Upper Floridan

Aquifer

Ground-Water

Flow Direction 

Sinkhole Undifferentiated Sand and Clays Confining Unit

River

Dairy Farm

Cropland

Poultry

Farm

Spring

Spring Run

U.S. GEOLOGICAL SURVEY

Water-Resources Investigations Report 99-4252

Prepared in cooperation with the

SUWANNEE RIVER WATER MANAGEMENT DISTRICT

Sources and Chronology of

Nitrate Contamination in Spring Waters,

Suwannee River Basin,

Florida
U.S. GEOLOGICAL SURVEY

Water-Resources Investigations Report 99-4252

Sources and Chronology of Nitrate

Contamination in Spring Waters,

Suwannee River Basin, Florida

Prepared in cooperation with the

SUWANNEE RIVER WATER MANAGEMENT DISTRICT

By Brian G. Katz, H. David Hornsby, Johnkarl F. Bohlke, and Michael F. Mokray

Tallahassee, Florida

1999
U.S. DEPARTMENT OF THE INTERIOR

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY

Charles G. Groat, Director

Copies of this report can be 

purchased from:

U.S. Geological Survey

Branch of Information Services

Box 25286

Denver, CO 80225-0286

888-ASK-USGS

The use of firm, trade, and brand names in this report is for identification purposes only and 

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

For additional information

write to:

District Chief

U.S. Geological Survey

Suite 3015

227 N. Bronough Street

Tallahassee, FL 32301

Additional information about water resources in Florida is available on the World 

Wide Web at http://fl-water.usgs.gov
Contents III

CONTENTS

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

Introduction ........................................................................................................................................................................... 2

Purpose and Scope....................................................................................................................................................... 4

Description of Study Area ........................................................................................................................................... 5

Acknowledgments ....................................................................................................................................................... 8

Study Methods....................................................................................................................................................................... 8

Sample Collection and Analysis.................................................................................................................................. 8

Isotopic Tracers ................................................................................................................................................. 9

Age Dating Waters............................................................................................................................................. 9

Estimation of Age of Spring Waters and Average Residence Time of Ground Water ...................................................10

Estimation of Nitrogen Inputs from Nonpoint Sources............................................................................................... 12

Fertilizers........................................................................................................................................................... 12

Animal Wastes................................................................................................................................................... 13

Septic Tank Effluent .......................................................................................................................................... 13

Atmospheric Deposition.................................................................................................................................... 13

Chemistry of Spring Waters and Water from Wells............................................................................................................... 14

Major Ions and Nutrients............................................................................................................................................. 14

Dissolved Gases........................................................................................................................................................... 16

Sources of Spring Water and Water from the Upper Floridan Aquifer ................................................................................. 18

Sources of Nitrate in Spring Water and Water from the Upper Floridan Aquifer ................................................................. 20

Ages and Residence Times of Spring Water and Water from the Upper Floridan Aquifer................................................... 22

Chlorofluorocarbons.................................................................................................................................................... 22

Tritium ......................................................................................................................................................................... 31

Comparative Estimates of Spring-water Ages............................................................................................................. 31

Relation Between Nitrate, Dissolved Oxygen Concentrations, and Age of Spring Water.................................................... 33

Chronology of Nitrate Contamination of Ground Water....................................................................................................... 38

Nitrogen Inputs Associated with Land-use Changes, 1950-97 ................................................................................... 38

Relations Between Nitrate Concentrations in Spring Waters and Sources of Nitrogen .............................................. 45

Suwannee County Springs................................................................................................................................. 45

Lafayette County Springs .................................................................................................................................. 47

Summary and Conclusions .................................................................................................................................................... 49

References ............................................................................................................................................................................. 51

FIGURES

1. Map of study area showing location of sampled springs and wells........................................................................ 3

2. Schematic of idealized flow system in Suwannee River Basin .............................................................................. 7

3. Concentrations of CFC-11, CFC-12, and CFC-113 in the atmosphere, 1940-1997 and 

tritium concentrations in rainfall collected at Ocala, Florida, decay corrected to 1997 ......................................... 10

4. Dissolved nitrogen and argon concentrations in spring waters and water from wells............................................ 16

5. Deuterium and oxygen-18 content of spring water, ground water, and rainfall compared 

to the global meteoric water line............................................................................................................................. 19

6. Graph of δ15N versus NO3

-

-N concentrations for spring water and water from zones in the 

Upper Floridan aquifer......................................................................................................... ................................... 22

7. Graphs showing estimated average residence times for ground water calculated from ranges 

of hydrologic properties of the Upper Floridan aquifer using a reservoir model ................................................. .. 23
IV Contents

8. Graphs showing comparison of residence times calculated using an exponential model with 

apparent ages calculated using a piston-flow model using CFC-11 and CFC-113 concentrations 

at a recharge temperature of 22 °C ......................................................................................................................... 28

9. Binary-mixing model scenarios for ground water recharged post-1993 with waters recharged in 

1940 and before, 1965, and 1980............................................................................................................................ 29

10. Graph showing discharge of Suwannee and Santa Fe Rivers, rainfall during 1997-98, and dates of 

collection of spring-water samples ......................................................................................................................... 30

11. Graph showing concentrations of 3

H and CFC-113 in water from springs and wells relative to 

estimated apparent age (piston-flow model), average residence time (exponential model), and 

fraction of post-1993 water mixed with water recharged in 1940 and before (binary-mixing model) .................. 32

12. Graph showing concentration of CFC-11 and CFC-113 in water from springs and wells relative to 

estimated apparent age (piston-flow model), average residence time (exponential model), and fraction 

of post-1993 water mixed with water recharged in 1940 and before (binary-mixing model)................................ 33

13. Graph showing nitrate-N concentration against apparent age of spring waters from piston-flow model.............. 34

14. Graph showing nitrate-N concentration against average residence time of spring waters from 

exponential model................................................................................................................................................... 35

15. Nitrate-N concentration against fraction of post-1993 recharge waters from binary-mixing model ..................... 35

16. Nitrate-N concentrations versus spring-water discharge........................................................................................ 36

17-21. Graphs showing estimated annual N inputs and relative percentage of total inputs of nitrogen from 

fertilizers, animal wastes, atmospheric deposition, and septic tanks for the years 1955-97 in:

17. Alachua County .............................................................................................................. ............................... 39

18. Columbia County ............................................................................................................. .............................. 40

19. Suwannee County ............................................................................................................. ............................. 41

20. Lafayette County............................................................................................................................................ 42

21. Gilchrist County............................................................................................................ ................................. 43

22. Graphs showing long-term records of nitrate-N concentrations in spring waters, Suwannee County, 

relative to inputs of nitrogen from fertilizers and major nonpoint sources to ground-water recharge 

modeled using piston-flow and binary-mixing models with a mixing fraction of 0.4 post-1993 water 

and a 3-year lag of recent and old (1940 and older) water, exponential model with 50 percent of 

fertilizer nitrogen contribution to ground-water recharge and average residence time of 10 to 

60 years, and exponential model with 50 percent of nitrogen from all sources contributed to ground-water 

recharge and average residence time of 10 to 60 years .......................................................................................... 46

23. Graphs showing long-term records of nitrate-N concentrations in spring waters, Lafayette County, 

relative to inputs of nitrogen from fertilizers and major nonpoint sources to ground-water recharge 

modeled using piston-flow and binary-mixing models with a mixing fraction of 0.4 post-1993 water and 

a 3-year lag of recent and old (1940 and older) water, exponential model with 50 percent of fertilizer 

nitrogen contribution to ground-water recharge and average residence time of 10 to 60 years, and 

exponential model with 50 percent of nitrogen from all sources contributed to ground-water recharge 

and average residence time of 10 to 60 years ......................................................................................................... 48

TABLES

1. Location of springs and wells sampled in study, springflow discharge, and estimated spring catchment area ..... 6

2. Percentage of land-use types in Alachua, Columbia, Gilchrist, Lafayette, and Suwannee Counties..................... 8

3. Chemical characteristics, concentrations of major ions and other dissolved species, calcite and dolomite 

saturation indices for spring waters and ground water ........................................................................................... 15

4. Dissolved gas data, calculated recharge temperature, and excess air for samples of spring water and water 

from wells ............................................................................................................................................................... 17

5. Isotopic composition of spring waters and ground water....................................................................................... 20

6. Concentration of CFC-11, CFC-12, and CFC-113 in water from springs and wells, calculated atmospheric 

partial pressure, and model CFC recharge dates .................................................................................................... 24

7. Comparison of apparent ages of spring waters from piston-flow model, average residence times calculated 

using exponential-flow model, and fraction of post-1993 water calculated using binary-mixing model .............. 27

8. Spearman’s Rho correlation coefficients, probability level of exceeding Rho, and number of observations........ 37
Abstract 1

Sources and Chronology of Nitrate Contamination in

Spring Waters, Suwannee River Basin, Florida

By Brian G. Katz, H. David Hornsby, Johnkarl F. Bohlke, and Michael F. Mokray

Abstract

A multi-tracer approach, which consisted of 

analyzing water samples for naturally occurring 

chemical and isotopic indicators, was used to 

better understand sources and chronology of 

nitrate contamination in spring waters discharging 

to the Suwannee and Santa Fe Rivers in northern 

Florida. During 1997 and 1998, as part of a cooperative study between the Suwannee River Water 

Management District and the U.S. Geological 

Survey, water samples were collected and analyzed from 24 springs and two wells for major 

ions, nutrients, dissolved organic carbon, and 

selected environmental isotopes [18O/16O, D/H, 

13C/12C, 15N/14N]. To better understand when 

nitrate entered the ground-water system, water 

samples were analyzed for chlorofluorocarbons 

(CFCs; CCl3F, CCl2F2, and C2Cl3F3) and tritium 

(

3

H); in this way, the apparent ages and residence 

times of spring waters and water from shallow 

zones in the Upper Floridan aquifer were determined. In addition to information obtained from 

the use of isotopic and other chemical tracers, 

information on changes in land-use activities in 

the basin during 1954-97 were used to estimate 

nitrogen inputs from nonpoint sources for five 

counties in the basin. Changes in nitrate concentrations in spring waters with time were compared 

with estimated nitrogen inputs for Lafayette and 

Suwannee Counties.

Agricultural activities [cropland farming, 

animal farming operations (beef and dairy cows, 

poultry, and swine)] along with atmospheric deposition have contributed large quantities of nitrogen 

to ground water in the Suwannee River Basin in 

northern Florida. Changes in agricultural land use 

during the past 40 years in Alachua, Columbia, 

Gilchrist, Lafayette, and Suwannee Counties have 

contributed variable amounts of nitrogen to the 

ground-water system. During 1955-97, total estimated nitrogen from all nonpoint sources (fertilizers, animal wastes, atmospheric deposition, and 

septic tanks) increased continuously in Gilchrist 

and Lafayette Counties. In Suwannee, Alachua, 

and Columbia Counties, estimated nitrogen inputs 

from all nonpoint sources peaked in the late 

1970’s, corresponding to the peak use in fertilizer 

during this time. Fertilizer use in Columbia, 

Gilchrist, Lafayette, and Suwannee Counties 

increased substantially during 1993-97.

 The heavy use of fertilizers in the basin is 

corroborated by nitrogen isotope data. Values of 

δ15N of nitrate (δ15N-NO3) in spring waters range 

from 2.7 per mil (SUW718971) to 10.6 per mil 

(Poe Spring) with a median of 5.4 per mil. The 

range of values indicates that nitrate in the sampled spring waters most likely originates from a 

mixture of inorganic (fertilizers) and organic (animal wastes) sources; however, higher δ15N values 

for Poe and Lafayette Blue Springs indicate that an 

organic source of nitrogen probably is contributing 

nitrate to these spring waters. Water samples from 

two wells sampled in Lafayette County have high 

δ15N-NO3 values of 11.0 and 12.1 per mil, indicating the predominance of an organic source of 

nitrate. These two wells are located near dairy and 

poultry farms, where leachate from animal wastes 

may contribute nitrate to ground water. Dissolvedgas data (nitrogen, argon, and oxygen) indicate 
2 Sources and Chronology of Nitrate Contamination in Spring Waters, Suwannee River Basin, Florida

that denitrification has not removed large amounts 

of nitrate from the ground-water system. Thus, 

variations in δ15N-NO3 values of spring waters 

can be attributed to variations in δ15N-NO3 values 

of ground-water recharge, and can be used to 

obtain information about source(s) of nitrate.

Extending the use of age-dating techniques 

(CFCs and 3H) to spring waters in complex karst 

systems required the use of several different 

approaches for estimating age and residence time of 

ground water discharging to springs. These 

approaches included the use of a simple reservoir 

model, a piston-flow model, an exponential model, 

and a binary-mixing model. When age data (CFC11, CFC-113, and 3

H) are combined for all springs, 

models that incorporate exponential mixtures seem 

to provide reliable estimates of average residence 

times of ground water discharging to springs. 

Whereas, data for some individual springs fit a 

binary-mixing model with more than 50 percent 

young water (recharged within the past 5 years), 

data from other individual springs fit a piston-flow 

model with a water age of about 25 years. The 

young ages of several spring waters (such as 

SUW718971, SUW725971, and Ginnie Spring) 

indicate the high vulnerability of the springs to contamination. For most springs, CFCs suggest that a 

large fraction of the water is more than 20 years old. 

Springs with lower flows tend to have young ages 

(shallow ground-water flow systems), whereas 

springs with higher flows tend to have older ages 

(deep ground-water flow systems). 

The chemical composition of spring waters 

can be used as a qualitative indicator of age and 

ground-water residence time. Nitrate-nitrogen 

concentrations and dissolved oxygen in spring 

waters are inversely related to the apparent ages of 

spring waters and ground-water residence time in 

the basin. Silica concentrations increase as the age 

of spring waters increase. 

Long-term trends in nitrate concentrations 

in selected spring waters were compared with 

estimated inputs of nitrogen from various sources 

in Suwannee and Lafayette Counties. In both 

counties, trends in nitrate concentrations in spring 

waters closely followed the estimated contribution 

of nitrogen from fertilizers. Decreasing nitrate 

concentrations in spring waters from Suwannee 

County followed the decrease in estimated fertilizer use from the mid-1970’s to the early 1990’s. 

Increasing nitrate concentrations in spring waters 

from Lafayette County followed the steady 

increase in fertilizer use from the early 1960’s to 

the mid-1990’s.

The relation between the concentration of 

nitrate in ground water and the amount of nitrogen 

that is added to a ground-water contributing area 

for a spring is controlled by complex interactions 

among hydrogeologic, land-use, and climatic 

factors, as well as several other land-management 

factors. Spring waters represent mixtures of 

converging flow paths that contain ground water 

with a range of ages. Even if nitrogen inputs were 

reduced substantially, it may take decades for 

nitrate concentrations in the ground-water system 

to return to near background levels. 

INTRODUCTION

Springs provide not only sources of potable 

water, as well as recreational and cultural value, but 

they afford a way to assess ground-water quality 

because their discharge spatially and temporally 

integrates ground water from large parts of an aquifer. 

Agricultural activities and other land uses have 

impacted the quality of spring waters by contributing 

large quantities of nutrients to ground-water recharge 

in many parts of the world (Dietrich and Hebert, 1997; 

Focazio and others, 1998; Kendall, 1998). During the 

past 40 years, nitrate-nitrogen (N) concentrations in 

water from several springs have increased substantially 

from near background concentrations of nitrate-N (less 

than 0.1 milligram per liter (mg/L)) (Katz, 1992; Maddox and others, 1992) to more than 5 mg/L (Hornsby 

and Ceryak, 1999) in the Suwannee River Basin in 

northern Florida (fig. 1). 

Discharge of water from springs into the Suwannee and Santa Fe Rivers has contributed substantial 

loads of nitrate-N to both rivers (Hornsby and Mattson, 

1998). For example, along a 53-kilometer (km) reach 

of the Suwannee River, nitrate-N loads increased 

downstream from 2,300 kilograms per day (kg/d) just 

downstream of Dowling Park to 6,000 kg/d at Branford 

during base-flow conditions (Pittman and others, 

1997). Nearly 90 percent of the increase in nitrate load 

occurred in the lower two-thirds of the studied reach, 

and was attributed mainly to discharge from spring 

flow and upward diffuse leakage of ground water 

through the riverbed (Pittman and others, 1997). 
Introduction 3

83°15′ 83°00′ 82°30′

30°15′

29°15′

30°00′

45′

30′

45′

0 40 KILOMETERS

0 40 MILES

Location

of

Study

Area

SUWANNEE

BAKER

GILCHRIST

MADISON

COLUMBIA

ALACHUA

UNION

LEVY

DIXIE

TAYLOR

LAFAYETTE

MARION

BRADFORD

 

 

Rocky Hill

Fire Tower

Branford

GULF OF MEXICO

Santa Fe

River

Suwannee

River

Suw

anne

e

Ri ver

SUW718971

 SUW725971

Ichetucknee

Blue Hole

Spring

Mearson Spring

LAF718972

Blue

 Spring

Telford Spring

Charles Spring

Running 

Springs

Pothole Spring

Hornsby Spring

Poe Springs

Trail Springs

GIL917971

Columbia

Spring

Rock Bluff

Spring

Fannin

Spring

Manatee

Springs

Little

River

Springs

Ruth

Troy Spring

Spring

Ginnie Springs

& July Spring

Well-6011

Well-9001

Dowling Park

Live

Oak

SUWANNEE

RIVER

BASIN

Figure 1. Map of study area showing location of sampled springs and wells.
4 Sources and Chronology of Nitrate Contamination in Spring Waters, Suwannee River Basin, Florida

Along this middle reach of the Suwannee River in 

north Florida, mean nitrate-N concentrations increase 

nearly four-fold from 0.15 mg/L at Suwannee Springs, 

(242 km from mouth of river) (Katz and others, 1997) 

to 1.38 mg/L at Branford, (123 km from river mouth) 

(Hornsby and Mattson, 1999). This increase in nitrate 

in river water at Branford is attributed to ground-water 

discharge because there are no major stream inputs to 

the middle Suwannee River in this region. Also, the 

concentration of nitrate in river water is inversely 

related to discharge; that is, concentrations are higher 

during base-flow conditions when the contribution 

from ground water is greatest (Hornsby and Mattson, 

1996). Elevated concentrations of nitrate in rivers can 

cause eutrophication, which can result in algal blooms 

and depletion of oxygen that can lead to fish kills. 

Increases in nitrate concentrations from human activities may be causing adverse ecological effects, which 

are indicated by an increase in periphyton biomass 

along the middle and lower reaches of the Suwannee 

River (Hornsby and Mattson, 1996). Also of concern 

are the effects of high nitrate concentrations on the 

Suwannee River system’s estuary which, in addition to 

the Suwannee River, has been designated as an outstanding Florida waterbody and a State Aquatic 

Preserve and National Wildlife Refuge. 

Another important concern is elevated nitrate-N 

concentrations that have been measured in water from 

wells open to shallow parts of the Upper Floridan 

aquifer (less than 40 meters (m) below land surface) in 

parts of Suwannee and Lafayette Counties. Water from 

the Upper Floridan aquifer is the source of water supply in the study area. Numerous domestic and monitoring wells have yielded water with nitrate-N concentrations that exceed 10 mg/L, which is the maximum 

contaminant level set by the United States Environmental Protection Agency. This limit was set because 

of the health risk of high-nitrate water to infants, who 

can contract methemoglobinemia (Mueller and Helsel, 

1996). 

Identifying the sources of nitrate and understanding the chronology and processes affecting nitrate 

contamination of ground water are needed to develop 

effective management practices to prevent further degradation of water quality. At present, little information 

exists to determine the source(s) and fate of nitrate in 

the Upper Floridan aquifer in the Suwannee River 

Basin of northern Florida. The occurrence of many 

possible sources of nitrate in the basin makes it difficult 

to determine the relation between nitrate concentrations in ground water and the timing of nitrate introduced from a particular source or from multiple 

sources. Previous studies have identified animal 

wastes associated with dairy (Andrews, 1994) and 

poultry farming operations (Hatzell, 1995) and fertilizers applied to cropland (Hornsby, 1994) as important 

sources of nitrate to ground water in the Suwannee 

River Basin. 

Purpose and Scope

This report describes the results of a cooperative 

study between the Suwannee River Water Management 

District (SRWMD) and the U.S. Geological Survey 

(USGS). The study was designed to evaluate sources 

and chronology of nitrate contamination in water from 

24 selected springs and from two wells open to shallow 

zones of the Upper Floridan aquifer in the Suwannee 

River Basin. The study used a multi-tracer approach, 

which consisted of collecting and analyzing water samples for naturally occurring chemical and isotopic indicators, to better understand the sources and chronology 

of nitrate contamination of ground water in parts of the 

Suwannee River Basin in Florida. 

Naturally occurring chemical and isotopic tracers provide important information on geochemical and 

hydrologic processes. For example, the concentrations 

of major ions, nutrients, and dissolved organic carbon 

give information on sources and processes that affect 

the concentration of solutes in the Upper Floridan aquifer. The stable isotopes of water, oxygen (18O/16O) and 

hydrogen (D/H) provide information on the origin of 

the water and mixing of different sources of water in 

springs and in zones of the aquifer. Measurements of 

carbon isotopes (13C/12C) provide information to 

assess rock-water interactions, evaluate the mixing of 

water from different sources, and quantify mass transfer associated with microbially mediated processes (for 

example, degradation of organic matter with associated 

terminal electron-accepting processes such as ferric 

iron reduction, sulfate reduction, and methanogenesis), 

Nitrogen isotopes (15N/14N) are used to evaluate 

sources of nitrate in ground water. 

To better understand when nitrate entered the 

ground-water system, water samples were analyzed for 

chlorofluorocarbons (CFCs; CCl3F (CFC-11), CCl2F2

(CFC-12), and C2Cl3F3 (CFC-113)) and tritium (3

H) to 

assess the apparent ages (residence time) of spring 

waters and water from the Upper Floridan aquifer. 

Anthropogenic activities, such as industrial processes 
Introduction 5

and atmospheric testing of thermonuclear devices, have 

released CFCs and 3

H, respectively, into the atmosphere 

in very low but measurable concentrations. Precipitation, which incorporates CFCs and 3

H from the atmosphere, infiltrates into the ground and carries a particular 

chemical or isotopic signature related to atmospheric 

conditions at the time of recharge. The concentration of 

CFCs and 3

H in ground water along a flow path or water 

from a spring provides information on ground-water age 

and residence time of water in the aquifer. 

In addition to information on sources of nitrate 

obtained from the chemical and isotopic tracers, information on the hydrology of the Upper Floridan aquifer 

and present and historical land uses are presented to 

obtain a more complete understanding of the factors 

affecting the concentration of nitrate in springs and in 

parts of the aquifer. Substantial changes in land use 

have occurred along with associated inputs of nitrogen 

to the study area over the past 40 years. Estimates of 

nitrogen loading from nonpoint sources are provided 

for five counties in the study area (Suwannee, Lafayette, Gilchrist, Columbia, and Alachua) during 1950-

97. Important nonpoint sources of nitrogen in the study 

area include fertilizers applied to cropland, atmospheric deposition, animal wastes, and discharges from 

septic tank systems.

Description of Study Area

The springs sampled in this study discharge 

ground water to the Suwannee and Santa Fe Rivers, 

which are located in the Suwannee River Basin in the 

north and north-central parts of peninsular Florida 

(fig. 1). The basin is characterized by karstic wetland 

and lowland topography, a small number of tributary 

streams, and an abundance of Upper Floridan aquifer 

springs. In a recent survey during low-flow conditions, 

Hornsby and Ceryak (1999) identified 197 springs in 

the Suwannee River Basin, of which, only 65 were previously reported (Rosenau and others, 1977). In addition to springs, other common karst features in the 

basin include numerous sinkholes, other solution cavities (caves, conduits), siphons, and disappearing 

streams. As a result of these features, interactions 

between ground water and surface water constitute a 

single dynamic system in most parts of the basin (Katz 

and others, 1997).

Spring-water flow commonly is classified by 

Meinzer’s magnitude of discharge system (Rosenau 

and others, 1977). A first-magnitude spring has 

average flow greater than 2.8 cubic meters per second 

(m3

/s) (100 cubic feet per second (ft3/s)); a secondmagnitude spring has average flow ranging between 

0.28 and 2.8 m3

/s (10 and 100 ft3/s); and a third-magnitude spring discharges from 0.028 to 0.28 m3

/s (1 to 

10 ft3

/s) (Rosenau and others, 1977). Seven of the 

18 first-magnitude springs in the basin were sampled in 

this study: Troy, Fannin, Manatee, Ichetucknee Blue 

Hole, Hornsby, Columbia, and July (fig.1, table 1). 

Second-magnitude springs sampled included Ginnie, 

LAF718972, Poe, Pothole, Rock Bluff, Mearson, Ruth, 

Running, Little River, Charles, Lafayette Blue, and 

Telford (fig. 1, table 1). Water samples also were 

collected from the following third-magnitude springs: 

GIL917971, SUW718971, SUW725971, and Trail 

(fig. 1). Measured discharge values for sampled 

springs are presented in table 1.

Springs in the study area generally are categorized as water-table springs because the Upper Floridan 

aquifer is predominantly unconfined (Crane, 1986). 

Many of central Florida’s large springs are categorized 

as artesian because water is confined in permeable 

sediments beneath impervious confining beds and is 

under sufficient hydrostatic pressure to rise to the 

surface through natural breaches in the overlying confining units (Rosenau and others, 1977). Ground-water 

discharge to springs results from the difference 

between the hydrostatic head at the spring vent and the 

recharge area, and from a system of solution cavities 

and conduits that are present in the limestone that 

forms the Upper Floridan aquifer. 

Several climatic, geologic, and hydrologic 

factors control the amount of spring flow such as the 

amount and frequency of rainfall (recharge), the porosity and permeability of the aquifer, the size of the 

ground-water contributing area, the hydrostatic head 

within the aquifer, and the hydraulic gradient. The permeability of the Upper Floridan aquifer in the study 

area varies greatly due to differences in the size and 

type of openings in the water-bearing limestone that 

range from networks of small solution openings along 

joints or bedding planes to large cavernous openings 

developed in modern karst or paleokarst areas. Differences in morphology and geometry of saturated caves 

and conduits associated with specific springs also can 

influence the discharge rate of spring waters (Kincaid, 

1999). Large conduit systems that have been mapped 

for many springs in the basin can extend for many 

kilometers into the rocks of the Upper Floridan aquifer 

(Hornsby and Ceryak, 1999). The variability in 

permeability is reflected by transmissivities that range 

from 2,800 to greater than 93,000 square meters per 

day (m2

/d) (30,000 to greater than 1,000,000 square 

feet per day (ft2

/d) (Bush and Johnston, 1988). 
6 Sources and Chronology of Nitrate Contamination in Spring Waters, Suwannee River Basin, Florida

Table 1. Location of springs and wells sampled in study, springflow discharge, and estimated spring catchment area

[ft3/s, cubic feet per second; m3/s, cubic meters per second; km2, square kilometers; NA, not applicable, R, recharge rate, in m/yr]

Spring/well site name Date of 

sampling Latitude Longitude County

Discharge 

ft3/s

Discharge 

m3/s

Date of discharge 

measurement

Estimated spring catchment area, km2

R=0.5 R=0.2 R=0.8

Charles Spring 07/15/97 301002.0 831350.0 Suwannee 16.4 0.46 7/15/97 29 73 18

Lafayette Blue Spring 07/15/97 300733.0 831334.0 Lafayette 84.6 2.40 7/15/97 151 378 95

Little River Springs 07/15/97 295947.0 825759.0 Suwannee 76.1 2.16 7/15/97 136 340 85

Telford Spring 07/16/97 300624.0 830957.0 Suwannee 41.6 1.18 7/16/97 74 186 46

Troy Spring-97 07/15/97 300021.0 825951.0 Lafayette 138 3.91 7/15/97 247 617 154

Ichetucknee Blue Hole Spring 08/14/97 295709.0 824710.0 Columbia 117 3.31 8/14/97 209 523 131

Fannin Spring 08/18/98 293514.3 825607.7 Levy 109 3.09 7/16/98 195 487 122

GIL917971 08/20/98 295441.1 825032.5 Gilchrist 2 0.06 7/10/98 3.6 8.9 2.2

Ginnie Springs 08/21/98 295008.9 824200.0 Gilchrist 51 1.44 6/15/98 91 228 57

Hornsby Spring 08/21/98 295100.4 823536.0 Alachua 200 5.66 7/28/98 357 894 223

July Spring 08/21/98 295009.8 824150.0 Columbia 117 3.31 11/4/97 209 523 131

LAF718972 08/19/98 300021.2 830016.1 Lafayette 11 0.31 7/18/97 20 49 12

Manatee Springs 08/18/98 292921.3 825837.4 Levy 202 5.72 7/16/98 361 903 226

Poe Springs 08/21/98 294933.1 823859.9 Alachua 54 1.53 7/22/98 97 241 60

Pothole Spring 08/20/98 294837.8 825609.8 Dixie 32 0.91 9/23/97 57 143 36

Rock Bluff Spring 08/20/98 294755.8 825507.3 Gilchrist 45 1.27 8/17/98 80 201 50

Ruth Spring 08/19/98 295944.1 825837.2 Lafayette 13 0.37 8/24/98 23 58 15

Running Springs 08/18/98 300614.9 830657.0 Suwannee 22 0.62 6/24/97 39 98 25

SUW718971 08/19/98 300350.4 830343.7 Suwannee 9 0.25 7/10/98 16 40 10

SUW725971 08/19/98 300337.8 830312.0 Suwannee 8 0.23 7/10/98 14 36 9

Trail Springs 08/20/98 295353.4 825200.9 Gilchrist 9 0.25 9/16/97 16 40 10

Troy Spring-98 08/19/98 300021.1 825951.4 Lafayette 102 2.89 8/24/98 182 456 114

Mearson Spring 08/19/98 300228.0 830132.0 Lafayette 62 1.76 8/24/98 111 277 69

Columbia Spring 08/20/98 295113.0 823643.0 Columbia 210 5.95 8/20/98 375 938 235

Well-9001 07/16/97 300351.0 830748.0 Lafayette NA NA NA NA NA NA

Well-9001 03/09/98 300351.0 830748.0 Lafayette NA NA NA NA NA NA

Well-6011 07/16/97 300319.0 830650.0 Lafayette NA NA NA NA NA NA

Well-6011 03/09/98 300319.0 830650.0 Lafayette NA NA NA NA NA NA

The hydrogeology of the Suwannee River Basin 

is directly related to the physiography. In the northeastern part of the basin in the Northern Highlands, surfacewater features are common, and the area is characterized by land-surface altitudes ranging from greater than 

30 m to 70 m. Clayey sediments of the intermediate 

confining unit, which overlie the Upper Floridan aquifer, retard the infiltration of rainwater. Recharge rates 

to the Upper Floridan aquifer are less than 30 centimeters per year (cm/yr) in areas where the aquifer is confined (Grubbs, 1998). In contrast, the southern and 

southwestern parts of the basin lie within the Gulf 

Coastal Lowlands physiographic division, which is 

characterized by land altitudes that are less than 30 m 

and carbonate rock exposed at land surface or underlying the shallow subsurface. Where the Upper Floridan 

aquifer is unconfined in this area, recharge rates range 

from 40 to 80 cm/yr (Grubbs, 1998). 

Ground-water flow toward most springs in the 

Suwannee River Basin typically follows intermediate 

to long flow paths-- localized flow systems generally 

do not exist except for small springs with low flow rates 

(fig. 2). Regional simulation of the ground-water-flow 

system demonstrated that major springs receive a small 

percentage of water from upward leakage from the 

lower part of the Upper Floridan aquifer (Bush and 

Johnston, 1988). The regional ground-water flowmodel corroborated the importance of flow systems 

characterized as intermediate flow systems (Toth, 
Introduction 7

1963) and the lack of local flow systems (which the 

regional model was not able to simulate). Bush and 

Johnston (1988) estimated that ground-water discharge in the lower Suwannee River Basin was a mixture of the following components, in percent: spring 

flow and discharge to surface-water bodies, 87; diffuse 

upward leakage, 9; and pumpage, 4. It is likely that 

ground-water flow toward springs represents a mixture 

of water from shallow and deeper flow systems. 

Faulkner (1973) estimated that ground-water flow 

toward Rainbow Springs and Silver Springs, which are 

first-magnitude springs in central Florida, was a mixture of about 92 percent shallow water and 8 percent 

deep water, resulting in an intermediate flow system in 

Toth’s (1963) classification scheme. Using tritium, 

carbon-14, and carbon-13 data from Toth (1999), it was 

estimated that ground-water flow toward Ponce de 

Leon, Rock, and Wekiva Springs (in east-central Florida) was a mixture of about 70 to 100 percent shallow 

water (from intermediate flow paths) and 0 to 30 percent deep water (from regional flow paths).

In most parts of the basin, channels of the 

Suwannee and Santa Fe Rivers have cut into the aquifer 

rocks, thus springs discharge water to these two rivers 

because the river level is lower than that of the water 

table. During high river stage, however, many springs 

become flooded with river water and reversals in flow 

can occur if the hydraulic head of the spring (Upper 

Floridan aquifer) is lower than the hydrostatic pressure 

of the river (Rosenau and others, 1977; Kincaid, 1998; 

Hornsby and Ceryak, 1999). 

The climate in the basin is subtropical and is 

characterized by long, warm summers and mild winters. Rainfall averages 132 cm/yr in the study area 

(Crane, 1986); however, there are large variations 

between locations and from year to year (Owenby and 

Ezell, 1992). Approximately 50 percent of the average 

annual rainfall occurs from June through September, 

but the shorter rainy season from late February through 

late April typically produces some of the highest stages of 

the Suwannee and Santa Fe Rivers and their tributaries.

Major land uses in the Suwannee River Basin 

include forest, agriculture (row crops; dairy, poultry, 

and swine farming operations, pasture), and wetlands. 

Percentages of each land-use type in the basin vary 

considerably by county. For example, in 1997, for the 

five counties that are contiguous with the middle reach 

of the Suwannee River and the Santa Fe River (Alachua, Columbia, Gilchrist, Lafayette, and Suwannee), 

the percentage of agricultural land ranged from 11 in 

Lafayette County to 35 in Suwannee County (table 2). 

Agricultural land use includes cropland, pasture lands, 

confined feeding operations (poultry, dairy, livestock, 

and swine). The percentage of agricultural land use has 

decreased in all five counties based on a comparison of 

recent data (1997) and that from the mid-1970’s, which 

was based on a land-use and land-cover classification 

system for remote sensor data (Anderson and others, 

1976; Mitchell and others, 1977). Based on 1997 data, 

forested land ranged from about 39 percent in Alachua 

County to 56 percent in Columbia County. Much of the 

forested land is planted by the paper industry for silviculture. Wetlands covered between 2 and 36 percent in 

Suwannee and Lafayette Counties, respectively. 



WEST EAST

Gulf of Mexico Major Stream Major Spring Land Surface

North-central Florida springs area to the Gulf of Mexico

modified from Bush and Johnston (1988)

Freshwater-Saltwater Interface

FLORIDAN AQUIFER SYSTEM

SHOWS DIRECTION OF INTERMEDIATE

 GROUND-WATER FLOW

EXPLANATION

SHOWS DIRECTION OF REGIONAL

 GROUND-WATER FLOW

Figure 2. Schematic of idealized flow system in Suwannee River Basin.
8 Sources and Chronology of Nitrate Contamination in Spring Waters, Suwannee River Basin, Florida

Acknowledgments

This study was funded jointly by the Suwannee 

River Water Management District and the U.S. Geological Survey. The authors thank Robert L. Michel, 

USGS, for tritium analyses; Julian Wayland, Gerry 

Casile, Scott McBride, and Willie Ray Hunter 

(SRWMD) for help with water sampling; Michael 

Hallock-Solomon for compiling data on historical 

fertilizer use; and Niel Plummer and Marian Berndt for 

their review comments that were helpful in revising this 

report. 

STUDY METHODS

Study methods are described below for sample 

collection and analysis, estimation of age of spring 

waters and average residence times of ground water, 

and estimation of nitrogen inputs from various nonpoint sources. More detailed information about these 

topics can be found in the references listed in each section. 

Sample Collection and Analysis

Samples of water from 24 springs were collected 

during low-flow conditions along the Suwannee and 

Santa Fe Rivers in July and August 1997 and in August 

1998 (fig. 1). Troy Spring was sampled in 1997 and in 

1998. These springs were selected for sampling 

because previous data indicated that high loads of 

nitrate were being discharged to the Suwannee and 

Table 2. Percentage of land-use types in Alachua, 

Columbia, Gilchrist, Lafayette, and Suwannee Counties

[Upper number in each cell is for 1977 data, lower number is for 1995; 

km2

, square kilometers]

County

(area in km2)

Land use, in percent

Agriculture Forest Urban Wetlands Other

Alachua

(2,335)

35

24

43

39

5

14

11

17

6

6

Columbia

(2,063)

22

17

60

56

2

6

15

19

0

1

Gilchrist

(916)

34

29

55

47

1

9

9

13

1

1

Lafayette

(1,412)

15

11

51

49

0

2

33

36

0

1

Suwannee

(1,786)

47

35

49

52

1

9

1

2

0

1

Santa Fe Rivers in this area (Hornsby and Mattson, 

1996; Hornsby and Ceryak, 1999). Water samples also 

were collected during low-flow conditions (August 

1997) and during high-flow conditions (March 1998) 

from two wells open to shallow zones within the Upper 

Floridan aquifer. The wells were located in an agricultural area near the Suwannee River in Lafayette County 

(fig. 1). Well-9001 is screened in the interval from 7 to 

13 m below land surface (bls), and well-6011 is 

screened in the interval from 26 to 32 m bls. Both wells 

were part of a statewide network designed to study the 

effects of different land uses on ground-water quality. 

These two wells were chosen for this study because of 

differences in depth and both previously yielded water 

samples having consistently high nitrate-N concentrations (10 to 20 mg/L in water samples collected in 1994 

and 1996).

Samples of spring water were collected by 

lowering a positive displacement dual-piston (Bennett) 

pump head about 5 to 17 m into the spring vent. Water 

was pumped at approximately 0.06 liter per second 

(L/s) through 0.63-cm diameter copper (refrigerationgrade) tubing. Sampling methodology varied somewhat depending on accessibility to the spring and site 

characteristics. Water samples were collected from a 

boat at 14 springs, whereas samples were collected 

from the adjacent shoreline at the other nine springs. 

During the 1997 sampling, specific conductance, pH, 

dissolved oxygen, and temperature were measured by 

using a closed flow-through chamber to prevent contact 

of the spring water at depth with the atmosphere. During the 1998 spring-water sampling, a YSI multi-probe 

unit was lowered down into the spring vent to 

measure these four water properties in-situ. After field 

readings of these properties had stabilized, samples of 

spring water were collected for major element chemistry, dissolved organic carbon (DOC), 3

H, D, 18O, 15N, 

13C dissolved inorganic carbon (DIC), and CFCs 

(CFC-11, CFC-12, and CFC-113). 

Water samples from wells -9001 and -6011 were 

collected after a minimum of three well-bore volumes 

of water had been purged and readings of specific conductance, pH, dissolved oxygen, and temperature had 

stabilized. A closed flow-through chamber was used to 

measure these properties to prevent contact of the 

ground water with the atmosphere. At well-9001, the 

Bennett pump intake was positioned approximately 1 

to 2 m above the open interval in the well after three 

casing volumes were purged using a submersible 

pump. At well-6011, water samples were collected 

from the existing water-supply system upgradient of 
Study Methods 9

the pressure tank by using an in-line submersible 

pump. Samples of ground water were collected for 

major element chemistry, DOC, 3

H, 2H, 18O, 15N, 13C 

(DIC), dissolved gases (N2, Ar, CH4), and CFCs (CFC11, CFC-12, and CFC-113). 

Water samples were collected for major element 

chemistry and DOC by using standard techniques 

(Brown and others, 1970; Wood, 1976; Koterba and 

others, 1995) that included field filtration using a 

0.45-μm membrane filter for major ions, nutrients, 

silica, and using a 0.45-μm silver filter for dissolved 

organic carbon. These samples were preserved in the 

field and analyzed at the U.S. Geological Survey laboratory in Ocala, Fla., following standardized procedures (U.S. Geological Survey, 1999). 

Isotopic Tracers

Isotopic values are reported using standard delta 

(δ) notation (Gonfiantini, 1981), as defined by the 

following expression: 

δ (per mil) = [(Rsample/Rstandard) - 1] X 1,000.

For δ18O, R = 18O/16O; for δD, R = D/H; for δ13C, 

R = 13C/12C, and for δ15N, R = 15N/14N. Results for 

oxygen and hydrogen isotopes are reported in per mil 

relative to Vienna Standard Mean Ocean Water 

(VSMOW), and are normalized on scales such that the 

oxygen and hydrogen isotopic values of Standard 

Light Antarctic Precipitation (SLAP) are -55.5 and -

428 per mil, respectively (Coplen, 1994). The 2σ precision of delta oxygen-18 (δ18O) and delta deuterium 

(δD) results is 0.2 and 2 per mil, respectively. The 2σ

precision for the analytical procedure is 0.2 per mil 

(Coplen, 1994). Samples for analysis of δ13C in DIC 

were collected by direct precipitation in the field using 

ammoniacal-SrCl2 solution followed by filtering, drying, and acidifying the SrCO3 precipitate to produce 

CO2, which was analyzed by mass spectrometric methods (Hassan, 1982). Values of δ13C are reported relative to VPDB, Vienna Pee Dee Belemnite (Coplen, 

1994). Stable isotope analyses of water samples were 

performed by the USGS Isotope Fractionation Laboratory in Reston, Virginia. Values of δ13C were determined by mass spectrometry, and are reported as the 

per mil deviation from the VPDB standard (Coplen, 

1994).

Water samples for nitrogen isotope analysis were 

filtered (0.45 μm membrane) into 1-liter (L) plastic 

bottles and kept chilled at 4 oC. Analytical techniques 

are described by Bohlke and Denver (1995) and Bohlke 

and Coplen (1995). Values of δ15N for nitrate-N 

concentrations above 0.5 mg/L are normalized to 

values of +0.4 per mil for IAEA-N1 and +180.0 per mil 

for USGS-32 (Bohlke and Coplen, 1995) with analytical uncertainties of approximately +/- 0.1 per mil. 

Age Dating Waters

The transient tracers, tritium and chlorofluorocarbons (CFCs), were used to estimate the age of 

ground water. Tritium (3

H) analyses provide estimates 

of the time of ground-water recharge, by comparing 

measured 3

H concentrations in ground water with the 

long-term 3

H input function of rainfall measured at the 

International Atomic Energy Agency (IAEA) precipitation monitoring station in Ocala, Florida (Michel, 

1989) (fig. 3). 3H activity is reported in tritium units 

(TU), with 1 TU equal to 1 3

H atom in 1018 hydrogen 

atoms and 7.1 disintegrations per minute per gram of 

water. Water samples for 3H analyses were enriched 

electrolytically and analyzed by liquid scintillation 

counting in the low-level 3

H laboratory of the U.S. 

Geological Survey in Menlo Park, Calif., using a modification of procedures described by Thatcher and others (1977). Analytical uncertainties for 3H using the 

low-level counting procedure is approximately +/- 0.15 

to 0.30 (R.L. Michel, USGS, 1998, written commun.).

The CFC age-dating technique was used in this 

study for assessing the apparent age of spring waters 

and water from two zones in the Upper Floridan aquifer. This technique, which has been used to age-date 

ground water (Upper Floridan aquifer) in karst areas of 

northern Florida (Katz and others, 1995a) and southern 

Georgia (Plummer and others, 1998a), is based on four 

main assumptions: (1) the partial pressures of CFCs are 

the same in both the soil (unsaturated zone) and the tropospheric atmospheres, which is valid for unsaturated 

zones less than 10 m in thickness (Busenberg and 

Plummer, 1992); (2) the aquifer has not been contaminated by local, near-surface sources of CFCs commonly found near or in urban areas; (3) the CFC 

concentration in recharge water is in equilibrium with 

the CFC partial pressure in the soil atmosphere; and (4) 

the CFC concentrations in the aquifer have not been 

altered by biological, geochemical, or hydrologic processes. The stability of CFC compounds in the hydrosphere has led to their effective use as tracers to age 

date ground water that has been recharged during the 

past 50 years (Plummer and others, 1993). 
10 Sources and Chronology of Nitrate Contamination in Spring Waters, Suwannee River Basin, Florida

The apparent age of spring waters and water 

from zones in the aquifer is determined from a CFCmodeled recharge date (as defined by Busenberg and 

Plummer, 1992). A CFC-modeled recharge date is 

determined on the basis of the equilibrium partitioning 

between rainwater and the partial pressures of trichlorofluoromethane (CCl3F, CFC-11), dichlorodifluoromethane (CCl2F2, CFC-12), and trichlorotrifluoroethane (C2Cl3F3, CFC-113) in the troposphere or soil 

atmosphere (fig. 3). The CFC dating method actually 

provides three independent ages, which are based on 

measured concentrations of each of the three CFC compounds. Ideally, the three independent ages that are 

derived can be used as a cross-check on the sampling 

and analytical methods. An apparent age of the sampled water is modeled based on the comparison of the 

measured concentration of each CFC compound with 

calculated equilibrium partial pressures using solubility data for each compound with their respective atmospheric growth curve (fig. 3). The concentration of 

CFCs in ground water is a function of the atmospheric 

partial pressures and the temperature at the base of the 

unsaturated zone during recharge. Recharge temperature and quantity of dissolved excess air (Heaton and 

Vogel, 1981) were determined from gas-chromatography analyses of nitrogen (N2) and argon (Ar) gases in 

the headspace of water samples collected in the field 

(Busenberg and others, 1993). 

Estimation of Age of Spring Waters and

Average Residence Time of Ground Water

In previous studies where CFCs have been used 

to age-date ground water, hydrologic systems have 

been relatively well characterized and water samples 

typically were collected from short depth intervals. 

Extending the CFC age-dating technique to complex 

karst systems, where both conduit and diffuse flow 

exist, requires an integration and comparison of several 

approaches that are based on different conceptualizations of how water moves through the aquifer. In this 

report, the age and residence times of ground water discharging to springs were estimated by using the following conceptual models of ground-water flow to springs: 

a simple reservoir, piston flow, exponential flow, and 

the binary mixing of waters from two reservoirs. A 

simple reservoir model uses aquifer properties and 

rates of recharge to the ground-water system to estimate the residence time of ground water discharging to 

TRITIUM IN RAINFALL,

Ocala, Florida,

Decay Corrected

to 1997

CFC-12

(CCl2F2)

CFC-11

(CCl3F)

CFC-113

(C2Cl3F3)

1940 1950 1960 1970 1980 1990 2000

600

0

100

200

300

400

500

CFC CONCENTRATION,

IN PARTS PER TRILLION BY VOLUME

TRITIUM CONCENTRATION, IN TRITIUM UNITS

YEARS

1

200

5

10

50

100

Figure 3. Concentrations of CFC-11, CFC-12, and CFC-113 in the atmosphere, 1940-1997 and tritium concentrations in 

rainfall collected at Ocala, Florida, decay corrected to 1997.
Study Methods 11

springs. Steady-state flow conditions are assumed for 

the aquifer; that is, recharge rates and other aquifer 

properties remain constant at least on an inter-annual 

basis. The model also assumes that annual recharge is 

equal to annual discharge, and that the areal extent of 

the aquifer is equivalent to the watershed or contributing area to the spring. Average ground-water residence 

times can be approximated by using the following 

equation (after Focazio and others, 1998):

T = bn/R , (1)

where T is the average residence time of ground water 

in years, b is the aquifer thickness in length units, n is 

the porosity (unitless), and R is the annual recharge (or 

discharge) rate, expressed in length per time units. T 

also is equivalent to the turnover time, which can be 

defined as the volume of mobile water in the groundwater system divided by the volumetric flow rate of 

water through the system (Zuber, 1986). Using this 

relation, the mean age of water leaving the groundwater system is equivalent to the turnover time. The 

residence time of ground water is proportional to the 

amount of void space in the aquifer. From expression 

(1), longer residence times would be independently 

associated with a higher porosity, a larger thickness of 

aquifer, and a lower recharge rate. 

Following from the physical definition of the 

ground-water flow system, input and output concentrations of naturally introduced tracers (CFCs and 3

H) can 

be used to provide information about the age of ground 

water and springs issuing from the flow system. Input 

concentrations are obtained from relatively wellknown atmospheric input curves (fig. 3) and output 

concentrations are those measured at a particular time 

(t) in samples of spring water and ground water. 

Lumped-parameter models typically are used to interpret radioisotope and other age-related tracer data in 

ground-water hydrology studies (Richter and others, 

1993). In these models, the aquifer is assumed to be 

homogeneous and spatial variations are ignored. They 

have an advantage over numeric simulation approaches 

in that few hydrologic parameters are needed, consequently, results from these models tend to represent 

idealized conditions. The two lumped-parameter models used to interpret age information in this report are 

the piston-flow and exponential-flow models. A brief 

explanation of each model follows along with some 

assumptions and limitations; a more detailed description of these models can be found in previous studies, 

such as Maloszewski and Zuber (1982), Zuber (1986), 

and Richter and others (1993). 

For ground-water-flow systems that are assumed 

to be at steady state conditions, the relation between 

time-variable tracer input and output concentrations 

can be expressed mathematically for a lumped-parameter model (Zuber, 1986) as:

, (2)

where Cout(t) and Cin(t) are the output and input concentrations, respectively; t′ is the transit time of the 

tracer, and g(t′) is a weighting function or system 

response function for a given injection-detection mode, 

and t is the calendar time.

 The most simplified lumped-parameter 

approach used is the piston-flow model, which assumes 

that a particular tracer moves like a slug through the 

aquifer, thus corresponding to plug flow in a single 

flow tube. That is, after a tracer is isolated from the 

atmosphere (at the time the ground-water system is 

recharged), it becomes incorporated in a parcel of water 

that moves from the recharge area with the mean velocity of ground water (Zuber, 1986). All flow lines are 

assumed to have similar velocities, and hydrodynamic 

dispersion and molecular diffusion of the tracer are 

assumed to be negligible. One of the limitations to 

using this method is the assumption that dispersion and 

mixing do not occur as water moves through the aquifer. In the case of springs that spatially and temporally 

integrate water from large parts of an aquifer, the ages 

of ground water determined by using a piston-flow 

model may in some cases, underestimate the actual age 

or residence time of spring water. The weighting function, g(t′), for a piston-flow model is δ(t′-τ), where τ is 

the residence time. For a piston-flow model, the concentration of 3H in a water parcel separated since the 

time of recharge or entry into the ground-water system 

can be defined as:

C(t) = C(o) exp(-λt) , (3)

where C(t) is the concentration at a given time (t), C(o) 

is the input concentration, and λ = 0.05626. For nonradioactive tracers in the piston-flow model, such as 

CFCs, the concentration at any given time (t) is equal 

to the input concentration at time, t=0. 

Cout( )t Cin( ) t′ –λ( ) t t – ′

gt t ( ) – ′ dt′ –∞

∞

∫ =
12 Sources and Chronology of Nitrate Contamination in Spring Waters, Suwannee River Basin, Florida

A second approach for estimating average residence time (turnover time) of water in an aquifer 

involves the use of an exponential model (EM), which 

assumes that transit times in the aquifer are exponentially distributed and are related to the distribution of 

flow velocities. The model assumes steady-state flow 

conditions, areally distributed recharge to the aquifer, 

and constant recharge rates and other aquifer properties. The EM is mathematically similar to the wellknown model of a well-mixed reservoir (that is, complete mixing within the aquifer). However, the exponential model also has been applied to aquifers where 

mixing of stratified ground-water flow occurs as discharge to springs (Vogel, 1967; Maloszewski and 

Zuber, 1982; Focazio and others, 1998; Richter and 

others, 1993). The weighting function, g(t′), for the 

exponential model is (1/τm)exp(-t′/τm), where τm is the 

average residence time. 

A third approach used in this study to assess the 

ages of spring waters involves a binary-mixing model, 

which assumes that the age of water issuing from a 

spring can be represented by a simple mixture of relatively young water (recently recharged) from a relatively shallow flow system with older water originating 

from a deeper flow system. Two-component mixtures 

of spring waters would contain CFC compounds that 

yield different estimates of age resulting from the various fractions of young and old waters in the mixture. 

Mixing models typically are restricted to two endmember waters. With more than two waters mixing, 

the problem becomes essentially unsolvable because 

2X-1 tracers would be needed for X separate and 

distinct waters mixing in springs. There are not enough 

well-established tracers that could be used for age 

dating (Plummer and Busenberg, 1999).

Estimation of Nitrogen Inputs from 

Nonpoint Sources

As a way of assessing how changes in land use in 

the basin during the past 50 years have affected nitrate 

concentrations in ground water discharging to springs, 

nitrogen inputs from various nonpoint sources were 

estimated. Methods for compiling and estimating 

nitrogen inputs for each of five selected counties 

(Suwannee, Lafayette, Gilchrist, Columbia, and Alachua) in the study area are discussed below. For each 

county, estimated N input loads were computed for fertilizers, atmospheric deposition, animal wastes (dairy 

and beef cows, layer and broiler chickens, and swine), 

effluent from septic tank systems, and atmospheric 

deposition. 

Fertilizers

Estimates of N from fertilizers applied to cropland were computed by using two sources of information: (1) fertilizer sales records compiled annually by 

the Florida Department of Agriculture and Consumer 

Services (DACS), and (2) recommended rates of fertilizer application for various crop types by the Florida 

Institute of Food and Agricultural Sciences (Jones and 

others, 1974; Kidder and others, 1998). To obtain estimates of N inputs from fertilizer sales information, the 

amount of N was estimated for various materials sold 

as fertilizers. These dominant materials [with percentage by weight of N in parentheses (Farm Chemicals 

Handbook, 1998)] included anhydrous ammonia (82), 

ammonium nitrate (34), ammonium sulfate (21), cyanamid (21), potassium nitrate (16), nitrogen solutions 

(33), and urea (46). The relative amounts of these 

materials varied annually; however, the largest inputs 

of N were from the fertilizer materials anhydrous 

ammonia, ammonium nitrate, and nitrogen solutions. 

The amount of each material sold was multiplied by the 

percentage of N and summed to obtain the annual estimates of N from fertilizers sold in each county. Fertilizer sales data tended to overestimate the amount of N 

applied because not all fertilizer sold in a particular 

county was used in that county. There was no reliable 

way to track the amount of fertilizer sold in a particular 

county that was transported in or out of a county or that 

which was stockpiled in any given year. Therefore, 

estimated inputs of N from fertilizer sales information 

may represent maximum values of N inputs. 

Another estimate of N inputs from fertilizers was 

obtained from recommended application rates for various crop types and published information on crop 

acreage in each county. N inputs from fertilizer applications for years 1954, 1959, 1964, and 1969 were calculated by multiplying the total amount of fertilizer 

applied per unit area by the area on which fertilizer was 

applied (from Florida Agricultural Census data) by the 

average percentage of N by weight for fertilizers sold 

in each county. The average percentage of N by weight 

was calculated from historical sales data for each 

county for these 4 years. The percent N in each fertilizer mixture was weighted by amount sold for quantities greater than 22,700 kilograms (kg) or 25 tons. This 

represented approximately 94 percent of the total 

weight of mixed fertilizers sold annually. The remaining 6 percent was assumed to have the same percentage 

of N as the known amount. Estimates of N loading 

from fertilizer application in 1974 and for the 5 individ-
Study Methods 13

ual years following 1974 were based on crop acreage 

information from the Florida Agricultural Census. 

Starting in 1974, data for major crop acreage were 

compiled for each county. The number of acres of harvested crops was multiplied by the recommended rates 

of fertilizer application for each crop type (Kidder and 

others, 1998) and summed to obtain the total N for each 

county for a given year. Since the sum of the harvested 

cropland acreage was less than the reported total acres 

on which fertilizers were applied (DACS Census data), 

the total N loading from fertilizers was adjusted by 

multiplying the N-loading amounts for the harvested 

cropland acreage by the ratio of total acres of fertilizer 

applied to the sum of the harvested cropland acreage. 

 Animal Wastes

Estimates of N inputs from animal manure were 

calculated for 1955-95 from the numbers of beef cattle, 

dairy cattle, swine, broiler chickens, and egg-laying 

chickens in each county (U.S. Department of Commerce, 5-year intervals from 1946-97), and from estimates of the nutrient content of manure for each of 

these animals, as described below. Estimates of losses 

due to volatilization and other processes resulting from 

various waste management practices are discussed in a 

later section. 

Nutrient contributions, in kilograms per day 

 (kg/day) of nitrogen, from animal wastes were as follows, based on 454 kg (1,000 pounds) of animal: dairy 

cows, 0.18; beef cows, broiler chickens, 0.50; egg-laying chickens, 0.38; and swine, 0.24. Average weights, 

in kg, of animals used in the calculations were: broiler 

chickens, 0.9; egg-laying chickens, 1.4; beef cows, 

364; dairy cows, 635; and swine, 61 (American Society 

of Agricultural Engineers, 1996). 

Nitrogen loading to ground water from poultry 

manure and poultry litter (combination of manure and 

bedding materials) during 1954-97 was estimated by 

using information on the annual estimates of the number of broilers (also called fryers or frying chickens) 

from Florida agricultural census data (DACS, 1954-97) 

and from estimates of the amount of annual manure 

production and its average total N content. An annual 

N-loading estimate of 0.022 kg per broiler is based on 

a manure production rate of 1.1 kg with an average total 

N content of 2 percent (Sloan and others, 1992; Vest 

and Merka, 1994). 

Septic Tank Effluent

Annual N inputs from domestic septic tank 

systems were estimated by multiplying the average 

mass loading of N, 4.09 kg N per capita per year (Otis 

and others, 1993), by the number of people that are 

contributing wastes to septic tank systems in each of 

the five counties (U.S. Bureau of the Census, 1990). 

The percentage of the population contributing wastes 

to septic tanks or cesspools in each county was calculated from sewage disposal data from the 1990 Census 

of Housing: Alachua (27), Columbia (72), Gilchrist 

(88), Lafayette (83), and Suwannee (76). Sewage 

disposal data were available only from the 1990 Census 

of Housing (U.S. Bureau of the Census, 1993). To estimate N inputs from septic tanks prior to 1990, it was 

assumed that the percentage of the population on septic 

systems remained constant back to 1955. The average 

mass N loading assumes a water use of 170 liters per 

capita per day (Otis and others, 1993). Nitrogen loading from other human waste sources, such as commercial septic tank effluent and recreational vehicle 

wastewater, were assumed to be negligible in this 

study. 

Atmospheric Deposition

Estimates of N loading from atmospheric deposition were obtained from measurements of nitrate 

(NO3) and ammonium (NH4) in rainfall collected at the 

long-term NADP/NTN station in Bradford Forest, in 

Bradford County, which is located in the eastern part of 

the Suwannee River Basin (fig. 1). Annual wet deposition rates of total inorganic nitrogen (NO3-N and 

NH4-N) for the the Bradford Forest station for 1979 to 

1997 ranged from 2.01 to 3.99 kilograms per hectare 

(kg/ha ). The nitrogen load from atmospheric deposition (wetfall plus dryfall) for each county was obtained 

by multiplying the annual wet deposition rates of inorganic N by the county area and by the dry-deposition 

enrichment factor. Using a mean total dry-deposition 

flux of nitrogen (0.96 times the wet-deposition flux, 

+/- 0.14) for northern Florida (Baker, 1991) results in 

atmospheric deposition rates for 1979-97 that range from 

3.94 to 7.82 kg/ha. Dry-deposition fluxes of N were 

based on particulate and gas fluxes of nitric oxide 

(NO2) and nitric acid (HNO3) (Baker, 1991). The average of N loading estimates from atmospheric deposition during 1979-97 was assumed to represent N 

loading from atmospheric deposition during 1954-78. 
14 Sources and Chronology of Nitrate Contamination in Spring Waters, Suwannee River Basin, Florida

CHEMISTRY OF SPRING WATERS AND 

WATER FROM WELLS

In this section, the concentrations of major ions, 

nutrients, and dissolved organic carbon are used to provide information on the sources and processes that 

affect solute concentrations in water from the Upper 

Floridan aquifer. Also, dissolved gas data, in particular 

Ar and N2, are used to describe recharge processes in 

the ground-water system.

Major Ions and Nutrients

Water from springs and shallow zones in the Upper 

Floridan aquifer is classified as a calcium-bicarbonate 

(Ca-HCO3) type having low dissolved solids concentrations in spring waters ranging from 150 to 250 mg/L 

(table 3). Dissolved organic carbon concentrations in 

spring water and ground water are low, typically less than 

or equal to 1 mg/L (analytical method reporting limit of 

0.1 mg/L) with one exception; the DOC concentrations in 

water from Columbia Spring was 13 mg/L. Saturation 

indices of the spring waters with respect to calcite and 

dolomite were typically less than 0.0 (calcite SI ranged 

from -0.35 to 0.14, and dolomite SI ranged from -2.46 to 

-0.20) indicating that the waters are slightly undersaturated with respect to these two minerals which form the 

limestone rock matrix (table 3). Saturation indices with 

respect to calcite and dolomite were calculated by using 

the computer-based thermodynamic model WATEQFP 

(Plummer and others, 1994), which assumes that all dissolved species are at equilibrium with one another. When 

ground water is undersaturated with respect to calcite and 

dolomite, there is the potential for continued dissolution 

of the aquifer rock. 

Nitrate-N concentrations in spring waters were 

quite variable. In some waters from springs such as 

Columbia, Ichetucknee Blue Hole, Poe, and Hornsby 

(table 3), nitrate-N concentrations were less than 

1.0 mg/L, but still elevated above background nitrate 

concentrations of about 0.05 mg/L for the area (Katz, 

1992; Maddox and others, 1992). Nitrate-N concentrations in other spring waters clearly show the impact of 

agricultural activities in the basin; in particular springs 

SUW718971 (29 mg/L) and SUW725971B (38 mg/L) 

likely receive recharge water from an area dominated 

by cropland (approximately 2,000 hectares (ha)) that is 

extensively fertilized and irrigated. Likewise, the 

nitrate-N concentration in water from spring 

GIL917971, which receives water from an area with 

cropland and animal farming operations was 26 mg/L. 

In most cases, nitrate-N concentrations in water samples collected from springs during this study were 

slightly higher than nitrate concentrations found in 

water samples previously collected from these same 

springs (Hornsby and Mattson, 1996). 

During low-flow conditions in July 1997, nitrateN concentrations in water from wells -9001 and -6011, 

tapping shallow zones of the Upper Floridan aquifer in 

Lafayette County, were 18 and 20 mg/L, respectively 

(table 3). In contrast, following a period of prolonged 

rainfall during the winter and early spring of 1998, 

nitrate-N concentrations decreased substantially in 

water samples from well-6011 (13 mg/L) and well9001 (10 mg/L). A similar trend between high-flow 

and low-flow conditions was reported by Katz and others (1997) for ground-water samples collected in 1990 

and 1991 from wells tapping the Upper Floridan aquifer 

in this area. The decrease in nitrate concentrations can 

be attributed in part to denitrification reactions (Katz and 

others, 1997); however, dilution with recharge water low 

in nitrate is another important factor.

The concentrations of other dissolved species, 

such as sulfate, potassium, and magnesium, also are 

higher in spring water and water from the two zones in 

the Upper Floridan aquifer than background concentrations of these species found in the Upper Floridan aquifer in this area (Katz, 1992; Maddox and others, 1992). 

Sulfate concentrations in spring waters and ground 

water are elevated by a factor of 3, with the exception 

of water from Ichetucknee Blue Hole Spring, which 

has sulfate concentrations below the median background concentration of 5.3 mg/L (Katz, 1992). 

Concentrations of potassium in water from the two 

wells in the agricultural area are more than an order of 

magnitude higher than background concentrations 

(0.49 mg/L; Katz, 1992). Magnesium concentrations 

are slightly higher than background (6.1 mg/L; Katz, 

1992) in spring waters, but are 3 to 4 times higher in 

ground water collected from the two zones of the 

Upper Floridan aquifer. There is a significant (p<0.05) 

correlation between nitrate and potassium concentrations, as well as between chloride and potassium, and 

between sulfate and chloride in spring waters. The elevated concentrations of sulfate, chloride, potassium, and 

magnesium in spring water and ground water indicates 

the likely contribution from anthropogenic sources, such 

as fertilizers and leachate from animal wastes. Even 

though large amounts of phosphorus are applied to 

cropland in the basin, phosphate concentrations in
C

hemistryofSpringWatersandWaterfromWells15

Table 3. Chemical characteristics, concentrations of major ions and other dissolved species, calcite and dolomite saturation indices fo[Concentrations of elements and species are in milligrams per liter, unless otherwise noted; °C, degrees Celsius; SC denotes specific conductance in microsiemens pedissolved organic carbon; SiO2, dissolved silica; DS, dissolved solids; SI, saturation index]

SPRING OR WELL NAME

Sample 

date

Temperature, 

oC

pH

Specific 

conductance

Dissolved 

oxygen

Ca Mg Na K NO3-N NH3-N Organic N HCO3 Cl SO4 F SiO

SANTA FE RIVER SPRINGSGIL917971 8/20/98 22.0 7.32 462 3.28 78.5 5.2 6.6 2.03 26.00 0.030 <0.2 197 9.8 15.6 0.10 5.Ginnie Springs 8/21/98 22.3 7.51 305 4.64 54.0 4.7 2.4 0.23 1.20 0.030 <0.2 175 5.1 7.8 0.10 6.July Spring 8/21/98 22.3 7.39 368 4.53 64.1 6.1 3.4 0.39 1.70 0.030 <0.2 208 6.3 11.5 0.12 6.Poe Spring 8/21/98 22.3 7.45 387 1.56 67.3 5.2 5.2 0.56 0.82 0.030 <0.2 220 8.9 10.2 0.13 7.Hornsby Spring 8/21/98 22.2 7.36 404 1.10 64.3 6.8 8.2 0.87 0.80 0.030 <0.2 193 11.9 32.2 0.18 10.Columbia Spring 8/20/98 24.6 7.29 332 2.71 48.8 7.9 7.7 1.05 0.42 0.050 0.4 132 11.4 41.0 0.19 11.Trail Spring 8/20/98 21.6 7.40 345 2.14 60.4 4.3 3.6 2.13 3.80 0.030 <0.2 177 6.2 13.7 0.10 4.Ichetucknee Blue Hole 

Spring

8/14/97 21.7 7.39 262 2.42 51.0 4.7 2.8 0.37 0.72 0.010 <0.2 176 4.7 4.3 0.13 9.

median value 22.25 7.39 357 2.57 62.3 5.2 4.4 0.7 1.01 0.03 <0.2 185 7.63 12.55 0.13 6.LAFAYETTE CO. SPRINGSTroy Spring 7/15/97 22.4 7.20 362 0.38 59.8 6.5 3.0 0.99 2.70 0.013 <0.2 196 6.1 11.0 0.1 6.Troy Spring 8/19/98 21.2 7.53 367 1.74 62.6 6.7 3.5 1.11 2.80 0.060 <0.2 203 6.2 10.8 0.10 6.Mearson Spring 8/19/98 21.2 7.43 362 1.90 59.8 8.8 3.2 0.59 1.70 <0.01 <0.2 209 5.4 10.8 0.12 6.Lafayette Blue Spring 7/15/97 21.8 7.11 423 0.87 64.9 10.5 5.5 0.70 2.00 0.012 <0.2 237 8.7 11.0 0.1 5.Ruth Spring 8/19/98 20.9 7.25 405 1.96 68.5 6.0 5.0 2.67 5.50 <0.01 <0.2 205 8.7 13.2 0.10 5.LAF718972 8/19/98 21.0 7.28 383 0.45 66.4 6.2 3.8 1.76 3.00 0.020 <0.2 214 7.6 8.4 0.10 4.median value 21.2 7.27 375 1.3 63.8 6.6 3.7 1.1 2.75 0.0 <0.2 206.9 6.9 10.9 0.1 6.LOWER SUWANNEE SPRINGSPothole Spring 8/20/98 21.6 7.25 451 2.32 78.6 0.1 3.5 0.71 1.50 0.040 <0.2 242 7.0 32.5 0.13 6.Fannin Springs 8/18/98 22.3 7.28 452 3.03 79.3 5.2 4.3 2.38 4.50 0.010 <0.2 231 9.7 18.0 0.11 6.Manatee Springs 8/18/98 22.1 7.20 460 2.00 83.1 5.8 3.7 0.99 1.70 0.010 <0.2 239 8.0 31.0 0.10 5.Rock Bluff Spring 8/20/98 21.8 7.43 300 2.85 56.1 3.0 2.6 0.39 1.10 0.030 <0.2 167 4.5 10.8 0.10 5.median value 22.0 7.27 451.5 2.6 79.0 4.1 3.6 0.9 1.60 0.0 <0.2 234.8 7.5 24.5 0.1 5.SUWANNEE CO. SPRINGSRunning Springs 8/18/98 21.2 7.50 344 2.20 54.3 13.0 2.3 0.39 2.10 0.020 <0.2 209 5.2 14.0 0.16 6.Little River Springs 7/15/97 21.9 7.27 373 1.95 61.8 6.9 2.6 0.63 1.50 0.014 <0.2 201 5.9 18.3 0.1 6.SUW718971 8/19/98 21.0 7.41 455 5.52 58.5 16.2 5.4 3.49 29.00 0.070 <0.2 167 14.1 23.2 0.18 6.SUW725971A 8/19/98 21.1 7.42 481 5.90 61.8 14.9 6.0 4.20 37.00 <0.01 <0.2 149 16.1 27.0 0.17 5.Charles Spring-1 7/15/97 21.6 7.08 370 1.50 57.0 10.0 2.8 0.54 2.20 0.013 <0.2 200 5.1 17.0 0.1 6.Telford Spring 7/16/97 21.3 7.22 461 3.38 63.8 17.4 3.1 0.41 2.50 0.015 <0.2 226 5.9 43.6 0.2 7.SUW725971B 8/19/98 21.1 7.42 481 5.90 61.8 14.9 5.6 4.20 38.00 0.040 <0.2 149 16.1 27.0 0.17 5.Charles Spring-2 7/15/97 21.6 7.08 370 1.50 56.0 10.0 2.8 0.54 2.30 0.013 <0.2 200 5.1 17.0 0.1 6.median value 21.25 7.34 414 2.79 60.15 13.95 2.96 0.59 2.35 0.02 <0.2 200.5 5.89 20.73 0.17 6.LAFAYETTE CO. WELLSWell-6011 7/16/97 22.6 7.21 560 4.38 70.5 16.6 8.3 6.19 20.00 0.018 <0.2 203 15.2 19.6 <.1 6.Well-9001 7/16/97 23.2 7.12 570 4.10 58.1 23.2 11.6 6.40 18.00 0.014 <0.2 225 14.1 15.8 0.1 7.Well-6011 3/9/98 22.3 7.00 501 2.14 65.0 14.2 6.4 4.89 13.00 0.094 <0.2 200 11.5 17.9 0.11 6.Well-9001 3/9/98 23.0 7.27 319 1.52 62.3 18.4 7.6 4.89 10.00 0.024 <0.2 224 16.4 14.6 0.13 4.
16 Sources and Chronology of Nitrate Contamination in Spring Waters, Suwannee River Basin, Florida

spring waters and ground water from the Upper Floridan aquifer are low, typically less than 0.05 mg/L, 

except water from Poe, Hornsby, and Columbia 

Springs, which range from 0.18 to 0.24 mg/L. Low 

phosphate concentrations in water from the Upper 

Floridan aquifer are related to the low solubility of 

phosphate, which is controlled by coprecipitation and 

adsorption reactions with metal oxides, particularly 

ferric and manganese oxyhydroxides (Hem, 1985). 

Dissolved Gases

Dissolved gas data, particularly Ar and N2, can 

provide valuable information on recharge processes in 

ground-water systems. The solubilities of Ar and N2

vary substantially as a function of temperature (Weiss, 

1970); consequently, concentrations of these gases in 

ground water can indicate the temperature of the unsaturated zone during recharge. The recharge-water temperature and amounts of excess air (Heaton and Vogel, 

1981) can be evaluated from a plot of dissolved Ar versus N2 concentrations (Busenberg and others, 1993). 

In figure 4, the solubilities of Ar and N2 are shown for 

temperatures ranging between 0 and 30 o

C at sea level 

along the zero-excess-air line, which is labeled water in 

equilibrium with air. The lines labeled 5, 10, 15, and 

20 represent the equilibrium concentrations of Ar and 

N2 with 5, 10, 15, and 20 cubic centimeters per liter 

(cm3/L) of air added in excess of the equilibrium concentrations. Other information on the evolution of dissolved gases can be obtained from this plot. For 

example, adding radiogenic Ar will shift the sample 

composition vertically as shown in the lower right hand 

corner of the plot (fig. 4). Denitrification reactions 

involving the microbially mediated reduction of nitrate 

to N2 will shift the dissolved-gas composition horizontally. Excess air added during recharge processes will 

shift the composition diagonally. The recharge-water 

temperature of a sample plotted on fig. 4 is obtained by 

extending a diagonal line parallel to the excess air lines 

from the sample to the recharge-water temperature axis 

on the right-hand side of the diagram (Busenberg and 

others, 1993). The excess-air content of the sample is 

obtained by following a diagonal line (parallel to the 

excess-air lines) from the sample to the excess-air axis 

on the top of the diagram.

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

10 15 20 25 30 35 40 45 50

0

5

10

15

20

EXCESS AIR IN

CUBIC CENTIMETERS

0

5

10

15

20

25

30

RECHARGE

TEMPERATURE

IN o

C 

WATER IN

EQUILIBRIUM

WITH AIR LINE

DISSOLVED NITROGEN, IN MILLIGRAMS PER LITER

ARGON, IN MILLIGRAMS PER LITER

NO3 Reduction to N2

Excess Air

Radiogenic Air

Figure 4. Dissolved nitrogen and argon concentrations in spring waters and water from wells.
Chemistry of Spring Waters and Water from Wells 17

The concentrations of N2 and Ar in spring-water 

samples are consistent with atmospheric equilibration 

during ground-water recharge with minor amounts of 

excess air added either during recharge or as a result of 

sampling methods. The apparent recharge temperatures are 22 ± 3 oC (assuming an 8-m elevation for 

recharge and 100 percent humidity at the water table) 

with about 0 to 7 cm3

/L of excess air during recharge. 

This calculated temperature agrees closely with a mean 

annual air temperature of 20.4 oC at Live Oak, Fla., for 

the period 1961-90 (Owenby and Ezell, 1992). Also, 

δ15N values of dissolved N2 generally are consistent

with air equilibration during recharge and with minor 

amounts of excess air (0.6 to 0.8 per mil). Water 

samples from well-6011 and from Troy and 

GIL917971 springs have slightly lower δ15N (N2) values (0.3 to 0.4 per mil), which could indicate small 

amounts of non-atmospheric N2; however, some of 

those samples may have leaked slightly. Of the three 

sites, water from well-6011 seems most likely to have 

excess N2, based on the isotope and N2 concentration 

data (possibly as much as 2 to 3 mg/L). With the 

exception of water from well-6011, denitrification 

reactions likely have not reduced nitrate concentrations 

in the ground-water system.

Table 4. Dissolved gas data, calculated recharge temperature, and excess air for samples of spring water and water from wells

[mgL, milligrams per liter; mmol/L, millimoles per liter; °C, degrees Celsius; cm3, cubic centimeters] 

 Spring/Well Name Concentration in mg/L Concentration in mmol/L Recharge 

Temp. 

Excess 

air

CH4 N2 Ar CH4 N2 Ar oC cm3

Santa Fe River Springs:

GIL917791 0.000 17.832 0.569 0.000 0.637 0.014 25.7 4.2

GIL917791 0.000 17.897 0.572 0.000 0.639 0.014 25.4 4.2

Ginnie 0.000 16.016 0.567 0.000 0.572 0.014 20.8 1.2

Ginnie 0.000 16.095 0.562 0.000 0.575 0.014 21.7 1.6

Hornsby 0.001 19.130 0.615 0.000 0.683 0.015 21.6 4.6

Hornsby 0.001 17.441 0.583 0.000 0.623 0.015 22.1 3.0

July 0.000 15.786 0.549 0.000 0.564 0.014 23.1 1.6

July 0.000 15.843 0.553 0.000 0.566 0.014 22.6 1.5

Poe 0.001 16.752 0.572 0.000 0.598 0.014 22.1 2.3

Poe 0.001 16.727 0.573 0.000 0.597 0.014 21.8 2.2

Trail 0.000 17.450 0.583 0.000 0.623 0.015 22.1 3.0

Trail 0.000 17.400 0.577 0.000 0.621 0.014 22.9 3.2

Columbia 0.005 15.319 0.542 0.000 0.547 0.014 22.9 1.1

Columbia 0.006 15.226 0.535 0.000 0.544 0.013 23.8 1.2

Ichetucknee Blue Hole 0.000 17.729 0.601 0.000 0.633 0.015 20.1 2.8

Ichetucknee Blue Hole 0.000 17.738 0.599 0.000 0.633 0.015 20.4 2.9

Lafayette County Springs:

Troy 1998 0.000 19.062 0.605 0.000 0.680 0.015 23.1 4.9

Troy 1998 0.000 19.103 0.608 0.000 0.682 0.015 22.7 4.8

Troy 1997 0.001 17.825 0.604 0.000 0.636 0.015 19.9 2.8

Troy 1997 0.001 17.689 0.599 0.000 0.631 0.015 20.3 2.8

Mearson 0.001 17.736 0.588 0.000 0.633 0.015 22.2 3.3

Mearson 0.001 17.710 0.590 0.000 0.632 0.015 21.8 3.2

Lafayette Blue 0.002 17.675 0.601 0.000 0.631 0.015 20.0 2.7

Lafayette Blue 0.002 17.751 0.605 0.000 0.634 0.015 19.6 2.7

Ruth 0.000 17.792 0.590 0.000 0.635 0.015 22.0 3.3

Ruth 0.000 17.829 0.593 0.000 0.636 0.015 21.5 3.3

LAF718972 0.004 19.278 0.611 0.000 0.688 0.015 22.7 5.0

LAF718972 0.004 19.380 0.615 0.000 0.692 0.015 22.3 5.0

Lower Suwannee Springs:

Fannin 0.000 17.466 0.565 0.000 0.623 0.014 25.3 3.8

Fannin 0.000 17.587 0.568 0.000 0.628 0.014 25.1 3.9

Pothole 0.008 17.831 0.608 0.000 0.637 0.015 19.3 2.7

Pothole 0.008 17.858 0.608 0.000 0.637 0.015 19.3 2.7
18 Sources and Chronology of Nitrate Contamination in Spring Waters, Suwannee River Basin, Florida

SOURCES OF SPRING WATER 

AND WATER FROM THE UPPER 

FLORIDAN AQUIFER

Stable isotopes of oxygen (δ18O) and hydrogen 

(δD) were used in this study to determine the origin of 

water and to characterize possible mixing of different 

sources of water in springs and in zones of the aquifer. 

Values of δ18O and δD in spring waters cluster along the 

global meteoric water line (Craig, 1961), δD = 8δ18O + 

10, (fig. 5), which indicates that springs receive ground 

water that is recharged from rainfall that undergoes little 

or no evaporation. The more or less random deviations 

from the meteoric water line indicate that the spring 

waters are affected little by evaporation or mixing with 

surface waters that would have enriched isotopic signatures due to evaporation. Since spring water is representative of ground water that is integrated both temporally 

and spatially with rainfall (meteoric water) that 

recharges the Upper Floridan aquifer, one would expect 

the stable isotopic composition of the spring waters to 

plot near the global meteoric water line. The observed 

small differences in isotopic composition likely are 

related to storm track origin, the number of evaporation 

and condensation cycles (Dansgaard, 1964), and temperatures of infiltrating rain in recharge areas for the 

springs. Similar values of δ18O and δD were observed 

for monthly samples of rainfall (fig. 5) collected from 

June 1995 through June 1996 at the Rocky Hill Fire 

Tower in Suwannee County (fig. 1) (Katz and others, 

1998). Monthly samples were composited from weekly 

samples collected by a wet/dry atmospheric deposition 

collector. Values of δD and δ18O are slightly enriched 

(higher) in water from well-6011 (table 5) compared to 

the isotopic composition of other ground-water samples, 

indicating either that water recharging the aquifer at this 

site may have undergone some evaporation or that mixing has occurred with surface water that contains an 

enriched isotopic signature. 

Lower Suwannee Springs: (Continued)

Manatee 0.001 17.603 0.581 0.000 0.628 0.015 22.9 3.4

Manatee 0.001 17.899 0.586 0.000 0.639 0.015 22.9 3.7

Rock Bluff 0.001 17.238 0.590 0.000 0.615 0.015 20.5 2.4

Rock Bluff 0.002 17.377 0.589 0.000 0.620 0.015 21.0 2.7

Suwannee County Springs:

Running Spring 0.000 17.235 0.595 0.000 0.615 0.015 19.7 2.2

Little River 0.000 17.836 0.614 0.000 0.637 0.015 18.4 2.4

Little River 0.000 18.079 0.618 0.000 0.645 0.015 18.5 2.7

SUW 725971 0.000 16.253 0.570 0.000 0.580 0.014 21.0 1.5

SUW 725971-B 0.000 17.274 0.598 0.000 0.617 0.015 19.3 2.1

SUW 725971-B 0.000 16.478 0.579 0.000 0.588 0.014 20.1 1.5

SUW 718971 0.000 16.749 0.562 0.000 0.598 0.014 23.7 2.7

SUW 718971 0.001 17.006 0.567 0.000 0.607 0.014 23.6 2.9

Charles 0.001 17.157 0.591 0.000 0.612 0.015 20.2 2.2

Charles 0.001 17.152 0.588 0.000 0.612 0.015 20.6 2.3

Telford 0.000 16.757 0.587 0.000 0.598 0.015 19.7 1.7

Telford 0.000 17.096 0.595 0.000 0.610 0.015 19.3 2.0

Lafayette County wells:

Well-6011 (7/16/97) 0.008 17.045 0.559 0.001 0.608 0.014 25.2 3.3

Well-6011 (7/16/97) 0.019 17.143 0.558 0.001 0.612 0.014 25.5 3.5

Well-9001 (7/16/97) Leak 0.000 17.812 0.566 0.000 0.636 0.014 26.2 4.3

Well-9001 (7/16/97) 0.000 16.628 0.581 0.000 0.594 0.015 20.3 2.8

Well-6011 (03/09/98) 0.001 18.056 0.548 0.000 0.645 0.014 30.5 5.4

Well-6011 (03/09/98) 0.001 17.890 0.545 0.000 0.639 0.014 30.6 5.3

Well-9001 (03/09/98) 0.000 17.380 0.564 0.000 0.620 0.014 25.2 3.7

Well-9001 (03/09/98) Leak 0.000 25.923 0.710 0.000 0.925 0.018 25.1 12.2

Table 4. Dissolved gas data, calculated recharge temperature, and excess air for samples of spring water and water from wells 

(Continued)

[mgL, milligrams per liter; mmol/L, millimoles per liter; °C, degrees Celsius; cm3, cubic centimeters] 

 Spring/Well Name

Concentration in mg/L Concentration in mmol/L Recharge 

Temp. 

Excess 

air

CH4 N2 Ar CH4 N2 Ar oC cm3
Sources of Spring Water and Water from the Upper Floridan Aquifer 19

Spring water in the unconfined Upper Floridan 

aquifer evolves mostly under open-carbon-dioxide 

(CO2) conditions, as indicated by calculated values 

(WATEQF; Plummer and others, 1994) of the partial 

pressure of CO2 (expressed as log PCO2, in atmospheres) that exceed the atmospheric value of −3.5 and 

range from -2.29 to -1.77. Values of δ13C (DIC) are 

enriched (higher) in spring waters (-10.8 to -13.2 per 

mil) (table 5) compared to water from the surficial 

aquifer system (-18 to -25 per mil) in northern Florida 

(Katz and others, 1998). Rapid recharge rates in the 

study area, which result from the highly permeable 

sands, facilitate the downward transport of oxygen and 

DOC from the unsaturated zone. Heterotrophic bacteria present in the unsaturated zone aerobically degrade 

DOC in interstitial waters (δ13C ~ -24 per mil) to produce CO2 that is slightly enriched in its δ13C content 

(Deines, 1980). DOC concentrations in the Upper 

Floridan aquifer can range from 0.03 to 0.80 millimoles 

per liter (mmol/L), indicating that organic carbon is 

present for microbial degradation reactions (Katz and 

others, 1998). The PCO2, which has accumulated in 

the unsaturated zone to levels that exceed atmospheric, 

can persist in ground water because of the low rate of 

consumption of CO2 by the dissolution of silicate minerals present in the sediments overlying the Upper 

Floridan aquifer (Katz and others, 1995b). 

Most springs in the Suwannee River Basin in 

Florida intercept water from large parts of the aquifer, 

thus their 13C composition is derived from nearly 

equimolar amounts of carbon from CO2 (δ13C= -24 per 

mil from degradation of organic material) and from dissolution of calcite (δ13C= 0 per mil), according to the 

following reaction:

CaCO3 + H2CO3 = Ca2+ + 2HCO3

-

 . (4)

 The conversion of CO2 to HCO3

-

 during the dissolution of calcite (under conditions that remain open 

to CO2) was indicated by a decrease in PCO2 levels in 

water from the Upper Floridan aquifer compared to 

SANTA FE RIVER SPRINGS

LAFAYETTE COUNTY SPRINGS

LOWER SUWANNEE RIVER SPRINGS

SUWANNEE COUNTY SPRINGS

LAFAYETTE COUNTY WELLS

GLOBAL METEORIC WATER LINE

RAINFALL (6/95 - 6/96)

-24

-22

-20

-18

-16

-14

-12

-10

δ DEUTERIUM, PER MIL, VSMOW

-5.0 -4.5 -4.0 -3.5 -3.0 -2.5 -2.0 -1.5 -1.0

δ OXYGEN-18, PER MIL, VSMOW

Figure 5. Deuterium and oxygen-18 content of spring water, ground water, and rainfall compared to the global 

meteoric water line.
20 Sources and Chronology of Nitrate Contamination in Spring Waters, Suwannee River Basin, Florida

Table 5. Isotopic composition of spring waters and ground water

[δ, delta; CO., county; ND, not determined. Values are in per mil. Tritium concentrations 

are in tritium units]

Spring or well name Sample 

Date δ18O δD δ13C δ15N-NO3 Tritium

SANTA FE RIVER SPRINGS

GIL917971 8/20/98 -3.73 -19.7 -11.8 6.4 3.3

Ginnie Springs 8/21/98 -3.57 -17.3 -10.8 3.7 5.2

July Spring 8/21/98 -3.64 -18.0 -11.2 4.1 5.4

Poe Spring 8/21/98 -3.35 -16.3 -11.9 10.6 5.1

Hornsby Spring 8/21/98 -3.03 -15.0 -11.5 8.7 5.8

Columbia Spring 8/20/98 -3.32 -15.0 -12.1 ND 4.3

Trail Spring 8/20/98 -3.86 -17.8 -11.2 8.7 4.5

Ichetucknee Blue 

Hole Spring

8/14/97 -3.48 -18.6 -12.0 4.4 5.8

LAFAYETTE CO. SPRINGS

Troy Spring-97 7/15/97 -3.78 -20.1 -12.3 5.4 4.0

Troy Spring-98 8/19/98 -3.80 -18.8 -12.0 5.4 4.0

Mearson Spring 8/19/98 -3.77 -17.1 -11.7 4.5 4.2

Lafayette Blue Spring 7/15/97 -3.42 -18.0 -12.6 8.4 4.0

Ruth Spring 8/19/98 -3.49 -17.3 -11.9 6.9 3.0

LAF718972 8/19/98 -3.23 -17.5 -11.8 9.1 5.1

LOWER SUWANNEE SPRINGS

Pothole Spring 8/20/98 -3.14 -14.6 -11.7 4.8 5.2

Fannin Springs 8/18/98 -3.65 -18.1 -11.5 7.1 3.7

Manatee Springs 8/18/98 -3.55 -16.3 -11.4 5.8 4.5

Rock Bluff Spring 8/20/98 -3.71 -18.9 -11.5 3.2 4.8

SUWANNEE CO. SPRINGS

Running Springs 8/18/98 -3.94 -20.0 -11.5 6.2 5.3

Little River Springs 7/15/97 -3.87 -21.0 -12.0 3.9 4.2

SUW718971 8/19/98 -3.71 -19.4 -12.2 2.7 4.7

SUW725971A 8/19/98 -3.59 -17.7 -12.2 2.9 3.3

SUW725971B 8/19/98 -3.59 -17.7 -12.2 2.9 3.3

Charles Spring-1 7/15/97 -3.75 -19.3 -12.9 4.4 4.4

Charles Spring-2 7/15/97 -3.80 -19.9 -13.2 4.4 4.8

Telford Spring 7/16/97 -3.89 -20.3 -12.4 5.8 4.9

LAFAYETTE CO. WELLS

Well-6011 7/16/97 -3.07 -16.2 -9.7 12.1 4.4

Well-9001 7/16/97 -3.85 -22.2 -8.3 11.0 4.0

Well-6011 3/9/98 -2.00 -11.2 -8.5 12.8 3.1

Well-9001 3/9/98 -3.17 -17.7 -8.9 10.2 6.6

PCO2 levels in water from wells tapping the overlying 

surficial aquifer system in northern Florida (Katz and 

others, 1995b). Dissolution of calcite is indicated by 

other lines of evidence including: (1) water from 

springs and wells tapping shallow zones of the Upper 

Floridan aquifer was slightly undersaturated to equilibrium with respect to calcite (table 3); (2) the concentration of Ca2+ and HCO3

-

 (DIC) increased 

substantially compared to rain (recharge) water; 

(3) δ13C values (enrichment in 13C) increased; 

and (4) molar concentrations of Ca2+ and 

HCO3

-

 plotted along a line with slope of 2:1 

indicating reaction (2). Several spring waters 

(SUW781971, SUW725971, GIL917971) 

have HCO3

-

 /Ca2+ ratios that are substantially 

less than 2.0 (1.58-1.88), thus indicating that 

another source of Ca, such as gypsum 

(CaSO4-2H2O), is likely. When Ca concentrations in spring waters are adjusted by subtracting excess Ca resulting from dissolution 

of gypsum (containing equimolar amounts of 

Ca2+ and SO4

2), above that of background sulfate concentrations in ground water (8 mg/L) 

(Katz, 1992; Maddox and others, 1992), the 

HCO3

-

 /Ca2+ molar ratio approaches 2.0. 

These springs drain areas where high quantities of fertilizers and other soil treatments 

including gypsum have been applied. The low 

HCO3

-

 /Ca2+ molar ratio of 1.77 for water 

from Columbia Spring may indicate mixing of 

ground water with water from the Santa Fe 

River, which contains substantially lower 

concentrations of HCO3. Other evidence for 

surface-water mixing in this spring system is 

the high DOC concentration (13 mg/L) as 

compared to that in other spring waters.

SOURCES OF NITRATE IN SPRING 

WATER AND WATER FROM 

THE UPPER FLORIDAN AQUIFER

Values of δ15N of nitrate (δ15N-NO3) 

have been used to identify sources of nitrate 

contamination in ground water since the mid1970’s (Heaton, 1986). The method has been 

used successfully in areas with thin, permeable unsaturated zones and shallow water 

tables (Heaton, 1986), and in mantled karst 

aquifers (Wells and Krothe, 1989; Andrews, 

1994; Hornsby, 1994). Low δ15N-NO3 values (0 to 3 

per mil) generally indicate an inorganic nitrate source 

(artificial or synthetic fertilizer); whereas higher δ15NNO3 values (10 to 20 per mil) typically indicate an 

organic (animal waste--manure spreading or waste disposal) source of nitrate. Values of δ15N-NO3 that fall 

between 3 and 10 per mil likely are indicative of mixed 

inorganic and organic sources of nitrate or a soil organic 

nitrogen source. 
Sources of Nitrate in Spring Water and Water from the Upper Floridan Aquifer 21

An important assumption in using the nitrogenisotope method is that nitrate has not been removed 

from ground water due to denitrification reactions that 

may occur in the aquifer between recharge and discharge to springs. Denitrification can result in an 

enrichment of 15N of NO3 and an increase in N2, thus 

a corresponding increase in δ15N-NO3 values could 

lead to conclusions about N sources that are not consistent with actual values during recharge. Generally, 

concentrations of N2 and Ar are consistent with atmospheric equilibration during ground-water recharge 

with minor amounts of excess air added either during 

recharge or as a result of sampling methods. Consequently, denitrification reactions do not seem to be 

affecting nitrate concentrations in spring waters. 

Values of δ15 N-NO3 in water from the 

24 sampled springs (table 5) range from 2.7 per mil 

(SUW718971) to 10.6 per mil (Poe Spring). The 

median δ15N-NO3 value for all sampled spring waters 

is 5.4 per mil. The range of δ15N-NO3 values indicates 

a variation in sources of nitrate. Spring waters with 

lower δ15N-NO3 values (+2 to +6 per mil) and high 

NO3 concentrations (greater than 20 mg/L), such as 

SUW718971 and SUW725971, likely are discharging 

nitrate from fields receiving artificial fertilizers. These 

two springs probably receive recharge water from an 

area dominated by cropland in Suwannee County 

(approximately 2,000 ha) that is extensively fertilized 

and irrigated. Springs such as Mearson, Little River, 

and Rock Bluff, are discharging waters with low NO3

concentrations (less than 2 mg/L) and low δ15N-NO3

values that may represent nitrate derived from fertilized fields (diluted) and or natural soil organic matter 

sources. Spring waters with higher δ15N-NO3 values 

(> +9 per mil) generally indicate nitrate associated with 

manure spreading or waste disposal. The relatively 

high δ15N-NO3 values for Poe Spring (10.6 per mil) 

and Lafayette Blue Spring (8.4 per mil) indicate the 

likelihood that nitrate originates from an organic 

source. These two springs are located in county parks 

where waste-disposal systems operated near the 

springs may contribute nitrate to ground water. 

Water from wells-9001 and -6011 in Lafayette 

County sampled in 1997 have higher δ15N-NO3 values 

than spring waters, 11.0 and 12.1 per mil, respectively, 

indicating the likelihood of an organic (animal waste) 

source of nitrate. Even though nitrate concentrations 

decreased substantially in water from these two wells 

following a period of sustained rainfall, the δ15N-NO3

value in water samples from March 1998 decreased 

only slightly to 10.2 per mil in water from well-9001 

and increased slightly to 12.8 per mil in well-6011. 

These two wells are located near and downgradient 

from dairy and poultry farms, where leachate from animal wastes may contribute nitrate to ground water. 

No trend was observed between the concentration of nitrate and δ15N-NO3 values in spring waters 

(fig. 6), indicating that concentrations of nitrate in 

spring waters and in zones of the Upper Floridan aquifer are not likely altered after recharge. A trend of 

higher δ15N-NO3 values corresponding to lower nitrate 

concentrations would indicate denitrification reactions 

might be occurring. The presence of dissolved oxygen 

in all spring-water samples, low DOC concentrations 

(table 3), and little or no excess N2 (based on dissolved 

gas data for Ar and N2) (table 4) also indicate that denitrification reactions probably are not occurring. 

 Nitrogen isotope values obtained in this study 

correspond to the range in values obtained from previous 

studies in the basin. As part of a study of nitrate in 

ground water near four dairy farms in Lafayette and 

Suwannee Counties, Andrews (1994) reported the following values of δ15N-NO3 (per mil) for water samples 

collected in May 1993: Lafayette County Blue Spring, 

9.65; Telford Spring, 7.70; and Convict Springs, 8.90. 

Organic sources of nitrogen from leachate of livestock 

wastes and septic tanks were inferred to be the predominant source of nitrate to spring water (Andrews, 1994). 

The δ15N-NO3 values reported by Andrews (1994) are 

about 1 to 2 per mil higher for Lafayette Blue and Telford Springs than nitrogen isotope data collected during 

the present study, and may be indicative of differences in 

analytical procedures between laboratories. 

In a study of nitrate in ground water near dairy 

farms, Andrews (1994) concluded that the source of 

nitrate in shallow ground water tended to be leachate 

from livestock waste, whereas the source of nitrate in 

deeper ground water was attributed to leachate from 

synthetic fertilizers. Based on nitrogen-isotope analyses of water from 66 monitoring and drinking-water 

wells from the middle Suwannee River Basin, Hornsby 

(1994) found that water from four wells produced 

δ15N-NO3 values that were equal to or greater than 

10 per mil and indicated an organic N source. Hornsby 

(1994) found that three of the four wells were located 

downgradient from dairy or poultry operations, indicating the likelihood of localized sources of animal 

wastes. The majority of wells (44 of 66) yielded water 

with δ15N- NO3 values that were less than or equal to 

2 per mil, indicating that inorganic N (synthetic fertil-
22 Sources and Chronology of Nitrate Contamination in Spring Waters, Suwannee River Basin, Florida

izers) and/or soil nitrogen was the dominant source of 

N in the middle Suwannee River Basin. It is important 

to note that the depth of the 66 wells ranged from 14 to 

44 m (median depth 27 m) below land surface, and 

water samples from these wells were withdrawn from 

zones about 5 to 40 m deeper than the top of the aquifer, 

and therefore may not represent most recent nitrate 

contamination (Hornsby, 1994). 

AGES AND RESIDENCE TIMES OF 

SPRING WATER AND WATER FROM THE 

UPPER FLORIDAN AQUIFER 

A reservoir model (eq. 1; p. 11) was used to provide approximate ranges of residence times of ground 

water discharging to springs, and to provide a hydrogeologic framework for comparing estimated residence 

times and apparent ages obtained using data for CFCs 

and tritium. Even though the reservoir model cannot 

provide residence times for ground water discharging 

to a specific spring, one can obtain reasonable ranges of 

ground-water residence times for springs where some 

site-specific information is available (Focazio and others, 1998). Values of aquifer thickness, porosity, and 

recharge rates vary over the Suwannee River Basin, but 

reasonable ranges of these aquifer characteristics can 

be used to estimate their associated residence times 

(fig. 7). Residence times vary from about 5 to 80 years 

for a range of values for aquifer thickness (30 to 

100 m), and porosity (0.05 to 0.4). For a range of 

recharge rates determined for the study area, 0.2 to 0.8 

meter per year (m/yr) (Grubbs, 1998), along with aquifer thickness values of 30 and 100 m, calculated residence times range from about 5 to 50 years (fig. 7).

Chlorofluorocarbons

Assignment of age using concentrations of transient tracers, such as CFCs and 3

H, is based on an interpretation of ground-water flow conditions, and the 

assumption that CFC concentrations have not been 

altered by any biogeochemical processes other than 

0 5 10 15 20 25 30 35 40

NITRATE-N CONCENTRATION, IN MILLIGRAMS PER LITER

14

0

2

4

6

8

10

12

δ15N, PER MIL

Nitrate associated with

artificial fertilizer use

N in atmospheric deposition

Nitrate associated with

manure spreading or

waste disposal

SANTA FE RIVER

 SPRINGS

SUWANNEE RIVER

 SPRINGS IN

 LAFAYETTE COUNTY

LOWER SUWANNEE

 RIVER SPRINGS

SUWANNEE RIVER

 SPRINGS IN

 SUWANNEE COUNTY

GROUND WATER

 LAFAYETTE COUNTY

Well-9001

3/98

Well-6011

3/98

Well-6011

7/97

Well-9001

7/97

Figure 6. Graph of δ15N versus NO3

-

-N concentrations for spring water and water from zones in the Upper 

Floridan aquifer.
Ages and Residence Times of Spring Water and Water from the Upper Floridan Aquifer 23

those established by air-water equilibrium during 

recharge. The term "apparent age" is used because it is 

not possible to know unequivocably if CFC concentrations have been affected by these processes. The presence of measurable CFCs in ground water and spring 

water is indicative that some fraction of the spring 

water or water from shallow zones in the aquifer was 

recharged after 1940 because CFCs are not detectable 

in older water (Plummer and Busenberg, 1999). 

The apparent age of ground water collected from 

a well or a spring can have different connotations. For 

example, the age of water sampled from a well with a 

short screen interval is more indicative of a point along 

a ground-water flow path. In contrast, the age of spring 

water can be interpreted in different ways depending on 

how ground-water-flow conditions are conceptualized. 

Under simplified flow conditions, it can be assumed 

that ground water moves through the aquifer in a 

piston-like manner. The piston-flow assumption has 

been used to interpret the apparent age of water samples collected from wells and piezometers open to 

relatively narrow depth intervals of an aquifer in karst 

areas (for example, Katz and others, 1995a; Plummer 

and others, 1998b). Ground-water discharge from 

springs more likely integrates water from various converging flow paths, and therefore, the age of spring 

water represents a mean residence time of water from a 

part of the aquifer that contributes to spring flow 

(Focazio and others, 1998). 

Concentrations of CFC-11, CFC-12, and CFC113 in water samples reported in table 6 generally do 

not yield concordant piston-flow ages based on atmospheric equilibration data. Most springs have concentrations of CFC-11 and CFC-113 that are within the 

range expected for air-saturated waters that are less 

than 40 years old; however, the apparent equilibration 

years are different for these two compounds. Most 

spring waters have CFC-12 concentrations that are too 

high for air-saturated waters of any age, which likely 

represents contamination with a non-atmospheric 

source of CFC-12. The combined data from the three 

CFC compounds can be interpreted in several different 

ways, and different interpretations among springs are 

probable. In this report, apparent ages or residence 

times of spring waters have been estimated by using 

measured CFC-11 and CFC-113 concentrations 

(table 6) and three approaches for conceptualizing 

ground-water flow in the Upper Floridan aquifer:

(1) simplified piston flow, (2) exponential flow, and 

(3) binary mixing. These three approaches assume that 

CFC-11 and CFC-113 concentrations were not 

degraded in the aquifer because ground waters are oxic 

and contain low concentrations of organic carbon. 

Recent studies of the stability of CFCs in water from 

the Upper Floridan aquifer near Valdosta, Ga., found 

that CFCs were stable in ground water, which was 

attributed to the low content of particulate organic carbon (Plummer and others, 1998b). CFC-12 data were 

not used to estimate the apparent age of spring waters 

due to the large number of samples that were contaminated (CFC-12 concentrations were above the value for 

water in equilibrium with the CFC-12 concentration in 

modern (1998) air).

0

20

40

60

80

100

0

10

20

30

40

50

60

0 0.2 0.4 0.6 0.8 1

RESIDENCE TIME,

IN YEARS

RESIDENCE TIME,

IN YEARS

POROSITY, DIMENSIONLESS

RECHARGE RATE,

IN METERS PER YEAR

0 0.2 0.4 0.6

UPPER FLORIDAN AQUIFER

EXPLANATION

100

30

AQUIFER THICKNESS,

IN METERS 

AQUIFER THICKNESS (b)

TIMES POROSITY (n)

bn=3

bn=10

Figure 7. Graphs showing estimated average 

residence times for ground water calculated from 

ranges of hydrologic properties of the Upper Floridan 

aquifer using a reservoir model.
24 Sources and Chronology of Nitrate Contamination in Spring Waters, Suwannee River Basin, Florida Table 6. Concentration of CFC-11, CFC-12, and CFC-113 in water from springs and wells, calculated atmospheric partial pressure, a

[°C, degrees Celsius; pg/kg, picograms per kilogram; Elev., elevation of land surface; pptv, parts per trillon by volume; Contam., CFC concentrations in water samplerium with 1998 atmospheric concentrations; ERR, denotes analytical problems]

 Sample Name Sample number Sampling date Sampling time Recharge Concentration in solution Calculated atmospheric partial pressure in pptv 

Temp.

(°C)

Elev.

(meters)

pg/kg

CFC-11

pg/kg

CFC-12

pg/kg

CFC-113 CFC-11 CFC-12 CFC-113 

Charles Spring 2 07/16/97 1404 20.4 6 201.0 157.5 19.5 115.7 375.4 28.9 Charles Spring 4 07/16/97 1412 20.4 6 199.8 155.2 20.7 115.0 370.0 30.5 Charles Spring 5 07/15/97 1414 20.4 6 201.9 157.9 19.3 116.2 376.4 28.6 Lafayette Blue Spring 2 07/15/97 1616 19.8 6 213.9 315.7 19.0 119.9 735.0 27.2 Lafayette Blue Spring 4 07/15/97 1624 19.8 6 215.7 319.7 17.3 121.0 744.3 24.8 Lafayette Blue Spring 5 07/15/97 1628 19.8 6 222.9 318.0 11.5 125.0 740.5 16.4 Little River Springs 2 07/15/97 904 18.5 6 2518.3 302.4 103.0 1331.1 668.0 138.0 Little River Springs 4 07/15/97 912 18.5 6 152.5 181.9 14.9 80.6 401.9 20.0 Little River Springs 6 07/15/97 920 18.5 6 150.2 178.9 16.9 79.4 395.3 22.7 Telford Spring 2 07/16/97 759 19.5 6 186.9 136.9 17.5 103.4 315.1 24.7 Telford Spring 4 07/16/97 807 19.5 6 185.4 135.7 16.2 102.5 312.1 22.8 Telford Spring 5 07/16/97 811 19.5 6 193.6 140.1 17.7 107.1 322.3 25.0 Troy Springs 1 07/15/97 1005 20.1 6 109.3 141.7 9.7 62.1 333.8 14.2 Troy Springs 3 07/15/97 1013 20.1 6 99.5 121.5 13.5 56.5 286.2 19.6 Troy Springs 5 07/15/97 1021 20.1 6 100.1 126.5 9.9 56.9 298.1 14.5 Ichetucknee Blue 

Hole Spring

1 08/14/97

1130

20.0 6 119.6 64.7 76.6 67.7 151.8 111.0 

Ichetucknee Blue 

Hole Spring

3 08/14/97

1146

20.0 6 112.2 65.8 28.3 63.4 154.3 41.0 

Ichetucknee Blue 

Hole Spring

6 08/14/97

1216

20.0 6 111.9 65.7 18.5 63.3 154.1 26.7 

Well -9001 2 07/16/97 1019 20.3 6 277.3 178.8 35.4 158.9 424.6 52.0 Well -9001 4 07/16/97 1027 20.3 6 282.4 176.0 34.2 161.8 417.9 50.2 Well -9001 5 07/16/97 1031 20.3 6 295.9 184.4 30.2 169.6 437.9 44.4 Well -6011 2 07/16/97 1154 25.3 6 695.2 45620.3 36.0 491.1 130517.6 67.5 Well -6011 4 07/16/97 1204 25.3 6 690.6 44254.9 37.2 487.8 126611.2 69.8 Well -6011 5 07/16/97 1208 25.3 6 670.5 48078.5 37.4 473.7 137550.4 70.1 Well -9001 2 03/09/98 1210 22.0 6 93.2 66.5 9.2 59.0 173.0 14.8 Well -9001 5 03/09/98 1235 22.0 6 99.5 64.6 9.4 63.0 168.1 15.1 Well -9001 3 03/09/98 1220 22.0 6 94.3 69.5 ERR 59.7 180.7 ERR Well -6011 2 03/09/98 1500 22.0 6 357.5 48926.8 17.7 226.3 127312.8 28.4 Well -6011 4 03/09/98 1510 22.0 6 350.6 47363.3 19.4 221.9 123244.3 31.1 Well -6011 5 03/09/98 1515 22.0 6 343.9 50769.3 20.4 217.7 132107.2 32.6 
Ages and Residence Times of Spring Water and Water from the Upper Floridan Aquifer 25

Columbia Spring 2 08/20/98 1655 23.4 7 221.1 293.2 23.4 149.0 806.8 40.2 Columbia Spring 4 08/20/98 1705 23.4 7 217.1 294.0 24.0 146.2 808.8 41.1 Columbia Spring 5 08/20/98 1710 23.4 7 221.5 281.5 22.1 149.2 774.6 38.0 Fannin Spring 2 08/18/98 1225 25.2 7 2270.9 300.8 26.9 1650.7 885.8 50.2 Fannin Spring 3 08/18/98 1230 25.2 7 2288.0 301.3 29.3 1663.1 887.4 54.6 Fannin Spring 5 08/18/98 1240 25.2 7 1949.3 321.1 34.9 1416.9 945.6 65.2 GIL 917971 spring 2 08/20/98 1310 25.5 7 271.9 208.3 39.7 200.1 620.4 75.2 GIL 917971 spring 4 08/20/98 1320 25.5 7 255.9 177.4 33.5 188.3 528.4 63.5 GIL 917971 spring 5 08/20/98 1325 25.5 7 255.5 169.9 33.7 188.0 505.9 63.7 Ginnie Spring 2 08/21/98 915 21.2 7 302.0 210.5 24.4 184.7 531.2 37.5 Ginnie Spring 4 08/21/98 925 21.2 7 303.2 213.4 26.3 185.4 538.5 40.5 Ginnie Spring 5 08/21/98 930 21.2 7 308.8 209.2 27.5 188.9 527.9 42.4 Hornsby Spring 2 08/21/98 1435 21.0 7 167.6 845.3 7.1 101.6 2115.8 10.8 Hornsby spring 4 08/21/98 1445 21.0 7 164.6 839.2 6.8 99.7 2100.5 10.3 Hornsby spring 5 08/21/98 1450 21.0 7 165.5 862.6 7.1 100.3 2159.1 10.7 July Spring 2 08/21/98 950 22.8 7 495.7 318.1 31.9 325.4 855.0 53.1 July Spring 4 08/21/98 1000 22.8 7 501.4 326.6 33.1 329.1 878.0 55.2 July Spring 5 08/21/98 1005 22.8 7 495.1 313.8 32.1 325.0 843.4 53.4 LAF 718972 spring 2 08/19/98 1235 22.5 7 688.5 820.4 212.8 446.1 2179.8 349.3 LAF 718972 spring 4 08/19/98 1245 22.5 7 105.3 134.2 14.2 68.2 356.7 23.3 LAF 718972 spring 5 08/19/98 1250 22.5 7 103.4 129.5 12.9 67.0 344.1 21.2 Manatee Spring 2 08/18/98 1005 23.0 7 185.1 402.9 21.2 122.5 1091.6 35.7 Manatee Spring 4 08/18/98 1015 23.0 7 188.8 415.6 21.8 125.0 1125.9 36.6 Manatee Spring 5 08/18/98 1020 22.9 7 188.2 409.9 22.6 124.1 1106.2 37.9 Mearson Spring 2 08/19/98 1125 22.0 7 157.3 242.9 20.9 99.7 632.9 33.5 Mearson Spring 4 08/19/98 1135 22.0 7 158.2 243.8 20.5 100.2 635.1 32.7 Mearson Spring 5 08/19/98 1140 22.0 7 157.7 234.0 20.4 99.9 609.5 32.6 Poe Springs 2 08/21/98 1245 22.0 7 117.8 195.8 9.1 74.6 510.0 14.6 Poe Springs 4 08/21/98 1255 22.0 7 118.7 197.1 9.5 75.2 513.4 15.3 Poe Springs 5 08/21/98 1300 22.0 7 118.1 187.6 8.4 74.9 488.6 13.5 Pot Hole Spring 2 08/20/98 905 19.3 7 152.8 137.0 24.4 85.7 319.8 34.1 Pot Hole Spring 4 08/20/98 915 19.3 7 147.9 128.6 22.6 83.0 300.3 31.5 Pot Hole Spring 5 08/20/98 920 19.3 7 147.7 126.4 22.9 82.8 295.0 32.1 

Table 6. Concentration of CFC-11, CFC-12, and CFC-113 in water from springs and wells, calculated atmospheric partial pressure, a

(Continued)

[°C, degrees Celsius; pg/kg, picograms per kilogram; Elev., elevation of land surface; pptv, parts per trillon by volume; Contam., CFC concentrations in water sample

rium with 1998 atmospheric concentrations; ERR, denotes analytical problems]

 Sample Name Sample number Sampling date Sampling time Recharge Concentration in solution Calculated atmospheric partial pressure in pptv 

Temp.

(°C)

Elev.

(meters)

pg/kg

CFC-11

pg/kg

CFC-12

pg/kg

CFC-113 CFC-11 CFC-12 CFC-113 
26 Sources and Chronology of Nitrate Contamination in Spring Waters, Suwannee River Basin, Florida Rock Bluff Spring 2 08/20/98 815 20.7 7 232.2 190.1 28.9 138.8 470.0 43.3 

Rock Bluff Spring 4 08/20/98 825 20.7 7 229.7 186.4 28.4 137.3 460.9 42.6 Rock Bluff Spring 5 08/20/98 835 20.7 7 227.0 179.0 27.7 135.8 442.5 41.5 Running Springs 2 08/18/98 1750 20.2 7 232.5 197.8 14.8 135.9 479.2 21.7 Running Springs 4 08/18/98 1800 20.2 7 268.1 240.4 32.8 156.7 582.5 48.0 Running Springs 5 08/18/98 1805 20.2 7 235.0 197.1 14.8 137.4 477.7 21.7 Ruth Spring 2 08/19/98 1515 21.8 7 231.8 148.8 27.3 145.6 384.5 43.3 Ruth Spring 4 08/19/98 1525 21.8 7 220.6 131.0 24.0 138.6 338.6 38.0 Ruth Spring 5 08/19/98 1530 21.8 7 232.0 148.5 27.7 145.8 383.9 43.9 SUW 718971 spring 2 08/19/98 915 23.6 7 375.1 1106.9 35.3 254.9 3068.9 61.1 SUW 718971 spring 4 08/19/98 925 23.6 7 373.3 1079.7 35.4 253.6 2993.6 61.2 SUW 718971 spring 5 08/19/98 930 23.6 7 381.6 1104.4 35.6 259.3 3061.9 61.7 SUW 725971 spring 2 08/19/98 1005 19.8 7 401.6 513.3 48.1 230.5 1223.6 68.9 SUW 725971 spring 4 08/19/98 1015 19.8 7 398.7 509.2 48.2 228.8 1213.7 69.0 SUW 725971 spring 5 08/19/98 1020 19.8 7 401.1 510.8 46.0 230.2 1217.4 66.0 Trail Springs 2 08/20/98 1110 22.5 7 206.3 174.7 27.4 133.6 464.2 45.0 Trail Springs 4 08/20/98 1120 22.5 7 205.8 174.7 27.2 133.4 464.1 44.6 Trail Springs 5 08/20/98 1125 22.5 7 213.2 181.3 29.7 138.2 481.7 48.7 Troy Spring 3 08/19/98 1330 22.9 7 137.9 137.6 17.6 90.9 371.3 29.4 Troy Spring 4 08/19/98 1335 22.9 7 137.3 140.9 18.5 90.5 380.4 31.0 Troy Spring 5 08/19/98 1340 22.9 7 136.4 133.6 18.9 89.9 360.4 31.6 

Table 6. Concentration of CFC-11, CFC-12, and CFC-113 in water from springs and wells, calculated atmospheric partial pressure, a

(Continued)

[°C, degrees Celsius; pg/kg, picograms per kilogram; Elev., elevation of land surface; pptv, parts per trillon by volume; Contam., CFC concentrations in water sample

rium with 1998 atmospheric concentrations; ERR, denotes analytical problems]

 Sample Name Sample number Sampling date Sampling time Recharge Concentration in solution Calculated atmospheric partial pressure in pptv 

Temp.

(°C)

Elev.

(meters)

pg/kg

CFC-11

pg/kg

CFC-12

pg/kg

CFC-113 CFC-11 CFC-12 CFC-113 
Ages and Residence Times of Spring Water and Water from the Upper Floridan Aquifer 27

Using the piston-flow approach, apparent ages of 

spring waters range from 10 (GIL917971) to 24 years 

(Hornsby Spring) using measured CFC-113 concentrations (table 7). For most spring waters, apparent ages 

estimated using CFC-11 concentration data are 1 to 

12 years older than those obtained using CFC-113 data 

with ages, ranging from 16 (GIL917971 and Ginnie 

Springs) to 28 years (Troy Spring sampled in 1997). If 

the assumption is valid that CFC-11 and CFC-113 are 

stable in this system, then the lack of concordancy in 

apparent ages for spring waters obtained from CFC-11 

and CFC-113 concentrations may result from mixtures 

of ground water of different ages converging from various flow paths. In relatively simple hydrogeologic 

settings, such as aquifers comprised of unconsolidated 

sediments, CFC-11 and CFC-113 concentration data 

typically would yield concordant ages (Plummer and 

Busenberg, 1999).

One way to represent spring waters as a mixture 

of water from various flow paths with different associated ages is to assume an exponential age distribution 

for ground-water discharge to springs. Average residence times (or turnover times; Zuber, 1986) can be 

estimated by using a exponential model. The exponential model provides a simplified way of accounting for 

mixing of stratified ground waters (of different ages) at 

discharge (Zuber, 1986), which gives this model an 

advantage over other simple reservoir models, which 

assume all discharge to be the same age or piston-flow 

models that assume plug flow. Average residence 

times calculated from the exponential model using 

CFC-11 and CFC-113 concentration data have a much 

larger range than apparent ages for ground water from 

springs calculated using these compounds with the piston-flow model. For example, average residence times 

range from 7 (SUW725971A) to 77 years (LAF718972 

Table 7. Comparison of apparent ages of spring waters from piston-flow model (PFM), average residence times calculated 

using exponential-flow model (EM), and fraction of post-1993 water calculated using binary-mixing model (BMM)

[yrs, years; Contam., water sample was contaminated (CFC-11 concentrations were above those for water in equilibrium with CFC-11 in modern air)]

SPRING NAME

Tritium 

concentration, 

TU

CFC-11 

Apparent 

age (PFM), 

yrs

CFC-113 

Apparent 

age (PFM), 

yrs

CFC-11 

Turnover 

time (EM), 

yrs

CFC-113 

Turnover 

time (EM), 

yrs

Fraction of 

post-1993 

water (BMM), 

CFC-11

Fraction of 

post-1993 

water- (BMM), 

CFC-113

SANTA FE RIVER SPRINGS

GIL917971 3.3 16 10 22 13 0.69 0.70

Ginnie 5.2 16 14 16 21 0.68 0.45

Hornsby 5.8 25 24 43 110 0.32 0.13

July 5.4 Contam. 12 Contam. 15 Contam. 0.56

Poe 5.1 27 22 64 81 0.27 0.17

Trail 4.5 22 13 32 19 0.48 0.50

Columbia 4.3 21 14 29 25 0.52 0.45

Ichetucknee Blue Hole 5.8 27 15 69 27 0.22 0.35

LAFAYETTE CO. SPRINGS

Troy98 4.0 25 16 55 35 0.33 0.35

Troy97 4.0 28 18 77 66 0.21 0.18

Mearson . 25 15 45 30 0.36 0.38

Lafayette Blue 4.0 22 17 29 44 0.42 0.27

Ruth 3.0 21 13 27 21 0.52 0.50

LAF718972 5.1 27 19 77 51 0.25 0.25

LOWER SUWANNEE SPRINGS

Fannin 3.7 Contam. 11 Contam. 17 Contam. 0.63

Pothole 5.2 26 15 50 26 0.28 0.37

Manatee 4.5 23 14 36 28 0.43 0.44

Rock Bluff 4.8 22 13 27 19 0.50 0.50

SUWANNEE CO. SPRINGS

Running 5.3 21 17 24 36 0.49 0.33

Little River 4.2 25 18 48 43 0.28 0.24

SUW725971A 4.7 12 10 7 6 0.82 0.76

SUW718971 3.3 10 11 9 13 0.92 0.68

Charles 4.6 22 15 33 31 0.42 0.33

Telford 4.9 23 17 34 38 0.37 0.27
28 Sources and Chronology of Nitrate Contamination in Spring Waters, Suwannee River Basin, Florida

and Troy-97, sampled in 1997) using measured CFC-11 

concentration data, and from 6 (SUW725971A) to 110 

years (Hornsby Spring) using measured CFC-113 concentration data (table 7) (fig. 8). Differences in estimated average residence times for springs result from 

differences in measured CFC-11 and CFC-113 concentrations relative to their air-water equilibration values 

over time. The relation between apparent ages calculated by using a piston-flow model compared to average residence times of ground water estimated by using 

an exponential model is shown in figure 8 (modified 

from Focazio and others, 1998). For apparent ages less 

than or about 10 years, the average residence time is 

slightly less than the apparent age for CFC-11 and 

CFC-113. For apparent ages more than about 15 years, 

the exponential model residence times are significantly 

greater than the apparent ages for both CFC-11 and 

EXPONENTIAL

MODEL (EM)

EXPONENTIAL

MODEL (EM)

COMPARISON OF

PISTON-FLOW AND EXPONENTIAL-FLOW MODELS,

(22 oC) 

PISTON

FLOW (PF)

PISTON

FLOW (PF)

1 10 100 1,000

AGE (PF) OR RESIDENCE TIME (EM), IN YEARS

CFC-11 CONCENTRATION, IN

PICOGRAMS PER KILOGRAM

CFC-113 CONCENTRATION, IN

PICOGRAMS PER KILOGRAM

0

100

200

300

400

500

0

10

20

30

40

50

60

Figure 8. Graphs showing comparison of residence times calculated using an 

exponential model with apparent ages calculated using a piston-flow model using 

CFC-11 and CFC-113 concentrations at a recharge temperature of 22 °C.

CFC-113 (fig. 8). The exponential model is particularly sensitive to small changes in CFC-113 concentration data due to the smaller range of CFC-113 

concentrations in water that are in equilibrium with 

atmospheric concentrations compared to CFC-11. 

Another approach that accounts for differences 

in apparent ages and residence times obtained by using 

CFC-11 and CFC-113 assumes that spring water is a 

simple binary mixture of ground water from two 

sources (Plummer and Busenberg, 1999): a shallow 

reservoir that contains young, recently recharged water 

and a deeper reservoir containing older ground water, 

recharged more than 50 years ago. Binary-mixing 

models were developed to test three scenarios that 

involve the mixing of a young water (recharged post1993) with waters recharged in 1980, 1965, and 1940 

and before. Since the atmospheric input concentrations 

for CFC-11 and CFC-113 have 

essentially flattened out after 

1993, the model would not be 

able to distinguish recharge 

occurring in 1993 from more 

recent recharge, so recent 

recharge is considered as post1993. Similarly, since the concentrations of CFC-11 and 

CFC 113 in the atmosphere were 

near zero in 1940, the model cannot distinguish between waters 

recharged in 1940 with those 

recharged prior to 1940. Hence, 

the term older ground water refers 

to that recharged prior to 1940. 

Curves shown in figure 9 were 

constructed based on methods in 

Plummer and Busenberg (1999) 

and recalculated for 22 o

C 

recharge temperature. 

When the apparent 

recharge dates (based on pistonflow assumption) for CFC-11 and 

CFC-113 (determined from measured concentrations of CFC-11 

and CFC-113) are plotted on the 

various mixing curves (fig. 9), the 

closest agreement between mixing fractions determined by using 

measured concentration data for 

both CFC-11 and CFC-113 is for 

mixtures of young (post-1993) 
Ages and Residence Times of Spring Water and Water from the Upper Floridan Aquifer 29

water with old (pre-1940) water. Using apparent 

recharge dates determined from CFC-11, the mixing 

fraction of recent water (f1993) ranged from 0.21 (Troy 

Spring 1997 sample) (fig. 9) to 0.92 (SUW718971); 

whereas f1993 ranged from 0.13 (Hornsby Spring) to 

0.76 (SUW725971A) using CFC-113 data (table 7). 

Generally, f1993 values calculated by using CFC-11 and 

CFC-113 data agreed within +/- 0.05; however, the 

largest discrepancies between calculated f1993 values 

using CFC-11 and CFC-113 concentration data were 

for spring waters with extreme (very young or old) 

apparent ages or residence times, such as SUW718971 

and Hornsby (table 7). For these two springs, the 

apparent ages calculated from CFC-11 and CFC-113 

data are concordant (agree within one year); therefore, 

the concept of binary mixtures may not be appropriate 

as there may be only one water (SUW718971) or possibly more than two waters (Hornsby) in a mixture. The 

1940

1950

1960

1970

1980

1990

2000

APPARENT RECHARGE DATE

(a) MIXING 1993 WATER WITH 1940 WATER, 22 oC

CFC-11

CFC-113

TROY SPRING-1998

GIL917971

1940

1950

1960

1970

1980

1990

2000

APPARENT RECHARGE DATE

(b) MIXING 1993 WATER WITH 1965 WATER, 22 oC

1940

1950

1960

1970

1980

1990

2000

0 0.2 0.4 0.6 0.8 1.0

APPARENT RECHARGE DATE

(c) MIXING 1993 WATER WITH 1980 WATER, 22 oC

MIXING FRACTION OF POST-1993 WATER

Figure 9. Binary-mixing model scenarios for ground water recharged post-1993 with 

waters recharged in (a) 1940 and before, (b) 1965, and (c) 1980.
30 Sources and Chronology of Nitrate Contamination in Spring Waters, Suwannee River Basin, Florida

range of recent (post-1993) recharge in the presumed 

binary mixtures, which presumably originates from shallow flow systems, is similar to the estimate of 92 percent 

shallow water for the Rainbow Springs-Silver Springs 

drainage areas (Faulkner, 1973). This estimate was based 

on differences in sulfate concentrations in water from the 

top of the Upper Floridan aquifer, near the bottom of the 

aquifer (the middle of the Lower Floridan), and from Silver Springs. 

The apparent age of water from wells (-6011 and 

-9001) open to the two shallow zones in the Upper 

Floridan aquifer ranges from 8 to 18 years using the 

piston-flow model approach. Higher than present-day 

concentrations of CFC-11 and CFC-12 in water from 

well-6011 may indicate a local source of contamination 

of CFCs, possibly from human wastes (Busenberg and 

Plummer, 1992). Elevated concentrations of N2 in 

water from well-6011 (above what would be expected 

from water in equilibrium with N2 and Ar in air) (table 

4) indicate that nitrogen species, such as nitrate from 

septic tanks or animal wastes, are being transformed to 

nitrogen gases, possibly due to denitrification reactions 

in the aquifer (Katz and others, 1997). 

It is important to note that the estimated apparent 

ages, mean residence times, and fraction of post-1993 

recharge water mixtures for spring waters or water 

from zones in the aquifer are specific for the hydrologic 

conditions at the time of sampling. Samples were collected during low-flow conditions for the Suwannee 

and Santa Fe Rivers (fig. 10); however, flows in these 

two rivers can vary considerably depending upon the 

amount of rainfall (fig. 10). Spring water collected 

during low-flow or base-flow conditions in the Suwannee River (periods of low rainfall) probably yield older 

ages than spring water collected during high-flow conditions in the river (extended periods of high rainfall). 

The effect of hydrologic conditions on ages of spring 

waters probably results from the greater contribution of 

waters from longer ground-water-flow paths during 

low-flow conditions than during high-flow conditions 

when the contribution of younger waters from shorter 

ground-water-flow paths probably would be greater. 

Studies of the relative ages of spring waters in the 

Chesapeake Bay region determined that spring waters 

are younger during high-flow conditions than during 

low-flow conditions (Focazio and others, 1998). 

Some of the hydrologic and hydraulic factors 

that affect the relative ages of spring water and water 

from shallow zones in the Upper Floridan aquifer are 

(1) the relative contribution of recharge from different 

parts of the basin; for example, nearby points of 

focused recharge such as sinkholes (relatively recent 

20

2,000

30

40

50

70

100

200

300

400

500

700

1,000

DISCHARGE, IN CUBIC METERS PER SECOND

J FMAM J J A SON D J FMAM J J A SOND

1997 1998

0

10

20

30

40

RAINFALL, IN CENTIMETERS

SUWANNEE

RIVER

Discharge

at

Branford

SANTA FE RIVER

Discharge at Fort White

SAMPLING PERIOD

RAINFALL AT

 Live Oak

 High Springs

Figure 10. Graph showing discharge of Suwannee and Santa Fe Rivers, rainfall during 1997-98, and dates of 

collection of spring-water samples.
Ages and Residence Times of Spring Water and Water from the Upper Floridan Aquifer 31

recharge) versus water moving along long groundwater-flow paths (relatively old recharge), (2) conduit 

size, extent, and distribution pattern, (3) the relation 

between the heads in the aquifer, the hydraulic gradient, and water levels in the Suwannee River, and (4) the 

size of the contributing area to a spring. Keeping these 

factors in mind, the relative ages of spring waters may 

provide useful information about the size of the contributing area to a spring system. For example, ground 

water discharging to springs with longer residence 

times probably indicates that older water from a deeper 

flow system is mixing with more recently recharged 

water that results in a mixture of ages. 

The size of the catchment area for the springs 

sampled in this study was calculated from flow information (table 1), assuming that the measured discharge 

represents a long-term average flow for each spring and 

the recharge rate. This calculation also assumes that 

there are no other discharges from the catchment area 

besides that from the spring. The catchment area for 

sampled springs varies from 3.6 km2

 (GIL917971) to 

375 km2

 (Columbia) based on an average recharge rate 

of 0.5 m/yr (table 1). For the range of recharge rates 

determined for the study area, 0.2 to 0.8 m/yr (Grubbs, 

1998), the size of catchment areas for springs varies 

from 2.2 (GIL917971) to 938 km2

 (Columbia) (table 1). 

These estimated catchment areas are considerably lower 

than a value of 1,890 km2

 estimated for Silver Springs, 

which has a long-term average flow of 23 m3

/s and an 

average recharge rate of 39 cm/yr (Faulkner, 1973). 

Tritium

Tritium (3H) concentrations measured in spring 

water and water from two wells (in shallow zones of 

the Upper Floridan aquifer) were very similar and 

ranged from about 4.0 to 5.8 TU (table 5). Concentrations of 3

H in ground water in the study area reflect the 

passing of the 3H transient through the hydrologic system (fig. 3). Prior to the advent of the atmospheric testing of fusion weapons in 1953, 3H concentrations were 

on the order of 2 TU or less in this region (Thatcher, 

1962). Atmospheric weapons testing during the late 

1950’s through the mid-1960’s increased 3H concentrations in rainfall in this area to a maximum of several 

hundred TU during the mid-1960's, followed by a sharp 

decline in concentrations after the moritorium in atmospheric nuclear testing. As pre-nuclear testing water 

would have a maximum concentration of 0.2 TU at this 

time (1998), it is evident that spring-water discharge 

(tritium values in table 5) is of relatively recent origin, 

and almost certainly from the period of the falling limb 

of the 3H transient (fig. 3). Accurate dating of the 

water is not possible due to the lack of a long-term 

record of the 3

H transient in this area, as well as the 

slow change in 3

H concentrations over the past decade. 

Also, measured 3

H concentrations can be affected by 

hydrodynamic dispersion and mixing of different age 

waters (Solomon and Sudicky, 1991; Reilly and others, 

1994). However, 3

H concentrations found in spring 

and ground waters are compatible with the estimates of 

high recharge rates (20 to 80 cm/yr; Grubbs, 1998) and 

the high transmissivity of the Upper Floridan aquifer in 

the study area (Crane, 1986; Bush and Johnston, 1988). 

A comparison of measured 3H concentrations in 

spring water with the 3H record for rainfall collected at 

Ocala, Fla., was used to further constrain ground-water 

ages obtained by using the aforementioned models and 

data for CFC-11 and CFC-113. The majority of springs 

can be represented by exponential mixtures with average residence times of around 10 to 80 years (fig. 11). 

Tritium and CFC-113 data also are consistent with a 

binary-mixing model that includes a modern component (post-1993 recharge water) and an older component (mid-1970’s recharge water) (fig. 11). However, 

if the assumption is not valid that CFC-11 and CFC-113 

are stable in this aquifer system, then the 3H data alone 

would indicate that spring-water ages would range 

between 0 and about 25 years, but many spring waters 

would not be reliably distinguishable from modern 

rainfall that recharges the ground-water system. 

Comparative Estimates 

of Spring-water Ages

When most springs are combined into a single 

data cluster and data on CFC-11, CFC-113, and 3H are 

integrated (figs. 11 and 12), exponential-model mixtures seem to provide reliable estimates of average residence times of ground water discharging to springs. 

CFC and tritium data for the majority of spring waters 

cluster along a curve generated for the exponential 

model (figs. 11 and 12). However, for some springs, 

different conclusions can be drawn. For example, 

CFC-11 and CFC-113 data for the springs GIL917971, 

Rock Bluff, Manatee, Pothole, Trail, LAF718972, 

Troy-98, and Ruth (also tritium data) fit better with a 

two-component mixing model with more than 50 percent young water (post-1993) indicated by all three 

tracers (fig. 11). Tritium and CFC-113 data for 

Hornsby, SUW718971, GIL917971, and Columbia 
32 Sources and Chronology of Nitrate Contamination in Spring Waters, Suwannee River Basin, Florida

spring waters fit better with a piston-flow model; however, the data also are close to fitting an exponential 

model (fig. 11). One important conclusion that seems 

to fit most springs is that the CFCs indicate that spring 

waters likely have large fractions of water that are more 

than 20 years old.

Several hydrochemical processes could alter the 

concentrations of CFCs in the ground-water system, 

thereby affecting the interpretation of age and average 

residence times of spring waters. The age interpretations 

are predicated on the assumption that CFC-11 and CFC113 are stable in this ground-water system, which may 

not be entirely valid. For the few spring waters that seem 

not to have been contaminated with non-atmospheric 

CFC-12, some degradation of CFC-11 was possible. 

Also, the low dissolved oxygen concentrations in some 

spring waters (table 3) could signify mixing components 

that have been subjected to sufficiently reducing conditions that could cause some degradation of CFC-11 and 

possibly CFC-113. If this is the case, the apparent ages 

and average residence times presented in table 7 are an 

overestimation of the actual ages and residence times of 

ground water in this system. If degradation of only 

CFC-11 has occurred, the older apparent recharge dates 

obtained by using CFC-11 data compared to those 

obtained by using CFC-113 data would not be realistic, 

and would preclude the use of CFC-11 data in binarymixing models to estimate young and old fractions of 

mixing components for spring waters. Another possibility, albeit remote, is that CFC-113 concentrations are elevated due to contamination, as was the case for CFC-12 

in many spring waters. If CFC-113 contamination has 

occurred, the apparent ages and average residence times 

presented in table 7 would be an underestimation of the 

actual ages or residence times. To resolve some of these 

issues, it would be worthwhile to conduct a comparative 

study of CFCs and trituim/helium-3 (3

H/3He) age-dating 

techniques at selected springs to more effectively quantify mixing of waters of different ages. The 3

H/3He and 

CFC age-dating techniques were used to date the young 

fraction in ground-water mixtures from the Upper Floridan aquifer near Valdosta, Ga., where old regional water 

receives recharge of young water from sinkholes and 

overlying confining beds (Plummer and others, 1998b). 

SUW725971

Well-9001

(3/98) 

July

Well-6011 (8/97)

GIL91

SUW718971

Fannin Well-9001 (8/97)

Rock Bluff

Trail

Ginnie

Ichetucknee

Pothole

Ruth

Columbia

Manatee

Mearson

Well-6011

(3/98)

Running

Charles

Troy-98

Little

River

L.

Blue Troy-97

LAF718972

Hornsby

Poe

Telford

10

20

20

30

40

50

70

100

30 PISTON-FLOW MODEL

EXPONENTIAL MODEL

BINARY-MIXING MODEL

SPRINGS

AVERAGE RESIDENCE TIME OF

 SPRING WATER

20

20

100%

Post-1993

Water 100%

Mid-1970's

Water

APPARENT AGE OF SPRING-WATER 

0

2

4

6

8

10

TRITIUM, IN TRITIUM UNITS

CFC-113 CONCENTRATION, PICOMOLES PER LITER

0 0.1 0.2 0.3

Figure 11. Graph showing concentrations of 3H and CFC-113 in water from springs and wells relative to estimated 

apparent age (piston-flow model), average residence time (exponential model), and fraction of post-1993 water mixed 

with water recharged in 1940 and before (binary-mixing model).
Relation Between Nitrate, Dissolved Oxygen Concentrations, and Age of Spring Water 33

RELATION BETWEEN NITRATE, 

DISSOLVED OXYGEN CONCENTRATIONS, AND AGE OF SPRING WATER

Nitrate-N concentrations in water from springs 

and wells are inversely related to the apparent ages 

obtained from the piston-flow model and average residence times of spring waters obtained from the exponential model. Spearman’s Rho statistic was used to 

determine if the concentration of nitrate and other 

chemical constituents were correlated with springwater ages determined from the various models. This 

nonparametric statistic measures the strength of a 

monotonic increasing or decreasing correlation 

between two variables (Helsel and Hirsch, 1992). A 

correlation is considered to be statistically significant if 

the probability level (p) is less than 0.05. A significant 

inverse correlation was found between nitrate concentrations and apparent ages determined by using CFC113 data and the piston-flow model. A correlation 

coefficient of -0.39 (p<0.058) indicated a marginally 

significant inverse relation between nitrate-N concentrations and average residence times estimated by using 

CFC-113 and the exponential model (table 8). Given the 

relatively small sample size (24), the result is of interest 

even though it is above the commonly used p value of 

<0.05. A significant positive correlation was found 

between nitrate concentrations and the fraction of post1993 water obtained by using CFC-113 and the binarymixing model (table 8). Several different patterns can 

produce the same correlation coefficients (Helsel and 

Hirsch, 1992); however, nitrate concentrations

0

1

2

3

4

0 0.05 0.1 0.15 0.2 0.25 0.3

CFC-113 CONCENTRATION, PICOMOLES PER LITER

CFC-11 CONCENTRATION, PICOMOLES PER LITER

July

Well-6011

SUW718971 SUW725971

GIL917

Well-9001

Rock Bluff

Trail

Pot Hole 

Ichetucknee Troy-98

Mearson

Manatee

Columbia

Ruth

Ginnie

Running

Lafayette

Charles

Telford

Little

River

Hornsby

Poe

LAF917971

Troy-97

Well-9001

PISTON-FLOW MODEL

EXPONENTIAL MODEL

BINARY-MIXING MODEL

SPRINGS

AVERAGE RESIDENCE TIME OF

 SPRING WATER

20

20

APPARENT AGE OF SPRING-WATER 

10

10

20

20

30

30

40

50

70

100

0% Post-1993 Water

100%

Post-1993

Water

Figure 12. Graph showing concentration of CFC-11 and CFC-113 in water from springs and wells relative to estimated 

apparent age (piston-flow model), average residence time (exponential model), and fraction of post-1993 water mixed 

with water recharged in 1940 and before (binary-mixing model).
34 Sources and Chronology of Nitrate Contamination in Spring Waters, Suwannee River Basin, Florida

0

5

10

15

20

25

30

35

40

0

5

10

15

20

25

30

35

40

10 15 20 25 30

CFC-11 APPARENT AGE, IN YEARS

NITRATE-N CONCENTRATION,

IN MILLIGRAMS PER LITER

NITRATE-N CONCENTRATION,

IN MILLIGRAMS PER LITER

5 10 15 20 25

CFC-113 APPARENT AGE, IN YEARS

Figure 13. Graph showing nitrate-N concentration against apparent 

age of spring waters from piston-flow model.

and the age of spring waters (apparent age, average residence time, and fraction of post-1993 water) show a 

strong linear monotonic relation (figs. 13-15). This 

trend may indicate that recent recharge water contributes relatively higher concentrations of nitrate to 

ground water discharging from springs. Nitrate concentrations also are inversely related (p< 0.001) to the 

amount of spring discharge (table 8), indicating that the 

contributing area and hydraulic properties of the aquifer (conduit and diffuse flow) near the spring also are 

important factors. 

Nitrate-N concentrations are significantly 

inversely correlated with spring-water discharge 

(table 8). While it may seem that this significant trend 

is controlled by the three spring waters with high nitrate 

concentrations (SUW718971, SUW725971, and 

GIL917971), when nitrate and discharge data for an 

additional 36 springs (Hornsby and Ceryak, 1999) are 

combined with data for springs presented in this report, 

there is a substantial increase in nitrate concentrations 

below a discharge value of 0.3 m3

/s (fig. 16). Ten of the 

combined 60 spring waters contain nitrate concentrations above 2.5 mg/L. This combined data set included 

only water samples collected during July 1997 and 

August 1998 for the 36 additional springs located 

throughout the Suwannee River Basin (Ceryak and 

Hornsby, 1999) to correspond with samples presented 

herein. Spring-water discharge is significantly correlated to the apparent ages of spring waters; that is, 

springs with lower flows tend to have young ages, 

whereas springs with higher flows tend to have 

increased ages. Springs, such as SUW718971, 
Relation Between Nitrate, Dissolved Oxygen Concentrations, and Age of Spring Water 35

NITRATE-N CONCENTRATION,

IN MILLIGRAMS PER LITER

NITRATE-N CONCENTRATION,

IN MILLIGRAMS PER LITER

0 20 40 60 80 100

0 20 40 60 80 100 120

0

5

10

15

20

25

30

35

40

0

5

10

15

20

25

30

35

40

AVERAGE RESIDENCE TIME (CFC-11),

IN YEARS

AVERAGE RESIDENCE TIME (CFC-113),

IN YEARS

Figure 14. Graph showing nitrate-N concentration against average 

residence time of spring waters from exponential model.

y = 178.82x2 - 116.59x + 18.388

R2 = 0.8367

y = 102.18x2 - 67.847x + 12.343

R2 = 0.7663

0

10

20

30

40

0

10

20

30

40

0 0.2 0.4 0.6 0.8 1.0

0 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80

FRACTION OF POST-1993 WATER, CFC-113

FRACTION OF POST-1993 WATER, CFC-11

NITRATE-N

CONCENTRATION,

IN MILLIGRAMS

PER LITER

NITRATE-N

CONCENTRATION,

IN MILLIGRAMS

PER LITER

Figure 15. Nitrate-N concentration against fraction of post-1993 recharge waters from 

binary-mixing model.
36 Sources and Chronology of Nitrate Contamination in Spring Waters, Suwannee River Basin, Florida

SUW725971, and GIL917971, with smaller amounts 

of flow ranging from 0.06 to 0.25 m3

/s (2 to 9 ft3/s), are 

receiving water from relatively shallow ground-waterflow systems that contain a relatively high proportion 

of recent recharge. In contrast, first-magnitude springs, 

such as Hornsby, Little River, and Troy, with flows 

ranging from 2.1 to 5.6 m3

/s (76 to 200 ft3/s), receive 

water from large contributing areas with deep flow systems that contain a relatively higher proportion of older 

water with low concentrations of nitrate. Similar 

trends between nitrate concentrations and age were 

noted in spring waters discharging from a Triassic karst 

aquifer in Germany (Dietrich and Hebert, 1997); nitrate 

concentrations decreased with increasing age and proportion of old water. 

Dissolved oxygen concentrations were inversely 

correlated (significant at p<0.003) with all estimates of 

spring-water age (table 8). This strong inverse correlation between dissolved oxygen indicates that dissolved 

oxygen could be used as a qualitative tracer of groundwater age. Recently recharged waters have relatively 

high dissolved oxygen concentrations, and as the residence time of water in the system increases, the dissolved oxygen concentrations decrease. Similar trends 

for dissolved oxygen in spring waters discharging from 

a Triassic karst aquifer in Germany were noted by 

Dietrich and Hebert (1997). 

Marginally significant positive correlations were 

observed between silica and spring-water ages, but 

only for apparent ages estimated by using CFC-113 

(p<0.054) and with older fractions of water estimated 

by using CFC-113 in binary-mixing models (p<0.052) 

(table 8). This increase in silica with spring-water age 

is consistent with older waters having more opportunity to incorporate silica from mineral dissolution, and 

may represent waters recharging from areas where the 

aquifer is semiconfined with overlying clays and 

clayey sands. Dietrich and Hebert (1997) also noted 

that silica was a usable qualitative marker for residence 

time of spring waters in a karst system. They also 

found that fluoride, dissolved solids, and δ13CDIC were 

effective markers of ground-water residence times for 

springs; however, similar concentrations of these constituents in water samples from most springs precluded 

their effectiveness as qualitative indicators of springwater age in this study. 

Correlations between spring-water age and springwater flow were not as strong as one might expect. Half 

of these were statistically significant at p <0.05, while the 

other half were significant at p levels ranging between 

0.05 and 0.11 (table 8). Variability in spring-water flow 

may provide enough scatter that results in a weaker correlation between spring-water ages than that observed 

between certain chemical indicators (dissolved oxygen 

and nitrate) and the age of spring waters. 

Data from this study

Data from Hornsby and Ceryak (1998)

NITRATE-N CONCENTRATION,

IN MILLIGRAMS PER LITER

0

5

10

15

20

25

30

35

40

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

SPRING DISCHARGE, M3/S

Figure 16. Nitrate-N concentrations versus spring-water discharge.
R

elationBetweenNitrate,DissolvedOxygenConcentrations,andAgeofSpringWater37

Table 8. Spearman’s Rho correlation coefficients (top number in each cell), probability level of exceeding Rho (for the null hypothesis number in each), and number of observations (bottom number in each cell)[Statistically significant and marginally significant correlations (p <0.054)are shown in boldface. Spring-water apparent ages estimated using piston-flow model for C(PFMC113), residence times estimated using exponential model for CFC-11 (EMC11) and CFC-113 (EMC113), and fractions of post-1993 waters calculated using b(F93C11) and CFC-113 (F93C113). Chemical constituents are denoted as follows and are expressed in mg/L unless otherwise noted: tritium in TU (TRIT), nitrate-N delta carbon-13 in per mil (13C), specific conductance in microsiemens per centimeter (SPECOND), fluoride (F), silica (SiO2), dissolved solids (DS), sulfate (SO4), p(CALSI), dolomite saturation index (DOLSI). Spring discharge is denoted by FLOW]

TRIT NO3N O2DISS 13C SPECOND F SiO2 DS SO4 PO4 

PFMC11 0.33356 -0.28810 -0.59456 -0.11118 -0.08427 -0.01062 0.22392 0.09997 -0.22363 -0.14536 

0.1395 0.1935 0.0035 0.6223 0.7093 0.9626 0.3165 0.6580 0.3171 0.5186 

21 22 22 22 22 22 22 22 22 22 

PFMC113 0.37164 -0.44562 -0.75542 -0.21510 -0.20551 0.03135 0.39926 0.09045 -0.16171 0.05418 

0.0808 0.0291 0.0001 0.3128 0.3354 0.8844 0.0533 0.6743 0.4503 0.8015 

23 24 24 24 24 24 24 24 24 24 

EMC11 0.24544 -0.33201 -0.68155 -0.07825 -0.19700 -0.13631 0.25419 0.03809 -0.26995 -0.04785 

0.2836 0.1312 0.0005 0.7292 0.3795 0.5453 0.2537 0.8664 0.2244 0.8325 

21 22 22 22 22 22 22 22 22 22 

EMC113 0.26759 -0.39338 -0.81836 -0.25273 -0.14273 -0.03446 0.35763 0.15221 -0.14155 0.02488 

0.2170 0.0572 0.0001 0.2335 0.5058 0.8730 0.0862 0.4777 0.5094 0.9081 

23 24 24 24 24 24 24 24 24 24 

F93C11 -0.35496 0.37839 0.69285 0.10887 0.20803 0.09397 -0.29929 -0.04804 0.27816 0.05582 

0.1144 0.0825 0.0004 0.6296 0.3529 0.6775 0.1760 0.8319 0.2101 0.8051 

21 22 22 22 22 22 22 22 22 22 

F93C113 -0.33739 0.45312 0.76873 0.26525 0.18732 0.00233 -0.40149 -0.09735 0.13233 -0.06735 

0.1154 0.0262 0.0001 0.2103 0.3808 0.9914 0.0518 0.6509 0.5376 0.7545 

23 24 24 24 24 24 24 24 24 24 

FLOW 0.20089 -0.63632 -0.27012 0.08801 -0.30890 0.09427 0.58573 -0.06372 0.00784 0.04561 

0.3580 0.0008 0.2018 0.6826 0.1419 0.6613 0.0026 0.7674 0.9710 0.8324 

23 24 24 24 24 24 24 24 24 24 
38 Sources and Chronology of Nitrate Contamination in Spring Waters, Suwannee River Basin, Florida

CHRONOLOGY OF NITRATE 

CONTAMINATION OF 

GROUND WATER

The information obtained from the chemical 

and isotopic tracers provides an important step in 

better understanding the sources of nitrate in springs 

and in shallow zones of the Upper Floridan aquifer. 

However, this information alone does not provide a 

complete understanding of the sources of nitrate and 

chronology of nitrate inputs to the ground-water 

system. Knowledge of other factors, such as 

changes in land uses with time, need to be integrated 

with the chemical and isotopic data to obtain a more 

complete understanding of the factors affecting the 

concentration of nitrate in springs and in parts of the 

Upper Floridan aquifer. There are four main sources 

of nitrogen to ground water: fertilizers applied to 

cropland, animal wastes from dairy and poultry 

operations, atmospheric deposition, and septic tank 

effluent. Changes in loading from these sources during the years 1955 to 1997 are presented below. 

Nitrogen Inputs Associated with Land-use 

Changes, 1950-97

Estimated nitrogen inputs from various nonpoint sources in each county have varied considerably during the past 40 years as a result of changes 

in agricultural activities in the Suwannee River 

Basin. Variable fertilizer N inputs in each county are 

related to changes in the amount of cropland fertilized and crop type. During the past 40-year period, 

the amount of cropland fertilized varied considerably by county with Alachua and Suwannee Counties having the greatest (more than 100,000 ha) and 

Lafayette County the least (40,000-50,000 ha). The 

temporal distribution of fertilized cropland has varied annually for Alachua, Columbia, Gilchrist, and 

Suwannee Counties with the peak amount of fertilizer use occurring in the late 1970’s. Peak fertilizer 

use in Lafayette County occurred in the late 1980’s, 

but recent use during the past 5 years has increased 

substantially. Crop types also have varied during the 

past 50 years with corn being the dominant harvested 

crop until the early to mid 1980’s when soybeans and 

hay became the dominant harvested crop types 

(DACS Census data). 

During 1954-1997, ranges in total estimated 

nitrogen inputs, in million kilograms per year, for 

each county were: Alachua, 3.88 to 10.3; Columbia, 

2.19 to 5.77; Suwannee, 2.75 to 10.9; Lafayette, 1.36 

to 5.36; Gilchrist, 1.01 to 6.09; and (figs. 17-21). In 

the five counties, atmospheric deposition accounted 

for about 25 to 60 percent of the total N input during 

the 1950’s and early 1960’s. In Alachua, Columbia, 

Gilchrist, and Suwannee Counties, the relative contribution of nitrogen from fertilizers (based on fertilizer sales data) increased continuously from the 

1950’s, peaked around 1980, and decreased slightly 

after the early 1980’s. In Lafayette County (fig. 20), 

the relative contribution of nitrogen from fertilizer 

use has increased continuously from the mid-1940’s 

to the around 1990, decreased for a short time, then 

increased back to levels in the late 1980’s. In 

Suwannee, Gilchrist, Lafayette, and Columbia 

Counties, fertilizer use has increased substantially 

from 1993 to 1997. In all five counties, the shape 

and pattern of the curve for estimated total N inputs 

from all nonpoint sources closely matches that for 

fertilizer use (figs. 17-21). The relative contribution 

of estimated N inputs from animal wastes (dairy and 

beef cows, poultry, and swine) to total estimated N 

inputs varied from about 15 to 30 percent for Columbia and Gilchrist Counties to about 27 to 49 percent 

for Lafayette County.

The relative contributions of nitrogen inputs 

from the various nonpoint sources in Suwannee and 

Lafayette Counties are given special emphasis in this 

report because of the high concentrations of nitrate 

in springs in these counties. Also, estimated N 

inputs from these two counties represent two 

extremes with regards to the relative amounts of fertilizer used and animal wastes. Prior to 1960, atmospheric deposition contributed the majority of the 

estimated total N inputs. In Suwannee County, the 

relative contribution of N from fertilizers increased 

from about 23 percent in 1955 to more than 60 percent in 1980 (fig. 19). Beginning in the early 1980’s, 

the relative contribution of N from animal wastes 

(poultry, dairy and beef cows, and swine) began to 

increase, and by the late 1980’s, estimated N inputs 

from animal wastes (poultry and dairy) contributed 

about 30 percent of the total estimated N inputs. 

During 1955-95, the contribution of estimated N 

inputs from animal wastes ranged from about 21 to 

42 percent of the total estimated N inputs. 
Chronology of Nitrate Contamination of Ground Water 39

Alachua County - Nitrogen Inputs and Nitrogen Fertilizer Sales

Alachua County - Nitrogen Inputs, Percent and Sales

1940 1950 1960 1970 1980 1990 2000

0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

YEAR

YEAR

NITROGEN INPUTS, AS PERCENTAGE OF TOTAL

NITROGEN, IN MILLIONS OF KILOGRAMS

ATMOSPHERIC DEPOSITION

BEEF CATTLE

DAIRY CATTLE

POULTRY

SEPTIC TANKS

SWINE

FERTILIZER - Sales

TOTAL NITROGEN INPUT

1955 1960 1965 1970 1975 1980 1985 1990 1995 2000

0

10

20

30

40

50

60

70

80

90

100

TOTAL NITROGEN INPUTS,

IN MILLIONS OF KILOGRAMS

0

2

4

6

8

10

12

ATMOSPHERIC DEPOSITION

BEEF CATTLE

DAIRY CATTLE

POULTRY

SEPTIC TANKS

SWINE

FERTILIZER - Sales

FERTILIZER - Recommended

 Application Rate

NOTE: Open symbols denote interpolated

 or estimated data.

Figure 17. Estimated annual N inputs and relative percentage of total inputs of nitrogen from fertilizers, animal wastes, 

atmospheric deposition, and septic tanks for the years 1955-97 in Alachua County (interpolated data for missing years 

are denoted by open symbols).
40 Sources and Chronology of Nitrate Contamination in Spring Waters, Suwannee River Basin, Florida

Columbia County - Nitrogen Inputs, Percent and Sales

YEAR

NITROGEN INPUTS, AS PERCENTAGE OF TOTAL

ATMOSPHERIC DEPOSITION

BEEF CATTLE

DAIRY CATTLE

POULTRY

SEPTIC TANKS

SWINE

FERTILIZER - Sales

TOTAL NITROGEN

 INPUT

1955 1960 1965 1970 1975 1980 1985 1990 1995 2000

0

10

20

30

40

50

60

70

80

90

100

TOTAL NITROGEN INPUTS,

IN MILLIONS OF KILOGRAMS

0

1

2

3

4

5

7

6

Columbia County - Nitrogen Inputs and Nitrogen Fertilizer Sales

1940 1950 1960 1970 1980 1990 2000

0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

YEAR

NITROGEN, IN MILLIONS OF KILOGRAMS

ATMOSPHERIC DEPOSITION

BEEF CATTLE

DAIRY CATTLE

POULTRY

SEPTIC TANKS

SWINE

FERTILIZER - Sales

FERTILIZER - Recommended

 Application Rate

NOTE: Open symbols denote interpolated

 or estimated data.

Figure 18. Estimated annual N inputs and relative percentage of total inputs of nitrogen from fertilizers, animal wastes, 

atmospheric deposition, and septic tanks for the years 1955-97 in Columbia County (interpolated data for missing years 

are denoted by open symbols).
Chronology of Nitrate Contamination of Ground Water 41

Suwannee County - Nitrogen Inputs and Nitrogen Fertilizer Sales

1940 1950 1960 1970 1980 1990 2000

0

1

2

3

4

5

6

7

YEAR

NITROGEN, IN MILLIONS OF KILOGRAMS

ATMOSPHERIC DEPOSITION

BEEF CATTLE

DAIRY CATTLE

POULTRY

SEPTIC TANKS

SWINE

FERTILIZER - Sales

FERTILIZER - Recommended

 Application Rate

NOTE: Open symbols denote interpolated

 or estimated data.

Suwannee County - Nitrogen Inputs, Percent and Sales

YEAR

NITROGEN INPUTS, AS PERCENTAGE OF TOTAL

ATMOSPHERIC DEPOSITION

BEEF CATTLE

DAIRY CATTLE

POULTRY

SEPTIC TANKS

SWINE

FERTILIZER - Sales

TOTAL NITROGEN INPUT

1955 1960 1965 1970 1975 1980 1985 1990 1995 2000

0

10

20

30

40

50

60

70

80

90

100

TOTAL NITROGEN INPUTS,

IN MILLIONS OF KILOGRAMS

0

2

4

6

8

10

12

Figure 19. Estimated annual N inputs and relative percentage of total inputs of nitrogen from fertilizers, animal wastes, 

atmospheric deposition, and septic tanks for the years 1955-97 in Suwannee County (interpolated data for missing years 

are denoted by open symbols).
42 Sources and Chronology of Nitrate Contamination in Spring Waters, Suwannee River Basin, Florida

Lafayette County - Nitrogen Inputs and Nitrogen Fertilizer Sales

Lafayette County - Nitrogen Inputs, Percent and Sales

1940 1950 1960 1970 1980 1990 2000

0

0.5

1.0

1.5

2.0

2.5

3.0

YEAR

YEAR

NITROGEN INPUTS, AS PERCENTAGE OF TOTAL

NITROGEN, IN MILLIONS OF KILOGRAMS

ATMOSPHERIC DEPOSITION

BEEF CATTLE

DAIRY CATTLE

POULTRY

SEPTIC TANKS

SWINE

FERTILIZER - Sales

TOTAL NITROGEN INPUT

1955 1960 1965 1970 1975 1980 1985 1990 1995 2000

0

10

20

30

40

50

60

70

80

90

100

TOTAL NITROGEN INPUTS,

IN MILLIONS OF KILOGRAMS

0

1

2

3

4

5

6

ATMOSPHERIC DEPOSITION

BEEF CATTLE

DAIRY CATTLE

POULTRY

SEPTIC TANKS

SWINE

FERTILIZER - Sales

FERTILIZER - Recommended

 Application Rate

NOTE: Open symbols denote interpolated

 or estimated data.

Figure 20. Estimated annual N inputs and relative percentage of total inputs of nitrogen from fertilizers, animal wastes, 

atmospheric deposition, and septic tanks for the years 1955-97 in Lafayette County (interpolated data for missing years 

are denoted by open symbols).
Chronology of Nitrate Contamination of Ground Water 43

Gilchrist County - Nitrogen Inputs and Nitrogen Fertilizer Sales

Gilchrist County - Nitrogen Inputs, Percent and Sales

1940 1950 1960 1970 1980 1990 2000

0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

YEAR

YEAR

NITROGEN INPUTS, AS PERCENTAGE OF TOTAL

NITROGEN, IN MILLIONS OF KILOGRAMS

ATMOSPHERIC DEPOSITION

BEEF CATTLE

DAIRY CATTLE

POULTRY

SEPTIC TANKS

SWINE

FERTILIZER - Sales

TOTAL NITROGEN INPUT

1955 1960 1965 1970 1975 1980 1985 1990 1995 2000

0

10

20

30

40

50

60

70

80

90

100

TOTAL NITROGEN INPUTS,

IN MILLIONS OF KILOGRAMS

0

2

4

6

8

10

12

ATMOSPHERIC DEPOSITION

BEEF CATTLE

DAIRY CATTLE

POULTRY

SEPTIC TANKS

SWINE

FERTILIZER - Sales

FERTILIZER - Recommended

 Application Rate

NOTE: Open symbols denote interpolated

 or estimated data.

Figure 21. Estimated annual N inputs and relative percentage of total inputs of nitrogen from fertilizers, animal wastes, 

atmospheric deposition, and septic tanks for the years 1955-97 in Gilchrist County (interpolated data for missing years 

are denoted by open symbols).
44 Sources and Chronology of Nitrate Contamination in Spring Waters, Suwannee River Basin, Florida

In Lafayette County, the relative contribution of 

N from atmospheric deposition decreased from about 

62 to 27 percent from 1955 to 1975, respectively, but 

remained the major source of N (fig. 20). N inputs 

from atmospheric deposition were calculated for the 

entire county area. The western part of the county consists of low-lying, poorly drained swamps referred to as 

San Pedro Bay, and ground water from this area typically does not flow toward springs that discharge into 

the Suwannee River. Therefore, the contribution from 

atmospheric deposition is somewhat overestimated relative to the other sources that mainly occur in the eastern part of the county. Based on ground-water-flow 

patterns inferred from potentiometric surface maps of 

the Upper Floridan aquifer, the eastern part of the 

county contributes most ground water to springs that 

discharge into the Suwannee River. From 1955 to 

1985, the relative contributions of N from fertilizers 

and animal wastes (dairy cows, poultry) steadily 

increased. The estimated contribution of N from 

fertilizer continued to increase until 1990, while the 

relative contribution of N from animal wastes (dairy 

and poultry) decreased slightly during this period. 

From 1990-97, the contribution of N from animal 

wastes represented about 40 percent of the total N 

inputs. 

Estimated inputs of nitrogen presented in figures 

17-21 do not account for losses of nitrogen that may 

occur from several processes. For example, losses of N 

through volatilization of ammonia can vary considerably depending upon specific fertilizer and waste-handling management practices at a given farm. In an 

intensively farmed area in the mid-Atlantic coastal 

plain, Bohlke and Denver (1995) estimated that 

approximately 20 to 35 percent of the N applied as fertilizer could account for the observed nitrate concentrations in the ground-water reservoir and variations in 

nitrate concentrations with time. Manure from dairy 

and beef cows may be stored wet or dry, flushed with 

water into a holding pond or lagoon, spread fresh on 

land, or spread at some later time (Van Horn and others, 

1991). As a result of different storage and land application methods, a range of N loading values to ground 

water was estimated. Minimum estimates of N loading 

to ground water (6.6 percent of the total N excreted) 

were obtained by assuming a tightly managed manure 

system that includes a cropping system and a land 

application system that are in nutrient balance (Van 

Horn and others, 1991). Maximum estimates of N 

leaching to ground water (60 percent of the total N 

excreted) were obtained from estimates of N volatilization losses in handling and assuming that the manure 

was stored in an open lot (Van Horn and others, 1991). 

Other estimates of nitrogen loss under certain conditions can decrease the content of nitrogen in manure by 

25 to 80 percent (Kay and Hammond, 1985). These 

estimates of nitrogen losses do not consider recycling 

of nitrogen that may occur when animals consume 

grains grown within the basin (Asbury and Oaksford, 

1997). 

The composition and amount of poultry litter and 

manure can vary widely in both physical and chemical 

composition. Several factors affect the N composition 

such as type of birds raised, number of birds per unit 

area, nutrient composition of the feed, type and amount 

of bedding material, time in use, moisture content, and 

other management practices (Vest and Merka, 1994; 

Jacobs and others, 1996). Losses of N from poultry 

wastes can result from volatilization of ammonia during stockpiling, composting, or disposal in a lagoon. 

Approximately 30 to 60 percent of the total nitrogen in 

manure or litter that is incorporated into soil after 

spreading is available for crop utilization (Vest and 

Merka, 1994). However, the release of N from soil is 

dependent on the nitrogen content of manure, the form 

of N present, and the ratio of carbon to nitrogen. 

Depending upon methods of handling after production, 

the amount of nitrogen leached to ground water from 

poultry manure or litter could vary substantially from 

almost none to possibly as much as 30 percent. Some 

loss of N also may result from the transport of manure 

or litter away from the farm to other locations for application as fertilizer or as a food source for cattle (Jacob 

and Mather, 1997). 

Under optimal site and operating conditions, 

conventional septic tank systems can remove nearly all 

biodegradable organic compounds, suspended solids, 

and fecal coliforms (Otis and others, 1993). However, 

septic tanks remove only about 30 percent of the nitrogen in raw domestic wastewater (University of 

Wisconsin, 1978). Further losses of N from septic tank 

effluent can result from adsorption, volatilization, mineralization, nitrification, denitrification, and biological 

uptake. For example, approximately 20 percent of 

nitrogen was lost from wastewater percolating through 

soil as a result of denitrification reactions (Jenssen and 

Siegrist, 1988). However, most of the ammonium, 

which comprises about 75 percent of the N in the effluent, and organic nitrogen are converted to nitrate by 

microorganisms under aerobic conditions. Based on 
Chronology of Nitrate Contamination of Ground Water 45

soil lysimeter studies in southwest Florida, reductions 

in total Kjeldahl nitrogen (organic N plus ammonia N; 

TKN) concentrations were in excess of 97 percent in 

samples collected 0.6 and 1.2 m (2 and 4 ft) below the 

infiltrative surface, but nitrate-N concentrations 

increased to about 50 percent of the original TKN concentrations in septic tank effluent due to nitrification 

reactions (Otis and others, 1993). Nitrate can move 

freely in ground water with little or no attenuation. 

Relations Between Nitrate Concentrations 

in Spring Waters and Sources of Nitrogen

In an effort to assess the influence of the various 

N sources on nitrate contamination of spring waters 

with time in Suwannee and Lafayette Counties, a comparison was made between long-term trends in nitrate 

concentration in water from selected springs and the 

estimated inputs of nitrogen contributed by the various 

sources. For this comparison, springs in Lafayette and 

Suwannee Counties were selected that had water samples analyzed for nitrate over a period of 15 years or 

more. The amount of nitrate-N load contributed to 

ground-water recharge from the different sources of 

nitrogen was estimated as follows: first, N inputs for 

each source were converted to a N flux expressed as 

grams per meter squared per year (g/m2/yr). This was 

done by dividing the N input data by the county area in 

square kilometers-- for Suwannee County, the area was 

1,786 km2

; and for Lafayette County, the area was 

854 km2 for the agriculture corridor along the Suwannee River (which was about 60 percent of the total land 

area of 1412 km2). Second, N flux values (g/m2/yr) 

were converted to nitrate-N concentrations, in mg/L, in 

recharge water by dividing the flux values by the average annual recharge rate of 500 liters per meter squared 

per year (L/m2

/yr) for areas where the Upper Floridan 

aquifer is unconfined in the Suwannee River Basin in 

Florida (Grubbs, 1998). 

Suwannee County Springs

In Suwannee County, N inputs from fertilizers 

have contributed more than 50 percent of the nitrogen 

load to ground water from 1960-97. This estimate is 

consistent with δ15N-NO3 values that indicate fertilizers are the dominant source of nitrate in spring waters 

and in ground water (Hornsby, 1994) from Suwannee 

County. The amount of N applied to fields per unit area 

increased by a factor of 6 to 8 between 1945 and the late 

1970’s. If all the fertilizer N were converted to nitrate 

and dissolved in the average annual recharge water (500 

L/m2

/yr), the resulting ground-water concentrations 

would be approximately 2 to 3 times the observed 

nitrate concentrations in spring waters. This indicates 

that about 33 to 50 percent of the N applied as fertilizers 

(using county-wide estimates) accounts for nitrate concentrations in the ground-water reservoir and their variations with time. If N from all sources were converted 

to nitrate and dissolved in recharge water, the resulting 

ground-water concentration would be approximately 3 

to 4 times the observed nitrate concentrations in spring 

waters. 

Four springs have historical records of nitrate 

concentrations (Hornsby and Ceryak, 1999): Charles 

(1973-97), Little River (1980-97), Running (1980-98), 

and Telford (1980-97). Nitrate concentrations in these 

spring waters generally have remained constant or have 

decreased slightly with time. The trends in nitrate concentrations with time from these spring waters have 

similar shapes and for the most part are proportional to 

the fertilizer-use curve (fig. 19). However, the increasing trend in fertilizer use from 1993-97 is not matched 

by an increase in nitrate concentrations in spring waters 

during this same period. A lack of response to 

increased N inputs may result from a delay or lag of 

several years between N fertilizer applications and 

resulting nitrate contamination of ground water. This 

delay may result from temporary storage in the unsaturated zone, dilution due to increased rainfall during this 

time period, or other recharge processes. 

Curves of nitrate concentrations in ground water 

were generated by using historical N input data for all 

sources and the three previously-cited models (piston 

flow, exponential flow, and binary mixing) to estimate 

apparent ages or residence times of spring waters 

(fig. 22). Simulated curves of ground-water nitrate 

concentrations obtained from these models were 

compared to the observed nitrate concentrations in the 

four spring waters to determine the consistency of age 

information obtained by using CFCs and nitrate and to 

further characterize the cycling of nitrate in the groundwater system. For these simulations, it was assumed 

that no nitrate is removed from the ground-water 

system (for example, by denitrification reactions).

The importance of N inputs contributed by fertilizers to ground water is demonstrated by both binarymixing and exponential models. For example, 

observed temporal trends in nitrate concentrations 

observed in spring waters can be accounted for by a 

binary-mixing model that contains mixtures of about 
46 Sources and Chronology of Nitrate Contamination in Spring Waters, Suwannee River Basin, Florida

40 percent young water, a recharge lag-time of 3 years 

between fertilizer application and subsequent groundwater contamination, and 60 percent old water 

(recharged prior to 1940) (fig. 22). Also, nitrate trends 

with time can be accounted for with the exponential 

model. Two possible scenarios were investigated using 

this model to simulate changes in nitrate concentrations 

in spring waters with time: (1) 50 percent N loading 

from fertilizer applications, and (2) 50 percent of N 

loading from all sources as the input function. The first 

scenario resulted in average ground-water residence 

times ranging from 10 years for Telford and Running 

Springs to about 40 years for Little River Spring, 

whereas the second scenario resulted in average residence times that ranged from 40 to 60 years. Average 

residence times estimated using exponential models 

with CFC-11 and CFC-113 data ranged from 24 to 

38 years for Running, Charles, and Telford Springs and 

from 43 to 48 years for Little River Spring. Compared 

to residence times estimated using CFCs, the fertilizerN input data alone tends to slightly underestimate average residence times whereas N input data from all 

sources tend to overestimate average residence times. 

The position of the residence-time curves (generated from exponential model) relative to the temporal 

trend lines for nitrate concentrations in spring waters is 

very sensitive to the N loading percentage used in the 

exponential models. For example, if values below 

50 percent N loading are modeled, the curves of the 

average residence time shift downward relative to the 

temporal spring-water nitrate trends. This results in an 

apparent decrease in the average residence time for 

0

1.0

2.0

3.0

4.0

5.0

6.0

1970 1975 1980 1985 1990 1995 2000

0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0

1

2

3

4

5

(a) PISTON-FLOW AND BINARY-MIXING MODELS NITROGEN LOADING -

 FERTILIZER SALES*0.5

BINARY MIX - 3 YEAR LAG

 (0.4 YOUNG WATER)

CHARLES SPRING

LITTLE RIVER SPRING

RUNNING SPRING

TELFORD SPRING

(b) EXPONENTIAL MODEL- FERTILIZER SALES N*0.5 EXPLANATION

EXPLANATION

EXPLANATION

CHARLES SPRING

LITTLE RIVER SPRING

RUNNING SPRING

TELFORD SPRING

10 YEARS

25 YEARS

40 YEARS

60 YEARS

(c) EXPONENTIAL MODEL- ALL SOURCES OF N*0.5 CHARLES SPRING

LITTLE RIVER SPRING

RUNNING SPRING

TELFORD SPRING

10 YEARS

25 YEARS

40 YEARS

60 YEARS

NITRATE-N CONCENTRATION, IN MILLIGRAMS PER LITER

RESIDENCE TIME

RESIDENCE TIME

Figure 22. Long-term records of nitrate-N concentrations in spring waters, Suwannee County, relative to inputs of 

nitrogen from fertilizers and major nonpoint sources to ground-water recharge modeled using (a) piston-flow and 

binary-mixing models with a mixing fraction of 0.4 post-1993 water and a 3-year lag of recent and old (1940 and 

older) water, (b) exponential model with 50 percent of fertilizer nitrogen contribution to ground-water recharge and 

average residence time (RT) of 10 to 60 years, and (c) exponential model with 50 percent of nitrogen from all 

sources contributed to ground-water recharge and average residence time (RT) of 10 to 60 years.
Chronology of Nitrate Contamination of Ground Water 47

ground water discharging to springs. Conversely, when 

N loading values greater than 50 percent are modeled, 

the curves of average residence time shift upward relative to the temporal nitrate trends for spring waters, 

which results in an apparent increase in the average 

residence time for ground water discharging to springs. 

The actual N loading percentage cannot be determined 

accurately, given the resolution of the temporal nitrate 

data for spring waters. Results from both the binarymixing and exponential models indicate that substantial nitrate concentrations probably are being added to 

the ground-water system in water that is being 

recharged on a time-scale of about 1 to 5 years, even 

though the overall ground-water residence time for 

spring waters is on the order of 20 to 40 years. 

Lafayette County Springs

In Lafayette County, even though estimated N 

inputs from fertilizers did not contribute more than 

50 percent of the total estimated load of N to ground 

water, fertilizers still were the dominant contributor of 

N to ground water since the mid-1970’s (fig. 20). The 

amount of N applied to fields increased by a factor of 8 

to 10 during the period from 1950 to 1997 (fig. 20). As 

was the case in Suwannee County, if all the fertilizer N 

were converted to nitrate and dissolved in recharge 

water, the resulting ground-water concentrations would 

be approximately 2 to 3 times the observed nitrate concentrations in spring waters. Therefore, nitrate concentrations in the ground-water reservoir and variations in 

nitrate levels with time can be accounted for by about 

33 to 50 percent of the N applied as fertilizers. If N 

from all sources were converted to nitrate and dissolved in recharge water, the resulting ground-water 

concentration would be approximately 4 to 5 times the 

observed nitrate concentrations in spring waters. 

Three springs receiving ground water from 

Lafayette County have long-term records of nitrate 

concentrations (Hornsby and Ceryak, 1999): Troy 

(1960-98), Lafayette Blue (1980-97), and Mearson 

(1980-98). Nitrate-N concentrations have increased 

monotonically in water from Troy Spring from 

0.06 mg/L in 1960 to 2.8 mg/L in 1998. The increase 

in nitrate concentrations in Troy Spring track the trend 

in fertilizer N inputs, which contribute substantially to 

the overall trend in total N inputs (fig. 23). Fertilizer 

sales in Lafayette County increased substantially 

during 1992-97 similar to the observed trend in Suwannee County. During this period, the increasing trend in 

fertilizer use during 1993-97 is seen by a relatively 

large increase in nitrate concentrations in Troy Spring, 

but only small increases in nitrate concentrations in 

water from Lafayette Blue and Mearson Springs 

(fig. 23). As was the case with springs in Suwannee 

County, a small response to increased N inputs may 

result from a delay or lag of several years between N 

fertilizer applications and resulting NO3 contamination 

of ground water. This delay for springs from Lafayette 

County also may result from temporary storage in the 

unsaturated zone, dilution due to increased rainfall 

during this time period, aquifer characteristics (such as 

porosity), or other recharge processes. 

Nitrate-N concentrations in ground water in 

Lafayette County were simulated by using historical N 

input data and the piston-flow, exponential-flow, and 

binary-mixing models (fig. 23). For Troy Spring, 

results from the exponential model simulated changes 

in nitrate concentration that generally matched 

observed changes in nitrate concentrations with time. 

However, the estimated average residence time of 

10 years using 50 percent of N loading from fertilizers 

alone tended to underestimate the ground-water average residence time compared to estimates from CFC-11 

and CFC-113 (35 to 77 years). When 50 percent of N 

loading from all sources is included in the exponential 

model, the N data for the three springs plot between the 

lines representing the 40- and 60-year average residence times (fig. 23), which more closely match average residence times obtained by using CFC-11 and 

CFC-113 data. Curves of nitrate concentrations simulated by a binary-mixing model tended to underestimate the observed changes in nitrate concentrations in 

water from Troy Spring. A piston-flow model using 

50 percent of the N inputs from fertilizers also produces a curve that has a shape and a range of nitrate 

concentrations that are similar to the observed nitrate 

concentrations. Even though it is difficult to resolve 

the exact temporal relation between N inputs and 

resulting nitrate contamination of ground water contributing to spring flow due to limited data on nitrate in 

spring waters with time, all three models indicate the 

relative importance of the fertilizer inputs of N to the 

ground-water-flow system. Also, results from the 

models indicate that water containing high nitrate 

concentrations (dominated by fertilizer N) is recharging the ground-water system on a time-scale of less 

than 10 years. These findings are corroborated by the 

relatively low δ15N-NO3 values for Troy and Mearson 

Springs, 5.4 and 4.5 per mil, respectively. The higher 

δ15N-NO3 value for Lafayette Blue Spring (8.4 per 
48 Sources and Chronology of Nitrate Contamination in Spring Waters, Suwannee River Basin, Florida

mil) may indicate a mixed source of inorganic and 

organic nitrogen, possibly from septic tank effluent. 

The complex relation between the concentration 

of nitrate in spring waters and the amount of nitrogen 

that is added to a ground-water-contributing area to a 

spring is controlled by several factors: hydrogeology, 

land-use, land-management practices, and climate. 

The amount and timing of rainfall relative to fertilizer 

applications can strongly influence the amount of N 

loading to ground water in any given year. Variations 

in nitrate concentrations in spring waters from Suwannee and Lafayette Counties during the past 5 to 10 

years likely are related to large fluctuations in annual 

and monthly rainfall (and hence recharge) during this 

period. The amount of annual rainfall can vary by more 

than 100 percent. For instance, the amount of rainfall 

measured at Live Oak, Fla. (fig. 1) in Suwannee 

County, was about 84 cm in 1995 compared to 189 cm 

in 1991. Rainfall for any given month also varies considerably from one year to the next. For example, during 1991 and 1995, measured rainfall in June was 27.1 

and 7.57 cm, respectively. Variations in the nitrogen 

concentrations of recharge waters that enter the subsurface with time also are related to natural processes that 

might lower the concentration of nitrate (dilution) or 

remove nitrate from the system (denitrification), and 

the distance, direction, and time between areas of 

recharge to and discharge from the ground-water 

system. 

0

0.5

1.0

1.5

2.0

2.5

3.0

1950 1960 1970 1980 1990 2000

0

1

2

3

4

5

6

(a) PISTON FLOW and BINARY-MIXING MODELS

LAFAYETTE COUNTY 

(b) EXPONENTIAL MODEL

FERTILIZER SALES N*0.5

(c) EXPONENTIAL MODEL

ALL SOURCES OF N*0.5

0

1

2

3

4

5

6

7

NITRATE-N CONCENTRATION, IN MILLIGRAMS PER LITER

NITROGEN LOADING

 ALL SOURCES*0.5

NITROGEN LOADING -

 FERTILIZER SALES*0.5

BINARY MIX - 3 YEAR LAG

 (0.3 YOUNG WATER)

BLUE SPRING, 

 L:AFAYETTE COUNTY

MEARSON SPRING

TROY SPRING

EXPLANATION

EXPLANATION

EXPLANATION

BLUE SPRING,

 LAFAYETTE COUNTY

MEARSON SPRING

TROY SPRING

10 YEARS

25 YEARS

40 YEARS

60 YEARS

BLUE SPRING,

 LAFAYETTE COUNTY

MEARSON SPRING

TROY SPRING

10 YEARS

25 YEARS

40 YEARS

60 YEARS

RESIDENCE TIME

RESIDENCE TIME

Figure 23. Long-term records of nitrate-N concentrations in spring waters, Lafayette County, relative to inputs of 

nitrogen from fertilizers and major nonpoint sources to ground-water recharge modeled using (a) piston-flow and binarymixing models with a mixing fraction of 0.4 post-1993 water and a 3-year lag of recent and old (1940 and older) water, (b) 

exponential model with 50 percent of fertilizer nitrogen contribution to ground-water recharge and average residence time 

(RT) of 10 to 60 years, and (c) exponential model with 50 percent of nitrogen from all sources contributed to groundwater recharge and average residence time (RT) of 10 to 60 years.
Summary and Conclusions 49

SUMMARY AND CONCLUSIONS

A cooperative study between the Suwannee 

River Water Management District (SRWMD) and the 

U.S. Geological Survey (USGS) has evaluated sources 

of nitrate in spring waters and water from shallow 

zones in the Upper Floridan aquifer in the Suwannee 

River Basin. A multi-tracer approach, which consisted 

of analyzing water samples for naturally occurring 

chemical and isotopic indicators, was used to better 

understand the sources and chronology of nitrate contamination in spring waters discharging to the Suwannee and Santa Fe Rivers. During 1997 and 1998, water 

samples from 23 springs and two wells were collected 

and analyzed for major ions, nutrients, dissolved 

organic carbon (DOC), and selected environmental isotopes [18O/16O, D/H, 13C/12C, 15N/14N]. Water samples were analyzed for chlorofluorocarbons (CFCs; 

CCl3F, CCl2F2, and C2Cl3F3) and tritium (3H) to assess 

the apparent ages and residence times of spring waters 

and water from shallow zones in the Upper Floridan 

aquifer. In addition to information obtained from the 

use of isotopic and other chemical tracers, information 

on changes in agricultural activities in the basin from 

1954-97 were used to estimate N inputs from nonpoint 

sources for five counties in the basin.

Agricultural activities (cropland farming, animal 

farming operations (beef and dairy cattle, poultry, and 

swine) along with atmospheric deposition have contributed large quantities of nitrogen to ground water in 

the Suwannee River Basin in northern Florida. 

Changes in agricultural land use during the past 40 

years in Alachua, Columbia, Gilchrist, Lafayette, and 

Suwannee Counties have contributed variable amounts 

of nitrogen to the ground-water system. From 1955-97, 

the total estimated N from all nonpoint sources (fertilizers, animal wastes, atmospheric deposition, and septic tanks) increased continuously in Gilchrist and 

Lafayette Counties. In Suwannee, Alachua, and 

Columbia Counties, estimated N inputs from all nonpoint sources peaked in the late 1970’s corresponding 

to the peak in fertilizer use during this time. Fertilizer 

use in Columbia, Gilchrist, Lafayette, and Suwannee 

Counties has increased substantially during the past 

5 years, based on records obtained from the Florida 

Department of Agriculture and Consumer Services. 

Even though estimated N inputs from animal wastes 

have increased in Suwannee and Lafayette Counties 

during the past 40 years, the relative contribution from 

fertilizers remains high. 

 Nitrate-N concentrations in spring waters were 

variable. Nitrate-N concentrations were less than 

1.0 mg/L, but elevated above background nitrate concentrations of about 0.05 mg/L, in waters from Columbia, Ichetucknee Blue Hole, Poe, and Hornsby springs. 

In other spring waters, elevated nitrate-N concentrations clearly show the impact of agricultural activities 

in the basin, in particular, SUW718971 (29 mg/L) and 

SUW725971 (38 mg/L). These two springs likely 

receive recharge water from an area dominated by 

cropland that is extensively fertilized and irrigated. 

The heavy use of fertilizers in the basin is corroborated by nitrogen isotope data, with values of δ15N 

values of NO3 in spring waters ranging from 2.7 per 

mil (SUW725791) to 10.6 per mil (Poe Spring) with a 

median of 5.4 per mil for all sampled spring waters. 

The range of values indicates that nitrate in the sampled 

spring waters most likely originates from a mixture of 

inorganic (fertilizers) and organic (animal wastes) 

sources; although higher δ15N values for Poe and 

Lafayette Blue Springs indicate that an organic source 

of nitrogen probably is contributing nitrate to these 

spring waters. Water samples from the two wells sampled in Lafayette County have higher δ15N-NO3 values 

of 11.0 and 12.1 per mil than values for spring waters, 

indicating the likelihood of an organic source of nitrate. 

These two wells are located near dairy and poultry 

farms, where leachate from animal wastes may contribute nitrate to ground water. Dissolved oxygen concentrations generally were in the range of 1 to 6 mg/L, 

indicating that ground water contributing to springs is 

not depleted in oxygen. Concentrations of dissolved 

gases, N2 and Ar, are consistent with atmospheric 

equilibration during ground-water recharge with an 

apparent recharge temperature of 22 +/- 3oC. The gas 

data indicate that denitrification has not removed large 

amounts of nitrate from the ground-water system. 

Variations in δ15N-NO3 values of spring waters were 

not affected by denitrification, and can be attributed to 

variations in the δ15N-NO3 values of ground-water 

recharge, thus provide reliable information about 

source(s) of nitrate.

Several models, including a well-mixed reservoir model, piston-flow model, exponential model, and 

a binary-mixing model were used to estimate ages and 

residence times of ground water discharging to springs. 

When most springs are combined into a single data 

cluster, and data on CFC-11, CFC-113, and tritium are 

simultaneously integrated, exponential mixtures seem 

to provide reliable estimates of average residence times 
50 Sources and Chronology of Nitrate Contamination in Spring Waters, Suwannee River Basin, Florida

of ground water discharging to springs from converging flow paths. However, some data for individual 

springs fit better with a binary-mixing model with more 

than 50 percent young water for all three tracers, 

whereas data from other individual springs fit with a 

piston-flow model with an age of about 25 years. The 

young ages of several spring waters, such as 

SUW718971, SUW725971, and Ginnie Spring indicate their high vulnerability to contamination. One 

important conclusion that seems to fit most springs is 

that the CFCs indicate that spring waters have large 

fractions of water that likely are more than 20 years old. 

To further quantify fractions of young and old water in 

spring waters, it would be useful to conduct a comparative study using CFCs and the tritium/helium age-dating technique for selected springs. 

Spring-water discharge is related to the apparent 

ages of spring waters, that is springs with lower flows 

tend to have young ages whereas springs with higher 

flows tend to have increased ages. Springs, such as 

SUW718971, SUW725971, and GIL917971, with 

smaller amounts of flow, 0.06-0.25 m3/s (2 to 9 ft3/s), 

receive water from relatively shallow ground-waterflow systems and smaller contributing areas that likely 

contain a relatively high proportion of recent recharge. 

In contrast, first- and second-magnitude springs, such 

as Hornsby, Little River, and Troy, with flows of 2.1 to 

5.6 m3

/s (76 to 200 ft3/s), receive water from large contributing areas with deep flow systems that contain a 

relatively higher proportion of older water with low 

concentrations of nitrate. 

The chemical composition of spring waters can 

be used as a qualitative indicator of age and groundwater residence time. Nitrate-N concentrations and 

dissolved oxygen in spring waters are inversely related 

to apparent ages of spring waters and ground-water residence time in the basin. Both nitrate and dissolved 

oxygen concentrations decrease with increasing residence time of ground water in the aquifer. Silica concentrations increase with age of spring waters. Similar 

concentrations of other chemical constituents, such as 

dissolved solids, delta δ13C, and fluoride, that correspond to a wide range of spring-water ages preclude 

their use as effective markers of relative age of spring 

waters in this study area, even though they have been 

used as qualitative indicators of age and mixing proportions of young and old waters of spring waters in other 

karst areas. 

A comparison was made of long-term trends in 

nitrate concentrations in selected spring waters with 

estimated inputs of nitrogen from various sources in 

Suwannee and Lafayette Counties. In both counties, 

nitrate concentrations in spring waters closely followed 

the estimated contribution of nitrogen from fertilizers 

to ground water; nitrate concentrations decreased with 

time in four spring waters from Suwannee County, and 

increased with time in three spring waters from Lafayette County. Results from binary-mixing and exponential models indicate that water containing high nitrate 

concentrations (dominated by fertilizer N inputs) is 

recharging the ground-water system on a time-scale of 

less than 10 years, even though the overall average residence time of ground water discharging to springs can 

be on the order of 20 to 40 years.

The complex relation between the concentration 

of nitrate in ground water and the amount of nitrogen 

that is added to a ground-water contributing area to a 

spring is controlled by hydrogeologic, land-use, climatic, and several other land-management factors. 

Variations in the nitrogen concentration of water that 

enters the subsurface with time are related to changes 

in land-use practices, natural processes that might 

remove nitrate (denitrification) or lower (dilution) its 

concentration, and the distance, direction, and time 

between recharge to and discharge from ground water. 

Large quantities of nitrogen from fertilizers and other 

nonpoint sources currently are added to the ground 

water system. Spring waters represent mixtures of converging flow paths that contain ground water with a 

range of ages. Even if nitrogen inputs were reduced 

substantially, it may take decades for nitrate concentrations in the ground-water system to return to concentrations near background levels. 
References 51

REFERENCES

American Society of Agricultural Engineers, 1996, Cooperative standards program, 37th ed., p. 583-584. 

Anderson, J.R., Hardy, E.E., Roach, J.T., and Witmer, R.E., 

1976, A land use and land cover classification system 

for use with remote sensor data: U.S. Geological 

Survey Professional Paper 964, 28 p.

Andrews, W. J., 1994, Nitrate in ground water and spring 

water near four dairy farms in north Florida, 1990-93, 

U.S. Geological Survey Water-Resources Investigations Report 94-4162, 63 p.

Asbury, C.E., and Oaksford, E.T., 1997, A comparison of 

drainage basin nutrient inputs with instream nutrient 

loads for seven rivers in Georgia and Florida, 1986-90: 

U.S. Geological Survey Water-Resources Investigations Report 97-4006, 7 p.

Baker, L.A., 1991, Regional estimates of atmospheric dry 

deposition, in Charles, D.F., ed., Acidic deposition and 

aquatic ecosystems: Regional case studies: New York, 

Spring-Verlag, app. B., p. 645-653.

Bohlke, J.K., and Coplen, T.B., 1995, Interlaboratory 

comparison of reference materials for nitrogen-isotoperatio measurements, in Reference and intercomparison 

materials for stable isotopes of light elements, International Atomic Energy Agency, IAEA TECDOC 825, 

p. 51-66.

Bohlke, J.K., and Denver, J.M., 1995, Combined use of 

groundwater dating, chemical and isotopic analyses to 

resolve the history and fate of nitrate contamination in 

two agricultural watersheds, Atlantic coastal plain, 

Maryland: Water Resources Research, vol. 31, no. 9, 

p. 2319-2339.

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

Methods for collection and analysis of water samples 

for dissolved minerals and gases, laboratory analysis: 

U.S. Geological Survey Techniques of Water Resources 

Investigations, book 5, chap. C1, 160 p.

Busenberg, E. and Plummer, L.N., 1992, Use of chlorofluoromethanes (CCl3F and CCl2F2) as hydrologic tracers 

and age-dating tools: Example- the alluvium and 

terrace system of central Oklahoma: Water Resources 

Research, vol., 28, p. 2257-2283.

Busenberg, E., Weeks, E., Plummer, L.N., and Bartholemay, 

R.C., 1993, Age dating ground water by use of chlorofluorocarbons (CCl3F and CCl2F2), and distribution of 

chlorofluorocarbons in the unsaturated zone, Snake 

River Plain aquifer, Idaho National Engineering Laboratory, Idaho: U.S. Geological Survey Water Resources 

Investigations Report 93-4054, 47 p.

Bush, P.W., and Johnston, R.H., 1988, Ground-water 

hydraulics, regional flow, and ground-water development of the Floridan aquifer system in Florida and parts 

of Georgia, South Carolina, and Alabama. U.S. Geological Survey Professional Paper 1403-C, 80 p.

Coplen, T.B., 1994, Reporting of stable hydrogen, carbon, 

and oxygen isotopic abundances: Pure and Applied 

Chemistry, vol. 66, p. 273-276.

Craig, H., 1961, Isotopic variations in meteoric waters: 

Science, vol. 133, p. 1702-1703.

Crane, J.J., 1986, An investigation of the Geology, Hydrogeology, and Hydrochemistry of the Lower Suwannee 

River Basin: Florida Geological Survey Report of 

Investigations No. 96: 205 p.

Dansgaard, W., 1964, Stable isotopes in precipitation: Tellus, 

vol. 16, p. 436-468.

Deines, P., 1980, The carbon isotope composition of reduced 

organic carbon, in Fritz, P., and Fontes, J.C., eds., Handbook of Environmental Isotope Geochemistry, vol. 1A, 

Elsevier, Amsterdam, p. 329-406.

Dietrich, P.G., and Hebert, D., 1997, Regional discharge of a 

Triassic artesian karst aquifer: Mixing and age of spring 

waters in the Thuringian basin, Germany, estimated by 

isotope methods, in Gunay, G., and Johnson, A.I., eds., 

Karst Waters and Environmental Impacts, A.A. 

Balkema, Rotterdam, p. 221-229.

Farm Chemicals Handbook, 1998, Vol. 84: Meister Publishing Company, Willoughby, Ohio, p. 1-446.

Faulkner, G.L., 1973, Geohydrology of the Cross-Florida 

Barge Canal area with special reference to the Ocala 

vicinity: U.S. Geological Survey Water-Resources 

Investigations, 73-1, 117 p.

Focazio, M.J., Plummer, L.N., Bohlke, J.K., Busenberg, E., 

Bachman, L.J., and Powars, D.S., 1998, Preliminary 

estimates of residence times and apparent ages of 

ground water in the Chesapeake Bay Watershed, and 

water-quality data from a survey of springs: U.S. 

Geological Survey Water-Resources Investigations 

Report 97-4225, 75 p.

Gonfiantini, R., 1981 , The δ-notation and the massspectrometric measurement technique, in J.R. Gat, 

and R. Gonfiantini, eds., Stable isotope hydrology: 

Deuterium and oxygen-18 in the water cycle, chap. 4, 

p. 35-84, International Atomic Energy Agency, Vienna, 

Austria.

Grubbs, J.W., 1998, Recharge rates to the Upper Floridan 

aquifer in the Suwannee River Water Management 

District, Florida: U.S. Geological Survey Water 

Resources Investigations Report 97-4283, 30 p.

Hassan, A.A., 1982, Methodologies for extraction of dissolved inorganic carbon for stable carbon isotopes 

studies: Evaluation and alternatives: U.S. Geological 

Survey Water-Resources Investigations Report 82-6, 

51 p.

Hatzell, H.H., 1995, Effects of waste-disposal practices on 

ground-water quality at five poultry (broiler) farms in 

north-central Florida, 1992-93: U.S. Geological Survey 

Water-Resources Investigations Report 95-4064, 35 p.

Heaton, T.H.E., and Vogel, J.C., 1981, Excess air in groundwater: Journal of Hydrology, vol. 50, p. 201-216.
52 Sources and Chronology of Nitrate Contamination in Spring Waters, Suwannee River Basin, Florida

Heaton, T.H.E., 1986, Isotopic studies of nitrogen pollution 

in the hydrosphere and atmosphere: A review: Chemical Geology, vol. 59, p. 87-102.

Helsel, D.R., and Hirsch, R.M., 1992, Statistical methods in 

water resources: Studies in Environmental Science 49, 

Elsevier, Amsterdam, 522 p.

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

characteristics of natural water: U.S. Geological Survey 

Water-Supply Paper 2254, 263 p.

Hornsby, H.D., 1994, The use of δ15N to identify non-point 

sources of nitrate-nitrogen beneath different land uses: 

Gainesville, University of Florida, Master’s Thesis, 

58 p.

Hornsby, D., and Ceryak, R., 1999, Springs of the Suwannee 

River Basin in Florida: Suwannee River Water Management District Report WR99-02, 178 p.

Hornsby, H.D., and Mattson, R., 1996, Surface water quality 

and biological monitoring network: Suwannee River 

Water Management District Annual Report WR-96-02, 

130 p.

Hornsby, D., and Mattson, R., 1998, Surface water quality 

and biological monitoring network: Suwannee River 

Water Management District Annual Report WR-98-02.

International Atomic Energy Agency, 1981, Statistical treatment of environmental isotope data in precipitation: 

Technical Report Series No. 206, 256 p.

Jacob, J.P., and Mather, F.B., 1997, Florida’s commercial 

broiler industry: Gainesville, University of Florida, 

Florida Cooperative Extension Service, Institute of 

Food and Agricultural Sciences Fact Sheet PS-20, 5 p.

Jacobs, R.D., Sloan, D.R., and Jacob, J.P., 1996, Cage layer 

manure: An important resource for land use: Gainesville, University of Florida, Florida Cooperative Extension Service, Institute of Food and Agricultural 

Sciences Fact Sheet PS-9, 2 p.

Jenssen, P.D., and Siegrist, R.L., 1988, Nitrogen removal 

from wastewater in soil infiltration systems, in Fjerning 

Av Nitrogen I Avlopsvann, Technical University of 

Trondheim, Trondheim, Norway.

Jones, D.W., Freeman, C.E., Johnson, J.T., and Whitty, E.B., 

1974, Fertilizer recommendations for agronomic crops 

in Florida: Soil and Crop Science Society of Florida, 

vol. 33, p. 43-45.

Katz, B.G., 1992, Hydrochemistry of the Upper Floridan 

aquifer in Florida. U.S. Geological Survey Water 

Resources Investigations Report 91-4196, 37 p. 

Katz, B.G., Catches, J.S., Bullen, T.D., and Michel, R.L., 

1998, Changes in the isotopic and chemical composition of ground water resulting from a recharge pulse 

from a sinking stream: Journal of Hydrology, vol. 211, 

p. 178-207.

Katz, B.G., DeHan, R.S., Hirten, J.J., and Catches, J.S., 

1997, Interactions between ground water and surface 

water in the Suwannee River Basin, Florida: Journal of 

the American Water Resouces Association, vol. 33, 

no. 6, p. 1-18. 

Katz, B.G., Lee, T.M., Plummer, L.N., and Busenberg, E., 

1995a, Chemical evolution of groundwater near a sinkhole lake, northern Florida: 1. Flow patterns, age of 

groundwater, and influence of lakewater leakage: Water 

Resources Research, vol. 31, no. 6, p. 1549-1564.

Katz, B.G., Plummer, L.N., and Busenberg, E., Revesz, 

K.M., Jones, B.F., and Lee, T.M., 1995b, Chemical 

evolution of groundwater near a sinkhole lake, northern 

Florida: 2. Chemical patterns, mass-transfer modeling, 

and rates of chemical reactions: Water Resources 

Research, vol. 31, no. 6, p. 1565-1584. 

Kay, F., and Hammond, C., 1985, Land application of livestock and poultry manure: Athens, Cooperative Extension Service, The University of Georgia College of 

Agriculture, Leaflet 378, 4 p.

Kendall, C., 1998, Tracing nitrogen sources and cycling in 

catchments, in Kendall, C., and McDonnell, J.J., eds., 

Isotope tracers in catchment hydrology, Elsevier 

Science B.V., Netherlands, chap. 16, p. 519-576.

Kidder, G., Hanlon, E.A., and Chambliss, C.G., 1998, UF/

IFAS standard fertilization recommendations for 

agronomic crops: Gainesville, University of Florida, 

Florida Cooperative Extension Service, Institute of 

Food and Agricultural Sciences SL-129, 7 p.

Kincaid, T.R., 1998, River water intrusion to the unconfined 

Floridan aquifer, Environmental and Engineering Geoscience, vol. 4, no. 3, p. 361-374.

Kincaid, T.R., 1999, Morphologic and fractal characterization of saturated karstic caves: University of Wyoming, 

Ph.D. dissertation, 174 p.

Koterba, M.T., Wilde, F.D., and Lapham, W.W., 1995, 

Ground-water-data-collection and documentation of 

water-quality samples and related data: U.S. Geological 

Survey Open-File Report 95-399, 113 p.

Maddox, G.L., J.M. Lloyd, T.M. Scott, S.B. Upchurch, and 

R. Copeland, eds., 1992, Florida’s Ground-Water 

Quality Monitoring Network Program: Background 

Hydrogeochemistry, Florida Geological Survey Special 

Publication no. 34, 364 p.

Maloszewski, P., and Zuber, A., 1982, Determining the 

turnover time of groundwater systems with the aid of 

environmental tracers 1. Models and their applicability: 

Journal of Hydrology, vol. 57, p. 207-231.

Michel, R.M., 1989, Tritium deposition in the continental 

United States 1953-1983: U.S. Geological Survey 

Water Resources Investigations Report 89-4072, 46 p.

Mitchell, W.B., Guptill, S.C., Anderson, K.E., Fegeas, R.G., 

and Hallam, C.A., 1977, GIRAS--A geographic information retrieval and analysis system for handling land 

use and land cover data: U.S. Geological Survey 

Professional Paper 1059, 16 p.
References 53

Mueller, D.K., and Helsel, D.R., 1996, Nutrients in the 

Nation’s waters-Too much of a good thing?: U.S. 

Geological Survey Circular 1136, 24 p.

Otis, R.J., Anderson, D.L., and Apfel, R.A., 1993, Onsite 

sewage disposal system research in Florida: Report prepared for Florida Department of Health and Rehabilitative Services, Contract No. LP-596, Ayres Associates, 

Tampa, Florida, 157 p.

Owenby, J.R., and Ezell, D.S., 1992, Monthly station 

normals of temperature, precipitation, and heating and 

cooling degree days, 1961-90: U.S. Department of 

Commerce, National Oceanic and Atmospheric 

Administration, Climatography of the United States 

No. 81, 26 p.

Pittman, J.R., Hatzell, H.H., Oaksford, E.T., 1997, Spring 

contributions to water quantity and nitrate loads in the 

Suwannee River during base flow in July 1995: U.S. 

Geological Survey Water-Resources Investigations 

Report 97-4152, 12 p.

Plummer, L.N., and Busenberg, E., 1999, Chlorofluorocarbons, in Cook, P.G., and Herczeg, A., eds., Environmental Tracers in Subsurface Hydrology, chap. 15, 

Kluwer Academic Press. 

Plummer, L.N., Busenberg, E., McConnell, J.B., Drenkard, 

S., Schlosser, P., and Michel, R.L., 1998a, Flow of river 

water into a karstic limestone aquifer, 1. Tracing the 

young fraction in groundwater mixtures in the Upper 

Floridan aquifer near Valdosta, Georgia: Applied 

Geochemistry, vol. 13, no. 8, p. 995-1015.

Plummer, L.N., Busenberg, E., Drenkard, S., Schlosser, P., 

Ekwurzel, B., Weppernig, R., McConnell, J.B., and 

Michel, R.L., 1998b, Flow of river water into a karstic 

limestone aquifer, 2. Dating the young fraction in 

groundwater mixtures in the Upper Floridan aquifer 

near Valdosta, Georgia: Applied Geochemistry, vol. 13, 

no. 8, p. 1017-1043.

Plummer, L.N., Michel, R.L., Thurman, E.M., and Glynn, 

P.D., 1993, Environmental tracers for age-dating young 

ground water, in Regional Ground-Water Quality, 

W.A. Alley, ed., Van Nostrand Reinhold, New York, 

p. 255-294.

Plummer, L.N., Prestemon, E.C., and Parkhurst, D.L., 1994, 

An interactive code (NETPATH) for modeling net 

geochemical reactions along a flow path: U.S. Geological Survey Water-Resources Investigations Report 91-

4169, 130 p.

Reilly, T.E., Plummer, L.N., Phillips, P.J., and Busenberg, E., 

1994, The use of simulation and multiple environmental tracers to quantify groundwater flow in a shallow 

aquifer: Water Resources Research, vol. 30, p. 421-433.

Richter, J., Szymczak, P., Abraham, T., and Jordan, H., 1993, 

Use of combinations of lumped parameter models to 

interpret groundwater isotope data: Journal of Contaminant Hydrology, vol. 14, p. 1-13.

Rosenau, J.C., Faulkner, G.L., Hendry, C.W., Jr., and Hull, 

R.W., 1977, Springs of Florida: Florida Bureau of 

Geology Bulletin No. 31, 461 p.

Sloan, D.R., Kidder, G., and Jacobs, R.D., 1992, Poultry 

manure as a fertilizer: Gainesville, University of 

Florida, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences Fact Sheet PS-1, 

2 p.

Solomon, D.K., and Sudicky, E.A., 1991, Tritium and 

helium-3 isotope ratios for direct estimation of spatial 

variations in groundwater recharge: Water Resources 

Research, vol. 27, p. 2309-2319.

Spechler, R.M., and Schiffer, D.M., 1995, Springs of Florida: U.S. Geological Survey Fact Sheet FS-151-95, 2 p.

Thatcher, L.L, 1962, The distribution of tritium fallout in 

precipitation over North America: Bulletin International Association of Hydrological Sciences, vol. 7, 

p. 48-58.

Thatcher, L.L, Janzer, V.J., and Edwards, K.W., 1977, 

Methods for determination of radioactive substances in 

water and fluvial sediments, in Techniques of WaterResources Investigations of the U.S. Geological 

Survey, Chapter A5, p. 79-81.

Toth, D.J., 1999, Water quality and isotope concentrations 

from selected springs in the St. Johns River Water Management District: St. Johns River Water Management 

District Technical Publication SJ99-2, 67 p.

Toth, J., 1963, A theoretical analysis of groundwater flow in 

small drainage basins: Journal of Geophysical 

Research, vol. 68, no. 16, p. 4795-4812.

U.S. Bureau of Census, 1990, Census of population and 

housing: Bureau of the Census, Washington, D.C.

U.S. Department of Commerce, 1946-97, Agriculture census 

data for Florida, 1945, 1954, 1959, 1964, 1969, 1974, 

1979, 1984, 1989, 1994: U.S. Department of Commerce, Bureau of the Census, Washington, D.C.

U.S. Bureau of the Census, 1993, Census of population and 

housing: Bureau of the Census, Washington, D.C.

U.S. Geological Survey, 1999, Comprehensive Quality 

Assurance Plan, Quality of Water Service Unit, Ocala, 

Florida, 144 p.

University of Wisconsin, 1978, Management of small wastewater flows: U.S. Environmental Protection Agency 

Report No. EPA-600/7-78-173, U.S. EPA, Cincinnati, 

Ohio.

Van Horn, H.H., Nordstedt, R.A., Bottcher, A.V., Hanlon, 

E.A., Graetz, D.A., and Chambliss, C.F., 1991, Dairy 

manure management: Strategies for recycling nutrients 

to recover fertilizer value and avoid environmental pollution: Gainesville, University of Florida, Florida 

Cooperative Extension Service, Institute of Food and 

Agricultural Sciences Circular 1016, Florida, 16 p.
54 Sources and Chronology of Nitrate Contamination in Spring Waters, Suwannee River Basin, Florida

VanDyne, D.L., and Gilbertson, C.B., 1978, Estimating U.S. 

livestock and poultry manure and nutrient production: 

U.S. Department of Agriculture Report ESCS-12, 

150 p.

Vest, L., and Merka, W., 1994, Poultry waste: Georgia’s 

50 million dollar forgotten crop: University of Georgia 

College of Agricultural and Environmental Sciences, 

Cooperative Extension Service Leaflet 206, 11 p.

Vogel, J.C., 1967, Investigation of groundwater flow with 

radiocarbon: Isotopes in Hydrology, International 

Atomic Energy Agency, Vienna, Austria, p. 355-368.

Weiss, R.F., 1970, The solubility of nitrogen, oxygen, and 

argon in water and seawater: Deep Sea Research, v. 17, 

p. 721-735.

Wells, E.R., and Krothe, N.C., 1989, Seasonal fluctuation in 

δ15N of groundwater nitrate in a mantled karst aquifer 

due to macropore transport of fertilizer-derived nitrate: 

Journal of Hydrology, v. 112, p. 191-201.

Wood, W.W., 1976, Guidelines for collection and field analysis of ground-water samples for selected unstable constituents: U.S. Geological Survey Techniques of Water 

Resources Investigations, book 1, chap. D2, 24 p.

Zuber, A., 1986, Mathematical models for the interpretation 

of environmental radioisotopes in groundwater systems, in Fritz, P., and Fontes, J.C., eds., Handbook of 

Environmental Geochemistry, vol. 2, The terrestrial 

environment: New York, Elsevier Publishing Company, 

p. 1-59.