 5

0

5

10

15

20

25

30

35

40

DETECTION FREQUENCY, IN PERCENT

NUMBER OF VOLATILE ORGANIC

COMPOUND DETECTIONS PER SAMPLE
26 Factors Affecting Water Quality in Domestic Wells in the Upper Floridan Aquifer, Southeastern United States, 1998-2005 

(59 percent) of 1,2-dichloropropane, a fumigant, were noted 

by Tesoriero and others (2001) in samples collected from an 

unconfined shallow aquifer in Washington State. 

Concentrations of VOCs typically were low with 

94 percent of the detections at concentrations less than 

1.0 μg/L (fig. 22). Concentrations of the three most frequently 

detected VOCs—chloroform, carbon disulfide and 1,2-

dichloropropane—ranged from 0.01 to 1.14 μg/L. The highest 

concentration of any VOC was 1,430 μg/L for tetrahydrofuran 

in a sample from the semiconfined area in South Carolina 

(fig. 22). Tetrahydrofuran is a solvent that is used in the 

preparation of adhesives, lacquers, printing inks, fat oils, and 

unvulcanized rubber (U.S. Department of Labor, 2008), and its 

occurrence at such a high concentration in this sample may be 

related to usage of this compound near the well. There are no 

human-health guidelines established for tetrahydrofuran. 

Comparison to Water-Quality Benchmarks 

for Human Health

Comparison of concentrations of VOCs to human-health 

benchmarks (MCLs and HBSLs) was made to determine the 

potential adverse health effects of these compounds. 

Chloroform concentrations were compared to the MCL for 

in southeastern Georgia (fig. 20). The percentage of samples 

in the Upper Floridan aquifer containing one or more VOCs 

was slightly higher than the findings from national studies 

in which 51 percent of the wells sampled contained one or 

more VOCs at an assessment level of 0.02 g/L (Zogorski and 

others, 2006, p. 16). The finding that one or more VOCs were 

detected in nearly two-thirds of the samples demonstrates the 

vulnerability of this aquifer to low-level VOC contamination.

The most frequently detected VOCs were chloroform 

(29 percent of samples), carbon disulfide (27 percent), and 

1,2-dichloropropane (11 percent) (fig. 21). Tetrahydrofuran, 

acetone, tetrachloroethene (PCE), and dichlorodifluoromethane (CFC-12) were detected in 2 to 4 percent of samples and 

the remaining VOCs were detected in less than 2 percent of 

samples (fig. 21).

Chloroform is a trihalomethane compound and is a 

byproduct of the chlorination process. The source of 

chloroform in groundwater is attributed, in part, to the 

recycling of chlorinated waters to aquifers (Zogorski and 

others, 2006). Other potential sources of chloroform and other 

trihalomethanes to domestic wells are laundry wastewater 

containing bleach and the well disinfection practice of shock 

chlorination. Chloroform was also the most frequently 

detected VOC in groundwater nationwide (Zogorski and 

others, 2006). Carbon disulfide is a solvent used in a number 

of industrial applications (U.S. Environmental Protection 

Agency, 1994a), but it can also occur natrually due to the 

action of microorganisms in marine sediments and 

sediments in marshes (U.S. Environmental Protection 

Agency, 1994b). Chloroform and carbon disulfide were 

both frequently detected in monitoring and public supply 

wells from recent studies in the Tampa Bay area (Metz and 

others, 2006; Katz and others, 2008). Frequent detections 

0 5 10 15 20 25 30

Chloroform

Carbon disulfide

Tetrahydrofuran

Acetone

Toluene

Bromodichloromethane

cis-1,2-Ethylbenzene

Chloromethane

Dichloroethene

Dibromochloromethane

Dichloromethane

1,1-Dichloroethane

1,1-Dichloroethene

1,4-Dichlorobenzene

Styrene

o-Xylene

Ethylmethylketone

m- + p-Xylene

DETECTION FREQUENCY,

IN PERCENT

Tetrachloromethane

Trichloroethene (TCE)

1,2-Dichloropropane

Tetrachloroethene (PCE)

Dichlorodifluoromethane (CFC-12)

Methyl tert-butyl ether (MTBE)

Figure 20. Detection frequencies for samples with volatile 

organic compound detections in each sampling network.

Figure 21. Detection frequencies for volatile organic 

compounds detected in more than 3 percent of samples.

0

20

40

60

80

PERCENT OF SAMPLES WITH

ONE OR MORE VOLATILE ORGANIC

COMPOUNDS DETECTED

U-GA

(Unconfined

southwestern

Georgia,

acfbsus1)

U-FL

(Unconfined

northern

Florida,

gaflsus2)

S-FL

(Semiconfined

northern

Florida,

gaflsus3)

S-SC

(Semiconfined

southern

South Carolina,

santsus2)

C-GA

(Confined

southeastern

Georgia,

gaflsus4)
Occurrence and Distribution of Selected Contaminants 27

total trihalomethanes of 80 μg/L (U.S. Environmental 

Protection Agency, 2006) and no concentrations exceeded this 

level. The maximum concentration of carbon disulfide was 

1.14 μg/L, which was not within an order of magnitude of the 

HBSL of 700 μg/L (Toccalino and others, 2007). No MCLs 

or HBSLs were exceeded for any of the compounds, although 

several compounds had benchmark quotient values greater 

than 0.1 (concentrations within an order of magnitude of the 

0.001 0.01 0.1 1 10 100 1,000 10,000

0.001 0.01 0.1 1 10 100 1,000 10,000

0.001 0.01 0.1 1 10 100 1,000 10,000

CONCENTRATION, IN MICROGRAMS PER LITER BENCHMARK QUOTIENT VALUE

"

UNCONFINED SOUTHWESTERN

GEORGIA–acfbsus1

U

gaflsus2

SEMICONFINED NORTHERN

FLORIDA–gaflsus3

SEMICONFINED SOUTHERN

SOUTH CAROLINA–santsus2

CONFINED SOUTHEASTERN

GEORGIA–gaflsus4

NCONFINED NORTHERN

FLORIDA–

BENCHMARK QUOTIENT VALUE IS THE RATIO OF THE DETECTED

CONCENTRATION OF A PARTICULAR COMPOUND TO THE

MAXIMUM CONTAMINANT LEVEL (MCL) (regulated compounds) or

HEALTH-BASED SCREENING LEVEL (HBSL) (unregulated compounds).

COMPOUND NAMES IN BOLD

ARE PESTICIDES

Chloroform

Carbon disulfide

1,2-dichloropropane

Tetrahydrofuran

Tetrachloroethene (PCE)

Atrazine

Deethylatrazine

Metolachlor

Alachlor

Simazine

Tebuthiuron

Chloroform

Carbon disulfide

1,2-dichloropropane

Tetrahydrofuran

Tetrachloroethene (PCE)

Atrazine

Deethylatrazine

Metolachlor

Alachlor

Simazine

Tebuthiuron

Chloroform

Carbon disulfide

1,2-dichloropropane

Tetrahydrofuran

Tetrachloroethene (PCE)

Atrazine

Deethylatrazine

Metolachlor

Alachlor

Simazine

Tebuthiuron

0.000001 0.00001 0.0001 0.001 0.01 0.1 1 10

0.000001 0.00001 0.0001 0.001 0.01 0.1 1 10

0.000001 0.00001 0.0001 0.001 0.01 0.1 1 10

EXPLANATION

BENCHMARK QUOTIENT VALUE OF 1.0–Values

greater than this line indicates concentrations

of potential human-health concern

BENCHMARK QUOTIENT VALUE OF 0.1–Values

greater than this line are within an order of

magnitude of a MCL or HBSL

Figure 22. Concentrations and benchmark quotient values for volatile organic compounds and pesticides 

detected in 3 percent or more of samples.

MCL or HBSL) (fig. 22), including chloroform and 1,2-dichloropropane. These concentrations within an order of magnitude 

of MCLs or HBSLs may be considered to be “of potential 

human-health concern” in drinking water. Information about 

detection of VOCs at these concentrations could be valuable 

with respect to future monitoring plans and groundwater 

protection policies (Zogorski and others, 2006). 
28 Factors Affecting Water Quality in Domestic Wells in the Upper Floridan Aquifer, Southeastern United States, 1998-2005 

Factors Affecting Volatile Organic 

Compound Detections

The frequency of VOC detections by confinement 

category differed among VOCs. Chloroform was detected 

most frequently in samples from the unconfined category 

and was detected in nearly 50 percent of samples from this 

category (fig. 23A). In contrast, carbon disulfide was detected 

most frequently in the confined category and was detected in 

more than 60 percent of samples from this category 

(fig. 23A). Chloroform is associated with the presence of 

dissolved oxygen in groundwater, and dissolved oxygen 

concentrations were mostly greater than 0.5 mg/L in unconfined areas (fig. 17B). Carbon disulfide has been more 

frequently detected in the anoxic groundwater of the Upper 

Floridan aquifer (concentrations of dissolved oxygen less than 

0.5 mg/L) in an urban area in Tampa, Florida, than in oxic 

waters of overlying aquifers (Katz and others, 2008). The 

detections of 1,2-dichloropropane only in samples from the 

unconfined and semiconfined categories is similar to results 

from Tesoriero and others (2001) where 1,2-dichloropropane 

was detected in 59 percent of samples collected from an 

unconfined aquifer. In this study, groundwater in the Upper 

Floridan aquifer from oxic zones (dissolved oxygen greater 

than 0.5 mg/L) had a substantially higher detection frequency 

of 1,2-dichloropropane than samples from low-oxygen 

zones. The most frequently detected VOCs (chloroform, 

carbon disulfide, and 1,2-dichloropropane) were detected in 

20 percent or more of samples in two or more land-use 

categories (fig. 23B), indicating the widespread production 

or usage of these compounds in several land-use settings. 

Although carbon disulfide is a solvent, it also can occur 

naturally, which may explain its occurrence in samples from 

confined areas of the Upper Floridan aquifer.

Statistical comparisons were made to assess the effect 

of selected factors on the occurrence of chloroform and 

carbon disulfide in the Upper Floridan aquifer. The Wilcoxon 

rank-sum test was used to compare the medians for selected 

chemical factors (dissolved oxygen, nitrate and dissolved 

organic carbon) and anthropogenic factors (land-use and 

population density) between samples that contained 

detections of chloroform and carbon disulfide and samples 

without detections of either compound. For chloroform, 

results of the Wilcoxon rank-sum test indicated that population 

density and urban land use for the 500-m-radius area 

surrounding the well site, as well as dissolved oxygen, 

dissolved organic carbon and nitrate concentrations in 

groundwater from samples with chloroform detections 

(43 samples) were significantly different from samples without 

chloroform detections (105 samples). High median values 

were noted in the samples with chloroform detections for 

population density, percent urban land use, and concentrations of dissolved oxygen and nitrate (table 14). The median 

concentration of dissolved organic carbon was slightly higher 

in samples without chloroform detected (0.44 mg/L) than in 

samples with chloroform detections (0.33 mg/L). Chloroform 

was detected more frequently in samples from the unconfined 

category (46 percent of samples) than in the semiconfined 

(20 percent of samples) or confined categories (10 percent of 

samples) (fig.23A). These results suggest that groundwater 

in oxic unconfined aquifers within densely populated urban 

areas could be more susceptible to chloroform occurrence than 

groundwater in other areas.

CHLOROFORM

CARBON DISULFIDE

1,2-DICHLOROPROPANE

CHLOROFORM

CARBON DISULFIDE

1,2-DICHLOROPROPANE

DETECTION FREQUENCY, IN PERCENT

CONFINED

(30 sites)

SEMICONFINED

(60 sites)

UNCONFINED

(58 sites)

0

10

20

30

40

50

60

70

0

10

20

30

40

50

60

70

AGRICULTURE

(43 sites)

URBAN

(14 sites)

MIXED

LAND USE

(54 sites)

UNDEVELOPED

(37 sites)

A

B

DETECTION FREQUENCY, IN PERCENT

Figure 23. Detection frequencies for chloroform, carbon 

disulfide, and 1,2-dichloropropane by A, confinement 

category and B, land-use category.

Figure 24. Detection frequencies for samples with 

one to five pesticide detections per sample.

NUMBER OF PESTICIDE

DETECTIONS PER SAMPLE

0

10

20

30

40

1 2 3 4 5

DETECTION FREQUENCY, IN PERCENT
Occurrence and Distribution of Selected Contaminants 29

For carbon disulfide, results of the Wilcoxon rank-sum 

test indicated that population density in the 500-m-radius area 

surrounding the well, and dissolved oxygen and dissolved 

organic carbon concentrations in groundwater, were 

significantly different between the samples with carbon disulfide detections (40 samples) and those without carbon disulfide 

detections (108 samples) (table 15). In contrast to chloroform, 

the population density in the 500-m-radius area and dissolved 

oxygen concentrations in groundwater were lower for samples 

with carbon disulfide detections than for those samples without carbon disulfide detections. The median dissolved oxygen 

concentration for samples with carbon disulfide detections 

was 0.1 mg/L, indicating waters are mostly anoxic (oxygen 

less than 0.5 mg/L). The median dissolved organic carbon 

concentration was higher (1.05 mg/L) in samples with carbon 

disulfide detections compared to samples without detections 

(0.33 mg/L). Additionally, carbon disulfide was detected less 

frequently in samples from the unconfined category (7 percent 

of samples) than in samples from semiconfined category (28 

percent of samples) or confined areas (63 percent of samples) 

(fig.23A). These results suggest that groundwater in anoxic 

confined aquifers with elevated dissolved organic carbon 

could be more susceptible to carbon disulfide occurrence than 

groundwater in other areas.

Pesticides

The results of pesticide analyses are evaluated with 

respect to the occurrence and distribution of pesticides, waterquality benchmarks for human health, and factors affecting 

pesticide detections. Appendix 2 lists all the pesticides and the 

minimum reporting level for each pesticide analyzed for this 

study.

Occurrence and Distribution

Pesticides were detected in 31 of the 148 samples (about 

21 percent). Thirteen samples (about 9 percent of samples) 

contained 1 pesticide detection per sample, 10 samples (about 

7 percent) contained 2 pesticide detections, 3 samples (about 

2 percent) contained 3 pesticide detections, 2 samples (about 

1 percent) contained 4 pesticide detections, and 3 samples 

(about 2 percent) contained 5 pesticide detections (fig. 24). For 

Table 15. Results of the Wilcoxon rank-sum test comparing median values for selected chemical and anthropogenic factors between 

samples with carbon disulfide detections and samples without carbon disulfide detections.

[mg/L, milligrams per liter; p-values less than the alpha value of 0.05 indicate significant difference and are shown in bold; NS, correlation not significant at an 

alpha value of 0.05 ] 

 

Carbon disulfide detected Carbon disulfide not 

detected

40 samples

27.0 percent of samples

108 samples

73.0 percent of samples

Variable Median Median p-value

1990 population density, people per square kilometer 9.72 16.22 0.0224

Urban land use, percent 0.11 1.09 0.0908 (NS)

Dissolved oxygen, mg/L 0.10 2.10 <.0001

Dissolved organic carbon, mg/L 1.05 0.33 0.0012

Table 14. Results of the Wilcoxon rank-sum test comparing median values for selected chemical and anthropogenic factors between 

samples with chloroform detections and samples without chloroform detections.

[mg/L, milligrams per liter; p-values less than the alpha value of 0.05 indicate significant difference and are shown in bold; NS, difference not significant at an 

alpha value of 0.05 ] 

Chloroform detected Chloroform not detected

43 samples

29.1 percent of samples

105 samples

70.9 percent of samples

Variable Median Median p-value

1990 population density, people per square kilometer 18.81 11.64 0.1360 (NS)

Urban land use, percent 3.54 0.34 0.0295

Dissolved oxygen, mg/L 4.29 0.27 <.0001

Dissolved organic carbon, mg/L 0.33 0.44 0.0593 (NS)

Nitrate, mg/L 1.08 < 0.06 <.0001
30 Factors Affecting Water Quality in Domestic Wells in the Upper Floridan Aquifer, Southeastern United States, 1998-2005 

the unconfined area in southwestern Georgia, about 

69 percent of samples contained pesticide detections, 

compared to less than 20 percent of samples in the other 

four sampling networks (fig. 25).

 The most frequently detected pesticides were the 

herbicides atrazine, detected in about 10 percent of samples, 

deethylatrazine (a degradate of atrazine) detected in about 

9 percent of samples, metolachlor detected in 7 percent of 

samples, and alachlor detected in about 5 percent of samples 

(fig. 26). A degradate is a compound formed by the trans-formation of a pesticide by chemical, photochemical, or biological reactions. Simazine was detected in about 4 percent of 

samples and tebuthiuron was detected in about 3 percent 

of samples. The remaining pesticides were detected in 

1 percent or less of the samples. Atrazine, deethylatrazine, 

and metolachlor were also the pesticides most frequently 

detected in groundwater from NAWQA studies across the 

Nation, and their frequency of occurrence is attributed to their 

high application rates and their mobility and persistence in the 

hydrologic system (Gilliom and others, 2006).

Comparison to Water-Quality Benchmarks 

for Human Health

Concentrations of pesticides typically were low with 

nearly 99 percent of the detections at concentrations less than 

0.6 μg/L. Maximum concentrations for the most frequently 

detected pesticides (all herbicides) were 0.02 μg/L for atrazine, 

0.03 μg/L for deethylatrazine, 0.03 μg/L for metolachlor, and 

0.52 μg/L for alachlor (fig. 22). The maximum concentration 

of any pesticide was 2.09 μg/L for tebuthiuron, in a sample 

from the unconfined area in southwestern Georgia (fig. 22). 

This concentration is several orders of magnitude lower than 

the HBSL of 1,000 μg/L (Toccalino and others, 2007). Comparison of pesticide concentrations to human-health benchmarks (MCLs and HBSLs) showed that no pesticides were 

detected at concentrations greater than the MCL or HBSL 

although one pesticide, alachlor, had a benchmark quotient 

value greater than 0.1 (within 10 percent of the benchmark) in 

a sample from the unconfined area in southwestern Georgia.

Factors Affecting Pesticide Detections

The herbicides atrazine, deethylatrazine, metolachlor, 

and alachlor were most frequently detected in samples from 

the unconfined category and the agriculture land-use category 

(fig. 27A-B). These herbicides were detected in about 12 to 

25 percent of samples from the unconfined category, compared to less than 5 percent in the semiconfined and confined 

categories. Because many of the sites in unconfined areas are 

also designated in the agriculture category, detection rates for 

these herbicides were also greatest in the agriculture category. 

Detection rates for atrazine, deethylatrazine, metolachlor, and 

alachlor ranged from about 10 to 21 percent for agriculture 

category (fig. 27B). In the urban category, detection rates for 

atrazine and deethylatrazine were about 7 percent, reflecting 

the use of atrazine for weed control in residential and urban 

settings. In the undeveloped land-use category, atrazine was 

detected in 8 percent of samples and deethylatrazine and 

metolachlor were detected in 5 percent of samples.

Statistical comparisons were made to assess the effect 

of selected factors on the occurrence of atrazine in the Upper 

Floridan aquifer. The Wilcoxon rank-sum test was used to 

compare the medians for selected chemical factors (dissolved 

oxygen, nitrate and dissolved organic carbon) and anthropogenic factors (land-use, nitrogen sources, and pesticide 

applications) between samples that contained detections 

of atrazine and samples without atrazine detections. Figure 26. Detection frequencies for pesticides detected in 

1 percent or more of samples.

Figure 25. Detection frequencies for samples with pesticide 

detections in each sampling network.

0

20

40

60

80

SAMPLES WITH ONE OR MORE

PESTICIDES DETECTED, IN PERCENT

U-GA

(Unconfined

southwestern

Georgia,

acfbsus1)

U-FL

(Unconfined

northern

Florida,

gaflsus2)

S-FL

(Semiconfined

northern

Florida,

gaflsus3)

S-SC

(Semiconfined

southern

South Carolina,

santsus2)

C-GA

(Confined

southeastern

Georgia,

gaflsus4)

0 2 4 6 8 10 12

Atrazine

Deethylatrazine

Metolachlor

Alachlor

Tebuthiuron

Simazine

2,6-Diethylaniline

Chlorpyrifos

Diazinon

Prometon

Napropamide

Trifluralin

DETECTION FREQUENCY, IN PERCENT
Occurrence and Distribution of Selected Contaminants 31

Because atrazine detections only occurred in the unconfined 

category (fig. 27A), comparisons were only made for samples 

from this category. About 87 percent of the 15 samples with 

atrazine detections also had deethylatrazine detections, and 

40 percent had metolachlor detections (table 16). Statistical 

results indicated that samples with atrazine detections had 

significantly higher nitrate concentrations, dissolved oxygen 

concentrations, and nitrogen fertilizer loads and significantly 

lower dissolved organic carbon concentration than samples 

without atrazine detections. The samples with atrazine 

detected also had significantly higher amounts of atrazine 

application rates within the 500-m-radius area surrounding 

the well than samples without atrazine detections (table 16). 

For the 15 samples with atrazine detections, the median 

application rate of atrazine within the 500-m-radius area was 

2.5 kg/yr compared to atrazine application rates of about 

0.6 kg/yr for the samples without atrazine detections 

(table 16). The atrazine application rate is derived from the 

pesticide use per county and apportioned based on the 

percentage of land area in row crops, small-grain crops, and 

fallow lands (Nakagaki and Wolock, 2005). These results 

indicate that atrazine detections in the Upper Floridan aquifer 

are associated with high atrazine applications, shallow wells, 

and oxic groundwater (dissolved oxygen greater than 

0.5 mg/L), conditions that were observed in the unconfined 

area in southwestern Georgia.

Table 16. Results of the Wilcoxon rank-sum test comparing median values for selected chemical and anthropogenic 

factors between samples with atrazine detections and samples without atrazine detections.

[mg/L, milligrams per liter; p-values less than the alpha value of 0.05 indicate significant difference and are shown in bold; 

NS, correlation not significant at an alpha value of 0.05]

 

Atrazine detected

15 samples

Atrazine not detected

43 samples

Deethylatrazine detection rate, in percent 86.7 0.0

Metolachlor detection rate, in percent 40.0 9.3

Variable Median Median p-value

Atrazine, kilograms per year 2.5 0.6 0.0112

Nitrogen load from agriculture,

average 1987-2001, kilograms

2671 937 0.0025

Agricultural land, percent 64 53 0.2867 (NS)

Well depth, feet below land surface 140.0 80.5 0.0081

Nitrate, mg/L 2.46 1.15 0.0013

Dissolved oxygen, mg/L 5.90 2.80 0.0021

Dissolved organic carbon, mg/L 0.24 0.33 0.0029

DETECTION FREQUENCY, IN PERCENT DETECTION FREQUENCY, IN PERCENT

0

5

10

15

20

25

30

0

5

10

15

20

25

30

ATRAZINE

DEETHYLATRAZINE

METOLACHLOR

ALACHLOR

CONFINED

(30 sites)

SEMICONFINED

(60 sites)

UNCONFINED

(58 sites)

AGRICULTURE

(43 sites)

URBAN

(14 sites)

MIXED

LAND USE

(54 sites)

UNDEVELOPED

(37 sites)

A

B

Figure 27. Detection frequencies for atrazine, deethylatrazine, 

metolachlor, and alachlor by A, confinement category and 

B, land-use category.
32 Factors Affecting Water Quality in Domestic Wells in the Upper Floridan Aquifer, Southeastern United States, 1998-2005 

Summary

Water-quality analyses of samples from 148 domestic 

wells completed in the Upper Floridan aquifer indicated that 

the water in this aquifer is of good quality. Maximum 

contaminant levels (MCLs) for drinking water were only 

exceeded for fluoride, where 2 samples (about 1 percent of 

samples) had concentrations greater than the MCL of 4 mg/L 

(milligrams per liter). The Upper Floridan aquifer is used as 

drinking water for about 10 million people, mostly in Florida 

and Georgia. Results of the inorganic geochemistry analyses 

indicate that the dissolution of calcite and dolomite controls 

the major ion chemistry. Cation exchange of major ions in 

water with underlying sediments was evident in South 

Carolina. Effects of aquifer confinement on groundwater 

geochemistry are seen in the higher concentrations of 

dissolved oxygen in groundwater in unconfined areas 

compared to semiconfined or confined areas. Median concentrations of dissolved oxygen were highest in samples from 

unconfined areas in southwestern Georgia (5.7 mg/L) and 

northern Florida (2.2 mg/L). Median concentrations were 

less than 1 mg/L for samples from the three networks in 

semiconfined or confined areas.

Results of trace element analyses indicated that most 

concentrations are low or not detected at a reporting level of 

1.0 μg/L (micrograms per liter), except for strontium and a 

few samples (13 and 3 percent) that had iron or manganese 

concentrations greater than the Secondary Maximum 

Contaminant Levels (SMCLs) of 300 and 50 μg/L, respectively. Radon concentrations were variable and ranged from 

10 to 6,000 pCi/L (picocuries per liter), and about 40 percent 

of samples had concentrations greater than the proposed MCL 

of 300 pCi/L. The variable concentrations did not appear 

related to aquifer confinement and probably reflect spatial 

differences in lithology of the aquifer materials or overlying 

materials.

The effects of confinement and land use were evident in 

the occurrence and distribution of nitrate in the Upper Floridan 

aquifer. Overall, nitrate concentrations were low (median less 

than 0.06 mg/L), but were significantly higher in one sampling 

network in the unconfined area in southwestern Georgia than 

in the other four sampling networks, including samples from 

a sampling network in an unconfined area in northern Florida. 

Although the percentage of agricultural land surrounding wells 

was similar in the two unconfined networks, the estimated 

amount of nitrogen applied as fertilizer was twice as high in 

the network in the unconfined area in southwestern Georgia 

than in the network in the unconfined area in northern Florida. 

This finding reflects the greater intensity of agricultural 

practices (related to crop types and their associated fertilizer 

requirements) in the unconfined area in southwestern Georgia 

relative to the unconfined area in northern Florida. Dissolved 

oxygen concentrations were mostly greater than 0.5 mg/L in 

these unconfined areas and were not correlated with nitrate 

concentrations; specifically, higher dissolved oxygen 

concentrations were not associated with higher nitrate 

concentrations.

The occurrence and distribution of volatile organic 

compounds (VOCs) and pesticides also were related to the 

effects of confinement and land use. Overall, most 

concentrations of both groups of compounds were low (less 

than 1 μg/L) and no drinking-water standards were exceeded. 

More VOCs were detected (24 compounds) than pesticides 

(12 compounds); VOCs were detected in more samples 

(55 percent) than pesticides (21 percent); and concentrations 

of most VOCs were about an order of magnitude higher than 

most pesticide concentrations. Concentrations of the three 

most frequently detected VOCs—chloroform, carbon disulfide 

and 1,2-dichloropropane—ranged from 0.01 to 1.14 μg/L. 

Chloroform, a byproduct of chlorination, was detected in 

29 percent of samples and was most frequently detected 

in unconfined areas and in urban areas, which is similar to 

nationwide NAWQA results. In contrast, carbon disulfide, 

a solvent that also has natural sources, was detected in 

27 percent of samples and was most frequently detected in 

confined areas.

The most frequently detected pesticides were the 

herbicides atrazine, detected in about 10 percent of samples, 

deethylatrazine (a degradate of atrazine) detected in about 

9 percent of samples, metolachlor detected in 7 percent of 

samples, and alachlor detected in about 5 percent of samples. 

Maximum pesticide concentrations for the most frequently 

detected pesticides (all herbicides) were 0.02 μg/L for 

atrazine, 0.03 μg/L for deethylatrazine, 0.03 μg/L for metolachlor, and 0.52 μg/L for alachlor. Atrazine and alachlor were 

only detected in unconfined areas and atrazine, deethylatrazine, metolachlor, and alachlor were most frequently detected 

in agricultural settings where the estimated atrazine 

application rate near well sites was greatest. Results of 

nonparametric statistics showed that, among samples from 

unconfined areas, samples with atrazine detections had 

significantly higher nitrate, dissolved oxygen, and applications of nitrogen fertilizer and atrazine than those without 

detections.
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Appendixes 35

Appendixes

Appendixes
36 Factors Affecting Water Quality in Domestic Wells in the Upper Floridan Aquifer, Southeastern United States, 1998-2005 
Appendixes 37

Analyte Minimum 

reporting limit 

(μg/L)

Carbon disulfide 0.075

Chlorobenzene 0.028

Chloroethane 0.12

Chloroform 0.024

Chloromethane 0.25

cis-1,2-Dichloroethylene 0.038

cis-1,3-Dichloropropene 0.09

Dibromochloromethane 0.18

Dibromomethane 0.05

Dichlorodifluoromethane 0.27

Dichloromethane 0.16

Diethyl ether 0.17

Diisopropyl ether 0.1

Ethyl methacrylate 0.18

Ethyl tert-butyl ether 0.054

Ethylbenzene 0.03

Hexachlorobutadiene 0.14

Hexachloroethane 0.19

Isopropylbenzene 0.032

m- and p-Xylene 0.06

Methyl acrylate 1.4

Methyl acrylonitrile 0.57

Methyl iodide 0.12

Methyl methacrylate 0.35

Naphthalene 0.25

n-Propylbenzene 0.042

o-Ethyl toluene 0.06

o-Xylene 0.038

sec-Butylbenzene 0.032

Styrene 0.042

tert-Butyl methyl ether (MTBE) 0.17

tert-Butylbenzene 0.06

tert-Pentyl methyl ether 0.11

Tetrachloroethene (PCE) 0.1

Tetrachloromethane 0.06

Tetrahydrofuran 2.2

Toluene 0.05

trans-1,2-Dichloroethylene 0.032

trans-1,3-Dichloropropene 0.09

trans-1,4-Dichloro-2-butene 0.7

Trichloroethene (TCE) 0.038

Trichlorofluoromethane 0.09

Vinyl chloride 0.11

Analyte Minimum 

reporting limit 

(μg/L)

1,1,1,2-Tetrachloroethane 0.03

1,1,1-Trichloroethane 0.032

1,1,2,2-Tetrachloroethane 0.09

1,1,2-Trichloroethane 0.064

1,1,2-Trichlorotrifluoroethane 0.06

1,1-Dichloroethane 0.035

1,1-Dichloroethylene 0.044

1,1-Dichloropropene 0.026

1,2,3,4-Tetramethylbenzene 0.23

1,2,3,5-Tetramethylbenzene 0.2

1,2,3-Trichlorobenzene 0.27

1,2,3-Trichloropropane 0.16

1,2,3-Trimethylbenzene 0.12

1,2,4-Trichlorobenzene 0.19

1,2,4-Trimethylbenzene 0.056

1,2-Dibromo-3-chloropropane 0.21

1,2-Dibromoethane 0.036

1,2-Dichlorobenzene 0.048

1,2-Dichloroethane 0.13

1,2-Dichloropropane 0.029

1,3,5-Trimethylbenzene 0.044

1,3-Dichlorobenzene 0.03

1,3-Dichloropropane 0.12

1,4-Dichlorobenzene 0.05

2,2-Dichloropropane 0.05

2-Butanone 1.6

2-Chlorotoluene 0.026

2-Hexanone 0.7

3-Chloropropene 0.07

4-Chlorotoluene 0.056

4-Isopropyl-1-methylbenzene 0.07

4-Methyl-2-pentanone 0.37

Acetone 7.1

Acrylonitrile 1.2

Benzene 0.021

Bromobenzene 0.036

Bromochloromethane 0.044

Bromodichloromethane 0.048

Bromoethene 0.1

Bromoform 0.06

Bromomethane 0.26

Butylbenzene 0.19

Appendix 1. Volatile organic compounds analyzed in water samples.

[Compounds in bold were detected in this study; μg/L, micrograms per liter]
38 Factors Affecting Water Quality in Domestic Wells in the Upper Floridan Aquifer, Southeastern United States, 1998-2005 

Analyte Minimum reporting 

level, micrograms 

per liter

Pesticides analyzed in all 148 samples, 1998-2005

2,6-Diethylaniline 0.006

Acetochlor 0.006

Alachlor 0.005

Atrazine 0.007

Azinphos-methyl 0.05

Benfluralin 0.01

Carbaryl 0.041

Carbofuran 0.02

Chlorpyrifos 0.005

cis-Permethrin 0.006

Cyanazine 0.018

Dacthal 0.003

Deethylatrazine 0.014

Desulfinylfipronil 0.012

Desulfinylfipronil amide 0.029

Diazinon 0.005

Dieldrin 0.009

Disulfoton 0.021

EPTC (S-Ethyl dipropylthiocarbamate) 0.004

Ethoprophos 0.012

Fipronil 0.016

Fipronil sulfide 0.013

Fipronil sulfone 0.024

Fonofos 0.0053

Malathion 0.027

Metolachlor 0.006

Metribuzin 0.028

Molinate 0.003

Parathion-methyl 0.015

Pendimethalin 0.022

Phorate 0.055

Prometon 0.01

Propanil 0.011

Propargite 0.023

Propyzamide 0.004

Simazine 0.005

Tebuthiuron 0.016

Terbufos 0.017

Thiobencarb 0.01

Trifluralin 0.009

 

Analyte Minimum reporting 

level, micrograms 

per liter

Pesticides analyzed in 118 samples from unconfined and 

semiconfined areas, 1998-2002 

alpha-HCH 0.005

Butylate 0.004

Ethalfluralin 0.009

Lindane 0.004

Linuron 0.035

Napropamide 0.007

p,p'-DDE 0.003

Parathion 0.01

Pebulate 0.004

Propachlor 0.01

Terbacil 0.034

Tri-allate 0.006

Pesticides analyzed in 30 samples from confined areas, 2005 

1-Naphthol 0.0882

2-Chloro-2,6-diethylacetanilide 0.0065

2-Ethyl-6-methylaniline 0.01

3,4-Dichloroaniline 0.0045

3,5-Dichloroaniline 0.012

4-Chloro-2-methylphenol 0.005

alpha-Endosulfan 0.011

Azinphos-methyl-oxon 0.042

Chlorpyrofos, oxygen analog 0.0562

cis-Propiconazole 0.013

Cyfluthrin 0.053

Cypermethrin 0.046

Diazinon, oxygen analog 0.006

Dichlorvos 0.013

Dicrotophos 0.0843

Dimethoate 0.0061

Disulfoton sulfone 0.014

Endosulfan sulfate 0.022

Ethion 0.016

Ethion monoxon 0.021

Fenamiphos 0.029

Fenamiphos sulfone 0.053

Fenamiphos sulfoxide 0.04

Hexazinone 0.026

Iprodione 0.026

Appendix 2. Pesticides analyzed in water samples.

[Compounds in bold were detected in this study; μg/L, micrograms per liter]
Appendixes 39

Analyte Minimum reporting 

level, micrograms 

per liter

Isofenphos 0.011

lambda-Cyhalothrin 0.014

Malaoxon 0.039

Metalaxyl 0.0069

Methidathion 0.0087

Myclobutanil 0.033

Oxyfluorfen 0.017

Paraoxon-methyl 0.019

Phorate oxygen analog 0.027

Phosmet 0.0079

Phosmet oxon 0.0511

Prometryn 0.0059

Tebuconazole 0.0136

Tefluthrin 0.0033

Terbufos oxygen analog sulfone 0.045

Terbuthylazine 0.0083

trans-Propiconazole 0.034

Tribufos 0.035

 

Appendixes

Appendix 2. Pesticides analyzed in water samples.—Continued

[Compounds in bold were detected in this study; μg/L, micrograms per liter]